Advances in Urethane Science and Technology

Advances in Urethane Science and Technology Editors: D. Klempner K.C. Frisch Advances in Urethane Science and Technolo...

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Advances in Urethane Science and Technology Editors: D. Klempner K.C. Frisch

Advances in Urethane Science and Technology

Daniel Klempner and Kurt Frisch

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2001 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2001, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

ISBN: 1-85957-275-8

Typeset by Rapra Technology Limited Printed and bound by Lightning Source UK

Dedication

In memory of Kurt C Frisch One of the founding Fathers of polyurethanes

January 15th 1919 to October 21st 2000

Contents

1 Dimensional Stabilising Additives for Flexible Polyurethane Foams ................ 3 1.1 Introduction ............................................................................................. 3 1.2 Experimental Procedures .......................................................................... 6 1.2.1 Materials ....................................................................................... 6 1.2.2 Handmix Evaluations .................................................................... 8 1.2.3 Machine Evaluation ...................................................................... 9 1.3 TDI - Flexible Moulded Additives .......................................................... 15 1.3.1 Dimensional Stability Additives for TDI ...................................... 16 1.3.2 Low Emission Dimensional Stability Additives ............................ 42 1.4 MDI Flexible Moulded Foam Additives ................................................. 63 1.4.1 Dimensional Stability Additives for MDI .................................... 64 1.4.2 Low Emissions Dimensional Stability Additives in MDI.............. 67 1.5 TDI Flexible Slabstock Low Emission Additives ..................................... 73 1.5.1 Reactivity .................................................................................... 74 1.5.2 Standard Physical Properties ........................................................ 74 1.5.3 TDI Flexible Slabstock Foam Review .......................................... 74 1.6 Foam Model Tool Discussions ................................................................ 75 1.6.1 TDI and MDI Moulded Foam Model .......................................... 75 1.6.2 TDI Flexible Slabstock Foam Model ........................................... 78 1.7 Conclusions ............................................................................................ 81 2 Demands on Surfactants in Polyurethane Foam Production with Liquid Carbon Dioxide Blowing .................................................................... 85 2.1 History of Polyurethane Foams .............................................................. 85 2.1.1 Environmental Concerns in Relation to Flexible Foam Density ... 86

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Advances in Urethane Science and Technology

2.2 Current Liquid Carbon Dioxide Technologies for Flexible Slabstock ......... Polyether Foam Production .................................................................... 88 2.2.1 Machinery ................................................................................... 88 2.2.2 The Foaming Process ................................................................... 90 2.2.3 Additional Tasks of Silicone Surfactants in Flexible Slabstock Foam Production ......................................................................... 95 2.2.4 Chemistry of a Silicone Surfactant in Flexible Slabstock Foam Production ......................................................................... 99 2.2.5 A Surfactant Development Example .......................................... 101 3 Polyurethane Processing: Recent Developments ........................................... 113 3.1 Industrial Solutions for the Production of Automotive Seats Using Polyurethane Multi-Component Formulations ........................... 113 3.1.1 Market Requirements ................................................................ 113 3.1.2 Dedicated Solutions: Metering Equipment ................................ 114 3.1.3 Dedicated Solutions: Mixing Heads........................................... 116 3.1.4 Dedicated Solutions ................................................................... 121 3.2 ‘Foam & Film’ Technology - An Innovative Solution to Fully Automate the Manufacture of Automotive Sound Deadening Parts ..... 130 3.2.1 The Problem .............................................................................. 131 3.2.2 The Approach to a Solution ...................................................... 131 3.2.3 The Film .................................................................................... 133 3.2.4 Industrial Applications .............................................................. 135 3.2.5 Applications .............................................................................. 137 3.2.6 Advantages ................................................................................ 138 3.3 InterWet - Polyurethane Co-injection ................................................... 138 3.3.1 Glass-Reinforced Polyurethanes, a Well-Known Technology ..... 139 4 Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels for Super Insulation Applications ...................................................... 157 4.1 Introduction ......................................................................................... 157 4.2 Some General Properties of Open Cell PU Foams for Vacuum

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Contents

Insulated Panels .................................................................................... 158 4.3 Vacuum Issues in the Selection of VIP Components .............................. 163 4.3.1 Vacuum Properties of the Open Cell Foams .............................. 163 4.3.2 Vacuum Properties of the Barrier Film ....................................... 167 4.3.3 The Getter Device ...................................................................... 179 4.4 Vacuum Panel Manufacturing Process and Characterisation ................ 188 4.4.1 Some Manufacturing Issues ....................................................... 188 4.4.2 Characterisation of Vacuum Panels ........................................... 191 4.5 Insulation Performances of Open Cell PU-Filled Vacuum Panels .......... 196 4.6 Examples of VIP Applications and Related Issues................................. 199 4.6.1 Household Appliances ............................................................... 199 4.6.2 Laboratory and Biomedical Refrigerators .................................. 203 4.6.3 Vending Machines ..................................................................... 204 4.6.4 Refrigerated/Insulated Transportation ....................................... 205 4.6.5 Other Applications .................................................................... 206 4.7 Near Term Perspectives and Conclusions.............................................. 206 5 Modelling the Stabilising Behaviour of Silicone Surfactants During the Processing of Polyurethane Foam: The Use of Thin Liquid Films ................. 213 5.1 Introduction ......................................................................................... 213 5.2 Film Drainage Rate: Reynold’s Model and Further Modifications ........ 216 5.2.1 Rigid Film Surfaces .................................................................... 216 5.2.2 Mobile Film Surfaces ................................................................. 217 5.2.3 Surface Viscosity ........................................................................ 217 5.2.4 Surface Tension Gradients ......................................................... 218 5.3 Experimental Investigation of Model, Thin Liquid Polyurethane Films and the Development of Qualitative and Semi-Quantitative Models of Film Drainage ...................................................................... 219 5.3.1 Experimental Details ................................................................. 221 5.3.2 Qualitative Description of Polyurethane Films .......................... 223

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Advances in Urethane Science and Technology

5.3.3 Quantitative Measurement of Film Drainage Rates: Bulk and Surface Effects ............................................................ 226 5.4 The Development of Theoretical Models of Vertical, Draining Thin Liquid Model PU Films ................................................................ 236 5.4.1 Rigid-Surfaced Collapsing Wedge Model ................................... 236 5.4.2 Deforming Film Models ............................................................ 239 5.4.3 Tangentially-Immobile Films ..................................................... 242 5.4.4 Finite Surface Viscosity .............................................................. 245 5.4.5 Adding Surfactant Transport ..................................................... 249 5.5 Summary .............................................................................................. 254 6 Synthesis and Characterisation of Aqueous Hybrid Polyurethane-UreaAcrylic/Styrene Polymer Dispersions ............................................................ 261 6.1 Preface .................................................................................................. 261 6.2 Introduction ......................................................................................... 261 6.2.1 General Considerations ............................................................. 261 6.2.2 Acrylic Dispersions and Polyurethane Dispersions (DPUR) ....... 264 6.2.3 Hybrid Acrylic-Urethane Dispersions ........................................ 266 6.3 Concept of the Study ............................................................................ 268 6.3.1 Selection of Starting Materials ................................................... 268 6.3.2 Assumptions for Synthesis of Hybrid Dispersions ..................... 269 6.4 Methods of Testing ............................................................................... 276 6.4.1 Dispersions ................................................................................ 276 6.4.2 Coatings .................................................................................... 277 6.4.3 Films .......................................................................................... 278 6.5 Experimental results ............................................................................. 279 6.5.1 Characterisation of Starting Dispersions Used for Synthesis of MDPUR ................................................................................ 279 6.5.2 Synthesis of MDPUR and MDPUR-ASD ................................... 288 6.5.3 Investigation of the Effect of Various Factors on the Properties of Hybrid Dispersions ............................................... 290

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Contents

6.5.4 Additional Experiments ............................................................. 312 6.6 Discussion of results ............................................................................. 320 6.6.1 Estimation of the Effect of Various Factors on the Properties of Hybrid Dispersions and Films and Coatings Made from Them ................................................................................ 320 6.6.2 Mechanism of Hybrid Particle Formation ................................. 326 6.7 Summary ................................................................................................ 330 7 Adhesion Behaviour of Urethanes ................................................................ 335 7.1 Introduction ......................................................................................... 335 7.2 Surface Characteristics of PU Adhesive Formulations ........................... 335 7.2.1 Experimental ............................................................................. 336 7.2.2 Results and Discussion .............................................................. 338 7.2.3 Conclusions ............................................................................... 347 7.3 Acid/Base Interactions and the Adhesion of PUs to Polymer Substrates 347 7.3.1 Experimental ............................................................................. 348 7.3.2 Results and Discussion .............................................................. 351 7.4 The Effectiveness of Silane Adhesion Promoters in the Performance of PU Adhesives .................................................................................... 355 7.4.1 Experimental ............................................................................. 356 7.4.2 Results and Discussion .............................................................. 358 7.4.3 Conclusions ............................................................................... 364 8 HER Materials for Polyurethane Applications ............................................. 369 8.1 Introduction ......................................................................................... 369 8.2 Experimental Conditions ...................................................................... 370 8.2.1 Chain Extenders ........................................................................ 370 8.2.2 Prepolymers ............................................................................... 370 8.2.3 Preparation of Cast Elastomers ................................................. 372 8.2.4 Physical and Mechanical Properties Determination ................... 372 8.3 HER Materials Synthesis and Characterisation .................................... 373 v

Advances in Urethane Science and Technology

8.4 Cast Poly(Ether Urethanes) ................................................................... 375 8.4.1 Pot Life Determination .............................................................. 375 8.4.2 Polyurethane Castings ............................................................... 376 8.4.3 Calculation of Hard and Soft Segment Contents ....................... 376 8.4.4 Hard Segment Versus Hardness ................................................. 378 8.4.5 Tensile Properties ....................................................................... 378 8.4.6 Tear, Compression Set and Rebound Properties......................... 380 8.4.7 Differential Scanning Calorimetry ............................................. 381 8.4.8 Dynamic Mechanical Analysis ................................................... 383 8.5 Cast Poly(Ester Urethanes) ................................................................... 390 8.5.1 Pot Life Determination .............................................................. 390 8.5.2 Tensile Properties ....................................................................... 390 8.5.3 Tear, Fracture Energy, Compression Set and Rebound Properties ................................................................... 390 8.5.4 Differential Scanning Calorimetry Analysis ............................... 393 8.5.5 Dynamic Mechanical Analysis ................................................... 395 8.6 Cast Polyurethanes from HER/HQEE Blends ....................................... 397 8.6.1 Freezing Point Determination of HER/HQEE Blends ................ 397 8.6.2 Cast elastomers and Their Properties......................................... 398 8.7 High Hardness Cast Polyurethanes ....................................................... 401 8.7.1 Cast Elastomers and Their Hard and Soft Segment Contents .... 401 8.7.2 Hardness, Tensile, Tear, Compression Set and Rebound Properties ................................................................... 401 8.7.3 FT-IR Analysis of Cast Polyurethanes ........................................ 403 8.7.4 Differential Scanning Calorimetric Analysis .............................. 405 8.7.5 Dynamic Mechanical Analysis ................................................... 405 8.8 High Thermal Stability Polyurethane with Low Heat Generation ........ 405 8.8.1 Hardness Measurements ............................................................ 408 8.8.2 Tensile Measurements ................................................................ 408 8.8.3 Differential Scanning Calorimetric Analysis .............................. 410

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Contents

8.8.4 Dynamic Mechanical Analysis ................................................... 412 8.9 Conclusions .......................................................................................... 416 9 Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes ......................................................................................... 421 9.1 Introduction ........................................................................................... 421 9.2 MDI/BDO Cured Elastomers Based on Ultra-Low Monol PPG Polyols . 424 9.2.1 Effect of Monol Content on 4,4´-Methylene Diphenylmethane Diisocyanate (MDI)/1,4-Butanediol (BDO) Cured Elastomers... 424 9.2.2 Processability and Property Latitude of Elastomers Based on Ultra-Low Monol PPG Polyols .................................................. 429 9.2.3 Processing Latitude Improves by Incorporating Oxyethylene Moieties ................................................................ 434 9.3 One-Shot Elastomer System Based on EO-Capped, Ultra-Low Monol PPG Polyols .............................................................................. 436 9.3.1 Effect of Primary Hydroxyl Concentration on One-Shot Elastomer Processability ............................................................ 436 9.3.2 Effect of Monol Content on One-Shot Elastomer Processability and Properties............................................................................ 438 9.3.3 Processability and Property Latitude of Elastomers Based on EO-Capped, Ultra-Low Monol Polyols ................................ 440 9.4.1 MDI/BDO Cured Elastomers: Acclaim Polyol 3205 Versus ............. PTMEG-2000 ............................................................................ 445 9.4.2 Enhanced Elastomer Properties Utilising Ultra-Low Monol PPG/PTMEG Blends .................................................................. 447 9.5 Polyol Molecular Weight Distribution Effect on Mechanical and Dynamic Properties of Polyurethanes ............................................ 449 9.5.1 TDI Prepolymers Cured with Methylene Bis-(2-Chloroaniline) [MBOCA] .................................................................................. 450 9.5.2 Moisture-Cured TDI Prepolymers ............................................. 454 9.5.3 Aqueous Polyurethane/Urea Dispersion Coatings ...................... 456 9.5.4 MDI Prepolymers Cured with BDO .......................................... 459 9.6 Conclusions .......................................................................................... 461

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Advances in Urethane Science and Technology

APPENDIX........................................................................................................ 465 Laboratory Preparation of 2,4-TDI and 4,4´-MDI Prepolymers ................... 465 Laboratory Casting of 4,4´-MDI Prepolymers Cured with BDO .................. 465 Laboratory Casting of One-Shot Elastomers Based on CarbodiimideModified MDI, Polyol, and BDO ................................................................. 465 Laboratory Casting of 2,4-TDI Prepolymers Cured with MBOCA .............. 466 Laboratory Moisture-Curing of 2,4-TDI Prepolymers ................................. 466 Laboratory Preparation of Aqueous Polyurethane/Urea Dispersions using the Prepolymer Mixing Process ........................................................... 466 Abbreviations .................................................................................................... 469 Contributors ...................................................................................................... 473 Author Index ..................................................................................................... 477 Main Index ........................................................................................................ 483

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Preface

This is a landmark issue of ‘Advances in Urethane Science and Technology’. Not only is this the first volume of the new millennium, but it is the first to be published by Rapra Technology. On a more solemn note, one of the editors, Kurt C. Frisch, passed away shortly before publication. Dr. Frisch, founder of the University of Detroit Mercy’s Polymer Institute, was one of the pioneers of polyurethanes and was responsible for the successful introduction of polyether polyurethane flexible foams into commerce in the mid-1950s. Let us not only mourn the loss of, but also celebrate the life of this great scholar by continuing to further the frontiers of urethane science and technology. This volume is a good example of this progress. Polyurethanes continue to be one of the most versatile of all polymers, finding applications in foams (flexible, rigid, and in-between), elastomers, coatings, sealants, adhesives, paints, textiles, and films. This volume presents some of the major advances in polyurethanes, both from the materials and research side of things as well as processing and applications, and includes studies on foams (additives, vacuum panel applications, blowing and processing), elastomers, adhesion behaviour and new urethane raw materials. I would like to take this opportunity to express my gratitude to the authors who contributed to this book and to the University of Detroit Mercy for its encouragement of this effort. I would also like to thank the staff of Rapra, in particular, Frances Powers, Claire Griffiths and Steve Barnfield. Daniel Klempner, Ph. D. Polymer Institute,University of Detroit Mercy July, 2001

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Advances in Urethane Science and Technology

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Gary D. Andrew, Jane G. Kniss, Mark L. Listemann, Lisa A. Mercando, James D. Tobias and Stephan Wendel

1.1 Introduction The issues that an automotive seat manufacturer faces when formulating and producing seats are escalating. Physical properties such as tensile and tear strengths, compression set and wet set are critical when meeting specific mechanical performance requirements as defined by the original equipment manufacturer (OEM). As new requirements for comfort and durability are instituted, tests such as dynamic creep testing, long term vibration characterisation and repeated compression tests under various atmospheric and load conditions have been used to characterise foam performance for comfort. Comfort properties are best controlled by the polyols used to produce the polyurethane foam cushion. Significant changes in polyol technology to meet these dynamic comfort properties have had an impact on the processing of polyurethane foam and on physical properties. Increased tightness of the foam article resulting from changes in these raw materials has focused more attention by foam producers on crushing methods. Flexible moulded polyurethane foam requires some type of mechanical crushing to prevent shrinkage and ultimately maintain part stability. With recent changes made to polyol technology, mechanical methods of crushing do not always provide the consistency required to produce a part that is dimensionally stable. Additionally, producers of polyurethane articles are continually building more complexity into their seat designs to meet the aesthetic values required by today’s consumers. These complex seat designs place more emphasis on crushing capability due to the nature of the designs. With all these changes, additives needed to be developed which provide a wider processing latitude and increased breathability to the polyurethane article. Wider processing latitudes should reduce scrap and repair rates on the foam production line and improve economics for the polyurethane producer [1]. The formation of moulded foam is a complicated chemical process which involves several reactions occurring simultaneously. There are rapid volume and temperature increases and the concurrent development of phase separated polymer networks. To understand how foam properties can be affected by catalyst and surfactant chemistries several techniques are used to identify key performance benefits and issues. A force to crush

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Advances in Urethane Science and Technology (FTC) detection device was used to measure the force required to crush a part to 50% of its thickness for determination of cell openness. Mass-loss/rate-of-rise was run to understand rate of rise and height measurements, weight loss from carbon dioxide generation and temperature profiles. A scanning electron microscope (SEM) was used to determine differences in cell structure and cell distribution caused by changes in the catalyst and surfactant chemistries. Physical properties were also tested using ASTM test methods for flexible cellular polyurethane. A novel chemical reaction foam modelling technique was also used to determine the selectivity of the catalyst packages, compared to industrial standard controls [2]. In the past it was thought that the cell structure of polyurethane foam is controlled by the type and amount of surfactant used. Dabco DC5043 (Air Products and Chemicals, Inc.) was developed to enhance cell wall drainage to better enable cell opening during crushing cycles. It was also thought that surfactant technology was the best way to provide improved crushing techniques; therefore, catalyst technology was ignored [3]. As mentioned earlier, with new polyol technology development more emphasis was placed on crushing. New additive technology needed to be developed that would open cells during the foam formation and reduce the requirement and criticality of the crushing processes. The technology had to go beyond providing easier cell opening at crush to providing more open cells during the polyurethane formation. The real challenge in polyurethane foam formation is to control the chemical and physiochemical processes up to the point where the material finally sets. The sequence and the rate of the chemical reactions are predominately a function of the catalyst and the reactivity of the basic raw materials, polyol and isocyanate. The physiochemical contribution to the overall stability and processability of a system is provided by the silicone surfactants. Optimum foaming results will be achieved only if the correct relationship between chemistry and physics exists [4]. Another rapidly increasing environmental concern is over the emission of volatile organic compounds (VOC) during and following the production of industrial and consumer goods. This has stimulated a great deal of effort within the chemical industry to reduce and/or control the ways in which such emissions may occur. In the polyurethane foam industry, efforts to reduce VOC emissions have greatly impacted the technologies used in manufacturing processes, especially for the use of organic auxilliary blowing agents such as chlorofluorocarbons. In addition, the ultimate fate of additional foaming additives, including surfactants and catalysts, is now coming under increased global scrutiny. As a result, foam manufacturers have expressed a desire for polyurethane additives that, among other things, do not exhibit the degrees of fugitivity common to many of the additives that are currently used in polyurethane foam production today.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams Polyurethane foams are prepared from the simultaneous reactions of diisocyanate with water and with polymeric diols and/or triols to form hydrogen-bonded urea (hard) segments and polyurethane networks (soft segments). The commercial production of polyurethanes via isocyanate poly-addition reactions requires the use of one or more catalysts. Tertiary amines are widely accepted in the industry as versatile polyurethane catalysts. Amine catalysts are generally stable in the presence of standard polyurethane formulation components and can have an impact on both the blowing (water-isocyanate) and gelling (polyol-isocyanate) reactions. Although the use of catalysts in the manufacturing of polyurethane foam both speeds the production of the foamed article and, through the judicious choice of catalyst package, allows control of the physical properties of the product, there are some problems associated with the use of these additives. A number of commonly used tertiary amine catalysts can volatilise under certain conditions. Release of tertiary amines during foam processing and from consumer products is generally undesirable. Therefore, identifying alternatives to standard tertiary amine catalysts which have no or low volatility, yet exhibit the same type of activity in isocyanate poly-addition reactions, is desirable. The non-fugitive catalysts reported in this chapter address the problems associated with the use of polyurethane catalysts by reducing the odour and volatility of these materials and by eliminating the ability of these additives to escape from finished foam products. One strategy has involved functionalising the catalysts to render the species reactive toward isocyanates, thereby covalently attaching the catalysts to the polymer network. This strategy not only renders the catalytic material non-fugitive in the final product, but also reduces the odour and volatility of the catalyst through increases in molecular weight and polarity. These non-fugitive catalysts also provide equivalent or improved physical properties when compared to industry standards, whereas conventional reactive amine catalysts as well as metal catalysts cannot always meet todays ever increasing manufacturer and consumer performance requirements. These increasingly evolving requirements have led to the development of both novel non-fugitive catalysts and new cell-opening non-fugitive catalysts for flexible foam. These new low emission additives have been developed to meet the challenge of optimised foaming and result in little or no emissions. Several of the non-fugitive catalysts possess cell-opening capability. This new technology allows the manufacturer of polyurethane foam to optimise their system to achieve the best processing latitude for their foam process. These new additives maintain, or in some cases, improve key physical properties while providing a more open foam.

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Advances in Urethane Science and Technology

1.2 Experimental Procedures Data presented herein was derived from a combination of handmix and high pressure impingement-mixing machine produced foam. Foams were prepared using several general types of formulations for toluene diisocyanate (TDI) and two general types of formulations for methylenediphenyl diisocyanate (MDI) which are representative of currently utilised formulations in the automotive interior component industry. In addition, an all water blown formulation was used to represent the flexible slabstock industry.

1.2.1 Materials The materials used are shown in Table 1.1.

Table 1 Materials used in experimental work Trade name

Formulation

Manufacturer

Dabco 33LV

33% TEDA in DPG

APCI

Dabco BL-17

Delayed action tertiary amine blowing catalyst.

APCI

Dabco BL-53

Newly developed tertiary amine, which provides blowing and cell opening capabilities.

APCI

Dabco B-16

Tertiary amine surface cure catalyst.

APCI

Polycat 15 (PC-15) Balanced reactive amine catalyst.

APCI

Dabco BLV

Dabco 33-LV/Dabco BL-11 in a 3:1 catalyst blend.

APCI

Dabco T-9

Stannous octoate catalyst.

APCI

Dabco NE1060

Newly developed non-fugitive gelling catalyst for APCI flexible moulded applications.

XF-N1085

Newly developed non-fugitive cell opening blowing catalyst.

Dabco NE500

Newly developed non-fugitive gelling catalyst for APCI flexible slabstock foam from APCI.

Dabco NE600

Newly developed non-fugitive intermediate blowing catalyst for flexible slabstock applications.

APCI

Dabco NE200

Newly developed non-fugitive intermediate blowing catalyst for flexible slabstock applications.

APCI

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APCI

Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1 Continued Trade name

Formulation

Manufacturer

XF-O11006

Newly developed non-fugitive cell opening gelling catalyst.

APCI

Dabco DC5169 Silicone copolymer surfactant for cold cure systems.

APCI

Dabco DC5043 Silicone copolymer surfactant for TDI cold cure systems.

APCI

Dabco DC2585 Silicone copolymer surfactant for MDI cold cure systems.

APCI

Dabco DC2517 Silicone copolymer surfactant.

APCI

Dabco DC2525 Silicone copolymer surfactant.

APCI

Dabco DC5258 Silicone copolymer surfactant.

APCI

XF-N1586

Newly developed silicone copolymer surfactant, which promotes open cells.

APCI

XF-N1587

Newly developed silicone copolymer surfactant, which promotes open cells.

APCI

DEOA-LF (Dabco)

Diethanolamine Liquid Form (85% DEOA: 15% water)

APCI

Arcol E848

Conventional polyol with an OH# of 31.5

Lyondell Chemical

Arcol E851

43% solids copolymer polyol with an OH# of 18.5 Lyondell Chemical

NC-630

Polyol with an OH# of 31.4

NC-700

41% solids copolymer polyol with an OH# of 21.0 Dow Chemical

Voranol 3512

Polyether polyol with an OH# of 48.3

Dow Chemical

Polyol A

High functionality triol with an OH# of 32.5

Dow Chemical

Polyol B

41% solids copolymer triol with an OH# of 23.8

Dow Chemical

Polyol C

Polyether polyol with an OH# of 28

Dow Chemical

Polyol D

Cell opening polyol

PRC-798

Solvent-based release agent

Dow Chemical

Chem-Trend

APCI: Air Products and Chemicals, Inc. DEOA: Diethanolamine DPG: Dipropylene glycol TEDA: Triethylene diamine

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Advances in Urethane Science and Technology

1.2.2 Handmix Evaluations 1.2.2.1 Flexible Moulded Foam Handmix Procedure Handmix experiments were carried out using the following procedure. Formulations were blended together for approximately 10 minutes using a mechanical mixer equipped with a 7.6 cm diameter, high shear, mixing blade, rotating at 5000 rpms. Premixed formulations were maintained at 23 ± 1 °C using a low temperature incubator. Mondur TD-80 (Bayer; a blend of 2,4-TDI and 2,6-TDI isomers in the ratio of 4:1) or modified MDI was added to the premix at the correct stoichiometric amount for the reported index for each foam. The mixture was blended together with a Premier Mill Corporation Series 2000, Model 89, dispersator for approximately five seconds. The foaming mixture was transferred to an Imperial Bondware #GDR170 food container or ‘chicken’ bucket and allowed to free rise in order to obtain the processing data.

1.2.2.2 Flexible Slabstock Foam Handmix Procedure Handmix experiments were carried out using the following procedure. A premix consisting of polyol, surfactant and water was prepared by blending the components in a shaker for approximately 20 minutes. The premix was allowed to stand for 2 hours prior to making the foam to allow for degassing of the mixture. A measured amount of premix was poured into a 1.9 litre paper cup; the required stoichiometric amounts of amine and tin catalysts were added to the contents of the cup and mixed for 20 seconds using a Premier Mill Corporation dispersator equipped with a 5.5 cm diameter, high shear, mixing blade, rotating at 6,000 rpm. The corresponding amount of Mondur TD-80 to provide for a 110 index (isocyanate index, which is the amount of isocyanate used relative to the theoretical equivalent amount [5]) was measured into a 400 cm3 beaker. Methylene chloride in the correct proportion was added to the beaker containing the Mondur TD-80; the beaker was carefully swirled for 4 or 5 seconds and the contents poured into the paper cup. The mixture was blended together for 6-7 seconds and the foaming mixture poured into a paper bucket for up to 12 seconds and allowed to free rise with the processing data being recorded. Reactivity profiles were determined from hand-mix foams prepared in 5.68 litre paper buckets. Foams for physical properties were prepared in 35.6 x 35.6 x 25.4 cm cardboard boxes. Identical procedures were followed for both reactivity and physical property experiments.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

1.2.3 Machine Evaluation 1.2.3.1 TDI Flexible Moulded Foam Procedure Machine runs for the TDI flexible moulded foam were carried out on a Hi Tech SureShot MHR-50 (Hi-Tech Industries, Inc.), cylinder displacement series, high pressure machine. Fresh premixes, consisting of the appropriate polyols, water, crosslinker, surfactants and catalysts for each formulation were charged to the machine. Mondur TD-80 was used throughout the entire study. All chemical temperatures were held at 23 ± 2 °C via the machine’s internal temperature control units. The foam was poured into an isothermally controlled, heated aluminium mould maintained at 71 ± 2 °C. The mould was a typical physical property tool designed with internal dimensions of 40.6 cm x 40.6 cm x 10.2 cm. The mould has five vents, each approximately 1.5 mm in diameter, centred 10.0 cm from each edge and the geometric centre of the lid. The mould was sprayed with a solvent-based release agent, Chem-Trend PRC-798, prior to every pour and allowed to dry for one minute before pouring. The foam premix was puddle poured into the centre of the mould with a wet chemical charge weight capable of completely filling the mould and obtaining the desired core density. Minimum fill requirements were established for each formulation evaluated. The foam article was demoulded at 240 seconds after the initial pour. After demoulding, the foam was placed through a mechanical crusher, tested for FTC measurements, or left uncrushed and set aside for 24 hour shrinkage measurements described in Section 1.2.3.2c. All foams to be tested in each catalyst set were mechanically crushed 1 minute after demoulding using a Black Brothers Roller crusher set to a gap of 2.54 cm. Crushing was carried out three times on each part, rotating the foam 90 degrees after each pass through the roller. All parts produced for physical testing were allowed to condition for at least seven days in a constant temperature and humidity room (23 ± 2 °C, 50 ± 2% relative humidity). Three to four specimens were produced for any given set of conditions. Four test specimens were die-cut from each foam pad and evaluated for each physical property listed in subsequent data tables. All results were included in calculating averages and standard deviation. Each test was carried out as specified in ASTM D3574 [5]. For each formulation evaluated, duplicate free rise ‘chicken’ buckets were poured at the same shot size to determine overall reactivities and foam shrinkage. Data recorded were cream time (the time between the discharge of the foam ingredients from the mixing head and the beginning of the foam rise [5]), top-of-cup (TOC; the time between the discharge of the foam ingredients from the mixing head and when the centre of the foam reaches the same height as the top of the chicken bucket), string gel (the time between

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Advances in Urethane Science and Technology pouring of the mixed liquids and the time that strings of viscous material can be pulled away from the surface of the foam when it is touched with a tool [5]), full rise time and final height. The free rise buckets were again tested for final heights after 24 hours. Measurements of height were made using a Mitutoyo height gauge. In addition to all the standard tests, several more unique tests were performed where indicated, and are described in Section 1.2.3.2.

1.2.3.2 Tests 1.2.3.2a Maze Flow Mould Test Description A common type of isothermally heated mould was used to determine the flowability of formulations with each of the catalyst candidates. This maze mould is shown in Figure 1.1. Machine foam was poured into the mould at the top left corner of the open cavity as indicated by ‘pour spot’ on the figure. The lid was then closed and clamped tightly. Foam was allowed to free flow consecutively through each of the five gates for the standard 4

Figure 1.1 Diagram of Maze Flow Mould (Top View)

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Dimensional Stabilising Additives for Flexible Polyurethane Foams minutes prior to demould. Minimum fill was first determined by completely filling the cavity with little or no extrusion through the vent at the end of the fifth gate. Mathematical reduction of the shot size was performed to obtain the first of three systematically scaled down foam fill weights. This first foam should have a fifth leg (the foam in gate 5 of the maze flow mould, see Figure 1.1) which barely touches the front cavity wall. The second reduction in foam fill weight produced a foam that flowed approximately halfway through the fifth gate. The third reduction in foam fill weight was equivalent to the step change from foam 1 to foam 2. Shot times were held constant for each of the three foam fill weights as compared to the control determined standard shot time in any given solids level formulation. These three foams were weighed for total foam pad and fifth leg weight, and measured for fifth leg length to obtain a range of flow values for each of the experimental catalysts compared to the control additives.

1.2.3.2b Dimensional Stability Test Foam dimensional stability is essentially the result of a balance between external and internal forces. The external forces are defined as the ambient pressure along with any additional applied loads. The internal forces are the strength of the polymer matrix and the internal cell pressure [6]. Basically, if the sum of the internal forces is greater than the external forces, the foam will expand. Consequently, if the sum of the external forces is greater than the internal forces the foam will shrink. Any expansion or shrinkage will impact on the internal and/or external forces until an equilibrium is obtained. It is the internal forces, i.e., cell pressure and strength of the polymer matrix as defined by ‘green strength’ or cure, which will have an impact on the dimensional stability performance of the moulded polyurethane. Dimensional stability can be measured on a freshly demoulded part by determining the amount of force required to open cells, as measured by FTC. FTC measurements were made thirty seconds after demoulding. The foam pad was removed from the mould, weighed and placed in the FTC apparatus. The force detection device is equipped with a 2.2 kg capacity pressure transducer mounted between the 323 cm2 circular plate cross head and the drive shaft. The actual force is shown on a digital display. This device mimics the ASTM D3574, Indentation Force Deflection Test [6] and provides a numerical value of the freshly demoulded foam’s initial hardness or softness. The foam pad was compressed to 50 percent of its original thickness at a cross head velocity of 275 mm per minute with the force necessary to achieve the highest compression cycle recorded in whole Newtons. Several compression cycles were completed. A cycle takes approximately 30 seconds to complete. Values are reported as the FTC value for the foam based on the assumption that the lower the FTC values the better the dimensional stability of the foam. This test requires the foam to be fully cured at demould. A dimensionally stable

11

Advances in Urethane Science and Technology foam will exhibit little or no tendency to shrink after demoulding. Poor dimensional stability can result in numerous defects of the polyurethane article, such as lack of fit of a polyurethane piece to the substrate. These defects will ultimately cause loss of revenue to the polyurethane manufacturers because of increased repair and/or scrap rates. Additionally the degree of cell openness of polyurethane foam can be measured directly by the air flow physical properties of the polyurethane part. Higher air flow values measured for a particular foam would indicate that the foam has less of a tendency to shrink and therefore be more dimensionally stable as compared to a foam with lower air flows. Additionally, higher air flows may also indicate that the foam was much easier to crush-out thereby breaking many of the cell windows. Dimensionally stable foam should reduce scrap and rework by allowing the foam to conform to near its original moulded shape or at least return to its original shape after being crushed.

1.2.3.2c Shrinkage Test An additional dimensional stability evaluation was done with a shrinkage template apparatus (Figure 1.2).

Figure 1.2 Diagram of Shrinkage Template Apparatus

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Dimensional Stabilising Additives for Flexible Polyurethane Foams It was designed to measure the average foam pad shrinkage of an uncrushed foam. This apparatus consists of two 432 mm long x 432 mm wide x 6.35 mm thick, plexiglass plates, mechanically pinned in each corner with threaded bolts to maintain the plates at a constant 102 mm spacing. The single uncrushed foam from each of the catalyst sets was aged 24 hours prior to being placed between the two plates. Both top and bottom plates each contain 5 mm diameter holes evenly spaced, diagonally from corner to corner, 25 mm apart in an X-shaped pattern. Nineteen holes are contained in each leg of the X, for a total of 37 holes per plate. Measurements were made with a digital caliper by inserting the end down through each hole to just touch the foam surface with the indicated value being recorded. All measurements were normalised to discount the plexiglass plate thickness and subsequently averaged to a single mould cavity and lid value.

1.2.3.2d Time Pressure Release Test Time Pressure Release (TPR) is the opening of the mould during the curing cycle to release the internal pressure and then re-closing for the duration of the cure time [8]. The sudden release of the internally generated pressure bursts the cell windows, thereby improving the crushability of the foam. The tool is opened only a few millimetres and for a specific time. TPR can be applied at any time during the curing cycle, however, care must be taken not to perform the operation too early or too late since surface quality issues may occur. A ‘simulated’ TPR process was carried out during this study, whereby the tool lid was opened approximately 1.5 mm for a three second duration. TPR was applied at various time intervals throughout the evaluation. Two mechanical clasps affixed to the top and bottom halves of the tool precisely controlled the gap opening. These clasps were manually opened and closed at the desired TPR time interval.

1.2.3.2e Pail Test Another test was devised to evaluate foam bulk stability in a free-rise mixed foam. Foams prepared from TDI formulations were poured directly from the machine head into a large open pail (the pail is a common high density polyethylene plastic with approximate dimensions of 365 mm high and 290 mm in diameter) at a targeted mass. Several pours were carried out to ensure equivalent catalyst activity amongst each formulation. Foams were allowed to stand for 24 hours prior to removal from the pail. Each foam was weighed to obtain total individual mass. Subsequently, a 25 mm slice was cut directly through the geometric vertical centre of the foam. Foam slices were examined for cell structure.

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Advances in Urethane Science and Technology 1.2.3.2f Dynamic Fatigue Test Dynamic Fatigue Constant Pounding testing was carried out using standard testing procedures outlined in ASTM D3574-95 [6]. A 60 minute recovery time and 80,000 cycles were used for each test sample.

1.2.3.2g Fogging Test Gaseous foam emissions were compared in the standard fogging test procedure outlined in SAE J1756 [9]. A standard 7.6 cm thick piece of foam was preconditioned for 48 hours, then tested in the Hart fogging apparatus at 100 °C for 3 hours. Glossmeter readings of the foam were taken after 60 minutes at room temperature.

1.2.3.2h Headspace Analysis Test Foam emissions were also evaluated by cutting equivalent portions, approximately 1 gram, of foam from the geometric centre of the moulded foam articles 60 minutes after demoulding. Each sample was inserted into a 20 cm3 Kimble glass crimp top vial with a Teflon seal. Several vials were sealed without foam to be used as blanks to ensure all emissions had eluted through the gas chromatograph prior to the injection of a second gas sample. All vials were loaded into a Tekmar 7000 Headspace Autosampler tray for sequential heating for 1 hour at 54 °C. After heating and temperature equilibration, the headspace of the vial was sampled and directly injected within a closed loop system onto a Hewlett Packard 5890 Series II Plus gas chromatograph containing a HP-5 (5%-diphenyl/95%dimethylsiloxane copolymer) stationary phase column (30 m, 0.25 mm internal diameter, 1.00 mm film thickness). A standard oven heating profile was used to separate the gas components for detection with a Hewlett Packard 5972 Series Mass Selective Detector. Elution peaks were individually identified by comparison to standard libraries.

1.2.3.3 MDI Flexible Moulded Foam Procedure Machine runs for the MDI flexible moulded foam were conducted on a Krauss-Maffei, cylinder displacement series, high pressure machine. Fresh premixes, consisting of the appropriate polyols, water, crosslinker, surfactants and catalysts for each formulation were charged to the machine. Modified MDI was used throughout the entire study. All chemical temperatures were held at 25 °C ± 2 °C via the machine’s internal temperature control units. Foam pours were made into an isothermally controlled heated aluminium mould maintained at 60 °C ± 2 °C. The mould was a typical physical property tool designed with

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Dimensional Stabilising Additives for Flexible Polyurethane Foams internal dimensions of 40.6 cm x 40.6 cm x 10.2 cm. The mould has two vents each approximately 1.0 mm in diameter centred 10.0 cm from each edge and the geometric centre of the lid. The mould was sprayed with a solvent-based release agent prior to every pour and allowed to dry for one minute before pouring. The foam premix was puddle poured 15 cm away from the geometric centre of the mould with a wet chemical charge weight capable of completely filling the mould with the appropriate core density. Minimum fill requirements were established for each formulation evaluated. The foam article was demoulded at 300 seconds after the initial pour. Upon demoulding, the foam was placed through a mechanical crusher or tested for FTC measurements. The foams were mechanically crushed 1 minute after demoulding using a roller crusher set to a gap of 3.0 cm. Crushing was carried out three times on each part. All parts produced for physical testing were allowed to condition for at least seven days in a constant temperature and humidity room (23 °C ± 2 °C, 50% ± 2% relative humidity). Three to four parts were produced for any given set of conditions. Four test specimens were die-cut from each pad and evaluated for each physical property listed. All results were included in calculating the averages and standard deviation. Each test was conducted as specified in ASTM D3574 [5]. For each formulation evaluated, free rise cup foams (see Section 1.2.2.1) were poured to determine reactivities and foam shrinkage. Data recorded were gel time, full rise time and final height. The free rise cup foams were tested for final heights and free rise density after 24 hours. Height measurements were carried out using a Mitutoyo height gauge. All experimental formulations reported in this work were matched by rise profile to each control formulation. FTC measurements were conducted 90 seconds after demoulding. The foam pad was removed from the mould, weighed and placed in the FTC apparatus (Instron 4502). The force detection device is equipped with a 5.0 kN capacity pressure transducer. The actual force is shown on a digital display. This device mimics the ASTM D3574, Indentation Force Deflection Test and provides a numerical value of freshly demoulded foams initial hardness or softness. The pad was compressed to 70 percent of its original thickness at a cross head velocity of 380 mm per minute with the force necessary to achieve the highest compression cycle recorded in Newtons. Values are reported as the FTC value for the foam based on the assumption that the lower the FTC values the better the dimensional stability of the foam.

1.3 TDI - Flexible Moulded Additives The automotive industry has placed increased pressures on OEM suppliers to improve their productivity, quality and cost of the polyurethane articles which they produce. Styling changes, complex designs and OEM productivity demands for automotive seats have necessitated the need to produce more open foam in all-water blown TDI- and

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Advances in Urethane Science and Technology MDI-based systems. Furthermore, changes in polyol technologies designed to improve seat comfort factors can have a negative impact on foam openness and dimensional stability. Specifically, the comfort of high resilience foam may be related to the degree of dimensional stability or foam openness. Flexible moulded polyurethane foam requires mechanical crushing to open foam cells, which in turn prevents shrinkage and improves overall dimensional stability. Current mechanical methods for cell opening consist mainly of roller crushing, vacuum rupture and TPR. However, mechanical methods do not always result in complete or consistent cell opening and require a flexible moulded foam producer to invest in additional machinery. Additionally, if the polyurethane article is not crushed properly, dimensional stability suffers which can cause an increase in repair and scrap rates resulting in a negative impact on the cost of production. A chemical method for cell opening would be preferred.

1.3.1 Dimensional Stability Additives for TDI When producing flexible high resilience foam, it is important to provide a wider TPR window to expand processing latitude and at the same time maintain or improve physical properties. This should result in reduced scrap and/or repair rates, providing improved economics for the polyurethane producer. The most commonly used catalyst and surfactant package for all water blown TDIbased moulded foam production is a blend of Dabco 33-LV and Dabco BL-11, coupled with Dabco DC5043 silicone surfactant. Additionally, the acid-blocked counterparts of these two catalysts, Dabco 8154 and Dabco BL-17, can also be used for the production of high resilience moulded foam. A combination of Dabco 33LV and Dabco BL-17 is used to facilitate a short delay in the reactivity of the polyurethane foaming process. A combination of silicone surfactants, Dabco DC5043 and Dabco DC5169, are utilised to provide good foam stabilisation, improve cell regulation and cell wall drainage. These combinations of catalysts and surfactants served as the control additives to which the experimental additives were compared and contrasted. A newly developed cell opening catalyst, Dabco BL-53, was evaluated to determine its impact on dimensional stability and general processability. Dabco BL-53 affords all the benefits of Dabco BL-11 or Dabco BL-17, with the added advantage of cell opening and slightly delayed initiation times. Dabco BL-53 is not a chemical equivalent for Dabco BL-11 or Dabco BL-17; however, it will provide similar performance. For rapid demoulding systems, it is recommend that Dabco BL-53 be used at 0.12 to 0.22 pphp, with the optimum level at 0.16 to 0.19 pphp, in combination with a Dabco 33-LV level at 0.30 to 0.32 pphp.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams As polyurethane seating design changed from the relatively simple configurations of the early 1990s to the more complex designs of today, the need to improve cell opening or dimensional stability has intensified. Accordingly, silicone surfactants providing improved foam stabilisation, cell regulation and cell wall drainage were needed to enable polyurethane manufacturers to achieve their production goals. Two experimental silicone surfactants, X-N1586 and X-N1587, were developed to provide open foam and promote dimensional stability. A TDI cushion formulation, with a density of 45 kg/m3 and a TDI back formulation, with a density of 35 kg/m3 were used in the TDI automotive study. All of these formulations were modified accordingly with the appropriate crosslinker, water, and additive levels for the chosen density range. These formulations are shown in Tables 1.2 and 1.3.

Table 1.2 ~45 kg/m3 cushion formulation Formulation

Control

BL53

Exp. SSF

BL53/Exp. SSF

Identification

I

II

III

IV

Components

pphp

pphp

pphp

pphp

Polyol A

68.00

68.00

68.00

68.00

Polyol B

32.00

32.00

32.00

32.00

Water

2.77

2.77

2.77

2.77

Dabco 33LV

0.30

0.30

0.30

0.30

Dabco BL-17

0.18

-

0.18

-

Dabco BL-53

-

0.20

-

0.20

Dabco DC-5043

0.75

0.75

-

-

Dabco DC-5169

0.25

0.25

-

-

X-N1586

-

-

0.3

0.3

X-N1587

-

-

0.7

0.7

Dabco DEOA-LF

1.53

1.53

1.53

1.53

TDI (100 index)

37.3

37.3

37.3

37.3

Exp. SSF: experimental silicone surfactant pphp: parts per hundred polyol

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Advances in Urethane Science and Technology

Table 1.3 ~35 kg/m3 back formulation Formulation

Control

BL53

Exp. SSF

BL53/Exp. SSF

Identification

V

VI

VI I

VIII

Components

pphp

pphp

pphp

pphp

Polyol A

80.00

80.00

80.00

80.00

Polyol B

20.00

20.00

20.00

20.00

Water

3.27

3.27

3.27

3.27

Dabco 33LV

0.30

0.30

0.30

0.30

Dabco BL-17

018

-

0.18

-

Dabco BL-53

-

0.20

-

0.20

Dabco DC-5043

0.75

0.75

-

0.75

Dabco DC-5169

0.25

0.25

-

-

X-N1586

-

-

0.3

0.3

X-N1587

-

-

0.7

0.7

Dabco DEOA-LF

1.53

1.53

1.53

1.53

TDI (100 index)

42.2

42.2

42.2

42.2

1.3.1.1 Reactivity 1.3.1.1a Handmix Mass-Loss/Rate-of-Rise Formulations used in the mass-loss/rate-of-rise are summarised in Tables 1.2 and 1.3. Surfactant and catalyst additives were changed according to the formulation being studied. Foams were run at the optimum index as they were during the machine study. All experiments were duplicated. Each mixed formulation was poured into a ‘chicken’ bucket equipped with a thermocouple positioned at the centre of the bucket resting on a Mettler PM 30,000 balance. The centre height of the rising foam was recorded every second using a DAPS QA Model #2500 rate-of-rise apparatus. Knowing the foam mass, the rate-of-rise and using the ideal gas law, it is possible to calculate the carbon dioxide generated or trapped over time [10, 11]. Figures 1.3 and 1.4 show height versus time achieved for the cushion and back formulations using the control formulations I and V, as well as the experimental formulations II, III, IV, VI, VII and VIII. The foam height versus time graphs clearly indicate higher rates for control formulations I and V in both cushion and back formulations. Cushion and back

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.3 Foam Height versus Time - Cushion Formulation

Figure 1.4 Foam Height versus Time - Back Formulation

formulations, which contained the new additives in formulations II, III, IV, VI, VII and VIII did not achieve the same foam height when compared to the control formulations. Several things could cause the differences observed in the foam heights. First, reactivity rates for control formulations might be faster than the experimental catalyst or surfactants. Second, overall foam stability could be compromised for the experimental catalyst and surfactants. Lastly, carbon dioxide might be diffusing from the reacting polyurethane foam

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Advances in Urethane Science and Technology at an accelerated rate. All of these possibilities were explored. Data generated and observations will be reported which demonstrate how these new additives promote dimensional stability through increased carbon dioxide diffusion. Temperature profiles for control formulations I and V indicate reaction temperatures which fall in the middle of the temperature profiles compared to the new additives denoted in Figures 1.5 and 1.6. These temperature profiles clearly demonstrate that carbon dioxide

Figure 1.5 Temperature versus Time - Cushion Formulation

Figure 1.6 Temperature versus Time - Back Formulation

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Dimensional Stabilising Additives for Flexible Polyurethane Foams conversion is occurring at the same rate in the control as in the experimental formulations. The fact that carbon dioxide conversion is occurring at the same rates would not account for the lower foam heights observed in Figures 1.3 and 1.4. In the control formulations, I and V, the amount of carbon dioxide diffused was less than the amount of carbon dioxide diffused using the new additives. Figures 1.7 and 1.8 were generated using the ideal gas law from data generated with the mass-loss/rate-of-rise apparatus. Diffusion

Figure 1.7 Carbon Dioxide Trapped versus Time - Cushion Formulation

Figure 1.8 Carbon Dioxide Trapped versus Time: Back Formulation

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Advances in Urethane Science and Technology of carbon dioxide from cells as they form in the free rise reaction apparently keeps the reacting foam from reaching the same foam height when using these experimental additives. Reactivity profiles for these three new additives are essentially the same. Data discussed later in Section 9.3.1.1b, Tables 1.4 and 1.5, further supports the fact that there is no

Table 1.4 Machine mix free rise reactivity comparison using cushion formulation Control

BL53

Exp. SSF

BL53/Exp. SSF

I

II

III

IV

Cream (Seconds)

4. 7

4.6

4.9

4.9

Top of Cup (Seconds)

24.1

23.6

23.9

23.4

String Gel (Seconds)

55.4

53.7

54.3

54.2

Full Rise (Seconds)

100.4

103.8

105.3

105.6

Rise: Gel Ratio

1.81

2.01

1.94

1.94

Final Height (mm)

278.4

279.6

280.8

272.6

24 Hour Height (mm)

263.4

265.5

265.5

257.6

5.4

5.0

5.4

5.5

Formulation

% Shrinkage

Table 1.5 Machine mix free rise reactivity comparison using back formulation Control

BL53

Exp. SSF

BL53/Exp. SSF

V

VI

VII

VIII

Cream (Seconds)

4.8

4.5

4.8

4.8

Top of Cup (Seconds)

23.5

22.9

25.4

26.5

String Gel (Seconds)

53.2

52.1

55.6

55.9

Full Rise (Seconds)

109.5

108.7

111.9

111.2

Rise/Gel Ratio

2.06

2.08

2.02

1.99

Final Height (mm)

273.6

275.7

268.3

270.1

24 Hour Height (mm)

245.6

246.9

236.1

235.9

% Shrinkage

10.2

10.4

12.0

12.7

Formulation

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Dimensional Stabilising Additives for Flexible Polyurethane Foams significant change in reactivity when comparing the control formulations to the experimental formulations.

1.3.1.1b Machine Free Rise Reactivity The machine mix free rise reactivity comparison of all formulations are shown in Tables 1.4 and 1.5. This experimental data illustrates that the overall free rise foam reactivity for both the cushion and back formulations remains relatively the same for the beginning of the reaction. The full rise reactivities in cushion formulations II, III and IV and back formulations VII and VIII start to deviate slightly from the control reference formulations I and V. Percent shrinkage remains fairly consistent within the cushion formulations, I, II, III and IV. Increased foam shrinkage was observed with back formulations VII and VIII. This could be attributed to better cell wall drainage efficiency, providing more open foam and/or an overall increased carbon dioxide diffusion through the polymer network.

1.3.1.2 Foam Physical Properties 1.3.1.2a TPR Effect on Machine Run Moulded Foam FTC When producing polyurethane, manufacturers use some type of mechanical crushing to open cells and insure the polyurethane article does not lose dimensional stability. Several techniques can be used to provide the needed mechanical cell opening. Manufacturers will use TPR, which has been described in Section 1.2.3.2d, along with mechanical roller crushing. Some producers will rely exclusively on the roller crushing and vacuum crushing techniques to provide the mechanical cell opening required. In both cases reducing FTC values and improving foam openness is important for producing polyurethane articles that are dimensionally stable. If TPR is carried out too soon during the polyurethane moulding cycle, the article will collapse (blowout) as indicated in Figure 1.9. This is indicative of the foam being insufficiently cured or lacking enough green strength when TPR was applied. If TPR is conducted too late in the manufacturing process scalloping (concave surface areas on the foam article) and tight foam (insufficient number of open cells within the foam article that causes the hot gas to be trapped and upon cooling forces the entire foam part to shrink) may also occur. When scalloping occurs the foam article must be repaired or scrapped. When tight foam occurs dimensional stability will suffer and there will be a negative impact on physical properties as denoted in Figure 1.10. The foam pad in Figure 1.10 was produced at a 140 second TPR without crushing using formulation V.

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Advances in Urethane Science and Technology

Figure 1.9 Example of a Collapse/Blowout Moulded Foam (Reproduced with permission from APCI)

Figure 1.10 Example of Tight Foam with Shrinkage (Reproduced with permission from APCI)

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Dimensional Stabilising Additives for Flexible Polyurethane Foams To understand the benefits of these new additive technologies that provide reduced FTC values, a TPR range from 70 to 150 seconds was run for each of the formulations in Tables 1.2 and 1.3. At a 70 second TPR all formulations suffered blowout since the foam was not sufficiently cured and thus lacked green strength. At an 80 second TPR, no formulations evaluated experienced blowouts or collapse; however, slight distortions and imperfections were evident on the foam surfaces to varying degrees of severity. To complete the TPR window, TPR cycle times were continually ramped up in this fashion to determine the upper limit at which TPR could be applied for each formulation. The upper limit is reached for a given formulation when the foam displays the obvious signs of scalloping and/or ‘dishing’ (concave surface areas of the foam). When this occurs the foam is usually very tight and cannot be used as a functional part. Additionally, parts were produced without utilising TPR during the production cycle in order to compare the difference in foam crushability when TPR is used.

1.3.1.2b Cushion Formulation Machine Evaluation Utilising TPR Cushion control formulation I, which is listed in Table 1.2, was evaluated at a 90-100 second TPR. Initial FTC values of 156 N/323 cm2 for a 90 second TPR and 165 N/323 cm2 for an 100 second TPR were observed. These values were acceptable and produced foam parts of good quality. The new additives in formulations II, III and IV produced maximum initial FTC values of 160 N/323 cm2 at TPR of 90 to 100 seconds. Foam produced at an 80 second TPR for the control formulation I and formulations II, III and IV containing the new additives resulted in minor problems with foam quality, i.e., scalloping. At a 70 second TPR, control and experimental formulations failed because of severe blowout. Figures 1.11 and 1.12 show no significant difference of FTC for all formulations evaluated. When TPR values were increased to 120 seconds for control formulation I, initial values increased to 623 N/323 cm2 and scalloping or foam quality suffered (Figure 1.13). However, increasing the TPR time for cushion formulations II, III and IV to 120 seconds, produced maximum initial FTC values of 205 N/323 cm2 (Figure 1.13). Foam surface quality was very good. Increasing the TPR to 140 seconds increased initial FTC values to a maximum of 543 N/323 cm2 and 534 N/323 cm2 for formulations II and III, respectively (Figure 1.14). Foam quality was still very good. Formulation IV, which utilises a combination of both surfactant and catalyst technologies, achieved a lower initial FTC value at a 140 second TPR of 191 N/323 cm2. This was only slightly higher than the FTC values of the control formulation at a 90 second TPR. Figures 1.11, 1.12, 1.13, and 1.14 show the results of FTC through the entire TPR range applied in this study. These figures clearly demonstrate that use of the new additives can reduce FTC values and maintain dimensional stability over the applied TPR range.

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Advances in Urethane Science and Technology

Figure 1.11 FTC at 90 TPR - Cushion Formulation

Figure 1.12 FTC at 100 TPR - Cushion Formulation

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.13 FTC at 120 TPR - Cushion Formulation

Figure 1.14 FTC at 140 TPR - Cushion Formulation

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Advances in Urethane Science and Technology Figures 1.15, 1.16, and 1.17 further demonstrate the reduction of FTC values that can be achieved when the new catalyst and surfactant technologies are utilised. These figures clearly show a large difference achieved from plotting initial FTC to the final FTC cycle versus increasing TPR cycle times. Moreover, the lower delta FTC values obtained indicate an improved crush-out capability. The optimum TPR range for control formulation I is 90-100 seconds, while the optimum range for formulations II and III is increased to 120

Figure 1.15 Initial FTC versus TPR Time - Cushion Formulation

Figure 1.16 Delta Difference of 1st to 3rd FTC versus TPR Time - Cushion Formulation

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.17 Delta Difference of Initial to Final FTC versus TPR Time - Cushion Formulation

seconds. Formulation IV provided reduced FTC values up to a 140 second TPR. Formulation I did not provide acceptable foam surface quality above 100 second TPR. Formulations II, III and IV provided good surface quality and acceptable physical properties throughout the entire TPR range.

1.3.1.2c Back Formulation Machine Evaluation Utilising TPR Back formulation evaluations were carried out in the same manner as the cushion formulation study. Because of lower solids and higher water content necessary to obtain specific densities and physical properties, this system was more sensitive to processing and TPR range than the cushion formulation. Back formulations V, VI, VII and VIII found in Table 1.3, were all run at TPR times of 70-150 seconds and no TPR. Collapsed foam was encountered at TPR times of 70 seconds for all formulations. FTC values for the back control formulation V had an initial FTC value of 138 N/323 cm2 at a 90 TPR and 160 N/323 cm2 at a 100 second TPR time. Formulations VI, VII and VIII produced a maximum initial FTC value at a 90 and 100 second TPR of 138 N/323 cm2. Control formulation V and experimental formulations VI, VII and VIII provided acceptable FTC values at 90 and 100 second TPR times (Figures 18 and 19). No significant difference in FTC was realised with these formulations at these TPR times. The foam quality at this TPR range was also very good. When TPR times were increased to 120 seconds for control formulation V, initial FTC increased to 645 N/323 cm2. Experimental formulations VI, VII and VIII maintained acceptable initial

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Advances in Urethane Science and Technology

Figure 1.18 FTC at 90 TPR - Back Formulation

Figure 1.19 FTC at 100 TPR - Back Formulation

FTC values of 334 N/323 cm2, 245 N/323 cm2 and 156 N/323 cm2, respectively, (Figure 1.20). At a 120 second TPR, control formulation V exhibited scalloping while the experimental formulations VI, VII and VIII continued to produce parts with good foam quality. Initial FTC values for the control formulation V increased to 1,045 N/323 cm2 at a 140 TPR and severe shrinkage was observed. FTC values for formulations VI and VII reached maximum values of 649 N/323 cm2, respectively, and 627 N/323 cm2 at a

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.20 FTC at 120 TPR - Back Formulation.

Figure 1.21 FTC at 140 TPR - Back Formulation.

140 second TPR (Figure 1.21). Foam produced for these two formulations at a 140 second TPR still produced acceptable quality foam. Formulation VIII achieved lower FTC values than any of the back formulations evaluated at a 140 second TPR range. The initial FTC value for formulation VIII was 405 N/323 cm2 with foam surface quality maintained (Figure 1.21). Figures 1.18, 1.19, 1.20, and 1.21 show the results of FTC values through the entire TPR range utilised in this study.

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Advances in Urethane Science and Technology Figures 1.22, 1.23, and 1.24 further demonstrate the reduction of FTC values that can be achieved in the back formulations when the new catalyst and experimental silicone surfactant technologies are utilised. At a 90 and 100 second TPR, control formulation V produces good quality foam. When the TPR is increased above 100 seconds foam tightness and surface quality issues arise with the control formulation. Formulations VI, VII and VIII yield good quality foam at all TPR cycle times used in this study. As in the cushion formulation, these lower FTC delta values demonstrate how these newly developed additives enhance crush-out capabilities. Furthermore, this effectively illustrates that the TPR window can be extended as compared to the control formulation.

Figure 1.22 Initial FTC versus TPR Time - Back Formulation

Figure 1.23 Delta Difference of 1st to 3rd FTC versus TPR Time - Back Formulation 32

Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.24 Delta Difference of Initial to Final FTC versus TPR Time - Back Formulation

1.3.1.2d Cushion and Back Formulation Machine Evaluation Without TPR Several polyurethane manufactures do not utilise TPR in their production process. The formulations evaluated in this study were designed to utilise TPR when producing polyurethane articles. However, indications of reduced force required to crush a pad using roller crushing and/or vacuum rupture can be understood from the data generated when TPR is not applied to the process. The initial FTC values recorded for the control cushion and back formulations I and V were 1,379 N/323 cm2 and 1,299 N/323 cm2, respectively. Using formulations II, III, VI and VII, FTC values were reduced to 1,152 N/ 323 cm2 and 1,090 N/323 cm2 for the cushion formulation and 1036 N/323 cm2 and 1023 N/323 cm2 for the back formulation. Only minimal shrinkage was observed with the surface quality being maintained. Using formulations IV and VIII, FTC values could further be decreased to 1032 N/323 cm2 and 943 N/323 cm2, respectively. Again, foam surface quality was good with only minimal shrinkage being observed. Figures 1.25 and 1.26 show the FTC values obtained when no TPR was applied during this study.

1.3.1.2e Machine Physical Property Data Comparisons of Various TPR Times Physical properties were evaluated for all the formulations found in Tables 1.2 and 1.3 with TPR being applied at various times throughout the moulding cycle as previously discussed. Additionally, physical properties were evaluated with no TPR being applied. The 90 second TPR time was chosen to be the minimum time to perform TPR for all formulations. Physical property pads were produced at this TPR time. Extended TPR

33

Advances in Urethane Science and Technology

Figure 1.25 FTC at No TPR - Cushion Formulation

Figure 1.26 FTC at No TPR - Back Formulation

times were achieved with the experimental formulations. Physical property pads for physical testing were produced at these extended times. Various TPR windows were identified for the formulations outlined in Tables 1.2 and 1.3. The control formulations I and V both had an upper TPR range of 100 seconds. At a 110 second TPR both formulations I and V had scalloping and surface distortion issues (see Section 1.3.1.2a). Thus the effective TPR range for formulations I and V is 90 to 100 34

Dimensional Stabilising Additives for Flexible Polyurethane Foams seconds. This provides a very narrow process range when utilising TPR with these specific formulations. However, when using the Dabco BL-53 catalyst, or X-N1586/X-N1587 experimental silicone surfactants, both cushion and back formulation TPR times can be extended well beyond the 100 second TPR upper limit of the control formulations I and V.

1.3.1.2f Physical Property Comparison at 90 Second TPR Tables 1.6 and 1.7 provide the physical property comparison for all formulations I-VIII at a 90 second TPR time. The data clearly demonstrates that physical properties are maintained, and in several cases improved, compared to the control formulations. For example, airflow can be improved by as much as 20% as compared to both cushion and back control formulations, when using Dabco BL-53 and experimental silicone surfactants

Table 1.6 Physical properties at 90 TPR: cushion formulation Physical Property

Control

BL53

Exp. SSF

I

II

III

BL53/ Exp. SSF IV

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

175

2.2

181

1.5

185

3.1

184

2.8

65%

455

3.5

468

2.1

492

2.4

487

4.1

25% Return

154

1.5

159

1.8

160

1. 5

163

1.8

Ball Rebound (%)

61

(1.5)

63

0.6

61

0.9

62

0.6

Airflow (SLM)

38

5.8

47

4.2

50

5.1

57

6.2

3

Density (kg/m )

44

0.8

44

0. 4

43

0.8

44

0.7

Tensile (kPa)

179

8.4

183

3.1

187

5.4

180

3.2

Tear (N/m)

254

12.1

261

8.9

291

13.3

292

9.4

Elongation (%)

176

7.2

182

6.1

182

5. 8

179

5.1

Wet Set (%)

22

(1.2)

21

0.9

20

0.9

18

1.7

50% Compression Set

5

0.3

5

0.2

5

0.3

4

0.1

50% Humid Aged Compression Set

12

0.9

10

0.4

9

0.3

8

0.5

SD: Standard Deviation ILD: indentation load deflection SLM: standard litres per minute

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Advances in Urethane Science and Technology

Table 1.7 Physical properties at 90 TPR: back formulation Physical Properties

Control

BL53

Exp. SSF

V

VI

VII

BL53/ Exp. SSF VIII

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

125

3.0

134

3.6

144

1.8

155

1.7

65%

35 4

3.9

370

5.8

396

5.9

422

6.0

25% Return

10 8

1.7

116

2.9

122

1.7

128

3.1

Ball Rebound (%)

60

2.1

60

0.9

60

1.2

60

1.2

Airflow (SLM)

37

2.5

48

1.2

49

4.1

50

3.8

3

Density (kg/m )

35

0.6

36

1.2

36

1.2

36

1.2

Tensile (kPa)

137

6.3

145

5.1

155

9.7

163

10.1

Tear (N/m)

245

9.6

254

10.5

268

11.5

275

11.8

Elongation (%)

165

8.2

176

4.7

170

8.9

178

10.5

Wet Set (%)

24

1.1

22

1.1

18

1.2

19

1.2

50% Compression Set

8

0.5

8

0.2

4

0.2

4

0.5

50% Humid Aged Compression Set

14

1.2

10

0.3

10

0.4

9

0.5

SD: Standard Deviation ILD: indentation load deflection SLM: standard litres per minute

X-N1586/X-N1587 in combination (formulations IV and VIII). Furthermore, Japanese wet set values [6] can be improved by 5 to 8% with the Dabco BL-53, and experimental silicone surfactants X-N1586 and X-N1587. Wet set values are improved nearly 20% when the two additives are utilised in combination as evidenced by Formulations IV and VIII. Equally important, humid aged compression set, tensile and tear physical properties display a positive improvement trend as compared to the control formulations.

36

Dimensional Stabilising Additives for Flexible Polyurethane Foams 1.3.1.2g Physical Property Comparison at 130 and 150 Second TPR Tables 1.8 and 1.9 illustrate the physical properties for selected formulations at TPR times of 130 and 150 seconds. Control physical properties were not evaluated at these extended TPR times since the control formulations I and V had visual surface distortions and severe scalloping. The data in Tables 1.8 and 1.9 demonstrate that the TPR window can be extended by using these newly developed additives without negative impact to the physical properties. In fact, the data indicates that several of the physical properties for the experimental formulations exceed the control formulation properties at the 90 second TPR time. For example, airflow measurements are improved when utilising the extended TPR times. Improvements are greater than 10% with the Dabco BL-53 catalyst and experimental silicone surfactant combinations. Additional improvements are also observed with wet set and 50% humid aged compression set values.

Table 1.8 Physical properties at 130 TPR and 150 TPR: cushion formulation Physical Properties

130 TPR BL53 II

150 TPR Exp. SSF III

BL53/Exp. SSF IV

ILD (N)

AVG

SD

AVG

SD

AVG

SD

25%

175

1.8

164

3.5

174

1.1

65%

434

2.5

457

1.0

467

4.5

25% Return

138

1.8

143

1.5

152

1.1

Ball Rebound (%)

61

0.5

62

0.5

61

0.4

Airflow (SLM)

43

3.2

46

4.7

53

7.4

3

Density (kg/m )

43

0.8

42

0.8

43

0.7

Tensile (kPa)

174

4.1

180

5.1

170

3.5

Tear (N/m)

255

7.4

274

10.5

279

12.1

Elongation (%)

180

3.9

180

3.5

173

3.3

Wet Set (%)

21

1.5

19

1.3

19

0.3

50% Compression Set

5

0.5

5

0. 1

4

0 .3

50% Humid Aged Compression Set

10

0.8

10

0.8

8

0.3

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Advances in Urethane Science and Technology

Table 1.9 Physical properties at 130 TPR and 150 TPR: back formulation Physical Properties

130 TPR BL53

Exp. SSF

VI

VII

150 TPR BL53/ Exp. SSF VIII

BL53/ Exp. SSF VIII

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

111

2.1

145

4.2

156

0.7

155

1.2

65%

344

5.1

396

3.5

435

8.0

430

3.0

25% Return

96

2.2

123

2.2

132

0.6

129

2.1

Ball Rebound (%)

59

1.4

59

1.5

62

0.7

60

0.2

Airflow (SLM)

49

5.5

45

2.3

49

1 .8

52

2.1

3

Density (kg/m )

35

0.8

36

1.0

36

0.8

36

0.2

Tensile (kPa)

136

5.1

148

5.4

152

7.9

147

6.1

Tear (N/m)

254

9.1

261

8.0

248

8.0

239

5.8

Elongation (%)

170

9.3

166

5.3

163

5.6

175

4.9

Wet Set (%)

22

1.1

19

0.8

19

0.5

19

0.8

50% Compression Set

9

0.2

5

0.3

4

0.2

4

0.6

50% Humid Aged Compression Set

10

0.5

10

0.6

9

0.5

9

0.8

1.3.1.2h Physical Property Comparison with No TPR Tables 1.10 and 1.11 illustrate the physical property performance of all formulations I through VIII without utilising TPR. Given that the specific formulations were designed to be used with the TPR process, foam pads produced under these conditions were very tight upon demoulding. Mechanical crushing at 30 seconds after demould was required instead of the one minute time frame; otherwise, irreversible shrinkage may have occurred prior to placing the foam through the roller crusher. The data in Tables 1.10 and 1.11 indicate that utilising Dabco BL-53 catalyst and/or the experimental silicone surfactants X-N1586 and X-N1587, have a positive influence on the physical properties even when no TPR is applied. Moreover, if the properties of the new additives are compared to the control formulations I and V at the 90 second TPR time (Tables 1.5 and 1.6), physical properties are not adversely affected. In some instances the properties are still maintaining their improvement over the control formulations. Control formulations run at no TPR exhibited surface quality distortions.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.10 Physical properties at no TPR: cushion formulation Physical Properties

Control

BL53

Exp. SSF

I

II

III

BL53/ Exp. SSF IV

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

145

0.5

150

1.2

157

0.5

157

2.1

65%

395

0.5

407

2.2

428

3.0

428

4.2

25% Return

127

0.5

131

1.1

136

0.5

138

2.1

Ball Rebound (%)

60

0.7

61

1.4

61

1.0

62

0.7

Airflow (SLM)

34

2.6

42

2.5

47

6.2

49

1.3

3

Density (kg/m )

42

0.8

42

0.5

41

0.6

41

0.6

Tensile (kPa)

159

5.3

160

5.4

175

3.5

169

4.5

Tear (N/m)

241

6.7

252

6.7

269

10.6

268

11.1

Elongation (%)

174

4.1

180

4.2

174

4.5

174

4.3

Wet Set (%)

24

0.6

22

0.9

22

0.2

20

0.3

50% Compression Set

6

0.2

5

0.1

5

0.2

5

0.2

50% Humid Aged Compression Set

12

0.2

10

0.1

10

0.7

9

0.1

1.3.1.2i Physical Property Review All of the physical property data generated at the higher TPR cycle times illustrates that the window for TPR can be extended when using Dabco BL-53 catalyst and X-N1586, XN1587 experimental silicone surfactants. Physical properties were not adversely affected when compared to the control samples. By extending the effective TPR window, polyurethane moulders who utilise this process will enjoy a greater latitude for their processing, providing more freedom to troubleshoot difficult tooling and moulding operations. Additionally, polyurethane manufacturers that do not practice the TPR process can also benefit with improvements realised for physical properties and dimensional stability, as illustrated by many of the physical properties enhancements found in Tables 1.9 and 1.10. Finally, the question remains whether the stability, especially shear stability, suffers from the significantly improved breathability and dimensional stability provided with these new additives. Examination of cellular structures of the machine mixed free rise ‘chicken’ bucket

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Advances in Urethane Science and Technology

Table 1.11 Physical properties at no TPR: back formulation Physical Properties

Control

BL53

Exp. SSF

V

VI

VII

BL53/ Exp. SSF VIII

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

103

0.5

113

2.1

127

0.7

138

5.0

65%

297

5.0

330

7.7

355

0.6

386

8.4

25% Return

89

0.5

98

2.5

107

0.6

115

3.5

Ball Rebound (%)

58

0.7

61

0.1

61

0.1)

61

1.4

Airflow (SLM)

39

1.1

46

3.1

47

5.1

46

3.8

3

Density (kg/m )

35

0.5

35

0.8

35

0.8

35

0.8

Tensile (kPa)

123

4.9

135

8.8

132

5.7

140

9.9

Tear (N/m)

235

6.5

245

6.3

252

11.6

239

5.9

Elongation (%)

161

8.2

175

7.0

165

6.1

170

9.2

Wet Set (%)

23

1.8

22

1.6

19

0.2

21

1.0

50% Compression Set

9

1.0

9

0.4

6

0.4

5

0.3

50% Humid Aged Compression Set

14

1.8

11

0.5

11

0.6

10

0.6

foams demonstrated no evidence of elongated cells or side shear in the outer edges of the cup. Furthermore, when the TPR ramp study was being carried out it was determined that all formulations had similar lower limit TPR times of approximately 90 seconds. Essentially, this early TPR time would mimic a poorly sealed mould so that the shear forces become obvious in and around the gap or space created by opening the mould. The foam mixture, if not sufficiently stabilised or adequately cured, would be accompanied by a pressure relief which leads to significant mechanical stress, causing collapse. The fact the Dabco BL-53 catalyst, X-N1586 and X-N1587 experimental silicone surfactants did not have blowouts or shear collapses at the 90 second TPR illustrates shear stability is not compromised.

1.3.1.3 Scanning Electron Microscopy A scanning electron microscopy study of moulded foam parts was carried out to understand what affect the additives might have on the morphology of the foam cell structure. It is clear from the SEM photomicrographs in Figures 1.27, 1.28 and 1.29 that there is no 40

Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.27 Formulation I, Control (Reproduced with permission from APCI)

Figure 1.28 Formulation II, BL-53 (Reproduced with permission from APCI)

Figure 1.29 Formulation III, Exp. SSF (Reproduced with permission from APCI)

41

Advances in Urethane Science and Technology visual effect on the cell structure of foam produced with the new additives as compared to the control. The additives did not appear to have an affect on the cell morphology of a crushed foam. However, it was not possible to determine differences in cell morphology from an uncrushed part, since the parts were mechanically crushed to prevent normal distortion from foam shrinkage. Formulations I, II and III were used for the SEM study.

1.3.2 Low Emission Dimensional Stability Additives Another emerging need in the automobile market segment is for lower and ultimately no emissions of polyurethane foam additives into the environment. Current efforts revolve around automotive passenger compartment air quality improvements by reduction of the emissions typically from the current migratory additives used to produce interior foam components. These emissions can cause a variety of problems in the final application of the foam, ranging from odour to vinyl staining and window fogging. The migratory nature of current additives is accelerated by sunlight and heat. Thermally induced dehydrohalogenation of the foam/covering interface is assisted by the migration of fugitive tertiary amine catalysts causing vinyl staining (colour change). Breakdown of the vinyl clad covering subsequently releases other chemical compounds, such as plasticisers, which then contribute to window fogging. Consumers and automakers are becoming more conscious of automotive interior air quality, interior vinyl degradation and film formation (windshield fogging) [12]. This section of the chapter reports on the development of non-fugitive gelling and blowing polyurethane catalysts to address the aforementioned emission issues. These catalysts chemically bind to the polyurethane foam matrix (contain active hydrogens) rendering them incapable of migrating back out of the foam after the reaction is complete, while still producing a quality product. Equivalent or improved physical properties such as airflow, compression set and crushability, must be maintained as compared to the industry standards used in TDI-, MDI-based systems, and TDI/MDI blend systems. Presented in this section is work using these non-fugitive catalysts in a variety of flexible polyurethane foam applications, including MDI and TDI automotive and flexible slabstock formulations. The results show that these new additives can give dimensional stability and processability to the foam, while providing non-fugitive and non-fogging benefits. Additionally, improvements to the physical properties of the polyurethane article when using the new additives, occurs. Accordingly, these non-fugitive catalysts provide good to excellent foam stabilisation, cell regulation and cell wall drainage and are needed to enable polyurethane manufacturers to maintain their standard production goals. Two experimental non-fugitive catalysts, XF-N1085 and XF-O11006 were developed to provide open foam and promote

42

Dimensional Stabilising Additives for Flexible Polyurethane Foams dimensional stability. During the development of non-fugitive catalysts, it was observed that TDI formulations, in general, tend to be more closed cell with the use of these catalysts compared to MDI systems. Therefore, the technology was advanced to incorporate non-fugitivity and cell opening characteristics within a single catalyst. XF-N1085 is a novel cell opening blowing, non-fugitive catalyst which affords all the benefits of the well known Dabco BL-11, with the added advantage of cell opening and no foam emissions. XF-N1085 is not a drop in replacement for Dabco BL-11. For rapid demoulding of TDI systems, it is recommended that XF-N1085 be used at 0.2 to 0.4 php, with the optimum level at 0.25 to 0.35 pphp, in combination with an XF-O11006 level at 0.6 to 0.9 pphp. XF-O11006 is a novel cell opening gelling non-fugitive catalyst which gives all the benefits of the well known Dabco 33-LV, with the added advantage of cell opening, low volatility and no foam emissions. XF-O11006 is not a drop in replacement for Dabco 33-LV. Several representative industry cushion and back formulations, listed in Tables 1.121.15, were utilised to compare multiple additives. Dabco NE200 was compared against

Table 1.12 ~45 kg/m3 TDI Lyondell cushion formulation Formulation identification

IX

X

XI

XII

Components

pphp

pphp

pphp

pphp

Arcol E848

50.00

50.00

50.00

50.00

Arcol E851

50.00

50.00

50.00

50.00

Water

2.34

2.34

2.34

2.34

Dabco 33LV

0.25

-

-

-

Dabco BL-11

0.10

-

-

-

Dabco NE1060

-

0.08

0.53

-

Dabco NE200

-

0.64

-

-

XF-N1085

-

-

0.26

0.26

XF-O11006

-

-

-

0.68

Dabco DC5043

0.75

0.75

0.75

0.75

Dabco DEOA-LF

1.76

1.76

1.76

1.76

TDI (100 index)

32.74

32.74

32.74

32.74

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Advances in Urethane Science and Technology

Table 1.13 ~35 kg/m3 TDI Lyondell back formulation Formulation Identification

XIII

XIV

XV

XVI

Components

pphp

pphp

pphp

pphp

Arcol E848

70.00

70.00

70.00

70.00

Arcol E851

30.00

30.00

30.00

30.00

Water

3.24

3.24

3.24

3.24

Dabco 33LV

0.25

-

-

-

Dabco BL-11

0.10

-

-

-

Dabco NE1060

-

0.08

0.53

-

Dabco NE200

-

0.64

-

-

XF-N1085

-

-

0.26

0.26

XF-O11006

-

-

-

0.68

Dabco DC5043

1.00

1.00

1.00

1.00

Dabco DEOA-LF

1.76

1.76

1.76

1.76

TDI (100 index)

41.87

41.87

41.87

41.87

Table 1.14 ~45 kg/m3 TDI Dow cushion formulation Formulation identification

XVII

XVIII

XIX

XX

Components

pphp

pphp

pphp

pphp

NC-630

46.00

46.00

46.00

46.00

NC-700

54.00

54.00

54.00

54.00

Water

2.34

2.34

2.34

2.34

Dabco 33LV

0.25

-

-

-

Dabco BL-11

0.10

-

-

-

Dabco NE1060

-

0.22

0.70

-

Dabco NE200

-

0.64

-

-

XF-N1085

-

-

0.27

0.25

XF-O11006

-

-

-

0.80

Dabco DC5164

1.0

1.0

1.0

1.0

Dabco DC5169

0.5

0.5

0.5

0.5

Dabco DEOA-LF

1.76

1.76

1.76

1.76

TDI (100 index)

32.70

32.70

32.70

32.70

44

Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.15 ~35 kg/m3 TDI Dow back formulation Formulation identification

XXI

XXII

XXIII

XXIV

Components

pphp

pphp

pphp

pphp

NC-630

68.00

68.00

68.00

68.00

NC-700

32.00

32.00

32.00

32.00

Water

3.24

3.24

3.24

3.24

Dabco 33LV

0.25

-

-

-

Dabco BL-11

0.10

-

-

-

Dabco NE1060

-

0.27

0.58

-

Dabco NE200

-

0.52

-

-

XF-N1085

-

-

0.32

0.24

XF-O11006

-

-

-

0.77

Dabco DC5164

0. 2

0.2

0.2

0.2

Dabco DC5169

0.6

0.6

0.6

0.6

DEOA-LF

1.76

1.76

1.76

1.76

TDI (100 index)

41.92

41.92

41.92

41.92

Dabco BL-11 for equivalent blowing efficency. Dabco NE1060 was compared against Dabco 33LV for equivalent gelling efficiency. Required blow:gel ratios are listed in each table. Additionally, XF-N1085 and XF-O11006 were specifically developed as dimensional stability/cell opening additives and they are also compared here.

1.3.2.1 Reactivity Tables 1.16 and 1.17 illustrate that the overall free rise foam reactivity for both the cushion and back formulations remains relatively the same for the entire foaming reaction. This was achieved by balancing the experimental catalyst levels, blow:gel ratios, and matching them to the control Dabco BL-11 and Dabco 33LV levels in IX, XIII, XVII and XXI.

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Advances in Urethane Science and Technology

Table 1.16 Machine mix free rise reactivity comparison using TDI cushion formulations Formulation

IX

X

XI

XII

XVII

XVIII

XIX

XX

Cream (Seconds)

4.9

4.8

4.8

4.5

3.5

3.8

4.5

4.9

Top of Cup (Seconds)

37

36

39

35

32

35

36

37

String Gel (Seconds)

56

55

56

53

53

53

55

56

Full Rise (Seconds)

83

81

84

78

71

75

81

83

Table 1.17 Machine mix free rise reactivity comparison using TDI back formulations Formulation

XIII

XIV

XV

XVI

XXI

XXII

XXIII

XXIV

Cream (Seconds)

3.8

3.8

3.8

3.4

4.2

3.2

3.2

3.2

Top of Cup (Seconds)

30

33

32

31

32

37

36

37

String Gel (Seconds)

58

56

59

57

54

55

57

56

Full Rise (Seconds)

82

85

94

91

75

86

85

88

1.3.2.2 Flexible Moulded Foam Machine Physical Property Data To understand the benefits of these new additive technologies that provide low volatility, no amine emissions, no fogging and, in some cases, cell opening, all TDI moulded formulations were run on the Hi Tech machine. Several foams were produced for each catalyst combination and formulation to obtain physical property pads, FTC pads, shrinkage pads, and flow evaluations.

46

Dimensional Stabilising Additives for Flexible Polyurethane Foams 1.3.2.2a Standard Physical Properties Tables 1.18 to 1.21 provide the physical property comparison for the TDI flexible moulded formulations IX-XXIV. The data clearly demonstrates that physical properties are maintained, and in several cases improved, compared to the control formulations, depending on the formulation and the use of experimental cell-opening catalysts. For example, ILD properties were comparable or improved over the control in both low and high solids formulations, especially in the Dow polyol system. This is attributed to the slightly different polymer matrix resulting from non-fugitive catalysis which can produce higher load properties. In almost all cases, the support factor was comparable to the control. Tensile values in the Dow system were matched to the control using XF-N1085 in combination with Dabco NE1060, formulations XIX and XXIII. Dry and humid compression sets (50% dry, 50% humid aged, and 75% humid aged) were, in many formulations, slightly elevated, with the Dabco NE1060/XF-N1085 catalyst combination (XIX and XXIII) closest to the control. The compression set data illustrates that this

Table 1.18 Physical properties for Lyondell TDI cushion formulations Physical Property

IX

X

XI

XII

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

180

-

170

-

182

-

181

-

65%

534

-

486

-

522

-

517

-

25% Return

157

-

150

-

160

-

158

-

Ball Rebound (%)

64

1.0

65

0.8

64

1.2

65

0.7

Airflow (SLM)

48

5.7

45

8.5

62

11.3

40

2.8

3

Density (kg/m )

42

1.3

44

0.5

43

1.0

44

1.0

Tensile (kPa)

143

10.1

136

66.7

163

7.8

176

6.5

Tear (N/m)

193

12

186

7

217

23

193

7

Elongation (%)

98

7.0

102

6.3

97

6.1

102

10.6

Wet Set (%)

18

1.0

20

1.2

20

0.9

22

1.4

50% Compression Set (%)

5

0.4

8

1.1

6

0.1

7

0.7

50% Humid Aged Compression Set (%)

9

0.2

16

0.8

6

0.5

17

1.2

Hysteresis (%)

19

0.1

18

0.1

19

0.1

20

0.4

Support Factor

3.0

-

2.9

-

2.9

-

2.9

-

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Advances in Urethane Science and Technology

Table 1.19 Physical properties for Lyondell TDI back formulations Physical Property

XIII

XIV

XV

XVI

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

103

-

97

-

99

-

103

-

65%

315

-

287

-

289

-

310

-

25% Return

87

-

84

-

86

-

89

-

Ball Rebound (%)

62

0.8

64

0.56

64

1.1

62

0.7

Airflow (SLM)

65

5.7

62

8.5

71

11.3

59

2.8

Density (kg/m3)

31

1.0

30

0.9

31

1.3

30

0.9

Tensile (kPa)

104

4.5

119

6.8

112

8.7

121

4.4

Tear (N/m)

243

19

224

21

259

16

259

19

Elongation (%)

93

5.7

97

7.6

100

4.2

100

7.4

Wet Set (%)

27

0.9

25

0.9

26

1.0

26

0.9

50% Compression Set (%)

6

0.7

8

0.4

7

0.53

8

0.4

50% Humid Aged Compression Set (%)

16

0.6

20

1.2

20

1.1

17

1.0

Hysteresis (%)

21

0.1

18

0.2

20

0.2

20

0.1

Support Factor

3.1

-

3.0

-

2.9

-

3.0

-

property is very system dependent when using non-fugitive catalysts. For performance optimisation, one may want to consider reformulation to produce the desired physical properties. The use of XF-N1085 and XF-O11006 will improve or maintain physical properties compared to the control for TDI systems. The Japanese wet set data was determined to be comparable or, in the case of the Lyondell back formulation, slightly improved when compared to the control foam. The use of cell opening non-fugitive catalysts was found to improve wet sets in the Lyondell high solids formulation. In addition, the cell opening catalysts were also found to decrease hysteresis and increase tensile strength in the back formulations using Lyondell polyol compared to the control formulation.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.20 Physical properties for Dow TDI cushion formulations Physical Property

XVII

XVIII

XIX

XX

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

106

-

180

-

180

-

193

-

65%

321

-

51 3

-

518

-

551

-

25% Return

92

-

155

-

156

-

166

-

Ball Rebound (%)

56

0.6

55

1.9

56

0.8

57

0.8

Airflow (SLM)

17

2.8

23

5.7

17

2.8

20

2.8

Density (kg/m3)

45

0.4

44

0.6

44

0.8

44

0.7

Tensile (kPa)

215

14.3

176

6.8

205

4.0

171

13.7

Tear (N/m)

210

11

210

16

235

18

221

14

Elongation (%)

100

7.0

93

6.6

89

6.9

103

5.8

Wet Set (%)

16

0.2

21

1.4

20

0.6

17

0.6

50% Compression Set (%)

6

0.6

9

0. 1

8

0.3

8

0.3

50% Humid Aged Compression Set (%)

13

0.5

17

0.8

20

2.6

18

0.7

Hysteresis (%)

21

0.1

21

0.1

21

0.6

22

0.1

Support Factor

3.1

-

2.9

-

2.9

-

2.9

-

1.3.2.2b Dynamic Fatigue Results Table 1.22 shows the dynamic fatigue test results for Lyondell and Dow cushion foams tested for 80,000 cycles and 60 minute recovery time. Results indicate that equivalent dynamic fatigue values are obtainable with use of the new non-fugitive and cell opening catalysts, based on nominal test error of ± 3.

1.3.2.2c FTC Results In both the Lyondell cushion and back formulations IX through XVI listed in Tables 1.11 and 1.12, addition of XF-N1085 and XF-O11006 result in a reduction in FTC values (Tables 1.22 and 1.23) throughout the initial several repetitions of the crushing cycle illustrated in Figures 1.30 and 1.31. At the end of the ten complete repetitions, all three experimental catalyst packages, resulted in the same final FTC value. Without the

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Advances in Urethane Science and Technology

Table 1.21 Physical properties for Dow TDI back formulations Physical Property

XXI

XXII

XXIII

XXIV

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

108

-

100

-

110

-

115

-

65%

315

-

299

-

313

-

311

-

25% Return

94

-

87

-

95

-

100

-

Ball Rebound (%)

63

0.8

60

1.7

60

1.9

63

0.8

Airflow (SLM)

48

2.8

62

8.5

45

5.7

59

5.7

Density (kg/m3)

33

1.0

33

0.4

33

0.9

32

0.3

Tensile (kPa)

145

5.4

127

7.4

141

5.8

139

4.8

Tear (N/m)

249

9

242

4

247

16

263

23

Elongation (%)

106

11.0

98

8.0

109

12.7

105

4.9

Wet Set (%)

21

0.9

16

1.3

22

1.7

19

1.9

50% Compression Set (%)

5

0.4

7

0.6

8

0.6

7

0.4

50% Humid Aged Compression Set (%)

15

0.9

31

2.1

21

0.2

26

1.8

Hysteresis (%)

19

0.1

18

0.2

20

0.1

19

0.2

Support Factor

2.9

-

3.0

-

2.9

-

2.7

-

Table 1.22 Dynamic fatigue ASTM D3574-95 [5] (80,000 cycles) Original 40% Deflection Force (N)

Final 40% Deflection Force (N)

% Change (%)

IX

282

244

13.4

X

258

207

19.9

XI

273

222

18.6

XII

272

238

12.7

XVII

302

228

24.4

XVIII

261

206

20.9

XIX

264

200

24.3

XX

280

297

29.5

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.23 FTC values for TDI cushion in Lyondell system Lyondell High Solids (N) Frequency

FTC Value

FTC Value

FTC Value

FTC Value

IX

X

XI

XII

1

767

923

738

698

2

356

540

345

349

3

242

316

234

249

4

198

236

189

205

5

167

193

162

178

6

149

167

160

171

7

140

151

158

167

8

138

145

156

160

9

138

140

158

160

10

133

127

156

160

Figure 1.30 FTC Graph for TDI Cushion in Lyondell System (Ref. Table 1.22)

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Advances in Urethane Science and Technology

Figure 1.31 FTC Graph for TDI Back in Lyondell System (Reference Table 1.23)

use of these cell opening catalysts, the Dabco NE1060/Dabco NE200 non-fugitive catalyst package tightens the foam resulting in slightly elevated values during several initial FTC footplate depressions. In the cushion Dow formulations XVII through XX, the XF-N1085 and XF-O11006 show similar trends to the Lyondell formulations with reduced FTC (Figure 1.32 and Table 1.24), thereby producing a more dimensionally stable foam. In contrast, for the low solids back formulations XXI through XXIV, all catalyst combinations exhibited no significant difference to the control catalyst (Figure 1.33 and Table 1.25). Inherent tightness in this particular back formulation did not allow the same degree of catalyst differentiation to occur in these foams. Adjustment to the catalyst loading levels would be required to demonstrate the same reduced FTC performance as previously described in other formulations. Figures 1.34 and 1.35 further demonstrate the reduction of FTC values that can be achieved with these new catalyst technologies. The graphs indicate the results achieved by plotting the delta of the initial and third FTC value for every formulation. Moreover, lower delta values illustrate an improved crush-out capability.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.32 FTC Graph of TDI Cushion in Dow System (Reference Table 1.24)

Table 1.24 FTC values for TDI back in Lyondell system Lyondell Low Solids (N) Frequency

FTC Value

FTC Value

FTC Value

FTC Value

XIII

XIV

XV

XVI

1

830

858

698

683

2

474

558

427

409

3

294

360

251

245

4

196

236

169

165

5

151

182

129

129

6

118

136

107

105

7

100

11 6

91

91

8

85

96

80

80

9

76

85

76

71

10

73

80

69

64

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Advances in Urethane Science and Technology

Figure 1.33 FTC Graphs for TDI Back in Dow System (Reference Table 1.25)

Table 1.25 FTC values for TDI cushion in Dow system Dow High Solids (N) Frequency

FTC Value

FTC Value

FTC Value

FTC Value

XVII

XVIII

XIX

XX

1

1014

1234

959

843

2

654

885

620

516

3

471

641

454

383

4

369

503

360

309

5

314

414

305

267

6

274

347

280

240

7

254

298

256

225

8

236

247

229

211

9

216

249

207

196

10

207

220

187

191

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.26 FTC values for TDI back in Dow system Dow Low Solids (N) Frequency

FTC Value

FTC Value

FTC Value

FTC Value

XXI

XXII

XXIII

XXIV

1

936

881

901

905

2

500

505

516

500

3

309

311

311

298

4

227

231

211

209

5

171

178

158

167

6

138

140

127

138

7

116

111

107

118

8

100

96

93

105

9

87

82

82

98

10

80

69

76

89

Figure 1.34 Delta Difference Graph of First and Third FTC Values in Lyondell System

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Advances in Urethane Science and Technology

Figure 1.35 Delta Difference Graph of First and Third FTC Values in Dow System

1.3.2.2d Pail Test Results Visual examination of the foam slice’s cellular structure showed no evidence of elongated cells or side shear collapse near the outer edges. The appearance of these elongated cells or side shear collapse is a phenomenon created by the frictional drag of the plastic pail (see Section 1.2.3.2e) on the rising foam. Each slice was measured for maximum height, normalised by weight and displayed in Figure 1.36 for comparison of final foam recession to the control. It is evident from the data that no significant difference exists in bulk stability when using any of the cell opening or non-fugitive catalysts.

1.3.2.2e Maze Mould Test Results Data in the Lyondell system, formulations IX through XVI, generated using the previously described maze flow mould is summarised in Tables 1.27 and 1.28 and

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.36 Graph Comparing 24 Hour Pail Height

Table 1.27 Maze flow mould data for Lyondell TDI cushion formulations IX - XII Formulation Identifier

Pad wt. (g)

Flow past 5th gate (g)

% of Total pad wt.

MINIMUM 5th Gate IX

405

30

8.0

X

404

30

8.0

XI

405

35

9.5

XII

409

30

7.9

MEDIAN 5th Gate IX

474

101

27.1

X

472

104

28.3

XI

478

109

29.5

XII

479

105

28.1

MAXIMUM 5th Gate IX

524

150

40.1

X

526

160

43.7

XI

531

161

43.5

XII

528

155

41.6

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Advances in Urethane Science and Technology

Table 1.28 Maze flow mould data for Lyondell TDI back formulations XIII-XVI Formulation Identifier

Pad wt. (g)

Flow past 5th gate (g)

% of Total pad wt.

MINIMUM 5th Gate XIII

619

92

17.5

XIV

617

102

19.8

XV

615

108

21.3

XVI

616

107

21.0

MEDIAN 5th Gate XIII

667

143

27.3

XIV

666

153

29.8

XV

665

160

31.7

XVI

668

156

30.5

MAXIMUM 5th Gate XIII

718

193

36.8

XIV

715

202

39.4

XV

716

206

40.4

XVI

718

208

40.8

shown graphically in Figure 1.37. The graph utilises a xy scatter plot with linear regression trendlines and R2 values (coefficient of determination) reported for each formulation group. It compares estimated and actual y-values and ranges in value from zero to one. Since these values are very close to one, there is a near perfect correlation in the samples. However, if the coefficient of determination approaches zero, the regression equation is not helpful in predicting a y-value. The data trends suggest that increasing part weight, regardless of formulation type, shows no significant penalty in flow compared to the control formulations IX and XIII. Similar maze flow mould results are also obtained with the Dow cushion and back formulations summarised in Tables 1.28 and 1.29 and shown graphically in Figure 1.38. In these Dow formulations, XVII through XXIV, the data suggests slightly improved linear regression trendlines and R2 values. Again, no significant penalty in flow is observed as compared to the control formulations XVII and XXI.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Figure 1.37 Maze Flow Mould Graph for Formulations IX-XVI

Table 1.29 Maze flow mould data for Dow TDI cushion formulations XVII-XX Formulation Identifier

Pad wt. (g)

Flow past 5th gate (g)

% of Total pad wt.

MINIMUM 5th Gate XVII

404

40

11.0

XVIII

403

36

9.8

XIX

399

34

9.3

XX

404

25

6.6

MEDIAN 5th Gate XVII

457

93

25.6

XVIII

457

92

25.2

XIX

458

100

27.9

XX

453

69

18.0

MAXIMUM 5th Gate XVII

509

149

41.4

XVIII

508

140

38.0

XIX

508

149

41.5

XX

513

136

36.1

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Advances in Urethane Science and Technology

Table 1.30 Maze flow mould data for Dow TDI back formulations XXI-XXIV Formulation Identifier

Pad wt. (g)

Flow past 5th gate (g)

% of Total pad wt.

MINIMUM 5th Gate XXI

568

65

12.9

XXII

566

70

14.1

XXIII

565

64

12.8

XXIV

569

83

17.1

MEDIAN 5th Gate XXI

617

122

24.7

XXII

616

120

24.2

XXIII

614

112

22.3

XXIV

618

132

27.2

MAXIMUM 5th Gate XXI

668

168

33.6

XXII

665

172

34.9

XXIII

666

164

32.7

XXIV

670

183

37.6

Figure 1.38 Maze Flow Mould Graph for Formulations XVII-XXIV

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Dimensional Stabilising Additives for Flexible Polyurethane Foams 1.3.2.2f Shrinkage Test Results Shrinkage results displayed in Figures 1.39 and 1.40 were generated with the shrinkage apparatus previously described in the experimental section. Overall, the cavity shrinkage is much greater as compared to the lid shrinkage, evidenced in both Lyondell and Dow polyol systems. The use of Dabco NE1060/Dabco NE200 produced foams with more shrinkage than the control catalysts. However, utilising non-fugitive cell opening catalysts, XF-N1085 and XF-O11006 resulted in shrinkage comparable to the control and in some cases even less. As expected, less shrinkage was observed in both Dow and Lyondell cushion formulations due to the increased solids loading as compared to their respective back formulations.

Figure 1.39 Uncrushed Foam Shrinkage for Lyondell System

Figure 1.40 Uncrushed Foam Shrinkage for Dow System

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Advances in Urethane Science and Technology 9.3.2.2g Fogging Test Results Several machine foam samples were tested in accordance with fogging test method SAE J1756-94 [9]. Results are listed in Table 1.31. The results obtained in the Lyondell polyol system showed improved percentage reflectance over the control indicating reduced emissions from the foam. Comparable results were obtained in all the Dow polyol system samples regardless of catalyst use.

Table 1.31 Fogging results System Identifier

% of Reflectivity

IX

87

X

97

XI

98

XII

96

XVII

93

XVIII

92

XIX

93

XX

93

1.3.2.2h Headspace Analysis and Vinyl Staining Test Results Headspace analysis in series with gas chromatography and mass spectrometry has indicated no emissions from the experimental non-fugitive catalysts mentioned in this paper. Emissions from the fugitive control catalysts were measured and identified. These results support the fact that these experimental catalysts will result in no VOC emissions which will ultimately yield no amine fogging and no vinyl staining from the final polyurethane foam article. Additional volatility measurements have indicated that these experimental catalysts volatilise 3 to 10 times less than the current industry standards (Dabco 33LV and Dabco BL-11) in the temperature range of 0 °C to 150 °C. Vinyl staining tests using commercial formulations and various grades and types of vinyl have indicated that the experimental catalysts mentioned in this chapter do not contribute to vinyl staining. Compared to industry standard amine catalysts, use of these experimental non-fugitive catalysts will result in significant reduction (ΔE of less than 2) in staining measurements [12]. DE is the numerical total colour difference, using lightness and chromaticity factors, between a sample and a known colour standard.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams 1.3.2.2i TDI Flexible Moulded Foam Review The most commonly used catalyst packages for all water blown TDI flexible foam production are blends of Dabco 33LV and Dabco BL-11 at a typical ratio of approximately 3:1. These new non-fugitive catalysts, Dabco NE1060, Dabco NE200, XF-N1085, and XF-O11006 are recommended for TDI formulations at various gel:blow ratios depending upon the selectivity desired. Ratios from 2:1 to 8:1 can be used to tailor the reaction profile to the preferred selectivity for optimisation of physical properties and adaptation to various processing conditions. The physical properties obtained with these new nonfugitive catalysts are similar to the physical properties generated with standard industry catalysts. However, slight formulation adjustments may be necessary to improve dry and wet compression set values in certain instances. Improved crush-out capability can also be achieved when incorporating the new cell-opening non-fugitive catalysts, XF-N1085 and XF-O11006. Complementary methods of emission analysis, headspace and fogging, both suggest reduced emissions from the foam with these new non-fugitive catalysts.

1.4 MDI Flexible Moulded Foam Additives The current trend toward lower foam densities, faster demould times, and the increased use of complex metal and plastic insert frames in the construction of automobile interior components has increased the difficulty of controlling the numerous process and chemical variations a polyurethane manufacturer encounters on a daily basis. To successfully produce moulded high resilience polyurethane foam, the manufacturer must maintain a critical balance between foam over stabilisation, resulting in foam shrinkage, and under stabilisation, resulting in internal defects, such as basal cell formation and shear collapse. This often difficult to achieve balance is referred to as processing latitude. Variations on either side of the processing latitude can result in costly increases for the producer in scrap and repair rates. To help minimise production losses, polyurethane manufacturers are essentially forced to over stabilise their foam formulations on a regular basis. Subsequently, the closed cells which normally result from this over stabilisation are opened via standard mechanical crushing techniques [13]. New surfactant technologies have been developed which promote improved cell wall drainage and contribute to the final level of cell openness of the foam product without causing any of the aforementioned negative attributes typical to a foam process. Due to this unique cell opening action of these new additive technologies, the foam crushing portion of the process can be greatly simplified or potentially eliminated. Figure 1.41 shows a variety of flexible moulded Dabco surfactants which can be utilised in TDI and/or MDI formulations. Using Figure 1.41, proper selection of the surfactant can be made, enabling polyurethane formulators to expand their processing latitude, cell openness, and bulk stability of the foam article in their critical formulations.

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Advances in Urethane Science and Technology

Figure 1.41 Flexible Moulded Surfactant/Property Relationships

1.4.1 Dimensional Stability Additives for MDI An increasing North American trend in the moulded polyurethane foam industry today is the use of MDI. While the advantages and disadvantages of MDI versus TDI are still being debated, the fact remains that most polyurethane producers are using MDI in at least part of their daily flexible foam production. The surfactant requirements for MDIbased formulations can vary dramatically, dependent upon the type of MDI and polyols being used. Most MDI formulations are inherently quite stable; therefore minimal stabilising contributions are necessary from the surfactant. Conventional TDI high resilience moulded surfactants are far too potent for MDI formulations. For example, utilisation of Dabco DC5043 or Dabco DC5164 in an MDI seating formulation may result in over stabilisation and significantly contribute to the shrinkage of the foam article. To demonstrate the effectiveness of these dimensional stabilising additives, Dabco DC2517 and Dabco DC5244, several moulded high resilience foam experiments were carried out using the formulation illustrated in Table 1.32, to compare with two non-cell opening surfactants Dabco DC2585 and an additional surfactant (control). Much of the work described next utilises proprietary formulations and only a general description of some of the components can be disclosed.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.32 MDI flexible moulded formulation Components

pphp

Polyether polyol, diol/triol blend

100

Water

3.5

Diisopropanol amine

2.0

Dabco 33LV

0.3

Dabco BL-11

0.25

Dabco B-16

0.3

Surfactants

1.0

MDI prepolymer (2.3 functionality, 26% NCO)

100 Index

1.4.1.1 Reactivity - Handmix Rate of Rise Data generated in the rate-of-rise comparison tests are summarised in Table 1.33. Foams were run at an optimum index of 100 and all experiments were duplicated. Each mixed formulation was poured into a ‘chicken’ bucket equipped with a thermocouple positioned at the centre of the bucket resting on a Mettler PM 30 balance. The centred height of the rising foam was recorded in millimetres every second using a DAPS (data acquisition and plotting system) QA Model #2500 rate-of-rise aparatus. Reactivity profiles for these new additives are essentially the same. Data discussed next further supports the fact that there is no significant impact on reactivity or physical properties when comparing the control to the Dabco DC2517 and Dabco DC5258 formulations.

Table 1.33 Handmix rate of rise reactivity comparison Formulation Profile Top of Cup 1 (seconds)

12.4

Top of Cup 2 (seconds)

48.1

String Gel (seconds)

53.4

Full Rise (seconds)

101.8

Full Rise Height (mm)

395.1

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Advances in Urethane Science and Technology

1.4.1.2 Standard Physical Properties To understand the benefits of these new additive technologies that provide low volatility and cell opening, all MDI moulded formulations were run in handmix foam to demonstrate performance. Several foams were produced for each surfactant formulation to obtain physical property pads, FTC pads, shrinkage pads, and flow evaluations. Table 1.34 illustrates moulding foams at a density of 47-48 kg/m3; similar physical properties are obtained for ILD, resilience, airflow, 50% dry sets and extensive properties. However, slight nominal improvements in 50% humid aged compression sets (HACS) and wet sets are achieved with Dabco DC2517 and Dabco DC5258.

Table 1.34 Physical properties for MDI moulded formulations Physical Property

Control

DC2517

DC2585

DC5258

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

302

3.4

295

4.5

292

5.2

300

1.8

65%

764

9.9

762

8.2

759

7.2

753

1.3

25% Return

223

3.5

213

4.7

212

3.9

219

1.0

Ball Rebound (%)

44.3

0.4

43.7

0.7

43.7

0.7

44.0

0.5

Airflow (SLM)

22.5

2.1

24.3

9.1

23.3

5.0

28.1

10.4

Tensile (kPa)

165

5.1

171

9.8

165

11.7

167

8.6

Tear (N/M)

305

11.5

303

15.7

309

11.2

330

19.8

Elongation (%)

95

7.4

94

5.5

93

6.8

97

4.5

Wet Set (%)

18.5

1.3

16.6

2.6

19.1

0.2

16.4

0.6

50% Compression Set

10.1

1.1

10.2

1.1

10.4

0.9

9.1

0.8

50% HACS

28.0

0.2

26.6

1.2

25.7

0.7

24.3

1.6

1.4.1.3 FTC Results The FTC graph shown in Figure 1.41 illustrates an approximate 14% decrease in initial FTC values and continues throughout the remaining FTC cycles. This improvement in FTC represents foam articles that will shrink less immediately after demoulding when cooling of the trapped gas begins, allowing more time to complete the post crushing

66

Dimensional Stabilising Additives for Flexible Polyurethane Foams procedure or potentially eliminating the procedure all together. Several foams were prepared with each surfactant and allowed to cool for 24 hours in an uncrushed state, to confirm the relationship between FTC and foam shrinkage. Foams prepared with Dabco DC2517 and Dabco DC5258 showed significantly less shrinkage, approximately 1015% less, as compared to the control and Dabco DC2585. This is readily comparable to the differences in FTC values shown in Figure 1.42.

Figure 1.42 FTC Graphs for MDI Dimensional Stability Additives

1.4.2 Low Emissions Dimensional Stability Additives in MDI The control catalyst combination for the MDI seating is a standard commercial catalyst package. The balanced non-fugitive catalyst package Dabco NE1060/Dabco NE200 and the low emission catalyst package Dabco NE1060/Dabco BL-11 use levels were set in order to match performance to the commercial catalyst control package for the MDI formulations. The silicone surfactant control was Dabco 2525. Cell opening non-fugitive catalysts were not needed for this application based on the reported FTC values discussed later in this chapter. An MDI formulation, with a density of 40 kg/m3 and an MDI formulation, with a density of 55 kg/m3 were used in the MDI automotive study with the formulations shown in Table 1.35. For rapid demoulding of non-fugitive MDI systems, it is recommended that Dabco NE1060 be used at 0.5 to 2.0 pphp, with the optimum level at 1.0 to 1.5 pphp with 0.3 to 0.7 pphp of Dabco NE200. For low emission systems the authors recommend Dabco NE1060 be used at 1.0 to 2.0 pphp, with the optimum level at 1.0 to1.6 pphp with 0.03 to 0.1 pphp of Dabco BL-11.

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Table 1.35 Flexible moulded MDI formulations Formulation Identification

XXV

XXVI

XXVII

XXVIII

XXIX

XXX

XXXI

XXXII

XXXIII

XXXIV

Components

pphp

pphp

pphp

pphp

pphp

pphp

pphp

pphp

pphp

pphp

Polyol C

100

100

100

100

100

100

100

100

100

100

Polyol D

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

Water

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

Control Catalyst Package

0.70

-

-

-

-

0.70

-

-

-

-

Dabco BL-11

-

0.08

-

0.05

-

-

0.08

-

0.05

-

Polycat 15

-

0.30

0.30

-

-

-

0.30

0.30

-

-

Dabco NE1060

-

1.00

0.55

1.50

1.20

-

1.00

0.55

1.50

1.20

Dabco NE200

-

-

0.60

-

0.40

-

-

0.60

-

0.40

Dabco DC2525

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

Crosslinker

0.60

0.60

0.60

0.60

0.60

0.60

0.60

0.60

0.60

0.60

90

90

90

90

90

105

105

105

105

105

Modified MDI Index

1.4.2.1 Low Emissions Moulded Foam Machine Standard Physical Properties For complete comparison of these new additive technologies that provide low volatility, no amine emissions and no fogging, all moulded formulations were run on a Krauss-Maffei machine. Several foams were produced for each catalyst combination and formulation to obtain physical property pads and FTC pads, shown in Tables 1.36 to 1.41. Tables 1.36 and 1.37 provide the physical property comparison for the ~46 kg/m3 MDI flexible moulded formulations XXV-XXXIV. The data clearly demonstrates that physical properties are maintained, and in several cases improved, compared to the control formulations, depending on the formulation and the use of experimental cell-opening catalysts. For example, the data in Table 1.36 illustrates that at an index of 90, all physical properties are comparable to the control formulation. When the index is increased to 105, ball rebound and airflow are improved, with a slight decrease in indentation force deflection (IFD) properties when the non-fugitive catalyst (XXXIV) is compared to the control formulation (XXVI). When Dabco NE1060/Dabco NE200 is used (XXIX), the physical properties are equal to the control at an index of 90. Improved Japanese wet sets (see Section 1.3.1.2f) and slightly decreased IFD values are observed at an index of 105 compared to the control formulation (XXX).

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Dimensional Stabilising Additives for Flexible Polyurethane Foams Table 1.36 Physical properties for 46 kg/m3 MDI moulded formulations at an index of 90 Physical Property

XX V

XXVI

XXVII

XXVIII

XXIX

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

159

-

159

-

14 9

-

15 5

-

15 5

-

65%

459

-

450

-

43 6

-

45 0

-

44 8

-

25% Return

127

-

126

-

11 9

-

12 3

-

12 3

-

51

0.6

51

0.6

51

0.6

51

0.6

51

1.2

Airflow (SLM)

Ball Rebound (%)

22.4

7.6

24.4

6.5

29

7.4

34

7. 9

17

5.0

Density (kg/m3)

46

3.4

44

1.0

43

2.4

46

3.7

45

0.9

Tensile (kPa)

129

10.6

298

14.4

125

6.7

133

5.9

127

9.9

Tear (N/m)

212

12

201

16

197

12

217

16

205

20

Elongation (%)

114

9.9

118

6.1

112

4.6

120

1.3

118

9.3

Wet Set (%)

13

0.6

16

1.2

10

0.5

13

0. 4

13

1.3

50% Compression Set (%)

9

1.6

10

1.0

9

1.6

10

0.5

10

0.8

50% Humid Aged Compression Set (%)

13

0.5

13

2.0

13

1.5

12

1. 3

14

1.0

Table 1.37 Physical properties for 46 kg/m3 MDI moulded formulations at an index of 105 Physical Property

XX X

XXXI

XXXII

XXXIII

XXXIV

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

275

-

260

-

264

-

261

-

251

-

65%

768

-

731

-

739

-

738

-

71 6

-

25% Return

209

-

197

-

201

-

198

-

190

-

Ball Rebound (%)

43

0.6

45

1.2

45

0.0

45

1. 0

47

1.2

Airflow (SLM)

40.8

8.5

31.4

7.1

38

27.2

33

9.9

46

8.5

Density (kg/m3)

46

1.1

47

4.4

45

0.6

46

3.2

44

1.3

Tensile (kPa)

176

9.8

409

19.2

173

6.8

168

6.3

169

11.7

Tear (N/m

263

8

252

4

25 6

12

279

16

244

12

Elongation (%)

105

4.2

109

4.1

102

6.7

105

4.3

103

6.2

Wet Set (%)

15

0.8

15

0.4

11

0.2

9

0. 7

17

0.2

50% Compression Set (%)

8

0.6

9

1.3

8

0.3

10

0.9

9

1.0

50% Humid Aged Compression Set (%)

13

2.0

13

1.8

12

1.0

14

1. 7

14

1.0

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Advances in Urethane Science and Technology Table 1.38 Physical properties for 55 kg/m3 MDI moulded formulations at an index of 90 Physical Property

XXV

XXVI

XXVII

XXVIII

XXIX

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

235

-

238

-

218

-

26 0

-

23 5

-

65%

637

-

636

-

59 7

-

731

-

64 1

-

25% Return

188

-

190

-

17 4

-

197

-

18 7

-

Ball Rebound (%)

46

0.6

44

1.0

46

0.6

45

1.2

42

0.6

Airflow (SLM)

21

4.0

35

11.9

35

5.4

31.4

7.1

27

12.5

Density (kg/m3)

53

3.4

50

3.6

50

3.5

47

4. 4

52

1.9

Tensile (kPa)

148

24.7

164

8.0

161

3.2

149

19.2

162

11.0

Tear (N/m

207

12

220

12

197

4

252

4

20 5

12

Elongation (%)

117

6.0

116

7.9

120

1.7

109

4.1

118

4.1

Wet Set (%)

11

1.4

13

0.9

15

1.3

15

0. 4

10

0.3

50% Compression Set (%)

8

0.9

7

1.5

9

1. 0

9

1. 3

9

0.4

50% Humid Aged Compression Set (%)

11

1.5

14

2.1

13

1.3

13

1.8

12

0.9

Table 1.39 Physical properties for 55 kg/m3 MDI moulded formulations at an index of 105 Physical Property

XXX

XXXII

XXXI

XXXIII

XXXIV

ILD (N)

AVG

SD

AVG

SD

AVG

SD

AVG

SD

AVG

SD

25%

398

-

38 0

-

38 8

-

38 3

-

381

-

65%

1066

-

1029

-

1039

-

1042

-

1032

-

25% Return

303

-

288

-

295

-

289

-

289

-

Ball Rebound (%)

46

1.0

43

0.6

44

0.6

43

0.6

45

1.2

Airflow (SLM)

54

17.3

62.9

21.8

48

15.0

67

9.3

48

4.5

Density (kg/m3)

52

3.7

52

1.2

54

0.7

54

0. 4

55

0.5

Tensile (kPa)

217

17.0

218

7.0

207

12.9

217

6.6

223

10.5

Tear (N/m)

299

12

283

24

307

24

29 9

16

315

16

Elongation (%)

105

4.2

108

1.4

102

1.1

111

4.1

109

2.1

Wet Set (%)

14

1.4

12

0.7

13

0.7

11

0. 6

15

0.9

50% Compression Set (%)

6

1.1

8

0.8

8

0. 6

9

1. 1

9

0.4

50% Humid Aged Compression Set (%)

14

1.9

14

0.8

13

0.9

12

1. 2

14

0.8

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Dimensional Stabilising Additives for Flexible Polyurethane Foams Tables 1.38 and 1.39 provide the physical property comparison for the ~55 kg/m3 MDI flexible moulded formulations XXV-XXXIV. The data clearly demonstrates that physical properties are maintained, and in several cases improved, compared to the control formulations, depending on the formulation. For example, the data in Table 1.38 illustrates that all physical properties are matched to the control formulation (XXV) at an index of 105 with the exception of a slightly lower ball rebound for the full non-fugitive catalyst package. At an index of 105, all physical properties for the experimental non-fugitive catalysts formulations were determined to be similar to the control (XXX). When Dabco NE1060/BL-11 is used as a low emission catalyst package (XXXIII), all physical properties at the index of 105 are comparable to the control foam. At an index of 105 (XXXIII), airflow, elongation and Japanese wet sets and HACS are improved over the control (XXX) and 50% dry compression sets are slightly elevated.

1.4.2.2 FTC Tables 1.40 and 1.41 show numerical values which are shown graphically in Figures 1.43 and 1.44. FTC values are dramatically improved versus the control (XXV), shown

Table 1.40 FTC values for MDI formulations at an index of 90 FTC (N/323cm2) 46 kg/m3

FTC (N/323cm2) 55 kg/m3

XXV

52

76

XXVI

14

24

XXVII

35

63

XXVIII

8

13

XXIX

4

12

Formulation

Table 1.41 FTC values for MDI formulations at an index of 105 FTC (N/323cm2) 46 kg/m3

FTC (N/323cm2) 55 kg/m3

XXX

2.6

4.5

XXXI

1.3

1.9

XXXII

1.9

3.2

XXXIII

1.9

3.2

XXXIV

1.3

1.9

Formulation

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Advances in Urethane Science and Technology

Figure 1.43 FTC at an Index of 90 for MDI Formulations

Figure 1.44 FTC at an Index of 105 for MDI Formulations

in Figure 1.43, for the foams with an index of 90. FTC values for all of the formulations at an index of 105 are very similar to the control (error is ± 6.4 N/323cm2), as illustrated in Figure 1.44.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

1.4.2.3 MDI Flexible Moulded Foam Review The non-fugitive catalyst package Dabco NE1060/Dabco NE200 and the low emission catalyst combination Dabco NE1060/Dabco BL-11 closely match the entire reactivity profile demonstrated by the standard control catalysts typically used in MDI flexible foam. Additionally, the Dabco NE1060/Dabco NE200 combination demonstrates a higher selectivity towards the blowing reaction as compared to a standard 2.5:1 ratio of Dabco 33LV to Dabco BL-11. This higher blowing selectivity can help improve cell opening and produce a more dimensionally stable polyurethane article. These non-fugitive catalysts also provide a reduction in the force necessary to crush freshly demoulded foam without adversely affecting the remaining physical properties of the foam in MDI formulations over a range of formulation indices.

1.5 TDI Flexible Slabstock Low Emission Additives Handmix rate of rise comparison tests were performed using the same equipment and process as described for TDI and MDI hand mix foam. The formulations used are listed in Table 1.42 and the rate of rise data reported in Table 1.43. Dabco BLV with Dabco T-9 was used as the industry standard catalyst control for the water blown flexible slabstock foam formulations. The silicone surfactant used in the reported flexible slabstock formulations was DC5160. All water blown formulations were made at densities of 23 kg/m3.

Table 1.42 Flexible slabstock formulations Formulation Identification

XXXV

XXXVI

XXXVII

Components

pphp

pphp

pphp

Voranol 3512

100

100

100

Water

4.60

4.60

4.60

Dabco BLV

0.12

—

—

Dabco NE500

—

0.06

0.16

Dabco NE600

—

0.12

—

Dabco T-9

0.25

0.25

0.26

Dabco DC5160

0.90

0.90

0.90

TDI (100 index)

57.4

57.4

57.4

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Advances in Urethane Science and Technology

Table 1.43 Hand mix free rise reactivity comparison using flexible slabstock formulations Formulation

XXXV

XXXVI

XXXVII

Cream (Seconds)

13

12

13

Top of Cup (Seconds)

25

24

24

String Gel (Seconds)

67

67

64

Full Rise (Seconds)

112

1 11

109

1.5.1 Reactivity Formulations from Table 1.42 were used in rate of rise catalyst comparisons and the results are summarised in Table 1.43. The data suggests there is no significant change in reactivity when comparing the control formulation (XXXV) to the experimental formulations.

1.5.2 Standard Physical Properties Table 1.44 lists the physical property data for the flexible slabstock formulations. The experimental non-fugitive catalyst Dabco NE500 can be used with a slight increase in Dabco T-9 catalyst level (shown in Table 1.42, formulation XXXVII). Formulation XXXVII compares very well to the control (XXXV). Formulation XXXVI illustrates the use of Dabco NE500 and Dabco NE600 balanced non-fugitive catalyst combination, to match a Dabco BLV control. In this case, all physical properties were also matched, with the exception of slightly decreased airflow and improved ILD properties. Overall, either the non-fugitive catalyst combination Dabco NE500/Dabco NE600 or the use of Dabco NE500 with increased Dabco T-9 can replace Dabco BLV in all water blown flexible slabstock formulation.

1.5.3 TDI Flexible Slabstock Foam Review Dabco NE600 in combination with Dabco NE500 or Dabco NE600 as a sole amine catalyst used with increased levels of T-9 provides for equal or improved performance compared to Dabco BLV in all water-blown flexible slabstock foam. In all cases, these catalyst combinations can completely replace Dabco BLV and eliminate amine emissions from the flexible slabstock foam.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams

Table 1.44 Physical properties for flexible slabstock foam Physical Property

XXXV

XXXVI

XXXVII

ILD (N)

AVG

SD

AVG

SD

AVG

SD

25%

198

-

211

-

200

-

65%

401

-

421

-

401

-

25% Return

126

-

134

-

128

-

Ball Rebound (%)

41

0.1

40

0.1

40

0.1

Airflow (SLM)

108

5.3

72

6.0

94

6.8

Density (kg/m3)

22

0.3

22

0.4

22

0.2

Tensile (kPa)

71

0.6

73

0.5

75

0.5

Tear (N/m)

200

-

200

-

200

-

Elongation (%)

97

7.5

106

6.7

104

6.1

90% Compression Set (%)

6

0.5

7

0.5

6

0.5

1.6 Foam Model Tool Discussions Details of the TDI and MDI foam model systems have been previously published [2]. The models require the use of mono-functional reactants that are quantitatively analysed to correlate structure-activity relationships for various classes of catalysts. A realistic thermal profile is produced through the imposition of an external exotherm. Urethane, urea, allophanate and biuret reaction products are quantified by liquid chromatographic analysis of quenched reaction samples. The models effectively account for such nonideal conditions as reactant depletion at variable rates, temperature and concentrationdependent catalyst activity, and catalyst selectivity as a function of isocyanate distribution.

1.6.1 TDI and MDI Moulded Foam Model The information below highlights the features characteristic of the TDI and the MDI flexible moulded models. Table 1.45 illustrates the conversion of the formulations from Tables 1.12-1.15 into a TDI model system. Table 1.46 illustrates the conversion of the formulations from Table 1.35 into an MDI model system. Since the TDI and MDI automotive polyols are highly ethylene oxide (EO) tipped the reactive hydroxyl group can be represented by a primary alcohol. The electronic effect of

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Advances in Urethane Science and Technology

Table 1.45 TDI flexible moulded foam model system Formulation

Mass (g)

Polyether Polyols

100

Model Compounds

Mass (g)

Diethylene glycol methyl ether CH3OCH2CH2OCH2CH2OH

10.43

Diglyme CH3OCH2CH2OCH2CH2OCH3

95.89

Diethanolamine (water free basis)a

1.74

Dibutylamine

2.14

Water

4.20

H2O

4.20

Surfactant

No surfactant required for model

Catalyst Package

0.35

Catalyst Package

TDI 80 Index

105

40 mole% PhNCO 60 mole% o-TolylNCO

a

0.35 Varied

Diethanolamine hydroxyls included with diethyleneglycol methyl ether mass.

Table 1.46 MDI flexible moulded foam model system Formulation Polyether Polyol

Mass (g) 100

Model Compounds

Mass (g)

Diethylene glycol methyl ether CH3OCH2CH2OCH2CH2OH

7.51

Diglyme CH3OCH2CH2OCH2CH2OCH3

93.54

Diisopropanolamine (water free)a

0.85

Dibutylamine

0.81

Water

2.90

H2O

2.90

Dabco DC-2525

1.00

No surfactant required for model

Catalyst Package

1.60

Catalyst Package

Mondur MR Index a

95

80 mole% PhNCO 20 mole% o-TolylNCO

1.60 Varied

Diisopropanolamine hydroxyls included with diethyleneglycol methyl ether mass.

76

Dimensional Stabilising Additives for Flexible Polyurethane Foams the ether two carbons away from the alcohol is accounted for by using an ether alcohol. Thus both stearic and electronic effects on the reaction kinetics can be accurately reflected in the model compound. The unreactive polyether backbone of the polyol can be represented by the dimethyl ether of diethylene glycol. Dipropylene glycol dimethyl ether would be more representative of the polyether backbone of a typical EO-tipped propylene oxide (PO) polyol, but the rate and selectivity measurements are minimally affected by the choice of model polyol backbone. Note the sum of the masses of the model alcohol and the diglyme slightly exceeds 100 grams, the total polyol mass, since the hydroxyls from the alkanolamine crosslinkers are included in the model alcohol mass. Nonetheless, the concentration of reactive groups is comparable to that in the actual foam. Water and catalysts are the same for both formulation and model. The model system does not require a surfactant however, since a foam is not actually produced. TDI 80 and Mondur MR are represented by 40:60 mole% and 80:20 mole% mixtures of phenyl and orthotolyl isocyanate, respectively. Phenyl isocyanate represents the 4-position in 2,4-TDI or 4,4´-MDI, and tolyl isocyanate represents the more sterically hindered 2- and 6-positions in the 2,4- and 2,6-TDI, or the internal rings of MDI oligomers. It is important to include both isocyanate types because using only the more reactive phenyl isocyanate significantly overestimates the reactivity of the system. Detailed procedures for an individual model run can be found in the literature [14]. For these runs a masterbatch mixture containing the alcohol, ether, water, catalyst and the 3,3´-dimethylbiphenyl internal standard was prepared in advance. The masterbatch mixture was charged to a 50 cm3 roundbottom flask equipped with a glass thermocouple well, septum, and jacketed mechanical stirrer with gas inlet and a septum. For the TDI model the flask was placed under a slow argon purge in a sand bath capable of raising the internal temperature from 60 °C to 120 °C in 4 minutes in the absence of any reactions. The isocyanate was added via a syringe when the internal temperature reached 60 °C. For the MDI runs the flask was wrapped with insulation but not otherwise heated externally, and the isocyanate was added at 25 °C. Samples were then withdrawn via a syringe at 30 second intervals for eight minutes. The samples were quenched with dibutylamine and analysed by liquid chromatography to determine the yields of the urea and urethane reaction products as well as the unreacted isocyanates, analysed as the dibutyl ureas. The flexible moulded chemical foam model provides a detailed look at the performance of the industry standard catalyst package Dabco 33LV/Dabco BL-11 compared to the non-fugitive catalyst packages Dabco NE1060/Dabco NE200 and Dabco NE1060/XFN1085. The foam results presented previously can be explained in terms of the overall catalyst performance. The catalysts were characterised with a model system because urea, urethane and isocyanate can be quantified as a function of time, so this approach provides the highest level of detail. However, a model system is only relevant to the

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Advances in Urethane Science and Technology extent that it accurately reproduces the environment in an actual foam. The mixing in the model systems is less energetic, so this may influence the relative rates of catalyst hydrolysis versus reaction with isocyanate. However, if catalysts are compared under a consistent set of conditions, the relative performance differences should be meaningful. The most convenient way to compare catalyst performance is to use blow to gel selectivity. Selectivity as a function of time (t), is defined next. Normalised blowing rate = (% yield of ureas at time, t) / (limiting urea yield) Normalised gelling rate = (% yield of urethanes at time, t) / (limiting urethane yield) Blow to gel selectivity = (Normalised blowing rate) / (Normalised gelling rate) Catalyst selectivity is defined as the ratio of the normalised amount of blowing (urea formation) to the normalised amount of gelling (urethane formation). A selectivity of 1.0 means that the normalised amounts of blowing and gelling are equal at that point in the reaction. A selectivity substantially below 1.0, for example about 0.3, is indicative of a strong gelling catalyst. A selectivity greater than 1.0 is indicative of a blowing catalyst. However, it has been shown that a catalyst with a blow-to-gel selectivity greater than 0.8 can still serve to balance a strong gelling catalyst such as TEDA, and a good quality foam can be produced [14]. Note the limiting urea and urethane yields are simply the molar equivalents of water and alcohol, respectively, from Tables 1.45 and 1.46. Selectivities are plotted as a function of isocyanate conversion in Figures 1.45 and 1.46.

1.6.2 TDI Flexible Slabstock Foam Model Details on the development and use of foam model systems for flexible moulded foam have been published [14]. In this section the features characteristic of slabstock foams will be highlighted. Table 1.47 illustrates the conversion of the formulations from Table 1.42 into a model system. Since the Voranol 3010 polyol is fully PO tipped, the reactive hydroxyl group can be represented by a secondary alcohol. The unreactive polyether backbone of the polyol can be represented by the dimethyl ethers of dipropylene glycol and diethylene glycol. The diglyme represents the wt% EO incorporation into the polyol. Note the sum of the masses of the model alcohol and the two dimethyl ethers equals 100 grams, the total polyol mass. This ensures that the concentration of reactive groups is comparable to that in the actual foam. Water and catalysts are the same for both formulation and model.

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Figure 1.45 TDI Flexible Foam Model Selectivity versus NCO Conversion

Figure 1.46 MDI Flexible Foam Model Selectivity versus NCO Conversion

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Table 1.47 Slabstock foam model system Formulation Voranol 3010

Mass (g) 100

Model Compounds 1-Methoxy-2-propanol CH3OCH2CH(CH3)OH

9.01

Dipropyleneglycol dimethyl ether (CH3O(CH3)CHCH2)2O

82.99

Diglyme CH3OCH2CH2OCH2CH2OCH3 Water

2.00-6.00

Mass (g)

H 2O

8.0 2.00-6.00

Dabco DC5160

0.75

No surfactant required for model

Dabco BLV

0.20

Dabco BLV

0.20

Dabco T-9

0.50

Dabco T-10

0.50

TDI 80 Index

90, 105, 120 40 mole% PhNCO 60 mole% o-Tolyl NCO

Varied

Once again the model system does not require a surfactant, since a foam is not actually produced. As noted previously, TDI 80 is represented by a 40:60 mole% mixture of phenyl and ortho-tolyl isocyanate. Detailed procedures for an individual model run can be found in the literature [14]. For these runs a masterbatch mixture containing the alcohol, ethers, water, Dabco BLV catalyst and the 3,3´-dimethylbiphenyl internal standard was prepared in advance. The masterbatch mixture was charged to a 50 cm3 roundbottom flask equipped with a glass thermocouple well, septum, and a jacketed mechanical stirrer with gas inlet. The flask was placed under a slow argon purge in a sand bath capable of raising the internal temperature from 50 °C to 120 °C in 4 minutes in the absence of any reactions. Isocyanate was added via syringe when the internal temperature reached 50 °C, and the T-9 was injected from a microliter syringe immediately afterwards. Samples were then withdrawn via the syringe at 30 second intervals for eight minutes. The samples were quenched with dibutylamine and analysed by liquid chromatography to give yields of the urea and urethane reaction products as well as the unreacted isocyanates, analysed as the dibutyl ureas. The chemical flexible slabstock foam model provides a detailed look at the performance of the industry standard catalyst Dabco BLV compared to the non-fugitive catalysts Dabco NE500 and the Dabco NE500/Dabco NE600 package, keeping the Dabco T-9 level constant. Selectivity and conversion can be calculated as described previously. Note that

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Dimensional Stabilising Additives for Flexible Polyurethane Foams the limiting urea and urethane yields are simply the molar equivalents of water and alcohol, respectively, from Table 1.47. Dabco 33LV is considered to be a strong gelling catalyst, but under slabstock conditions it is actually more of a blowing catalyst, with an initial selectivity near 2. This reflects the difficulty of catalysing the secondary alcohol-isocyanate reaction in competition with the water-isocyanate reaction. Tertiary amine catalysts are highly sensitive to stearic hindrance near the reaction site. Secondary alcohols actually become less reactive than water, even in the presence of a strong gelling catalyst. This is why tin catalysts are critical in flexible slabstock formulations because they are less sensitive to stearically hindered reactions. Dabco BLV contains some bisdimethylaminoethyl ether (Dabco BL11), which is a very strong blowing catalyst. Thus the initial selectivity is even higher, near 4. The non-fugitive catalysts give selectivities comparable to that of Dabco BLV. Note that the non-fugitive catalyst packages are more blowing selective than both the TDI and MDI controls. Modern polyol technology tolerates and in some cases even benefits from higher initial blowing selectivity because of the reduced levels of monol and diol relative to triol. Thus network formation is more efficient, and correspondingly less gelling is required to produce a superior urethane network. Higher blowing selectivity can promote improved cell opening, which is now more critical in the face of improved network formation. In a sense the control formulations are over gelled, but older formulations could tolerate high gelling due to weaker urethane networks and the frequent use of Dabco BL-11, which is a potent blowing catalyst. Although the selectivity curves are different, the catalyst packages are still rate matched. Rate matching in foam is usually accomplished by comparing rise profiles, which are largely determined by the blowing rate. Thus the non-fugitive catalyst packages provide blowing comparable to the controls, but provide a lower extent of urethane formation early in the foaming process.

1.7 Conclusions Incorporation of these newly developed additives in the production of polyurethane foam results in polyurethane articles that give dimensional stability, low emissions and wider processing latitude. These dimensional stability additives provide a significant reduction in the force necessary to crush freshly demoulded foam without adversely affecting the physical properties of the foam. As increasing demands are placed on the foam producers to meet specific comfort and durability requirements, physical properties and environmental concerns, the need for these additives will continue to be a key criterion for polyurethane manufacturers.

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Acknowledgments The authors would like to thank and acknowledge the efforts and support of the following people at Air Products and Chemicals, Inc.: Mark A Eckert, Steven E Robbins for physical property testing of foams generated in this study, and Ilean S Ruhe for preparation and editing of this work. We would also like to thank Air Products and Chemicals, Inc., for support of this work and permission to publish it.

References 1.

J. D. Tobias, G. D. Andrew, Presented at the SPI Polyurethanes Expo ‘98, Dallas, TX, 1998, p.445.

2.

M. L. Listemann, A. C. L. Savoca and A. L. Wressell, Journal of Cellular Plastics, 1992, 28, 4, 360.

3.

R. G. Petrella and S. A. Kushner, Presented at the SPI Polyurethanes ‘90 Conference, Orlando, FL, 1990, p.186.

4.

G. Burkhart and M. Klincke, Presented at the SPI Polyurethanes ‘95 Conference, Chicago, IL, 1995, p.297.

5.

Flexible Polyurethane Foam, The Dow Chemical Company.

6.

ASTM D3574-95 Standard Test Methods for Flexible Cellular Materials - Slab, Bonded, and Molded Urethanes Foams.

7.

L. J. Gibson and M. F. Ashby, Cellular Solids, Structure and Properties, Pergamon Press, Oxford, 1988, Chapters 5 and 6.

8.

K. D. Cavender, Presented at the SPI, Magic of Polyurethane Conference, Reno, NV, 1985, p.314.

9.

SAE J1756 Test Procedure to Determine the Fogging Characteristics of Interior Automotive Materials, 1994.

10. M. S. Vratsanos, Presented at the SPI, Polyurethanes ’92 Conference, New Orleans, MS, 1992, p.248. 11. K. F. Mansfield, Air Products & Chemicals, Inc., unpublished results.

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Dimensional Stabilising Additives for Flexible Polyurethane Foams 12. R. G. Petrella and J. D. Tobias, Presented at the SPI Polyurethanes ‘88 Conference, Philadelphia, PA, 1988, p.28. 13. D. G Battice and W. I. Lopes, Presented at the SPI Polyurethanes: Exploring New Horizons Conference, Toronto, Canada, 1986, p.145. 14. M. L. Listemann, K. R. Lassila, K. E. Minnich and A. C. L. Savoca, inventors; Air Products and Chemicals, Inc., assignee; US Patent 5,508,314, 1996.

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Demands on Surfactants in Polyurethane Foam Production with Liquid Carbon Dioxide Blowing Andreas Weier and Georg Burkhart

2.1 History of Polyurethane Foams The history of polyurethane foams and of polyurethanes in general is not that long for a plastic material. It has been characterised by one special feature: the consistent changes of the industry. When Otto Bayer started his research on this new class of plastic material built from the addition reaction of isocyanates and hydroxyl containing materials in the years 1936/1937 he envisioned a plastic material which could be tailored to many different applications by the broad variety of acidic hydrogen containing compounds already available. But there were also setbacks to this concept. The first major drawback was the fact that the necessary diisocyanates were not available on an industrial scale and large scale synthesis of these compounds seemed to be a rather high hurdle. A second major point was an obstacle that Bayer and his group faced again and again. When they tried to make the polyurethanes they envisioned in solid form, like films, etc., they always ended up with gas bubbles in their plastics. That was due to the inefficiency of the methods available to synthesise pure water free material. After encountering this effect for nearly four years Bayer made a bold move in line with the concept ‘use what you cannot prevent’ – in this case the formation of gas bubbles [1, 2]. Thus Bayer’s group started to make polyurethane foams. Although these new lightweight porous materials were envisioned as supportive materials as well as insulative materials in the base patents [3, 4] their market development remained slow. This can be seen by the fact that even in 1952 polyisocyanates, mainly toluene diisocyanate (TDI), were available worldwide in quantities of less than 100 tonnes. After this rather hesitant start of polyurethane history and the first major switch from solid materials to porous foamed plastics, the industry has been characterised by significant changes in concept and the resulting industrial application of these switches. The first major switch resulted from the basic research enabling the technical production of soft polyurethane foams in the early 1950s. This technology was mostly focused on polyester polyols as raw materials. About five years later polyether polyols entered the

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Advances in Urethane Science and Technology market as yet another major change within the polyurethane foam industry. Since the early 1950s the foaming machines for the industrial production of polyurethane foams had been discontinuous machines. With the use of polyalkyleneglycols as polyols, the importance of continuous foam production grew significantly, for both performance and commercial reasons. In the early 1950s, the necessary stabilisers for the production of polyurethane foams had been silicone oils, with the major suppliers being Bayer, Dow Corning, General Electric and ICI. In the late 1950s the picture changed with the broad application of the continuous process, first with prepolymers, then the one-shot-process. For this type of polyurethane foam production a different class of stabilisers was needed: the new silicone-polyether copolymers. The larger variability of the available polyetherpolyols laid the foundation for a significant broadening of the performance range of polyurethanes – and all this with lower costs than before. The growth of polyurethanes after this chemical change was fast [5]. Even in 1960 more than 45,000 tons of flexible polyurethane foam were produced. Besides its major application as insulation materials and comfort products in furniture applications, new usage areas have been continuously identified and tailor-made polyurethane materials have been supplied. Nowadays the range covers very high density contour parts as well as very low density packaging foams, flexible foams as well as semi-rigid and rigid foams, thermoplastic polyurethanes as well as integral skin foams or reaction injection moulded (RIM) materials [6]. In the beginning of the 1990s an estimated 500,000 people had jobs in the polyurethane industry, either within production of raw materials or in the conversion to final products [7].

2.1.1 Environmental Concerns in Relation to Flexible Foam Density By definition the production of a foam depends on the formation and stabilisation of gas bubbles in a liquid. That is true even for polyurethane foam when the additional curing of the liquid results in an elastic or rigid solid material. One of the central questions is how the gas bubbles are generated and how long this generation takes. The basic chemical gas forming process in polyurethane foaming is the very exothermic reaction of water and isocyanate, resulting in the generation of carbon dioxide, urea and heat. This method as the sole method to generate the gas bubbles of a polyurethane foam is limited to the production of a rather small range of density/hardness combinations. The achievable minimum density is limited by the tolerable heat in a foam bun before scorching or even self-ignition occurs. Increasing water levels lead to the generation of increasing amounts of carbon dioxide and therefore to lower densities. Increasing water levels will also result in higher hardness as more urea is generated. During reaction the urea appears in a variety of associated forms before it will ultimately separate as a solid in flexible slabstock polyether foams [8, 9, 10]. 86

Demands on Surfactants in Polyurethane Foam Production… During the history of polyurethane flexible slabstock foam production the accessible density/ hardness range was continuously broadened with a wide variety of techniques. The most obvious one was the use of a physical blowing agent to generate the gas volume necessary to expand or generate gas bubbles without generating additional heat during the process. Due to its inert character and its low biological toxicity, CFC-11 (Freon) was the liquid blowing agent of choice for many years in Europe, North America and Asia. That changed after recognition of the fact that CFC-11 had both a high global warming potential (GWP) and a high ozone depletion potential (ODP). The result was the inception of the Montreal Protocol and subsequently the exploration of a broad variety of alternative blowing agents (ABA), including hydrochlorofluorocarbons (HCFC), acetone, cyclopentane and methylene chloride. With the use of ABA, both the available densities and the available foam hardnesses were lowered in comparison to the purely water blown foams, as in these cases gas volume was produced without generation of urea. Softening of foam was also achieved chemically, either with the use of an additive to change the urea morphology [13] or with the use of crosslinkers in combination with a low isocyanate index [14]. A few years later machinery modifications enabled foamers to produce low density foams by the use of increased water levels while reducing traditional blowing agent levels. This reaction preferentially utilises the reaction generating carbon dioxide as a blowing agent. When the water in the formulation is increased, the foam bun temperature can become too high for safe processing. This problem has been tackled by forced cooling equipment generating an air exchange in the foam bun directly after production and thereby cooling the bun to prevent scorch or fire problems. There is a variety of technologies for the forced cooling of foams, including Envirocure [13, 14], Rapid Cure [15], Reeves Brothers [16] and lateral cooling [17]. For the production of very soft foam grades some physical blowing agent is still needed. Also there are always some safety concerns over possible equipment malfunctions when making these hot foam grades. A safer way to control flexible foam density and hardness is the variation of production pressure. Making high as well as low pressures available in the foam production environment can give access to higher density hard grades as well as to low density foams without the use of physical blowing agents. The two commercial processes addressing this approach are Variable Pressure Foaming (VPF) and Foam One. As foam production is done in a closed chamber, volatile materials can be trapped rather efficiently. This creates the possibility of a clean and safe, as well as ABA free foam production. A setback is the high capital investment cost, especially for a VPF unit. Therefore this equipment only becomes cost effective in very high volume production plants. Review of the many production techniques reveals that the production of a broad range of flexible foam qualities with reduction of environmental hazards (ODP as well as GWP) is possible by numerous options. All of these current technologies offer benefits, but provide no clear economical solution for total ABA elimination.

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Advances in Urethane Science and Technology Within all the blowing agents available today, carbon dioxide probably offers the best combination of zero ODP, low GWP and low price. As it is abundant, it doesn’t have to be generated. For capturing and transporting only a minor amount of energy is needed. Merely the point of emission is changed. On top of all that it is also non-flammable, which minimises safety concerns in the foam plant. Therefore many attempts have been made to produce polyurethane foams with carbon dioxide that is not generated by a chemical reaction during the foaming process. The difficulty is that carbon dioxide is a gas at room temperature under atmospheric pressure. The only way to utilise carbon dioxide not generated chemically is to mix the carbon dioxide under pressure or in a liquid state with the chemical components of the foam formulation. Then the pressure is subsequently released causing the frothing or preexpansion. Carbon dioxide as a non-reactive blowing agent in the frothing technique has been suggested for quite some time already [18, 19]. The pressurised reaction mixture is ejected at atmospheric pressure causing a turbulent vaporisation of the blowing agent. This allows the manufacture of a foam with reduced density, but the cell structure is of very inconsistent quality due to irregular shaped and oversized cells or bubbles being present.

2.2 Current Liquid Carbon Dioxide Technologies for Flexible Slabstock Polyether Foam Production 2.2.1 Machinery The situation changed when Cannon introduced liquid carbon dioxide foaming technology for the industrial production of polyurethane flexible foams with its CarDio process. This process was introduced to the polyurethane foam production industry in 1994. Today foam manufacturers worldwide consider liquid carbon dioxide as a blowing agent and make this process available for the industry. The major point of this technology is the control of the frothing that occurs as soon as the pressurised liquid carbon dioxide as part of the foam formulation leaves the pressurised equipment and the liquid foam mixture is released into atmospheric pressure. The controlled release of the mixture into the comparatively low atmospheric pressure conditions is essential to all the liquid carbon dioxide foaming techniques. In the CarDio process this is accomplished with the specifically designed lay down device [20, 21] which is called a ‘Gate Bar’ (see Figure 2.1). The gate bar essentially consists of a metal bar with a pressure drop slot connecting the outside with a feeding tube inside the bar. Through a liquid mix injection point formulation components with the pressurised liquid carbon dioxide enter the gate bar.

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Demands on Surfactants in Polyurethane Foam Production…

Figure 2.1 Cannon Gate Bar Principle

The frothing mixture flows along the frothing cavity and through an outlet aperture acting as a pressure drop zone onto a substrate, e.g., the moving belt of the foam machine. A variation of the original gate bar design was introduced with the CarDio 2000 featuring a pressure adjustable lay down device [22]. It allows adjustment of the slot width in response to the output and the liquid carbon dioxide percentage (see Figure 2.2).

Figure 2.2 CarDio Froth laydown

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Advances in Urethane Science and Technology Another patented technology for liquid carbon dioxide foaming is the NovaFlex process developed by Hennecke and Bayer [23]. The NovaFlex process uses a creamer dispensing the froth on to the machine. It focuses on the same task of pressure reduction as the CarDio gate bar, except dispensing the froth occurs at one single spot on the belt and is not distributed over nearly the whole belt width. The same is true with the third version of liquid carbon dioxide machinery, the Beamech CO-2 equipment. Obviously, both the NovaFlex as well as the Beamech CO-2 process use pressure reduction devices different from the gate bar of the CarDio process as they are said to show a different response to the use of fillers or generally solid particles within the foam formulations. One modification of the original NovaFlex process is the MultiStream configuration. It allows the use of liquid carbon dioxide with a range of polyol types in all different ratios by utilising one liquid carbon dioxide addition point in one high pressure polyol stream which is then combined with additional polyols by pass streams [24, 25].

2.2.2 The Foaming Process 2.2.2.1 A Comparison of Rise Profiles A comparison of the foaming profile found in a standard flexible slabstock foam production with the one seen during liquid carbon dioxide foaming clearly shows a large difference in the rise profiles, especially at the lay down (Figure 2.3).

Figure 2.3 Foaming profiles of standard flexible foam production and liquid carbon dioxide foaming.

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Demands on Surfactants in Polyurethane Foam Production… In standard foam production, a mixture of liquid materials is dispensed and the gas bubbles within the foam structure are only generated by a rather slow chemical reaction. In the case of liquid carbon dioxide foaming the mixture is already dispensed in a two-phase process. No matter which particular pressure drop system is utilised, the pressure drop is very fast due to the large flow speeds in the system. The fast pressure drop between the mixing head and the dispensing compresses this time span for bubble formation to fractions of a second. As all the gas bubbles resulting in foam cells of the final foam have to be generated in this short instant, clearly the nucleating powers of the formulation ingredients are challenged severely. This is especially true when looking at the foam stabilisers used as they are the main formulation ingredients determining the nucleation.

2.2.2.2 The Nucleation of Gas Bubbles As early as 1969, Kanner and Decker [26] showed by photomicrography that self-nucleation is essentially absent in a polyurethane foam system. Their results indicate that bubbles are introduced by the process of mixing and that the presence of a silicone surfactant increases the volume of air bubbles introduced during the mixing. Nucleation in this context means the formation of gas bubble cores on which a bubble might grow above its critical bubble radius. The critical bubble radius is the smallest bubble radius for which the addition of more gas will result in a net decrease in free energy. A bubble which is smaller than the critical radius will not spontaneously grow in size because the addition of more gas results in a net increase in free energy. Obviously a decrease in the surface tension of a foaming mixture by the silicone surfactant would decrease the energy needed to generate bubbles of the necessary critical radius. Even under conditions of a supersaturation pressure of 2.03 MPa carbon dioxide in a liquid with a surface tension of 0.025 N/m and a temperature of 25 °C, the critical bubble radius is 2.4 x 10-5 cm. It has been proven that silicone surfactant can aid nucleation by lowering surface tension [27]. However the surface tension lowering of a silicone surfactant might just be one effect helping to induce new bubble formation by liquid carbon dioxide. The surfactant is entirely soluble in the liquid and therefore cannot act as a heterogeneous nucleating agent. Besides that other surfactants like fluorocarbons can also lower the surface tension but still don’t act like a silicone surfactant does in polyurethane foam production. A possible explanation of this difference might be related to the exceptional solubility of oxygen and nitrogen in silicones as demonstrated by Arkles [28, 29]. It is well known in the polyurethane industry that insufficient air loading of raw materials like the polyols results in coarse cell structures even in the conventional foaming process. Obviously a sufficient amount of nitrogen and/or oxygen is needed for good nucleation. As silicones show a high permeability and solubility for these gases, it is reasonable to assume that

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Advances in Urethane Science and Technology the ease of formation of nuclei for gas bubbles is higher in silicone-like surroundings provided along the siloxane backbone of a silicone surfactant. An effect not accessible to Kanner and Decker, and therefore not a subject of their experimental work, was the high nucleation demand found in the new liquid carbon dioxide foaming technologies. Their finding that bubbles are introduced during the process of mixing by the shear energy of the mixer might reflect the observation that the amount of nuclei generated during mixing has always been sufficient for a standard polyurethane foaming process. The rather large timescale between mixing and appearance of gas bubbles large enough to be visually detectable obviously gave carbon dioxide molecules sufficient time to diffuse to a nucleation site as they were only generated slowly within the foam formulation. With the new liquid carbon dioxide foaming technologies, a significantly higher number of nucleation sites is needed for the carbon dioxide to find a gas bubble or a gas bubble core during the short time frame of the pressure decrease in the pressure reduction area within the equipment used. Whereas the former trend in some polyurethane (PU) markets has been to sacrifice more and more of a surfactant’s nucleating power to gain higher and higher activity, liquid carbon dioxide foaming has led to a new description of the needed performance profile. This is recognising the fact that quite a number of the very high active surfactants used at the time of the introduction of the liquid carbon dioxide processes resulted in unacceptable foam structures. Irrespective of the liquid carbon dioxide system, the high activity surfactants generated coarse foams. That is in contrast to the findings of Kanner and Decker in standard foam systems. Obviously under these conditions the mixing energy of the machine system has not been high enough to generate sufficient nuclei or gas bubbles to guarantee a regular fine celled foam structure. Obviously under these conditions the main quality issue of a surfactant is its nucleation power, and not the stabilisation activity. Not surprisingly a well established medium active surfactant was used during the development of the first commercial liquid carbon dioxide process, the CarDio process. It provided a good balance between nucleation and processing as well as activity. Therefore this type of surfactant was still the number one choice for processes even four years after the introduction of commercial liquid carbon dioxide foaming. Still, due to the high demands in nucleation, the broad variety of foam grades and the flame retardant (FR) demands of some markets a desire for optimising the different surfactant performance aspects could clearly be seen. It also provided a focus of interest for the flexible slab polyurethane community for quite a few years.

2.2.2.3 Emulsification of Raw Materials In addition to nucleation, there is one further objective to be fulfilled or at least supported by the silicones used. That is the emulsification of the raw materials used in the foam 92

Demands on Surfactants in Polyurethane Foam Production… formulation. An obvious example for this performance aspect is the chemical reaction of water and isocyanate, i.e., the formation of carbon dioxide and urea. The different polarities of isocyanate and water should normally prevent their reaction with each other in the short time frame of a PU foam reaction and even the use of polyols as the major formulation component will not always help to overcome the tendency for phase separation. Even the polyols used in some special foam types can give rise to a number of problems. One of the challenges for the emulsification power of silicone surfactants in flexible slabstock foam production for example is the combination of a standard polyol with a hypersoft polyol. The consistency of the foaming results and the cell structure obtained can be severely influenced by the choice of silicone surfactant used. It can be easily rationalised that the rise profile and physical foam properties like tensile strength and elongation will be influenced by tendencies for phase separation after the mixing and dispensing of the formulation ingredients.

2.2.2.4 Stabilisation of the Foam During Foam Rise Probably the most obvious task of any silicone surfactant in flexible polyurethane foam production is to stabilise the foam during the foam rise. If the surfactant used is contaminated or is in too low a concentration, settling of the foam or even foam collapse will occur. Two possible reasons for the foam instability during the rise time are discussed in the literature, and lead to different theories on the roles of silicone surfactants used. The first explanation of the role of silicone surfactants in flexible polyurethane foam is based on the fact that as soon as the volume fraction of the gas bubbles exceeds 74%, the spherical bubbles will distort into multi-sided polyhedrals. This means that cell windows with Gibb’s plateau borders are formed (a polyhedral foam consists of cell windows and struts. Another term for struts is Gibb’s plateau borders). A pressure difference due to capillary pressure will cause liquid in the cell window to drain into the struts since the pressure inside the plateau borders is lower than that in the cell windows. Without adding silicone surfactants this drainage rate will be very fast, so that film rupture and bubble coalescence occur rapidly. It has been shown that due to the surface tension gradient generated by a silicone surfactant the cell window drainage rate is lower [30]. Therefore different surface structures do have different effects on cell window drainage. That has been said to result in a distribution of different cell window thicknesses at the time of cell opening [31]. The critical factor in this picture is the surface elasticity. It is known to be caused by a surface tension gradient along the cell window upon expansion and it will retard the cell window drainage rate. This effect is called Gibbs-Marangoni effect and is reviewed by Scriven and Sternling [32]. Owen and co-workers also studied the dynamic surface tension of a series of surfactant solutions and stressed the importance of surface elasticity [33]. Similar work has been performed by Zhang and co-workers [34].

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Advances in Urethane Science and Technology A different view point of the mechanism of flexible polyether polyurethane foam stabilisation by silicone-based surfactants has been published by Rossmy and co-workers [8, 9, 10]. The authors investigated the urea precipitation phenomenon in polyurethane foams in greater detail. By adding a defoaming agent to a foam formulation they showed that urea precipitation turned a clear foam mix opaque. They also noted that the cell rupture always occurred just after urea precipitation. In light of this observation the hypothesis is that precipitation of polyurea destabilises the foam mix and leads to cell opening. The surfactant then aids in stabilising the foam by incorporating the urea precipitate in the foam matrix, adding integrity to the foam. Today it is not doubted any more that there really are urea domain structures in polyurethane foams and they have been subject to intensive research [35, 36].

2.2.2.5 Control of Blow Off Another major point of consideration is obviously the processing latitude of silicone surfactants. This phrase describes the variability of catalyst concentration without running into either closed, dead foam or observing first signs of foam instability indicated by splits in the foam. Not only will a different silicone surfactant result in a different processing window, e.g., amount of stannous octoate variation between the two extremes, but also the position of the processing window might be different. Figure 2.4 shows the remaining fragments of a cell window after cell opening as found in the fully cured foam. It is very obvious that the opening of a flexible PU foam is not a

Figure 2.4 Opening of a cell window

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Demands on Surfactants in Polyurethane Foam Production… pressure induced rupture of a foam bubble. Otherwise there would be no explanation why the cell opening process clearly starts at a number of locations around the periphery of the cell window. Obviously weak points in the cell window offer starting points for the opening of the liquid film. Then drainage occurs, as can be seen by the thin strings remaining between the opened areas in the cell window. The actual timing of this opening process is critical as it determines the physical properties of the foam and the foam economics. As the viscosity of the liquid material is rising very sharply due to crosslinking at the end of the foaming profile, there is only a very short time frame in which the material is crosslinked enough to support the foam without a high settling once the gas bubbles have opened and no internal gas pressure is supporting the foam anymore. On the other hand a further delay in gas bubble opening would lead to an incomplete recession of the material into the foam struts so that the foam will be showing a rather low air flow or even shrinkage.

2.2.3 Additional Tasks of Silicone Surfactants in Flexible Slabstock Foam Production 2.2.3.1 Influence on burn performance of the foam As soon as the foam is produced and cured the silicone surfactants used are not needed or desired any more. One exception might be the production of hydrophilic or hydrophobic foams where a surface active material like a silicone will in some cases have a recognisable influence. But with standard foams as well as with liquid carbon dioxide blown foams, additional performance aspects besides the stabilisation during the foam rise might be important for surfactant selection. These criteria therefore have to be addressed as well and deserve some consideration as they might also affect the number of key additives a foam operation has to handle. An effect clearly seen in the final foam is the undesirable effect the silicone surfactant has on the burn performance of the resulting foam. Especially in liquid carbon dioxide foaming, suitable conventional (or non-FR) surfactants have been available right from the beginning. But the development of optimised universal or non-FR-surfactants for liquid carbon dioxide foaming took considerable effort and time. To understand the effect of the silicone surfactant on burning it is interesting to have a look at a theory for flame spread development. The simplified illustration in Figure 2.5 shows that the heat decomposes the organic material at the surface during an endothermic procedure. Pyrolysis products are created and because of an exothermic reaction with oxygen at the boundary layer of the flame, the formation of even higher reactive decomposition products takes place. These aggressive radicals are responsible for an accelerated degradation of the polymeric surface. As long as the result of the energy is positive, there is thermal feedback to the endothermic process

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Figure 2.5 General assumption about flame spread

at the surface and the combustion keeps going. It is believed that the formation of hydrogen radicals and hydroxyl radicals from the organic material are the key factors initiating and supporting the combustion phenomena. The working mechanisms of FR address these phenomena. The effect of a silicone surfactant on the burning behaviour of the polyurethane foam can be quite significant even though they normally represent less than 1% of the plastic material. It is due to the fact that in any case the decomposition of a polyurethane foam starts at the surface. Because of the surface activity of the foam stabilisers it is easy to rationalise their enrichment on the surface and it is the surface that is the most influential part of the polymer regarding flame spread development. This point can be further highlighted if we look at the influence of surfactants on the FR-performance of a polyurethane foam with 1 weight percent of surfactant externally applied (Figure 2.6). To characterise the burn performance in this case, a horizontal burn test was used and the burn length was measured. Surfactants A, B, C and D are rigid foam surfactants with different structural parameters in the silicone as well as in the polyether chains. It can be seen in Figure 2.6 that all the different types of silicone surfactants used do have a negative influence on the burn performance of the foam. The reason for this behaviour can be mainly attributed to the structure of the siloxane backbone within the siliconepolyethercopolymers [37]. To characterise the effect of the polydimethylsiloxane (PDMS) chain used as the backbone of such a siliconepolyethercopolymer, interesting tests have been made by external application of pure PDMS in different weights as well as the use of molecules of different average chain length (Figure 2.7).

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Figure 2.6 Influence of surfactants on FR-performance - external application of 1% surfactant

Figure 2.7 Effect of 1% PDMS - applied externally

Not only do increasing amounts of externally applied PDMS lead to increasing burn length in, but more astonishingly, this effect is obviously correlated to the average chain length of the applied siloxane. It can be seen that with longer chain length of the PDMS, the detrimental effect on the FR-performance of the foam increases. This effect far out balances the pure weight effect that would explain why a higher amount of PDMS leads to higher burn length value due to the larger amount of materials supporting the decomposition. To address the question of whether the different volatility of the different PDMS chains is the main reason for this effect, an additional test with silica was carried

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Advances in Urethane Science and Technology out. It is easy to expect that solid materials, especially if inorganic, will have a positive effect on the FR-performance of the foam and that they will lead to a decreasing burn length in a burn test. Depending on the chemical nature they could be either expected to be a heat sink, a material to catch free radicals, to char the burn front of the foam or to liberate non-gaseous products to dilute the oxygen at the burn front. Astonishingly, silica, a chemically inert material, under these conditions increased the burn length of the foam (Figure 2.8). The assumption here is that on the surface of the silica-particles either catalytic effects on the heat initiated oxidation processes take place or the silica particles themselves act like a wick in a candle, providing a means to transport melted organic material from the surface and increase the diffusion flame front. If the formation of silica-particles on the burn front is indeed the major detrimental effect of the silicone containing polymer, it should be helpful to decrease the average length of any unmodified PDMS chain part between any two modifying groups. Although in practice this is also an efficient tool to improve the burn performance of the resulting foam, it negatively affects the nucleation power of the silicone surfactant. Therefore this way of addressing FR-performance behaviour is detrimental to the nucleation efficiency of any surfactant for liquid carbon dioxide foaming [38]. That is the main reason why conventional (non-FR) silicones for the liquid carbon dioxide processes were available before their optimised universal or even pure FR-counterparts.

Figure 2.8 Addition of solid material

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2.2.4 Chemistry of a Silicone Surfactant in Flexible Slabstock Foam Production As outlined previously the main reason for the interest in optimised surfactants for the liquid carbon dioxide processes lies in their role as nucleation promoters. The silicone surfactants can be viewed as PDMS-polyether-copolymers which are mainly based on a combination of just three structural units: the methyl substituted siloxane backbone as well as a sophisticated ratio and arrangement of ethylene oxide and/or propylene oxide forming the attached polyethers and, in some cases, additional modifications. A typical structure of a silicone surfactant is shown in Figure 2.9.

Figure 2.9 Building blocks of a surfactant

The molecules generally have a siloxane backbone formed by dimethylsiloxane units, substituted methylsiloxane units and some endgroups. Polyether groups and/or additional modifications can be attached to the siloxane backbone. Not only are a wide variety of those additional modifications possible but also the polyether groups can vary in quite a number of ways. Although they mostly consist of ethylene oxide and propylene oxide units they obviously can be different in chain length, i.e., their molecular weight. They can also have the different monomers distributed along the polyether chain in an either random or a blockwise fashion as well as with alternating blocks. The polarity can be either evenly or unevenly distributed. A more polar side could be either close to the attachment point to the siloxane or far away from it. As this is true for each of the polyethers used and flexible

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Advances in Urethane Science and Technology slabstock surfactants generally have more than one type of polyether attached, this gives rise to virtually millions of different combinations for surfactant molecules. Also, in most foaming applications the pendant polyethers have to be unreactive to isocyanates so that they do not act like crosslinkers which would lead to tight or shrinking foam. In addition there are two general ways to link the polyethers to the siloxane chains (see Figure 2.9). This opens the basis for two separate product lines, each having its special advantages over the other.

2.2.4.1 Si-C-Surfactants Versus Si-O-C-Surfactants The Si-O-C products resulting from the reaction of chlorosiloxanes and hydroxyl groups of polyethers provide an extremely good processing and superior consistency combined with being hydrolytically stable under the water-amine conditions found in polyurethane foaming. The Si-C products offer a more beneficial access to high activity and lead to an easier production of flame retardant foams. These products are derived from the addition of an Si-H functionality at the siloxanes to a double bond, a process called hydrosilylation. Examples of how this variety of structural parameters affects the development of silicone surfactants especially for liquid carbon dioxide blown foams, are many [39, 40, 41, 42] and this latest drive in surfactant development provides good examples of the important performance issues and how they can be addressed.

Figure 2.10 Synthesis of Si-C and Si-O-C linked siloxane-polyether-copolymers

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2.2.5 A Surfactant Development Example In surfactant developments for liquid carbon dioxide PU foaming, the described complexity of structural parameters in a surfactant led to the desire to characterise nucleation performance of a surfactant without the need to judge the final, cured foam. One important reason is the fact that a coarse or even irregular cell structure in the final foam does not necessarily indicate the reason for such a ‘bad’ cell structure. It could arise from a deficiency in nucleation performance so that the number of gas bubbles formed at the beginning of the foaming reaction was not sufficient (lack of nucleation). Another possible scenario could be that the initial number of bubbles formed was rather high, but many of the gas bubbles were lost during the foam rise due to gas bubble coalescence. That would mean that the stabilising power of the surfactant would be insufficient and had to be improved. One of the first published attempts to get an indication of the nucleation performance of a silicone surfactant was the froth test [39, 40, 41]. In this test polyol and surfactant are stirred in a reproducible fashion and the resulting froth density as well as the time needed for the foam to collapse is measured. After some experience with industrial surfactants used in the market these products with known performance in industrial liquid carbon dioxide foaming were subjected to the froth test. No direct correlation could be found between the obtained data and the performance of the products on industrial machines (Figure 2.11).

Figure 2.11 Performance of surfactants (Tegostab B 8228 and B 8220) in the froth test Comp.: competitive surfactant; EP-H-18: experimental product

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Advances in Urethane Science and Technology As can be seen from Figure 2.11, both products Tegostab B 8228 and Tegostab B 8220 show nearly identical performance in this mixing test although industrial experience has shown that Tegostab B 8220 results in significantly finer cell structure than Tegostab B 8228, even if both of them yield a very regular cell structure. Even more confusing is the picture resulting from the comparison of a widely used surfactant and an experimental product called EP-H-18 by Burkhart and co-authors [42]. Both of the products again resulted in identical values in the froth stability test, but differed significantly on an industrial production machine. Whereas the competitive material resulted in a coarse cell structure, the experimental product yielded a fine and regular foam.

A (30 s)

B (30 s)

C (30 s)

A (150 s)

B (150 s)

C (150 s)

bad

good

better

Figure 2.12 Video images of foam formulations with commercial surfactants of different performance in liquid carbon dioxide processes

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Demands on Surfactants in Polyurethane Foam Production… Because of these results a video microscopy check on the homogeneity of the gas bubbles in the liquid mixture was performed. For this test the ingredients of a standard foam formulation were mixed and immediately poured onto a polished metal plate. The creaming of the foam was then filmed by a video camera with a 10x magnification. Thirty and 150 seconds after the stirring commenced, a video printout was made and the number of formed gas bubbles as well as their size homogeneity was characterised (see Figure 2.12). This procedure is referred to as the video test.

2.2.5.1 Possible Silicone Structures Not taking cyclic molecules into account, the general structures of industrial silicone surfactants for flexible slabstock foam production can be seen in Figure 2.13. The main building blocks of these materials are a PDMS backbone and attached polyethers based on ethylene oxide and propylene oxide addition products. The siloxane backbones can either be linear or branched and can have their polyether substituents attached in an either pendant or terminal location. These four general structures are outlined in Figure 2.13).

Figure 2.13 General structures of silicone surfactants (schematic depiction)

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Advances in Urethane Science and Technology The simplest type of the possible structures is a linear A-B-A-copolymer with a straight siloxane backbone and polyether chains at the termini. This type of structure is described as ST (for straight, terminal). With a straight siloxane backbone there is also the possibility of attaching polyether groups as pendant chains. This combination is denoted SP (straight, pendant). If the siloxane is branched as well it could have polyether groups attached as terminal or as pendant groups (BT and BP). These four types of structures gave different results in the video imaging test (see Figure 2.14). The ST type of structure resulted in only a small number of gas bubbles being formed in the early stages of the reaction. Somewhat better was the combination of branched silicone with pendant polyether groups. The best video imaging results were reported with the branched siloxane, combined with terminal polyether groups followed closely by the SP type of molecule.

ST

SP

–

+

BT

BP

+(+)

o

Figure 2.14 Video imaging test, comparison of different silicone structures -, o, +, +(+) are a non numerical quality rating of degree of nucleation. - is poor and +(+) is very good

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Demands on Surfactants in Polyurethane Foam Production… These results matched, more or less, with industrial experience. Whereas the ABA type of copolymer (ST-structure) is known to be a rather poor performing surfactant for polyurethane production, the SP and the BT type of structures are commonly used. The structure that performed best in the video imaging test (the BT type structure) is the standard structural type that can be found in commercial SiOC-PDMS copolymers obtained by the reaction of a chlorosubstituted PDMS with the hydroxyl group of polyether (see previously). Although these SiOC products (named after the silicone-oxygencarbon bond linking the PDMS and the polyether side chain) provide a very good combination of broad processing, good nucleation and consistent quality, their main disadvantage is their very recognisable negative effect on the burning behaviour of the produced polyurethane foam. Therefore these types of products are commonly denoted as conventional or non-FR-silicones. In contrast, the second best type of structure seemed to be the SP type. These structures are the fundamental means to build SiC-products which are used as universal surfactants, for example in North America. Although SP structures seemed to be the obvious choice to build FR or universal type of surfactants (having a less negative effect on FR-performance) they did not seem to show as good a nucleation as the BT type of molecules. The fourth structural type, BP products, no known experience or industrial application was recognised, and due to the combination of not extremely good nucleation (according to the video test) and the complicated synthesis necessary to make them, no reported work was undertaken with these types of molecules. Besides the type of branching in the siloxane backbone and the attachment points of the polyether side groups, another factor characterising the siloxanes is the number of unmodified PDMS groups for each of the modified methylsiloxane groups in the molecule. This ratio is often denoted as the P-value and is especially important in regards to the burn performance of the resulting foams (see Figure 2.10 and discussion above). As that is another variable that has to be taken into account when characterising a silicone surfactant for polyurethane foam production, this factor was screened for as well. As can be seen from the video test results, there seems to be a slight improvement in the nucleation efficiency with increasing P-value. However the drawback is that the higher P-values result in surfactants which will lead to a considerably negative effect on the burn performance of the resulting foam [43]. The summary of the data at this point seemed to indicate that the combination of good nucleation efficiency with the BT-structures and the advantage of high P-values seem to

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P-value Figure 2.15 Video imaging test; nucleation efficiency with increasing P-value

correlate with the fact that, especially based on SiOC type of chemistry, it is easier to build a non FR-surfactant structure than to build a universal or true FR-silicone surfactant for liquid carbon dioxide foaming. In the case of good FR-surfactants therefore, a compensation for the unavailability of high P-values had to be found in regard to nucleation efficiency of the molecule. Because of a suggestion in the literature [44] that the high solubility of carbon dioxide in certain aromatic solvents might be due to interaction between the molecules, the influence of aromatic structures in surfactant molecules was screened. Whereas the standard siloxane backbone in PU surfactants is normally consisting of a PDMS chain between the substituted silicon atoms, high temperature silicone resins are used with phenylmethylsiloxane groups within the chain. Although these types of molecules seemed to be viable candidates for testing, they did not seem to be attractive options for industrial use due to the complicated synthesis and the high costs associated with them. Another option to incorporate aromatic structures in organomodified silicones has been the use of styrene oxide as a monomer for the synthesis of the polyether side chain within the copolymer structure [45] (see Figure 2.16). Comparison of structural siliconepolyether analogues with and without styrene oxide incorporation in surfactant test molecules showed an indication of a positive trend in the video test regarding the nucleation efficiency of those aromatic group substituted silicone polyether copolymers. Surprisingly enough in a real foam test these molecules performed rather badly, resulting in a considerably coarser foam than the reference (see Figure 2.17).

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Figure 2.16 Aromatically substituted polyether

Surfactant without aromatic groups

with aromatic groups

Cell structure in foam test: 10 cells/cm

8 cells/cm

Figure 2.17 Comparison of standard and aromatically substituted siliconepolyether

That observation was rationalised with the following assumption. If indeed aromatic rings would increase the solubility of carbon dioxide then these types of structures would be more soluble in a liquid carbon dioxide containing environment. This would result in a decreased tendency to move to the surface of a such a liquid phase. The obvious result would be a decrease of surface activity, so the net result could very well be a higher nucleation efficiency combined with a lower activity during the rise time of the foam, leading to gas bubble coalescence.

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Advances in Urethane Science and Technology If that is true, the polarity and therefore solubility of the molecule would have to be adjusted to lower values. As explained previously the increase of the P-value was not an option for the generation of a surfactant with good FR-performance. Therefore to further decrease the solubility of a surfactant in a formulation for flexible slabstock PU-foam, the polarity of the surfactant molecule was decreased by lowering the polarity of the polyether side chain with the use of higher alkylene oxides to have more non-polar groups along the polyether chain (Figure 2.18). This would lead to a higher carbon:oxygen ratio in the polyether side chain. Since it has been known for more than 30 years [46] that the polyether polarity affects the cell size distribution of flexible slabstock PU foams that is a very good method for fine-tuning surfactants. Foaming tests showed that the carbon:oxygen content had a significant impact on the overall activity of the surfactant, resulting in decreasing foam height with increasing carbon:oxygen ratio, especially when a carbon:oxygen ratio of about 2.6 was exceeded.

Figure 2.18 Influence of carbon to oxygen ratio in polyether side chains

The utilisation of the outlined principles and the continuous work with the many possible variables by all the additive suppliers again and again has brought new high performance surfactants, supporting the continuously changing demands in the innovative polyurethane industry and most probably will continue to do so. Therefore this industry has all the fascination that comes from the interaction of chemistry and physics, industrial application and theoretical interest, commercial importance and environmental awareness. So even more than 50 years after its beginning it will remain an area of new challenges and performance oriented efforts. 108

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References 1.

No inventor; I.G. Farben AG, assignee; German Patent 728,981, 1937.

2.

A. Höchtlen and W. Droste, inventors; Farbenfabriken Bayer AG, assignee; German Patent 913,474, 1941.

3.

P. Hoppe, inventor; Farbenfabriken Bayer, assignee; German Patent 851,851, 1948.

4.

W. Droste and A. Höchtlen, inventors; Farbenfabriken Bayer, assignee; German Patent 860,109, 1952.

5.

Chemical and Engineering News, 1961, 39, 11, 62.

6.

K. Moser, Kunststoffe, 1983, 73, 12, 764.

7.

F. K. Brochhagen in Handbook of Environmental Chemistry, Vol. 3, Part G, Ed., O. Hutzinger, 1991, Springer Verlag, Berlin, p.73.

8.

G. Rossmy, H. J. Kollmeier, W. Lidy, H. Schator and M. Wiemann, Journal of Cellular Plastics, 1981, 17, 6, 319.

9.

G. Rossmy, H. J. Kollmeier, W. Lidy, H. Schator and M. Wiemann, Journal of Cellular Plastics, 1977, 13, 1, 26.

10. G. Rossmy, W. Lidy, H. Schator, M. Wiemann and H J. Kollmeier, Journal of Cellular Plastics, 1977, 15, 5, 276. 11. G. Burkhart, H-H. Schlöns and V. Zellmer, inventors; Th. Goldschmidt AG, assignee; US Patent 5 132 333, 1992. 12. R. Ricciardi, Presented at the Polyurethane Foam Association conference in Scotsdale, AZ, 1996. 13. M. A. Ricciardi, D. J. Smudin, R. D. Wagner, M. Pcolinsky and J. E. Chaya, inventor; no assignee, US Patent 3,890,414, 1975. 14. M. A. Ricciardi and D. G. Dai, inventors; Crain Industries, Inc., assignee, US Patent 5,171,756, 1992. 15. H. Stone, inventor; PMC, Inc., assignee, US Patent 5,128,379, 1992. 16. A. A. Grizwold, inventor; Reeves Brothers, Inc., assignee; US Patent 4,537,912, 1985.

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Advances in Urethane Science and Technology 17. J. L. Drye and G. C. Cavenaugh, inventors; Trinity American Corporation, assignee, US Patent 5,188,792, 1993. 18. P. Merriman, inventor; Dunlop Rubber Co., Ltd., assignee; US Patent 3,184,419, 1965. 19. E. N. Doyle and S. Carson, inventors; no assignee, US Patent 5,120,770,1992. 20. C. Fiorentini, M. Griffith and A. Charles, inventors; Krypton International SA, assignee; European Patent 0645226A2, 1995. 21. C. Fiorentini, M. Taverna and T. Griffiths, Journal of Cellular Polymers, 1994, 13, 5, 361. 22. CarDio Newsletter No.4, September 1997, Article no. 97037. 23. R. G. Eiben, Presented at Utech ’96, The Hague, The Netherlands, 1996, Paper No.31. 24. Innovations, 1998, October, 801, 15. 25. Novaflex Multistream, Hennecke leaflet PI125, October 1998. 26. B. Kanner, T. G. Decker and G. Thomas, Journal of Cellular Plastics, 1969, 5, 1, 32. 27. B. Kanner, W. G. Reid and I. H. Petersen, Industrial Engineering Chemistry, Product Research & Design, 1967, 6, 2, 88. 28. B. Arkles, Silanes & Silicones catalogue, Petrach Systems, Bartram Road, Bristol, PA, 19007, USA, 1987, p 87. 29. B. Arkles, Chemtech, 1983, 13, 542. 30. X. D. Zhang, C. W. Macosko, H. T. Davis, A. D. Nikolov and D. T. Wasan, Journal of Colloid and Interface Science, 1999, 215, 2, 270. 31. K. Yasunaga, R. A. Neff, X. D. Zhang and C. W. Macosko, Journal of Cellular Plastics, 1996, 32, 5, 427. 32. L. E. Scriven and C. V. Sternling, Nature, 1960, 187, 186. 33. M. J. Owen, T. C. Kendrick, B. M. Kingston and N. C. Lloyd, Journal of Colloid Interface Science, 1967, 24, 2, 141.

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Demands on Surfactants in Polyurethane Foam Production… 34. X. D. Zhang, C. W. Macosko and H. T. Davis in Polymeric Foams, Ed., K. C. Khemani, Chapter 9, ACS Symposium Series 669, American Chemical Society, Washington DC, 1997. 35. J. P. Armistead, G. L. Wilkes and R. B. Turner, Journal of Applied Polymer Science, 1988, 35, 3, 601. 36. M. W. Creswick, K. D. Lee, R. B. Turner and L. M. Huber, Presented at the SPI 31st Annual Technical/Marketing Conference, Philadelphia, PA, 1988, p.11. 37. A. Weier, G. Burkhart, M. Klincke, Presented at Industrial Applications of Surfactants IV, Suffolk, UK, 1998, p.260. 38. G. Burkhart, R. Langenhagen and A. Weier, Presented at the Polyurethanes Expo ’98, Dallas, TX, 1998, p.129. 39. S. B. McVey, B. L. Hilker and L. F. Lawler, Presented at the Polyurethanes Foam Association conference, San Antonio, CA, 1995. 40. S. B. McVey, B. L. Hilker and L. F. Lawler, Presented at Utech 96, The Hague, Paper No.38. 41. G. Burkhart, V. Zellmer and R. Borgogelli, Presented at the SPI Polyurethanes Expo ’96, Las Vegas, NV, 1996, p.144. 42. G. Burkhart, R. Langenhagen and A. Weier, Presented at the Polyurethanes Expo ’98, Dallas, TX, 1998, p.129. 43. A. Weier, G. Burkhart and V. Zellmer, Presented at the SPI, Polyurethanes ’94 Conference, Boston, MA, 1994, p.202. 44. J. H. Hildebrand, J. M. Prausnitz and R. L. Scott, Regular and Related Solutions: the Solubility of Gases, Liquids and Solids, Van Nostrand Reinhold Co., New York, 1970. 45. No inventors; Th. Goldschmidt AG, assignee; German Patent 19, 726, 653, 1999. 46. R. J. Boudreau, Modern Plastics, 1967, 44, 5, 133.

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3

Polyurethane Processing: Recent Developments Max Taverna

This chapter highlights three aspects of manufacturing technology, which have recently brought significant benefits to the producers of automotive seats, reinforced parts and generic moulded items. These different manufacturing areas have been grouped here under a single title in order to cover a wider spectrum of reader’s interests and to illustrate the broad area of improvement and innovation that can still be found in this steady-growing segment of industry.

3.1 Industrial Solutions for the Production of Automotive Seats Using Polyurethane Multi-Component Formulations Cannon has developed complete systems for the manufacture of moulded polyurethane automotive seating elements made with varying combinations of several raw materials. The resultant cushions – although produced in a random sequence on the same moulding line - are characterised by mechanical properties tailored to the specific application of each part [1]. This section describes the components of this technology which are: •

dedicated multi-component, high-pressure metering machines with closed-loop control of both the output and pour pressures

•

dedicated mixing heads capable of processing six components (all with high-pressure recirculation), including low-viscosity toluene diisocyanate (TDI) and water-based additives

•

dedicated mould-handling systems for a flexible manufacturing concept based on just-in-time methods

3.1.1 Market Requirements A recent analysis (internal Cannon Report) of the automotive seat-manufacturing market segment, to identify the future needs and trends of this group highlighted a number of interesting considerations for a producer of polyurethane processing equipment.

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Advances in Urethane Science and Technology First, the number of suppliers is getting smaller and smaller, due to the extensive merger and acquisition campaign run in recent years by the two largest players (Lear Co., and Johnson Controls). This concentration creates an opportunity for making strategic choices regarding the ‘make or buy’ decision with respect to manufacturing solutions. These large manufacturers have grown significantly thanks to their own successful development of both chemical formulations and moulding plants. Providing them with new machinery sometimes means accepting their request to manufacture a concept that was developed by their own engineering department. The second aspect concerns the growing demand for simpler, high-efficiency plants, with a reduced number of operators and a high degree of operating flexibility. It must be possible to produce several different parts on the same moulding line so that, in the case of a sudden change in production plans, complete projects can be switched from one production line to another within a very short time frame. This increased flexibility requires careful design of the metering equipment, mix heads and mould carriers because they must be able to perform very different tasks in sequence or by project. A specific request involves the potential to process multi-component formulations, where a wide range of foams can be produced on one machine. The third aspect, potentially in conflict with the previous concept, involves the development of families of formulations based on specific chemicals: all diphenylmethane diisocyanate (MDI), MDI/TDI in various percentages, all TDI and special polyols. This means dealing with very different demould times and moulding conditions, that render a generic ‘seat plant’, that was still so useable just a few years ago, obsolete. In this case, specific packages must be available, which conflicts with the concept of high flexibility expressed above.

3.1.2 Dedicated Solutions: Metering Equipment A high-pressure multi-component metering unit was designed for automotive seating producers, with a pump-dosing system capable of precisely dosing TDI and other lowviscosity components. Output adjustment on-the-fly and closed-loop control, easy to obtain with piston-driven metering units, were engineered for pump-driven machines as well. The number of main components (polyols and isocyanates) can be set, as necessary, since the design is modular: each dosing line includes a dedicated tank, its temperature control system, high- and low-pressure filters, recirculation valves, a dosing pump and motor and a portion of the control panel (see Figure 3.1). The modular design of this unit allows for easy addition of further components to a formulation. Storage, metering, temperature-time control modules and feed lines are assembled in modules and can be added as needed. Computerised process control easily integrates the new chemicals in the formulation.

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Figure 3.1 A high-pressure multi-component metering unit

Two methods were used to enable fast, precise changing of the output and to ensure closed-loop control of the machine, which is essential to guarantee continuous, accurate output of low-viscosity chemicals: •

via an inverter mounted on the pump motor

•

via a programmable step-by-step motor that changes the setting of the pump.

The output control on the closed-loop machine operates continuously, comparing the set output value of the inverters with the real output as measured by the volumetric flow transducers. The system enables pouring only when the parameter is within the limits set by the operator via the keyboard and when it is possible to change the pump speed in less than half a second. The unit also includes maintenance and alarm menus. Through the maintenance menu, it is possible to set limitations on the following parameters: •

number of shots

•

material consumption

•

working hours

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Advances in Urethane Science and Technology Once these set values are reached, the corresponding warning message informs the operator to perform the required maintenance operation. The programmable stepping device is very precise - the pump can be set at 256 discrete positions - and can be easily programmed through the machine programme logic controller (PLC). Since the switching time between formulations can be as low as 0.6 of a second, multiple ratios can be set within the same pour program to produce multi-hardness and multi-density foams with repetitive results.

3.1.3 Dedicated Solutions: Mixing Heads Cannon has designed a new mixing head, capable of mixing six components, each with individual recirculation control. The new mixing head was specifically designed to meet the needs of the automotive seat producers who wished to mould TDI-based flexible foams with a maximum of flexibility in their formulation. Several pure ingredients are kept separate up to the point of injection and it is possible to operate each stream on demand, provide highpressure recirculation and avoid the contamination of components (see Figure 3.2) [1, 2].

Figure 3.2 The new Cannon Ax Head, specifically designed for multi-component polyurethane formulations. It can mix up to six chemicals all with a high-pressure recirculation feature. 116

Polyurethane Processing: Recent Developments Although this solution is currently available on the market, competitive mix heads are limited to four streams, their dimension and weight require heavy-duty pour robots while providing speed and pour pattern limitations during operation and they have complex pressure set-up and regulation procedures.

3.1.3.1 Multi-Component Operation The innovative aspect of this mixhead is that TDI - and eventually a TDI-compatible sixth component - are fed axially into the mixing chamber through the small piston that cleans the mixing chamber. The other four components, polyols, other catalysts, additives, flame retardants, are fed radially into the mixing chamber. A seal on the small piston provides a permanent separation between the polyol and TDI feeding areas so that any crossover of the low-viscosity components is avoided. The main advantage of the new mixing head is its reliability. It can operate for millions of shots because of the remote position of the TDI feeding area. TDI is fed in at a pressure of only 1 MPa, with perfect mixing efficiency (see Figures 3.3, 3.4 and 3.5).

Figure 3.3 Section of the mixing-chamber’s cleaning piston. Four chemicals are fed radially in the mixing chamber, while two are fed through a hole drilled in the cleaning piston.

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Figure 3.4 During the high-pressure recycle phase all six components flow in the grooves carved in the mixing chamber’s piston and return to their storage tanks.

Figure 3.5 When the injection signal is received, the mixing chamber’s piston retracts, the four components fed radially meet the two that are fed axially through the mixing chamber’s piston. The mixed blend reaches the discharge duct, which is positioned at an angle of 90° to the mixing chamber and leaves the head through the pouring hole.

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Polyurethane Processing: Recent Developments Defining ‘Formulation #1’ as being composed of polyol #1 and isocyanate and ‘Formulation #2’ as polyol #2 and isocyanate, an example of a working sequence is as follows: 1. Pour ‘Formulation #1’: polyol #1 and isocyanate recycle through the mixing chamber’s piston grooves and are poured in the open mould 2. ‘Formulation #2’ remains dormant since polyol #2 continues to recirculate through a high-pressure nozzle positioned close to the mixhead 3. The formulation change is implemented by: • closing the mixing chamber’s piston (the self-cleaning piston is kept open) • opening the polyol #2 high-pressure nozzle and closing the polyol #1 nozzle • re-opening the mixing chamber’s piston with the new ‘Formulation #2’ The maximum response time, which occurs between two consecutive shots with different formulations, is 0.6 seconds.

3.1.3.2 Pour Pressure Control The pressure control of the component injectors, which is very important for ensuring the proper mixing of the different liquid streams, has been achieved via three different approaches: •

closed-loop control of the injection pressure via hydraulic servo valves and feedback control from the pressure gauges

•

open-loop control via hydraulic servo valves

•

fixed-position control via various hydraulic valves, preset at different values

The package supplied with the head to achieve pressure control (see Figure 3.6) includes: •

hydraulically-operated nozzles to be installed on the mixhead for the components that are fed radially; each nozzle has a double function: - selection of the component to be used in the specified formulation - control of the re-circulation/injection pressure during the pour

•

hydraulic unit for operation of nozzles

•

valves for control of the oil flow; each valve has three positions (off, injection pressure #1 and injection pressure #2)

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Figure 3.6 Scheme of the circuit used to guarantee a closed-loop control of the pouring pressure of each chemical.

•

high-performance recycle stream distributors with relevant pressure control to maintain non-specified components in high-pressure recirculation, ready to be injected within 0.6 seconds

•

one set of high-pressure flexible hoses to connect the proportional valves to the injector nozzles

•

appropriate controls

3.1.3.3 Variable Geometry Mixhead To ensure proper mixing conditions for a wide range of formulations - which can differ in chemical composition, viscosity, ratio and output - it is important to provide appropriate backpressure in the mixing area. This can be obtained with the adjustable geometry regulation of the mixing area: the mix head’s larger piston can be mechanically set to partially block the outlet of the mixing chamber when it is fully retracted. This occlusion increases the turbulence in the mixing area, causing more efficient mixing to be obtained.

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3.1.3.4 Advantages A very compact design: its outside dimensions of 40 x 20 cm and 22 kg weight translate into a high degree of manoeuvrability. •

The pour pattern can be defined very precisely, according to the design of the mould and the position of any inserts.

•

The pouring operation can be executed in an open mould without fear of collision with the upper mould half.

•

The robot carrying the head can achieve high acceleration and speeds without mechanically stressing the moving elements.

The large number of chemical components that can be handled simultaneously provides a high degree of flexibility in formulations, allowing for optimum use of the moulding line. Several different parts can be produced on the same moulding line and different types of foam can easily be produced in random sequence without forcing the operator to use pre-defined sequences of moulds.

3.1.4 Dedicated Solutions 3.1.4.1 Foaming Robots The basic features of a typical Cannon double-arm pour robot, normally supplied to carry two heads (see Figure 3.7), are: •

two-axes Cartesian robot with two arms moving independently of each other,

•

arm movements (along the ‘X’ and ‘Y’ axes) achieved via a rack-and-pinion system driven by electronic variable-speed motors,

•

racks fitted with position encoders to detect the relevant ‘X’ and ‘Y’ coordinates.

The Technical Specifications include: -

maximum speed for X axis: maximum speed for Y axis: maximum acceleration: maximum load on the arm: maximum ‘X’-axis stroke: maximum ‘Y’-axis stroke: minimum distance between 2 arms: available working area:

2.4 m/s 2.4 m/s 3.5 m/s2 120 kg 250 cm 115 cm 55 cm 307 x 115 cm

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Figure 3.7 A double-arm Cartesian robot, designed to perform multi-hardness PU foaming in open-mould pouring processes for automotive seat moulding.

3.1.4.2 Mould Carriers The concept of the dedicated flexible foam moulding line relies on a mould carrier having a simple solid structure, with a minimum number of mechanical parts, which require maintenance. It consists of two upper and two lower sections linked by means of a hooking system. The presence of two separate lower platens allows the operator to compensate for any difference in mould thickness without incurring any problem by mounting two moulds on the same carrier (see Figure 3.8). Insertion, centring and fixation of various moulds within the mould carrier are achieved by means of tensioning screws. Each mould carrier is usually provided with two pneumatic cylinders for opening and closing. Upon demand, the closing system can be actuated hydraulically or mechanically. Mould carrier tilting, for optimum evacuation of air during the filling phase, is accomplished with a cam system. Each mould carrier can have two tilting positions: either horizontal or frontally inclined by 20 degrees (see Figure 3.9). It is possible to programme different angles of opening and tilting to correspond with the type of mould currently being used. Mould identification – necessary in order to transmit the appropriate mould/formulation information to the dosing system - is achieved with a code reader. The electrical controls

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Figure 3.8 Independently-moving lower platens allow for the use of different thickness of moulds as a double mould carrier.

Figure 3.9 A different degree of tilting can be obtained for each mould carrier, to optimise the evacuation of air from the moulds during the filling/ polymerisation phase.

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Advances in Urethane Science and Technology for the moulding line are interfaced to the metering system controls in order to coordinate all the working processes. The typical working sequence can be defined as follows: •

mould identification: the control system on the moulding line reads the code coming from the field

•

pour programme selection: inside the mixhead manipulator control unit, a schedule - assigning a specific pour programme to the different moulds - has to be loaded

•

pour programme execution: the manipulator control unit sends the input to the machine control unit; when the mould under consideration requires multi-hardness foaming, both mix heads are activated, while in the case of a single-hardness seat, only one mixhead is activated

Stroking of the Platens As stated previously, the lower half of the mould carrier is composed of two independent platens. They are fitted with a guiding system to ensure maximum parallelism between the platens. In order to ensure equal clamping pressure and complete closure of the moulds over the entire parting line, pneumatically inflated tubes stroke each platen. They are inflated after the mould carrier has been closed and deflated just before opening. The total stroke is a few centimetres. The clamping force afforded by this system is 12,700 kg, using a working pressure for the air bags of 0.3 MPa.

Mould Temperature Control System Individual thermoregulator units are used, one for each press. They are mounted on the rear of each mould carrier. This solution affords complete autonomy to each press, making it possible to control the temperature of the moulds prior to being placed on the production line. Subsequently, when a press is placed on the moulding line, it is ready to commence production. The thermoregulators work in a closed circuit; the relevant refilling has to be performed off-line. The control panel for each thermoregulator is mounted on the operator side for easier access.

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Polyurethane Processing: Recent Developments Mould Carrier Exchange The mould carrier concept for fixing and centring is designed for quick mould carrier removal or exchange. The mould carrier can be removed as follows: • • •

disconnect the electrical plug (positioned on the operator side) disconnect the air by removing the relevant joint mounted on the mould carrier remove the complete mould carrier (with its thermoregulator) using a fork lift

The estimated time to perform this operation is less than five minutes.

Service Station A dedicated station where service operations on moulds and mould carriers can be performed, without disturbing the production cycle, is foreseen for each moulding line (see Figure 3.10).

Figure 3.10 All maintenance and setting operations on moulds and mould-carriers can be executed off-line, in a dedicated service station that performs all the line’s movements and operative functions.

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Advances in Urethane Science and Technology The service station comes with one complete mould carrier on its freestanding frame. It is equipped with a dedicated thermoregulator for warming of moulds destined to be inserted on the moulding line, along with all the required safety fences/light barriers, controls, and connections for air, water and electrical power.

3.1.4.3 Transport Systems Cannon provides different mould-carrying systems, each customised to meet the customer’s needs. The most common solutions are conveyors and turning tables. In the last part of this section some innovative concepts, which provide a more compact layout, a minimised investment requirement and a maximum degree of flexibility in a changing manufacturing scenario, are presented.

Oval Conveyor The conveyor is made of a central steel frame, which is positioned on the floor. It is composed of a series of straight-line modules with a curved module at each end and comes complete with a number of carriages running on an oval steel track (also mounted on the floor). On each carriage, mainly composed of a rigid steel frame positioned on four pivoting wheels, a single lid mould-carrier is mounted. Four idle wheels, running along a central guide plate, maintain the system on its set path (see Figure 3.11). Continuous movement is achieved by means of one drive system being placed between every two carriages. Each driving system has two entrainment wheels running along the guide plate. The entrainment wheels – operated via an AC motor – are maintained in traction along the guide plate by a spring. An inverter is used to adjust the speed from 4 m to a maximum of 9 m/min, while the AC motor is fitted with a brake so that, in case of an emergency, the plant can stop within a few centimetres of travel. This system can easily be expanded to incorporate any new business obtained for that range of moulded products. Extensions are achieved by fitting an even number of new carriers to the line and extending the supporting frame accordingly.

Mould Carrying Systems: FlexiDrum Developed in the early 1980s as a revolutionary tool for the production of foamed refrigerator doors, the well-known concept of the rotary polymerisation system is now being proposed as a compact and simple mould-carrying system for the production of automotive seats (see Figure 3.12).

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Figure 3.11 A modular oval conveyor for flexible foam moulding. It can be easily extended, adding modules of track and pairs of mould carriers, when higher productivity is required.

Figure 3.12 Flexidrum, a compact moulding line for medium-low volume of automotive seat production. 1a Service station: demoulding, cleaning, insert positioning, release agent application; 1b Open mould view of 1a; 2 Foaming station, with robot on a platform; 3-6 Polymerisation stations; 7 Six arm rotating structure to support mould carriers; 8 Central collector of signals, air and warm water; 9 Supporting structure (and rotor, not shown)

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Advances in Urethane Science and Technology Six or more mould carriers can be fixed on the surfaces of a wheel that rotates vertically – similar to a Ferris wheel, rather than horizontally, as with a merry-go-round. The opening and closing movements can be hydraulic or pneumatic. Mould conditioning can be accomplished using a rotating collector that feeds each mould carrier with water at the desired temperature. Manual service operations are performed on the lower mould half in the first station. Foaming is achieved in the subsequent station using a platform-mounted robot. Polymerisation takes place in the remaining four (or more) elevated stations as they are passed through a compact suction hood that removes the escaping fumes. Several advantages can be highlighted for this new version of mould carrying system: •

simplified mechanical construction that does not incorporate wheels which roll along the floor picking up and transporting scrap foam and/or dirt

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optimised layout which will work even when space is limited

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reduced power cost thanks to its vertical layout which minimises the suction area, requiring only one fan for extraction of the fumes

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limited number of operators required

Carousel for In Situ Foaming A number of dedicated moulding lines have been designed for the production of in situ moulded foams. In situ moulding technology adds a delicate operation to the list of conventional operations (mould cleaning, release agent spraying, insert positioning, foaming and demoulding) that must be executed to mould a standard foamed item: the manual positioning of the textile container into which the foam will be dispensed. This is a delicate operation that requires some time, yet should not penalise the cycle. A practical solution consists of a carousel line with a row of service positions where the operators can work on moulds that have been temporarily taken off-line (see Figure 3.13). When the press leaves the curing area, it passes in front of the first free operator and is automatically disengaged by the dragging system. The moulds can be serviced, taking all the time required, then, when the textile inserts have been positioned, the carrier can be reinserted in the first available position in the line.

Multi-Hardness and Multi-Density Foams with Natural Carbon Dioxide The availability of multi-component mixheads with variable geometry opens the path to another interesting option: the addition of natural carbon dioxide for expansion of the

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Figure 3.13 Oval moulding line with 16 double mould carriers, designed for in situ foaming of textiles. 1 to 4 Service stations, where mould carriers are taken temporarily off-line for mould service; 5 Spare station; 6 Foaming station; A Pouring robot; B Metering equipment, on mezzanine; 7 to 16 Polymerisation stations; C Mould carrier changing station; D Suction hoods

foam with an environmentally friendly blowing agent and reduction of the foam density by as much as 20-25%. With this new mixhead, it is possible to use two different approaches to add natural carbon dioxide to the formulations: •

CannOxide [3-12] - a technology developed to meter natural carbon dioxide at the point of injection, at the desired percentage, into one of the polyol streams

•

EasyFroth [1, 2, 4, 11, 12, 13,14] for carbon dioxide – a technology that allows the premixing of given percentages of blowing agent in one of the two components, usually the isocyanate.

Both methods are currently in industrial production, each having operating and investment ‘pro’s and con’s’ that must be evaluated according to the production volumes and flexibility required. For example, a car seat with three different hardnesses can be produced using two different formulations for the hard and soft parts - (see areas ‘A’ and ‘B’ in Figure 3.14) - and incorporate some natural carbon dioxide to decrease the seat density below the thighs (area ‘C’).

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Figure 3.14 Triple hardness automotive seat A – Soft foam; B – Hard foam; C – Soft and lower-density foam, blown with natural carbon dioxide

3.2 ‘Foam & Film’ Technology - An Innovative Solution to Fully Automate the Manufacture of Automotive Sound Deadening Parts Until now, one of the major limitations in the polyurethane moulding process has been the necessity to interrupt the working sequence between each moulding, to remove foam scrap and apply release agent prior to foaming. The introduction of ‘Foam & Film’ technology makes manual intervention unnecessary, removing the one factor that has always been a major weakness when working with polyurethanes in comparison to other injected or extruded plastics [15-19]. The main idea behind this new approach consists of thermoforming a thermoplastic or polyurethane film as part of the moulding sequence. By using a vacuum effect, this film adheres perfectly and smoothly to the mould cavity, without any creases or wrinkle formation. The mould is equipped with a dedicated frame device, specifically designed to hold the film. A heating system ensures that the film reaches the desired temperature prior to the thermoforming phase and subsequent injection of polyurethane into the mould.

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Polyurethane Processing: Recent Developments Several industrial applications have been found and a number of fully automated plants currently incorporate this technology. The manufacture of sound deadening parts for the automotive industry represents one of the most exciting applications for this innovative technology.

3.2.1 The Problem Mould cleaning is required because of the chemical nature of the polyurethane process. The objective in developing the ‘Foam & Film’ technology was to eliminate the tedious, time-consuming manual operations, which must be performed on the moulds as part of a discontinuous polyurethane foaming process. The need to properly vent the mould, to avoid air entrapment, often results in a thin flash of polyurethane being formed around the moulded part, which needs to be manually removed afterwards via a simple trimming operation. These manual operations involve both cleaning of the mould and spraying of the release agent. It is necessary to demould parts in the shortest possible time, when the foam is not yet fully polymerised, because the thin cross-section of the flash makes them very fragile during the first few minutes following the demould. Often small pieces of flash will break off when the part is extracted, falling back onto the mould. If they are not removed from the mould surface, they could mar the surface of the subsequent parts or they could allow rising foam to leak from the mould if left along the seal. More extensive cleaning operations are required every few shifts in order to remove deposits of release agent from the mould surface and there require production to be stopped. Various technologies have been applied to minimise the required downtime, but so far a valid solution has not been found. The adhesive force between polyurethane and metal (either aluminum or steel) requires a release agent to be sprayed onto the moulds every single cycle (or every few cycles) to enable easy removal of the part. In addition, various internal mould release technologies are available, but they do not apply to all formulations. As stated previously, these operations take a long time, are expensive and are typically executed manually and it is impossible to have a fully automated line without getting rid of them. To create a fully automated line, both the need to manually clean the mould and the need to spray release agents on it had to be eliminated.

3.2.2 The Approach to a Solution The new approach to polyurethane moulding is to introduce a thermoplastic or polyurethane film thermoforming process on one or both halves of the mould as part of

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Advances in Urethane Science and Technology the overall sequence. By using a vacuum, this film adheres perfectly and smoothly to the mould cavity, without any creases or wrinkle formation. The system’s key concept is a specially designed and patented frame, integrated into the mould, which is specifically designed to hold the film in position during this thermoforming process. This frame can be either two or three dimensional, to better follow the shape and the cavities of the mould (see Figure 3.15).

Figure 3.15 ‘Foam and Film’ concept 1. Unrolling and cutting plastic film to required length; 2. Infrared (IR) heating of film on holding frame; 3. Thermoforming the film in the mould cavity; 4. The same process (1-3) is carried out in the other half of the mould at the same time; 5. Robotised foam deposition in mould; 6. Mould closed for expansion/polymerisation. Parts are removed ‘wrapped’ in the film.

The frame receives the film after it is unrolled by pinchers and cut dimensionally. It then moves it in front of an infrared lamp for several seconds to heat it to the required temperature. A control system ensures that the film reaches the desired temperature prior to the thermoforming phase. Special infrared heaters, Cannon’s MVL heaters, are used to ensure high efficiency and very low thermal inertia. Once the film is heated, the frame moves into the mould where the film, held in place along its four edges, is vacuum-formed onto it. The use of the frame during the vacuum forming prevents the film from folding or wrinkling (see Figure 3.16).

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Figure 3.16 A patented frame holds the releasing film in position so that when thermoformed over the mould, it is kept under tension and does not form wrinkles.

3.2.3 The Film There are two different types of ‘Foam & Film’ processes available, based on the different types of film used: adhesive and releasing. •

In the adhesive type, the film sticks to the part and is unloaded with it, granting an aesthetic finish and a waterproof covering (see Figure 3.17).

•

The releasing film, on the contrary, remains vacuum-formed to the mould for several shots (5-15, dependent upon the process and the materials utilised) and it is then replaced when it begins to wear.

Obviously, with the adhesive type, the film has a high adhesion coefficient with polyurethane, while in the releasing version the poorer the adhesion, the better. In both cases, the film prevents the mould from ever being put in contact with the polyurethane foam. That is why both cleaning and spraying are no longer necessary and all operations can be automated. 133

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Figure 3.17 Two shock absorbing inserts for automotive doors moulded in semi-rigid EAfoam with the ‘Foam & Film’ technology as they appear immediately after demoulding. The film protects the foaming against moisture and degradation. No release agents or mould cleaning operations are needed to produce this part in a highly automated moulding plant.

3.2.3.1 Adhesive Film Solution Basically, three types of film can be used in the adhesive Foam & Film process: • Thermoplastic polyurethane (TPU) • Polyethylene (PE) • Thermoplastic film (TP) TPU is a good film for both cold and hot processes, providing good adhesion to the foam and excellent mechanical properties but the cost is high (around 0.65 US$ per square metre). PE film can only be used in cold processes, it requires a special treatment for perfect adhesion and it gives an overall poorer performance but it has the advantage of being very inexpensive. TP film is only suitable for hot processes, it requires no treatment, and it guarantees good adhesion at a low cost (less than 0.2 US$ per square metre). This last type of film is what has been developed in depth, obtaining very good tear and impact resistance, flexibility, elongation and welding ease. This is the solution that has been used for most automated Foam & Film plants supplied so far. Using this film, General Motors makes sound-deadening parts with fully automated equipment that has a productivity of 8000 parts/day, running 24 hours per day with zero operators [17, 18].

3.2.3.2 Releasing Film Solution The second type of film, the releasing one, sticks to the mould and not to the polyurethane part. In the releasing Foam & Film technology, there are two main types of film that can be

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Polyurethane Processing: Recent Developments used: PE and TP. PE, once again, is very common and inexpensive, but gives poor release from the foam, which minimises the benefits of this technology. TP film provides an easy process with good release from the foam for several shots at a very reasonable price.

3.2.4 Industrial Applications A good example of such a continuous moulding process that incorporates ‘Foam & Film’ technology is a system developed for the production of industrial vehicle carpets. These products are usually made out of a sandwich of two or more layers of different materials. Individually, they provide different features: aesthetics and function (a textile carpet or a synthetic mat), sound deadening (a layer of polyurethane foam) and protective (a cheap layer to protect the foam from moisture and degradation). INSOTEC, a technology designed by Cannon for the manufacture of sound-deadening automotive components, offers a variety of manufacturing alternatives that provide significant benefits such as improved quality parts, reduced costs, shorter process times, as well as regular and consistent production cycles (see Figure 3.18).

Figure 3.18 A large full-automated production plant for truck floor-covering mats using the ‘Foam & Film’ technology. The parts are made of thermoformed PVC (or thermoplastic elastomer) foam-backed with PU, which is kept separate from the mould with a film (adhesive type). 135

Advances in Urethane Science and Technology These excellent results have recently been improved with the introduction of the new ‘Foam & Film’ technology. The technology makes manual intervention unnecessary, removing the one factor, which has always been a major weakness with polyurethane processing and added a significant cost to the parts that are perceived to be an economical component of a vehicle. These components have a surface area of about 3 m2 and consist of a surface layer of polyvinyl chloride (PVC) or thermoplastic elastomer (referred to as the ‘heavy layer’), an intermediate sound-deadening layer of flexible, medium-to-low density polyurethane foam and a lower thermoplastic film. The film is designed to prevent the formation of flash during the moulding process, eliminate permanent residue usually left in the mould, and to act as a release agent (once the part is in the vehicle, this film inhibits water absorption, which is a very frequent problem with industrial vehicles). In the past, polyurethane films, which were strong mechanically and performed well but were quite expensive were used. With the new ‘Foam & Film’ technology, it is possible to replace this polyurethane film with a thinner, less costly thermoplastic one, resulting in high quality production at a lower cost. The system is composed of two shuttle-bed clamps served by one metering unit, which dispenses the pre-heated heavy layer in the mould. Pre-heating is carried out using a special infra-red heater, which incorporates easily adjustable, special low thermal-inertia resistances. When a vacuum is applied to the mould, the material adheres to the lower mould half, taking on its shape and embossed design. The protective film is automatically unrolled from an overhead source using a vertical traversing frame with pinchers that pulls an appropriate length of film over the frame and cuts it to length. The frame is positioned over the edge of the mould. Another bank of heaters slides laterally in front of the mould and warms the film to the correct forming temperature; a vacuum is applied at the end of the heating phase to conform the film to the mould. The use of two presses, as opposed to one, means that slack periods are eliminated and use of the cycle time is maximised. While the film and heavy layer are being placed in one of the moulds and pre-heated, foaming and polymerisation are taking place on the other. The polyurethane dosing and foaming section of this plant is equipped with a moulding technology that allows chlorofluorocarbon (CFC) blowing agents to be replaced with liquid carbon dioxide. This helps reduce the density of the polyurethane considerably and thus saves on material costs. The overall production time is just two and a half minutes per part. Being a twostation plant, this equipment can produce close to 50 finished parts per hour – without a dedicated operator.

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3.2.5 Applications This technology has already been used on turnkey plants for sound insulation parts, carpet back foaming and seat cushions. No limitations for this technology are foreseen and, actually, ‘Foam & Film’ can be implemented in almost every kind of polyurethane process needing either release agent or a cover/surface film. The system requires the presence of an open mould where the film-holding frame can be inserted to position the film prior to the vacuum-forming phase. Obviously, this technology is more easily applied to new equipment and new moulds since, most of the time, existing moulds must be modified to provide the vacuum and hold the frames. When one surface of the part is covered by an aesthetic or functional layer (carpet, plastics, etc.), obviously the film is only applied to the opposing side where the foam would be in contact with the mould (see Figure 3.19).

Figure 3.19 Turnkey installations using the ‘Foam & Film’ technology are producing numerous parts for the automotive industry.

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3.2.6 Advantages ‘Foam & Film’ offers significant benefits, such as quality parts, reduced costs, shorter process times, as well as regular and consistent production cycles. •

operator intervention is no longer required for mould cleaning and application of release agent; consequently this leads to increased productivity, uniformity and the cycle regularity which comes with a completely automatic line - a fully automated foaming process is now possible.

•

polyurethane or TP film can be used, the latter being thinner and cheaper but giving the same performance.

•

films which adhere to the product can be used, becoming an integral part of the finished component; this can be a very useful feature for non-aesthetic parts that are to be mounted in hidden positions and will benefit from this extra protection against humidity, oil, aggressive chemicals and foam-aging agents such as oxygen or other gases.

•

films, which adhere to the mould, can be re-used several times as a substitute for release agent.

•

the availability of a wide range of film sizes means no dimensional limitations on the parts to be moulded.

To summarise, ‘Foam & Film’ Technology: •

automates the production of polyurethane moulded parts,

•

eliminates mould cleaning,

•

eliminates spraying of release agent, saving its cost plus those of all the relevant dispensing equipment and special fume extraction systems (although regular fume extraction must be maintained for the polyurethane process),

•

offers a low operating cost.

3.3 InterWet - Polyurethane Co-injection The combined use of polyurethane and reinforcing agents or fillers has been common practice for a long time. A recently introduced technology - simultaneously injecting foam and glass fibre in open moulds - did not give satisfactory results, according to some of the early users. Cannon has developed an industrial solution - named InterWet - that

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Polyurethane Processing: Recent Developments improves both manufacturing performances and mechanical properties of moulded parts [20-27]. In addition, it allows for the use of other reinforcing fibres (natural and synthetic) and opens the way to the easy addition of powder and granulate fillers.

3.3.1 Glass-Reinforced Polyurethanes, a Well-Known Technology Glass-reinforced polyurethane has been used for many years, utilizing different technologies: reinforced reaction injection moulding (RRIM), open-mould pouring over a flat mat (LD-SRIM), closed-mould injection on preformed glassmat-sandwiches (SRIM). Cannon has for many years dealt with glass-handling polyurethane technologies, developing in the early 1970s a special RRIM mixing head able to mix formulations containing high percentages of milled fibre, and launching in the 1980s the HE metering machines, closed-loop electronically-controlled piston-metering systems able to cope with the abrasion deriving from glass and mineral charges. Later in the 1980s the Compotec preformers were introduced, for the production of preformed glass mats required by SRIM - structural-moulding applications. A new technology has been introduced by Krauss-Maffei, long fibre injection (LFI) [28], that simultaneously pours polyurethane and chopped glass roving on the surface of open moulds; after the shot the press is immediately closed to allow for the expansion of the foam, which surrounds all the fibres and produces a lightweight, resistant composite panel. Most recently, Hennecke has introduced FipurTec [29, 30], a technology where the fibre is chopped outside the head and projected into the flow of reacting chemicals, where this touches the surface of the mould. In essence these systems carry out a job similar to that performed for very many years in the polyester ‘chop-and-spray’ applications, the only differences being that polyurethane has a different profile of reactivity and viscosity build-up than polyester and that, at least in one case, the glass roving is conducted through the mixing device instead of being fed outside of it. The interest behind this technology lies in the fact that a thin, resistant composite part can be moulded in only one operation. Using preformers requires more equipment, space, investments, etc. In addition, glass roving costs roughly half that of glass mat, and this can represent a considerable saving when producing large, highly reinforced parts.

3.3.1.1 Practical Problems That Needed to be Addressed A few practical problems have been identified in this technology, mainly deriving from the design of the mixing equipment currently available. In one case the chopped fibre is

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Advances in Urethane Science and Technology fed through a pipe that is co-axially positioned in a conventional ‘L-shaped’ head and then the mixed resin is conveyed around the pipe that feeds the chopped fibre. In the other case the fibre is projected separately from the polyurethane; in this case the glass meets the polyurethane only at the end of the head’s final discharge duct. The separation of the fibre and polyurethane before the exit of the head results in wetting of the fibres not being optimal; this is visible during moulding, where part of the glass gets stuck vertically into the base of rising foam, visibly dry. The main problem comes from the dry chopped glass that - leaving the nose of the head still not wet - flies everywhere near the mould surface. This results in negative effects on workers, on cleaning and maintenance operations, and on part quality. This problem can seriously hinder the application of this technology.

3.3.1.2 The Cannon Approach Cannon started looking into this technology in the summer of 1997, at the urging of some car parts producers who were unsatisfied with the performances of the existing ones. The main objectives of the project were: •

To supply in the shortest possible time a reliable solution for the injection of glassreinforced polyurethane foams: with as little as possible an impact on the working environment, with the best possible quality,

•

To provide the lightest possible mixing/dispensing equipment, in order to reduce the investment in head-carrying robots,

•

To design a multi-purpose solution, not limiting the choice of charge to glass roving only, but including a wider range of natural and artificial fibres, as well as pulverised fillers,

•

To provide maximum flexibility in charge feed, so that products with different content of charges - in the same moulding or between subsequent moulds - can be produced,

•

To provide maximum number of automatic checks on the charge’s feeding line, to avoid the troubles experienced during automatic production cycles.

Due to the short delivery time required, it was not realistic to conceive totally new equipment for this application, and it was decided to concentrate on the use of existing, proven pieces of hardware. Performance of the foaming section had been optimised in previous years, and the main obstacle was the proper handling of glass and other fillers.

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3.3.1.3 The Technologies Involved The project was split into three parallel lines of development, to speed up results and make use of the experience of various existing specialists for the specific engineering tasks: •

Charge/Reinforcement (handling, dosing and feeding)

•

Polyurethane (dosing, mixing and laydown)

•

Robotics.

Handling Reinforcements and Charges This project required a multi-functional solution, able to accommodate various types of fibres, fillers and charges; accordingly, the design of the whole handling section would have had to respect this future necessity. Since the most urgent need called for the use of glass roving, the first development focused on the design of a simple, reliable device to store, meter and chop conventional type glass-roving rolls. A proper technical specification was drafted to provide optimum handling of the types of glass most suitable for this peculiar application. Various qualities of roving are available on the market for different end-uses: the diameter of the basic threads (bunches of individual glass fibres) and of the final roving (the rope made with various threads) define the weight and the field of application, while the type of coating (a thin layer of special resin applied on the fibres after the glass-extrusion phase) determines the best compatibility of the roving with the polymeric matrix. A specific type of Owens Corning Fibreglass (OCF) roving was selected as the reference, having a 2400 Tex specific weight, i.e., the roving weighs 2,400 grams per kilometre, with high ‘softness’ of the thread. Other types would have been easier to cut (or to be broken, since a fibre of glass is not shear-cut - it is bent beyond its critical radius until it breaks), but their individual fibres were more agglomerated in the thread and would have been more difficult to wet. All parts were designed to cope with a wider range of roving. The glass-feeding device was conceived to be installed over the polyurethane mixing head, and its design was made in strict cooperation with the team in charge of designing the head. It is basically composed of: •

A pulling/cutting device: two opposed rolls that rotate and trap between them one or more roving, pulling them from their storage. A blade - held in one of the two rolls forces the fibres to bend beyond their critic radius until they break;

•

A conducting system: a flow of compressed air that pushes the chopped fibre from the cutting place to the mixing head.

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Advances in Urethane Science and Technology A few interesting technical features characterise this roving feeding-chopping section: •

A hydraulic motor, to drive the glass-roving pulling rolls, was chosen to provide proper speed, torque and easy variation of the driving speed. Its high power/weight ratio made it preferable to an electric motor;

•

A special cutting device was conceived, based on a rotating blade-holder (very easy to access and maintain) capable of accommodating up to four blades. By fixing more than one blade in the proper holders one can chop the roving in different lengths, the longer size being that equal to the circumference of the rotating blade-holder. The part had to be dimensioned to resist the very high operating speed, up to 5,000 rpm, without suffering from the presence of glass debris around rotating shafts. This design proved useful later, in the further development of a variable-length chopping system;

•

A glass-guide system using compressed air was designed to take away all the cut roving from the chopping device and bring it through the mixing head down onto the mould, with the additional task of providing a final purge of the discharge duct.

The Mixing Head Most of the development work was concentrated in designing a mixing head able to wet all the solid components before they left, to ensure the projection of a very well wetted mixture rather than a blend of liquid only coupled with a dry, flying filler. The mechanism which could ensure a thorough wetting of high percentages of solid fillers with a mixture of polyurethane is to provoke high turbulence in the mixing head’s area where liquid and solid meet, and then reduce the turbulence to allow for splash-free open-mould distribution of the blend obtained. It was in fact observed that, with heads where the junction between liquid and solid was in a turbulence-free zone, the wetting was not optimal. By using the Cannon FPL head, an existing, two-decades-proven, piece of hardware it would have been possible to obtain the best available mixture of the polyurethane component with a turbulence-free laydown in the mould. Its internal geometry is L-shaped, with two cylindrical chambers of different diameters connected at a 90º angle. The turbulent flow created by impinging of two components in the small mixing chamber is quickly converted into a smooth laminar flow as the chemicals are diverted through 90 degrees downwards into a large discharge duct and then leave the head. At the end of the pour the smaller mixing chamber is sealed and automatically cleaned using a hydraulic piston. By using the original mixing chamber for the liquid components and boring a hole through the main cleaning piston - the plunger - so that a solid component could be fed through it, a wide range of fillers can be used (see Figure 3.20). When the mixing operation begins, the 142

Polyurethane Processing: Recent Developments

Figure 3.20 The Cannon FPL head used for InterWet incorporates in its upper section the glass cutting mechanism and relevant safety checks.

main piston is retracted until it clears the discharge duct at the junction of the two cylindrical chambers. In a controlled sequence, the small piston sealing the mixing chamber retracts and the two chemical components are fired at high pressure against each other through injectors in the mixing chamber before leaving it through the discharge duct. The metering and feeding operation of the solid component from the head’s upper part is controlled mechanically, assisted by an intense flow of compressed air. Its feed is synchronised with the arrival of the liquid blend from the mixing chamber. The solid component meets the liquid blend and together they are co-injected into the mould. Following co-injection, the small piston seals the mixing chamber, the large piston seals and cleans the discharge duct and the feed of solid component is interrupted, although the flow of air is maintained for a short time in order to clean any residual polyurethane - which could interfere with the next pouring operation - from the nose of the head.

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Figure 3.21 Open-mould co-injection of PU and glass fibre is executed at high speed and in a clean working environment, with an InterWet machine.

The innovative concept in this solution is that the solid component meets the liquid formulation just in front of the mixing chamber, where the turbulent flow of the mixture is directed through a 90° angle downwards creating a laminar flow. In this way, the kinetic energy from the pressurised liquids is used to wet the stream of solid component thoroughly and efficiently at a point just 20-25 mm from the point of impingement. The blend of polyurethane and filler leaves the head already well blended, and there is no evidence of ‘flying glass’ out of the head (see Figure 3.21). This special design and technique ensures that the solid component is thoroughly internally wetted, hence its name, InterWet.

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Polyurethane Processing: Recent Developments Robotics The development of a proper device to carry the mixing head during the pouring operation over open moulds required relatively short time. Basic requirements were multi-axes capacity, speed, precision and easy programming. The designers concentrated on the use of commercial polar robots, operating up to 6 axes at high speed and precision (see Figure 3.22). The limited weight of the Cannon FPL mixing head allowed for the selection of a mediumsized model able to carry on the wrist 125 kg of payload at up to 2 m/s, with acceleration of 3 m/s2. This robot was fitted with two storage boxes for the roving, mounted on the elbow area (actually this model allows to install there up to 3 glass-roving rolls, for a maximum 60 kg payload). This is a very convenient glass-storage solution for laboratory and small production applications. When high quantities of glass are required, a solution able to reduce rolls-replacement times must be provided. A proper device has been designed to accommodate the 1,000 kg of glass demanded for industrial heavy-duty tasks. In this case the glass rolls - that could be cascade-joined one another - must be positioned the closest possible to the head, so that the way the roving must take to reach the chopper is not restrained by long guides.

Figure 3.22 One of Cannon’s R&D units devoted to the development of InterWet technology.

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Advances in Urethane Science and Technology First Results The assembly of the pieces of hardware was completed in less than eight weeks from the beginning of the project, and immediately trials were scheduled in the laboratory using a flat test-plate mould. A conventional Cannon HE - a piston-driven, electronically closed-loop controlled dosing unit available in the laboratory - was connected to a new mixing head, that had been fitted to the wrist of the robot by means of proper flanges. The glass was carried from its storage to the chopper by means of flexible plastic pipes, and the path was designed so that there would be no obstructions and friction for the roving. Glass roving X900A, a 2400 Tex, very soft roving with average attitude to the cut, was supplied by Owens Corning Fibreglass. A chemical formulation supplied by Dow, characterised by cream time of 12 seconds, gel time of 40 seconds, demoulding time of 3 minutes and free rise density of 50-55 kg/m3 was used. The first tests were run using approximately 30 parts of glass over 100 parts of polyurethane. Surprisingly, the results were extremely satisfactory from the very beginning (see Figure 3.23).

Figure 2.23 Homogenous wetting of all fibres was obtained since the very first trials with InterWet, thanks to the interval geometry of the mixing head. 146

Polyurethane Processing: Recent Developments After fixing minor problems with the cutting control, an optimum distribution of blend was obtained, and the glass fibres were all wet (this was clearly visible because polyurethane was pigmented in black, and absolutely no white spot of dry glass was shown at the end of an open-mould pouring operation). Being all wet, the fibres did not tend to stick vertically in the rising foam, therefore less possibility remained for the air to remain trapped in the mixture: the moulded parts were, in fact, totally free of air bubbles. A number of trials were satisfactorily run using thin, transparent polyethylene film as liner for both mould halves, so that when the liner was peeled off it was possible to evaluate the quality of the compound and its surface (see Figure 3.24). The second series of trials was run on a mould for production of automotive door panels where a layer of expanded or compact PVC sheet was unrolled on the lower mould-half and cut to measure, its edges were blocked with a frame and it was vacuum-formed to perfectly adhere to the mould surface. After this operation the blend of polyurethane and glass was poured on the rear side of the PVC liner, and the mould carrier closed for polymerisation. On this occasion - since the first results were very encouraging - the

Figure 3.24 No holes are visible in this InterWet non-presses laboratory sample, photographed against a strong light. The diffuser-mounted on the InterWet provides a very distribution of material all over the mould.

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Advances in Urethane Science and Technology percentage of glass was increased to 35% of the total blend, i.e., approximately 60 parts of glass to 100 parts of polyurethane. Again, very nice parts were obtained at the early stages (see Figure 3.25).

Figure 3.25 Nice parts, free from air entrapments between reinforced PU and outer skin were obtained at a very early stage of the InterWet development work.

First Improvements •

Improved distribution [25]

One aspect of the laydown - the width of the pouring path - was worth immediate attention; since the reactive blend leaving the head covered only a few centimetres of mould at each pass, a rather high number of passes was required to properly cover the whole surface which forced use of the robot at very high speed to complete the whole pattern before creaming started. To avoid the overlapping of two contiguous layers of mixture, sometimes a narrow gap remained between the layers. This did not create problems but the distribution of glass in those thin strips was probably different than in the other areas of the mould.

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Polyurethane Processing: Recent Developments To overcome these problems, a specially-designed pneumatic device was added at the exit of the mixing head, that deviates, under controlled conditions, the flow of mixture from its natural, vertical direction and distributes it over a wider pattern and, consequently, in a thinner section. By doing so it is irrelevant if two contiguous layers do overlap (since the layers are thinner the effect is negligible) and fewer passes are required to cover the same surface. The robot can be used at lower speed (or a simpler, less expensive robot can be used) or more parts can be made in the same unit of time. This could be relevant thinking to carousel-based operations, where the robot would have to perform different pouring patterns in a sequence, on different moulds. Another advantage derived from this pneumatic distributor is that the jet of air pushes even further down the fibres, which results in a very thin layer of mixture applied on the mould surface. This means that the male half of the mould - when closing over the female part - does not ‘wash down’ the vertical walls from the layer of reacting foam and glass which covers them, since this layer is quite thin and does not constitute an obstacle for the lowering plug. Parts as thin as 1.5 mm can be moulded with optimum surface aspect and homogeneous distribution of glass across the whole surface.

•

Variable fibre length, with ‘on-the-fly’ change

The first fibre-cutting device, as stated above, had the possibility of accommodating more blades. Therefore, the length of cut fibre obtained was equal to the circumference of the cylindrical blade-holder when only one blade was installed, or half of its circumference with two blades, or one-quarter of it with four blades. Since this was foreseen in the original wish list, the option of allowing the user to vary the length of glass fibre at will, even ‘on-the-fly’ on the same moulding, was soon taken into consideration. A simple, sturdy mechanical device was designed to allow for this variable cutting. This mechanism which operates via an impulsion given by the robot’s programme at a given point of the pouring pattern, cuts the roving in shorter sections; this variablelength cutter provides different cuts of fibre: from L to L/12, where L is the longest possible choice and the combinations depend on the use of the 12 blades installed on the holder.

•

Variable glass fibre output

Operated by a hydraulic motor, the pulling device (two opposed rolls that rotate and trap between them one or more roving, pulling them from their storage) is able to provide variable output of glass. By electronically controlling an inverter or a proportional valve it is quickly possible to vary the motor speed, providing more or less fibre in different

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Advances in Urethane Science and Technology parts of the moulding. The motor can be even stopped (thus feeding only polyurethane foam) or left running after the end of the polyurethane shot (providing dry, unblended glass fibre over some areas of the mould where an extra reinforcement would be desired).

•

Extended safeties

The main concerns in operating this kind of plant derive mostly from the irregular feed of glass to the head. For this reason the latest InterWet machines have been equipped with detectors able to command an immediate stop to the machine in case of: •

Empty storage of glass roving

•

Roving blocked in its roll

•

Roving blocked somewhere along the feeding line

•

Roving not properly fed into the head after the cutter and forming a ‘birds nest’

•

Malfunction of a single blade.

3.3.1.4 Advantages of the InterWet Process These new developments have resulted in a higher degree of flexibility for the InterWet machines whose advantages versus the existing and available similar methods can be summarised as follows: •

Optimum overall mechanical properties, due to an optimum glass fibre wetting with PU, because of the mix-head design

•

Better distribution of the mechanical properties of the moulded part, i.e., no areas without or with little glass, due to a more homogeneous glass fibre distribution in the mould, because of the special distribution system.

•

Programmable variation of mechanical properties on the same part, since the distribution of the composite’s mixture on the surface of the mould is fully programmable. One can have in different areas of the same part:

•

PU and glass in fixed proportion

•

PU and a varying proportion of glass

•

PU only

•

Extra glass over an already filled area.

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Polyurethane Processing: Recent Developments •

Easy operation because it uses a standard (modified) FPL mix-head with obvious consequences in terms of simplicity of maintenance, reliability in use and availability of spare parts when required.

•

Flexibility in operation because it is designed for more than one type of additive component: it can easily be converted to process fillers or fibres or chopped scrap foam. Its range of application includes continuous reinforcing fibres, natural fillers (mineral or organic), pulverised plastics, etc.

•

All the operational advantages deriving from the use of a light mixing head (robot size, speed, productivity, etc).

•

Economics, because - being based on commercially available components - its mechanical parts can be less expensive than specially developed ones.

When compared with other glass-reinforcing technologies the following advantages must be added: Versus some RRIM applications: •

Economics, since it is no longer necessary to carry out the expensive dispersion of solids in the polyol component - now, by adding them directly in the mixing head, it is possible to eliminate all the traditionally associated problems, i.e., possible absorption of polyol into the filler with the resulting slow release of reactive components in the polymer matrix, abrasion, clogging of lines and tanks, fluctuating percentages of filler in the liquid, etc.

Versus some mat applications: •

Economics: in glass (roving costs less than mat) in equipment (one more machine, storage, handling, floor space, etc.) in scrap (preformed parts - although optimised - require punch-cutting that produces scrap trim) in stock (stored preforms can be damaged by sunlight, dust, moisture, and often must be thrown away) in manpower (to handle, position, punch, etc.)

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Advances in Urethane Science and Technology •

Quality (foam mixes better when co-injected with glass roving rather than when injected into a pressed mat preform)

•

Flexibility: it’s easy to mount on a carousel various moulds and produce small series exploiting the flexible properties of the robot.

It is evident that there are applications where both RRIM and mat preforms are still preferred because of their specific advantages. InterWet has been conceived to fill the gap between the two technologies, and allow for more extended use of reinforcements and fillers in applications where neither RRIM nor mat preforms could be used [31].

3.3.1.5 Applications When this technology was first introduced it immediately appealed - for the above mentioned reasons – to automotive and transportation end users. Volumes, cycle times and final properties meet this market segment’s requirements, but until the currently available solutions address some of the above-illustrated problems this industry would not select it for mass production. A market segment where this application would be really interesting is the manufacture of special sanitary-ware parts, for at least three different purposes. The quickest to understand for its immediate advantages is the combination of an outer plastic shell with either aesthetic or functional purposes - back-foamed with reinforced polyurethane; examples include whirlpool tubs, bath tubs, shower trays, and vertical elements for shower cabins integrated with accessories. These products are characterised by medium-small production series, wide range of models, and large surfaces. The high operating flexibility allowed by the programmable robot is very adaptable for the production of this type of moulding: the larger the part, the more advantages can be found in this technology. Medium-deep-draw shaped parts can be easily thermoformed from large thermoplastic sheets (even in medium-small series) and immediately reinforced in an integrated production line including thermoformer, foaming station and curing area. Dimensions, productivity and investment must be tailored to the project’s requirements. Another area of interest for this new technology is the re-use of chopped foam scrap, either flexible or rigid; the design of the solid component’s feeding device - as foreseen in the project’s objectives - has been made in a way that, using an appropriate volumetricfeeding mechanism, powders, granulates and recycled foams can be dosed in the mixing chamber through the same channel bored in the mixing head’s cleaning plunger. It is possible to add roughly pulverised scrap foam to the virgin formulation directly in the mixing head. This would solve a recycling problem, as well as lowering the cost of

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Figure 3.26 In addition to glass roving a number of other materials can be co-injected with PU using the InterWet technology, scrap PU foams, sand, ground cork and other fillers that would be difficult to add in the component tank.

formulation. Positive trials have been made in a development phase, with filler loading as high as 10% of the final part’s weight (see Figure 3.26). Another interesting potential application is the use of continuous fibres, cut in mediumlong size, to reinforce thin pads and cushions moulded with flexible foams. The continuous development of lighter, thinner parts - where weight-reduction and volume-exploitation is the object of intense development - requires the use of formulations characterised by very high mechanical properties that unreinforced foam cannot meet. The use of long natural or synthetic fibres to reinforce these flexible mouldings had applied for a long time, but only under the form of textile inserts manually inserted in the mould prior to the foaming operation. Being able to co-inject foam and long fibres - applying longer or shorter sizes according to the mechanical properties required - represents an advantage that comes at a very attractive investment cost.

Note: The Cannon FPL Mixing Head is covered by several international patents.

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References 1

M. Taverna and B. Pile, Presented at the Polyurethanes Expo ’99 Conference, Orlando, FL, USA, 1999, p.37.

2

Modern Plastics International, 1999, 29, 3, 127.

3

M. Taverna, Presented at the Recycle ’94 Conference, Davos, Switzerland, 1994, Paper No.35.

4

L. White, Urethanes Technology, 1995, 12, 2, 3.

5

Plastics and Rubber Weekly, 1995, No.1612, 10.

6

D. Smock, Plastics World, 1995, 53, 11, 20.

7

British Plastics and Rubber, 1995, November, p.10.

8

D. Smock, Plastics World, 1995, 54, 10, 32.

9

M. Taverna, Presented at the Utech Asia ’97 Conference, Suntec City, 1997, Paper No.12.

10

D. Smock, Plastics World, 1996, 54, 9, 23.

11

C. Fiorentini, M. Taverna and J. Luca, Presented at the Polyurethanes ’95 Conference, Chicago, IL, USA, 1995, p.476.

12

Plastiques Flash, 1998, 304/5, 92.

13

M. Taverna, P. Corradi and B. Biondich, Presented at the Polyurethanes ’95, 1995, Chicago, IL, p.484.

14

M. Taverna and P. Corradi, Presented at the Utech Asia ’96 Conference, The Hague, 1996, Paper No.10.

15

P. Mapleston, Modern Plastics International, 1997, 27, 8, 30.

16

Plastics and Rubber Asia, 1997, 12, 72, 34.

17

British Plastics and Rubber, 1997, July/August, 19.

18

Plastics and Rubber Weekly, 1997, No.1687, 17.

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M. Taverna, Presented at the Polyurethanes Expo ’98 Conference, Dallas, TX, USA, 1998, p.685.

20

P. Mapleston, Modern Plastics International, 1997, 27, 11, 41.

21

Plastics Rubber Weekly, 1998, No.1729, 13.

22

Macplas International, 1998, February, 46.

23

G. Graff, Modern Plastics International, 1998, 28, 11, 38.

24

British Plastics and Rubber, 1998, November, 20.

25

M. Taverna and A. Bonansea, Presented at the Polyurethanes Expo ’98, 1998, Dallas, TX, 687.

26

European Plastics News, 1999, 26, 4, 48.

27

Composites – French/English, 1999, 36, 50.

28

J. Stark and F. Peters, Presented at the Utech ’99 Conference, 1999, Singapore, Paper No.6.

29

Plastics and Rubber Asia, 1997, 12, 70, 34.

30

D. Smock, Plastics World, 1996, 54, 12, 35.

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4

Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels for Super Insulation Applications Paolo Manini

4.1 Introduction Rigid polyurethane (PU) foam is the preferred insulator material in a wide range of applications encompassing household, industrial and commercial appliances (refrigerators, freezers, display cases, vending machines), transportation (refrigerated trucks and reefers, shipping containers), insulation in buildings and in industrial plants. The replacement of chlorofluorocarbons (CFC) with environmentally more benign chemicals, as recently mandated by the Montreal Protocol and subsequent revisions, has caused a reduction of the insulation efficiency of the PU foams, since the new blowing agents available, like hydrocarbons, hydro-fluorocarbons (HFC) or carbon dioxide have worse gas thermal conductivity properties. In applications, the lower insulation efficiency of the CFC-free foams generally leads to higher energy consumption levels unless specific compensation actions are taken. This issue is particularly severe for the household appliance industry, which has also been called upon over the past years to progressively reduce the energy consumption of their products. With the energy efficiency targets getting more and more demanding and only marginal improvement obtainable from the optimisation of conventional technologies, the appliance manufacturers are seriously looking at alternatives which may provide additional benefit in terms of energy savings. The appliance industry is not the only one involved in the debate on energy consumption reduction. The growing attention to the global warming issue is forcing policy makers to take actions to cut carbon dioxide emissions and minimise the greenhouse effect. With this in mind, the Kyoto Conference has posed ambitious targets for energy saving, the achievement of which calls for improvements in many fields, from transportation, to refrigeration and building insulation. The ban on CFC and the demand for a more efficient use of energy, both issues coming from environmental concerns, are therefore posing serious challenges to the Industry as a whole.

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Advances in Urethane Science and Technology Vacuum Insulated Panels (VIP), having a thermal conductivity three to five times lower than conventional PU foams, allow the achievement of superior insulation performance. They can be used to partially replace the conventional insulation materials to provide a more efficient insulation structure, which allows energy saving without the need to increase the insulation thickness. Alternatively, VIP can be used in those applications where it is important to reduce the insulation thickness to a minimum value without loosing thermal performances. A VIP is obtained by packaging a microporous low conductivity filler material inside a highly impermeable gas barrier bag. The filler is then evacuated to a proper vacuum level and the bag sealed. A gas adsorbent, normally referred to as a getter, inserted in the bag before sealing, is also necessary in most panel designs to ensure the proper vacuum level during the lifetime of the panel. Several fillers have been proposed in the past, either in the form of compressed powders or fibres, however all have some disadvantages in terms of cost, process complexity and/ or weight. This has prevented VIP technology from finding widespread acceptance in most applications. The recent development of open cell rigid foams, first introduced into the market in the early 1990s has sparked off new interest in this technology. However, in order to fully exploit the unique properties of these insulating materials, the reevaluation and re-design of the other key elements of a vacuum panel, i.e., the barrier film, the gas adsorbent and the manufacturing cycles, has also been necessary. This process, which is still ongoing and has potential for further improvements, has required the commitment of several sectors of the industry in the last five to six years. In this chapter, the present status of the open cell PU foam-filled vacuum panel technology is discussed and some recent developments in film, getter and processing technologies are reviewed. Special focus is given to the vacuum issues, which are key for the proper selection and treatment of the VIP components. Some specific aspects, related to the manufacturing, characterisation and practical use of vacuum panels, as well as the achievable benefits, are also considered. Examples of applications where this technology is finding its place in the market and areas where further work is necessary to make vacuum panels more cost effective are presented.

4.2 Some General Properties of Open Cell PU Foams for Vacuum Insulated Panels Vacuum panels have been studied as a means to improve thermal insulation for a long time. Several insulating fillers, such as silica and perlite powders [1, 2, 3], fibre glass [4, 5], and aerogels [6, 7, 8] have been proposed as core materials for VIP, each of them

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… having specific advantages and disadvantages in term of weight, thermal conductivity, handling, processing, vacuum properties and cost. To overcome some of the drawbacks associated with the use of these materials, new families of fillers, based on PU [9, 10, 11, 12] and, more recently, polystyrene (PS) [13] open cell foams, have been developed and proposed to the market. Both open cell PU and polystyrene foams are now being considered as interesting options due to their moderate outgassing, good thermal insulation values, low weight, ease of handling and cost effectiveness. This chapter will focus on the open cell PU foams, even though many general aspects and recent developments of the technology, dealing with films, getters and manufacturing processes, can be also applied, with minor changes, to other micro porous fillers. The thermal conductivity, or λ factor (mW/m-k), for some of the most popular filler materials is shown in Figure 4.1 as a function of pressure (Pa). The thermal conductivity of cyclopentane (CP) blown closed cell PU foam is also given for comparison.

Figure 4.1 Thermal conductivity as a function of pressure for some filler materials

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Advances in Urethane Science and Technology Regardless of the type of filler, all curves have a similar behaviour, with a low pressure region, where the λ factor is constant, followed by a region where the λ factor increases with increasing pressure. This common trend can be explained considering that the total apparent thermal conductivity in micro porous materials is the sum of four physical contributions, i.e., the thermal conductivity through the solid, λs , the thermal conductivity through the gaseous phase, λg, the thermal conductivity by thermal radiation, λr and by gas convection, λc: λ = λs + λr + λg + λc

(1)

The last term of Equation (1) can be neglected for core material with cell size smaller than 1 mm. An extensive treatment of the above physical mechanisms and their role in heat transfer in low density closed cell foams has been provided by Glicksman [14]. In the case of the open cell PU foams detailed expressions for the first two terms of Equation (1) have been given by Kodama and co-workers [9]. Both terms are independent of pressure and strictly related to the foam density and morphology (cell size, structure and degree of anisotropy). In particular, λs is directly proportional to the thermal conductivity Ks of the foam and inversely proportional to the cellular anisotropy η according to the formula [9]:

[(

) ]

λ s = 1 − Vg2 / 3 K s / η

(2)

where Vg is the fraction of foam volume occupied by the gaseous phase. To reduce the heat transfer through the solid it is therefore necessary to increase the cellular anisotropy and to lower the thermal conductivity of the polymer structure. Following Kodama and co-workers, the heat transfer by radiation is given by: λ r = 2 / 3 Vg1/ 3 H r d

(3)

where d is the cell size and Hr is the coefficient of thermal conduction by radiation which depends on the emissivity of the polymer and the morphology of the cell. To minimise the λr contribution, the cell size and Hr have to be reduced. The former can be achieved by adjusting the formulation of the foam, the latter by reducing the opening size of cell membranes to make radiation transmission through the cellular windows less efficient.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… The heat transfer by gas conduction, λg, becomes progressively more important as the pressure increases, as shown by the following set of equations [15]:

(

)

λ g = λ goΠ / (1 + 2L / d)

(4)

where λgo is the thermal conductivity of air at atmospheric pressure, Π is the porosity of the foam and L is the mean free path of air, linked with pressure P by: L = (kT ) / ⎛ 2 π P σ 2 ⎞ ⎝ ⎠

(5)

where T is the absolute temperature, k the Boltzmann constant and σ the collision diameter of air (about 4 x 10-10 m2). The λg contribution vanishes for L>>d. For currently available foams, having an average cell size in the 100-200 μm range, this means that pressure has to be kept at 1.0 Pa or below to completely eliminate the heat transfer by gas conduction. To ensure this condition the foam has to be properly evacuated and all gas sources deteriorating the vacuum during the panel life effectively compensated by a suitable adsorbent, or getter, having sufficient gas capacity and efficiency. As shown by Equations (1-5), the insulation properties of an open cell foam depend on a complicated interplay of different parameters such as the PU thermal conductivity, foam density, cell size and cell size distribution, cell morphology and anisotropy, which have to be optimised during the foam preparation to obtain the best trade-off among the three thermal conductivity contributions, λs, λr and λg. In this process, the mechanical properties of the foam cannot be neglected, being essential to ensure structural stability of the vacuum panel. In fact, after sealing, the vacuum panel is exposed to the hydrostatic load of the atmospheric pressure and has to withstand it for a long time, which can be 15-20 years or even more depending on the application. Dimensional stability tests carried out on open cell PU foam-filled vacuum panels showed that creep problems should not occur for properly prepared panels, provided the open cell foam preparation has been optimised [15]. The open cell PU cores are produced according to various preparation technologies, such as lamination and block foaming. In both cases, after the foam has been grown, a cutting operation is usually necessary to remove the outer skin of the material, which

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Advances in Urethane Science and Technology contains a high percentage of closed cells, and to slice the PU into slabs of the proper size and thickness. The removal of the outer skin may be a labour intensive process which produces waste, lowers the process yield and increases the overall production costs, this issue being particularly important in the case of the lamination process. Great attention is therefore now being paid to design and the open cell foam production process, to reduce waste and increase productivity. A different approach to making open cell foam slabs has been proposed recently [11], which is based on the use of the PU fluff obtained from the recycling of used refrigerators (Recycled Urethane Fluff, or RUF panel). During the recycling process, the rigid insulation foam contained in the old refrigerators or freezers is mechanically ground into a fine powder, to completely separate and recover the CFC present in the foam. The resulting fluff, composed of completely open cells, is dried and introduced into a binding machine where it is sprayed with a certain amount of isocyanate (15-20% by weight) and thoroughly mixed. The blend is then transferred to a mould, heated at 120 °C and compressed at 0.5 MPa for 10 minutes to consolidate it and remove residues of the process. After de-moulding and curing, the RUF panel can be used as core material in a VIP. The fluff obtained from recycling one single refrigerator can be used to produce enough vacuum panels to insulate a new appliance, thus generating a virtual recycling loop. Due to the use of very fine compressed powder, the density of this open cell foam is three to four times higher than those produced by lamination or block foaming. Selected physical properties of some open cell PU foams are listed in Table 4.1.

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Table 4.1 Some physical properties of open cell PU foams Ref. [56]

Ref. [56]

Ref. [9]

Ref. [12]

Ref. [11]

Lamination

Block foam

-

Block foam

RUF

60-64

52-58

52

60-65

180-200

100

130-170

140

-

-

Perpendicular

-

-

75

-

-

Parallel

-

-

205

-

-

Thermal conductivity (mW/m-K) @ 5 Pa

7

5.5-7.5

4.5-5

7

8-10

Production process Density (kg/m3) Cell size(μm) Average

Compressive strength (MPa ) Average

0.25

0.23-0.35

0.26

-

-

Parallel

-

-

0.383

-

-

Perpendicular

-

-

0.133

-

-

Pre-treatment

120 °C x 30 min

120 °C x 30 min

140 °C x 1 hour

120 °C x 30 min

120 °C x 30 min

100

100

10 0

100

100

Open cell content (%)

4.3 Vacuum Issues in the Selection of VIP Components 4.3.1 Vacuum Properties of the Open Cell Foams To fully exploit the insulating performances of the open cell PU foams, pressure in the panel has to be kept preferably below 1.0 Pa during its life. To achieve this demanding target, the foam must be 100% open celled and with a very low outgassing rate. In spite of the increasing use of open cell PU foam, data from the literature on its vacuum properties are quite scarce. To estimate the outgassing contribution, specific tests can be carried out using high vacuum benches equipped with a quadrupole mass spectrometer.

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Figure 4.2 Scheme of the stainless steel high vacuum bakeable bench equipped with mass spectrometer for outgassing tests on vacuum components. IG: ionisation gauge.

To quantify the relatively small amounts of gas species released from the sample, it is necessary to use very clean, bakeable, high or ultra-high vacuum systems made with stainless steel and/or glass components. This ensures better leak-tightness and a negligible gas emission from the bench. A sketch of a typical experimental apparatus, having two separate pumping groups and base pressure at 1 x 10-6 Pa or lower, is shown in Figure 4.2. A practical technological implementation of the scheme shown in Figure 4.2 is illustrated in Figure 4.3. The sample to be analysed is mounted in a glass bulb which is connected to the test bench and evacuated for 10 minutes with a turbo and a rotary pump to a final pressure of 10-4 Pa. The bulb is then isolated from the pumps and the total pressure increase, due to the sample outgassing, is monitored with a capacitance manometer for some days. The use of the capacitance manometer avoids changes in the gas composition which might occur if a hot filament pressure gauge is used. From time to time a small amount of gas is sampled from the test volume, through valves V4 and V6, and passed to the mass spectrometer for partial pressure measurement. Before running the outgassing tests

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Figure 4.3 A stainless steel outgassing bench equipped with a quadrupole mass spectrometer and an external oven for vacuum bake-out (Reproduced by courtesy of SAES Getters SpA, Italy).

on the PU sample, a blank run has to be carried out. The result of the blank run is then subtracted from the outgassing test to get the actual gas emission from the PU sample. The use of two distinct pumping groups is essential to minimise the system contamination (mainly water vapour) which occurs when the bulb is opened to the air, to mount the specimen. In fact, during this operation, valve, V4, is kept closed, reducing the surface area of the bench which can absorb water and atmospheric gases. Before being used in a vacuum panel, the open cell PU foam needs a preliminary heat treatment in air, generally carried out at 120-150 °C for 10-60 minutes to remove water and other volatile species which otherwise would desorb and rapidly cause the vacuum to deteriorate. The result of a typical outgassing test carried out at 23 °C on a foam sample baked at 120 °C for 30 minutes is shown in Figure 4.4 for all desorbed gases but water. Water is difficult to quantify since it sticks to the walls of the system and only partially reaches the mass spectrometer. Water can be estimated as the difference between the total absolute pressure and the sum of the partial pressures of the other gas species, which can be accurately quantified with the mass spectrometer.

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Figure 4.4 Outgassing rate for an open cell PU foam. Water is not shown.

For each gas species, the amount released by the sample at a given time is obtained by simply multiplying the test volume (V) by the gas partial pressure (qi(t) = Pi V, qi(t) is the outgassing rate (Pa-l/h) at time t, Pi being the measured i-th partial pressure). The experimentally determined qi value for each gas species can be interpolated over time according to the semi-empirical law quoted in the literature [16]:

q i (t) = q0i (t − αi )

(6)

where qi(t) and q0i are the outgassing rate (Pa-l/h) at time t (hour) and the desorbed amount (Pa-l) after 1 hour of the i-th gas species, respectively. The dimensionless parameter αI is related to the desorption mechanism of the i-th gas species, its value usually ranging from 0.5 to 1, depending on the gas species and the material considered [16, 17]. Integration over time of Equation (6), provides an estimate of the PU sample outgassing load Qi for each gas species after a given time. As an example, the estimated gas released after 1 to 20 years for the sample of Figure 4.4 and for a 50 x 50 x 2 cm3 panel size is given in Table 4.2. The main gases, besides water, are carbon dioxide, nitrogen, carbon monoxide and hydrogen. It is interesting to note that, given the nature of the desorption

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Table 4.2 Outgassing parameters and estimated gas load Qi (Pa-l) after 1, 5 and 20 years for a 50 x 50 x 2 cm VIP Gas

q0i (Pa-l/h)

αi

Qi (1) (Pa-l)

Qi (5) (Pa-l)

Qi (20) (Pa-l)

CO2

8 x 10-3

1

22

25

27

CO

2 x 10-3

1

54

64

75

N2

9 x 10-3

1

10.8

12.8

15

H2

-5

0.5

2.3

6.2

13

5 x 10

process (Equation (1)), most of the gas is given off during the initial period of operation of a VIP. Results shown in Figure 4.4 and Table 4.2 can vary from sample to sample depending on the open cell PU foam preparation technique, its microstructure and density and the pre-treatment. Since desorption is a thermally activated process [18], the outgassing rates increase as the temperature increases. The outgassing contribution has therefore to be carefully evaluated in all those applications where the vacuum panels operate continuously at temperatures higher than room temperature, e.g., 60-80 °C, or have to withstand high temperature peaks, for example 100 °C, even for a relatively short period of time. Examples of such applications are presented and discussed in Section 4.6. An additional factor which may have an impact on the vacuum properties is the closed cell content of the PU foam. All the open cell PU foams so far presented in the literature for VIP applications are quoted to be 100% open cell based on pycnometric measurement. On the other hand, the accuracy of this test method is generally close to ± 0.2% [15], so that the possibility of a small fraction of closed cells cannot be completely ruled out and has to be considered as an additional potential source of pressure build-up in the panel. Closed cells might be present due to a non optimal foam preparation or to the incomplete skin removal.

4.3.2 Vacuum Properties of the Barrier Film The barrier film plays an important role in vacuum panel technology since it has the task of minimising air and moisture penetration into the vacuum core. The barrier must be

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Advances in Urethane Science and Technology durable and able to resist to puncturing and abrasion. It must be functional over a wide temperature range and retain its physical properties, such as dimensional stability, flexibility and sealability for years. Thermal conductivity of the skin should also be very low to minimise the heat transfer through the panel edges which can partially reduce the insulation efficiency of the evacuated panel (‘edge effect’, see Section 3.2.1). These properties have to be coupled with very low gas permeation and outgassing rates. Gas permeation through the barrier envelope is one of the most important factors responsible for the pressure increase in a panel during its life. Depending on the structure of the barrier film and the materials used, gas ingress can preferentially take place through the whole surface of the barrier film (permeation through the surface) or through the heat-sealed plastic VIP flanges (permeation at the edges), or both.

4.3.2.1 Gas Permeation Through the Surface Several types of barrier materials, having different structures and gas transmission rates, are commercially available from the food, packaging and electronic industry. Gas barrier requirements for vacuum panels are however much more demanding. Composite plastic films, obtained by laminating several polymeric sheets have been proposed in the past for precipitated silica-filled panels, where pressures as high as 5001000 Pa can be tolerated without excessive degradation of the thermal conductivity. In the case of open cell PU-filled panels, requiring much lower pressure values, this barrier structure is generally inadequate, if a lifetime exceeding one to two years is needed. To improve the permeation properties, barrier materials, such as aluminium and silicon oxide, can be vacuum deposited on the polymer surface in the form of a thin film (0.010.05 μm thick) [19]. However, most commercially available metallised samples do not provide enough barrier properties for long term applications, mainly due to water transmission through the large density of pin holes and micro-defects present in the sputtered aluminium layers [20]. To overcome this problem, barrier films obtained by laminating a thicker continuous metal foil with various polymeric sheets, such as polyethylene terephthalate (PET), Nylon and polyester (PE) have been proposed to the appliance industry some years ago and are now commercially available [15, 21]. Aluminium is the preferred choice for

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… the metal, mainly due to its good workability, which allows production and lamination of virtually pin hole-free foils having thickness of 6 μm or less [21]. The presence of the aluminium foil dramatically improves the gas barrier property of the film, well beyond the sensitivity limits of the analytical techniques commercially available for the measurement of gas permeation. In the case of water, which is one of the most important permeating species, two standard methodologies are at present widely used to measure its transmission rate through a material, as described by ASTM E96-00 [22]. According to the first procedure (‘the desiccant method’) the barrier sample is sealed to the open mouth of a test plate, which is cylindrical in shape, in which the desiccant is placed, and the assembly kept in a humidity and temperature controlled environment. The permeation of water through the sample is measured by the weight increase of the assembly. In the second procedure (‘water method’) the test plate contains water and the transmission rate through the barrier sample into the controlled environment is measured by the weight decrease of the assembly. A variation of the first method, frequently used in practice, is based on periodically weighing vacuum panels, containing of desiccant, and kept in a given temperature and humidity-controlled environment. Another very common technique is based on the use of an infrared sensor as described by ASTM F1249-90 [23]. The barrier sample is sealed between a dry and a wet chamber kept at known temperature and relative humidity. The two chambers make up a diffusion cell which is placed in a test station where the dry chamber and the top of the barrier are flushed with dry air. Water vapour which penetrates through the barrier film blends with the dry air and is transported to a pressure-modulated infrared sensor. The infrared radiation is absorbed by the water molecules and the sensor produces an electrical signal which is proportional to the concentration of water in the gas phase. The intensity of the signal is then compared with the signal generated by measurement of a sample having a known water transmission rate. This allows the determination of the water transmission rate through the test specimen. The detection limit for water transmission generally quoted for commercially available instruments based on this principle is in the 0.01 g/m2 day range (at 90% relative humidity and 23 °C). These techniques are generally more than adequate for many plastic or standard quality metallised films but can hardly discriminate between samples containing aluminium foil, which generally have water transmission rates much lower than this value. To better estimate gas permeation for high quality barriers a novel technique has been developed [24, 25], which is based on the measurement with a quadrupole mass spectrometer of the helium transmission rate through the sample.

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Figure 4.5 Scheme of the experimental bench for the helium permeation test.

A sketch of the experimental bench is shown in Figure 4.5. A picture of the bench is shown in Figure 4.6. The bench is a stainless steel ultra-high vacuum apparatus equipped with rotary, turbomolecular and non-evaporable getter (NEG) pumps. The sample, a circular coupon of about 30 cm2 area, is mounted between two flanges and the helium pressure is applied (generally in the 0.0001-0.1 MPa range) on one side of the sample, the other side being in view of the quadrupole mass spectrometer. A variable conductance C (l/s) is mounted between the mass spectrometer and the evacuation group, so that the helium flow rate F (Pa-l/s) through the conductance can be measured with the mass spectrometer according to the equation:

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Figure 4.6 High vacuum bench for helium permeation tests (Reproduced by courtesy of SAES Getters SpA, Italy).

F = C (P1 - P2)

(7)

where P1 and P2 are the pressures before and after the conductance C, respectively. Under equilibrium conditions, which can be reached after some minutes or some hours, depending on the nature of the sample, the gas flow, F, provides the helium transmission rate. A typical graph showing the achievement of steady state conditions is given in Figure 4.7.

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Figure 4.7 Achievement of the equilibrium condition in a helium permeation test for a laminated film composed of 15 μm Nylon, 12 μm PET, 6 μm aluminium, 50 μm high density polyethylene (HDPE) (or Nylon 15 μm/PET 12 μm/Al 6 μm/HDPE 50 μm).

The use of the mass spectrometer ensures very high sensitivity for helium, better than 10-11 Pa-l/(s m2 Pa), mainly due to the negligible presence of helium as a constituent of the gas background. Due to its high sensitivity and the relatively short measuring time, this technique can be used effectively to support the development of improved laminates and also for quality control in a production environment. The estimation of the transmission rates for gases other than helium can also be obtained, provided a preliminary calibration is carried out. This is particularly important for water, which is the main gas permeating through the barrier skin. To run the calibration procedure, the helium transmission rate is measured for various samples having different and known permeation rates for water. The linear correlation between helium and water transmission rates is then established, as shown in Figure 4.8. The correlation factor for water was found to be close to 500 for most of the PE/PET/ Nylon-based films analysed by the authors, either metallised or incorporating an aluminium foil [26]. It was then possible to estimate the order of magnitude of the water

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Figure 4.8 Experimental determination of the correlation factor between helium and water transmission rates.

transmission rate for an unknown low permeation rate sample by simply measuring the helium transmission rate and multiplying it by 500. Table 4.3 provides a comparison between the helium permeation rates in two films, a laminate incorporating a 6 μm aluminium foil, PET 12 μm/Aluminium 6 μm/HDPE 50 μm (Film B) and a multi-layered barrier composed of four aluminium-sputtered PET sheets laminated onto a 50 μm PE (Film A). Tests were run at 24 °C. From the measurement of

Table 4.3 Helium and water permeation rates through films Sample

Permeation rate (MPa s-1cm-2 Pa-1)

Estimated water amount in a VIP after 15 years

helium

water

Film A

910 x 10-12

460 x 10-9

100 g

Film B

-15

-13

0.02 g

130 x 10

670 x 10

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Table 4.4 Thermal conductivity of some materials used as gas barriers and/or structural layers in barrier films. Material

Thermal conductivity (μ /m-K)

Typical thickness (μm)

PET

0.15

10-15

HDPE

1.05

40-60

PVDC

0.13

10-20

Nylon

0.43

10-20

Aluminium

273

6-7

PVDC: Polyvinylidene chloride

the permeation rates, the quantity of water permeating in 15 years in a 50 x 50 x 2 cm3 vacuum panel has been estimated. The metallised barrier here considered cannot be used for long term applications, where 10-15 years or more are targeted, since the amount of desiccant necessary to compensate for the water transmission would be too large. A much lower permeation rate is provided by Film B, which is therefore well suited for long term applications. In spite of the extremely good gas barrier properties, VIP prepared with such a film suffers from an intrinsic limitation, i.e., the high thermal conductivity of the aluminium foil, as shown in Table 4.4. A fraction of the heat flow is, in fact, transferred from the hot to the cold panel surface by the aluminium foil through the panel flanges, rather than through the core material (so called ‘edge effect’). As a result, the average insulation value of the panel is less than the expected value based on the actual insulating properties of the core material (centre of the panel thermal conductivity), this difference being more remarkable the smaller the panel size. The edge effect can be evaluated, as a function of the panel size, the laminate structure and the aluminium thickness, by numerical methods, such as the Finite Element Analysis (FEA). Results of such an analysis are given in one specific example in Figure 4.9, where the average thermal conductivity of a vacuum panel using a PET 12 μm/Aluminium 6 μm/ HDPE 50 μm laminate is plotted against the panel area.

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Figure 4.9 Thermal conductivity versus panel area.

The thermal conductivity and the thickness of the PU core material considered here are 6 mW/m-K and 2 cm, respectively. Figure 4.9 shows that the panel thermal conductivity approaches the value of the core material only for a sufficiently large panel area (≈ 1-10 m2). For very small panels, the insulating properties of the core material are spoiled by the edge effect and the energy savings in the real application may be minimal. For this reason, efforts are now being made to improve the aluminium–based laminates and/or to develop products not containing the aluminium foil but still having sufficiently good gas barrier properties, so as to achieve an acceptable trade-off between energy saving performances and acceptable lifetime. This challenging objective can be addressed by selecting and properly combining together suitable polymers having improved gas barrier properties. A typical example of this process, recently reported by Lamb and Zeiler [27], is given in Table 4.5, which shows how significant improvements can be obtained in non aluminiumfoil containing skins working on the film structure.

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Table 4.5 Oxygen transmission (OTR) and water (WTR) transmission rates for some film structures [27] OTR x 10-10 m2 day (STP)

WTR g/m2 day

PET

34.8

43.4

PET + PVDC

2.3

9.3

PET + PVDC + metallised layer

0.04

0.62

0.00015

< 0.15

Film structure

Mylar 200RSBL300 STP: Standard, temperature, pressure

A comparison of some different types of improved metallised and aluminium foilcontaining barriers, also including a preliminary investigation of the effect of temperature on the gas transmission rates, has been recently reported by Bonekamp [28]. Due to the still developmental nature of most of these barriers, more work is necessary to assess their usability for long term applications. Extensive characterisation of the mechanical, sealability and stress resistance properties are also important issues to consider.

4.3.2.2 Gas Permeation Through the Flanges The permeation of atmospheric gases through the polymeric sealed flange of a vacuum panel can be an important contribution to the pressure increase in a VIP for long term applications. The gas permeation rate depends on the pressure gradient across the vacuum panel, the type of polymer used as a sealant, the flange geometrical parameters (exposed surface and width) and temperature. Polyethylene is at present one of the preferred sealing materials due to the good trade-off achieved among sealability, mechanical properties, gas permeation, outgassing rates, reliability and cost. Gas permeability values quoted in the literature for PE are scattered over an order of magnitude, depending on the actual density of the sample and the gas considered [29, 30, 31]. Typical permeability values corresponding to medium and high density PE are shown in Table 4.6. Results of the calculation for argon, nitrogen, oxygen and water and the total pressure, are given in Figure 4.10 and show the role of permeation in deteriorating the vacuum level in the panel. The width and height of the PE flange here considered are 10 and 0.1 mm, respectively and the environment is air at 21 °C and 50% relative humidity.

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Table 4.6. Permeation rates for gases (cm3/m2d, STP) and water (g/m2d) in a 25 μm thick PE sheet Gas

MDPE x 10-10

HDPE x 10-10

Temperature °C

N2

387

96.75

21

O2

1548

425.7

21

Ar

3096

774

21

H2O

10.85

4.65

38, 90% RH

MDPE: medium density PE; RH: relative humidity

Figure 4.10 Pressure increase due to gas permeation at the panel PE flanges.

In the case of panels which are encapsulated, i.e., surrounded by a closed cell PU foam, as happens for panels in household refrigerators, freezers or vending machines (see Section 4.6), the picture is more complicated. The gas environment surrounding the vacuum panel will depend on the type of encapsulating foam and will change with time as a consequence of the gas out-diffusion from the closed cells and the progressive air penetration from the outside, thus making predictions more difficult. This ageing process

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Advances in Urethane Science and Technology will also depend on the temperature and the hermeticity of the case surrounding the encapsulating foam as shown in various papers [32, 33]. Permeation of blowing agents through the seals, even though not a major contribution, has also to be considered. The use of a different PE grade or a different sealant polymer will provide different pressure build-up curves in the panel both in terms of total and partial gas pressures. High barrier polymers, such as polyvinylidene chloride (PVDC), polyacrylonitrile (PAN), polyester, acid copolymers and ionomers have been proposed and are under evaluation as a replacement for PE. However, they present some drawbacks in terms of sealability and/or mechanical properties and/or outgassing rates. Cost of these polymers is also generally higher.

4.3.2.3 Outgassing Properties of the Film In general, very little data have been published on the outgassing properties of the skins for vacuum panels, even though they can contribute, in some cases, in a non-negligible way to the deterioration of the pressure inside the VIP. This can be due to the outgassing properties of the materials used as barrier layers and/or the lamination process, which may introduce volatile substances or trap gases in between the various sheets. The outgassing properties of a barrier sample can be measured using the same experimental equipment shown in Figure 4.2 and have to be taken into account to estimate the total gas load in the panel. For the same film having the barrier property in Table 4.3, the extrapolation of the outgassing experimental data to 20 years in a 50 x 50 x 2 cm3 size panel is shown in Table 4.7. Water, which is also given off by the barrier, was not quantified in this test.

Table 4.7 Outgassing parameters and gas load for a barrier incorporating a 6 μm aluminium foil Gas

q0 (Pa-l/h)

α

Q (Pa-l)

H2

35 x 10-4

0.5

29

N2

-1

1.3

133

178

43 x 10

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4.3.3 The Getter Device As discussed in the previous sections, gas desorption from the surfaces exposed to the vacuum (filler and barrier) and gas permeation through the bag contribute to increase the pressure in a VIP during its life. These contributions, as well as the residual gases left in the panel after the exhaust and seal-off process, have to be taken into account to provide an accurate estimation of the pressure increase in a VIP as a function of time. Figure 4.11 shows the total pressure increase, not including water, for some panel sizes, when data from Tables 4.2, 4.3, 4.6 and 4.7 are used.

Figure 4.11 Estimated total pressure increase in some open cell foam-filled VIP as a function of time. The additional pressure increase due to water vapour is not considered in this calculation.

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Advances in Urethane Science and Technology For given temperature and environmental conditions, the pressure build-up depends on the specific size of the panel, and in particular on the ratio between the perimeter and the thickness, the lower the latter the higher the pressure increase. A barrier film incorporating an aluminium foil and a panel seal-off pressure of 5 Pa have been here considered. In this example, water has been assumed to be completely absorbed by a proper amount of desiccant and not to contribute to the pressure increase. In the case of a typical 50 x 50 x 2 cm3 size VIP, the pressure build-up exceeds 100 Pa after a few years and even not considering water, the total pressure after 20 years exceeds 250 Pa. The deterioration of the thermal conductivity for such a panel, a direct consequence of the increase of the internal pressure, is shown in Figure 4.12, for a PU foam following the curve of Figure 4.1. Due to the nature of the thermal conductivity versus pressure curve, the deterioration of the λ factor is negligible at the beginning but increases steadily with time.

Figure 4.12 Increase of the thermal conductivity in a vacuum following the pressure build-up as per Figure 4.11. Water is supposed to be absorbed by a desiccant and not contribute to the pressure rise.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… In some applications, such as in domestic refrigerators and freezers, an additional gas burden may be also generated in the panel during the appliance manufacturing process or by the appliance operating conditions, which may even accelerate the deterioration of the thermal conductivity. These examples show that for medium and long term applications (> 2-3 years) a simple desiccant is not enough and specific getter materials have to be added to sorb the extra amount of gases generated in the panel. The wide spectrum of gases present in a panel, including carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen and water, as well as traces of blowing agents, also calls for an absorption system having high sorption capacities and adequate pumping speed for all these gases. Zirconium-based NEG alloys, which are widely used in a variety of vacuum applications [34, 35, 36, 37] cannot be used in plastic evacuated panels due to their limited sorption capacity at room temperature and the need to be heat activated at relatively high temperature (> 350 °C) prior to their use, this process being clearly not compatible with the panel polymeric components. Very large area physical adsorbents, like molecular sieves, zeolite or activated charcoal [38, 39] have very good efficiency for water and some organics but present serious limitations in sorbing carbon monoxide, hydrogen, nitrogen and oxygen at the temperature and pressure conditions typically encountered in VIP applications. Therefore, they have beneficial effects during the very initial life of the VIP without being able to ensure long lifetime, as required, for example, by the appliance industry. They are also sensitive to the sorption temperature, i.e., the higher the temperature the lower the sorption performance, which spoils sorption performances in high temperature applications, e.g., 60-100 °C. Physical adsorbents have also a second drawback which is related to their treatment, when mass production quantities have to be handled. To fully take advantage of their sorption capacity, in fact, a relatively high temperature pre-treatment process which cleans the surface by promoting gas desorption is required. However, after this treatment, the adsorbents are exposed again to the ambient environmental conditions during the subsequent handling and mounting operations. Due to their reactivity, they will rapidly sorb some gases from the environment and this will reduce their capacity in a poorly reproducible way, critically dependent on the environmental conditions, temperature and exposure time. Fluctuations in their sorption performances as well as in the VIP insulating efficiency could be the final result. To eliminate the above problems and address the lifetime issues posed by the appliance industry, a novel getter device, the COMBOGETTER, has been recently proposed [40]. 181

Advances in Urethane Science and Technology At the heart of this device is a barium-lithium alloy, in a 1 to 4 atomic ratio [41], able to efficiently chemically absorb a large amount of nitrogen at room temperature, up to more than 2500 Pa-l (N2)/g (alloy) [42]. This very specific feature allows the getter to compensate for the air inlet coming from permeation, thus ensuring long lifetime requirements. High efficiency calcium oxide and a metal oxide are also added to barium-lithium to absorb moisture, hydrogen and some of the most common blowing agents, such as R141 b and cyclopentane which could permeate through the VIP during its life. All these materials are prepared according to proprietary processes, which confer upon them unique properties in terms of sorption efficiency and capacity. The active powders are compressed in a stainless steel cup according to a configuration which allows optimum sorption performances and ease of use (Figure 4.13). The getter device does not need to be heat activated or pre-treated before being used in VIP and can be handled in air for a reasonable period of time (several minutes), during the panel manufacturing process, without affecting its sorption capacity. Mounting can be accomplished by inserting the getter in a recess cut in the filling material. Alternatively, if the filler is sufficiently soft, as is the case for open cell PU, the getter can be simply pressed into it.

Figure 4.13 Picture of the COMBOGETTER device for VIP.

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Table 4.8 Gas sorption capacity of the COMBOGETTER Gas type

Gas capacity, Pa-1

Air

530

CO/CO2

800

H2

9300

H2O

80000

Blowing Agents

130

Gas capacity data for the getter are summarised in Table 4.8. A typical sorption curve for nitrogen at room temperature is given in Figure 4.14 (sorption throughput, Pa-l/s, as a function of the sorbed amount, Pa-l).

Figure 4.14 Sorption curve for nitrogen for the COMBOGETTER (at room temperature)

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Figure 4.15 Sorption curve for nitrogen for the COMBOGETTER (in the 20-70 °C range)

This throughput is adequate to compensate for the relatively slow gas inlet rate occurring during the VIP lifetime. The effect of the temperature on the sorption performance is shown in Figure 4.15 in the range 20-70 °C. These data show that the getter is effective also at relatively high temperatures, where physical adsorbents become less efficient. The getter in VIP technology has several roles, as demonstrated by the following examples: a) Life insurance Permeating and outgassed species (carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen, water) which can significantly increase the pressure are irreversibly fixed by the getter, as shown by tests of actual panels (Figure 4.16). Pressure in VIP has been recorded by means of a viscous pressure gauge, as explained in Section 4.4.2.2. The resulting thermal conductivity of gettered panels is therefore much lower in the long term, as shown in Figure 4.17, which compares the λ factor in gettered and ungettered panels after almost 3 years of ageing.

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Figure 4.16 Measurement of the pressure in two vacuum panels (with and without getter)

Figure 4.17 Measurement of the thermal conductivity in two panels (with and without COMBOGETTER) aged at room temperature

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Advances in Urethane Science and Technology b) VIP manufacturing process The ability of the getter to sorb air can be also used to reduce the evacuation time in panels, this contribution being particularly interesting for medium and large size panels, e.g., > 0.5 m2, where achieving base pressure in the low 1 Pa can be time consuming for open cell foams with very small cell openings, which limit the evacuation efficiency. For each application, a trade-off between the getter capacity needed to reduce the evacuation time and that necessary to ensure long lasting service performance has to be found. A typical example of the getter’s ability to act as a chemical ‘in situ’ pump to sorb residual gases after the evacuation process is shown in Figure 4.18, which compares the pressure trend in panels with and without the getter immediately after the evacuation and seal-off.

Figure 4.18 Pressure decrease in a VIP due to the in situ pumping effect of the COMBOGETTER.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… c) Process yield and heavy duty conditions The COMBOGETTER normalises the vacuum level in the production cycle by compensating for fluctuations in the quality of the VIP components and the manufacturing process. It can also compensate for the gases generated in the panel during ageing or heat treatment, as happens during storage in the warehouse, the foaming process in a refrigerator cabinet or the high temperature operating conditions required by several specific applications. Experimental data on vacuum panel performances under these conditions are presented and discussed in Section 4.6. For some specific VIP sizes and short term applications (for example one to two years) a getter for air may not be necessary, since the permeation contribution is less relevant here. In these cases water is usually the most important gas to be sorbed and the use of a simple dryer is sufficient to keep the pressure at the required 10 Pa value. Calcium oxide is one of the preferred desiccants due to its availability, low cost, good environmental features and large water sorption capacity. The production process influences the calcium oxide physical structure (particle size, porosity and morphology) and thus the water sorption efficiency may be actually an important parameter to consider in short-term applications (for example in shipping containers, where lifetimes as short as a few months or less, can be of interest). A comparison between a commercially available product and a highly efficient calcium oxide, prepared by SAES according to a proprietary process is given in Figure 4.19. Also physical absorbents, like molecular sieves and silica gel or activated carbon, can be used for short-term applications.

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Figure 4.19 Water sorption test on two calcium oxide samples. Samples were exposed at 100-200 Pa water partial pressure at 23 °C. Sample A was prepared by SAES according to a proprietary process to increase material surface area and porosity.

4.4 Vacuum Panel Manufacturing Process and Characterisation 4.4.1 Some Manufacturing Issues Before being inserted in the barrier bag, the open cell foam needs preliminary pretreatment in air to remove water and the residue of the foam production process. Volatile materials are in fact used to blow the foam chemicals and remain partially trapped in the foam matrix or condense as a liquid on the foam surface after the cells have been opened. Even though part of these volatile substances will desorb at room temperature, the remaining quantity will generate an unacceptable pressure build-up in the panel [43]. The baking process is also necessary to remove the water absorbed by the PU foam when exposed to humid air (about 1.8 % by weight, at 23 °C, 50% relative humidity), this being particularly effective to reduce the evacuation time, as shown by tests of actual panels [44]. Pre-treatments generally range from 150 °C for 10 minutes to 120 °C for up to a few hours [9, 12, 43], depending on the foam type.

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Figure 4.20 Water re-absorption curve of a small PU sample after baking.

After the foam has been thoroughly baked, it is important to reduce its exposure time to the working environment to avoid significant moisture re-absorption, which takes place when PU foam cools down. As an example, the weight increase due to water re-absorption during cooling to room temperature is shown in Figure 4.20 for a small PU sample, dried at 150 °C for 20 minutes and then exposed to 50% relative humidity. These data indicate the need to design the production process and equipment in such a way that the foam is exposed to air for a very short time, not exceeding a few minutes. A limited exposure is also beneficial to reduce the evacuation time of the panel, which strongly depends on the desorption rate of physisorbed water. To further minimise reabsorption of water on the foam it is advisable to process the PU in a dry or humiditycontrolled area. The water content picked-up by the foam can be measured in the finished panel by means of residual gas analysis (RGA) with the mass spectrometer, as described in Section 4.4.2.2. In general, the proper pre-treatment of the foam followed by adequate handling ensures better initial insulation performance and shorter evacuation cycles which also mean higher productivity and lower costs per panel. The open cell foam slabs are generally easier to handle and process than powder-based core materials like precipitated silica or perlite. However, precautions have to be adopted

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Advances in Urethane Science and Technology during handling of the foam to avoid powder generation which could have an impact on the quality and reliability of the seals. Care also has to be taken during the production process to ensure adequate VIP flatness, which is an important factor for the subsequent proper mounting of the panel in various applications. Appropriate film handling and processing is also the key to produce reliable and good quality panels. Gas barrier properties of films, in fact, mainly depend on the number of defects, such as cracks and pinholes, present in the layered structure. This number increases as a consequence of the mechanical stresses the film undergoes during its handling. Moreover, the high load applied by the atmospheric pressure on the vacuum panel, this value being particularly high in the corners and along the edges, stretches the layers and locally increases the defect density. A preliminary investigation of these effects has been carried out by Sugiyama and coauthors [21] for some laminated plastic films incorporating a 6 μm aluminium foil. Films have been analysed as such and after having been twisted to simulate handling conditions. VIP have also been prepared and samples cut from the flat surface of the panel, the edges and the corners to check for any local increase of the permeation rate. Tests were carried out at 23 °C on samples of 2.5 x 10-3 m2 area applying 0.0505 MPa helium pressure. The structure of the analysed films was Nylon 15 μm/vacuum metallised PET 12 μm/aluminium 6 μm/HDPE 50 μm. The results, shown in Table 4.9 clearly indicate that the twisting process is responsible for significantly larger permeation rates. Samples cut at the corners also show higher permeation rates, which confirms that stress conditions in these areas increase the local pin-hole density in the aluminium foil.

Table 4.9 Helium permeation rates through barrier films submitted to different mechanical treatments Sample number

Type of sample

He permeation rate (Pa-l/s) x 10-11

1

Reference film (no treatment)

<7

2

Sample cut from the flat surface of the vacuum panel

7

3

Sample cut from the corner of the vacuum panel

450

4

Sample mechanically twisted

230

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… Even though the total permeation rate is still acceptable, data suggests that suitable precautions have to be adopted to limit as much as possible stress and film deterioration. Similar behaviour is also expected for metallised films and has to be addressed accordingly.

4.4.2 Characterisation of Vacuum Panels The rapid development of the open cell foam-filled vacuum panel technology has required parallel development and improvements of the analytical techniques necessary to assess VIP performance and reliability. This latter aspect is key for the widespread adoption of the technology. The selection of the best components, foam, bag and adsorbent, as well as the careful control of the manufacturing cycle, minimises the chance of having poorly performing insulating panels. However, since the potential risk of defective seals or microleaks cannot be completely ruled out, several techniques have been developed to either support and establish the VIP manufacturing cycle or to assess their quality after production. In the following sections, some techniques to check the insulating performance of vacuum panels are illustrated, a few of them also being able to be used as a tool for Quality Assurance (QA) and Quality Control (QC) for the manufacturing process.

4.4.2.1 Measurement of the VIP Thermal Conductivity The direct measurement of the thermal conductivity or λ factor is one of the most common ways to check the quality of a vacuum panel. Several test methods are available, such as the heat flow meter [45], guarded hot-plate and guarded-calibrated hot box [46, 47] procedures, which measure the heat transferred under steady state conditions through the sample whose surfaces are kept at two different given temperatures. The last two techniques are absolute methods since they do not require the use of reference or calibration specimen. However they are relatively expensive and time-consuming and difficult to adapt to rapid testing. In the heat flow meter technique, the sample is placed between two plates controlled at different temperatures. Thermocouples placed just below the plate surfaces measure the temperature drop across the sample. One (or more) heat flow transducers mounted on the plate measures the voltage which is proportional to the heat flow through the sample. Knowing the relationship between the voltage signal of the transducer and the heat flow through it, which can be obtained by calibrating the transducer to a known thermal conductivity standard, the thermal conductivity for the specimen can be obtained.

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Advances in Urethane Science and Technology Heat flow meter test results generally agree well with those generated by absolute methods. The technique, which is easier to use and is a universally recognised standard test procedure [45], is also usable either for high accuracy measurements to support VIP analytical modelling/development or for fast tests for QA/QC programs. This latter requirement is becoming more and more important as VIP technology moves closer to the market. Recent improvements have been based on the use of a novel dual heat flow transducer design and a tight control of plate temperatures [48]. They allow a pass/fail test on a vacuum panel to be completed within a few minutes, thus providing the possibility for an extensive quality check during the manufacturing cycle. This is particularly important not only for VIP manufacturers but also for the final users, to verify continued panel integrity after handling and transportation and before mounting. Another relatively novel development in thermal conductivity measurement equipment is based on a transient modified hot wire (MHW) heat reflectance technique [49]. In this technique, heat is generated by a constant electrical current in an element that has one side in contact with the surface of the specimen and the other side in contact with a backing material. The temperature at the interface is measured and the rate of temperature rise is related to the thermal conductivity of the sample. Quantitative data can be obtained by calibrating the instrument with known standards in a given environment (pre-set temperature and relative humidity). Preliminary investigations by Dixon and Mathis [50] suggest that the MHW method has the potential to discriminate between panels having different vacuum levels, at least in the 100 and 1000 Pa range. Thanks to the short measuring time (~5 minutes are required between two measurements), this technique also seems promising as a pass/fail test for QA/QC applications.

4.4.2.2 Measurement of the Pressure Since the thermal performance of the panel is dependent on its internal pressure, an alternative approach is to measure the pressure evolution as a function of time. The spinning rotor gauge (SRG) technique [51], which is based on the measurement of the deceleration rate caused by gas friction on a freely spinning spherical metallic ball, is particularly well suited to monitor the pressure in sealed-off devices and has been proposed in the recent years also for vacuum panel applications [5, 40, 52, 53]. The ball, or rotor, is suspended by magnetic forces between two mutually attracting permanent magnets and additional coils provide the rotating magnetic field to spin it.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… After the rotor has been accelerated and has achieved constant speed, the powering coils are switched off, allowing the rotor to slow down. A correlation between the deceleration rate and the pressure can be drawn, provided the density and the radius of the rotor and the mean velocity of the gas molecules impinging the rotor surface is known. Positive features of this technique are the wide pressure range covered, from 10-4 to 105 Pa, the low-cost sensor and the possibility of operating several sensors with a single electronic unit. The sensor can be connected to the VIP using a high vacuum compatible resin, as shown in Figure 4.21. In the advanced SRG models, such as the SAES Getters SpiroTorr unit [54], the instrument can operate basically at any spatial orientation on both the sensor and the gauge head.

Figure 4.21 Diagram of the VIP mounting the spinning rotor gauge. Vacuum tightness is ensured by a high vacuum epoxy resin.

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Advances in Urethane Science and Technology In particular, by mounting the sensor horizontally in the panel it is then possible to combine the pressure readings with thermal conductivity measurements on the same specimen. The measuring equipment is shown in Figure 4.22. Pressure readings are not very much dependent on the gas type for pressure in the range or below 10 Pa, so that the precise knowledge of the gas composition is not strictly necessary to get a fairly accurate pressure measurement. For higher pressures, the ball (which is suspended by the magnetic field) deceleration rate becomes more dependent on the gas species and a precise reading requires the knowledge of the main gases involved [55]. Typical results obtained using the SRG are shown in Figure 4.16 and Figure 4.18.

Figure 4.22 Picture of the SAES Getters SpiroTorr SRG connected to a vacuum panel.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… Due to the need to mount the sensor, this technique cannot be used as a quality control method. Its role is more in the area of basic investigation and technology assessment. Another way to measure the pressure is by puncturing the panel and analysing the gas atmosphere with a mass spectrometer. The sample is connected to the analytical bench of Figure 4.3 by a glass tube which is sealed with a high vacuum epoxy resin, as in Figure 4.23. The tubing is then evacuated, the panel punctured, using for example a metallic hammer, and the internal atmosphere expanded to the mass spectrometer for the residual gas analysis. This approach allows measurement of both total and partial pressures in the panel, thus providing useful information on the gas ratios, this being useful as an R&D tool or to monitor and improve the manufacturing process. However, since this is a destructive and relatively time-consuming test it can only be used for one-off samples. A nondestructive technique to measure total pressure in a vacuum panel has been proposed recently, which is based on the use of a laser beam source coupled to a detection system.

Figure 4.23 Pictorial view of the method used to connect the analytical bench to the vacuum panel for RGA analysis.

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Advances in Urethane Science and Technology The panel is placed in a vacuum chamber and the laser beam impinging on the panel surface is reflected by the laminate and recorded by the detector. The chamber is then slowly pumped down. When the pressure in the chamber is lower than the internal pressure in the panel, the volume of the bag increases by gas expansion, thus causing the laminate surface to move and deflect the laser beam in a different direction. This will generate an electrical feedback signal which is recorded by the equipment as the internal pressure of the vacuum panel. Present sensitivity of such types of equipment, although not well established, is around some hundreds of Pa. It is likely that this value can be lowered by calibrating the laser sensor against an absolute pressure gauge. This measuring technique is quite fast (about one panel per minute) and can therefore be used as a QC/QA tool in a manufacturing line.

4.5 Insulation Performances of Open Cell PU-Filled Vacuum Panels The actual insulation efficiency of PU-based vacuum panels depends on the intrinsic core material conductivity, the vacuum level, the type of barrier envelope, the panel size and arrangement in the insulating structure. All these aspects have to be optimised to get full benefits from VIP technology. As previously discussed, laminates incorporating a continuous aluminium foil provide, at present, the best gas barrier properties for VIP. Their main drawback is the loss of insulation properties at the panel flanges due to the relatively high thermal conductivity of the aluminium foil which reduces the overall insulating performance, this effect being directly related to the panel size and the aluminium foil thickness. The influence of the edge effect on the insulating performance of the panel in actual applications can be better appreciated by introducing the ECR (Energy Consumption Reduction) factor: ECR = (Kpu – Kvip )/ Kpu

(8)

where Kpu and Kvip (W/m) are the thermal conductivity of the conventional closed cell foam and of the vacuum panel, respectively. The ECR factor provides the percentage increase in the insulation efficiency obtained by completely replacing the conventional foam with a vacuum panel having given thermal conductivity Kvip. Calculation of Kvip is not easy, since it is a complicated function of the size of the panel, its geometry, the thermal conductivity of the core material and the thermal conductivity of the laminate (which is basically the aluminium foil, due to its high thermal conductivity). The heat flow through the laminate and through the open cell foam, both necessary to get Kvip, can be calculated by FEA.

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Figure 4.24 ECR values as a function of the aluminium thickness for VIP of different sizes.

Once Kvip is known, the ECR factor is obtained by Equation (8). The result of the ECR calculation is shown in Figure 4.24 as a function of the aluminium thickness for some different panel sizes. It is clear from Figure 4.24 that high ECR values can only be obtained by reducing the aluminium thickness as much as possible and using relatively large vacuum panels, i.e., > 0.5 m per side. If the panel is encapsulated, as actually happens in several applications, the conventional foam surrounding it also has to be taken into account in the ECR calculation, since it may contribute to heat dissipations, its effect being particularly important around the VIP flanges. The ECR values for encapsulated panels of different sizes prepared with a laminate incorporating a 6 μm aluminium foil (Table 4.3, Film B) are given in Table 4.10. The calculation has been made assuming a foam thermal conductivity of 6 mW/m K and a thickness of 0.5 cm of additional PU foam (thermal conductivity of 20 mW/m K) for a total thickness of 0.025 m.

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Table 4.10 ECR values in encapsulated VIP of different sizes VIP size (cm)

ECR (%)

40 x 40 x 2

16.5

50 x 50 x 2

26.7

50 x 100 x 2

36.7

100 x 100 x 2

46.1

The ECR values achievable with the aluminium foil-based laminates are very interesting for large size panels, where the edge effect is not severe. For small panels, which are also important, since they can be easily adopted in a variety of applications, from household appliances to shipping containers, the improvement in the insulation efficiency is not as remarkable. The energy consumption reduction in two refrigerator cabinets, each using one panel, 100 x 50 x 2 cm3 large, covering 40% and 60% of the total surface is shown in Table 4.11. The same assumptions used in Table 4.10 have been made for the PU thermal conductivity and thickness of the barrier foil.

Table 4.11 ECR values for two refrigerators with varying VIP coverage VIP size (cm)

Refrigerator/freezer surface coverage

ECR

100 x 50 x 2

60%

22%

100 x 50 x 2

40%

14.7%

In general, for household appliances, depending on the surface coverage of the cabinet and the panel thickness, energy savings from 10% to 30% have been reported using open cell PU foam-filled panels packaged in a 6 μm aluminium foil-based barrier [12, 15, 52, 56].

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… The influence of the vacuum panel thickness to the total insulation thickness ratio as well as the change in performance, by increasing the foam λ value, has been determined by Hamilton [54]. In general, results can be optimised by adjusting the VIP size and thickness to provide the most cost effective option. Further improvements can be obtained by decreasing the aluminium foil thickness or using a metal foil-free barrier and/or decreasing the thermal conductivity of the foam. The latter can be achieved by careful control of the foam microstructure and in particular the cell shape, orientation and size, which play an essential role in determining the foam thermal conductivity and its dependence on gas pressure. Conductivity values close to 5 mW/m K have been quoted for properly prepared open cell foam samples [9].

4.6 Examples of VIP Applications and Related Issues 4.6.1 Household Appliances Refrigerators and freezers account for about 20% of the total electricity consumption of household appliances. For this reason the appliance industry is under pressure to improve the energy efficiency of their products to cope with the need to reduce carbon dioxide emissions, as recently mandated by the Kyoto Conference. This objective has to be achieved without penalising product performance. Several options to decrease the energy consumption are under evaluation, ranging from high efficiency compressors to the adoption of intelligent electronic devices [57, 58, 59]. Improvement of the insulation through the use of vacuum insulated panels filled with silica powder or glass wool as core materials has been evaluated, and adopted in limited amounts, by most refrigerator manufacturers [60, 61, 62], cost being the main obstacle to large sales of these products. The lower cost of the open cell foams, combined with the low density and good handling properties, has generated renewed interest for this technology as an environmentally friendly option to energy consumption reduction. A second driving force for the appliance industry is the possibility of increasing the internal volume of refrigerators and freezers without increasing the outer dimensions. This aspect is particularly important in Europe and Japan, where the built-in appliance market is an important segment and space constraints play a role. Adoption of PU-based vacuum panels in refrigerators and freezers requires the proper handling of the foaming process, which may influence the structural and vacuum properties of the panel.

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Advances in Urethane Science and Technology In fact, during the appliance manufacturing cycle, the panels are glued to the cabinet walls and then foamed in place with a conventional closed cell PU foam, this process being necessary to further improve the overall structural and thermal insulating properties of the cabinet. To allow a better flowability of the injected chemicals, the cabinets are generally pre-heated at about 45 °C for some minutes. The presence of the panels, which reduces the free flow and expansion of the foam, requires special care to avoid generation of air-filled void volumes which would deteriorate the insulation performance [56]. To prevent this, and provide the best integration between VIP and conventional PU foams, specific systems have been recently developed to be used in conjunction with VIP [10]. As a combined effect of the cabinet pre-heating and the exothermicity of the foaming reaction, temperatures even higher than 100-120 °C can be reached on the VIP surface, leading to temperature-enhanced gas desorption. A typical result using a 50 x 50 x 2 cm3 panel is shown in Figure 4.25.

Figure 4.25 Measurement of the temperature increase experienced by the two panel surfaces during the foaming process.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… This gas contribution has to be evaluated case by case since it depends strictly on the actual foaming conditions, such as the chemicals used, the process variables, the refrigerator design and VIP geometry. However, under some circumstances, the pressure increase can be a measurable fraction of the maximum acceptable level, thus causing a deterioration of the VIP thermal insulation properties from the very beginning of the refrigerator life. As an example, Figure 4.26 shows the effect of a thermal treatment carried out at 50, 70 and 90 °C on a VIP prepared with an open cell foam and an aluminium foil-containing laminate. The foam was pre-baked at 150 °C for 20 minutes and no absorbent was used. After the preparation, the panel was put in a oven and kept at the indicated temperature for 15 to 20 minutes. The panel was then removed and the total pressure measured as a function of time. To allow the continuous monitoring of the pressure, a SpiroTorr spinning rotor gauge was mounted in the panel as shown in Figure 4.21. Desorbed gases after the thermal treatments are only partially reabsorbed by the foam and generate a measurable pressure build up in the VIP.

Figure 4.26 pressure evolution in a panel submitted to various heat treatments. Pressure readings are taken by means of a SpiroTorr SRG (see Figure 4.3).

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Table 4.12 Residual Gas Analysis in encapsulated panels with and without the getter Encapsulated VIP

Total Pressure (Pa)

Partial Pressure (Pa) Air

CO/CO2

H2O

H2

Without getter

40

6

29

5

0.1

With getter

0.7

0.3

0.01

0.3

0.1

The effect of the temperature increase resulting from the foaming process, on the vacuum level in a panel can also be seen in Table 4.12 which shows the result of an RGA carried out on two test panels encapsulated by an appliance manufacturer. In the getterless panel, the pressure immediately after the foaming is already exceeding the 10 Pa target value and close to the maximum acceptable value (generally set at 50 Pa). The deterioration of the vacuum during the foaming process has been recently addressed by Kücükpinar and co-workers who ran specific tests aimed at quantifying this effect in open cell foam-filled VIP with and without getters [53]. As shown in this study, the COMBOGETTER was able to compensate for the extra gas load generated during the foaming process, thus confirming the results in Table 4.12. Further specific outgassing studies are necessary to understand the mechanisms of gas generation during the foaming process, even though this investigation presents several difficulties because of the large number of parameters involved. The foaming process may also stress the laminate due to the combined effect of temperature and applied pressure on critical areas such as the panel edges and the corners. Defects generated here may in turn increase the gas diffusion inside the panel, reducing the designed lifetime. Another important issue in refrigerator/freezer applications is longevity, since 15 to 20 years lifetime is targeted by most manufacturers. Durability tests on panels are therefore necessary to assess VIP reliability and performance over time. Figure 4.27 shows the pressure values in some encapsulated panels, as measured by RGA. Panel A, containing a COMBOGETTER, was aged at room temperature for some weeks and then kept at 40 °C for more than 30 months. Panels B and C did not have any adsorbent. The latter was aged at 40 °C for about 3 months, while the former was unaged.

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Figure 4.27 Residual gas analysis in panels encapsulated and aged for different times. Panel A contains one COMBOGETTER, panels B and C are without.

The pressure in panel A is well below the 10 Pa target value as compared to panel C whose pressure is one order of magnitude higher. Pressure in panel B, which was just encapsulated, was slightly lower than in panel C. For comparison, the calculated pressure increase in the panel, as predicted based on the model discussed in Section 4.3, is also shown. This result confirms the role of the getter to compensate for the encapsulation process and to ensure VIP longevity and also supports the gas load model previously described. Still, a better assessment is necessary through longer term tests and, possibly, through accelerated tests able to provide experimental data within a shorter time. Activity is ongoing in SAES Getters Laboratories, as well as in various other research centres to provide such an assessment.

4.6.2 Laboratory and Biomedical Refrigerators This is very special equipment designed to operate at very low temperatures, e.g., from -30 °C to -86 °C, to age samples or to store valuable and perishable goods, like organs and tissues, biological and medical samples or vaccines. Vacuum panels are used mainly to increase the internal storage volume without increasing the energy consumption.

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Advances in Urethane Science and Technology Since the conventional insulation must be very thick to ensure the achievement of such low temperatures, a partial replacement of the conventional insulator with VIP contributes significantly to increase the internal volume, from 20% up to 40% or more in specific models. As a second advantage, panels provide an overall superior passive insulation, which means that the temperature rise in the case of power failure is less steep and more time is necessary before a given critical temperature is reached. This provides extra-safety for delicate articles which may rapidly deteriorate when their temperature exceeds a given value. Due to the generally higher insulation thickness, the foaming process here may be even more exothermic than in domestic appliances. Means have therefore to be taken to keep the VIP temperature as low as possible during the encapsulation. Ultra-low temperature freezer models using foam filled vacuum panels were successfully placed on the market by a leading company some years ago and other companies are expected to follow soon.

4.6.3 Vending Machines Vending machines are especially popular in Japan, Korea and the Far East. It is estimated that in Japan about 2.5 million of these appliances are in the field with a replacement market of about 0.4 million pieces/year. VIP are finding widespread use in vending machines since most manufacturers are extensively using one or two panels to separate the hot and cold beverage compartments. Also in this case, the main driving force for VIP adoption has been the possibility to increase the internal volume for the storage of beverages, rather than the improvement in energy efficiency. Vending machines are quite a demanding application for VIP, since the panel operating temperature is cycled between room temperature and 60-70 °C and the lifetime is 5 years. As already discussed, the high temperature promotes higher diffusion and outgassing rates and provides additional mechanical stress to the envelope. Ageing tests have been carried out by vending machines manufacturers to assess VIP usability and reliability in this application. Results obtained in SAES Getters Laboratories for 50 x 50 x 2 cm3 panels aged for several months at 60 °C are shown in Figure 4.28.

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Figure 4.28 Measurement of the thermal conductivity as a function of time for panels aged at 23 and 60 °C, with and without the COMBOGETTER.

Again, the deterioration of the thermal conductivity, due to the pressure increase, is quite dramatic in the getterless panels both at 23 and 60 °C. As expected, the thermal conductivity increase is much more rapid for the panels aged at 60 °C than for the panel kept at room temperature. Only minimal differences are measured in the panels containing the COMBOGETTER, which could compensate for the increased gas load at both temperatures.

4.6.4 Refrigerated/Insulated Transportation Large vacuum insulated containers, ships and trucks are under evaluation by various companies, mainly in Europe and Japan. The main advantages offered by using VIP are space saving and better insulation, so as to keep the temperature increase rate as low as possible even in the absence of a power source. The lifespan is generally from 10 to 15 years. Since very large VIP, e.g., 2-3 m2 size are considered for this application, the edge effect (see Section 4.3.2.1) is here a marginal issue.

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Advances in Urethane Science and Technology Small commercial shipping containers to store and deliver pharmaceuticals, frozen food and valuable temperature-sensitive products is also another interesting area for the use of VIP. Long lifespan is not a problem, since most of these shipping containers have a projected life of the order of weeks or months. Therefore, it is even possible to use the VIP more than once, the specifications on the permeation properties of the barrier film are less tight and the getter not necessary. A specific dryer, to adsorb water, may be used in most cases. The small size VIP and the limited projected life suggest the possibility of using metallised barriers for the skin of the panel in these applications, at least on one side, so as to overcome the edge effect, which, in this case is very critical.

4.6.5 Other Applications A variety of other potential applications exist for VIP, from cold stores to insulation in buildings and in industrial plants, e.g., industrial reactors or liquid natural gas tanks. Also super insulation at relatively high temperature applications for water heaters, heat pipes or boiling pots have been considered and are under evaluation. The key issue of the adoption of VIP in this last family of applications is the possibility to bend the open cell PU filled VIP into a round or cylindrical shape. The combination of high temperature and bending provides additional challenges to VIP technology and further improvements in the material selection and processing might be required.

4.7 Near Term Perspectives and Conclusions The open cell PU foam is a very promising filler material for VIP. It shows good vacuum compatibility, thermal conductivity, light weight, ease of handling and processing and moderate cost. To allow its use, significant improvements have been made recently in the barrier and getter technology, as well as in the panel manufacturing and testing procedures. Laminated barriers, incorporating a 6 μm continuous aluminium foil, are providing a reasonable trade-off between permeation properties and energy saving, especially in large size panels (~1 m2), where the edge effect is less important. These films can therefore be used in applications such as refrigerators, freezers, vending machines and insulated transportation, where 10-20 years lifetime and relatively large panels are required. Traditional metallised or multilayered plastic barriers, even though more appealing from the energy saving point of view, are not advisable in long term applications, due to their poorer gas barrier properties. However, the need to improve performance and VIP cost effectiveness is pushing the film industry to further improve their products either by

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… reducing the aluminium foil thickness to 5 μm and below or developing aluminium-free composite barriers. If successful, this latter solution would dramatically increase the vacuum panel insulation efficiency. New families of improved metallised products have been developed very recently and proposed for testing and are under evaluation in several laboratories. The outgassing and permeation data, on both components and finished panels, and the use of models quoted in the literature, allow the evaluation of the total gas load and the pressure increase in a vacuum panel as a function of time, size and operating conditions. This in turn is beneficial for designing the most suitable solution for the absorbent. A high capacity getter system, the COMBOGETTER, capable of adsorbing air, moisture and the other gases of interest without the need for any pre-treatment has been specifically developed for VIP application. Its roles are many since it chemically absorbs residual, outgassed and permeating species and provides the means to shorten the evacuation process and increase the manufacturing yield. Mass production technologies to manufacture high quality open cell foam panels are on the market. Several techniques are also becoming available for the fast measurement of the thermal conductivity. This is a very important issue to build a QA/QC system able to ensure the quality and the reliability of the products, not only after production but also immediately before use. As far as the applications are concerned, open cell PU foams are finding their place in the market for both low and high temperature appliances, from refrigerators to vending machines. Present production volumes are still limited but near term perspectives are encouraging, especially in Asia and Europe. The strong push for energy reduction and the parallel continuous refinement of the technology should provide further motivation to VIP adoption. Cost is still the main obstacle to the widespread adoption of this technology. A preliminary qualitative cost comparison between silica and open cell PU, which includes raw material, panel manufacture and panel installation costs [10] shows that a cost reduction of about one-third over silica could be achieved. This has to be improved to bring the cost of the vacuum panel down further, so as to really interest more segments of the insulation industry. Very recent breakthroughs in the foam mass production technologies seem to indicate that this target is reachable. Even though far from being complete, the material presented in this chapter shows that significant technological achievements have been obtained during the last few years.

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Advances in Urethane Science and Technology Thanks to this, vacuum panel technology is becoming a technically viable and cost effective solution to the need to reduce energy consumption in household appliances and in commercial and industrial applications. To successfully achieve this target, additional efforts are necessary to further improve the component quality and reliability (foam, film and adsorbent), to optimise panel production and to reduce costs.

Acknowledgements The author would like to thank Dr. Paolo della Porta, President and CEO of SAES Getters SpA, for his continuous support and long lasting commitment to VIP technology. The author also acknowledges Dr. Bruno Ferrario, Corporate R&D Director, for fruitful discussions and suggestions. Special thanks are given to Dr. Roberto Caloi and Dr. Enea Rizzi for their valuable technical support in the development of theoretical models and in running experimental measurements on VIP components and finished devices.

References 1.

H. A. Fine, Journal of Thermal Insulation, 1989, 12, 183.

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D. W. Yarbrough, R. S. Graves, F. J. Weaver and D. L. McElroy, presented at ORNL/ CON-215, Oak Ridge National Laboratory, Oak Ridge, TN, Sept. 1986, p.1.

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L. R. Glicksman and N. Solomou, Presented at the SPI Polyurethanes Conference, Boston, MA, 1994, p.182.

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R. L. Tabor, Presented at the SPI Polyurethanes Conference, Boston, MA, 1994, p.288.

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G. Biesmans, D. Randall, E. Francais and M. Perrut, Presented at the Polyurethanes World Congress ’97, Amsterdam, The Netherlands, 1997, p.294.

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K. Kodama, K. Yuge, Y. Masuda and Y. Tanimoto, Presented at the Polyurethanes World Congress, October 1993, p.140.

10. R. De Vos and I. D. Rosbotham, Cellular Polymers, 1994, 13, 2, 147. 11. K. W. Dietrich and D. W. McCullogh, Presented at UTECH 96, The Hague, The Netherlands, 1996, Paper No.64. 12. W. H. Tao, W. F. Sung and J. Y. Lin, Journal of Cellular Plastics, 1997, 33, 3, 545. 13. P. Pendergast and B. Malone, Vuoto, 1999, 28, 1-2, 27. 14. L. R. Glicksman in Low Density Cellular Plastics: Physical Basis of Behaviour, Ed., N. C. Hilyard and A. Cunningham, Chapman & Hall, London, 1994, Chapter 5, p.104. 15. W. Wacker, A. Christfreund, D. Randall and N. W. Keane, Presented at the Polyurethanes Expo ’96, Las Vegas, NV, 1996, p.35. 16. A. Roth, Vacuum Technology, North Holland Publishing Company, Amsterdam, 1982, 186. 17. F. Sciuccati, G. Gasparini and B. Ferrario, Vuoto, 1988, 17, 4, 345. 18. P. A. Redhead, Vacuum, 1962, 12, 203. 19. P. Mercea, L. Muresan and V. Mecea, Journal of Membrane Science, 1985, 24, 297. 20. T. A. Beu and P. V. Mercea, Materials Chemistry and Physics, 1990, 26, 309. 21. A. Sugiyama, H. Tada and M. Yoshimoto, Vuoto, 1999, 28, 1-2, 51. 22. ASTM Standard E96-00 Standard Test Methods for Water Vapor Transmission of Materials. 23. ASTM Standard F 1249-90 (1995) Standard Test Method for Water Vapour Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor. 24. P. Manini, inventor; Saes Getters SpA, assignee; WO 9803850A1, 1998. 25. P. Manini, Journal of Cellular Plastics, 1999, 35, 5, 403.

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Advances in Urethane Science and Technology 26. P. Manini and E. Rizzi, Poster presented at the European Workshop on Vacuum Panel Technology for Super Insulation in Domestic Appliances and Industrial Applications, Milano, Italy, 1998. 27. W. Lamb and R. Zeiler, Presented at the Vacuum Insulation Panel Symposium, Baltimore, MD, 1999. Paper available from the SAES Getters web site, http:// www.saesgetters.com. 28. J. Bonekamp, Presented at the Vacuum Insulation Panel Symposium, Baltimore, MD, 1999. Paper available from the SAES Getters web site, http:// www.saesgetters.com. 29. R. Juran, J. Covington and J. Carley, Modern Plastics Encyclopedia, McGraw Hill, NewYork, 1987, 64, 10A. 30. Polymer Handbook, 3rd Edition, Ed., J. Brandrup and E. H. Immergut, John Wiley and Sons, New York, 1989. 31. S. J. Adam and C. E. David, Presented at the 23rd International SAMPE Technical Conference, New York, NY, 1991. 32. K. E. Wilkes, W. A. Gabbard and F. J. Weaver, ORNL/CP-95971, Oak Ridge National Laboratory, TN, 1998. 33. G. Biesmans, R. De Vos and I. D. Rosbotham, Presented at the Polyurethanes World Congress 1993, Vancouver, BC, 1993, 498. 34. A.Barosi in International Symposium on Residual Gases in Electron Tubes, Ed., T. A. Giorgi and P. della Porta, Academic Press, London, 1972, 221. 35. C. Boffito, B. Ferrario, P. della Porta and L. Rosai, Journal of Vacuum Science and Technology, 1981, 18, 1117. 36. B. Ferrario, A.Figini and M.Borghi, Vacuum, 1984, 35, 13. 37. C. Boffito, B. Ferrario, L. Rosai and F. Doni, Journal of Vacuum Science and Technology, 1987, A5, 3442. 38. D. W. Breck, Zeolite Molecular Sieves, R. E. Krieger Publishing Company, Malabar, Florida, 1984, Chapter 8. 39. J. S. Mattson and H. B. Mark, Activated Carbon: Surface Chemistry and Adsorption from Solution, Marcel Dekker, New York, 1971.

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Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels… 40. P. Manini, Vuoto, 1997, 27, 2, 45. 41. C. Boffito and A. Schiabel, inventors; Getters SpA, assignee; European Patent 0514348 A1, 1991. 42. R. M. Caloi, P. Manini, S. Valdrè, E. Magnano, J. Kovac, E. Narducci and M. Sancrotti, Journal of Vacuum Science and Technology, 1999, A17, 2696. 43. R. De Vos, I. D. Rosbotham and J. Deschaght, Presented at the Polyurethanes World Congress, 1994, Boston MA, p.194. 44. J. Akita, Vuoto, 1999, 28, 1-2, 59. 45. ASTM Standard C518-98, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. 46. ASTM Standard C236-89 (1993) e1, Standard Test Method for Steady-State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box. 47. ASTM Standard C177-97, Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the GuardedHot-Plate Apparatus. 48. S. Smith and C. Urso, Vuoto, 1999, 28, 1-2, 64. 49. N. E. Mathis, Measurement of Thermal Conductivity Anisotropy in Polymer Materials, Chemical Engineering Department, University of New Brunswick, Fredericton, NB, Canada, 1996, Ph.D. Thesis. 50. C. Dixon and N. Mathis, Presented at the Polyurethanes Expo ‘99, 1999, p.607. 51. J. K. Fremerey, Journal of Vacuum Science and Technology, 1985, A3, 3, 1715. 52. E. Kücükpinar, H. Güclü, A. S. Akkurt and F. Özkadi, Vuoto, 1999, 28, 1-2, 31. 53. E. Kücükpinar, F. Özkadi , A. Soyal and H. Güclü, Presented at the Polyurethanes Expo ‘99, 1999, p.339. 54. C. Boffito, M. Moraja and G. Pastore, Journal of Vacuum Science and Technology, 1997, 15, 4, 2391. 55. B. E. Lindenau and J. K. Fremerey, Journal of Vacuum Science and Technology, 1991, A9, 5, 2737.

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Advances in Urethane Science and Technology 54. A. J. Hamilton, Vuoto, 1999, 28, 1-2, 27. 55. H. A. Fine and J. Lupinacci, Energy-Efficient Refrigerator Prototype Test Results, EPA-43D-R-94-011, June 1994. 56. E. A. Vineyard, J. R. Sand and R. H. Bohman, ASHRAE Transactions, 1995, 1, 1422. 57. E. A. Vineyard and J. R. Sand, presented at ORNL/CP-97450, Oak Ridge National Laboratory, TN, 1998. 58. G. J. Haworth, R. Srikanth and H. A. Fine, Presented at the 44th Annual International Appliance Technical Conference, Ohio, USA, 1993, p.1. 59. E. A. Vineyard, T. Stovall, K. E. Wilkes and K. W. Childs, ASHRAE Transactions, 1998, 104, 2. 60. T. F. Potter, D. K. Benson and L. K. Smith, Presented at ACEEE 1988, Proceedings from the Panel on Appliances and Equipment,1998, 4, 4.86.

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Modelling the Stabilising Behaviour of Silicone Surfactants During the Processing of Polyurethane Foam: The Use of Thin Liquid Films Steven A. Snow, Udo C. Pernisz, Benjamin M. Nugent, Robert E. Stevens, Richard J. Braun and Shailesh Naire

5.1 Introduction The commercial production of polyurethane (PU) foam requires a tremendous diversity of ingredients. Common ingredients include isocyanates, polyether polyols, water, catalysts and stabilisers (surfactants). Most of the surfactants used in the foaming process are silicone surfactants, the major suppliers being Th. Goldschmidt AG, Witco and Air Products and Chemicals, Inc. Worldwide volume for silicone surfactants in polyurethane foam has been recently estimated at 30,000 tonnes/year [1]. In practice, a wide range of silicone surfactant structures are necessary to produce PU foam with a sufficiently broad range of physical properties. This structural diversity also reflects the need for the surfactant to perform many different tasks in foam formulations. The physical behaviour of these surfactants in the PU foam formulation process [2] has recently been reviewed. These roles of the surfactant include reducing surface (interfacial) tension [3-11], altering surface viscoelasticity [5, 7, 11, 12-15], and modifying polymer reaction kinetics and morphology [16-21]. The focus of this chapter is on the mechanism of PU foam stabilisation and how it is affected by silicone surfactants. In order to understand a complicated physical mechanism such as PU foam stabilisation, one must first be concerned with the thermodynamics and kinetics of the process. As a PU (or any) foam rises, energy is absorbed by the system and converted into a combination of excess gravitational and surface energy. For PU foams, this excess energy is created by both high-shear mixing and energy transfer from the chemical polymerisation reaction (between isocyanates and either polyols or water). One function of the silicone surfactant is to stabilise this foam until it is a self-supporting, gelatinous froth. This froth can further harden, yielding elastomeric or rigid foam products. Once hardened, the foam still contains excess gravitational and surface energy; however, the rate of dissipation of this energy is extremely low. Therefore, when discussing foam ‘stability’, one is specifically considering the rate of foam degradation. Foam degradation involves both physical and energetic changes in the foam. Four physical processes are commonly associated with PU foam degradation:

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a decrease in foam volume,

•

a decrease in the number of bubbles (cells),

•

an increase in average cell size, and

•

a decrease in the dispersity of cell sizes.

These physical processes involve the release of excess gravitational and surface energy. Excess gravitational energy is released during the drainage of liquid from the foam. The liquid can drain from between the bubbles, thinning the liquid membrane known as the lamella. The liquid can also drain from the plateau borders in the foam. These borders are the relatively wide channels in the foam created at the intersection point of three lamellae. Mysels, Frankel and Shinoda [22] estimated that, in aqueous foams, the amount of gravitational energy released during degradation was five orders of magnitude less than the amount of surface energy released. It is assumed that this estimation roughly holds for the degradation of PU foams; therefore, the focus here will be on processes that reduce the surface energy of PU foams. Two processes, the diffusion of gas from small to larger bubbles (Ostwald Ripening) [23] and bubble coalescence, reduce the surface area of the foam and, therefore, the surface energy. Ostwald ripening involves the diffusive molecular transport of gas from small to large(r) bubbles. Ostwald ripening in foams leads to the counterintuitive phenomena of large bubbles in a foam expanding as smaller ones shrink. This process results both in a reduction in the number of bubbles and in the dispersity of bubble sizes. However, Owen [12] has demonstrated that Ostwald ripening plays only a minor role in PU foam stability; therefore, this chapter will focus on the process of bubble coalescence. Bubble coalescence involves the merging of two distinct bubbles into one, due to the rupture and recession of the lamella. Early in the PU foaming process, the rupture of a lamella at the edge of the PU foam will cause bubble collapse as the gas inside is lost to the surrounding atmosphere. This process will reduce the number of bubbles present and the foam volume. Lamella rupture within the interior of the foam will cause bubble coalescence, decreasing the number of bubbles, increasing the average bubble size, but not necessarily decreasing the foam volume. Later in the foaming process, as the PU gel network sets up, the lamella may rupture but the two bubbles can retain their separate identities. This process is known as cell opening. If many of the lamellae have ruptured in this fashion, the object is more correctly called a sponge instead of a foam. In this case, air can easily be pushed out of the sponge if it is loaded with weight (as for example when one sits on PU foam padding on a car seat). Therefore, the mechanical properties of the PU foam depend greatly on the degree of cell opening present. There is general agreement that a strong stabilising effect from the surfactant is necessary early in the

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Modelling the Stabilising Behaviour of Silicone Surfactants… foam process. The stabilising role, if any, of the surfactant late in the foaming process remains a controversial issue [2, 19-21, 23-27]. This chapter will focus on the earlier stages of the foaming process, before the gelling of the liquid PU phase. The mechanism of rupture depends upon the thickness of the lamella film. Ultra-thin films, with thicknesses in the 1-20 nm range, are believed to rupture due to the amplification of thermally-generated surface waves [28-30]. However, studies by Artavia and Macosko [31] and Akabori and Fujimoto [32] demonstrated that most of the ruptured lamellae in a PU foam are in the 200-1000 nm thickness range. Therefore, it is assumed here that the mechanism of rupture of these thicker films is most important when considering PU foam. It has been extensively demonstrated that ‘thick’ liquid films rupture due to the formation (nucleation) of a pinhole in the film which spreads outward radially until the film is destroyed. The nucleation of the pinhole could result from either the presence of fluid instabilities in the film (for example, Marangoni, Rayleigh-Taylor or Benard instabilities) [33] or else by the presence of immiscible microphases which could serve as nucleation sites. For PU foams, it has been proposed that film rupture is nucleated by the phase separation of polyurea segments [19-21, 24-26]. It was postulated that the phase separation process could be retarded by the presence of silicone surfactants containing a high percentage of polyoxyethylene. The polyoxyethylene segment of the surfactant could interact strongly with polyurea via the formation of a hydrogen bonding network. However, in cases where the silicone contains lesser amounts of polyoxyethylene, such as in many commercial foam stabilisers, a direct effect of the concentration and structure of the silicone surfactant on this phase separation process has not been found [2, 26]. Circumstantial evidence of the influence of Marangoni instabilities on PU foam stability, and presumably film stability and the nucleation of pinholes, has been amply demonstrated [34]. Marangoni instability in a fluid film refers to a situation where a gradient of surface tension exists, stimulating compensating surface flows. For example, the stretching of the film reduces the surface concentration and, therefore, increases the surface tension. Furthermore, this stretched and thinned spot would be prone to rupture. However, within a certain range of surfactant concentration, this thin spot can re-thicken due to the diffusion/convection of surfactant and underlying bulk liquid to the thin spot (Marangoni flow). Once nucleated, the film hole can spread out radially at a rate inversely proportional to the bulk viscosity of the film. Spreading can occur until the film is destroyed. For example, in typical aqueous films, the rate of spreading is so high that high-speed photography is necessary to capture the event. However, in a gelling PU foam, the rate

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Advances in Urethane Science and Technology of spreading can be very low, and, in fact, films can have multiple holes in them without being destroyed [24-26, 32]. Detailed investigations on the effect of silicone surfactants on the nucleation and growth of holes in PU films have not been reported. Furthermore, that is not the topic of this chapter which is specifically concerned with the effect of the silicone surfactant on the rate of drainage of PU films. However, this topic is highly related to film rupture as it has been shown that the probability of film rupture is an inverse squared function of the film thickness [35]. The time-dependent film thickness is a function of the drainage rate of the film.

5.2 Film Drainage Rate: Reynold’s Model and Further Modifications 5.2.1 Rigid Film Surfaces Thin film drainage has been investigated in detail both theoretically and experimentally [22, 28-30, 35]. An extensively applied physical model of a draining thin film is the one of Reynold’s, a form of which is shown in Equation 1: VRe = 2Fk3 3πμR 4

(1)

where VRe is the velocity of film thinning, F is the external force on the film (causing drainage), k is the film thickness, μ is the dynamic viscosity, and R is the film radius. The Reynold’s model assumes that: •

the liquid inside the film is Newtonian in its rheological behaviour.

•

the surfaces of the film are circular, immovable, parallel plates; surface velocity equals zero, surface viscosity is infinite.

•

the flow inside the film is laminar.

•

the flow inside the film is driven by gravity and/or pressure gradients and is resisted by the bulk viscosity of the liquid.

The dependencies of drainage (flow) rate on the variables of film thickness, area and bulk viscosity are quantified in Equation 1. For example, with decreasing film thickness k, the drainage rate decreases in proportion with k3; furthermore the drainage rate decreases with increasing viscosity.

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Modelling the Stabilising Behaviour of Silicone Surfactants… Previous discussions [34] of PU film drainage rate (and ultimately foam stability and cell opening) focused on the dependence of rate on bulk viscosity. Per Reynold’s model, factors which increased the bulk viscosity, such as catalyst concentration and polyol reactivity, presumably decreased film drainage rate and yielded a more stable, less porous foam.

5.2.2 Mobile Film Surfaces The Reynold’s model and equation have also been modified to account for surface effects on film drainage rate [29]. To accomplish this, the assumption (boundary condition) of a surface velocity of zero must be relaxed. This change also decreases the surface viscosity from an infinite to a finite value. This process yields Equation 2. Allowing for a finite surface velocity increases the film drainage rate from that which would be expected under the Reynold’s conditions. V / VRe = 1 + 1 / e f

(2)

Where V/VRe is the measured velocity of thinning divided by the velocity of thinning under the conditions of the Reynold’s equation and ef is the surface mobility factor. The surface mobility factor is analysed in detail in [36]. This analysis demonstrates that two factors control the degree of surface mobility: the surface viscosity and the presence of surface tension gradients.

5.2.3 Surface Viscosity The rates of thinning of vertically-supported, thin liquid films of polyol solutions of various silicone surfactants have been measured [37]. It was found that the rate for films stabilised by a trimethylsilyl-capped polysilicate (TCP; a highly branched silicone not containing polyethers), was much lower than that for the films stabilised by common silicone polyether copolymer surfactants. The retardation of drainage rate was correlated with an increase in surface viscosity. Furthermore, it was noted that PU foams prepared using TCP were significantly more stable than those containing the commercial surfactants. Surface viscosity scales directly with the surfactant surface concentration, the intermolecular cohesion between the surface molecules and the intermolecular adhesion between the surfactant molecules and the underlying bulk liquid layer. Increasing the surface concentration decreases the average distance between molecules, therefore increasing the sum value of the attractive molecular forces between them. In

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Advances in Urethane Science and Technology many cases, high surface concentration yields a low film drainage rate and a highly stabilised foam. The intermolecular cohesion is the result of the balance of attractive (van der Waals, polar bonding) and repulsive (ionic) intermolecular forces. The total cohesive energy is a product of the total area of cohesion and the cohesive energy per unit area. For example, the cohesion of the polyether chains of a typical silicone surfactant yields most of the surface viscosity contribution in PU films and foams [38]. Although it has not been investigated, one could speculate that the surface viscosity would scale with the length of the polyether chain. Increasing the chain length increases the potential area of contact between the chains. The unit area cohesive energy is a function of the strength of the intermolecular bonds present. For example, silicone surfactant molecules could possibly cohere via van der Waals bonding of the methyl groups of the siloxane backbone or else via polar bonding between the ethylene oxide units in the polyether chain. The van der Waals bonding between methyl groups is known to be weak; consequently, the surface viscosity of unsubstituted PDMS is close to zero [15, 38]. However, the polar bonding between the polyether chains would be much stronger and, therefore, the source of surface viscosity measured in silicone polyether copolymers. Intermolecular adhesion between the polyether chains of a typical silicone surfactant and the underlying ‘polyurethane’ liquid matrix could increase the surface viscosity contribution in PU films and foams. In order to maximise surface viscosity, and therefore minimise the drainage rate, the surfactant concentration, the intermolecular cohesion and adhesion should all be high. Later in the chapter evidence is presented correlating silicone surfactant concentration and structure (which influences the intermolecular cohesion and adhesion) to surface viscosity and film drainage rate.

5.2.4 Surface Tension Gradients An idealised model of the surface of a thin liquid film is one of a monolayer of evenlydistributed surfactant molecules. However, a more realistic model is one where the molecules are not evenly distributed; therefore, the surface concentration depends on surface position. The result of this heterogeneous distribution is that gradients of surface concentration, and therefore surface tension, are present. One example of this was pointed out in Section 5.1 on the effect of Marangoni instabilities on film rupture. Regarding film drainage, a surface tension gradient exerts a surface stress that can either impede or

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Modelling the Stabilising Behaviour of Silicone Surfactants… accelerate the underlying bulk flow (drainage). In thin films, these gradients typically make the surface rigid, retarding flow and therefore decreasing, drainage rate. The gradients can be relieved by the diffusion of surfactant from areas of high to low concentration; the drainage rate of the film is proportionately increased. Diffusive surfactant fluxes are functions of the intensity of the concentration gradient and the diffusion coefficient of the surfactant. The intensity of the concentration gradient depends upon the overall surfactant concentration. At low or high concentrations, these gradients are relatively weak. At intermediate concentrations, they are quite strong. The diffusion coefficient of the surfactant is a function of its size and shape. The most simple and common case to analyse is where the surfactant assumes a spherical shape. In this case, the diffusion coefficient of the surfactant scales inversely with the solvated molecular volume (Stoke’s law). Overall, to accelerate film drainage, it is useful to have a high concentration of surfactant whose surface partition coefficient is also high. The surface partition coefficient is a measure of the tendency of the surfactant to adsorb at the surface instead of remaining in the bulk. In the case of silicone polyether surfactants, the coefficient scales with the ratio of silicone to polyether. Under these conditions (high surfactant concentration, high ratio of silicone to polyether) the flux of surfactant to the surface, relieving surface tension gradients, will also be high. A large diffusion coefficient (for the surfactant molecule, a small size and a compact shape) is also helpful. Later in this chapter correlations of silicone surfactant concentration, partition coefficient, and diffusion coefficient to film drainage rate will be discussed.

5.3 Experimental Investigation of Model, Thin Liquid Polyurethane Films and the Development of Qualitative and Semi-Quantitative Models of Film Drainage This section begins with a qualitative description of thin liquid PU films. This initial investigation had five goals in mind: to confirm that stable, vertically-oriented, thin liquid films could be prepared using mixtures of ingredients designed to model a PU foam, to study the hydrodynamic phenomena in the films, to compare the physical behaviour of these films to the behaviour of the more common aqueous soap films, to observe specific surfactant effects on the properties of these films, and to extrapolate conclusions about the behaviour of these films to operational PU foam. After this qualitative description, an accurate measurement of the drainage rates of these films was sought in order to study the effect of bulk and surface variables on the rate. In

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Advances in Urethane Science and Technology order to make an accurate measurement, a novel interferometric method was developed and implemented. This method was validated by the discovery that film drainage rate scaled with the reciprocal of bulk viscosity as predicted by Reynold’s equation. Once this method was validated, the effect of silicone surfactant concentration and structure on the drainage rate of the films was investigated. Generally, the drainage rate displayed a maximum as a function of surfactant concentration. This maximum was consistent with a physical model where the two major influences on drainage rate are surface tension gradients and surface viscosity. As surfactant concentration increased, the retarding effect of gradients decreased, and the film drainage rate increased. However, above a certain concentration, the drainage retarding effect of surface viscosity overcame the gradient effect. The dependence of film drainage rate on silicone surfactant molecular structure was also systematically investigated. In order to understand this correlation, three physical parameters of the film affected by surfactant structure must be considered. These parameters are the surface partition coefficient, the surfactant molecular diffusion coefficient and the degree of intermolecular cohesion within the surface layer. Specifically, as the length of the polyether (solvophilic) portion of the surfactant increased, the surface partition coefficient decreased, the diffusion coefficient decreased, and the degree of cohesion increased. This resulted, at constant surfactant concentration, in a complex effect on the film drainage rate. A quantitative physical model of a draining vertical thin film was developed from first principles. The starting point was the Navier-Stokes equation. The initial model featured a fixed-surface, wedge-shaped vertical film, with immobile surfaces. This is essentially the Reynold’s model modified to this film shape and orientation. Good agreement of the predictions of this model with experimental data was obtained. The next model relaxed the condition of fixed film shape (allowed for curvature in the film surface) and analysed the effects of the menisci on the film drainage. Analysis of the Navier-Stokes equation was simplified by the application of the lubrication approximation. The results from this analysis agreed extremely well with experimental values both in terms of film drainage rate and the changes in film shape with time. The development and growth of bulges and waves on the bottom of the film were particularly intriguing. This phenomenon was experimentally observed in these films. Finally, models were developed where the condition of infinite surface viscosity was relaxed. This allowed the analysis of surfactant effects on film drainage, in particular surface viscosity and surface transport. Specifically, the model predicted the decrease in drainage rate as surface viscosity increases, as expected from the qualitative models and measured experimentally. The effect of surface transport was significantly less than that of surface viscosity.

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Modelling the Stabilising Behaviour of Silicone Surfactants…

5.3.1 Experimental Details 5.3.1.1 Film Formulation Unless noted otherwise, the experimental work in this chapter involved a model flexible slabstock PU foam formulation [39] at the instant of mixing (see below): •

20.00 g VORANOL 3137 (3100 MW polyether copolymer, The Dow Chemical Company),

•

5.46 g toluene,

•

0.20 g DABCO DC 198 (surfactant Air Products and Chemicals, Inc.)

For simplicity’s sake, chemical reactions were avoided. This was accomplished by substituting toluene for toluene diisocyanate (TDI) in the formulation. This formulation had a nominal viscosity of about 0.8 Pa-s.

5.3.1.2 Description of the Experimental Interferometer The following experimental variables were addressed during the design and construction of the interferometer: •

control of film area,

•

control of the withdrawal rate of the film,

•

control of spurious vibration potentially leading to film rupture,

•

control of solvent evaporation,

•

precise vertical alignment of the film,

•

optical control of the interferometric measurement (illumination with plane parallel light, use of monochromatic light, uniform illumination of the film surface),

•

Imaging of the film including magnification, presentation on a video screen, and video recording capabilities, and

•

Precise measurement of film lifetimes.

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Advances in Urethane Science and Technology Consideration of these points led to the experimental setup sketched in Figure 5.1. The film is formed between two narrow blades set by 10 mm apart and supported in a surrounding frame; a film of 20 mm height could be formed between the blades. The frame was vertically clamped on an optical bench. A glass cuvette with rectangular cross section was fabricated with a closely fitting lid through which the clamp rod fits tightly in order to prevent evaporation of the solvent and at the same time to allow the cuvette to be lowered and raised to adjust the bulk liquid level or to draw a film. The cuvette stands on a small pedestal which was driven by a computer-controlled stepper motor assembly allowing film size control at velocities of the receding or advancing bulk liquid level up to 25 mm/s. The sample holder assembly was mounted on an optical bench. The two nearly plane-parallel surfaces of the film constitute a Fizeau interferometer which produces interference fringes in the reflected light [40]. The fringes are lines of constant film thickness. Film drainage data were acquired by generating monochromatic light by means of an interference filter (λ = 505 nm) or with a HeNe laser (λ = 632.8 nm).

Figure 5.1 Schematic of the thin liquid film interferometer.

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Modelling the Stabilising Behaviour of Silicone Surfactants…

Figure 5.2 Depiction of film formation. The film is formed within the boundaries of the inner frame by the lowering of the liquid level at a controlled rate. The frame and liquid are within the transparent walls of the enclosed glass cuvette.

5.3.1.3 Film Formation The process of film formation is depicted in Figure 5.2. Films were formed by lowering the sample liquid from a point where the vertical frame was completely immersed to a point where it was only partially immersed. In most experiments, the vertical film so formed was left in direct contact with the bulk liquid. Films were withdrawn from the bulk liquid at a rate varying from 0.25 to 50 mm/s.

5.3.2 Qualitative Description of Polyurethane Films In a typical experiment, stable, vertical, liquid films were formed from the model PU formulation (see Section 5.3.1.1). These films were stable for two to five minutes. The series of photographs in Figure 5.3 depicts many of the physical features of these films. Thirty to sixty seconds after film formation, dark horizontal interference fringes were observed that initially appeared at the top of the film and steadily moved downward. Over time, the number of fringes decreased. Flow patterns, including eddy currents, fingering patterns and swirls, appeared at the bottom and sides of the films and rose upwards. The rate and amount of these flows decreased as the lifetime of the film increased. Films were subjected to a rapid raising and lowering of the bulk liquid level. These motions strongly stimulated the surface flows previously mentioned.

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Figure 5.3 The visual sequence of a draining film. The first photograph in the sequence is in the top left hand corner and the sequence of photographs proceeds clockwise. Photograph 2: one minute after formation. The dark horizontal lines faintly present are the interference fringes. Photograph 4: 3-5 minutes after film formation. Fewer fringes are present and span the height of the film. The reduction in the number of fringes versus Photograph 2 is consistent with a significant amount of film drainage. Photograph 3: 8-12 minutes after film formation. Photograph 1: approximately 15 minutes after film formation.

The pattern of horizontal fringes observed on the face of the films suggested a vertical wedge shape, with the film thickness increasing as one descends the film. The wedge shape of the vertical PU films has a number of implications for operational PU foam. Any cell window in PU foam does not have uniform thickness. This non-uniformity has been reported in at least two studies [24-26, 32]. In addition, it would be expected that the rupture of the cell window would occur at its thinnest point [24-26, 32]. As seen in Figure 5.3, the interference fringes progress down the face of the film and the distance between the fringes increases as the film continues to drain. This indicates a decrease in the film thickness gradient during the drainage of the film. The drainage of the wedge-shaped film can be visualised in that the sides of the wedge, intersecting at the

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Modelling the Stabilising Behaviour of Silicone Surfactants… top of the film, pivot inwards as fluid drains out of the bottom. This is a collapsing inwards of the wedge. As the film is draining, interference fringes radiate outwards from the top centre of the film (photographs 2-4 in Figure 5.3). This radiation suggests that both horizontal and vertical flow processes are present. These flow processes are driven by both gravity (vertical flow) and suction into the Plateau borders of the film (horizontal flow). Once the fluid has entered the Plateau border, it flows down a relatively wide channel into the bulk liquid. The Plateau borders in these films can be seen in Figure 5.3. They are the dark regions between the film and the frame. The width of the Plateau channel increases as one descends the face of the film, giving it roughly a triangular shape. Plateau borders are also present in operational polyurethane foams. In fact, horizontally-aligned films within these foams can only drain due to Plateau border suction. The individual Plateau borders within the PU foam form a network which allows for liquid drainage down the foam. Ultimately, this network becomes the structural ‘struts’ of the foam. As reported in the literature, interference fringes in many vertical aqueous films are horizontal with little curvature. These films also displayed ‘mobile’ surfaces, rapid and extensive surface flows and rapid rates of drainage [22, 28-30]. These features were all linked to the presence of a low surface viscosity. Based on our experimental observations, the PU films stabilised by DABCO DC 198 have low surface viscosities. This conclusion is supported by reports of measurements of low surface viscosities in polyol solutions containing silicone polyether surfactants [12, 15, 38]. The phenomena of edge turbulence, fingering and upward flows in aqueous soap films have been extensively investigated [22, 28-30, 41-46]. These hydrodynamic phenomena have been defined as marginal regeneration and gravity convection. Marginal regeneration refers to a process where, simultaneously, thick films are sucked into the Plateau border and thinner films are pulled out. This exchange results in a net increase of material in the Plateau border, essentially draining the film. Within the film, after the material exchange, the new thin spots then migrate upward (gravity convection) until they reach a height where their thickness equals the film thickness. Instead of describing these phenomena in PU films as ‘gravity convection ‘, it is proposed that this flow near the surface of the film is driven by surface tension gradients. These are examples of Marangoni flows, and would proceed from an area of high surface tension to one of low surface tension. Marangoni flows are extremely common and one well-known example is the phenomenon of ‘tears’ in a glass of port wine or brandy. This view is also gaining some acceptance in the field of aqueous thin liquid films. Stein [4244] recently proposed that the flows observed in mobile-surfaced, aqueous soap films are Marangoni flows.

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Advances in Urethane Science and Technology The surface tension gradients necessary to drive this type of flow can arise from three different physical phenomena: surface tension gradients are necessary to support the weight of the vertical film, flows in the film or the channel of the Plateau border impose stresses on the surface monolayer which are balanced by a surface tension gradient, and extension or compression of the surface layer can create transient surface tension gradients. Regarding the cell windows of operational PU foam, it is apparent that surface tension gradients are constantly present, due to all three mechanisms discussed previously. In particular, the constant stretching of the bubbles during the process of foam growth acts as a potent stimuli for these flows.

5.3.3 Quantitative Measurement of Film Drainage Rates: Bulk and Surface Effects 5.3.3.1 A Method to Measure Film Drainage Rates: The ‘Collapsing Wedge’ Model The distribution of the interference fringes in a liquid film can be viewed as a contour map of the film thickness, with each successive fringe representing a section of constant film thickness (isopach). Therefore, the distance between fringes throughout the film gives the gradient of film thickness. In order to precisely measure PU film drainage rates, a physical model correlating the time change in the fringe density (ds/dt; the quantity that can be measured in the laboratory) to the time change in volume (dV/dt, the film drainage rate) of the film must be applied. Qualitatively it appears that the drainage of the film collapsed the sides of the film wedge inwards. During this collapsing process, the two sides of the wedge act as if they were hinged together at the top of the film. With this geometric model of film shape, the drainage rate of the film, dV/dt, can be expressed mathematically by Equation 3. dV c ds = WL2 dt 2 dt

(3)

Where V is the film volume, t is time, c is the change in film thickness per interference fringe, W and L are the film width and height, respectively, and s is the fringe density. If both sides of Equation 3 are divided by the cross-sectional area (WcsL) of the film at the bottom Equation 4 is obtained:

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Modelling the Stabilising Behaviour of Silicone Surfactants… (1/WScL) dV/dt = L/2s ds/dt

(4)

The quantity on the left of Equation 4 is the flux. If it is assumed that the flux is a hyperbolic function of time, namely, (5)

(1/WScL) dV/dt = b/t + t0 where b is a drainage constant and t0 is the time where hyperbolic drainage begins. Solving the differential equation for s yields Equation 6. s = k0 (t + t0 )

m

with m = −

2b L

(6)

where k0 is the fringe density at t = t0 As Equation 6 shows, a log-log plot of fringe density versus time should be linear for t > t0 with a slope of m (<0) and an intercept of logs0 = logk0 + mlogt0. This prediction was realised experimentally with high degrees of precision and accuracy. Therefore, the instantaneous film drainage rate is explicitly given by Equation 7. dV / dt =

c WL2 ⋅ mk0 t m −1 = −cWLbk0 t m −1 2

(7)

5.3.3.2 The Effect of Bulk Dynamic Viscosity on Film Drainage Rate The drainage rate measurement described in Section 5.3.3.1 was novel and required experimental verification of its accuracy and precision. This was accomplished by measurement of the dependence of film drainage rate on the reciprocal of bulk viscosity (see Figure 5.4). The Reynold’s equation (Equation 1) predicts that this correlation should be linear. As seen in Figure 5.4, a linear correlation was obtained. It should be noted that this form of the Reynold’s equation assumed an infinite surface viscosity. This condition was not able to be achieved in practice. However, it was assumed that the surface viscosity was constant throughout the range of experiments and that Equation 1 was valid under this condition. The excellent correlation achieved between film drainage rate and reciprocal bulk viscosity motivated the measurement of the correlations between drainage rate and surface variables.

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Figure 5.4 The experimental dependence of film drainage rate on bulk viscosity. The bulk viscosity was varied by varying the ratio of toluene to polyol in the formulation

5.3.3.3 The Effect of Surface Variables on Film Drainage Rate 5.3.3.3.1 Analysis of Commercial Silicone Surfactants Initially, the drainage rates of PU films stabilised by various commercial silicone surfactants were measured. These surfactants are applied in the marketplace to stabilise flexible slabstock PU foam. These data are given in Table 5.1. The surfactants listed in Table 5.1 span a range of drainage rates of 1.5 to 2. The narrowness of this range is not surprising because the PU foam stability (in terms of foam volume and porosity) achievable with these commercial surfactants does not vary to a great extent. By comparison, as described in Section 5.3.3.3.2, rigid-surfaced films have been observed which have drainage rates of less than 1% of those listed in Table 5.1. Within this context, the drainage rates in Table 5.1 are rather fast, consistent with the previously discussed low surface viscosity of the films. This is also consistent with

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Table 5.1 A comparison of commercial flexible slabstock PU foam surfactants in PU thin liquid films (1 pph surfactant) Surfactant

dV/dt x 104 (mm3/min) (Error ± 10%)

DC 198a

165

DC 5950a

190

a

170

DC 5039 L620

b

BF2370

173 c

107

DC: Dabco a Air Products and Chemicals, Inc. b Witco Specialties c Th. Goldschmidt AG

the consideration that these surfactants have been optimised to provide sufficient drainage rate to produce porous, flexible slabstock PU foam. Incidentally, one surfactant investigated (FC 171; 3M Company) gave a significantly higher drainage rate (271 x 10-3 mm/min) than the surfactants shown in Table 5.1. This surfactant, a non-ionic fluorocarbon-based material, is not sold as a stabiliser for PU foam. Although foam tests were not attempted with this stabiliser, one must presume, based on its high drainage rate, that these foams would be quite unstable.

5.3.3.3.2 The Effect of Surfactant Concentration on Film Drainage Rate Physical models for the potential effect of surfactant concentration on film drainage rate have been discussed in detail in Section 5.2. It was predicted that surface tension gradients and surface viscosity affect film drainage rate. It was expected that the drainage rate would increase with an increase in surfactant concentration due to the lessening of the intensity of surface tension gradients. However, It was also expected that the drainage rate would decrease as the surfactant concentration increased due to the increase of surface viscosity. This correlation was examined for two different silicone surfactants as illustrated in Figure 5.5. These plots show a maximum in film drainage rate as a function of surfactant concentration, consistent with the concept that the overall rate is the result of two opposing

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Figure 5.5 The effect of surfactant concentration on film drainage rate: Comparison of the two surfactants DC 198 (solid line) and DC 5950 (dotted line) 10 mm high films were drawn from the standard PU film formulation at a rate of 0.25 mm/s

effects. At lower concentrations, the increase in drainage rate observed with increase in surfactant concentration is consistent with a decrease in the intensity of surface tension gradients in the film due to increased bulk-to-surface surfactant flux. The decrease in drainage rate seen at higher surfactant concentrations is consistent with an increase of surface viscosity. These conclusions are supported by several published experimental studies. In their studies of three-phase capillary air slug (bubble) flow, Stebe and Maldarelli [47] correlated surfactant concentration to equivalent flow rate invoking similar reasoning. At a higher surfactant concentration, flow rate increased with surfactant concentration, due to the relief of surface tension gradients at the air bubble surface. When using protein-based surfactants, retardation of flow due to surface viscous effects (flow rate decreased with an increase in protein concentration) was observed.

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Modelling the Stabilising Behaviour of Silicone Surfactants… Following the work of Kanner and co-workers [37], the drainage of films stabilised by a TCP was examined. As previously mentioned, they observed a significantly low film drainage rate and attributed it to the high surface viscosity measured for these solutions. In studies of high TCP concentrations, rigid-surfaced films with very slow drainage (as detected by the movement of interference fringes) and surface flows were observed. At low TCP concentrations, the films were mobile-surfaced and fast draining. These observations are consistent with the concept that the surface viscosity increases with surface concentration. Further reinforcement of this concept is given by the data in Table 5.2. Assuming that surface viscosity is proportional to surfactant concentration, the inverse dependence of film drainage rate on surface viscosity is quite apparent from this data.

Table 5.2 The rate of film drainage of PU thin films as a function of the concentration of TCP [TCP] (ppm)

dV/dt x104 (mm3/min)

10

69 5

60

31 1

120

33

330

7

ppm: parts per million

Previously, in the literature, the ‘extremes’ of mobile and rigid surfaces were much discussed without significant mention of ‘intermediate’ behaviour. Intermediate behaviour was studied here by simple titration of the appropriate amount of TCP into the film solutions. In this regime, the film drainage rates were intermediate between those of mobile and rigid-surfaced films, as were the rate and extent of surface flows. For both Dabco DC 198 and Dabco DC 5950 (Air Products and Chemicals, Inc.), the surfactant concentration (approximately 1 pph) leading to maximal drainage rate, is similar to that optimally used in manufactured PU foam. Presuming that the surfactant concentration needed for effective film stabilisation is similar to that needed for effective foam stabilisation, it appears that a maximal drainage rate aids in the optimal production of flexible slabstock polyurethane foam.

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Advances in Urethane Science and Technology 5.3.3.3.3 The Effect of Surfactant Molecular Structure on Film Drainage Rate Once the effect of surfactant concentration on film drainage rate had been measured, the effect of molecular structure was investigated. It was hypothesised that the surfactant structure would affect the intensity of surface tension gradients within the film and the surface viscosity. Within this framework, the effect of molecular structure on the diffusion and surface partition coefficients, and on the degree of intermolecular cohesion in the surface layer was investigated. For TCP mentioned in the previous section, an accurate structural characterisation of the material was not available. However, it was known that it was a complex threedimensional siloxane structure, capped by Me3SiO- or -OH units. By comparing the films prepared from TCP solutions to those from solutions of highly purified (Me3SiO)8Si8O12, it was deduced that the source of the surface rigidity was the OH (silanol) groups on TCP. These groups can hydrogen bond to each other, thus forming a highly cohesive network at the surface and yielding a high surface viscosity. A similar effect was also noted in aqueous films containing carbohydrate functional organic surfactants [48]. The source of OH groups in these surfactants are the carbohydrate functionalities. These results motivated a more careful look at the specific effects of surfactant structure on film drainage rate using traditional silicone PU foam stabilisers. In order to accomplish this, the drainage rates of PU films stabilised by a series of surfactants of the general molecular structure (Me 3 SiO)(SiMe 2 0) 100 (SiMeRO) 10SiMe 3 (R= -(CH 2) 3 (EO) x (PO)xOC(O)Me, EO = ethylene oxide, PO= propylene oxide, X= 6, 12, 18, 21, 30) were investigated. These five surfactants differed only in the length of the alkylene oxide chains. Therefore, for the rest of this chapter, they will be referred to only by their X value. Relevant measured physical properties are listed in Table 5.3. Film drainage rate data is shown in Figure 5.6. Unfortunately, it was not possible to obtain accurate surface viscosity data for these materials. Analysis of the data in Table 5.3 and Figure 5.6 begins with four hypotheses regarding the physical consequences, to a silicone/polyether copolymer, of an increase in polyether length (X) at constant siloxane chain length. An increase in X leads to a copolymer with: 1) A higher ratio of solvophilicity (through the polyether portions) to solvophobicity (through the silicone portion) and therefore, a higher solubility in the liquid (polyether/ toluene mixture) phase. 2) A larger solvated volume and therefore a lower diffusion (through the liquid phase) coefficient.

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Table 5.3 Characterisation data for silicone surfactants Γ∞ × A Kad D 2 2 17 11 (x 10 mol/cm ) (A ) x 10 (cm) x 10 (cm2/s) 6 1.76 94 33.2 723 12 1.26 132 22.3 574 18 0.98 169 12.8 424 21 NM NM NM 377 30 1.04 160 21.0 66 × = length of alkylene oxide chains Γ∞ = surface concentration at surface saturation, also known as the surface adsorption A = area per molecule at the surface Kad = surface partition constant D = diffusion coefficient NM = not measured X

10

Figure 5.6 Film drainage rate as a function of surfactant concentration and structure (X = length of alkylene oxide chain)

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Advances in Urethane Science and Technology 3) A larger area at the liquid/air interface. 4) A larger area of polyether chain cohesion in the interfacial layer. This increase in cohesion area should result in an increase in cohesion energy per molecule and therefore, at surface saturation, a higher surface viscosity. Generally, the experimental data in Table 5.3 are consistent with these hypotheses. Firstly, the surface partition coefficient, Kad, is the ratio of the surface saturation concentration (Γ∞) to the overall amount of surfactant needed to saturate the surface (keeping in mind that a significant amount of the surfactant remains solubilised). Therefore, at relatively constant Γ∞, Kad should decrease as the solubility of the copolymer increases. This can be seen in Table 5.3. Secondly, the solvated volumes were not directly measured; however, the diffusion coefficients were measured and they decreased as X increased. It is generally known that the diffusion coefficient varies as the inverse of the solvated volume [49]. From this relationship, one would conclude that the solvated volumes of the copolymers increased with X. Thirdly, the interfacial areas of the copolymers generally increased as X increased. Finally, we were not able to measure the surface viscosities of these materials in solution. However, all of them yielded mobile-surfaced films. This means that their surface viscosities were relatively low, as expected. It is unclear why the X = 30 copolymer did not give data consistent with the expected trends in the data observed for X = 6-18. It would have been helpful to have had more data on the X = 21 species; perhaps, the deviations observed for the X = 30 species would have also been recognisable for the X = 21 data. For X = 30, one possible factor which could have yielded non-compliant data would have been an unexpected configuration of the polyether groups. After all, they are quite long (total of 60 monomer units). Perhaps they assumed an unusual coiled or spiral configuration both in bulk solution and at the interface. This would be consistent with an unexpected solvated volume, degree of solvation, interfacial area and area of polyether cohesion. Previous work with silicone polyethers has demonstrated unexpected results with exceptionally long polyethers [50]. One goal of this study was to correlate the data in Table 5.3 with the experimental drainage rate data for these surfactants, displayed in Figure 5.6. In order to interpret these correlations, the following hypothesis were developed. •

Low values of X correspond to a high tendency of the surfactant to adsorb at the surface (high Kad) and a large diffusion coefficient (D). This favours rapid relief of surface tension gradients. Furthermore, the surface viscosity values should be minimal under these conditions. Therefore, the films should drain rapidly and be relatively unstable.

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Modelling the Stabilising Behaviour of Silicone Surfactants… •

As X increases, Kad and D should decrease, which decreases the efficiency of relieving surface tension gradients. Also, the surface viscosity should increase. Both factors should cause a relative decrease in film drainage rate and furthermore, an increase in film stability.

The following conclusions can be made on analysing Figure 5.6. The profile for X = 6 is rather complex. Ignoring the points at very low concentrations (which are difficult to interpret), the profile has only the ‘first half’ of the expected parabolic shape. At moderately low concentrations, the drainage rate increases with concentration, consistent with the decrease in the intensity of surface tension gradients. Incidentally, this decrease in gradient is also reflected in the fact that the film lifetimes decrease in this range. In fact, above a certain concentration, films are too unstable to allow for the measurement of the drainage rate. This high degree of (Marangoni) instability, only observed for the X = 6 species, is consistent with an overly-efficient relief of surface tension gradients in the film. This efficiency is due to the high bulk-to-surface flux of this surfactant, which itself is due to its high values of Kad and D. At high surface concentrations, the films could potentially be stabilised by a high surface viscosity. However, for X = 6, the surface viscosity would be expected to be low, and this is borne out by the experimental data, i.e., the films are so unstable that drainage rate measurements could not be made. The drainage rate for X = 6 should be the highest of any of the surfactants. This is not confirmed by experiment. It is possible that the films where this would be demonstrated (at higher surfactant concentrations) were simply too unstable to be investigated. The expected parabolic shape of the drainage profile is observed for X = 12, and at a relatively low concentration, as might be expected. These films were more stable than those exhibited for X = 6, due to a less efficient relief of surface tension gradients. This would be expected, as Kad and D should be lower than for X = 6. The decrease in drainage rate at higher concentrations is due to the build up of surface viscosity. A higher surface viscosity than for X = 6 would be expected, as the area of polyether cohesion has increased. The drainage rate profile for X = 18 is at a lower value than either X = 6 or X = 12, consistent with stronger surface tension gradients and higher surface viscosity. The fact that film stabilities were higher than for either X = 6 or X = 12 is consistent with this conclusion. It is intriguing that, for X = 30, the ‘first half’ of the drainage profile is not observed at all. Perhaps it could exist at very low surfactant concentrations, but these films prematurely rupture. A sufficiently high concentration of surfactant is necessary at the surface to allow for the presence of the surface tension gradients that stabilise films in this regime 235

Advances in Urethane Science and Technology (in other words, ‘high’ but not ‘too high’). These low gradients would result in a high drainage rate and, a marginally stable film. As the concentration of this surfactant is increased, the drainage rate is strongly retarded due to the build up of surface viscosity. Viscous effects would be expected to be particularly strong in this system due to the long polyether chains, and the resulting large area of cohesion. Overall, the limitations of analysing these data in terms of simple, qualitative models motivated the development of more refined, quantitative models of the draining film. The development of these models is the topic for the next section.

5.4 The Development of Theoretical Models of Vertical, Draining Thin Liquid Model PU Films In quite a few of the experiments, there was an extension region in the upper part of the film that was essentially two-dimensional. In particular, this occurred in the early part of the experiments (typically several minutes, at least) and in the upper half to two-thirds of the film for nearly the entire width. Under these circumstances, the interference fringes were essentially horizontal and parallel. These were also the conditions under which the exponent for the drainage rate was measured. This leads us to consider two-dimensional models for film drainage; they can be considered as a cross-sectional slice down the centre of the film perpendicular to the surface of the film. All of the theory described in this section will be for this two-dimensional case. To develop a quantitative model, the problem was approached in stages. The first model assumed that the surface of the film was wedge-shaped; the narrowing with time was predicted in the tangentially-immobile case [51]. The film was then modelled with a deforming surface; the film still thins, but it takes on more complicated shapes predicted by the solutions of non-linear partial differential equations. These generalisations have resulted in a series of models that gradually incorporate more of the experimental behaviour. A few details and some results from these models will now be discussed.

5.4.1 Rigid-Surfaced Collapsing Wedge Model The simplest model was a theoretical version of the collapsing film wedge. As discussed previously (Section 5.3.3.1), the collapsing wedge geometry was imposed on the analysis of the experimental data to generate instantaneous film drainage rates. Therefore, it seemed reasonable to use this same shape in the development of the first theoretical model [51]. This led to the condition that the shape of the film was invariant with

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Modelling the Stabilising Behaviour of Silicone Surfactants… time. The other key assumptions applied in the construction of this model were that all of the flow in the film was downward (parallel flow) and that the surface of the film was tangentially immobile (infinitely large surface viscosity). Therefore, outside of surface viscosity, there was no specific surfactant effects on the drainage rate of the film. In fact, no surfactant transport or concentration gradient effects were incorporated in the model. A diagram of the model construction is shown in Figure 5.7. The film shape is represented by the solid line; the wedge shape that replaces the deforming film shape is sketched with the dashed line.

Figure 5.7 Schematic representation of the model film studied. The dashed-dotted line is the centreline of the film; the film is assumed to be symmetric around it. The solid curve is a sketch of the actual film surface; the dashed line is a wedge approximation to the film shape. Differences are exaggerated to clarity.

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Advances in Urethane Science and Technology Within the wedge, the momentum in the vertical direction is conserved by that component of the Navier-Stokes equations:

ρ

∂w ∂p ∂2 w = ρg − +μ 2 ∂t ∂z ∂c

(8)

To simplify matters, it is assumed that the flow is so slow and gradually changing that the left hand side of the equation can be neglected. The flat sides of the wedge along with the slow flow assumption allow the pressure gradient term to be neglected. The differential equation for the velocity profile in the film is then obtained: ∂2 w ρg + =0 μ ∂x2

(9)

The solution to this equation is subject to ∂w/∂x = 0 in the centre of the film and ω = 0 on the surface of the film, x = k (z, t). The velocity profile is given by

w( x, t ) =

ρg 2 k − x2 2μ

(

)

(10)

and the average velocity of the film is then found to be w=

ρg k2 3μ 4

(11)

This equation clearly indicates that drainage of the film is accelerated by increasing density and inhibited by increasing viscosity. Further analysis of this model yields a prediction that the slope of the plot of log (fringe density) against log (time) should be -0.5. These predicted results are in excellent agreement with experimental results where a rigid-surfaced film was investigated; experimental data gave a slope of -0.47. For the mobile-surfaced films investigated, the value of the shape ranged from -0.6 to -0.92; the more negative the value, the faster the film drainage. At this point, theory could be developed only for the rigid case; the development of theory for the mobile cases requires more complicated mathematical models, which are described next.

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5.4.2 Deforming Film Models A schematic of the draining film useful for setting up the mathematical theory is given in Figure 5.8. The free surface of the film, k(z, t), can now deform along its length with time, and partial differential equations for this and other dependent variables of interest will be developed. The mathematical problems in this section are specified in the following way, unless stated otherwise. The Navier-Stokes equations govern the motion of the incompressible, Newtonian fluid inside the film [52]. The film is assumed to be symmetric about its vertical centreline (the z-axis). On the free surface of the fluid, several equations must be satisfied: the kinematic condition, and the normal and tangential stress conditions [53, 54]. An equation governing the transport of surfactant in the free surface by both advection (fluid

Figure 5.8 Schematic representation of the model studied

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Advances in Urethane Science and Technology motion) and diffusion also holds [55, 56]. The surface tension, σ (in mks units: N/m), of the film-air boundary is assumed to vary linearly with the surface concentration, Γ: σ = σm +

∂σ Γm (Γ − Γm ) ∂Γ

(12)

The subscript m denotes the reference values of σ and Γ; these reference values are chosen for convenience in matching up with experiments. The additional parameters that show up in the boundary conditions on the free surface are the surface shear viscosity and surface dilatational viscosity, represented by μs and κs, respectively, and the surface diffusivity of the surfactant denoted by Ds. No fluid can enter or exit through the wire frame at the top, and the film surface is pinned there. At the bottom, the film drains into the bath (or pool) in the cuvette, from which the film is drawn. The bath is otherwise inert, and it is assumed that the film shape tends to a static meniscus shape over the bath. This assumption will allow boundary conditions over the bath to be specified. The details of the mathematical specification of the problem appear elsewhere [57-60]; in this section, the focus is on simplified models and comparing results from them with the experimental results discussed previously. The equations mentioned above will be made dimensionless with the length scales

d=

μV0 , D= ρg

σ and l = d1 / 3 D2 / 3 ρg

(13)

d, D and l are the film thickness, the static meniscus radius, and an intermediate scale, respectively. The horizontal length scale is d, while the length scale along the film is l . Of particular importance is that, for the PU solvent of interest here, the ratio of the lengths d/ l is a small quantity. The ratio δ 2 ≡ d / l << 1

(14)

is used to reduce the original mathematical fluid dynamics problem to a simpler set of partial differential equations; the mathematical method is a multiple scale analysis, and when it is applied to thin fluid layers, it is called lubrication theory. For recent reviews of this type of approach, see references [29, 61]. We choose V0 =

240

ρgd 2 μ

(15)

Modelling the Stabilising Behaviour of Silicone Surfactants… as the velocity scale; in mks units, V0 is in m/s. The time scale is l / V0 and the pressure scale is based on viscous shear, μV0 / ( lδ 4 ). The surface concentration is made dimensionless with the reference value, Γm. Several dimensionless groups appear in the non-dimensional equations; these are given in Table 5.4. These non-dimensional groups serve to highlight the relative importance of the myriad parameters in the problem; the interpretation of these groups in terms of ratios of physical forces are also given in Table 5.4. These parameters will be referred to as appropriate in the following sections. For the typical experiment of interest, the Reynolds number is very small; this will help make the simplifications that follow valid. The Péclet number (P) is large; it may be as large as 104 at the start of some experiments, according to our estimates. In all the computations presented here, P = 102 will be used. The typical value of δ is 0.1 to 0.2; the capillary number is then smaller than 10-4. The other quantities will be varied in the results to follow.

Table 5.4 Table of non-dimensional parameters and their interpretations. The third column gives an interpretation of the physical forces represented in the numerator compared to the physical forces represented in the denominator (Note that in our scalings C = σ6) Number

Definition

Reynolds

ℜ=

Modified Boussinesq

S=

Marangoni

Péclet

ρV0 l μ

Inertial Viscous shear

(κ + μ) V0 / l2 μV0 / d

Surface viscous shear Viscous shear

μV0 σm

Viscous shear Surface tension

C=

Capillary

M=

Comparison of forces

Γm

∂σ Γm δ 2 ∂Γ μV0

p=v

V0 l Ds

Shear from concentration gradients Viscous shear Advective transport Diffusive transport

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5.4.3 Tangentially-Immobile Films For high concentrations of some TCP in PU, the surfactant develops structure on the film surface and the surface of the film still deforms but doesn’t move tangentially. This ‘rigid’ case [22] is the slow draining limit of the experiment. In that case, the tangential stress condition may be replaced by the condition that the tangential velocity component is zero. We have been able to show mathematically that this case is achieved in the limit of large surface viscosity [59, 60] or S → ∞; this will be discussed further in the next section. When lubrication theory is used in this context, a single, nonlinear partial differential equation for the film shape k(z,t) is obtained. In one approach, which is called the ‘whole film’ model, the equation obtained is:

⎡k ⎤ kt + ⎢ 3 (κ z + 1)⎥ = 0 ⎣3 ⎦z

(16)

which holds over the domain 0 < z < L. The subscripts t and z denote partial differentiation; for example κz = δk/δz. Here

κ=

kzz

(1 + δ k ) 4

2 z

(17)

3 /2

is the curvature of the film. At the top of the film, z = 0, the film is pinned (k(0,t) = 1) and no fluid leaves or enters, by requiring that

κ z = −1

(18)

At the bottom of the film, we specify the slope, kz(L,t) = C, and the second derivative is specified via ⎛ 1 ⎞ C 4 2 kzz (L,t ) = 2⎜ 2 − ⎟ 1+ δ C 4 2 ⎝δ 1+ δ C ⎠

(

)

3 /2

≡ s1

(19)

This approach is based on methods for including the static meniscus in film models that have been developed by Ruschak [62] and Kheshgi and co-workers [63]; equation (12) is a first integral of the equation for the static meniscus that is the surface of the bath.

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Modelling the Stabilising Behaviour of Silicone Surfactants… The initial condition used for all calculations was a linear shape for k. This was a convenient shape, and for later times in the computations, the results are rather insensitive to the initial conditions. This problem is readily solved using numerical methods described in references [57]. Numerical results are given in Figure 5.9; one can see in the figure that distinct regions of the film developed: (i) a meniscus at the top of the film (at the wire frame); (ii) a long flat middle region where effects from the average surface tension were negligible; (iii) a region where bumps and dips developed near the bottom of the film and (iv) a static meniscus at the bottom of the film where it joined the bath.

Figure 5.9 Film shapes k(z,t) versus z with L = 37.5, h = 0.05, kz(L) = 5 and δ = 0.2. The initial slope of the film was 0.23

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Figure 5.10 Computed thickness for models without and with surface tension, solid and dashed lines, respectively, at t = 15 on the right and t = 231.9 on the left; here δ = 0.25. The case with surface tension is computed with kz(L,t) = 0 and neglecting all terms with d in equations (17) and (using C = 0 in Equation (19)); see [57] for details

A number of interesting possibilities arise. The bumps and dips at the bottom of this film could be the precursor to complicated three-dimensional behaviour observed at the bottom of films in experiments. Only further work will verify this conjecture, and we are currently pursuing that line of inquiry. The long middle region of the film offers the possibility of still further simplification. If one rescales the variables in the problem again, a much simpler equation for the (rescaled) free surface shape K(Z,T) arises, which we shall call the ‘flat film’ model. This model is

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Modelling the Stabilising Behaviour of Silicone Surfactants… KT + K 2 K z = 0

(20)

with the single boundary condition specifying K(0,T). Here Z =z/δ and T =t/δ are the rescaled space and time variables, respectively. Solutions found numerically for the film shapes from Equation (20) and a more complicated model for the film shape [57] are given in Figure 5.10. As time increases, the agreement between the approximations improves. In the middle part of the film, no matter which simplification is used, the numerical results indicate that the film is flattening out with t-1/2 behaviour at long times. This simple equation also has an exact solution, which has similarity behaviour for long times given by K = Z / T [57, 61]. Similar solutions have been found by Mysels and co-workers [22] and Moriarty and co-workers [64]. In this way, it can be found analytically that the rate of flattening of the film is proportional to t-1/2 for long times, in good agreement with the slowest-draining experimental case of t-0.47. Based on that success, this kind of simplification will be attempted with more complicated models to see if similar results can be achieved.

5.4.4 Finite Surface Viscosity When there is finite surface viscosity, the surface of the film is mobile. Lubrication theory for our vertical draining film gives a system of two partial differential equations describing the evolution of the film surface k(z,t) and the velocity of the fluid at the surface w(S)(z,t). These equations are ⎡ ⎤ k3 1 + κ z )⎥ = 0 kt + ⎢ kw( s ) + ( 3 ⎣ ⎦z

(21)

k (1 + κ z ) = 0 NS

(22)

wzz( s ) + Here N≡

1 1 + δ 4 kz2

(23)

The boundary conditions at the top are k(0, t) = 1, κ z (0, t) = −1 and w ( s ) (0, t) = 0

(24)

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Advances in Urethane Science and Technology and at the bottom are kz (L,t) = C, kzz (L,t) = s1 and w ( s ) (L,t) = 0

(25)

Here C is the specified slope and a consistent s1 can be calculated with the first integral of the equilibrium meniscus shape [62, 63], as in the rigid case discussed above. In Equations (21) and (24), κ is the curvature of the film given in Equation (27). Previous work [62, 63] has shown that it is possible to keep the full curvature in the normal and tangential stress conditions and integrate the partial differential equation for the free surface k through the matching region (where the film meets the bath) onto the static meniscus; this model relies on those results. While we do not rigorously apply matched asymptotic expansions in this work, the equations contain all of the terms necessary to match the film onto a static meniscus (for the bath surface) and it has been shown that the terms neglected are uniformly small for δ<<1 [65]. By introducing surface velocity, w(S) (z,t), as a dependent variable in the model, we avoid computing a fifth order derivative of k. This formulation greatly simplifies our task; a complete description of the above derivation can be found in references [58, 60, 65]. During this work such a formulation was applied in a similar situation [66, 67]. Some computed results are shown in Figures 5.11 and 5.12. For large S, the film drains slowly; the downward parabolic shape in the middle region (Figure 5.11; free surface shapes for varying S) agrees with the typical rigid film profiles given in Chapter III of Mysels and co-workers [22]. In the limit of S → ∞, w(S) approaches zero along the whole film and the free surface shapes tend to that of the tangentially-immobile case [57]. It is also found that for very large S, the film thickness in the middle region tends to decrease with the power law t -0.5 as before; this is very close to the experimental rate of t-0.47 obtained at Dow Corning [68] for their most rigid film and agrees with results from Mysels and co-workers [22]. As S decreases, drainage is much faster and the film thins more rapidly (Figure 5.11); free surface shapes for smaller S, take on the typical shapes for mobile films given in Chapter IV of Mysels and co-workers [22]. In the limit of small S, t-1 thinning in the middle of the film is found which agrees well with the maximum experimental rate of t-0.92. This model is able to span the film drainage behaviour from rigid to mobile films. If the simplification for the flat middle part of the film is pursued. In that case, the simplified equations become

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Figure 5.11 Film shape at t = 16 for several values of S (shown in the upper right). The film is much thinner for small values of S because the free surface is mobile. For large values of S, the free surface becomes tangentially immobile.

⎡ K3 ⎤ KT + ⎢ KW ( s ) + ⎥ =0 3 ⎦z ⎣

(26)

K =0 S

(27)

Wzz

( s)

+

Once again the capitalised variables are rescaled versions of those in Equations (25-29) [60]. These are subject to the boundary conditions:

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Figure 5.12 w(s) with t = 16 and kz(L,t) - 10 varying S shown in the upper right. The surface velocity decreases as S increases

and

K = W ( s ) = 0, at Z = 0

(28)

WZ( s ) = 0, at Z = L

(29)

The initial condition for K is K(Z, 0) = 1 + 0.23Z

(30)

and a consistent W(s)(Z,0) is found from its equation. The consistent value of W(S)(Z,0) is required for the numerical solution of the problem [60].

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Modelling the Stabilising Behaviour of Silicone Surfactants… By studying these equations for large and small S, the slow and fast draining limits of the experiment have been obtained using analytical expressions [60]. For large S, solutions of the following form are found:

K≈

f1 (Z) f (Z) , and W ( s ) ≈ 2 1 / 2 1 /2 T T

(31)

where f1(Z) and f2(Z) satisfy ordinary differential equations. The solutions approximate computed solutions for the rigid film case very well. In this way the rigid case for large surface viscosity within lubrication theory is analytically recovered. For small S, we can find solutions of the form

K≈

f3 (Z) f (Z) , and W ( s ) ≈ 4 T T

(32)

where f3(Z) and f4(Z) also satisfy ordinary differential equations. In this way, the fast draining limit of the experiments, where the measured exponents for the fastest thinning case are -0.92, is analytically recovered. Surfactant transport will now be included in the model, in an effort to obtain intermediate thinning rates as observed experimentally.

5.4.5 Adding Surfactant Transport Surfactant transport is also important in the vertical draining film; surfactant concentration gradients may develop, which may in turn strongly affect the fluid flow via the Marangoni effect. When surfactant transport is considered, lubrication theory then gives three nonlinear partial differential equations for the free surface shape k(z,t), the surface velocity w(S)(z,t) and the surface concentration of surfactant Γ(z,t). The mathematical problem to be solved for these dependent variables is: ⎡ k3 ⎤ k2 + ⎢ kw( s ) + (1 + κ z ) ⎥ = 0 3 ⎦z ⎣ wzz( s ) +

k M (1 + κ z ) + 2 Γz = 0 NS N S

(33)

(34)

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Advances in Urethane Science and Technology ⎡ ⎤ 1 Γt + N 2 ⎢Γw ( s ) − N 2 Γz ⎥ = 0 P ⎣ ⎦z

(35)

inside the film. Boundary conditions are, at the top: k(0, t) = 1, w ( s ) (0, t) = Γz (0, t) = 0, κ z (0, t) = −1

(36)

and at the bottom: kz (L,t) = C, kzz (L,t) = s1 and w ( s ) (L,t) = Γz (L,t) = 0

(37)

where L is the lower end of the computational domain; C is specified and a consistent s1 can be calculated with the first integral of the equilibrium meniscus shape [62, 63], as discussed previously. Some normalisation factors in the surface transport equation for G that ordinarily would become unity in lubrication theory at leading order have also been retained. While we do not rigorously apply matched asymptotic expansions in this work, the equations contain all of the terms necessary to match the film onto a static meniscus (for the bath surface) and it has been shown that the terms neglected are small for δ<<1 [65]. Marangoni effects have a substantial impact for smaller values of S, retarding surface drainage and enhancing film thickness; Figure 5.13 shows free surface shapes for several M with S = P = 102 and δ = 0.1. For a small M, the Marangoni effect is too weak to retard drainage and the film thins rapidly. As M increases, the Marangoni effect become more prominent, retarding drainage and the film is considerably thicker at the same time in the computation. As M → ∞, w(S) → 0 all along the film and the surface becomes rigid. A rigid fluid surface associated with a large Marangoni number was observed in a levelling problem [69] and in a similar geometry to this work. Interesting free surface shape features are shown in Figure 5.14, for M = -10, S = 102 with varying t. There are parts of the free surface shape that appear to be draining rapidly, as if they are mobile, and other parts that drain slowly, as if they were rigidsurfaced. Concave-out profiles above an upward-propagating wave indicate a mobile surface while an essentially downward-parabolic profile following the wave indicates a rigid surface. These assertions are supported by the thinning rates of t-1 above the wave and t-1/2 below it. The wave that separates the two regions is driven by surface tension gradients that are shown in Figure 5.15. The localised gradient of the surfactant concentration on the surface is sufficient to drag fluid up the film; the gradient is preserved

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Modelling the Stabilising Behaviour of Silicone Surfactants…

Figure 5.13 Film shape at t = 16 with S = 102 fixed for several values of M (shown in the upper right). The film is much thinner for small values of M. For large values of M, Marangoni effects are stronger, drainage is retarded and the free surface becomes tangentially immobile.

because the Péclet number is large, and so it is not smeared out by diffusion. The associated surface velocity profiles are shown in Figure 5.16; the negative surface velocity clearly shows that fluid is moving up the film on its surface, as a result of the Marangoni effect. The wave seen in Figure 5.14 propagates up the entire length of the film. Similar behaviour is seen in the experiments of Snow and co-workers [70, 71] with PU films, where what appears to be bumps propagate up the edges of the film. On that basis, it is believed that the wave observed may be related to those structures observed in the three-dimensional films. Work is currently being done to verify this conjecture.

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Figure 5.14 Film shape with S = 102 and M = -10 fixed for several values of time t (shown in the upper right). A wave driven by surface tension gradients propagates up the film. Thinning proportional to t-1 and concave-out profiles above the propagating wave indicate a mobile surface; below the wave, t1/2 thinning and the downward parabolic shape indicate a rigid surface.

If S < 102, it is found that the film drainage is typically too rapid to allow for much effect from surface tension gradients (and thus Marangoni stresses). For S > 104, we find that the films behave, in large part, as if they were rigid [70]. It is clear that the maximum impact of the Marangoni effect occurs at an intermediate value of the surface viscosity. These results lend support to the assertion that the greatest foam stability occurs when the Marangoni and surface viscous effects are at intermediate value, as given in the first section of this chapter.

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Modelling the Stabilising Behaviour of Silicone Surfactants…

Figure 5.15 Surfactant distribution with S = 102 and M = -10 fixed for several values of time t (shown in the upper right). The surface tension gradients driving the wave up the film are easily seen.

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Figure 5.16 Surface velocities with S = 102 and M = -10 fixed for several values of time t (shown in the upper right). Negative values indicate flow up the film, and flow up the film is caused by the surfactant concentration gradients.

5.5 Summary The stabilising behaviour of silicone surfactants during the formation of PU foam can be investigated in detail, both experimentally and theoretically by the use of a foam model system, specifically vertical, thin liquid PU films. These films were investigated by a combination of direct observation and interferometric measurements. The formation and temporal evolution of structures on the surface of the film, including fingering patterns, was a result of the Marangoni effect. Experimentally, these films demonstrate a wedge shape and the gravity-driven drainage of these films causes a collapsing inwards of the wedge. The evolution of structures and the drainage of these films were functions of the concentration and molecular structures of the surfactants. The two film physical

254

Modelling the Stabilising Behaviour of Silicone Surfactants… parameters correlated to film structures, film drainage and the properties of the surfactant are: (1) the strength and lifetime of surface tension gradients, (2) the surface viscosity. Two extremes of film behaviour were identified. Firstly, a rigid-surfaced film where the rate of drainage and structural evolution were low. Secondly, a mobile-surfaced film where the rate of drainage and structural evolution were high. Intermediate cases were also detected and measured. The theoretical and numerical model for vertical liquid film drainage that has been developed reproduces a number of features described in these experiments. These features include film shapes and thinning rates. Complex, time-dependent, three-dimensional structures were observed in the region where the film meets the bath (away from the edges of the film). It is also believed that the localised bumps and dips at the bottom of the film in these two-dimensional results, may be the precursor of those observed motions in the experiments; this notion is currently being investigated. Future work will include adding intermolecular forces that act across the film and extending our work to three-dimensions in order to make closer comparison with experiment. Furthermore we endeavor to obtain accurate independent measurements of surface viscosity and correlate them to both film properties and surfactant parameters.

Acknowledgements RJB and SN gratefully acknowledge support from Dow Corning Corporation for this work. RJB is also grateful for the support of the National Science Foundation via grants DMS9623092 and DMS-9631287. SAS and UCP would like to thank Ben Vesper, Mike Owen, Andy Goodwin, John Frey, Randy Hill, Dave Battice, Mike Stanga, Kathy Goudie, Joan Sudbury-Holtshlag, Mike Reuter, and Gloria Lyon for many helpful discussions and their support.

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Advances in Urethane Science and Technology 34. R. M. Herrington, and R. B. Turner, Advances in Urethane Science and Technology, 1992, 11, 1. 35. A. J. De Vries, Recueil, 1958, 77, 383. 36. I. B. Ivanov, in Thin Liquid Films, Surfactant Science Series No.29, Marcel Dekker Inc., New York, 1988, p.419. 37. B. Kanner, B. Prokai, C. S. Eschbach and G. J. Murphy, Journal of Cellular Plastics, 1979, 15, 6, 315. 38. M. J. Owen, Industrial Engineering Chemistry, Product Research and Development, 1980, 19, 67. 39. R. Herrington, K. Hock, Flexible Polyurethane Foams, Dow Plastics, Midland, MI, 1991, 9. 40. Photonics Dictionary, Volume 3, The Photonics Design & Applications Handbook, in the chapter on ‘Interpretation of Interferograms’. Laurin Publishing Co., Oakbrook Terrace, IL, 1991. 41

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42. J. B. M. Hudales and H. N. Stein, Journal of Colloid and Interface Science, 1991, 138, 2, 354. 43. H. N. Stein in Surfactants in Solution, Volume 11, Eds., K. L. Mittal and D. O. Shah, Plenum Press, New York, 1991, 481. 44. H. N. Stein, Advances in Colloid and Interface Science, 1991, 34, 175. 45. K. Koczo, J. K. Koczone and D. T. Wasan, Journal of Colloid and Interface Science, 1994, 166, 1, 225. 46. J. B. M. Hudales and H. N. Stein, Journal of Colloid and Interface Science, 1990, 137, 2, 512. 47. K. J. Stebe and C. Maldarelli, Journal of Colloid and Interface Science, 1994, 163, 1, 177. 48. S. A. Snow, B. M. Nugent and F. L. Baines, Unreported observations.

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Modelling the Stabilising Behaviour of Silicone Surfactants… 49. Principles of Colloid and Surface Chemistry, 2nd Edition, Ed., P. C. Heimenz, Marcel Dekker, Inc., New York, 1986, p.65 and p.83. 50. T. E. Gentle and S. A. Snow, Langmuir, 1995, 11, 2906. 51. U. C. Pernisz and S. A. Snow, The Drainage of Liquid Polyurethane Films, Dow Corning Corporation Research Report, 1999, 1999-I000-46182, Midland, MI. 52. G. K. Bachelor, An Introduction to Fluid Dynamics, Cambridge University Press, Cambridge, 1987. 53. D. A. Edwards, H. Brenner and D. T. Wasan, Interfacial Transport Processes and Rheology, Butterworth-Heinemann, Boston, 1991. 54. J. C. Slattery, Interfacial Transport Phenomena, Springer-Verlag, New York, 1990. 55. H. A. Stone, Physics of Fluids A, 1990, 2, 1, 111. 56. H. Wong, D. Rumschitzki and C. Maldarelli, Physics of Fluids, 1996, 8, 11, 3203. 57. R. J. Braun, S. A. Snow and U. C. Pernisz, Journal of Colloid and Interface Science, 1999, 219, 2, 225. 58. S. Naire, R. J. Braun and S. A. Snow, Dow Corning Corporation Research Report, 1999, 1999-I0000-47199. 59. S. Naire, R. J. Braun and S. A. Snow, Journal of Colloid and Interface Science, 2000, 230, 1, 91 . 60. S. Naire, R. J. Braun and S. A. Snow, SIAM Journal on Applied Mathematics, 2000, 61, 3, 889. 61. A. Oron, S. H. Davis and G. B. Bankoff, Reviews of Modern Physics, 1997, 69, 931. 62. K. J. Ruschak, AIChE Journal, 1978, 24, 705. 63. H. S. Kheshgi, S. F. Kistler and L. E. Scriven, Chemical Engineering and Science, 1992, 47, 683. 64. J. A. Moriarty, L.W. Schwartz and E.O. Tuck, Physics of Fluids A, 1991, 3, 5, 733.

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Advances in Urethane Science and Technology 65. S. Naire, R. J. Braun and S. A. Snow, An Insoluble Surfactant Model for a Vertical Draining Free Film II. Film Plus Meniscus in preparation, 2000. 66. L. W. Schwartz and R. V. Roy, Journal of Colloid and Interface Science, 1999, 218, 309. 67. O. E. Jensen, Physics of Fluids, 1994, 6, 3, 1084. 68. S. A. Snow, B. M. Nugent and U.C. Pernisz, Dow Corning Corporation Research Report, 1996, 1996-I0000-42699. 69. L. W. Schwartz, R. A. Cairncross and D. E. Weidner, Physics of Fluids, 1996, 8, 7, 1693. 70. S. A. Snow, U. C. Pernisz and R.E. Stevens, Dow Corning Corporation Research Report 1995, 1995-I0000-40859. 71. S. A. Snow, U. C. Pernisz and R. E. Stevens, Presented at the Polyurethanes Expo ‘98, Dallas, TX, 1998, p.217.

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea-Acrylic/Styrene Polymer Dispersions Janusz Kozakiewicz, Anita Koncka-Foland, Jan Skarzynski and Izabella Legocka

6.1 Preface Because of several limitations in using solvent-based systems worldwide, there is a growing demand for new types of aqueous polymer dispersions for a wide application range (coatings, adhesives, leather or textile finishing compounds). Especially interesting seems to be combining the excellent application properties of vinyl/acrylic polymer and copolymer dispersions such as adhesion, suitable viscosity, high solids content and surface wettability with excellent properties of films made of polyurethane dispersions (DPUR) such as mechanical strength, elasticity, solvent resistance and abrasion resistance. Since blending of both types of dispersions did not result in truly positive effects, the solution of choice must be synthesis of combined (hybrid) dispersion systems containing both polymers. This chapter will be devoted to a description of methods of synthesis and a discussion of the structure/properties relationships of such systems, especially when applied as coatings.

6.2 Introduction 6.2.1 General Considerations There are strict ecological limitations relating to allowable volatile organic compounds (VOC) content in coatings introduced in many regions and countries, e.g., the California Rule of 1995 [1] or special provisions in the EU, and which force the manufacturers to search for new, more environment-friendly formulations for particular products which usually requires replacing of the substantial binder (polymer or resin). Undoubtedly the products for which the development has been the most significant in the field of coatings over the last few decades are aqueous dispersion-based and powder coatings. In the former group, DPUR, which are presently being widely used as binders for coating formulations have already taken a leading position [2-5] and compete successfully with acrylic dispersions despite the distinctly higher market prices. Both blends of DPUR and acrylic dispersions [6], and (recently) hybrid acrylic-urethane dispersions [7-12], obtained by synthesis, and not simple blending are now available. The latter products are much 261

Advances in Urethane Science and Technology more interesting from both a scientific and practical point of view since the synthesis of a hybrid system may lead to new polymer architectures which may result in considerable improvement of properties. Especially attractive is the possibility of synthesising interpenetrating polymer network (IPN) systems based on polyurethanes and acrylic polymers using this principle since it has been shown that in such systems synergistic effects may occur [12], i.e., the properties of a hybrid may be better than those of a blend of its components. At the same time, hybrid dispersion particles usually have quite complex morphology (in most cases a ‘core-shell’ type) which affects properties of dispersions and films and coatings made of them [13]. If a comparison were made between hybrid dispersion systems taking into account the kind of dispersion particles they contain, it would appear that the best properties should generally be observed for systems obtained by synthesis for which the particles have a uniform structure (they may be called ‘true hybrid’ particles) and the worst – for simple blends of the two dispersions [14] (see Figure 6.1). Obviously, both types of dispersion particle morphology presented in Figure 6.1 (‘coreshell’ and ‘true hybrid’) should be considered as idealised cases. In practice, a variety of different particle morphologies may be observed. Three of them are shown in Figure 6.2. The most important factor that affects the morphology of hybrid dispersion particles seems to be the method of preparation of such hybrids. Generally, the methods of preparation of aqueous hybrid polymer dispersions may be classified into four groups [17]: 1) blending of two dispersions (a) addition of polymer A dispersion to polymer B dispersion (b) addition of polymer B dispersion to polymer A dispersion 2) synthesis of polymer A in a dispersion of polymer B 3) synthesis of polymer B in a dispersion of polymer A 4) ‘in situ’ preparation of hybrid dispersions (a) co-emulsifying of polymers A and B in water (b) co-emulsifying the mixture of polymers A and B (c) co-emulsifying the system consisting of polymers A and B: • IPN composed of polymers A and B (or a precursor of such IPN) • chemically linked polymers A and B (or a precursor of such system) 262

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a) Blend of dispersions ⇓ no synergistic effect • poor stability • matte and hazy films and coatings • always two Tgs • poor mechanical properties of films

b) ‘Core-shell’ ⇓ synergistic effect possible • good stability • usually matte and hazy films and coatings • usually two Tgs • good mechanical properties of films

c) ‘True-hybrid’ ⇓ synergistic effect probable • excellent stability • usually glossy and transparent films and coatings • usually one Tg • excellent mechanical properties of films

Figure 6.1 Comparison of hybrid dispersion systems obtained by (a) blending, (b) and (c) by synthesis, taking into account particle type.

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‘engulfed’ or ‘englued’ morphology [15] (the particles of one polymer are ‘glued’ to the particles of the other)

‘gradient’ morphology [15] (there is a gradient of concentration of one polymer in the other in the hybrid particle)

‘fruit-cake’ morphology [16] (particles of one polymer are distributed in a matrix of the other)

Figure 6.2 Examples of different particle morphologies of hybrid dispersions obtained by synthesis (such morphologies may be considered intermediate between ‘core-shell’ and ‘true hybrid’ morphology shown in Figure 6.1).

In practice, methods (2) and (3) seem to be the most commonly used. Recently, yet another method of preparation of hybrid polymer dispersions, called an ‘active blending’ has been reported [18]. The components of such ‘active blends’ or ‘active mixtures’ remain inactive when in the presence of water, and cross-react during film formation. The other factors that may influence hybrid dispersion particle morphology will be discussed in detail in Section 6.3.2 of this Chapter.

6.2.2 Acrylic Dispersions and Polyurethane Dispersions (DPUR) Aqueous acrylic dispersions have been used as binders for coatings for over 60 years. Their main advantages include: low cost and excellent tolerance of additives or pigments; however, they have several drawbacks such as the presence of relatively large amounts of water-sensitive additives (protective colloids, emulsifiers, etc.) and the need to use coalescing agents when a continuous film is desired. Aqueous DPUR appeared on the market much later (in the 1960s), and after only a couple of years achieved the quality that allowed them to be widely applied as coating binders. Substantial advantages of DPUR include excellent mechanical properties and solvent resistance. Their drawbacks are relatively high cost and poor tolerance of additives and pigments.

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(a)

(b)

- polyol chain - hydrophilic moiety - hard segment (urethane or urea) Figure 6.3 Main structures of polyurethane or polyurethane-urea chain in DPUR. (a) Hydrophilic groups are not incorporated into the polymer chain (are present as pendant groups); (b) Hydrophilic groups are incorporated into the polymer chain

Methods of preparation of aqueous acrylic dispersions are well known [19] and will not be discussed here. Emulsion polymerisation of acrylic monomers is usually carried out using a water-soluble initiator or a ‘redox’ system. In this process, relatively high levels (up to several %) of emulsifiers and protective colloids are used. The dispersed polymer generally has a linear structure and a high molecular weight (MW). The properties of dispersions and films are mostly affected by the kind of monomer(s) used [20]. DPUR can be prepared in many ways [21, 22], which eventually lead to one of the structures (polyurethane or polyurethane-urea) shown in Figure 6.3. In practice, the structures shown in Figure 6.3 are partly or fully crosslinked. The two main methods which lead to these structures are the ‘acetone process’ (historically older) [23] and the ‘prepolymer-ionomer process’ [24] that is commonly used to manufacture DPUR at present. A combination of these two methods can also be used. In the ‘acetone process’, the first step is the synthesis of NCO-terminated urethane prepolymer which is then dissolved in acetone and reacted with a di-hydroxy or diamino compound containing groups which are the precursors of ionic groups, e.g., -SO3, -COOH, N≡. The next steps are the emulsification of the resulting product in water combined with the formation of ionic groups and distillation of acetone.

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Advances in Urethane Science and Technology In the ‘prepolymer-ionomer process’, the compound which contains the moieties which are the precursors of ionic groups is incorporated in the polymer chain already at the stage of urethane prepolymer formation. Then they are neutralised, which results in the formation of a ‘prepolymer-ionomer’ which is emulsified in water and eventually crosslinked. In this process, the prepolymer-ionomer is usually dissolved in a small amount of water-miscible solvent of high boiling point, e.g., N-methylpyrolidone, which plays the role of coalescing agent in the process of film formation. It is then possible to obtain DPUR which contain either ‘cationic DPUR’ with a pH of less than 7 (cationic moieties are attached to the polyurethane or polyurethane-urea chain) or ‘anionic DPUR’ with a pH of greater than 7 (anionic moieties are attached to the polyurethane or polyurethaneurea chain). If non ionic hydrophilic moieties are attached to or incorporated in the polyurethane or polyurethane-urea chain, then a ‘nonionic DPUR’ may be obtained. Properties of DPUR depend on several factors. The most important are structure of the polyurethane or polyurethane-urea chain, degree of crosslinking and the amount of coalescing agent [25].

6.2.3 Hybrid Acrylic-Urethane Dispersions Information on the preparation of hybrid acrylic-urethane dispersions by synthesis, published so far is quite scarce. More detailed data on this process are available only in the patent literature. The basic process was first patented by Inmont, USA, in 1982 [9]. They simply claimed polymerisation of vinyl polymers in DPUR. In 1986, a patent for another method of preparation of hybrid acrylic-urethane dispersions (diluting of prepolymer-ionomer with monomers, emulsifying the resulting solution in water and polymerisation) was granted to Witco, USA [26]. In subsequent years the number of patents dealing with hybrid acrylic-urethane dispersions increased rapidly, and approximately 30 patents have been granted so far worldwide, the majority in Japan. Despite the large number of patents, at the time acrylic-urethane hybrid dispersions entered the market (in about 1990), their properties, and specifically their structure/properties relationships and particle morphology, had not been described in the scientific literature until 1992, when Gruber from Air Products presented a paper at the ACS Meeting in San Francisco [27] that dealt with the general characterisation of such dispersions. In 1996, Hegedus and Kloiber (also from Air Products) published an important paper [28] where they presented the properties not only of dispersions but also of coatings made on their basis. However, there was no information on composition and chemical structure of products

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… referred to in their article. To prepare hybrid dispersions, they diluted a prepolymer-ionomer with acrylic monomers, emulsified them in water and carried out emulsion polymerisation of the monomers. They found, amongst other things, that unlike films obtained from blends of acrylic dispersion and DPUR, the film obtained from the hybrid dispersion showed only one glass transmission temperature (Tg), but no information on the particle morphology was given. Some data on particle morphology was presented in 1997 in two publications by Zeneca [29, 30] who described the properties of hybrid acrylic-urethane dispersions crosslinked by formation of azomethine groups from the reaction between carbonyl groups present as pendant groups in the polyurethane-urea chain and amino groups from the polyamine used for crosslinking the dispersion by reaction with free NCO groups of the prepolymer-ionomer. In one of these articles [30] the properties of this kind of dispersion were compared with the properties of its analogue crosslinked additionally by oxidation of double bonds present in polyurethane-urea chain (air-drying) and it was concluded that the former resulted in higher coating hardness. The authors of both publications presented their schematic drawings showing the ‘core-shell’ morphology of their dispersion particles, but did not reveal any experimental evidence that would confirm such morphology. Another interesting paper dealing with hybrid vinyl/acrylic-urethane dispersions was published in 1996 by Cytec [31]. They prepared hybrid dispersions by swelling DPUR particles with monomers at a polyurethane:vinyl component ratio of 3:1 (w/w) and then carrying out emulsion polymerisation. The resulting hybrid dispersions were additionally crosslinked by the reaction of epoxy groups (when glycydyl methacrylate was used as one of the monomers) with carboxylic groups attached to the polyurethane-urea chain or by using TMI (unsaturated aliphatic isocyanate; Cytec industries, USA) as one of the monomers. The authors suggested that IPN structures were formed in these reactions. The paper contains very good photographs of dispersion particles, but no information on particle morphology is given. The effect of type of diisocyanate and polyol used in synthesis of hybrid PU/acrylic polymer dispersions was recently reported by authors from Lamberti [32]. A recently published patent [33] from Zeneca described in detail the methods of preparation of hybrid polyurethane/vinyl polymer dispersions but neither discussed the effect of various factors on the properties of the hybrid dispersions nor provided any information on their particle morphology. Based on this brief literature survey, it seems quite clear that not all features of the hybrid acrylic-urethane dispersions have been studied so far. The purpose of the present study is to clarify the effect of various factors on the properties of acrylic/styrene-urethane dispersions. Some results of preliminary investigations of the particle morphology will also be revealed.

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6.3 Concept of the Study 6.3.1 Selection of Starting Materials It was decided that synthesis of DPUR would be carried out according to the ‘prepolymerionomer’ method generally described in Section 6.2. The scheme of the reactions leading to DPUR is presented in Figure 6.4.

Step I - polyaddition

Step II - neutralisation

Step III - emulsifying + crosslinking

Figure 6.4 Schematic representation of the reactions leading to DPUR using the ‘prepolymer-ionomer process’

For the synthesis of DPUR, a cycloaliphatic diisocyanate, isophorone diisocyanate (IPDI) from Hüls, was used since unlike aromatic isocyanates, it does not result in yellowing of the coatings and films. The polyols used were polytetramethylene glycol (PTMG) with a MW of 2000

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… supplied by BASF and commercially available polyesterdiols commonly used in coatings (Bester 190 and 195, supplied by Polychimica) with hydroxyl numbers equal to 32 and 122, respectively. Polyesterdiol containing double bonds (Irpurate 240Tz from InterRokita, Poland, with hydroxyl numbers equal to 276) was also used in some experiments. Both polyether- and polyester-diols were used to look at the effect of structure of the polyurethane-urea chain on the properties of the dispersions, films and coatings. N-methylpyrrolidone (NMP) from Sidaco was used as cosolvent (coalescing agent) and 2,2´-dimethylolpropionic acid (DMPA) from Angus was used as the source of pendant carboxylic groups in the polyurethane-urea chain. Polymerisation of acrylic monomers was carried out according to the standard procedure using two different initiating systems: (a) Water-soluble initiator – potassium persulphate (b) ‘redox’ system – cumene hydroperoxide (CHP) + Rongalit with the structure:

Rongalit

The choice of initiating systems was made by considering their possible effect on particle morphology (see section 6.3.2). The other additives (emulsifiers, protective colloids) were standard materials commonly used in emulsion polymerisation of acrylic monomers. It was assumed that three monomers would be used in this study: methyl methacrylate, butyl acrylate and styrene. The possiblity of using styrene seemed to be especially interesting because of its very low cost compared to the other monomers. The other two monomers were selected for the significant differences in their polarity and solubility in water that might affect the particle morphology and the Tg of poly(methyl methacrylate) and poly(butyl acrylate). Difunctional acrylate monomer (1,4-butanediol dimethacrylate) was also used in some experiments since it encouraged the formation of an IPN structure.

6.3.2 Assumptions for Synthesis of Hybrid Dispersions 6.3.2.1 General Considerations Based on the literature survey (see Section 6.2) and the author’s preliminary experiments the following methods of synthesis of hybrid dispersions (after this point in the chapter they will be called modified DPUR (MDPUR)) were selected:

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Advances in Urethane Science and Technology 1) radical polymerisation of monomers in DPUR obtained as presented in Figure 6.4 and then diluted with water. a) with continuous feeding of monomers b) after swelling of DPUR particles with monomers for 20 hours 2) using the monomers as active diluents (co-solvents) of prepolymer-ionomer followed by their polymerisation after emulsifying the prepolymer-ionomer in water 3) emulsifying the prepolymer-ionomer in diluted acrylic-styrene dispersion (ASD) It was assumed that the method of MDPUR synthesis would have be the major factor that would determine particle morphology. In Figures 6.5-6.7, the methods (1) – (3) applied in this study are presented schematically. Hybrid dispersions obtained by polymerisation, i.e., according to methods 1a, 1b and 2, will be designated as MDPUR-ASD and hybrid dispersions obtained by synthesis of PUR in ASD, i.e., according to method 3, will be designated as MDPUR.

Figure 6.5 Method 1 (radical polymerisation of monomers in DPUR

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

Figure 6.6 Method 2 (using the monomers as active diluents for prepolymer-ionomer)

Figure 6.7 Emulsifying the prepolymer-ionomer in diluted ASD

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6.3.2.2 Analysis of the Process of MDPUR Synthesis Using Dispersion Particle Morphology •

Method 1: Polymerisation of monomers in DPUR

Before the polymerisation starts, the system will consist of the following components: (a) DPUR particles that will be only slightly swelled with monomers (in Method 1a – continuous feeding of the monomers) or fully swelled with monomers (in Method 1b – swelling of DPUR particles before polymerisation)

(b) Monomer droplets which will be obviously more numerous in Method 1a

(c) Monomer dissolved in water

~ ~~~ ~~ ~ ~~ ~~~~~~~~ ~~ ~ ~ (d) Micelles of emulsifier

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… When the process is carried out according to Method 1a it may be assumed that the kind of initiating system (water-soluble initiator or redox system) does not have any significant effect on the morphology of hybrid dispersion particles since the polymerisation proceeds mainly on the surface of DPUR particles and monomer droplets and in the micelles of emulsifier. In the case of water-soluble initiator (potassium persulphate) the polymerisation starts in water where the ion-radicals are formed from the monomer molecules dissolved in water. These ion-radicals can diffuse to DPUR particles, monomer droplets and emulsifier micelles. After this happens the system will consist of: (a´) DPUR particles partly swelled with monomer, with adsorbed ion-radicals

~ ~~~~ ~ ~~ ~~ ~ ~ ~~~ ~ ~~ ~ ~

(b´) monomer droplets of the size much smaller than before polymerisation and with the adsorbed ion-radicals

~

~

~

(c´) micelles of emulsifier with the adsorbed ion-radicals

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Advances in Urethane Science and Technology After polymerisation is completed both acrylic/styrene polymer particles (a) and the hybrid particles (b1 and b2) may be formed:

(a) Hybrid particles of uniform structure (‘true hybrid’) (b1) or hybrid particles of ‘coreshell’ structure where the core is composed of polyurethane-urea (b2):

(b1)

(b2)

Obviously, other morphologies of dispersed particles are also possible (see Section 6.2). The factor which probably has quite a significant effect on formation of separate particles of the acrylic/styrene polymer is concentration of initiator. The higher it is, the higher is the probability that polymerisation would proceed in monomer droplets. If it is relatively low, this chance is diminished since monomer droplets diffuse into DPUR particles. Probability of formation of hybrid particles of more uniform structure increases with increase in solubility of monomer in water since longer-lived ion-radicals may be formed and their adsorption on DPUR particles is easier. The second important factor affecting the hybrid particle morphology is solubility of monomer in polyurethane-urea which constitutes the particles of DPUR. If it is higher, swelling of DPUR particles with monomer is easier and the probability of formation of hybrid particles of more uniform structure is higher. When polymerisation is carried out after initial swelling of DPUR particles in monomers (Method 1b), then formation of separate particles of acrylic/styrene polymer is also possible, but much less probable since the amount of monomer present in the system as monomer droplets is very low in this case (assuming that the amount of monomer used to swell DPUR particles was not higher than the maximum amount resulting from the equilibrium swell curve (see Section 6.5.4)). It can be anticipated that in this method of MDPUR synthesis many more hybrid particles of much more uniform structure will be formed than when the monomers are continuously added to the reaction system (Method 1a), especially when a

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… ‘redox’ type of initiating system is used. Furthermore, formation of hybrid particles of reversed morphology (core – acrylic/styrene polymer, shell – polyurethane-urea) cannot be excluded, especially when the polarity of monomer will be low:

If this happens, then gelling of the whole reacting system becomes very probable.

•

Method 2: Using monomers as active diluents for prepolymer-ionomer

In this case it can be assumed that polymerisation will proceed in a similar way as described previously for Method 1b. The only difference may be that both structure and size of DPUR particles formed from the prepolymer-ionomer diluted with monomers will differ from the ‘standard’ case, i.e., when NMP is used as diluent for prepolymer-ionomer. Moreover, some fraction of the monomer molecules may be so closely ‘trapped’ in the crosslinked polyurethane-urea structure in DPUR particles that their participation in polymerisation will be difficult. This may lead to a decrease in average MW of acrylic/ styrene polymer which will be formed inside DPUR particles.

•

Method 3: Emulsifying prepolymer-ionomer in diluted acrylic/styrene dispersion

In this case, the course of MDPUR synthesis seems to be much simpler than in Methods 1 and 2 because no polymerisation of monomers will proceed in the system. Before the process starts, only ASD particles are present in the system:

After addition of prepolymer-ionomer and crosslinking it with polyamine some of these particles may remain untouched, but both new hybrid ‘core-shell’ particles where acrylic/ styrene polymer will constitute the core and polyurethane-urea will form the shell:

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as well as DPUR particles:

will appear in the system. It may be assumed that the factors that would decide the course of the process and the fraction of hybrid particles would be: (a) Difference in surface tension of prepolymer-ionomer and the emulsifier used in synthesis of ASD. If the surface tension of the former is higher, then formation of ‘core-shell’ hybrid dispersion particles is also higher since adsorption of prepolymerionomer on acrylic/styrene polymer dispersion particles is easier. If the surface tension of the latter is higher, then adsorption is more difficult and a greater number of separate DPUR particles may be formed. (b) Polarity of acrylic/styrene polymer. If it is high, then formation of ‘core-shell’ hybrid particles is less probable because of difficult adsorption of prepolymer-ionomer. If it is low, then it is expected that formation of hybrid dispersion particles will be easier.

6.4 Methods of Testing 6.4.1 Dispersions The viscosity of prepolymer-ionomers and dispersions was determined at 25 °C using a Hoppler Rheoviscometer. The NCO content of the prepolymer-ionomer was measured by titration. MW was determined by gel permeation chromatography (GPC) according to ISO/TC61/ISC5 using tetrahydrofuran (THF) as solvent and Shimadzu CR 4A

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… Chromatopac apparatus. Free monomer content in the dispersions was measured by gas chromatography (GC) (Giede 18.3 apparatus) Mechanical stability of dispersions was determined using a laboratory centrifuge with rotational speed of 3700 rpm. Particle size, particle size distribution and zeta potential of the dispersions was determined using Malvern Zeta Sizer 4 equipment. Minimum film formation temperature (MFFT) was measured using a Coesfeld apparatus. To produce photographs of dispersion particles, transmission electron microscopy (TEM) was used. Photographs were taken at the Lomonosov University, Moscow, (see Figures 6.12, 6.16 and 6.23-6.25) and the Institute of Chemical Physics of the Russian Academy of Sciences in Moscow), see Figures 6.31-6.33. A special method developed in this Institute was used for investigating dispersion particle morphology. According to this method, a drop of dispersion (very diluted, 0.5-1.0%) and stained with osmium tetroxide) was placed between the ‘thermal electrodes’ of a ‘thermodiffusion’ apparatus constructed at the Institute. The temperature gradient between the electrodes was 35 °C and the time taken to settle the dispersion particles was 30-40 minutes. The substrate with the settled particles was then washed with water and dried. Photographs were taken using a TEM-EM 301 apparatus (Philips). It was shown in previous studies [34] that the substrate does not affect the dispersion particle morphology. The maximum degree of swelling of DPUR particles with monomers was determined using the equilibrium swelling method described in Section 6.4.3.

6.4.2 Coatings Coatings were made from dispersions by applying them onto degreased glass plates with a 120 μm gap applicator. The resulting coating was dried at room temperature for 72 h (for hardness and adhesion tests) and at 40 °C (for water resistance test). Water resistance was determined by examination of the coating after 24 hours immersion. It was designated by numbers (1-4) and letters (A-D) where the higher number means a higher number of bubbles and the ‘higher’ letter means a higher dissipation of bubbles on the plate [35]. Adhesion was determined by a ‘double-cutting knife’ method and designated by numbers (1-4). Higher numbers mean lower adhesion. For determination of hardness the pendulum method (Persoz) was applied [36]. The drying time of the coatings was determined according to the relevant Polish standard (corresponding to DIN 53150 [37]). Times needed to achieve 1° of hardness (touch dry) and 3° of hardness (hard dry) were measured.

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6.4.3 Films Films were obtained from dispersions by casting them onto glass Petri dishes and allowing them to dry for 14 days at room temperature. Mechanical properties, i.e., tensile strength (dr), stress at 100% elongation (d100), elongation at break (er) and Shore A hardness were determined according to ISO 527 [38] (strip type specimens were tested). Percentage swell in water, methylethylketone (MEK) and xylene was determined for square specimens (25 x 25 mm) after 2 weeks of immersion at room temperature and calculated from the following equation: 3

⎛ L⎞ % swelling = 100 ⋅ ⎜ ⎟ − 100 ⎝ 25 ⎠ where L is maximum length. The Tg was determined by differential scanning calorimetry (DSC) according to ASTM D3418-82 [39]. Surface free energy was calculated from experimental values of dynamic contact angle determined using a tensiometer constructed in the Institute of Chemical Physics of the Russian Academy of Sciences. The degree of crosslinking was calculated from the maximum degree of swelling determined according to the following procedure: samples of film weighing approximately 0.1 g each were immersed in toluene and the degree of swelling was calculated from the equation: α(τ) =

m(τ) − m0 m0

where m(τ) – mass of the sample after time τ α(τ) – degree of swelling, and m0 – initial mass of the sample. After the maximum degree of swelling was reached (determined graphically), the sample was weighed and dried to constant solids content (mc). Content of the soluble fraction of the sample (S, %) was calculated according to the equation:

S=

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m0 − mc m0

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… Crosslinking density (j) was calculated from the equation: j=

1 S+ S

and fraction of active chains (V ), from the equation: S

Vs = (1 − S)2 (1 − 2jS)(1 + js)

6.5 Experimental results 6.5.1 Characterisation of Starting Dispersions Used for Synthesis of MDPUR Table 6.1 shows the properties of DPUR used for synthesis of MDPUR (MDPUR-ASD) together with the properties of prepolymer-ionomers used as intermediates in their preparation. Figures 6.8 and 6.9 show the MW distribution for standard prepolymer-ionomers used in DPUR synthesis, obtained from polyetherdiol (MW = 2000) and a mixture of polyesterdiols (MW = 920 and 3190), respectively. The weight average MW (Mw) and number average MW (MN) of these prepolymer-ionomers were (not counting free IPDI): -

-

using polystyrene standard MN

MW

MW/MN

Polyetherdiol-based

6083

13604

2,24

Polyesterdiol-based

6619

16010

2,41

using polyethylene glycol (PEG) standard MN

MW

MW/MN

Polyetherdiol-based

5215

11153

2,1

Polyesterdiol-based

6999

16811

2,40

For DPUR the MW distribution could not be determined by GPC as the films did not dissolve completely.

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Polyol

3

3 0

4

PTMG-2000 and Irpurate

PTMG-2000

PTMG-2000

PTMG-2000

Bester 190, 195 Irpurate

PTMG-2000 and Irpurate

PPUR 289

PPUR 333

DPUR 345

DPUR 346

DPUR 354

DPUR 356

4.53

4.32

3.24

2.74

2.75

3.66

2.50

2.75

3.23

Theor.

4,23

4,72

3.04

2.32

2.48

3.96

2.68

2.15

2.59

Pract.

NCO, %

a the numbers refer to the laboratory sample number b different crosslinking polyamine was used

17.8

0

3.2

PTMG-2000

PPUR 288

15.3

PTMG-2000

0

NMP Content %

DPUR 258b

DPUR 237 PTMG-2000 (240, 244, 332)

Designationa of DPUR (PPUR)

3,800

6,300

7,500

38 0

-

-

-

4,720

7,500

28.4

39.0

31.5

35.9

-

-

-

36.1

30.9

Viscosity Solids mPa-s %

Properties of prepolymerionomers

15.5

108

20

200

-

-

-

315

20

Viscosity mPa-s

7.1-7.4

7.1-7.4

7.6

7.6-7.9

-

-

-

7.0-7.3

7.9

pH

>9 0

>90

60

>9 0

-

-

-

>90

>90

Mechanical Stability, minutes

30-850

30-140

75-770

110-1,200

-

-

-

120-420

37-55 75-130

Particle size distribution

Properties of DPUR

75.1

56.2

215.5

211.1

-

-

-

161.8

58.3

nm

Average Particle size

-98.8

-60.6

-4.5

-7.9

-

-

-

-21.2

-54.6

nm

Zeta potential mV

<0

<0

<0

<0

-

-

-

<0

<0

MFFT °C

Table 6.1 Properties of DPUR used as starting materials for MDPUR-ASD obtained according to methods 1a and 1b (see Section 6.3.2) and of the prepolymer-ionomers (diluted with NMP or acetone) used in their preparation. Properties of prepolymer-ionomers diluted with monomers (designated as PPUR) used as intermediates in synthesis of MDPUR-ASD according to method 2 have also been included in this table.

Advances in Urethane Science and Technology

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

Figure 6.8 MW distribution of prepolymer-ionomer synthesised from polyetherdiol (PTMG 2000, MW = 2000) as determined by GPC

Figure 6.9 MW distribution of prepolymer-ionomer synthesised from a mixture of polyesterdiols (Bester 190 and 195, MW 920 and 3190, respectively)

Particle size distribution and zeta potential determined for one of the DPUR from Table 6.1 are shown in Figures 6.10 and 6.11, respectively. TEM photographs of the particles of the same dispersion are shown in Figure 6.12. Properties of films and coatings made from DPUR included in Table 6.1 are shown in Tables 6.2 and 6.3, respectively. DSC thermogram for a typical DPUR from Table 6.1 (DPUR-237) is presented in Figure 6.13. Properties of ASD used as starting materials in synthesis of MDPUR according to method 3 are presented in Table 6.4. 281

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Figure 6.10 Particle size distribution for typical DPUR synthesised in this study (DPUR 268 = DPUR 237 from Table 6.1)

Cell type AZ104 Cross Beam Mode F(ka) = 1.50 (Smoluchowsky) Data taken on 20/11/96 at 14:48:58 Cell voltage 138.1 V Conductivity 0.03 mS Viscosity 8.95 x 10-4 Pa-s Count rate 491761.9

Current 0.1 mA Temperature 24.8 Dielectric Constant 79.0 pH 3.91

f(ka) 1.50

Cell position 85.40

Zeta Potential -54.6 mV StDev 5.7

Zeta

Intensity

Zeta

Intensity

Zeta

Intensity

-150.0 -137.5 -125.0 -112.5 -100.0 -87.5 -75.0 -62.5

0.0 0.0 0.0 0.0 0.0 0.0 5.1 38.8

-50.0 -37.5 -25.0 -12.5 -0.0 12.5 25.0 37.5

43.8 12.1 0.1 0.0 0.0 0.0 0.0 0.0

50.0 62.5 75.0 87.5 100.0 112.5 125.0 137.5

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Peak 1 mean -54.6 width 5.7

Figure 6.11 Zeta potential for typical DPUR synthesised in this study (DPUR 268 = DPUR 237 from Table 6.1)

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Figure 6.12 Particles of typical DPUR synthesised in this study (DPUR 237 from Table 6.1). Photograph was taken using TEM technique. Reproduced with permission from Professor I. A. Grickova at Lomonosov University, Moscow.

Table 6.2 Properties of films from DPUR used as starting materials for synthesis of MDPUR-ASD according to methods 1a and 1b. Properties of films Designation of DPUR

δ 100

δr

εr

MPa

MPa

%

H2O

MEK

Xylene

Tg °C

T

3.2

19.0

680

Wh, Pieces

Pieces

Pieces

-79.1

DPUR 345

T

7.05

7.9

130

Nt1 Wh

174.4 Br

130 Br

-71.6

DPUR 346

T, OrP

Nt1

Nt1

Nt1

Nt1

Nt1

Nt1

-71.1

DPUR 354

T, Cracks

-

-

-

-

-

-

-15.7

DPUR 356

T, El

-

16.3

27

12

199 T, Br

109.7 T, Br

<0

DPUR 237

Appear.

(240, 244, 332)

% swell

T - transparent, El - elastic, Br - brittle, Wh - white, OrP - orange peel, Nt1 - not tested because of too high adhesion to substrate

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Table 6.3 Properties of coatings from DPUR used as starting materials for synthesis of MDPUR-ASD according to methods 1a and 1b. Properties of coatings Designation of DPUR

Drying time, min Hardness (1°)

Hardness (3°)

Hardness (Persoz)

Adhesion

Water resistance

DPUR 237 (240, 244)

10

15

0.12

4

No bubbles

DPUR 345

180

340

0.164

1

1D

DPUR 346

25

50

0.178

3

Nt2

DPUR 354

25

25

0.25

None

-

DPUR 356

40

60

0.36

4

1C, does not whiten

Nt2 - not tested because of uneven surface See section 6.4.2 for an explanation of water resistance codes

Figure 6.13 DSC thermogram for the film obtained from a typical DPUR synthesised in this study (DPUR 237 from Table 6.1).

284

As above

BA: butyl acrylate S: styrene MM: methyl methacrylate Nt: not tested

ASD 266

K2S2O8

S

BA/MM/S (1/1.32/ 1.64)

ASD 20

ASD 264

BA

22.5

21.4

K2S2O8

23.3

24.9

24.9

24.2

Solids, %

CHP/ Rongalit

K2S2O8

K2S2O8

MM

ASD 17

ASD 17

K2S2O8

Initiator

BA/MM/S (1/1.32/ 1.64)

Monomer

ASD 15

Design of ASD

6

5

5

6

14

20

Viscosity mPa-s

6.2-6.5

6.2-6.5

>90

>90

45

45

5.0-5.3

45

4.0-5.0

45

Mech stability min

4.0-5.0

6.2

pH

Nt

Nt

0.24

0.53

1.00

0.04/0.1/ 0.1

Free monom., %

Nt

Nt

40-180

30-140

35-85

40-145

Particle size distr nm

Properties of ASD

Nt

Nt

79.5

59.7

54.0

39.7

Average part size nm

Nt

Nt

-55.5

-41. 3

-43.4

-12.9

Zeta potential mV

>+50

>+50

>+50

>+50

>+50

MFFT °C

nt

nt

+98.0

-88.0

+123.1

+58. 0

Tg °C

Table 6.4 Properties of ASD used as starting materials for synthesis of MDPUR according to method 3

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

285

Advances in Urethane Science and Technology In Figures 6.14, 6.15 and 6.16 the particle size distribution, zeta potential and TEM micrograph of dispersion particles of typical ASD from Table 6.3 (ASD sample number 15) are presented.

Figure 6.14 Particle size distribution for typical ASD synthesised in this study (ASD 15 from Table 6.3).

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

Figure 6.15 Zeta potential for typical ASD synthesised in this study (ASD 15 from Table 6.3).

Figure 6.16 TEM micrograph of dispersion particles of ASD (see Table 6.3). Reproduced with permission from Professor I. A. Grickova at Lomonosov University, Moscow. 287

Advances in Urethane Science and Technology

6.5.2 Synthesis of MDPUR and MDPUR-ASD Hybrid polyurethane-urea-acrylic/styrene polymer dispersions were prepared according to methods ‘1a’, ‘1b’, ‘2’ and ‘3’ described in Section 6.3.2. Dispersions designated as MDPUR-ASD were made by polymerisation of monomers in DPU according to the methods 1a, 1b and 2 while dispersions designated as MDPUR were made by synthesis of DPUR in ASD according to method 3. In all syntheses the ratio of polyurethane-urea to acrylic/styrene polymer in the hybrid was 2:1. Tables 6.5, 6.6, 6.7 and 6.8 show the compositions of MDPUR-ASD prepared according to methods 1a, 1b, 2 and 3, respectively.

Table 6.5 Composition of MDPUR-ASD prepared according to method 1a (polymerisation of monomers in DPUR with continuous feeding of the monomers) Designation of MDPUR-ASD

Designation of starting DPUR (see Table 6.1)

Monomer

Coalescent (NMP) content in MDPURASD, %

MDPUR-ASD 22

DPUR 240

MM

0

MDPUR-ASD 23

DPUR 244

BA

0

MDPUR-ASD 24

DPUR 244

BA, MM, S

0

MDPUR-ASD 14

DPUR 244

S

0

MDPUR-ASD 259

DPUR 345

BA, MM, S

7

MDPUR-ASD 268

DPUR 346

BA, MM, S

2

MDPUR-ASD 274

DPUR 346

BA, MM, S

2

MDPUR-ASD 300

DPUR 354

MM, S

0

MDPUR-ASD 307

DPUR 356

BA, MM, S

3.3

MDPUR-ASD 309

DPUR 356

BA, MM, S

3.3

Table 6.6 Composition of MDPUR-ASD prepared according to method 1b (polymerisation of monomers in DPUR after swelling DPUR particles with monomers) Designation of starting DPUR (see Table 6.1)

Monomer

Coalescent (NMP) content in MDPURASD, %

MDPUR-ASD 270

DPUR-346

BA, MM, S

2

MDPUR-ASD 272

DPUR-346

BA, MM, S

2

MDPUR-ASD 304

DPUR-356

BA, MM, S

3.2

Designation of MDPUR-ASD

288

PPUR 288

PPUR 289

PPUR 333

MDPUR-ASD 98

MDPUR-ASD 102

MDPUR-ASD 198 PTMG-2000 Irpurate

PTMG-2000

PTMG-2000

PTMG-2000

Polyol

BA,MM,S

BA,MM,S

BA,MM,S

BA,MM,S

Monomer

-

3

3

3

Coalescent (NMP) content in MDPUR-ASD %

2.75

3.66

2.48

3.96

2.68

2.68

2.50 2.50

Pract.

Theoret.

NCO, %

Nt

730

760

760

Viscosity at 25 °C, mPa-s

Properties of starting prepolymer-ionomer

PTMG-2000

PTMG-2000

PTMG-2000

PTMG-2000

MDPUR 245

MDPUR 247

MDPUR 250

MDPUR 347

PTMG-2000

PTMG-2000

MDPUR 243

MDPUR 348

Polyol

Designation of MDPUR

3.6

3.6

11.6

11.6

11.6

11.6

Coalescent (NMP) Content in MDPUR, %

ASD 266

ASD 264

ASD 20

ASD 18

ASD 17

ASD 15

Designation of starting ASD

3.12

3.12

2.74

2.74

2.74

2.74

theoret.

NCO, %

2.79

2.85

2.51

2.24

2.34

2.64

Pract.

5000

5700

4000

4200

4500

4000

Viscosity at 25 °C, mPa-s

Properties of starting prepolymer-ionomer

Table 6.8 Compositions of MDPUR prepared according to method 3 (synthesis of DPUR in ASD). Properties of starting prepolymer-ionomers are also included

Nt – not tested

PPUR 288

MDPUR-ASD 97

Designation of MDPUR-ASD

Designation of starting prepolymerionomer (PPUR)

Table 6.7 Composition of MDPUR-ASD prepared according to method 2 (diluting of the prepolymer-ionomer in monomers, polymerisation and crosslinking of prepolymer-ionomer). Properties of starting prepolymerionomers are also included. Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

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6.5.3 Investigation of the Effect of Various Factors on the Properties of Hybrid Dispersions 6.5.3.1 Method of Hybrid Dispersion Synthesis and Kind of Initiating System The results of investigations of the effect of method of hybrid dispersion synthesis (1a, 1b, 2 or 3 – see Section 3.2) on the properties of dispersions as well as of films and coatings made from them are presented in Tables 6.9 to 6.11 (dispersions prepared using water-soluble initiator) and in Tables 6.12 to 6.14 (dispersions prepared using ‘redox’ initiating system). In all dispersions the chemical structure of the polyurethane-urea and acrylic/styrene polymer component was the same (see relevant tables in Section 6.5.2). All the dispersions contained a similar low level (2-3.6%) of NMP. Particle size distribution and zeta potential of a typical hybrid dispersion obtained using a water-soluble initiator are presented in Figures 6.17 and 6.18, respectively.

290

34.2 31.4 41.6

MDPURASD 270

MDPURASD 97

MDPUR 348

1b

2

3 200

10

10

9

Viscosity MPa-s

7.4-7.7

6.8-7.1

6.8-7.1

7.1-7.4

pH

15

15

15

15

Mechanical Stability Min

Nt

0.7/0.02/ 0.015

Nt

Nt

Free monomer, BA/MM/S, %

55-520

50-120

50-670

30-230

Particle size distribution

Properties of dispersions

115.4

108.4

9 4. 4

61. 9

Average part size nm

-23.0

-60.7

-39.1

-18.4

Zeta pot MV

<0

<0

<0

<0

MFFT °C

T, El T, El M, El

MDPUR-ASD 270

MDPUR-ASD 97

MDPUR 348

1b

2

3

14.1

6.5

9.2

8.2

14.5

10.0

13.3

10.8

δr MPa

δ100 MPa

117

210

180

164

%

εr

13 Wh, El

13 Wh

0 Wh, El

13 Wh, El

H2O

Properties of films

T- transparent, El - elastic, M - Mat, Wh - white, Br - brittle, G - gelatinous

T, El

MDPUR-ASD 268

1a

Appear.

Designation of Hybrid Dispersion

Method of Synthesis

198 T, Br

341 T, G

347 T, Br

347 T, Br

MEK

% swell

198 T, Br

310 T, G

224 T, Br

198 T, Br

Xylene

-75.7

-56.6

-73.4

-73.4

Tg °C

Table 6.10 Properties of films made from hybrid dispersions of the same chemical composition prepared according to different methods using water-soluble initiator (K2S2O8)

Nt – not tested

30.7

MDPURASD 268

1a

Solids, %

Designation of hybrid dispersion

Method of Synthesis

Table 6.9 Properties of hybrid dispersions of the same chemical composition prepared according to different methods using a water-soluble initiator (K2S2O8)

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

291

292

MDPUR-ASD 270

MDPUR-ASD 97

MDPUR- 348

1b

2

3 20

35

25

30

1°

25

60

45

50

3°

Drying time, min

0.20

0.12

0.20

0.22

Hardness (Persoz)

1

4

4

4

Adhesion

Properties of coatings

2A whitening disappears after 1 h

2B whitening disappears after 1 h

2C whitening disappears after 1 h

1A whitening disappears after 1 h

Water resistance

29.3 31.4 32.5

MDPURASD 272

MDPURASD 98

MDPUR 347

1b

2

3

Nt – not tested

29.7

MDPURASD 274

1a

Solids %

Designation of hybrid dispersion

Method of Synthesis

48

8

10

7

Viscosity MPa-s

7.6-7.9

6.8-7.1

7.1-7.4

6.8-7.1

pH

15

15

15

15

Mechanical Stability Min

60-260

105-165

<0.02/ <0.008/ <0.001 Nt

85-650

35-310

Particle size distribution

Nt

Nt

Free monomer, BA/MM/S %

Properties of dispersions

141.7

130.6

90.4

68.0

Average part size nm

-25.4

-39.0

-32.6

-39.3

Zeta potential MV

<0

<0

<0

<0

MFFT °C

Table 6.12 Properties of hybrid dispersions of the same chemical composition prepared according to different methods using a ‘redox’ initiator

See section 6.4.2 for an explanation of water resistance codes

MDPUR-ASD 268

Designation of hybrid Dispersion

1a

Method of Synthesis

Table 6.11 Properties of coatings made from hybrid dispersions of the same chemical composition prepared according to different methods using a water-soluble initiator (K2S2O8)

Advances in Urethane Science and Technology

MDPUR-ASD 272

MDPUR-ASD 98

MDPUR 347

1b

2

3 M, el

T, El

T, El

T, El

Appear.

12.5

5.3

7.6

6.3

15.0

10.6

12.0

11.2

δρ δρ MPa

δ100 MPa

187

285

210

250

%

εr

0 Wh, El

40 Wh, El

0, Wh, El

26 El

H2O

Properties of films

130 T, Br

374 T, G

280 T, Br

445 T, Br

MEK

% swell Xylene

174 T, Br

409 T, Br

280 T, Br

374 T, Br

-77.2

-70.6

-72.2

-68.7

Tg °C

MDPUR-ASD 272

MDPUR-ASD 98

MDPUR 347

1b

2

3 30

40

25

25

1°

40

120

50

45

3°

Drying time, min

See section 6.4.2 for an explanation of water resistance codes

MDPUR-ASD 274

Designation of hybrid dispersion

1a

Method of Synthesis

0.19

0.08

0.16

0.18

Hardness (Persoz)

1

1

4

3

Adhesion

Properties of coatings

4A whitening disappears after 1 h

1A whitening disappears after 1 h

3C whitening disappears after 1 h

2D whitening disappears after 1 h

Water resistance

Table 6.14 Properties of coatings made from hybrid dispersions of the same chemical composition prepared according to different methods using a ‘redox’ initiator.

T- transparent, El - elastic, M - Mat, Wh - white, Br - brittle, G - gelatinous

MDPUR-ASD 274

Designation of Hybrid Dispersion

1a

Method of Synthesis

Table 6.13 Properties of films made from hybrid dispersions of the same chemical composition prepared according to different methods using a ‘redox’ initiator

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Figure 6.17 Particle size distribution for a typical polyurethane-urea-acrylic/styrene hybrid dispersion synthesised in this study (MDPUR-ASD 97 from Table 6.7 prepared according to method 2 using a water-soluble initiator).

Figure 6.18 Zeta potential for a typical polyurethane-urea-acrylic/styrene hybrid dispersion synthesised in this study (MDPUR-ASD 97 from Table 6.7 prepared according to method 2 using a water-soluble initiator)

294

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… The DSC thermogram of the film obtained from a typical hybrid dispersion obtained using a water-soluble initiator is presented in Figure 6.19. Particle size distribution and zeta potential of a typical hybrid dispersion obtained using a ‘redox’ initiator are presented in Figures 6.20 and 6.21, respectively. The DSC thermogram of the film obtained from a typical hybrid dispersion obtained using ‘redox’ initiator is presented in Figure 6.22.

Figure 6.19 DSC thermogram for typical hybrid polyurethane-urea-acrylic/styrene dispersion synthesised in this study (MDPUR-ASD 97 prepared according to method 2 using water-soluble initiator)

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Figure 6.20 Particle size distribution for typical polyurethane-urea-acrylic/styrene hybrid dispersion synthesised in this study (MDPUR-ASD 98 from Table 6.7 prepared according to method 2 using a ‘redox’ initiator)

Figure 6.21 Zeta potential for typical polyurethane-urea-acrylic/styrene hybrid dispersion synthesised in this study (MDPUR-ASD 98 from Table 6.7 prepared according to method 2 using a ‘redox’ initiator)

296

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

Figure 6.22 DSC pattern for typical hybrid polyurethane-urea-acrylic/styrene dispersion synthesised in this study (MDPUR-ASD 98 prepared according to method 2 using a ‘redox’ initiator)

6.5.3.2 Concentration of Coalescing Agent (NMP) The properties of hybrid dispersions prepared using different methods (1a, 2 and 3 – see Section 6.3.2) and different levels of coalescent (NMP) as well as of films and coatings made from these dispersions are presented in Tables 6.15 to 6.17. In all dispersions the chemical structure of the polyurethane-urea and acrylic/styrene polymer component was the same (see relevant Tables in Section 6.5.2). All dispersions were prepared using a water-soluble initiator.

297

298

Content of NMP % 0 2.0 11.6 0 3.0 3.6 11.6

MDPURASD 241

MDPURASD 268

MDPURPUM 259a

MDPURASD 198

MDPURASD 97

MDPUR 348

MDPUR 243

1a

1a

1a

2

2

3

3

a dispersion gelled during synthesis Nt - not tested

Method of synthesis

Designation of hybrid dispersion

35.1

41.6

31.4

38.3

-

30.7

34.3

Solids %

Nt (settling)

200

10

21

-

9

63

Viscosity mPa-s

7.6

7.4-7.7

6.8-7.1

6.2-6.5

-

7.1-7.4

6.5-6.8

pH

45

15

15

15

-

15

45

Mechanical Stability Min

0.1/0.17/ 1.64

Nt

0.7/0.02/ 0.015

<0.02/0.005/ 0.008

-

Nt

<0.02/0.001/ 0.008

Free monomer %, BA/MM/S

33-133

55-520

50-120

30-290

-

30-230

24-688

Particle size distribution nm

Properties of dispersions

59.4

115.4

108.4

261.3

-

61.9

134.5

Average particle size nm

-14.4

-23.0

-60. 7

-63.6

-

-18. 4

-42. 2

Zeta potential mV

Table 6.15 Properties of hybrid dispersions of the same chemical composition differing only in the concentration of coalescent (NMP)

Nt

<0

<0

<0

-

<0

<0

MFFT °C

Advances in Urethane Science and Technology

0

2.0 11.6 0 3.0 3.6

11.6

MDPUR-ASD 24

MDPUR-ASD 268

MDPUR-ASD 259a

MDPUR-ASD 198

MDPUR-ASD 97

MDPUR 348

MDPUR 243

1a

1a

1a

2

2

3

3

14.1

Nt

M, El Film not even

6.5

5.6

-

8.2

Nt

14.5

10.0

11.2

-

10.8

15.8

MPa

MPa 9.5

δr

δ100

M, El

T, El

T, El

-

T, Br

T, El Partly cracked

Appearance

dispersion gelled during synthesis T- transparent, El - elastic, M - Matte, Wh - white, Br - brittle, G - gelatinous Nt - not tested

a

Content of NMP %

Designation of Hybrid Dispersion

Method of Synthesis

Nt

117

210

255

-

164

215

%

εr

12 Wh, Br

13 Wh, E

13 Wh

0 Wh, E

-

13 Wh, Br

0 T

H2O

Properties of films

300 T, Br

198 T, Br

341 T, G

280 T, G

-

347 T, Br

T, G Pieces

MEK

Swelling

251 T, Br

198 T, Br

310 T, G

374 T, G

-

198 T, Br

T, G Pieces

Xylene

-66.3

-75.2

-56.6

-59.0

-

-73.4

-60.8

°C

Tg

Table 6.16 Properties of films made from hybrid dispersions of the same chemical composition differing only in the concentration of coalescent (NMP).

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

299

300

MDPUR-ASD 24

MDPUR-ASD 268

MDPUR-ASD 259

MDPUR-ASD 198

MDPUR-ASD 97

MDPUR 348

MDPUR 243

1a

1a

1a

2

2

3

3 11.6

3.6

3.0

-

11.6

2.0

-

120

20 165

25

60

45

35 35

nt

50

50

Hardness (3°)

nt

30

30

Hardness (1°)

Drying time, min

nt - not tested See section 6.4.2 for an explanation of water resistance codes

Designation of Hybrid Dispersion

Method of Synthesis

Content of NMP %

0.14

0.20

1

1

4

3

0.12 0.12

nt

4

4

Adhesion

nt

0.22

0.14

Hardness (Persoz)

Properties of coatings

nt surface not even

2A

2B whitening disappears after 1 h

1B whitening disappears after 1 h

nt whitening disappears after 1 h

1A whitening disappears after 1 h

2C whitening disappears after 1 h

Water resistance

Table 6.17 Properties of coatings made from hybrid dispersions of the same chemical composition differing only in the concentration of coalescent (NMP)

Advances in Urethane Science and Technology

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

6.5.3.3 Chemical Structure of the Polyurethane-Urea Portion of the Hybrid (a) Effect of polyol type In Tables 6.18 to 6.20 the properties of two hybrid dispersions prepared using the same method 1a (see Section 3.2) and having the same structure of the acrylic/styrene part of the hybrid but based on different polyols, as well as of films and coatings made from them are presented. Neither dispersion contained coalescent (NMP)

(b) Effect of introducing double bonds to the polyurethane-urea part of the hybrid Properties of hybrid dispersions prepared according to the different methods (1a, 1b and 2 - see Section 6.3.2) and based on the same polyol (PTMG 2000), but differing in the presence or absence of double bonds in the polyurethane-urea part of the hybrid, as well as of films and coatings made of them, are presented in Tables 6.21 and 6.22. All dispersions have a similar low level (2.0-3.3%) of coalescent and have the same structure of the acrylic/styrene part of the hybrid. ‘Redox’ initiator was used in the synthesis of dispersions according to the method 2, and in all other dispersions presented in these tables a water-soluble initiator was applied.

301

302

Polyetherdiol (PTMG-2000)

Polyesterdiol (Bester 190 + Bester 195)

MDPUR-ASD 24

MDPUR-ASD 300 39.5

34.3

Solids %

18

63

Viscosity MPa-s

6.5-6.8

6.5-6.8

pH

15

45

Mechanical Stability Min

Nt

<0.02/0.001/0.008

Free monomer, BA/MM/S %

78-197

24-688

Particle size distribution nm

Properties of dispersions

175.6

134.5

Average particle size nm

-62.8

-42.2

Zeta pot mV

T, El T, pieces

Polyetherdiol (PTMG-2000)

Polyesterdiol (Bester 190 + Bester 195)

MDPUR-ASD 24

MDPUR-ASD 300

-

-

15.8

MPa

MPa 9.5

δr

δ100

T- transparent, El - elastic, M - matte, Wh - white, Br - brittle, G - gelatinous

Appear.

Polyol

Designation of hybrid Dispersion

-

215

%

εr

-

0.9

H2O

Properties of films

-

0.8 pieces

MEK

Swelling

-

T, G, pieces

Xyl e n e

Table 6.19 Properties of films made from hybrid dispersions differing only in the structure of polyurethane-urea part of the hybrid

Nt – not tested

Polyol

Designation of hybrid Dispersion

Table 6.18 Properties of hybrid dispersions differing only in the structure of the polyurethane-urea part of the hybrid

-

-60.8

°C

Tg

+18

<0

MFFT °C

Advances in Urethane Science and Technology

25

MDPUR-ASD 300

Double bonds in polyurethaneurea + + +

Designation of hybrid dispersion

MDPUR-ASD 268

MDPUR-ASD 307

MDPUR-ASD 270

MDPUR-ASD 304

MDPUR-ASD 98

MDPUR-ASD 102

Method of synthesis

1a

1a

1b

1b

2

2

25

50

Hardness (3°)

0.3

0.14

Hardness (Persoz)

2

4

Adhesion

Water resistance

1B no whitening

33.6

31.4

34.4

34.2

35.3

30.7

Solids %

9

8

18

10

24

9

Viscosity mPa-s

5.9-6.2

6.8-7.1

6.5-6.8

6.8-7.1

6.5-6.8

7.1-7.4

pH

>9 0

15

15

15

15

15

Mechanical Stability Min.

25-190

105-165

<0.02/<0.008/ <0.001 <0.02/<0.008/ <0.001

30-70

66.8-422.1

35-180

43.8-174.4

Particle size distribution nm

-

-

-

-

Free monom. %, BA/MM/S

Properties of dispersions

66.9

130.6

32.5

94.4

81.7

61.9

Average particle size nm

-33.1

-39.8

+0.7

-39.1

-31.2

-18.4

Zeta potential mV

<0

<0

<0

<0

<0

<0

MFFT °C

2C whitening disappears after 1 h

Properties of coatings

Table 6.21 Properties of hybrid dispersions differing only in the presence of double bonds in the polyurethane-urea part of the hybrid

See section 6.4.2 for an explanation of water resistance codes

30

Polyesterdiol (Bester 190 + Bester 195)

Hardness (1°) Polyetherdiol (PTMG-2000)

Polyol

Drying time, min

MDPUR-ASD 24

Designation of hybrid Dispersion

Table 6.20 Properties of coatings made from hybrid dispersions differing only in the structure of the polyurethane-urea part of the hybrid

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

303

304 + + +

MDPUR-ASD 268

MDPUR-ASD 307

MDPUR-ASD 270

MDPUR-ASD 304

MDPUR-ASD 98

MDPUR-ASD 102

1a

1a

1b

1b

2

2 T, E l

T, El

T, El

T, El

T, Cracks

T, El

Appearance

-

5. 3

-

9.2

-

17.5

10.6

11.0

13.3

-

10.8

MPa

MPa 8. 2

δr

δ100

T - transparent, El - elastic, M - matte, Wh - white, Br - brittle, G - gelatinous

Double bonds in polyurethaneurea

Designation of Hybrid Dispersion

Method of Synthesis

95

285

63

180

-

164

%

εr

374 T, G T, G 224

26 Wh, El

174 T, Br

56 Wh, Br 14 Wh, El

347 T, Br

-

347 T, Br

MEK

Swelling

0 Wh, El

-

13 Wh, El

H2O

Properties of films

174 T, El

409 T, G

151 T, Br

224 T, Br

-

198 T, Br

Xylene

-51.3

-70.6

-63.2

-73.4

-54.1

-73.4

°C

Tg

Table 6.22 Properties of films made from hybrid dispersions differing only in the presence of double bonds in the polyurethane-urea part of the hybrid

Advances in Urethane Science and Technology

MDPUR-PUM 268

MDPUR-PUM 307

MDPUR-PUM 270

MDPUR-PUM 304

MDPUR-PUM 98

MDPUR-PUM 102

1a

1a

1b

1b

2

2 +

-

+

-

+

-

Double Bonds in polyurethaneurea %

25

40

20

25

15

30

Hardness (1°)

60

120

40

45

40

50

Hardness (3°)

Drying time, min

See section 6.4.2 for an explanation of water resistance codes

Designation of Hybrid Dispersion

Method of synthesis

0.16

0.08

0.38

0.19

0.42

0.22

Hardness (Persoz)

1

1

4

4

4

4

Adhesion

Properties of coatings

2C, whitening disappears after 1 h

1A, whitening disappears after 1 h

1C, whitening disappears after 1 h

2C, whitening disappears after 1 hr

1C, whitening disappears after 1 h

1A, whitening disappears after 1 h

Water resistance

Table 6.23 Properties of coatings made from hybrid dispersions differing only in the presence of double bonds in the polyurethane-urea part of the hybrid

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6.5.3.4 Chemical structure of the acrylic/styrene part of the hybrid The properties of hybrid dispersion prepared according to two different methods (1a and 3 – see Section 3.2) and having the same chemical composition and differing only in the chemical structure of the acrylic/styrene polymer component are presented in Tables 6.24 to 6.26. All dispersions were synthesised using a water-soluble initiator. Dispersions prepared according to method 1a did not contain any coalescent while dispersions prepared according to method 3 contained 11.6% of NMP. TEM photos of the particles of two hybrid dispersions prepared using less hydrophobic and more hydrophilic monomer prepared according to method 1a are presented in Figures 6.23 and 6.24. A TEM micrograph of the particles of a hybrid dispersion prepared according to method 3 using more hydrophobic monomer is presented in Figure 6.25.

6.5.3.5 Crosslinking of the Acrylic/styrene Part of the Hybrid The properties of hybrid dispersions prepared using method 1a (see Section 6.3.2), having the same chemical composition and differing only in the presence or absence of partial crosslinking of the acrylic/styrene polymer component are presented in Tables 6.27 to 6.29. Dispersions contained 3.3% of NMP.

Figure 6.23 Particles of hybrid polyurethane-urea-acrylic/styrene dispersion prepared according to method 1a using less hydrophobic monomer and water-soluble initiator (MDPUR-ASD 22). Photograph was taken using TEM. Reproduced with permission from Professor I. A. Grickova, Lomonosov University, Moscow. 306

MDPURASD 23

MDPURASD 22

MDPURASD 14

MDPURASD 24

MDPUR 247

MDPUR 245

MDPUR 250

MDPUR 243

1a

1a

1a

1a

3

3

3

3

nt - not tested

Designation of hybrid dispersion

Method of synthesis

BA/MM/S 1/1.32/1.64

S

MM

BA

BA/MM/S 1/1.32/1.64

S

MM

BA

Monomer

35.1

35.2

35.6

35.1

34.3

34.0

34.5

32.6

Solids %

nt (settling)

36

44

40

63

13

70

75

Viscosity mPa.s

7.6

7.3-7.6

8.2

8.2-8.5

6.5-6.8

6.2-6.5

7.1-7.4

6.2-6.5

pH

45

90

45

45

45

45

45

45

Mechanical Stability Min.

0.1/0.17/1.64

-/-/0.11

-/0.92/-

0.09/-/-

<0.02/0.001/ 0.008

-/-/0.02

-/1.15/-

0.3/-/-

Free monom. %, BA/MM/S

33-133

40-250

35-130

40-270

24-688

16-525

18-360

15-450

Particle size distribution nm

Properties of dispersions

59.4

87.1

61.1

73.8

134.5

96.3

83.7

93.7

Average particle size nm

-14.4

-27.9

-31.6

-31.3

-42.2

-26. 6

-19.3

-27.6

Zeta potential mV

nt

<0

nt

<0

<0

<0

<0

<0

MFFT °C

Table 6.24 Properties of hybrid dispersions differing only in the chemical structure of the acrylic/styrene part of the hybrid

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

307

308 T, El

S BA/MM/ S1/1.32/ 1.64

MDPURASD 14

MDPURASD 24

MDPUR 247

MDPUR 245

MDPUR 250

MDPUR 243

1a

1a

3

3

3

3

M, El Uneven film

AB/MM/ S1/1.32/ 1.64

nt

14.5

nt

6.7

15.8

nt

nt

nt

201

nt

230

215

nt

nt

136

%

εr

310 T, Br

224 T, Br

26 Wh, El 12 Wh, Br

nt

483 T, G

13 Wh nt

T, G

nt

nt

347 T, G

MEK

Swelling

0 T

nt

nt

13 Wh, El

H2O

Properties of films

T - transparent, El - elastic, M - Matte, Wh - white, Br - brittle, G - gelatinous nt - not tested

nt

10.4

Wh, El

S

4.3

9.5

nt

nt

nt

T, El

T, Pieces

4.2

MPa

MPa 3.5

δr

δ100

T, Pieces

MM

BA

Wh Pieces

MM

MDPURASD 22

1a

T, El

BA

MDPURASD 23

1a

Appear.

Monomer

Designation of Hybrid Dispersion

Method of Synthesis

251 T , Br

251 T , Br

nt

341 T, G

T, G

nt

nt

409 T, G

Xylene

- 66.3

- 67.7

- 66.6 +77.0

- 77.37 - 47.38

-60.8

- 54.0 +72.1

- 59.8 +71.4

- 57.6 +79.6

°C

Tg

Table 6.25 Properties of films made from hybrid dispersions differing only in the chemical structure of the acrylic/styrene part of the hybrid

Advances in Urethane Science and Technology

See section 6.4.2 for an explanation of water resistance codes nt - not tested

16 5

120

BA/MM/S 1/1.32/1.64

MDPUR 243

3

50

40

S

MDPUR 250

3

nt

MM

nt Cracks

MDPUR 245

3

60

BA

MDPUR 247

3

50

50

50

50

Hardness (3°)

35

30

1a

35

BA/MM/S 1/1.32/1.64

MDPUR-ASD 14

1a

MM

35

MDPUR-ASD 24

MDPUR-ASD 22

1a

BA

Hardness (1°)

Drying time, min

35

MDPUR-ASD 23

1a

Monomer

S

Designation of Hybrid Dispersion

Method of synthesis

0.14

0,14

nt

0.06

0.14

0.12

0.27

0.09

Hardness (Persoz)

1

4

nt

4

4

1

4

1

Adhesion

1B

2C

Water resistance

nt

1D whitening disappears after 1 h

nt

2B whitening disappears after 1 h

2C whitening disappears after 1 h

1D whitening disappears after 1 h

Properties of coatings

Table 6.26 Properties of films made from hybrid dispersions differing only in the chemical structure of the acrylic/styrene part of the hybrid

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

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Figure 6.24 Particles of hybrid polyurethane-urea-acrylic/styrene dispersion prepared according to method 1a using more hydrophobic monomer and water-soluble initiator (MDPUR-ASD 14). Photograph was taken using TEM. Reproduced with permission from Professor I. A. Grickova, Lomonosov University, Moscow.

Figure 6.25 Particles of hybrid polyurethane-urea-acrylic/styrene dispersion prepared according to method 3 using more hydrophobic monomer and water-soluble initiator (MDPUR 250). Photograph was taken using TEM. Reproduced with permission from Professor I. A. Grickova, Lomonosov University, Moscow. 310

+

MDPUR-ASD 307

MDPUR-ASD 309 17

24

35.3 35.6

Viscosity MPa-s

Solids %

6.8-7.1

6.5-6.8

pH

45

15

Mechanical Stability Min 25-150

35-200

Particle size distribution nm

Properties of dispersions

50.0

81.7

Average particle size nm

-64.5

-31.2

Zeta pot mV

<0

<0

MFFT °C

+

MDPURASD 309 T, El

T, cracks

Appear.

-

11.1

-

MPa

MPa -

δr

δ100

T - transparent, El - elastic, M - matt, Wh - white, Br - brittle, G - gelatinous Nt - not tested

-

Crosslinking of Acrylic/styrene part of hybrid

MDPURASD 307

Designation of Hybrid Dispersion

53

-

%

εr

56 Wh, Br

-

H2O

Properties of films

Nt Pieces

-

MEK

Swelling

130 T , Br

-

Xylene

- 57.5

- 54. 1

°C

Tg

Table 6.28 Properties of films made from hybrid dispersions differing only in the presence or absence of crosslinking of the acrylic/styrene part of the hybrid

Crosslinking of Acrylic/ styrene

Designation of hybrid Dispersion

Table 6.27 Properties of hybrid dispersions differing only in the presence or absence of crosslinking of the acrylic/styrene part of the hybrid

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

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Table 6.29 Results of investigation of degree of crosslinking of hybrid dispersions using the equilibrium swelling test. Results obtained for the starting DPUR and ASD dispersions are also included for comparison Designation of starting dispersion

Method of hybrid synthesis

αmax

MDPUR-ASD 22

DPUR 240

1a

MDPUR-ASD 23

DPUR 244

MDPUR-ASD 24 MDPUR-ASD 14

Designation of hybrid dispersion

MDPUR 243

Soluble %

Crosslinking density, j

Fraction of active chains, Vs

230

41.0

0.95

0.11

1a

310

0

∞

1.00

DPUR 244

1a

170

86.0

2.8

0.49

DPUR 237

1a

320

7.5

2.9

0.59

ASD 15

3

220

4.0

4.2

0.71

%

fraction, S,

MDPUR 245

ASD 17

3

1320

25.5

1.3

0.25

MDPUR 247

ASD 18

3

290

25.0

1.8

0.25

MDPUR 250

ASD 20

3

75

21.5

1.5

0.29

-

DPUR 237

-

140

41

0.95

0.11

-

ASD 15

-

210

30

1.0

0.19

-

ASD 17

-

D

D

D

D

-

ASD 18

-

D

D

D

D

-

ASD 20

-

D

D

D

D

D - film dissolved in toluene

6.5.4 Additional Experiments Additional experiments were performed in order to better clarify the mechanism of the process of formation of hybrid dispersions and the effect of various factors on their properties.

6.5.4.1 Investigations of the Course of Swelling of DPUR Particles with Monomers This experiment was designed in order to further clarify the mechanism of hybrid dispersion particles formation. In this experiment, monomer was continuously added to DPUR (of the composition corresponding to DPUR-237 – see Section 6.3.1) at a rate of 1 ml/h. After each hour, samples of the dispersion were taken and the following parameters were determined: particle size, particle size distribution and zeta potential. Changes of particle size distribution in the course of swelling are shown in Figure 6.26, and changes of average particle size and zeta potential in Figure 6.27.

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

After 5 h

Initial sample

After 2 h After 1 h After 9 h

After 3 h

Figure 6.26 Changes of particle size distribution in the course of swelling of DPUR particles with monomer

313

Advances in Urethane Science and Technology Taking advantage of the capabilities offered by the Malvern Zeta Sizer apparatus the computer analysis of total volume of smaller and bigger particles (it may be noticed that on the curves showing particle size distribution in Figure 6.26 that there are two peaks visible). Results are:

Time of sampling Initial sample

Volume of small particles (peak 1)

Volume of big particles (peak 2)

100%

After 1 h

98%

2%

After 2 h

94%

6%

After 3 h

66%

34%

After 4 h

40%

60%

After 5 h

50%

50%

Figure 6.27 Changes of average particle size and zeta potential during addition of monomer to DPUR

314

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

6.5.4.2 Determination of the Maximum Degree of Swelling of DPUR Particles This experiment was aimed at clarifying what is the maximum amount of monomer that would be able to swell polyurethane-urea, i.e., would be able to take part in the formation of hybrid dispersion particles, assuming that enough time is allowed to achieve equilibrium. Films made from DPUR 258 were used in this experiment. The following results were obtained: Monomer MM Styrene MM/Styrene = 1/1

Maximum degree of swelling αmax, % 88.14 88.21 89.0

The changes of the mass of polyurethane-urea film in the course of swelling with monomers is shown in Figures 6.28 to 6.30.

Figure 6.28 Change of the mass of polyurethane-urea film (made from DPUR 258) in the course of swelling with MM

315

Advances in Urethane Science and Technology

Figure 6.29 Change of the mass of polyurethane-urea film (made from DPUR 258) in the course of swelling with styrene

Figure 6.30 Change of the mass of polyurethane-urea film (made from DPUR 258) in the course of swelling with MM/S = 1/1 (w/w) mixture

316

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

6.5.4.4 Investigation of the Effect of the Method of Synthesis of Hybrid Dispersions on their Degree of Crosslinking The degree of crosslinking of the starting dispersions (DPUR and ASD) and the hybrids was determined using the equilibrium swelling test described in Section 4.3. The following parameters were calculated based on the results of this experiment: • • • •

Maximum degree of crosslinking, amax (%) Content of soluble fraction, S (%) Crosslink density (number of crosslinks per polymer molecule), j Fraction of active chains (composed of segments linked with crosslinks at both ends), Vs

The results are shown in Table 6.29

6.5.4.5 Investigation of Hybrid Dispersion Particles Morphology The morphology of dispersion particles was investigated using the method described in Section 6.4.1. Examples of different morphologies of particles of hybrid dispersions synthesised in this study according to methods 1a, 1b and 3 are presented in Figure 6.31 in comparison with particles of the starting dispersion of BA/MM/S copolymer. The contrast was selected so that in pictures ‘d’ and ‘c’ white colour represents the polyurethane-urea part of the hybrid and in picture ‘b’ the same colour represents the acrylic/styrene part of the hybrid. The morphology of particles of hybrid dispersion synthesised according to method 2 using water-soluble and ‘redox’ initiators is presented in Figures 6.32 and 6.33, respectively. Both pictures show both the single particles and the coalesced particles to demonstrate what happens to the particle morphology in the process of film formation. White colour represents the polyurethane-urea part of the hybrid.

6.5.4.6 Determination of Surface Free Energy of Films Made from Hybrid Dispersions The surface free energy of films made from hybrid dispersions was determined in order to look at the values of its non-polar and polar component which could provide some information on whether polyurethane-urea or acrylic/styrene polymer is on the film surface (see Table 6.30).

317

Advances in Urethane Science and Technology

(a)

(b)

(c)

(d)

Figure 6.31 Examples of different morphologies of particles of hybrid polyurethaneurea-acrylic/styrene dispersions synthesised in this study. Micrographs were taken using TEM. (a) ASD dispersion (BA/MM/S) (shown for comparison); (b) hybrid MDPUR-ASD dispersion (the same monomers) prepared according to method 1b (see section 3.2); (c) hybrid MDPUR-ASD dispersion (the same monomers) prepared according to method 1a (see Section 3.2); (d) hybrid MDPUR-ASD dispersion (the same monomers) prepared according to method 3 (see Section 3.2) Reproduced with permission from Professor A. E. Czalych, Institure of Chemical Physics of the Russian Academy of Sciences, Moscow.

Figure 6.32 Morphology of particles of hybrid polyurethane-urea-acrylic/styrene hybrid dispersion prepared according to method 2 (See Section 6.3.2) using water-soluble initiator (MDPUR-ASD 97). Micrograph was taken using TEM. Both single particle and coalesced particles are shown. Reproduced with permission from Professor A. E. Czalych, Institure of Chemical Physics of the Russian Academy of Sciences, Moscow.

318

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

Figure 6.33 Morphology of particles of hybrid polyurethane-urea-acrylic/styrene hybrid dispersion prepared according to method 2 (See Section 6.3.2) using ‘redox’ initiator (MDPUR-ASD 98). Micrograph was taken using TEM. Both single particle and coalesced particles are shown. Reproduced with permission from Professor A. E. Czalych, Institure of Chemical Physics of the Russian Academy of Sciences, Moscow.

Table 6.30 Surface free energy (γ) and its polar (γp) and non-polar (γd) components of films made from hybrid dispersions prepared according to method 3. Results obtained for films made from DPUR and starting acrylic/styrene dispersions are also shown for comparison Designation of hybrid dispersion

Acrylic/styrene part of the hybrid

γ mJ/m2

γp mJ/m2

γd mJ/m2

MDPUR 243

BA/MM/S 1/1.32/1.64

35.4

4.0

31.4

MDPUR 245

MM

34.7

3.3

31.4

MDPUR 247

BA

31.3

6.3

25.0

MDPUR 250

S

36.7

0.7

36.0

ASD 15

BA/MM/S 1/1.32/1.64

22.5

18.0

4.5

ASD 18

BA

23.0

1.8

21.2

-

33.7

2.3

31.4

DPUR 258

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Advances in Urethane Science and Technology

6.6 Discussion 6.6.1 Estimation of the Effect of Various Factors on the Properties of Hybrid Dispersions and Films and Coatings Made from Them 6.6.1.1 Method of Hybrid Dispersion Synthesis and Type of Initiator •

Properties of dispersions

When the properties of hybrid dispersions having the same composition but prepared according to different methods using water-soluble and ‘redox’ initiator (Tables 6.9 and 6.12) are compared with the properties of starting DPUR and ASD (Tables 6.1 and 6.4) it can be noted that: a) The type of initiator does not affect the properties of hybrid dispersions significantly except for free monomer content which is distinctly lower when a ‘redox’ initiator is used. This suggests that the mechanism of hybrid particle formation is similar in both cases. This is strongly supported by the direct evidence that there is no difference in morphologies of particles of hybrid dispersions obtained according to method 2 (see Section 6.3.2) using water-soluble or ‘redox’ initiator (compare Figures 6.32 and 6.33). b) Neither method of synthesis nor type of initiator have any influence on mechanical stability, viscosity or MFFT of hybrid dispersions. However, the following observations can be made: • mechanical stability of all hybrid dispersions is slightly lower than that of the starting dispersions, but nevertheless it is good enough for successful application of hybrid dispersions in practice (there was no settling of dispersions during storage). Relatively high negative values of zeta potential observed for all hybrid dispersions confirm that their stability should be good. For all hybrid dispersions MFFT is < 0 °C (just as for starting DPUR) which should undoubtedly be considered as their great advantage when compared to ADS. • the viscosity of all hybrid dispersions is quite low and similar to the starting DPUR a) The average particle size of hybrid dispersions depends strongly on the method of synthesis (regardless of the initiator). The lowest values of average particle size are observed for MDPUR-ADS prepared according to method 1a, i.e., with continuous feeding of monomers to DPUR, and the highest for MDPUR prepared according to method 3, i.e., emulsifying of prepolymer-ionomer in ASD followed by crosslinking

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… with polyamine. This result is in good correlation with the assumptions concerning the mechanism of hybrid particles formation presented in Section 6.3.2. If the dispersion is prepared according to method 1a, smaller ASD particles can be formed along with bigger hybrid particles which may have various morphologies. If the dispersion is prepared according to method 3, only bigger ‘core-shell’ particles can be formed. Detailed comparison of the size of particles of starting DPUR (DPUR 346) and hybrid dispersions prepared from it according to method 1a (MDPURASD 268 and MDPUR-ASD 274) confirms this assumption that the average particle size for starting DPUR is higher than for hybrid dispersions prepared from it. b) The particle size distribution observed for hybrid dispersions is the lowest for MDPURASD prepared according to method 2, i.e., diluting prepolymer-ionomer with monomers, emulsifying this solution in water, crosslinking with polyamine and polymerisation of monomers. This can be explained by the fact that in this method, formation of hybrid dispersion particles proceeds ‘in situ’, i.e., both prepolymer-ionomer (precursor of polyurethane-urea component of the hybrid) and the monomers (precursors of acrylic/ styrene component of the hybrid) are emulsified at the same time, which means that the chance of formation of hybrid dispersion particles having similar morphology and size is very high. Higher uniformity of the whole system in the case when the hybrid dispersions are prepared according to method 2 is also confirmed by the higher negative values of zeta potential observed for these dispersions.

•

Properties of films

From a practical point of view it is very important that, regardless of the type of initiator and method of synthesis, the films made from hybrid dispersions are transparent (this proves that their structure is quite uniform), strong, elastic and resistant to water (very good resistance) and organic solvents (good resistance). When the properties of films made from hybrid dispersions having the same composition but prepared according to different methods using water-soluble and ‘redox’ initiator (Tables 6.10 and 6.13) are compared with the properties of films made from the starting DPUR (Table 6.2) it can be noted that: a) Type of initiator practically does not have any influence on mechanical properties, water resistance or solvent resistance of films made from hybrid dispersions. Slightly higher values of elongation at break observed for films made from hybrid dispersions prepared using ‘redox’ initiator (regardless of the method of synthesis) can result from the fact that less grafting (or crosslinking) proceeds in the course of polymerisation.

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Advances in Urethane Science and Technology b) The effect of method of synthesis on the mechanical properties, water resistance and solvent resistance of the films can be clearly seen when the results obtained for the films made from hybrid dispersions prepared according to method 3 and according to other methods are compared. In the cases where method 3 has been applied, the films are distinctly stronger, less elastic and better resistant to water and organic solvents. It is also interesting that their Tgs are lower (closer to the one observed for DPUR). The explanation of this phenomenon is not clear since taking into account the assumed hybrid particles formation mechanism presented in Section 6.3.2 one could rather expect that not only Tg, but also the mechanical properties of the films made from hybrid dispersions prepared according to method 3 would resemble those of films made from DPUR (high elasticity and water/solvent resistance) more than the properties of the films made of dispersions prepared according to the other methods. Nevertheless, when this problem is more deeply analysed, it can be anticipated that the reason for this phenomenon is the very high (much higher than in dispersions prepared according to the other methods) number of particles of ‘pure core-shell’ structure in dispersions prepared according to method 3. In the film made of hybrid dispersion prepared according to this method, a very regular heterogeneous structure is formed during coalescence of particles. In such a structure, acrylic/styrene polymer (which constitutes the core in dispersion particles) is the dispersed phase while polyurethane-urea (which constitutes the shell in dispersion particles) is the matrix. Acrylic/styrene polymer then plays the role of toughener while polyurethaneurea increases the water and solvent resistance of the film and provides its low Tg. The second Tg corresponding to acrylic/styrene polymer, should also be detected in this particular system, but DSC does not have the resolution which would be needed here. However, when the torsion pendulum method was applied we were able to spot this second Tg quite easily. We believe that this particular hybrid system is very interesting and requires further studies to explain the mechanisms of both hybrid particles and film formation.

•

Properties of coatings

Regardless of the initiator and method of synthesis, all coatings made from hybrid dispersions show very short drying time, reasonable hardness and water resistance. Based on the results presented in Tables 6.11 and 6.14 it can be concluded that the best properties of coatings have been achieved with dispersions prepared according to method 3, i.e., by emulsifying the prepolymer-ionomer in ASD, and the worst – for dispersions prepared according to method 2, i.e., by diluting the prepolymer-ionomer with monomers. The properties of coatings made from hybrid dispersions are generally much better than those of coatings made from starting DPUR (starting ASD dispersions did not form coatings since they did not contain any coalescent).

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

6.6.1.2 Concentration of Coalescent in Hybrid Dispersions •

Properties of dispersions

As can be seen in Table 6.15, concentration of coalescent has a significant effect on the properties of hybrid dispersions, regardless of the method of synthesis. Very high levels of coalescent lead to gelling in the course of polymerisation (see MDPUR-ASD prepared according to method 1a) or partial settling of dispersion during storage (see MDPUR 243 prepared according to method 3). It can also be noted that for dispersions prepared without coalescent according to methods 1 and 2, average particle size is distinctly higher and particle size distribution broader than when the process is carried out in the presence of coalescent. This phenomenon can be explained by the positive influence of small amounts of coalescent on the process of hybrid particle formation in the course of polymerisation. It can be assumed that the particles of starting DPUR are partly swelled by coalescent which helps the monomers to diffuse into them. When hybrid dispersions have been prepared according to method 3, the reverse effect is observed. Here, increasing the coalescent concentration results in diminishing both the average particle size and particle size distribution range. The reason for this may be easier formation of separate DPUR particles at lower levels of coalescent.

•

Properties of films

The effect of coalescent on the properties of films made from hybrid dispersions seems to be not quite significant (see Table 6.16) which is strange when taking into account the important role of coalescent in the process of film formation. The only observation that can be made here is that, as has already been stated in Section 6.6.1.2.1, high levels of coalescent cannot be recommended in preparation of hybrid dispersions.

•

Properties of coatings

Here, the effect of coalescent is clearly visible (see Table 6.17), but only if the coalescent content is high (MDPUR 243). Then, drying time increases and coating hardness decreases. However, no differences can be observed between the properties of coatings made from hybrid dispersions which do not contain any coalescent and those which contain only small amounts of coalescent.

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6.6.1.3 Chemical Structure of the Polyurethane-Urea Component of the Hybrid 6.6.1.3a Substituting Polyetherdiol for Polyesterdiol •

Properties of dispersions

As can be seen from Table 6.18, the chemical structure of the polyurethane-urea part of the hybrid does not affect the properties of hybrid dispersions, obviously not counting the MFFT which is much higher if polyesterdiol has been used as a starting material for the prepolymer-ionomer synthesis (compare MDPUR-ASD 300 and MDPUR-ASD 24). •

Properties of films

Here, the effect of chemical structure of the polyurethane-urea part of the hybrid is, of course, substantial (see Table 6.19). For hybrid dispersion synthesised without coalescent using polyesterdiol as a starting material for the prepolymer-ionomer, the Tg is so high that films cannot be obtained. On the other hand, if polyetherdiol is applied as a starting material in the synthesis of the same dispersion, films of very good mechanical properties are obtained. This was the reason for using polyetherdiol rather than polyesterdiol as the starting material for synthesis of dispersions in this study. •

Properties of coatings

It can be clearly seen from Table 6.20 that the hybrid dispersion prepared using polyesterdiol (MDPUR-ASD 300) forms faster-drying coatings of better hardness, adhesion and water resistance. This information may be important from the practical point of view.

6.6.1.3b Introducing Double Bonds to Polyurethane-Urea •

Properties of dispersions

As can be seen from Table 6.21, introducing double bonds into the polyurethane-urea component of the hybrid does not affect the properties of hybrid dispersions regardless of the method used for their synthesis. •

Properties of films

Based on the data contained in Table 6.22 it can be concluded that the effect of introducing double bonds to polyurethane-urea on the properties of films made from hybrid dispersions

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… is exceptionally strong. Regardless of the method of hybrid dispersion synthesis, the presence of double bonds in polyurethane-urea results in a distinct increase in Tg and a drastic increase in film stiffness (decrease in film elasticity), and, an increase in resistance to water and organic solvents. This may mean that grafting or even crosslinking takes place during polymerisation and/or crosslinking via oxidation of double bonds suggested earlier [29], proceeds during air-drying of the films.

•

Properties of coatings

The presence of double bonds in polyurethane-urea has very significant effects on the properties of coatings made from hybrid dispersions (see Table 6.23). It leads to drastic (two-fold) increase in coating hardness and to distinct shortening of drying time. However, there is no effect of double bonds in polyurethane-urea on adhesion or water resistance of the coatings. Generally, based on the results shown above, it can be concluded that introducing double bonds into polyurethane-urea is undoubtedly one of the ways to obtain coatings of excellent properties based on hybrid polyurethane-urea-acrylic/styrene dispersions.

6.6.1.4 Chemical Structure of the Acrylic/styrene Part of the Hybrid •

Properties of dispersions

Table 6.24 shows that the chemical structure of the acrylic/styrene part of the hybrid has no effect on the macroscopic properties of hybrid dispersions, although the appearance of the dispersion particles may be quite different (compare Figures 6.23 and 6.24). Difference in the appearance of particles may result from the degree of hydrophobicity of the monomer (this effect is explained in Section 6.3.2.1).

•

Properties of films

The chemical structure of the acrylic/styrene part of the hybrid has a substantial effect on the properties of films made from hybrid dispersions (see Table 6.24). When the monomers that form polymers of high Tg (styrene or MM) are used and the synthesis is carried out according to method 1a, no film is obtained. For the same monomers but a different method of dispersion synthesis (method 3), a film was obtained only for styrene and only when a high level of coalescent was applied. In this case, the mechanical properties, water and solvent resistance of the film were quite good, but the film was not transparent, which

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Advances in Urethane Science and Technology indicated that the uniformity of the system, i.e., the compatibility of polystyrene with polyurethane-urea was rather poor. Based on the results of investigations of the effect of styrene on the morphology of the model polyurethane-urea-acrylic/styrene hybrid systems [41] this assumption can be proved. When BA or a mixture of monomers (BA/MM/S) was used for synthesis of hybrid dispersions, transparent films of very good mechanical properties and water resistance were obtained (the best properties were obtained when the dispersions were prepared according to method 3). Solvent resistance was good only for films made from the dispersion prepared according to method 3.

•

Properties of coatings

Using data presented in Table 6.26 it can be concluded that the type of monomer used in the synthesis of hybrid dispersions has no effect on the drying time of coatings made from these dispersions, but has a significant effect on coating hardness. The highest hardness can be achieved with coatings made from dispersions prepared using MM as a starting monomer, if it is possible to make a coating (compare MDPUR-ASD 22 and MDPUR 245 in Table 6.26). It is noteworthy that it was not possible to make any coating from MDPUR 245 (prepared according to method 3), despite the fact that it contained a high level of coalescent.

6.6.1.5 Crosslinking of the the Acrylic/styrene Part of the Hybrid Table 6.27 shows that the properties of hybrid dispersions prepared with additional crosslinking of acrylic/styrene polymer do not differ from the properties of dispersions of the same composition prepared without additional crosslinking. Also the properties of films made from both dispersions were very similar (see Table 6.28). However, lack of positive effects of additional crosslinking of the acrylic/styrene part of the hybrid may result from improper selection of starting materials for hybrid synthesis. Therefore, we intend to investigate this effect further in another study along with further studies on the effect of introducing double bonds to the polyurethane-urea part of the hybrid.

6.6.2 Mechanism of Hybrid Particle Formation Based on the results of supplementary investigations (see Section 6.5.4) that were carried out to better understand the mechanism of hybrid particle formation and to determine particle morphology, the following observations can be made:

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… (a) Investigations of the kinetics of the process of swelling the DPUR particles with monomers (see Section 6.5.4.1) have shown that the number of ‘bigger’ particles (particles swelled with monomer and monomer droplets) increases gradually while the number of ‘smaller’ particles (DPUR particles not swelled with monomers) decreases. After some time (about 5 hours), equilibrium is reached and then a bimodal particle size distribution occurs. This direct observation of the process of swelling DPUR particles with monomers is very interesting because it does not correlate with the course of swelling the film with monomers (compare with Section 6.5.4.2). In the process of swelling the film with monomers, equilibrium is reached after a much longer time (12 hours) regardless of the monomer used. This happens at a much higher degree of swelling (approximately 90%) as compared to the process of swelling DPUR particles (maximum 40% assuming that all monomer diffused into the DPUR particles). In practice, a significant amount of the monomer remains in the form of monomer droplets suspended in DPUR. It is also interesting to note that during the feeding of the monomers, the zeta potential becomes less negative which indicates that the uniformity and stability of the whole system is diminishing. Investigations of swelling of DPUR particles with monomers indicate that although all monomer being fed into the system may theoretically enter DPUR particles, since the maximum degree of swelling is more than twice that of the monomer/ DPUR solids w/w ratio used commercially in synthesis of hybrid dispersions, in practice there will always be a natural tendency of the system to reach equilibrium between monomer droplets, non-swollen DPUR particles and swollen DPUR particles. Therefore, when hybrid dispersions are prepared according to method 1a, 1b or 2 (see Section 6.3.2) the result will always be a mixture of particles of hybrid structure with DPUR and ASD particles, as has been explained in Section 6.3.2.1). (b) The results of investigations of crosslink density of films made from hybrid dispersions (see Section 6.5.4.2) show that when the values of the crosslink density of the films determined by swelling in toluene are compared it can be concluded that regardless of the method used for synthesis of hybrid dispersions the films are all crosslinked, or at least they behave as if they were crosslinked. This may mean either that the polymers in the films are actually crosslinked or very strongly branched (by the grafting of monomers on polyurethane-urea) and/or that there is strong domination of ‘core-shell’ structure with a ‘shell’ made of polyurethane-urea. The latter possibility is supported by the properties of films made from hybrid dispersions prepared according to method 3 (emulsifying of prepolymer-ionomer in ASD and then crosslinking with polyamine). It may also be additionally supported by:

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Advances in Urethane Science and Technology • the experimentally proven existence of an outer polyurethane-urea shell in the particles of hybrid dispersions prepared not only according to method 3 (which seems quite obvious), but also according to method 2 (see Figures 6.32 and 6.33). • the values of the polar and non-polar (dispersed) components of surface free energy determined for the films made from hybrid dispersions being very similar to the values found for the films made from DPUR, not ASD (see Table 6.30). This may be considered as evidence for ‘hiding’ of the acrylic/styrene part of the hybrid under the film surface, which results presumably from the ‘core-shell’ structure of the coalesced dispersion particles that form the film, where polyurethane-urea constitutes the shell and acrylic/styrene polymer the core. (c) The morphology of hybrid dispersion particles shown in Figure 6.31 looks quite interesting. It can be clearly seen that regardless of the method of synthesis, resulting in particles of different morphologies, they all have ‘core-shell’-like structures and differ substantially from the particles of ASD, which have a uniform structure. Deeper analysis of these structures leads to the conclusion that for MDPUR-ASD prepared according to Method 1b, the particle morphology resembles that described as ‘engulfed’ [15] for other types of hybrid dispersions (see Figure 6.1). For our hybrid dispersion, small round-shaped particles of acrylic/styrene polymer are ‘engulfed’ or ‘englued’ in the bulky core made of polyurethane-urea. The most interesting result seems to be, however, the specific morphology of the particles of hybrid dispersions prepared according to method 2 shown in Figures 6.32 and 6.33. It is shown schematically in Figure 6.34. Here, the acrylic/styrene polymer constitutes an under-surface sphere ‘embedded’ in a particle made of polyurethane-urea. This sphere is approximately 25-40 nm thick and is situated approximately 15-20 nm from the particle surface. After coalescence of the particles this specific structure is retained (see coalesced particles in Figure 6.32) so that the film made from a dispersion of this particular particle morphology should have the structure presented schematically in Figure 6.35. This kind of structure is undoubtedly very interesting since it is a ‘nanostructure’ where ‘nanospheres’ of acrylic/styrene polymer are embedded in a polyurethaneurea matrix. Such a structure suggests that very specific properties of the film could be obtained if the components of the hybrid were especially selected.

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Figure 6.34 Schematically presented ‘embedded sphere’ morphology of the particles of hybrid dispersions prepared according to method 2 (diluting the prepolymer-ionomer with monomers, emulsifying it in water, crosslinking with polyamine and polymerisation)

Figure 6.35 Schematically presented structure of film made from a hybrid dispersion of the particle morphology shown in Figure 6.34

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6.7 Summary In this study, aqueous hybrid polyurethane-urea-acrylic/styrene polymer dispersions were obtained using four different methods. The dispersions are stable and their viscosity, and pH as well as average particle size and particle size distribution are similar to those observed for DPUR. Most of the hybrid dispersions formed transparent films of good mechanical properties, water resistance and organic solvent resistance. Detailed investigations of coating properties of hybrid dispersions proved that they could form transparent coatings of reasonable hardness, adhesion and water resistance. The effect of various factors on the properties of hybrid dispersions as well as of films and coatings made from them was determined. The results are summarised in Table 6.31. Based on the data in Table 6.31 the following dependencies may be concluded: •

the properties of hybrid dispersions: dependent on method of hybrid synthesis and chemical structure of the polyurethane-urea part of the hybrid

•

the properties of films made from hybrid dispersions: dependent on method of hybrid synthesis, chemical structure of the polyurethane-urea part of the hybrid, presence of double bonds in the polyurethane-urea and the chemical structure of the acrylic/ styrene part of the hybrid

•

the properties of coatings made from hybrid dispersions: dependent on chemical structure of the polyurethane-urea part of the hybrid and presence of double bonds in the polyurethane-urea

The morphology of dispersed particles revealed by TEM appeared to be very interesting. It was found that the method of hybrid synthesis had a substantial influence on the particle morphology and that usually ‘core-shell’ or ‘englued’ morphologies described earlier in the literature were observed. However, in one case the unusual ‘embedded sphere’ morphology was seen. In these particles, the core made of polyurethane-urea is surrounded by a 25-40 nm thick sphere made of acrylic/styrene polymer and covered by a 15-20 nm thick outer layer of polyurethane-urea. The structure of film made from such dispersions is very interesting since it is a twophase structure where ‘nanospheres’ of acrylic/styrene polymer are suspended in a polyurethane-urea matrix. The supplementary experiments, and especially investigations of the process of swelling of dispersion particles with monomers and film surface free energy determinations, allowed confirmation of the assumed mechanism of hybrid particle formation.

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+

-

-

-

-

Coalescent content

Chemical structure of polyurethane-urea part of hybrid

Presence of double bonds in polyurethane-urea part of hybrid

Chemical structure of acrylic/styrene part of hybrid

Crosslinking of acrylic/styrene polymer

+: some effect was observed -: no effect was observed ++: distinct effect was observed

-

-

Type of initiator

-

-

-

-

++

-

Particle size and size distribution

Method of hybrid synthesis

Stability Viscosity pH

-

-

-

++

-

-

-

MFFT

Properties of dispersions

-

++

++

++

-

-

+

Mechanical properties

-

+

++

++

-

-

+

Water and solvent resistance

Properties of films

-

++

++

++

-

-

-

Tg

-

-

++

+

+

-

+

Drying

-

+

-

+

-

-

+

Water resistance

-

+

++

++

+

-

+

Hardness

Properties of coatings

-

+

-

+

-

-

+

Adhesion

Table 6.31 Effect of various factors on the properties of hybrid polyurethane-urea-acrylic/styrene polymer dispersions synthesised in this study Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea…

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Acknowledgements The authors wish to thank Professor I. A. Grickova (Lomonosov University, Moscow) and Professor A. E. Czalych (Institute of Chemical Physics, Russian Academy of Sciences, Moscow) for valuable consultations. The assistance of our co-workers from the Industrial Chemistry Research Institute in carrying out the experiments and testing is also gratefully acknowledged. This study was carried out under project 3TO9811410 sponsored by the Polish National Committee for Scientific Research.

References 1.

R. R. Roester and R. W. Rimmer, Paint and Coatings Industry, 1991, 7, 3, 22.

2.

J. Kozakiewicz in Adhesion 15, Ed., K. W. Allen, Elsevier Applied Science Publishers, London, 1991, Chapter 6.

3.

J. Kozakiewicz, Aqueous Polyurethane Dispersions as Coating Binders, presented at Ekolak Symposium, Ustron, Poland, 1993.

4.

J. W. Rosthauser and K. Nachtkamp, Journal of Coated Fabrics, 1986, 16, July, 39.

5.

J. Goldsmith, Journal of Coated Fabrics, 1988, 18, July, 12.

6.

Waterborne PU Dispersions, ICI leaflet.

7.

H. C. Goos and G. C. Overbeek, inventors; Polyvinyl Chemie Holland BV, assignee; EP 0308115A2, 1989.

8.

M. Hartung, M. Grabbe and P. Mayenfels, inventors; BASF Lacke + Farben, German Patent DE 4 010 176 A1, 1991.

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M. Guagliardo, inventor; Inmont Corporation, assignee; US 4318833, 1982.

10. J. T. Huybrechts, inventor; E.I. DuPont de Nemours and Company, assignee; US 4888383, 1989. 11. R. G. Coogan and R. Vartan-Boghossian, inventors; ICI Americas, Inc., assignee; US 4927876, 1990. 12. P. Penczek in Polymer Blends, Eds., E. Maruscelli, R. Palumbo and M. Kryszewski, Plenum Press, New York, 1986, p.333.

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Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea… 13. S. Muroi, H. Hashimoto and K. Hosoi, Journal of Polymer Science: Polymer Chemistry, 1984, 22, 6, 1365. 14. J. Kozakiewicz, Presented at the 7th European Polymer Federation Symposium on Polymeric Materials, Szczecin, 1998, p.94. 15

D. E. Mestach and F. Loos, Presented at the 23rd FATiPEC Congress, Brussels, 1996, Paper No.B-91.

16

N. Subramaniam, M. J. Monteiro, J. R. Taylor, A. Simpson-Gomes and R. G. Gilbert, Presented at the 7th European Polymer Federation Symposium on Polymeric Materials, Szczecin, 1998, p.53.

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J. Kozakiewicz, Hybrid Dispersion Systems as Coating Binders, Presented at the Seminar of Polifarb Cieszyn-Wroccaw, 9.05.1998, Ustron-Zawodzie.

18

U. Desor, B. Lean and R. Kuropka, Presented at the 25th FATiPEC Congress, Torino Lingotto, 2000, Volume 2, p.117.

19

Z. Florianczyk and S. Penczek, Polymer Chemistry, Warsaw Technical University Press, Warsaw, 1995.

20

M. Schwartz and H. Kossmann, European Coatings Journal, 1998, 3, 134.

21

J. W. Rosthauser and K. Nachthamp, Advances in Urethanes Science and Technology, 1987, 10, 121.

22

J. Kozakiewicz, Presented at the 2nd Symposium on Aqueous Dispersions and Solutions of Polymers, Ustron, Poland, 1993, p.27.

23

D. Dieterich, Angewandte Makromolekulare Chemie, 1981, 98, 133.

24

R. E. Tirpak and P. H. Markusch, Journal of Coatings Technology, 1986, 58, 738, 49.

25

I. Bechara, Journal of Coatings Technology, 1998, 70, 236.

26. P. Loewrigkeit and K. A. Van Dyk, inventors; Witco Corporation, assignee; EP 0189945B1, 1990. 27. B. Gruber, Presented at the ACS Meeting, San Francisco, CA, 1992, Paper No.108.

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Advances in Urethane Science and Technology 28. C. R. Hegedus and K. A. Kloiber, Journal of Coatings Technology, 1996, 68, 860, 39. 29. R. Tennebroek, J. M. Geurts, A. Overbeek and A. Hermsen, European Coatings Journal, 1997, 11, 1016. 30. J. M. Geurts, R. Tennenbroek, A. Overbeek and Y. W. Smak, Presented at the 23rd FATiPEC Congress, Brussels, Belgium, 1996, Paper No.D-259. 31. H. R. Lucas, W. E. Mealmaker and N. Giannopous, Progress in Organic Coatings, 1996, 27, 1-4, 133. 32. M. Locatelli, V. Alonzo, S. Giordano, M. Piccinini and E. Sironi, Presented at the 25th FATiPEC Congress, Torino Lingotto, 2000, Volume 2, p.135. 33. G. C. Cornelius, Y. W. Smak and E. Martin, inventors; Zeneca Resins BV, assignee; WO Patent 9916805A1, 1999. 34. A. E. Czalych, unpublished results 35. ISO 1522 Paints and Varnishes - Pendulum Damping Test, 1998. 36. ISO 2812-1 Paints and Varnishes - Determination of Resistance to Liquids - Part 1 - General Methods, 1993. 37. DIN 53150 Testing of Paints, Varnishes and Similar Coating Materials; Determination of the Drying stage of Coatings (Modified Bandow Wolff Method), 1995 38. ISO 527-1, Plastics – Determination of Tensile properties – General Principles, 1993. 39. ASTM D3418-99, Standard Test Method for Transition Temperatures of Polymers by Differential Scanning Calorimetry. 40. G. C. Overbeek, Y. W. Smak and E. Martin, inventors; Avecia BV, assignee; EP1015507A1, 1999. 41. I. Legocka, J. Kozakiewicz and A. Czalych, unpublished results.

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7

Adhesion Behaviour of Urethanes Ashok Sengupta and H.P. Schreiber

7.1 Introduction Two-component polyurethanes (PU) are applied in many areas of advanced technology. For example PU are used as architectural/maintenance coatings or as adhesives for a variety of substrates including glass and polymers such as polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene (ABS) and polymer blends, e.g., XenoyTM. Two component PUs are created by the reaction of a polyisocyanate and a polyol or coreactant. The strength and permanence of bonds forged by the adhesive is a vital part of the system’s performance. A comprehensive understanding of factors influencing the polymer/PU bond is therefore important. The adhesion behaviour of urethanes depends not only on their composition but also on the contribution of physico-chemical factors, in particular on the surface properties of urethanes. Two specific aspects of surface behaviour are considered in this chapter, which is arranged in three sections: the first concerns the ability of some polymers, including PU to adopt a variety of surface structures or orientations, as dictated by preparation conditions and the medium in contact with the urethane polymer. The ability to undergo surface restructuring is a factor in the well documented difference between surface and bulk properties of all polymers. A second aspect of surface behavior, exerting major influence on the adhesion of urethanes to a variety of plastics or glass, is the acid-base interaction between the adhesive polymer and the substrate. Acid-base interactions are a subject of discussion in the second section of this chapter. Finally, the role of silane modifiers in affecting the adhesion behavior of defined urethane adhesive formulations is considered.

7.2 Surface Characteristics of PU Adhesive Formulations This study involves surface analyses on formulations typical of PU adhesives. The objective is to examine the surface properties of such polymers; to investigate the possibility of surface changes occurring when the adhesive, cured to different degrees, is contacted with orienting fluids notably water; and to indicate the degree to which surface restructuring may influence the bonding characteristics of PU adhesives.

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7.2.1 Experimental 7.2.1.1 Materials The major portion of this work used as reference a two-part soft-segment PU formulation. A prepolymer of methylene bis-4-cyclohexylisocyanate (MDI) with a large amount of 2,4-isomer constituted Part A. Part B was a mixture of two tri-functional polyols with molecular weights of 900 and 500. These were used in weight ratios of 70/30. Part B also contained about 0.08 wt% of a tin catalyst (UL-28, Witco Chemicals). The formulations were cured to various degrees. In one cure procedure, the polymer was first hardened at 100 oC for 1 hour and then exposed to ambient conditions for five days. In a number of cases, this was followed by further curing at 50 °C/50% relative humidity. In an alternative procedure, the initial 100 °C/1 hour exposure was followed by conditioning for varying lengths of time at room temperature and 50% RH. These alternative post-cure processes were followed to produce a range of crosslink levels and to vary also the degree of enolisation occurring during cure.

7.2.1.2 Surface Analyses In order to produce PU films, parts A and B were mixed to give ratios of NCO:OH of 1.03, and in some cases, 1.13. These mixtures were cast onto previously degreased glass microscope slides and drawn down with doctor blades to give final polymer coating thicknesses of 1.2 mm. Samples were cured by the regimes outlined previously. Surface energies were obtained from experimental determinations of contact angles for sessile drops, using a Rame-Hart goniometer. All experiments were conducted at 30 °C. Data were obtained for wetting by dispersion force fluids, represented by a series of n-alkanes, and by formamide, glycerol, ethylene glycol, and water (liquids with appreciable non-dispersive forces). In all cases, the dead space around the enclosed sample was in equilibrium with the vapour of the contacting liquid, and contact angles at zero contact time were obtained by extrapolation of data at finite times of contact. This avoided the risk of changes due to any possible liquid-substrate interactions. The procedures of Gent and Schultz [1] were used for determinations of dispersive and non-dispersive contributions to the overall surface energies, γsd and γsnd. These were obtained with experimental uncertainties of less than 6%. Following initial surface energy evaluations, the PU films were immersed in water at controlled temperatures and for controlled times. Following immersion, films were dried under vacuum (room temperature, 24 hours) and surface energies were redetermined.

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Adhesion Behaviour of Urethanes Analyses of chemical structure in cured films, before and after immersion, were carried out by IR spectroscopy, utilising a Bonem-Michelson-100 FTIR instrument. Another aspect of surface analysis involved the use of inverse gas chromatographic (IGC) techniques [2]. Specifically, IGC was used to evaluate the acid-base interaction potential of PU surfaces and of substrates against which the adhesive was to be bonded. In the IGC determination, the polymers are the non-volatile phase, while the volatile phase included n-alkane vapours and vapours defined as acids and bases by contemporary theories. Such as the theory of Gutmann [3], applied in the present case along with developments reported separately by Saint Flour and Papirer [4] and by Deng and Schreiber [5]. In this protocol [5], chloroform represents a pure acid vapour; tetrahydrofuran and diethylether are strong bases; while acetone may be used as an amphoteric vapour. The IGC procedure, described in detail elsewhere [2, 4, 5], yields interaction parameters for the test solids. The parameter acceptor number (AN) is a measure of surface acidity; the donor number (DN), evaluates the base interaction potential; and an averaged interaction potential is given by Ki = DN – AN.

(1)

Positive values of Ki designate a surface which is predominantly basic. Predominant surface acidity will produce a negative Ki. Generally, polymers show finite values of both AN and DN, with different groups of the surface region acting as electron donors and acceptors. A surface is considered to be amphoteric when AN and DN are finite and of similar values, with Ki ~ 0. When both AN and DN are zero, the surface is considered to be neutral. For IGC work with PU, the polymer was cured against Teflon sheet, peeled off, and powdered by pestle and mortar. About 1.0 g of powder was packed into previously washed and dried steel columns, 15 cm long and 1.2 cm in diameter. Polymer substrates, such as PVC and ABS, were deposited onto Chromosorb support (acid-washed, 60/40 mesh diatomaceous earth) following well-established procedures [6]. Small, nanoliter amounts of the volatile phase were injected into the chromatograph, with helium as the carrier gas at a flow rate of 10 cm3/min.

7.2.1.3 Adhesion Bond strengths of PU adhesives were obtained from single lap-shear determinations. Substrate/adhesive/substrate joints were made with ABS and PVC coupons, 8 x 2.5 cm, as substrates; the overlap area was 4.5 cm2. The reference PU here was compounded with glass beads to provide adhesive layers with uniform thicknesses of 1.27 mm. Lap

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Advances in Urethane Science and Technology shear data were obtained with an Instron tester at 25 mm/min jaw separation speed. At least three, and in most cases five to eight, separate determinations were carried out for each of the systems, with averaged results presented here. The experimental uncertainty in these determinations ranged from 8 to about 20%.

7.2.2 Results and Discussion 7.2.2.1.Surface Analyses Throughout this section, reference will be made to PU specimens cured to varying degrees. For convenience, cure processes will be coded as indicated by the entries in Table 7.1.

Table 7.1 Summary of PU cure procedures Cure Code

Procedure

C0

0.5 h, 100 °C

C1

1 h, 100 °C

C2

1 h, 100 °C + 5 days, RT

C3

1 h, 100 °C + 5 days, RT + 1 day, 50 °C, 50% RH

C4

C2 + 2 days, 50 °C, 50% RH

C5

C2 + 5 days, 50 °C, 50% RH

C6

C2 + 10 days, 50 °C, 50% RH

C7

C2 + 14 days, 50 °C, 50% RH

C8

C1 + 1 day, RT, 50% RH

C9

C1 + 2 days, RT, 50% RH

C10

C1 + 5 days, RT, 50% RH

C11

C1 + 10 days, RT, 50% RH

C12

C1 + 14 days, RT, 50% RH

RT = Room temperature RH = relative humidity Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Table 1. © 1991, VSP BV.

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Adhesion Behaviour of Urethanes The capability of (partially) cured PU surfaces to restructure under the influence of orienting fluids is illustrated in Figure 7.1. Here, the polymer cross-linked by procedure C2 initially shows a non-dispersion force contribution of about 2mJ/m2 to a measured total surface energy of about 38 mJ/m2. If the surface and bulk of the polymer are identical in composition, γnd is lower than an expected value near 98.5 mJ/m2. The reference polymer has a 41 wt% contribution from the polar hard segment, leading to the conclusion that as initially prepared, the polymer surface has an excess concentration of the soft-segment moiety. Essentially, this is a hydrocarbon with an expected surface energy in the 35 mJ/m2 range. This finding confirms those of Chen and Ruckenstein [7] and further substantiates results from earlier studies [5]. As shown in [5], a marked increase in γnd takes place on contact with water. The nondispersive component rises toward a plateau value of about 8.5 mJ/m2, again in

Figure 7.1 Time-dependent increase of non-dispersive surface energy for polyurethane immersed in water. Polymer cure code C2 (Table 7.1). Immersion at (▲ ) 38.5 °C, (●) 60 °C, and (● ) 81 °C. Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Figure 1. ©1991 VSP BV

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Advances in Urethane Science and Technology agreement with data in reference [5], and the rate of change is strongly temperaturedependent. Activation energies for an assumed diffusion-dependent process may be calculated from the results in Figure 7.1. The value obtained, 11.4 kcal/mol, is in excellent agreement with expected values for diffusion processes of chain molecules through polymer matrices, reported to be in the 10-12 kcal/mol interval [8]. By this interpretation, contact with water (γnd = 52 mJ/m2) leads to the diffusion of hard segments into the surface region of the PU, driven by the thermodynamic need to minimise the interfacial tension. In the new equilibrium state, the polymer has a considerably higher total surface energy, since as shown in Figure 7.2, the dispersionforce component is not greatly affected by contact with water. When in the conformation dictated by water, the total γs of the PU surface is 42.5 mJ/m2, more than 10% greater than when this polymer is in its ‘air-conformation’ state.

Figure 7.2 Response of the polyurethane dispersion-force surface energy to water immersion at (●) 38.5 °C, (● ) 60 °C, and (■) 81 °C. Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Figure 2. ©1991 VSP BV

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Adhesion Behaviour of Urethanes While the emphasis above is on structural reconformation as the cause of changes in γsnd, other mechanisms may be considered. They include: (i) plasticisation of the polymer by water; and (ii) enolisation, by water, of NCO linkages in the polymer. The former of these may be rejected, since the data in Figure 7.1 were shown to be independent of the drying time following removal of samples from water. Vacuum-drying for 5 hours, for the standard 24 hours, and 48 hours produces samples in which γsnd was unchanged within experimental error. The observed surface energy effects are therefore unlikely to be caused by the physical incorporation of water in the polymer. The second mechanism, that of enolisation, may be investigated using IR data, focusing on the NCO absorption peak at 2300 cm–1 and on the OH signal at 3300 cm–1. Pertinent results are given in Table 7.2 for specimens prepared under the full range of cure conditions.

Table 7.2 IR Spectra of PU adhesive: effect of cure variables Peak Area (cm2) -NCO (2300 cm-1) -OH (3300 cm-1) Cure Code a C0 1.31 2.22 C1 1.06 2.17 C2 0.84 1.88 C3 0.77 1.37 C4 0.64 1.72 C5 0.70 1.58 C6 0.43 1.60 C7 0.52 1.44 C8 0.93 2.04 C9 0.74 2.15 C10 0.61 1.73 C11 0.68 1.87 C12 0.59 1.70 a For definitions, see Table 7.1 Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Table 2. © 1991, VSP BV.

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Advances in Urethane Science and Technology Comparison of the NCO and OH absorption peak areas shows that chemical changes continue to take place as the urethane polymer ages. Specimens made by routes C0 and C1 show that an additional 30 minutes at 100oC reduces both signals, and more particularly that for NCO. Additional reductions are observed for samples cured via routes C2 and C3. The series C3-C7 shows continued decreases in the OH peak and a slight downward tendency for the NCO peak. The sequence C8-C12, referring to room temperature exposure at controlled humidity, follows similar patterns, though the magnitude of the reductions in peak areas is somewhat diminished. It is concluded that chemical changes do indeed take place in the curing polymer, and that even after 14 days at 50 °C these changes have not gone to completion. The speculation on enolisation effects therefore appears to be supported analytically. The relative contribution of the supposed enolisation effects to observed changes in surface energy must now be questioned. One aspect of the question is considered by the results in Figure 7.3. This shows the response of γnd of the reference polymer to immersion in a non-polar, dispersion-force fluid

Figure 7.3 Response of the non-dispersion force surface energy of reference polyurethane to immersion in heptane, 60 °C. Polymer previously immersed in water. Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Figure 3. ©1991 VSP BV 342

Adhesion Behaviour of Urethanes (n-heptane). Prior to immersion in heptane at 60 °C, the polymer had been immersed in water at the same temperature for 48 hours to bring the surface state into equilibrium with the orienting fluid. Consequently, the ordinate in Figure 7. 3, namely the difference between γnd of the polymer in the ‘water’ and the ‘air’ steady states, originates at about 6 mJ/m2. If this increase, shown in Figure 7.1, were due entirely to surface restructuring, then immersion in heptane would restore the ‘air’ steady-state and Δγnd = 0. Instead, however, the nondispersive surface energy levels out at about 1.8 mJ/m2. Conformational and reversible restructuring accounts for the major portion of surface energy changes in the partially cured PU. A smaller, yet significant irreversible portion, however, arises from changes in the surface chemistry of the polymer when it is allowed to condition in aqueous media. Another view of the relationship between surface mobility and the level of cure in the PU is shown in Figure 7.4. This follows the γsnd on immersion in water at 60 °C of samples cured under procedures C1, C5, and C10 (see Table 7.1). Clearly, in each case there is a significant rise in γnd, but the magnitude of the increase varies with the cure procedure.

Figure 7.4 Effect of the degree of cure on the surface restructuring of polyurethane on water immersion, 60 °C. Cure states coded according to Table 7.1. (● ) C1, (■ ) C5; (●) C10. Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Figure 4. ©1991 VSP BV 343

Advances in Urethane Science and Technology The most drastic cure in this group was applied to sample C5, which produces the lowest restructuring effect; C10 and C1 represent progressively milder cure strategies and the restructuring tendency correspondingly increases. Thus, surface chain dynamics of PUs are a function of the degree of cure, but under each of the procedures used in this work the restructuring effect remains sufficiently important, potentially, to affect adhesion, and related properties of the polymer surface.

7.2.2.2 Adhesion Performance The lap-shear experiments described previously, clearly show that the adhesive bond between PU and a polymer substrate is not uniquely defined by the composition of the materials in the joint. This is shown by the results in Table 7.3 which gives the adhesive bond strengths of ABS/PU joints, in which the adhesive was cured by the various routes

Table 7.3 Adhesive strength of ABS/PU joints (all data are lap-shear tests at 30 °C) Cure Code a

Wa (MPa)

C1

3.6± 0.3

C2

2.1± 0.3

C3

1.7± 0.3

C4

1.5± 0.3

C5

1.2± 0.3

C6

1.2± 0.3

C7

1.2± 0.3

C8

3.6± 0.3

C9

3.1± 0.3

C10

2.3± 0.3

C11

2.4± 0.3

C12

2.2± 0.3

a

For definitions, see Table 7.1

Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Table 3. © 1991, VSP BV.

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Adhesion Behaviour of Urethanes described in Table 7.1. A wide variation in bond strengths is noted; the major trend is toward reduced adhesion with increasing duration of cure, and thus also with increasing contact time between PU and ABS. In this case, of course, the orienting medium for the adhesive is the ABS surface. The results, therefore, suggest that the steady-state conformation of PU relative to ABS is less favourable to strong bonding than is an initial, transient conformation of the adhesive surface. This somewhat surprising result may be rationalised with a knowledge of the acid-base characteristics of the polymer. IGC determinations of AN, DN, and Ki values for ABS, PVC, and PU are summarised in Table 7.4, all data being at 30 °C.

Table 7.4 AN, DN and Ki values AN

DN

Ki

ABS

5.1

8.5

3.4

PVC

9.4

2.1

-7.3

PU

2.4

4.4

2.0

These results show ABS to be a moderate base and PVC a strong acid. PU, measured here in its surface conformation relative to air, is mildly basic. There is, therefore, no tendency for specific non-dispersive forces to act across the ABS/PU interface, the basicity of the materials being unfavourable to the establishment of strong, short-range forces of this kind. On prolonged contact, the initial tendency for dispersion-force soft segments to populate the PU surface preferentially should become more pronounced, accounting for the data sequence in Table 7.3. Of course, the trends should be reversed with PVC, for in this case acid-base attraction should promote adhesive bond strength. Only fragmentary experimental results are available to test the hypothesis, but as shown in Figure 7.5, these support it. They are also consistent with the recent work of Berger [9], showing the influence of non-dispersion forces on the lap shear strength of polymer/metal joints. A matter of much practical importance is the durability of bond strength when joints are exposed to aggressive environments. A cursory examination of this point was carried out as follows: joints of PVC/PU and ABS/PU were prepared by cure procedure C2. Several of these were aged at 50 °C/50% relative humidity for up to 10 days and lap-

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Figure 7.5 Effect of ageing at 50 °C, 50% RH on lap-shear adhesion of polyurethane/ PVC (▲ ) and polyurethane/ABS (● ) joints. Polyurethane adhesive cured according to code C2 (Table 7.1) Reproduced with permission from A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947, Figure 5. ©1991 VSP BV

shear evaluations were carried out as described previously. The results in Figure 7.5, however, are limited for the most part to single specimen evaluations. A lower degree of confidence therefore applies to them. The effects seen in the figure, however, are entirely consistent with the preceding analyses. With ABS, as before, a slight deterioration takes place in bond strength. The initial bond strength with PVC as substrate is significantly higher than that with ABS, and, moreover, increases with ageing, as called for by the acid-base concept. The orienting potential of polymers including PU therefore may arise from the nature of acid-base interactions at the point of contact between materials. These effects may have serious consequences on the performance parameters of materials such as PU adhesives.

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7.2.3 Conclusions The results of this study lead to the following conclusions: (1) The surface energy of a PU formulation has been shown to vary with the nature of the medium in contact with the polymer. The variations are attributed to the nondispersive part of the overall surface energy. The observed trends follow closely earlier observations on two-part, linear PU surfaces. (2) Thermodynamic or acid-base interaction forces appear to be the dominant driving force for surface restructuring in PU, leading to the surface energy effects. (3) Chemical changes in the PU surface following prolonged exposure to high temperatures and/or high humidity contribute to a lesser degree to variations in the surface energy. (4) The adhesive bond strength of PU varies with the degree of acid-base interaction with the substrate and with the conformational state of the adhesive molecule at the time of testing. Being basic, PU develops stronger bonds with acidic substrates (PVC in this case) than with basic ones, this is exemplified by ABS.

7.3 Acid/Base Interactions and the Adhesion of PUs to Polymer Substrates As noted in the preceding section, IGC is an excellent tool for measurements of surface properties and of acid base interaction potentials of macromolecular solids (2, 10, 16). In this section the importance of acid-base interactions relative to the adhesion of PUs is considered in greater detail. IGC has been applied to a series of PU adhesives and to selected polymer substrates, allowing quantitative measurements to be made of the acid/base (electron donor-acceptor) interaction parameters applicable to the surfaces of these materials. Acid base pair-interaction parameters for substrate/PU combinations have been calculated. The bond characteristics of polymer/PU combinations have been measured, in part by conventional lap-shear procedures and in part, by the more recent constrained blister detachment method [11, 12]. Possible relationships between bond properties and acid base interactions have been considered, and a comparison of the two adhesion tests has been made.

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7.3.1 Experimental 7.3.1.1 Materials Six PU adhesives, modelled on practical formulations, have been studied. They are coded PU1 – PU6. All were two-part compounds, denoted as A and B in Table 7.5. In all cases, part B contained a tin catalyst. In each of these formulations A and B were thoroughly mixed, applied to the substrate and bonded in less than 3 minutes following mixing. The substrates were rigid PVC, ABS impact resin and the GE polyblend Xenoy. All were commercial samples, and all were injection moulded to form 8.0 cm x 2.5 cm rectangles, 1.5 mm in thickness.

Table 7.5 Formulation of PU adhesives PU1-PU6 PU1

A Aromatic polyisocyanate (Mondur XP-744) with NCO:OH of 1.03. B A mixture of castor oil and polycaprolactone-based triol.

PU2

A Aromatic polyisocyanate as in PU1. B A mixture of polycaprolactone diol (MW 1250) and 1,4-butane diol

PU3

A Aromatic polyisocyanate (XP-744, obtained from Bayer Corporation) with NCO:OH of 1.03. B OH-terminated epichlorohydrin polyol (MW 1000) and 1,4-butane diol.

PU4

A Rubinate 9225 Polyamine, Mn = 1000; B Polyamine Mn = 1000. The two mixed at a ratio of 20:5.2 by weight.

PU5

As PU1 but with addition to B of silane adhesion promoter at 1.4 g/100 parts of B.

PU6

As PU4 but with addition to B of silane adhesion promoter at 0.8 parts/100 parts of polyol in B.

Mn = number-average molar weight

7.3.1.2 Procedures For IGC work, PU and substrate polymers were coated onto Chromosorb AW support (60/80 mesh) from solution. From 6.0 – 11 wt% of polymer was supported on the

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Adhesion Behaviour of Urethanes Chromosorb. The coated support was packed in stainless steel columns (0.5 cm id, about 0.8 m long) and served as the stationary phase in IGC. The stationary phases were contacted with selected vapours at very high dilution and described procedures were followed [6, 11] to measure the net retention volume, Vn, of the vapours probing the deposited solids. A Perkin-Elmer Sigma-2 chromatograph with hot wire detector was used. All determinations were in the range 30 – 60 °C, and values of Vn were measured in at least triplicate, with a reproducibility better than 4%. The use of IGC data to obtain acid and base interaction parameters for polymer solids has been described in detail elsewhere [2,13,14]. The procedure calls for a reference relationship between RTln Vn and the normal boiling point of mobile-phase materials able to interact through dispersion forces only, represented in this work by n-alkanes (C6-C9) . Vapours known to be acids and bases were then injected into the columns, again at extreme dilution. Following the theories of Gutmann [3], diethyl ether (DEE) and chloroform (CHL) were used as reference base and acid, respectively. As shown in Figure 7.6 for PVC as stationary

Figure 7.6 Retention volume of vapour probes interacting with PVC as function of normal boiling point of probe liquids. Data are for 40 °C. Reproduced with permission from PU Congress.

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Advances in Urethane Science and Technology phase, the points for DEE and CHL both deviate from the straight line defined by the alkanes. Thus, PVC interacts with both the acidic and basic probe, but with a pronounced prevalence of interaction with DEE. The polymer is therefore a net acid. Following the principles of reference [1], electron acceptor and donor indices, AN and DN for the polymers are calculated from the behaviour of the DEE and CHL vapours. The adhesion at substrate/PU interfaces was measured by lap-shear and by constrained blister test methods [11, 12]. For lap-shear testing, adhesives were mixed with glass beads, Class 4A from Ferro Microbeads, and deposited on PVC and Xenoy substrates to yield dry films 0.35 mm thick. The adhesives were cured by exposure of 3 days at room temperature and 50% relative humidity, followed by 4 days at 50 °C, 0% relative humidity. The lap-shear specimen arrangement si shown in Figure 7.7a. All determinations were

Figure 7.7 Schematic of arrangements for adhesion tests. (a) Specimen construction for lap-shear test. (b) Specimen construction and installation for blister detachment test.

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Adhesion Behaviour of Urethanes made in triplicate with an Instron tester, at a separation speed of 50 mm/min. The blister test arrangement, used with all substrates, is shown schematically in Figure 7.7b. The support polymer here had a central hole, 6 mm in diameter, which connected to a pumping system able to channel distilled water onto the polymer/adhesive assembly. During application of the adhesive this opening was blocked by a Teflon plug, removed after the urethane was cured. Adhesive films, again 0.35 mm thick, were deposited on the PVC, ABS and Xenoy surfaces and cured as above. Bond energies, Ea, were obtained by following protocols described in recent literature [4, 5]. These require measurement of the pressure generated by the liquid pumped against the joint at which the adhesive debonds from the substrate. Bond energy data were reproducible to better than 8%; lap-shear determinations carried a similar uncertainty.

7.3.2 Results and Discussion 7.3.2.1 IGC Determinations Acid-base interaction parameters for the materials of this research are given in Table 7.6. As anticipated by the results in Figure 7.7, and in agreement with the results shown in the preceding section, these show PVC to be predominantly acidic. ABS has a moderately basic surface and Xenoy is an amphoteric solid with nearly equal AN and DN indices. All of the PU systems function as both donors and acceptors, with PU5 and PU6 skewed to acidity, PU1 and PU4 to basicity, while PU3 and (to a lesser degree) PU2 are amphoteric.

Table 7.6 Acid/base indexes of PU and polymer substrates (all at 30 °C). Material

AN

DN

ABS

3.2

7.0

Xenoy

6. 6

6.8

PVC

8.5

2.1

PU1

2.7

4.3

PU2

3. 7

5.3

PU3

6.6

6.9

PU4

3.9

7.6

PU5

8. 2

5.3

PU6

7.2

3.9

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Advances in Urethane Science and Technology The acidic adhesives would be expected to interact strongly with ABS, the basic ones with PVC. The magnitude of acid-base forces is open to question when amphoteric surfaces such as Xenoy are involved. Any attempt to relate acid/base interactions between adhesives and substrates to the bond properties of the relevant pair depend on a quantitative evaluation of a pair interaction parameter. There are no broadly accepted theoretical guidelines to such an evaluation, but a pair interaction parameter, Isp, can be defined by Isp = ([AN]1 [DN]2)1/2 + ([AN]2 [DN]1)1/2 Here subscripts 1 and 2 designate adherent and adhesive, respectively. Values of Isp for the polymer/adhesive pairs are given in Table 7.7.

Table 7.7 Pair interaction parameters at 30 °C Isp

PVC

ABS

Xenoy

PU1

8.4

8.0

9.6

PU2

9.5

9.2

10.9

PU3

11.4

11.5

10.4

PU4

10.9

10.1

13.8

PU5

12.6

11.7

14.4

PU6

9.7

10.6

12.1

7.3.2.2 Adhesive Bond Properties A summary of lap-shear and blister detachment test data is presented in Table 7.8. The six adhesives perform differently with each of the substrates, suggesting a link with specific interactions at the adherent/adhesive interface. This is considered in the next section. Although lap-shear evaluations are more familiar, they do not necessarily evaluate the strength of adherent/adhesive interfaces. Cohesive failure of the weaker component of an assembly can be a complicating factor, tearing of the PU layer being a possibility in certain of the present measurements. In principle, the detachment of the adhesive in the blister test is a pure interfacial event and one that can be quantified provided parameters

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Table 7.8 Lap shear and blister detachment values at 30 °C Substrate

PVC

ABS

Xenoy

LS

Ea

LS

Ea

LS

Ea

PU1

20.5

37.5

-

40.8

27.0

42.5

PU2

26.0

42.5

-

40.5

28.5

44.0

PU3

27.2

47.5

-

48.2

29.7

46.5

PU4

25.8

47.0

-

44.7

31.5

49.7

PU5

28.0

49.0

-

47.8

31.2

50.3

PU6

25.5

42.5

-

47.2

30.0

48.0

Adhesive

LS: lap shear in J/m-2 Ea: bond energy in J/m-2

such as adhesive thickness and the geometry of the detaching blister are known [11, 12]. The two test methods may therefore complement each other and the existence of quantitative correlations between the test data is a point of interest. The theory is tested by means of Figure 7.8. Although the results for PVC and Xenoy substrates are separated by a significant margin, in both cases the correlations are very satisfactory. The correlation coefficient for the more strongly bonded Xenoy-PU system is 0.988, that for the weaker PVC-PU bonds is 0.935. Thus it is concluded that for assemblies of the type used here, lap-shear and blister detachment test methods produce sets of internally consistent data which correlate in a statistically acceptable manner. The choice of test procedure, therefore, may rest on convenience and on the proven relevance of test results to field performance.

7.3.2.3 Adhesion and Acid/Base Interaction As already noted, the adhesion test data suggests the existence of significant contributions to bond strengths from non-dispersion forces acting at the adhesive/ substrate interface. Indeed, quantitative correlations have been confirmed between Isp and either of the two sets of adhesion performance results. A demonstration is given in Figure 7.9, using the more complete blister test data. The correlation coefficient for this linear plot is 0.982.

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Figure 7.8 Correlation between lap-shear and blister detachment evaluations of bond strengths in Xenoy/PU and PVC/PU assemblies.

The value is in fact surprisingly large when it is noted that no account has been taken of contributions made to bond strengths by dispersion (van der Waals) forces. Acid/base forces appear to play a determinant role when bonding occurs at interfaces of materials with pronounced electron donor and acceptor tendencies. This alone seems to justify concern for expressing these tendencies quantitatively, and for favouring the IGC technique as a convenient route to the objective. Another noteworthy feature in Figure 7.9 is the inclusion of all polymer-substrate pairs is the relationship. This encourages speculation that Isp is an objective measure of the interactions generated by electron donor and acceptor sites in the surfaces of the present polymers. It also suggests that IGC determinations of Isp may be useful for screening purposes in designing PU adhesives for optimum performance with specified polymeric substrates. These suggestions form the origin of further research which is being carried out.

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Figure 7.9 The dependence of equilibrium bond energies for PU/polymer assemblies on acid/base pair interaction. ● - PU1, ● - PU2, ▲ - PU3, ■ - PU4

7.4 The Effectiveness of Silane Adhesion Promoters in the Performance of PU Adhesives One of the most frequent procedures in the formulation of adhesives is the addition of adhesion promoters. Of these, silanes are by far the most frequently used. The silanes form a multi-membered family of chemicals. Numerous publications have described aspects of silane chemistry and the applications of silanes [15-18]. Although many silanes are readily available, in many industrial formulations of adhesives the most frequently used of these are the primary amine and diamine versions because of their relatively low cost. However, these do not necessarily produce optimum benefits in adhesion for all applications. Guidelines for the selection of silanes best suited for specific applications would therefore be useful. These should be based on a fundamental understanding of the manner in which silane compounds affect the interface and interphase in adhesively bonded systems. The

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Advances in Urethane Science and Technology last section relates to this objective by describing the effect of various silanes on the adhesion performance of a model PU adhesive in joints involving selected polymers and glass. The adhesion and its retention under aggressive ageing conditions is interpreted in terms of acid-base interactions at the adhesive/substrate interface. The interpretative basis appears suited for the selection of preferred silanes in the cases under study.

7.4.1 Experimental 7.4.1.1 Materials The adhesive used was a model semi-structural PU, similar to one described in an earlier work [7]. The two component formulation consisted of methylene bis-4-cyclohexyl isocyanate prepolymer and trifunctional polyols with molecular masses in the range 500900. A small amount of tin catalyst (UL-28, Witco Chemicals) was also present. The PU was used as control without additives and was modified by the following silanes: APS - aminopropyl triethoxy silane CS - chloropropyl trimethoxy silane VS - vinyl trimethoxy silane MS - mercaptobutyl trimethoxy silane ES - epoxy trimethoxy silane The silanes were obtained from Dow-Corning Corporation and were added at 0.5 wt% levels. All formulations were cured by a standard procedure of 1 hour at 100 °C, followed by 5 days at 50 °C and 50% relative humidity. As in the preceding section, the polymers to be bonded were PVC, ABS, and the thermoplastic polyester blend Xenoy. In addition microscope slide glass was also studied. Surfaces of these materials have been shown to be capable of widely differing acid-base interactions. The PVC was an unplasticised polymer (MW = 54,000 g/mol) from Synergistic Chemicals, Inc. It contained 5 phr of Advastab TM-821SP thermal stabiliser. The ABS (Cycolac AR 3501), was a moulding grade, commercial product of GE Plastics, Inc. The same source supplied the Xenoy blend; the exact composition of this material was not determined.

7.4.1.2 Procedures The establishment of silane concentration in modified PU adhesives was based on the tendency of PU to restructure in response to the orienting strength of media in contact

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Adhesion Behaviour of Urethanes with the polymer [5, 19]. The process involves the diffusion-controlled accretion of polar moieties from the bulk to the surface layer of the polymer. It has been shown earlier in this chapter that surface restructuring can be followed by monitoring γnd, the nondispersive contribution to the polymer surface energy, when a film, initially dried in air, is immersed in a polar medium. Static contact angles were measured for n-decane, methylene iodide and water, on a freshly prepared film specimen of pure PU, and also on films of PU containing up to 2 wt% APS and ES. The harmonic mean procedure [20] was used to compute γnd. The measurements were repeated after each of the films had been exposed for 7 days to water at 90 °C. Preliminary work showed that this was adequate to allow the PU surface to equilibrate with respect to the aqueous environment. Whilst the silane-free film restructured in water to increase γnd by about 5 mJ/m2, the presence of silane sharply reduced the effect. The silane apparently anchors surfacelocalised moieties and restricts their ability to adopt new conformations. Of the silanes, ES appeared to be more successful in inhibiting the restructuring effect, but both ES and APS, when added at a concentration of 0.5 wt%, were found to greatly reduce restructuring. This was, therefore, chosen as the concentration level in subsequent work. For testing bond properties, single lap-shear specimens were assembled from polymer and glass coupons cut following the practice outlined in reference [19]. The polymers were moulded to a thickness of 1.5 mm and cut to 8 x 2.5 cm size. No mould release agents were used in moulding operations. For convenience, the selected overlap area of 4.5 cm2 was bonded with adhesives drawn to a thickness of 0.4 mm. Following conditioning (24 hours, room temperature), initial bond strengths were measured in triplicate with an Instron apparatus at a draw speed of 10 mm/min. To ascertain property retention, certain of the systems were exposed in air ovens to 60 °C, 100% relative humidity for up to 3 weeks under an applied stress. To generate stress, small holes were drilled near the ends of the lap-shear coupons, one of the holes being used to suspend the test pieces from a horizontal beam within the oven. Consistent with certain industrial procedures, stress was generated by a 200 g weight, suspended from the free end of the joint. Repeat evaluations of lap-shear bond strengths were carried out after various ageing times. Interfacial properties in assemblies were represented by acid-base interactions, as evaluated by methods of IGC [2], as described earlier. PU and polymer stationary phases were prepared as described in preceding sections. Since grinding microscope slides proved to be ineffective for the production of a solid with sufficiently high surface area, powdered glass beads were used as a representative glass substrate. Following well-established procedures [2, 6], the solids were investigated at infinite dilution with vapours including n-alkanes (C6 to C10), and with acidic and basic probes chosen on the basis of Gutmann’s classification [3]: chloroform and benzene as acids, diethyl ether, tetrahydrofuran as bases and acetone as an amphoteric substance. The temperature throughout IGC determinations

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Advances in Urethane Science and Technology was in the range 35-65 °C. Net retention volumes, Vn, were obtained from at least triplicate vapour injections, generally with a reproducibility better than 4%. The protocols leading to the determination of acid-base interaction potentials in this portion of the work were somewhat different from those documented earlier. Here the interaction potential of solids is described by the acid and base interaction constants, Ka and Kb. The derivation of these is given in detail in recent publications [4, 14, 21]. As before, an initial requirement is the establishment of a linear plot of log Vn against the normal boiling point, Tb, of the alkane vapours [9]; this represents the dispersion-force interactions between solid and vapour. The slope of the linear plot may also be used to evaluate the dispersion contribution to the solid’s surface energy, γsd. The position of Vn for the acid and base probes, at their respective Tb, defines the contribution of acid-base interactions to the free energy of adsorption, ΔGab. When measured over a specified temperature range, the corresponding enthalpy, ΔHab, is also obtained. The enthalpy is then used to estimate Ka and Kb from the expression [4, 14, 21]: -ΔHab/AN = KaDN/AN + Kb Here AN and DN are Gutmann’s values [3] for the acidity and basicity of the vapour probes. When Ka and Kb values for substrates and adhesives are known, an acid-base pair interaction value Isp is calculated: Isp = (Ka)1(Kb)2 + (Ka)2(Kb)1 where subscripts 1 and 2 refer to adhesive and polymer (or glass) substrate, respectively. In this work, Isp values were primary tools for the generation of correlations between component interactions and the performance of the bonded systems. In so doing, the relevance to adhesion of a parameter related to the acid-base contribution to the enthalpy of component interaction is established.

7.4.2 Results and Discussion The acid-base interaction potentials of polymer substrates and of the various adhesive formulations are given in Table 7.9 as are the relevant values of Isp. The IGC experiments show the model PU to interact as both mild acid and base, with these tendencies well balanced. The presence of APS has no pronounced effect on the electron acceptor capacity of the surface, but shifts the balance to basicity, presumably because of the surface localisation of the silane’s primary amine groups. Each of the CS, VS and MS additives produces the opposite trend, the adhesive surface now displaying stronger acidity, but without significant change in the basic interaction potential. Addition of ES raises both

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Table 7.9 Interaction constants and pair interaction parameters for polyurethane adhesives and substrates A: Acid-base interaction constants Material

Ka

Kb

Ka + Kb

PVC

8.5

2.1

10.6

ABS

3.2

7.0

10.2

Xenoy

6.6

6.8

13.4

Glass

2.6

1.9

4.5

PU

3.1

3.8

6.9

+ APS

3.7

6.0

9.7

+ CS

4.9

4.0

8.9

+ MS

5.0

4.2

9.2

+ VS

4.6

3.9

8.5

+ ES

5.0

6.2

11.2

PVC

ABS

Xenoy

Glass

PU

39

34

46

16

+ APS

59

45

65

23

+ CS

44

47

60

20

+ MS

46

48

62

20

+ VS

41

45

57

19

+ ES

64

55

74

26

B: Pair interaction, Isp

See text for definition of Silane codes Reproduced with permission from H. P. Schreiber, R. Qin and A. Sengupta, Journal of Adhesion, 1998, 68, 1-2, 31, Table 1. © 1998, Gordon and Breach Science Publishers.

the acid and base interaction potential with a predominance of basicity. Furthermore, the total interaction potential, given by the sum Ka + Kb, is most affected by the ES adhesion modifier. As expected, the PVC surface is strongly acidic, that of ABS is basic, and Xenoy is amphoteric with excellent balance between donor and acceptor capabilities. The glass surface is a weak acid. As a result of choosing the Gutmann concept of acidity/ basicity, all of the surfaces retain both acidic and basic interaction potentials.

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Advances in Urethane Science and Technology The resulting Isp data in Table 7.9, show ES and APS to be the most effective promoters of interaction between adhesives and PVC. APS is not an effective interaction modifier for ABS substrates; all the other surface modifiers exert a greater effect. Indeed, for each of the four substrates in this work, ES produces the maximum increase of interaction, although for glass the interaction level with these adhesives remains low when compared with the polymeric substrates. The effects of silane additives on the bond characteristics of assemblies is summarised in Table 7.10. The initial bond strength, prior to accelerated ageing, is reported. The contribution made by the silanes is variable, ranging from a few percentage points, e.g., PVC/PU + MS, to a doubling of the bond strength (Glass/PU + ES). The largest increment in bond strength is generated by ES. As seen in Table 7.9, this confers the largest increase to the total interaction potential of the adhesive surface, while maintaining a balance between donor and acceptor characteristics. For the systems selected for study in this work, ES is closest to a ‘universal’ adhesion modifier. In other cases, the effects of acid/base functionality are evident. Thus, the acidic substrate (PVC) benefits from the presence of the basic APS modifier, but does not respond well to silanes which accentuate the acidity of the adhesive surface. ABS demonstrates the analogous effect of responding more strongly to the acidic modifiers than it does to APS. In joints based on Xenoy, a strongly amphoteric substrate, the choice of silane is less critical, although ES maintains a slight advantage over the remainder. The strong performance of the epoxy silane notwithstanding, it seems doubtful whether a ‘universal’ silane promoter can be identified for application to a wide range of substrates.

Table 7.10 Initial bond strength of assemblies joined by PU adhesive modified by silane additives Lap-shear bond (J/m2) for substrate and adhesive

PVC

ABS

Xenoy

Glass

PU no additive

27

20

28

14

+ APS

34

24

35

21

+ CS

28

24

34

19

+ MS

30

287

34

19

+ VS

31

29

31

17

+ ES

39

34

39

27

All silanes at 0.5wt%; see text for definition of silane codes Reproduced with permission from H. P. Schreiber, R. Qin and A. Sengupta, Journal of Adhesion, 1998, 68, 1-2, 31, Table 2. © 1998, Gordon and Breach Science Publishers.

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Adhesion Behaviour of Urethanes The critical question of interdependence between initial lap-shear bond strengths and the Isp parameters is addressed in Figure 7.10. Although each of the substrates tends to define a specific envelope of behaviour (more particularly evident for glass), the first order regression line drawn through all of the points has a correlation coefficient of 0.915, sufficient to justify the claim of a true relationship. A more rigorous correlation is probably unrealistic, since no account is taken here of varying chemical interactions between the silanes and the substrates, or of variations in the contributions to bonding made by longer range dispersion forces. Acid-base interactions at the adherent/adhesive interface thus make important contributions to the initial lap-shear bond strength of these systems. The extrapolation to Isp = 0 suggests that dispersion forces make a net contribution of about 12 J/m2 to the bond property. Within experimental error (~10%) this is the initial bond strength for glass joined by unmodified PU, even though an Isp value of 16 is reported for this pair in Table 7.7.

Figure 7.10 Initial bond strengths as a function of pair interaction parameter Reproduced with permission from H. P. Schreiber, R. Qin and A. Sengupta, Journal of Adhesion, 1998, 68, 1-2, 31, Figure 2. ©1998 Gordon and Breach Science Publishers.

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Table 7.11 Effect of ageing (60 °C, 100% relative humidity) on lap-shear bond strength Age time (h)

0

1.3

3.5

6.9

11

22.4

R

PVC/PU

27

22

17

14

14

9

0.33

+ APS

34

33

30

27

24

22

0.65

+ VS

31

28

23

21

17

14

0.45

+ ES

38

37

37

33

30

28

0.74

ABS/PU

20

18

16

12

8

7

0.35

+ APS

24

21

21

16

17

13

0.54

+ VS

28

25

24

22

20

19

0.68

+ ES

34

34

30

29

27

26

0.76

Xenoy/PU

28

23

19

16

16

12

0.43

+ APS

35

35

32

28

28

25

0.71

+ VS

31

29

27

24

19

17

0.55

+ ES

38

36

35

32

32

29

0.77

Glass/PU

14

12

7

0

0

0

0.00

+ APS

21

20

16

11

11

7

0.33

+ VS

18

17

12

9

7

4

0.22

+ ES

27

25

25

20

17

13

0.48

Bond strength (J/m2)

See text for definition of silane codes R represents the ratio of final to initial bond strength Reproduced with permission from H. P. Schreiber, R. Qin and A. Sengupta, Journal of Adhesion, 1998, 68, 1-2, 31, Table 3. © 1998, Gordon and Breach Science Publishers.

Evidently at this level of Isp, the strength of specific interactions is insufficient to influence the lap-shear bond strength as measured by the chosen procedure. Of equal interest to the initial bond strength is the ability of the assembly to withstand aggressive ageing. Joints using as adhesives the unmodified PU and the PU with 0.5 wt% APS, VS or ES adhesion promoters were subjected to the ageing protocol of this work.

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Adhesion Behaviour of Urethanes Results of the procedure are presented in Table 7.11. Ageing periods are shown as the square root of exposure hours, in the expectation that property loss would be a diffusioncontrolled process due to the intrusion to the interface of water. In Table 7.11, R is defined as R = (Bond Strength)f /(Bond Strength)i where the subscripts f and i represent the bond strength after three weeks of ageing and the initial value, respectively. Each of the systems is adversely affected, but R varies significantly among the systems as well as within them. Qualitatively, a relationship between R and the initial bond strength becomes apparent on inspection of the data. The ES modified PU, which has the strongest acid-base interaction with each of the substrates, produces the strongest initial bond, also produces the smallest property loss. A clearer view of the apparent correlation is presented in Figure 7.11. This indicates that below initial bond strengths of about 13-14 J/m2, corresponding to Isp < 18-20, the selected ageing regime will totally destroy the bonded assembly. The best fit of the data is obtained from the second order regression curve shown, with a coefficient of 0.932. However, a first order linear fit to the data (not shown) with a coefficient of 0.917 is not much inferior and has the attraction of being extrapolatable to R = 1.0. If the linear fit is accepted as realistic, then the extrapolation predicts that an assembly with an initial lap-shear bond strength > 45 J/m2 would withstand the chosen ageing exposure indefinitely, without loss of bond strength. As the correlation in Figure 7.11 makes evident, the degree of acid/base interaction at the bonded interface also should correlate with the residual bond property. The matter is put to the test with results shown in Figure 7.12. The resulting linear fit to experimental data has a correlation coefficient of 0.938, with much of the scatter contributed by the glass joints, where acid-base interactions are feeble and most subject to uncertain determination. An extrapolation to R = 1.0, identifies Isp to be greater than or equal to 85, as the strength of acid-base interactions required to ensure long-term resistance to the selected ageing procedure. The forecasts should be useful in optimising the choice of silane (and other) adhesion promoters in the formulation of adhesives such as the present PU. However, the physico-chemical analyses on which these are based cannot specify the chemical approaches to be followed. Moreover, the general applicability of the physico-chemical analyses followed here calls for the study of a broader range of adhesive/additive/substrate systems.

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Figure 7.11 Bond strength ratio versus initial bond strengths Reproduced with permission from H. P. Schreiber, R. Qin and A. Sengupta, Journal of Adhesion, 1998, 68, 1-2, 31, Figure 3. ©1998 Gordon and Breach Science Publishers.

7.4.3 Conclusions 1. A strong correlation has been shown to exist between initial bond strengths displayed by polymer and glass surfaces bonded by PU adhesives containing a variety of silane adhesion promoters and the acid-base interaction at substrate/adhesive interfaces. 2. Silane additives change the surface interaction potential of PU in distinct ways; APS accentuates surface basicity, while CS, MS and VS accentuate surface acidity. The addition of ES results in significant increases in both surface acidity and basicity. 3. Within the scope of the present work, ES was found to be the most effective adhesion promoter for each of the substrates. APS was particularly useful with the acidic PVC substrate, but the CS, MS and VS were preferred additives for the basic ABS substrate. 364

Adhesion Behaviour of Urethanes

Figure 7.12 Dependence of pair interaction parameter on bond strength ratio Reproduced with permission from H. P. Schreiber, R. Qin and A. Sengupta, Journal of Adhesion, 1998, 68, 1-2, 31, Figure 4. ©1998 Gordon and Breach Science Publishers.

4. A correlation has been developed between the residual adhesion of bonded assemblies following accelerated ageing and the magnitude of acid-base interfacial interactions. The strength of interactions needed to avoid property loss under the chosen ageing conditions can be estimated. This capability represents a guideline to the selection of preferred silane additives for the adhesion of PU adhesives to substrates with known acid-base interaction potentials. 5. IGC was shown to be a convenient and effective tool for evaluation of acid-base interactions at substrate and adhesive surfaces.

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References 1.

A. Gent and J. Schultz, Journal of Adhesion, 1972, 3, 4, 281.

2.

Inverse Gas Chromatography, Eds., D. R. Lloyd, T. C. Ward and H. P. Schreiber ACS Symposium Series No. 391, American Chemical Society, Washington, DC 1989.

3.

V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York, 1978.

4.

C. Saint Flour and E. Papirer, Industrial and Engineering Chemistry Product Research and Development, 1982, 21, 4, 666.

5.

Z. Deng and H. P. Schreiber in Contemporary Topics in Polymer Science, Vol. 6, Ed., W. M. Culbertson, Plenum Press, New York, 1989, p.385.

6.

D. G. Gray, Progress in Polymer Science, 1977, 5, 1.

7.

J. H. Chen and E. Ruckenstein, Journal of Colloid and Interface Science, 1990, 135, 496.

8.

V. Stannett in Diffusion in Polymers, Ed., J. Crank Butterworths, London, 1968, Chapter 2.

9.

E. J. Berger, Journal of Adhesion Science and Technology, 1990, 4, 5, 373.

10. J. M. Braun and J. E. Guillet, Advances in Polymer Science, Volume 21, Springer Verlag, Berlin, 1976, p.108. 11. A. N. Gent, and L. H. Lewandowski, Journal of Applied Polymer Science, 1987, 33, 5, 1567. 12. Y-S. Chang, Y-H. Lai and D. A. Dillard, Journal of Adhesion, 1989, 27, 4, 197. 13. C. Saint Flour and E. Papirer, Journal of Colloid and Interface Science, 1983, 91, 63. 14. U. Panzer and H. P. Schreiber, Macromolecules, 1992, 25, 14, 3633. 15. E. P. Plueddemann, Interfaces in Polymer Matrix Composites, Academic Press, New York, 1974, p.173. 16. E. P. Plueddemann, Silane Coupling Agents, Plenum Press, New York, 1982.

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Adhesion Behaviour of Urethanes 17. D. E. Leyden, Silanes, Surfaces and Interfaces, Gordon and Breach Science, New York, 1986. 18. H. Ishida in The Interfacial Interactions in Polymeric Composites, Ed., G. Akovali, NATO ASI Series E, Volume 230, Kluwer Academic Publishers, The Netherlands, 1993, p.169. 19. A. Sengupta and H. P. Schreiber, Journal of Adhesion Science and Technology, 1991, 5, 11, 947. 20. D. T. Sawyer and D. J. Brookman, Journal of Analytical Chemistry, 1968, 40, 1847. 21. J. Schultz, L. Lavielle and C. Martin, Journal of Adhesion, 1987, 23, 1, 45.

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8

HER Materials for Polyurethane Applications Raj B. Durairaj

8.1 Introduction Polyurethane elastomers have a wide range of industrial applications due to their unique combination of valuable physical and mechanical properties. These elastomers, in general, are composed of phase separated flexible soft segments and crystalline hard segments. The soft segment is based on polyether or polyester type aliphatic diols and the hard segment is the result of reaction between aromatic diisocyanate and low molecular weight diol chain extenders [1]. The soft segments have low polarity and are longer than the hard segments and provide a soft and flexible matrix. On the other hand, the hard segments are shorter, highly polar and have a strong tendency to aggregate. The thermodynamic incompatibility of these two segments leads to phase separation, which ultimately influences the physical and mechanical properties of urethane elastomers. In the development of cast polyurethane elastomers, various chain extenders are used with toluene diisocyanate (TDI) and 4,4´-diphenylmethane diisocyanate (MDI) terminated prepolymers based on polyether or polyester polyols. With MDI-terminated prepolymers, 1,4-butanediol is the most commonly used chain extender. In order to maintain desired mechanical properties at elevated temperature and to improve hardness and tear resistance, aromatic diols such as bis-(β-hydroxyethyl) ether of resorcinol (HER) or hydroquinone (HQEE) are often used. One of the difficulties with HQEE is its processing problem associated with a high melting point (~102 °C). On the other hand, the structurally similar isomer HER offers a lower melting point (89 °C), thus facilitating more forgiving chemistry and processing ease. Previous studies [2-5] have shown that HER chain extender offers significant processing advantages over HQEE and the properties of HER extended elastomers are comparable to those of HQEE cured elastomers. With the development of technical grade HER (HER TG), the processing temperature of the HER was further lowered without sacrificing the physical and mechanical properties of the final cured cast elastomers. The major difference between HER HP and HER TG chain extenders is the presence of some higher molecular weight (MW) reactive aromatic diol monomer in the technical grade material. With this unique

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Advances in Urethane Science and Technology product, HER TG, there is no need to add plasticisers, diluents or aliphatic diols to lower the melting points of aromatic diol chain extenders. It is also important to note that the presence of about 10 weight percent of this higher MW aromatic diol monomer does not appear to affect the performance of the cast elastomers. With this discovery, it was decided to investigate different grades of HER materials with varying levels of higher MW monomer and to study their performance in cast elastomers based on MDI terminated polyether prepolymer. The following are some of the objectives of this chapter: 1. To develop low melting and highly processable aromatic diol extenders from resorcinol and ethylene carbonate (HER materials). 2. To determine the nature of products or by-products formed by the resorcinol-ethylene carbonate reaction. 3. To investigate how the high MW aromatic diol monomer affects the phase separation behavior of polyurethane elastomers. 4. To examine the effect of various HER chain extenders on the physical, mechanical and dynamic mechanical properties of cast polyurethanes made with MDI-terminated prepolymers. 5. To study both polyether- and polyester-based polyurethane systems.

8.2 Experimental Conditions 8.2.1 Chain Extenders HER materials used as chain extenders in this work were prepared based on INDSPEC Chemical Corporation’s patented technology and the product details are given in Table 8.1. Hydroquinone (di-β-hydroxyethyl) ether (HQEE) and 1,4-butanediol (BD) were obtained from commercial sources.

8.2.2 Prepolymers Polyether and polyester-based MDI prepolymers obtained from commercial sources were used. The details are given in Table 8.2 370

> 99 100/0 >99 555-560 566

HER content (wt. %, by GC)

HER/H(OCH2CH2)nO-R-O(CH2CH2O)nH (n = 2 and 3; Theoretical; wt. ratio)

HER/H(OCH2CH2)nO-R-O(CH2CH2O)nH ( n = 2; by NMR; wt. ratio)

Hydroxyl number : Determined : Theoretical, calculated

COM: Commercial DEV: Developmental nd: Not determined HP: High purity TG: Technical grade GC: Gas chromatography

COM

89.1

Peak temperature (°C)

Status

86.7

88-92

Onset temperature (°C)

DSC analysis (endotherm)

Capillary melting point (°C)

nd

Crystalline solid

Physical form

Viscosity (Pa-s, 25 °C)

HER-HP

Extender Type

COM

545-550 554

96/4

93/7

93

85

81.6

78-90

nd

Molten solid

HERTG210 (TG-210)

DEV

525-535 535

85/15

83/17

82

80.6

71

76-85

nd

Molten solid

HERTG225 (TG-225)

DEV

500-520 506

69/31

68/32

68

71.7

65.6

65-78

nd

Waxy solid

HERTG250 (TG-250)

DEV

470-480 469

60/40

54/46

54

35-65

3 PEAKS

63-65

nd

Pasty solid

HERTG275 (TG-275)

Table 8.1 Development of HER materials for polyurethane applications

DEV

379 382

nd

0/100

13

nd

nd

20

Liquid

HER-LIQ

HER Materials for Polyurethane Applications

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Table 8.2 MDI-Prepolymer

Supplier

% NCO

Polyester 1. Baytec MS-242 2. Baytec MS-090

Bayer Bayer

6.6 9.0

Polyether 1. VIBRATHANE B 625 2. Baytec ME-050

Uniroyal Chemical Bayer

6.4 5.8

8.2.3 Preparation of Cast Elastomers All elastomers used in this project were prepared by the following standard procedure. A known quantity of prepolymer was weighed in a glass jar. Separately, in another glass jar, the chain extender was weighed to achieve 95 or 90% stoichiometry. Both jars were then placed in a vacuum oven at the desired temperatures and held under vacuum until the bubbles disappeared indicating that all the dissolved gases and any moisture, if present, was removed. This process required about 1 mm Hg vacuum and 2.5 to 3.0 hours at about 80-110 °C depending on the chain extenders (HER materials or HQEE) used for casting. After degassing, both the prepolymer and extender were mixed thoroughly for one minute using a stirrer to avoid bubbles. The mix was poured into a heated stainless steel plate mould pretreated with Teflon or silicone mould release agent. After pouring, the mould was placed in a programmable oven and cured for 16 hours. The samples, after curing, were allowed to cool slowly to room temperature. Test specimens were cut from the cured sheet for determination of tensile, tear strength and dynamic mechanical analysis (DMA) and then stored for at least 7 days at 22 °C, 50% relative humidity conditions before testing. Compression set samples were prepared separately, following the degassing procedure similar to that for plate casting, and aged similarly before testing.

8.2.4 Physical and Mechanical Properties Determination The following test methods were used to determine the properties of cast polyurethane elastomers: •

Pot life determinations were done using a Brookfield viscometer at controlled temperatures.

•

Shore Hardness, Durometer A/D (ASTM D2240) [6].

•

Tensile Modulus, strength and % elongation (ASTM D412) [7].

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HER Materials for Polyurethane Applications •

Tear Resistance, Die C (ASTM D624) [8].

•

Bashore Rebound (ASTM D2632) [9].

•

Compression Set % (ASTM D395) [10], Method B.

•

DMA was determined using a Rheometrics RMS-800 instrument at 1 Hz frequency with a heating rate of 2-10 °C/minute.

•

Differential scanning calorimetry (DSC) analysis of the cast elastomers was performed on a Perkin Elmer (DSC-7) instrument at a heating rate of 10 °C/minute under a nitrogen atmosphere.

•

Fourier transform-infra-red (FT-IR) analysis was performed on a Perkin Elmer System2000 instrument.

8.3 HER Materials Synthesis and Characterisation High purity (HP) and technical grade HER materials were synthesised by the reaction of various ratios of resorcinol and ethylene carbonate. Previous studies had demonstrated that the HER TG has a performance equal to that of the HER HP material [5]. It was also observed that, in the cast polyurethanes, HER TG could be processed at a lower temperature than the HER-HP. These interesting experimental facts prompted the development of various HER-TG materials and an investigation into their effectiveness as chain extenders to enhance the processing of cast and other polyurethanes. By varying the molar ratios of resorcinol and ethylene carbonate, the technical grade materials such as HER TG-210, HER TG-225, HER TG-250, HER TG-275 and HER LIQ were developed. HER TG-210 is the same as HER-TG reported in prior work and in this chapter. The major difference between the HP and TG materials is the presence of higher MW reactive diol monomers present in the TG materials. From the resorcinol and ethylene carbonate reaction, depending upon the excess of ethylene carbonate, two types of higher MW diols can be expected: HOH2CH2CO-R-OCH2CH2OCH2CH2OH and HOH2CH2C(OH2CH2C)n-O-R-O(CH2CH2O)nCH2CH2OH where R = meta phenylene, n = 1 and 2

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Advances in Urethane Science and Technology On the basis of theoretical calculations and also from the gas chromatography (GC) analysis, hydroxyl number and nuclear magnetic resonance (NMR) analysis results, it has been confirmed that the reactive high MW diol present in the technical grade materials has the structure: HOH2CH2C(OH2CH2C)nO-R-O(CH2CH2O)nCH2CH2OH where n = 1 and 2 For HER TG-210, TG-225 and TG-250 materials, the reactive high MW diol has predominantly n = 1 and is present at approximately 7, 17 and 32 weight percent, respectively. With higher molar ratios of ethylene carbonate:resorcinol, high MW diol with n = 1 and 2, having MW of 286 and 374, respectively, are observed to be present. In HER LIQ, the HMW diol monomers were estimated to be as high as 87 weight percent. It was suspected that the presence of these reactive diol monomers in the HER could have several advantages. Since they have primary hydroxyl groups, their reactivity towards isocyanate groups is similar to that of other reactive diol chain extenders. The molecular weight of these high MW diols is still low enough that they act as chain extenders and not as polyols. In addition, they can act as reactive plasticisers in various polyurethanes and other polymers. Due to the presence of this high MW material, the melting point of HER is lowered, thereby increasing its processability in polyurethane applications. Laboratory observations indicated that HER and other technical grade HER materials have supercooling tendencies. In fact, HER TG-275 stays liquid at room temperature for a period of 2-3 months. The TG-250 remains liquid at room temperature indicating that processing could be easier with this material alone and it would be a good candidate for mixing with other small chain extenders, such as butane diol, and polyols (polyether and polyester) at lower temperatures. The melting points determined by the capillary method and DSC analysis are given in Table 8.1. The varied melting point determined by the capillary is associated with the high MW diol impurity. To determine the melting point by DSC, measurements were done at a heating rate of 5 °C/minute under a nitrogen atmosphere. HER-HP showed one sharp endothermic peak. With HER TG-210, TG-225 and TG-250 materials, the major endothermic peak shifted to lower temperatures and also showed one minor peak, indicating one impurity. For HER TG- 275, three endothermic peaks were observed between 35-65 °C indicating three components. For this material, GC analysis showed two major peaks in addition to the HER peak.

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8.4 Cast Poly(Ether Urethanes) 8.4.1 Pot Life Determination Before making the castings, pot life determinations were performed to determine the processing window of the prepolymer and chain extender materials. As a standard practice, both the prepolymer and chain extender were preheated to the desired temperatures, degassed thoroughly, mixed at the desired stoichiometry (90 or 95%) and poured into a thermostatted vial. The viscosity was measured, using a Brookfield viscometer, as a function of time. The measurements were terminated when either the time of measurement reached 20 minutes or the viscosity of the mix reached 0.1 Pa-s. Figure 8.1 shows the viscosity-time plots of VIBRATHANE B 625 extended with HQEE and various HER materials. As can be seen, the pot life temperatures for HER materials are considerably lower than HQEE. The shorter pot life for HQEE is due to the higher processing temperature. If the processing temperature were lower than 110 °C, then HQEE would crystallise out from the solution. Premature crystallisation is a major problem encountered by most users of HQEE. But these problems are eliminated with HER-HP and other HER-TG materials due to their supercooling behaviour.

Figure 8.1 Pot-life curves for Vibrathane B-625-HER materials

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8.4.2 Polyurethane Castings The prepolymer % NCO content and the equivalent weight of the chain extender materials were determined to calculate the amount of each material to achieve 95% stoichiometry. After careful degassing and mixing, the material was poured into the stainless steel mould and cured for 16 hours continuously at 110 °C. Table 8.3 summarises the experimental details relating to cast elastomer preparation. In order to compare the properties of cast polyurethanes, conditions relating to castings, ageing and testing were kept constant throughout this work.

8.4.3 Calculation of Hard and Soft Segment Contents HER-HP has a purity of >99%. The other HER-TG materials contained at least one or two high MW reactive diol monomers. The presence of these reactive diol monomers can play a major role in the formation of hard segments, hard segment content and phase separation behavior of the polyurethanes, which will ultimately dictate the final cured physical and mechanical properties of the cast elastomer materials. A previous study on HER-LIQ showed that the cast elastomers based on this extender produced softer material compared to HER-HP [5]. The elastomeric materials developed using HER-LIQ appeared to be more transparent indicating that the high MW diols present in this chain extender were responsible for this change. The MW of these high MW diols are still low enough to produce a hard segment in the polyurethanes, but the nature of the hard segment can be different from that formed between the isocyanate and HER due to a longer and more flexible chain. Based on the extensive study made on HER and HQEE chain extenders, it is well known that HER can produce hard segments with MDI and other isocyanate monomers, which are crystalline in character [4]. Due to their high flexibility, the high MW reactive diol monomers can produce hard segments which may be more amorphous than crystalline in character. The amorphous hard segment formed can mix with the soft segment of the polyurethane elastomers and ultimately change the physical and mechanical properties of the elastomeric materials. Therefore, it was decided to calculate the amount of hard and soft segment contents of the polyurethanes resulting from the VIBRATHANE B 625 prepolymer and HER materials containing various levels of the high MW reactive diols. VIBRATHANE B 625 is a polyether prepolymer based on polytetramethylene glycol (PTMEG) and MDI raw materials. Based on the 6.5% NCO content, it can be assumed that this was prepared using 1 mole of PTMEG-1000 polyol and 2.2 moles of MDI. It can also be assumed that the HER chain extender produces crystalline hard segments

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16

Cure, (h/110 °C)

11.35 13.04 14.64 3419 538 67 127 21 55 96

100% Modulus

200% Modulus

300% Modulus

Tensile strength

% Elongation

Energy to break, (kN/m)

Tear strength, DIE C, (kN/m)

Compression set, %

Bashore rebound, %

Hardness, Shore-A (D)

Tensile Properties (MPa)

95

Stoichiometry, (% theoretical)

100/14.6

110

Extender temperature, (°C)

Mix ratio (Prepolymer/extender)

110

HQEE

Prepolymer temperature, (°C)

HER Materials

95

58

15.4

11 5

79

559

3816

17.33

13.71

10.56

16

95

100/14.5

100

10 0

HER-HP

95 (48)

60

17.4

102

70

47 6

3937

19.08

14.57

10.71

16

95

100/14.9

90

90

HERTG210

92 (44)

60

17

97

53

462

3712

15.6

11.77

8.84

16

95

100/15.5

90

90

HERTG225

90 (43)

54

17.5

86

49

413

4037

16.35

11.35

7.98

16

95

100/16.1

90

90

HERTG250

89 (40)

46

29.4

72

40

372

3682

16.79

10.44

9.94

16

95

100/17.2

90

90

HERTG275

78 (30)

49

9.1

44

20

321

2301

11.55

6.63

3.99

16

90

100/20.7

80

80

HERLIQ

Table 8.3 Cast elastomers based on Uniroyal B-625 (polyether) prepolymer elastomers performance with various HER-TG materials

HER Materials for Polyurethane Applications

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Advances in Urethane Science and Technology and the high MW diols produce amorphous hard segments. Taking into account the amount of HER and high MW diols present in the chain extender, for the 95% stoichiometry, the crystalline hard segment, amorphous hard segment and polyether soft segment contents were calculated for different cast polyurethanes made using the VIBRATHANE B 625 prepolymer. These calculations were based on the reaction scheme (8.1) shown opposite and an idealised polyurethane structure from this scheme and the details are given in Table 8.4. As can be seen from Table 8.4, the crystalline hard segment content is decreased from 48.7% to 35.4% when the high MW diol content increases from 0% to 87%. Concurrently, the amorphous hard segment content increases from 0% to 16.6%. At the same time, the soft segment content resulting from the polyol was affected only slightly (changed from 51.3% to 48%). Since the high MW diol changed the concentrations of the hard and soft segments, it is interesting to see how this will affect the physical and mechanical properties of the cast elastomers. In general, the contributions of both the soft and hard segments in the polyurethanes can be correlated with the properties observed. The soft rubbery block primarily affects resiliency, wear, tear, compression set and low temperature properties, while the hard block affects hardness, modulus and tensile properties [11].

8.4.4 Hard Segment Versus Hardness The Durometer hardness was measured after the cast elastomers were aged for at least 7 days at 23 °C and 50% relative humidity. The details are given in Table 8.3. From the results, it is evident that as the crystalline hard segment content of the elastomer is decreased, the elastomer hardness also decreases. By analysing the soft and hard segments and their relation to hardness, it appears that the amorphous hard segment behaves like the polyether soft segment.

8.4.5 Tensile Properties The tensile data obtained from the cast elastomers are summarised in Table 8.3, plotted and shown in Figure 8.2. From this figure, it can be seen that the 100% modulus and % elongation decreased for the HER-TG materials compared to HQEE and HER-HP chain extenders. On the other hand, the 200% and 300% modulus values were higher for HER-HP and HER TG-210 extenders compared to HQEE and the values were lower for the other technical grade materials. Except for HER-LIQ extender, all HER based

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93 82 68 54 13

3. Vibrathane B-625 + TG210

4. Vibrathane B-625 + TG225

5. Vibrathane B-625 + TG250

6. Vibrathane B-625 + TG275

7. Vibrathane B-625 + HER-LIQ 87

46

32

18

7

0

0

H(OCH2CH2)nORO(CH2CH2O)nH (%)

HER Materials Analysis

35.4

43.1

44.4

46.9

(n = 1-3, chain extender)

48

50.2

50.5

50.9

51.1

51.3

58.7

Soft Segment (%)

Crystalline Hard Segment

Soft Segment

Reaction scheme 8.1

where R - diphenyl methane and R´-1,3-phenylene groups.

|—

| —|

| |

| | |— Amorphous (Hard) Segment —|

O-H2CH2CO-R′-OH2CH2C-OOCHN-R-NHCOO-(Polyether)-OOCHN-R-NHCOO(CH2CH2O)2-3-R′-(OCH2CH2)2-3-O

(Prepolymer)

16.6

6.7

5.1

2.2

0.9

0

48.7 48

0

41.3

Amorphous

Hard Segment (%) Crystalline

Theoretical Calculation

OCN-R-NHCOO(Polyether)OOCHN-R-NCO + HO(H2CH2CO)n-R′-(OCH2CH2)n-OH

100

No extender

HER Content (%)

2. Vibrathane B-625 + HER-HP

1. Prepolymer (MDI/PTMEG-1000) (NCO=6.5%; Mole ratio = 2.2/1)

Polymer-chain Extender

Table 8.4 Distribution of hard and soft segments in cast polyurethanes

HER Materials for Polyurethane Applications

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Figure 8.2 Tensile properties of cast polyurethanes

extenders showed either equal or better tensile strength values than the HQEE extender. In general, the HER-TG materials showed good tensile properties.

8.4.6 Tear, Compression Set and Rebound Properties These properties are summarised in Table 8.3. Tear strength is generally a very important property in many applications. By comparing the tear strength values of the technical grade materials against HQEE and HER-HP, they can be seen to decrease continuously as the hardness of the elastomers is decreased. HER-HP had a lower compression set than the HQEE extended elastomer. This value increases when the concentration of high MW aromatic diols content is increased in the HER-TG extenders. In fact, HER TG-275 extender with about 46% high MW aromatic diol showed a high compression set of 29.4%. On the other hand, when this high MW aromatic diol concentration reached 87% in HER-LIQ, the elastomer based on this chain extender produced a very low compression set material.

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HER Materials for Polyurethane Applications The most important property to look for in choosing materials for applications such as skate wheels is resilience or rebound [12]. The ether-based elastomers, in general, give higher rebound resilience. The measured Bashore technique rebound values, show that HER-HP, HER TG-210 and HER TG-225 extenders have slightly better resilience than HQEE. The rebound values appeared to be lower for HER TG-250- and HER TG-275based elastomers. This value was higher for HER-LIQ (49%) when compared to TG275 (46%) extender. Since the rebound is generally measured at room temperature, its value would relate to tan δ data given in Table 8.5. A comparison of the two indicates a correlation between tan δ and the rebound values. Obviously, HER-LIQ extender showed lower tan δ (0.044) compared to HER TG-275 (0.076) material.

8.4.7 Differential Scanning Calorimetry The influence of the high MW diols present in the HER chain extenders on the transition behavior of cast elastomers was studied by DSC analysis. Figure 8.3 compares the DSC curves of HQEE, HER-HP and different HER-TG extended polyurethanes. The transitions, which are observed to occur above room temperature, are associated with the melting of

Figure 8.3 DSC curves of cast polyurethanes from Vibrathane B-625-HER materials

381

382 43

at 150 °C 22

30

90.2 1.9 0.89

Peak modulus

at 25 °C

at 150 °C

160

0.055

1300

800 500

at 25 °C

at 150 °C

700

1700; 1300

1100; 700

Peak compliance

Peak temperature (°C)

Loss Compliance, J′′ (MPa) -53.3

0.044

0.02

-53.3

0.038 0.016

0.039

at 25 °C

at 150 °C

at 10 Hz /23 °C (frequency)

0.255

-63.3 0.198

Peak temperature, Tg (°C) -61.4

0.35

1.1

95.8

-70.8

19.9

Peak tan delta

Tan Delta

-70.9

Peak temperature, Tg (°C)

Loss Modulus, G′′ (MPa)

170 39.5

Temperature (°C)

G′, MPa x 108

Thermal Stability (Temperature maximum for constant G′)

47

at 25 °C

Storage Modulus, G′ (MPa)

48

0

800

1400

1800; 1600

-49.4

0.042

0.016

0.038

0.251

-59.2

0.31

1.1

90.9

-68.8

17.4

1100

2100

3900

-39.3

0.05

0.016

0.043

0.307

-51.4

0.25

0.88

89.6

-67

12.6

160

14

19 160

21

2.2

46.9

28

0.9

48.7

Crystalline HS content (%)

Amorphous HS content (%)

TG225

1900

3300

6400

-24.9

0.057

0.018

0.047

0.295

-43.4

0.18

0.69

83.3

-65.3

8. 8

160

9.8

14

5.1

44.4

TG250

Dynamic Mechanical Analysis Results TG210

HQEE HER-HP

HER Materials

2300

5600

5600

5.4

0.092

0.02

0.076

0.246

-45.3

0.16

1.0

76.9

-63.5

7.7

155

8.4

14

6.7

43.1

TG275

5600

11100

32600

-14.8

0 .0 7 2

0.018

0.044

0 .4 7 1

-35.4

0 .0 5

0 .1 7

6 5 .4

-59.2

2 .9

160

3 .1

39

16.6

35. 4

HER-LIQ

Table 8.5 Cast elastomers based on Uniroyal B 625 (polyether) prepolymer elastomers performance with various HER-TG materials

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HER Materials for Polyurethane Applications

Table 8.6 DSC results on cast polyurethanes from Vibrathane B 625 - HER materials Endothermic Peak Temperature First Heating

Second Heating

Temperature (°C)

ΔH (J/g)

Temperature (°C)

20 5

17

205, 234

186, 199

22

184, 224

3. HER-TG210

195

19

155, 192

4. HER-TG225

183

13

162, 192

5. HER-TG250

181

9

150

6. HER-TG275

173

13

143

7. HER-LIQ

nd

nd

nd

Chain Extender 1. HQEE 2. HER-HP

hard segment domains. The melting behaviour of the hard segments can be seen as an endothermic peak in the DSC curve. The hard segment melting temperatures and heats of transitions observed from the DSC analysis are given in Table 8.6. From the heats of transition data (ΔH), it is clear that when the hard segment content is decreased, the energy associated with the melting of the hard segments is also decreased. In addition to this, the presence of amorphous hard segments appeared to decrease the melting temperatures of the crystalline hard segment. Since the temperature at which the first transition takes place above room temperature in the DSC curve is considered to indicate the limit of the thermal stability of the polyurethane elastomer, the thermal stability of these elastomers was found to be decreased when the high MW diol contents were increased. This might be due to the reduction of hard segment content. Higher hard segment transition in the DSC indicates higher thermal stability of the resulting polyurethane.

8.4.8 Dynamic Mechanical Analysis DMA is an analysis technique used to determine the dynamic properties of the elastomers [13, 14]. Dynamic properties of the elastomeric materials are important because they influence the performance of certain parts such as wheels and tyres. This method determines the storage modulus G´ (elastic behaviour), loss modulus G´´ (energy dissipation), tan δ, loss compliance J´´ and glass transition temperature (Tg) values. The Tg of the soft segment can determine the low temperature behaviour of polyurethane elastomers. This is not only influenced by the nature of the soft

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Advances in Urethane Science and Technology segment (polyether) but also the degree of phase separation between the hard and soft segments. In this work, the DMA method was utilised to understand the effect of HQEE, HER-HP and various HER-TG materials on the cast elastomers made using VIBRATHANE B 625 (PTMEG-Polyether) prepolymer. The results are summarised in Table 8.5.

8.4.8.1 Storage Modulus G´ Property The storage modulus, G´, values determined by the DMA for the elastomers extended with HER chain extenders are given in Table 8.5, and the storage modulus curves are shown in Figure 8.4. Storage modulus quantitatively measures the material’s elastic properties and also qualitatively determines the elastomer’s stiffness and hardness. Since the hardness of the elastomer is related to its hard segment content, the DMA storage modulus was found to decrease as the hardness of the elastomers was decreased due to the increasing amounts of high MW diols present in the HER-TG materials. From the DMA curve, it was observed that the G´ was considerably decreased for HER-LIQ extended elastomer which contained about 87% of the high MW diol

Figure 8.4 Storage modulus (G′) curves of cast polyurethanes

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HER Materials for Polyurethane Applications monomers. It has been observed and reported in the literature [13, 15] that elastomers based on PTMEG polyol exhibit a secondary transition (additional dispersity) which appears as a step in the temperature—shear modulus function curves. The same kind of behaviour is observed for elastomers based on HQEE, HER-HP and HER TG-210 chain extenders at temperatures below 0 °C. A sharp transition in the G´ curve may be associated with good phase separation of the elastomer. In this respect, a gradual transition occurs with HER TG-225, TG-275 and HER-LIQ compared to HQEE, HER-HP and HER-TG-210 extended elastomers indicating that poor phase separation occurrs in these elastomers.

8.4.8.2 Thermal stability of Elastomers The thermal stability of urethane elastomers can be determined by the DMA method. The temperature at which the storage modulus (G´) decreases significantly in the rubbery region is considered to be the limit of thermal stability of the elastomers [16]. From the G´ values of the elastomers, it was determined that HER extended materials were stable up to 160 °C whereas HQEE extended elastomer showed a 10 °C higher stability.

8.4.8.3 Loss Modulus Property (G´´) The loss modulus, G´´, is a quantitative measure of energy dissipation in the elastomers. Low values of G´´ are indicative of low energy dissipation, low hysteresis and, consequently, low heat build up, an important requirement for applications such as in-line skate wheels [17]. Figure 8.5 shows the plots of loss modulus as a function of temperature. The loss modulus values are high at low temperature and found to decrease as the temperature is increased. The curves show maxima corresponding to the T g of the soft segments of the elastomers which gradually shift to higher temperatures indicating the effect of high MW diol monomer of the HER materials. In the temperature range between 0 and 160 °C, the loss modulus decreases into a long plateau and then starts to increase when the temperature reaches beyond 170 °C and 180 °C. This might be due to the melting of the hard segments. Figure 8.6 shows the plots of storage modulus (G´) and loss modulus (G´´) as a function of various technical grade HER materials along with HQEE and HER HP measured at 25 and 150 °C. The curves clearly demonstrate that G´ and G´´ decrease as the hard segment content is decreased in these elastomers.

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Figure 8.5 Loss modulus (G′′) curves of cast polyurethanes

Figure 8.6 Effect of HER materials on shear modulus properties

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8.4.8.4 Tan Delta Property (tan δ) The measure of tan δ is a qualitative tool to determine the hysteresis or heat build up in an elastomer during dynamic flex conditions. Figure 8.7 shows the plot of tan δ as a function of temperature and the values obtained from this determination are shown in Table 8.5. As the hard segment content is decreased, the tan δ peak height is increased indicating higher hysteresis of the elastomers associated with the higher amounts of high MW diols. The peak widths are also observed to increase, corresponding to the decrease in the hard segment content of the elastomers. By comparing the tan δ peak (Tg) to that of loss modulus peak (Tg) temperatures, the tan δ temperatures appeared to be higher. The Tg associated with the soft segment were found to shift higher as the amorphous hard segment content of the elastomers increased. This temperature shift might be associated with the restriction of mobility of the soft segments due to phase mixing. Also, the greater the amount of hard segment dissolved in the soft segment phase, the higher the expected Tg of the soft segment.

Figure 8.7 Tan delta curves of cast polyurethanes

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Advances in Urethane Science and Technology The Tg of the elastomers increased gradually when the hard amorphous content of these materials was increased. Since this hard amorphous segment is the result of reaction between the isocyanate and high MW aromatic diol monomers containing ether linkages, there is a possibility that this amorphous hard segment can mix effectively with the soft polyether polyol segments of the elastomers. As the amorphous hard segment content is increased, it is possible that the effect of phase mixing between this phase and the soft polyol segment is enhanced, thereby increasing the Tg of the soft segment of the resulting polyurethane elastomers. The Tg of the HER-LIQ extended elastomer is increased by as much as 26 °C with 16.6% amorphous hard segment content. These results indicate that the high MW aromatic diol monomers present in the HER-TG materials, influence the low temperature properties of the cast polyether elastomer materials.

8.4.8.5 Loss Compliance Property (J´´) Loss compliance quantifies the heat generated during dynamic loading in applications like tyres, wheels and rollers. Polyurethane elastomers with lower loss compliance values will experience less heat build up and, consequently, may suffer fewer field failures [14]. Figure 8.8 shows the graph of loss compliance as a function of temperature for the cast elastomers made with HER extended materials. The results are summarised in Table 8.5. Loss compliance curves show peak maxima at the soft segment Tg. The values appear to be lower for elastomers containing higher hard segment contents. In the case of HQEE, HER-HP and HER-TG 210 extended elastomers, loss compliance curves showed two peaks at the Tg. The two peaks appear to merge into a single peak as the amorphous hard segment of the elastomers is increased. Based on the loss compliance values, it appears that HQEE based elastomers show less heat generation than the HER materials containing higher levels of high MW diols. However, HER-HP and HER TG-210 based elastomers behave similarly and, therefore, comparable performance can be expected in their applications. In order to compare the Tg observed from the peak temperatures of loss modulus (G´´), tan δ and loss compliance (J´´) curves, the peak temperatures are plotted as a function of the amorphous hard segment and shown in Figure 8.9. As can be seen in the figure, the amorphous hard segment produced by the high MW aromatic diol extenders has a pronounced effect on the soft segment Tg which, in turn, ultimately dictate the low temperature properties of the elastomers made from them.

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Figure 8.8 Loss compliance (J′′) curves of cast polyurethanes

Figure 8.9 Amorphous hard segment versus peak temperatures

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8.5 Cast Poly(Ester Urethanes) Polyurethane elastomers containing polyester soft segments are used in applications such as wheels, rollers, gaskets, seals, sheet goods, belting, cyclone liners and forklift tyres due to their high tensile strength, tear strength, abrasion resistance, oil resistance and heat-aging properties. In this work, MDI-terminated polyester prepolymer based on ethylene adipate polyol (Baytec MS-242) was used to prepare cast poly(ester urethane) materials. The HERTG materials, HER-HP and HQEE were used as chain extenders. As discussed in section 8.4, technical grade HER materials contain various levels of high MW diols as reactive impurities. Therefore, as with poly(ether urethanes), the high MW diols are expected to play a major role in the performance of poly(ester urethanes) made with these materials.

8.5.1 Pot Life Determination The experimental details on the pot life determination and elastomer casting conditions are given in Table 8.7. Pot lives exceeding 20 minutes can be easily achieved with HER materials due to their low melting and processing temperatures. Because of their supercooling behaviour and tendency to stay liquid after cooling to room temperature, further enhancement in the pot life can be achieved by lowering the mixing temperature of TG-250 and TG-275 materials.

8.5.2 Tensile Properties The data obtained from the tensile measurements are summarised in Table 8.7 and the graphs are shown in Figure 8.10. From the results, it can be seen that 100% modulus and %elongation are decreased and the tensile strength increased as the high MW diols content of the chain extender is increased.

8.5.3 Tear, Fracture Energy, Compression Set and Rebound Properties These properties are summarised in Table 8.7 and the values are plotted against elastomer hardness and shown in Figure 8.11.

390

HER/HQEE Materials Pot-life Determination Temperature (°C) Pot-life (minutes) Pot-life viscosity (Pa-s) Casting Conditions Prepolymer temperature (°C) Extended temperature (°C) Mix ratio (prepolymer/extender) Stoichiometry, % theoretical Cure, (h/110 °C) Cast Polyurethane (Calculated) Hard segment (%) Soft segment (%) Amorphous hard segment (%) Tensile Property (MPa) 100% Modulus 200% Modulus 300% Modulus Tensile strength % elongation Energy to Break (kN/m) (area under stress-strain curve) Tear Strength, Die C (kN/m) Compression set (%) Bashore rebound (%) Hardness, Shore-A (D) HER-HP 100 20 10.8 100 100 100/15 95 16 37.3 62.7 0 11.13 14.93 18.8 29.6 699 122 144 30.4 39 46 D

HQEE 110 7.5 10.1 110 110 100/14.7 95 16 ND ND ND 11.53 14.1 16.39 28.42 726 107 148 26.2 41 50 D

137 32.4 34 44 D

100

10.55 14.49 19.13 34.93 579

36.6 62.5 0.9

85 85 100/15.1 95 16

85 >20 4.7

HER-TG210

110 51.9 33 94 ( 40 )

75

8.9 12.59 17.3 34.49 499

35.4 62.3 2.3

85 85 100/16 95 16

85 >2 0 6.15

HER-TG225

10 4 55.5 28 89 ( 39 )

80

7.09 10.14 15.62 39.85 45 8

33. 7 61. 8 4. 5

85 85 100/17 95 16

85 >20 5. 5

HER-TG250

Table 8.7 Cast elastomers based on Bayer MS-242 (polyester) prepolymer Effect of HER - HQEE Composition on Elastomer Properties

77 51 15 84( 34 )

40

5.19 7.48 10.73 34.17 43 0

31. 8 61. 4 6.8

85 85 100/17.6 95 16

85 >2 0 5. 3

HER-TG275

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Figure 8.10 Tensile property of Baytec MS-242 prepolymer extended with HER chain extenders

Figure 8.11 Variation of cast polyurethanes properties as a function of hardness

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HER Materials for Polyurethane Applications The hardness of cast polyurethanes is decreased as the high MW diols content of the chain extender is increased. This eventually affects some of the most important elastomer properties such as the tear strength, 100% tensile modulus and fracture energy. The presence of high MW diols in the TG materials also increases the amount of amorphous hard segment which, in turn, interferes with the phase separation of hard and soft segments of the polyurethane elastomers. The continuous reduction in the rebound property is associated with the formation of higher amorphous hard segments from these TG materials.

8.5.4 Differential Scanning Calorimetry Analysis During the DSC analysis, the elastomer sample is first heated to 225 °C (first heating), cooled back to room temperature (cool down) and then heated again to 250 °C (second heating) at 10 °C/minute heating and cooling rates. The endothermic transitions observed above room temperature during the heating are associated with the melting of hard segments and exothermic peaks are due to the crystallisation of the hard segment upon subsequent cooling. DSC studies made on the segmented polyurethanes revealed three hard segment-associated endotherms at temperatures of 60-80 °C, 120-190 °C and above 200 °C [18-21]. These endothermic transitions are due to short-range, long-range and microcrystalline ordering of hard segment domains. The endothermic peak temperature determined from the second heating is taken to be the thermal stability limit of the elastomer. The hard segment melting and crystallisation temperatures and their heats of transitions determined from the DSC analysis are given in Table 8.8. Figure 8.12 shows the DSC curves obtained during the heating and cooling of cast elastomers based on HER-HP and HER TG-250 chain extenders. From the second heating endothermic peak temperatures, HER-HP extension resulted in a thermal stability at 177 °C and the stability decreased to 135 °C for an elastomer made from TG-275 extender. An interesting observation was also made in the DSC analysis during cooling. An exothermic peak associated with the crystallisation of hard segment was seen for the elastomers made from HER-HP, TG-210 and TG-225 extenders. Although this exothermic peak is not seen for TG-250 and TG-275 extenders during the DSC cooling curve, this exothermic behaviour is seen during the second heating. This strongly suggests that the higher amorphous hard segments formed from the isocyanate and high MW diols interfere with the crystallisation of the crystalline hard segment. From the endothermic transition energy, HER-HP extension produced a higher hard segment content compared to the other TG materials.

393

394

ND = Not detected

6. HER-TG275

5. HER-TG250

4. HER-TG225

3. HER-TG210

ND 217

Minor

215

Minor Major

170

204

Minor Major

185

ND

Minor Major

189

203, 211

Minor Major

188

Major

201

Minor

2. HER-HP

190

Major

1. HQEE

2

14

15

23

33

28

J/g

°C

Peaks

HER/HQEE Material

Energy

ND

ND

66

73

83

102

°C

Peak

ND

ND

6

10

14

10

J/g

Energy

Exotherm

Endotherm Peak

Cooling Down

First Heating

135

143

140

168

140

173

177

218

188

°C

Peak

3

4

6

7

13

9

10

J/g

Energy

Endotherm

74

71

ND

ND

ND

ND

°C

Peak

5

6

ND

ND

ND

ND

J/g

Energy

Exotherm

Second Heating

Table 8.8 DSC results on cast polyurethanes from MS-242-HER materials

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HER Materials for Polyurethane Applications

Figure 8.12 DSC curves of cast polyurethanes from Baytec MS-242

8.5.5 Dynamic Mechanical Analysis The storage modulus (G´), thermal stability, Tg and loss compliance (J´´) properties obtained from the DMA are given in Table 8.9. Poly(ester urethanes) based on HER chain extenders behaved in the same way as poly(ether urethanes). When the high MW diol content of the extender is increased, the storage modulus is decreased due to the reduction of the hard segment content. As was seen from thermal analysis, the thermal stability of the HER-HP extended elastomer is slightly higher than the technical grade materials. In addition to storage modulus reduction, a continuous reduction in the Tg, an increase in tan δ and increase in the loss compliance values are also noticed when the amorphous hard segment content of the elastomers is increased.

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Table 8.9 DMA results of cast elastomers from Baytec MS-242 HER Materials

HQEE

HER-HP

TG210

TG225

TG250

TG275

at 25 °C

51.5

30.3

28.7

23.8

12.4

6.5

at 100 °C

36.7

18.2

16.4

12.1

6.3

3

at 150 °C

33.5

15.7

10.6

7.4

4.1

2.2

Temperature (°C)

170

170

160

160

150

15 0

G′, MPa

20.6

7.8

8.7

4. 7

4.1

2.2

-29.6

-27.7

-26

-27.6

-19.8

-13.9

Peak modulus

152

166

173

15 9

104

14 6

at 25 °C

3.29

1.64

1.79

2.18

1.27

0.54

at 100 °C

1.48

4.62

0.37

0.5

0.16

0.05

at 150 °C

1.59

4.19

0.37

0.51

0.24

0.1

Peak temperature, Tg (°C)

-21.5

-20.1

-19.8

-19.4

-7.6

0.5

Peak tan delta

0.27

0.467

0.452

0.409

0. 5

0.749

at 25 °C

0.064

0.054

0.062

0.091

0.102

0.085

at 100 °C

0.04

0.025

0.022

0.04

0.026

0.019

at 150 °C

0.047

0.027

0.035

0.07

0.058

0.043

at 10 Hz /23 °C (frequency)

0.085

0.077

0.083

0.124

0.154

0.22

0.9

-11.9

-9.9

10.5

5.7

10.1

Peak compliance

1660

4060

3960

4100

1200

350 0

at 25 °C

1230

1780

2160

3820

8150

128 0

at 100 °C

1090

1400

1380

3460

4070

630 0

at 150 °C

1410

1690

3320

9400

1420

193 0

Storage Modulus, G′ (MPa)

Thermal Stability (Temperature maximum for constant G′)

Loss modulus, G′′ (MPa) Peak temperature, Tg (°C)

Tan Delta

Loss compliance, J′′ (MPa) Peak temperature (°C)

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8.6 Cast Polyurethanes from HER/HQEE Blends Although the cast polyurethanes made using HQEE chain extender produce high performance materials, processing problems are always encountered with this material. In order to use HQEE, the processing temperatures must be as high as 120-130 °C. If the temperatures are lower than this, then ‘starring’ (after curing a ‘snowflake’ type material embedded in the continuous matrix, (cast elastomer) can be seen) occurs due to the crystallisation of HQEE out of the elastomer system. To overcome this processing problem, HQEE is often blended with various aliphatic diols such as ethylene glycol, butanediol, and so on [22-24]. The addition of aliphatic diols into HQEE, therefore, is expected to affect the high temperature physical and mechanical properties of the resulting elastomers. These problems can be easily overcome if an aromatic diol chain extender like HER is used in the place of aliphatic diols. Due to the supercooling behaviour of HER, this extender offers a unique opportunity to lower the melting point of HQEE without the customary degradation of properties.

8.6.1 Freezing Point Determination of HER/HQEE Blends To study the eutectic nature of the HER/HQEE blends, freezing point determinations were made. HER/HQEE blends were prepared at different weight ratios and melted at elevated temperatures. Then the melt was allowed to cool slowly under controlled conditions. The temperature at which the first crystallisation was observed was considered to be the freezing point of the melt corresponding to that particular HER/ HQEE composition. The plot of freezing point versus HER/HQEE blend is shown in Figure 8.13. As can be seen from the plot, HQEE melting can be considerably reduced with the help of an HER extender.

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Figure 8.13 Apparent freezing points of HER/HQEE blends

8.6.2 Cast elastomers and Their Properties The experimental details on the cast elastomers prepared using Baytec MS-242 and Baytec ME-050 prepolymers and their physical, mechanical and dynamic mechanical properties are given in Tables 8.10 and 8.11. In the case of poly(ester urethanes), HER and HQEE extended elastomers showed comparable tensile, tear, glass transition temperature (Tg) and hysteresis (tan δ) properties. Compared to HER, HQEE extension produced elastomer with about 2 Shore-D units higher hardness. An improvement in the tensile strength value was observed for the HER/HQEE blend at 25:75 weight ratio. In general, all the physical and mechanical properties were slightly affected or lowered with HER/HQEE blends compared to either HER or HQEE extended material. In the case of poly(ether urethanes), HQEE extended elastomer had slightly higher hardness, tensile strength and tear strength values than the HER extended material. HER extension produced elastomer with a lower compression set compared to HQEE extension. DMA properties of HER and HQEE extended elastomers are similar. All the other physical and mechanical properties of the elastomers from the HER/HQEE blends were lower than the HER or HQEE extenders.

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Table 8.10 Cast elastomers based on Baytec MS-242 (polyester) prepolymer. Effect of HER-HQEE composition on elastomer properties HER/HQEE (wt. ratio)

100/0

75/25

50/50

25/75

0/100

Prepolymer temperature, (°C)

110

103

110

105

110

Extender temperature, (°C)

110

110

110

110

110

Mix ratio (prepolymer/chain extender)

100/15.0

100/15.0

100/15.1

100/15.3

100/14.7

Stoichiometry, (% theoretical)

95

95

95

95

95

Cure, (h/110 °C)

16

16

16

16

16

100% Modulus

11.13

8.65

8.91

10.47

11.53

200% Modulus

14.93

11.6

11.86

13.08

14.1

300% Modulus

18.8

15.6

15.35

17.2

16.4

Tensile strength

4293

4421

3393

4718

4122

% Elongation

699

648

507

634

726

Tear strength , Die C (kN/m)

144

126

120

135

148

Compression set, (%)

30.4

29.2

28.5

27.1

26.2

52

50

50

52

54

Tan delta at 10 Hz / 23 °C (frequency)

0.073

0.106

0.097

0.092

0.076

Tan delta at 25 °C (temperature sweep)

0.054

0.072

0.069

0.063

0.056

Tg (tan delta peak), °C

-21.9

-16.2

-15.5

-18.1

-24.1

Tg (G′′ peak), °C

-29.4

-23.8

-25.8

-25.9

-29.7

Tensile Property (MPa)

Hardness, Shore-D Dynamic Mechanical Analysis

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Table 8.11 Cast elastomers based on Baytec ME-050 (polyether) prepolymer. Effect of HER-HQEE composition on elastomer properties HER/HQEE (wt. ratio)

100/0

75:25

50:50

25:75

0:100

Prepolymer temperature, (°C)

110

105

105

105

110

Extender temperature, (°C)

110

110

110

110

110

Mix ratio (prepolymer/chain extender)

100/12.5

100/12.1

100/12.9

100/12.6

100/12.7

Stoichiometry, (% theory)

90

90

90

90

90

Cure, (h/110 °C)

16

16

16

16

16

100% Modulus

9. 3

8.3

8.8

9

10.2

200% Modulus

12.1

11.9

11.3

11.3

12

300% Modulus

15.3

15.6

14.8

13.5

13.6

Tensile strength

3412

3434

3931

3787

3732

% Elongation

605

509

528

561

632

Tear strength, Die C (kN/m)

110

98

101

109

118

Compression set, (%)

14.6

16.4

15.9

18.3

16.4

47

48

50

50

50

Tan delta at 10 Hz / 23 °C (frequency)

0.038

0.039

0.038

0.037

0.033

Tan delta at 25 °C (temperature sweep)

0.032

0.03

0.05

0.034

0.029

Tg (tan delta peak), °C

-62.9

-59.8

-57.1

-57.5

-61.4

Tg (G′′ peak), °C

-70.4

-69.5

-67

-69.3

-69.3

Tensile Property (MPa)

Harness, Shore-D Dynamic Mechanical Analysis

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8.7 High Hardness Cast Polyurethanes High hardness polyurethanes are used in applications such as rolls and rollers. When the hardness of the elastomer is increased, certain other physical and mechanical properties such as the tensile strength, tear strength, compressive strength and tensile modulus are also increased. To develop materials with the best possible properties such as high hardness, solvent, oil and abrasion resistance, MDI-terminated prepolymers with NCO contents of greater than 9% are often used. Hard polyurethane elastomers, produced from the higher NCO containing prepolymers, are generally more hydrolytically stable than the soft ones, since the hard segment is hydrophobic and is less rapidly attacked [16]. With the high NCO containing prepolymers, HER chain extended materials are expected to produce harder polyurethane elastomers due to longer chain length than the conventional 1,4-butanediol chain extender. On the other hand, the presence of a high MW diol chain extender impurity in the HER-TG materials would affect the physical and mechanical properties of the resulting polyurethanes. The main objective of this work was to investigate and study the properties and performances of hard cast polyurethane elastomers obtained from the HER chain extenders.

8.7.1 Cast Elastomers and Their Hard and Soft Segment Contents Cast elastomers were prepared using Baytec MS-090 prepolymer and HER chain extenders with 95% stoichiometry. The experimental details of the elastomer castings and the mix ratios of the prepolymer and chain extender are given in Table 8.12. Since the cast urethane elastomer properties are greatly influenced by the soft and hard segment contents and their phase separation behaviour, they are theoretically calculated from the molar ratios of the prepolymer and chain extenders and are reported in Table 8.12. Table 8.12 shows the hard segment content of the elastomer is 45.5% for HER-HP and is reduced to 38.2% for TG-275. This reduction in the hard segment content is due to the formation of 8.8% amorphous hard segment from TG-275 chain extender.

8.7.2 Hardness, Tensile, Tear, Compression Set and Rebound Properties These properties are summarised in Table 8.12.

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Table 8.12 Properties of cast elastomers HER Materials

HER-HP

TG-210

TG-225

TG-250

TG-275

Prepolymer temperature, (°C)

105

100

100

100

100

Extender temperature, (°C)

105

100

100

100

100

Mix ratio (prepolymer/chain extender)

100/20.4

100/20.2

100/21.1

100/21.8

100/24

95

95

95

95

95

16/110

16/110

16/110

16/110

16/110

Hard segment (%)

45.5

44.5

43

40.6

38.2

Soft segment (%)

54.5

54.3

54

53.5

53

0

1.2

3

5.9

8.8

100% Modulus

20.44

20.24

17.97

15.42

10.98

200% Modulus

23.32

23.66

22.3

21.26

17.02

300% Modulus

25.82

26.59

27.5

29.44

27.01

Tensile strength

26.42

26.79

29.14

32.04

33.96

% Elongation

45 4

389

403

345

355

Fracture energy (kN/m) (area under stress-strain curve)

105

81

82

73

55

Tear strength, Die C, (kN/m)

149

142

135

166

91

Compression set, (%)

28

36.4

37.9

45.4

47.8

Bashore rebound, (%)

31

31

28

26

24

Hardness, Shore-A (D)

62 D

62D

98 (56D)

97(56D)

95(48D)

Casting Conditions

Stoichiometry, (% theoretical) Cure, (h/110 °C) Cast polyurethanes (calculated)

Amorphous hard segment (%) Tensile Property (MPa)

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HER Materials for Polyurethane Applications HER-HP and TG-210 chain extenders produced elastomer hardnesses in excess of 60 Shore D, but the hardness decreased to 48 Shore D for the elastomer extended with TG275 material. By correlating the elastomer hardness with that of other physical and mechanical properties, a clear trend can be seen showing that as the hardness is decreased, the 100% tensile modulus, fracture energy and tear strength values are also decreased. On the other hand, a gradual increase in tensile strength and compression set values is observed for cast elastomers containing more amorphous hard segment contents. Polyurethane elastomers with higher hard segment content result in more hard segments mixed into the soft phase [25-27]. Also, polyester soft segments are more compatible with urethane hard segments. The decrease in Bashore rebound values of TG-225, TG250 and TG-275 extended materials is associated with the lower degrees of phase separation in the polyurethanes.

8.7.3 FT-IR Analysis of Cast Polyurethanes In order to understand the effect of high MW diols on the formation of amorphous hard segments and their interaction with the crystalline hard segments, FT-IR analysis of the cast polyurethanes was performed. Samples were cast as films on a NaCl plate from a hot dimethyl sulphoxide (DMSO) solution. The crystalline structure of polyurethanes is controlled by the formation of hydrogen bonds between NH and C=O groups of the urethane linkages [28, 29]. From the IR analysis, the hydrogen bonded C=O present in the crystalline domain can be identified by the peak absorbance at 1699-1706 cm-1 and the non-bonded or free C=O group shows peaks at 1735 cm-1 and 1748 cm-1. Figure 8.14 shows the C=O stretching vibration bands of FT-IR spectra of cast polyurethanes obtained from the HER materials. The appearance of peaks at 1695 cm-1 and 1705 cm-1 strongly suggests hydrogen bonded C=O groups exist with all the HER extended materials. The intensities of these two peaks are gradually reduced when the hard segment content of the elastomer is decreased. The complete disappearance of the 1695 cm-1 peak might be associated with the strong interaction or interference of the amorphous hard segment in the phase separation of elastomers. IR analysis also showed a peak at 1736 cm-1 associated with the free C=O groups of the elastomers. From the IR absorbances of bonded and free C=O groups, the hydrogenbonding index, ‘R,’ which is the ratio of Abonded C=O/A nonbonded C=O, can be calculated [30].

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Figure 8.14 FT-IR stretching vibration bands of HER materials extended cast polyurethanes

The greater the value of R indicates an increase in participation of the C=O group in the hydrogen bonding. The values of R calculated from the absorbance of hydrogen bonded C=O (1705 cm-1) and free C=O (1736 cm-1) peak values are given in Table 8.13. The values clearly suggest an increased hydrogen bonding with HER HP extended material due to high hard segment content. The reason for the higher R value for TG-225 extender is not known at the present time.

Table 8.13 Value of nydrogen bonding index for cast polyurethanes Chain Extender

Hydrogen Bonding Index R = A1705/A1736

HER HP

0.80

HER TG-210

0.67

HER TG-225

0.83

HER TG-250

0.60

HER TG-275

0.50

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8.7.4 Differential Scanning Calorimetric Analysis The crystalline hard segment melting, crystallisation and heats of transition observed during the DSC analysis are given in Table 8.14. From the endothermic melting temperatures, HER-HP and TG-210 extended elastomers show a thermal stability of 180 °C. Increase in hard segment content in the elastomer results in an increase in hydrogen bonding between adjacent hard segments which may result in higher transition energy. By comparing the energies of endothermic peaks observed during the first and second heating DSC experiments, HER-HP extended elastomer has the highest hard segment content. The presence of amorphous hard segment appeared to affect the crystallisation behaviour, during cooling, and also the thermal stability of the urethane elastomers.

8.7.5 Dynamic Mechanical Analysis The DMA analysis results are summarised in Table 8.15 Due to higher hard segment content, HER-HP and TG-210 extended elastomers had high storage modulus (G´) compared to the other HER chain extenders. By comparing the tan δ, Tg and loss compliance properties, it is seen that HER-HP and TG-210 extended elastomers behaved similarly.

8.8 High Thermal Stability Polyurethane with Low Heat Generation It is well known that conventional polyester-based urethane elastomers extended with butanediol can withstand continuous use temperatures of about 80 °C. At higher temperatures, a reduction in the physical and mechanical properties is seen due to degradation of the material. The thermal stability of the polyurethanes is related to the nature of the starting materials such as the aromatic diisocyanate and diol chain extender. The hard segment of the urethane elastomer is primarily responsible for temperature resistance, and the soft segment determines the material’s performance at low temperature. Butanediol chain extender has four aliphatic carbon atoms in the chain. On the other hand, HER is an aromatic diol chain extender with the following structure:

HOH2CH2CO

OCH2CH2OH

405

406

nd = Non-detected

HER-TG275

HER-TG250

HER-TG225

166 216

Minor

nd

Minor Major

171

173,163

Minor Major

188

16 4

Minor Major

193,180

Major

nd

Minor

HER-TG210

186

Major

HER-HP

1

10

20

19

25

31

J/ g

°C

Peaks

HER/HQEE Material

Energy

nd

nd

58

72

84

°C

Peak

nd

nd

9

15

18

J/ g

Energy

Exotherm

Endotherm Peak

Cooling Down

First Heating

142

148

152

175

152

181

144

180

°C

Peak

5

6

8

20

1

15

J/ g

Energy

Endotherm

81

71

57

nd

nd

°C

Peak

11

10

1. 5

nd

nd

J/g

Energy

Exotherm

Second Heating

Table 8.14 DSC results on cast polyurethanes from MS-090-HER materials

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Table 8.15 DMA results of cast elastomers based on MS-090 prepolymer HER Materials

HER-HP

TG210

TG225

TG250

TG275

at 25 °C

98.6

93.6

75.4

48.7

29.5

at 150 °C

33.7

25.8

22.2

12.9

8.5

Temperature (°C)

165

150

150

150

150

G′, (MPa)

33

26

22

13

9

-12.7

-14.4

-8.53

-2.5

5.4

Peak modulus

138

143

147

147

146

at 25 °C

10.5

10.3

10

8.9

11.9

at 150 °C

0.79

0.67

0.47

0.22

0.14

-6.4

-6.2

-0.4

7.1

14.4

Peak tan delta

0.259

0.246

0.3

0.4

0.57

at 25 °C

0.106

0.11

0.133

0.183

0.396

at 150 °C

0.023

0.026

0.021

0.017

0.016

at 10 Hz /23 °C (frequency)

0.14

0.14

0.18

0.3

0.61

Peak temperature, (°C)

29.5

29.9

29.5

19.8

24.7

Peak compliance

1070

1190

1750

3730

11700

at 25 °C

1060

1160

1730

3630

11700

at 150 °C

700

1000

960

1340

1900

Storage Modulus, G′ (MPa)

Thermal Stability(Temperature Maximum for Constant G′)

Loss modulus, G′′ (MPa) Peak temperature, Tg (°C)

Tan Delta Peak temperature, Tg (°C)

Loss compliance, J′′ (MPa)

Based on the structures of butanediol and HER, it can be expected that the HER-based hard segment would have a longer chain length than the butanediol-containing hard segment. The domains containing these longer hard segments are also expected to show a higher melting temperature than the shorter ones. Therefore, HER based elastomers are expected to be more thermally stable than the butanediol extended material.

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Advances in Urethane Science and Technology In order to demonstrate that HER compared to butanediol-based elastomer has better physical and mechanical properties at elevated temperatures, cast elastomers with 95% stoichiometry have been made from an MDI terminated polyester prepolymer (Baytec MS-242) extended with butanediol and HER-HP (from INDSPEC Chemical Corporation). Test specimens were cut from a sheet post cured at 110 °C for 16 hours, for the tensile, tear, DSC and DMA determinations. In the preparation of material for the dry heat aging evaluation, tensile, DSC and DMA samples were placed in an air oven with the temperature controlled within ± 2 °C of the set point and aged for 28 days at 100 °C, 21 days at 120 °C and 14 days at 135 °C. Then, all the samples were stored for at least seven days at room temperature and 50% relative humidity before testing.

8.8.1 Hardness Measurements The Durometer hardness measured on the elastomers before and after the heat aging is given in Table 8.16. Since hardness is the measure of hard segment content of the elastomer, the unaged HER elastomer showed a 96A (Shore) hardness compared to 88A for the butanediol elastomer clearly suggesting a higher hard segment content with HER extension. On heat ageing, the HER elastomer maintained its hardness even after 21 and 14 days of aging at 120 °C and 135 °C, respectively. The butanediol extended elastomer’s hardness was reduced by 4 Shore A units at all heat-ageing temperatures suggesting material deterioration during the heat treatment.

8.8.2 Tensile Measurements Polyurethane elastomers are known for their high elongation, tensile strength and modulus properties. The combination of these properties provides toughness and durability in fabricated parts. Cast elastomers extended with butanediol can maintain these tensile properties when the use temperature is about 80 °C. When these elastomers are subjected to higher temperatures, reduction in the tensile properties are observed due to the weakening of physical bonds in the elastomer. The tensile properties measured on the unaged and heat-aged elastomer materials made from HER and butanediol are given in Table 8.16. The 100, 200 and 300% tensile modulus values of unaged elastomers are higher for HER than for the butanediol elastomer. Analysing the 100, 200 and 300% modulus values of 100 °C heat-aged samples showed that HER elastomer retained about 95-97% of its original unaged values compared to 75-76% for butanediol. At 120 °C, there was 93-94% modulus retention for HER whilst 85-88% was observed for butanediol. When

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14.53 (100) 18.46 (100) 28.19 (100) 619 110 142

200% Modulus

300% Modulus

Tensile strength

% Elongation

Energy to break (kN/m) (area under stress-strain curve)

Tear strength, Die C (kN/m) nd

124

691

27.28 (97)

17.35 (94)

13.75 (95)

10.38 (95)

94

28

100

nd

111

635

26.15 (93)

17.17 (93)

13.66 (94)

10.35 (94)

96

21

120

nd

104

646

21.95 (78)

15.97 (88)

12.84 (88)

9.86 (90)

96

14

135

105

92

534

46.72 (100)

15.04 (100)

9.51 (100)

6.58 (100)

88

None

RT

nd

100

642

38.80 (83)

11.40 (76)

7.35 (77)

4.93 (75)

84

28

100

nd

98

600

42.33 (90)

13.24 (88)

8.04 (85)

5.34 (81)

84

21

120

1,4-Butanediol

Note: The values in the parenthesis are the percentages of properties retained after the heat ageing. nd: Not determined

10.96 (100)

96

None

RT

100% Modulus

Tensile property (MPa)

Hardness, Shore-A

Days at ageing

Temperature (°C)

Heat Ageing Conditions

HER-HP

Table 8.16 Heat ageing and mechanical properties of cast elastomers from Baytec MS-242

nd

91

619

35.61 (76)

11.38 (76)

7.37 (77)

5.0 (76)

84

14

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Advances in Urethane Science and Technology the aging temperature is increased to 135 °C, HER elastomer showed better modulus retention property than the butanediol (87% versus 77%). Although the butadiene elastomer has an unaged tensile strength of 46.72 MPa, this value drops to 38.80 MPa (83% retention) when this material is heated for 28 days at 100 °C. On the other hand, the HER extended elastomer retains 97% of its original (unaged) tensile strength under the same temperature heat-aging conditions, indicating its excellent high temperature stability. In spite of the severe aging conditions, HER extended elastomer maintained its excellent (greater than 600%), tensile elongation property. The combination of higher modulus and good elongation of the HER-based elastomer clearly demonstrates its ability to withstand higher tensile loads without the risk of failure. Tensile measurements can also be used to determine the fracture energy of the material. The area under the stress-strain curve is the measure of fracture energy which is recognised as a measure of toughness. As can be seen in Table 8.16, HER extended elastomer showed higher fracture energy before and after heat aging suggesting that it has a higher toughness than the butanediol elastomer. Tear strength is a property commonly used to determine the cut growth propagation in an elastomeric material. The unaged tear strength is much higher for HER than the butanediol elastomer. The combination of fracture energy and tear strength properties clearly predicts that the HER elastomer will have better wear resistance than the butanediol extended elastomer.

8.8.3 Differential Scanning Calorimetric Analysis From the DSC analysis, the thermal stability of polyurethanes can be determined by the endothermic transition temperatures associated with the melting of hard segment domains. The results are summarised in Table 8.17 and also shown in Figure 8.15. The first heating DSC endothermic peaks for HER extended elastomer are broader and appeared at higher temperatures than the butanediol elastomer. Higher endothermic peak energy suggests that larger size crystalline hard segment domains exist within the HER elastomer. Considering the molecular structures of HER and butanediol chain extenders, there is no doubt that the higher thermal stability of the HER elastomer is associated with the formation of large size hard segments from the aromatic isocyanate and extender reaction. On cooling the heated samples, exothermic peaks, due to the crystallisation of hard segment domains, were seen in the DSC curves. Again, the higher exothermic peak temperature suggested the existence of larger size hard segments for HER elastomer.

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HER Materials for Polyurethane Applications

Table 8.17 DSC results of heat aged cast elastomers from Baytec MS-242 prepolymer First Heating

Cooling Down

Second Heating

Endotherm

Exotherm

Endotherm

Peak

Energy

Peak

Energy

Peak

Energy

°C

J/g

°C

J/g

°C

J/g

173

10

70

5

160

4

220

1

100

176

11

70

5

16 8

2

120

175

9

72

2

16 6

3

135

180

7

ND

ND

217

<0.5

17 0

2

RT

176

17

204

<0.5

100

179, 196

24

215

<0.5

178, 193

20

Chain Extender: 1,4 Butanediol Ageing Temperature (°C) RT

Chain Extender: HER-HP Ageing Temperature (°C)

120 135

179, 195

21

76

12

179

11

82

12

187

11

21 5

<0.5

187

11

219

<0.5

186

11

81 80

11 8

DSC Experiment: 1. Heat the sample from 25 to 225 °C at 10 °C/min (first heating) 2. Cool from 225 to 25 °C at 10 °C/min (cooling down) 3. Heat the sample from 25 to 225 °C at 10 °C/min (second heating)

During the second heating, DSC curves showed that a well-defined endothermic melting peak appeared at higher temperature confirming the higher HER elastomer thermal stability. Based on the DSC results, HER extension produced about 20 °C higher hard segment melting temperature indicating the superior high temperature properties of HER compared to butanediol elastomer.

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Figure 8.15 DSC curves of 135 °C heat aged urethane elastomers from Baytec MS-242

8.8.4 Dynamic Mechanical Analysis DMA measures the ability of the material to store and dissipate mechanical energy. DMA properties of the elastomeric materials are important because they often correlate to actual field performance. This method determines the storage modulus (G´, elastic behaviour), loss modulus (G´´, energy dissipation), tan δ, loss compliance (J´´) and Tg values. In this work, the DMA method was effectively utilised to understand the advantages of HER over the butanediol extended elastomer. The results are summarised in Table 8.18.

8.8.4.1 Storage Modulus Property (G´) Storage modulus quantitatively measures the material’s elastic properties and also qualitatively determines the elastomer’s stiffness and hardness. The unaged elastomer storage modulus values are higher for HER at 25, 100 and 150 °C temperatures compared to butanediol. On heat ageing at 100 °C, the HER elastomer retained about 85-86% of its original G´ value compared to 60-85% for butanediol. After ageing for 28 days at

412

21.2 14.4

at 100 °C

at 150 °C

171

Peak modulus

0.392 0.072 0.019 0.029

Peak tan delta

at 25 °C

at 100 °C

at 150 °C

2160 900 2030

at 25 °C

at 100 °C

at 150 °C

Peak area: -50 —> 150 °C 28

22

2580

Peak compliance

Peak area: 0 —> 150 °C

-11.6

Peak temperature, (°C)

Loss compliance, J′′ (MPa)

-23.9

Peak temperature, Tg (°C )

Tan Delta

-29.8

Peak temperature, Tg (°C)

Loss Modulus, G′′ (MPa)

33.1

RT

at 25 °C

Storage Modulus, G′ (MPa)

Ageing Temperature (°C)

41

34

4210

1780

2770

3000

-12.1

0.052

0.032

0.08

0.404

-23.7

168

-29.6

12.4

18

28.9

100

41

35

4040

1850

2750

2810

-9.4

0.051

0.033

0.083

0.401

-21.8

167

-30

12.7

18

30

120

HER-HP

73

67

8940

5000

3090

2600

-13

0.08

0.064

0.091

0.386

-23.6

165

-29.6

8.9

12.8

29.4

135

82

57

4450

3350

4720

1460

-9.3

0.027

0.024

0.051

0.657

-19.3

167

-27.4

6.1

7.2

10.9

RT

216

190

20000

1580

9220

1640

-6

0.075

0.081

0.087

0.628

-16

117

-25.6

3.7

5.1

9.3

100

185

160

1690

1190

8210

1530

-7.7

0.066

0.064

0.08

0.65

-17.8

164

-28

3.9

5.3

9.7

120

1,4-Butanediol

Table 8.18 DMA of heat aged cast polyurethane from Baytec MS-242 prepolymer

316

292

32000

25100

11200

15100

-3.6

0.091

0.097

0.103

0.623

-17.7

159

-26

2.8

3.9

9.1

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Advances in Urethane Science and Technology 100 °C temperature, a very sharp decline in G´ values (60%) for butanediol and a high retention for HER were observed at 150 °C. For 120 °C heat-aged samples, 85-90% retention of G´ values for HER compared to 6389% for butanediol were seen. The same kind of trend is seen with 135 °C heat-aged elastomer storage modulus values. Thus the HER extended elastomer has better retention of storage modulus at higher temperatures than the butanediol elastomer.

8.8.4.2 Loss Modulus Property (G´´) Loss modulus measures the energy dissipation in the elastomer. The Tg measured from the loss modulus curves (Table 8.18) showed lower values for the HER than butanediol elastomer suggesting good phase separation.

8.8.4.3 Tan Delta Property (tan δ) The soft segment Tg is often defined as the maximum in the tan δ versus temperature curve which determines the low temperature behavior of urethane elastomers. The results obtained are summarised in Table 8.18. The Tg obtained from the tan δ curves confirms again that a good phase separation existed with HER elastomer. As a consequence, the HER-based polyurethane has a higher flexibility at low temperature. Although the unaged 25 °C tan δ value appeared to be higher for HER, the effect of heat ageing showed an increase of 26% compared to 101% for BD. Similarly, the 100 °C and 135 °C heat-aged samples showed very high percentages of elevated temperature (100 °C and 150 °C) tan δ value increases in butanediol-based elastomer as compared to the HER material. These experimentally determined tan δ values from the DMA analysis strongly suggest that more heat generation problems can be anticipated with butanediol-based elastomer when they are exposed to temperatures above 80 °C than with HER elastomer.

8.8.4.4 Loss Compliance Property (J´´) Figures 8.16 and 8.17 show the plots of loss compliance for butanediol and HER elastomers, respectively and the results are summarised in Table 8.18. The loss compliance values of unaged and heat-aged butanediol extended elastomer are about 3-5 times greater than the HER elastomer.

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HER Materials for Polyurethane Applications

Figure 8.16 Heat ageing study - loss compliance of MS-242-butanediol elastomer

Figure 8.17 Heat ageing study - loss compliance of MS-242-HER elastomer

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Advances in Urethane Science and Technology In addition to the loss compliance data at various temperatures, the area under the loss compliance curve can also be used to predict the tendency of the elastomeric materials to convert mechanical energy to heat. A smaller area means less mechanical energy is converted to heat, indicating better dynamic performance of the elastomer over the temperature range of interest. By comparing the loss compliance peak areas of heat-aged samples, there is no doubt the HER extended elastomer is far superior in performance to butadiene elastomer. Based on the current study, HER extended elastomers has the following room temperature and high temperature physical and mechanical properties advantages over butanediolbased elastomers: •

Excellent retention of hardness.

•

Retention of high tensile modulus and elongation.

•

Higher fracture energy.

•

High tear strength.

•

Higher thermal stability due to high crystalline hard segment melting.

•

High flexibility due to low Tg.

•

High storage modulus.

•

Lower hysteresis and compliance values.

•

Broader service temperature.

8.9 Conclusions The HER materials developed for polyurethane and other applications have a wide range of melting characteristics. Lower melting temperatures of these materials can provide improved processability in the development of cast and thermoplastic polyurethane elastomers. The high MW diols present in the HER materials not only reduce the melting point of the HER extender, but also exhibit a profound influence on the concentration of crystalline hard segments and, as a result, ultimately affect the physical and mechanical properties of the elastomers. Polyurethanes with varying properties can be obtained by using HER materials containing different concentrations of high MW diols. Since the high MW diols can act as plasticisers, the addition of external plasticisers can be avoided by use of these materials. Cast polyurethanes show good tensile, tear, compression set and hysteresis properties with HER-based chain extenders. This results in a wide choice in the selection of a suitable aromatic diol chain extender from the HER materials family.

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Acknowledgements The author would like to thank Jason Burchianti and Stephen J. Ondrey for preparing elastomer samples and for conducting pot life measurements, Richard F. George for the DMA, physical properties and DSC work and Vaughn J. Romell for the FT-IR analysis work. My special thanks to Fred M. Covelli, V.P. Technology, and Joseph de Almeida, Marketing Manager of Specialty Products, for their continuous encouragement in this work.

References 1.

K. C. Frisch and S. L. Reegen in Advances in Urethane Science and Technology, Volume 2, Eds., K. C. Frisch and D. Klempner, Technomic Publishing Co., Inc., Lancaster, PA, 1973, 29.

2.

D. Klempner and K. C. Frisch in Advances in Urethane Science and Technology, Volume 8, Eds., K. C. Frisch and D. Klempner, Technomic Publishing Co., Inc., Lancaster, PA, 1981, 93.

3.

J. E. Tiedemann, Presented at the Fall PMA Meeting, Miami, FL, USA, 1993.

4.

M. A. Mendelsohn, F. W. Navish and D. Kim, Rubber Chemistry and Technology, 1986, 58, 5, 997.

5.

R. B. Durairaj, J. Burchianti, S. J. Ondrey, R. F. George and J. de Almeida, Paper Presented at the Fall PMA Meeting, Baltimore, MD, USA, 1998.

6.

ASTM D2240-00 Standard Test Method for Rubber Property-Durometer Hardness.

7.

ASTM D412-98a Standard Test Methods for Vulcanized Rubber and Thermoplastic Rubbers and Thermoplastic Elastomers-Tension.

8.

ASTM D624-00e1 Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers.

9.

ASTM D2632-96 Standard Test Method for Rubber Property-Resilience by Vertical Rebound.

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Advances in Urethane Science and Technology 10. ASTM D395-98 Standard Test Methods for Rubber Property-Compression Set. 11. Polyurethane Handbook, 2nd Edition, Ed., G. Oertel, Hanser Publications, Munich, 1994. 12. R. Palinkas, Paper Presented at the Fall PMA Meeting, Cincinnati, OH, USA, 1990. 13. J. E. Doyle, Paper Presented at the PMA Meeting, Milwaukee, WI, USA, 1994. 14. S. M. Clift, Paper Presented at the Fall PMA Meeting, Charlotte, NC, USA, 1991. 15. J. R. Lin and L. W. Chen, Journal of Applied Science, 1998, 69, 8, 1575. 16. C. Hepburn, Polyurethane Elastomers, 2nd Edition, Elsevier Applied Science, London, 1992. 17. S. Madan, S. Franyutti, K. Recker and R. S. Pantone, Presented at the Polyurethanes World Congress ‘97, Amsterdam, The Netherlands, 1997, 381. 18. R. W. Seymour and S. L. Cooper, Journal of Polymer Science, Part B, Polymer Letters, 1971, 9, 7, 689. 19. T. R. Hesketh, J. W. C. van Bogart and S. L. Cooper, Polymer Engineering and Science, 1980, 20, 3, 190. 20. L. M. Leung and J. T. Koberstein, Macromolecules, 1986, 19, 3, 706. 21. G. Pompe, A. Pohlers, P. Potschke and J. Pionteck, Polymer, 1998, 39, 21, 5147. 22. Eastman HQEE-80, As a Chain Extender for Liquid Castable Urethane Elastomers, Industrial Chemical Division, Eastman Chemical Products, Inc., Kingsport, TN-37662, USA, Publication No. D-161, December 1982. 23. M. Palmer, Paper Presented at the Spring PMA Meeting, Kansas City, KY, 1983. 24. M. Palmer, J. H. Davis, T. L. Douglas and M. R. Wilhelm, Paper Presented at the Fall PMA Meeting, Chicago, IL, USA, 1977. 25. J. W. C. van Bogart, A. Lilaonitkul, L. E. Lerner and S. L. Cooper, Journal of Macromolecular Science B, 1980, 17, 2, 267. 26. J. W. C. van Bogart, P. E. Gibson and S. L. Cooper, Journal of Polymer Science: Polymer Physics, 1983, 21, 1, 65.

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HER Materials for Polyurethane Applications 27. C. B. Hu, R. S. Ward, Jr., and N. S. Schneider, Journal of Applied Polymer Science, 1982, 27, 6, 2167. 28. R. Bonart, L. Morbitzer and E. H. Muller, Journal of Macromolecular Science, 1974, B9, 3, 447. 29. J. Blackwell, J. R. Quay, M. R. Nagarajan, L. Born and H. Hespe, Journal of Polymer Science: Polymer Physics, 1984, 22, 7, 1247. 30. R. W. Seymour, G. M. Estes and S. L. Cooper, Macromolecules, 1970, 3, 5, 579.

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9

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes Stephen D. Seneker, Nigel Barksby and Bruce D. Lawrey

9.1 Introduction Polyoxypropylene glycols (PPG) have been available since the early 1960s. Polyurethanes based on these polyols are used in a diverse range of applications including flexible foams (moulded and slabstock), sealants and adhesives. Until recently, their use in high-performance applications such as cast and thermoplastic urethane elastomers has been limited due to their inferior physical properties and processing characteristics. These problems can be largely attributed to the considerable amount of monol (monofunctional hydroxyl-containing species) present in these polyols. Monol acts as a chain terminator, which severely reduces both the molecular weight (MW) build during polymer formation (processability) and the ultimate polymer MW (physical properties). Conventional polyoxypropylene diols are produced commercially through the basecatalysed (KOH) propoxylation of glycol starters such as propylene glycol. However, the base catalyses not only the addition of propylene oxide to the growing polyol molecule, but also a side reaction in which propylene oxide isomerises to allyl alcohol [1, 2, 3]. Allyl alcohol acts as a monofunctional starter resulting in propoxylated allyl alcohol (‘monol’) as shown in Figure 9.1. Since each monol chain contains a terminal double bond, the amount can be quantified by measuring the unsaturation level. The level of unsaturation is typically reported in milliequivalents of unsaturation per gram of polyol (meq/g). Table 9.1 shows the relationship between conventional polyoxypropylene diol MW and unsaturation level, functionality, and monol content (mole%). As the MW of conventional PPG increases, there is a dramatic increase in the monol content. For example, a PPG with a MW of 2000 typically has an unsaturation or monol level of about 0.03 meq/g, corresponding to a functionality of 1.94, while a diol with a MW of 4000 has a monol content of 0.09 meq/g and a functionality of only 1.69. A conventional diol with a MW of 4000 is considered to be a mixture of about 70% diol and 30% monol (molar basis). This is why the practical upper limit for commercial PPG is a 2000 equivalent weight (4000-MW diol or 6000-MW triol).

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Figure 9.1 Reaction of propylene oxide to form polyoxypropylene diol and monol

Table 9.1 Monol (unsaturation), functionality and mole % monol of conventional PPG polyols PPG Diol

OH#

Unsat. (meq/g)

Functionality

Mole % Monol

1000 MW

111

0.01

1.99

1

2000 MW

56

0.03

1.94

6

3000 MW

37

0.05

1.86

14

4000 MW

28

0.09

1.69

31

8000 MW*

14

—*

—*

—*

*Cannot produce an 8000-MW PPG with conventional KOH catalyst using commercially feasible reaction conditions. OH# is the hydroxyl number, which is a measure of the concentration of hydroxyl groups. It is related to the polyol equivalent weight by the equation: 56,100/OH# = equivalent weight.

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes Advances in catalyst technology demonstrated that PPG could be prepared with significantly lower monol contents using double metal cyanide (DMC) catalysts such as zinc hexacyanocobaltate [4, 5, 6, 7, 8]. This technology was originally developed by General Tire Inc., [9, 10, 11]. These DMC catalysts produce PPG with monol contents typically in the range of 0.015 to 0.020 meq/g, which corresponds to a diol with a MW of 4000 with a functionality of 1.94. The synthesis and utility of these low monol PPG polyols in polyurethanes have been reported extensively [6, 7, 8, 12, 13, 14]. Although the physical properties and processability of polyurethanes improved significantly with low monol PPG polyols, the cost to performance ratio was still considered unacceptable; therefore, these polyols achieved limited commercial success. Further improvements in catalyst technology resulted in Lyondell (formerly ARCO Chemical Company) commercialising a new family of PPG with ultra-low monol contents designated Acclaim Polyols. Table 9.2 summarises the unsaturation, functionality, and monol contents of these polyols. Comparing Tables 9.1 and 9.2, one can clearly see the dramatic improvements in functionality. For example, a 4000-MW, ultra-low monol PPG (Acclaim Polyol 4200) has a functionality of 1.98 versus 1.69 for a conventional 4000-MW PPG diol. In this chapter, the dramatic effect that monol content or polyol functionality has on processability and properties of polyurethane cast elastomers is discussed. This effect is shown for elastomers prepared by both the prepolymer and one-shot processes. Further improvements in elastomer processability and formulating latitude can be achieved by incorporating oxyethylene moieties into the polyol backbone. For one-shot elastomer processes, ultra-low monol PPG polyols capped with ethylene oxide have been commercialised.

Table 9.2 Monol (unsaturation), functionality and mole % monol of Acclaim polyols (ultra-low monol PPG polyols) Polyol (MW)

OH#

Unsat. (meq/g)

Functionality

Mole % Monol

1000-MW PPG*

112

0.005

1.995

0.5

2000

56

0.005

1.99

1

3000**

35

0.004

1.99

1

4000

28

0.005

1.98

2

8000

14

0.005

1.96

4

* Experimental sample ** Polyoxypropylene-oxyethylene diol containing 20 weight percent oxyethylene moieties. OH#: hydroxyl number

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Advances in Urethane Science and Technology This chapter also focuses on the effect of the polyol molecular weight distribution (MWD) or polydispersity on mechanical and dynamic properties of polyurethanes and polyurethane/ureas. Ultra-low monol PPG polyols are unique in that they have a very narrow MWD in comparison to other high-performance polyols such as polytetramethylene ether glycol (PTMEG) or polyester polyols. Narrow polydispersity results in lower viscosity polyols as well as lower viscosity isocyanate-terminated prepolymers based on those polyols. However, a not so obvious effect is that polyols with a narrow MWD result in polyurethanes with significantly different mechanical and dynamic property profiles. These differences will be highlighted in various polyurethane and polyurethane/urea systems and explained using dynamic mechanical thermal analysis (DMTA). By understanding the effect of polyol MWD, we will illustrate how the polydispersity can be adjusted in order to maximise the mechanical and dynamic properties obtained from ultra-low monol PPG polyols. The experimental procedures utilised to prepare the various polyurethane and polyurethane/urea polymers described in this chapter are discussed in the Appendix.

9.2 MDI/BDO Cured Elastomers Based on Ultra-Low Monol PPG Polyols 9.2.1 Effect of Monol Content on 4,4´-Methylene Diphenylmethane Diisocyanate (MDI)/1,4-Butanediol (BDO) Cured Elastomers The importance of monol content in 4,4´-MDI/BDO cured elastomer systems was determined by comparing 4000-MW PPG diols prepared via ultra-low monol technology, DMC and potassium hydroxide. They are designated as ultra-low monol, low monol and conventional and have monol contents of 0.005, 0.016 and 0.085 meq/g, respectively. This corresponds to functionalities of 1.98, 1.94 and 1.71. We prepared 6% NCO 4,4´-MDI prepolymers and chain extended with BDO at an isocyanate to hydroxyl ratio (NCO:OH) of 1.03 [15]. Table 9.3 summarises the monol effect on elastomer processing characteristics (pot life and demould time), and physical properties. The monol content has a major effect on elastomer processability. Lower monol content results in a faster molecular weight build, which reduces demould time. Elastomers based on the 4000-MW, ultra-low monol PPG had demould times as short as 20 minutes, whereas, the low monol PPG gave demould times of over one hour. The elastomer based on conventional polyol had a demould time greater than three hours. Lower monol content clearly has a positive effect on all elastomer physical properties. Dramatic property improvements are even seen when comparing elastomers based on ultra-low monol and low monol polyol. This is particularly surprising considering the

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Table 9.3 Comparison of polyurethane elastomers based on ultra-low monol, low monol and conventional 4000-MW diols (6% NCO MDI prepolymers; BDO cured) Monol Content (meq/g)

0.005

0.016

0.085

Polyol Functionality

1.98

1.94

1.71

20 °C

8.5

7.5

8.1

40 °C

2.2

1.9

2.0

60 °C

0.78

0.69

0.73

80 °C

0.38

0.32

0.38

1-3

1-3

1-3

20-45

60-120

>180

72A

68A (66A)*

51A (38A)*

Pendulum Rebound, %

70

62

48

Ultimate Elongation, %

910

1020

130

Tensile Strength, MPa

20.6

12.4

1.4

100% Modulus, MPa

3.4

3.0

1.6

200% Modulus, MPa

4.9

4.3

—

300% Modulus, MPa

6.3

5.5

—

400% Modulus, MPa

7.8

6.7

—

Die C Tear, kN/m

67

49

10

Compression Set, % (22 h @ 70 °C)

25

40

53

5%

0.8

0.4

0.1

10%

1.2

0.8

0.2

15%

1.7

1.2

0.3

25%

2.8

1.9

0.8

Prepolymer Viscosity, Pa-s

PROCESSING CHARACTERISTICS Pot life (minutes) Demould Time (minutes) ELASTOMER PROPERTIES Hardness, Shore A

Compression Deflection, MPa

*

Shore Hardness needle slowly penetrated into the sample to the value indicated in parenthesis

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Advances in Urethane Science and Technology small difference in functionality (1.98 versus 1.94). For example, Shore A hardness increased from 68 to 72; rebound from 62 to 70%; tensile strength from 12 to 21 MPa; Die C tear strength from 49 to 67 kN/m; and compression set decreased from 40 to 25%. The load bearing capabilities improved significantly as shown by the compression deflection results. The stress/strain curves in Figure 9.2 clearly show that the elastomer based on ultra-low monol PPG exhibits much greater modulus build with increasing elongation.

Figure 9.2 Elastomer stress/strain curves (6% NCO MDI/4000-MW diol prepolymers; BDO cured)

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes Reducing the monol content also improves the dynamic properties of cast elastomers. Figure 9.3 shows the DMTA response for the two polymers described above. The polymer derived from the ultra-low monol polyol has a flatter rubbery plateau region. The higher modulus in the rubbery plateau is consistent with the polymer’s higher tensile modulus. The substantial reduction in tan delta (δ) across the entire temperature range should also be noted. Lower tan delta translates into improved performance in dynamic applications due to lower heat buildup and improved rebound as noted above.

Figure 9.3 Elastomer DMTA curves (6% NCO MDI/4000-MW diol prepolymers; BDO cured)

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Advances in Urethane Science and Technology All these improvements indicate that significant changes are occurring in the elastomer as the monol content is reduced and the polyol functionality increased from 1.94 to 1.98. We believe the benefits are directly attributable to an increase in the polymer molecular weight. Figure 9.4 shows the relationship between functionality and theoretical elastomer molecular weight (calculated from the Carothers equation). Raising the functionality from 1.94 to 1.98 increases the ultimate polymer molecular weight by a factor of three from 82,000 to 270,000. This increase in MW could easily account for the observed improvements in processability and properties.

Figure 9.4 Effect of polyol functionality on theoretical elastomer molecular weight

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

9.2.2 Processability and Property Latitude of Elastomers Based on Ultra-Low Monol PPG Polyols Ultra-low monol PPG diols are commercially available in molecular weights from 2000 to 12,000. A product summary is given in Table 9.4. To determine the formulating range of the various diols in MDI/BDO cured elastomers, we prepared MDI prepolymers based on the 2000, 4000 and 8000 MW diols at a wide range of isocyanate contents. These prepolymers were chain extended with BDO to produce elastomers with a broad range of properties. Tables 9.5 to 9.7 summarise the prepolymer compositions and viscosities, processing characteristics and physical properties of these elastomers.

Table 9.4 Acclaim polyol product line Acclaim Polyol

Molecular Weight

Composition

Functionality

Hydroxyl Type

2200

2000

PO Diol

1.99

Secondary

3205

3200

EO/PO Diol

1.99

Secondary

4200

4000

PO Diol

1.98

Secondary

8200

8000

PO Diol

1.94

Secondary

12200

12000

PO Diol

1.92

Secondary

6300

6000

PO Triol

2.97

Secondary

2220

2250

EO-Capped Diol

1.99

Primary (>85%)

4220

4000

EO-Capped Diol

1.97

Primary (>85%)

6320

6000

EO-Capped Triol

2.94

Primary (>85%)

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Advances in Urethane Science and Technology

Table 9.5 Polyurethane elastomers based on 2000-MW ultra-low monol PPG (MDI prepolymers; BDO cured) PREPOLYMER COMPOSITION % NCO Prepolymer

4.0

6.0

8. 0

10.0

12.0

NCO/OH

2.2

3.0

3.8

4.8

6.0

4,4´-MDI (pbw)

27.7

36.9

47.7

60.2

75.0

2000-MW PPG (pbw)

100

100

10 0

100

10 0

25

15

7.5

4. 0

3.3

Prepolymer Viscosity, Pa-s 20 °C 40 °C

4.6

2.7

1.45

0.93

0.58

60 °C

1.45

0.85

0.49

0.33

0.22

0.74

0.40

0.25

0.15

0.1

80 °C PROCESSING CHARACTERISTICS

Prepolymer @ 65 °C; BDO @ RT; NCO/OH = 1.03; dibutyltin dilaurate catalyst; Moulds @ 100 °C; cured 16 hours @ 100 °C

Prepolymer (pbw)

100

100

10 0

100

10 0

1,4-butanediol, BDO (pbw)

4.2

6. 2

8.3

10.4

12.5

Pot life (minutes) Demould Time (minutes) ELASTOMER PROPERTIES Hardness, Shore A

1-3

1-3

1-3

1- 3

1- 3

20-45

20-45

20-45

30-60

60-12 0

Samples conditioned 4 weeks @ 23 °C/50% relative humidity 57A

68A

80 A

89A

97A (45D)

Pendulum Rebound, %

60

61

61

56

54

Ultimate Elongation, %

810

710

70 0

640

58 0

Tensile Strength, MPa

15.9

22.1

26.2

25.5

23.4

1.4

3.5

5. 7

8.5

11.7

100% Modulus, MPa 200% Modulus, MPa

1.8

5.3

8.1

10.9

13.7

300% Modulus, MPa

2.5

7.4

10.3

13.0

15.4

400% Modulus, MPa

3.5

9.9

12.8

15.4

17.5

Die C Tear, kN/m

26

58

74

91

10 7

Compression Set, % (22 h @ 70 °C)

29

22

18

16

20

5%

0.3

0.5

0. 9

1.4

2.1

10%

0.6

1.0

1.9

2. 8

4.1

Compression Deflection, MPa

15%

0.8

1.6

2. 6

4.1

6.1

25%

1.5

2.8

4.7

6. 8

10.4

pbw: parts by weight; RT: room temperature

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Table 9.6 Polyurethane elastomers based on 4000-MW, ultra-low monol PPG (MDI prepolymers; BDO cured) PREPOLYMER COMPOSITION % NCO Prepolymer

4.0

6. 0

8.0

10.0

NCO/OH

3.3

4. 7

6.3

8.2

4,4´-MDI (pbw)

20.6

29.4

39.5

51.3

4000-MW PPG (pbw)

100

100

10 0

100

16

8.5

4.5

3.1

Prepolymer Viscosity, Pa-s 20 °C 40 °C

3.8

2. 2

1.25

0.89

60 °C

1.32

0.78

0. 5

0.36

0.63

0.38

0.25

0.18

80 °C PROCESSING CHARACTERISTICS Prepolymer (pbw)

Prepolymer @ 65 °C; BDO @ RT; NCO/OH = 1.03; dibutyltin dilaurate catalyst; Moulds @ 100 °C; Cured 16 hours @ 100 °C 100

100

100

100

1,4-butanediol, BDO (pbw)

4.2

6.3

8. 3

10.4

Pot life (minutes)

1-3

1- 3

1-3

1-3

20-45

20-45

30-60

60-120

Demould Time (minutes) ELASTOMER PROPERTIES Hardness, Shore A

Samples conditioned for 4 weeks @ 23 °C/50% humidity 60A

72A

82 A

91A

Pendulum Rebound, %

70

70

65

60

Ultimate Elongation, %

890

910

970

690

Tensile Strength, MPa

15.9

20.7

17.2

15.2

100% Modulus, MPa

1.9

3.4

4. 5

6.6

200% Modulus, MPa

2.9

4.9

6. 2

8.7

300% Modulus, MPa

4.0

6.3

7. 6

10.1

400% Modulus, MPa

5.3

7. 8

8.9

11.3

Die C Tear, kN/m

47

67

75

88

Split Tear, kN/m

4.7

8.8

9.6

14.2

Compression Set, % (22 h @ 70 °C)

19

25

19

21

5%

0.3

0.8

1.0

1.5

10%

0.6

1.2

1. 8

3.0

15%

0.9

1.7

2.5

4.3

25%

1.7

2.8

4. 0

7.3

Compression Deflection, MPa

pbw: parts by weight; RT: room temperature

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Table 9.7 Elastomers based on 8000-MW ultra-low monol PPG: Effect of 6000MW ultra-low monol triol addition (MDI prepolymers; BDO cured) PREPOLYMER COMPOSITION % NCO Prepolymer NCO/OH 4,4´-MDI (pbw) 6000-MW PPG triol (pbw) 8000-MW PPG diol (pbw) Prepolymer Viscosity, Pa-s 20 °C 40 °C 60 °C 80 °C PROCESSING CHARACTERISTICS Prepolymer (pbw) 1,4-butanediol, BDO (pbw) Pot life (minutes) Demould Time (minutes) ELASTOMER PROPERTIES Hardness, Shore A Pendulum Rebound, % Ultimate Elongation, % Tensile Strength, MPa

2. 0

3.0

4.0

2.9 9.9 2.6 97.4

3.1 9.6 — 100

4.0 13.5 2.6 97.4

4.2 13.2 — 100

5. 1 17.4 2.6 97.4

5.5 17.1 — 100

72

50

40

35

24

22

5. 9 18 12 10 9 6.4 6.7 4.5 3.7 3. 3 2.4 2.3 3.2 1.7 1.6 1. 5 1.1 0.83 Prepolymer @ 65 °C; BDO @ RT; NCO/OH = 1.03; dibutyltin dilaurate catalyst; Moulds @ 100 °C; Cured 16 hours @ 100 °C 100 100 10 0 100 100 100 2.1 2.1 3.1 3.1 4.2 4.2 1-3 1-3 1- 3 1-3 1-3 1-3 30-60 >180 20-45 20-45 20-45 20-45 Samples conditioned for 4 weeks @ 23 °C/50% humidity 34A 27A 49A 45A 58A 56A (32A)* (16A)* (41A)* (53A)* 59 55 68 65 71 69 1030 >1300** 1130 1190 920 960 4.0 2.9 12.7 10.1 13.2 11.8

100% Modulus, MPa

0.5

0.5

1. 2

1.2

1.7

1.6

200% Modulus, MPa 300% Modulus, MPa 400% Modulus, MPa

0.7 0.8 1.0

0.5 0.6 0.7

1.8 2.4 3. 1

1.7 2.2 2.9

2.5 3.4 4.4

2.3 3.1 4.0

16

13

42

42

49

47

34

53

22

40

19

21

0.1

0.1

0. 2

0.2

0.3

0.3

0.3

0.3

0.4

0. 4

0 .6

0.6

Die C Tear, kN/m Compression Set, % (22 h @ 70 °C) Compression Deflection, MPa 5% 10%

15% 0.4 0.4 0.6 0. 6 1.0 0.8 25% 0.7 0.7 1.0 1. 0 1.7 1.5 * Shore Hardness needle slowly penetrated into the sample to the value indicated in parenthesis ** Samples did not break during testing

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes A review of the elastomer processability and physical property data indicates that there is an optimum hardness range for each polyol MW. Based on the demould time and the percent compression set, the following elastomer Shore hardness ranges are recommended. The lower hardness limits of these ranges can be extended by addition of low levels (less than 10%) of 6000-MW, ultra-low monol triol (Acclaim Polyol 6300). Acclaim Polyol 2200 (2000-MW diol) Acclaim Polyol 4200 (4000-MW diol) Acclaim Polyol 8200 (8000-MW diol)

70 to 90 Shore A 55 to 75 Shore A 50 to 65 Shore A

The rebound of the elastomers based on 2000-MW, ultra-low monol PPG ranges from 55 to 60%. For high-performance dynamic applications, use of higher MW diols are recommended, which give rebound values up to 10 units higher, ranging from 65 to 70%. Figure 9.5 shows the DMTA response for the 70 Shore A elastomers based on the 2000, 4000 and 8000 MW diols. The glass transition temperature (Tg) for the elastomers based on the higher MW diols is about 15 °C lower, which probably accounts for the higher rebound of these materials.

Figure 9.5 DMTA curves of 70 Shore A elastomers: Effect of polyol molecular weight (6% NCO MDI prepolymers; BDO cured)

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Advances in Urethane Science and Technology The 8000-MW, ultra-low monol PPG is used particularly in the development of soft, plasticiser-free elastomers [16, 17]. The use of this high MW, low polydispersity polyol allows for the preparation of low viscosity prepolymers with low isocyanate contents. Initial evaluations of these very soft elastomers showed lower than expected physical properties. The lower properties can be attributed to the very low hard-segment content of these polymers. The lack of hard segment (physical crosslinks) can be compensated for by the incorporation of low levels of chemical crosslinks (triol) [18]. This is accomplished by the addition of a 6000-MW, ultra-low monol triol (Acclaim Polyol 6300) into the polymer matrix. Table 9.7 shows the triol effect on prepolymer viscosities, elastomer processability and physical properties. It should be noted that very low levels of crosslinking are needed to improve the elastomer properties.

9.2.3 Processing Latitude Improves by Incorporating Oxyethylene Moieties The ultimate goal for MDI/BDO cured elastomers was to design a polyol that would have a combination of good processability and high rebound in the mainline hardness range of 80 to 95 Shore A. This was an elusive goal as one can see from the elastomer data in Tables 9.5 and 9.6. A 90 Shore A elastomer based on the 4000-MW diol had poor processability but a high rebound, whereas, the 90 Shore A elastomer based on the 2000-MW diol was processable but had a lower rebound. We attributed the poor processability of the 90 Shore A elastomer based on the 4000MW diol to the excessive phase-separation of the MDI/BDO hard segment [19]. To alleviate the hard segment phase-separation problem, the compatibility of the MDI/BDO hard segment in the soft-segment phase was increased by incorporating oxyethylene moieties into the polyol backbone. This technique dramatically enhanced the elastomer processability. The polyol MW and oxyethylene content were varied to obtain the best possible combination of processability, high hardness and high rebound. This optimisation process led to the development of Acclaim Polyol 3205, a 3200-MW diol containing approximately 20 weight percent oxyethylene moieties. Table 9.8 shows the enhanced processing latitude of elastomers based on this polyol. In addition to enhanced processability, elastomer physical properties were improved. Acclaim Polyol 3205 is a unique material in that it allows the formulation of MDI/BDO cured elastomers from 75 to 95 Shore A with excellent processability and high rebound.

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Table 9.8 Polyurethane elastomers based on Acclaim polyol 3205 (MDI prepolymers; BDO cured) PREPOLYMER COMPOSITION % NCO Prepolymer

4.0

6.0

8. 0

10.0

12.0

NCO/OH

2.9

4.0

5.3

6.9

8.7

MDI (pbw)

22.4

31.2

41.5

53.5

67.7

Acclaim Polyol 3205 (pbw)

100

100

100

100

100

14

8.5

4. 6

3.0

1.4

Prepolymer Viscosity, Pa-s 20 °C 40 °C

3.2

1.9

1.0

0.8

0.65

60 °C

1.0

0. 7

0.41

0.32

0.26

80 °C

0.5

0. 3

0.19

0.16

0.14

PROCESSING CHARACTERISTICS

Prepolymer @ 65 °C; BDO @ RT; NCO/OH = 1.03; dibutyltin dilaurate catalyst; Moulds @ 100 °C; cured for 16 hours @ 100 °C

Prepolymer (pbw)

100

100

100

100

100

1,4-butanediol, BDO (pbw)

4.2

6.2

8. 3

10.4

12.5

Pot life (minutes) Demould Time (minutes) Green Strength @ 30 minutes

1-3

1-3

1- 3

1-3

1-3

15-40

15-40

15-40

20-50

30-60

excellent

excellent

excellent

good

average

ELASTOMER PROPERTIES

Samples conditioned for 2 weeks @ 23 °C/50% relative humidity

Hardness, Shore A

43A (36A)

68A

82A

90 A

95A

Pendulum Rebound, %

64

69

65

60

55

Ultimate Elongation, %

650

720

960

850

810

Tensile Strength, MPa

12.3

24.0

24.0

24.0

21.9

1.5

3.0

4. 9

7.1

9.8

100% Modulus, MPa 200% Modulus, MPa

2.1

4.7

6.7

8.9

11.4

300% Modulus, MPa

3.2

6.6

8.4

10.8

12.7

400% Modulus, MPa

4.7

8.9

10.3

12.6

14.1

Die C Tear, kN/m

30

54

70

89

93

Compression Set, % (22 h @ 70 °C)

60

42

19

21

24

5%

0.2

0. 5

1.0

1.6

3.2

10%

0.4

1.0

2. 1

3.3

5.8

Compression Deflection, MPa

15%

0.6

1.4

3.0

4.7

7.8

25%

1.1

2.5

5.0

7. 3

11.5

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Advances in Urethane Science and Technology

9.3 One-Shot Elastomer System Based on EO-Capped, Ultra-Low Monol PPG Polyols The lower reactivity of secondary hydroxyl groups on the initial ultra-low monol PPG polyol products limited their use to mainly prepolymer systems. PTMEG and polyester polyols have primary hydroxyl groups, allowing them to be easily formulated with curatives such as 1,4-butanediol and reacted with polyisocyanates using a ‘one-shot’ or ‘quasi-prepolymer’ approach. For this reason, high-reactivity versions of ultra-low monol PPG were developed. These polyols give the performance enhancements expected from ultra-low monol PPG with the added benefit of being one-shot processable [20, 21]. The higher reactivity is achieved by capping the propylene-oxide based polyols with ethylene oxide (EO) resulting in products with a high percentage of primary-hydroxyl groups. A significant amount of work has been reported on polyurethanes based on EO-capped low monol PPG [20, 21, 22, 23, 24, 25].

9.3.1 Effect of Primary Hydroxyl Concentration on One-Shot Elastomer Processability A key factor in determining whether or not a polyol can be used in a one-shot process is reactivity. As a general rule, the polyol reactivity needs to be nearly equal to or greater than the reactivity of the curative. For example, most one-shot elastomers based on MDI use BDO as a curative which contains primary hydroxyl groups. A typical PPG polyol contains about 95% secondary hydroxyl groups. If one attempts to process a blend of MDI, BDO and PPG, the MDI will react preferentially with the BDO to form an MDIBDO hard segment. The MDI-BDO hard segment is incompatible in the system and phase separates before the PPG polyol has a chance to react. The end result is a low MW, two-phase system in which the MDI-BDO hard segment is dispersed in partially reacted polyol, which results in very poor physical properties. Therefore, successful one-shot processing requires that the polyol and curative have similar reactivities. The reactivity of PPG polyols can be increased by reacting with EO, which ring opens to form a primary hydroxyl group. A 100% primary hydroxyl content cannot be achieved using this approach since EO has a tendency to form chains instead of distributing evenly to each secondary hydroxyl group; however, it is well known that a 100% primary content is not required to achieve one-shot processability. The effect of the primary hydroxyl content on the processability of a one-shot elastomer system is discussed next. To determine the effect of the polyol primary content on the processability of a one-shot system, two 4000-MW, ultra-low monol PPG diols, which were EO-capped to obtain primary contents of 80 and 87 percent were prepared. These were compared with a

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes 2000-MW PTMEG-2000, which has 100 percent primary hydroxyl content. These polyols were evaluated in a one-shot elastomer system consisting of carbodiimide-modified MDI (29.3% NCO), polyol and BDO formulated to an 80 Shore A hardness. Processability was assessed by measuring the hardness build up with time. The reaction rate was held constant by adjusting the dibutyltin dilaurate catalyst concentration to achieve a constant pot life of 90 seconds. Pot life is defined as the work life of the solution, which is the time period until the solution becomes too viscous to be poured into a mould. The elastomer solution was poured into moulds preheated to 100 °C and then placed into a 100 °C vented oven until the elastomer had developed enough integrity to be demoulded. After demoulding, the elastomer hardness was measured at regular intervals and placed back in the oven. The final hardness measurement was made after 16 hours cure at 100 °C. All the data were normalised to 100% using the final hardness measurement. The normalised hardness versus time were then plotted. The hardness build of 80 Shore A, one-shot elastomers based on polyols with primary contents of 80, 87 and 100 percent are shown in Figure 9.6. As expected, the elastomer

Figure 9.6 Hardness buildup of 80 Shore A one-shot elastomers (carbodiimide-modified MDI/polyol/BDO)

437

Advances in Urethane Science and Technology based on PTMEG-2000 has the fastest hardness build, achieving 90% of its final hardness after only 30 minutes. The 87% primary content, 4000-MW, EO-capped diol gave the next fastest hardness build as it took 60 minutes to achieve 90% of its final hardness. Surprisingly, there was a large difference between the 80 and 87% primary content 4000-MW, EO-capped diols as the time to achieve 90% of its final hardness went from 60 minutes to over 4 hours. Based on this information, it was decided that the primary-hydroxyl content needs to be at least 85%. The previous studies led to the development of three products, a 2250-MW diol (Acclaim Polyol 2220), 4000-MW diol (Acclaim Polyol 4220) and a 6000-MW triol (Acclaim Polyol 6320). Table 9.4 shows the typical properties of these new higher-reactivity, ultra-low monol PPG polyols. The behaviour of these products in one-shot systems is discussed below.

9.3.2 Effect of Monol Content on One-Shot Elastomer Processability and Properties The effect of monol content on the processability and properties of one-shot elastomers is shown in Table 9.9. We prepared 80 Shore A elastomers based on carbodiimidemodified MDI and BDO using a 4000-MW, EO-capped, ultra-low monol diol containing 3% monol content and a conventional EO-capped diol containing 15% monol. These polyols had comparable primary hydroxyl concentrations of 87 and 89%, respectively. As expected, the monol content of the EO-capped PPG diols has a significant effect on the elastomer processability. The Shore hardness of the conventional elastomer was only 57A after 16 hours cure at 100 °C, whereas the elastomer based on ultralow monol PPG had a hardness of 78A. It took three weeks for the conventional elastomer to achieve its final hardness of 82A. Virtually all elastomer properties improved with lower monol contents (see Table 9.9). Of particular note, the rebound increased from 59 to 68 percent, the elongation from 470 to 550%, tensile strength from 10 to 19 MPa, tear strength from 49 to 68 kN/m, compression set decreased from 64 to 23%, and the Taber abrasion loss (ASTM D4060-95 [26]) decreased from 220 to 80 mg loss/1000 cycles. The improvement in the stress/strain curves is shown in Figure 9.7. Lower monol content results in significantly higher ultimate polymer MW, which results in improved mechanical properties.

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Table 9.9 One-shot elastomer comparison: Ultra-low monol versus conventional EO-capped PPG (cardodiimide-modified MDI/polyol/BDO) POLYOL COMPOSITION Polyol

Conventional

Ultra-Low Monol

Monol Content, molar %

15

3

Primary Hydroxyl Content, %

89

87

42.9

43.0

ELASTOMER COMPOSITION CD-MDI (pbw) Polyol (pbw)

100

100

1,4-Butanediol, BDO (pbw)

10.9

10.9

Pot life (minutes)

1.5

1.5

Demould Time (minutes)

20

20

Good

Excellent

57 A

78A

82A

82A

Pendulum Rebound, %

59

68

Ultimate Elongation, %

470

550

Tensile Strength, MPa

10.4

19.4

100% Modulus, MPa

4.3

5.3

200% Modulus, MPa

6. 0

7.8

300% Modulus, MPa

7.6

10.0

400% Modulus, MPa

9.4

12.5

Die C Tear, kN/m

49

68

Compression Set, % (22 h @ 70 °C)

64

23

Taber Abrasion*, (mg loss/1000 rev)

220

80

5%

1.2

1.3

10%

2.1

2.3

15%

3.0

3.3

25%

4.4

5.2

PROCESSING CHARACTERISTICS

Green Strength @ 30 minutes Initial Shore Hardness (16 h cure, 100 °C) ELASTOMER PROPERTIES Hardness Shore A

Compression Deflection, MPa

*

Elastomer contained 1 wt% Irganox 1076 (Ciba-Geigy)

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Advances in Urethane Science and Technology

Figure 9.7 Stress/strain curves of 80 Shore A one-shot elastomers based on 4000-MW, EO-capped PPG: Ultra-low monol versus conventional

9.3.3 Processability and Property Latitude of Elastomers Based on EO-Capped, Ultra-Low Monol Polyols The processability and property latitude of elastomers based on EO-capped, ultra-low monol polyols were determined by preparing a series of elastomers at varying hardsegment contents. The percent hard-segment content is defined as: (weight MDI + weight BDO)/total elastomer weight x 100. The elastomers were prepared using the one-shot process by the hand-casting technique at a pot life of 1.5 minutes. The compositions, processability and properties of elastomers prepared using the oneshot process are given in Tables 9.10 to 9.12. For the 4000-MW, EO-capped diol, the upper hardness processability limit via hand casting is about 85 Shore A as indicated by the green strength in Table 9.10. Machine casting using a shorter pot life (30 to 45

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Tables 9.10 One-shot elastomers based on 4000 MW, EO-capped, ultra-low monol PPG (carbodiimide-modified MDI/polyol/BDO) ELASTOMER COMPOSITION Hard-Segment Content

15

20

25

35

40

45

CD-MDI (pbw)

15.2

20.8

27.2

43.0

53.2

64.4

Acclaim 4220 Polyol

100

100

100

10 0

100

100

1,4-Butanediol, BDO (pbw)

2.5

4.2

6.1

10.9

13.9

17.5

PROCESSING CHARACTERISTICS Pot life (minutes)

1.5

1.5

1.5

1. 5

1.5

1.5

Demould Time (minutes)

20

20

20

20

20

20

Excellent

Excellent

Excellent

Excellent

Good

Poor

50A

60A

70 A

80A

90A

95A (45D)

Pendulum Rebound, %

68

69

70

68

60

56

Ultimate Elongation, %

640

610

58 0

550

500

450

Green Strength ELASTOMER PROPERTIES Hardness, Shore A

Tensile Strength, MPa

4.9

7.6

10.7

19.4

20.4

21.4

100% Modulus, MPa

1.1

1.8

2.4

5.3

7.6

10.0

200% Modulus, MPa

1.5

2.5

3.6

7.8

10.7

13.1

300% Modulus, MPa

1.9

3.4

5.0

10.0

13.1

15.9

400% Modulus, MPa

2. 4

4.6

6.6

12.5

15.9

18.7

Die C Tear, kN/m

21

35

47

69

70

85

Compression Set, % (22 h @ 70 °C)

40

33

30

25

20

22

5%

0.2

0.4

0.6

1.3

1.2

3.0

10%

0.4

0.7

1. 0

2. 3

2.8

5.3

15%

0. 6

1.0

1.5

3.3

4.2

6.9

25%

1.0

1.8

2.6

5.2

7.3

10.4

Compression Deflection, MPa

seconds) could extend the hardness limit to the low to mid-90s. The lower hardness limit as indicated by the increase in compression set is about 60 Shore A. The lower hardness limit can be extended down to at least 40 Shore A by replacing some of the 4000-MW

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Advances in Urethane Science and Technology

Table 9.11 One-shot elastomers based on ultra-low monol, EO-capped, 4000-MW diol/6000-MW triol blends (CD-MDI/polyol/BDO) ELASTOMER COMPOSITION Hard-Segment Content

10

10

10

10

10

CD-MDI (pbw)

10.2

10.2

10.2

10.2

10.2

Acclaim Polyol 4220 (pbw)

100

95

90

85

80

Acclaim Polyol 6320 (pbw)

0

5

10

15

20

1,4-Butanediol, BDO (pbw)

0.9

0.9

0. 9

0.9

0.9

Pot life (minutes)

1.5

1. 5

1.5

1.5

1.5

Demould Time (minutes)

20

20

20

20

20

Excellent

Excellent

Excellent

Excellent

Excellent

40 A

42A

43A

45A

45A

Pendulum Rebound, %

70

72

73

75

75

Ultimate Elongation, %

880

600

450

320

270

Tensile Strength, MPa

2.5

2. 3

1.9

1.7

1.5

PROCESSING CHARACTERISTICS

Green Strength ELASTOMER PROPERTIES Hardness, Shore A

100% Modulus, MPa

0.8

0. 9

0.9

0.9

1.0

200% Modulus, MPa

0.9

1.2

1.2

1.2

1.3

300% Modulus, MPa

1.1

1.4

1.5

1.6

—

400% Modulus, MPa

1.1

1.6

1.7

—

—

Die C Tear, kN/m

15

14

12

11

9

Compression Set, % (22 h @ 70 °C)

45

38

20

15

12

0.1

0.1

0.1

0.1

0.1

Compression Deflection, MPa 5% 10%

0.3

0. 3

0.3

0.3

0.3

15%

0.4

0.4

0. 5

0.5

0.5

25%

0.8

0.8

0.9

0. 9

0.9

diol with the 6000-MW, EO-capped, ultra-low monol triol as shown by the reduction in compression set in Table 9.11. Thus, using combinations of 4000-MW diol and 6000MW triol, a broad hardness range from about 40 to 85 Shore A can be achieved.

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes The 2250-MW (50 OH#) EO-capped, ultra-low monol diol was designed to enhance processability at higher Shore hardnesses. The processability and properties of one-shot elastomers based on this polyol are shown in Table 9.12. One-shot elastomers with excellent processability and properties can be formulated from an 80 Shore A up to about a 55 Shore D. Thus, a broad range of Shore hardnesses (40A to 55D) can be achieved using EO-capped, ultra-low monol polyols.

Table 9.12 One-shot elastomers based on 2250 MW, EO-capped, ultra-low monol PPG (CD-MDI/polyol/BDO) ELASTOMER COMPOSITION Hard-Segment Content

25

35

45

55

65

CD-MDI (pbw)

28.5

44.2

65.6

96.6

145.2

Acclaim Polyol 2220 (pbw)

100

100

100

100

100

1,4-Butanediol, BDO (pbw)

4.8

9.6

16.2

25.6

40.5

Potlife (minutes)

1.5

1.5

1. 5

1.5

1.5

Demould Time (minutes)

20

20

20

20

20

Excellent

Excellent

Excellent

Excellent

Average

67A

82A

94A (44D)

52D

60D

Pendulum Rebound, %

69

65

56

51

53

Ultimate Elongation, %

660

510

430

320

200

Tensile Strength, MPa

10.7

19.4

22.1

22.8

25.6

100% Modulus, MPa

2.6

5. 1

9.7

15.9

19.4

200% Modulus, MPa

3.7

7.6

13.1

19.4

25.6

PROCESSING CHARACTERISTICS

Green Strength ELASTOMER PROPERTIES Hardness, Shore A

300% Modulus, MPa

5.4

10.4

16.6

21.4

—

400% Modulus, MPa

8.3

14.5

20.0

—

—

Die C Tear, kN/m

44

67

88

97

127

Compression Set, % (22 h @ 70 °C)

38

28

24

nd

nd

nd: not determined

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Advances in Urethane Science and Technology 9.4 Comparison of Ultra-Low Monol PPG Polyols with PTMEG When comparing ultra-low monol PPG polyols with PTMEG it is essential to keep in mind the fundamental differences between these two high-performance polyether polyols in order to obtain the maximum benefit in urethane applications. Table 9.13 summarises these differences in terms of their chemical structure, hydroxyl type (reactivity), MWD and crystallisability.

Table 9.13 Comparison of fundamental characteristics of ultra-low monol PPG polyols and PTMEG Chemical Structure Hydroxyl Type Molecular Weight Distribution Crystallisability

Ultra-Low Monol PPG

PTMEG

-O(-CH2-CH-O)x-CH3 Tg ≈ -75 °C

-O(-CH2CH2CH2CH2-O)xTg ≈ -84 °C

Secondary Hydroxyl (Lower Reactivity)

Primary Hydroxyl (Higher Reactivity)

Narrow: Mw/Mn = 1.02-1.15 (Acclaim 2200: 0.165 Pa-s @ 40 °C)

Broad: Mw/Mn = 1.7-2.4 (PTMEG-2000: 12 Pa-s @ 40 °C)

Non-crystallisable (Liquid; Pour Point: -33 °C)

Crystallisable (Waxy Solid; mp: 28-40 °C)

mp: melting point

Both PPG and PTMEG have polyether backbones, which inherently impart excellent hydrolytic stability and microbial resistance to polyurethanes. The Tg of PPG approaches that of PTMEG resulting in polyurethanes that can be used in cold, harsh environments. PTMEG has primary hydroxyl groups, whereas, ultra-low monol PPG are available with secondary hydroxyl groups or EO-capped versions for primary hydroxyl contents in excess of 85%. Ultra-low monol PPG polyols have narrow MWD, which gives polyols that are an order of magnitude lower in viscosity than PTMEG. The storage and handling requirements for PPG polyols are minimal since they are non-crystallising liquids with pour points of about –30 °C. Additionally, polyurethanes based on non-crystallising polyols will not ‘cold harden’ like some of those based on crystallisable polyols. A comparison of ultra-low monol PPG versus PTMEG in MDI/ BDO cured elastomers is given below.

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

9.4.1 MDI/BDO Cured Elastomers: Acclaim Polyol 3205 Versus PTMEG-2000 For a comparison of PTMEG in MDI/BDO cured elastomers the 3200-MW ultra-low monol PPG diol was chosen which contains about 20 wt% EO (Acclaim Polyol 3205) since it gives the best overall properties in this type of system. A 2000-MW PTMEG was used since it gives the best performance at an 80 Shore A hardness. Elastomers of 80 Shore A hardness were prepared using the prepolymer process. Table 9.14 summarises their prepolymer viscosities, elastomer processing characteristics and physical properties. The initial surprise was to find that it takes an 8% NCO MDI/Acclaim 3205 prepolymer to make a 80 Shore A elastomer, whereas, PTMEG-2000 requires a 6% NCO prepolymer. This finding will be explained in the section on the effect of polyol MWD (see section 9.5.4). Note the dramatic difference in prepolymer viscosities in Table 9.14. The prepolymer based on ultra-low monol polyol has a viscosity almost ten times lower than that of PTMEG-2000. This is due to a combination of the narrower MWD and the higher % NCO of the Acclaim 3205 prepolymer. Lower viscosity prepolymers offer significant advantages in processability. This gives the polyurethane manufacturer much more latitude in terms of processing conditions such as temperature and catalyst combinations. Elastomers based on Acclaim Polyol 3205 have demould times and green strengths comparable to those based on PTMEG-2000. When hand casting elastomers using a 1 to 3 minute pot life, this can give demould times of significantly less than 30 minutes. As expected, the fundamental differences in the polyols also have a significant impact on the elastomer physical properties. Although elastomers based on ultra-low monol PPG have pendulum rebounds significantly higher than conventional or low monol polyols, they are about 10 units lower that PTMEG at an equal Shore hardness. It is believed this is due to fundamental differences in the polyol backbone structure. PPG polyols have a pendant methyl group which may inherently absorb more energy than the linear methylene chain of PTMEG. Elastomers based on Acclaim Polyol 3205 have ultimate elongations that are twice that of PTMEG. It is believed that this is due to the fact that PTMEG stress crystallises as it is stretched. Crystallites behave like crosslinks and lower the elongation. To test this theory, a series of MDI/BDO cured elastomers were prepared using 2000-MW crystallisable polyesters (polybutylene adipate and polycaprolactone) and non-crystallisable polyesters (polyethylenebutylene adipate and poly-2-methyl-1,3-propylene adipate). Figures 9.8 and 9.9 compare the stress/strain curves of the crystallisable versus non-crystallisable polyethers and polyesters, respectively. From these figures the importance of the polyol stress crystallisability in determining the elastomer stress/strain profile can be seen. 445

Advances in Urethane Science and Technology

Table 9.14 Acclaim 3205 elastomers compared with PTMEG-2000 at 80 Shore hardness (MDI prepolymers: BDO cured) PREPOLYMER COMPOSITION Polyol Type

Acclaim 3205

PTMEG-2000

% NCO Prepolymer

8.0

6.0

NCO/OH

5.3

3.0

20 °C

4.6

42

40 °C

1.0

11

60 °C

0.41

3.7

80 °C

0.19

1.6

Prepolymer Viscosity, Pa-s

PROCESSING CHARACTERISTICS

Pot life (minutes)

Acclaim Prepolymers @ 65 °C; PTMEG Prepolymers @ 80 °C; BDO @ RT; NCO/OH = 1.03; dibutyltin dilaurate catalyst; Moulds @ 100 °C; cured for 16 hours @ 100 °C 1-3

1-3

15-40

15-40

82A

83A

Pendulum Rebound, %

65

75

Ultimate Elongation, %

960

510

Tensile Strength, MPa

24

31

100% Modulus, MPa

4.9

5.3

200% Modulus, MPa

6.7

7.1

300% Modulus, MPa

8.4

9.9

400% Modulus, MPa

10.3

14.7

Die C Tear, kN/m

70

70

Compression Set, % (22 h @ 70 °C)

19

15

Demould Time (minutes) ELASTOMER PROPERTIES Hardness, Shore A

Compression Deflection, MPa

446

5%

1.0

0.8

10%

2.1

1.7

15%

3.0

2.6

25%

5.0

4.6

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Figure 9.8 Stress/strain curves of 80 Shore A elastomers: Crystallisable versus non-crystallisable polyethers

The tensile strengths of ultra-low monol based elastomers are lower than that of PTMEG. However, if one considers the total energy to break as measured by the area under the stress/strain curve, the elastomers based on Acclaim Polyol 3205 are significantly tougher than those based on PTMEG. The tear strengths of the elastomers are comparable.

9.4.2 Enhanced Elastomer Properties Utilising Ultra-Low Monol PPG/PTMEG Blends Previously, it was shown that elastomers based on non-crystallising polyols such as ultralow monol PPG have twice the ultimate elongation of crystallising polyols such as PTMEG. The next experiments were aimed at determining how much ultra-low monol PPG it would take to eliminate the stress crystallisability of PTMEG [19]. A series of 80 Shore A elastomers

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Figure 9.9 Stress/strain curves of 80 Shore A elastomers: Crystallisable versus non-crystallisable polyesters

were prepared using MDI prepolymers based on 4000-MW, ultra-low monol PPG/ PTMEG-2000 blends chain extended with BDO. The results of the addition of 0, 10, 40 and 100 weight percent 4000-MW PPG based on the total polyol weight are given in Table 9.15. It was found that only about 10 weight percent of 4000-MW PPG is required to break up the stress crystallisability of PTMEG-2000 as shown in Table 9.16 with the elongation increasing from 510 to over 1000%. Surprisingly, it was found that 4000-MW PPG addition gave a synergistic response in terms of elongation, tensile and tear strength. Even more surprising was that the synergy doesn’t reach a maximum until up to 40 weight percent of 4000-MW PPG. This elastomer has a 1270% elongation, 35 MPa tensile strength and a 77 kN/m tear strength. Additionally, the rebound was only three units lower than the PTMEG elastomer system and the prepolymer viscosity was significantly lower which enhances processability. 448

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Table 9.15 Effect of 4000-MW PPG addition to 80 Shore A elastomers based on PTMEG-2000 (MDI prepolymers: BDO cured) PREPOLYMER CHARACTERISTICS Wt% 4000-MW PPG

0

10

40

100

% NCO Prepolymer

6

6

6

8

20 °C

42

30

22

4.5

40 °C

11

7.7

5.9

1.3

60 °C

3.7

2.7

2.1

0.5

80 °C

1.6

1.2

0.9

0.3

83A

82A

82A

82A

Pendulum Rebound, %

75

72

72

65

Ultimate Elongation, %

510

1040

1270

970

Tensile Strength, MPa

31

33

35

17

100% Modulus, MPa

5. 3

5.0

4.5

4.5

300% Modulus, MPa

9.9

8.4

7.9

7.6

70

81

77

75

Prepolymer Viscosity, Pa-s @

ELASTOMER PROPERTIES Hardness, Shore A

Die C Tear, kN/m

So one can maximise the benefit from ultra-low monol PPG polyols in urethane systems such as MDI/BDO cured elastomers by understanding their fundamental differences in comparison to other high-performance polyols like PTMEG. A detailed look at the effect of the polyol MWD on the mechanical and dynamic properties of polyurethanes follows.

9.5 Polyol Molecular Weight Distribution Effect on Mechanical and Dynamic Properties of Polyurethanes A less well-understood factor, which affects the properties of polyurethanes, is the polyol MWD. Ultra-low monol PPG polyols have very narrow MWD (polydispersity < 1.1) compared to other high-performance polyols such as PTMEG or polyester polyols (polydispersity: 1.7 to 2.4) as illustrated by the gel permeation chromatography (GPC) curve in Figure 9.10. As expected, a narrower MWD results in an order-of-magnitude

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Advances in Urethane Science and Technology

Figure 9.10 GPC comparison of 4000-MW ultra-low monol PPG to 2000-MW PTMEG

reduction in polyol and prepolymer viscosities [7]. Additionally, it was found that polyol MWD has a dramatic effect on the polymer mechanical and dynamic properties, particularly in polyurethane/urea systems [19, 27].

9.5.1 TDI Prepolymers Cured with Methylene Bis-(2-Chloroaniline) [MBOCA] In formulating polyurethane/urea systems using ultra-low monol PPG polyols, it was found that polyol MWD has a major effect on polymer properties. A broadened MWD is required to achieve a property profile similar to a PTMEG-based system. The MWdistribution effect is illustrated using TDI prepolymers cured with MBOCA, TDI moisturecured prepolymers and aqueous polyurethane/urea dispersion coatings. Traditionally, high-performance TDI/MBOCA-cured elastomers are based on PTMEG. They are prepared by reacting TDI with PTMEG at a 1.7:1 to 2:1 NCO to OH ratio to form a prepolymer and then extend the polymer chain with MBOCA. For these studies, prepolymers were prepared using pure 2,4-TDI reacted with polyol at a 2:1 NCO to OH ratio. The properties of a MBOCA-extended prepolymer based on a 1000-MW PPG compared to a 1000-MW PTMEG are shown in columns 1 and 2 of Table 9.16. Surprisingly, the

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes elastomer properties were much lower than expected as shown by the hardness (83A versus 96A); rebound (15 versus 45%); tensile strength (9.2 versus 45 MPa) and tear strength (39 versus 110 kN/m). After several attempts to improve the properties via processing conditions, catalyst, etc., it was decided to artificially broaden the MWD of an ultra-low monol PPG polyol to approximate that of PTMEG-1000. This was achieved by blending a 4000-MW PPG with a low-MW glycol diethylene glycol (DEG), to an average MW of 1000. The property profile of this elastomer is shown in the last column of Table 9.16. Interestingly, the property profile is now closer to the elastomer based on PTMEG-1000 and is very similar to a commercial PTMEG-based 93A product, Airthane PET-93A, which is a product of Air Products and Chemicals, Inc.

Table 9.16 Mechanical properties of MBOCA-cured TDI prepolymes Polyol Type

PPG-1000

PTMEG-1000

Airthane PET-93A*

4000-MW PPG/DEG Blend

83A

96 A

93A

92A

Pendulum Rebound, %

15

45

58

55

Ultimate Elongation, %

340

480

380

310

Tensile Strength, MPa

9.3

44.8

37.9

21.7

100% Modulus, MPa

2.8

14.2

11.0

11.0

Die C Tear, kN/m

39

110

72

88

Hardness, Shore A

*

Data taken from Engineering Properties of Castable Polyurethane Elastomers, Air Products and Chemicals, Inc., 1994. Airthane is a registered trademark of Air Products and Chemicals, Inc.

It is thought that PTMEG gives a higher hardness/modulus polymer because a significant amount of the low-MW tail (see Figure 9.10) ends up in the hard-segment phase. Also, the PTMEG-based elastomer gives a higher rebound because the soft-segment phase consists of PTMEG with MW significantly higher than 1000. The above conclusions are supported by the DMTA curves shown in Figure 9.11. The storage modulus in the rubbery plateau region is significantly higher for the elastomer based on the PPG/DEG blend, supporting the theory that DEG is contributing to the hard segment. Also, note the transition at 130-150 °C, which is likely to be the softening point of the DEG/TDI/ MBOCA hard segment. The dramatic lowering of the soft-segment Tg from 0 °C to -42 °C also supports the view that the PPG polyol and DEG are going into separate phases. 451

Advances in Urethane Science and Technology

Figure 9.11 DMTA curves of TDI prepolymers cured with MBOCA-PPG-1000 versus 4000-MW PPG/DEG blended to 1000 MW

In Figure 9.12, the effect of varying the glycol chain length on the DMTA profiles is shown by using propylene glycol (PG), dipropylene glycol (DPG), tripropylene glycol (TPG) and a 400-MW PPG. The short-chain glycols (PG, DPG, TPG) all gave similar storage-modulus profiles, while the 400-MW PPG gave a dramatically different profile. This indicates that if the MW of the glycol is too high it becomes miscible in the softsegment phase and increases the Tg, thus reducing the elastomer rebound and dynamic performance. In some cases it may not be practical or cost effective to perform DMTA analysis on all samples. It was found that materials can be screened by measuring hardness and rebound at a series of temperatures (-10 °C, 23 °C and 105 °C) and these results are shown in Table 9.17. Small changes in properties imply a broad temperature use range for the polymer. These results are consistent with the DMTA findings. In Table 9.18 the effect of a higher average MW polyol is shown by comparing 2000MW ultra-low monol PPG with a blended product (4000-MW PPG/DEG). Again, the differences in hardness, rebound and modulus are dramatic and are consistent with the above trends.

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Figure 9.12 Effect of glycol chain length on DMTA curves of TDI prepolymers cured with MBOCA

Table 9.17 Mechanical properties of MBOCA-cured TDI prepolymers (effect of glycol chain strength) Polyol Type – 1000 MW

Shore Hardness @ -10/23/105 °C

Bashore Rebound @ -10/23/105 °C

Elongation %

Tensile Strength MPa

100% Modulus MPa

Die C Tear Strength kN/m

PPG-1000

74D/83A/69A

51/14/42

340

9.3

2. 8

39

4000/PG

94A/92A/88A

27/42/62

410

29.0

10.9

93

4000/DPG

95A/94A/91A

28/48/62

350

24.8

12.0

93

4000/TPG

95A/92A/89A

28/42/58

36 0

29.0

13.0

97

4000/PPG400

69D/58D/86A

46/38/35

360

28.3

12.3

86

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Advances in Urethane Science and Technology

Table 9.18 Mechanical properties of MBOCA-cured TDI prepolymers Polyol Type Hardness, Shore A

2000-MW PPG

Airthane PET-80A

4000-MW PPG DEG Blend

68A

80A

80 A

Pendulum Rebound, %

41

62

58

Ultimate Elongation, %

480

550

63 0

Tensile Strength, MPa

6.4

34.5

17.9

100% Modulus, MPa

2.1

4. 1

4.3

Die C Tear, kN/m

33

61

67

*

Data taken from Engineering Properties of Castable Polyurethane Elastomers, Air Products and Chemicals, Inc., 1994. Airthane is a registered trademark of Air Products and Chemicals, Inc.

9.5.2 Moisture-Cured TDI Prepolymers To determine if the polyol MWD effect can be seen in other polyurethane/urea systems, a moisture-cured film was prepared using a 2:1 prepolymer based on 2,4-TDI and PPG1000 and this was compared with one based on 4000-MW PPG blended with DEG to a MW of 1000. Table 9.19 shows the mechanical properties of the films in bold type. The dramatic increases in 100% modulus (1.3 to 3.6 MPa), tensile strength (14 to 35 MPa) and tear strength (18 to 58 kN/m) obtained by broadening the MWD of the polyol can

Table 9.19 Effect of 4000-MW PPG/DEG blend MW on mechanical properties of moisture-cured TDI prepolymers PPG-1000

4000-MW PPG/Diethylene Glycol Blends

Polyol Blend MW

1000

3000

1600

1000

770

Prepolymer % NCO (theory)

6.23

2.54

4.24

6.23

7.53

Elongation, %

730

1280

850

720

540

Tensile Strength, MPa

13.7

10.3

22.8

35.2

39.0

100% Modulus, MPa

1.3

0. 4

2.0

3.6

8.3

300% Modulus, MPa

2.1

0. 6

4.0

5.6

14.6

Die C Tear, kN/m

18

12

40

58

93

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes clearly be seen. The DMTA curves of these materials show the same trends as with the MBOCA-cured TDI prepolymers as seen in Figure 9.13. A formulation with 4000-MW PPG decreases the soft-segment Tg from –16 °C to –58 °C greatly enhancing the low temperature performance of this system. The versatility of using this approach can be clearly seen by blending 4000-MW PPG and DEG to a wide range of MW (770 to 3000) as shown in Table 9.19. By simply blending these two components, one can vary the 100% modulus from 0.4 to 8.2 MPa. Traditionally, hardness would be adjusted by using various polyol MWs; however, when using this approach there is a reduction in the low-temperature flexibility as shown by the DMTA curves in Figure 9.13.

Figure 9.13 DMTA curves of TDI moisture-cured prepolymer films - PPG-1000 versus 4000-MW PPG/DEG blended to 1000 MW

Using the blend approach, the hardness can be varied over a wide range while retaining the low-temperature flexibility as shown by the DMTA curves in Figure 9.14. Note that with increasing amounts of DEG, there is a transition beginning at about 100 °C that becomes more pronounced and shifts to about 80 °C. This transition is likely

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Advances in Urethane Science and Technology

Figure 9.14 Effect of 4000-MW PPG/DEG blend MW on DMTA curves of TDI moisture-cured prepolymer films

associated with the Tg of the TDI/DEG/urea hard segment. Similar property profiles are observed in isophorone diisocyanate-based (IPDI), moisture-cured systems utilised in deck-coating materials.

9.5.3 Aqueous Polyurethane/Urea Dispersion Coatings We also determined the effect of polyol MWD in aqueous polyurethane/urea dispersion coatings by first comparing a 2000-MW, ultra-low monol PPG with PTMEG-2000. The dispersions were prepared by the prepolymer-mixing process using the formulations given in Table 9.20 [15]. The prepolymer was based on IPDI, polyol and dimethylol propionic acid (DMPA). It also contained 10 weight percent N-methyl pyrrolidone (NMP) as a coalescing aid. Triethylamine was used as a neutraliser and ethylene diamine as chain extender. All aqueous dispersions were prepared to 40% solids content. A detailed experimental procedure is given in a previous article [28].

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Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

Table 9.20 Aqueous polyurethane/urea dispersion formulations Polyol

2000 MW PPG

PTMEG-2000

4000 MW PPG/PPG

2000

2000

2000

Polyol Blend, MW PREPOLYMER COMPOSITION

g (equiv.)

Isophorone Diisocyanate

65.2 (0.587)

65.2 (0.587)

65.1 (0.586)

Polyol

195.6 (0.196)

195.6 (0.196)

188.9 (0.095)

—

—

6.8 (0.101)

9.2 (0.137)

9.2 (0.137)

9.2 (0.137)

N-Methylpyrrolidone

30

30

30

Total Weight

300

300

300

NCO/OH

1.76

1.76

1.76

% NCO (theoretical)

3.6

3.5

3.6

% NCO (actual)

3.25

3.22

3.30

% Carboxylate

1.12

1.10

1.11

nd

3.7

0.7

Dipropylene Glycol Dimethylolpropionic acid

PREPOLYMER CHARACTERISTICS

Viscosity @ 80 °C, Pa-s DISPERSION AND CHAIN EXTENSION Neutraliser

Triethylamine

Triethylamine

Triethylamine

Ethylene diamine

Ethylene diamine

Ethylene diamine

Solids Content, wt %

40

40

40

Cosolvent Content, wt %

4.0

4.0

4.0

Chain Extender DISPERSION CHARACTERISTICS

The stress/strain curves in Figure 9.15 show that the film based on the 2000-MW PPG has a significantly lower hardness than that based on PTMEG-2000. To take into account the differences in polyol MWD, we then prepared an aqueous dispersion using 4000MW PPG and DPG blended to a 2000 MW. Figure 9.15 shows that these stress/strain curves are now very similar up to 200% elongation. At higher elongations, the film based on PTMEG-2000 begins to stress crystallise resulting in increased modulus and reduced ultimate elongation. The DMTA curves in Figure 9.16 show that the film based on the blend of 4000-MW PPG and DPG has a profile more similar to PTMEG-2000 than to that based on 2000-MW PPG.

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Figure 9.15 Stress/strain curves of aqueous polyurethane/urea dispersion films

Figure 9.16 DMTA curves of aqueous polyurethane/urea dispersion films 458

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes

9.5.4 MDI Prepolymers Cured with BDO Table 9.21 demonstrates the polyol MWD effect by comparing BDO cured 6% NCO 4,4´-MDI prepolymers based on 2000-MW PPG and PTMEG-2000. The elastomer based on the narrow-MWD polyol has a significantly lower hardness (68A versus 83A) and rebound (60 versus 75%). This is consistent with PTMEG-2000 containing some shortchain diol, which phase separates into the hard segment promoting higher hardness. Also, the soft-segment Tg of the PTMEG-2000 elastomer would be lower since the soft segment actually consists of PTMEG with MW significantly higher than 2000. Based on our previous discussion, to best approximate the property profile of a broad-MWD polyol such as PTMEG, one needs to use a higher MW ultra-low monol PPG polyol with some additional chain extender (BDO). Table 9.21 illustrates this effect by increasing the PPG MW from 2000 to 4000 and the prepolymer % NCO from 6 to 8. This increases the hardness to the same level as the PTMEG-based elastomer and increases the rebound to 65%. As an aside, increasing the prepolymer % NCO from 6 to 8 represents a polyol/BDO blend of 2160 MW. We conclude with some remarks regarding recommendations as to which short chain diols to use with polyurethanes versus polyurethane/ureas. In elastomeric materials, optimal dynamic performance is obtained in a well phase-separated system (a low softsegment Tg and a high softening or melting-temperature hard-segment). For polyurethane systems, optimal dynamic performance is achieved by maintaining symmetry in the hard segment. This is accomplished by using a symmetrical diisocyanate (such as pure 4,4´MDI) with a symmetrical chain extender (such as ethylene glycol, BDO, DEG or 1,6hexanediol). This symmetry results in a high-melting-temperature crystalline hard-segment phase, enhancing phase separation in the polymer. Complete phase separation results in a low soft-segment Tg and excellent rebound or dynamic performance. Disrupting the symmetry of the hard segment potentially reduces the rebound or dynamic performance. In formulating with ultra-low monol PPG in polyurethane systems, optimal properties are attained using a high MW polyol with additional symmetrical chain extender. Use of mixed chain extenders will disrupt the hard-segment symmetry and lead to a reduction in dynamic properties. In polyurethane/urea systems, phase separation and optimal dynamic performance is driven by the hydrogen bonding of the urea linkage and not by the hard-segment symmetry. So when formulating ultra-low monol PPG polyols in polyurethane/urea systems, there is more latitude in the choice of short-chain glycols. However, it is undesirable to choose an extender that is so short or symmetrical that the isocyanate adduct is incompatible in the isocyanate-terminated prepolymer. Additionally, the glycol MW must be limited to prevent it from phase mixing with the soft segment.

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Table 9.21 Elastomer properties of MDI prepolymers cured with BDO PREPOLYMER COMPOSITION Polyol Type

2000-MW PPG

PTMEG-2000

4000-MW PPG

% NCO Prepolymer

6.0

6.0

8.0

NCO/OH

3.0

3.0

6.3

4,4´-MDI (pbw)

36.9

36.9

39.5

Polyol (pbw)

100

100

100

20 °C

15

42

4.5

40 °C

2.7

11

1.25

60 °C

0.85

3.7

0.5

80 °C

0.4

1.6

0.25

Prepolymer (pbw)

100

100

100

1,4-butanediol, BDO (pbw)

6.2

6.2

8.3

Pot life (minutes)

1-3

1-3

1-3

20-45

20-45

30-60

Prepolymer Viscosity, Pa-s

PROCESSING CHARACTERISTICS

Demould Time (minutes ELASTOMER PROPERTIES Hardness, Shore A

Samples conditioned for 4 weeks @ 23 °C/50% relative humidity 68A

83A

82A

Pendulum Rebound, %

61

75

65

Ultimate Elongation, %

710

510

970

Tensile Strength, MPa

22.1

31.0

17.2

100% Modulus, MPa

3.5

5.3

4.5

200% Modulus, MPa

5.3

7.1

6.2

300% Modulus, MPa

7.4

9.9

7.6

400% Modulus, MPa

9.9

14.7

8.9

Die C Tear, kN/m

58

70

75

Compression Set, % (22 h @ 70 °C)

22

15

19

5%

0.5

0.8

1.0

10%

1.0

1.7

1.8

15%

1.6

2.6

2.5

25%

2.8

4.6

4.0

Compression Deflection, MPa

460

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes The benefits from using ultra-low monol PPG polyols in polyurethane or polyurethane/ urea systems can be realised by understanding the fundamental differences between them and other high-performance polyols such as PTMEG. In particular, when considering the polyol MWD, PTMEG should be regarded as a high MW polyol blended with a low MW chain extender (or pseudo chain extender). In contrast, ultra-low monol PPG contain no low-MW fraction, therefore, to mimic the behavior of PTMEG it is necessary to add additional low MW glycol into the prepolymer or resin components. Because of this feature, ultra-low monol PPG have a broader formulating latitude, since chain extender can be added to tailor the property profile to the specific application.

9.6 Conclusions Ultra-low monol PPG polyols have excellent properties and ease of processing. The elimination of monol enables polymers to be produced having much higher final MW, i.e., improved properties, as well as significantly faster MW buildup during polymer formation, i.e., shorter demould times and excellent green strength. Another important feature of these polyols is their narrow MWD. Low polydispersity results in lower viscosities in both the polyol and isocyanate-terminated prepolymers. Polyol MWD has a significant effect on the mechanical and dynamic properties of polyurethane and polyurethane/urea systems. Broad-MWD polyols such as PTMEG or polyester polyols contain significant amounts of hard-segment diol, which limits their softness. Acclaim polyols contain no hard-segment diol and thus have inherent superior properties in soft systems. They can be made to approximate broad-MWD polyols such as PTMEG by incorporation of a low MW glycol.

Acknowledgements We would like to thank Steve Bailey, Daphne Hale, Beth Lambert, Tony Loveday and Mike Robinson, who carefully and conscientiously prepared the samples and carried out the physical property testing.

References 1.

D. M. Simons and J. J. Verbanc, Journal of Polymer Science, 1960, 44, 144, 303.

2.

G. Gee, W. C. E. Higginson, K. J. Tailor and H. W. Trenholme, Journal of the Chemical Society, 1961, 4298.

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Advances in Urethane Science and Technology 3.

W. H. Snyder, K. J. Tailor, N. S. Chu and C. C. Price, Transactions of the New York Academy of Science, Series II, 1962, 24, 341.

4.

R. J. Herold in Macromolecular Syntheses, Volume 5, Ed., E. L. Wittbecker, John Wiley and Sons, Inc., New York, 1974, 9.

5.

H. van der Hulst, G. A. Pogany and J. Kuyper, inventors; Shell Oil Company, assignee; US Patent 4,477,589, 1984.

6.

J. L. Schuchardt and S. D. Harper, Presented at the 32nd Annual Polyurethane Technical/Marketing Conference, San Francisco, CA, USA, 1989, p.360.

7.

R. L. Mascioli, Presented at the 32nd Annual Polyurethane Technical/Marketing Conference, 1989, San Francisco, CA, USA, p.139.

8.

J. W. Reisch and D. M. Capone, Presented at the 33rd Annual Polyurethane Technical/Marketing Conference, Orlando, FL, USA, 1990.

9.

J. Milgram, inventor; General Tire Corporation, assignee; US Patent 3,278,457, 1966.

10. R. J. Belner, inventor; General Tire Corporation, assignee, US Patent 3,278,458, 1966. 11. R. J. Herold, inventor; General Tire Corporation, assignee, US Patent 3,278,459, 1966. 12. R. J. Herold and R. A. Livigini, Polymerization Kinetics and Technology, Ed., N. A. J. Platzer, Advances in Chemistry Series No.128, American Chemical Society, Washington, DC, 1973, 208. 13. C. P. Smith, J. W. Reisch and J. M. O’Connor, Presented at the Polyurethane World Congress, 1991, Nice, France, p.313. 14. N. Barksby, G. L. Allen, Presented at the Polyurethane World Congress, 1993, Vancouver, BC, Canada, p.445. 15. N. Barksby, S. D. Seneker and G. L. Allen, Presented at the Polyurethane Manufacturing Association Conference, Pittsburgh, PA, November 1995. 16. S. D. Seneker and N. Barksby, Presented at the Utech 96 Conference, The Hague, The Netherlands, March 1996, Paper No.46. 17. N. Barksby, S. D. Seneker and G. L. Allen, Urethanes Technology, 1996, 13, 1, 36.

462

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes 18. R. J. Tuinman, T. L. Fishback and C. J. Reichel, Presented at the Polyurethanes Expo ’98 Conference, Dallas, TX, 1998, p.535. 19. S. D. Seneker, N. Barksby and B. D. Lawrey, Presented at the Polyurethanes Expo ’96 Conference, Las Vegas, NV, 1996, p.305. 20. S. D. Seneker, C. C. Shen and N. Barksby, Presented at the Polyurethanes World Congress ‘97, Amsterdam, The Netherlands, 1997, p.568. 21. S. D. Seneker, C. C. Shen and N. Barksby, Presented at the Polyurethane Manufacturing Association Conference, San Diego, CA, March 1998. 22. R. J. Tuinman, R. L. Fishback and C. J. Reichel, Presented at the Polyurethanes World Congress ‘97, Amsterdam, The Netherlands, 1997, p.560. 23. T. L. Fishback and C. J. Reichel, Presented at the Polyurethanes Expo ‘96 International Conference, Las Vegas, NV, 1996, p.282. 24. A. T. Chen, R. R. Wells, C. P. Smith, J. W. Reisch, M. M. Emmet and J. M. O’Connor, Presented at the Polyurethanes World Congress 1993, Vancouver, BC, Canada, 1993, p.388. 25. C. P. Smith, J. W. Reisch and J. M. O’Connor, Journal of Elastomers and Plastics, 1992, 24, 4, 306. 26. ASTM D4060-95, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser. 27. S. D. Seneker, N. Barksby and B. D. Lawrey, Presented at the Polyurethanes Expo ’98 Conference, Dallas, TX, 1998, p.195. 28. S. D. Seneker, N. Barksby and G. B. Ellerbe, Presented at the Utech Asia ‘97 Conference, Suntec City, Singapore, 1997, Paper No.41.

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APPENDIX

Laboratory Preparation of 2,4-TDI and 4,4´-MDI Prepolymers Isocyanate prepolymers were prepared in glass reaction vessels in a laboratory fume hood. The reaction vessel was equipped with a stirrer, temperature controller and nitrogen inlet. The reaction was carried out under a dry-nitrogen atmosphere to minimise exposure to atmospheric moisture and polyol oxidation. Isocyanate is added first to the reaction vessel and heated to approximately 50 to 60 °C. Polyol is then added to the isocyanate, carefully controlling the addition rate to ensure the reaction temperature does not exceed 85 °C. Higher temperatures can cause harmful side reactions that can affect prepolymer performance. After reacting at 80 °C for approximately 4 hours, the extent of reaction was determined by measuring the percent isocyanate content (% NCO) of the prepolymer. Heating was continued until the prepolymer was slightly below the theoretical % NCO value.

Laboratory Casting of 4,4´-MDI Prepolymers Cured with BDO MDI/BDO cast-elastomer systems were made by mixing and reacting two components, the MDI-terminated prepolymers (described previously) and BDO. The elastomers in this chapter were prepared using the hand-mixing procedure. The MDI prepolymer was preheated to ~65 °C and degassed under vacuum (130 to 665 Pa) until foaming stopped. Catalyst (25-50 ppm dibutyltin dilaurate) was added to the prepolymer and mixed thoroughly to give the desired pot life of 1 to 3 minutes. The BDO was added (isocyanate to hydroxyl ratio (NCO:OH) of 1.03) and mixed thoroughly using a Jiffy mixer until the blend was homogeneous. This solution was immediately poured into preheated moulds at 100 °C treated with mould release. After demoulding, the samples were post-cured at 100 °C for 16 hours. The polymer samples were conditioned at 23 °C and 50% relative humidity for at least four weeks prior to testing.

Laboratory Casting of One-Shot Elastomers Based on CarbodiimideModified MDI, Polyol, and BDO One-shot elastomers were prepared by mixing and reacting three components: carbodiimide-modified MDI (CD-MDI), high primary-hydroxyl-content polyol and BDO. The polyol was preheated to 60 °C and degassed under vacuum (130 to 665 Pa). The 465

Advances in Urethane Science and Technology CD-MDI, polyol and BDO were charged to a 250 ml glass jar. Dibutyltin dilaurate catalyst (25-50 ppm) was added to adjust the pot life to approximately 90 seconds. These components were mixed thoroughly and poured into preheated moulds at 100 °C, treated with mould release. After demoulding, the samples were post-cured at 100 °C for 16 hours. The polymer samples were conditioned at 23 °C and 50% relative humidity for at least four weeks prior to testing.

Laboratory Casting of 2,4-TDI Prepolymers Cured with MBOCA The TDI/MBOCA cured elastomers were prepared by mixing and reacting two components: 2,4-TDI prepolymers (described previously) and MBOCA. The elastomers in this chapter were prepared using the hand-mixing procedure. The 2,4-TDI prepolymer was preheated to 90 °C and degassed under vacuum (130 to 665 Pa) until foaming stopped. The MBOCA was melted at 120 °C. The 2,4-TDI prepolymer and MBOCA were stirred thoroughly using a Jiffy mixer at an NCO:NH ratio of 1.05 without catalyst. The solution was immediately poured into preheated moulds at 100 °C treated with mould release. After demoulding, the samples were post-cured at 100 °C for 16 hours. The polymer samples were conditioned at 23 °C and 50% relative humidity for at least four weeks prior to testing.

Laboratory Moisture-Curing of 2,4-TDI Prepolymers The 2,4-TDI prepolymers were cast as films on glass plates using a 0.762 mm drawdown bar (a draw-down bar is a metal bar with a gap, which is pulled down a glass plate leaving a thin film of prepolymer. The prepolymer is then moisture-cured to form the polymer film). The films were cured/conditioned at 23 °C and 50% relative humidity for at least four weeks prior to testing.

Laboratory Preparation of Aqueous Polyurethane/Urea Dispersions using the Prepolymer Mixing Process Isocyanate prepolymers were prepared in glass reaction vessels in a laboratory fume hood. The reaction vessel was equipped with a stirrer, temperature controller, and nitrogen inlet. The reaction was carried out under a dry-nitrogen atmosphere to minimise exposure to atmospheric moisture and polyol oxidation. The stepwise procedure is: Charge polyol, DMPA, (stabiliser), NMP, (coalescing aid) in a reaction vessel equipped with a thermocouple (thermometer) and agitator. Care should be taken to keep 466

Ultra-Low Monol PPG: High-Performance Polyether Polyols for Polyurethanes moisture away from the reaction mixture by using a drying tube or a positive pressure of dry nitrogen. Heat the mixture to 95 °C and apply vacuum to remove any residual water (DMPA and polyol stored in the laboratory may contain high water levels). Cool the mixture to 50-60 °C, then weigh in isocyanate (IPDI, etc.) Heat the mixture to 80 to 100 °C for aliphatic diisocyanates or 60 to 80 °C for aromatic diisocyanates. If necessary, a small amount of a tin catalyst, (e.g., 25 to 100 ppm of stannous octoate) can be added. React this mixture until the % NCO content is 0.2-0.5 units lower than the theoretical % NCO. Reaction time is usually 6 to 12 hours depending on the % NCO of the prepolymer. Cool the mixture to 50-60 °C, and then add neutraliser, i.e., triethylamine, to the prepolymer. Mixing is critical during this step. The neutralisation forms the waterdispersible carboxylate ammonium salt. Heat the prepolymer amine salt to 60 to 80 °C and slowly pour into a resin kettle charged with a known amount deionised water at 25 to 50 °C with strong agitation. Add the prepolymer to give the desired solids content. Immediately add the chain extender (ethylene diamine), which has been diluted in water slowly to the aqueous polyurethane/urea dispersion. Typically chain extend only 8090% of the actual free NCO groups because some will be lost due to reaction with water. As an alternative, the amine extender may be added to the water prior to dispersing the neutralised prepolymer. Cook out the residual excess isocyanate by heating the aqueous dispersion at 60-80 °C for two hours. Excess unreacted isocyanate in an aqueous polyurethae/urea dispersion will lead to carbon dioxide gas and pressure buildup in the container.

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Abbreviations and Acronyms

ABA ABS AN APCI APS ASD ASTM AVG BA BD BDO CD-MDI CFC CHL CHP COM CP CS DAPS DC DEE DEG DEOA DEOA-LF DMA DMC DMPA DMSO DMTA DN DPG DPUR

Alternative blowing agents Acrylonitrile butadiene styrene Acceptor number Air Products and Chemicals, Inc. Aminopropyl triethoxy silane Acrylic-styrene dispersion American Society for Testing and Materials Average Butyl acrylate 1,4-butadiene 1,4-butanediol Carbodiimide-modified MDI Chloroflurorocarbons Chloroform Cumene hydroperoxide Commercial Cyclopentane Chloropropyl triethoxy silane Data acquisition and plotting system Dabco Diethyl ether Diethylene glycol Diethanolamine Diethanolamine liquid form Dynamic mechanical analysis Double metal cyanide Dimethyl propionic acid Dimethyl sulphoxide Dynamic mechanical analysis Donor number Dipropylene glycol Polyurethane dispersions

469

Advances in Urethane Science and Technology DSC ECR EO ES FEA FR FTC FT-IR GC GPC GWP HACS HCFC HDPE HER HER-TG HFC HP HQEE id IFD IGC ILD IPDI IPN IR KOH LD-SRIM LFI-PU MBOCA MDI MDPE MDPUR MEK MFFT MHW MM Mn

470

Differential scanning calorimetry Energy consumption reduction Ethylene oxide Epoxybutyl trimethoxy silane Finite element analysis Fire retardant Force-to crush Fourier transform infra-red spectroscopy Gas chromatography Gel permeation chromatography Global warming potential Humid aged compression sets Hydrochloroflurocarbon High density PE Bis-(beta-hydroxyethyl) ether of resorcinol HER Technical grade Hydrofluorocarbons High purity Hydroquinone (di-beta-hydroxyethyl) ether Internal diameter Indentation force deflection Inverse gas chromatography Indentation load deflection Isophorone diisocyanate Interpenetrating polymer network Infrared Potassium hydroxide Low density structural RIM Long fibre injection PU Methylene bis- (2-chloroaniline) Methylene diphenyl isocyanate Medium density PE Modified polyurethane Methyl ether ketone Minimum film formation temperature Modified hot wire Methyl methacrylate Number average molecular weight

Abbreviations and Acronyms MOCA mp MS MS MW MWD NCO ND nd NEG NM NMP NMR OCF ODP OEM OH# OTR PAN Pbw PDMS PE PEG PET PG PLC PO PPG PPH pphp ppm PPUR PS PTMEG PTMG PU, PUR PVC PVDC

Methylene bis – (2-chloroaniline) Melting point Mass spectrometer Mercaptobutyl trimethoxy silane Molecular weight Molecular weight distribution Isocyanate Not detected Not determined Non-evaporable getter Not measured N-methyl pyrrolidone Nuclear magnetic resonance Owens Corning fibre glass Ozone depletion potential Original equipment manufacturer Hydroxyl number Oxygen permeation rate Polyacrylonitrile Parts by weight Polydimethylsiloxane Polyethylene Polyethylene glycol Polyethylene terephthalate Propylene glycol Programme logic controller Propylene oxide Polyoxypropylene glycol Parts per hundred Parts per hundred polyol Parts per million Polyurethane prepolymer Polystyrene Polytetramethylene ether glycol Polytetramethylene glycol Polyurethane Polyvinyl chloride Polyvinylidene chloride

471

Advances in Urethane Science and Technology QA QC R&D RGA RH RIM RRIM RT RUF S SAE SD SEM SLM SRG SRIM SSF STP TCP TDI TEDA TEM Tg TG THF TOC TP TPG TPR TPU VIP VOC VPF VS WTR

472

Quality assurance Quality control Research and Development Residual gas analysis Relative humidity Reaction Injection Moulding Reinforced reaction injection moulding Room temperature Recycled urethane fluff Styrene Society of Automotive Engineers Standard deviation Scanning electron microscope Standard litres per minute Spinning rotor guage Structural RIM Silica surfactant Standard temperature and pressure Trimethylsilyl-capped polysilicate Toluene diisocyanate Triethylene diamine Transmission electron microscopy Glass transition temperature Technical grade Tetrahydrofuran Top-of-the-cup Thermoplastic Tri propylene glycol Time pressure release Thermoplastic PU Vacuum insulated plate Volatile organic compounds Variable pressure foaming Vinyl trimethyl silane Water permeation rate

Contributors

Gary D. Andrew Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown, PA 18195 USA Nigel Barksby Bayer Corporation South Charleston Technical Center 3200 Kanawha Turnpike Building 727 South Charleston WV 25303 USA Richard J. Braun Department of Mathematical Science University of Delaware Newark DE 19716 USA Georg Burkhart Goldschmidt AG Goldschmidtstrasse 100 D-45127 Essen Germany Raj B. Durairaj Director of Research Indspec Chemical Corporation Pittsburgh PA 15238 USA

Daniel Klempner Polymer Institute University of Detroit Mercy 4001 W. McNichols Road Detroit MI 48219-0900 USA Jane G. Kniss Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown, PA 18195 USA Anita Koncka-Foland Industrial Chemistry Research Institute 8, Rydygiera Street 01-315 Warsaw Poland Janusz Kozakiewicz Industrial Chemistry Research Institute 8, Rydygiera Street 01-315 Warsaw Poland Bruce D. Lawrey Bayer Corporation South Charleston Technical Center 3200 Kanawha Turnpike Building 727 South Charleston WV 25303 USA

473

Advances in Urethane Science and Technology Izabella Legocka Industrial Chemistry Research Institute 8, Rydygiera Street 01-315 Warsaw Poland Mark L. Listemann Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown, PA 18195 USA Paolo Manini SAES Getters SpA Viale Italia 77 20020 Lainate (MI) Italy Lisa A. Mercando Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown, PA 18195 USA Shailesh Naire Department of Mathematical Science University of Delaware Newark DE 19716 USA Benjamin M. Nugent Central Research and Development Mail # CO43C1 Dow Corning Corporation Midland MI 48686-0994 USA Udo C. Pernisz Central Research and Development Mail # CO43C1

474

Dow Corning Corporation Midland MI 48686-0994 USA Arturo Plana Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown PA 18195 USA H.P. Schreiber Chemical Engineering Department Ecole Polytechnique P.O. Box 6079 Stn. Centre-Ville Montreal Quebec Canada H3C 3A7 Stephen D. Seneker Anderson Development Company 1415 E Michigan street Adrian MI 49221 USA Ashok Sengupta Corporate Research & Development 3M Canada Company London Ontario Canada N6A 4T1 Jan Skarzynski Industrial Chemistry Research Institute 8, Rydygiera Street 01-315 Warsaw Poland

Contributors Steven A. Snow Central Research and Development Mail # CO43C1 Dow Corning Corporation Midland MI 48686-0994 USA Robert E. Stevens Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown PA 18195-1501 USA Max Taverna Cannon Group Via C Colombo 49 20090 Trezzano s/Naviglio Milano Italy

James D. Tobias Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown, PA 18195 USA Andreas Weier Degussa AG Karl-Arnold Plazt 1a D-40474 Dusseldorf Germany Stephan Wendel Air Products and Chemicals, Inc. Robert Koch Strasse 27 22851 Norderstedt Germany

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Author Index

Adam, S.J. 210 Akabori, K. 215, 257 Akita, J. 211 Akkurt, A.S. 211 Allen, G.L. 462 Allen, K.W. 332 Alonzo, V. 334 Andrew, G.D. 82 Arkles, B. 91, 110 Armistead, J.P. 111 Artavia, L.D. 215, 257 Ashby, M.F. 82 Bachelor, G.K. 259 Baines, F.L. 258 Bankoff, G.B. 259 Barksby, N. 462, 463 Barosi, A. 210 Battice, D.G. 83 Bayer, O. 85 Bechara, I. 333 Belner, R.J. 462 Benson, D.K. 212 Berger, E.J. 366 Beu, T.A. 209 Biesmans, G. 208, 210 Biondich, B. 154 Blackwell, J. 419 Boffito, B. 210 Boffito, C. 210, 211 Bohman, R.H. 212 Bonansea, A. 155 Bonart, R. 419 Bonekamp, J. 176, 210 Borghi, M. 210 Borgogelli, R. 111

Born, L. 419 Boudreau, R.J. 111, 256 Bovenkerk, H.P. 208 Brandrup, J. 210 Braun, J.M. 366 Braun, R.J. 259, 260 Breck, D.W. 210 Brenner, H. 259 Brochhagen, F.K. 109 Brookman, D.J. 367 Bundy, F.P. 208 Burchianti, J. 417 Burkhart, G. 82, 102, 109, 111 Cairncross, R.A. 260 Caloi, R.M. 211 Capone, D.M. 462 Carley, J. 210 Carson, S. 110 Cavenaugh, G.C. 110 Cavender, K.D. 82 Chang, Y-S. 366 Charles, A. 110 Chaya, J.E. 109 Chen, A.T. 463 Chen, J.H. 339, 366 Chen, L.W. 418 Childs, K.W. 212 Christfreund, A. 209 Chu, N.S. 462 Clift, S.M. 418 Clunie, J.S. 257 Coogan, R.G. 332 Cooper, S.L. 418, 419 Cornelius, G.C. 334 Corradi, P. 154

477

Advances in Urethane Science and Technology Covington, J. 210 Creswick, M.W. 111 Cunningham, A. 209 Czalych, A.E. 319, 334 Dahm, M. 256 Dai, D.G. 109 David, C.E. 210 Davis, H.T. 110, 111 Davis, J.H. 418 Davis, S.H. 259 de Almeida, J. 417 De Vos, R. 209, 210, 211 De Vries, A.J. 258 Decker, T.G. 91, 92, 110, 256 della Porta, P. 210 Deng, Z. 337, 366 Deschaght, J. 211 Desor, U. 333 Dieterich, D. 333 Dietrich, K.W. 209 Dillard, D.A. 366 Dixon, C. 192, 211 Doni, F. 210 Douglas, T.L. 418 Doyle, E.N. 110 Doyle, J.E. 418 Droste, W. 109 Drye, J.L. 110 Dubjaga, J.E. 256 Dubyaga, E.G. 256 Durairaj, R.B. 417 Edwards, D.A. 259 Eiben, R.G. 110 Ellerbe, G.B. 463 Emmet, M.M. 463 Eschbach, C.S. 258 Estes, G.M. 419 Evans, D.F. 257 Feinerman, A.E. 256 Fenton, W.N. 257 Ferrario, B. 209, 210

478

Figini, A. 210 Fine, H.A. 208, 212 Fiorentini, C. 110, 154 Fishback, T.L. 463 Florianczyk, Z. 333 Francais, E. 208 Frankel, S.P. 214, 257 Franyutti, S. 418 Fremerey, J.K. 211 Fricke, J. 208 Frisch, K.C. 417 Fujimoto, K. 215, 257 Gabbard, W.A. 210 Gasparini, G. 209 Gee, G. 461 Gent, A. 336, 366 Gentle, T.E. 259 George, R.F. 417 Geurts, J.M. 334 Giannopous, N. 334 Gibson, L.J. 82 Gibson, P.E. 418 Gilbert, R.G. 333 Giordano, S. 334 Glicksman, L.R. 160, 208, 209 Goddard, E.D. 256 Goldsmith, J. 332 Goodman, J.F. 257 Goos, H.C. 332 Grabbe, M. 332 Graff, G. 155 Graves, R.S. 208 Gray, D.G. 366 Grickova, I.A. 283, 287, 306, 311 Griffith, M. 110 Griffiths, T. 110 Grizwold, A.A. 109 Gruber, B. 266, 333 Guagliardo, M. 332 Güclü, H. 211 Guillet, J.E. 366

Author Index Gutmann, V. 337, 349, 357, 359, 366 Hamann, H. 256 Hamilton, A.J. 199, 212 Harper, S.D. 462 Hartung, M. 332 Hashimoto, H. 333 Haworth, G.J. 212 Hegedus, C.R. 266, 334 Heimenz, P.C. 259 Hepburn, C. 418 Hermsen, A. 334 Herold, R.J. 462 Herrington, R.M. 258 Hesketh, T.R. 418 Hespe, H. 419 Higginson, W.C.E. 461 Hildebrand, J.H. 111 Hilker, B.L. 111 Hill, R.M. 255 Hilyard, N.C. 209 Höchtlen, A. 109 Hock, K. 258 Hoogendoorn, P. 208 Hoppe, P. 109 Hosoi, K. 333 Hu, C.B. 419 Huber, L.M. 111 Hudales, J.B.M. 258 Hutzinger, O. 109 Huybrechts, J.T. 332 Immergut, E.H. 210 Ingram, B.T. 257 Ishida, H. 367 Ivanov, I.B. 257, 258 Jensen, O.E. 260 Juran, R. 210 Kanner, B. 91, 92, 231, 256, 258 Kanner, T.G. 110 Keane, N.W. 209 Kendrick, T.C. 110, 256 Khemani, K.C. 111

Kheshgi, H.S. 242, 259 Kim, D. 417 Kingston, B.M. 110, 256 Kistler, S.F. 259 Klempner, D. 417 Klincke, M. 82, 111 Kloiber, K.A. 266, 334 Koberstein, J.T. 418 Koczo, K. 258 Koczone, J.K. 258 Kodama, K. 160, 209 Kollmeier, H.J. 109, 257 Komarova, A.B. 256 Kopusov, L.I. 256 Kossmann, H. 333 Kovac, J. 211 Kozakiewicz, J. 332, 333, 334 Kryszewski, M. 332 Kücükpinar, E. 202, 211 Kulkarni, R.D. 256 Kuropka, R. 333 Kushner, S.A. 82 Kuyper, J. 462 Lai, Y-H. 366 Lamb, W. 175, 210 Langenhagen, R. 111 Lassila, K.R. 83 Lavielle, L. 367 Lawler, L.F. 111 Lawrey, B.D. 463 Lean, B. 333 Lee, K.D. 111 Legocka, I. 334 Lerner, L.E. 418 Leung, L.M. 418 Lewandowski, L.H. 366 Leyden, D.E. 367 Lidy, W. 109, 257 Lilaonitkul, A. 418 Lin, J.R. 418 Lin, J.Y. 209

479

Advances in Urethane Science and Technology Lindenau, B.E. 211 Lipatova, T.E. 256 Listemann, M.L. 82, 83 Livigini, R.A. 462 Lloyd, D.R. 366 Lloyd, N.C. 110, 256 Locatelli, M. 334 Loewrigkeit, P. 333 Loos, F. 333 Lopes, W.I. 83 Luca, J. 154 Lucas, H.R. 334 Lupinacci, J. 212 Macosko, C.W. 110, 111, 215, 257 Madan, S. 418 Magnano, E. 211 Maldarelli, C. 230, 258, 259 Malone, B. 209 Manini, P. 209, 210, 211 Mansfield, K.F. 82 Mapleston, P. 154, 155 Mark, H.B. 210 Markusch, P.H. 333 Martin, C. 367 Martin, E. 334 Maruscelli, E. 332 Mascioli, R.L. 462 Masuda, Y. 209 Mathis, N.E. 192, 211 Mattson, J.S. 210 Mayenfels, P. 332 McCullogh, D.W. 209 McElroy, D.L. 208 McVey, S.B. 111 Mealmaker, W.E. 334 Mecea, V. 209 Mendelsohn, M.A. 417 Mercea, P.V. 209 Merriman, P. 110 Mestach, D.E. 333 Milgram, J. 462

480

Miller, C.A. 257 Minnich, K.E. 83 Monteiro, M.J. 333 Moraja, M. 211 Morbitzer, L. 419 Moriarty, J.A. 245, 259 Moser, K. 109 Muller, E.H. 419 Muresan, L. 209 Muroi, S. 333 Murphy, G.J. 258 Mysels, K.J. 214, 245, 246, 257 Nachtkamp, K. 332, 333 Nagarajan, M.R. 419 Naire, S. 259, 260 Narducci, E. 211 Navish, F.W. 417 Neff, R.A. 110, 257 Neogi, P. 257 Nikolov, A.D. 110 Nugent, B.M. 258, 260 O’Connor, J.M. 462, 463 Oertel, G. 418 Ondrey, S.J. 417 Oron, A. 259 Overbeek, A. 334 Overbeek, G.C. 332 Owen, M.J. 93, 110, 214, 256, 257, 258 Özkadi, F. 211 Palinkas, R. 418 Palmer, M. 418 Palumbo, R. 332 Pantone, R.S. 418 Panzer, U. 366 Papirer, E. 337, 366 Pastore, G. 211 Pcolinsky, M. 109 Penczek, P. 332 Penczek, S. 333 Pendergast, P. 209 Pernisz, U.C. 259, 260

Author Index Perrut, M. 208 Peters, F. 155 Petersen, I.H. 110, 256 Petrella, R.G. 82 Piccinini, M. 334 Pile, B. 154 Pionteck, J. 418 Platzer, N.A.J. 462 Plueddemann, E.P. 366 Pogany, G.A. 462 Pohlers, A. 418 Pompe, G. 418 Potschke, P. 418 Potter, T.F. 212 Prausnitz, J.M. 111 Price, C.C. 462 Prokai, B. 258 Pugh, R.J. 257 Qin, R. 360, 361, 362, 365 Quay, J.R. 419 Randall, D. 208, 209 Recker, K. 418 Redhead, P.A. 209 Reed, D. 255 Reegen, S.L. 417 Reichel, C.J. 463 Reid, W.G. 110, 256 Reisch, J.W. 462, 463 Ricciardi, R. 109 Rimmer, R.W. 332 Ritter, J. 256 Rizzi, E. 210 Roester, R.R. 332 Rosai, L. 210 Rosbotham, I.D. 209, 210, 211 Rossmy, G. 94, 109, 256, 257 Rosthauser, J.W. 332, 333 Roth, A. 209 Roy, R.V. 260 Ruckenstein, E. 339, 366 Rumschitzki, D. 259

Ruschak, K.J. 242, 259 Saint Flour, C. 337, 366 Sancrotti, M. 211 Sand, J.R. 212 Sanger, G. 256 Savoca, A.C.L. 82 Sawyer, D.T. 367 Schator, H. 109, 257 Schiabel, A. 211 Schlöns, H-H. 109 Schneider, N.S. 419 Schreiber, H.P. 337, 338, 341, 342, 344, 347, 359, 360, 361, 362, 366, 367 Schuchardt, J.L. 462 Schultz, J. 336, 366, 367 Schwartz, L.W. 259, 260 Schwartz, M. 333 Schwarz, E.G. 256 Sciuccati, F. 209 Scott, R.L. 111 Scriven, L.E. 93, 110, 259 Seneker, S.D. 462, 463 Sengupta, A. 338, 341, 342, 344, 347, 359, 360, 361, 362, 365, 367 Seyffert, H. 256 Seymour, R.W. 418, 419 Sheinina, L.S. 256 Shen, C.C. 463 Shinoda, K. 214, 257 Simons, D.M. 461 Simpson-Gomes, A. 333 Sironi, E. 334 Slattery, J.C. 259 Smak, Y.W. 334 Smith, C.P. 462, 463 Smith, L.K. 212 Smith, S. 211 Smock, D. 154, 155 Smudin, D.J. 109 Snow, S.A. 251, 255, 257, 258, 259, 260 Snyder, W.H. 462

481

Advances in Urethane Science and Technology Solomou, N. 208 Soyal, A. 211 Srikanth, R. 212 Stannett, V. 366 Stark, J. 155 Stebe, K.J. 230, 258 Stein, H.N. 225, 258 Sternling, C.V. 93, 110 Stevens, R.E. 255, 260 Stone, H. 109 Stone, H.A. 259 Stovall, T. 212 Strong, H.M. 208 Subramaniam, N. 333 Sugiyama, A. 190, 209 Sung, W.F. 209 Tabor, R.L. 208 Tada, H. 209 Tailor, K.J. 461, 462 Tanimoto, Y. 209 Tao, W.H. 209 Tarakanov, O.G. 256 Taverna, M. 110, 154, 155 Taylor, J.R. 333 Tennebroek, R. 334 Thomas, G. 110 Tiedemann, J.E. 417 Tirpak, R.E. 333 Tobias, J.D. 82 Trenholme, H.W. 461 Tuck, E.O. 259 Tuinman, R.J. 463 Turner, R.B. 111, 258 Urso, C. 211 Valdrè, S. 211

482

van Bogart, J.W.C. 418 van der Hulst, H. 462 Van Dyk, K.A. 333 Vartan-Boghossian, R. 332 Veltkamp, B. 208 Vengerovskaya, S.H.G. 256 Verbanc, J.J. 461 Vineyard, E.A. 212 Vratsanos, M.S. 82 Wacker, W. 209 Wagner, R.D. 109 Ward Jr., R.S. 419 Ward, T.C. 366 Wasan, D.T. 110, 258, 259 Weaver, F.J. 208, 210 Weidner, D.E. 260 Weier, A. 111 Wells, R.R. 463 Wennertröm, H. 257 White, L. 154 Wiemann, M. 109, 257 Wilhelm, M.R. 418 Wilkes, G.L. 111 Wilkes, K.E. 210, 212 Wittbecker, E.L. 462 Wong, H. 259 Wressell, A.L. 82 Yarbrough, D.W. 208 Yasunaga, K. 110, 257 Yoshimoto, M. 209 Yuge, K. 209 Zeiler, R. 175, 210 Zellmer, V. 109, 111 Zhang, X.D. 93, 110, 111, 257 Zharkov, V.V. 256

Main Index

Main Index

A Acclaim elastomers 446 Acclaim polyol 423, 429, 435, 438, 445, Acclaim polyols Acetone process 265 Acid copolymers 178 Acid/base interactions 347 Acrylic dispersions 261 Acrylic polymer 262 Activated charcoal 181 Additives 3, 15, 16, 42, 63, 64, 67, 73 Dimensional stability 3, 16, 64 Flexible slabstock low emission 73 For TDI 16 Low emission dimensional stability 42 in MDI 64, 67 MDI flexible moulded foam 63 TDI flexible moulded 15 Adhesion 218, 325, 337, 344, 353 Acid/base interaction 353 Bond properties 352 Determination 277 Performance 344 Polyurethane 355 Strength 344 Tests 350 Adhesion promoter 355, 364 Silane 355 Adhesive Ageing 362 Air flow values 12 Alloy 182 Barium-lithium 182 Aluminium 168 Sputtered 168

Analysis 174, 189, 195, 336, 373, 395, 403, 405, 410, 412 Differential scanning calorimetric 373, 381, 383, 393-395, 405, 406, 410 Dynamic mechanical 373, 383, 395, 396, 405, 407, 412 Finite element 174, 196 Fourier transform-infra red 373, 403 Gel permeation chromatography 450 Head space 14, 62 Inverse gas chromatography 345, 347-349, 354, 357-358, 365 Residual gas 189, 195 Surface 336, 338 Anisotropy 160 Cellular 160 Appliance 198 Household 198 Applications 135, 187 Industrial 135 Shipping containers 187 Arcol 7 ASTM 15 D3574, Indentation force deflection test 15 Test methods 4

B Bag 158 Gas barrier 158 Barrier 168, 174, 198, 206 Aluminium foil-based 198

483

Advances in Urethane Science and Technology Gas 168 Metallised 174, 206 Multilayered plastic 206 Film 167 Vacuum properties 167 Properties 168 Bashore rebound 373 Beamech CO-2 90 Blister detachment 350, 352, 353, 354 Block foaming 161 Blowing agents 4, 87, 88, 136, 178 Alternative 87 Carbon dioxide 87, 88 CFC-11 87 Chloroflurorcarbon 187 Physical 87 Bond strength 337, 360, 361, 364 Boundary 240 Conditions 240, 245 Film-air 240 Bubble coalescence 214 Bubble formation. See Nucleation Bulk stability 13 Bulk viscosity 217 Butyl acrylate 269

C Calcium oxide 182 CannOxide 129 Capillary number 241 Carbon dioxide 88, 128, 166 Liquid 88 Carbon monoxide 166 CarDio 88, 89, 90, 92 Carrier 122 Mould 122, 123 Cast elastomers 372, 377, 382, 391, 398402, 409, 411 Hard segment content 401 Heat ageing 409, 411 Mechanical properties 409

484

Preparation 372 Properties 398, 402 Soft segment content 401 Cast poly(ester urethanes) 390 Cast poly(ether urethanes) 375 Cast polyurethanes 379, 380, 392, 393, 397, 401, 404, 413 Analysis 403 Hard segments 379 Hardness 392, 393 Heat aged 413 High hardness 401 HER/HQEE blends 397 Properties 392 Soft segments 379 Tensile properties 380 Catalyst 16, 43, 52, 75, 81 Activity 75 Cell opening 16, 52 Cell opening blowing 43 Non-fugitive 43, 52, 81 Selectivity 75 Cell opening 4, 93-94, 214, 217 Blow off 94 Cell structure 101, 107 Cell wall drainage 4 Chain extenders 370 Chamber 118 Mixing 118 Chem-Trend 7 Chem-Trend PRC-798 9 Chemical foam model 75 Flexible cellular polyurethane 4 Chicken buckets 9 Chromatography 276, 337 Gel permeation 276 Inverse gas 337, 345. See also IGC Closed cells 162 Coalescing agent 297-300 NMP 297-300 Coating 277 Binder 264

Main Index DPUR 284 Drying time 277 Properties 292-293, 300, 303, 305, 320, 322-326 Coating Coatings 456 Aqueous polyurethane/urea dispersion 456 Coefficient 232 Diffusion 232 Surface partition 232 Cohesion 218, 220 Intermolecular 218 Failure 352 Collapse 23 Collapsing wedge model 226 COMBOGETTER 181-187, 202, 203, 207 for VIP 182 Comfort properties 3 Compression set 373, 380, 390, 401 Tests 3 Container 206 Shipping 206 Contamination 165 System 165 Control 119 Pour pressure 119 Conveyor 126 Oval 126 Cooling 87 Lateral 87 Creep 161 Crosslinking 278 Degree of 278 Density 279 Crusher 9 Black Brothers Roller 9 Cumene hydroperoxide 269

D Dabco 6, 80

Deforming surface 236 Dehydrohalogenation 42 Thermally induced 42 Density 17 Differential scanning calorimetry 373, 381, 393 Diffusion 214, 215, 219, 220, 240 Diffusion coefficient 232, 234 Dimensional stability 11-12 Dispersion 264, 266, 267, 276, 279 Acrylic 264, 267 Characterisation 279 Hybrid 267 Hybrid acrylic-urethane 266 Polyurethane 264 Properties 323, 324, 325 Systems 263 Hybrid 263 DMA see Analysis DMTA 427, 433, 452, 453, 454, 458 DPUR particle 312, 315 Swelling of 312, 315 Drainage rate 218, 219, 220, 229, 236 DSC see Analysis Dynamic creep testing 3 Dynamic fatigue 14, 49 Dynamic mechanical analysis see Analysis

E EasyFroth 129 Edge effect 174 Effect 226 Bulk 226 Surface 226 Elastomer processability 424, 438 One-shot 438 Elastomers 385, 424, 433 MDI/BDO cured 424 rebound 433 Thermal stability 385 Electron microscopy 277

485

Advances in Urethane Science and Technology Transmission 277 Embedded sphere 329 Emissions 4 Emulsion polymerisation 265 Energy 218 Cohesive 218 Efficiency 157 Envirocure 87 Environmental concernS 86 Equipment 114 Metering 114, 115

F FEA See Analysis Filler 158 Aerogels 158 Compressed powder 158 Fibre 158 Fibre glass 158 Perlite 158, 189 Precipitated silica 189 Silica 158 Film 134, 178, 213, 215-217, 219, 221, 226, 228, 231, 278, 317 Adhesive 134 Bulk viscosity 215 Crosslink density 327 Drainage rate 216, 217, 226, 227, 228 Formation 223 Formulation 221 Free surface 239 Mechanical properties 278 Mobile surface 217 Models 239 Deforming 239 Outgassing 178 Properties 291, 293, 299, 302, 304, 308-309, 311, 320-321, 323-325 Releasing 134 Rigid surface 216, 228, 231

486

Film rupture 215, 216, 218, 221 Film stability 235 Surface free energy 317 Tangentially-immobile 242 Thin liquid 213 Thin liquid polyurethane 219 Wedge 236 Collapsing 236 Film drainage 219, 255 Rate 227-229, 232, 235 Surface variables 228 Vertical liquid 255 Fingering pattern 223, 254 FipurTec 139 Fire-retardants 96, 97, 98, 108 Flexible moulded foam 14, 46, 63, 73 MDI 14, 68, 73 TDI 63 Flexible slabstock 73-74, 88, 90, 95, 99 Polyether 88 Polyurethane 229 FlexiDrum 126 Flux 219, 227, 230 Foam 74, 76, 78-80, 85, 88, 107, 128, 157159, 161, 163, 177, 179, 201, 213, 254 CFC-free 157 Cooling 87 Density 86 Hardness 86 Minimum 86 Encapsulating 177 Evacuated 161 Flexible slabstock 75 Formation 86 Hardness 87 Soft 87 MDI flexible 79 Moulded 76 Mechanical properties 161 Multi-density 128 Multi-hardness 128 Open cell 163, 179, 201

Main Index Open cell PU 158, 163 Physical properties 163 Polyether 88 Polystyrene 159 Polyurethane 85, 96, 213 Porosity 161 Rise 93 Stabilisation 93 Slabstock 80 TDI flexible 79 Moulded 76 Slabstock 74, 78 Foam & film 133 Foam One 87 Foaming 87, 90, 128 Carousel 128 In situ 128 Liquid carbon dioxide 90 Variable pressure 87 Fogging 14, 62 Force to crush 3, 23, 66, 71, 72 Formulations 72 MDI 72 Fracture energy 390 Freezer 177, 204 Ultra-low temperature 204 Froth 101, 213 Density 101 Gelatinous 213 FTC see Force to crush Full rise time 15

G Gas permeation 168, 176, 177 Flanges 176, 177 Gas transmission 168 Gate Bar 88 Gauge 192 Spinning rotor 192 Gel time 15 Getter 158, 170, 181, 183, 185, 193

Device 179 Material 181 Non-evaporable 170 SpiroTorr 193, 194, 201 Technology 206 Gibbs-Marangoni 93 Global warming 157 Goniometer 336 Rame-Hart 336 GPC See Analysis Gradient 230 Gravitational energy 214 Gravity convection 225 Greenhouse effect 157 Guarded hot-plate 191

H Handmix procedure 8 Mass-loss 18 Flexible moulded foam 8 Flexible slabstock foam 8 Hard segment 378 Content 376, 378 Hardness 86-87, 378, 401, 408 Determination 277 Heat flow meter 191 Heat reflectance 192 Modified hot wire 192 HER materials 369, 370, 371, 373 Characterisation 373 Polyurethane applications 369, 371 Synthesis 373 HER/HQEE blends 397, 398 Freezing point determination 397 High vacuum benches 163 Hot box 191 Guarded-calibrated 191 Hybrid 301, 306 Chemical structure 306, 325 Crosslinking 306, 326 Polyurethane-urea 301

487

Advances in Urethane Science and Technology Hybrid acrylic-urethane dispersions 261 Hybrid dispersion 269, 320 Chemical structure 324 Coalescent 323 Crosslinking 312, 317 Film 328 Initiator 320 Particle size 320, 321 Properties 290, 291, 292, 298, 302, 303, 307, 311, 320, 330 Surface free energy 319 Synthesis 269, 290, 320, 322 Hybrid dispersion particles Morphology 317, 328 Hybrid dispersion system 262 Preparation 262 Hybrid polymer dispersion 262 Hydrogen 166

I IGC See Analysis INSOTEC 135 Instabilities 215, 218, 235 Fluid 215 Benard 215 Marangoni 215, 235 Rayleigh-Taylor 215 Marangoni 218 Insulation 196 Interference fringe 222, 223, 224, 225, 226, 231, 236 Interferometer 221 Interpenetrating polymer network 262 InterWet 138, 143, 144, 145, 146, 147, 150, 153 Advantages 150 Applications 152 Improvements 148 Ionomers 178 Isopach 226 Isophorone diisocyanate 268

488

K Krauss-Maffei 14 Kyoto conference 157

L Laboratory moisture-curing 466 2,4-TDI prepolymers 466 Lamella 214, 215 Laminate 201 Aluminium foil 201 Lamination 161, 162 Lap-shear 337, 344, 347, 352, 353, 354, 362 Liquid carbon dioxide blowing 85 Long fibre injection 139 Long-term vibration 3 Loss compliance 388, 414, 415, 416 Loss modulus 385, 386, 414 Lubrication theory 240, 242, 245, 249

M Machine 9 Hi Tech SureShot MHR-50 9 Machine evaluation 9, 25, 29, 33 Back formulation utilising TPR 29 Back formulation without TPR 33 Cushion formulation utilising TPR 25 Cushion formulation without TPR 33 TDI flexible moulded foam 9 Marangoni Effect 249, 250, 251, 252, 254 Flows 215, 225 Marginal regeneration 225 Mass spectrometer 163, 164, 189 Quadrupole 163 Mass-loss/rate-of-rise 4 Maze flow mould 10, 56 MDI Prepolymers 459-460, 465 Cured with BDO 459, 460

Main Index Laboratory casting 465 Laboratory preparation 465 Mechanical crushing 3 Mechanical properties 372 Melting points 374 Metal foil 168 Methyl methacrylate 269 Micelles 273 Micro-leaks 191 Microscopy 4, 40 Scanning electron 4, 40 Mill Premier Mill Corporation dispersator 8 Mixing head 116, 120, 142 Variable geometry 120 Molecular sieves 181 Mondur TD-80 8 Monolayer 218, 226 Monomer 272, 275 as active diluents 275 Polymerisation 272 Montreal Protocol 157 Morphology 262, 264, 267, 272 Dispersion particle 262, 272 Engulfed 264 Fruit-cake 264 Gradient 264 Particle 267 Mould 56, 60, 124 Carrier 129 Carrier exchange 125 Carrying system 126 FlexiDrum 126 Maze flow 56, 57, 58, 59, 60 Temperature control system 124 Moulded foam 3, 68 Low emission 68 Moulded formulation 69, 70 MDI 69, 70, 71 Moulding 127, 130 Flexible foam 127

Polyurethane 130 Multi-component operation 117

N N-methylpyrrolidone 269 Navier-Stokes equation 220, 238, 239 NEG alloy 181 Zirconium-based 181 Newtonian fluid 239 Nitrogen 166 NovaFlex 90 Nucleation 91, 101, 105, 215 Nylon 168

O OEM Specifications 3 One-shot elastomer 436-437, 439-443, 465 Hardness buildup 437 Laboratory casting 465 Processability 436 Properties 438 Open cell 158, 159 PU 159 PU foam 158 Ostwald ripening 214 Outgassing 163, 165, 202 Load 166 Oxyethylene moieties 434 Oxygen transmission 176

P Pail test 13, 56 Panel 158, 177, 207 Encapsulated 177 Lifetime of 158 Open cell foam 207 Partial pressure 164

489

Advances in Urethane Science and Technology Particle 327 Morphology 330 Swelling 327 Particle formation 326 Particle size 277, 313, 314 Distribution 277, 313 PDMS 218 Péclet number 241, 251 Permeation 170, 171, 172 Helium 170, 171, 172 Physical properties 3, 23, 33, 35, 37-39, 47, 66, 68-70, 74, 372 Flexible slabstock foam 75 Foam 23 MDI 69, 70 Machine data 33 Review 39 TPR 35, 37 Plateau borders 93, 214, 225, 226 Gibb’s 93 Polarity 5 Polyacrylonitrile 178 Polycat 6 Polyester 168, 178 Polyether chains 218 Polyethylene 176 Polyethylene terephthalate 168 Polyisocyanate 85 Polymer dispersions 261 Polymerisation 270 Radical 270 Polyol 75, 77, 78, 449 EO-tipped propylene oxide 77 Ethylene oxide tipped 75 Molecular weight distribution 449 Polyolefin tipped 78 Polyol functionality 428 Polyoxyethylene 215 Polyoxypropylene glycol 421 Ultra-low monol 421 Polysilicate 217 Trimethylsilyl-capped 217

490

Polytetramethylene glycol 268 Polyurea 215 Polyurethanes 85, 92, 113, 138, 139, 157, 262, 335, 347, 405, 449

Adhesion 347 Adhesive 335, 360 Formulations 335 IR spectra 341 Silane 356 Silane additives 360 Co-injection 138 Cure 338 Dynamic properties 449 Filled 196 Open cell 196 Film 216, 236 Draining thin liquid 236 Rate of drainage 216 Vertical 236 Foam 166, 200 Closed cell 200 Open cell 166 Insulation 157 Glass-reinforced 139 High thermal stability 405 History 85 Markets 92, 113 Mechanical properties 449 Multi-component formulations 113 Two-component 335 Polyurethane castings 376 Polyurethane catalysts 42 Blowing 42 Non-fugitive gelling 42 Polyurethane elastomer 430, 431 Polyurethane Foams 3, 5, 93 Catalysts 5 Flexible 3, 93 Non-fugitive catalysts 5 Tertiary amine catalysts 5

Main Index Polyurethane-urea 324 Double bonds 324 Polyurethane-urea-acrylic dispersions 261 Polyurethane/urea dispersions 466 Formulations 457 Laboratory preparation 466 Polyvinylidene chloride 178 Pot life 372, 375 Determination 375, 390 PPG polyols 424, 429, 436 One-shot elastomer system 436 Processability 429 Processing latitude 434 Property latitude 429 Ultra-low monol 424, 444 PPG/PTMEG blends 447 Elastomer properties 447 Prepolymer-ionomer 265, 275, 276 Processing 113, 167, 179 Desorption 166 Exhaust 179 Latitude 3 Polyurethane 113 Seal-off 179 Thermally activated 167 Yield 187 Propionic acid, dimethylol 269 PU see Polyurethane PVC 136

Q Quality assurance 191 Quality control 172, 191

R Rapid Cure 87 Rate 169 Helium transmission 169 Water transmission 169

Rate-of-rise 18 Raw materials 92 Emulsification 92 Reactive amine catalysts 5 Reactivity 18, 23, 45, 65, 74 Handmix rate of rise 65 Machine free rise 23 Rebound properties 380, 390, 401 Recycling 162 Refrigerators 162 Reeves Brothers 87 Refrigerator 177, 203 Biomedical 203 Laboratory 203 Reinforcement 141 Handling 141 Reynold’s Equation 220, 227 Model 216 Number 241 Rigid foam 158 Open cell 158 Robots 121, 145 Cartesian 122 Foaming 121 Robotics 145

S Scalloping 23, 25 Seating 17 Automotive 113, 122, 130 Back formulation 19-22, 30-34, 36, 38, 40, 44-46, 48, 60, 113 Cushion formulation 19-22, 26-29, 34-35, 37, 39, 43-44, 46, 49, 57, 59 Polyurethane 17 PU back formulation 18 PU cushion formulation 17 Service station 125 Shear modulus 386 Shore hardness 372

491

Advances in Urethane Science and Technology Shrinkage 12, 61 Foam 61 Template 12 Silanes 356 Additive 364 Silica 98 Silicone surfactant 4, 95, 99, 213, 215217, 219-220, 228-229, 254 Burning 95 Characterisation data 233 Chemistry 99, 103 Dabco 4 Stabilisation 93 Soft segment content 376 Solvated volume 232, 234 Sound deadening 130 Sponge 214 Stoke’s law 219 Storage modulus 384, 385, 412 Stress conditions 239, 246 Normal 239, 246 Tangential 239, 246 Stress/strain curves 426, 440, 447, 458 Structure 174 Laminate 174 Styrene 269 Polymer dispersions 261 Surface characteristics 335 Surface energy 213, 214 Surface free energy 278, 317 Surface partition coefficient 219, 220 Surface tension 91, 230, 276 Gradient 218-220, 225-226, 229, 232, 234-235, 250, 252-253 Surface viscosity 217 Surfactants 64, 91-92, 101, 108, 213, 229, 232 Activity 92 Concentration 229 Development 101 Emulsification 92 Flexible moulded 64

492

Molecular structure 232 Non-ionic fluorocarbon 229 Silicone 91, 92, 100, 213 Synthesis 288 MDPUR 288 MDPUR-ASD 288

T Tan delta 387, 414 TDI 80, 465 Laboratory preparation 465 MBOCA 450, 453 Mechanical properties 454 Moisture-cured 454 Prepolymers 450, 453, 454, 466 Laboratory casting 466 Tear 380, 390, 401 Resistance 373 Strength 410 Technology 130 Foam & film 130-132, 134-138 Tegostab 102 Tensile 401, 408 Tensile Measurements 410 Modulus 372 Properties 378, 390, 392 Thermal conductivity 159, 174, 175, 180 Thermoregulation 124 Time pressure release test 13 Toluene diisocyanate 85 TPR 23 TPR times 33 Transmission rate 169 Helium 173 Water 169, 173 Transport 126 Systems 126 Treatment 165 Heat 165

Main Index

U Urethane elastomers 413 Heat aged 413 Urethanes 335 Adhesion behaviour 335

V Vacuum panel 158, 161, 188, 191 Characterisation 188 Insulating performance 191 Manufacturing process 188 Vacuum panel technology 158 Van der Waals bonding 218 Vending machines 177, 204 Vibrathane 375-376, 378, 383 Video imaging 106 Video microscopy 103, 105 Vinyl staining 42, 62 VIP 190, 193, 197, 202, 203 Longevity 203 Performance 202 Reliability 202

Flange 168 Viscosity 217-218, 220, 227-228, 245, 276 Bulk 220 Bulk dynamic 227 Finite surface 245 Surface 217-218, 220, 225, 227-232, 235-237, 242, 255 Surface dilatational 240 Surface shear 240 Volatile organic compounds 4 Voranol 7, 80

W Water resistance 277 Water transmission 176 Window fogging 42

Z Zeolite 181 Zeta potential 314

493

Advances in Urethane Science and Technology

494

ISBN: 1-85957-275-8

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