Handbook for Critical Cleaning

Handbook for

CRITICAL CLEANING Barbara Kanegsberg Associate Editor Edward Kanegsberg Editor-in-Chief

CRC Press Boca Raton London New York Washington, D.C. © 2001 by CRC Press LLC

Library of Congress Cataloging-in-Publication Data Handbook for critical cleaning / Barbara Kanegsberg, editor ; Edward Kanegsberg, associate editor. p. cm. Includes bibliographical references and index. ISBN 0-8493-1655-3 (alk. paper) 1. Integrated circuits--Cleaning. 2. Manufacturing processes. 3. Electronic packaging. 4. Surface preparation. 5. Surface chemistry. I. Kanegsberg, Barbara. II. Kanegsberg, Edward. TK7874 .H348 2000 621.381′046--dc21

00-048568 CIP

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The Editors Barbara Kanegsberg, President of BFK Solutions, is a consultant who works with components and parts manufacturers to resolve cleaning, contamination control, and environmental issues. She has over 25 years of involvement in process development. Her projects cover a range of areas of critical cleaning including metals, electronics, optics, semiconductors, motion picture film, and medical devices. She has conducted a number of product development projects for manufacturers of solvents, cleaning agents, and industrial equipment. Prior to establishing BFK Solutions, she managed the substitution of ozone-depleting chemicals at Litton Industries. Barbara has a background in biology, biochemistry, and clinical chemistry. She developed laboratory tests at BioScience Laboratories. Barbara is a recipient of the U.S. EPA Stratospheric Ozone Protection Award for her achievements in implementing effective, environmentally preferred processes. She is a member of the Editorial Advisory Board of Precision Cleaning Magazine and has been appointed to the University of Massachusetts Lowell Toxics Use Reduction Institute (TURI) Surface Cleaning Laboratory Advisory Committee. She has published extensively in surface preparation, contamination control, manufacturing process development, and regulatory issues. She has organized and participated in numerous seminars and conferences. She has a B.A. degree in Biology from Bryn Mawr College and an M.S. degree in Biochemistry from Rutgers University. She can be reached at (310) 459-3614 or by e-mail at [email protected].

Ed Kanegsberg, Ph.D., is a physicist with BFK Solutions and a member of the technical staff at Litton Guidance and Control (Woodland Hills, CA). He has over 30 years of experience in precision instrument development and technology transfer to the production facility. He addresses production yield issues through both process improvement and component modification. In addition to numerous papers and presentations, he has six patents in the area of optical instrumentation. He has a B.S. degree in Physics from MIT and a Ph.D. degree in Physics from Rutgers University. Ed can be reached at (310) 459-3614 or [email protected].

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Contributors John W. Agopovich Draper Labs

Ray A. Cull Dow Corning

David E. Albert North American Science Associates (NAmSA)

Phil Dale Layton Technologies, Ltd.

Stephen O. Andersen U.S. Environmental Protection Agency Sami B. Awad Crest Ultrasonics Mohan Balagopalan South Coast Air Quality Mgmt. District (SCAQMD) Matt Bartell Forward Technology Mark Beck Product Systems Inc. Michael Beeks Brulin Corp.

John Durkee Creative EnterpriZes Eric Eichinger Boeing Reusable Space Systems Division Max Friedheim PDQ Precision, Inc. F. John Fuchs Cleaning Technology Resources Christine Geosling Litton Guidance and Control Systems Arthur Gillman Unique Equipment

Rick Bockhorst Brulin Corp.

Don Gray University of of Rhode Island Department of Chemical Engineering

John Burke Oakland Museum Conservation Center

Ross Gustafson Florida Chemical Company, Inc.

Ahmed A. Busnaina Northeastern University

Steve R. Henly Layton PLC

Michael S. Callahan Jacobs Engineering

Barbara Kanegsberg BFK Solutions

Frank Cano Vatran Systems

David Keller Brulin Corp.

Mantosh K. Chawla Photo Emission Technologies, Inc.

Kenroh Kitamura Asahi Glass Co., Ltd.

John Chu SVC/Shipley

Jana Koran Litton Guidance and Control Systems

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Edward W. Lamm Branson Ultrasonics

Stephen P. Risotto Halogenated Solvents Industry Alliance (HSIA)

Carole LeBlanc Massachusetts Toxics Use Reduction Institite (TURI)

Reva Rubenstein U.S. Environmental Protection Agency

Joe McChesney Detrex Corp.

John F. Russo Separation Technologists

Abid Merchant DuPont Chemicals

Joe Scapelliti Detrex Corp.

Toshio Miki Asahi Glass Co., Ltd.

Ronald L. Shubkin Albemarle Corp.

William Moffat Yield Engineering Systems (YES)

P. Daniel Skelly Occidental Chemical Corp.

William M. Nelson Illinois Waste Management Research Center

Stephen P. Swanson Dow Corning

John G. Owens 3M Co. Michael Pedzy Zenith Ultrasonic, Inc. Richard Petrulio B/E Aerospace Galley Prods. Robert L. Polhamus RLP Associates Lou Rigali Ardency, Inc.

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Mahmood Toofan Semiconductor Analytical Services (SAS) Masaaki Tsuzaki Asahi Glass Co., Ltd. James L. Unmack Unmack Corp. Daniel J. VanderPyl Sonic Air Systems, Inc. Donald J. Wuebbles University of Illinois–Urbana

To our most valuable collaborative efforts: Deborah Joan Kanegsberg and David Jule Kanegsberg And to the memory and positive influence of: Israel Feinsilber Jule Kanegsberg Dr. Jacob J. Berman

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Contents Introduction What is Critical Cleaning? Barbara Kanegsberg SECTION 1: CLEANING AGENTS Chapter 1.1 Overview of Cleaning Agents Barbara Kanegsberg Chapter 1.2 Solvents and Solubility John Burke Chapter 1.3 Aqueous Cleaning Essentials Rick Bockhorst, Michael Beeks, and David Keller Chapter 1.4 Review of Solvents for Precision Cleaning John W. Agopovich Chapter 1.5 Hydrofluoroethers John G. Owens Chapter 1.6 Hydrofluorocarbons Abid Merchant Chapter 1.7 normal-Propyl Bromide Ronald L. Shubkin Chapter 1.8 Vapor Degreasing with Traditional Chlorinated Solvents Stephen P. Risotto Chapter 1.9 Volatile Methylsiloxanes: Unexpected New Solvent Technology Ray A. Cull and Stephen P. Swanson Chapter 1.10 Benzotrifluorides P. Daniel Skelly

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Chapter 1.11 HCFC-225: Alternative Precision and Electronics Cleaning Technology Toshio Miki, Masaaki Tsuzaki, and Kenroh Kitamura Chapter 1.12 d-Limonene: A Safe and Versatile Naturally Occurring Alternative Solvent Ross Gustafson SECTION 2: CLEANING SYSTEMS Chapter 2.1 Cleaning Equipment Overview Barbara Kanegsberg Chapter 2.2 The Fundamental Theory and Application of Ultrasonics for Cleaning F. John Fuchs Chapter 2.3 Ultrasonic Cleaning Mechanism Sami B. Awad Chapter 2.4 Higher-Frequency and Multiple-Frequency Ultrasonic Systems Michael Pedzy Chapter 2.5 Megasonic Cleaning Action Mark Beck Chapter 2.6 Equipment Design Edward W. Lamm Chapter 2.7 Cold and Heated Batch Solvent Cleaning Systems P. Daniel Skelly Chapter 2.8 Flushing Systems Richard Petrulio Chapter 2.9 Solvent Vapor Degreasing — Minimizing Waste Streams Joe McChesney and Joe Scapelliti Chapter 2.10 Vapor Degreaser Retrofitting Arthur Gillman

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Chapter 2.11 Enclosed Cleaning Systems Don Gray and John Durkee Chapter 2.12 Precision Cleaning and Drying Utilizing Low-Flash-Point Solvents Matt Bartell Chapter 2.13 Dense-Phase CO2 as a Cleaning Solvent: Liquid CO2 and Supercritical CO2 William M. Nelson Chapter 2.14 Carbon Dioxide Dry Ice Snow Cleaning Frank Cano Chapter 2.15 Gas Plasma: A Dry Process for Cleaning and Surface Treatment Lou Rigali and William Moffat Chapter 2.16 Super-Heated, High-Pressure Steam Vapor Cleaning Max Friedheim Chapter 2.17 Making Decisions about Water and Wastewater for Aqueous Operations John F. Russo Chapter 2.18 Overview of Drying: Drying after Solvent Cleaning and Fixturing Barbara Kanegsberg Chapter 2.19 Aqueous Parts Drying Daniel J. VanderPyl Chapter 2.20 Liquid Displacement Drying Techniques Robert L. Polhamus, Steve R. Henly, and Phil Dale SECTION 3: CONTAMINATION CONTROL; ANALYTICAL TECHNIQUES, COMPATIBILITY Chapter 3.1 How Clean Is Clean? Measuring Surface Cleanliness and Defining Acceptable Level of Cleanliness Mantosh K. Chawla

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Chapter 3.2 Contamination Control and Analytical Techniques Christine Geosling and Jana Koran Chapter 3.3 Material Compatibility Eric Eichinger SECTION 4: PROCESS SELECTION AND MAINTENANCE Chapter 4.1 Evaluating, Choosing, and Implementing the Process: How to Get Vendors to Work with You Barbara Kanegsberg Chapter 4.2 Optimizing and Maintaining the Process Michael S. Callahan SECTION 5: SPECIFIC AREAS OF CLEANING Chapter 5.1 Surface Cleaning, Particle Removal Ahmed A. Busnaina Chapter 5.2 Cleaning Metals: Strategies for the New Millennium Carole LeBlanc Chapter 5.3 Very High Performance, Complex Applications Barbara Kanegsberg Chapter 5.4 Cleaning Solutions in the Semiconductor Wafer Manufacturing Process Mahmood Toofan and John Chu Chapter 5.5 Biomedical Applications: Analytical Characterization for Biocompatibility David E. Albert SECTION 6: REGULATORY/SAFETY CONSIDERATIONS Chapter 6.1 Safety and the Environment — Some Editorial Thoughts Barbara Kanegsberg Chapter 6.2 Critical Cleaning — Working with Regulators — From a Regulator’s Viewpoint Mohan Balagopalan

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Chapter 6.3 Lessons Learned from the Phaseout of Ozone-Depleting Solvents Stephen O. Andersen Chapter 6.4 Screening Techniques for Environmental Impact of Cleaning Agents Donald J. Wuebbles and Reva Rubenstein Chapter 6.5 Health and Safety James L. Unmack SECTION 7 Glossary of Common Terms and Acronyms Contributors: Background and Contact Information

Color Figures

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Introduction What is Critical Cleaning? Barbara Kanegsberg

CONTENTS Soil Cleaning Identifying the Cleaning Operation Critical Cleaning Why Should One Be Concerned about Critical Cleaning? Performance, Reliability Costs Safety and Environmental Regulatory Requirements Overview of this book Philosophy Organization Section 1: Cleaning Agents Section 2: Cleaning Systems Section 3: Contamination Control; Analytical Techniques, Compatibility Section 4: Process Selection and Maintenance Section 5: Specific Areas of Cleaning Section 6: Regulatory/Safety Considerations Section 7: Glossary/Contributors’ Information Conclusions Acknowledgments References Critical cleaning is required for the physical manifestation of technology. We are in the information age, an age of thought, ideas, communication, but the underlying technology is based on physical objects, parts, or components. Many of these objects require precision cleaning or critical cleaning either because they are intrinsically valuable or because they become valuable in the overall system or process in which they are used. Some parts or components require critical cleaning not because of the inherent value of the part itself but instead because of their place in the overall system. For example, inadequate cleaning of a small inexpensive gasket can potentially lead to catastrophic failure in an aerospace system. © 2001 by CRC Press LLC

Nearly all companies that manufacture or fabricate high-value physical objects (components, parts, assemblies) perform critical cleaning at one or more stages. These range from the giants of the semiconductor, aerospace, and biomedical world to a host of small to medium to large companies producing a dizzying array of components. SOIL The concepts of contamination, cleaning, and efficacy of cleaning are open to debate and are intertwined with the overall manufacturing process and with the ultimate end use of the assembled product. Contamination or soil can be thought of as matter out of place.1 During manufacture, parts or components inevitably become contaminated. Contamination can come from the environment (dust, smog, skin particles, bacteria), from materials used as part of fabrication (oils, fluxes, polishing compounds), as a by-product of manufacturing, and as residue from a cleaning agent ostensibly meant to clean the component. CLEANING Cleaning processes are performed because some sort of soil must be removed. In a general sense, we can consider cleaning to be the removal of sufficient amounts of soil to allow adequate performance of the product, to obtain acceptable visual appearance as required, and to achieve the desired surface properties. You may notice that surface properties are included because most cleaning operations probably result in at least a subtle modification of the surface. If a change in the cleaning process removes additional soil and if as a result the surface acquires some undesirable characteristic (e.g., oxidation), then the cleaning process is not acceptable. Therefore, surface preparation and surface quality can be an inherent part of cleaning. IDENTIFYING THE CLEANING OPERATION Cleaning processes and the need for cleaning would seem to be trivial to identify. If you had a child who appeared in the doorway covered with mud, you would do a visual assessment of the need for cleaning and perform site-directed immersion or spray cleaning in an aqueous/saponifier mixture with hand drying. However, people perform critical cleaning operations without knowing it. This lack of understanding can be detrimental to process control and product improvement. Recognizing a cleaning step when it occurs is probably one of the major challenges in the components manufacturing community. Cleaning is often enmeshed as a step in the overall process rather than being recognized as a concept in itself. It may be considered as something that occurs before or after another process, but not as a process to be optimized on its own.2 A cleaning process is often not called a cleaning process. For example, optics deblocking (removing pitches and waxes), defluxing, degreasing, photoresist stripping, edge bead removal in wafer fabrication, and surface preparation prior to adhesion, coating, or heat treatment can all be thought of in terms of soil removal (cleaning). Sometimes the cleaning process is identified only by the name of the engineer who first introduced it. The sociological and psychological bases for this aversion to discussing cleaning are no doubt fascinating, but are beyond the scope of this book. The important thing is for you to recognize a cleaning process when you see it. © 2001 by CRC Press LLC

There are several reasons. One obvious reason is process control. A second is troubleshooting or failure analysis. If the product fails and you need to fix the process, it is crucial to identify not only where soil might be introduced but also what steps are currently being taken in soil removal. If the chemical being used comes under regulatory scrutiny, identifying cleaning is even more important. If a supplier provides the component and a problem arises, it is important to be able to recognize where the cleaning steps occur. Finally, identifying the cleaning steps allows you to apply technologies developed in other industries to your own process. CRITICAL CLEANING Defining critical cleaning or precision cleaning is a matter of ongoing debate among chemists, engineers, production managers, and those in the regulatory community. Certainly the perceived value or end use of the product is a factor, as are the consequences of remaining soil. The level of allowable soil remaining after cleaning is a consideration. Precision cleaning has been defined as the removal of soil from objects that appear to be clean in the first place.3 In some instances, however, high levels of adherent soil are involved in the processing of critical devices. Precision cleaning was once euphemistically said to be YOUR cleaning process for YOUR critical application, whereas everyone else’s process could be considered as general cleaning.4 In one sense, there is some truth that the manufacturer is often the best able to understand process criticality. At the same time, recognizing general cleaning and critical cleaning as parts of other operations can lead to overall industrial process improvement. WHY SHOULD ONE BE CONCERNED ABOUT CRITICAL CLEANING? Critical cleaning issues are becoming increasingly important. Competitive pressure is increasing. Higher demands are being placed on industry. A clean component produced efficiently and in an environmentally preferred manner (or at least in an environmentally acceptable manner) is a given in today’s economy. Performance, Reliability Products are becoming smaller, with tighter tolerances, and higher performance standards. Some products, such as implantable biomedical devices, are expected to perform for decades without a breakdown. Small amounts of soil and very tiny particles can irreparably damage the product. To remove the soils successfully, you have to understand the various cleaning chemistries and cleaning equipment, and how they are combined and meshed with the overall build process. Costs Pressure to keep costs down increases constantly. The costs of effective processes have tended to increase. Choosing the best option for the application can keep costs down.

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Safety and Environmental Regulatory Requirements The manufacturing community needs a wide selection of chemicals and processes to achieve better contamination control at lower costs. However, our understanding of health and the environment has led to restrictions on chemicals and processes. In order to foresee future trends, the manufacturer needs an understanding of atmospheric science and of the approaches used by regulatory agencies. OVERVIEW OF THIS BOOK Philosophy In setting out to put together this comprehensive book on critical cleaning, the editor sought inputs from the experts in the field. Frequently, these are people associated with vendors of cleaning equipment and/or cleaning agents. Naturally, each person’s viewpoint is somewhat colored by his or her own portion of the market. However, on the whole, the scope and fairness of the material submitted was impressive. An attempt has been made to minimize use of brand names. In some cases, there are several contributors in a similar area. In general, the editorial philosophy has been to include all but the most blatantly commercial material. By having a large number of contributors, a wide range of products and viewpoints are presented; the reader is expected to weigh the advantages and/or disadvantages of each approach for his or her own application. Organization This book is organized into seven sections: • • • • • • •

Cleaning Agents Cleaning Systems Contamination Control, Analytical Techniques, and Compatibility Process Selection and Maintenance Specific Areas of Cleaning Regulatory/Safety Considerations Glossary/Contributors’ Information/Index

Following is a capsule summary of each of the book chapters. Section 1: Cleaning Agents An overview of cleaning agents is presented by the editor, Barbara Kanegsberg. Included in this are agents that are discussed in detail by other authors as well as some that are not. John Burke, from the Oakland (California) Museum of Art, presents a scholarly discourse on solubility and the techniques used to classify solvents. It becomes clear why certain solvents are applicable to removing certain types of soil. Water, the most common cleaning agent, is discussed first. Rick Bockhorst, Michael Beeks, and David Keller from the Brulin Corporation, a producer of aqueous cleaning equipment, give a comprehensive review of aqueous cleaning essentials. Many of the subjects discussed are also applicable to nonaqueous solvent cleaning. John W. (Bill) Agopovich from the C. S. Draper Laboratories presents a detailed © 2001 by CRC Press LLC

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overview of solvents, discussing requirements, methods, and environmental issues, as well as reviewing the different types of available solvents. John Owens from 3M thoroughly presents the hydrofluoroethers (HFEs), a class of solvents that has been introduced in the past few years as replacements for ozone-layerdepleting chemicals (ODCs). Abid Merchant from DuPont has a full discussion of the hydrofluorocarbons (HFCs) another class of recently introduced ODC replacements. Ron Shubkin of Albermarle Corporation gives particulars on normal-propyl bromide (nPB), a recently introduced substitute for the aggressive ODC solvent, 1-1-1-trichloroethane. Steve Risotto from the Halogenated Solvents Industry Association (HSIA) presents a detailed discussion of the chlorinated solvents, a group of traditional solvents that is seeing a resurgence of use in certain applications. Ray Cull from Schneller Inc. and Steve Swanson from Dow Corning have an informative presentation of volatile methylsiloxanes (VMSs), a group of silicon-based chemicals that have been found to have cleaning abilities in a number of areas. Dan Skelly from Occidental Chemical Corp. has a comprehensive chapter on the benzotrifluorides, a group of potentially useful solvents. Toshio Miki, Masaaki Tsuzaki, and Kenroh Kitamura, and all from Asahi Glass, give a good overview of HCFC-225, another solvent with utility for electronics and other precision cleaning applications. Ross Gustafson from Florida Chemical ends the cleaning agent section by specifying critical cleaning uses of the orange peel–derived d-limonene.

Section 2: Cleaning Systems The cleaning systems section is the largest, at least in terms of numbers of chapters, which reflects the wide range of process choices. Drying, an important aspect of cleaning processes, is covered in this section as are those systems, such as CO2 cleaning, where the cleaning agent and the cleaning equipment are inseparable. Barbara Kanegsberg gives an overview of cleaning systems. As with the overview for cleaning agents, this reviews processes that are treated in this book by other authors, as well as those for which there are not dedicated chapters. There are four chapters dealing with ultrasonics and the closely related megasonics technologies. John Fuchs of Cleaning Technology Resources and Sami Awad of Crest Ultrasonics each present an informative overview of ultrasonics. This is such a complex and widely used technology that it was felt that both authors had useful insight. Michael Pedzy of Zenith Ultrasonics expands on this by discussing higher-frequency and multiplefrequency ultrasonics. Mark Beck of Prosys introduces the interesting and efficacious technology of megasonics. Edward Lamm from Branson provides a practical chapter on optimizing the equipment design, covering solvent, aqueous, and semiaqueous cleaning equipment as well as rinsing, drying, automation, and other ancillary equipment. Dan Skelly of Occidental Chemical Corp. in addition to the chapter on benzotrifluorides, has written a useful chapter on equipment for cold and heated batch solvent cleaning, i.e., where cleaning agents are used below their boiling point. Richard Petrulio of B/E Aerospace contributes a very readable chapter on the design of flushing systems. This is one example where a company was able to design equipment for its own cleaning application. © 2001 by CRC Press LLC

Joe McChesney and Joe Scapelliti of Detrex Corporation discuss important techniques for minimizing waste streams in solvent vapor degreasers. Included are methods for calculating the size or capacity of the required equipment. Art Gillman of Unique Equipment talks of retrofitting vapor degreasers to allow use of different cleaning chemicals or to meet newer emission-control standards. This is an option that sometimes avoids major capital expense. Don Gray from the University of Rhode Island and John Durkee of Creative EnterpriZes present an informative chapter on contained airless and airtight solvent systems, one approach to using costly or heavily regulated solvents. Matt Bartell of Forward Technology lucidly writes about low-flash-point cleaning systems, an option that extends vapor degreasing to flammable chemicals. In some cases, the cleaning agent and the cleaning equipment are inseparable. Several examples are provided in the next four chapters. Bill Nelson from the Hazardous Waste Research and Information Center (HWRIC), University of Illinois explains the theory and application of supercritical and liquid CO2 cleaning. Frank Cano of Va-tran Systems clearly describes the use of carbon dioxide in the solid form, CO2 snow cleaning, a gentle approach for light soil loads. Lou Rigali of Ardency, Inc. and Bill Moffat of Yield Environmental Systems (YES) discuss an important solvent-free approach to removing organics, plasma cleaning. Max Friedheim of PDQ Mini-Max presents interesting uses of steam vapor cleaning to critical cleaning applications. After you have cleaned, there is the question of disposing of the cleaning agent. John Russo of Separation Technologies discusses selecting the best waste water treatment for aqueous operations. This comprehensive chapter also discusses pretreatment and water recycling techniques. Cleaning with liquids frequently means that drying is part of the process. The final three chapters in the Cleaning Systems section deal with this sometimes neglected process. Barbara Kanegsberg provides an overview of drying. Dan VanderPyl of Sonic Air Systems details physical methods of drying. Bob Polhamus of RLP Associates with Steve Henly and Phil Dale of Layton Engineering clearly present chemical displacement drying techniques. Section 3: Contamination Control, Analytical Techniques, and Compatibility As important as cleaning is knowing when to clean and knowing when the part is clean enough. Also important is knowing whether what you are using to clean with is compatible with your parts. The four chapters in this section address these topics. Mantosh Chawla of Photo Emission Technology (PET) discusses the important topic of measuring surface cleanliness and defining acceptable level of cleanliness. Christine Geosling and Jana Koran of Litton Guidance and Control Systems provide an informative explanation of contamination control, analytical techniques, and clean room standards and practices. Eric Eichinger of Boeing North America concludes with a thoughtful discussion of the critical issue of materials compatibility both for metals and nonmetals. Section 4: Process Selection and Maintenance This overview section contains two chapters. Barbara Kanegsberg discusses evaluating, choosing, and implementing the process, including productive interaction with vendors. This has previously been presented under the title “How to Survive a Trade Show.” Techniques for information gathering and communication with suppliers are emphasized. Mike Callahan from Jacobs Engineering provides valuable advice on optimizing and main© 2001 by CRC Press LLC

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taining the process. A number of topics such as fixturing, process monitoring, and process improvements are discussed. Section 5: Specific Areas of Cleaning A number of specific applications are presented in this section. Ahmed Busnaina from Northeastern University explains particle removal. This is an effective treatment of surface physics presented in a readable and understandable manner. Carole LeBlanc of the Toxics Use Reduction Institute (TURI) at the University of Massachusetts presents a study of aqueous cleaning of metals, which outlines the benefits of developing practical, industrially oriented studies in the academic community and provides valuable and logical guidelines for the individual manufacturer. Barbara Kanegsberg of BFK Solutions discusses very high performance and biomedical applications. Specific examples including aerospace and electronics are given. John Chu SVC and Mahmood Toofan of SAS provide a thoughtful presentation of challenges faced by the semiconducting wafer fabrication industry. David Albert of NAMSA concludes this section with a lucid explanation of analytical characterization for biocompatibility in biomedical applications. This is a must for anyone involved in biomedical-related products. Section 6: Regulatory and Safety Considerations Cleaning almost always is involved with using materials or processes with concerns about environmental or exposure effects. Dealing with regulations and regulators has become part of the overall picture. This final section treats a number of these issues. Barbara Kanegsberg provides an overview of the safety and environmental issues. Mohan Balagopalan of Southern California’s South Coast Air Quality Management District (SCAQMD) presents a thoughtful section on working with regulators—from a regulator’s viewpoint. Steve Andersen from the U.S. EPA rationally discusses how industry and government can work together. He presents lessons learned from the ozone-depleting chemical (ODC) phaseout. Don Wuebbles from the University of Illinois and Reva Rubenstein of the U.S. EPA teamed effectively to submit a readable, informative overview of screening techniques for the environmental impact of cleaning agents, with implications for industry. Last, Jim Unmack of Unmack Corporation Services outlines critical health and safety aspects associated with cleaning processes. Section 7: Glossary/Contributors’ Information/Index This supporting section contains a glossary of many of the commonly used terms and acronyms. An alphabetical list of contributing authors with their backgrounds and contact information is also included. CONCLUSIONS A diverse assortment of components and assemblies requires critical or precision cleaning. Some examples include: Accelerometers Automotive parts Biomedical/surgical/dental devices (e.g., pacemakers) © 2001 by CRC Press LLC

Bearings Computer hardware (metal, plastic, other composites—the insides of your computer and printer) Consumer hardware (telephones) Digital cameras Disk drives Electronics components Flat panel displays Gaskets Gyroscopes Motion picture film Optics Space exploration hardware Wafers/semiconductors/microelectronics Weapons, defense systems (missiles) While each application is very site specific, contamination control problems cut across industry lines. At the same time, each industry still tends to work in a separate world. It is hoped that this book will provide a synthesis of cleaning approaches.

ACKNOWLEDGMENTS This book is the result of a phenomenal level of effort by those involved in the worlds of critical cleaning, surface preparation, and environmental issues. The information, expertise, and guidance provided by the contributing authors is invaluable. Dr. Ed Kanegsberg, business associate and spouse, provided support, encouragement, and invaluable participation in the editing process. He also provided the viewpoint and experiences of a physicist and practicing engineer. Bob Stern and the staff at CRC Press provided excellent guidance throughout the process. I would also like to thank family members Deborah Kanegsberg, David Kanegsberg, Ruth Feinsilber, and Mimi and Murray Steigman for their patience and encouragement. Finally, I would like to thank Dr. Shelley Ventura-Cohen, a wise colleague and adviser She tells the story of her grandmother, who, on observing Shelley staring blankly at a cookbook while an inert, raw chicken sat on the counter, exclaimed: Look at the chicken, not the book. Dear reader, critical cleaning, surface preparation, and contamination control are complex subjects, but they are also intensely practical subjects that relate to a product— your product. My suggestion, therefore, is to look at this book, and at the same time look at the chicken. REFERENCES 1. Petrulio, R. and B. Kanegsberg, Back to basics: the care and feeding of a vapor degreaser with new solvents, in Proc. Nepcon West ‘98, Anaheim, CA, 1998. 2. Rosa, D., A2C2 Magazine, personal communication. 3. LeBlanc, C., Toxics Use Reduction Institute, personal communication. 4. Kanegsberg, B., Options in the high-precision cleaning industry: overview of contamination control working group XIII, presented at International CFC & Halon Alternatives Conference, Washington, D.C., October, 1993.

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SECTION 1

Cleaning Agents

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CHAPTER 1.1

Overview of Cleaning Agents Barbara Kanegsberg

CONTENTS Introduction Aqueous Cleaning Agents Simple Additives Commercial Blends, Aqueous Water-Soluble Organics Solvent-Based Cleaning Agents Classic Organic Solvents Restricted or Low Availability Ozone-Depleting Chemicals Solvents on the Horizon Oxygenated Solvents Esters Hydrocarbon Blend (Mineral Spirits), Soy-Derived Cleaning Agents d-Limonene Blends, α-Pinene Blends Cyclic Volatile Methyl Siloxanes Perfluorinated Compounds Solvent Blends, Azeotropes, Cosolvents Stabilization Extending the Solvency Range or Moderating the Solvency Cosolvents Surfactants Emulsions: Macroemulsions, Structured Solvents, Microemulsions Mystery Mixes Solvency and Physical Properties; Other Parameters Kauri-Butanol Number Wetting Index Other Physical Properties; Regulatory Features Costs Overall Considerations References

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INTRODUCTION Choosing the cleaning agent appropriate to the job at hand can be a traumatic experience, even for those with some background in chemistry. While the cleaning agent section of this book highlights a number of different types of cleaning agents, it is by no means exhaustive. This chapter provides an overview of cleaning agents and highlights a few additional cleaning agents not discussed in other chapters. Cleaning agents are generally divided into aqueous and solvent. When most people involved in cleaning refer to the term solvent, they actually mean organic solvent. Organic solvents are not those processed from, say, free-range, herbicide-free lemons. Instead, organic refers to materials that have the element carbon in them. However, when you think about it, both organic and aqueous-based cleaning agents can be thought of as solvents. If you dissolve sugar in tea, the water is acting as the solvent; the sugar is the solute. Because many oils are carbon based, organic solvents have been classically used for very heavy degreasing jobs. However, other mechanisms such as saponification (as described in the chapter covering aqueous cleaning agents) have been successfully used to lift heavy grease off parts. Certainly, water-based cleaning is widely used; most of us have successfully removed oils and greases from dishes using semiautomated aqueous cleaning, not a vapor degreaser. Although a segment of the industry has traditionally been carried out by aqueous cleaning, until the last 10 to 15 years, most cleaning was conducted using classic chlorinated solvents or with ozone-depleting compounds (ODCs). In fact, many products were designed specifically around the solvency properties of CFC-113 and 1,1,1-trichloroethane (TCA). The loss of ODCs has led to upheaval in the manufacturing world; and, because there are no true drop-in substitutes for ozone depleters, the market has fragmented.1 This fragmentation has continued for a number of reasons, including: • • • • • • • •

Lack of understanding of how to use the new methods Dissatisfaction with selected new approaches High cleaning costs Development of new solvent and aqueous blends Increasingly stringent and ever-changing regulatory conditions Differences in regulations in various geographic locations Increasingly stringent cleaning and performance requirements product miniaturization

To make matters even more complex, the line between cleaning agent and cleaning equipment or cleaning is often blurred. Sometimes the cleaning agent is generated in use, for example CO2 (solid, liquid, and supercritical) and plasma. Where solid or abrasive media2 are used, in a sense, they can also be considered as the cleaning agent. No one product or class of products is likely to satisfy all cleaning requirements. The cleaning agent must be matched to the soil, the substrate (the component or part to be cleaned), the cleaning requirements, drying requirements, and other performance and environmental constraints. Inorganic soils are often referred to as hydrophilic; they dissolve effectively in water. Organic-based soils, often referred to as hydrophobic, tend to dissolve more effectively in organic solvents. The choice of cleaning agent should, ideally, be based on technical considerations. Unfortunately, chemists, engineers, production people, and those involved in regulatory agencies may themselves become hydrophilic or hydrophobic. Although we all have cleaning agent prejudices, irrationally ruling out one class of cleaning agents can result in inadequate cleaning and can be economically and ecologically detrimental. © 2001 by CRC Press LLC

Another reason to keep an open mind and to try to understand both approaches is that the line between aqueous and organic cleaning tends to blur, so even if you and your firm are unalterably devoted to organic solvents, it will be helpful to learn about aqueous cleaning, and vice versa. Some inert organic cleaners are blended with small amounts of surfactants to improve removal of soils. Many aqueous cleaners contain significant amounts of organic additives. Some may be basically blends of water-soluble organic compounds. In fact, certain similar organic solvent blends may be classified as semi-aqueous or cosolvent only in that very small formulation differences allow for rinsing in water (for semiaqueous) or another solvent (for cosolvent).

AQUEOUS CLEANING AGENTS Simple Additives Water removes some soils. With appropriate cleaning action and constant rinsing to remove soils, water alone can clean. However, additives improve performance. Some blends are relatively simple and are accomplished by in-house blending. Such blends can be particularly desirable where residue is an issue either for the product or for disposal of the spent cleaning agent. Small amounts of peroxide (0.5%) have been added to water to clean and remove bacteria. Dilute hypochlorite (bleach) is often used to prevent biological contamination. Ammonia is often used for simple cleaning where residue is of concern. Alcohol and acetone are sometimes added to boost cleaning power and promote rapid drying. Sodium bicarbonate may be added. Acid washing and acid etching with Piranha, chromic acid, and other strong acids can be thought of as selective cleaning. Although the solutions are simple, process control, process monitoring, employee safety, potential flammability, and environmental regulatory issues must all be considered.

Commercial Blends, Aqueous Commercially available aqueous cleaning agents contain additive blends, often consisting of a dizzying array of organic and inorganic compounds. Some additive packages are totally inorganic; most are a mixture. Although a few companies disclose the additive package, more typically, for competition-sensitive issues and other factors, the exact formulation is a closely held secret. A few examples of additives are provided in Table 1. This summary is provided for several reasons. Well-designed aqueous formulations are complex, sophisticated, and are specifically designed to remove certain soils. Also, be aware that even though aqueous cleaning agents are used in dilute form, they are not formulated from organic carrot juice. As with other cleaning agents, aqueous cleaning agents must be used with understanding and respect. For general metals cleaning, aqueous formulations with relatively broad range of acceptability for substrate and soil have been found. However, for most high-precision applications the aqueous cleaning agent must be specifically matched to the soil, the expected soil loading, the substrate, and the expected end use of the product. For example, in some applications, phosphate or silicate residue is not acceptable. In addition, in cleaning certain metals, notably aluminum, careful selection of the aqueous cleaning agent and process is required. © 2001 by CRC Press LLC

Table 1 Some Additives Used in Aqueous Formulations Additive

Function

Description, Examples

Surfactants

• Wettability • Soil displacement/dispersion • Solubilization

Defoamers

• Control excessive foam • Allow use in high-pressure spray applications etc.

Solvents, assorted

• Decrease surface tension • Adjust pH • Improve solubility range

Corrosion inhibitors passivating

• Prevent corrosion of metals

Corrosion inhibitors, nonpassivating

• Prevent corrosion of metals

Builders

• Promote efficacy of cleaning by surfactants • Sequester water hardness • Maintain pH (acidity, alkalinity) • Decrease metal, lead content of waste stream • Promote solubility of organics in presence of high levels of inorganic salts

• Single molecule with hydrophilic and hydrophobic portions • May have long chain organic portion, many carbons in a row • Example: alcohol ethoxylates • Poorly soluble in bath at operating temperature • Impart slight oil-like quality • Usually nonionic surfactants • Example: nonionic block copolymers • Typically soluble in water • May be VOCs • Examples: butyl cellosolve, pyrollidone,morpholine, glycol ethers, alcohols • React with metal surface to reduce reactivity • Typically oxidizing agents • Examples: chromates, nitrates, permanganates, chlorates • Some reducing agents, e.g., Na sulfite • Adsorption, formation of protective films • Examples: silicates (most common), pyrophosphates, carbonates, amines, gelatin, tannic acid, thiourea • Typically salts • Chelating agents • Examples: sodium tripolyphosphate, sodium hexametaphosphate, sodium citrate • Precipitating builders (e.g., carbonates) • Important with nonorganic surfactant packages • Examples: toluene sulfonates, short-chain alcohols, benzoate salts • Example: hydrogen peroxide

Hydrotropes

Oxidizers

• Corrosion inhibitors • Adsorption, dissolution in soils, oxygen release • Better soil removal

Source: Adapted from Cala, F.R. and A.E. Winston, Handbook of Aqueous Cleaning Technology for Electronic Assemblies, Electrochemical Publications, 1996.

Water-Soluble Organics Some cleaning agents, nominally referred to as aqueous cleaning agents are primarily or significantly high in organic solvent blends including long-chain nonlinear alcohols or d-limonene. They can be rinsed with water, or in some cases with either water or solvents. Providing both solvent and aqueous cleaning in the same process has advantages. However, recovery of the waste stream and carryover can become a problem. © 2001 by CRC Press LLC

SOLVENT-BASED CLEANING AGENTS As in aqueous cleaning, there are mystery blends. However, it is often easier to identify the components of solvent-based cleaning agents. With the proliferation of new organic solvents, a number of by-products and natural products have been or are now used in cleaning processes. Cleaning agents based on orange, pine, cantaloupe, and grapes have been developed—an entire fruit basket of possibilities. Some solvents have been discussed in detail; others are alluded to in discussions of cleaning equipment or of specific cleaning agents. A few additional solvents and solvent categories are worth noting. In general, one must consider that while many of these solvents have been used for years, long-term inhalation toxicity data may not be available. In addition, blends and azeotropes of both new and well-established solvents may have their own solvency, compatibility, or toxicity properties. This holds true for organic and water-based cleaning agents. The best advice remains to test the solvent or blend in the application under consideration and to be conservative— minimize employee exposure and minimize loss of cleaning agent to the environment. In addition, compounds with higher boiling points and fairly complex blends with certain additives may leave undesirable residue, so rinsing is often required. Classic Organic Solvents Examples of classic organic solvents include toluene, hexane, heptane, benzene, and xylene. Flammability, worker exposure, air toxics, and company liability issues reduced the use of these solvents when ODCs were available. They have remained popular for specialized uses or as blends, particularly for specific, high-value applications. With the decrease in availability of ODCs and an increase in low-flash-point and well-contained cleaning systems, these solvents have enjoyed a resurgence in popularity. They have a wide solvency range, but they are particularly oil-like. If they are to be used, appropriate environmental and safety controls must be employed. Restricted or Low-Availability Ozone-Depleting Chemicals Chlorofluorocarbon 113 (CFC 113) and 1,1,1-trichloroethane (TCA) set the standards for cleaning for decades. CFC 113 has low to moderate solvency, TCA is an aggressive solvent. Both can be used for liquid/vapor phase degreasing, but both have high ozone depletion potentials. In the United States, they have been phased out of production. It is still possible to obtain recycled material, but at a high cost. Hydrochlorofluorocarbon 141b (HCFC 141b) has moderate solvency and a fairly low boiling point. It was first considered as a replacement for other ODCs, but was found to have an ozone depletion potential similar to that of TCA. It will be phased out of production. In the United States, HCFC 141b is highly restricted and falls under a usage ban in cleaning applications.3 It should be noted in general that regulation and availability of various cleaning agents vary from country to country and often from city to city. Regulation of Class I ODCs and of HCFC 141b differs in various parts of the world. Solvents on the Horizon The landscape of available cleaning agents and processes varies as advances in technology, requirements of build processes, and regulatory drivers change the perceived appeal of © 2001 by CRC Press LLC

various options. Because the assortment of products will inevitably change, it is hoped that the reader will gain an appreciation not just of the specific cleaning agents, but of the underlying, commonsense approaches to successful cleaning and contamination control. Having said this, it is important to mention emerging solvents that are being used to develop new cleaning agents. For example, Nippon Zeon Co. Ltd. is marketing a hydrofluorocarbon (HFC) that may have favorable formulation qualities. Two additional HFCs, HFC-365mfc and HFC-245fa, will be discussed in greater detail. Both were initially introduced as foam-blowing agents, and most of the information must be extracted from literature geared to the foam industry. However, both may have uses, alone and in blends, as replacements for HCFC 141b, and for other solvents that may be heavily regulated. HCFC 141b is under a federal usage ban for most cleaning applications. There are some exceptions. For example, HCFC 141b could be used under some conditions as an aerosol wasp repellent. Suffice it to say that there is anecdotal evidence of noticeable continuing use of HCFC 141b and of a significant number of components manufacturers who, shall we say, must be having severe problems with wasps. Those manufacturers should be aware (1) of the general usage restrictions on HCFC-141b as well as (2) the imminent production phaseout of HCFC-141b. These newer solvents may provide viable alternatives to HCFC-141b and additional options for other applications. It is important to be aware that while both materials are HFCs, they behave differently from the HFCs you may be more familiar with (described elsewhere in this book by Merchant). One product is produced and imported to the United States by Solvay.4 It is marketed under the trade name Solkane® 365mfc5 and it will be sold to other cleaning agent suppliers for formulation. DuPont recently began introduction of several blends of HFC 365 with other HFCs and with trans-1,2,-dichloroethylene. Other companies and formulators are also developing products based on 365mfc. It should be noted that, unlike other HFCs commonly used as solvents (Merchant), HFC 365 has a very low flash point. In nonflammable blends, and even in azeotropes, there is the potential for flammable mixtures to develop in use. Therefore end users should work with advisors and with responsible cleaning agent suppliers to evaluate their own production situation carefully. HFC-245fa is produced by Honeywell6, 7; it was also developed primarily as a foam-blowing agent. There is interest in use as a solvent for aerosols, and it has potential applicability as a cleaning solvent in some applications. With a 15°C boiling point, many end users will find HFC-245fa to be rather evanescent; and it would not be suitable for use in classic, unmodified vapor degreasers. The material will be more suitable for use in newer vapor degreasers with subzero chilling and auxiliary cooling coils. The material is also expected to be usable in many airless and airtight systems and in flushing applications. The very low boiling point can be advantageous for deposition and other applications where rapid evaporation is desirable. HFC245fa appears to have reasonable hydrolytic stability, and good materials compatibility with metals and with many plastics. It is miscible with some hydrocarbons, and it shows partial (over 10%) to full miscibility with a number of other solvents, including methanol, isopropyl alcohol, trans-1,2-dichloroethylene, and perfluoropolyethers. It shows good to very good miscibility with polyolester oils, limited miscibility with fluorinated hydrocarbon oils, and poor miscibility with mineral oil and silicone oil. Both HFC-365mfc and HFC-245fa are very mild solvents, similar to the currently more familiar HFCs and hydrofluoroethers (HFEs), and costs are expected to be somewhat lower. Of the two, HFC-245fa should have better solubility for polyolester oils. With a boiling point of 40°C, HFC-365mfc has a significantly higher boiling point than HFC-245fa. These boiling points may be compared with the 32°C boiling point for HCFC-141b. The differences may not seem significant, but remember that as a rule of thumb chemical processes double in rate with every 10°C increase. For both materials, some plastics © 2001 by CRC Press LLC

compatibility studies have been conducted. Because synergistic behavior can occur, you would be well advised to confirm the compatibility of any proposed blend to the specific mix of materials and in the application at hand (time of exposure, temperature, force of cleaning action). A comparison of HFC-365mfc and HFC-245fa is presented in Table 2. Oxygenated Solvents Examples of oxygenated solvents include the short-chain alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), and acetone. Compared with, for example, hexane or heptane, the addition of oxygen makes these compounds more polar, that is, more like water. They are more suited to polar or inorganic soils. Despite the low flashpoint, some oxygenated compounds, such as isopropyl alcohol (IPA) can be used in appropriately designed vapor degreasing systems. However, it should be noted that oxygenated solvents cannot systematically be used to replace chlorinated solvents. After it was delisted as a volatile organic compound (VOC), there has been increased interest in acetone. Although acetone has been adopted for some processes, the extremely rapid evaporation rate, aggressiveness to some plastics, and low flash point have limited the extent of substitution of acetone. Long-chain alcohols such as tetrahydrofurfuryl alcohol (THFA) are used alone and in blends. The longer the carbon chain, the more they become fatlike (able to dissolve oil). However, the alcohol portion confers some waterlike qualities. Often, these alcohols can be part of aqueous, semiaqueous (rinse with water), or cosolvent (rinse with solvent) blends. N-Methyl pyrrolidone (N-methyl-2-pyrrolidone, NMP) is a high boiling (295°C), highflash-point (91°C, 196°F) solvent that is used alone and in blends. Because it is water soluble, it can be blended for removal of both rosin and organic acid flux, and it is used in photoresist systems. Other pyrrolidones are being developed, notably n-octyl pyrollidone (NOP) and n-hydroxy ethyl pyrrolidone (HEP). Used alone and in blends, they may serve to extend the range of cleaning in degreasing. For example, NOP has a longer carbon chain Table 2 Comparison of Characteristics of HFC-365mfc and HFC-245fa Characteristic

HFC-365mfc

HFC-245fa

Structure Molecular weight Boiling point (°C) Flashpoint (°C) UEL/LEL (% by volume) ODP Atmospheric lifetime VOC status SNAP status, solvents

CF3CH2CF2CH3 148 40.2 Below 27 3.8 to 13.3 Zero 10.8 years Exempt Acceptable

Toxicology

90-day subacute inhalation study in progress

CF3CH2CF2H 134 15 None None Zero Low to moderate Exempt Not yet submitted for solvent applications 90-day sub-acute inhalation study complete; under evaluation by AIHA WEEL committee; suggested inhalation levels not yet published

UEL/LEL  upper/lower explosion limit; ODP  ozone depletion potential; VOC  volatile organic compound; SNAP  Significant New Alternatives Policy.

© 2001 by CRC Press LLC

and is therefore more oil-like, so it could be used in formulations for degreasing, paint stripping, and deinking of paper. HEP shows promise in photoresist removal.8 Relatively recently, dimethyl sulfoxide (DMSO) has also been used in some photoresist applications as well as in other processes requiring a fairly aggressive solvent. DMSObased products are in the process of being developed and tested.

Esters Various monobasic and dibasic esters and notably lactate esters are used alone and in blends in semiaqueous and cosolvent applications. They have proved particularly effective in removal of pitches, waxes, and other difficult, mixed soils. The esters have a fairly strong odor. They have high boiling points and must be rinsed in high-precision applications. Esters may also be used in coatings formulations. Two of the esters, t-butyl acetate (VOC status pending) and methyl acetate, have relatively low tropospheric reactivity and may therefore be particularly useful where VOCs are an issue. In precision cleaning of electronics assemblies, t-butyl acetate has been found to provide a good complement to acetone. t-Butyl acetate has a higher boiling point. Unlike acetone, it does not dissolve nitrile gloves, and in one test evaluation the assemblers found the odor to be acceptable.9

Hydrocarbon Blend (Mineral Spirits), Soy-Derived Cleaning Agents Hydrocarbon blends (mineral spirits or Stoddard solvent or kerosene) consisting of a petroleum cut of hydrocarbons with a range of molecular weights have been used in cleaning applications for many years. Given the high boiling point and the potential for contaminants in some formulations, care must be taken in use and removal, and lot-to-lot variability may impact process control. It is possible to obtain very pure mineral spirit blends with a narrow, defined range of hydrocarbon chain lengths. Such well-defined hydrocarbon blends are better suited to high-precision applications, both for cold cleaning and, in the appropriately designed system, even for vapor-phase cleaning. Methyl soyate is a soy-derived substitute for mineral spirits. It is being developed for cleaning applications, and may prove to a renewable-resource alternative to hydrocarbon blends. d-Limonene Blends, -Pinene Blends In addition to d-limonene (citrus derived), which is discussed in more detail elsewhere in this book, cleaning agents have been based on -pinene (pine tree derived). Both have noticeable odors; both can leave appreciable residue, depending the application, and must be rinsed completely. In practice, d-limonene-based cleaners have proved more useful. As with many of the esters and other specialty cleaning agents, long-term inhalation exposure studies have not been conducted. Cyclic Volatile Methyl Siloxanes Linear volatile methylsiloxanes (VMSs) have been developed for critical cleaning processes and are discussed in Chapter 1.9 by Cull and Swanson. Cyclic VMSs alone and © 2001 by CRC Press LLC

in blends, have also been used in cold-cleaning applications where higher-boiling solvents are preferred. The cyclic VMSs do not have as favorable a toxicity profile as do linear VMSs. One supplier of cyclic VMS blends (QSOL) indicates a 10-ppm recommended inhalation level. Therefore, they should be used in enclosed, well-vented cleaning systems. Perfluorinated Compounds Perfluorinated compounds (PFCs) contain fluorine and carbon, but no chlorine or bromine. They are exceedingly mild, inert cleaners that can be used for removal of fluorinated lubricants and as rinsing and drying agents. PFCs are very effective for particulate removal. They are not ODCs. They are still sold alone and in various formulations. However, because of the global warming potential associated with a long atmospheric lifetime, often ranging in the thousands of years, industry has been under what might be termed strong regulatory encouragement to find substitutes. HFCs and HFEs can replace PFCs in many if not most applications. SOLVENT BLENDS, AZEOTROPES, COSOLVENTS Aqueous additives are used for such purposes as improving wettability, improving solubililization and removal of soils, compensating for water quality, and preventing corrosion. Blends involving solvents are a much more diffuse concept. Solvents are blended for a number of reasons. Blends can blur the lines of demarcation among various categories of cleaning agents. Stabilization Water and acidicity are the enemy of many halogenated solvents. Stabilizer packages are added to many chlorinated solvents (including TCA) and to n-propyl bromide (nPB) to prevent acid formation, breakdown, and reactivity with metals. Effective stabilization is important in degreasing (liquid/vapor cleaning). Stabilization becomes even more challenging in airtight and airless systems because the solvent is reused without replenishment over a much longer period than in traditional open-top degreasers. Extending the Solvency Range or Moderating the Solvency Solvent blends can provide custom-made, fine-tuned cleaning options. Sometimes, blends provide surprises. Azeotropes are strongly preferred over blends for vapor degreasing applications. An azeotrope is a constant-boiling mixture of two or more compounds. An extreme example of a nonazeotrope blend would be sugar in water. On heating, the water is boiled away, and the sugar remains. In contrast, in specific proportions, IPA and cyclohexane form a constant- boiling azeotrope. This means that the vapors contain both components in a constant proportion that does not change over the life of the blend. Azeotropes have to be managed with care. Even azeotropes can vary in composition if they are used at a temperature not in the azeotropic range. Blends that are not azeotropes will lose various components to evaporation at different rates. This means that the relative concentrations in the liquid and remaining blend will vary with time. Cleaning capability, compatibility, and flash point can all change. Blends © 2001 by CRC Press LLC

that are not true azeotropes should be viewed cautiously, particularly in vapor degreasing applications. In addition, azeotropes have been known to behave synergistically (nonadditively) in terms of performance and compatibility. These properties are not necessarily predicted by solvency parameters. In other words, while two solvents may each separately show acceptable materials compatibility with a given component, the blend could produce component deformation. Therefore, even if one thinks the solvency and compatibility issues of each component in an azeotrope are understood, it is prudent to test the mixture. Solvent blends are also used to modify or extend the solvency range in cold cleaning applications. An aggressive solvent can be toned down and a mild solvent made more aggressive. For example, HFE has been blended with NPB to tone down the aggressiveness of NPB. VMS may be blended with an alcohol to boost solvency. Cosolvents The terminology of cosolvents is a bit confusing. Most often, cosolvents are thought of as two chemicals used sequentially. However, strictly speaking, any blended solvent could be thought of as a cosolvent system. Cosolvents can be blends that are primarily aqueous or primarily solvent. Supercritical or liquid CO2 cleaning can also be accomplished with cosolvents. Surfactants Surfactants are used in solvent blends to provide some qualities similar to aqueous cleaners, to change emulsifying qualities, and to allow the solvent to be readily rinsed in water (as in cosolvent processes). Some blended cleaning agents are offered as similar formulations, with or without the surfactant. Emulsions: Macroemulsions, Structured Solvents, Microemulsions Oil and water do not mix, except in emulsions when they may coexist in a transient or relatively permanent form. An oil-and-vinegar salad dressing is typically a transient macroemulsion. Mayonnaise is a more permanent emulsion. Emulsifying qualities are used in aqueous blends, solvent blends, or both. In aqueous formulations, organic chemicals may be part of the mix only under certain conditions, such as temperature. For example, the separation of the organic phase in an aqueous cleaner at the operating temperature may serve to defoam the blend, allowing for spray applications. In the same way, immiscible organic chemicals may be used as emulsions, transient or permanent. Transient macroemulsions can be used to transfer the soil from one chemical to the other; the part is then rinsed in more of the chemical with very low solubility for the soil in question. Macroemulsions are typically cloudy. Sometimes one of the phases is aqueous; in other processes both are solvent. Microemulsions, structured solvents, liquid crystals, or continuous-phase emulsions have been introduced as cleaning agents. Structured solvents are stable mixtures of organic solvent, water, and coupling agents. The continuous phase may be solvent or water. They appear clear and may be primarily water or primarily solvent. Structured solvents can be made to separate during the application process. Such products are useful for mixed soils where both solvent and waterlike characteristics are desirable.10 © 2001 by CRC Press LLC

Mystery Mixes Blended solvents, particularly the high-boiling blends involving hydrocarbons, esters, and nonlinear alcohols, greatly extend the specificity of cleaning that can be obtained. However, blended solvents can be particularly difficult for the components manufacturer to evaluate. As with aqueous cleaning agents, many manufacturers consider the formulations to be highly proprietary and competition sensitive. Many of the comments regarding mystery mixes apply to both aqueous- and nonaqueous-based formulations. Some areas of concern in using complex mystery mixes include unexpected compatibility issues, regulatory constraints on one or more component, and unscheduled formulation changes. In such cases, it may be desirable to set up confidentiality agreements so that the ingredients are understood in detail. At the very least, particularly where the product is used in a process requiring high levels of validation and testing, it is prudent to obtain an agreement with the vendor that the product will not be changed. In addition, supposed improvements in formulations can have unintended consequences. This author has observed many instances where a blended product was “improved” in such a manner as to impact the process adversely. SOLVENCY AND PHYSICAL PROPERTIES; OTHER PARAMETERS Kauri-Butanol Number A number of solvency systems have been described in Chapter 1.2 by J. Burke. In addition, other solvency systems are in use. One cloud-point test, the Kauri-butanol (KB) number, is often referred to. The KB number is determined by the volume of solvent required to produce a defined degree of turbidity when added to standard solutions of Kauri resin in n-butyl alcohol. As a general rule, the higher the value, the stronger the solvent. The system was developed to indicate the relative solvent power of hydrocarbons, and it is not valid for oxygenated solvents. The KB number should be considered along with the boiling point, because, if the solvent can be heated to higher temperatures, more entropy is introduced into the system and better solvency occurs. Estimating solvency by mixing a cleaning agent with t-butyl alcohol and tree sap is a rather unsophisticated approach. However, the KB number remains widely used, and it is somewhat predictive of solvency.11 Table 3 lists the KB number and boiling points of several representative cleaning agents. Wetting Index The wetting index has been used as a guideline to the ability of a cleaning agent to penetrate closely spaced components. The wetting index is directly proportional to the density and inversely proportional to the surface tension and viscosity. In general, many of the vapor degreasing solvents have a higher wetting index than water or hydrocarbon blends. As with other indications, however, wetting index alone does not determine efficacy of cleaning. The wetting index of a few common cleaning agents are provided in Table 4.12 Other Physical Properties; Regulatory Features Physical properties of solvents constrain the range of choices. Other physical properties such as boiling point, flash point, and evaporation rate must be considered in choosing © 2001 by CRC Press LLC

Table 3 KB Number and Boiling Point (BP), Representative Cleaning Agents Cleaning Agent

KB No.

BP,°C

CFC-113 1,1,1-Trichloroethane HCFC 141b Methylene chloride Trichloroethylene n-Propyl bromide d-Limonene Parachlorobenzotrifluoride (PCBTF) HCFC 225 HFC 43–10 HFC 43–10 blend including trans-1,2,-dichloroethylene HFE 569sf2 VMS OS-10

32 124 56 136 129 125 68 64 31 9 30 10 17

48 74 32 40 87 71 150 139 54 55 37 76 100

a solvent, and the solvent must be considered in the particular regulatory microclimate where the process is being carried out. Tables 5a and 5b list some physical properties of some commonly used solvents, and a few regulatory considerations.13 The concept of a VOC-exempt solvent refers to the U.S. federal regulatory designations at the time of writing. Solvents that are VOC exempt are judged to have negligible reactivity relative to ethane. Many solvents, including those that are VOC exempt, can be used in vapor-phase cleaning applications. Those with low flash points, however, must be used in specially designed equipment. Such equipment has a high initial capital cost. Many solvents do not have a flash point but do have an upper explosion level (UEL) and a lower explosion level (LEL); this must be considered in specialized operations and in selecting and maintaining emission control equipment. The boiling point must be high enough to allow efficient cleaning, but not so high as to damage materials of construction or slow the build cycle. A very high boiling point may preclude use of the solvent in a standard vapor-phase degreasing operation. The evaporation rate must be sufficiently rapid to allow rapid drying, but not so rapid that the solvent is immediately lost. These considerations are all relative to the operation in question.

Table 4 Examples, Wetting Index Cleaning Agent Generally desirable CFC 113 TCA IPA NPB HCFC 225 HFE 449 sl Hydrocarbon blend Water Saponifier solution, 6% aqueous

© 2001 by CRC Press LLC

Density, g/cm3 High 1.48 1.32 0.785 1.33 1.40 1.52 0.84 0.997 0.998

Surface Tension, dyne/cm Low 27.4 25.9 21.7 25.9 16.8 14 27 72.8 29.7

Viscosity, cp Low 0.70 0.79 2.4 0.49 0.61 0.6 2.8 1.00 1.08

Wetting Index High 121 65 15 105 145 181 11 14 31

Table 5a Physical Properties, VOC, ODC Status

Cleaning Agent 1,1,1-Trichloroethane (ODC) CFC-113 HCFC-141b Stoddard solvent, typical (hydrocarbon blend) (VOC) n-Propyl bromide (VOC) Methylene chloride, VOC-exempt hazardous air pollutant Perchloroethylene, VOC-exempt hazardous air pollutant HCFC 225, VOC exempt HFE 569sf2, VOC exempt HFE 449s1, VOC exempt HFC 43-10mee, VOC exempt Water

Boiling Point, °C (°F)

Flash Point

UEL/LEL, %

Comments, Evap. Rate (ref. for evap rate)

74 (165)

None

15/7.0

5 (buac  1)a

48 (118)

None (TOC)

NA

32 (90) 152 (305)

None 40.6 (106)

17.7/7.6 6.1/1/1

0.45 (buac  1)  1 (ether  1) Hydrocarbon blend, VOC

71 (160)

None

8/3

4.5 (buac  1)

40 (104)

NA

19/12

NA

121 (250)?

None (TCC)

None

2.1 (buac  1)

54 (130)

None

None

0.9 (ether  1)

76 (169)

NA

—

NA

—

55 (131)

None (TCC, TOC) None (TCC, TOC) None (TOC)

NA

—

100 (212)

None

None

—

61 (142)

Note: These data were obtained from various standard publicly available references, primarily MSDS from the Cornell University Program Design Construction Web site (http://msds.pdc.cornell.edu/ISSEARCH/ MSDSsrch.htm); University of Vermont Web site, with some confirmation by Lange’s Handbook of Chemistry, 13th ed. McGraw-Hill, New York, and Dangerous Properties of Industrial Materials, 3rd ed., N. Irving Sax, Reinhold Book Corp). They should be used as guidelines only—the evaporation rate data in particular is prone to inconsistency among references. Boiling points rounded to nearest integer. Please confirm all information with current MSDS. buac  butyl acetate; NA = not available.

a

Costs Costs are relative. Few solvents are inexpensive, particularly if total process costs14 are considered. The most important considerations are cleaning agent quality and product stewardship by the chemical supplier.15,16 In terms of organic solvents, pound per pound, traditional solvents such as IPA, acetone, and the chlorinated solvents are relatively inexpensive. nPB and the VMSs are moderately priced, and the engineered solvents (HCFC 225, HFEs, and HFCs) are the most costly. Blended high-boiling solvents can vary markedly in price. The costs may be perceived © 2001 by CRC Press LLC

Table 5b Physical Properties, VOC, ODC Status, Low-Flash-Point Solvents

Cleaning Agent

Boiling Point, °C (°F)

Methyl acetate, VOC exempt t-Butyl acetate, proposed, VOC exempt para-Chlorobenzo trifluoride, VOC exempt Di-siloxane VMS, VOC exempt Tri-siloxane, VOC exempt Cyclo-tetrasiloxane, VOC exempt Acetone, VOC exempt

Flash Point

UEL/LEL, %

55.8 –58.2 (132 –137)

13 (9)

16/3.1

98 (208)

15 (59)

NA/1.5

139 (282)

43 (109)

10.5/0.9

100 (212)

3 (27) TCC

18.6/1.25

152 (306)

34 (94) TCC

13.8/0.9

205 (401)

76 (170) TCC

56 (134)

20 ( 4)

13/2.5

Comments, Evap. Rate (ref. for evap rate) Recently exempt, 5.3 (buac  1) Recently proposed, VOC exemption

Dow VMS OS10, 3.8 Dow VMS-OS20, 0.7 Dow VMS-OS245, not calc. 6

Note: These data were obtained from various standard publicly available references, primarily MSDS from the Cornell University Program Design Construction Web site (http://msds.pdc.cornell.edu/ISSEARCH/ MSDSsrch.htm), University of Vermont Web site, with some confirmation by Lange’s Handbook of Chemistry, 13th ed., (McGraw-Hill, New York), and Dangerous Properties of Industrial Materials, 3rd ed., N. Irving Sax, Reinhold Book Corp. They should be used as guidelines only—the evaporation rate data in particular is prone to inconsistency among references. Boiling points rounded to nearest integer. Please confirm all information with current MSDS. a buac  butyl acetate; NA = not available

as high in applications where soil loading is a problem and frequent solvent change-out is required. With heavy soil loading, it may be more effective to perform initial cleaning in a relatively inexpensive product, and then to conduct subsequent steps in the more sophisticated cleaning agent. Aqueous cleaning agents must be compared against each other in the intended application. Assume that two concentrates are under consideration and that one is twice as costly as the other. If the inexpensive cleaning concentrate must be used at a 1:4 dilution while the other provides equivalent performance at a 1:20 dilution, the picture changes.15 –16 Filtration17 markedly influences bath life and therefore modifies the overall cost of the cleaning agent. OVERALL CONSIDERATIONS One of the problems in developing a manufacturing process is the rather daunting list of considerations and provisos. To cope with the problem, there is the tendency to think linearly and to attempt to find the perfect cleaning agent. There is no perfect cleaning agent. However, we persist in our search for unattainable perfection. All too often, when a cleaning process is being developed, a cleaning agent selection committee is established to screen out all undesirable applicants. The safety/environmental group is likely to rule out any environmentally challenged cleaning agent, even if it © 2001 by CRC Press LLC

could be used nonemissively. Company management and sometimes the customer may submit a series of “don’t” lists. Whole classes of cleaning agents may be ruled out as being unacceptable on general environmental principles. The materials and process chemists may insist that for any cleaning agent to be considered, it should be able to be in contact with all materials of construction at some elevated temperature for, say, 24 hours. The purchasing department may insist that only one or two cleaning agents be selected—period. The manufacturing engineers may insist on an extremely rapid process time, instant drying, and aqueous cleaning. What’s left? Sometimes nothing, sometimes a class of cleaning agents that is totally unsuited to the cleaning application at hand. Perfection aside, for nearly every cleaning application, there are several workable solutions. Some of the considerations in cleaning agent selection are indicated in Table 6. The cleaning agent has to be considered in the context of the cleaning process and, indeed, in the context of the overall manufacturing process. The factors indicated in Table 6 are meant to provide a starting point. It usually becomes very apparent which factors are the most important in a given manufacturing situation. It is more productive to proceed with a nonlinear approach that considers performance, costs, cleaning agent, cleaning equipment, suitability to the workforce, worker safety, and the local regulatory microclimate. Table 6 Overall Considerations, Choosing Cleaning Agents Factor

Process Consideration

Cleaning properties, cleaning performance

• • • • • • • • • •

Materials compatibility

Residue

Cycle time

Cleaning equipment

© 2001 by CRC Press LLC

• • • • • • • • • • • • • • • • • • • •

Cleaning requirements of your process (how clean is clean enough) Performance under actual process conditions Solubility characteristics relative to soil of interest Wetting ability Boiling point Evaporation rate Soil loading capacity Ability to be filtered Ability to be redistilled Compatibility under actual process conditions (temperature, time of exposure) Product deformation at cleaning, rinsing, drying temperatures Nonvolatile residue (NVR) level Rinsing requirement Process time impact Cleaning Rinsing Drying Product cool-down Component fixturing Loading and unloading equipment Product rework Suitability with current cleaning equipment Ability, costs of retrofit Costs of new cleaning equipment Auxiliary equipment required Maintenance, repair Automation, component handling Footprint (length, width, height) Equipment weight Component fixturing (continued )

Table 6 Overall Considerations, Choosing Cleaning Agents (continued ) Factor

Process Consideration

Flash point

• • • • • • • •

Toxicity

Worker acceptance

Cleaning agent management

Regulatory, air

Regulatory, water Company, customer, product performance requirements

Costs

© 2001 by CRC Press LLC

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Choice of cleaning equipment Process control Control of proximal processes, activities Choice of auxiliary equipment Choice of emissions control Acute Long term Anticipated exposure under process conditions (including sprays, mists) Employee monitoring Inhalation Skin adsorption Method of application Drying speed Similarity to current process Automation Computer skills Perceived loss of control of process Odor Water preparation In-process filtration (water, organic, or aqueous cleaning agent) Wastewater disposal On board redistillation Global, national, local (ODC, VOC, HAPs, GWP) Neighborhood concerns Environmental justice issues Production phaseout Usage bans Disposal of waste stream Global, national, local Disposal of waste streams Contractual requirements, restrictions Company policy Testing, acceptance qualification required In-house safety, environmental policy Insurance company issues Cleaning agent Cleaning agent preparation and disposal Costs as-used (dilution) Capital equipment Disposables Sample handling Total process time Rework Insurance Regulatory permitting Process qualification Employee education, training Process monitoring

Table 6 Overall Considerations, Choosing Cleaning Agents Factor

Process Consideration

Supplier stewardship, cleaning equipment supplier

• • • • • •

Responsive distributor, supplier Supportive technical staff Clear, understandable MSDS Provides required technical information Provides required regulatory information Supports process development

HAP  hazardous air pollutant; GWP  global-warming potential; MSDS  Material Safety Data Sheet.

REFERENCES 1. Kanegsberg, B., Precision cleaning without ozone depleting Chem. Ind., 20, 787 –791, 1996. 2. Kanegsberg E. and B. Kanegsberg, Cleaning by abrasive impact A2C2 Mag, May 2000. 3. Dibble, C., EPA SNAP program update: solvent cleaning, presented at Nepcon West 2000, February 29, 2000, Anaheim, CA. 4. Information provided by K. Neugebauer, V.P. Specialty Fluorides, Solvay Fluorides, Inc. 5. Zipfel, L. and P. Dournel, HFC-365mfc, the key for high performance rigid polyurethane foams, presented at UTECH 2000, The Hague, The Netherlands, April 2000. 6. Information provided by G. Knopeck, Manager, Fluorocarbon Technical Services, Honeywell International. 7. Knopeck, G., Pentafluoropropane: an HFC solvent for aerosols, presented at SATA (Southern Aerosol Technology Association) Spring Meeting, Atlanta, GA, April 2000. 8. Waldrop, M.W., BASF, communications and technical summaries, January 2000. 9. Elias, W.G., Real-life applications with environmentally compliant solvents for electronics, Proc., Nepcon West 2000, Anaheim, CA. 10. Shick, R.A., Formulating cleaners with structured solvents, in Proc., Precision Cleaning 96, Anaheim, C.A., 1996, 285 –289. 11. Kenyon, W.G. and B. Kanegsberg, Accelerating the Change to Environmentally Preferred, CostEffective Cleaning Processes, Tutorial with Precision Cleaning ‘95, Chicago, May 1995. 12. Kenyon, W.G., Wetting Index, personal communication. 13. Kanegsberg, B., Economic and environmental costs of process conversion, presented at Nepcon West 2000, February 29, Anaheim, CA, 2000. 14. Kanegsberg, B. and C. LeBlanc, The cost of process conversion, (in Report to Toxic Use Reduction Institute, B. Kanegsberg Proc., CleanTech‘99, Rosemont, IL, May 1999. 15. O’Neill, E., A. Miremadi, R. Romo, M. Shub, and B. Kanegsberg, Four steps to process conversion, Parts Cleaning Mag., May 2000. 16. O’Neill, E., A. Miremadi, A. Guzman, R. Romo, M. Shub, and B. Kanegsberg, Simplifying aqueous cleaning, the value of practical experience, Products Finishing Mag., August 2000. 17. Kanegsberg, E., Liquid filtration in critical cleaning, A2C2 Mag., April 2000.

© 2001 by CRC Press LLC

CHAPTER 1.2

Solvents and Solubility John Burke

CONTENTS Introduction Solutions Molecular Attractions Cohesive Energy Solubility Parameters Three Component Parameters The Teas Graph Visualizing Solubility Solvent Mixtures References INTRODUCTION Solvents are ubiquitous. Not a day goes by when we don’t rely on one solvent or another, to accomplish some essential task. Given such frequent experience with solubility, we might conclude that we all would have a pretty good idea of how solvents work. And yet, who among us hasn’t tried in vain to remove one substance from another, guided by rules of thumb such as “like dissolves like” or vague concepts of solvent “strength.” While this approach may often succeed, it also might be risky if, for example, we needed to dissolve one material selectively while leaving other materials completely unaffected. Or, it would be clearly inefficient if, at the same time, we were also trying to control evaporation rates, solution viscosity, material costs, or environmental and health effects. The selection of a solvent or solvent mixture in the face of complex criteria moves beyond trial and error, and of necessity must rely on a system that can organize and predict solubility behavior. While this could be accomplished empirically, by simply testing the effects of specific solvents on specific materials, a universal system that could encompass solubility behavior in general would be immensely useful. Although understanding such a system may seem dauntingly complex, the practical application of solubility theory is actually quite straightforward. In fact, many solubility interactions can be predicted on a simple triangular graph. But before we can accomplish this feat, a certain amount of both theoretical and historical background is in order. © 2001 by CRC Press LLC

SOLUTIONS When two liquids are mixed together, simply stated, they either stay together or they don’t. They may stay together because of a chemical reaction that changes the liquids into some other compound, usually by an exchange of electrons (in which case it may be difficult to retrieve the original materials). Fortunately, for the purposes of this discussion, we can ignore solutions of this kind, since ionic and even water-based solutions are beyond the scope of simple solubility. Another reason a mixture may stay together is because the two materials are mutually soluble, such as gin and tonic. Simple solubility implies that the individual materials remain essentially unchanged even while mixed, and can usually be separated with some ease, say, by distillation. Solutions of this kind usually include solvents and are easier to characterize and predict. But to say this kind of interaction is simple may be misleading. Not just any two or three solvents may be successfully combined. When asked why oil and water do not mix, most people will reply that the oil is “lighter” than the water. Yet their mutual repulsion is not at all due to gravity. And at the limits of solubility solutions may become clear, cloudy, or separate out again after the slightest change in concentration. How can these seemingly complex behaviors be accounted for? Actually, dissolving a thing is very similar to evaporating it—but to understand that remark we first need to know a bit of what happens on the surfaces of molecules.

MOLECULAR ATTRACTIONS Liquids and solids differ from gases basically because their molecules stick together, resisting the tendency to evaporate completely into space. This must mean that the molecules that make up liquids and solids are somehow attracted to each other, that there is some kind of intermolecular stickiness that prevents them from flying apart into a gas. These sticky forces between molecules were first described by Johannes van der Waals in 1873, and thus bear his name. Originally thought to be small gravitational attractions, van der Waals forces are actually caused by electromagnetic interactions between molecules. The outer shell of an atom is composed entirely of a cloud of negatively charged electrons, completely enclosing the positively charged nucleus within. Molecules, because they are composed of atoms, are also covered by a cloud of electrons. In both cases we can imagine a positively charged center surrounded by a negatively charged outer shell. These positive and negative charges essentially balance out, with the result that the atom or molecule as a whole is neutral. It’s easy to see that the negatively charged surfaces of adjacent molecules should repel each other like the negative poles of two magnets (and it’s a good thing too, or the universe would collapse), but we were speaking earlier of intermolecular stickiness. How can we account for this anomaly? In reality, the electron cloud is never evenly distributed around a molecule. A single molecule, because of its structure, can exhibit a variety of electromagnetic charges on its surface, some strong and some weak, some which cancel out, and some which reinforce each other. The resulting sum of all the charges is what is known as the dipole moment of the molecule. Molecules that have permanent dipole moments are said to be polar, while molecules in which all the dipoles cancel out (zero dipole moment) are said to be nonpolar. Some atomic elements attract electrons more vigorously than others, causing the electrons to be unequally shared between the individual atoms in a molecule. If the molecule © 2001 by CRC Press LLC

is symmetrical, these charges may cancel out. If, on the other hand, the electron density is permanently imbalanced, with some atoms in the molecule harboring a greater share of the negative charge distribution, the molecule itself will be polar. The polarity of a molecule is related to its atomic composition, its geometry, and its size. A particularly strong type of polarity, for example, occurs in molecules where a hydrogen atom is attached to an extremely electron-hungry atom such as oxygen, nitrogen, or fluorine. In these cases, the sole electron of hydrogen is drawn toward the electronegative atom, leaving the strongly charged hydrogen nucleus exposed. In this state the exposed positive nucleus can exert a considerable attraction on electrons in other molecules, forming what is called a protonic bridge that is substantially stronger than most other types of intermolecular attractions. This special kind of polar attraction between molecules is called hydrogen bonding, and plays a major role in solubility as we shall see. But what about nonpolar molecules, where electron clouds are evenly distributed? The source of electromagnetic attractions in this case stems from the random movement of the electron cloud. From instant to instant, random changes in electron distribution give rise to polar fluctuations that shift about the molecular surface. Since the distribution of the electron cloud is uneven (maybe thicker in one place and thinner in another), small local charge imbalances are created. The parts of the molecule with a greater electron density will be more negatively charged, and the electron-deficient parts will be more positively charged. Although no permanent polar configuration is formed and the molecule is essentially nonpolar, numerous temporary poles are created constantly, move about, and disappear. When two molecules are in proximity, the random polarities in each molecule tend to induce corresponding polarities in one another, causing the molecules to fluctuate together. This allows the electrons of one molecule to be temporarily attracted to the nucleus of the other, and vice versa, resulting in a play of attractions between the molecules. These induced attractions are called dispersion forces. The degree of dispersion forces that these temporary dipoles confer on a molecule is related to surface area: the larger the molecule, the greater the number of temporary dipoles, and the greater the intermolecular attractions. Molecules with straight chains have more surface area, and thus greater dispersion forces, than branched-chain molecules of the same molecular weight. Deviations in electron shell density, therefore, result in minute magnetic imbalances, so that each molecule as a whole becomes a small magnet, or dipole. These electron density deviations depend on the physical architecture of the molecule. Certain molecular geometries will be strongly polar. Other molecules may be nonpolar, but are still capable of dispersion forces. It is these electromagnetic effects that account for the intermolecular stickiness holding liquids and solids together. Now we can explore the intriguing relationship among vaporization, van der Waals forces, and solubility. COHESIVE ENERGY To bring a liquid to its boiling point, we usually add energy in the form of heat. This heating raises the temperature of the liquid until it finally begins to boil. Once the liquid reaches its boiling point, however, the temperature of the liquid will not continue to increase. Any subsequent heat added is used up in continuing the boiling and separating the molecules of the liquid into a gas. Only when all the liquid has been completely vaporized will the temperature again begin to rise. If we measure the amount of energy that we add between the moment that boiling starts and the point when all the liquid has boiled away, we will have a direct indication of the © 2001 by CRC Press LLC

amount of energy required to separate that amount of liquid into a gas. Interestingly, this is also a measure of the amount of van der Waals forces that held the molecules of that liquid together. It is important to note here that the temperature at which the liquid begins to boil is not important, but rather the amount of heat that has to be added to separate the molecules. A liquid with a low boiling point may require considerable energy to vaporize, while a liquid with a higher boiling point may vaporize quite readily, or vice versa. Regardless of the temperature at which boiling begins, the liquid that vaporizes readily has fewer intermolecular attractions than the liquid that requires considerable addition of heat to vaporize. The energy required to vaporize the liquid is called, not surprisingly, the heat of vaporization, and reflects the attractions that exist between its molecules. Here is the connection we have been looking for: vaporization and solubility are similar because the same intermolecular attractive forces have to be overcome to vaporize a liquid as to dissolve it. This can be understood by thinking about what happens when two liquids are mixed. The molecules of each liquid have to be physically separated by the molecules of the other liquid. For any solution to occur, solvent molecules must be separated from each other to penetrate between the molecules of the solute. At the same time, the solute molecules must also overcome their own intermolecular stickiness to allow solvent molecules between and around them. This is similar to the molecular separations that need to occur during vaporization. The same intermolecular van der Waals forces must be overcome in both cases. So it stands to reason that for two materials to be soluble in each other their internal energies must be similar. The molecules of a polar liquid (such as water) with strong intermolecular attractions just will not be separated by the molecules of a nonpolar liquid (such as oil) that are held together only by weak dispersion forces.

SOLUBILITY PARAMETERS If we wanted to give a number to these attractions independent of temperature (basically to put everything on an even playing field), we can derive a value called the cohesive energy density from the heat of vaporization by the following formula: RT   c  H V

(1)

m

where c  cohesive energy density H  heat of vaporization R  gas constant T  temperature Vm  molar volume In 1936, Joel H. Hildebrand, in his landmark book on the solubility of nonelectrolytes, proposed the square root of the cohesive energy density as a numerical value indicating the solvency behavior of a specific solvent.





RT   δ  c  H V m

1/2

(2)

It was not until the third edition in 1950 that the term solubility parameter was proposed for this value, represented by the symbol δ. Subsequent authors have proposed that the © 2001 by CRC Press LLC

Table 1 Hildebrand Solubility Parameters Solvent

Parameters

n-Pentane n-Hexane Freon ® TF n-Heptane Diethyl ether Cyclohexane Amyl acetate 1,1,1-Trichloroethane Carbon tetrachloride Xylene Toluene Ethyl acetate Benzene Chloroform Trichloroethylene Tetrahydrofuran Cellosolve acetate Acetone Ethylene dichloride Methylene chloride Diacetone alcohol Butyl Cellosolve Morpholine Pyridine Cellosolve n-Butyl alcohol Ethyl alcohol Dimethyl sulfoxide n-Propyl alcohol Dimethylformamide Methyl alcohol Propylene glycol Ethylene glycol Glycerol Water

(7.0) 7.24 7.25 (7.4) 7.62 8.18 (8.5) 8.57 8.65 8.85 8.91 9.10 9.15 9.21 9.28 9.52 9.60 9.77 9.76 9.93 10.18 10.24 10.52 10.61 11.88 11.30 12.92 12.93 11.97 12.14 14.28 14.80 16.30 21.10 23.5

Sources: Hildebrand values from Hansen.6 Values in parenthesis from Crowley et al.3

term hildebrands be adopted for these units, in recognition of Dr. Hildebrand’s contribution. Table 1 lists solvents arranged according to their solubility parameter. In looking over a table of Hildebrand solubility parameters, it becomes apparent that by ranking solvents according to solubility parameter a solvent spectrum is obtained, with solvents occupying positions in proximity to other solvents of comparable “strength.” For example, if acetone dissolves a particular material, it may likely be soluble in neighboring solvents, like diacetone alcohol or methyl ethyl ketone, since these solvents have similar internal energies. It may not be possible to achieve solutions in solvents farther from acetone on the chart, such as ethyl alcohol or cyclohexane—liquids with very different internal energies. Theoretically, there will be a contiguous group of solvents that will dissolve a particular material, while the rest of the solvents in the spectrum will not. Some materials © 2001 by CRC Press LLC

will dissolve in a large range of solvents, while others might be soluble in only a few. A material that cannot be dissolved at all, such as a thermosetting resin, might exhibit swelling behavior in precisely the same way. Another interesting aspect of the solvent spectrum is that the Hildebrand value of a mixture can be determined by averaging the values of the individual solvents by volume. For example, a mixture of two parts toluene and one part acetone will have a Hildebrand value of 18.7 (18.3  2/3  19.7  1/3), about the same as chloroform. Theoretically, such a 2:1 toluene/acetone mixture should behave similarly to chloroform. Thus, for example, if a resin was soluble in one, it would probably be soluble in the other. What’s attractive about this approach is that it attempts to predict the properties of a mixture using only the properties of its components. No empirical information about the mixture is required. But, as you might expect, this is not completely accurate. Figure 1 plots the swelling behavior of a dried linseed oil film in various solvents arranged according to Hildebrand number. Of the solvents listed, chloroform swells the film to the greatest degree, about six times as much as ethylene dichloride, and over ten times as much as toluene. Solvents with greater differences in Hildebrand value have less swelling effect, and the range of peak swelling occupies less than 2 Hildebrand units. Theoretically, we would expect any solvent or solvent mixture with a Hildebrand value between 19 and 20 to swell a linseed oil film severely. But careful examination of the graph reveals an anomaly. Two solvents with Hildebrand values right in the middle of the severe swelling range, methyl ethyl ketone (19.3) and acetone (19.7), actually cause very little swelling behavior. How can this be? According to the theory, liquids with similar cohesive energy densities should have similar solubility characteristics, and yet actual behavior in this instance does not bear this out. To understand the reason for this discrepancy we need to recall our previous discussion about van der Waals forces. Remember the point about different molecular architectures giving rise to different kinds of polarity? Some materials are made of molecules that

Figure 1

The swelling behavior of linseed oil films in solvents arranged according to solubility parameter. (Adapted from Feller et al.4)

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are strongly polar, especially molecules capable of hydrogen bonding. Other materials are made of molecules that are essentially nonpolar, with intermolecular attractions due entirely to dispersion forces. It is precisely these differences in polarity that also must be taken into account if we want to improve our solubility predictions. The inconsistencies in Figure 1 stem from a difference in hydrogen bonding between chlorinated solvents and ketones. The intermolecular forces in linseed oil are primarily due to dispersion forces, with practically no hydrogen bonding involved. These polar configurations are perfectly matched by the intermolecular forces between chloroform molecules, thus encouraging interpenetration and swelling of the linseed oil polymer. Acetone and methyl ethyl ketone, however, are more polar molecules with moderate hydrogen bonding capabilities. Even though the total cohesive energy densities, and therefore Hildebrand solubility parameters, are similar in all four solvents, the differences in polarity, primarily hydrogen bonding, lead to differences in solubility behavior. A scheme to overcome the inconsistencies caused by hydrogen bonding was proposed by Harry Burrell in 1955.2 His solution was to segregate the solvent spectrum into three separate lists depending on hydrogen bonding capability. This is briefly summarized as follows: 1. Weak hydrogen bonding: Hydrocarbons, including chlorinated and nitrocompounds; 2. Moderate hydrogen bonding: Ketones, esters, ethers, and glycol monoethers; 3. Strong hydrogen bonding: Alcohols, amines, acids, amides, and aldehydes. Accordingly, solvents with similar Hildebrand numbers and similar polarity (especially hydrogen bonding) should exhibit similar solubility. This system of classification does in fact improve the prediction of solvent behavior, and is still widely used in practical applications.

THREE COMPONENT PARAMETERS But it was only a matter of time before researchers needing even greater precision in quantifying solvent behavior began to look at the cumulative effects of several different kinds of van der Waals forces. It was found that by using separate values for dispersion forces, polarity, and hydrogen bonding even greater accuracy was achievable. But the addition of a third value created practical problems in portraying this information, since it could not be presented in a simple list. For this reason most three-component parameter systems were represented as slices through three-dimensional solubility models. Unfortunately, this made developing formulations using solvents outside the slice impractical. The most widely accepted three-component system was developed by Charles M. Hansen in 1966. Hansen took the existing three values farther by relating all three to the total Hildebrand value. This was done by first getting the dispersion parameter of a solvent from the Hildebrand value of the nonpolar molecule most closely resembling it in size and structure (as n-butane would be to n-butyl alcohol). Hansen then used trial-and-error experimentation on numerous solvents and polymers to separate the polar value into polar and hydrogen bonding component parameters best reflecting empirical evidence. Hansen also used three-dimensional models but also found that, by doubling the dispersion parameter axis, an approximately spherical volume of solubility could be formed. This volume, being spherical, could then be described in a simple manner: the coordinates at the center of the solubility sphere could be located by means of the three component © 2001 by CRC Press LLC

parameters, along with the radius of the sphere, which he called the interaction radius (R). The mathematics involved are inconvenient, however, especially where solvent blends are concerned. THE TEAS GRAPH Given the awkwardness of presenting three-component data, Jean P. Teas devised a triangular graph in 1968, on which polymer solubility areas could be drawn in their entirety. Because of its clarity and ease of use, the Teas graph has found increasing application in problem solving, documentation, and analysis, and is an excellent vehicle for understanding complex solubility behavior. To plot all three parameters on a single planar graph, however, a departure had to be taken from established theory. Teas constructed his graph on the fantastic hypothesis that all materials have the same Hildebrand value. According to this assumption, solubility behavior is determined, not by differences in the Hildebrand value, but by the relative amounts the three component forces (dispersion force, polar force, and hydrogen bonding force) contribute to that value. This allows us the convenience of related percentages rather than unrelated sums. But because Hildebrand values are not the same for all liquids, it should be remembered that the Teas graph is somewhat more empirical than theoretical. Fortunately, this does not prevent it from being an accurate and useful tool, and perhaps the most convenient method by which solubility information can be illustrated. Hansen derived his parameters from the Hildebrand value: when all three Hansen parameters for a solvent are added together, their sum will be the Hildebrand value for that solvent. Teas parameters, also called fractional parameters, are mathematically derived from Hansen values by calculating the relative amount that each Hansen parameter contributes to the whole. In other words, when all three of Teas’s fractional parameters are added together, their sum will always be 100. Once this step has been taken, the rest is easy. Now the intersection of these three values can be easily located on a triangular grid (Figure 2).

Figure 2

Any point on a triangular Teas graph is the intersection of three percentage values.

© 2001 by CRC Press LLC

If we examine a Teas graph containing the locations of many solvents we can see that the alkanes, whose only intermolecular bonding is due to dispersion forces, are located in the far lower right corner, the corner corresponding to 100% dispersion forces. Moving toward the lower left corner are solvents with increasing hydrogen bonding contribution. Moving from the bottom of the graph upward are solvents with polarity due less to hydrogen bonding than to an increasingly greater dipole moment of the molecule as a whole. Overall, the solvents are grouped closer to the lower right apex than the others. This is because the dispersion force is present in all molecules, polar or not, and determining the dispersion component is the first calculation in assigning Hansen parameters, from which fractional parameters are derived. Unfortunately, this greatly overemphasizes the dispersion force relative to polar forces, especially hydrogen bonding interactions. It can also be seen that increasing molecular weight within each class shifts the relative position of a solvent on the graph closer to the bottom right apex (Figure 3). This is because, as molecular weight increases, the polar part of the molecule that causes the specific character identifying it with its class, called the functional group, is increasingly “diluted” by progressively larger, nonpolar “aliphatic” molecular segments. This gives the molecule as a whole relatively more dispersion force and less of the polar character specific to its class. This trend toward less polarity with increasing molecular weight within a class also accounts for the observation that lower-molecular-weight solvent’s are often “stronger” than higher-molecular-weight solvents of the same class, although determinations of solvent strength must really be made in terms of the solvent’s position relative to the solubility area of the solute. (Another reason for low-molecular-weight solvents seeming more active is that smaller molecules can disperse throughout solid materials more rapidly than their bulkier relatives.) The only class in which increasing molecular weight places the solvent farther away from the lower right corner is the alkanes. As previously stated, the intermolecular attractions between alkanes are due entirely to dispersion forces, and, accordingly, Hansen parameter values for alkanes show zero polar contribution and zero hydrogen bonding contribution. Since fractional parameters are derived from Hansen parameters, we would expect all the alkanes to be placed together at the extreme right apex. Observed behavior indicates, however, that different alkanes do have different solubility characteristics, perhaps because of the tendency of larger dispersion forces to mimic

Figure 3

Within each class, solvents with higher molecular weight tend to be closer to the lower right axis.

© 2001 by CRC Press LLC

slightly polar interactions. For this reason, Teas adjusted the locations of the alkanes to correspond to empirical evidence. Several other solvent locations were also shifted slightly to reflect observed solubility characteristics properly. The position of water on the chart is uncertain, because of the ionic character of the water molecule, and the placement in this paper is according to Teas.12 The presence of water in a solvent blend, however, can alter dramatically the accuracy of solubility predictions. VISUALIZING SOLUBILITY Now that solvent positions are located on the Teas graph, we can discuss (and visualize) complex solubility behavior. For example, it is easy to describe the solubility of a material by testing solvents on it and coding the results. Active solvents could have their positions marked with a star, marginal solvents with a filled circle, and nonsolvents with hollow circles. Once this is done, a solid area of stars will be seen, possibly with a border of filled circles. This would define the solubility window of the material (Figure 4). Not every solvent would need to be tested in this way; only enough to circumscribe the area. The boundaries of this solubility window could be more accurately defined by using two liquids near the edge of the solubility window, one within the window and one outside. The material could then be tested in various mixtures of these two liquids, and the mixture just producing solubility noted on the graph. If this procedure is repeated in several locations around the edge of the window, its boundaries may be accurately determined. Interestingly, some composite materials (such as rubber/resin pressure-sensitive adhesives, or wax/resin mixtures) can exhibit two or more separate solubility windows, more or less overlapping, that reflect the degree of compatibility and the concentration of the original components. The solubility window of a material will have a specific size, shape, and placement on the Teas graph depending on its polarity and molecular weight, and the temperature and concentration at which the measurements are made. Most published solubility data are derived from 10% concentrations at room temperature. Heat has the effect of increasing the size of the solubility window, because of an increase in the disorder (entropy).

Figure 4

The solubility window of a hypothetical material.

© 2001 by CRC Press LLC

Concentration also has an effect on solubility. Most polymer solubility windows are determined at 10% concentration of polymer in solvent. Because an increase in concentration also causes an increase in disorder, solubility information can be considered accurate for solutions of higher concentration as well. Solvent evaporation as the solution dries serves to increase concentration, thus ensuring that the two materials stay mixed. Solutions of less than 10% may become immiscible, however, especially with solvent combinations at the edge of the solubility window. Solution viscosity of a polymer will also vary depending on where solvent is located in the solubility window of the polymer. We might expect viscosity to be at a minimum when a solvent near the center of a polymer solubility window is used. However, this is not the case. Solvents at the center of a polymer solubility window dissolve the polymer so effectively that the individual polymer molecules are free to uncoil and stretch out. In this condition molecular surface area is increased, with a corresponding increase in intermolecular attractions. The molecules thus tend to attract and tangle on each other, resulting in solutions of slightly higher than normal viscosity. When dissolved in solvents slightly off center in the solubility window, polymer molecules stay coiled and grouped together into microscopic clumps, which tend to slide over one another, resulting in solutions of lower viscosity. As solvents nearer and nearer the edge of the solubility window are used to dissolve the polymer, however, these clumps become progressively larger and more connected and viscosity again increases until ultimately separation occurs as the region of the solubility boundary is crossed. The position of a solvent in the solubility window of a polymer has a marked effect on the dried film characteristics of the polymer as well. Because of the uncoiling of the polymer molecule, films cast from solvent solutions near the center of the solubility window exhibit greater adhesion to compatible substrates. This is due to the increase in polymer surface area that comes in contact with the substrate. Many other properties of dried films, such as plastic crazing or gas permeability, are related to the relative position that the original solvent occupied in the solubility window of the polymer. The degree of both crazing and permeability is predictably less when solvents more central to the solubility window have been used.

SOLVENT MIXTURES The Teas graph is particularly useful for creating solvent mixtures for specific applications. Solvents can easily be blended to exhibit critical solubility behavior such as dissolving one material but not another. The use of the Teas graph can reduce trial-and-error experimentation to a minimum, by allowing the solubility behavior of mixtures to be predicted in advance. This is because the solubility parameters of a mixture can be simply calculated by averaging its components. We can then expect the mixture to behave more or less like the single solvent with that solubility parameter. Determining the solubility parameters of a mixture can be done either by calculating from the fractional parameters of the individual solvents, or in the case of a binary mixture, by simply drawing a line between its two solvents and measuring their ratio. To calculate the solubility parameters from the individual components, the fractional parameters for each liquid are multiplied by the fraction that the liquid occupies in the blend, and the results for each parameter added together. Or, graphically, a mixture can be located by drawing a line connecting the two solvents and determining the distance that represents their ratio. © 2001 by CRC Press LLC

Figure 5

A mixture (M) of two nonsolvents (A and B) may act as a true solvent if the parameters of the mixture fall within the solubility window.

What is interesting about visualizing solvent blends on a Teas graph is the control with which effective solvent mixtures can be formulated. For example, two liquids that are nonsolvents for a specific polymer can sometimes be blended in such a way that the mixture will act as a true solvent (Figure 5). This is possible if the graph position of the mixture lies inside the solubility window of the polymer, and is most effective if the distance of the nonsolvents from the edge of the solubility window is not too great. This phenomenon is also valuable when selective solvent action is required, such as in selectively dissolving one material while leaving other materials unaffected, particularly if the solubilities of the materials involved are very similar. In this case it is helpful first to plot the solubility windows of all the materials in question. Once this has been done, it is easy to see the overlap of solubilities, and any areas where solubilities are mutually exclusive. A solvent blend can then be formulated that actively dissolves the proper material, while positioned as far away from the solubility window of the other material as possible (Figure 6). It is important to remember that differences in evaporation rates can shift the solubility parameter of the blend as the solvents evaporate, and this must be taken into account. Additionally, while a material may not show immediate signs of solution in a solvent or solvent blend, the solvent may still adversely affect the material, for example, by softening it or leaching out partial components. A further advantage of blending solvents is the ability to design mixtures with similar solubility characteristics and lower toxicity. In such cases, it should be pointed out that the similarity between solvents and blends having the same numerical parameters decreases as the distance between the components of the blend increases. Where alternate blends are effective, however, the use of a less toxic replacement can be a sensible choice, and the Teas graph a useful tool (Figure 7).

© 2001 by CRC Press LLC

Figure 6

In situations where one material must be dissolved while another remains unaffected, a solvent blend that falls within the solubility window of the first and outside the window of the second may be effective.

Figure 7

The Teas graph (numbers indicate solvents listed in Table 2).

© 2001 by CRC Press LLC

Table 2 Fractional Solubility Parameters Position

Solvent

100d

100p

100h

Alkanes 1 1 1 1 2 3 4

n-Pentane n-Hexane n-Heptane n-Dodecane Cyclohexane VMP naphtha Mineral spirits

100 100 100 100 94 94 90

0 0 0 0 2 3 4

0 0 0 0 4 3 6

8 7 5 8 4 3 0

14 13 12 22 18 10 3

21 19 12 12 2 19 17 10

20 14 21 20 13 11 8 0

13 19 7 22 20 18 21 23 18

23 26 26 39 38 36 35 29 36

32 [30] 28 27 24 22 20

21 [17] 17 17 21 20 18

Aromatic Hydrocarbons 5 6 7 8 9 10 11

Benzene Toluene o-Xylene Naphthalene Styrene Ethylbenzene p-Diethylbenzene

78 80 83 70 78 87 97 Halogen Compounds

12 13 14 15 16 17 18 19

Methylene chloride Ethylene dichloride Chloroform Trichloroethylene Carbon tetrachloride 1,1,1-Trichloroethane Chlorobenzene Trichlorotrifluoroethane

59 67 67 68 85 70 65 90 Ethers

20 21 22 23 24 25 26 27 25

Diethyl ether Tetrahydrofuran Dioxane Methyl Cellosolve Cellosolve Butyl Cellosolve Methyl carbitol Carbitol Butyl carbitol

64 55 67 39 42 46 44 48 46 Ketones

28 29 30

31 32

Acetone Methyl ethyl ketone Cyclohexanone Diethyl ketone Mesityl oxide Methyl isobutyl ketone Methyl isoamyl ketone

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47 [53] 55 56 55 58 62

Table 2 Fractional Solubility Parameters (continued ) Position

Solvent

100d

100p

100h

33

Isophorone Di-isobutyl ketone

51 [67]

25 [16]

24 [17]

36 38 18 [37] 12 39 13 15 12 12 15 21 20

19 14 31 [24] 24 19 27 25 25 28 34 35 32

45 41 47 43 37 36 26 15 19 32 30 [12] 42 32

16 15 13 13 13 12 18 28 31 20 32 [24] 30 27

8 36

4 23

22 18 16 [16] 15 [16] 16 12 13

48 46 44 [43] 42 [40] 36 38 41

Esters 34 35 36

37 38 39 40

Methyl acetate Propylene carbonate Ethyl acetate Trimethyl phosphate Diethyl carbonate Diethyl sulfate n-Butyl acetate Isobutyl acetate Isobutyl isobutyrate Isoamyl acetate Cellosolve acetate Ethyl lactate Butyl lactate

45 48 51 [39] 64 42 60 60 63 60 51 44 40

Nitrogen Compounds 41 42 43 44 45 46 47 48 49 50 51 52

Acetonitrile Butyronitrile Nitromethane Nitroethane 2-Nitropropane Nitrobenzene Pyridine Morpholine Aniline N-Methyl-2-pyrrolidone Diethylenetriamine Cyclohexylamine Formamide N,N-Dimethylformamide

39 44 40 44 50 52 56 57 50 48 38 [64] 28 41

Sulfur Compounds 53 54

Carbon disulfide Dimethyl sulfoxide

88 41 Alcohols

55 56 57 58 59

60 61

Methanol Ethanol 1-Propanol 2-Propanol 1-Butanol 2-Butanol Benzyl alcohol Cyclohexanol n-amyl alcohol

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30 36 40 [41] 43 [44] 48 50 46

Table 2 Fractional Solubility Parameters (continued ) Position

Solvent

62

Diacetone alcohol 2-Ethyl-1-hexanol 2-Ethyl-1-butanol

100d 45 50 48

100p

100h

24 9 10

31 41 42

18 23 16 29 28

52 52 50 40 54

15 23 18 20 [28] [22] [14] [13] 17 15 14 14 11 0

39 16 5 5 [39] [38] [24] [22] 17 18 17 16 14 0

Polyhydric Alcohols 63 64 65 66 67

Ethylene glycol Glycerol Propylene glycol Diethylene glycol Water

30 25 34 31 18 Miscellaneous Liquids

68 69 70 71

1

Phenol Benzaldehyde Turpentine Dipentene Formic acid Acetic acid Oleic acid Stearic acid Linseed oil Cottonseed oil Neets foot oil Pine oil Sperm oil Mineral oil

46 61 77 75 [33] [40] [62] [65] 66 67 69 70 75 100

Note: Numbers in left column refer to solvent positions in Teas graph, Figure 7. Sources: Values from Gardon and Teas.5 Values in brackets derived from Hansen’s original 1971 parameters recalculated by the author.

REFERENCES 1. Burrell, H., Solubility parameters, Interchem. Rev., 14, 13–16 and 31–46, 1955. 2. Burrell, H., Solubility parameters for film formers, Off. Dig. Paint Varn. Prod. Clubs, 27, 726, 1955. 3. Crowley, J.D., G.S. Teague, Jr., and J.W. Lowe, Jr., A three dimensional approach to solubility, J. Paint Technol., 38 (496), 1966. 4. Feller, R.L., N. Stolow, and E.H. Jones, On Picture Varnishes and Their Solvents, The Press of Case Western Reserve University, Cleveland, 1971. 5. Gardon, J.L. and Teas, J.P., Solubility parameters, in Treatise on Coatings, Vol. 2, Characterization of Coatings: Physical Techniques, Part II, Myers, R.R. and J.S. Long, Eds., Marcel Dekker, New York, 1976. 6. Hansen, C.M., The three dimensional solubility parameter—key to paint component affinities. I. Solvents plasticizers, polymers, and resins, J. Paint Technol., 39(505), 1967. 7. Hansen, C.M., The three dimensional solubility parameter—key to paint component affinities. II. Dyes, emulsifiers, mutual solubility and compatibility, and pigments, J. Paint Technol., 39(511), 1967. 8. Hansen, C.M., The three dimensional solubility parameter—key to paint component affinities. III. Independent calculations of the parameter components, J. Paint Technol., 39(511), 1967. 9. Hansen, C.M., Solubility in the coatings industry, Skand. Tidskr. Faerg. Lack, 17, 69, 1971 (English). 10. Hildebrand, J.H., The Solubility of Non-Electrolytes, Reinhold, New York, 1936. 11. Teas, J.P., Graphic analysis of resin solubilities, J. Paint Technol., 40(516), 1968. 12. Teas, J.P., Predicting resin solubilities. Columbus, Ohio, Ashland Chemcial Technical Bulletin, No., 1206, 1971. © 2001 by CRC Press LLC

CHAPTER 1.3

Aqueous Cleaning Essentials Rick Bockhorst, Michael Beeks, and David Keller

CONTENTS Introduction Cleaning Overview Cleaning Parameters Temperature Agitation Concentration Time Required for Cleaning Rinsing Redeposition Protecting the Substrate Equipment Controlling the Cleaning Line Improving Bath Life Water Physical Properties of Water Impurities Water Pretreatment Softening General Principles of Water Softening Deionization Reverse Osmosis General RO Components Cleaning Formulations Cleaning Chemistry Acid Cleaners Alkaline and Neutral Cleaners Ingredients Alkaline Builders © 2001 by CRC Press LLC

Water Conditioners Surface-Active Agents Corrosion Inhibitors Additional Ingredients Hydrocarbon Solvents Rinsing Importance of Rinsing Part Cleanliness Required Production Levels and Dragin Incoming Water Quality Number of Rinse Tanks Rinse Tank Design and Placement Dragin and Final Rinse Quality Disposal Conclusion Acknowledgment References

INTRODUCTION Largely as a result of environmental pressures, parts cleaners have been forced to explore alternatives to the various solvent cleaning processes and especially vapor degreasing. These pressures have come in the form of international, federal, state and local regulations, but the most significant influence has probably been the Montreal Protocols. While aqueous cleaning is almost as old as humanity, parts cleaners have long held that certain soils could be cleaned adequately only by nonaqueous methods. The reality is that many cleaning applications that were once strictly the province of nonaqueous cleaning methods are now being done quite successfully with aqueous processes.

CLEANING OVERVIEW * With few exceptions there are certain principles, treated generally here, that apply to all types of cleaning. Cleaning processes combine mechanical, thermal, and chemical energy sources to remove a soil from a substrate. The total energy needed is the sum of these energy sources over a given period. Within these parameters the following general guidelines apply: 1. Cleaning efficacy and rate improve as temperature increases. 2. Agitation improves the rate and efficacy of soil removal. Agitation provides mechanical energy to remove soils physically and assures that fresh cleaner will continuously contact the soil. 3. Cleaner solutions generally have a performance vs. concentration curve. A minimum level of cleaner is generally necessary for effective cleaning. Cleaning improves with incremental increases of cleaner up to some point, where further increases result in little or no further improvement in performance. 4. Time is the controlling factor in total energy input. If cleaning requires X mechanical energy for T time and the energy level is then decreased, to compensate the time must be increased proportionally. * Reference 16, Chapter 3. © 2001 by CRC Press LLC

5. Rinsing is necessary to remove any cleaner or soil residue remaining on the parts after washing. • Rinse type and quality are dependent on the cleanliness requirements of the application. • Multiple small rinses are generally more efficient and cost-effective than one large rinse. • An agitated rinse is generally more effective than a still rinse. • Final part cleanliness or, conversely, residue on the part is limited by rinse quality. 6. Soil must be prevented from redepositing on parts. The most obvious answer is to remove the soils from contact with the substrate. This may be accomplished by various methods, including: • Emulsification; • Emulsification followed by demulsification and physical removal; • Flocculation; • Ultra- or microfiltration. Additionally, redeposition can be controlled by: • Choosing cleaners formulated with “antiredeposition” properties; • Using cleaning tanks of sufficient size to disperse the soil and slow the rate of increase of contamination concentration. 7. The cleaning method or solution should not harm the item (substrate) being cleaned. 8. Precleaning to remove bulk soils may be an economical and commonsense way to increase cleaner life. 9. Cleaning systems should be designed as a unit. That is, the cleaner and the cleaning equipment should be chosen to work together and address the particular cleaning application. Typical concerns that should be addressed include: • Cleaning temperature and its effect on the cleaner as well as the parts being cleaned. Part of this consideration includes method of heating, insulation, and evaporation. • Equipment design, which should include an evaluation of cleaner and part compatibility with regard to materials of construction, economy of operation, electrochemistry, OSHA and other regulatory guidelines, and ease of service. • The compatibility of the mechanical energy input, which must be addressed in terms of effectiveness of removing soil from the substrate, controlling foaming tendencies of the cleaner, avoiding mechanical damage to the parts, and avoiding degradation of the cleaner. Aqueous cleaners can generally be categorized as being acidic, alkaline, or pH-neutral. Alkaline cleaners are by far the predominant of the three used in all commercial/ industrial cleaning applications. Thus, this discussion will mainly cover alkaline cleaning, but the principles are applicable to all types of cleaners. Agitation techniques represent the greatest variation in cleaning methods. The most important factor is that it costs money in equipment and/or labor to provide high levels of agitation. The equipment must be designed to meet the objective of providing adequate-tosuperior agitation for soil removal at the lowest cost. The major limitations for providing adequate agitation are equipment costs, equipment size (i.e., how big of a “footprint” does the equipment have), excessive foam generation, excessive generation of mist/spray, toxic vapors, or flammable gases. © 2001 by CRC Press LLC

Now that we have taken a broad look at some cleaning principles, let us look a little more closely at each. CLEANING PARAMETERS Temperature The effect of temperature depends on the type of soil being removed and the cleaner. The first consideration is what type of soil needs to be removed. Temperature is very important in speeding the removal of fats, greases, oils, and waxes. Increased temperature reduces the viscosity of oils and greases, making them more mobile, and therefore easier to displace from the substrate. Fats and waxes are often solids at room temperature. It is critical to melt these fats and waxes to remove them by aqueous methods. If the melt range of the fat/wax is above the boiling point of the cleaner, aqueous cleaning may not be effective on this type of soil. There is a well-established principle that the rate of a chemical reaction is doubled for each 10°C (18°F) increase in temperature. If the cleaning process works by reaction between a fatty acid/oil and alkali, by a paint coating undergoing chemical decomposition, or by an acid chemically removing rust and scale, then this reaction rate relationship is applicable. It is possible to remove solid fats/waxes if they can chemically react with the cleaner without melting them; however, the rate of removal may not be adequate. On the other hand, excessively high temperatures could “set” proteinaceous soils or may cause an undesirable reaction between the soil and the substrate, making the soil more difficult to remove. Just as increasing temperature will increase the rate of cleaning, it will increase the rate of undesired reactions. Most corrosion inhibitors work by forming a loose barrier on the clean metal surface. Excessively high temperature can disrupt this barrier and result in chemical attack, usually seen as discoloration and etching. The majority of industrial cleaning is carried out at 140 to 180°F. Agitation As has been previously stated, agitation techniques represent the greatest variation in aqueous cleaning systems. It is usually possible to find an aqueous cleaner to remove a soil from a substrate. Thus, one of the biggest problems users run into is inadequate cleaning performance because of inadequate or improper choice of agitation. The method of agitation should be matched to the size and shape of the part. For example, while spray washing may be very effective for cleaning large relatively flat parts, it may not be suitable for parts with blind holes where direct impingement is problematic. Relatively flat objects and components that do not have hidden areas can be cleaned by immersion or spray wash. Typically, parts too small or too large cannot be cleaned by spray wash; an exception for small parts is possible when specialty mounting racks are built. Spray wash cleaning is also limited on the chemical side because the formula must not be foamy. Eliminating foam restricts the choices of raw materials a chemist can use in formulating a spray wash cleaner. Parts that can be damaged from spray impingement should only be cleaned by immersion. Virtually all parts can be cleaned by immersion. Ultrasonic cleaning is the most effective method of agitation for immersion cleaning, but is restricted by cost (very expensive equipment) and size (not as effective on tanks above
1000-gal capacity). Spray-under immersion and turbulation are the next most effective methods of immersion agitation. Spray-under immersion and turbulation can sometimes create excessive foam if care is not

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taken in choosing a cleaner. General agitation from pump circulation can be adequate for noncritical cleaning applications, but is not usually suitable for precision cleaning. Air sparging can very effectively agitate an aqueous cleaner, but it has several restrictions: 1. The cleaner must be a low-foaming to nonfoaming product. Air sparging may cause foam to overflow the tank. Basically, cleaners designed for spray washing should only be used with this form of agitation. 2. The cleaning tank should be covered to eliminate generation of mist and spray. The popping of the bubbles will generate mist and spray that will create an exposure problem for workers. The only cure for this is to cover the tank or place the tank within a cabinet to contain the mist/spray. It is usually impractical to put a cover on most immersion tanks because of the mechanics of moving the cover and parts in and out of the tank. Adding on a cabinet increases the cost of the cleaning system and will likely increase the “footprint” taken up in the facility. 3. Air sparging can shorten the life of alkaline cleaners, especially heavy-duty caustic cleaners, by neutralization with carbon dioxide (CO2), a weak acid. Acids and bases will neutralize each other. Even though it makes up only a fraction of a percent of the atmosphere (0.035% measured at Mauna Loa Observatory, 1990, as reported in Handbook of Chemistry and Physics, pp. 14–25, 199620), the large volume of air passed through the tank will expose the cleaner to a significant amount of CO2. The CO2 will neutralize the alkaline builders, especially sodium or potassium hydroxide. Heavy-duty caustic cleaners are especially prone to this problem, whereas mildly alkaline cleaners are much less sensitive. The CO2 reacts with free hydroxide ions (OH  , the cause of alkaline pH) to form bicarbonate ions: CO2  OH → HCO3 The bicarbonate then goes on to react with more hydroxide ions to form a carbonate ion and water: HCO3  OH → CO32  H2O Thus, as the hydroxide ions (OH ) are consumed, the pH drops in the cleaner. This can adversely affect alkaline cleaners, especially heavy-duty caustic cleaners used in descaling operations. Acidic and pH-neutral cleaners are not affected by this problem. Finally, soaking the substrate in a stagnant tank is unacceptable, even for crude cleaning applications.

Concentration Concentration, also called use dilution can affect multiple attributes of the cleaning process. In many cases minimum or maximum cleaner concentrations can control corrosion characteristics, chemical etching, or the deposition of protective barriers as well as cleaning efficacy. The necessary cleaner concentration will vary with the type of agitation and temperature. As an example, in the absence of foaming problems, it may be possible to obtain similar cleaning performance from the same alkaline cleaner at 5 to 10% by immersion, 3 to 5% by spray, 1 to 3% by steam cleaning or high-pressure hot spray, or 2 to 4% in

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a high-pressure, room-temperature spray. For each of these applications, increasing the cleaner concentration will give better cleaning performance up to a certain point and then level off. This leveling-off point will be dependent on the specific chemistry used in the cleaner, the soil being removed, the agitation technique, and temperature. Time Required for Cleaning The cleaning time is dependent on the concentration of the cleaner, the specific chemistry used in the cleaner, the soil being removed, the agitation technique, and the temperature. It is important to emphasize that cleaning is not instantaneous; some time is necessary for the detergent to perform its work on the soil. In a stagnant bath, cleaning may take from 5 min to over an hour to occur, if at all. Most immersion cleaning situations do not exceed 10 min, although numerous exceptions can be found. Spray washes typically take no more than 5 min. One general rule of thumb does exist for ultrasonic cleaning; if it takes more than 5 min to clean, either there is something deficient in the cleaner or the process itself is not suited for the application. One notable exception to the “5-min rule for ultrasonics” is the cleaning of used automobile cylinder heads and other major engine components. Many automobile repair facilities have adopted ultrasonic processes as an alternative to heavyduty caustic or solvent cleaning tanks. The heavy-duty caustic tanks have fallen out of favor because they cannot be used on aluminum parts; nearly all major engine parts are now made of aluminum. Corrosion hazards and waste disposal considerations also have contributed to the decline of the caustic stripping tank. Solvents have been on the decline mainly because of environmental regulations that govern volatile organic content (VOC), ozone-depleting substances (ODSs), and hazardous air pollutants (HAPs). The few solvents (cresylic acids) that are effective on baked-on soils also pose serious health risks. Ultrasonic cleaning with highly concentrated, moderately alkaline cleaners has been found to be effective at removing most of the baked-on soils. The drawbacks include long processing times, often 15 to 45 min, and some cavitational erosion. The cavitational erosion appears as a “star-” or “Y-shaped” pattern on the metal. It must be stressed that the time available for cleaning is very closely related to the economics of the cleaning operation. The increased cost in equipment, energy, and chemicals to reduce time must be weighed against the economic gains in increased production. Additionally, consideration must be given to effects on reject rates and customer satisfaction. Rinsing No matter what cleaning method has been employed, the surface of the freshly cleaned part will contain some amount of soil and cleaner residue. In some situations this residue may present no problem, but in many others that residue must be removed to yield acceptable parts. This is an important issue. Just as with solvent rinses, water rinses may contain impurities that can dry on the parts. Water quality issues will be discussed later. The value of pressure sprays and mechanical action in rinsing is often neglected. Experience shows that direct spraying is far more effective in flushing away the loosened soil than just soaking the piece in an immersion tank. The use of a short spray rinse followed by a soak or agitated rinse to reduce contamination level is most effective at reducing contamination. Static or slow-moving rinses in which there is improper/inadequate flow usually result in parts that need to be recleaned or discarded. © 2001 by CRC Press LLC

Another important consideration in rinsing is the number of rinses performed; two rinse steps are more effective than one, three are better than two, etc. Multiple rinses can be of shorter individual duration and still be more effective than single rinses because of the exponential dilution of contaminants as the parts proceed from one rinse to the next. Unfortunately, multiple rinses can lead to higher costs in equipment (i.e., number of tanks needed, footprint taken up on the plant floor). The cost can be offset by counterflowing the water from the last rinse back into each previous rinse. This significantly reduces water consumption; some of the overflow can be used as add-back into the cleaning tank to replace evaporated water. Studies have shown that a counterflowed, triple rinse has the optimum balance of reducing water consumption, obtaining clean parts, and capital costs in the rinse step; more than three rinses yields diminishing returns. It is possible in some situations to equip the multistage rinse with a set of deionizing resin beds and an activated carbon filter. Closing the rinsing loop by deionizing the overflow water can reduce water consumption, replacing only the water lost to evaporation. Users with significant wastewater disposal costs should consider this kind of setup. For further reading, see Peterson13 and Spring.16 Another important consideration is the quality of water used in the rinse step. The quality of rinsing can only be as good as the quality of water used in the last rinse. Unsoftened water obtained from a municipal source or well often contains varying levels of hard water ions, carbonates, phosphates, and organic by-products from treatment processes. The water hardness can often be extremely high, leading to hard water deposits and soap scum residue. Softening the water to remove hard water ions (calcium and magnesium) will eliminate those hard water salts but leaves other impurities and therefore may not be adequate. Using deionized, distilled, or reverse-osmosis purity water gives the best rinsing performance. As usual, as the quality of water increases, so does the cost. The level of performance must meet the requirements of the application. The application requirements must be evaluated for each system.

Redeposition The design of the tank and cleaner is an important factor in reducing/eliminating redeposition of soils. The tank must be of sufficient capacity to provide room for the soils to move away from the parts. The size is also important to moderate the rate at which soil loading increases; too small of a tank can result in the cleaner becoming saturated in a matter of hours or days. Bag filters can be used to remove gross particulate matter. The cleaner may incorporate phosphates, silicates, specialty surfactants, and synthetic polymers, which remove and suspend soils in solution. Use of aqueous cleaners that can “splitout” oils instead of emulsifying them in combination with an oil coalescer/skimmer will slow down, possibly even prevent, the soil from reaching a saturated condition in the cleaner. This oil splitting followed by physical removal will lead to longer tank life and prevent redeposition. Self-emulsifying oils are difficult to handle in preventing redeposition. These lubricants contain their own surfactants that form stable emulsions. In general, aqueous cleaners cannot break these emulsions without being consumed themselves. The only viable alternative is to use microfiltration in the 0.1 to 0.5
m pore size range to remove the emulsion. The filter will not remove all of the emulsion but will remove some or most. The cleaner must also be designed to be able to pass through the filter. Micro- or ultrafiltration has been successfully done but generally results in some loss of cleaner constituents. Thus, monitoring and adjusting the tank becomes difficult and may not be an

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economically viable option. A good rinse must always follow the cleaning step to prevent soil redeposition. Waxes typically require a hot rinse to prevent resolidification on the clean surface. Protecting the Substrate The cleaner must be compatible with the substrate. Unless these effects are necessary, the cleaner must not discolor, etch, cause hydrogen embrittlement, cause stress cracking, or otherwise damage the substrate. Thus, the cleaner must contain additives to protect the substrate from these effects. For example, aluminum cleaners may either contain silicates and have an elevated pH or have a pH of around 7 to 9 to prevent corrosion. Stainless steel cleaners must not contain chlorides because they will cause stress cracking, especially in acidic cleaners. Rusting on steel may be avoided with strongly alkaline cleaners, especially cleaners containing amine-based corrosion inhibitors. High-chrome steels are especially prone to rusting and often need to have a corrosion inhibitor added to the rinse tanks to prevent rusting during the rinse and dry cycles. Acid cleaners sometimes require inhibitors to minimize corrosion of the base metal without reducing the efficacy on hard water or metal oxide scales. Good rinsing, careful drying, or use of inhibitors may avoid tarnishing. Copper and copper-based alloys are notorious for tarnishing during the drying cycle. Transfer time between cleaning and rinsing tanks should be minimized to avoid drying of cleaner residue on the substrate. Equipment Some of the greatest advances in the art of cleaning in the past decade relate to improvements in equipment. Intelligent, appropriate use and care of equipment may be the key to proper cleaning in many instances. Controlling the Cleaning Line It is necessary to determine when the cleaner is nearly exhausted so that fresh cleaner can be prepared or the old cleaner can be rejuvenated. This is not always easy to determine. Measuring properties such as alkalinity, conductivity, and pH are useful in determining the state of the cleaner, but it is not uncommon for tanks to fail even when the above-mentioned test results are within specifications. The properties that should be measured are those that are critical to the specific process. For example, silicate-based aluminum cleaners should be monitored mainly for pH; silicate testing should be done if affordable. The silicates protect aluminum from corrosion and they participate in soil anti-redeposition. The problem with silicates is that their solubility in water decreases as the pH drops in the cleaner. At some point, the silicate level will drop below the minimum level necessary to protect aluminum. When this happens, spotting or etching may occur. Heavy-duty caustic cleaners are best monitored by active and total alkalinity titration methods. Acid cleaners are best monitored by total acidity titration methods. Cleaners that undergo microfiltration to remove emulsified oils present the greatest difficulty in monitoring. The pH may need to be monitored if the cleaner contains silicates. Alkalinity titration is useful, but does not detect many of the surfactants that are stripped out by the filtration process. The emulsified oils may have ingredients that will severely impact the pH and skew the alkalinity titration. Monitoring how much the solution bends light, called the refractive index, can help in tracking loss of cleaner to stripping by the filter.

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Unfortunately, this measurement requires that the solution be relatively clear; cloudy solutions are difficult to impossible to measure. Some tanks are so difficult to maintain that the user must seek out a chemical maintenance firm or change some of the manufacturing processes that precede the cleaning step to eliminate some of these problems. Improving Bath Life One way of improving bath life is to have two cleaning tanks in series. Most of the soil is removed in the first tank. The part going into the second tank is relatively clean. After the first tank becomes heavily contaminated it can be discarded and the cleaner in the second tank pumped over to the first tank. In this manner the cleaners can be used for long periods and, if handled intelligently, it is possible to operate with the first bath very heavily contaminated so that cleaner is seldom discarded. Contamination of the second tank can be reduced by employing a short rinse after the first cleaning operation, the rinsing water either going to sewer or back to the first tank. Besides better performance and economy, this procedure may be necessitated by governmental regulation of effluent that prohibits or limits the discarding of cleaning baths. The single biggest problem in obtaining long tank life is understanding that cleaning is part of the manufacturing process and must be evaluated as part of the whole. The design of the cleaning process must consider the soils and their effects not only on processing but on disposal. The choice of cleaning equipment and of cleaner should be made as a coordinated effort that takes into account soil removal from the part and from the bath. These choices must also consider the ultimate costs of operation, including the costs of disposal. It is in this area that chemical and equipment manufacturers and their customers most often fail. Great strides can be made that will allow aqueous baths with suitable replenishment to be useful for multiple years, but cooperation of chemical manufacturers, equipment manufacturers, and industrial users is critical.

WATER For most aqueous cleaners, water comprises 80 to 99% of the cleaning solution and is used in practically all rinsing steps. Although most people do not think of it in this way, water is actually a solvent in aqueous cleaners. A major key to understanding the efficacy of aqueous cleaners lies in the role played by water, its natural properties, and impurities. Water has been vital to life and nature from the beginning of time. The basic cycle by which water evaporates, condenses, and flows along the surface of the Earth governs all animal and plant life. Approximately 61.8% of the human body is water. Water covers almost 70% of the Earth’s surface, most of it in oceans, with the balance found in lakes, rivers, the atmosphere, and absorbed in soil and rocks. Water is never absolutely pure in nature and its impurities are the factors of concern in industrial applications. Humans have contributed to impurities found in water sources. One concern of aqueous cleaning is disposal of spent cleaning solutions. When an aqueous cleaner is used to remove contaminants from a surface, the water is basically the solvent in which the cleaning takes place. The importance of its function cannot be overstated. As the solvent, water is able to dissolve and disperse the soils being removed. Additives such as acids, alkalis, chelants, and detergents significantly augment the cleaning process. These additives are not nearly as effective by themselves unless they are dissolved in a solvent, i.e., water. The combination of these additives with water yields the powerful, synergistic effects that are exploited today. © 2001 by CRC Press LLC

Physical Properties of Water Pure water is colorless, odorless, and tasteless. Its chemical formula is H2O, which shows that it is made from the two elements, hydrogen and oxygen, in a ratio of 2:1. These two specific elements combined in a 2:1 ratio yield physical properties unmatched by any other molecule: 1. 2. 3. 4.

Very small size. Not flammable. Very high boiling point for its size. The two elements that make it up are so different that they impart a high polarity to the molecule. 5. The high polarity of water accounts for the high boiling point. It also accounts for: a. The high level of thermal energy that it can absorb per degree of temperature increase (called the heat capacity) and to get it to boil (called the heat of vaporization). The high heat of vaporization is what makes water so effective in steam boiler heat exchanging systems. b. Ability to dissolve numerous substances, especially minerals and other polar substances. c. Inability to dissolve nonpolar substances like fats, greases, and oils. The very high boiling point gives aqueous cleaning the flexibility of using a wide temperature range. The temperature of choice can be fine-tuned to the properties of the soil and substrate. This property also minimizes solvent loss due to evaporation. Water loss due to evaporation may become a concern especially at temperatures above 150°F. The very high boiling point, 212°F, is beneficial in that most aqueous cleaning operations do not exceed 180°F so outright boiling is not a problem. Many substrates cannot tolerate the extreme heat of boiling water without suffering from discoloration, etching, or mechanical deformation. The high heat capacity of water makes it very effective in heating metal parts up to the cleaning temperature of the bath while having minimal impact on the bath temperature. Because metals have a low heat capacity, very little energy, relatively speaking, is expended in raising them to the bath temperature. Traditional organic solvents have low heat capacities like metals so they are more prone to temperature fluctuations when used as heated immersion cleaners. The high polarity of water can be viewed as a double-edged sword. The high polarity makes it possible for water to dissolve many inorganic compounds, such as caustic soda, caustic potash, borates, carbonates, phosphates, and silicates. Water is also an effective solvent for many surfactants used in formulating aqueous cleaners. Unfortunately, this polarity results in water also being contaminated by numerous impurities both from the Earth’s crust and from anthropogenic pollution. Some of the impurities of the starting water are identical to cleaning ingredients, i.e., carbonates and phosphates. The key is which impurities are beneficial and which are detrimental. Contaminant levels in water used to make aqueous cleaners are usually low so one must focus on which impurities are detrimental. Elimination/suppression of these impurities is essential in preventing problems including reduced cleaner performance, longevity, corrosion, contaminated surfaces, and water spotting. Chemical manufacturers and industrial users spend millions of dollars annually on water-conditioning equipment to reduce/remove the impurities as part of preventive maintenance. Many users remain uninformed about their water quality needs. Many of these users suffer from increased cost of recleaning and rejects as a result of the impact that poor water quality has on the whole cleaning process. A discussion of water treatment options will follow. © 2001 by CRC Press LLC

An often forgotten property of water is its ability to dissolve oxygen gas. Oxygen gas in water can be corrosive and will attack metals. High chrome steels are exceptionally prone to rusting in these situations. When these parts are damp and left exposed to the air, flash rusting will occur. The boiler water treatment industry knows all too well how detrimental dissolved oxygen is in boiler systems. They have to treat these systems with what are known as “oxygen scavengers” to remove the oxygen gas from water chemically. Oxygen scavengers are not normally used in aqueous cleaning but corrosion inhibitors may have to be used to combat the corrosive effects of oxygen and other ingredients dissolved in water. Impurities If water were H2O and nothing else, or if all waters carried the same impurities, the use of water for industrial applications would be simple and straightforward. But natural waters, even rain, snow, sleet, and hail, as well as all treated municipal supplies contain some impurities. The type and amount of contaminants in natural waters depend largely on the source. Well and spring waters are classed as groundwaters, rivers and lakes as surface waters. Groundwater picks up impurities as it seeps through the rock strata, dissolving some part of almost everything it contacts. But the natural filtering effect of rock and sand usually keeps the water free and clear of suspended matter. Surface waters often contain organic matter, such as leaf mold, and insoluble matter, such as sand and silt. Pollution from industrial waste and sewage is frequently present. Stream velocity, amount of rainfall, and where this rain occurs on the watershed can rapidly change the character of the water. Below is a list of the more common and troublesome impurities: Turbidity—Suspended insoluble matter, including coarse particles (sediment) that settle rapidly on standing. Amounts range from zero, in most groundwaters, to some surface supplies of over 6% or 60,000 parts per million (ppm) in muddy and turbulent river waters. Hardness—Water content of soluble calcium and magnesium salts equals hardness expressed as calcium carbonate equivalents in gpg (grains per gallon) or ppm. 1 gpg  17.1 ppm. These salts, in order of their relative average abundance in water, are (1) bicarbonates, (2) sulfates, (3) chlorides, and (4) nitrates. Calcium salts are about twice the concentration of magnesium salts. Hardness is undesirable because the salts become less soluble and drop out of solution as the water is heated and upon drying, producing a hard, stony water spot that can be difficult to remove. Iron—The most common soluble iron in groundwater is ferrous bicarbonate (black iron). Although some water is clear and colorless when drawn, on exposure to air ferrous bicarbonate can cloud up and deposit a yellowish or reddish-brown sediment that stains everything it contacts. Iron can also shorten the life of a water softener, contaminating the resin. Although the majority of iron-bearing waters have less than 5 ppm, as little as 0.3 ppm can cause trouble. Manganese—Although rarer than iron in water, manganese occurs in similar forms and can form deposits in pipelines and tanks very rapidly with as little as 0.2 ppm. Silica—Most natural waters contain silica ranging from 1 to over 100 ppm. When silica spotting occurs, it can be very difficult, if not economically impossible, to remove. © 2001 by CRC Press LLC

Mineral acidity—Surface waters contaminated with mine drainage or trade wastes will contain sulfuric acid, plus ferrous, aluminum, and manganous sulfates. These contaminants are corrosive and therefore waters contaminated with mineral acidity are unfit to use without a pretreatment system. Carbon dioxide—Free carbon dioxide is found in most natural supplies. Surface waters have the least, although some rivers contain up to 50 ppm. In groundwaters (wells), it varies from zero to concentrations so high that carbon dioxide bubbles out when pressure is released (as in “sparkling” or seltzer water). Most well waters contain from 2 to 50 ppm. Carbon dioxide is also formed when bicarbonates are destroyed by acids, coagulants, or heating the water. This can reduce the pH of an alkaline cleaning solution. Carbon dioxide is corrosive and accelerates the corrosion properties of oxygen. Oxygen—Found in surface and aerated waters. Deep wells contain very little oxygen. The oxygen content of water is inversely proportional to the temperature, meaning that the hotter the water, the less oxygen is present. (Note that in elevated temperatures, the water contains less oxygen; however, what oxygen remains is much more aggressive and corrosive.) Oxygen is very corrosive to iron, zinc, brass, and other metals. Flash rusting of metals can be a problem when hot parts are rinsed in cold water that contains higher amounts of oxygen.

WATER PRETREATMENT Softening General The impurities that cause the most trouble in aqueous cleaning processes are the inorganic salts. Salts are ionic compounds, which are chemicals that have a positively charged species called a cation and a negatively charged species called an anion. It must be pointed out that the term salt has commonly meant sodium chloride, i.e., table salt. The chemical definition is “salts are ionic compounds that contain any negative ion except the hydroxide ion and any positive ion except the hydrogen ion.” An ionic compound that contains the hydrogen ion is called an acid and an ionic compound that contains the hydroxide ion is a base. Specific examples of common ionic impurities that are encountered as impurities in water are given in Table 1. It must be pointed out that silicates are usually discussed/represented as silicon dioxide, SiO2, when they are actually present in natural waters as the ions listed in Table 1. There are more complex forms of phosphate and silicate ions and numerous other trace impurities that can be present in natural, untreated water but the above examples represent the bulk of the impurities that the cleaning industry must be concerned with.

Principles of Water Softening The most problematic impurities are the cations. The most economical method for removing them is by passing the water through an ion-exchange column, better known as a water softener. A standard water softener contains polystyrene beads that have been modified such that the surface of each bead has numerous negatively charged sites. Nature © 2001 by CRC Press LLC

Table 1 Dissolved Impurities in Water Ion Type Cations

Anions

Impurity 2

Ca Mg2 Fe2 and Fe3 Mn2 and Mn4 Na K CO32 HCO3 PO43 SiO44 and SiO32 Cl NO32

Property Hardness Hardness Iron stains Manganese stains and scales Too much sodium in rinse water can cause spotting Too much potassium in rinse water can cause spotting Alkalinity, carbonates form hard water deposits with calcium, magnesium, iron, and manganese Alkalinity, bicarbonates form hard water deposits with calcium, magnesium, iron, and manganese Alkalinity, ortho-phosphates form hard water deposits with calcium, magnesium, iron, and manganese Silicates can form the most tenacious of deposits, especially in the presence of calcium, magnesium, iron, and manganese Chlorides promote corrosion on aluminum, iron, and steel, too much chloride in rinse water can cause spotting Too much nitrate in rinse water can cause spotting

requires that charge must be balanced; which is accomplished by pairing each negatively charged site with a sodium cation. As the impure water passes through the water softener, the hard water ions become attached to the resin beads and displace the sodium cations. The number of displaced sodium cations equals the charge of the hard water ion trapped in the softener. The hard water ions are bound more tightly to the resin because higher positively charged cations bind more strongly to negatively charged surfaces. Basically, the water softener removes highly charged cations and replaces them with sufficient sodium cations to maintain the balance of charge. Thus, sodium contamination increases but it does not cause nearly as much trouble as calcium, magnesium, iron, and manganese. There are only a finite number of resin beads in a water softener so there is a point where the softener becomes saturated. At this point, the softener must be recharged. This is accomplished by passing a saturated salt solution, usually sodium chloride, through the softener. The overwhelming quantity of sodium cations slowly replaces the calcium and magnesium ions, which returns the softener back to working order. Iron and manganese are more difficult to remove from a water softener. Iron and manganese can have very high positive charges, 3 and 4, respectively, which make them bind so tightly to the resin bead that the mass action of the regenerating salt solution cannot knock them off. This is typically called “iron poisoning.” These cations can be washed out of the softener if their charge can be lowered first. Reducing agents can be used to lower the charge of the iron and manganese ions, typical “reducing agents” are sodium sulfite, sodium hydrosulfite, and sodium thiosulfate. Deionization Basic water softening removes only the undesirable cations by replacing them with less problematic cations. It is also possible to replace anions by the same technique, only this time the charge on the surface of the resin bead is positive and the charge is balanced by pairing up with an anion. The chloride anion is an economical choice for balancing charge

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in an anion exchanger. However, excessive chloride content in water can lead to stress cracking of certain stainless steels and promote corrosion on aluminum and mild steels. When industry has to be concerned with both cation and anion contaminants, it is easier to perform a process called “deionization” than it is to replace undesirable cations with sodium and the anions with chloride. Deionization involves passing impure water through a series of cation and anion exhange resins where the cations are replaced by hydrogen ions (H  acid) and the anions are replaced by hydroxide ions (OH base). The liberated acid and base then neutralize each other to form water: H  OH → H2O When deionization is performed, theoretically an equal amount of acid and base is liberated. The neutralization reaction should then result in pH-neutral water. This is usually not the case. Deionized water typically is slightly acidic; pH of 4.5 to 5.5. The mild acidity is caused by the carbonate impurities initially present in the water. As the carbonate passes through the cation exhange column, carbonic acid is formed. The examples below will assume that the carbonates are passing through as sodium salts: Na2CO3  2 HResin → H2CO3  2 NaResin NaHCO3  HResin → H2CO3  NaResin Carbonic acid is not stable in water so it self-destructs to form carbon dioxide, CO2, and water: H2CO3 → CO2  H2O Carbon dioxide has some solubility in water so unless the water is boiled to drive off the CO2 after deionization, the reverse reaction can occur, which liberates some acid. This causes the low pH. CO2  H2O → H2CO3 H2CO3 → H  HCO3 Both water softening and deionizing systems are very effective at removing impurities, but they are not perfect. The exchange of ions is really an equilibrium process so some material can work its way through a column before the saturation state is reached. Imperfectly sealed control valves and incorrect flow rates can result in some impurities never coming in contact with the resin so that exhange never occurs. The ion exchange columns will not effectively remove nonionic impurities. Incorporation of an activated carbon filter will remove the nonionic impurities.

Reverse Osmosis General Reverse osmosis (RO) involves separating water from a solution of dissolved solids by forcing water through a semipermeable membrane. As pressure is applied to the solution, water and other molecules with low molecular weight pass through micropores in the membrane. The membrane retains larger molecules, such as organic dyes, cleaners, oils, © 2001 by CRC Press LLC

metal complexes, and other contaminants. RO membrane systems feature cross-flow filtration to allow the concentrate stream to sweep away retained molecules and prevent the membrane surface from clogging or fouling. In the past, RO applications for industrial operations were mostly limited to final treatment of combined wastewater streams. Such applications typically involved discharging permeate to a publicly owned treatment works (POTW) and returning the concentrate to the head of the wastewater treatment system. Because of the high flow rates associated with treating combined wastewater streams, large, costly RO units were required. More recent applications in cleaning involved installing RO units in specific process operations (such as wash tank or rinse water maintenance), allowing return of the concentrate to the process bath, and reuse of permeate as fresh rinse water. By closing the loop, process contaminations are removed and fresh water is recycled. Furthermore, a waste stream is eliminated that would otherwise be discharged to the POTW. RO systems have been successfully applied to a variety of industrial operations, reducing the cost of waste treatment and disposal. RO Components The essential components of an RO unit include strainer, pressure booster pump, cartridge filter, and the RO membrane modules. The strainer removes large, suspended solids from the feed solution to protect the pump. The booster pump increases the pressure of the feed solution. Typical operating pressures range from 150 to 800 psi. Commercially available cartridge filters are used to remove particulates from the feed solution that would otherwise foul the units. Cartridge filter pore sizes are typically between 1 and 5 m. Cleaning Formulations Water has been used as a cleaner for centuries. The first water-soluble soaps were a blend of lye and animal fat. The chemical reaction of this mixture is a process defined as saponification. The addition of heat made the soap work better at removing the oils and greases of the day, which were also made from animal fat. As industry advanced and metal processing became more sophisticated, various organic and inorganic salts were found to enhance detergency by combining with metal ions to prevent them from reacting with the soap. These salts are termed “builders” and include phosphates, carbonates, silicates, and gluconates, just to name a few. The development of synthetic detergents as a substitute for soap in wartime has completely changed the cleaning industry. Today, true soaps make up only a small portion of the surfactants used in either industrial or consumer cleaning applications. By the mid-1970s, government regulations were starting to be felt at the job site. OHSA and Material Safety Data Sheets (MSDS) became common terms within the industrial arena. Chemicals came under increasing scrutiny for worker safety. During the following decades, environmental issues have played an increasingly important role in chemical evaluation. The terms EPA, chlorofluorocarbons (CFCs), the Montreal Protocol, global warming, SARA Reportables, and air quality boards are all commonplace. No cleaning process can completely escape the concerns of health, safety, and the environment. CLEANING CHEMISTRY 21 Aqueous cleaners are acid, neutral, or alkaline. Acid products, which have a pH of less than 6, are used for removal of inorganic soils and to pickle or passivate a metallic surface. Neutral and alkaline cleaners have a pH range from 6 to above 13. These products are very © 2001 by CRC Press LLC

effective on organic oils and greases. Additional ingredients are frequently added for increased effectiveness on inorganic soils as well. When defined as the removal of soil or unwanted matter from a surface to which it clings, cleaning can be accomplished by one or more of the following methods: Wetting: Through the use of surface active agents, the cleaning penetrates and loosens the substrate–soil bond by lowering surface and interfacial tension. Emulsification: Once wetting occurs, two mutually immiscible liquids are dispersed. Oil droplets are coated with a thin film of surfactant, thus preventing them from recombining and floating to the surface. Solubilization: The process by which the solubility of a substance is increased in a certain medium. The soil is dissolved in the cleaner bath. Saponification: The reaction between any organic oil containing reactive fatty acids with free alkalies to form soaps. Insoluble Fatty Acid
Alkali  Water Soluble Soap Deflocculation: The process of breaking the soil into very fine particles and dispersing them in the cleaning media. The soil is then maintained as a dispersion and prevented from agglomerating. Displacement: Soil is displaced by mechanical action. Movement of the workpiece or fluid enhances the speed and efficiency of soil removal. Sequestration: Undesirable ions such as calcium, magnesium, or heavy metals are deactivated, thus preventing them from reacting with material that would form insoluble products (i.e., hard water soap scum).* Water-based cleaners are generally divided into five major pH groups as follows: Caustic, pH 12 to 14 High alkaline, pH 10 to 13 Low alkaline, pH 8 to 10 Neutral, pH 6 to 8 Acid, pH 1 to 6 Acid Cleaners Acid cleaners are generally not used for the removal of organic oily soils. A typical acidic solution with a pH of 4.5 could include citric acid and nonylphenol ethoxylate. It would be effective for removing metal oxides or scale prior to pretreatment or painting. Systems using acid cleaners generally require constant maintenance because the aggressive chemistry attacks tank walls, pump components, and other system parts, as well as the materials to be cleaned. Inhibitors can be used to reduce this attack. Acid cleaners often suffer from rapid soil loading, particularly metal loading. This loading leads to frequent decanting and dumping of the cleaner solution. Both of these disadvantages lead to relatively high operating costs compared with alkaline cleaners. Alkaline and Neutral Cleaners Ingredients Ingredients frequently contained in alkaline cleaners include alkaline builders, water conditioners, surface-active agents, corrosion inhibitors, fragrances and/or dyes, defoamers or foam stabilizers, and water. Occasionally, hydrocarbon solvents are also added to a formulation. * See Acknowledgment at end of chapter. © 2001 by CRC Press LLC

Alkaline Builders Alkaline builders are selected based on the pH, detergency, corrosion inhibition, and/or cost limitations required or desired for a specific formulation. Environmental or process restrictions must also be considered. These builders may include one or more of the items listed in Table 2. Neutral-pH cleaners contain little or no alkalinity builder(s) or the alkalinity reserve is neutralized with an organic or mineral acid. Water Conditioners Sequestrants or chelators are frequently used to deactivate undesirable ions such as calcium, magnesium, or heavy metals. These ions or heavy metals are then no longer free to react with bath substances that would subsequently form undesirable compounds, such as hard water soap scum. Some of the more commonly used sequestrants include: EDTA: Ethylenediamine tetraacetic acid NTA: Nitrilotriacetic acid HEEDTA: Hydroxyethylenediamine triacetic acid STPP: Sodium tripolyphosphate ATMP: Amino tri(methylene) phosphoric acid HEDP: 1-Hydroxyethylidine-1, 1-diphosphonic acid Sodium gluconate Sodium glucoheptanate Low-molecular-weight polyacrylates EDTA has maximum effectiveness in tying up calcium and magnesium, thereby softening the water used to dilute the cleaner bath. Sodium gluconate or glucoheptanate has maximum effectiveness in tying up heavy metals. Complex phosphates are extremely costeffective, but have come under environmental pressure since the late 1960s. Lowmolecular-weight polyacrylates have only found a limited market to date. Table 2 Alkaline Builders 22 Component

Advantage

Disadvantage Caustic Cleaners (pH 12 to 14)

Hydroxides

Cost-effective

Corrosive

High Alkaline Cleaners (pH 10 to 13) Amines Carbonates Hydroxides Phosphates Silicates

Detergency corrosion inhibition Detergency, soil holding, low cost Cost-effective Detergency, sequestration, corrosion inhibition Detergency, corrosion inhibition

More costly Consumable Corrosive Environmental restrictions Residues, restricted use

Low Alkaline Cleaners (pH 8 to 10) Amines Borates Sulfates

Detergency, corrosion inhibition, sequestration Corrosion inhibition Filler, carrier

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More costly Limited effect Restricted use

Surface-Active Agents Surface-active agents, also known as surfactants, are used to reduce the surface or interfacial tension on aqueous solution. Selection of the surfactant package used in cleaner formulations depends on the performance characteristics desired. Surface active ingredients frequently used in water-based cleaners include fatty acid soaps or organic surfactants. These surfactants are classified into four basic types: Anionic: Negatively charged ions that migrate to the anode Cationic: Positively charged ions that migrate to the cathode Nonionic: Electronically neutral ions Amphoteric: Ions charged either negatively or positively, depending on the pH Physical properties affected by surfactants include the cloud point, the foaming characteristics, and the detergency, emulsifying, or wetting mechanisms used to facilitate the cleaning process. Often a combination of ingredients is used to obtain the specific properties desired. Corrosion Inhibitors Corrosion inhibitors are also contained in some alkaline cleaners, depending on the application involved. If a wide variety of substrates is involved, a combination of inhibitors may be used. These inhibitors are water soluble and therefore are removed with a thorough rinse if desired. Inhibitors frequently added to aqueous cleaner formulations include, but are not limited to, aldehydes, amine benzoates, borates, carboxylates, molybdates, nitrites, thiols, triazoles, and urea. Phosphates and silicates could also be added to this list. The intended cleaner application dictates the type of inhibitor package selected. As the alloying of metals and composites becomes more complex, there is a greater need for sophisticated inhibitor packages, which provide protection on a broad spectrum of substrates. The synergism of chemicals allows the formulator to obtain the inhibiting properties desired and may be limited only to the imagination of the formulator and the cost restrictions of the chemicals selected for this use.

Additional Ingredients Aqueous cleaner formulations may also contain a broad spectrum of ingredients designed to affect the appearance, odor, or physical properties of the composition. These include dyes, fragrances, thickeners, defoamers, foam stabilizers, or fillers for cost reduction. Again, the intended cleaner application will dictate final composition of a formula. Hydrocarbon Solvents A variety of hydrocarbon solvents have been blended with surfactants to make emulsion or semiaqueous cleaners. Glycol ethers have been added to stabilize formulations or to increase the cleaning efficiency of a composition. Environmental regulations have identified these ingredients as VOCs, which are regulated by air quality boards. In addition, certain glycol ethers, including 2-butoxyethanol, have been identified as health hazards. © 2001 by CRC Press LLC

RINSING Rinsing is often the most overlooked aspect of cleaning. While a process may employ the best cleaning equipment money can buy and an optimized cleaning solution, without adequate rinsing the overall result may be unsatisfactory. Rinsing is no more than a reduction in contamination by dilution. Adding mechanical or thermal energy may enhance rinsing. Although it is important to use adequate water in rinsing, the key to economy is to use no more than is necessary for acceptable parts. Importance of Rinsing Rinsing is a science in itself. Many factors enter into the design of a rinse system. Let us start with the end in mind. Below is a partial list of considerations: • • • • • • •

Part cleanliness required Production levels required Type of contamination to be removed (amount and type of dragin and residue) Incoming water quality Treatment capabilities Number of rinse tanks, size, layout Water usage and disposal

Each concern will be discussed further. Part Cleanliness Required This consideration is the controlling factor in the whole process of rinsing. If the manufacturer is only concerned with gross contamination, then perhaps rinsing is not even necessary. At the other extreme, if the slightest residue leads to failure, then perhaps the final rinse may need a conductivity of 50 microsiemens/cm (S/cm) or less. Obviously, final quality must be determined before other decisions can be made.

Production Levels and Dragin Production levels and dragin will determine what measures are needed to meet the above determined quality requirements. Dragin will depend on many factors including part configuration, orientation, temperature, drain time, etc. Production levels will determine the amount of dragin per time. Rinsing equations must deal with these factors to predict final rinse quality. Incoming Water Quality Incoming water quality can vary from naturally soft water with few contaminants to water that contains many hundreds of ppm hardness. Water hardness is generally measured either in ppm of equivalent calcium carbonate, CaCO3, or in grains of hardness. Values greater than 120 ppm are considered to be hard water, with values greater than 180 ppm considered to be very hard. Hardness can have many deleterious effects in the cleaning process. Hardness can react with the surfactants to deactivate them, cause corrosion to © 2001 by CRC Press LLC

increase on steel surfaces, and leave deposits on the cleaned surfaces. These residues may cause paint adhesion problems, plating problems, or aesthetic problems, just to name a few. Fortunately, as has been discussed, there are alternatives. Whether softened water, deionized, distilled, or RO water is chosen will depend on the application needs. Number of Rinse Tanks Generally speaking, one large rinse is less effective than multiple small rinses. As an intermediate step, a single rinse may be adequate, but, as a final rinse, that is seldom true. Experience has shown that past a certain point, generally three rinses, increasing the number of rinses is of little value. Rinse Tank Design and Placement Rinse tank design and placement can greatly affect rinsing efficiency. Rather than an afterthought, rinsing needs should be addressed early in the planning of the cleaning line. Considerations should include tank geometry and composition, rinse flow, rinse temperature, and necessary final quality of the parts. It should be noted that rinsing can only be effective if it reaches the parts. Part orientation, loading, and rinse flow dynamics are important and often overlooked considerations. The rinsing needs are quite different for the manufacturer of circuit board components and the rebuilder of motorcycle engines. In the first case the choice might be a heated cascaded triple rinse with a deionized water source and, for the latter, perhaps a quick dip in a cold tap water tank is sufficient. A whole chapter could be easily devoted just to basic rinsing concepts. Certain basic principles should be considered in any case. • Multiple rinses are more efficient than single rinses with the optimum balance being about three rinses. • Water usage can be minimized by cascading the final rinse overflow into the previous rinse and that rinse into the one before. • The final rinse quality is the determining factor in final residue on the part. • Rinsing will not be effective if it does not reach the parts. For the interested reader, further reading should include Peterson’s Practical Guide to Industrial Metal Cleaning.13 Dragin and Final Rinse Quality Knowing and understanding how much and what kind of dragin occurs from the previous step is crucial to predicting rinsing needs. Quite simply dragin over some time equals the quantity that must be diluted to some lower specified level. Solve for the quantity of diluent. In a dynamic situation at equilibrium the flow rate of overflow rinse water must equal the dragin times the process chemical concentration divided by rinse chemical concentration. If this equation is extended to multiple rinse tanks, the dragin is obviously reduced in each successive tank. If the overflow for each rinse tank is cascaded to the previous tank, for all practical purposes, the equation takes the form Overflow  Dragin (process chemical concentration/rinse chemical concentration)1/number of rinse tanks. © 2001 by CRC Press LLC

This cascading obviously reduces water use dramatically over either single rinsing or multiple rinses that are not cascaded to achieve the same final rinse quality. The final rinse quality should be maintained just as carefully as the processing tanks. After all, the rinse is the last liquid the parts will see. It does not make sense to go to great lengths to clean them only to recontaminate them with a contaminated rinse. Acceptable rinse quality will depend on the needs of the application. As such, the steps to measure rinse quality will vary with the needs. Generally speaking, cleaning needs only be adequate to eliminate subsequent problems. While rinse quality is relative, some general guidelines may be helpful. Some sources would classify applications into general cleaning, critical, and very critical cleaning. Although different standards may be proposed, one suggestion is to use conductivity as a guide and divide these applications as follows: • General rinsing operations having a residual level of 1000 S/cm • Critical rinsing with a residual of 500 S/cm • Very critical at less than 50 S/cm Most plating operations call for a high-quality final rinse of less than 50 S/cm as a minimum rinse quality. It can be seen that at these higher-quality rinses, higher-quality water must be used to achieve the desired cleanliness. These guidelines are suggested as a place to start when determining final rinse quality. DISPOSAL Oftentimes local regulations limit or prohibit the discharge of any process water to sewers. Thus, water conservation becomes an increasingly important issue. Bath life and water conservation become very important issues. The result is that overall planning and coordination includes a cradle-to-grave approach for planning and setting up a cleaning line including the rinse and its disposal. For that reason, closed cycle systems for water treatment may actually be economical long-term alternatives. At the very least water use minimization is an important consideration.

CONCLUSION Aqueous cleaning is an increasingly important segment of the cleaning industry. That importance will probably increase with time and the development of improved cleaning systems. The emphasis must be on cleaning systems, as the interactions between parts, soils, equipment, cleaner, and water are much more complex than they appear on the surface. Environmental, economic, and other business concerns demand that industry obtain acceptable parts with minimal impact on the environment and at the least possible cost. The challenge to the cleaner manufacturer and the equipment manufacturer is to develop effective cleaners and equipment that meet those criteria and are compatible with each other. The newer generation of aqueous cleaners is designed to reject contaminants rather than emulsify soils. This feature allows the cleaner to be filtered routinely without significant adverse effect on the cleaner chemistry. These newer formulations can be replenished with routine chemical additions of the cleaner concentrate, according to the maintenance procedures recommended by the chemical supplier. Extension of cleaner bath life obtained with regular bath maintenance results in © 2001 by CRC Press LLC

reduced chemical consumption, reduced waste generation, reduced waste liability, and reduced cleaning costs. Very often the newer cleaning processes also yield cleaner parts as well. Aqueous cleaning is changing to meet the economic and environmental needs of the times. Clearly the future progress of aqueous cleaning will require close cooperation between the chemical and equipment industries. It is also apparent that water quality is a make-or-break issue in critical cleaning, especially as it applies to rinsing. This area is consistently the most neglected aspect of aqueous cleaning. ACKNOWLEDGMENT Portions of the text were reprinted from Metal Finishing, September 1995, J.A. Quitmeyer, The Evolution of Aqueous Cleaner Technology, pp. 34–39, copyright 1995, with permission from Elsevier Science. REFERENCES 1. Archer, W., Reactions and inhibition of aluminum in chlorinated solvent systems, in Corrosion 1978 Conference Proceedings, 1978. 2. Betz, Handbook of Industrial Water Conditioning, 8th ed., Betz Laboratories, Inc., Trevose, 1980. 3. Durkee, J.B., The Parts Cleaning Handbook, Gardner Publications, Cincinnati, 1994. 4. Farrell, R. and Horner, E., Metal cleaning, Metal Finishing, 96, (1), 1998. 5. Gruss, B., Cleaning and surface preparation, Metal Finishing, 96, (5A), 1998. 6. Hanson, N. and Zabban, W., Plating, 46, 1959. 7. Hirsch, S., Deionization for electroplating, Metal Finishing, January, 145 –149, 1997. 8. Kanegsberg, B.F., Aqueous cleaning for high-value processes, A2C2 Mag., 2, (8), 1999. 9. Metals Eng. Q., November 1967. 10. Mohler, J.B., The rinsing ratio applied to practical problems, Part 1, Metal Finishing, May 1972. 11. Nelson, W., The key to successful aqueous cleaning is water, Precision Cleaning, Flemington, April 1996. 12. Permutit, Water and Waste Treatment Data Book, 18th printing, U.S. Filter/Permutit, 1993. 13. Peterson, D.S., Practical Guide to Industrial Metal Cleaning, Hanser Gardner, Cincinnati, 1997. 14. PPG Handbook, Vapor Degreasing, 1986. 15. Schrantz, J., Rinsing: a key part of pretreatment, Ind. Finishing, June 1990. 16. Spring, S., Industrial Cleaning, Prism Press, Melbourne, 1974. 17. Vapor zone solvent cleaning systems, Precision Cleaning, Flemington, December 1996. 18. Wolf, K. and Morris, M., Ozone depleting solvent alternatives: have you converted yet?” Finishers’ Manage., June/July 1996. 19. Zavadjancik, J., Aerospace manufacturer’s program focuses on replacing vapor degreasers, Plating Surf. Finishing, April 1992. 20. CRC Handbook of Chemistry and Physics, 77th ed., CRC Press, Boca Raton, FL, 1996. 21. Quitmeyer, J., The Evolution of Aqueous Cleaner Technology, Metal Finishing, Tarrytown, September 1995. 22. Quitmeyer, J., All Mixed Up: Qualities of Aqueous Degreasers, Precision Cleaning, Fleminton, September 1997.

© 2001 by CRC Press LLC

CHAPTER 1.4

Review of Solvents for Precision Cleaning John W. Agopovich

CONTENTS Review of Solvents for Precision Cleaning Background General Requirements of a Precision Cleaning Solvent Cleaning Methods for Solvents Environmental Issues with Solvents Volatile Organic Compounds Global Warmers Flammability SNAP Approval Discussion of the Different Solvent Classes Flammable Solvents Hydrocarbons (Aliphatic) Hydrocarbons (Aromatic) Ketones Alcohols Halogenated Solvents Perfluorocarbons Hydrofluoroethers HFEs HFE Azeotropes and Blend Hydrofluorocarbons HFCs HFC Azeotropes and Blends Chlorinated Solvents Nonozone Depleters Ozone Depleters Other Solvents References

© 2001 by CRC Press LLC

REVIEW OF SOLVENTS FOR PRECISION CLEANING Background Since the early 1990s, organizations have been actively searching for alternatives for ozone-depleting chemicals (ODCs). CFC-113 and 1,1,1-trichloroethane (also called methyl chloroform) were the major ODCs utilized for precision cleaning processes. Before the Montreal Protocol mandates, these two materials were excellent for cleaning a wide range of contaminants from many different devices. CFC-113 was a good general cleaner for a wide range of contaminants, including hydrocarbon- and halogenated-based oils and particles. 1,1,1-Trichloroethane was required for degreasing of parts contaminated with heavier oils and residual greases, along with solder flux. Both CFC-113 and 1,1,1-trichloroethane were available in high-purity grades required for precision cleaning operations and readily evaporated after use. Neither material is highly toxic and neither has a flash point. 1,1,1Trichloroethane does have flammable limits in air, however. This chapter discusses the several classes of solvents that have been implemented as replacements for these two materials in precision cleaning. For purposes of definition in this chapter, a solvent is defined as a cleaning agent that readily evaporates after use, when cleaning a component. No follow-up cleaning is required after the use of this class of material. These solvents will evaporate even after cold-cleaning. This will generally mean that the solvent has a vapor pressure of greater than 25 torr at ambient temperature. Another better quantifier of volatility is the evaporation rate compared to n-butyl acetate. n-Butyl acetate is set at unity (ether and trichloroethylene are also used as reference materials). The majority of solvents discussed in this chapter all have an evaporation rate  n-butyl acetate or 1. Another parameter related to volatility is the heat of vaporization, the heat required when a solvent transforms to the vapor phase from the liquid phase. When possible, it is best to choose solvents that have a low heat of vaporization, as these materials will evaporate without absorbing heat (the part cools). Solvents with high heats of vaporization cool a part as more heat is absorbed from the environment. Often water condenses on a device that is cleaned, if the environment is humid. This is not desirable in precision cleaning. Volatility and ease of vaporization have drawbacks. These include issues of containment, flammability, toxicity, local regulations on emissions, and cost. These issues are discussed later in the chapter. Cleaners, such as any aqueous based terpenes and hydrocarbons in the combustible range (flash point 100°F), do not fall under the definition of a solvent and will not be discussed in this chapter. Critical precision cleaning areas that use solvents under this definition include but are not limited to medical devices, directional devices (gyroscopes, accelerometers, and components within), computer components (disk drives), precision ball bearings, oxygen transport systems, and circuit boards. General Requirements of a Precision Cleaning Solvent In addition to the need for volatility discussed previously, a precision cleaning solvent must meet other critical requirements. Obviously, the contaminant requiring removal must have a finite solubility in the cleaning solvent. This definition varies as factors such as time, temperature, and agitation can be altered. Generally speaking, increasing the temperature of a solvent will improve cleaning effectiveness, as will increasing the exposure time. As an example, 3M defines “soluble” when a material dissolves in another in the range of 5 to 25 g/100 g of solvent at room temperature.1 In another article, solubility has been defined as © 2001 by CRC Press LLC

50 g/100 ml of solvent.2 There are many other ranges of definitions. The solubility and cleaning effectiveness required will vary depending on how contaminated the part is, as well as the user’s final requirements. Other solubility parameters include a Kauri-butanol (KB) number. The KB numbers for CFC-113 and 1,1,1-trichloroethane are 31 and 124, respectively. Generally speaking, the highly chlorinated compounds have the higher reported KB values. The KB number reflects the ability of a solvent to dissolve heavy hydrocarbon greases. Specifically, it is a measure of ability to dissolve a solution of butanol and Kauri resin. ASTM D 1133-90 describes the standard test method for determining the KB value of hydrocarbon solvents. This procedure has been extended to evaluate ODC replacements discussed in this chapter. It is not applicable to oxygen-containing solvents. However, the author believes that as a first approximation, the “like dissolves like” concept is very useful. This means that polar solvents dissolve polar contamination and nonpolar solvents dissolve nonpolar contaminants. Hydrocarbon solvents will dissolve hydrocarbon oils and fluorocarbon-based solvents dissolve fluorocarbon oils and greases. To clean solder flux residues from printed circuit boards, polar oxygen-containing solvents like alcohols or chlorinated solvents are required. In precision cleaning, it is beneficial to have a solvent with a low viscosity and low surface tension. This property will allow solvents to enter very narrow gaps in a complicated device, to clean a contaminant. Particle removal is often a critical part of precision cleaning operations. A solvent with a low viscosity and low surface tension also facilitates particle removal. A high-density solvent provides additional momentum to remove particles from surfaces. Another critical requirement is chemical stability during use and also a solvent having a long or infinite shelf life. This was been a problem with the use of 1,1,1-trichloroethane without stabilizers. Some azeotropes and mixtures discussed later require stabilizers. Of even more importance is compatibility with the component one is cleaning. There cannot be a chemical reaction or a physical change such as irreversible swelling or extraction of the materials of construction of the component being cleaned. CFC-113 and 1,1,1trichloroethane were compatible with most materials. ODC alternative solvent manufacturers are very cognizant of the concerns of customers about solvent compatibility. Extensive solvent/materials compatibility tests are performed on a wide range of materials when a new solvent is introduced to the public. If one still has a question of the compatibility of material and solvent, it is best to have the solvent user perform the compatibility test in one’s particular application. Another critical solvent property is the nonvolatile residue (NVR). It is critical that, when a solvent evaporates from a surface after cleaning, no residue is left behind. Solvent manufacturers typically have NVR specifications in the range of 1 to 10 parts per million (ppm) for precision cleaning. Very often the reported NVR of a given batch of solvent is well under the company-set specification. The issue of NVR is also important when expensive solvents such as the perfluorocarbons (PFCs), hydrofluoroethers (HFEs), and hydrofluorocarbons (HFCs) are reclaimed and recycled. Any recycling process (such as distillation) must produce a product with the NVR meeting the original manufacturer (OEM) specifications. Recycling is desirable for cost savings when using expensive solvents. The halogenated-containing solvents (PFCs, HFCs, and HCFCs) are particularly expensive. Recycling of used solvents is possible when solvents are used in cleaning and subsequently contaminated with particles or high-boiling oils and greases. A simple strip or low theoretical plate distillation can be performed to purify the reclaimed solvent, to obtain NVR levels equal or to better than the OEM specifications. © 2001 by CRC Press LLC

Table 1 Physical Properties of CFC-113 and 1,1,1-Trichloroethane

Solvent

Vapor Boiling Pressure, Density Point mm Hg Flash TWAa (g/cc), (°C) at 25°C Point (ppm)b 25°C

CFC 113 48 1,1,1-Trichloroethane 74

334 121

None None

1000 350

1.56 1.32

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

0.68 0.80

17.3 25.0

Note: Physical properties compiled from published information. a Time-weighted average. b American Conference of Governmental Industrial Hygienists (ACGIH) 8-h TWA (1998).

Later in this chapter, the topic of azeotropes is discussed. The use of azeotropes is beneficial if the user requires expanding the capabilities of an existing single-component cleaning solvent. In the days when CFC-113 was used, many azeotropes of this material were available and popular for a wide range of applications. In many cases, an azeotrope will allow the ability of a precision cleaning agent to remove contaminants not possible when using the pure component. An example is using a PFC/hydrocarbon azeotrope that can clean light hydrocarbon oils. Many existing PFC alternatives form azeotropes with alcohols, chlorinated solvents (nonozone depleters), and hydrocarbons. The alcohol azeotropes are useful for removing ionic contamination from solder flux residuals. Listed in Table 1 are typical physical properties of CFC-113 and 1,1,1-trichloroethane. This table is presented for comparison purposes to the upcoming discussion of the recent generation of ODC replacement solvents. Cleaning Methods for Solvents These solvents can be used in vapor degreasers (both single component and as a cosolvent), ultrasonic cleaners, power spray (low- and high-pressure), dip, and wipe cleaning. The issues with the flammable solvents and toxicity in equipment must be addressed in light of local regulation requirements and standards set by each manufacturer. Environmental Issues with Solvents Unfortunately, even though most of the solvents discussed in this chapter do not deplete the ozone layer, there are other environmental issues that must be dealt with. Some ODC alternatives are classified as volatile organic compounds (VOCs), hazardous air pollutants, (HAPs), global warmers, or fall under National Emission Standards for Hazardous Air Pollutant (NESHAP) regulations. Volatile Organic Compounds Any compound that is released in the troposphere can undergo chemical reactions via hydroxyl radical extraction. According to the Environmental Protection Agency (EPA), all solvents are under the definition of VOC unless specifically exempted. VOCs can lead to the formation of ground-level, tropospheric ozone. The formation of ozone can in turn lead to the formation of smog via a series of complex free radical and photochemical reactions. Many compounds are exempted from the VOC restrictions, when it is determined that the release of this material in the troposphere will not lead to ozone formation. © 2001 by CRC Press LLC

Global Warmers There is an environmental concern about materials such as PFCs that have no mode of breakdown in the atmosphere after release. These materials do not react with hydroxyl radicals or ultraviolet (UV) radiation and break down. They simply accumulate and have atmospheric lifetimes on the order of hundreds or thousands of years. These materials can trap heat (infrared radiation) from escaping the troposphere. Alternatives to PFCs have been developed and have much shorter lifetimes. Another measurement of atmospheric lifetime is global warming potential (GWP). Typical materials are compared with carbon dioxide. The carbon dioxide GWP is set at 1.0 for a 100 year integration time horizon (ITH), per the Intergovernmental Panel on Climate Change (IPCC).3 Flammability CFC-113 and 1,1,1-trichloroethane had the advantage of not being flammable materials. Many of the alternatives discussed in this chapter possess flash points. Others do not flash, but have flammable limits in air, like 1,1,1-trichloroethane. Some azeotropes and azeotrope-like mixtures that will be discussed contain a high percentage of a nonflammable solvent (inerting agent). SNAP Approval The EPA has required that the Significant New Alternatives Policy (SNAP) program must approve all alternatives to ODCs. This program was formalized in 1994. Unless specifically mentioned in this chapter, all solvents discussed are SNAP approved. Some materials are SNAP approved with conditions. An example is the PFCs. The PFC materials can only be used if no other solvents can be used for technical or safety concerns. Other materials are conditionally SNAP approved for safety reasons, such as requiring that personal exposure limits be met. This is especially the case with chlorinated solvents. The safety issues with solvents, such as flammability and exposure limits, should be verified by a user by reviewing the Material Safety Data Sheets (MSDS) and product information literature from the solvent manufacturer.

DISCUSSION OF THE DIFFERENT SOLVENT CLASSES Flammable Solvents Hydrocarbons (Aliphatic) Aliphatic hydrocarbons such as n-hexane, n-heptane, and isooctane have utility in cleaning a wide range of hydrocarbon-based contaminants. These include hydrocarbonbased oils and greases. These materials can be used as wipe solvents to remove residual hydrocarbons and fingerprints. A list of physical properties is shown in Table 2. All these are exceedingly flammable with flash points well below room temperature. Hydrocarbon solvents are SNAP approved. These materials have KB values in the range of 30, from comparison to mineral spirits. Cyclohexane is added to this group as a saturated cyclic hydrocarbon.

© 2001 by CRC Press LLC

Table 2 Physical Properties of Aliphatic Hydrocarbons

Solvent

Boiling Point (°C)

n-Hexane n-Heptane Isooctane Cyclohexane

69 98 99 81

Vapor Pressure (torr) at 20°C 124 36 41 78

Flash Point (TCC), °F 15 25 10 17

TWA (ppm)a 50 400 300b 300c

Density (g/cc), 20°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

0.66 0.68 0.69 0.78

0.31 0.41 0.50 1

18 (25°C) 20.3 18.8 25

Note: Physical properties compiled from published information. TCC  tag closed cup. a ACGIH 8-h TWA (1998). b TWA for gasoline. c Planned to be reduced to 200 ppm per the ACGIH, 1998 TLVs ® and BEIs ® Threshold Limit Values for Chemical Substances and Physical Agents, “Notice of intended changes for 1998,” p. 74.

Hydrocarbon solvents are not terribly aggressive cleaning solvents. However, they are compatible with a wide range of materials. Flammable hydrocarbons must be used per VOC and HAP regulations, as required. All should be high-performance liquid chromatography (HPLC)-grade solvents or equivalent. Hydrocarbons (Aromatic) Aromatic hydrocarbons such as toluene often display increased cleaning effectiveness as compared with aliphatic hydrocarbons. Toluene is used instead of benzene for the obvious toxicity reasons. The physical properties of toluene are shown in Table 3. Aromatic hydrocarbons have KB numbers greater than the aliphatic hydrocarbons and mineral spirits. The KB number of toluene is 105. Aromatic solvents are not as popular as the aliphatic materials because of toxicity reasons. Toluene is classified as a VOC and HAP. Toluene is SNAP approved. The HPLC grade or equivalent should also be used. Ketones Acetone and methyl ethyl ketone (MEK) are popular cleaning solvents where aggressive cleaning is required. These materials are used for removal of polar contaminants and also solder flux and conformal coatings. These materials are sometimes too aggressive for general cleaning operations so care must be taken if these are used. Compatibility verification with organic polymers (plastics, elastomers, etc.) is required for each application. Physical Table 3 Physical Properties of Toluene

Solvent

Boiling Point (°C)

Vapor Pressure (torr) at 20°C

Flash Point (TCC), °F

Toluene

111

29

35

TWA (ppm)a

Density (g/cc), 20°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

50

0.87

0.59

28.5

Note: Physical properties compiled from published information. TCC = tag closed cup. a ACGIH 8-h TWA (1998).

© 2001 by CRC Press LLC

Table 4 Physical Properties of Acetone and MEK

Solvent

Boiling Point (°C)

Vapor Pressure (torr) at 20°C

Acetone MEK

56 80

185 74

Flash Point (TCC), °F 0 30

TWA (ppm)a

Density (g/cc), 20°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

500 200

0.79 0.80

0.36 0.43

23.3 24 (25°C)

Note: Physical properties compiled from published information. a ACGIH 8-h TWA (1998).

properties of acetone and MEK are shown in Table 4. Acetone and MEK are SNAP approved. HPLC grades are adequate. Acetone has recently been exempted as a VOC and is not a HAP. KB values are not applicable for acetone and MEK as they are oxygen-containing solvents. Alcohols Short-chain alcohols such as methanol (MeOH), ethanol (EtOH), and isopropyl alcohol (IPA) are also more polar than hydrocarbons and can therefore clean a wider range of contamination. Many water-soluble materials are miscible in alcohols. Also, alcohols can be used to follow up an aqueous cleaning process to remove traces of water left behind. Alcohols are popular in mixtures and azeotropes for cleaning solder flux and ionic residues. These mixtures are discussed later. KB values are not applicable for alcohols. Physical properties are shown in Table 5. Alcohols are SNAP approved and must be used per VOC and HAP regulations, as required. Halogenated Solvents This section discusses a popular class of solvents that are heavily halogenated containing fluorine and/or chlorine. These solvents are all expensive cleaning materials and range from $10 to $20/lb.

Table 5 Physical Properties of Short-Chain Alcohols

Solvent Methyl Ethyl (200 proof) Isopropyl

Boiling Point (°C)

Vapor Pressure (torr) at 20°C

Flash Point (TCC), °F

TLV/ TWA (ppm)a

Density (g/cc), 25°C

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

65 78

97 45

54 58

200 1000

0.79 0.79

0.55 1.10

22.6 22 (25°C)

82

32

53

400

0.78

2.40

21.8 (15°C)

Note: Physical properties compiled from published information. TCC
tag closed cup. a ACGIH 8-h TWA (1998).

© 2001 by CRC Press LLC

Figure 1

Perfluoro-N-methyl morpholine.

Perfluorocarbons (PFCs) 3M has had a range of PFCs under the name Fluorinert™ for several years. As the replacements of ODCs became an issue, 3M introduced a line of solvents referred to as Performance Fluids (PFs). Some of these were from the Fluorinert line (FC) of fluids that did not meet the tight requirements for these FC applications, but were acceptable for cleaning requirements. Examples are FC-84 and PF-5070. The predominant material in both is perfluoroheptane, but the PF-5070 is acceptable for cleaning. PFCs have a high liquid density, a low viscosity, and low surface tension. These physical properties make these materials excellent replacements for ODCs in critical particle removal applications. Because these are fluorinated, they are not miscible with hydrocarbon oils. Therefore, they are not candidates for general cleaning operations (fingerprints, oils, etc.). Properties of PFCs have been discussed elsewhere.4 Other popular PFCs for cleaning are PF-5060 (predominant component is perfluorohexane) and perfluoro N-methyl morpholine (PF-5052). The structure of PF.-5052 is shown in Figure 1. The ring carbons are completely fluorinated. PFCs are excellent solvents for PFPEs (perfluoropolyethers such as Krytox™ fluids), which are used as lubricants in computer disk drives and precision ball bearings. PFCs are used as carriers to deposit PFPEs on a computer disk drive and also clean them from surfaces. PFCs are also excellent solvents for cleaning the Halocarbon Corporation product line of chlorotrifluoroethylene (CTFE)-based fluids, as these materials are heavily fluorinated. Additional details of the use of PFCs in precision cleaning have been discussed previously.4 When PFCs are used for critical particle removal applications, the solvent may have to be filtered before use. This will depend on the application. PFCs are relatively inert, nontoxic, and compatible with essentially all materials. They are not classified as VOCs. PFCs have a KB number of 0 as they have no miscibility with the Kauri resin. PFCs are SNAP approved with conditions when they are the only materials available for technical and safety reasons. The physical properties of PFCs are shown in Table 6. Hydrofluoroethers HFEs The HFEs are the 3M second generation of replacements for ODCs. HFEs are ethers with an n-butyl/isobutyl fluorocarbon group and a shorter hydrogen-containing alkyl

© 2001 by CRC Press LLC

Table 6 Physical Properties of PFCs

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

Flash Point

PF-5060 PF-5070 PF-5052

56 80 50

232 79 274

None None None

TWA (ppm)

Density (g/cc), 25°C

Viscosity (cS), 25°C

Surface Tension (dyn/cm), 20°C

N/A N/A N/A

1.68 1.73 1.70

0.4 0.6 0.4

12.0 13.0 13.0

Note: Physical properties taken from recent 3M product information. N/A — not applicable.

group. The HFE-7100 (3M™ Novec™ HFE-7100) has a methyl group and HFE-7200 (3M™ Novec™ HFE-7200) has an ethyl group. The structures are shown below. HFE-7100

HFE-7200

CH3–O–C4F9 CH3CH2–O–C4F9

The atmospheric lifetime, although still significant, is reduced drastically from that of the PFCs because of the hydrogen-containing moiety (carbon–hydrogen bonds). This becomes a site in the molecule for hydroxyl radical attack and breakdown in the troposphere. These materials are SNAP approved by the EPA without conditions. HFEs are therefore the logical replacements for PFCs. The HFEs have cleaning characteristics similar to PFCs as they are solvents for PFPEs and CTFE fluids. The hydrocarbon side chain allows for increased cleaning effectiveness for light hydrocarbon oils, especially in the case of HFE-7200, and at elevated temperatures. Because of these solubilities, these materials have KB numbers greater than zero but still very low, on the order of 10. Physical properties of the HFE-7100 and HFE-7200 are summarized in Table 7. These materials still have the low viscosity, high density, and low surface tension (like PFCs) to assist particle removal from surfaces. For some critical particle removal applications, the HFEs may require filtering before use. 3M has recently introduced a high-purity HFE-7100DL (disk lubricant) material, which has controls on particles, ions, metals, etc. HFEs are compatible with most materials2 and are not classified as VOCs. Both materials have no flash points. HFE-7200 has flammable limits in air. Other details about HFEs are also discussed in Reference 2, 3M product literature, and in Chapter 1.5 by Owens. Table 7 Physical Properties of HFE-7100 and HFE-7200

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

Flash Point

HFE-7100 HFE-7200

61 76

202 109

None None

TWA (ppm)a

Density (g/cc), 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 20°C

750 200

1.52 1.43

0.61 0.61

13.6 13.6

Note: Physical properties taken from recent 3M product information. a HFE-7100 exposure guideline from the American Industrial Hygiene Association. 3M exposure guideline for HFE-7200.

© 2001 by CRC Press LLC

Table 8 Physical Properties of HFE Azeotropes and Blend

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

HFE-71DE HFE-71DA HFE-71IPA

41 40 55

383 381 207

Flash Pointa

TLV/ TWA (ppm)b

Density (g/cc), 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

None None None

See below See below See below

1.37 1.33 1.48

0.45 0.45 nlc

16.6 16.4 14.5

Note: Physical properties taken from recent 3M product information. a The HFE-71IPA material has no closed-cup or open-cup flash point. See 3M literature about details of the flash points of these materials. b

3M reports separately the level of HFE-7100 (750 ppm) and the other components (components were mentioned earlier in chapter).

c

nl — not listed in literature reviewed.

HFE Azeotropes and Blend Azeotropes are defined as mixtures that maintain an essentially constant relative proportion in the vapor and liquid phase as the mixture components boil. 3M has two commercially available azeotropes and one azeotrope-like blend to improve cleaning effectiveness of hydrocarbon-based materials. These include HFE-71DE, HFE-71IPA, and HFE-71DA, whose physical properties are summarized in Table 8. HFE-71DE is a 50/50 mixture by weight of HFE-7100 and trans-1,2-dichloroethylene. This material can be used to clean a wider range of hydrocarbon-based oils. The KB value of this azeotrope is 27, which approaches the KB number of mineral spirits. In general, HFE-71DE is compatible with most materials. However, the high percentage of trans-1,2dichloroethylene warrants users to verify the compatibility of this azeotrope-like mixture with certain organic materials. Another product is a ternary azeotrope, HFE-71DA. This is similar to HFE-71DE, but has a low percentage of ethyl alcohol to remove ionic materials in flux removal applications. HFE-71IPA is an azeotrope-like mixture of HFE-7100 and approximately 5% IPA. The small percentage of IPA increases solubility of light oils and hydrocarbon oils. The HFE azeotropes have no flash points. The HFE-71DA and HFE-71IPA have a flammability range in air. The HFE-71DE does not have a flammability range in air. Hydrofluorocarbons HFCs HFCs are PFCs where selected fluorines are replaced in the molecule by hydrogen. In actuality, it is usually not chemically possible to replace fluorines in a PFC molecule. The HFC molecule is synthesized via a different route than a fluorocarbon, allowing a selected few hydrogens in the molecule. As with the HFEs, the HFC have a weak area in the molecule where hydroxyl radical attack can occur to break these molecules down in the troposphere. The atmospheric lifetime (global warming potential) of this molecule is much less than a PFC and they are SNAP approved without conditions. DuPont introduced an HFC material with the trade name Vertrel® XF. This solvent is perfluoropentane, with fluorine replaced by hydrogen at the second and third carbons:

© 2001 by CRC Press LLC

CF3CF2CHFCHFCF3 This material is still very similar to a PFC for cleaning effectiveness. PFPEs and CTFE fluids are exceedingly soluble in this material. However, XF has limited utility for dissolving hydrocarbon materials, as noted by a KB value of 9. This material can be used for particle removal also, as it still has a relatively high density, low viscosity, and low surface tension. HFC Azeotropes and Blends There are several azeotropes of XF available, expanding the cleaning capability of these products. These include XM, XE, SMT, and MCA. MCA Plus and XMS Plus are blends. The XM and XE azeotropes have a small amount of methyl alcohol and ethyl alcohol, respectively. These azeotropes can be used for light oil cleaning and particle removal. The remaining four cleaners contain trans-1,2-dichloroethylene, analogous in concept to the HFE azeotropes. MCA is a binary azeotrope of XF and trans-1,2-dichloroethylene. The SMT is a ternary azeotrope containing a small amount of methyl alcohol in addition to the trans-1,2-dichloroethylene, making it useful for cleaning ionic and solder flux contamination from circuit boards. MCA Plus is a ternary blend useful for heavy oil and grease cleaning. It contains trans-1,2-dichloroethylene and cyclopentane. XMS Plus is a quaternary blend and is similar to MCA Plus, but contains a small amount of methyl alcohol. It therefore has potential cleaning effectiveness of both the SMT and MCA Plus. DuPont has the details of the percentages of each component of the above-mentioned mixtures. Physical property and toxicity information is shown in Table 9. All details of XF formulations (compatibility, environmental issues) can be found in the latest DuPont product specification literature. Discussions of some DuPont HFC materials are also presented in Reference 2 and in Chapter 1.6 by Merchant. Another HFC material, benzotrifluoride (BTF), is toluene except the methyl group is completely fluorinated. The fluorinated methyl group results in this material having a lower KB number than toluene (49 compared with 105). However, it evaporates more Table 9 Physical Properties of HFC Azeotropes and Blends

Solvent

Boiling Point (°C)

Vapor Pressure, mmHg at 25°C

Flash Pointa PMCC, °F

AEL Limit, ppm

Density (g/cc), 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

XF XM XE MCA SMT MCA plus XMS plus

55 48 52 39 37 38 38

226 298 250 464 471 461 470

None None None None None None None

200b 200c 235d 200c 192d,e 214d 197d,e

1.58 1.49 1.52 1.41 1.37 1.33 1.34

0.67 0.63 0.73 0.49 0.47 0.49 0.46

14.1 14.1 14.1 15.2 15.5 16.1 14.9

Note: Physical properties taken from recent DuPont product information. a Pensky-Martens closed cup except for XE and XM, which are tag-open-cup tested. b Dupont AEL/8- and 12-h TWA. c trans-1,2-Dichloroethylene and methyl alcohol have a 200-ppm TLV/ 8-h TWA per ACGIH. d ACGIH calculation for TLV of mixtures. e Small amount of stabilizer required.

© 2001 by CRC Press LLC

Table 10 Physical Properties of BTFa

Solvent

Boiling Point (°C)

Vapor Pressure (mmHg) @ 20°C

Flash Point TCC, °F

BTF

102

30

54

CEL (ppm)

Density (g/cc), 20°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 20°C

100

1.19

nl

23

Note: Physical properties taken from OxyChem product literature. TCC
tag closed cup, nl
not listed in literature reviewed. a BTF is SNAP approved per the condition of the 100-ppm exposure limit.

quickly after use, is less toxic, and has a higher density and lower surface tension than toluene. This KB value is higher than any other pure, single-component HFC, likely because of the presence of aromatic hydrogens. This material forms azeotropes with a wide range of materials and has a flash point in the flammable range6. Other details can be found by review of literature from OxyChem and in Chapter 1.10 on benzotrifluorides by Skelly. Properties are shown in Table 10. BTF is also known as Oxsol® 2000. Chlorinated Solvents Nonozone Depleters Chlorinated solvents are popular because of their excellent solvency for cleaning a wide range of contaminants. These materials do not have flash points but all have flammable limits in air, similar to 1,1,1-trichloroethane. When 1,1,1-trichloroethane was available for use, it was the preferred degreasing chlorinated solvent. All of these have high KB numbers, ranging from 90 for perchloroethylene to 136 for methylene chloride. Methylene chloride Trichloroethylene Perchloroethylene (PCE) trans-1,2-Dichloroethylene (trans)

CH2Cl2 CCl2
CHCl CCl2
CCl2 CHCl
CHCl

In a recent issue of Parts Cleaning, an article details the toxicity and carcinogenicity of these three solvents: trichloroethylene, methylene chloride, and perchloroethylene.5 All these materials are suspected of being carcinogenic. Even though these materials contain carbon–chlorine bonds, none is an ozone depleter because of their short atmospheric lifetimes. However, chlorinated ethylenes are VOCs. trans is not a HAP. These materials are SNAP approved with the condition of meeting personal exposure limits. The chlorinated ethylene materials are not completely stable, need to be protected from moisture, and some require the presence of stabilizers. A listing of physical properties is shown in Table 11. They are also discussed in Chapter 1.8 by Risotto. Ozone Depleters AK-225 is a product offered by AGA chemicals. Of all the solvents discussed in this chapter, this material is most similar to CFC-113 in physical properties and therefore cleaning effectiveness. Some of these details are discussed in Reference 2. They have identical KB numbers of 31. AK-225 is a solvent for hydrocarbon oils but because it contains fluorine and chlorine, it is also a solvent for heavily fluorinated materials such as the PFPEs and CTFE © 2001 by CRC Press LLC

Table 11 Physical Properties of Non-Ozone Depleting Chorinated Solvents

Solvent Methylene chloride Trichloroethylene Tetrachloroethylene trans-1,2Dichloroethylenec

Vapor Pressure (torr) at 20°C

Flash Point TCC, °F

40

350

None

50

87 121 48

47b 18b 324b

None None 36d

50 25 200

Boiling Point (°C)

Viscosity (cP), 20°C

Surface Tension (dyn/cm), 20°C

1.33

0.44

28.1

1.46 1.62 1.26

0.57 nl nl

29.5 nl nl

TLV/ Density TWA (g/cc), (ppm)a 20°C

Note: Physical properties compiled from published information; nl  not listed in literature reviewed. a ACGIH TWA (1998). b At 25°C. c This material is generally used as a part of a mixture or azeotrope with HFEs and HFCs. d Closed-cup flash point from 7/6/94 Dupont MSDS.

fluids. This material is a mixture of the ca and cb isomers. The chemical structures of the two isomers are as follows: HCFC-225ca isomer HCFC-225cb isomer

CF3CF2CHCl2 CF2ClCF2CFHCl

This material is a Class II ozone depleter, with an ODP of 0.03 (CFC-11 is 1.0 on this scale). It is SNAP approved and is not classified as a VOC. These materials are not scheduled for start of phaseout of production until the year 2015, with complete phaseout by 2030. Interestingly, the ca isomer is more toxic than the cb isomer, necessitating the conditions placed upon the SNAP approval. The SNAP conditions that a company-set exposure limit of 25 ppm for the ca isomer is required (the level for the cb isomer is 250 ppm). The exposure limit for the mixture was set at 50 ppm by the manufacturer. Physical properties of AK225 are shown in Table 12. Note at the writing of this chapter that AGA chemicals has mentioned that a material designated as AK-225 G will possibly be made available for critical government applications. This material is the purified cb isomer, which is less toxic. Several azeotropes or mixtures with HCFC-255ca/cb exist and are discussed in product literature. These can be used for heavier oil and grease removal or solder flux removal. Some are discussed in Reference 2.

Table 12 Physical Properties of AK-225

Solvent

Boiling Point, °C

Vapor Pressure, kg/cm2 at 25°C

AK-225 (HCFC-255ca/cb)

54

283

Flash Point

TLV/ Density TWA (g/cc), (ppm) 25°C

Viscosity (cP), 25°C

Surface Tension (dyn/cm), 25°C

None

50

0.59

16.2

1.55

Note: These physical properties are taken from recent AK-225 product information and are further discussed in Chapter 1.11 by Miki et al.

© 2001 by CRC Press LLC

Other Class II ozone depleters include HCFC-141b and HCFC-123. The SNAP program deemed HCFC-141b unacceptable for precision cleaning because of its high ODP, which is similar to 1,1,1-trichloroethane. HCFC-123 has a similar ODP to AK-225, but has a boiling point near room temperature, making it difficult to handle. It can have utility in properly designed vapor degreasing equipment. It is SNAP approved with the condition of meeting the 30 ppm exposure limit. Other Solvents This group includes: n-Propyl bromide (nPB) CH3CH2CH2Br Volatile methyl siloxane (VMS) (CH3)3–Si–O–Si–(CH3)3 para-Chlorobenzotrifluoride (PCBTF) There are three other solvent-type precision cleaners that merit discussion. n-Propyl bromide is a replacement for 1,1,1-trichloroethane in degreasing operations. As of this writing, this material has not yet been SNAP approved but is listed as SNAP pending and therefore can be legally sold and used. The EPA has petitioned inputs from users of this material to assist in the SNAP-approval process. This material has a low ODP. The exact value is being reexamined because of uncertainties in the model when the material has a short atmospheric lifetime, such as n-propyl bromide. It likely will range from 0.006 to 0.027. The EPA also has questions about the toxicity of this material. It will likely be SNAP approved with conditions. n-Propyl bromide is discussed in Chapter 1.7 by Shubkin. The n-propyl bromide material has a KB value of 129, which is similar to 1,1,1trichloroethane. It is therefore an aggressive solvent. Physical properties of this material are shown in Table 13. Dow Corning has a series of VMSs (volatile methyl siloxanes) under the trade name Ozone Safe (OS) fluids. The OS-10 material has a vapor pressure in the solvent range. OS10 is SNAP approved, compatible with many materials, and not classified as a VOC. It is flammable as noted in Table 13. The KB values of these materials are low, with OS-10 at 16.6. It is suitable for precision cleaning of materials with light to medium hydrocarbon and silicone oil contamination. VMS is further discussed in Chapter 1.9 by Cull and Swanson. Table 13 Physical Properties of Other Solvents

Solvent n-Propyl (nPB) bromide OS-10 para-Chlorobenzotrifluoride (PCTBF)

Boiling Point, °C

Vapor Pressure, mmHg at 25°C

Flash Point CC, °F

TWA (ppm)

Density (g/cc), 25°C

Viscosity 25°C

Surface Tension (dyn/cm), 25°C

70

110

None

TBDa

1.33

0.49 cP

26 (20°C)

100

42

27

200b

0.76

0.65cS

15.2

109

25c

1.34

0.79cP

25

139

7.9



Note: Physical properties compiled from published information. nPB data from EnSolv ; OS-10 properties from Dow Corning; PCTBF data from OxyChem. a To be determined, EPA estimates 50 to 100 ppm b Dow Corning limit. c The OxyChem Corporate exposure limit for 8-h TWA is also 25 ppm (May 5, 1998 MSDS).

© 2001 by CRC Press LLC

OxyChem produced para-chlorobenzotrifluoride (PCBTF), which is a replacement for 1,1,1-trichloroethane. (Oxsol® 100) has a KB value of 64, intermediate for a wide range of cleaning operations. It is compatible with a wide range of organic polymers. It was SNAP approved with the condition of a 25-ppm exposure limit. It is exempt from VOC restrictions, is not an ODC, and is not an HAP. The vapor pressure of PCTBF is lower than solvents discussed previously and has a flash point in the combustible range, but does have utility as a cleaner if used safely to comply with the low exposure limit. Other details can be found by review of OxyChem literature or in Chapter 1.10 on benzotrifluorides by Skelly. [Editor’s note: As of press time, OxyChem is exiting the market and is ceasing production of benzotrifluorides. Therefore, the future availability of PCBTF is uncertain. — B.K.] REFERENCES 1. 3M Fluorinert™ Liquids Product Manual, 1991. 2. Agopovich, J.W., PFC alternatives analyses, Precision Cleaning, March 1997 and references cited therein. 3. IPCC, Radiative Forcing of Climate Change, The 1994 Report of the Scientific Assessment Working Group of the IPCC. 4. Agopovich, J.W., Fluorocarbons and supercritical carbon dioxide serve niche needs, Precision Cleaning, February 1995, and references cited therein. 5. Reynolds, R., TCE and cancer fact and fiction, Parts Cleaning, March 1999. 6. Ostrowski, P., Benzotrifluoride: a new HFC for cleaning, Precision Cleaning, April 1997.

© 2001 by CRC Press LLC

CHAPTER 1.5

Hydrofluoroethers John G. Owens

CONTENTS Introduction Properties of Segregated HFEs and Their Impact on Cleaning Processes General Properties Physical Properties Solvency and Mixtures Safety Considerations Environmental Considerations Materials Compatibility Cleaning Systems and Equipment Neat Cleaning System Azeotrope Cleaning System Cosolvent Cleaning System Parts Cleaning in HFE Cleaning Systems Drying/Water Removal Processes Operation Practices to Maximize Solvent Containment References INTRODUCTION The identification of suitable long-term alternatives to ozone-depleting substances (ODSs) is challenging because of the complex combination of performance, safety, and environmental properties required. Researchers investigating alternatives have evaluated hundreds if not thousands of compounds. This effort has significantly increased the understanding of the structure–property relationship for a number of compound classes. Much of the initial research has focused on various halogenated alkanes (i.e., organic molecules with a carbon–carbon backbone substituted with fluorine, chlorine, or bromine). However, some researchers believe that the requirement that a long-term alternative be both nonflammable and nonozone depleting effectively eliminates compounds containing chlorine or bromine.1 Recently, a number of researchers have investigated the new class of

© 2001 by CRC Press LLC

hydrofluoroethers (HFEs).2 –7 This class of compounds is nonozone depleting since the compounds contain neither chlorine nor bromine.8 A variety of HFE structures have been investigated including hydrofluoropolyethers.9 The insertion of an ether oxygen atom into the backbone of the molecule was often done to modify the thermophysical properties of a compound for specific end uses. However, one of the principal advantages of the HFE class is that certain HFE structures lead to significantly improved environmental properties. Results on numerous segregated HFE compounds demonstrated that they could have significantly shorter atmospheric lifetimes and, as a result, lower global warming potentials (GWPs) when compared with alkanes such as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).10 Segregated HFEs are those in which all of the hydrogen atoms reside on carbons with no fluorine substitution and are separated from the fluorinated carbons by the ether oxygen, i.e., CxFyOCmHn. This segregated structure maximizes the effect of the ether oxygen in reducing the atmospheric lifetime of the compound. The research also showed that segregated HFEs combined these environmental attributes with the performance and safety properties required in a suitable alternative solvent.7 The HFE solvents commercialized to date have all been of the segregated HFE class. These materials have been shown to be low in toxicity.7 They also have physical properties similar to the solvents they replace. Although their inherent solvent power is relatively low, HFEs are readily mixed with other components to produce nonflammable, high-strength solvents. As a result, HFEs have been identified as a class of compounds capable of replacing ODSs, high-GWP solvents, and chlorinated solvents for a number of new as well as conventional industrial cleaning processes. PROPERTIES OF SEGREGATED HFEs AND THEIR IMPACT ON CLEANING PROCESSES General Properties The novel structure of the segregated HFEs results in a unique combination of properties. HFEs are clear, colorless liquids that have very little odor. This class of compounds offers a wide range of boiling points from 34°C to well over 100°C. The materials have very low freezing points, often well below 100°C. The liquid HFEs are low in toxicity, nonflammable, noncorrosive, thermally stable, and electrically nonconductive. Like the solvents they replace, these fluids have high densities, but low viscosity and surface tension. To date, two HFEs compounds have been commercialized. Each consists of two inseparable isomers with essentially identical properties. The structures and several identifying names and numbers are listed in Table 1. Physical Properties The physical properties that determine the performance of solvents in typical end-uses are listed in Table 2 in comparison with CFC-113. The higher boiling points of the HFEs mean that they have lower vapor pressures, resulting in easier containment of the fluid in cleaning systems. These fluids have extremely low freezing points, which allows the use of low-temperature cooling coils to enhance fluid containment further. The wide liquid range of the HFEs also enables their use in other applications such as heat transfer fluids. Like many of the materials they are intended to replace, HFEs are nonflammable, having no open or closed cup flash point. © 2001 by CRC Press LLC

Table 1 Structures and Names of Commercially Available HFEs Chemical Structures Common name CAS numbers CAS names

Halocarbon numbers Simplified halocarbon numbers Commercial names

C4F9OCH3

C4F9OC2H5

Methyl perfluorobutylether 163702-07-6 163702-08-7 2-(Difluoromethoxymethyl)1,1,1,2,3,3,3-heptafluoropropane 1,1,1,2,2,3,3,4,4-Nonfluoro-4methoxybutane HFE-449sccc1 HFE-449scym1 HFE-449s1

Ethyl perfluorobutylether 163702-05-4 163702-06-5 1-Ethoxy-1,1,2,2,3,3,4,4,4nonfluorobutane 2-(Ethoxydifluoromethyl)1,1,1,2,3,3,3heptafluoropropane HFE-569sfccc2 HFE-569sfcym2 HFE-569sf2

3M™ Novec™ HFE-7100a

3M™ Novec™ HFE-7200

a

Registered trademark of 3M Company, St. Paul, Minnesota, USA.

The solubility of water in the HFEs is very low, which limits their ability to absorb humidity from the air. And their solubility in water is extremely low, minimizing the potential to contaminate contacting water streams. The high density of the HFEs and their very low viscosity and surface tension are important properties in cleaning applications, particularly for penetrating and cleaning components having complex geometry. Some researchers in the cleaning industry have suggested that a useful parameter for assessing the potential performance of a precision cleaning solvent is the “wetting index,” which is defined as the ratio of the density of the solvent to its viscosity and surface tension.11 The higher the wetting index, the better the ability of a solvent to wet component surfaces and penetrate into tight spaces, especially for the removal of particulate contamination. The HFEs’ solvent properties indicate that they are well suited for precision cleaning applications with wetting indices exceeding that of CFC-113 (see Table 2). Another advantage of superior wetting capability is that it provides good drainage of the solvent from components at the end of the cleaning cycle. This property, combined with Table 2 Physical Properties of HFEs Compared with CFC-113

Structure Boiling point, °C Freezing point, °C Flash point, °C, open or closed cup Solubility for water, ppmw Solubility in water, ppmw Density, ρ, g/ml Viscosity, µ, cp Surface tension, γ, dyn/cm Heat of vaporization, cal/g Wetting index (1000* / * )

© 2001 by CRC Press LLC

CFC-113

HFE-449s1

HFE-569sf2

CCl2FCClF2 48 31 None

C4F9OCH3 61 135 None

C4F9OC2H5 76 138 None

110 170 1.56 0.7 17 35 131

95 12 1.52 0.6 14 30 181

92 3 1.43 0.6 14 30 170

the low heat of vaporization, allows the HFEs to dry rapidly from part surfaces and minimizes fluid dragout from the cleaning process. Measurements of solvent dragout have confirmed the effectiveness of a solvent with a high wetting index. If a part is subjected to a typical cleaning cycle with a dwell in the vapor zone, it will carry residual solvent when it enters the freeboard zone (the area between the top of the vapor zone and the lip of the machine). Thus, it is usually necessary to hold parts in the freeboard for a period of time prior to removal from the cleaning system to minimize solvent dragout. The amount of liquid remaining on a representative, high-surface-area part is shown in Figure 1* for several solvents as a function of residence time in the freeboard. Solvents with high wetting indices, such as the HFEs, drain more completely from the part surface and dry faster. HFEs exhibit lower dragout compared with the ozonedepleting solvents they are intended to replace. The difference is even greater when compared with conventional chlorinated solvents such as trichloroethylene (TCE) and methylene chloride (MeCl). Solvency and Mixtures The segregation of the hydrogen and fluorine atoms on the HFE molecule leads to higher solvency than if the hydrogen atoms had been placed on a simple fluorochemical backbone. However, the pure HFEs are relatively mild solvents as indicated by the solvent parameters listed in Table 3. They do not have the hydrocarbon solvency to match the ODS solvents, but since they can be readily mixed with other materials to form nonflammable azeotropes or cosolvent systems, they can be just as effective in cleaning. Azeotropes are mixtures that are inseparable by boiling. As a result, azeotropes have found significant use in cleaning processes. For example, the binary azeotropic mixture of C4F9OCH3 with trans-1,2-dichloroethylene (t CClH  CClH) as well as the ternary azeotrope of C4F9OCH3 with trans-1,2-dichloroethylene and ethanol are nonflammable and have Kauri-butanol values similar to CFC-113. trans-1,2-Dichloroethylene is used in a number of azeotropic solvent mixtures since it has a wide margin of safety with an exposure Table 3 Solvency Properties of HFEs Compared with CFC-113 CFC-113 Structure Hildebrand solubility parameter Kauri-butanol (KB) value Solubility for mineral oil (weight %) Solubility for silicone oil (weight %) Solubility for fluorinated oil (weight %)

HFE-449s1

HFE-569sf2

Azeotrope 1

CCl2FCClF2 7.3

C4F9OCH3 6.5

C4F9OC2H5 6.3

7.7

7.8

31

10

10

27

33

Mc

1

1

20

20

M

1

1

M

M

M

M

M

M

M

a

Azeotropic mixture of 50% by weight of trans-1,2-dichoroethylene in C4F9OCH3, commercially available as HFE-71DE from 3M. b Azeotropic mixture of 44.6% by weight of trans-1,2-dichoroethylene and 2.7% by weight of ethanol in C4F9OCH3, commercially available as HFE-71DA from 3M. c M  miscible in all proportions. * Chapter 1.5 Color Figure 1 follows page 104. © 2001 by CRC Press LLC

Azeotrope 2

a

b

guideline of 200 ppm, is nonozone depleting, and its flammability is easily inerted by mixing with a nonflammable solvent such as an HFE.12 The HFEs are miscible with a wide range of organic solvents and form homogeneous, azeotropic mixtures with numerous compounds.13 Since they are highly fluorinated, HFEs have very high solubility for fluorocarbons and other halogenated compounds. Fluorinated oils and greases are typically completely miscible in an HFE solvent. Safety Considerations A wide range of safety considerations is necessary for proper design of cleaning solvents. As indicated in Table 2, the commercially available HFE compounds are nonflammable. These products as well as the commercially available azeotropes have been assigned NFPA (National Fire Protection Association) flammability indices14 of zero to one similar to the ODS solvents. These products do not become flammable under any conditions of normal use.12 Beyond flammability, the next most important safety consideration is the toxicity of the solvent. Under normal conditions, workers can be exposed to a small amount of a solvent during its use. The risk of greater exposure also exists in the event of a solvent spill, leak, or equipment failure. Extensive toxicological tests should be completed to determine if a solvent is safe in its intended applications. A number of the key findings for the HFEs15 are listed in Table 4 in comparison to CFC-113. The HFE solvents have very high acute lethal concentrations. That is, workers can be exposed to the solvent in very high doses for short periods of time without adverse effects. The HFEs were found to be nonmutagenic, are not cardiac sensitizers, and are dermally and ocularly nonirritating. The high exposure guidelines and lack of exposure ceiling indicate that the HFEs have a wide margin of safety. Another important safety consideration is the thermal stability of the solvent. The HFEs are capable of being continuously refluxed without degradation. Even in the presence of air and metals there has been no evidence of peroxide formation, which is common to many hydrocarbon ethers. All solvents can degrade if severely overheated. This degradation can produce byproducts that are more hazardous than the original solvent. For example, it was known that Table 4 Toxicological Properties of HFEs15 Compared with CFC-113

Structure Acute lethal conc., 4 h LC50, ppmv Mutagenicity Cardiac sensitization threshold, ppmv Ocular irritant Dermal irritant Exposure guideline 8-h TWA, ppmv Exposure ceiling, ppmv

© 2001 by CRC Press LLC

CFC-113

HFE-449s1

HFE-569sf2

CCl2FCClF2 55,000

C4F9OCH3  100,000

C4F9OC2H5  92,000

Negative 10,000

Negative  100,000

Negative  18,900

No No 1000

No No 750

No No 200

None

None

None

Table 5 Environmental Properties of HFEs Compared with CFC-113

Structure Atmospheric lifetime, years1 Ozone depletion potential,1 [CFC-11  1] Global warming potential,1 [CO2  1] Photochemical smog precursor18

CFC-113

HFE-449s1

HFE-569sf2

CCl2FCClF2 85 0.8

C4F9OCH3 4.1 (19) 0

C4F9OC2H5 0.77 0

6000

320 (20)

55

No

No

No

CFC-113 could produce decomposition products such as HCl and HF if overheated.16 Similarly, the HFEs can generate hazardous decomposition products such as HF if severely overheated (e.g., exposure to temperatures 150°C or higher).17 Conventional cleaning equipment has safety interlocks incorporated into its design to prevent overheating the solvents. In addition, the high temperatures required to decompose an HFE solvent (90°C or more above its boiling point) provide a wide margin for safe use. Solvent manufacturer’s recommendations should be followed when recycling and recovering solvent for reuse. The HFEs also have a high degree of chemical stability. The materials are hydrolytically and oxidatively stable under normal use conditions. The pure HFEs are stable when refluxed in the presence of water or strong aqueous base. Contact with many other relatively strong acids and bases produces little, if any, reaction. Exceptions are reactions with amines such as piperidine. As with all halogenated solvents, the HFEs should not be contacted with finely divided active metals, alkali, or alkaline earth metals (i.e., groups IA and IIA of the periodic table). Environmental Considerations An increasing number of environmental properties need to be considered when selecting cleaning solvents, such as those listed in Table 5. These properties affect both local environmental issues, such as smog formation, and global issues, such as ozone depletion and global warming. The atmospheric lifetimes of the HFE solvents are in a very desirable range. They are long enough not to contribute to formation of photochemical smog (i.e., volatile organic compound, VOC, exempt18) yet short enough to preclude concerns of accumulation in the atmosphere.1,19 These short atmospheric lifetimes, compared with PFCs, lead to lower global warming potentials.1,20 Since the HFEs contain no chlorine or bromine, they have no ozone depletion potential. The HFEs are not considered hazardous air pollutants since they are low in toxicity. The stability of the HFEs allows their recovery and recycling for repeated use in cleaning processes. Used HFE solvents are not classified as hazardous waste; however, the soils present in them may change that classification (e.g., metals from a defluxing operation would typically be considered hazardous waste). For this reason, manufacturers have established return programs to assist users in disposal of used solvents. Materials Compatibility The pure HFE solvents are compatible with essentially all common metals, most plastics and a number of elastomers. Table 6 lists a number of the specific materials that have been tested with the HFEs. Test coupons of the materials listed in Table 6 were exposed to © 2001 by CRC Press LLC

Table 6 Materials Compatibility with HFE Solvents

Aluminum Copper Carbon steel 302 stainless steel Brass Zinc Molybdenum Tantalum Titanium Tungsten Acrylic Polyethylene Polypropylene Polycarbonate Polyester Nylon Epoxy PVC PET ABS PTFE Butyl rubber Natural rubber Nitrile rubber EPDM Fluoroelastomer Polychloroprene

HFE-449s1

HFE-569sf2

C4F9OCH3

C4F9OC2H5

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2

Azeotrope 1 C4F9OCH3t-CClH  CClH 1 1 1 1 1 1 1 1 1 1 3 2 2 3 2 1 2 3 3 3 1 2 3 3 2 2 2

1. Compatible over extended exposures to the solvent with less than 5% changes in weight or volume. 2. Compatible with limited exposure to the solvent (i.e., short time exposures such as a cleaning cycle). 3. Typically incompatible.

the HFE solvents for 7 days at the boiling point of the solvent and subsequently examined for weight, volume, and appearance changes. The HFE azeotropes containing trans-1,2dichloroethylene exhibit compatibility similar to the pure HFEs with all metals but due to their higher solvency have limited compatibility with most polymeric materials. Compatibility of the components to be cleaned as well as materials of construction for the cleaning equipment should be evaluated with the various cleaning fluids during process selection.

© 2001 by CRC Press LLC

CLEANING SYSTEMS AND EQUIPMENT Several different cleaning processes exist that employ HFE solvents. These include: 1. Neat cleaning systems using pure HFEs 2. Azeotrope cleaning systems using azeotropic mixtures 3. Cosolvent cleaning systems using zeotropic mixtures The appropriate cleaning system is selected based upon the soil to be removed, the materials of construction of the part to be cleaned, and a number of other factors as indicated in Table 7. All of the HFE cleaning processes can be conducted in conventional cleaning equipment such as a vapor degreaser (Figure 2) as well as in-line cleaning systems. Neat Cleaning System The neat cleaning process, employing pure HFE solvents, is used when a single-component, mild solvency cleaning fluid is required. This process is conducted in a conventional vapor degreaser (Figure 2) and can effectively clean light hydrocarbon and silicone oils, particulate contamination, and halogenated lubricants, oils, and greases from parts. The HFE solvent can be used in a single or multiple sump system. Parts cleaning with this process is conducted in a manner similar to that used with conventional vapor degreasing solvents. Both vapor-phase and liquid-phase cleaning is possible depending upon the parts and soils to be removed. The mechanism of cleaning with neat HFE systems can be by dissolving or displacement of the soil.

Table 7 HFE Cleaning Process Selection Guidelines Process KB value of cleaning solvent Soils

Neat Cleaning Process

Azeotrope Cleaning Process

Cosolvent Cleaning

10

27 –33

20 to  150

Light hydrocarbon and silicone oils, halogenated oils, particulate

Medium oils, lubricants, release agents, some waxes and fluxes

Heavy oils, greases, buffing compounds, heavy flux

Substrates Plastic partsa Metal parts Circuit boards Coils Ball or roller bearings Cathetersa Elastomer partsa Glass parts a

OK OK Not appropriate OK OK OK OK OK

Not appropriate OK OK OK OK May be applicable May be applicable OK

OK OK OK OK OK Not appropriate OK OK

Careful evaluation of the compatibility of the parts with the various cleaning fluids should be considered during process selection.

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Freeboard

Freeboard Secondary Coils Primary Coils Cleaning Agent Vapor

Cleaning Agent

Single Sump Figure 2

Water Separator

Cleaning Agent Vapor

Cleaning Agent

Cleaning Agent

Multiple Sump

HFE cleaning equipment.

Azeotrope Cleaning System The azeotrope cleaning process uses an HFE azeotropic mixture, such as those listed in Table 3, as the cleaning fluid. This process is used in applications requiring a stronger solvent mixture. Since the solvents used in this process are true azeotropes, the compositions of the cleaning and rinse sumps remain essentially constant throughout use. The mechanism of cleaning with azeotrope systems is almost exclusively via dissolving the soil. The equipment required is similar to the conventional degreasers used with CFCs and their azeotropes. The HFE azeotropic mixtures can be used in single or multiple sump equipment in the same manner as the neat cleaning system described above. This process effectively cleans many oils, waxes, greases, and fluxes depending upon the azeotropic solvent used. Cosolvent Cleaning System A cosolvent process combines two different fluids to conduct the cleaning process. One is a low-volatility, high-solvency organic solvent that is used to dissolve the soil from a part’s surface. The second component, the HFE, functions as a rinsing agent since it is used to rinse the solvating agent from the component. This process typically uses solvating agents that are miscible with the HFE. These mixtures are not azeotropes (i.e., they are zeotropes) since the components separate when boiled, resulting in very different compositions in the cleaning sump and rinse sump. The process operates analogously to a twosump vapor degreaser, with a mixture of the solvating agent and rinsing agent in the boil sump and pure HFE in the rinse sump (see Figure 3). Because of the large difference in boiling points between the two components, very little of the solvating agent is distilled into the rinse sump. The rinse sump contains essentially pure HFE throughout the process. The mechanism of cleaning in this process is most often by dissolving the soil into the mixture of solvating and rinsing agents in the first sump. A wide variety of high-boiling, combustible solvents can be used as solvating agents in this process, provided that their flash points are well above the operating temperature of the system. Use of the HFE rinse agent renders the solvating agent nonflammable during

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. . . . . .

. . . . . .

. . . . . .

. . . . . . . . . . . . . . . . . . Rinsing . . . . . Agent . . . Vapor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Solvating Agent + Rinsing Agent Mix

Figure 3

Rinsing Agent

HFE cosolvent cleaning equipment.

use. Higher-solvency mixtures are created by increasing the ratio of solvating agent to rinsing agent in the boil sump. The process is controlled by monitoring the boiling temperature of this mixture since the operating temperature is determined by composition in the boil sump as shown in Figure 4. Temperature and composition control can be accomplished manually through periodic measurements of the boiling temperature followed by solvent additions when necessary or via an automated system. Since this process requires immersion into the solvent mixture in the cleaning sump, vapor cleaning is not possible. The HFE cosolvent process is capable of effectively cleaning a very wide variety of soils including heavy oils, greases, fingerprints, waxes, and flux. Cosolvent processes offer a great deal of flexibility by selecting a solvating agent and rinsing agent combination that best meets the needs of a particular cleaning application. The process can provide sufficient solvency for a given soil while maintaining compatibility with the part’s materials of construction. The higher solvency mixtures of the solvating and rinsing agents can accommodate higher soil loading levels than the neat or azeotrope systems.

Figure 4

Operating temperature of HFE cosolvent cleaning process. Example using C4F9OCH3 and methyl decanoate as a solvating agent.

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Parts Cleaning in HFE Cleaning Systems HFE solvents can be used with a variety of cleaning equipment designs such as conventional vapor degreasers, in-line systems, enclosed systems, and vacuum systems. The most common equipment employed with HFE solvents is a vapor degreaser. Both single and multiple sump systems are available (see Figure 2). Single sump systems are generally limited to vapor-phase cleaning and may require some form of spray rinsing. Multiple sump systems typically require immersion in a cleaning sump followed by rinsing in one or more rinse sumps. Efficient fluid containment requires holding the cleaned parts in the vapor zone followed by a dwell in the freeboard zone (the space between the vapor zone and the top of the machine) prior to exiting the equipment. This allows the maximum amount of solvent to drain and evaporate from the part while it is in the equipment, minimizing fluid dragout. The dwell times during the various steps of the process (cleaning sump, rinse sump, vapor zone, freeboard zone) are typically only a few minutes each, resulting in a relatively short cleaning cycle. Several basic steps are followed in most cleaning processes using HFE solvents to ensure the parts are thoroughly cleaned, rinsed, and dried in the specified cycle time. An example of a liquid immersion cleaning process would include: 1. Maintain cleaning system at operating temperature (cleaning sump, fluids boiling, full vapor zone established, condensate flow into the rinse sump, ultrasonic generators operating, etc.). 2. Arrange parts to maximize the flow of solvent around them. Avoid shadowing the parts from direct spray. Prevent the nesting of parts and the cupping of fluids. 3. Slowly lower the basket of parts into the cleaning sump. The parts must be completely covered by the cleaning fluid. The parts must remain covered for the entire immersion time. Ultrasonics or recirculating pumps may enhance cleaning in some applications. 4. Raise the basket of parts above the boil sump and allow excess cleaning solvent to drain back into the sump. 5. Move the basket over the rinse sump and completely immerse it into the rinsing fluid. 6. Let parts dwell in the rinse sump for a specified time. 7. Slowly raise the basket into the vapor zone, dwelling in the vapor until fluid drainage has stopped. Rapid ascent of the basket can result in increased vapor losses. 8. Slowly raise the basket into the freeboard zone. Dwell in the freeboard should last until the solvent has visibly evaporated from basket and parts. 9. Ensure that soil loading in the cleaning solvent is maintained below the level determined for process efficacy. Soil loading is typically monitored via the boiling temperature in the cleaning sump (the boiling temperature increasing with soil loading). Additional factors to consider when selecting the appropriate combination of cleaning solvent and equipment are listed in Table 8.

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Table 8 Equipment Comparisons for HFE Cleaning Processes

Vapor-phase cleaning Single sump Boil-up ratea Ultrasonics Distillation equipment

Water separator Pump seals Heaters

Filtrationd

System piping

HFE Neat Cleaning Process

HFE Azeotrope Cleaning Process

HFE Cosolvent Cleaning Process

Possiblea

Possiblea

Not possible

Possibleb 1–3 times rinse sump volume/h Suggested for use where possible Recommended (external)

Possibleb 1 –3 times rinse sump volume/h Suggested for use where possible Recommended (external)

Decanter PTFE or seal-less Sized to field voltage with 15% safety margin Recommended with immersion sumps Welded or compression fittings recommended

Desiccant PTFE or seal-less Sized to field voltage with 15% safety margin Recommended with immersion sumps Welded or compression fittings recommended

Not possible 1 –3 times rinse sump volume/h Suggested for use where possiblec Recommended with control features (internal) Decanter PTFE or seal-less Sized to field voltage with 15% safety margin Recommended with all sumps Welded or compression fittings recommended

a

Depends on part geometry, soil, soil loading, throughput, and part type. Single sump systems should be limited to vapor-phase cleaning and may require spray rinsing. c May be necessary in both clean and rinse sumps. d As required by part cleaning specifications. b

Drying/Water Removal Processes HFE solvents are sometimes used as a drying fluid following processes such as aqueous cleaning or metal plating. The drying processes can be conducted in equipment similar to vapor degreasers used for cleaning applications. Water can be absorbed from the surface of a component using HFE/alcohol solutions such as the azeotrope that forms between C4F9OCH3 and isopropanol (6.7% isopropanol by weight). These mixtures are typically useful for applications with relatively low water removal needs since the solutions can become saturated with water, reducing their efficacy. The solutions can be distilled to remove the water but care must be taken to avoid extracting the alcohol into the water and removing it from the drying solution. Applications demanding larger water removal capability typically use HFE/surfactant mixtures to displace the water from the component’s surface. Parts to be dried are immersed into a dilute solution of a silicone- or fluorochemical-based surfactant in the HFE solvent. The surfactant allows the solvent to wet the surface preferentially. This wetting action causes the water to bead up and displace from the surface, floating to the top of the solution. The part is then immersed in one or more sumps of pure HFE solvent to rinse the surfactant from its surface. This process is capable of efficiently removing water from complex components in short cycle times.

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Operation Practices to Maximize Solvent Containment In nearly all applications, the HFE solvent emissions are significantly lower than those of the ODS it replaces.21 However, to ensure economical operation of the cleaning system as well as reduce emissions of solvent to the environment, the following practices are recommended to maximize containment of the solvent: 1. Eliminate drafts near the cleaning equipment. Drafts can increase vapor losses by causing disturbances in the vapor–air interface. 2. A sliding cover is recommended to reduce losses while the system is idling (i.e., the system is operating but parts are not being processed) and during downtime. Hinged or plug-type lids are typically not as effective since their operation can cause disturbances in the vapor–air interface. 3. Cleaning equipment should incorporate extended freeboard ( 120%) and low temperature (20°C) secondary cooling coils to minimize diffusive losses of solvent. 4. Do not begin cleaning operation until the equipment is at the operating temperature. This will ensure the parts are adequately heated and that the solvent will dry in the freeboard zone prior to exiting the equipment. 5. The cycle times established for specific parts should be strictly followed. Inadequate dwell times in any of the zones (cleaning sump, rinse sump, vapor zone, or freeboard zone) can have a deleterious effect on process performance. Suitable residence times should be established to ensure that the parts are completely cleaned, rinsed, and dried. 6. Use of spray wands should be limited to only those parts where absolutely necessary and then only below the cooling coils since misdirected spray can increase losses. 7. Parts should be arranged so that solvent can efficiently drain and trapping or cupping of fluid is avoided. Tumbling of parts before removing them from the vapor zone can help remove excess solvent and reduce dragout. 8. Parts should not be raised above the vapor zone until the end of the cleaning cycle. 9. Use automated hoists and transfer equipment where possible. 10. Perform periodic checks for fluid leaks and follow equipment maintenance procedures. 11. Soil-loaded solvent should be recovered and recycled to every extent possible. Solvents used in the neat, azeotrope, or cosolvent cleaning processes can be distilled to be efficiently recovered for reuse following the manufacturer’s directions. Often this distillation can take place within the cleaning equipment. REFERENCES 1. WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project, Rep. 44, WMO, Geneva, 1999. 2. Cooper, D. L., Cunningham, T. P., Allan, N. L., and McCulloch, A., Tropospheric lifetimes of potential CFC replacements: rate coefficients for reaction with hydroxyl radical, Atmos. Environ., 26A, 1331, 1992. 3. Cooper, D. L., Cunningham, T. P., Allan, N. L. and McCulloch, A., Potential CFC replacements: tropospheric lifetimes of C3 hydrofluorocarbons and hydrofluoroethers, Atmos. Environ., 27A, 117, 1993.

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4. Bartolotti, L. J. and Edney, E. O., Investigation of the correlation between the energy of the highest occupied molecular orbital (HOMO) and the logarithm of the OH rate constant of hydrofluorocarbons and hydrofluoroethers, Int. J. Chem. Kinetics, 26, 913, 1994. 5. Zhang, Z., Saini, R. D., Kurylo, M. J., and Huie, R. E., Rate constants for the reactions of hydroxyl radical with several partially fluorinated ethers, J. Phys. Chem., 96, 9301, 1992. 6. Owens, J. G. and Minday, R. M., Update on hydrofluoroethers alternatives to ozone-depleting substances, presented at the International CFC and Halon Alternatives Conference, Washington, D.C., October, 1995. 7. Koenig, T. A. and Owens, J. G., The role of hydrofluoroethers in stratospheric ozone protection, presented at the International Conference on Ozone Protection Technologies, Washington, D.C., October, 1996. 8.Wallington, T. J., Schneider, W. F., Sehested, J., Bilde, M., Platz, J., Nielsen, O. J., Christensen, L. K., Molina, M. J., Molina, L. T., and Wooldridge, P. W., Atmospheric chemistry of HFE-7100 (C4F9OCH3): reaction with OH radicals, UV spectra and kinetic data for C4F9OCH2 and C4F9OCH2O2 radicals, and the atmospheric fate of C4F9OCH2O radicals, J. Phys. Chem. A, 101, 8264, 1997. 9. Marchionni, G., Silvani, R., Fontana, G., Malinverno, G., and Visca, M., Hydrofluoropolyethers, J. Fluorine Chem., 95, 41, 1999. 10. Owens, J. G., Segregated hydrofluoroethers: low GWP alternatives to HFCs and PFCs, presented at Joint IPCC/TEAP Expert Meeting on Options for the Limitation of Emissions of HFCs and PFCs, Petten, the Netherlands, May 26–28, 1999. 11. Kenyon, W. G., New ways to select and use defluxing solvents, presented at Nepcon West, Anaheim, 1979. 12. Owens, J. G., Hydrofluoroethers: a growing family of alternatives to ozone-depleting compounds, presented at the International Conference on Ozone Protection Technologies, Baltimore, November, 1997. 13. Flynn, R. M., Milbrath, D. S., Owens, J. G., Vitcak, D. R., and Yanome, H., U.S. patent 5,827,812, Minnesota Mining and Manufacturing Company, 1998. 14. NFPA (National Fire Protection Association) 49, Hazardous Chemicals Data, 1994 ed., NFPA, Quincy, MA. 15. Product Toxicity Summary Sheets: HFE-7100 and HFE-7200, 3M Company, 1998. 16. Ellis, B. N., Cleaning and Contamination of Electronics Components and Assemblies, Electrochemical Publications Limited, Ayr, Scotland, 1986, 175. 17. Material Safety Data Sheet: HFE-7100, 3M Company, 1998. 18. U.S. Fed. Regist., 62, August 25, 44900, 1997. 19. Wallington et al. (1997) reported that the n- and i-isomers of HFE-7100 were expected to have similar reactivity with OH based upon the observation that there was no difference in isomer reactivity toward Cl and F radicals. The n-C4F9OCH3 was reported in Wallington et al. (1997) as  5 years and was requoted in WMO Report No. 44 as 5.0 years. The value calculated to two significant figures is 4.7 years. Later measurements by Molina et al. at MIT on the pure i-C4F9OCH3 determined the lifetime to be 3.7 years. The commercial HFE-7100 is an approximate 60/40 mixture by weight of the iso and normal isomers, which results in an average lifetime of 4.1 years. 20. WMO Report 44 reports the GWP of HFE-7100 as 390 over a 100-year integration time horizon using the lifetime of 5.0 years. Calculation using the 4.1-year lifetime of the commercial product yields a GWP of 320 (100 year ITH). 21. Warren, K. J., Use of hydrofluoroethers in electronics cleaning applications, presented at the International Conference on Ozone Protection Technologies, Baltimore, November, 1997.

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CHAPTER 1.6

Hydrofluorocarbons Abid Merchant

CONTENTS Introduction Structure and Properties of HFC-43-10mee Chemical Structure and Nomenclature Properties Electrical Properties Replacement of Ozone-Depleting Substance (ODS) and High Global Warming Gases— Opportunities and Alternatives Ozone-Depleting Substances PFC Alternatives Alternatives to ODS Toxicity Environmental Regulatory Considerations Compatibility of Materials of Construction Plastics and Elastomers Metals Compatibility Chemical and Thermal Stability Selective Solvent Power Applications of HFC-43-10mee, Neat Carrier Fluid Particulate Removal Rinsing Agent Fingerprint Solvent Displacement Drying Application HFC-43-10mee Formulations Binary Alcohol Azeotropes Binary Heptane Azeotrope Binary 1,2-trans-Dichloroethylene Azeotropes Multicomponent Azeotrope Azeotrope-Like Formulations Nonazeotropic Blends

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Equipment Design Considerations for Low Emission Emission Measurement Data Summary References INTRODUCTION Hydrofluorocarbons (HFCs) are a family of compounds containing carbon, fluorine, and hydrogen. These compounds were developed and commercialized as a result of the Montreal Protocol to replace ozone-depleting substances (ODS), such as chlorofluorocarbons (CFCs) for refrigeration, foams, propellants, and solvent applications. The absence of chlorine in the HFC molecules makes them nonozone-depleting substances. The presence of hydrogen atom(s) reduces atmospheric life, and therefore these compounds have significantly lower GWP than the similar fully fluorinated or CFC compounds. The presence of a large number of fluorine atoms tends to make these compounds nonflammable, low in toxicity, stable to heat, low in reactivity, and compatible with most materials of construction. In mid-1990s, an HFC containing 5 carbon, 2 hydrogen, and 10 fluorine atoms, CF3CFHCFHCF2CF3 was introduced to replace CFC-113, perfluorocarbons (PFC) (C6F14, C7F16), and hydrochlorofluorocarbons (HCFC-141b and HCFC-225) in some solvent applications. This new HFC, called HFC-43-10mee in ASHRE (American Society of Heating, Refrigeration and Air Conditioning/Engineers) nomenclature, has physical properties similar or better than CFC-113 and C6F14. Compared with CFC-113, this HFC has very low surface tension, higher boiling point, and lower heat of vaporization so that it dries rapidly without leaving any residue. These properties combined with nonflammability, chemical and thermal stability, low toxicity, and ease of recovery by distillation make this HFC ideal for a broad range of applications. Solvency lies between CFC-113 and PFC and can be enhanced significantly by use of appropriate azeotropes and blends with alcohol, hydrocarbons, esters, and hydrochlorocarbons. STRUCTURE AND PROPERTIES OF HFC-43-10MEE Chemical Structure and Nomenclature HFC-43-10mee is a straight-chain HFC. Its molecular structure and that of CFC-113 are shown in Figure 1. The absence of chlorine in the HFC molecule requires a large number of carbon (5), fluorine (10), and some hydrogen (2) atoms to achieve the desired boiling point, good environmental properties, and nonflammability similar to CFC-113. In the IUPAC (International Union of Pure and Applied Chemistry) nomenclature this compound is 1,1,1,2,3,4,4,5,5,5-decafluorpentane or 2,3-dihdrodecafluorpentane. It is marketed under a trade name of Vertrel XF. Because of the position of hydrogens on the second and third carbons, it consists of an identical pair of diesteriomers with very similar properties.

F

F

F

F

F

F

C

C

C

C

C

F

H

H

F

F

F F

HFC-43-10mee

Figure 1

Structures of HFC-43-10mee and CFC-113.

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F

C1

C

C

C1

F

CFC-113

F

Properties HFC-43-10mee is a clear, dense, colorless liquid with a faint ethereal solvent odor like CFC-113. Table 1 gives a list of physical, transport, and thermodynamic properties of the HFC along with those of CFC-113. The HFC has a boiling point of 55°C, which is higher than that of CFC-113. The freezing point is 80°C which is lower than the freezing point of CFC-113. Thus, it can be used as a liquid over a broader temperature range than CFC-113. The higher boiling point also reduces emissions in the existing degreasing equipment, making it environmentally more friendly. The high liquid density helps to displace soils and particulate from the surfaces of parts being cleaned and to float these soils to the surfaces of the solvent. One of the clear advantages of CFC-113 was its low surface tension compared with other solvents such as chlorocarbons, hydrocarbons, alcohols, and water. The surface tension of the HFC is lower than that of CFC-113, which makes it easier to wet surfaces and thus assist in removal of soils through small crevices and openings found in Table 1 Physical Properties Propertya

HFC-43-10mee

CFC-113

Molecular weight Boiling point, °C (°F) Vapor pressure, mmHg (psia) Freezing point, °C (°F) Liquid density, g/cc (lb/gal) Surface tension, dyn/cm Viscosity, cPs Solubility in water, ppm Solubility of water, ppm Critical temperature, °C (°F) Critical pressure, psia (atm) Critical volume, cc/mol Heat of vaporization (at boiling point), cal/g (Btu/lb) Specific heat at 20°C (68°F), cal/g°C (Btu/lb·°F) Diffusivity, cm2/s Thermal conductivity Btu/h ft °F Vapor Liquid Refractive index Flash point Closed cupb Open cupc Flammable range in air Autoignition point in air

252 55 (130) 226 (4.4) 80 (112) 1.58 (13.2) 14.1 0.67 140 490 181 (357) 331.9 (22.6) 433 31.0 (55.7)

187 47.6 (117.6) 334 (6.46) 35 (31) 1.56 (13.1) 17.3 0.68 170 110 214 (417) 495 (33.7) 325 35.1 (63.1)

0.27 (0.27)

0.21 (0.21)

0.066

0.068

0.0057 0.036 1.24

0.0043 0.043 1.35

None None None Noned

None None None None

a

At 25°C (77°F) except where indicated. Pensky –Martens closed cup tester (ASTM D 93). c Tag open cup tester (ASTM D 1310). d None detected up to 540°C. b

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Table 2 HFC-43-10mee Density and Vapor Pressure Change with Temperature Temperature, °C (°F) 20 (4) 10 (14) 0 (32) 10 (50) 20 (68) 30 (86) 40 (104) 50 (122) 60 (140) 70 (158) 80 (176) 90 (194) 100 (212) 110 (230) 120 (248) 130 (266)

Density, g/cc (lb/g)

Vapor Pressure, mmHg (psia)

1.70 (14.2) 1.68 (14.0) 1.66 (13.8) 1.62 (13.5) 1.60 (13.3) 1.57 (13.1) 1.55 (12.9) 1.51 (12.6) 1.49 (12.4) 1.46 (12.2) 1.43 (11.9) 1.40 (11.7) 1.38 (11.5) 1.34 (11.2) 1.32 (11.0) 1.30 (10.8)

16 (0.3) 36 (0.7) 62 (1.2) 109 (2.1) 176 (3.4) 284 (5.5) 434 (8.4) 641 (12.4) 921 (17.8) 1288 (24.9) 1753 (33.9) 2343 (45.3) 3072 (59.4) 3961 (76.6) 5032 (97.3) 6309 (122.0)

surface mount printed wiring boards and close tolerance precision inertial guidance components. The energy consumption of a recirculating solvent system in a degreaser is a direct function of the heat of vaporization of the solvent. The heat of vaporization is lower than that of CFC-113, and much lower than that of alcohol, hydrocarbons, chlorocarbons, and water, making it more energy efficient in use. It has no flash point by both open and close cup methods and no flammable limits in air. Also, it does not have autoignition temperature in air measured up to 540°C. Table 2 gives both vapor pressure and specific gravity data as a function of temperature. From Antoine’s equation for vapor pressure of HFC-43-10mee,

where a  7.03668 b  1093.094 c  208.3936 P  millimeter of mercury T  °C

b Log10 P  a   (c  T)

Electrical Properties Electrical properties are given in Table 3. The dielectric constant is slightly higher than that of CFC-113 and the breakdown voltage is lower than that of CFC-113. Thus, the dielectric strength of the HFC is lower but still considered acceptable. The volume resistivity is in the most desirable range to minimize electrostatic discharge (ESD). ESD has become very important in the computer and electronic industry. The high-density disk drives use new technology consisting of giant magneto resistive (GMR) heads, which are extremely sensitive to small changes in current flow through the devices. ESD can result in a momen-

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Table 3 Electrical Properties of HFC-43-10mee Resistivity Dielectric constant Dissipation factor Breakdown voltage

2.9  109 ohms-cm 7 0.14 17.8 kV

tary current surge, which could as a minimum cause severe damage to the magnetic properties or, at most, a complete failure by the fusion of the GMR head. Therefore, it is desirable for all materials used in the manufacturing process to have low propensity to generate ESD. The ESD properties of a solvent can be measured by measuring its volume resistivity. The resistivity can be classified per EIA-541 and other specifications as: Conductive Less than 1  105 ohm/sq. Dissipative Between 1  105 and 1 x 1010 ohm/sq. Insulative Above 1  1012 ohm/sq. Lower value of the resistivity means lower propensity to generate ESD. As can be seen, the HFC has a resistivity number in the range called the “dissipative,” which is considered most desirable to minimize ESD generation and at the same time have adequate insulative properties. REPLACEMENT OF OZONE-DEPLETING SUBSTANCE (ODS) AND HIGH GLOBAL WARMING GASES—OPPORTUNITIES AND ALTERNATIVES Ozone-Depleting Substances HFC-43-10mee was primarily developed to replace ODS’s CFC-113 and methyl chloroform in applications where not-in-kind alternatives and other alternative solvents were not acceptable because of safety, process, or material incompatibility problems. To aid understanding of solvent markets and applications in 1989, before ODSs were restricted, solvent uses of CFC-113 and methyl chloroform are provided in Figures 2 and 3, respectively. With the implementation of the Montreal Protocol, the manufacture of both of these ODSs for solvent applications ceased in January 1996 (except in a few Article 5 countries). It is estimated that the current production of these ODS substances in these countries is less than 5% of the peak production of 1989. PFC Alternatives PFCs such as C5F12, C6F14, C7F16 and C8F18 were introduced as substitutes for the ODS CFC-113 in the late-1980s to early 1990s, and their uses grew rapidly as the phaseout of CFC113 began. Prior to this, PFCs were used in small quantities in niche applications. PFCs were heavily promoted as a substitute for CFC-113 in such applications as carrier fluid for fluorinated lubricants for the computer hard disk drives; flush fluid for particulate removal in precision cleaning; drying fluid in displacement drying application; coolants in other electronic components; and rinsing agents in a cosolvent process for cleaning printed circuit boards and mechanical components containing oil, grease, and other soils. The PFC uses for 1998 are estimated to be around 2500 MT of C6F14 (PFC-5060) and 500 to 1000 MT for other PFCs.1

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Medical Uses Miscellaneous 3% 9% Dielectric/ Coolant 2% Carrier 4%

Electronic 40%

Particulate Removal 4%

Drying 8%

Metal Cleaning 30%

Figure 2

CFC-113 uses in 1989, 251,000 metric tons (MT). (Breakdown by DuPont internal data, AFEAS Report, 1998.)

Alternatives to ODS The use of CFC-113 and methyl chloroform in electronic applications, especially defluxing, has been primarily replaced by three not-in-kind technologies: aqueous, semiaqueous, and “no-clean.” However, there are a few instances where these not-in-kind alternatives have not performed satisfactorily or were rejected because of compatibility, safety, flexibility, or reliability reasons and other solvent alternatives have been considered. HFC solvent (neat and in formulations) is used in very small niche specialty applications. It is estimated that total HFC-43-10mee use will be less than 1000 to 2000 MT and is equal to less than 1% of the CFC-113 market in 1989. The vast majority of the CFC-113 uses have been replaced by not-in-kind technology such as aqueous, no clean, and hydrocarbon.

Aerosols 13%

Coatings 10%

CPI 7% Adhesives 13%

Electronics 4%

Vapor Degreasing/ Cold Cleaning 53%

Figure 3

Methyl chloroform uses in 1989, 706,000 MT. (From ECSA, 1996.)

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Fluorinated Solvents in 2004 (Estimated 5% of 1989)

HFC projected use in 2004 (<1%)

Total CFC-113 use in 1989 (251 M Metric Tons)

Figure 4

Fluorinated solvent market size from 1989 to 2004. (From DuPont estimate.)

The ODSs, CFC-113 and methyl chloroform have been replaced primarily by non-HFC alternatives such as no-clean flux, aqueous, and semi-aqueous cleaning processes. It is estimated that the total organic solvent market is only 10% of the pre-phaseout solvent uses and consists of hydrocarbons, chlorocarbons, bromocarbons, HCFC, PFC, HFC, HFE, and HFC.2 Although several HFC solvents have been developed, only HFC-43-10mee has been commercially available since mid-1995. The volume of HFCs as a solvent substitute is extremely small. Figure 4 provides an estimate of the fluorinated and HFC solvent market, 1999 to 2004. It is estimated that the HFC uses will be less than 1% of the CFC-113 uses in peak year 1989, before the phaseout of CFC-113 began. Also, HFC-43-10mee is an ideal substitute for highly global warming PFC (C6F14, C7F18) compounds and is successfully replacing them in many applications. Although HFEs have slightly lower global warming potential (GWP), they may not have all the desired performance attributes to be substituted for the HFCs in many applications. For many cleaning applications, HFC-43-10mee blends and azeotropes are preferred. The most common azeotrope or azeotrope-like blends are of HFC with hydrochlorocarbons, alcohols, and hydrocarbons. These will be discussed in greater detail in other sections of this chapter. Toxicity HFC-43-10mee has a low order of toxicity on both an acute and chronic basis. DuPont has established an allowable exposure limit (AEL) for HFC-43-10mee of 200 ppm (v/v: 8h time weighted average). The AEL is the atmospheric concentration of an airborne chemical to which nearly all workers can be exposed repeatedly, day after day, during an 8-h day, or 40-h week without any adverse effect. It has low acute inhalation toxicity as estimated by the lowest concentration that causes mortality in experimental animals, the approximate lethal concentration (ALC). The 4-h ALC is 10,000 ppm in rats. In a 90-day inhalation study (6 h/day, 5 days/week) in rats exposed to 0, 500, 2000, and 3500 ppm, no effects on body weight, hematology, organ weight, or histopathology were observed. The 500 ppm was a no-observed-adverse-effect level (NOAEL). Since there was no exposure level between 500 and 2000, a subsequent 2-week repeated inhalation study (6 h/day, 5 days/week) was conducted at 1000 ppm. No adverse clinical signs of toxicity were evident at this 1000 ppm level throughout this study.

© 2001 by CRC Press LLC

Several genetic toxicity studies have also been completed and included an in vitro Ames assay, a chromosomal aberration study with human lymphocytes and an in vivo mouse micronucleus. No mutagenic changes were observed in any of these studies. Therefore, the evidence from these studies suggests that HFC-43-10mee is not genotoxic. Inhalation of certain other compounds followed by intravenous injection of epinephrine, to stimulate human stress reaction, can result in a cardiac sensitization response in experimental screening studies with dogs. For the HFC, no cardiac sensitization response was observed at 1000 and 5000 ppm. At 10,000 ppm seizurelike behavior was observed in the dogs, thereby preventing detection of a cardiac sensitization response. The results from a developmental toxicity study show that this material does not have embryotoxic or teratogenic effects in rats. The results from skin and eye exposure studies show that it is a slight skin and eye irritant. In summary, HFC-43-10mee has low toxicity, an AEL of 200 ppm, and a ceiling limit of 400 ppm. Environmental Regulatory Considerations The environmental properties of HFC-43-10mee and CFC-113 are given in Table 4. Because HFC has no chlorine, it has zero ozone depletion potential. The presence of hydrogen in the molecule reduces the atmospheric lifetime to about 17 years, low compared to 100 years for CFC-113 and 3200 years for PFC-5060. The reactivity with hydroxyl ions in the lower atmosphere is very low; therefore, emissions to the atmosphere do not contribute to formation of the smog or air pollution. Therefore, the U.S. Environmental Protection Agency (EPA) has exempted HFC-43-10mee from the volatile organic carbon (VOC) regulations. The HFC-43-10mee molecule is recently developed and hence required TSCA listing from the EPA for commercial applications. This approval was obtained and the substance was added to the TSCA inventory listing in 1995. Since 1995, similar approvals have been obtained in other industrialized countries of the world and the HFC has been added to the chemical inventories of Canada, Japan, European Common (EC) Market countries, Korea, Taiwan, Singapore, and Australia. Countries that are not listed above either do not require formal application and/or accept approval of the United States and the EC as sufficient for permitting its use in their countries. The EPA, under the Significant New Alternatives Program (SNAP), also accepted HFC43-10mee as a substitute for ODS like CFC-113 for various application categories. It is also not considered as a hazardous waste (RCRA) nor a hazardous air pollutant (HAP) and therefore not subject to NESHAP regulations or SARA-III reporting requirements. Table 5 summarizes regulatory status for HFC-43-10mee. The Montreal and Kyoto Protocols are interconnected because HFCs, developed as significant substitutes for some important uses of ODSs, are included in the basket of greenTable 4 Environmental Properties Property Formula Atmospheric lifetime, years Ozone depletion potential (ODP) Global warming potential (CO2  1) VOC

© 2001 by CRC Press LLC

HFC-43-10mee C5H2F10 17.1 0.0 1700 Exempt

CFC-113 C2Cl3F3 100 0.8 4800 Exempt

Table 5 HFC-43-10mee Regulatory Status Regulations

Status

SARA III HAP RCRA Halogenated NESHAP SNAP VOC ODP

No No No No Approved Exempt Zero

house gases (HFC, PFC, SF6, N2O, CO2, and CH4) to be controlled by the Kyoto Protocol. HFCs are effective substitutes in that they are safe in use, they are very energy efficient, and they have unique properties that help meet key needs of society—such as refrigeration, air conditioning, some areas of health care, and precision cleaning of high tech equipment. When considered total “greenhouse gases” the fluorocarbon gases contribute less than 2% of total carbon equivalent greenhouse gas emission.2 The HFC-PFC task force sponsored by the Montreal Protocol to resolve differences in Montreal and Kyoto protocol concluded that some HFC uses as ODS substitutes are critical and must be continued. The task force also recommended that the HFC uses must be responsible and continued where they provide the greatest overall value in terms of energy efficiency, environmental protection, performance, and safety. COMPATIBILITY OF MATERIALS OF CONSTRUCTION Two types of compatibility testing are conducted, short term and long term. Short-term compatibility tests, generally a 15-min exposure under a given set of conditions, are useful for evaluating compatibility with materials of construction in articles being cleaned. Longer-term compatibility tests, generally a 1- to 2-week period, at or above the boiling point, are useful for evaluating compatibility of the materials of construction of cleaning equipment. HFC-43-10mee is a milder solvent than CFC-113 and its compatibility with most plastics, elastomers, and metals is as good as or better than that of CFC-113. Plastics and Elastomers The effect of solvents on plastics or on elastomers (such as swelling, shrinkage, extractables, and hardness) depends on the nature of polymer, the compounding method, the curing or vulcanizing conditions, the presence of plasticizers or extenders, and other factors. For these reasons, it is difficult to make generalizations on the effects of any solvents on these materials. Testing in the proposed application is particularly important. Table 6 summarizes the effects of the HFC on a large variety of plastics in the unstressed state exposed at 50°C in a sealed tube tests for a period of 2 weeks. The plastic specimens were examined before and after the exposure tests for physical change as well as weight gain, and were assigned empirical ratings from 0 to 2; i.e., 0 means no effect, 1 means borderline, 2 means incompatible. These tests show that the HFC has minimal effects on most commonly used plastics except acrylic. Fluoropolymer plastics show high weight gain; however, the weight returns to normal after air drying. The 2-week test data should be used for selecting the vapor degreaser construction material and not for assessing the compatibility of the parts to be cleaned. The tests also suggest that the HFC may © 2001 by CRC Press LLC

Table 6 Plastic Compatibility with HFC-43-10mee Sealed Tubes (2 weeks at 50°C, 122°F) Plastic

Common Brand Name

HDPE PP PS PVC CPVC PTFE ETFE PVDF Ionomer Acrylic ABS Phenolic Cellulosic Epoxy Acetal PPO PEK PEEK PET PBT Polyarylate LCP Polyimide A PB PAI PPS Polysulfone Polyaryl sulfone

Alathon Tenite Styron

Teflon® Tefzel® Kynar Surlyn® Lucite® Kralastic Ethocel Delrin® Noryl Ultrapek Victrex Rynite® Valox Arylon®

Vespel® Ultem Torlon Rython Udel Rydel

Rating

Weight Gain, %

0 0 0 0 0 1a 1 0 0 2 0 0 1c 0 0 0 0 0 0 0 0 0

0.3 0.5 0.3 0.1 0.1 3.5 1.4 0.4 0.5 —b 0.0 0.0 4.7 0.0 0.2 0.2 0.1 0.1 0.2 0.0 0.0 0.1

0 0 0 1 0 0

0.0 0.1 0.0 2.7 0.1 0.1

Rating: 0  compatible; 1  borderline; 2  incompatible. Physical change: amore flexible; bsample dissolved; csome extraction.

dissolve and extract plasticizer from flexible, highly plasticized PVC tubing and contaminate the solvent. Therefore, flexible PVC tubing use should be minimized if plasticizers are of concern in the cleaning cycle. Table 7 gives the effects of the HFC on a large variety of elastomers exposed at 50°C in a sealed tube test for a period of 1 to 2 weeks. The elastomer specimens were examined before and after the tests for linear swell, hardness change, and physical appearance. The samples were assigned 0 to 2 empirical ratings similar to the plastics as discussed above. The fluoroelastomers as a group, and two others (“Vamac” and “Alcryn”), were considered incompatible because of excessive linear swells and hardness change. Fluoroelastomers swelling and shrinking will, in most cases, revert to within a few percent of the original size after air-drying. © 2001 by CRC Press LLC

Table 7 Elastomer Compatibility with HFC-43-10mee, Sealed Tubes (2 weeks at 50°C, 122°F) Elastomer

Rating

Natural rubber Butyl rubber Nordel® EDPM Neoprene CR Buna S Nitrile Rubber Buna-N NHBR Vamac® EA Hypalon® CSM Fluoroelastomer Viton® A Viton® B Zalak® Kalrez® Fluorinated silicone Silicone Epichlorohydrin Homopolymer Copolymer Adiprene U FA polysulfide Thermoplastic Alcryn® Santoprene Geoplast Hytrel® Polyester

Linear Swell, %

Units Hardness Change

0 0 0 0 0

0.6 1.0 1.0 0.2 0.7

1 1 2 1 0

0 0 2a 0

0.6 3.9 13.9 1.3

2 8 12 0

2 2 2a 2 2 0

17.3 22.8 13.7 21.6 14.1 0.5

14 34 13 20 11 4

0 0 1a 0

0.5 0.0 2.7 1.5

1 2 2 0

2a 0 1a 0

1.2 0.1 0.5 0.3

13 0 3 0

Rating: 0  Compatible; 1  Borderline; 2  Incompatible. a Noticeable extraction affecting rating.

Metals Compatibility Table 8 summarizes the effect of the HFC on most commonly used metals exposed at 100°C in a sealed tube test for a period of 2 weeks. The tests were conducted under two conditions: dry solvents and solvents fully saturated with water. The metal coupons were examined for change in appearance and weight, and solvent was tested for any reaction products. All metals were judged to be compatible, although a slight discoloration of zinc, brass, and copper was seen with the solvent saturated with water. Chemical and Thermal Stability HFC-43-10mee is stable to most soils and chemicals commonly encountered in solvent uses. It does not hydrolyze with water or moist air. Therefore, no stabilizer is required. Like all hydrogen-containing halocarbons, it does react with strong bases and amines, particularly in the presence of alcohol. Therefore, contact with such materials should be avoided. © 2001 by CRC Press LLC

Table 8 Metal Compatibility with HFC-43-10mee (2 weeks at 100°C in sealed tube test) Rating Metal 1

Zinc Stainless steel Brass Aluminum Copper

Dry

Wet

0 0 0 0 0

0a 0 0a 0 0a

a

Slight discoloration.

HFC-43-10mee has excellent thermal stability. In laboratory tests using an accelerating rate calorimeter, no decomposition could be detected at temperature reaching 300°C. Selective Solvent Power HFC-43-10mee is a relatively mild solvent, with selective solvency for soils, oils, and greases. Some commonly used tests to measure the solvency are Hansen and Hildebrand solvency parameters and Kauri-butanol number (KB). Hansen and Hildebrand solubility parameters3 for HFC-43-10mee as well as those for C6F14 and CFC-113 are given in Table 9. Based on these data, its is apparent that the HFC has overall solvency between C6F14 and CFC-113. Unlike C6F14, the HFC is completely miscible with most esters, ketones, ethers, ether-alcohols, and the lower alcohols such as methanol, ethanol, and isopropanol. The lower hydrocarbons, such as pentane, hexane, and heptane, have good solubility in HFC43-10mee as well as most chlorocarbons and fluorocarbons, including high-molecularweight fluorocarbon lubricant such as Krytox® and Fomblin®. Thus, HFC-43-10mee is used as an application carrier fluid or to remove these kinds of compounds. Unlike CFC-113, the neat HFC-43-10mee has limited solvency for many higher-molecular-weight materials such as hydrocarbon oils, silicon oils, fluxes, waxes, and hydrocarbon greases. However, many HFC formulations containing chlorocarbons, hydrocarbons, esters, and alcohols have enhanced solubility and cleaning efficiency to remove these soils. Applications of HFC-43-10mee, Neat Carrier Fluid HFC-43-10mee has excellent solvency for high-molecular weight fluorocarbon lubricants. Thus, it is ideally suited to replace PFC C6F14, (PFC®-5060), which is currently used in applying fluorolubricant to computer hard disks. The lube application process consists of Table 9 Solubility Parameters Compound

Dispersive

Polar

Hydrogen Bonding

Hildebrand

5.6 7.2 6.3

0 0.8 2.2

0 0 2.6

5.6 7.2 7.8

C6F14 CFC-113 HFC-43-10mee

© 2001 by CRC Press LLC

Table 10 Wetting Indexes of Solvents Cleaning Agents Freon® TF (CFC-113)

Density 1.48

Surface Tension 17.4

Viscosity 0.7 0.79

Wetting Index 122

1,1,1-Trichloroethane (TCA)

1.32

25.9

Isopropyl alcohol (IPA)

0.79

21.7

DI water

0.997

72.8

1.00

(6% water)

0.998

29.7

1.08

31

HFC-43-10mee

1.58

14.1

0.67

167

24

65 15 14

Saponifier solution

dipping the disks in a solvent bath containing a small amount of the high-molecularweight, high-boiling fluorocarbon lubricant. The lubricated disks are either removed from the bath or the bath is pumped out of the chamber. The solvent from the disk surface evaporates, leaving a desirable thin film of the lubricant on the surface. A similar lubrication process is also used to apply fluorolubricant to bearings. Also, a dilute lubricant mixture in HFC-43-10mee is being used to apply the lubricant to a specific joint, shutter, or moving part in precision equipment such as cameras, videos, tape decks, etc. Particulate Removal The physical properties such as low surface tension, low viscosity, and high density make the HFC very efficient for particulate removal. One common measure of particulate removal efficiency is wetting index, which is defined as a ratio of density over viscosity and surface tension. Table 10 gives wetting indexes for selected solvents. A high wetting index means the solvent readily wets the soil and easily rinses away contaminant such as particulate. The wetting index for HFC-43-10mee is higher than many common solvents. HFC-43-10mee is commercially used in several particulate removal applications. One such application is removal of particulates from camera-original motion picture negatives prior to making a large number of prints for use in theaters. Another such application involves particulate removal (size range from 0.5 to 60 µm) from miniature ion pumps used in the Shuttle Hardware Program.4 Wipes saturated with HFC-43-10mee are used on previously cleaned parts of the shuttle hardware to remove particulates before final assembly, and by electricians to clean cable surfaces prior to splicing the wires. Rinsing Agent As discussed earlier, HFC-43-10mee has limited solvency for high-molecular-weight hydrocarbon oils and greases. However, HFC-43-10mee is fully miscible with many aggressive organic solvents such as hydrocarbons, esters, ester-ethers, and ether-alcohols. Unfortunately, these solvents (also called solvating agents) are flammable, combustible, or have high boiling points. A mixture of HFC-43-10mee with one of these solvents can provide increased solvency, suppression of flammability, and rapid drying. This process is called a cosolvent process and is carried out in a conventional two-sump degreaser with a minimum modification. Typically, the first sump of the degreaser is charged with a mixture of HFC-43-10mee and a solvating agent, and the second sump is filled with pure HFC as a rinsing agent. The first sump primarily removes the soil and the second sump provides a

© 2001 by CRC Press LLC

Table 11 HFC-43-10mee Solvating Agents Dibasic esters (DBE) Diisobutyl DBE Methyl decanoate Isopropyl myristate N-Methyl-2-pyrrolidone (NMP) Tetrahydrofurfuryl alcohol (THFA)

Aliphatic hydrocarbons Aliphatic alcohols Dipropylene glycol butyl ether Propylene glycol N-propyl ether Dipropylene glycol monomethylether

clean rinse of fluorocarbon solvent to remove traces of high-boiling solvating agent left on the components to be cleaned. Table 11 gives a list of potential solvating agents. Fingerprint Solvent Ninhydrin dissolved in CFC-113 is commonly used by law enforcement agencies to develop latent fingerprints from porous surfaces such as papers. This CFC-113-based formulation is now being modified to use HFC-43-10mee by many law enforcement authorities to comply with the Montreal Protocol. The HFC-based new formulation works as well or better than the CFC-113 formulations. Displacement Drying Application CFC-113 containing a surfactant was commonly used for spot-free drying after aqueous cleaning. HFC-43-10mee, with physical properties similar to those of CFC-113, makes an attractive replacement in this application. HFC drying fluid with a proprietary surfactant is now commercially available and effectively used in many drying applications. The solvent drying process is carried out by dipping the wet parts in a boiling solvent containing a small amount of surfactant. The surfactant and the dense solvent lift the water from the surface of the parts. Water floats to the top of the solvent and is pushed out into a compartment by the circulating solvent, where it is decanted. The parts are subsequently rinsed in one or two baths of the pure solvent to remove residual surfactant. The whole process typically takes 3 to 10 min. This solvent drying process is more energy efficient than conventional oven or hot air drying processes. Two recent case studies5 indicate effectiveness of HFC-based displacement drying. One study involves drying of optical glass products (changing from a conventional oven dryer to the solvent displacement dryer). The HFC-based process reduced the reject rate (depending on the parts size) from 23 to 65%, and reduced the drying cycle time from 45 to 5 min. The second case study involved drying of microprocessor lids where the HFC-4310-based formulation replaced PFC-5060 (a high global warming fluid). HFC-43-10MEE FORMULATIONS Since the chemistry of soils varies greatly, the chemistry of a cleaning agent must be similarly adjusted to remove them. HFC-43-10mee can be effectively blended with a variety of other solvents to form specialized cleaning agents for widely divergent cleaning applications. The resulting formulations are used in vapor degreasing, aerosol, or cold cleaning processes. A thorough search was conducted to select materials for HFC-43-10mee formulations. The materials qualified for blending must have properties that would enhance the performance of the neat HFC-43-10mee for removal of certain soils and must also have acceptable

© 2001 by CRC Press LLC

Table 12 HFC-43-10 Formulations Vertrel

Components

Composition, wt%

Applications

Azeotropes XM

43-10/Methanol

94/6

XE

43 10/Ethanol

96/4

XP

43-10/IPA

97/3

KCD-9566 (XH)

43 10/Heptane

93/7

MCA

43 10/t-DCE

62/38

Ionic and particulate removal Pneumatic system cleaning Ionic and particulate removal Ionic and particulate removal Light oil and particulate removal Oxygen system Precision cleaning Cleanliness verification

Near Azeotropes SMT

43-10/t-DCE/Methanol

53/43/4

MCA Plus

43–10/t-DCECyclopentane 43–10/t-DCE/ MeOH/Cyclopentane

50/45/5

XMS Plus

50.9/43/4/2

Defluxing Precision cleaning Precision cleaning Oil and grease removal Defluxing Precision cleaning

Nonazeotropes Xsi

43–10/OS-10

57/43

X-B3 X-P10

43 –10/Butyl Cellusolve 43 –10/IPA

97/3 90/10

X-DA

43 –10/Surfactant/ antistatic

99.4/0.1/0.5

Silicone deposition and removal Swelling of silicone tubing Cutting/drilling fluid Ionic and particulate removal Absorption drying Drying

environmental and toxicity profiles. The candidates were further screened to pick those that would either form azeotropes or behave like azeotropes. Based on these criteria the candidates that met these requirements were alcohols (methanol, ethanol, and isopropanol), hydrocarbons (cyclopentane and heptane) and 1,2-trans-dichlotoethylene. Butyl Cellusolve, surfactants, and hexamethyldisiloxane were chosen for some specialty blends. There are three basic types of HFC-43-10mee formulations: azeotropes, azeotrope-like blends, and specialty mixtures. Table 12 gives their trade names, components, compositions, and key applications, Table 13 gives their physical, thermodynamic, and environment properties, and Tables 14 and 15 give their compatibility with plastics and elastomers, respectively.

© 2001 by CRC Press LLC

Binary Alcohol Azeotropes Three binary azeotropes of HFC-43-10mee with alcohol have been commercialized: XM for methanol azeotrope, XE for ethanol azeotrope, and XP for isopropanol (IPA) azeotrope. The presence of alcohol makes these azeotropes superior in removing particulates and ionic contamination. The methanol-based azeotrope is used for cleaning laser disks and flushing pneumatic lines. The ethanol azeotrope is used for precison cleaning as well as removal of trace moisture from thin tubes. The isopropanol azeotrope is effectively used for particulate removal from the large satellite mirrors and disk drive components. Also, the isopropanol azeotrope meets the California South Coast Air Management District VOC exemption guidelines of 50 g/l. Binary Heptane Azeotrope HFC-43-10mee forms an azeotrope with n-heptane. The azeotrope has improved solvency compared to neat HFC-43-10mee for light oils. The compatibility of this azeotrope is the same as neat HFC-43-10mee, which makes this material suitable for cleaning polycarbonates and polyurethane polymers. This azeotrope is marketed as a developmental product as KCD-9566. Binary 1,2-trans-Dichloroethylene Azeotropes Although 1,2-trans-dicholoethylene (trans-DCE) contains chlorine, the presence of hydrogen and the double bond makes the molecule reactive in the lower stratosphere. Therefore, it has near zero ozone depletion potential. trans-DCE has excellent solvency for higher-weight-molecular hydrocarbon oils and greases and for many fluxes. The permissible exposure limit of 1,2-trans-DCE is 200 ppm, the same as that of HFC-43-10mee. 1,2transDCE is flammable, but the presence of the HFC in the azeotrope makes the azeotropic mixture nonflammable. The azeotrope is marketed under the trade name MCA. This azeotrope is effective for precision cleaning of mechanical components, bearings, and compressor parts. It is also used as a verification fluid in the NASA shuttle hardware program and other aerospace applications.6 Because 1,2-trans-DCE is somewhat aggressive to plastics and elastomers it is important that the compatibility of the component materials to be cleaned should be tested. Multicomponent Azeotrope HFC-43-10mee, 1,2-trans-DCE, and methanol form a ternary azeotrope. A small concentration of nitromethane is added to prevent free radical reaction of alcohol with active metal (marketed as SMT). This azeotrope is effective in removing residual flux and ionic impurities in defluxing printed circuit boards and precision cleaning electronic components. Processes using this azeotrope have met or exceeded the cleaning requirement for ionic impurities for the military and the surface insulation resistance.7

© 2001 by CRC Press LLC

Table 13 HFC-43–10 Formulations: Physical, Thermodynamics, and Environmental Properties Blend Property

XM

XE

XP

XH

MCA

SMT

MCA Plus

XMS Plus

XSi

X-B3

X-P10

X-DA

Physical Boiling point (°C) Liquid density (g/ml) Liquid density (lb/gal) KB Vapor pressure (psia at 25°C) Surface tension (dyn/cm) Freezing point (°C) Heat of vaporization at bp (cal/gm) Heat capacity at 25°C (cal/g C) Viscosity (cps) Molecular wt.

48 1.49

52 1.52

52 1.53

53 1.47

39 1.41

37 1.37

38 1.33

38 1.34

56.6 1.0473

61 1.54

54 1.42

55 1.58

12.43 9.5

12.68 9.4

12.76 9.4

12.26 9.7

11.76 20

11.43 38

11.12 29

11.15 32

8.73 17

12.84

11.84 10.5

13.18

5.8

4.8

3.9

9.0

9.1

8.90

9.10

2.6

4.4

4.6

4.4

14.1  80

14.1  80

15.1  80

14.4  80

15.2  50

15.5  50

16.1  50

14.9  50

14  50

14.1  80

14.1 80

43

35

tbd

35

43

53

51

54

38

0.27 0.63 178.5

0.27 0.73 212

tbd 0.7 228

0.29 0.64 230.9

0.27 0.49 155.5

0.28 0.47 128

0.28 0.49 136.2

0.29 0.46 125

0.6 203.4

31 0.27 0.75 190.8

0.27 0.67

Environmental AEL (ppm) ODP GWP (100 year ITH) HGWP (vs. CFC-11) VOC (g/l) Flash point (closed cup) LEL (vol%) UE (vol %)

200 0 1222 0.235 89

235 0 1248 0.240 61

213 0 1258 0.240 50

217 0 1209 0.233 103

200 0 806 0.155 536

192 0 688 0.132 645

214 0 650 0.130 665

197 0 662 0.130 658

200 0 741 0.143 Exempt

92 0 1261 0.243 46

238 0 1170 0.225 142

186 0 1292 0.249 8

None 9 11

None None None

None None None

None None None

None None None

None tbd tbd

None 6 11

None 4 14

None 5

None None None

None 5 11

None None None

Negligible contribution from other compounds. a Condensation—could not determine. tbd  to be determined.

© 2001 by CRC Press LLC

a

Table 14 HFC-43-10 Formulations: Plastic Compatibility, 15 Min. @ Room Temperature (22°C) Compatibility Tradename

Generic

Alathon Tenite Styron

HDPE Polypropylene Polystyrene PVC CPVC TFE ETFE Ionomer Acrylic ABS Phenolic Ethyl cellulose Epoxy Acetal PPO PEK PEEK PET PBT

Teflon Tefzel Surlyn Plexiglas

Ethocel Delrin Noryl Ultrapek Victrex Rynite Valox Arylon Zenite Vespel Ultem Torlon Ryton Udel Radel

LCP PI PEI PAI PPS PSO PES

0  Compatible 1  Borderline 2  Incompatible

© 2001 by CRC Press LLC

XF

X-P

XE

XM

XSi

MCA

MCA Plus

SMT

XMS Plus

0 0 0 0 0 1 0 0 2 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 1 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 2 0 0 0 0 0 2 2 0 2 2 2 0 2 0 0 0 0 0 0 0 0 0 1 0

0 0 2 0 0 0 0 0 2 2 0 0 2 0 0 2 0 0 0 0 0 0 0 0 0 1 0

0 0 2 1 1 0 0 0 2 2 0 0 2 0 0 2 0 0 0 0 0 0 0 0 0 2 0

0 0 2 0 0 0 0 0 2 2 0 2 0 0 2 0 0 0 0 0 0 0 0 0 0 2 0

Table 15 HFC-43-10 Formulations: Elastomer Compatibility, 15 Min. @ Room Temperature (22°C) Compatibility Tradename

Generic

Adiprene Krynac Plioflex

Nordel Hypalon Natural Rubber Neoprene Butyl Silopren Thiokol FA Viton

X-P

XE

XM

X-DA

XSI

MCA

Polyurethane Nitrile Styrene/ Butadiene Ethylene Ethylene/ Propylene Chlorosulfonated Polyethylene Isoprene

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

2 2 0

2 2 0

2 2 0

2 2 0

0

0

0

0

1

0

0

1

1

0

0

0

0

0

2

2

2

2

0

0

0

0

0

1

1

1

1

Chloroprene Isobutyl isoprene Silicone Polysulfide Fluoroelastomer

0 0 0 0 0

0 0 0 0 1

0 0 0 0 1

0 0 0 0 1

0 0 0 0 1

0 2 1 2 2

1 2 1 1 2

1 1 1 1 2

1 2 1 1 2

0  Compatible 1  Borderline 2  Incompatible

© 2001 by CRC Press LLC

MCA Plus

SMT

XMS Plus

Azeotrope-Like Formulations Two azeotrope-like formulations of HFC-43-10mee are available for precision cleaning and defluxing, respectively. The first azeotrope-like formulation is a ternary consisting of HFC-43-10mee, 1,2-trans-DCE, and cyclopentane and is marketed as MCA Plus. This formulation has higher solvency for hydrocarbon oils and greases than MCA and is used for heavier oils and greases. This formulation is used for precision cleaning components and machined parts with close tolerances. The second formulation is a four-component azeotrope-like mixture consisting of HFC-43-10mee, 1,2-trans-DCE, cyclopentane, and methanol, and is marketed as XMS Plus. A small amount of nitromethane is added to protect against free radical reaction of alcohol with active metals. This formulation has a slightly better solvency for the fluxes and improved compatibility with PWB components. Therefore, it is used in defluxing PWB and precision cleaning of metal components.

Nonazeotropic Blends There are four nonazeotropic blends of HFC-43-10mee commercially available for specialty applications. 1. A proprietary blend of HFC-43-10mee with hexamethyldisiloxane is marketed as Xsi. It has excellent solvency for the high-molecular-weight silicones and, the same time, good compatibility with polycarbonates and polyurethane. The solvent is used as a carrier fluid to apply or remove silicone lubricant from the surfaces of medical devices used for insertion into the human body. It is also used as a swelling media for silicone rubber tubings. Both components of this formulation are VOC exempt and therefore the blend is a non-VOC. This blend is flammable and in use can separate into flammable compositions; hence, it should be used in flammable-rated equipment. 2. A proprietary blend of HFC-43-10mee with 2-butoxyethanol (butyl Cellosolve) is marketed as X-B3. This blend replaces a CFC-113 blend with 2-butoxyethanol that was used to serve a lubricant and heat removal function in cutting and drilling thick aluminum sheets or plates. The HFC blend was developed to serve a similar purpose. 3. A proprietary blend of HFC-43-10mee with 10% isopropanol is marketed as X-P10. The higher alcohol helps in the removal of water and ionic impurities from nonporous surfaces and for absorption drying. Although the blend is nonflammable as formulated, because it is nonazeotropic, its composition may shift in use and it could become flammable. 4. A proprietary blend of HFC-43-10mee with a small amount of fluorosurfactant additive ( 0.1 % by wt), an antistatic additive (0.5 %), is commercially available as X-DA. This formulation works extremely well as a displacement drying fluid. As discussed earlier, this blend offers one-step, low-energy, spot-free drying that is efficient, safe to use, and environmentally responsible.

© 2001 by CRC Press LLC

EQUIPMENT DESIGN CONSIDERATIONS FOR LOW EMISSION With increased environmental awareness and increased cost of solvents, it is imperative that the equipment designed to operate with the HFC solvents have the state-of-theart emission reduction technology. The primary enhancement features8 recommended are as follows: • • • •

An extended deep freeboard: freeboard to width ratio of 1.2 to 2.0 A secondary condenser for vapor diffusion control operating at20 to30°F Piping systems containing welded or soldered joints to minimize joint leaks Hoods and/or sliding doors on the top-entry machines

Some additional enhancements, although costly, may further reduce emission: • Automated work transport facilities • Facilities for superheated vapor drying The emission measurement tests9 run with an unmodified degreaser and with the primary enhancement features showed emission rate reduction from 0.075 to 0.0155 lb/hr ft2, or about 80% reduction in emission loss. A rough calculation indicates the cost of retrofitting an existing degreaser can be recovered in reduced solvent consumption in as little as 6 months of operation. Emission Measurement Data Several vapor-in-air concentration measurements have been made for HFC-43-10mee solvents in the working space immediately adjacent to the degreasers with and without enhancement features.9 In all cases, the emission has been well below the allowable exposure limit of 200 ppm. Table 16 gives the actual emission measurement data under various operating conditions around an enhanced Ultronix-modified Barons –Blakeslee 120 model degreaser (higher freeboard and a secondary low-temperature coil). As one can see, the average emission is less than 10 ppm.

Table 16 HFC-43-10mee Vapor Concentration in Air Measured Adjacent to a Modified BBI-MSR-120 Vapor Degreaser Vapor-in-Air Conc, ppm HFC-43-10mee Sample Location

Operating Conditions

Front and behind Front 24 in, behind degreaser

Degreaser idling with lid open Degreaser idling with lid open Degreaser idling with lid open Cleaned six basket loads of parts/lid open at all times Cleaned four basket loads of parts/lid closed during idling period

© 2001 by CRC Press LLC

Range of Values

Average Values

2.3 –18 2.3 –12 4.9 –18 1.6 –31

7 6 9.6 8.4

0.6 –38

8.5

SUMMARY HFC-43-10mee is an option that is critical but only in a very small segment of the previous CFC-113 markets. The replacement for the ODS solvents and/or high-GWP PFCs should be selected based on a holistic assessment of performance, environmental protection, safety, and health impacts. Responsible use of any alternatives including HFC-4310mee is strongly encouraged. Where possible, equipment should be upgraded with emission-reduction technologies. REFERENCES 1. Harnisch J. et al., Primary aluminum production: climate policy, emission and cost, presented at Joint IPCC/TEAP Meeting on Options for the Limitation of Emission of the HFC and PFCs, Petten, the Netherlands, 1999. 2. Report of the HFC and PFC Task Force of Technology and Economic Assessment Panel (TEAP) of the Montreal Protocol, October 1999. 3. Fritz, H. L., DuPont KSS report. 4. Jones, D. W., Evaluations of ODS-free particulate removal from miniature ion pumps, presented at Int. CFC and Halon Conf., October 1995. 5. Westbrook, G. A., et al., Solvent-based drying system offer fast spot-free results, Precision Cleaning, June 1999. 6. Wittman, C. L., et al., The search for a replacement for CFC-113 in the precision cleaning and verification of shuttle hardware, presented at Int. CFC and Halon Conf., October 1995. 7. Hanson, M. R. B., et al., Performance testing of HFC azeotropes for precision cleaning, presented at Int. CFC and Halon Conf., October 1995. 8. Ramsey, R. B., et al., Considerations for the selection of equipment for employment with HFC-4310mee, presented at Int. CFC and Halon Conf., October 1995. 9. Ramsey, R. B., et al., DuPont KSS report.

© 2001 by CRC Press LLC

CHAPTER 1.7

normal-Propyl Bromide Ronald L. Shubkin CONTENTS Introduction Historical Development normal-Propyl Bromide Physical Properties Cleaning Power Drying Compatibility Plastics and Elastomers Metals Drums and Drum Linings Thermal Stability Hydrolytic Stability Comparison of nPB Properties with Other New Cleaning Solvents Special Formulations—Electronics Compatibility with Metals Compatibility with Plastics and Elastomers Removal of Ionic Residues from Integrated Circuit Boards Prevention of Silver Tarnish Health, Safety, Environmental, and Regulatory Issues Toxicology—Acute Mammalian Genetic Toxicity Mammalian Metabolism Aquatic Acute Toxicology Recommended Exposure Limit Environmental and Health Regulatory Status Environmental and Health-Related Parameters Compared with Other New Solvents Summary Case Histories 1. Electronic Equipment 2. Electric Motor Stators and Refrigeration Coils © 2001 by CRC Press LLC

3. Implantable Body Parts 4. Aluminum Parts for Optical Applications 5. High-Performance Inertial Navigation Systems References

INTRODUCTION The cleaning of complex parts to exacting specifications has become a major consideration in the manufacture of a wide variety of machines, appliances, and instruments. The introduction of chlorinated solvents provided manufacturers and fabricators a convenient and economical way to clean a host of difficult soils contaminating strategic parts. Efficient cleaning, rapid drying, low flammability, residue-free parts, and relatively low solvent costs all contributed to the popularity of chlorocarbon and hydrochlorocarbon fluids. However, many chlorine-containing solvents have now been banned or restricted because of environmental and/or health considerations. With the mandated elimination of the most popular cleaning solvents, many manufacturers switched to aqueous or semiaqueous cleaning systems. Although these proved to be viable solutions in many applications, they were not suitable for all situations. In the search to find more appropriate alternatives, a wide variety of new solvents were developed.1 Many of the new solvent cleaners do excellent jobs, but still suffer from one or more deficiencies relative to the overall cost/performance of the chlorinated materials they replaced. Some of the newer solvents, such as some hydrochlorofluorocarbons (HCFCs), have been shown to have environmental problems and either have been banned from use in cleaning applications or are scheduled for phaseout. In other cases, the newly introduced solvents meet the environmental and toxicological requirements, but do not meet the performance standards. The need in specialized applications for a high-performance cleaning agent that could be used in a safe and efficient manner led to the development of cleaning systems based on the solvent normalpropyl bromide (nPB).

HISTORICAL DEVELOPMENT In 1991, the total U.S. market for 1,1,1-trichloroethane (TCA) as a cleaning solvent was 700 million lb. Another 200 million lb were sold as emissive solvents. TCA was, and still could be, a very effective and popular cleaning solvent, but it suffers from two important environmental drawbacks. It has a relatively high ozone depletion potential (ODP) and a relatively high global warming potential (GWP). The manufacture of TCA was banned by the Montreal Protocol effective January 1, 1996. A host of new cleaning systems have been introduced in recent years to meet the challenges presented by today’s industrial cleaning requirements. All of the new systems offer advantages in specified niche applications, but none has met all the requirements of the marketplace. Some of the important cleaning options are as follows: • Alternative chlorocarbon solvents. Most chlorinated solvents are effective cleaning agents. However, most have been banned from manufacture, or restricted to specified applications, or scheduled for phaseout, or have regulations restricting emissions and requiring extensive reporting.

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• Hydrocarbons and oxygenated hydrocarbons. These solvents have the obvious advantage of a historical low cost. However, most are readily flammable and present serious hazards when used in cleaning operations. Many also have toxicological problems. • Hydrochlorofluorocarbons (HCFCs). As with many chlorocarbon solvents, the most popular of these (HCFC-141b) has been restricted to noncleaning applications. HCFC-225 is still being used in cleaning, but it is scheduled for phaseout. Other types of HCFC molecules are too volatile, have only moderate cleaning ability, or are not considered cost-effective in specific application areas. • Fluorocarbons. Fluorocarbons are nontoxic, nonflammable, and very safe to use. However, they tend to be expensive and have poor solvency for most soils. The notable exception is their ability to solubilize highly fluorinated oils and greases. • Hydrofluorocarbons (HFCs). HFCs have low or moderate solvency for most soils of interest and historically have been expensive. They are excellent for niche applications, and they are often blended with more aggressive cleaning solvents. The environmental acceptability of the blend tends to be dependent on the second component. • Hydrofluoroethers (HFEs). HFEs are similar to HFCs in solvency, cost, and overall performance. Again, blends are sometimes used to boost cleaning performance. • Volatile methyl siloxanes (VMSs). VMSs, such as hexamethyldisiloxane, are low in toxicity and contain no halogen atoms. They are chemically very stable. On the other hand, they have flash points and only moderate solvency. • Semiaqueous systems. The problem of proper disposal of semiaqueous systems is often overlooked. Because of the high organic content, it is not appropriate (or legal in most cases) to dispose of these systems down the drain. Separation and recycle of the organic phase is usually difficult and is not cost-efficient. Slow drying and potential corrosion problems may also come into play. • Aqueous systems. Aqueous systems are very inexpensive in terms of detergent cost, but they are not suitable for all applications. High capital investment, multistep processing, a large equipment footprint, and high energy costs are often reported. Residuals on the clean parts and difficult drying are also problems. Corrosion of metal parts may become a factor. Finally, electrical and electronic applications often cannot tolerate the presence of remaining traces of water. • No-clean systems. Some manufacturers have eliminated the need to clean at various stages of manufacture. Sometimes this requires a change in the manufacturing process or the order of assembly.

normal-PROPYL BROMIDE In the mid-1990s, a new solvent/cleaner based on nPB was developed to meet the needs of those who require the cleaning efficiency of the chlorinated solvents but who must meet the strict environmental standards for a replacement solvent.2 The new solvent/cleaner does not suffer from many of the shortcomings of other alternative solvents that have been offered to the market. nPB is an effective cleaning agent. It is safe to use under the proper conditions, has a low ODP and a low GWP, and it is not regulated under most environmental and worker safety rules. It is compatible with metals, has a low tendency to cause corrosion, and may be used in most current vapor degreasing equipment. It is easily recycled and is moderately priced.

© 2001 by CRC Press LLC

Table 1 Physical Property Comparisons

Boiling point, °C Specific gravity, 25°C Viscosity, 25°C, cP Vapor pressure, 20°C, torr Specific heat, 25°C Latent heat of vapor, cal/g Solubility in water, g/100 g water Solubility of water, g/100 g solvent Surface tension, 25°C, dyn/cm Flash point, TCC, °C Flammability limits, vol %

n-Propyl Bromide

1,1,1-Trichloroethane

Trichloroethylene

HCFC-141b

HCFC-225

71 1.35 0.49 110.8

74 1.32 0.79 100

87 1.46 0.54 57.8

32 1.24 0.43 593

54 1.55 0.59 285

0.27 58.8

0.25 57.5

0.22 57.2

— 52.3

0.25 33

0.24

0.07

0.11

0.18

0.033

0.05

0.05

0.03

0.042

0.03

25.9

25.6

None 4–7.8

None 7–13

26.4 None 8–10.5

19.3 None 7.6–17.7

16.2 None None

PHYSICAL PROPERTIES Table 1 compares the physical properties of nPB to two hydrochlorocarbons and two HCFCs. HCFC-141b is CH3–CCl2F, and HCFC-225 is a mixture of the two isomers CF3–CF2–CHCl2 and CF2Cl–CF2–CHClF. The physical properties of the nPB are similar to all four of these solvents, but particularly so to TCA. CLEANING POWER Indicators that relate to the cleaning ability of a solvent are the solubility parameters. These are the Hildebrand parameter, the Kauri-butanol number (KB) and the Hansen parameters. As indicated by the data in Table 2, the values for nPB compare quite well with the common chlorocarbons. Although solubility parameters provide a guide to cleaning power, they do not take the place of experimental results. To compare the cold cleaning ability of neat nPB to neat chlorinated solvents, a simple test procedure was devised.3 Solutions of typical soil contaminants (30 wt%) were prepared in solvent. Steel wool wedges were weighed and then soaked in the contaminated solvent, drained, and dried at 100°C for 30 min. The wedges were reweighed and the weight of retained soil recorded. The impregnated wedges were then placed in short glass tubes and washed with 3 ml of the test solvent. The wedges were drained, dried, and weighed as before. The grams of soil lost per milliliter of test solvent give a measure of cleaning power. Figure 1, the cleaning power of chlorinated solvents relative to that of neat nPB, shows that nPB-based cleaners have fractionally lower cleaning ability than TCA when used to clean mineral oil, equivalent performance on grease and silicone oil, and superior performance in removing polyol esters. These experimental results are consistent with the © 2001 by CRC Press LLC

Table 2 Solubility Parameters

Hildebrand parameterb KB

n-Propyl Bromide

1,1,1TCAa

Trichloroethylenea

Methylene chloridea

Perchloroethylenea

18.2 125

17.4 124

18.8 129

19.8 136

19.0 90

18.2 6.3 6.1

19.0 6.5 2.9

Hansen parameters b

Nonpolar Polarb Hydrogen bondingb

16.0 6.5 4.7

17.0 4.3 2.1

18.0 3.1 5.3

a

From CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd ed., Allan F. M. Barton, Ed., CRC Press, Boca Raton, 1991. b Data presented as (MPa)1/2.

Figure 1

Relative cleaning ability, at ambient temperature, of nPB and chlorinated solvents.

Hansen parameters, which show that nPB has a slightly lower value for nonpolar materials and higher values for polar and hydrogen-bonding compounds. The cleaning power of nPB-based cleaners is clearly equivalent to the popular chlorinated solvents that have been banned or restricted. Comparisons to the new alternative solvents that have been introduced to replace the chlorinated materials, however, are more relevant to today’s needs. Again, it is instructive first to compare the relative solvency power of nPB with some of the new solvents that are being offered in the cleaning market. Table 3 compares the KB of nPB with that of several alternative solvents that have been introduced in recent years. By this one measure, superior performance is expected from the Table 3 Comparisons of Solvency Characteristics Solvent

KB

nPB HFC-43-10 mee HFE (methyl ether) HCFC-225 VMS (hexamethyldisiloxane)

125 9 10 31 17

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cleaning formulations based on nPB. The abbreviations used here and throughout the rest of this chapter are as follows: • • • • •

nPB: normal-propyl bromide HCFC-225: dichloropentafluoropropane (two isomers) HFC-43–10mee: decafluoropentane HFE: perfluorobutyl alkyl ethers (especially the methyl ether) VMS: volatile methyl siloxanes (especially the dimer, hexamethyldisiloxane)

Of course, the KB is only an indication of cleaning performance. Figure 2 shows the results of experiments that were conducted in the same manner as those carried out to compare nPB with the chlorinated solvents. For these experiments, performance was compared for the removal of mineral oil, silicone oil, and a complex mixture of common soils that simulates the ASTM 448 Standard Soil (some minor substitutions were made). These experimental results indicate clearly superior cleaning performance for nPB-based cleaners. It is for these reasons that the other solvents are often sold as blends with more aggressive solvents that for one reason or another cannot be sold by themselves as cleaning solvents. Cold cleaning is suitable for a variety of cleaning needs, but the very difficult soils usually require more severe conditions. For this reason, difficult soils are often removed using vapor degreasing equipment. In this equipment, the parts may be cleaned in the hot vapors of the solvent. For the most difficult soils, especially those that have been “baked on,” the part to be cleaned is often immersed in the boiling solvent. Because cold cleaning performance does not always reflect performance at higher temperatures, a new cleaning test was designed. Steel coupons (C1010) were weighed and coated with the selected soil (0.1 g). The coupons were placed in an oven at 250°C for 1 h, cooled, and reweighed. The coupons were then immersed in boiling solvent for 5 min, dried, and reweighed. Each trial was conducted in triplicate. The weight percent of soil removed from each coupon was recorded and averaged for the three trials. Because formulation plays a part in total cleaning efficiency, commercial cleaning solvents were used for this comparison. Five formulated solvents were chosen. The solvents were an nPB-based cleaner (Albemarle ABZOL™ VG Cleaner), HFC-43–10 mee (DuPont Vertrel ® XF), HFE (3M HFE-7100), HCFC-225 (Asahi Glass AK-225), and VMS (Dow

Figure 2

Relative cleaning ability, at ambient temperature, of nPB and four newer solvents.

© 2001 by CRC Press LLC

Figure 3

Cleaning of tough, baked-on soils. Coupon immersed in boiling solvent for 5 min.

Corning OS-10). The soils chosen were mineral oil, a polyol ester, and a rosin-based solder flux. The results that are shown in Figure 3 indicate that the nPB cleaner removes the baked-on mineral oil much more efficiently than the other cleaning solvents. The baked-on polyol ester and the rosin-based solder flux remain nearly untouched by the other solvents, but are mostly removed after 5 min in boiling nPB cleaner. DRYING From the standpoint of raw material costs, the most economical class of substitutes for chlorinated solvents would appear to be aqueous-based systems. However, aqueous cleaning procedures suffer from a number of difficulties, one of which is the slow drying. Drying rates can be improved by the installation of specialized drying equipment, but this is costly and usually contributes significantly to the process time. Cleaning systems based on nPB have drying rates that are comparable to the chlorinated solvents. To compare evaporation rates, loss of weight from 2 ml of solvent at room temperature (24°C) was measured after 5 min. Figure 4 compares the relative rate of evaporation of nPB with four common chlorinated solvents.

Figure 4

Relative evaporation rates of nPB and four common chlorinated solvents.

© 2001 by CRC Press LLC

An alternative, but popular, benchmark is the evaporation rate relative to butyl acetate at room temperature. For 1,1,1-TCA, the rate is 5.8 times that of butyl acetate. For nPB, the relative evaporation rate is 6.2—marginally faster than 1,1,1-TCA. COMPATIBILITY Because nPB is a very aggressive cleaner base, care must be taken to ensure that it is compatible in the short term and at the appropriate temperature with all of the components of the parts being cleaned. Similarly, care must be taken that the nPB-based cleaner is compatible for the long term with the materials of construction for the cleaning equipment and the storage and handling facilities. Plastics and Elastomers Plastics and elastomers that pass short-term compatibility tests with nPB (immersion in boiling solvent for 15 min) include the following: Plastics

Elastomers

Acculam™ epoxy glass Alathon™ HDPE Delrin™ acetala Kynar™ polyvinyl fluoridea Nylon™ (6 and 6.6) Phenolicsa Polyester (filled and unfilled) Polypropylene Teflon™ PTFEa Tefzel™ ethylene/PTFEa XLPE™ cross-linked PE

Adiprene™ polyurethane Aflas™ PTFE Buna-N™ rubber Kalrez™ fluorelastomera Neoprene™ polychloroprene Viton-A™ fluoroelastomerb Viton-B™ fluoroelastomerb

a These materials are also compatible for long-term (2 months) immersion at elevated temperature (65°C). b The Viton fluorelastomers are marginal for long-term (2 months) immersion at elevated temperature (65°C).

Plastics and elastomers that were found to be unsuitable (U) or marginal (M) for contact with nPB at elevated temperature for short periods (15 min) include: Plastics

Elastomers

Low-density polyethylene (M) Ultem™ polyether imide (M)

Butyl rubber (M) NBR nitrile rubber (M)

EPDM-60 (U) Silicone (U)

Metals nPB must be formulated with passivating agents to be used with many metals. Because the type and concentration of the passivators determines the performance, the following information applies only to ABZOL® VG Cleaner (VG). It was tested for compatibility with © 2001 by CRC Press LLC

metals according to Mil-T-81533A 4.4.9. This is a metal corrosion test that was originally designed to test the suitability of TCA for military applications. The metal coupon is held half-submerged in the refluxing cleaning fluid for 24 h. It is then examined for signs of corrosion. All of the following metals passed this test: Nickel Brass Monel Stainless steel 316L Stainless steel 304L

Inconel Copper Aluminum Carbon steel 1010

Titanium Zinc Tantalum Magnesium

Fresh aluminum surfaces react immediately with TCA at room temperature. nPB is much less reactive toward aluminum. If an aluminum coupon is scratched beneath the surface of TCA, which contains no metal passivators, there is an immediate formation of a dark, brownish red color. If the test is repeated using neat nPB, no color formation is observed. At reflux, some small dark spots are observed on the edges of the aluminum coupons after 3 to 4 hours. Similar tests were done with magnesium. Fully formulated nPB is completely safe for use with aluminum, magnesium, and other active metals. Drums and Drum Linings In addition to the corrosion test, 2-month immersion studies were carried out at 130°F with carbon steel 1010, stainless steel 316, and high-baked phenolic linings. All of these materials were shown to be suitable for long-term storage of cleaning fluids containing nPB. Most perfluorinated plastics are also suitable for storage. THERMAL STABILITY Knowledge of the thermal stability and thermal degradation products of a new solvent is important for safety considerations during use. Buildup of contaminants on heating elements, for example, can cause localized hot spots that may degrade the solvent. It is important to know at what temperature this will occur and to ensure that the products of the degradation are not dangerous or highly toxic. Two different approaches were taken to determine thermal stability. Thermal degradation studies were conducted by Columbia Scientific using the accelerating rate calorimetry (ARC) method. This method identifies the temperature for the onset of degradation by detecting the accompanying exotherm. For a fully formulated commercial cleaner based on nPB (VG), a significant exotherm occurred at 226.5°C. The test was terminated at 347°C. For nPB containing no additives, no exotherm was reported up to 395°C. However, the final pressure of the bomb after it was cooled was considerably higher than for the VG cleaner. Scrutiny of the temperature and pressure data shows that the temperature curve for the pure nPB flattened briefly at 226.5°C and that the rate of increase in pressure increased simultaneously. A possible interpretation of the data is that nPB thermally degrades at 226.5°C, but the event is either endothermic or it is slightly exothermic and is not detected by the ARC instrument. In the case of the VG cleaner, which contains stabilizers, the products of the degradation react exothermically with one or more of the stabilizers present.

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The degradation products formed in the experiments described above were trapped in a stainless steel bomb and analyzed by gas chromatography/mass spectrometry (GC/MS). The products are essentially the same from both the stabilized and the unstabilized nPB, although the ratios of the products are somewhat different. No free bromine or HBr is detected. Although trace amounts of methyl bromide and benzene are found, no products of a highly toxic nature form in significant quantities. Unlike chlorinated solvents, it is chemically impossible to produce an extremely toxic compound such as phosgene, which contains two chlorine atoms. The second approach to determining thermal stability was to simulate a real-world failure of a heating element in a vapor degreaser.4 A coiled nichrome wire was immersed in VG cleaner in the bottom of a 250-ml flask. The flask was connected to a dry ice/acetone trap that was vented to a laboratory hood. Electric current was passed through the nichrome wire until the exposed part glowed red hot. The cleaning solution boiled vigorously. The vaporized products were collected in the trap and analyzed by GC/MS. Unlike the ARC experiment that was conducted in the absence of air, some of the products formed in this experiment contain oxygen. The decomposition products detected after the two experiments were as follows: ARC Method

Submerged Nichrome Wire

Propane Isobutane Butane Methyl bromide 2-Methyl butane Pentane Ethyl bromide Branched C6H14 isomers Isopropyl bromide

Propene Methyl bromide Ethyl bromide Benzene Toluene Dipropyl ether 1,3,5-Trioxacycloheptane 4-Bromo-2-butanol 4-Bromo-1-butanol

Excessively high temperature “hot spots” on the interior walls of a vapor degreaser can occur if the heating elements short-circuit. The thermal degradation studies indicate that the use of nPB-containing cleaners creates no risks that are not normally encountered in the event of catastrophic equipment failure. HYDROLYTIC STABILITY Laboratory tests show that nPB is subject to a small degree of hydrolysis when contacted with water for extended times, particularly at elevated temperatures. In laboratory tests, an nPB cleaning solvent (VG) was compared with TCA cleaning solvent. After refluxing with water for 164 h, the layers were separated and analyzed. The nPB formulation showed two to three times as much hydrolysis as the TCA. Other tests were then performed to determine the relative corrosivity of HBr and HCl. In these tests, dilute HBr was six to seven times less corrosive at 50°C than was an equimolar concentration of HCl.2 Hydrolysis of nPB is less likely to cause corrosion than hydrolysis of TCA.

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Table 4 Comparison of Properties for New Cleaning Solvents

Boiling point, °C Flash point, °C Flammability limits, vol% Density, g/m

nPB

HCFC-225

HFC-43-10

HFE

VMS

71 None 3–8 1.35

54 None None 1.55

55 None None 1.58

60 None None 1.43

100 2.77 1.2 –18.6 0.76

Gb/P Gb/P G Gb/P G

Gb/P Gb/P G Gb/P G

G/F G/F P G F

G G/Fe G

G G/Fe G

G G/F/P P

Cleaning Wide range of soils High soil loading in liquid Fluorinated oils and greases Silicone oils and greases Fast drying

a

G G Gb/P G G

Gb/F Gb/F G Gb/P G Compatibility

Metals Plastics and elastomers Liquid oxygen—direct contact

c

G G/F/Pd P

Gc G/F/P P

G  good, F  fair, P  poor. Applies to blend or azeotrope, not pure solvent. c Must be properly formulated for metals compatibility. d G/F/P  compatibility varies significantly with different plastics and elastomers. e Fluorinated solvents may have poor compatibility with fluoroplastics and elastomers. a b

COMPARISON OF NPB PROPERTIES WITH OTHER NEW CLEANING SOLVENTS Table 4 is a brief summary of the physical performance characteristics of the relatively new entries in the cleaning solvent application area.5 Data are for the pure solvents, not the commercial blends. SPECIAL FORMULATIONS—ELECTRONICS The electronics industry faces some cleaning challenges not found in other types of cleaning applications. In particular, it is of utmost importance that ionic residues remaining on integrated circuit boards after soldering operations be removed to very low levels. In addition, conventional formulations for nPB-based solvents have a tendency to tarnish silver and silver-plated leads. To address these specific challenges, a formulation has been introduced that is designed specifically for the electronics industry. The new formulation,6 designated EG (electronics grade), is exceptionally efficient at the removal of ionic residues. Compatibility with Metals The EG of nPB-based cleaner was tested for compatibility with metals using the Mil-T81553A protocol. Metal coupons were polished with a fine emery cloth and then exposed to boiling solvent (liquid and vapor) for 24 h. The coupons were examined for signs of corrosion. The metals that passed this test were as follows: © 2001 by CRC Press LLC

Zinc Copper Brass C1010 steel Al 2024

Monel Magnesium Inconel 6000 Galvanized steel 304 Steel

Titanium-2 Tin plate Nickel 2000 316L steel

The only metal that failed this test was C12L14 leaded carbon steel. Compatibility with Plastics and Elastomers Plastics and elastomers that pass short-term compatibility tests (immersion in boiling solvent for 15 min) with EG cleaner are shown below. Those that were (M)arginal or (U)nsatisfactory after 2 weeks at 65°C are marked accordingly. The marked materials may be cleaned but should not be incorporated in equipment where they may contact the hot cleaning solvent for extended periods. Plastics

Elastomers

Acculam™ epoxy glass (M) Alathon™ HDPE Delrin™ acetal Kynar™ polyvinyl fluoride Low-density PE Phenolics Polypropylene

Teflon™ PTFE Teflon™ PTFE Tefzel™ ethylene/PTFE Ultem™ polyether imide XLPE™ cross-linked PE

Adiprene™ polyurethane (U) Aflas™ PTFE Lucite™ (U) Natural rubber (U) NBR polyether imide Neoprene™ polychloroprene (U) Viton-A™ fluoroelastomer (M)

Removal of Ionic Residues from Integrated Circuit Boards The testing of EG cleaner was carried out in conjunction with Contamination Studies Laboratories, Inc. (CSL). CSL prepared IPC B-24 boards fluxed with Higrade RMA liquid flux and processed through a standard wave solder system. The boards were then shipped to the Albemarle Technical Center where they were cleaned using a Branson 5-gal laboratory vapor degreaser. The cleaning cycle was as follows: 1. 2. 3. 4.

Vapor zone—2 min Boil sump—3 min Rinse sump—1 min Vapor zone—2 min

Table 5 Removal of Ionic Residues with EG nPB Cleaner Board No. 1 2 3 Average

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Omega Meter, g/in.2 4.3 5.2 4.1 4.5

Ion Chromatography, chloride ion, g/in.2 2.37 2.49 2.08 2.31

The cleaned boards were returned to CSL for evaluation. Three techniques were used: 1. Resistivity of solvent extract (ROSE) using an omega meter; 2. Ion chromatography (IC) after extraction; 3. Surface insulation resistance. The results of the ROSE (omega meter) tests and the IC tests are given in Table 5. Note that the IC values are for chloride ions only. Bromide ions were also found, but control tests showed that these were most likely extracted from the brominated flame retardants incorporated in the board material. The IC test employs a very vigorous extraction technique that involves heating the boards at 80°C for 1 h in a 3/1 IPA/water solution. The CSL report concludes that “The ionic residue testing of ROSE (Resistivity of Solvent Extract—Omega Meta) and Ion Chromatography show that the samples have low residues.” They also report, “The SIR (Surface Insulation Resistance) data all pass the SIR testing protocol for good product performance.” At a presentation for the IPC/SMTA Electronics Assembly Expo in October of 1998, Howard Feldmesser of The Johns Hopkins University Applied Physics Laboratory reported that both the VG and EG formulations were superior to AK-225 AES in the removal of ionic residues from circuit boards. In addition, the EG formulation was superior to the three other nPB-based formulations that he included in his evaluation.7 Prevention of Silver Tarnish Reports from the field indicated that nPB-based cleaning formulations were tarnishing silver and silver-plated contacts. A study of the additive technology resulted in a solution that was then incorporated into the nPB EG formulation.8,9 Silver-plated steel coupons were placed vertically into beakers of several commercial nPB-based solvents such that the solvent came approximately one third of the way up the coupon. The beaker was then heated on a hot plate to boiling and held at the boiling temperature for 10 min. The heat-up time was approximately 5 min. The coupons were removed from the boiling solvent, dried, and examined. All of the commercial grades badly tarnished the coupons, but the EG cleaner showed no tarnish. A second experiment involved the cleaning of lead frames that are made of copper and are tipped with silver. One frame was cleaned with a typical nPB-based formulation and the other frame with EG cleaner. The cleaning cycle included 3 min in an ultrasonic sump, 3 min in the boil sump, and 4 min in the vapor zone. The difference is dramatic and obvious. A solvent cleaner based on nPB and formulated in the usual way for vapor degreasing severely tarnishes silver and silver plate. The EG formulation, on the other hand, does not tarnish silver when used in a normal cleaning cycle (including ultrasonics and/or submersion in the boiling solvent). HEALTH, SAFETY, ENVIRONMENTAL, AND REGULATORY ISSUES10 Toxicology—Acute nPB is somewhat toxic, and it should be handled with appropriate precautions. Below is a compilation of the current acute toxicology data for nPB. Current Material Safety Data Sheets (MSDSs) should be obtained from the manufacturer for additional and/or the latest information.

© 2001 by CRC Press LLC

Mammalian Genetic Toxicity nPB is negative for dominant lethal activity in rats at 400 mg/kg/day given for 5 days to male rats prior to breeding once weekly for 8 successive weeks. No difference in mating performance was noted in treated males. Their frequency of fertile matings, mean numbers of corpora lutea, number of implants per female, number of live embryos per female, and the dominant lethal index were comparable to the negative control group at weeks 1, 2, 3, 4, 5, 6, 7, and 8 after treatment. The frequency of dead implants was higher at week 8 of treatment compared to the control group, but no increase was observed in the dominant lethal index at that or any other time. The frequency of dead implants in the treated group was comparable with the control group at weeks 1, 2, 3, 4, 5, 6, and 7. Mammalian Metabolism The half-life of nPB in the rat is very short (approximately 2 h). The majority of the administered dose is eliminated rapidly in expired air as the unchanged parent compound. The remainder is metabolized and excreted in the urine (predominant route) or in the expired air as CO2 (minor route). Following a single intraperitoneal dose (200 mg/kg), the initial rate of excretion of unchanged 14C-labeled parent compound in the expired air of the rat was rapid; 2 h after administration, 56% of the administered dose was exhaled as the parent compound. After 4 hours, 60% had been exhaled; only trace amounts were detected in expired air after this time. An earlier study also reported the elimination of the unchanged parent compound in expired air. Oxidation to CO2 occurred only to a minor extent. Only 1.4% of the total dose (or 3.5% of the metabolized dose) was exhaled as CO2 over 48 hours. Approximately 40% of the total intraperitoneally-administered dose was available for metabolism in the rat and excretion in the urine. Aquatic Acute Toxicology The solubility of nPB in is approximately 0.25 g/100 ml water at 20°C. The 96-h LC50 in flathead minnows is 67,300 mg/l. Recommended Exposure Limit Based on 90-day inhalation studies and preliminary two-generation inhalation reproductive tests on rats, the Albemarle Workplace Exposure Guideline (AWEG) was set for workers using nPB on an 8-h shift, five shifts per week. The AWEG has been set at 25 ppm. At this level, solvent/cleaners based on nPB can be used safely with modern vapor degreasers and in other applications with proper handling. As of this writing, the U.S. EPA has not made a final decision on nPB. [Editor’s note: At the time of publication, cooperative studies by the Bromine Solvents Consortium (BSOC) to support determination of an appropriate inhalation level for nPB are in progress. BSOC is working closely with the U.S. EPA. Studies will also be conducted under the U.S. National Toxicology Program. It should be noted that producers of nPB differ somewhat in their interim recommendations. For example, another major producer, which, like Albemarle Corporation, is a member of BSOC, recommends that exposure levels be controlled to below 50 ppm pending evaluation of the BSOC studies and resolution of areas of uncertainty. — B.K.]

© 2001 by CRC Press LLC

Table 6 Environmental and Health Regulatory Status Regulation a

SARA HAPb NESHAPc RCRAd HGWPe ODPf Atmospheric Lifetimeg PELh VOCi SNAPj

n-Propyl Bromide

1,1,1-Trichloroethane

Trichloroethylene

No No No No 0.0001 0.0019, 0.02711 15 days10

Yes Yes Yes Yes 0.023 0.1 5.4 years

Yes Yes Yes Yes Almost zero? Almost zero? NA

25 ppm (additional data under review) Yes Pending

350 ppm

50 ppm (25 ppm in CA)

No Unacceptable

Yes Acceptable

a

SARA—Superfund Amendments and Re-authorization Act. This act requires reporting of inventories and emissions of listed chemicals and groups. nPB is not regulated. Most common chlorinated solvents are regulated. b HAP—Hazardous Air Pollutant. A listing of chemicals that the EPA has declared hazardous. nPB is not on the list, but chlorinated solvents are. c NESHAP—National Emission Standard for HAP. Sets standards for use of HAPs. Since nPB is not a HAP, these standards do not apply. d RCRA—Resource Conservation Recovery Act. Defines hazardous wastes and how to manage them. nPB is not regulated under this act. Chlorinated solvents are regulated. e GWP and HGWP—Global Warming Potential, Atmospheric Lifetime, and Ozone Depletion Potential calculations were carried out by Atmospheric and Environmental Research, Inc. GWP is calculated relative to CO2, whereas HGWP (Halocarbon GWP) is calculated relative to CFC-11. GWP calculations were done using different integration time horizons. By the HGWP method, CFC-11 is 10,000 times more detrimental as a global warming agent than nPB. By the GWP method, it is 14,000 times worse than nPB, and nPB is only one tenth as bad as CO2. GWP Compound

HGWP

20 years

100 years

500 years

CFC-11 nPB

1.0 0.0001

4500 1.01

3400 0.31

1400 0.1

f

Atmospheric Lifetime—Ozone Depletion Potentials (as well as GWP) depend on the atmospheric lifetime of the substance in question. Ozone depletion takes place in the stratosphere. In order for a substance to have a high ODP, it must be able to work its way to the stratosphere. The atmospheric lifetime of nPB is only 15 days. In comparison, TCA has a lifetime of 5.4 years.

g

ODP—Ozone Depletion Potential. Models for calculating ODPs have generally been based on the assumption that the chemicals are relatively long-lived in the atmosphere. Because of the short atmospheric lifetime of nPB, certain assumptions had to be made relative to the rate of transport of the molecules and the free radicals that are formed when they dissociate. Two different models were used, resulting in ODPs of 0.0019 and 0.027 (Atmospheric and Environmental Research, Inc.). For comparison, TCA has an ODP of 0.1. Refinement of this modeling for short-lived chemicals is ongoing. h PEL—Permissible Exposure Limit. These are exposure guidelines for workers using the given chemical. PELs may be set by EPA or OSHA. For nPB, neither agency has set a PEL at the time of this writing but studies are in progress. The value given has been assigned by Albemarle Corporation and is referred to as the Albemarle Workplace Exposure Guideline (AWEG). Based on inhalation studies, Albemarle set the exposure level guideline at 25 ppm for a time-weighted 8-h exposure. i VOC—Volatile Organic Compound. All VOCs are classified as VOCs until there is experimental evidence that they do not contribute to the formation of smog. Therefore, nPB is currently classified as a VOC and must be used in accordance with local regulations regarding VOCs. Studies have been conducted to determine the degree of photoreactivity of nPB and the types of photochemical products formed. The data has been supplied to the EPA. At the time of this writing, the classification is still under review. j SNAP—Significant New Alternatives Policy. This is the policy under which EPA gives approval for the marketing of a replacement for an ozone depleting chemical. EPA may approve, disapprove, or give restricted approval within 90 days of the application. Alternatively, EPA may choose to delay any ruling on an application. If the EPA does not rule within 90 days, as is the case with nPB, the replacement chemical may be commercialized, pending final determination. On February 18, 1999, the EPA published an “Advance Notice of Proposed Rulemaking (ANPR)” on nPB, requesting further input. No final action has been taken at the time of this writing.

© 2001 by CRC Press LLC

Table 7 Comparison of Environmental Parameters for New Cleaning Solvents

Atmospheric lifetime VOC ODP GWPb Halocarbon GWPd Exposure guideline, TWA, ppm

nPB

HCFC-225

HFC-43-10

HFE

VMS

15 days

2.7–7.9 years

17.1 years

4.1 years


30 days

Yesa 0.002, 0.027 0.31 0.0001

No 0.03 170/690c 0.04/0.06c

No None 1700 0.33

No None 320 0.06

No None Low Low

25e

50

200

750

200

a

Application for delisting as VOC has been filed with EPA. GWP (CO2)  1, 100-year ITH). c Different values for the two isomers. d HGWP (CFC 11  1.0). e Please see Editor’s Note in the section “Recommended Exposure Limit.” b

Environmental and Health Regulatory Status Worker safety, public safety, and environmental protection are paramount in the development of any new product. The status at the of nPB and solvent/cleaner systems based upon it is given in Table 6. The table includes information on 1,1,1-TCA and TCE for comparison. Explanations of the regulations follow the table. Environmental and Health-Related Parameters Compared with Other New Solvents It is also useful to compare the environmental impact and safe usage factors of the various new solvents. As one can see by studying the data in Table 7, there remain difficult conflicts in deciding which solvent has the least impact on both environmental and health and safety issues. The ODP and the GWP and HGWP are often in conflict. The atmospheric lifetime is an important consideration, although there remains debate about how to calculate ODP of molecules with short half-lives. One must also consider whether the compound is a VOC, i.e., a compound that causes smog formation. SUMMARY Solvent/cleaner systems based on nPB have been introduced as replacements for chlorinated solvents in cleaning applications. nPB is an aggressive, fast-drying solvent that is suitable for a variety of difficult cleaning and degreasing applications. The use of nPBbased solvents is not regulated under SARA, HAP, NESHAP, or RCRA, and they are approved for sale under SNAP (further review by EPA is under way). nPB has low potentials for ozone depletion and for global warming, but it is currently classified as a VOC. The Albemarle Workplace Exposure Guideline is 25 ppm, which indicates the use of proper precautions for worker safety. In comparative performance testing, nPB-based formulations have been demonstrated to be as effective as chlorinated solvents and more effective than HCFCs, HFCS, HCES, and VMSs.

© 2001 by CRC Press LLC

The newest developments in nPB-based cleaning solvent evolution include the extension of the formulation technology to solve specific requirements in specialty niche applications. One such example is for the electronics industry. In addition to the expected advantages for a nPB-based cleaning solvent, the EG formulation has the advantages of providing enhanced efficiency in the removal of ionic residues while not tarnishing silver. CASE HISTORIES 1. Electronic Equipment A company that produces electronic components for clinical equipment used in biomedical applications had used CFC-113 blends in both liquid and vapor phase defluxing.12 Exacting performance standards, rate of throughput, space limitations, limited capital equipment budget, and lack of an adequate industrial water system were major constraints on a changeover to a new cleaning system. The company attempted to switch to a system based on d-limonene. Residue from the cleaning agent, buildup of rosin flux, and reactivity to produce assorted oxidation products made the electronic components unsuitable for use. An unacceptable green residue remained on the assemblies and there was an increase in product failures. After switching to VG cleaner, the company found that the new cleaner did a better job of removing flux than the CFC-113 blend. There were no residue problems as there were with d-limonene. Some of the plastic components did show some discoloration, but this problem was solved by shortening the exposure time—an added benefit that increased production throughput. 2. Electric Motor Stators and Refrigeration Coils The Galley Products Division of B/E Aerospace has a need to clean burned oils from used electric motor stators. They sent two such stators for test cleaning. One was clean and the other was covered in oil. Both were cleaned using VG cleaner. They were first lowered into the vapor zone of a vapor degreaser and held there until condensation of the vapor on the parts ceased. This required about 5 min. They were then lowered into the ultrasonic bath for 10 min and back into the vapor zone for 5 min. The parts were returned to B/E for examination. B/E Aerospace reported that the parts were “perfect.” No residual oils or other contaminants were found, and there was no damage to the electrical wiring or casings. This finding led B/E Aerospace to examine other applications for VG cleaner. It has since devised a flushing station for cleaning the long coils of copper tubing used in refrigeration units. B/E presented a paper at the CleanTech’98 conference outlining the extensive selection process that led it to choose VG for its tough cleaning problems.13,14 3. Implantable Body Parts A company manufactures artificial body parts, such as hip joints, for implantation. The parts consist of a titanium bone replacement and an ultrahigh-molecular-weight polyethylene (UHMWPE) cartilage replacement. Standards for cleanliness are, of course, very high. In addition, the company expressed concern about retention of solvent in the UHMWPE parts. A final criterion is that the cleaning solvent must kill at least 50% of the bacterial spores on an artificially inoculated UHMWPE part. There were two cleaning solvents that © 2001 by CRC Press LLC

the company wished to replace in its process. The first was based on HCFC-141b. The second was based on trichloroethylene. The initial set of experiments was for part cleanliness, and these were performed at Baron Blakeslee, Inc. (Long Beach, CA). UHMWPE parts were exposed to VG vapors for 90 s. Both cleaned and uncleaned parts were returned to the company, which found the cleaning to be satisfactory. Acetabular components and hip stems were contaminated with buffing compound. These were cleaned using the Baron Blakeslee AutoBatch vapor degreaser. The cycle consisted of 30 s in the vapor, 5 min immersion in the ultrasonic sump at 130°F, 2 min TopHat drying, and 2 min freeboard dwell. The parts were reported to be completely dry with no solvent dragout. Baron Blakeslee returned the parts to the company, which again determined that the cleaning was satisfactory. In a third experiment, a femoral part with fingerprints was exposed to vapor for 90 s. Evidence of fingerprints remained after this test. The second company concern was retention of the solvent by the UHMWPE parts. nPB (VG) was compared directly with commercial cleaning grades of HCFC-141b and TCE. The test procedure was to place five parts in the boiling solvent for 3 min. The parts were removed and placed in the same solvent at ambient temperature for 2 h. The parts were then removed and placed in an open dish. The dish was left in a fume hood with the fan going until it was time to take the measurements. To obtain the quantity of headspace vapors, the parts were placed in a sealed glass chamber and allowed to equilibrate for 1 h. The vapors were then analyzed using GC/MS and quantified against a standard. Measurements were taken at 24 and 96 h for the nPB. For the other two solvents, the measurements were at 24 and 106 h. The vapor in the headspace is reported in ppm by volume. Concentration of Solvent in Headspace, ppm

nPB (VG) HCFC-141b Trichloroethylene

24 h

96 or 106 h

30 169 469

3.7 4.4 27

These experiments indicate that the nPB is retained in the UHMWPE parts to a lesser extent than either the HCFC-141b or the TCE. The third criterion was that the solvent/cleaner reduce bacterial spore counts on the UHMWPE parts by at least 50%. The company supplied two sets of parts (six parts each) contaminated with Spordex® Bacillus subtilis (globigii) spores. Each set was divided into two sets of three parts each—one set to be cleaned and one set as a control. The cleaning procedure was as follows: 1. Test parts were placed in the basket of a laboratory vapor degreaser and covered with a metal screen to prevent the parts from floating to the surface. The degreaser contained nPB (VG). 2. The basket was lowered into the vapor zone and remained there until condensation stopped. 3. The basket was then lowered into the boil-up sump (71°C) for 3 min (Set A) or 1.5 min (Set B). 4. The parts were placed in the rinse sump for 1 min, followed by the vapor zone for 1 min. The basket was allowed to hang in the freeboard zone for an additional minute to assure that the parts were dry. © 2001 by CRC Press LLC

The two sets of cleaned parts along with the accompanying control sets were returned to Smith & Nephew for analysis. The bioburden validation was done at Axios, Inc. (Kennesaw, GA). The results were as follows: Sample Sample A Control Cleaned Sample B Control Cleaned

Spore Count

% Reduction

5.5  106 1.5  106

73%

8.8  106 2.8  106

68%

4. Aluminum Parts for Optical Applications A manufacturer of optical equipment that uses anodized aluminum components requested these tests. There were two areas of concern. First, aluminum is a very active metal. It is especially active toward halogenated materials. The manufacturer was concerned about possible interactions of the aluminum with nPB-based cleaners. The second concern involved the lettering and other markings that the parts have on them. The customer requested an evaluation to make sure that the markings would not be damaged in the normal cleaning process. It sent four sets of components, each set containing six different parts. Two sets were cleaned in nPB (VG) by immersing in the boil-up sump for 10 min followed by 1 min in the rinse sump. The other two sets were immersed for only 3 min in the boil-up sump and then 1 min in the rinse sump. All 24 parts were returned to the customer for examination. No damage to the parts was observed.

5. High-Performance Inertial Navigation Systems The Guidance & Control Systems Division of Litton Industries builds high-performance navigation systems. Systems include gyroscope instruments and associated electronics assemblies. Producing inertial navigation systems involves exacting, multistep cleaning of complex subassemblies. Each subcomponent of a system may require various cleaning steps. Fluxes, oils, and flotation fluids must be removed from an array of materials of construction including a wide assortment of metals, plastics, and epoxies. For this application, soil residues, cleaning agent residues, and water are not acceptable. When faced with the prospect of having to replace 1,1,1-TCA and CFC-113, Litton undertook an ambitious program to evaluate cosolvent systems, fluorinated solvents, and nPB-based cleaners.15 Litton listed a number of requirements for new cleaning systems. These included: • • • • • • • • •

Replacement of ozone-depleting chemicals, Process improvement, Maintenance of superior performance, Minimization of contamination, Cost-effective operation, Efficient processing, Compliance with national and local regulatory requirements, Assurance of worker safety, Production to meet exacting customer standards.

© 2001 by CRC Press LLC

A variety of systems were evaluated. Based on feedback from the assemblers in the plant, the cleaning sequences were refined and implemented. The approach initially adopted was a cosolvent system consisting of initial cleaning with a hydrocarbon blend containing various alcohols followed by 2 to 3 rinses with IPA. This new process allowed elimination of TCA cleaning. Some perfluorinated material continues to be used as a final rinse to assure thorough removal of fluorolube. Overall, the number of process steps was reduced. In some cases, 18 steps were reduced to 4 to 6 steps. There were still problems, however. Because IPA was found to react with beryllium periodically, intermittent residues were found. Eventually, the IPA was replaced with VMS. The cosolvent system developed to replace TCA was still far from optimal. The subassemblies are very complex, with close tolerances and blind holes. While the cleaning agent can be removed with careful process control, extreme and constant care is required to assure that no cleaning agent residue is left. In addition, the hydrocarbon blends were costly, some of the operators found the odor to be disagreeable, and the blends were flammable. Cleaners based on nPB had been introduced to the cleaning market by this time, and Litton undertook an extensive evaluation. One consideration was that the nPB is a very aggressive solvent that could be expected to have cleaning performance similar to TCA. In the end, VG cleaner was chosen as the most suitable for the Litton requirements. In addition to the cleaning capability, the reliability and the environmental and safety acceptability of the VG, Litton found improved processing time and lower cleaning agent usage. Litton has reported that the processing time has been reduced by over 40%. The cleaning agent usage has been reduced to one third of the previous amount. REFERENCES 1. Kanegsberg, B., Precision cleaning without ozone depleting chemicals, Chem. Ind., #20, 787, 1996. 2. Shubkin, R. L., A new and effective solvent/cleaner with low ozone depletion potential, in 1996 International Conference on Ozone Protection Technologies, Proc., Presentation, Washington, D.C., October 21 –23, 1996. 3. Unless otherwise noted, experimental procedures and results referred to in this chapter were performed at the Albemarle Technical Center, Baton Rouge, LA, or by contract laboratories under the direction of Albemarle technical personnel. 4. Shubkin, R. L. and Liimatta, E. W., A new cleaning solvent based on n-propyl bromide, NEPCON West ‘97 Conference, Anaheim, CA, February 23 –27, 1997. 5. Shubkin, R. L., Solvent cleaning into the next century and beyond, presented at CleanTech99, Rosemont, IL, May 19, 1999. 6. Shubkin, R. L. and Liimatta, E. W., n-Propyl bromide based cleaning solvent and ionic residue removal process, U.S. patent 5,792,277, to Albemarle Corp. August 11, 1998. 7. Feldmesser, H. S., Loyd, K. M., Clausen, M., and Karvar, P., Examining the compatibility of electronic assembly materials with cleaning solvents, presented at IPC/SMTA Electronics Assembly Expo, Providence, RI, October 25 –29, 1998, and at the Ninth Annual Solvent Substitution Workshop, Scottsdale, AZ, December 1–4, 1998. 8. Shubkin, R. L., Method for inhibiting tarnish formation when cleaning silver with ether stabilized, n-propyl bromide based solvent systems, U.S. patent 5,990,071, to Albemarle Corp., November 23, 1999. 9. Shubkin, R. L., Method for inhibiting tarnish formation during the cleaning of silver surfaces with ether stabilized, n-propyl bromide based solvent systems, U.S. patent allowed to Albemarle Corp. 10. Shubkin, R. L. and Smith, R. L., normal-Propyl bromide: formulation technology and product stewardship for the electronics industry, presented at NEPCON West ’99 Conference, Anaheim, CA, February 23 –25, 1999.

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11. Nelson, D. D., Jr., Wormhoudt, J. C., Zahniser, M. S., Kolb, C. E., Ko, M. K. W., and Weisenstein, D. K., OH reaction kinetics and atmospheric impact of 1-bromopropane, J. Phys. Chem. A, 101, 27, 4987 –4990. 12. Kanegsberg, B., Cleaning high value components for biomedical and other applications, in 1996 International Conference on Ozone Protection Technologies, Proc., Washington, D.C., October 21 –23, 1996. 13. Petrulio, R. and Kanegsberg, B. F., Practical solutions to cleaning and flushing problems, presented at CleanTech ’98, Rosemont, IL, May 19 –21, 1998. 14. Petrulio, R. Kanegsberg, B. F., and Chang, S.-C., A practical search solves aerospace cleaning quandary, Precision Cleaning Mag., 6(8), August, 1998. 15. Carter, M., Anderson, E., Chang, S.-C., Sanders, P. J., and Kanegsberg, B. F., Cleaning high precision inertial navigation systems, a case study and panel discussion, presented at CleanTech ’99, Rosemont, IL, May 18 –20, 1999.

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CHAPTER 1.8

Vapor Degreasing with Traditional Chlorinated Solvents Stephen P. Risotto

CONTENTS Overview Physical and Chemical Information Environment and Worker Health Considerations Environment Worker Health Regulatory Overview General NESHAP Requirements Life Cycle Assessment Objectives and Methodology Interpretation of the Results Cleaning Processes Vapor Degreasing Cold Cleaning Summary

OVERVIEW Today’s manufacturing engineers and plant managers can face a difficult challenge when choosing among the many available surface cleaning options. Aqueous, semiaqueous, flammable solvents, and new fluorinated and brominated solvents are just a few of the possibilities. Among these many choices, however, one process stands out for its ability to produce a clean, dry part at a reasonable price—vapor degreasing with the chlorinated solvents. Trichloroethylene (TCE, TRI), perchloroethylene (PCE, PERC), and methylene chloride (MC, METH), have been the standard for cleaning performance in precision parts cleaning for more than 50 years. Today, the development of new equipment and processes that minimize emissions and maximize solvent recovery makes TCE, PCE, and MC more effective than ever. © 2001 by CRC Press LLC

Despite their superior performance attributes, however, some companies have replaced TCE, PCE, or MC with other solvents or processes. Their decision often was based on misperceptions about the regulatory status, continued availability, and safety in use of the chlorinated solvents. The facts are as follows: • TCE, PCE, and MC have not been banned. Among the commonly used chlorinated solvents, only 1,1,1-trichloroethane (methyl chloroform) was phased out of production, because of its ozone-depletion potential. Meanwhile, the U.S. Environmental Protection Agency (EPA) issued a 1994 decision under its Significant New Alternatives Policy (SNAP) program that the other three chlorinated solvents are viewed as acceptable substitutes for ozone-depleting solvents. • Chlorinated solvents will continue to be available. TCE and PCE demand has remained steady or increased in recent years as a result of their use as raw materials in the production of refrigerant alternatives to chlorofluorocarbons (CFCs). MC continues to be used in a wide variety of applications. The producers of these solvents remain committed to serving their markets for many years to come. • TCE, PCE, and MC can be used safely. From the point of view of health and the environment, the chlorinated solvents are among the most thoroughly studied industrial chemicals. Animal tests and epidemiological studies indicate that when the solvents are handled, used, and disposed of in accordance with recommended and mandated practices, they do not cause adverse health or environmental effects. • The potential impacts of the solvents can be minimized. Environmental, health, and safety regulations governing the chlorinated solvents are strict, but manageable. In complying with these regulations companies can get help from several sources—EPA, the Occupational Safety and Health Administration (OSHA), state and local agencies, producers and distributors of solvents and degreasing equipment, and organizations like the Halogenated Solvents Industry Alliance (HSIA). PHYSICAL AND CHEMICAL INFORMATION TCE, PCE, and MC are clear, heavy liquids with excellent solvency. All are virtually nonflammable, since they have no flash point as determined by standard test methods. Each has its own advantages for specific applications, based on its physical profile (see Table 1). These solvents work well on the oils, greases, waxes, tars, lubricants, and coolants generally found in the metal-processing industries. They are widely used in the vapor degreasing process. TCE has been long recognized for its cleaning power. TCE is a heavy substance (12.11 lb/gal) with a high vapor density (4.53 times that of air) that allows for relatively easy recovery from vapor degreasing systems. The ability of the solvent to provide constant pH and to protect against sludge formation has helped make it the standard by which other degreasing solvents are compared. Its high solvency dissolves soils faster, providing high output. TCE is used extensively for degreasing zinc, brass, bronze, and steel parts during fabrication and assembly. It is especially suited for degreasing aluminum without staining or pitting the work, because its stabilizer system protects the solvent against decomposition. For cleaning sheet and strip steel prior to galvanizing, TCE degreases more thoroughly and several times faster than alkaline cleaning, and it requires smaller equipment that consumes less energy. PCE has the highest boiling point, weight (13.47 lb/gal), and vapor density (5.76 times that of air) of the chlorinated solvents. The high boiling point of PCE gives it a clear © 2001 by CRC Press LLC

Table 1 Typical Properties of the Chlorinated Solvents Methylene Chloride Chemical formula Molecular weight Boiling point °F (°C) at 760 mmHg Freezing point °F (°C) Specific gravity at 68°F (g/cm3) Pounds per gallon at 77°F Vapor density (air  1.00) Vapor pressure at 77°F (mmHg) Evaporation rate at 77°F Ether  100 n-Butyl acetate  1 Specific heat at 68°F (BTU/lb/°F or cal/g/°C) Heat of vaporization (cal/g) at boiling point Viscosity (*cps) at 77°F Solubility (g/100 g) Water in solvent Solvent in water Surface tension at 68°F Kauri-butanol (KB) value Flash point Tag open cup Tag closed cup Flammable limits (% solvent in air) Lower limit Upper limit

Perchloroethylene

CH2Cl2 84.9

C2Cl4 165.8

104 (40) 139 (95)

250 (121) 9 (23)

Trichloroethylene C2HCl2 131.4 189 (87) 124 (87)

1.33 10.99 2.93

1.62 13.47 5.76

1.46 12.11 4.53

436

18.2

74.3

71 14.5

12 2.1

30 4.5

0.28

0.205

0.225

78.9 0.41

50.1 0.75

56.4 0.54

0.17 1.70 28.2 136

1.01 0.015 32.3 90

0.04 0.10 29.5 129

None None

None None

None None

13 23

None None

8 9.2

advantage in removing waxes and resins that must be melted to be solubilized. The higher temperature also means that more vapors will be condensed on the work than with other solvents, thus washing the work with a larger volume of solvent. PCE is effective in cleaning lightweight and light-gauge parts that would reach the operating temperature of lower-boiling solvents before cleaning is complete. When cleaning parts with fine orifices or spot-welded seams—especially if there is entrapped moisture—the high boiling point of PCE is essential for obtaining good penetration. Inherently more stable than other chlorinated (and brominated) solvents, PCE also incorporates a multicomponent stabilizer system that provides the greatest resistance to solvent decomposition available in the industry. While it can be used to degrease all common metals, PCE is especially applicable to cleaning those that stain or corrode easily, including aluminum, magnesium, zinc, brass, and their alloys. MC has the lowest boiling point of the chlorinated solvents, as well as the lightest vapor density (2.93 times that of air) and weight (10.98 lb/gal). MC is uniquely suited for © 2001 by CRC Press LLC

use as a vapor degreasing solvent in applications where low vapor temperatures and superior solvency are desirable. The low boiling point of MC makes it a popular choice for cleaning temperature-sensitive parts such as thermal switches or thermometers. Vapor degreasing with MC allows more rapid processing and handling, particularly when cleaning large, heavy parts. The more aggressive nature of MC is especially useful when degreasing parts soiled with resins, paints, or other contaminants that are difficult to remove. ENVIRONMENT AND WORKER HEALTH CONSIDERATIONS The potential health effects of TCE, PCE, and MC have been very well studied. Each can cause acute health effects at elevated exposure levels, but these effects have been found to be reversible. The primary concern with these solvents has been their potential to cause cancer, based on the results of laboratory animal tests showing an increase in certain tumors following lifetime exposure to the solvents. Scientific questions have been raised regarding the relevance of these animal tumors to human health, however, and epidemiology studies of workers exposed to the chlorinated solvents over extended periods of time have failed to produce a consistent pattern of increased cancer incidence. The animal data, nevertheless, have traditionally been viewed by regulatory agencies as indicating a potential for risk in humans. Environment Chlorinated solvents have been found as contaminants in soil and groundwater as a result of past handling and disposal practices. Releases of the chlorinated solvents to land and water are minor, however, in comparison with atmospheric emissions. The residence times of TCE, PCE, and MC in the atmosphere are very short and, despite many years of use, concentrations in the ambient air are very low. TCE and PCE are oxidized to carbon dioxide, water, and hydrogen chloride in the lower atmosphere by reaction with either oxygen (ozone) or hydroxyl (OH) radical. Methylene chloride is oxidized by OH only, forming the same naturally occurring organic breakdown products. Because of their relatively short lifetimes in the atmosphere, TCE, PCE, and MC are not considered to contribute to depletion of the stratospheric ozone layer. Similarly, the chlorinated solvents have negligible global warming potential. TCE is photochemical reactive and is believed to contribute to the formation of ozone (smog) in the lower atmosphere under certain conditions. The decomposition of PCE and MC contributes only negligibly to the formation of ozone. In reviewing the acceptability of TCE, PCE, and MC in its SNAP review, the EPA noted that these compounds are regulated under several other environmental laws and regulations, including the occupational limits and national emission standards described elsewhere in this chapter. The agency concluded that compliance with these regulations will significantly reduce the potential for environmental releases and worker exposure from degreasing operations. As a result, the SNAP program did not impose further use restrictions on the three solvents in degreasing. Worker Health As with all industrial chemicals, occupational exposure to the chlorinated solvents should be kept as low as practical. OSHA has set permissible exposure limits (PELs) for © 2001 by CRC Press LLC

chlorinated solvents. The PEL for PCE and TCE is 100 parts per million (ppm) for an 8-h time weighted average (TWA). The limits for MC are 25 ppm for an 8-h TWA and 125 ppm for a 15-min short-term exposure limit, or STEL. In addition to the TWA and STEL, the OSHA standard for MC imposes several additional requirements. The American Conference of Governmental Industrial Hygienists (ACGIH) also recommends exposure limits for the chlorinated solvents. The solvent producers recommend maintaining workplace exposure levels within the OSHA limits or the ACGIH levels, whichever is lower (see Table 2). The OSHA Hazard Communication (HAZCOM) standard specifies a minimum element of training for people working with hazardous materials, including the chlorinated solvents. This includes how to detect the presence or release of a solvent, the hazards of the solvent, and what protective measures should be used when handling it. The OSHA HAZCOM standard also requires labeling of all hazardous chemicals and preparation of a Material Safety Data Sheet (MSDS). Labels must contain a hazard warning, the identity of the chemical, and the name and address of the responsible party.

REGULATORY OVERVIEW U.S. federal regulations affecting the use, handling, transportation, and disposal of chlorinated solvents can be found under the Clean Air Act, the Clean Water Act, the Resource Conservation and Recovery Act (RCRA), and the Comprehensive Environmental Response Compensation and Liability Act (CERCLA, or Superfund). State and local regulations also exist for the purpose of controlling emissions. Although numerous, these regulations are manageable and companies can obtain compliance assistance from numerous sources. The major federal regulations pertaining to the chlorinated solvents are summarized below.

Table 2 Workplace Limits for the Chlorinated Solvents (ppm) Methylene Chloride

Perchloroethylene

Trichloroethylene

OSHA Permissible Exposure Limitsa 8-h TWA 15-min STEL Ceiling Peak

25 125 — —

100 — 200 300

100 — 200 300

ACGIH Threshold Limit Valuesb 8-h TWA 15-min STEL

50 —

25 100

50 100

a An 8-h TWA is an employee’s permissible average exposure in any 8-h work shift of a 40-h week. The short-term exposure limit (STEL) is a 15-min TWA exposure that should not be exceeded at any time during the day. The acceptable ceiling concentration is the maximum concentration to which a worker may be exposed during a shift, except that brief excursions to the Acceptable Maximum Peak are permissible. b Threshold Limit Values (TLVs) are established by the American Conference of Governmental Industrial Hygienists (ACGIH).

© 2001 by CRC Press LLC

General Volatile organic compound (VOC) regulations under the Clean Air Act apply to TCE and limit its emissions to reduce smog formation, particularly in ozone nonattainment areas. Exact requirements vary by state, but generally include obtaining a permit allowing a specific amount of VOC emissions from all sources within a facility. PCE and MC, however, are exempt from VOC regulations in most states. The Clean Air Act also calls for the three chlorinated solvents to be regulated as hazardous air pollutants (HAPs). The EPA has issued National Emission Standards for Hazardous Air Pollutants (NESHAP) for solvent cleaning with halogenated solvents, which are discussed below. Other NESHAPs govern dry cleaning with PCE and the use of MC in aerospace manufacture and rework, wood furniture manufacture, and polyurethane foam manufacture. The Clean Water Act defines chlorinated solvents as toxic pollutants and regulates their discharge into waterways. Under RCRA, wastes containing chlorinated solvents from solvent cleaning operations are considered hazardous. Generators, transporters, and disposers of such hazardous waste must obtain an EPA identification number and must comply with “cradle-to-grave” management requirements for these wastes. The Superfund law requires that if a reportable quantity of a chlorinated solvent or other hazardous chemical is released into the environment in any 24-h period, the federal, state, and local authorities must be notified immediately. Reportable quantities are 1000 lb for MC and 100 lb for PCE and TCE. NESHAP Requirements The EPA NESHAPs for new and existing halogenated solvent cleaning operations govern emission standards for chlorinated solvent degreasing operations. These standards cover both vapor degreasing and cold cleaning with TCE, PCE, and MC. In developing the standards, the EPA focused on equipment and work practice requirements that permit a level of control between 50 and 70%. Companies operating batch or in-line degreasers are given three options for compliance: (1) Installing one of several combinations of emission control equipment and implementing automated parts handling and specified work practices; (2) meeting an idling-mode emission limit, in conjunction with parts handling and work practice requirements; or (3) meeting a limit on total emissions. The multiple compliance options in the NESHAP recognize the vast number of different industries and operating schedules associated with the use of halogenated solvent cleaners. The EPA standard allows companies considerable flexibility in complying with the control requirements. The alternative idling and total emissions limits allow the use of new and innovative technologies to achieve a level of control equivalent to the available equipment combinations. When a company chooses the first option, it may choose from a series of combinations of two or three procedures, which include freeboard ratio of 1.0, freeboard refrigeration device, reduced room draft, working-mode cover, dwell, and superheated vapor. In addition to these options, solvent cleaning processes must include an automated hoist or conveyor that carries parts at a controlled speed of 11 ft/min or less through the complete cleaning cycle. Compliance with one of the control options for batch or in-line vapor equipment is demonstrated by periodic monitoring of each of the control systems chosen. Work practices are also required as part of the new EPA standards. Rather than require direct monitoring of

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work practice compliance, however, EPA has developed a qualification test, included as an appendix to the standard. The test is to be completed by the operator during inspection, if requested. A company choosing to comply with the second option, the idling-emission limit (0.045 lb/ft 2/h for batch vapor equipment, 0.021 lb/ft 2/h for in-line equipment), is required to demonstrate initial compliance by using the EPA idling reference test method 307. Data from the equipment manufacturer may be used, provided the unit tested is the same as the one for which the report has been submitted. Compliance with the idlingemission limit also requires installation of an automated parts-handling system and compliance with work-practice requirements. In addition, the company must show that the frequency and types of parameters monitored on the solvent cleaning machine are sufficient to demonstrate continued compliance with the idling standard. Complying with the third option, the limit on total emissions, requires the company to maintain monthly records of solvent addition and removal. Using mass-balance calculations, the company determines the total emissions from the cleaning machine, based on a 3-month rolling average, to ensure they are equal to or less than the established limit for the cleaner (30.7 lb/ft 2/month for small-batch vapor machines, 31.4 lb/ft 2/month for large batch vapor machines, 20.37 lb/ft 2/month for in-line machines). For new machine designs without a solvent/air interface, the EPA has established an emission limit based on cleaning capacity ( 330  (vol)0.6). Companies meeting the total emission limit requirements do not need to conduct monitoring of equipment parameters, but must maintain records of their solvent usage and removal of waste solvent. According to the EPA, this compliance option provides an incentive for innovative emission control strategies to limit solvent use. For some cleaning machines, the EPA calculates that the alternative total emission limit could be more stringent than the equipment specifications. In particular, the EPA expects that this alternative standard will be more difficult to meet for larger machines, for machines operating more than one shift, and for machines cleaning parts with difficult configurations.

LIFE CYCLE ASSESSMENT Substitution of chlorinated solvents for cleaning metal parts is frequently proposed by regulators and others. Alternatives, such as aqueous cleaning with detergents, are often perceived as having less environmental impact than cleaning with chlorinated solvents used in a vapor cleaning process. Objectives and Methodology A life cycle assessment (LCA) was conducted in 1996 by the European Chlorinated Solvents Association (ECSA) to provide robust data relating to the environment impact of metal parts cleaning in TCE vapor degreasing and aqueous processes. Five major environmental indicators were considered: nonrenewable resource depletion, greenhouse effect, air acidification, eutrophication (water impacts), and solid waste. For a viable comparison, the cleaning processes were studied assuming the same performance (the “functional unit”) and a clear definition of what is and is not included in the system (the “system boundary”). As requested by standard LCA methodologies, the study also incorporated the environmental impacts caused by the manufacturing of the cleaning agents, upstream of their use in cleaning installations.

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The functional unit adopted in this study was the complete removal of 46.8 g (1.6 oz) of grease from 1 m2 (10.7 ft 2) of metal parts. Thus, a single level of contamination is assumed in all the cases studied, as well as the removal of the whole quantity of grease by the cleaning step. To allow for viable comparisons, the same boundaries were used for all the cleaning processes assessed, as required by the standard methodology for LCAs. In particular, both for solvent cleaning and for aqueous cleaning, the following were included in the life cycle assessment. • The environment impacts incurred by the manufacturing of the cleaning agents • The environmental impacts of the cleaning steps themselves • The environmental burdens associated with the treatment of the cleaning residues or effluents Seven cleaning scenarios were chosen, three for solvent cleaning and four for aqueous cleaning. Data on the environmental impacts of metal parts cleaning were collected at five cleaning sites in Europe. The sites were selected to be representative of various technologies across different countries.

Interpretation of the Results Each cleaning technology was found to have potentially significant environmental impact (see Figures 1 through 6) The primary disadvantage of TCE, air pollution (i.e., air acidification), can be minimized with emission controls. The water pollution disadvantages of aqueous cleaning, however, remain significant even after significant physiochemical  biological treatment of the cleaning residues. With aqueous cleaning, impact on water was between 200 and 2000 times higher than with TCE degreasing, depending on the site under consideration. For cleaning and drying metal parts, solvent technology has a lower overall environmental impact than aqueous technology. This is true even without the use of carbon recovery of solvents in the vapor phase, provided that equipment meets the NESHAP requirements and is operated to best practice. However, air acidification can be higher as a result of solvent releases. In the ECSA investigation, the air pollution impact of solvent cleaning varied from 8.5 times greater to 5 times less than that for aqueous cleaning with a drying step. Generally, a given solvent machine can treat a wider range of metal parts than a given detergent installation. This occurs because a detergent is often specific to a kind of contamination and a shape of metal part. A solvent technology without carbon recovery is competitive with an aqueous technology from an environmental viewpoint, provided that solvent emissions are limited through other means.

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Figure 1

Nonrenewable resource depletion (kg/year) results of LCA (per square meter of metal part cleaned) Scenarios: VDG1—open-top degreaser without NESHAP-compliant controls; VDG2—NESHAP-compliant degreaser with on-site distillation; VDG3—NESHAP-compliant degreaser with on-site distillation and carbon adsorption; AQ1—aqueous cleaning equipment with primary wastewater treatment; AQ2—aqueous cleaning equipment with primary and secondary wastewater treatment and drying. (Two additional aqueous scenarios are not included in the graphs.) (From CA Comparison of Metallic Parts Degreasing with Trichloroethylene and Aqueous Solutions, prepared for the European Chlorinated Solvents Association by Ecobilan, December 1996).

Figure 2

Total energy use (MJ) results of LCA (per square meter of metal part cleaned).

© 2001 by CRC Press LLC

Figure 3

Greenhouse effect (gram equivalent CO2) results of LCA (per square meter of metal part cleaned).

Figure 4

Solid waste (kg) results of LCA (per square meter of metal part cleaned).

© 2001 by CRC Press LLC

Figure 5

Air pollution (gram equivalents H) results of LCA (per square meter of metal part cleaned).

Figure 6

Water pollution (gram equivalents PO43  ) results of LCA (per square meter of metal part cleaned).

© 2001 by CRC Press LLC

CLEANING PROCESSES The chlorinated solvents have been used traditionally in both vapor degreasing and cold cleaning applications. Recent advances in vapor degreasing help to ensure that worker and environmental regulations and concerns can be effectively addressed. Vapor Degreasing The vapor degreasing process is the ideal technology for high-quality cleaning of parts. It is able to remove the most stubborn soils. It reaches into small crevices in parts with convoluted shapes. Parts degreased in chlorinated solvent vapors come out of the process dry, with no need for an additional drying stage. Vapor degreasing is particularly effective with parts that contain recesses, blind holes, perforations, crevices, and welded seams. Chlorinated solvent vapors readily penetrate complicated assemblies as well. Solid particles such as buffing compounds, metal dust, chips, or inorganic salts contained in the soils are effectively removed by the washing action of the solvent vapor. Vapor degreasing can be carried out in either a batch or an in-line degreaser. The traditional batch degreaser is a covered tank, with cooling coils at the top, into which the dirty parts are lowered. Solvent in the bottom of the tank is heated to produce vapor. On contacting the cooler work, the vapor condenses into pure liquid solvent. The condensation of solvent dissolves the grease and carries off the soil as it drains from the parts into the solvent reservoir below. This process continues until the parts reach the temperature of the vapor, at which point condensation ceases and the parts are lifted out of the vapor, clean and dry (Table 3). Table 3 Operating Parameters for the Chlorinated Solvents

Vapor thermostat setting, °F (°C) Boil Sump Thermostat Setting,a °F (°C) Steam pressure (psi) Solvent condensate temperature,b °F (°C) Cooling coil outlet temperature range (°F)

Methylene Chloride

Perchloroethylene

Trichloroethylene

95 (35) 110 (43) 1–3 100 (38) 75 –85

180 (82) 260 (127) 40 –60 190 (88) 100 –120

160 (71) 195 (91) 5–15 155 (68) 100–120

a

Maximum boiling temperature, based on 25% contamination with oil.

b

To facilitate effective separation of the solvent from the water.

Many degreasers contain one or several immersion tanks below the vapor zone, so that parts can be lowered into liquid solvent—often in a tumbling basket—before being raised into the vapor for final rinsing. Ultrasonic cleaning can be added to remove heavy oil deposits and solid soils by installing transducers in the degreaser. When ultrasonic energy is transmitted to a solution, it imparts a scrubbing action to the surface of soiled parts through cavitation—the rapid buildup and collapse of thousands of tiny bubbles. Several types of conveyorized equipment provide in-line vapor degreasing. These large, automatic units, which can handle a volume of work and are enclosed to provide minimal solvent loss, include the monorail and the cross-rod degreasers. They are particularly valuable when production rates are high.

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Although conveyorized degreasers are enclosed, there is still some solvent loss through the openings where work enters and leaves the equipment. Consequently, some companies have found it cost-effective to install one of the advanced types of degreasers that have no air/vapor interface (see Chapter 2.11 by Gray and Durkee). These sealed units were first introduced in Europe, but have become available in the United States in recent years. Typically these degreasers perform the cleaning operation in a sealed chamber into which solvent is introduced after the chamber is closed. Solvent vapor then performs the final drying stage, and all vapors are exhausted after each cycle and passed into a solvent recovery system. With the sealed chamber, control of solvent loss exceeds 90%. Operation is programmed and automated, permitting a variety of cleaning programs, including hot solvent spray. Although these sealed units can be costly and may not be effective for some cleaning jobs, a few U.S. plants have installed them to ensure compliance with safety and environmental regulations. Cold Cleaning The manufacturers of the TCE, PCE, and MC generally do not recommend the use of methylene chloride, trichloroethylene, and perchloroethylene in hand wipe and other cold (room-temperature) cleaning applications. In circumstances where workplace exposure, NESHAP, and other requirements can be met, these solvents may provide a viable option for companies searching for an effective cold cleaning solvent.

SUMMARY Among the surface cleaning options available, one process stands out for its ability to produce a clean, dry part at a reasonable price—vapor degreasing with the chlorinated solvents. TCE, PCE, and MC are clear, heavy liquids with excellent solvency. All are virtually nonflammable, and can effectively remove the oils, greases, waxes, tars, lubricants, and coolants generally found in the metal processing industries. The potential health effects of TCE, PCE, and MC have been very well studied. Environmental, health, and safety regulations governing these chlorinated solvents are strict, but manageable. While cleaning with these solvents is often perceived as having greater environmental impacts, life cycle assessment suggests that TCE, PCE, and MC may have a lower overall environmental impact in many situations. The chlorinated solvents have been used traditionally in both vapor degreasing and cold cleaning applications. Recent advances in vapor degreasing help to ensure that worker and environmental regulations and concerns can be effectively addressed.

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CHAPTER 1.9

Volatile Methylsiloxanes: Unexpected New Solvent Technology Ray A. Cull and Stephen P. Swanson

CONTENTS Overview Chemistry and Properties Environmental/Regulatory Profile Health and Safety Testing Performance Assessment Compatibility Silicone Conformal Coating Removal Process Considerations Summary References

OVERVIEW A new class of fluid chemistry has been introduced to the precision and industrial cleaning markets, based on linear and cyclic volatile methylsiloxane (VMS). The use of a silicone-based cleaner may seem “counterintuitive” to some people, since the low surface tension of silicone contamination has historically made it very difficult to remove. However, in a cleaning solvent the low surface tension becomes a definite asset, since it helps wet out and undercut soils. Dragout is also reduced, because of the low viscosity of the liquid. One of the keys to success with what appears at first to be an unlikely technology is the ability to manufacture VMS that dries with ultralow nonvolatile residue (NVR) so that it will evaporate completely, leaving behind a clean surface. While VMS materials are new as cleaning solvents, they have been commercially available since the 1950s, primarily used as the building blocks for higher-molecular-weight, nonvolatile silicone fluids and polymers. In addition, VMS fluids are widely used in the personal care industry, including many antiperspirant, hair care, and skin care products. The majority of the personal care applications employ VMS materials with a cyclic structure, designated “cyclomethicones” by the Cosmetic Toiletry and Fragrance Association. In © 2001 by CRC Press LLC

Figure 1

Chemical structure of linear VMS.

contrast, industrial and precision cleaning fluids are made primarily with linear VMS fluids, which have a faster rate of evaporation and higher recommended exposure levels, but higher cost than the cyclics. As shown in Figures 1 and 2, both the linear and cyclic materials are relatively simple, and contain little opportunity for reaction or breakdown into harmful compounds. Common traits for nearly all “neat” VMS materials include low toxicity and practically no odor. They are compatible with a variety of surfaces, and can be used on metals, glass, polycarbonate, acrylic, and other plastics. With their Kauri-butanol (KB) value, pure VMS fluids are not very aggressive cleaners, however, making them primarily effective on nonpolar contaminants like silicones, oils, and light greases. VMS fluids can be blended with other, more polar solvents to increase cleaning performance. [Editor’s note: Cyclic VMS fluids are being introduced in cold cleaning applications (e.g., automotive) where VOCs are an issue. – B.K. (See Chapter 1.1 by Kanegsberg.)] Patented azeotropes have been developed that improve cleaning effectiveness on more difficult soils such as rosin solder flux. The primary limitation of the linear VMS products is their flash points. The materials are available in a range of drying rates, and each has a corresponding flash point. The drying rates of the di-, tri-, and tetrasiloxane materials are comparable to acetone, isopropyl

Figure 2

Cyclic VMS structure.

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alcohol (IPA), and mineral spirits, respectively. The cyclics typically dry more slowly than mineral spirits, but have a higher flash point. Regardless of molecular weight, all VMS fluids have the advantage of rapid, natural degradation in the environment. Their atmospheric life is just 10 to 30 days,1,2 and they do not contribute to smog. The U.S. EPA has declared VMS materials as a class to be exempt from the federal VOC regulations, and to date they have also been granted VOC-exempt status in 48 states (including California). Several commercial versions of linear VMS have also earned Clean Air Solvent certification by the California South Coast Air Quality Management District. Based upon their low-odor, good-toxicity profile, VOC-exempt status, and flash point limitations, most of the applications for VMS cleaning technology have been in benchtop work and other hand cleaning operations. Most VMS fluids can also be used in flammablerated cleaning equipment, with some minor changes to the fire detection system. In general, any operations involving the removal of greases and oils or the cleaning of silicones would be a good fit for VMS fluids, because of their inherent ability to dissolve many silicone oils and residues, as well as to soften cured sealants and coatings. CHEMISTRY AND PROPERTIES VMS materials are siloxanes with relatively low molecular weight (600) and high vapor pressure. When distilled and handled to ensure ultrahigh purity, these liquids remove light oil, silicone residue, and other nonpolar contaminants. For more aggressive cleaning, a recently patented VMS azeotrope helps remove polar soils, such as rosin flux and liquid crystal residue. Overall, VMS chemistry has demonstrated broad compatibility with metals, plastics, elastomers, glass, and many other materials. As an example, a 1995 Battelle Memorial Institute study found that VMS fluids were compatible with gyroscope materials, including adhesives and sensitive metal components.3 Individual VMS compounds have different evaporation rates and flammability. (See Table 1 for typical properties.) Drying of disiloxane resembles acetone, for example, and it has a 3°C (24°F) flash point vs. the more flammable acetone flash point of 18°C (1°F). Trisiloxane dries similarly to IPA, with a flash point of 34°C (94°F), while the tetrasiloxane has a 57°C (135°F) flash point and dries like mineral spirits. Because the commercial azeotrope is based on disiloxane, their flash points are about the same. The di- and trisiloxane materials are considered flammable, and tetrasiloxane is classified as combustible. The key requirement when using a VMS-based material for precision cleaning operations is low NVR, usually less than 5 ppm in a typical lot. Although VMS fluids have been used as commodity industrial products for decades, it has been only recently that commercial grades have become available that are tested and certified at the low NVR levels required for precision cleaning operations. This is especially critical in applications where secondary bonding or coating is required, since even 50 ppm residue may cause problems in adhesive and coating operations. ENVIRONMENTAL/REGULATORY PROFILE VMS compounds offer a very favorable environmental profile. They were declared exempt from VOC regulation at the federal level in 1994,4 and so far 48 states (including California) have followed suit. These SNAP-approved materials are not considered hazardous air pollutants (HAPs) or ozone-depleting compounds (ODCs), and are not regulated under NESHAP or Clean Air Act National Ambient Air Quality standards. © 2001 by CRC Press LLC

Table 1 Typical Properties of VMS Cyclotetrasiloxane (244 fluid) Flash point, closed cup (°C) Freezing point (°C) Boiling point (°C) Evaporation rate (ASTM D 1901) Viscosity, cSt at 25°C Specific gravity at 25°C Surface tension, dyn/cm at 25°C Heat of vaporization, cal/gm at 25°C KB value VOC content, weight %

Cyclopentasiloxane (245 fluid)

Disiloxane OS-10

Trisiloxane OS-20

Tetrasiloxane OS-30

Azeotrope OS-120

55

76

3

34

57

4

172 0.2

205

68 100 3.8

82 152 0.7

68 194 0.15

68 98 3.5

2.5

4.2

0.65

1.0

1.5

0.65

0.953

0.956

0.76

0.82

0.85

0.77

17.8

18.0

15.2

16.5

17.3

16.3

42

39

46a

44a

36a

61

0

16.6 0

15.1 0

13.4 0

18.5 11

14.5 0

a

Estimated.

Further, VMS fluids are not controlled under U.S. EPA Title III, Air Toxics, nor are they classified as EPA Criteria Pollutants. In 1996, the California South Coast Air Quality Management District (responsible for air quality in Los Angeles) ranked linear VMS as the number 1 preferred cleaner for substrate and precision cleaning, an option second only to no-clean technology for electronic applications.5 In 1998, several commercial versions of VMS technology (Dow Corning® OS Fluids) were awarded Clean Air Solvent Certificates by the California South Coast Air Quality Management District. Because of the volatility of VMS fluids, the most likely route for environmental exposure is through evaporation. VMS compounds degrade quickly via natural oxidation in the presence of ultraviolet light; they have an atmospheric life span of less than 30 days. The ultimate degradation products are water, dissolved silica, and carbon dioxide, all of which are benign compounds that occur abundantly in nature. Because VMS fluids are effectively oxidized long before reaching the stratosphere, they are believed to have no impact on the Earth’s protective ozone layer and negligible contribution to global warming (see Figure 3). VMS emissions from industrial sources are very limited, because the compounds are manufactured and used as intermediates in enclosed systems. Although VMS fluid volatilizes into the atmosphere from consumer applications, the amount released per use is extremely small, and environmental emissions are therefore very diffuse. Small amounts of VMS can also end up in wastewater treatment plants (WWTPs), primarily incidental quantities from industrial wastewater. If these essentially insoluble compounds should end up in surface waters with treated effluent, they have been shown to rise to the surface as a result of their low specific gravity (less than 0.9) and evaporate to the atmosphere. With its short lifespan, the atmospheric content of VMS is expected to remain far below the No Observable Effect Level (NOEL). Because they contain no chlorine, fluorine, or bromine atoms and degrade quickly in the troposphere,1,2 VMS materials are considered to have no potential to deplete (or even © 2001 by CRC Press LLC

Figure 3

OS fluids life cycle.

reach) the stratospheric ozone layer. They do not contribute significantly to global warming in comparison with organic compounds, which have much higher carbon levels that are converted to CO2. Recent studies demonstrate that VMS compounds have negligible potential to impact urban air quality adversely. Specifically, these studies show that VMS materials do not contribute to the formation of ozone in the urban atmosphere.6 Similar conclusions were drawn when atmospheric reactivity data were used to calculate the Photochemical Ozone Creation Potential (POCP) of VMS materials using the Harwell Photochemical Trajectory Model.7 VMS compounds are not persistent in the atmosphere. No significant contribution has been observed from VMS materials to ground-level ozone generation6 or aerosol formation.8 When partial oxidates of VMS are deposited or scrubbed out of the atmosphere, they are not expected to present any threat to aquatic or terrestrial biota because of their decreased lipophilicity. Used VMS fluids from precision cleaning applications can typically be purified to extend service life and increase cost-effectiveness. Techniques include distillation, filtration, and desiccant water removal. At least one supplier has established a no-charge fluid return program for customers, under which returned material is fuel blended for its BTU value and silica content in making Portland cement. The technique produces a necessary ingredient in cement manufacturing, captures energy from the used VMS fluid, and releases no hazardous by-products. The cyclic tetramer [(CH3)2SiO]4 (octamethylcyclo-tetrasiloxane, or OMCTS) has been extensively tested for ecotoxicological properties as part of an industry testing program carried out under the U.S. EPA Toxic Substances Control Act.9 A recent regulatory-driven effort evaluated the possible transport of OMCTS from consumer products to aquatic ecosystems and the potential effects on aquatic organisms. The testing defined the NOEL for this material, and the program (under a consent order negotiated with the EPA) © 2001 by CRC Press LLC

included physicochemical and environmental fate studies, as well as extensive aquatic toxicity testing. Additional voluntary work sponsored by the industry through the Silicones Environmental, Health, and Safety Council (SEHSC) included broad-scale environmental fate modeling, WWTP monitoring, and the development of a comprehensive aquatic risk assessment. Although the studies indicated that toxic aquatic effects can be observed in laboratory tests,10 a subsequent exposure assessment and exposure monitoring at WWTPs11 concluded that levels in the aquatic environment can be conservatively estimated at 64 to 444 times below the NOEL.12 For benthic organisms, 157- to 1080-fold margins of safety13 have been estimated. Rapid volatilization and additional dilution in most aquatic environments will increase this ratio even further. Because of their physical properties (low water solubility, low density, rapid volatilization) and short lifetimes, OMCTS and other VMS compounds are not expected to reach ecologically significant levels in the aquatic environment. Concentrations are expected to be low and transient in water and sediments. No adverse effects are expected in the aquatic environment from the use of VMS in consumer products. HEALTH AND SAFETY TESTING Linear VMS fluids have favorable toxicological characteristics as demonstrated in oral, inhalation, and dermal exposure testing. In acute studies, no adverse effects have been observed, even at the maximum achievable doses in laboratory animals. However, the materials may result in mild, transient discomfort upon direct eye contact.14 Subchronic oral and inhalation studies have revealed no toxicological responses significant to human health. Tissue culture biocompatibility studies have assessed whether these fluids can damage or destroy cells, and no adverse effects have been observed from any of the materials tested. The linear VMS liquids tested have been found nongenotoxic in short-term evaluations for DNA damage or mutation.14 Threshold limit values (TLVs) have not been established for VMS fluids by the American Conference of Governmental Industrial Hygienists (ACGIH). However, based on a 90-day study, an Industrial Hygiene Guideline (IHG) has been set by Dow Corning at 200 ppm for linear VMS, defined as the maximum average worker exposure level over an 8-h workday. In studies that included worker exposure monitoring, hexamethyldisiloxane (the major ingredient of the azeotrope formulation) has been measured at two typical electronics assembly facilities (see Table 2). The results of these studies demonstrate the long-term safety of manual cleaning from pressure-dispensed containers. Since the disiloxane measured is the most volatile of the linear VMS fluids, this represents a worst-case scenario. Other linear VMS fluids would have even lower exposure levels as a result of lower volatility. For the 14 different operator stations tested, vapor exposures remained at less than 10% of the 200 ppm guideline at all times. Unlike more traditional cleaning solvents such as alcohol, acetone, and terpenes, VMS fluids have little or no odor, a characteristic especially welcomed in benchtop operations. PERFORMANCE ASSESSMENT A variety of surface analysis techniques have been used to evaluate the cleaning performance and residue levels of commercially available, ultrahigh-purity VMS fluids. Techniques have included electron spectroscopy chemical analysis (ESCA) and contact © 2001 by CRC Press LLC

Table 2 Operator Exposure to Hexamethyldisiloxane (measured as an 8-h, time-weighted average) Operator, Facility A

Hexamethyldisiloxane, ppm

1 2 3 4 5 6 7 8

9.3 0.4 1.7 7.5 2.2 10.0 1.2 3.1

Operator, Facility B 1 2 3 4 5 6 — —

Hexamethyldisiloxane, ppm 7.5 1.6 2.5 1.2 16.0 0.7 — —

Note: 200 ppm is maximum recommended level.

angle testing. Standard coatings tests (including appearance and crosshatch adhesion tests such as ASTM D 3359 78B) have been conducted on steel and aluminum panels. Test coatings have been observed to adhere well to the ultrahigh-purity VMS-cleaned panels. In another evaluation, clean metal panels were dipped in commercial linear ultrahighpurity VMS fluids and allowed to air-dry. Lap shear specimens were then made from the panels using an epoxy, a urethane, and a quick-set adhesive. As shown in Table 3, panels exposed to the ultrahigh-purity VMS had as good or better adhesion than the clean control panels, which were wiped with IPA. Independent testing of eight cleaning fluids on threaded stainless steel fasteners further demonstrated the cleaning ability of VMS. By measuring particle removal in ultrasonic cleaning equipment, researchers found that the high-purity, linear VMS fluids were more effective than CFC-113 or IPA, and were outperformed only by a cleaning agent/surfactant mixture.15 High-purity VMS materials offer desirable evaporation rate, typically with less than 5 ppm NVR. Their low surface tension allows penetration into small spaces, and promotes cleaning, rinsing and drying. With their ability to undercut and displace soils, VMS fluids are most effective on nonpolar contaminants, including greases, oils, dirt, and uncured silicone residue (see Table 4).16 Even though VMS fluids have a low KB value, they are miscible in most solvents. They can be used in conjunction with more polar fluids, providing a solution with greater versatility than either type alone. Often the combination results in a formulation that cleans a broad range of contaminants, yet reduces the amount of the more aggressive solvents used.

Table 3 Lap Shear Adhesion Testing with Ultrahigh-Purity VMS Fluids (in psi) Sample Control Disiloxane (OS-10) Trisiloxane (OS-20) Tetrasiloxane (OS-30)

Epoxy 1213 1233 20 1440 50 1478 100

Urethane 344 494 20 383 395 

Quick-Set Adhesive 475 50 581 640 507

Note: Lap shears were based on soaking the substrate panels in VMS fluids, then allowing them to air-dry. Control samples were cleaned with IPA and wiped.

© 2001 by CRC Press LLC

Table 4 Cleaning Performance of OS Fluids (% of soil removed from steel substrate)

Motor oil Quenching oil Cutting oil Gyroscope fill fluid (Newark AFB) 200® Fluid 350 cst 200® Fluid 100,000 cst Grease HD #2 Dow Corning 550 Fluid Dow Corning 710 Fluid Uncured silicone conformal coating

Disiloxane

Trisiloxane

Tetrasiloxane

99 100 100 100 100 94 84 99 97 100

100 100 100 100 100 88 75 99 96 99

99 100 100 100 100 89 76 99 100 99

COMPATIBILITY The broad compatibility of VMS has been demonstrated in studies with a number of materials, using an approach similar to the one described by the National Center of Manufacturing Sciences. For example, results of immersion testing with 16 different plastics in linear VMS showed negligible changes in weight, volume, and appearance. Even acrylic and polycarbonate, which typically experience clarity loss (clouding) from solvent exposure, displayed no change in optical properties from VMS immersion, as determined by colorimeter.17 Tests on 1  2 in plastic substrates immersed in VMS fluid for 3 h at 50°C (122°F) have revealed negligible changes in specimen weight, volume, or appearance. Colorimeter assessment of acrylic and polycarbonate materials (often susceptible to crazing from solvent exposure) have shown no effect on optical properties, even after exposure at 100°C (212°F). Similar testing of various elastomers revealed little or no change in most cases. However, exposure to VMS fluids has produced swelling in some low-consistency silicone sealants, natural rubber, and neoprene. Because of their compatibility, VMS fluids are suited to a variety of cleaning applications, including sensitive optical devices, gyroscopes, fiber optics, and electronic components. They can be used on most plastics and sensitive metals, and make effective agents for precleaning circuit boards or other surfaces before coating, sealing, or bonding operations. A patented VMS azeotrope is especially useful for benchtop cleaning of electronic assemblies and removing rosin flux and liquid crystal residue. The hexamethyldisiloxanebased formulation includes propylene glycol ether for enhanced cleaning capability. SILICONE CONFORMAL COATING REMOVAL VMS-based fluids are also effective at swelling and softening cured silicones so they can be lifted from substrate materials. This allows electronics manufacturers to remove silicone conformal coatings from circuit boards and components to rework failed units.16 The boards can then be recoated after repair, and very low NVR helps ensure a good bond with adhesives and coatings, whether organic or synthetic based. The ultrahigh-purity VMS fluids are well suited for applications requiring secondary bonding or coating, and will not cause adhesion problems in subsequent operations, even on surfaces previously coated with silicone.

© 2001 by CRC Press LLC

In the past, the electronics industry has typically used nonpolar solvents such as toluene and xylene to remove silicone conformal coatings from circuit boards when repairs are necessary. Although they are very effective removal agents, these compounds are also hazardous air pollutants. Diphenyl oxide disulfonate formulations are also popular for coating removal. The chemical structure of these compounds actually depolymerizes the silicone coating, causing a partial reversion toward a gel state. In fact, diphenyl oxide disulfonates provide effective silicone removal from both primed and unprimed surfaces. However, the caustic nature of the formulations also deteriorates FR4 board and other plastics used in manufacturing printed circuits. Any process involving conformal coating removal with these compounds requires close monitoring of exposure times to prevent damage to boards and components. Further, the strong odor of diphenyl oxide disulfonates can be extremely unpleasant in the workplace. In contrast, the plastics compatibility of VMS allows circuit boards to soak in the fluid for an unspecified period of time without damage, and the low odor helps avoid worker discomfort.

PROCESS CONSIDERATIONS When used in applications other than benchtop cleaning, VMS fluids require equipment and systems designed to accommodate their flash points. One such process employs a pneumatically controlled machine that requires no electrical components. Air-driven pumps are used to circulate the cleaning fluid, with the compressed gas supply directed to the final stage to speed drying. Multitank systems are also in successful operation with methylsiloxane fluids.

SUMMARY When manufactured to ultralow NVR levels, VMS fluids and VMS-based azeotropes have been proved to be effective cleaning agents for precision and industrial applications, and for removal of cured silicone coatings. Although the low KB value of straight VMS limits its cleaning performance to nonpolar contaminants such as greases, oils, and silicones, the inherent low surface tension helps undercut and lift many soils. Since VMS fluids are miscible in most solvents, they can be used in conjunction with more polar fluids, providing a solution with greater versatility than either type alone. Often the combination results in a formulation that cleans a broad range of contaminants, yet reduces the content of the more aggressive solvents. These ozone-safe materials demonstrate a favorable health and environmental profile, and their use is not likely to be restricted by foreseeable legislation. VMS fluids can usually be reprocessed using established technology, depending on the specific contaminants. Spent fluid is currently being fuel blended in cement manufacturing, used for its silica content and BTU value. Equipment designed to handle flammable liquids safely can typically be used for cleaning with VMS. Developed as an alternative to conventional solvent technology, these fluids help contribute to greater worker comfort and safety. While no drop-in replacement has yet been found to equal the cleaning performance and cost of ODCs, VMS fluids offer an exceptional balance of performance and ecological properties.

© 2001 by CRC Press LLC

REFERENCES 1. Atkinson, R., Kinetics of the gas-phase reactions of a series of organosilicon compounds with OH and NO3 radicals and O3 at 297 2 K, Environ. Sci. Technol., 25, 863, 1991. 2. Sommerlade, R., Parlar, H., Wrobel, D., and Kochs, P., Product analysis and kinetics of the gasphase reactions of selected organosilicon compounds with OH radicals using a smog chambermass spectrometer system, Environ. Sci. Technol., 27, 2435, 1993. 3. Battelle Lab Report to Newark Air Force Base, Experimental Evaluation of the Adhesive Degradation and Corrosion Potential of Silicone Fluids, Contract F09603-90-D-2217-Q805, January, 1995. 4. U.S. EPA, Fed. Regist., 192, 50693 –50696, 1994. 5. Iwata, T., Motavassel, F. and Perryman P., Ozone Depleting Compounds Replacement Guidelines, California South Coast Air Quality Management District, Office of Stationary Source Compliance, January, 1996. 6. Carter, W.P.L., Pierce, J.A., Malkina, L.L., and Duo, D., Investigation of the Ozone Formation Potential of Selected Volatile Silicone Compounds, final report from the University of California to Dow Corning Corporation, November 20, 1992. 7. Jenkin, M.E., and Johnson, C.E., Photochemical Ozone Creation Potentials of Volatile Siloxanes, AEA Technology Consulting Services (AEA/CS/16411030/001 Issue 1), August, 1993. 8. Carter, W.P.L., Luo, D., Malkina, I., and Venkataraman, C., Screening Experiments to Evaluate the Aerosol Forming Potential of Selected Volatile Silicone Compounds, final report from the University of California, Riverside to Dow Corning Corporation, June 16, 1994. 9. U.S. EPA, Testing Consent Order for Octamethylcyclotetrasiloxane, Fed Regist., 54, 818 –821, 1989. 10. Sousa, J.V., McNamara, P.C., Putt, A.E., Machado, M.W., Surprenant, D.C., Hamelink, J., Kent, D.J., Silberhorn, E.R., and Hobson, J.F., Effects of octamethylcyclotetrasiloxane (OMCTS) on freshwater and marine organisms, Environ. Toxicol. Chem., 14, 1639, 1995. 11. Mueller, J.A., DiTorio, D.M., and Maiello, J.A., Fate of octamethylcyclotetrasiloxane (OMCTS) in the atmosphere and in sewage treatment plants as an estimation of aquatic exposure, Environ. Toxicol. Chem., 14, 1657, 1995. 12. Hobson, J.F., and Silberhorn, E.M., Octamethylcyclotetrasiloxane (OMCTS), a case study: summary and aquatic risk assessment, Environ. Toxicol. Chem., 14, 1667, 1995. 13. Kent, D.J., McNamara, P.C., Putt, A.E., Hobson, J.F., and Silberhorn, E.M., Octamethylcyclotetrasiloxane in aquatic sediment toxicity and risk assessment, Ecotoxicol. Environ. Saf., 29, 372, 1994. 14. Dow Corning OS Fluids Product Stewardship Summary, Dow Corning form 10-678-96. 15. Ambati, R.R. and Kaiser, R., Silicone Particle Removal Study for Gyro Components, Entropic Systems, report prepared for Defense Construction Supply Center, June, 1994. 16. Witucki, B.A. and Cull, R.A., Evaluation of volatile methylsiloxane fluids as potential replacements for ozone depleting solvents, presented at the 213th ACS National Meeting, San Francisco, paper TECH-06, April 13–17, 1997. 17. Swanson, S., Cull, R., Bryant, D., and Moore, J., Cleaning Performance and New Technologies Based on Volatile Methyl Siloxanes, Dow Corning Corporation paper, presented at SAMPE Seattle, November, 1996.

© 2001 by CRC Press LLC

CHAPTER 1.10

Benzotrifluorides P. Daniel Skelly

CONTENTS Overview Properties of the Benzotrifluorides Solvent Toxicity Worker Protection Cleaning Systems for Benzotrifluorides General Considerations Size and Shape of Parts Materials of Construction Volume of Parts to Be Cleaned and Amount of Soil They Contain Soil Loading Cold Cleaning Mechanical Agitation Heated Cleaning Solvent Blends Compatibility Compatibility with Metals Compatibility with Polymers and Elastomers Approved Military and Aerospace Applications Reclamation and Disposal References

[Editor’s note: Many factors impact the cleaning options available to components manufacturers. The situation with the benzotrifluorides illustrates the impact of overall business plans of chemical producers. Recently, as this book was going to press, Occidental Chemical, the U.S. producer of benzotrifluorides, announced its intention to exit the market, ceasing production of parachlorobenzotrifluoride (PCBTF) and offering the business for sale. At this point, the future of PCBTF and related compounds as cleaning agents is not known, although some imported PCBTF may be available.]

© 2001 by CRC Press LLC

Despite the uncertainty, this chapter has been included for two reasons. For one thing, PCBTF has been found to be a valid option for some components manufacturers. In addition, Mr. Skelly has written a thoughtful approach to cleaning options. PCBTF has been introduced as a substitute for mineral spirits, and it is particularly valuable in areas of poor air quality. In Southern California, some local regulations explicitly depend on PCBTF for cold cleaning. In addition, because it is neither a VOC or a HAP, PCBTF has been used along with acetone in the reformulation of coatings to meet regulatory constraints. This is perhaps an object lesson to all of us, including end users, formulators, equipment manufacturers, and regulators that depending on a single or primary chemical to resolve major problems may not be prudent. PCBTF is produced throughout the world, but it is primarily a chemical intermediate, not a cleaning agent. In the interest of assuring options, it is hoped that the material will continue to be made available with the appropriate technical support and product stewardship.—B.K.] OVERVIEW Three commercially available benzotrifluorides, benzotrifluoride (BTF), parachlorobenzotrifluoride (PCBTF) and 3,4-dichlorobenzotrifluoride (DCBTF), have potential as replacements for ozone-depleting compounds and other organic solvents. Because it is exempt as a volatile organic compound (VOC), PCBTF is used in cleaning applications, particularly in areas with high regulatory constraints. “Likes dissolves like.” This is why, over the years, people have used organic solvents to clean oils, greases, and other organic contaminants off their parts and assemblies. However, most organic solvents used in cleaning operations are VOCs, which contribute to groundlevel ozone or smog formation. As a result, restrictions have become burdensome and permits difficult to obtain, especially in metropolitan areas where air pollution is a significant problem. Although the three benzotrifluorides have been commercially produced in the United States since the 1960s, until the mid-1990s, their use was limited to chemical intermediates for the agricultural and pharmaceutical industries. Interest in their use as cleaning agents developed when it was discovered that the atmospheric lifetimes of the benzotrifluorides are short enough to avoid ozone depletion. The compounds are not listed as hazardous air pollutants (HAPs). The low tropospheric reactivity of PCBTF led to VOC exemption by the U.S. EPA. PROPERTIES OF THE BENZOTRIFLUORIDES The structures of BTF, PCBTF, and DCBTF are indicated in Figure 1. Occidental Chemical Corporation (OxyChem®) is currently the only U.S. producer of these compounds, and they are currently sold under the OXSOL® 2000, OXSOL 100 and OXSOL 1000 trademarks, respectively.* Some physicochemical properties are summarized in Table 1; their regulatory status is indicated in Table 2. In addition, properties of the benzotrifluorides obviate many of the issues associated with aqueous cleaning. Water pretreatment and wastewater disposal are not issues, nor are problems of flash rusting. Because the benzotrifluorides are organic, they have an affinity for a wide range of organic contaminants. The fluorine content provides a low surface tension, which allows penetration into blind holes and crevices. Although the efficiency of most processes tends to increase with use of multiple tanks, multiple cleaning and rinsing steps are not required. While parts will still come out wet after cold cleaning, drying is relatively fast. Cycle times are comparable with many traditional organic solvent systems. *OXSOL and OxyChem are registered trademarks of Occidental Chemical Corporation. © 2001 by CRC Press LLC

Figure 1

Structures of the benzotrifluorides.

SOLVENT TOXICITY Because no official exposure levels have been established by either OSHA or the ACGIH, Occidental Chemical Corporation has set a CEL (corporate exposure limit), 8-h TWA (time-weighted average) for each of the three benzotrifluoride molecules. These values, which appear in Table 2, are as follows: BTF  100 ppm, PBCTF  25 ppm, and DCBTF  5 ppm. These values were established as a safe level of exposure for an average 8-h workday. Additional studies have been completed and are available. WORKER PROTECTION Benzotrifluorides can be used with appropriate consideration given to worker protection. The cleaning system must be designed to minimize hazards to the worker and the environment. This may include mechanical controls such as tank covers and auxiliary cooling coils to condense solvent vapors, or fans and exhaust hoods to remove solvent vapors from the workstation. The flash point must also be considered in appropriate equipment design. CLEANING SYSTEMS FOR BENZOTRIFLUORIDES General Considerations1 No single cleaning system can be used universally, and what works for a neighbor or a sister plant may be ill-fitted for your requirements. Educate yourself on the options, then choose the solvent(s) and equipment that best meet your needs. The system should be capable not only of removing your soils efficiently and economically, but also of meeting the regulatory and safety requirements of today and the future. A critical first step in selecting the cleaning system is to address, “How clean is clean?” Only you, the owner of the parts that need cleaning, can properly evaluate if a given system will meet the majority of your needs and expectations. Size and Shape of Parts Where there are blind holes, lap joints, or other small crevices, the solvent must not only have a good solubility for the soil, but also a low viscosity and low surface tension. Unlike water, the benzotrifluorides have a naturally low surface tension allowing them to penetrate and clean these hard-to-reach areas. © 2001 by CRC Press LLC

Table 1 Physical Properties of the Benzotrifluorides Physical Properties

BTF

PCBTF

DCBTF

CAS number Chemical formula Molecular weight Boiling point (BP) at 760 mmHg (°C) Density, liquid at 25°C (lb/gal) Density vapor (air  1) Dielectric breakdown (kv) Dielectric constant at 25°C Electrical resistivity M  (M -cm) Evaporation rate at 25°C (n-BuAc  1) Flammability Flash point, Tag Closed Cup, °C (°F) Fire point, Tag Open Cup, °C (°F) Autoignition temperature (°C) Upper flammability limit (% vol) Lower flammability limit (% vol) Limited oxygen index (LOI) (% vol) Freeze point (°C) Hansen solubility parameter (MPa)1/2 Nonpolar Polar Hydrogen bonding Kauri-butanol value (KB) Latent heat of vaporization at BP (cal/g) Refractive index at 25°C Solubility Solvent in water at 20°C (ppm) Water in solvent at 25°C (ppm) Specific gravity at 25/15.5°C Specific gravity correction factor (per °C) Specific heat, liquid at 20°C (cal/g/°C) Surface tension in air at 25°C (dynes/cm) Vapor pressure at 20°C (mmHg) Viscosity, liquid at 25°C (cps) Volatiles (% wt)

98-08-8 C7H5F3 146.11 102 9.88 5.0 — 11.5 2.5 (330) 2.8

98-56-6 C7H4F3Cl 180.56 139 11.16 6.2 49 — 20 (2640)a 0.9

328-84-7 C7H3F3Cl2 215.00 173.5 12.3 7.4 — — 20 (2640)a 0.2

12 (54) 23 (74) — — — 18.2 29 16.1 13.0 9.5 0 49 53.9 1.4131

43 (109) 97 (207) 500a 10.5 0.9 26.2 36 17.7 13.9 9.9 4.7 64 49.6 1.4444

77 (170) 114 (238) 500a 7.8 2.9 29 12.4 18.2 14.1 10.0 5.7 69 46.6 1.4736

a

250 290 1.185 0.0013 0.31 23 30 0.53 100

29 240 1.3380 0.00146 0.27 25 5.3 0.79 100

12 153 1.478 0.00146 — 29 1.6 1.52 100

Limitation of test equipment.

Materials of Construction The selection of a strong solvent may be good at removing the soil, but you must also be sure that it does not damage the substrate or your equipment. Based on the Kauributanol (KB) scale, all three benzotrifluorides have a moderately high solvency power. They have about twice the cleaning power of mineral spirits and half that of toluene. This makes them effective for a wide range of organic soils, but not so strong that they damage most plastics or elastomers. © 2001 by CRC Press LLC

Table 2 Regulatory Summary of the Benzotrifluorides Regulatory Issue

BTF

PCBTF

DCBTF

Regulated as an ozone depleter Regulated as a VOC Regulated as an HAP SARA Title III, Sect. 313 Reportable Global warming potential Suspected carcinogen Corporate exposure limit, 8-h TWA (ppm) DOT hazard class OSHA hazard class RCRA hazardous waste number

No Yesa No No Low No 100 Flammable Flammable D001

No No No No Low No 25 Not regulated Combustible D001

No Yes No No Low No 5 Not regulated Combustible Not regulated

a

Petitioned the Federal EPA for VOC exemption on 3/11/97. Still pending at the time of this writing.

If a heated process is planned, the substrate must be able to withstand the prescribed temperature. For vapor degreasing, keep in mind that the boiling points of PCBTF and DCBTF are 139 and 173.5°C, respectively. These relatively high temperatures could damage some parts. Volume of Parts to Be Cleaned and Amount of Soil They Contain A process involving occasional cleaning of a few small parts is very different from a critical process in a high-volume assembly line operation. Do not shortchange yourself. You do not want to pay for excess equipment capacity, but make sure to account for future growth potential. Fabricating additional sheet metal tanks does not generally significantly increase capital costs, and additional and/or slightly larger tanks can make a big difference in the capability and capacity of the equipment. Soil Loading You cannot clean parts with dirty solvent. If only a small amount of soil needs to be removed, your solvent will remain relatively clean for a long time. However, if there is a large amount of contaminant, you might want to consider multiple dips in progressively cleaner solvent, and/or on-site distillation. With solvent recovery, a stable, single-component cleaner (such as PCBTF) is more easily maintained and recycled than is a complex proprietary blend. In most applications, once the soil loading exceeds 30%, it is recommended that it be reclaimed or disposed. Soil loading may often be approximated by measuring its specific gravity at a given temperature. In the case of PCBTF, a simple graph was developed at three different temperatures and various levels of mineral oil. Once such a chart is developed for your “typical” contaminant, you can then approximate the soil loading with a quick comparison to a chart similar to Figure 2. It will be up to end users to determine what soil loading level is tolerable in their system for adequate cleaning performance. Cold Cleaning Recent regulations have focused on the VOC content of mineral spirit solvents traditionally used in these systems. PCBTF, alone or in blends, may be an appropriate replacement in recirculating overspray applications where aqueous cleaning is ineffective. © 2001 by CRC Press LLC

Figure 2

Specific gravity vs. oil loading in PCBTF2 (From OXSOL 100—Used as a Solvent Flush in Refrigeration Conversions, pending bulletin BCG-OX-A/C, 8/95, Occidental Chemical Corp., Niagara Falls, NY. With permission.)

In some cases, single or multiple tank immersion units can be adapted for use with PCBTF and DCBTF with only minor modification, with the proviso that all equipment must be designed for use with low flash point solvents. However, it is important to assure that top enclosures and side workload entry be included to minimize worker exposure and to eradicate possible solvent odors. Because of its relatively high vapor pressure and the resulting solvent losses, BTF would not generally be used in immersion dip tanks. Mechanical Agitation The cleaning operation is generally enhanced with some level of mechanical agitation to help carry the soil away from the surface of the part. Where ultrasonics are used with the benzotrifluorides, the possible elevation of the solvent bath temperature must be taken into consideration. Heated Cleaning With waxes, buffing compounds, and similar soils, heating (from slightly above ambient temperature to the boiling point) may be required. Unless the equipment is properly designed for elevated temperatures, solvent losses will increase. Further, given the flash points, cleaning systems designed for flammable solvents must be used. Because of its volatility and flammability, BTF is ill-suited for this cleaning method, and both DCBTF and PCBTF use is limited in heated dip tanks. Benzotrifluorides must not be used in a standard, unmodified vapor degreaser. PCBTF and DCBTF could be used in approved, specifically designed low flash point systems with sufficient containment and automation to maintain employee exposure well below the recommended limit (see Chapter 2.12 by Bartell). Initial capital expenditure will be significant for such systems. © 2001 by CRC Press LLC

Automation is desirable to distance the worker from the cleaning operation, thereby minimizing unnecessary solvent exposure. Solvent Blends Although a single-solvent system is generally cheaper and easier to maintain, in some applications a blend of the benzotrifluorides with other compounds is desirable. A system containing a mixture of solvents can have advantages in areas such as solubility parameters, flash point suppression, odor reduction, and evaporation rates. However, depending on the additive selected, you may add to the regulatory requirements, complexity of solvent recovery or disposal, and total system cost. COMPATIBILITY Compatibility with Metals In one study run for the U.S. Navy, PCBTF passed the ASTM F 483 total immersion corrosion test with no visible corrosion on any of the following alloys: AMS 4037, 4041, and 4049 aluminum, AMS 5046 grade 1020 steel, cadmium-plated 1020 steel, AMS 4911 titanium, and AMS 4377 magnesium. Compatibility with Polymers and Elastomers The solvency power of the benzotrifluorides ranges between that of mineral spirits and perchloroethylene (dry cleaning solvent). As a result, these moderately strong solvents have the ability to dissolve a wide range of uncured resins (of interest to the paint and adhesive markets) and selectively remove many organic soils from the workload. At the same time, they are generally mild enough to preserve the integrity of the articles being cleaned. A representative sample of a study of commercially available polymers and elastomers exposed to PCBTF is summarized in Table 3. Similar data are available for the other benzotrifluorides. In the study, test strips were weighed before and after submersion in PCBTF at room temperature for 24 h. They were removed, blotted dry, and reweighed. The differences in the weights are reported in Table 3 as “wt% Initial” (% gain or loss). These same test strips were then allowed to air-dry at room temperature for 4 days and were again reweighed and the results reported as “wt% Overall” (% gain or loss). APPROVED MILITARY AND AEROSPACE APPLICATIONS In a study contracted by Occidental Chemical Corporation for the U.S. Navy, PCBTF passed or conformed to the following corrosion/compatibility tests: • ASTM F 483—Total Immersion Corrosion (see section on Compatibility) • ASTM F 945—Stress Corrosion of Titanium Alloys by Aircraft Maintenance Materials –Method A • ASTM F 519—Test Method for Mechanical Hydrogen Embrittlement Testing of Plating Processes and Aircraft Maintenance Chemicals As a result of these favorable test results, both the U.S. Navy and Air Force have approved the use of PCBTF in several protective coatings for military aircraft applications. © 2001 by CRC Press LLC

Table 3 Examples of PCBTF Compatibility with Polymers and Elastomers

Material Tested Acrylic Butyl rubber Carboxylic rubber Chloroprene rubber Chlorosulfonyl PE EPDM EPD terpolymer Fluorocarbons PTFE PVDF VF-HFP copolymer Ionomer Nitrile butadiene rubber Polyamide Polycarbonate Polyester PBT PET Polyethylene, high density Polyphenylene sulfide Polypropylene Polysulfone Polyurethane Polyvinyl chloride CPVC PVC Silicone Styrene Styrene butadiene rubber

PCBTF ( wt%)

Brand or Common Name

Initiala

Overallb

Lucite® Bucar NBR Neoprene W Hypalon® 40 Nordel® 6962 Nordel (Elastomer)

16.5 83.1 120.4 73.6 74.5 69.7 61.2

16.6 9.0 0.5 1.8 3.1 6.6 5.1

Teflon® Kynar® Viton® A Surlyn® Buna N Nylon 6/6 PC

0.1 0.1 30.6 1.8 79.9 0.0

0.0 0.5 6.5 0.6 10.4 0.0 12.4

Valox® Rynite® HDPE Ryton® PP Polysulfone Adiprene® L

0.1 0.1 1.8 0.0 17.5 0.1 29.1

0.0 0.1 0.6 0.0 0.2 0.1 6.3

— — Silicone — Buna S

0.0 0.1 77.3

0.0 0.0 27.2

c

d

124.3

d

3.7

Initial  initial weight change after 24-h solvent soak. Overall  overall weight change after a 4-day drying period. c Attacked and cracked d Test strips dissolved. Adiprene® L is a registered trademark of Uniroyal Chemical Corporation; Delrin®, Hypalon®, Lucite®, Nordel®, Rynite®, Surlyn®, Teflon®, Tefzel®, and Viton® are registered trademarks of I.E. du Pont de Nemours and Company; Kynar® is a registered trademark of Pennwalt Corporation; Noryl®, Ultem®, and Valox® are registered trademarks of General Electric Company; Panacea® is a registered trademark of Prince Rubber & Plastics Co., Inc; Ryton® is a registered trademark of Phillips Petroleum Company; Thiokol® FA is a registered trademark of Morton International, Inc. Source: Modified from OXSOL Solvents, Compatibility with Polymers and Elastomers, BCG-OX-20 5197, Occidental Chemical Corp., Dallas, TX, 1997, 2–4. With permission. a b

Parachlorobenzotrifluoride has also been approved by the U.S. Air Force under T.O. 1-1-8 for use as a cleaning solvent on aircraft, as a wipe solvent for primer reactivation and as a paint thinner. After an extensive study to replace 1,1,1-trichloroethane (TCA), Alliant TechSystems (formerly Hercules Aerospace) has approved the use of PCBTF as a wipe cleaning solvent on the Navy’s Trident II D/5 first- and second-stage rocket motors. © 2001 by CRC Press LLC

RECLAMATION AND DISPOSAL Recovery of the benzotrifluoride containing waste stream4 may be done by a competent and properly permitted contractor, or by investment in on-site explosion-proof distillation equipment. Most stills available today have efficiencies of 90% or greater. Recovery increases to 95% or more with a high-efficiency, thin-film evaporation still. To increase the amount of reclaimed solvent further, and reduce the volume of waste that must be disposed, stills can be equipped to employ steam stripping or vacuum. Because additive stabilizer packages are not required, the distillate can easily be returned into the cleaning tank for reuse; recycling equipment may have a short payback period. The still bottoms from a typical on-site distillation operation contain between 1 and 10% solvent and must be disposed of according to proper Resource Conservation and Recovery Act (RCRA) hazard classifications. Although DCBTF is not regulated under RCRA, pure BTF and PCBTF have flash points of less than 140°F, which qualifies them as D001 “Characteristic Hazardous Wastes.” Because they do not contain any listed concentrations of compounds recognized by RCRA as hazardous wastes, many state and local regulations allow these still bottoms to be added to other combustible products and incinerated as fuel oils. This can minimize costly hazardous waste disposal fees. Fuels blending programs for cement kilns are preferred as the flames are generally hotter and the alkalinity of the cement will neutralize the acid gases that will be generated. An alternative disposal method is hazardous waste incineration in licensed equipment capable of handling HCl and HF. The heat of combustion values for BTF, PCBTF, and DCBTF are 8060, 7700, and 4830 BTU/lb, respectively. Carefully analyze the characteristics of your particular waste prior to selecting any waste disposal option. Recycling, disposal. Is the spent cleaning solution classified as a hazardous waste? This needs to be determined on an individual basis, taking into consideration the components and characteristics of the waste stream. REFERENCES Information was taken from the following technical bulletins produced by Occidental Chemical Corporation: 1. OXSOLs for Metal Cleaning, BCG-OX-19 1/95. 2. OXSOL 100—Used as a Solvent Flush in HFC Refrigeration Conversions, pending bulletin BCGOX-A/C 8/95. 3. OXSOL Solvents—Compatibility with Polymers and Elastomers, BCG-OX-20 5/97. 4. Disposal of OXSOL Degreasing Wastes, BCG-OX-11 6/97.

© 2001 by CRC Press LLC

CHAPTER 1.11

HCFC-225: Alternative Precision and Electronics Cleaning Technology Toshio Miki, Masaaki Tsuzaki, and Kenroh Kitamura

CONTENTS Introduction Fundamental Properties Toxicology Summary Applications Precision Cleaning Other Sequential Cleaning Applications Defluxing of Printed Circuit Boards for Automobiles Special Applications Regulatory References

INTRODUCTION CFC-113 has been widely used as a cleaning agent in various industrial applications such as electronics cleaning, precision cleaning, metal cleaning, and dry cleaning, due to such factors as excellent chemical inertness, thermal stability, nonflammability, and low toxicity. Various alternatives to CFC-113 such as hydrocarbons, alcohols, aqueous and semiaqueous cleaning have been commercialized. These alternatives can easily replace some of the CFC113 applications. However, their properties are different from CFC-113. Asahi Glass has developed and commercialized HCFC-225 (a mixture of two isomers, HCFC-225ca and HCFC-225cb, sold as Asahiklin™ AK-225) as an alternative to CFC-113.1,2 HCFC-225 has most of the characteristics of CFC-113 as a cleaning agent, such as physical and chemical properties and nonflammability, and can be applied to the range of CFC-113 applications with few changes in the process or cleaning equipment. HCFC-225 has been applied as a replacement of CFC-113 in applications where other alternatives cannot be used. HCFC-225 has a very low ozone depletion potential (ODP) and a very low global warming potential (GWP). It has been widely used for more than 10 years. In this chapter,

© 2001 by CRC Press LLC

the fundamental properties, range of applications, and typical cleaning processes of HCFC225 and its blends are discussed. The emphasis is on applications used by manufacturers in Japan. FUNDAMENTAL PROPERTIES The physical properties of HCFC-225, HCFC-225AES, CFC-113, and CFC-113AES are summarized in Table 1. The boiling point, surface tension, and Kauri-butanol (KB) value are the key properties of a cleaning agent. A boiling point in the range from 40 to 60°C is important to clean parts without raising their temperature to the point where the temperature-sensitive elements or materials are affected. Low surface tension enables a cleaning agent to penetrate into small gaps, and a KB value of approximately 30 to 35 allows removal of soils without damage to metals, plastics, or elastomers. Latent heat of vaporization is also a key property in vapor cleaning. A low latent heat of vaporization results in less energy consumption in vaporization and rapid drying. As shown in Table 1, the physical properties of HCFC-225 and its blends are very similar to those of CFC-113 and its blends. HCFC-225 and its blends can be used with a few or no changes of the existing cleaning equipment and procedure for CFC-113 and its blends. Compatibility of HCFC-225 with representative materials of construction is summarized in Table 2. In general, HCFC-225 is equivalent to CFC-113; in some cases it is somewhat more aggressive. Some plastics such as stressed polycarbonate and ABS resin may crack, craze, or swell. TOXICOLOGY SUMMARY HCFC-225ca and HCFC-225cb were individually tested by PAFT (Programme for Alternative Fluorocarbon Toxicity Testing). Testing was conducted on the separate isomers to allow characterization of the toxicity of each isomer and of the mixed isomer products. Table 1 Physical Properties of HCFC-225 Products HCFC-225 Boiling point (°C) 54 Freezing point (°C) 131 Density (g/cm3)b 1.55 Viscosity (cP)b 0.59 Surface tension (dyne/cm)b 16.2 Latent heat of vaporization (cal/g, b.p.) 34.6 Relative evaporation rate (ether=100) 90 Specific heat (cal/g °C)b 0.24 Solubility of water (wt%)b 0.031 Solubility in water (wt%)b 0.033 Flash point (°C) None KB value 31 ODP (CFC-11  1.0) 0.03 GWP (CO2  1.0, 100 years) 370 a

Azeotrope of CFC-113 and ethanol. At 25°C. c At 20°C. b

© 2001 by CRC Press LLC

HCFC-

CFC-113

CFC-113AESa

52 138 1.49 0.61 16.8 40.6 81 0.27 0.33 0.053 None 41 0.03 310

47.6 35 1.57 0.65 17.3 36.1 123 0.229 0.109 0.017 None 31 0.8 5000

46.5 41.8 1.51c 0.66 18.5c 42.9 120 0.272c 0.25 — None 39 0.8 4800

Table 2 Material Compatibilities of HCFC-225

Plastic

a

Elastomerb Metalc

HCFC-225

HCFC-225AES

Similar to CFC-113; exception: acrylic Similar to CFC-113 Similar to CFC-113

Similar to CFC-113/EtOH; exception: acrylic Similar to CFC-113/AES Similar to CFC-113/AES

a

21 kinds of plastics tested. 10 kinds of elastomers tested. c 17 kinds of metals tested. b

Data from acute toxicity studies demonstrate that HCFC-225ca and HCFC-225cb have very low acute toxicity. Neither isomer causes eye irritation or dermal toxicity in standardized tests; skin application of both isomers at high doses (2000 mg/kg body weight) produces no adverse effects. Oral administration of either isomer at high doses (5000 mg/kg body weight) does not cause any mortality. Therefore the oral LD50s are greater than 5000 mg/kg body weight. Both isomers also have very low acute inhalation toxicity as measured by the concentration that causes 50% mortality in experimental animals, the LC50. The 4-h exposure LC50s for both isomers are approximately 37,000 ppm in rats. Anesthetic-like effects are observed in rats at high inhalation concentration (greater than 5000 ppm). As with many other halocarbons and hydrocarbons, inhalation of HCFC-225ca and HCFC-225cb followed by intravenous injection of epinephrine, which simulates human stress reactions, results in a cardiac sensitization response in experimental screening studies with beagle dogs. This cardiac sensitization response is observed at approximately 15,000 ppm for the mixture of HCFC-225 at ca/225cb (45/55 weight percent) and 20,000 ppm for the isomer 225cb, which are levels well above expected exposures. By comparison, a cardiac sensitization response is observed with CFC-113 at 5000 ppm. In 28-day inhalation studies with rats, the activity and responsiveness of the animals were reduced at exposures of 5000 ppm or greater for each isomer. Toxicity was otherwise confined to the liver. To investigate the biological relevance of the liver toxicity to humans, comparative repeated inhalation studies have been conducted with rats, hamsters, guinea pigs, and marmosets. The results indicated that although liver effects were observed in the rats, the effects did not transcend to higher primates. In conclusion, HCFC-225 is not a carcinogen, a teratogen, does not cause birth defects, and is not harmful to unborn children. From the results of the toxicological findings, an 8-h/day/40-h workweek exposure was set at 50 ppm (AEL). Emergency exposure limit guidelines have been set at 1000 ppm for a period of 15 min, and 2000 ppm limit for a 1-min time period. This information is considered supplemental and applies to the material without additives. Please review the Material Safety Data Sheet for any chemical before using. APPLICATIONS Of the HCFC-225 produced, 70% is used in Japan. About 70% is used neat (without additives) in precision cleaning, for rinsing and drying in cosolvent systems, as a carrier solvent, and as a solvent for chemical reaction.

© 2001 by CRC Press LLC

Blends are used when the solvency of HCFC-225 is insufficient for the cleaning application. In some cases, including precision cleaning applications, cleaning agents are used sequentially, with HCFC-225 typically used as the rinsing and/or drying agent; an advantage is that rinsing and drying times typically are reduced. Precision Cleaning HCFC-225 has been used as an effective cleaning agent for removal of oils, greases, dusts, and so on. Especially, it has been applied for degreasing and dust removing of precision components such as bearings, miniature motors, coils, relays, and connectors. These components usually have to avoid contact with water, and they can hardly be dried with alternatives other than HCFC-225 because of their minute and complicated shapes. Cleaning procedures of HCFC-225 are quite similar to those of CFC-113 as shown in Figure 1. The procedure consists of immersing a work load into the warm solvent, rinsing with cold solvent, and drying in solvent vapor. An existing vapor degreaser for CFC-113 can be used for HCFC-225 as described above. Agitation and ultrasonic cleaning are often used together in the immersing step. Total cleaning time may vary from less than 1 min to 30 min, depending on the required cleanliness level. Typical cleaning time is a few minutes in total. In certain cases, rust preventative is added into HCFC-225, and then parts to be cleaned are immersed into HCFC-225 to clean and prevent them from corroding at the same time. Other Sequential Cleaning Applications In some processes, parts are cleaned with hydrocarbon, and then rinsed with HCFC225 vapor. In other typical semiaqueous/cosolvent processes, HCFC-225 is added to the end of the process to improve drying and spotting. Parts are cleaned with an aqueous or semiaqueous cleaning agent and then rinsed with water. Residual water is removed with alcohol. Finally, alcohol is removed with HCFC-225. In both cases the content of hydrocarbon or alcohol should be monitored by determining the specific gravity or boiling point of the mixture. The isopropyl alcohol mixture will

Figure 1

Schematic diagram of six-sump type cleaning equipment used by bearing manufacturer.

© 2001 by CRC Press LLC

become flammable at greater than 20% by weight alcohol. It is necessary to maintain the alcohol content at or below 10% by weight by monitoring the boiling point. The content can also be monitored by specific gravity of the mixture. Defluxing of Printed Circuit Boards for Automobiles High reliability and durability are required for printed circuit boards (PCBs) installed in automobiles. The manufacturing process of the PCBs consists of soldering electronic devices, cleaning, and coating. Given the high production volume, rapid cleaning is required. Many manufacturers use HCFC-225AES for defluxing. Most use fluxes and solder pastes specifically suited to the solvent to maximize process efficiency (Table 3). In one application, a three-sump process is used (Figure 2). The procedure is the same as that using HCFC-225 as described above. The tact (labor) time is 15 s, total process time is 1 min. In a second application, a vertical two-sump is used (Figure 3). PCBs are cleaned with warm solvent (40 to 45°C), then rinsed by spraying with cold solvent and dried in solvent vapor. The tact time may be 10 s or more; total process time is 1 min. It is important to note that in both cases, less solvent is used than in the original CFC113 process. Manufacturers no longer clean all PCBs. Instead, they evaluate the necessity of cleaning, and they only clean for essential applications. They also use recovery and/or distillation equipment to reduce solvent usage. Special Applications HCFC-225 is used in special cleaning and noncleaning applications. Such special cleaning includes refrigeration cycle cleaning to replace CFC-11, liquid oxygen cleaning for aerospace, and iron manufacturing to replace CFC-113 or carbon tetrachloride. It is used as a carrier solvent for silicones to coat syringe needles and for lubricant to coat hard disks. Furthermore, it is used as a solvent in chemical reactions. HCFC-225 is gradually replacing CFC-113 for some dry cleaning processes. Specifically designed dry cleaning equipment is available from several equipment manufacturers. Surfactant additives for HCFC-225 are also available in Japan. Table 3 Fluxes and Solder Pastes for HCFC-225AES Manufacturer

Flux

Solder Paste

Alphametals

SM4592C-10T, PRM615-15, R5186 AGF-200FZ — JS-64-ND-3 — NS-829 PO-4003-K3, PO-F-210-K4, SR-210-K2

SR-9100-39

Asahi Chemical Res. Harima Chemicals Koki Nihon Handa Nihon Superior Senju

Tamura

© 2001 by CRC Press LLC

CRF-225-20

— F6, F16, F27 types SE4-A228 RX-463-AK-3, RX-462-AK-3 SM63RA-FMQ7S OZ 63-606F-AA-10.5, OZ 2062-606F-AA-10.5, OZ 63-633F-42-10, OZ 2062-633F-42-10 SQ-1040AK-7, SQ-1030AK-7, SQ-2030AK-7

Figure 2

Schematic diagram of three-sump-type cleaning equipment. (A) Warm sump (15 s), (B) cold sump (15 s), (C) vapor (15 s).

REGULATORY HCFC-225 is considered a transitional alternative that will be phased out by the year 2020 based on the Montreal Protocol. However, as described in the report of the 15th meeting of the open-ended working group of the parties to the Montreal Protocol,4 for certain high-reliability applications, the use of this chemical has remained important in view of the lack of appropriate alternatives with good technical or environmental performance.

Figure 3

Schematic diagram of vertical two-sump-type cleaning equipment. (1) Warm sump immersion (15 s), (2) rinse with spray (10 s), (3) vapor (20 s).

© 2001 by CRC Press LLC

REFERENCES 1. K. Kitamura et al., in Proceedings of the 1994 International CFC & Halon Alternatives Conference, October, Washington, D.C., 1994, 585. 2. K. Kitamura et al., in Proceedings of the 1995 International CFC & Halon Alternatives Conference, October, Washington, D.C., 1995, 779. 3. K. Kitamura et al., in Proceedings of the 1996 International Conference on Ozone Protection Technologies, October, Washington, D.C., 1996, 669. 4. UNEP, Report of the 15th meeting of the open-ended working group of the parties to the Montreal Protocol, INEP/Ozl.Pro./WG.1/15/5, 12 June, 1997.

© 2001 by CRC Press LLC

CHAPTER 1.12

d-Limonene: A Safe and Versatile Naturally Occurring Alternative Solvent Ross Gustafson

CONTENTS Introduction Typical Properties Safety and Environmental Concerns Effectiveness Stand-Alone Parts Washers Automatic Parts Washers Conversion of Vapor Degreasers Circuit Board Cleaning Aerosol Applications Sample Formulations Case Studies Other Uses Other Concerns Conclusion References

INTRODUCTION With the production phaseout of chlorofluorocarbons (CFCs) and other ozonedepleting chemicals and increased awareness of workplace safety, many different cleaning solvents have been introduced. d-Limonene, a naturally occurring substance extracted from citrus rind during the juicing process, has shown great effectiveness in the cleaning market and is experiencing a growing acceptance as the solvent of choice in a number of different applications. d-Limonene is a non-water-soluble solvent. It can be used straight or blended with an emulsification system to produce a water dilutable/rinsable product. It is capable of effectively removing organic dirt loads ranging from light cutting oils and lubricants to heavy greases, such as cosmoline. © 2001 by CRC Press LLC

+ d-limonene Figure 1

Structure of d-limonene.

d-Limonene is in the chemical family of terpenes. These are products, produced by all plant life, based on isoprene. All terpenes are combinations of two or more isoprene molecules. The d-limonene structure is shown in Figure 1 as the addition of two isoprene molecules. Some common terpenes besides d-limonene are pinene, menthol, camphene, and b-carotene. d-Limonene is extracted at two different points in the citrus-juicing process. During the pressing of the fruit to remove the juice, a significant quantity of the peel oil is also pressed out. This floats on top of the juice, and is separated by decanting. This fraction is called cold-pressed oil, and contains many flavor and fragrance compounds along with the d-limonene. The cold-pressed oil can be distilled to separate the d-limonene fraction from the flavor and fragrance compounds. The separated d-limonene is termed food-grade d-limonene in the industry. After the fruit has been juiced, the peels are sent to a steam extraction step where more d-limonene is recovered. During this process, the peels are exposed to steam, which carries the d-limonene to a condenser. Here the water and d-limonene separate into aqueous and oil phase, and the d-limonene is recovered. This product is termed technical-grade d-limonene. Historically, orange oil and d-limonene have been used extensively as flavor and fragrance ingredients in a wide variety of products, including perfumes, soaps, and beverages. d-Limonene has also been used to make paint solids through a polymerization process. The first uses of d-limonene as a cleaning solvent began in the 1970s, but with the increased environmental awareness in the 1980s and 1990s, use of d-limonene as a cleaner has shown a tremendous increase.

TYPICAL PROPERTIES d-Limonene is a thin, relatively colorless liquid. The typical physical and chemical properties for technical grade are: Color Odor Specific gravity (25°C) Refractive index (20°C) Optical rotation (25°C) Flash point Boiling point Freezing point Evaporation rate Water solubility Vapor pressure (20°C)

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Slight yellow to water white Orange aroma 0.83800.843 1.4710 to 1.4740 96° to 104° 115°F 178°C (310°F) 96°C (140°F) 0.05 vs. butyl acetate Insoluble 1.4 mmHg

SAFETY AND ENVIRONMENTAL CONCERNS From a personal-safety standpoint, d-limonene is a much safer product for use than most other solvents. The oral LD50 of d-limonene is greater than 5000 mg/kg body weight. The product also is classified as a food additive and has been granted the Food and Drug Administration GRAS (Generally Recognized as Safe) status. For comparison, the typical mineral spirit LD50 is around 2000 mg/kg body weight. A formal threshold limit value (TLV) or permissible exposure limit (PEL) has never been established for d-limonene. d-Limonene is also noncaustic and nonreactive to metal surfaces. It has been classified as a slight skin irritant, because it can remove the naturally occurring oils from skin, but has not been shown to cause lasting damage. It is not carcinogenic or mutagenic, and is currently being evaluated for its chemopreventative and chemotherapeutic properties. d-Limonene is not itself and does not contain any ozone-depleting chemicals. It is currently regulated as a volatile organic compound (VOC). The evaporation rate of d-limonene is relatively low, so the actual VOC emissions are small. d-Limonene is not considered an air toxic or hazardous air pollutant (HAP), and is not regulated under the Clean Air Act or SARA Title III. It is an approved solvent substitute under SNAP (Significant New Alternatives Program). The issue of global warming as it pertains to the recovery and use of d-limonene is difficult, and no reliable estimate has been completed. When plants create d-limonene, or any terpene, they use carbon dioxide and water. When a terpene is destroyed or degrades, carbon dioxide and water are produced. So the creation and destruction of d-limonene would result in a net zero global-warming effect. Generally, d-limonene solutions are not heated when used. In those applications there would be essentially no global-warming effect. At some point in a cleaning process, a small amount of energy may be used for heating drying air or rinse tanks, or at least circulation of fluids. This will have a global-warming impact, but in general is not significant. The real difficulty lies in estimating the impact of producing the fruit and extracting the product from the rind. Since d-limonene is truly a byproduct of the juicing industry, very little or none of the impact from growing and processing the fruit should be attributed to d-limonene production. Citrus growers do not grow fruit for the express purpose of producing d-limonene. They are in the business of producing fruit and juice. So the global-warming impacts in the form of fertilizers, irrigation, transportation, and other energy uses associated with agriculture are attributable to fruit production, but not to d-limonene production. The major source of global-warming potential attributable to d-limonene production is the energy required to perform the extraction. Although this has not been formally calculated or estimated, one would not expect to see a significant amount of energy used and one could therefore consider production of d-limonene to have a very small global-warming potential. The closed cup flash point for d-limonene has been established at 117° F. This makes d-limonene a Class III combustible under Department of Transportation (DOT) regulations. For transportation purposes this means that when shipping d-limonene by ground, the vehicle must be placarded as hazardous if a single container contains over 110 gal of d-limonene. For storage purposes, sprinkler systems in the storage area are required. It is also recommended to use explosion-proof pumps and wiring. When d-limonene/surfactant/water systems are made, the closed cup flash point will generally rise to about 130°F. These solutions will have a very high open cup flash point, and will not support a flame at any temperature below boiling. Under RCRA regulations, d-limonene is classified as a characteristic ignitable hazard, and must be disposed of as a hazardous waste. However, water/d-limonene emulsions will

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have no flash point at less than 1.5% d-limonene concentration. So when using a water/ d-limonene system and rinsing the cleaned materials, the rinse water can generally be disposed of as a nonhazardous material if the concentration is below this amount. EFFECTIVENESS In many cleaning applications, the soils to be removed are organic oils and greases. Because of the inherent chemical differences between organic and inorganic materials, such as polarity and ionic effects, organic solvents tend to perform much better for cleaning these types of soils than water-based solutions. To compare organic solvent strengths, the Kauri-butanol (KB) value, an ASTM method (D1133-97), has been established. The more toxic chlorinated solvents and benzene and its related compounds are all extremely effective cleaning solvents and have high KB values. The KB value of d-limonene is a bit lower, but higher than that of petroleum-derived products (Table 1). It is not possible to perform the KB test on oxygenated compounds, so there is no listed value for methyl ethyl ketone (MEK) or acetone. STAND-ALONE PARTS WASHERS The typical tub-and-basin parts washers found in various mechanical maintenance shops can be adapted to use d-limonene as the washing solvent, and in most cases cleaning effectiveness will be increased and the time necessary for cleaning will be reduced. Typically the parts washer holds 18 to 20 gal of solvent and the solvent is pumped through a nozzle or brush into the tub for cleaning the parts. As the solvent is used and soil loading increases, the effectiveness of the solvent diminishes. The solvent is replaced at some predetermined point. When using d-limonene as the solvent, an in-line cartridge filter can be installed. A wound cotton filter preferentially absorbs the oils and greases, allowing the d-limonene to maintain its cleaning effectiveness for extended periods of time. The filters are changed out every 1 to 3 weeks, depending on workload, and approximately 1 gal of fresh d-limonene is added to the parts washer monthly to make up for dragout, which is Table 1 KB Values, Comparative Strength (Solvency) of Industrial Solvents (Higher Values  Higher Dissolving Power) Solvent Methylene Chloride Trichlorethylene Benzene Toluene Xylene Perchloroethylene d-limonene Mineral Spirits Naphtha Kerosene Stoddard Solvent MEK Acetone

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KB Value 136 129 107 105 98 92 67 37 34 34 33 N/A N/A

solution lost by adhesion to the parts being cleaned when they are removed from the bath. While a petroleum-based product may need replacement every 1 to 2 weeks, parts washers using d-limonene as the solvent have stayed in service for up to 3 years without a complete replacement of the solvent. The used filters are incinerated by a waste disposal company. Because these filters are the sole waste to dispose of, a company’s waste status may change to a small-quantity generator. AUTOMATIC PARTS WASHERS In much the same manner, d-limonene can be used in batch-loaded spray and immersion systems. In many of these systems, a water-soluble detergent is used as the cleaning agent. Typically, these detergents have an elevated pH and are used at temperatures well above 120°F. This combination of temperature and pH can have adverse effects on metal parts, such as corrosion. Depending on the temperature and cleaning system, appropriate controls may be required in consideration of the flash point. In addition, a hot caustic solution can potentially cause injuries to personnel. Water usage is also a concern. d-Limonene can be used in automatic parts washers in much the same manner as in stand-alone parts washers. By inserting a filter in-line on the system, the solvent can be used for extended periods of time. It can also be used effectively at room temperature, reducing costs associated with heating the solution. If water rinsing of the parts is necessary to remove residue of soil or cleaning agent, an emulsifier can be added to the d-limonene, which will allow the parts to be water-rinsed. CONVERSION OF VAPOR DEGREASERS The easiest conversion is the one most likely to be used. In some applications, dlimonene can be substituted directly into the line as a replacement solvent. One issue is that d-limonene will not work as a vapor-phase cleaner. The solution must be brought into direct contact with the parts being cleaned. Such conversions are suited to applications where rinsing to remove d-limonene is not required. The approach to conversion depends partly on the design of the original cleaning system. In systems where a conveyor carries parts into the vapor cleaning zone, it can be modified so that parts are lowered into the liquid instead of into the vapor. The dwell time for the parts in contact with the cleaner will be about the same, or a little less. The parts can be air-dried just as they are with a vapor cleaning solvent. The existing equipment can probably be used with only minor modifications, as long as the parts conveyor system can be fitted to allow direct contact with the solvent and the pumps and piping are checked for material compatibility with the solvent. Another method of conversion for a straight solvent substitution is to build a spray system into the cleaning line. This requires installing an enclosed booth with spray headers directed at the parts. The parts can then be air-dried. Some new equipment and line modifications are needed (the spraying time is determined by testing), but it is not an extensive change. In both these cases, the d-limonene is used neat, or undiluted. It is not water soluble; it is not rinsed; it will not cause rusting or oxidation of any materials. As the d-limonene is used, soil loading of oils and greases from the parts being cleaned will increase. Also, due to dragout, d-limonene will need to be periodically replaced to maintain the initial volume.

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Once the d-limonene has reached the limits of its effective lifetime, it must be disposed of properly. Ideally, waste d-limonene is incinerated. The d-limonene has a fuel value of 18,000 BTU/pound, and is valuable as a secondary fuel. Alternatively, the d-limonene can be distilled for reuse. d-Limonene blended with surfactants can be used in aqueous emulsion. In diluting the d-limonene with water, the cost of cleaning agent is reduced. Usually these solutions in use are 10 to 25% d-limonene/surfactant and the remainder is water. This requires more extensive equipment changes when converting from a vapor-type cleaning system. Because of the nature of the mixture of d-limonene, surfactants and water, the parts must be waterrinsed after cleaning to remove any residue that may remain on the parts. In general, most vapor cleaning equipment can be converted to a wash tank, as suggested above, for using the water emulsion. Again, the line must be altered to allow direct contact of the parts with the cleaning solution, but no additional equipment is necessary at the wash step. A new piece of equipment needs to be inserted after the wash tank to rinse the parts, usually a water-spray system. After rinsing, the parts can then be dried or go on for further processing. Periodic addition of cleaning agent is needed to replace the dragout, and filtration can be used to keep the solution usable. When this solution can no longer be used, again it is best to have it incinerated. A number of incinerators are equipped to deal with the presence of water in the solution. Because of the mixture created by the surfactants and water, this mixture cannot be distilled and regenerated. CIRCUIT BOARD CLEANING d-Limonene has been used for flux removal on circuit boards. The d-limonene can be substituted directly into the washing equipment and used either in the spray or flushing method. d-Limonene does not dry as quickly as the CFCs that have historically been used for this process. But, if a cleaning system designed for use with low-flash point solvents is being used, by following the d-limonene washing stage with a rinse of either acetone or isopropyl alcohol (IPA), drying times and residue levels are comparable. In such applications, low flash point cleaning systems must be adopted. High-purity/low-residue grades of dlimonene are being introduced for printed circuit board applications with some success, although cost of this material may be twice that of regular d-limonene. AEROSOL APPLICATIONS d-Limonene has been used as an aerosol for various cleaning applications, including electronics and parts cleaning. It can be sprayed on motor windings and allowed to dry by evaporation and sprayed on contacts, switches, and connections and wiped or let air-dry. It can also be sprayed into tight areas or onto bolts to facilitate removal. In many cases the residue left after evaporation does not interfere with the process. d-Limonene can also be combined with 10% IPA or acetone to promote quicker drying times and lower residue. When packaging d-limonene in an aerosol container, carbon dioxide at 15 to 20% can be used as the propellant. A water-rinsable product can be made by combining d-limonene in a 90/10 ratio of d-limonene to emulsifier. Various types of emulsifiers can be used for this, including ethoxylated alcohols, alkylamine dodecylbenzene sulfonates, coconut diethanolamides, and sodium xylene sulfonates. The emulsified product can then be sprayed on the surface to be cleaned and rinsed off with water. Again, carbon dioxide is the propellant of choice.

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It is also possible to make aqueous dilutions of the d-limonene/emulsifier blends or foaming products for aerosol packaging. These types of products require propane/isobutane propellant. d-Limonene can be combined in aerosol formulations to impart a pleasant citrus odor. Care must be taken to evaluate proper packaging materials. d-Limonene may cause swelling of gaskets and valves of some conventional dispensers. Viton and neoprene may be some of the best choices for aerosol stem gaskets (better than butyl or buna). Valves and cans should have an epoxy coating. Aerosol packagers and gasket suppliers should be consulted as to materials recommended for d-limonene. SAMPLE FORMULATIONS A few examples of possible formulations for various applications for critical cleaning are presented here. The emulsifier in the formulations can be a wide variety of products, or a blend of several different products, depending on the properties desired. A general-purpose concentrate for water dilution: 85 to 90% d-limonene 10 to 15% emulsifier A water dilutable/rinsable printing press cleaner: 90% d-limonene 7% emulsifier 3% tripropylene methyl glycol ether This product can be diluted to 70 to 75% water to clean soy- and water-based inks. The following guidelines can be used to formulate products for various applications. In this case the d-limonene and emulsifier or surfactant blend should be mixed in equal proportions. Application Engine degreaser Tar/asphalt removal Adhesive removal Marine vessel cleaner Industrial metal cleaner

% d-Limonene/Emulsifier

% Water

25 50 15 10 20

75 50 85 90 80

These ratios are meant as guidelines. CASE STUDIES The following case studies show a variety of uses and benefits that have been found from switching to d-limonene-based cleaning systems. Martin Marietta Astronautics has replaced 1,1,1-trichloroethane (TCA) and MEK with a terpene cleaner for hand-wiping operations.1 The terpene cleaner was selected after 16 months of extensive testing of citrus- and alkaline-based compounds. Workers prefer the

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citrus-based cleaner because it is more efficient. The terpene cleaner leaves less residue resulting in higher coating bond strength. Martin Marietta estimates the change has reduced toxic emissions by thousands of pounds per year. Research costs were $350,000 to find a suitable replacement for MEK and TCA. Estimated savings are $250,000/year. In a joint research effort, the U.S. EPA and APS Materials, Inc., have investigated the use of a limonene cleaner to replace TCA and methanol.2 APS Materials, Inc. is a metal-finishing company that plasma-coats parts for use in hostile environments. In the biomedical parts division, cobalt/molybdenum and titanium parts are coated with a porous titanium layer for use as orthopedic implants. APS Materials has converted to the terpene cleaner as a result of the investigation. Cleaning efficacy is excellent with a slight increase in bonding strength for the limonene-cleaned parts. Changing to the aqueous system required the addition of rinse and dry stations. The new system cost $1800 to install with annual operating expenses of $850. Net savings are $4800/year. GE Medical Systems of Waukesha, Wisconsin, is a manufacturer of medical diagnostic equipment. Spray cleaning (degreasing) of parts using TCA resulted in fugitive air emissions. GE Medical Systems eliminated fugitive TCA emissions by converting to a terpene cleaner.3 With TCA, 800 gal of solvent were purchased annually, all of which was lost to atmosphere. Because terpene cleaner is much less volatile, only 30 gal are purchased per year. In addition, terpene cleaner is recycled. No capital expenditure was required. Northern Precision Casting of Lake Geneva, Wisconsin, switched to a citrus-based solvent for cleaning the wax patterns used in making molds.4 Previously, they used TCA. TCA fugitive emissions amounted to 18,000 lb in 1988. The terpene solvent is water soluble and is discharged to a publicly owned treatment works. No capital costs were incurred for the change. Maintenance and operating costs are equivalent. The Marine Corps Air Station Naval Aviation Depot, Cherry Point, North Carolina, is responsible for the complete maintenance/rebuilding of naval aircraft. In 1990 the depot used 8000 gal of CFC-113 and 15,600 gal of 1,1,1-TCA. By the end of 1992, CFC-113 usage had been reduced to 500 gal annually and TCA usage had been cut to about 4800 gal annually. Terpene cleaners were used as one of a number of approaches.5 Approaches included soap bubbles for leak checks; aqueous power washers for electronics, motor, and engine shop use; terpene cleaners for hand wiping; steam cleaning or wet sodium bicarbonate blasting for soil and carbon removal; and plastic media blasting for paint removal. AT&T has reduced usage of CFC-113 by converting to a semiaqueous chemistry for cleaning surface mount assemblies.6 Parts are carried by conveyor into a power washer consisting of wash and rinse/dry modules. Low- and high-pressure sprays of a terpene cleaner are followed by nitrogen knives to reduce cleaning solution dragout and blanket the washer with an inert atmosphere to prevent fire. In the second module, the parts are rinsed with low-pressure, then high-pressure water sprays to remove the terpene cleaner. Rinsing is followed by water removal by air knives within the same module. Care must be taken in selecting materials of construction in the surface-mount components because the terpene cleaner swells some plastics and elastomers. AT&T has found that the new cleaning method is more economical than the previous CFC-113 method. In 1988, the Motorola Corporation had 29 flux-removal cleaning systems using 250,000 lb of CFC-113 annually. By August 1991, Motorola had eliminated CFC-113 usage. Many of the printed circuit board assemblies are now assembled using a no-clean flux. Assemblies that require cleaning now use terpenes and water.7 Benefits reported include cleaner assemblies, lower production downtime, and decreased cleaning cost. Cleaning costs are now about $8/h using the terpene/water vs. $38/h for CFC-113. Crown Equipment Corporation, New Bremen, Ohio, manufactures electric lift trucks and television antenna rotors. Parts cleaning involves mild steel, aluminum, cast iron, and © 2001 by CRC Press LLC

copper. In 1988 Crown used 208,000 lb of TCA in cold-cleaning (immersion) and vapor degreasing operations. Hand dipping now uses a water-based cleaner with rust inhibitor added for corrosion resistance, and 100% d-limonene spray cleaner has replaced TCA for hand-wiped parts.8 An alkaline aqueous immersion cleaner has replaced one degreaser (with inhibitor added for ferrous parts). The other degreaser was replaced with an aqueous power washer that uses heat, agitation, and forced air drying to produce clean parts. The payback period for capital expenses was 10 months. In 1989, Crown saved $100,000 in chemical costs. Crown Equipment has switched to water-based cleaning with no decrease in production. Employees prefer the water-based cleaner for hand dipping. The Bureau of Engraving, Industrial Division, manufactures printed circuit boards. In 1990, it decided to eliminate the use of methylene chloride and TCA, which were being used at the rate of 681,000 lb/year. Several changes in the manufacturing process were necessary to accomplish this goal, including the use of water-based and terpene-based cleaners. The Bureau of Engraving, Industrial Division, saves $250,000 annually in purchase cost and $20,000 in maintenance, energy, and disposal costs.9 OTHER USES Several other cleaning applications exist for d-limonene. d-Limonene is also used extensively in the asphalt industry for clean up and aggregate analysis, as an ink cleaner in printing operations, in the oil and gas fields for maintenance and as a well recovery solvent. It is used in hand cleaners and a wide variety of other general cleaning and household applications. In general, if an organic soil is to be removed, d-limonene may perform as well or better than other solvents. In addition to cleaning applications, d-limonene has been found to have thermodynamic properties that make it a very good heat transfer fluid, especially for cryogenic applications, below 100°C. It has also been used as a carrier solvent in paints and similar coatings, as well as adhesives. Studies have also shown d-limonene to be an effective pesticide and bactericide, although these are not yet approved uses. These are in addition to the best-known uses of d-limonene and other citrus derivatives as flavor and fragrance components. OTHER CONCERNS A “perfect” solvent probably does not exist, and d-limonene is no exception. d-Limonene can cause a certain amount of swelling of polymers, so the plastic materials used with a d-limonene system must be chosen with caution. Viton is the best seal to use in joints, and nylon-braided PVC seems to be the most acceptable material for hoses. As with any solvent, gloves should be worn when working directly in the solution. The best material for these is nitrile latex. CONCLUSION d-Limonene is an extremely effective and relatively safe cleaner and solvent for use in many industries. It can be used in a wide variety of applications and in most cases will perform better and longer than the classic solvents. Although it is not perfect, it is a good option to be considered when choosing a cleaning system or looking for an effective solvent replacement.

© 2001 by CRC Press LLC

REFERENCES 1. Dykema, K.J., and G.R. Larsen. 1993. Shifting the environmental paradigm at Martin Marietta Astronautics, Pollut. Prev. Rev., Spring, 205. 2. Brown, L.M., J. Springer, and M. Bower. 1992. Chemical substitution for 1,1-trichloroethane and methanol in an industrial cleaning operation, J. Haz. Mater., 29:179 –188. 3. Wisconsin Department of Natural Resources, Case Study: GE Medical Systems; Replacing 1,1,1Trichloroethane with Citrus-Based Solvents, PUBL-SW-168 92, Hazardous Waste Minimization Program (SW/3), Madison, WI. 4. Wisconsin Department of Natural Resources, Case Study: Northern Precision Casting; Replacing 1,1,1-Trichloroethane (TCA) with Citrus-Based Solvents, PUBL-SW-161 92, Hazardous Waste Minimization Program (SW/3), Madison, WI. 5. Fennell, M.B., and Roberts, J.M., Naval Aviation Depot: hazardous minimization—saving time, money , and the environment, in Proceedings of the Aerospace Symposium, Lake Buena Vista, FL, 1993, 39 –46. 6. Terpene Cleaning of Surface Mount Assemblies, Aqueous and Semi-Aqueous Alternatives for CFC-113 and Methyl Chloroform Cleaning of Printed Circuit Board Assemblies, EPA/400/ 1 –91/016, June 1991, 51–60. 7. Terpene Cleaning of Printed Circuit Board Assemblies, Aqueous and Semi-Aqueous Alternatives for CFC-113 and Methyl Chloroform Cleaning of Printed Circuit Board Assemblies, EPA/400/ 1 –91/016, June 1991, 61–62. 8. Kohler, K., and A. Sasson, Case studies: multi-industry success stories to reduce TCA use in Ohio, Pollut. Prev. Rev., Autumn, 407 –409, 1993. 9. Currie, W.T., Vice President, Facilities and Environmental Affairs, Bureau of Engraving, Inc., Industrial Division, 500 South Fourth Street, Minneapolis, MN, 55415, MnTAP, 1993 Governor’s Awards for Excellence in Pollution Prevention.

© 2001 by CRC Press LLC

SECTION 2

Cleaning Systems

© 2001 by CRC Press LLC

CHAPTER 2.1

Cleaning Equipment Overview Barbara Kanegsberg

CONTENTS Why Do We Need Cleaning Equipment? Performance The Chicken or the Egg? Cleaning Action Drying Action Materials Compatibility Fixturing, Parts Handling, Automation Process Efficiency, Process Costs, Environmental and Safety Concerns Plant Facilities, Floor Space, Growth Employee Involvement, Employee Education In-House Equipment Design Ultrasonic Cleaning Other Cleaning Systems Spray Cleaning Systems Reel-to-Reel or Continuous Web Cleaning Centrifugal Cleaner Spinners Microclusters Industrial Cleaners/Cabinet Washers, Dishwashers, Spray Cabinets Semiaqueous and Cosolvent Systems Wet Benches Impingement Cleaning Conclusion References WHY DO WE NEED CLEANING EQUIPMENT? Anyone tasked with choosing new cleaning equipment asks this question, at least facetiously. Cleaning equipment often requires a substantial capital investment, and process change may be accompanied by comments and input from company management, the © 2001 by CRC Press LLC

insurance company, the fire department, governmental regulatory agencies, the company facilities/maintenance group, the in-house environmental health and safety department, and last but not least from the technicians and assemblers who have to watch over the process. So why do we need cleaning equipment? There are a number of reasons, the most important of which should be maximizing cleaning performance. Other reasons for choosing a particular type of equipment include decreasing process time, protecting the environment, and protecting the individual worker. In any cleaning system, it is important to consider both the cleaning agent and the cleaning action. A cleaning agent may have very high solvency and may be effective in dissolving the soil. However, it is important to have appropriate cleaning action to assure that the cleaning and rinsing agents reach all surfaces and to assure that the soil is carried away from the surface. As a chemist, this author’s first thought in terms of cleaning action is plopping a magnetic stirrer into a beaker of cleaning agent, and cranking up the rheostat. This is not practical for large-scale processes. Similarly, pilot or specialized operations may involve hand-wiping parts with a soft cloth, hand-dipping each part, or scrubbing individual portions with a cotton swab. For more systematic cleaning, ultrasonics, megasonics, spray in air, spray under immersion, and turbulation all add effectiveness to the process. In addition, weirs and filters1 prolong the life of the cleaning agent in general industrial processes. In high-precision applications, filtration with in-line particle monitoring may be required for adequate contamination control. PERFORMANCE Setting aside for a moment worker safety, environmental regulatory issues, and minimization of cleaning agent loss (notice, this says “for a moment,” not permanently), performance involves matching the cleaning agent to the cleaning equipment, cleaning action, drying action, materials compatibility, and fixturing (including sample handling and automation). In addition, one must take into account a host of additional site-specific considerations, including process costs, safety, and regulatory issues. The Chicken or the Egg? Integrating the cleaning system, the drying system, and overall sample handling can be crucial in maintaining process control and process efficiency. For this reason it is important to match the cleaning agent or agents with the cleaning equipment. The question is often asked: Should I first look at the cleaning agent or at the cleaning equipment? Cleaning agent manufacturers typically say: Look first at the cleaning agent. Cleaning equipment manufacturers, on the other hand, often say: Don’t worry, just buy the cleaning system, you can use many different sorts of cleaning agents in it. In the opinion of this author the most productive approach is to consider both factors at the same time. Looking at the cleaning agent without considering the cleaning system can lead one to rule out what could be the optimal approach to cleaning. Pragmatic experience leads this author to conclude that attempting to emulate a sophisticated cleaning system at the benchtop level, by cleaning coupons or scrap parts in beakers, is all too often not informative and can lead the user to discard as unworkable what could be an effective, economical cleaning process. At the same time, some engineers have been known to contact cleaning equipment manufacturers, demand cleaning equipment be fabricated to exacting specifications, and then, only after the equipment has been built and delivered, inquire about the © 2001 by CRC Press LLC

appropriate cleaning solution to be used. Such scenarios have led one colleague to abandon a career as manager of an applications laboratory.

Cleaning Action If soils are considered in a general sense as “matter out of place,” then cleaning action is inherently important to assist in: • • • • • •

Bringing the cleaning agent in contact with the parts to be cleaned Assisting in solubilization Assisting in removal of particles Keeping the soil away from the parts or components Maintaining cleanliness of the cleaning solution In rinsing, assisting in removal of cleaning agent and/or continuation of soil removal

Cleaning action is often discussed in terms of aqueous systems, as a means of assisting in emulsifying soils. However, cleaning action cannot be ignored in solvent, semiaqueous, and cosolvent systems.

Drying Action Drying is considered a separate topic and is discussed in detail elsewhere. In considering possible drying techniques, it must be emphasized that the drying technique must be integrated with the cleaning system. Again, drying is most often considered in terms of aqueous systems. Often, in solvent systems drying is assumed to be an inherent part of the process. However, particularly with complex, ornate components, the capability of the system to remove residual solvent rapidly and effectively without damage to the component must be considered.

Materials Compatibility Typically, cleaning agent compatibility is considered in static systems, in beakers that are perhaps heated. However, as is pointed out in Chapter 3.3 covering compatibility by E. Eichinger, the interaction of cleaning agent(s) with materials of construction can be markedly enhanced by cleaning action. Ultrasonic cleaning can produce a sonochemistry effect that may both enhance cleaning and adversely impact materials of construction (Chapter 2.2 by J. Fuchs). Such effects are reportedly particularly pronounced with some aqueous surfactant packages. It should be pointed out that other types of cleaning action, such as forceful sprays, also have the potential to damage parts by deformation and erosion. Such erosion may not be visible, but may show up in altered tolerances or gravimetric changes. For example, in testing alternative defluxing systems, electronics component simulators were designed with brass coupons used to simulate components raised to varying heights.2 Effectiveness of removal of rosin flux from under the components was determined gravimetrically. In a few cases, over 100% apparent removal of flux was observed; this was determined to be due to erosion of the coupons. © 2001 by CRC Press LLC

Fixturing, Parts Handling, Automation Choosing the appropriate fixturing and sample handing techniques are important in: • Maximizing contact with the cleaning agent • Draining and removal of the cleaning agent • Avoiding parts deformation or damage For example, even in solvent-based systems, parts rotation is often used in conjunction with ultrasonic cleaning to boost cleaning effectiveness. Inattention to fixturing and parts handling can result in inadequate cleaning and drying, as well as in parts damage. It must be continuously emphasized that the use of fixture and appropriate fixture design is important for all cleaning systems, including solvent and aqueous systems. Some of the factors involved in appropriate fixture design are discussed in other chapters of this section and in Chapter 4.2 by Callahan, and are summarized in the overview of drying (Chapter 2.18 by Kanegsberg). In addition, one must consider several other factors. Long-term compatibility of materials of construction with the cleaning agent and with the cleaning technique (ultrasonics, spray systems) are essential. If worn or damaged fixtures corrode or oxidize or shed particles, soil can be inadvertently introduced during the process. Automation of the process can improve performance and consistency, minimize loss of cleaning agent, minimize worker exposure to chemicals, and may have desirable environmental impacts.2,3 The concept of automation is a matter of degree. Certainly, even a process consisting of a simple open-top degreaser with a manual hoist could be said to be more automated than one where assemblers hand-scrub each part or component individually. However, one generally thinks of automation as being a bit more sophisticated. In general, parts handling is either batch or in-line. In classic in-line processes, samples are carried along a conveyor belt through various cleaning and rinsing solutions; in such cases, drying is typically through air or nitrogen nozzles. In batch-automated cleaning, overhead hoists or robotics are used to carry parts from to various cleaning, rinsing, and drying chambers. A batch automation system generally consists of a mechanical superstructure, a drive system, a control package, and an operator interface.3 Engineers accustomed to in-line processes may be reluctant to try batch cleaning, in part, because they may associate batch cleaning with inconsistent, nonautomated processes. Batch cleaning actually can provide more control and more flexibility than in-line cleaning because with in-line cleaning one can either vary the length of the process chambers (and this is predetermined during equipment design) or the speed of the conveyor belt. Each step of the process is therefore inherently tied to the next. Because the duration of each phase of the process can be varied separately, batch automation can actually provide much more process flexibility. By using several robotic arms, loads can be processed sequentially to maintain process flow (i.e., one load of parts can be at the drying stage while another is in the wash stage). Cleaning is also typically automated in more-sophisticated single-chamber tanks. In such batch processes, the part remains in the chamber for all cleaning steps. Such processes can be very flexible, but, since only one load can be processed per chamber, throughput of parts is limited by the size of the chamber and by the number of chambers. An automated system may or may not provide a more rapid cycle time. Often, assemblers are accustomed to speeding up the process, by quickly submerging and then removing the part to be cleaned. However, there are benefits to automation, such as:

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• • • • • •

Improved process control Improved cleaning consistency Regulatory, safety, or quality compliance Lower consumption of cleaning agent Lower solvent emissions Lower exposure of employees to potentially toxic cleaning agents

In designing the automation process, it is important to consider not only the immediate cleaning process but also those before and after. In a number of manufacturing facilities, the cleaning process itself is automated, but other aspects of assembly are carried out by hand. Or, the fixturing and sample handling for the cleaning process may not mesh well with the surrounding processes. One then observes technicians spending appreciable time re-racking and re-fixturing samples to go from one piece of equipment to the next. A bit of advance planning in designing the entire build process can alleviate the problem. Even in automation, the human factor is critical. Automated systems can reduce exposure of employees to cleaning agents in terms of both inhalation and skin adsorption. There are other safety issues. One of my colleagues consistently showed up at meetings with a bump on his head with ever more interesting stories of attempts to install an overhead hoist. The systems must be designed to prevent employee injury during operation, and they are best designed and installed by experts in the field. The other aspect of the human factor is employee education and training. All the thoughtful planning and programming can be undone if the technicians accelerate the process speed to undesirable levels. This is a real problem, particularly where costs and production pressure have built up.

PROCESS EFFICIENCY, PROCESS COSTS, ENVIRONMENTAL AND SAFETY CONCERNS Overall process costs and efficiency are very difficult to determine. It may be necessary to take educated guesses. In terms of costs, one must consider such factors as initial capital costs, costs of disposables, cleaning agents costs (concentrated vs. effective dilution), bath life, loss of cleaning agent through evaporation/dragout/dragin, disposal costs for the spent cleaning agent, costs of safety and environmental controls, regulatory costs, energy efficiency, and rework costs. Process costs are site specific. While efficiency claims are prevalent, hard data are not. Further, studies of process costs, whether by governmental agencies or by cleaning agent or cleaning equipment suppliers, are inevitably influenced by the economic interests and political agendas of those sponsoring the studies. All studies have validity; all must be looked at in context. A summary of factors in process costs has been reported;4 studies are ongoing. Environmental requirements often inherently determine the menu of available process options in a given area. In addition, one must consider the impact not only of the cleaning agents but also of the process on worker safety. The same increased cleaning forces that boost cleaning efficiency can magnify materials compatibility concerns—including not only the materials that make up the part to be cleaned but also the materials that make up the workers in the area. Safety has to be considered in terms of the chemical, the process, and the interaction with other processes conducted in the workplace.

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Plant Facilities, Floor Space, Growth In addition to all the other considerations, one must not forget the physical limitations of the production plant, anticipated growth, and flexibility. In evaluating cleaning systems, it is important to consider utilities (e.g., water, electricity, nitrogen lines). In addition, one must consider floor space. It is important to look at overall equipment dimensions. Many are careful to look at length and width; fewer consider height, and fewer still consider the additional three-dimensional space needed for the robotic arm. One must also consider wall and door dimensions relative to cleaning equipment as well as total weight. Placement is important in terms of maintaining and cleaning the cleaning equipment itself. Based on a number of process remodeling anecdotes (e.g., “skylights” to accommodate the unanticipated height of robotic arms), which are amusing only in retrospect, the most realistic advice is to involve the facilities/maintenance department at the beginning of the anticipated process change. One must also consider growth and flexibility. Purchasing a marginally–sized cleaning system is a false economy. A cleaning system that just barely manages current throughput will not be effective if business improves. All estimates of process throughput with a given system, particularly vendor-generated estimates, must be critically examined to see if they are overly optimistic. In addition, while it is unrealistic to expect a cleaning system to handle all possible chemistries, one should avoid a cleaning system designed for only a single cleaning chemistry. The situation may change, and it may be necessary to use another cleaning agent. Possible changes include: • • • • • • •

Product design Material modification in the component Cleanliness standards modification Cost of cleaning agent Composition of cleaning agent Availability of cleaning agent Regulatory changes

If several cleaning systems are essentially equivalent in performance, in general, it is better to select the more flexible system.

Employee Involvement, Employee Education The newer, sophisticated automated cleaning systems provide very thorough, consistent cleaning. However, employee education (as opposed to attempted rote training) is necessary to maintain process quality and to assure employee safety. Equipment maintenance is also more complex with sophisticated equipment.4 Sophisticated computer programming or at least reasoned following of prearranged steps can degenerate into a bravado of desperate button-pushing by a terrified employee—this can be disastrous. Initial education when the equipment is installed is helpful; follow-up training at intervals is the key to ongoing success. In addition, in this author’s observation, the dedicated and resourceful employee intent on cutting corners and speeding up the process can override even the most-sophisticated interlock system. Ongoing monitoring of employee performance and behavior is essential.

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In-House Equipment Design In the course of evaluating sophisticated but deceptively simple equipment, many engineers, in an attempt to control costs and/or achieve customization, are tempted to design the equipment themselves. The general advice in such cases is that if the equipment is similar in design to the product being manufactured, there is a chance of success. Occasionally, in-house equipment design can be very successful (see Chapter 2.8 by Petrulio). In most cases, however, the advice is: Don’t design your own equipment. Some factors involve: • Experience: You don’t have to be a rocket scientist to design a quality vapor degreaser, but you do need practical experience in the area. Most rocket scientists probably do not have this experience. • Cost: In looking at a half-million dollar box, you may be tempted to design one. However, you must consider your time and research effort. Each commercially produced cleaning system may be cost-effective for the manufacturer to produce, but the engineering effort to produce the initial model is typically significant. • Product support: If the equipment breaks, who is going to fix it? If you designed it, and if you break it, you have to fix it. • Permitting, regulatory issues: Often, regulators and fire inspectors are more comfortable with a standard equipment design, which may be certified to meet certain design standards.

ULTRASONIC CLEANING Ultrasonic and megasonic cleaning provide manufacturers very powerful cleaning techniques. The question why so many chapters are needed in this book arises, particularly because manufacturers may consider the choice of ultrasonic equipment to be generic. This is probably, in part, due to the large number of variables to consider. There are several reasons ultrasonic cleaning should be emphasized. For one thing, ultrasonic cleaning is useful over a very wide range of applications. It is a technique that is nearly certain to become increasingly important both to extend the range of applications amenable to aqueous cleaning and to clean products with increasing miniaturization, complexity, and close tolerances. In addition, despite the advances in theoretical understanding of ultrasonics, much needs to be learned about the mechanism of action. Ultrasonic techniques are controversial; opinions differ markedly and are also influenced by the expert’s view of the relative efficacy of aqueous vs. solvent cleaning. Therefore, some of the statements in various chapters may be contradictory. If this makes readers somewhat uneasy, the unease reflects the fact that ultrasonic and megasonic cleaning is an exciting, dynamic, and sometimes contentious area of technology. The chapters provide a snapshot of at least a portion of current understanding of ultrasonic and megasonic techniques. New techniques are constantly under development. For example, processes have been proposed that do not involve added chemistry or that use water alone.5 Very specialized ultrasonic techniques involving no cleaning agent, only contact between the ultrasonics source and the component to be cleaned, are also possible, in specialized situations.

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Lack of standardization is a real issue. One reason that ultrasonic systems are too often thought of as generic is the absence of any standardized way of comparing performance of various systems. There is always a concern with transducer degradation over time as well as a lingering questions of comparisons of systems produced by different vendors or even of two identical models produced by a given equipment supplier. Detection and quantitation of ultrasonic systems are very difficult. Holding an aluminum foil coupon in the tank and then inspecting it for the characteristic orange peel design that indicates reasonable cavitation (and hoping not to see erosion of the foil, indicating a hot spot or a potentially undesirable cleaning solution) can provide hours of play value. However, results are not quantitative. The slurry wand provides a simple and economically accessible means of determining that cavitation is occurring, as well as relative frequency, hot spots, and dead zones (see Chapter 2.4 by Pedzy). Quantitative probes to estimate ultrasonic performance within the tank have been proposed. One such probe is coming into favor. The meter can be used to confirm and monitor cavitation energy and frequency within the tank.6 Meters of this type would not necessarily be cost-effective for smaller, less critical applications. However, the meter does show promise for monitoring behavior of ultrasonic tanks as well as determining relative efficiency of cavitation of various cleaning agents. In one study, efficacy of wafer cleaning was related to cavitation energy as quantitated with the quantitative meter.7 OTHER CLEANING SYSTEMS Cleaning systems undergo constant changes, refinement, and development. This book does not cover all cleaning systems available. Instead, some key cleaning techniques have been highlighted. It is anticipated that from the examples, principles, and reasoning processes, the practicing engineer will be able to extrapolate to evaluate other cleaning systems. A few additional cleaning systems are summarized. Spray Cleaning Systems* The spray cleaning process has been in use for many years with high reliability, small mechanical, optics, microelectronics, and other extremely sensitive and demanding components. Most often, it is used in those areas of production, such as military, aerospace, and medical applications, where components and subassemblies must be totally free of all organic and inorganic contaminants. Spray systems have successfully met exacting cleaning requirements for delicate wire bonds without damage, to clean both blind and throughholes of extremely small diameters (e.g., 0.0005), and the removal of both organic and inorganic contaminants. Such systems utilize a gentle agitation via the fine, usually venturi type, spray and the molecular weight of the chosen solvent. Often in this type of process, when more traditional volatile of solvents are utilized, there is little or no waste product and virtually no effluent to handle. The solvents are generally used at a rate of 0.2 fluid oz/s, a minimal amount at best, and because the cleaning cycles tend to be very short in duration, an average of 5 s of solvent spray, the solvent is evaporated when these types of systems are properly used. This provides several advantages, the most important of which may be the potential to add carbon absorption in gas phase to minimize solvent emissions. This fume and solvent extraction process can be con*The description of this technology was contributed by Rebecca Overton, who has had many years of experience in this technology, most recently at Cobehn.

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tained in a small unvented chamber, thereby eliminating the exhausting of waste to the atmosphere through existing fume exhausting systems, or the expense of modifying the physical plant. The spraying process can totally replace all other processes only in small R&D type facilities and is generally not intended for high-production-requirement facilities. Where requirements are less than 100 components per day it is often the ideal solution to an otherwise highly expensive problem.

Reel-to-Reel or Continuous Web Cleaning Reel-to-reel systems are used for such diverse processes as cleaning metal wire, strips of lead frames, and motion picture film. In such systems, it is helpful to picture giant spools of thread (with the thread being made of the material to be cleaned). The part is a continuous filament running from one spool, through a cleaning solution, through a drier, and then onto another spool. Ultrasonics or other agitation may assist in cleaning; rollers or brushes may assist in removing excess cleaning agent prior to drying. Such cleaning systems can be aqueous or solvent-based. Often, 1,1,1-trichloroethane or another chlorinated solvent was used. Where chlorinated solvents are used, manufacturers initially found it difficult to comply with the Halogenated Solvents NESHAP (National Emissions Standard for Hazardous Air Pollutants). The initial NESHAP simply did not consider specific issues associated with reel-to-reel cleaning. National regulations specifically covering such applications are being developed.8 Motion picture film cleaning is a very specialized, fascinating challenge in reel-to-reel cleaning, involving cleaning performance, speed, materials compatibility, and long-term storage. Motion picture film has high aesthetic, technical, and informational value. The substrate is complex, composed of multiple layers of plastics and other synthetics. Nearly all current film-cleaning equipment is designed for use with solvents. Classically, motion picture film was cleaned with TCA. Because of the ODP production phaseout, most film is currently cleaned with perchloroethylene. Regulatory and technical issues have generated great interest in finding other cleaning alternatives. The delicacy and complexity of materials of construction, and long-term storage issues make acceptance of appropriate cleaning systems for motion pictures a difficult task. A list of solvent options for film cleaning which have acceptable physical properties and maintain acceptable image stability9 is updated periodically. Water-based cleaning in redesigned equipment has been tested, but acceptance has been limited at best because of problems with materials compatibility and lengthy, possibly incomplete, drying. Alternative cleaning systems, some with redesigned equipment, have been introduced and use such solvents as isopropyl alcohol, hydro-treated naphtha, and the newer engineered fluorinated solvents.10 One promising system was discussed at a recent technical conference. It uses HFE 7200 (packaged for film cleaning as 8200) in Lipsner-Smith cleaning equipment which has been redesigned with additional, powerful ultrasonic transducers to boost cleaning power of this very mild solvent along with more effective solvent containment. Results of initial testing by an end user were very encouraging.11 After over a decade of effort, a cleaning agent matching all of the desirable properties of TCA has not yet been found. Many end users would prefer a cleaning agent that could be readily adapted to existing cleaning equipment. In addition, there are ongoing issues of whether video or some other format will totally replace motion picture film. If this were to © 2001 by CRC Press LLC

happen, cleaning issues could become a moot point. However, motion picture film remains a medium of choice, at least for master copies. Centrifugal Cleaner In centrifugal systems, parts are cleaned in a single centrifuge chamber filled with cleaning agent. Cleaning at low centrifugal force promotes agitation due to Coreolis mixing. Initial costs may be high, and throughput is often lower than conventional methods, but equipment is often designed for use with many different cleaning agents. Centrifugal cleaners may use water- or solvent-based cleaning agents. Spinners Spinners are used in cleaning and surface preparation of wafers and optics. The component is placed on a rapidly moving turntable and various cleaning and rinsing solutions are applied to the surface to be cleaned. Sequential cleaning with various cleaning agents as well as rinsing and spot-free drying is carried out in place. These systems are typically designed to be used in clean rooms where contamination is an issue. Avoiding particles, residue from additive packages, and excess foaming are considerations. Spinners may use water and/or solvent-based cleaning agents within a single process. Microclusters Microcluster cleaning is a specialized line-of-sight technique that has been demonstrated to remove submicron particles. Microclusters are produced by atomizing a conducting liquid (typically a mixture of n-methyl pyrollidone and water), which is then exposed to high electric fields. The microcluster dimensions are in the range of that of the contaminants to be removed. The technique is proposed for wafer cleaning, but may have other high-end applications.12 Industrial Cleaners/Cabinet Washers, Dishwashers, Spray Cabinets This category includes a vast array of cleaning systems most often associated with general industrial cleaning.13 Except where indicated, they are primarily used with aqueous cleaning agents. Some resemble consumer-variety dishwashers, perhaps with stainless steel interiors. Sink-on-a-drum cleaning systems are used in general metals-cleaning applications with mineral spirits, or, particularly in areas that are heavily regulated because of poor air quality, with an aqueous-based cleaning agent or a VOC-exempt solvent. They can be constructed of metal or plastic. Some solvent-based systems have been adapted to provide onboard recycling of the cleaning agent (Chapter 2.7 by Skelly). Cabinet washers are typically tall, cylindrical systems. The parts (engines, etc.) are typically placed on a turntable and sprayed with hot surfactant solution. In spray cabinets, the part to be cleaned is placed inside a boxlike container and sprayed typically with an aqueous-based cleaning chemistry. Spray cabinets may be manual or automated. The manual models resemble a glove box. Automated models, such as are used by the automotive industry, can be quite large and sophisticated and may include in-line (conveyor belt) or overhead robotics, with automated monitoring of the cleaning solution. Some aqueous systems have even been fitted with high-pressure spray for tube-cleaning.14 © 2001 by CRC Press LLC

As with other cleaning systems, the cleaning chemistry, temperature, force of cleaning action, filtration, rinsing, and, in some cases, drying all impact cleaning quality. It is important to match the equipment to the application. In many general metals-cleaning applications, a single cleaning tank with either solvent or aqueous-based material is sufficient. The features, capabilities, prices, and quality of construction vary by orders of magnitude. Particularly because such systems will be subjected to extremes of temperature and of cleaning chemistry, purchasing a well-designed system of high quality can result in longterm benefits. The user must consider initial costs, ongoing costs (including disposables such as filters), and labor and rework costs. Some of the more-sophisticated systems require a high level of ongoing maintenance.4

Semiaqueous and Cosolvent Systems Semiaqueous and cosolvent systems are related. In semiaqueous systems, a high-boiling solvent-based cleaning agent is used for primary removal of soils. This step is followed by several rinsing and drying steps. Cosolvent cleaning can imply two or more solvents, used in a single tank or sequentially. Technically, any solvent blend or azeotrope could be considered cosolvent cleaning. This discussion, however, considers cosolvent cleaning to be a process in which a high-boiling solvent blend is rinsed in a second solvent, solvent blend, or azeotrope. In cosolvent cleaning, typically a lower-boiling solvent is chosen to serve as a rinsing and vapor-phase drying agent. In some cases, a supplier may offer two very similar blends based on hydrocarbon, d-limonene, or ester, which differ in subtle changes in the additive packages to make them more readily removable with water or with solvent. Other products are based on complex, modified alcohols. Many high-boiling solvent blends are considered competition sensitive by the manufacturers and, therefore, unfortunately are shrouded in mystery. This, of course, makes rational process design a challenge. In such situations, the end user would be well advised to set up comprehensive product support arrangements. While water is the rinsing agent in semiaqueous cleaning, the rinse/vapor phase/drying agent can be any of a number of lower-boiling solvents, such as HFC, HFE, isopropyl alcohol, even isopropyl alcohol/cyclohexane azeotrope (which would, one supposes, constitute a co-cosolvent process). Both semiaqueous and cosolvent systems have potential advantages and drawbacks. In both cases, because compounds and mixtures with widely different solvency characteristics are used for cleaning and rinsing (we are considering rinse water as a cleaning agent), the rinse phase can in a sense be considered part of the cleaning phase. Both types of systems can extend the range of soils that can be removed; both can allow the use of high-boiling cleaning agents that may themselves leave residue on the part and/or may not dry sufficiently rapidly. Both types of systems can use agitation, including ultrasonic cleaning, to improve performance. In some cases, the cleaning agent is designed to form an emulsion with the rinsing agent, either for initial cleaning or as final rinsing. The emulsion can be stable or transient (i.e., the emulsion exists only during agitation). Alternatively, the cleaning and rinsing agents may be miscible. For stable emulsions as well as with miscible cleaning and rinsing agents, the issues of recovering the cleaning and rinsing agents and of waste stream management become more complex. Depending on the situation, reverse osmosis may be needed for recovery. Multiple filtration of the waste stream is typically needed in semiaqueous systems. © 2001 by CRC Press LLC

Initially, semiaqueous and cosolvent systems were widely and enthusiatically adopted. However, these systems are relatively complex and sophisticated. Like other newer cleaning systems, they require maintenance, employee education, and process monitoring. In both cases, proper fixturing of the product is crucial to assure optimal cleaning and to avoid excessive carryover of cleaning agent into the rinse tank. This author recalls implementing a semiaqueous cleaning system with in-line automation that initially performed very acceptably. Then came the phone call: the system does not work; the cleaning agent is gone; all of the filters for the wastewater are “dead.” The reason turned out to be cultural and historical, and involved the legendary third shift. As it happened, the facility was located in a town that held monthly auto racing. Over the years, the third shift had become accustomed to cleaning their carburetors in the vapor degreaser. With vapor degreasing, because final cleaning is in freshly distilled solvent, the extraneous soils did not present a problem. A semiaqueous system is not as forgiving. When carburetors were placed on the conveyor belt of the semiaqueous system, there was carryover of excessive amounts of cleaning agent into the rinse tank, resulting in system failure. The scope of cleaning allowed in semiaqueous, cosolvent, or any other cleaning system is the prerogative of management. The point is that semiaqueous and cosolvent systems are not forgiving of mediocre process control. It should also be noted that the physical properties, including flammability, have to be carefully considered in designing a system. A semiaqueous cleaning system requires multiple rinse tanks (for in-line systems, a fairly long portion of the conveyor belt should be devoted to rinsing). Depending on the product and the next step in the process, drying will be required. Even though the primary cleaning occurs below the boiling point of the cleaning agent, a typical cosolvent system is designed more like a multitank degreaser, to allow for vapor-phase rinsing and drying. In some cases, vapor degreasers have been converted to cosolvent systems. However, it should be remembered that if a low-flash-point solvent is employed, a standard vapor degreaser would not be suitable and would pose a potential fire hazard. Wet Benches Biomedical devices, optics, semiconductors, and microelectronics are often processed and cleaned in wet benches. This is a broad, generic term for a series of cleaning tanks that may contain aqueous, semiaqueous, or solvent-based cleaners. In addition, etching with strong acids and bases may take place. While such processes might better be considered as surface modification, they are certainly related to cleaning. It should be noted that appropriate process controls, such as titrators, are desirable and may be crucial to maintain process control. In addition, with strong acids such as hydrofluoric acid, process automation, vapor monitors, and controlled bath neutralization may be needed for adequate employee protection. Impingement Cleaning Impingement cleaning covers an array of processes, including line-of-sight high-force solvent and aqueous sprays, CO2 snow, and dry bicarbonate. Impingement cleaning has been used in critical applications for many years. In metal finishing and deburring, a variety of nonsolvent, nonaqueous media are used. The equipment manufacturer often considers this equipment as separate from cleaning, but there is overlap. For example, when faced with the choice of using chemical stripping © 2001 by CRC Press LLC

or mechanical stripping of paint, a facility that repairs pumps chose media blast for a variety of safety, environmental, and economic considerations.15 Increasingly, other forms of impingement cleaning are being adapted from general cleaning to meet critical cleaning and surface finishing requirements. Examples include plastic pellets, walnut shells, sand, diamond dust, nails, aluminum oxide, garnet, small nails—the list goes on and on. The impingement or rubbing action of the media itself may be achieved with blast, agitation, centrifugation, or ultrasonics, and cleaning may be dry or in an aqueous or aqueous-surfactant media.16 Although some forms of impingement cleaning are becoming widely adopted, other media cleaning may become more important in the future for high-precision, critical applications. For example, selective removal of high-value coatings in optics and wafer fabrication bears some relationship to selective removal of paint. As with other cleaning approaches, there will be provisos in expanding media blast. Speed, potential part damage, and preventing residual blast media from recontaminating the part are among the considerations. CONCLUSION Cleaning applications and requirements are exceedingly diverse. To meet these needs, an array of cleaning agents and cleaning equipment has been developed. This chapter summarizes a few aspects of cleaning systems; other chapters discuss the specifics of cleaning systems in much greater detail. In evaluating various cleaning processes, the reader must remember that, inherently, cleaning involves a melding of cleaning agent, cleaning action, and overall process equipment. In developing processes, some argue that the cleaning agent should be selected prior to the cleaning equipment, or vice versa. There is some validity to either approach, as long as it moves us along the path to ADS (actually doing something). However, given the complexity of performance, economics, and environmental requirements, it is often more productive to consider the cleaning agent and the cleaning system in parallel, then making the choices. In addition, even though some cleaning processes may not be widely used in a particular industry, the reader is urged to at least skim through all of the available process choices. By looking at processes creatively and in a more encompassing manner, it may be possible to adapt processes from one area of manufacturing to another. This kind of adaptation fosters overall progress and, to the thoughtful manufacturing company, specific competitive advantage. REFERENCES 1. E. Kanegsberg, Liquid filtration in critical cleaning, A2C2 Mag., April, 2000. 2. B. Kanegsberg, Successful Cleaning/Assembly Processes for Small to Medium Electronics Manufacturers, half-day workshop, Nepcon West ‘97, Anaheim, CA, February 27, 1997. 3. J. Aries, Moves toward automation, Parts Cleaning Mag., February, 1998. 4. B. Kanegsberg and C. LeBlanc, The cost of process conversion, CleanTech ‘99, Rosemont, IL, May, 1999. 5. J. Baker and J. Durkee, Rethinking cleaning processes III, A2C2 Mag., 3, 39 –40, 2000. 6. L. Azar, ppb Inc., personal communication.

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7. Y. Wu, C. Franklin, M. Bran, and B. Fraser, Acoustic Property Characterization of a Single Wafer Megasonic Cleaning, presentation and proceedings, Electrochemical Society 196th Meeting, Honolulu, HI, October, 1999. 8. Fed. Regist., 64, (232), 67793–67803; 1999 DOCID:fr03de99-30, amendments to the National Emission Standards for Hazardous Air Pollutants: Halogenated Solvent Cleaning, Docket A-92-39. Available at http://www.epa.gov/fedrgstr/EPA-AIR/1999/December/Day03/a31356.htm. 9. Eastman Kodak, Film Cleaning Solvents, Film Cleaning Solvent Options, available at http://www.kodak.com/US/en/motion/hse/solvent.jhtml?id=0.1.4.5.16.6&lc=en. 10. L. Smith, Clean Technology News, available at http://www.rti-us.com/newsletters/ clnthnws.html. 11. J. Banks, An Alternative to Chlorinated Solvents for Deep Film Cleaning in Telecine Suites, Archives, and Film Laboratories, presentation at the 142nd SMPTE Technical Conference and Exhibition, Pasadena, CA, October 20, 2000. 12. J. Perel, C. Sujo, and J.F. Mahoney, Microclusters make an impact on wafer cleaning, Precision Cleaning Mag., 7, 18 –24, 1999. 13. J.B. Durkee, The Parts Cleaning Handbook, Gardner Publishers, Inc., Cincinnati, OH, 1994. 14. S.J. Adam, Aqueous Tube Cleaning Advances at McDonnell Douglas Aerospace, in Proceedings of the 2nd Aerospace Environmental Technology Conference, Huntsville, AL, August. 6 –8, NASA Conference Publication 3349, 1996, 145. 15. P. Maluso and B. Kanegsberg, Hydrostatic Pump Rebuild: Implementing Aqueous, Steam and Solvent Free Processes, in Proc. Tenth Annual International Workshop on Solvent Substitution and the Elimination of Toxic Substances and Emissions, Scottsdale, AZ, September 13 –16, 1999. 16. E. Kanegsberg and B. Kanegsberg, Cleaning by abrasive impact, A2C2 Mag., May 2000.

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CHAPTER 2.2

The Fundamental Theory and Application of Ultrasonics for Cleaning F. John Fuchs

CONTENTS Introduction What Is Ultrasonics? The Theory of Sound Waves Sound Wave Generation The Nature of Sound Waves Benefits of Ultrasonics in the Cleaning and Rinsing Processes Ultrasonics Speeds Cleaning by Dissolution Ultrasonic Activity Displaces Particles Complex Contaminants A Superior Process Ultrasonic Equipment Ultrasonic Generator Square Wave Output Pulse Frequency Sweep Frequency and Amplitude Ultrasonic Transducers Magnetostrictive Piezoelectric Ultrasonic Cleaning Equipment Maximizing the Ultrasonic Cleaning Process Parameters Maximizing Cavitation Importance of Minimizing Dissolved Gas Ultrasonic Power Ultrasonic Frequency Maximizing Overall Cleaning Effect Ultrasonic Power Conclusion

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INTRODUCTION Cleaning technology is in a state of change. Vapor degreasing using chlorinated and fluorinated solvents, long the standard for most of industry, is being subjected to increased regulatory requirements in the interest of the ecology of our planet. At the same time, cleaning requirements are continually increasing. Cleanliness has become an important issue in many industries where it never was in the past. In industries such as electronics where cleanliness was always important, it has become more critical in support of growing technology. It seems that each advance in technology demands greater and greater attention to cleanliness for its success. As a result, the cleaning industry has been challenged to deliver the needed cleanliness and has done so through rapid innovation over the past several years. Many of these advances have involved the use of ultrasonic technology. The cleaning industry is currently in a struggle to replace solvent degreasing with alternative “environmentally friendly” means of cleaning. Although substitute waterbased, semiaqueous, and petroleum-based chemistries are available, they are often somewhat less effective as cleaners than the solvents and may not perform adequately in some applications unless a mechanical energy boost is added to assure the required levels of cleanliness. Ultrasonic energy is now used extensively in critical cleaning applications to both speed and enhance the cleaning effect of the alternative chemistries. This chapter is intended to familiarize the reader with the basic theory of ultrasonics and how ultrasonic energy can be most effectively applied to enhance a variety of cleaning processes. WHAT IS ULTRASONICS? Ultrasonics is the science of sound waves above the limits of human audibility. The frequency of a sound wave determines its tone or pitch. Low frequencies produce low or bass tones. High frequencies produce high or treble tones. Ultrasound is a sound with a pitch so high that it cannot be heard by the human ear. Frequencies above 18 kHz are usually considered to be ultrasonic. The frequencies used for ultrasonic cleaning range from 20,000 cycles per second or 20 to over 100 kHz. The most commonly used frequencies for industrial cleaning are those between 20 and 50 kHz. Frequencies above 50 kHz are more commonly used in small tabletop ultrasonic cleaners, such as those found in jewelry stores and dental offices. THE THEORY OF SOUND WAVES To understand the mechanics of ultrasonics, it is necessary first to have a basic understanding of sound waves, how they are generated and how they travel through a conducting medium. The dictionary defines sound as the transmission of vibration through an elastic medium which may be a solid, liquid, or a gas. Sound Wave Generation A sound wave is produced when a solitary or repeating displacement is generated in a sound-conducting medium, such as by a “shock” event or “vibratory” movement (Figure 1). The displacement of air by the cone of a radio speaker is a good example of vibratory sound waves generated by mechanical movement. As the speaker cone moves back and forth, the air in front of the cone is alternately compressed and rarefied to produce sound waves, which travel through the air until they are finally dissipated. We are probably most © 2001 by CRC Press LLC

Figure 1 Vibratory and shock events.

familiar with sound waves generated by alternating mechanical motion. There are also sound waves that are created by a single “shock” event. An example is thunder, which is generated as air instantaneously changes volume as a result of an electrical discharge (lightning). Another example of a shock event might be the sound created as a wooden board falls with its face against a cement floor. Shock events are sources of a single compression wave that radiates from the source.

The Nature of Sound Waves Figure 2 uses the coils of a spring similar to a Slinky® toy to represent individual molecules of a sound-conducting medium. The molecules in the medium are influenced by adjacent molecules in much the same way that the coils of the spring influence one another. The source of the sound in the model is at the left. The compression generated by the sound source as it moves propagates down the length of the spring as each adjacent coil of the

Figure 2

Coils of a spring representing individual molecules of a sound-conducting medium.

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spring pushes against its neighbor. It is important to note that, although the wave travels from one end of the spring to the other, the individual coils remain in their same relative positions, being displaced first one way and then the other as the sound wave passes. As a result, each coil is first part of a compression as it is pushed toward the next coil and then part of a rarefaction as it recedes from the adjacent coil. In much the same way, any point in a sound conduction medium is alternately subjected to compression and then rarefaction. At a point in the area of a compression, the pressure in the medium is positive. At a point in the area of a rarefaction, the pressure in the medium is negative. In elastic media such as air and most solids, there is a continuous transition as a sound wave is transmitted. In nonelastic media such as water and most liquids, there is continuous transition as long as the amplitude or “loudness” of the sound is relatively low. As amplitude is increased, however, the magnitude of the negative pressure in the areas of rarefaction eventually becomes sufficient to cause the liquid to fracture because of the negative pressure, causing a phenomenon known as cavitation. As shown in Figure 3, cavitation “bubbles” are created at sites of rarefaction as the liquid fractures or tears because of the negative pressure of the sound wave in the liquid. As the wave fronts pass, the cavitation “bubbles” oscillate under the influence of positive pressure, eventually growing to an unstable size. Finally, the violent collapse of the cavitation bubbles results in implosions, which cause shock waves to be radiated from the sites of the collapse. The collapse and implosion of myriad cavitation bubbles throughout an ultrasonically activated liquid result in the effect commonly associated with ultrasonics. It has been calculated that temperatures in excess of 10,000°F and pressures in excess of 10,000 psi are generated at the implosion sites of cavitation bubbles.

Cavitation bubble growth in negative pressure

Maximum bubble size

Bubbles collapse in compression

Cycle repeats new bubble growth

Figure 3 Cavitation and implosion.

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BENEFITS OF ULTRASONICS IN THE CLEANING AND RINSING PROCESSES Cleaning in most instances requires that a contaminant be dissolved (as in the case of a soluble soil), displaced (as in the case of a nonsoluble soil), or both dissolved and displaced (as in the case of insoluble particles being held by a soluble binder such as oil or grease). The mechanical effect of ultrasonic energy can be helpful in both speeding dissolution and displacing particles. Just as it is beneficial in cleaning, ultrasonics is also beneficial in the rinsing process. Residual cleaning chemicals are removed quickly and completely by ultrasonic rinsing. Ultrasonics Speeds Cleaning by Dissolution In removing a contaminant by dissolution it is necessary for the solvent to come into contact with and dissolve the contaminant. The cleaning activity takes place only at the interface between the cleaning chemistry and the contaminant. (See Figure 4.) As the cleaning chemistry dissolves the contaminant, a saturated layer develops at the interface between the fresh cleaning chemistry and the contaminant. Once this has happened, cleaning action stops as the saturated chemistry can no longer attack the contaminant. Fresh chemistry cannot reach the contaminant (Figure 5). Ultrasonic cavitation and implosion effectively displace the saturated layer to allow fresh chemistry to come into contact with the contaminant (Figure 6) remaining to be removed. This is especially beneficial when irregular surfaces or internal passageways are to be cleaned.

Figure 4

Ultrasonic Activity Displaces Particles Some contaminants comprise insoluble particles loosely attached and held in place by ionic or cohesive forces. These particles need only be displaced sufficiently to break the attractive forces to be removed (Figure 7). Cavitation and implosion as a result of ultrasonic activity displace and remove loosely held contaminants such as dust from surfaces. For this to be effective, it is necessary that the coupling medium be capable of wetting the particles to be removed (Figure 8).

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Figure 5

Figure 6

Figure 7

Figure 8

Complex Contaminants Contaminants can also, of course, be more complex in nature, consisting of combination soils made up of both soluble and insoluble components. The effect of ultrasonics is substantially the same in these cases, as the mechanical microagitation helps speed both the dissolution of soluble contaminants and the displacement of insoluble particles. Ultrasonic activity has also been demonstrated to speed or enhance the effect of many chemical reactions. This is probably caused mostly by the high energy levels created as high pressures and temperatures are created at the implosion sites. It is likely that the superior results achieved in many ultrasonic cleaning operations may be at least partially attributed to the sonochemistry effect. A Superior Process In the above illustrations, the surface of the part being cleaned has been represented as flat. In reality, surfaces are seldom flat, instead, they comprise hills, valleys, and convolutions of all description. Figure 9 shows why ultrasonic energy has proved to be more effective at enhancing cleaning than other alternatives, including spray washing, brushing, turbulation, air agitation, and even electrocleaning in many applications. The ability of ultrasonic activity to penetrate and assist the cleaning of interior surfaces of complex parts is also especially noteworthy.

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Figure 9

ULTRASONIC EQUIPMENT To introduce ultrasonic energy into a cleaning system requires an ultrasonic transducer and an ultrasonic power supply or “generator.” The generator supplies electrical energy at the desired ultrasonic frequency. The ultrasonic transducer converts the electrical energy from the ultrasonic generator into mechanical vibrations. Ultrasonic Generator The ultrasonic generator converts electrical energy from the line which is typically alternating current at 50 or 60 Hz to electrical energy at the ultrasonic frequency. This is accomplished in a number of ways by various equipment manufacturers. Current ultrasonic generators nearly all use solid-state technology (Figure 10). There have been several relatively recent innovations in ultrasonic generator technology which may enhance the effectiveness of ultrasonic cleaning equipment. These include square wave outputs, slowly or rapidly pulsing the ultrasonic energy on and off, and modulating or “sweeping” the frequency of the generator output around the central operating frequency. The most-advanced ultrasonic generators have provisions for adjusting a variety of output parameters to customize the ultrasonic energy output for the task. Square Wave Output Applying a square wave signal to an ultrasonic transducer results in an acoustic output rich in harmonics. The result is a multifrequency cleaning system that vibrates simultaneously at several frequencies which are harmonics of the fundamental frequency. Multifrequency operation offers the benefits of all frequencies combined in a single ultrasonic cleaning tank.

Figure 10 Generation of ultrasonics.

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Figure 11 Pulse operation.

Pulse In pulse operation, the ultrasonic energy is turned on and off at a rate that may vary from once every several seconds to several hundred times per second. The percentage of time that the ultrasonic energy is on may also be changed to produce varied results. At slower pulse rates, more rapid degassing of liquids occurs as coalescing bubbles of air are given an opportunity to rise to the surface of the liquid during the time the ultrasonic energy is off. At more rapid pulse rates the cleaning process may be enhanced as repeated high energy “bursts” of ultrasonic energy occur each time the energy source is turned on (Figure 11). Frequency Sweep In sweep operation, the frequency of the output of the ultrasonic generator is modulated around a central frequency, which may itself be adjustable (Figure 12). Various effects are produced by changing the speed and magnitude of the frequency modulation. The frequency may be modulated from once every several seconds to several hundred times per second with the magnitude of variation ranging from several hertz to several kilohertz. Sweep may be used to prevent damage to extremely delicate parts or to reduce the effects of standing waves in cleaning tanks. The frequency of sweep may be varied randomly to prevent damage to parts susceptible to resonating at or near the sweep rate frequency. Sweep operation may also be found especially useful in facilitating the cavitation of terpenes and petroleum-based chemistries. A combination of pulse and sweep operation may provide even better results when the cavitation of terpenes and petroleum-based chemistries is required. Frequency and Amplitude Frequency and amplitude are properties of sound waves. Figure 13A to C demonstrate frequency and amplitude using the spring model introduced earlier. If Figure 13A is the base sound wave, Figure 13B with less displacement of the media (less intense compression and rarefaction) as the wave front passes represents a sound wave of less amplitude

Figure 12 Frequency sweep.

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Figure 13 Demonstration of frequency and amplitude using the spring model. (A: base, B: lower amplitude, C: higher frequency.)

or “loudness.” Figure 13C represents a sound wave of higher frequency indicated by more wave fronts passing a given point within a given period of time. Ultrasonic Transducers There are two general types of ultrasonic transducers in use today: magnetostrictive and piezoelectric. Both accomplish the same task of converting alternating electrical energy to vibratory mechanical energy but do it using different means. Magnetostrictive Magnetostrictive transducers utilize the principle of magnetostriction in which certain materials expand and contract when placed in an alternating magnetic field. Alternating electrical energy from the ultrasonic generator is first converted into an alternating magnetic field through the use of a coil of wire. The alternating magnetic field is then used to induce mechanical vibrations at the ultrasonic frequency in resonant strips of nickel or other magnetostrictive material that are attached to the surface to be vibrated. Because magnetostrictive materials behave identically to a magnetic field of either polarity, the frequency of the electrical energy applied to transducer is one half of the desired output frequency. Magnetostrictive transducers were the first to supply a robust source of ultrasonic vibrations for high-power applications, such as ultrasonic cleaning (Figure 14). Because of inherent mechanical constraints on the physical size of the hardware as well as electrical and magnetic complications, high-power magnetostrictive transducers seldom operate at frequencies much above 30 kHz. Piezoelectric transducers, on the other hand, can easily operate well into the megahertz range. Magnetostrictive transducers are generally less efficient than their piezoelectric counterparts. This is due primarily to the fact that the magnetostrictive transducer requires a dual energy conversion from electrical to magnetic and then from magnetic to mechanical. Some efficiency is lost in each conversion. Magnetic hysteresis effects also detract from the efficiency of the magnetostrictive transducer. Piezoelectric Piezoelectric transducers (Figure 15) convert alternating electrical energy directly to mechanical energy through use of the piezoelectric effect in which certain materials change dimension when an electrical charge is applied to them. Electrical energy at the ultrasonic frequency is supplied to the transducer by the ultrasonic generator. This electrical energy is applied to piezoelectric element(s) in the © 2001 by CRC Press LLC

Mechanical Output Frequency = 2F

Output Face Laminated nickel strips attached to output diaphram by silver brazing

F

Electrical coil wrapped around nickel strips Oscillating magnetic field

Figure 14 Magnetostrictive transducer.

transducer, which vibrate. These vibrations are amplified by the resonant masses of the transducer and directed into the liquid through the radiating plate. Early piezoelectric transducers utilized such piezoelectric materials as naturally occurring quartz crystals and barium titanate, which were fragile and unstable. Early piezoelectric

Ultrasonically Active Liquid

Stainless steel nose piece

Attachment of the nose piece by vacuum brazing with copper

Ground connection Aluminum coupling mass

Electrode

Electrical insulator

Piezoelectric driving elements Steel back mass Compression bolt

Figure 15 Piezoelectric transducer.

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Figure 16 Small, self-contained cleaner.

transducers were, therefore, unreliable. Today’s transducers incorporate stronger, more efficient, and highly stable ceramic piezoelectric materials, which were developed as a result of the efforts of the U.S. Navy and its research to develop advanced sonar transponders in the 1940s. The vast majority of transducers used today for ultrasonic cleaning utilize the piezoelectric effect. Ultrasonic Cleaning Equipment Ultrasonic cleaning equipment ranges from the small tabletop units often found in dental offices or jewelry stores (Figure 16) to huge systems with capacities of several thousand gallons used in a variety of industrial applications. Selection or design of the proper equipment is paramount in the success of any ultrasonic cleaning application. The simplest application may require only a small heated tank cleaner with rinsing to be done in a separate container. More sophisticated cleaning systems include one or more rinses, added process tanks and hot air dryers. Automation is often added to reduce labor and guarantee process consistency. The largest installations utilize immersible ultrasonic transducers that can be mounted on the sides or bottom of cleaning tanks of nearly any size. Immersible ultrasonic transducers offer maximum flexibility and ease of installation and service. Small, self-contained cleaners are used in doctor’s offices and jewelry stores. Heated tank cleaning systems are used in laboratories and for small batch cleaning needs (Figure 17). Console cleaning systems integrate ultrasonic cleaning tank(s), rinse tank(s), and a dryer for batch cleaning (Figure 18). Systems can be automated through the use of a

Figure 17 Heated tank cleaning system.

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Figure 18 Console cleaning system.

PLC-controlled material handling system. A wide range of options may be offered in custom-designed systems. Large-scale installations or retrofitting of existing tanks in plating lines, etc., can be achieved by using modular immersible ultrasonic transducers. Ultrasonic generators are often housed in climate-controlled enclosures (Figure 19). MAXIMIZING THE ULTRASONIC CLEANING PROCESS PARAMETERS Effective application of the ultrasonic cleaning process requires consideration of a number of parameters. While time, temperature, and chemical remain important in ultrasonic cleaning as they are in other cleaning technologies, there are additional factors that must be considered to maximize the effectiveness of the process. Especially important are those variables that affect the intensity of ultrasonic cavitation in the liquid. Maximizing Cavitation Maximizing cavitation of the cleaning liquid is obviously very important to the success of the ultrasonic cleaning process. Several variables affect cavitation intensity.

Figure 19 Large-scale installation.

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Temperature is the most important single parameter to be considered in maximizing cavitation intensity. This is because so many liquid properties affecting cavitation intensity are related to temperature. Changes in temperature result in changes in viscosity, the solubility of gas in the liquid, the diffusion rate of dissolved gasses in the liquid, and vapor pressure, all of which affect cavitation intensity. In pure water, the cavitation effect is maximized at approximately 160°F. The viscosity of a liquid must be minimized for maximum cavitation effect. Viscous liquids are sluggish and cannot respond quickly enough to form cavitation bubbles and violent implosion. The viscosity of most liquids is reduced as temperature is increased. For most effective cavitation, the cleaning liquid must contain as little dissolved gas as possible. Gas dissolved in the liquid is released during the bubble growth phase of cavitation and prevents its violent implosion, which is required for the desired ultrasonic effect. The amount of dissolved gas in a liquid is reduced as the liquid temperature is increased. Importance of Minimizing Dissolved Gas During the negative-pressure portion of the sound wave, the liquid is torn apart and cavitation bubbles start to form. As negative pressure develops within the bubble, gases dissolved in the cavitating liquid start to diffuse across the boundary into the bubble. As negative pressure is reduced by the passing of the rarefaction portion of the sound wave and atmospheric pressure is reached, the cavitation bubble starts to collapse because of its own surface tension. During the compression portion of the sound wave, any gas that diffused into the bubble is compressed and finally starts to diffuse across the boundary again to reenter the liquid. This process, however, is never complete as long as the bubble contains gas since the diffusion out of the bubble does not start until the bubble is compressed. And once the bubble is compressed, the boundary surface available for diffusion is reduced. As a result, cavitation bubbles formed in liquids containing gas do not collapse all the way to implosion but rather result in a small pocket of compressed gas in the liquid. This phenomenon can be useful in degassing liquids. The small gas bubbles group together until they finally become sufficiently buoyant to come to the surface of the liquid (Figure 20). The diffusion rate of dissolved gases in a liquid is increased at higher temperatures. This means that liquids at higher temperatures give up dissolved gases more rapidly than those at lower temperatures, which aids in minimizing the amount of dissolved gas in the liquid. A moderate increase in the temperature of a liquid brings it closer to its vapor pressure, meaning that vaporous cavitation is more easily achieved. Vaporous cavitation, in which the cavitation bubbles are filled with the vapor of the cavitating liquid, is the most effective form of cavitation. As the boiling temperature is approached, however, the cavitation intensity is reduced as the liquid starts to boil at the cavitation sites. Ultrasonic Power Cavitation intensity is directly related to ultrasonic power at the power levels generally used in ultrasonic cleaning systems. As power is increased substantially above the cavitation threshold, cavitation intensity levels off and can only be further increased by using focusing techniques. Ultrasonic Frequency Cavitation intensity is inversely related to ultrasonic frequency. As the ultrasonic frequency is increased, cavitation intensity is reduced because of the smaller size of the © 2001 by CRC Press LLC

Negative pressure cavitation bubble growing

Atmospheric pressure bubble starts to collapse

Positive pressure bubble continues to collapse

Maximum pressure

Figure 20

cavitation bubbles and their resultant, less violent implosion. Higher frequencies are used to eliminate cavitation erosion on delicate parts. Ultrasonic frequencies above the traditional 25 and 40 kHz have also been demonstrated more effective at removing submicron-sized particles from silicon wafers and coated optics. Other applications may also benefit from the use of higher frequencies. Maximizing Overall Cleaning Effect Cleaning chemical selection is extremely important to the overall success of the ultrasonic cleaning process. The selected chemical must be compatible with the base metal being cleaned and have the ability to remove the soils that are present. It must also cavitate well. Most cleaning chemicals can be used satisfactorily with ultrasonics. Some are formulated especially for use with ultrasonics. However, the nonfoaming formulations normally used in spray washing applications should be avoided. Highly wetted formulations are preferred. Many of the new petroleum-cleaners, as well as petroleum and terpene-based semiaqueous cleaners, are compatible with ultrasonics. Use of these formulations may require some special equipment considerations, including increased ultrasonic power, to be effective. Temperature was mentioned earlier as being important to achieving maximum cavitation. The effectiveness of the cleaning chemical is also related to temperature. Although the cavitation effect is maximized in pure water at a temperature of approximately 160°F, optimum cleaning is often seen at higher or lower temperatures because of the effect that © 2001 by CRC Press LLC

Figure 21 Temperature effect.

temperature has on the cleaning chemical. As a general rule, each chemical will perform best at its recommended process temperature regardless of the temperature effect on the ultrasonics. For example, although the maximum ultrasonic effect is achieved at 160°F, most highly caustic cleaners are used at a temperature of 180 to 190°F because the chemical effect is greatly enhanced by the added temperature. Other cleaners may be found to break down and lose their effectiveness at these high temperatures; for example, some should not be used above 140°F. The best practice is to use a chemical at its maximum recommended temperature, but not exceeding 190°F. (Figure 21). Degassing of cleaning solutions is extremely important in achieving satisfactory cleaning results. Fresh solutions or solutions that have cooled must be degassed before proceeding with cleaning. Degassing is done after the chemical is added and is accomplished by operating the ultrasonic energy and raising the solution temperature. The time required for degassing varies considerably, based on tank capacity and solution temperature, and may range from several minutes for a small tank to an hour or more for a large tank. An unheated tank may require several hours to degas. Degassing is complete when small bubbles of gas cannot be seen rising to the surface of the liquid and a pattern of ripples can be seen. Ultrasonic Power The ultrasonic power delivered to the cleaning tank must be adequate to cavitate that entire volume of liquid with the workload in place. Watts per gallon is a unit of measure often used to measure the level of ultrasonic power in a cleaning tank. As tank volume is increased, the number of watts per gallon required to achieve the required performance is reduced. Cleaning parts that are very massive or that have a high ratio of surface to mass may require additional ultrasonic power. Excessive power may cause cavitation erosion or “burning” on soft metal parts. If a wide variety of parts is to be cleaned in a single cleaning system, an ultrasonic power control is recommended to allow the power to be adjusted as required for various cleaning needs (Figure 22). Part exposure to both the cleaning chemical and ultrasonic energy is important for effective cleaning. Care must be taken to ensure that all areas of the parts being cleaned are flooded with the cleaning liquid. Parts baskets and fixtures must be designed to allow penetration of ultrasonic energy and to position the parts to assure that they are exposed to the ultrasonic energy. It is often necessary to individually rack parts on a specific orientation or rotate them during the cleaning process to clean internal passages and blind holes thoroughly. © 2001 by CRC Press LLC

Figure 22

CONCLUSION Properly utilized, ultrasonic energy can contribute significantly to the speed and effectiveness of many immersion cleaning and rinsing processes. It is especially beneficial in increasing the effectiveness of today’s preferred aqueous cleaning chemistries and, in fact, is necessary in many application to achieve the desired level of cleanliness. With ultrasonics, aqueous chemistries can often give results surpassing those previously achieved using solvents. Ultrasonics is not a technology of the future—it is very much a technology of today.

© 2001 by CRC Press LLC

CHAPTER 2.3

Ultrasonic Cleaning Mechanism Sami B. Awad

CONTENTS Introduction Cavitation Formation Mechanism Matching the Frequency to the Process Transducers Enhanced Transducers Precision Cleaning Ultrasonic Cavitation and Surface Cleaning Ultrasonic Cleaning Equipment Cleaning Chemistry Contaminants Mechanism of Cleaning Cleaning Chemistry and Particles Conclusion References INTRODUCTION Ultrasonic cleaning is used in such diverse applications as automotive components, optics, disk drives, semiconductors, electronics, medical/pharmaceutical products, surface preparation for plating and precision coating, aerospace, general metals cleaning, precision bearings, and a variety of consumer products from jewelry to guns. To understand the power and utility of ultrasonics, it is important to understand cavitation implosion.1 This unique phenomenon occurs when high-energy ultrasonic waves (20 kHz to about 500 kHz, at about 0.3 to 1 W/cm2) travel in a liquid or a solution. Ultrasonic waves interact with the liquid media to generate a highly dynamic agitated solution, producing microvapor/vacuum bubbles. The bubbles grow to maximum sizes inversely proportional to the applied ultrasonic frequency and then implode, releasing energy. The higher the frequency, the smaller the cavitation size and the lower the implosion energy.

© 2001 by CRC Press LLC

Figure 1

Scrubbing forces.

CAVITATION FORMATION MECHANISM The ultrasonic cleaning model (Figure 1) illustrates generation of cavitation through nucleation, growth, and violent collapse or implosion. Transient cavities (also referred to as vacuum bubbles or vapor voids), ranging from 50 to 150 m in diameter at 25 kHz, are produced during the half cycles of the sound waves. During the rarefaction phase of the sound wave, the liquid molecules are extended outward against and beyond the liquid natural physical elasticity/bonding/attraction forces, generating vacuum nuclei, which continue to grow. A violent collapse occurs during the compression phase. It is believed that the compression phase is augmented by the enthalpy of the medium, the degree of mobility of the molecules, and the hydrostatic pressure of the medium. Cavitation generates high forces in very brief bursts. Generation time of cavitation is in the order of microseconds. At 20 kHz, pressure is estimated at approximately 35 to 70 kPa, transient localized temperatures are about 5000°C, with the velocity of microstreaming around 400 km/h (Figure 2). A number of factors influence the intensity and abundance of cavitation in a given medium The ultrasonic waveform, frequency, and the power amplitude are important. Other influential factors include physical properties of the liquid medium (viscosity, surface tension, density, and vapor pressure); temperature; and liquid flow (static, dynamic, or laminar); and dissolved gases. High-intensity ultrasonics can grow cavities to the maximum diameter prior to implosion in the course of a single cycle. At 20 kHz the bubble size is roughly 170 m in diameter (see Figure 2). The vacuum bubble size becomes smaller at higher frequencies as a function of the wavelength. For example at 132 kHz it is estimated to be about half the size of cavitations generated at 68 kHz. At 68 kHz, the total time from nucleation to implosion is estimated to be about one third of that at 25 kHz. © 2001 by CRC Press LLC

Figure 2

Ultrasonic frequency and cavitation size and population.

At higher frequencies, the minimum amount of energy required to produce ultrasonic cavities is higher and must be above the cavitation threshold. In other words, the ultrasonic waves must have enough pressure amplitude to overcome the natural molecular bonding forces and the natural elasticity of the liquid medium in order to grow the cavities. For water at ambient temperature the minimum amount of energy needed to be above the threshold was found to be about 0.3 and 0.5 W/cm2 of the transducer radiating surface for 20 and 40 kHz, respectively.

MATCHING THE FREQUENCY TO THE PROCESS Selecting the proper frequency for a particular application is critical. Estimates of cavitation abundance at various ultrasonic frequencies have shown that the number of cavitation sites is directly proportional to the ultrasonic frequency. For example, about 60 to 70% more cavitation sites per unit volume of liquid are generated at 68 kHz than at 40 kHz. The average size of cavities is inversely proportional to the ultrasonic frequency. Therefore, one would expect that at the higher frequency, at a given energy level, the scrubbing intensity would be milder, particularly on soft and thin or delicate surfaces. Because a lower number of cavitations of larger size and higher energy are generated at frequencies of 20 to 35 kHz, systems with lower frequencies are appropriate for cleaning large or heavy components. As the frequency increases, denser cavitation with moderate or low energies is formed. Therefore, frequencies of 60 to 80 kHz are recommended for delicate surfaces; frequencies of 132 and 200 kHz are recommended for cleaning ultradelicate and tiny components. The guidelines hold for both cleaning and rinsing.

TRANSDUCERS The transducers most commonly used for generating ultrasonic vibrations are piezoelectric, magnetostrictive, electromagnetic, pneumatic, and other mechanical devices. The piezoelectric transducer (PZT) is the most widely used technology in cleaning and welding applications. It offers a wide range of frequencies from about 20 kHz to the megasonic range. © 2001 by CRC Press LLC

PZT transducers are typically mounted on the bottom and/or sides of the cleaning tanks. The transducers can be mounted in various designs and sizes of sealed stainless steel containers or immersed in the cleaning solution/liquid (immersibles). Ultrasonic transducers should be placed on the longer sides and/or on the bottom of the tank, to provide maximum distribution of the sonic energy through the cleaning solution. A new transducer design developed by Crest Ultrasonics2 provides greater sound energy transmission with very low acoustic impedance at high frequencies. Benefits include high-quality surface cleanliness and efficient submicron particle removal. Another recent design, the push –pull transducer rod, is an immersible transducer. The push–pull is made of two PZT transducers mounted on the ends of a titanium rod. The generated ultrasonic waves propagate perpendicularly to the resonating surface. The waves interact with liquid media to generate cavitation implosions. ENHANCED TRANSDUCERS Since its inception about 40 years ago, the conventional PZT transducer assembly has consisted of sandwiching a PZT crystal under compression between two metals. A newer design2 was recently developed in which one or both metals are replaced with a ceramic material having twice or higher acoustic inductance. One important benefit is that the new transducer assembly produces sharply defined primary and tertiary resonant frequencies, including new ones not available using the classic design. A second improvement is a higher transmission coefficient of ultrasonic waves into liquid, estimated at 20 to 30%. The enhanced transducer design has been shown to improve cleaning efficiency. A study at Clarkson University, (New York) by A. Busnaina et al.3 compared one system with the conventional transducer design with a second with the enhanced transducer. Results indicate that, at 68 kHz, efficiency of removal of small particles from wafer substrates increases from 84 to 93% with the enhanced transducer system. Efficiency was also influenced by frequency; higher efficiency of particle removal (97%) was observed at 132 kHz. PRECISION CLEANING Precision or critical cleaning of components or substrates is the complete removal of undesirable contaminants to a preset level, without introducing new contaminants in the process.1 This preset level is typically the minimum level at which no adverse effects take place in a subsequent operation. In attempting to clean, it is critical not to introduce new contaminant(s). For example, in an aqueous cleaning process, it is important to have high-quality rinse water and a minimum of two rinse steps. Otherwise, new contaminants will be introduced by residual detergent and/or ionics in the rinse water. Recontamination of cleaned parts with outgassed residues produced from packaging or storing materials is another source of contamination.1 To meet production and quality demands, choosing the appropriate cleaning chemistry and process is essential. Rejected parts are the curse of the assembly line and improper cleaning methods are often to blame. Even beyond the factory floor, improper or inadequate cleaning of a component could directly affect warranty claims.4,5 ULTRASONIC CAVITATION AND SURFACE CLEANING The energy released from an implosion in close proximity to the surface collides with and fragments or disintegrates the contaminants, allowing the detergent or the cleaning © 2001 by CRC Press LLC

solvent to displace them at a very fast rate. The implosion also produces dynamic pressure waves, which carry the fragments away from the surface. The implosion is also accompanied by high-speed microstreaming currents of the liquid molecules. The cumulative effect of millions of continuous tiny implosions in a liquid medium is what provides the necessary mechanical energy to break physically bonded contaminants, speed up the hydrolysis of chemically bonded ones, and enhance the solublization of ionic contaminants. The chemical composition of the medium is an important factor in speeding the removal rate of various contaminants. Cleaning with ultrasonics offers several advantages over other conventional methods. Ultrasonic waves generate and evenly distribute cavitation implosions in a liquid medium. The released energies reach and penetrate crevices, blind holes, and areas that are inaccessible to other cleaning methods.6 ,7 The removal of contaminants is consistent and uniform, regardless of the complexity and the geometry of the substrate.

ULTRASONIC CLEANING EQUIPMENT Ultrasonic aqueous batch cleaning equipment consists of at least four steps: ultrasonic wash, a minimum of two ultrasonic separate (or reverse cascading) water rinse tanks, and heated recirculated clean air for drying. The last drying step is not included if the postcleaning operation includes an aqueous process, as in electroplating or electroless plating. Ultrasonic transducers are bonded to the outside bottom surface, or to the outside of the sidewalls, or they are provided as immersibles and placed inside the tanks. Immersibles are usually the preferred method for large tanks. Two types of immersibles are commercially available in various sizes and frequencies. The first is the traditional sealed metal box containing a multitransducer system. The second is the cylindrical push–pull immersible, powered by two main transducers, one at each end. Prior to selecting equipment, it is imperative that an effective cleaning process be developed. Then the number and size of the stations are determined based on required yield, total process time, and space limitation. Typical tank size ranges from 10 to 2500 liters based on the size of the parts, production throughput, and the required drying time. The tanks are typically constructed of corrosion resistant stainless steel or electropolished stainless steel. Titanium nitride or a similar coating such as hard chrome or zirconium is used to extend the lifetime of the radiating surface in the tanks or the immersible transducers. Advantages of automation are numerous, including consistency, achieving desired throughput, and full control of process parameters.8 Automation includes a computerized transport system able to run different processes for various parts simultaneously as well as data monitoring and acquisition. The entire cleaning system can be enclosed to provide a clean room environment meeting Class 10,000 down to Class 100 clean room specifications. Process control and monitoring equipment consists of flow controls, chemical feed pumps, in-line particle counter, TOC (total organic carbon) measurement, pH, turbidity, conductivity, refractive index, etc. The power requirement for most ultrasonic cleaning applications using PZTs, expressed in terms of electrical-input wattage to the transducers, ranges from 50 to 100 W/gal of cleaning fluid, or 2.8 to 3.6 W/in.2 of transducer radiating surface. © 2001 by CRC Press LLC

CLEANING CHEMISTRY It is important to realize that the use of ultrasonics does not eliminate the need for the proper cleaning chemicals and implementing and maintaining the proper process parameters.9 Cleaning fluids are selected on the basis of the chemical and physical nature of the contaminants, substrate material(s), environmental considerations, and cleanliness specifications. Aqueous and solvent cleaning have advantages and disadvantages. With appropriate additives, aqueous cleaning is universal and achieves better cleaning results. Cleaning with ultrasonics using only plain water is workable, but only for short time. The question then is how long a system will work before cleaning action stops. The chemical composition of the cleaning medium is a critical factor in achieving the complete removal of various contaminants, without inflicting any damage to the components. In fact, cleaning is more complex than just extracting the contaminants from the component and moving them away from the surface. Soil loading and encapsulation/dispersion of contaminants are determining factors in the effective lifetime of the cleaning medium and therefore in effective cleaning of the part. Requirements for the selected chemistry are many and no one chemistry is universal. For example, solvents are appropriate for removing organic contaminants but not for removing inorganic salts.11 The solvent must cavitate well with ultrasonics and be compatible with components to be cleaned. Other properties such as wettability, stability, soil loading, oil separation, effectiveness, dispersion or encapsulation of solid residues, ability to rinse readily, and disposal considerations must be all addressed in choosing the appropriate chemistry. With so many factors to consider, an expert in the field may be better able to make this decision. The role of additives in aqueous chemistries is multifaceted: to displace oils, to solubilize or emulsify organic contaminants, to encapsulate particles, and to disperse and prevent redeposition of contaminants. With appropriately formulated aqueous cleaning chemistries, ferrous and nonferrous metals (for example, aluminum, copper, brass, steel, and stainless steel) can be cleaned in the same bath without interaction. Special additives are used to assist in the process of breaking chemical bonding, removal of oxides, preventing corrosion, enhancing the physical properties of the surfactants, and enhancing the surface finish. Ultrasonic rinsing with deionized water or reverse osmosis (RO) water is important to achieve spot-free surfaces. A minimum of two rinse steps is recommended. Drying and protection of steel components are valid concerns. However, the current available technologies offer effective ways to alleviate these concerns. CONTAMINANTS Three general classes of common contaminants are organic, inorganic, and particulate matter (organic, inorganic, or a mixture). Contaminants of any class may be water soluble or water insoluble. Most organic contaminants such as oils, greases, waxes, polymers, paints, print, adhesives, or coatings are hydrophobic. Organic contaminants can be classified into three general classes: long-chain, medium-chain, and short-chain molecules. The physical and chemical characteristics are related to their structure and geometry. Insoluble particulate contaminants can be divided into two groups, hydrophilic and hydrophobic. Examples of the first group include water-wettable particles, such as metals, metal oxides, minerals, and inorganic dusts. Examples of the second include non-waterwettable particles such as plastics, smoke and carbon, graphite dust, and organic chemical © 2001 by CRC Press LLC

dusts. Similarly, substrate surfaces can be divided into hydrophilic and hydrophobic groups. With few exceptions, inorganic materials or salts are insoluble in water-immiscible solvents. However, water-insoluble inorganics, such as polishing compounds made of oxides of aluminum, cerium, or zirconium, require a more elaborate cleaning process. MECHANISM OF CLEANING Two main steps take place in surface cleaning. The first is contaminant removal; the second is prevention of re-adherence. The removal of various contaminants involves different mechanisms, based on the nature and/or the class of the contaminant. Organic contaminants are removed by two primary mechanisms. The first is solublization in an organic solvent. The second is by displacement with a surfactant film followed by encapsulation and dispersion. The mechanism of removal of organic contaminants by detergent involves wetting both contaminant and substrate. According to Young’s equation, wetting increases the contact angle () between the contaminant and the surface, thus decreasing the surface area wetted with the hydrophobe, and reducing the scrubbing energy needed for removal (Figure 3). SB  SO  COS    OB

Aqueous additives contain one or more surfactants. Surfactants are long-chain organic molecules with polar and nonpolar sections. Surfactants may be ionic or nonionic. When diluted with water, surfactants form aggregates called micelles at a level above the critical micelle concentration (CMC). The micelles, composed of aggregates of hydrophilic and hydrophobic moieties, act as a solvent encapsulating the contaminants, thus preventing redeposition. Ultrasonic cavitation plays an important role in removal of hydrophobic contaminants. The shock wave (and the microstreaming currents) greatly speed up the breaking of adhered contaminants, enhancing displacement with the detergent film. The contaminants

Figure 3

Liquid soil removal.

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are then encapsulated in the micellic aggregates, thus preventing redeposition. The net result is that ultrasonic cavitation accelerates displacement of contaminants from the surface of the substrate and facilitates their dispersion. CLEANING CHEMISTRY AND PARTICLES Theoretically, adhesion forces, including van der Waals, electrical double layer, capillary, and electrostatic, are directly proportional to the size of the particle. One would expect the energy of detachment to decrease with the size of particles. However, smaller particles are always more difficult to detach, mainly because small particles tend to get trapped in the valleys of a rough surface. According to the Gibbs adsorption equation, the mechanism of particle removal involves shifting the free energy of detachment to slightly above or less than zero. Surfactants play a very important role in decreasing the adsorption at particle and substrate interfaces. Ultrasonic cavitation provides the agitation energy for detachment (i.e., the removal force). At 40 kHz, the detachment or removal efficiency of 1-m particles is 88%. Efficiency increases to 95% at high frequencies (60 to 70 kHz), equalling the efficiency of megasonics of approximately 850 kHz. This is expected in light of the fact that cavitation size is smaller at higher frequencies and can reach deeper into the surface valleys. One would then anticipate that a combination of high-frequency ultrasonics at 65 to 70 kHz with appropriate chemistry would further improve efficiency of particle removal. Inhibiting redeposition of contaminants involves formation of a barrier between the suspended contaminant and the cleaned surface. In solvent cleaning, a film of solvent adsorbed to both substrate and contaminant forms the barrier. In aqueous cleaning, an effective surfactant system encapsulates contaminants in the micellic structure as depicted in Figure 4. Redeposition of the encapsulated contaminants (soils) is prevented via stearic hindrance (nonionic surfactants) or via electrical repulsion (anionic surfactants). Depending on the surfactant system, encapsulation can be permanent or transient. Transient encapsulation is preferable to emulsification, as it allows better filtration and/or phase separation of contaminants. Allowing soil loading to reach the saturation point significantly decreases cleaning agent efficiency; cleaning action may cease. To ensure

Figure 4

Antiredeposition.

© 2001 by CRC Press LLC

consistent cleaning, the dispersed contaminants must be removed by continuous filtration or separation of contaminants, and the recommended concentration of the cleaning chemical must be maintained. The physical properties of the substrate, including surface finish, are important factors in submicron particle removal.13,14 For example, a silicon wafer surface differs from that of an aluminum disk, in their physics, topography, and finish. The inherent static charges of plastics are another challenge when dealing with submicron particles. CONCLUSION Cleaning with the assistance of ultrasonic cavitation has numerous advantages, most importantly consistency in results. Advantages: • Efficient cleaning in recessed areas and blind holes • Capability of cleaning assemblies or devices • Removal of micro- and submicrocontaminants • The proper chemistry → exceptional and consistent cleaning • Shorter process time • Full automation and controls, batch and continuous processes For best cleaning results, selection of the ultrasonic frequency or the cleaning medium (solvent or aqueous) for an application must be precise and specific. REFERENCES 1. S.B. Awad, Ultrasonic cavitations and precision cleaning, Precision Cleaning, Nov. 1996, p. 12. 2. J.M. Goodson, U.S. Patent 05,748,566. 3. A. Busnaina et al., Microcontamination Research Lab, Clarkson University, Potsdam, NY, 1998, 1999, results to be published elsewhere. 4. H.A. Bhatt, How now, Parts Cleaning, May 1998, p. 17. 5. J.B. Durkee, The Parts Cleaning Handbook, Gardner Pub. Inc., Cinncinati, OH, 1994. 6. M. O’Donoghue, The ultrasonic cleaning process, Microcontamination, 2 (5), 1984. 7. F.J. Fuchs, Ultrasonic cleaning principles for parts cleaning potential, Parts Cleaning Mag., December 1997, p. 14. 8. J. Harmon, Ultrasonic applications in the life sciences, A2C2 Mag., March 1999, p. 7. 9. S.B. Awad, Ultrasonic cleaning of medical and pharmaceutical devices and equipment, A2C2 Mag., Feburary 2000. 10. S.S. Seelig, The chemical aspects of cleaning, Precision Cleaning, 1995, p. 33. 11. B. Kanegsberg, Aqueous cleaning for high-value processes, A2C2 Mag., September 1999, p. 25. 12. Figures 1, 2, and 5 were reproduced from Precision Cleaning Mag., Witter Publishing Co., Inc., 1966. 13. S.B. Awad, Ultrasonic aqueous cleaning and particle removal of disk drive components, Datatech, 1999, p. 59. 14. K.L. Mittal, Surface contamination concepts and concerns, Precision Cleaning, 3 (1), 17, 1995.

© 2001 by CRC Press LLC

CHAPTER 2.4

Higher-Frequency and Multiple-Frequency Ultrasonic Systems Michael Pedzy

CONTENTS Introduction Ultraprobe and High Frequency Systems Development of a Multiple-Frequency Ultrasonic System Hurdles to Overcome User Experiences Conclusion

INTRODUCTION Multiple-frequency ultrasonics systems incorporate the use of more than one operational frequency in the same cleaning bath. They provide the cleaning characteristics of more than one frequency, allowing a significantly broader range of particle sizes and contaminant types to be addressed than single-frequency ultrasonic systems. Thus, multiplefrequency ultrasonics systems represent a significant development in the field of ultrasonic cleaning technology. Although these systems were developed for the requirements of high-technology industries such as computer hard disk, semiconductor, and optical manufacturing, multiple-frequency systems are now also finding utility in the heavy-duty manufacturing. For example, the technique has found application in manufacture of ball screw mechanisms, ball bearings, and solenoid valve bodies. Since more than one frequency is present in the liquid medium, the part being cleaned is exposed to the cleaning characteristics of all frequencies included in the system. Each ultrasonic frequency has its own set of characteristics, which are produced by the physical development of cavitational energy, the means by which all ultrasonic systems clean. Use of any particular frequency has advantages and disadvantages. However, by combining more than one operating frequency, the negative characteristics of both frequencies are often reduced or eliminated, thereby drastically improving cleaning performance.

© 2001 by CRC Press LLC

The development of multiple-frequency ultrasonic systems actually began during the time which will be termed The High Frequency Revolution. Prior to 1992, many considered that one of the major negative side effects of all ultrasonic cleaning systems, standing waves, had been eliminated with the development of 40-kHz ultrasonics, and piezoelectric transducer elements capable of operating at these frequencies. Standing waves are produced by the ultrafast compression and expansion cycles produced by the ultrasonic transducers typically mounted on the cleaning tank bottom. This causes a cleaning action that is distributed as thin bands oriented perpendicular to the stroke direction of the transducers. In areas where standing waves occur, the bands do not move, and only a small percentage of cavitation is produced between these bands. The end result is a cleaning pattern of hot spots, where high-intensity cavitation is produced, and dead spots, where little energy is present. The consequence of using systems with standing waves is inefficient cleaning; some areas on the parts are cleaned, others are not. Smaller holes or detailed part areas may be missed, or receive only a small amount of cavitational activity. This is one of the most common complaints about single-frequency ultrasonic cleaning systems in the lower-frequency ranges.

ULTRAPROBE AND HIGH FREQUENCY SYSTEMS The ultraprobe, a patented device that can be used to indicate the presence and quality of cavitation, was first demonstrated in September 1992 at the International Manufacturing Technology Show in Chicago, Illinois. Edward Pedzy developed the Ultraprobe as a means of visualizing frequency and cavitation and of differentiating higher frequency (80-kHz) systems from those with lower frequency to components manufacturers and other end users. As with many significant developments, the Ultraprobe was discovered accidentally. The vibration created by an ultrasonic tank caused a container filled with a particular metal powder to fall into the bath. The particles of powder immediately separated, indicting a strange bandlike pattern. Noticing the faint bands of patterns in the tank, Pedzy filled a quartz test tube with the fluid, and submerged the tip into an ultrasonic bath; the bands were clearly visible. Within 2 h, the first Ultraprobe was produced. The probe consists of a 24-in. quartz test tube, filled with a proprietary compound composed of ultrafine metallic particles, and an opaque carrier fluid. When the tip of the instrument is submerged into an ultrasonic cleaning bath, the alternating compression and expansion pulses cause the metallic particles to suspend in areas of cavitational activity, creating a visual “picture” of the cleaning action that parts will receive in the cleaning tank. Tests with the Ultraprobe indicate that the sweep frequency circuit does not eliminate standing waves in many systems. Tests also indicate that the cleaning action produced by elevated-frequency systems was more evenly distributed within the fluid, with less dead area, and a greater number of standing waves. The 80-kHz system produces standing waves only 14 in. apart, while the 40-kHz sweep frequency system produces standing waves 1 2 in. apart. Over the next 2 years, many major ultrasonic cleaning system manufacturers offered cleaning systems with frequencies above 40-kHz. Industrial ultrasonic cleaning systems are now available with operating frequencies of 200-kHz and higher. One might consider that ultrasonic cleaning technology has seen more improvements over the past 8 years than at any other time in the history of the cleaning process. High-technology industries began investigations of higher-frequency ultrasonics, because the cleaning action produced by the elevated frequencies allowed cleaning of their sensitive components without damage. Some of the early studies produced by these hightechnology companies indicated that elevated-frequency systems not only produced a © 2001 by CRC Press LLC

more evenly distributed cleaning pattern, but they also clearly had greater overall particle removal counts. Many high technology manufacturers utilize a particle removal test to provide cleanliness data. Essentially, parts are artificially contaminated with particles of known size to represent the expected contaminant. The parts are then cleaned in ultrasonic baths with different operating frequencies, and a liquid particle counter is used to tally the number of particles in the liquid bath. The results indicate which system removes more particles in general, as well as providing information on how well each frequency addressed particles of a given size. In the applications tested, high-frequency systems removed substantially greater numbers of particles than did lower-frequency systems. This might be expected given the nature of the cleaning action produced by high-frequency systems. Most of the contaminant removed during the ultrasonic cleaning process is not actually removed by the implosion of cavities themselves, but rather by the blast area produced by the imploding cavity. The actual implosions strike the part, causing the liquid jet to spread out over the surface of the part, thereby removing neighboring contamination. Since higher-frequency systems produce significantly greater numbers of imploding cavities, these systems would be expected to remove a greater amount of contamination more rapidly than their low-frequency counterparts. In addition, each frequency tends to remove particles within a particular size range. The 40-kHz system tested demonstrated a tendency to remove particles larger than 0.7 m in size in greater number than the 80-kHz system, while the 80-kHz system demonstrated a tendency to remove particles 0.2 m and smaller in greater number than the 40-kHz system. Further elevated frequency ultrasonic systems tend to produce less cavitational erosion, both on parts being cleaned and on the transducer radiating diaphragm, the surface to which the transducers are mounted. Cavitational erosion is damage caused by the cavitational action produced by an ultrasonic cleaning system. The scrubbing action produced by the ultrasonic system has the potential to erode the surface of the part itself. The radiating diaphragm is continuously eroding in all ultrasonic cleaning systems, which releases microscopic particles of stainless steel into the bath, particles that could potentially result in part failure or system failure if left on the surface of a computer hard disk or other highly sensitive hardware. Higher-frequency systems, however, produce a much more evenly distributed cleaning pattern, with less energy produced at standing wave locations. An additional advantage of higher-frequency systems is increased efficacy of cleaning in small spaces and with very complex geometries. This is because high-frequency systems have a greater number of standing waves and have shorter compression/expansion cycles, producing cavities that are much smaller in size.

DEVELOPMENT OF A MULTIPLE-FREQUENCY ULTRASONIC SYSTEM Because ultrasonic cleaning systems with different operating frequencies are expected to produce vastly different cleaning characteristics, development of a multiple-frequency ultrasonic system was undertaken. It was hypothesized that an ultrasonic system capable of producing more than one ultrasonic frequency would result in more rapid and effective cleaning. It was further expected that by combining different frequencies, some of the negative effects of each frequency could be eliminated. A system with both low- and high-frequency transducers would not only remove heavy contamination, but would be capable of cleaning ultrafine detail. Designing a multiple-frequency ultrasonic system has involved overcoming a number of hurdles. At the time of original conception, it was thought impossible to mount © 2001 by CRC Press LLC

transducers of differing frequencies onto the same radiating diaphragm without destroying ultrasonic components, or eliminating some of the ultrasonic energy by simple cancellation of wave energy. To overcome this potential problem, the first multiple-frequency ultrasonic systems devised included ultrasonic submersible transducer packs, each with a different operating frequency. To avoid transducer damage, transducers of differing frequencies are isolated from one another. Testing of the newly developed systems was performed by manufacturers in the critical coating and plating industries and compared multiple-frequency with single-frequency systems. Their test results indicate that cleaning with the multiple-frequency systems resulted in significantly fewer rejected parts and significantly more consistent results.

HURDLES TO OVERCOME Tests of the early multiple-frequency systems did expose shortcomings in the original system design. Since each frequency was being emitted from different sides of the cleaning tank, one side of the part would be cleaned by only one ultrasonic frequency, while the other side would be exposed to a different frequency. When cleaning a large batch of dense and heavy parts, shadowing of the ultrasonic energy would occur, and one side of the basket received the effects of one frequency, the other side would receive the effects of the other. The best multiple-frequency ultrasonic system would be one that could emit all included frequencies from the same radiating surface. If the system design were possible, the benefits would be significant. Each radiating surface would emit all frequencies, providing all parts with an evenly distributed exposure to all included ultrasonic frequencies. Smaller tanks could also be equipped with multiple-frequency ultrasonic systems, increasing utility to the disk and semiconductor industries. Development of such a system was another story. Several major design hurdles immediately became apparent. First, transducers are typically mounted to radiating surfaces with differing thickness. The lower the frequency, the thicker the diaphragm. In fact, some 20-kHz systems utilize diaphragms up to 38 in. thick to overcome the effects of cavitational erosion. However, higher-frequency systems, having transducers with extremely short stroke lengths during expansion, must be mounted to thinner diaphragms, since thicker materials would completely prevent the transducer from oscillating effectively. Interference between neighboring transducers was also a hurdle. Since each transducer operates at a different frequency, the effects of one transducer would cancel the effects of neighboring transducers at specific moments. The intense interaction would potentially loosen the bonds used to attach the transducers to the radiating surface, or crack the transducer itself. In one early system designed to address these problems, transducers of identical frequency are mounted to a tank, and connected to a generator capable of producing signals with more than one frequency. During activation, all transducers are activated with the same frequency of ultrasonic energy for a given period of time, after which all transducers are activated with a second frequency. This cycling of frequencies does indeed produce the cleaning effects of all included frequencies. Although these systems produce multiple-frequency cleaning effects, this author, as the manufacturer and patent holder of a different system, considers that the above systems have certain shortcomings. Each transducer has a very specific optimum operating frequency, based on the size of the transducer itself. Even the smallest difference in transducer size will affect the optimum operational frequency. In addition, the frequency range of © 2001 by CRC Press LLC

effective operation is very narrow. For example, if an 80-kHz transducer is activated with ultrasonic energy at 40-kHz, ultrasonic efficiency is reduced to less than 40%. The farther the input signal is from the optimum operating frequency of the transducer, the greater the power loss. Only one of the frequencies will operate at optimum efficiency. To overcome this loss of power, manufacturers have included devices that increase the power to the ultrasonic transducer to make up for the loss of energy that is produced when transducers are powered by signals far from their optimum operating frequency. Another side effect produced by this system is limited individual frequency exposure. Each frequency operates for only a fraction of the total cleaning time. For example, if two frequencies are present, and a 10-min cycle is used, each frequency is activated for only 5 min. Although a drastic improvement over single-frequency systems, transducers and generators used in these systems are exposed to electrical extremes to overcome efficiency losses inherent in this design. To overcome the shortcomings of the modified-frequency design mentioned earlier, this author felt it would be necessary to manufacture a system that would incorporate transducers operating at different frequencies to provide superior performance. The two major challenges were to eliminate destructive transducer interaction and to resolve the problems associated with transducer diaphragm thickness. Overcoming the thickness issue was rather easy. The key was to develop transducer mounts with significantly less cavitational erosion potential, allowing them to be mounted to thinner diaphragms next to high-frequency transducers. The other possibility was to grind away material under the higher-frequency transducers, thereby allowing each transducer to emit its energy through a diaphragm with a thickness most efficient for that particular operating frequency. The final patented design includes both developments. Eliminating the destructive interaction between transducers with different frequencies was more difficult, and more important to overcome, than the diaphragm thickness issue, because of the possibility of permanent damage to components. However, after more than 2 years of experimentation, a patented, proprietary technology was developed that allows simultaneous operation of transducers with a variety of operating frequencies without destructive interference, even when mounted directly next to one another. Today, transducers of differing frequencies are staggered between one another, allowing the diaphragm to produce multiple-frequency output from any location on the radiating surface. To improve the design even further, ultrasonic frequencies were selected to take advantage of harmonic frequencies. Ultrasonic systems, regardless of frequency, produce not only sound waves of the primary operating frequency of the transducers, but harmonic frequencies, which are multiples of the original operating frequency. The first harmonic is by far the most powerful, and produces energy sufficiently powerful to produce cavitation. Transducer combinations can be selected to prevent overlapping of harmonics. For example, originally, a common multiple-frequency combination for industrial use once was 40 and 80-kHz. However, since 40-kHz has its first harmonic at 80-kHz, better cleaning action would be produced if the 80-kHz were changed to a slightly different frequency to prevent this overlapping of frequencies, thus increasing the number of frequencies producing cavitation in the cleaning bath.

USER EXPERIENCES The high-technology sector was largely responsible for the development of multiplefrequency ultrasonic systems, and represents the largest share of users of multiple-frequency systems. The disk industry utilizes these systems to remove diamond slurry, which © 2001 by CRC Press LLC

is used to texture the disks prior to a successive plating/coating operation. Should a single particle remain on the surface, the disk fails, and becomes essentially waste material. In fact, if one of the multiple-frequency ultrasonic systems used to remove this slurry fails to remove every single particle from the surface of at least 10,000 disks, the system is taken off-line and serviced. Hundreds of these systems are in use 24 h a day, with long-term reliability that equals or exceeds that of single-frequency systems. As a result of their installation, the disk yields at these locations have improved 2 to 3%, saving the corporations millions of dollars each year. The systems are also beginning to find utility in industrial processes, particularly for applications with high levels of contamination, requiring low-frequency ultrasonics, and recessed areas, more efficiently cleaned with elevated frequencies. Other applications requiring the removal of thick layers of contamination are also better addressed by multiple-frequency ultrasonic systems. The multiple-frequency system includes the lower-frequency components to remove the bulk of the contaminant, while also including transducers with slightly higher operating frequency to better address contaminants that are hidden in blind holes or missed by the standing waves of the lower-frequency system. Yet another segment that has discovered the benefits of high-frequency ultrasonic systems is the coal-processing industry. Ultrasonics at the 80-kHz frequency have been used for years to process coal slurry to remove hazardous by-products from this material, thereby allowing the slurry to be disposed of easily. After experimenting with multiple-frequency ultrasonics at 40/90-kHz combinations, a significant improvement in waste extraction was produced. The slurry is passed over a series of transducers at both frequencies, at an unbelievable flow rate of over 400 gal/min, through a reactor with only 12 gal of liquid volume. The multiple-frequency system had the ability to remove contaminant that was missed by the 80-kHz transducers, producing an effluent that was significantly cleaner. CONCLUSION Development of the Ultraprobe has enabled visualization of differences in frequency and of potential problems in ultrasonic cleaning, such as dead zones. Development of multiple-frequency ultrasonic systems has been a major challenge, but has yielded many realized and potential benefits to the end user. Although in many cases single-frequency ultrasonics are appropriate, in other cases ultrasonic cleaning processes can be greatly improved by the use of multiple-frequency ultrasonics.

© 2001 by CRC Press LLC

CHAPTER 2.5

Megasonic Cleaning Action Mark Beck

CONTENTS Introduction Overview of Megasonic Cleaning Megasonic Cleaning Compared with Ultrasonic Cleaning Application of Megasonic and Ultrasonics Discussion of Underlying Physics Properties of Piezoelectric Transducers Particle Attraction and Removal Forces Principal Mechanisms of Megasonic Cleaning Acoustic Cavitation Acoustic Streaming Significance of the Boundary Layer Cleaning Chemistry and Other Factors Cleaning Chemistries Other Cleaning Factors Design Considerations for Megasonic Systems Conclusion References INTRODUCTION Megasonics has been a widely accepted cleaning method for contamination-sensitive products for nearly 20 years. Megasonics was initially developed in the early 1940s as a result of U.S. Navy research into advanced sonar instrumentation for antisubmarine warfare. In the late 1970s, RCA adapted this technology for wafer cleaning, and by 1982 commercial megasonic cleaning equipment was being delivered to the semiconductor industry. More recently, advances have been made in this acoustic cleaning technology, through a better understanding of high-frequency acoustic streaming, and controlled acoustic cavitation, the megasonic cleaning technique has proved effective for removing submicron particles from silicon and other substrates without damage. As a result, growing numbers of manufacturers in the integrated circuit, hard drive, raw silicon, mask, flat panel © 2001 by CRC Press LLC

display, and other industries have been turning to megasonic cleaning to help meet stringent cleaning requirements. Megasonic cleaning is now increasingly accepted by industry as a cost-effective, efficient, and safe method for the removal of nanoscale particles from contamination-sensitive products.

Overview of Megasonic Cleaning Megasonics utilizes the piezoelectric effect at high frequencies to generate controlled acoustic waves in a liquid bath to enable removal of submicron particles from substrates. In megasonic cleaning (Figure 1), a piezoelectric crystal array transducer converts alternating electrical energy directly to mechanical energy using the piezoelectric effect, in which certain materials change dimension when an electrical charge is applied. A ceramic piezoelectric crystal is excited by high-frequency AC voltage, between 500 and 2000 kHz, causing the ceramic material to change dimension rapidly, or vibrate. These vibrations are transmitted by the resonant masses of the transducer, and directed into the liquid through a resonating plate, producing acoustic waves in the cleaning fluid. Acoustic cavitation, produced by pressure variations in the sound waves moving through the liquid, and the effects of acoustic streaming cause particles to be removed from the material being cleaned.

Figure 1

Megasonic cleaning uses the piezoelectric effect to produce acoustic waves that move through the cleaning liquid.

© 2001 by CRC Press LLC

Megasonic Cleaning Compared with Ultrasonic Cleaning There are two types of acoustic cavitation: transient cavitation and stable, or controlled, cavitation. Ultrasonic cleaning frequencies, between 20 and 350 kHz, produce transient acoustic cavitation. Transient cavitation is characterized by transient bubbles that exist for only a few acoustic cycles, after which they collapse violently, producing very high local temperature and pressure. Transient acoustic cavitation generates shock waves that are powerful enough to erode solid surfaces nearby,1 and to damage some substrate surfaces. Megasonic cleaning operates at much higher frequencies, 500 to 2000 kHz, which produce controlled acoustic cavitation. Controlled cavitation is characterized by stable bubbles that are relatively permanent, can exist for many acoustic cycles, and do not cause damage to substrate surfaces,2 because the cavitation radii are much smaller at higher frequencies and have less energy upon collapse. Thus, megasonic-controlled acoustic cavitation is best suited for sensitive substrate surfaces that cannot withstand the heat and pressure of transient cavitation. In addition, ultrasonics simultaneously cleans all surfaces of a submerged object. This means that ultrasonic cleaning subjects all areas of the substrate, including areas that may not need to be cleaned to the previously described effects of transient acoustic cavitation. Megasonics accomplishes line-of-sight cleaning; it affects only those surfaces of the object that are in the path of the acoustic wave. Application of Megasonics and Ultrasonics The mechanical effects of both ultrasonic and megasonic cleaning can be helpful in speeding particle dissolution and in displacing particles. Both ultrasonics and megasonics have also been demonstrated to speed or enhance the effect of many chemical reactions. In addition, residual cleaning chemicals can be removed quickly and completely by either ultrasonic or megasonic rinsing. However, there are applications for which megasonic cleaning clearly would be favored. The effects of ultrasonics and megasonics on substrate surfaces and particle removal results provide the basis for identifying the best applications for each process. Ultrasonic cleaning is most appropriate for strong, heat-tolerant substrate materials requiring multisurface cleaning. Ultrasonics is also well suited for the removal and/or dissolution of large particles from chemically tolerant substrates. Megasonics is most appropriate for heat- or chemical-sensitive substrates that cannot withstand the heat and pressure of transient cavitation and for applications requiring lineof-sight-dependent cleaning. Parts that cannot be cleaned with ultrasonics, because they are sensitive to the frequency or transient cavitation effects can often be cleaned with megasonics. Megasonics cleaning is also the application of choice for the removal and/or dissolution of small particles (less than 0.3-m, Figure 2). For example, this cleaning technique has been proved effective for removing 0.15-m particles from silicon wafers and other cavitation-sensitive products, without causing substrate damage. Table 1 summarizes the relative strengths of megasonic and ultrasonic cleaning. Positive Environmental Effects of Megasonics The use of megasonic cleaning yields several positive environmental results. The high pressure and temperatures produced by ultrasonic cleaning result in the evaporation of large volumes of both chemicals and ultrapure water. This has two negative effects. First, © 2001 by CRC Press LLC

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Particle size vs. frequency for megasonic and ultrasonic cleaning.

the chemical compositions of cleaning solutions cannot be maintained at constant levels. Second, large amounts of chemical vapors are released, increasing the loads on clean-air exhaust systems. The lower pressures and temperatures produced by megasonic cleaning enable processes that drastically reduce both chemical vapor evaporation and the load on air exhaust and replacement systems. In addition to their environmental benefits, megasonic cleaning methods optimize the use of cleaning fluids and reduce the costs associated with the acquisition and disposal of toxic substances. DISCUSSION OF UNDERLYING PHYSICS Properties of Piezoelectric Transducers A basic characteristic of the piezoelectric crystal is that when a sine wave is applied to it, through the application of AC voltage, it expands. For the purpose of megasonic cleaning, the molecules in the piezoelectric crystal have been aligned, or poled, in the thickness extension mode. Upon application of the sine wave, the first expansion takes place to the side of the crystal, a second expansion takes place to the end of the crystal, and the third expansion takes place in the thickness of the crystal (Figure 3). The frequency at which this third expansion, which is the first thickness expansion, takes place is known as the fundamental frequency. The fundamental frequency occurs at approximately 1000 kHz (Figure 4), with harmonic frequencies at 3 and 5 MHz.

Table 1 Strengths of Megasonics and Ultrasonics Applications for Megasonic Cleaning

Applications for Ultrasonic Cleaning

Cavitation-sensitive substrates Small particle removal/dissolution (0.3 m) Chemically sensitive substrates Line-of-sight-dependent cleaning Heat-sensitive material

Strong substrates Larger particle removal/dissolution Chemically tolerant substrates Multisurface cleaning Heat-tolerant material

© 2001 by CRC Press LLC

Figure 3

Piezoelectric crystal transducer rapidly changes dimensions or vibrates with the application of high-frequency AC voltage.

The most efficient transfer of the energy generated by crystal expansion would occur from direct contact between the piezoelectric crystal and the liquid bath. However, cleaning solutions can damage the piezoelectric crystal, and the piezoelectric crystal is not pure and can add impurities to the cleaning solution. To prevent this, a resonator is adhered to the top of the crystal, between the crystal and the fluid. Ideally, the resonator should have no effect on the energy being transferred, that is, there would be no energy loss or frequency distortion. The velocity of sound in the resonator is an important factor in approaching this ideal. Based on acoustic KLM

Figure 4

Piezoelectric transducer characteristic of impedance as a function of frequency. Impedance changes near fundamental frequencies.

© 2001 by CRC Press LLC

transmission theory, the resonator should be designed to be half the wavelength thickness and should operate optimally at the fundamental frequency. V

  fL where   wavelength of sound in resonator VL  velocity of sound in the resonator, mm/s f  frequency of sound, MHz Particle Attraction and Removal Forces Megasonics cleaning is able to overcome the attraction forces that hold very small particles to a surface. Particle adhesion force is a function of the type of medium surrounding the particle and the surface. In general, it is weaker in liquid media than in gas media,3 and it increases linearly with an increase in particle diameter1 (Figure 5). Although adhesion force is lower at smaller particle diameters, small particles are more difficult to remove. The weight of the particle decreases as a function of the diameter cubed, and for small particles the adhesion force can easily exceed the gravitational force by a factor of 103 or more.1,3 In addition, van der Waals attractive forces (Fvw) vary depending upon the composition of the particle. They are about ten times larger for silicon particles than for polystyrene latex (PSL) particles, for example, indicating that some particles may require much greater removal forces than others.1 PRINCIPAL MECHANISMS OF MEGASONIC CLEANING An understanding of exactly how particles are removed when megasonic cleaning techniques are used has been the subject of increased investigation during the past decade.

Figure 5

van der Waals forces vs. particle size.

© 2001 by CRC Press LLC

To date, researchers have not been able to explain precisely why megasonics works, and there is disagreement on whether controlled acoustic cavitation or acoustic streaming is the more effective mechanism in removing particles. The effect of various parameters on particle removal can be determined, but whether removal is the result of acoustic cavitation or acoustic streaming, or both, is not always clear.2 However, it is accepted that controlled acoustic cavitation, acoustic streaming, and reduction of the boundary layer are the principal particle removal mechanisms in megasonic cleaning. Acoustic Cavitation Acoustic cavitation is the generation and action of cavities, or bubbles, in a liquid. Acoustic waves moving through a liquid produce variations in the liquid pressure. When the liquid pressure drops momentarily below the vapor pressure during the low-pressure portion of the acoustic wave, small evacuated areas, or cavities, are formed that quickly become filled with gas (a foreign contaminant such as dissolved oxygen or air) and/or vapor (a gaseous form of the surrounding liquid).4 These tiny bubbles are set in motion by the acoustic wave. The bubbles may be suspended in the liquid medium, or they may become trapped in voids either in the boundary surface of the liquid or in solid particles suspended in the liquid. The tiny bubbles can expand and contract in the liquid. Bubble expansion can be caused by reducing the ambient pressure in the liquid, either by static or dynamic means. The bubbles can then become large enough to be seen by the unaided eye. The bubbles may contain gas or vapor or a mixture of both. If the bubbles contain gas, then their expansion can be caused by rectified diffusion, pressure reduction, or an increase in temperature.3 Rectified diffusion is the diffusion of dissolved gas from the liquid into the bubble, and vice versa, with the pressure oscillations resulting in a net diffusion into the bubble. This net inward diffusion occurs because the bubble surface area increases during inward diffusion and decreases during outward diffusion; a higher surface area leads to more diffusion.2 If the ambient liquid is not saturated with gas, then rectified diffusion must compete with ordinary diffusion from the bubble to the liquid. In that case, the sound pressure amplitude must exceed a certain value in order for the bubbles to increase significantly in size.2 The pressure oscillations that created the bubbles can also cause them to expand and contract. If the pressure variation is great enough to reduce the local liquid pressure down to, or below, the vapor pressure in the negative parts of the acoustic cycle moving through the liquid, any minute cavities or bubbles that are present will grow larger. If the range of the pressure variation is increased to produce zero and then negative pressures locally in the liquid, then bubble growth is increased. Gas from the liquid diffuses into a bubble during expansion, and leaves the bubble during contraction. When the bubble reaches a size that can no longer be sustained by its surface tension, the bubble will expand and then collapse, or implode, which is an important action of the cavitation phenomenon. The bubble action of cavitation has sufficient energy to overcome particle adhesion forces and to dislodge particulates attached to substrates in the stream of bubbles. Essentially, imploding cavitation bubbles generate shock waves that dislodge particles from substrate surfaces. Cavitation breaks down the molecular force by which a particle is held to a surface either by direct impact from bubble implosion or by the fatiguing action caused by repeated bombardment.3 Cavitation implosion force varies with the size and contents of the bubble. Larger bubbles are unstable and implode with larger force; smaller bubbles are stable and collapse with less force. Vapor collapses more quickly, resulting in larger implosion force, whereas gas cushions and slows the collapse, resulting in smaller implosion force. © 2001 by CRC Press LLC

Cavitation does not occur until a specific threshold is reached.3 The cavitation threshold is defined as the minimum pressure amplitude required to induce cavitation.2 A number of methods have been developed for detecting cavitation, including acoustic emissions, visual observations, sonoluminescence (SL), and surface erosion. Of these, SL is believed to be the most suitable method for characterizing cavitation in a megasonic tank,1 because it is related to the cavitation collapse of bubbles. The intensity and effect of cavitation on materials being cleaned are related to the type of acoustic cavitation produced. Two types of acoustic cavitation have been identified and studied: transient cavitation and stable cavitation. Transient acoustic cavitation is produced by ultrasonic cleaning frequencies, between 20 and 350 kHz, which transform lowenergy-density sound waves into high-energy-density collapsing bubbles. In transient cavitation, the mostly vapor-filled bubbles exist for only a few acoustic cycles, followed by a rapid and violent collapse. This type of cavitation is likely to produce violent events in the acoustic field, such as radiation of light (SL) and shock waves. The level of violence produced is believed to be dependent on the maximum size of transient bubbles, which is related to the acoustic frequency.1 Because transient cavitation concentrates energy into very small volumes and tends to produce very high local temperatures and pressure, it can cause surface erosion and damage to sensitive substrates. Bubble size decreases as acoustic frequency increases, and the smaller the maximum bubble size, the less violent the cavitation produced.1 The high frequencies used in megasonic cleaning, 500 to 2000 kHz, produce controlled acoustic cavitation, which is characterized by mostly small, gas-filled cavities. Unlike the violent implosion associated with vapor-filled cavities in transient cavitation, controlled cavitation bubbles exhibit less violent collapse,4 producing lower temperatures and pressure. As a result, megasonic cleaning substantially minimizes surface erosion and damage to substrates being cleaned. Stable cavitation produces light in the visible range (violet), while the light produced by transient cavitation is primarily in the ultraviolet range (with a peak at 270 to 290 nm).4 The bubble action of controlled acoustic cavitation is believed to be a primary particle removal mechanism in megasonic cleaning. Acoustic Streaming Acoustic streaming is considered another primary particle removal mechanism of megasonic cleaning. Acoustic streaming is time-independent fluid motion generated by a sound field. This motion is caused by the loss of acoustic momentum by attenuation or absorption of a sound beam. Acoustic streaming enhances particle dissolution and the transport of detached particles away from surfaces,4 thereby decreasing particle redeposition. It also produces a much thinner boundary layer (less than 1 m) than would be found in a cleaning tank without megasonics. Acoustic streaming velocity is a function of energy intensity, geometry, energy absorption, liquid density and viscosity, and sound speed in the liquid. Streaming velocity has been found to increase linearly with acoustic intensity (power). Velocity also increases linearly with frequency. Streaming velocity also decreases with distance from the source, due to attenuation.2 Acoustic streaming comprises several important effects: (1) bulk motion of the liquid, (2) microstreaming, and (3) streaming inside the boundary layer. The primary effect of acoustic streaming is bulk motion of the liquid, the strong localized flow of cleaning solution. The shear force of the bulk liquid motion is the primary particle removal agent. In a closed tank, forces due to sound pressure variation create this bulk fluid motion, which carries particles away from the substrate once the molecular attraction © 2001 by CRC Press LLC

of the particle to the surface is broken and the particle is dislodged. Bulk fluid motion increases linearly with acoustic intensity. The bulk fluid motion shear force combines with the other effects of acoustic streaming to increase particle removal. A second effect of acoustic streaming is microstreaming. Microstreaming, also known as Eckart streaming, occurs near oscillating bubbles, or any compressible substance in the liquid. Microstreaming occurs at the substrate surface, outside the boundary layer, because of the action of bubbles as acoustic lenses that focus sound power in the immediate vicinity of the bubble. This is a powerful type of streaming, in which the bubbles scatter sound waves and generate remarkably swift currents in localized regions. The currents are most pronounced near bubbles that are undergoing volume resonance and are located along solid boundaries. Microstreaming aids in dislodging particles and contributes to megasonic cleaning.4 Most of the flow induced by acoustic streaming occurs in the bulk liquid outside the boundary layer. However, there is a third effect of acoustic streaming, called Schlichting streaming, which is associated with cavitation collapse and is believed to assist in the removal of small particles and their transport away from surfaces. Schlichting streaming occurs outside the boundary layer and is characterized by very high local velocity and vortex (rotational) motion. The vortices are of a scale much smaller than the wavelength. Schlichting streaming results from interactions with a solid boundary. Steady viscous stresses are exerted on the boundaries where this type of rotational motion occurs, and these stresses may contribute significantly to removal of surface layers.2 The combined effects of acoustic streaming produced in megasonic cleaning may slide, roll, or lift a particle from its initial position on a substrate, depending on the size and shape of the particle, as well as the nature of the hydrodynamic force being applied. Acoustic streaming, both inside and outside the boundary layer, clearly enhances cleaning and other chemical reactions. Particle transport is aided significantly by the strong currents and small boundary layer thicknesses that result from acoustic streaming.2 Significance of the Boundary Layer During megasonic cleaning, the cleaning solution flows swiftly past the substrate being cleaned, forcing chemistry into contact with contaminant particles, removing them from the surface, and carrying them away. On a microscopic scale, during acoustic cleaning, fluid friction at the surface of the substrate being cleaned causes a thin layer of solution to move more slowly than the bulk solution. This layer of slow-moving fluid at the surface is called the boundary layer (Figure 6). The boundary layer effectively shields the substrate surface from fresh chemistry and shields contaminant particles from the removal forces of the bulk fluid. Within the boundary layer, van der Waals attractive forces have been shown to be substantially stronger than the removal forces that result from acoustic pressure oscillations, acoustic velocity oscillations, or bulk fluid motion associated with acoustic streaming. Megasonic cleaning has proved especially effective at removing submicron particles in part, because it reduces the boundary layer. The higher frequencies of megasonic cleaning reduce the boundary layer to less than 0.5 m, compared to the boundary layer of 2.5 m produced by ultrasonic cleaning frequencies. The primary effect of acoustic streaming is the bulk fluid motion of the cleaning solution. The thickness of the boundary layer decreases as the velocity of bulk fluid motion increases. Reduction of the boundary layer yields several benefits. It allows fresh chemistry to come closer to the substrate,2 and come into contact with smaller particles. This higher chemistry refresh rate results in faster cleaning. Boundary layer reduction increases the © 2001 by CRC Press LLC

Figure 6

Comparison of boundary layer in ultrasonic and megasonic cleaning.

effectiveness of the acoustic streaming removal forces by allowing the cleaning solution to rush past the substrate closer to the substrate surface, forcing chemistry onto particles, removing them from the surface, and carrying them away. The small, controlled cavitation bubbles generated by megasonics are able to remove contaminants within the thinner boundary layer. This effect is especially important in removing small particles and accessing small surface features. Reducing the boundary layer results in increased removal of submicron particles, particles that were previously protected by the boundary layer, as well as increased particle removal overall. In megasonic cleaning, the combined results of boundary layer reduction, acoustic streaming, and controlled acoustic cavitation are very effective at enabling smaller particles to be removed. CLEANING CHEMISTRY AND OTHER FACTORS Several additional factors contribute to the effectiveness of megasonic cleaning. These include cleaning chemistries, fluid temperature, process time, and power. Cleaning Chemistries Megasonics cleaning may be used with a variety of chemistries, including water, neutral aqueous solutions, alkaline aqueous solutions, acidic aqueous solutions, ethyl lactate, alcohol, acetone, N-methyl pyrollidone, dibasic esters, and glycol ethers. Although megasonic cleaning is used primarily for particle removal, it can also be used to increase the efficiency of chemical cleaning with surfactants or detergents. Efficacy of removal of other contaminants depends on the solution in the tank. Cleaning chemistries play a significant role in megasonic cleaning, because the chemical composition of the cleaning solution may affect how quickly the cavitation threshold is reached. In megasonic cleaning (as contrasted with ultrasonic cleaning), it is believed that operating at or below the cavitation threshold produces better cleaning results. The cavitation threshold, defined as the minimum pressure amplitude required to induce cavitation, has been found to increase with increasing hydrostatic pressure (under © 2001 by CRC Press LLC

most conditions) and to decrease with increasing surface tension, with increasing temperature, and with an increasing number of solid contaminants. A reduction in the number of hydrophobic ions (such as C and F) will also decrease cavitation threshold, since these ions collect at bubble surfaces and prevent cavitation bubbles from dissolving.2 A lower cavitation threshold allows cavitation to occur more readily. This suggests that cavitation could be mitigated under the following conditions: low surface tension, high hydrostatic pressure, low temperature, and the presence of as few solid surfaces and contaminants as possible.2 The original RCA Standard Clean consists of sequential immersion in two chemicals: Standard Clean 1 (SC-1) and Standard Clean 2 (SC-2). The formula for SC-1 is one part H2O2, one part NH34OH, and five parts H2O. The formula for SC-2 is one part H2O2, one part HCl, and five parts H2O. The addition of megasonic cleaning to the SC-1 solution substantially enhances particle removal.5 Chemists have succeeded in getting very dilute solutions to clean effectively with the addition of megasonics to the cleaning process. For example, in statistically designed experiments on semiconductor wafer cleaning, megasonic power was observed to be the dominant factor for particle removal using SC-1 type chemistries. Both the bath temperature and the ratio of ammonium hydroxide to hydrogen peroxide were found to modify the effect of megasonic power on particle removal. Using substantially diluted chemistries, together with high megasonic input power and moderate to elevated temperatures, resulted in very high cleaning efficiencies for small particle removal.6 Table 2 presents typical chemicals used in the wet cleaning of silicon wafers. Other Cleaning Factors Cleaning fluid temperature, process time, and power are additional factors that can affect megasonic cleaning results. In general, sound speed decreases with increasing temperature. The optimum temperature for the cleaning fluid will vary with the type of substrate being cleaned and with the type of particle that must be removed. The choice of temperature will also depend on the specific cleaning solution being used and how effective it is to begin with at room temperature. In megasonics cleaning, exposure time and megasonic power are the most significant variables. The combination of megasonic controlled cavitation and acoustic streaming enables typical substrate exposure times of 1 to 30 min, with most exposure times between 10 to 30 min. As megasonic power or exposure time increases, particle redeposition decreases. Increasing the power level directly affects bulk streaming. Higher power levels increase the microstreaming component of megasonic cleaning, Table 2 Typical Chemicals for Wet Cleaning of Silicon Wafers Contaminants

Chemicals

Organics

SPM (H2SO4/H2O2) APM (NH4OH/H2O2)  SC-1 APM (NH4OH/H2O2)  SC-1 HPM (HCl/H202/H2O)  SC-2 SPM (H2SO4/H2O2) DHF (HF/H2O) DHF (HF/H2O) BHF (NH4F/HF/H2O)

Particles Metallics

Native oxides

© 2001 by CRC Press LLC

reducing the boundary layer, and can shorten the process time required. Megasonic power is affected by array geometry, manufacturing method, and bath geometry. DESIGN CONSIDERATIONS FOR MEGASONIC SYSTEMS Initial megasonic systems developed for general industry use had transducer array lifetimes of a few months. Current technology has increased reliabilities to tens of thousands of hours (years). For overall megasonic cleaning effectiveness, one must take tank design, fluid circulation and filtration, and system electronics into consideration. The tank size should be small, to minimize the amount of cleaning fluid used. Additional fixturing may be required to position the substrate accurately in the bath; only those surfaces located within the acoustic stream will be cleaned. The system should incorporate efficient fluid circulation and filtration, to assist in final particle removal from the fluid. Acoustic cavitation dislodges the particles, and acoustic streaming carries them away, but they must be removed from the fluid to prevent their redeposition on the surface being cleaned. The electronics that drive the resonator are crucial. Piezoelectric impedance is very dynamic over frequency, temperature, and age. If computer-controlled electronics with positive feedback are not used to supply the RF power source to the piezoelectric material, reliability is seriously impaired. Additional important considerations when choosing a megasonic cleaning system are choosing the appropriate power level and resonator for the fluid type to be used and the type of particle to be removed. CONCLUSION Megasonics provides several advantages over ultrasonics for damage-sensitive substrates. Megasonic-controlled cavitation and high-power acoustic streaming enable substrate exposure times of 1 to 30 min and provide effective submicron particle removal without the substrate damage typically associated with ultrasonics. The lower pressures and temperatures produced in megasonic cleaning reduce substrate surface erosion while also providing significant environmental benefits. REFERENCES 1. Gouk, R., Experimental Study of Acoustic Pressure and Cavitation Fields in a Megasonic Tank, M.S. thesis, University of Minnesota, Minneapolis, 1996, 47. 2. Gale, G., Physical and Chemical Effects of High Frequency Ultrasound (megasonics) on Liquid Based Cleaning of Si 100 Surfaces, Ph.D. thesis, 1995, 4. 3. Zhang, D., Fundamental Study of Megasonic Cleaning, Ph.D. thesis, University of Minnesota, Minneapolis, 1993, 18. 4. Gale, G., Busnaina, A., Dai, F., and Kashkoush, I., How to accomplish effective megasonic particle removal, Semiconductor Int., 133, August 1996. 5. Hottori, T., Trends in wafer cleaning technology, in Solid State Technology, Penwell Publishing, Nashua, NH, May 1995, S8. 6. Resnick, P.J., Adkins, C.L.J., Clews, P.J., Thomas, E.V., and Korbe, N.C., A study of cleaning performance and mechanisms in dilute SC-1 processing, in Ultraclean Semiconductor Processing Technology and Surface Chemical Cleaning and Passivation, Liehr, M., Heyns, M., Hirose, M., and Parks, H., Eds., Materials Research Society, Pittsburgh, 1995, 21.

© 2001 by CRC Press LLC

CHAPTER 2.6

Equipment Design Edward W. Lamm

CONTENTS Introduction Choosing the Correct Equipment Path Solvent Equipment Open-Top Vapor Degreasers Closed-System Degreasing Semiaqueous and Aqueous Similarities Aqueous Equipment Water Rinsing Rinse Tank Design Drying Semiaqueous Separation Stage Ancillary Equipment Oil Skimming and Filtration Media Filtration Membrane Filtration Physical Principles of Coalescing Coalescing Equipment Coalescing Elements Liquid/Liquid Systems Water Quality Automation Mechanical Superstructure Reference INTRODUCTION In the late 1980s as the concern about the effects of CFCs on the ozone layer came to a head, solvent cleaning was subjected to severe scrutiny. During the next few years, most cleaning applications were evaluated regarding their ability to be converted to aqueous or © 2001 by CRC Press LLC

semiaqueous. With this new activity, the aqueous marketplace began to explode with new manufacturers eager to gain a piece of the market that always seemed solvents were granted by royal decree. The new regulations now offered a slice of the fief that was previously out of reach. The manufacturers of chemicals that could be rinsed with water (semiaqueous) and had minimal emissions also began to see a crack in the solvent armor. They began probing into this new potential growth market. Those applications that could not undergo the conversion to aqueous or semiaqueous were tested with new solvents that had almost no ozone depletion potential, but emissions continued to be an issue. As this market began to grow, new equipment was required to meet the tighter environmental regulations that were instituted to assure solvent loss be kept to a minimum. All these events changed the distribution of the cleaning market and the available equipment. The new designs that were born will be explored along with the original designs and the ancillary support equipment that enhances their performance. CHOOSING THE CORRECT EQUIPMENT PATH To look at the types of equipment, it is first necessary to understand how the decision to select a cleaning process is reached. Essentially there are a few questions that must be answered (Figure 1). The questions basically help the engineer decide the approach to take in regard to the cleaning agent to be used. This will then determine the type of equipment to be investigated, solvent or aqueous. Once the selection of the cleaning agent category has been completed, the field of available designs is significantly reduced. This is a great start because there are over 100 equipment companies that provide in excess of 200 products.1 SOLVENT EQUIPMENT Equipment designed for cleaning with solvents is divided into two simple groups, cold batch cleaning and vapor degreasing. The first group, cold batch, is somewhat a throwback to the paintbrush and coffee can era with a bit of scale-up and sophistication. A sink on a

Figure 1

Equipment path questions.2

© 2001 by CRC Press LLC

barrel is a good example. This system is quite labor intensive, but simple to operate. With the addition of an agitation lift, the process is enhanced; however, it is still relegated to fairly noncomplex parts and far from the tight tolerances of precision cleaning. Vapor degreasers on the other hand can run the gamut of low-end open-top to extremely sophisticated closed designs depending on the cleanliness required. The basics of the concept are the same for all operations. Vapor degreasers are designed not only to vaporize solvent for cleaning and drying the parts, but also to confine and recycle the solvent and solvent vapor to maintain a healthful environment and to keep cleaning costs low. Vapor degreasing equipment must provide for: • Cleaning to remove soluble and particulate soil • Recovery of solvent by distillation for repeated use • Concentration of the soils

Open-Top Vapor Degreasers These requirements are met by the basic degreasing unit, which is an open rectangular tank with a pool of solvent in the bottom (Figure 2). The solvent is heated in the boil sump and vaporized into a dense vapor layer (specific gravity greater than 1, heavier than air) that lies above the liquid and constitutes the vapor cleaning zone. The condensing coils, which are installed high on the inside periphery of the tank, condense the vapor reaching that level. The solvent condensate is returned to the solvent boil sump via the condenser trough and water separator. The very important vertical extension of the degreaser wall is the freeboard. The freeboard provides a stationary air zone above the vapor level. It shields the normal vapor zone

Figure 2

Solvent cycle. (1) Heaters boil solvent to make vapors; (2) vapors condense on parts; (3) condensate drips into boil sump; (4) excess vapors condense on coils; (5) condensate flows to water separator where water is removed; (6) dry condensate overflows to ultrasonic sump; (7) solvent overflows from ultrasonic sump to boiling sump.

© 2001 by CRC Press LLC

from moderate drafts, which could carry the vapors into the work environment. In addition, a very thin solvent film evaporates from the work, as it is slowly withdrawn from the vapor zone, and the freeboard area confines these vapors. The degreaser work opening must be adequate to handle the work dimensions and cleaning cycle, but should be kept to a minimum to maintain economical operation and acceptable working conditions. Even when vapor loss is controlled and no work is passed through, every square foot of exposed surface permits loss of a given amount of solvent related to the equipment design and the solvent used. Since vapor in the center of the tank must be passed to the walls for condensation, extending the tank width increases turbulence, which causes entrainment of air resulting in vapor loss. As narrow a tank as feasible is recommended. Freeboard is the distance from the top of the vapor line to the top of the confining side wall at the top of the tank. The freeboard zone reduces vapor disturbance caused by air motion in the work area. The freeboard zone also permits drainage of the work being removed, evaporation of residual solvent, and drying of the part with a minimum of solvent loss as well as reduced solvent emissions into the air. In degreasers of extreme length, the height of the freeboard is increased. Generally speaking, the higher the freeboard, the lower the solvent consumption. The basis of effective vapor degreaser design is control of the vapor level. The control of the vapor zone provides for cleaning in freshly distilled solvent and also helps minimize solvent loss. This control is best done by use of condensing coils. Condensing coils are located within the degreaser tank at a height above the boiling solvent equal to the work height plus allowances for clearance below the work and a 3 to 9 in. vapor layer above the work (Figure 3). The usual design of a degreaser provides for the normal vapor level to be at the midpoint of the vertical span of these coils. Thus, the positioning of the condensing coils also establisheds the freeboard height in a given tank.

Figure 3

Vapor degreaser.

© 2001 by CRC Press LLC

To prevent excessive condensation of atmospheric moisture on the coil surfaces above the vapor line, the temperature of the water leaving the coils should be above the dew point of the ambient air. To accomplish this, the water should flow into the lowest coil and out the top coil. The refrigerated freeboard chiller is designed to reduce solvent emissions at the solvent vapor–air interface by placing a cool, dry layer of air above the vaporzone. This cool air blanket assists in confining the solvent vapors. The refrigerated freeboard chiller consists of a coil placed on the inside perimeter of the unit, immediately above the primary condensing coils. An external refrigeration unit supplies the coils with the necessary cooling. The chiller unit will condense, and in some cases freeze, atmospheric moisture onto the coils. The additional water from these coils should be handled by placing a separate trough under the coils, draining to its own water separator for a holding tank. If an additional trough is not placed on the equipment, the coil should drip into the solvent condensate trough, but a larger separator would then be considered for effectively separating the water from the solvent. Additional water in the solvent may cause corrosion and shorten equipment life. Water enters a degreaser from several sources: • • • •

Condensation of atmospheric moisture on the condenser coils Moisture on the work being cleaned Steam or cooling water leaks Water-soluble cutting oil

Water can form a boiling mixture with the solvent (an azeotrope) that is vaporized, causing equipment corrosion, decreased solvent life, and increased vapor losses. All degreasers should be equipped with a properly sized water separator. In the water separator, the condensed solvent–water mixture drops into a trough below the condenser coils and flows by gravity to the separator. The mixture enters the separator below the solvent level. The water with a lower specific gravity and insolubility rises to the top and is discharged through a water drain. Relatively moisture-free solvent is then discharged through the solvent return line to the degreaser. This separation requires time. Since 5 min is a practical minimum, the separation chamber should have a capacity of at least 112 the hourly solvent condensing rate. A deeper separator is more efficient to operate than a shallow one of equal volume, because the solvent –water interface area is smaller in the deeper design. To minimize solvent loss induced by air turbulence over the vapor zone, a degreaser should be placed away from excessive air currents, open windows or doors, heating and ventilating equipment, and any device causing rapid, uncontrolled air displacement. Typically, the usual air circulation is sufficient to dilute small quantities of vapor that normally escape from the degreaser. When the degreaser must be placed in an unfavorable location, a baffle on the windward side will divert drafts and protect the vapor level.

Closed-System Degreasing The contained or closed degreaser (Chapter 2.11 by Gray and Durkee) offers all the benefits of the open-top design, but enhances the process by eliminating the solvent–air interface. This is accomplished by conducting the cleaning in a sealed chamber, thereby preventing emission. Another benefit of the closed chamber is the addition of a vacuum step during processing. This feature aids in soil displacement from blind holes and © 2001 by CRC Press LLC

complete removal of solvent during drying. The major concern when evaluating this technology is the associated cost and low throughput as compared with open-top systems. The major difference in design between the open and closed systems is the parts movement. Unlike the open-top degreaser, when items are placed in the closed chamber to be cleaned in closed-system degreasing, the parts never leave the chamber until the process is complete. The cleaning solvent is brought to the parts where they are cleaned, rinsed, and dried. This eliminates the possibility of a disturbed or collapsed vapor zone, which contributes to solvent consumption and emissions. In addition, a number of agitation options, which include spray, ultrasonics, and rotation, can be employed during any of the process steps (Figure 4*). These systems typically include multiple feed tanks for a variety of solvent cleanliness levels. For reclamation of the solvent and to provide a fresh uncontaminated rinse source, a still is an integral part of the design. As elimination of emissions is a key principle, heat exchangers are installed to remove solvent from the discharge of the vacuum system. SEMIAQUEOUS AND AQUEOUS SIMILARITIES In cleaning applications where water does not have a negative impact, both aqueous and semiaqueous systems have been given substantial consideration. There are a number of similarities between semiaqueous and aqueous equipment. The major one is that they both use water as the medium to remove the wash medium, which provides for a huge similarity in the last two thirds of the process. With both using the same rinse design, the drying options for elimination of moisture are identical for these processes. Obviously, the ancillary equipment is also similar. Pumping, filtration, and water purification are all handled by equipment of identical design. AQUEOUS EQUIPMENT The process of aqueous cleaning can be divided into three specific components: cleaning, rinsing, and drying. This is no different from solvent cleaning except that each process component is conducted in a different piece of equipment, each section is generally more sophisticated than a section of the vapor degreaser. Of course, the cost associated in providing the added detail is significant for aqueous design. When washing a part, the contaminant is often removed through the introduction of cleaning chemistry and mechanical force. Rinsing involves the removal of any residual soil and chemistry that remain after washing. It is important to perform this task without introducing new contaminants, such as dust or impurities in the water. Drying is the process by which residual rinse liquid is removed without introducing any new contaminants. In the cleaning step, a detergent that is typically diluted in water actually bonds to the soil (oil, grease, or particulate). To be effective, the detergent requires temperature and mechanical activity to loosen the dirt. Both of these are important when evaluating the design of the equipment. Mechanical force is typically used in both the cleaning and rinsing stages. There are a number of options available (Table 1). Spraying is a fairly effective, low-cost method for large parts without intricate details or holes. However, for smaller components or parts with blind holes, immersion with an additional source of agitation is required. All the options provide a relative level of removal of both soluble and particulate soils and must be evaluated for the type of contaminant to be removed. Of course, all come at a price and must be judged on their need and effectiveness. *Chapter 2.6 Color Figure 4 follows page 104. © 2001 by CRC Press LLC

Table 1 Mechanical Forces—Separating the Soil from the Substrate2 Method Spray Immersion Agitations Bubbler Lift Propeller Ultrasonics

Relative Energy

Solubles Removal

Particle Removal

Relative Cost

High Low

Good OK

OK Poor

Low Low

Low Med High High

OK Good Good Excel

Poor Good Good Excel

Low Med Med High

When people think of cleaning applications, the focus is typically placed on the washing stage of the operation. In precision cleaning, however, rinsing becomes a much more important step. The allowable contamination levels are lower, and spot-free drying is almost always a requirement. WATER RINSING Rinsing is a technology, just as washing is. It is measurable, controllable, and directly contributes to the effectiveness of the cleaning process. Effective rinsing can improve yield, reliability, and appearance; it is also an important factor in containing operation costs. Rinsing removes two basic types of soils: (1) solubles, which encompass washing chemistries and other soils that dissolve in the cleaning media, and (2) insolubles, consisting of particulate dispersed throughout the cleaning media. Rinsing is based on the principle of dilution. To develop an effective rinsing process, three questions must be answered: 1. What soils are present? 2. How much soil is there? 3. How much residue is acceptable? What soils and how much soil there is can be determined with analytical testing. How much residue is acceptable is a more difficult question—one that must often be answered empirically by the end user. Often, acceptable residue levels are defined by testing a cleaned part for acceptable performance in its next operation or use. If the part performs acceptably after being put through the cleaning process, the cleanliness level is assumed to be acceptable. It is important to minimize contamination in the rinsing steps and to allow the use of less rinse water. Two rinsing techniques commonly used to minimize rinse water volume are spray rinsing and countercurrent immersion flow rinsing. Spray rinsing, as the name implies, uses spray nozzles to direct the flow of rinse water over the parts. This type of rinsing can be very effective, using much less water than a typical flowing rinse. Proper application of spray rinses is necessary to ensure that all areas of the parts can be rinsed and also that the spray is only activated when the part is present to be rinsed. Effective immersion flow rinsing is based on the successful completion of two tasks: first, the soils must be separated from the part; then, the soils must be prevented from redepositing onto the part. This can be accomplished by several means—for example, sparging the surface to remove buoyant soils, filtering the solution for particulate, and maintaining continuous dilution of solubles and fine particulate. © 2001 by CRC Press LLC

Separating the soil often requires mechanical energy, especially with parts having complex shapes, or those that are “nested” in blind holes and/or crevices. Table 1 lists several options to accomplish this. If ultrasonic agitation is used in the wash, it might also be helpful in the rinse. Many times, a higher frequency is used in the rinse than has been used in the wash. This facilitates removal of smaller particles and reduces the potential for part damage. Continuous filtration of the rinse baths is very important in precision rinsing. The level of retention of the filter should reflect the level of cleanliness required. In systems with multiple rinse tanks, the filter retention level is often reduced with each succeeding bath. Continuous dilution is also a method of preventing redisposition and involves four key elements: 1. Concentration of tank chemistry in dragout (C), which is measured in parts per million (ppm) (1 oz/gal  approximately 7500 ppm). 2. Volume of dragout (V), which is the volume of water/chemistry moved (with the parts and carrier) from the wash to the rinse stage. 3. Flow rate of rinse water (F), which is measured in gallons per hour (gph). 4. Rinse tank equilibrium concentration (E), which is a function of flow rate and dragout, to the point at which incoming and outgoing chemistry levels are equal. These four rinsing factors are related by the following formula: CVFE C  V defines the amount of chemistry entering the rinses. Precision rinsing generally requires low E values, therefore, high F values (or overflow rates) are required. Figure 5 illustrates the process of continuous dilution. One method for improving rinsing is the use of several rinse tanks in a series (Figure 6). The rinse formula applies to each successive tank. This allows a significant reduction in equilibrium concentration with a fixed overflow rate (F). This arrangement increases the

Figure 5

Continuous dilution.2

© 2001 by CRC Press LLC

Figure 6

Continuous dilution—several rinse tanks.2

capital costs of achieving a particular cleanliness level by requiring more rinse tanks, but it reduces operating costs by lowering the required overflow rate. It is important to note that the water flow is in the opposite direction of the work flow. This is called “counter cascade” rinsing. In applications with high dragout, a spray rinse may be used to remove the gross chemistry before the first immersion rinse, further increasing efficiency. In precision applications, the quality of the rinse water itself can be a factor in the effectiveness of the rinsing stage. In most cases, deionized (DI) water is required. In the deionization process, organics are removed by carbon, and special functional exchange resins remove the ions. Biological growth is controlled with ultraviolet lights and special filtration. One method of measuring rinse water quality is through resistivity or conductivity (Table 2). This is a measure of the electrical insulation properties of the water. Dirty water and tap water may contain many ions that conduct electricity, lowering the resistivity. Rinse Tank Design In addition to the process variables, rinse tank design can impact the effectiveness of rinsing. The flow pattern of the water can be important in rinsing, and this pattern is a function of the tank design. The most common design for rinse tanks is the single-sided, overflow weir design. This design depends on dilution for effectiveness and has “dead spots” in the corners where mixing does not take place, thereby reducing its effectiveness.

Table 2 Deionized Water Quality2 Resistance (Megohms) 18.2 10.0 4.0 1.0 0.4

© 2001 by CRC Press LLC

Conductance (Microsiemens) 0.055 0.100 0.250 1.000 2.500

Total Dissolved Solids (ppm) None (higher quality) 0.115 0.288 1.150 2.875 (lower quality)

Figure 7

Four-sided overflow weir with 360° saw-tooth weir design.

Another, more effective rinse tank design, which has become popular in precision rinsing, is the four-sided overflow model (Figure 7). This design utilizes a laminar upflow of water, which improves mixing and eliminates dead spots. This design is also very effective at sweeping fine particulate off the surface, preventing redeposition on the parts. The overflow design is often augmented by the use of high-flow recirculation filtration, which further increases the sweeping action. The return of the filtered water typically enters the bottom of the rinse tank. Return manifolds that are specifically configured for the fixture are often used. One of the important variables in rinsing is the cleanup rate. This is defined as “the time it takes for the contamination in the rinse tank to return to a steady level, after the parts enter the bath.” An experiment was run to determine the cleanup rate for a four-sided overflow with recirculation/filtration and a single-sided rinse with recirculation/filtration and a sparger. The four-sided overflow rinse had a faster cleanup rate than the single-sided overflow rinse. There are several factors that contribute to the improved efficiency. The high internal flow and mixing in the four-sided design enhances solubility. The high-volume laminar flow is efficient for particle removal and minimizes redeposition. In the foursided design, the distance to the overflow is minimized, improving the sweeping action. The four-sided overflow design can provide up to 60% advantage in rinsing over the conventional single-sided design. It has improved efficiency for both soluble soils and particulate. For fixed overflow rate, process throughput, and cleanliness level, fewer rinse tanks may be required with a four-sided design. Alternately, for a fixed number of rinse tanks, a higher throughput may be possible. Figures 8 and 9 show the cleanup rate for the two designs, using soluble soils and particulate. DRYING The drying stage of aqueous cleaning has the objective of removing the residual water that is carried over from the last rinse. Removal of the remaining water can be quite difficult depending on the geometry of the part being processed. The two basic additives required to increase water evaporation are temperature and flow of air. The mechanical methods used to produce these effects include:3 © 2001 by CRC Press LLC

Rinse Analysis - Rate of Removal - Solubles

Conductance (microsiemens)

15

Single overflow

10

5 4-Way overflow 0 0

1

1 gpm DI Make-up flow rate

Figure 8

2

3

4

5

6

TIme (minutes)

Equipment considerations. Cleanup rate using solubles.

Compressed air blow-off Infrared lamp bank Recirculating air oven

Vacuum oven Centrifuge Solvent displacement

If moisture can be tolerated, an air blow-off or centrifugation of the parts may be all that is needed. If the parts need to be dry to the touch, infrared lamps or oven drying may be required. If a higher level of dryness is needed prior to subsequent processing, a combination of drying steps may be used. Parts configuration, the substrate involved, and the degree of dryness will dictate which drying method or methods are most suitable for the majority of parts involved. Plastics, copper, and aluminum containing alloys may have temperature restrictions that need to be considered. Parts with blind holes, threads, depressions, and narrow cavities may require special handling. Small parts tightly nested together also offer special drying challenges when considering the best or most efficient design. Compressed air is economical and can be used directly over a process tank to minimize dragout. It is especially effective on large flat surfaces. This method is ideal if some moisture can be tolerated or if used in conjunction with another drying process. Air velocity Rinse Analysis - Rate of Removal - Particulate

Particles (cumulative/ml)

40000 30000

Single overflow

20000 10000

4-Way overflow

0 0

1

1 gpm DI Make-up flow rate

Figure 9

2

3

4

TIme (minutes)

Equipment considerations. Cleanup rate using particulate.

© 2001 by CRC Press LLC

5

6

dictates the percentage of moisture removed as droplets and the percentage removed by evaporation. A lower velocity results in a greater amount of evaporated moisture. Infrared heat lamps are designed to focus heat where needed. A full line of area and chamber heated lamps are available. Infrared heat is clean, fast, and controllable. Sample part configurations of delicate construction respond best to this type of drying. Recirculating hot air ovens are commonly used by industry to dry parts cleaned with water. Drying times are directly related to air velocity and temperature. These ovens are ideal if a high degree of dryness is required. Vacuum ovens should be used only as a polishing step as required to get the last little bit of moisture off the parts. Vacuum of 1 T or greater is used at temperatures of 120°F or greater. Centrifugal dryers are used for small parts with simple configuration. Parts are spun at speeds approaching 1000 rpm for up to 10 min. The liquid can be recovered and returned to the process or rinse tank as desired. This type of drying process requires little space, and operation costs are relatively low. This process is, however, for small parts and it is not adequate if a high level of dryness is required. Parts with complicated internal components or blind cavities may require final moisture removal using a water-displacing solvent. Any solvent immiscible with water can be used for this process. Molecular structure and physical characteristics all must be considered carefully. The primary cost of parts drying is energy. For that reason it is prudent not to dry parts any more than is essential for subsequent processing. Less energy is required to run a centrifuge than an air compressor needed for forced air blow-off. If heat is added to the drying process, the cost increases as the temperature rises. In addition, depending on circumstances, manual labor could be the most costly part of the drying equation.

SEMIAQUEOUS In semiaqueous cleaning, there are basically two distinct categories of agents. They differ based on the miscibility of the cleaning agent in water or the boiling points, i.e., the method used to separate the cleaning agent from the rinse water.4 This is conducted either by gravity or by difference in boiling point. Separation by gravity is based on the immiscibility of the solvent and rinse water. Boiling point differences are separated by distillation of the water-soluble solvents and rinse water. As in any industry, there are a number of semiaqueous formulations created from different solvent bases. Typically, the semiaqueous cleaning agent suppliers are the manufacturers of the main components included in their products. In the fundamental semiaqueous process, parts are cleaned of soil with a suitable solvent that often may contain a detergent. The solvent is then removed from the parts by washing with progressively cleaner water. The parts are dried with hot forced air. To be economical, the cleaning agent must be separated from the rinse water by gravity or distillation. The rinse water may be purified further for recycle with membranes that reject organic materials. Amajor advantage of the semiaqueous process is the high degree of waste recovery—the only direct waste is a concentrate of the soil in the cleaning agent. A major disadvantage is equipment complexity. Relative to a vapor degreaser, semiaqueous equipment is expensive. The cleaning tank is designed similarly to those for other cleaning agent systems. The operating temperature is from ambient to as high as 180°F, because of the high flash point of semiaqueous cleaning agents. Soil concentration at equilibrium should be no more than © 2001 by CRC Press LLC

5 to 10 wt%. Cleaning time typically runs from 30 s to 5 min. Ultrasonic cleaning is often used for removal of particulates. The next stage is called emulsion cleaning. The parts are removed from the tank and contacted with a rapidly moving stream of air (air knife) to blow off liquid cleaning agent from the parts. This is done for two reasons: (1) to adjust the soil concentration in the cleaning stage and (2) to adjust the cleaning agent concentration in the next emulsion cleaning stage. Too little blow-off will harm cleaning performance by raising the soil concentration in the cleaning stage and reducing the cleaning agent concentration in the next emulsion cleaning stage. Water is sprayed onto the parts in the emulsion cleaning stage. Again, this is done for two reasons: (1) to remove cleaning agent and (2) to continue the cleaning process with a water emulsion of the cleaning agent. The water emulsion is often a better cleaner than the concentrated semiaqueous chemistry used in the cleaning stage because little soil is present in the emulsion cleaning stage. The temperature is increased slightly. Cleaning time is in the same range. Cleaning agent concentration in water is from 1 to 10%. This is deliberately low to minimize organic cleaning agent flow to the final rinsing stages. Separation Stage The separation stage is not part of cleaning per se, but refers to recovery of the semiaqueous cleaning agent or to removal of oils from aqueous cleaning agents. Since the separation stage is the keystone of a semiaqueous process, the opportunity to avoid problems in that stage is worthwhile. The term gravity separation refers to the driving force that controls the rate of separation. That is the density difference between water and the cleaning agent, and is typically 0.15 to 0.2 g/cc. The emulsion is fed to a decanter for separation (in the gravity-separation process) and to a distillation column (in the distillation-separation process). Conditions in the decanter are deliberately different from those in the cleaning and rinse tanks; usually the temperature in the decanter is higher by 20 to 40°F. The separation should take place in between 5 and 30 min. An interface monitor in the decanter is used to activate pumps that withdraw the top organic phase and the bottom water phase. Removal is usually done in batch mode to maintain the organic/water interface between prescribed levels. Problems occur in a decanter system when the withdrawal of one phase becomes contaminated with the other phase. A change in soil chemistry is a major potential cause of contamination. Another potential problem is foaming in the rinse tank, which can occur if spray nozzles are not correctly sized and positioned. Distillation separates chemicals based on differences in their boiling points. For most solvents of interest, the difference between the boiling point of the solvent and of water is more than 70°C. That is well above the minimum of the 10 to 15°C acceptable for good operation. Further, boiling points of soil are typically 200°C above the boiling point of water. The key advantages of a distillation separation system are reproducible and forgiving separation of soil from the rinse water and of water from the cleaning solvent. Operation could be with batch or continuous mode, depending on cleaning load. Batch distillation systems probably are less expensive. Both types of separation schemes have been used in a variety of industrial situations. Decanters and distillation columns commonly are used in chemical plants and refineries. If the successful cleaning situation is one in which two solvents can be used—one of each separation type—the distillation option will work best. Distillation requires more capital ($5000 vs. $2000) and consumes more energy than does operation of a decanter. However, © 2001 by CRC Press LLC

distillation is a more positive separation approach than decantation. It can be more easily monitored, and is less affected by changes in soil chemistry. ANCILLARY EQUIPMENT Cleaning solution chemistry can be as benign as hot water or can be a mixture of water and cleaning chemicals.5 Cleaning chemicals are typically used where heavier soils such as oils need to be removed. Hot water is used where water-soluble contaminants (such as water-soluble fluxes) need to be removed from the part. The recovery and reuse techniques described apply to chemical-based cleaning solutions. Those cleaning solutions comprising water only can be dealt with using the techniques applicable to the recovery and recycling of rinse water. The key to minimizing the disposal of cleaning solutions lies in extending their useful life. At some point, the cleaning solution becomes too concentrated in contaminants for the cleaner to perform adequately. The contaminants that cause a cleaning solution to become spent include both organic compounds such as free and emulsified oils and inorganic components such as dissolved metal, which are introduced into the solution as part of the process. They may also be components inherent in the cleaning chemistry or makeup water, which build up over time. Processes that are used for recovering aqueous cleaning solutions include oil skimming, media/membrane filtration, and coalescing. Oil Skimming and Filtration Oils removed from parts during cleaning can either be emulsified or “free,” depending upon the cleaning chemical formulation. Some cleaners are formulated to reject soils, which allows the soils (typically oils) to float on the surface of the solution. Skimmers are used to remove these free oil layers. For those cleaners that are formulated to emulsify oils, the oil can be removed via a coalescing-type filter or membrane filtration. Media Filtration Media filtration (e.g., cartridges, bags, and sand) is used to remove suspended solids from cleaning solutions and associated wastewater. No dissolved materials are removed and these total dissolved solids (TDS) remain in the water. Membrane Filtration Membrane filtration processes are pressure driven and are used for various aqueous separations. Several types of membrane processes are used (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis) depending upon the size of the contaminant to remove. The two most important membrane separation processes used in the recovery and reuse of aqueous cleaning solutions are microfiltration and ultrafiltration. The limitations on these processes are those created by the presence of material that can foul, scale, or damage the membrane. Physical Principles of Coalescing Liquid/liquid coalescing technology is used to accelerate separation of an emulsion. The principal driving force for coalescing action in either a gas or liquid stream is the © 2001 by CRC Press LLC

interfacial tension of the droplets. Interfacial tension is the excess free energy due to the existence of an interface at the surface of a droplet, arising from unbalanced molecular forces. A relatively small interfacial tension value is typically required to obtain a coalescence rate low enough for practical application. In a carrier stream of dispersed liquid droplets, the total interfering effect of surface active agents, particulate masking, or electrical charge is not great enough to render the dispersion permanent. The interfacial tension value between the two liquids is neither drastically reduced nor destroyed. Therefore, the dispersed droplets can be physically induced to agglomerate and the natural process of fluid coalescing can be mechanically accelerated to separate economically the liquids making up the emulsion. This provides the basis for liquid/liquid coalescing technology. There are several different methods available to promote coalescence in an industrial process. Three primary mechanisms of coalescence are generally observed: impaction, Brownian diffusion, and turbulent field coalescence. Impaction occurs when the momentum of a droplet in the carrier stream causes it to collide with a droplet attached to a fiber or surface media, resulting in coalescence. The second mechanism occurs when the Brownian motion of a droplet in the carrier stream causes it to collide either with another droplet in the carrier stream or with a droplet attached to a fiber or surface media. In turbulent field coalescence, drops that have associated in pairs are pushed through the small capillary passage of the bed or barrier, resulting in turbulence in the carrier stream. The associated droplets eventually coalesce as a result of their relative motion when passing through the capillary. Coalescing Equipment Industry uses a variety of mechanical means to effect fluid coalescing. A settling tank reduces the velocity of a liquid emulsion and provides a quiescent zone. At low velocity, the dispersed droplets agglomerate and form a second continuous phase because of differences in specific gravity. Additional techniques are used to improve the coalescing rate in settling tanks, including directional flow inducers and baffles. System modifications may include recycling the excess dispersed phase and flowing the emulsion through beds of coarse, porous media, such as wire mesh or fiberglass. Similar methods are used to effect gas/liquid coalescing. Surge tanks are used to reduce the velocity of the gas stream, encouraging the agglomeration of liquid droplets. After the droplets settle, they are removed from the system. In many instances, vessels use devices to induce centrifugal flow and create abrupt changes in the direction of flow. Coalescing Elements Using elements with a medium of engineered surface and pore-size characteristics can augment coalescing of fluids. Several factors need to be considered when selecting the most effective fluid coalescing element. 1. The size and range of the openings (pores) in the porous material. 2. The relative surface tension value of the fluids. 3. The degree of wetting of the porous material exhibited by the fluid. (This is related to the surface tension value between the liquid and porous media.) 4. The fluid pressure drop across the coalescing media. 5. The chemical compatibility of the fluid system and the coalescing element. © 2001 by CRC Press LLC

Liquid/Liquid Systems A liquid/liquid system that is a candidate for coalescing is generally in the form of an unstable emulsion. An emulsion is a dispersion of fine droplets of one liquid in a second in which the first liquid is completely immiscible or incompletely miscible. Generally, emulsions are formed by the mixing or mechanical agitation of liquids. The dispersed fine droplets will rise or fall in the continuous liquid column as a result of differences in liquid densities. The droplets may impact other droplets, agglomerate, and become larger (coalesce). However, interfering factors usually retard or prevent natural coalescing at an acceptable rate.

WATER QUALITY Water of defined quality is needed for controlled cleaning. High-purity water is usually needed for precision cleaning; 18.3 M-cm is considered the measure of perfection most commonly sought the world over when talking about water purity. The only commonly available way to achieve this resistivity level is by use of deionization. To appreciate fully what deionization is and how it works, one must first look at the contaminants found in water and what purification processes are needed, in addition to deionization, to provide water purity for a specific application. Because pure water is the “supreme” solvent, it actively gathers contaminants from everything it passes over or through, including, potentially, the parts that are trying to be cleaned. Dissolved ionized solids such as sodium (Na), calcium (Ca), and chloride (Cl) are stripped from rock and soil. Organic molecules are gathered from decaying debris and environmental pollutants. Particulates include organic debris, dirt and rust from soil and piping; bacteria and microbials (including pyrogens) from normal growth in water; dissolved gases such as chlorine (Cl) and carbon dioxide (CO2) from water treatment and organic decay, and colloids from rock and sand. All these contaminants are present in varying concentrations in water. Each presents different problems depending upon the application. Deionization alone can allow achievement of 18.3 M-cm resistivity, guaranteeing water free of ionic contaminants, but it does not remove organics, particulate, bacteria, or microbials. To remove these contaminants, other types of purification are used in conjunction with deionization. Activated carbon is used to remove organics and chlorine gas. Filtration is used to remove particulate and bacteria. Ultrafiltration is used to remove microbials, including pyrogens. Resistivity is the measure of how much electrical current will pass between two electrodes at a specific distance. When an electrical current is passed through a solution such as water, ionic molecules are used as stepping stones by the electrical current. The fewer stepping stones, the more difficult the passage becomes, and the higher the resistivity reading. Most organic and bacteria are not adequate stepping stones to change the resistivity of water appreciably. The temperature of water will also have an impact on its resistance. For this reason most water systems incorporate a meter that will automatically compensate temperatures to 25°C, the standard for water purification. The maximum achievable resistivity reading of water at 25°C is 18.3 M-cm. Ionic contaminants exist dissolved within the chemical structure of water. Dissolved ionized solids and dissolved ionized gases are removed using ion-exchange resins, which act like tiny magnets stripping ions from water, replacing them with H and OH ions, which ultimately join to form water (H2O). © 2001 by CRC Press LLC

Ion-exchange resins are for the most part synthetic polymers with several ionexchange sites attached to the surface. Two basic types of ion-exchange resins are used. Cation removal resins have several hydrogen ions (H  ) attached to their surface, capable of exchanging for positively charged ions. Anion resins have several hydroxyl groups (OH  ) attached to their surface, each capable of exchanging for negatively charged ions. In a two-bed cartridge, these reactions occur separately with the cation removal resin being used first, followed by anion removal resin. A two-bed cartridge is used to remove the bulk of ionic contaminants, because when the two resins are separated, the cartridge has higher effective capacity for ionic molecules. However, a two-bed system cartridge cannot fully remove all the ionic contaminants because the reaction is never completed. To achieve totally deionized water, a mixed-bed cartridge is required. The mixed-bed cartridge is configured so that the cation and anion resin are mixed. When a reaction takes place in a mixed-bed cartridge, the by-products of one reaction are picked up by the corresponding reaction, thus taking it to its completion. As previously explained, deionization alone may not be enough for a specific application. This is the reason a system should incorporate more than one method of purification to deliver water free of any and all contaminants. The system should employ a pretreatment cartridge that utilizes a combination of macroreticular resin and carbon to prepare the water for the deionization that takes place in the following steps. The feed water first passes through the carbon to remove organics and chlorine. These components could potentially reduce the effectiveness of the ion-exchange resin. From the carbon, the water passes through a layer of macroreticular colloids. Colloids are very slightly ionized, extremely small particles that both clog conventional filtration and reduce the ability of the resin to produce high-purity water. This would be followed by a two-bed high-capacity cartridge to remove the majority of ionic contaminants as a preparation for the ultrapure mixed-bed cartridge. An ultrapure mixed-bed cartridge is then employed to remove all remaining ionic contaminants yielding up to 18.3 M water. Organics, which are still present after initial carbon adsorption and deionization, are removed now using high-efficiency synthetic carbon. Membrane filtration is used as the final treatment to remove bacteria and particulate, which have passed through the previous steps. A 0.2-m hollow fiber filter attached to the faucet block performs the final filtration. For most applications, water after this step is sufficiently pure for use.

AUTOMATION Whether the cleaning system is aqueous, semiaqueous, or solvent, automated parts handling can add enormous value to the process in terms of throughput, total output, and ease of equipment operation. The obvious requirement for a mechanical assist to moving parts through a system is the sheer weight of the load. However, there are other benefits that automation provides. In addition to eliminating the labor cost required if the unit were to be operated manually, automation increases consistency in the process, provides a traceable process, and permits the use of static process control.6 Automated systems are composed of four main components: the mechanical superstructure, the drive systems, the control package, and the operator interface. All these subsystems need to mesh with the entire tank line, which includes the tanks themselves, and environmental equipment. Consideration should also be given to up- and downstream production. © 2001 by CRC Press LLC

The most visible feature that differentiates the various automation options of a system is the mechanical superstructure. A number of standard designs are available and described below. Mechanical Superstructure The basic objective for horizontal and vertical travel should be a clean design with minimal moving parts, especially over the tank line. The automation system needs to be rigid, durable, adaptable to available footprint, and compatible with the chemistry in use. Concerns include overhead clearance and accessibility for the tank line to operators. Overhead conveyors are chain or belt pulley systems mounted laterally over the centerline of the tanks. Stated tank lengths are typically exaggerated to allow for the transitions for vertical travel. There is no flexibility in altering processing and the only variability in throughput is by altering the speed of the conveyor. Tank level conveyors use powered rollers to move payloads between stations and vertical movement is implemented by lifts in each tank. This can be an efficient approach to automation. However, processing flexibility is limited, tanks are significantly oversized, and it may not be appropriate for delicate parts. A walking beam is typically a top- or side-mounted fixture that indexes payloads simultaneously. It can be advantageous in single-recipe, high-volume applications. It has the same limitations in flexibility and throughput described above. Additionally, these systems limit tank design in that all stations must be the same distance apart and all station process times are identical. I beam or cable systems employ suspended independent head(s), which travel over the centerline of the tanks. The only advantage to these systems is where ceiling clearances are an issue. By design, the moving parts of the heads inherently create potential for contamination of the payload. Alternatives for low-ceiling applications include motion multipliers, or where footprint constraints require front-to-back tank layout, three-axis automation. However, given the potential for contamination, caution is required in clean room installations. Cantilevered design has one horizontal frame mounted behind the tank line along which one or more heads travel and execute vertical movement. Properly designed, this concept is considered optimal for general applications since it creates the least contamination, uses the smallest footprint, and affords unimpeded operator access to the front of the tank line. Multiple heads, which overlap travel zones, can be an efficient way to increase throughput, especially during “dead travel” with no payload. Any head can lift more than one payload at a time for simple high-throughput applications (use of a “gang fixture”), although as in a walking beam the distance and processing time between stations must be equal. Gantry/rim runners are two horizontal frames, one along each long axis of the tank line. From here the system can be essentially two I beam systems with associated contamination concerns, or mated cantilevered heads sharing weight distribution of the payload. The main disadvantage to this concept is that access to the front of the tank line is limited.

© 2001 by CRC Press LLC

REFERENCE 1. Reynolds, R., Cleaning Equipment Directory, Precision Cleaning Magazine, Witter Publishing, February 1997. 2. Genet, C., Key requirements for proper rinsing in precision applications, CleanRooms East ‘99, Philadelphia, PA, Penn Well Publishing, Nashua, NH, March 1999, 125–143. 3. Quitmeyer, J.A., Aqueous cleaning process challenges, in Precision Cleaning ‘96 Proceedings, Anaheim, CA, Witter Publishing, Flemington, NJ, May 1996, 275–284. 4. Durkee, J.B., The Parts Cleaning Handbook: How to Manage the Challenge without CFCs, Section II, Semi Aqueous Cleaning, Gardner Publications, Cincinnati, OH 1994, 36–42. 5. Riley, C.T., Reduction/recycle/reuse concepts for aqueous cleaning process, in CleanTech ‘98 Proceedings, Rosemont, IL, Witter Publishing, Flemington, NJ, May 1998, 128–136. 6. Aries, J., Automation: designing the right system for your cleaning equipment and production integration, Precision Cleaning ‘97 Proceedings, Cincinnati, OH, Witter Publishing, Flemington, NJ, April 1997, 296–305.

© 2001 by CRC Press LLC

CHAPTER 2.7

Cold and Heated Batch Solvent Cleaning Systems P. Daniel Skelly

CONTENTS Introduction The Ideal Solvent Cold Cleaning Pail and Scrub Brush Hand Wipe Aerosol Spray Recirculating Overspray(“Sink-on-a-Drum”) Parts Washer Immersion Cleaning, Single-Dip Tank, with Manual Parts Handling Automated Immersion Cleaning, Multiple-Dip Tanks Heated Solvent Cleaning Methods Heated Dip Tank Vapor Degreasing Summary References

INTRODUCTION In light of current and expected regulations, the trend of the 1990s has been to adopt aqueous (water-based) cleaning systems. In some applications, this may be the best choice. However, in other applications, water just does not work. Some considerations and problems include requirements for pretreatment of water supply, waste stream handling requirements and costs, limited efficacy of cleaning due to low solvency for many soils of interest and high surface tension, energy costs of heating and drying, requirements for rinsing and drying, high total cycle time, compatibility/flash rusting, complicated bath maintenance, high capital equipment costs, high maintenance costs, and large equipment footprint.

© 2001 by CRC Press LLC

THE IDEAL SOLVENT When evaluating a new or replacement cleaning system, the ideal solvent would have the following properties: • Environmentally friendly —Does not create air or water pollution —Biodegradable • Not regulated at the federal, state, or local levels —Not implicated in ozone depletion —Exempt from VOC regulations —Not a HAP —Not implicated in global warming —Not on the SARA 313 or other regulatory lists —Not a RCRA Hazard • Solubility parameters match those of the contaminant to be removed • Works well as a single-component solution to avoid complex proprietary blends • Widely available at a reasonable cost • Compatible with all construction materials in the operation • Stable, does not readily break down in the presence of heat, metals, or chemical contact, and does not require the addition of stabilizers to achieve this goal • Nonflammable at operating and handling temperatures • Easily (and inexpensively) distilled or recycled • Low toxicity (a high PEL), with extensive animal testing and a long application history • Low or pleasant, yet detectable odor • Worker exposure easily controlled under the prescribed conditions of use • Fast evaporation rate for quick dry times • Low vapor pressure to minimize solvent losses Unfortunately, no chemicals have every desirable property, and development of an ideal solvent is unlikely. Therefore, the end user must evaluate the particular cleaning requirements as well as specific regulatory constraints. Solvents are often characterized by their degree of perceived toxicity and rated as low, moderate, or severe. However, it is possible that the largest category, especially for the newgeneration products, should be “unknown” or “unsure.” Classic solvents, including the aliphatic and aromatic hydrocarbons, alcohols, ketones, and chlorinated solvents have each been studied by numerous organizations and testing laboratories. Even with this sizable database, scientists, toxicologists, and regulators seldom agree on the significance of their results. It is wise to assume that all chemicals have some degree of toxicity and a priority should be to minimize emissions and worker exposure. Once the solvent options have been reviewed, the cleaning method must be chosen. Since there are no completely nontoxic solvents available for cleaning applications, the system must be designed to minimize hazards to the worker and the environment. This may include mechanical controls such as tank covers and auxiliary cooling coils to condense solvent vapors, or fans and exhaust hoods to remove solvent vapors from the workstation. Each of the solvent alternatives can be used safely with an appropriately controlled cleaning system. With organic solvents, the choice of cleaning methods generally falls into one of three categories: ambient temperature (cold cleaning), elevated temperature (hot liquid dip), or vapor degreasing (cleaning in boiling solvent vapors and often immersion in the liquid solvent). © 2001 by CRC Press LLC

COLD CLEANING Cold cleaning with organic solvents and solvent blends is often used when water is detrimental or ineffective, when the soils are of an oily or greasy nature, or when the capital costs of vapor degreasing cannot be justified. Generally speaking, the majority of the industrial cleaning applications can be accomplished in a cold solvent system. If cold cleaning provides results that meet expectations, use it. This method will ordinarily be the simplest, most trouble-free, have the lowest utility requirements, and be the least capital intensive of the cleaning system options. Cold cleaning methods are as varied as the solvent choices that go with them. The most significant limitations to cold cleaning are decreased cleaning efficiency as a function of workload, absence of a drying system, difficulty in controlling flammability, potential worker exposure hazards, and regulatory compliance. However, these limitations can be countered by the solvent selection and by equipment design. Pail and Scrub Brush This method is very basic and has a low capital investment. However, solvent losses and worker exposure may be excessive, particularly with solvents having a high vapor pressure and low allowable exposure limits. Brushing provides some abrasive action, but is generally not effective on small or intricate parts. A rinse in clean solvent is often necessary after brushing, and there is generally no means for reclaiming the solvent once it becomes contaminated. Hand Wipe Hand-wipe cleaning can be accomplished by carefully pouring solvent on a reusable rag, or the purchase of presaturated disposable wipers. Mechanical rubbing with the wipe provides some abrasive action, but unless the soil loading is low, it is likely to leave a thin residue film. Aerosol Spray Aerosol cleaning is effective for removing soluble soils and the spray action helps to flush away insoluble particulates mechanically. However, it is generally inefficient in solvent utilization and is therefore reserved for small bench-scale and precision cleaning applications. Depending on the solvent selected, there is a potential concern for flammability and/or worker exposure to high levels of the atomized solvent. Recirculating Overspray (“Sink-on-a-Drum”) Parts Washer This is a standard method for garage and maintenance shops, and has reasonable cleaning potential until the solvent becomes dirty. The solvent (traditionally a mineral spirits blend) is often replaced under a service contract, but it is necessary to assure that the solvent will be replaced often enough to meet the soil loading requirements. In addition, the convenience of this service generally comes at a high price. These systems are not generally suitable for high-vapor-pressure, low-flash-point solvents. © 2001 by CRC Press LLC

Figure 1

Recirculating overspray parts washer with vacuum distillation. (Machine by SystemOne, Miami, FL. With permission.)

It may be worth considering some of the new systems with built-in vacuum distillation for on-site solvent recovery (Figure 1). Such a system can reduce overall solvent usage and minimize off-site waste disposal. In addition, freshly distilled solvent is available on a regular basis and the need for frequent solvent change-out is eliminated, a particular consideration in heavy-duty operations. With solvents and solvent blends where there are concerns for worker exposure and odor, the unit should be equipped with a hood and exhaust fan for proper ventilation. In areas of poor air quality, recent regulations have focused on the VOC content of solvents traditionally used in sink-on-a-drum systems. As a result, water-based cleaners have been the suggested replacement. Where organic solvents are required for performance, using a recirculating system with a hood and exhaust fan and with exempt solvents such as parachlorobenzotrifluoride (PCBTF) or volatile methyl siloxanes (VMSs) may provide an additional option. Immersion Cleaning, Single-Dip Tank, with Manual Parts Handling Immersion cleaning is often the most economical cold cleaning method. These are simple cleaning systems where the workload is lowered and raised hydraulically, mechanically, or manually into liquid solvent. Agitation generally increases efficiency. Air agitation is not recommended because of high solvent losses to the atmosphere, but ultrasonic agitation is often recommended because of its powerful scrubbing action. Mechanical agitation can be supplemented with a pump and filter. Standard single-dip cleaning systems are offered by many equipment manufacturers for aqueous cleaning. With only minor modifications, these units can sometimes be adapted for use with organic solvents (Figure 2). Where worker inhalation exposure and odor must be controlled, top enclosures and side workload entry can be added. © 2001 by CRC Press LLC

Figure 2

Single immersion dip cleaning system.2 (Machine design by Magnus Equipment, Willoughby, OH. From BCG-OX-36, Occidental Chemical Corp., May 1996. With permission.)

In Figure 2, the parts are manually loaded on a roller conveyor, fed through a side opening on the machine, then immersed and hydraulically agitated in solvent. At the end of the cleaning cycle, the deck is raised to the top position and the parts are allowed to dry. Drying is accomplished by passing a stream of ambient or heated air over the basket. This design is useful for light workloads and is adaptable to a wide variety of parts. Addition of a still would enhance removal of oil. Automated Immersion Cleaning, Multiple-Dip Tanks A multiple-dip system (typically two to four tanks) is recommended for applications having high soil loading. Sequential dipping into progressively cleaner dip tanks provides for efficient solvent usage, and the final rinse is in the cleanest solvent. Automated parts handling is recommended to maximize process control and reduce worker exposure (Figure 3). The system is generally unsuitable for containing solvents that have a high

Figure 3

Automated multiple dip cleaning system with cascade overflow. (Machine design by Finishing Equipment, Eagan, MN. From BCG-OX-36, Occidental Chemical Corp., May 1996. With permission.)

© 2001 by CRC Press LLC

vapor pressure and low boiling point. Depending on the regulatory and toxicological profile of the solvent, additional controls may be needed. In the design in Figure 3, an automated hoist controlled by a microprocessor picks up the workload at the operator station (far left) and processes the part(s) through a cleaning cycle, a hot air drying chamber, and then returns clean dry parts to the operator station. The operator has the option of controlling duration of immersion, number of immersions, rotation, drying time, and drying temperature. A distillation system could be added to remove oils and keep the final dip tank supplied with fresh clean solvent. HEATED SOLVENT CLEANING METHODS In applications where parts are not adequately cleaned with a cold solvent, a combination of temperature and solvency may be required. For example, buffing compounds, spinning compounds, and waxes are solids at room temperature and must be converted to a liquid for effective removal. Heated Dip Tank Although solvents are generally more effective cleaners when they are heated, there are a significant number of disadvantages. Depending on the solvent selected, flammability may be a concern, solvent losses increase, and there is an increased potential for worker exposure. To address these issues, extensive safeguards may be required, equipment design becomes more complex, and costs increase. Vapor Degreasing Although the capital investment can be significant, vapor degreasing (Figure 4) is a very effective and forgiving technology. Cleaning can be accomplished by immersion in hot solvent with agitation and ultrasonics. The final cleaning takes place in freshly distilled solvent. This vapor blanket also helps to minimize solvent loss. The most important problems relate to the additional engineering controls required to comply with environmental regulations and to control solvent loss, to minimize worker exposure, and to use specific equipment design for low-flash-point solvents. In addition, for certain solvents, buildup of water and acidity must be controlled, so the process has to be monitored. SUMMARY Aqueous cleaning is not suitable for all applications; some solvent cleaning is appropriate. There is not now and there is never likely to be an ideal solvent. With appropriate controls and subject to the particular regulatory climate, solvents can be used responsibly in a variety of cold cleaning, heated cleaning, and vapor degreasing systems. The end user must consider his or her specific application to select the best option.

© 2001 by CRC Press LLC

Figure 4

Open-top vapor degreaser with still, hood, automated conveyor, and inert atmosphere. (From BCG-OX-36, Occidental Chemical Corp., May 1996. With permission.)

REFERENCES The following references were taken from technical bulletins produced by Occidental Chemical Corporation: 1. OXSOLs for Metal Cleaning, BCG-OX-19, January 1995. 2. Cleaning Systems for OXSOL 100, BCG-OX-36, May 1996.

© 2001 by CRC Press LLC

CHAPTER 2.8

Flushing Systems Richard Petrulio What makes flushing so different from the mainstream of cleaning processes? It is truly a unique process with subtleties that can make it difficult to develop. So what is flushing? Webster’s says it is to “cleanse with a rush of water.” From this simple definition the basic idea of a fluid moving across a surface to remove soils mechanically can be visualized. However, what is missing is the idea of enclosed, basically inaccessible surfaces that need to be cleaned. This goes beyond blind holes and slots to the fact that parts requiring flushing to clean internal surfaces are unable to utilize surface inspection to verify cleanliness. Thus, the process must be reliable. The inability to verify cleanliness of a part without dissecting it is what drove the manufacturer of custom equipment for airline galley refrigeration to develop its own flushing process and equipment. Refrigeration systems such as these circulate relatively small volumes of refrigerant (hydrofluorocarbons) and oil through a closed-loop system. Both a precision compressor and an electric motor, which drives it, are sealed within the system and thus are exposed to air that is circulated with the refrigerant. Valves with small orifices and heat exchangers with many feet of tubing join in as main parts of the fluid loop. Soils such as metal chips, moisture, or incompatible chemicals can do catastrophic damage to a refrigeration system. Unfortunately, poor cleaning in this application will not show up until the equipment has been assembled and operated. Having a compressor lock up or an electric motor burn up is a very expensive method of cleanliness checking. The question may still be asked, so why is flushing required? The answer comes in two forms: because you know what’s inside a part or you don’t know what’s inside a part. Mostly, removal of soils, known or unknown, from complex internal surfaces is driven by reliability. As with refrigeration equipment, metal fines and solid particles can damage moving parts or foul critical passages. Other soils may combine chemically within enclosed surfaces and slowly degrade the material. This activity is seen within a refrigeration system when water is left inside the parts. Water, when exposed to heat and refrigerant, can react to form acid. The acid will remove copper from the walls of the tubing and redeposit it on the surfaces of the compressor parts. Deposits of copper will grow on the moving parts and violate the clearances required for proper operation. Thus, after the equipment has been in service for a short period of time, it grinds to a painful halt, leaving the customer hot and the manufacturer with a tarnished reputation. Soils left inside parts may not cause catastrophic failure; instead, they could limit performance. Passages that require fluid flow can become partially or totally blocked. Small © 2001 by CRC Press LLC

valves with delicate or precise sealing surfaces can fail to seal. Surfaces that transfer heat may become insulated by soils causing loss of heat transfer rate. For many products these problems will cause annoying or embarrassing performance situations. Other products perform tasks that are safety-critical. For these, performance and reliability are not just desires; they are requirements. Examples of such products can be found aboard the U.S. space shuttle in the liquid oxygen systems. Of course, the issue driving the need for flushing may have nothing to do with safety, performance, or reliability. It may only hinge on aesthetics. Any issue that forces the need to flush must be addressed for its own requirements. However, if the product can operate with full performance, reliability, and safety while looking good doing it without flushing, then stop there. Do not flush time and money down the drain. Now, if the decision to use a flushing process has been put in place, the details must be fleshed out. Determining a level of cleanliness is the next logical step in developing the process. The goal is to clean only as much as required to consistently meet the product need. Develop the criteria and final results desired of the flushing process. The example refrigeration system required flushing to remove particles that could do mechanical damage to a compressor or block critical valve passages. In addition, fluids that could form sludge or acid had to be removed. The next problem is how to determine the goal has been met. Since the nature of parts that require flushing is that they have inaccessible internal surfaces, verification is difficult. Two directions can be taken for flushing process verification. The first is continuous inspection of parts for results. The other is to develop the process so that it will obtain the desired results without inspection. Although these methods could apply to almost any process, flushing presents a substantial inspection challenge. To gain visibility of internal surface cleanliness would require special equipment such as a boroscope, methods such as chemical analysis of cleaning fluid, or dissection of the part. Dissection of a part can only be used for spot or batch inspection because of its destructive nature, although for high-volume, low-cost parts this can be effective. Fluid sampling can give results for 100% inspection via analysis of samples for each part. Such a method would be suitable for small quantity parts, which require high precision and consistency of cleanliness. The downside of this method lies in the tracking required for each sample and the potentially long turnaround time for results. Immediate results can be obtained by using sophisticated equipment such as a boroscope. However, such equipment is expensive and requires proper training to be used effectively. Additionally, the interiors of some parts are not conducive to accepting the boroscope. For these reasons, developing the flushing process to do the job right every time becomes an attractive method. In the case of refrigeration equipment heat exchangers and plumbing, developing the flushing process to ensure consistent results proved to be the best method. Now that the forest view of flushing needs has been seen, it is time to look at some trees. The soils to be removed play an important part in developing the verification method. All known and potential soils should be listed. Each of the soils should be evaluated to determine if it actually needs to be removed. This is another opportunity to choose a no-clean option. Again, some of the soils may have no impact on the performance or reliability of a part. For those soils that do need to be removed, the process to flush them may not remove them all in one step. Aqueous flushing will require multiple steps to clean out the soils as well as rinse out the cleaning solution. Solvent flushing may also require additional steps to rinse out the main solvent. An early flushing technique used on the refrigeration heat exchangers employed a Stoddard solvent-based cleaner that had been punched up with perchloroethylene and methylene chloride. The solvent cleaned well but © 2001 by CRC Press LLC

was incompatible with the refrigerant and oil used in the final product. Thus, it required a rinse step using HCFC-141b to flush out the Stoddard solvent. Both aqueous and solvent flushing chemistries will likely require a drying step. The drying step is most difficult for aqueous and solvents that do not readily evaporate. As mentioned earlier, moisture is an enemy of refrigeration systems. Therefore, flushing with water did not look like a good candidate for the heat exchangers and plumbing. As the methods and chemistry for flushing are being narrowed down, it is important to consider material compatibility. In the same manner that soils were listed and their potential effects on the parts explored, compatibility of the cleaning compounds and process with the materials of construction must be evaluated. Substantial effort should be put forth to ensure that all materials that will be exposed to the flushing process would not be degraded in either the short or long term. Consideration for cleaning level, soils to remove, and material compatibility has now been given. But, what thoughts have been given to the environment? Remember that cleaning is a dirty business. A manufacturing facility must be able to provide a safe and appropriate environment for the flushing process to be performed. Further, the process must be designed to have as minimal impact on its surroundings as possible. The solvent flushing process used for the heat exchangers required a location with ample space, electrical power, and ventilation. Safety for the operators as well as governmental emission limits required that the process maintain a tight lid on release of the chosen solvent. These considerations were made a part of the design for the equipment as well as the process steps. Even so, upon initial operation of the system, air monitoring was conducted to ensure the safety of the operators and nearby employees. Long-term monitoring consists of emission logs to track any losses of solvent. Ultimately, common sense and sound ethics will dictate what equipment and steps are required to build a safe and environmentally sound process. Before the equipment is built and dropped on the production floor, put the whole process together virtually. Nail down the chemistry desired to fit with the projected requirements. Do not forget to have a backup. Next, envision the equipment needed to use the chosen chemistry. From these write down each step of the procedure from start to finish. Decide what skill level of operator is required to perform that procedure. Then, project ahead to when the process has matured some and look at who will be in charge of the process and equipment. Will the equipment be reliable? Who will be responsible for maintenance? What about record keeping and follow-on training? Building a virtual process will help shake out some of the bugs and shed light on potential pitfalls. Now it is time to make the process a reality. Equipment can be obtained outside or developed in-house. Since flushing is unique even to the cleaning industry, finding a turnkey system that is off the shelf is nearly impossible. Custom-designed systems can be fabricated but are generally very costly. In addition, the fabricator may not fully understand the process, thereby making it difficult to get the system desired. For these reasons, it may be justified to develop a system in-house. In support of this phase of the process, it is worth employing outside help to provide industry contacts and keep the sales glitz to a minimum. Time spent interviewing the industry with a well-versed consultant along is well worth the cost. The flushing process developed for cleaning heat exchangers and plumbing in the galley refrigeration equipment was designed and built completely in-house. The unique requirements of refrigeration eliminated the ability to use an aqueous process comfortably and not flushing could not be an option. Thus, the use of a solvent-based flushing process was the only option. The level of cleaning for the flushing process was difficult to determine. It was clear that moisture of any magnitude could not be trapped in the system. Past failures due to copper fines being ingested into the compressor led to the discovery that © 2001 by CRC Press LLC

forming oils within heat exchangers and complex tube shapes held these fines in place. These were the main challenges for flushing of new equipment parts. However, the repair of used equipment presented the challenge of cleaning out coked oil following a burnout of a compressor motor. A burnout occurs when the electric motor overheats or shorts out and causes the refrigerant and oil to burn. This generally will coat the entire refrigerant system with the tough black residue of the burned oil. To meet these cleaning needs, a very aggressive flushing system was going to be required. Fortunately, there had been a history of flushing activity used for the cleaning of refrigeration equipment parts. The downfall of these past methods was caused by the increased environmental awareness. Without the creation of new processes to take over for the old unacceptable methods, product performance and reliability suffered severely. One of the first methods employed for flushing heat exchangers was to blast liquid refrigerant R12 through the tubes and out to the parking lot through a hole in the wall. Verification of cleanliness required allowing some of the R12 to flow through a white towel. If the towel remained white, the flushing was done. Although this method worked very well, it was destined to be eliminated. The replacement method used the previously mentioned punched-up Stoddard solvent to break down the oils and loosen debris. A flush of 141b using the spray wand within a degreaser rinsed out the Stoddard solvent residue. Finally, shop air was blown through the heat exchanger to remove and evaporate the 141b. Again, the method worked but it was costly and rather unsound environmentally. It was at this point that a systematic approach to providing a flushing process was initiated. The understanding of soils and materials had been investigated and the time had come for professional assistance. With the help of a top consultant, numerous solvent chemistries and equipment options were investigated. As a result, n-propyl bromide was chosen to perform the cleaning task. However, that still left the equipment end open. After numerous conversations, demonstrations, and some hard-to-swallow quotes, it was clear that the only way to obtain the flushing equipment needed was to design and build it in-house. The result would be a safe, effective system with a reasonable price tag. During the virtual process phase, at least five different systems were penned out. Each focused on the need to introduce solvent into the parts, flush out the soils, and then remove the solvent without exposing the operator to the chemical. The largest challenge was removal of the solvent from the part such that it was not vented to the atmosphere and yet the part was left clean and dry. A sophisticated system was prototyped and given to the repair department for evaluation. Although it worked, a couple of the premises needed to be revisited. The system was modified on paper for creation of the first full-scale system. Its operation employs a vapor degreaser to provide clean hot solvent. Pumps draw the solvent out of the degreaser and direct it through the part being flushed. Solvent is passed through filters to collect particulate soils while the degreaser separates out the oils from the solvent. Once the automatic pump timers shut off the solvent circulation cycle, the solvent left in the part can be pushed out with carefully controlled nitrogen flow. The mixture of nitrogen and solvent is delivered to a separating tank to allow the solvent to be captured and the nitrogen expelled. Each of the components needed for this process was selected from commercially available hardware to ensure reasonable cost and allow for future replacement. Electrical controls and safeties were employed to allow for simple operation and control. As the system took physical shape, a comprehensive procedure and maintenance manual was written to be available as soon as the equipment was put into operation. Initial use of the system demonstrated the success of the process. The development effort had paid off. However, not all was rosy. A couple of valves did not operate as expected, technicians found ways to let small parts be ingested into the pumps, and the solvent separation left something to be © 2001 by CRC Press LLC

desired. Minor changes corrected the valve and debris issues; however, the separation issue took some thought. Because the temperature of the solvent was close to its boiling point when it entered the separation tank, some of it would exhaust with the nitrogen. The solution turned out to be a significant feature of the total flushing system. Refrigeration was used to subcool the solvent prior to reaching the separation tank. With the solvent now in a fully liquid state, it separated from the nitrogen effectively. Two complete systems have been in operation with continuous use. Cleaning results from the flushing process have been consistent and effective. The equipment combined with thorough procedures, training, and a maintenance program has allowed an effective and reliable flushing process.

© 2001 by CRC Press LLC

CHAPTER 2.9

Solvent Vapor Degreasing— Minimizing Waste Streams Joe McChesney and Joe Scapelliti

CONTENTS Introduction Vapor Degreasing Fugitive Solvent Emissions Reclamation Using Carbon Adsorption Overview of Fugitive Solvent Emission Process Adsorption Desorption Cool Down Calculations of Solvent Emissions/System Design Size of CAS System Operational Cost Estimates Electrical Water Return on Investment Summary Distillation Process—Requirements and Calculations Formulas Examples

INTRODUCTION Every industry, from the huge automobile plant down to the smallest specialty manufacturer, is impacted by the need to comply with environmental regulations, reduce amounts of chemical fluids used, and decrease waste streams. With today’s ever-changing rules and regulations concerning solvent cleaning systems, the cleaning industry has been greatly influenced by process control, from the standpoint of both equipment design and operational and maintenance procedures. The inclusion of these tighter processes on solvent vapor degreasers has created a substantial price increase in the original cost of the equipment. However, the savings resulting from © 2001 by CRC Press LLC

reduced chemical usage and reduced waste streams will offset the price increase, usually within the first year of operation. Waste streams typically consist of fugitive emissions and/or dragout losses as well as contaminated solvent residue in the distillation system commonly referred to as “still bottoms.” This chapter first describes the solvent cleaning process. Guidelines and calculations for reduction of fugitive solvent emissions by means of carbon adsorption are discussed as is recycling of the contaminated residue by means of distillation. VAPOR DEGREASING Vapor degreasing is a process that has been used in the United States since the early 1930s. It became a popular method for cleaning metal parts for both precision and general metal cleaning during the World War II and still remains widely used today. In the beginning, the solvents used in the process were basically the chlorinated solvents such as methylene chloride, trichloroethylene, and perchloroethylene. In the 1960s through the mid-1990s, other solvents such as 1,1,1-trichloroethane, CFCs, HCFCs, HFCs, and HFEs helped expand the process into electronic assembly cleaning, the cleaning of medical implant devices (i.e., artificial joints and pacemakers), and aerospace hardware. In the last few years a new solvent, n-propyl bromide, has become a part of the process for applications in both the general metal and precision cleaning market. The vapor degreasing process is very efficient. It provides excellent cleaning and drying in one tank. The distillation of the cleaning solvent within the system is continuous. This means the solvent is continually cleaned of soluble contaminants and used again in the degreasing system. The life of the chemistry in the system is potentially infinite so long as it is managed properly. The floor space and energy required to run a degreasing process are generally less than running other cleaning processes. Parts are cleaned in the degreasing process by vapor alone or vapor in combination with spray, or immersion and vapor, or immersion and vapor in combination with spray. Adding ultrasonics or mechanical agitation to the process can enhance certain cleaning operations. The parts to be cleaned can be manually processed through a degreaser by an operator using a hoist or they can be processed automatically using a robot or conveying system. A typical vapor degreaser is shown in Figure 1.

Free Board

Vapor Level

Flexible Hose Spray Lance

Condensing Coils Water Separator

Water Jacket Condensate Trough Boiling Sump

Heater

Figure 1

Typical vapor degreaser.

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Condensate Reservoir Spray Pump

The solvent used to clean the dirty parts is placed in the boiling sump and condensate reservoir. Heat is applied to the solvent in the boil sump bringing the solvent to its boiling point. As the solvent is boiled, a vapor is created and fills the machine. The vapor is maintained in the machine by condensing coils. The condensing medium normally is recirculated tower water. For low-boiling–point solvents, recirculated chilled water or direct refrigeration is used. The condensing coils convert the solvent vapor into liquid. The liquid is collected in the condensate trough and flows into the water separator. The condensed solvent flows from the water separator into a condensate reservoir. Excess solvent overflows from the condensate reservoir into the boil chamber completing the distillation cycle. Degreasing in vapor is a simple process. The parts to be cleaned are lowered into the degreaser and allowed to dwell in the vapor. The part that enters the degreaser must be cooler than the vapor temperature of the solvent. The solvent vapor begins to condense on the cool surface of the dirty part. As the solvent condenses on the part, it dissolves the oils or greases on the part. The dissolved oils and greases flow off the part and into the boil sump of the degreaser. This process continues until the work temperature and the vapor temperature are equal and then the cleaning stops. Spraying the part with distilled solvent from the condensate reservoir will enhance the cleaning process. Spraying the part removes insoluble debris such as chips, fines, and dirt. Spraying the part also cools the part below the vapor temperature allowing additional vapor rinsing to take place until the vapor and part temperature once again reach equilibrium. When the part is removed from the degreaser, it will be both clean and dry. This process is suited for parts of simple geometry that can be racked so that the parts to be cleaned do not have surface contact with each other. Examples of parts that can be cleaned in this process are flat sheets of metal, bar stock, simple stampings, molds, dies, machinery parts, transmission parts, engine parts. Parts of a more complex geometry or parts that are nested together in baskets or carriers will require more than vapor or vapor/spray cleaning. For applications such as these an immersion/vapor degreaser is required. See Figure 2. Examples of parts that would best be cleaned in an immersion degreaser are screw machine parts, heater cores, tubing, medical implants (i.e., artificial joints and pacemaker components), electronic assemblies, cosmetic cases, and fasteners. Both cleaning cycles can be augmented with features to enhance the performance of the degreasing system. Features such as ultrasonics can be added to assist in removing solids

Free Board

Vapor Level

Condensate Trough

Condensing Coils Water Jacket

Boiling Sump Water Separator Condensate Reservoir

Steam

Figure 2

Immersion/vapor degreaser.

© 2001 by CRC Press LLC

Figure 3

Conveyor and transporter.

from the surface of the parts being cleaned. Conveyors and transporters (Figures 3 and 4) can be used to move the work through the cleaning cycle. Sometimes it is desirable to tumble the baskets of parts as they move through the degreaser. Tumbling allows cleaning solvent to fill and then drain from cavities within the part. Tumbling is also helpful in dislodging chips and fines on the surface of a part. Filtration equipment will remove solid debris. Distillation equipment will remove oil from the system on a continuous basis. Carbon adsorption recovers solvent from airstreams, keeping it out of the environment and returning it to the degreasing system for reuse.

Figure 4

Conveyor and transporter.

© 2001 by CRC Press LLC

FUGITIVE SOLVENT EMISSIONS RECLAMATION USING CARBON ADSORPTION Overview of Fugitive Solvent Emissions Most manufactured products must be cleaned to remove lubricants, cutting oils, drawing compounds, miscellaneous contaminants, etc. used in the fabrication process. When the cleaning process involves typical solvents, it is practical, efficient, and sometimes mandatory that the emissive solvent vapors be recovered and possibly reclaimed. Carbon adsorption is one of the most efficient and cost-effective pollution control/solvent recovery processes available today. Carbon adsorption reclaims solvent vapors that would normally be dissipated to the atmosphere. Carbon is the preferred material used in adsorption systems because it exhibits unique surface tension properties. Because of its nonpolar surface, activated carbon will preferentially attract other nonpolar materials such as organic solvents rather than polar materials like water. The granular multifacet geometry of carbon also possesses tremendous surface area (with 1 lb having an area greater than 750,000 ft2). This characteristic allows carbon to adsorb up to 30% of its own weight in solvent. Solvent recovery consists of passing solvent-laden air through an activated carbon bed (Figure 5). The activated carbon captures the solvent molecules allowing residual denuded air to be exhausted to the atmosphere.

Process Adsorption Solvent-laden air is directed from the exhaust source to the activated carbon bed by a blower/fan assembly (Figure 6, left). The carbon adsorbs the solvent vapor and residual purified air is exhausted through the ventilation duct. This process continues until the entire carbon bed is near saturation.

Figure 5

Solvent recovery.

© 2001 by CRC Press LLC

Adsorption

Desorption Solvent Vapor

Damper

Damper Open

Laden Air In Damper

Damper Closed

Condenser

Activated Charcoal Bed Reclaimed Solvent

Clean Air Out

Figure 6

Steam In

Water Separator Waste Water

Solvent adsorption/desorption.

Desorption At the end of the time allowed for adsorption, the unit will automatically switch the incoming airflow from the first carbon bed to a second carbon bed. This will allow incoming solvent-laden air to flow through a fresh activated carbon bed while the first bed is desorbed or stripped (Figure 6, right). The first bed is now injected with steam, which passes through the carbon bed vaporizing the adsorbed solvent. Additionally, the physical characteristics of the steam condensate passing through the carbon assist in removing solvent residue. The mixture of steam condensate and solvent then passes through a water-cooled heat exchanger, which cools the solution. This allows for gravity separation to occur in a water separator device because of the difference in specific gravity of the liquids. The reclaimed solvent is now ready for reuse or disposal. The water discharge is channeled for treatment or disposal. Cool Down As hot wet carbon will not readily adsorb solvent, the carbon must be dried and cooled before the next adsorption cycle. Ambient air or process air is drawn through the bed for a preset period of time, which dries and cools the carbon. At the end of this cycle, the unit shifts into a standby mode ready for the next adsorption cycle. Calculations of Solvent Emissions/System Design The proper sizing of a carbon adsorption system (CAS) for a particular solvent application depends on two main factors: 1. The volume of recoverable solvent that is to be directed to the CAS 2. The amount of air mixture that is to be directed to the CAS With respect to the first factor, obviously, if the fugitive emissions that are lost cannot be picked up in an airstream and directed toward a carbon adsorption system, these fugitive © 2001 by CRC Press LLC

Table 1 Capacity of Activated Carbon (lb solvent/lb carbon at 80°F) Solvent Level (ppm)

TCE

MC

PCE

CFC 113

20 50 100 200 500 1000 2000 5000

0.052 0.066 0.077 0.091 0.110 0.124 0.138 0.153

0.00413 0.00935 0.0154 0.022 0.037 0.049 0.0633 0.08

0.069 0.092 0.099 0.112 0.129 0.142 0.152 0.167

0.033 0.041 0.050 0.060 0.077 0.091 0.102 0.125

ppm  parts per million; TCE  trichloroethylene; MC  methylene chloride; PCE  perchloroethylene.

emissions are not recoverable by normal means. The best way to determine the recoverable loss is to measure the amount of solvent in the airstream that is at present being emitted from the source, which is usually through a vent duct. This can be done with a variety of meters on the market today, but is most accurately performed with a recording device connected to a properly calibrated meter. It should be emphasized that solvent losses may not be constant. This necessitates either continuous monitoring via a recorder connected to the meter or periodic sampling. For the best accuracy, all factors should be gathered, starting with the total solvent purchases, and then determination made of how much is used, disposed as waste, and lost as a fugitive emission. With respect to the second factor, accurate equipment measuring solvent/air mixture is useless without knowing the exact amount of airflow. This can be best determined by using a precise air-measuring instrument. In most cases, lower airflow rates are preferred to reduce the amount of fugitive emissions being generated and to increase concentration of the mixture. This will allow the CAS to be more efficient. Calculation examples for determining CAS size make use of the carbon capacity with the solvent of interest (Table 1), and the rate of loss of the solvent (Table 2). Size of CAS System Customer vent system airflow is measured to be 2500 cfm. The solvent is trichloroethylene. The daily stack loss average measurement is 1500 ppm for the first 8 h and for the next 16 h is 800 ppm; the operation runs 6 days a week. Referring to Table 2:

Table 2 Loss Factors Trichloroethylene lb/h loss  ppm  cfm  0.00002175 Perchloroethylene lb/h loss  ppm  cfm  0.00002756 CFC 113 lb/h loss  ppm  cfm  0.0000286 Methylene chloride lb/h loss  ppm  cfm  0.00001414 cfm  cubic feet per minute.

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lb/hr loss  ppm  cfm  0.00002175 For first 8 h 1500  2500  0.00002175  81.375 lb/h For next 16 h 800  2500  0.00002175  43.4 lb/h Or for total day 81.375  8  651.0 43.4  16  694.4 1345.4 lb/day The user’s records indicate machines connected to this vent are using 70 drums/month and they operate 26 days/month, which means they are losing through the stack 26 days/month  1345.4 lb/day  34,980.4 lb/month but are using 70 drums/month  660 lb/drum  46,200 lb/month. Therefore, 34,980.4/46,200 or 76% is directly available for recovery. A CAS is approximately 95 to 98% efficient, so one can expect to recover a maximum of 0.95  34,950.4  33,231 lb/month. Referring to Table 1, at the rate of 1500 ppm, a unit will hold approximately 0.13 lb of solvent per pound of carbon. Therefore, with a system containing 1200 lb of carbon, 0.13  1200  156 lb of solvent can be recovered before desorption is required. At this high rate, the CAS will require desorption in a little over 2 h so the timers can be set to desorb every 2 h during the first 8 h. At the lower rate, i.e., 800 ppm, approximately 0.12 lb per pound or 0.12  1200  144 lb of solvent can be recovered between desorbs and since only 43.6 lb/h is lost during the remainder of the day, the timers can be set for 4 h between desorption for the 16 h the CAS is on low rate, thus saving water and steam. Thus, by calculating or obtaining the following information: • • • • • • •

Operational hours Amount of solvent to be adsorbed Adsorption characteristics of applied solvent Pounds of carbon required to adsorb incoming solvent amount Airstream velocity/volume Discharge limits to atmosphere Safety factor

a carbon adsorption system size can be determined that will safely handle the solvent emissions. Operational Cost Estimates The CAS system described above, with 1200 lb of carbon, needs 500 lb/h of steam, 2400 gal/h of water, a 5 HP motor for the steam blower, and a 3 HP motor for the condenser water to recover 4.5 gal/h of trichloroethylene. Electrical Steam power: 500 lb/h  945 BTU/lb  138 kW 3415 BTU/kW A 5 HP blower motor (460 V/3-phase/60 Hz) needs 6.04 kW; similarly, a 3 HP motor will have an energy use rate of 3.8 kW. Therefore, © 2001 by CRC Press LLC

138 kW  6.04 kW  3.8 kW  52 kW (misc.)  200 kW total loading Using the Tennessee Valley Authority (TVA) general power rate for a 200 kW total load  24 h operation  31 days/month  148,800 kWh at 100% loading. GP-12 rating for these conditions per TVA area is $8.708/h average cost. Water 40 gpm at 85°F inlet  2400 gph At a rate of $0.0013/gal, water cost is $3.20/h. Return on Investment Electrical costs: 8.708/h Water: 3.20/h 11.91/h operating costs Using trichloroethylene at $5.87/gal × 4.6 gal/h (recovered)  $27.00/h (recovered) less $11.91 operation cost  $15.09/h net payback. Therefore, $33,275.00 (system cost)  2205 h payback or 138 days operating 16 h/day × $15.09/h The recovery system can therefore pay for itself in well under half a year. Summary Carbon adsorption systems are ideal for solvent recovery. Numerous systems exist in the field today reclaiming various solvents with high efficiency. Recovery efficiency as high as 95 to 98% of the incoming solvent-laden airstream can be achieved or exceeded. In reclaiming this solvent, the system quickly pays for itself in solvent savings. Typical reduction in gross solvent purchases due to reclamation is 50%. In addition, carbon adsorbers help comply with EPA and OSHA regulations while providing a better workplace environment for employees. DISTILLATION PROCESS—REQUIREMENTS AND CALCULATIONS The process of distillation occurs when a fluid is heated to its boiling point and converted to a gas. The gas is then condensed back to a liquid and contained in such a manner as to remove it from the area of the mixture containing the liquid phase and other contaminants of fluids. Distillation is therefore a method through which contaminated or mixed process fluids may be separated, purified, and reused. Although it is not feasible in all cases, distillation should be given due consideration when practical. The main factor to be considered when determining the feasibility of distillation is the cost/benefit ratio. Components to the analysis should include the following: • Fluid purchase costs—How much can the purchased volumes be reduced? Will this increase the fluid unit cost by eliminating volume discounts?

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• Waste disposal costs—What part of the waste stream can be reduced or eliminated? • Capitol equipment expenditures—What will distillation equipment cost? Unit price can vary depending on the size and complexity of the unit and process. • Installation costs—Installation costs can be a significant percentage of the capital equipment purchase price; 25% is usually a good budgetary estimate. • Floor space—What is the floor space required and what will it cost? • Maintenance costs—Will maintenance costs increase, decrease, or remain the same? When associated with a cleaning process, maintenance costs can actually go down in some cases. • Energy costs—Is there an energy source readily available? Will this source add to the energy requirements or can it be supplied from another process or source that might otherwise be wasted? Additional energy costs will be associated with condensing the vapor back to a liquid. • Process control—Will the process be in better control if online distillation is added or if fluids are changed more frequently? • Environmental—In some instances the environmental issues outweigh all the other factors combined. These are the main factors to be considered when evaluating a distillation process; there may be others for a particular situation. These are included here as a starting point. Types of process fluids are essentially divided into two classes: • Flammables, which include alcohols, acetone, petroleum distillates, and many others. • Nonflammables, which include water and other types of solvents. Flammable fluids have special considerations when selecting equipment and processes. Nonflammable fluids by their very nature pose less risk in most instances. Processes of distillation include three types. They are classed by the pressure at which they operate: • Conventional or atmospheric stills • Pressure stills • Vacuum stills Some systems operate with a combination of pressures or levels of vacuum in order to extract different fluids. All distillation systems require at least four basic components: • Containment vessel—To provide for containment of the solution. This vessel can be of any one of a number of configurations. • Heat source—To boil the solution. This energy can be provided by any one of a number of sources including steam, electrical, hot water, natural gas, solar, and others. • Condenser—To provide condensing of the gas back to liquid. This condenser can be cooled by air, water, brine, refrigeration, and many other sources of conductive media. • Process controls—To ensure the process is producing the desired results. Controls can include temperature controls, pressure controls, fluid flow controls, and other analytical devices as necessary. © 2001 by CRC Press LLC

In addition, some systems may incorporate one or more of the following: • • • • • •

Vacuum pumps Fractional distillation columns Distillate analyzer Feed pumps Internal agitators Thin-film applicators

For purposes of discussion, this chapter deals with atmospheric stills. Upon initial start-up, the system requires sufficient time to heat the contents and the containment vessel to the boiling point before any appreciable amount of vapor is produced. Additionally, the air volume in the vessel must be displaced by vapor before distillation flow is stabilized. Formulas The basic formula for calculating heat input for a given distillation rate is as follows: H  D  ((T  Sh)  Lhv)  (Hrl  A) where H  total heat input in BTUs/hour D  total amount of solution in pounds distilled per hour Sh  specific heat of the solution in BTUs/lb/°F (This factor is readily available for pure solutions; however, the specific heat of a mixture will vary. This must be taken into consideration if the distillation rate is critical; otherwise using the Sh of the majority component is usually acceptable.) T  the difference in feed temperature and the boiling point of the solution in °F Lhv  the latent heat of vaporization of the solution in BTUs/lb (As with specific heat, the actual Lhv may vary depending on the different components of the feedstock.) Hrl  the heat lost from radiation to the surroundings in BTUs/ft 2/h. (The radiation losses may vary due to the containment vessel, boiling point of the solution, whether the vessel is insulated or not.) A  the total area radiating heat to the surroundings in square feet. This must include all the heated surfaces capable of radiating to their surroundings. Distillation rate for a given heat input can be obtained from the same equation as D  (H  (Hrl  A))/(T  Sh  lhv) Examples Table 3 contains density and boiling point data for the four solvents listed in Tables 1 and 2. A distillation unit made of 12 gauge steel designed to distill 2200 lb of trichloroethylene/h will require a certain amount of heat. The heat requirement is calculated as follows: © 2001 by CRC Press LLC

Table 3 Density and Boiling Point Data for Four Solvents lb/gal 12.22 TCE 13.55 PCE 11.07 MC 13.16 CFC 113

Boiling Point, °F 188 240 104 117

H  D((T  Sh)  Lhc)  (Hrl  A) H  (2200 lb  (((188°F  75°F  0.225 BTU/lb h °F)  103 BTU/lb))  ((315 BTU/ft 2)  (64 ft 2))  302,695 BTU  302,695 BTU/3414 BTU/kW  89 kW A similar unit designed with a fixed heat input of 60 kW of electric heat should distill a specific amount of trichloroethylene. Expected distillation rate can be calculated as follows: D  (H  (Hrl  A))/(T  Sh  Lhv) D  (60 kW  3414 BTU/kW)  ((315 BTU/ft 2)  (64 ft 2))/ (188°F  75°F)  (0.225 BTU/lb/°F)  (103 BTU/lb))  1438 lb/h Addition of insulation can lower the BTU loss from radiation significantly. Radiation loss in this example is from a bare metal surface and is approximately 7% of the total heat required. This can be lowered to less than 1% with proper insulation. The formulas should work for most fluids considered recoverable by distillation provided the necessary factors are available. Payback calculations for a distillation unit can be made in a similar manner to those made for carbon adsorption systems.

© 2001 by CRC Press LLC

CHAPTER 2.10

Vapor Degreaser Retrofitting Arthur Gillman

CONTENTS Introduction Economics Safety Retrofitting Freeboard Ratio Major Emission Reduction Devices Freeboard Chiller Carbon Superheat Additional Emission Control Devices Cover Controlled Speed Hoist Retrofit Sources INTRODUCTION The first question would be why? If a unit is in good working order, and there are no particular complaints, why make costly changes? There are good reasons, and they include regulation, including federal (NESHAP), state, and regional. There are also economic and safety issues. The Halogenated Solvents NESHAP (National Emission Standard for Hazardous Air Pollutants) is a federal regulation that specifically regulates vapor degreasers using trichloroethylene, perchloroethylene, l,l,l-trichloroethane, and methylene chloride (two additional solvents not typically used in vapor degreasing are also part of the NESHAP). It is made up of a series of emission reduction choices. Assuming the vapor degreaser is in relatively constant use, retrofitting is most often the best choice. ECONOMICS Where low-cost chlorinated solvents either cannot be used or are not a good choice, © 2001 by CRC Press LLC

there are new so-called exotic solvent and solvent blends to choose from. One of the common threads among many of these newer solvents is cost. They are expensive per pound, per gallon, or per drum. They are so expensive, costing perhaps $10,000 to $15,000/drum, that unnecessary solvent losses are worth preventing. Proper operating procedure, combined with a decent retrofit, can produce operating costs on a par with the older solvents. SAFETY Each solvent has a toxicity listing called a threshold limit value (TLV). Many of the newer solvents are more toxic (lower TLV) than the solvents they replaced. Further, the chlorinated solvents are being reevaluated and may see lower limits set. One of the newer solvents, normal-propyl bronide (nPB), has a recommended exposure rate but it has not been firmly established. The government-approved rate has not been set as of this date. This all means that reducing operator exposure makes good sense. RETROFITTING Retrofitting means making physical changes and additions to the vapor degreaser. Although there are theoretically many things to be done, here are the tried-and-true “best of the list.” Freeboard Ratio This is defined as the distance from the point where the boiling solvent vapor idles (usually around the middle of the cooling coils or cooling jacket) to the top of the machine opening. This dimension must be at least equal to the narrowest width of the overall vapor area. Example: If vapor depth measures 20 in. and the tank measures 24  48 in., then the freeboard ratio must be raised to the narrowest dimension of 24 in. or an increase of at least 4 in. This is a standard that has been changing. The early vapor degreasers were typically manufactured with a freeboard ratio of 50%. Later, the federal government dictated that this ratio should be raised to 75%. The federal NESHAP rules now dictate a ratio of 100%. The question is often asked, “Will the ratio be moved higher?” This author’s opinion is no. The reason is that a ratio above 100% does not improve idle losses by much and the continual raising of the freeboard causes operating problems, including interference with hoist mounting and ceiling heights. There remains the question of how much freeboard is “best.” The answer is, the more the better, but be practical! Even if one is not affected by NESHAP, a freeboard ratio of 100%, or greater, is going to reduce idle solvent losses. Freeboard ratio can be accomplished by installing a stainless steel collar of the appropriate height. Make certain that the collar is sealed and that the top is flat and sturdy enough to support a proper cover. We recommend that, where possible, the top should have a lip that is turned in horizontally toward the tank opening and then formed down toward the vapor. Major Emission Reduction Devices Freeboard Chiller This consists of a second set of cooling coils, powered by a separate refrigeration compressor condenser, using an EPA-approved refrigerant such as 404A. The coils are mounted © 2001 by CRC Press LLC

as close to the primary cooling coils, or jacket, as practical. Freeboard coils can be mounted one side of the tank wall or around all sides. The combination of refrigeration power (motor horsepower) and coil surface area must produce a temperature at the center of the tank, and center of the coil system, that does not exceed 40% of the boiling point of the solvent. The larger the opening of the vapor degreaser, and the higher the boiling point of the solvent, the more power and surface area is required to achieve the desired temperature. We recommend using finned tubing and mounting on all four sides for best results. There are situations where this is not practical because the addition of either finned tubing, or the mounting on all four sides, chokes off the tank area too severely. In this case, do what you need to get results. This might include mounting finned coils on one side only or using straight, nonfinned tubing, and raising the number of coil wraps to increase the surface area. Carbon Activated carbon systems have been used for decades and can be quite effective. A system consists of one or two specifically sized canister(s), a lip exhaust, a heat source to release the solvent from the carbon, and a condensing system to collect the condensate. The idea is to draw off vapor from the top of the vapor degreaser and trap (adsorb) it in the carbon canister. When loaded, the canister is desorbed by heating the carbon, causing the trapped solvent to turn to vapor where it is condensed. A two-canister system allows for continuous operation. Carbon systems typically are chosen only for very large vapor degreasers. Cost is the reason. Carbon systems can easily cost $100,000 and up. For this reason, carbon is warranted only when there is no other economic choice. Super Heat Superheat involves the addition of a heated surface placed in the vapor zone. By raising the temperature of the solvent vapor above the boiling point of the solvent, liquid solvent that is entrained in the parts is boiled off and the result is less solvent dragout. Heating must be carefully controlled because surface temperatures that are too high can damage the solvent. Each solvent has its own limits. The most common heating method is circulating hot oil. The problem with super heat as a retrofit device is that it significantly reduces tank area. For that reason it is not as popular as a retrofit choice and is most often considered a major emission control device on new equipment. Additional Emission Control Devices Cover Here is a simple test to determine if the cover style is adequate. Can one open and close the cover quickly and not disturb the vapor? If not, then a non interfering-style cover is necessary. Cover styles that do not disturb the vapor include roll top, sliding, and pivot. Some of these styles are available in both manual and automatic versions. Power covers are best in two situations. The first is with large vapor degreasers where reaching across the opening presents a risk and unnecessary exposure. The second situation is any vapor degreaser utilizing a programmable hoist or material handling system. The reason is because it is often possible to integrate the automatic open/close function as part of the hoist or material-handling controller. The bottom line advantage is solvent saving, knowing that the cover will be closed after each cycle and during idle periods. © 2001 by CRC Press LLC

Controlled Speed Hoist The most overlooked emission control device is the controlled speed hoist. This device guarantees that the speed of the workload will always be at or below the 3.3 m/min maximum mandated by NESHAP and other regional rules. This is the speed limit determined to be necessary to prevent dragging out vapors. With large vapor degreasers and heavy loads, a hoist seems obvious. But with smaller systems it is often overlooked. The problem is that an operator has no concept of what 3.3 m/min means. Even if the operator did, there is another problem and that is that the typical basket handle puts an operator’s arm at an awkward and uncomfortable angle. Going that slow is almost impossible. The result is that most solvent losses occur during load insertion and withdrawal. That means nothing anyone can do will save more solvent than automating the speed of the parts in and out of the vapor degreaser. Hoist systems can be as simple as pendent-controlled chain hoists, costing around $1000, to microprocessor hoists (Figure 1) that can automate all of the movement involved in the cleaning cycle, as well as automatically turn on/off various vapor degreaser accessories, such as automatic covers, ultrasonics, and pump/filter systems. Cost of these automated systems can range from approximately $12,000 to $50,000, depending on weight and complexity of function. Retrofit Sources Contact the vapor degreaser supplier, as well as the solvent supplier. In addition, there are independent retrofit suppliers who specialize in this area.

© 2001 by CRC Press LLC

Figure 1

Automated hoist. (Courtesy of Unique Equipment Corporation.)

© 2001 by CRC Press LLC

CHAPTER 2.11

Enclosed Cleaning Systems Don Gray and John Durkee

CONTENTS Background and Definitions Another Distinction A Dynamic Field Rationale Principles of System Design Airtight Systems Airless Systems Externally Sealed Systems Summary, Enclosed Systems Regulation of Enclosed Systems Federal Regional Costs Analysis Hidden Costs Why Purchase an Enclosed System? Summary References

BACKGROUND AND DEFINITIONS While enclosed cleaning systems have been used for specific applications for several generations, their popularity as a solution to broader industrial cleaning problems has only emerged in the United States in the 1990s. At the heart of every enclosed cleaning system is a cleaning process. The general purpose of the enclosure is to protect the environment from emissions from the cleaning process. A second purpose is to implement some unique cleaning process within the enclosure, which could not be implemented outside the enclosure.

© 2001 by CRC Press LLC

Enclosed cleaning systems are of three general types. They differ by the degree and method by which they are sealed from the ambient environment. As one would expect, sealing is the key issue in defining enclosed cleaning systems. The three types are Airtight—These systems are sealed to contain a light pressure above ambient. Typically, maximum pressure is around 0.5 psig. Airless—These systems are sealed to contain either full vacuum (1 mmgHg) or a pressure significantly elevated above ambient (800 to 10,000 mmHg). The word has been used generically, and later as a trademark. Externally Sealed—These systems are not sealed to contain either pressure or vacuum. Rather they are sealed to restrict interaction of the internal environment with the ambient environment.

Another Distinction All enclosed cleaning systems bring the value of keeping the cleaning solvent “in the tank.” There are two very different methods by which this is accomplished. Usually both are incorporated in any enclosed cleaning system—however, one is the dominant method of emission control. Reliance on each method has very different consequences for users. The first method is described by environmental engineers as “tailpipe control.” Generally this means solvent vapors leave the cleaning system in a stream of air and pass through a bed of activated carbon to be adsorbed prior to discharge to the environment. Nearly every enclosed system uses carbon treatment for “tailpipe control” to meet environmental standards. The second method would be similarly described as “pollution prevention.” This means that the operating process has steps through which solvent liquid and vapor are recovered and not allowed to leave the cleaning systems. Although not necessarily practical, excellent internal recovery of solvent could mean that external carbon treatment is not required.

A Dynamic Field This subject is a “moving target.” The commercial application of enclosed cleaning systems is affected by environmental regulations, investment at purchase, perception of economics in use, competitive offerings, customer needs, availability and price of solvents, and quality of design. As this is written, all factors are in flux—especially environmental regulations and investment at purchase. New and existing firms are providing new offerings of enclosed cleaning systems. Existing firms, currently offering enclosed cleaning systems, are retrenching. Prices in the United States and attitudes about enclosed cleaning systems are also in a state of flux. Consequently, a comparison by supplier of offerings would be obsolete within a year or so. For example, such a comparison written in 1997 would not have included the impact on the marketplace of the LAER/BACT regulations (lowest achievable emission rate/best achievable control technology) and would be of little value to current readers. So, this chapter will focus on basic differences among enclosed cleaning systems, general principles of operation, common process steps, and lasting disadvantages and advantages of their use. © 2001 by CRC Press LLC

Rationale In a sense, the initial applications of enclosed systems were chemical reactors, autoclaves, or storage vessels. Only very seldom would a process engineer consider completion of a chemical reaction in an open system, and depend on controlling atmospheric diffusion rates to keep the feeds and products of reaction out of the ambient environment. Similarly, when it became necessary to contain emissions from cleaning systems it was natural for process engineers to turn to sealed vessels. In traditional, liquid/vapor degreasers, diffusion-based controls (high side walls with refrigeration) are used with open-top cleaning systems because they are low cost, not because they produce high containment efficiency. Here the establishment of effective sealing mechanisms offers much higher efficiency of containment. When this degree of containment is demanded by environmental regulations, or for other reasons, cleaning experts turn to enclosed systems. PRINCIPLES OF SYSTEM DESIGN Design of enclosed systems is partially based on what is known about the equivalent cleaning process in an open-top vapor degreasing system. The designer of any enclosed system must consider the following principles: 1. Most of the same processes as practiced in an open-top vapor cleaning system can be well used in an enclosed cleaning system. That is, almost any cleaning process (immersion, sonics, hot rinse, superheat, dry, etc.) practiced in an open-top system can be converted to an enclosed system for the purpose of emission reduction. 2. In addition, other process features may be added or subtracted. For example, vapor spray onto parts often leads to unacceptable emissions in an open-top system, but is a normal cleaning technique practiced in enclosed systems. 3. Environmental contaminants must not be allowed to enter the cleaning chamber of the enclosed system, or additional precleaning process steps will be necessary. Basically, the items of concern in the outside environment are humidity (water), noncondensables (nitrogen and possibly oxygen), and airborne particulates. Because the system is sealed, the process designer must be careful to eliminate the entry of impurities that are not normally purged from the enclosed system. For example, retention of water and oxygen can lead to rapid deterioration of the solvent. Finally, any process in an enclosed system must allow for separation of the solvent from the internal environment prior to release of that environment. 4. The environment inside the chamber of the enclosed cleaning system must not be allowed to enter the ambient atmosphere. Since this environment is rich (or possibly saturated) in solvent, the result would be significant air pollution. One cannot simply “open the door” in the enclosed chamber when the cleaning cycle is complete, because the chamber is loaded with solvent. This chapter discusses each of the types of enclosed cleaning systems, and provides some examples where they have been successfully and unsuccessfully used. AIRTIGHT SYSTEMS Compared with other enclosed cleaning systems, airtight systems are simpler to design, cheaper to construct, and more inexpensive to operate. Usually, the cleaning cycle © 2001 by CRC Press LLC

in an airtight system is rapid. In fact, a cleaning cycle in an airtight system may well be shorter than the same cycle in a open-top system. Here is an example of a cleaning cycle for parts with a low thermal mass (ballpoint pen refills, catheter wires, electronic connectors, gold foils, eyeglass frames, etc.). Example—Airtight 1 Total Elapsed Time (min:s) A. B. C. D. E. F. G.

Parts loaded in racks; racks loaded in chamber Chamber sealed; cycle selected and started Hot liquid solvent sprayed on cold parts, parts heat rapidly Superheated solvent vapor sprayed on hot parts to dry them Solvent vapors displaced with dry forced hot air Hot air displaced with cool air to cool parts Cycle complete; chamber unsealed automatically

0:00 0:15 1:45 3:15 4:30 5:15 5:30

The equivalent process in an open-top vapor degreaser would be immersion in a single sump followed by spray-drying with hot vapor. Steps E and F are required to satisfy the fourth principle, to keep solvents from escaping. The process equipment required for step F is a holdup chamber followed by a huge carbon absorption column. The holdup chamber is mandatory because the hot air solvent mixture is forced from the cleaning chamber at a rate higher than solvent can be adsorbed by the carbon absorber. This is an important point. It can be used to distinguish low-cost enclosed cleaning systems of poor design from enclosed cleaning systems offering real value. Those who would choose this process: 1. 2. 3. 4. 5. 6. 7.

Desire solvent cleaning to avoid mineral residues (water spotting) Require extraordinarily low solvent emissions Greatly value short and controlled cycle time Have a low level of soil on their parts Have a high throughput and a highly automated process Are able to rack parts for exposure to liquid, vapor, and air sprays Require good to excellent drying

The first example illustrates a short cleaning cycle. As a second example, envision a situation where the soil is difficult to remove and the parts have a high thermal mass. Such a process is similar to the first in respect to points 1, 2, 6, and 7, but could take nearly 50 min, because long soaking in hot solvent is required to remove soil (e.g., wax/gum, soot, or buffing compound). Ultrasonic cleaning may be required during the immersion process. In this case, step C, from the previous case, is replaced by a step lasting 34:45 where the parts are immersed in hot boiling liquid solvent. The remainder of this process is similar to Example Airtight 1 except that the parts are elevated above the immersion vessel prior to the start of drying steps D and E. In step F, 45 s rather than 15 s might be needed to cool parts with a higher thermal mass. These users would also need to monitor solvent quality frequently because, in contrast to design principle 3, water and oxygen enter the sealed chamber. While equipment is quite flexible, there is one common operation that cannot be completed in an enclosed cleaning system: true boiling of the solvent. True boiling happens when the vapor pressure of the solvent equals the total pressure of the atmosphere. The former is a property of the solvent molecules. The latter is atmospheric pressure in open space, © 2001 by CRC Press LLC

or some pressure in an enclosed chamber. Air has a partial pressure. The sum of partial pressures of the two vapors (air and solvent) must equal the total pressure. The presence of a diluent (air) means that the partial pressure of the solvent can never equal the total pressure. Thus, the solvent can never truly “boil.” If the temperature in the chamber is raised, the partial pressure of air is raised per Dalton’s law; and the partial pressure of solvent follows the vapor pressure curve upward. Is it possible to have high solvent evaporation rates without boiling? Absolutely! But the net rate of vapor generation will not be as high as that in true boiling. The vapor condensation rate on the parts is likewise impeded by the presence of air, which, with no place to go, cannot be displaced by a vapor blanket as in an open-top degreaser. The solvent now must diffuse to the solid surface through the air surrounding the part—thus adding a significant resistance to condensing heat transfer. There are no inherent limitations to which solvent can be used. Naturally, the solvent should be chosen to match the soil. Newer, engineered solvents such as hydrofluorocarbon (HFC), hydrofluoroether (HFE), and hydrochlorofluorocarbon (HCFC) are well suited for the first application (short cycle time) because of their rapid evaporation rate. However, trichloroethylene (TCE), perchloroethylene (PCE), and normal-propyl bromide (nPB) are often used to remove drawing oils and ink residues from ballpoint pen components. Flammable solvents are not commonly used with air-spray processes because of the absence of the control of sparks. But they could be used readily with the second application (soaking).

AIRLESS SYSTEMS Airless systems offer the most capability and power for customization of cleaning operations, albeit with the highest price tag. The capability of pressure (vacuum) greatly expands the possibilities for use of airless systems. The initial airless systems were developed for three applications: use of highly regulated toxic solvents, cleaning of large parts, and cleaning/drying of complex parts. Airless systems are commonly used in Europe, chiefly with hydrocarbon solvents. In addition, there are many untapped, unrecognized applications. Typically, an airless system is a vacuum system, although a few systems use methylene chloride or other solvents1,2 under pressure in such applications as paint stripping. Temperature and pressure are linked because the boiling point of a solvent decreases as the pressure is reduced. The reverse is also true. For example, it is quite possible to use TCE (boiling point 189°F) at the temperature at which CFC-113 is used (117°F). The vapor pressure curve indicates what pressure should be selected to attain that temperature. Several commercial processes will be described below. The first3,4 is typical of an approach to cleaning and drying highly porous or complex parts. Examples include metal structures impregnated with grease for lubrication, multiport injectors, or aluminum/alloy honeycomb structures used as panels in construction of aircraft. The process schedule is described below. Example—Airless 1 Elapsed Time A. Parts loaded in racks; racks loaded in chamber B. Chamber sealed; cycle selected and started

© 2001 by CRC Press LLC

0:00 0:15

Elapsed Time C. Air removed by vacuum pump-down to 1 mmHg (to remove water and oxygen; see design principle 3) D. Parts sprayed with hot solvent vapor (which raises total pressure); the vapor condenses on the cold parts E. Solvent vapors are removed by vacuum; parts are naturally cooled F. Step D is repeated (solvent vapor spray) G. Step E is repeated (vacuum evacuation) H. Step D is repeated (solvent vapor spray) I. Step E is repeated (vacuum evacuation) J. Chamber is filled and flushed with air (fed to carbon absorption column) K. Step E is repeated (vacuum evacuation) L. Chamber is filled and flushed with air (fed to carbon absorption column) M. Cycle complete; chamber unsealed automatically

3:30 5:30 7:30 9:30 11:30 13:30 15:30 16:30 18:30 19:30 20:00

Note that step C reduced the air concentration in the cleaning chamber to 1300 ppm (v/v) (equal to 1 mmHg/760 mmHg), the oxygen concentration to around 250 ppm (20% of 1300), and the water concentration in a warm, humid ambient environment (100% relative humidity at 90°F) to 50 ppm. For additional control of the process environment, the evacuated chamber could be filled with clean dry nitrogen and again vacuum-evacuated to 1 mmHg. That would reduce each concentration by a factor of 759/760. The above process provides three separate stages of vapor degreasing. Parts are cooled between stages by vacuum removal of vapor. Obviously, this process could be shortened by use of fewer wash stages. A cycle time of 15:00 to 20:00 is typical. Another example where airless systems can provide excellent value is with hydrocarbon-based solvents. Hydrocarbon-based solvents are used in Europe because of an aversion to chlorinated solvents. Hydrocarbons are high boiling (some are at 400°F), have a flash point above 200°F, are excellent solvents for hydrocarbon-based soils, have low odor, low skin irritation, etc. The drawback is evaporation/drying rates are very low. Airless systems overcome the drying problem: (1) nitrogen is added to increase the pressure and dilute the hydrocarbon concentration, and (2) the chamber is vacuum-evacuated. When the total pressure is quickly reduced to around 10 mmHg, the “oily” hydrocarbons “fly off” the parts. Some vendors have excellent videos showing this effect. If drying is not sufficient, the cycle may be repeated. The process schedule is shown below. Steps A through F are omitted because the operations and timing are similar to Example Airless 1. At step D, parts are sprayed, then immersed in hot solvent, and ultrasonic agitation may be used. Example—Airless 2 Elapsed Time A–F. As above except step D as noted N. Step E is repeated (vacuum evacuation; first drying O. The chamber is filled with hot nitrogen and the pressure increased to 500 mmHg P. Step E is repeated (vacuum evacuation); final drying Q. Chamber is filled and flushed with air (fed to carbon absorption column) R. Cycle complete; chamber unsealed automatically

© 2001 by CRC Press LLC

9:30 11:30 12:30 14:30 15:30 16:00

That this technique is only seldom practiced in the United States does not mean it is not of value. Here, improved technology is employed to allow use of an excellent and environmentally sound solvent—while overcoming the normal drawback of this solvent. A recently commercialized process5,8 provides good value for cleaning of small parts. In the previous system,3 air is removed by vacuum before solvent is added. This is the desirable system (according to principle 4). In this example, the vacuum step is only used for final drying. Instead, at step C, air and water are displaced by flushing with hot nitrogen. Note that this system is not a “true” vacuum system in the sense that the cleaning is not done at reduced pressure—under vacuum. But if the cleaning process downstream of step D is appropriate, the user should receive clean dry parts. Example—Airless 3 Elapsed Time A. B. C. D. E. F. G. H. I. J. K. L. M. N. O.

Parts loaded in racks; racks loaded in chamber Chamber sealed; cycle already started Chamber flushed with hot nitrogen (to displace oxygen) Chamber filled with oxygen-free hot nitrogen after purging Hot solvent liquid introduced for immersion cleaning Immersion cleaning with ultrasonics Drain liquid and flush with hot clean solvent Second immersion cleaning with ultrasonics Drain liquid and flush with hot clean solvent Continuous flushing with hot clean solvent Drain liquid solvent from tank Blow hot nitrogen across parts to dislodge liquid Evacuate chamber to 1 mmHg, to dry parts Replace chamber environment with clean dry air Cycle complete, chamber unsealed automatically

0:00 0:00 1:00 1:30 2:00 4:00 4:30 6:30 7:00 9:00 9:30 10:30 13:30 14:45 15:50

A final example involves an unusual solvent—water.9 This technique could also be used with other solvents. Vacuum technology is used to overcome the major limitation of aqueous cleaning—drying. Evaporation of water at atmospheric pressure is slow, and soluble mineral salts are left behind on the parts as imperfections, stains, scars, or spots. The evaporation rate is raised by the huge partial pressure difference between water on the part surface and the water in vapor space. Naturally, the partial pressure of water in the vapor space is low because the vacuum pump is continually removing all vapor from the chamber. Evaporation of water under vacuum is quick, but brings an unexpected problem: ice. Remember, evaporation involves both a transfer of mass (water) as well as a transfer of heat. This is true for evaporation in a vacuum or under pressure. If water (or any other liquid) is evaporated, the heat of vaporization must be supplied. In this case heat comes from the surroundings. Typically, without process modification, the parts become chilled, and the remaining water becomes frozen. This situation is much more critical with water than with organic solvents because the heat of vaporization of water is 1000 BTU/lb and that for organic solvents is 200 BTU/lb. The modification is to add hot air, hot water, or radiant heat, so that the heat of vaporization is supplied externally. Using the system for drying only has a cycle time of about 14 min. © 2001 by CRC Press LLC

Water spots are usually oxides and salts of metal ions. The metal ions are soluble in water—that is the reason the final rinse is often with metal-free (deionized) water. The oxygen component is thought to come from the air. No metal salts are left on the surface since the oxygen has been removed from the chamber prior to evaporation of the water, and since metal-to-nitrogen bonds are exceedingly difficult to form. What happens to the metal ions left on the parts by the last rinse with water? These authors do not know, but believe that the ions remain on the parts as ionic contamination. EXTERNALLY SEALED SYSTEMS Basically, these systems are open-top systems in an isolation chamber. Thus, the solvent is effectively separated from the atmosphere. Exhaust is vented through a carbon absorption trap. In some cases, the isolation chamber is retrofitted on an existing open-top vapor degreaser.9 Additional designs have recently become available. Externally sealed systems have been designed around traditional vapor degreasers as well as low-flash-point systems. In externally sealed systems, loading and unloading of parts requires either more labor or additional capital for automation. In addition, cycle time may be increased. SUMMARY, ENCLOSED SYSTEMS The above information is summarized in Table 1. REGULATION OF ENCLOSED SYSTEMS Two major air pollution regulations cover enclosed systems. One is federal, the other is regional. The federal regulation applies to all users in the United States. The regional regulation, which is more stringent, applies to U.S. firms in that region, or in regions that have been defined by the EPA as having similar characteristics. Federal The federal regulation is one of many National Emission Standards for Hazardous Air Pollutants, known by its acronym NESHAP. The halogenated solvent NESHAP was published in December 1994, and took effect in December 1997. This NESHAP covered cleaning operations using the chlorinated solvents 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), methylene chloride (MC), and perchloroethylene (PCE), and two others not used normally in cleaning operations. The basis for this standard was maximum achievable control technology (MACT) as defined in the 1990 Clean Air Act1 for these solvents. This was embodied in the engineering requirements for compliance as 50 to 70% control efficiency. This NESHAP does not cover other halogenated and nonhalogenated solvents. The adjectives used here: airless, airtight, and externally sealed, were not in common use when the NESHAP for chlorinated solvents was developed. These terms are not mentioned in the NESHAP. However, these three types of cleaning systems are covered by interpretation of other language. In the NESHAP, airless systems are included under “solvent systems without an air–solvent interface.” Although airtight systems are full of air © 2001 by CRC Press LLC

Table 1 Comparison of Types of Enclosed Systems Item

Airtight

Airless

Externally Sealed

Operating pressure

Low pressure

Atmospheric

Operating Temperature

 Normal boiling point Volatile

Vacuum or pressure Any

Choice of solvents Type of parts Best type of process Drying quality Estimated investment (smaller units) Estimated operating costs10 (with TCE and PCE) Supplies

Best with low thermal mass Hot soaking in liquid

Any (volatile or nonvolatile) Any

Normal boiling point Volatile

Normal with volatile solvents 2.0  open top

2.5  open top

Best with low thermal mass Traditional vapor degreasing Normal with volatile solvents 1.25  open top

10%  open top

 open top

 open top

One to two

Five

Two (plus two with flammables)

Hot vapor spray Vacuum quality

and solvent, as are externally sealed open-top systems, airtight systems are classified in the same manner as airless systems.1,2 Unfortunately, the NESHAP was written approximately 4 years ago when externally sealed systems were not recognized. The EPA only considered “systems without an air–solvent interface” as those “that do not expose the cleaning solvent to the ambient air during or between the cleaning of parts.”1,3 The authors’ interpretation is that this definition includes only airless (and airtight) systems, but does not include externally sealed open-top systems. The EPA has recently confirmed this understanding.1,4 Regional Los Angeles and surrounding counties have a significant problem with smog caused by emissions of volatile organic compounds (VOCs). The EPA defines this region, regulated by the South Coast Air Quality Management District (SCAQMD) as “non-attainment” for federal VOC guidelines. The concepts of lowest achievable emission rate (LAER) or best achievable control technology (BACT) apply in all nonattainment areas. These concepts are beyond the scope of this chapter. The situation is contentious and, as of this time, subject to continuing interpretation. Given these extreme problems, manufacturers wishing to use nonexempt solvents in new operations (this includes changing locations and changing solvents in a given operation) might do well to consider an enclosed cleaning system with documented, demonstrated emissions values. Other regions of the United States are nonattainment areas. If solvent cleaning in nonattainment areas is contemplated, one should insist on a commercial-scale demonstration or a supplier certification prior to purchase of a new or rebuilt solvent cleaning system. In addition, the regional environmental regulatory agency should provide written approval of this evidence to satisfy compliance requirements.

© 2001 by CRC Press LLC

In summary, although all enclosed systems should easily meet the NESHAP requirements, in areas determined by the EPA to have poor air quality, performance-based evidence should be required from the supplier and accepted by the regional regulatory body before enclosed systems are purchased for use in nonattainment regions. COSTS Enclosed systems of all types are more expensive than open-top liquid/vapor degreasers, in part because there are relatively few producers of enclosed cleaning systems. Technical justification for choosing the more complex systems, beyond cleaning and emissions control, may include better drying and operation at a lower temperature with less potential damage to parts. The authors estimate that the average ratio of initial investment costs relative to opentop systems is 2.5 for airless, 2.0 for airtight, and 1.3 for externally sealed systems for commonly used, smaller systems. In general, the larger the cleaning chamber, the more costly the system. However, the differential relative to open-top systems begins to converge for large systems (approximately 75 ft3 chamber volume). These are estimates; additional process control and parts handling may increase the initial investment. Analysis Although some data have been provided by equipment suppliers, the analysis, and conclusions are those of the authors. Are the costs of enclosed systems justified? Do enclosed systems pay for themselves? Over what period? Unbiased answers are difficult to obtain. Nearly all studies are based on estimates or forecasts.1,5 With chlorinated solvents, the decrease in solvent purchases with 99.X% control efficiency system over 70% control efficiency probably will not pay for the additional 2.5 times greater investment needed from an enclosed system. For more costly solvents, a stronger case may be made for enclosed cleaning systems. What if a solvent costing $15/lb is used? For HFC-43-10, HFE-7100, or AK-225, the authors estimate that the enclosed airless system costs less to operate than does the open-top cleaning system. Studies based on customer experience are difficult to apply to other situations because of the narrow focus of the customer’s application, the customer’s youth on the learning curve, and the small number of systems constructed to date in the United States (75). Additional cost issues do not normally register on a cleaning cost sheet: reduction of hazardous solvent-based waste and reduced cost of obtaining and complying with an environmental permit. One reason to purchase an enclosed system is to reduce labor costs. The main component of operating costs (approximately 80% for very large systems) for all enclosed systems is capital payback. Additional costs include, in decreasing order of significance, labor, solvent, and miscellaneous (power/waste disposal). In contrast, for open-top systems, capital payback is much less significant, and operating labor is much more significant. In the authors’ surveys, capital investment was found to be the main barrier to implementation of enclosed airless systems. Total cost of ownership (capital payback  labor  solvent purchase  solvent disposal  miscellaneous) should be significant in the minds of users. In all of the analyses, open-top cleaning systems show less annual cost of ownership than do airless enclosed cleaning systems. The authors do not have adequate experience

© 2001 by CRC Press LLC

with airtight and externally sealed enclosed systems. However, because they require less investment than airless enclosed systems, it seems reasonable, at worst case, to assume their cost of ownership is no different from that of open-top systems. Hidden Costs Not all costs are readily quantified. First, there are the environmental/regulatory costs. What does it cost in time and legal fees to get a permit for an open-top system in locations where no permit is required for an enclosed airtight system because emissions are below a de minimis value? What does it save to avoid the need for environmental monitoring, where it might not be required with an externally sealed system? What if one must use PCE in a process, and only an airless system will meet the regulatory emissions requirements? Can the operating costs of an opentop system be justified if a solvent costing $15/lb is necessary? Costs of quality are also difficult to quantify. What is vacuum drying worth, if it comes “free” with purchase of an airless system? Will an enclosed system allow one to match the solvent precisely with the soil rather than compromise based on what can be readily contained in an open-top system? What is it worth to be able to clean and dry repeatedly a complex structure that would retain residual solvent when processed in a nonvacuum system?

WHY PURCHASE AN ENCLOSED SYSTEM? That question puzzles users, especially since the cost of ownership (with low-priced solvents) is slightly higher for an enclosed system. Regulators easily find an acceptable answer: to control solvent loss. However, opinions vary. The view of the authors is that an enclosed system is preferred over open-top systems for nearly all cleaning operations. For low-price solvents, which pose no environmental concern, the open-top system is slightly more cost-effective. However, because most of these solvents dry very slowly, why not purchase an airless enclosed system that provides vacuum drying? For low-price solvents, with environmental issues, the open-top system is slightly cheaper to operate. But, either now or in the near future some type of enclosed system is likely to be needed to meet environmental regulations Why not purchase an enclosed system and save the second purchase investment? For medium- and high-price solvents, enclosed systems are readily justified on the basis of reduced total operating cost. What if an operation is located in an area of poor air quality that is heavily regulated? An enclosed system is nearly certainly needed. Even where the predicted cost of ownership of an open-top system is less than that of an enclosed system, what about the hidden costs? In summary, the strength of the case for the open-top cleaning system is diminishing relative to the strength of the case for some type of enclosed cleaning system.

SUMMARY The sole purpose of an enclosed system is to conduct some cleaning process within a contained environment. Enclosed cleaning systems are relatively new and not commonly used commercially. They were developed in the United States for a few specific applications that were difficult to compete satisfactorily with open-top cleaning systems. Operation with reduced emissions is the principal motivation for purchase. Other factors

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include excellent drying performance and cleaning of unusual shapes or structures. Airless systems also have significant potential to conduct additional cleaning processes. The three types of systems include airtight, airless (a vacuum chamber), and externally sealed (an open-top system with an airlock). The airless system is the most costly and is suitable for certain unique applications. The airtight system operates with reduced emissions, cleans components, and is priced intermediately. The externally sealed system operates with reduced emissions and is the lowest priced. High purchase investment has restricted commercial adoption of enclosed cleaning systems. The authors’ cost analysis indicates that total operating costs for enclosed systems are close to those for open-top systems, even with low-cost solvents. With high-cost solvents, users rapidly recover the higher capital investment. REFERENCES 1. Grant, D.C.H., Solvent Recovery and Reclamation System, U.S. patent 5,232,476, August 3, 1993. Assignee is Baxter International. 2. Grant, D.C.H., Method for Cleaning with a Volatile Solvent, U.S. patent 5,304,253, April 19, 1994. Assignee is Baxter International. 3. Gray, D.J. and Gebhard, P.T.E., Cleaning Method and System, U.S. patent 5,469,876, November 28, 1995. Assignee is Serec. 4. Gray, D.J. and Gebhard, P.T.E., Solvent Cleaning System, U.S. patent 5,538,025, July 23, 1996. Assignee is Serec. 5. Tanaka, M. and Ichikawa, T., Cleaning System Using a Solvent, U.S. patent 5,193,560, March 16, 1993. Assignee is Tiyoda. 6. Tanaka, M. and Ichikawa, T., Cleaning Method Using a Solvent While Preventing Discharge of Solvent Vapors to the Environment, U.S. patent 5,051,135, September 24, 1991. Assignee is Tiyoda. 7. Grant, D.C.H., Emission Control for Fluid Compositions Having Volatile Constituents, and Method Thereof, U.S. patent 5,106,404, April 21, 1992. Assignee is Baxter International. 8. Turicco, T., Pressure Controlled Cleaning System, U.S. patent 5,449,010, September 12, 1995. 9. Nafzifer, C.P., Single Chamber Cleaning, Rinsing, and Drying Apparatus, and Method Therefor, U.S. patent 5301701, April 12, 1994. Assignee is Hyperflo. 10. Grant, D.C.H., Vacuum Airlock for a Closed-Perimeter Solvent Conversation System, U.S. patent 5,343,885, September 6, 1995. Assignee is Baxter International. 11. Basis: 5-year use of capital at 8% annual interest; cost components are capital use, solvent, operating labor, disposal, and miscellaneous, which includes power. 12. Durkee, J.B., NESHAP recap—”Dr. PC” explains all, Precision Cleaning, April 1995, p. 39. 13. Almodovar, P., U.S. EPA, personal communication, August 14, 1997. 14. U.S. EPA, Guidance Document, Part 2, Section 1.2, 1995. 15. Almodovar, P., U.S. EPA, personal communication, April 16, 1999. 16. Office of Air Quality Planning and Standards at Research Triangle Park, Impact Analysis of the Halogenated Solvent Cleaning NESHAP, U.S. EPA-453/D93-058, November 1993. This analysis was based on 15-year recovery of capital cost and 10% cost of capital.

© 2001 by CRC Press LLC

CHAPTER 2.12

Precision Cleaning and Drying Utilizing Low-Flash-Point Solvents Matt Bartell

CONTENTS Solvent Overview Other Solvents Cosolvents Solvent Costs Process Overview Applications Equipment Configuration Heating System Cooling System Automation Solvent Containment Safety Features Final Thoughts SOLVENT OVERVIEW A low-flash-point solvent is defined by the NFPA (National Fire Protection Association) as any solvent having a flash point below 100°F. Precision cleaning and drying with low-flash-point solvents such as isopropyl alcohol (IPA), cyclohexane, and acetone can be a very effective cleaning strategy. However, properly configured equipment and a well-designed cleaning process are essential to the successful and safe implementation of low-flash-point solvent processes. The three most common low-flash-point solvents used in precision cleaning processes are IPA, cyclohexane, and acetone. These solvents are extremely effective cleaning agents, and are priced well below engineered solvents such as azeotropes or blends of hydrofluorocarbons or hydrofluoroethers. • IPA is typically used for the removal of particle contamination and inorganic films such as salts, fingerprints, and highly activated fluxes. © 2001 by CRC Press LLC

• Cyclohexane is effective in removing particle contamination and heavy organic films including oils and greases. • Acetone has been proved effective in the removal of many types of inks and adhesives. Acetone also has an advantage in areas of poor air quality in that it is not federally regulated as a volatile organic compound (VOC). Although a welldesigned solvent cleaning system should lose only a very small amount of solvent to fugitive emissions, the VOC status of acetone is a distinct and environmentally friendly advantage, especially in highly regulated geographic locations. Other Solvents An azeotrope of alcohol and cyclohexane has been demonstrated to be effective in removing most contaminants, and is highly effective in removing resin-activated fluxes. Azeotropes are solvent mixtures that have a constant composition in the liquid and vapor phases over a certain temperature range, and so take on common cleaning properties. Once combined, they are inseparable via normal distillation during cleaning processes. Many other solvents, such as heptane, ethanol, and volatile methyl siloxanes, may also be used in low-flash-point systems. Use of such solvents should first be discussed with the equipment manufacturer to ensure safe and effective operation. Cosolvents A low-flash-point solvent may be used to rinse a higher-boiling solvent within a single specially designed cleaning system. An example of such a process, deemed cosolvent, would be the use of N-methyl pyrillodone to remove thermally or ultraviolet cured adhesives from various substrates, followed by a rinse of IPA or acetone. These cosolvent processes require specific temperature parameters and control to prevent the solvents from mixing within the cleaning system. Solvent Costs Solvent cost is typically $3 to 4/gal (IPA). Solvent waste may be incinerated for energy generation. Disposal cost for IPA is typically $2 to 3/gal. This low solvent cost provides a significant benefit, especially when compared to many of the engineered solvents currently available. PROCESS OVERVIEW The vaporization and condensation of the solvent act as the driving force to move solvent throughout the system. This process is commonly known as the “reflux cycle.” In a well-designed system, the dirtiest solvent is concentrated in an offset boil sump, and parts are never exposed to this offset boil sump. The condensed distillate drains via gravity through the immersion sump or sumps and into the offset boil sump. The final rinse and dry take place in a saturated superheated solvent vapor blanket. A superheated zone is defined here to be a vapor zone maintained at approximately 20 to 50°F above the boiling temperature of the solvent being used. Any liquid solvent remaining on the parts in the vapor zone will flash dry due to the superheated zone, allowing dry parts to emerge. This © 2001 by CRC Press LLC

on-board distillation process not only provides the solvent vapor needed for complete drying of the parts being processed, but also acts to clean the solvent continuously in the immersion tanks. A well-designed system will reflux the solvent at a minimum rate of one immersion tank volume per hour. A schematic view of this type of process is shown in Figure 1. A typical low-flash-point precision cleaning and drying process includes two solvent immersion tanks, one or both of which may include ultrasonics and filtered recirculation of solvent and a superheated vapor drying zone. Additional options to enhance cleaning performance include spray under immersion, vertical basket oscillation, and a final spray flush of distillate solvent.

APPLICATIONS Low-flash-point solvent precision cleaning processes have been in use for many years. A multitude of companies have found solvents such as the azeotrope of IPA/cyclohexane to be a low-cost and effective alternative to engineered solvent for precision cleaning operations. Typical applications include the removal of contaminants from the following: • • • • •

Printed circuit boards/hybrid circuits/MCMs/C4 packages Disk drive components Precision mechanical/electromechanical components Optical instruments Medical devices and components

In addition to cleaning, low-flash-point solvents may be used to dewater parts following aqueous cleaning. IPA is widely used throughout the computer hard disk industry to provide spot-free drying following wet-bench cleaning processes.

TRANSPORT SYSTEM

SUBZERO FREEBOARD COOLING COILS

INNER DOOR

FREEBOARD ZONE VAPOR LINE

CONDENSING COILS

LOAD LOCKTM ACCESS CHAMBER

PROCESS PART

SUPERHEATED VAPOR ZONE OUTER DOOR SUPERHEATER

Figure 1

IMMERSION

IMMERSION

SUMP 2

SUMP 1

OFFSET BOIL SUMP

Reflux of a solvent in a well-designed system.

© 2001 by CRC Press LLC

EQUIPMENT CONFIGURATION The safe and effective use of low-flash-point solvent requires careful attention to specific system design criteria. Heating System Indirect heating systems hold a distinct advantage in that circulating hot water through heat exchange coils in the boil sump and immersion tanks provides an effective means of heating the solvent, without the possibility of exposing the solvent to a runaway heater element, either immersed or mounted to the side of the tank. Indirect heating eliminates both the safety concerns and the possibility of thermally degrading the solvent because of contact with the high surface temperature associated with most contact heaters. Cooling System Every BTU (British Thermal Unit) introduced into a solvent cleaning system must be removed by the cooling system of the system. Two options for cooling media are chilled water and refrigerant. Chilled water, typically run at 40°F, may be used to condense the solvent vapor, and is especially effective when solvents with a relatively high freezing point are used. Refrigerated systems provide a higher level of emissions control, but require a higher level of design expertise to handle the varying heat load encompassed within most precision cleaning equipment. Automation The use of PLCs (programmable logic controllers) and mechanical transport systems adds a valuable layer of process control and repeatability when compared with a manual system. A well-designed automated system will allow the operator simply to load the basket into the system and start the cycle. From this point forward, all other machine actions, such as moving the baskets from tank to tank, and controlling system temperatures, should be controlled by the system PLC. This is true for all cleaning systems; however with lowflash-point systems, design considerations are of particular importance. The transport system itself must be designed to comply with all NFPA guidelines associated with the zone in which it is operating. Typically, the area immediately above the process tank will be classified as NFPA Class One, Division One. This will require all components used in this area to be classified as explosion-proof. Beyond the safety concerns associated with the transport system, other points of consideration in the choice of an automated system include payload capacity, maintainability, particle generation, and the overall ruggedness of the design. Solvent Containment One of the most important considerations when choosing a solvent cleaning system is the reduction of solvent emissions and operator solvent exposure. A well-designed solvent system will be based on solid vapor degreaser principles, utilizing the reflux cycle of boiling and then condensing the solvent to minimize solvent vapor losses, combined with a high freeboard ratio. If solvent is used at ambient temperature, in an open-top immersion © 2001 by CRC Press LLC

tank with no saturated vapor blanket, the solvent will quickly evaporate. The use of the solvent reflux cycle greatly slows down this evaporation process. A well-designed “open top” cleaning system, engineered to use a low-flash-point solvent such as IPA can be expected to emit approximately 1 gal/day. In an attempt to improve on the already impressive emissions reduction results of the aforementioned open-top cleaning systems, many other methods of solvent containment have been incorporated into various systems. One particularly effective solution has been the addition of a sealed upper enclosure and load-lock loading chamber to the existing vapor degreaser type design. This overall machine design offers a single solvent emission loss point based on the number of loads passing through the load-lock loading chamber per hour. This load-lock loading chamber acts as a true air lock, separating the process environment from the bay environment surrounding the cleaning system. This single exhaust point emits one load-lock volume of air contaminated with solvent vapors per basket. This solvent-contaminated air is directed to the facility exhaust stream where it may be further treated before being emitted to the environment. With a properly designed cleaning and drying cycle, such a cleaning system can yield low solvent emissions of 1 lb/day. SAFETY FEATURES The single item differentiating a system designed for low-flash-point solvents from other solvent-based cleaning systems is the level of safety features. When considering any system for use with these solvents, safety should be the first consideration. The safety design of a well-designed low-flash-point system should be based on the principles found in the fire safety triangle. To support combustion, three ingredients must be present, fuel, ignition sources, and oxygen. Any system designed to comply with NFPA regulations must eliminate two of these components in all areas of the system. The systems with the highest levels of safety will eliminate two of the three combustion requirements in all areas of the system. As a minimum, the following safety features should be included in any system intended for use with low-flash-point solvents. • Compliance with all applicable NFPA guidelines. • Indirect heating system, to prevent heater “runaway.” • Electrical signals wired through intrinsic barriers, when located in the classified area. • High-voltage items must be wired in sealed, explosion-proof conduits, in the classified areas. • Safety checks in a given area of the machine should include redundant backups. • The secondary containment pan should include a monitored leak-detection system. • An integral fire detection and CO2 fire suppression system should be installed as part of the system. • Only systems bearing independent third-party approval, such as by Factory Mutual Research Corporation, should be considered. FINAL THOUGHTS Although there has been a trend toward aqueous cleaning, use of solvents offers advantages over aqueous cleaning in many applications. Some factors include a greater solvency range for soils of interest such as activated solder flux, lower viscosity for © 2001 by CRC Press LLC

components with blind holes and complex geometries, improved spot-free drying for optics, compatibility with readily oxidized metals, and low conductivity in cleaning motor windings. In addition, compared with aqueous cleaning, which requires multiple rinsing and drying steps, solvent systems tend to have a smaller footprint. Finally, a well-designed low-flash-point system minimizes solvent losses due to dragout and emissions, resulting in very low solvent consumption, compared with hundreds of gallons of deionized rinse water usage and waste processing. Familiar solvents such as IPA, cyclohexane, and acetone can solve a broad spectrum of cleaning challenges. Examples include cleaning titanium medical components, defluxing circuit boards, cleaning sputtering targets, water removal, industrial degreasing (TCE replacement), debonding, and precision ink and lubricant removal. Why do low-flash-point solvents work? Low-flash-point solvents have advantages that make them ideal for a broad spectrum of cleaning and drying challenges. Effective solvency for a range of soils. Low-flash-point solvents efficiently remove fluxes, oils, most inks and dyes, particulates, and other impurities. Low cost. Low-flash-point solvents are very inexpensive ($2 to $3/gal) and have low disposal costs. Some can also be used as fuels. Low toxicity. Many low-flash-point solvents are common chemistries found in homes and businesses. This wide acceptance makes low-flash-point solvents an excellent choice where there are considerations of employee safety and low usage restrictions. Low residual. Low residue (solvent or otherwise) on the part delivers spot-free drying and a high level of cleanliness. So why doesn’t everyone use low flashpoint solvents? There are two main objections to low-flash-point solvents. The first, surprisingly enough, centers on price. “It can’t work; it’s too cheap.” The answer to this concern is that low-flash-point solvents do work (even if they are inexpensive). The second centers on safety concerns. Although a flammable substance, the hazards involved are virtually nonexistent with properly designed equipment and are comparable to driving a car fueled with gasoline. Low-flash-point solvents are an excellent choice for many applications. Their established track record including excellent cleaning, low cost, low regulatory requirements, and accepted safety makes them the ideal choice for many applications. Give them a look; you may be pleasantly surprised with what you find.

© 2001 by CRC Press LLC

CHAPTER 2.13

Dense-Phase CO2 as a Cleaning Solvent: Liquid CO2 and Supercritical CO2 William M. Nelson

CONTENTS Introduction Need for New Solvents Liquid and Supercritical Carbon Dioxide Carbon Dioxide Phases of CO2 Liquid CO2 Supercritical CO2 Industrial Cleaning Condensed-Phase CO2 Cleaning Supercritical Fluid Extraction Supercritical CO2 Engineering Condensed-Phase CO2 Cleaning Details of Condensed Phases Solubility Cosolvents Substrate Compatibility Effect of Mixing on Supercritical CO2 Cleaning Economics Conclusions References

INTRODUCTION The use of carbon dioxide (CO2), either liquid CO2 (LCO2) or supercritical CO2 (SCCO2), cleaning has been shown through extensive laboratory and pilot testing to be potential alternatives for manufacturers looking for new precision and parts cleaning systems. The use of carbon dioxide as an extraction solvent has slowly found acceptance, and also close

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to commercialization are applications for supercritical and liquid CO2 in the electronics industry. Although much of the potential for supercritical or liquid carbon dioxide lies in fairly exotic applications, CO2 solvent technology is already commercial in the comparatively mundane fields of dry cleaning and spray painting. The technology is especially well suited for precision cleaning applications in which parts have intricate geometries or for applications in which parts are sensitive to water or high temperature. CO2 cleaning appears to be compatible with a majority of substrates encountered in manufacturing, including most metal and glass, and many plastics. Mixing generally appears to improve the efficiency of the CO2 cleaning process, as does the incorporation of cosolvents with condensed-phase CO2. Condensed-phase CO2 cleaning has relatively low operation and maintenance costs, and does not generate additional waste streams as a result of operations. These low operation and maintenance costs are offset by a relatively high initial capital cost that has prevented the technology from being truly cost-competitive with other options. If the savings from regulatory fees and/or wastewater treatment costs from manufacturing processes are considered, the economics become more appealing.

NEED FOR NEW SOLVENTS Increasing concern regarding the dissemination of chemical waste (both aqueous and organic) into the environment has prompted considerable interest in new technologies aimed at reducing current waste streams. Cleaning technologies (preparing the surface of a material for subsequent steps in an industrial process) can have a significant effect on its overall environmental impact. Over the past decade, concern for the environment, economic competitiveness, and technological advances all converged to cause both industry and government to reevaluate manufacturing processes. Changes from traditional solvent cleaning to alternative methods set into motion recent trends toward zero discharge of pollutants into the air, water, and soil. With ozone-depleting chemicals (ODCs) being phased out, many manufacturers are struggling to find efficient and effective replacement solvents and cleaning agents. The ODC phaseout and a host of other environmental and safety concerns have prompted the development of alternative cleaning agents. However, there are no drop-in substitutes for ODCs. The quality and suitability of the cleaning process is heavily dependent upon the quality of the solvent utilized. The solvent is either an active agent in the process or is the stage on which the process occurs.1 Solvents “clean” by producing species from contaminants that have a higher affinity for the cleaner than for the surface to which they adhere. In so doing, the cleaning agent can separate the contaminant from the surface. Solvent use in cleaning will continue to be pervasive. The challenge to the cleaning industry will be to adopt the most environmentally benign and efficacious technology. Probably one of the most critical components of a cleaning process is the identity of the solvent(s) used. The available solvents have given individuals in the cleaning industry a great set of tools. This is, however, only the beginning. Careful reasoning must enter into the choice of cleaning technologies. Condensed-phase CO2 is a true solvent and it can serve as the dissolving media for cleaning processes. This chapter examines: (1) the solubility character of CO2 in its liquid and supercritical phases, (2) the use of cosolvents and entrainers to enhance solubility, and (3) the equipment and engineering necessary to implement this cleaning technology. Within the discussion, references to case studies and test results are provided. © 2001 by CRC Press LLC

LIQUID AND SUPERCRITICAL CARBON DIOXIDE Carbon Dioxide Carbon dioxide is a colorless gas, which was first recognized in 1577 by Van Helmont who detected it in the products of both fermentation and charcoal burning. CO2 is used in solid (dry ice), liquid, and gaseous form in a variety of industrial applications such as beverage carbonation, welding, chemicals manufacture, and cleaning. It occurs in the products of combustion of all carbonaceous fuels and can be recovered from them in a variety of ways. CO2 is also a product of animal metabolism and is important in the life cycles of both animals and plants. It is present in the atmosphere in small quantities (0.03%, by volume). CO2 is not very reactive at normal temperatures. It does, however, form carbonic acid, H2CO3, in aqueous solution. This will undergo the typical reactions of a weak acid to form salts and esters. A solid hydrate, CO2 . 8H2O separates from aqueous solutions of CO2 that are chilled at elevated pressures. It is very stable at normal temperatures but forms CO and O2 when heated above 1700°C. CO2 has several advantages: environmental acceptability, nonflammability, and noncorrosivity. Additionally, CO2 has no ozone-depletion potential, and while it does have some global warming potential, its use in cleaning operations would contribute insignificantly to global warming in comparison with, for example, automobile emissions, coalburning electric generation, steel smelting, etc. In addition, as Frank Cano points out in Chapter 2.14 on CO2 snow cleaning, commercially produced CO2 is recycled from other industrial processes and repesents a delayed emission rather than a new source. Condensed-phase CO2 cleaning systems assume several forms. Cleaning with CO2 is advantageous in that, after cleaning, the only waste streams generated are the isolated contaminants that were removed from the part that was cleaned. There are no large, liquid streams to treat (as there are with aqueous cleaning) or airstreams to treat (as is the case with some solvent cleaning solutions). Phases of CO2 A pressure –temperature (P–T) phase diagram, shown in Figure 1*, illustrates the phase changes of CO2, where the three phases of solid, gas, and liquid are indicated. The substance remains a liquid, as long as the temperature and pressure fall within the CO2(1) region. CO2 is supercritical when its pressure and temperature are beyond the critical point. Notice that the triple point of carbon dioxide is well above 1 atm. Notice also that at 1 atm CO2 can only be the solid or the gas. Liquid CO2 does not exist at 1 atm. Dry ice (solid CO2) has a temperature of 78.5°C at room pressure, which is why one can get a serious burn (actually frostbite) from holding it. Liquid CO2 Although CO2 liquid does not exist at normal room pressures, it does exist at slightly elevated pressure. A laboratory cylinder of CO2 will contain LCO2 at a pressure of about 75 psi at room temperature. On a particularly hot day (above 88°F) the liquid CO2 will pass though its critical point and the contents of the cylinder will exist as the supercritical fluid. The LCO2 cleaning technology used alone under these conditions is a solvent much like room-temperature 1,1,1-trichloroethane (TCA), As such, it will remove many but not all types of contaminants. Contaminants that are not soluble in LCO2 alone can be *Chapter 2.13 Color Figure 1 follows page 104. © 2001 by CRC Press LLC

solubilized or otherwise separated by employing proprietary additives, modifiers, or mechanical adjuncts in the process.2 Liquefied CO2 can be used as a solvent for cleaning. Process temperatures generally range between 50 and 70°F, and process pressures range from 750 to 1200 psi. The area in the CO2 phase diagram where the substance is liquid is shown in Figure 1. Finally, the LCO2 immersion cleaning process can meet a variety of cleanliness requirements, ranging from visually clean to more rigorous quality standards requiring such sophisticated test methods as nonvolatile residue analysis, infrared spectroscopy, or scanning electron microscopy. Functional testing, such as that measuring weld joint porosity and adhesive strength, has also been conducted to evaluate the technology. Supercritical CO2 Gases become “supercritical” when they are heated above their critical temperature— the point beyond which they cannot be liquefied—and compressed (see Figure 1). CO2 becomes supercritical at temperatures above 87.8°F (31.1°C) and pressures above 1072 psi (73.8 bar).3 Supercritical CO2 applications typically operate at temperatures between 90 and 120°F (32 and 49°C) and pressures between 1070 and 3500 psi. There are two unique points on this phase diagram. The lower point is called the “triple point” and is the unique combination of temperature and pressure at which all three phases exist simultaneously. The easiest way to imagine this is to think of boiling ice water. If we remember that the word boiling has nothing to do with “hot,” then it is easy to imagine lowering the pressure far enough to have ice water boil. In fact, the pressure need only be about 4.58 mm, which is quite easy for a simple vacuum pump. Notice that this temperature is slightly above the normal melting point because of the retrograde nature of the melting point curve. The second unique point is stranger than the first. It can be shown experimentally that for every liquid there is a point along the boiling point curve where the line between the liquid and gaseous phases disappears. This is called the critical point. At temperatures higher than this point, we can no longer think of two phases; there is only a single phase that is a very dense gas, or frequently called a critical fluid. Another way of thinking about this is to remember that at the critical temperature or above we can no longer compress the material to a liquid no matter how much pressure we apply. These critical fluids have extremely unique properties and are now used for many commercial processes. For example, supercritical CO2 is used for the extraction of caffeine from coffee and tea. Industrial Cleaning A cleaner, by definition, removes dirt or other extraneous material from a surface. An effective cleaner is able to perform its task because of its ability to: • • • •

Wet the surface Penetrate the soil Lift and remove the soil Hold soil in suspension (so a surface can be wiped or rinsed)

Cleaning produces a clean surface that contains no significant amounts of undesired material. The degree of surface cleanliness must meet the following two criteria: (1) it must be sufficient for subsequent processing, and (2) it must be sufficient to ensure the future © 2001 by CRC Press LLC

reliability of the device or system. Beyond this, there are numerous factors that affect the quality of the cleanliness. In terms of cleaning, a solvent is a substance, single or multicomponent, capable of dissolving other substances to form a homogeneous system (solution). The criteria for what determines a good solvent will vary, depending upon the use and ultimate level of cleanliness sought during the process. The development of knowledge of solutions reflects to some extent the development of chemistry itself. A solvent may be defined in rough terms as any liquid that serves as a carrier for another substance or as a means of extracting or separating other substances. In practice, many solvents are mixtures rather than pure compounds. The most common solvent is water, but next in importance comes a group of organic liquids and their mixtures. At present, there is a concern for use of environmentally benign solvents, and there is increasing interest in alternatives to the more traditional solvents. Precision and parts cleaning of manufactured metal parts has relied heavily on the use of conventional chlorofluorocarbon (CFC) solvents. Condensed-Phase CO2 Cleaning The growth in interest in liquid and supercritical CO2 in industrial applications over the past two decades has resulted from several key characteristics, which are relevant to both academic and industrial communities. Three drivers, or forces, have contributed to the recent attention given to these solvents (Table 1). The availability of inexpensive, nontoxic solvents such as liquid or supercritical CO2 and their attractive properties has renewed interest in the applicability of these solvents, especially in the area of cleaning. Supercritical Fluid Extraction Supercritical fluid (SCF) extraction has been developed extensively as both a cleanup technique and as an analytical technique for liquid and solid environmental samples.4,5 We can regard cleaning with condensed-phase CO2 as an extraction process: the extraction of contaminants from the surface of interest. Condensed CO2, when regarded as a solvent, may benefit from the enormous data of the science of solvents.6 SCF extraction (SFE), like any cleaning technology, works better on certain classes of soils. SFE has been employed successfully to remove oils such as hydrocarbons, esters, silicones, perfluoropolyethers, halocarbon substituted triazines, etc. SFE is an elegant cleaning method with many environmental benefits, but one that is not at present cost-effective. Metal cleaning in an industrial setting is typically an environmentally hazardous activity. Until recently, vapor degreasers utilizing halogenated solvents were prevalent throughout industry. It is readily apparent that the reliance of industry on halogenated solvents as metal cleaners has resulted in a technologically easy way to clean metals. A change to a

Table 1 Drivers Influencing Adoption of Condensed-Phase CO2 Cleaning Driver

Description

Environmental

The environmental problems associated with common industrial solvents (mostly chlorinated hydrocarbons) The increasing cost of regulated solvent use The inability of traditional techniques to provide the necessary separations needed for emerging new industries (microelectronics, biotechnology, etc.)

Economic Technological

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different technology will necessitate some adaptations. As in any reaction medium it must be engineered for greater productivity by manipulating the temperature and pressure. Supercritical CO2 While liquid CO2 has come to see increased use,2 further discussion of supercritical CO2 will illuminate characteristics of condensed-phase CO2. An extensive discussion of supercritical media is to be found in the review by Savage et al.7 SCFs are effective cleaning agents because of their ability to penetrate substrates and small interstitial spaces rapidly. After dissolving any contaminants, the critical fluid is easily and completely removed because it lacks surface tension. The critical fluid of choice for surface cleaning applications is most often CO2, either pure or in combination with a small amount of cosolvent. Solvent properties can be adjusted by small changes in temperature and pressure, allowing CO2 to dissolve a range of organic compounds. Supercritical CO2 actually has physical properties somewhere between those of a liquid and a gas. SCFs are able to spread out along a surface more easily than a true liquid because they have lower surface tensions than liquids. At the same time, an SCF maintains the ability of a liquid to dissolve substances that are soluble in the compound, which a gas cannot do. In the case of supercritical CO2, this means oil and other organic contaminants can be removed from a surface even if the surface has an intricate geometry or includes cracks and crevices. In general, this process cannot remove contaminants that do not dissolve in CO2 (Table 2). It has been shown that high-pressure supercritical CO2 shows potential as a cleaning medium for removing hydrocarbon machine coolants from metal substrates.8 In addition, supercritical CO2 alone can be tuned to remove the contaminants listed in Table 3. As was mentioned previously, these condensed-phase fluids have both liquid and gaslike properties. This property will confer highly desirably solubility characteristics. This allows the liquid or fluid to penetrate very small gaps and complex assemblies, which will enhance the potential range of substrate geometries (Table 4). The supercritical phenomenon has been known since the middle of the 19th century, but industrial applications—mostly extraction based—emerged only in the early 1980s. Today, a number of companies operate extraction facilities based on supercritical CO2. These include General Foods, which runs a coffee decaffeination plant in Houston; companies in Washington’s Yakima Valley, such as Yakima Chief and John I. Haas, which extract flavor from hops; and natural flavor producers such as Finland’s Cultor and Germany’s SKW Trostberg.9 As the range of chemicals soluble in CO2 expands, wider frontiers will become available for supercritical and liquid CO2. Chemical manufacturing uses of CO2, as well as cleaning and materials processing applications, are likely to become the initial industrial-scale uses of condensed-phase CO2. Table 2 Contaminants Not Removed by Supercritical CO2 • • • • • •

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Rust Scale Lint or dust Ionic Species Metal salts Many (but not all) fluxes

Table 3 Contaminants Removed by Supercritical CO2 • • • • • • • • •

Silicone oils Flux residues Petroleum oils Machining oils Dielectric oils Lubricants Adhesive residues Plasticizers Fats and waxes

ENGINEERING CONDENSED-PHASE CO2 CLEANING To achieve condensed-phase CO2, the CO2 gas needs to be pressurized and heated (see Figure 1). Parts are placed into a pressure vessel into which CO2 gas is introduced. The temperature and pressure are then raised until the supercritical state is reached. A basic system consists of six components. 1. 2. 3. 4. 5. 6.

Compressor Heat exchanger (heating) Extraction vessel (pressure vessel) Pressure control valve (expansion) Heat exchanger (cooling) Separation vessel

Figure 2 shows the basic components that comprise a condensed-phase CO2 cleaning system. CO2, which may be stored as a gas or in liquid form, is compressed above its liquid or critical pressure by a pump. The compressed CO2 is then heated to its liquid phase or to above its critical temperature in a heater, or sometimes in the cleaning chamber. Any parts in the cleaning chamber are cleaned by exposure to the liquid or fluid. Typically, the cleaning chamber will include an impeller to promote mixing. Further work is being done on Table 4 Examples of Substrates for Supercritical CO2 Cleaning • • • • • • • • • •

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Missile gyroscopes Accelerometers Thermal switches Nuclear valve seals Electromechanical assemblies Polymeric containers Special camera lenses Laser optics components Porous ceramics Metal parts

Heating unit

Cleaning vessel

Pump

Liquid CO2

CO2 Recycle

Cooling unit

Separator

Figure 2

Components of a condensed-phase CO2 cleaning system.

cosolvents and entrainers, which will enhance the solubility characteristics of CO2 (either liquid or supercritical). Condensed-phase CO2 (containing dissolved contaminants) is then bled off to a separator vessel, where the liquid/fluid is decompressed and returned to a gaseous state. The contaminants remain in liquid form and are collected out the bottom of the separator, while the gaseous CO2 is sent through a chiller to return it to a liquid form for storage to be reused again. This closed-loop recycling of the CO2 means only a small portion of the cleaning solution has to be replaced over time due to system leakage. The now-clean parts can be removed from the chamber and are usually immediately ready for the next step in the manufacturing process, since no drying or rinsing is required to remove residual cleaning solution. With some plastics, which can absorb CO2, a bakeout may be needed. Process temperatures may range from 95 to 149°F (35 to 65°C). Pressures vary from about 1070 to 4000 psi. Nonmetallic materials must be tested for compatibility. The process works well for removing trace fluids. Some suppliers also claim effective removal of particle contamination. It may be possible to fine-tune the operating pressure and temperature to match the soil being removed. Parts that cannot be subjected to elevated atmospheric pressures cannot be cleaned with SCFs. This process has been developed for the precision cleaning industry. With further development it may become more broadly applicable. DETAILS OF CONDENSED PHASES Solubility An experimental determination of the solubilities of solid mixtures in supercritical fluids was made.10 Examples of the solubility measurements are becoming more common in the scientific literature (Table 5). Cosolvents If condensed-phase CO2 is going to be implemented on a commercial scale to remove contaminants from material surfaces, the costs associated with this process must be reduced and the range of solubilities must be broadened. These goals can be made more

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attainable with the use of an entrainer (or cosolvent) to increase the solubility of the solute in the supercritical phase, reducing the size of the required extractor and/or lowering the pressure needed to effect the desired extraction. An example of this is the cleanup of soils contaminated with organics by extraction with supercritical CO2. It can be influenced decisively by additional substances or entrainers. In most cases, the contaminated soil already contains water as a substance that can alter the extractability of these contaminants. In particular, the effects of soil moisture, as a kind of discontinuous addition of water, on the extraction of polycyclic aromatic hydrocarbons (PAHs) from soil with supercritical CO2 were examined. On the other hand, humidifying the supercritical CO2 used a continuous addition of water by humidifying the supercritical CO2. The improvement of the extraction yield by moisture indicates additionally that the extraction is limited by adsorption and not by diffusion effects. However, the contaminant is more accessible and is transported faster out of the soil with water.18 Substrate Compatibility While case studies from successful industry implementation of supercritical CO2 cleaning are helpful, the number of successful industrial case studies for supercritical CO2 cleaning is small. Although the majority of published matter is laboratory- or pilot-scale test results, the lessons learned from true, long-term implementation of a technology are always the most valuable to others considering applying the technology themselves (Table 6). Although test results show the technology works quite effectively in many cases, there are some published case studies from industry that resulted in decisions not to pursue the technology for full-scale application. In one instance, tests of supercritical CO2 cleaning were performed for a manufacturer on metal disks contaminated with oil-type residues in a one-liter system at 180°F (82°C) and 2000 psi.28 • The cleaning was acceptable (below the specified “clean” level and comparable with what a trichloroethylene vapor degreaser could obtain). • Carbon residue on parts was reduced by 50%. • Mixing did not improve the cleaning efficiency in this test.

Table 5 Examples of Solubility Measurements in Supercritical CO2 Issue

Result

Ref.

Improving polymer solubility Low solubility of organophosphorous compounds Solubilities of amorphous polymers Separation of vegetable cuticular waxes Effect of cosolvents Determine effects of methanol as cosolvent Determine effects of n-pentanol as cosolvent

Surfactants improved solubility All compounds solubility improved

11 12

Describes conditions for solubility Enhanced separation

13 14

Measured effects Measured effects

15 16

Measured effects

17

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Table 6 Examples of Compatibility Tests Issue

Result

Ref.

Concerns over supercritical CO2 cleaning polymers

Supercritical CO2 cleaning can be adjusted to have no detrimental effect on crystalline polymers Supercritical CO2 cleaning removed 97 to 99.95% of the oil Supercritical CO2 good for water sensitive or high-temperaturesensitive parts Supercritical fluids can be applied to these substrates Supercritical CO2 used; excellent payback and 90% reduction in ODS Supercritical CO2 works well for parts with complex shapes

19

Supercritical CO2 shown to work more efficiently for contaminated plastic than cleaning with CFC-113 Supercritical CO2 is equally effective

4, 8, 24 –27

Oil removal from rings, washers, and plates Different substrates, including aluminum, glass, copper, brass, stainless steel, and epoxy boards Soils and other solid materials, containing residual pesticides Various metals, plastics, and epoxies needed to be precision cleaned, using CFC-113 Vapor degreaser used to clean gyroscope parts contaminated with machining coolants, silicone oils, and damping fluids High cleanliness standards required for precision cleaning

Replace perchloroethylene (PERC) as a dry cleaning solvent

20 21

22 23

24

9

• Supercritical CO2 cleaning was not pursued because of the predicted long payback time (10 years) of the investment, and the high operating pressure of the system (which was viewed as a safety hazard). In another study, AT&T researchers tested a number of different cleaning alternatives to replace 1,1,1-trichloroethane vapor degreasing. • Aluminum parts contaminated with cutting oils and protective fluids were tested. • Supercritical CO2 cleaning efficiency was more than 98% for several of the contaminants. • The requirement for additional additives and cosolvents to obtain acceptable cleaning results led the researchers to eliminate supercritical CO2 from consideration.29 As can be seen from these examples, while supercritical CO2 cleaning appears to be effective in a number of instances from a technological standpoint, some technical limitations combined with the economies of the process, which will be discussed more below, have resulted in a slow rate of implementation in the private industrial sector. Effect of Mixing on Supercritical CO2 Cleaning Test results indicate that increasing the internal agitation and temperature in the cleaning chamber reduces the time needed to clean the metal parts. The supercritical cleaning © 2001 by CRC Press LLC

process, when run at optimum conditions, appears to use less energy than conventional vapor degreasing operations. Furthermore, cleaning results attained with supercritical CO2 plus mixing compare favorably with conventional solvent cleaning.27 Two studies that discuss the effect of mixing on supercritical CO2 cleaning resulted in somewhat different conclusions. As mentioned earlier, a manufacturer of metal disks was not able to determine that mixing had any effect on cleaning efficiency, but that a larger sample size may have given a more definitive result.28 A more in-depth study specifically designed to examine the effect of fluid turbulence on supercritical CO2 cleaning concluded that mixing has an effect. Researchers at Pacific Northwest National Laboratory completed the study. The researchers recommended that agitation be used whenever possible for supercritical CO2 cleaning applications to help maximize cleaning efficiency, and that mixing rates can be optimized to minimize power costs.30 Previous studies by Phasex Corporation, the company with the longest experience with supercritical fluids for cleaning, have demonstrated that supercritical CO2 is an excellent solvent for oils such as hydrocarbons, esters, silicones, perfluorpolyether, halocarbonsubstituted triazines, and organosilicones with various reactive functionalities; many of these oils are associated with the manufacture of precision components such as gyroscopes and accelerometers.4 The ability to dissolve a particular oil or polymer at any given pressure will greatly depend on the molecular weight and structure of the material. The ability of supercritical fluids to dissolve many types of oils and organic materials, coupled with the ability to penetrate minuscule pores and interstices of metal, ceramic, and composite parts, suggests that these fluids could partially replace CFCs. As the use of CFCs and other halogenated solvents is phased out because of their role in stratospheric ozone depletion, condensed-phase CO2 is the most promising of the alternative technologies because of its low cost, low toxicity, nonflammability, and environmental acceptability. The CO2 cleaning process, the CO2 cleaning system operation, technical factors and life-cycle costs, commercial applications, and future developments have been considered.4

ECONOMICS As has been pointed out, the economics behind condensed-phase CO2 use in cleaning has been largely prohibitive when applied in the cleaning industry. Equipment for supercritical fluids cleaning tends to be costly, and process development is very applicationspecific.31 Researchers at Pacific Northwest National Laboratory during a supercritical CO2 cleaning market assessment completed in 1994 identified some of the reasons for lack of demand in the private sector for supercritical CO2 cleaning (Table 7).32 The initial cost of a condensed-phase CO2 system might seem prohibitive, but when the environmental benefits (health and safety and regulatory) and the energy savings overall are considered, the return-on-investment looks rosier. The high capital cost of supercritical CO2 cleaning systems can be attributed, in part, to the high-pressure cleaning chamber and the valves and instrumentation required for the system. The system cost is also high because there are no vendors that mass-produce supercritical CO2 systems. Lack of mass production does not allow vendors to realize the economies of scale that could be obtained if demand for the system were higher. Unless demand increases, it will be difficult to reduce the purchase costs to a point where the basic payback for supercritical CO2 cleaning is rapid enough to attract a significant number of manufacturers. © 2001 by CRC Press LLC

Table 7 Potential Barriers to Acceptance of Supercritical CO2 • • • • • •

Higher capital costs for supercritical CO2 systems relative to other cleaning technologies Lack of awareness of supercritical CO2 cleaning technology Substrate to be cleaned lacks compatibility with supercritical CO2 or with high pressures A perception that supercritical CO2 cleaning does not remove particulates effectively The requirement for a continuous process (supercritical CO2 cleaning is a batch process) The existence of established aqueous-cleaning technologies to replace solvent vapor degreasers

CO2 cleaning systems have lower capital costs, require less labor, do not release any CFCs, take up less space, and, above all, the solvents cost less. The time required for cleaning and stripping processes can be reduced by as much as 80 to 90%. Published investigations into the economics of supercritical CO2 cleaning show that operational costs of supercritical CO2 cleaning are quite reasonable and often lower than solvent vapor degreasers or aqueous cleaning systems. CO2 is relatively inexpensive and can be reused in most supercritical CO2 cleaning systems. Waste treatment is minimal, as no waste stream is generated beyond the actual contaminants removed from the parts being cleaned. Unfortunately, the initial capital costs for supercritical CO2 systems are usually higher than other alternatives—sometimes by a significant amount. Findings on economics include: • Researchers at Los Alamos Laboratory conducted a study to measure the total electrical costs associated with operation of a 60-l capacity supercritical CO2 system at temperatures between 30 and 50°C and pressures between 1500 and 3500 psi. The researchers concluded that utility costs for the unit will be relatively insignificant when compared with operations and maintenance labor costs.33 • An economic analysis done after bench-scale testing based on cleaning 150 bearings/year showed annual savings of only $1400 on a $75,000 to $100,000 investment. It appears that a much higher volume application would be required to justify the supercritical CO2 system.34 CONCLUSIONS In terms of the triad of drivers affecting the adoption of any cleaning technology (technological, regulatory, and economic), condensed-phase CO2 is favorably positioned in all but the economic arena. As the economics of this cleaning technology become more acceptable, cleaning with liquid and/or supercritical CO2 will become more prevalent. REFERENCES 1. Nelson, W.M., Art in science: utility of solvents in green chemistry, in Green Chemistry: Frontiers in Benign Chemical Syntheses and Processes, Anastas, P.T. and Williamson, T.C., Eds., Oxford University Press, Oxford, UK, 1998, 200. 2. Jackson, D. and Carver, B., Liquid CO2 immersion cleaning, Parts Cleaning, April, 1999. 3. Smith, J.M. and Van Ness, H.C., Introduction to Chemical Engineering Thermodynamics, 4th ed., McGraw-Hill, New York, 1975, 54. 4. Gallagher, P.M. and Krukonis, V.J., Precision parts cleaning with supercritical carbon dioxide, in Solvent Substitution for Pollution Prevention, U.S. Department of Energy and U.S. Air Force, Noyes Data Corporation, Park Ridge, NJ, 1993, 76.

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5. Yamauchi, Y., Supercritical fluid extraction and chromatography, J. Syn. Org. Chem. Jpn., 54, 395, 1996. 6. Connors, K.A., Chemical Kinetics: The Study of Reaction Rates in Solution, VCH Publishers, Inc., New York, 1990, 480. 7. Savage, P.E., Gopalan, S., Mizan, T.I., Martino, C.J., and Brock, E.E., Reactions at supercritical conditions—applications and fundamentals, AICHE J., 41, 1723, 1995. 8. Salerno, R.F., High pressure supercritical carbon dioxide efficiency in removing hydrocarbon machine coolants from metal coupons and components parts, in Solvent Substitution for Pollution Prevention, U.S. Department of Energy and U.S. Air Force, Noyes Data Corporation, Park Ridge, NJ, 1993, 98. 9. McCoy, M., Industry intrigued by CO2 as solvent: “green” processes based on supercritical carbon dioxide are moving out of the lab, Chem. Eng. News, June 14, 11, 1999. 10. Liu, G.T. and Nagahama, K., Solubility of organic solid mixture in supercritical fluids, J. Supercritical Fluids, 9, 152, 1996. 11. McClain, J.B., Betts, D.E., and Canelas, D.A., Design of nonionic surfactants for supercritical carbon dioxide, Science, 274, 2049, 1996. 12. Meguro, Y., Iso, S., and Sasaki T., Solubility of organophosphorus metal extractants in supercritical carbon dioxide, Anal. Chem., 70, 774, 1998. 13. O’Neill, M.L., Cao, Q., Fang, R., Johnston, K.P., Wilkinson, S.P., Smith, C.D., Kerschner, J.L., and Jureller, S.H., Solubility of homopolymers and copolymers in carbon dioxide. Ind. Eng. Chem. Res., 37, 3067, 1998. 14. Stassi, A. and Schiraldi, A., Solubility of vegetable cuticular waxes in supercritical CO2 isothermal calorimetry investigations, Thermochim Acta, 246, 417, 1994. 15. Anitescu, G. and Tavlarides, L.L., Solubilities of solids in supercritical fluids 2. Polycyclic aromatic hydrocarbons (PAHs) plus CO2/cosolvent, J. Supercritical Fluids, 11, 37, 1997. 16. Souvignet, I. and Olesik, S.V., Solvent-solvent and solute-solvent interactions in liquid methanol/carbon dioxide mixtures, J. Phys. Chem., 99, 16800, 1995. 17. McFann, G.J., Johnston, K.P., and Howdle, S. M., Solubilization in nonionic reverse micelles in carbon dioxide, AICHE J., 40, 543, 1994. 18. Schleussinger, A.O. and Ingo, B.R., Moisture effects on the cleanup of PAH-contaminated soil with dense carbon dioxide, Environ. Sci. Technol., 30, 3199, 1996. 19. Sawan, S.P., Evaluation of the Interactions between Supercritical Carbon Dioxide and Polymeric Materials, Los Alamos National Laboratory, Los Alamos, NM, 1994. 20. Novak, R.A., Cleaning of precision components with supercritical carbon dioxide, International CFC and Halon Alternatives Conference, U.S. Environmental Protection Agency, Washington, D.C., 1993. 21. Williams, S.B., Elimination of Solvents and Waste by Using Supercritical Carbon Dioxide in Precision Cleaning, LA-UR-94-3313, Los Alamos National Laboratory, Los Alamos, NM, 1994. 22. Knez, Z., Riznerhras, A., Kokot, K., and Bauman, D., Solubility of some solid triazine herbicides in supercritical carbon dioxide, Fluid Phase Equilibria, 152, 95, 1998. 23. Hunt, D., How one of the largest Air Force users is getting out of CFCs, in Proceedings of the 1992 International CFC and Halon Alternatives Conference: Stratospheric Ozone Protection for the 90’s, Washington, D.C., 1992. 24. Weber, D.C., McGovern, W.E., and Moses, J., Precision surface cleaning with supercritical carbon dioxide: issues, experience, and prospects, Metal Finishing, 93, 22, 1995. 25. McGovern, W.E., Moses, J.M., and Weber, D.C., The use of supercritical carbon dioxide as an alternative for chlorofluorocarbon (CFC) solvents in precision parts cleaning applications, in Proceedings Air Pollution Control Association Annual Meeting, 1994. 26. Purtrell, R., Rothman, L., Eldridge, B., and Chess, C., Precision parts cleaning using supercritical fluids, J. Vac. Sci. Technol., 11, 1696, 1993. 27. Silva, L.J., Supercritical fluid for cleaning metal parts, Haz. Waste Consultant, 13, 1.25, 1995. 28. Supercritical Fluid Extraction Cleaner Application: Texas Instruments Incorporated, Toxics Use Reduction Institute, Lowell, MA, 1994.

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29. Gillum, W.O., Replacement of chlorinated solvents for metal parts cleaning, in Precision Cleaning ‘94, Witter Publishing, Rosemont, IL, 1994. 30. Phelps, M.R., Waste Reduction Using Carbon Dioxide: A Solvent Substitute for Precision Cleaning Applications, Pacific Northwest National Laboratory, Richland, WA, 1994. 31. Kanegsberg, B., Precision cleaning without ozone depleting chemicals, Chem. Ind., 787, 1996. 32. Pacific Northwest National Laboratory, Richland, WA, 1994. 33. Barton, J.C., The Los Alamos Super Scrub: Supercritical Carbon Dioxide System Utilities and Consumables Study, Los Alamos National Laboratory, Los Alamos, NM, 1994. 34. Licis, I.J., Pollution Prevention Possibilities for Small and Medium-Sized Industries—Results of the WRITE Projects, U.S. Environmental Protection Agency, Washington, D.C., May, 1995, 127.

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CHAPTER 2.14

Carbon Dioxide Dry Ice Snow Cleaning Frank Cano

CONTENTS Introduction Properties Cleaning with CO2 Momentum Transfer Solvent Effect Thermophoresis Freeze-Fracture Effect Applications Condensation Electrostatic Discharge Redistribution of Contaminants Thermal Shock Too Much Blast for Sensitive Parts Surface Properties Cost of CO2 Cleaning Safety/Environmental Conclusion INTRODUCTION Jan Baptist van Helmont, the 17th-century Belgian chemist who first recognized carbon dioxide, could never have imagined the varied applications of carbon dioxide today. In chemical extraction, decaffeination of coffee, carbonation of beverages, uses as a refrigerant, use in fire extinguishers, use in cleaning applications, and many other commercial uses, carbon dioxide has important applications in the modern world. PROPERTIES Carbon dioxide is a minor component of Earth and makes up about 0.04% of the total atmosphere. Plants employ it in the photosynthesis process. CO2 is formed in various ways: © 2001 by CRC Press LLC

in the combustion of carbon-containing materials, in fermentation, and in the respiration of animals. Commercially it is recovered from flue gasses, as a by-product in the synthesis of ammonia, from the cracking of petroleum, and other processes. CO2 is colorless in its liquid and vapor state, is about 53% heavier than air, and will displace oxygen. Heavy concentrations must be avoided in areas not sufficiently ventilated. Asphyxiation is a real danger when CO2 is used in large amounts in confined areas. It is nontoxic, which makes it ideal for many commercial uses. CO2 is inert at normal atmospheric temperatures; however, potassium will burn violently if heated in a CO2 atmosphere. Under specific conditions, pressurized CO2 can combine with water to form carbonic acid (H2CO3), a relatively weak dissociated acid. CO2 can exist in three phases—vapor, liquid, or solid. The state of CO2 is dependent upon the temperature and the pressure at which it exists. At atmospheric temperatures and pressure CO2 exists as a gas but when cooled and pressurized it converts to its liquid form. Liquid CO2 cannot exist at atmospheric pressures. At 21°C (69.8°F) it must be compressed to 850 psia (58.6 bar) to remain a liquid. Thus, pressurized cylinders of CO2 can be stored for an indefinite time at normal room temperature and still maintain a liquid phase. The highest temperature at which liquid CO2 can exist is 31.1°C (87.9°F). This is known as its “critical temperature.” The highest pressure at which CO2 will liquefy is 1070.6 psia (73.8 bar). This is known as its “critical pressure.” Below 56.7°C (69.8°F) and below 75 psia (5.2 bar) CO2 exists as a solid. The condition at which all three forms of CO2 exist simultaneously is known as its “triple point.” This phenomenon occurs when CO2 is at 56.7°C (69.8°F) and 75 psia (5.2 bar). If pressurized liquid CO2 is allowed to expand to atmospheric pressure through a constriction, it passes through its triple point and a portion of the liquid is converted to dry ice particles. When the “dry ice” warms, it converts directly to vapor bypassing the liquid phase. This is known as sublimation. It leaves no residue in this process. The temperature of “dry ice” at atmospheric pressure is 78.5°C (109.3°F). This low temperature accounts for its wide use as a refrigerant. CLEANING WITH CO2 One of the latest uses of CO2 is in cleaning applications. CO2 is an effective cleaning agent in its liquid and supercritical states and as dry ice particles, flakes, or pellets. This chapter will deal with dry ice snow cleaning only. When the liquid CO2 is released to atmospheric pressure, flocculent particles of “snow” are formed. “Snow flakes” are formed when the particles of dry ice agglomerate. The particles of dry ice snow are central to the cleaning process of CO2. Dry ice particles and flakes clean in a variety of ways. These mechanisms combine to create an effective means to clean particulate and light hydrocarbon contamination from surfaces quickly and efficiently. The solid phase of CO2 sublimes directly to a vapor at atmospheric pressures leaving no residues. MOMENTUM TRANSFER Contaminating particles in the 2-m range and smaller are difficult to remove by conventional blowoffs with compressed air or dry nitrogen. The smaller the particle, the greater the percentage of its total area in contact with the surface to be cleaned and therefore the lesser the percentage of its surface exposed to aerodynamic drag forces generated by gas or liquid flowing over the contaminated surface. There are also electrostatic and © 2001 by CRC Press LLC

bonding adhesive forces that hold contaminants to surfaces. Even if there were no adhesive forces holding the contaminant to the surface, there would still be removal problems because of the boundary-layer phenomenon. The boundary-layer phenomenon pertains to the fact that a fluid flow (gas or liquid) is only effective at some finite distance from the surface being cleaned. As a flow gets nearer a surface, its velocity decreases. The laws of fluid dynamics state that fluid flow velocity at a surface must be zero. Thus, small particles below this boundary layer are minimally affected by pressurized gas blowoffs. One way to penetrate the boundary layer is to introduce a mass into the cleaning flow allowing the mass to transfer its energy to the contaminant and knock it free. Many methods have been used to introduce a mass such as sand, walnut shells, or talc. The residue left by these methods often outweighs their benefits. CO2 introduces a mass to the cleaning flow but leaves no residue. Contaminants are knocked free by the particles of CO2 snow and are carried away in the vapor flow of the CO2 (Figure 1). By changing nozzle configurations the spray pattern, size, and force of the dry ice particles can be adjusted. Microparticles can be generated in the 1-mm-diameter range with a velocity from 150 to 1000 fps that produces an aggressive dry ice storm capable of removing light oils, light greases, and hydrocarbons. Fingerprints can be removed from many surfaces. Other nozzles may be employed generating flakes up to 0.5 cm that produce a gentle snow fall capable of cleaning unadhered particles down to 0.1 um without disturbing delicate substrates. The force of the gas pressure is not the only means of transference of energy. As the very cold particle of dry ice approaches a much warmer surface, the side of the particle closest to the surface rapidly changes phase. When the CO2 sublimes there is a rapid, virtually explosive, expansion as the particle of CO2 changes from a solid to a gas. This is a source of shear stress energy that can dislodge a contaminant. SOLVENT EFFECT CO2 in its liquid phase acts as an excellent solvent. The solvent action occurs when a thin layer of liquid CO2 forms at the collision interface of the dry ice particle and the surface being cleaned. The liquid is generated at the moment of impact when the dry ice particle is deformed (Figure 2). Surface pressure on the dry ice particle rises above the triple point pressure of 75 psia (5.2 bar). At this pressure, all three phases of CO2 are present: solid,

Figure 1

Particle removal by momentum transfer.

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Figure 2

Solvent effect from transient liquid due to impact pressures.

liquid, and vapor. It is the liquid that acts as a solvent dissolving organic contaminants and carrying them off in the vapor flow.

THERMOPHORESIS Thermophoresis refers to the temperature gradient in a particle of CO2. As the cold particle approaches a warmer surface, the side of the particle closest to the surface warms more rapidly than the side away from the surface. Warm air molecules have the tendency to push contaminants toward the colder side of the particle of dry ice. The greater the temperature difference, the stronger the force. Contamination becomes entrapped in the dry ice particle and is again carried off in the vapor flow. The contamination remains entrapped in the dry ice particle until it has fully sublimed. It is then redeposited at the locations where the dry ice sublimed. This factor should be considered when cleaning with CO2 dry ice. Adequate measures should be taken to exhaust the vapor and dry ice flow to prevent the redeposition of contaminants.

FREEZE-FRACTURE EFFECT As the supercold flow of CO2 blankets a surface, certain organics may freeze into a hardened state. Liquid CO2 is forced through surface pores in the organic material. In the explosive rapid phase change, the frozen, brittle materials are shattered and carried away (Figure 3).

APPLICATIONS The many applications where CO2 dry ice cleaning can be used are diverse. Some of the more typical include the following: Optics—From a 400-in. telescope mirror to 20  20 mm photoelectric cells, CO2 cleaning removes dust particles and light hydrocarbon contamination. Laser interferometers and mirrors—Cleaning with dry ice particles provides contaminant-free optical paths. © 2001 by CRC Press LLC

Figure 3

Freeze-fracture effect.

Silicon wafers—The speedy cleaning effect allows contamination as small as 0.1 um to be removed in an effective manner. Ceramics—Light oils, hydrocarbons, and fingerprints can be removed from ceramic surfaces. Substrates—Flat-panel-display substrates and glass prior to coatings are effectively cleaned of the most minute contamination. Semiconductors—Printed circuit boards and hybrid circuits can be cleaned to a precision level without disturbing delicate wire bonds or damaging substrates. Read/write heads—Removing microscopic contamination is quick and thorough with CO2 dry ice cleaning. Medical products—Cardiac and ophthalmic shunts are cleaned to a precision level. Medical tools and instruments have dust, light oils, and fingerprints removed. Decontamination—Semiconductor instruments can be reclaimed by having arsenic deposits removed from difficult-to-reach areas (threads, orifices). No measurable arsine gas has been detected after CO2 cleaning. Micromechanical assemblies—Gyroscopes and microvalving components can be cleaned to remove microscopic contamination. Many other applications where precision cleaning to a microscopic level is necessary may be served by CO2 dry ice cleaning systems. CO2 cleaning is not the panacea for all cleaning applications and consideration for several aspects of CO2 use must be taken. CONDENSATION Because of the very cold nature of solid CO2, parts being cleaned that have a small thermal mass often can be cooled until they drop below the dew point of the ambient air. The result is that moisture is drawn from the surrounding air and condenses on the cooled parts. This may lead to additional cleaning problems. There are several ways to eliminate or reduce this moisture problem. Parts may be cleaned in a “dry box” where ambient air is purged and replaced with a dry inert gas, such as nitrogen, argon, or CO2 vapor. This allows the ambient humidity to be reduced so that moisture does not form on the cleaned surfaces. © 2001 by CRC Press LLC

Another method is to heat the parts to be cleaned. Raising the temperature with a hot plate, infrared lamp, or heat gun allows the part to be cleaned without dropping its temperature below the dew point, thus reducing the condensation effect. ELECTROSTATIC DISCHARGE Precautions must be taken to protect parts vulnerable to electrostatic discharge (ESD). CO2 is itself nonconducting, but as particles of CO2 leave the nozzle of a cleaning mechanism friction results in an electrostatic charge. Many techniques can be used to reduce or eliminate the ESD. Grounding parts is one method. A voltage probe inserted into the CO2 flow can dissipate the ESD effect. Ionized airflow convergent with the CO2 is also an effective means to reduce the ESD and may have the added benefit of reducing the condensation effect. REDISTRIBUTION OF CONTAMINANTS As contaminants are knocked from the surface of a part, they are picked up in the flow of CO2 vapor and solid and can be redeposited wherever the CO2 sublimates. If there are orifices, nooks, crannies, or corners where the CO2 snow can accumulate, this is where the contaminants will be deposited. There are various solutions to this problem. Cleaning can be done under a laminar flow allowing the contaminants to be carried off. Often parts are cleaned upside down to allow the particles of dry ice and the contamination to drop free of the part. Sometimes parts are heated to create a chimney effect of rising hot air to carry the contaminants free of the part. Dry ice cleaning is a line-of-sight cleaning. Any surface that is struck by the CO2 particle will be scoured. Often surfaces not meant to be cleaned are struck by the dry ice particles, cleaned of contaminants, and then those contaminants are deposited on the surface originally intended to be cleaned. Dry ice particles can rebound off walls around surfaces and deposit contaminants from those walls. Again, adequate exhaust flow or laminar flow will often prevent this occurrence. THERMAL SHOCK Parts that are sensitive to rapid temperature drops or cold temperatures may be affected by CO2 cleaning. This may not be a problem, however, since cleaning with dry ice snow is very rapid. Particles are removed in an action so fast that parts often do not have a chance to drop significantly in temperature. TOO MUCH BLAST FOR SENSITIVE PARTS Users may be concerned about the effect of the pressurized flow of CO2 striking their sensitive parts. Because of the nature of the particle of CO2 at the moment of impact, damage rarely happens. A pressure distribution exists across the particle–surface interface. When the local pressure exceeds the yield pressure of the dry ice, the dry ice will yield. The yield stress point is equal to the triple point pressure of 75 psia. At pressures above the triple point a liquid phase will form that causes it to yield, resulting in an increased impact area. Spreading the impact force over a larger area limits the stress and permits thorough © 2001 by CRC Press LLC

cleaning without destructive forces. By using interchangeable nozzles, users may also vary the spray pattern, force, and speed of the dry ice particles. SURFACE PROPERTIES Although CO2 dry ice cleaning can remove light oils and hydrocarbons, surface properties may affect its cleaning potential. If a surface is porous and the contaminants are absorbed into the surface material, CO2 cleaning may be ineffective. Oils have a tendency to etch themselves into soft materials and coatings like gold, silver, or aluminum. Unless cleaned in a timely manner, a more abrasive means to clean these etched surfaces must be used. COST OF CO2 CLEANING Cleaning to a precision level requires that no other contaminants be introduced to the cleaning process. With CO2 cleaning this means that ultraclean CO2 must be used. Typically, many grades of CO2 are available from industrial gas suppliers. These are supplied under many names (Food Grade, Coleman, Medical, Research, or Laboratory to name a few) but these grades should always be specified by their purity, 99.99% (four nine), 99.999% (five nine), etc. Gas suppliers should be able to provide guaranteed specifications for the liquid-phase CO2 that includes total hydrocarbon, water, and nonvolatile residue content. The price for the purest grade of liquid phase CO2 may be negotiated with the supplier but will typically be from $2 to $15 lb. Availability of high-purity CO2 may vary from region to region. CO2 snow cleaning systems, depending upon the manufacturer, may consume from 30 lb/h to over 100 lb/h under continuous duty. With CO2 snow the cleaning operation is generally done in short 2- to 3-second bursts. Hydrocarbon and fingerprint removal may take a longer spray duration. Rarely is continuous-duty operation performed. If operating costs become significant, CO2 purification equipment is available that allows the user to purchase low-quality “welding grade” CO2 and process it into ultrapure 99.999999% quality. Welding-grade vapor CO2 could then be purchased in the $0.08 to $0.18/lb range, thus cutting the operating costs significantly. The purifiers work through a process of distillation removing contaminants and nonvolatile residues. Because of the vast differences in the condensing pressures and temperatures for CO2 and these impurities, purifiers eliminate these contaminants and only condense pure CO2 into its liquid phase. SAFETY/ENVIRONMENTAL CO2 is an inert gas and human beings have a very high tolerance of exposure. OSHA requirements effective March 1, 1990 specify a PEL-TWA of 5000 ppm and a STEL of 30,000 ppm. In real English, this means that a person can be exposed to an average concentration of 5000 ppm over an entire 8-h workday. A person can also be exposed to a concentration of 30,000 ppm when exposure is averaged over 15 min. The threshold for humans is well below both of these limits. At moderate concentrations, a sharp odor or taste will be detected. If there is a concern regarding the accumulation of CO2 in the working environment, several high-quality monitors are available for CO2 detection or oxygen depletion. CO2 should only be used in well-ventilated areas where CO2 vapor will not build beyond the tolerance levels. © 2001 by CRC Press LLC

The blast of dry ice snowflakes or flow of liquid CO2 should never be aimed at any person. The dry ice is very cold (78.5°C). Momentary contact with human skin is harmless but prolonged contact may cause frostbite. Users of dry ice snow cleaning equipment should always wear eye protection. Parts being cleaned should always be secured to prevent them from being blown around by the vapor pressure of CO2. Liquid CO2 is stored under high pressure. Cleaning equipment should be manufactured to withstand these high pressures. Safety devices like rupture disks and relief valves should be standard to prevent overpressurization. If high-pressure cylinders are used, they should always be adequately restrained in an approved cylinder rack or securely fastened to a structural wall or bench clamp unit to prevent any possibility of tipping over. CO2 does not contribute to the depletion of the ozone layer. However, it is considered a “greenhouse gas.” Commercially sold CO2 is recycled CO2 that has already been produced as a result of some other industrial process. If CO2 were not recaptured for cleaning, cooling, and other uses, it would have been vented to the atmosphere at the time it was generated from the manufacture of ammonia and hydrogen, from natural gas production, and from the cracking of petroleum to make gasoline and other petroleum products. Its use therefore is a delayed release and has no net additional effect on the environment. Also the amounts released for this application are exceedingly small compared with the amounts generated by the combustion of fossil fuels. CONCLUSION CO2 dry ice cleaning is a fast and effective way to remove particulate contamination and light hydrocarbon contamination down to 0.1 um from silicon wafers, hybrid circuits, optics, disk drive assemblies, medical instruments, metal parts, and many other components. It leaves no residue.

© 2001 by CRC Press LLC

CHAPTER 2.15

Gas Plasma: A Dry Process for Cleaning and Surface Treatment Lou Rigali and William Moffat

CONTENTS History Technology Surface Effects Semiconductor Applications Wafer Fabrication Packaging and Assembly Die Bonding Wire Bonding Encapsulation Marking Fluxless Soldering Nonsemiconductor Applications Removal of Vacuum Grease from Machined Copper and Stainless Steel Parts Equipment Summary References

HISTORY The first commercial application of gas plasma was ashing animal tissue. The technique was used to remove the organic matrix and leave the inorganic residue for subsequent analysis. A chemist, Steve Irving, at the Signetics wafer processing company reasoned, “If plasma could remove organic material, and photo resist was an organic material, then plasma should be able to remove resist from wafers.”1 This was in about 1965 and it did not take long for plasma technology to develop and expand into many areas of wafer fabrication and to spawn companies such as Applied Materials, Lam Research, and many others whose total sales are hundreds of millions of dollars. © 2001 by CRC Press LLC

In the 1970s, companies like Raytheon and Rockwell were manufacturing hybrids and found that plasma cleaning bond pads improved bond strengths by more than 70% and plasma treating the die pad improved adhesion of the die.2 Now, probably more than 80% of all hybrid manufacturers use plasma cleaning as part of the process.

TECHNOLOGY Plasma is a state of matter, a so-called fourth state of matter along with gases, liquids, and solids. This chapter discusses a low-temperature plasma where a significant but low fraction of the gas is ionized (an ion is a gas atom that has become charged by losing or gaining an electron). Such a plasma is not in thermal equilibrium because the temperature of the gas is only about 30 to 50°F above ambient, while the electrons are much hotter. A neon light is an example of this kind of plasma. The sun would be an example of a hightemperature plasma where the gas is all ionized and the gases and the electrons are both at very high temperatures. Typically, a low-temperature plasma is formed under vacuum conditions ranging from 100 to 1000 mT, although there are a number of publications that describe an atmospheric process (not a corona or dielectric discharge).3 The energy to dissociate the gas can be either DC voltage or radio frequency from several thousand hertz up to microwave frequencies of 10 GHz. It can be argued that the chemistry of the plasma is the same independent of the power source; however, each method of dissociation has its merits and disadvantages. Besides ions, a plasma contains free radicals, which are atomic and molecular specie in excited energy states. Many of these specie can react chemically or physically under relatively mild conditions. For example, paper which will “burn” or oxidize rapidly at about 800°C, and will undergo the same reaction at about 30 to 60°C in a plasma. The types and nature of the specie formed will depend on the gases used. The most important active agent is atomic oxygen in an oxygen plasma. This is a free radical and will react chemically with organic material to form CO2 and H2O. When argon is used as a gas, an argon ion can be accelerated in a field and has enough physical energy to break carbonto-carbon bonds or to displace by sputtering other elements on a surface. Whichever gases are used, the reaction is at the surface and material is removed on the molecular level at rates of angstroms per second or minute. There is substantial chemistry that can take place at the surface, especially with organic and polymeric material. Plasma can be used to introduce functional groups into a polymer chain or to actually deposit polymers. These applications are important but not within the scope of this chapter, which attempts to describe some applications where dry plasma is effective and can be used instead of volatile solvents and corrosive chemicals.

SURFACE EFFECTS The plasma process affects the surface of materials usually ranging up to 100 to 300 Å. One can etch material and as the material is removed, the new surface is exposed, and the process continues so that plasma can be used to etch (ash) microns of material. Many applications, however, do not involve removal of the material but the modification of a surface. Treatment with plasma can change the surface energy. The lower the contact angle, the higher the surface energy and bondability. Plasma is an effective technique for etching and treatment of material that involves no organic solvents nor any acids or caustic agents. © 2001 by CRC Press LLC

Figure 1

Contact angle.

A convenient way to measure surface energy is by measuring the contact angle of a deionized water droplet (Figure 1). When the contact angle is small, the surface energy is large and the inactivated surfaces are called hydrophobic. Good adhesion can be obtained when the contact angle is less than 10°. Many applications are related to the semiconductor or electronics industry, but the modifications are of broader use and can extend into many other areas, such as polymer chemistry and material sciences. Semiconductor Applications Wafer Fabrication There are many terms that are used to describe the plasma process. It is difficult to define these terms in many examples because people will use the same term when describing different processes. In the semiconductor industry, the terms ashing, stripping, and etching can all mean the removal of photoresist from a wafer. This is a major application and replaces the use of hot sulfuric acid. Removing metal and metal oxides is usually referred to as an etching process. Thin organic and inorganic films such as nitrides can be deposited on wafers (and other surfaces). Most applications involve making significant changes in the surface, extending several microns into the surface. Packaging and Assembly Unlike wafer fabrication, most of the applications in this section involve only the first few 100 Å of the surface. The topography of the surface is not drastically changed and most applications involve improving adhesion. Adhesion to a surface is usually good if the surface has a high surface energy. Terms such as cleaning, ablation, treatment, or roughening generally describe the sample process. © 2001 by CRC Press LLC

Die Bonding Plasma cleaning/treatment of substrates will always improve the adhesion of the epoxy and provide a better bond between the die and the substrate. The better bond provides better heat dissipation. Studies have also shown that there is less delamination at the die with samples that have been plasma treated.4 Wire Bonding ESCA (electron spectroscopy for chemical analysis) has shown that the presence of carbon on a surface will limit the quality of the wire bond.5 The relative level of carbon contamination on a copper surface, for example, can be determined from the ratio of the area of the carbon (C) and copper (Cu) peaks in ESCA. Since water drop measurements are much easier to perform, it is good to know that the measured ESCA C/Cu ratios correlate extremely well with water drop contact angles measured on the same surface. Bonding pads on the substrate are also subjected to various and inconsistent levels of contamination. One source of contamination on the die surface is fluorine ion, most likely left as a residue on the wafer or die during the fabrication process.6 Although the presence of this contamination may not show wire bond degradation immediately, there is evidence showing correlation of the presence of fluorine element and bond failure due to the migration of fluorine, causing embrittlement of the wire.7 Removal of this contaminant may require the use of argon bombardment. This is an important application of plasma, i.e., the removal of trace amounts of impurities such as inorganic ions by the use of argon. Encapsulation In this process the molding compound is asked to provide, in addition to all the other required properties that are important, good adhesion to a number of different surfaces. Depending on the type of package, the molding compound must adhere to the substrate material, solder mask, die, and the metal bond pads. There are many applications that involve the bonding of one material to another, plastic to plastic, metal to plastic, etc. In each of these bonding operations, plasma improves the quality of the bond (Figure 2) as shown by the testing and measurement done in the study by Herard.8 Marking The replacement of organic solvent-based inks with aqueous-based inks does not provide the same consistency of adhesion. Even heat or hydrogen flame treatment is inconsistent. Again, plasma-treated surfaces always provide a better surface. Some situations will show better results than others because of the nature of the ink, the encapsulant, and the marking process. Plasma treatment ensures uniformity of the process and decreases the variability of the results. Printing or marking on many different types of surfaces is especially improved with use of aqueous-based inks. Fluxless Soldering A plasma process, patented by MCNC, has been shown to give good welding results with typical lead and gold solder formulations, without the use of flux, and hence without the requirement of a flux removal step.9 © 2001 by CRC Press LLC

treated nontreated

Stress (MPa)

171 Composite rupture Yield point

65

Strain Figure 2

Improved adhesion.

Nonsemiconductor Applications Removal of Vacuum Grease from Machined Copper and Stainless Steel Parts A manufacturer of a very sophisticated piece of medical diagnostic equipment that operates under vacuum conditions requires that the machined metal subassemblies that go into product must be absolutely clean from contamination, in particular, but not limited to, carbon. In addition many of these same machined parts have to be periodically replaced. The wet cleaning process that was used included the use of acetone and water. The disadvantage was that there was always some residue of the solvent that compromised the vacuum, and a company directive indicated that wherever possible the use of all organic solvents be eliminated. A specific cleaning process was developed using O2/CF4 plasma in a batch plasma system (March PX-2400). The test samples were contaminated by taking a finger smear of vacuum grease and spreading it on the parts. No attempt was made to measure the amount of contamination, but it would appear that it was about 1000 times greater than would normally occur during actual use. Process time varied as one would suspect, because of the indeterminate levels of contamination, from about 10 to 30 minutes. There are many other application that are outside the semiconductor industry. The following list gives a brief summary of the types of current applications. Medical—Treatment of catheters Environmental—Low-temperature combustion toxic agents Manufacturing—Removal of machine oils from various parts Plastic—Treatment of films to improve surface properties Disk media—Improving the uniformity of deposited thin films Film—Microvia etching10 © 2001 by CRC Press LLC

EQUIPMENT One can select manual batch, automated batch, or fully automated processes. Plasma systems have been developed to meet most requirements from small development tools costing less than $10,000 to automated systems costing about $250,000. Most systems operate under a soft vacuum and require a vacuum chamber and, of course, a vacuum pump. A relatively new development3 involves the use of an atmospheric plasma, not a corona or dielectric discharge. SUMMARY Plasma, a dry nontoxic cleaning/treatment process, has been around for more than 30 years and is being used to replace wet chemicals in many industries. REFERENCES 1. Irving, S.M., Kodak Photoresist Seminar, 2, 26, 1968. 2. Bonham, H.B. et al, Plasma cleaning for improved wire bonding on thin-film hybrids, Electr. Packaging Prod., Feb., 1979. 3. Roth, J.R. et al, Experimental generation of a steady-state glow discharge at atmospheric pressure, Proc. IEEE International Conference on Plasma Science, 1992, 170. 4. Oren, K., A case study of plastic part delamination, Semiconductor Int., April, 1996, 109. 5. Djennas, F. et al., Investigations of plasma effects on plastic packages: delamination and cracking, presented at Electronic Components and Technology Conference, Orlando, FL, June 1–4, 1993. 6. Goodman, J, et al., Fluoride contamination from fluoropolymers in semiconductor manufacture, Solid State Technology., July, 1990, 65 –68. 7. Gore, S., Degradation of thick film gold bondability following argon plasma cleaning, ISHM Proc., 1992, 737 –742. 8. Herard, L., Surface treatment for plastic ball grid array assembly and its effect on package reliability, Proc. Workshop on Flip Chip and Ball Grid Array, Berlin, Germany, Nov. 11 –15, 1995. 9. Koopman, N. et al, Solder flip chip developments at MCNC, presented at ITAP, 1996. 10. Fisher, J., ITRI Reports on Micro-Vias, Printed Circuit Fabrication, 1997, 58 –60.

© 2001 by CRC Press LLC

CHAPTER 2.16

Super-Heated, High-Pressure Steam Vapor Cleaning Max Friedheim

CONTENTS Introduction Operation Additives Drying Safety Waste Stream Management Energy and Water Usage General Applications Cost Saving Estimates Case Studies Electronics Assembly, General Cleaning Final Surface Preparation prior to Laser Welding, Biomedical Application Detail Cleaning of Refrigeration Equipment Printing Equipment Conclusions References

INTRODUCTION Steam, a natural phenomenon used in cooking and medication, is known to all. Steam drives ships, blows whistles, and in general is part of our lives. We take it for granted. Heat up some water above 100°C (212°F) and we get steam. Traditional steam cleaning has been used successfully for a number of years for janitorial and food service applications. This chapter, however, discusses a patented cleaning technique based on superheated highpressure steam vapor. The basis of steam/vapor cleaning or aqueous/waterless cleaning is readily explained by analogy with steam from a tea kettle. Almost everyone at one time or another has put a

© 2001 by CRC Press LLC

hand over the tea kettle into the flow of steam vapor, felt the warmth, and marveled in the delight. But how many have thought to ask, “How come we are not scalded by boiling water?” Simply put, the steam vapor is composed of single molecules of water, not water droplets. Water droplets have a high specific heat and can store much heat energy, which can potentially cause the burns or scalds when the common old-fashioned steam jenny or steamer is used.

OPERATION This new technology sometimes enables the use of water without an additive package as the sole primary cleaning technique. In the steam vapor cleaner, distilled or deionized water is drawn from a reservoir. A metered amount of liquid is injected into the patented chamber and instantaneously is converted to steam vapor under pressure. The equipment is activated via a hand or foot switch, and the equipment discharges high-pressure, dry steam through a wand or handle. Efficiency of cleaning is based on: • Heat • Pressure • Vapor-phase water Heat, when applied to the contaminant such as oil or grease, helps to liquefy the soil so that it is more readily removed. In addition, the pressure used in the range of 200 to 300 psi mechanically assists in dislodging and removing contamination from the surface. For many benchtop applications, steam vapor cleaning offers advantages over traditional hand-wipe cleaning. There is no wipe cloth, swab, or solid applicator. Instead, the combination of hot steam and pressurized blast enables cleaning in tight spaces, complex geometries, blind holes, and under closely spaced components. The equipment can be small and highly portable. As such, the technique can be used for small-scale benchtop cleaning or even in field repair, where the wand is handheld. The duration of the steam blast is controlled by the operator. The operator typically activates generation of steam vapor by pressing an activation switch. The resulting flow of highpressure steam vapor continues for approximately 10 to 20 s, allowing the part to be cleaned. In the smaller models, a short recovery time is required for more steam to be generated. After the chamber has purged itself of all liquid, the user again activates the system. With some practice, the operator is able to time the bursts to provide a nearly continuous flow of steam. In addition, the technology can be adapted for automated, continuous generation of steam vapor for large-scale operations.

ADDITIVES In some cases, cleaning and drying can be accomplished with water alone. With heavily soiled surfaces, cleaning can be enhanced by application of surfactant solution directly to the part to be cleaned. Steam vapor is then used to heat and remove the cleaning agent and contaminant. Avoiding corrosion during cleaning of carbon steel is another potential problem. In such cases, an appropriately designed rust inhibitor can be added for applications where potential corrosion of the part is an issue. © 2001 by CRC Press LLC

DRYING While many consider liquid aqueous cleaning to be environmentally preferable in terms of pollution prevention, one potential drawback to liquid aqueous cleaning is the need for a drying step. Drying may add to the cost of the process in terms of: Equipment Drying time Drying temperature Cooldown time Energy Hold up in production flow In the case of steam vapor cleaning, because cleaning is accomplished with heated, pressurized water vapor, the cleaned parts are typically dry and ready for the next step in the manufacturing process, be it plating, painting, or further fabrication.

SAFETY Because the steam vapor is generated almost instantaneously, no steam is stored under pressure. This provides safer operation for the worker. Because steam vapor cleaning is free of water droplets, the operator gains a measure of safety with steam vapor cleaning, even at the high pressures employed. However, gloves may be considered depending on the specific application, and goggles are recommended to protect the eyes from debris. In addition, the equipment may be used in a waste management cabinet to minimize worker exposure to debris from the part being cleaned, as well as entrapment of the residue being removed.

WASTE STREAM MANAGEMENT Waste stream management is a concern in any cleaning process. It is generally understood that organic solvents must be handled as hazardous waste. However, liquid aqueous systems must also be managed as waste streams. The additive package itself may be unsuitable for discharge to a sewer line. In addition, even where the surfactant package is said to be biodegradable, soils and trace metals may result in the need to treat wash and rinse water as hazardous waste. Even with filtration and evaporative techniques, this can add significant costs to the process. In contrast, because the temperature at which the steam vapor is being produced is 500°F the vapor evaporates, leaving only the residue of contaminants for disposal. The residue is typically collected on rags or absorbency pads, resulting in a relatively concentrated, manageable waste.

ENERGY AND WATER USAGE Compared with many other cleaning techniques, steam vapor technology is very low in energy consumption because only a small mass of water is being heated. Many cleaning systems require constant heating of a fairly large mass bath. In steam vapor cleaning, © 2001 by CRC Press LLC

Table 1 Estimated Water Consumption, Steam Vapor Cleaning Size Steam Vapor Cleaner

Water (average) use, gal/8 h

Small Medium Large, continuous steam capability

1 2 5

electrical current is drawn only when the chambers are heating. In addition, one cannot take the availability of water for granted. In many areas, water is a costly commodity. In steam vapor cleaning, the average water consumption per 8-h shift is relatively low (Table 1). GENERAL APPLICATIONS Solvent-based cleaning, for all the safety and environmental problems involved, has advantages of rapid solubilization of a wide range of soils. With liquid, aqueous-based cleaning, it is sometimes necessary to use additional cleaning approaches to achieve the desired cleanliness. Steam vapor cleaning has utility not just on its own but also as an adjunctive technique to complement solvent or liquid aqueous approaches. In some applications, a chemical alternative or solvent must be used because the contaminant is such that only a chemical agent can provide the needed solubility or “softening” power to allow it to be readily removed. Examples include burned-on carbon deposit, paint, and heavy industrial grease. In such cases, steam vapor can be used as a final rinsing, drying, or detailing process. In such cases, steam vapor is used to remove the final traces of cleaning agent residue, soil, or surface oxidation. In addition, while a large-scale cleaning operation may be adaptable to most of the parts being cleaned, there may be parts that do not lend themselves to the standard cleaning process, or must be cleaned rapidly, on short notice. Vapor steam cleaning can be used to add flexibility to the general cleaning system. No cleaning system is perfect. With many cleaning systems in use today, a certain proportion of parts are not cleaned acceptably. This, in turn, necessitates costly time-consuming hand-probing-type detailing. Adding steam vapor technology as a final detailing tool aids in final inspection and acceptance of parts, and may allow manufacturers to do their jobs more efficiently and economically. Other cleaning systems may rely on applying abrasion or force to remove contaminants. Pressure washers use water blasting; ultrasonic cleaning uses implosions to loosen dirt; enclosed-type cleaners use spray and chemical under high pressure to dislodge soils. Steam vapor technology has essentially no abrasive quality as the vapor consists of individual water molecules, so it may be preferable for fragile, delicate applications. COST SAVING ESTIMATES A U.S. Navy report1 by its Fleet Activity Support Technology Transfer (FASTT–P-2) using this technology as a viable alternative to solvent cleaning and degreasing of weapons, automotive parts, electronics, printed circuit boards, ground support equipment, and other items estimates a capital cost of approximately $8300, an annual saving of nearly $400,000 with a payback period of under 1 year, actually less than 10 days.

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CASE STUDIES Electronics Assembly, General Cleaning Steam vapor cleaning has been evaluated for removal of flux and other contaminants for surface-mounted assemblies. Test were performed for the U.S. Navy at Crane, IN. Ten motherboards and 26 interface cards were cleaned with steam vapor technology. No damage due to heat or electrostatic discharge (ESD) was detected. The U.S. Navy has authorized use of the steam vapor technology with avionics and other applications.

Final Surface Preparation prior to Laser Welding, Biomedical Application A manufacturer of stainless steel needles for biomedical applications used steam vapor cleaning to improve surface cleanliness prior to laser welding. The overall process of unwinding the roll of stainless steel strip, bending, shaping, and laser-welding the product requires 4.5 h. Prior to laser welding, the original process called for the strip to be run through solvent, then wiped dry between paper towels. It is important to remove all traces of solvent; any residual solvent interferes with laser welding. Unfortunately, residual solvent produced welding “misses,” resulting in an unacceptable reject rate. An automated steam vapor cleaner was implemented prior to laser welding. The reject rate was reduced to negligible levels, production was increased by over 30%, and solvent usage was eliminated.

Detail Cleaning of Refrigeration Equipment B/E Aerospace, Galley Products Group, in Anaheim, CA produces some 90% of the airline galley refrigeration equipment in use worldwide. B/E also repairs refrigeration inhouse and specifies options for field repair. After some process optimization, B/E has introduced steam vapor cleaning to replace some mineral spirits cleaning of segments of refrigeration tubing.2 Initially, assemblers accustomed to cleaning with mineral spirits were unfamiliar with the new technology. By making the equipment available in the shop, operators found a number of applications for steam vapor cleaning. It is currently in regular use in assorted applications as a final cleaning technique.

Printing Equipment The Los Angeles Daily News is subject to stringent requirements for solvent elimination mandated by the South Coast Air Quality Management District (SCAQMD). The Los Angeles Daily News implemented steam vapor cleaning technology for an assortment of printing-related cleaning applications. As a result, solvent tank cleaning at the facility was eliminated. Emissions of volatile organic compounds (VOCs) were reduced from 20 to 2.2 tons annually.

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CONCLUSIONS Steam vapor cleaning has found wide application in such diverse areas as electronics, aircraft, ground equipment, plant maintenance, and biomedical uses. Because of the diversity and relatively low capital investment, it has been adopted by the U.S. military, Fortune500 companies, and small to medium-sized manufacturers. Still, there are many facets to using steam vapor technology, some of which are yet to be discovered. An in-depth evaluation3 by the Naval Air Warfare Center Aircraft Intermediate Maintenance Facility at Coronado, CA concludes that there are so many potential applications for steam vapor cleaning that “we haven’t even scratched the surface yet.” To date, the manufacturer of this equipment has observed no injury to products and no personnel injury. Further, plastics such as conformal coating on electronics assemblies are not damaged or removed when steam vapor is applied. Note: Technology and applications of steam vapor cleaning refer to various models of the PDQ Mini-Max Steam Vapor Cleaner. REFERENCES 1. U.S. Navy report by Fleet Activity Support Technology Transfer (FASTT –P-2), available at http://web.dandp.com/n451/aboutus/about.cfm. 2. Petrulio, R. and B. Kanegsberg, Practical solutions to cleaning and flushing problems, presented at CleanTech ‘98, 1998. 3. Naval Air Warfare Center Aircraft Intermediate Maintenance Facility at Coronado, CA.

© 2001 by CRC Press LLC

CHAPTER 2.17

Making Decisions about Water and Wastewater for Aqueous Operations John F. Russo

CONTENTS Introduction Typical Cleaning System Operational Situations of a Typical User Determining the Water Purity Requirements Measuring Water Purity Undissolved Contaminants Dissolved Contaminants Undissolved and Dissolved Contaminants Other Conditions Determining the Wastewater Volume Produced Source Water Treatment No Treatment Removal by Mechanical Filters, Adsorptive Filters, and an Oxidation Method Mechanical Filters Adsorptive Filters Oxidation Method Water Softening Water Softener Capacity Calculation Dissolved Solids and Ionic Removal Reverse Osmosis Process Deionization Process DI or RO or Both Other Methods No-Wastewater-Discharge Options Closed-Loop Method Zero-Wastewater-Discharge Method Wash Chemical Rinse Water

© 2001 by CRC Press LLC

Wastewater Discharge Options Fat, Oil, and Grease Biological Oxygen Demand and Chemical Oxygen Demand Hazardous Metals Determining the Wastewater Treatment for a New Process Source Water Treatment No-Wastewater-Discharge Design Wastewater-Discharge Design Overcapacity of Current Wastewater Treatment System Case Histories Case 1—Manufacturer Unable to Reduce the High Failure Rate of Plated Parts Case 2—Large Computer Manufacturer Buys a System from Local Supplier Case 3—Large New England Military Contractor Decides to Build Its Own System and Makes a Large Investment Case 4—Small Contractor Conclusion References

INTRODUCTION Water is the essential liquid in aqueous cleaning processes. Purity of the water is an integral part of the cleaning process. With water, one must be concerned about the condition of the water at each stage of the process to finish with a usable product. Also of concern is the condition of the water at the end of the process, i.e., the wastewater. This chapter discusses water purification and conditioning techniques both for the cleaning process itself and for the wastewater. In many cases, the wastewater from one stage of an operation is the source water for another stage. It is notable that discussions of water source treatment processes are often integrated with those from wastewater since, in many cases, the principles and techniques are the same. Usually this subject is discussed by describing several general water treatment systems. But the author has decided to take the user’s viewpoint to make this chapter a more usable reference. Even with minimal knowledge of water processes, the reader can refer to the section “Operational Situations of a Typical User,” review the specific area of interest, and devise a plan of action. As new technologies are introduced, users have more options in source water and wastewater treatment than ever before. This adds to the complexity of decision making, especially if the most cost-effective solution is necessary. Typical water treatment terms are defined and various water processes are explained and compared. The main objective of this chapter is to introduce new users to the water treatment field and to serve as a quick, easy-to-use reference guide for experienced users

TYPICAL CLEANING SYSTEM Essentially all cleaning operations use one or more of the sequence of operations shown in Figure 1 (washer only). The schematic shows a parts washing unit where the © 2001 by CRC Press LLC

Washer Equipment

Parts

Wash

Figure 1

K An i i r f e

Drag - Out Reduction

K An i i r f e

Rinse

Final Rinse

Conveyorized washer schematic. Note. The schematic represents a conveyorized washer. It can also be visualized as a multistage cabinet washer where all of the parts remain stationary and are subjected to each cleaning step or a diptank cleaning process where the parts are moved manually or automatically from one cleaning step to another.

parts move along a conveyor to different stages of washing, rinsing, and drying. Also, this same schematic can be visualized as a cabinet washer in which the parts remain stationary while they are subjected to one or more of the same cleaning stages as in a conveyorized washer. Many of the following discussions apply to this schematic. OPERATIONAL SITUATIONS OF A TYPICAL USER There are seven general, operational situation considerations: 1. 2. 3. 4. 5. 6. 7.

Determining the water purity requirement Determining the wastewater volume produced Source water treatment No-wastewater discharge options Wastewater discharge options Determining the wastewater treatment for a new process Overcapacity of the current wastewater treatment system

In most cases, a user may have to consider more than one of the above situations. The first two are the most critical and greatly affect the others. It is not unusual for minor differences in conditions between one user and another to have a major impact on a user’s final decision. Determining the Water Purity Requirements In some cases, determining the water purity requirements is not easy and some investigation is necessary. Information from trade associations, competitors, or related processes is helpful. If these sources are inadequate, the user may have to experiment on a small scale or make the determination during the actual production process. The latter decision has a downside risk of too many part failures. The user may then have to rent a system on short notice to reduce the failure rate. In certain cases, purchasing new equipment with a vendor buyback if the equipment is later found to be unnecessary is a good option.

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Measuring Water Purity In many applications the user must be concerned about measuring those characteristics of source water (tap water) from a lake, river, well, or from wastewater that affect the quality of the parts being cleaned. In the great majority of cases, two characteristics are measured: undissolved and dissolved contaminants. Undissolved Contaminants Undissolved contaminants are contaminants in water that do not affect its electrical properties. These contaminants can be measured by different methods, depending upon specific requirements, and any or none of these might have to be monitored. Fat, oil, grease (FOG) measurements are used to determine whether a user complies with the discharge regulations of municipal sewer districts, called publicly owned treatment works (POTWs). Total suspended solids (TSS) is a measure usually of the amount of suspended particles with sizes over 0.45 m. Fat, oil, grease (FOG) is a measure of any compound (vegetable or animal fats, petroleum and synthetic oils, lubricants and some sulfur compounds) extracted by a fat-soluble solvent. Dissolved Contaminants Dissolved contaminants such as ionic compounds including sodium chloride, calcium carbonate, and many others that form ions in water, are measured by a total dissolved solids (TDS), conductivity, or resistivity meter. Dissolved contaminants such as sugar, starches, and other water-solubilizing organic compounds are not ionic, do not conduct an electrical current, and are not detected by electrical measurements. These measurements are not usually used by POTWs to determine compliance with discharge regulations but can interfere with some cleaning processes if not detected and removed. Typically, measurement of dissolved contaminants is made with a TDS meter to make a quick approximation of the capacity of ion-exchange resins and reject capability of nanofilter and reverse osmosis (RO) membranes. The readings are in ppm (parts per million) of ions in water. Each meter manufacturer might use a different algorithm to convert the electrical measurement to a TDS reading so it is possible that different meters might give different results. Without this measurement, a user would need a complete water analysis, which is time-consuming and expensive. Such an analysis is done primarily when a high degree of accuracy is required. The higher the dissolved ionic content of the water, the higher the conductivity. Source water (tap water) typically has a conductivity from 40 to 1000 S/cm. A conductivity meter is the measurement instrument of choice for water typically above about 10 S/cm. Conductivity readings of about 1 S/cm are near the limit of accuracy for this type of measurement. For a conductivity meter to be useful as a TDS meter, the conductivity reading has to be converted to an approximate amount (ppm) of ions in the water. The conversion factor (0.4 to 0.6) was determined by averaging the readings calculated from a complete water analysis of many samples of well, river, or lake water supplies throughout the United States. For wastewater, which may contain ions that differ substantially from natural water supplies, this conversion range might be less accurate. For simplicity, all TDS readings used in this chapter are determined by multiplying the conductivity readings by a conversion factor of 0.5 (e.g., a conductivity of 1000 S corresponds to approximately 500 ppm of TDS). © 2001 by CRC Press LLC

Table 1 Resistivity, Conductivity, and TDS Conversion Chart Resistivity Ohm-cm @25°C

Conductivity Dissolved Solids Microsiemens/cm Parts per Million @25°C (ppm)

Approximate Grains/Gallon (GPG) as CaCO3

18,000,000 15,000,000 12,000,000 10,000,000 5,000,000 2,000,000 1,000,000 500,000 300,000 200,000 100,000 50,000 30,000 20,000 10,000 5,000 3,000 2,000 1,000 500 300 200 100

0.056 0.067 0.084 0.100 0.200 0.500 1.00 2.00 3.33 5.00 10.0 20.0 33.3 50.0 100.0 200 333 500 1,000 2,000 3,330 5,000 10,000

0.00164 0.00193 0.00240 0.00292 0.00585 0.0146 0.0292 0.0585 0.0971 0.146 0.292 0.585 0.971 1.46 2.92 5.85 9.71 14.6 29.2 58.5 97.1 146 292

0.0277 0.0333 0.0417 0.0500 0.100 0.250 0.500 1.00 1.67 2.50 5.00 10.0 16.7 25.0 50.0 100 167 250 500 1,000 1,670 2,500 5,000

Note: Approximate grains/gallon calculated by dividing ppm column by 17.1. Source: From Owens, D.L., Practical Principles of Ion Exchange Water Treatment, Tall Oaks Publishing, Colorado, 1985. With permission.

For lower conductivities (purer water), the inverse of conductivity, resistivity, is measured. A convenient conversion number to remember is that a conductivity reading of 1 S/cm is equal to a resistivity reading of 1 M-cm. Table 1 lists typical conversions of conductivity and resistivity and TDS. From an accuracy standpoint, readings from 10 to 18.2 M-cm (the highest purity possible) can only be made with a resistivity meter with its cell inserted into a pipe in a flowing stream of water. Undissolved and Dissolved Contaminants There are several measurements that are made on water for both undissolved and dissolved contaminants. Total organic carbon (TOC) is a measure of the total amount of oxidizable organic matter (oxidized by ultraviolet radiation).

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Biological oxygen demand (BOD) is a measure of the amount of oxygen that bacteria need to oxidize biodegradable organic matter over a given period of time. Chemical oxygen demand (COD) is a measure of the amount of oxygen required to oxidize reducing compounds such as sulfides, salts of metals, etc. and organic compounds into carbon dioxide and water. TOC measurements are usually used for critical high-purity water applications. BOD and COD measurements are usually used to determine whether a user complies with discharge regulations of POTWs. Other Conditions pH is a measure of the acidity, neutrality, or basicity of water and is expressed as the negative log of the hydrogen ion concentration, or
log [H]. A pH reading below 7 is an acid condition, 7 is a neutral condition, and above 7 is a basic condition. The pH of source (tap) water for certain wash chemical preparations and of rinse water in certain applications can be very important. Determining the Wastewater Volume Produced Determining the amount of wastewater produced by a cleaning process is very important because it has a major influence on the user’s strategy and decision making. For example, for small volumes, cleaning processes generating less than about 25 to 75 gals/week, it is probably best to haul away the wastewater unless there is an existing treatment system. Depending upon the cost, some form of evaporation, like solar evaporation, might be less expensive. A determination of whether the wastewater is hazardous or not is required to comply with federal, state, and local regulations. Hauling a hazardous waste can cost as much as $1000/55-gal drum, whereas for a nonhazardous waste the cost can be less than $50/55-gal drum. For large volumes, other wastewater reuse processes should be employed and are discussed in later sections. Source Water Treatment All aqueous processes require a minimum initial charge of water from a well, river, lake, or a transported supply of water (bottled or from a tanker truck). Many operations might need a continuous supply. Typically, a closed-loop system uses the lowest amount of makeup water, while a cleaning process without any water reuse requires the largest amount of water. There are five typical options in general order of decreasing amounts of suspended solids and dissolved minerals: 1. 2. 3. 4. 5.

No treatment Mechanical, adsorptive and oxidation Water softening Reverse osmosis (RO) Deionization (DI)

There are exceptions to this ranking, for example, a water supply with no treatment could be as good as another water supply that is softened. In some cases, a source water could be

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even better than river water treated with RO, if the criterion is ionic content. Also, the amount of particles passing through an RO is far less than from DI, but the ionic content from DI can be far less than RO. No Treatment In some cases the source (tap) water is of sufficient quality that no treatment is necessary. If a water purity specification is not available, the required purity might be determined by testing on a small or a pilot production scale. If a pilot scale is not practical, it may be necessary to go to a full production scale with a backup plan to treat the source water as quickly as possible should this option prove to be insufficient. Removal by Mechanical Filters, Adsorptive Filters, and an Oxidation Method Mechanical filters depend on a physical barrier for contamination removal. Adsorptive filters use large surface areas to remove contamination. An oxidation method uses oxygen to convert dissolved ions into particles that are removed mechanically. Mechanical Filters Mechanical filtration is one of the most common methods used to remove particles from water and wastewater in cleaning processes. They are ranked from coarse to fine removal with some overlap of removal capability of one method with another. See Table 2 for a chart of the different types of contaminants and the separation technology used to remove each one. Granular media filters are composed of single media or multimedia with various grades of sand and other minerals, used primarily to remove suspended particles from 20 to 40 m (micrometers or microns) in size, but can remove finer particles as well. As a reference point, a grain of table salt is about 125 m. Bag filters are manufactured from feltlike materials both woven and nonwoven and typically have a higher contaminant loading and a lower cost per pound of contamination removal than cartridge filters. Cartridge filters are commonly used filters made from a wide variety of plastic and natural fibers, such as polypropylene and cotton, in a large variety of designs such as molded, fiber wound, and pleated papers. Generally, cartridge filters are most often used for lower flow rates and higher-efficiency applications, whereas bag filters are used for lower-efficiency and high flow rate applications. For high-flow-rate and high-volume applications, granular filters are most often used first, then frequently followed by the other two methods. Membrane filters are manufactured from a variety of plastic and inorganic materials with different shapes (flat sheets, tubes, spiral wound tubes). They are designed to remove very small particles and organic molecules from a liquid stream. Microfilters (MFs) are rated at about 0.05 to 1.0 m. Ultrafilters (UFs) essentially remove all particles and molecules from about 10,000 to 1,000,000 MW (molecular weight; sometimes referred to as Daltons) from water. There is neither an industry-wide micrometer rating that demarcates microfilters and ultrafilters nor an industry-wide filtration efficiency rating standard. So it is not uncommon for a microfilter from one manufacturer to be called an ultrafilter by another manufacturer. To compare one membrane with another, a user must determine from the

© 2001 by CRC Press LLC

Table 2 Relative Size of Small Particles Mol. Wt.

Å 107


m (micron) 1000 800

General filtration

600

106

• Sewing needle diameter

400 • Razor blade thickness 200 100

Drizzle

Beach sand

80

• Human hair Carbon diameter black • Smallest visible 40 particle 20 • Ragweed pollen 10 Bacteria 8 60

105

6 Microfiltration

4 10

4

8,000

2 1 0.8

Red blood cell Yeasts and fungi

Mist

White light microscopy

Pollens Jewelers rouge Syrups

Emulsions (Latex)

0.6

300,000

400

• Serratia Electron marcescens micros• Pseudomonas 0.2 Mycoplasm copy diminuta; DOP 0.1 Tobacco 0.08 smoke Colloids 0.06 Proteins 0.04

100,000

100

0.02 0.01

80

0.008

30,000

40

0.004

10,000

10

0.002 0.001

4,000 1,000,000

1000 800

Reverse Osmosis

Ultrafiltration

500,000

50,000

500

0.4

Endotoxins Virus • Albumin (Pyrogen) 0.006 (60,000 MW)

Soluble salts (ions)

Metal ions

50

Differential pressure increases with reduced micron ratings; dirt-holding capacity and relative flow rates decrease with reduced micron ratings. Å, Angstrom = 10-8cm; mm, micrometer (micron)=104 Å; 1 mil = 0.001 in. = 25.4 mm. Note: Nanofilters, a newer technology, is between reverse osmosis and ultrafiltration. Source: Gelman Sciences, Ann Arbor, MI, ©1987. © 2001 by CRC Press LLC

manufacturer the test method for the rating. This rating problem can be extreme, for example, a membrane manufactured from a plastic material, such as polysulfone, polypropylene, or nylon, rated at 0.2 m can reject 99.9999% of all bacteria, whereas a ceramic membrane with the same rating may have a far lower removal efficiency. Neither of these types of membranes removes ions from water, but they do remove colloids and other high-molecular-weight substances such as surfactants. Microfilter membranes have holes and are coarser than ultrafilters, and both are used to recycle wash chemicals (alkaline cleaners). Ultrafilter membranes do not have physical holes and are even more effective than microfilters in removing large organic molecules and low-molecular-weight petroleum products. Adsorptive Filters Activated carbon is a granular medium made by heating carbon-containing materials, such as coal, coconut shells, and similar substances, in the absence of air, producing a porous material with a large surface area. This large surface area allows the attachment of large organic molecules. Typically, it is used as a pretreatment method to remove chlorine and long-chained organic molecules prior to ion-exchange resins and some RO systems. It acts as a catalyst to eliminate the oxidizing power of the chemicals by reducing them to other ions. It is also used to remove low levels of oil and grease (petroleum and synthetic) products. Oxidation Method Oxidation is a chemical process that changes the state of the dissolved species, such as iron or manganese, to a particulate form that is removed by mechanical filtration. This is an important pretreatment process before RO or ion-exchange resins for iron- and manganese-bearing water. Oxidation is sometimes used alone to treat water just before a cleaning process. The oxidation is achieved by a chemical or air. Water Softening Water softening is a process of removing hardness minerals such as calcium and magnesium cations from water without reducing the TDS content of the water. The key component of a water softener is the ion-exchange resin contained inside a tank. The tank can have manual or automatic controls to regenerate the ion-exchange resin (Figure 2). Ion-exchange resin is manufactured from polystyrene that is cross-linked with divinylbenzene. It consists of small plastic spheres about the size of the head of a common pin. The resin has positively charged sodium cations held on the resin surfaces by electrostatic charges. The sodium cations are exchanged for cations of calcium, magnesium, and dissolved iron in the water. Once all of the sodium cations are exchanged, the resin is exhausted. It must be replaced with new resin or be regenerated (reversing the process) by flowing concentrated sodium chloride brine through the resin during a multistage process, performed manually or automatically, within the tank. Even though ions are being exchanged for other ions, there is essentially no change in the TDS of the water as measured by a conductivity or TDS meter.

© 2001 by CRC Press LLC

Figure 2

Water softener.

Water Softener Capacity Calculation Water treatment chemists can predict the probable number of gallons of soft water a water softener will produce. For example, the “ppm” (expressed as CaCO3) of the water has to be converted to grains per gallon because most ion-exchange resins are rated on the basis of grains expressed as CaCO3/ft3. The term “grain” is an old unit of weight measurement, originally referring to grains of wheat, and is used in the water industry. There are 7000 grains per pound and 1 gr/gal = 17.1 ppm. To convert a reading, for example, 100 ppm of hardness to grains/gal of hardness, the following proportion is used: 1 gr/gal 17.1 ppm

=

X 100 ppm

X = 5.8 gr/gal

Most water softeners with cation resin have a capacity of about 30,000 gr (expressed as CaCO3/ft3) of resin. If water supply has a total hardness of 5.8 gr/gal, a user can expect a softener with 1 ft3 of cation resin to produce close to 5172 gal/ft3 (30,000 gr/ft3
5.8 gr/gal = 5172 gal) of soft water before the cation resin has to be regenerated again. There are factors such as regenerant concentration, iron fouling of the resin, and others that can significantly influence the actual capacity of the resin. Dissolved Solids and Ionic Removal The most common industrial processes used for reducing dissolved solids and ions in water are deionization (DI) and reverse osmosis (RO). Nanofiltration (NF), a membrane process very similar to RO, can remove dissolved solids and ions to a much lesser extent, and is used in far fewer applications than RO. While RO removes dissolved solids and ions down to about 200 MW NF removes down to about 300 MW. Distilled water is not as economical to use unless it is purchased in low volumes. Electrodeionization, a newer ion-exchange process, produces high-purity water of less than about 0.4 ppm (1 M-cm resistivity) as sodium chloride without the use of chemical regeneration.

© 2001 by CRC Press LLC

Reverse Osmosis Process RO is a membrane process that removes essentially all particles, and most molecules and ions about 200 MW and larger from water. RO (Figure 3) is a process in which a pump is used to force water through a membrane barrier to produce water with a lower dissolved solids content. The key component of an RO unit is the membrane, which is made from a thin film of plastic, most often in the form of a spiral or “jelly roll.” Membranes vary in size from 2 in. diameter  10 in. long up to about 12 in. diameter and about 5 ft long. Water pressure up to 1000 lb/in2 forces water through the membrane. A complete RO system can consist of a pretreatment stage using mechanical filter (cartridge or multimedia filter), adsorptive media (activated carbon), and/or antiscaling (chemical, pH treatment, water softening), a high-pressure pump, RO membrane, storage tank (optional), and post-treatment (ultraviolet light, repressurization pump, and deionization). The selection of these processes depends upon a source water analysis and the specific objectives of the user. A double-pass RO is an RO followed by another RO. The RO membrane separates the water into two streams: contaminants into a reject stream (wastewater to a sewer) and lower-ionic-content water into a permeate stream (usable for process). About 25 to 85% of the total water in a single-pass RO becomes a reject stream containing all of the contaminants. Therefore, 15 to 75% of all source water becomes permeate water ready for use in the process. This percentage range is the practical limit for a single-pass RO and the actual percentage depends upon the RO design and a water analysis. The membrane removes essentially all particles including microorganisms and rejects 70 to 99+% of the dissolved solids and ions down to about 200 MW. It rejects essentially the same percentage of ions whether the incoming stream has thousands or hundreds of parts per million of dissolved solids. For example, if the TDS of the wastewater to the RO doubles, the TDS of the permeate water will about double, and if ion-exchange is used as posttreatment after the RO, the ion-exchange cost will about double. The ionic weight, shape, and amount of the charge determine the degree of rejection. The water purity of the permeate (usable) water typically ranges from 50,000 to 600,000 -cm and can be estimated with a source water analysis. As the membrane ages, its ability to reject dissolved solids decreases, resulting in a practical life of the membrane of about 3 years. Higher-purity water, which has a lower TDS and higher resistivity, can be attained with a double-pass RO (replacing the single-pass RO), DI, or electrodeionization of the RO product water being required. DI or electrodeionization is necessary as a post-treatment process to RO whenever the user requires a higher water purity than 1 M-cm resistivity.

Figure 3

Single-pass RO system.

© 2001 by CRC Press LLC

Deionization Process DI is a process using ion-exchange resin to remove ionized solids (cations and anions) from water. The key component of a DI unit is the ion-exchange resin. A two-bed deionizer consists of two tanks in series: a tank with cation resin followed by a tank with anion. The cation resin is the same as the resin used in a water softener except that it has hydrogen cations instead of sodium cations on the functional groups of the ion-exchange resin. Another type of deionizer, a mixed-bed, has both cation and anion resins intimately mixed in one tank. There are three basic deionizer designs: 1. Two-bed 2. Mixed-bed 3. Tri-bed (two-bed followed by a mixed-bed) Usually there are three basic operating options: 1. Disposable resin 2. Regenerable resin (rental or owned) 3. On-site regenerable deionizers For the disposable resin option, the resin is used once and discarded. For the rental or owned resin option, the user rents or owns the tanks with resin and the vendor takes the exhausted tanks back to its facility and regenerates the resins with strong acid and caustic chemicals. For the on-site regenerable deionizer option, the resins are regenerated inside the tanks with the same chemicals used in the rental or owned tank option, but the user might have to treat the wastewater produced by the regeneration process for pH and/or heavy metals. When resin is regenerated repeatedly, its capacity to remove ions after each regeneration decreases. The rate of this decrease depends upon a number of variables, such as the type and amount of foulants, oxidizing power of the contaminants, temperature, and other factors in the water. The capacity decrease rate is usually greater for wastewater applications than for source water (tap water) treatment. With the disposable and rental or owned tank options, there is no waste stream to treat at the user’s facility since the contaminants are held on the resin beads. An RO system, by comparison, always has a wastewater stream that goes to a sewer. This is the key reason resin systems lend themselves more easily to closed-loop treatment, whereas membrane systems generally do not. Occasionally, DI is referred to as demineralization, an older term used infrequently today. Technically, deionized water is any water treated by a deionizer from which dissolved solids are removed and the water resistivity increases. There is no specific water purity measurement that defines the term deionized. DI removes ions, positively charged cations and negatively charged anions, from water using ion-exchange resins in the hydrogen and hydroxyl form. Even though RO removes dissolved solids similar to DI and often can produce similar water with resistivity below 1 M-cm, it is not referred to as deionized water, but RO water. Ion-exchange resins have specific capacities, that is, the ability to remove ions from a given number of gallons of water and it is inversely proportional to the TDS of the water. For example, if tripling the TDS, the capacity of the resin will be decreased to about one third of its capacity. If the TDS is too high, the cost of replacing or regenerating the resin can be uneconomical. © 2001 by CRC Press LLC

Sometimes the only way to make deionization economical is to use an RO membrane as pretreatment for the deionizer (discussed below with closed-loop systems). The type of ions, ionic charge, and the concentration of each ion can affect the capacity of the resin differently. Mixed-bed ion-exchange resin has a nominal capacity of approximately 10,000 grains/ft 3 to an end point of 1 M
-cm resistivity. The actual capacity of the resin depends upon a number of factors such as amount of chemical used to regenerate the resin, the type and concentration of each ion in the water, the amount and type of foulants in the water, the flow rate, the cross-sectional area of the resin surface in the tank, the depth of the resin in the tank, and the temperature of the water. For a two-bed deionizer, the resin capacity is calculated from the capacity of the cation and anion resin. The cation resin has a nominal capacity of 30,000 grains/ft 3 and a strong base anion resin at about 20,000 grains/ft 3. A weak-base anion resin (another option) has a capacity 50 to 100% greater than a strong base because it does not remove dissolved carbon dioxide and silica. Table 3 shows the key differences between the performance of four types of deionization systems. DI or RO or Both Generally, DI is • Preferred when wastewater has a TDS less than about 100 ppm because operating costs are lower; • Required when higher water purity is needed than an RO alone can produce; • Able to maintain the same water purity even if the feed water quality varies substantially; • A simpler system to operate for low-flow-rate applications using rental tanks.

Table 3 Types of Deionizer Designs vs. Water Characteristics Type of Deionizer Design Water Characteristics

Two-Bed Weak Base

Two-Bed Strong Base

Mixed-Beda

Tri-Bed

Purity (M
-cm)

0.02–0.6

0.1–1.0

1.0–18.2

(see previous column “Mixed-Bed”)

pH Carbon dioxide and silica removal BOD and COD reduction

6 or lower

8.0

5.5–8.5

No

Yes ¨

b

Yes Essentially none Æ

a A mixed-bed followed by one or more mixed-bed tanks is used when (1) a polisher is necessary to remove residual ions that might get through only one mixed-bed tank and when 18.2 M
-cm water purity (highest water purity available) is required and (2) added capacity is required; the higher the water purity, the closer the pH is to 7.0 b These are typical ranges for each process.

Source: Otten, G., American Laboratory, July 1972.

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Generally, RO is • Preferred when wastewater has a TDS above about 100 ppm because operating costs are lower; • Preferred when lower water purity is required, unless low flow rates are used; • Not able to maintain the same water purity if the feed water quality varies substantially unless DI post-treatment is used; • A more complicated system to operate for low-flow-rate applications. Even though these reasons are typical for choosing DI or RO, there are exceptions: • Even though the initial cost of an RO system and its operating costs are significantly higher than DI in a low-TDS case, the capability of an RO system to remove microorganisms and other fine particles might be more desirable. • The required use of strong acids and caustics when using a regenerable unit at the user’s site may be too hazardous. • Even though RO may be preferable in a higher-TDS application, the simplicity of renting a DI system with minimal operating costs may be preferred. In summary, both of these technologies are used together whenever the water purity required is higher than an RO can produce and the TDS of the wastewater is too high to make DI alone cost-effective. When comparing closed-loop and zero-discharge wastewater treatment systems, it is important to consider that RO always has a reject stream, whereas DI might have a wastewater stream. Other Methods The following methods have limited use in providing high-purity water for cleaning operations. • Distillation is a process that heats water until it vaporizes and condenses into water with a purity up to about 1 M-cm. Distillation is capable of removing dissolved and undissolved minerals and some organics, but is not generally used for industrial water purification of tap water. As compared with RO and DI, it has a higher operational cost because it is an energy-intensive process. However, it is an inexpensive source for low-volume applications if purchased in bottled quantities. Using bottled water is an economical way of testing what water purity is required by a cleaning process. • Electrodeionization is an ion-exchange process that uses an electrical current on a membrane barrier embedded with ion-exchange resin. This process, usually requiring pretreatment of a source water with a membrane process like RO, can increase the resistivity of the water purity to 1 M-cm and as high as 15 Mcm. This is a newer technology primarily used to eliminate safety hazards from using strong acids and caustics when regenerating mixed-bed deionizers on site. Flow rates can range from low to high volume.

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Figure 4

Washer and closed-loop wastewater system interface schematic.

No-Wastewater-Discharge Options The key to any “no-wastewater-discharge” option is the reuse of the wastewater. Sometimes wastewater from one application can be considered as acceptable source water for another process. Cascade counterflow rinses are used very often and are a good example of wastewater reuse in the same process. In this method, the purest water is used to rinse at the end of the process and the wastewater flows opposite to the parts being cleaned as it cascades to the previous step in the process. As many as four cascade rinses are not unusual. Each time the wastewater is reused, the overall cost of water for the process decreases as compared with using new water for each rinse stage. A user may decide not to discharge any wastewater because: • There is a desire or policy to reduce the chance of future liability for contamination. • The local community prohibits discharge of any industrial wastewater. • There is a high cost of monitoring contamination to a septic system and/or a prohibitive cost of possible future remediation of the groundwater. • There is uncertainty of water availability. Closed-Loop Method A closed-loop process can be defined as a wastewater treatment process that has no wastewater discharged to a sewer, with the wastewater recycled to the same or another process. A closed-loop is not easily attained, but for some processes it is the most cost-effective, ideal solution. This is the design standard for the electronics assembly cleaning industry (see Figure 4). In this application, the capital cost for a closed-loop system is about 20% more than a non-closed-loop system that discharges all the wastewater to a sewer. However, the operating cost for a closed-loop system is usually so favorable that it has a positive operating cash flow. The low TDS, below about 20 ppm, is the key to making this process economical. The lower the TDS, the greater the return on the user’s investment. For many non-electronic-assembly applications, the capital cost difference might be similar but the operating cost for a closed-loop may be prohibitive because of the high dissolved solids in the wastewater. © 2001 by CRC Press LLC

Washer Equipment

Recycled Deionized Water

Parts

Wash

Zero-Discharge Equipment

Wash Chemical Recycle -Microfiltration

Figure 5

K An i i r f e

Drag out Reduction

Make-Up To Wash

K An i i r f e

Wastewater

1 Evaporation and Haul

Final Rinse

Rinse

Wastewater Source Water

Pre-Treatment -Reverse Osmosis 2

3

4 Closed-Loop Deionizer

Washer and zero-discharge-wastewater system interface schematic.

Several aspects of the electronics assembly application might be applicable to other user applications. In this application, a manufacturer takes printed circuit boards, inserts a variety of electronic devices on the boards, fluxes the boards, and then solders the devices onto the board. The flux might be left as is on the board or sometimes is removed with either source water or DI water and the wastewater discharged to a sewer or treated with a closed-loop system. This closed-loop process accomplishes the following: • • • • • • • • •

No water pollution (no wash or rinse water goes down the drain) No wastewater tests, permits, inspections, and reports Reduction of energy and water usage by at least 90% Essential elimination of the continuous need for water Water purity ranging from low to high depending upon the process requirements Solid waste contaminants that are not hazardous except in unusual cases Wastewater converted to hot, deionized water Wastewater that can be recycled indefinitely Pretreatment of water performed by equipment

The typical electronics assembly closed-loop design uses a combination of particulate removal, organic removal media, and ionic removal media to allow the water to be completely reused. Water purity levels start at 15 M-cm and higher and, as the contaminants accumulate on the ion-exchange resin, the water purity decreases to the minimum acceptable water purity. This process operates economically whenever the water purity is allowed to decrease to about 1 M-cm. However, the operating costs would be about half as much if the water purity were allowed to decrease to 1000 to 20,000 -cm. This latter design has the highest potential positive cash flow as compared with a non-closed-loop system. Once the particulate, granular organic, and ionic removal media are exhausted, the solid waste generated is usually nonhazardous, according to the federal Toxic Characteristic Leaching Procedure (TCLP) test.

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Zero-Wastewater-Discharge Method Even though the closed-loop process is the ideal process because it has the greatest probability of yielding the largest return on investment, it can be used only in limited applications. For applications where it is not feasible, a zero-discharge method can be used. This design allows no wastewater to be discharged to the drain and uses a combination of microfiltration and reverse osmosis membrane, ion-exchange (closed-loop), and evaporation. When comparing this design with a closed-loop recycling, the additional capital equipment is about double and it is more expensive to operate than a closed-loop system. Figure 5 shows a possible zero-discharge design that represents the cleaning stages of a typical conveyorized or batch-type cleaner (parts remain stationary). In this design, the wash chemical might be recycled with a microfiltration membrane system. Most often, the final rinse water from the same cleaning process cannot be recycled economically in an ionexchange closed-loop system because of the excessive TDS, usually above about 75 ppm. This is caused by the dragout from the wash tank. If the same ion-exchange closed-loop process discussed above were used, a pretreatment method such as RO would be required; otherwise, the operating costs would be prohibitive. The following paragraphs describe in detail each part of the zero-discharge design and evaluate the user’s decision-making factors, starting with the chemical wash (alkaline) stage, from left to right. Wash Chemical There are three methods of handling the wash chemical: hauling, evaporation, and recycling. If the wastewater is not hazardous, it can be hauled by a standard commercial vehicle. If it is hazardous, it must be manifested to an authorized facility. This might be used as a temporary measure until other solutions are implemented. Evaporation may be an alternative, when hauling large volumes of wash chemical is not appropriate and recycling is not cost-effective. The user has more cost-effective options for treating wastewater from a low-volume than a high-volume application. For example, in cleaning processes producing less than about 75 gal/week of wastewater, it is more costeffective to haul the wastewater unless there is an existing wastewater treatment system. For large volumes of wastewater, the hauling option is not usually cost-effective. Evaporation is an energy-intensive process and the cost of the energy must be considered. It is a method of separating a liquid from its solids typically by heating the liquid (gas, electricity, solar energy) or by using a vacuum distillation unit. This can greatly reduce the amount of wastewater to be disposed of by 70 to 95%. If there are other processes in a plant producing excess energy, or if solar energy is available, evaporation can be economical for large volumes. After evaporating the volatiles, the remaining contaminant might become a solid waste containing hazardous metals or have a high pH, which makes it a hazardous waste. For some of these processes, the vapors might be regulated and a permit might be required. The water vapor from any of these evaporative devices might have a distilled water quality (100,000 to 700,000 -cm resistivity) that can be reused in the process. However, in most instances, the cost of condensing the water vapor is greater than treating the source water. It is noteworthy that the cost of hauling and evaporation usually is not significantly affected by the concentration of dissolved minerals or hazardous metals in the wastewater. In addition to evaporating the spent wash chemical, an evaporator can treat the reject wastewater stream from an RO membrane in the next stage of the treatment process.

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Membrane recycling is a relatively new treatment process. As mentioned earlier, all membrane processes have a wastewater reject stream containing all contaminants in a concentrated form that usually goes to a sewer. However, some processes, like membrane recycling of wash chemicals, can reuse the reject stream in a closed-loop manner. When membrane recycling of wash chemicals is used, the contaminants are continuously concentrated and eventually must be processed or disposed. This membrane recycle process uses microfilters or ultrafilters and permits the reuse of a chemical cleaner by allowing most of it to pass through the membrane, while at the same time removing the fine particles and emulsified oils. The term oil refers generically to both petroleum and synthetic products that are oils, greases, lubricants, and similar products. This separation process is imperfect and sometimes some or many of the key ingredients of the cleaner are removed. The critical balance of this membrane recycling process is to achieve the separation of the oil from the wash chemical while not removing too many of the key ingredients of the chemical cleaner. Even in the best-balanced process, some chemical cleaner is removed and the critical ingredients might be replaced periodically with small amounts of additional chemical. Experience from operating such systems has shown that the life of a chemical cleaner can be extended from three to ten times. There are multiple benefits from this process, including increased life of wash chemical with resulting less chemical consumption, less water used, lower hauling costs, and less labor and downtime. There can also be an increased consistency of wash chemical with a much lower average concentration of emulsified oil and a much lower average particulate level. Table 4 Zero-Discharge-Wastewater System Designs Using Different TDSs of Wastewater to Produce Low- and High-Purity Rinse Water Sampling Point

Case A: Low-purity rinse water Case B: Low- and high-purity rinse water Case C: High-purity rinse water

1 TDS of Wastewatera (ppm)

2 Pretreatment Equipment

3 Resistivity of Water after Pretreatment (-cm)

4 Resistivity of Final Rinse Water (-cm)

Up to about 5000

(1) Single-pass RO (2) Double-pass RO

(1) About 62,000b (2) About 900,000

No DI closed-loop; same as column 3

Less than about 20

None

Same as column 1

After a DI closedloop, from 1000 to 5,000,000

Up to about 5000

Dragout reductionc (1) About 30,000 with either (1) single- (2) About 1,000,000 pass RO or (2) double-pass RO

After a DI closedloop (1) 15,000,000 (2) 15,000,000

Note: The water sampling points for 1, 2, 3, and 4, are shown in Figure 5. a TDS of the wastewater going to drain from the wash chemical tank (excluding wash chemical) and is the wastewater treated by next column. The conversion from TDS (ppm) to conductivity is 0.5 (as calcium carbonate) = 1 S/cm. b If 98% reduction is used for the second RO, the calculated resistivity would be 3 M-cm (3,000,000 -cm). However, the water purity is sensitive to any dissolved solids, which can most likely reduce the resistivity to below 1 M-cm. c RO rejecting 98% of the TDS was used for these calculations. The percentage may as high as 99% but not for all dissolved solids. Dragout refers to mechanical methods used to reduce the amount of dissolved solids going to the next process. The dragout could be a prerinse section in the in-line cleaner, or a time delay between two dip tanks to allow drainage of the parts or other similar method.

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A user’s ability to achieve these benefits depends on a careful evaluation of the user’s process and suppliers of the cleaning chemical, parts cleaner, membrane unit, and oil/lubricant/grease contaminants. Once a user is convinced recycling might be cost-effective, a demonstration test should be performed on the wash chemical to determine its recyclability and the cleanliness of the recycled product. This would be followed by a pilot test at the user’s facility to corroborate the benefits of recycling. Sometimes, other chemicals and micrometer-rated membranes are required to achieve optimum results. Rinse Water After the parts are washed, the next step in the cleaning process is rinsing. There are several possible methods to treat the wastewater depending upon the volume of wastewater produced. The designation of a low- and high-volume application is arbitrary, and there can be a large overlap between the two in actual applications. Hauling (even with evaporation) is usually not economical for processes producing thousands of gallons of rinse wastewater daily. To make hauling cost-effective, reuse methods like RO can reduce the amount of wastewater requiring further treatment by up to about 75%. The RO can provide the additional benefit of treating the source water to make up for any water losses from drying parts or the reject wastewater from the RO. The last consideration for a zero-discharge-wastewater design is the effect on the design by the user’s requirement for either a low- or high-purity rinse water. Low- and high-purity water are arbitrary terms that can have a wide range of meanings depending upon the user industry. For this discussion, water with a resistivity below 1 M-cm is considered low purity and water above 1 M-cm is considered high purity. RO as pretreatment is required to attain both levels of water purity, unless the TDS is 20 ppm or lower. For low-purity rinse water, a single-pass RO might produce the required water purity. For a high-purity rinse water, a dragout reduction step plus a single-pass RO, or double-pass RO, might be required before a final rinse DI closed loop. The amount of pretreatment depends upon the TDS of the wastewater being dragged out from the wash chemical tank by the parts being cleaned, racks, conveyor, and other handling equipment used in a dip tank operation, conveyorized in-line cleaner, or a cabinet washer. As a first step for any type of cleaning process, it is important to orient the parts to allow more time to drain off the wash chemical. These pretreatment methods will assure a lower operating cost for a closed-loop system if used to polish the water up to 15.0 Mcm and higher. Table 4 provides a guideline for the kind of pretreatment equipment and the expected water purity for the final rinse. As shown, the lower the TDS of the wastewater, the less extensive the pretreatment equipment required. Case A: Depending upon the TDS of the wastewater before the RO and the water purity requirement, a single-pass RO alone might achieve a user’s goal for a low-purity-water rinse (below 1 M-cm). If the water purity is not sufficient, a double-pass RO will produce a higher water purity than a single-pass RO. To achieve a zero-wastewater-discharge system, the reject wastewater stream from either RO process is hauled or evaporated and hauled. Case B: As discussed above, no pretreatment is required for the economical operation of a zero-discharge wastewater system if the TDS of the wastewater is below about 20 ppm just before a final rinse DI closed-loop. The key difference between operating a closed-loop system for a low (below 1 M-cm) and high (above 1 M-cm) water purity application is that for a low-purity application the water purity is allowed to degrade to a resistivity of about 1000 to 20,000 -cm, which is about the range of the purity of source water through© 2001 by CRC Press LLC

out the United States. The control of this process can be accomplished simply with a conductivity or TDS meter. When the maximum conductivity or TDS allowed by the process is reached, the ion-exchange resin is replaced. This reduces the operating costs of the system by one half to one third compared with a high-purity application. For a high-purity application, the higher the minimum water purity required by the process, the higher the operating cost, because the ion-exchange media will have to be replaced more frequently. For some applications, high-purity water may be too corrosive to the parts being cleaned, especially steel, galvanized, or brass parts. A DI closed-loop process produces only granular media disposed of or regenerated at a vendor’s plant. The different operating conditions of this closed-loop process might compare more favorably to hauling whenever RO is used. Case C: When the wastewater TDS of the stream feeding the RO membrane is about 5000, the RO is followed by a final rinse DI closed-loop with a dragout reduction before the RO to reduce the TDS. Dragout reduction refers to mechanical methods used to reduce the amount of contamination dragged out of a wash chemical tank going to the next process. Its purpose is to concentrate the dragout from the wash chemical tank into the smallest volume of water possible to minimize the size of the RO. For an in-line cleaner, the dragout reduction step is usually a prerinse. For a dip tank, the amount of dragout can be controlled by letting the wash chemical drain from the parts into the wash tank before going to the rinse tank, a brief rinse spray, or by using a still rinse tank of water (even source water might be adequate). For a conveyorized cleaner, a good design is air spray the parts and conveyor belt to blow off excess wash chemical before it enters the dragout reduction step that has a water spray and follow with another air spray to blow off excess water. For the cabinet washer without a conveyor, the most practical way is to let the parts drain off excess wash chemical and, if necessary, have one or more short rinses with a drain-out step. After the dragout reduction step, the next TDS reduction process is the RO. A doublepass RO without dragout reduction might be an alternative to a dragout reduction with a single-pass RO. To determine which alternative is most cost-effective, compare the cost of installing a dragout reduction in a washer along with a single-pass RO and evaporating the RO reject wastewater with the cost of using a double-pass RO without dragout reduction in a washer and evaporating the RO reject wastewater. The additional benefit of a doublepass RO is that there is a higher probability of achieving a higher water purity that might eliminate the need for a final rinse DI closed-loop. If the wastewater to the RO has significant amounts of oil or surfactants, the life of the RO membrane can be reduced. To protect the RO, pretreatment such as activated carbon or bag filters for low oil concentration applications or an ultrafilter (UF) or microfilter (MF) membrane for higher concentrations can be used. The activated carbon does not have a reject stream creating more wastewater to handle while a UF or MF membrane process does. There are low-challenge applications for which a UF or MF membrane can be used as a dead-end unit (without any reject stream) and taken off line and cleaned periodically. In summary, low-purity (below 1 M-cm) rinse water is sufficient for some applications. A single-pass or double-pass RO is adequate to produce this purity. But for higher purity (above 1 M-cm) rinse water, the amount of wastewater treatment depends upon the TDS of the wastewater and the required resistivity of the final rinse water. For low-TDS wastewater, no pretreatment is necessary before a DI closed-loop that produces a high-purity-water final rinse. For high-TDS wastewater, pretreatment before the final rinse DI closed-loop is required. The dragout wastewater from a wash tank along with the rinse wastewater might be recycled through a single RO or double-pass RO. In other cases, a dragout reduction step prior to the RO may be a better choice to achieve the required water purity. If a dragout reduction step followed by a single-pass RO does not © 2001 by CRC Press LLC

achieve the desired water purity, a double-pass RO might provide the additional removal of the dissolved solids. For high-purity-water requirements above about 1 M-cm for the final rinse, a DI closed-loop may be necessary. Wastewater Discharge Options Federal, state, and local regulations determine a user’s program of action for which of the contaminants and how much of the contaminants to treat. Each user must comply with the federal regulations at a minimum. After this requirement, the state regulations, which may be the same or even more stringent than the federal, must be followed. Finally, the local community regulations, which must be as restrictive as the state and federal regulations at a minimum, might be still more stringent. Local compliance issues can vary greatly throughout the United States. It is very important for any user planning to discharge any industrial wastewater to obtain a permit from the local regulatory agency, a POTW). Even though the wastewater is in compliance with the discharge regulations, discharges from small batch-type cleaners, like a household dishwasher, are considered industrial wastewater discharges subject to permitting before any discharge is allowed. In the past, the testing point usually was the end of the sewer pipe from the building. However, in increasingly more states, the wastewater is tested in the building at the source of discharge as it comes from the equipment. This makes compliance more difficult. The typical regulation requirements pertain to FOG, pH, BOD, and COD, and heavy metals. General treatment methods will be discussed for each of these conditions. Fat, Oil, and Grease Most POTWs regulate the amount of these three contaminants in wastewater. These contaminants come from petroleum and synthetic compounds from the parts being cleaned. The ability to remove them depends on numerous factors and conditions, including the condition of oil (free, dispersed, chemically or physically emulsified), temperature, amount removed per unit of time, types of petroleum and synthetic compounds, amount of TSS and TDS, available space, maintenance, and other operating conditions. There are a number of removal methods selected on the basis of the specific application. Membranes, both MF and UF remove contaminants by preventing them from penetrating the membrane and allowing water to pass through. Dissolved air flotation (DAF) uses air that attaches to free or dispersed oil and facilitates its rise to the surface of the wastewater for easy removal. Chemicals are often used to enhance the process efficiency. Chemical precipitation causes separation of the contaminants by precipitation. Centrifugation spins the wastewater at high velocities, forcing the heavier particles and high-molecular-weight compounds to separate from lighter molecules or particles. A coalescer is a device constructed of materials that allows the adherence of very small droplets of contaminants that grow in size and are released to the surface of the water. An oil skimmer includes a belt, disk, or other mechanical device or other methods such as a thinfilm technology to remove contaminants from the surface of the wastewater. A decanter (gravity separator) allows the separation to the surface of contaminants lighter than water, which then, under low turbulence conditions, spill over a weir into a waste container. The following guideline shows an approximate order for the effectiveness of each process according to its ability to remove petroleum and synthetic compounds from wastewater.

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• Membranes —UF —MF • Dissolved air flotation • Chemical precipitation • Centrifugation • Coalescers • Oil skimmers • Decanters

(most effective)

(least effective)

pH Typically, the wash tank of a cleaning operation contains an alkaline cleaner with a pH higher than the local discharge limit. This condition can be corrected by using an acid pH chemical control system. When there are regulated hazardous metals, the user must comply with the federal, state, and local regulations when disposing of the waste. Treatment of hazardous metals is discussed below. Biological Oxygen Demand and Chemical Oxygen Demand BOD is a test method that uses microorganisms to determine the amount of oxygen required to oxidize organic contaminants in water. COD is a test that uses a chemical oxidant to determine indirectly the amount of oxygen required to oxidize both organic and inorganic contaminants in water. Sometimes, state and local regulatory agencies have limits for BOD and COD. The removal methods for petroleum and synthetic contaminants may achieve sufficient reduction of these two measurements to meet these discharge limits. However, BOD and COD not only measure these contaminants, but also other oxidizable compounds that the FOG test does not. A packaged biological wastewater treatment system reduces the BOD levels to meet the discharge limits. It is a natural process that uses microorganisms to achieve the degradation of the organic contaminants and is used by essentially all POTWs in the United States. An equivalent industrial design is based on the amount of wastewater being processed and is usually much smaller than what a POTW would use. Since COD is composed of both inorganic and organic contaminants and microorganisms effectively oxidize only organic contaminants, an insufficient amount of the inorganic contaminants might be oxidized to meet the discharge limits. To reduce the COD further, a chemical oxidant, carbon adsorption, ultraviolet oxidation, ozonation, or other means is required. A membrane process such as ultrafiltration, nanofiltration, and RO could be used, but they are more often used in a recycling process where the permeate would be reused. The reject stream for either of these two processes increases the concentration of the contamination from 10 to 100 times. Hazardous Metals The eight hazardous metals that are federally regulated are cadmium, lead, selenium, mercury, barium, chromium, silver, and arsenic. In addition, some states and local agencies might list others. Any of the four following methods might be used to reduce the metal concentration in the wastewater to meet discharge limits:

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1. 2. 3. 4.

Mechanical filtration (particulate only) Chemical precipitation (particulate and dissolved) Ionic removal (dissolved only) Membrane (particulate and dissolved)

The choice depends greatly upon the metal and its state (dissolved, particle, colloidal), flow rate, total flow per day, and other factors. For example, if a user is cleaning cadmium-plated parts and must comply with an FOG and cadmium metal regulation, a UF might achieve both so long as the dissolved cadmium metal is not beyond the regulatory limit. The membrane does not effectively remove dissolved low-molecular-weight contaminants. In some cases, the processed water could be reused instead of being disposed. The reject stream containing the concentrated metal and oils would be hauled as hazardous waste. Determining the Wastewater Treatment for a New Process This is one of the most difficult applications. To reduce the uncertainty of wastewater treatment decisions, the user should determine the local source water conditions, similar processes in the industry (competitors), availability of hauling, potential discharge waivers, and piloting the process, all of which can aid in limiting overdesigning costs. The less that is known about a process, the greater the margin of safety that is usually necessary to ensure a treatment system that meets the user’s requirements. The user should try to maintain maximum flexibility before buying a permanent system. This section examines three possible decision-making areas. Source Water Treatment If a water sample is available, it is best to have it analyzed especially if high-purity water is necessary. It is best to wait for the results of the analysis before renting a long-term system or buying a permanent system unless the uncertainty of the treatment process is minimal. No-Wastewater-Discharge Design It is difficult to achieve an economical wastewater treatment system for a nowastewater-discharge design because of the unknowns: type of wash chemical, specific contamination generated by the process, surface quality of the parts, and other conditions. For small-volume applications, the entire wash tank and rinse water could be hauled. For large volumes of wastewater, where hauling might be a problem and the user is on a municipal sewer, it may be possible to discharge it with minimal treatment on a waiver. If on a septic system, river, or other body of water, hauling may be the only practical way. Another alternative for any of the above could be a temporary treatment system alone or along with hauling until enough data are gathered to define the final permanent treatment system. Wastewater-Discharge Design If the user has decided to discharge to a POTW, it is necessary to obtain the discharge regulations to determine the wastewater conditions that must be met and to obtain a permit. It is easier to prepare for this application than for a zero-discharge design because there are far fewer conditions affecting the final design. For example, for most alkaline cleaning applications, pH and oil are the two key concerns. For the pH adjustment, equipment is © 2001 by CRC Press LLC

usually easily obtainable on relatively short notice. The amount of oil in the wastewater is more difficult to assess and could lead to a large, unnecessary initial expenditure if a large margin of safety is required, such as considering a UF membrane or chemical treatment system. In such cases, a discharge waiver from a POTW would be of great value until the final effluent is tested. Overcapacity of Current Wastewater Treatment System In such applications, usually recycling at the source of the discharge can become a primary solution. The reason is that the cost of expanding the entire wastewater treatment system is usually much more than trying to reduce the amount of wastewater going to the treatment system by using a point-source treatment system. A careful evaluation of all discharge sources is made to determine which are the most viable from a cost standpoint. It is unusual for the expansion of the central wastewater treatment to be the most economical choice. For temporary overcapacity applications, hauling may be most economical. CASE HISTORIES Case 1—Manufacturer Unable to Reduce the High Failure Rate of Plated Parts Situation “We are replacing our wash chemical weekly, but part spotting is still a substantial problem that causes post-cleaning plating part failures.” Discussion The user was replacing the 600 gal of wash chemical weekly because neither coalescing nor skimming was capable of removing the emulsified oil, causing part spotting. To consider a wash chemical membrane recycling system, the user had to try another type of wash chemistry. After a successful match of a new wash chemistry with a recycling system, the incidence of spotting was essentially eliminated. After the new equipment was installed, the user’s costs from product defects, chemical purchases, haulage of spent chemicals, and labor totaling about $120,000/year were eliminated. With the new system the concentration and cleanliness of the wash chemical are maintained at a relatively constant level where, previously, the emulsified oil would build up toward the end of the weekly cycle of replacement of wash chemical. Lesson New technologies can sometimes help solve problems that existing methods cannot and, in addition, can yield additional unforeseen benefits.

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Case 2—Large Computer Manufacturer Buys a System from Local Supplier Situation “I have a local wastewater treatment company that said it could do it.” Discussion The manufacturing engineer was not familiar with wastewater treatment and believed the local company. The installed system cost in excess of $50,000 and required essentially a full-time operator trained in chemical wastewater treatment practices and a 20 20 ft floor space. This type of system is very typical in a printed circuit board fabrication facility. High operating costs, floods, and high volumes of water discharged to drain characterized the first year’s operation before a major design change eliminated one of the three major problems (floods). Another vendor with extensive experience with these systems had informed the engineer that it was not economically feasible to operate a closed-loop system without major changes in the way the cleaner had to operate. Several months later, the engineer left the company under unknown circumstances, and a supervisory engineer involved in the decision making was reassigned. Several months later, the company purchased another closed-loop wastewater recycling system for about $35,000 with a specially designed cleaner specified by the recycling vendor. The system only required an operator once every 3 to 4 weeks for 2 h for normal maintenance. Lessons 1. The engineer lacked the fundamental knowledge necessary to judge the technical merits of the two competing companies. 2. The local vendor had no operating systems experience or knowledge of these systems, despite its other wastewater treatment experience. Case 3—Large New England Military Contractor Decides to Build Its Own System and Makes a Large Investment Situation “I can do it myself for less money.” Discussion A manufacturing engineer believed that a large central system would be more costeffective than a vendor’s recommendation of several standard packaged systems. The user hired a water treatment consultant to assist with the design of the system. After talking to a number of different water treatment suppliers, none of whom was familiar with board assembly wastewater recycling, the engineer bought components and integrated them into a system with the help of the above consultant at a cost of $200,000 and the time of outside engineering consultants. The footprint was ten times what the packaged systems would have required, making an addition to the existing building necessary. At this time,

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the expensive, overdesigned system does not achieve the specified water quality or the operating economies used to justify the investment. Lesson A careful evaluation of alternative technologies and vendors is important before making a final decision. Case 4—Small Contractor Situation “If we did it over again, we would have spent less money, and saved 160 hours of engineering time and liability concerns with a local water purification company servicing a waste treatment application.” Discussion Upper management decided that a local water purification company, not experienced in waste treatment, could perform the service less expensively. The user purchased a water treatment system at a substantial cost without the necessary functional features. In addition, the user was not aware of the liability issues concerning the possible misuse of leadcontaminated ion-exchange resin by vendors servicing both waste treatment systems and high-purity systems such as medical facilities, laboratories, and other sensitive customers. If these other customers knew that their vendor was supplying them with resins that had been exposed to wastewater containing lead and other contaminants, they would immediately discontinue their business relationship. Lesson The engineering and design of closed-loop wastewater recycling systems were seriously underestimated and liability issues were completely overlooked. CONCLUSION The current general trend is increasing stringency of discharge regulations. This requires continual vigilance by users in maintaining their knowledge of current water and wastewater practices. Selecting the best source water and wastewater treatment processes for a cleaning application requires a methodical approach. In the case of solving an immediate cleaning problem, it is usually best to take a systems approach by evaluating the entire cleaning process each time because of the interdependency of each part of the cleaning process. Sometimes a simple change in the cleaning or water/wastewater process can alter the entire economic equation, transforming a previously uneconomical solution into an economical or, perhaps, even the best choice.

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REFERENCES 1. Water Quality Association, WQA Glossary of Terms, 1993. 2. McPherson, L., Correlating conductivity to PPM of total dissolved solids, Water/Engineering & Management, August 1995. 3. Owens, D.L., Practical Principles of Ion Exchange Water Treatment, Tall Oaks Publishing, Colorado, 1985. 4. Kunin, R, Ion-Exchange Resins, Robert E. Krieger, 1985. 5. American Water Works Association, Water Quality and Treatment, McGraw-Hill, New York, 1990. 6. Byrne, W. Reverse Osmosis: Practical Guide for Industrial Users, Tall Oaks Publishing, Colorado, 1995. 7. Quitmeyer, J., Sifting through filtration options, Precision Cleaning, December 1997. 8. Russo, J.F. and Fischer, M., Operating cost analysis of PWB aqueous cleaner systems: zero discharge water recycling system vs. once-through, presented at Third Int. SAMPE Electronics Conference, June 20 –22, 1989. 9. Kieper, T. and Russo, J.F., Closed-loop alkaline recycling proves an award winning application, Parts Cleaning, May 1999. 10. Rajagopalan, N., Lindsey, T., and Sparks, J., Recycling Aqueous Cleaning Solution, Products Finishing, July 1999.

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CHAPTER 2.18

Overview of Drying: Drying after Solvent Cleaning and Fixturing Barbara Kanegsberg

CONTENTS Introduction Solvent Drying Fixturing The Human Factor Conclusion References INTRODUCTION Drying is a critical step in the cleaning process. The drying process must be chosen carefully with consideration of the product, process flow, and ultimate use of the product. In planning a cleaning process, to assure an effective process with optimal process flow, budget some money for an effective drying system. Particularly with aqueous cleaning, many manufacturers have found it to be the most time-consuming step in the process. This is because drying requires that the product be heated and agitated. If the product cannot tolerate a high temperature, for physical drying, the drying step must be longer and slower. In addition, if a fairly large component must be heated to a high temperature, it may be too hot to handle for longer than may be tolerable to achieve efficient process flow. This chapter considers drying after solvent cleaning, the importance of fixturing, and the critical human factor. SOLVENT DRYING Two chapters in this section are concerned with drying in aqueous processes, and this is indeed appropriate because water is, well, wet. Water has a much higher boiling point and higher surface tension than many solvents, so it is more difficult to get rid of. Perhaps the reader is thinking: “I am purchasing a highly sophisticated solvent system. Drying will not be a problem.” © 2001 by CRC Press LLC

Think again! Those considering solvent systems also need to plan carefully to achieve adequate drying in an efficient, cost-effective manner. Some of the principles (although not all of the specifics) discussed in the chapters on removal of water are very applicable to designing a good solvent drying system. The author suggests that particular attention be paid to discussions involving vacuum drying and the need to avoid recontamination of the part during the drying step. In general, all drying systems, solvent and aqueous, should be properly vented to minimize employee exposure to undesirable vapors. Considering that understanding of what is undesirable is still limited, and considering that many different solvent residues may be present, it is best to err on the side of caution even when removing seemingly benign solvents. At the same time, solvent traps may be needed to assure environmental compliance, neighborhood safety, and worker safety. Those experienced in cleaning regularly observe problems associated with solvent drying. Some are equipment related; others are related to employee education and inappropriate handling of components. Inadequate and improper drying in solvent cleaning processes can result in: • Product damage • Production slowdown • Solvent loss With aqueous systems, drying is often optional. In solvent systems, the drying portion of the process may be so integrated a part of the cleaning process that it is indistinguishable as a separate set of features. It is important that those features related to drying be identified and then which features will be most important in an application be determined. Some features associated with solvent drying include: • • • • • •

Hoists Superheated vapor zones Freeboard Sample rotation Fixture design Vacuum-assisted removal of solvent

Solvent drying is often thought of as automatic. However, drying is relative. One needs to consider not only how clean is clean enough, but also, how dry is dry enough. The concept of adequate drying is relative to process requirements. Perhaps it is not necessary to dry. In industrial applications, a very light coating of mineral spirits or of a light oil can protect the part from corrosion. However, with components containing plastics and those having complex geometries, one needs to consider the consequences of not removing residual solvent. Residual solvent can interfere with subsequent process steps such as coating and physical assembly. One must also consider the consequences of outgassing. Outgassing can be thought of as a subtle form of solvent incompatibility or cleaning agent residue. Minute crevices in components can entrap solvent. Plastics and epoxies can adsorb solvent. The extent of solvent adsorption depends on the material, the chemical, and the application conditions (such as temperature, pressure, duration of cleaning). At ambient temperature, the solvent can be released over a period of days, weeks, or even years. It is particularly critical to avoid outgassing in sealed systems, which are expected to last for decades. In a sealed © 2001 by CRC Press LLC

gyroscope, released solvent may chemically react with the flotation fluid, producing a medium that corrodes delicate coil windings, resulting in unexpected, catastrophic product failure.1 One can think of other sealed, critical systems, such as pacemakers, where neither the doctor nor the recipient wants to deal with catastrophic product failure.2,3 Even in nonsealed systems, for biomedical implantables and other critical applications, avoiding solvent outgassing would seem to be a reasonable policy on general principles. One also might want to avoid outgassing where the part is sealed or bagged. If metal parts with residual water are placed in plastic bags for storage or shipment, they can corrode or discolor. Similarly, solvents can gradually escape or outgas from parts in plastic bags, perhaps interacting with the plastic, perhaps recondensing on the parts—in general making a mess. How does one know when there is outgassing? There are analytical techniques such as residual gas analysis (RGA) and head-space gas chromatography (GC). In both cases, a protocol is established to speed up the outgassing process, systematically collect the vapors, and then analyze them. However, only users and their co-workers can make the decision regarding where outgassing is an important factor. If so, one needs to dry. As with removal of water, drying to avoid outgassing can be accomplished by both chemical and physical methods. Users may decide to use both methods. Removal of solvent can be accomplished by chemical methods. Isopropyl alcohol (IPA) is sometimes used to displace water. IPA can also be used as a second rinsing and drying step with high-boiling solvents such as those based on mineral spirits, ester blends, longchain alcohols, and terpenes,2 However, for some applications, with very complex components, particularly those containing complex composites and plastics, IPA may leave the component wet, for the purpose of the process under consideration.4,5 In such cases, the IPA would be classically displaced by a perfluorinated material (PFC). Today, because of concerns about using materials with a long atmospheric lifetime, hydrofluorocarbons (HFCs) or hydrofluoroethers (HFEs) are preferred over PFCs in such applications. Solvent drying (i.e., removal of excess solvent) can also be accomplished by physical methods, including spin-drying and vacuum, and vacuum with heat. In manual systems, as might be used with very low volume, high-precision processes, vacuum bakeout in a small oven can be useful. In larger, airless or airtight cleaning operations, the vacuum drying system may be an integral part of the cleaning system.6 Appropriate vacuum drying can complete the process of cleaning under vacuum. In all cases, the process must be carefully adjusted to the product in question and to the total volume of parts to be processed. Where low-flash-point solvents are involved, the drying system must have appropriate engineering controls to prevent fires or explosions. In a liquid/vapor-phase cleaning or degreasing system, the components must be held in the vapor zone and then above the vapor zone to achieve adequate final cleaning, rinsing, and drying. It is critical to handle solvent properly to avoid solvent loss. In addition to technical considerations, there are both regulatory and economic factors related to solvent drying. For certain solvents, notably in the Federal Halogenated Solvents NESHAP,7 the dwell time and rate of removal are spelled out. Local regulations may also call out certain requirements to avoid emissions of air toxics (HAPs) and volatile organic compounds (VOCs). Readers are reminded to explore the fascinating, and often contradictory and Byzantine, world of solvent control as it applies to the regulatory climate in their location. Even for relatively unregulated solvents, it is important to remember that cleaning agents are not free. Some of them cost more than $15/lb. Particularly with large-scale operations, containing the solvent can be an important economic consideration even for relatively inexpensive solvents. Even in well-designed, automated systems, assuring complete drying of the parts is a critical part of solvent containment. © 2001 by CRC Press LLC

FIXTURING Appropriate fixturing and positioning of the components cannot be overemphasized in achieving adequate drying. This applies to aqueous, semiaqueous, and solvent systems. If the parts are not adequately exposed to the drying agent or the drying environment, there will be problems. In aqueous systems, if the parts are not adequately exposed, water can recondense on the parts, and drying may not be achieved within a reasonable time. Designing the fixturing system is very process specific, and the difficulties are not limited to aqueous processes. Time and again, the author has observed that in the first trial run of even very well-contained, sealed solvent systems with vacuum drying, after processing the first batch of product, visible solvent is present. This does not mean that the system is performing poorly. Often, it means that the parts have to be positioned so that the solvent flows out of the part. Part rotation can be helpful in this regard. Some of the factors may be summarized as in Table 1. THE HUMAN FACTOR Employee education and the input and feedback of production personnel, particularly technicians, are critical in optimizing the drying systems. This factor is often neglected in solvent drying processes. In manual systems, there is the tendency for the operator to speed up the degreasing process by dunking the parts in the solvent, yanking the basket out, holding it briefly in the now-destroyed vapor zone, and then racing across the floor while rapidly shaking the parts. This phenomenon is popularly referred to as the “dunky-do” approach. To counter this, employee training and, more important, employee education are crucial.7

Table 1 Summary: Impact of Fixturing on Drying Fixturing Factor

Positive Impacts on Drying

Provisos

Basket design, large proportion of mesh or screen

Adequate exposure of parts to heat, forced air, or drying solvent Avoid trapping water or drying agent Able to withstand heat Must have long-term compatibility with chemical drying agent Adequate size allows flexible processing

Large solid, nonmesh metal areas of fixtures may trap water or drying agent

Materials of construction

Size, strength relative to produce load

Part rotation

Achieve thorough drying Decrease drying time

Sample positioning

Decrease drying time

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Exercise care in adapting existing baskets to new drying process Overloading baskets can mechanically damage fixturing, producing inadequate drying Parts can be mechanically damaged during rotation particularly when not immersed in solvent Improper positioning can cause water or solvent to remain trapped

Certainly, good equipment design and system automation are important. The increasing emphasis on cost-containment and rapid manufacturing tends to counteract even the most thoughtfully designed solvent cleaning system. All too often, this author has observed that a relatively sophisticated cleaning system will be purchased and cleaning and drying times set appropriately. The employees are then “trained,” and manuals outlining complex procedures are installed. Then, within a few weeks, there are complaints of solvent loss, solvent odor, and inadequately cleaned parts with pockets of solvent. An investigation usually reveals that someone (typically someone on that infamous third shift, the equivalent of “the butler” in mystery stories) has taken shortcuts. In aqueous processes, employees may remove the parts from the drier sooner than would be desirable, or even skip the drying step completely. They may contaminate the parts by attempting to speed up drying using air hoses, or they may damage parts by increasing drying temperature. Typical undesirable shortcuts in both aqueous and solvent systems include: • Speeding up the drying cycle • Shortening the entire process • Overloading baskets of parts The importance of employee buy-in cannot be overemphasized. One may think one has purchased a fail-safe, totally automated system. In the author’s experience, the creative and determined employee can override a fail-safe system and destroy process control almost as quickly as it takes the average 4-year-old to remove a childproof medicine cap. Usually, employees have an immediate reason for shortcutting the system, and it is typically related to total processing time and immediate perceived profits. An employee may intellectually understand that a new aqueous or solvent process will take, for example, 20 min for complete cleaning and drying. If the old process only took 10 min, or if there has been a sudden increase in production volume, that employee may independently or even under direct orders from the supervisor decide to cut back on the process time. More often than not, this means cutting back on the drying time on the grounds that drying is an extra, nonessential part of cleaning. Often, in cutting back on drying, the employee has compromised the most important step. It is critical to ask, are we unwittingly rewarding supposed efficiency at the expense of adequate process control? The only way to combat the problem of shortcutting the drying step is to reward good process control to at least the same extent as rapid production rates. CONCLUSION In summary, drying is a critical, time-consuming portion of the cleaning process for aqueous, semiaqueous, and solvent cleaning.8 It is also a part of the process that is overlooked and underfunded on a regular basis. Once the question of how dry is dry enough has been answered, it is important to follow through with appropriate choices in drying method and fixturing, and then with appropriate ongoing employee training and education.

© 2001 by CRC Press LLC

REFERENCES 1. Kanegsberg, B., Abbink, B., Dishart, K.T., Kenyon, W.G., and Knapp, C.W., Development and implementation of non ozone depleting, non-aqueous high precision cleaning protocols for inertial navigation subassemblies. in Microcontamination ‘93 Proceedings, San Jose, CA, 1993. 2. Kanegsberg, B., Mallela, H., Dominguez, H., and Kenyon, W.G., Integrating precision de-oiling and defluxing processes in high volume manufacturing systems, in IPC Proceedings, San Diego, CA, 1995. 3. Kanegsberg, B., Cleaning for biomedical applications, in Proc. Precision Cleaning 97, Cincinnati, OH, 1997. 4. Kanegsberg, B., Cleaning systems for low flashport solvents, Precision Cleaning Mag., 3(3), 21 –28, 1995. 5. Carter, M., Andersen, E., Chang, S.C., Sanders, P.J., and Kanegsberg, B., Cleaning high precision inertial navigation systems a case study and panel discussion in Proc. Clean Tech ‘99, Rosemont, IL, 1999. 6. Ohkubo, M., An airtight argument: vacuum solvent cleaning systems work, Precision Cleaning Mag., 7, 24 –29, 1999. 7. Petrulio, R., and Kanegsberg, B., Back to basics: the care and feeding of a vapor degreaser with new solvents, in Nepcon West ‘98, Anaheim, CA, 1998. 8. Seelig, S., Adequate water removal for aqueous based operations presented at Tenth Annual Workshop on Solvent Substitution, Scottsdale, AZ, 1999. 9. U.S. EPA, National Emission Standards for Hazardous Air Pollutants, Fed. Regis., December 2, 1994. 40 CFR Parts 9 and 63 (AD-FRL-5111-3) RIN 2060-AC31, available at http://www.epa.gov./ fedrgstr/EPA-AIR/1994/December/Day-02/pr-184.html

© 2001 by CRC Press LLC

CHAPTER 2.19

Aqueous Parts Drying Daniel J. VanderPyl

CONTENTS Definition of Drying for Aqueous Parts Historical Perspective on the Drying Process Types of Physical Drying Techniques for Aqueous Cleaning Technology Centrifugal Spin Drying Desiccant Bulk Drying Forced Air Drying without Heat Forced Air Drying with Heat High-Velocity Air Blowoff (Compressed Air and Blowers) Compressed Air Blowers Low-Velocity Air Drying Radiant Heat Spot Drying (Vacuum Hoses and Compressed Air Blowoff Nozzles) Vacuum Hoses Compressed Air Blowoff Nozzles Vacuum Chamber Drying Integration of Drying Systems with Cleaning Systems DEFINITION OF DRYING FOR AQUEOUS PARTS There are two primary categories of parts-drying technologies for aqueous cleaning systems. There is chemical displacement drying, where typically organic solvents are used to displace water from a component, and physical drying, the topic of this chapter. This chapter focuses on the most widely used methods, i.e., those that use an exchange of air throughout the component by various means to blast, strip, or evaporate moisture from the exterior and interior of a component. Air is the primary element interacting with the moisture on the surface of the component. Air drying is therefore defined as any mechanical action to remove water/moisture from the surface that does not rely solely on natural evaporative drying. The following pages delineate several types of air-drying systems with the related applications and the © 2001 by CRC Press LLC

impact of component design on dryness standards, production rates, operating costs, and worker safety. HISTORICAL PERSPECTIVE ON THE DRYING PROCESS The continuing evolution of manufacturing technology has led to a wide range of purpose-built machinery for automated and semiautomated processes in parts cleaning and drying. Prior to today’s prominence of aqueous cleaning technology, those using solvent/chemical cleaning and degreasing processes were unconcerned about drying. Often, the process resulted in cleaning and subsequent evaporation simultaneously. The natural process of chemical cleaning resulted in a clean, spot-free, generally high-quality part, although maintaining product cleanliness was complicated by a continuous need to filter and replenish chemicals. Little was known about the health or environmental effects of such chemical use. Drying was an afterthought in the early aqueous cleaning systems. As such, approaches to drying consisted of a wide variety of methods that were often inefficient or ill suited to the application. As the impact of various drying methods on product cleanliness became apparent, the focus on drying quickly sharpened. The 1987 Montreal Protocol initiated the phaseout of ozone-depleting chemical cleaning processes, and its signers began a movement that took aqueous cleaning from a cleaning option to a cleaning standard. The printed circuit board industry led the way. It pioneered new aqueous cleaning equipment and effective chemical replacement technologies. Throughout the 1980s, product quality in the printed circuit board industry was the driving force while manufacturing efficiency was a much lower priority. Today, this scenario has reversed itself in the industry and most other high-volume production environments; process costs and throughput rates are now scrutinized just as much as quality. Currently, aqueous cleaning systems are effective and able to match the cleanliness of most chemical-based cleaning systems. Customer knowledge of cleaning/drying processes has increased tremendously. Customers have learned that cleaning and drying go hand-in-hand; one cannot exist without the other. Today, there is more specific drying required than ever before. As always, the requirements of the drying system are intimately connected to the needs of the subsequent manufacturing steps whether they be parts inspection, component assembly, painting, ink jet coding, or packaging. Equipment manufacturers have developed machinery and technology to match the increasingly stringent cleanliness requirements. The U.S. military specifications (mil-specs) were a driving force in many of these refinements, which then spurred equipment manufacturers to identify drying as the weak link to total product cleanliness. With total product cleanliness the focus, process costs soared. Drying systems were cumbersome, operating costs were high, and throughput capacity was poor. As the world economy continued to evolve and prices in the technology sector (i.e., computers and communications equipment) dropped, competitive forces dictated that improvements in manufacturing efficiency were the key to tomorrow’s profits. Thus evolved a menu of drying options. The secret of their best use lies in matching the appropriate technology to the cleaning method and to the part being cleaned to attain the desired levels of cleanliness and productivity at the least cost.

© 2001 by CRC Press LLC

TYPES OF PHYSICAL DRYING TECHNIQUES FOR AQUEOUS CLEANING TECHNOLOGY The following drying systems will be covered in this chapter. • • • • •

• • •

•

Centrifugal spin drying Desiccant bulk drying Forced air drying without heat Forced air drying with heat High-velocity air blowoff —Compressed air —Blowers Low-velocity air blowoff Radiant heat Spot drying —Vacuum hoses —Compressed air blowoff nozzles Vacuum chamber drying

There are many good options to achieve efficient drying. Making the optimal choice is difficult. Just as there is no ideal, one-size-fits-all cleaning technique, the drying system must be suited to the components and to the overall manufacturing processes. Therefore, the pros and cons, the applications and misapplications, are indicated. Centrifugal Spin Drying Centrifugal spin drying involves high-speed centrifugal spinning of up to hundreds of batched components to remove moisture from component surfaces following cleaning, cooling, and plating. Commercially available spin dryers are rated by loading capacity and range from one to several hundred pounds per basket. Components are transported in a basket and either manually or automatically hoisted from the final rinse process to the centrifugal spin dryer. Cycle time can last from 2 to 20 min, depending on component complexity. At speeds up to 1500 rpm, care must be taken not to damage precision or delicate components. Ideal Application. Centrifugal drying is best suited to components of less than 1 in.3 in size having simple geometry and few if any blind holes or crevices that do not require critical drying, such as those on plating lines. Misapplication. The technique is not well suited for drying complex machined components requiring absolutely dry and spot-free surfaces. Advantages. Advantages include low operating cost and a small footprint for batch applications with adequate dwell time at the drying step. In addition, the noise level is low. Disadvantages. On the other hand, because baskets must be constantly loaded and unloaded, the method is relatively labor intensive. With heavily loaded baskets, there is the potential for worker injury. As production rates increase, the tendency is to load baskets more and to add more spin-drying units, rather than upgrade drying technology. Impact of Component Design. Centrifugal drying is more likely to become ineffective for parts with complex shapes or with densely packed batches. It can only be used for durably designed components.

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Drying Effectiveness. Component geometry influences drying effectiveness in centrifugal spin dryers more than most other drying methods, and moisture retention can be a problem. Forced air heating may be used to supplement drying capacity. Drying Impact on Cleanliness. The potential for component damage makes spin drying unsuitable for precision components. Clean components wearing against one another can result in surface damage, deformation, or dislodging of particles, thus defeating the cleanliness requirement of more critical applications. Equipment Menu. The primary equipment required is a spin dryer (0.5 to 7.5 HP). Larger dryers equipped with heated air circulation are also available. Ancillary equipment includes an automated or semiautomated hoist. Associated Equipment Requirements; Labor Costs. Loading and unloading time can make labor costs significant. The number of components per batch impacts the energy cost per component dried. As with all forms of drying, there is an optimum point of effectiveness with the spin dryer; overloading the spin basket leads to diminished drying effectiveness and increased per part drying costs. Safety and Environmental Issues. The greatest potential issue for employees is the physical handling of baskets of components, both during loading and unloading of the chamber and emptying the basket of components for the next process. Desiccant Bulk Drying Desiccant bulk drying is one step better than nature’s own evaporative process. Desiccant material draws moisture from ambient air and is often used as both a curing and a drying process. Storage in a large chamber or room with desiccant material would be the final drying step to achieve as close to zero moisture content as possible. Frequently, it is used for removing moisture from compressed air sources and in large batch drying of agricultural products. It is the least often used method to dry manufactured parts. Ideal Application. Desiccant drying is ideally suited for products where a moisture content of 1 to 5% as a portion of total weight is needed for maximum part stability or shelf life and where heat or other aggressive drying methods may be detrimental to the parts. For example, any assembly with a latex component may benefit most from desiccant drying. Misapplication. Desiccant drying is not appropriate for a continuous manufacturing process where drying cycle time must be kept to a minimum. It is not suitable for applications where absorbed moisture is not the issue. Advantages. It is much easier to control the uniformity of dryness among all the components with desiccant drying than with radiant heat or forced air drying. Radiant heat and forced air create different rates of drying within the same batch or process. Disadvantages. The drying chamber typically has a significantly large footprint, the cycle time is long, and labor costs associated with product handling are high. Impact of Component Design. Desiccant drying performance is impacted by the porosity of the product. The more porous, the longer it will take to draw out the moisture. Typically, the goal is to reduce the moisture content of a given part to between 1 and 5% of moisture as a percentage of the weight of the component. Drying Effectiveness. The level of drying using this method is measured by the humidity in the chamber. Desiccant drying in large bulk processes makes it easy to measure the level of retained moisture. Drying Impact on Cleanliness. Outside elements that could compromise the level of product cleanliness are not usually a factor. The cleanliness of porous products has more to do with the forming of the product, and water may be part of that process. For example, in molded parts such as latex, water is an integral part of product molding and additional cleaning is not a required process. © 2001 by CRC Press LLC

Equipment Menu. The primary equipment consists of a sizable container or room, from 1 to 1000 ft3 in volume. Ancillary equipment includes a fan for air circulation and a dehumidifying unit. Associated Energy and Labor Requirements. Throughput is lowest with desiccant drying. Labor rates are among the highest because product must be manually moved in and out of the chamber or room. Safety and Environmental Issues. Desiccant drying itself poses relatively few worker hazards. Hazards associated with product composition are a potential problem. Forced Air Drying without Heat Forced air drying (without heat) is based on the exchange of large volumes of air through an enclosed zone to extract ambient moisture. It is an accelerated evaporation process that can also use heated air. A dehumidification unit can shorten the drying cycle. Ideal Application. The most effective use is with simply designed components that have surface and/or core temperatures higher than ambient. By drawing away air with forced air drying, humidity and moisture are removed from all but the surface, encouraging more evaporation. Without air circulation, such a heated product would make the whole chamber humid. For example, in the tire industry, using a Banberry rubber extrusion process, a continuous ribbon of rubber product exits at 350°F. It is then immersed in a cold water quench. Forced air accelerating the evaporative process can help remove the water on the rubber strip. Misapplication. Forced air drying is not well-suited to batch-drying applications where part configuration and part stacking result in numerous water pockets within the batch. In such situations, the drying time may increase unacceptably. Advantages. Initial capital outlay is minimal, as no specialized equipment is needed. Noise levels are low, and environmental impact is minimal. Disadvantages. The throughput rate for forced air drying without heat is low, and the drying zone may be large, resulting in a large footprint. There can be problems in process control. For one thing, there can be difficulty in controlling drying effectiveness where ambient temperatures may fluctuate. In addition, product quality may be adversely impacted due to the potential for part recontamination from unclean air. Impact of Component Design. Typically, forced air drying is used only for simply designed components at low production rates. Drying Effectiveness. Forced air drying is effective for industrial drying. For example, it can dry rubber products well enough to eliminate potential voids in the stamping or molding phase of production. However, forced air drying without heat is ineffective for parts with complex surfaces requiring a high drying standard. Drying Impact on Cleanliness. Forced air drying can contaminate the component if the air source is unclean. However, cleanliness is a function of the total manufacturing process, and forced air drying is not used in applications involving critical cleaning. Equipment Menu. The primary equipment consists of a low-pressure fan assembly adjacent to the product, most commonly an axial or box fan simply blowing large volumes of air across the surface. Ancillary equipment includes a dehumidification unit to decrease drying time. Associated Energy and Labor Requirements. Where the process is conveyorized (as in most applications of this method), labor costs are low. However, forced air drying may also be used for batch cleaning, where labor costs may be significant. Energy costs are low since blowers use much less energy than heaters. Safety and Environmental Issues. Worker safety and environmental concerns are minimal, since humid air can be expelled with a roof-mounted exhaust fan. Noise is typically not a problem. © 2001 by CRC Press LLC

Forced Air Drying with Heat In such systems, components are indexed into a drying zone using a circulating fan that warms air up to 200°F above ambient temperature. The heat source is most often electric, but natural gas-fired heat and waste steam are also used. Overhead conveyor systems also use forced air drying with heat prior to electrostatic powder paint zones. Ideal Application. Forced air drying with heat is the method commonly used in batch ultrasonic cleaning systems. Using sliding beam systems, baskets of components are transferred into drying chambers after the final rinse. The forced air with heat provides an accelerated evaporative drying process. Misapplication. Misapplications of forced air drying are related to the thermal stability of the component as well as to required process time, process flow, and part handling/worker training problems. In a continuous conveyorized process, forced air drying with heat can be either a benefit or detriment depending on what happens after the drying process. The time taken to stabilize the parts at room temperature is the price paid to dry those parts with heated forced air. Attempts to speed up the process can cause problems. For example, in a batch type paint process of cleaning/drying/painting, components are often manually handled in inspection or assembly, and high-temperature parts may burn workers. In addition, increased drying temperatures cause thermal expansion of the component, pushing a precision machined part out of inspection tolerances. Advantages. Forced air drying with heat is versatile and can be used over a wide range of part and component sizes. Because the air is mixed evenly, the method results in uniform drying. Compared with desiccant drying, it can dry relatively large batches of parts more effectively in a much shorter period of time. In some applications, the waste heat from other processes can be used to create steam for forced heat drying. Disadvantages. The drying cycle time is directly proportional to variations in batch size and component orientation. If an elevated core temperature is required, drying time and cost will increase. Parts may become too hot to handle in the next step of production. Heat may expand components beyond inspection tolerances. Heat may bake on contaminants depending on the particle count in final rinse stage. Impact of Component Design. Particularly in ultrasonic batch cleaning systems, batch size component geometry and orientation in each basket must be consistent for reproducible results. Complex, ornate components with blind holes present the most difficult challenge in maintaining drying/cleanliness consistency. Drying Effectiveness. For both batch and in-line continuous processes, variations in component design, volume of parts per batch, drying rate, and any thermal constraints of the product directly affect drying quality. Drying Impact on Cleanliness. Forced air drying with heat results in accelerated evaporation of moisture from components. Any particulate suspended in the surface liquid will ultimately be baked onto the surface of the component as it is dried. Therefore, the water quality of the final rinse zone in terms of dissolved contaminants and the filtration needed to maintain water quality and eliminate particulates are integral parts of final product cleanliness. Equipment Menu. The primary equipment required includes a squirrel cage fan/blower (high volume, low pressure) and an in-line duct heater. The equipment is predominantly electric. However, natural gas may be used in a continuous conveyor system. High efficiency particulate arrestance (HEPA) filters are commonly used for some precision and all critical cleaning applications. For batch processes, the drying zone is typically an enclosed chamber placed subsequent to the ultrasonic cleaning and rinsing tanks.

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Associated Energy and Labor Requirements. Process costs depend on the drying cycle required for each component. Process costs for batch processes can be considerable. For example, running 10 batches a day over an 8-h day requires a fan and a heat source. The lowest temperature likely to result in effective drying would be approximately 200°F. If production levels were to increase to 20 batches a day, to achieve the same level of drying either the temperature would have to increase or the number of drying chambers would have to be doubled. While the energy consumed per component dried might not increase, total energy usage would likely increase considerably. In addition, for batch processes, labor costs are incurred for loading and unloading. Lag time for cooling may add extra handling and labor costs. In-line applications generally reduce labor costs. However, conveyor length and lag time in the production flow may involve added costs. The throughput rate can be anything desired within the limits of the temperature constraints of the parts. The utilizable temperature becomes the limiting factor. Safety and Environmental Issues. Workers must be protected against burns from the heat chamber and from heated components. Environmental impact can vary according to the application.

High-Velocity Air Blowoff (Compressed Air and Blowers) A high-velocity air blowoff is any air stream directed at the surface of a product to create a sheer force that strips liquid from the product. It is one of the most widely used drying methods. The air source can be generated from high-pressure plant air systems, compressed nitrogen, or self-contained blower systems. High-velocity airstreams can generate static charge. While the moisture being removed from the part counteracts this tendency, static buildup in parts-drying applications is of concern. Compressed Air Compressed air systems are defined as drying systems using a compressed air generator with a minimum 10 psi (pounds per square inch) and supplying air through air knives and nozzles. Ideal Application. Compressed air is best for the small-scale drying of parts. The approach is practical for parts of less than 6 in.2 of cross-sectional area and traveling single file at less than 5 ft/min. Misapplication. Compressed air, when used to dry components with more than 36 in.2 of surface area on any one side, will generally result in very high operating costs. Also, components with critical cleanliness requirements must be protected from oil and condensation produced by the compressed air system. Advantages. Compressed air systems of adequate capacity are often already in place. Equipment is available from a range of suppliers and the systems consume relatively little space. Air knives and nozzles are compact; piping systems are generally less than 1 in. in diameter. Disadvantages. Energy consumption can be up to 75% more than with blowers. Because most compressors are oil lubricated, filtration of the airstream is required to prevent recontamination of the parts. In addition, compressors condense liquid from the airstream, resulting in part contamination. Finally, compressed air produces a low-frequency noise that is audible at considerable distances from the blowoff zone. Workers may find the noise uncomfortable or unacceptable. © 2001 by CRC Press LLC

Impact of Component Design. Component design and fixturing considerations during cleaning and drying impact the choice of compressed air vs. a blower system. As a rule of thumb, compressed air can be effectively used for drying parts measuring less than 6  6 in. of cross-sectional area with blind holes and crevices that measure less than 1/2 in. diameter with hole depth a minimum five times hole diameter. Components with large, smooth surface areas and simple geometry are poor candidates for compressed air, as operating costs will skyrocket. Drying Effectiveness. The ability of air to strip moisture from complex or critical surfaces relies on the air having a straight path to the respective surface or area. A part can be easy to dry, but if parts are stacked ten on a rack, airflow and effectiveness are restricted. Moisture condensation is a potential problem. Drying Impact on Cleanliness. Compressed air may carry particles of oil and dirt that would compromise precision cleaning. Also, condensation from the high-pressure air can recontaminate the parts with moisture. Therefore, compressed air is typically used for industrial processes, but not for precision cleaning. Equipment Menu. The primary equipment consists of an electrically operated dryer, an oil separator, a piping system, a compressed air knife or nozzle, available plant air, and a receiver tank, which is placed as close to the blowoff point as possible. If the drying process is not continuous, this tank can be used as a reservoir for compressed air. Ancillary equipment includes an enclosure, tunnel, or chamber where blowoff can take place, to reduce ambient noise and provide an opportunity to exhaust moisture-laden air as the product is dried. Associated Energy and Labor Requirements; Additional Cost Considerations. A wide range of compressors and accessory items, nozzles, knives, and other blowoff devices are available, but the end user is generally left to decide how many of what configuration are required. It is important to note that compressor costs are commonly higher than for blower systems and use approximately 75% more energy. The decision whether to use compressors or blowers must be made on a trade-off basis, considering existing resources and return on investment (e.g., is sufficient compressed air already available?). In general, if neither blowers nor compressors are currently available, a blower will generally cost less in terms of initial cost and operating cost. In addition, for more exacting processes, while compressed air is available with very stringent air-drying capabilities, in most cases the cost of filters, separators, oil filters, and energy use is prohibitive. Finally, for larger parts or precision cleaning, where workers must carefully and individually clean parts, added labor costs must be considered. Safety and Environmental Issues. Noise is a problem with compressed air. It generally operates at a lower frequency range than high-velocity air blowers, and this allows sound levels to travel greater distances than blower system air sounds. For example, at 100 ft, a blower system might read in the high 70 dbA range. A compressed air nozzle at 90 dbA, located 100 ft away, would still generate 85 dbA. The high-frequency whistling noise that occurs as air is blown off surfaces causes random spikes of impact noise that often exceed 100 dbA. The cost of the materials capable of absorbing lower-frequency sound levels is generally very high. Small bits of debris blown from blind holes can be a worker safety problem, and workers should wear protective goggles. Sound levels often require hearing protection. Contaminants may be blown into the environment, and condensation can blow into associated work areas, causing slippery floors or fouling of work sites. If the systems cannot be adequately enclosed or ventilated, operators must also be protected from breathing atomized contaminants.

© 2001 by CRC Press LLC

Blowers High-velocity air blowoff can be achieved using a dedicated blower producing between 0.5 and 5.0 psi, with the air directed through air nozzles or air knives. Blower systems with optional infrared lamps are the most widely applied method of drying components in conveyorized precision cleaning and plating systems. In addition, high-velocity air is often used for critical cleaning applications such as semi-conductor components and medical devices. Ideal Application. High-velocity air blow-off with air blowers is most effective with any component size greater than a 6  6 in.2 surface area, having simple to moderate surface complexity, and for production rates greater than 100/h. Misapplication. Blowers are not suitable for extremely small components, complex geometries and blind holes, low production rates, and short cycle times. Process equipment design may restrict nozzle or air knife access, necessitating small piping and unacceptably high pressure. However, adding heat can increase drying capability. Advantages. Blowers can be sized to accommodate a wide variety of components and production rates . High-velocity blowers provide the most energy efficient of blowoff air methods. The heat of compression assists in the drying. Air can be supplied free of oil and moisture, and self-contained air delivery without air pressure fluctuations is achievable. Disadvantages. Blowers cannot remove liquid from complex parts with unexposed surfaces. Exhaust air requirements are much higher than for compressed air. Large 2- and 3in.-diameter feed and connecting piping for air knives and nozzles takes up additional space. Impact of Component Design. Small intricate components and those with blind holes more than three times the depth of the hole diameter are not dried effectively. Drying Effectiveness. As compared with compressed air, the greater range of air volume that can be used in blowers allows greater versatility. Drying Impact on Cleanliness. Sealed bearing blowers can deliver oil-free air. However, any oil-lubricated blower is vulnerable to seal failure resulting in contamination of the entire cleaning system. On a positive note, because the high velocity strips away all moisture, the system does not create enough pressure to produce condensation that might recontaminate the part. In addition, use of high-impact air effectively eliminates baking on of particulates entrained in surface liquids. Equipment Menu. The primary equipment consists of a self-contained motor assembly blower, an air filter/silencer assembly to filter out ambient air and dirt particles, and air knives. Ancillary equipment includes electric in-line heaters, a recirculating blower air with piping and filters, and exhaust fans connected to drying chambers. The option of a recirculating blower air should be considered to cut process costs and to avoid contamination. Air introduced into a drying chamber must be properly exhausted to avoid introducing heat, moisture, and contaminants into the surrounding work environment. The heat introduced can be comparable to leaving a door open in a factory all day long. Associated Energy and Labor Requirements. High-velocity air blowers are the most efficient means of forced air blowoff. While little labor is associated with the drying process, complex parts may require spot blowoff following exit from the drying chamber. Throughput rates for blower systems are generally high and and are based on matching blower size to throughput demands. Assistance of an experienced equipment vendor can be very helpful in giving specific application advice in designing the system and evaluating the appropriate size and/or number of blowers to support demand.

© 2001 by CRC Press LLC

Safety and Environmental Issues. Noise with high-velocity blowers can be significant. The blower system operates at the higher end of the audible frequency range, i.e., 90 dbA. But at 100 ft, that same blower system might read in the high 70 dbA range (every point is a multiple of 10, therefore, there is a tremendous difference between 85 and 80 dbA). Proper enclosure can reduce sound impact. In-line heated air or recirculated blower air can increase surface temperature of the components being dried. Subsequent handling by operators must be considered. If air from the blower is not recirculated, an exhaust system must be installed. Controls depend on specific regulatory requirements. Low-Velocity Air Drying Low-velocity air-knife drying systems involve large volumes of air introduced through an air-knife plenum exiting at velocities no greater than 10,000 ft/min. Although initial velocities are low, because of the wider air path and greater volume of air, the drop in pressure is lower compared with other systems. This effectively results in a longer air path. Ideal Application. The ideal application for low-velocity air drying is conveyorized cleaning of components with simple geometries and with low throughput rates. In the early days of aqueous cleaning, parts cleaners frequently used low-air-velocity drying systems. They worked well because the component geometry was simpler and throughput rates and conveyor speeds were slower. The low velocity was effective for many applications, as the dwell time within the drying zone was correspondingly much greater than with the higher production speeds of today. Today, higher-impact velocity will compensate for higher throughput rates by creating greater shear force for more effective drying despite shorter dwell time. Misapplication. Despite advances in the technique, low-velocity drying is not readily adaptable to parts with complex geometries. Low-velocity air drying is unsuitable for high-speed throughput of anything but the smoothest of surfaces. Advantages. Advantages include low initial capital outlay, low operating costs, low maintenance, and low noise levels. Disadvantages. The method is slow and is ineffective with complex parts. Equipment size is large, particularly relative to drying capacity. Impact of Component Design. Simple component designs are best suited to low-velocity air drying. Drying Effectiveness. Low-velocity air drying is effective for simple parts at low speeds. It can work effectively at higher throughput rates if critical levels of drying are not required or with supplementary in-line heating. Drying Impact on Cleanliness. Filters must be increased appropriately to filter air effectively while minimizing pressure drop. The design also does not lend itself to overcome the pressure drop associated with high-capacity inlet filters or in-line HEPA filtration. Equipment Menu. The primary equipment is a centrifugal fan assembly (1 to 10 HP), an air distribution manifold, flex-hose and piping, and air-knife plenum. Ancillary equipment includes a shroud enclosing the air-knife assembly with an optional hood to draw moisture-laden air from air-knife zone. Associated Energy and Labor Requirements. While total energy consumption of low-velocity air knives is low, energy usage per component tends to be comparable with highervelocity systems with higher production rate equipment. In addition, with supplementary in-line heating, low-velocity air drying then becomes an energy-efficiency problem when compared with other types of technology and may become an issue.

© 2001 by CRC Press LLC

Safety and Environmental Issues. Even with the low-velocity system, undesirable aerosols must be vented and, if required to meet environmental regulations, appropriately trapped. Radiant Heat Radiant heat is the process (typically in-line) by which a heat source (generally infrared tube lamps, IR) is used to flash-dry very thin layers of moisture on the surface of components. It is often used after high-velocity systems that remove the bulk of the liquid. In some batch processes it is used for final drying following forced air drying. Ideal Application. The ideal application is for final drying of parts with complex geometries where air knives may not completely dry the parts. Misapplication. IR heat is inappropriate as the primary or sole drying source for products having more than a trace amount of liquid residue. Advantages. Advantages include relatively low capital outlay and the absence of noise or environmental concerns. Disadvantages. When used without initial drying to remove gross moisture, operating costs and cycle time can be high. The footprint for exclusive IR drying can be high. Dissolved or suspended contaminants can bake onto surface, and excessive heating can result in throughput, handling, and inspection problems. Impact of Component Design. Complex shapes and surfaces with close dimensional clearance between components in an assembly, i.e., circuit boards, are best suited to the IR drying method. IR provides better control and greater simplicity. Drying Effectiveness. An IR heater element, like any heater element, is either on or off. The cycle time of the element determines the amount of radiant heat produced. It can be difficult to maintain consistent heating. Drying Impact on Cleanliness. When IR is the second stage of drying to an air-knife system, most contaminants entrained in the surface liquid are carried away with the air velocity. However, if the percentage of surface moisture is more than 5% or the level of entrained contaminants is high, the likelihood of undesirable residue increases. Equipment Menu. Primary equipment consists of IR heater tubes in a shielded or shrouded chamber with insulation to minimize surface heat on the exterior of the chamber along with a heater control system to ensure even element cycle times. Ancillary equipment consists of initial drying with fans, blowers, and/or air knives. Associated Energy and Labor Requirements. IR heater elements used as the only drying method are likely to result in excessive electrical demands. But as a second stage, they can be a very energy efficient drying method for complex components. Automated processes are available. Safety and Environmental Issues. If components are raised above 125°F, part handling issues must be considered. If preceded by adequate rinsing, there are no obvious environmental issues related to IR drying. Spot Drying (Vacuum Hoses and Compressed Air Blowoff Nozzles) A wide range of products have drying requirements for specific areas of the product, but do not necessarily require a completely dry part. In such cases, capital outlay may not be justified relative to labor costs.

© 2001 by CRC Press LLC

Vacuum Hoses Localized vacuum drying is a manual process used for very low production rates, or for component inspection where only certain portions require drying. A vacuum drying system could be a central system or a dedicated vacuum unit. Ideal Application. The ideal application is for simple components in industrial-grade cleaning. Misapplication. Examples include complex parts and blind holes where air cannot be exchanged adequately to draw liquid to the vacuum source, or critical applications where contact with the vacuum nozzle could compromise cleanliness. Advantages. Advantages include low equipment and energy costs. The technique may avoid scattering of large amounts of moisture and contaminants. In addition, it may be possible to combine the drying and inspection steps. Disadvantages. Contact with the vacuum hose and/or bristled pickup nozzles may result in contamination. Process control is difficult. Impact of Component Design. Localized vacuum is ineffective with complex components. Drying Effectiveness. Process control is operator dependent; effectiveness is variable. The greatest use of localized drying is in quality control operations adjunctive to critical inspection of machined components. Equipment Menu. Equipment consists of vacuum systems or portable vacuum units. Associated Energy and Labor Requirements. Vacuum units require manual handling of each component. With low production rates, labor costs may be acceptable. The energy consumption for a vacuum is 1 to 3 HP; energy per component is highly variable. A vacuum source running continuously with sporadic parts spot drying may be more costly to operate than an on-demand compressed air nozzle. Safety and Environmental Issues. When using a portable vacuum, residual chemicals may cause problems as potential fire hazards. Compressed Air Blowoff Nozzles Compressed air blowoff is the most common spot drying method because of the availability of compressed air and its ability to blow moisture from confined spaces. Ideal Application. Spot drying with air blowoff is uniquely suitable for blind holes or crevices where moisture is randomly trapped. The technique is used for low-volume, intermittent production and for inspection. Misapplication. The most common misapplication is where compressed air nozzles are operated by line workers repeatedly for a nonvariable product line. These applications are much better suited to high-velocity blowoff systems. Advantages. The small handheld nozzles are very maneuverable for working with complex parts. Disadvantages. The equipment is noisy and can scatter moisture and contaminant. Process control is difficult. Equipment Menu. Equipment consists of localized blowoff nozzles or custom nozzle and tube assemblies connected to a compressed air line. Associated Energy and Labor Requirements. The method is labor intensive. On-demand use of compressed air generally minimizes the power requirements of the compressed air system applied for spot drying, unless the blowoff nozzles are operated on a continuous basis.

© 2001 by CRC Press LLC

Safety and Environmental Issues. The potential for debris or moisture to be sprayed throughout the work area poses potential worker exposure issues (eyes, inhalation). Scattered contaminant on floors may produce slipping hazards. The equipment can produce intermittent, unpleasant noises. Vacuum Chamber Drying Vacuum drying is the process by which components are placed into a sealed chamber where a vacuum is pulled on the component to lower the water vapor point. The moisture becomes an aerosol in the chamber. A filtration system pumps the moisture from the air, returning dry air to the chamber to repeat the process. Ideal Application. Vacuum chamber drying is best suited to complex geometries and to porous metal components that must be completely dried, such as prior to epoxy resin impregnation. It is also useful for metals that must be completely dried to prevent corrosion. Misapplication. Where the above considerations are not a concern, vacuum drying may not be the most efficient or rapid approach. Advantages. Because high heat is not required, vacuum drying is less likely to damage temperature-sensitive components. Also, energy costs may be less for critical metal components. Disadvantages. Equipment costs are relatively high; with manual systems, component handling (loading) increases labor costs and process times. Drying Effectiveness. With proper equipment maintenance, the technique is very effective for most components. Drying Impact on Cleanliness. The process does not typically produce any contaminants. Equipment Menu. The primary equipment is a vacuum chamber of appropriate dimensions. Ancillary equipment includes hoists and semiautomatic systems. Associated Energy and Labor Requirements. The vacuum pump and the air circulation/moisture extraction system can require from fractional to 15 to 20 HP, depending on chamber volume. Labor costs must be included in considerations of manually loaded chambers. Safety and Environmental Issues. The chamber itself, even though under high vacuum, generally presents no immediate worker safety issues. Loading and unloading of the chamber must be done to minimize worker injury.

INTEGRATION OF DRYING SYSTEMS WITH CLEANING SYSTEMS Integration of drying with the aqueous cleaning process is determined, in part, by the type of system employed, in-line vs. batch cleaning. In conveyorized in-line cleaning, cleaning occurs in one zone, rinsing in a second zone, and drying in the last zone. Batch cleaning may be single zone, where all steps take place sequentially in a single chamber; or multiple zone batch, where cleaning, rinsing, and drying steps occur in separate chambers. Integration is also dependent on the level of drying required. Industrial drying demands only visual cleanliness. If it looks clean, then it must be. This level of drying is associated with a broad range of heavy industry manufacturing requirements. Precision cleaning is the standard in a wide array of manufacturing, from metals to electronics, where surface contaminants remaining after cleaning and drying are measured by weight, electrical conductivity, optical scanners, and other methods. Critical cleaning has the most stringent cleaning and drying needs. Products made with critical cleanliness standards are

© 2001 by CRC Press LLC

generally those of the highest-technology sector. Any of these drying levels can be achieved using any of the aforementioned methods of throughput. A single-zone batch cleaning/drying process can work for automotive machine parts as well as for medical fiber-optic components. The key to effective integration of cleaning and drying is the correct evaluation of user needs. This can best be done by starting at the end result (with cleanliness requirements and throughput) and working backward to develop the proper equipment menu. With today’s fast-paced changes in technology and production capacity, users must continuously revisit the process to validate that the method used today still meets current quality and production criteria. Increases in production and throughput, the ability to measure contaminants more accurately, and increasing understanding of the effect of contamination level on the quality of the end product all drive cleaning/drying integration. Unless a component is cleaned and dried in a bubble, where all elements are controlled to absolute values, the potential for contamination exists the moment the component enters a new environment. Critical cleaning applications are performed in clean rooms where the humidity level is controlled, yet humidity can increase just from people breathing, compromising the cleanliness of the part. Handling components through conveyorized processes or having air inadequately filtered from the compressed air or blower source compromises the cleaning method. Every engineer responsible for parts cleaning must ask the question, “Does the drying method maintain the results of the parts cleaning method?” If it does, then the integration of the drying and cleaning processes has been successful.

© 2001 by CRC Press LLC

CHAPTER 2.20

Liquid Displacement Drying Techniques Robert L. Polhamus, Steve R. Henly, and Phil Dale

CONTENTS Introduction Water-Soluble Displacement Vapor-Phase Process Dual-Vapor Drying Systems Liquid/Vapor Process Liquid Displacement Liquid More Dense Than Water Displacement Fluid with Surfactan Displacement Fluid with Alcohol Liquid Less Dense Than Water Summary

INTRODUCTION As the old expression goes, “The job ain’t over ‘til the paper work’s done.” This axiom can also be applied to cleaning. A great deal of effort has been expended this far in determining the appropriate cleaning chemistry and its suitability for the soil encountered and its environmental impact. In addition, the appropriate rinsing of the chemical has been discussed. Having fully analyzed and tested the cleaning and rinsing aspects, the method of drying the part needs equally dedicated review. It would appear obvious that if a simple cleaning chemistry can be used with a simple rinse system, an equally simple drying technique can be employed. Unfortunately, this is not always the case. When evaluating a potential drying technology, one must remember that the product is clean prior to entering the dryer, and the dryer can only recontaminate the product. It would be impractical to expend any amount of time in cleaning a part beyond the capabilities of the drying system. As a result, it is critical that the drying technology be matched to the application. Various drying technologies offer advantages while suffering from some disadvantages. Any of the three basic drying techniques: evaporative, mechanical displacement, and © 2001 by CRC Press LLC

liquid displacement can be optimized to provide the proper balance of technical performance and cost. As in the evaluation of the cleaning and rinsing chemical, once a thorough investigation has determined technical requirements of the application, several drying processes can be evaluated on their technical merit to meet the needs of the application. Having determined the acceptable drying technology, the equipment to implement the process is fairly self-evident and, subsequently, the cost is appropriate to the need. The use of liquid displacement dryers has historically centered around value-added cleaning applications. These applications are those in which a measurable cleaning requirement is established. In other words, there is a reason to clean the components that provides value to the product, whereas if they were not clean they would most likely not perform adequately in the final application. As more users become more definitive in their cleaning requirements, the need for precision drying opportunities will grow. In addition, more products are being designed and fabricated to support the development of small geometry and close tolerances. Once water is entrapped in these devices, it becomes increasingly difficult to remove it by any method. Although liquid displacement drying is most closely associated with water removal, it can also be used in areas of solvent cleaning. As with water-based cleaning, a multitank solvent system may have one or more chemicals that are good cleaning agents but, with low volatility, are difficult to evaporate. These products can undergo a rinsing phase with a more volatile chemical and, eventually, either the rinsing chemical or an additional solvent is evaporated from the surface, leaving the product surface dry. For the purposes of this discussion the removal of water from the substrate will be the subject, although these same principles can be applied to solvent cleaning systems. Two liquid displacement processes are commonly utilized. One process involves chemicals that are soluble with water. The second process uses chemicals that are insoluble with water, and these are characterized as either more or less dense than water. These processes will be described.

WATER-SOLUBLE DISPLACEMENT Water-soluble displacement drying is primarily accomplished through two methods. The predominant technique is vapor-phase using isopropyl alcohol (IPA). Another process uses an immersion technology in cold high-purity water with an alcohol vapor layer in the same process chamber. In either case, it is the molecular interaction between the two chemicals that makes the process successful. IPA and water are perfectly miscible materials. That is, they will dissolve or take each other into solution in a nearly unlimited capacity. After the drying process, the effluent from the system will be a homogeneous mixture of alcohol and water. As a result, the alcohol is considered “consumed” in the process and is inappropriate for further drying unless the IPA can be purified and returned to the system. There are a variety of techniques suitable for this purpose, and they must be evaluated relative to the potential payback based upon the application. With a few exceptions, all liquid displacement techniques utilize batch processing. The following process descriptions center on the batch concept. Vapor-Phase Process The vapor-phase process is the most widely used of the liquid displacement processes. The equipment is simple in nature and the process relies very heavily on the principles of © 2001 by CRC Press LLC

physical chemistry. The equipment configuration consists of a single chamber where the entire process is performed. The main features of the process chamber are heating elements, a vapor-containment cooling coil, a liquid effluent capture tray, a freeboard zone, and a robotic lift mechanism (Figure 1). The equipment must be prepared to induce and contain the process of vapor displacement. Liquid IPA is introduced into the process chamber to a preset level. Cooling water is circulated through the containment coils at an appropriate temperature and flow rate. If all conditions are acceptable, heat is applied to the liquid bath. The IPA is heated until it reaches its boiling point. IPA vapors are generated from the boiling liquid and begin to rise upward in the process chamber. Through a combination of condensation on the sidewalls and a vapor density greater than air, the vapors move slowly upward from the liquid level displacing air in a “plug flow.” The vapors will continue to rise upward in the chamber until they contact the containment coil. The containment coil will cause the vapors to condense on its surface, and as the vapors condense, more vapor is attracted to the coil to produce a continuous condensation action. Convection of the vapors toward the coil will produce a limit or ceiling on the height to which the vapors will rise. As the vapors condense, they are directed back downward along the sidewalls and returned to the liquid volume. Upon return, the liquid is available to be regenerated into vapor. This process continues indefinitely in the process-ready or idle mode until a drying cycle is initiated. Once the vapor zone has been fully established and stabilized, the unit is ready to process work. Prior to the implementation of a vapor-phase process, careful SLIDING LID

CONTAINMENT COIL

COMPONENT

CATCHMENT TRAY

IPA SUPPLY IPA

HEATER

WASTE IPA / WATER

Figure 1

Vapor-phase process.

© 2001 by CRC Press LLC

consideration should be given to product orientation and fixture design. Both factors must take into consideration the accessibility of the parts to the vapor and the ability of the parts to drain freely. The critical nature of these factors will become self-evident from further discussion of the process. The process contains three zones: the liquid level, vapor zone, and freeboard area. The liquid level is the amount of liquid IPA contained in the process chamber. In most cases this level is no more than a few inches deep. The second zone is called the vapor zone. This is the area from the top of the liquid to the top of the vapor blanket at the point where it is captured by the containment coil. The containment coil is located a specified distance from the top opening of the process chamber. The third zone is the area from the top of the vapor zone to the top of the process chamber, called the freeboard zone. This dimension should be greater than or equal to the height of the vapor zone. This freeboard plays a critical role in the containment of the vapors and the cost-effectiveness of the system. The work and fixture (load) are placed on the robotic hoist platform or suspended from the robot arm. The temperature of the load must be below the boiling point of IPA, and, in general, the cooler the better. Once loading is complete, the process is initiated and the load is lowered from the home position through the freeboard zone into the vapor zone. This begins the first phase of drying called the displacement phase. Once the load penetrates the vapor zone, it becomes a preferential condensation site for the vapors vs. the containment coil, and the vast majority of the vapor condenses on the load. The rate of condensation on the coil observed during the idle mode will show significant, if not complete, reduction upon insertion of the load. The IPA will condense and mix with the water on the surface of the load. As the IPA liquid accumulates on the surface, it will fall from the surface by gravity, taking dissolved water with it. In many cases, the water that is on the surface of the load is high-purity deionized water, which has a relatively high surface tension. The IPA has a relatively low surface tension so it will “wet” the surface and also displace water from the load. The liquid condensate and water fall from the product and are collected on a liquid effluent capture tray and directed to waste. Through the combination of solvation and displacement, all water is removed from the surface. After a period of time, based on water loading and vapor generation rate, the surface becomes water free. Even though the surface is water free, as long as the load is below the boiling point of the liquid, condensation will continue. The water layer on the parts has been displaced by a layer of IPA condensate. As the IPA condenses, it transfers energy to the load. Over time, the load temperature begins to rise. As the temperature approaches the boiling point of IPA, the rate of condensation slows and eventually almost stops. This is the second phase of the process and is referred to as the thermal equilibrium phase. This phase begins when the water is displaced from the surface and ends when the load reaches a near equilibrium with the vapor, where equal volumes of condensate and liquid are changing state on the surface. An indication that thermal equilibrium has been reached is when the condensate dripping from the containment coil has reached a rate equal to or close to the precycle idling rate. At this point in the process the load is covered with a microlayer of IPA liquid. Although the surface may appear to be dry, this microlayer exists on the surface in equilibrium with the vapor. To “dry” the surface, the load is removed from the vapor zone into the freeboard area. Once in the freeboard area, and even beginning during the transitional phase from the vapor zone into the freeboard area, the microlayer is allowed to flash-dry from the surface. The IPA vapors that flash from the surface are more dense than air and, if allowed to remain undisturbed, will fall back into the vapor zone. Residence time in the freeboard area will allow these vapors to be recaptured as well as allow the load to cool prior to removal © 2001 by CRC Press LLC

from the system. After the freeboard residence has timed out, the load is returned to the home position. The vapor-phase drying system is quite effective but, as can be implied from the process description, works very well on certain product configurations. Since the vapor phase provides little if any mechanical agitation, the process relies strictly on the affinity of IPA for water. If product or fixture configurations trap or hold water, the process is less effective. This is the most significant drawback to vapor phase and must be taken into consideration. Vapor phase does provide two significant advantages over its competitive liquid displacement techniques. First, the process is extremely simple. It contains limited moving parts and simpler component design. It utilizes the laws of physics to transport the chemical in contact to the load and, in the vapor phase, has the ability to provide equal contact to all surfaces simultaneously. The second significant advantage is that it can maintain chemical purity levels superior to other techniques. The process generates contaminant-free chemistry by the production of the vapor zone. By definition, the vapor that condenses into liquid must contain no nonvolatile constituents since all components of the vapor must be volatile to exist in the vapor zone. The liquid in the sump remains pure since any condensate that will be mixed with water is immediately removed from the process chamber after it has been captured on the liquid effluent capture tray. This process prevents any build-up of contamination from the influent alcohol or from the water carried into the dryer.

Dual-Vapor Drying Systems The drive to continually improve drying techniques and the ability to design processes for specific applications are never ending. Recently, a technique has been introduced that uses two immiscible fluids that create a constant-boiling blend vapor phase. This technique is referred to as the dual-vapor drying system. The chemicals chosen for this process have the commonality of being relatively high in volatility with similar boiling points and relative vapor pressures. However, in order for this process to be successful, the fluids must be essentially immiscible with each other. By utilizing one chemistry that is soluble with water and one that is not, the amount of chemical that can be returned directly to the system without contamination is increased, thus reducing overall chemical consumption while still providing effective drying. An additional advantage of this system is that if one chemical is chosen that is nonflammable, it can be used with a flammable solvent to make the combined vapor phase nonflammable. One such system on the market combines a perfluoronated chemical (PFC) with the conventional drying fluid IPA. The process utilized is very similar to conventional IPA vapor-phase drying discussed above with minor differences in chemical management (Figure 2). From a drying perspective, the parts that can be effectively dried and the duration of the process are essentially unchanged relative to conventional IPA drying. The significant difference between this process and conventional IPA drying is that less IPA is consumed in the process, the vapor is nonflammable, and gravimetric separation recovers the PFC for reuse in the process. A volume of liquid that is approximately 50% PFC and 50% IPA (v/v) is placed in the bottom of the process chamber. Since the liquids are immiscible and have a significant difference in density, the PFC will settle to the bottom and the IPA will form a layer on the top of the PFC. Heat is applied to the liquid. As the PFC liquid begins to boil, the PFC vapor must percolate through the IPA layer, heating the IPA, which although not boiling will © 2001 by CRC Press LLC

SEALED LID

CHILLED FREEBOARD

CONTAINMENT COIL

IPA / PFC VAPOR

COMPONENT SEPARATOR

CATCHMENT TRAY

IPA

PFC

HEATER

SETTLING TANK

Figure 2

Dual vapor process.

contribute vapor to the system. As previously described, the vapor is more dense than air and will displace all air out of the process zone to create a 100% vapor blanket above the liquid to the height of the condensing coil. The relative vapor pressures of each chemical contribute to a mixture of vapor that is approximately 50/50 by volume and is nonflammable. During idling conditions, the vapor will condense on the coil with the liquid condensate being directed back down the chamber walls to return to the boiling solution. Upon the introduction of a load, the vapor will preferentially condense on the parts to be dried. The liquid condensate will drip off the parts and be collected on a condensate trough described in the conventional IPA process. The condensation of both vapors will contribute heat to the substrate and, as the product approaches the temperature of the vapor, the process will eventually cease. Leaving the product in the vapor zone will eventually produce an equilibrium of vapor/condensate on the surface of the parts with its associated microlayer of liquid coating the parts. The liquid that condenses on the parts is captured by a saucer tray as described in the earlier discussion of IPA vapor phase. However, in this case the liquid will separate into two phases, a PFC and IPA/water mixture. This liquid effluent is sent to a holding tank where the liquid is allowed to settle and separate into two layers. The more dense PFC will settle to the bottom where it can be extracted and recirculated back into the process chamber. The IPA/water mixture is decanted off for reclamation or disposal. Since the PFC is not chilled, it retains much of its thermal content and is returned to the process chamber at elevated temperatures ready to begin the vapor cycle again. This conserves heat energy by not extracting the thermal value as with the waste IPA/water effluent. Since IPA is consumed in the process, the level of IPA in the tank will be reduced as a function of process load volume. A special system of liquid level floats is included in the © 2001 by CRC Press LLC

design to make up the IPA volume continually from a sealed reservoir. By maintaining a relatively consistent ratio of IPA to PFC in the liquid phase, the composition of the vapor will remain constant, providing reproducible results. In a conventional IPA process, the parts are removed from the vapor zone into the freeboard area above the condensing coil where final flash evaporation takes place. After a relatively short time, the parts are removed from the system. Any residual IPA that is left on the parts or that is distributed into the atmosphere around the unit is of a sufficiently low concentration to avoid any operator exposure or flammability concerns. A significant difference between the conventional and dual-vapor process is that the cost of the PFC chemical is such that even minor losses will have cost impact on the process. The dual-vapor system is designed with a chilled freeboard to increase solvent retention. Above the primary condensing coil is a refrigerated plate operating at a temperature below the freezing point of water. This low temperature over a boiling environment creates a temperature inversion producing a dense layer of cold air above the vapor. The parts are raised from the vapor zone into the freeboard, where the vapor pressure of either constituent is essentially zero. As the condensate layer on the parts is flashed off the surface, it is immediately cooled and, as its density is significantly higher than air, it will fall back into the vapor zone for recapture. The refrigerated freeboard area works to reduce the amount of the expensive PFC that will exit the system as well as to reduce significantly the amount of IPA evolved from the system. Although the cost consideration is not as critical for the IPA, it is a volatile organic compound (VOC) and is tightly regulated by many federal, state, and local environmental laws. The design of this system is such as to produce very low solvent emissions. The unit can also be supplied with a lid that seals during the process to lower solvent emissions even further. In addition to lowering overall vaporous emissions, the dual-phase system reduces the amount of chemical consumed in the process. In the conventional IPA dryer, the entire vapor zone is composed of IPA and it condenses on the parts, which solvates and displaces water from the surface. The volume of liquid IPA generated during the vapor-phase drying process is contaminated with water. In some cases, the water content will be a little as 1 to 2% or as high as 12 to 15%. In either case, the IPA is unusable in this state and must be extracted from the system for disposal or reclamation. The difficulty of removing IPA from the water is exacerbated because IPA and water will form an azeotrope that makes simple, single-plate distillation inappropriate to provide solution suitable for use. Multiplate distillation or membrane separation is usually required, often with significant cost impact. The dual-vapor system reduces the amount of water-laden IPA waste by simply making less IPA available to be contaminated. Since the vapor phase is roughly 50/50 PFC to IPA, and water is immiscible in PFC, all of the water removed must be contained in the liquid IPA effluent. This automatically doubles the water loading in the IPA by halving the volume condensed. The major advantage of this process is the ability to use IPA efficiently as the drying fluid, which has established itself as a product of choice. The long history of IPA in this application provides confidence in the performance, whereas this technique eliminates some of the negative aspects. By providing a vapor phase that is nonflammable the safety aspects of using IPA are addressed. Also, since the system has emission control technology targeted at the maximum level of solvent retention, it reduces the potential environmental impact from using IPA in communities sensitive to VOC emissions. In addition, the total amount of chemical consumed in the process can be reduced through effective recirculation of the PFC solvent. One drawback to this process is the potential of increased cost to add the solvent retention technology. Another potential drawback may be in extended process times © 2001 by CRC Press LLC

necessitated by increased residence time in the chilled freeboard to accommodate solvent retention criteria. Liquid/Vapor Process A second process that uses a combination of liquid immersion followed by a vapor deposition is called the “Marangoni” process. This process is generally used following a high-purity deionized (DI) water final rinse stage. The principle for displacement differs significantly from the previously described technique (Figure 3). The fundamental principle for displacement of the water relies on the differential surface tension between the DI water and the IPA. Following a high-purity rinse stage, the substrates are immersed in a bath of DI water at ambient or slightly above ambient temperature. The substrates are slowly removed from the water either by gently raising the substrates from the bath or, as more generally used in production, slowly draining the water from the tank. At this point, the substrates are usually removed from the carrier through a lift mechanism that suspends them in the liquid bath providing minimal contact points. The slow withdrawal of the DI water creates a sheeting effect on the surface to strip away any residue. A meniscus is formed that trails the liquid withdrawal. As the liquid level is being lowered, IPA vapor is introduced into the system. Some of the vapor coats the exposed surface as some of it is absorbed into the surface layer of the DI water. At this point, an intriguing phenomenon occurs. A discovery by Lord Kelvin’s brother, James Thompson, identified a reaction in fluids of dissimilar surface tensions as related to a teardrop effect witnessed on vessels containing alcoholic beverages. He identified this in

N2

N2

IPA VAPOR

COMPONENT

DI IMMERSION

Figure 3

Marangoni process.

© 2001 by CRC Press LLC

a paper published in 1855. Subsequent investigations by a fluid dynamics investigator, Marangoni, provided the theory and the name for the process. As the IPA is absorbed into the microlayer of liquid on the surface of the substrate, it lowers the surface tension relative to the bulk liquid in the process tank. The physics of the two dissimilar surface tensions drive the system to equilibrium by forcing the low-surfacetension liquid toward the liquid of higher surface tension. This produces a flow that essentially strips the water layer from the substrate. The bulk liquid is continually purged with pure DI water to maintain its high surface tension. Once the substrate has been completely removed from the liquid, a purge of drying atmosphere, usually nitrogen gas, is admitted to the system and any residual IPA that may be on the surface of the substrate is evaporated. Small amounts of IPA are consumed in the process as it is absorbed into the bulk liquid or evaporated. The DI water must be completely drained from the system and refilled between cycles since it has become contaminated with the IPA. Fixturing, substrate topography, and fluid motion are all critical to the success of this process. In most commercial systems, the substrate is removed from the carrier during the water-removal phase. This is a result of requiring a smooth flow of liquid across the substrate surface, which could be impaired or negated by contact points or irregular configurations in the carrier. The product surface must also be extremely smooth to guarantee successful water removal. Finally, the motion of the water across the surface must be controlled to eliminate any turbulence. Agitation at the interface of the IPA-rich layer and bulk liquid will result in a disruption of the surface tension differential and negate the effects of the Marangoni principle. The technical requirements stated above make the product configuration much more complex than other dryer designs. More moving parts and the possibility of disturbances in the flow of fluid across the surface make the system less forgiving in its operation. The process also consumes large volumes of DI water since the entire tank volume must be exchanged after each cycle. Even though there is only a small amount of IPA dissolved in the liquid, it is quite difficult to recover in situ and must either be thrown out or recycled. The major advantage of Marangoni over other principles is that it essentially uses water to dry itself. The small amount of alcohol used is fairly insignificant from a chemical consumption perspective. Also, since the process takes place at room temperature or slightly higher, and the water is removed from the surface without evaporation, the results should be spot-free drying and minimal energy consumption.

LIQUID DISPLACEMENT The previously discussed techniques relied on the solubility of water with the drying chemical (IPA) for success.As was mentioned, the solubility of water in alcohol enhanced the removal process, but rendered the IPA unsuitable for reuse in the system. There exist several chemicals that can successfully remove water from the surface without being soluble. The advantage of these chemicals is that they will reject the water down to part per million levels allowing the drying chemistry to be recycled in the system. Closed-loop recycling of the chemical will significantly reduce the waste stream volume and enhance system economics. The chemicals that will dry while rejecting the water are also highly volatile and can be recirculated in the system through distillation. Although these various chemicals use the same principles described below, they differ significantly in their behavior with the water once it is removed. Two types of chemicals are used in the liquid displacement technique: one is more dense than water and the other is less dense. Both techniques will be discussed. © 2001 by CRC Press LLC

Liquid More Dense Than Water Fluids more dense than water have been used for many years in industrial applications. These products were based on chlorofluorocarbon (CFC) chemicals. Two methods dominated the process. One process involved using a CFC with a surfactant; the other blended the CFC with alcohol. The chemical management within the system was slightly different and so was the process. As a result of the terms of the Montreal Protocol, the CFC chemistries were banned worldwide for use in this application. Several chemical companies have developed replacement chemistries for this process. These chemicals are hydrofluorocarbons (HFCs), hydrofluoroethers (HFEs), and some brominated hydrocarbons. All of these chemicals are being developed with surfactant additives to assist in the drying process. These products are also being blended with alcohols to replace CFC products in nonsurfactant drying applications. Displacement Fluid with Surfactant Unlike the previously discussed techniques that used vapor-phase alcohol, the liquid displacement technique requires a minimum of two process tanks. One process tank contains the drying chemical and the surfactant, while the second chamber contains pure drying chemical distillate. The chemical management is strikingly similar to the conventional solvent cleaning system, and this makes sense if water is considered a contaminant. In the boil sump, usually situated on the left-hand side of the equipment (Figure 4), the drying chemical and surfactant are blended. This mixture is boiled to create a vapor as described in previous techniques. The surfactant is nonvolatile or of extremely low vapor pressure at the boiling point of the drying chemical to contribute little if any vapor. The vapor is condensed on a containment coil and the liquid returned to the right-hand immersion

DRYING FLUID SURFACTANT / DRYING FLUID

BOIL SUMP

Figure 4

IMMERSION SUMP

More dense than water—surfactant/displacement fluid.

© 2001 by CRC Press LLC

sump. This pure distillate stream fills the immersion sump and overflows a weir to return to the boil sump. The drying process takes place in a manner similar to conventional solvent cleaning. The product is lowered into the unit over the boil sump. As it enters the vapor zone, solvent vapors condense on the part and begin to displace the water on the surface. This liquid falls from the surface into the boil sump. The product continues to be introduced into the equipment until it is fully immersed in the boil sump. When the product is fully immersed, the combination of the drying chemical and surfactant displaces the water from the surface, including blind holes and other areas where water may be trapped. The water that is displaced rises to the surface of the sump since it is insoluble and less dense than the drying fluid. The water displaced from the product will accumulate at the surface of the boil sump. To prevent recontamination of the surface of the parts, the surface of the fluid is continually purged by a sparging mechanism to skim the surface layer off the sump and force it into a water separator. The water separator is a series of baffled chambers that will allow the water to separate from the drying fluid and surfactant mixture for removal from the system. The pure drying fluid/surfactant mixture is returned to the boil sump through the sparging system to continue the process. The water is decanted from one of the baffled chambers and sent to drain or collected for proper disposal. After the product has been removed from the boil sump and suspended in the vapor zone to allow bulk liquid to drip off the product, the parts are transferred to the pure distillate immersion sump. The parts are coated with a microlayer of drying fluid/surfactant. In the immersion sump, the surfactant is removed from the surface by its solvency in the drying fluid. The mechanism in this sump will be to remove all surfactant from the surface, disperse it throughout the bath, and redeposit it on the surface at the equilibrium concentration of the mixture in the bath. This fluid is continually returned to the boil sump by overflow of the weir, which returns the surfactant to the boil sump for reuse. Since the surfactant is essentially nonvolatile, the concentration in the system will remain fairly constant across a long period of time and will produce consistent performance. After a specified period of time, the parts are removed from the immersion sump and suspended in the vapor zone above the immersion sump. The temperature of the immersion sump is below the boiling point of the drying fluid. While the surfactant rinse is occurring, the parts cool down. When they are suspended in the vapor zone after the surfactant removal, they will experience a final distillate rinse. Pure distillate from the vapor zone will condense on the parts until they reach the microlayer of thermal equilibrium, previously described, and the residual layer will be flashed off in the freeboard area for the final “dry” cycle. Water-free and solvent-free parts are removed from the system. This process offers several advantages over the previously described techniques. The primary advantage is its ability to dry complex geometries by going into full immersion while maintaining chemical purity through the ability of the drying fluids to reject the water for rapid recycling. Another advantage is that it performs at lower temperatures than vapor-phase alcohol dryers. It also has lower chemical consumption than the previously described techniques. The major disadvantage of the system is that the use of a surfactant can raise the suspicion of residue on the product after drying. Although this can usually be addressed by multiple rinse sumps, it could be considered unacceptable in some applications. Other disadvantages are the relatively large footprint of the equipment and the cost of the drying fluid. The drying fluids are quite expensive relative to alcohol and require sophisticated equipment design features to minimize emissions. © 2001 by CRC Press LLC

Displacement Fluid with Alcohol The fluids discussed above are also being blended with alcohol to eliminate the need for the surfactant. In this process alcohol is blended with the drying fluid in the boil sump. The formulation is generally 10% IPA and 90% drying fluid. Although the alcohol is soluble in the drying fluid, it does not form an azeotrope at that concentration but will form an azeotrope at a very low percentage as it is boiled and generated into a vapor. The vapor generated from the boil sump will form an azeotrope with the alcohol and, in a manner described earlier, will be condensed on a cooling coil and sent to an immersion sump (Figure 5). The parts to be dried are immersed in the immersion sump. Here, the small amount of alcohol with the high-density drying fluid will combine to remove the water from the surface. The alcohol is preferentially soluble in the water and produces a phase separation creating an insoluble layer of alcohol and water. This liquid layer is less dense than the drying fluid and therefore rises to the top of the sump as previously described in the surfactant system. The surface is sparged to force the insoluble layer into the water separator. The water/alcohol layer is separated from the drying fluid and the drying fluid is returned to the boil sump via the sparging system. Since no surfactants are used and all constituent chemicals are volatile, an immersion rinse after immersion in the drying sump is not required. The drying cycle is essentially identical to the vapor-phase drying cycle described earlier. The chemical composition of the vapor is an azeotrope of the drying fluid/alcohol and will condense on the parts and heat the substrate until condensation ceases. This process will tend to displace any liquid dragout from the water removal step and leave the product free of water and liquid solvent. The major advantage of this process is that it is able to use alcohol as a drying fluid without the concern for flammability while also being able to reject the water rapidly from the system. Although this process maintains the advantageous aspect of immersion drying, it produces a waste stream of alcohol and water not generated in the surfactant process. It

SPARGER ALCOHOL / DISPLACEMENT FLUID AZEOTROPE ALCOHOL / DISPLACEMENT FLUID BLEND

BOILING FLUID

WATER SEPARATOR

IMMERSION SUMP

Figure 5

More dense than water—alcohol/displacement fluid.

© 2001 by CRC Press LLC

does eliminate the concern of surfactant residue but, since alcohol is consumed in the process, requires that the alcohol level in the boil sump be monitored and adjusted to produce consistent drying performance. Another aspect of this technique is that the top layer of the immersion must be adequately purged to prevent the redeposition of water from this layer. If this is a critical concern, an additional sump may be used for safety. Liquid Less Dense Than Water Liquid displacement with fluids less dense than water and insoluble with water has been used for many years. The established techniques usually utilize one of an assortment of hydrocarbons that displaces the water from the surface and then is air-dried. Many of these techniques are not able to produce residue-free surfaces because of nonvolatile fractions in the hydrocarbon. With water displacing oils, a significant fraction of the solution is nonvolatile with the intended purpose of leaving behind a residual material to protect the surface from oxidation or other aspects of contamination. These dryers are usually quite simple in design, generally consisting of a single immersion tank (Figure 6). The process is also simple. The parts to be dewatered are placed in the immersion bath and usually suspended off the bottom with fixturing or a work rest. Some form of mechanical energy is applied, usually vertical agitation in the fluid, air agitation, or ultrasonics. The water is displaced from the surface, and since it is more dense than the displacement fluid, it sinks to the bottom of the tank. The bottom of the tank below the work rest or below the suspended parts is usually necked down to produce a funnel-shaped section where the water is collected. The water level in the tank can be observed through the use of a sight glass or other methods. When the water accumulates to a proper level, it can be drained from the system off the bottom of the tank. When it is drained, a small layer should be left in the tank to prevent draining of any hydrocarbon into the water effluent. The advantage of this technology is its simplicity both in process and design. The equipment is very inexpensive as are the displacement fluids. It has a disadvantage in that

U/SONIC

U/SONIC

DISPLACING OIL

WASTE WATER

Figure 6

Less dense than water—oil displacement.

© 2001 by CRC Press LLC

its ability to remove all water totally from the surface is suspect since it relies strictly on unenhanced chemical incompatibility. Without affecting the substrate affinity for water, it cannot prevent redeposition of water should the opportunity arise. Displacement chemicals that are volatile, less dense than water, and insoluble in water offer an additional drying option. Much of the development work in this area has centered around volatile methyl siloxane (VMS) fluids. These chemicals are insoluble in water and are combined with a surfactant to displace water from the surface of the substrate. The development of this technology is relatively recent and is still being tested to optimize its performance. The process for these chemicals differs from the more-dense-than water technique. Although the process still uses a vapor-phase drying step, the equipment is considerably different from the systems described above (Figure 7). The equipment is a two-sump design, but the chambers are isolated in that they do not share a common vapor zone. The primary water displacement takes place in a chamber using room-temperature or slightly above ambient drying fluid. The parts are immersed in a bath of displacement fluid and surfactant. The water is removed from the surface, and as it is more dense than the drying fluid, sinks to the bottom of the liquid bath. After a specified period of time, the parts are removed from the liquid bath and suspended above the fluid. The parts are coated with the liquid from the immersion sump, which is a combination of displacement chemistry and surfactant. The bulk liquid is allowed to drip off the parts. The parts are then sprayed with recycled displacement fluid, which will help to knock off any of the dragout layer coating the parts. After the spray has timed out, the parts are removed from the dewatering chamber and transferred to the drying chamber. During the dewatering cycle, the liquid immersion sump is continually recycled through a water separation system that strips the water from the solution and returns it to the chamber. This material is also used as the source for the spray following the immersion dewatering.

CONTAINMENT COIL

U/SONIC

U/SONIC

SPRAY BARS C/W FAN NOZZLES

HEATER

DISPLACING FLUID

WATER SEPARATOR TANK "1" DISPLACEMENT FLUID / SURFACTANT

Figure 7

TANK "2" VAPOR RINSE

Less dense than water—surfactant/displacement fluid.

© 2001 by CRC Press LLC

The drying chamber is a single-sump design very similar to the vapor-phase drying system discussed previously, and it essentially performs the same process. In this chamber the displacement chemical without surfactant is boiled to create a vapor. The vapor is contained with a cooling coil to define a vapor zone, and the cooling coils are located at a depth in the chamber to provide acceptable freeboard. The vapor that condenses on the cooling coil is directed downward along the side of the chamber walls to be returned to the liquid level to complete the distillation cycle. The displacement fluid-wet parts from the dewatering step are introduced into the drying chamber. They are immersed into the vapor zone and the displacement chemical condenses on the surface. This liquid condensate flushes off the displacement fluid and the liquid falls into the liquid level. The vapors continue to condense on the parts until the parts approach the boiling point of the displacement chemical, completing the thermal equilibrium phase of the process. After thermal equilibrium is reached, the parts are elevated into the freeboard area where the residual microlayer of displacement chemistry is flashed off. The product can be removed from the system free of water and displacement chemical. During the vapor rinse phase the vapor condenses on the parts and removes the displacement fluid, which falls into the sump. This liquid contains displacement fluid and surfactant as well as the pure distillate from the vapor zone. The surfactant is nonvolatile so as the solution boils it does not contribute to the vapor zone, and as a result the vapor zone remains pure regardless of how much product is processed. The introduction of wet parts into the drying chamber, and the collection of the displaced fluid in the liquid level, result in a net increase in liquid volume in the drying chamber. Liquid level floats monitor the level in the chamber, and periodically liquid from the sump is returned to the dewatering chamber. This fluid contains the displacement chemistry as well as surfactant that has accumulated in the drying chamber. This action returns the surfactant to the dewaterng chamber to prolong its useful life. The most significant advantage of this process is that the water leaves the surface and falls to the bottom of the tank. Unlike the more-dense-than-water technique where the water rises, this process eliminates the necessity to remove dewatered product through a layer of liquid that may contain some water. The process is suited to complex geometries because of its immersion step. These chemicals are also more moderately priced than the more-dense-than-water products and have essentially no adverse environmental impact associated with global warming or ozone depletion. The major drawback of this system is that the chemicals are flammable; however, they are not VOCs which may make them acceptable in specific environmental situations. The equipment also has a relatively large footprint. SUMMARY The attempt in this limited evaluation is not to make inference that all liquid drying techniques are applicable across the board, but merely to show that the availability of drying techniques is almost as diverse as the applications they address. In any case, it is the responsibility of users to provide the necessary insight into cleanliness standards, evaluation criteria, costs, environmental impact, and other specific considerations relative to particular requirements. They will utilize this information in collaboration with the chemical and equipment suppliers to develop their appropriate process. As it is with all things, however, change will occur and any decisions made today must be considered tentative. Careful consideration should also be given to future needs and changes in technology so an optimized position can be established that satisfies today’s needs while being flexible enough to accommodate tomorrow’s. © 2001 by CRC Press LLC

SECTION 3

Contamination Control, Analytical Techniques, and Compatibility

© 2001 by CRC Press LLC

CHAPTER 3.1

How Clean Is Clean? Measuring Surface Cleanliness and Defining Acceptable Level of Cleanliness Mantosh K. Chawla

CONTENTS Introduction Definitions Types of Contamination Why Monitor Cleanliness? Issues Affecting the Selection of a Cleanliness Measurement Method Types of Cleanliness Measurement Methods Indirect Methods Direct Methods Analytical Methods Most Common Verification/Measurement Methods and Their Principles of Operation Indirect Methods Nonvolatile Residue Ultraviolet Spectroscopy Optical Particle Counter Direct Methods Magnified Visual Inspection Black Light Water Break Test Contact Angle Optically Stimulated Electron Emission Total Organic Carbon Measurement and Analysis of Surfaces by Evaporative Analysis Analytical Methods Cost Impact of Cleanliness Levels Methods for Defining Acceptable (Optimum) Level of Cleanliness

© 2001 by CRC Press LLC

Controlled Experiment Production Testing In-Process or Online Surface Cleanliness Monitoring Summary References

INTRODUCTION This section focuses primarily on surface contamination/cleanliness, as opposed to airborne or other type of contamination. Specifically, this section will cover: • Brief descriptions of the types of contamination encountered, various techniques for verifying/measuring surface cleanliness, their strengths, weaknesses, skill level required to operate, and approximate price for available instruments. In addition, most common analytical techniques are also listed. • Ways to establish an acceptable level of cleanliness that can become a basis for evaluating alternative cleaning processes, optimizing existing cleaning processes and ongoing monitoring of cleaning process to assure that acceptable level of cleanliness is achieved.

DEFINITIONS The following list of definitions has been included to facilitate subsequent discussion and convey a consistent understanding of the information presented in this chapter. Analytical methods: Any technique that gives information about the type/species and level of contamination in relative or absolute terms. Contamination: Molecular and particulate surface material that has the potential to degrade the appearance or performance of a part, component, or assembly. Direct methods: Any technique that gives a direct and relative measure of some surface characteristic that relates to the level of surface cleanliness. Indirect methods: Any technique that gives an indirect indication of the level of surface cleanliness. Molecular contamination: Nonparticulate contaminant material (film) without definite dimension; volatile species that may be physically or chemically absorbed on surfaces. This includes corrosive and noncorrosive films resulting from oil, greases, chemical residues, fingerprints, heat and vacuum applications, chemical action, and incompatible materials, such as films from outgassing. Nonvolatile residue: Soluble or suspended material and insoluble particulate matter remaining after controlled evaporation of a filtered volatile liquid. Optimum cleanliness Level: A level of cleanliness that minimizes the total cost of cleaning and cost of nonconformance/failures due to poor surface cleanliness. Particulate contamination: Contaminant material with observable length, width, and thickness. In practice an observable size will be about 0.1 m. © 2001 by CRC Press LLC

TYPES OF CONTAMINATION There are several types of contamination that can be present on the part surface that may be undesirable for product performance. Some of the common types of contaminants are listed below. Particle Contaminants. Contamination present in the form of foreign particles on the surface, such as dust, hair, fibers, metallic microfragments. Thin-Film Contaminants. Contamination present in the form of a thin film on the surface. This type of contamination includes both organic and inorganic thin-film contamination, such as skin oil, greases, processing fluids, surfactant/chemical residues, rinsing residues, oxides, and other unwanted thin films on surfaces. Microbial Contaminants. Contamination present in the form of microbes on the surface, such as spores, bacilli, etc. There are other types of contaminants, such as radioactive, heavy metal, etc. Discussion of these types of contaminants is beyond the scope of this chapter. The primary focus of this chapter is thin-film contaminants.

WHY MONITOR CLEANLINESS? Presence of contamination can degrade the performance of parts, components, and systems, and result in nonconformance and, in the worst case, product failure. Molecular contamination of surfaces can drastically affect the performance of the parts. Thin-film contamination on surfaces can result from inadequate or incomplete cleaning methods, from oxide growth during the time between cleaning and performing the next operation, or from failure to protect cleaned surfaces properly from oxide growth during the time between clearing and performance of the next operation, or from failure to protect cleaned surfaces properly from oils, greases, fingerprints, release agents, or deposition of facility airborne molecules generated by adjacent manufacturing or processing operation. Cleaning is part of many manufacturing operations. Parts may require cleaning before they can be electroplated or painted, before they can be soldered, or before they can be packaged and shipped for end use. Thus, cleaning is necessary for various reasons to assure desirable product appearance or performance. Since there is a need for cleaning parts for various reasons, it makes sense to monitor cleanliness on an ongoing basis to assure consistent cleaning and part performance. In most cases, control of cleaning processes is achieved by specifying the operating parameters of the cleaning process, e.g., chemical concentration, temperature, water pressure, or the amount of time the parts are washed or rinsed. This approach defines how “clean” a part should be by specifying the process used to do the cleaning (i.e., dip Part A in Cleaning Solution B at Temperature C for X minutes), without regularly checking how clean parts actually are. This approach takes advantage of knowledge gained through experience with the cleaning process or through measurements taken during initial testing of the cleaning process. This method, while practical and good most of the times, cannot be consistently relied upon for precision cleaning. This type of procedure generally also specifies the properties of the cleaning solution and replenishment of the chemicals on a periodic basis. This approach does not take into account the number of parts that may go through the process in a given period of time. The more parts go through the cleaning process, the more contamination is removed from the parts, which is mixed in the cleaning solution. It also does not take into account the amount of contamination present on each part. The type and amount of contamination on each part varies from time to time and from © 2001 by CRC Press LLC

vendor to vendor. Thus, this approach to assuring cleanliness works only if the average number of parts and the average level of contamination on each part are consistent during a given period of time. If this condition is not achieved, the part cleanliness level will deteriorate below the acceptable level. Without the use of a surface cleanliness monitoring method, the lower level of contamination will not be detected until there are problems downstream. Hence, in many cases, it is more effective for a level of cleanliness to be specified and to have that level checked by measuring cleanliness on a portion or all of the parts. This is especially true in precision cleaning applications. Another approach used is to monitor the contamination levels in the cleaning solution to determine when the solution needs to be replaced. If a level of cleanliness is to be specified, then a method of verification must be specified at the same time for that level to have meaning. This generally leads to the question, what do we mean by “clean”? How clean is clean? Even so-called “clean” parts have certain amount of contamination, even if it is at a microscopic level. With advances in technology, more and more applications are moving toward the need for a higher level of cleaning. Increasingly, precision cleaning cannot afford to rely on old methods of verifying or assuring cleanliness. Measuring cleanliness not only helps ensure product quality; it is an essential part of implementing pollution prevention approaches related to cleaning. Cleanliness measurement/verification methods can be utilized to (1) evaluate performance of any existing or alternative cleaning process, (2) optimize the cleaning process by analyzing parts during initial implementation of a new cleaning process, or (3) determine if better parts handling or other innovation may allow the cleaning process to be eliminated entirely. In addition, measuring cleanliness often prevents pollution by reducing rejects. To specify a desired level of cleanliness, it is important to specify a method of measuring surface cleanliness that will help in assuring the desired level of cleanliness. Once a method for measuring cleanliness has been selected, it can be used to establish the level of cleanliness achieved by any existing process. This level of cleanliness can be used as a benchmark to make changes to existing process to see if those changes improve the achieved level of cleanliness. It can also be used to evaluate alternative cleaning processes prior to implementation.

ISSUES AFFECTING THE SELECTION OF A CLEANLINESS MEASUREMENT METHOD There are a wide variety of cleanliness measurement methods. To determine which method is right for a given application, many issues must be considered. Some of the issues that affect the choice of method are as follows: • Type of contaminants to be monitored—The method selected must be able to detect the contaminants of interest. For example, some methods will detect only organic contamination and not inorganic contamination. If inorganic contamination is of concern, then such methods would not help. Some methods detect only a certain type of contaminants. Such methods would be good if only certain types of contaminants are always expected to be on the surface. In general, it is better to have a method that can detect both organic and inorganic types of contamination and not be restricted to a certain type of contamination. This helps to assure surface cleanliness, even if there is a change in any aspect of the production upstream. © 2001 by CRC Press LLC

• Type of substrate being checked—If the part is being inspected directly, then the method must be compatible with the material being measured, without causing any damage. For example, certain cleanliness measuring methods deposit some type of “measuring media” on the surface to measure the cleanliness. Care should be taken to make sure that the measuring media deposited on the surface is not going to affect the surface of the part. Care must also be taken to make sure that measuring media do not contaminate the part surface. • Level of cleanliness that must be measured—The method must be able to detect the contaminants at the minimum and maximum level of interest. Each measurement method has a certain range of detection, and in most cases the minimum level of contaminant that can be detected is important for precision cleaning applications. • Accuracy and precision required—(i.e., how critical is it that the parts are cleaned to narrow specifications?)—Some methods provide gross estimates of contamination, even if they can detect contamination at very low levels, while others provide very precise concentration data for evaluation. The method selected must be appropriate for the application. • Features of the measurement method—Some methods have certain features that may or may not be desirable. For example, some methods have to contact the surface or deposit something on the surface to make a measurement. It may be desirable not to contact the surface or deposit anything on the surface. Whether the method is noncontact, nondestructive, and/or noninvasive should be considered in selecting the right method. • Speed of measurements—In most cases, it is not necessary to inspect every part. A representative sample at preset intervals is generally sufficient to track the performance of the cleaning process over time. Hence, the number of measurements that each method can complete per unit of time becomes important in selecting the right method. The method selected must be able to make analyses/measurements at the desired rate. • Acquisition and operating costs—The more precise and automated measurement methods tend to be very expensive. In addition to acquisition cost, the operating costs, such as cost of any disposable supplies or costs of required operating skill, must also be considered. The total cost/benefit of the measurement method must be evaluated. • Skill level required—The required skill level to utilize the technique and interpret the results varies a great deal among various methods available. The ongoing cost of operating is higher for the more-sophisticated techniques, particularly analytical techniques. For a given cleaning process, it may be possible that more than one method is required to verify/measure all of the parameters of interest. There are many measurement methods that can be used to evaluate cleanliness in a manufacturing environment.

TYPES OF CLEANLINESS MEASUREMENT METHODS The wide range of verification/measurement/analytical methods available can be differentiated in many ways. One simple way to divide these methods is their mode of operation and the type of measurement yielded. Following is the classification of various techniques based on these criteria. © 2001 by CRC Press LLC

Indirect Methods Most indirect methods of cleanliness measurement depend on a solvent of some type to dissolve any contaminants left on the part and the solvent is then analyzed for contamination. This requires that the solvent used be stronger than the solvent that was originally used in the cleaning to remove any residual the original cleaning solvent was not able to remove. Historically, these methods use solvents that are the type many manufacturers are trying to eliminate from their cleaning processes. Recently, more environmentally benign alternatives have begun to be evaluated for this class of measurement methods. Indirect methods that use solvents to extract contamination are usually only practical for small parts because of the large volume of extraction solvent that would be needed for larger parts. Still, this method can analyze larger parts compared with some direct methods such as contact angle where very small parts must actually be able to fit in the equipment. Also, when extraction is used, none of the geometric limitations exists as they do for contact angle and some other direct methods.

Direct Methods Direct methods actually measure cleanliness on the part of interest by analyzing the surface of the part directly. Direct methods, therefore, avoid many of the problems inherent in collecting contaminants off the part to be analyzed indirectly. However, since the part is being analyzed directly, there may be a limitation on the size or geometry of the parts that can be checked with some direct measurement equipment. It is preferred, wherever possible, to measure the cleanliness of the part directly since the surface of the part is the one that is of direct interest.

Analytical Methods Analytical methods are those that analyze the part surface or small piece of the part surface by studying the species of contaminants on the surface. Although these techniques are often direct techniques, they may also be indirect; and they are classified separately because of their ability to identify the type of contaminants on the surface. Generally this type of technique utilizes high vacuum for operation, provides information about the type of contaminant on the surface, which cannot be provided by any of the indirect or direct methods. These systems are generally laboratory types of instruments and cannot be used on the shop floor/production area. The cost of this type of equipment is very high, generally in the range of $60,000 to high six figures. A very high skill level is required to operate the system and interpret the results. These techniques are very powerful and very useful in determining the type of contaminants present on the surface. The knowledge of the type of contaminants on the surface is very useful in locating and possibly eliminating the source of contamination.

MOST COMMON VERIFICATION / MEASUREMENT METHODS AND THEIR PRINCIPLES OF OPERATION Some of the most common indirect, direct, and analytical methods, with a brief discussion of their principles of operation, are presented below. © 2001 by CRC Press LLC

Indirect Methods Nonvolatile Residue This method, which is a gravimetric measurement, requires a highly sensitive balance that can weigh parts to an accuracy of plus or minus 1 mg, or better. The nonvolatile residue (NVR) method uses a volatile chemical, such as trichloroethylene, to flush the part. There are two ways to determine the level of contamination. One is to weigh the parts after cleaning. Use a strong cleaning agent to flush the part and weigh the part again after it is dry. The difference between the initial weight and postflushing weight is attributed to the residual contamination that was left on the part by the regular cleaning process. If there is no difference in these two weights, the part is considered clean. This approach is good for small parts. If the area of the part is known, then contamination/cleanliness level can be specified in some weight per unit of area. Another way, which is particularly good for large parts or surfaces, requires a container for collecting flushed fluids and a volatile chemical to flush the surface. The technique involves weighing the container, flushing a portion of the surface with volatile chemicals, preferably a known area, i.e., square foot, and collecting the flushed fluid in the container. After the volatile chemicals have evaporated, the container is weighed again. The difference in the weight of the container before and after collecting the flushed fluids is considered the amount of contamination left on the surface by the regular cleaning process. Once again, the level of cleanliness can be specified in weight per unit area, e.g., mg/ft2, etc. This is a good gross measurement method when extremely high accuracy is not required. A small laboratory is needed to conduct these tests. Ultraviolet Spectroscopy This method has been used to measure flux residue left on printed circuit boards in the electronics industry, and has also been adapted to detect oils and greases on metal parts. This method requires the use of extraction equipment and an ultraviolet (UV) spectrometer, which are moderately expensive. In addition, the method requires that the contaminant to be analyzed has a unique absorption wavelength that can be identified in the UV spectrum. A calibration curve then is created by measuring samples of the solvent containing known concentrations of the contaminant at the unique wavelength. The method is only usable in the concentration ranges where the calibration curve is straight. Parts that are to be analyzed are extracted in a known amount of solvent to remove any of the contaminant. Typically, agitation or sonication is required during the extraction, which must be done in the same manner for each sample for the results to be meaningful. The solvent extract is then analyzed in the UV spectrometer at the unique wavelength. The absorbance then is compared to the calibration curve to find the concentration of the contaminants in the extraction solvent. Based on the total volume of solvent and this concentration, the actual amount of contamination is derived. This method must be used in a laboratory and requires a skilled operator. Optical Particle Counter An optical particle counter (OPC) gives both a count and size of particles in the solution measured, and can therefore be used to find very specific information about the nature of the contaminants on the part. The method is typically useful when particle size is of interest, because, for example, particles below a certain size may be acceptable as residual © 2001 by CRC Press LLC

contamination while those above the designated size are not. OPC requires extraction equipment to prepare a sample for analysis, and the OPC equipment itself. Two major techniques are available for particle counting—light extinction and light scattering. Light extinction (also called light blocking) uses a light source to shine a beam of light through a flow channel. Particles that pass through the beam block some of the light. The blockage of light creates an electrical pulse that is proportional to the particle size. A microprocessor counts and sorts the pulses according to size. Light extinction can measure particles as small as 1 m. Light scattering is a more sensitive method that measures the light “scattered” by a particle as it passes through a light beam. Light scattering detects particles as small as 0.1 m or even smaller but does not work for particles bigger than about 25 m. The sensitivity of OPC comes at a price. OPC is very expensive and requires skilled operators.4 Direct Methods Magnified Visual Inspection Visual inspection using a magnifying glass or low-power microscope can be used to examine a part made of any material directly and observe any gross contamination that may not be visible to the naked eye but that is still larger than the micron range. The method requires the part be removed from the production area and taken to the inspection area and out of production to be inspected. It is a pass/fail measurement method that may be used as a cross-check in precision cleaning applications that also use a more precise measurement method, or as a primary method in noncritical cleaning applications where only gross contamination need be removed. Magnified visual inspection is only effective from a practicality standpoint with smaller parts that can be handled by an operator, and inspected in several short scans. It has the advantage of requiring minimum equipment. However, an area separate from production, such as a small laboratory, is almost a requirement, and inspectors must be well trained and thorough. Black Light This test requires a dark room and a black light source for direct visual inspection of parts. This method is a pass/fail test that will work on any material with a contaminant that fluoresces under black light, provided the part itself does not fluoresce. The operator simply places the part under the black light and visually inspects the part. This method has most of the same application issues that magnified visual inspection does, except, since the contaminants fluoresce, if present they are even easier to notice. Although this method can be used on large surfaces, it is generally practical for testing smaller parts. Experience shows that the detection capability, even for the best operator, is limited to contamination of more than 100 mg/ft 2, which is approximately equal to 10,000 Å. This level of contamination is generally too much for precision cleaning requirements. Therefore, this method is only good for detecting gross fluorescing contaminants. Water Break Test This simple method takes advantage of the fact that many contaminants of interest are hydrophobic. In this pass/fail test, which is typically used for metal surfaces, water is flowed over the part. If it sheets off the surface evenly, the part is “clean.” If the water channels or beads on certain areas, the part is rejected or sent for additional cleaning. This test © 2001 by CRC Press LLC

can be done in production areas or as a batch test and is also usable on very large parts, such as airplane wings. To be effective, the water used in the test must be free of surfactants or other contamination that would cause the water to flow evenly, even in the presence of contamination, and the parts must be of a geometry that allows water to flow across the surface of interest. This test will not detect inorganic contaminants and is not likely to detect small amounts of contamination or water-soluble contaminants. Contact Angle This method can be thought of as a more-sophisticated water break test, as it also takes advantage of the fact that most contaminants cause water to bead up because of their hydrophobic nature. The test requires a small laboratory area and a highly skilled operator. To use this method a contact angle goniometer (an instrument that can be used to measure contact angle) is required, and the part to be analyzed must be flat and of small size (about 3  3 in., or less). In addition, distilled water must be used, and other parameters must be carefully controlled, such as static electricity and humidity. The test is performed by applying a distilled water droplet of reproducible size to the test surface. After waiting a couple of minutes for the drop to equilibrate, the operator examines the droplet using the goniometer and records the angle of contact the drop has with the surface. An idealized, perfectly clean metal surface would have a contact angle of 0°, which is impossible to obtain in laboratory air. A contaminated metal part would have a high contact angle, such as 30° or more. Some parts, such as plastics, have positive contact angles even when “clean,” so the method is not typically used for cleanliness analysis for these materials. While a number is obtained from this test, the contact angle, the test still is nonquantitative in terms of the contaminants on the part. This method is very subjective and not capable of detecting scattered or small amounts of contamination. Water-soluble contaminants are not likely to be detected by this test. This method will also not detect inorganic contamination. Optically Stimulated Electron Emission The surface to be tested is illuminated with UV light of a particular wavelength. This illumination stimulates the emission of electrons from the surface. These electrons are collected and measured as current by the instrument. Contamination reduces the electron emissions and, therefore, the current measured. The inspected surface must emit electrons, i.e., the material must be photoemissive, for the technique to work. However, most materials of engineering importance do emit electrons. The technique is simple to operate, fast, and relatively inexpensive. In addition, it is quantitative, nondestructive, and noncontact. The optically stimulated electron emission (OSEE) sensor can be handheld and placed directly on the surface to measure the cleanliness of the area of interest in a few seconds. This technique detects both organic and inorganic contamination, such as oxides, and can be used on any shape of parts as long as the geometry of the part is presented to the sensor in a consistent manner. This system lends itself to scan small parts or large surface areas. This technique can be used in the production environment and for online real-time measurement of surface cleanliness. For example, an OSEE sensor has been mounted online in steel mills to measure the cleanliness of steel sheet moving as fast as 1000 ft/min prior to phosphate coating of the steel sheet. Total Organic Carbon Total organic carbon (TOC), also known as direct oxidation carbon coulometry (DOCC), is a technique that works by oxidizing the sample surface to convert any carbon © 2001 by CRC Press LLC

compounds present into carbon dioxide, and then detecting and measuring the carbon dioxide. The detection of carbon implies that there was some contamination that had carbon as its constituent. The level of TOC detected determines the level of cleanliness of any part. Since the TOC analyzers only detect carbon, the compound of interest must contain some carbon in a detectable quantity, in order for the analysis to function. This method works on a variety of materials and is surface-geometry independent. The method works only on small parts or pieces of larger parts. Because of the high temperature in the combustion chamber (more than 750°F) the method is not suitable for parts sensitive to high temperature. This method can be used in a laboratory but is adaptable to production environment. Measurement and Analysis of Surfaces by Evaporative Analysis (MESREN) This technique utilizes a radioactive decay to quantify organic contamination on a surface. This technique can be used to analyze the surface directly, or indirectly by using solvent extraction. The extent of radioactivity depends on the level of surface contamination present. This technique will not detect any inorganic contamination. Analytical Methods Analytical techniques are defined as those techniques that can analyze a surface to determine the type/species of contaminants and, in most cases, the absolute amount of contamination on the surface. These types of instruments/systems generally use high vacuum chambers, and are very costly to acquire and operate. In addition, a very high skill level is required to operate these types of systems and to interpret the results. Most of these techniques can only examine small samples. This may require cutting a piece off the part to be inspected. Usually careful sample preparation is required prior to analysis. All these techniques are laboratory techniques and cannot be used on the shopfloor. The samples have to be sent to the laboratory for analysis. Testing time, in most cases, is quite long, thus limiting the number of samples that can be tested. In some cases it is important to know the species of contamination on the surface. For example, when developing a new cleaning process, the knowledge of the species of contamination helps in designing the most effective cleaning process. It is also possible, if the species is known, to trace the source of contamination and minimize it or eliminate it. Because of the high cost and the high level of skill required, it is recommended that these techniques be used for development or investigative applications. These techniques very seldom provide “real-time” information to be used on a daily basis. Some of the most common analytical techniques are listed below. For more information on these analytical techniques, please refer to Chapter 3.2 by Geosling and Koran. Electron Spectroscopy for Chemical Analysis (ESCA) is often called X-ray photoelectron spectroscopy (XPS). This highly sophisticated and expensive measurement method uses special equipment to bombard the surface of interest with X-rays under vacuum conditions, causing electrons to be released from the surface. Since each type of element (i.e., carbon, oxygen, etc.) releases a unique amount of electrons under these conditions, the actual elemental composition of the surface can be quantified. The test requires a very small, flat surface and is not only expensive but lengthy. Its application is limited mostly to research and development, but it can be used to calibrate and evaluate other

© 2001 by CRC Press LLC

less-sophisticated measurement methods. XPS equipment is typically only present in specialized university laboratories and larger industrial research laboratories. Fourier Transform Infrared Spectroscopy (FTIR) uses infrared light focused on the part surface. Reflected light is detected and analyzed for the absorption of specific light frequencies. This information can be used to identify the type of organic materials that are on the surface. Secondary Ion Mass Spectrometry (SIMS–Static) is often called TOF SIMS (time-of flight mass spectrometry). An energized primary ion beam is used to bombard a surface in high vacuum. This causes the ejection of surface atoms as secondary ions. This technique identifies masses according to a gated arrival time at a detector. From the arrival time the mass of the various species present can be identified. The typical sampling depth ranges from 3 to 6Å. Secondary Ion Mass Spectrometry (SIMS –Dynamic) is inherently a profiling technique. It uses O2 or Cs ions to bombard a surface in high vacuum. This technique identifies masses according to a gated arrival time at a detector. From the arrival time, the species of masses can be identified. Auger Electron Spectroscopy (AES) The Auger electrons, named after the discoverer of the process, are ejected when the surface is bombarded with electrons in a high vacuum. The energy level of Auger electrons gives information about the species of contamination. Scanning Electron Microscopy (SEM) is a surface imaging technique, performed in high vacuum. SEM utilizes a beam of electrons that is passed over a very small area of surface. This beam scatters when it strikes the surface. The “backscattering” carried in by the return beam of electrons is measured with a microscope. This technique is well suited for identifying particulate and potentially nonuniform or thick films of contaminant. It is not very sensitive to very thin, uniform films. It also cannot test large areas. COST IMPACT OF CLEANLINESS LEVELS For every level of contamination, there is a corresponding level of performance (i.e., failure/nonconformance). There is cost associated with achieving a certain level of cleanliness just as there is a cost associated with the failure/nonconformance rate corresponding to that level of cleanliness. These two cost components can be combined to assess total cost of cleaning. An acceptable level of cleanliness is the one that minimizes the total cost. A minimum total cost can only be accomplished by balancing the cost of incremental cleaning with the reduced cost of corresponding failure/nonconformance rate. Surface cleaning cost is directly proportional to the desired surface cleanliness level. Experience shows that the higher the desired level of surface cleanliness, the greater the cost of cleaning to achieve that level. Beyond a certain level of cleanliness, the cost to achieve incremental cleanliness increases exponentially. Experience also shows that the higher the level of surface cleanliness, the lower the failure/nonconformance rate due to surface cleanliness and the lower the cost due to failure/ nonconformance. Hence, the cost of failure/nonconformance is inversely proportional to the level of failure/nonconformance. If both of these curves are plotted, the graph would be similar to the one shown in Figure 1. The acceptable level of cleanliness is the one that minimizes the total cost. That cost is associated with the “optimum” level of cleanliness. Since all processes have some variation, there is bound to be some variation in the level of cleanliness achieved. An acceptable variation around the optimum level of cleanliness, where the total cost is minimum, would define the “acceptable level” level of cleanliness.

© 2001 by CRC Press LLC

NO

MA

NC

E

CL

N

OPTIMUM LEVEL OF CLEANLINESS COST

Figure 1

EA

CLEANLINESS

OR

SS

NF

NE

O

LI

LEVEL OF NON-CONFORMANCE

N

-C

INCREASING

Optimum level of cleanliness.

METHODS FOR DEFINING ACCEPTABLE (OPTIMUM) LEVEL OF CLEANLINESS Required cleanliness levels for desired performance can be determined by utilizing some means of quantifying the surface cleanliness/contamination testing to correlate product performance with contaminant level, thereby establishing the degree of contamination that can be tolerated. Call such a level of cleanliness the “optimum” level of cleanliness. Once the optimum level of cleanliness is defined, a method is needed to measure quantitatively the level of cleanliness achieved during production to assure that the surface meets the established cleanliness requirements. To define the optimum level of cleanliness, the cleanliness level must be correlated with the failure/nonconformance rate of the subsequent process. A controlled experiment or production cleanliness monitoring can determine this correlation.

Controlled Experiment One way to define quantitatively the level of cleanliness needed is to perform a controlled experiment. At a minimum, this experiment should involve preparing parts with different levels of cleanliness, measuring the surface cleanliness of each part, and correlating a measure of the “success” of the subsequent operation to the level of part surface cleanliness. For example, if two parts are to be bonded together, the success of this operation is a good bond. The measure of bond strength can be correlated to surface cleanliness. If the parts are to be coated after cleaning, then the adhesion strength of the coating should be correlated to surface cleanliness. For the purpose of discussion, assume that the parts are to be coated and that the adhesion of the coating is measured by the peel strength. Parts with various degrees of cleanliness can be prepared either by altering some factors of the cleaning process or by applying contaminants. The contaminants can be applied by mixing known weights of contaminants with volatile chemicals and air-brushing the mixture over the clean parts. The parts can be weighed when clean, and after the contaminants have been applied. The difference in part weight divided by the part surface area gives a measure of contamination in weight per unit area, such as milligram per square foot or similar units. Thus, by varying the weight of contaminants added to a given amount of volatile chemical, parts with various and known levels of contaminants can be prepared. After the parts have been prepared, surface cleanliness measurements for each part should be taken and recorded. If possible, make several measurements per part. The next © 2001 by CRC Press LLC

step is to apply the coating to these parts. After the coating has been applied and cured/dried, make as many measurements of the peel strength as possible for each part, and record these values. The mean surface cleanliness reading for these parts should be correlated with the mean measurement of peel strength. Figure 2 graphically depicts the typical result of correlating the peel strength of coating adhesion to the level of cleanliness of the surface. The cleanliness level that correlates with the target level of adhesion strength measurement then becomes the required minimum level of surface cleanliness. Production Testing Another way to establish the optimum level of surface cleanliness is to measure and monitor the surface cleanliness level achieved in production and to correlate the achieved cleanliness level to the incident rate of product failure due to surface contamination. A cleanliness measuring method can be used to establish the level of cleanliness currently achieved by the cleaning process. It is also important to note the level of nonconformance due to this level of cleanliness. This level can be used as a benchmark by comparing the level of nonconformance resulting from surface contamination as the level of cleanliness changes from the benchmark level. Changes in cleanliness level occur periodically. Thus, by monitoring the currently achieved level of surface cleanliness and correlating this current level of cleanliness to nonconformance rate attributed to surface cleanliness can indicate if the total cost of cleaning has increased or decreased. For example, if the failure/nonconformance rate is too high, the surface cleanliness level will have to be improved to reduce the failure rate. On the other hand, a very low failure rate implies that the surface may be “overcleaned.” If no failures occur due to surface contamination, then it may be desirable to optimize the cleaning process by comparing the cost of nonconformance with the cost of reduced surface cleanliness. It is also possible to change the surface cleanliness level up or down by altering cleaning process parameters in response to the actual failure/nonconformance rate. This way the relation between the level of surface cleanliness and cost of nonconformance due to poor surface cleanliness can be established. Deliberately changing the level of surface cleanliness can help in establishing this relationship much faster than waiting for the cleaning process to give a different level of surface cleanliness.

X

TARGET PEEL STRENGTH

MINIMUM SURFACE CLEANLINESS

0

Figure 2

SURFACE CLEANLINESS

Surface cleanliness vs. peel strength.

© 2001 by CRC Press LLC

X

Table 1 Comparing Salient Features of Various Methods of Cleanliness Verification/Measurement

Method

Type of Part Operator Contaminant Relative Measurement Geometry Skill NonNonArea Detected Cost Time Quantitative Limitation Level contact destructive Inspected Limitations

NVR

Organic

Low

Few minutes

Yes

Some

Low

No

Yes

Limited

UV spectroscopy Optical particle counter Magnified visual inspection

Some organic High

Few minutes

Yes

Yes

High

Yes

Yes

No limit

Particulate

High

Few minutes

Yes

Yes

High

No

Yes

No limit

Organic

Low

Few seconds

No

Yes

High

Yes

Yes

No limit

Black light

Some organic Low

Few seconds

No

No

High

Yes

Yes

No limit

Water break test

Organic

Low

Few minutes

No

Some

Low

No

Yes

No limit

Contact angle

Organic

Medium

Few minutes

Yes

Flat surface

Medium

No

Yes

Small

OSEE

Organic and inorganic

Medium

Few seconds

Yes

No

Low

Yes

Yes

No limit

MESREN

Organic

Medium

Few minutes

Yes

Flat surface

Medium

No

Yes

Limited

TOC or DOCC

Organic

Medium

Few minutes

Yes

No

High

Yes

Yes

Limited

© 2001 by CRC Press LLC

Generally small parts Fluorescing contaminants only Large particle contamination only Only gross level of contamination detected Only fluorescing gross contaminants Only detects hydrophobic contaminants Only detects hydrophobic contaminants Does not detect particle contamination Does not detect inorganic contamination Subjects the part to high temperature

In-Process or Online Surface Cleanliness Monitoring Once the optimum cleanliness level has been established, a surface cleanliness measurement system can be used to monitor the process and assure that the desired cleanliness level is being achieved. By monitoring the surface cleanliness to an established level of cleanliness, the nonconformance due to surface contamination can be minimized or eliminated. Another advantage of in-process or online monitoring of surface cleanliness is that replenishment of chemicals or cleaning agents will only be done when needed, and not done according to a predetermined, somewhat arbitrary schedule. This replenishment schedule is usually time-dependent. In reality, the amount of contamination can vary considerably from part to part. In addition, the number of parts being cleaned during a given time frame can also vary considerably. Thus, a time-dependent replenishment schedule is not the best way of controlling the cleaning process. The required level of chemical or cleaning agent concentration in the cleaning solution can be objectively determined and maintained by using a surface cleanliness verification system. SUMMARY A summary table comparing factors and aspects verification/measurement methods is presented in Table 1.

of

various

cleanliness

REFERENCES 1. Gause, R., NASA Marshall Space Flight Center, A Non-Contacting Scanning Photo Electron Emission Technique for Bonding Surface Cleanliness Inspection. 2. Chawla, M.K., How clean is clean? Measuring surface cleanliness, Precision Cleaning, June, 11 –14, 1997. 3. Pacific Northwest Pollution Prevention Center Web site, available as www.pprc.org/pprc/ p2tech/measure. 4. Precision Cleaning Web site, available as www.cleantechcentral.com/. 5. Mittal, K.L., Treatise on Clean Surface Technology, Vol. 1, Plenum Press, New York, 1987.

© 2001 by CRC Press LLC

CHAPTER 3.2

Contamination Control and Analytical Techniques Christine Geosling and Jana Koran

CONTENTS Contamination Control Clean Room Operations and Practices Definition of a Clean Room Clean Room Operation and Monitoring Standards Clean Room Classifications Clean Room Standard Evaluation Clean Room Certification Clean Room Monitoring Airborne Particle Counting Temperature Uniformity and Humidity Monitoring Velocity and Airflow Patterns Contamination Control What Is Contamination? Sources of Contamination Forms of Contamination Contaminant Classification Contamination Prevention Key Elements That Reduce Clean Room Contamination Key Elements That Minimize Overall Contamination Clean Room Housekeeping Rules Gowning Requirements Gowning Procedures Clean Room Restricted Materials Clean Room Restricted Activities What Is a Clean Surface? Cleanliness Verification Methods Contact Angle Measurement Particle Counting Methods Surface Analytical Techniques © 2001 by CRC Press LLC

Sample Preparation SIMS (Static) SIMS (Dynamic) ESCA Auger Electron Spectroscopy SEM/EDS or SEM/EDX FTIR Summary References CONTAMINATION CONTROL As the new high-technology geometries are getting smaller, the sensitivity to particulate and thin-film contamination is increasing. Controlling contamination is a top-priority task for high-technology science and manufacturing operations. Implementing comprehensive clean room operation strategies and optimizing process lines have significant impacts on process yields and product reliability. A whole segment of technology and manufacturing supports high-technology industries with a variety of products designed to minimize or eliminate sources of contamination. Clean room systems and products are being developed alongside new high-tech marvels of today and tomorrow. Clean Room Operations and Practices Definition of a Clean Room A clean room is a controlled environment in which all incoming air, water, and chemicals are filtered to meet high standards of purity. Temperature, humidity, and pressure are also controlled. Clean Room Operation and Monitoring Standards Federal Standard 209 is published by the U.S. General Services Administration (GSA). Revision E defines clean room classification in terms of the concentration of airborne particles. The class name defines the maximum allowable concentration of airborne particles in units of cubic feet that are 0.5 m in diameter or larger. This standard also defines how a clean room classification must be verified. It spells out the number of locations that must be sampled, the minimum number of independent samples that must be taken at one location, and the minimum volume of air that must be sampled in one measurement at a location. It also includes statistical procedures for analyzing and interpreting the particle count data. The federal standard is soon to be replaced by ISO TC209, which will be phased in over time.1 Clean Room Classifications Class 1 Class 10 Class 100 Class 1,000 Class 10,000 Class 100,000 © 2001 by CRC Press LLC

Class 1 means that aerosol particles with diameter of 0.5 m or larger can be present in concentrations no greater than 1 particle/ft 3. Similarly, Class 10 means that the concentration of particles of 0.5 m or larger cannot exceed 10 particles/ft 3, and so on. See Table 1 for the complete federal standard description of the clean room classes. See Table 2 for the complete ISO TC209 description. Clean Room Standard Evaluation The principal tests to evaluate clean room operational parameters are as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

HEPA filter installation leak test Airflow parallelism test Recovery test Room pressurization test Lighting level and uniformity test Noise level test Temperature uniformity and humidity test Vibration test Enclosure induction leak test Main air supply and makeup air supply volume and reserve capacity test Airborne particle count Particle fallout (optional) Overall quality of workmanship

Clean Room Certification Newly built clean room facility operational conditions have to be certified. Clean room certification must be performed as built, at rest, and in operation. Clean Room Monitoring When the clean room facility becomes operational, the clean room parameters have to be monitored as required per Federal Standard 209, Revision E. Taking an occasional single reading at a single location arbitrarily selected inside a clean room is not appropriate procedure for monitoring the performance of the clean room and verification of the cleanliness classification. Airborne Particle Counting The particle counting method is a very useful tool to measure airborne contamination levels. Particle count data provide information about the size and quantity of airborne particles present in the room. Taken regularly, they give an indication about the effectiveness of clean room operations and housekeeping practices. Most optical counters used today are instruments based on the light scattering effect. The instruments detect particles by measuring the light scattered by a single particle as it passes through a light beam. Temperature Uniformity and Humidity Monitoring Uniform temperature and humidity across the clean room are very important. Static electric charge buildup in dry air attracts particles to surfaces. Excessive moisture from © 2001 by CRC Press LLC

Table 1 Airborne Particulate Cleanliness Classes Class Limits Class Name

0.1 m volume units

SI

m3

ft3

m3

ft3

m3

350 1,240 3,500 12,400 35,000 — — — — — — — —

9.91 35.0 99.1 350 991 — — — — — — — —

75.7 265 757 2,650 7,570 26,500 75,700 — — — — — —

2.14 7.50 21.4 75.0 214 750 2,140 — — — — — —

30.9 106 309 1,060 3,090 10,600 30,900 — — — — — —

English

M1 M1.5 M2 M2.5 M3 M3.5 M4 M4.5 M5 M5.5 M6 M6.5 M7

1 10 100 1,000 10,000 100,000

Source: Federal Standard 209E.

© 2001 by CRC Press LLC

0.2 m volume units

0.3 m volume units ft3 0.875 3.00 8.75 30.0 87.5 300 875 — — — — — —

0.5 m volume units

5 m volume units

m3

ft3

m3

ft3

10.0 35.3 100 353 1000 3,530 10,000 35,300 100,000 353,000 1,000,000 3,530,000 10,000,000

0.283 1.00 2.83 10.0 28.3 100 283 1,000 2,830 10,000 28,300 100,000 283,000

— — — — — — — 247 618 2,470 6,180 24,700 61,800

— — — — — — — 7.00 17.5 70.0 175 700 1,750

Not available due to copyright; see printed book to view the table. Source: Copyright by the International Organization for Standardization (ISO). This material is reprinted from ISO 14644-1 with permission of the American National Standards Institute (ANSI) on behalf of the International Organization for Standardization. Not for resale. No part of ISO 14644-1 may be copied or reproduced in any form, electronic retrieval system or otherwise or made available on the Internet, a public network, by satellite or otherwise without the prior written consent of the American National Standards Institute, 11 West 42nd Street, New York, NY 10036.

ambient air forms a water layer on surfaces, causing metal oxidation. Moisture also traps chemical contaminants. Relative humidity below 45% and temperature around 25°C help to minimize the presence of static charged particles and the interaction of air moisture with product surfaces to cause defects. Humidity transmitters measure relative humidity from 0 to 100% and temperature from 40°C to 80°C. Velocity and Airflow Patterns Velocity and airflow pattern choices depend on the process requirement for which the clean room is being used. Any velocity is acceptable; the 90 fpm criterion is no longer the rule. Air-filtering units have to be capable of handling large volumes of air with the required velocity. Do not attempt to save costs on energy use by reducing the air volume, temperature, or humidity during off-hours. Operating conditions must be maintained around the clock to ensure proper containment. Contamination Control2 Understanding the complexity of the science behind contamination control and the knowledge of its basic principles is an effective tool to meet higher production performance goals. What Is Contamination? Contamination is unwanted matter or energy present on the surface or in the material causing interference or dysfunction. The nature and origin of contaminants can vary greatly.

© 2001 by CRC Press LLC

Sources of Contamination • • • •

Personnel Material, chemicals Processes Equipment

By far, the biggest source of clean room contamination is clean room personnel. Contamination is generated in the form of particles from saliva, shedding skin flakes, human hair, cosmetics, street clothing, perspiration, and smoking. Other sources of contamination are fabrication processes, materials, and chemicals. A chemical reaction that produces by-products is one of the most challenging problems the industry faces today. Chemical changes during the fabrication process can cause unwanted absorption or diffusion of impurities to surfaces. Tooling and equipment can cause particulate contamination due to abrasion, oxidation, peeling, flaking, braking, or fracturing. Forms of Contamination Contamination can occur from a number of sources: • • • •

Chemical Microbiological Radioactive Light

Contaminant Classification Particle contaminants include dust, lint, hair, fibers, paint and metal chips, corrosion flakes, skin flakes, water spots, burrs, and lapping and polishing compound residue. Organic thin films include fingerprints, grease and lubricant oil residue, pump oil vapors, plasticizers, solvent residue, wax, and ambient atmosphere contaminants (primarily hydrocarbons and moisture). Inorganic thin films include water-soluble salts and acids, metallic ions, plating residue, and metal oxides. Microbiological contaminants include bacteria, spores, and bacilli. Organic and inorganic film contamination form during chemical reactions of the surface material with surrounding media. Outgassing of heated plastic material (plasticizers, additives), solvent evaporation, vacuum pump exhaust, fluxes, lubricants and grease leave layers of organic film. Detergent dragout leaves a residue of chemical complexes formed during the cleaning process between organic contaminants and anions from cleaning agents. Stagnant water in deionized water lines and tanks is prone to accumulation of organics and bacteria growth. Contamination Prevention Experience shows how challenging it can be to prepare clean surfaces and keep them clean. An effort to produce and maintain product purity should be focused on clean room operational conditions, manufacturing process design, and transportation and storage. Another important aspect in the contamination prevention process is well-trained clean room personnel and implementation of a clean room personnel system. A comprehensive © 2001 by CRC Press LLC

clean room personnel system defines clean room behavior training and compliance, hygiene, gowning, the clean room entry process, clean room housekeeping rules, clean room monitoring, and auditing procedures. For high precision optics manufacturing in a 10,000-class clean room, a witness sample test for monitoring particulate contamination was found to be a useful housekeeping evaluation tool. The test is based on the use of clean microslides exposed for the duration of 24 hours to the various clean room working areas, including laminar-flow benches. The microslides are then inspected under a microscope at a magnification of 10 to identify particles by their size, color, and shape. Some particles are easy to identify; some require Scanning Electron Microscopy/Energy Dispersive X-ray spectroscopy (SEM/EDX) or micro-Fourier transform infrared (micro-FTIR) analysis. More than seven particles collected on one microslide are considered a high level of contamination. Key Elements That Reduce Clean Room Contamination Key clean room elements that reduce contamination include: • Air showers • High-tack, washable, contamination control mats at the room’s entrance • Flooring—vinyl floor curved at the base of the wall to avoid dust collecting in corners; inside and outside wall corners rounded for easy cleaning • Sterilants for the deionized (DI) water system • Ergonomic clean room seating • Clean room doors • HEPA filters • Air ionizers • Central vacuum system for cleaning floors and equipment • Pressurized pass-through windows in the wall between cleaning and main assembly area • Washable bench surfaces • Laminar-flow benches • A suspended ceiling system for easy surface wiping • Dust-proof garments, caps, and shoe covers • Powder-free gloves or finger cots • Clean room relative humidity below 45%; temperature of 25°C • Positive room pressurization Key Elements That Minimize Overall Contamination A number of guidelines to reduce contamination generally include: 1. For processes, use material and equipment with no particulate or chemical contamination: • Water (resistivity 18 M) • Chemicals with purity requirements • Particle-free, outgassing-free storage and transportation containers • Extraction-free, outgassing-free piping and tubing material • Dry pumps, turbo pumps • Water-based lubricants • Water-based fluxes • Durable equipment material resistant to oxidation, abrasion © 2001 by CRC Press LLC

2. Prevent by-product formation from occurring by optimizing process conditions (temperature, pressure, and time). 3. Follow clean room housekeeping procedures and rules. 4. Keep “dirty” processes outside of the clean room. Optimize your own contamination control management system and develop a spirit of ownership in solving contamination control problems. Clean Room Housekeeping Rules Good housekeeping is of paramount importance. The clean room cleaning frequency depends on the clean room class, level of use, and density of personnel working in the clean room (how many personnel and how many hours per week). Sequence for cleaning vertical clean rooms: 1. 2. 3. 4.

Ceiling Walls, doors Equipment Floor

Sequence for cleaning horizontal clean rooms: 1. 2. 3. 4. 5.

HEPA wall Ceiling Walls Floors Wall opposite HEPA

Cleaning methods: 1. Vacuuming 2. Mopping 3. Wiping Cleaning agents have to be clean room and environmentally compatible. A recommended cleaning solution for work surfaces in ambient or laminar-flow controlled environments is a mixture of 70% isopropyl alcohol and 30% DI water. Rules: 1. Do not walk over freshly mopped floors. 2. When equipment, furniture, etc. is moved around during cleaning, do not forget to clean cart wheels, trash can bottoms, etc. Gowning Requirements Class 100 Class 10,000

Full hood, mask, coverall (inside boots), boots, nonpowdered gloves Hair cover, facial hair cover, coverall, shoe cover or appropriate shoes

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Class 100,000

Hair cover, facial hair cover, a general protective suit (smock), shoe covers or appropriate shoes

Gowning Procedures In general, gowning procedures for various clean room grades are the same. Gowning is a top-to-bottom procedure, a safeguard against contamination transfer from personal street clothing to the clean room environment. Below are three gowning procedures. The differences between columns reflect the clean room class from less restrictive (Class 100,000) on the left to most restrictive (Class 100) on the right. Class 100,000

Class 10,000

Class 100

Hair cover Shoe covers Smock

Hood Boots Coverall Face mask Gloves

Hairnet Shoe covers Gowning gloves Hood Face mask Coverall Boots Goggles Production gloves

Gowning procedure takes place in a gown room. The gown room is usually divided into separate zones for cleaning hands prior to clean room entrance, gown storage, and the gowning process itself. The gown room has to be kept clean and organized to eliminate cross-contamination. Clean room garments have to be stored in bins, lockers, bags, or by hanging to prevent cross-contamination. Clean room garments have to be changed periodically; change frequency depends on particular clean room class and job requirements. Attention and time should be paid to training clean room personnel in gowning procedures. Gowning procedure audits should be part of the clean room performance evaluation. Passing through air showers installed at the entrance to the clean room keeps fully gowned personnel from bringing particulate contamination into the clean room. A minimum 45-second stay in the air shower is required to make the process of eliminating particles effective. Clean Room Restricted Materials All Classes

Classes 1–10,000 Classes 1–1000

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No jewelry No cosmetics No food products, drinks No cardboard, foam, unsealed wood No external medication No aerosol products No writing tools (except ballpoint pens with no cover) No paper products (except clean room paper) No outside clothing into clean room

Clean Room Restricted Activities Fast motion Sitting or leaning on equipment or working surfaces Writing on equipment or garments Removing items from under the clean room garment Wearing the garment outside the clean room Wearing soiled or torn garments Touching or scratching exposed skin or hair What Is a Clean Surface? A definition of a clean surface depends on particular cleanliness requirements. Without substantial analytical evidence it is hard to determine the surface cleanliness level. See the section below on Analytical Testing. The cleanliness at any point of the surface can be related to its number of particles, film thickness, or charge demand. For most of the hightech industry, a clean surface is one that is free of all but a few percent of a single monolayer of foreign atoms, either absorbed on or substantially replacing surface atoms of the parent material. For less-demanding industries a clean surface is one that contains no significant amounts of undesired material that will affect measurements or further processing of the surface. Considerable variations in the cleanliness of the components do occur and frequently lead to manufacturing difficulties and poor instrument performance. Cleanliness Verification Methods Cleanliness verification methods implemented at various critical steps of the fabrication process are a very important tool to monitor quality parameters. For critical cleaning applications, this is generally done with instrumental methods. The section below on Analytical Techniques summarizes some of the available methods and sampling procedures. Some of the limiting factors in choosing a cleanliness verification protocol may include the complexity of the component, its size, its chemical composition, and the destructive nature of some methods. If the verification process is to be done in-house, then costly equipment and specific operator skills may be required. Many of the more expensive techniques can be performed by a vendor on a periodic basis. Cleanlinessrelated product failures may require more extensive analytical testing than a day-today verification protocol. The choice of instrumentation used to measure cleanliness depends on: 1. The degree of precision needed. For gross contamination level information, contact angle measurement or particle counts are very useful and inexpensive techniques. In many cases, however, material characterization methods such as SEM/EDX, SIMS, ESCA, AES, or FTIR (acronyms will be spelled out in the relevant sections below) are used to quantify the level of cleanliness further and reveal the chemical composition of the contaminant. 2. The extent of quality control required. Cleaning process efficiency can be determined by analyzing a representative sample with chosen analytical techniques. From these data collected over a period of time and compared with the performance of samples as final products, a cleanliness acceptance level can be established and used as a pass/fail cleaning process efficiency evaluation tool.

© 2001 by CRC Press LLC

Cleanliness level verification is entirely end-user dependent. For example, contact angle measurements, optical contact seals, bore flush residue FTIR analyses, and bore flush particle-counting methods were found to be useful monitoring tools in laser gyroscope production. When complemented with SEM, ESCA, and/or SIMS techniques, they provide valuable information about the cleanliness of various surfaces. Spectra obtained by using surface sensitive techniques reveal not only the elemental composition of the contaminant but also help to narrow down what exactly is left on the surface and determine its source. The analyses are expensive and the size of the sample is limited, making the use of these three techniques impractical for everyday cleanliness monitoring. However, they are useful tools when the efficiency of the cleaning process is being questioned. Contact Angle Measurement When a drop of liquid is placed on a solid, the drop will spread uniformly over the surface or form a lens shape. The contact angle of a liquid on a solid surface is defined as the angle between a substrate surface and the tangent line at the point of contact of the liquid droplet liquid/solid interface. The contact angle depends on the relationship of the surface tension forces of the liquid droplet and the solid surface. If the contact angle is high, the surface is considered to be contaminated with hydrophobic matter (organic). Low contact angle values represent clean surfaces. However, some contaminants, such as a detergent residue, are water soluble and produce a low contact angle. Contact angle is the most widely used and quickest technique to evaluate surface contamination levels. It makes a great initial pass/fail test to determine if a part should be recleaned or sent on for more critical analysis. Particle Counting Methods Particle counting methods are based on light absorption and/or light scattering from a sample surface. The most quantitative method of accomplishing this is by extracting any potential contaminants from the cleaning fluid or the substrate surface. A photodetector diode is able to detect the degree of light scattering (small particles scatter less light). The methods have wide range application from batch sampling to in-line monitoring. Problems associated with particle counting include nonspherical natures of most particles and the presence of bubbles, which may alter the scattered light characteristics of the sample. Light scattering techniques are more commonly used for small or light-colored particles. SURFACE ANALYTICAL TECHNIQUES This section discusses some of the more common surface analysis techniques, which can be valuable in the identification of surface contamination or the qualification of a clean surface. It is not intended to be exhaustive but rather a guide to practical, real-life problem solving. The topics to be discussed include: 1. Sample preparation 2. Secondary ion mass spectrometry (SIMS)–static or time-of-flight SIMS (TOF SIMS) 3. SIMS–dynamic 4. Electron spectroscopy for chemical analysis (ESCA) 5. Auger electron spectroscopy (AES) 6. Scanning electron microscopy (SEM) 7. Fourier transform infrared (FTIR) Table 3 provides a summary of the techniques, which are discussed below. © 2001 by CRC Press LLC

Table 3 Surface Analysis Techniques for Cleanliness Evaluation

Technique Static secondary ion mass spectrometry Dynamic secondary ion mass spectrometry Electron spectroscopy for chemical analysis Auger electron spectroscopy Scanning electron microscopy with energy dispersive X-ray spectroscopy Fourier transform infrared spectroscopy

Acronym

Probe Beam

Analyzed Species

Analysis Depth

Spot Size

Element Sensitivity Range

Threshold

Comments

ppm

Very sensitive, nondestructive, few monolayers Depth profiling (deep or indiffused contamination)

TOF SIMS

69

Ga ions

Ions

1 –3 monolayers

0.01 m

SIMS

O2 ions Cs ions

Ions

10 –300 Å  profile

30 m

10,000 amu H to U H to U

ESCA

X-rays

Photoelectrons

30 Å

10 m–2 mm

Li to U

0.01–1 atom%

Quantitative, chemical environ. oxidation state

AES

Electrons

Auger electrons

30 Å

0.02 m

Li to U

SEM/EDS

Electrons

Electrons/ X-rays

5,000–30,000 Å

15–50 Å (30 m for EDS)

B to U

0.1–1 atom% 0.1 –1 atom%

Quantitative, small spot size Microimaging element analysis

FTIR

Infrared light

Infrared light absorption

1,000 –10,000 Å

10 m

Molecular bonds

0.1–1 wt%

Identifies organics and polymers

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ppb –ppm

Sample Preparation In the preparation of highly clean surfaces, there are usually two analytical problems. First, what sort of dirt (contamination) is on the surface? Second, when is the surface sufficiently clean? Other, secondary, questions like “Where did the contamination come from?” are often easily answered once identification is made. The evaluation of a “sufficiently clean” surface is usually empirical. That is, the part in question is put to work in its application, and a well-performing part is then subjected to a selected analytical technique to establish a “baseline” contamination level. In all the analytical activity described in this section, it is absolutely imperative to maintain the state of the surface, whether it is contaminated or pristine. If this is not done, the simple question, “Where did the contamination come from?” becomes “What is the true contaminant, and what did we introduce in the analysis?” Obtaining a sample may be as simple as plucking a part from a production line. Alternatively, it may entail removing a part from a functioning or failed assembly. Once a surface has been selected for analysis, it should be touched by absolutely nothing. Care should also be taken in what touches the periphery of the surface. In the simple case of a production line part, it should be analyzed as soon as possible after removal from the line. Ambient air always contains hydrocarbons, which immediately deposit on the surface as “adventitious carbon.” This is seen in several of the analytical techniques described in this section. The longer the part is exposed to air, the more adventitious carbon accumulates. One way to account for this phenomenon is to analyze a control sample. More on this later. The part should be handled only by solvent-cleaned, dry tools (tweezers, forceps, etc.) or powder-free gloves or finger cots. Human fingers are notorious sources of contamination. Some contaminants of human origin are sodium (salt) and silicones. Silicones are ingredients in almost all personal care products, and have become one of the signature contaminants of skin exposure to a surface. In the vacuum atmosphere of many of the surface analysis techniques, the silicone materials, even if deposited locally, can spread into patches or form a monolayer. Another human contaminant family is that of cosmetics, often insidious particle sources. A word of caution about gloves and finger cots: they are only useful on a diligent, aware wearer. A quick brushing of a tuft of hair from one’s face or an inadvertent scratch of a nose itch makes any finger cot or glove look just like a bare hand to the analyte surface. Just a little common sense, like not pulling the gloves out of their wrapping by the fingers, may save an analysis. Powdered gloves, of course, can introduce particles into an analysis, but even powder-free gloves can pose a problem. Calcium is an unexpected ingredient in laboratory gloves that has been found in surface analyses (TOF SIMS) of parts handled with them. Packaging a sample for analysis is almost as important as the analysis itself and often the most difficult part of the overall task. The whole object is to preserve the surface state of the sample without introducing additional contamination. Obviously, one cannot touch the surface to be analyzed, so the problem becomes one of mounting, enclosing, and protecting the sample. One common mistake in packaging for analysis is the use of polyethylene bags, boxes, or other plastic materials, which contain plasticizers. Dioctyl phthalate is one of the more common plasticizers used to give plastic materials the softness and workability required for molding, rolling, or other shaping procedures. Plasticizers are usually relatively volatile and will outgas, depositing on what would otherwise be well-controlled surfaces in the package. They are easily detected by TOF SIMS as discussed below, but do become part of the adventitious carbon layer, which burdens the analysis. It is often a problem to determine if contamination of this sort is adventitious or part of the original failure mechanism.

© 2001 by CRC Press LLC

Some effective packaging methods can in fact include plastic materials. Nylon bags (not polyethylene) propped open with a clean piece of metal or glass and baked overnight in a clean oven at approximately 80°C have been used to minimize outgassing. These are often backfilled with dry nitrogen or other inert gas before being heat-sealed. Cleaned metal containers are a good choice. Wrapping parts in cleaned aluminum foil, being careful not to touch the analysis surface, is also good practice. Note that ordinary food service foil must be solvent-cleaned before use in this application because it has a thin film of lubricating oil from the foiling (rolling) process. Foil without this contaminant can be purchased, but it is expensive; usually a solvent wipe is sufficient to remove the oil. If the sample is small and needs to be mounted to protect the analyte surface, mounting must be done with as little impact as possible. Clamping with metal clamps is inherently non-outgassing, but care must be taken not to generate particles mechanically. Some analysts use double-sided adhesive tape in their vacuum systems, but they have usually characterized any outgassing components in their own machines. It is best to check with the analyst before using double-sided tape on a sample bound for vacuum. The final caveat in packaging is to be aware of the particular analysis requirements of a sample. If, for instance, the failure of a component is suspected to be due to aluminum particles, one would not package that component in aluminum foil, or in any type of aluminum container, for that matter. The different types of samples that will require analysis are as varied as the readers of this book. It would be futile to claim one or even several packaging methods as optimum. The best method for any sample will be determined by the scientist, engineer, or technician closest to it, as long as the guidelines are followed: 1. 2. 3. 4.

Do not touch the analysis surface in mounting or with any packaging material. Do not add contaminant—outgassing or particles. Seal the protected sample as best as possible from ambient conditions. Eliminate any analysis-sensitive materials from the packaging.

SIMS (Static) Static SIMS is often called TOF SIMS. The time-of-flight mass spectrometer identifies masses according to a gated arrival time at a detector, the lightest masses arriving most quickly. (Dynamic SIMS uses either a quadrupole or magnetic sector mass spectrometer for mass detection.) TOF SIMS is carried out on a surface by bombarding it in high vacuum with pulses of gallium (69Ga) ions. This is a very gentle bombardment, which can be thought of as similar to hitting a concrete wall with basketballs as opposed to cannonballs, which would be the analogy for dynamic SIMS. As a result, this is an extremely surface-sensitive technique. Only one or two monolayers are affected by the bombarding ions. Atoms, molecules, and molecular fragments are ejected from this monolayer as ionic or neutral species. Only a small percentage of the ejected materials are ions, but these are the species analyzed downstream. The masses are counted at the detector and catalogued electronically as a mass spectrum, sensitive from the lightest mass (m/e 1, hydrogen) to masses approaching 10,000. What is actually measured is a mass to charge (m/e) ratio, the vast majority of ions being singly charged. A small portion can also be doubly charged, so the same ion, if it is abundant, can give a second peak at exactly half the value of its major peak. For example, a surface with a large amount of calcium present may give a large peak at m/e 40 and a smaller one at m/e 20. A good analyst will recognize this and distinguish it from a neon peak, also at m/e 20. Isotopes can be distinguished as well. A TOF SIMS instrument has fractional mass resolution, which aids in specific compound identification. The beauty of TOF SIMS is its noninvasive nature. It is often the first analysis carried © 2001 by CRC Press LLC

out in a series. It is sensitive to all the stable elements to ppm levels. It is extremely valuable in identifying thin layers of organic materials on a surface because the ion bombardment is gentle enough to preserve organic fragments and even parent molecular ions. For this reason, it is susceptible to adventitious contamination, detecting even small monolayers or patches of organic material from the air or packaging. Control samples should always be run with the desired analysis samples to distinguish these contaminants. TOF SIMS (and dynamic SIMS as well) can yield quantitative information if standards are used. Even without standards, evaluation can be made fairly accurately of relative amounts of the same material on different samples analyzed at the same time. Concentration maps can be made of small areas of a surface, as shown in Figure 1*. SIMS (Dynamic) Dynamic SIMS is inherently a profiling technique. It is like hitting a concrete wall with cannonballs. The bombarding ions are usually O2
for electropositive elements or Cs
for electronegative elements. In high vacuum the ions hit the surface with sufficient force to break bonds and “sputter” the constituent material. As bombardment proceeds, a crater is formed, and the material ejected is a time function of the crater depth—hence the designation as a “depth profile.” As mentioned above, the ejected material is a mixture of ions and neutrals, and the ions, although a minority, are the species analyzed. In its normal mode, dynamic SIMS is not highly sensitive for analyzing surface contamination and cleanliness because it quickly eats through the evidence. However, a surface survey scan can be useful. This is a very quick scan, sputtering only a very shallow depth, and capturing the full spectrum of ion peaks. SIMS is sensitive to all the elements, and this can be a good starting point if it is not known what one is looking for. In fact it is always done before a profile, since at most 10 or 12 elements can be followed in profile mode, and they must be selected beforehand. SIMS is sensitive to charging induced in nonconducting samples, but this can be offset by use of an electron beam (“flood gun”) in the chamber. The surface can also be coated with a thin conducting layer of material such as gold, but this destroys the surface for subsequent analyses. ESCA This technique, Electron Spectroscopy for Chemical Analysis, is often called X-ray Photoelectron Spectroscopy, or XPS. X-rays with a discreet frequency, v, are directed to surface of the sample and excite photoelectrons from the core electron shells of the atoms present. The photoelectrons are captured at a detector. This is all done in high vacuum. The kinetic energies of the ejected electrons are measured, and binding energies are deduced from Ek  h  Eb , where h is Planck’s constant. Since electrons in the various shells of each element have discrete energies, the elements can be identified by the characteristic binding energies of the emitted photoelectrons. The photoelectrons contain not only the binding energy of their element of origin, but that value is shifted to slightly higher or lower energies as function of the oxidation state or chemical environment of the atom. This is called the chemical shift. Thus, ESCA can identify not only the constituent elements of a surface but their chemical state as well. ESCA is a surface-sensitive technique—approximately the top 30 Å of a surface depending on the escape depth of the photoelectrons from the various elements. It is sensitive to all of the elements except H and He. Some elements, Li, for example, have poor sensitivity but can be detected. ESCA detects carbon well. In fact the layer of adventitious carbon is always seen (Figure 2). It is important to do reference samples with ESCA to identify the portion of the carbon layer © 2001 by CRC Press LLC

Figure 2

ESCA spectrum of a lithium aluminosilicate glass ceramic surface with an adventitious carbon peak.

that is from the air or packaging. High-resolution scans of the carbon peak can identify a number of organic functional groups. The main advantage of ESCA is its quantitative nature. Surface compositions in atom% can be determined to about 1%. The less adventitious carbon, the more accurate the determination. Its disadvantage is its low overall sensitivity, with detection levels of 0.1 to 1% depending on the element. ESCA is useful for detecting thin-film contamination on a surface, since its inherent spot size is too large to focus on particles. Large particles may, however, show up as a fraction of the elemental composition within the analysis area. Auger Electron Spectroscopy This technique (pronounced oh zshay) has the acronym AES and is also a high-vacuum technique. It can be simplistically described as electrons-in/electrons-out. The Auger electrons, named after the discoverer of the process, are ejected when electrons from an outer shell fill the core vacancies produced under electron bombardment. The Auger electrons, in essence, leave the atom with the excess energy from this electron transition. AES is very surface sensitive (the top few monolayers) and has the advantage of the tightest focus of any of the techniques discussed here. Its spatial resolution is on the order of 20 nm. It is thus extremely useful for identifying the composition of small particles or tiny areas of surface contamination. AES is ideal for samples that are conductors or semiconductors, but sample charging limits its use on insulators. A technique of tilting the sample to minimize charging effects allows analysis of insulating samples in many cases. Scanning Auger microscopy (SAM) can be used to give elemental composition maps of surfaces to show areas of contamination. Its detection limit is not high (approximately 0.1 atom-%), but it is sensitive to elements from Li through U.

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SEM/EDS or SEM/EDX SEM is a surface imaging technique, but with energy dispersive X-ray spectroscopy it can identify elements in the near surface region. It is the tool of choice for imaging particles, with a lateral resolution of 15 to 50 Å. EDS mode, however, requires a spot on the order of 3000 Å. EDS is also not particularly surface sensitive. The interaction depth for EDS is actually a pear-shaped volume from about 0.5 to 3 m in depth. Only contamination this thick or in the form of large (3 m) particles can be analyzed. It is sensitive to elements from B to U as a result of the beryllium window used between the vacuum and the detector. Windowless instruments do exist but are not common. A disadvantage of SEM/EDS is its restriction to metallic or semiconductor samples or the need to deposit a thin conductive coating onto an insulator’s surface to prevent charging. If further analysis is required on an insulating surface, one should not use SEM as an initial technique in a sequence. FTIR FTIR is used to detect organic and polymeric materials on surfaces. Light in the infrared region of the spectrum is focused onto a sample surface (or residue removed from a sample), and absorption of specific light frequencies is detected. These frequencies are the vibrational frequencies of the various molecular bonds in organic (and some small inorganic) molecules. From the spectrum of absorbed vibrational frequencies in a sample, a good picture of the constituents of organic molecules can be pieced together like a puzzle. Samples can be examined in transmission, reflectance, and total-internal-reflectance modes. Very complicated organic molecules and polymers can be identified this way. Databases of thousands of molecules are available to match and identify spectra of unknown contaminants. FTIR is one of the lower-sensitivity techniques, on the order of 0.1 to 1 wt%, and its depth resolution is 0.1 to 1 m. Its power lies in the unambiguous identification of larger organic molecules. SUMMARY There are a large variety of techniques available for detection of surface contamination. The ones described in this section, with the exception of FTIR, are even capable of producing surface maps of contamination. This may aid in the identification of the origin of the contamination. The success of any analysis will depend on the selection of the best techniques, the proper order of analysis, and the skill in sample preparation and packaging. The large surface analysis companies will provide state-of-the-art equipment and expertise on each technique. Smaller, specialized, analysis firms may be preferred for some jobs, particularly where proximity to one’s own laboratory is desired. Consulting the Thomas Register or a “Buyer’s Guide” published by some of the technical magazines such as Physics Today may locate one close by. In the end, however, it is the scientist, engineer, or technician closest to the process who is the most important ingredient in the analysis. He or she will have the knowledge of process pitfalls and will be able to eliminate dead-ends in the analysis path. When certain contaminants are identified, it will be this individual with the “ah HAH” who will relate them to a failure in the process.

© 2001 by CRC Press LLC

REFERENCES 1. R.P. Donovan, Clean Rooms Mag., 13(5), 9, 1999. 2. K. Mittal, Contamination control, presented at Precision Cleaning ‘97, 1997.

© 2001 by CRC Press LLC

CHAPTER 3.3

Material Compatibility Eric Eichinger

CONTENTS What Is Meant by Compatibility? Small Investments in Compatibility can Eliminate Unwanted Problems Preventing Damage to the Hardware Preventing Effects That Alter Hardware Performance Protecting the Processing Equipment and Preserving the Processing Fluid Identifying Optimum Materials and Processes Who Should Consider Compatibility? Compatibility Data Are Most Useful When Considering a Change Changing the Cleaning Process Changing the Cleaning Fluid Changing the Parts Finding and Interpreting Good Compatibility Data Can Be a Challenge Finding the Data Interpreting the Data Examples of Common Compatibility Test Data Compatibility Testing Is Easy Obtaining Good Test Material Setting up a Good Test Immersion Testing Other Types of Compatibility Tests Example of a Successful Compatibility Test WHAT IS MEANT BY COMPATIBILITY? Compatibility is a concept that everyone is familiar with at some level, but it is exactly this familiarity that leads to a wide range of precision cleaning problems. One person’s idea of compatible may indeed be very different from another’s. Take for example some new cleaning device. A solvent manufacturer may look it over and say that its material is compatible with the equipment. However, what does that mean? Does it mean the solvent will function in the unit? Will the solvent increase the potential for the unit to corrode? Will © 2001 by CRC Press LLC

the seals soften or swell? All of the above? Obviously the answer in this example is that we simply do not know. Yet many will adopt their own definition of compatibility and assume that was what the solvent manufacturer meant. Thus, the compatibility road is filled with pitfalls stemming from both what we do not know and, perhaps more importantly, what we think we already know. For the purposes of this discussion, compatibility is the ability of two or more things to combine and/or remain together with no after effects. Although the definition is simple, the “things” in question must be specifically identified. Subsequent sections will describe what is meant by “combine” and what exactly an “after effect” is. In precision cleaning, the “things” involved are usually one of two types. First are the things that we can control, and second the things that, for whatever reason, are beyond our control. Examples of things that can be controlled include cleaning equipment, cleaning processes, and the cleaning environment. Examples of things that may be beyond control include the hardware we are cleaning and the service environment of the hardware. Therefore, the trick to solving compatibility problems is to manipulate the things that can be controlled to eliminate potential after effects. SMALL INVESTMENTS IN COMPATIBILITY CAN ELIMINATE UNWANTED PROBLEMS So what are these after effects and why do we try so hard to prevent them? There are at least four answers to this question, and each must be considered independently. Preventing Damage to the Hardware The amount and variety of damage to hardware due to poor compatibility are surprising. Nonmetallic seals and other parts may swell, soften, gain or lose weight, embrittle, craze, or otherwise degrade when exposed to a cleaning compound. Metallic parts may rust/corrode, or become more susceptible to stress fracture. Coatings and lubricants are likely to raise compatibility concerns because of their ability to dissolve, soften, debond, discolor, and generally degrade in performance. In addition, these effects are dependent on temperature, contact time, and other factors such as the pressure and presence of oxygen. Higher temperature will generally amplify the effects, as shown in Figure 1. Extended contact time will also amplify effects, but will generally do so according to Figure 2. In addition to the cleaning solvent selected, there are other process variables that can damage hardware. The best example of this is the use of ultrasonic energy to enhance cleaning. Those same cavitation bubbles that do such a great job at removing tenacious soils also can erode soft metals such as aluminum, or can remove coatings. Lower ultrasonic frequencies, such as 27 kHz, generally cause more damage than the higher frequencies (40 kHz and above). Many ultrasonic baths have so-called hot spots, where standing wave patterns inside the tank increase the threat to parts. Certain sensitive part configurations, such as small bellows, may not be compatible with many ultrasonic processes. Other process variables that can damage hardware include drying temperature, which if too hot, can degrade chemical conversion coatings on aluminum among other things. Preventing Effects That Alter Hardware Performance There are some cases in which the precision cleaning process does not damage a part, yet the after effects can be severe. It is important to understand how the part operates as © 2001 by CRC Press LLC

20 18

Observable Effect

16

Sample Measurement

14 12 10 8 6 4 2 0 Ambient

Medium

High

Very High

Temperature

Figure 1

Compatibility effect vs. temperature.

well as where it operates. Here are a few examples that are associated with complex valves. A valve cleaned with a water-based solvent is vacuum dried. However, rather than evaporate away, the water entrapped in the valve freezes during the vacuum dry process and forms a block of ice. Now the valve will no longer function, and even if the ice melts, the part will still be wet. If the same valve is cleaned with a flammable solvent like isopropyl alcohol (IPA), a small amount of IPA may remain trapped behind a secondary seal after drying. The valve is then placed in service with liquid oxygen. The combination of the service fluid and an incompatible flammable solvent creates an explosion risk that will likely jeopardize the entire system. In this example, liquid oxygen is a known compatibility concern, but milder service fluids should also be considered. Take the case above, but switch the service fluid from liquid oxygen to hydraulic fluid (Mil-H-5606, to be specific). This relatively unreactive fluid

6

Observable Effect

4 2 0 8 6 Sample Measurement

4 2 0 Week 0

Figure 2

Compatibility effect vs. time.

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Week 1

Week 2

Week 3

has an acrylic constituent that can precipitate out in the presence of IPA. The precipitate can clog fluid passageways and jeopardize the entire system. Protecting the Processing Equipment and Preserving the Processing Fluid Who is not familiar with the axiom “Look before you leap”? Yet, given the immediate need to replace obsolete equipment or solvents coupled with the high cost of “downtime” there is a lot more leaping than looking going on. When the new wonder solvent dissolves the cleaning system seals and ends up on the floor, the high cost of downtime will arrive with the high cost of cleaning solvents and high cost of equipment repair. What can be done about this? Start by selecting either a cleaning solvent or a cleaning device. If the solvent is selected first, evaluate cleaning devices that can handle the solvent. Remember the story of the inventor who brought forward a flask of the universal solvent? The king recognized the false claim because the solvent did not dissolve (read, was compatible with) the flask. A great solvent will usually be incompatible with many things, which is often the reason it is such a great solvent. Keep in mind all the concerns mentioned above. This may be a case where exposure time really makes a difference. Ask the cleaning device manufacturer if that view port is really glass or is it Plexiglas? Compatibility problems cannot always be avoided by selecting a water-based cleaner. High-pH detergents (10) can damage aluminum, and even certain nonmetallics such as Vespel rubber. When the cleaning device is selected before the solvent, the compatibility review works in reverse. List the materials of construction of the device and then find the cleaning solvent that is most compatible. Identifying Optimum Materials and Processes The three sections above can all be summarized by saying that initial ignorance of compatibility can lead to high-cost headaches and irritated customers. However, compatibility consideration is an effective way to save money as well. A good compatibility assessment as well as some performance data will identify all the potential cleaning materials and processes available. Then, a selection can be based on implementation and operation costs. Usually the more tools there are in the toolbox, the better the final result will be. WHO SHOULD CONSIDER COMPATIBILITY? One can just as easily consider the question of who should avoid fatty foods because the answer is the same. Everybody! Yet, as with fatty foods, there are special “at-risk” groups that must be the most vigilant. The two at-risk groups in terms of compatibility are those working with sensitive materials, and those working with things that can entrap cleaning solvents. There is no compendium on what a sensitive part is, but if there were it would include some metallics, such as plated surfaces. There would be lots of nonmetallics, perhaps with polycarbonate at the top. A good rule of thumb with nonmetallics is “guilty until proven innocent.” Parts with coatings will likely have special needs. Even if the coating does not visibly change during cleaning, there is no guarantee that its performance will be unaffected. For example, corrosion-inhibiting chromates will leach out of coatings into © 2001 by CRC Press LLC

pure water with no visible change. This may, however, upset the customer when the part corrodes, not to mention the facilities folks who now must dispose of pure water with carcinogenic chrome in it! Then, there is the at-risk group that “cleans” parts with lubrication requirements. Bearing assemblies are good examples of this type of hardware. In terms of compatibility this type of cleaning is more of an art than a science and is far beyond the scope of this chapter. The other at-risk group is staff working with entrapment areas. Whoever said, “What goes in must come out” never saw this type of hardware. Hardware with a high potential for incompatibility may contain fraying surfaces, blind holes, seals, threaded inserts, or seat assemblies, which can trap or soak up and retain cleaning solvents. Just because one cleans at the “piece part” level does not mean that there is not potential for entrapment. A piece part may just be the lowest logical level of disassembly. Only when working with a noncomplex, monomaterial item do the fears of entrapment fade. So to summarize, an operator is at risk and should definitely consider compatibility if it is: 1. 2. 3. 4. 5. 7.

Cleaning plated surfaces Cleaning nonmetallic surfaces Cleaning coated surfaces Cleaning hardware with lubricity properties Cleaning hardware with entrapment areas Changing the cleaning process (see next section)

COMPATIBILITY DATA ARE MOST USEFUL WHEN CONSIDERING A CHANGE One nice thing about compatibility worries is that they only need to be dealt with during changes associated with the precision cleaning process, cleaning fluid, or parts. Sometimes, however, recognizing the change can be the most difficult part. The next three sections summarize some common changes that trigger the need for evaluating compatibility, as well as some less obvious changes.

Changing the Cleaning Process These are usually the easiest changes to cope with. Process engineers have the ability to control what the new process will be like. The supplier usually has a large amount of literature to assist in decision making, and often can demonstrate a similar process in operation. Compatibility data are a great tool for distinguishing between two otherwise acceptable processes. Make sure that the supplier has performed all the necessary tests with the appropriate materials. If it has not, it may be wise to augment some of the testing (see later sections). Compatibility should still be considered even when replacing one piece of equipment with something almost identical. Certain types of precision cleaning equipment tend to have their own personality. For example, two 27-kHz ultrasonic tanks with the same power rating will not be exactly the same from a compatibility standpoint. Each tank will have its own unique “hot spots” where part damage potential is highest. If sensitive parts are used in this tank, the hot spots should be mapped out and avoided.

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Pay close attention to the “incidental” process materials that would not obviously contribute to compatibility problems. For example, many cleaned parts are sealed in polyethylene bags. Polyethylene is compatible with almost everything and is usually factored out from compatibility considerations. However, certain additives to the polyethylene bags can cause compatibility issues. In one case, silver-plated parts tarnished when an antistatic agent from the bag contacted the part surface. In another case, a company received a new shipment of bags that contained an elevated quantity of plasticizer. The plasticizer is a compound added to keep the bags flexible and easy to work with. The extra plasticizer deposited out onto the hardware during weeks of storage (storage could be considered a change), however, forcing extensive and expensive recleaning and delaying final assembly. Changes like this are very difficult to identify, but the consequences can be severe. Changing the Cleaning Fluid Whoever coined the phrase “nothing in life is easy” could very well have been changing cleaning fluid. As described above, cleaning performance and compatibility are necessarily at odds with one another. Thus, performance and compatibility evaluations are best performed together. Remember to evaluate compatibility under actual use conditions. Also remember to consider compatibility (toxicity) with the people who have to work with the cleaner. Frequently, the best cleaner is a compromise among performance, compatibility, and toxicity. Just as with the changes in cleaning processes, there are some subtle changes with cleaners to be aware of. For example if two cleaners, each with great compatibility, are utilized in series, what happens when the cleaners are mixed? A second example concerns mixed (more-than-one-constituent) solvents. Beware of mixed solvents. Often these solvents require trace amounts of stabilizers, coupling agents, or antioxidizers. These “secret ingredients” can be manipulated by the manufacturer and the user will be the last to know. A perfect example of “secret ingredient” manipulation involves a cleaner considered for use on space hardware that had to be compatible with liquid oxygen. The cleaner demonstrated oxygen compatibility and was near implementation. Then, the vendor decided to address complaints received from other market sectors by changing a stabilizer. The product name did not change, nor were the users aware of the change. Yet, the cleaner was no longer compatible with oxygen. Fortunately, a test identified this change and the cleaner was not implemented. In the case of mixed solvents used for critical processes, compatibility testing may be required on a lot-by-lot basis. Changing the Parts The most difficult types of changes to respond to are those associated with the part or the service environment of the part. Any design changes in configuration or material selection should prompt a compatibility assessment. These changes are often difficult to respond to because they are not usually made by the group that does the cleaning. The key again is awareness, and awareness relies upon good channels of communication between the redesign group and the cleaning group. Compatibility between the precision cleaning process and the ultimate service environment of the part is essential. Consider the potential for cleaning fluid entrapment and other compatibility effects. In one case a logistics depot decided it would be more efficient if all its valves were cleaned to the strictest cleanliness level. This would allow flexibility when the valves were needed by a particular subsystem since they would always be clean enough. However, this decision was short lived, because the cleaning fluid in the strict process was not compatible with all of the subsystem service fluids. © 2001 by CRC Press LLC

FINDING AND INTERPRETING GOOD COMPATIBILITY DATA CAN BE A CHALLENGE A good compatibility assessment may cost more time and money than is available, so the best thing to do first is try to find existing data. The plethora of compatibility data available seems at first a blessing, which can turn to a curse as one wades hopelessly through a mountain of data that do not address the specialty application. Finding the Data Vendors often have compatibility literature that is applicable. However, it will not have the unbiased flavor of data generated by a “third party.” The third-party data can be found, with a little library or Internet searching, in books and periodicals. The majority of these data cover cleaner–material interactions (especially at room temperature). Thus, if one knows the materials involved, these books should help select a cleaner. They will also assist in the selection of things like plastic gloves and storage containers. However, there is less information available on compatibility with ultrasonic equipment, and other processrelated factors. Interpreting the Data Generally, compatibility data are a measurement of a change in properties over a period of time. The time may be very short as in an impact test, or the time can be much longer as in immersion compatibility tests. Specifically, the data must be representative to be meaningful. Representative data usually means the materials tested are the same or very similar to the material in question. Also, the exposure time should represent a worst-case scenario (remember to consider entrapment potential). Temperature, pressure, and other environmental factors should also be factored in. Subtle differences can potentially determine whether the data from a given study are useful in evaluating the specific application. For example, the amount of head space in the sample container will affect the amount of oxygen available to immersion test specimens. Thus, there is almost always a degree of subjectivity in compatibility data because it is not common to find directly representative data. As if compatibility data were not subjective enough, much of the published data are qualitative. Rather than list quantitative data, such as a linear swelling of 5%, publications often assign a letter rating, and then somewhere in fine print explain an “A” is fine for longterm storage, “B” is fine for short term, “C” materials should be avoided, “D” means “run for your life” or some other threatening note. It is best to allow only the amount of subjectivity merited by the criticality of the hardware. If the criticality is high, compatibility testing may be in order. Examples of Common Compatibility Test Data The different compatibility tests (described in more detail in the next section) measure a variety of properties. Often multiple types of data will be required to make a thorough assessment. The next section briefly summarizes the most common compatibility tests used to quantify degradation (yes, the data are usually quantitative and then reported in a qualitative form). Note that there are many specific types of compatibility tests, such as breakthrough time for gloves, that are not described below, but may be distant cousins to some of the tests. © 2001 by CRC Press LLC

Metals are the most straightforward. Immersion testing is often performed either with single materials, or with galvanic couples (two metals in contact with each other). Immersion frequently assumes covering the sample to some degree with a liquid, but can also include immersion to a controlled environment such as an oven or an atomic oxygen chamber (which simulates an Earth orbit environment). Metals sensitive to stress corrosion, such as titanium, usually require a more complex (stressed) specimen and may require elevated temperatures. Important factors include exposure time, the percentage of the specimen that is immersed, and the amount of oxygen available. Data taken from these specimens include the difference between the starting and ending weights (pay attention here to the specimen size), amount of oxide formed, and depth of corrosion (frequently reported in the rather odd units of mils per year—1 mil  0.001 in.). Elastomers, like metals, are frequently immersion tested. Testing can be done on “chunks” of material, actual parts, such as “O-rings,” or parts in a service environment (under compression, specific temperature, etc.). Weight loss data are again common, but there are two other performance-related measurements that are also common. “Shore hardness” is measured by pressing a blunt pin against the elastomer before and after exposure. Frequently, solvents will cause a noticeable drop in shore hardness of elastomers. The shore hardness recovery may also be important. Over time, after the immersion compatibility test is over, an elastomer usually will gradually recover some or all of its original shore hardness. A second frequently reported elastomer compatibility result is “percent swell.” Not surprisingly, this is the dimensional change associated with a period of immersion. The dimensions of an elastomer can begin to change within the first 5 min, and may take an hour to a month to stabilize. There is no universal “acceptable amount” of swelling used to generate the qualitative rankings published, but a good rule of thumb is that 10% swell is a problem. Just as in the hardness test above, an elastomeric compatibility specimen will usually try to return to its original dimension. This is not always a good thing, however, because if swell is induced by a high-vapor-pressure solvent (very volatile), uneven shrinkage is likely to occur as the edges shrink faster than the center. Elastomers can crack and break as a result of uneven shrinkage. Coatings compatibility tests are usually performed either by immersion or wiping. Data are gathered visually, or by using tape to measure changes in adhesion (ASTM D 3359), or by measuring the ability of a series of sharpened pencils to break the coating (ASTM D 3363). Sometimes it is important to assess whether a particular liquid will be degraded upon contact with another liquid or a solid. Liquid compatibility is assessed by immersion of solid specimens or by mixing two liquids. Immersion results will demonstrate whether there are changes in purity, or if the liquid extracts something from the solid. Extraction is frequently quantified as nonvolatile residue (NVR). The NVR is what is left behind after the liquid is evaporated away. A good thing about NVR is it can often be identified through a variety of instrumental methods. Liquid mixing is an excellent way of quantifying reactive compatibility effects. In this test two liquids are placed in a sealed pressure vessel. Temperature and pressure changes can be recorded to assess whether the liquid mixing is a catastrophic threat, or a potential nuisance. Again, at the conclusion of the test, chemical breakdown products and NVR can be quantified and identified. This is a very useful test to evaluate the effect of entrapped processing media on the ultimate service media. Table 1 summarizes the common types of compatibility tests. These tests will be described in more detail in the next section.

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Table 1 Compatibility Tests Material Type

Test

Important Variables

Result Reported

Metallic

Immersion

Time, oxygen availability

Elastomers

Immersion

Coatings

Immersion

Time, specimen configuration Time, coating age

Liquid

Wipe Solid Immersion Mixing

Coating age Time Relative amounts, time

Weight loss/gain, amount of corrosion Weight loss/gain, shore hardness, % swell Visual change, adhesion loss, pencil hardness change Visual change Purity change, NVR Temperature change, pressure change

COMPATIBILITY TESTING IS EASY So after spending some time in the library and on the phone it is clear that there is not enough relevant compatibility data to say whether the new wonder solvent will be a future workhorse or a Trojan horse. Perhaps the solvent is too new or too obscure. Whatever the case, the best bet is to collect enough data to ensure there are no unwelcome surprises. The following description of common compatibility tests is not intended to provide the “how to” details, as those can be left to ASTM. However, the quality and interpretation of the data obtained from a compatibility test can be heavily influenced by the detailed approach. Thus, the following is intended to point out key considerations during test setup, or good questions to ask when reviewing existing data. Obtaining Good Test Material Since many compatibility tests are performed before implementation, the need for a test sample frequently arises. Pay careful attention to the sample! Where did it come from? How similar is it to what will be used? Answer these questions first, or the test may be doomed before it is even started. For example, it is not uncommon for small samples to originate in a laboratory rather than a manufacturing line. Laboratory samples are usually more pristine with fewer impurities and no traceability. These pristine samples may not represent the production material very well. The sample should represent what will actually be used in the actual product to be cleaned. Following this logic, materials are often procured under one requirement, and then discarded when they no longer meet a second use requirement. A good worst-case compatibility test should consider material at the low end of the use specification as well as the high end. Setting Up a Good Test “Never ask a question for which you are not prepared for the answer.” In the world of compatibility this platitude suggests that a pass/fail criterion always be established. The difference between passing and failing may hinge on how the test is set up.

© 2001 by CRC Press LLC

Immersion Testing Immersion tests are often the simplest meaningful compatibility tests available. The variables to measure have already been covered in the previous section, but the test conditions are every bit as important. The exposure to air is a good example of an important condition. Fully immersed specimens will be exposed to dissolved air. To control dissolved air, the sample container should be sealed with a minimum of head space. For partially immersed specimens, the air –liquid interface will often behave differently from the fully immersed portion. Finally, if wet –dry cycles are likely, an alternate immersion test should be considered. Alternate immersion tests are especially well suited to measuring stress corrosion. In addition to simulating environmental factors, immersion tests can also simulate exposure to service fluids. When there is the potential for entrapment, it may be worth considering if any reaction will occur between the processing material and the service material. Changes in pressure, temperature, or color will indicate whether a reaction occurs. The ratios of the two test fluids are subjective, but should be based on worst case entrapment. Since reactivity usually increases with temperature, run the test at the highest anticipated fluid temperature. However, since the amount of dissolved air decreases at higher temperature, it may also be necessary to repeat the test at a lower temperature if the effects of dissolved air could be important. Other Types of Compatibility Tests Although immersion testing is the single most common test, there are many other tests to consider. These tests can be selected based on the specific configurations of the parts and the processing equipment involved. For example, if there are entrapment locations, or capillary passageways, a “sandwich corrosion” test (ASTM F 1110) may be in order. A small amount of sample is placed on a flat sheet with another placed on top to make the “sandwich”. The sandwich is put into some sort of hostile environment, most commonly a salt fog chamber, and then checked for damage. This test has been used effectively to identify aqueous products that are safe to use on aluminum. Coatings can be tested by a simple wipe test, but the age of the coating and the number of wipes are important. Metals can be stressed by bending into “U” or “C” type specimens and then placed in a variety of environments (including simple immersion). In this case the alloy is very important, and should be representative, or one that is sensitive to stress corrosion cracking. In summary, the difference between “passing” or “failing” a compatibility test can be determined by how the test was done. The tests can be simple, but should always be as representative as possible. EXAMPLE OF A SUCCESSFUL COMPATIBILITY TEST To summarize this chapter, look at the example of “Clevis McKleen’s” new degreasing system. He would like to replace an old vapor degreaser with a new precleaning process. The better to understand his options, Clevis examines the things he cannot control about his process first. He knows he must remove light hydrocarbon and particulate contamination from oxygen regulators for diving equipment. The regulators have the potential to entrap his cleaner. Furthermore, he has several Teflon O-rings, and an anodized aluminum

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housing that are assembled into the oxygen regulators. Clevis must select an appropriate cleaning agent and process that safely clean the regulators. The first step, Clevis decides, is to set some boundaries based on the things he has no control over. Since the parts are used in a breathing air system, and can entrap solvent, he decides that he must use a cleaner that is relatively nontoxic. The entrapment potential also precludes the use of highly acidic or basic cleaners. He can consider flammable materials if he takes some safety precautions, and can use a volatile organic compound (VOC) if he can keep the emissions low. Water-based products and ultrasonic immersion baths are attractive options. Given the above conditions, Clevis selects a mild water-based cleaner he already uses for another process, and a 40 kHz ultrasonic bath for consideration. Clevis knows he needs compatibility data before implementing this process, and contacts the appropriate vendors. The vendor for the cleaning agent has compatibility data that indicate the cleaner will not damage any of Clevis’s materials, even at the proposed use temperature of 150° F and concentration of 10%. However, all of the data were taken from flat immersion panels. Since Clevis is worried about entrapment, he decides to augment the vendor data with a sandwich corrosion test of his own. The ultrasonic bath vendor also has compatibility data, but these data suggest that the 40-kHz bath may damage the anodized aluminum housing. The vendor offers an 80-kHz bath as a safer alternative. Clevis takes a scrapped regulator to the vendor for trial processing and finds that the anodize is not harmed by the 80-kHz process, but the higher frequency is unable to remove his particulate contamination. To make matters worse, Clevis gets his sandwich corrosion specimens back and the data do not look good. Even the mild cleaner caused corrosion to the aluminum specimens. Clevis decides to consider an organic solvent, perhaps in an immersion cleaning bath. He steers away from the ozone depleters, and the ones with poor solvency. He also must consider his facility’s VOC concern and rules out solvents with a vapor pressure over 50 mmHg. IPA appears to be a good candidate. A brief review of the published compatibility data on IPA indicates no problems with any materials. Clevis decides to run a simple immersion test with the Teflon O-rings, just to be sure. After 2 days exposure he observes a 2% weight gain and no change in shore hardness. However, during his literature review, Clevis finds that IPA can fuel an explosion in high-pressure oxygen systems. This makes the complete removal of the IPA essential, so Clevis decides to place the regulators in a vacuum drier at the end of the process. Clevis learns that his immersion bath is too high in VOCs, but that a recirculating flush with IPA will reduce the VOCs to an acceptable level and minimize the fire hazard. Knowing that he plans to use IPA, Clevis reviews all of the materials in the recirculation system with the equipment vendor for compatibility. No issues are identified, and Clevis confidently implements his IPA recirculation system for cleaning his regulators.

© 2001 by CRC Press LLC

SECTION 4

Process Selection and Maintenance

© 2001 by CRC Press LLC

CHAPTER 4.1

Evaluating, Choosing, and Implementing the Process: How to Get Vendors to Work with You* Barbara Kanegsberg

CONTENTS Introduction Before the Show During the Show Cleaning Agents Equipment Drying Follow-Up Conclusions References INTRODUCTION What is the best process for an application? All chemical and equipment vendors honestly see their own offering as the best available. Cleaning and process equipment can no longer be considered commodity items; there are choices. A trade show provides an unparalleled opportunity to compare those choices efficiently. There are simple approaches to reaching the goal of gaining information as quickly as possible at the show, then changing the process to make it better, faster, more cost-effective, safer, more environmentally sound. While enjoying the bells, whistles, and popcorn, it is possible to visit the exhibits efficiently and to determine the best of the new technologies for current and future process needs. At the same time it is possible to choose the best vendors to work with and set up efficient test protocols that provide the needed answers. * This chapter is based on a paper, “Getting the Most Out of Precision Cleaning ‘97—How to Get Vendors to Work with You,” presented at CleanTech ‘97, a conference sponsored by the Cleaning Technology Group, Witter Publishing Corp., April 1997, Cincinnati, OH. © 2001 by CRC Press LLC

Some key factors in success are: • • • •

Understanding the process requirements Communicating those requirements to the chemical and equipment manufacturers Determining the most-promising process options Following through after the show BEFORE THE SHOW

There are arrays of cleaning agents and cleaning equipment, with more possibilities appearing seemingly daily. Chemicals and process equipment can no longer be considered commodity items. Therefore, what is worthwhile depends on a number of basic factors. Step back for a moment and take a look at the product line. Estimate throughput and product mix and flow (now and in 3 years). Consider the major regulatory requirements for the area as well as critical company policy and customer requirements. Making a list of major process considerations prior to the show helps to clarify requirements both to the manufacturers’ representatives and to you. One hopes you do not have to consider all facets of process change on your own. Approaching process change as a team effort has been discussed on a number of occasions.1 The team approach can save wasted time and effort, particularly where studies by chemical and equipment vendors are required.2 If you are working with a team, you are fortunate; it will be easier to optimize the process. Based on team input and on your own considerations, a number of factors to consider are likely to emerge; some are indicated in Tables 1 through 4. Obviously, if you were to consider all of these factors while trying to walk through the exhibits, you probably would not Table 1 Preliminary Process Considerations: The Product Line (Suggestion: Circle the Factors Most Important for Your Process) Factor

Considerations

Component, subassembly, part characteristics

Size (l  w  h) or continuous feed

Materials of construction

Spacing of components (e.g., standoff of components from circuit board) Complex build, blind holes Product mix

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High throughput/high mix High throughput/low mix Intermittent changes in product line Major changes in product line expected

Implications, Examples Minimum tank size Optimal dimensions of cleaning tanks Type of equipment Compatibility with cleaning, rinsing agents Maximum acceptable temperature Acceptability of ultrasonics Wettability Rinseability Force of cleaning action (ultrasonics, turbulation, spray) Type of drying equipment Cleaning agent Drying system Automation

Table 2 Preliminary Considerations: Throughput, Cleanliness Requirements (Suggestion: Circle the Factors Most Important for Your Process) Factor Sample throughput

Considerations Approximate throughput Expected changes in 1 to 5 years Sample handling requirements

Cleanliness requirements

Visual Performance Specific process requirements Surface soils Particulates (size range) Specific customer requirements

Implications, Examples Maximum cycle time Drying requirements Soil loading Tank size Conveyor belts Overhead robotics Fixturing Overall process flow (beyond cleaning steps) Specific testing needed Laboratory support from vendor Ongoing surface monitoring Recirculating cleaning agent with real-time particle monitor

Table 3 Preliminary Considerations: Budget, Workspace, Utility Constraints (Suggestion: Circle the Factors Most Important for Your Process) Factor

Considerations

Budget

Chemicals Equipment Consumables

Water purification Solvent containment Waste stream

Workspace limitations

Current workplace utilities

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Closed loop; recycling Total footprint (l  w  h) of process equipment

Electrical Water Compressed air Nitrogen lines

Implications, Examples Initial cost Expected usage Basic cost Cost for full system Water filters Ion-exchange resin Carbon filters Other consumables Deionizers Higher freeboard Fully contained system Evaporators Holding tanks Steam or vacuum distillation Cleaning, rinsing, drying Conveyors Hoists, robotics Water recycling Evaporators Holding, separation tanks Solvent recyclers May limit your equipment choices

Table 4a Safety, Regulatory, Company and Customer Requirements and Requests (Suggestion: Circle the Factors Most Important for Your Process) Factor Environmental regulatory

Safety regulatory

Corporate policy; customer policy or requirement Insurance

Specifics Federal State Local VOCs Hazardous air pollutants (HAPs) Record-keeping requirements Effluent requirements cleaning agents; soils, trace metals Right to know Chemical composition and handling Equipment Ozone-depleting chemicals HAPs VOCs Other specific requirements Containment of low-flash-point solvents Containment of toxics

Table 4b Additional Company Requirements Factor Requests, requirements, strong prejudices Management Safety/environmental Production workers Workforce sophistication

Specifics Cleaning agent odor Overwhelming positive, or negative regarding aqueous, solvents, ultrasonics, low flash point, new technology Other considerations Customers Chemical handling Adjustment to major process change Computer literacy

get very far. These tables are simply to help you focus on those factors that are most important for your particular application. I suggest you look over the tables, circling those things that are of most concern, and keep them in mind while touring the exhibits. There is a proviso here. While it is necessary to avoid a totally random approach, one important factor in attending the show is to educate yourself about a range of cleaning agents, cleaning equipment, sample handling equipment, and advanced processes such as supercritical and near-supercritical cleaning, plasma cleaning, and laser ablation. If management, the safety/environmental people, or, most importantly, you box yourself in too much, too many cleaning options may be eliminated, to the detriment of company productivity. In some cases, the company may prohibit entire classes of cleaning agents. Often, the real problem is that additional record keeping and containment may be required; you may need to ask some pointed questions to determine the true regulatory requirements. It is important to choose the safest, most benign, environmentally preferred process possible. However, because successful cleaning action inherently implies a degree of reactivity, virtually all processes have some safety and/or environmental baggage. With the proper containment and chemical handling systems, a range of products can be used responsibly. © 2001 by CRC Press LLC

DURING THE SHOW As a start, it is a good idea to take notes, either on a general checklist or questionnaire, or in a bound notepad. Even the backs of business cards are okay, as long as you remember to keep them all in one place. You might ask, why take notes when all you have to do is let them swipe your card and brochures will appear in the mail? There are several good reasons. For one thing, taking notes helps you focus and prioritize after the conference. By the time several weeks (or months) have passed, when you have finally sorted through the brochures that now clutter your desk, the superior technologies may no longer be obvious. Notes supplement initial impressions of the technology, the vendor, and likely technical support from the vendor. Taking notes lets you document and track vendor claims. In my experience, talking with notepad in hand tends to make vendors a bit more specific and careful in their claims. If nothing else, taking notes conveys a sense of purpose, allowing you to gather essential information while simultaneously engaging in the all-important collection of canvas bags, balsa airplanes, pens, markers, and candy bars.

Cleaning Agents What used to be a narrow choice in cleaning agents and processes has exploded into an array of possibilities,3 leaving many users in a state of semipermanent confusion. Unfortunately, engineers have been known to adopt a new cleaning agent and even report the process change at public forums, while having no idea if they were using an aqueousbased, hydrocarbon blend, nonlinear alcohol, or some other chemistry. While it is not necessary to have an advanced degree in chemistry, an understanding of the basic nature of the cleaning agents being offered is essential to optimize any process and to achieve appropriate chemical handling. Types of cleaning agents can be classified as: • Aqueous (with either an inorganic or an organic solvent base) • Aqueous with significant amounts of organic solvents • Solvent blends (typically relatively high boiling), which can be rinsed with water and/or solvent depending on the specific formulation • Oxygenated solvents • Solvents for liquid/vapor phase cleaning The first thing you have to ask the person at the exhibit booth is, exactly what are you selling? What the chemical manufacturer calls a degreasing agent may mean a blend of chlorinated solvents, a low-flash-point solvent, hydrocarbons, orange or pine terpenes, esters, nonlinear alcohols, hydrofluorocarbons, hydrofluoroethers, n-propyl bromide, parachlorobenzotrifluoride, aqueous/saponifiers, or some other chemistry. A checklist of some important questions to ask the chemical vendors is indicated in Table 5. If a vendor offers a degreasing agent, it is important to determine exactly what that implies in terms of process change. Often, the engineer is advised to convert liquid/vaporphase cleaning equipment into a dip tank of the new cleaning agent. For some general metals cleaning applications, this approach may be successful. However, other users have had problems with such a changeover, including impaired product performance. High-boiling materials like water and terpenes may not dry rapidly enough. They may leave significant cleaning agent residue; a single-tank system does not provide for rinsing.4 While manufacturers may © 2001 by CRC Press LLC

Table 5 Initial Evaluation, Cleaning Agent Checklist Company (Chemical Vendor): Date: Contact:

Phone: What cleaning agents do you offer?

Aqueous/saponifier Bicarbonate (liquid/solid) Semiaqueous Orange terpene Pine terpene Hydrocarbon blend Ester blend Tetrahydrofurfuryl alcohol n-Methyl pyrollidone Alcohol (specify) Other solvent blend (specify) Microemulsion

Solvent (specify) Chlorinated (specify) Brominated (specify) Combustible Flammable Parachlorobenzotrifluoride PFC HFC HFE HCFC 225 VMS NMP Other: ______

If it is proprietary, can we obtain the information after signing a confidentiality agreement? Regulatory status: Ozone depletion potential (ODP) SNAP status: approved? pending? Volatile organic chemical (VOC)? Hazardous air pollutant (HAP)? Combustible? Flammable? Inhalation studies? acute? 90 day? Please provide: MSDS

inhalation level?

Technical/Applications Sheet Based on major soils and materials of construction, which products do you recommend? Costs, performance: Cost per pound/per gallon Soil loading Cleaning/performance studies available? Compatibility information What is the evaporation rate? Similar to alcohol? Similar to water? Can I use liquid/vapor cleaning? Can I use my current degreaser? Do I need to rinse for my application? Do I need a rust inhibitor in the rinse water? Is the cleaning agent miscible in the rinse agent (i.e., will it dissolve, or will it form two layers)? What process equipment do you recommend? Is the equipment displayed at this show?

© 2001 by CRC Press LLC

Table 5 Initial Evaluation, Cleaning Agent Checklist (Continued) Can I retrofit my current equipment? Cleaning Drying Ultrasonics Solvent containment Spray in air Recycling/closed loop Spray under immersion Other Rinsing Drying options: Air knives Moisture absorbing materials Forced hot air Drying by solubilization (alcohols, etc.) Vacuum drying Drying by displacement (surfacant-based solvents) Centrifugal drying Convection ovens What kind of solvent containment will I need? Vendor support: Can you supply a sample for in-house testing? Do you have an applications lab? Or can you perform testing at my site? Name of chief chemist: Phone: Fax: Can you do cleaning/soil loading studies for me? Disposal of spent material Waste stream treatment? On-site recycling? Take-back of spent solvent?

claim that rinsing is not necessary, some users have found that rinsing is required for highreliability applications.5 Further, aqueous or viscous blends may not penetrate spaces in low-standoff components, and it may be difficult to achieve cleaning and drying for complex assemblies with blind holes. Some cleaning agents have toxicity and/or regulatory issues. Many of the newer designer solvents require well-contained systems to be costeffective. You should be very suspicious of supposedly safe mystery mixtures. Insist on knowing what you are working with. Typically, you simply cannot clean parts in chicken soup, and even chicken soup might have a substantial volatile organic compound (VOC) content. If the cleaning agent supplier will not conveniently and cheerfully supply a confidentiality agreement covering your company and pertinent consultants including: • Lawyers • Environmental advisors • Process development specialists I suggest finding a different supplier. Equipment In evaluating any new piece of cleaning equipment, you need a ballpark idea of: • Price • Footprint • Suitable cleaning agents © 2001 by CRC Press LLC

Because cleaning equipment is so specialized and given the fear of sticker shock, the equipment manufacturer may be reluctant to provide pricing information, sometimes even claiming confidentiality. Both the price and footprint can vary, particularly for processes where parts handling, rinsing, drying, and recycling/closed loop are issues. Rinsing and drying modules can double or triple both costs and space requirements. A checklist of information to obtain from cleaning equipment manufacturers is indicated in Table 6. Again, it is neither necessary nor productive to ask all of these questions to all equipment manufacturers. Simply mark the most important factors for your application. For large-scale processes, sample handling equipment can add more than intended height to the overall footprint. A number of components manufacturers have required lastminute workplace remodeling because the ceilings were too low. One needs to allow room for the overhead hoists and robotics to move. In some cases such as ultraviolet/ozone cleaning, plasma cleaning, and laser cleaning, the cleaning agent or cleaning action is effectively generated by the process. For liquid CO2, supercritical CO2, or CO2 snow, the main concern is cleanliness of the CO2 (or other gas or gas mixture under consideration). In most cases, however, the optimal cleaning process is a combination of the cleaning agent and the cleaning equipment. To make an informed decision, it is important to learn as much about both aspects of the process at the same time. Because equipment manufacturers are often geared to the mechanical aspects of cleaning, they may downplay the importance of the chemical aspects. Therefore, it is important to determine what cleaning agents can be used with the particular equipment in question. Drying Given that many parts nest or have complex structure, drying can be difficult. It is a factor that is often not considered in simple dip-tank conversions. In fact, inadequate drying can lead to recontaminated or damaged, parts,6 and drying often becomes the limiting step in nonsolvent production processes.7 –9 As outlined in Table 6, a number of approaches to drying are available; all have their positive and negative features.10 FOLLOW-UP After the show, your notes should assist you in sorting through the wealth of brochures and catalogs that are certain to follow. However, stacks of notes and brochures alone will not result in process change. This is a good point to obtain input from: • • • • • •

In-house environmental/safety group Insurance company Company management Local regulatory agencies Federal agencies such as the EPA SNAP group Fire department

If any of the information seems unclear or doubtful, it is wise to follow your instincts and check further. With this input, a relatively short list of promising possibilities should emerge. © 2001 by CRC Press LLC

Table 6 Initial Evaluation Checklist, Cleaning Equipment Company (Cleaning Equipment Vendor): Date: Contact:

Phone:

Type of equipment: Cleaning agents/cleaning sequences that can be used: Please give me some typical examples Cycle time (minutes): range average Costs, Availability, Training: Base cost: Cost “fully loaded”: Average system cost: Shipping, installation costs: Employee training availability, costs: Time to build from receipt of order (weeks): Appearance/footprint, tank size Overall impression Materials of construction Coved tank? Welding Footprint (l  w  h, in.) Base equipment Total equipment (including robotics, recycling, separation tanks) Cleaning chamber capacity (l  w  h, in.) Chamber capacity (gallons) Number of tanks or chambers needed Fixturing: sizes, materials of construction Vendor support and reliability: Can you supply a loaner model for in-house testing? Do you have an applications lab? Or can you perform testing at my site? Name of chief chemist or engineer: Phone: Fax: Can you do cleaning/drying/soil loading studies for me? Name, location of repair person Warranty UL/FM approval Have you worked with my local regulators? Can I get permitting in my area? What support can you provide? Cleaning, rinsing, drying: Heat (adjustable temperature?) Over 100 kHz Spray (force of spray) Megasonics Rotation Rinse action Mechanical agitation Weir/sparger Spray under immersion Drying Centrifugal Air knives Immersion Forced air Ultrasonics Vacuum Standard or sweep Other 25, 40, 60, kHz, other © 2001 by CRC Press LLC

Table 6 Initial Evaluation Checklist, Cleaning Equipment (Continued) Auxiliary equipment, pollution prevention: Do I need, do you recommend: Vapor containment Meets NESHAP? Closed loop Hoist Batch robotics Parallel robotics Solvent/cleaning agent filtration On-board recycling Vents Chillers Atmosphere inerting Fire suppression (nebulizers, CO2) Separation tanks Waste stream treatment Deionization Reverse osmosis Other Facilities requirements for Electrical Air Nitrogen Deionized water Water for chilling Venting Other requirements

Working with the vendors is the next challenge. You must estimate how well the new process will perform for your application. This is no easy task; many engineers, chemists, and economists all struggle with this issue—there is no foolproof approach. Further, if the equipment and chemical purchases are small, vendors may not be able to economically justify testing. There are still several actions you should take, which are critical not only for small but for larger equipment installations. Ask for an in-house demonstration. Ask for references; then, call those components manufacturers. Ask to visit some similar equipment installations. In the author’s experience, components manufacturers can provide a wealth of frank information about process performance, vendor response to crisis, repairs, maintenance, and downtime. If you are still not ready to make a commitment, ask about leasing the equipment. One problem in setting up tests with vendors is avoiding a generic approach and a generic response. Often, the vendor will say, “Of course, we can clean your component.” You carefully pack up the samples, send them in for test. The applications laboratory cleans the samples with the cleaning juice they most commonly use (never mind what you asked for) at the default settings (never mind that the temperature is too high for a critical component) and reports back that the parts were successfully cleaned. Even more frustrating, with the current pressure for process changeover, some components manufacturers have waited months for test results. A few suggestions to avoid such delays and meaningless testing include: © 2001 by CRC Press LLC

• • • • • • • • •

Describe soils, materials of construction Outline cleaning standards Provide some parts which you judge to be acceptably clean as a standard Explain constraints and limitations (e.g., maximum temperature) Be very specific about desired cleaning agents, cleaning sequences Make up a standard cleaning form Do not abuse the test laboratory by asking for large numbers of cleaning studies Provide packaging and shipping instructions Get to know the applications chemist CONCLUSIONS

While regulatory constraints were the initial motivation for process change, precision cleaning has become an integral part of ongoing process improvement. It is possible to survive, even enjoy a trade show. More importantly, by outlining requirements, asking specific questions, taking notes, understanding the chemistry and technology, and following through with your process team, it is possible to make your company more productive. REFERENCES 1. Kanegsberg, B., The chemical balance: performance, economy, and safety, in Nepcon West ‘95 Proc., March, 1995. 2. Kanegsberg, B., H. Mallela, H. Dominguez, and W.G. Kenyon, Integrating precision de-oiling and defluxing processes in high volume manufacturing systems, in IPC Proc., May, 1995. 3. Kanegsberg, B., Precision cleaning without ozone depleting chemicals, Chem. Ind., October, 21, 1996. 4. Kanegsberg, B., Resolving compatibility issues with aqueous cleaning of ferrous metals, specialty alloys, and complex composites, in Proc., Int. CFC & Halon Alternatives Conf., Washington, D.C., October, 1995. 5. Kanegsberg, B., Cleaning high value components for biomedical and other applications, Proc., Int. Conf. Ozone Protection Technol., Washington, D.C., October, 1996. 6. Schreitmueller, R., Watching the boards dry, SMT Mag., August, 1996. 7. LeBlanc, C., Exploring cleaning options with an eye on quality control, Precision Cleaning Mag., March, 1995. 8. Durkee, J., Why is drying so hard with aqueous cleaning technology? Products Finishing, September, 1995. 9. VanderPyl, D.J. and K. McGlothlan, Precision drying completes precision cleaning, Precision Cleaning Mag., March, 1996. 10. Kanegsberg, B. and S. Seelig, Precision cleaning options for the‘90’s, in Precision Cleaning ‘96 Proc., Anaheim, CA, May, 1996.

© 2001 by CRC Press LLC

CHAPTER 4.2

Optimizing and Maintaining the Process Michael S. Callahan

CONTENTS Introduction Good Operating Practices Improving Part Drainage and Racking Practice Monitoring Bath Performance Monitoring Bath Quality Loss Prevention Practices Process Improvements Use Demineralized Water for Makeup Increase Tank Agitation Eliminate Use of Air Sparging Remove Soils from the Bath Employ Two-Stage Cleaning References INTRODUCTION* The adoption of pollution prevention practices in the field of cleaning should follow a prescribed sequence or hierarchy of investigation. The first step is to eliminate the need to clean by eliminating upstream contamination or by relaxing an overly critical cleanliness requirement. If this is not possible, and it usually is not to a large extent, the part or contaminant may be modified to enable the use of a less hazardous cleaning material or method. The replacement of a heavy cutting oil with a water-based lubricant is an example of contaminant modification. Solvent may be required to remove the cutting oil while the lubricant may be wiped off or rinsed off with water. The remaining steps of the hierarchy include:

* Portions of this material exerpted from Callahan, M.S. and Green, B., Hazardous Solvent Source Reduction, McGraw-Hill, New York, 1995. With permission. © 2001 by CRC Press LLC

• Select the least hazardous cleaning material and/or method that achieves the desired level of cleanliness; • Optimize and maintain the process so that the greatest amount of soil is removed for a given amount of cleaning material consumed; • Keep all resulting wastes segregated to promote their recyclability; and • Recycle and/or reuse spent cleaners to the extent practical. Solvents may be recycled via distillation; aqueous cleaners via ultrafiltration. Spent cleaner from a critical cleaning operation might be used for maintenance cleaning. The following sections discuss the optimization and maintenance of the cleaning process by means of good operating practice and the improvement of the cleaning system. Good operating practice typically involves a change in procedure while improvement of the system often involves equipment modification. Discussions of ways to recycle spent solvent and aqueous cleaners are presented in several chapters.

GOOD OPERATING PRACTICES While much has been said and written about good operating practices, they are still the most cost-effective but under-used methods for improving efficiency. The enforcement of good operating practices can result in reduced chemical use, reduced emissions, improved cleaning efficiency, and a safer working environment. Implementation can often be achieved at little or no cost. Various methods include defining worker responsibilities and providing training; following proper start-up, shutdown, maintenance, and repair procedures; locating equipment away from areas where it will be prone to upsets; and segregation of the wastes produced to promote the potential for recycling. With regard to vapor degreasing, many of these methods must be practiced by law.

Improving Part Drainage and Racking Practice Dragout, the cleaning solution that remains on parts as they are pulled out of the bath, is one of the major reasons for material loss. High levels of dragout represent an expensive loss of cleaner. Since the presence of cleaning chemicals on the part may interfere with subsequent processing operations, removal of dragout by rinsing is often required. When all is said and done, the best place for the cleaner to be is in its own tank where it belongs. Unfortunately, the complete elimination of dragout is not possible. The physical shape of some parts will catch and trap fluids that can only be removed by rinsing with another fluid, typically water. This is common when attempting to clean bent tubing or components with narrow passages. Specific cures may be possible, but what works for one part may not work on others. Facilities engaged in maintenance cleaning, where a wide variety of parts must be handled, are faced with an ever-changing challenge. Hence, the search for ways to reduce dragout should be viewed as an attempt to achieve the optimum answer as opposed to the absolute best answer. To find ways to reduce dragout, it is often helpful to understand how dragout occurs. Dragout may be viewed as two sequential processes involving liquid removal and drainage. The thickness of a liquid film that clings to a flat vertical surface as it is removed from a bath is mainly a function of the inertia or resistance to flow of the liquid (i.e., its viscosity) and the speed of removal. © 2001 by CRC Press LLC

Parts removed quickly from a process bath will have much more liquid cling than parts removed slowly. Equations have been proposed for the modeling of withdrawal losses due to dragout on flat panels,1 and the one developed by Kushner2 is presented below. Kushner developed his model by means of dimensional analysis using dragout data presented by Soderberg.3 While the Kushner model may not be as robust as other more complex models, it can be used to explain many observations made in the field. The equation relates the volume of liquid removed to three part parameters (area, vertical length, and speed of withdrawal) and two liquid parameters (viscosity and density) as shown: V  0.02 . A .

l   t.  .

(1)

where V  volume of liquid, cm3 A  area of piece, cm2 l  vertical length, cm   viscosity of liquid, poise t  time of withdrawal, s   density of liquid The major point to note in the equation above is that for a given fluid held at constant temperature and concentration, withdrawal losses are a function of area, length, and withdrawal speed. The slower a part is withdrawn from a bath, the less liquid will cling. For flat parts with no recesses or crevices, a slow rate of withdrawal will minimize dragout and eliminate the need for prolonged draining. The issue of withdrawal speed vs. drainage time will be discussed shortly. To illustrate the impact withdrawal rate plays in the determination of initial dragout, data presented by Soderberg may be employed. Soderberg reported a dragout value of 10.8 ml of a zinc cyanide plating solution for a 4  12 in. panel. The panel was removed from the plating solution at a rate of 0.5 s/f (2 f/s) and allowed to drain for 0.5 s following removal. Solution loss that may have occurred during the 0.5-s drainage period may be ignored. The 10.8 ml value equates to a dragout of 4.28 gal/1,000 ft 2 of wetted surface. Knowing the volume of solution removed and the speed of withdrawal, and holding all other variables constant such as area, length, viscosity, and density, Equation 1 may be rearranged as shown and the system constant determined.

1t

(2)

K  V . t

(3)

VK.

Plugging the dragout and withdrawal speed values into Equation 3, a K value of approximately 3.0 is derived. Therefore, the withdrawal loss for this system as a function of withdrawal speed may now be determined. Assuming a speed of 10 s/ft (0.1 ft/s), dragout is estimated to be 0.96 gal/1,000 ft 2. This represents a 78% reduction in dragout from the 4.28 gal/1,000 ft 2 value. Reduced withdrawal speeds may be accomplished by reprogramming conveyorized systems or by regearing hoists. Manual operations are most difficult to control. The last thing an operator wants to do is to remove a 20 lb. part slowly from a bath of liquid. Not only is this uncomfortable, it places a tremendous strain on the operator. This is where the use of an overhead hoist is essential. Following withdrawal, drainage is the next part of the process that determines overall liquid loses due to dragout. All of the previously mentioned factors apply including the © 2001 by CRC Press LLC

surface tension of the liquid. Surface tension may cause liquids to be retained within crevices and small openings, hence increasing dragout. To overcome the effect of surface tension, wetting agents may be added to the cleaner and rinse water. To illustrate the effect increased drainage time plays in the determination of overall losses, the dragout data developed by Soderberg is again employed. Table 1 lists three sets of data representing 0.5, 2, and 10 s of drainage following withdrawal at various withdrawal speeds. The 2- and 10-s drainage data are from Soderberg while the 0.5-s data are derived from Equation 3. The data shows that increased drainage time can have a pronounced effect depending on the initial amount of dragout present (i.e., dependent on the withdrawal speed). Drainage time is most critical when parts are removed rapidly from the bath. When parts are removed slowly, drainage time is of lesser importance. The data show that similar amounts of dragout occur for a 0.5 s/ft withdrawal speed and 10-s drainage period (10.5 s overall for a 1-ft part) as they do for a 5-s/ft withdrawal speed and 0.5-s drainage (5.5 s overall). The benefit of long drainage times is negligible when flat panels are removed at a rate slower than 4 to 6 s/ft. When processing flat parts that do not retain liquid, always favor withdrawal time over drainage time. Unfortunately, few parts are truly flat and free draining. The real-world importance of drainage time may be seriously understated in the work conducted with flat panels. Liquid may pool on a flat horizontal surface and be carried out of the bath. Hollow recesses may fill with liquid and be difficult to drain. Such parts may require rotation following withdrawal from the bath to allow the trapped liquid to drain out. Soderberg investigated the effect part configuration plays in the determination of overall losses due to dragout. His often-quoted estimates of dragout for various part configurations are presented in Table 2. By following proper racking practices, dragout can be reduced by 50% or more. Proper racking may involve the proper orientation of parts inside a basket so as not to collect and retain liquid. Drinking glasses placed inside a dishwasher should always be placed upsidedown to promote drainage. Dishes are best cleaned by stacking them vertically. Why should the cleaning of industrial parts be different? For complex shapes, the determination of the most optimum racking position will likely require experimentation. Modification of the part to promote drainage (e.g., by adding several drainage holes) may be a possibility, but it tends to be a limited solution in practice. Bigge and Graham4 present an example where 0.25 in. holes were added to the ends of a “wrap-around” automobile bumper. The hole in the lower bend allows cupped solution to drain while the Table 1 Effect of Withdrawal Speed and Drainage on Dragout Dragout (gal/1000 ft2) per Drainage Time (s) Withdrawal (s/ft)

0.5a

2b

10b

0.5 2 4 6 8 10 15 20

4.28 2.14 1.51 1.24 1.07 0.96 0.78 0.68

2.32 2.12 1.68 1.21 1.10 0.86 0.69 0.60

1.35 1.31 1.26 1.17 1.10 0.86 0.69 0.60

a

Predicted dragout values derived from Kushner equation. See text for explanation. Dragout values derived from Soderberg for a zinc cyanide plating solution draining from a 4  12-in. panel. Source: Callahan, M. S. and Green, B., Hazardous Solvent Source Reduction, McGrawHill, New York, 1995. With permission. b

© 2001 by CRC Press LLC

Table 2 Effect of Part Configuration and Drainage on Dragout Parts

Dragout Loss (gal/1000 ft2)

Vertical, well drained Vertical, poorly drained Vertical, very poorly drained Horizontal, well drained Horizontal, very poorly drained Cup shaped, very poorly drained

0.4 2.0 4.0 0.8 10.0 8 to 24

Source: Soderberg, G., in Proceedings of the Twenty-Fourth Annual Convention of the American Electroplaters Society, 24, 233–249, 1936. With permission.

upper hole allows trapped air to vent from the underside of the bend. Another example was the replacement of a hollow channel fabricated from rolled steel with a die-cast channel. The die-cast channel, being solid, eliminated the problem of solution entrapment. While each part may represent a unique challenge, the guidelines presented in Table 3 have been found to be effective at many facilities. In a manual operation handling small racks or baskets of parts, installation and usage of drainage bars may help promote liquid drainage and reduce dragout. Such bars run the length of the tank and are used to hold the parts while they drain. This eliminates much of the physical strain placed on the operator. To remove and recover most of the dragout and to minimize the potential for spotting, aqueous cleaned parts may be rinsed off by spraying with demineralized water. Spray rinsing is conducted over the cleaning tank by means of a small, hand spray bottle. For parts cleaned in solvent, a quick blast of compressed air may be helpful for removing entrapped liquid as the parts drain over the cleaning tank. Monitoring Bath Performance Rigorous monitoring of bath performance is a given practice in precision cleaning. Many online techniques and devices have been developed in recent years to ensure critically clean parts. While the cost of these techniques and devices continues to drop, most are still outside the realm of practicality for maintenance and industrial cleaning operations. For this reason, they are excluded from this discussion. Table 3 General Guidelines to Minimize Dragout • Favor slower withdrawal speed over longer drainage time. For a fixed time cycle, spend two thirds of the time withdrawing the part from the bath and one third of the time draining over the bath. • Drain parts over the tank or use drainage boards to extend the effective length of the tank. Drainage boards may be installed inside manual tanks to serve as a scrubbing table. Make sure drainage boards slope back toward the tank so that drained liquid is returned. • Do not rack parts directly over one another. Drippage from the top parts may dirty the parts below and cause spotting or staining. • Orient part surfaces as close to vertical as possible. Rack parts with their lower edge tilted from the horizontal. Runoff should be from a corner rather than an entire edge. Burp bars may help to knock solution off the part after removal from the tank. • Ensure that all cavities and recessed pockets are oriented downward. Rotation of parts during withdrawal and drainage may help. • Use the correct size and type of basket to minimize the wetted surface area of the basket. A wire mesh basket can retain much liquid if the mesh is too tight. Never use porous materials such as ropes or cloth bags to hold parts. Source: Callahan, M. S. and Green, B., Hazardous Solvent Source Reduction, McGraw-Hill, New York, 1995. With permission. © 2001 by CRC Press LLC

Given the diverse nature of cleaning activities, it is not surprising to find that dozens of methods have been developed to test cleaning performance. Most of the methods employed test only the effectiveness of soil removal and not the actual level of surface cleanliness. Common test methods include visual inspection and paper tissue wiping, water break and spray atomizer testing, copper dipping, paint adhesion, gravimetric measurement, and solvent monitoring. Cleaning bath performance may be monitored by periodically running test coupons through the bath and noting any change in cleaning efficiency. Coupon tests are commonly used to rule out the cleaning bath as a source of trouble following a sudden increase in rejects. Visual inspection and paper tissue wiping are two highly subjective but widely used methods to denote cleaning effectiveness. The tests are limited to visible soils, mainly grease, soot, and particulate matter. For visual inspection, the use of a microscope by a trained individual increases the validity of test results. Determination of particulate contamination by the paper wipe method is sensitive to the pressure applied while wiping. The method is most sensitive when performed on a wet surface rather than a dry surface. Water break and water spray atomization are quick and effective ways to determine the cleanliness of a cleaned part. The method is based on the observation that water will not bead on a clean surface; rather, it will form a continuous film. Cleaned parts may be dipped into water and the flow behavior of the water as it drains from the part noted. In the spray method, the formation of beaded areas denotes the presence of hydrophobic soils. Both of these methods are best performed on flat parts by trained observers. Copper dipping is often used to test the effectiveness of a cleaning operation that will precede an electroplating step. The method utilizes an acid copper sulfate solution (often referred to as copper flash) to lay down a layer of copper on the clean part. Improperly cleaned areas are revealed by either poor adhesion of the copper plating or discoloration of the copper. The method is very sensitive but is limited to ferrous metals and requires an experienced operator to produce consistent and reliable results. The paint adhesion method is based on the assumption that an improperly cleaned part will not provide a good surface for paint adhesion. Clean, dry parts are painted and allowed to cure for a specified length of time. A baking cycle may be employed to speed curing. The paint is then cross-cut with a razor blade and a piece of adhesive tape placed over the area. The cuts should be of equal width, forming a checkerboard pattern. The depth of cut should be down to the substrate. The tape is lifted off at a steady rate of pull and the number of squares remaining serves as an indication of part cleanliness. Gravimetric measurement involves the weighing and soiling of a test piece with a known contaminant and then weighing the piece both before and after cleaning. Cleaning effectiveness may be reported as the weight or percentage of contaminant removed. The direct measurement of removal has good sensitivity but does not truly indicate surface cleanliness. Residual films of cleaning solution, metal and cleaner reaction products, or metal removal and etching by the cleaner can affect weighing measurements. Monitoring Bath Quality In addition to soil removal efficiency and cleaning effectiveness, there are a number of other measurements that may be of use in monitoring bath quality. These measurements include soil loading, acid acceptance, and alkalinity. Since bath quality is monitored by measuring a bath sample directly, these tests may show up potential problems before an increase in rejects occurs. Field test kits have been developed by the military for monitoring soil loading in maintenance cold-cleaning baths.5 The military found that leaving the decision to replace © 2001 by CRC Press LLC

solvent up to the operator either resulted in excessive solvent use or questionable cleaning performance. Some operators would tolerate very high levels of contamination, while others would change-out solvent as soon as they noted a slight decrease in the rate of cleaning. The intent of developing a simple field kit was to place the decision for solvent replacement on a more quantifiable and consistent basis. The most reliable combination of tests identified for Stoddard solvent (PD-680) included light transmittance, electrical conductivity, and specific gravity. For solvents used in vapor degreasing, soil loading may be determined by a change in specific gravity, by gravimetric analysis, and by monitoring the boiling sump temperature. High soil loading often requires an increase in temperature to maintain adequate vapor generation. For industrial cleaning, degreaser solvents may be replaced when they become loaded with 25% or more of oil. This loading typically equates to a 6°F increase in boiling point. To monitor the effectiveness of particulate contamination removal, the solvent used for cleaning may be passed through a membrane filter and a particulate count performed. This method can also be used for testing the particulate removal efficiency of aqueous cleaners (the testing of oil and grease removal requires the cleaner to be evaporated, which would result in erroneous weighing due to mineral salts). For testing the removal efficiency of ionic contaminants, deionized water rinses may be monitored for resistivity. This method is commonly used to test for the removal of ionic fluxes from soldered printed circuit boards. Resistivity measurements may also be used to check for ionic surfactant contamination following the water break test. Halogenated solvents used in vapor degreasers should be routinely checked for acid acceptance. Both 1,1,1-trichloroethane and methylene chloride are highly susceptible to breakdown in the presence of catalytic contaminants such as aluminum or zinc metal fines. Other causes of acid formation include degreasing of parts wet with water or water-based fluids and failure to remove routinely lost metal parts, oils, fines, and sludge from the boiling sump. When solvent breakdown occurs, hydrochloric acid is formed that may etch parts and cause extensive equipment damage. To prevent breakdown, stabilizers are added to the virgin solvent by the producer. A number of causes may result in stabilizer loss and the most direct way to determine solvent stability is to perform an acid acceptance test. This test should be performed monthly or quarterly as facility conditions warrant. Users of perchloroethylene and trichloroethylene should also perform acid acceptance tests if their solvent is reclaimed or used for extensive lengths of time. While much less of a problem in terms of part etching or cleaning equipment damage, acid formation may also occur in nonhalogenated solvents. Exposure to air and the failure to remove sludge routinely from the cleaning bath may lead to the formation of organic acids. Many of the soils typically encountered in an industrial setting are acidic and these too can lead to acid buildup. Monitoring of the acid acceptance value of a given organic solvent may be used to monitor the quality of the solvent during use. Acid acceptance is typically reported as the milligrams of potassium hydroxide needed to neutralize the acid in 1 g of solvent. The titration is often performed on a water extract of the solvent (i.e., the solvent is mixed with an equal volume of water and the separated water phase is then titrated to determine acidity). A water extraction can only be performed when testing water immiscible solvents such as halogenated solvents and petroleum distillates. Emulsion cleaners may be tested directly. The monitoring of aqueous cleaners is typically limited to the determination of alkalinity or acidity by titration. Since many alkaline cleaners have a reserve capacity for producing hydroxide ions, titrations often involve determining both the free and total alkalinity of the cleaning bath. Titrations are performed by first adding a few drops of © 2001 by CRC Press LLC

indicator to a fixed volume of cleaner and then adding a standard acid solution until a color change takes place. The amount of acid added is a measure of free alkalinity. A second indicator is then used and the acid addition repeated to determine total alkalinity. By comparing the level of free alkalinity to total alkalinity, changes in the quality of an alkaline cleaner can be monitored. Such changes can be due to loading of the cleaner with acidic soil, excessive dragout, use of hard water, and failure to make proper additions of replacement cleaner. Monitoring of bath strength can also be used to determine if undue dilution of the cleaner is occurring. This is a common problem with poorly maintained automatic makeup controls. A slow leak of makeup water into the tank can result in excessive loss of cleaner and poor cleaning performance. By checking bath strength both before and after a period of inactivity, such as over a weekend or holiday, problems associated with makeup valve leakage can often be identified. Before leaving this section, it should be pointed out that the cleaning performance of an aqueous bath is a function of concentration, temperature, processing time, and agitation. Failure to maintain and control all four variables can result in poor performance. Increasing any one variable can improve efficiency, but there are limitations. Increasing the concentration of cleaning agent can improve the rate of cleaning, increase the soil loading capacity of the bath, and reduce the need for frequent bath additions. On the negative side, high concentrations of cleaner can result in tarnished or etched parts and increased dragout as the result of an increase in bath viscosity. If the cleaner does not rinse readily from the part, rejects may increase as the cleaning agent now becomes a soil in subsequent processing operations. In some critical cleaning operations, where the amount of soil to be removed is very slight, using a very low concentration of cleaner offers the best overall performance. As with concentration, increasing bath temperature also speeds cleaning. Solid soils such as fats and waxes must be heated above their melting point before they can be removed. A higher operating temperature can often be used to improve a poorly operating bath. However, operating a bath at too high a temperature can warp parts or set certain soils, making them harder to remove. A higher operating temperature also results in increased energy usage and greater evaporative loss. Loss Prevention Practices The inadvertent spillage of a solvent, caustic, or acidic chemical represents a direct loss in process efficiency and often results in the generation of hazardous waste. While efforts should be made to recover as much of the spilled material as possible so that it may be used for its intended purpose, this is not always possible. The recovered materials may be too contaminated for use or they may be in a form too difficult to recover. Spills and leaks represent a decrease in process efficiency since more raw materials must be purchased for a given amount of cleaning. In addition to the costs associated with raw material procurement, cleanup, and waste disposal, serious leaks and spills may result in fines due to regulatory enforcement actions. While the potential for leaks and spills can never be completely eliminated, there are numerous ways in which this potential can be reduced and mitigated. One way is to implement and support a strong maintenance program, whether preventive or predictive. Good maintenance programs can be instrumental in cutting production costs stemming from expensive equipment repairs, excessive waste generation and disposal due to upsets, and business interruptions. Other ways to reduce the potential for leaks and spills are presented in Table 4. © 2001 by CRC Press LLC

Table 4 General Guidelines to Prevent Leaks and Spills Bulk Storage • Equip all storage and process tanks with overflow alarms and test them routinely • Maintain the physical integrity of storage tanks over time; perform monthly visual inspections of all tanks and vessels, especially the weld seams; take corrective action at the first sign of weeping or corrosion • Set up administrative controls for all loading, unloading, and transfer operations • Store volatile solvents in small pressure vessels or large floating roof tanks as opposed to fixed roof tanks; compared to a fixed roof tank, floating roof and pressure tanks may reduce solvent emissions by as much as 95 to 100% • Install sufficient secondary containment facilities to hold the stored materials in the event of equipment failure or accident Drum Storage • Store drummed materials and wastes inside a well-lit and secured area, on pallets to prevent rusting due to concrete sweating; use of self-contained storage pallets is preferable; maintain adequate row spacing to allow for visual inspection of each drum • Store different chemicals in different areas to prevent cross-contamination in the event of a spill; fire codes mandate that incompatible materials cannot be stored together • Provide adequate berming to prevent product or waste migration and contamination of the surrounding area in the event of a spill; consideration must also be given to containing or handling the large volume of water that might enter the bermed area in the event of a fire or accidental activation of the fire suppression sprinkler system Process Piping • Adopt a good piping layout whereby valves are easy to operate, are properly identified, and appropriate interlock and lockout devices are included • Implement a valve quality program to purchase only valves that are certified to be leak-free or zero-emission types • Adopt the use of magnetically driven seal-less or canned pumps in place of mechanically sealed pumps Source: Callahan, M. S. and Green, B., Hazardous Solvent Source Reduction, McGraw-Hill, Inc. New York, 1995. With permission.

PROCESS IMPROVEMENTS Process improvements often involve physical modification of the existing equipment or installation of additional equipment. Many of these methods can provide substantial increases in process efficiency, but the ability to implement them is not always present. Improvements requiring substantial physical modification to the equipment may not be possible because of the questionable integrity of the existing system. In these cases, replacement of the system with a new one may be a more viable and cost-effective approach. Use Demineralized Water for Makeup With the use of aqueous and semiaqueous cleaners, makeup water is required to compensate for losses due to evaporation and dragout. Use of demineralized water is preferred over use of tap water since tap water may have a very high mineral or dissolved solids content. As the water evaporates from the tank, these minerals are left behind. To prevent their precipitation, many cleaning formulations employ additives such as chelators that tie up © 2001 by CRC Press LLC

and maintain these minerals in solution. This is fine as long as the additives are active and if their use does not create problems elsewhere. For example, chelators may complex with heavy metals such as lead and make their treatment and removal from rinse water much more difficult. In addition to these additives, sodium hydroxide (caustic) will react with the minerals and serve to soften the water as it is added to the tank. It has been reported that as much as 10 to 25% of the cleaner may be consumed by this effect.6 This leads to more frequent bath replacement and higher operating costs. Use of softened (or preferably demineralized) water avoids this problem and makes monitoring of bath quality easier since it eliminates a potential source of bath contamination. Rinsing operations can also benefit from the use of demineralized water. Use of demineralized water for rinsing is often considered to be costly and impractical because of the large quantities of water involved. However, use of demineralized water may allow for countercurrent and closed-loop rinsing techniques, which can substantially reduce the need for water. In the electroplating industry, some shops have been able to eliminate all rinse water discharges by use of demineralized water and closed-loop rinsing techniques. An important point to note is that much of the hazardous sludge produced by a wastewater treatment system consists of nonhazardous minerals. These minerals enter the system by way of the raw water supply used for rinsing. Chemical analysis of wastewater treatment sludge from shops using tap water for rinsing often shows the sludge to consist of 80% or more nonhazardous calcium and magnesium minerals. By removing these minerals from the water before it is used, the generation of hazardous wastewater treatment sludge is minimized. Increase Tank Agitation Mechanical agitation, in the form of a liquid jet pump, spraying, basket rotation, basket raising and lowering, ultrasonic cavitation, or manual brushing, is an effective way to increase cleaning efficiency. Many solvents used in cold cleaning applications are replaced when the speed at which they clean is no longer acceptable to the operator. This may correspond to a contaminant level of 2 to 3%. By providing mechanical agitation, soil loading levels as high as 10% can be achieved. Disadvantages of mechanical agitation include higher electrical costs, more equipment to maintain, and increased worker exposure depending on the method employed. One development in the field of tank agitation is the use of small, seal-less, in-tank agitation pumps. By placing the pump inside the tank, energy losses due to piping pressure drops and the potential for solution spillage due to pipe or seal leakage is eliminated. Reusable filter units may be attached to the pump inlet to provide filtration and agitation at the same time. These pumps are available from a number of sources. To achieve even greater levels of agitation, ultrasonic agitation may be considered. This technique uses high-frequency sound waves to create a cavitation effect in the liquid. Small vapor bubbles are formed in the liquid by the rarefaction (low-pressure) sound waves. The subsequent pressure waves then result in bubble collapse. Very high pressures and temperatures are created within the bubbles as they collapse and implode. This cavitation effect blasts the soil away from all surfaces of the part in contact with the liquid. Ultrasonic agitation is more difficult to implement on an existing tank than in-tank agitation. Ultrasonic transducers must be tightly coupled to the tank to minimize mechanical stress and tank wall erosion. Holding fixtures are often prone to attack due to repeated exposure. For a new system, erosion and deterioration due to ultrasonic agitation are directly dependent on weld and fabrication quality. System life should be at least 5 years © 2001 by CRC Press LLC

for properly built equipment. The cost for an ultrasonic system varies widely depending on tank size and level of control. Small tabletop or desk-sized systems may be purchased for less than $1000 (no pumps, filters, etc.). Parts fabricated from materials that are poor conductors of sound may not be effectively cleaned in an ultrasonically agitated system. Instead, they may absorb the sonic energy and deaden the effect. The same is true for the materials used to fabricate baskets and part holders. Parts held in a glass beaker are effectively cleaned since the glass walls of the beaker will transmit the sonic waves. With a polyethylene beaker, the intensity of the sound waves will be attenuated or reduced by 25 to 50%. Similar effects may be noted with metal mesh baskets. Fine-mesh (300 mesh) and coarse-mesh baskets will allow the sound to pass while medium-mesh (40 to 60 mesh) baskets tend to absorb the most energy. When erosion or part damage due to ultrasonic agitation is a problem, higher-frequency acoustic waves may be used. This process is sometimes called acoustic streaming or megasonic agitation. The difference between ultrasonic and megasonic agitation is the range of frequencies used. Ultrasonic systems typically operate in the 20 to 40 kHz range while megasonic agitation operates in the range of 700 to 1000 kHz. The higher frequencies do not cause cavitation of the fluid and hence eliminate the potential for erosion. Megasonic agitation is primarily used for particle removal and its effect is line of sight. Eliminate Use of Air Sparging The reader should note that the use of air sparging to agitate an aqueous cleaning bath was deliberately excluded from the previous discussion. While air sparging is relatively easy to implement, there is not much more to support its use. Air sparging is an inefficient method and it does not provide the level of agitation truly needed to ensure complete mixing. The introduction of air beneath the parts can lift them off the rack. Entrapped air can prevent the cleaning solution from reaching recessed areas, hence increasing rejects. Air supplies that are not oil-free can soil parts and defeat the purpose of cleaning. Air agitation also results in very high operating costs for energy and water. Cleaning chemical usage and spent bath treatment costs can also increase. The air leaving the surface of the bath is saturated with water vapor, which carries away a large amount of heat (i.e., the heat of vaporization). Extra energy must then be used to replace this heat loss and to heat the incoming makeup water. As more makeup water is added, mineral impurities build up and consume cleaner. These mineral impurities are also responsible for much of the sludge generated during treatment. In most new installations, air agitation is not employed. Remove Soils from the Bath Common ways to remove solid soils from cleaning baths include gravity separation, decantation, centrifugation, and filtration. These techniques are employed in both solvent and aqueous-based cleaning systems to remove solid soils and extend the life of the bath. The process of filtration removes insoluble particulate matter from a fluid by means of entrapment in a porous medium. It is often used to extend the life of a cold cleaning bath or to remove metal fines and sludge continuously from a vapor degreaser sump. Common styles include bag and disposable cartridge. The expanding use of ultrafiltration and microfiltration allows finely emulsified oils to be removed from aqueous cleaning solutions. With solvents, no degree of filtering will remove dissolved soils from the bath. To recover a clean solvent, the spent bath may often be recycled via distillation. © 2001 by CRC Press LLC

While standard filtration does not remove soluble contaminants such as dissolved oils from a solvent, it can be used to remove solid dirt and grease particles. Passing the dirty solvent through a fine metal screen may remove these contaminants before they have a chance to dissolve and load the bath. Routine screening and removal of undissolved contaminants can be an effective way to extend the life of a cold cleaning bath. Microfiltration systems are filtration technologies that can remove soils to a much finer degree than standard filtration. In the field of precision cleaning, their use is essential. Typically, vapor degreasers are equipped with a 5 or 10 m filter for removal of particulate. The smaller particles that are not removed accumulate in the sump and eventually contaminate the solvent vapor and hence the assemblies being cleaned. The use of a microfiltration system can remove particulate down to less than 0.1 m in size. This minimizes the potential for particulate contamination of the solvent vapor. Because of the fine filtration capability of the filter, removal of water, organic acids, and other soils from the solvent is feasible. Moving beyond microfiltration, membrane filtration (which includes ultrafiltration) is capable of removing emulsified oil and grease from aqueous cleaning solutions. Membrane filtration is sometimes so effective that it will also remove surfactants and other special additives from the cleaner. The removal of particles as fine as 0.003 m and organic molecules with molecular weights exceeding 500 is possible. Employ Two-Stage Cleaning Two-stage countercurrent cleaning can effectively reduce the amount of solvent used in cold cleaning. The process involves soaking the part in a tank of dirty solvent followed by rinsing and final cleaning in a tank of clean solvent. The dirty solvent is used to remove the bulk or gross contamination while the clean solvent is used to ensure that the part is clean. When the clean solvent finally becomes too loaded with dirt and grease, the dirty solvent is disposed of and the clean solvent is transferred into the dirty tank. Fresh solvent is then added to the empty clean tank. As an example of the type of reduction possible by converting a cold cleaning tank into a two-stage cleaner, assume that a 100-gal tank of solvent is disposed of weekly when the level of oil contamination reaches 2%. When the contamination level exceeds 2%, the parts will drag out enough oil to be considered not clean enough for subsequent use. The parts being cleaned are small; consequently, the existing 100-gal tank can be converted into two 50-gal tanks by welding a divider inside. The parts are first soaked in the dirty solvent where 80% of the oil is removed followed by a rinse and spray flush in the clean solvent. Given that the loading rate of oil into the cleaner is 2 gal/week and that the first stage removes 80% of the oil, it will take 2.5 weeks for the first stage to reach 8% contamination and the second stage 2%. Disposing of the dirty solvent and transferring the clean solvent into the dirty stage, the clean stage will then be filled with fresh solvent. To provide a margin of safety, the facility elects to change out the dirty stage every 2 weeks. Therefore, 50 gal will be disposed of every 2 weeks with a two-stage cleaner as compared to 100 per gal week from a single-stage cleaner. This represents a 75% reduction in solvent use provided air emissions remain the same in both cases. REFERENCES 1. Tallmadge, J.A. and Gutfinger, C., Entrainment of liquid films, drainage withdrawal and removal, Ind. Eng. Chem., 59, 11, 1967. 2. Kushner, J.B., a three part series of articles on rinsing, in Metal Finishing, part I: 49(11), 1951; part II, 49 (12), 1951; and part III, 50 (1), 1952. © 2001 by CRC Press LLC

3. Soderberg, G., Drag-out, in Proceedings of the Twenty-Fourth Annual Convention of the American Electroplaters Society, Cleveland, OH, 24, 233 –249, 1936. 4. Bigge, D.M. and Graham, A.K., Design for plating, in Electroplating Engineering Handbook, 3rd ed., A.K. Graham, ed., Van Nostrand Reinhold Company, New York, 1971, chap. 2. 5. Joshi, S.B. et al., Use of solvent test kits to monitor solvent condition and maximize solvent utilization, in Proceedings of Process Technology ‘88, The Key to Hazardous Waste Minimization, Sacramento, CA, sponsored by Air Force Logistics Command, August 15 –18, 1988. 6. Spring, S., Metal Cleaning, Reinhold, New York, 1963. 7. Callahan, M.S. and Green, B., Hazardous Solvent Source Reduction, McGraw-Hill, New York, 1995.

© 2001 by CRC Press LLC

SECTION 5

Specific Areas of Cleaning

© 2001 by CRC Press LLC

CHAPTER 5.1

Surface Cleaning, Particle Removal Ahmed A. Busnaina

CONTENTS Introduction Adhesion Forces Particle Removal Hydrodynamic Removal Laminar Turbulent Ultrasonic Removal Megasonic Removal Brush Cleaning Particle Removal Mechanism Lifting Sliding Rolling Chemical Cleaning References INTRODUCTION Surface contamination by small particles and other contaminants is a major problem in many industries, such as semiconductor, storage, imaging, aerospace, pharmaceutical, automotive, food, and medical equipment. Contaminant particles range in size from several hundred microns to less than 0.1 m. Surface contamination can result from particle deposition in the manufacturing environment as well as particle generation by the manufacturing process or process tool. Improving the clean environment further or isolating products can only solve part of the contamination problem. There is always a need for effective and economical techniques for surface cleaning a variety of substrates. Adhesion of small particles to substrates presents a serious problem to many industries. Particulate surface contamination is one of the reasons for yield problems in these industries. The adhesion forces of these particles are greatly affected by many of the processes that the substrate may go through, such as cutting, polishing, etching, rinsing, © 2001 by CRC Press LLC

and drying, that follow the particle deposition. The adhesion forces to be considered in the process include van der Waals, electrostatic forces, and chemical bonds. Chemical bonds are usually orders of magnitude larger than van der Waals bonds. As the size of the circuit line width approaches 0.1 m in the semiconductor industry, the situation will become much more serious with respect to very small particles. There is a need for efficient and reliable particle removal techniques capable of removing very small particles without causing surface damage. Many studies have been conducted using various methods to detach particles from surfaces.18 –45 In this chapter the most common and widely used particle removal (surface cleaning) techniques will be reviewed.

ADHESION FORCES Adhesion forces are the forces responsible for adhering a particle to a surface. It is important to know the adhesion forces for particles that need to be removed to ensure that the removal force applied is sufficient for the particle removal. Adhesion forces depend on the particles and substrate material and the medium they are in (water, air, etc.). The adhesion force is also a function of size; it is proportional to the radius of the particles. The reason smaller particles are more difficult to remove than large particles is not because the adhesion force is larger. On the contrary, it is smaller. But since the removal forces applied depend on the area (R2) of the particles (such as removal using hydrodynamic, megasonic, etc.) and particle mass (R3) (such as removal using centrifugal, gravity, vibration, etc.) the force that can be applied to a particle decreases much faster than the adhesion force. Adhesion forces have been categorized by Krupp1 into three classes: Class I: Includes van der Waals and electrostatic forces which act in the periphery of the adhesive area as well as in the contact area.2 –5 Class II: Includes various types of chemical bonds as well as intermediate bonds (hydrogen bonds). Chemical bonds are usually an order of magnitude stronger than van der Waals bonds. Class III: Includes sintering effects such as diffusion and condensation and diffusive mixing. These forces are usually known as interfacial reactions. Class I forces, i.e., van der Waals and electrostatic forces, are the major contributors to particle adhesion. Dry uncharged surfaces in contact with dry uncharged particles will experience van der Waals and electrostatic double-layer forces as the only adhesion forces. Charged particles and surfaces will introduce an additional force (electrostatic image force). Wet surfaces, on the other hand, can shield these forces and thus reduce them significantly. The van der Waals forces can be reduced by a factor of two and the electrostatic forces can be more or less eliminated. Visser 6 –8 has published several papers on particle adhesion. A detailed review of the adhesion forces and particle–surface interaction is presented in other references.9 –15 Class II forces (chemical bonds and intermediate bonds) can also occur on silicon substrates following certain conditions of treating the substrates chemically followed by rinse-and-dry processes. Adhesion forces resulting from chemical bonds are usually orders of magnitude larger than van der Waals adhesion forces. The van der Waals force (an intermolecular adhesion force acting between molecules that arises because of the polarizability of the molecules) is the dominant adhesion force for small particles (less than 50 m). It arises from the short-period movement of the electrons in the atoms or molecules giving rise to momentary areas of charge concentrations called dipoles.14 © 2001 by CRC Press LLC

The van der Waals force (vdW) is given as AR Fvdw = 6 z 2

(1)

where A is the Hamaker constant, R is the radius of the spherical particle, and z is the separation distance between the particle and the substrate. The average separation distance (z) between the two surfaces is taken as 4 Å (for smooth surfaces). For the ideal case in which both the spherical particle and surface are not deformed, the vdW is proportional to the radius of sphere as shown in Equation 1. However, when deformation occurs, the magnitude of the adhesion force will also depend on the contact area between the particle and the surface. When a sphere and a flat substrate come into contact with each other, the attractive force deforms the interface and a circular adhesion area is formed. The total adhesion force consists of two additive components, the force acting between the adherents before deformation, and the force acting on the contact area due to the deformation, F(vdW · deform).12,15 AR a2 Fvdw = 6 z 2 (1 R z )

(2)

Krishnan and Busnaina showed that the adhesion-induced deformation is a plastic in the case of polystyrene latex (PSL) particles on silicon and that it may take up to 70 h to reach equilibrium.12,15 Adhesion-induced deformation will occur when the particle is softer than the substrate, or vice versa. Electrostatic force constitutes the main force of attraction for particles larger than 50 m in diameter. For dry particle–substrate system the electrostatic force becomes important. The presence of electrostatic charge can drastically alter the total adhesion force. Zimon13 reported that the force of adhesion could be increased by a factor of two when the net number of unit charges per particle on 40 to 60 m particles increases from 700 to 2500. Most particles carry some electric charge, and some may be highly charged. Particles at low humidity were found to retain their charge and are held to surfaces by an attractive electrostatic force.14 A charged particle experiences an electrostatic force in the vicinity of a charged surface or other charged particle. The charge on a particle can be negative or positive, depending on whether the particle has an excess or deficiency of electrons. If a charged particle carrying a charge Q comes in contact with an uncharged plate, the charged particle induces an equal and opposite charge on the surface; this is known as the coulombic or the electrostatic image force (Fcl) and is given as:2–4 Q Fcl = 6(D+z )2 0

(3)

where D is the particle diameter, and z0 is the separation distance. This electrostatic force will deteriorate with time because of the dissipation of the charge. When moisture is present in the air medium, condensation can take place between the particle and substrate. The capillary condensation gives rise to a capillary force (FC). Which is given by: 2 –4 FC = 4 πr LV

(4)

where LV is the surface tension (for the liquid –vapor interface), and r is the radius of the spherical particle. Equation 4 is applicable to smooth surfaces and represents the maximum force that could be experienced as a result of capillary condensation. However, the force of © 2001 by CRC Press LLC

adhesion approaches the values predicated by Equation 4 only at relative humidity near 100%.16 The capillary force is made up of two components: surface tension at the perimeter of the meniscus and the capillary due to the difference in pressure between the liquid and vapor phases. The existence of tension in a liquid–gas interface causes a difference in hydrodynamic pressure across the interface if the interface is curved. The capillary force depends on several parameters such as particle size, the surface tension of the condensed fluid, the wettability (contact angle) of the substrate surface. Capillary forces are proportional to particle size and the adhesion of large particles is found to increase with the relative humidity of the air. Equation 4 applies only to perfectly smooth spheres and in a saturated atmosphere. Zimon13 found that, in air with relative humidity near 100%, the majority of particles are held with forces less than those predicated by Equation 4. Kordecki and Orr17 observed that capillary condensation begins to appear at relative humidity above 50%. Luzhnov18 reported that adhesion due to capillary force occurs when the relative humidity exceeds 70%. The same effect was also reported by Zimon,13 who concluded that at relative humidity of 50%, and particularly at humidity below 50%, capillary forces have no effect on the adhesion force. But all agreed that at relative humidity between 50% and 65%, the capillary force starts to have an effect on the total adhesion force. Zimon went on to conclude that between 70% and 100% relative humidity, the capillary force dominates the other adhesion forces and should be the only adhesion force considered. Busnaina and Elsawy19 showed that the effect of relative humidity on the adhesion and removal for the 22 m PSL particles on silicon substrates was very significant. The removal of PSL particles was very low at high and low relative humidity. The lowest adhesion force (highest removal efficiency, 49%) occurs at 45% relative humidity as shown in Figure 1. PARTICLE REMOVAL Hydrodynamic Removal Hydrodynamic removal is the removal of a particle by using the inertia of a moving fluid on a particle. This can be done using a jet, overflow, or spin rinse, or using many other

Figure 1

The effect of relative humidity on the adhesion force (PSL particles/silicon).

© 2001 by CRC Press LLC

applications. The hydrodynamic force on a particle is applied through the drag and the lift force the fluid applies on any body in its path. The hydrodynamic forces depend on the cross-sectional area of the particle, fluid velocity, and density. The force is directly proportional to these parameters. Drag on a sphere in a uniform flow can be obtained by utilizing available experimental data.2 –4 For low Reynolds number flows Oseen’s approximation can be used. Submicron particles exist in the boundary layer and usually in that part known as the viscous sublayer. To calculate the correct drag on the sphere, the sphere is broken into small discrete cylinders. The velocity across the cylinder is arrived at using boundary-layer analysis. For parts of the sphere that lie within the viscous sublayer, standard law of the wall is used. By using this velocity and expressions for the drag coefficient, a local drag force is found. These local drag forces are then summed over the sphere producing a total drag force. The hydrodynamic lift force is calculated in the same manner.2 –4,20 Visser,21 in addition to his later work in theoretical aspects of adhesion, conducted experiments in 1970 concerning particle removal. The apparatus consisted of two concentric cylinders, the outer one was fixed and the inner one was capable of rotating at a maximum of 5000 rpm. The adhering system involved 0.21 m carbon black particles deposited on cellulose film on the inner cylinder. Visser21 assumed the criterion of 50% removal as a measure of the adhesion force. Musselman and Yarbrough22 used a model of viscous drag from a high-velocity spray to predict the drag force on particles at different spray nozzle pressures. They explain the difficulties in hydrodynamic drag removal due to “particle hideouts” in the boundary layer. Although free stream velocities may be substantial, the local fluid velocity at the particle is small because of its proximity to the wall. Both the turbulent and laminar boundary layers cause this problem. Drag force on the particle was calculated by a summation of the local drags at different heights on the sphere. Musselman and Yarbrough predicted the drag vs. particle size at different nozzle pressures. Kurz et al.5 used a rotating disk (silicon wafer) to generate hydrodynamic force to remove 1-m or larger particles. They used PSL spheres on bare silicon in deionized water as the medium. Removal rates above 90% were reported for particles larger than 2.0 m. Taylor, Busnaina, and co-workers2 –4 measured the removal force (using hydrodynamic drag and lift forces) of submicron particles on silicon substrates and correlated it with the theoretical adhesion force. The results indicate that the theoretical adhesion force (using the Hamaker equation) was in agreement with the experimental measurements. Most of the hydrodynamic removal was effective at removing micron-size particles or larger. The efficiency of submicron particle removal has been shown to be small.2 –4 There are three hydrodynamic factors acting on the adhering sphere; a Saffman20 lift force, Stokes drag, and turbulent bursts. Saffman lift results from the gradient in the shear flow. Drag originates due to a pressure difference across the sphere. In the case of a very slow flow around a sphere, Stokes drag provides adequate formulation. Turbulent bursts are present in turbulent flows and act to move fluid very rapidly from one section of the flow. They are influenced by vortex patterns among other things. The bursting activity is not yet wholly understood. Laminar Rizk and Elghobashi provide these formulas: Stokes drag: FD · 3 π  d V © 2001 by CRC Press LLC

Saffman lift: 1 du 1 FL = 1.615d2 ( ν dx ) 2 V where V is the relative velocity between the sphere and the fluid. Turbulent In their analysis of particle detachment, Cleaver and Yates23 specify two types of fluid force working on the contaminant. The first is the drag force, FD, given by dU* F0 = 8ρν2( ν )2 The other force acting against adhesion is the Saffman lift force and it is given by dU* FL = 10.1ρν2( ν )3 where d is the particle diameter and U* is the shear velocity. By defining a Reynolds number based on the shear velocity as dU* Re* = ν it can be seen that for Re*  1 the drag force dominates the lift force. As Re* increases the lift becomes a more dominant factor. Cleaver and Yates also contend that the turbulent bursts are a main mechanism in particle removal. They present an equation for the time between bursts or burst time, tb, ν tb = 75 U* While the authors present an in-depth discussion about the role of bursting and the parameters involved in bursting, they do not actually propose a bursting force. Cleaver and Yates state that the lift force is due to the bursting activity. The impulse force generated by the burst is left undefined.

Ultrasonic Removal Ultrasonic cleaning, where transducer frequencies operate between 25 and 200 kHz, is widely used in many aerospace, automotive, electronic hardware, medical industries. In reality, any frequency higher than 17 kHz is considered ultrasonic. However, because the dominant cleaning mechanism is different at low frequency (25 to 200 kHz) as compared to frequencies higher than 360 kHz, the lower frequency cleaning is known in the industry as ultrasonic cleaning while the high frequency is known as megasonic cleaning. The boundary between the two has not been clearly defined yet, but it lies somewhere between 200 and 360 kHz. © 2001 by CRC Press LLC

Ultrasonic cleaning has been an accepted cleaning method for decades and still proves to be valuable today. A typical ultrasonic cleaning system consists of piezoelectric transducers attached to the bottom of a tank. These transducers typically vibrate at a single frequency. The vibration energy generated by these transducers is transmitted to the cleaning solution, creating longitudinal pressure waves. The key to ultrasonic cleaning is the physical implosion of gas or vapor bubbles in a cleaning solution. The major drawback to ultrasonic cleaning is the damage to a surface caused by cavitation. Cavitation forms when the tensile strength of a liquid is exceeded as a result rapid alternation between positive and negative pressure of the sound wave propagating through the liquid. The cavitation bubbles are formed when the pressure is in the negative area. There are two types of cavities formed: transient and stable. The transient cavities occur at low frequency, change in size, and implode, whereas stable cavities change very little in size and do not implode.36 –38 The transient cavities undergo a series of expansion and contraction within the fluid until they reach a critical size. At the critical size the bubble implodes, creating a high-velocity jet and increasing the local temperature to well above thousands of degrees Kelvin. It has been calculated that the velocity of the jet and the local temperature during the implosion can reach up to 130 m/s and 3000 K, respectively. These implosions remove particulate or film contaminants on the surfaces to be cleaned. These implosions can also cause damage by creating erosion.32 –34 Many studies on cavitation impact have been done and found that cavitation damage can be minimized by changing the liquid property, such as temperature and gas content, as well as the transducer frequency sweep. Cavitation is the dominant cleaning mechanism in ultrasonic cleaning, but it is not the only one. Acoustic streaming, which is the dominant cleaning mechanism in megasonic cleaning, is equally as important to particle removal in ultrasonic cleaning. Figure 2* shows the effect of ultrasonic cleaning time and temperature on the removal of submicron PSL particles using 68 kHz frequency and 400 W of input power. The figure shows that the optimum temperature occurs at an intermediate value in the considered range used and that the cleaning time is optimum after 15 min of cleaning.

Megasonic Removal Megasonic cleaning, where transducer frequency typically operates between 360 and 1200 kHz, is widely used in many semiconductor (wafer cleaning) and the hard disk industries. Cavitation implosion does not occur in megasonic cleaning and therefore the dominant cleaning mechanism is the acoustic streaming. The acoustic streaming removes particles by exerting a hydrodynamic removal force (drag and lift) but at much higher velocity near the surface as compared with typical hydrodynamic cleaning using a jet or spin rinse. Recently, megasonic cleaning at high frequencies near 1 MHz has gained attention as an efficient, although poorly understood, Si surface cleaning technique. Olaf 24 made early observations of sonic cleaning of glass surfaces in the range from 15 kHz to 2.5 MHz. Rosenberg25 used ultrasonic cleaning for removing contaminant films and concluded that the removal was due to cavitation. McQueen26,27 recognized the importance of acoustic streaming in decreasing the boundary layer thickness, based upon his studies of removing small particles from surfaces. Megasonic cleaning applications were first described in detail by RCA engineers.28,29 Kashkoush and Busnaina and co-workers30 –38 studied ultrasonic and megasonic particle removal, focusing on the effects of acoustic streaming. Removal percentage increased with power. Their results also indicated different removal efficiencies for PSL, silica (SiO2), and silicon nitride (Si3N4) particles. Greatly enhanced * Chapter 5.1 Color Figure 2 follows page 104. © 2001 by CRC Press LLC

particle removal efficiency on Si from megasonics in SC-1 and SC-2 solutions (hydrochloric acid and hydrogen peroxide mixtures used in the RCA process for silicon wafers) was reported by Syverson et al.40 Again, removal increased with increasing power, up to a maximum tested value of 150 W. Wang and Bell41 performed experiments using megasonics for cleaning after RIE planarization. Of the parameters they tested, power had the greatest influence on the results. Cleaning improved with increased power up to the maximum tested value of 300 W,41 another result consistent with what was observed by Kashkoush, Busnaina, and Gale.30 –38 Megasonic power exerts a greater influence on particle removal efficiency than does solution temperature, both in water and in SC-1 solution. Removal efficiency increases with increasing power up to an intermediate point above which it decreases slightly. In deionized (DI) water, removal efficiency decreased slightly at temperatures above 50°C, whereas in SC-1 solution it was generally highest at temperatures above 50°C. Although SC-1 removes particles more efficiently than DI water, particularly at lower megasonic powers, it was still possible to achieve 100% removal in DI water under the proper conditions. SC-1 solutions, which are significantly more dilute in NH4OH content than the standard 5:1:1 recipe, work well in the presence of megasonic energy. Particle removal efficiency decreases when the ammonia content is decreased slightly from the 5:1:1 ratio, but increases again as ammonia content is further decreased. The efficiency then remains high even for R as low as 0.01. Figure 3* shows the effect of cleaning time and temperature on megasonic cleaning of polished thermal oxide wafers using silica slurry using 760 kHz frequency and 640 W of input power. Figures 4 through 6 show the effect of input power, cleaning time, and temperature on the removal of submicron silica slurry particles using megasonic cleaning at 760 kHz. Figure 4 shows the effect of cleaning time using 41.5°C and 345 W of input power. The figure shows that the optimum cleaning time is 20 min after which the cleaning efficiency goes down. This is due to particle redeposition on the substrate. Figure 5 shows the effect of input power using 41.5°C and 20 min of cleaning time. The figure shows that the optimum power is about 500 W. This is due to the fact that high power generates many more bubbles that interfere with the acoustic streaming, thereby decreasing the removal efficiency. Figure 6 shows the effect of the cleaning temperature

Figure 4

The effect of cleaning time on megasonic cleaning using 760 kHz frequency, 41.5°C, and 345 W of input power.

* Chapter 5.1 Color Figure 3 follows page 104. © 2001 by CRC Press LLC

Figure 5

The effect of input power on megasonic cleaning using 760 kHz frequency, 41.5°C, and 20 min of cleaning time.

using 20 min of cleaning and 345 W of input power. The figure shows that the optimum cleaning temperature is higher than 40°C. Brush Cleaning Brush cleaning works by using a soft brush that engulfs a particle and removes it by applying a torque through the brush rotation. The pressure helps the brush engulf the particle and the rotation applies the torque that overcomes the adhesion moment and removes the particle. Brush cleaning is widely used in the industry, especially following chemical –mechanical polishing processes of silicon or metal substrates. There are few scientific published studies on the effectiveness of brush cleaning in removing small particles adhered by van der Waals forces or chemically bonded. There has also been some recent work on cleaning oxide silicon wafers using PVA (polyvinyl acetate) brush, DI water, basic chemistry, or sur-

Figure 6

The effect of cleaning temperature on megasonic cleaning using 760 kHz frequency, 20 min of cleaning, and 345 W of input power.

© 2001 by CRC Press LLC

factants.43 Brush cleaning can be effective if applied properly by optimizing the water flow, the rotational speed, and brush pressure. Using chemistry during brush cleaning can enhance particle removal.42 Research shows that the brush pressure is one of the most important parameters in removing particles.42 The pressure helps the brush engulf the particles to be removed. The rotation of the brush applies the torque that will overcome the adhesion moment and remove the particles. Complete removal using the brush scrubber with DI water is achieved in cleaning thermal oxide silicon wafers dipped in STI silica slurry by Busnaina et al.43 They showed that intermediate brush pressure, speed, and time gave the best overall particle removal efficiency. High pressure and long cleaning time will cause scratches (more defects) in the substrate. Figure 7* shows the effect of cleaning time and brush speed on brush cleaning using a PVA brush at 40 psi pressure between the brush and the substrate. The figure shows that the optimum cleaning time is longer than 30 s at 40 psi brush pressure.43 Particle Removal Mechanism This section discusses the removal mechanisms of particles. Three different mechanisms may contribute to particle removal: lifting, sliding, or rolling. Consider deformed PSL particles on silicon substrates; the magnitude of the adhesion force is several orders larger than removal forces.12,15,45 –47 Lifting Particles will be removed from the surface if the lift force acting on particles is larger than the adhesion force. FL  Fa

(5)

Sliding Particles will also be removed by sliding if the drag force,45 lift force, and adhesion force satisfy the following equation: FD  (Fa  FL)

(6)

where κ is the coefficient of friction. The ratio of drag force over adhesion force, RS, is defined to judge whether detachment by instantaneous sliding occurs or not (if it is assumed that the lift force is very small and can be neglected compared with the adhesion force). D RS  F F

(7)

a

If RS , particles will be removed by sliding. Rolling Hubbe48 evaluated the torque balance on a spherical particle in contact with the surface. Sharma et al.49 further included a factor of 1.399 in Equation (8) since the drag force and the hydrodynamic torque on a particle near the wall could be substituted by an * Chapter 5.1 Color Figure 7 follows page 104. © 2001 by CRC Press LLC

Figure 8

The lift force FL, drag force FD, and adhesion force FA acting on a particle in a shear flow.

effective force at a distance of 1.399R from the surface. When large deformation (a/R 0.1) and the lift force are considered, the torque balance equation about point O can be described as follows (as shown in Figure 8.): (1.399R  ) FD  (Fa  FL )a

(8)

where a is the contact radius, and   R  (R 2  a 2)0.5 is the relative approach between the particle and the substrate. The particle will be removed instaneously when the removal forces are applied, if the removal force overcomes adhesion force Fa. The ratio of the hydrodynamic rolling moment to the adhesion resisting moment, RM, is given by (neglecting a very small lift force): D (1.399R  ) RM  F Fa

(9)

a

When RM 1, particles are removed by the drag force instantaneously. The relationship between RS and RM is given by R M  1.399 R RS a

(10)

Chemical Cleaning Chemicals such as detergents, surfactants, etchers, etc. are often used to enhance physical cleaning. The chemical can be used to increase the wetting, wash and dissolve organic contaminants, or change the charge on particles to make them more repulsive to facilitate the removal process. In this chapter only chemicals used to remove particles are discussed. Basic chemistry is often used in particle removal from silicon and metal substrate. It is usually accompanied by megasonic cleaning or overflow rinse. Basic chemistry has been used to remove silica, alumina, PSL, and silicon nitride particles.30 –40 The basic chemistry is used mainly to increase the repulsive charge ( potential) between the particle and the substrate. The potential of the particle and the substrate has to be known to use the proper © 2001 by CRC Press LLC

solution pH that provides the maximum repulsion. A number of investigators have studied the effects of potential on deposition of particles onto surfaces.50 –57 Marshall and Kitchener44 examined the deposition of carbon black particles from dilute aqueous suspensions onto glass. They observed that deposition was greatest when the potentials of the particles and the substrate were of opposite sign. Hull and Kitchener50 studied deposition of PSL particles onto a rotating disk. They found that when the particles and the substrate have opposite charge, deposition followed the expected diffusion-limited behavior. However, when the particles and substrate had like charges (repulsive interaction) there was considerably less deposition. Ali55 measured the potentials of a number of different particle types in semiconductor processing liquids with an emphasis on applications to ultrafiltration of semiconductor chemicals. Albaugh and Reath56 correlated particle counts in a process bath with surface counts following deposition from the bath onto hydrophilic wafers. They demonstrated the strong influences of pH and ionic strength on deposition. The dependence of potential on pH plays a significant role in surface cleaning. potential decreases as pH increases; it is typically positive at low pH, and negative at high pH. The point at which the potential of a solid surface is zero is referred to as its isoelectric point or point of zero charge (pzc). The pzc of different solids depends upon the H and OH ion concentrations in the solution, and therefore occur at different pH values (pH  log[H ]). At high pH, the particle can release H ions into solution, resulting in a negative charge for the particle. At the pH of water, a silicon surface with a native oxide has a negative potential (the pzc for a hydrophilic silicon substrate is approximately 2.6).55 Thus, negatively charged particles will be repulsed from a hydrophilic substrate at this pH, and even more strongly at higher pH. In DI water, silica and PSL particles are both negatively charged, whereas silicon nitride (Si3N4) particles typically carry a positive charge.57 Thus, silica and PSL will be repelled from a wafer surface in water while silicon nitride will be attracted. The reduction of potential at high pH contributes to the success of the SC-1 solution (pH  11) as a particle removal chemistry. Figure 9 shows the potential (particle charge) of colloidal silica particles as a function of pH.37,38 Ionic strength of the liquid also affects electrical double layer interaction. When ionic strength is high, the Debye length decreases and the strength and range of double layer

Figure 9

The ζ potential (particle charge) of silica particles as a function of pH.

© 2001 by CRC Press LLC

interactions are significantly reduced.57 Thus, where repulsion between particles and surfaces is expected, an increase in ionic strength will increase deposition. Similarly, low ionic strength gives rise to a thicker double layer and more repulsion between the particle and the substrate. REFERENCES 1. Krupp, H., Adv. Colloid Interface Sci., 1, 111 –140, 1967. 2. Taylor, J., Measurement of Detachment Forces for Submicron Particles on a Silicon Wafer, M.S. thesis, Clarkson University, Potsdam, NY, December 1990. 3. Taylor, J., Busnaina, A.A., Kern, F.W., and Kunesh, R., in Proceedings, IES 36th, New Orleans, LA, April 23 –27, 1990, 422 –426. 4. Busnaina, A.A., Taylor, J., and Kashkoush, I., J. Adhesion Sci. Technol., 7, 5, 441, 1993. 5. Kurz, M., Busnaina, A.A., and Kern, F.W., in Proceedings, IES 35th, Anaheim, CA, May 1 –5, 1989, 340 –347. 6. Visser, J., Adv. Colloid Interface Sci., 2, 331 –363, 1972. 7. Visser, J., Adv. Colloid Interface Sci., 15, 157 –169, 1981. 8. Visser, J., Adv. Colloid Interface Sci., in Surface and Colloid Science, E. Matijevic, ed., Vol. 8, John Wiley and Sons, New York, 1976. 9. Hamaker, H.C., Physica, 4, 1937. 10. Tabor, D., J. Colloid Interface Sci., 58, 1977. 11. DeMejo, L.P., Rimai, D.S., and Bowen, R.C., J. Adhesion Sci. Technol., 2, 331 –337, 1988. 12. Rimai, D.S. and Busnaina, A.A., J Particulate Sci. Technol., 13, 249, 1995. 13. Zimon, A.D Adhesion of Dust and Powder, Plenum Press, New York, 1969. 14. Hinds, W.C., Aerosol Technology, John Wiley & Sons, New York, 1982. 15. Krishnan, S., Busnaina, A.A., Rimai, D.S. and DeMejo, D.P., The adhesion-induced deformation and the removal of submicrometer particles, J. Adhesion Sci. Technol., 8 (11), 1357–1370, 1994. 16. Davies, C.N., Aerosol Science, Academic Press, New York, 1966. 17. Kordecki, M.C. and Orr, C., Jr., Arch. Environ. Health, 1, 7, 1960. 18. Luzhnov, Yu., M., Research in Surface Forces, Consultants Bureau, New York, 1971. 19. Busnaina, A.A. and Elsawy, T.M., The measurement of particle adhesion forces in humid and dry environments, in Adhesion Society Proceedings 21st Annual Meeting, Feb. 22–25, Savannah, GA, 1998. 20. Saffman, P.G., J. Fluid Mech., 22, 385 –400, 1965. 21. Visser, J., J. Colloid Interface Sci., 34, 1970. 22. Musselman, R.P., and Yarbrough, T.W., J. Environ. Sci., 51 –56, 1987. 23. Cleaver, J. and Yates, B., J. Colloid Interface Sci., 44, 1973. 24. Olaf, J. , Acustica, 7, (5), 253, 1957. 25. Rosenberg, L.D., Ultrasonic News, Winter, 16, 1960. 26. McQueen, D.H., Ultrasonics, 24, 273, 1986. 27. McQueen, D.H., Ultrasonics, 28, 422, 1990. 28. Schwartzman, S., Mayer, A., and Kern, W., RCA Rev., 46, 81, 1985. 29. Mayer, A. and Schwartzman, S., J. Electr. Mater., 8, 855, 1979. 30. Kashkoush, I., Busnaina, A., Kern, F., and Kunesh, R., in Particles on Surfaces 3: Detection, Adhesion, and Removal, K. L. Mittal, Ed., Plenum Press, New York, 1991, 217 –237. 31. Kashkoush, I., and Busnaina, A., in Proceedings, IES 38th Annual Meeting, San Diego, CA, May 6 –10, 1991, 861 –867. 32. Kashkoush, I. and Busnaina, A., Particulate Sci. Technol., 11, 11, 1993. 33. Busnaina, A. and Kashkoush, I., Chem. Eng. Commun., 125, 47, 1993. 34. Kashkoush, I. and Busnaina, A., in Proceedings, IES 40th, 1993, 356. 35. Kashkoush, I., Ph.D. thesis, Clarkson University, Potsdam, NY, 1993. 36. Gale, G., Busnaina, A., and Kashkoush, I., in Proceedings, Precision Cleaning ‘94, Rosemont, IL, May 17 –19, 1994, 232 –253. © 2001 by CRC Press LLC

37. 38. 39. 40. 41. 42. 43.

44. 45. 46.

47. 48. 49. 50. 51. 52. 53. 54. 55.

56. 57.

Busnaina, A.A., Kashkoush, I.I., and Gale, G.W., J. Electrochem. Soc., 142, 8, 2812 –2817, 1995. Gale, G.W. and Busnaina, A.A., J. Particulate Sci. Technol., 1995. Hirota, Y., Appl. Surf. Sci., 60, 619, 1992. Syverson, W., Fleming, M., and Schubring, P., in Second International Symposium on Cleaning Technology in Semiconductor Manufacturing, Electrochemical Society Proceedings, PV92-10, 1992, 10. Wang, P. and Bell, D., in Third International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, Electrochemical Society Proceedings, PV94-7, 1994, 132. Roy, S.R., Ali, I., Shinn, G., Furusawa, N., Shah, R., Peterman, S., Witt, K. and Eastman, S., J. Electrochem. Soc., 142, 1, 216 –226, 1995. Busnaina, A.A., Moumen, N., and Piboontum, J., Contact post-CMP cleaning of thermal oxide wafers, in Proceedings of the VLSI Multilevel Interconnection Conference (VMIC), Santa Clara, CA, February 8–12, 1999. Marshall, J.K. and Kitchener, J.A., J. Colloid Interface Sci., 22, 342, 1966. Zhang, F. and Busnaina, A., Particle adhesion and removal in chemical mechanical polishing (CMP) and post-CMP cleaning, Electrochem. Solid-State Lett., (in press), 1999. Zhang, F. and Busnaina, A.A., The effect of particle adhesion on chemical mechanical polishing (CMP) removal rate an post-CMP cleaning, in Adhesion Society Proceedings, 21st Annual Meeting, Panama City, FL, February 21 –24, 1999. Zhang, F. and Busnaina, A., The role of particle adhesion and surface deformation in chemical mechanical polishing, Electrochem. Solid-State Lett., 1, (4), 1998. Hubbe, M.A., Colloid Surf., 12, 1984. Sharma, M.M., Chamoun, H., Sarma, D., and Schechter, R., J. Colloid Interface Sci., 149, 1992. Hull, M. and Kitchener, J.A., Trans. Faraday Soc., 65, 3093, 1969. Clint, G.E., Clint, J.H., Corkill, J.M., and Walker, T., J. Colloid Interface Sci., 44, 121, 1973. Ruckenstein, E. and Prieve, D., J. Chem. Soc. Faraday II, 69, 1522, 1973. Prieve, D. and Ruckenstein, E., J. Colloid Interface Sci., 60, 337, 1977. Brouwer, W. and Zsom, R., Colloids Surf., 24, 195, 1987. Ali, I., Electrokinetic Characteristics of Particulate/Liquid Interfaces and Their Importance in Contamination from Semiconductor Process Liquids, Ph.D. thesis, University of Arizona, Tucson, 1990. Albaugh, K.B. and Reath, M., in Proceedings, Microcontamination 1991, San Jose, CA, October 16 –18, 1991, 603. Riley, D.J. and Carbonell, R.G., J. Colloid Interface Sci., 158, 259, 1993.

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CHAPTER 5.2

Cleaning Metals: Strategies for the New Millennium Carole LeBlanc

CONTENTS Introduction Goals Early Observations Cleaning Test Plan Test Results Interpreting the Results Surface Analysis Techniques Fluorescence FTIR Spectrometry Goniometry Gravimetric Analysis Microscopy with Photographic Capabilities OSEE Cleaning Test Database Cleaning Steel Cleaning Aluminum More about Metal Cleaning Acknowledgments References

Editor’s Note: The following study outlines the approach used by the Surface Cleaning Laboratory at the University of Massachusetts Lowell to develop alternative cleaning applications. The study is a good example of a success story outlining the benefits of developing practical, industrially oriented studies in the academic community. On a more universal scale, the approach and findings provide valuable and logical guidelines for the individual manufacturer. In the following study, results and trends refer to testing performed by the Surface Cleaning Laboratory during the period 1994 through 1999 — B.K.

© 2001 by CRC Press LLC

Trichloroethylene, Trichloroethane TCE, TCA

Glycol Ether 3%

Toluene / Heptane 6%

Other 7%

Acetone 2%

23%

Aerosol Ineffective 5% Detergents 7%

Alcohols 12%

Perchloroethylene 'Perc' 14%

Unspecified Degreaser 7%

Mineral Spirits NMP* 8% 4%

Dichlorofluoroethane 4%

*N-Methyl

Figure 1

Pyrrolidone

Metal-cleaning solvents assessed for replacement.

INTRODUCTION The passage of the Toxics Use Reduction Act (TURA) in 1989 by the Massachusetts legislature marked the creation of the Surface Cleaning Laboratory (SCL) at the University of Massachusetts Lowell. Fully operational since 1994, SCL is the research and testing facility of the Toxics Use Reduction Institute (TURI). That same year, the U.S. Environmental Protection Agency Toxic Release Inventory (TRI) revealed that many of the state’s industries such as metalworking and electronics use several hazardous chemicals recognized for their excellent solvating powers. GOALS The objective of the laboratory is to develop and promote safer alternatives to these hazardous materials, primarily organic and chlorinated solvents used to clean metal surfaces (Figure 1), without causing economic hardship or a loss in cleaning performance. Specifically, reductions in ozone depletion, global warming, and volatile organic compound (VOC) emissions are sought as well as decreases in exposures to flammable, carcinogenic, and other toxic substances.1 Termed environmental indicators, these factors are important not only to workers directly involved in surface cleaning but to the communities in which the processes are conducted and, ultimately, the consumer. The Institute determines the potential chemical damage associated with any given cleaning solvent by using the state-of-the-art databases of its Technology Transfer Center (TTC). EARLY OBSERVATIONS Prior to beginning a project, a brief questionnaire (Figure 2) is filled out so that testing can be tailored to meet the needs of the individual firm. Designed with templates from both industry and government, this format also assists plant personnel involved with cleaning to become familiar with all aspects of the process. © 2001 by CRC Press LLC

1. Please print or type. Be as thorough as possible. 2. Attach MSDS of present relevant chemistries. 3. Do not send any samples/parts without first contacting SCL. Test to be witnessed? No Yes DESCRIBE THE PART/PRODUCT TO BE CLEANED Materials of construction: Metal Plastic Other Please specify type: List percentages cleaned (if more than one substrate): (for example, 60% of parts are aluminum; 40% are 304 stainless steel) Surface (circle two): Rough or Smooth Hard or Soft Approx. size (dimensions in inches): Geometry: Simple (e.g., flat) OR Complex (contains inaccessible areas) Gram weight: Min. Max. What is this part/product used for? DESCRIBE THE CURRENT CLEANING PROCESS Contaminants to reduce or eliminate (circle all that apply) Oil Grease Wax Flux Dirt Combination (describe): Other: Are samples of contaminants available? No Yes (if available, attach MSDS)

Salts

Manufacturing step immediately before cleaning: Manufacturing step immediately after cleaning: Number of parts cleaned per week (or shift, etc.): per batch: Equipment in use (circle all that apply): Vapor degreaser Agitation/air sparging unit Immersion/soak/dip tank Ultrasonics Pressure spray washer (approx. psi) Other: Specify vendor if possible: Cleaning Chemical(s): (attach MSDS) Concentration: % Time: min. Temp. deg.F Water source, if applicable: DI (deionized)/Tap Rinse Cycle, if any: Time: min Temp.: deg.F Water source: DI (deionized)/Tap Drying Cycle, if any: Method Time: min. Temp: deg.F Any problems with present cleaning system? After cleaning, parts are (circle one): Used Immediately OR Stored If stored, How: How long: What is the purpose of cleaning (i.e., desired product specifications)? Methods employed for evaluating cleanliness: None Visual Microscopic Other performance test, if any (please describe): Comments or Areas of Concern: Return any samples/parts? No Yes

Figure 2

UV

Cleaning test request form.

Figure 3 presents an overview of business sectors where evaluations have been conducted during the past five years. A further breakdown of surface substrates and common soils (i.e., surface contaminants) under investigation is found in Figures 4 and 5, respectively. As indicated in Figures 4 and 5, over 80% of the evaluations have involved metal cleaning applications. CLEANING TEST PLAN Prior to the search for safer and greener chemical cleaners, solvent cleaning was often performed via single-species vapor degreasing. In addition to immersion, vapor © 2001 by CRC Press LLC

Paper Medical 1% Recycling 7% 3% General Mfg 18%

Metalworking 34%

Plastics 2%

Optical 3%

Electronics 9% Aircraft 4% Consulting 2%

Figure 3

Adhesive 12%

State/Military 5%

Evaluation by business sector, 1994–1999.

degreasers were employed because of the superior ability of traditional solvents to dissolve organic matter. In vapor-phase cleaning, the final “rinsing” of the surface was accomplished by the mere condensation of solvent vapors. Unlike these chlorinated organic solvents, environmentally friendlier aqueous (i.e., water-based) detergents may not depend on their penetrability for their cleaning efficiencies, especially for the removal of petroleum-based surface debris. They rely instead on a number of chemical processes such as solubilization, wetting, emulsification, deflocculation, sequestration, and saponification.2 To complete these tasks, aqueous cleaners can be complex mixtures of surfactants, emulsifiers, and other additives in an alkaline (pH  7.0) formula. The basic outline of a cleaning test plan is shown is Figure 6. Because of their decrease in chemical energy relative to chlorinated organic solvents, several aspects of an aqueous cleaning system must be optimized. These include but are not limited to agitation methods, cleaner concentration, temperature, and cycle time as reflected in Phase II of Figure 6. Textile Liquid* Electronics Glass Other 1% 1% Related 1% 3% 4% Plastics Related 4% Steel 57% Non-Steel Metals 29% *Analysis only Figure 4

Substrate categories tested, 1994 –1999.

© 2001 by CRC Press LLC

200 180

Number of Tests

160

140 120 100 80 60 40

Figure 5

* None

Stickies

Preclean

Ink

Rust Preventer

Wax

Flux

Sealant

Rust

Lubricants

Fingerprints

Dust

Metals

Grease

Oil

Dirt

Urethane Coat

Machining Fluid

Carbon Black

Buffing Compound

Asphalt

0

Adhesives +

20

*Refers to applications related to surface cleaning

Surface contaminants studied vs. number of cleaning tests performed, 1994 –1999.

As this Figure 6 illustrates, metal surfaces initially cleaned are flat coupons (Phase I) matched to the parts’ materials of construction, followed by the eventual cleaning of actual product (Phase IV). Phase IV is concerned with the significance of part shape (screw configurations and blind holes and so forth) on metal cleaning. Note that in this model, mechanical energies (Phase III) are critically examined only after the proper cleaners, concentrations, and temperatures (Phase II) are ascertained. Other testing protocols exist that may be as effective; it is essential that the experimental design be logical and consistent.

I. Brainstorm Compatibility and "Lift" Studies

II. Temperature and Concentration Studies

Helps to 'scope' project more efficiently Determine substrate surface / chemical cleaner reactivity issues (use MSDSs, Technical data Sheets, etc.) Monitor the effect of drops of selected detergent concentrates on grossly contaminated coupons over time (Hansen method if no coupons available*)

Chemical field may be narrowed / changed from Phase I Follow chemical manufacturers' recommendations for both parameters Equalize time Minimize same source agitation*

ambient conditions; chemical ( no mechanical) energy subjective; visual

*chemical comparison tool; first use of mechanical energy; first round of scientific trials: gravimetric analysis

IV. Actual Product Cleaning Studies

III. Mechanical Energy Studies

Geometries and sizes of parts important to cleaning efficiency Duplicate optimal Phase III cleaning conditions Duplicate optimal Phase III cleanliness testing

Number of chemical cleaner candidates further decreases from Phase II

V. Pilot Plant / Scale-up Feasibility Studies Production volume or throughout dictated by MANUFACTURING PROCESS DRYING

Figure 6

Application-specific Economically-sensitive Training-dependent Space-limiting

scientific study, may employ a variety of analytical tools for cleanliness evaluation

Phases of an aqueous-based surface cleaning test.

© 2001 by CRC Press LLC

Ultrasonics 28%

Extracting / Blasting 2% Air Sparging 4% Tank Agitation 41%

Spray Wash 2% Manual 15%

Other 2%

Lift 'Peel' Test* 4%

Figure 7

Immersion 2%

*Analysis only

Cleaning mechanisms used with alternative processes.

TEST RESULTS The increased dependence on mechanical energy of replacement cleaners during laboratory testing is depicted in Figure 7. On average, in the author’s experience, three tests are conducted per company, each test involving experimentation with as many as six chemical cleaners, over a period of 2 to 8 weeks.3 While Phase V (see Figure 6) deals with metal cleaning as it relates to the entire manufacturing process and drying as the rate-limiting cycle, it is equally important to consider rinsing requirements in the selection of chemical cleaners and equipment.4 Figure 8 outlines laboratory test results for both rinsing and drying regimes. During the course of an investigation, companies receive the latest edition of “Industrial Cleaning Survey: Directory of Vendors,” which lists the suppliers of cleaning chemicals, equipment, and related items worldwide. Vendors are responsible for the information contained in the catalog, which is maintained on file by responding to the Institute’s Vendor Survey Questionnaires. This publication prepares staff for any time-sensitive purchases that may be required due to process changes. Example entries are found in Table 1.

DI Water 4% Tap & DI Water 39%

Oven 16%

Tap Water 54%

Tap & Rinse Aid 3%

RINSING

Figure 8

Rinsing and drying methods.

© 2001 by CRC Press LLC

Laminaire Hood 27%

Air Knife 38%

Chemical Displacement 1% Ambient Air Infrared Heat Gun 5% 7% 6%

DRYING

Table 1 Typical Entries in Cleaning Vendor Directory, Technical Report 15 Product (Address, Phone, Contact)

Classification

Industrial Applications

Contaminant Removal

Compatibility

Chemical cleaner x

Alkaline, water-based degreaser

Metal finishing/ fabrication

Buffing, lapping compounds

High/low pressure spray

Cleaning equipment z

Manual parts washer

Automotive/ Greases/ machine shop lubricants

© 2001 by CRC Press LLC

All aqueous/ most semiaqueous

Physical Properties pH 9.0 VOCs 0 FP N/A VP N/A

Dimensions N/A

4 (h)  3 (w)  4 (l) 150 lb

Known Components 50% NaOH 3% Surfactant

Options

Cost

N/A

Per lb

Oil skimmer

Per unit

Possible funding sources for the project and the potential appointment of a University of Massachusetts graduate student intern to the in-house, scale-up portion (Figure 6, Phase V) of the project are also discussed with individual companies. In addition to laboratory publications, periodicals, and referrals to conference proceedings, other technology transfer assistance is provided through cleaning and degreasing workshops held on campus and in off-campus training sessions. INTERPRETING THE RESULTS Surface Analysis Techniques If the purpose of solvent substitution testing is to identify a safer, greener cleaning process that is at least as proficient as its solvent-based counterpart, then benchmarking becomes necessary. Benchmarking is achieved by conducting scientific comparisons of variously treated surfaces via acceptable analytical methods. Depending on the application, ASTM (American Society for Testing and Materials) and other standards such as military specifications may also be employed. Figure 9 details the surface inspection tools used by SCL and a brief description of the laboratory’s major techniques follows. Fluorescence Some contaminants, in particular lubricants, naturally fluoresce. Examination under black light reveals the location and extent of this type of surface contamination. Artificial fluorescence is possible with the addition of chemical tags, similar to those used in forensics. This is a limited application, however. FTIR Spectrometry Fourier transform infrared spectroscopy correlates vibrational energy to the molecular signature of a compound. Similar to other high-tech methods such as GC (gas 200 46

150

6/95-11/97

100 145

1/94-5/95

86

50

Figure 9

Water Break

Visual 1/94-5/95

Peel Test

OSEE

Microscopy

Gravimetric

FTIR

Flame Test

Contact Angle Goniometery

Characteristic Tests

20

Black Light

0

Types of surface analyses conducted vs. number of cleaning trials performed, 1994 –1999.

© 2001 by CRC Press LLC

chromatography), the curves generated in this analytical technique are both quantitative for species identification (the placement of the curve on the electromagnetic spectrum) and qualitative for amounts (the area under the curve). A relatively expensive instrument, an FTIR spectrometer requires special training and care in sample preparation. Not all contaminants can be analyzed this way and interpretation of graphs can be difficult because of the presence of interfering peaks. It may be used in clean rooms or disk drive manufacture where the origins of contamination may be entirely unknown and the amounts of contamination very low. Goniometry Like optically stimulated electron emission (OSEE) described below, laser or optical contact angle goniometry is the measurement of a secondary effect to extrapolate surface cleanliness. A small drop of deionized (DI) water is placed on the substrate of interest. A light is shown to reflect the interface of the droplet with the surface. Usually, the higher the contact angle (that is, the height of the bubble), the greater the contamination. Conversely, water dropped on a clean surface generates a much smaller, flatter contact angle. An example of this effect is noticeable after waxing and then washing a car; the remaining wax acts as a contaminant and the residual water on the surface of the car “bubbles up.” The technique is limited since only the cleanliness under the tiny drop is measured so that several readings must be taken. Flat surfaces are more conducive to accuracy. Gravimetric Analysis Properly employed, gravimetric analysis can be the most inexpensive and revealing of all surface measurement techniques. Ideally, the part or test coupon is weighed a total of three times with the same analytical balance and under the same atmospheric conditions. Weights are taken (1) before artificial contamination, (2) after artificial contamination, and (3) after cleaning. These tests should be duplicated a number of times to ensure reproducibility of results. Percent soil removal and standard deviations can then be calculated. Some difficulty may arise in arriving at a precontamination weight under actual plant/production settings, although estimates may be possible. Care must be taken in selecting nearidentical substrate pieces and applying the contaminant in a consistent manner. These problems are largely avoided using test coupons. Microscopy with Photographic Capabilities From SEM (scanning electron microscopy) with magnifications as high as several thousand that provide actual surface morphology to light microscopy with magnifications as low as decimal fractions, there is a magnification range to suit almost every surface cleanliness application. Parts cleaning, as opposed to precision cleaning, can be adjudicated with a stereoscope and magnifications well under 1000. Computer software packages are available that “count” the soil load per photographic frame and store the information to disk for a permanent record. OSEE Optically stimulated electron emission or photo electron emission (PEE) is based on the principle that metals and certain surfaces emit electrons upon illumination with ultraviolet (UV) light. These electrons can be collected, measured as current, converted to a © 2001 by CRC Press LLC

Table 2 Partial List of Successful Glass-Cleaning Tests Conducted by SCL

SCL Number 95-409-01-2 95-409-02-2 95-409-03-2 96-435-01-8 97-550-01-3

Substrate Glass Glass Glass Glass Glass

Soil Wax Wax Wax Rosin Grease

Mechanism Ultrasonics Immersion Ultrasonics Ummersion Spray

Cleaner Mfg Oakite Alconox Alconox Occidental Alconox

voltage, and digitally displayed. A surface contaminant will either enhance or attenuate this signal, depending on it own photoemissive nature. While OSEE will not identify a contaminant, it is a good comparative tool to determine the degree of contamination. The method is best suited for thin films (oils, etc.) and not particulate matter (dust, for example).5 Figure 9 confirms a trend toward more scientific surface evaluation. As the number of tests for which the evaluation laboratory was able to conduct gravimetric analysis increased, visual inspection as the sole means to measure surface cleanliness decreased proportionately. Other similar, subjective inspection methods, for example, wipe glove and water break tests, are considered unacceptable by SCL for most modern metal-cleaning applications. Cleaning Test Database To arrive at meaningful data in a more timely fashion, the laboratory developed a database of trial outcomes, based on 5 years of predominantly metals (86%) cleaning, searchable by four fields: surface substrate, surface contaminant, chemical cleaner, and cleaning equipment. To demonstrate the use of this program, known as the Effective Test Conditions (ETC) Database, the information in Table 2 was obtained as a result of the inquiry, “What recommendations can be made to a manufacturer of opto-mechanical devices to clean lenses dealing with stain sensitivity?”6 CLEANING STEEL Steel is defined as “an iron-based alloy, malleable in some temperature ranges as initially cast, containing manganese, usually carbon, and often other alloying elements.”7 In carbon steel and low-alloy steel, the maximum carbon is approximately 2%; in the highalloy version, this concentration is about 2.5%. Carbon steel has no minimum quantity for any alloying elements other than manganese, silicon, and copper. It contains only incremental amounts of substances other than those mentioned and sulfur and phosphorus. The differentiating line between low- and high-alloy steels is generally 5% metallic alloys. Steels may be separated from cast irons, the large family of cast ferrous metals (containing at least 2% carbon, plus silicon and sulfur with or without other alloying elements) and low-carbon pure iron. In very low-carbon steels, the manganese content is the primary difference. Steel usually contains at least 0.25% manganese and ingot iron much less. Coldrolled sheets are milled from a hot-rolled, pickled coil that has been given substantial cold

© 2001 by CRC Press LLC

reduction at room temperature. This results in a product requiring further processing but with improved characteristics and uniformity. Of the many different kinds of steel, the majority of tests conducted by the laboratory with this substrate were categorized as stainless steel (in a few cases, companies were unaware of the steel classification of parts from suppliers). These steels contain 12 to 30% chromium as the alloying element and usually exhibit passivity in aqueous settings. Some trends in conducting effective cleaning trials on steel substrates in the laboratory are revealed in Table 3. In two of the above case studies, several potential replacement cleaners performed proficiently under almost identical operating conditions. This suggests that cleaning practitioners should source equipment and chemicals separately. A chemical cleaner proffered by a cleaning equipment vendor may reflect an economic partnership, rather than the optimal selection for a particular application.

CLEANING ALUMINUM The silvery-white, ductile metallic element aluminum is used to form many hard, light alloys. Nevertheless, fewer tests were performed by SCL on this metal than on steel (see Figure 4). Cleaning aluminum substrates can be challenging because of its proclivity to etch. Etching occurs when some of the metal is dissolved, along with the contaminant, as a consequence of cleaning. Properly conducted gravimetric analysis can prove most useful during cleaning trials on aluminum surfaces since cleaned weights may be less than original (i.e., precleaned) weights. Table 4 illustrates this effect. The assumption is made that no cleaner is capable of removing more than 100.00% of a soil. In this application, all three soils are present on the surface of aluminum parts. A cleaner must be found to remove the contaminants without damaging the substrate. In the first trial, Cleaners B, C, and D were relatively successful on the viscous and difficult-toremove lubricant mix. In subsequent testing on the easier-to-remove vanishing oil and drawing compound, however, Cleaners A, B, and C pose moderate (light-gray-shaded area) to substantial (dark-gray-shaded areas) etching risks. This is especially true if not all cleaning-cycle durations and temperatures can be accurately monitored at all times. Cleaner D is the best selection since it removed the vast majority of the three contaminants without etching in any of the test replicates.

MORE ABOUT METAL CLEANING Cleaning of other metallic substrates, including brass, bronze, copper, gold, molybdenum, nickel and nickel alloys, silver, tin, and titanium, was tested in the search for safer, greener chemical solvents. Some of those results are generalized in Table 5. In closing, tests conducted at the laboratory involve all aspects of the cleaning process. They confirm that the future of metal cleaning depends upon establishing industry standards or a ranking system for the energy and water efficiency of related equipment as well as a more complete understanding of the environmental and health consequences of newly developed chemical cleaners.8

© 2001 by CRC Press LLC

Table 3 Successful Cleaning, Rinsing and Drying Stages with Analyses (Representative Steel Samples) Steel Part and Contaminants *316 Stainless steel heat exchangers Fingerprints and light oils Stainless steel pump seals Coolant and metal fines

Carbon steel jet engine parts Rust preventative and quenching oil **1010 and 1020 cold rolled steel Lubricant and metal chips Unspecified steel rachet handles Dirt, metal fines, grease, buffing compound

Cleaner Types and Concentrations (vol %)

Cleaning Methods, Temperatures, and Cycle Times

Rinsing Methods, Temperatures, and Cycle Times

Drying Methods, Temperatures, and Cycle Times

5% aqueous to neutral pH

Ultrasonics (25 kHz) at 140°F for 10 min

Tap Water at 140°F or DI water at ambient temp for 2 min each

Convection oven at 100°F or IR heat lamp for 30 min each

Visual

4–10% several aqueous to alkaline pH

Ultrasonics (40 kHz) or spray wash at 110 –150°F for 5 –15 min

Tap Water at 130 –150°F or DI water at ambient temp. for 2–5 min each

Photomicrography

100% terpene/ semiaqueous and hydrocarbon

Immersion/soak at ambient temp. for 2 min

None

Convection oven at 140 –145°F for 30 –60 min or air knife at ambient temp. for 2 min Air Dry at ambient temp. overnight

3–5% several aqueous to alkaline pH

Ultrasonics (40 kHz) or tank agitation at 100 –130°F for 2–5 min Immersion/agitation at 150°F for 5 min

Tap water with/ without rust prohibitor at 120° for 0.5 min Tap water at 120°F for 0.5 min

IR heat lamp for 1 min

FTIR

Air dry at ambient temp. overnight

Customer performance test

5% aqueous to alkaline pH

*Alloy designation (refers to steel’s state of composition, annealing, hardness, etc.). **Customer-reported grade designation.

© 2001 by CRC Press LLC

Surface Analyses

Customer performance test

Table 4 Gravimetrically Based Contaminant Removal Rates Reveal Potential for Surface Damage Due to Chemical Etching Trial I

Percent Soil Removal, Lubricant Mix

Aqueous Cleaner

A

B

C

Coupon #1

87.15

96.04

99.26

98.60

Coupon #2

91.99

99.77

93.17

99.58

Coupon #3

78.57

100.00

94.97

99.25

85.90

98.60

95.80

99.14

6.80

2.22

3.13

0.50

Average Std deviation Trial II

D

Percent Soil Removal, Vanishing Oil

Aqueous Cleaner

A

B

C

Coupon #1

100.32

100.77

101.26

99.79

Coupon #2

100.54

100.05

101.24

99.68

Coupon #3

100.76

99.72

99.70

99.16

100.54

100.18

100.73

99.54

0.22

0.54

0.89

0.33

Average Std deviation Trial III

D

Percent Soil Removal, Drawing Compound

Aqueous Cleaner

A

B

C

D

Coupon #1

100.76

99.58

99.36

99.62

Coupon #2

100.57

100.00

99.18

99.72

Coupon #3

100.50

99.17

99.77

99.89

100.61

99.58

99.44

99.74

0.13

0.41

0.31

0.14

Average Std deviation

ACKNOWLEDGMENTS The author wishes to express gratitude to Jason Marshall of the Toxics Use Reduction Institute’s Surface Cleaning Laboratory for his vital contributions to graphics, databases, and the bench chemical testing that form the cornerstones of this chapter.

Table 5 Examples, Evaluation of Other Metal-Cleaning Applications Metal Part

Surface Contaminants

Present Chemical Cleaner

Recommended Chemical Cleaner

Copper tubes Nickel engine parts Inconel turbine blades

Machining oil Oil, grease, wax Penetrating oil

TCE Acetone None (new system)

Alkaline aqueous Semiaqueous Alkaline aqueous

© 2001 by CRC Press LLC

REFERENCES Note: Additional details may be obtained by contacting the laboratory Web site, http://scp.rti.org/lab.htm. 1. LeBlanc, C., The Toxics Use Reduction Act of 1989: Lessons Learned, in Annual Conference Proceedings: Precision Cleaning, Cincinnati, OH, April 1997. 2. Grace Metal Working Fluids, Aqueous Cleaning Handbook, Lexington, MA, 1995. 3. Marshall, J., Toxics Use Reduction Institute, Surface Cleaning Laboratory Notebook, University of Massachusetts, Lowell, 1998. 4. McLaughlin, C. and Zisman, A., The Aqueous Cleaning Handbook, Morris-Lee, New Jersey, 1998. 5. Green Seal, Degreasing Agents: Proposed Standard for Aberdeen Proving Ground, MD, Washington, D.C., 1999. 6. Toxics Use Reduction Institute, Environmental Strategies for the Next Millennium: Laboratory Services Dedicated to Surface Treatment, Cleaning and Analysis to Ensure Product Excellence, University of Massachusetts, Lowell, June 1999. 7. Boyer, H. and Gail, T., Metals Handbook, American Society for Metals, 1985. 8. Kanegsberg, B. and LeBlanc, C., The cost of process conversion, in Annual Conference Proceedings: CleanTech, Chicago, IL, May 1999.

© 2001 by CRC Press LLC

CHAPTER 5.3

Very High Performance, Complex Applications Barbara Kanegsberg

CONTENTS Introduction Examples of High-Precision Cleaning Aerospace and Related Industries High-Precision Navigation Systems Optics Removal of Blocking Agents Additional Issues and Suggestions Cleaning for Biomedical Applications Cleaning Issues, Biomedical Applications Case Studies Cleaning Electronics Assemblies Product Design Soils (Fluxes et al.) Build Process Cleaning Process Issues and Conclusions References

INTRODUCTION Defining high precision cleaning is difficult. Precision cleaning has been defined as cleaning of products of perceived high value, submicron level particulate removal, or cleaning products where the results of improper cleaning could be catastrophic. Precision cleaning has also been described as cleaning something that did not look particularly soiled in the first place.1 On the other hand, those involved in processing optics could argue that optics may be embedded in heavy pitch and wax, which then have to be completely removed.

© 2001 by CRC Press LLC

At one point, this author explained that everyone knew the difference between high precision cleaning and ordinary cleaning. High precision cleaning was simple to define: that would be my manufacturing process. Ordinary cleaning, on the other hand, encompasses everyone else’s manufacturing process.2 The author went on to explain that understanding as much as possible about the manufacturing process in question, and then collaborating with others who were also faced with stringent cleaning requirements, is crucial to advancing the science and art of contamination control. Over perhaps a quarter of a century, aerospace applications were highly dependent on solvents, notably the ozone-depleting chemicals (ODCs), CFC 113 (commonly referred to by the trade name Freon) and 1,1,1-trichloroethane (TCA). In the ODC era, given the time required to validate new processes, many high-precision, high-value processes changed only to develop more and more steps. Over the years, if there were problems at a particular stage that could conceivably be traced to contamination, the engineer in charge might recommend additional cleaning with CFC 113, TCA, or isopropyl alcohol (IPA), with some IPA and perhaps acetone for drying. Over the past decade, increasingly stringent environmental regulations covering volatile organic compounds (VOCs), air toxics, and ODCs have driven many process changes. The process of change has produced a number of false starts and inefficient results. On the bright side, where process development has been approached with an eye to better performance rather than simply coping with the regulation of the moment, more efficient processes with fewer steps and lower usage of cleaning agents have been adopted.

EXAMPLES OF HIGH-PRECISION CLEANING In this chapter, three examples will be discussed: aerospace applications, biomedical applications, and cleaning of electronics assemblies. Certainly, examples of high-precision cleaning abound and are discussed in other chapters. For example, cleaning of motion picture film, another critical cleaning application with unresolved issues, is discussed in the overview of cleaning equipment (Chapter 2.1).

Aerospace and Related Industries Considering issues of product reliability, military requirements, competition, and costly testing that may take years to complete, aerospace has been understandably conservative in adopting new processes. At the same time, aerospace has been fearlessly inventive in evaluating and eventually choosing from among the range of new solvents, mixtures, and cleaning techniques. Cleaning techniques adopted include supercritical CO2, aqueous cleaning, cosolvent systems, proprietary blends, and acetone and other low-flashpoint cleaning systems. In LOX systems (systems that will be exposed to liquid oxygen), the issue has been to find cleaning agents that pass stringent tests to prevent fires or explosions during use. For LOX cleaning, a range of cleaning agents may be allowed for initial cleaning, then, at the final stage, a level of defined but noninterfering residue is tolerated. The following example is one in which a particular application was modified first by adopting a cosolvent system (solvent cleaning followed by rinsing in another solvent), and then by evaluating a solvent. The study is presented to show the logic of the approach used in a given situation, not to indicate that this is the desirable solution for all aerospacerelated applications.

© 2001 by CRC Press LLC

High-Precision Navigation Systems* This example is based on experiences at Litton Guidance and Control Systems Division in modifying processes for a beryllium-based instrument. As with many such applications, process modification was required for environmental/regulatory considerations, notably the phaseout of ODCs. Because new cleaning agents and processes have been developed, process modification has been multistep and is ongoing. Navigation systems consist of gyroscopes and/or accelerometers along with the surrounding electronics. While cleaning problems associated with electronics may seem daunting, cleaning is a much more complex and diverse problem for high-accuracy instruments than it is for typical electronics assemblies.3 In electronics assembly, there are relatively limited materials of construction and configurations, and there are widely accepted industry standards. Definition and control of residue, residue of soil and of the cleaning agent itself, are major issues. The rosin mildly activated (RMA) flux was known to produce an inert residue; very small amounts of this residue, should they occur, would not be acceptable. While low-solids fluxes have been implemented in some hand-cleaning operations in other high-precision builds, process control and reliability testing were judged impractical for this application. One reason is that in some cases, the sealed system is in an atmosphere of a polyhalogenated flotation fluid; as such, there must be no residue of cleaning agent. Use of water is a controversial issue in this and in some other high-precision applications. Many of the components are water sensitive and difficult to dry, so aqueous cleaning is unacceptable at many stages. Even under clean room conditions, building precise instrumentation inherently generates testing, rebuild, and, doing so, generates an array of soils including: • • • • • • •

Greases Oils RMA flux Water-soluble flux Fingerprints Particles Polyhalogenated flotation fluids

In assembling classic gyroscopes and accelerometers, one finds a much larger range of materials of construction than are found in the typical electronics assembly. For inertial navigation systems, an array of materials of construction is used in instrument build. An entire list of materials of construction would require half a dozen pages. Some representative materials of construction are indicated in Table 1, along with potential cleaning and contamination problems. It is important to note that for Litton’s applications, it is not enough to consider each individual material separately. The entire assembly or subassembly must be evaluated to avoid potential issues of cleaning agent residue, galvanic interaction, outgassing, and product deformation. For this reason, the feedback of experienced instrument assemblers was crucial. The other issue is determining how “clean is clean,” or “how clean is clean enough.” When the product may be expected to perform continuously and reliably for a quarter of a century, addressing these ultimately unanswerable questions becomes increasingly important. * This subsection is based on a paper presented at CleanTech ‘99, a conference sponsored by the Cleaning Technology Group, Witter Publishing Corp., May 1999, Rosemont, IL.3

© 2001 by CRC Press LLC

Table 1 Representative Materials of Construction, Gyroscopes, Process Concerns Materials of Construction

Primary Concerns

1. Beryllium, aluminum 2. Stainless steel, other ferrous metals 3. Complex assemblies containing 1 and/or 2, and combinations of magnesium, gold, tungsten carbide, copper, Hy-MU-80 (nickel alloy) 4. Sapphire, specialized glasses 5. Kapton, plastic coatings, coated and uncoated epoxies

Oxidation, erosion, cleaning agent Corrosion erosion, cleaning agent Same as 1 and 2, plus galvanic interactions

6. Flotation fluid

7. Complex assemblies, many materials of construction (1–5), complex soils

Cleaning agent residue, subtle surface changes Solid cleaning agent residue, outgassing of vapors, softening, deformation, solubilization of material of construction Becomes a soil under test, rework conditions; residue undesirable, especially if reacts with cleaning agent (nucleophilic substitution reaction, SN2, with alcohols) Same as 1–5, plus entrapment of cleaning agent; reactivity of cleaning agents with nonmetals and residue of soils; situation exacerbated in sealed systems

Whereas in the electronics world there are established cleaning standards, for precision cleaning the goals are often pragmatic, based on expected use of the product; a combination of analytical testing of residue with performance is often used. The goal is to minimize contamination and residue to its lowest level. Those actually using inertial navigation systems, particularly aircraft pilots, have expressed enthusiastic support for aggressive contamination control. Process modification was multistep and involved a team approach by in-house engineering and production staff as well as ongoing collaboration with manufacturers of cleaning agents and cleaning equipment. Initially, an array of cleaning agents and cleaning sequences were evaluated in-house. Some more-promising cleaning sequences including various hydrocarbon and d-limonene (orange terpene) blends followed by self-rinsing or rinsing in IPA or perfluorinated materials, were then tested using the facilities and personnel of a major cleaning agent manufacturer, in cooperation with a number of other cleaning agent manufacturers. Results were evaluated visually and by analytical techniques, Fourier transform infrared spectroscopy (FTIR) and electron spectroscopy chemical analysis (ESCA). Some in-house residual gas analysis (RGA) was also incorporated in the study. Surprisingly, given the stringent process requirements, at least half a dozen promising cleaning sequences were found. The most promising cleaning sequences were then tested against the most exacting critics of all: experienced production assemblers. Based on assembler input, the cleaning sequences were refined and implemented. The approach adopted was a cosolvent system consisting of initial cleaning with a hydrocarbon blend containing various alcohols followed by two to three rinses with IPA. This new process allowed elimination of TCA cleaning; some perfluorinated material continues to be used as a final rinse to assure thorough removal of fluorolube. Overall, the number of process steps was reduced; often an 18-step process was reduced to four to six steps. Even the process introduced initially required refinement, because IPA was found to react with beryllium periodically; intermittent residues were found. Eventually, IPA was replaced with volatile methyl siloxanes (VMS). At the time, the new processes represented the best option to replace ODCs. © 2001 by CRC Press LLC

The cosolvent processes, while allowing replacement of TCA, were far from optimal. The subassemblies are very complex, with close tolerances and blind holes. While the cleaning agent can be removed with careful process control, extreme and constant vigilance is required to assure that no cleaning agent residue is left. The hydrocarbon blends are costly, and some of the operators found the odor to be disagreeable. The search for a more reliable process continued. After extensive testing, a specific n-propyl bromide (nPB) formulation was adopted to replace some of the multistep cleaning. The VG formulation has been implemented for cleaning during instrument build where cleaning activity similar to TCA is required. The cleaning agent was chosen for a number of reasons, including solubility parameters, low residue, liquid- and vapor-phase cleaning characteristics, and compatibility with materials of construction. Operations engineers conducted the primary evaluation of the VG cleaning agent. The materials and processes group conducted some preliminary evaluations and found it to be very effective for removal of a wide range of soils, including flux, without the requirement for rinsing with another cleaning agent. While nPB was adopted because of its aggressive solvency, which is similar to 1,1,1trichloroethane, there is always a balance between solvency and compatibility. Beryllium metal, used in some of Litton’s inertial navigation systems because of its relatively high ratio of strength to weight, is also a relatively reactive metal, forming beryllium oxide. To put it into perspective with more commonly used metals, beryllium can be thought of as rather temperamental aluminum. Beryllium coupons supplied by Litton were tested by the cleaning agent manufacturer to determine the impact of exposure to the VG formula. After 24 h of exposure at the boiling point, beryllium submerged in the solvent showed no sign of discoloration or tarnish.5 It should be emphasized that the results of these studies should be used to indicate the approach to be used in evaluation, not to recommend a specific process or product. Having indicated the results of these studies, it should be noted that, with reactive metals and aggressive cleaning agents, each formulation and each application should be considered as a separate case. Given the aggressive nature of nPB, collaboration with the manufacturer is crucial. In the case of Litton, the industrial hygienist provided training, site testing, and recommendations for appropriate engineering controls and personnel protection to minimize exposure via inhalation or skin adsorption. At Litton, a combination of carbon tubes and colorimetric indicator tubes was used. Employee exposure by inhalation of nPB in all the applications measured was below 10 parts per million (ppm). Area samples, used in some cases to measure the concentration of nPB at specific locations in the workroom to predict “worst-case” potential exposures, were also below 10 ppm. The observed exposure depends on the type of cleaning, cleaning action, ventillation, and worker education and awareness. In this case, activities included ultrasonic cleaning, spray and flushing systems. As with compatibility, worker exposure is very application and site specific and should be evaluated on a case-by-case basis. Because of changing regulations and given the ongoing development of new products and cleaning methods, process modification and process improvement are often best achieved in a multistep manner. This has certainly been the case at Litton. The interim process eliminated Class I ODCs but was cumbersome to use. Adding a more aggressive cleaning agent with a higher evaporation rate along with reevaluating the necessity of various cleaning and rinsing steps has significantly simplified the process. In addition to cleaning capability and reliability, the new process has provided improved processing time and lower cleaning agent usage. Litton’s processing time has been reduced by over 40% and cleaning agent usage has been reduced to essentially one third (Tables 2 and 3). Process modification would not have been successful without the input, cooperation, © 2001 by CRC Press LLC

Table 2 Summary: Litton Process Modification Interim Process

Simplified, Modified Process

Hydrocarbon blend (two used, depending on soil to be removed) Volatile methyl siloxane rinse Perfluorinated A Perfluorinated B, particulate removal

VG, self-rinse Perfluorinated B, particulate removal

and collaboration of the production assemblers. For many high-precision processes, the end product is produced on a very small scale and may be based on hundreds of assembly and cleaning steps. Automated cleaning processes may be impractical. In such cases, build and cleaning processes are highly specialized, involving the input of skilled and experienced assemblers.4 Particularly in situations with repeated hand-cleaning processes involving skill and judgment, it is important to involve those who will work with the alternative cleaning processes on a day-to-day basis. Often, the assemblers themselves are able to detect potential problems that are not picked up even with sophisticated analytical testing. For example, in flux removal, a pilot project was undertaken in which individuals performing overhaul and rework and hand-cleaning operations evaluated several hydrocarbon blends and orange terpenes. While initial laboratory-scale evaluation indicated that all the products provided for pilot test cleaned equivalently, the technicians reported subtle differences. Some said they could see differences in cleanliness, and saw problems with certain of the cleaning agents. More-detailed, costly testing including surface analysis and outgassing confirmed that, indeed, certain cleaning agents were leaving a previously undetected residue. Attempting to conduct all possible analytical testing initially would have been time-consuming, costly, and probably unproductive. In general, where the production people have been involved at various stages in new process development, a much more robust process has resulted. Optics Optics covers a range of applications from eyeglasses and contact lenses to an array of specialized sensors and components of sophisticated devices. The substrate is sometimes a plastic, but is more often than not a specialized glass. This section deals primarily with high-precision glass optics. Some of the considerations apply to plastics. With plastics, one must also factor in issues of compatibility with some of the more aggressive solvents. Processing of optics is shrouded in mystery. Competition-sensitive issues and military concerns result in secrecy, vagueness, and lack of communication. While many of these concerns are no doubt justified, the result has been an array of complex, Byzantine processes that are difficult to control and troubleshoot. Because of the many fabrication steps involved, processes are often performed for traditional reasons; it is often impractical Table 3 Summary: Benefits, Process Modification Typical Process Interim, with hydrocarbon blends Replacement, with VG

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Cleaning Time (min)

Cleaning Steps

Solvent Usage (gal/week)

35

4

45

20

3

15

to take the time and effort to justify a particular operation. We know things have to be very clean, and we often perform complex laboratory analysis. In the end, it comes down to maintaining the process so that the next fabrication step can be successfully completed and that the final device operates acceptably. Optics processing inherently develops into a tightly woven sequence of processes. A supposedly minor change to one process, perhaps mandated by some regulatory requirement, may impact other parts of fabrication such that extensive modification and reevaluation are needed. In processing optics, the substrate is first machined and grossly shaped or sliced. Various surfaces are then polished with specific slurries, and the surfaces may be etched and/or acid-treated. Finally, specific coatings are applied, often by vacuum deposition. At various stages, cleaning has to occur in such a manner as to remove the soils without damaging the surface, which has just been carefully polished, treated, or coated. Cleaning must be accomplished without redepositing soil, adding residue of cleaning agent, depositing particles, or changing the surface in some undesirable manner. With optics and with other sensitive materials it should be remembered that water quality and composition can impact surface properties. Contaminants can be deposited on and react with the surface. In addition, ultrapure deionized (DI) H2O can leach materials out of surfaces and alter the surface. A vast, often fanciful assortment of materials, which will be referred to as blocking compounds, is used to hold the substrate in place for polishing, or to protect certain portions of the optical system from treatment. Blocking compounds may include such materials as: • • • • • • • •

Pitch (asphaltum) Soft wax Beeswax Rosins Nail polish Epoxies Thermoplastics Mixtures of plastics, rosin, and waxes of variable composition

The materials are chosen for a variety of reasons, often relating to the forces needed for polishing. In some cases, optics may be set into a 3 mm or more thick base of an organic-based blocking compound on a larger base plate. This mosaic-like object is then polished in a specific slurry. At the end, the blocking compound and polishing compound must be removed. Classically, TCA was used for many optics operations. The optics could be soaked in TCA, either at ambient or at elevated temperature; the blocking compound would dissolve, leaving a light residue. Final cleaning would then be accomplished by vapor-phase cleaning. With the advent of the ODC problem and the TCA phaseout, a number of other chemicals and processes have been used with varying success. Issues involve: • • • •

Inadequate solvency Cleaning agent residue Scratching of the substrate Etching of the substrate

Some blocking agents currently in use are summarized in Table 4. Some details follow. In addition, it might be pointed out that, in general, particularly for rinsing and final cleaning of optics, there is no subsitute for high-quality, low-particulate cleaning agents. Where possible, © 2001 by CRC Press LLC

Table 4 Summary: Some Considerations, Examples of Newer Deblocking Compounds General Characteristics

Performance Concerns

Regulatory/Safety Issues

Chlorinated solvents

• Liquid- and vaporphase cleaning • Self-rinsing • Aggressive solvency • Rapid process

• High boiling point (PCE) may be result in components damage • Must be tested with individual application to assure no surface changes

• Engineering controls required for personnel protection • Air toxics • Solvent containment, reporting required

nPB

• Liquid- and vaporphase cleaning • Similar performance to TCA

• Must be tested with individual application to assure no surface changes

• VOC • Low ODP • Engineering controls required for personnel protection • Regulatory status may change, pending evaluation

d-Limonene (orange terpene)

• Good solvency for many blocking compounds • Liquid-phase cleaning • Some formulations can be rinsed with water

• Must be tested with individual application to assure no surface changes • Leaves significant residue for most high-end applications • Residue from additives may interfere with subsequent applications • Rinsing with solvent or water required • Some blends can oxidize

• VOC • Distinct odor can be an issue

Esters (e.g., ethyl lactate, di-basic esters) alone or in blends

• Good solvency for many blocking compounds • Liquid-phase cleaning • Formulations available for aqueous rinsing

• Must be rinsed

• VOC • Distinct odor can be an issue

N-Methylpyrrolidone

• Moderate to good solvency for many blocking compounds • Liquid-phase cleaning

• Must be rinsed • Often somewhat longer processing time

• VOC

Deblocking Agent

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Table 4 Summary: Some Considerations, Examples of Newer Deblocking Compounds (Cont’d) Deblocking Agent

General Characteristics

Performance Concerns

Regulatory/Safety Issues

• Often less prone to leaving residue than d-limonene or ester blends Proprietary blends, water-soluble organics

• Moderate to good solvency for many blocking compounds

• Require rinsing

• May contain significant amounts of VOCs • Appropriate mixed waste stream handling needed

Acetone

• Good to aggressive solvency • Can be used as heated liquid vapor phase (with proper equipment) • Useful in combination with other deblocking agents, including aqueous

• Exceedingly rapid evaporation

• • • •

Alcohols (IPA, methyl alcohol)

• Good to aggressive solvency • Cold cleaning • Can be used as heated liquid vapor phase (with proper equipment) • Useful in combination with other deblocking agents, including aqueous

• Solvency often limited for blocking agents of interest

• VOCs • Low flash point • If heated, must be used in specially designed equipment

Hot water

• Low solvency, acts by melting • Useful with soft wax • Rinse agent

• Limited solvency • May require multiple rinses • Must control water quality

• Disposal of waste streams

Cold shock

• Nonchemical

• May damage substrate

Aqueous/surfactant

• Moderate to good solvency for many blocking compounds • Liquid-phase cleaning

• May require multiple rinses • Additives may produce subtle surface changes, not immediately evident

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VOC exempt Not an air toxic Very low flash point If heated, must be used in specially designed equipment

• Disposal of waste streams • Additive packages may have significant VOCs or require special handling

electronics grade, HPLC grade, or the equivalent should be used. Certification of desired properties by the supplier and some assurance that the formulation will not be changed without notification are crucial. Removal of Blocking Agents Chlorinated, brominated solvents: Perchloroethylene (PCE) and other chlorinated and brominated solvents have been tested, with varying success. PCE has an inherent limitation in that the higher boiling point may produce bubbling of the blocking compound. The bubbling, boiling action may jar the optics, effectively undoing the previous polishing process. nPB has also been used effectively in some applications, and it has the advantage of allowing for final cleaning in the vapor phase. Many of the chlorinated solvents are covered by a federal NESHAP (National Emissions Standard for Hazardous Air Pollutants), so that careful record keeping and reporting are required. As of the time of writing, nPB is acceptable for use pending evaluation by the U.S. EPA SNAP group. With all aggressive solvents, the changing regulatory picture and, in some areas, local restrictions on smogproducers (VOCs, or volatile organic compounds) or air toxics, may require that alternative processes be developed. Acetone, alcohols, low flash-point blends: Acetone, IPA and methyl alcohol are widely used in fabrication of optics. Acetone has a certain appeal, particularly in areas of poor air quality, in that is VOC exempt. In addition, many low-flash-point solvents are of low cost and can be obtained in high quality. The authors cannot stress enough that low-flash-point solvents can be used safely and with confidence in appropriately designed equipment. There have been semidisastrous attempts to heat low-flash-point solvents or to adapt existing equipment in-house. Even though equipment for low-flash-point solvents is costly, the decrease in solvent usage and in disposal costs may result in a short payback period. Hot water or cold shock: Either approach depends on physical rather than chemical removal of the blocking agent. In the case of hot water, one is simply melting the wax or rosin. While heat is important in many cleaning processes both to boost solvency and to promote melting, one must be aware of the inherent limitations of heat in the absence of solvency. Cold shock, either by freezing the optics or by using a CO2 snow gun, has had limited success in releasing the optics via changes in thermal expansion or by physically cracking the blocking material. More often than not, however, particularly in production situations, the process has resulted in damaged substrate. Aqueous/surfactant: Water-based cleaning is becoming increasingly popular because of safety and environmental concerns. Adopting water-based cleaning agents often requires a change in the blocking agent. Heavy pitches and very hard waxes are often difficult if not impossible to remove with water-based cleaning agents. In addition, aqueous/surfactant blends require increased cleaning action such as mechanical agitation or spray, ultrasonics, additional heat, and careful rinsing. Where agitation is used, it is important to protect the substrate. Rather than struggle through evaluation of a succession of aqueous cleaning agents and processes, it is often more productive to change the blocking agent to one more readily removed with water-based cleaners. Effect of additives: Glass is soft. In modifying older, solvent-based processes, one might be aware that additives both in aqueous cleaning agents and water-soluble organics have the potential to produce subtle surface changes. These changes may not be immediately apparent but may manifest as some other problem many steps later in the process. Additional Issues and Suggestions Deblocking is a common problem in optics. There are additional issues that might be considered. © 2001 by CRC Press LLC

Cleaning vs. surface modification: Many optics preparation processes are old and steeped in the rich tradition of the company. (Translation: no one knows why they work, but if one attempts to change anything, the process fails, and the person who made the change is blamed.) Part of the problem relates to subtle changes in the surface of the substrate. This means that cleaning problems may become entangled with problems of surface preparation. For example, optics are often cleaned in mixtures of very strong acids and salts. In one instance, in an attempt to reduce usage of a chromic acid mixture, solvents were tested for cleaning. The solvents were judged unacceptable on the grounds of a change in contact angle measurement. One might suspect, however, that the acid was modifying the surface and so changing the contact angle. The problem was not one of cleaning but probably rather one of surface modification. Troubleshooting: Optics processing is notorious for intermittent, inexplicable problems. With so many steps in the fabrication process, precise control of every factor in every operation is difficult. If one is in the oh, so fortunate position of being put in charge of troubleshooting an optics fabrication process, it is often difficult to determine the source of the problem. One must become a detective. Suppose that a coating process is suddenly no longer successful. One would, of course, look at the coating operation itself and factors in the immediate vicinity of the suspect problem including: • Analytical testing to determine the nature of the contaminant (coated surface, surface prior to coating) • Coating equipment operation • Personnel changes or dissatisfaction • Chemical changes • Water purity, if applicable • Clean room conditions • Surface preparation • Storage conditions prior to coating If careful examination of these factors does not yield the source of the problem, one needs to venture farther afield, considering factors mentioned above as they apply to earlier operations, as well as: • • • • • • •

Earlier cleaning, polishing, and etching steps Changes in any chemical Changes in any process Equipment condition Recent equipment repair or overhaul New lot or new source of substrate Modification in initial slicing or polishing operations

Above all, see if the investigation can be turned into a team effort, involving all of the people who might be influenced or who might be the source of the problem. It can be a real challenge to keep communication open and maintain a no-fault atmosphere, even within the facility. Of course, it is always possible that someone is purposely breaking clean room discipline (a term the author finds to be unproductive and unprofessional). It is more likely that someone or several people have made changes that they honestly felt were the equivalent to the status quo, or that they just were not aware of. It can be a real challenge to persuade people to communicate. Many people who become involved in high-technology,

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clean room operations like optics fabrication do so because they would prefer to work quietly, out of the public eye. Some new idea may have been quietly implemented which, while wonderful in isolation, does not quite mesh with the whole operation. The important thing is to keep the atmosphere open, productive, and as free as possible from finger-pointing. This is a real art, and it is an art the author is still developing. Side benefits include more-up-to-date, well-documented processes in that production workers may confide what they are actually doing. They may also have some wonderful ideas for improvement. Quality circles and teams may seem a generation out of date, but honest communication and respect for the ideas of others can really pay off. Teaming ideally involves the vendor community as well. If subcontractors are involved, it is necessary to find out as tactfully as possible whether or not they have made changes. The same holds for cleaning agent and cleaning equipment vendors and for any outsourced items such as maintenance of the clean room or of the water system. Maintaining and improving the process: With optics fabrication, as with all multistep, high-precision processes, maintaining the process is crucial. This involves: • Using the highest quality (chemicals, equipment, water, disposables, substrates) • Documentation of chemical quality through on-site testing and/or vendor certification • Control of disposables, storage conditions • Maintaining records of product and chemical lots • Documentation of water quality • Clean room control • General thorough record keeping • Employee education • Valuing the workers, listening to their ideas In essence: invest in quality, trust nothing, strive for consistency, but do not become a tyrant. Someday, we’ll all get this right! Cleaning for Biomedical Applications* Consider the phrases high-precision cleaning, high-value cleaning, or critical cleaning. The applications that immediately come to mind include wafer fabrication, computer subassemblies, precision optics, microelectronics, and high-accuracy inertial navigation systems. The biomedical field brings an entirely new dimension to the concept of critical cleaning applications, but it is an area that has not typically been discussed. Cleaning and manufacturing for biomedical applications, in the context of this discussion, include • Implants (metals and plastics) for long-term use in humans • Catheters and other devices for use in animal experimentation • Surgical instrumentation, which may include long, and extremely fine-bore tubing

* This subsection is based on a paper presented at CleanTech ‘97, a conference sponsored by the Cleaning Technology Group, Witter Publishing Corp., April 1997, Cincinnati, OH.6

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• Assorted plastics and metal disposable and nondisposable materials • Subassemblies for clinical instrumentation, including automated equipment for clinical laboratory testing (blood tests, etc.) The biomedical community is justifiably concerned about quality processes, but each group tends to work in isolation and secrecy that surpasses that of many military applications. There is an overwhelming reluctance to discuss either successes or difficulties in a public forum. A number of factors promote secrecy. It is difficult and costly to bring a new product to the marketplace, or to change the manufacturing process for an existing product. The manufacturer faces not just the hurdles of coping with strict and often conflicting local and national air quality, water quality, and employee safety regulations, but there are also the issues of biocompatibility, pyrogens, bacterial growth, and other harmful residues. In addition, with biomedical applications there may be environmental regulatory concerns as well as the challenge of working with the Food and Drug Administration (FDA). Gaining FDA approval is often a major hurdle. Some companies reportedly fear that emerging FDA guidelines will render their complex, costly testing inadequate. Therefore, even groups doing careful, thoughtful testing may be reluctant to present findings lest they call attention to themselves by the FDA or by any other regulatory agency. The FDA does not regulate or specify cleaning per se. However, the FDA does regulate the effect of the manufacturing process on the finished device. This would then be part of the 510K Premarket Notification. One FDA spokesperson notes that while the 510K format is standard, because there must be tens of thousands of different devices, across-the-board testing requirements would be difficult to define. Because companies are fearful of incurring increased inspection by either the competition or by government agencies, they keep both their successes and failures to themselves. In fact, some individuals declined to discuss issues involved in manufacturing biomedical instrumentation, even anonymously, on the grounds that their phraseology would be recognized. The author would like to thank those colleagues, clients, and associates throughout the United States who shared their concerns and success stories. It should be noted that in the following examples and case studies some names of companies and of individuals have been pointedly omitted to assure candor and to avoid competition-sensitive issues. Cleaning Issues, Biomedical Applications Many in the biomedical community developed cleaning processes using ODCs or other classic solvents. As in the inertial navigation world and other high-value-added applications, they knew the product was clean based on years of operational experience. With new processes, the issue becomes more complex. Some issues of importance to the biomedical community in modifying their cleaning process are indicated in Table 5. Many of these factors are also important to other critical cleaning applications. Manufacturers in the biomedical field have been justifiably focused on biocompatibility. The Association for the Advancement of Medical Instrumentation (AAMI) has produced guidelines for reprocessing devices.7 Some groups also adapt the approach for use in manufacturing of components. Given the range of new cleaning manufacturing processes being implemented, there may also be a need to address issues related to process control and to impact of the process on quality and consistency of the finished device. Additional cleaning issues are discussed in Chapter 5.5 by Albert.

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Table 5 Examples, Potential Contamination Problems, Biomedical Applications Potential Contamination

Potential Problems

Possible Solutions

Soils (greases, oils, polishing compounds)

• • • •

Toxicity, biocompatibility Increase in bioburden Blockages Mechanical malfunctions

Cleaning agent residue from solvating agent or aqueous/saponifier

• • • •

Toxicity Variable changes in bioburden Lot-to-lot variability Mechanical malfunctions

Solvent or rinse agent residue

• Variable residue on component • Outgassing leading to toxicity; damage to components in sealed devices • Materials compatibility issues • Unsuitability for use in sterile situations • Mechanical breakdown of clinical equipment

• More aggressive cleaning agent • Enhance cleaning action (turbulation, ultrasonics) • Monitor soil loading • Avoid cleaning agents with significant nonvolatile residue • Provide more rinsing than is typically used • Monitor the process to avoid excess carryover of cleaning agent • Choose a solvent not readily adsorbed by materials of construction • Provide adequate drying

Microbial contamination

• Choose a more aggressive solvent • More aggressive cleaning, longer cleaning • Clean at a higher temperature

Case Studies One company, which produces a variety of plastic and metal components for biomedical applications, switched from TCA to PCE in a contained solvent system from Pero. The selection process involved many months of comparison studies and testing including bioburden testing. Determination of bioburden involves contaminating the product with microorganisms, cleaning, then testing to determine the amount of remaining microorganisms relative to the control or reference cleaning method. For this particular group, given the configuration and spacing of components, entrapment of solvent is an issue. While cleaning in IPA or with IPA azeotropes has been used for many biomedical applications, in this case, low-flash-point solvents were not removed rapidly or completely enough to allow adoption of a flammable liquid/vapor-phase system. Even using high-temperature cleaning with PCE, specially designed fixtures were crucial to assuring adequate cleaning and drying. While this success story utilizes a system from Pero, it should be noted that other contained solvent systems (e.g., from Serec, Baron Blakeslee, Branson, Durr, Hyperflo, Unique, and Tiyoda) are being evaluated and/or adopted. Aqueous cleaning has always been a popular option, particularly where compatibility issues make solvent cleaning unwise. Aqueous cleaning can be successfully adopted where careful attention is paid to process design.8 The force of action of water may be enough to clean some soils of concern. Where companies have been accustomed to using aqueous cleaning, extending the use of aqueous cleaning has been a relatively simple, successful © 2001 by CRC Press LLC

process. One company eventually successfully implemented both solvent cleaning to replace TCA and aqueous cleaning to replace CFC 113. The group found that, initially, it was more difficult to ramp up the new aqueous process because blind holes in some of the disposable plastics components made drying a limiting part of the process. Their drying technique had to be improved. Another group makes sealed implantables for relatively long-term application in humans. The device must of necessity be extremely compact, so all components are very closely spaced. They use surface-mount electronics, and have been replacing CFC 113 for cleaning rosin flux. They also have an array of bonding problems, including bonding of plastics to metals. If there is any flaw in surface preparation, subsequent bonding may not be successful. Perhaps even more potentially distressing, any flaw in the surface can result in subsequent adhesive failure in a warm, saline environment. One of the engineers notes that, “like everyone else, they tried orange terpene alone, but were not successful.” This maker of implantables has adopted cold cleaning with IPA cleaning, and aqueous cleaning prior to coating as an interim measure. IPA alone is not efficient for removing most rosin fluxes. The process is admittedly inefficient and labor-intensive. Their cleanliness standards are • Ionograph measurements • Coating adhesion at a subsequent step (i.e., the parylene sticks) The group would like to adopt aqueous cleaning for environmental reasons. They expect that some solvent usage may be required. However, they are concerned about the capital outlay and chemical-handling issues associated with use of low-flash-point solvents. In another example, a new process provided performance superior to the existing ones. A manufacturer of titanium bone replacement implants and ultrahigh-molecular-weight polyethylene (UHMWPE) cartilage replacement implants wished to replace cleaning processes utilizing HCFC-141b and trichloroethylene. They found that a stabilized nPB formulation met the company cleaning requirements, including: • Acceptable removal of buffing compound • Low concentration of retained solvent in the UHMWPE • Reduction of bacterial spore count in the UHMWPE by 50% or greater Minimum solvent outgassing is required. Retained solvent was measured by gas chromatography/mass spectrometry. Results for nPB were compared with HCFC 141b and trichloroethylene. After 24 h of drying at ambient temperature with good airflow, 30 ppm of nPB were detected. By comparison, the level of retained solvent was over five times as high for HCFC 141b and over 15 times as high for PCE. After 96 h of drying, the level of retained nPB decreased to 3.7 ppm. Measurements made at 106 h were 4.4 ppm for HCFC 141b and 27 ppm for PCE. Bioburden studies indicated spore count reduction of 68 and 73% after cleaning with nPB. Details of these studies have been reported. Yet another group has had good success in using HCF XM, an HFC blend from DuPont, to clean plastic and metal surgical devices. HFC has low surface tension and provides increased wettability along with compatibility with a wide range of materials. The blend was chosen because it has a somewhat higher polarity and therefore a greater solvency range than the HFC alone. The engineer in charge of the project notes that one of the major challenges is that cleanliness is not defined for the medical industry; the FDA has no information as to how clean is clean. The process had to be developed and justified to © 2001 by CRC Press LLC

company management based on what would be logical standards considering the end-use application. In this case, residual particulate material of greater than 5 µm is considered crucial. HFCs (as well as HFEs and many VMS) are particularly effective in removing particles. Cleaning Electronics Assemblies* Cleaning of electronics assemblies is a relatively well studied area that has been the topic of a number of books,8,11 trade associations (e.g., IPC), and conferences (e.g., IPC, Nepcon). Because of the number of studies defining materials of construction and recommending cleaning agents and cleaning equipment, some consider electronics cleaning to be a fait accompli, needing little or no additional study. However, as designs change, environmental restrictions increase, and performance and economic demands become ever higher, many manufacturers continue to be faced with issues of cleaning and contamination control. Some groups have chosen to outsource the entire design and assembly process. However, the basic issues of design, manufacture, and cleaning remain. In electronics assembly, industry has successfully varied a number of parameters to improve manufacturing capabilities. The approach could be used in other industries. It is possible to modify: • • • •

Product design/materials of construction Soils Build process Cleaning process

Products can be modified for easier assembly. Unfortunately, many design engineers, often with good reason, take the approach of automatically attempting to pack 10 lb of stuff in a 5-lb bag. There may be good reasons, such as some technical requirement for a compact or miniaturized product. However, the more closely spaced are the components, the more difficult the product is to clean. Product Design The effect of component density on ability of a particular cleaning system to work effectively was illustrated by W. Machotka, C Knapp, and B. Kanegsberg in a study at Litton Industries in which leadless, 1-in. square component simulators were placed at varying standoff distances.10 That is, components were spaced at 0.003, 0.005, 0.008, and 0.015 in. from the surface of the board. The component simulators were dipped in RMA flux. Parts were charred by heating immediately before cleaning to simulate soldering. They were then cleaned in actual production batch and in-line cleaning systems using a variety of cleaning agents. Efficiency of removal of RMA flux was estimated gravimetrically.. Cleaning equipment and cleaning agents are identified only by code. This experiment was not completely balanced in that the cleaning agents tested in a particular piece of equipment depended on what was being used at the time in a particular production situation. In that sense, the options tested were considered acceptable, but all available options had not

* Significant portions of this section have been adapted from “Successful Cleaning/Assembly Processes for Small to Medium Electronics Manufacturers,” a tutorial presented by the author at Nepcon West ‘98, Anaheim, CA, March 3, 1998.10

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Table 6 Flux (%) Removed Relative to Standoff Equipment

Cleaning Agent

In-line a

1,1,1-Trichloroethane azeotrope HCFC d-Limonene A, semiaqueous d-Limonene A, semiaqueous Hydrocarbon blend, semiaqueous Aqueous/saponifier A Aqueous/saponifier A Aqueous saponifier B Aqueous/saponifier B

In-line b In-line c Batch a In-line d In-line e Batch b In-line f In-line g (older model)

0.003 in.

0.005 in.

0.008 in.

0.015 in.

98 88 79 69

100 100 100 100

100 100 100 100

100 100 100 100

56 79 65 71

100 93 96 100

100 100 94 100

100 100 100 100

26

38

63

73

Notes: Five samples were run per test, Batch systems were manually operated; in-line systems were automated.

been tested by the production facility. As indicated in Table 6, below a 5 mil (0.005 in.) standoff, it is difficult to clean with less aggressive solvents, semiaqueous, and aqueous methods. In addition, automated systems tend to be more effective in soil removal. Results provide additional evidence that system design can influence cleaning; the very old cleaning system was not nearly as effective as the newer designs. This author has empirically observed that 5 mil tends to be the borderline level between relatively straightforward and relatively difficult cleaning applications for all manner of assemblies, not just electronics. Properly motivated design engineers, on being presented with such cleaning information, might modify the assembly to allow easier fluxing and defluxing. It is typically helpful to involve the design engineers in process modification plans, keeping them on any teams or at least keeping them up-to-date on proposed changes. Conversely, those involved in assembly process development would do well to look at the next generation of assemblies to determine if they can be cleaned using the methods under consideration. Soils (Fluxes et al.) In electronics assembly, solder flux is the primary soil. Flux is, in a sense, a cleaning/surface modification agent in that it facilitates soldering by preventing buildup of oxides. However, once used it must itself be cleaned. Many manufacturers have made significant strides in manufacturing by modifying the soil, in this case, the flux. Choices in flux include: • Rosin based • Water soluble • Low solids, the so-called no-clean fluxes All have their advantages and difficulties. Rosin-based flux is formed of pine tree sap with additives, including activators, some of which are acids. RMA flux is classically used in military and other high-end applications. Rosin-based fluxes have had their problems. As naturally occurring materials, they can show wide variations in soil from lot to lot, because, for one thing, groves of pine trees vary © 2001 by CRC Press LLC

in composition of the sap. Flux residue can be a problem, particularly if the flux cures after soldering and prior to cleaning. This most often happens with significant delay prior to the cleaning process. In addition, many of the polar additives are difficult to remove; they may leave complex mixtures known as the infamous white residue. Because flux is a mixture of polar and nonpolar components, cleaning agents with a wide solvency spectrum are preferred, or azeotropes, such as IPA/cyclohexane azeotrope, have proved effective. Flux residue of any sort is not considered desirable. Residues can form crystals (dendritic growth) that can impair product function or even result in product failure. Dendritic growth is a sort of microscopic stalactite or stalagmite that appears on assemblies after cleaning, often after aging in a humid atmosphere. Acid or salt residue can be corrosive, damaging materials of construction. While manufacturers claim that they want no detectable residue, it has become widely accepted that rosin fluxes may leave a residue that is less damaging to the product than are some other types. Rosin flux, when heated in the soldering operation and cured due to delayed cleaning or further heating with inadequate cleaning, may leave trace residues under components. However, these residues are often hard and jewellike, and they appear to be nonreactive. One would suspect that such flux residues form a sort of artificial amber. Amber is petrified tree sap, and, in fact, several jewelers have quietly confided that realistic, artificial amber can be produced by heat-curing rosin flux. Other fluxes are widely used because of pressures to avoid organic solvent cleaning or to avoid cleaning all together. Organic acid (OA) fluxes or water-washable fluxes can be very effective and can obviate the need for solvent cleaning. They are meant to be cleaned with water-based cleaning agents, not with solvents. While some organic acid fluxes are synthetic, others are based on lemon juice or apple juice. This has caused some confusion among manufacturers who confuse lemon-based flux with d-limonene (orange-peel-derived) cleaner. Changes in the soldering procedure are needed, and care must be taken in the cleaning process to avoid residues that can produce dendritic growth or other interfering or corrosive residue. Low-solids or no-clean fluxes are designed with minimal solids and minimal residue to avoid the need for cleaning altogether. Control of the overall assembly and soldering process is typically more exacting with no-clean fluxes. An inert atmosphere may be required, and there is generally a much narrower process window. Because of the high level of process control, the most successful initial implementation of low-solids fluxes occurred in very large scale manufacturing facilities where ongoing training and process control could occur. Because solder joints may not be as aesthetically pleasing with no-clean processes, it is often necessary to modify in-house or customer requirements from visual requirements to functional requirements. In addition, there are applications where even the small amount of residue left by low-solids fluxes is unacceptable. Because of this, examples of cleaning the no-clean flux are increasingly seen. Sometimes, only a very dilute aqueous-based cleaning agent is needed, or water without additives may be acceptable. It should be pointed out that some low-residue fluxes are more amenable to being cleaned with water than are others. Some no-clean flux residue can result in product degradation in a humid atmosphere, a distinct disadvantage if the product is to be used in New Jersey or Texas in July. It is the preference of this author to use a cleanable no-clean flux where possible. While most manufacturers think of flux residue as the primary contaminant, other contaminants can cause problems. Often, contamination is introduced by a components supplier and may not be detected until some problem arises after final assembly. Other residues include oils, sulfur-based compounds from machining fluids, and metal or ceramic particles. Determining residue of flux and other contaminants can involve a host of visual, microscopic, and analytical testing such as surface insulation resistivity and ion chromatography. © 2001 by CRC Press LLC

Build Process It should be noted that it is also possible to modify the build process. Alternative solders and soldering techniques are being developed. Epoxies may be used, or laser ablation may replace soldering. Cleaning Process The type of cleaning process depends on multiple factors including the type of flux, the design of the assembly, expected product end use, customer requirements, worker preferences, cleaning agent/cleaning equipment costs, and the local mix of safety and environmental regulations. It should also be remembered that, for many high-end applications, electronics assembly is often much more than defluxing. An assembly may contain mixed OA and rosin fluxes, machining, oils, and lapping compounds. Subvendors may change the process, resulting in changes in soil residue and in cleaning agent residue. An array of cleaning processes has been used with electronics assemblies including noclean, water, aqueous/saponifier, water with large amounts of organic additives including nonlinear alcohols and unidentified proprietary additives, semiaqueous, cosolvent, classic chlorinated solvents, brominated solvents, engineered solvents alone and as blends, and flammable solvents. For every successful application of a given flux and cleaning process, an unsuccessful one could be cited. For all the standardization, cooperative testing, and understanding of materials compatibility, cleaning electronics assemblies is a very site-specific effort. It should also be remembered that eliminating electrostatic discharge (ESD) is important not only to avoid assembly failure but also to achieve contamination control. With no-clean fluxes the process window is not as large as it is with rosin or organic acid fluxes. Where no-clean flux was sucessful in a hand-soldering operation, the manufacturing engineer noted a number of critical factors that basically add up to understanding what is being soldered: • • • • •

Condition and materials compatibility of the components and bare boards Cleanliness of alloys and base materials (essential for adequate wetting) End use of the product Signal-to-noise requirements Component and design requirements

The company worked both with clients and materials suppliers to choose the proper materials. Issues of the composition of various alloys and thickness of application over the base metal can affect solderability and shelf life. The engineer emphasized that it is a constant educational process to make vendors and customers knowledgeable.12 ISSUES AND CONCLUSIONS The world is demanding increasingly high standards for high-value processes. Increased miniaturization and expected longevity and reliability (e.g., in pacemakers) implies designs that will be more difficult to assemble, increasingly difficult to clean, and where residual soils can be catastrophic. Standards are needed in clean rooms, chemicals, and process performance. Standards are set, without necessarily knowing whether those standards are relevant, sufficient, or overkill. © 2001 by CRC Press LLC

Fear is probably one of the biggest deterrents to progress in precision cleaning. Fear results in unrealistic, dogmatic processes that experienced assemblers simply ignore. Fear results in ever-increasing process steps, which are not only inefficient but may actually produce contamination through excess product handling. Fear blocks communication, including communication with the workforce, the engineers, with subcontractors and vendors, and with those involved in other, often seemingly unrelated applications. Some general keys to successful process implementation include: • • • • • • • • •

Testing the actual process in the proposed equipment Comparing results with the control method Providing appropriate automation Involving the production team (including management and assemblers) in process development and decision making Providing for more than one cleaning option Assuring thorough rinsing Providing for rapid drying Educating (not just training) the production people Understanding that process optimization is an ongoing issue, one of continuous improvement, with potential high production rewards11

High-value components may involve processes that are so specialized that standards may be nearly impossible to define. Certainly, understanding the performance of related processes and even seemingly unrelated product lines can be helpful in avoiding problems. In all, the best guidelines remain: Logic Choice of quality cleaning agents and processes Documentation of ongoing processes An educated production force Good communication REFERENCES 1. LeBlanc, C., Toxics Use Reduction Institute, personal communication. 2. Kanegsberg, B., Cleaning options in the high precision cleaning industry: overview of contamination control working group XIII, in Proc., The 1993 International CFC and Halon Alternatives Conference, October 20 –22, Washington, D.C., 1993, 943. 3. Carter, M., Andersen, M.E., Chang, S., Sanders, P.J., and Kanegsberg, B., Cleaning high precision inertial navigation systems, a case study and panel discussion, in Proc., CleanTech ‘99, May 18 –20, Rosemont, IL, 1999, 294 –301. 4. Kanegsberg, B., Abbink, B., Dishart, K.T., Kenyon, W.G., and Knapp, C.W., Development and implementation of non-zone depleting, non-aqueous high precision cleaning protocols for inertial navigation subassemblies, in Microcontamination ‘93 Proc., 1993. 5. Shubkin, R., Albemarle Corporation, Test Results, Beryllium Compatibility, 1998. 6. Kanegsberg, B., Cleaning for biomedical applications, in Proc., Precision Cleaning ‘97, Cincinnati, OH, April 15–17, 1997. 7. Arscott, E. et al., Validating Reusable Medical Devices: An Overview, Medical Device & Diagnostic Industry, January, 1996. 8. Cala, F.R. and Winston, A.E., Handbook of Aqueous Cleaning Technology for Electronic Assemblies, Electrochemical Publications, 1996.

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9. Shubkin, R.L., A highly effective solvent/cleaner with low ozone depletion potential, in Precision Cleaning ‘97 Conf. and Proc., April, 1997 (studies performance with ABZOL (TM) VG. 10. Kanegsberg, B., Successful Cleaning/Assembly Processes for Small to Medium Electronics Manufacturers, tutorial, Nepcon West ‘98, Anaheim, CA, March 3, 1998. 11. Tautscher, C.J., The Contamination of Printed Wiring Boards and Assemblies, Omega Scientific Services, Bothell, WA, 1976. 12. Kanegsberg, B., Choosing the process and tracking success—case studies, in Proc., Nepcon West ‘97, Anaheim, CA, 1997.

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CHAPTER 5.4

Cleaning Solutions in the Semiconductor Wafer Manufacturing Process Mahmood Toofan and John Chu

CONTENTS Introduction Basic Operations in Wafer Fabrication Photoresist Chemistry Photolithography and Masking Process Radiation-Sensitive Polymers Comparison of Positive and Negative Resists Negative-Acting Photoresists Positive-Acting Photoresists Solvents Sensitizers Additives Photoresist Performance Factors Resolution Adhesion Capability Exposure Speed and Sensitivity General Wafer Cleaning Techniques FEOL Cleaning Processes Cleaning Process Optimization Temperature Effect Ultrasonic and Megasonic Effect FEOL Cleaning Processes Sulfuric-Peroxide Chemistry Sulfuric Acid and Ammonium Persulfate Chemistry RCA Chemistry Quaternary Ammonium Hydroxides/Choline–Surfactant Chemistry TMAH Chemistry Ozone–Water Mixtures BEOL Cleaning Processes

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Chemistry of Positive Photoresist Strippers NMP-Based Strippers Non-NMP-Based Organic Strippers Chemistry of Negative Photoresist Strippers Chemistry of Post-Plasma-Etch-Polymer Removers HA/Amine Chemistry HF/Glycol Chemistry Challenges of the Future Technology Copper Interconnects Low-k Dielectric Material References

INTRODUCTION In semiconductor device manufacturing, silicon wafers are processed to fabricate very large-scale integration (VLSI) or ultra-large-scale integration (ULSI) circuits. Since the early stages of the semiconductor wafer processing in the 1960s, significant improvements and advancements have been made in chip manufacturing. However, the chemistry of wafer cleaning material and basic cleaning operations have remained fundamentally unchanged. During recent years, the geometry of the microcircuits, the diameter of the silicon wafers, and the processing equipment and methods have been significantly improved and updated. In early stages, simple immersion tanks of cleaning solutions were employed with manual agitation. Today, more-advanced cleaning solutions are applied on sophisticated wet benches and spray tools with automated chemical delivery systems and robotic arm movements for displacement of wafers. In the past three decades, wafer fabrication technology has made significant advancements in terms of density of microcircuits, reduction of feature size, and increase in wafer diameters. Diameters of the wafers have increased from 2 to 3 in. to 8 to 12 in. On the other hand, as the diameter of the wafers has increased, the geometry of microcircuits and interconnects have been reduced from 6 to 8 m (106 m) to 0.2 to 0.3 m. These dramatic changes in wafer processing technology require more precise cleaning solutions with ultrahigh purity and advanced cleaning formulations that otherwise would not be use. The cleanliness of the wafer surfaces and the purity of cleaning chemicals used in wafer fabrication processes are essential requirements to yield improvement in microelectronic device manufacturing. To meet the required specifications of sub-half-micron substrate geometry in wafer processing, the surface cleaning chemistry must meet stringent quality of clean room packaging, filtration, and ultrahigh purity of sub-ppb (part per billion) ionic contamination. Trace ionic impurities, such as sodium or potassium cations and chloride anions and particulates, are especially detrimental if present on wafer surfaces during thermal processing.

BASIC OPERATIONS IN WAFER FABRICATION Wafer fabrication is the series of processes used to create the semiconductor devices on a silicon wafer surface. The polished silicon wafers with blank surfaces undergo hundreds of process steps and end up producing hundreds of thousands of chips with multiple and diverse functions. The designs of the devices and circuits are based on different transistor structures. Among the major structure designs, bipolar and MOS (metal oxide semiconductor) © 2001 by CRC Press LLC

transistors are the most widely manufactured and used with numerous variations. Furthermore, there are several choices of processes and materials available to create each individual layer of any particular device structure. Regardless of the process diversity and hundreds of varieties of process options, only four major operations are performed during the fabrication process. These major operations are layering, patterning, doping, and heat treatment. Layering is the operation used to add thin layers of materials to the surface of the wafer. The layers are added to the surface in multiple major techniques: growing a silicon oxide or silicon nitride layer on the wafer using a thermal process, and chemical vapor deposition (CVD). Rapid thermal operation (RTO) or rapid thermal process (RTP) technology is a natural choice for the growth of oxides used in MOS devices. Other techniques such as evaporation, physical vapor deposition (PVD), spin-on deposition, and sputtering are also used to add layers on the wafers. Patterning is the series of steps that results in the removal of selected portions of the added surface layers (Figures 1a and b). After removal, a pattern of the layer is left on the wafer surface. The material left or removed may be in the form of a hole in the layer or just a remaining island of the material. The patterning process is named photomasking, photolithography, or microlithography. Photolithography is a multistep pattern transfer process similar to stenciling or photography. In photolithography the required pattern is first formed in photomasks and transferred into the surface layers of the wafer through the photomasking steps. The polymeric materials used in photolithography to transfer patterns to the wafer are called photoresist. Figure 2 shows the ten-step process of pattern transfer to the wafer surface using photomasking process for a negative acting photoresist. The foregoing was a brief description of some of the basic operations in wafer fabrication.

Figure 1a

Figure 1b

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Figure 2

Pattern transfer process.

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Since the focus of this chapter is a review of cleaning technology in semiconductor wafer fabrication, the emphasis of the following sections will be on the chemistry of photoresist, its cleaning solutions, and cleaning processes in wafer operations, especially advanced cleaning methods for film removal, such as photoresist strippers and post-plasma-etch polymer removers. For additional information on fabrication processes, readers are encouraged to refer to Microchip Fabrication by Peter Van Zant.1 Photoresist Chemistry Photoresists have been used in the printing industry for over a century. In the 1920s photoresists found wide range of application in the printed circuit board industry. The semiconductor industry adopted this technology for wafer fabrication in the 1950s. Photoresists specifically designed for semiconductor use were first developed by the Eastman Kodak Company. In the late 1950s it introduced Kodak Photo Resist (KPR), Kodak Metal Etch Resist (KMER), and Kodak Thin Film Resist (KTFR)—negative photoresists. At around the same time, the Shipley Company introduced a line of positive-acting photoresists. Since that time, some other companies also have entered the market with photoresists designed to keep pace with increasing demand in the industry for printing narrower lines in fabrication of fine-geometry integrated circuits. Today, different manufacturers offer a wide range of products designed to match a variety of applications. Photoresists are used in the masking process for patterning the wafers in the process of photolithography. Other terms used in industry for these steps are photomasking, masking, or microlithography. Photolithography and Masking Process Photolithography is one of the most critical operations in semiconductor manufacturing processes. It is the patterning process that sets two-dimensional horizontal patterns on the various parts of the circuit design on the wafer. The photoresist materials perform the function of transforming a two-dimensional circuit design into a three-dimensional electric circuit. The photoresist materials used in photolithography are generally formulated from polymeric materials with photosensitive additives. Most photoresist materials consist of four basic ingredients, each having a different function. Table 1 shows the basic components of photoresists. Radiation-Sensitive Polymers The photosensisitive ingredients of the photoresist material are special polymers. Polymers are macromolecules containing carbon, hydrogen, and oxygen atoms that are Table 1 Photoresist Components and Their Functions Component

Function

Polymer

Changes structure due to reaction with radiation energy (polymerization or photosolubility) Used as thinner to allow application of a thin-film layer of the spun material Control modification of chemical reaction when exposed to light For special purposes

Solvent Sensitizers Additives

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formed by repeated patterns of their monomers or simple molecules. Most plastics are a form of polymer. Photoreactive polymers are radiation sensitive and react with some type of light energy, ultraviolet (UV) or laser. Those photoresists that contain these type of polymers are called optical resists. Other resists respond to X-ray radiation or e-beams, which are i-line or j-line resists. Comparison of Positive and Negative Resists Up to the mid-1970s, negative resist was dominant in the masking process. The advent of VLSI circuits and image sizes in the 2- to 5-m range strained the resolution capability of negative resists. Positive resists had been around for over 20 years, but their poorer adhesion properties were a drawback and their superior resolution capability and pinhole protection were not needed. By the 1980s, positive resist became the resist of choice. The transition was not easy. To switch a fabrication line from negative to positive resist requires changing the polarity of the masks or reticles from clear field to dark field. Unfortunately, it is not a simple matter of reversing the fields in the mask-making process. The dimensions have to be adjusted to accommodate the different characteristics of the positive resist. The determination of the correct mask of reticle dimensions is a lengthy procedure. Positive resists have a higher aspect ratio compared with negative resists. In other words, have a better resolution capability and can resolve smaller geometry such as wire lines and via openings. Another problem with negative resists is oxygenization. This is a reaction of the resist to oxygen in the atmosphere, and can result in a thinning of the resist film by as much as 20%. Positive resists do not have this property. Cost is always an important consideration. Negative resists sell for about one third of the cost of positive resists. Developing characteristics differ between the two types of resists. Negative resists develop in readily available solvents and possess wider developer process latitude. Positive resists require carefully prepared developer solutions and temperature control of the process. The next-to-last step in the masking process is photoresist removal, which can take place in chemical solutions or in plasma systems. Generally, the removal of positive resists is easier and takes place in chemicals that are more environmentally sound. While positive photoresists are the resists of choice for fabrication areas processing state of the art circuits, there are many lines still producing devices and circuits with image sizes greater than 5 m. A great many of these lines use negative resists. Table 2 shows a comparison of properties of the two resists. Table 2 Comparison of Negative and Positive Resists Parameter Aspect ratio (resolution) Adhesion Exposure speed Pinhole count Step coverage Cost Developers Strippers Oxide steps Metal steps

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Negative

Positive Higher

Better Faster

Organic solvents Acid Chlorinated solvent compounds

Lower Better Higher Aqueous Acid Simple solvents

Figure 3

Chemistry of negative photoresist (isoprene monomer).

Negative-Acting Photoresists Negative photoesists are normally based on polyisoprene-type polymers. Polyisoprene polymers naturally occur in rubber material. The Hunt Corporation developed the first synthetic polyisoprene polymer structure (Figure 3). Before exposure to the light, the negative resist polymers exist in their unpolymerized condition (under which the polymers are not chemically linked to each other). When the photoresist is exposed to proper light or energy, the polymers become cross-linked or, in chemical term, polymerized. This process may also be achieved when the photoresist materials are exposed to heat and/or visible light. To prevent this deterioration, the photoresist material is normally packaged in amber glass bottles or dark color, brown or black plastic packaging. During the application process, to prevent accidental exposure, photomasking and resist processing areas use yellow filters or yellow lighting. Depending on the response of the photoresists to the type of energy or radiation, photoresists are normally referred to by their general category, such as UV, deep UV, X-ray, I-line, etc. Positive-Acting Photoresists Positive-acting photoresists are based on the phenol-formaldehyde polymer, also called phenol-formaldehyde novolak resin (Figure 4). The novolak resin within the unexposed photoresist is relatively insoluble. After exposure to the proper radiation energy, the photoresist converts to a more soluble state. This reaction is called photosolubilization. Table 3 contains a list of commonly used photoresist polymers used for the photolithography process in the semiconductor industry.

Figure 4

Phenol-formaldehyde novolak resin structure.

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Table 3 Commonly Used Photoresist Materials Sensitivity (C/cm2)

Exposure Source

     

3–5  105 5  10 5 1  10 5 2  10 6 8  10 7

UV UV E-beam E-beam/deep UV E-beam E-beam

 

5  10 7 2  10 7

E-beam/X-ray E-beam

Resist

Polymer

Polarity

Positive Negative PMMA PMIPK PBS TFECA COP (PCA) PMPS

Novolak (M-cresoformaldehyde) Polyisoprene Poly-(methyl methacrylate) Poly-(methyl iso-propenyl ketone) Poly-(butene 1-sulfone) Poly-(trifluoro-ethyl chloroacrylate) Copolymer-(cyano ethyl acrylateamido ethyl acrylate) Poly-(2-methyl pentene-1-sulfone)

Source: Van Zant, P., Microchip Fabrication, McGraw-Hill, New York, 1990 with permission.

Solvents The largest ingredient by volume in photoresist composition is the solvent. It is the solvent that converts the solid resist material to a liquid and allows the liquid photoresist to be applied to the wafer surface as a thin layer by spinning. Photoresist is analogous to paint, which is composed of the coloring pigment and polymer dissolved in an appropriate solvent. It is the solvent that allows the application of the paint onto a surface in a thin layer. For negative photoresist, the solvent is an aromatic hydrocarbon such as xylene. In positive resist, a variety of solvents are used depending on the type of polymer. The most commonly used solvents are ethoxyethyl acetate (EEA), 2-methoxy propyl acetate (propylene glycol monomethyl ether acetate, PGMEA), and ethyl lactate (ELS). Sensitizers Chemical sensitizers are added to the resists to cause or control certain reactions of the polymer. In negative resists, the untreated polymer responds to a certain range of the UV spectrum. Sensitizers are added to either broaden the response range or narrow it to a specific wavelength. In negative resists, a compound called bisaryldiazide is added to the polymer to provide light sensitivity. In positive resists, the sensitizer is O-naphthaquinonediazide. Additives Various additives are mixed with resists to achieve particular results. Some negative resists have dyes that are intended to absorb and control light rays in the resist film. Photoresist Performance Factors The selection of a photoresist starts with the dimensions required on the wafer surface. The resist must first have the capability of producing those dimensions. Beyond that, it must also function as an etch barrier during the etching step, a function that requires a certain thickness for mechanical strength. In the role of etch barrier, it must be free of pinholes, which also requires a certain thickness. In addition, it must adhere to the top wafer surface or the etched pattern will be distorted, just as a paint stencil will give a sloppy image if it is not taped tight to the surface. These, along with process latitude and step coverage capa© 2001 by CRC Press LLC

W T

Resist

Aspect Ratio = Figure 5

W T

Aspect ratio.

bilities, are resist performance factors. In the selection of a resist, the process engineer often must make trade-off decisions between the various performance factors. Resolution The smallest opening of space that can be produced in a photoresist layer is generally referred to as its resolution capability. The smaller the line produced, the better the resolution capability. Generally, smaller line openings are produced with thinner resist film thickness. However, a resist layer must be thick enough to function as an etch barrier and to be pinhole-free. The selection of a resist thickness is a trade-off between these two goals. At present, a more-advanced photoresist is the 0.193-m resist with a resolution sensitivity of 193 nm. There is also a more-advanced line of positive resists in the UV range with an exposure wavelength as low as 154 nm. The capability of a particular resist relative to resolution and thickness is measured by its aspect ratio (Figure 5). The aspect ratio is calculated as the ratio of the resist thickness to the image opening. Positive resists have a higher aspect ratio compared with negative resists, which means that for a given image-size opening, the resist layer can be thicker. The ability of positive resist to resolve a smaller opening is due to the smaller size of the polymer. It is similar to the requirement of using a smaller brush to paint a thinner line.

Figure 6

Advanced aspect ratio.

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Visible

Name Wavelength (cm)

Gamma Rays 10-11

X-Rays

Ultra violet (UV)

Infrared (IR)

Short Radio Waves

Broadcast Radio Waves

10-8

10-6

10-3

102

104

10-4

Figure 7

Electromagnetic spectrum.

Advanced photoresists can generate a via opening with an aspect ratio of up to 10:1 in copper interconnect technology. Figure 6 shows some advanced device structure with high-aspect-ratio via. Adhesion Capability In its role as an etch barrier, a photoresist layer must adhere well to the surface layer to transfer the resist opening faithfully into the layer. Lack of adhesion results in distorted images. Resists differ in their ability to adhere to the various surfaces used in chip fabrication within the photomasking process; there are a number of steps that are specifically included to promote the natural adhesion of the resist to the wafer surface. Negative resists generally have a higher adhesion capability than positive resists. Exposure Speed and Sensitivity The primary action of a photoresist is the change in structure in response to an exposing light or radiation. An important process factor is the speed at which that reaction takes place. The faster the speed, the faster the wafers can be processed through the masking area. Negative resists typically require 5 to 15 s of exposure while positive resists take three to four times longer. The sensitivity of a resist relates to the amount of energy required to cause the polymerization or photosolubilization to occur. Further, sensitivity relates to the energy associated with specific wavelength for the exposing source. Understanding this property requires a familiarization with the properties of the electromagnetic spectrum (Figure 7). Within nature, are a number of different types of energy: light, short and long radio waves, X-rays, etc. In reality they are all electromagnetic energy (or radiation) and are differentiated from each other by their wavelengths, with the shorter-wavelength radiations having higher energies. Common positive and negative photoresist responds to energies in the UV and deep ultraviolet (DUV) portion of the spectrum (Figure 8). Some are designed to respond to particular wavelengths (peaks) within those ranges. Resist sensitivity refers to the specific wavelengths to which the resist reacts. This property is also called the spectral response characteristic of the resist. Figure 9 is the spectral response characteristic of a typical production resist. The peaks in the spectrum are regions (wavelengths) that carry higher amounts of energy. GENERAL WAFER CLEANING TECHNIQUES Impurities on the surface of the silicon wafers come from various sources at different stages of the manufacturing process. These impurities must be removed following each © 2001 by CRC Press LLC

Visible 400 - 700 Near UV 250 - 400 UV 100 - 250 DUV 4 - 100 250

500

1000

Wavelength (nm) Figure 8

UV and visible spectrum.

Figure 9

Exposure response of a positive photoresist.

process step to keep the substrates clean for the next process. Depending on the type of surface contamination or impurities, different cleaning solutions and techniques need to be applied.1 Wafer cleaning solutions, depending on the process and their functions, are classified as 1. Cleaning solutions of bare silicon and oxidized wafers involving premetal processes, or so-called front end of line (FEOL) cleaning processes; 2. Cleaning solutions of postmetal processes used in different stages of metallization, or so-called back end of line (BEOL) processes.2 © 2001 by CRC Press LLC

Surface conditioning or FEOL is a premetal process and normally uses acids and oxidants to precondition and clean the wafer surfaces. Postmetal processing, or BEOL, which includes photoresist stripping, post-plasma-etch residue removing, and post-CMP (chemical mechanical polishing) slurry removal, uses sophisticated solvent formulations that are not the traditional cleaners. These two processes can be distinguished as residual or contamination cleaning vs. bulk material removing, such as resist stripping. The solvent formulations for bulk material removal or BEOL processes are either semiaqueous solvent formulations or aqueous solutions of more advanced cleaning solutions that will include corrosion inhibitors and wetting agents for protection of fine geometry in metal layers, and high aspect ratio via openings. Depending on whether a semiconductor device is DRAM (dynamic random access memory) or logic, a wafer can undergo 25 to 30 different steps, including ash processing followed by a cleaning step using one of two chemistries. FEOL Cleaning Processes In FEOL cleaning, a universal method is treatment by sulfuric acid and hydrogen peroxide (piranha) following an ash process, which takes place in either a spray tool or an immersion wet bench. There is little single-wafer processing application of the spray chemistry in FEOL processes. Other cleaning chemistries used in FEOL processes include RCA clean, choline chemistry, and quaternary ammonium hydroxide or tetramethyl ammonium hydroxide (TMAH) based chemistry in either spray or immersion equipment. These will be discussed in more detail shortly. Cleaning Process Optimization Depending on the chemistry of the cleaning solution involved, the type of wafers, and the process conditions, such as process temperature, and process time, and the equipment used may vary from one fabrication site to another. Process engineers optimize their cleaning process to achieve high yields and low defects. Temperature Effect An important factor in cleaning wafers is the bath temperature of the processing material. In an FEOL cleaning process such as RCA clean chemistry the chemical is normally heated to an optimum temperature (typically 55 to 60°C), to achieve the best results. Photoresist developers and edge bead removers (EBR) in BEOL process are normally applied at ambient temperature. These materials only dissolve the uncured or soft-baked photoresist, which is not cross-linked or polymerized. In the resist stripping process, however, the cross-linked and polymerized resist may require heated stripping solutions and a longer time for complete removal of the hardened photoresist. If plasma-treated photoresist residues (post-plasma polymers) are not oxygen plasma ashed, they are even more difficult to clean and will need more aggressive solutions at higher temperatures. In any case, whether an FEOL or BEOL process is contemplated, the temperature of the immersion tank or a spray tool is optimized and preset to safe operational conditions. Ultrasonic and Megasonic Effect For certain cleaning applications such as metal-lift-off processes or stripping of an ionimplanted and deep-UV-cured photoresist, without a plasma ashing, ultrasonic or mega© 2001 by CRC Press LLC

Figure 10 Megasonic tank configuration. (After Kern, W., Ruzyllo, J., and R. Novak, Proc. Electrochem. Soc., 90-9, 5–15 and 67–68, 1989.)

sonic agitation may be necessary for complete dissolution. In FEOL cleaning using RCA clean process, megasonic energy has shown significant improvement on cleanliness of the wafers and particle removal efficiency of the solution. Figure 10 shows the configuration of a megasonic wafer-cleaning tank. FEOL CLEANING PROCESSES Sulfuric-Peroxide Chemistry Early cleaning processes of silicon wafers involved using concentrated inorganic acids such as boiling nitric acid, aqua regia, concentrated hydrofluoric acid, and mixtures of phosphoric, acetic, and sulfuric acids. Mixtures of sulfuric acid and hydrogen peroxide, or so-called piranha solutions, are still being used in FEOL wafer cleaning applications. In terms of sulfuric chemistry and acid-to-peroxide mix ratios, process engineers use their own selections. Instead of having a premixed, stabilized mixture, one can prepare two chemicals on site, as needed in different ratios. Oxidizing agents such as mixtures of sulfuric acid and chromic acid were also used as a general-purpose glass cleaner or silicon wafer surface cleaner. This type of cleaners, however, caused ecological toxic pollution and waste disposal problems. Sulfuric Acid and Ammonium Persulfate Chemistry Hydrogen peroxide (H2O2) is essentially unstable and readily disassociates to water and oxygen at elevated temperatures. H2O2 → H2O  1/2 O2 An alternative oxidizing agent used in wafer cleaning is ammonium persulfate (AP) in sul© 2001 by CRC Press LLC

furic acid mixtures. AP is added to sulfuric acid baths in a concentrations of 40 to 80 g/l. Since AP is less reactive at room temperature than H2O2, it is and therefore safer to store and has a longer shelf life. Being less reactive, AP ensures a more stable and steady release of oxygen to the cleaning bath and more consistent and stable bath life to the chemical. Sulfuric acid –AP mixture is used in FEOL cleaning process, for general wafer cleaning and also in resist stripping processes of nonmetalized wafers. RCA Chemistry The first systematically developed cleaning process for unprocessed or oxidized silicon wafers, called RCA clean, was developed at RCA and published in 1970. The RCA cleaning process involved a two step process of peroxide treatment with a high-pH alkaline solution (normally ammonium hydroxide mixed with hydrogen peroxide) as RCA1 or SC-1, followed by a treatment with a mixture of hydrochloric acid and hydrogen peroxide as SC-2.3 The solutions are made using ultrafiltered deionized (DI) water, electronic-grade ammonium hydroxide (29% wt/wt% as NH3), electronic-grade hydrochloric acid (37 wt/wt%), and high-purity unstabilized hydrogen peroxide. In the first treatment step, the wafers are exposed to a hot mixture of water-diluted hydrogen peroxide and ammonium hydroxide. This procedure was designed to remove organic surface films by oxidative breakdown and dissolution to expose the silicon or oxide surface for concurrent or subsequent decontamination reactions. In this treatment, metal impurities such as copper and zinc are dissolved and removed by a complexing agent of ammonia, for example, in forms of [Cu (NH3)4]2 amino complex. The second treatment is designed to remove alkali ions, cations such as Al 3, Fe3, and 2 Mg that form water-insoluble hydroxides in SC-1 ammonia solution. The volume ratios for the RCA standard clean 1, SC-1 clean used in first treatment step are H2O:H2O2 (30%):NH4OH (29% as NH3) as 5:1:1 and the volume ratios for the RCA standard clean 2, SC-2 clean used in second treatment step are H2O:H2O2:HCl as 6:1:1. The processing temperature should be kept at 75 to 80°C to activate the mixture sufficiently without causing thermal decomposition due to higher temperatures. The original RCA cleaning processes was based on a simple immersion technique. Several different improved techniques have been introduced over the years. More advanced automated wet-bench immersion systems for large-scale production are now available and being offered in the industry by equipment manufacturers such as FSI International.

Quaternary Ammonium Hydroxides/Choline—Surfactant Chemistry Among the other alternative alkaline cleaning solutions that have been studied on silicon wafers, choline (2-hydroxyethyl trimethyl ammonium hydoroxide),5 a strong base with a chemical formula [N(CH3)3CH2CH2OH]OH, and pK b  5.06, which is free of metal ion, has drawn much attention.5–7 Immersion of HF-etched silicon wafers in a choline solution followed by a water rinsing has shown very clean results. Muraoka et al.5 have developed several techniques to clean silicon wafers using choline. They have reported that dilute aqueous solutions of choline with selected nonionic surfactants can remove heavy metals from the silicone wafer surface and prevent replating of these metals from solution on the wafer. Harri and Hockett 6 compared wet cleaning involving choline with that of other solutions and concluded better electrical properties with the choline process. © 2001 by CRC Press LLC

TMAH Chemistry Another strong base that is relatively stable at ambient temperature and is also free of metal ion is TMAH, N(CH3)4OH. TMAH is widely used as a positive photoresist developer in a relatively low concentration (2.5 wt%) in aqueous solution with surfactants. TMAHbased photoresist developers have replaced the traditional alkaline developers because of their low ionic impurities and high polymer dissolution capability. In recent years TMAH-based formulations in organic solvents have been used for positive photoresist stripping applications.8 –10 Even though the TMAH-based strippers are now commercially available and are being used in the industry, they have shown some drawbacks and do not provide a robust resist stripping process. A major disadvantage of TMAH-based strippers is the high pH value of the product that makes it corrosive to sensitive metals, specially Al and Al alloys. Aluminum reacts with alkaline solution in aqueous media, which results in etching of the aluminum lines, especially in the submicron geometry. 2Al  2OH  2H2O → 2AlO2  3H2 (g) Another disadvantage of TMAH strippers is the instability of the quaternary ammonium hydroxide at elevated temperatures. An independent laboratory study of a commercially available TMAH/NMP-based stripper (NMP  N-methylpyrrolidone) using proton nuclear magnetic resonance (NMR) spectroscopy and gas chromatography/mass spectrometry (GC/MS) has indicated the following: at high temperatures, TMAH disassociates to trimethylamine and methyl alcohol in aqueous media. N (CH3)4 OH → N(CH3)3 (g)  CH3OH (g) at t  80°C The bath life of a typical TMAH/NMP stripper is approximately 2 to 4 h at 85°C. At ambient temperatures, however, the TMAH-based developers are widely used in the industry for positive photolithography process.

Ozone –Water Mixtures Another oxidizing agent that has been historically used for wastewater treatment, drinking water sterilization, and in swimming pools is ozone in water. In recent years ozone has been introduced into microelectronic industry in both wafer cleaning (FEOL) applications and in photoresist residue removal (BEOL) processes. Ozone has basically the same role in oxidizing and cleaning organic residues as H2O2 has in the RCA clean. Ozone and H2O2 decompose virtually the same way: H2O2 → O  H2O O3 → O  O2 The biggest advantage of ozone over RCA clean is that ozone leaves no harmful decomposition residues or by-products. It is partially soluble in water, especially at lower temperatures. Generally, ozone is about ten times more soluble in water than oxygen. The lower the water temperature, the higher the ozone solubility. The half-life of ozone in high-purity DI water is about 20 min. © 2001 by CRC Press LLC

Table 4 SIMS Analysis of RCA Clean vs. Ozone Clean (atoms/cm2) Relative Elemental Abundance

Clean starting wafers Modified RCA clean Ozone clean Contaminated starting wafers Modified RCA clean Ozone clean

F

Na

140 200

225 105

145 250

330 125

K

Cu

Mg

Al

Ca

65 25

20 20

22 10

1575 1064

113 98

160 20

20 20

N/A N/A

N/A N/A

N/A N/A

Table 5 TXRF Analysis of RCA Clean vs. Ozone Clean (atoms/cm2)

Clean starting wafers Modified RCA clean Ozone clean Contaminated starting wafers Modified RCA clean Ozone clean

Mn

Fe

Zn

Br

Cr

Co

Cu

0.4 0.4

0.3 0.3

0.2 0.2

1.4 0.3

0.6 0.6

0.4 0.4

0.2 0.2

0.4 0.5

0.3 0.3

0.2 0.2

1.8 0.3

0.6 0.6

0.4 0.4

0.2 0.2

In recent years ozone chemistry has been receiving considerable attention because it has a potential to be used in both FEOL and BEOL and to eliminate or reduce the usage of organic solvents. In a study comparing the effectiveness of ozone chemistry and modified RCA clean, FSI International used its centrifugal spray processors to clean silicon wafers. The wafers were then examined for their metal ionic contamination and change in particles before and after the cleaning process. Residual metals following the cleaning processes were measured using both secondary ion mass spectroscopy (SIMS) and total reflection X-ray fluorescence (TXRF) methods. Results of those studies are presented in Tables 4 and 5.4 BEOL CLEANING PROCESSES Bulk material removing processes normally take place following three major process steps in wafer fabrication: 1. Photolithography process (photoresist stripping and edge bead removal) 2. Metal and oxide etched by plasma or reactive ion (polymer removing) 3. CMP, chemical mechanical polishing (slurry removal process) The major photoresist residues resulting from the photolithography process are normally cleaned using a photoresist-stripping process involving organic solvents. The EBR process utilizes the following solvents: 1. 2. 3. 4.

PGMEA based Ethyl lactate based Organic solvents as ketones and esters (acetone, MEK, MIBK, and nBAc) Environmentally preferred alternatives (VOC exempt)

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After plasma-etch of metal and oxide, the tough sidewall polymers are cleaned by advanced formulations of post-etch polymer removers, or by ashing with oxygen followed by wet clean. After CMP, the slurry particles are rinsed away with water-based, diluted solvents. In many instances, scrubbing with a brush and/or megasonic agitation is needed to dislodge the much heavier slurry particles. CHEMISTRY OF POSITIVE PHOTORESIST STRIPPERS NMP-Based Strippers Among other aprotic solvents, N-methyl pyrolidone (NMP) has attracted particular attention in photoresist stripping formulations. In some applications, pure NMP is used for stripping soft-baked resists. For relatively hard and cross-linked resists, a more aggressive alkaline mixture with high-pH values is needed to mix with NMP. Organic amines have been shown to possess the desired characteristics when mixed with the aprotic solvents. During the past two decades the NMP/amine-based strippers have dominated the positive resist stripping market because of their low toxicity and their resist cleaning efficiency. NMP/amine solutions are not only effective in cleaning hard-baked cross-linked resists at elevated temperature but they are also 100% water soluble and biodegradable, which makes these formulations particularly popular, as opposed to the more toxic phenolic or chlorinated solvents. For more-advanced applications, different wetting agents or nonionic surfactants, such as poly-alkylene glycol (ethylene glycol or propylene glycol), are added to the stripper formulations.11 –15 Non-NMP-Based Organic Strippers Other aprotic solvents such as tetra hydrothiophrene 1,1-dioxide (Sulfolane), dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), and dimethyl acetamide (DMAC) are also used in different stripping formulations.11,12 Several other stripping solutions using dibasic esters, alcohols, ketones, glycol ether, or other organic solvents have also been reported and are being used. Aqueous-based (water/surfactant) strippers using dibasic esters as active ingredient with neutral pH values have also shown favorable results in stripping soft-baked and hard-baked bulk resist.13 –14 CHEMISTRY OF NEGATIVE PHOTORESIST STRIPPERS Negative photoresists are polymerized rubber and are normally soluble in aromatic hydrocarbons or phenolic solvents. The most commonly used solvent/thinner for negative photoresist is xylene mixture. In the early 1960s, the first organic stripper, containing chlorinated aromatic hydrocarbon as solvent and an alkylbenzene sulfonic acid as surfactant, was introduced to the market by a company called Industri Chem. This formulation utilized phenol to create a water-rinsable solution; as such, the first organic resist stripper J-100 was born. Stripping photoresist by J-100 requires a heating bath in the range of 90 to 120°C, followed by a series of post-strip rinse solutions and a DI water rinse and spin dry. Since the introduction of J-100, a number of suppliers have developed similar products, some © 2001 by CRC Press LLC

designed for direct water rinse. Other manufacturers have offered similar products containing dodecylbenzene sulfonic acid, phenol, and chlorinated benzene solvents.15 –17 These products were successfully used as workhorse strippers for two decades. However, in the 1970s the environmental concerns over the toxic ingredients in these formulas led to the development of nonphenolic and nonchlorinated solvents with fewer waste disposal difficulties. CHEMISTRY OF POST-PLASMA-ETCH-POLYMER REMOVERS Tough sidewall polymers are created during the plasma etch process (Figure 14). The cross-linked and hardened polymers need to be cleaned by advanced chemical formulations. These advanced formulations are normally referred to as post-plasma-etch-polymer removers. In general, after plasma etch, the wafers are normally ashed in oxygen plasma ashers, in which the majority of the photoresist material is oxidized and removed from the surface of the wafer. However, the organometallic polymer formed on the sidewall of the metal layer (Figure 11) or inside the via polymer openings (Figure 12) does not react and remains. If the polymer is not removed properly, the residue will cause failure in connections, and subsequently the device will fail the electrical test. A number of post-etch-polymer removers have been introduced to the market by chemical manufacturers and have been used since the early 1990s. EKC Technologies first introduced hydroxylamine (HA) chemistry for post-etch-polymer removal applications. After that, Ashland Chemical Co. and a number of other chemical manufacturers followed the lead and offered various formulations. Among the many products available commercially, the following basic formulations have been used most frequently: Hydroxyl amine chemistry HF/glycol chemistry Other organic alternative solvents

Figure 11 SEM images of sidewall polymers on metal lines, before and after clean.

Figure 12 SEM images of via polymers, before and after clean. © 2001 by CRC Press LLC

HA/Amine Chemistry Hydroxylamine, NH2OH free base, commercially available as 50% by weight in water, is a strong reducing agent and a weak base with pH about 8.0 to 8.5. Commercially used for polymer removal application, EKC-265 manufactured by EKC Technology and ACT935 manufactured by Ashland Chemical are HA and organic amine (diglycolamine or monoethanolamine) mixtures. Although these products are widely used in the industry for post-plasma-cleaning applications, the products have a number of disadvantages and drawbacks. HA/amine mixtures are not good resist strippers and do not strip the photoresist if it is not ashed. The products have a short shelf life and are not stable at elevated temperatures. To increase their efficiency and improve corrosiveness, the manufacturers add chelating agents such as catechol up to 5 wt%. Despite problems associated with HA chemistries, most DRAM manufacturers have been using HA/amine-type products for metal and via polymer cleaning applications. The metal stacks on those wafers are typically Al-Cu/Ti/TiN and the dielectric layers on vias are silicon dioxide. HA/amine products with some process modifications can be adapted to those cleaning processes with few process difficulties.18 In new technologies of copper metallization and with increasing applications of different low-k dielectric material, usage of HA chemistry has become very limited and, therefore, more-advanced formulations have become necessary for those processes. HA/amine mixtures are corrosive to copper and dissolve copper layers to form a water-soluble complex of Cu(NR3)4. Other sensitive metal alloys are also being used, such as tungsten (W), in via interconnects as tungsten plugs, which are also susceptible to amine corrosion. Figure 13 presents a via structure with exposed W openings, before and after a cleaning process with a noncorrosive polymer remover. HF/Glycol Chemistry Other formulations used in cleaning post-plasma polymers are hydrofluoric acid (HF) and ethylene glycol (EG) mixtures. Low concentrations of HF in EG are not corrosive to sensitive alloys, have strong residual cleaning capability and can be used as an alternative to HA chemistry. However, HF attacks the silicon oxide layer on the wafer and hence is not suitable for via cleanings. For metal cleaning with controlled temperature and process latitude, HF/EG mixtures can be used successfully with limited oxide loss. Ashland Chemical Co. manufactures different products of HF/EG mixtures for various cleaning or oxide etching applications. Another similar product that is also commercially available from ACSI (Advanced Chemical Systems International), another specialty chemical company, is called NOE (natured oxide etchant), which is buffered HF with ammonium bifluoride in a polyglycol mixture.

Figure 13 Exposed W via with sidewall polymer on Al metal, before and after clean. © 2001 by CRC Press LLC

Figure 14 SEM images of sidewall polymers on via and metal lines, before and after polymer removal process. (Courtesy of Silicon Valley Chemlabs, SVC, Inc.)

CHALLENGES OF THE FUTURE TECHNOLOGY The real challenge for the future of wafer cleaning technology is the integration of copper interconnects and new low-k dielectric material in wafer manufacturing. As we enter the Third Millennium, major advancements and dramatic changes are taking place in manufacturing and applications of electronic products. Computer manufacturers are motivated to come up with more powerful systems having more complex power transistors in smaller sizes. Copper Interconnects In chip manufacturing, a dramatic shift from aluminum to copper interconnects is taking place. IBM was the first to produce products with 100% copper wires with substantial shipments of the Power PC 750 microprocessor starting in late 1998. Other major memory and DRAM manufacturers are in various stages of making this transition from Al to Cu processes. Semiconductor equipment manufacturers such as Applied Materials and © 2001 by CRC Press LLC

Novellus and R&D institutions such as SEMATECH, a consortium of major semiconductor manufacturers in Austin, Texas, are following the lead to implement copper as replacement for aluminum. Chip performance is the motivation for this transition since copper has significantly better conductivity (60 to 70%) than aluminum. There is a continuous improvement in transistor performance, which is about 20 to 30% per technology generation. As the performance is improving, the geometry of interconnects including the wires is shrinking about 30% per generation to provide a constant cost per circuit reduction. As the result, the problem appears that increasingly power transistors must be wired together with thinner metal lines (wires) that are getting smaller and less conductive, creating limited chip performance. The shift from Al to Cu, which is 70% more conductive than aluminum, is helping solve this problem. Integration of copper technology requires a completely different line of equipment and material, including cleaning chemicals. Low-k dielectric material is a major part of this emerging technology transition. CMP slurries and polishing material and post-CMP cleaners are also part of this transition. Low-k Dielectric Material In recent years several different low-k dielectric materials have been developed and introduced to the market by chemical manufacturers. These products are known as trade names HOSP, FLARE, HSQ, FOX, and SiLK. Allied Signal has offered HOSP and FLARE, Dow Corning offers HSQ and FOX, Dow Chemical manufactures SiLK. These products, having lower dielectric constant k compared with silicon dioxide SiO2, replace the traditional spin-on glass (SOG) or other thermally grown oxide material. These, together with other newly invented low-k material, present challenges to the cleaning chemicals. The traditional solvent and amine chemistry may interact with these materials and change their low-k property. They could have been left behind on the wafers, causing contamination and via poisoning. Fortunately, new chemistry has been invented to clean these low-k materials without the associated adverse effects of the traditional polymer strippers.

REFERENCES 1. Van Zant, P., Microchip Fabrication, A Practical Guide to Semiconductor Processing, 2nd ed., McGrawHill, New York, 1990. 2. Braun, A.E., “Photoresist Stripping”, Semicond. Int., Oct. 1999. 3. Kern, W., Ruzyllo, J., and Novak, R.E., Semiconductor cleaning technology/1989, Proc. Electrochem. Soc., 90-9, 5–15 and 67–78, 1989. 4. Kern, W., Ruzyllo, J., and Novak, E., Cleaning technology in semiconductor device manufacturing, Proc. Electrochem. Soc., 92-12 11–27. 5. Muraoka, H., Hiratsuka, H., and Usami, T., Abstract 238, The Electrochemical Society Extended Abstracts, 81-2, October 1981, 570. 6. Harri, A., and Hockett, R.H., Semicond. Int., 8, 74, 1989. 7. Gould, G., and Irene, E., J. Electrochem. Soc., 135, 1535, 1988. 8. Haq, N., et al., U.S. patent 4,744,834. 9. Steppan, H. et al., U.S. patent 4,776,892. 10. SVC Labs, Technical Note, Instability of TMAH Based Strippers. 11. Ward, I. et al., U.S. patent 5,554,312.

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12. 13. 14. 15. 16. 17. 18.

Sizensky, J. et al., U.S. patent 4,617,251. Sahbari, J. et al., U.S. patent 5,741,368. Sahbari, J. et al., U.S. patent 5,909,744. Schwartzkopf, F. et al., U.S. patent 5,308,745. Corbey, W. et al., U.S. patent 3,673,099. Thomas, E. et al., U.S. patent 4,791,043. Lee, W. et al., U.S. patent 4,824,763.

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CHAPTER 5.5

Biomedical Applications: Analytical Characterization for Biocompatibility David E. Albert

CONTENTS Introduction Cleaning Process Choice of Disinfecting Chemicals Alcohols Aldehydes Halogens and Halogen-Containing Compounds Quaternary Ammonium Compounds Biguanide Disinfectants Phenol and Phenolic Derivatives Analytical Testing Chromatographic Analysis Spectrophotometric Analysis Infrared Ultraviolet/Visible Spectroscopy Conclusion References

INTRODUCTION In the United States alone, nearly 9 million people work in the health care professions.1 Consequently, when proper infection control measures are not followed, the risk of disease transmission to the community results. Infection control does not begin and end with animate objects but includes inanimate objects as well. Environmental surfaces may become contaminated by human pathogens. Extensive environmental contamination has been demonstrated in rooms housing patients with antibiotic-resistant Staphylococcus aureus and Enterococcus.2 Microbes (bacteria, fungi, and viruses) can be carried from one person to another on the surface of any equipment that is shared between them unless the equipment is decontaminated between use. They can also be carried on the skin surface, © 2001 by CRC Press LLC

which is the reason hand washing between examining patients is important. Microbes gain access to the body through open wounds, inhalation of infected secretions, or by close contact with mucous membranes. The process by which microbes are passed from one infected person to cause infection in another is known as “cross-infection.” Although most institution-acquired infections result from a patient’s endogenous flora or person-to-person transmission, contaminated surfaces have been linked to nosocomial infections.3 The assurance of proper cleaning, disinfection, and sterilization of instruments and medical devices in health care practice is a vital issue for all infection control programs. Obviously, there is a clear need for safe and effective cleaners and disinfectants worldwide. Decontamination of medical equipment, devices, and instruments involves the destruction or removal of any organisms present to prevent infecting other patients or hospital staff. Cleaning, disinfection, and sterilization are all procedures that are used in the decontamination process. Decontamination reduces the risks of cross-infection and helps maintain the useful life of equipment, while helping control hospital-acquired infection. The issues of cleaning medical instruments and devices have become very controversial, and to add fuel to this controversy is the ever-increasing practice of hospitals and health care facilities to reprocess disposable medical devices for reuse.4 The dual pressures of cost containment and demand for health care services are driving this practice. Consumers are demanding more and better services while the federal government has reduced Medicare funding by $44 billion as a consequence of the Balanced Budget Act of 1977. According to 29 CFR 1910.1030(d)(4)(ii), OSHA requires that equipment and surfaces be cleaned and disinfected after contact with blood or other potentially infectious material (OPIM).5 OSHA continues to require the use of a tuberculocidal disinfectant for the decontamination of blood or body fluids. According to OSHA, decontamination is defined as the use of physical or chemical means to remove, inactivate, or destroy bloodborne pathogens on a surface or item to the point where they are no longer capable of transmitting infectious particles and the surface can be rendered safe for handling, use or disposal. Inherent in this definition is the idea that decontamination includes inactivation of all pathogens (bacteria, fungi, viruses) capable of producing disease and not just those found in the blood. For disinfection and sterilization purposes, reusable patient care equipment will be classified and processed according to recommendations of the Centers for Disease Control and the Association for Practitioners in Infection Control Guidelines on the Selection and Use of Disinfectants.6,7 Before implementing any cleaning, disinfection, and sterilization of patient care equipment, the appropriate category under which the article to be cleaned, disinfected, or sterilized must be classified. Class I—Critical. Equipment in this category includes any instrument that will be introduced into the patient’s bloodstream, through the patient’s skin, or into other normally sterile areas. Examples include surgical instruments, implanted devices, cardiac catheters, pacemakers, and so forth. Sterility is required for these instruments. Class II—Semicritical. Equipment in this category includes any instrument that will come into contact with intact mucous membranes and does not penetrate body surfaces. Such instruments include noninvasive endoscopes, endotracheal tubes, MacGill forceps, oropharyngeal airways, endotracheal tube stylets, anesthesia masks, Ambubag masks, thermometers, laryngoscope blades, and so forth. Sterility is not essential. However, at a minimum, a high-level disinfection procedure that can be expected to destroy vegetative microorganisms, most fungal spores, tubercle bacilli, and small, nonlipid viruses is recommended. Meticulous physical cleansing followed by an appropriate high-level disinfection treatment gives a reasonable degree of assurance that the items are free of pathogens. © 2001 by CRC Press LLC

Class III—Noncritical Equipment in this category comes in contact with patients and their intact skin and rarely, if ever, is implicated in the transmission of disease. Items in this category include crutches, bed boards, blood pressure cuffs, stethoscopes, and so forth. Routine cleansing with soap and water and an EPA-approved disinfectant is sufficient to reduce the number of microorganisms on the surface of this equipment. Alcohol may also be used to clean the surface of items if they are not visibly soiled. Table 1 gives examples of instruments, levels of disinfection, and the procedures recommended. The process of cleaning and disinfection/sterilization must be properly validated. To ensure this process, the following issues must be considered. All surfaces of the device must come into contact with the cleaning and disinfecting agents to reduce the bioburden adequately. The device materials should be compatible and unaffected by the process. After processing the device must function as designed by the manufacturer. The cleaning, disinfection, and sterilization processes must allow for complete removal of the cleaning and disinfecting agents. Once the cleaning and disinfecting process has been implemented and validated, it becomes necessary to demonstrate that chemicals used in this process are not left on the medical device or equipment at a level that may cause harm to patients. To demonstrate that this is indeed the case, numerous analytical procedures have been developed and used to measure residual levels of disinfectants and chemical sterilants. Sterilization validation must demonstrate that all device surfaces have a sterility assurance level (SAL) of at least 106 and can be maintained.8 This chapter will first examine the various kinds of chemical disinfectants used in the medical device industry and then explore various analytical techniques to measure their presence or absence from these medical devices and equipment. Table 1 Instruments, Levels of Disinfection, Recommended Procedures Instruments Class I—Critical Includes all invasive instruments (e.g., surgical instruments, intravenous catheters, implanted devices, etc.) Class II—Semicritical All instruments that contact mucous membranes (e.g., endotracheal tubes, endoscopes, airways, etc.) Class III—Noncritical

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Level of Disinfection/ Sterilization

Procedure

Sterility required

Moist heat, dry heat, or ethylene oxide

High level required, must be disinfected between patients

Moist heat, 100°C for 30 min, aqueous 2% glutaraldehyde for 20–30 min, 1:10 dilution of bleach for 20 min

Low level required

Chemical disinfectants include ethyl or isopropyl alcohol (70–90%), phenolic germicidal detergent solutions, iodophores for intermediate level, an exposure time of at least 10 min is required, for low level, all the above plus quaternary ammonium compounds

CLEANING PROCESS The way equipment is cleaned and stored may present issues that impact the success of infection control practices. The strategy for cleaning instruments and medical devices essentially is dictated by the inaccessibility of the surfaces to be cleaned. The efficacy of cleaning and disinfecting agents is contingent upon direct contact with the pathogens. Organic material (bioburden) such as blood, pus, feces, and tissues, when developed by a surface not only prevents direct contact with the disinfectant, but can inactivate the disinfectant as well. Chlorine and iodophores are particularly susceptible to such inactivation.9 A perfect example of a very difficult convoluted instrument to clean is the flexible scopes (i.e., endoscopes, bronchoscopes, and sigmoidoscopes). Flexible scopes are normally classified as minimally invasive or semicritical instruments that touch only mucous membranes.10 As such, the procedure for flexible scopes is to clean them in preparation for using high-level disinfection. As with sterilization, the preliminary procedures needed to prepare an item for high-level disinfection are paramount for a successful result. Exposing all surfaces of the instrument or device to the cleaning agent or agents is imperative if the cleaning process is going to be successful. Proper preparation of the scope begins before it even gets to the CS (central services) department or area. After use on the patient, the scope must be flushed out immediately with an enzymatic solution.11 The outside must also be wiped off with the solution to remove gross soil. Transportation of the scope must be done as quickly as possible to prevent drying. If the soil dries on the instrument or device, then a lengthy soaking procedure will be needed to loosen this material. Once the instrument has reached the decontamination area, it should be flushed with an enzymatic solution from the distal to the proximal end. This would be a reverse flow in the instrument and would loosen soil that tends to build up in layers. By flushing in the opposite direction, there is a greater chance that soil will lift off. A cleaning brush must then be applied to the internal surfaces. This brush must be a good fit for the channel, not too small or too large, without flat spots or missing bristles, and should soak in solution before use. To design effective and competitive cleaning products, producers combine several different chemicals. Surfactants form the base of many cleaning products, removing dirt and soil through a physicochemical process.12 These products contain hydrophobic and hydrophilic components that work together to loosen absorbed or chemically bound entities. The old saying, “You can clean without sterilization, but never sterilize without cleaning,” is true. Every effort must be made to reduce bioburden to as low a level as possible. The cleaner items are when entering the sterilizing solution or sterilizer, the better the chances of successful sterilization. Soaps and detergents are only mildly microbicidal. Their use aids in the mechanical removal of microorganisms by breaking up the oily film on the skin (emulsification) and reducing the surface tension of water so it spreads and penetrates more readily. Detergents may be anionic or cationic.12 Anionic (negatively charged) detergents, such as laundry powders, mechanically remove microorganisms and other materials but are not very microbicidal. Cationic (positively charged) detergents alter membrane permeability and denature proteins. They are effective against many vegetative bacteria, some fungi, and some viruses. However, endospores, Mycobacterium tuberculosis, and Pseudomonas species are usually resistant. They are also inactivated by soaps and organic materials like excreta. Cationic detergents include the quaternary ammonium compounds (Zepharin, Diaprene, Roccal, Ceepryn, and Phemerol). Zepharin may be used to disinfect instruments (20-min soaks) but have no effect against the tubercle bacillus and are inactivated by soaps. Cleaning agents can be made even more effective by the addition of enzymes.13 Enzymes are protein biological catalysts that are extremely efficient and speed up normal © 2001 by CRC Press LLC

biochemical reactions. Proteases are the most widely used enzymes in the detergent industry. They hydrolyze proteins and break them down into more soluble polypeptides or free amino acids. As a result of the combined effect of surfactants and enzymes, stubborn stains can be removed. The inefficiency of nonenzymatic detergents in removing proteins can result in permanent stains due to oxidation and denaturing caused by bleaching and drying. Blood, for example, will leave a rust-colored spot unless it is removed before bleaching. While protein stains can be easily digested by proteases, oily and fatty stains were problematic and continued to be a problem. However, in the late 1980s, genetically engineered proteins (lipases) were developed and added to detergent formulations.13

CHOICE OF DISINFECTING CHEMICALS Several factors dictate how and with what chemicals a device is to be processed to ensure that it is clean, decontaminated, and/or sterile. For those devices that do not come into contact with bodily fluids, cleaning only may be sufficient. Decontamination should be carried out in accordance with the manufacturer’s instructions. Decontamination agents must be compatible with the article to ensure that decontamination does not change the properties or damage the item. Product deterioration (deterioration of materials) and consequently function are extremely important considerations.16 Various changes, depending on how the disinfection process is carried out, can occur such as weak spots, changed material (becomes more brittle), reduced performance, and an accumulation of biological material, which can interfere with performance (e.g., balloon catheters). Chemical treatments and detergents may remove some of the nonpolymer components of the plastic (plasticizers, antioxidants, and fillers) and may alter the polymers.17,20 Prolonged exposure to 70% alcohol can disrupt adhesives, damage seals, and denature some plastics. After cleaning, the item may be disinfected or sterilized. Items compatible with the high temperatures and pressures comprising steam sterilization cycle should be sterilized in an appropriate autoclave. For items where exposure to high temperature is detrimental to the device, it may be possible to sterilize using ethylene oxide. The manufacturer must be consulted before either method of sterilization is used. Those items which cannot be sterilized may be disinfected using either low-temperature steam or immersion in liquid chemical disinfectants. Consideration should be given to the requirements for the safe use of such chemicals as alcohols, and aldehydes such as glutaraldehyde and peracetic acid. The following agents are commonly used as disinfectants, antiseptics, and sterilants in the medical community.

Alcohols Aliphatic alcohols are antimicrobial in varying degrees by denaturing proteins.12 Ethanol (ethyl alcohol) in 70% concentration is bactericidal in 1 to 2 min at 30°C, but less effective at lower and higher concentrations. Ethyl alcohol, 70%, and isopropanol (isopropyl alcohol, IPA), 90%, are at present the most satisfactory general antiseptics for skin surfaces. They may be useful for sterilizing instruments but have no effect on spores, and better agents are available for this purpose.21 Alcohols are often combined with other disinfectants, such as iodine, mercuirals, and cationic detergents for increased effectiveness. © 2001 by CRC Press LLC

Aldehydes Formaldehyde in a concentration of 1 to 10% effectively kills microorganisms and their spores in 1 to 6 h. It acts by combining with and precipitating protein. It is too irritating for use on tissues, but it is widely employed as a disinfectant for instruments.22 Formaldehyde solution USP contains 37% formaldehyde by weight, with methyl alcohol added to prevent polymerization, is extremely active, and kills most forms of microbial life. Glutaraldehyde has been available for more than 30 years and its use was first regulated by the EPA. Standards of care for the use of glutaraldehyde products are available from specialty organizations such as the Society of Gastroenterology Nurses Association (SGNA) and the Association of Operating Room Nurses (AORN), which have significant experience and interest in the safe and effective use of liquid chemical disinfectants/sterilants.23 These guidelines all recommend a 20 min soak in a 2% glutaraldehyde solution after a meticulous manual precleaning with an appropriate detergent. When used with the guidelines and precautions available, glutaraldehyde is an appropriate and effective highlevel disinfectant.24 Glutaraldehdye as a 2% alkaline solution in 70% isopropanol (pH 7.5 to 8.5) serves as a liquid disinfectant for most optical and other instruments and for some prosthetic materials. It kills viable microorganisms in 10 min and spores in 3 to 10 hours, but the solution is unstable, and tissue contact must be avoided. It must be rinsed from instruments and other items with sterile water before use. It is the “cold” sterilant of choice for lensed instruments.

Halogens and Halogen-Containing Compounds Elemental iodine is an effective germicide and is most often used as a skin disinfectant.25 Its mode of action is not definitely known but does combine with cell protein and is an active germicidal agent with a moderate activity against spores. Iodine tincture USP contains 2% iodine and is the most effective antiseptic available for intact skin.26 Its principal disadvantage is the occasional dermatitis that can occur in hypersensitive individuals. Iodine can be complexed with polyvinylpyrrolidone to yield povidone-iodine USP, an iodophore. This is a water-soluble complex that liberates free iodine in solution. Like iodine, povidone-iodine is widely employed as a skin antiseptic and used as a surgical scrub.26 Povidone-iodine solutions can become contaminated with Pseudomonas and other aerobic Gram-negative bacteria. Iodine and povidone-iodine both are rarely used to disinfect instruments or medical devices. Chlorine has been used for many years as a disinfectant and has generally been associated with treatment of swimming pool water and water supplies. It exerts its antimicrobial action in the form of undissociated hypochlorous acid (HOCl) and acts by oxidizing the cell membrane.25 Hypochlorous acid is formed when chlorine is dissolved in water. Chlorine concentrations of 0.25 parts per million (ppm) are effectively bactericidal for many microorganisms except for mycobacteria.27 Organic matter greatly reduces the antimicrobial activity of chlorine. Instrument disinfectants containing chlorine dioxide, known as Tristel, Dexit, and Medicide, are commercially available. These products contain two components, a base and an activator, requiring addition and dilution in accordance with the manufacturers’ instructions. Freshly prepared chlorine dioxide is highly effective and rapidly destroys bacterial spores.28 Sporicidal activity is maintained for 7 to 14 days provided the disinfectant is stored in sealed containers with minimal head space above the solution. Unfortunately, © 2001 by CRC Press LLC

chlorine dioxide is also more damaging to instrument and processor components than aldehydes. A discoloration of the black plastic casings of flexible endoscopes has been reported, but this change may be only cosmetic. Quaternary Ammonium Compounds Cationic surface-active agents (quaternary ammonium compounds) such as benzalkonium chloride USP and cetylpyridinium chloride USP are both detergents and bactericidal agents.26 These agents are bactericidal probably by altering the permeability characteristics of the cell membrane.29 Benzalkonium chloride (Zepharin) may be used to disinfect instruments. A 20-min soak is recommended. However, their toxicity seems to be somewhat higher than that of the biguanides described below. Biguanide Disinfectants All of the biguanides have a wide spectrum of antibacterial activity against both Grampositive and Gram-negative bacteria.30 They have a high kill rate, but toxicity towards mammalian cells is very low and irritancy is so insignificant that the biguanide disinfectants can be used on the sensitive mucosal surfaces. Chlorohexidine is a bisdiguanide antiseptic that disrupts the cytoplasmic membrane, especially of Gram-positive organisms. It is employed as a skin cleanser, as a constituent of antiseptic soaps, and as a mouthwash for combating plaque-inducing bacteria. Chlorohexidine is rarely used on inaminate objects and is neutralized by alcohol, therefore the two agents should not be used in combination.31 Another biguanide-type disinfectant that has recently been used in contact lens solutions is polyaminopropyl biguanide (PAPB). This disinfectant appears to be very effective with a wide spectrum of antibacterial activity against both Gram-positive and Gram-negative bacteria. Toxicity toward mammalian cells is very low and irritancy is so insignificant that the biguanide antiseptics can be used on sensitive mucosal surfaces and the eye. Phenol and Phenolic Derivatives Phenol (5 to 10%) was the first disinfectant commonly used.12 However, because of its toxicity and odor, phenol derivatives are now generally used instead.32 These include orthophenylphenol, hexachlorophene, and hexylresorcinol.33 Phenol denatures protein. Concentrations of at least 1 to 2% are required for antimicrobial activity, whereas a 5% concentration is strongly irritating to tissues. Therefore, phenol is used mainly for the disinfection of inanimate objects and excreta. Table 2 presents a summary and comparison of various liquid disinfectants used in the medical community. ANALYTICAL TESTING Although the retention of functionality is recognized as one of the most-pressing issues to deal with in a decision to reuse, there is a scarcity of detailed scientific studies in the literature that have investigated the deterioration of materials and function in reused disposables. © 2001 by CRC Press LLC

Table 2 Liquid Disinfectants Used in the Medical Community Class

Recommended Use

Method of Action

Advantages

Alcohols (70% IPA or ethanol)

Cleaning some instruments Cleaning skin

Fairly inexpensive and readily available.

Chlorine compounds

Spills of human body fluids Bactericidal—Good Fungicidal—Good Sporicidal—Good at 1000 ppm

Changes protein structure of microorganism. Presence of water assists with killing action Free available chlorine combines with contents within microorganism, reaction by-products cause its death Need 500 to 5000 ppm

Aldehydes (e.g., glutaraldehyde)

Bactericidal—Good Fungicidal—Good Tuberculocidal— Excellent Virucidal—Good Sporicidal—Good

Coagulates cellular proteins

Iodophors (iodine with carrier)

Disinfecting some semicritical medical equipment Bactericidal—Very good Fungicidal—Excellent Virucidal—Excellent

Phenolic compounds

Bactericidal— Excellent Fungicidal—Excellent Tuberculocidal— Excellent Virucidal—Excellent Ordinary housekeeping (e.g., floors, furniture, walls) Bactericidal— Excellent Fungicidal—Good Virucidal—Good (not as effective as phenols)

Free iodine enters microorganism and binds with cellular components Carrier helps penetrate soil/fat Need 30 to 50 ppm Probably acts by disorder of protein synthesis due to hindrance and/or blocking of hydrogen bonding. Overt protoplasmic poison Disrupts cell walls Precipitates proteins Low concentrations inactivate essential enzyme systems Affects proteins and cell membrane of microorganism Releases nitrogen and phosphorus from cells

Quaternary ammonium compounds (QUATS)

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Kills hardy viruses (e.g., hepatitis) Kills a wide range of organisms Inexpensive Penetrates well Relatively rapid microkill Nonstaining, relatively noncorrosive Usable as a sterilant to plastics, rubber, lenses, stainless steel, and other materials that cannot be autoclaved Kills broad range of organisms Highly reactive Low tissue toxicity Kills immediately rather than by prolonged period of of time

Nonspecific concerning bactericidal and fungicidal action

Acts as a detergent to loosen soil Rapid action Colorless, odorless Nontoxic Highly stable

Medical devices are commonly made of polymers or plastics of varying density. Lowdensity plastics are less resistant to heat than high-density ones. However, the properties of plastics are also obtained from additives that may stabilize the material to heat and light or reduce costs (fillers), enhance beneficial properties such as abrasion, resistance and strength, or provide lubrication, flexibility, or antifungal properties. Some of these additives are susceptible to reprocessing and reuse and can be leached out, or their composition can be altered through exposure to light.20 Mechanical testing can be used to help determine the functional integrity of devices. It may be difficult to simulate in an experimental setting the various stresses a device may be exposed to in a clinical environment. Even if all the variables related to stress and strain were known, there is no one set of tests that is suitable for all devices. However, a key mechanical test is the tensile test, which measures the force required to stretch a device through a range of extensions. The most widely used instrument for stress –strain measurement is the Instron Tensile Tester. This instrument is essentially a device in which a sample is clamped between grips and jaws, which are pulled apart at constant stress rates.17 A variety of parameters are determined such as elongation, elongation at break, breaking strength, and tensile modulus of elasticity. The tensile modulus is defined as the ratio of stress to strain, and is determined from the initial slope of the stress–strain curve. The modulus of a material is a measure of the ability of a specimen to resist deformation. Tensile modulus is also referred to as Young’s modulus. Some plastic materials can be weakened by the process of sterilization or disinfection that leads to a decrease in the tensile strength of the material. Tensile strength data collected before and after cleaning, decontamination, or sterilization can help predict any changes in the strength of the material.16 Hardness testing is another mechanical test useful to determine the functional characteristics of material used in a medical device or instrument. Hardness is generally defined as an indication of the resistance to indentation, scratch resistance, and/or rebound resilience. International Standards Organization (ISO) standards report three methods for measuring hardness: Shore hardness, a ball indentation method, and Rockwell hardness.34,35 It is important to emphasize that hardness values obtained from one method in general cannot be compared with those derived from another, although data can be empirically compared. Generally, hardness is used as an end-performance property of material used in a device. The instrument used to obtain the measurement is called a durometer. Thermal analysis, the response of a polymer to controlled heating processes, is a family of techniques widely used in the development and characterization of materials, including plastics and elastomers. Characterizing the glass transition temperature, melting point, and extent of crystallinity of a polymer is important and often used to produce materials whose properties are tailored to the ultimate application of the product.36 The primary thermal analysis techniques for certifying product quality are differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Thermal analysis tests for certifying product quality are listed in Table 3. Table 3 Thermal Analysis Techniques for Materials Test

Technique

Melting point Degree of crystallinity Glass transition temperature Component quantification

DSC DSC DSC, TGA TGA

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Differential scanning calorimetry results can give rapid measures of thermal characteristics, such as the polymer melting point and glass transition temperature. The nature of these transitions, in addition to identifying characteristics unique to each polymer, can also provide information about its phase structure, thermal history, and purity. The loss of polymer additives can have a dramatic effect on these thermal properties. Chromatographic Analysis The various chromatographic methods, such as gas, liquid, paper, and thin-layer, have become indispensable aids in the isolation, separation, and determination of chemicals. Gas and liquid chromatography, especially high-performance liquid chromatography (HPLC), have become powerful analytical tools to characterize additives in polymeric materials used in medical devices, as well as disinfectants and liquid sterilants adsorbed onto surfaces.16 HPLC is undeniably one of the fastest growing and most useful of all the analytical separation techniques. The reason for this growth is attributable to the sensitivity of the method, its ready adaptability to accurate quantitative determinations, and its suitability for separating nonvolatile species or thermally fragile ones. Liquid chromatography, because of its great flexibility and wide-spread applicability, can be used for the analysis of more than 80% of all known organic compounds.17 Spectrophotometric Analysis Infrared Infrared (IR) instruments measure the vibrational spectrum of a sample by passing IR radiation through it and recording which wavelengths have been absorbed and to what extent. Since the amount of energy absorbed is a function of the number of molecules present, the IR instrument provides both qualitative and quantitative information. Since the IR spectrum of a chemical compound is perhaps its most characteristic physical property, IR finds extensive application in identifying substances and their respective concentrations. Many cleaning and disinfecting agents have distinctive IR spectra that can be used to identify residuals on the surface of devices and instruments.37 If residual glutaraldehyde is present on a medical device, then it can be extracted in a solvent and the solvent analyzed for glutaraldehyde. Most detergents and enzyme (protein) solutions can be identified by this analytical method. Ultraviolet/Visible Spectroscopy Much like IR, the ultraviolet (UV) and visible spectrum of a chemical can be used to identify and determine the concentration of analytes in an extract. Enzymes, which are proteins, are very easily detected and their concentration determined by their distinct UV spectrum.20 Most proteins absorb maximally at 280 nm. CONCLUSION Successful cleaning, decontamination, and sterilization/disinfection of medical devices and instruments requires both the careful selection of materials and consistent monitoring of the procedures used to process them. Chemical characterization of residues © 2001 by CRC Press LLC

that may be adsorbed onto surfaces and mechanical testing to ensure functionality should provide sufficient information to evaluate the potential success of cleaned and disinfected/sterilized medical devices. By using a combination of chemical and mechanical analysis techniques, both manufacturing and decontamination processes can be optimized to ensure a safe and effective product. It is important to note that no test, however foolproof its design, can ever be considered a definitive predictor of clinical performance. REFERENCES 1. Hart, P.D., Decontamination compliance of porous surfaces—is it attainable? Infect. Control Steril. Technol., 5(3), 43 –44, 1999. 2. Caballes, N., de Guzman, D.R., and Voorhis, J.V., An infection control perspective of antibiotic resistance, Infect. Control Steril. Technol., 3(10), 18 –26, 1997. 3. Avalos-Bock, S., Small hospitals face special challenges in infection control, Infect. Control Steril. Technol., 4(1), 24 –27, 1998. 4. Designing, Testing, and Labeling Reusable Medical Devices for Reprocessing in Health Care Facilities: A Guide for Device Manufacturers, AAMI TIR 12-1994, Association for the Advancement of Medical Instrumentation, Arlington, VA, 1995. 5. U.S. Department of Labor, Occupational Safety and Health Administration. Occupational Exposure to Blood Borne Pathogens; Final Rule,. 29 C.F.R. Part 1910, 1030, December 6, 1991. 6. Spaulding, E.M., Chemical Disinfection of Medical and Surgical Materials. Disinfection, Sterilization and Preservation, 1986, 517 –531. 7. Alvarado, C.J., Revisiting the Spaulding classification scheme, in Chemical Germicides in Health Care, Proc. Int. Symp., Rutala, W.A., Ed., Cincinnati, May, 1994, , 203 –209. 8. DeSchutter, C. and Ritz, S., Processing minimally invasive instruments, Infect. Control Steril. Technol., 2(12), 26 –28, 1996. 9. Block, S.S., Disinfectant, Sterilization, and Preservation, 4th ed., Lea & Febiger, Philadelphia, 1991. 10. Jendresky, L., Garcia, R., Clarke, A., and McDonagh, W., Performance improvement of disinfection and sterilization systems: a case study, Part II, Infect. Control Steril. Technol., 3(4), 44 –53; 1997. 11. Rutula, W. et al., FDA labeling requirements for disinfection of endoscopes: a counterpoint, Infect. Control Hosp. Epidemiol., 231 –235, 1995. 12. Jawetz, E., Disinfectants and Antiseptics in Basic and Clinical Pharmacology., 4th ed., Appleton & Lange, Norwalk, CT, 1989. 13. Miner, N., Reuse of single-use only devices, Biomed. Instrum. and Technol., 33(1), 5 –6, 1999. 14. Weller, I.V.D., Williams, C.B., Jeffries, D.J., Leicester, R.J., Gazzard, R.G., Axon, A.T.R., Hanson, P.J.C., Ayliffe, G., Barrison, I., and Neumann, C. Cleaning and disinfection of equipment for gastrointestinal flexible endoscopy: interim recommendations of a working party of the British Society of Gastroenterology, Gut, 29, 1134 –1151, 1988. 15. Vaughn, S., Disinfectants and their use, in Proceedings of MARE Seminars, 1993. 16. Albert, D.E. and Wallin, R.F., A practical guide to ISO 10993 –14: materials characterization, Med. Dev. Diag. Ind., 20, 96 –99, 1998. 17. Albert, D.E., Materials characterization as an integral part of global biocompatibility, Med. Plast. Biomater., 4, 16 –23, 1997. 18. Feldman, L. and Hui, H., Method of Protecting and Sterilizing Aluminum Surfaces on Medical Instruments, U.S. patent pending, 1997. 19. Sloan, F., Chemical attack of graphite/epoxy by hydrogen peroxide, Appl. Spectros., 46, 524–528, 1992. 20. Albert, D.E., and Hintz, M.A., in Material Properties and Bioreactivity in Spinal Drug Delivery, T. Yaksh, Ed., Elsevier, New York, 1999, 395 –406. 21. Prescott, Harley, and Klien, Microbiology, 2nd ed., Wm. C. Brow Publishers, Dubuque, IA, 1993. 22. Basett, R.C., Formaldehdye in Biological Monitoring Methods for Industrial Chemicals, 1980, 144 –147. 23. Casey, R.L., Glutaraldehyde: concern for use, Infect. Control Steril. Technol, 4(2), 12 –16, 1998. © 2001 by CRC Press LLC

24. Mbithi, J.N., Springthorpe, V.S., and Sattar, S.A., Bactericidal, mycobactericidal and virucidal activities of reused alkaline glutaraldehyde in an endoscopy unit, J. Clin. Microbiol., 31, 2988 –2995, 1993. 25. Frobisher, M., Hinsdill, R.D., Crabtree, K.T., and Goodheart, C.R., Sterilization and disinfection: practical applications, in Fundamentals of Microbiology, 1974, 302 –318. 26. U.S. Pharmacopeia, Vol. 23, NF18, U.S. Pharmacopeia Convention, Washington, D.C., March 8–10, 1990, published by the Board of Trustees, January 1, 1995. 27. Holton, J., Nye, P., and McDonald, V., Efficacy of selected disinfectants against mycobacteria and cryptosporidia, J. Hosp. Infect., 27, 105 –115, 1994. 28. Ossia-Ongagna, Y. and Sabatier, R., Comparison of in vitro activity of six disinfectants on bacteria from contamination in haemodialysis water, J. Pharm. Belg., 48, 341 –345, 1993. 29. Tennant, J.M., Lyon, B.R., Gillespie, M.T., May, J.W., and Skurray R.A., Cloning and expression of Staphylococcus aureus plasmid-mediated quaternary ammonium resistance in Escherichia coli, Antimicrob. Agents Chemother., 27, 79 –83, 1985. 30. Brecx, M. et al., Efficacy of Listerine, Meridol and chorohexidine mouth rinses on plaque, gingivitis and plaque bacteria vitality, J. Clin. Periodontol., 17, 292 –296, 1990. 31. Vigeant, P., Loo, V.G., Bertrand, C., et al., An outbreak of Serratia marcescens infections related to contaminated chlorohexidine, Infect. Control Hosp. Epidemiol., 19, 791 –794, 1998. 32. Best, M., Sattar, S.A., Springthorpe, V.S., and Kennedy, M.E., Efficacies of selected disinfectants against Mycobacterium tuberculosis, J. Clin. Microbial., 28, 2234 –2239, 1990. 33. U.S. Pharmacopeia XXIII. 34. ASTM D2238, D2583, in Annual Book of ASTM Standards, 08.02—Plastics (II), American Society for Testing and Materials, Philadelphia, 1991. 35. ISO, International Standard 868, 1985. 36. DiVito, M.P., Fielder, K.J., Curran, G.H., and Feder, M.S., Recent advance in routine thermal analysis instrumentation, Am. Lab., 24(1), 30 –36, 1992. 37. Palley, I. and Signorelli, A.J., Physical testing, in A Guide to Materials Characterization and Chemical Analysis, Sibilia, J.P., Ed., VCH Publishers, New York, 1988, 273 –284.

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SECTION 6

Regulatory/Safety Considerations

© 2001 by CRC Press LLC

CHAPTER 6.1

Safety and the Environment— Some Editorial Thoughts Barbara Kanegsberg

CONTENTS Introduction Environmental vs. Safety Regulations Beyond the Lists MSDS et al. Known Hazards vs. the Great Unknown Air Monitoring, Process Monitoring, Emergency Supplies Your Input Is Important Communicating with Safety Professionals and Environmental Regulators Additional Environmental Issues Hazardous Air Pollutants Volatile Organic Compounds Conclusion References

INTRODUCTION Components and parts manufacturing takes place in a highly regulated world. We should be maximizing process performance and process efficiency. All too often, the goal of manufacturing takes a back seat to the need to satisfy the environmental crisis du jour. By the time all the environmental controls and safety controls are in place, the efficiency of cleaning may be so compromised that orders of magnitude more cleaning agent or energy may be used. The various controls required may in fact be at odds with one another. Sometimes, manufacturing in a particular location becomes so onerous that the decision is be made to ship the process or even the entire manufacturing plant to a geographically remote location. One can only imagine the environmental impact of such a decision.1 At the same time, environmental crises starting with the depletion of stratospheric ozone have led to new technology in cleaning chemistries, cleaning equipment, and overall processing, which have provided additional valuable options to manufacturers. © 2001 by CRC Press LLC

ENVIRONMENTAL VS. SAFETY REGULATIONS Manufacturers must be concerned with the environment, but they must also protect the individual worker. One point of confusion is that many engineers think of environmental regulations as being synonymous with safety regulations. In terms of impact on the manufacturing process, worker safety and environmental regulations may actually be at odds. Let us look at a plausible, if somewhat exaggerated scenario. Let’s assume you are a manufacturing engineer. You enter an assembly area to be greeted by the breathtaking aroma of fluoro-iodo propyl daffodilic acid (a fictional strong organic acid) bubbling merrily out of the contained safety bath and onto the floor and, currently, splashing all over you. At the same time, you spot the warning light on your degreaser full of hexamethyl nitro chicken wire, a fictional VOC (volatile organic compound) and HAP (hazardous air pollutant); the light indicates that someone on the notorious third shift has managed to override all the safety switches—the room is full of those lovely vapors, as well. How do you handle this? One good way might be to sound the alarm, get yourself and everyone else out of the work area, and dive under the safety shower while, in an attempt to reenact Woodstock, stripping away your clothing. At the same time, someone else turns on the fans full-blast to exhaust the vapors from the room. Then the crew dons protective gear and cleans up the mess on the floor. You have protected yourself and your workers. At the same time, depending on where you work and on other environmental controls, you have perhaps increased air and water pollution. Granted, this was an emergency. However, it is important to be aware that policy impacting the ongoing operation of a process may be based on ongoing protection of the neighborhood environment or the global environment, not on protection of the worker. It is important to work with the appropriate health and safety professionals and to be yourself aware of potential problems. Just because a chemical is not on an environmental hit list does not mean you can automatically assume it will be safe for your workers.

BEYOND THE LISTS Let me repeat. Just because a product is not on a safety or environmental list does not mean it is safe. You must evaluate your own manufacturing situation carefully and thoughtfully. The SNAP program, for example, is involved with determining allowable substitutes for ozone-depleting compounds (ODCs). In your particular process, the solvent that best suits your needs may be acceptable as an ODC substitute, and it may be delisted as a VOC. However, it may have also a much lower allowable worker inhalation level than your current solvent. Or, the chemical could itself have a very high allowable exposure limit, but the vapors may react with acids or bases used in a nearby process. Without looking at worker safety in terms of the full process and the overall manufacturing environment, you would not necessarily be adequately protecting the individual worker. If the material has a low flash point, it would be unsuitable for use in standard heated equipment. Yet, this author has observed what can best be described as doubtful proposals to use isopropyl alcohol to replace an ozone depleter or acetone to replace VOCs without considering the engineering controls required in the process. Even cleaning agents without a flash point can oxidize during use, and azeotropes (and certainly, many “near-azeotropes”), if not stored appropriately, can change in composition, sometimes resulting in significant amounts of flammable vapors being produced. © 2001 by CRC Press LLC

Particularly in areas of poor air quality, VOCs and, in fact, all solvents are discouraged by some regulatory groups in favor of water-based processes. Suppose you avoid solvents and adopt a water-based process. Such a process may resolve ODC, VOC, and HAP issues. However, even environmentally friendly aqueous cleaners require appropriate handling to minimize hazards to workers. This was observed with some “beyond-compliant” so-called Clean Air Solvents (CAS). The CAS are cleaning agents that have been analyzed by South Coast Air Quality Management District (SCAQMD) in California and found to meet its stringent environmental requirements for VOCs, ODCs, GWPs (global-warming potential), and air toxics. The additives in certain CAS, particularly when the mixture is heated, could produce skin irritation. This does not mean the products cannot be used; the CAS provides an extra measure of reassurance to the end user. It does mean that all products must be used with careful regard for the safety of the individual worker. With organic additives, heated mists may release significant amounts of solvent vapors. In addition, the force of cleaning action can be sufficient to cause injury to workers. In some cases, cabinet washers may be designed to be linked to a sink-on-a-drum cleaner for economic reasons. In such cases, it is important to restrict the maximum operating temperature of the cabinet washer so as not to scald the worker who unknowingly begins to operate the attached sink-on-a-drum for hand cleaning. It is also important to walk through the plant to see how the process is being used. If the worker is supplementing the aqueous process with aerosol sprays, it is important to determine the content of the sprays and to evaluate potential worker exposure, perhaps with vapor monitoring. In addition, it is important to look at the spent cleaning solution in terms of soils and unexpected solvent additives in terms of impact on the individual employee and on the potential for water pollution. Appropriate impervious gloves, goggles, aprons, etc. become important. In summary, there is no substitute for a well-designed process run by a well-educated workforce.

MSDS ET AL. The Material Safety Data Sheet (MSDS) has become a standard part of the workplace. Unfortunately, an MSDS can be difficult to read and to interpret, the information format is not standardized, and the degree of information and recommendations provided may vary from one manufacturer to the next. The author likes to compare the MSDS from several suppliers. A bit of time spent looking over these sleep-inducing documents can save headaches in the long run. You should beware of an overabundance of proprietary data. If you suspect a hazard, ask questions. Sign a confidentiality agreement, if necessary, but get as full a picture as possible. If you cannot get disclosive information and are suspicious the author suggests you find another supplier. It is also important to be aware that you may have to go beyond the immediate MSDS, depending on the way the cleaning or processing agent is used. For example, an MSDS may indicate that Chemical A dissolved in Chemical B will release vapors of C and D. The MSDS may vaguely indicate that the vapors of C and D are harmful but give no further details of these latter chemicals. If you are using Chemicals A and B separately and in portions of the plant that are isolated from each other, there may be no significant problems. However, if you are indeed dissolving A in B, you would be well advised to obtain safety information on Chemicals C and D. The mere presence of books of MSDS at the work site, while helpful, is not a substitute for an educated, aware, and informed workforce. Some workers look at the MSDS as a sort of talisman. If there is a problem, they will wave the book of MSDS in the general direction © 2001 by CRC Press LLC

of the chemical spill or source of solvent vapors, with the expectation that the problem will be solved. Mere ownership of MSDS is not enough. Workers must be able to read them, understand them, and apply the information to day-to-day process operation. KNOWN HAZARDS VS. THE GREAT UNKNOWN We sometimes fear an abundance of data without considering the alternatives. A solvent that has been used for decades and with a well-established toxicity may have a relatively low allowable inhalation level. People may be fearful of using the chemical. Or, because it is on a list of hazardous air pollutants, ozone depleters, or VOCs, there may be regulatory restrictions or company policy restrictions on using the product. However, suppose an alternative cleaning agent or an alternative cleaning blend has a list of ingredients indicating that no inhalation level has been set. Does this mean you should be blithely breathing great gulps of it or drinking it as a liqueur? Of course not. Not all possible hazards to the worker or the environment have been evaluated. There are simply not the resources available to do so. It is preferable to exercise reasonable prudence. On the other hand, the argument has also been made that some of the older, more established air toxics have well-known hazards and can therefore be used more readily. However, this does not mean that they are necessarily relatively safe. The closest analogy that can be made is to a speeding train. We understand the power, we understand the utility in getting us from one place to the next. However, this does not mean we can stand on the tracks. Older chemicals are said to be safer because there have been decades of use and decades of epidemiological data, whereas newer chemicals only have data based on animal exposure. In the first place, it is difficult to evaluate epidemiological data. In the second place, we now ought to know enough to understand that all chemicals should be used less emissively and with less exposure to workers than they were in the past. In the third place, let’s not put our production workers in the position of unwittingly supplementing testing in laboratory rats. In summary, all chemicals, aqueous and solvent-based, have potential safety and environmental hazards, and should be handled to minimize exposure. AIR MONITORING, PROCESS MONITORING, EMERGENCY SUPPLIES Designing and maintaining an effective, profitable cleaning process is very site specific. Similarly, assuring that a process is environmentally sound and minimizes employee exposure to chemical is also very site specific. A detailed treatment is beyond the scope of this book. However, a few general guidelines are appropriate. Monitoring emissions of organic solvents to the environment is often heavily mandated and inspected. In the same manner, it is important to monitor employee exposure. This involves taking a dispassionate look at the process in terms of all of the workers in the area as well as particular workers who might have more exposure. For cleaning processes, monitoring may include: • • • •

Overall air quality in the manufacturing plant Overall air quality in the work area Air quality proximal to the sample-handling area of the cleaning system Individual employee exposure

Monitoring may be appropriate not only at process start-up but also at regular intervals. Often, a process may be set up to assure that parts are dry and free of trapped solvent. Over © 2001 by CRC Press LLC

time, day-to-day considerations of the production schedule may abbreviate process time, resulting in increased solvent loss and potential employee exposure. If the solvent has an obvious odor, such losses may be apparent. Choosing a cleaning agent because it has an additive to mask odors may be unwise. The human nose is an extremely sensitive detection instrument; if there is no emission, there will be no odor. However, workers can become desensitized to odors, so one cannot depend on odor alone for solvent detection. Selection of the solvent detection device is also important. As with analytical techniques one must consider the lowest detection limit and specificity for the solvent of interest. In addition to working with the safety/environmental professional, the cleaning agent supplier and safety supply distributors can be good sources of information. With all processes, whether aqueous or solvent-based, ongoing process monitoring to minimize employee exposure is crucial. In most cases, it is productive to manage this along with overall process control. Clear, unambiguous process instructions, employee education and training, and accountability are required.2 Emergency supplies also have to be considered. If your process involves chemicals, such as hydrofluoric acid, with the potential for acute, extraordinary hazards, it may be wise to have specific emergency materials beyond the typical eyewashes and safety showers on hand in case of spills or employee exposure. How much to keep on hand depends on the process as well as on the capabilities of outside emergency teams. The best way to design an effective cleaning process is to involve not only the environmental staff and advisors but also health and safety professionals during process design.

YOUR INPUT IS IMPORTANT Most of us have had the experience of having company management or an environmental advisor present us with the latest environment hit list, with the orders to stop using whatever is on the list. Or they may provide a “safe” list with an indication to use only chemicals or products on that list. The detrimental consequences to your production line can be economic, performance, or safety related. Rather than respond with an “Oh, no, I can’t possibly do this,” or “ Great! More unreasonable regulations. This means career security,” perhaps you should consider some proactive work. Environmental policy may be based on research by the greatest minds in the world. However, based on experience, the author thinks we could set more realistic, holistic, and environmentally sound policy if you, the manufacturing engineer, tracked environmental policy and contributed your own views. A perusal of advisory committees to regulatory agencies from the local to the national and even the international level is likely to show a reasonable membership by employees of or advisers to various chemical companies and/or manufacturers of cleaning equipment. This includes aqueous and solvent-based processes. It is laudable and valuable that such involvement take place. After all, the technical people who design cleaning agents or cleaning processes are often in the best position to explain their products to regulators and policy makers. You might also see participation by advisors, advocates, and lobbyists. No matter how impartial the participants, the author does not think anyone can completely operate out of the context of his or her company. Each participant has a viewpoint that must inherently be influenced by his or her own training, background, and experience. Let us look, for example, at the membership in the United Nations Solvent and Adhesives Technical Options Committee (STOC). This committee provides a great deal of input into worldwide environmental policy, which may ultimately influence the cleaning agents that are available to you. An explanation of the committee charter2 and a list of committee members3 © 2001 by CRC Press LLC

are available on the Web site of a manufacturer of equipment for aqueous cleaning. The manufacturer maintains the site as a service to the United Nations. A perusal of the list of STOC members will indicate a host of affiliations. Given the independent nature of funding, some of the more active members include individuals employed by or affiliated with particular chemical and/or equipment companies. At a local level, one might observe that those involved in regulatory advisory committees concerned with solvent usage often include a large proportion of manufacturers of cleaning agents and cleaning equipment, often aqueous cleaning. Equipment and chemical suppliers associated with solvent cleaning are often also involved, and many of them are interested in fostering their own particular technology. Many of these technologies deserve to be fostered. In her view, the author also sees the need for you, the manufacturing engineer to become directly involved. Even where companies are represented on such groups, very often, individuals involved in regulatory compliance rather than people concerned in the technical aspects of the manufacturing process are involved. Sometimes, trade associations send advisors. Such views are valuable in providing input from various overall sectors. Even if there is a trade association involved, by the time issues percolate from the work area, to the environmental group, to the legal advisor, to the association committees, to the regulatory advisory group, and then back down to the production area again, the message may become diluted, lost, homogenized, or otherwise compromised. Regulatory advisory committees aside, at conferences you might find it instructive to look, listen, and observe not only during the formal presentations and question and answer sessions, but also during the informal questions and comments to representatives of regulatory agencies. You might observe representatives of particular cleaning technologies explaining how their approach is environmentally far superior to that of the competition. The author is not suggesting that you in the manufacturing world don an imported designer suit, grab a cell phone, and head for Washington, D.C. You have more important things to do. However, it is desirable to be proactive. If your trade association or environmental advisor asks for your input, even though you may be preoccupied with the immediate production schedule, my advice is to help them. If you are in a small company, keep track of the regulatory issues and impending policy decisions. Take your safety officer to lunch; take your lawyer to lunch—find out what is going on. If you see a regulator at a conference, you do not need to explain details of a process, but if contemplated, impending regulations are going to leave you without reasonable options, say so, before you are burdened with an unreasonable regulation. If you are asked to write a note, do so—working with your company management where appropriate. One to two pages is plenty; regulators have lots to read in their spare time. Your actions just might help. The interplay of the chemistry, physics, biology, toxicology, environmental science, and politics will ultimately impact your engineering and economic choices. Further, because unintended consequences of environmental policy may be detrimental to workers or to other aspects of the environment, your input will allow policy makers to set a regulatory atmosphere that, ultimately, better protects the environment.

COMMUNICATING WITH SAFETY PROFESSIONALS AND ENVIRONMENTAL REGULATORS Having said that we should regularly communicate with regulators, this author must add the obvious: many engineers run, hide, and otherwise avoid communicating process plans with the safety and/or environmental community. Why?

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Some of the fear is based on previous unpleasant, inconvenient experience. A chemical may be needed to perform the process, but the safety advisor may, for reasons of his or her own convenience (e.g., maybe it requires more paperwork), discourage its use. Recollections of regulatory problems or permitting hassles can also produce a “run for the hills” response. In some cases, the company safety/environmental group or the company lawyer may strongly advise technical people to have no communication with the outside regulatory community. Each individual manufacturing situation is different; it is important to evaluate your own legal status. Under ordinary circumstances, however, a more open communication with the regulatory world is probably productive. With no communication, restrictive regulations that do not allow the company to operate effectively are more likely to be enacted. Companies may want to choose advisers who, while protecting company privacy and security, also provide a clear, proactive presentation to regulators. ADDITIONAL ENVIRONMENTAL ISSUES A dizzying array of environmental concerns regarding air, water, and soil contamination can potentially impact manufacturing processes. Environmental issues are complex, because the underlying science, economics, and politics are complex. As such, environmental issues are subject to change. Future regulations are likely to reflect impacts on both air and water. With this understanding, two additional pressing environmental issues, current as of the time of writing, are noted. Hazardous Air Pollutants HAPs are a list of nearly 200 compounds that the U.S. federal government considers damaging to air quality. Presence on the list does not mean that the compound cannot be used safely. Absence from the list does not mean that there are no potential toxic issues. One major impact on components manufacturing is that certain chlorinated solvents are regulated under the National Emissions Standard for Hazardous Air Pollutants (NESHAP) for Halogenated Solvents. Many of the newer cleaning systems are inspired by the requirements of this NESHAP. The Aerospace NESHAP impacts volatility of cleaning agents used in hand-cleaning operation. In the future, additional NESHAPs with additional impact on industry are likely. Volatile Organic Compounds VOCs are smog producers. VOCs react with nitrogen oxides in the presence of light to form photochemical smog, which contributes to poor air quality. All organic compounds (carbon-containing compounds) are VOCs. Even trees and other vegetation can produce solvent emissions that contribute to smog. Current VOC policy is the subject of extensive study.5 However, not all VOCs are created equal; some are more reactive than others. A number of scales indicating reactivity of various organic compounds have been developed.6,7 Some are based on smog chamber studies; others are models that include actual smog episodes. As of this writing, the EPA policy classed certain organic compounds, those few organic compounds with reactivity judged less than ethane, as being negligibly reactive. Ethane is the “line in the sand.” Organic compounds more reactive than ethane are all considered VOCs, even though reactivity may vary by orders of magnitude. Reactivities of the solvent vapors are considered, not the speed of volatilization. The goal of the policy was to

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encourage that these negligibly reactive compounds be used in place of more reactive ones. A few compounds are now classified as being negligibly reactive. The current policy is being reconsidered at the U.S. federal level. The scientific basis of measurement and detection in the urban environment is currently under review. The resulting policy could be more or less stringent, or a qualitatively different set of requirements could emerge. Input from the manufacturing community has been requested. Components manufacturers who are concerned about the number of process options available might be well advised to provide input. CONCLUSION There is no such thing as a free lunch. All cleaning agents, all cleaning processes inherently have some environmental and safety concerns. In this author’s opinion, the situation is not likely to change. When we manufacture, we basically remove or redistribute soils. Soils are matter out of place. Oils, greases, cleaning agents all have some underlying similarity to the compounds that make us alive, that make the grass green, that form the basis of our ecosystems. Regulations often still seem to be on a compound-by-compound basis. There seems to be an attitude that if only compounds A, B, and C were not used, or if we could restrict the manufacturing community to using only processes Q, R, and S, all would be well. During the phaseout of ozone depleting chemicals (ODCs), components manufacturers responded to an environmental crisis with a variety of innovative and creative process changes. This author has been very much involved in the changes. However, it seems safe to say that most, if not all, of these changes have had their own environmental consequences. Despite the best efforts of regulators to control adverse safety impacts, there have doubtless been impacts relating to employee exposure. At the microlevel, you as the individual components manufacturer have to evaluate the menu of process options available in terms of performance, economics, and employee safety. At the same time, environmental regulation is becoming an increasingly complex task. The regulators and policy makers need your input. Otherwise, we will all continuously need to reformulate and redesign, moving from one crisis to the next, without necessarily seeing a net environmental improvement.7 REFERENCES 1. Kanegsberg, B. and LeBlanc, C., The cost of process conversion, Proc. CleanTech, Chicago, IL, May 1999. 2. Petrulio, R. and Kanegsberg, B., Back to basics: the care and feeding of a vapor degreaser with new solvents, in Clean Tech ‘98, Rosemont, IL, May 21, 1998. 3. UNEP STOC, the Statute of the Committee, available at http://www.protonique.com/unepstoc/stocfile/stocstat.htm. 4. Available at http://www.protonique.com/unepstoc/stocfile/memblist.htm. 5. Dimitriades, B., Scientific basis of an improved EPA policy on control of organic emissions for ambient ozone reduction, J. Air Waste Manage. Assoc., 49, 631, 1997. 6. Carter, W.P.L., Current status of reactivity research, presented at the Photochemical Reactivity Workshop Durham, NC, May 12 –14, 1998. 7. Carter, W.P.L., Development and evaluation of an updated detailed chemical mechanism for VOC reactivity assessment, presented at the A&WMA 93rd Annual Conference and Exhibition, Salt Lake City, UT, June 18 –22, 2000. 8. Kanegsberg, B., Safety, the environment, and profitable manufacturing, presented at EIA/EIC Conference, San Francisco, CA, October, 1994. © 2001 by CRC Press LLC

CHAPTER 6.2

Critical Cleaning—Working with Regulators—From a Regulator’s Viewpoint Mohan Balagopalan

CONTENTS History Case Situation Communication Gaps Conclusion HISTORY Communication is the one of the biggest challenges faced by all parties concerned with critical cleaning applications. In the past, when the use of chlorofluorocarbons (CFCs) and 1,1,1-trichloroethane (TCA) were ubiquitous as cleaning solvents and their usage was encouraged by the air agencies, the regulatory issues were much simpler to communicate. These solvents were considered unreactive organic gases, were considered not responsible in the formation of photochemical smog (ozone) in the air basins, and, therefore, were exempt from most of the regulations. Permits for degreasing equipment using these solvents were issued by the air agencies and there were no restrictions on usage, especially for CFC-113. TCA typically contained a cosolvent, 1–4-dioxane, approximately 5% by weight, and this cosolvent was considered photochemically reactive. The reactive hydrocarbon emissions calculated from the degreaser using TCA were based on 1–4-dioxane weight fraction and TCA usage. The physical chemistries of these solvents—low boiling point, low toxicity, nonflammable, fast drying time—resulted in large consumption of the solvents in the cleaning industry. The air permits for degreaser using CFCs and TCA were fairly standard and issued without much difficulty. However, in 1974 , two scientists presented a paper on the role of CFCs in the depletion of the ozone layer and this launched an international debate on the destruction of the ozone layer and further studies and experiments. The Food and Drug Administration (FDA) was the first U.S. agency to act and on October 15, 1976, announced a proposal to phase out all nonessential uses of CFC propellants in food, drug, and cosmetic products. The following year, the FDA, EPA, and the Consumer Product Safety Commission (CPSC) announced

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that CFC use in spray cans would be phased out and banned within 2 years, except for some limited use such as in asthma medication and few other essential products. This ban did not have much immediate effect on industries using CFCs for solvent degreasing. Eventually, as more data were gathered, further impacts of the CFCs in causing global warming were presented. To prevent further deterioration of the ozone protective layer, representatives from 27 countries including the United States signed an agreement in 1987 pledging to reduce and eventually stop the production of solvents that deplete the ozone layer. This global agreement, which was quite unprecedented with the participation of many industrialized nations and developing countries, was known as the Montreal Protocol as the conference was held in Montreal, Canada. Responding to the data on the destructive nature of the CFCs on the protective ozone layer in the stratosphere, the U.S. Congress in 1990 also directed the EPA to protect the ozone layer through several regulatory and voluntary programs in Title VI of the Amendments to the Clean Air Act of 1990. In addition, under Title VI, Section 612 of the Clean Air Act, the EPA established the Significant New Alternatives Policy (SNAP) Program. The SNAP mandate is to identify alternatives to ozone-depleting substances (ODS) and to publish a list of acceptable and unacceptable substitutes. These substitutes are reviewed on the basis of ozone-depletion potential, global-warming potential, toxicity, flammability, and exposure potential. The SNAP program makes decisions on a particular substitute in a particular end use within the sectors identified by the EPA: refrigerants, foam blowing, solvent cleaning, fire and explosion protection, aerosols, sterilants, tobacco expansion, adhesives, coatings, inks and pesticides. What is unique about the SNAP program, in this author’s opinion, is the consideration of all the effects of the substitute solvent and not a piecemeal approach, which the air agencies have been accused of. The approval or disapproval of a substitute solvent through the SNAP program can be lengthy, allowing the vendor of the substitute solvent to market the solvent if no decision is made within 90 days after the petition for acceptance is made to the EPA with the proper documentation. Unfortunately, because of this interim legality to market, in some cases solvents have been marketed as “EPA approved” even though they were only pending approval, and were eventually disapproved by the EPA. This causes more problems for the industry as users would have to start anew looking for a substitute solvent if they had started using it. This is another example of miscommunication, where all the facts about the SNAP program were not communicated or understood. In the United States, the production phaseouts of CFCs, halons, carbon tetrachloride, and methyl chloroform (TCA) were accelerated and were to be phased out by December 31, 1995. With the accelerated phaseout of the CFCs and TCA, the prices of these solvents, still legal to use but no longer produced, started to rise and companies were forced to scramble to find cheaper alternatives that met regulatory requirements. Traditionally, before the 1990 Amendments to the Clean Air Act, the emphasis of air quality regulations had been on the control of the “criteria” pollutants that included reactive hydrocarbons, oxides of nitrogen, oxides of sulfur, carbon monoxide, particulate matter less than 10 µm, ozone, and seven hazardous air pollutants (HAPs). The reactive organic solvents used in the cleaning industry were thus regulated. This included perchloroethylene (PCE), trichloroethylene, and acetone. Methylene chloride and TCA were also used as degreasing solvents, but since these were unreactive organic solvents, their use was actually encouraged until much later when methylene chloride was identified as a toxic air contaminant by the California Air Resources Board and as an HAP by EPA and TCA was phased out as an ODS. Ironically, PCE and acetone, which were regulated as reactive hydrocarbons, are now classified as unreactive hydrocarbons and PCE use as a degreasing solvent has increased. Acetone is used in cleaning solvent formulations to reduce the over© 2001 by CRC Press LLC

all volatile organic compound (VOC) content of the mixture, but because of its high flammability properties, its use is limited, unlike PCE. PCE use in solvent cleaning is regulated in the National Emission Standard for Hazardous Air Pollutant for Solvent Cleaning (NESHAP). In the NESHAP for vapor degreasers, there are a number of options to comply with the equipment standards (increased freeboard height, superheated system, increased dwell time, freeboard refrigeration, etc.), but there are no limits on the actual usage of the solvents regulated. In the South Coast Air Quality Management District (SCAQMD) in Southern California, PCE was regulated as a toxic air contaminant as of September 8, 1998, which drastically limited its emissions, practically making it impossible to use unless in an airtight or airless cleaning system and unless a health risk assessment is conducted. Methylene chloride and trichloroethylene were listed as toxic air contaminants in June 1990 and their use has been restricted since. The SCAQMD regulation for the control of VOC emissions (Rule 1122) from solvent cleaning operations and Best Available Control Technology Requirements (BACT) has been modified and now has some of the most stringent requirements anywhere in the country. Solvent containing VOC greater than 50 g/l used in a degreaser must be used either in an airless or airtight system for cold and vapor cleaning. This rule may be further modified. With the multitude of restrictions on the use of solvents for critical cleaning operations, it becomes even more important that proper communication occurs among the industry, the regulatory agencies, the environmental organizations, and the public. So how can communication be improved? The Internet is the new communication medium and there are a prolific number of Web sites that have information on solvent cleaning. The EPA and most air quality management districts have their own Web sites but oftentimes the information is scattered in these sites and there is no central repository for specific information, such as “What are the permitting requirements for vapor degreasing in [place]?” The degreaser equipment manufacturers and solvent manufacturers have their own Web and portal sites with information on the criteria for the selection of their equipment or their solvent properties and chemistries, in addition to offering complete solutions for critical cleaning. However, very few Web sites are available yet that make it easy for someone to determine the regulatory and fiscal impact of a solvent selection. Another useful source of information is attendance at trade shows, seminars, and workshops, where specific information can be obtained. At these events, the vendors, the manufacturers, and regulators meet, discuss the issues, and share information. While all these portals of information are good sources, frequently it is best to go to the source for the regulations, the regulators. There is some reluctance to do so because the agencies are perceived to be bureaucratic and it is sometimes difficult to find the “right person” to talk to. So how does one contact a regulator and approach the agency for information and get connected with the person or persons most knowledgeable on the cleaning processes and regulations? Asking the wrong questions or talking to the wrong persons may lead to inaccurate or misleading information that could prove costly in the long run.

CASE SITUATION In the SCAQMD, the use of methylene chloride (MC) was encouraged by permitting engineers, as a replacement for CFCs and TCA, as it is an unreactive organic solvent and it did not contribute to the formation of photochemical smog. Its use was later restricted (June 1990) when MC was listed as a toxic air contaminant in the agency‘s new source rule for air toxics and the cancer risk from its usage was considered high even for a moderate © 2001 by CRC Press LLC

user of the solvent. Permits that were issued prior to the effective date that MC was listed as a toxic air contaminant were considered “grandfathered” and were not affected. It only affected the permitting of new cleaning machines or those increasing their usage. In this situation, permits were issued for the use of solvent that was later determined not to be an ideal choice. An opposite situation to this occurred with the use of perchloroethylene (PCE) as a cleaning solvent. PCE was considered a reactive hydrocarbon and it use was regulated as such, whereby emission offsets were required and technology standards and practices were required. It was identified as a hazardous air pollutant by the Clean Air Act of 1990 as a probable human carcinogen. In California, the Air Resources Board listed PCE in 1991, as a toxic air contaminant pursuant to its Air Toxics Program (AB 1807) and Proposition 65. At the local (SCAQMD) permitting agency level, PCE was not listed as a toxic air contaminant until September 1998 and subject to health risk assessment. In 1994, the EPA adopted the National Emission Standards for Hazardous Air Pollutants for Solvent Cleaning. This regulated the use of PCE and established emissions standards and equipment design and work practices. In 1996 the EPA delisted PCE as a volatile organic compound and the SCAQMD followed suit in June 1997. When this delisting occurred at the SCAQMD, industries looking for a suitable substitute rushed to PCE, as there were no regulations limiting its use, in the interim, until it was added to the toxic list, September 1998. Industries that switched to it were looking for a drop-in replacement without any obstacles in obtaining an air permit. In this situation, permits were issued because the regulations during this period, June 1997 to September 1998, allowed them even though it was known that PCE was a listed toxic air contaminant. In the first situation, when the use of MC was encouraged, its toxicity and cancer-causing potential were relatively unknown to the permitting staff. In the second situation, the potency of PCE was known and it was, one hopes, communicated to the permit applicant that there were other ramifications to its use, such as the NESHAP and OSHA permissible exposure limit. COMMUNICATION GAPS What role can the regulator play in the process of solvent and equipment selection? Should the regulator be consulted and made a partner early in the selection process? Can the regulator be trusted to be impartial and forthcoming in providing information? This is one area where, due to poor communication, the regulator is oftentimes ignored, considered to be an adversary, and not considered early in the selection process. The regulator frequently becomes involved only when he or she has to work on the permit application for approval of the permit with the chosen solvent. The operator may have chosen the solvent based on its solvency, its cost, the capital cost of new equipment if needed, the ongoing maintenance and repair cost, the cost of insurance, employee training, and customer satisfaction and with some assurance that the permit would be approved. A permitting engineer responsible for issuing permits may have limited knowledge of the cleaning processes or the reasons behind the selection of the cleaning solvent; however he or she is normally familiar with the rules and regulations and evaluates the permit application accordingly. A compliance inspector during a normal site inspection is concerned mainly with verifying compliance with the permit conditions and whether the requirements for record keeping are met.

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Typically, unless asked, the engineer does not volunteer information and there is no expectation of the regulator to offer advice or provide information unless specifically requested. Alas, in many cases the requests are not made. Similarly, due to lack of communication and trust, the learning and the gathering of knowledge by the permitting engineer or compliance inspector of the various cleaning processes from the regulated source are not maximized. Thus, the opportunity for both sides to benefit from each other about the processes and regulations is lost. To bridge the communication gap, both parties have to trust each other and take the initial steps in getting together to meet and confer on the issues. From the regulator, the industry should ask for participation and assistance and to be a partner in the selection of a suitable solvent. From the industry, the regulator should learn the process and the different items that have to be considered in the selection of a solvent. From the regulators, there must be better customer relationship management and more willingness to impart information and share knowledge. From the industry, there should be more trust and openness to ask questions and to solicit information. Quite often, it appears to the regulator that the decision to select a cleaning system or a solvent is made strictly for economic reasons when evaluating a permit application. In reality numerous other considerations, such as health and safety of the workers and insurance cost, may have been factored in the decision to select a cleaning process or solvent. Some forums where informal communications occur are in seminars, workshops, conferences, and at association meetings. A strong association with active members in the regulatory arena that liaisons between the regulated community and the regulators is an effective way to communicate the concerns and needs of the cleaning industry. It is possible for misinformation and misleading information to be provided by the regulator unintentionally, because of lack of knowledge or misunderstanding of the questions posed. Sometimes there is the “shopping around” for answers from different regulators from consultants or a company representatives until they get the information they want to hear. This can create problems and confusion later down the road. CONCLUSION Facilities that integrate environmental compliance as a strategic component of business have a better advantage over those that do not since the former would have consulted with the regulatory agency in a proactive manner to determine the necessary steps to ensure continued compliance. This could be in the form of self-audit programs, participation in International Organization for Standardization (ISO) 14000, no-fault compliance inspections that are available from some agencies, or in periodic meetings and communications with the regulators. Improving industry–government relations by having more open communication, sharing solutions, and working together to solve problems fosters development of sound regulations that facilitates permit issuance and compliance.

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CHAPTER 6.3

Lessons Learned from the Phaseout of Ozone-Depleting Solvents Stephen O. Andersen

CONTENTS Introduction EPA Congratulates Industry for Ozone Layer Protection After the Montreal Protocol Was Signed, Industry Changed Direction Remarkable Partnerships Helped Protect the Ozone Layer Military Leadership Was Important Electronics Companies, Associations, and Committees Became Environmental Leaders No-Clean Soldering Was a Remarkable Team Success Developing Countries Are Doing Their Part Governments Credit Industry with Ozone Layer Protection Lessons from the Montreal Protocol for Climate Change Proactive Companies Are Already Working to Protect the Climate Conclusions and Climate Leadership Opportunities INTRODUCTION This chapter describes how electronics and metal product manufacturers have innovated to protect the stratospheric ozone layer. It outlines how the lessons from the Montreal Protocol can be applied to the challenge of protecting the fragile climate under the Kyoto Protocol. EPA CONGRATULATES INDUSTRY FOR OZONE LAYER PROTECTION Cleaning and assembly experts have earned a fully deserved global reputation for ozone layer protection. Electronics, aerospace, dry cleaning, and other experts participated in the United Nations Environment Programme Technology and Economics Assessment Panel (UNEP TEAP). Industry worldwide developed, shared, and implemented technology so rapidly that almost no regulations specific to solvents were needed. © 2001 by CRC Press LLC

AFTER THE MONTREAL PROTOCOL WAS SIGNED, INDUSTRY CHANGED DIRECTION In the early 1980s, industry initially opposed ozone layer protection in much the way some industry opposes climate protection today. During the debate before the signing of the Montreal Protocol, manufacturers of chlorofluorocarbons (CFCs) and many of their industrial customers fought aggressively against regulations. Industry argued that scientists had not yet proved that CFCs destroyed stratospheric ozone, that the products made with or containing CFCs were absolutely vital to society, that there were no safe substitutes, and that potential substitutes would be ineffective and costly. At the 1987 Montreal meeting, only the U.S. Air Force and a few small businesses expressed technical optimism that sufficient alternatives and substitutes could be successfully introduced to satisfy the 50% reduction in CFC use and the freeze in halon production prescribed by the first Protocol control schedule. Fortunately, industry then began listening more carefully to the scientific findings linking ozone depletion to CFCs, halons, and the other potent ozone-depleting substances (ODSs). In late 1987 and early 1988 leading multinational companies began announcing corporate goals to help protect the ozone layer by halting their use of ODSs. They mobilized their technical experts to seek solutions, motivated suppliers to offer alternatives, and commercialized and implemented new technology that eliminated the need for ODS. Corporate support and the availability of technical solutions enabled the parties to the Montreal Protocol to make strong political decisions to expand the list of controlled substances and to accelerate the schedules for phasing them out. REMARKABLE PARTNERSHIPS HELPED PROTECT THE OZONE LAYER Remarkable partnerships were formed as corporate and military technical experts began working more closely with regulators. These partnerships allowed business to adopt more cost-effective alternatives more rapidly than would have been possible using a “command-and-control” approach, or other traditional regulations. Corporate leadership included a variety of unprecedented initiatives. These included: • Dramatic announcements of CFC-free products—such as the announcement as early as 1975 by S. C. Johnson Company that it was phasing out CFCs as aerosol propellants and switching to hydrocarbons. • Public announcements that CFCs destroy the ozone layer—such as the aboutface by DuPont in 1986, when it accepted that there was sufficient scientific evidence and began to advise customers to seek alternatives to ODSs. • Breaking ranks with industry associations—such as the 1988 announcement by AT&T that a new solvent made from oranges cleaned as effectively as CFC-113. • Setting challenging goals—such as in public pledges by Nortel/Northern Telecom and Seiko Epson in 1988 to phase out CFCs faster than was required. • Starting up organizations to speed the commercialization of alternatives, such as the Industry Cooperative for Ozone Layer Protection (ICOLP) and the Japan Industrial Conference on Ozone Layer Protection (JICOP). The EPA helped organizations to qualify under the Cooperative Research Act, allowing collaboration to protect the ozone layer with protection from antitrust concerns.

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MILITARY LEADERSHIP WAS IMPORTANT The U.S. military also surprised skeptics by acknowledging its responsibility as customers and spearheading technical innovation, green procurement, and market influence. Department of Defense (DoD) manufacturing standards requiring the use of CFC-113 had contributed to a global dependence on this potent ozone-depleting solvent. Many companies manufactured all their sophisticated, high-reliability products with CFCs to qualify for military sales. In response to this manufacturing customer demand, suppliers to electronics manufacturing perfected components, equipment, flux, solder, and other products to optimize performance. CFC-113 manufacturers offered valuable training and troubleshooting services to their customers. Because the DoD specifications became globally respected for quality assurance, many companies and some countries cited these specifications in manufacturing products for civilian markets. In effect, the DoD standard was a de facto global standard in 1987 when the Montreal Protocol was agreed. The DoD joined with the EPA and the Institute for Interconnecting and Packaging of Printed Circuits (IPC) in organizing the Ad-hoc Solvent Working Group to change the prescriptive standard requiring CFCs to a performance standard that would allow and ultimately encourage CFC-free assembly. In less than 2 years, the working group created a test board and procedure for measuring cleaning potential of solvent alternatives, benchmarked cleaning with CFC-113, established a test verification team to observe the cleaning tests, persuaded DoD to accept solvents that cleaned “as well or better than” the CFC benchmark, and verified the first solvents to pass the demanding test. As this project was proceeding, the working group experts concluded that solvent-free assembly with conductive adhesives and “no-clean” flux could be an additional alternative to CFC solvents. By the time the first solvents were passing the test, DoD had revised the standard to encourage CFC-free assembly and had written the performance standard to accommodate innovative solvent-free technology. Later, DoD prohibited cleaning with CFCs unless highlevel DoD exceptions were granted. DoD was so technically confident in the CFC phaseout that it cosponsored technical seminars in cooperation with the North Atlantic Treaty Organization (NATO). By 1992, leading military experts had recognized ozone layer depletion as a national security threat requiring the strongest actions. NATO took the unprecedented step of writing directly to the executive director of the United Nations Environment Program, advocating a complete phaseout of substances that deplete the ozone layer. ELECTRONICS COMPANIES, ASSOCIATIONS, AND COMMITTEES BECAME ENVIRONMENTAL LEADERS Nortel/Northern Telecom and Seiko Epson were technically optimistic in 1988 when they became the first electronics and precision manufacturers to pledge a phaseout of CFC solvents, but they soon realized that they needed the creative force of the entire industry to achieve their goals. AT&T, Nortel, and the EPA founded the ICOLP to encourage competing companies to cooperate on the development and implementation of environmentally protective industrial technologies. The members included respected multinational companies that together exercised extraordinary market clout: AT&T, Boeing Company, British Aerospace, Compaq, Digital Equipment Corporation, Ford, Hitachi, Honeywell, Hughes Aircraft Company, IBM, Lockheed-Martin, Matsushita Electric, Mitsubishi Electric, Motorola, Nortel/Northern Telecom, Ontario Hydro, Seiko Epson, Texas Instruments, and Toshiba. ICOLP helped to fast-track implementation of innovative technologies by foster-

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ing a spirit of collaboration rather than competition among industry rivals and then transferred their successes by sharing their findings with the electronics industry worldwide. Digital Equipment Corporation (DEC) and Nortel/Northern Telecom were the first companies to donate patented technology to the public domain to speed ozone layer protection. Dozens of companies opened new production facilities that were ODS-free. Public tours and technical cooperation projects helped promote use in all countries. Industry participation was also key to the success of the United Nations Environment Programme (UNEP) Solvents, Coatings and Adhesives Technical Options Committee (TOC), which advised the parties to the Montreal Protocol on alternatives to replace CFC-113 and 1,1,1-trichloroethane (methyl chloroform). The Solvents TOC created a forum for identifying and documenting promising technology, in addition to increasing global awareness of the role of the electronics industry in ozone layer protection. Its contributions have been critical in the development of domestic ozone protection regulations by many country governments.

NO-CLEAN SOLDERING WAS A REMARKABLE TEAM SUCCESS The Solvents TOC exemplifies the power of proactive thinking and cooperation. In 1989, the German company SEHO demonstrated a controlled atmosphere soldering technology to the Solvents TOC, which immediately recognized its potential but realized that the equipment developers had only basic soldering skills. Upon close examination, the Solvents TOC realized that the component parts were oxidized, the flux was mismatched, and the soldering wave was poorly formed. However, the promise of the technology was so compelling that Solvents TOC members from AT&T, Ford, and Nortel/Northern Telecom persuaded their companies to experiment with the technology. Some experts concentrated on the flux composition, while others believed that the flux was chemically suitable and worked on ways to apply it to the board more precisely. Still others concentrated on the mixture of gases in the soldering chamber. Several months into development, one company discovered that gas monitoring and control calibrations were critical. Better calibrations dramatically improved the soldering quality. The team consulted with flux suppliers who grasped the opportunity for developing and commercializing new products and intensified development in cooperation with the electronics manufacturers. The EPA encouraged the work by documenting and publishing the global environmental advantages that such a technology could provide. Meanwhile, engineers at AT&T Bell Laboratories were developing state-of-the-art spray fluxing machines to apply precisely the optimal amount of flux to the locations in the printed circuit boards where necessary. Nortel/Northern Telecom was developing equipment to verify flux concentrations on production boards, and Motorola was experimenting with soldering ultraminiaturized circuits with hybrid components including optical devices and flexible connectors. One by one, companies satisfied internal quality controls and moved from laboratory scale to pilot scale and finally to full implementation. During implementation, experts from the intercompany team continued to cooperate to debug operations and optimize performance. It is impossible to say just when the engineering team realized that its no-clean technology would revolutionize electronics assembly. Engineers who had cautiously reported as-good performance, began to report improved performance. Line managers cautiously increased the speed of soldering to rates never achieved with conventional soldering and found that in some cases defect rates actually decreased. The EPA in cooperation with ICOLP/ICEL published the first no-clean handbook to make the expertise and technology available worldwide. © 2001 by CRC Press LLC

DEVELOPING COUNTRIES ARE DOING THEIR PART Although the Montreal Protocol allowed a 10-year grace period for developing countries, some countries set their own, more ambitious schedules. In 1989, Mexican environmental and industry leaders, concluding that protecting the ozone layer was too important to wait, jointly declared that they preferred to proceed at the same pace as developed countries; this would enable them to leap-frog inferior and obsolete CFC equipment and use cutting-edge technology. Mexico formed a technology cooperation partnership with Nortel to transform its electronics industry. Therefore, its electronics industry halted CFC uses faster than most European companies and built some of the world’s first new CFC-free factories— including AT&T and Nortel plants that manufactured products without using any solvents. Thailand went one step farther. The government of Thailand and the UNEP Industry and Environment Regional Office surveyed domestic use of ODS and presented the results at a conference in Singapore in 1991, finding that foreign companies were responsible for more than three quarters of the ODS use in Thailand. Approximately 50% was by Japanese companies, 25% by U.S. companies, 5 to 10% by European companies—leaving the remaining 15 to 20% to Thai companies and such local uses as automobile air-conditioning service. The Thai government therefore asked foreign companies to propose a cooperative solution. A plan emerged to ask multinational companies operating in Thailand to pledge to phase out ODS there no later than 1 year after they did so in their home company or country. Workshops were organized to demonstrate CFC-free technology, and suppliers were challenged to offer it to Thai companies. GOVERNMENTS CREDIT INDUSTRY WITH OZONE LAYER PROTECTION The Montreal Protocol is an extraordinary diplomatic achievement that was made possible by excellent scientific research and advocacy. National governments are also justifiably proud of their contributions to the ODS phaseout. However, industry, and industry alone, deserves credit for the technical innovation and global business cooperation that is making the Protocol an environmental success. Throughout the world governments have given credit to industry for this leadership, and in the United States, the EPA created its first international award to recognize exceptional contributions to ozone layer protection. LESSONS FROM THE MONTREAL PROTOCOL FOR CLIMATE CHANGE This experience with the Montreal Protocol holds several lessons for the world as it confronts the problem of climatic change. • The controls originally introduced under the Montreal Protocol in 1987 motivated industry. Although little technology had yet been identified, industry moved quickly to commercialize alternatives and substitutes. The December 1997 Kyoto Protocol on Climate Protection may prove to be a similar precondition to gaining support from industry and to developing technology. • Corporate environmental goals focus company priorities and motivate suppliers—companies are beginning to set such goals for climate protection. British Petroleum has heeded the science and announced that it is now time to act to protect the climate. Dow has set a goal of improving energy efficiency by 2% a year and Mitsubishi Electric and Philips have announced targets of increasing it by 25% by 2010. General Motors has introduced an electric vehicle, Honda has an

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ultralow-emission vehicle, Toyota is marketing a hybrid vehicle, and Mitsubishi has commercialized direct fuel injection. ENRON has developed a portfolio of climate-friendly energy supply technology for its customers. Whirlpool has commercialized the most-energy-efficient refrigerator in the world and is advocating stringent regulation to motivate its competitors. • Partnerships and associations between industry and government attract companies taking leadership. The ICOLP has been reorganized as the International Cooperative for Environmental Leadership with a climate protection agenda. The organizers of the Alliance for Responsible Atmospheric Policy have formed the International Climate Change Partnership to guide climate negotiations toward flexible, performance-based regulations. • Information is critical. Industry experts on the Montreal Protocol Assessment Panel catalogued and evaluated the best technologies. The UNEP Industry and Environment Programme office in Paris provided publications, databases, and online access to information—and organized networks and regional offices to guide the selection of technology. The Convention on Climate Change has now reached the stage where increased industry participation is needed and it is considering how UNEP can repeat its success in providing information that helped to protect the ozone layer. PROACTIVE COMPANIES ARE ALREADY WORKING TO PROTECT THE CLIMATE Encouraged by environmental nongovernment organizations (NGOs), governments, and the public, companies have developed a well-earned reputation for their leadership and technology cooperation. Some of the extraordinary accomplishments on climate protection include the following. The semiconductor production process uses PFC and SF6. Partnerships in Japan and the United States began working together several years ago to promote research and development to reduce these gases. This spring, the World Semiconductor Council announced that companies representing over 90% of global semiconductor manufacturing have pledged to reduce PFC emissions 10% by 2010, even as semiconductor output will increase substantially. In solving environmental problems, action by industry is essential, and the announcement of corporate leadership helps avoid command-and-control regulation. Voluntary programs such as Green Lights and Energy Star are now being globalized from their EPA origin. CONCLUSIONS AND CLIMATE LEADERSHIP OPPORTUNITIES Technical experts and their companies who have protected the ozone layer can build on their global reputation by contributing to climate protection. Electronics are the key to improvements in manufacturing process efficiency, key to products that save energy, and key to efficient organization to conserve resources. Furthermore, business can help government with market transformation to more sustainable commerce. A “Watch List” of technologies important to the environment and the bottom line include:

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1. New HFE and HFC solvents—safe for the ozone layer, performing like CFC-113, and with the prospect for climate impact (emissions effect plus energy consumption); 2. Vistion’s Superintegration™ of no-clean soldering, flexible circuits, and product structural surfaces in ways that reduce weight and materials use, save energy, simplify assembly and disassembly, accelerate electronic functions, and provide design and manufacturing flexibility, which allows solutions that were previously impossible; and 3. E-commerce, telecommuting, and information systems that reduce travel, improve productivity, and enhance quality of life. Remember, technologies alone cannot build the momentum necessary to protect the climate. People, as always, are the force of decision, persuasion, innovation, and implementation. People protected the ozone layer and people can protect the climate. Electronics engineers and managers are invited to spearhead that noble effort.

© 2001 by CRC Press LLC

CHAPTER 6.4

Screening Techniques for Environmental Impact of Cleaning Agents Donald J. Wuebbles and Reva Rubenstein

CONTENTS Introduction Focus of this Chapter The EPA SNAP Program Good Ozone vs. Bad Ozone Air Quality and VOCs Concerns about Stratospheric Ozone: Ozone Depletion Potentials Concerns about Stratospheric Ozone: Equivalent Chlorine Loading Concerns about Climate Change: Global Warming Potentials Summary and Conclusions Acknowledgment References INTRODUCTION* To many in the manufacturing world, environmental policy may seem to be a series of lists of cleaning agents and compounds that can no longer be used, of compounds that can be used only with extensive and costly controls, or, as in the case of ozone-depleting compounds (ODCs), of compounds that will be available for a limited time, at high cost, and of perhaps doubtful quality. In fact, determining the environmental impact of a given compound, weighing the relative risks to human health and to community health, and then deciding on the appropriate regulatory status of that compound is a monumental task. Scientists in specialized diverse fields must evaluate the compound in question. Science is not the only factor considered. Certainly economics and, in this author’s (BFK) opinion, politics, also plays a role. Scientists determine the ozone depletion potential (ODP) and impact by the best available modeling and experimental techniques. Then Congress, in the United States, and an * The introduction has been written by B. Kanegsberg at the request of Drs. Wuebbles and Rubenstein.

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international consortium of scientists, policy makers, and representatives of chemical manufacturers in United Nations policy committees, globally, ultimately determine what ODP level will be significant. However, it must be emphasized that scientific input has a very strong influence on environmental policy. Scientists and policy makers use a variety of approaches and techniques to analyze the potential environmental effects of chemicals, including cleaning agents. A number of factors affect the screening approaches that need to be applied. These factors then drive a series of questions to be addressed: • Could the agent potentially affect stratospheric ozone? Does it contain chlorine, bromine, or iodine? • Is the substance toxic? To what degree, and what is the nature of the toxicity? • What is the vapor pressure of the agent (will it go to a gas at standard temperature and pressure)? • What is the atmospheric lifetime of the agent? • Will the agent be a volatile organic compound (VOC) and thus potentially affect urban/regional ozone? • Is the agent a greenhouse gas? Could the agent potentially affect climate? • In what processes will the agent be used? • Will the agent be used emissively or can it be contained for repeated use? • If introduced, is the agent likely to replace agents with a more favorable environmental profile? • What is the likely production level of the agent? This chapter is a collaboration of two noted and influential scientists, one an atmospheric scientist and the other a toxicologist. The goal is to focus on just a few of the issues related to substitutes for Class I ODCs, particularly the chlorofluorocarbons (CFCs) and 1,1,1-trichloroethane (TCA). The depth of understanding and the requirements to evaluate, meld, and determine the relative importance of various factors should provide the manufacturer with a flavor of the difficulty involved in determining environmental policy. Further, determining the likely risk to the world of a given compound requires looking into the future. It means taking the best guess possible. This vexes some engineers who want definite answers based on completely defined parameters. Thus, individuals will continue to write that depletion of the ozone layer has not been proved, that air quality may not be impacted by VOCs. As someone with a background in biology, this author can only say: get over it. Unlike physics, atmospheric science and biology are sufficiently multifactoral and complex that one needs to evaluate available and often circumstantial evidence. In doing this, policy makers must be a bit conservative. After all, at present, we have only one Earth to experiment with.

FOCUS OF THIS CHAPTER This chapter focuses primarily on the areas of concern to the EPA SNAP program in evaluating replacement compounds, namely, concerns about toxicity, effects on urban and regional ozone, effects on stratospheric ozone, and effects on climate. Scientific analyses of current and possible future changes in ozone are largely based on complex, numerical models of the chemical and physical processes controlling the atmosphere. Similarly, analyses of possible future changes in climate largely depend on complex, numerical, three-dimensional models of the global climate system (atmosphere, oceans, © 2001 by CRC Press LLC

and land surface) that attempt to represent the many processes controlling climate. These models are computationally expensive and, given current computer capabilities, severely limited in the number of calculations that can be and have been examined. As a result, several approaches have been developed to examine and evaluate gases for their potential effects on global atmospheric ozone and climate. The concepts of ozone depletion potentials and global-warming potentials are discussed.

THE EPA SNAP PROGRAM As solvents or solvent blends that are composed of or contain ODSs are phased out under the Clean Air Act (CAA), it is critical that replacements be found. These replacement solvents must have reduced (in the case of HCFCs, interim substitutes for CFCs) or no ODP and must not pose other unacceptable risks to human health or the environment. In particular, CAA Section 612(c) directs the EPA to prohibit users of CFC-containing solvents from replacing these ODSs with any substance that the EPA has determined may present adverse effects to human health and the environment. These replacement substances, or “substitutes,” are defined as any chemical, product substitute, or alternative manufacturing process, existing or new, that could replace a substance that has the potential to deplete the stratospheric ozone layer. In evaluating the acceptability of substitutes the EPA must review the overall risk to both the environment and to human health resulting from exposure to consumers, workers, and the general population for any proposed substitute. The important factors considered by the EPA in these evaluations are (1) the ability of the chemical to deplete the stratospheric ozone layer and/or to contribute to global warming; (2) the environmental fate and transport of substitutes, including information on bioaccumulation, biodegradation, adsorption, volatility, and transformation; and (3) whether the substitute is a VOC and thus a potential contributor to tropospheric smog. To evaluate these risks, the EPA has established the Significant New Alternatives Policy (SNAP) program to ensure that substitutes for ozone-depleting solvents pose lower aggregate levels of risk than the substances that they replace. To accomplish this goal, the EPA conducts a screening-level assessment of the health and environmental risks of the ozonedepleting solvents for which substitution is proposed. These risk screens combine conservative assumptions about worker/consumer/general population exposure levels and the toxicity of the substance to estimate human health risks resulting from use of the substitute in a particular application, such as solvent cleaning. The future atmospheric impacts of substances with reduced or no ODP are assessed relative to a “no substitution” baseline that assumes continued use of the ODS. These atmospheric impacts are modeled by the Atmospheric Health Effects Framework (AHEF), which forecasts changes in stratospheric ozone concentrations based on estimated future ODS emissions. The model then predicts changes in future skin cancer and cataract incidence and skin cancer mortality based on increased ultraviolet (UV) exposure. Because substitutes usually have ODPs less than the materials they replace, the atmospheric impacts of these substitutes are generally found to be less than the continued use of the ODS. Many proposed substitutes have zero ODP but may lead to other adverse health effects. To assess the overall risks of exposure from these substitutes to workers, consumers, and the general population, the EPA considers a wide range of factors. Information from a variety of animal models is required on the acute and chronic toxicity of a substitute chemical and its potential impurities. The EPA requests a minimum submission of the following mammalian tests: a range-finding study conducted using the appropriate exposure © 2001 by CRC Press LLC

pathway for the specific end use (e.g., inhalation, oral, etc), and a 90-day subchronic repeated dose study in an appropriate rodent species (e.g., rats or mice). For some substitutes, a cardiotoxicity study, usually measuring cardiotoxic effects in dogs, is also required. The EPA may request additional toxicity tests on a case-by-case basis, depending on the particular substitute and application being evaluated. In addition to evaluation of primary literature, the EPA also reviews various occupational and consumer exposure guidelines, deriving missing parameters where possible. Typically, the following parameters are reviewed for each chemical: short- and long-term occupational exposure limit, cardiotoxic lowest-observed adverse effect level (LOAEL) and no observed adverse effect level (NOAEL), reference concentration, and cancer slope factor. If the initial risk screen indicates a potential health or environmental risk, in-depth evaluations are conducted to ascertain whether the risk was accurately estimated and if management controls could reduce the risk to acceptable levels. In cases where a substitute poses a significant risk that does not seem likely to be abated by alternative methods, the EPA can examine the risk screen analyses and decide to classify the substitute as unacceptable in certain end uses.

GOOD OZONE VS. BAD OZONE Ozone, O3, is composed of three oxygen atoms and is a gas at atmospheric pressures and temperatures. Most of the ozone (about 90%) exists in the stratosphere, the layer of the atmosphere about 10 to 50 km above the Earth’s surface. The remaining ozone is in the troposphere, the lower region of the atmosphere extending from the Earth’s surface up to roughly 10 km at mid-latitudes and 16 km in the tropics. Ozone in the stratosphere is largely formed naturally following the dissociation of oxygen molecules by sunlight. In the lower atmosphere, ozone is also a major component of photochemical smog in urban areas. While the ozone in the troposphere and stratosphere is chemically identical, it has very different effects on life on the Earth depending on its location. Stratospheric ozone, the “good” ozone, plays a beneficial role by absorbing solar UV radiation (UV-B), preventing biologically harmful levels of UV radiation from reaching the Earth’s surface.9,13,15 It is the absorption of solar radiation by this ozone that explains the increase in temperature with altitude in the stratosphere. It is also the concern about increased UV-B from the decreasing levels of ozone that has been the driver for policy actions to protect the ozone layer. Closer to the Earth’s surface, ozone displays its destructive side. Ozone is a strong oxidizer, hence, direct exposure to high levels of ozone has toxic effects on human health and plant viability.9 Thus, the “bad” ozone formed in urban areas is also of significant concern to policy makers. In recent years it has become progressively clearer that human activities have been affecting the amount of ozone in the global atmosphere, including the formation of the ozone “hole” a significant decrease in the amount of ozone roughly over Antarctica during August through October. Atmospheric measurements indicate that the amount of ozone in the global stratosphere is decreasing, while that in the troposphere appears to have increased in the past but is relatively unchanged in the last decade.17 –19 Although ozone formed in urban smog is not thought to play a significant role in the global distribution of ozone, it may be affecting the troposphere at larger scales than the urban regions it is being formed in. Overall, the vertically integrated ozone column is decreasing globally; however, because of the effects of international policy to control the production and emissions of compounds that are affecting ozone, the maximum decrease should be reached within the next few years. Understanding the changes occurring to ozone and determining the appropriate © 2001 by CRC Press LLC

societal response have presented important challenges to scientists and to policy makers. Atmospheric measurements and associated analyses have clearly implicated chlorine and bromine from CFCs, halons, and other compounds as the primary cause of the decreases in stratospheric ozone that have occurred over recent decades. Although chlorine does exist in the stratosphere as a result of natural sources, the source of this chlorine, predominantly from methyl chloride (CH3Cl), can explain only about 0.6 ppbv (parts per billion by volume) of the 3.6 ppbv of reactive chlorine in the current stratosphere. Although many additional natural sources of chlorine exist on Earth (including sea salt and chlorine in volcanic emissions), most of this chlorine is water soluble and is washed out of the atmosphere by precipitation processes before reaching the stratosphere. On the other hand, CFCs are very longed lived, with atmospheric lifetimes of 50 years or longer, implying they are essentially inert and their concentrations well mixed in the troposphere. Once they are transported into the stratosphere, however, the presence of highenergy UV light allows photolysis to destroy them, thus releasing the chlorine they contain. It is this active chlorine atom which takes part in the catalytic reactions responsible for destroying ozone. The chlorine catalytic cycle can occur thousands of times before the catalyst is converted to a less reactive form. Because of this cycling, relatively small concentrations of reactive chlorine can have significant impact on the amount and distribution of ozone in the stratosphere. In the lower stratosphere, atmospheric and laboratory measurements indicate that heterogeneous chemistry on sulfate particles is leading to enhanced effects on ozone from chlorine by converting less reactive chlorine compounds to the reactive forms such as Cl and ClO.18 These heterogeneous reactions are a significant factor in explaining the enhanced ozone destruction in the lower stratosphere. The other major factor in the lower stratosphere comes from the effects of bromine chemistry and human sources of bromine compounds that reach the stratosphere. Bromine is potentially far more destructive toward stratospheric ozone than is chlorine. The bromine catalytic cycle is about 60 times more efficient than the chlorine catalytic mechanism at destroying ozone. However, since the emissions and amounts of brominated compounds in the atmosphere are much smaller than those of the chlorinated compounds, the impact from bromine on the current atmosphere is smaller, although not negligible, than the effects from increasing chlorine. CFC-11 and CFC-12 are of primary concern as they account for roughly half of the organic chlorine loading of the current atmosphere. Brominated compounds of importance include methyl bromide, CH3Br, and several halons (brominated halocarbons), such as H-1211 (CF2ClBr) and H-1301 (CF3Br). The total atmospheric burden of a halocarbon is determined both by its release rate and the atmospheric lifetime. The long atmospheric lifetimes and past increases in the rate of use have contributed to a sustained trend of increases in the concentration of CFCs. The recognition of deleterious effect of chlorine and bromine on ozone spawned international action to restrict the production and use of CFCs and halons, and to protect stratospheric ozone, such as the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer11 and the subsequent 1990 London Amendment14 and 1992 Copenhagen Amendment.16 These agreements called for elimination of CFC consumption in developed countries by the end of the decade. A November 1992 meeting of the United Nations Environment Program held in Copenhagen resulted in substantial modifications to the existing protocols because of large observed decrease in ozone, and called for phaseout of CFCs, carbon tetrachloride (CCl4), and methyl chloroform (CH3CCl3) by 1996 in developed countries. Production of these compounds is to be totally phased out in developing countries by 2006. Production of halons was stopped in 1994 in the developed countries. Human-related production and emissions of methyl bromide are not to increase after 1994. A UNEP-sponsored © 2001 by CRC Press LLC

meeting in Montreal in 1997 has further refined the Montreal Protocol, particularly in relation to future emissions of methyl bromide. AIR QUALITY AND VOCs In general, VOCs are defined as any compound containing carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions. The real concern with VOCs occurs, however, if the emission of a given VOC can contribute to the formation of ground-level ozone. For that reason, emissions of certain organic compounds, termed VOCs in the regulatory process, have been subject to controls for a number of years. The emissions of certain VOCs are also controlled or banned because they are toxic, can deplete the ozone layer, can contribute to formation of particulate matter, or for other impacts on the environment. The impact of a VOC on formation of ozone or other measures of air quality is often referred to as its atmospheric “reactivity.” VOCs vary by large factors in their ability to affect ozone. Because some VOC compounds have almost no effect in producing ozone, the EPA has had a policy of exempting some compounds from VOC regulations. However, the vast majority of compounds have been regulated as if they essentially had the same effect on ozone. Because of the recognized inadequacies of the current approach, a team of experts8 has been coordinating with the EPA in defining areas of research to assist policy makers in developing VOC emission control strategies based on the reactivity of individual species of VOC. At the time this was written, the recommendations of this working group were not yet complete. CONCERNS ABOUT STRATOSPHERIC OZONE: OZONE DEPLETION POTENTIALS The cleaning agents most likely to have a potential effect on stratospheric ozone are those containing chlorine, bromine, or iodine. These atoms react extremely efficiently with ozone in catalytical processes. Thus, chlorine, bromine, or iodine released in or transported to the stratosphere can destroy thousands of ozone molecules. The concept of ODPs arose as a means of determining the relative ability of a chemical to destroy ozone. Thus, a key screening approach relative to stratospheric ozone is to ask whether the chemical being considered contains chlorine, bromine, or iodine. If it does, then the ODP needs to be determined. If a compound does not contain chlorine, bromine, or iodine, then it most likely will not affect the stratosphere. Exceptions may occur if the compound could achieve such significant use that it could become a source of stratospheric nitrogen oxides, hydrogen oxides, or sulfuric particles. Such exceptions are unlikely in the case of solvents unless the chemical also gained extremely wide use for other applications. The concept of ODP17–21 provides a relative cumulative measure of the expected effects on ozone of the emissions of a gas relative to CFC-11, one of the gases of most concern to ozone change. The ODP of a gas is defined as the integrated change in total ozone per unit mass emission of the gas, relative to the change in total ozone per unit mass emission of CFC-11. Alternatively, the ODP can be derived by using a constant emission calculated to steady state relative to the same for CFC-11. Numerically, the two approaches are equivalent. As a relative measure, ODPs are subject to fewer uncertainties than estimates of the absolute percentage of ozone depletion caused by different gases. ODPs are an © 2001 by CRC Press LLC

integral part of national and international considerations on ozone-protection policy, including the Montreal Protocol and its Amendments and the U.S. Clean Air Act. ODPs provide an important means for analyzing the potential for a new chemical to affect ozone relative to CFCs, halons, and other replacement compounds. ODPs are currently determined by two different means: calculations from models, primarily from models of the global atmosphere,18,19 generally from zonally averaged twodimensional models, and the semiempirical approach developed by Solomon et al.10 The two approaches give similar results. The numerical models attempt to account for all of the known chemical and physical processes affecting chemical species in the troposphere and stratosphere. The compounds are assumed to enter the atmosphere at ground level, be transported in the atmosphere by dynamic processes, and react by a variety of pathways, depending on their molecular structure. The compounds may undergo photolytic breakdown by UV or near-UV light, react with OH in the troposphere and stratosphere, or react with atomic oxygen. Products resulting from these reactions, such as atomic chlorine and bromine, can react in the modeled atmosphere, which in turn may affect the calculated distribution of ozone. A major uncertainty in the models is the amount of tropospheric hydroxyl, OH. Since few reliable measurements of OH are available, the global distribution has not been directly measured. As a result, given the importance of the atmospheric lifetime in determining the ODP for a substance, those gases where reaction with tropospheric OH is the primary loss mechanism, as it is for many of the replacement compounds, a scaling to the partial lifetime of CH3CCl3 due to its reaction with tropospheric OH is used. Table 1 shows a partial list of derived ODPs for several compounds, including HCFCs and other gases where reaction with tropospheric OH is the primary loss mechanism. These ODPs are derived from various sources based on different scaling relative to the partial tropospheric OH lifetime of CH3CCl3. Table 1 does not contain any of the HFCs as they are thought to have insignificant effects on ozone.18,19 In the column after the list of gases, the ODPs from the WMO assessment18 are shown. These ODPs are based on a scaling relative to the partial tropospheric OH lifetime of CH3CCl3 of 5.7 years as has been recommended in recent studies27 and adapted in the most recent international assessment of stratospheric ozone.19 The effect is a significant decrease in the ODPs for these compounds relative to the earlier recommendations. Note that CF3I is included in Table 1 even though its destruction primarily occurs through photolysis and is therefore unchanged by the different scaling changes. Also, studies of the ODPs of chlorobromomethane (CBM) and 1bromo propane (n-propyl bromide, nPB) suggest that the best ODPs, with a scaling of 5.7 years, will be 0.083 to 0.098 for CH2ClBr and 0.026 for 1-C3H7Br.28 These values are appreciably different from those derived in an earlier study.25 However, a workshop sponsored by the EPA and NASA in March 199926 concluded that ODPs for short-lived atmospheric compounds (those with atmospheric lifetimes less than 6 months, such as nPB) need to be defined as a function of the location of their emissions. A recent study by Wuebbles et al.29 is the first to follow these recommendations. A summary of its preliminary findings for nPB, coupling the results of two- and three-dimensional models of the global atmosphere is given in Table 2. The semiempirical approach10,18 for determining ODPs is based on direct measurements of select halocarbons and other trace species in the stratosphere. The observed fractional dissociation is used to determine the amount of chlorine and bromine released and is then compared with the observationally derived ozone loss distribution, with the assumption that the ozone loss results only from halogen chemistry. The correlation between different compounds is determined on the basis of their relative reactivity in the troposphere and stratosphere. This semiempirical method avoids some of the demanding requirements of accurate numerical simulation of source gas distributions (i.e., of the CFCs, © 2001 by CRC Press LLC

Table 1 Steady-State ODPs Gas

Formula

ODP

Chlorofluorocarbons CFC-11 CFC-12 CFC-113 CFC-114 CFC-115

CCl3F CCl2F2 CCl2FCClF2 CClF2CClF2 CF3CClF2

1.0 0.82 0.90 0.85 0.40

Bromocarbons Methyl bromide Halon-1301 Halon-1211

CH3Br CF3Br CF2ClBr

0.37 12.0 5.1

HCFCs HCFC 22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HCFC-225ca HCFC-225cb

CHClF2 CF3CHCl2 CF3CHClF CH3CCl2F CH3CClF2 CF3CF2CHCl2 CClF2CF2CHClF

0.034 0.012 0.026 0.086 0.043 0.017 0.017

Others Carbon tetrachloride Methyl chloroform Methyl chloride

CCl41.20 CH3CCl3 CH3Cl

1.20 0.11 0.02

Source: WMO (1999).19

Table 2 Estimated ODPs for nPB as a Function of Location of Emissions Location of Emissions

Estimated ODP

Global land masses (60°S to 70°N) North America (to 70°N) United States North America, Europe, China, Japan (to 70°N) India, Southeast Asia, and Indonesia

0.033–0.040 0.018–0.019 0.016–0.019 0.021–0.028 0.087–0.105

Note: Estimates are based on combined results from analyses with the UIUC two-dimensional model and the MOZART2 three-dimensional model. Emissions were evenly distributed over the landmasses based on their representation in the three-dimensional model. The primary values are based on scaling using the tropopause burdens between the two models, while the values in parentheses are based on an alternative approach using stratospheric burden scaling. The differences between these is representative of some of the remaining uncertainties in determining these ODPs. Source: Based on Wuebbles et al. (2000).29

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HCFCs, and other compounds) and of the resulting ozone destruction. However, the semiempirical approach also depends on the accuracy of the measurements used with this approach. As mentioned earlier, the results from current models compare well with the semiempirical derivation of the ODPs. Since ODPs are defined in terms of the steady-state ozone change (or alternatively as the integrated cumulative effect on ozone), they are not representative of the relative transient effects expected for shortlived compounds during the early years of emission. Timedependent ODPs can also be defined that provide information on the shorter timescale effects of a compound on ozone. However, the steady values generally are preferred and are used in regulatory considerations. By definition, the ODP for CFC-11 is 1.0. The calculated ODPs for other CFCs being banned are all greater than 0.4. The Clean Air Act currently calls for policy actions on compounds whose ODPs are greater than 0.2. The ODPs for halons are all extremely large, much greater than 1.0, reflecting the reactivity of bromine with ozone. The ODPs for the HCFCs being used or considered as replacements are generally quite small relative to CFCs. The effect on ozone from a unit mass emission of one of these HCFCs would correspondingly be less than a hundredth of the effect on ozone than the CFC or halon they would replace. ODPs for all the HFCs, PFCs, and for sulfur hexafluoride are near zero because of the low reactivity of their dissociation products with ozone.

CONCERNS ABOUT STRATOSPHERIC OZONE: EQUIVALENT CHLORINE LOADING Although ODPs provide a useful guide to the relative effects on ozone from different gases, other analyses are also useful in fully evaluating the potential effects of a chemical on ozone. For example, it is useful to consider how much of an ozone decrease would really be expected to occur over the coming decades from the emissions of a new chemical like nPB. The concept of equivalent chlorine loading (ECL) provides a straightforward means of evaluating these effects.7,17,18 ECL can be used as a proxy for the approximate amount of ozone depletion resulting from a given set of emissions of chlorine- and bromine-containing gases at ground level. With the ECL concept one can examine the effects of different assumptions about changing emissions of these gases on the ozone layer over the coming decades. The concept of ECL provides a measure of the total amount of chlorine and bromine reaching the stratosphere that is available to affect ozone. This concept has proved to be a powerful tool in evaluating the total potential effects on ozone from the use of CFCs and halons and their replacement compounds. The concept of ECL combines the knowledge gained from atmospheric observations with analyses from atmospheric models, to evaluate the total amount of chlorine and bromine transferred from the troposphere to the stratosphere, where these halogens can react with ozone. ECL is directly proportional to the surface emissions of the halocarbons, their reactivity as reflected in their atmospheric lifetimes, and the number of chlorine and bromine atoms released per molecule. Model calculations and laboratory measurements indicate that bromine is much more reactive with ozone than chlorine. To represent bromine loading equivalent’ to chlorine loading, ECL includes a multiplicative factor, α (taken to be a value of 55), on the amount of bromine released into the stratosphere to account for its larger reactivity with ozone. The factor of α  40 has been used in recent assessments,18 but is likely to be a low estimate. The ECL values presented here assume that α has a value of 55, to account for the greater reactivity of each bromine emitted with stratospheric ozone compared with chlorine. This value is based on integration of the relative

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effects of chlorine and bromine on ozone from the zonally averaged, chemical-radiativedynamic model of the global atmosphere.5,24,27 A useful form of ECL is referred to as equivalent effective stratospheric chlorine (EESC). EESC accounts for the chlorine and bromine released in the lower stratosphere where a majority of the ozone destruction has occurred over recent decades. Figure 1* shows the EESC due to past emissions. The authors have recently updated the model for ECL (and EESC) to account better for the actual observed changes in concentrations of CFCs halons, and other halocarbons. This was needed as some of the CFCs and other halocarbons have changed more rapidly than was originally expected from the Montreal Protocol. The impacts of CFCs and other halocarbons on the EESC are shown individually in Figure 1, with the topmost curve representing the sum of all of the individual components. Similar results are also presented in the latest international assessment of stratospheric ozone.19 Recent measurements19 indicate that the reduction in most CFCs plus several other halocarbons (such as methyl chloroform) is occurring at a faster rate than the Montreal Protocol, but the authors have attempted to account for this in the scenario evaluated. Figure 1 also shows the EESC for future emissions assuming global emissions corresponding to the Montreal Protocol and its modifications under the latest amendments. From Figure 1, the EESC should have reached its peak value by 1998 (at a value of about 3.4 parts per billion of air by volume, or ppbv) and should slowly recover to levels observed before the extensive human-related emissions in recent decades. The smaller effect on projected ECL from HCFCs is related both to their much shorter atmospheric lifetimes and to the expected halt in their production by 2030 in support of the regulatory actions called for by the current amendments under the Montreal Protocol. As demonstrated in several published studies (e.g., Wuebbles and Calm23), there are a few situations where this halt in production, such as, for example, relating to the use of HCFC-123 in large chiller units, may not be in the best interest of environmental policy making. Based on the measurements from the Nimbus 7 TOMS instruments,29 the depletion of global mean total column ozone between 1979 and 1995 was ~5%. Assuming a linear relationship between EESC and the total column ozone, Figure 2 shows the change in total ozone corresponding to the scenario evaluated in Figure 1. The highest and lowest values for ECL and ozone reductions are reached at about 1998, reversing thereafter, although it is not until about 2040 that the 1979 level is again obtained. One could then evaluate other scenarios, for example, of the assumed global emissions of a new solvent, to then evaluate its potential effects on ozone relative to this baseline scenario.

CONCERNS ABOUT CLIMATE CHANGE: GLOBAL WARMING POTENTIALS Greenhouse gases and other radiatively active substances in the atmosphere are important influences on climate, the aggregation of the weather generally expressed in terms of averages and variances of temperature, precipitation, and other physical properties. Greenhouse gases in the atmosphere absorb infrared radiation emitted by the Earth that would otherwise escape to space. This trapped radiation warms the atmosphere, creating a positive forcing on climate, called radiative forcing, which in turn warms the Earth’s surface. The concept of radiative forcing provides an estimate of the potential effect on climate from greenhouse gases. For the given concentration of a gas, the radiative forcing depends primarily on the infrared absorption capabilities of the gas. Once the infrared absorption cross sections have been measured in a chemical laboratory, the radiative forcing can be * Chapter 6.4 Color Figure 1 follows page 104. © 2001 by CRC Press LLC

4 3 2

Percent (%)

1 0 -1 -2 -3 -4 -5 -6 1980

1990

2000

2010

2020

2030

2040

2050

Year

Figure 2

Total column ozone change base case.

determined by using computer models of the physics affecting radiative transfer in the Earth’s atmosphere. All of the halocarbons are greenhouse gases. In fact, any gas containing a C–F or C –H bond will be a greenhouse gas. The effects of such gases on climate will depend on their radiative absorption properties and the amount of the compound in the atmosphere. Global–warming potentials (GWPs) provide a means for comparing the relative effects on climate expected from various greenhouse gases. The concept of GWPs is used to estimate the relative impact of emission of a fixed amount of one greenhouse gas compared with another for globally averaged radiative forcing of the climate system over a specified timescale. GWPs are a better measure of the relative climatic impacts than radiative forcing alone as they also account for the atmospheric lifetime, and thus the change in concentration for a given emission, of the gases. GWPs have been evaluated for a number of replacement compounds and have been reported in the international IPCC and WMO assessments. The GWP concept is also being used in policy-making considerations associated with concerns about global warming from greenhouse gases. Policy makers have generally chosen to use the GWP values associated with a 100-year integration. GWPs are expressed as the time-integrated radiative forcing from the instantaneous release of a kilogram of a gas expressed relative to that of a kilogram of the reference gas, carbon dioxide, following the following equation: GWPX (t) 

t t ∫ 0 FXe /τXdt t

∫ 0 FCO2R(t)dt

where FX is the radiative forcing per unit mass of species of X, x is the atmospheric lifetimes of species X, FCO2 is the radiative forcing due to CO2, and R(t) represents the response function that describes the decay of an instantaneous pulse of CO2. The numerator and the denominator represent the absolute global warming potential (AGWP) of species X and © 2001 by CRC Press LLC

CO2, respectively. Recently WMO19 revised the formula for radiative forcing due to a pulse of CO2 from the one used in previously based on new radiative transfer analyses. This resulted in about 12% decrease in the forcing compared to the IPCC3 value. This decrease manifests itself in lower values of CO2 AGWPs. This decrease in CO2 AGWPs would lead to slightly larger GWPs for other gases. IPCC3 reported the GWPs for a number of replacement compounds evaluated relative to CO2. The latest assessment by WMO19 reported the GWPs for the previously evaluated species and expanded the list to add some newer compounds. These values are greater than the IPCC3 values due to the above-mentioned decrease in CO2 forcing. Naik et al.6 and Jain et al.,4 have reevaluated the GWPs for the replacements in a consistent manner (more so than the international assessments) using the model-derived atmospheric lifetimes and radiative forcings relative to the revised CO2 AGWPs. Shown in Table 3 is the most up-to-date evaluation of GWPs, based on Jain et al.4 These are compared with the values from the WMO assessment1 for the 100-year integration. Note that CO2 has GWPs equal to a value of one by definition. Policy makers in general are most concerned about the GWPs that are quite large, such as those for the perfluorocarbons. However, it is a combination of the emission level and the GWPs that are being used in policy considerations. It should also be mentioned that there remain a number of uncertainties with GWPs and their use (e.g., see Wuebbles22). SUMMARY AND CONCLUSIONS This chapter summarizes many of the environmental issues of concern in considering replacement solvents, focusing particularly on the concerns about global changes in ozone and climate. ACKNOWLEDGMENT Work at the University of Illinois for this study was supported in part by the U.S. Environmental Protection Agency and by the National Aeronautics and Space Administration Atmospheric Chemistry Modeling and Analysis Program.

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Table 3 Estimated GWPs Time Horizon in Years 20 Gas CH4 N2O

100

This Study 72 296

This Study 28 340

WMOa 24 360

500 % Difference

This Study

14 6

9 188

2 0 4 0 1 13

1700 5200 17000 2700 8700 12700

22 12

600 50

Chlorofluorocarbons (CFCs) CFC-11 CFC-12 CFC-13 CFC-113 CFC-114 CFC-115

6100 9800 10000 5800 7100 6000

4700 10600 14600 6000 9700 9100

4600 10600 14000 6000 9800 10300

Chlorocarbons (CCs) CCl4 CH3CCl3

2700 500

1800 160

1400 140

Hydrochlorofluorocarbons (HCFCs) HCFC-22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HCFC-225ca HCFC-225cb

4900 280 1800 2000 4300 460 1600

1900 90 590 690 2000 140 500

1900 120 620 700 2300 180 620

0 33 5 1 15 28 24

590 28 180 220 640 45 160

24 20 12 0 11 6 7 26 — 14 1 17

15900 350 1400 390 560 110 2100 46 2 1500 7400 270

1 16

33200 10600

Hydrofluorocarbons HFCs HFC-23 HFC-32 HFC-125 HFC-134 HFC-134a HFC-143 HFC-143a HFC-152a HFC-161 HFC-227ea HFC-236fa HFC-245ca

15000 3500 6700 3400 4400 1100 6900 480 25 6300 7200 2600

19600 1100 4300 1200 1800 350 5800 150 8 4400 9500 870

14800 880 3800 1200 1600 370 5400 190 — 3800 9400 720

Perfluorocarbons (PFCs) SF6 CF4

14600 4400

22500 6800

22200 5700

continued

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Table 3 Estimated GWPs continued Bromocarbons (BCs) H-1211 H-1301 CH3Br CH2Br2 CH2F2Br

2900 6800 11 10 1200

1100 6300 4 3 390

1300 6900 5 1 470

18 10 25 67 20

340 2500 1 1 120

0 —

1 1

Iodocarbons (ICs) CF3I CF3CF2I

1 1

1 1

1 —

Note: The percent difference in GWPs for 100-year time horizon evaluated in this study and those reported by WMO (Granier et al., 1999) are also given. a Granier et al. (1999).1 Source: Based on Jain et al. (2000).4

REFERENCES 1. Granier, C., K.P. Shine, J.S. Daniel, J.E. Hansen, S. Lal, and F. Stordal. Climate effects of ozone and halocarbon changes, in Scientific Assessment of Ozone Depletion, WMO Global Ozone Research and Monitoring Project, Rep. 44, Geneva, Switzerland, chapt. 10, 1998. 2. IPCC (Intergovernmental Panel on Climate Change), Climate Change 1994: Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emissions Scenarios, Houghton, J.T., Filho, L.G.M., Bruce, J., Lee, H., Callander, B.A., Haites, E., Harris, N., and Maskell, K., Eds., Cambridge University Press, Cambridge, UK, 1995. 3. IPCC (Intergovernmental Panel on Climate Change), Climate Change 1995: The Science of Climate Change. Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change, Houghton, J.T., Meira Filho, L.G., Callander, B.A., Harriss, N., Kattenberg, A., and Maskell, K., Eds., Cambridge University Press, Cambridge, UK, 1995. 4. Jain, A.K., B.P. Briegleb, K. Minschwaner, and D.J. Wuebbles, 2000: Radiative forcings and global warming potentials of thirty-nine greenhouse gases. J. Geophy. Res., submitted. 5. Kinnison, D.E., K.E. Grant, P.S. Connell, D.A. Rotman, and D.J. Wuebbles, The chemical and radiative effects of the Mount Pinatubo eruption, J. Geophys. Res., 99, 25705 –25731, 1994. 6. Naik, V., A.K. Jain, K.O. Patten, and D.J. Wuebbles, Consistent sets of atmospheric lifetimes and radiative forcings on climate for CFC replacements, J. Geophys. Res., 105, 6903–6914, 2000. 7. Prather, M., P. Midgley, F.S. Rowland, and R. Stolarski, The ozone layer—the road not taken, Nature, 381, 551 –555, 1996. 8. Reactivity Research Work Group, VOC Reactivity Policy White Paper, October 1, 1999. 9. SCOPE (Scientific Committee on Problems of the Environment), Effects of Increased Ultraviolet Radiation on Biological Systems, SCOPE Secretariat, Paris, France, 1992. 10. Solomon, S., M.J. Mills, L.E. Meiht, W.H. Pollack, and A.F. Tuck, On the evaluation of ozone depletion potentials, J. Geophys. Res., 97, 825 –842, 1992. 11. UN (United Nations), Montreal Protocol on Substances That Deplete the Ozone Layer, United Nations, New York, 1987. 12. UN (United Nations), Kyoto Protocol to the United Nations Framework Convention on Climate Change, United Nations, New York, 1997. 13. UNEP (United Nations Environment Programme), Environmental Effects Panel Report, Nairobi, Kenya, 1989. 14. UNEP (United Nations Environment Programme), London Amendments to the Montreal Protocol, Nairobi, Kenya, 1990. © 2001 by CRC Press LLC

15. UNEP (United Nations Environment Programme), Environmental Effects of Ozone Depletion: 1991 Update, Nairobi, Kenya, 1991. 16. UNEP (United Nations Environment Programme), Report of the Fourth Meeting of the Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer, Copenhagen, November 23 –25, Nairobi, Kenya, 1992. 17. World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 1991, Global Ozone Research and Monitoring Project Rep. 25, Geneva, 1992. 18. World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 1994, Global Ozone Research and Monitoring Project Rep. 37, Geneva, 1995. 19. World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project Rep. 44, Geneva, 1999. 20. Wuebbles, D.J., The relative efficiency of a number of halocarbons for destroying stratospheric ozone, Lawrence Livermore National Laboratory Rep. UCID 18924, 1981. 21. Wuebbles, D.J., Chlorocarbon emission scenarios: potential impact on stratospheric ozone, J. Geophys. Res., 88, 1433 –1443, 1983. 22. Wuebbles, D.J., Weighing functions for ozone depletion and greenhouse gas effects on climate, Annu. Rev. Energy Environ., 20, 45 –70, 1995. 23. Wuebbles, D.J. and J.M. Calm, An environmental rationale for retention of endangered chemical species, Science, 278, 1090 –1091, 1997. 24. Wuebbles, D.J., D.E., Kinnison, K.E., Grant, and J. Lean, The effect of solar flux variations and trace gas emissions on recent trends in stratospheric ozone and temperature, J. Geomag. Geoelectr., 43, 709 –718, 1991. 25. Wuebbles, D.J., A. Jain, K. Patten and P. Connell, Evaluation of ozone depletion potentials for chlorobromomethane (CH2ClBr) and 1-bromo-propane (C3H7Br), Atmos. Environ., 32, 107–114, 1997. 26. Wuebbles, D.J. and M.K.W. Ko, Summary of EPA/NASA Workshop on the Stratospheric Impacts of Short-Lived Gases, March 30–31, Washington, D.C., 1999. 27. Wuebbles, D.J., A.K. Jain, R. Kotamarthi, V. Naik, and K.O. Patten, Replacements for CFCs and halons and their effects on stratospheric ozone, in Recent Advances in Stratospheric Processes, Nathan, T.R., and E. Cordero, Eds, Research Signpost, Kerala, India, 1999a. 28. Wuebbles, D.J., R. Kotamarthi, and K.O. Patten, Updated evaluation of ozone depletion potentials for chlorobromomethane (CH2ClBr) and 1-bromo-propane (CH2BrCH2CH3). Atmos. Environ., 33, 1641 –1643, 1999b. 29. Wuebbles, D.J., K.O. Patten, M.T. Johnson, and R. Kotamarthi, The new methodology for ozone depletion potentials of short-lived compounds: n-propyl bromide as an example, J. Geophys. Res., in press.

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CHAPTER 6.5

Health and Safety James L. Unmack

CONTENTS Introduction Identifying Hazards Mechanical Trauma Thermal Trauma Chemical Effects Flammable and Reactive Materials Personal Protective Equipment Respiratory Protection Levels of Protection Using PPE Donning Procedures Doffing Procedures Selection Matrix INTRODUCTION Safety is having knowledge of the hazards associated with the intended activities and applying that knowledge to minimize the risk of injury or damage. Safety describes bringing those resources, information, and equipment to the activity to improve the likelihood of a successful outcome and reduce the risk of failure. The concept of safety implies having available the appropriate information to provide the basis for decisions regarding processes, materials, and equipment to accomplish the objectives. Safety is more than just risk avoidance, for when decisions are optimized, job performance is enhanced significantly. Appropriate information provides the basis for achieving the objectives. The safety information when working with hazardous materials or processes must include: • Information about the processes What are the hazards associated with the processes? What are the reasonable and customary procedures to control the hazards? © 2001 by CRC Press LLC

How can the job be done safely? • Information about the materials The identities of the hazardous materials and their properties How to identify a release What to do about a release Signs and symptoms of exposure Knowledge of the hazards associated with a process and the materials used in the process helps guide decisions on the selection of processes. As technologies are evaluated to achieve the cleanliness objective, the safety of the object to be cleaned and the effect of the technology on the object are prime considerations. The effect of the technology on the global environment and the people who use this technology must also be considered. It is because of their effect on the global environment that chlorinated and photoreactive solvents are avoided. When considering the effect on the user, the situation is somewhat different. Rather than prohibiting the use of a hazardous material or process, consider cost trade-offs. The more hazardous the processes and materials, the greater the degree of isolation. The greater the degree of isolation, the more automated the process becomes. IDENTIFYING HAZARDS The ways in which a person may be injured may be categorized in the following ways: • Mechanical trauma Hitting, squeezing Cutting, puncturing Abraiding, rubbing, scuffing Shaking, vibrating Pressure, vacuum, blast • Thermal trauma Heat Cold • Chemical effects Toxic Irritating Corrosive A material is considered hazardous if it is flammable or reactive. Mechanical Trauma Mechanical trauma usually results from moving parts of equipment and machinery. Standards have been developed for guarding power transmission and point of operation. As a rule of thumb, the moving parts of machinery must be guarded so that not even a finger can touch a moving part of a machine that would cut, squeeze, or strike with such force as to cause injury. The standards for protection against shaking, vibration, and pressure are less precise. The threshold to cause injury is less well defined.

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Thermal Trauma The temperature comfort zone for bare hand handling of metal objects is 12 to 43°C (55 to 110°F). Momentary contact with metal objects hotter than 54°C (130°F) may produce tissue injury. Metal objects colder than 0°C (32°F) may freeze moisture and stick to the skin.

Chemical Effects Materials that are corrosive, irritating, or toxic are hazardous. While there is no bright line between hazardous and nonhazardous, several government agencies have developed guidelines (see Table 1). The toxicity of a material is rated on its ability to kill. The toxicity of a material depends on its route of entry. The usual routes of exposure in the workplace are inhalation for materials that are airborne, skin contact with liquids and solids, and ingestion, based on transferring materials from work surfaces to hands to the mouth. For many materials, the most hazardous exposure is ingesting the material, either in food or water. Oral toxicities are usually measured on laboratory rats and expressed as the amount of material taken at one meal required to kill 50% of the rats. This is expressed as the lethal dose-50, LD50. Toxicities are expressed as milligrams of the poison per kilograms of body weight of the laboratory animals. The guiding principal is the larger the animal, the more poison needed to kill it. In the workplace, the most common route of entry is inhalation. Laboratory rats are exposed to various airborne concentrations of the material to determine the lethal concentration that statistically would kill half the rats. This value is expressed as LC50. For materials that are hazardous to your skin, the toxicity is determined by placing a measured quantity of the material on the shaved skin of a rabbit. OSHA considers a material corrosive if after 4 hours of contact the skin is destroyed. If the skin is inflamed, reddened, or exhibiting a rash, the material is considered an irritant. If the material is absorbed through the skin to produce systemic effects, the material is considered toxic. Environmental effects must be considered by the Environmental Protection Agency. The degree of hazard of a material getting into rivers, lakes, or oceans is rated by its aquatic toxicity. The EPA rates aquatic toxicity by killing fathead minnows, rainbow trout, or golden shiners. The aquatic toxicity is the concentration in water that kills half of the fish in 96 hours.

Flammable and Reactive Materials The definitions of flammability and reactivity are purposely broad. To further confuse matters, the enforcement agencies loosely apply these definitions. For example, ammonia is considered a flammable gas for good reason. Many ice houses which used ammonia as the refrigerant have had serious explosions when a source of ignition was brought too close to an ammonia leak. Technically, ammonia does not fit the definition of an explosive gas. The range of explosive concentrations of ammonia is 16 to 25%. The lower explosive limit (LEL) for ammonia is higher than 13% and the explosive range (difference between the upper and lower exposure limits) is less than 12%. What is a reactive material? A reactive material is an unstable substance which in the pure state, or as produced or transported, will vigorously polymerize, decompose, condense, or will become self-reactive

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Table 1 Comparison of Hazardous Materials Definitions OSHA 29 CFR 1910

EPA 40 CFR

U.S. DOT 49 CFR

Toxic

§1910.1200 LD50  500 mg/kg oral rat LD50  1000 mg/kg skin rabbit LC50  2000 ppm inh rat LC50  20 mg/m3 inh rat

§261.24 LD50  5000 mg/kg oral rat LD50  4300 mg/kg dermal rabbit LC50  10,000 ppm LC50  500 mg/L fathead minnows

Corrosive

§1910.1200 Irreversible tissue damage  4 h

Flammable

§191.1200 LEL  13%, UEL  LEL  12% FP  100°F (37.8°C) Pyrophoric §1910.1200 Vigorously polymerize, decompose, condense, or become self-reactive under conditions of shocks, pressure, or temperature

§261.22 pH 2.0 pH 12.5 Corrodes steel  6.35 mm/year (0.25 in./year) §261.21 Ignitable FP  60°C (140°F) Pyrophoric Oxidizer §261.23 Unstable, undergoes violent change without detonating Water reactive Forms explosive mixtures with water Generates toxic gas Cyanide or sulfide Detonates or explodes DOT forbidden explosive §261.3 Listed or not excluded by EPA Administrator in 40 CFR 261

§173.132 LD50  500 mg/kg liquid LD50  200 mg/kg solid LD50  1000 mg/kg dermal LC50  10 mg/L dust/mist LC50  5000 mL/m3 gases and vapors §173.136 Corrodes  6.35 mm/year (0.25 in./year) steel or aluminum Destroys skin  60 min

Reactive

Other

§1910.1200 Presents significant safety or health hazard

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§173.121, FP  60°C (140°F) §173.120, Temperature  FP LEL  13%, UEL  LEL  12% §173.115, FP  100°F (37.8°C) §173.56 Explosive

§171.8 Determined by the Secretary of Transportation to be capable of posing an unreasonable risk to health, safety, and property when transported in commerce, and which has been so designated; the term includes hazardous substances, hazardous wastes, marine pollutants, and elevated temperature materials

under conditions of shocks, pressure, or temperature. Commercial and military explosives, blasting agents, and pyrotechnics are considered reactive materials, as well as other unstable substances which can violently release energy without the need for other substances to react with. Fuels are not considered reactive materials because they need an oxidizer to react with. Table 1 shows that the three agencies have different agendas. The mission of OSHA is to ensure the health and safety of people at work. The assumption OSHA makes is that people work for 8 hours per day, 5 days per week, for a total of 40 hours per week. Inherent in this assumption is that people have time to recover from the effects of the exposure each day before being exposed again. EPA may assume that environmental exposures are continuous, 24 hours per day, without time for recovery each day. Therefore, the criteria used by EPA must be more protective. Notice that a material such as table salt, sodium chloride, with an LD50 of 1200 mg/kg oral rat, would not be considered toxic by OSHA, but would be considered toxic by EPA. The criteria used by DOT are intended to protect cleanup workers at a transportation accident and also to protect the infrastructure, vehicles, and equipment. Notice that an isotonic brine solution could be a corrosive to DOT for its corrosive effects on steel, but not a corrosive to OSHA because it is not corrosive to skin. In fact, an isotonic brine is used medicinally to protect skin from corrosive effects. The answer to the question of what is a hazardous material is not simple. To give a good answer, we need to know who is asking the question and why. Are hazardous materials really that dangerous? By knowing the hazards and taking reasonable precautions, most hazardous materials can be handled with comparatively little risk. In fact, many more injuries in the workplace are caused by trauma than by exposure to hazardous substances. The leading causes of fatal injuries to workers are automobile accidents, violence, and falls. The ranking of these three leading causes depends on the business sector in question. Violence is number one in retail sales; automobile accidents are first among workers who travel to work sites outside their permanent work location. Over half of the fatal falls are falls to the same level. The number and severity of slips, trips, and falls can be reduced by keeping all walking surfaces clean, dry, and clear of obstructions and debris. Wintertime ice on walking surfaces is a major hazard in colder climates and contributes to more falls than any other hazard. What are considered reasonable precautions? Reasonable precautions are determined by a hazard assessment. An inventory is developed to include all the hazardous materials in the workplace. For each material on this list, the opportunities for exposure are identified. Where exposure may result in injury or illness, means of preventing the exposure must be identified. The preferred way of preventing exposure is through what are known as engineering controls. Engineering controls include process or material substitution, process enclosure, process automation, local exhaust ventilation, and general dilution ventilation. When all practical engineering controls have been applied and still there exists a significant possibility of exposure, personal protective equipment is needed. Personal protective equipment includes those devices and clothing that are worn to protect and prevent direct contact. Some of the more common items of personal protective equipment include gloves, aprons, faceshields, hearing protectors, hardhats, and respirators. Many other types of personal protective equipment have been developed for specific hazards.

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In the workplace, most exposures occur through inhalation of gases, vapors, or aerosols and by direct skin contact with liquids or solids on the hands. Respirators protect against inhalation hazards and require special attention to ensure their adequate functioning. OSHA requires a written respiratory protection program to ensure all critical aspects of the program are met. PERSONAL PROTECTIVE EQUIPMENT The selection of personal protective equipment (PPE) depends on the characterization and analysis of the hazardous materials, job tasks, physical hazards of the work location, intended use, and duration of potential employee exposure. Maintenance and storage of PPE, decontamination, donning and doffing procedures, inspection and monitoring of effectiveness, and limitations are outlined in this section. Respiratory Protection All employees whose jobs may require the use of respiratory protection should be certified medically fit to use a respirator before being fit tested and issued a respirator. Respirators put an extra load on the heart and lungs which is not well tolerated by some people. All employees whose jobs may require the use of respiratory protection should be certified annually as medically fit to use a respirator. Only employees who have successfully completed respiratory protection training should be allowed to use respiratory protection. Respiratory protection training includes how to wear and maintain respirators properly, the proper use and limitations of respirators, and familiarization with respirators to be used at the job. Employees should be fit tested using a fit testing protocol recognized by OSHA. Qualitative and quantitative fit testing protocols are well described in the appendices of the OSHA respiratory protection standard, 29 CFR 1910.134, Appendix A. The objective of the fit testing is to determine that the protection factor exceeds a minimum value. The protection factor is defined as the ratio of the concentration of the contaminant outside the respirator to the concentration inside. The fit testing should be conducted before issuing a respirator to ensure the selected respirator will provide an adequate seal on the employee’s face. The fit test should be repeated annually to ensure that a good seal is maintained. The contours of the face change with age and loss or gain of weight. If an employee has difficulty in breathing during the fit test or during use, he should be evaluated medically to determine if he can wear a respirator safely while performing assigned tasks. No employee should be assigned to tasks requiring the use of respirators if, based upon the most recent examination, a physician determines that the health or safety of the employee will be impaired by respirator use. There is much controversy regarding the wearing of contact lenses in areas with the potential for exposure to hazardous concentrations of airborne contaminants. Contact lenses are clearly inappropriate in dusty atmospheres unless protected with a cover goggle. Restrictions for hazardous vapors and gases are less clear-cut. Hydrophilic gases, such as ammonia, formaldehyde, and hydrogen chloride, may cause more harm to contact lens wearers than non-wearers, and therefore, contact lenses are not recommended where these gases may reach hazardous concentrations. Contact lenses are well protected by full face respirators. OSHA has reversed its long held position and now permits contact lenses with full-face respirators. However, contact lenses should not be worn with half-face respirators. Facial hair that might interfere with a good facepiece seal or proper operation of the respirator is prohibited. The seal that a respirator makes when seating on facial hair is unreliable. © 2001 by CRC Press LLC

A favorable level of protection in a fit test is not a good indication of seal performance during actual work conditions. Facial hair that might interfere with a good facepiece seal or proper operation of the respirator must automatically disqualify respirator use. Only a respirator for which an employee has been successfully fit tested should be made available to the employee. To ensure an adequate level of protection against airborne contaminants, the selected respirator should be approved by National Institute of Occupational Safety and Health (NIOSH) for protection against the identified hazards. Many respirator manufacturers have a variety of styles and sizes. When the selection is based on cost, life-cycle costing techniques should be used. The cost of wearing a respirator includes the purchase price of the respirator plus all the accessories purchased over the life of the respirator, and cost of labor to maintain the respirator. The respirators must be properly cleaned, maintained, and stored when not in use to preserve the level of protection provided. Air purifying respirators (APRs) should not be used in heavily contaminated atmospheres where the protection factor is likely to be exceeded. When the fit test is performed with qualitative methods, the maximum protection factor is 10. The nature and concentration range of the contamination must be known before an APR may be selected for use. There is no clear guidance for how often the air-purifying cartridges should be replaced. Cartridges must be replaced when breakthrough or increased breathing resistance is detected. However, many users recommend replacing the air-purifying cartridges at the end of each shift. Few cartridges have an end-of-life indicator to show when the cartridge needs to be replaced. PAPR cartridges will be changed when flow falls below 4 cfm through the cartridge. Positive and negative pressure tests must be performed each time the respirator is donned. Air-supplied respirators should be assembled according to manufacturer’s specifications. Hose length, couplings, valves, regulators, manifolds, and all accessories should meet ANSI and the manufacturer’s requirements. Respirators should be cleaned and sanitized daily after use. Respirators should be inspected during cleaning. Worn or deteriorated parts should be replaced. Respirators maintained onsite for emergency response should be inspected monthly by a trained technician. The person responsible for the respiratory protection program should check the users periodically to ensure they are properly wearing and maintaining their respirators and that the respiratory protection is adequately protecting them. Regulations advise that a health and safety professional, such as an industrial hygienist, should evaluate the respiratory protection program annually to ensure the continuing effectiveness. Levels of Protection The U.S. Environmental Protection Agency defines levels of protection based on the protection afforded. • Level A Protection is a gas-tight suit. Level A provides protection to the skin and lungs against gases, vapors, dusts, and mists. Level A is often called the “moon suit” and is the ultimate in protection. • Level B Protection provides full protection to the lungs from gases, vapors, dusts, and mists, but only provides splash protection for the skin. Level B is defined by © 2001 by CRC Press LLC

the supplied air respirator, such as a self-contained breathing apparatus (SCBA) or an airline respirator. • Level C Protection provides limited protection for the lungs against gases, vapors, dust, or mists. Level C is defined by the air-purifying respirator. The selection of the respirator depends on the airborne contaminant. Air-purifying filters, cartridges, and canisters are generally specific to a particular airborne contaminant and can be overwhelmed by high concentrations. Most gases and vapors are limited to 1000 ppm or 10 times the OSHA permissible exposure limit, whichever is lower, when air-purifying respirators are used. • Level D Protection is defined by ordinary work clothes appropriate to the job. Level D protection may include coveralls, gloves, or apron to protect the skin from contact with liquid or solid hazardous materials. Using PPE All persons entering the work area where hazardous materials are used should put on the required personal protective equipment according to established procedures to minimize exposure potential. When leaving the work area, personal protective equipment should be removed according to these established procedures to minimize the spread of contamination. Donning Procedures To minimize the spread of contamination, the following donning procedures have been developed and will serve as a good model. 1. Remove street clothes and store in a clean location. 2. Put on underwear and coveralls. Underwear should be absorbent and may be cotton or disposable. 3. Put on boots and boot covers. Secure boot covers to coveralls with tape. If boot covers are loose, adjust fit with wraps of tape. 4. Put on under gloves. 5. Put on chemical- and abrasion-resistant outer gloves. Secure gloves to coveralls with tape. 6. Don respirator and check for secure fit. 7. Put hood or head covering over the respirator. 8. Put on remaining protective equipment, such as hard hat, safety glasses, etc. One person should remain outside the work area to check that each person entering has the proper protective equipment. No persons should be allowed to enter an exclusion improperly attired. Doffing Procedures When the potential for contamination with hazardous materials is severe, a good procedure to exit the work area and leave the hazardous materials behind is essential. For highly contaminating work areas, the following procedure will provide an adequate level of protection. Whenever a person leaves the work zone, the following proper decontamination sequence will be followed:

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1. Upon entering the Contamination Reduction Zone, rinse contaminated mud and debris from boot covers. Then remove boot covers. 2. Clean reusable protective equipment. 3. Remove protective garments and equipment except respirator on the contaminated side of shower. All disposable clothing should be placed in plastic bags and labeled as contaminated waste. 4. Take shower: begin washing hair, neck, and face. 5. Remove respirator in shower and finish washing. 6. Dry with fresh towel. 7. Proceed to the clean area and dress. 8. Clean respirator and prepare for next use. 9. Proceed to the sign-out point. All disposable equipment, garments, and personal protective equipment should be bagged in two 6-mil plastic bags, properly labeled, and disposed. Selection Matrix The level of personal protection (Table 2) can be based on measurements of the work environment when such measurements can be made in real time. When the assessment of the work environment depends on laboratory analysis of samples collected, then the selection of PPE must be made on professional judgment of possible or expected exposures.

Table 2 PPE Selection Matrix EPA Protection Level

Airborne Contaminant Level

Personal Protective Equipment

D C B A

 PEL  10x PEL and  1000 ppm  1000x PEL and  10% LEL  10% LEL

Work clothes Air-purifying respirator Supplied air respirator Total encapsulating, gas-tight suit

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SECTION 7

Glossary of Common Terms and Acronyms Contributors: Background and Contact Information

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Glossary of Common Terms and Acronyms Compiled by Ed Kanegsberg

[Note: This is intended as an explanation of some of the more commonly used terms and acronyms related to cleaning chemicals and processes; it is not a complete list of all technical terms or abbreviations used in this book. In general, the included terms are referred to in more than one chapter. These definitions are to be considered to be descriptive rather than formal definitions.] Abrasive media: Materials used to remove soil via the momentum of impact. ACGIH: American Conference of Governmental Industrial Hygienists; sets TLV values. Airless: A description of an enclosed cleaning system that is sealed to contain either full vacuum (
1 mmHg) or a pressure significantly elevated above ambient (
800 to 10,000 mmHg). Airtight: A description of an enclosed cleaning system that is sealed to contain a light pressure above ambient, typically about 0.5 psig. Aliphatic: A compound that is not aromatic; i.e., it lacks a particular arrangement of atoms in its molecular structure. Apriotic: A substance that can neither donate nor accept protons (hydrogen atoms). Aqueous: Water-based. Aromatic: A molecule or compound that has special stability and properties due to a closed loop of electrons. Not all molecules with ring (loop) structures are aromatic. ASTM: American Society for Testing and Materials; a group that establishes testing standards. Azeotrope: A solvent blend that, over a limited range of temperatures, maintains the same relative concentrations as the mixture components evaporate. Benchtop cleaning: Generally referred to as a small volume, labor-intensive, nonautomated cleaning process performed in the open rather than in specially designed cleaning tanks; examples include overhaul and repair and spot cleaning. CAA: Clean Air Act; the U.S. legislation that regulates air quality standards including the phaseout of ODCs. CAS: Clean Air Solvent; cleaning agents which have been analyzed by South Coast Air Quality Management District (SCAQMD) in California and found to meet their stringent environmental requirements for VOCs, ODCs, GWPs, and air toxics. CAS can also refer to Carbon Adsorption System or to Chemical Abstract Services, a division of the American Chemical Society, which assigns a unique registry number, referred to as a CAS number, to each chemical. For example, since they are actually the same substance, acetone and dimethyl ketone have the same CAS number. © 2001 by CRC Press LLC

Cavitation: Vacuum “bubbles” created by negative pressures in ultrasonic and megasonic processes. CFC: Chlorofluorocarbon. Cold cleaning: A cleaning process in which the cleaning solvent is below its boiling point (as distinguished from vapor degreasing). Contaminant: Material that has the potential to degrade the appearance or performance of a part, component, or assembly. Cosolvent: A sequential process using a different solvent for a rinse. Cyclic: Organic molecules with ring structures. d-Limonene: A citrus-derived organic cleaning solvent. DMSO: Dimethyl sulfoxide; a cleaning solvent. Dragin: Material (cleaning chemicals and contaminants) brought in from a previous cleaning step. Dragout: Material (cleaning chemicals and contaminants) carried over to a subsequent cleaning step. EPA: Environmental Protection Agency; the U.S. government agency responsible for setting and administering air and water standards. ESCA: Electron spectroscopy chemical analysis; an analytic technique for determining surface contamination. Flammable: Term used to describe a combustible material that ignites very easily, burns intensely, or has a rapid rate of heat spread. Flash point: The lowest temperature of a flammable liquid at which vapors are given off to form a flammable mixture with air, near the surface of the liquid or within the container. FOG: Fat, Oil, Grease. Measurements of water purity to determine compliance to discharge regulations. Freeboard: A term used in vapor degreasers defined as the distance from the point where the boiling solvent vapor idles to the top of the machine opening. Freon: A trade name (DuPont) for CFC-113; sometimes applied generically to CFCs. FTIR: Fourier transform infrared spectroscopy; a surface analytic technique utilizing reflected infrared light to identify types of surface contaminants. Greenhouse gas: A gas that persists in the stratosphere and acts to trap re-radiated heat from the Earth’s surface. GWP: Global-warming potential; a relative measure of a material’s heat trapping ability as a greenhouse gas. HAP: Hazardous air pollutant. HEPA: High Efficiency Particulate Arrestance (or Arrestor or Air). A class of fine mesh air filters. HFC (or HCFC): Hydrofluorocarbon; a class of chemicals developed as ODC replacements. HFE: Hydrofluoroether; a class of chemicals developed as ODC replacements. Hydrophilic: Water-soluble. Hydrophobic: Water-insoluble; usually soluble in organic solvents. IPA: Isopropyl alcohol, a common organic solvent. KB: Kauri-butanol; a number used to compare the solubility of heavy oils in a particular solvent. It is the volume of solvent required to produce a defined degree of turbidity when added to standard solutions of Kauri resin in n-butyl alcohol. Kyoto Protocol: International agreement to limit emissions of greenhouse gases responsible for global warming. LEL: Lower explosion level; the lowest concentration at which a mixture can explode. © 2001 by CRC Press LLC

Linear: Organic molecules without ring structures or branches. MC (Meth): Methylene chloride. Megasonics: A cleaning technique utilizing sound waves at frequencies higher than those for ultrasonics, from 500 kHz to 2 MHz. Montreal Protocol: International agreement to limit or eliminate production of ozonedepleting compounds (ODCs). MSDS: Material Safety Data Sheet. Neat: A term meaning pure or undiluted. NESHAP: National Emission Standards for Hazardous Air Pollutants; a series of U.S. federal regulations involving chemicals that can cause air pollution. NMP: N-Methyl pyrillodone; a cleaning solvent. NPB (or nPB): n-Propyl bromide; a cleaning solvent. NVR: Nonvolatile residue; solid material left behind when a solvent evaporates. ODC: Ozone-depleting compound, known to persist in the stratosphere and cause depletion of the ozone layer. ODP: Ozone depletion potential; a relative cumulative measure of the expected effects on ozone of the emissions of a gas relative to CFC-11. ODS: Ozone Depleting Substance (see ODC). Organic: Any substance which contains the element Carbon. OSEE: Optically stimulated electron emission; a surface analytic technique that measures the degree (but not the nature) of contamination by using UV light to stimulate the surface to emit electrons. OSHA: Occupational Safety and Health Agency; the U.S. government agency responsible for setting and administering worker safety standards. Particulates: Contaminant material with observable length, width, and thickness. In practice an observable size will be about 0.1
m or larger. PCE (Perc): Perchloroethylene. PEL: Permissible exposure limit; these are exposure guidelines for workers using the given chemical. PELs may be set by EPA or OSHA. PFC: Perfluorinated compounds containing fluorine and carbon but not chlorine or bromine. POTW: Publicly owned treatment works; a local water treatment facility. RCRA: Resource Conservation Recovery Act; defines hazardous wastes and how to manage them. RO: Reverse osmosis; a filtering mechanism through a semipermeable membrane. Saponification: The reaction between any organic oil containing reactive fatty acids with free alkalies to form soaps. SARA: Superfund Amendments and Re-authorization Act; this act requires reporting of inventories and emissions of listed chemicals and groups. SCAQMD: South Coast Air Quality Management District; the air quality regulating agency in southern California. SEM: Scanning electron microscopy; a surface analytic technique involving imaging a surface by means of an electron beam. Semiaqueous: A sequential process using both organic solvent and water rinse. SIMS: Secondary ion mass spectroscopy; a surface analytic technique using atoms ejected from a surface to identify contaminants. SNAP: Significant New Alternatives Policy; an EPA effort to identify CFC replacement chemicals. Soil: Matter out of place, contamination. Solvent: Organic (carbon-containing) liquid; usually distinguished from aqueous. © 2001 by CRC Press LLC

STEL: Short-term exposure limit; a 15-min TWA exposure that should not be exceeded at any time during the work day (see TWA). STOC: Solvent and Adhesives Technical Options Committee; a United Nations UNEP committee that provides a great deal of input into worldwide environmental policy on cleaning. Stoddard solvent: A common hydrocarbon blend used for cleaning oils. Stratosphere: The atmospheric layer above the troposphere; considered to be above about 7 miles. Surfactant: A material added to water or a solvent to increase wettability. TCA: 1,1,1-Trichloroethane (also called methyl chloroform). TCE (TRI): Trichloroethylene. TDS: Total dissolved solids; a measure of concentration of dissolved contaminants. TLV: Threshold limit value; a concentration level above which there may be adverse health risks on exposure; usually set by the American Conference of Governmental Industrial Hygienists (ACGIH). TOC: Total organic carbon; a measure of concentration of organic matter in water. Troposphere: The lower layer of the atmosphere. TWA: Time-weighted average; an employee’s permissible average exposure in any 8-hour work shift of a 40-hour week (see STEL). UEL: Upper explosion level; the maximum concentration at which a mixture can explode. Ultrasonics: A cleaning technique utilizing sound waves from 20 kHz to over 100 kHz. UNEP: United Nations Environment Programme; a United Nations group that includes the STOC committee. Vapor degreasing: A cleaning process in which, at least for the final cleaning, the part is suspended above a boiling solvent and is cleaned by the condensate of freshly distilled solvent vapor. VMS: Volatile methyl siloxane; a silicon-based cleaning solvent. VOC: Volatile organic compound; responsible for smog formation in the troposphere. VOC exempt (or de-listed): An organic compound which has been specifically determined by the EPA to be a sufficiently low threat to smog formation that it can be used in areas with VOC restrictions.

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Contributors: Background and Contact Information Dr. John W. Agopovich has worked at Draper Laboratories since 1983 and has been involved in various projects including the replacement of ozone depleting chemicals in critical cleaning operations. He received his undergraduate and graduate training at Rensselaer Polytechnic Institute. Contact: 555 Technology Square MS 48, Cambridge, MA 02139 Phone: (617) 258-3068 E-mail: [email protected] David E. Albert is a Senior Scientist at NAMSA (North American Science Associates). He is also an adjunct professor at the University of Toledo and Lourdes College where he teaches pharmacology, pathophysiology, and biochemistry. He has been in the medical device industry for 20 years. He holds a B.S. degree in Pharmaceutical Sciences from University of Toledo, an M.S. degree in Biochemistry from Bowling Green State University, and a Doctorate in Podiatric Medicine from Ohio College of Podiatric Medicine. Contact: 2261 Tracy Road, Northwood, OH 43619 Phone: (419) 666-9455 E-mail: [email protected] Stephen O. Andersen is Director of Strategic Climate Projects at the Climate Protection Partnerships Division of the U.S. EPA. He has served as Co-Chair of the Montreal Protocol Technology and Economic Assessment Panel (TEAP) and worked on the teams that changed military solvent specifications and commercialized no-clean soldering. He has helped organize solvent substitution workshops in 11 countries worldwide. Contact: 401 M St., SW, Mail Code 6202J, Washington, D.C. 20460 Phone: (202) 564-9069 E-mail: [email protected] Sami B. Awad is Vice President of Technology at Crest Ultrasonics Corp. He has more than 18 years of experience in developing new chemistries and processes for general and ultrasonic precision cleaning and surface preparation/treatment. He has a Ph.D. degree in Organic Chemistry and served as a professor of chemistry on the faculties of Drexel University, Philadelphia and Cairo University, Egypt. Contact: P.O. Box 2766, Trenton, NJ 08628 Phone: (609) 883-4000 E-mail: [email protected] Mohan Balagopalan is a Senior Engineer with the South Coast Air Quality Management District (SCAQMD). With 15 years of experience at SCAQMD, he is involved in streamlining permitting activities and has worked in rule development. Mohan has a B.S. degree in Mechanical Engineering and an M.B.A. Mohan is an instructor at the University of California, Riverside for the Certificate Program for Air Quality and Geographic Information Systems and has helped develop several courses for that program. Contact: 21865 East Copley Dr., Diamond Bar, CA 91765 Phone: (909) 396-2704 E-mail: [email protected] © 2001 by CRC Press LLC

Matt Bartell is Solvent Systems Product Manager at Forward Technology Industries. He writes machine technical specifications, prepares proposals, makes sales presentations, and manages current solvent projects. He has been with Forward since 1992 and has worked as a project engineer and project manager on both aqueous- and solvent-based precision systems. Contact: 13500 County Road 6, Minneapolis, MN 55441 Phone: (612) 557-2671 E-mail: [email protected] Mark Beck is the CEO of Product Systems Inc. He has over 20 years of experience in semiconductor processing and equipment design. Mark is graduate of University of California–Berkeley with a B.S. degree in Mechanical Engineering. Contact: 1745 Dell Ave., Campbell, CA 95008 Phone: (408) 871-2500 E-mail: [email protected] Michael Beeks is Technical Director, R&D, at Brulin Corporation. He has 31 of years experience in the cleaning industry, as formulator and technical manager of household and industrial detergents. He has a Bachelor's degree from Yankton College. Contact: P.O. Box 270, Indianapolis, IN 46206 Phone: (800) 776-7149 E-mail: [email protected] Rick Bockhorst is the Industrial Marketing Manager at Brulin Corporation. He has over 30 years of experience in industrial chemical sales, service, and marketing. He has a B.S. degree in Business from Hillsdale College, Michigan. Contact: P.O. Box 270, Indianapolis, IN 46206 Phone: (800) 776-7149 E-mail: [email protected] John Burke is currently Chief Conservator for the Oakland Museum of California. He has been active in the field of art conservation for over 28 years. He is an Adjunct Professor of Conservation at John F. Kennedy University where he teaches a graduate course in preventive conservation and is a Director on the Board of the American Institute for Conservation. Contact: 1000 Oak St., Oakland, CA 94607 Phone: (510) 238-3806 E-mail: [email protected] Ahmed A. Busnaina, Ph.D., is the William Lincoln Smith Chair Professor and Director of the Microcontamination Research Laboratory at Northeastern University. He authored more than 200 papers in journals, proceedings, and conferences. He specializes in wafer cleaning technology, particle adhesion and removal, and chemical and particulate contamination in LPCVD and sputtering processes. He won a Fulbright Senior Scholar Award. Contact: Dept. of Mechanical, Industrial, & Manufacturing Engineering, Boston, MA 02115 Phone: 315-268-6574 E-mail: [email protected] Michael S. Callahan is a Principal Chemical Engineer with Jacobs Engineering. During his 20 years at Jacobs, he has been involved in a variety of projects, including development of an EPA waste minimization manual. He is co-author of a book titled Hazardous Solvent Source Reduction. He has served on a California State Panel for Solvent Certification. He received a BSCHE from UCLA and is a professional engineer registered in the State of California. Contact: 1401 E. Willow St., Signal Hill, CA 90806 Phone: (562) 528-2204 E-mail: [email protected]

© 2001 by CRC Press LLC

Frank Cano is a Technical Representative with Vatran Systems. He has over 20 years of experience in design engineering. He attended California State University–Los Angeles. Contact: 677 Anita St., Suite A, Chula Vista, CA 92911 Phone: (619) 423-4555 E-mail: [email protected] Mantosh K. Chawla is President and co-founder of Photo Emission Tech., Inc. He has over 35 years of industrial experience including 16 years in surface cleanliness monitoring. He has a B.S. degree in Mechanical Engineering from Bradford University, England and an M.B.A. in Finance from John Carroll University, Ohio. Contact: 3255 Grande Vista Dr., Newbury Park, CA 91320 Phone: (805) 499-7667 E-mail: [email protected] John Chu is Application Manager at SVC/Shipley, where he works on polymer cleaning for Cu/Low-k technology and multi-layer metals. He has about 20 years of experience in IC manufacturing. He has a Master’s degree from CWRU. Contact: 245 Santa Ana Ct., Sunnyvale, CA E-mail: [email protected] Ray A. Cull is the Director of Product and Business Development at Schneller, Inc. He has an M.S. degree in Materials Engineering and a B.S. degree in Chemical Engineering. While at Dow Corning, Ray was responsible for the commercialization of volatile methyl siloxane technology as a solvent. Contact: 6019 Powdermill Road, P.O. Box 670, Kent, OH 44240 Phone: (330) 673-6063 E-mail: [email protected] Phil Dale is Operations Director at Layton Technologies, LTD, and has had 22 years of experience in the manufacturing business with extensive knowledge of design and manufacture of complex precision industrial equipment. Contact: Unit 6, Imex Technology Park, Trentham Lakes, Stoke-on-Trent, Staffordshire, England ST4 8LJ Phone: 011-44 1785-815732 E-mail: [email protected] John Durkee heads Creative EnterpriZes and has over 20 years of experience in chemical and engineering businesses. He is a registered Professional Engineer in Texas. He is an innovator with patents and corporate recognition. He has B.S. and M.S. degrees and a Ph.D. in Chemical Engineering from Lehigh University. Contact: 145 Oyster Creek Dr., Lake Jackson, TX 77566 Phone: (979) 471-7707 E-mail: [email protected] Eric Eichinger is a Materials and Process Engineer at Boeing’s Reusable Space Systems Division. He has over 10 years of experience in material substitution and source reduction for pollution prevention. He is the Chairman of the Aerospace Chromate Elimination Team with experience in replacing ozone depleters and volatile organic compounds. He holds a Bachelor’s degree from the University of California–Irvine. Contact: 5301 Bolsa Ave. Mail Code H021-F225, Huntington Beach, CA 92647 Phone: 714-372-5197 E-mail: [email protected] Max Friedheim is President of PDQ Precision Inc. He has been in the tool supply business since 1947. He is extensively involved in product development and is the holder of numerous patents. In 1997, on behalf of his company, he accepted the Clean Air Award for Technology from SCAQMD. Contact: P.O. Box 99838, San Diego, CA 92169 Phone: (619) 581-6370 © 2001 by CRC Press LLC

F. John Fuchs heads Cleaning Technology Resources. Having many years of experience with ultrasonics, he has been involved in developments in ultrasonic cleaning and other related technologies. He has authored numerous educational articles on ultrasonics and presented major papers. Recently he has been developing advanced aqueous cleaning techniques and processes to replace those using CFC solvents. Mr. Fuchs holds a B.S. degree in Industrial Engineering from the University of Michigan. Contact: 315 North Main St., Jamestown, NY 14702-2480 Phone: 716-487-9828 E-mail:[email protected] Christine Geosling is a member of the Senior Technical Staff in the Systems Development & Technology Division of Northrop Grumman. She has created analytical procedures to assess contamination and cleanliness on critical parts for laser gyroscopes, integrated optics, and MEMS devices. She holds a B.A. degree in Chemistry from Northwestern University and a Ph.D. degree in Physical Inorganic Chemistry from the University of Southern California. Contact: 21240 Burbank Blvd. m/s 19, Woodland Hills, CA 91367-6675 Phone: (818) 715-4050 E-mail: [email protected] Arthur Gillman is President of Unique Equipment and has over four decades of practical and theoretical experience in an array of critical cleaning applications. He has developed equipment for both aqueous- and solvent-based processes. He has been an advisor on several SCAQMD committees. He has assisted numerous components and parts manufacturers, in areas ranging from benchtop to very large airless applications. Contact: 2029 Verdugo Blvd. M/S 1005, Montrose, CA 91020-1636 Phone: (818) 409-8900 E-mail: [email protected] Dr. Don Gray has been a full-time faculty member in the Chemical Engineering Department at the University of Rhode Island for 20 years. He has spent 10 years designing environmentally safe solvent processing equipment. Dr. Gray has 10 patents or patent-pending designs including those related to pollution prevention methods and/or equipment for metal degreasing, carbon desorption, cavitation enhancement of mass transfer, and dry cleaning techniques. Contact: 205 Crawford Hall, Kingston, RI 02881 Phone: (401) 874-2655 E-mail: [email protected] Ross Gustafson is the Technical Director for Florida Chemical Company, Inc. He has over 12 years of experience working with citrus terpenes in the cleaning industry. He has a B.A. degree in Chemistry from Gustavus Adolphus College, and an M.S. degree in Chemistry from the University of Colorado–Boulder. Contact: 401 Somerset Dr., Golden, CO 80401 Phone: (303) 216-9420 E-mail: [email protected] Steve R. Henly is Group Managing Director of Layton PLC. He has several years experience in critical cleaning and drying applications in electronics and semiconductor technology. He holds several patents in semiconductor wafer drying using inerted low flashpoint solvents. Contact: The Bridges Business Park, Horsehay, Telford, Schropshire England TF4 3EE Phone: 011-44 1952-503200 E-mail: [email protected]

© 2001 by CRC Press LLC

Barbara Kanegsberg is President of BFK Solutions. With over 25 years of experience, she has been involved in numerous projects related to process development, failure analysis, and product development. She is the author of many articles related to critical cleaning, process development, and related safety and environmental concerns. She has a B.A. degree in Biology from Bryn Mawr College and an M.S. degree in Biochemistry from Rutgers University. Contact: 16924 Livorno Dr., Pacific Palisades, CA 90272 Phone: (310)459-3614 E-mail: [email protected] David Keller has been the Industrial/Process Chemist at the Brulin Corporation since October 1997. Previously, David was employed for 3 years at Wayne Chemical Corporation as a formulating chemist in the food sanitation industry. David has a B.S. degree in Chemistry from Lawrence Technological University in Michigan and has performed graduate work in synthetic inorganic chemistry at Indiana University. Contact: P.O. Box 270, Indianapolis, IN 46206 Kenroh Kitamura is Director of the Technology and Process Development Center of Asahi Glass Company Limited. He received his Master’s degree in Applied Chemistry at Keio University. He joined Asahi Glass in 1985. He is now in charge of the entire research and development activities for applications of CFC alternatives. Contact: 10, Goikaigan, Ichihara-shi, Chiba 290, Japan Phone: 81-436-23-3151 E-mail: [email protected] Jana Koran is an Engineer Specialist in the Material and Processes Department at NorthropGrumman, Electronic Systems, Navigation Systems Division. She has been involved in the cleaning and contamination control field for more than 20 years. She was primarily involved in helping the company make the transition from solvent-based cleaning to alternative cleaning techniques. She holds an M.S. degree in Chemical Engineering and an M.B.A. degree from the Technical University, Prague, Czech Republic. Contact: 5500 Canoga Ave. MS 53, Woodland Hills, CA 91367 Phone: (818) 715-3975 E-mail: [email protected] Edward W. Lamm is Worldwide Technology Manager for Precision Processing at Branson Ultrasonics Corp. After 15 years of experience in the petrochemical industry, he has been active since 1989 in both chemical and equipment manufacturing in the precision cleaning field. He holds a B.A. degree in Chemistry and an M.S. degree in Chemical Engineering. Contact: 41 Eagle Rd., P.O. Box 1961, Danbury, CT 06813 Phone: (203) 796-0392 E-mail: [email protected] Carole LeBlanc is the Director of the Surface Solutions Lab at Toxics Use Reduction Institute University of Massachusetts, Lowell. She has research experience in both industry and academia. She has a B.S. in Biology and Chemistry from Boston College and a Ph.D. in Sustainable Development and Management from Erasmus University’s, (Rotterdam, the Netherlands) Centre for Environmental Studies. Contact: One University Ave., Lowell, MA 01854 Phone: (978) 934-3249 E-mail: [email protected]

© 2001 by CRC Press LLC

Joe McChesney is Technical Director for the Detrex Parts Cleaning Technologies Division. Joe has 25 years of experience in engineering, design, and field applications of all types of cleaning equipment—both aqueous and solvent. Joe has an engineering degree from Western Kentucky University. Joe has several published technical papers and participates in numerous organizations in regards to technological and environmental issues. Contact: P.O. Box 5111, Southfield, MI 48086-5111 Phone: (248) 358-5800 E-mail: [email protected] Abid Merchant has been with DuPont for over 30 years and is a lead technical person in the Cleaning Agent Group. He played a key role in developments of the HFC cleaning agents and holds many patents. He is a member of the UNEP's Solvent Technical Options Committee, and served as an expert to a task force to reconcile the Kyoto and Montreal Protocols. Abid received an M.S. degree in Chemical Engineering and an M.B.A. degree. Contact: Fluorochemicals Laboratory, CRP-711, Wilmington, DE 19880-0711 Phone: (302) 999-4269 E-mail:[email protected] Toshio Miki is an Engineer at Asahi Glass Company Limited. He has over 7 years of experience in research and development for applications of HCFC-225 and HFC refrigerants. He received his Master’s degree in Materials Science and Engineering from Kyushu Institute of Technology, Japan. Contact: 10, Goikaigan, Ichihara-shi, Chiba 290, Japan Phone: 81-436-23-3151 E-mail: [email protected] William Moffat is CEO of Yield Engineering Systems (YES). He has been with YES for 17 years and has been involved in equipment design and use for 30 years. He is the holder of numerous patents related to processes for plasma stripping and plasma cleaning. He has higher national certificates in mechanical engineering, electrical engineering, and electronic engineering from Stockport Technical College and Manchester Technical College in the U.K. Contact: 2119 Oakland Road, San Jose, CA 95131 Phone: (408) 954-8353 E-mail: [email protected] William M. Nelson is the Process Evaluation Chemist at the Illinois Waste Management and Research Center. He has been active in green chemistry for over 6 years, and is a recognized expert in the area of green solvents. He earned a Ph.D. degree in Organic Chemistry from the Johns Hopkins University. Contact: One E. Hazelwood Dr., Champaign, IL 61820 Phone: (217) 244-5521 E-mail: [email protected] John G. Owens is a Senior Research Specialist with the Specialty Materials Laboratory of the 3M Company. He has over 13 years of experience in the development of fluorinated compounds and their use in applications such as precision cleaning. He has B.S. and Master’s degrees in Chemical Engineering from the University of Minnesota and the University of Virginia, respectively. Contact: 3M Center Bldg., 236-3A-03, St Paul, MN 55144-1000 Phone: (651) 736-1309 E-mail: [email protected] Michael Pedzy is President and Chief Equipment Design Engineer of Zenith Ultrasonics Inc. He has over 18 years of experience designing and applying ultrasonic systems for both industrial and hightechnology, ultra-critical cleaning applications He has a background in environmental studies from Montclair State University. Contact: 85 Oak St., Norwood, NJ 07648-0412 Phone: 800-432-SONIC E-mail: [email protected] © 2001 by CRC Press LLC

Richard Petrulio is the Chief Engineer at B/E Aerospace, Inc, Anaheim Facility. He has 14 years of experience in designing, manufacturing, and testing specialized compact refrigeration systems. He has extensive hands-on experience in cleaning process development. He earned a B.S. degree in Mechanical Engineering from California State Polytechnic University in Pomona. Contact: 3355 East La Palma Ave., Anaheim, CA 92806 Phone: (714) 666-4253 E-mail: [email protected] Robert L. Polhamus is Principal for RLP Associates, involved with sales of industrial cleaning equipment, and is a marketing consultant for industrial cleaning equipment. He has over 23 years of industrial experience and has served on several industrial committees including ad hoc CFC elimination with the U.S. EPA. He holds a B.A. degree in Chemistry and an M.B.A. degree. Contact: 25 Split Rock Dr., Chester, NY 10918 Phone: (845) 469-6956 E-mail: [email protected] Lou Rigali was founder and CEO for March Instrument and has had ties to the semiconductor industry for more than 25 years. He is now CEO of Ardency, Inc., an e-commerce company located in Northern California dealing with art and music. Contact: 247 4th St. #403, Oakland, CA 94607 Phone: (925) 827-1240 Stephen Risotto is the Executive Director for the Halogenated Solvents Industry Alliance (HSIA). He coordinates HSIA’s regulatory, legislative, and stewardship activities. He received a Stratospheric Ozone Protection Award from the U.S. EPA for work in finding alternatives to 1,1,1-trichloroethane for cleaning. He has a B.S. degree in Biology from Cornell and an M.S. degree in Marine Science from L.S.U. Contact: 1300 Wilson Blvd. Arlington, VA 22209 Phone: (703) 741-5780 E-mail: [email protected] Reva Rubenstein is an Environmental Health Scientist and Science Advisor in the Stratospheric Protection Division of the U.S. Environmental Protection Agency. She has been a Diplomate of the American Board of Toxicology since 1980. She holds a Doctorate in Physical Chemistry from Polytechnic Institute of Brooklyn. Her EPA experience is in both science and program policy related to risk assessment. She is responsible for assessing the risks to human health and the environment of substitute chemicals to protect the ozone layer. Contact: 401 M St., SW 6205J, Washington, D.C. 20460 Phone: (202) 564-9155 E-mail: [email protected] John F. Russo is the President and founder of Separation Technologists. He has worked in the high purity water, filtration, and wastewater treatment field for more than 25 years. He has authored more than ten technical papers. He is a recipient of a U.S. EPA Stratospheric Ozone Protection Award for leadership in closed-loop water recycling. He holds an Associate’s degree in Engineering and a B.S. degree in Chemistry. Contact: 100 Griffin Brook Park, Methuen, MA 01844 Phone: (978) 794-1170 E-mail: [email protected] Joe Scapelliti is National Marketing Manager for the Detrex Parts Cleaning Technologies Division. Joe has over 30 years of experience in cleaning equipment and chemistries, both solvent and aqueous applications. Joe has an engineering degree from Lawrence Technical College in Michigan. Contact: P.O. Box 5111, Southfield, MI 48086 Phone: 248) 358-5800 E-mail: [email protected] © 2001 by CRC Press LLC

Ronald L. Shubkin is Manager of Technical Services at Poly Systems USA, Inc. He has thirty-five years of experience in industrial research and development and has spent the last eight years developing formulations and applications for solvent systems based on normal-propyl bormide. He holds 30 U.S. patents. He received a B.S. degree in chemistry from the University of North Carolina, a Ph.D. degree in Inorganic Chemistry (Organometalics) from the University of Wisconsin, and an MBA from Michigan State University Contact: 18723 W. Piney Point Ave., Baton Rouge, LA 70817 Phone: (225) 751-7778 E-mail: [email protected] P. Daniel Skelly has held a wide variety of technical positions over the past 19 years, most recently as a Senior Technical Service Specialist at Occidental Chemical Corporation in Niagara Falls, NY. Dan has worked extensively in cleaning applications with chlorinated solvents and the Benzotrifluorides. He received his B.S. degree in Chemical Engineering from the University of Illinois and an M.B.A. degree from Niagara University. Contact: P.O.Box 344, Niagara Falls, NY 14302 Phone: (716) 278-7344 E-mail: [email protected] Stephen P. Swanson is the Applications Engineer for the Dow Corning OS Fluids. His responsibilities include training Dow Corning's customers, sales, and other technical professionals on solvent replacement. He also runs an applications lab for precision cleaning. Swanson has a B.S. degree in Chemical Engineering. Contact: 5 Corporate Park, Suite 280, Irvine, CA 92606 Phone: (949) 757-5000 E-mail: [email protected] Mahmood Toofan is the Technical Director of Semiconductor Analytical Services (SAS), an analytical service laboratory offering consulting and cleaning services to the semiconductor industry. He has designed and developed a spray washing system and particle-free wafer washing and drying equipment. Dr. Toofan has presented papers and holds patents in electrochemistry and microcontamination analytical processes. He has a B.S. degree from National University of Tehran and a Ph.D. degree from University of California, Davis. Contact: 2133 Cuesta Dr., Milpitas, CA 95035 Phone: (408) 994-2559 E-mail: [email protected] Masaaki Tsuzaki is Senior Engineer of the Technology and Process Development Center of Asahi Glass Company Limited. He joined Asahi Glass in 1990. He is now in charge of all research and development activities for applications of CFC-113 alternatives, especially HCFC-225. He received his Bachelor’s degree in Environmental Science from Tokyo University of Agriculture and Technology. Contact: 10, Goikaigan, Ichihara-shi, Chiba 290, Japan Phone: 81-436-23-3151 E-mail: [email protected] James L. Unmack heads Unmack Corporation. He has more than 30 years of experience in protecting workers' health and safety through the application of good science to the work environment. He is a registered Professional Engineer in the State of California for the practice of safety engineering. Mr. Unmack is also a Certified Industrial Hygienist and a Certified Safety Professional. He is a graduate of the University of California at Berkeley in Electrical Engineering and Santa Clara University in Bioengineering. Contact: 1379 Park Western Drive, #282, San Pedro, CA 90732 Phone: (310) 422-4340 E-mail: [email protected] Daniel J. VanderPyl is the President of Sonic Air Systems, Inc. He has been involved with indus© 2001 by CRC Press LLC

trial blower and air knife drying, application engineering, and product development for 23 years and has authored a variety of technical articles and industry presentations on parts drying. Contact: 1150 W Central Ave., Brea, CA 92621 Phone: (714) 255-0124 E-mail: [email protected] Donald J. Wuebbles is head of the Atmospheric Sciences Department and Director of the Environmental Council at the University of Illinois–Urbana. He is an internationally known expert on atmospheric chemistry who serves on numerous national and international panels related to global atmospheric change. Contact: 105 South Gregory, Urbana, IL 61801 Phone: (217) 244-1568 E-mail: [email protected]

© 2001 by CRC Press LLC