Polymer Dispersions and Their Industrial Applications

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

Polymer Dispersions and Their Industrial Applications Edited by Dieter Urban and Koichi Takamura

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

Polymer Dispersions and Their Industrial Applications

edited by Dieter Urban and Koichi Takamura

IV

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

Editors Dr. Dieter Urban Dr. Koichi Takamura

BASF Corp. 11501 Steele Creek Road Charlotte, NC 28273, USA

This book was carefully produced. Nevertheless, editors, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.:

applied for British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek – CIP Cataloguingin-Publication Data

A catalogue record for this publication is available from Die Deutsche Bibliothek © 2002 Wiley-VCH Verlag GmbH, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Scanning electron micrograph of a hollow sphere created by the deposition of 7.9 µm polystyrene particles on a nitrogen bubble during their preparation in the microgravity environment of the Space Shuttle Challenger (courtesy of the Emulsion Polymers Institute, Lehigh University, Bethlehem, PA, USA).

Cover photograph

Printed in the Federal Republic of Germany Printed on acid-free paper Typesetting

TypoDesign Hecker GmbH,

Leimen Printing betz-druck GmbH, Darmstadt Binding Großbuchbinderei J. Schäffer

GmbH & Co. KG, Grünstadt ISBN 3-527-30286-7

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

Contents Preface

XIII

1

Introduction

1.1 1.2 1.3 1.4 1.5

Names and Definitions 1 Properties of Polymer Dispersions 3 Important Raw Materials 8 Commercial Importance of Polymer Dispersions Manufacturers of Polymer Dispersions 12 References 14

1

10

2

Synthesis of Polymer Dispersions 15

2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.3 2.3.1 2.3.2 2.3.3 2.3.4

Introduction 15 Chemistry 17 Mechanism of Emulsion Polymerization 17 Major Monomers 23 Functional Monomers 26 Surfactants 27 Initiator Systems 30 Other Ingredients 32 Manufacturing Processes 34 Types of Process 34 Influence of Process Conditions on Polymer/Colloidal Properties Equipment Considerations 39 Safety Considerations 40 References 40

3

Characterization of Aqueous Polymer Dispersions 41

3.1 3.2 3.2.1 3.2.2 3.2.3 3.2.4

Introduction 41 Polymer Dispersions 42 General Characterization of Dispersions 42 Characterization of Polymer Particles 48 Residual Volatiles 56 Aqueous Phase Analysis 57

37

V

VI

Contents

3.3 3.3.1 3.3.2 3.3.3

Polymer Films 58 Film Formation 59 Macroscopic Characterization of Polymer Films Microscopic Characterization of Polymers 68 References 72

60

4

Applications in the Paper Industry 75

4.1 4.2 4.3 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.5

Introduction 75 The Paper Industry 76 Surface Sizing 79 Paper Coating 81 Coating Techniques 84 Pigments used in Coating Colors 86 Co-binders and Thickeners used in Coating Colors Binders used in Coating Colors 90 Test Methods 97 Concluding Remarks 100 Acknowledgments 100 References 101

5

Applications for Printing Inks 103

5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.3 5.3.1 5.3.2 5.3.3 5.4 5.4.1 5.4.2 5.5 5.5.1 5.5.2 5.5.3 5.6 5.6.1 5.6.2 5.7

Introduction 103 Flexographic Ink 104 Gravure Ink 106 Ink Composition 106 Pigment Dispersion 108 Emulsion Vehicle 109 Solution Vehicle 112 Waterborne Wax Emulsions and Powders 113 Ink Additives 113 Physical Properties and Test Methods 114 Typical Properties 114 Application Tests 115 Test Method Abstracts 115 Inks for Flexible Substrates (Films) 117 Surface Print Film 118 Lawn and Garden Bags 118 Inks for Paper Board Substrates 118 Folding Cartons 118 Direct Print Corrugated Packages 119 Pre-print Corrugated Packages 119 Inks for Poly-coated Board 120 Milk Cartons 120 Cup and Plate 120 Inks for Paper Products 120

87

Contents

5.7.1 5.7.2 5.7.3 5.7.4

Multiple Wall Bags 121 Gift Wrap and Envelopes Newspapers 121 Towel and Tissue 122 References 122

121

6

Applications for Decorative and Protective Coatings 123

6.1 6.1.1 6.1.2 6.1.3 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.4 6.4.1 6.4.2 6.4.3 6.5 6.5.1 6.5.2 6.5.3 6.6 6.6.1 6.6.2 6.6.3 6.7 6.7.1 6.7.2 6.7.3 6.8 6.8.1 6.8.2 6.8.3 6.8.4 6.8.5 6.8.6

Introduction 123 Market Overview 123 Coating Industry Trends 124 Coatings Provide Decoration and Protection 124 Overview of Coating Formulations 125 Volume Solids and Pigment Volume Content 125 Polymer Matrix 127 Film Formation 128 Typical Polymer Compositions 129 Pigments, Extenders, and Additives 132 Decorative Coatings 137 Emulsion Polymers in Decorative Coatings 137 Polymer Compositions used for Emulsion-based Decorative Coatings 137 Regional Distinctions in Decorative Coatings 138 Market Size of Decorative Coatings 138 Interior Decorative Coatings 139 Key Performance Features 139 Interior Decorative Coating Formulations 140 Standard Application and Performance Tests 142 Exterior Decorative Coatings 146 Key Performance Features 146 Exterior Decorative Coating Formulations 147 Standard Application and Performance Tests 147 Elastomeric Wall Coatings 149 Key Performance Features 149 Typical Elastomeric Wall Coating Formulations 150 Standard Application and Performance Tests 151 Primer Coatings 151 Key Performance Features 152 Primer Formulations 152 Standard Application and Performance Tests 153 Protective and Industrial Coatings 154 Copolymers used in Protective and Industrial Coatings 154 Market Size 155 Industrial Maintenance Coatings 155 Key Performance Features 155 Formulation Characteristics for Industrial Maintenance Coatings 156 Standard Application and Performance Tests 156

VII

VIII

Contents

6.9 6.9.1 6.9.2 6.9.3 6.9.4

Traffic Marking Paints 158 Description of Traffic Paint Market 158 Key Performance Features 159 Typical Traffic Paint Formulation 159 Standard Application and Performance Tests 159 References 161

7

Applications for Automotive Coatings 163

7.1 7.1.1 7.2 7.2.1 7.2.2 7.2.3 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.3.6 7.4 7.5 7.6 7.6.1 7.7

Introduction 163 History of Automotive Coating 164 Automotive Coating Layers 166 Electrocoat 170 Primer 172 Basecoat 173 Properties of Water-borne Binders used for Automotive Coatings Emulsion Polymers 176 Microgels 177 Miniemulsions 177 Selection of Monomers, Initiators, and Surfactants 178 Secondary Acrylic Dispersions 179 Secondary Polyurethane Dispersions 179 Rheology 181 Crosslinking 183 Application Properties 185 Metallic Effect 186 Environmental Aspects and Future Trends 186 References 187

8

Applications in the Adhesives and Construction Industries 191

8.1 8.2 8.2.1 8.2.2 8.2.3 8.3 8.3.1 8.3.2 8.3.3 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.4.5 8.4.6

Introduction 191 Pressure-sensitive Adhesives 193 Self-adhesive Labels 194 Self-adhesive Tapes 207 Test Methods 210 Laminating Adhesives 217 Flexible Packaging 217 Glossy Film Lamination 219 Furniture and Automotive 222 Construction Adhesives 224 Floor-covering Adhesives 224 Sub-floor and Wall Mastics 231 Sealants 233 Ceramic Tile Adhesives 238 Polymer-modified Mortars 241 Waterproofing Membranes 244

176

Contents

8.4.7

Elastomeric Roof Coatings Acknowledgments 250 References 251

247

9

Applications in the Carpet Industry 253

9.1 9.2 9.3 9.4 9.5 9.5.1 9.5.2 9.5.3 9.5.4 9.5.5

Introduction 253 History of Carpet 253 Present Day Carpet Business 255 Carpet Backing Binders 256 Carpet Laminating 259 Background 259 Carpet Terminology 260 Back-coating Applications 261 Back-coating Formulations and Ingredients 262 Industry Issues 264 References 266

10

Non-wovens Application 267

10.1 10.2 10.2.1 10.2.2 10.3 10.4

Introduction 267 Manufacturing Systems 270 Web Formation 271 Web Consolidation 272 Polymer Dispersions for Chemical Bonding Application Test Methods 275 References 281

11

Applications in the Leather Industry 283

11.1 11.2 11.3 11.3.1 11.3.2 11.3.3 11.3.4 11.3.5 11.3.6 11.3.7 11.4 11.4.1 11.4.2 11.4.3 11.5 11.5.1 11.5.2 11.5.3

Introduction 283 Market Situation 284 Leather Finishing 286 Modern Finishing 287 General Construction of Finishing Coats Spray Dyeing 287 Grain Impregnation 287 Base Coat 287 Pigment Coat 288 Top Coat 288 Application Methods 288 Spraying 289 Roll Coating 289 Curtain Coater 289 Binders 291 Polyacrylate Dispersions 291 Polybutadiene Dispersions 291 Polyurethane Dispersions 292

273

287

IX

X

Contents

11.6 11.6.1 11.6.2 11.6.3 11.6.4 11.7 11.7.1 11.7.2 11.7.3 11.7.4 11.7.5 11.7.6 11.7.7 11.7.8 11.7.9

Production of Selected Leather Articles 292 Shoe Upper Leather 292 Apparel Leather 293 Automotive Leather 294 Furniture Leathers 295 Test Methods in Leather Finishing 296 Flexing Endurance 297 Rub-fastness 298 Dry and Wet Adhesion 299 Fastness to Ironing 299 Hot Air Fastness 299 Aging resistance 299 Fogging test 300 Light-fastness 300 Hot light aging 300 References 300

12

Applications for Asphalt Modification 301

12.1 12.2 12.2.1 12.2.2 12.3 12.3.1 12.3.2 12.3.3 12.3.4 12.3.5 12.4 12.5

Introduction 301 Hot Mix Asphalt Paving 303 Asphalt Specification 304 In-line Injection (Pump-in) 311 Paving with Asphalt Emulsion 313 Applications of Asphalt Emulsions 314 Asphalt Emulsion Tests 317 Polymer Honeycomb Structure in Cured Asphalt Emulsion 317 Asphalt Emulsion Residue Characterization 319 Application Tests for Chip Seal and Microsurfacing 321 Eco-efficiency Analysis 323 Concluding Remarks 326 Acknowledgement 326 References 326

13

Applications of Redispersible Powders

13.1 13.2 13.3 13.4 13.4.1 13.4.2 13.4.3 13.4.4 13.4.5 13.4.6 13.5

Introduction 329 Manufacturing of Redispersible Powders 330 Dry Mortar Technology 332 Markets and Application Areas of Redispersible Powders 333 Adhesives for Ceramic Tiles 334 Tile Grouts 340 Exterior Insulation and Finish Systems and Top Coats 341 Self-leveling Underlayments 345 Patch and Repair Mortars 346 Waterproof Membranes 350 Summary 353

329

Contents

References

354

14

Applications for Modification of Plastic Materials 355

14.1 14.2 14.2.1 14.3 14.3.1 14.3.2 14.4 14.4.1 14.4.2

Introduction 355 Emulsion Polymerization and Isolation Technology 356 Isolation Technology 357 Processing Aids 358 Processing Aids for PVC 359 Processing Aids for Other Resins 366 Impact Modifiers 367 Impact Modifiers for PVC 368 Engineering Resins 375 Acknowledgment 378 References 379

15

Applications for Dipped Goods 383

15.1 15.2 15.3 15.4 15.4.1 15.4.2 15.4.3 15.4.4 15.5 15.5.1 15.5.2

Introduction 383 Polymers Used by the Dipping Industry 384 Principles of Dipping 385 Dipping Synthetic Polymer Emulsions in Practice 386 Former Design 386 Mix Design 388 Coagulant 390 The Dipping Process 390 The Testing of Synthetic Gloves 395 Non-safety-critical Gloves 395 Safety-critical Gloves 396 References 398 Index 399

XI

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

Preface Aqueous polymer dispersions are important raw materials used in a variety of industrial processes. They consist of very small polymer particles dispersed in water and appear as milky fluids. When finally processed and providing the function for which they were selected, they are barely visible. Polymer dispersions are used to protect metal, wood, and leather against water and microorganisms, and are used as binders for pigments, fillers, and fibers and to finish the surfaces of metal, wood or paper. Protecting, binding, and finishing are the essential effects achieved by use of polymer dispersions. In most applications the water will be evaporated and a functional polymer remains. This can be hard or tacky, plastic or elastic, transparent or opaque. Accordingly, they are used for coatings or as adhesives, for binders or foams, for clear coat varnishes or opacifiers. It is even possible to reconcile these classically contradictory properties by proper design of a single dispersion or by mixing several. Even small amounts of polymer dispersion are able to improve considerably the properties of different binders, e.g. starch, bitumen, or cement. The huge variety of applications continues into the area of solid plastic materials – impact modifiers are added to improve the properties of plastic materials. Dipping goods, e.g. gloves, and latex foams for mattresses are polymeric materials which are made directly from polymer dispersions. Finally, there are also applications in which polymer dispersions remain in their liquid form – they are used as drug carriers, in medical diagnosis, and in liquid soap. This book focuses on the applications of aqueous polymer dispersions. The chapters on synthesis and characterization should be regarded as an introduction and should aid understanding of the applications. The applications of aqueous polymer dispersions have developed differently, both historically and regionally. Regulatory issues have contributed to these differences. The strongest development of polymer dispersions occurred in Europe and North America in the middle of the 20th century. The differences between these two regions are emphasized. We are specially grateful to all the authors who helped us make this global comparison and acknowledge the authors’ companies, for approving and supporting this work. Charlotte, North Carolina, USA

Dieter Urban Koichi Takamura

XIII

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

List of Authors Peter Blanpain

Dr. Christoph Hahner

7834 Covey Chase Drive Charlotte, NC 28210, USA

Wacker Polymer Systems, L. P. 3301 Sutton Road Adrian, MI 49221, USA

Dr. Mary Burch

Rohm & Haas Company 727 Norristown Road Spring House, PA 19477, USA Dr. Chuen-Shyong Chou

Rohm & Haas Company Rt. 413 and Old Rt. 13 Bristol, PA 19007, USA Dr. Dieter Distler

BASF Aktiengesellschaft GKD - B1 D-67056 Ludwigshafen, Germany Dr. Johannes Peter Dix

BASF Aktiengesellschaft EVL/I – G100 D-67056 Ludwigshafen, Germany Dr. Luke Egan

BASF Corporation 11501 Steele Creek Road Charlotte, NC 28273, USA Dr. Onno Graalmann

BASF Nederland B.V. Westervoortsedijk 71 NL-6827 AV Arnhem, The Netherland Dr. Sunitha Grandhee

BASF Corporation 26701 Telegraph Road Southfield, MI 48034, USA Richard Groves

Synthomer LTD Central Road, Templefields, Harlow, Essex, CM20 2BH, UK

Dr. Do Ik Lee

The Dow Chemical Company 1604 Building Midland, MI 48674, USA Dr. Hermann Lutz

Wacker Polymer Systems GmbH&CoKG Johannes-Hees-Str. 24 D-84489 Burghausen, Germany Dr. Werner Kirchner

BASF Aktiengesellschaft EV/CS – H201 D-67056 Ludwigshafen, Germany Andrew Lanham

Synthomer Ltd. Central Road, Templefields, Harlow, Essex, CM20 2BH, UK Dr. Brough Richey

Rohm & Haas Company 727 Norristown Road Spring House, PA 19477, USA Dr. Jürgen Schmidt-Thümmes

BASF Aktiengesellschaft GKD/S – B1 D-67056 Ludwigshafen, Germany Dr. Elmar Schwarzenbach

BASF Aktiengesellschaft EDP/MB – H201 D-67056 Ludwigshafen, Germany Richard Scott

BASF Corporation 475 Reed Road NW Dalton, GA 30720, USA

XV

XVI

J. Arthur Smith

BASF Nederland B.V. Westervoortsedijk 71 NL-6827 AV Arnhem, The Netherland K. Spenceley

Synthomer Ltd. Central Road, Templefields, Harlow, Essex, CM20 2BH, UK Barna Szabo

Flint Ink Corporation 4600 Arrowhead Drive Ann Arbor, MI 48105, USA Dr. Koichi Takamura

BASF Corporation 11501 Steele Creek Road Charlotte, NC 28273, USA Jim Tanger

BASF Corporation 11501 Steele Creek Road Charlotte, NC 28273, USA Michael A. Taylor

BASF Corporation 11501 Steele Creek Road Charlotte, NC 28273, USA Dr. Dieter Urban

BASF Corporation 11501 Steele Creek Road Charlotte, NC 28273, USA Dr. Jane E. Weier

Rohm & Haas Company Rt. 413 and Old Rt. 13 Bristol, PA 19007, USA Dr. Harm Wiese

BASF Aktiengesellschaft GKD/N – B1 D-67056 Ludwigshafen, Germany Marilyn Wolf

BASF Corporation 11501 Steele Creek Road Charlotte, NC 28273, USA

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic) Color Plates

Color Plates Fig. 1-3

Particle morphologies.

Raspberry structure

Core/shell structure

Acorn structure

Uncoated grade, supercalendered

Coated grade, supercalendered

Fig. 4-7

Effect of coated paper on offset printing.

XVII

XVIII

Color Plates

Coated gravure paper

Uncoated gravure paper

Fig. 4-8

Effect of coated paper on rotogravure printing.

Coating head

Steam Dryer Laminating station Release liner

Schematic representation of PSA label coater.

Backing

Fig. 8-9

Unwind

Rewind

Latex Polymer Network

Photomicrograph demonstrating spontaneous formation of polymer network upon curing of the CRS-2 asphalt emulsion modified with 3 % cationic SBR latex.

Fig. 12-15

50 µm

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

1

Introduction Dieter Urban and Dieter Distler

1.1

Names and Definitions

Most precisely the subject of this book is called “aqueous synthetic organic polymer colloids”. The term “polymer colloid” defines a state of subdivision in which polymolecular particles dispersed in a medium have at least in one direction a dimension of roughly between 1 nm and 1000 nm [1]. The term “organic” needs to be added to exclude inorganic polymers like silica. To be more precise the term “synthetic” will be added, if organic polymers of natural origin like natural rubber should be excluded. Finally, the term “aqueous” ensures that the continuous medium is only water, excluding e.g. organic solvents. However, depending on the language, the geographical region and the field of application there are many other names commonly used (Fig. 1-1). In general the term “dispersion” characterizes a two phase system consisting of finely dispersed solid particles in a continuous liquid phase. An example of a dispersion is whitewash, calcium hydroxide above the solubility limit in water. If the finely dispersed phase and the continuous phase, both are liquid, the term “emulsion” will be used. An example is milk, which essentially consists of fat droplets in water; the droplets are stabilized by proteins. In both cases, in dispersions and emulsions, the continuous phase is therefore a liquid; in all of our examples, the liquid is water. In dispersions, the finely disperse substance is solid, while in emulsions it is liquid. Dealing with organic polymers being the dispersed substance it is difficult to define precisely whether they are solid or liquid. Depending on the glass transition temperature (Tg) and chain length, polymers are viscous liquids at low Tg and low molecular weight or they will be tough to brittle solids, if Tg and molecular weight are high. The temperature and stress duration are other important factors. At temperatures below the glass transition temperature or in the case of very short stress duration, polymers behave like glasses, while above this temperature or in the case of long stress times, they are viscous or elastic materials. This behavior of polymers between liquid and solid is one reason why aqueous synthetic organic polymer colloids are referred to as dispersions (Danish, Dutch, Finnish, German, Greek, Hungarian, Japanese, Korean, Norwegian, Polish, Portuguese, Romanian, Russian, Spanish,

1

2

1 Introduction

Fig. 1-1

colloids.

Commonly used names for aqueous synthetic organic polymer

1.2 Properties of Polymer Dispersions

Swedish, Turkish) and emulsions (Arabic, Chinese, English, Indonesian, Italian, Malay). Another reason for the use of emulsion or emulsion polymer comes from the most important production process for these products, the emulsion polymerization. The products are referred to as emulsion polymers or simply emulsions. In contrast to this the name latex (Latin: latex, liquid; Greek: λαταξ, droplet) is derived from the naturally occurring rubber milk and is most widely used for aqueous synthetic organic polymer colloids, especially for the substitution products of natural latex, butadiene-styrene copolymer emulsions. The Union for Pure and Applied Chemistry recommends two names: Latex and polymer dispersion [2]. Latex is defined as “A colloidal dispersion of polymer particles in an aqueous medium. The polymer may be organic or inorganic.” Since we will not cover inorganic dispersions, this book should have been called “Organic Latices and Their Industrial Applications”, which seems to be a pleonasm because the use of latex is generally associated with organic material. Polymer dispersion is defined as “A dispersion in which the disperse phase consists of polymer particles.” The continuous phase can be a liquid, solid or gas. If we want only water to be the continuous phase, aqueous is added. In industrial applications non-aqueous polymer dispersions are negligible. Therefore this book has been called “Polymer Dispersions and Their Industrial Applications”. However, according to the preference of the authors the terms “polymer dispersion”, “dispersion”, “emulsion polymer”, “emulsion” and “latex” are used synonymously.

1.2

Properties of Polymer Dispersions

The aggregate state of a polymer dispersion is thermodynamically unstable. The very large internal surface area of up to 100 m2 mL–1 of dispersion requires stabilization of the particle surfaces in order to suppress phase separation and coagulation. Driving force for the agglomeration of particles is the gain of energy by reducing the internal surface. Finally a polymer block and a substantially polymer-free water phase will be formed. This coagulation can be accelerated by salts, acids, solvents, freezing, shear, etc. To obtain highly stable polymer dispersions, the particles are usually provided with ionic groups, for example by adsorption of anionic or cationic surfactants, or by incorporation of ionic groups into the polymer. Another, nonionic type of stabilization takes place via hydrophilic groups on the particle surface, for example by aminoor hydroxyl-containing monomers or protective colloids. Polymer dispersions used in industry usually are stabilized by both mechanisms (ionic and nonionic). The special nature of the particle surface, which differs from the particle interior, plays an important role in all applications. Industrially important polymer dispersions usually contain 40–60 % of polymer in water. Each mL of dispersion contains about 1015 particles with diameters of 50–500 nm. One particle contains 1–10 000 macromolecules, and each macromolecule contains about 100–106 monomer units (Fig. 1-2).

3

4

1 Introduction

Fig. 1-2

What is a polymer dispersion?

These figures give an impression of the possible variation, if just the molecular weight (or molecular weight distribution) and particle size (or particle size distribution) of homo-polymers will be considered. The random incorporation of various monomers in the chains, the possibility of cross-linking between the polymer chains and finally separated phases of different polymers in a particle allow a virtually unlimited variety in this product class. Polymer dispersions normally consist of spherical particles. The dispersed particles scatter the light and are the cause of the milky appearance. This Mie scattering is utilized for particle size measurement. Very small polymer particles hardly scatter visible light at all, those polymer dispersions have a translucent appearance. If all the particles are of the same size, the term “monodisperse dispersions” will be used. They are frequently recognized from a certain particle size merely from the iridescent appearance, which is caused by Bragg scattering at a crystalline superstructure of close packing of the particles. Polymer dispersions with a heterogeneous particle structure – a special particle morphology consisting of a number of phases – have recently become of interest. Examples are particles with a core/shell structure or two coexistent polymer phases, particles with a raspberry structure, etc. The particle morphology may be thermodynamically preferred; in the case of polymers with reduced chain mobility or even in the case of relatively low cross-linking, it is mostly kinetically controlled morphologies that are frozen. This enables product properties with even contradictory requirements to be achieved better, for example low film formation temperature, but maximum blocking resistance or hardness of the polymer (Fig. 1-3). The flow behavior is also an important parameter. The flow property of polymer dispersions is a particular advantage of this aggregate state. Dispersions can have a polymer content which is a multiple higher than polymer solutions, yet still be freeflowing. Besides the polymer content, particle size, particle size distribution and electrolyte content, the viscosity is also affected by dissolved constituents in the aqueous phase. The water phase of many polymer dispersions contains a whole range of water-soluble oligomers, auxiliaries and additives which contribute to the application properties as well.

1.2 Properties of Polymer Dispersions Fig. 1-3

Particle morphologies.

Raspberry structure

Core/shell structure

Acorn structure

To obtain readily free-flowing dispersions with low viscosity at high polymer contents of >60 % by volume, very broad or bimodal particle size distributions are needed (Fig. 1-4).

Electron photomicrograph of a bimodal polymer dispersion. Fig. 1-4

This can be achieved during the polymerization or by partial agglomeration, for example, by means of a shear gradient, by freezing or by addition of an agglomeration aid, so that significantly larger agglomerates are present alongside the small primary particles. The viscosity of polymer dispersions is usually dependent on the shear rate. A distinction is made between pseudoplastic behavior (viscosity decreases with increasing shear), possibly with a flow limit, thixotropic behavior (viscosity decreases with in-

5

6

1 Introduction

creasing shear time) and dilatant behavior (viscosity increases with increasing shear). The rheology of concentrated polymer dispersions is complex, often being dependent on the shear rate and previous history. Owing to the content of surface-active substances, the foaming behavior is an important property for many applications. Antifoam agents reduce foaming, while further emulsifiers and rheology modifiers increase the foaming or stabilize the foam once formed. The biodegradability of many additives makes the dispersions susceptible to attack by microorganisms (bacteria, yeast). Most dispersions are therefore provided with biocides. In most applications, the water is evaporated from the dispersions. Depending on the composition and/or processing temperature, a polymer film or powder is formed. The properties of the polymer now come into play: strength, elongation at break, elasticity, transparency, solvent and environmental resistance, glass transition temperature, tack, etc. These properties are determined by the chemical composition of the copolymers, the molecular weight and the molecular weight distribution, by the morphology of the polymer particles and by the morphology of the polymer film. Important polymer classes are: Styrene/butadiene dispersions are used for their elastic properties since molecular weight and cross-linking of the polymer can be adjusted widely by choosing the degree of conversion and the amount of chain transfer agents. They are used as synthetic rubber for tires and molded foam. When styrene is replaced by acrylonitrile, elastic and solvent resistant polymers are obtained, which are used for dipping goods. Carboxylated styrene/butadiene (XSB) dispersions contain acrylic, methacrylic, maleic, fumaric or itaconic acid. The carboxylic groups provide stabilization of the polymer particles and a good interaction with fillers (calcium carbonate, clay) and pigments. The main applications are paper coating and carpet backing. The remaining 1,2 and 2,3 double bonds of butadiene favor autoxidation of the polymer, it becomes yellow and brittle. This is prevented by adding antioxidants. This polymer class is resistant to hydrolysis at all pH values since it does not contain ester units which tend to hydrolyze especially at very high pH. Acrylic dispersions (pure acrylics and styrene acrylics) are extremely versatile. The big variety of available acrylic and methacrylic esters together with styrene offer almost unlimited opportunities to choose for the glass transition temperature and the hydrophilic/hydrophobic properties. Acrylic esters tend to form cross-linked polymers by abstraction of the α-hydrogen atom, methacrylic esters in contrast form polymer chains which are not cross-linked. Acrylics are resistant against oxidation by air and degradation by light. The main application areas are coatings and adhesives. Vinyl acetate dispersions are widely used for coatings and adhesives as well. To stabilize the polymer particles often polyvinyl alcohol is used as protective colloid. Most common co-monomers are ethylene, versatic esters, vinyl chloride or acrylic esters. The polymer dispersions are spray dried to obtain a polymer powder, which is widely used in construction industry. Ethylene/vinyl acetate copolymers form elastic films and are fairly resistant to oxygen and light.

1.2 Properties of Polymer Dispersions

Polymer dispersion with a high content of vinylidene chloride form polymer films with crystalline areas. These PVDC films are highly impermeable for both, oxygen and water vapor, and are used as barrier coatings in packaging materials, especially for food packaging (Fig. 1-5).

Permeability of polymer films.

Fig. 1-5

Polymer dispersions with a high amount of acrylic/methacrylic acid convert to aqueous solutions or gels when pH is increased. They are used as thickeners. Films made from polyurethane dispersions combine elastic properties with high tensile strength. Polystyrene dispersions have a glass transition temperature of 105 ºC. They are used in paper coating to improve gloss, in liquid soaps to provide opacity and in medical diagnosis as carrier for active ingredients. Films of acrylic dispersions, which are cross-linked with metal ions and re-dispersible with an aqueous solution of ammonia, are used as temporary protective films. All those examples elucidate that polymer dispersions are used in both big volume and small volume applications. They are both commodities and specialties. And the use of polymer dispersions is increasing worldwide. The main reasons for this are: the variety of polymer properties achievable by emulsion polymerization is virtually unlimited, emulsion polymerization is an inexpensive production process for these products, the fluid form of polymer dispersions is easy to handle, and water is environmentally friendly. The complex colloidal and chemical behavior of polymer dispersions is an interesting working area for many scientific disciplines and is important for many applications. In addition to excellent reviews [3–13], a whole range of periodicals focuses on polymer dispersions [14–18].

7

8

1 Introduction

1.3

Important Raw Materials

The most important production process for polymer dispersions is emulsion polymerization [19]. This process is started by preparing a monomer emulsion consisting of monomer droplets in water. The monomer droplets are stabilized by emulsifiers and/or protective colloids. When adding an initiator polymerization is started converting the monomers into polymer particles (Chapter 2). The production of polymer dispersions by emulsion polymerization requires deionized water, free-radical-polymerizable monomers, emulsifiers and/or protective colloids and initiators. Further auxiliaries, such as chain transfer agents, buffers, acids, bases, anti-aging agents, biocides, etc., can be used. The most important source of the main monomers used or their precursors is petroleum chemistry, with the steam cracker as reactor. Liquid hydrocarbons (naphtha or liquefied natural gas LNG) are broken down (“cracked”) into short-chain hydrocarbons at 800–850 °C with addition of steam as diluent (Fig. 1-6) [20].

Fig. 1-6

Steam cracker products.

There are currently about 200 steam crackers worldwide. In Europe, Latin America and South-East Asia, the starting material is mostly naphtha, while in North Africa, the Middle East and North America, predominantly ethane and propane from natural gas are used. The largest plants have an annual capacity of more than 800 000 tons of naphtha. Ethene, the most important petrochemical feedstock today, reached a world capacity of about 80 million tons per year in 1995. Almost half is polymerized to give polyethylene. It plays only a secondary role for emulsion polymerization in vinyl acetateethene copolymers and in polyethylene waxes. It is important, however, in this connection as a feedstock for the production of vinyl chloride, styrene and vinyl acetate.

1.3 Important Raw Materials

Propene cannot be polymerized by means of free radicals. It is, however, a feedstock for acrylic acid, acrylates and acrylonitrile. Butadiene is extracted from the C4 fraction from the steam cracker, and can be used directly for emulsion polymerization. The principal monomers butadiene, styrene, vinyl acetate, (meth)acrylates and acrylonitrile essentially determine the material properties of films made from the corresponding dispersions: the glass transition temperature, the water absorption capacity, the elasticity, etc. Auxiliary monomers, which are only used in a small proportion, usually <5 %, control important properties such as colloid-chemical stabilization (acrylic acid, methacrylic acid, acrylamide, methacrylamide), crosslinking within the particles (difunctional acrylates, divinylbenzene, etc.) or hydrophilic properties (OH-containing monomers, such as hydroxyacrylates). Reactive monomers which still contain a latently reactive group even after incorporation into the polymer, for example glycidylmethacrylate or N-methylol(meth)acrylamide, can form a network between various particles and polymer molecules after film formation. These specific polar groups are frequently not distributed homogeneously over the particle cross-section, but are preferentially moved to the area of greatest effectiveness, for example the particle surface. Besides the monomers, the emulsifiers are important constituents. Emulsifiers (surfactants) consist of a long-chain hydrophobic group (dodecyl, hexadecyl or alkylbenzene) and a hydrophilic end group. The hydrophilic group may be anionic (sulfate, sulfonate, sulfosuccinic acid, phosphate, carboxylate) or cationic (quaternary ammonium salts) or have a zwitterionic structure (betaine groups). In addition, there is a whole series of nonionic emulsifiers and protective colloids, which are frequently used in combination with ionic emulsifiers. Ethylene oxidepropylene oxide block copolymers, amphiphilic 2- and 3-block copolymers, polyvinyl alcohols, polyvinyl-pyrrolidone, alkylpolyglycol ethers, etc. For the polymerization to start and maintain, a free-radical initiator which forms free radicals at elevated temperatures (60–100 ºC) is needed, for example sodium peroxodisulfate, hydrogen peroxide, organic peroxides or azo compounds, or a redox system, for example hydrogen peroxide/ascorbic acid with Fe2+ salts. The polymerization can also be initiated by UV, γ-rays, electron beams or strong sound or shear fields, although these, apart from UV initiation, have not yet been used in practice. The combination of initiator- and surface-active properties (inisurf) or surfaceactive and monomer properties (surfmer) in a single molecule is possible, but is so far mainly of academic interest.

9

10

1 Introduction

1.4

Commercial Importance of Polymer Dispersions

Polymers were discovered in the 1920s. During World War II large industrial scale production was established and since the 1950s production and use of polymers have grown strongly (Fig. 1-7). Million Metric tons

200 180

189

160 140 120 114

100 80 60

68

40 20 0 Fig. 1-7

8

1960

32

1970

1980

1990

2000

Growth of plastics production.

This growth is ongoing, and production of synthetic polymers has reached about 189 million metric tons with a total value of more than US$ 200 billion worldwide by the year 2000. This growth is due to two factors: the ability of polymers to combine properties such as light weight, strength, electrical insulation, etc., and the extremely low energy content (as product and in production). The possibility of energy recovery, recycling of the raw material or even of the polymer after use conserves resources. We encounter a wide range of polymers every day in the form of fibers, materials, films, etc., in virtually all products we use in everyday life. Combinations of the various product classes make a significant contribution toward the variety of end products made of plastic materials and synthetic fibers. The variety of functional polymers is even greater than for plastics and fibers. Functional polymers are used as polymer solutions, polymer dispersions or polymer powders. They essentially perform the functions of protecting, binding, bonding and finishing. The major polymer classes – polyolefins, polyvinyl chloride and polystyrene – are defined by their monomers; ethene, propene, vinyl chloride and styrene (Fig. 1-8). These three groups together account for 64 % of synthetic polymers. The class of polymer dispersions is only described by the state of aggregation, but not by the chemical composition. In the chapters dealing with applications, we will also see that for a particular application a number of polymer classes are suitable; the specific

1.4 Commercial Importance of Polymer Dispersions

PVC 14% Polyolefin 43%

Polyester 14%

Polystyrene 7%

Polyurethane 4%

Other 14%

Fig. 1-8

Polymer Dispersions (dry) 4%

Production by polymer class [21].

state of aggregation of the dispersions is consequently often more important than the monomer combinations. 4 % polymer dispersions correspond to about 7.5 million metric tons (dry) polymer, or 15 million metric tons (wet), assuming an average polymer content of 50 %. The most important dispersion, natural latex from Hevea brasiliensis with about 6 million metric tons (dry), is, as a natural product, not included here. The majority is coagulated and used predominantly in the tire industry, only about 15 % is sold as latex with a solids content of 60 %. These figures also omit impact modifiers for plastics. They are not sold as dispersions, but further processed directly by the manufacturers. About 1 million metric ton (dry) of impact modifiers is produced worldwide. The most important product classes of polymer dispersions are butadiene-styrene copolymers, vinyl acetate homopolymers and copolymers, and polyacrylates. Other polymer dispersions contain copolymers of ethylene, styrene, vinyl ester, vinyl chloride, vinylidene chloride, chloroprene and polyurethane (Fig. 1-9).

11

12

1 Introduction

Vinylacetate 28% Acrylate 30%

Other 5% Styrene Butadiene 37%

Fig. 1-9

Aqueous polymer dispersions by product class.

1.5

Manufacturers of Polymer Dispersions

Worldwide there are far more than 500 companies producing and offering polymer dispersions. However, only 20 companies account for about half of the global market. The leading 3 suppliers – BASF, DOW Chemical, Rohm and Haas – have an annual production capacity of more than 1 million metric tons (wet) and cover about 20 % of the world market. In Fig. 1-10 major suppliers of polymer dispersions are listed in alphabetical order. The product lines are defined by the main monomers used. Acrylic dispersions include pure acrylics and styrene acrylics, specialty dispersions consist of monomers like vinyl pyridine, vinyl chloride, vinylidene chloride, chloroprene, etc.. The product lines as well as the information about the main application areas and the trade names were mainly taken from the company’s web sites [22–49].

1.5 Manufacturers of Polymer Dispersions Company

Product lines

Applications

Air Products [1-22]

VAc, EVA, A

Asahi Kasei [1-23] Avecia[1-24]

A, Sp A, PU

BASF [1-25]

A, SB, PU, Sp

Clariant [1-26] Dow [1-27]

VAc, EVA, A, SB, A, VAc, PU, A, VAc

Adh, Coat, Con, I/GA, Pap, Tex Airbond, Airflex, Flexbond, Flexcryl, Valbond, Valtac, Vancryl, Vinac Adh, Coat Polytron, Sun Wrap Adh, Coat, I/GA NeoCryl, NeoRes, NeoPac, NeoRad, Haloflex Adh, Coat, Con, Pap, Tex Acronal, Basonal, Butofan, Butonal, Diofan, Emuldur, Luhydran, Luphen, Styrofan, Styronal Adh, Coat, Con, Tex Mowilith, Mowiplus, Appretan Coat, Con, Pap, Tex Dow Latex, UCAR Latex

Eastman Chem. [1-28] Elf Atochem [1-29] Enichem [1-30] Goodyear [1-31] BFGoodrich [1-49] now Noveon JSR Corporation [1-32] S.C. Johnson [1-33] Mitsubishi Chem [1-34]

A, EVA, VAc SB, NB A, SB, Sp A, NB, SB, PU, Sp

National Starch [1-35]

A, SB A, PU A, EVA, VAc, PU A, EVA, VAc,

Nitriflex [1-36]

NB, SB, Sp

Trade names

Adh, Coat, I/GA

Eastek, Eastarez, Waterborne Polymer Adh, Coat, Con, I/GA, Pap, Tex Repolem Adh, Pap, Tex Intex, Europrene, Latice Adh, Coat, Con, Tex Pliolite, Pliotec Adh, Coat, Con, Tex Aqueous XPD, Carbotac, Carboset, Carbobond, Goodrite, Hycar, Hystrech, Sancure, Vycar Adh, Coat, Con, Pap Glasca, Dynaflow Adh, Coat, I/GA Joncryl, SCX Adh, Coat, Con, Pap, Tex Rikabond Adh, Coat, Con, I/GA, Pap, Tex Vinamul, Dur-o-set, Dur-o-cryl, Nacrylic, Resyn Nitrilatex

Adh, Tex

Zeon Corp. [1-37]

A, SB, NB

Adh, Coat, Con, I/GA, Pap, Tex Nipol

Omnova [1-38]

A, SB, NB, VAc, Sp

Polymer Latex [1-39]

Revertex [1-42] Rhodia [1-43] Rohm&Haas [1-44]

A, NB, SB, PU, Sp A, SB, Vac A, EVA, NB, SB, VAc, Sp EVA, VAc A, VAc, SB A, VAc, PU

Solutia Inc. [1-45] Synthomer [1-46] UCB [1-47] Wacker [1-48]

A A, NB, SB A, PU EVA, VAc, Sp

Adh, Coat, Con, I/GA, Pap, Tex AcryGen, GenFlo, SunCryl, AcrylGen, AcrylPrint, GenCal, GenCryl, GenTac, OmnaBloc, Sequabond, Sunbond Adh, Coat, Con, Pap, Tex Acralen, Baystal, Baypren, Bunatex, Lipaton, Lipolan, Plextol, Perbunan Pap, Tex Raisional Adh, Coat, Con, I/GA, Pap, Tex Elvace, Pace, Plyamul, Synthemul, Tylac Adh Durabond Adh, Coat, Con, Pap, Tex Rhodopas, Rhodotak, Rhoximat Adh, Coat, Con, I/GA, Pap, Tex Lucidene, Primal, Polyco, Rhobond, Rhopaque, Rhoplex, Rovace Adh Gelva Adh, Con, I/GA, Pap, Tex Adh, Coat Ucecryl, Ucecoat Adh, Coat, Con Vinnapas, Wacker SMK

Raisio Group [1-40] Reichold [1-41]

Fig. 1-10 Major suppliers of aqueous polymers dispersions. Product lines: A acrylic dispersions, SB styrene butadiene dispersions, NB acrylonitrile butadiene dispersions, VAc vinyl acetate dispersions, EVA ethylene vinyl acetate

dispersions, PU polyurethane dispersions, Sp specialty dispersions. Applications: Adh adhesives, Coat coatings/paints, Con construction/building products, I/GA inks/graphic arts, Pap paper, Tex carpet/textile/non-woven.

13

14

References 1 Everett, D. H., Pure Appl. Chem. 31(4), 2

3 4 5 6

7

8

9

10 11

12

13

14 15 16 17

579–638, 1972. IUPAC Proposal for The nomenclature for Polymerization Processes and Polymers in Dispersed Systems. See also ISO 12000 Plastics/rubber-Polymer dispersions and rubber latices – Definitions and review of test methods. Blackley, D. C., High Polymer Lattices, two volumes, MacLaren, London, 1966. Warson, H., The Application of Synthetic Resin Emulsions, Benn, London, 1972. Piirma, I., Emulsion Polymerisation, Academic Press, New York, 1982. Blackley, D. C., Emulsion Polymerisation, Theory and Practice, Applied Science, London, 1975. Hölscher, F., Dispersions of Synthetic High Polymers, Part I, Properties, Preparation, Testing, Springer, Berlin, 1969. Reinhard, H., Dispersions of Synthetic High Polymers, Part II, Use, Springer, Berlin, 1969. Buscall, R., Corner, T., Stagemann, J. F., Polymer Colloids, Elsevier Applied Science, London, 1985. Athey, R. D., Emulsion Polymer Technology, Marcel Dekker, New York, 1991. Poehlein, G., Encyclopedia of Polymer Science and Engineering; Volume 6, Emulsion Polymerisation, J. Wiley, New York, 1986. Lovell, P. A., El-Asser, M. S., Emulsion Polymerisation and Emulsion Polymers, J. Wiley, New York, 1997. Asua, J. M., Polymeric Dispersions: Principles and Applications, (NATO ASI Series E: Appl. Science, Vol. 335), Kluwer Academic Publishers, Dordrecht, 1997. Colloid Polym. Sci., Steinkopf. Colloids Surf., Elsevier. J. Colloid Interface Sci., Academic Press. Langmuir, ACS Journal of Surfaces and Colloids, American Chemical Society.

18 J. Dispersion Sci. Technol., Coden. 19 Gilbert, R., G., Emulsion Polymerisation,

20

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

49

A Mechanistic Approach, Academic Press, London, 1995. Weissermel, K., Arpe, H.-J., Industrial Organic Chemistry, Major Organic Precursors and Intermediates, Verlag Chemie, Weinheim, 1994. P. Baum, J. Engelmann, Nachrichten aus der Chemie, 49/3, 368f, 2001. http://www.airproducts.com http://www.asahi-kasei.co.jp/asahi/ english/kasejusi.htm http://www.avecia.com/neoresins/ http://www.basf.de/de/dispersionen/ products http://www.clariant.com http://www.dow.com/emulpoly/ index.html http://www.eastman.com/ http://www.atofina.com/ http://www.enichem.it/english/ http://www.goodyear.com/ http://www.jsr.co.jp/main/english/ http://www.scjohnsonwax.com/ http://www.m-kagaku.co.jp/ http://www.Vinamulpolymers.com/ http://www.nitriflex.com.br/ http://www.zeon.co.jp/ http://www.omnova.com/ http://www.polymerlatex.de/ http://www.raisiogroup.com/ http://www.reichhold.com/ http://www.revertexfinewaters.com/ http://www.rhodia.com/ http://www.rohmhaas.com/ http://www.solutia.com/ http://www.synthomer.com/ http://www.ucb.be/ http://www.wacker.com/vip/ produktion/wacker/website/polymersystems/index_en.html http://www.bfgsolutions.com

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

2

Synthesis of Polymer Dispersions Mike A. Taylor

2.1

Introduction

The intent of this chapter is to give a short overview of the chemistry and manufacturing processes involved in the synthesis of emulsion polymers. While the equipment used in preparing an emulsion polymer is relatively simple, and the mechanism of the important reactions are fairly well understood, the development of new and improved products is often still carried out in a somewhat empirical fashion. Recipe and process conditions can frequently be designed, based on a theoretical knowledge, to produce specific polymeric and colloidal properties, but there are still large gaps in the knowledge needed to translate this into application behavior. In general, scale-up from laboratory to manufacturing gives good duplication of polymeric and colloidal properties, and laboratory equipment normally consists of simple stirred reactors, usually glass for non-pressure polymerizations, with a means of maintaining temperature control of the exothermic reaction. With non-pressure reactors, ingredients may be added under gravity, while pumps or inert gas pressure may be used for pressurized systems. Two important process features that are not reproduced well between small and large-scale reactors are heat transfer and shear. Laboratory reactors, with their large cooling surface to volume ratio and the large heat capacity of the reactor relative to the contents, do not normally pose any problems for cooling. In fact, heat losses often exceed heat generated by the reaction, necessitating heat input to maintain reaction temperature. Heat transfer, on the other hand, often limits production rates in large-scale reactors. In order to achieve a similar degree of mixing in vessels of different sizes, the most important scale-up criteria is usually to maintain the same power input per unit volume. Unfortunately, this translates to a higher agitation speed as reactor size reduces, a consequence of which is increased shear on the emulsion. Therefore, for the study of process characteristics, laboratory reactors have significant limitations. Figures 2-1 and 2-2 show modern laboratory facilities for non-pressure and pressure emulsion polymerization. Both batch and semi-batch reactions are regularly carried out on laboratory scale. The larger quantities involved in continuous poly-

15

16

2 Synthesis of Polymer Dispersions Typical laboratory apparatus for emulsion polymerization at atmospheric pressure (photograph courtesy BASF Corporation).

Fig. 2-1

Laboratory equipment for emulsion polymerization at high pressures (photograph courtesy BASF Corporation).

Fig. 2-2

merization generally rule out this process for laboratory scale reproduction, although the kinetics of a chain of multiple continuous stirred-tank reactors can be simulated with a batch reaction (Sect. 2.3.1). Reactions at low temperatures require the provision of refrigerated coolant. A simple recipe, which could be used to demonstrate the influence of ingredients and process on polymer and colloidal properties, is shown in Tab. 2-1. Subsequent sections of this chapter give greater detail on materials used to produce emulsion polymers. This recipe could be utilized for investigating both batch and semi-batch emulsion polymerization at a range of temperatures. With just these two monomers and one functional monomer, a very wide range of polymers with significant differences in polymer and latex properties can be produced (soft/hard, low/high molecular weight, tacky/non-tacky, stable/unstable, etc.)

2.2 Chemistry Tab. 2-1 Model system for the study of some aspects of emulsion polymerization.

Ingredient

Quantity (phm1)

Influence

Water Styrene

100–150 0–95

n-Butyl acrylate

0–95

Methacrylic acid

0–5

Sodium lauryl sulfate

0.5–3.0

Ammonium persulfate

0.1–1.0

t-Dodecyl mercaptan Divinylbenzene

0–1.0 0–0.5

Solids content; viscosity Glass transition temp; minimum film-forming Glass transition temp; minimum film-forming Colloidal stability; viscosity; reaction kinetics Particle size; colloidal stability; reaction kinetics Particle size; colloidal stability; viscosity; reaction kinetics; molecular wt. Molecular wt.; reaction kinetics Cross-linking/gel

1 Parts per hundred parts of monomer

2.2

Chemistry 2.2.1

Mechanism of Emulsion Polymerization

Strictly speaking, emulsion polymerization can take place in a system with only three components, a monomer that forms the structure of the polymer, water that acts as the continuous medium in which the polymer particles are dispersed, and an initiator that produces free radicals which start and maintain the polymerization process. However, at the very least the system will almost invariably contain a fourth ingredient, surfactant, which can provide the initial site, from which polymer particles subsequently grow, and/or give stability to the growing particles. In addition, most commercial recipes would normally include other ingredients to impart specific properties to the final polymer or emulsion, for example, a modifier to control the molecular weight of the polymer or a cross-linking agent to control the amount of gel. In many cases, ingredients used to control polymerization behavior will also exert their own influence on application properties of the final emulsion. Particularly, surfactants, while often determining the number of particles and their stability, can also have significant effects on such properties as adhesion, rheology, filler tolerance and many others. The overall formulation of an emulsion polymer is therefore often a compromise to obtain an optimum balance of properties. Rarely can the best of everything be achieved. The basic building block of any polymer is the monomer, characterized as a molecule containing at least one carbon-carbon double bond, C=C, and which, through a

17

18

2 Synthesis of Polymer Dispersions

free radical mechanism, can add on to itself, ultimately forming very large molecules of repeating units. Many different monomers (Sect. 2.2.2) are in use commercially for producing emulsion polymers, either as the sole monomer or, more usually, as combinations of monomers to give specifically desired properties. Polymerization is started when a free radical, originating from the decomposition of the initiator (Sect. 2.2.5), comes into contact with a monomer molecule and adds on at the site of the C=C double bond. This creates a monomer unit that is then itself a free radical and can in turn add on to another monomer molecule. The process continues, building up long chains of monomer units, until the free radical at the end of the chain comes into contact with some species other than a monomer molecule, normally another free radical, at which time the growing polymer chain is terminated. The free radical that terminates the chain can be an original radical, from the decomposition of the initiator, or a “polymeric radical” when the ends of two propagating chains terminate each other. Other species, such as inhibitors and short-stopping agents if present, can also cause termination to occur. These three main stages of polymerization are termed initiation, propagation and termination and can be denoted schematically as follows: Initiation Propagation

Ι → 2R• (decomposition of initiator) M + R• → R–M• R–M(n)• + M → R–M(n + 1)•

or transfer to polymer leading to branching Termination or

R–M(n)• + R–M(m)–R → R–M(n) + R–M•(m)–R R–M(n + 1)• + R• → R–M(n + 1)R R–M(n)• + R–M(m)• → R–M(n + m)–R

In such a system, the average molecular weight of the polymer chains is controlled primarily by the temperature of polymerization and the quantity of initiator. To exert additional control over molecular weight, a molecular weight modifier (chain transfer agent) is used. With chain transfer, a growing polymer chain is terminated but at the same time another radical is generated which can initiate polymerization of a further monomer unit, thus starting another polymer chain. Widely used chain transfer agents are the mercaptans, R–SH, where R is typically a twelve to fourteen hydrocarbon (t-dodecyl or n-dodecyl being the most common). Chain Transfer

R–M(n)• + R–SH → R–M(n)–SH + R•

These four mechanisms are common to all types of free-radical polymerizations, for example bulk, solution, suspension and emulsion. The difference between the processes is the environment. In bulk polymerization there exists only one phase, initially the monomer, then as polymerization progresses a solution of the polymer in its own monomer. Both polystyrene and poly(methyl methacrylate) are produced in large quantities by bulk polymerization. Solution polymerization is similar in that there is only one phase present, but in this case the monomer is diluted with a fully miscible solvent and the final polymer is in solution in the solvent. Polyacrylic acid, with the solvent being water, is produced by this technique. In suspension polymer-

2.2 Chemistry

ization, the monomer is dispersed in droplet form in a continuous medium that is usually water. The size of the droplets is typically in the range ten to one hundred microns. This process would be favored where an aqueous based polymer is required, but where the polymer is insoluble in the monomer. Polyvinyl chloride dispersions are made in this way. Emulsion polymerization is also carried out in a continuous water phase, but in this case the site of polymerization is a far smaller entity than dispersed monomer droplets, as is the size of the final polymer particles. Harkins [1, 2] developed a quantitative theory describing emulsion polymerization in an ideal system. This early model is still basically accepted today, and is described briefly as follows. In this process, monomer is “solubilized” within clusters of surfactant molecules, termed micelles, which form the nucleus of the polymer particle. or a pre-formed polymer particle of very small size, usually less than 50 nm, which is used as the seed for further polymerization. In the case of micellar nucleation, many surface active agents, when dissolved in water above a certain concentration (Critical Micelle Concentration or CMC), will form ordered clusters of molecules, with the hydrophobic portion of the molecule oriented toward the center of the cluster and the hydrophilic portion toward the outside. The size of these micelles is typically about 4 nm, the general shape being either spherical or lamellar. When a sparingly water-soluble monomer (which describes most of the monomers used in emulsion polymerization) is added to an aqueous solution containing these micelles, it becomes distributed in three sites; relatively large monomer droplets stabilized by surfactant molecules at the droplet surface, monomer molecules in solution in the water, and monomer molecules that diffuse into the micelles. The inside of the micelle, with the high concentration of the hydrophobic portions of the surfactant, provides an attraction for the hydrophobic monomer that diffuses through the water and swells the micelle. These monomer-swollen micelles are limited in size by hydrodynamic forces and interfacial tension. The number of monomer-swollen micelles in such a system is orders of magnitude greater than the number of monomer droplets present, and as a consequence the ratio of the surface areas is similarly large. For example, a dispersion of 50 weight percent monomer droplets in water would contain typically about 1010 monomer droplets per liter, whereas a system containing water, soap solution at a concentration greater than the CMC, and monomer could contain 1017–1019 monomer-swollen micelles per liter. This represents a total surface area of the swollen micelles approximately 105 times that of the monomer droplets. The consequence of this is that when free radicals are produced in the aqueous phase of a system containing water, surfactant and monomer, the free radical has a far greater probability of entering a micelle and initiating polymerization than it has of entering a monomer droplet. Also, the overall rate of polymerization, which is the rate per particle multiplied by the number of polymerizing particles, is greatly enhanced in the micellar system. In most cases, the initiators used in emulsion polymerization are water soluble, and the decomposition, either thermal or with the use of a reducing agent, to produce free radicals takes place in this phase. It is most probable that polymerization also starts in the aqueous phase, with free radicals initiating monomer molecules in

19

20

2 Synthesis of Polymer Dispersions

Fig. 2-3

Emulsion polymerization [7].

solution in water. As monomer units are added on, these “oligomeric radicals” increase in hydrophobicity and hence the probability of entering a monomer-swollen micelle increases. A radical which enters a micelle will then continue to add on monomer using the reservoir within. As long as there is a source of monomer outside the micelles, such as monomer droplets, the monomer within a growing particle will be replenished by diffusion from the droplet through the aqueous phase and into the particle, the driving force being the affinity of the monomer for the polymer. The solubilization of the monomer in the micelles and the mechanism of growth of the polymer particles are depicted in Fig. 2-3. Polymerization will continue within the particle until either all of the monomer has been depleted or another radical enters the particle and terminates the growing chain. If termination occurs, the particle will then remain “dead” until another radical enters and initiates a new chain. With polymerization taking place within a particle and fresh monomer entering, the particle obviously increases in size during the process. Stability is maintained by further adsorption of surfactant molecules at the surface, along with other mechanisms discussed in Sects 2.2.3 and 2.2.4. Other sparingly water-soluble ingredients, such as chain transfer agents, follow the same route, diffusion through the aqueous phase, to enter the growing polymer particles. Diffusion of molecules into particles is not usually a limiting step in the overall rate of polymerization, but can be a limit on other processes (Sect. 2.2.6). An alternative to micellar nucleation, and much practiced today in industry, is the use of preformed polymer particles of very small and uniform size, normally less than 50 nm, which act as the nucleus for further polymer growth. This is known as seeded emulsion polymerization.

2.2 Chemistry

Within an individual particle, assuming an active radical is present, the rate of polymerization is dependent on the particular monomer and the concentration of monomer in the monomer-polymer mixture. The rate of addition of a monomer molecule onto a growing polymer chain is known as the propagation rate of the monomer, kp, this being temperature dependent with increasing temperature giving increasing propagation rate. Thus in a system with N total particles, and with an av– , the overall rate of polymerization erage number of radicals per particle denoted by n is given by: R = k p ⋅ N ⋅ n ⋅ [M ] [M], the monomer concentration in the swollen particles, is normally expressed as mol L–1, giving the overall rate of polymerization in mol s–1. It is evident that, with a constant number of particles at a constant temperature, the overall rate will change according to the average number of radicals per particle and the monomer concentration in the particle. Smith and Ewart [3] developed an early quantitative theory to predict the rate of polymerization in an emulsion system, where they describe three regions. Typically there is a short induction period as the flux of free radicals builds up, following which a period occurs during which the entry rate of free radicals into particles is less than the exit rate (region 1). During this period the average number of radicals per particle can be much less than unity. Region 2 is quickly reached, where exit of radicals from particles becomes negligible. While the particles are small and still have high concentrations of monomer, diffusion of radicals within the particles and mobility of the polymer chains is unrestricted. Under these circumstances, a maximum of one growing radical is thought to exist per particle, that is, when a radical enters a particle which already contains a growing polymer radical, termination of the chain will occur almost instantly. On average, therefore, only one half of the total number of particles will be actively polymerizing at any given instant, that is the average number of radicals per particle is about one half. It then remains at this value until overall conversion reaches 50–60 %. As particles grow larger and the polymer/monomer ratio increases, distances within the particle become greater, viscosity of the mixture increases, and chain entanglement and cross-linking all contribute toward reduced mobility within the particle. In this case termination is not instantaneous, and an entering radical can co-exist with an already growing chain. This gives rise to an increase in the overall rate of polymerization in the system (region 3), and is referred to as the gel effect. In a styrene-butadiene system, the average number of radicals per particle does not usually exceed two, but with butyl acrylate polymerization values of twenty and higher often occur. Figure 2-4 shows this relationship. Monomer concentration starts off at one hundred percent in the monomerswollen micelles, then drops rapidly when polymerization begins. The polymer formed is not infinitely swellable, the swollen size of the particles being limited by entanglement and crosslinking of polymer within the chain and by hydrodynamic forces and interfacial tension. Typically the weight fraction of monomer in the monomer-polymer mixture is limited to about 0.45 maximum. As long as there is a

21

2 Synthesis of Polymer Dispersions Typifying the variation of average number of radicals per particle with conversion. (SB system).

Fig. 2-4

n - Av. radicals/particle

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

20

40

60

80

100

% Conversion

greater quantity of monomer in the total system, the weight fraction in the particles will remain at 0.45, with the excess in the form of monomer droplets. When the monomer droplets have been exhausted, the weight fraction of monomer in the particles will reduce, reaching zero at one hundred percent conversion. This is depicted in Fig. 2-5. Weight Fraction Monomer M/(M+P)

22

1 0.8 0.6 0.4 0.2 Typical monomer concentration in the polymer particles as a function of conversion.

Fig. 2-5

0 0

20

40

60

% Conversion

80

100

It can be seen in Figs. 2-4 and 2-5 that, very shortly after the start of polymeriza– and [M] become constant, usually to beyond 50–60 % conversion. The retion, both n sult of this is a constant rate of polymerization over this period. The normal type of conversion-time curve for a batch polymerization is shown in Fig. 2-6. After a short – increases. This is followed by a induction period, the rate of reaction increases as n constant rate period. At around sixty percent conversion, the rate often shows an in– has a greater influence than decreasing [M]. Finally the crease, where an increasing n decreasing monomer concentration has the biggest influence on rate, which thereafter decreases. In his book on emulsion polymerization, Blackley [4] gives a comprehensive review of the development of the theory of the subject.

2.2 Chemistry Conversiontime curve for a typical batch emulsion polymerization.

100 % Conversion

Fig. 2-6

80 60 40 20 0 0

1

2

3

4

5

6

7

8

9

10 11

Time h

2.2.2

Major Monomers

The major monomers are considered as those that make up the bulk of the final polymer chains, being normally greater than five percent of the final polymer composition. Not included here are the so-called functional monomers, discussed in Sect. 2.2.3, which are generally used at levels of less than five percent of the total composition, and which are used to impart certain special properties to the latex or polymer. A large number of major monomers are used in emulsion polymerization, either by themselves to give homopolymers containing recurring monomer units of the same type or, more frequently, as mixtures giving copolymers (two different monomer units), terpolymers (three different monomer units) or polymers with even higher order. Generally, free-radical polymerization is a random process with the different monomer units distributed randomly in the polymer molecules. However, the different reactivities of free radicals with different monomers does lead to uneven distribution of monomers throughout the polymer chains. One of the major determining factors in the choice of a monomer is the glass transition temperature, Tg, of the homopolymer. This is the temperature at which the polymer changes from a glassy state to an elastomeric material, a change that takes place over a relatively narrow temperature range. Table 2-2 lists a number of widely used major monomers in order of increasing Tg. The Tg of polymers made up from mixtures of different monomers can be approximated by use of the Fox equation [5]: 1 Wm1 Wm2 W = + + … + mn Tg Tg1 Tg2 Tgn

where Tg refers to the final polymer, Tg1, Tg2 … refer to the individual homopolymers, and Wm1, Wm2 … are the weight fractions of the different monomers making up the final polymer composition. It can be seen that, with 1,3-butadiene and methyl methacrylate as monomers, a copolymer can be made with any desired Tg in the

23

24

2 Synthesis of Polymer Dispersions Tab. 2-2

Some major monomers used in emulsion polymerization.

Monomer

Structure

1,3-Butadiene n-Butyl acrylate 2-Ethylhexyl acrylate

CH2=CH–CH=CH2 CH2=CH–C(O)–O–(CH2)3–CH3 CH2=CH–C(O)–O–CH2– CH(CH2CH3)–(CH2)3–CH3 CH2=CH–C(O)–CH3 CH2=CH–O–C(O)–CH3 CH2=CH–Cl CH2=CH–CN CH2=CH–(C6H5) CH2=C(CH3) CH3C(O)–O–CH3

Methyl acrylate Vinyl acetate Vinyl chloride Acrylonitrile Styrene Methyl methacrylate

Normal b.p. (°C)

Tg of homopolymer (°C)

–4.4 148

–85 –54

216 80 73 –13 77 145 100

–50 10 32 81 97 100 105

range –85 °C to +105 °C. One of the important attributes of polymers which is related to the Tg is the film-forming temperature, normally very close to the Tg. As a polymer latex dries and the water evaporates, if the polymer is at a higher temperature than the Tg then the molecules in an individual particle have enough freedom of movement to penetrate and intertwine with molecules in an adjacent particle. In this way, the polymer can form a coherent film. Below the Tg, the movement of molecules is too restricted to allow this interpenetration between particles, and a coherent film cannot form. Of course in the design of a polymer, the choice of monomers is not only made on the basis of the required Tg. Many other polymer properties are of importance. For example, vinyl chloride may be used where fire retardency is required; acrylonitrile can impart solvent resistance; acrylates tend to give good heat and light aging properties. All of the monomers listed in Tab. 2-1, apart from one, are characterized by having only one C=C double bond, and are known as vinyl monomers. However, 1,3butadiene is a member of the diene group of monomers, characterized by having two C=C double bonds. Isoprene is another common diene monomer. The presence of this second double bond results in both differences in the polymerization mechanism of a diene relative to a vinyl monomer and in the subsequent behavior of the polymer. During free-radical polymerization, the butadiene molecules become incorporated into the polymer chain through one of the C=C bonds. This can occur in one of three different ways: First, by a process of electron transfer, the molecule can be linked into the chain through carbon atoms 1 and 4, the remaining C=C double bond being between carbon atoms 2 and 3. This repeat unit is called 1,4 and there are two possible isomeric forms, cis-1,4 with the carbon atoms of the double bond both on the same side of the backbone chain, and trans-1,4 with the carbon atoms of the double bond on opposite sides of the chain. A third alternative, with the butadiene linked into the chain through carbon atoms 1 and 2, has the C=C double bond hanging off the chain as a pendant vinyl group. This incorporation is known as 1,2. Figure 2-7 shows these three possibilities. Isoprene can be incorporated in four different ways.

2.2 Chemistry Possible methods of incorporation of butadiene during free-radical polymerization. Fig. 2-7

*

cis -1,4

* H

H H

C

H H C H C C C H H * C

*

C n

C

H

H C C

H H

1,2 (pendant vinyl)

H C

*

H

C

H Hn trans -1,4

H

n

H

*

The presence of these additional C=C bonds in the polymer, generally referred to as unsaturation, can be of benefit or it can be deleterious to polymer properties. The double bonds in the backbone chain can be used to give controlled crosslinking between chains. The process of vulcanization uses controlled amounts of sulfur to achieve a desired degree of crosslinking and producing a thermosetting polymer. Also, during free-radical polymerization, the 1,2 pendant vinyl groups compete with monomer for addition onto a growing free radical, so that crosslinking actually occurs during propagation. This can be controlled to some degree by the choice of process conditions (Sect. 2.3.2), but cannot be completely eliminated. On the negative side, the presence of unsaturation in diene polymers leads to inferior heat and light aging properties relative for example to acrylics, the residual double bonds being attacked by oxygen, UV radiation etc., eventually leading to yellowing and brittling of the polymer. The general characteristics that control a polymer’s behavior are basic chemical composition, crystallinity, glass transition temperature, molecular weight and distribution, gel and crosslinking. Some ways in which desired properties can be achieved are shown in Tab. 2-3. Of course this Table only shows a few possibilities in polymer Tab. 2-3

Some aspects of polymer design through composition.

Desired property

Possible polymer design

Stiffness Soft hand Tackiness Water resistance

Use methacrylates, acrylonitrile, styrene Use n-butyl acrylate, ethyl acrylate, butadiene Use 2-ethylhexyl or hexyl acrylate Use crosslinking monomers N-methylol(meth)acrylamide. Use hydrophobic monomers like n-butyl acrylate or styrene Crosslinking monomers and/or acrylonitrile High Tg monomers like styrene, acrylonitrile or methyl methacrylate Low Tg monomers like n-butyl acrylate or butadiene Avoid crosslinking. Use thermoplastic monomers like styrene Use high amounts of polymerizable acids like acrylic acid

Resistance to organic solvents High tensile strength High elongation Thermoformability High alkali swellability

25

26

2 Synthesis of Polymer Dispersions

design. There are vast numbers of different potential combinations of monomers available to choose from, each with variations in molecular weight, branching, crosslinking etc., giving almost infinite possibilities in the balance of properties obtained. 2.2.3

Functional Monomers

Certain monomers are characterized as functional monomers, so called because in addition to having the polymerizable C=C double bond they contain a functional group such as a carboxylic acid or amide. These monomers are important because they can impart special properties to both the polymer and the colloidal system. They are normally used in relatively small amounts, typically 2–5 % of the dry polymer. Table 2-4 lists some of the commonly used functional monomers. Acrylic and Methacrylic acids are the most widely used monobasic carboxylic acids, with Itaconic and Fumaric acids as common dibasic acids. These acids, through the C=C bond, participate in the free-radical polymerization and become incorporated in the main polymer, but due to the highly polar carboxyl group (COOH) tend to be at the surface of the polymer particles (polymer-water interface) with the carboxyl group orientated toward the aqueous phase. The acid group is ionized in water, so that the particle surface has a negative charge at each acid site (–COO–). The negative charge at the surface imparts a high degree of stability to the polymer particles, particles repelling each other due to the like charges. This stabilizing influence is the same as that produced by surfactants, but with the added advantage that the carboxylic acid is bound into the polymer chains, not just adsorbed at the particle surface. To maintain overall electrical neutrality across the interface, the layer of negative ions is balanced by an adjacent layer of cationic counterions. The ions and counterions are referred to as the electric double layer and the thickness of this layer is very dependent on the pH of the continuous medium. At low pH (high H + concentration) the layer is compressed and at its minimum thickness. As the pH is increased (reducing H + concentration), the layer expands outward from the particle. The thickness of this double layer contributes to the effective diameter of the latex particle, and is one reason for increasing viscosity as pH increases. It should be noted that the presence of watersoluble polymer in the latex could also contribute strongly to viscosity increase with increasing pH, due to stretching of the chains. Tab. 2-4

Commonly used functional monomers.

Functional monomer

Structure

Acrylic acid Methacrylic acid Itaconic acid Fumaric acid Hydroxyethyl acrylate Acrylamide

CH2=CH–C(O)–OH CH2=C(CH3)–C(O)–O–H CH2=C(C(O)–OH)–CH2–C(O)–O–H H–O–C(O)–CH=CH–C(O)–O–H CH2=CH–C(O)–O–CH2–C(OH)H2 CH2=CH–C(O)–NH2

2.2 Chemistry

In addition to the greatly enhanced mechanical stability imparted to the emulsion by these functional monomers, stability to electrolytes is generally improved, as is filler tolerance of the latex. Mechanical strength of the polymer films is increased, and in fact can be increased further by the use of, for example zinc oxide, which gives an ionic crosslink between carboxyl groups. The presence of the carboxyl groups also allows crosslinking through the use of urea-formaldehyde, phenol-formaldehyde, melamine-formaldehyde and various epoxy resins. Polymerization of the acid functional monomers is usually carried out under conditions of relatively low pH. Neutralization of the acid favors partitioning in the aqueous rather than the organic phase, reducing incorporation into the polymer and at worst, where homo-polymerization of the acid is a possibility, as with acrylic and methacrylic acids, the formation of polyacrylic acid salts in the aqueous phase. These high molecular weight polyelectrolytes can act as very effective coagulants for the latex. 2.2.4

Surfactants

As first discussed in Sect. 2.1.1, surface-active materials are normally an essential ingredient in emulsion polymerization. They can be used in any or all of the following roles: – micellar solubilization of monomers, forming the primary sites for nucleation – stabilization of growing polymer particles – enhancement of application properties of the finished latex A single surfactant may satisfy all three roles, or there may be a requirement for multiple surfactants. The optimum choice of surfactant for one role may produce undesirable performance in other roles, and there may be positive or negative synergism exhibited when multiple surfactants are used. As with many other aspects of emulsion polymerization, a compromise is often required in the choice of surfactant. A key phenomenon observed with surfactants is a marked change in a number of physical properties of an aqueous solution that takes place at a certain critical concentration. For example, with ionic surfactants, equivalent conductivity exhibits a sharp reduction, surface tension reaches a minimum then begins to increase, interfacial tension reaches a minimum and osmotic pressure almost plateaus. Below this concentration, surfactant molecules are in normal random solution in the water. As the critical concentration is reached, the surfactant molecules aggregate into ordered clusters known as micelles, normally considered to be spherical but other geometry such as lamellar is also possible. The break in the behavior of solution properties represents the change from a true solution to a colloidal solution. The concentration of surfactant at which this change takes place is known as the Critical Micelle Concentration (CMC), and the number of molecules of surfactant which form one micelle is called the agglomeration number. Addition of further surfactant above the CMC all goes toward increasing the number of micelles. The main characteristic of a surfactant is that its molecular structure consists of two parts, a lyophobic (solvent hating) portion and a lyophilic (solvent loving) por-

27

28

2 Synthesis of Polymer Dispersions

tion. When the solvent is water, these two groups are referred to respectively as hydrophobic and hydrophilic. The hydrophobic portion of a surfactant is usually a longchain hydrocarbon or oxygenated hydrocarbon, although other structures are possible. The hydrophilic group is either ionic or highly polar. Surfactants are classified according to the nature of the hydrophilic group of the molecule: – Anionic, where the hydrophilic group has a negative charge. Examples are carboxylic, sulfate, sulfonate and phosphate groups. – Cationic, where the hydrophilic group has a positive charge. Long chain quaternary ammonium salts and long chain amines and amine salts, with or without incorporation of ethylene oxide units. – Non-ionic, with no charge on the hydrophilic group, often polyoxyethyleneated long chain alcohols or alkylphenols, the hydrophobic/hydrophilic balance being controlled by the number of moles of ethylene oxide. – Zwitterionic, with both positive and negative charge on the hydrophilic group. Anionic and cationic surfactants are not compatible with one another. Non-ionic and zwitterionic types are compatible and can be used with either anionics or cationics. Certain zwitterionics become cationic at low pH and anionic as the pH is increased. Some of the structures that can make up the hydrophobic group are straight or branched long alkyl groups, long chain alkylbenzene residues, alkylnaphthalene residues, rosins and high molecular weight propylene oxide polymers. The alkyl groups are generally 3–20 carbon atoms long, and in most cases, because of the source of the alkyl group, a particular surfactant will actually be a mixture of various chain lengths. The properties of the surfactant depend on the length of the hydrophobic group, branching or unsaturation, the presence of an aromatic group or the incorporation of propylene oxide units. Rosen [6] discusses the influence of different hydrophobic and hydrophilic groups in detail. Table 2-5 gives an example of the influence of some different structures on agglomeration number and CMC. The effect of increasing linear alkyl chain length in a series of sodium alkyl sulfonates is Tab. 2-5

Some structural influences on surfactant properties (Rosen [6]).

Surfactant

Formula

Agglomeration number / T (°C)

CMC at 40 °C (mol L–1)

Sodium octyl sulfonate Sodium decyl sulfonate Sodium dodecyl sulfonate Sodium tetradecyl sulfate Sodium hexadecyl sulfate Sodium dodecyl sulfate Branched sodium alkyl sulfate Sodium dodecyl ethoxylate (2EO) Dodecyl alcohol ethoxylate (5EO) Dodecyl alcohol ethoxylate (7EO) Dodecyl alcohol ethoxylate (8EO)

C8H17SO3– Na+ C10H21SO3– Na+ C12H25SO3– Na+ C14H29SO3– Na+ C16H33SO3– Na+ C12H25SO4– Na+ C12H25CH(SO4– Na+)C3H7 C12H25(OC2H4)2SO4– Na+ C12H25(OC2H4)5OH C12H25(OC2H4)7OH C12H25(OC2H4)8OH

25 / 23 40 / 30 54 / 40 80 / 60

1.6 × 10–1 4.0 × 10–2 1.1 × 10–2 2.5 × 10–3 7.0 × 10–4 8.6 × 10–3 1.7 × 10–3 2.8 × 10–3 5.9 × 10–5 7.3 × 10–5 9.3 × 10–5

2.2 Chemistry

seen, increasing carbon number giving rise to reducing agglomeration number and CMC. The sulfate group is seen to give a lower CMC than the sulfonate. Inclusion of the sulfate anion on a non-terminal carbon atom increases the CMC, and the introduction of polyethylene oxide (2 mol) into the sulfate reduces CMC. Finally, increasing the moles of ethylene oxide in the non-ionic series of polyoxyethyleneated straight chain alcohols is seen to increase the CMC. In emulsion polymerization, anionic and non-ionic surfactants are the most common choice during the polymerization stage. Cationics are used in polymerization for some applications, but the use of cationics and anionics in the same equipment is generally avoided. It is possible to produce an emulsion polymer with either an anionic or a cationic surfactant, and subsequently switch to an oppositely charged species along with a controlled pH change. Zwitterionics are not common in emulsion polymerization. If the surfactant is serving the dual purpose of providing nucleation sites and subsequently stabilizing the growing particles, balancing the two requirements can be difficult. For micelle formation, the concentration of surfactant must be at or above its CMC. Then, as a general rule, for a specific surfactant the number of polymer particles initiated is approximately proportional to [surfactant concentration]0.6. It is therefore often the case that the amount of surfactant required to give the desired ultimate particle size is insufficient to provide continued stabilization as the particles grow. This can occur almost anywhere within the normal particle size range, and as a consequence it is usually necessary to add additional surfactant as polymerization progresses. Addition of too much surfactant during polymerization can, if the CMC is exceeded, cause another family of particles to be initiated. The difficulty of balancing nucleation and stability is exacerbated by the fact that many factors influence nucleation, for example temperature, concentration of initiator/surfactant/electrolyte, pH and any impurities that either retard or increase polymerization. Seeded processes significantly reduce this variation and eliminate the requirement for the initial surfactant. Overall, with seeded processes, the total amount of surfactant is often considerably less than with micellar nucleation. Some common surfactants used in emulsion polymerization are: – Sodium and potassium salts of naturally occurring fatty acids (oleic, linoleic) and rosin acids. These soaps are used in large quantities in the production of styrenebutadiene latex for both dry rubber production and latex applications. These materials are only useful at pH values greater than 7, normally being used at pH 10–12. Below pH 7, the insoluble acids are precipitated, and all stabilizing function is lost. – Salts of sulfated linear alcohols, for example sodium lauryl sulfate, are widely used in the emulsion polymerization of functionalized styrene–butadiene polymers and many acrylic esters. – A range of the salts of alkylbenzene sulfonates and alkylnaphthalene sulfonates, which give improved electrolyte stability and are not subject to hydrolysis in acid media as are the sulfated alcohols. Sodium dodecylbenzene sulfonate is widely used. – Salts of alkylphosphates, usually polyoxyethyleneated.

29

30

2 Synthesis of Polymer Dispersions

– non-ionic surfactants in wide use in emulsion polymerization include polyoxyethyleneated alkylphenols and straight chain alcohols, where the length of the alkyl group and the moles of ethylene oxide can be varied, and polyoxyethyleneated polypropylene glycols, block copolymers where the moles of ethylene and propylene oxides can be varied to adjust the hydrophilic/hydrophobic balance. This list is by no means exhaustive, there being an almost limitless choice of surfactants or combinations of surfactants available. From the aspect of particle stabilization during the emulsification process, and even to a large extent nucleation, the choice of surfactant is usually not too critical. By far the biggest factor in the choice of surfactant is the application performance of the final product. Unfortunately as a general rule, the presence of surfactant in the final dry polymer causes reduced water resistance. Also, there is a tendency for surfactant molecules to diffuse to the polymer/air or polymer/substrate interface, where deleterious effects (cloudiness at the surface, loss of tack, etc.) are often caused. This again demonstrates the compromise often necessary in emulsion polymer synthesis. In an attempt to mitigate against migration of surfactant, there are certain products available, known as “polymerizable surfactants”, where the molecule contains a polymerizable C=C double bond. (This is a functional monomer because it does not form micelles.) Examples are the Noigen and Hitenol series of products from DaiIchi Kogyo Seiyaku (polyethoxylated alkylpropenyl phenyl ethers and polyethoxylated alkylpropenyl phenyl ether sulfates respectively). 2.2.5

Initiator Systems

The initiator system in emulsion polymerization is the source of free radicals. There are two major types of system used, substances which thermally decompose to produce free radicals and substances which produce free radicals when part of a redox system. Light or other radiation can generate free radicals, but is not widely used for emulsion polymerization. By far the most common thermal systems are peroxy compounds; ammonium, sodium and potassium persulfate and a wide range of organic peroxides and hydroperoxides. The rate of decomposition of these materials is usually specified by the “half-life”, defined as the time taken at a particular temperature for the concentration of a solution of the material to reduce to one half of its initial value through thermal decomposition. The three persulfates have a similar half-life and their effectiveness in emulsion polymerization is therefore also similar. However, the lower water solubility of the potassium salt makes it less commonly used than the others. Persulfates are generally used for polymerization in the temperature range 50–100 °C, and production of free radicals takes place in the aqueous phase of the emulsion. At higher temperatures, decomposition is usually too fast to give efficient use of the free radicals due to radical recombination. At lower temperatures, persulfates can still be used in conjunction with a reducing agent such as sodium bisulfite. The organic peroxides and hydroperoxides cover a wide range of half-life, and can provide an appropriate choice for most normal polymerization temperatures. The

2.2 Chemistry Tab. 2-6

A range of thermally dissociating initiators (from manufacturers’ product literature). Half-life (h)1

Substance

Dicyclohexyl peroxydicarbonate Ammonium persulfate Dilauryl peroxide Dibenzoyl peroxide t-Butyl peroxybenzoate Dicumyl peroxide Cumene hydroperoxide t-Butyl hydroperoxide

40 °C

50 °C

70 °C

18

4.1 192 50

0.27 8.4 3.2 14

90 °C

0.55 0.29 1.2 70

110 °C

0.13 6 23 570

130 °C

150 °C

0.7 2.3 100 520

0.26 20 70

1

Approximate values only. pH and the presence of other components can significantly influence decomposition

differing solubilities in water also determine if the free radicals are produced in either the aqueous or the monomer phase. Table 2-6 lists some of the commonly used thermally dissociating initiators, along with their half-life. Most commonly, the peroxides and hydroperoxides are used at lower temperatures, 0–50 °C, as a part of a redox system. Often persulfates are chosen in preference to the organic peroxides because of the increase in colloidal stability that results from the sulfate end groups on the polymer chains. On the negative side, these sulfate groups also increase the water sensitivity of dried polymer films. The thermal decomposition of persulfate produces both sulfate and hydroxyl radicals, according to the mechanism: S2O82– → 2SO4–• SO4 + H2O → HSO4– + HO• 2OH• → H2O + 1⁄2O2 –•

It is generally accepted that the primary initiating species is the sulfate anion radical, and to the extent that termination is predominantly caused by another sulfate initiated radical species, it is expected that most polymer chains would contain two sulfur atoms. This is generally found to be the case. An organic peroxide decomposes as follows: ROOR → 2RO• and the reduction of a hydroperoxide by iron(II): ROOH + Fe2+ → RO• + HO– + Fe3+ During the nucleation stage of emulsion polymerization, the concentration of initiator exerts an influence on the number of polymer particles formed. An initiated particle grows very rapidly as polymer is formed. As the particle increases in surface area, it adsorbs surfactant from solution thus reducing the possible number of micelles in the system. Therefore, the faster that initiation occurs, then the greater will

31

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2 Synthesis of Polymer Dispersions

be the number of polymer particles formed before micellar surfactant is exhausted. Because the thermal decomposition of the initiator is first order with respect to its concentration, the higher the concentration, the greater will be the rate of radical production and the shorter the initiation period. The number of polymer particles formed is approximately proportional to the concentration of initiator to the power of 0.4. As discussed in Sect. 2.2.1, the overall rate of polymerization is proportional to the number of polymer particles, and therefore the consequent rate of polymerization is also proportional to [Initiator]0.4. However, after completion of the nucleation stage, subsequent changes in the initiator concentration (assuming a certain minimum) have little effect on the rate of polymerization. This is due to the fact that the number of particles remains fairly constant, and the average number of radicals per particle is dependent on the environment within the particle, not the rate of radical production. The molecular weight of the polymer is however influenced by the initiator concentration, because, neglecting other influences, the time available for a polymer chain to grow is dependent on the rate at which free radicals enter the particle. One entering radical initiates polymerization, the next normally terminates the chain. The higher the concentration of initiator, the higher the rate of production of radicals and the higher the rate of entry of radicals into a particle. Although this effect on molecular weight is significant, chain transfer agents often exerts a bigger influence. 2.2.6

Other Ingredients

The use of chain transfer agents in emulsion polymerization was briefly discussed in Sect. 2.2.1. As stated, the most commonly used chain transfer agents are the mercaptans (thioalcohols) RSH, although a wide range of other compounds also exert a modifying effect during polymerization, for example carbon tetrachloride, certain disulfides, rosin acid salts, 4-vinylcyclohexene (butadiene dimer) amongst many others, which may also include impurities in other raw materials. The effectiveness of a chain transfer agent is denoted by its transfer constant, ε, which is the ratio of rate of the chain transfer step to the propagation step: Ks R–M(n)–SH + R• R–M(n) • + R–SH → • Kp R–M(n) + M → R–M(n + 1)• Ks =ε Kp

It can be shown that, neglecting all other reactions of monomer and chain transfer agent, that a plot of log [S] against log [M], where [S] and [M] are the concentrations of modifier and monomer respectively, should be a straight line with slope ε. This linear relationship is generally found to hold true in emulsion polymerization, but the slope is often not equal to ε as determined from bulk polymerization. This is because the rate-determining step for mercaptan consumption can often be the diffusion of mercaptan through the aqueous phase into the reaction zone in the polymer particle. In general, the fewer the number of carbon atoms in the alkyl group of the

2.2 Chemistry

mercaptan, the closer the apparent transfer constant in emulsion polymerization becomes to that measured in bulk (faster diffusion). This means that the shorter chain mercaptans tend to disappear more quickly than the longer chains. Thus the short chain modifiers tend to exert a greater modifying influence during the early stages of a batch emulsion polymerization, whereas the longer chain alkyl groups tend to be more effective toward the end of polymerization. C12 chains often show the best balance between these extremes, hence the very common use of n-dodecyl and t-dodecyl mercaptans. Of course, the overall effect of the chain transfer agent can also be controlled by making injections at appropriate times during polymerization, or with semi-batch reactions by having continuous (linear or non-linear) feeds of modifier. Polymerizable cross-linking agents are often included in emulsion polymerization recipes, either to form cross-links during the polymerization (increasing the gel in the polymer, or increasing the so-called “green strength”) or to form cross-links subsequent to polymerization, by heat application or chemical means. As previously discussed, when a diene is one of the monomers in an emulsion polymerization, the second double bond in the diene will lead to a considerable degree of cross-linking during the polymer formation. In the absence of a diene, other monomers that are commonly used to produce cross-links during the polymer formation are divinyl benzene and a range if diacrylates and triacrylates, for example butanediol diacrylate and trimethylolpropane triacrylate. These materials would normally be incorporated at low levels, <0.1 weight percent of the monomer, so one can see that, on average, the number of cross-links introduced would be less than one per thousand monomer units. To introduce heat sensitivity into the polymer (cross-linking which occurs after polymerization, usually when a dried polymer film is heated), two commonly used monomers are N-methylolacrylamide and N-methylolmethacrylamide. The structure of N-methylolacrylamide is as follows: CH2=CH–C(O)–NH–CH2–OH Incorporation into the polymer backbone takes place through the vinyl double bond. Cross-linking then takes place between two methylol groups. |

|

CH2–CH–C(O)–NH–CH2–NH–C(O)–CH–CH2  Electrolytes, most often alkali metal phosphates or sulfates, are utilized in emulsion polymerization systems for a variety of reasons. When present in a micelleforming surfactant solution, electrolytes can increase the aggregation number of the micelles (the number of soap molecules per micelle). Thus with increasing electrolyte concentration the number of micelles is reduced, and this is an additional means of controlling the number of polymer particles formed. Electrolytes are also used to reduce the viscosity of the polymer emulsion, especially during the polymerization process, an effect that is achieved through compression of the electric double layer. Finally, electrolytes are often used as part of a buffer system to minimize pH variation during polymerization. High concentrations of electrolytes will generally cause de-stabilization and agglomeration of polymer particles.

33

34

2 Synthesis of Polymer Dispersions

It is common to include a chelating agent, normally a salt of ethylenediamine tetraacetic acid (EDTA), in emulsion polymerization recipes. Use of this compound to chelate metal ion impurities in the system, particularly calcium, magnesium and iron, generally leads to lower coagulum levels in the final latex, and often gives more consistent initiation of polymerization. It is fairly common practice in the manufacture of emulsion polymers at higher temperatures to use untreated (non-deionized) water as the continuous media. The use of a chelating agent protects against the “hardness” salts. In low temperature systems with redox initiators, a chelating agent is often a part of the redox system, used to complex the iron component and prevent precipitation.

2.3

Manufacturing Processes 2.3.1

Types of Process

Compared with many other types of chemical manufacture, the production of emulsion polymers is relatively simple in terms of the unit operations required. Typically there is a reaction stage, where the polymer is made, a purification stage to remove residual organic volatile components, some type of filtration or screening to remove any coagulum from the latex and a final stage where post additions of other ingredients may be made along with final adjustment of latex properties such as pH and solids content. Occasionally there may be a concentration of the latex to reduce the water content, and for some processes there may be some purification of raw materials, or recovery and recycling of raw materials. Industrial chemical processes are categorized as batch, semi-batch or continuous, and the manufacture of emulsion polymers is carried out in all these process types. The processes differ not only in equipment type and economics of operation but also in the specific properties imparted to the polymer and the emulsion. In a batch emulsion polymerization, all ingredients are added to a vessel, polymerization is initiated and the reaction proceeds to completion over a period of time. Thus, conditions in the reactor gradually change from monomer + water → polymer + water, passing through all intermediate ratios of monomer/polymer. A continuous polymerization may be carried out in a continuous, stirred-tank reactor, where the reactants continuously enter a stirred vessel and the product continuously exits. Here the conditions in the reactor remain constant with time, the composition being equal to the exit composition. There may be only one CSTR, or multiple CSTRs in a chain, the product from one being the feed for the next in the chain. The greater the number of CSTRs in a chain, the closer the properties approach those from a batch reaction. A plug-flow continuous reactor is one in which the reacting mixture passes through the reactor without any forward or backward mixing, as for example in a tubular reactor. In this type of system, at any given position in the reactor, the composition is constant with time. Distance along the reactor is equiva-

2.3 Manufacturing Processes

lent to time in the batch reactor. Although emulsion polymerization in a continuous tubular reactor has been the subject of much research in the past, this process is not commonly used because of the poor degree of mixing, heavy fouling of the reactor and the difficulties of cleaning. In a semi-batch process, only a portion of the total ingredients is added to the reactor initially, polymerization is initiated and the remainder of the ingredients is added over a period of time until the desired filling volume is reached. The material is then discharged and the process repeated. In the semi-batch process, conditions in the reactor change rapidly when the feeds start (monomer → monomer + polymer), remain relatively constant for the majority of the feed period, and change rapidly again when the feed stops (monomer + polymer → polymer). For safety reasons (Sect. 2.3.4) semi-batch is the preferred manufacturing process. Figure 2-8 shows the typical progression of the monomer/polymer composition profile in these main process types and Fig. 2-9 is a diagrammatic representation of the processes. (A)

(B) 100

% Conversion

% Conversion

100 80 60 40 20 0

80 60 40 20 0

0

1

2

3

4

5

6

0

1

2

3

(C)

5

6

(D)

70

100

60 CSTR 5

50

CSTR 4

40 30

CSTR 3

20 CSTR 2

10 CSTR 1

% Conversion

% Conversion

4

Time

Time

80 60 40 20 0

0

0

Reactor number in chain

1

2

3

4

5

6

Distance along reactor

Variation of monomer/polymer in different processes expressed as % conversion of added monomer: (A) batch; (B) semi-batch; (C) chain of five CSTRs; (D) continuous plug flow. Fig. 2-8

There are novel variations of these processes in use, as are combinations of the processes. The reactor type and the process conditions exert large influences on the resulting properties as discussed in Sect. 2.3.2. The product from emulsion polymerization reactors usually contains a small amount of non-reacted monomers, along with volatile impurities from many sources. Such impurities may include a range of solvents originating from various raw materials, dimers and co-dimers either present in monomers or formed during the polymerization, alcohols from hydrolysis of vinyl esters, products formed from

35

36

2 Synthesis of Polymer Dispersions Feed

Feed

Final Level a)

b) Initial Level

Product

Product

Product

c)

Feed

Feed

d) Product Types of process used for emulsion polymerization: (a) batch; (b) semi-batch; (c) continuous stirred tank (chain of three); (d) continuous plug flow (tubular).

Fig. 2-9

organic initiators, and a whole range of saturated and unsaturated organics coming from the monomers. In many cases, polymerization is taken to a high degree of conversion in the reactors (>99 %), so that the residual monomers are often <1 %. However, it is normal to further polymerize this residual monomer, often using a redox initiator system. Because of the low monomer concentration at this stage, the rate of polymerization is relatively slow, and with reactor time normally being at a premium this “chemical stripping” is carried out in separate, lower cost equipment. Of course many of the organic impurities are either not polymerizable or cannot be polymerized under typical emulsion polymerization conditions. To remove these contaminants, physical separation techniques are often employed. Steam distillation is the most widely used technique, either in batch strippers or in continuous processes such as a column stripper. In some cases, solvent extraction, membrane separation or adsorption processes may be used.

2.3 Manufacturing Processes

The production of styrene-butadiene rubber emulsions is one case where polymerization is deliberately stopped at a low conversion, typically 70–80 %, in order to limit the crosslinking reaction from the pendant vinyl groups in the butadiene units. With such large amounts of residual monomer, economics force the recovery and recycling of both butadiene and styrene. After the polymerization stage, residual butadiene is flashed off under vacuum, compressed, cooled and returned to the reactor feed, and styrene is steam stripped in a column stripper, condensed and also returned to the reactor. Coagulum formed during the manufacture of emulsion polymers can cause problems with application processes, and although much progress has been made in the industry to minimize these problems, (improved recipe design, better stabilization systems, improved control of process parameters), there still exists the need to remove coagulum during various stages of the process. All types of filtration equipment are used in the industry, from simple filter bags and static screens through wiped screens, band filters, vibrating screens, filter presses to quite complex selfcleaning filters of various types, both with and without the use of filter aids such as diatomaceous earth. Centrifugation is also used, although with many lattices, the density difference between polymer and the disperse medium is too small to make this an efficient process. Post additions to the latex product, and final adjustments to properties such as pH and solids content are carried out usually in simple stirred tanks, although continuous in-line mixing may also be practiced. 2.3.2

Influence of Process Conditions on Polymer/Colloidal Properties

As shown in Sect. 2.3.1, the three main types of reaction process differ in the monomer/polymer concentration profile throughout the reaction. Because many of the chemical reactions occurring during emulsion polymerization, such as branching, crosslinking and propagation, are competitive and dependent on the relative quantities of monomer and polymer at the reacting site, it can be seen that all of these will be influenced by reactor type. Branching and crosslinking are favored at high polymer concentrations. Therefore, given the same reaction temperature, a batch process, which has the highest average monomer concentration through the process, will give the least branched polymer with the lowest degree of crosslinking. The opposite end of the scale is represented by a single CSTR operating at a high conversion which will tend to give a highly branched and crosslinked polymer. Increasing the number of CSTRs in a chain will lead to a reduction in branching and crosslinking. A semi-batch process can be operated at both ends of the scale. With very fast feed rates, the system is monomer flooded and the product will be close in properties to the batch reaction. Slow feed rates (monomer starved) lead to high branching and crosslinking. In particular, in systems containing butadiene where the pendant vinyl groups contribute strongly to crosslinking, the influence of monomer/polymer ratio is highly significant. Crosslinking increases rapidly as con-

37

Relative X-linking

2 Synthesis of Polymer Dispersions

0.00045 0.0004 0.00035

Variation in the relative degree of crosslinking with percentage conversion during the emulsion polymerization of styrene/butadiene 25:75 in a batch system at 30 °C. (average number of cross-links per monomer unit in polymer)

Fig. 2-10

0.0003 0.00025 0.0002 0.00015 0.0001 0.00005 0 0

20

40

60

80

100

% Conversion version increases (Fig. 2-10) necessitating shortstopping of the polymerization at low conversion when a polymer with high elongation is required. Temperature similarly influences branching and crosslinking, both normally reducing with lower temperatures. Molecular weight tends to increase with reducing temperature. In a semi-batch and a CSTR, the influence of temperature is enhanced because reducing the temperature at a constant feed rate causes a reduction in polymerization rate and hence a reduction in the instantaneous conversion. The number of particles in the polymerization system influences the rate of reaction; the larger the number (smaller final particle size) the faster the overall rate. Therefore in a semi-batch reactor or a CSTR, for a given feed rate, the number of particles exerts an influence on instantaneous conversion and thus all of the properties previously discussed under conversion. Figure 2-11 shows this influence. This control over polymer properties by the number of particles in the system makes it critical to control particle number, and has been one of the driving forces to100 % Instantaneous Conversion

38

90 80 70 60 50 40 30 20 10 0 0

2

4

6

Time in Hours PD = 135 nm

PD = 155 nm

PD = 175 nm

8

Fig. 2-11 Influence of number of particles on the instantaneous conversion in a semi-batch emulsion polymerization of styrene-butadiene 45:55 with a feed time of 4.5 h at 85 °C.

2.3 Manufacturing Processes

ward the use of seed polymer. The use of seed reduces the variability of the nucleation stage, making it less dependent on temperature, surfactant concentration, electrolyte concentration etc. Of course this is in turn dependent upon the seed itself being a consistent raw material. 2.3.3

Equipment Considerations

The reactors used for the manufacture of emulsion polymers are normally relatively simple, agitated vessels, ranging in size from 5 m3 (less than this is normally considered to be pilot plant scale) to 200 m3, with the majority being in the range 15–100 m3. Carbon steel, glass-lined construction used to be the standard (the glass surface minimizing polymer deposition) for polymerization under acid or slightly alkaline conditions. For high pH systems (for example those based on the alkali metal salts of fatty acids and using redox initiator systems) carbon steel reactors are the standard. As emulsion stability has been improved over the years, and better cleaning techniques have been developed (high pressure water jets), it has become common to replace glass-lined reactors with either stainless steel or carbon steel with stainless steel cladding. Typically, to avoid contamination from metal ions, all pipework associated with feeds into and product from the reactor would be stainless steel construction. Pressure ratings of reactors range from atmospheric to >100 bar with temperature of operation in the range 5–200 °C. (in certain special cases sub-zero temperatures may be achieved through the use of anti-freeze agents in the emulsion). The most common temperature range for emulsion polymerization is 60–100 °C. To control the temperature of these highly exothermic reactions, reactors are fitted with a variety of cooling systems. This may be a jacket around the reactor (annular, dimpled, half-pipe); the reactor may be fitted with internal cooling coils or cooled baffles; there may be supplemental cooling systems such as reflux condensers, or heat exchangers of various types through which the reaction mixture is circulated. For agitation, a wide variety of impeller types are in use; both axial and radial flow turbines, propellers, paddles, anchor agitators and many other proprietary designs. The choice of impeller is very much dependent on the properties of the emulsion polymer being made (stability, viscosity etc.). Latex may be transferred through subsequent parts of the process either using inert gas pressure or with a variety of low shear pumps. It is normal to neutralize the latex after the reaction stage, thus greatly increasing stability. High-pressure equipment is not usually necessary downstream of the reactor, although some operations will require full vacuum rating. As discussed in Sect. 2.3.1, batch strippers, stripping columns, filtration equipment and mixing vessels are all in use to a varying extent downstream of the reaction stage. Because of the tendency of emulsion polymers to cause deposits in the process, the secret of successful operation is usually to keep equipment simple. Ease of cleaning is paramount, and “moving parts”, particularly if high shear is imparted to the latex, should be avoided where possible.

39

40

2 Synthesis of Polymer Dispersions

2.3.4

Safety Considerations

To carry out emulsion polymerization safely, the pressure which can develop under a worst case scenario should always be less than the design pressure of the reactor. As with any highly exothermic chemical reaction, if the generated heat is not removed from the system, the temperature (and in an enclosed system, the pressure) will increase in an exponential manner. The pressure can rapidly exceed the design pressure of the containing vessel, resulting in rupture of the vessel or its associated piping if the system is not protected by appropriate relief devices. It has to be questioned whether or not relief devices are ever really appropriate – in reality, although the reactor may be protected, a potentially hazardous situation is being transferred elsewhere (vent tanks, quench tanks, environment). The correct sizing of relief systems for emulsion polymerization reactors is also not a trivial exercise, in part because it is difficult to define a worst case scenario and in part because emulsion and polymer properties at extreme reaction conditions are not well known. Very large relief devices are often indicated, which in turn means a large problem outside the reactor. A major driving force for the change from batch to semi-batch polymerization processes, despite the loss in some polymeric properties which often accompanies this change, has been the inherently safer aspects of semi-batch, with smaller quantities of monomer in the reactor at any given time. Thus, the major capacity limitation in the process can be seen to be a function of the pressure rating of the reactor, the rate and efficiency of removing heat from the system, and the safety systems which are in place to ensure that the maximum reactor working pressure can never be exceeded.

References 1 W. D. Harkins, J. Am. Chem. Soc. 1947,

69, 1428. 2 W. D. Harkins, J. Polym. Sci. 1950, 5, 217. 3 W. V. Smith, R. H. Ewart, J. Chem. Phys. 1948, 16, 592. 4 D. C. Blackley, Emulsion Polymerisation Theory and Practice, Applied Science, 1975.

5 T. G. Fox, P. J. Flory, J. Appl. Phys.

1950, 21, 581. 6 M. J. Rosen, Surfactants and Interfacial

Phenomena, John Wiley and Sons, 1989. 7 The schematic diagram is a courtesy of C. D. Anderson, M. S. El-Aasser, Emulsion Polymers Institute, Lehigh University, Bethlehem Pennsylvania.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

3

Characterization of Aqueous Polymer Dispersions Harm Wiese

3.1

Introduction

Aqueous polymer dispersions and the polymer films that form from them exhibit a diverse and complex range of properties. Moreover, these systems possess a marked heterogeneity at the mesoscopic level and their characterization is therefore a difficult task. In addition to determining the macroscopic properties of the dispersion (Sect. 3.2.1), characterization requires investigating the polymer particles themselves (Sect. 3.2.2), the residual volatile content (Sect. 3.2.3) and the aqueous phase (Sect. 3.2.4). It is also important to understand the process of film formation (Sect. 3.3.1) and to be able to describe the macroscopic and microscopic properties of the film (Sects 3.3.2 and 3.3.3). Given the large number of parameters and the numerous techniques employed to measure them, this article can only hope to provide a broad overview of this vast subject area. For more detailed descriptions of the various measurement methods, the reader is referred to the literature. Beyond the general physical and chemical characterization of dispersions and polymer films, a large number of application-specific tests exist. In this chapter, space does not permit discussion of these tests nor of the wide variety of the formulations used in the different applications. Other areas which have had to be excluded are the on-line and off-line methods of monitoring emulsion polymerization [1] and techniques for determining the microbial contamination of polymer dispersions.

41

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3 Characterization of Aqueous Polymer Dispersions

3.2

Polymer Dispersions 3.2.1

General Characterization of Dispersions Solids content In most of the applications the determination of the solids content is the first part of any routine characterization of emulsion polymers, since it is the polymer and not the water which is used in the final product. Typically, the dispersion is dried to constant mass at a temperature of between 100 and 140 °C (see, for example, ISO 1625) and the solids content is then expressed as the percentage ratio of the dry matter to the total mass of the sample. The dry matter comprises the polymer, emulsifiers and inorganic salts (formed by the decomposition of the initiators and from neutralization). The volatile part includes the water and monomers which were not converted during the polymerization reaction. A comparison of the theoretical solids content (that is assuming complete monomer conversion) to the experimental value therefore provides a means of assessing how far polymerization has proceeded to completion. To accelerate the drying process, most modern laboratories make use of alternative drying techniques such as halogen lamps, microwaves and infrared radiators. Because of a possible thermal decomposition of the polymer or emulsifiers however the temperature of the sample must not be allowed to rise much above the temperature range specified above. Furthermore, the drying rate should not be too high since this may cause skin formation on the surface of the sample. If the skin bursts material can be lost from the sample tray. Coagulum and grit In many applications polymer dispersions must only contain a very small fraction of agglomerates with diameters greater than 1 µm. Coarse components may be removed by filtration. Mesh sizes of between 45 and 180 µm are typical. The filter residue is known as the coagulum content or sieve residue. A determination of sieve residue according to ISO 4576 may involve pre-diluting the dispersion with water before filtering it through stainless steel filters having the above mesh size. The residue is rinsed with water, dried and weighed. The amount of residue is expressed as a fraction of the original dispersion mass. The type of filter and the mesh size should be specified. Fine agglomerates or large polymer particles which cannot be separated by filtration but are still visible in the wet or dry polymer film are known as grit. The grit is undesirable in many applications, particularly in transparent coatings and must be prevented during the polymerization process. To characterize the fraction of grit present, the filtered dispersion is cast on to a glass plate using film applicators with specified gap size (for example 45 or 125 µm). When viewed with transmitted light, grit particles can be detected by their refraction and diffraction effects. Normally, the assessment is performed on the dry film. The number of grit particles per unit area

3.2 Polymer Dispersions

is counted or the grit pattern classified by comparison with standard samples. Possible sources of error in assessing grit content are wetting defects and occluded gas bubbles. pH, density and surface tension pH is an important factor in both the stabilization and the formulation of polymer dispersions. For example, dispersions that contain carboxylic acids are usually adjusted to a pH of between 7 and 9 in order to improve their stability and to increase viscosity. pH measurements can be performed using a standard combination electrode (see ISO 976). Problems however often arise due to film formation on the glass membrane. The densities of most polymer dispersions are close to 1 g cm–3 as the corresponding polymers (with the exception of polyvinyl chloride and poly(vinylidene chloride)) have densities in the range 1.0 to 1.2 g cm–3 [2]. Since the densities of the polymer particles almost match the density of the aqueous phase, sedimentation is usually only a problem in emulsion polymers if they contain very coarse particles. Density measurements have been used in the past to follow the course of emulsion polymerization reactions, because the density of the monomer is usually lower than that of the polymer (densitometry [1]). Densities can, for instance, be determined quite simply with a pycnometer (see ISO 2811). Very high precision density measurements (±5 × 10–6 g cm–3) are possible with a vibrating-tube densimeter [3]. In this method, the change in the resonant frequency of the tube, which depends on its total mass, is measured when the dispersion is placed in it. It is essential that the sample is wholly free of gas bubbles. The surface tension of a polymer dispersion is of major importance in the coating of substrates. Good wetting of the substrate is achieved with a dispersion of low surface tension (Sect. 3.3.2). A general approach to obtain information on the wetting properties of a dispersion is by measuring its surface tension in air, which is generally easy to determine experimentally. As a result of the emulsifiers used in emulsion polymerization, the surface tensions of polymer dispersions generally lie some 20 to 40 units below that for water (73 mN m–1). Surface tension measurements can be made by the Du Nouy ring method (see ISO 1409), the hanging drop method or by using a stalagmometer [4]. In the latter two techniques, the shape or volume of a drop of the dispersion as it emerges from a capillary is used to compute the surface tension. In the Du Nouy ring method, a thin ring of platinum wire, suspended in the dispersion parallel to the surface, is withdrawn from the dispersion and the tensile force exerted by the liquid lamella that extends from the ring to the bulk liquid is measured just before it ruptures. As this method requires relatively low dispersion viscosities (< 200 mPa s), it is often necessary to dilute the dispersion before measurement. The ring method enables the static surface tension of the dispersion to be determined. When polymer dispersions are applied on large-scale coating machines, it is also important how fast the surface tension of a freshly generated surface is able to decrease. A device which permits this dynamic surface tension to be measured is the maximum bubble pressure tensiometer [5]. In this method, gas bubbles are blown

43

44

3 Characterization of Aqueous Polymer Dispersions

through a capillary into the dispersion. The surface tension can be calculated on the basis of the pressure changes during bubble formation. By varying the gas speed, the dynamical processes during the growth of the surface can be accessed. It is worth noting that the hanging drop and stalagmometer techniques are also dynamic methods, because the dispersion is continuously emerging from the capillary, albeit much more slowly than the gas exiting the capillary in the maximum bubble pressure tensiometer. Flow behavior The flow behavior of a polymer dispersion or of its formulation is a central, and often critical, processing parameter [6]. Paints, for example, must be easy to apply, should form smooth surfaces, but should not sag when applied to a wall and should not spatter during brush application. In contrast to conventional liquids, the rheological properties of dispersions with a solids content above roughly 25 % are complex and strongly dependent upon the forces applied, developing, in certain cases, a “memory” of these forces. This behavior is caused by the particle interactions that become apparent when the solids content is high. Apart from solids content, particle size and particle size distribution play a crucial role. Other factors affecting flow behavior are the electrostatic charges of the polymer particles, their surface composition and water-soluble oligomers in the aqueous phase. In practice, particle charge has only a relatively minor influence because the ionic strength of the aqueous phase is generally high enough (due to the presence of ions arising from the decomposition of the initiator and from neutralization) to restrict to a few nanometers the range over which the electrostatic forces are effective. Even relatively small amounts of watersoluble polymers have a pronounced influence on the flow properties of a dispersion and this fact is used in practice to adjust the viscosity of the dispersion to the desired level (thickeners). In contrast to polymer solutions, molecular weight and polymer composition do not have a significant effect on the rheology of a polymer dispersion. A simple crude assessment of flow behavior can be achieved on-site using socalled flow cups – funnel-shaped vessels with specified orifices in their bases. The measurement variable is the efflux time, that is the time taken for a known volume of dispersion to exit the cup through the orifice. It is important to be aware of the fact that a variety of cups are in use and that each type of cup produces a different efflux time. The most common types are the ISO cups (complying with ISO 2431) and the Ford cups used in the ASTM D 1200 test procedure. The efflux time characterizes the low-shear flow behavior of a dispersion flowing under its own weight. In many industrial applications, however, much greater shear forces are applied (for example in coating machines) and these forces often have a strong effect on the rheological properties of the dispersion (non-Newtonian behavior, see below). The effect of shear forces can be investigated by measuring a flow curve with a rotational viscometer (Fig. 3-1 and ISO 3219). The dispersion is sheared in a cup by an immersed rotating cylinder (spindle). In the measurement the velocity gradient (or shear rate) between the outer surface of the cylinder and the inner surface of the stationary cup is varied. The flow curve is the plot of the torque acting on the cylinder (or the shear stress τ which can be derived from it) as a function of the

3.2 Polymer Dispersions

Rotational viscometer

Flow curve shear stress τ

shear stress

shear rate

shear rate D

Measuring a flow curve using a rotational viscometer. Fig. 3-1

0

viscosity η = τ / D

shear rate D. The (dynamic) viscosity is defined as the quotient of shear stress and shear rate at every point along the flow curve. As both low viscous aqueous-like and highly viscous dispersions are used, a measurement range of between one and several thousand mPa s must be accessible. Because of the technical limitations of the shear-force transducers, different cylinder/cup sizes are usually required in order to cover both low-viscosity to high-viscosity dispersions. Shear rates of up to about 1000 s–1 can be accessed with conventional rotational viscometers. The Brookfield type of viscometer, in which one of a number of different spindle types (RV, LV and so forth) is rotated in the sample dispersion, also enjoys widespread use for viscosity measurements (see ISO 2555 and 1652). The disadvantages associated with this type of viscometer are that the shear rate is not well defined and that the results of measurements made using different spindle types cannot be compared with one another. Figure 3-2 presents a number of τ/D and η/D curves which summarize the various phenomenological descriptions of how dispersion viscosity depends upon shear rate or time. In many cases, one observes shear thinning (viscosity decreases with increasing shear rate) and thixotropic behavior (viscosity falls with time at a constant shear rate). For this reason, the flow curve is recorded (as shown in Fig. 3-1) by measuring the shear stress both as a function of increasing shear rate and as a function of decreasing shear rate. The hysteresis visible in Fig. 3-1 is typical of thixotropic dispersions. Figure 3-3 shows a viscosity/shear rate dependence which is often observed for polymer dispersions. In this log-log plot, an high initial plateau at low shear rates is followed by a region of shear thinning which leads to a lower plateau at high shear rates. Increasing the shear rate further induces a strong dilatancy and possibly also coagulation. The shear thinning is assumed to be caused by the onset of ordering within the dispersion as the polymer particles align themselves in parallel layers (Fig. 3-3). The dilatancy is thought to be the result of the temporary formation of aggregates which can only pass by one another with difficulty.

45

46

3 Characterization of Aqueous Polymer Dispersions Phenomenological classification of the flow behavior of polymer dispersions (τ, shear stress; D, shear yield stress rate; η, viscosity; t, time).

Shear rate dependence τ

τ

Fig. 3-2

τ

τ

η D

Newtonian η

D

D

pseudoplastic, shear thinning η

dilatant, shear thickng. η

D

D

plastic η

D

D

D

Time dependence (constant shear rate) η

thixotropy

η rheopexy

t

t

log (viscosity) shear thinning

dilatancy

log (shear rate)

statistical distribution

Typical dependence of the viscosity of a polymer dispersion on the shear rate.

Fig. 3-3 shear-induced ordering

aggregation

When processing dispersions, a particular viscosity is often desired both at low and at high shear rates, and these viscosities may be very different. A key objective in the formulation and production of polymer dispersions is therefore to adjust the shear-rate profile to meet the demands of the particular application. One way in which this can be achieved is by the addition of polymeric thickeners, which influence the viscosity at low and at high shear rates differently depending upon their structure and molecular weight. Figure 3-4 shows schematically how the viscosity of a polymer dispersion varies as a function of the volume fraction φ of the particles. The volume fraction is normalized to unity. In rheology it is more commonly used than the alternative solids content of the dispersion. A characteristic steep increase in the viscosity is observed as one approaches a maximum volume fraction φm. Semi-empirical expressions

3.2 Polymer Dispersions Dependence of viscosity on the particle volume fraction. Fig. 3-4

viscosity shear rate: low high

φm

volume fraction φ

exist which provide more or less reasonable approximations of the experimental curves. For the purposes of illustration the Dougherty–Krieger equation is reproduced here:

η  φ  = 1 −  η0  φm 

−2.5φm

(3-1)

where η0 is the viscosity of the aqueous phase. The theoretical upper limit for the maximum volume fraction φm of monodisperse spheres is 0.74, which is the value associated with hexagonal close packing. However, the steep rise in viscosity can occur at smaller volume fractions (often at around 0.55 to 0.6), depending upon the type of packing and the distance over which the interparticle forces act. If the volume fraction is constant, decreasing particle size results in a decrease in the distance between the particles and an increase in the total particle surface area. This is the reason why dispersions containing fine particles have higher viscosities than those containing coarser ones. Low viscosity at high volume fractions can be achieved with a bimodal or broad size distribution where the interstitial spaces between the larger particles are filled with the smaller ones. Machine processing exposes dispersions not only to shear but also to tensile stresses (extensional flow). Because of the lack of commercially available test equipment, studies of the extensional flow of polymer dispersions are still in their infancy. Little use is also made of viscoelastic techniques where the sample is subjected to low-amplitude oscillatory shear and the amplitude and phase of the oscillating stress is measured (usually as a function of the frequency of the oscillation). Stability The production, transport and processing of polymer dispersions expose these materials to significant degree of mechanical and thermal stress, which can lead to coagulation, sedimentation, phase separation or changes in viscosity. These changes are generally due to instabilities of the polymer particles. To avoid these problems, polymer dispersions are routinely tested for mechanical and storage stability and, for certain applications, also subjected to freeze-thaw cycles. After testing, any changes can be inspected visually or quantified using the methods available for assessing the coagulum, the viscosity or the particle size distribution.

47

48

3 Characterization of Aqueous Polymer Dispersions

Mechanical stability: The dispersion is subjected to intensive, defined stirring (using a serrated stirring disk or rotor/stator units) as, for example in ISO 2006 where the sample is agitated for 10 min at 14 000 rotations min–1. Storage stability: Accelerated testing is achieved by storing the dispersion at enhanced temperature for a particular time (for example for 15 h at 80 °C). Freeze-thaw stability: This test provides information about the re-dispersibility of a dispersion after having been frozen. The test involves subjecting the dispersion to repeated freeze-thaw cycles (for example 16 h at –20 °C followed by 8 h at + 23 °C). See, for example ISO 1147. Stability with respect to additives: For many formulations, the stability of the dispersion with respect to various additives, such as electrolytes, solvents, fillers and pigments, must be tested. The additives are added either directly or, where necessary, appropriately diluted, with any changes of the dispersion being assessed as described above. Testing is often conducted on diluted dispersions to permit simple visual inspections to be carried out, though the conclusions that can be drawn from these qualitative assessments are naturally limited. Foaming behavior Because of the presence of emulsifiers, polymer dispersions tend to foam. For many applications (for example spray coating) foaming must be suppressed by the addition of defoaming agents. The tendency of a dispersion to foam can be assessed by a number of application oriented methods which can be used for relative measurements [7]. One common method uses a graduated cylinder whose base is sealed by a porous glass frit through which gas can enter the cylinder. A known quantity of the dispersion is placed on the frit, the gas flow initiated and the height of the foam within the cylinder is then recorded as a function of time. Good reproducibility requires careful temperature control and thoroughly clean cylinders and frits. An alternative approach is to measure the foam height after beating the dispersion within a cylinder with a perforated plate for a set time. 3.2.2

Characterization of Polymer Particles

This chapter restricts itself to a presentation of the methods used to characterize the size and the surface of the polymer particles. Analysis of the polymer itself or the particle morphology is usually performed on the polymer film or on the dried particles and is therefore treated later in Sect. 3.3.3. Particle size Polymer dispersions contain particles with diameters ranging from 10 to about 1500 nm. Typically, the particle size is between 100 and 250 nm. In the majority of applications, particle size and particle size distribution are highly significant factors that determine the properties of a polymer dispersion, such as its flow behavior or its stability (Sect. 3.2.1). Measuring particle size is thus an important element when developing polymer dispersions and is also used in in-process control. A broad range of

3.2 Polymer Dispersions

methods are available for determining particle size [8] of which only light-scattering and sedimentation techniques as well as modern fractionation methods will be discussed here. Electron microscopy is dealt with later in Sect. 3.3.3 which discusses the characterization of particle and film morphology. Light transmission A distinctive feature of polymer dispersions is their turbidity. It is caused by light scattering of the polymer particles due to the difference in the refractive indexes of the polymer (typically 1.4 to 1.6 [2]) and water (1.33), and provides a simple way of accessing the mean particle size in the dispersion. The link between the scattering behavior of a dispersion of spherical particles and their diameter is provided by Mie theory [9] and is shown in Fig. 3-5 for the relative transmission of white light through various 0.01 %, w/w dispersions. Transmission increases as particle size falls or with decreasing relative refractive index (refractive index of the polymer/refractive index of water). If the relative refractive index is known, Fig. 3-5 can be used to determine the mean average particle size from the observed relative light transmission. The measurement can be performed within a matter of seconds using a simple arrangement of lamp, cell and photocell detector. LT / % 100 80

Relative light transmission LT of 0.01 % polymer dispersions as a function of particle diameter for different relative refractive indexes, m. LT = transmission through water/transmission through dispersion (2.5 cm cuvette, white light). m = refractive index of the polymer/refractive index of water. Fig. 3-5

60 m =1.10 (polyacrylate) 40 m =1.20 (polystyrene)

20 0

0

100

m =1.15

200

300

400

500

600

diameter / nm

For polydisperse dispersions, the measured light transmission is the inverse geometric mean of the relative transmissions LT1, LT2, … of the respective mass fractions m1, m2, …: LT

−1

= LT1− m 1 ⋅ LT2− m 2 ⋅ …

(3-2)

Laser light scattering Of the many methods based on laser light scattering, dynamic light scattering (DLS, also called quasielastic light scattering QELS or photon correlation spectroscopy PCS) has established itself as the most important technique of measuring particle size in polymer dispersions [10]. The measurement (Fig. 3-6) involves directing a laser beam into a highly diluted sample of the dispersion and recording the scattered light impinging on a photomultiplier at a particular angle.

49

50

3 Characterization of Aqueous Polymer Dispersions laser

Dynamic light scattering. Experimental set-up and intensity fluctuations.

Fig. 3-6

sample

scattering angle polarizer

analyzer intensity

photomultiplier time

The intensity of the scattered light reaching the detector is determined by the mutual interference of the light waves scattered from the individual particles in the dispersion. Because laser light is highly coherent, the scattered waves have a fixed phase relationship to one another which is determined by the geometrical arrangement of the scattering particles. The Brownian motion of the particles causes a statistical variation of the phase relationship in time, producing corresponding fluctuations in intensity at the detector (Fig. 3-6). The mean frequency of these fluctuations, which in DLS is determined by autocorrelation of the scattering intensity, is proportional to the diffusion coefficient of the particles. A hydrodynamic particle diameter d can then be calculated from the measured diffusion coefficient, D, using the Stokes–Einstein equation: D=

kT 3πηd

(3-3)

where k is Boltzmann’s constant, T temperature, and η the viscosity of the aqueous phase. If the approximation of hard, non-interacting spheres is assumed, the hydrodynamic diameter is equal to the particle diameter. The measurement, which takes only a few minutes to perform, can be used to determine particle diameters of between 5 nm and 5 µm. In order to avoid complications due to multiple scattering of the laser light and due to particle interactions, which influence diffusion, the measurements must be carried out on highly dilute samples (10–5 to 10–2 %, w/w). DLS is used as a routine means of determining particle size in monodisperse polymer dispersions. Typical systems employ a red helium-neon laser (wavelength: 633 nm) and a scattering angle of 90°. However, the resolution achievable with such systems when measuring polydisperse samples is generally quite low. As a rule of thumb, the particle diameters of two fractions must differ by a factor of 3 or 4 if they are to be clearly differentiated. A further fact which complicates the analysis of polydisperse samples is that the diffusion coefficients are weighted according to the scattering intensity. According to

3.2 Polymer Dispersions

Mie theory, the scattering intensity of light on particles whose diameter d is approximately equal to the wavelength λ of the light is a complex function of d, λ, the refractive indexes of the particles and the scattering angle. This fact considerably complicates the calculation of the exact mass fractions. For this reason, most equipment manufacturers make use of simple approximate descriptions of the dependence of scattering intensity on particle size. A more accurate approach is to measure the absolute scattering intensities and intensity fluctuations at a number of angles and then to use Mie theory (assuming that the refractive indexes of the particles are known) to convert the measured data to a particle size distribution [11]. Compared with DLS, static light scattering, in which the absolute intensity of the scattered light is analyzed as a function of scattering angle, has become less relevant as a method of determining polymer particle size. Static measurements are today mainly used for characterizing dissolved macromolecules (with gyration radii <100 nm) and for particles with a diameter greater than 1 µm (Fraunhofer diffraction). The reader is referred to the literature for further details on these techniques [12, 13]. Centrifugation Centrifugation methods allow a detailed and comprehensive characterization of polymer dispersions. Figure 3-7 is a schematic view of an analytical ultracentrifuge (AUC) equipped with two types of optical detection systems (schlieren optics and turbidity measurement at fixed radial position, “turbidity optics”) [14]. Particle size determination with an AUC exploits the different sedimentation rates of the particles in the centrifugal field. According to Stokes’ law, the sedimentation time, ts, for the path between the radial position of the meniscus, rm, and the position of the detection optics r (Fig. 3-7) in a centrifuge rotating at a constant angular velocity, ω, is given by:

ts =

18η ln(r/rm )

(3-4)

( ρ – ρm )d 2ω 2

where η is the viscosity of the aqueous phase, ρ – ρm the difference in density between the particles and the aqueous phase, and d is the particle diameter. Thus particle size determination requires the precise knowledge of the particle densities. schlieren optics

turbidity optics photomultiplier

video camera schlieren plate

Schematic diagram of an analytical ultracentrifuge (ω is the angular velocity of the rotor). Fig. 3-7

ω

sample cuvette

rm r

lamp

laser

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3 Characterization of Aqueous Polymer Dispersions

The measurement of the particle size distribution (PSD) is performed on dilute samples (typical concentration: 0.05 to 2 %, w/w) in a so-called sedimentation velocity analysis using the turbidity optics (Fig. 3-7). At the start of the measurement the dispersion is uniformly distributed throughout the cell and the detector registers an attenuated laser beam. As soon as the first particle fraction has migrated under the influence of the centrifugal field out of the optical path, the signal at the detector increases. Particle size can then be determined by measuring the time at which the signal begins to rise. By applying Mie scattering theory (knowledge of particle diameter and refractive index required) the mass fraction of that particular particle fraction can be computed from the increase in signal amplitude. Measurements can be performed with high resolution in the diameter range between 20 and 2000 nm. Figure 3-8 illustrates the result of a sedimentation velocity analysis on a mixture of ten polystyrene calibration latexes. Measurements on such broadly distributed samples are usually performed with an exponentially increasing rotation speed and require centrifuges capable of reaching 60 000 rotations min–1 (Eq. 3-4); a measurement typically lasts 1 h. Machines designed to allow simultaneous determinations with eight sample cells per rotor are described in the literature [14].

Particle size distribution (differential and cumulative) of a mixture of ten polystyrene calibration latexes (sedimentation velocity analysis).

Fig. 3-8

By carrying out the sedimentation velocity analysis not only in H2O but also in D2O and in a 1:1 H2O:D2O mixture (H2O/D2O analysis), both the PSD and information on the density (and thus chemical uniformity) of the individual particle fractions may be obtained. Apart from the sedimentation velocity analysis, the AUC may also be used to perform a so-called density gradient analysis. In a density gradient analysis, a water-soluble substance of high density (CsCl or the iodinated sugar metrizamide) is added to the sample so that in the liquid phase a radial density gradient is established at equilibrium in the centrifugal field. The various particle fractions migrate along the gradient to the point having their own density, thus allowing the densities – as in the H2O/D2O analysis – to be determined. I this case the schlieren optics (Fig. 3-7),

3.2 Polymer Dispersions

which detect changes in the refractive index along the radial axis, is used for the analysis. In contrast to the turbidity optics, a photo of the entire cell is taken once equilibrium has been established. Normally between 10 and 20 h are needed to achieve equilibrium. The advantage of using the schlieren optics is that in addition to the particle fractions also dissolved macromolecules can be studied with respect to chemical composition and molecular weight. Like the polymer particles, the macromolecules migrate along the density gradient to their isodensity point. However, the small size of the macromolecules means that the bands are diffusion broadened. If the scaling law that relates the diffusion coefficient to the molecular weight is known, the latter can be calculated. The considerable amount of information obtainable by AUC analyses must be viewed in the light of the considerable technical expense and effort needed to run such a machine. At present, only a few laboratories have access to this technology. Disc centrifuges are a cost-effective alternative (rotation speeds of up to 15 000 rotations min–1). Because of the lower rotation speeds in a disc centrifuge a different analysis technique has to be employed. The cell is first filled with a spin fluid and then a sample layer is injected on top of the fluid while the disc is rotating. By this means the particle fractions migrate past the detection optics layer by layer according to their differing sedimentation velocities. Unfortunately, it is often difficult to achieve a uniform injection layer in practice (because of disruptions of the sample flow front). For this reason, and also because of the low density difference between the polymer particles and the aqueous phase, disc centrifuge sedimentometry is not widely used for the characterization of polymer dispersions. Modern fractionation methods In recent years a number of new fractionation techniques, such as capillary hydrodynamic fractionation (CHDF) [15] and field field-flow fractionation (F-FFF) [16], have established themselves as reliable alternatives to centrifugation in PSD analysis. Only CHDF will be discussed here. The technique involves injecting a small amount of the sample into an aqueous eluent containing an emulsifying agent. The eluent is pumped through a glass capillary tubing (inner diameter 7–10 µm) and in so doing adopts a laminar flow profile (Fig. 3-9). The larger the particles, the less able they are to approach the capillary wall during thermal Brownian motion. Large particles are therefore, on average, flowing in faster stream lines than smaller ones and are transported more rapidly through the capillary. The particle fractions are detected using a UV-detector. Complications due to specific interactions between the particles themselves or between the particles and the wall are eliminated by using a particular type and amount of emulsifier and working at low ionic strength. When the apparatus has been calibrated with particles of known size, the PSD of a sample can be determined from its elution curve. As is the case for AUC, calculating the mass fractions requires application of Mie scattering theory, but this is not implemented in CHDF equipment currently available on the market. The manufacturers content themselves with a relative conversion based on the extinction coefficients

53

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3 Characterization of Aqueous Polymer Dispersions

glass capillary

parabolic flow field

Capillary hydrodynamic fractionation (CHDF): the principle.

Fig. 3-9

inaccessible regions (shown for two different particle sizes)

particle

of polystyrene calibration latexes. Typically, CHDF is able to measure particle diameters in the range 10 to 400 nm. By using capillaries with a larger inner diameter, the range can be extended to include particles about 1 µm in diameter, but the resolution achievable at the lower end of the particle size range is then reduced. A measurement takes about 10 minutes to complete. Particle surface The surface characterization of a polymer particle involves investigating the adsorption of ions and amphiphilic molecules (emulsifiers, oligomers), determining the number of covalently bonded functional groups and acquiring information on the structure of the interfacial layer (swollen state or ‘hairy layers’). Presently this task can not be solved satisfactorily. The main methods used are titrimetric analyses on purified dispersions, soap titration and electrokinetics. Titrimetric methods Titrimetric analysis of polymer dispersions is mainly used to quantitatively determine acidic and basic groups covalently bonded to the particle surface (from initiators or comonomers). Before titration the dispersion has to be cleaned thoroughly, that is all traces of amphiphilic and ionic components have to be removed. The recommended purification technique employs a combination of anionic and cationic ion-exchange resin beads [17]. The beads have to be thoroughly purified themselves before use. After purification, the dispersion is titrated potentiometrically to determine the quantity of residual, that is covalently bonded, acid or base groups [17]. When titrating for acids, the different pKa values enable distinction of sulfuric/sulfonic acid and carboxylic acid. Fundamental questions that arise in connection with this method are (1) whether all of the bonded acid groups can be neutralized because of the high resulting charge density, and (2) to what extent the particle surface reorganizes during neutralization. The increasing hydrophilicity might, for instance, cause particle swelling and a migration of acid groups from the particle interior to the surface.

3.2 Polymer Dispersions

Soap titration Soap titration is employed to determine the emulsifier coverage of the polymer particles in the dispersion. Emulsifier coverage is defined as the percentage of the particles’ total occupiable surface area that is covered by emulsifier. In soap titration the surface tension of the dispersion is measured, for example using the Du Nuoy ring method [4], as a function of the emulsifier added (Fig. 3-10). The emulsifier molecules distribute themselves between the particle surfaces, the aqueous phase and the dispersion/air interface where the surface tension is measured. As a rule the equilibrium lies well over in favor of adsorption on the particle surface, so that if the surfaces are not fully covered, only a few of the added emulsifier molecules are found at the dispersion/air interface where, as a consequence, relatively high surface tension values γ are recorded. As more and more emulsifier is added, γ gradually decreases (Fig. 3-10). When the surface of the particles is completely covered, the excess emulsifiers must be taken up by the aqueous phase, leading eventually to the formation of micelles. From this point on the aqueous phase can accommodate large amounts of emulsifier and γ remains essentially constant. The sharp change in the gradient of the curve shown in Fig. 3-10 determines the critical micelle concentration (CMC) of the particular emulsifier in the dispersion under test. The soap titration is usually carried out at a series of solids contents (for example, 2.5, 5, 7.5 and 10 %, w/w) in order to eliminate the amount of emulsifier required for micelle formation. Plotting the resulting CMC values against the solids content produces a straight line whose slope is inversely proportional to the emulsifier coverage α (Maron plot, see [18, 19]). If the size of the particles is known, the effective molecular surface area of the emulsifier occupied on the particle can be calculated. Studies have shown that the emulsifier molecular surface area is determined not only by the type of polymer, but also by the way in which comonomers and initiator residues are incorporated into the particle surface. air emulsifier polymer particle

model dispersion

100% coverage of particle surface

micelle formation

surface tension

Fig. 3-10 Soap titration. Determination of the emulsifier coverage of the polymer particles.

cmc log(emulsifier conc.)

55

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3 Characterization of Aqueous Polymer Dispersions

The soap titration technique is strictly only applicable for dispersions which contain one type of emulsifier. However, many polymer dispersions are stabilized by a combination of emulsifiers, often both ionic and non-ionic types. One approach in such cases is to perform the study with the emulsifier mixture, though there is the problem of exchange processes occurring on the particle surfaces if one of the emulsifiers is preferentially adsorbed. The results may also be affected by adsorbed amphiphilic oligomers generated during the emulsion polymerization. Electrokinetics Electrokinetic measurements [20] are used to access the electrophoretic mobility µe of the polymer particles and thereby to get information on their charges. Because of the relatively small particle size of 100 to 250 nm, the measurement technique used for polymer dispersions is laser Doppler electrophoresis. Sample preparation and experimental set-up correspond to those of a dynamic light scattering experiment (Sect. 3.2.2, Fig. 3-6). The only difference is a pair of electrodes immersed in the sample between which the particles are moved backwards and forwards by an alternating voltage. The electrophoretic mobility, µe, is related to the zeta potential, ζ, which is defined as the electric potential at the surface of shear of the particles and is therefore a measure of their total charge. Unfortunately, the electrophoretic mobility of dispersion particles does not depend solely on the zeta potential, but also in a complex way on particle size and on the ionic strength and viscosity of the aqueous phase [21]. It is only at the limits of very high and very low ionic strength that ζ can be directly computed from the measured µe values (Helmholtz–Smoluchowski or Hückel approximations). These complex dependencies and some experimental difficulties (for example, due to electro-osmotic convection) are the reason why electrokinetic measurements are still of only minor importance in the characterization of polymer dispersions. On the other hand, the technique provides a simple means by which the adsorption of amphiphilic components (emulsifiers, protective colloids and so forth) on the particle surfaces can be followed at least qualitatively. 3.2.3

Residual Volatiles

The increased attention paid to ecological and environmental issues in recent years has lead to a growing significance of residual volatile determination in polymer dispersions. Depending upon the production process, polymer dispersions may contain small quantities of residual monomers, monomer impurities, substances formed by the decomposition of the initiator or from chemical reactions between the various components in the reaction mixture. The European Union has defined such substances as volatiles, if they have a boiling point below 250 °C. The determination of the residual volatiles is usually performed by capillary column gas chromatography [22]. Different sampling techniques are described. In the headspace technique (see ISO 13741-2) a diluted dispersion sample is mixed with an

3.2 Polymer Dispersions

internal standard and a polymerization inhibitor. The sample is then heated in a sealed vial (for example at 90 °C for 1 h) and, after equilibration, a small part of the headspace vapors is introduced into the chromatography column. In the direct liquid injection method (see ISO 13741-1) a diluted dispersion sample is mixed with an internal standard and directly injected on to the hot insert liner (temperature 150–200 °C) of the chromatograph causing the dispersion to vaporize instantly. In both techniques the column (typically coated with a 1 µm thick layer of polydimethylsiloxane, PDMS) is initially thermostatted at 50 °C causing the injected volatiles to condense at the entrance part of the column. The temperature of the column is then raised linearly to 250 °C and the component substances are fractionated by the column in the order of their volatility and detected for example by a flame ionization detector (FID). Careful calibration is necessary in order to assign elution time and signal height to the type and amount of the components. With this technique, the typical residual volatiles of polymer dispersions can be quantitatively determined in a range between 10 and approximately 10,000 ppm (measurement duration about 45 minutes). 3.2.4

Aqueous Phase Analysis

In common practice the aqueous phase, or serum, of a polymer dispersion is only investigated for its pH (Sect. 3.2.1). On the other hand, the aqueous phase contains a host of substances which play an important role in many applications. These substances include: (a) emulsifiers, (b) initiator residues, (c) electrolytes from the neutralization process or from initiator decomposition (for example sodium sulfate from sodium peroxodisulfate), (d) unreacted water-soluble monomers such as acrylic acid or vinyl sulfonic acid, and (e) water-soluble oligomers formed from this kind of monomers. To analyze the aqueous phase for any of these substances, it must first be separated from the polymer particles. Both flocculation and membrane filtration techniques can be used for this purpose and they are described in more detail below. The detection of the substances listed above can then be performed with the usual array of analytical methods used for characterizing aqueous media. For the determination of emulsifiers, electrolytes and water-soluble monomers, ion chromatography (IC) and high-performance liquid chromatography (HPLC) are particularly suitable. The techniques of choice for characterizing oligomers are gel permeation chromatography (GPC) and capillary electrophoresis (CE). As these analytical techniques are not specific to colloidal chemistry, they will not be described further here and the reader should consult the literature for more information. Serum separation techniques Flocculation techniques The dispersion is for instance flocculated by the addition of acids or salts (typically containing polyvalent ions). Examples of salts of this type are aluminum sulfate or

57

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3 Characterization of Aqueous Polymer Dispersions

the combination of K4Fe(CN)6 and ZnSO4 (Carrez precipitation). Subjecting the dispersion to freeze-thaw cycles also often proves successful. A further possibility is centrifugation. If the centrifugal forces are high enough, the dispersion flocculates at the base of the cell allowing the aqueous phase to be subsequently drawn off. In the case of well-stabilized dispersions, high-performance centrifuges are required. Two disadvantages of the flocculation methods should be mentioned. First, the flocculated polymer particles can release considerable amounts of emulsifier into the aqueous phase. Secondly, centrifugation may cause components in the aqueous phase to be flocculated along with the polymer particles. Membrane filtration techniques In this case, the polymer particles are separated from the aqueous phase by a membrane through which the particles cannot permeate. Suitable membranes include dialysis tubes (molecular weight cut-off: 10 000–15 000 g mol–1) or, for example, Nucleopore membranes, which are available with pore diameters from 15 nm to several micrometers. In dialysis the dispersion is placed in a well-sealed tube and immersed for several days in water, which should be changed regularly. Before being analyzed, the dialysate usually has to be concentrated. Changing the water and concentrating the dialysate can both be carried out easily if the dialysis tube is placed inside a Soxhlet apparatus. In the diafiltration method [23], which uses the Nucleopore membranes, the dispersion is filtered under pressure through the membrane. Like the dialysis method, diafiltration can be used not only to separate the aqueous phase, but also to ‘purify’ a polymer dispersion, that is to separate all the water-soluble components. When used for the latter purpose, the dispersion is continuously rinsed with water during the diafiltration process. Filter cake formation is prevented by adopting a cross-flow filtration arrangement in which, for example, a stirrer is used to create a convective current parallel to the surface of the membrane.

3.3

Polymer Films

In the typical applications such as paints, adhesives, textiles and non-wovens the dispersions or their formulations are subjected to a drying process. The properties of the dispersion itself are for this reason only of relevance during processing. It is the properties of the polymer film that are of importance to the end product, and these properties are essentially determined by the polymer itself. Characterizing the properties of the polymer films is thus a subject of central relevance to the typical dispersion applications. In the description of methods presented in this chapter, the focus is on pure polymer films. However, these methods are equally applicable to characterizing formulated films such as paints.

3.3 Polymer Films

3.3.1

Film Formation

In the drying stage at the end of water evaporation the particles adopt a hexagonal close-packed geometry. Good subsequent film formation requires a high level of polymer particle deformability and the rapid interdiffusion of polymer chains between the particles. Emulsion polymers therefore possess a so-called minimum film formation temperature (MFT), below which no compact film can be formed. The determination of the MFT is discussed below. Immediately after its formation, the properties of the polymer film are still mainly determined by the particulate structure of the dispersion. The interstitial regions will still house the water-soluble components (salts, emulsifiers, oligomers and so forth) and multiphase particles, for example, will initially give films with micro domains. The phases formed directly after drying are not in thermodynamic equilibrium with one another. Changes in these micro domains can occur gradually with time, or more rapidly if subjected to higher temperatures. An example of such changes is the tendency of the water-soluble components to group together or to migrate to the surface of the film. In multiphase films, the micro domains can merge to form macro domains. The quality of a polymer film is therefore influenced not only by the properties of the constituent polymer, but also by the conditions under which the dispersion is dried. To achieve reproducible results when characterizing polymer films, it is necessary to control such parameters as wet film thickness, drying temperature, air humidity, air convection currents, and drying and storage times. Rapid drying, in particular, can cause a skin to form on the surface of dispersion, thus hindering the controlled drying of the dispersion below. If low-volatility substances, such as certain film-forming agents, are present, thorough drying of the film is essential if the measurement results are to be meaningful. To create a film with a defined (dry film) thickness of up to about 200 µm, the dispersion is usually cast on to the substrate using either a drawdown film applicator or a roller applicator. Suitable substrates are glass, polyethylene, polyethylene terephthalate or teflon. Films with thicknesses in the millimeter range, such as are used for mechanical strength testing, can be formed by pouring the dispersion into flexible polyethylene or silicone rubber trays, which facilitate the removal of the film after drying. Minimum film formation temperature (MFFT) The minimum film formation temperature is determined according to ISO 2115 by spreading the dispersion at defined layer thickness (for example at 200 µm wet) on a plate along which a linear temperature gradient is established (for example from 0 to 40 °C). Commercial equipment usually has shallow channels engraved in the plate which facilitate the spreading of the dispersion. The drying has to be performed in a controlled atmospheric environment. Once completely dry, the film is visually inspected for the presence of cracks and cloudiness. The MFFT is the lowest temperature at which a homogeneous and crack-free film forms. The MFFT is either displayed by built-in temperature sensors or can be determined using a surface temper-

59

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3 Characterization of Aqueous Polymer Dispersions

ature probe. The method also enables the so-called white-point temperature to be determined. This is the temperature below which a cloudy film forms and above which a clear, transparent film results. The white-point temperature always lies a few degrees below the MFFT. As an aqueous dispersion can only dry above 0 °C, the MFFT and white-point temperature are only defined above this value. The control of the polymer layer thickness is crucial for the measurements. Mechanical stress may develop during film formation (particularly when crosslinking is involved) which leads to crack formation above a certain layer thickness. A further point which should be considered is that very short drying times are often used in dispersion processing, for example on coating machines. In this case, the MFFT may well lie above the value determined according to ISO 2115. The discrepancy is caused by kinetic limitations in water evaporation and polymer interdiffusion [24]. The main factors determining the MFFT of an emulsion polymer are the composition, molecular weight and crosslinking density of the main copolymer [24]. However, particle size and the water-soluble substances such as auxiliary monomers or emulsifiers also play an important role. The effect of these substances is to retard the rate at which water leaves the interstitial region. As long as the water is present the mobility of the polymer chains is increased and interdiffusion thus favored. The MFFT of a dispersion can therefore be lowered by inclusion of auxiliary monomers. In the case of multiphase polymer particles, the MFFT is strongly dependent upon morphology. An example of this type of system are the core-shell particles with copolymers of differing glass temperature discussed below. 3.3.2

Macroscopic Characterization of Polymer Films Thermal characterization Thermal characterization of an emulsion polymer essentially means the measurement of the glass transition temperature Tg, that is the temperature above which the hard, glass-like polymer film becomes viscous or rubber-like. Polymers whose Tg lies well above room temperature are designated as ‘hard’, those with a Tg much lower than room temperature as ‘soft’. Normally Tg is measured by differential scanning calorimetry (DSC [25]). In this technique, the difference between the heat absorbed per unit time by the polymer film to that absorbed by a thermally inert reference material is recorded during a linear temperature ramp. The sample and the reference are placed on a sensor plate of defined thermal resistance R, and the temperature difference ∆T between the sample and the reference is then recorded over the temperature ramp. Usually, the heat flow difference, which is the negative quotient of ∆T and R, is plotted as a function of temperature (Fig. 3-11). Figure 3-11 is a schematic representation of a DSC measurement in which a glass transition and a melting process are shown. A glass transition is not a second-order transition between two defined equilibrium states. It therefore occurs over a relatively wide temperature range and depends upon the rate of temperature change. For this reason a number of different definitions of the glass transition temperature can

3.3 Polymer Films

melting process heat flow difference β∆Hs increasing endothermicity

Fig. 3-11 DSC. Investigation of glass transitions and melting processes in polymer films (β, heating rate; Tg, glass transition temperature; ∆Cp, heat capacity difference of the polymer in the temperature regions below and above Tg; Tonset and Tpeak, different definitions of the melting point; ∆Hs, enthalpy of melting).

glass transition Tpeak β∆Cp

Tonset

Tg temperature

be found in the literature. The Tg shown in Fig. 3-11 is that of the so-called “midpoint” definition. The ISO 11357-1 standard specifies a heating and cooling rate of between 0.5 and 20 K min–1 and recommends the repeat heating of the sample (that is heat/cool/heat). This repeat heating helps to eliminate any influence of the thermal history and the drying process, for example due to the presence of residual water. Tg should always be determined during the second heating ramp. The investigated temperature range, in the case of soft adhesives, should start at –110 °C and, in the case of hard coatings, should extend to 150 °C. Melting processes are uncommon in the emulsion polymers described in this book. Exceptions are the melting and crystallization phenomena observed with ethylene oxide chains when highly ethoxylated emulsifiers or protective colloids are employed in the polymerization process. The glass transition temperature of an emulsion polymer is the temperature above which the polymer chains become mobile and it is therefore directly related to the minimum film formation temperature MFFT. In contrast to Tg, which is essentially determined by the main copolymer, the MFFT is influenced by the drying process. If, for instance, water is able to solubilize part of the copolymer during the coalescence of the particles at the end of the drying stage, the MFFT can be lowered significantly. This phenomenon, which is known as “Tg/MFFT splitting”, is typical of vinyl acetate emulsion polymers but also observed for other polymer types when large amounts of hydrophilic monomers are used in the polymerization process. “Tg/MFFT splitting” is important for all applications in which a hard film with a low MFFT is required. Tg values of several important homopolymers are listed in reference [2]. The values were determined on samples of non-crosslinked emulsion polymers. In crosslinked polymers, Tg is shifted to higher temperatures as a result of the restricted chain mobility. A number of approximations for calculating the Tg of copolymers have been proposed in the literature [26]. The Gordon–Taylor equation usually produces reliable results:

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3 Characterization of Aqueous Polymer Dispersions

Tg =

Tg(1)m 1 + αTg( 2)m 2 m 1 + αm 2

(3-5)

Here m1 and m2 are the mass fractions of the monomers 1 and 2 and α is defined as ∆β(2)/∆β(1), with ∆β the difference in the coefficient of expansion of the molten and glass states of the respective homopolymer. If α is not known, the Fox equation can be used to provide a simple estimate: 1 m1 m 2 = + Tg Tg(1) Tg( 2)

(3-6)

For statistical copolymers, the width of the glass transition corresponds approximately to that of the homopolymers. The transition broadens with increasing inhomogeneity of the monomer distribution within and between the polymer chains. Beyond enabling the glass transition temperature to be measured, differential scanning calorimetry also provides a simple means of investigating polymer compatibility and phase separation in polymer films. If a film contains two phases, this shows up as two glass transition regions in the DSC scan. The relative fraction of the phases can be determined by the ratio of the measured heat capacities. If, on the other hand, the constituent polymers are wholly compatible, only one glass transition is recorded and this lies between those of the individual components. In a similar way, the compatibility of the polymer to low molecular weight substances such as plasticizers can be examined. Mechanical characterization The mechanical characterization of a polymer film is performed on a free film. This requires drying of the dispersion on a substrate of low surface energy (such as Teflon or silicone rubber) from which it can be lifted without applying strong mechanical forces. Great care is required when preparing such free films as defects or deformations caused by mechanical stress have a detrimental effect on the reproducibility of the measurements. Mechanical characterization is typically performed by recording the stress-strain curve up until film rupture takes place (large deformations) or by dynamic mechanical analysis within the elastic limit (small deformations). Stress-strain measurements A stress-strain measurement on a free polymer film is performed as a uniaxial tensile test. The film (typical geometry: 250 µm thick, 30 mm long and 5–10 mm wide) is loaded into a tensile testing machine and the stress (force per unit area) recorded as a function of tensile strain (elongation over original length) at a constant drawing speed (typically 10–100 mm min–1) until the test sample ruptures [27]. Figure 3-12 shows a typical form of a stress-strain diagram measured for a polymer film. At small levels of deformation, the stress-strain curve is linear and the film behaves elastically. The gradient of the curve in this region is called the elastic modulus (or Young’s modulus) of the material under test. Other parameters available from this test are the tensile strength and the elongation at break. The integral under the curve

3.3 Polymer Films Fig. 3-12 Typical stress-strain curve for a polymer film.

stress

tensile strength

work of fracture

elongation at break strain

to failure represents the energy per unit volume required to rupture the sample (work of fracture or toughness). The stress-strain behavior shown in Fig. 3-12 is typical of the elastomeric response of a polymer film. Curves of this type are found in crosslinked films above the glass transition and in non-crosslinked films in the so-called entanglement region (see dynamic mechanical analysis below). Hard, highly crosslinked films below their glass transition temperature are characterized by their relatively small elongation at break and their high tensile strength. These materials show essentially elastic behavior up until rupture. On the other hand, non-crosslinked films (in the vicinity of Tg) are elastic at small elongations and start to deform plastically above a critical value. This phenomenon is known as necking. In this case, the tensile stress passes through a maximum after which it remains relatively constant over a certain deformation range (before rising again shortly before rupture). Stress-strain measurements are also a useful tool for studying film formation in polymer films. Such an investigation, in which the process of polymer chain interdiffusion in n-butyl methacrylate films was followed by monitoring the films work of fracture, has been reported elsewhere [28]. Dynamic mechanical analysis In dynamic mechanical analysis (DMA [27]) of a polymer film, a sample with the same dimensions as in the tensile stress-strain analysis described above is slightly pre-tensioned and then subjected to a low-amplitude and low-frequency sinusoidal deformation (typically 0.1 % and 1 Hz respectively). As the measurement is performed below the material’s elastic limit, the stress follows the strain in a sinusoidal manner. The amplitude ratio and the phase difference between the stress and strain oscillations enables the dynamic elastic modulus E* to be calculated:

E* = E′ + iE″

(3-7) ––––––

where E′ is the so-called storage modulus, E″ the loss modulus and i = √(–1). E′ is a measure of the (recoverable) energy stored in the film during deformation and E″ is the (irrecoverable) energy that is dissipated in the film as heat. In conventional DMA, the storage and loss moduli are recorded as a function of the oscillation frequency. Of more widespread application are DMA measurements

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3 Characterization of Aqueous Polymer Dispersions Dynamic mechanical analysis. Storage (E′) and loss (E″) moduli as a function of temperature for a polymer film of poly(2-ethylhexyl methacrylate).

Fig. 3-13

in which E′ and E″ are measured at a constant frequency over a temperature range. As a result of the time-temperature superposition principle, the temperature scan provides the same information as the frequency scan. Figure 3-13 shows a typical DMA measurement (temperature scan) on a non-crosslinked polymer film. The storage and loss moduli can be seen to vary over several orders of magnitude across the temperature range. A high storage modulus is measured in the glassy state. It decreases rapidly in the glass transition region as the film softens. The loss modulus passes through a maximum at the beginning of the glass transition region. This maximum can be used as an alternative definition of the glass transition temperature of the sample (compare with Sect. 3.3.2). After passing through the glass transition region, the moduli decrease more weakly with temperature as a result of polymer chain entanglement and crosslinking within the film. In the case of non-crosslinked polymers, a further increase in temperature causes the film to undergo plastic flow. For non-crosslinked polymers, the entanglement region is only observed above a critical molecular weight (typically between 2000 and 10 000 g mol–1). This molecular weight corresponds to the polymer chain length above which physical chain entanglement (temporary crosslinking) can occur (entanglement molecular weight). For crosslinked polymer films, the storage and loss moduli measured above the glass transition region remain relatively constant or exhibit a slightly positive temperature dependence (crosslinking plateau). E″ assumes significantly lower values than E′. According to the theory of rubber-elasticity, the storage and loss moduli in this region have the following values: E′ = 3ρRT/Mc; E″ = 0

(3-8)

where ρ is the film density, R the gas constant, T the temperature and Mc the average molecular weight between two crosslinking sites. Equation (3-8) shows that in this ideal case the storage modulus of a crosslinked film increases linearly with temperature and provides a direct means of accessing the crosslinking density ν of the polymer (ν = ρ/Mc).

3.3 Polymer Films

When analyzing multiphase samples, it may be possible to detect several glass transitions in a DMA measurement as was the case in the thermal characterization of multiphase polymer films described above. DMA is also able to provide information on the effects of plasticizers, resins and fillers on the polymer film. In the case of soft films which tend to flow it is easier to measure the dynamic shear modulus G* = G′ + iG″ than the elastic modulus E*. The advantage is that the film is placed between two plates rather than being clamped at its ends. G* is measured by exerting a small sinusoidal torsional displacement of one of the plates. The information content of the shear moduli curves corresponds to that of the elastic moduli ones. Optical characterization The transparency, gloss and color of a film are important in many applications. The complete optical characterization of a polymer film would require measuring the optical response of the film as a function of wavelength, angles of incidence and detection (relative to the surface normal), film thickness and type of substrate. Despite the fact that a multitude of optical techniques are available for such measurements (UVvisible spectroscopy, ellipsometry, laser scattering and so forth), in most applications simple techniques using white light are employed [29]. Film opacity is usually measured by the transmission of white light through a free film. The back-scattering power is determined using an integrating sphere photometer, that is diffuse illumination and detection of the scattered light at 0° to the film surface normal. Measurements of film gloss are performed by recording the intensity of light reflected at a specified angle to the normal (usually 20, 60 or 85°). In colormeasuring instruments, wavelength-dependent measurements are conducted at known angles of incidence and detection and the results then converted to color values. It is important to realize that when investigating films that are not wholly opaque to the wavelength concerned, the results will be influenced by film thickness and by the choice of substrate (color, transparency and so forth). For this reason, optical measurements on polymer films are often performed using black foils as substrate (for example pigment blackened PVC). Behavior with respect to liquids In a multitude of applications, polymer films get in contact with water or organic solvents. These liquids can wet, swell, permeate or even dissolve the film. To characterize these processes (with the exception of wetting) simple gravimetric methods are normally used. Wetting If a series of liquids with increasing surface tension γL are brought into contact with a polymer film, complete wetting will occur below a critical surface tension γC and partial wetting (that is droplet formation) will be observed above this value (see Fig. 3-14). The critical surface tension γC is a characteristic of the polymer film and a measure of its surface energy. Films with a high γC are easy to wet, those with a low γC value can only be wetted with difficulty.

65

66

3 Characterization of Aqueous Polymer Dispersions Determination of the critical surface energy γC of polymer films using the Zisman method (θ is the contact angle). Fig. 3-14

Wetting is quantified by measuring the contact angle, which is the angle subtended by the drop at the point of contact to the film. A contact angle of 0° reflects complete wetting. In contrast, a value of 180° represents complete non-wetting (see Fig. 3-14). The contact angle is measured either by image analysis (sessile drop method) or by using a Wilhelmy balance [4, 30]. In the Wilhelmy balance method, the polymer film is suspended vertically from the balance and then lowered slowly until it is in contact with the liquid. If the surface tension at the liquid-air interface is known, the contact angle can be calculated from the difference in sample weight when in and out of contact with the liquid. The Wilhelmy method can also be used to investigate dynamic wetting processes by recording the formation of the liquid lamella in time or by immersing and withdrawing the polymer film into and from the liquid at constant rate. Time-dependent measurements are also useful for examining cases in which liquid is taken up after the polymer film has been wetted or, conversely, in which the liquid dissolves film components such as emulsifiers. In addition to their use in determining the critical surface energy γC, contact angle measurements can also provide information on the polarity of the film surface. In this case the measurements are conducted with a series of liquids of different polarity (for example isopropanol-water mixtures). For evaluating the data a number of procedures have been published (see for example the Good-Girifalco-Fowkes method [30, 31]). Swelling, dissolution and permeation The usual means of characterizing swelling and dissolution processes involves storing weighed films in the solvent of interest (for example water or tetrahydrofuran). After a defined period of immersion (for example 24 h), the film is removed

3.3 Polymer Films

from the liquid, liquid adhering to the surface of the film is removed and the sample is weighed in its wet and dry state. The percentage increase of the wet film relative to its initial weight prior to immersion is known as the solvent or water uptake. The weight loss of the dried film compared to the initial sample weight specifies the extraction loss and is due to the partial dissolution (leaching) of film components in the liquid. Soluble and insoluble film parts are frequently referred to as the sol and gel fractions. Measurements conducted for different storage periods provide information on the kinetics of the sorption and dissolution processes. The speed with which the wet film dries may also be of significance for certain applications. Further parameters of interest are the volume changes that accompany the swelling and the subsequent drying. Interparticle crosslinking (that is crosslinking after film formation) reduces the swelling and dissolution of the polymer film strongly. Quantifying the solvent uptake and extraction loss is therefore a simple means for characterizing this type of crosslinking. In a crosslinked film, the mean molecular weight Mc between two crosslinking sites can be calculated from by the degree of swelling in a particular solvent using the Flory-Huggins equation [32]: Mc =

ρVS (Q 5/ 3 − Q / 2) 0.5 − χ

(3-9)

where Q is the swelling ratio by volume, ρ is the polymer density, VS the molar volume of the solvent and χ the Flory-Huggins interaction parameter for the polymersolvent pair (see also Eq. 3-8). Sorption and dissolution measurements on polymer films in various solvents are also the basis for determining the solubility parameters of a polymer [33], which are a measure of its solvent compatibility. In many applications, what is sought is the greatest possible compatibility or incompatibility between a polymer film and a particular solvent. In the case of a crosslinked polymer film, the greater the swelling the better the compatibility. In the case of a non-crosslinked polymer film, the greatest level of compatibility is achieved at the maximum solution viscosity. Many of the methods used for the characterization of the emulsion polymer macromolecules (see Sect. 3.3.3) require the polymer film to be dissolved in a solvent. Full dissolution is hindered if a gel fraction is present. The gel fraction is the result not only of covalent crosslinking between polymer chains, but also of the physical entanglement of the chains in these high-molecular-weight emulsion polymers. The gel fraction is often higher in polymer films which have been subjected to longer drying times as the chain segments then have the opportunity for greater interdiffusion. Swelling – particularly due to the uptake of water – often creates opacity within the film (whitening) which is undesirable in many applications. This phenomenon is caused by refractive index inhomogeneities created in the film when water penetrates the interstitial regions between the particles. Characterization of film whitening can be done with conventional techniques as discussed above. The permeation of a polymer film by a liquid can be investigated by filling the liquid into a container whose base is made of the polymer film under test. The loss of

67

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3 Characterization of Aqueous Polymer Dispersions

liquid is then recorded gravimetrically as a function of time. Such measurements are only reproducible if pore-free films can be produced. Films can be tested for the absence of pores by examining their gas tightness. Gas permeation The permeability of polymer films to vapors can be measured gravimetrically in analogy to liquid permeability (above), with the difference that the film now acts as the lid rather than the base of a container partially filled with the liquid forming the vapor. If, rather than being filled with a liquid, the container is filled with a material which acts as a strong absorber for a particular gas (for example sodium dihydrogen phosphate for water vapor or sodium hydroxide for carbon dioxide), gas permeation into the container can also be monitored. As an alternative to these gravimetric methods, conventional gas analytical techniques may be used to examine permeability, for instance by monitoring the pressure drop across the film or by the specific determination of a gas component that permeates the film. As in the liquid permeation studies, the film samples examined must be free of pores. 3.3.3

Microscopic Characterization of Polymers

Macromolecules Most of the methods used for the microscopic characterization of emulsion polymers in terms of their macromolecular composition, molecular weight and crosslinking require the removal of water. For this reason the investigations are performed on the dry polymer film or on freeze-dried samples. The methods employed are the standard techniques of polymer characterization [34–36]. Some of the measurements are performed on solutions of the polymer in organic solvents such as tetrahydrofuran or dimethylformamide. Because of their high molecular weight and their partial crosslinking, complete dissolution of an emulsion polymer is often difficult (see Sect. 3.3.2), and the information that can be provided by these methods is in such cases rather limited. Chemical composition The chemical composition of an emulsion polymer sample can for instance be determined by Fourier transform infrared (FTIR) spectroscopy [37]. The measurement is performed on a polymer film. Quantitative analysis involves comparison of the spectra obtained with those of standard calibration substances. An alternative or complementary method is pyrolysis gas chromatography. In this technique the polymer is rapidly heated causing depolymerization or decomposition and the products are separated and detected gas chromatographically [34]. Polymer composition can also be determined by 1H and 13C NMR [38] on dilute samples of the polymer in an organic solvent. NMR analysis also enables end group analysis and to a limited extent monomer sequence studies (for example in terms of triad distributions).

3.3 Polymer Films

In recent years there has been increased interest in using gradient HPLC techniques, such as gradient polymer elution chromatography (GPEC [39]), for determining the compositional distribution of copolymers. The solubility gradient is created by mixing a solvent in which the polymer dissolves well with one in which it does not dissolve (the so-called non-solvent). The copolymer is dissolved in the good solvent and then injected into the LC column with the non-solvent as eluent, with the result that the copolymer precipitates at the entrance of the column. During gradient elution, the amount of the good solvent in the eluent is gradually raised which leads to the re-dissolution and fractionation of the copolymer. Molecular weight The determination of the molecular weight of the polymer is also carried out in organic solution. A simple method is to measure the intrinsic viscosity [η] of the solution [34]. The measurement is normally made using a capillary viscometer and involves recording the solution viscosity as a function of polymer concentration c and then extrapolating the data to zero concentration (see ISO 1628-1). The dependence of the intrinsic viscosity and the molecular weight, M, is given by the Mark-Houwink equation:

 1 η( c ) − η0  α (3-10) [η ] = lim   = AM c →0  c η0   where c is the polymer concentration, η0 the solvent viscosity and A and α are quantities which are constant at specified temperature for the solvent-polymer pair. Normally, the c → 0 extrapolation is too involved for routine measurements. In such cases viscosity is only measured at one particular (low) concentration and used as a relative measure for the molecular weight of the investigated polymer. The Mark-Houwink equation (Eq. 3-10) assumes that the polymer in solution is present in the form of random statistical coils. For a given molecular weight, branching and crosslinking in the macromolecule lead to a lower viscosity. In doubtful cases, alternative methods of absolute molecular weight characterization (static light scattering, density gradient analysis in an analytical ultracentrifuge, membrane osmometry, end-group analysis, and so forth) should be used for comparison purposes [34–36]. A modern alternative is that of matrix-assisted laser desorption ionization mass spectrometry (MALDI MS [40]). In this technique the polymers are embedded in a matrix made of a strong UV absorber which enables the unfragmented ionization of the macromolecules by a UV laser pulse. Absolute molecular weight determination is achieved in this mass spectrometer by time-of-flight measurement. Because emulsion polymers are prepared by a radical polymerization process, the molecular weight distribution (MWD) is generally quite broad. MWD is usually characterized by gel permeation chromatography (GPC, also referred to as SEC, size exclusion chromatography). GPC fractionates a polymer solution according to coil size by passing it through a micro-porous gel with a defined pore size distribution [34]. In addition to simple UV and refractive index detectors, other techniques such as FTIR spectrometry and light scattering are now used to characterize the individual fractions as they elute from the column. The latter two detectors enable both the chemi-

69

70

3 Characterization of Aqueous Polymer Dispersions

cal composition and the molecular weight of the individual polymer fractions to be accessed directly. Crosslinking Internally crosslinked polymer particles (“micro-gels”) can be characterized by comparing hydrodynamic volumes and molecular weight. High molecular weights coupled with small hydrodynamic volumes indicate extensive crosslinking. The hydrodynamic volume is best accessed by viscosity measurements or dynamic light scattering, while molecular weight can be determined by the density gradient analysis in an analytical ultracentrifuge or by static light scattering. Micro-gel fractions are a common feature of emulsion polymers because of intraparticle crosslinking. On the other hand, interparticle crosslinking, which occurs after film formation, significantly reduces the solubility of the polymer film. In this latter case, crosslinking is characterized by performing swelling experiments in organic solvents. Film and particle morphology Polymer particles can be produced in a number of morphologies. Figure 3-15 shows examples of structures that have been observed.

Fig. 3-15

Morphologies of polymer particles.

The morphology of the film, directly after its formation, will be determined by the structure of the particles, but a significant restructuring of the phases can occur as a function of time (leading for example to larger domains). The major technique used to characterize particle and film morphology is transmission electron microscopy (TEM), which is described below. Other techniques are small angle X-ray and neutron scattering (SAXS [41] and SANS [42]), atomic force microscopy (AFM, [43, 44]) and NMR spin-diffusion and spin-relaxation techniques [45]. However these methods are not in widespread use and their ability to characterize the composition, size, shape and superstructure of the domains is somewhat limited. The reader is referred to the literature for further details. Transmission electron microscopy In transmission electron microscopy [46] the dry sample has to be transferred into ultrahigh vacuum and is illuminated by a high-energy beam of electrons (for example 100 keV). In an ideal case, a lateral resolution of around 1 nm is achievable. Since sample preparation is rather involved, TEM is not a routine technique. In order to examine individual particles, they have to be placed separately on a suitable

3.3 Polymer Films

substrate under conditions which prevent film formation (that means high dilution of the sample and drying below the minimum film formation temperature). For the TEM inspection of a polymer film a thin section containing only one particle layer is required (typical thickness <100 nm). The thin-cut can only be done at a temperature below the glass transition temperature of the polymer. Sometimes it is also possible to directly deposit a particle monolayer on a substrate by drying the dispersion at the right dilution. Transmission electron micrographs directly show the size and shape of the individual polymer particles. However, to draw any reliable conclusions on the distribution of particle size or shape the laborious counting of a large number of particles is required (>1000!). A fundamental problem of using electron microscopy to analyze polymer samples is their low electron density, which causes low contrast in the images. Improved contrast can be achieved by staining the polymer with heavy-metal compounds such as RuO4, OsO4 or uranyl acetate. These compounds are incorporated into the polymer network directly or via a suitable coupling agent. Staining agents which exhibit high selectivity for certain polymers also form the basis of morphology studies. For example, polystyrene and polybutadiene can be selectively stained with RuO4. Acrylates, on the other hand, require treatment with hydrazine and OsO4. A core-shell particle with a polystyrene core and an acrylate shell can thus be characterized by staining the core with RuO4. In the same way a possible phase restructuring taking place in the film of these particles can be studied. An alternative to the above preparation methods, albeit a rather involved one, is the freeze-fracture technique, in which the dispersion is shock frozen by being poured into liquid nitrogen. The freezing process has to be fast enough to avoid crystallization of the water phase. The sample is then cryo-transferred to the electron microscope where it is fractured. The fracture surfaces can then be imaged using, for example, replica techniques. Acknowledgments I wish to thank Dr J. Lamprecht, Dr W. Mächtle, Dr A. Zosel, Dr H. Nissler, Dr R. Baumstark, H.-J. Heiter, and S. Krause for their assistance in the preparation of the manuscript.

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References 1 W.-D. Hergeth in: Polymeric Dispersions:

2

3

4 5 6

7 8

9

10

11

12

13 14

15 16

Principles and Applications, J. M. Asua (ed.), Kluwer Academic Publishers, The Netherlands, 1997, pp. 267–288. E. Penzel in: Ullmann's Encyclopedia of Industrial Chemistry, Vol. 21, Verlag Chemie, Weinheim, 1992, pp. 157–178. R. S. Davis, W. F. Koch in: Physical Methods of Chemistry, Vol. VI: Determination of Thermodynamic Properties, B. W. Rossiter, R. C. Baetzold (eds), Wiley, New York, 1992, pp. 59–62. A. W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 1990. V. B. Fainerman, R. Miller, P. Joos, Colloid Polym. Sci. 272, 731 (1994). E. J. Schaller in: Emulsion Polymerization and Emulsion Polymers, P. A. Lovell, M. S. El-Aasser (eds), Wiley, New York, 1997, pp. 437–466. J. J. Bikerman, Foams, Springer, Berlin, 1973. E. A. Collins in: Emulsion Polymerization and Emulsion Polymers, P. A. Lovell, M. S. El-Aasser (eds), Wiley, New York, 1997, pp. 385–436. C. Bohren, D. Huffman, Absorption and Scattering of Light by Small Particles, Wiley, New York, 1983. W. Brown (ed.), Dynamic Light Scattering, Oxford University Press, Oxford, 1992. C. Wu, K. Unterforsthuber, D. Lilge, E. Lüddecke, D. Horn, Part. Part. Syst. Charact. 145–149 (1994). P. Kratochvil, Classical Light Scattering from Polymer Solutions, Elsevier, Amsterdam, 1987. H. G. Barth, Modern Methods of ParticleSize Analysis, Wiley, New York, 1984. W. Mächtle in: Analytical Ultracentrifugation in Biochemistry and Polymer Science, S. E. Harding, A. J. Rowe, J. C. Horton (eds), Royal Society of Chemistry, Cambridge, 1992, pp. 147–175. J. G. DosRamos, C. A. Silebi, J. Colloid Interface Sci. 135, 165 (1990). A. M. Botana, S. K. Ratanathanawongs, J. C. Giddings, J. Microcolumn Sep. 7, 395 (1995).

17 J. W. Vanderhoff, H. J. Van den Hul,

18 19 20 21

22 23

24

25

26

27 28 29

30

31 32

33

R. J. M. Tausk, J. T. G. Overbeek in: Clean Surfaces, G. Goldfinger (ed.), Marcel Dekker, New York, 1970. S. H. Maron, M. E. Elder, I. N. Ulevitch, J. Colloid Sci. 9, 89 (1954). J. G. Brodnyan, G. L. Brown, J. Colloid. Sci., 15, 76 (1960). R. J. Hunter, Zeta Potential in Colloid Science, Academic Press, London, 1981. R. J. Hunter, Foundations of Colloid Science, Vol. II, Clarendon Press, Oxford, 1989, Chapter 13. Gas Chromatography, J. P. Baugh (ed.), Oxford University Press, Oxford, 1993. S. Ahmed, M. S. El-Aasser, G. H. Pauli, G. W. Poehlein, J. W. Vanderhoff, J. Colloid Interface Sci. 73, 388 (1980). P. M. Lesko, P. R. Sperry in Emulsion Polymerization and Emulsion Polymers, P. A. Lovell, M. S. El-Aasser (eds), Wiley, New York, 1997, pp. 619–655. V. B. F. Mathot (ed.), Calorimetry and Thermal Analysis of Polymers, Hanser, Munich, 1994. H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges, J. I. Kroschwitz (eds) Encyclopedia of Polymer Science and Engineering, Vol. 7, Wiley, New York, 1987, p. 539. I. M. Ward, Mechanical Properties of Solid Polymers, Wiley, New York, 1983. A. Zosel, G. Ley, Macromolecules 26, 2222 (1993). J. V. Koleske (ed.), Paint and Coating Testing Manual, ASTM manual series, MNL 17, ASTM, Philadelphia, 1995, Chapters 40–42. P. C. Hiemenz, Principles of Colloid and Surface Science, Marcel Dekker, New York, 1986, Chapter 6. W. D. Bascom, Adv. Polym. Sci. 85, 89 (1988). P. J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, 1953. A. F. M. Barton, CRC Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press, Boca Raton, 1983.

References 34 C. Booth, C. Price (eds), Comprehensive

35 36

37

38

39

Polymer Science, Vol. 1, Polymer Characterization, Pergamon Press, Oxford, 1989. B. J. Hunt, M. I. James (eds), Polymer Characterization, Blackie, London, 1993. E. Schröder, G. Müller, K.-F. Arndt, Polymer Characterization, Hanser, Munich, 1988. D. O. Hummel, Atlas of Polymer and Plastics Analysis, Vol. 1, Polymer: Structures and Spectra, Verlag Chemie, Weinheim 1978. J. L. Koenig, Spectroscopy of Polymers, ACS Professional Reference Book, American Chemical Society, Washington, 1992. G. Glöckner, Gradient HPLC of Copolymers and Chromatographic Cross-Fractionation, Springer, Berlin, 1991.

40 H. S. Creel, Trends Polym Sci. 1, 336

(1993). 41 R. Grunder, G. Urban, M. Ballauff,

Colloid Polym. Sci. 271, 563 (1993). 42 R. H. Ottewill in: Polymeric Dispersions:

43

44 45

46

Principles and Applications, J. M. Asua (ed.), Kluwer Academic Publishers, The Netherlands, 1997, pp. 229–242. J. Didier, Y. Wang, J. Lang, O. Leung, M. C. Goh, M. A. Winnik, J. Polym. Sci: Part B: Polym. Phys. 33, 1123 (1995). S. Akari, D. Horn, W. Schrepp, Adv. Mater. 7, 549 (1995). K. Landfester, C. Boeffel, M. Lambla, H. W. Spiess, Macromolecules 29, 5972 (1996). L. C. Sawyer, D. T. Grubb, Polymer Microscopy, Chapman and Hall, London, 1987.

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Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

4

Applications in the Paper Industry Jürgen Schmidt-Thümmes, Elmar Schwarzenbach, and Do Ik Lee

4.1

Introduction

In 1998, the world demand for emulsion polymers (dry) was 7.4 million metric tons and is forecasted to increase to 8.8 million metric tons in 2003 with an annual growth rate of 3.6 % [1]. Of this 1998 world demand, about 35 % and 32 % were consumed in North America and Western Europe, respectively, while about 23 % was for paper and paperboard coatings. If the world-wide uses of emulsion polymers for both paper and paperboard coatings and paints and coatings are combined, they will account for about half of the world consumption of emulsion polymers. For this reason, the industry for paper and paperboard coatings is a core market for emulsion polymers, along with the industry for paints and coatings. The world demand for emulsion polymers in 1998 is shown by market and region in Fig. 4-1, while Fig. 4-2 shows the demand forecast in 2003 [1].

Other Markets (18%) Paints & Coatings (26%) Carpet Backing (11%) Adhesives Paper & Paperboard (22%) (23%)

By market

Other Regions (21%) North America (35%) Japan (12%) Western Europe (32%)

By region

The 1998 World Emulsion Polymer Demand: 7.4 Million Metric Tons Fig. 4-1

The 1998 world demand for emulsion polymers by market and region.

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4 Applications in the Paper Industry

Other Markets (17%) Paints & Coatings (26%) Carpet Backing (10%) Adhesives Paper & Paperboard (23%) (24%)

By market

Other Regions North America (24%) (34%) Japan (11%) Western Europe (31%)

By region

The 2003 World Emulsion Polymer Demand Forecast: 8.8 Million Metric Tons The 2003 world demand forecast for emulsion polymers by market and region.

Fig. 4-2

This chapter will cover the applications of emulsion polymers in the paper industry, especially in surface sizing and paper coating. Since information on the breakdown in the uses of emulsion polymers for surface sizing and paper coating is not readily available in the world markets, it is hoped that information available on the Western Europe market would provide some perspectives on their relative uses of emulsion polymers. Table 4-1 shows the amounts of emulsion polymers used for these applications in Western Europe: 3 % and 97 %, respectively [2]. Tab. 4-1 The Western European market for emulsion polymers in the paper industry [2].

Market segment

Amount of polymer dispersions in metric tons and percent (1997)

Surface sizing Paper coating Total

35 000 ( ~3 %) 1 150 000 (~97 %) 1 185 000

4.2

The Paper Industry 4.2.1

History

The precursors of paper were papyrus and parchment, which were used for writing as early as 3000 BC in Egypt. In China, strips of bamboo or wood were used for writing and drawing before the discovery of paper. The invention of paper has been attributed to Ts’ai Lun in AD 105, who produced a uniform writing-material paper from felted plant fibers [3]. The original paper was made in China from rags, bark

4.2 The Paper Industry

fiber, and bamboo. The plants were crushed in mortars and water was added to create a homogeneous fiber pulp. By dipping a hand wire screen into the suspension, a thin layer of the pulp was removed and then dried. Even today, these are still the fundamental steps in the papermaking process. The art of papermaking finally reached Central Asia by AD 751 and Baghdad by 793, and by the 14th century there were paper mills in several parts of Europe [4]. Later, it landed in the New Continent. With the invention of the printing press by J. Gutenberg in the middle of the fifteenth century, paper assumed a previously unimagined importance and there was a massive increase in the demand for paper. As a result of further discoveries, increasing levels of trade and for other reasons, the level of paper consumption continued to rise. Numerous raw materials were used for paper manufacture and there were rapid developments in industrial papermaking with the first papermaking machine being built in 1799. Nicholas-Louis Robert constructed the first papermaking machine. Using a moving screen belt, paper was made one sheet at a time by dipping a frame or mold with a screen bottom into a vat of pulp. A few years later, the brothers Henry and Sealy Fourdrinier improved Robert’s machine, and in 1809 John Dickinson invented the first cylinder machine [4]. 4.2.2

The Paper Industry Today

In 1998, the world production of paper and paperboard excluding newsprint and tissue totaled approximately 240 million metric tons and is expected to grow to approximately 290 million metric tons in 2003 with an annual growth rate of 4 % [1]. The main raw material used to make paper is wood. Both softwoods (long fiber) and quick-growing hardwoods (short fiber) are processed. The intermediate stage between the raw materials and the finished paper is socalled half stuff (pulp). Typically, this is: – Cellulose from which lignin, resins, and incrustations have been removed by the refining process to leave high-grade cellulose fiber that is particularly well suited to paper manufacture. – Mechanical pulp, which is produced from wood that has been ground or refined by mechanical means. This type of pulp is less well suited for paper manufacture, as the incrustations are still present to a large extent and the properties of the pulp are determined by fiber bunches and fragments inevitably present. – Paper made of pure cellulose is designated as “wood-free” paper, whereas that made from mechanical wood is called “wood-containing” paper. In an effort to protect wood resources, large amounts of recycled papers are also used in today’s papermaking industry. Modern technology combined with appropriate process chemicals enables this secondary raw material to be used not only for paperboard, but also for high-quality paper. The proportion of the chemical additives used as fillers in both paper and coatings is about 3 %, a surprisingly small amount compared to the other constituents such as recycled paper, cellulose, and pigments. Of this 3 %, synthetic additives comprise only about one third so that overall synthetic additives make up only about 1 % of the

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4 Applications in the Paper Industry

Chemical additives Chemical pulp

43%

3%

1.0%

Synthetic chemical additives

0.5%

Alum

1.5%

Starch

8% 35%

11%

Waste paper Mechanical pulp Pigment/Filler

The proportion of chemical additives relative to the total global raw material demand of the paper industry in 1996.

Fig. 4-3

total content (Fig. 4-3). The two most important groups of the synthetic additives are the synthetic binders (50 %) and the sizing agents (25 %), as shown in Fig. 4-4. While synthetic binders are composed of emulsion polymers, sizing agents can be monomeric or polymeric. In the latter case, they are in the form of polymer dispersions. Additives relevant to paper properties

Additives relevant to manufacturing process

(Functional chemicals)

(Process chemicals)

Synthetic binders 50% (Paper coating)

1% 1% 1%

Biocides Defoamers, deaerators Dispersants, cleaners

8% 3% 6% Sizing agents 25% Wet strength resins Bleaching chemicals Dyes, OBA

Fig. 4-4

5%

Retention drainage aids, curing agents, flocculants,etc.

8%

Process chemicals used in the papermaking.

The pulp is prepared for the paper machine in an upstream unit. In this unit, the wood is ground, washed, and sorted, and then fiber concentration and consistency are adjusted to the desired levels. The paper machine itself is a single continuous production line with a length that today may exceed 200 m and comprises the following main sections: headbox, wire section, pressing section, drying section, and finishing section.

4.3 Surface Sizing

In the headbox, the pulp suspension is spread across the entire width of the web and passed onto the wire mesh at the correct speed. The sheet is formed as the water drains from the mesh and the fibers are fixed into their final orientation while still in the wet mat stage. In the pressing section, water is driven out of the wet mat by applying pressure to the web. The web enters the pressing section with a dry content of about 20 % which increases to 40–50 % as the web leaves this section of the machine. On passing through the drying section, the web is dried to a final moisture content that is in equilibrium with the ambient air. The drying section is often equipped with additional devices which improve the surface properties of the paper or board. Examples of such devices are the size press, the machine calendering cylinder, various types of calenders, and coating equipment. The paper or board web is wound onto rolls in the finishing section which also contains roll handling and wrapping equipment. The size of today’s paper production lines is enormous. The state-of-the-art papermaking and finishing machines are up to 10 m wide in web widths and up to 2000 m min–1 (120 km h–1) in production speeds. A wide range of production and finishing processes guarantees that even the most demanding quality requirements can be met. The largest part of the papers produced today is for printing, also known as graphic arts, i.e. for printing paper and board. The requirements that these materials must meet include: – high degree of uniformity and smoothness – good optical properties of which brightness and gloss are the most important – high opacity and high strength In short, the physical characteristics of the paper or board must ensure both good processability and good printability. To meet these demands, the following two processing stages are incorporated into the drying section of the papermaking line: – surface sizing – paper coating The use of emulsion polymers in the paper industry is essentially restricted to these two processes which are described in more detail in the following sections.

4.3

Surface Sizing

Surface sizing means a pigment-free application of hydrophobicizing substances, the surface sizing agents, in combination with starch. The application will be on-line to the paper machine by either a size press or a film press. In relation to the paper mass usually 3 to 5 % (w/w) of starch and 0.1 to 0.25 % (w/w) of sizing agents, each calculated as solid, will be applied. So in a size press formulation starch clearly dominates with more than 95 % of the solids content. Starch enhances the strength of the paper, the surface sizing agent hydrophobicizes the paper sheet, thereby reducing the absorbency of the paper. Thus, the penetration and spreading of print colors are controlled and the loss of strength in the wet state is reduced.

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4 Applications in the Paper Industry

An alternative to surface sizing is internal sizing, the addition of a sizing agent to the wet end before the formation of the paper sheet. In this process step, however, exclusively low molecular weight sizing agents like rosin acids or alkyl ketene dimers (AKD) are applied. For surface sizing, mostly polymeric sizing agents are used. The most important product classes are acrylic copolymer dispersions stabilized by protective colloids. The particles of the sizing agent consist of a hydrophobic polymer core and a hydrophilic shell formed out of the protective colloid (Fig. 4-5).

Charged hydrophilic protective colloid

+ +

+

+ + +

+

+ +

Hydrophobic core

+ + + +

+ +

Charged hydrophilic protective colloid

70 - 200 nm Fig. 4-5

Structure of a polymer-based sizing agent.

The composition of the polymeric core influences hydrophobicity, glass transition temperature, viscous flow, and binding strength of the polymer. The hydrophilic shell is highly swollen in water and normally carries either an anionic or cationic charge. It renders stability to the dispersions during storage and against the high shear stress during application. It also plays an important role in the interaction between starch and sizing agent. The hydrophobic effect of surface sizing stems from the formation of a stable coherent film at the paper surface providing a halftone-like screen (raster) formed from well defined hydrophobic barriers and areas of hydrophilic character (Fig. 4-6). Polymer particles sitting at the interphase between starch film and fiber surface support the fixation of the starch film to the fiber. Polymer particles in the interior improve the wet strength and delay the dissolution of starch and the flow of aqueous media within the starch layer. Polymer particles at the surface reduce the wettability of the surface. Only dispersions stabilized by protective colloids are able to form such a coherent hydrophilic/hydrophobic raster. The protective colloid tightly fixed to the polymer core acts as a compatibilizer between starch and hydrophobic polymer core, preventing a rupture of the film during drying and shrinking of the starch.

4.4 Paper Coating Starch Polymer protective colloid Polymer core Hydrophilic

Hydrophobic

Fiber

Fig. 4-6

Formation of a starch-polymer film.

The hydrophilic/hydrophobic balance of the surface can be individually controlled by: – the ratio between starch and hydrophobic polymer – properties of the starch-like film formation, swelling, and water uptake – hydrophobicity, average particle size, and viscous flow of the polymer particles Also, in special paper applications like photocopy and ink jet papers, the addition of polymer dispersions to surface sizing formulations can lead to positive effects. The interaction between hydrophobic toner and polymer particle enhances the toner adhesion in case of photocopy papers. This proves to be very helpful in cases where the process conditions in the copier are insufficient to guarantee complete melting of the toner on the paper surface. Applied to ink jet papers, a hydrophilic/hydrophobic raster on the paper surface results in a highly accurate fixation of the dye right to the spot at the paper surface. Whereas the hydrophilic areas allow a fast dewatering of the printing ink, mostly to the interior of the paper sheet, the hydrophobic points prevent a spreading parallel to the paper surface. Additional modification of the starch/polymer film by cationic groups results in an additional fixation of the anionic dyes by ionic interaction. Thereby, color density and outline sharpness can be further improved.

4.4

Paper Coating

Paper coating is the most important surface finishing process for paper in terms of both the amount of paper that is coated and the quantity of emulsion polymers consumed in the coating process. The method involves coating the surface of the paper with a water-based pigmented coating color. The emulsion polymer used in the coating color formulation binds the individual pigment particles together and helps the entire pigment layer to adhere to the surface of the paper. Furthermore, emulsion polymers are also added to improve the processability and/or runnability of the coating color. Coating is typically applied onto paper and board for printing or packaging

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4 Applications in the Paper Industry

applications. Other specialty kinds of paper that undergo coating are labels, wallpaper, and non-printed silicone papers which act as the backing sheets for self-adhesive labels. Coating paper or board increases the homogeneity of the surface and considerably improves its optical characteristics such as gloss, smoothness, brightness, and opacity. The properties of the pulp severely limit the surface homogeneity achievable with uncoated papers. The surface of an uncoated paper will contain fibers which are approximately 1–3 mm (1000–3000 µm) long and approximately 10 µm thick. If this paper is printed by the halftone process using a 50 lines/cm screen, the dots (200 µm) are smaller than the dimensions of the fiber. The fibers are thus the limiting factor dictating image definition. In contrast, the pigments used in the coating color can be easily ground to a particle size of less than 1 µm. While the surface of an uncoated paper comprises numerous individual fibers of varying degrees of hardness, the surface of a coated paper is, by contrast, uniform and homogeneous in structure. Figures 4-7 and 4-8 demonstrate clearly the differences in the quality of offset and rotogravure printing on coated and uncoated paper surfaces, respectively.

Uncoated grade, supercalendered

Coated grade, supercalendered

Fig. 4-7

Effect of coated paper on offset printing.

For the reasons given above, coated paper exhibits more uniform ink receptivity and better holdout than uncoated papers. Coating also produces a much smoother paper surface that is particularly a significant factor when printing individual dots, especially when using a rotogravure process (Fig. 4-9). Low basis-weight papers require a high degree of opacity if show-through (i.e., when the printing on one side of the paper can be seen from the other side) is to be prevented. The opacity of an uncoated paper is determined by the cellulose fibers and any fillers it contains. While fillers are naturally better than cellulose in increasing opacity, they are unable to provide the opacity levels attainable by coating. The crucial factor determining opacity is the volume of the coating, since this determines

4.4 Paper Coating

Coated gravure paper

Uncoated gravure paper

Fig. 4-8

Effect of coated paper on rotogravure printing.

typical dotsize critical area for missing dots

Result of printing paper of insufficient smoothness by the rotogravure printing process. Fig. 4-9

the area of the pigment–air interface (within the coating layer) at which the scattering of incident light occurs.

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Gloss is a critical property when assessing the quality of printing. Calendering is only able to improve the gloss of an uncoated paper surface to a limited extent. The gloss of a coated paper surface can be varied over a wide range, either by purely mechanical means (calendering), by the coating process itself, or by controlled addition of gloss-imparting pigments. In this way, a full range of coated papers from highgloss to semi-gloss to matte are easily obtained. The degree of brightness can be controlled by selecting appropriate pigments, but can also be adjusted by the use of optical brighteners or toning dyes or both. The brightness of a coated paper also depends strongly on that of the base paper. If the base paper is of low brightness (e.g., unbleached), opacifying pigments such as titanium dioxide (TiO2) and special techniques such as double or triple coating are used. 4.4.1

Coating Techniques

A number of different coating machines exist for applying the coating color onto the base paper. Figures 4-10a–c illustrate the common coating methods, along with the range of coating weights that can be achieved, the required level of solid content, and the viscosity of the coating colors. Stiff blades are more commonly used in North America, while bent blades are more widely used in Europe. It is apparent that the various coating methods place different demands on the rheological properties of the coating color. These requirements must be taken into account when formulating a coating color for a particular application. The major components of a coating color are: – inorganic pigments to cover the surface of the base paper – co-binder and thickener for controlling the processing properties – binder (water-soluble or disperse systems or a combination of the two)

4.4 Paper Coating

Fig. 4-10 Coating equipment. (A) Stiff blade, bent blade, and roll blade; (B) Air-knife; (C) Pre-metered size press.

Quantitatively, pigments are the principal constituent of any coating color, binders being used in relatively small amounts. For every 100 parts pigment, there are typically about 5–20 parts binder and 0.1–3 parts of other additives. Coating color compositions common in both North America and Europe for sheet-fed offset and rotogravure printing processes are listed in Tab. 4-2. The quantities given here always refer to the amount of active ingredient required. A more detailed description of the constituents of a coating color is presented in Sects 4.4.2–4.4.4. Once coated, the paper is smoothed as part of the calendering process. Calendering involves subjecting the paper surfaces to high temperatures and pressures in or-

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4 Applications in the Paper Industry Tab. 4-2 Typical coating color compositions for sheet-fed offset and rotogravure printing processes in both North America and Europe.

Sheet-fed offset

Rotogravure

North American formulation 75 parts fine kaolin clay 25 parts fine ground calcium carbonate 12 parts emulsion polymer 3 parts starch 0.5 parts calcium stearate

North American formulation 85 parts delaminated clay 15 parts talc 5 parts emulsion polymer 2 parts starch 0.5 parts calcium stearate

European formulation 80 parts fine ground calcium carbonate 20 parts fine kaolin clay (high gloss clay) 12 parts emulsion polymer 0.5 parts co-binder 0.5 parts curing agent 0.5 parts optical brightener

European formulation 50 parts talc 50 parts kaolin clay (coarse or high aspect ratio) 5 parts rotogravure sole binder 0.75 parts calcium stearate

der to create a smooth, glossy surface. A distinction is made between soft-nip calendered and supercalendered papers. In the soft-nip calender process, the number of nips is kept low, and higher temperatures and lower pressures are used, compared to the supercalender process. The advantages of the soft-nip calendering are that it can be performed “on-line”, i.e., immediately after the coating process and that the bulk of the paper does not decrease as much as in supercalendering. By varying temperature and pressure in a controlled manner, a very broad range of gloss levels can be achieved. In the supercalender, the number of nips used to smooth the paper is greater, with typically twelve rolls in a supercalender stack. The smoothest and glossiest paper surfaces are achieved by supercalendering. 4.4.2

Pigments used in Coating Colors

The main constituents of a coating color formulation are the inorganic pigments, which serve to cover the surface of the base paper and thus to improve its optical properties. Coating pigments must therefore satisfy the following requirements: – high purity – high brightness and opacity – high refractive index – good dispersibility and desirable rheological properties – amount of binder required should be low The most important pigments are: – kaolin clay (often referred to simply as china clay) – calcium carbonate, natural or precipitated – titanium dioxide

4.4 Paper Coating

Nearly in all cases, not one but a combination of several pigments is used in coating color formulations. Kaolin clay and calcium carbonate are the most commonly used pigments. There are a great number of different types in each of the two pigment groups: the calcium carbonate grades being distinguished mainly by particle size, while the plate-like kaolin clays are classified according to their so-called aspect ratio (ratio of surface diameter to thickness) and particle size. In the recent years, the use of ground calcium carbonate pigments in North America has been steadily increased so that the differences in coating color formulations between North America and Europe are being narrowed. The pigments used in the preparation of coating colors are prepared as slurries. These are aqueous dispersions which by using dispersing agents such as tetrasodium pyrophosphate or sodium polyacrylate can have a solid pigment content of greater than 70 %. The different pigments require different amounts of binder in order to ensure adequate adhesion of the coating to the surface of the paper and sufficient binding between the pigment particles. For this reason, it is important to keep the specified binder-to-pigment ratios when formulating coating colors. The following table (Tab. 4-3) lists the amount of binder required by various pigments to achieve a given level of binding strength (pick strength) for sheet-fed offset printing paper. Tab. 4-3 Amount of binder in coating colors as a function of pigment type.

Pigment

Kaolin clay Ground calcium carbonate Precipitated calcium carbonate Titanium dioxide

Binder demand (%) Paper

Board

12 11 15 14

14 12 18 16

More binder is needed when coating board to ensure good glueability in folded cardboard boxes. 4.4.3

Co-binders and Thickeners used in Coating Colors

Pumping, transfer, re-circulation, and, most particularly, the actual coating method require certain rheological properties of the coating colors. Low-shear and highshear viscosities (shear rates of 10 to >106 s–1) and water retention values are highly important parameters. For example, in roll coating applications, the thixotropic behavior of the coating color is particularly important, whereas Newtonian or structurally viscous (i.e. pseudoplastic or shear-thinning) flow at high shear rates is important for all blade coating techniques. Coating colors are characterized by their viscosity, solid content, immobilization point, and water retention capacity.

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4 Applications in the Paper Industry

To adjust these properties to the required level, co-binders and thickeners are added to coating color formulations. If possible, these additives should be chosen to have a positive influence on the gloss, smoothness, printability, brightness, binding strength, and glueability of the paper, and they should certainly not have a detrimental effect on any of these properties. Typical amounts are 0.1–3 parts of co-binder or thickener to 100 parts pigment and approximately 12 parts binder. In addition to the emulsion polymers described in greater detail below, other substances are used as co-binders and thickeners. These include natural products such as starch and synthetic water-soluble polymers such as polyvinyl alcohol and carboxymethylcellulose. The chemical composition and the behaviors of co-binders and thickeners with respect to pH are shown in Fig. 4-11.

Dispersion

Solution

pH < 7

pH > 7

Hydrophobic polymer chains in form of small balls (dispersion particles)

- COOH

Alkali

- COO

- COO

- COOH

Anionic charges repel each other, polymer chains stretch and dissolve

Alkali

MAIN MONOMERS CH2=CH-COOH Acrylic acid CH2=C(CH3)-COOH Methacrylic acid CH2=CH-COO-R Acrylic acid esters Acrylonitrile CH2=CH-CN CH2=CH-O-CO-CH3 Vinyl acetate Fig. 4-11

Chemical structure of synthetic co-binders.

In contrast to the emulsion polymers used as binders, those employed as cobinders and thickeners contain large fractions of hydrophilic (typically carboxyl-rich) monomers. This high degree of hydrophilicity means that the particulate nature of the dispersion is lost when the acidic dispersion (pH < 7) is added to the alkaline environment of the coating color formulation (pH > 7). The resulting structures, which range from massively swollen polymer networks to polymer chains dissolved in the aqueous phase, influence the rheology of the coating color in a complex manner and are still not fully understood. Apart from the increase in the viscosity of the aqueous phase due to the dissolved polymer molecules, which are present as stiff chains at the pH of the coating color

4.4 Paper Coating

formulation, polymer bridges also form between the pigment particles (Fig. 4-12). These structures, which are strongly dependent on the state of shear in the coating color, result in a rise in its low-shear viscosity. If, in addition, one succeeds in incorporating hydrophobic side chains into the polymer, the resulting associative interaction between the dissolved chains enables the low-shear viscosity to be increased still further.

A

Dispersion + Alkali (pH>7)

Solution Carboxylate groups with anionic charges

Functional groups with high polarity

Additional hydrophobic side chains

Anionic charges repel each other

Adsorption on pigment surfaces

Associative interactions between polymers

Extended polymer chains in the aqueous phase

Polymer bridges between pigment particles

Additional network structures (micelles)

Viscosity in the aqueous phase

High viscosity at low shear

Very high viscosity at low shear

B

Latex particles

Extended polymer chains

Bridges between pigments

Associative thickening

Fig. 4-12 Thickening mechanisms with various types of alkali-soluble co-binders and thickeners. (A) Alkali solubilization and thick-

ening behaviors of various acrylate copolymers; (B) Various thickening mechanisms of synthetic co-binders and thickeners.

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All effects induced by the co-binder and thickener in the coating color are very strongly dependent on the shape, charge distribution, and size of the pigments used as well as on the solid content of the formulation. Choosing the right thickener or cobinder for a coating color which is to be formulated for use in a particular type of coating machine is a complex task that requires good product knowledge and a considerable degree of practical experience. 4.4.4

Binders used in Coating Colors

Both natural and synthetic binders are used in the paper coating. Binders from natural sources are used in the form of aqueous solutions and include: – starch – soy protein – cellulose derivatives such as carboxymethylcellulose (CMC). Synthetic binders, which are prepared as aqueous polymer dispersions, are: – styrene and butadiene – styrene and butyl acrylate – poly(vinyl acetate) – acrylates – vinyl ester and acrylic ester – ethylene and vinyl ester These synthetic binders commonly known as latexes are mostly modified with functional monomers such as vinyl acids, amides, acrylonitrile, etc. to improve the colloidal and rheological properties of coating color formulations and the printing and/or packaging properties of coated papers and paperboards. As a water-soluble substance, polyvinyl alcohol represents a special case among synthetic binders. When the coating of paper began more than one hundred years ago, animal glues and gelatin were used as binders. Partly because of their high price, these materials are no longer used today except in a few specialty applications (e.g., gelatin in the manufacture of photopaper). The natural products of more lasting significance were starch (from potatoes, corn, and rice) and casein (from milk). Both are binders which, like the synthetic sole binders, combine the characteristics of binder and cobinder. However, unlike the synthetic products, starch and casein cannot be added directly to the pigments, but must first be pre-processed. Table 4-4 presents a general comparison of natural and synthetic binders. The most important natural binder still in use today is starch, though it is now frequently used in combination with synthetic binders. Corn starch is more common in the USA, whereas potato starch is more prevalent in Europe. Native starch containing two fractions of amylose (linear chain) and amylopectin (branched chain) is not suitable for coating paper and board because the amylose fraction tends to undergo retrogradation and the viscosity of coating colors made with native starch is too high [5]. For these reasons, only treated (i.e., depolymerized) or chemically modified starches are used. Most paper mills carry out their own starch preparations in-house.

4.4 Paper Coating Tab. 4-4

Comparison of natural binders including polyvinyl alcohol with synthetic binders. Natural binders and polyvinyl alcohol

Synthetic binders

Sold as Quality consistency Dissolution/digestion needed Concentration in aqueous form Viscosity in aqueous form Film properties

Solid (powder) Good to poor Yes Maximum 10–20 % High Very hard and brittle

Tendency to foam Bacterial decay Water retention Binding strength (pick strength) Water resistance

Casein yes; starch no Yes High Medium high Poor

Dispersion Very good No 50 % Low Variable, ranging from soft to hard, thermoplastic, elastic Yes No Practically none High-very high Very good

The properties of the starch depend on how it is treated or modified. The best results are achieved by ethylation. However, the most economical method of modification is enzymatic degradation to prepare enzyme-converted starches. Also, oxidized starches are widely used in North America. Table 4-5 compares the advantages and disadvantages of using starch as a binder. Tab. 4-5

Evaluation of starch as a binder.

Advantages

Disadvantages

Low-price binder Improves runnability, particularly well-suited for roll coating (thixotropic coating colors can be prepared) Coating colors with a high solid content can be prepared

Low binding strength compared to synthetic binders Highly soluble in water, low wet-pick strength Not compatible with satin white Risk of non-uniform printing in an offset printing Variable quality of commercial products Liable to rot

In addition to starch, another natural binder still used in North America is soy protein. It is mainly used for recycled board coatings. Emulsion polymers were first used successfully as coating color binders in the nineteen forties. The advantages and disadvantages of these synthetic binders are summarized in Tab. 4-6. The binder in a coating color formulation must be capable not only of binding the pigment particles together, but also of securing them at the coating surface and of anchoring them to the base paper. The pigment particles at the coating surface must be held sufficiently tightly so that the coated paper can be smoothed in calendering and subsequently printed. The mechanical stress experienced by the surface of the paper depends very much on the printing process used and on the tack of the chosen

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Advantages and disadvantages of emulsion polymers as binders.

Advantages

Disadvantages

Binder properties can be optimized to meet requirements of printing process

High transportation costs (50 % water)

Does not affect coating color viscosity, high levels of solid content possible

Freeze-sensitive

Water resistance of coating is higher than that achieved with natural binders

No acceptor sites for optical brighteners

Better gloss and smoothness attainable Simple to use: – no digestion needed – feed can be controlled via a flow meter

printing ink. The amount of binder added to the coating color formulation must therefore be chosen appropriately. Special papers are an exception to this rule because in these materials the binder not only determines the paper’s printability, but also performs other functions such as controlling its oil- or water-resistance. Table 4-7 provides a rough guide to the amounts of binder required for the various types of printing process. Tab. 4-7

The dependence of binder quantity on printing process.

Printing process

Amount of binder per 100 parts of pigment

Letterpress printing Sheet-fed offset process Web offset process Flexographic printing Rotogravure process

8–15 parts 10–20 parts 10–18 parts 10–18 parts 4–10 parts

The amount of binder in coating colors used to coat board that is to be printed by rotogravure, flexography, or the sheet-fed offset process is usually somewhat greater than in coating color formulations for paper. More binder is needed for coated board in order to meet such additional requirements as folding strength and glueability. Using too much binder not only increases the price of a coating color unnecessarily, but also can be detrimental to quality. Large amounts of binder can cause the porosity of the coating to decrease so much that the printing ink does not transfer properly to the surface or, in extreme cases, is repelled by the surface. Drying times increase considerably as a result, causing set-off in the stack (i.e., the transfer of wet ink from a newly printed sheet to the reverse side of the following sheet). The binder accounts for approximately 15 to 40 % of the total cost of a coating color, depending on the printing process used.

4.4 Paper Coating

In addition to the amount of binder used, which is a dominant factor determining the binding strength, the type of binder is also of crucial significance in determining the properties that influence the appearance and classification of paper and board. These important properties are: – pick resistance (dry pick strength, IGT method, Pruefbau) – water resistance or wet pick resistance (wet pick strength, IGT method, Pruefbau) – gloss (specular reflection intensity) – brightness (reflection of visible light λ = 475 nm) – opacity (hiding opposite to transparency) – smoothness (Parker Surface Roughness Test, etc.) – porosity – compressibility (rotogravure) – stiffness (more important for light-weight papers) – drying/setting of printing inks – mottling (uneven uptake of ink) – water absorption capacity (the capacity of the paper to absorb water, thus permitting the transfer of inks to moist surfaces) – ink absorption capacity (the capacity of the paper to absorb ink and to prevent ink being transferred from the freshly printed areas to the rubber blanket of the following printing station) – blistering in web offset process (blister-free printing) – glueability of board and packaging paper In Sect. 4.4.5, the most important methods of testing coated papers will be described. The extent to which a coated paper needs to fulfil the various requirements listed above depends on the printing process to be used. The most exacting requirements on binder strength must be met by paper grades to be printed by the sheet-fed offset process. Because an aqueous fountain solution is used in the offset process, the wet pick strength (i.e. the binding strength of the moist paper) is crucially important. As the sheet-fed offset process is principally used to create high-quality prints, the demands made on optical parameters (brightness, gloss) and on printability (ink absorption, absence of mottling) are particularly stringent. As the printing inks used in a web offset press have less tack than those used in the sheet-fed offset process, the requirements on the binding strength for paper printed by the web offset process are not so high. However, the paper must exhibit high resistance to blistering. Once printed, the paper in a web offset press passes through a drier in order that the printing ink solvents and any residual water within the base paper can evaporate. If the porosity of the coating is too low, the water vapor can become trapped causing blistering and detachment of the coating layer. The rotogravure process uses inks with a very low viscosity. It therefore has the lowest requirements in terms of the pick strength of the paper. To guarantee the even and error-free transfer of the printing inks (i.e. low number of missing dots) from the rotogravure cells to the paper, requires paper which is both smooth and compressible.

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When choosing or developing a suitable binder for one of the various printing processes, one generally focuses on those four parameters whose effect on binder properties is sufficiently well known. These are: – nature of the constituent monomers – glass transition temperature – particle size and particle size distribution – molecular structure of polymers As mentioned at the beginning of this section, the binders used in coating color formulations are based on combinations of different monomers. The most common combinations are styrene with butadiene or acrylic esters and vinyl acetate combined with ethylene or acrylic esters. An important difference between styrene-butadiene binders and styrene-acrylic ester binders is the tendency of the binder to yellow under the influence of UV radiation or heat. Products containing a butadiene-based binder are considerably more susceptible to yellowing due to the much greater fraction of double bonds in the polymer. Acrylic ester copolymers are significantly less prone to thermal or UV-induced yellowing (as shown clearly in Figs 4-13 and 4-14) and these are the copolymers of choice for the production of high-quality, long-life prints. Generally speaking, binders based on polyvinyl acetate or on styrene-acrylate produce a more porous coating than do binders based on a butadiene copolymer. The glass transition temperature of a polymer is determined by the amounts of its different monomer constituents. Paper used in offset printing contains binders whose glass transition temperature lies between 0 °C and 30 °C. The high smoothness and compressibility required for paper grades used in the rotogravure printing process are achieved by using binders with a much lower glass transition temperature (<0 °C). Figure 4-15 shows the typical dependence of dry and wet pick strength, stiffness, gloss, porosity, and evenness of offset printing on the glass transition temperature.

Fig. 4-13 Thermal yellowing as a function of the chemical composition of the binder.

4.4 Paper Coating

Brightness Loss 14

After 8 hours of UV exposure

12 10 8 6 4 2 0

Base paper

Chemical Basis Styrene/Acrylate

Chemical Basis Styrene/Butadiene/Acrylate

Chemical Basis Styrene/Butadiene

Fig. 4-14 UV-induced yellowing as a function of the chemical composition of the binder.

- wet pick - paper gloss - porosity

Goal

Fig. 4-15 Dependence of paper properties on the glass transition temperature of the binder.

Goal

- dry pick - print gloss - printability

Glass transition temperature (Tg)

Particle size and particle size distribution are influenced by the choice and amounts of emulsifiers and protective colloids that a polymer dispersion contains. These components are added to stabilize the dispersion thus making it both processable (i.e., enabling it to be conveyed, metered, filtered, etc.) and storable. Variations in the emulsion polymerization process also have a major effect on the size and size distribution of the polymer particles. Typically, binders used in the paper coating process have particle sizes of between 100 and 300nm. Figures 4-16 and 4-17 demonstrate that both the viscosity of the coating color and the wet pick strength of the coated paper are strongly dependent on particle size. In contrast to the other possible monomer components, butadiene possesses two double bonds both of which can act as polymerization sites. Binders based on a styrene-butadiene combination therefore have a more cross-linked and branched polymer structure. The extent of cross-linking affects the dry and wet pick strength, the print gloss and the degree of blistering, which is a highly significant parameter in

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Viscosity, mPas 1200 1000 800 600 400 200 0 50

100

150

200

250

300

350

Particle size (D), nm Fig. 4-16

Dependence of the viscosity of binder dispersions on particle size.

Wet pick strength 100

50

Wet pick strength of paper and board coatings improves with decreasing particle size

0 100

150

200

250

Particle size (D), nm Fig. 4-17 Dependence of the wet pick strength of binder dispersions on particle size.

web offset printing. Unfortunately, binding strength and blister resistance tend to oppose one another and cannot therefore be optimized by the choice of binders alone (Fig. 4-18). A very similar dependence is observed with the styrene-acrylate binders (Fig. 4-19). In this case, binding strength and blister resistance show a mutually opposed dependence on the relative molecular weight of the polymers. The polymer structure, and thus the desired balance between binding strength and blister resistance, can be controlled in the two classes of binders by careful adjustment of the polymerization conditions and by the addition of a so-called chain transfer agent. Additional information on paper coating can also be found elsewhere [6–14].

4.4 Paper Coating

Blister resistance

Pick strength

Pick strength Blister resistance

Low

% Gel content

High

Fig. 4-18 Relationship between blister resistance and binding strength for styrene-butadiene binders.

Blister resistance

Fig. 4-19 Relationship between blister resistance and binding strength for styreneacrylate binders.

Dry pick strength

Molecular weight of Styrene/Acrylate copolymers

4.4.5

Test Methods

In this section, the most important methods of testing coated papers will be described. The printability and the final print quality can often be successfully predicted on the basis of these relatively simple tests. Coating strength tests The strength required for a paper surface is to a large part determined by the tack of the ink used in the printing process. Whether or not the paper is dampened prior to printing is also of considerable importance, particularly in the offset process. The following tests simulate the stresses experienced by the paper surface during the printing process, in particular ink splitting during the offset process (Fig. 4-20).

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4 Applications in the Paper Industry Fig. 4-20. Ink splitting in offset printing.

blanket printing ink

ink splitting

paper or board

Cylinder

Wet pick test This is a test to determine the water resistance of a coated paper. After wetting the test strip at constant speed and uniform pressure to create a precisely defined moisture content, the strip is printed with the testing ink while moving at constant or increasing speed through the press. Using a hole template to define ten separate measuring dots (representing precisely defined strip speeds), the color density of each of the ten dots is measured using a densitometer and then expressed relative to the full tone of the printed surface. When plotted as a function of printing speed, the color density values are a measure of the water resistance of the paper strip. When the paper strip is printed at constant speed, the measuring dots used to determine color density are chosen randomly. Dry pick test This test determines the tensile strength of the coating strip when subjected to ink splitting during the printing process: The test strip is printed at a precisely defined plate pressure while being accelerated through the printing zone. The location of the first picking point and the position at which picking is visible right across the test strip are determined by inspection and analyzed quantitatively with the aid of a computer program. However, since in practice, the paper passes through not one but several printing presses (4–8 in the offset process), a further test can be performed to examine the effects of this sequential stressing of the paper surface. This is the so-called offset test.

4.4 Paper Coating

Offset test This test simulates repeated ink splitting caused by contact between the printed area and the rubber blanket during the printing process. Ink is transferred to the test strip from a plate at an exactly specified plate pressure and a constant known printing speed. The printed region is subsequently reprinted five times at 10-s intervals using the same plate but without re-inking. The number of passes at which the first signs of picking become apparent is recorded. In North America, Paper-Ink Stability Test (P&I Test) is widely used [15]. The test measures the rate of ink setting by calculating the slope of the ink splitting force as a function of the number of impressions taken at a given time interval. Also, this test measures the number of passes-to-fail. In general, the faster the rate of ink setting, the lower the number of passes-to-fail. Therefore, for press runnability, the rate of ink setting and the number of passes-to-fail must be balanced. Printability tests Mottle test This test determines the evenness of the printing. Ink is transferred to the test strip from an inked plate at an exactly specified plate pressure and a defined constant speed, and subsequently split three times by an offset rubber blanket-covered cylinder. If the coating on the paper is unevenly distributed, the printed image may appear cloudy as a result. The inhomogeneity of the printed image is either ranked visually or with a mottle tester (which measures fluctuations in color density). Measurement of ink gloss A test strip is printed using a precisely defined plate pressure and constant printing speed. Once the test strip is dried, the ink gloss is measured using a gloss meter. Ink set-off test This test measures the speed at which the oils in the printing ink penetrate the coating of the paper during the drying (setting) process: A test strip is printed from an inked plate applied at a precisely defined pressure and a constant printing speed. A blank counter strip is pressed against the original strip once every 15 s during the duration of the test (900 s). The counter strip will pick up some of the non-dried printing ink from the original strip. The color density of any ink transferred to the counter strip is measured using a densitometer. The Paper-Ink Stability Test mentioned as an offset test can provide information on ink set-off in terms of the ink setting rate [15]. Rotogravure test This test determines the suitability of paper or board for printing by the rotogravure printing process: To perform this test, a gravure cylinder and blade must be added to a standard IGT tester. The gravure cylinder is inked and the excess ink removed from the non-printing areas by the blade. Printing is carried out at a constant speed

99

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4 Applications in the Paper Industry

and a defined force exerted by the impression cylinder. Each test strip is examined to determine the length at which 20 missing dots have occurred. This particular test method is known as Helio test.

4.5

Concluding Remarks

Science and technology in the fields of paper surface sizing and paper coating have been significantly advanced over the past two decades and will be continued to move forward to meet the needs in the paper industry. There are many challenges confronting the paper industry. Some of the challenges are conservation of raw materials, especially trees as fiber source, by improving the current basis weight vs. stiffness relationship as well as by using recycled and secondary fibers more effectively, environmentally friendly papermaking, coating, and finishing processes, better quality coated paper and paperboard products at lower costs, etc. These challenges will require us to continuously innovate in papermaking, coating, and finishing. It will be interesting to see how well the paper industry can succeed in the 21st century. Acknowledgments One of the co-authors (DIL) would like to thank his two other co-authors (JS-T and ES) for letting him contribute to this chapter. This chapter has been based on an English translation of the Paper Industry portion of Chapter 5 Anwendung in der Papier- and Graphischen Industrie, by Jürgen Schmidt-Thümmes, Elmar Schwarzenbach, and Berhard Prantl in Polymerdispersionen, Dieter Distler (ed.), Wiley-VCH, 1998.

101

References 1 P. A. Ita, World Emulsion Polymers, 2 3

4

5

6

7

The Freedonia Group, September, 1999. BASF Corporation. G. D. McGinnis, F. Shafizadeh, Cellulose and Hemicellulose, Chapter 1 in: Pulp and Paper: Chemistry and Chemical Technology, 3rd edn, Vol. I, J. P. Casey (ed.), John Wiley and Sons, 1980, pp. 1–38. Paper, The New Encyclopaedia Britannica, Vol. 9, Encyclopaedia Britannica, 1987, p. 126. R. L. Kearney, H. W. Maurer (ed.), Starch and Starch Products in Paper Coating, Tappi Press, 1990. L. Göttsching, Papier in unserer Welt, Econ Verlag, Düsseldorf, Vienna, New York, 1990. T. W. R Dean, The Essential Guide to Aqueous Coating of Paper and Board, Pita, Lancashire, UK, 1997.

8 G. L. Booth, Coating Equipment and

Processes, Lockwood, New York, 1970. 9 R.W. Hagemeyer, Tappi Monograph

Series, 1976, 38. 10 A. R. Sinclair, Tappi Monograph Series,

1975, 37. 11 E. J. Heiser, F. Kaulakis, Tappi Mono-

graph Series, 1975, 37, 22–63. 12 T. F. Walsh, L. A. Gaspar, Tappi Mono-

graph Series, 1975, 37, 98–119. 13 J. J. Latimer, H. S. De Groot, Tappi

Monograph Series, 1975, 37, 120–136. 14 R. C. Jezerec, G. P. Cogan, Tappi

Monograph Series, 1975, 37, 64–69. 15 N. P. Sandreuter, Tappi Coating Confer-

ence Proc., 211, 1994.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

5

Applications for Printing Inks Barna Szabo

5.1

Introduction

The major printing processes used worldwide are: lithographic, gravure, flexographic, screen, letterpress and digital. Flexography is the only printing process that consumes significant quantities of water based (aqueous) ink. Less than 23 000 tons per year of water based ink are used for gravure printing in the US and less than 14 000 tons in Japan; water based gravure volumes are negligible in Europe and Latin America. Solvent based ink is used mostly for printing gravure, although it is readily adaptable to aqueous ink. Oil- and solvent-based ink systems are used in the lithographic and letterpress processes. Lithography is the largest volume printing process. Letterpress is one of the smallest. Both water and solvent based inks are used in screen and digital printing; these consume comparatively small volumes of ink. In the United States, approximately two-thirds of flexographic ink is water based; one-third is solvent based in an estimated flexographic ink market of 200 000 tons. The major volume of water based flexo ink consumed in the United States is for printing corrugated containers. The total US ink market for all six printing processes listed above is estimated at over 1.1 million tons for year 2000 or approximately 4.5 billion $US. In Europe, approximately two-fifths of flexographic ink is water based; threefifths is solvent based in an estimated flexographic ink market of 180 000 tons. The total European ink market for all six printing processes is estimated at over 0.9 million tons for year 2000 or approximately 4.4 billion $US. In Latin America less than one-third of flexographic ink is water; greater than twothirds solvent based in an estimated flexographic ink market of 34 000 tons. The total Latin American ink market for all printing processes is over 125 000 tons for year 2000 or approximately 750 million $US. The flexo ink market in Japan is very small. Approximately one-half of flexographic ink consumed is water based; one-half solvent in an estimated flexographic ink market of 27 000 tons. The total Japanese ink market for all six printing processes is over 400 000 tons for year 2000 or approximately 1.5 billion $US.

103

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5 Applications for Printing Inks

The aqueous ink market in 2000 is summarized in Tab. 5-1. Tab. 5-1

The aqueous ink market in 2000.

US Europe Latin America Japan

Flexo ink Solvent-based Water-based

Total flexo ink × 103 tons, estimated Water and solvent

Total printing ink × 103 tons, estimated

2/3 2/5 <1/3 1/2

200 180 34 27

1100 910 125 400

1/3 3/5 >2/3 1/2

Emulsion polymers are used in flexographic and gravure ink for printing flexible packages, paperboard cartons, corrugated boxes, multi-wall bags, newspapers, and other flexible substrates (films) and paper products. Styrene acrylic based emulsion polymers are most commonly used in printing ink applications. Cost is a key factor in ink raw material suitability for most ink systems. The level of styrene monomer used in ink grade emulsion polymers is maximized due to film hardness requirements of paper and paperboard materials and its low price. Prior to the mid 1970s, rosin fumarates (i.e. sodium and amine rosin salts) were the main resins used in aqueous ink. Aqueous rosin based resin solutions provide the performance properties similar to use of rosin phenolic and rosin maleic pentaerythritol esters in a lithographic printing ink. Rosin based resins have been used in printing ink since the early days of letterpress printing. They are low cost, give inherently good pigment dispersion properties, and are easily modified to vary viscosity and hardness. When aqueous flexo and gravure printing began displacing solvent based systems, a wider range in properties than possible with rosin became important. Emulsion polymers provide a wide range in properties and low viscosity. Flexo and gravure inks are referred to as fluid inks because of their low viscosity. Molecular weight, Tg (glass transition temperature), and particle size distribution are key properties that are varied to meet specific ink requirements. Pigment dispersion stability, room temperature film formation, and ink re-solubility are important properties to consider in the design of emulsion polymers for printing ink. A “support” resin (Sect. 5.2.2) is used in most ink grade emulsions to maintain these properties. The “support” resin is a low to medium molecular weight styrene acrylic or other water soluble resin. It comprises up to 60 % solids content of the emulsion and replaces up to 90 % of surfactant stabilizers. 5.1.1

Flexographic Ink

A flexo ink is a low viscosity (fluid ink) suitable for transfer from an ink fountain via anilox roll to the plate cylinder and substrate. It dries by evaporation of the solvent (water). Most presses are equipped with air circulating dryers. A flexo ink is comprised of low boiling point solvents for low temperature evaporation and fast drying.

5.1 Introduction Fig. 5-1

Flexographic press [1].

The composition and solvency is limited to prevent swelling of rubber or photopolymer-based rolls. Water, alcohols, minimal concentrations of acetates (i.e. ethyl, isopropyl alcohol), and minor levels of aliphatics such as heptane are mostly used. Acetates and aliphatics are used to solubilize polyurethane and polyamide resins used in solvent based laminating ink. Only aqueous ink is discussed in this chapter. A diagram of a flexographic press is given in Fig. 5-1. A flexographic printing press consists of: – An ink fountain roll (rubber). The fountain roll rotates in a reservoir of ink and transfers a large volume of ink to the anilox roll. – An Anilox ink metering roll (chrome plated or ceramic coated). The anilox is engraved with cells (of inverted pyramid shapes) varying in density (size) between 80 to 1000 cells per linear inch (2.5 cm). The anilox supplies a precise volume of ink to the raised surface (print image) of the printing plate. A reverse angle doctor blade is used to wipe excess ink. – A printing plate cylinder (steel). A photopolymer printing plate is attached to the plate cylinder. The printing plate’s raised surface replicating the image contacts the substrate to transfer ink. – An impression cylinder (smooth and polished chrome). A smooth polished chrome cylinder that holds the substrate in contact with the printing plate.

105

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5.1.2

Gravure Ink

A gravure ink is a low viscosity (fluid ink) with rheological characteristics suitable for transfer “out of” small cells of an engraved cylinder to the substrate. A gravure ink is comprised of low boiling point solvents for low temperature evaporation and fast drying. Gravure utilizes various environmentally compliant solvents as required by specific printing applications. Only water based ink is discussed in this chapter. A diagram of a gravure press is given in Fig. 5-2.

Fig. 5-2

Gravure press [2].

A gravure printing press consists of: – A gravure print cylinder (chrome). The gravure cylinder is engraved with cells of varying sizes replicating the print image. The cylinder rotates through the ink fountain. Ink fills the cells. An excess is wiped by a doctor blade. Ink is transferred directly from cylinder to substrate. – An impression cylinder. The impression cylinder is a rubber coated cylinder that keeps the substrate in contact with the print cylinder. Its function is to control ink transfer. The ink is drawn out of the cells of the print cylinder by means of impression pressure and capillary action. An ESA (electrostatic assist) mechanism is sometimes used to assist in the capillary action. Ink rheology, electron charge and surface energy are key variables that effect transfer.

5.2

Ink Composition

An aqueous flexo or gravure printing ink is composed of polymer, pigment, solvent, wax, surfactant, crosslinker, and additives. The polymer (resin) functions as the “vehicle” for carrying the pigment. It is also a key component for achieving printing

5.2 Ink Composition

performance properties. The polymer is the material or compound for dispersing a pigment and preventing its re-agglomeration. It provides adhesion to the substrate. The branched network of the polymer provides hold-out on porous substrates. The smooth surface provides gloss for desirable visual effects. Oxidized or crosslinked polymer structures provide resistance to “chemicals” that contact printed packages. It provides resistance to scuffing or rubbing. It provides resistance to environmental conditions such as: heat and temperature, freeze–thaw, ozone, light, oxidation, moisture, etc. The polymer provides viscosity and rheology characteristics necessary for transfer of ink from press to substrate. The pigment provides the color. Organic pigments are used in most cases for lightfastness and transparency. Colorfastness and lightfastness is important to maintain desirable visual effects of printed materials. An average consumer would not purchase a food item (i.e. box of cereal) with faded or shifted colors. This effect would convey a message about its lack of freshness. Water (solvent) is used to solubilize the resin or polymer. Water is used mainly in flexo and gravure printing processes. Other solvents such as ethyl-isopropyl alcohol, ethyl-isopropyl acetate, toluene, and heptane are also used in flexo and gravure ink, but only aqueous flexo and gravure inks are discussed in this chapter. Illustrated in Tab. 5-2 is a generic formulation for an aqueous flexographic or gravure ink. A printing ink formula is frequently modified to meet exact color reproduction specifications and/or a customer’s changing performance requirement. Tab. 5-2

Generic aqueous flexo or gravure ink formulation.

Aqueous flexographic ink Component Pigment dispersion (Sect. 5.2.1)

Emulsion vehicle (Sect. 5.2.2) Solution vehicle (Sect. 5.2.3) Amine neutralizer Wax emulsion compound Wax powder Surfactant Crosslinking additive Silica additive Corrosion inhibitor Defoamer Other additives Total

Amount (%, w/w)

Gravure ink Component

Amount (%, w/w)

Dispersion varnish Organic pigment Total

55–70 30–45 100

35–50

25–35 10–20 0.5–1.5 2–5 0–2 1–1.5 0–2 2–5 0–1 0–1 0.25–0.5 100

Most commercially manufactured printing inks are made from intermediates such as: pigment dispersion (Sect. 5.2.1), emulsion vehicle (Sect. 5.2.2), solution vehicle(Sect. 5.2.3), and wax emulsion compound (Sect. 5.2.4). Using intermediates minimizes the number of raw materials handled at the ink manufacturing or blend-

107

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ing site. It facilitates the production of ink in blending plants with minimal equipment and at locations close to the customer and/or printer. Producing ink from intermediates offers flexibility in modifying formulas to meet color reproduction specifications and changing customer application requirements. 5.2.1

Pigment Dispersion

The pigment dispersion is made from dry pigment (which is surface treated) dispersed in an aqueous polymer solution. High speed mixers are used to disperse a pigment from a dried form or an aqueous slurry. The most common polymers used to disperse pigments are: low to mid-molecular weight styrene acrylics, SMA (styrene maleic anhydride), or rosin fumarate ester resins. The degree of stabilization of a pigment dispersion (which relates directly to color strength development), ink viscosity stability, transparency and pH drift are controlled by use of polymers such as those containing salt groups and hydroxyl functionality. Table 5-3 illustrates the physical properties of polymers used for dispersing pigments. Tab. 5-3

Solution polymers used for pigment dispersions.

Styrene acrylic I Styrene acrylic II SMA ester Rosin fumarate ester

Softening point (°C)

Glass transition temp., Tg (°C)

Mw (g mol–1)

Acid number (mg KOH g–1)

140–170 NA NA 125–145

70–125 15– 20 45–110 80– 90

12000– 18000 30000– 35000 50000–150000 2000– 10000

210–240 65– 70 165–285 115–200

The stabilization of organic pigment dispersions is achieved by use of polymeric anionic surfactants that provide strong adsorption on the polar surface of the pigment and hydroxyl groups for interaction with the aqueous phase. Non-polar intermediate sections of a polymeric anion add adsorbed layer thickness [3]. A typical organic pigment particle size ranges between 0.02 to 100 µm (aggregate size). The particle size distribution of particles in a pigment dispersion are typically 0.5 to 1.5 µm. Most emulsion polymers used in printing ink vary between 20 nm to 200 nm. Because of a pigments relatively large particle size and its wide range in particle size distribution, the smaller emulsion polymer particles are not suitable for forming stable dispersions of organic pigment particles and agglomerates. They do not have sufficient mobility to wet the surface area. Pigments used in aqueous flexo and gravure ink are supplied as a presscake, in dry form, or as chips. A presscake is a high solids dispersion of pigment in water. A presscake is dried by various drying processes to yield pigment agglomerates in a large range of particle sizes, between 0.02 to 100 µm. Chips are a dry form of dispersed pigment particles in a polymer. Most pigments used in pigment dispersions for printing ink are surface treated with a resin or polymer compatible with the pigment surface chemistry. The poly-

5.2 Ink Composition

mer is added as a dissolved aqueous solution during the “pigment striking” step. It adheres to the pigment surface via physical and/or chemical attraction. Surface treated pigments are known as “resinated” pigments. A pigment is surface treated or “resinated” as part of the pigment manufacturing process. A resinated pigment minimizes agglomeration, contributes to increased color value and improves the efficiency of the dispersion process. An explanation about the type of resins and polymers used for surface treatment of pigment is a separate segment of ink-pigment technology that is not covered in this chapter. It is important that resins or polymers used in the surface treatment of pigment, are compatible and do not react with the resins and polymers used in the pigment dispersion. Furthermore, they must be compatible with the emulsion and solution vehicles of the ink. Incompatibility between these components may cause: pigment re-agglomeration (resulting in a loss of color strength), increased low shear viscosity (known as “thixotropy”) leading to a change in ink rheology, and poor printability (e.g. variable ink transfer) performance. In addition there are various hybrid resins in use to disperse a pigment. They are used to achieve specific ink application properties not obtainable by the conventional resins and polymers discussed above. For example, a glycerol ester of fumarated rosin is further esterified with a styrene-allyl alcohol as taught in Westvaco Chemical Corporation’s patent – Rosin-Based Grind Resins for Aqueous Printing Ink. This type of resin has high softening point and gives relatively stable low viscosity ink [4]. A fumarated rosin polyamine condensation resin is explained in a second Westvaco patent – Modified Rosin Resins for Waterbased Inks. The condensation reaction product of polyamines with certain rosin-based polycarboxylic acids results in an efficient pigment dispersion resin and gives a stable viscosity over a wide range in pH, between 8.5 to 10.5 [5]. These resins and others not discussed provide alternatives for dispersing pigments. 5.2.2

Emulsion Vehicle

The emulsion vehicle provides the “workhorse” performance characteristics of an ink (i.e. adhesion, gloss, low viscosity, printability, re-solubility in water, heat, and chemical resistance at low cost). Most printing ink emulsion vehicles are polymer dispersions composed of styrene acrylics, terpolymers of styrene-α-methylstyrene and other acrylate monomers (i.e., ethyl acrylate, methyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate, methacrylic acid, acrylic acid, 2 hydroxyethyl acrylate, amino acrylate). Higher levels of styrene give higher Tg. Higher levels of butyl and 2-ethylhexyl acrylate give lower Tg, thus more flexible polymers. Compared to other polymers (i.e. polyurethanes) styrene acrylics do not give good alkali resistance, chemical resistance, or adhesion to films and lamination ink (cohesive) bond strength. Emulsion vehicles are prepared by emulsion polymerization in water in the presence of surfactant stabilizers. Viscosity is not dependent on molecular weight but only on solids content and particle size distribution. As a result, high molecular weights (>200 000 g mol–1) are achievable at low viscosity. Emulsions with small par-

109

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ticle sizes impart properties similar to solution resins with advantages of: low viscosity, near Newtonian rheology, pH stability, low polarity, and insolubility after drying for immediate water resistance. Most printing ink emulsions are “resin supported”. Printing ink emulsion polymers contain a “support” resin to reduce MFFT (minimum film forming temperature), and insure film coalescence. A “support resin” also decreases the need for surfactants. A support resin provides ink re-wettability, improves compatibility with pigment dispersions, and improves ink transfer and printability. Support resins are typically styrene acrylic polymers with acid functionality that are amine neutralized. A rosin fumarate ester can be used as a “support” resin for ink grade emulsions. According to Westvaco’s US Patent 5 216 064 [6], a fumarated rosin ester offers advantages such: as improved gloss, absence of residual glycol used in processing a styrene acrylic, and higher resin solids. A higher solids emulsion gives faster drying. Rosin based support resins are lower cost. Westvaco’s US Patent 5 656 679 teaches that a rosin fumarate reacted with an alkanolamine containing at least one secondary amine and one hydroxyl group is used as a support resin for ink grade emulsions for providing improved adhesion to films [7]. Polyurethanes are known to give excellent chemical and product resistance properties but increase cost. A water soluble polyurethane is made by adding acid modified monomer to the polymer backbone, the polymer is neutralized with amine, then used as a support resin in ink grade styrene acrylic emulsions. Polyurethane supported styrene acrylic emulsion polymers may be used to balance the high cost of polyurethanes and provide improved chemical resistance [8]. Emulsion polymers are supplied in bulk quantities at solids levels between 45 and 60 %. They are produced by emulsion polymer manufacturers such as: SC Johnson Polymer, Rohm & Haas, Avecia, Air Products, Westvaco, B.F. Goodrich and others. Outlined in Tab. 5-4 are: applications, emulsion characteristics, and physical properties of the styrene acrylic emulsion vehicles used in flexo and gravure ink. An emulsion vehicle comprises 25 to 35 % of the total ink formula. The table is sorted by increasing Tg. For printing inks that require specific properties not obtainable by conventional styrene acrylic emulsions, an aqueous dispersion of an acid functional polyurethaneepoxy acrylate hybrid (self crosslinking for improved chemical resistance) [9] patented by Air Products and Chemicals, Inc. or a self crosslinking styrene acrylic emulsion which reacts upon evaporation of water [10] patented by Akzo Nobel Resins BV, may be used. The Air Products novel dispersion contains a quaternary ammonium polyurethane acrylic hybrid carboxylate salt and pendant acrylate epoxide that selfcrosslink upon evaporation of water and ammonia. Akzo’s novel polymer contains a diacetone acrylamide reactive monomer and a bishydrazide. The crosslinking reaction between ketone groups and a bishydrazide proceeds rapidly at room temperature, after evaporation of water from the ink.

48–52

45–50

45–50

25–40

20–25

47–49

S.C. (%)

8.2–8.7

7.9–8.5

8.5–9.5

6.0–7.9

7.9–8.3

8.2–8.4

pH

150– 500

150– 500

150– 500

45–2500

2500–6000

700– 900

Viscosity (mPa s)

>200000

>200000

>200000

~100000

70000–100000

>200000

Mw (g mol–1)

40– 55

40– 55

40– 55

125– 50

120–130

35– 50

A.N. (mg KOH g–1)

95–105

95–105

40– 48

30– 35

10– 35

–30 to 1

Tg (°C)

P.S., number average particle size distribution in nanometer; S.C., solids content; A.N., acid number; MFFT, minimum film forming temperature

180–220

55– 65

Folding carton

Flexo news

40– 50

160–180

Direct print corrugated

Cup, plate, multi-wall bag, gift wrap

65– 75 120–140

Pre-print corrugated

P.S. (nm)

Surface print

Application

Tab. 5-4 Typical styrene acrylic emulsion vehicles used in flexo and gravure printing inks sorted by increasing Tg.

>60

>60

>45

<24

<24

< 7

MFFT (°C)

5.2 Ink Composition 111

112

5 Applications for Printing Inks

5.2.3

Solution Vehicle

Solution vehicles consist of water soluble polymers not manufactured by emulsion polymerization. The solution vehicle is an alkali soluble polymer in aqueous solution or a blend of polymers with combined properties into a single waterborne varnish. Soluble polymers are made by free radical polymerization in a processing solvent or as addition or condensation products with heat reaching temperatures up to 265 °C. Solution vehicles are mixtures of soluble resins unlike emulsion polymers. A solution vehicle is used to increase adhesion to film and improve ink printability or transfer to meet specific performance requirements. The solution vehicle provides pigment dispersion stabilization, transparency, low film forming temperature, gloss and re-solubility. An alkali soluble resin is a carboxylic acid functional polymer neutralized (solubilized) with ammonia, amine or sodium hydroxide. The acid numbers are generally above 100. Ammonia or volatile amines are used in most aqueous inks except for news print inks. After evaporation of the amine, the resin becomes insoluble and resistant to water spray or other water contact. The ink is re-solubilized with alkaline water for the clean-up cycle. For news print ink, the polymers are solubilized with sodium hydroxide to maintain re-solubility (open time) of the ink on the press. News print ink pressman prefer unlimited open time and fewer clean-up cycles. Water resistance is not required since ink penetrates the news print paper fibers. The key solution polymers (resins) used in printing ink are styrene or rosin based. Styrene-α-methylstyrene monomer and acid functional co-monomers (i.e. acrylic or methacrylic acid) comprise the bulk of styrene acrylic solution vehicles used in printing ink. Rosin acid reacted with fumaric acid gives a tri-functional “adduct”. The “adduct” is partially esterified with polyols such as pentaerythritol, glycerin, diethylene glycol, etc. to achieve a range of acidity, viscosity, Tg and molecular weight. The viscosity of aqueous polymer solutions is strongly dependant on molecular weight. High performance characteristics such as rub resistance and heat resistance are compromised since low ink viscosity is required for flexo and gravure fluid ink printing. The volatile amines used to neutralize acid functionality results in pH shifts, unstable viscosity, reduced pigment dispersion stability, and poor alkali resistance. Water soluble styrene acrylics are processed via free radical polymerization in glycol ether solvents. The solvent is stripped by conventional or proprietary processes. Rosin based resins are processed molten at high temperatures up to 265 °C. These materials are flaked or pelletized and packaged in bags or bulk storage for further conversion. There are various hybrid polymers and co-polymers in use to achieve specific ink application properties not obtainable by conventional resins and polymers. Water soluble fatty acid epoxy esters provide improved heat resistance. For example, an aqueous fatty acid-acrylic acid epoxy ester patented by Reichold Chemical, which crosslinks via heat and auto-oxidation is used to provide water and heat resistance [11]. Typical solution polymers are listed in Tab. 5-5.

5.2 Ink Composition Tab. 5-5

Typical solution polymers.

Surface print Folding carton Direct print corrugated Pre-print corrugated Alternative folding carton Direct print corrugated Pre-print corrugated Multi-wall bags and gift wrap Milk carton Cup and plate Towel and tissue

Resin/Polymer

Softening point (°C)

Tg (°C)

Styrene acrylic resin

145

73

4500– 7500 108–213

Rosin fumarate ester resin

125–145

80–90

2000–10000 115–200

Fatty acid (Castor oil)/ >125 acrylic epoxy ester Styrene acrylic Water and amine neutralizer

>100 75–100

Mw (g mol–1)

Acid number

30000–40000 50–60 6000–10000 200–230

5.2.4

Waterborne Wax Emulsions and Powders

Both natural and synthetic waxes are used in ink. Waxes provide increased block, rub, scuff and/or water resistance. Waterborne wax emulsions are produced in a range of particle sizes between 35 to 175 nm (number average particle size distribution) by Michelman, Shamrock Wax, and others. Polyethylene and Fischer-Tropsch emulsions improve the rub and scuff resistance of an ink. Carnauba paraffin and polypropylene emulsions are used to prevent blocking and improve water resistance of an ink. Certain waxes are micro-pulverized to yield particle sizes smaller than a dispersed pigment particle. A micro-pulverized wax is “stirred” into ink as a powder. Recycled PTFE (Teflon) is supplied in small particle powder form. 5.2.5

Ink Additives

An amine neutralizer is added to solubilize resins containing carboxylic acid functionality. The amine reacts with the resin carboxylic acid to form a water soluble salt. Volatile amines such as dimethylaminoethanol (DMAE), morpholine or ammonia are used to insure that a printed product becomes water resistant upon drying or evaporation of the amine. The type of amine used is selected based on press speed, pH requirement and evaporation rate and press drying capacity. Sodium hydroxide is commonly used in news print ink to maintain re-solubility (“open time”) of the ink on the press. A crosslinking compound is added to provide covalent branching to a polymer for enhanced printed film tensile strength and chemical resistance characteristics. Com-

113

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pounds such as zinc ammonium complexes or zinc oxide react with available carboxylic acid functionality. Self crosslinking emulsion polymers may be used as explained in Sect. 5.2.2. A surfactant is added to reduce the surface tension to give increased ink spreading and substrate wetting particularly when printing untreated or partially treated films. The surface tension of water is approximately 72 dynes cm–1 whereas a polyethylene film is approximately 30–40 dynes cm–1 after surface treatment. Surfactants increase the foaming tendency of an ink. Therefore levels of surfactant and defoamer are carefully balanced. A defoamer is added to reduce foaming. A fine particle size silica powder is added to increase the viscosity and modify print film slip or abrasion properties. A corrosion inhibitor is added to prevent corrosion of press parts made of steel.

5.3

Physical Properties and Test Methods

Viscosity, pH and color strength are the main properties that relate to press performance and print quality. Viscosity is critical for satisfactory ink transfer or printability. A gravure ink has slightly lower viscosity than a flexo ink. The pH is controlled since it effects viscosity, viscosity stability and compatibility with other components. The viscosity changes with change in pH, but is readily adjusted by adding amine or water. Color accuracy (ink color strength) is important for achieving satisfactory print color and to maximize profitability. The color strength of a pigment dispersion intermediate is carefully controlled to narrow tolerances. Therefore, color adjustments in the ink manufacturing step are minimized. 5.3.1

Typical Properties

The typical properties or specifications of aqueous flexo and gravure ink are: Viscosity, Zahn efflux cup (ref. ASTM D4212-99) @ 25 °C: Flexo ink shipping viscosity, Zahn #3 25–30 s Gravure ink shipping viscosity, Zahn #3 21–25 s Flexo ink printing viscosity, Zahn #3 18–22 s Gravure ink printing viscosity, Zahn #2 18–22 s Zahn #3 8–10 s pH 9.0–9.5 Color accuracy (ASTM D2244-93) <2.0 Delta E* (CIELAB total color difference) versus standard Fineness of grind, (ASTM D1316-93) <2.0 Residue (ref. ASTM F311-97) <15 mg per 100 g ink

5.3 Physical Properties and Test Methods

5.3.2

Application Tests

Application specific pass/fail tests are specified to guarantee that an ink shipment gives satisfactory performance. The application properties are measured relative to a standard sample. The results are reported as pass or fail versus the standard. The following application tests are performed on aqueous flexo and gravure ink: – Abrasion resistance, dry/wet rub resistance, Sutherland Rub Tester – Adhesion at surface tension of 38–44 dynes cm–1, Scotch 610 Tape Test – Block resistance – COF (coefficient of friction), ink to ink, static at 26.6° slide angle – COF (coefficient of friction), ink to ink, kinetic at 19.3° slide angle – Crinkle resistance at room temperature or that of ice water – Drying with a 1 millimicron or 2 mil fineness of grind gauge – Freeze-thaw resistance, two cycles – Heat resistance, 98 °C – Milk carton wet rub – Product resistance – acid, fertilizer, limestone, wood oil – Re-wetting – Rub test – metal corrugator – Surface tension of film – Viscosity, Zahn efflux cup – Water resistance, 24 h, immersion at 25 °C 5.3.3

Test Method Abstracts

– Abrasion resistance – Sutherland Rub Tester: A test strip (18.8 cm) is rubbed by a four pound test block with a 15 cm × 7.5 cm strip affixed. The test is run either: ink-to-ink or unprinted paper-to-ink, 20 to 40 rub cycles according to specifications. A subjective comparison is made to a photograph standard or control sample that is tested subsequently. This test method simulates scuffing that may occur during in-line filling, handling or transporting of a package. Wax emulsions or micro-pulverized powders are added to adjust the abrasion resistance properties of an ink. For heated abrasion resistance, the four pound test block is held in an oven for twenty minutes at a temperature specified. The test simulates scuffing that may occur on hot filling lines or under high friction conditions. For wet-rub resistance (approximately twelve drops) water is applied to a 18.8 cm test strip with a pipette. Un-printed paper is used to test paper or board substrates. A swatch of cotton material is used to test film substrates. The test simulates rubbing of a package by cotton clothing. – Adhesion/Scotch 610 Tape Test: A 2.5 cm wide 3-M 610 tape is attached to the ink and pulled off at an angle of 180 degrees. Ink removal of greater than 10 % is a failure. This test is performed to flag unusual problems associated with poorly treated films or ink composition errors

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– Block resistance – wet/dry – ink to ink and ink to substrate: The exposure time, pressure, and temperature are specified by the end use requirement (i.e. 3 min at 1034 bar, 50 °C for surface print ink (5.4) – The ink surface’s resistance to heat and pressure is subjectively measured. Ink properties that effect blocking results are: “hardness”, adhesion, cohesion, and slip. The polymer glass transition temperature (Tg), molecular weight, and surface compatibility effect the block resistance test. – Coefficient of friction measurement, TMI slip and friction tester: The peak angle (static) and average force (kinetic) are measured. An emulsion polymer with low glass transition temperature is required for high slide angles. – Crinkle resistance test at room temperature or at the temperature of ice water: The print is immersed in ice water for one hour (re-ice water crinkle). Two surfaces of ink are rubbed ink to ink for 10 cycles. Perform a subjective comparison between a test sample and a standard. – Drying test with a 1.0 mil (25 µm) or 2.0 mil (50 µm) fineness of grind gauge: Drying time is measured subjectively by finger tapping and a stopwatch. A polymer’s glass transition temperature, MFFT and emulsion particle size distribution effect drying. – Freeze–thaw test: A sample of ink is placed in a freezer at –15 °C for 4 h. Changes in viscosity, homogeneity, and seeding tendency are observed. – Heat resistance test at 100 °C – Sentinel heat seal tester: Set-up a Sentinel heat sealer according to heat pressure and time interval specified. A one by three inch (2.5 cm × 7.5 cm) print sample is folded ink surface-to-ink surface and placed between the sealer bars. The heat sealer is operated. After the sample has been cooled, the sheets are separated and a subjective comparison is made for cling, ink transfer and picking. The polymer glass transition temperature (Tg), molecular weight, and surface compatibility affect heat resistance. – Milk carton wet rub test: A test print is immersed in milk at 1 to 7 °C for 24 h. A rub resistance test is performed using a Sutherland rub tester (see abrasion resistance). Specific polymers (Sect. 5.6.1) are used to give resistance to milk. – Product resistance and/or chemical resistance – acid, fertilizer, limestone, wood oil, etc : Three drops of an appropriate chemical solution is applied using a 3-mL pipette. After a specified time interval, a cotton swab is rubbed through the drop over the print surface with moderate pressure. A comparison is made for discoloration, ink removal, or blistering. A polymer composition, branching structure and crosslinking density have the largest effect on chemical resistance of an ink. – Re-wetting test: A #4 Meyer bar drawdown is allowed to dry for 20 min at RT (room temperature). A drop of water is placed on the ink surface and subsequently wiped with a cloth. A subjective comparison is made versus a standard sample. Solution polymers are neutralized with volatile amines (Sect. 5.2.3) to prevent resolubilization after the ink print dries. – Rub test – metal corrugator: For pre-print liner board, this test simulates the effects of a corrugator. The extent of scuffing/marring is subjectively compared to a photographed standard. The emulsion polymer’s soft segment, glass transition

5.4 Inks for Flexible Substrates (Films)

temperature (Tg), and molecular weight have the largest effect on the rub resistance properties of an ink. – Surface tension of film: Accudyne level pens or solutions are used to estimate the surface tension of treated films. A targeted range of 38–42 dynes cm–1 is specified for most printing applications. The surface tension of most aqueous styrene acrylic based pigmented inks are greater than 38–42 dynes cm–1. – Viscosity, Zahn efflux cup (ref. ASTM D4212-99), seconds: A Zahn efflux cup is a fast and effective instrument for measuring viscosity of flexo and gravure ink. Viscosity is an important property for maintaining printability. For a flexo press, consistency of ink flow into the pan or well of the doctor blade system, ink-transfer to the anilox roller, and release of ink from anilox roller are largely effected by ink viscosity. On a gravure press the release of a consistent volume of ink out of the cylinder cell is effected by viscosity. Viscosity changes due to pH drift or evaporation of solvent (water) should be corrected immediately. – Water resistance, 24 h, immersion at 25 °C: A Crinkle test is performed. Two surfaces of ink are rubbed ink-to-ink for 10 cycles. A subjective comparison is made between a test sample and a standard.

5.4

Inks for Flexible Substrates (Films)

The ink used for printing flexible substrates (films) contains a soft film forming emulsion polymer based on styrene and co-monomers such as butyl acrylate, and 2-ethylhexyl acrylate. Solvent based inks have continued use in high performance printing applications such as packages requiring lamination bonds (i.e. candy wrappers, potato chip bags). Flexo and gravure printing companies have invested in solvent incinerators to remain compliant with environmental regulations. Examples of materials used for flexible packaging films are: LDPE Low density polyethylene (i.e. fruit and vegetable produce bags) MDPE Medium density polyethylene (i.e. department store merchandize bags) HDPE High density polyethylene (i.e. grocery item bags) LLDPE Linear low density polyethylene (i.e. department store merchandize bags) PP Polypropylene (i.e. salt, fertilizer bags) PP-EVA Ethylene vinyl acetate modified polypropylene OPP Oriented polypropylene PP-NO Non-oriented polypropylene PP-AC Acrylic coated polypropylene PP-PVDC Poly(vinylidene chloride)-coated polypropylene PET Polyethylene terephthalate PVDC Poly(vinylidene chloride) (Saran)

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5.4.1

Surface Print Film

Surface print inks are designed for printing on polyethylene substrates used in utility bags, department store merchandize bags, grocery bags, and general purpose surface film applications. The films are surface treated via corona discharge increasing to a surface tension of 38–41 dynes cm–1 before printing. A corona discharge induces ions and free radicals to oxidize the surface of a film to form polar functionality. This change in surface chemistry and roughness increases surface energy and improves wetting of ink on film. A surface print ink is composed of a soft film forming styrene acrylic emulsion vehicle to provide adhesion. A solution vehicle composed of low molecular weight styrene acrylic resin is added to adjust printability. Typical properties of the emulsion polymers and solution resins used in ink for surface print films are listed in Tabs 5-3 and 5-4. 5.4.2

Lawn and Garden Bags

Lawn and garden bag inks are designed for printing on polyethylene or polypropylene substrates used in fertilizer, salt, mulch, potting soil, manure, feeds and wood bark bags. Lawn and garden bags require resistance to: weak acids, bases, fertilizers, limestone dust, wood oils (i.e. cedar, pine), etc. A typical Lawn and garden bag ink has a similar composition to surface inks. A crosslinking compound (i.e. zinc oxide, carbodiimide, polyfunctional aziridine) adds covalent branching, increases modulus and enhanced print film adhesion and resistance to chemicals. As an alternative, a self crosslinking styrene acrylic emulsion which reacts upon evaporation of water may be used according to the Akzo Nobel Resins paper presented at the 44th NPIRI Technical Conference [10].

5.5

Inks for Paper Board Substrates 5.5.1

Folding Cartons

Folding carton inks are designed for printing on paper board (or boxboard) substrates used in: fast food carry out packages, pastry cartons, cereal boxes, mail containers, auto parts boxes, beverage carriers, milk and juice cartons, and other packages. Folding cartons are made from solid bleached kraft or solid bleached sulfate (SBS), unbleached kraft, solid unbleached sulfate (SUS), clay coated unbleached kraft, recycled paperboard, and coated paperboard. For beverage carriers, one substrate type is used consistently in most of the US market. Many beverage carriers are over-coated with a clear protective coating layer.

5.5 Inks for Paper Board Substrates

A folding carton ink is composed of a hard non-film forming styrene acrylic emulsion vehicle and a styrene acrylic solution vehicle. As an alternative, a film forming styrene acrylic emulsion vehicle and rosin fumarate ester solution vehicle are used. A plasticizer (i.e. propylene glycol) and solvent (i.e. n-propyl alcohol) are added to folding carton ink to improve printability and insure coalescence at room temperature. 5.5.2

Direct Print Corrugated Packages

Direct print corrugated inks are designed for printing on standard kraft paper board substrates. Corrugated board is manufactured by laminating flat sheets of paper to a corrugated inner layer to give an increased stiffness-to-weight ratio [12]. Direct print corrugated ink is made from styrene acrylic colloids. Colloids are polymers produced by emulsion polymerization followed by neutralization of available carboxyl functionality. Colloids have molecular weights in the range of 25 000– 100 000 g mol–1. They are used in low cost ink systems for printing on porous substrates such as corrugated paper board. Colloids are also supplied in solid form for dissolving in water by neutralization with an amine. This results in uncoiling of the colloidal particle to form a colloid solution. Colloid solutions give a high viscosity. They are supplied at low solids concentrations typically between 20 % to 40 %. A direct print corrugated ink is composed of a styrene acrylic emulsion colloid vehicle and a rosin fumarate ester and/or styrene acrylic solution vehicle. As an alternative, a mixture of colloid solution with a film forming emulsion provides the overall corrugated ink properties. Direct print corrugated ink specifications: Residue <15 mg per 100 g ink 1.07–1.11/8.9–9.3 Density (g cm–3/lbs gal–1) Dry rub 100 rubs/1.8 kg (4 lb) weight Grind, NPIRI <2 5.5.3

Pre-print Corrugated Packages

Pre-print corrugated inks are designed for printing on standard kraft, bleached kraft, mottled kraft, clay coated, and SBS coated paper board substrates. Many producers of corrugated packages have installed flexo presses for pre-printing linerboard before making the corrugated board. Printing on a smoother surface gives improved print quality. These inks must withstand heat (175–200 °C), pressure, and marring/scuffing conditions of the subsequent paperboard corrugating process. Most pre-print packages are coated with a clear, high gloss, heat resistant, and scuff resistant coating. The coating adds resistance properties and gloss. A pre-print corrugated ink is composed of a styrene acrylic emulsion colloid vehicle and a rosin fumarate ester resin and/or styrene acrylic resin solution vehicle. The physical properties of emulsions and resins used in pre-print corrugated ink are listed in Tabs 5.4 and 5.5 for.

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Plasticizers (i.e. propylene glycol) and coalescing solvents (i.e. glycol ether) are added to maintain satisfactory printability and satisfactory MFFT (minimum film formation temperature).

5.6

Inks for Poly-coated Board

Inks for polymer coated paper board are designed for printing on polyethylene coated SBS (solid bleached sulfate) paper board substrates used in milk cartons, ice cream cartons, beverage cups, and paper plates. A typical ink for coated board is composed of an epoxy ester based pigment dispersion, styrene acrylic emulsion vehicle, and an epoxy ester solution vehicle. The epoxy resin commonly used is an ester of drying or semi-drying fatty acid and acrylic acid (Sect 5.2.3). The system undergoes oxidation to give moisture and alkali resistance [11]. Alternative polymers that may be used in inks for coated board are: aziridine (ethylene imine) crosslinking styrene acrylic or a VAE (vinyl acetate ethylene) self crosslinking emulsion polymer. 5.6.1

Milk Cartons

Milk carton ink must have resistance to alkaline detergent based chain lubricants. A fatty acid acrylic epoxy ester [11] is the key resin used in this type of ink. Special pigment dispersions are used based on the same epoxy ester since conventional styrene acrylic based pigment dispersions are not stable in epoxy ester based systems. 5.6.2

Cup and Plate

Cup and plate inks must have tolerance for hot wax coatings and resistance to hot and cold fluids. They should meet the requirements for general purpose paper plate use. A hard non-film forming styrene acrylic emulsion combined with hot-air drying (by oxidation) fatty acid-acrylic epoxy ester [11] provide the resistance requirements.

5.7

Inks for Paper Products

A typical ink used for printing multiple wall bags, gift wrap, and envelopes are composed of a hard non-film forming styrene acrylic emulsion combined with a solution vehicle comprising a rosin fumarate ester to balance printability and coalescence. A polymer with high molecular weight and Tg fast drying via fast: resin-solvent (water) separation, penetration of paper and brightness of color via “holdout” of pigment particles on the paper surface.

5.7 Inks for Paper Products

5.7.1

Multiple Wall Bags

Inks for Multiple Wall bags are designed for printing on kraft: brown, or bleached, mottled (compressed thin layers of bleached pulp on top of brown pulp), clay coated, and uncoated paper substrates. Flexographic printing is the most used process for multi-wall bags. Multi-wall and other paper bags are made of one or more walls of paper glued together and treated to provide moisture resistance. Heavy duty bags used as shipping sacks have three or more plies. Bags used for packaging consumer products (i.e. pet food, sugar, flower) have two plies. The paper used in bags for packaging consumer products has been upgraded in recent years to bleached kraft and clay coated. 5.7.2

Gift Wrap and Envelopes

Gift wrap is used for seasonal gifts, retail wrapping, liquor boxes, candy and other products. The substrates used are 90 % paper and 10 % foil [12]. Inks for Gift Wrap are composed of a hard non-film-forming styrene acrylic emulsion combined with a solution vehicle to adjust printability and balance coalescence. Gravure is the most used process for printing gift wrap. Envelope ink is made from the same mix of polymers as gift wrap ink. 5.7.3

Newspapers

Newspapers were originally printed by the letterpress printing process. Low cost black ink was mostly used. A letterpress black ink is composed of carbon black dispersed in high boiling point aliphatic petroleum solvent (ink oil). By 1995 the majority of newspapers converted to the offset lithographic printing process. Offset news ink is composed of carbon black or organic pigments dispersed in high boiling point aliphatic petroleum solvent based vehicles. The vehicles are resin solutions in ink oil and/or vegetable oils (i.e. soybean oil). During the 1980’s the remaining letterpress printers began to switch to the flexographic process and water based ink. Aqueous flexo news ink is based on higher cost emulsion polymers than typical petroleum hydrocarbon resin based offset inks. Therefore it was necessary to target certain newspaper market segments with higher value products. Flexographic newspaper printing introduced brighter colors to comics and advertising, and overall increased color coverage per newspaper. A flexographic news ink contains a high molecular weight, styrene acrylic emulsion with high Tg (glass transition temperature) to provide fast drying. A typical flexographic news ink is composed of a hard semi-film-forming styrene acrylic emulsion vehicle.

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5.7.4

Towel and Tissue

The same styrene acrylic emulsions used for sizing tissue paper to provide wet strength are used in the ink. The colorants used are mostly dyes or pigments that are not dermatological irritants. The resin should give a waterproof bond and be low cost. The components of a towel and tissue ink are re-pulpable for ease in recovery of waste and/or off-grade material to achieve satisfactory economies of paper towel tissue converting.

References 1 Western Michigan University Web Site:

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3

4

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http://www.wmich.edu/ppse/flexo/, Flexographic Process Western Michigan University Web Site: http://www.wmich.edu/ppse/gravure/, Gravure Process, Wicks, Z. W. Jr., Jones, F. N., Pappas, S.P., Dispersions in Aqueous Media, in: Organic Coatings, Science and Technology, 2nd edn, Wiley-Interscience, 1999, Chapter 20.3, p. 395. Schilling, P. Westvaco Corporation, US Patent: 5,208,319, Rosin-Based Grind Resins for Aqueous Printing Ink. Zuraw, P. J., Westvaco Corporation , US Patent: 5,166,245, Modified Rosin Resins for Waterbased Inks, 1992. Rivera, M. A.; Zuraw, P. J., Westvaco Corporation, US Patent: 5,216,064, Rosin-Based Resin-Fortified Emulsion Polymers Hutter, G. F., Westvaco Corporation, US Patent: 5,656,679, Rosin Ester-Amide Support Resins for Acrylic Latexes.

8 Biggerstaff, J. A., Reuther, P. C., Jack-

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son, K., Westvaco Chemical Division, A Comparison of Water-borne Chemical Resistant Technologies and the Variables that Affect their Performance, 43rd NPIRI Technical Conference, September 15–17, 1999. Tien, C., Mao, C., Snyder, J. M., Beck, A., Air Products and Chemicals, Inc., US Patent 5,977,215, Low Temperature Self-Crosslinking, Aqueous Dispersions of Urethane–Vinyl Polymers for Coating Applications. de Krom, A., Mulder, H., Mestach, D., Akzo Nobel Resins BV, Self-Crosslinking Acrylic Dispersions Outperform Conventional Solventborne Liquid Inks, 44th NPIRI Technical Conference, October 18–20, 2000. Meeske, C. J., Van der Tuin, E. H., Racey, M. J., Reichold Chemicals, US Patent 4,166,054, Water Dispersible Epoxy resin Copolymers and Methods of Making Same, 1979. Eldred, R. N., Ph.D., Package Printing, Jelmar Publishing, 1993.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

6

Applications for Decorative and Protective Coatings Brough Richey and Mary Burch

6.1

Introduction

Decorative and protective coatings are used in a great variety of applications, ranging from the familiar such as coatings for buildings, furniture, automobiles, and large industrial structures, to less well known applications such as removable coatings, paper coatings, and specialized coatings for optical fibers and electronic components. Most coating applications have historically utilized solution polymers as the binder component. However, concerns over pollution, the toxicity of solvents, and ease of use and clean-up have driven the development of new emulsion polymer technologies to meet the needs of the coatings industry. The use of emulsion polymers for coating applications has increased tremendously over the past fifty years and they are now represented in nearly every segment of the industry. In fact, coatings based on emulsion polymers frequently set the performance standards and lead the market in many areas. The objective of this chapter is to give the reader an overview of the use of emulsion polymers in decorative and protective coatings. Because of the great variety of specific applications, it will not be possible to address them all, and we have chosen instead to focus on selected application areas which we believe will provide the reader with a useful foundation in a variety of coating applications. 6.1.1

Market Overview

The development, manufacture, sale and application of decorative and protective coatings comprise a large business and we estimate that approximately 20 billion liters of decorative and protective coatings are manufactured and applied each year, representing world wide sales of about $60 billion US [1]. Annual growth is typically the range of 1–3 % world wide, and is generally linked to the combined gross domestic product of the major industrialized and developing nations. Growth rates for

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water based coatings have been higher, in the 3–6 % range, with most of the extra growth due to the switch from solvent based coatings. We can obtain an estimate for the annual world wide production of emulsion polymers for decorative and protective coatings as follows. If we assume that water based coatings make up about 50 % of the total world wide volume, then approximately 10 billion liters of water based paint are manufactured each year. To estimate the amount of emulsion polymer manufactured to yield this amount of coating material, we can estimate an “average” water based paint has about 30 % volume solids, with the dry volume of binder representing about 50 % of this value. This yields an estimated 1.5 billion liters of solid emulsion polymer produced per year. Assuming an average polymer dry density of 1.1 kg L–1 and an average emulsion solids level of 50 % by weight, it follows that the world wide production of emulsion polymer for coating applications is about 3 billion wet kg (6–7 billion wet pounds) per year. While this is admittedly a rough estimate, it nevertheless serves to illustrate the enormous size of the water based coatings market and the large amount of polymer emulsion needed to supply it. 6.1.2

Coating Industry Trends

The most significant trend in the decorative and protective coatings markets has been the move to more environmentally friendly coating materials. The shift away from traditional solvent borne technologies to newer technologies based on waterborne emulsion polymers, high solids coatings, and powder coatings is a key consequence of this trend. This change has been driven by a variety of regulatory pressures aimed at reducing air pollution by lowering the volatile organic content (VOC) of coating materials. Related to these pollution concerns is the desire to increase the safety of the end use application process, particularly in the area of decorative coatings for homes and offices. Again, this has caused a shift away from traditional solvent borne technologies toward newer technologies with improved health and safety profiles. Finally, world wide economic conditions have led to consolidation in the paint manufacturing industry, and its associated raw material suppliers [2]. These economic factors have created a strong movement towards cost reduction and increases in production and distribution efficiency across the coatings industry. In our view, the combination of continued technical innovation with improved cost efficiency presents the central challenge to the coatings industry as we move into the twenty-first century. 6.1.3

Coatings Provide Decoration and Protection

It is common to divide the general class of coatings into two subclasses: decorative and protective coatings. While this division can be useful, it can be also be misleading; the vast majority of coatings systems provide both decoration and protection. Automotive coatings systems provide a familiar illustration of this dual role: they are

6.2 Overview of Coating Formulations

foremost a protective coating, and must protect the automobile body from damage by weathering and the environment; and yet, the color and appearance of automotive finishes are vital factors which influence the customer’s purchase decision. Interior decorative finishes provide an example at the other end of the decorative-protective continuum. While used primarily for decorative purposes, the stain resistance and cleanability of interior finishes are important performance characteristics which often provide differentiation in this competitive market.

6.2

Overview of Coating Formulations

Decorative and protective coatings consist of three main components: (i) pigment, which provides color and opacity to the coating (ii) binder, which holds the film together and provides coating integrity; and (iii) carrier liquid, which provides as the liquid character of the coating while in the wet state (before drying). In this chapter, we will focus on coatings utilizing emulsion polymer binders, and consequently, water will be the carrier liquid. These types of coatings are also commonly referred to as latex paints, presumably in reference to the similarities in appearance between the emulsion polymer binders and unprocessed natural rubber. Commercial paint formulations are usually complex and typically contain more than the three main components described above. These formulations usually have on the order of 10–20 raw material components, the specific nature of which depends on the intended application. A more realistic formulation would include: dispersed solids such as pigments and fillers stabilized by a polymeric dispersing agent, a dispersed emulsion polymer acting as the binder, a thickener to provide proper rheology, coalescents and co-solvents to promote film formation and optimize the drying process, surfactants to improve colloidal stability and promote substrate wetting, a biocide to prevent microbial spoilage, a defoamer to reduce foaming during manufacturing and application, and a neutralizing agent to adjust the pH. Many optimized formulations contain more than one member of each of these classes. The primary technical challenge of coatings formulation is to develop cost effective coating formulations which are stable indefinitely in the wet state, apply correctly to the substrate, dry into defect free films, and meet the appearance and performance requirements of the intended application. 6.2.1

Volume Solids and Pigment Volume Content

The relative ratios between the volumes of different formulation components control many of the key appearance and performance properties of a coating [3, 4]. Volume solids is the simplest of these relationships; it is the ratio between the volume of solid components in a coating and the total volume of the wet coating. It typically is reported as a percentage. Volume solids is a useful quantity because it allows one to

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calculate the thickness of a dried coating from the applied wet paint thickness, or from the spread rate (the volume of wet coating applied per unit of area). It can also be used to estimate the quality of a coating when comparing coating formulations of a given class. Since water is an inexpensive raw material, formulations high in water content (and low in volume solids) tend to have lower raw material costs. Typical emulsion polymer coatings for decorative and protective applications have volume solids in the range of 25–45 %, with values ranging up to 60 % for some specialty applications. The pigment volume content (PVC) is another useful volume relationship which is frequently used in coatings formulation development. It is the ratio of pigment and extender solids to the total coating solids (pigment, extender, and binder solids), and like volume solids, is usually reported as a percentage. Coatings with low PVCs have a high binder content, and coatings with high PVCs have a low binder content with higher levels of pigment and extender. It is important to recognize that PVC represents a property of the dry film, rather than the wet coating. PVC is an important quantity because it relates to many of the performance properties of a dry paint film. If several versions of a particular formulation are prepared with different PVCs, ranging from lower to higher values, a transition point will be observed at which many performance features of the coatings change abruptly. This point is termed the critical PVC, or CPVC, and, in conceptual terms, represents the PVC where the polymeric components of a film no longer form a continuous phase surrounding the pigment and extender particles. Above CPVC, the dry coating begins to develop small voids between the solid components of the film, leading to an abrupt change in the performance features of the coating. The value of CPVC for a coating depends somewhat on the property used to measure it, and thus it is not a truly fundamental characteristic of a film. Nevertheless, it has proven to be a useful and practical conceptual tool for coatings formulation development. Figure 6-1 is a scanning electron micrograph which provides a vivid illustration of the differences between above and below CPVC coatings: Figure 6-1A shows the smooth, polymer rich surface which is typical of a water based enamel formulated significantly below CPVC; the white spots are TiO2 particles sticking through the surface of the gray polymer matrix. Figure 6-1B shows the surface of a highly extended, above critical, flat ceiling coating at the same magnification. Note the high porosity and variety of extenders in the above CPVC coating. The differences in the surface features of these two emulsion polymer coatings are striking, and suggest that these two types of coatings would have very different performance profiles. The value of the CPVC for a particular formulation will depend on the chemical and physical nature of the pigments, extenders and the binder. To illustrate this, we consider two paints formulated with the same total volume of pigment and binder (equal PVC). In one case the paint is formulated with a smaller particle size (PS) pigment, and in the other case the paint is formulated with a larger PS version of the same pigment. The CPVC of the paint with the smaller PS pigment would be lower than the CPVC of the paint with the larger PS pigment. This is because the higher surface area of the smaller PS pigment will require a higher level of polymer to uniformly cover the pigment surfaces. This results in a higher binder demand, and low-

6.2 Overview of Coating Formulations

Field Emission Scanning Electron Micrographs of Below and Above CPVC Coatings. (A) shows the surface image obtained from a gloss enamel coating which has been formulated significantly below CPVC. (B) shows Fig. 6-1

the surface image obtained from a flat ceiling coating which has been formulated significantly above CPVC. Note the porosity and variety of extenders present in the coating formulated above CPVC.

ers the PVC at which CPVC is reached. Comparing the actual PVC of a coating to the CPVC can provide useful information regarding a coating’s physical properties and suitability for a particular application. Coatings formulated at PVCs below CPVC tend to have higher gloss, lower porosity, better flexibility and better barrier properties. Coatings formulated above CPVC generally have lower gloss, higher porosity, lower flexibility and lower total cost. Since PVC level is usually adjusted upward by increasing the levels of low cost extenders, PVC can provide a useful gauge of formulation cost (and quality). This measure is most useful for coatings formulated above CPVC. PVC is a less useful measure of quality in coatings formulated below CPVC, since these coatings contain little or no low cost extenders and consist primarily of higher cost resins and pigments. 6.2.2

Polymer Matrix

The polymer or binder component holds the coating together and provides many of the performance features needed for specific coating applications. A high molecular weight polymeric material is generally used as the binder in order to provide the toughness and resistance properties needed to protect the substrate and ensure a durable coating. In practical systems the minimum molecular weight of thermoplastic polymers targeted for coating applications is around 50 000 g mol–1 [5–7]. Polymers with molecular weight below this value generally do not have the required toughness properties needed for coating applications: lower molecular weight crystalline materials are generally too brittle and can chip or flake, while non-crystalline materials such as amorphous waxes do not have high enough moduli to provide the film integrity needed for most coating applications. Decorative and protective coatings are generally designed to perform their function over the temperature range of –20 °C to +45 °C. The polymers used for coating

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applications are usually random copolymers or terpolymers with monomer compositions such that the polymer glass transitions, Tg, fall in the middle to upper end of this range. Polymers with glass transition temperatures below 0 °C are not useful in most coating applications because their films are tacky and weak under normal ambient temperature conditions. Polymers with Tg significantly above 50 °C tend to be brittle and inflexible under normal ambient conditions and are less commonly used in coating applications. These generalizations are most applicable to thermoplastic polymers, but the concepts can also be applied in a less formalized way to thermoset coatings (temperature dependent reactive polymerization systems). 6.2.3

Film Formation

Coatings based on emulsion polymers exist as stabilized colloidal dispersions while in the wet state. Upon application to the substrate, water evaporates and the film dries and cures into the final coating. Properly designed and formulated, the wet paint is stable indefinitely; however, the drying and film formation processes are effectively irreversible and result in a final film which is a tough, pigment-polymer composite. The irreversibility of the film formation process is a key technical factor underlying the successful utilization of emulsion based polymers for decorative and protective coating applications. If film formation were reversible, then dried coatings could be degraded by contact with liquid water; the ubiquity of water in our environment would make the utility of such coatings very limited. Film formation from emulsion polymers is a complex process but for simplicity it can be taken to consist of three phases. In the first step, water evaporates from the continuous phase of the liquid coating and the polymer and pigment particles begin to crowd together. The effective volume solids of the coating rises significantly. In the second phase, the latex and pigment particles begin to pack together to create a contiguous film. In the third and final phase, interstitial water diffuses out of the film and the emulsion polymer particles coalesce into a continuous, inter-particle polymer network. The detailed physics and chemistry of the film formation process are still not completely understood and are affected by many factors [8–10]. Empirically, it has been observed that if the polymer Tg is higher than the ambient film formation temperature, the final coalescence step of the process may break down, resulting in a poor quality film with diminished integrity. The minimum temperature at which an emulsion polymer will form a good film is referred to as the MFFT (minimum film formation temperature) and is generally a few degrees lower than the polymer Tg. It should not be a surprise that there is a general relationship between the polymer Tg and the MFFT. Since film formation involves the interpenetration of polymer chains between adjacent polymer particles, a reasonable amount of chain mobility must exist for this process to proceed. However, the MFFT of an emulsion polymer is not a precisely defined physical quantity and its value can depend on several factors in addition to Tg including polymer molecular weight and composition, as well as the drying rate of the applied coating. Hence, MFFTs are usually determined empirically by moni-

6.2 Overview of Coating Formulations

toring film formation as a function of temperature under a set of standardized drying conditions. Most coatings are applied under ambient temperature conditions (either on a job site or in a factory) with typical application temperatures between 10 and 40 °C. To ensure good film formation at the lower end of this range, polymer Tg values would need to be in the 10–15 °C range. In practice, it has been found that polymers with higher Tg are usually needed to provide optimized performance in many coating applications, particularly for coatings formulated at lower PVCs where the binder content is high. This presents a problem: polymers with desirable dry film performance frequently have Tg which are too high to form a good film under typical drying conditions. To circumvent this problem, coatings formulators temporarily lower polymer Tg and MFFTs by use of coalescing agent. Coalescents work by partitioning into the emulsion polymer particles, disrupting the packing of the polymer chains, and thus lowering the effective polymer Tg. After the film is applied, the coalescent will slowly diffuse to the film surface and evaporate, allowing the effective Tg to rise and yielding a tougher, more useful coating. To be effective, the coalescent must be reasonably compatible with the polymer phase and relatively low in molecular weight in order to partition into the polymer matrix. It should also have a moderate vapor pressure: the coalescent needs to remain in the film long enough to optimize film formation, but should also evaporate from the film reasonably quickly in order to allow performance properties to develop. The choice of a coalescent depends on the polymer composition, the coating formulation and the intended application. Oxygenated solvents of moderate polarity are commonly used for this purpose; these include various ether-esters of propylene and ethylene glycols, and ester-alcohols. 6.2.4

Typical Polymer Compositions

A variety of polymer compositions are used as binders in decorative and protective coating applications. By definition, emulsion polymers are based on vinyl monomers, but even with this restriction there are a number of different polymer classes which can be used for a given application. The choice of polymer system depends on many factors, which we will highlight below in the context of specific examples of coating applications. In this section we give an overview of the major emulsion polymer classes and discuss their general performance characteristics. Styrene-butadiene copolymers Historically, styrene-butadiene copolymers were the first emulsion polymers to be used for coating applications. These polymers were based on technology developed for synthetic rubber production during WW II. Typical polymer compositions were 65 % styrene with 35 % butadiene. While paints based on styrene-butadiene emulsion polymers opened the door for the development of synthetic latex paints, their cost-performance profiles were not particularly competitive with the solvent borne coatings present at that time or with the other emulsion polymer technologies which would be developed later. They now occupy only a very small segment of the coatings market.

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Vinyl acetate copolymers Vinyl acetate (VA) homopolymers were also used in early latex paints, and like styrene-butadiene polymers, were also not particularly successful in the market. The main problems were that the high Tg of VA homopolymers made it difficult for these coatings to form strong, high quality films, and the mottling and loss of film integrity caused by hydrolysis of VA when applied over masonry (alkaline) substrates. However, vinyl acetate can be copolymerized with butyl acrylate (BA) in an emulsion polymerization process, resulting in internally plasticized copolymers with MFFTs in the ambient temperature range and improved resistance to hydrolysis. When large quantities of BA monomer became commercially available in the 1960s, the use of VA-BA emulsion polymers in coating applications increased substantially. Typical vinyl acetate-butyl acrylate copolymers compositions are 80 % VA with 20 % BA by weight. Because of their relatively low cost, VA-BA copolymers have proven to be very successful in interior decorative paint applications. While more resistant than VA homopolymers, VA-BA copolymers can still be degraded by alkaline hydrolysis and their polar character can yield films which are relatively water sensitive. These factors can limit the use of VA-BA copolymers in demanding exterior applications, although they are often used for less demanding exterior coatings when low raw material costs are a primary formulation factor. Vinyl acetate can also be copolymerized with ethylene (E) in an emulsion polymerization process. Again, ethylene serves as an internal plasticizer for VA, lowering the Tg of the copolymer into the useful ambient temperature range. Since ethylene is a gas at ambient temperature, VAE emulsion copolymers need to be manufactured in specialized reactors, designed for high pressure use. Typical VAE copolymers used in coating applications are about 90 % VA with 10 % ethylene by weight. While ethylene is a low cost monomer, the cost advantage relative to BA can be lost because of the higher manufacturing costs associated the use of pressurized reactor systems. The performance profiles of VAEs are similar to that of VA-BAs; like VA-BA copolymers, VAEs have been most successful in interior decorative coatings. Styrene acrylic copolymers Homopolymer styrene has a high Tg (100 °C) and thus needs to be copolymerized with a soft monomer for use in coating applications. Most frequently, butyl acrylate is chosen for this purpose, and styrene-butyl acrylate copolymers used in coating applications typically have a composition of around 50 % styrene with 50 % butyl acrylate by weight. Styrene is a relatively low cost monomer (although styrene costs have fluctuated widely over the years) which is produced widely around the world. Because styrene is relatively hydrophobic, paints based on styrene acrylic polymers tend to be resistant to water transport and provide good barrier properties, particularly in comparison to vinyl acetate polymers. However, styrene has a strong absorption band in the near UV region of the electromagnetic spectrum, and photons of this wavelength are energetic enough to induce photochemical processes which ultimately lead to polymer degradation and reduced exterior durability. In spite of this drawback, the exterior durability of styrene acrylics is often adequate to meet the performance requirements for many exterior applications, particularly those where low

6.2 Overview of Coating Formulations

formulation cost is a primary consideration. Their barrier properties and resistance to alkaline hydrolysis make styrene acrylics particularly popular in coatings for masonry applications, and often for stain blocking primers. Styrene acrylics are also commonly used in industrial maintenance applications, where their good barrier properties help provide effective corrosion resistance when applied over ferrous substrates. Acrylic copolymers Homopolymers of methyl methacrylate have a high Tg (100 °C) and, like styrene and VA, are too hard for typical coating applications. Again, butyl acrylate is commonly used to provide internal plasticization, and to bring the Tg of acrylic copolymers down into the ambient temperature range. Typical acrylic compositions for coating applications are around 50 % methyl methacrylate and 50 % butyl acrylate by weight. Unlike styrene, acrylic polymers do not absorb light in the near UV region, and thus they are resistant to photochemically induced polymer degradation processes. As a class, acrylics generally exhibit the best exterior durability of emulsion polymers commonly used in coating applications. However, methyl methacrylate is a higher cost monomer than VA or styrene, and coatings based on acrylic binders tend to have higher raw material costs. Acrylic copolymers are used in a wide variety of coating applications; they are most popular in exterior applications and can be engineered to provide cost-effective performance features for interior applications as well. Specialty monomers Most emulsion polymers used in coating applications are based on the general copolymer compositions outlined above. However, commercial polymers usually utilize small amounts of specialty monomers to provide added performance features desirable for specific applications. In the wet state, emulsion polymer coatings exist in the form of a densely crowded colloidal dispersion. Good colloidal stability (resistance to particle–particle aggregation processes) is required in order to provide long term storage stability and to deliver the intended performance features. Colloidal stability can be enhanced by utilizing coulombic or steric stabilization methodologies. Coulombic stabilization is the most commonly used method, and involves including a small amount of ionizable species (<10 %) in the polymer composition. Usually an acidic monomer such as acrylic or methacrylic acid is used, which upon neutralization with a suitable base, provides a layer of net negative charge on the particle surface. The coulombic repulsion between these negatively charged particles can provide an effective barrier to thermally induced aggregation processes. Most anionic surfactants associate with the surface of dispersed emulsion particles, and they can also be used to increase coulombic stability in emulsion polymer systems. Steric stabilization is based on attaching low molecular weight, water soluble polymers to the particle surface. This layer of soluble polymers on the particle surface provides an entropically based repulsive interaction between particles, thus conferring additional colloidal stability. Both coulombic and steric stabilization inhibit un-

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desired particle aggregation in the wet coating and this enhances storage and shear stability, as well as helping to optimize the film formation process. A wide variety of other specialty monomers are also used to provide specialized performance properties for coating applications. For example, amine functional monomers can be used to improve adhesion to aged alkyd substrates. Specialized monomers can also be used to improve exterior durability, for example VEOVA (vinyl ester of vesatic acid) monomers can improve the hydrolysis resistance of vinyl acetate polymers, and n-butyl methacrylate can be used to enhance the durability of BA-MMA acrylics. Polymer hydrophobicity can be fine tuned by varying the levels of hydrophobic and hydrophilic monomers in the composition and styrene or ethyl hexyl acrylate are used to increase film hydrophobicity and reduce water permeability in BA-MMA systems. Specialty monomers are also used to provide specific chemical functionality to polymer compositions. For example, hydroxyethyl methacrylate can be used to provide hydroxyl functionality to acrylic resins, allowing these polymers to be used in cross-linkable thermoset coatings which cure via melamine chemistry. While specialty monomers are used at relatively low levels in polymer compositions, they frequently provide the performance features needed for the successful application of emulsion polymers in many coating areas. 6.2.5

Pigments, Extenders, and Additives

While the polymeric binder is usually a major component of a coating formulation, it is important to recognize that the other components (pigments, extenders, and additives) also play a vital role in ensuring a coating will meet the desired cost and performance targets. In this section we will give a brief overview of these other components, with the aim of providing the reader with a background sufficient to understand the formulations and examples discussed later in this chapter. Pigments provide the color and hiding properties of a coating. They can be either organic materials, such as phthalocyanine blue and carbon black, or inorganic materials such as titanium dioxide (TiO2) and iron oxide [11]. For coatings based on emulsion polymers, pigments usually exists as a colloidal dispersion of sub-micron or micron sized particles. Pigments are dispersed into a liquid by a high shear rate grinding processes, usually in the presence of specific dispersing agents (specialized surfactants or low molecular weight polyacid resins) which provide colloidal stability and help optimize color efficiency. It is the selective absorption and scattering of visible light by the pigment particles which provides color and opacity to a coating. TiO2, the most common pigment used in coating applications, gives a white color and excellent hiding because its high index of refraction and carefully optimized particle size allow it to uniformly and efficiently scatters light across the visible spectrum. Colored pigments function by absorbing a portion of the visible light spectrum, and the unabsorbed spectral components are scattered back from the film, giving rise to its color. Pigments make up a substantial portion of a coating’s total raw material cost, and they are carefully processed and formulated in order to maximize their performance.

6.2 Overview of Coating Formulations

Extenders provide a low cost way to adjust the solids level of a coating formulation. For example, coatings formulated above CPVC generally contain high levels of extenders, because it would be cost prohibitive to raise the PVC this high by use of TiO2 alone. Extenders are inorganic materials which are processed to yield particle sizes on the micron scale. Functionally, extenders differ from inorganic pigments because they do not significantly absorb or scatter visible light. Like the binder and pigment components, the extenders in an emulsion based coating exist as a stable dispersion in the water phase. A high shear rate milling process, aided by dispersing agents, is used to create the dispersion. A variety of extender materials are commonly used in coating applications; these include: calcium carbonates, clays, feldspars, silicas, and talcs [12]. While extenders are relatively inexpensive, they can have a significant impact on the performance of a coating. Careful selection of extender components is needed in order to optimize the cost-performance balance of a coating formulation. Dispersants provide enhanced colloidal stability to pigment and extender particles when formulating coatings based on emulsion polymers. Dispersing resins also facilitate the wetting and breakdown of pigment and extender agglomerates in the initial milling and/or grinding process and help to stabilize and reduce the viscosity of the millbase (the millbase is a concentrated dispersion prepared from the pigment and extender powders). Dispersants are generally low molecular weight, water soluble, vinyl resins with high levels of acid functionality. They are usually neutralized with a base such as ammonium hydroxide, sodium hydroxide or potassium hydroxide, and are used at levels of 0.5 to 1.0 % by weight solids on pigment and extender solids. In coatings based on emulsion polymers, dispersants act by increasing the coulombic stability of the pigment particles; dispersing resins associate with polar functional groups on the pigment surface, and the ionized acidic groups on the resin backbone provide strong anionic stabilization to the particles under neutral or basic pH conditions. Thickeners are used to provide emulsion based coatings with the desired application rheology. Coatings formulated with emulsion polymers generally have volume solids in the range of 25–40 %, and in the absence of a thickening system, this range of volume solids would be too low to provide adequate viscosity for most coating applications. The proper choice of a thickener and optimizing its level allows the coating’s rheology to be adjusted to meet the needs of the intended application. There are three main classes of thickeners or rheology modifiers which are commonly used in emulsion polymer coatings: Cellulosic (a class of modified natural products, usually hydroxy ethyl cellulose, or HEC), HASE (a class of synthetic polymers termed hydrophobically modified alkali swellable emulsions) and HEUR (a class of synthetic polymers termed hydrophobically modified ethylene oxide urethanes). Cellulosics are relatively high molecular weight water soluble polymers which thicken by raising the viscosity of the water phase of the coating. HEC polymers were one of the original materials utilized for thickening water based coatings and they are still in common use, primarily in low sheen decorative coating applications. HASE thickeners are high molecular weight emulsion polymers which are activated, or swelled, by neutralization with a base such as ammonia. In contrast to conventional HEC, the

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HASE polymer backbone is modified by pendant hydrophobic functional groups, which can associate with hydrophobes from other thickener molecules, hydrophobes from surfactant molecules, and with hydrophobic domains on the surface of the emulsion polymer particles. This hydrophobic association gives HASE thickeners improved efficiency, and helps them resist volume exclusion flocculation, an undesirable aggregation process associated with high molecular weight, non-associating polymers. HASE thickeners are cost effective and they are used in a variety of decorative coating applications. However, the alkali swellable component of their composition can increase the water sensitivity of a film, and they are not generally preferred for demanding exterior applications. HEUR thickeners are also hydrophobically modified synthetic polymers, but they are lower in molecular weight (50 000 g mol–1) and do not have the alkali swellable component of the HASE thickeners. Because of their lower molecular weight and associative character, HEUR thickeners make a positive contribution to the colloidal stability of a coating. They are particularly useful for demanding applications of decorative and protective coatings where higher gloss, water resistance, and effective barrier properties are needed. Again, coating manufactures careful optimize the rheology modifier package in order to ensure that the coating applies correctly, and that it meets the appearance and performance needs of the intended application. Opacifying aids are frequently included in coating formulations to enhance the hiding performance of TiO2. At low TiO2 levels, the opacity or hiding power of a coating increases linearly with TiO2 content up to about 10 PVC. Above this level, individual TiO2 particles begin to crowd or interfere with each other, and while total hiding continues to rise, hiding efficiency (hiding scaled to the amount of TiO2) starts to fall off. In spite of this decrease in efficiency, coatings manufacturers generally utilize TiO2 levels in the range of 15–25 PVC to provide adequate wet and dry film opacity. Opacifying aids work by improving TiO2 efficiency, and thereby allow coating manufacturers to reduce formulated raw material costs while maintaining hiding performance. Common opacifying aids fall into two main classes: hollow sphere particles and small particle size extenders. Hollow sphere particles are sub-micron sized hollow polymer beads which enhance TiO2 hiding in the dry film by bringing a low index of refraction air void in close proximity to the TiO2 particle. (In the wet state before the film dries, the central air void is filled with water, and the hiding contribution is substantially reduced.) The air void effectively increases the difference in refractive index between the TiO2 scattering centers and their surrounding medium, thereby increasing scattering efficiency and improving hiding. Since TiO2 levels are usually reduced when hollow sphere opacifying aids are utilized, hiding efficiency is also improved through a reduction in TiO2 crowding. Hollow sphere particles also make a direct contribution to hiding, because their central air voids provide a certain amount of intrinsic scattering to the dry film. Figure 6-2 presents a transmission electron micrograph of a commercial hollow sphere opacifying aid. The sample is presented as seen from above, with the polymer shells appearing as a dark rings and the voids as the lighter cores. The particle size is quite uniform with particle diameters of roughly 300 nm.

6.2 Overview of Coating Formulations Fig. 6.2 Transmission Electron Micrograph of a Hollow Sphere Opacifying Aid. Sample was prepared by diluting polymer dispersion with water and then drying a small quantity on an electron microscope sample grid. Contrast was increased by staining with RuO4. Particles are approximately 300 nm in diameter. Polymer shells appear as dark rings, and the hollow cores appear as the light areas within the rings.

Small particle size extenders are the other class of opacifying aids; acting as spacers, they increase the average distance between TiO2 particles in a film, thereby reducing crowding effects. The primary particle size of these specialized extenders is quite small (typically <0.5 µm), and the colloidal components of a coating need to be properly formulated and stabilized in order for these materials to work effectively. Biocides protect coatings from attack by microbial organisms. In the wet state, water based coatings possess the basic ingredients needed to support microbial growth: water, a source of carbon and nitrogen, and trace minerals. The use of anti-microbial agents, or biocides, is generally required to prevent water based paints from spoiling while being transported and stored. A variety of materials are used as in-can preservatives for water based coatings; in general, preservatives used in these applications are electrophilic compounds which function by reacting with nucleophilic groups within the cell or on the cell surface, thereby disrupting the function of vital cellular components. Isothiazolones, and materials based on formaldehyde are most often used for this purpose, although other chemistries are used as well. Preservatives are used at low levels and generally do not affect the performance properties of a coating, unless they are inactivated and are unable perform their function. Paint film mildewcides are commonly used in exterior coatings formulations to prevent defacement of the coating surface by mold and mildew. The surface of an exterior coating can accumulate nutrient compounds from the local environment; they may leach out from the substrate, or they may be deposited by rain or from the atmosphere. These materials, and the coating itself, support microbial growth on the paint film surface. The microbial growth process can eventually lead to unsightly mildew and algae growth on the coating surface, seriously affecting decorative performance, and in severe cases, causing deterioration of the film itself. Coatings based on vinyl emulsion polymers have better resistance to microbial growth than traditional alkyd coatings, but paint film mildewcides are usually included in exterior formulations to provide additional protection. (Apparently, the natural oil components of alkyd resins make them more readily metabolized by microorganisms, leading to poorer intrinsic mildew resistance.) In the past, paint film mildewcides based

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on organomercury compounds provided cost effective protection in emulsion polymer coatings, but environmental and health concerns now strongly limit their use. Modern organic mildewcides have significantly improved environmental risk profiles; they partition strongly into the polymeric domains of the coating, and if they are released into the environment by the weathering process, they are present at extremely low concentrations so that they can be broken down and metabolized by microorganisms in the soil. The most common organic paint film mildewcides in use today are based on iodopropynylcarbamate, isothiazolone and chlorothalonil chemistries. Over the life of a film, these biocides slowly diffuse to the coating surface where they act to control microbial growth. Combinations of organic biocides are sometimes used to provide increased protection against a broad spectrum of microorganisms, and formulators commonly include zinc oxide in order to provide longer term protection from mildew growth. Used properly, modern paint film mildewcides offer a safe and cost effective means to significantly extend the service life of exterior coatings. Defoamers are used to prevent or to dissipate foam in coatings based on emulsion polymers. Small air bubbles can be introduced into the liquid coating during the manufacturing or application processes. If these bubbles are stabilized or long lived, they can interfere with the efficiency of manufacturing, or leave unwanted voids in the dried film. Unfortunately, the surfactants which are used in the manufacture of emulsion polymers and water based coatings can also act to stabilize foam. Defoamers are used in a formulation to destabilize and to speed the breakup of foam in the liquid coating. They are generally hydrocarbon or silicone based dispersions or emulsions which have limited compatibility with both the water and polymer phases of the coating. This marginal incompatibility is an important factor in defoamer effectiveness, and it must be properly balanced or it can result in film defects or loss of activity. While typically used at low levels (<1 % by weight), defoamers can significantly enhance the paint manufacturing process, as well as improving the application, appearance and performance characteristics of a coating. Wetting aids are used to improve the ability of water based coatings to form defect free films over a variety of substrates. Pure water has a high surface tension, 73 mN m–1 [9], and while the surfactants present in a water based coating can reduce this to values in the 25–30 mN m–1 range, this may not be adequate to allow proper wetting (and thus adhesion) to low surface energy substrates such as plastics or certain re-paint surfaces. Also, surfactants can potentially interact with all the colloidal materials in a coating formulation (pigments, extenders, thickeners and defoamers) this can affect the amount of free surfactant available to reduce the surface tension. The equilibrated surface tension of a formulated coating will ultimately depend on the complex distribution of surfactants molecules between these colloidal particles, the water phase and the coating surface, and the judicious use of wetting aids can be used to supplement colloidal stability and to promote proper wetting of the substrate. Typically wetting aids are surfactants or very low molecular weight oligomeric polymers. A wide variety of non-ionic and ionic surfactants are used, including hydrocarbon and silicone based materials. They are generally selected by a combination of previous experience and direct testing. Different wetting aids are used to ad-

6.3 Decorative Coatings

dress a variety of coating problems, including poor substrate adhesion, poor flow and leveling and poor color acceptance. While typically used at low levels (<1 %), they can play a vital role in enabling water based coatings to meet specific appearance and performance requirements.

6.3

Decorative Coatings

The primary role of decorative coatings is to enhance the esthetic appeal of homes, offices and other architectural structures by providing color, texture and sheen to interior and exterior surfaces. Decorative coatings are commonly classified by their intended application, and these include interior wall, interior trim, exterior wall and exterior trim. The sheen level of the coating is also commonly characterized, and these terms include flat, semi-gloss and gloss. These distinctions are not rigid or comprehensive, and coating manufactures commonly break these classes down further, or combine them, in order to enhance the marketability of their products. There are also many kinds of more specialized decorative coatings, such as masonry finishes, clear and stain finishes for wood, floor paints for masonry and wood, driveway sealers and arts and crafts finishes. Space limitations prevent a detailed discussion of these specialized coating applications, but much of the information we present here can be easily adapted to these coatings. 6.3.1

Emulsion Polymers in Decorative Coatings

The history of decorative coatings is long one, arguably extending back to prehistoric times with paintings on the walls of cave dwellings. Focusing on more recent times, solvent borne drying oils and alkyds were the predominant polymer technologies used throughout most of the twentieth century. However, rising environmental and health concerns, coupled with improvements in the performance of emulsion polymer coatings, have allowed water based coatings to move into the leading positions in the decorative application areas. While emulsion polymers are now the market leaders in most decorative applications, there are still a variety of technical issues which remain to be addressed by both raw material suppliers and paint manufacturers. Clearly, regulatory pressure to drive down VOC emissions continues to increase throughout the world, and the challenges of balancing the often conflicting objectives of product cost, product performance and product differentiation remain. 6.3.2

Polymer Compositions used for Emulsion-based Decorative Coatings

A variety of thermoplastic emulsion polymers are used in decorative coating applications, with VA-BAs, EVAs, styrene acrylics, and acrylics being most popular. VA-BAs, EVAs and styrene acrylics are commonly used for interior flat wall coatings, with the

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choices between them generally based on regional economic and performance characteristics. Acrylics and styrene acrylics are the preferred chemistries for exterior applications, again with regional economic factors and performance characteristics driving specific choices. Acrylics and styrene acrylics are both used for interior and exterior gloss and semi-gloss applications, while for interior semi-gloss applications, VA-BAs, acrylics and styrene acrylics are commonly used. These are generalizations, and the specific choices made by paint formulators are based on a complex mixture of factors, including regional custom, local availability, raw material cost, and specific performance needs. 6.3.3

Regional Distinctions in Decorative Coatings

There are significant regional distinctions in the formulation of decorative coatings. Differences in building materials and, consequently, the substrates to which coatings are commonly applied, underlie many of these distinctions. For example, North America (NA), Australia – New Zealand (ANZ) and Scandinavia all have relatively high levels of wood substrates, whereas masonry substrates are more common in Europe, Latin America (LA) and Asia. Substrate differences are a major factor in the preference of acrylics for exterior wood applications in NA, ANZ and Scandinavia, the use of elastomeric wall coatings for masonry applications in Europe, and the choice of styrene acrylics for masonry applications in LA and Asia. The availability of supplies of low cost monomers is another important factor affecting the choice of polymer composition; this drives the use of VA-BA polymers for the low cost interior flat segments of NA and ANZ, and the use of styrene acrylics for decorative segments in Europe and Asia. In Latin America, decorative paints are commonly designed for both interior and exterior applications, and styrene acrylics and VA-BA polymers predominate in these markets. Regional economic factors also influence the choice of polymer used in coating applications: differences in raw material costs, and in labor costs, can affect how end users balance the higher initial costs associated using a more durable, high performance coating, versus the higher deferred costs associated with using a less durable coating having a more frequent re-paint cycle. 6.3.4

Market Size of Decorative Coatings

Decorative coatings, as defined here, account for roughly 40 % of worldwide coatings production. The ratio can vary from country to country; heavily industrialized countries have a relatively higher proportion of protective and product finishes, while less industrialized countries often have a higher proportion of decorative finishes. Starting with the world wide coating production estimate of 20 billion liters per year [1], and taking the fraction of decorative finishes to be 40 %, would give the total production of decorative coatings to be roughly 8 billion liters per year worldwide. Assuming that about 50 % of this is based on emulsion polymer technology leads to an esti-

6.4 Interior Decorative Coatings

mated annual production of about 4 billion liters of water-based decorative coatings world wide. We estimate the North American market to be a third of this value or about 1.5 billion liters per year. If we estimate the average retail selling price to be $4–5 US per liter, this gives a total NA market value of $6–8 billion US per year for decorative coatings. The volume of the European decorative coating market is roughly comparable in size to the North American market. Taking the annual European production estimate to be also 1.5 billion liters per year, and assuming an average retail selling price of roughly 4–5 Euros per liter, would give a total European market value of about 6–8 billion Euros per year for decorative coatings. Again it is important to emphasize that these are very rough estimates, and are intended to give the reader a general picture of market size.

6.4

Interior Decorative Coatings

Interior decorative paints are designed to provide texture, sheen and color to the interior walls and ceilings of homes and offices. Paints based on emulsion polymer technology now account for most of this segment. The popularity of water based paints in interior decorative applications is due to many factors, including their ease of use, low odor, fast dry, good appearance and color stability, and the ease of soap and water clean-up. Paints for this application are designed for a variety of specific applications such as kitchen and bath, wall, trim and ceiling; they also provide different sheen levels such as flat, satin, semi-gloss and gloss. Coatings manufacturers produce a wide variety products to meet the different needs of this large market segment, and this in turn requires a correspondingly large number of formulations and raw materials. Because of space limitations, we will focus our discussion on two of the major segments of the interior decorative market, flat interior wall paints and interior trim enamels, and use them to highlight performance and formulation concepts of this market segment. 6.4.1

Key Performance Features

Interior flat wall coatings have a low or “flat” sheen, and they are usually formulated with an 85° gloss level below about 4 %. Low sheen levels are usually achieved by using large particle size extenders at relatively high PVCs. While low sheen levels help to hide defects in the substrate, differences in sheen and color are readily observable in these coatings, and thus sheen and color uniformity of the applied coating are key performance attributes. Good application characteristics are important as well; interior flat wall paints are frequently spray applied in new construction applications, and are generally applied by roller in re-paint applications. Formulating with associative thickeners (typically HASE) can help reduce roller splatter, an important characteristic in re-paint applications. End users expect decorative coating to have good hiding characteristics, and while true one-coat hiding remains an elusive goal, coat-

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ing manufacturers carefully balance hiding performance and cost in these formulations. Interior flat coatings also need to have good cleanability characteristics, since they are routinely applied to living areas (with higher potential for dirt and stains), and the coating’s ability to be easily cleaned can add significantly to its service life and user satisfaction. Enamel coatings are frequently applied to trim surfaces such as doors, window frames, decorative moldings around doors and windows, base boards, cupboards and shelving. These surfaces often have a high level of day to day human contact and a tougher, more resistant coating is needed to provide substrate protection, film integrity and optimum appearance characteristics. Trim enamels are frequently brush applied so good flow and leveling characteristics are important. Enamels are generally formulated at higher sheen levels, with values for semi-gloss and gloss coatings ranging from 30–85 % at 60°, respectively. They are applied over a variety of substrates, including aged alkyd, aged water based enamels, primed wood or metal, and thus they must have good adhesion characteristics. It is also desirable that the coating’s hardness develop quickly so that objects can be placed on painted surfaces without marking the film. Trim enamels should also have good block resistance so that adjacent painted surfaces can be pressed into contact without sticking to each other. Finally, because they are applied in high use areas such as doors, windows and kitchen areas, trim enamels need good stain resistance and cleanability. 6.4.2

Interior Decorative Coating Formulations Flat Interior Wall Coatings Cost is one of the most important factors for interior flat coatings and drives many of the choices in formulations and raw materials. Vinyl acetate and styrene based copolymers are most frequently used as binders for this market and cost considerations also drive the use of relatively high levels of low cost extenders. Thus, PVCs are generally high, and are usually above CPVC. Opacifying aids are commonly used to help achieve high hiding levels, and they also help to lower raw material costs by reducing TiO2 levels. Rheology modifiers are chosen to provide the desired rheology profile, with HASE and cellulosic thickeners being most commonly used. A coalescing agent is usually included to enhance film formation, although recently developed binders for interior flat coatings can often be formulated with very little or no added coalescent. Propylene glycol is commonly used as a pigment grinding aid, and to improve the freeze thaw resistance of the wet paint during storage. Finally, anionic and non-ionic surfactants are commonly added to optimize colloidal stability and enhance color development and uniformity. Table 6-1 provides an example of a typical interior flat coating formulation. Interior Enamels Polymer performance plays a more significant role in the formulation of enamel coatings than in flat coatings, since the polymer component makes up a higher volume fraction of the dried film. Acrylic and styrene acrylic emulsion polymers are

6.4 Interior Decorative Coatings Tab. 6-1

Interior decorative flat formulation.

Material

Weight %

Grind Water Propylene Glycol Dispersant Defoamer Biocide Aminomethylpropanol Titanium Dioxide Calcined Clay Extender Calcium Carbonate Extender Grind sub-total

13.56 4.07 0.52 0.18 0.16 0.18 10.73 13.65 9.04 52.09

Let down VA-BA Emulsion Polymer Hollow Sphere Opacifying Aid Coalescent Defoamer Water HEUR Thickener Ammonia (28 %) HASE Thickener Total Property Total PVC Volume Solids Weight Solids

Comments Prepare in a high speed disperser

19.72 6.84 1.34 0.18 16.58 1.72 0.18 1.36 100.00

Grind and freeze-thaw aid Polyacid Hydrocarbon dispersion Isothiazolone preservative Base, grind aid Interior grade Coarse grade

Add to grind with good agitation 55 % Solids Ester alcohol, film formation Hydrocarbon dispersion 25 % solids Base Emulsion

Value 63 % 33 % 47 %

most commonly used in this segment, with vinyl acrylics playing a role in the lower performance/lower cost end of the market. Performance considerations limit the PVCs of these coatings, and enamels are formulated significantly below critical PVC, generally in the range of 15–25 % PVC. Extenders and opacifying aids are not commonly used in gloss enamel coatings, and are used to a limited extent in semi-gloss coatings. HEUR and HASE thickeners are often used as rheology modifiers; they provide excellent flow and leveling with brush application and generally allow for higher gloss levels. The harder polymers used in enamel coatings generally require higher levels of coalescing agents to provide good film formation. Again, anionic and non-ionic surfactants are commonly added to optimize colloidal stability and enhance color development and uniformity. Table 6-2 provides an example of a typical interior/exterior gloss enamel formulation.

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Interior/exterior decorative gloss enamel.

Material

Weight %

Grind Propylene Glycol Water Biocide Dispersant Surfactant Defoamer Titanium Dioxide Grind sub-total

3.36 2.43 0.10 1.12 0.10 0.02 21.28 28.39

Let down Acrylic Emulsion Polymer Water Diethylene glycol butyl ether Coalescent Phosphate Surfactant HEUR Thickener A HEUR Thickener B Defoamer Total Property Total PVC Volume Solids Weight Solids

50.67 15.16 0.58 2.50 0.09 2.36 0.19 0.05 100.00

Comments Prepare in a high speed disperser Grinding aid Isothiazolone preservative Hydrophobically modified polyacid Non-ionic pigment wetting aid Silicone emulsion Universal Grade

Add to grind with good agitation Specialized gloss enamel vehicle Co-solvent Ester alcohol, film formation Wetting Aid 20 % solids 25 % solids Silicone emulsion

Value 21 % 32 % 45 %

6.4.3

Standard Application and Performance Tests

Many specialized tests have been developed to measure the application, appearance and resistance properties of interior decorative coatings. The reader is referred to the specific ASTM and ISO test methods given in Tab. 6-3, and the cited references [13–14] for a more detailed description of test method protocols. Our objective in this section, and in the subsequent application and performance test sections, will be to identify the key performance features and to give the reader an overview of how these tests are conducted. Decorative coatings are often stored for long periods of time under sub-optimum conditions before they are sold or applied. In the warmer regions of the world, coatings are frequently stored in warehouses where temperatures can reach 45 °C for extended periods of time. In the colder regions of the world, coatings can be subjected to repeated freeze-thaw cycling when stored at an unheated job site. Heat age and freeze-thaw stability testing protocols have been developed to assess the storage stability of coating products. Heat Age Stability is tested by placing the paint in an oven for a specific time and temperature, and then testing the coating for key performance

6.4 Interior Decorative Coatings Tab. 6-3

Selected coating applications test methods.

Application test

ASTM method*

ISO method**

Freeze-thaw resistance Heat age stability Low temperature film formation Gloss Color acceptance Hiding Block resistance Print resistance Adhesion – qualitative Adhesion – quantitative Scrub resistance Stain removal (top coat) Stain blocking (tannin) Durability – exterior Durability – accelerated Corrosion – exterior Corrosion – accelerated Tensile testing Permeability testing Early washout (traffic paint) No pickup test (traffic paint)

D2243 D1849 D3793 D523 D5326 D344, D2805 D4946 D2064 D3359, D6677 D2297, D4541 D2486, D4213 D3450, D4828 D6686 D660, D661, D662, D772, D3719, D4214 D4141, G26, G53, G151 D610, D1014 D2803, D4587, G85 D2370 D1653 D 1640 – Modified D711,D713

1147

2813 2814, 6504 3678 2409 4624 11998 4586 4628 4892, 11341 4628 7253, 11997 7783

** ASTM test methods can be obtained from the ASTM web site – http://www.astm.org ** ISO test methods can be obtained from the ISO web site – http://www.iso.ch

properties. Many different time-temperature protocols are used; two common ones are 10 days at 60 °C or 30 days at 50 °C. Freeze-thaw stability is tested by subjecting the coating to repeated freeze-thaw cycles, typically 3–5 cycles of temperature changes between –20 °C and room temperature; changes in viscosity and other key performance properties are then evaluated. Decorative coatings are commonly applied by brush, roller, or spray techniques, and coatings manufacturers generally design their products to perform well when applied by any of these methods. Testing protocols include laboratory testing under carefully controlled conditions, and field trials under realistic application conditions. The objective is to develop a coating which applies correctly, has a rheology profile which allows good flow and leveling, without excessive sagging, and provides a uniform coating to the substrate. Application properties are particularly important with products designed for professional painters, since these features are vital to reducing call backs and maintaining high productivity. Good initial appearance is an important factor in determining customer satisfaction in any coating application, but is particularly vital for decorative finishes. The applied coating is expected have the right color and the right sheen level. Defects in the applied coating should be minimal, and easily repairable. A variety of standardized laboratory instruments have been developed to measure sheen, hiding and color development (Tab. 6-3). Also, many specialized tests have been developed to as-

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sess appearance properties under specific application conditions. For example the color rub-up test assesses the color variability of a coating when applied by brush or roller application techniques. Since large interior wall areas are usually painted by brush application around the perimeters, and then filled in by roller application to the center sections, variations in color uniformity can have a significant impact on end user satisfaction. In laboratory testing of this property, a draw-down of uniform film thickness is first applied, and then a section of the coating is either rubbed with the finger or brushed until the coating starts to dry. Color differences between the low shear rate draw-down region and the high shear rate rub up or brushed region are then assessed subjectively, or measured quantitatively with a color spectrophotometer. Problems in this area are generally related to poor colloidal stability; a lightly flocculated pigment dispersion can be temporarily dispersed by the high shear conditions of brushing, and this can result in a color which appears different from adjacent areas coated under the lower shear rate of roller application. Resistance properties In contrast to the appearance and applications properties of a coating, resistance properties are not assessed by the user during and immediately after the application, but rather, they impact user satisfaction by affecting the service life of the coating. Laboratory testing plays a key role in assessing the resistance properties of coatings, because it provides a controlled and accelerated measure of performance features which may take years to become evident in actual end use applications. Adhesion is a key performance factor and several tests have been developed to measure this parameter. All adhesion tests follow the general protocol of applying the test coating to a defined substrate such as chalky or aged alkyd, steel or aluminum, allowing the coating to dry for a specified time, and then testing the adhesion of the applied coating by attempting to separate it from the substrate. Adhesion can be tested under wet or dry conditions (wet adhesion is usually a more severe test than dry adhesion), and the coating is usually scored or cut in order to minimize the confounding effects of film integrity. Cohesive failure occurs when the coating remains bound at the film–substrate interface and separation occurs within the coatings itself, or by destruction of the substrate. This is generally indicative of good adhesion performance, but it can sometimes be misinterpreted when a coating has extremely poor film integrity (giving a false positive reading). Adhesive failure occurs when the applied coating separates from the substrate at the interface between the coating and the substrate. Failures of this type can be assessed via subjective or quantitative measurements, such as using knife peel, cross hatch/tape pull, or a quantitative measurement of the force to peel. While it is desirable to have a coating exhibit cohesive failure in lab tests, coatings which exhibit adhesive failure often show good adhesion performance in actual exposure testing. Experience and careful comparisons against known standards are generally required in order to obtain useful performance predictions in these cases. The cleanability of a coating is generally assessed by stain resistance or scrub tests. In the stain resistance test, the coating is allowed to dry for a specified time (usually a week) and then common staining materials such as coffee, tea, fruit juice, mustard,

6.4 Interior Decorative Coatings

ketchup, pencil, pen or felt tip marker are applied to the surface. After a specified contact time, the coating is washed with a cleaning formulation and then is rated for stain removal relative to controls. The scrub test assesses cleanability differently, and measurers the ability of a coating to resist abrasion by a stiff brush and an abrasive cleaner, or cleaning solution. A coating of defined thickness is applied to a vinyl chart and dried for a specified time, usually a week. A specialized scrub testing machine is used to scrub the coating with a brush and a standardized abrasive medium. The operator assesses the number of scrub cycles needed to wear through the coating to the substrate. In another variation of the scrub test, the test coating is placed in the testing machine for a fixed number of cycles using either the abrasive medium or a cleaning solution, and then cleaned and dried. Performance is assessed by measuring the weight of coating lost during the scrub process. Both tests are subject to high levels of variability and carefully controlled experiments are needed to produce accurate and reproducible results. Block resistance is a measure of a coating’s ability to resist destructive self adhesion when placed into contact with itself. This is an important feature for coatings which are applied to windows and doors, since in these components, painted surfaces are placed in contact in routine operation. Dry time and contact pressure are important factors affecting block resistance and are the primary variables which are controlled in laboratory testing. The coating is cast on a non-rigid substrate such as a coated paper chart, and allowed to dry under controlled conditions for a specified time (typically ranging from 8 h to 4 weeks). Small squares of the coated substrate are then cut out and placed with the coated sides facing together. A defined pressure (generally in the form of a 0.5 kg weight on a surface area of approximately 5 cm2) is applied for a specified period of time and temperature (typically 12 h at 25 °C or 4 h at 50 °C). The test squares are then pulled apart and rated subjectively for self adhesion. Ideally, the two test squares separate with minimal force, leaving no film damage. Failure is noted when the film is visibly damaged upon separation. In a variation of this test, the coating can be applied to a rigid substrate such as glass or metal, and these test areas are placed together, under pressure, as described above. The force necessary to separate the test areas is then measured quantitatively. The print resistance test measures the ability of a coating to resist permanent imprinting caused by the placement of heavy objects, such as books or vases, on a horizontal coated surface. In this test, a coating is cast onto a metal substrate, and then a heavily textured object, such as a rough cloth is placed on the coated surface with a defined pressure (typically about 1 kg per 5 cm2). The test is allowed to progress for a specified time and temperature (usually one week at 25 °C, or one day at 50 °C), and then the coating is evaluated subjectively against controls for its ability to resist permanent marking or defacement.

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6.5

Exterior Decorative Coatings

Exterior decorative coatings are used to provide aesthetic and protective features to the exterior walls and trim of houses, apartments and offices. They share many of the application, appearance and resistance features characterizing interior decorative coatings, with the obvious and important difference that they are expected to provide these features while being subjected to the deleterious effects of UV radiation and weathering. In addition, exterior decorative coatings are expected to protect their substrates from the harmful effects of weathering for the lifetime of the coating. Historically, alkyd and oil based coatings were commonly used in exterior decorative applications, but over the past 30 years coatings based on emulsion polymers have advanced and are now the preferred technology for this application. The primary reason for this is the superior durability of acrylic emulsion polymers (and to a lesser extent vinyl-acrylic and styrene-acrylic polymers) in exterior applications. Alkyd and oil based coatings rely on oxidative cure processes to develop resistance properties. While the cure processes are quite efficient, they can eventually lead to film embrittlement and subsequent cracking over dimensionally unstable substrates. Additionally, most alkyd resins are made by the esterification of phthalic anhydride with unsaturated fatty acids or natural drying oils. The UV absorption characteristics of these materials make them quite susceptible to UV degradation, leading to poor tint retention and premature chalking. Finally, the ester linkages of oil and alkyd based coatings are susceptible to alkaline based hydrolysis, a polymer degradation process which can be accelerated by the basic pH conditions present in many masonry applications, particularly over freshly prepared concrete. Acrylic based polymers are more resistant to these different degradation processes and, consequently, have become the performance standards for exterior decorative coatings. 6.5.1

Key Performance Features

Exterior durability is the key factor which differentiates the performance of exterior decorative coatings. This is primarily manifested in three important areas: tint and gloss retention – the ability of a coating to maintain its original color and gloss level during exposure, chalk resistance – the ability of a coating to resist the surface powdering caused by UV and moisture induced polymer degradation, and resistance to cracking and adhesion loss – the ability of a coating to resist grain cracking and the subsequent flaking and loss of adhesion when applied over wood substrates. Good dirt pick-up resistance is also important; the coating should be resistant to darkening caused by adsorption of dirt and soot from the external environment. Adhesion to a variety of architectural substrates is also an important performance feature, particularly in re-paint applications where weathered substrates present particular challenges. Because exterior coatings are generally viewed from a distance, appearance properties such as flow and leveling are somewhat less important than they are in many interior applications. Of course, most of the other performance features de-

6.5 Exterior Decorative Coatings

scribed above for interior decorative coatings also apply to their exterior counterparts. 6.5.2

Exterior Decorative Coating Formulations

Acrylic emulsion polymers are generally preferred for exterior decorative applications because of their good exterior durability characteristics. Styrene acrylics and vinyl acrylics are also popular, particularly in regions of the world where lower labor costs reduce the economic barrier to a more frequent re-paint cycle. In contrast to interior flat paints, exterior flat paints are generally formulated below CPVC in order to provide improved durability. Because good moisture resistance generally is required for exterior applications, cellulosic and HEUR thickeners are generally chosen as rheology modifiers for exterior applications; cellulosics are commonly used in exterior flat formulations, while HEURs are commonly used in gloss and semi-gloss formulations. Durable grades of TiO2, coated with inorganic materials to provide improved UV resistance, are generally chosen for exterior applications. The choice of extenders can also have a significant impact on the performance of exterior coatings. Based on our experience, the tint retention and chalk resistance of exterior flat coatings can be enhanced by favoring the use of coarse silica or nephiline syenite over clays. Calcium carbonates generally have good tint retention, but they can degrade in regions with acid rain, sometimes leading to poor dirt pick-up resistance. Carbonates can also can show frosting (the appearance of a hard white exudate, visible on tinted films) in horizontal face down applications. Finally, a paint film mildewcide is generally included in exterior formulations, particularly in flat and satin formulations, to minimize mildew growth on the coating after application. A typical formulation for an exterior flat decorative coating is given in Tab. 6-4. 6.5.3

Standard Application and Performance Tests

Exterior exposure testing is the most direct and reliable method to evaluate the durability of exterior decorative coatings. The most representative and general exposure protocol is to actually apply the coating to test homes or buildings, and to then evaluate the performance over a long period of time. However, the cost and logistics of such large scale trials limit their use, and the vast majority of exterior exposure testing is done via controlled exposure experiments carried out at established exposure sites around the world. Most raw material suppliers and many coating manufacturers have set up their own exposure sites, or use commercial exposure services, to provide this capability. A typical experimental design for an exposure experiment would include experimental and control coatings which are applied, in replicate, to test areas (on the order of 15 cm × 30 cm) over a variety of representative substrates. The choice of substrates depends on the intended market segment and region, but normally would include several different types of wood and masonry in both new and re-paint applications. In the Northern hemisphere, South vertical exposures are

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Exterior decorative flat formulation.

Material

Weight %

Grind Cellulosic Thickener (2.5 %) Water Dispersant KTPP Non-Ionic Surfactant Defoamer Biocide Mildewcide Titanium Dioxide Zinc Oxide Nepheline Syenite Extender Functional Extender Grind sub-total

10.71 7.17 1.52 0.08 0.08 0.17 0.14 0.25 21.09 2.11 16.87 0.42 60.64

Let down Acrylic Emulsion Polymer (60 %)

22.86

Ester Alcohol Coalescent Ethylene Glycol Defoamer HEC Thickener Water Total Property Total PVC Volume Solids Weight Solids

0.83 0.25 0.17 8.07 7.17 100.00

Comments Prepare in a high speed disperser HEC thickener Hydrophobically modified polyacid Co-dispersant Pigment wetting aid Hydrocarbon dispersion Isothiazolone preservative Isothiazolone mildewcide Exterior universal grade Mildew protection Coarse grade Thixotropic clay

Add to grind with good agitation Enhanced adhesion to alkyd and chalky re-paint Film formation Co-solvent and freeze-thaw aid Hydrocarbon dispersion 2.5 % in water

Value 50 % 35 % 54 %

commonly chosen to accentuate failure modes which are linked to UV exposure; these include gloss loss, grain cracking, chalking and color fading. North vertical exposures are used to evaluate mildew resistance and discoloration by dirt pick up. Exterior exposure testing is not a rapid process; some failure modes can take several years to develop. Exposures at South 45° can be used to accelerate failure modes linked to UV radiation and moisture; this exposure angle increases the flux of UV energy incident on the sample, as well as increasing the intensity and duration of moisture contact brought about by the daily dew cycle and rain. However, 45° exposures are generally uncommon in real world applications, and the data obtained from these experiments should be interpreted with caution. By using specialized exposure techniques such as: exposing samples at South 45°, selecting woods with poor dimensional stability as substrates, and applying thinner layers of coating (one coat applications), coatings scientists can accelerate the exposure testing process. However, even with these methods, it can still take 2 years or more to develop a clear picture of a coating’s durability characteristics.

6.6 Elastomeric Wall Coatings

Accelerating the process for assessing the durability of exterior coatings is of obvious interest to raw material suppliers, coating manufacturers and end users. This is currently an area of active research and a variety of efforts are under way to improve the predictive ability of accelerated weathering protocols [15]. While a variety of exposure instruments and devices have been developed, and are commonly used to provide accelerated exposure information, we have found that these instruments do not always provide information consistent with exterior exposures. Accelerated exposure devices have proven to be most useful when evaluating specific failure modes, and comparing the performance of coatings of similar composition, applied over dimensionally stable substrates.

6.6

Elastomeric Wall Coatings

Specialized elastomeric wall coatings were first introduced into European markets in the early 1980s. Their function is to enhance the appearance and durability of exterior masonry surfaces present on large buildings such as apartments, hotels and offices. These types of structures often have large uniform surfaces which can be disfigured by small cracks and fractures, caused by uneven thermal expansion and contraction. Elastomeric coatings improve the appearance of masonry surfaces by covering these small cracks with a smooth elastic film, and they enhance the durability of masonry substrates by preventing the intrusion of water into these defects. 6.6.1

Key Performance Features

Elastomeric wall coatings are designed provide a high quality decorative and protective finish for large masonry surfaces. The elastic character of these coatings, along with the use of thicker applied films, allows them to bridge cracks in the substrate and to stretch and shrink with thermally driven building movement. Elastomeric coatings also prevent the penetration of wind driven rain and water into the substrate by sealing these cracks, thus improving the durability of the underlying masonry material. Emulsion polymers used for elastomeric wall coatings generally have low Tg (typically less than –20 °C) in order to provide the elastic character needed for effective crack bridging. The use of such soft polymers would normally lead to coatings with poor dirt pick up resistance and tackiness, but proprietary technologies are usually employed to address these problems. Elastomeric coatings can significantly enhance the durability of many masonry surfaces, and because masonry construction is widely used around the world, the use of elastomeric wall coatings has grown significantly over the past 20 years.

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6.6.2

Typical Elastomeric Wall Coating Formulations

Elastomeric coatings are formulated to yield tough films which maintain a balance of tensile strength and elongation characteristics across a broad temperature range (–10 to +30 °C). These coatings are generally formulated at relatively low PVCs (typically in the range of 30–45 %) to provide the dried film with good elasticity and barrier properties. These coatings are formulated with relatively low levels of hiding pigments, since thick coatings are generally utilized. They are usually extended with fillers such as calcium carbonate. Volume solids are typically high, in the in range of 50 % to 60 %, in order to provide for thicker dried films. For optimum long-term performance, it is recommended that the total dry film thickness be in the range of 300 to 500 µm, much thicker than typically used in architectural applications. Coatings of this thickness need a carefully optimized rheology profile in order to prevent sagging during the application and drying processes. Cellulosics, either alone or in combination with HEUR thickeners, are generally preferred for this application. An effective paint film mildewcide is also need in order to prevent discoloration of the coating surface by mildew growth. Elastomeric wall coatings are generally formulated to be applied by professional painters using either roller or spray application techniques. A typical elastomeric coating formulation is given in Tab. 6-5. Tab. 6-5

Elastomeric wall coating formulation.

Material

Weight %

Grind Water Ethylene Glycol Dispersant KTPP Defoamer Titanium Dioxide Calcium Carbonate Extender Zinc Oxide Grind sub-total

5.67 2.75 0.44 0.09 0.35 6.20 26.12 2.21 43.84

Let down Surfactant Acrylic Emulsion Polymer Defoamer Mildewcide HEC Thickener Water Total Property Total PVC Volume Solids Weight Solids

Comments Prepare with high speed disperser

0.55 54.20 0.09 0.18 0.27 0.89 100.00 Value 31 % 49 % 61 %

Grind aid and freeze-thaw Polyacid Co-dispersant Hydrocarbon dispersion Exterior Grade Fine grade Mildew protection

Add to grind with good agitation Wetting aid, non-ionic Specialized elastomeric vehicle Hydrocarbon dispersion Isothiazolone class Solid

6.7 Primer Coatings

6.6.3

Standard Application and Performance Tests

The tensile and elongation properties of an elastomeric coating are important performance features which characterize the coating’s ability to bridge cracks in the substrate. These properties are commonly evaluated in the laboratory at ambient and below ambient temperatures by use of a tensile testing instrument (Instron type or equivalent). Testing is performed at a constant rate of jaw separation with load cells adequate to measure the tensile forces generated. Testing sample dimensions are usually about 500 µm thick × 2 cm long × 1 cm wide, and samples are cut with a die from a dried film which was drawn down over a Teflon release plate. The sample is fastened between the jaws of the tester and is stretched apart at a constant strain rate until it breaks. Values for the percent elongation and tensile strength at break are then calculated. These values depend somewhat on the strain rate used; generally, lower strain rates yield higher values for elongation, coupled with lower values for tensile strength at break. The permeability of an elastomeric coating is crucial for determining whether the coating will allow adequate passage of water vapor through the coating. In the laboratory, permeability is evaluated by sealing a dried paint film of specified thickness over a cup of water and placing the assembly in a constant temperature and humidity room. Water loss from the cup, through the film, is measured by mass difference over the course of one week. Permeability is then calculated from the rate at which water is lost through the film.

6.7

Primer Coatings

Primer coatings are used to provide a functional boundary layer between the substrate and the topcoat. Coating manufacturers design a variety of specialized primers to enhance total system performance for a variety of coating applications. Primers generally provide many or all of the following features: (i) they promote effective adhesion to a variety of substrates; (ii) they prevent the transport of a variety of different types of colored stains from the substrate to the topcoat; (iii) they enhance corrosion resistance; (iv) they serve as a flexible linkage between dimensionally unstable substrates and the topcoat; and (v) they reduce irregularities and imperfections in the substrate, providing a smoother and more uniform surface for the topcoat. Decorative and protective primers generally have demanding performance specifications, and are usually formulated at relatively low PVCs and high volume solids. They usually are not designed to provide high hiding or intrinsic weathering resistance, since the topcoat can be independently optimized to provide these and other performance features.

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6.7.1

Key Performance Features

Primer vehicles based on emulsion polymer technology were introduced in the 1970s and have shown a steady increase in market share over the past 30 years. They are used in a variety of exterior and interior coating applications. Stain blocking primers are one of the most popular types, and are used to prevent discoloration of topcoats by a variety of materials, including tannin stains from wood, children’s marker stains, water stains and nicotine stains. Water based primers generally block the transport of stains by two principle mechanisms: (i) the primer acts as a physical barrier and blocks the migration of stains from the substrate to the topcoat (this is similar to the way in which solvent based primers block stain transport); and (ii) the primer is formulated with specialized ingredients, such as zinc oxide or other functionalized extenders, which lock (by interacting with) stain molecules into the film. While the stain may be visible at the surface of the primer coating after drying, it is effectively locked into the film and does not lead to discoloration of the topcoat. Binders for stain blocking primers are usually based on relatively hydrophobic emulsion polymers with acrylic or styrene acrylic compositions. Primers are expected to have excellent adhesion, and water based primers are formulated to adhere to a variety of substrates including metal, wood, and chalky or aged re-paint surfaces. Since this level of performance may not be needed, or achievable, in many topcoat formulations, the use of specialized primers offers a flexible and cost effective way to meet the performance needs of many different coating applications with a limited number of optimized topcoat products. 6.7.2

Primer Formulations

Primers based on emulsion polymers typically are formulated with PVCs in the range 25 to 45 % and volume solids in the range 30 to 40 %; lower PVCs and higher volume solids formulations are preferred because these characteristics provide tighter and more flexible films. Primer formulations frequently (but not always) contain some type of reactive pigment or specialized extender to enhance performance features: stain blocking primers frequently utilize functionalized extenders to lock stains while anti-corrosive primers utilize reactive pigments to passivate ferrous substrates. The performance characteristics of primers can be greatly affected by the choice of thickener and dispersant; HEUR rheology modifiers along with relatively hydrophobic dispersants are generally preferred in this application. In general, effective primers are formulated with a high degree of colloidal stability and reduced levels of water sensitive materials in order to improve performance characteristics. Table 6-6 illustrates a typical formulation used in stain blocking primer applications.

6.7 Primer Coatings Tab. 6-6

Stain blocking decorative primer formulation.

Material

Weight %

Grind Water Biocide Dispersant Defoamer Titanium Dioxide Calcium Carbonate Extender Dispersant Zinc Oxide Grind sub-total

4.90 0.15 0.89 0.17 14.67 4.90 0.21 1.18 27.07

Let down HEUR Thickener A Acrylic Emulsion Polymer Ester Alcohol Coalescent Ethylene Glycol Biocide Defoamer Thickener B Ammonia (28 %) Water HEUR Thickener A HEUR Thickener B Total Property Total PVC Volume Solids Weight Solids

Comments Prepare in a high speed disperser

0.25 65.16 1.40 2.73 0.19 0.39 0.20 0.10 1.24 0.25 1.03 100.00

Isothiazolone preservative Hydrophobically modified polyacid Hydrocarbon dispersion Exterior grade Coarse grade Acrylic acid type Functional extender – stain blocking

Add to grind with good agitation 25 % solids Specialized primer vehicle Film formation Co-solvent and freeze-thaw aid Isothiazolone mildewcide Hydrocarbon dispersion HASE (30 %) pH adjustment 25 % solids 20 % solids

Value 19 % 37 % 49 %

6.7.3

Standard Application and Performance Tests

Application testing for stain blocking primers is focused on the ability of the primer to block stains and to provide a suitable substrate for the topcoat. Laboratory testing for tannin stain resistance is performed by applying the primers, along with a suitable topcoat, to wooden panels with high tannin levels such as western red cedar or redwood. The panels are allowed to dry for a short time and they are then placed in a high moisture environment such as a fog box or mist chamber. Because of the high degree of panel to panel variability inherent with a natural substrate such as wood, a careful experimental design (comparisons on the same panel, replication, etc.) is needed to obtain meaningful results. After the panels are removed from the fog box and allowed to dry, the performance is rated by a visual comparison against the controls. Laboratory testing of marker stain resistance is done in a similar manner; the

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performance of primer-topcoat combinations are evaluated for their ability to resist discoloration relative to a set of pass and fail controls. Testing is usually carried out with several water based and solvent based markers, which are applied to draw-down charts. The trial primers are then drawn down over the test area, allowed to dry for a short period (usually 2–4 h) and then coated with a suitable topcoat. Samples dry overnight and are then rated relative to controls by a visual comparison.

6.8

Protective and Industrial Coatings

The distinctions between protective and decorative coatings are often a matter of degree, and for our purposes, we will define protective and industrial finishes as coatings which are applied to large industrial structures, such as bridges and factories, or to products produced in an industrial production process. We will not include automotive coatings in our current discussion, since they are covered in Chapter 7 of this volume. Even with these limitations, there are a great number of different types of application areas which use protective or industrial coatings; these include coatings for industrial structures, machinery and equipment, metal containers, wood furniture and flat stock, and more specialized applications such as coil coatings, marine coatings and traffic marking coatings. The performance requirements of coatings designed for protective and industrial applications are generally more demanding than those of decorative coatings. Because of this, and the wide variety of applications, the shift to coatings based on waterborne emulsion polymers has not been as pronounced as it has been in the decorative area. Solvent borne coatings are the historical market leaders in protective and industrial coating markets, and they still hold this position in most applications. However, new developments in polymer design and formulation technologies have allowed waterborne finishes to make significant inroads into many areas of protective and industrial coatings. This, coupled with an increasingly stringent regulatory environment, has led to significant growth in the use of waterborne coatings for these applications. 6.8.1

Copolymers used in Protective and Industrial Coatings

A variety of synthetic polymer resins are used in coatings for protective and industrial finishes, with solvent-borne alkyds, acrylics, urethanes, epoxies and polyesters being most common. Recently, high solids coatings (based on solvent borne polymers designed for formulation with low levels of solvent) and powder coatings have also made significant inroads into the protective and industrial coatings areas. In the context of waterborne coatings, acrylic emulsion polymers are the most common compositions chosen for these applications, although significant amounts of waterborne epoxies, polyurethanes and emulsified alkyds are also used. Acrylic emulsion polymers for protective and industrial coating applications are generally designed with Tg in the range 30 to 60 °C, significantly higher than binders used for decorative appli-

6.8 Protective and Industrial Coatings

cations. Thermosetting acrylic systems, which utilize melamine or other types of crosslinking chemistry, are also used in many factory applied, oven bake applications. Since most protective and industrial finishes are applied to dimensionally stable substrates, polymers with harder compositions can be utilized in order to provide improved performance characteristics, without suffering the drawbacks usually associated with using higher Tg polymers in many decorative applications. 6.8.2

Market Size

Based on the analysis presented in the earlier sections of this chapter, coatings used in protective and industrial (non-decorative) applications represent about 60 % of annual world wide production, or roughly 12 billion liters per year. Assuming that automotive and other applications outside the traditional finish applications represent roughly a third of this value, we estimate the world wide annual production to be approximately 8 billion liters per year for protective and industrial finishes, as we have defined them. North American production is estimated to be about 1/4 to 1/3 of this value, or roughly 2 to 3 billion liters per year. The penetration of waterborne coatings has been less in these markets than in decorative areas; assuming a value of 25 % yields an estimated annual production rate for waterborne protective and industrial finishes to be on the order of 2 billion liters world wide, and 0.5–0.7 billion liters per year in North America. Again, we emphasize the approximate nature of these estimates and provide them to give the reader a general idea of market size. 6.8.3

Industrial Maintenance Coatings

Industrial maintenance coatings are designed to provide corrosion control and extend the service life of large metal and concrete structures associated with manufacturing, chemical processing and transportation. Typical applications include bridges, storage towers, water tanks, chemical plants, oil refineries, water treatment plants, and electrical power plants. Solvent borne coatings predominate in this high performance application, with alkyds historically holding the largest share, and epoxies and urethanes showing very good growth in recent decades due to their strong performance profiles. The use of waterborne coatings in the light and medium duty segments of this market has grown significantly over recent years, and we estimate they now account for approximately 20 % of these segments in North America. 6.8.4

Key Performance Features

Early water based coatings for industrial maintenance applications utilized polymers developed for exterior decorative coatings. Although these polymers provided good gloss and tint retention in exterior applications, they were not optimized to provide good corrosion or chemical resistance. Better barrier properties were needed to im-

155

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6 Applications for Decorative and Protective Coatings

prove these performance features, and specialized maintenance binders were developed by reducing polymer molecular weight (to promote improved film formation) and utilizing more hydrophobic compositions (to provide better resistance to water and ion transport). A properly balanced composition is needed for optimum performance in maintenance applications; all acrylic polymers provide excellent durability characteristics, but are generally supplemented with more hydrophobic monomers such as styrene or ethylhexyl acrylate in order to provide improved barrier properties and corrosion resistance. Acrylonitrile is sometimes incorporated in order to improve chemical resistance, particularly in applications where resistance to gasoline and aromatic solvents is desirable. Good colloidal stability is particularly important in maintenance coatings; these coatings are often formulated with reactive pigments which can be difficult to stabilize, resulting in reduced gloss, or poor storage characteristics. In addition to these features, industrial maintenance coatings should show good adhesion to metal substrates, good hardness development and enamel-like resistance to marking and abrasion. 6.8.5

Formulation Characteristics for Industrial Maintenance Coatings

Industrial maintenance coatings are usually enamels, and are formulated in a manner similar to decorative gloss enamels: they are formulated significantly below critical PVC in order to enhance gloss, barrier properties and toughness. Urethane rheology modifiers are generally used to provide a more Newtonian rheology profile (good flow and leveling) as well as to avoid the weak flocculation and water sensitivity associated with cellulosic and HASE thickeners, respectively. A wide variety of reactive pigments are often used to enhance corrosion resistance, particularly in primer formulations. Many of these materials are inorganic salts, based on zinc, calcium or barium cations with anions of phosphate, borate, or metaborate. Effective coalescing agents are needed, since harder polymer compositions are generally used, and a combination of two or more coalescing agents is often used to optimize film formation, drying time and wetting characteristics. Finally, a variety of additives are commonly included in these formulations; like decorative enamels, these can include dispersants and surfactants for pigment incorporation and stabilization, defoamers, mar aids to increase abrasion resistance and glycols for improved pigment grinding and freeze-thaw stability. Table 6-7 illustrates a typical formulation used in industrial maintenance applications. 6.8.6

Standard Application and Performance Tests

Industrial maintenance coatings are evaluated by many of the standard stability, application and resistance tests used for decorative coatings. Like decorative finishes, they are applied in the field and need to have good heat-age and freeze-thaw stability. They should also have good appearance and application properties, and are generally evaluated for performance in spray, roller and brush application, as well as for

6.8 Protective and Industrial Coatings Tab. 6-7

Yellow industrial maintenance coating.

Material

Weight %

Grind Water Dispersant Defoamer Surfactant Ammonia (28 %) Titanium Dioxide Yellow Pigment Grind sub-total

5.34 0.75 0.21 0.21 0.11 6.94 6.94 20.49

Premix Styrene Acrylic Emulsion Polymer Ammonia (15 %)

68.51 0.32

Let down Add the grind to the above premix, then add the following ingredients Coalescent Propylene Glycol Methanol Sodium Nitrite (15 %) Ammonia (15 %) Thickener Total Property Total PVC Volume Solids Weight Solids

Comments Prepare in a high speed disperser

4.48 1.07 3.73 0.96 0.19 0.25 100.00

Hydrophobically modified polyacid Silicone emulsion Pigment wetting aid, non-ionic Base Universal grade Yellow iron oxide

Mix the following with good agitation Specialized maintenance vehicle

Add with good agitation Alkyl acetate ester Co-solvent, freeze-thaw aid Co-solvent, freeze-thaw aid Flash rust inhibitor Base HEUR (25 %)

Value 11 % 35 % 44 %

gloss, and color uniformity. Like exterior decorative finishes, the exterior durability of industrial maintenance coatings are commonly evaluated by exterior exposure testing, with particular emphasis being placed on corrosion resistance, gloss retention and color retention. In contrast to most decorative finishes, industrial maintenance coatings are also evaluated in the laboratory for corrosion resistance. Salt spray and prohesion testing are commonly used and both of these techniques use aqueous salt solutions and high humidity to accelerate the corrosion of steel test panels. Samples are prepared by coating steel test panels with a film of defined thickness and then drying for a specific time under controlled temperature and humidity conditions. The test panels are then scribed to expose bare steel beneath a portion of the coating, placed in the test chamber, and evaluated at periodic time intervals. Panels are subjectively rated, relative to controls, for blistering and corrosion on the face and at the scribe.

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6 Applications for Decorative and Protective Coatings

Salt spray and prohesion testing both suffer from the general problem discussed above in regard to accelerated exposure testing: corrosion resistance in laboratory tests does not always correlate well with performance in real world applications. Laboratory corrosion tests use a combination of aqueous salt spray and high humidity conditions to speed corrosion processes, and there is some question regarding whether they actually accelerate corrosion processes present under normal use conditions. Salt spray testing uses a high salt concentration (5 % NaCl by weight) and 100 % humidity at elevated temperature (35 °C) to provide a more aggressive testing environment. Prohesion testing utilizes lower salt levels (0.35 % ammonium sulfate and 0.05 % NaCl), and alternating cycles of salt spray and drying, at ambient temperature and 35 °C, respectively, to provide a somewhat more realistic, but slower evaluation. In spite of the simplicity and speed of these tests, it is generally acknowledged that realistic exterior exposures, preferably in the form of trials on actual metal structures such as bridges or storage tanks, are the best way to obtain useful information regarding the corrosion resistance of industrial maintenance coatings.

6.9

Traffic Marking Paints

Traffic marking paints are used to control the flow of vehicle and pedestrian traffic on a variety of surfaces such as roadways, parking lots, walkways and airport runways. Since the late 1980s, waterborne traffic markings have enjoyed strong growth in the United States and in parts of Europe and Asia. This is due not only to increased environmental sensitivity, but also is due to improved performance features which have been incorporated into waterborne acrylic emulsion polymers for traffic marking paints. We estimate the total world wide market for traffic marking paints to be around 400 million liters per year, with the North American market representing about 120 million liters of this total. Most of the traffic paint market in North America has shifted over to waterborne technology and the share of water based traffic coatings is now estimated to be greater than 80 %. 6.9.1

Description of Traffic Paint Market

Road-marking paints have been in use since the 1920s; however, traffic paint technologies have changed dramatically over this period. Early paints were based on oilmodified phenol-formaldehyde resins, which were later replaced by alkyds or chlorinated rubber blends. In the early 1990s, the technology shifted toward more environmentally friendly and higher performing waterborne acrylic systems. The enabling technology which allowed for the widespread use of waterborne traffic paints was the development of fast drying acrylic emulsion polymers in the late 1980s. These products use proprietary technology to provide waterborne traffic paints with the ability to dry rapidly under a wide range of relative humidity and airflow conditions. Due to their acrylic compositions, they also offer improved glass bead reten-

6.9 Traffic Marking Paints

tion, which leads to better retention of retro-reflectivity (the ability of a material to reflect light back towards the source from a variety of incidence angles) and nighttime visibility. 6.9.2

Key Performance Features

The success or failure of road-marking paints depends on their ability to: (i) dry quickly enough to prevent damage by traffic following the striping truck; (ii) adhere to the road surface (concrete or asphalt) during the expected lifetime of the coating; and (iii) retain a large percentage of the glass beads applied to the coating surface for nighttime visibility. In addition, various governmental agencies can mandate specific requirements for viscosity, color, VOC content, percent solids, opacity, etc. 6.9.3

Typical Traffic Paint Formulation

Formulations for waterborne traffic markings differ substantially from those of decorative coatings. The volume solids of waterborne traffic paints are quite high, around 60 %, in order to minimize the amount of water, and to thereby speed the drying process. Traffic paints are also formulated near or above CPVC, in the range of 55–60 %, to provide higher porosity and increased drying speed. They contain many of the components found in the waterborne coatings discussed previously, such as dispersants, defoamers, and rheology modifiers. As with any paint, these components must be chosen with care so as to not detract from the desired performance characteristics. Table 6-8 illustrates a typical formulation for paint used in traffic marking applications. 6.9.4

Standard Application and Performance Tests

Fast drying is an important characteristic for traffic marking paints, and a modification of the standard coating dry-through test (also called the early washout test) is used to evaluate the time needed for a traffic paint to become resistant to washing off with water (presumably rain). In this test, a draw-down with a film thickness of 330 µm wet is placed in a humidity chamber maintained at 90 % relative humidity with negligible air flow. The dry-through time is defined as the time required for there to be no surface deformation when the test operator’s thumb is twisted through an arc of 90° with minimal pressure on the paint film. While the coating has not dried in the conventional sense by this time (all moisture has not yet left the film), the values obtained in the dry-through test provide a useful measure of the time needed for a coating to become resistant to being washed away by rain.

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6 Applications for Decorative and Protective Coatings Tab. 6-8

Traffic marking paint.

Material

Weight %

Grind Acrylic Traffic Paint Emulsion Dispersant Wetting Agent Defoamer Titanium Dioxide Calcium Carbonate Extender Grind sub-total

32.40 0.36 0.20 0.21 7.11 54.07 94.35

Let down Methanol Coalescent Defoamer HEC Thickener Water Total

2.13 1.31 0.18 0.68 1.34 100.00

Property Total PVC: Volume Solids: Weight Solids:

Value 60 % 61 % 78 %

Comments Prepare in a high speed disperser Utilizes rapid set technology Ammonia neutralized, polyacid

White pigment Coarse grade

Mix grind for 15 minutes, and add with good agitation Co-solvent Ester-alcohol, film formation Hydrocarbon type 2.5 % in water.

A traffic marking paint also needs to quickly become resistant to tire pickup after application to the roadway. This property is commonly evaluated in the laboratory by the no-pick-up test. In this test, a steel cylinder, outfitted with rubber O-rings, is rolled over the surface of the drying paint film at specified times. The no-pick-up time is defined as the point at which paint does not adhere to the rubber rings when the cylinder is rolled across the film. The auto-no-track test is a complementary field evaluation method which also measures the time for a traffic marking paint to become resistant to tire pickup. This test is carried out by passing a moving automobile over a freshly applied transverse or diagonal marking line, and determining the minimum time required for there to be no indication of pick-up and re-deposition of the line by an observer standing at a distance of 15 m. Retro-reflectance is a quantitative measure of a traffic marking’s nighttime visibility. The retention of retro-reflectance is determined in the field, and is related to the ability of a traffic marking paint to retain small, reflective glass beads which are dropped onto the coating surface during the coating application process. It is measured either by a portable or truck-mounted retro-reflectomer. Often, specifications will call for initial minimum values as well as some minimum throughout the lifetime of the marking. Requirements can vary, but typical initial values for white markings are on the order of 250 mcd m–2 lux–1 while yellow markings are somewhat less (ca. 150 mcd m–2 lux–1) owing to their lower TiO2 content.

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References 1 Connolly, E., Ishikawa-Yamaki. M.,

2 3

4

5

6

7

8

9

Paint and Coatings Overview, Chemical Economics Handbook, SRI International, 592.5100 C, 1999. Tulo, A., Chem. Eng. News 2000, 78(41), 19–28. Patton, T.C., Paint Flow and Pigment Dispersion, 2nd edn, Wiley-Interscience, New York, 1979, pp. 126–204. Wicks, Z. W. Jr., Jones F.N., Pappas, S. P., Organic Coatings, Science and Technology, Vol. II, Wiley-Interscience, New York, 1994, 55–64. Wicks, Z. W. Jr., Jones F. N., Pappas, S. P., Organic Coatings, Science and Technology, Vol. I, Wiley-Interscience, New York, 1992, 35–48. Hare, C. H., Protective Coatings, Fundamentals of Chemistry and Composition, Technology Publishing Company, Pittsburgh, 1994, pp. 37–62. Friel, J. M., Acrylic Polymers as Coatings Binders in Paint and Coatings Testing Manual, 14th edn of the Gardner Sward Handbook, Koleske, J. V. (ed.), American Society for Testing and Materials, Philadelphia, 1995, pp. 39–52. Sperry, P. R., Snyder, B. S., O’Dowd, M. L., Lesko, P. M., Langmuir, 1994, 10, 2619–2628. Nicholson, J. W., in: Waterborne Coatings, in: Surface Coatings – 2, Wilson, A. D., Nicholson, J.W., Prosser, H. J. (eds), Elsevier Applied Science, London, 1988, pp. 1–38.

10 Wang, Y., Kats, A., Juhue, D., Winnik,

11

12

13

14

15

M. A., Sivers, R. R., Dinsdale, C. J., Langmuir, 1992, 8, 1435–1442. Lowell, J. H., Coatings, in: Encyclopedia of Polymer Science and Engineering, Vol. 3, Kroschwitz, J. I. (ed.), Wiley-Interscience, New York, 1985, pp. 615–675. Leman, A. A., Paint in: Encyclopedia of Chemical Technology, 4th edn, Vol. 17, Kroschwitz, J. I. (ed.), Wiley-Interscience, New York, 1996, pp. 1049–1069. Koleske, J. V. (ed.), Paint and Coatings Testing Manual, 14th edn of the Gardner Sward Handbook, American Society for Testing and Materials, Philadelphia, 1995. Annual Book of ASTM Standards, Vols 6.01 and 6.02, American Society for Testing and Materials, Philadelphia, 2001. Bauer, D. R., Martin, J. W. (eds), Service Life Prediction of Organic Coatings, A Systems Approach, American Chemical Society/Oxford University Press, Washington, 1999.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

7

Applications for Automotive Coatings Sunitha Grandhee

7.1

Introduction

The automotive coatings industry is faced with new challenges as we enter into the new millennium. Environment legislation, cost and quality in addition to globalization and market constraints are driving new developments. Increasingly stringent environmental legislation and tougher market conditions require that modern coating systems must bring, not just enhanced product performance but also reduced overall production costs to paint producers. Daimler-Chrysler, Ford and General Motors in the United States, has formed a consortium, the United States Council for Automotive Research (USCAR) which now is the umbrella for 11 other research consortia including the Low Emissions Paint Consortium (LEPC). LEPC is tackling the technical challenge of developing paint-related technologies to reduce or eliminate VOC from automotive coatings. The movement towards lower to zero VOC involves many types of coatings technologies. Need to reduce VOC has led to change in many raw materials that were traditionally used in coatings. Furthermore, manufacturers often want their coatings modified so that they can be used at faster production rates, baked at lower temperatures, or changed in color. An approach to this problem is by the appropriate modification of acrylic primary dispersions to suit automotive coating requirements or solvent-free secondary dispersions of conventional resin types like epoxies, polyesters, polyurethanes. Today, the latest water-borne coatings are much more robust in terms of usage or application friendliness and require significantly less heating or air-conditioning than two decades ago. They still require some additional dehydration or special kinds of flashes before going into ovens to help remove the water. The biggest manufacturing industry in the world today is the auto industry, with over seventy separate companies or subsidiaries, employing 4 million people. In the United States over 15 million cars are produced annually. It is estimated that motor vehicle production on a global basis will rise from 54.9 million in 1998 to some 59.3 million by 2003 [1].

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Europe $8.9 B Asia $3.1 B

Africa, Middle East, Latin America $1.6 B North America $7.5 B

Fig. 7-1

Original equipment manufacturers (OEM) coatings demand

1999.

Demand for motor vehicle coatings is forecast to rise by 1.8 % per year to 2.0 million metric tons by 2003, with OEM coatings posting 1.5 % growth per year to 1.2 million metric tons. In 1999, Europe was the largest market for coatings for OEM end markets at $8.9 billion. North America was second having used $7.5 billion in OEM end markets. Asia, with faster economic growth than Europe or North America, last year spent $3.1 billion on OEM coatings (Fig. 7-1). The rest of the world, which includes Africa, the Middle East, and Latin America, consumes $1.6 billion in OEM coatings, according to P.G. Phillips. The market for automotive OEM coatings alone is growing at 3.2 % per year [2]. 7.1.1

History of Automotive Coating

The history of automotive paint dates back to the beginning of the 20th century, when the mass production of automobiles started [3, 4]. Mixtures of ground pigments and linseed oil – like old wood coatings used for carriages and stagecoaches – were brushed on the surface and allowed to dry. The coating was then sanded smooth and refinished in the same manner. These products were not colorful. Henry Ford always said, “You can have a car any color you like as long as it is black.” In the 1920s nitrocellulose-based enamels were applied, offering a wider range of color choices to the market.

7.1 Introduction

During the early 30s the auto industry started using “stoving enamels” based on alkyd resins. These enamels were selected to improve gloss and gloss retention. The introduction of the spray gun technique made automotive coating much faster than using the brush method. It minimized sanding between coatings and applied the product evenly. Alkyds as a whole proved to be more durable and faster drying than nitrocellulose enamels. This product and process was the system of choice for most vehicle manufacturers until the 1950s [3, 4]. In the mid 1950s the next great technology leap happened. Acrylic lacquer, while not markedly better in terms of performance qualities over alkyd enamels, did possess one outstanding trait: it was incredibly fast drying as compared to enamels of the time. Automobile companies such as General Motors immediately saw the production time savings as a real plus. The coating was applied to the vehicle surface with a spray gun. At that point the product, still wet, contained a large amount of solvents. Baking the vehicle in a large oven caused the solvents to evaporate and the product to flow to a uniform smooth finish. In 1960 the Ford Motor Company went back to the stoving methods. They did this after realizing that consumers made a vehicle purchase using mainly their eyes: “shiny sells”. Ford also decided that they liked many of the properties that the early acrylic resins provided. They went to work with yet another new group of suppliers to create “acrylic stoving enamels”. This product was also applied with a spray gun. It had a very high gloss, was durable and was oven cured to produce a hard and colorful surface. Using polyisocyanates dramatically magnified and improved acrylic enamel’s performance qualities like gloss, hardness, durability [3, 4]. Throughout the 50s and 60s, corrosion was the major cause limiting automobile’s life span. Cationic electrodeposition of a protective coating was the major invention in the late 1960s, eliminating corrosion as a major cause of automotive failure [5]. In this process electrically charged paint particles are deposited from aqueous solution onto metallic substrates by application of an electrical field. Today, 99 % of all vehicles manufactured use some type of electrocoat process. During the 1970s Japanese and European major paint companies developed the next technology change, the application of two-coat acrylic painting systems: basecoat/clearcoat. They were also successful at providing the consumer with metallics or metal flake paints. German automakers influenced this change with their color-plus-clear combinations on such premium vehicles as Mercedes Benz. The technology is a one-stage acrylic flat basecoat followed immediately by a high gloss urethane crosslinked clearcoat. This results in excellent durability to corrosion and stone chips, and very high gloss [3, 4]. In the late 80s and early 90s new laws were enacted that governed the content and application of paints. The amounts of volatile organic compounds (VOC) were lowered using water-borne binder systems. Automotive paint systems are now well within VOC limits and comply with EPA standards for emissions. Today approximately 14 million vehicles are coated with water-borne technology each year. Out of the 99 North American assembly plants, 33 plants are using water-borne based coats [6]. The main suppliers of OEM coatings are PPG, Dupont, BASF, Nippon and Kansai Paint company.

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Another approach to VOC reduction is the use of powder coatings, however, there is a lot of cost in converting existing spray booths to suit powder application. Hence, a variation on this theme is to use slurries of powder in water as automotive clearcoats. A new clearcoat system has been introduced, that offers zero emissions, is powder dispersed, and is stabilized in water [7]. This method is used commercially by Daimler-Chrysler in Europe. Bayer recently won the Presidential Green Chemistry Challenge Award for its twocomponent water-borne polyurethane system. Generally, isocyanates, one of the raw materials for polyurethanes, are unstable in water. Trying to make water the carrier for isocyanates was no easy task. Bayer modified isocyanate molecules to stabilize them in water, so customers can apply them together with polyols, just prior to painting, to make polyurethane coatings [8].

7.2

Automotive Coating Layers

Current automotive coatings are made up of a number of distinct layers (Fig. 7-2), the coatings are either spray applied or electrodeposited. Clearcoat Basecoat Primer Electrocoat

(+)

( -) Automotive OEM coating layers.

Fig. 7-2

7.2.1

Spray Coating

OEM automotive coatings are those used for painting trucks and cars on fast moving assembly lines. Stringent conditions are established for surface preparation, application and curing. Typically the clearcoat, basecoat and primer are spray applied while the electrocoat is electro-deposited by dip application. Drying and curing usually involves energy input such as heat and UV radiation to produce coatings of higher toughness, solvent resistance and uniformity of appearance. The layers should cure to a desired finish and should have a wide tolerance for bake conditions. Low bake temperatures are desired to save energy cost. The various coating layers, with their thickness and functions, are shown in Tab. 7-1. Automobile bodies are generally fabricated from steel [9–11], therefore corrosion protection is one of the most important functions of automotive coatings. After the fabrication of the car body, the surface is coated (phosphate coating) by dipping into

7.2 Automotive Coating Layers Tab. 7-1

Function and typical thickness of automotive coating layers.

Layer

Thickness

Function

Clearcoat

40 µm

Basecoat Primer Electrocoat

15 µm 35 µm 20 µm

Withstand solar radiation, atmospheric pollution (acid rain, bird droppings, aggressive chemicals like road salts and caustic detergents) Optimum appearance and long lasting color Good adhesion and resistance to chipping Long-term corrosion protection

an aqueous solution containing primary zinc phosphate Zn2(H2PO4)2 as the major component. For parts of the exterior, plastic and rubber components are used, which in some cases are completely coated before assembly. The use of plastics as a substitute for metals began accelerating in the 1970s to achieve a weight reduction in order to improve fuel economy. Polymers used as binder Table 7-2 shows the emulsion polymers most often used in the various automotive layers. Tab. 7-2

Water-borne binders used for automotive coatings.

Coating

Resin chemistry

Dispersion

Type

Electrocoat Primer Basecoat Basecoat Basecoat Basecoat Basecoat

Epoxy-amine Polyesters Polyacrylics Polyacrylics Polyesters Polyurethanes Polyurethane-acrylics

Secondary Secondary Secondary Primary Secondary Secondary Secondary

Cationic Anionic Anionic Anionic Anionic Anionic/nonionic Anionic

Water-borne emulsion polymers are used in formulating automotive coatings for electrocoat, primers and basecoats. Clearcoat polymers are primarily still solventborne. A powder dispersed in water (powder slurry) has been developed as a binder for clearcoats [7]. For the purpose of this chapter’s discussion, electrocoat, is not referred to as primer since an additional layer is present before the vehicle is top coated. Most current automotive passenger cars are coated with four coating layers in addition to the phosphate coating, as shown in Fig. 7-2 and Tab. 7-1. Furthermore basecoat here, in this chapter is referred to an independent layer, even though basecoat is considered as part of the automotive topcoat system. Automotive OEM coatings are thermosetting coatings, i.e. a chemically crosslinked matrix is formed between the main resin molecules. This matrix cannot be returned to its original form by use of solvent or heat.

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7 Applications for Automotive Coatings

Aqueous polymer dispersions used in automotive water-borne coatings are classified as: – primary dispersions: polymerization is carried out in the presence of water by emulsion or mini emulsion polymerization. – secondary dispersions: the polymer is synthesized in an organic solvent and later dispersed in water. Based on the nature of the solubilizing group, the polymer is classified as anionic, cationic or nonionic. The higher the concentration of the polar groups, the greater is the solubility of the polymer. Incorporation of functional groups in the polymer skeleton is necessary for stabilization in aqueous phase [12]. Formulation ingredients The amounts of main ingredients present in a water-borne automotive coating formulation are shown in Tab. 7-3; the functions of these ingredients are summarized in Tab. 7-4. Tab. 7-3

Main ingredients (%) of for automotive coatings, RCA: rheology control agent. E-coat

Solids (resins, crosslinker(s), pigments) Solvents Additives (RCA, defoamer, wetting agents) Water

Tab. 7-4

17–24 2– 3 1– 2 71–80

Primer

35–45 5– 7 2– 3 45–58

Base coat Metallic

Solid color

23–25 10–15 2– 3 57–65

25–30 10–15 2– 3 52–63

Function of ingredients in an automotive coating formulation.

Ingredient

Function

Aqueous polymer dispersion Crosslinker

Film formation, mechanical properties Reacts with functional groups on main resin to form crosslinked matrix Provides color, hiding Provides appearance effect Reduces viscosity, controls rate of drying and film formation Inhibits degradation of film by sunlight Drives crosslinking reaction Pigment dispersion, adhesion promotion Decrease foaming tendencies Control leveling, sag, aligns aluminum flakes (metallic basecoats) Improve substrate wetting, flow, pop tolerance, etc.

Pigment dispersion Flake pigments Solvents UV absorber Catalyst Secondary resins Antifoamer/defoamer Rheology control agents (RCA) Other additives

Standard test methods The standard test methods for automotive water-borne coatings are summarized in Tab. 7-5.

7.2 Automotive Coating Layers Tab. 7-5

Test methods for automotive coatings. Test

Method

Description

E-coat Basecoat, primer Basecoat, primer Basecoat, primer Basecoat, primer E-coat, primer, basecoat

Stomer ICI cone/plate Brookfield Ford viscosity Dip type Film build

Viscosity Viscosity Viscosity Viscosity Viscosity Thickness

E-coat

Salt spray resistance

E-coat, primer

Gravelometer

ASTM D562 ASTM D4287 ASTM D2196 ASTM D1200 ASTM 4212 ASTM D1186, ISO 7253, ISO 4623 ASTM B117, SAE J2334, DIN 53167 J 400

E-coat E-coat E-coat, primer, basecoat

Hardness (pencil) Impact (Gardner) Humidity

Topcoat, primer

UV

ASTM D3363 ASTM D5420 ASTM D1735, ASTM D2247, ASTM D4585, DIN 53209 SAE J2020

Topcoat, primer

Xenon lamp

SAE J1960

Topcoat, primer

Sunshine/ carbon arc Tape test Exposure Exposure

ASTM G 152

Topcoat, primer Topcoat, primer Topcoat, primer

ASTM D3359 SAE J 951 SAE J 1976

Corrosion, J2334 preferred to B117 Chip resistance testing, testing coating flexibility, adhesion and overall resistance to chipping damage by stones Hardness Hardness Adhesion and appearance. Condensing humidity (D2247) and water soak (D1735) Appearance (gloss, cracking , chalking) Weathering test Accelerated test to check for gloss loss Accelerated test to check for gloss loss Adhesion Durability, Florida exposure Outdoor weathering of exterior materials

The J tests were developed by the Society of Automotive engineers, USA (www.SAE.org)

The test methods for automotive layers are designed to stimulate conditions likely to occur in the field and are of two types – appearance and performance. Appearance includes gloss, distinctness of image, mottle, orange peel. Performance properties include physical properties of hardness, flexibility, impact resistance, adhesion, stone-chip resistance, cold crack resistance i.e. stability to extremes of temperature and humidity, curing efficiency. Polymer dispersions are applied as coating binder mainly in three of the four layers: electrocoat, primer and basecoat. These layers are now described in more detail.

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7 Applications for Automotive Coatings

7.2.2

Electrocoat

Electrocoat or e-coat or Elpo is the first pigmented coating layer, which is applied over the phosphate coating of the fabricated car steel body. It serves as a bridge between the metal and the overlaying coating layers. This coating layer is applied in a dip application by cathodic electrodeposition to the steel automobile body. Dip application has the advantage to fill the smallest recessed areas of the automobile body. This complete coverage ensures excellent corrosion protection to the automobile. Cationic electrocoat applied by dipping process is used worldwide for coating autobodies and its adoption in 1970s and 1980s led to major improvement in the corrosion resistance of cars. Until mid 1970, electrocoat was of the anodic type [13–15]. In 1976, PPG introduced the first cathodic primer, and this technology, with continuous improvement, has become the standard of the automotive industry worldwide. Combined with the more recent introduction of galvanized sheet metal, car manufacturers are now able to offer ten years warranties against corrosion. Requirements Current systems are characterized by excellent corrosion protection, good throwing power and good filling properties at film thickness of ca. 20 µm. Polymers used The resins used in electrocoat, must have excellent hydrolytic stability and resistance to salt accelerated corrosion and the chemical composition of the resin also allows for excellent adhesion of the next coating layer. Most current cathodic systems are based on modified epoxy resins containing amino groups. These are dispersed in water by neutralizing the amino groups with organic acids such as formic, acetic or lactic acid. Bisphenol A epoxy resins are reacted with polyamines to yield a resin with amine and hydroxyl groups. The resulting polymer is reacted with a polyisocyanate, which is partially blocked with an alcohol (e.g. 2-ethylhexyl alcohol). Salts are formed with the amine groups with a low molecular weight carboxylic acid. The epoxy backbone is made flexible by various ways by incorporation of polyester and polyether diols [16], acrylic grafts [17]; low-Tg aliphatic epoxies [18]; fatty acids [19] and fatty monoepoxies [20]. The resins are crosslinked by blocked isocyanates, Mannich reactions or re-esterification. Blocked isocyanates which are used as crosslinking agent are stable in the slightly acidic water system, whereas melamine formaldehyde resins are not. During baking, the blocked isocyanate reacts with a hydroxyl group to form a urethane crosslink. Amine-substituted resin binders provide greater corrosion protection for steel, perhaps owing to strong interaction between the amine groups and the substrate surface that increases wet adhesion, which is the most critical factor for corrosion protection. Cationic polyurethane dispersions are obtained by incorporating tertiary amine functionality into the backbone, either by introducing tertiary amine groups in the

7.2 Automotive Coating Layers

diol, instead of a carboxylic group, followed by quaternization with an alkylating agent or protonation with a suitable acid [21]. Other coating systems contain cationically modified copolymers obtained by polymerization of acrylic monomers in presence of unsaturated polyurethane macromonomer [22], and water-dilutable dispersions of cationically modified and urethane modified methacrylic copolymers obtained by solution polymerization [23]. Composition Most of the commercial cathodic electrocoat formulas are two-pack formulas consisting of a pigment dispersion intermediate and the principal resin components [24].

Pigment dispersion: Aqueous dispersion resin (epoxy amine isocyanate adduct) Extenders Anticorrosive pigments Organometallic oxides Deionized water Principal resin component: Epoxy amine adduct Crosslinker Organic acid Organic solvent Deionized water

10–15 % 20–30 % 3–7 % 1–2 % 65–46 %

20–25 % 10–12 % 0.5–1.0 % 1–3 % 68–69 %

Pigments commonly used are titanium dioxide and extender pigments. Special lead-containing pigments e.g. silicates are used as anticorrosive agents. Lead also catalyzes the curing reaction, however the trend nowadays is eliminate lead from electrocoat. Solvent content is low (typically less than 2 % based on total volume). The solids content is ca. 20 %. The application technique allows coating of complicated shapes and even internal areas. A coating of uniform thickness is obtained after baking at 150–180 °C for ca. 20 min. Application The car body is coated on a production line by immersing the body in a tank containing the aqueous primer dispersion and subjecting it to a direct current charge. The applied voltage causes the dispersed particles and pigments to migrate to the car body. As they are deposited, the consequent transfer of electrons provides an electrically neutral film deposit. During the process electroendoosmosis occurs, squeezing the water out of the deposited coating and leaving it in a firm state. With this process, improved uniform coverage is achieved in recessed areas and on sharp edges as well as on flat surfaces. The body is baked to coalesce and cure the primer film with much less sagging occurring.

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Electrocoat bath: Coatings stable at a pH a little below 7 are preferred. Viscosity: 20–50 mPa s at 25 °C Bath solids: 15–25 % (1 h at 110 °C) Water: 64 % Deionized water Solvents: 1–4 % Organic solvent Bath pH: 5.0–6.0 Bath conductivity (20 °C): 0.8–1.5 mS cm–1 Cure schedule at metal temperature: 165–170 °C for 20 min 7.2.3

Primer

Primer or primer surfacer is spray applied over the electrocoat before applying the basecoat. The main function is to minimize surface roughness and improve adhesion of the basecoat. The uniform thickness provided by the e-coat, makes it smooth and glossy which makes the adhesion to basecoat very difficult. The primer also protects the light sensitive cathodic electrodeposition layer from exposure to light. Historically the basic function for spray primers has been corrosion resistance and preparation of a surface to receive a top coat. With the advent of electrocoat in the automotive industry corrosion resistance has become less of a issue for spray primers. The corrosion function of spray primers has centered around protecting against sanding cut throughs (sanding to bare metal) on the electrocoat primer. Presently the main purpose of spray primer is the preparation of a surface to receive a top coat (basecoat/clearcoat). Other properties that are presently of great concern for spray primers are yellowing, adhesion, chip resistance, sandability, leveling, UV durability and smoothness of the coating with respect to the surrounding coatings (electrocoat and topcoat). A current trend is to use color key primers; the colors are picked for use under a group of top coats with related colors. A chip-resistant primer called anti-chip is frequently applied over the electrocoat on the lower parts of the car body. It is designed to be especially resistant to impact by stones thrown up from a road against the car body. Requirements The primer must have good chip resistance and exterior durability. Weakness in the layer will lead to UV radiation degradation causing loss of adhesion of primer to basecoat/clearcoat and ultimately delamination. Polymers used The development of chip resistant water-borne primer-surfacers has benefited from the use of predominantly water-borne polyester and polyester-polyurethane resins. Polyesters which are dissolved or dispersed in water by neutralizing acid groups with amines are crosslinked with a suitable melamine resin. Aqueous polyurethane or acrylic modified polyurethane systems are also slowly entering the primer market.

7.2 Automotive Coating Layers

Water-borne polyesters used for automotive coatings have both hydroxyl and carboxylic groups as terminal groups. Usual acid numbers are in the range of 35–60 mg KOH g–1 resin, to give amine salt solutions in solvent that can be diluted with water to give reasonable stable dispersions of aggregates of resin molecules swollen with water and solvent. To give acid functionality, 2,2,-bis(hydroxymethyl)propionic acid is used as one of the diol components. Hydrolytic stability is also affected by the choice of the polyol. In addition to the steric effect, it has been shown that polyols with low water solubility give polyesters that are more stable against hydrolysis under basic conditions than those with higher water solubility, presumably because the polymers are more hydrophobic. Since many of the polymers are hydroxyl functional, they are cured with various melamines and blocked isocyanates. The polymers are mainly stabilized in the water phase by neutralization of anionic groups with amines which are volatile (dimethylethanolamine, 2-amino-2-methyl-1-propanol). To avoid emulsifiers, the polyesters are copolymerized with hydrophilic monomers. In the case of linear high molecular weight polyesters, often sodium 5-sulfoisophthalic acid, polyethylene glycol are copolymerized. However, the copolymerization increases the melt viscosity and decreases water resistance and adhesion. Water-borne polyesters which are of the acrylics-grafted type form stable aqueous dispersion. They consist of “core-shell” particles with a core of high molecular weight polyester. Small particle diameters were obtained by use of polyesters having the largest amount of unsaturated bonds unless gelation occurs [24–30]. Thermosetting water based polyester resin coating composition prepared from polyalkadienediol may be directly applied to wet electrodeposited coating [31]. Composition of primer Primers are highly pigmented systems, containing titanium dioxide in combination with extender pigments such as silicates or barium sulfate, carefully selected to improve the paint attributes (leveling, sanding, humidity resistance). UV absorbers and HALS stabilizers can be added to improve UV resistance. The pH of the system is maintained between 7 and 8. Application and testing Primer coating are spray applied onto the electrocoated substrate. Typical wet phase testing includes seed check, gloss, weight per gallon, viscosity, appearance, humidity sensitivity, settling , impact resistance, topcoat adhesion, solvent resistance, filterability and sand scratch telegraph. For the anti-chip-appearance, weight per gallon, viscosity, solids, sag and solvent pop and gravelometer-chip resistance test are used. 7.2.4

Basecoat

Basecoat is the layer which contains the color pigments. This layer is covered by the transparent coating (clearcoat).

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Basecoats – along with the clearcoat also called the topcoat system – form protective layers over the car body surface and are very important as decoration. They have the characteristics of: – full and deep gloss – highly brilliant solid or metallic color effects – long-lasting resistance against weather and chemical influences – easy to repair and polish In terms of appearance, a significant trend in automotive original equipment finishes has been influenced the dramatic growth in the use of basecoat-clearcoat finishes to replace single-stage pigmented topcoats. Basecoat-clearcoat finishes provide a “wet” appearance, previously associated with European vehicles, in that the appearance of the basecoat is enhanced by the transparent clearcoat. Because of the smoothness of the surface and clarity of the film of the clearcoats, the gloss and distinctness of the image of these multi-stage finishes has been widely accepted as the standard of appearance in both the automotive original equipment and refinish coatings markets. The basecoat/clearcoat system consists of a colored layer (basecoat) which is overcoated after a brief flash off time with a protective layer of clearcoat. Both layers are cured together at about 120–140 °C. The basecoat contains pigments which provide solid (straight) shades or metallic finishes. In order to reduce the emissions of VOC, water-borne basecoats have been developed, which may contain only up to 20 % co-solvents. Composition of basecoat The pH of the system is maintained between 7.0 and 8.0. Approximately 10–15 % by weight is comprised of solvents. The trend nowadays is to go with HAPS (Hazardous Air PollutantS) compliant solvents like monoethers of propylene glycol. Solvents help in achieving good flow and leveling of the coating after it is sprayed. They also help in proper alignment of the metal flakes of a metallic coating. A UV absorber in the top coat that strongly absorbs UV in the wavelength range of 290–380 nm also helps to protect the primer from degradation. HALS stabilizers are added to improve UV resistance. Water-borne basecoats contain crosslinker building a polymer network and ensuring film stability and durability. Water-borne basecoats also contain rheology modifiers e.g. polyurethane thickeners [32], polyacrylic acids [33] and pigment like additives (metasilicates, colloidal silicon dioxide). Metallic pigments are frequently incorporated into the basecoat to provide the appearance phenomenon known as the geometric metamerism or “color-travel”. Coatings with geometric metamerism display different hue and brightness when viewed at different angles. This effect is used by automotive stylists who specify metallic basecoat–clearcoat finishes to draw viewers eye the subtle contrast in hue and brightness found in styling lines and curvatures in the vehicle. Even though, water-borne coatings have been the most popular approach to VOC reduction and there has been a substantial reduction of solvents on going from solvent-borne systems to water-borne systems, the trend now is to eliminate solvents

7.2 Automotive Coating Layers

Solvent-borne basecoats

Water-borne basecoats

Resin 22% Resins 23%

Pigment 2%

Pigment 2% Organic Solvent 12% Water 63%

Organic Solvent 76%

Fig. 7-3

Solvent and water-borne basecoat compositions.

almost completely. Fig. 7-3 shows the differences in composition between solventborne basecoats and water-borne basecoats. Binders used Anionic coatings systems for water-borne topcoats are emulsion polymers, miniemulsion polymers, polyurethane dispersions, different types of dispersions of acrylic resins in water and amino resins, water-borne polyesters, polyurethanes. Many of the polymers are hydroxyl containing and cured with various melamines and blocked isocyanates. The polymers are mainly stabilized in the water phase by neutralization of anionic groups with volatile amines (2-amino-2-methyl-1-propanol). Cross-linkers like aminoplast resins, alkoxy silanes, blocked epoxy resins, carbodiimides can be used. Application and testing Typically, line speeds for spray application of basecoats are lower in Europe compared to United States. Furthermore, turbine pumps are used for circulation of water-borne basecoats in United States, while piston pumps are used in Europe. The main tests for basecoats paints are: solids content, viscosity, stability at room temperature/hot box 43 °C/56 °C. Basecoat–clearcoat testing depends strongly upon customer requirements, e.g. film failures (popping, cratering, seeding), flow and leveling, color matching, gloss and effect (in case of metallic systems). There are different stability tests: mechanical stability (hardness and flexibility) and stability against environmental influences (rain, humidity, light, higher temperature). To test the stability of topcoats against the effects of weather (mainly sunlight, humidity, temperature and fallout against pollution) coated panels are exposed for several years in special places (Florida, Arizona, Okinava, a southern Japanese isle and Alunga in northern Australia). For trials, there is the possibility to run shorter test times in accelerated weather machines (weather-o-meter) which try to model the natural conditions. There are many other requirements including: resistance to car brushes, bird drop-

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pings, acid rain, sudden thunder showers on a car that has been sitting in the hot sun, the impact of pieces of gravel striking the car, gasoline spillage and so on.

7.3

Properties of Water-borne Binders used for Automotive Coatings 7.3.1

Emulsion Polymers

Emulsion polymers are binders of choice for automotive water-borne basecoats applications, because of their low cost and processing requirements. A particularly valuable element of acrylic emulsion polymer chemistry is the ability to incorporate a broad range of functional chemical groups into the polymer chain via ester, amide carbamate, derivatives of acrylic or methacrylic acid. These functional groups can provide sites for crosslinking, adhesion, compatibility with other polymers, postpolymerization reactions, biochemical activity, etc. Acrylic latexes are increasingly becoming popular for basecoat automotive applications, because of their resistance to photodegradation and low cost, in addition to their well-known features including safe handling, low toxicity, low odor, and easy clean-up. A number of patents exist in the literature, wherein core shell polymers have been suggested to be used for waterborne basecoats. To achieve good flow properties, a low minimum film-forming temperature, good stability to agitation and good adhesion to other coating layers, multiphase emulsion polymers are used, where the latex particles are small in size, the core material is hard and the shell is soft and the latter is made of strongly hydrophobic monomers and a relatively large proportion of monomers carrying carboxyl groups [34–36]. Acrylic core-shell polymers have been used as principal polymers for aqueous metallic basecoat paints [37–40]. The anionic shell allows pseudoplastic flow behavior which ensures parallel orientation of the aluminium pigments in the wet paint film. This orientation and the low solids content are responsible for the metallic gloss and high color flop (change in color observed on varying the viewing angle) of the basecoats. For thermoset automotive coatings cross-linkable polymer dispersions are used [41–47]. Compositions containing methylol(meth)acrylamide can be used for very low VOC water-borne coatings. Relatively low-Tg polymers that coalesce well without coalescing solvents are applied and subsequent crosslinking will give the required film properties. Water-soluble or a water-dispersible alkylated melamine formaldehyde crosslinking agent or a polymeric partially methylated melamine formaldehyde resin having a degree of polymerization of approximately 1–3 are used frequently. Such compositions have been used for automotive quality clear coat and/or pigmented color coat for automobiles and for an automotive quality primer composition [48]. Functionalized latexes in baked coatings can be crosslinked with aminoplast resins, alkoxy silanes, blocked isocyanates, epoxy resins and many other cross-linkers.

7.3 Properties of Water-borne Binders used for Automotive Coatings

To improve the water resistance, branched vinyl esters with long hydrophobic chains can be used [49]. Emulsion polymerization is carried out as a semi-continuous batch process [50]. The polymer particle size is between 50 and 500 nm. 7.3.2

Microgels

Microgels are used as rheology control agents (RCA) for solvent-borne basecoats and clearcoats. They are crosslinked microparticles made by emulsion polymerization using monomers like ethylene glycol dimethacrylate, allyl methacrylate or divinylbenzene. They are insoluble in the aqueous medium and are stable towards gross flocculation. The chemical composition and degree of crosslinking of the microparticle may be such that it has a Tg below room temperature in which case the microparticles will be rubbery in nature, alternatively, it may be such that the Tg is above room temperature, that is to say the particles will be hard and glassy. The polymer micro-particles can be dispersed in the basecoat composition in a state in which, even at low solids contents, the dispersion contains a few if any multi-particle aggregates. The presence of the crosslinked polymer microparticles in the basecoat composition, confers upon the film derived from the latter, the desired ability to withstand subsequent application of the topcoat composition without disturbance of the film or of the pigmentation, in particular metallic pigmentation, which it contains and without which, therefore a successful basecoat/clearcoat system cannot be achieved. Microgel dispersions having a pseudoplastic or thixotropic character have been used for formulating metallic pigments in the basecoat composition. This gives the advantage of the flip tone effect as well as the gloss to produce the ever popular metallic finishes for the automotive industry [51]. 7.3.3

Miniemulsions

Significant advances have been made in recent years in applying miniemulsions for making water-borne polymers for the coating industry. Since the introduction of miniemulsion polymerization in the early 1970s [52] many investigators have studied the subject and have used many different methods to prepare miniemulsions [53–57]. Miniemulsions are routinely prepared using some kind of high shear device, in most cases this being an ultrasonifier or a microfluidizer [58]. Shork et al. have shown that incorporation of polyester into each acrylic latex particle, prepared via miniemulsion polymerization, leads to an effective in situ grafting of the acrylic and polyester systems [59]. The hydrophobic nature of the polyester resin makes it impossible to be accommodated by traditional emulsion polymerization due to mass-transfer limitations in crossing the aqueous phase to micellar nucleation sites. Thus, stable water-based latex coatings can be prepared that also have the ability to cure (by crosslinking).The above hybrid miniemulsion polymerization was successfully used to incorporate an oil modified polyurethane in the acrylic

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droplets to give stable miniemulsions which were polymerized to give hybrid latexes [60, 61]. The hybrid polyurethane modified miniemulsion latexes have been successfully used in formulating coatings for basecoats [62–64]. Additionally, miniemulsions made using a mixture of polyurethane and acrylic monomers were used to make latexes using a semi-continuous feed. This technique was successfully used to core-shell polymers for use in making water-borne basecoats [64, 65]. Stable aqueous dispersion of polymeric microparticles containing cellulose ester, and an acrylic polymer and a surfactant has been used for coating compositions. These coating compositions have good leveling and flow characteristics and exhibit good humidity resistance, appearance, adhesion and chip resistance when used in a “low bake repair” process as well as a good automotive quality finish [66]. These dispersions of microparticles are produced by high stress dispersion followed by polymerization of the vinyl monomers in the presence of cellulose ester within the micro-particles. 7.3.4

Selection of Monomers, Initiators, and Surfactants

Glass transition temperature is usually the first design property considered for the application. Hydrophilicity, hydrophobicity, acid-base properties, crosslinking ability are other properties [67, 68]. Acrylic acid (AA) or the somewhat less water-soluble methacrylic aid (MAA) are used in the order of 1–2 % (w/w) of the monomer charge. The effects of acid monomers on stability and viscosity are maximized when they are incorporated in the last part of the monomer feed and the polymerization medium is acidic. Sometimes di- or tri-functional cross-linking monomers are included. To improve acid rain etch resistance, carbamate functional monomers are included, wherein the carbamate groups crosslink with melamine, used for clearcoats. The aminoplast cured coating system combines acid resistance with excellent coating properties providing protection against etching by acid rain [69]. The most common initiators are peroxydisulfate salts, especially ammonium peroxydisulfate. Thermal initiation is preferred to redox initiation. Whitening of films may occur sometimes due to the hydrophilicity of the salts like ferrous thiosulfate. A water soluble azo initiator 4,4′-azo-bis(cyanovaleric acid) has also been used for making acrylic latexes [70]. Chain transfer agents are sometimes added to control the molecular weights and the distribution. Latexes and coatings are stabilized by biocides or water-miscible solvents to prevent microbiological contamination and deterioration [71]. Choice of surfactants are critical for automotive paint application due to their foaming tendency, ability to impart water sensitivity to paint films and change gloss characteristics. Typically a low particle size in the range of 50–300 nm is preferred, so in general anionic surfactants are used at levels of 0.5–2 % (w/w) based on polymer. Nonionic surfactants may be added in stabilizing the latex against coagulation during freeze-thaw cycling making it less sensitive to coagulation by salts, less sensitive to changes in pH. Choice of surfactant can also affect film formation temperature, since some nonionic surfactants plasticize the latex polymer, leading to lower Tg and hence, a

7.3 Properties of Water-borne Binders used for Automotive Coatings

lower temperature for coalescence. For example, nonylphenylethoxylate nonionic surfactants with less than nine ethoxy units reduce film formation temperature as compared to 20 to 40 ethoxylate units, but the higher ethoxylated surfactants are more effective latex stabilizers [72]. 7.3.5

Secondary Acrylic Dispersions

Automotive coatings containing acrylic resins as binders are well known. Organic solvents used are generally alcohols, glycol ethers and other oxygen-containing solvents that are soluble or miscible with water. Acrylic resins made in solvents (e.g. 1-(n-propoxy)-2-propanol, 2-butoxyethanol, butyl alcohols) are polymerized by freeradical mechanisms. Typical monomers used are MMA/BA/BMA. Azo initiators are typically used. Hydroxy monomers like HEMA, HEA are used, while acrylic acid monomers are used to impart water solubility. After polymerization the carboxylic acid groups are neutralized. Typically, these resins have acid numbers of 40–60 (acid number is determined by titration and is defined as mg of KOH required to neutralize 1 g of resin solids, equivalent weight equals 56 100/acid number). In the low-molecular-mass range (<6000 g mol–1), the Tg depends on the molecular mass. Subsequent cross-linking leads to an increase of Tg which is dependant on the cross-linking density. The choice of amine is crucial. 2-(dimethylamino)ethanol is widely used [73]. Thermoplastic latexes are of higher Tg (upwards of 60 °C) are used for higher toughness, mar and chemical resistance, since heat is available for film formation. The hydroxylic monomers can be incorporated for cure with melamine and isocyanate resins, the carboxyl monomers can provide cure with epoxies, aziridines and carbodiimides. 7.3.6

Secondary Polyurethane Dispersions

Another important class of materials used for OEM coatings are aqueous polyurethanes due to their versatility in their properties [74–80]. Since the introduction of polyurethane dispersions in 1960s, they have enjoyed considerable interest and commercial acceptance. Polyurethane dispersions can be classified into three main groups: – non-ionic type; – ionic type; and – dispersions containing both the non-ionic and the ionic groups. Anionic PU dispersions In the first step of the synthesis, a conventional polyether- or polyester-based isocyanate-terminated prepolymer is obtained by condensation polymerization of a diol and a diol containing a carboxyl function, preferably reacting the hydroxyl groups of dimethylol propionic acid with isocyanate groups. In the next step, the carboxyl groups are neutralized with an amine which is subsequently dispersed in water and

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chain extended in order to obtain high-molecular-weight materials by reacting with a diamine in further steps. Aliphatic isocyanates are preferred, because beside conferring good durability, they show lower reactivity with water and carboxylic groups. Further aliphatic isocyanates favor very rapid reaction with diamines in the chain extension step. As neutralizing bases, tertiary amines are preferred compared to other amines to prevent unwanted side reactions with isocyanates. A number of diols, isocyanates and amine raw materials can be used to adjust the mechanical properties, glass transition temperature and durability. Non-ionic Dispersions In the non-ionic types, the hydrophilic centers comprise of polyether chain segments [75, 81]. The different morphology exhibited by aqueous polyurethanes and acrylics explains why the minimum film forming temperatures of polyurethane dispersions (PUD) are lower than that of acrylics with equal hardness [82]. Polyurethane particles can exhibit core-shell morphology with the shell having higher molecular weight and higher urea functionality than the core. This effect was found to be quite pronounced with isophorone diisocyanate-based polyurethane dispersions [83]. A two coat one bake coating process which does not give environmental problems has been developed using aqueous PUD [84, 85]. Water-borne basecoats containing polyurethanes have been produced with a formulation containing less than three pounds per gallon and lower temperatures than solvent-borne systems [86]. Rapid-drying polyurethanes have been used for industrial finishing and automotive refinish with a well-balanced range of properties at low VOC level. High-performance OEM-clearcoats have been produced with good chemical resistance with excellent mar resistance. Some limitations of using anionic water-borne polyurethanes are, volatile amines used to salt these carboxylic acid-functional resins, which leaves during baking, thereby hindering the cure of the strong acid-catalyzed acrylic-melamine clearcoats, resulting in wrinkled finish and loss of DOI (Distinctness of image), and the limited rheological stability with metallic formulations which contain certain rheology control agents. In certain formulations, anionic PU resins have generally not given satisfactory application properties and paint stability. In response to these limitations, nonionic polyurethane dispersions were developed [87, 88]. Hybrid systems The blending of resins is a simple and useful technique for improving paint properties. While water-borne acrylic resins and polyurethanes have been widely used as polymers for automotive coatings, both water-borne resins are inferior to corresponding solvent based counterparts because of hydrophilic functional groups or surfactants which are introduced to impart dispersion stability to these resins. Table 7-6 shows some of the advantages and disadvantages of polyurethane and acrylic resins. The most widely utilized technique for making hybrids is to free radically polymerize a combination of monomers in the presence of a pre-formed polymer which may or may not be intrinsically dispersible. If the preformed polymer is water dispersible, it can be used directly as a seed for subsequent free radical polymerization.

7.4 Rheology Advantages and disadvantages of polyurethane and acrylic resins. Tab. 7-6

Acrylics

Polyurethanes

Advantages

Disadvantages

Hardness Weatherability Chemical resistance Gloss Affinity for pigments Cost Mar resistance Elongation Softness and adhesion

Toughness Mar resistance Elongation

Cost

Urethane acrylic aqueous dispersions prepared by an acrylic polymerization in the presence of an aqueous polyurethane can possess a range of advantages over the corresponding blends, e.g. reduced water sensitivity, ability to prepare in the cosolvent form. Since polyurethanes are generally more hydrophilic than the acrylic copolymer, the polyurethane concentrates at the particle surface. In a well designed system, the particles coalesce on film formation to give a film with a continuous polyurethane phase. “Hybrid” acrylic-urethane latexes have been made by simultaneous polymerization of acrylic monomers and chain extension of urethane prepolymers giving structures similar to interpenetrating network polymers, with mechanical properties exceeding those, if blended [89]. Combination of special emulsions, microgels and water-soluble resins have yielded excellent aqueous binders for various coatings [25].

7.4

Rheology

A major concern in developing water-borne automotive coatings is to achieve a distinct rheology profile providing good sprayability, sag resistance and leveling properties, simultaneously. In low solids solvent-borne systems, controlling the rate of solvent loss controls viscosity and sagging and metal flake orientation. In water-borne systems rheology control agents (RCA) are added to control sag and flake orientation. The viscosity of the paint must be very low at the spray gun in order to ensure a good and uniform atomization. After the paint meets the car body, however its viscosity needs to be very high so as to prevent sagging on vertical surfaces and clouding. The flow behavior of the aqueous basecoat therefore has to be adjusted by including a rheology additive, the thickener, in the formulation. The paint must have a good intrinsic viscosity, i.e. the viscosity must depend on the shear rate. With a high shear force, such as is present at the nozzle during the spraying process, the viscosity needs to be very low. If the paint then meets the car body, it is virtually unaffected

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by the shear strength and the viscosity must rise to very high values with almost the same solids content so that the paint does not sag and the aluminum platelets do not lose their alignment. A pseudoplastic (“shear thinning”) behavior is ideal for coating materials. Viscosity is fairly high at low shear rates which avoids settling in the can and gives good anti-sag properties. At higher shear rates, the viscosity is reduced which allows easy handling and application of materials. Interactions between thickener and latex particles in water-borne automotive coatings and the corresponding microstructures were investigated using dynamic mechanical spectroscopy, cryo-replication, TEM and analytical ultracentrifuge techniques [90]. Attempts to correlate various rheological parameters to good metallic appearance have not been found to be successful [91]. However, in the case of four samples with varying metallic flop index values, it has been observed that higher degree of pseudoplasticity led to a better flake orientation when compared to samples with lower metallic flop index values. (Fig. 7-4) Steady Shear Viscosity Profile 10000 Sample A

Viscosity [mPas]

182

Sample B Sample C

1000

Sample D

100

Viscosity versus shear rate, metallic flop index values: sam1000 ple A 62, sample B 57, sample C 58, sample D 49. Fig. 7-4

10 1

10

100

Shear Rate [1/s]

A Zeiss multiangle goniospectrophotometer MMK-111 with 45° illumination angle was used to measure the lightness values of the basecoat/clearcoat film at different angles. The degree of “travel” was estimated using the following mathematical calculation [92]: Metallic flop (MF) index = 50(L25 – L70)/L70 where L25 and L70 is the lightness value at 25° and 70° off of specular reflectance. Specular reflectance is the reflectance of the incident light. When incident light falls on the coating surface. Values of this parameter for basecoat/clearcoat typically range from 45 for a coating with poor travel to 70 for a coating with very good metallic flake orientation. When light strikes a surface, some of the light penetrates where it can then be absorbed, scattered, or even transmitted if the layer is sufficiently thin. Nevertheless, because of the change in refractive index between air and most substances, a certain

7.5 Crosslinking

proportion of the incident light is reflected directly from the surface. The angular distribution of this light depends upon the nature of the surface but light that is reflected at the opposite angle to the incident light is called specular reflectance. Light that is reflected by the substance itself is called body reflectance. Instead of examining the energy that passes through the sample, specular reflectance measures the energy that is reflected off the surface of a sample or its refractive index. By examining the frequency bands in which the rate of change in the refractive index is high, users can make assumptions regarding the absorbency of the sample. Acrylic microgels have been developed that impart thixotropic flow using swollen gel particles [52, 93–96]. Thixotropic agents are added to increase the viscosity at low shear rates to minimize sagging [97, 98]. In the final film, the index of refraction of the polymer from the microgel is nearly identical with that of the crosslinked acrylic binder polymer so that light scattering does not interfere with color flop. The effect of the gel particles depends on the interaction with the low molecular weight resin. The rheological properties of the systems are discussed elsewhere [99].

7.5

Crosslinking

Thermoset coatings (chemically crosslinked film) play a very important role in the automotive coatings industry. All coating layers uses resins which need to be crosslinked. By crosslinking the resin used in the coating, many properties can be greatly improved, such as hardness, mar resistance and solvent resistance. Crosslinking reactions became important in the 1950s with the introduction of acrylic resins in the automotive sector. A further impetus was given by increasingly stringent environmental legislation. Lower solvent content and the replacement of conventional solvent-borne paints by water-based paints meant that the molecular mass of the binders had to be lowered to a range where the required paint properties (film formation, hardness and flexibility) no longer exist. These properties therefore had to be obtained by increasing the molecular mass by crosslinking after application. Furthermore, the glass transition temperature and film hardness are increased, the chemical reaction after application also provides advantages of high molecular mass dispersions. A widely used method of cross-linking paint films consists of reacting of hydroxyl containing acrylates or carbamate containing acrylates with melamine-formaldehyde resins or urea-formaldehyde resins [100–102]. Crosslinking is carried out at ca.130 °C and is effected by acid catalysis (Fig. 7-5). The paints exhibit outstanding gloss and durability [103]. The possibility of making cross-linkable latexes by emulsion polymerization in the presence of etherified melamine-formaldehyde resins has been demonstrated by Jones et al. [104]. Dynamic mechanical measurements showed that films from slightly or moderately cross-linked particles behave like homogeneous networks in the linear viscoelastic range [105]. Melamine formaldehyde cross-linkers have been

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used with water-reducible acrylics, water-reducible polyester–polyurethanes and acrylic latexes [106–109] in automotive water-borne basecoats. A practical and effective crosslinking mechanism in cathodic electrocoating is done with polyfunctional blocked isocyanates. The mechanism involves the reaction of an isocyanate group (NCO) with the hydroxy group of the epoxy backbone and liberation of the blocking group. Other crosslinking mechanisms that have been studied are use of addition polymers, Mannich bases, Michael adducts [110] sulfonium stabilized polymers, transesterification reaction of hydroxy, alkoxy, amido and ester systems with hydroxy functional cathodic backbone (Fig. 7-5) [111–114]. Melamine-Hydroxyl reaction R' N

OR

N

+

N

N

HO

OR

N

R'

ROH

N OR

N

H+

R'

O

N R'

Alkoxylated Melamine

Hydroxyl-functional Acrylic

Melamine Ether

Isocyanate-Hydroxyl reaction

+

R NCO

HO

R

N

O O

Hydroxyl-functional Acrylic

Isocyanate

Urethane

Melamine-Carbamate reaction

OCNH2

OCNH2

O OR N N N

250 F

N N

OR

N

N

N N N

N N

Fig. 7-5

O

OH

O

Crosslink reactions.

C O N H OR

7.6 Application Properties

7.6

Application Properties

Water-borne coatings initially presented a number of difficulties. Water-borne viscosity characteristics are distinctly different and application, film formation and drying behavior are dependant on humidity and phase distribution of solvents as well as the usual factors of solids, temperature and air flow. Water-borne coatings themselves vary depending whether they are latexes, dispersion (semi-soluble) resins or a combination of the two. On account of the viscosity anomaly, water-borne paints based on water-soluble binders have a relatively low solids content (ca.30–40 %) and require relatively large amounts of organic solvents (up to 15 %) to ensure water solubility and film formation. They also have the advantage of a broad drying spectrum (physical, oven drying). As a result of the particulate nature of the acrylic latex polymers, only low gloss and, in some cases, only limited corrosion protection can be obtained. Water-borne coatings based on dispersions can be applied by spraying, however they are of only limited use for electrostatic coating and dipping applications due to their rapid drying properties. Aqueous dispersions generally contain a few per cent of high boiling solvents which act as temporary plasticizers and lower the minimum film-forming temperature, thus allowing film formation to occur. A film of sufficient hardness is obtained only after complete evaporation of solvent, which may take up to several days. The film may, however be somewhat hydrophilic due to the presence of carboxyl groups and emulsifier residues, this reduces its water resistance, gloss and gloss retention. Coating films formed from some water-soluble binders tend to be water-sensitive because of their hydrophilic solubilizing groups. They can be formulated to have a high gloss due to their good pigment wetting and stabilization. They can also have a high level of corrosion protection, which depends on the corrosion-inhibiting pigments used and the chemical nature of the binder. The latter determines adhesion to the substrate and diffusion of water and oxygen through the paint film. In the case of paints made from emulsion polymers, the required application viscosity of waterborne emulsion paints is generally obtained by adding a small volume of water. The evaporation behavior of polymer dispersions is similar to that of conventional solvent-based paints. In the production and application of water-borne paints, water is used as the solvent or diluent. The physical properties of water and organic solvents differ. Some of these properties have to be taken into account when water is used as a paint solvent. The water molecules have a high dipole moment and associate with one another. This means that water has a high boiling point and high latent heat of evaporation despite its low molecular mass. This in turn results in fairly long evaporations or in the need to supply energy in the form of heat to evaporate the water and dry the paint film. The high dipole moment of water is also responsible for its high surface tension. With substrates having a low critical surface tension (e.g. plastics or unsatisfactorily greased metals) this leads to inadequate wetting, unsatisfactory edge covering, and crater formation. Critical surface tension (at 20 °C) of water is 72.5 mN m–1.

185

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7 Applications for Automotive Coatings

Circulation studies of water-borne metallic basecoats demonstrate a few reasons why specular reflectance is lost during circulation. The flow induced stress of circulation reduces flake size and produces cycles in liquid surface tension. Surface tension in turn controls amount of picture framing and film thickness. If rust is present in the circulation system, it can react with the paint resins, creating gel lumps which under circulation trap the metal flake hindering flake alignment. Smaller flakes, thicker film thickness and poor aluminium alignment all reduce specular reflectance within the sprayed basecoat paint film [109–115]. 7.6.1

Metallic Effect

Typically in solvent-borne systems, the basecoat requires high solvent or low solids content in order to achieve perfect orientation of aluminium flakes, which results in the so-called flip effect. The key factor is the rapid rise in viscosity on the car which results in the aluminum flakes effectively frozen in an orientation parallel to the surface. The slower rate of evaporation of water means that a water-borne basecoat cannot rely upon water evaporation to achieve his rapid viscosity rise. ICI, through its patented microgel technology has managed to stimulate the rapid viscosity rise. The microgel pseudoplastic rheology means that the paint at the spray gun tip behaves like a thin liquid while the paint on the car panel is highly viscous. The microgel particles are acrylic-based and swell rapidly in the presence of small amounts of organic co-solvents such as butyl cellusolve in alkaline solution e.g. pH 7.6. These are supplied to the European and world automotive industry for production line applications. These basecoats are cured at 130–140 °C [116]. The lower film thickness of the basecoat and the flash-off time required before applying the clear top coat reduces the popping problem. Control of sagging during application requires that the waterborne basecoat is shear thinning, which also reduces surface distortion during subsequent application of the clear coat [117].

7.7

Environmental Aspects and Future Trends

Water-borne coatings for automotive applications have a broad application spectrum. The prospects for the increasing use of automotive water-borne coatings lies in their economic advantages and in the possibility of reducing solvent emissions during application to comply with legal requirements. Savings in organic solvents as diluents, savings in insurance premiums, lower energy consumption in spray cabins, ventilation zones, and drying ovens, all contribute to the overall economy of water-borne coating materials. Water-borne coatings can generally be classed as less toxicologically harmful than corresponding solvent-based paints. Nevertheless, lungpenetrating paint mists (aerosols) of water-based paints present a health hazard and appropriate protective measures (e.g. use of respirators) must be taken depending on the workplace concentration. The main problems arise due to the relatively high

7.7 Environmental Aspects and Future Trends

freezing point, high surface tension and low evaporation rates of water compared to organic solvents. Moreover the presence of water causes rusting problems of ferrous substrates and also make the water-borne systems very prone to attach by microorganisms. Although water-borne systems are not considered toxic, they require careful selection of resin/binders and additives such as biocides, cosolvents, coalescing agents, etc., in order to avoid toxicity caused by these components. Future approaches will encompass improvements in scratch and environmental etch resistance, better color options, better performance features all at a lower cost to the manufacturer. Acknowledgment The author wish to thank BASF Coatings Division for their support in writing this chapter.

References General M. S. El-Aasser, P. A. Lovell (eds) Emulsion polymerization and Emulsion Polymers, Wiley, New York, 1997. D. C. Blackley, Emulsion polymerization, Theory and Practice, Wiley, New York, 1975. J. W. Vanderhoff, J. Polym. Sci.: Polym. Symp. 1985, 72, 161–198. G. Fettis, Automotive Paints and Coatings, VCH, Weinheim, 1995. D. Stoye, W. Freitag, Paints, Coatings and Solvents, 2nd edn, Wiley-VCH, 1998. Z. W. Wicks, Jr, F. N. Jones, S. Peter Pappas, Organic Coatings Science and Technology, 2nd edn, 1998. 1

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9 B. M. Perfetti, Metal Surface Character-

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J. Appl. Polym. Sci. 2000, 76 (3), 350. T. Shimizu, S. Higashiura, M. Ohguchi J. Appl. Polym. Sci. 2000, 75 (9), 1149. T. Shimizu, S. Higashiura, M. Ohguchi H. Murase, Y. Akitomo, Polym. Adv. Technol. 1999, 10 (7), 446. T. Shimizu, S. Higashiura, M. Ohguchi, J. Appl. Polym. Sci. 1999, 72 (14), 1817–1825. T. Nishi, T. Takagi, Y. Okude, Eur. Pat. Appl. EP 849341 A2 (June 24 1998). DE 3606513, BASF L+F (1986); EP 0260430, AKZO (1987). DE 3630356, Asahi Glass (1986). S. C. Wieditz, J. Niemann, A. Dobbelstein (BASF), US Pat. 5635564 (Jun. 3, 1997). W. Jouck, B. Mayer, S. C. Wieditz, US Pat. 5322715 (Jun 21, 1994). A. J. Backhouse (ICI), US Pat. 4403003 (Sep 6, 1983). A. J. Backhouse, Eur. Pat. B-0015035 (1979). R. Buter, Eur. Pat. A 0238108 (1986). R. Buter, Eur. Pat. A-0273530 (1986). R. Buter, Eur. Pat. A-0287144 (1987). B. G. Bufkin, J. R. Grawe, J. Coat. Technol. 1978, 50 (641), 41. B. G. Bufkin, J. R. Grawe, J. Coat. Technol. 1978, 50 (643), 67. B. G. Bufkin, J. R. Grawe, J. Coat. Technol. 1978, 50 (644), 83. B. G. Bufkin, J. R. Grawe, J. Coat. Technol. 1978, 50 (645), 70. B. G. Bufkin, J. R. Grawe, J. Coat. Technol. 1978, 50 (647), 65. B. G. Bufkin, J. R. Grawe, J. Coat. Technol. 1979, 51 (649), 34. E. S. Daniels, A. Klein, Prog. Org. Coat. 1991, 19, 359. S. K. Nickle; E. R. Werner, Jr. (E. I. Du Pont de Nemours) US Pat. 5314945 (May 24, 1994). D. R. Bassett J. Coat. Technol. 2001, 73, 912, 65. Anon., Emulsion Polymerization of Acrylic monomers, Tech Bull. CM-104 A/cf, Rohm and Haas, Philadelphia. R. M. Christenson, T. R. Sullivan, S. K. Das, R. Dowbenko, J. W. Du, R. L. Pelegrinelli (PPG) US Pat. 4055607 (Oct 25 1977).

52 J. Ugelstad, M. S. El-Aasser, J. W. Van-

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Polym. Chem. Educ. 1979, 17, 3069. 56 B. J. Chamberlain, D. H. Napper, R. G.

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59 60

61 62 63

64 65 66 67 68 69

70 71 72 73 74

Gilbert, J. Chem. Soc. Faraday Trans. 1 1982, 78, 591. J. Delgado, M. S. El-Aasser, J. W. Vanderhoff J. Polym. Sci. Polym. Chem. Educ. 1986, 24, 861. Technical Bulletin on Microfluidizer, Microfluidics Corporation, Newton, MA, 1989. J. Tsavalas, J. W. Gooch, F. J. Schork, J. Appl. Polym. Sci. 2000, 75, 916. S. T. Wang, F. J. Schork, G. W. Poehlein, J. W. Gooch J. Appl. Polym. Sci. 1996, 60, 2069. W. Gooch, H. Dong, F. J. Schork, J. Appl. Polym. Sci. 2000, 76, 105. R. L. Martin, B. G. Piccirilli, D. L. Faler (PPG) US Pat. 5071904 (Dec 10, 1991). H. J. Drexler, F. Ebner, H. D. Hille, U. Roth (BASF Farben & Fasern AG) US Pat. 4489135 (1984). S. Grandhee (BASF) US Pat. 5569715 (Oct 29, 1996). S. Grandhee (BASF) US Pat. 5786420 (Jul 28, 1998). S. K. Das, S. Kilic, R. E. McMillan, (PPG) WO 9749739 (1998). M. K. Yousuf, Mod. Paint Coat. 1989, 79, 48. K. O’Hara, J. Oil. Colour Chem. Assoc. 1988, 71, 413. S. Swarup, D. L. Singer, G. J. McCollum, K. G. Olson, S. T. Stefko, R. J. Sadvary, R E. McMillan, M. A. Mayo, WO 9410212. M. Nair, Prog. Org. Coat. 1992, 20, 53. J. Gillatt, Pitture Vernici Eur. 1991, 67(11), 9. G. A. Vandezande, A. Rudin, J. Coat. Technol. 1996, 68 (860), 63. Z. Z. Jin, Y. Hu, Y. Zhu J. Coat. Technol. 1988, 60 (757), 31. J. W. Rosthauser, K. Nachtkamp, Water-borne Polyurethanes in: Advances in Urethane Science and Technology, K. C. Frisch, D. Klempner

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(eds), Technomic, Westport, CT 10, 1987, 121. R. Arnoldus, Surf. Coat. 3 (Waterborne Coat.). Wilson (ed.) Elsevier, London, 1990, 179. R. Arnoldus, J. Polym. Paint Colour 1991, 178, 4226, 860. D. Dietrich, Adv. Org. Coatings, Sci. Technol. Ser. 1979, 1, 55. D. Dietrich, Prog. Org. Coatings 1981, 9(3), 281. S. Paul, Surface Coatings: Science and Technology, John Wiley, Chichester, 1985. W. D. Davies, in: Additives for Waterbased Coatings, D. R. Karsa (ed.) Royal Society of Chemistry, 1990, p. 181. R. E. Tirpak, P. H. Markusch J. Coat. Technol. 1987, 58, 49. R. Satguru, J. McMohan, J. C. Padget, R. G. Coogan, J. Coat. Technol. 1994, 66 830, 47–55. H. T. Lee, Y. T. Hwang, N. S. Chang, C. C. T. Huang, H. C. Li, Proc. twenty second Interaction Waterborne, High-Solids and Powder Coating Symp., University of Southern Mississippi 22nd 1995, 224–233. U. Akimitsu, K. Teruaki (Nippon Paint Co. Ltd) Eur. Pat. EP 602497 (July 7, 1997). P. B. Jacobs, P. C. Yu, J Coat. Technol. 1993, 65 822, 45–50. R. R. Roesler, R. W. Rumer, WaterBorne, Higher-Solids and Powder Coatings Symp., Feb 1991, pp. 309–331. T. G. Savino, T. C. Balch, A. L. Steinmetz, S. E. Balatin, N. Caiozzo, US Pat. 4794147 (Dec 27, 1988). T. G. Savino, T. C. Balch, A. L. Steinmetz, S. E. Balatin, N. Caiozzo, US Pat. 4946910 (Aug 7, 1990). P. Walstra, Principles of Emulsion Formation, Conf. Preparation of Dispersions, J. Laven, H. N. Stein (eds) Veldhoven, The Netherlands, 1991, 77–92. N. Willenbacher, T. Frechen, H. Schuch, B. Lettmann, J. Eur. Coat. 1997, 9, 810. L. J. Boggs, H. Taniguchi, Int. Waterborne, High-Solids and Powder Coating Symp., Feb. 1998, pp. 30–39. D. Schmittmann, BASF internal communication.

93 J. M. Maklouf, S. Porter, US Pat.

4147688 (April 03, 1979). 94 M. S. Andrews, A. J. Backhouse,

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Etzell, US Pat. 4290932 (1981). 97 D. R. Bauer, L. M. Briggs, R. A. Dickie,

98 99 100 101 102 103

104 105 106 107 108 109 110

111 112 113 114 115

116 117

Ind. Eng. Chem. Prod. Res. Dev. 1985, 21, 686. L. G. Boggs, M. Rivers, S. G. Bike, J. Coat. Technol. 1988, 60 (766), 27. S. Ishikura, K. Ishii, R. Midzuguchi Prog. Org. Coat. 1988, 15, 373. W. S. Zimmt, Chemtech 1981, 11, 681. D, R, Bauer, R. A. Dickie J. Coat Technol. 1986, 58, 41. G. Y. Tilak, Prog. Org. Coat. 1985, 13, 333. L. W. Hill, A. Kaul, K. Kozlowski, J. O. Suter, Polym. Mater. Sci. Eng. 1988, 59, 283. Y. Huang, F. N. Jones Prog. Org. Coat. 1996, 28, 133. A. Zosel, G. Ley, Macromolecules 1993, 26(9), 2222. C. B. Fox, Proc. ESD/ASM Adv. Coat. Technol. Conf. 1991, 1991, 161. A. J. Backhouse, J. Coat. Technol. 3, 54 693 (1982) 83. Z. Vachlas, J. Oil. Color. Chem. Assoc. 1989, 72, 139. I. Wagstaff, Proc. ESD/ASM Adv. Coat. Technol. Conf. 1991 (1991) 43. S. L. Buchwalter, R. D. Jerabek, L. H. Chou, R. M. Christenson (PPG) US Pat. 4198331 (1978). J. T. Valko (PPG) US Pat. 4423167 (1981). J. T. Valko (PPG) US Pat. 4423168 (1981). J. T. Valko (PPG) US Pat. 4423169 (1981). P. E. Kordemenos, J. D. Nordstrom, J. Coat. Technol. 1982, 54 (686) 33–41. J. Meister, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 38(2), 638–639. C. B. Fox, Proc. ESD/ASM Adv. Coat. Technol. Conf. 1991 (1991) 161. Z. Vachlas, J. Oil Colour. Chem. Assoc. 72 (1989) 139.

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Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

8

Applications in the Adhesives and Construction Industries Dieter Urban and Luke Egan

8.1

Introduction

Adhesives are high-molecular-weight substances which bond materials to one another without significantly changing their structure. The action of adhesives is based on two key properties: they must firstly “wet” solid surfaces and adhere to them, and secondly, they must be cohesive, i.e. have internal strength. Adhesives of natural origin were mainly used prior to the beginning of the 20th century and by early civilizations as long ago as 2000 BC. These included animal glue, casein, natural rubber and starches. Today, specially developed adhesives based on semi- or fully synthetic products are used for a wide variety of bonding applications (Fig. 8-1). The development of synthetic adhesives paralleled the development of plastics which began in 1845 with the nitration of cellulose to give cellulose nitrate, the first semi-synthetic plastic, whose ethereal solution was used by the shoe industry in 1910 for bonding leather. The products discovered in 1872 by Adolf Baeyer by polycondensation of phenol with formaldehyde were the basis for the first fully synthetic plastic, Bakelite, which was obtained by Bakeland in 1909 by thermally curing reactive phenolic pre-condensates. But it was not until 1930 that phenol-formaldehydes and urea-formaldehyde condensates developed by C. Goldschmidt in 1896 (Kaurit) were used widely as adhesives. Polymerization processes discovered in the 1920s resulted in a large number of new thermoplastics and elastomers, of which, in particular, polychloroprene, and polyisobutylene were used as the basis for new adhesive technologies. Polyurethanes and epoxies – developed in the mid-1930s – further broadened the range of adhesive raw materials. Thermally stable plastics were more recently developed which enable adhesive bonds that withstand temperatures up to 350 °C. Concurrent development of new bonding technologies (e.g. sandwich construction) now makes it possible to adhere a multitude of various materials to one another. As such, classical connection meth-

191

192

8 Applications in the Adhesives and Construction Industries Natural adhesives Animal glue Gluten (hide glue, bone glue) Casein Blood albumin Vegetable glue Albumin, wheat, pectin Starches, artificial gum Natural gum, gum arabic, tragacanth Natural rubber, gum rosin, natural resins (colophony) Synthetic adhesives Physical binders Bonding by evaporation (water- and solvent-borne adhesives) Cellulose ester, cellulose ether Synthetic rubber (polychloroprene, copolymers of butadiene with acrylonitrile or styrene) Copolymers of acrylics, vinyl esters, vinyl ethers Derivatives of natural rubber (chlorinated or cyclized rubber) Polyurethane elastomers Bonding by cooling (hot melt) Ethylene copolymers Polyalkylene terephthalate Polyamide Butadiene or isoprene based block copolymers Chemical reacting adhesives Urethanes, epoxides, silicones Urea, melamine, and phenolic resins Cyanoacrylate, (meth)acrylates Fig. 8-1

Synthetic and natural adhesive raw materials.

ods, such as welding, riveting, screwing, sewing, etc., continue to be replaced by adhesive bonding techniques. Adhesive bonding has now become a routine joining method in a host of various industries – including automobile, furniture, shoe, construction and packaging. A particularly impressive example of new adhesive technology is in aircraft and rocket construction, where supporting structures are adhesively bonded on assembly lines [1–9]. In Western Europe, about 1.5 million tons of adhesives (dry weight) were used in 1996 [10] compared to 2.3 million tons in North America [11]. The world market in 1996 for adhesives and sealants was about US$ 21 billion compared to approximately US$ 6.4 billion in North America [11].

8.2 Pressure-sensitive Adhesives

8.2

Pressure-sensitive Adhesives

Pressure sensitive adhesives (PSA) are highly viscous, viscoelastic liquids which adhere to virtually all surfaces when pressed down gently. They are used in manufacturing “easy to apply” self-adhesive products, such as labels and tapes. Pressure sensitive adhesives typically have permanent tack and adequate cohesion, so that further curing operations after applying the tape or label are generally unnecessary. The first self-adhesive products were adhesive plasters [12], which were developed in the USA in 1845 by the medical doctor H.H. Day [13] and in Germany in 1882 by P. Beiersdorf [14]. The use of self-adhesive tapes for industrial purposes marked the beginning of “dry-adhesion” technology. A new branch of industry began to coat various support materials with pressure sensitive adhesives and to make self-adhesive products from them. In 1935, R.S. Avery [15] invented a coating unit for self-adhesive paper labels using a wooden cigar box filled with an adhesive solution. The adhesive dripped through holes cut in the bottom, on to a roll of paper that ran below. This was probably the first curtain coating process. Until the beginning of the 1970s, pressure sensitive articles were produced mainly by coating from organic solutions. Solvent-based rubber/resin mixtures and comparatively smaller amounts of acrylate solutions were processed almost exclusively. The first water-based acrylate dispersions were developed as long ago as the 1930s. They were produced on an industrial scale for the first time at the beginning of the 1940s. However, acrylate dispersions for pressure sensitive adhesives were not introduced into the market until the beginning of the 1950s. The first areas of application were self-adhesive book binding and map protection films. It was then possible for the first time to make flexible PVC films self-adhesive without the need for a primer. This excellent property also made it possible to use dispersions for electrical insulation tapes. The use of self-adhesive films in advertising applications (e.g. promotional graphics, signs) began at virtually the same time. Furthermore, acrylate dispersions satisfied the demand for removability without leaving a residue. A continuous increase in pressure sensitive adhesive dispersion production occurred throughout 1960s. The rapid development of a number of new applications accelerated this process. The crucial factor was the ground-breaking, systematic work in the 1970s on high speed coatings technologies [16]. The development of new application equipment and corresponding high-solids dispersions has enabled coating rates to be increased from less than 60 m min–1 to as high as 600–1000 m min–1 today. As so often it occurs in new technical developments, completely unexpected advantages became evident with newly developed acrylic dispersions. The inherent properties of polyacrylates made by emulsion polymerization enabled hitherto unknown speeds during down-stream converting operations (i.e. slitting, die cutting and stripping). It was also found that existing “solvent” coating units could be modified to permit economical processing of aqueous dispersions. This increased efforts to use aqueous polyacrylate dispersions as pressure sensitive adhesives.

193

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The total market demands for pressure sensitive adhesives (tapes and labels) is about 300 000 tons of polymer in North America and about 200 000 tons in Europe. Aqueous polymer dispersions have a share of about 30 % in North America and 40 % in Europe. About 30 % in North America and 45 % in Europe are applied from organic solution while hot melts have a share of 40 % in North America and 15 % in Europe [17, 18]. 8.2.1

Self-adhesive Labels

Self-adhesive labels (and decals) are pieces of paper, plastic film or metal foil coated with pressure sensitive adhesives which adhere to any solid surfaces after removal from a release liner. They are typically used to convey various kinds of information – for example, product or package contents, barcodes, price stickers, instructions, and technical data. The most important adhesive raw material compositions for adhesive labels are polyacrylate dispersions, styrene-butadiene rubber solutions, and styrene-butadienestyrene (SBS) and styrene-isoprene-styrene (SIS) hot-melt adhesives. Permanent and removable UV-cross-linkable acrylic hot-melt adhesives have recently also been introduced [19–22]. In the 1970s, Japanese paper label manufacturers started converting from rubber solutions to water-borne systems because of very strict environmental protection regulations, resulting in acrylic dispersions becoming the most important group of PSA raw materials in Japan. In North America, 85 % of the PSA label market was solvent born in 1975 [23] but in 1991 approximately two-thirds of the market was waterborne [24]. In Europe, the conversion was carried out in the 1980s and now the use of acrylic dispersions in self-adhesive products is widespread and well advanced. The importance of acrylic dispersions continues to increase for reasons of environmental protection (nonflammable, no hazardous solvents), high solids contents, good aging resistance, and excellent coating and processing properties. About 2.9 billion m2 of self-adhesive labels were produced in Europe in 1996, corresponding to 35 % of world production. At average application rates of 24 g m–2, this corresponds to a demand for adhesives of 70 000 dry tons year–1 in Europe. In North America 3.5 billion m2 of self adhesive labels were produced in 1999. Assuming the same coating weight, the adhesive demand in North America was 84 000 dry tons year–1. Polyacrylate dispersions Adhesion and cohesion of polyacrylate dispersions can be varied over a broad range and matched to many applications through the type and combination of low Tg (“soft”) and high Tg (“hard”) monomers, the choice of auxiliaries, and the control of the molecular weight and process parameters [26–29]. Some examples are given below. The influence of glass transition temperature (Tg) of acrylic homopolymers on tack (according to A. Zosel [30, 31]) at various temperatures is shown in Fig. 8-2.

8.2 Pressure-sensitive Adhesives

Tack of polyacrylates as a function of composition and temperature. Fig. 8-2

It can be seen that tack is greatest at temperatures 50–80 °C above the Tg of the homopolymers. In addition, the tack increases with increasing hydrocarbon chain length of the acrylic monomer. For use at room temperature, polyethylhexyl and polybutyl acrylate have the greatest tack [27]. Besides polymer composition, the molecular weight of the polymer also affects adhesion properties. A practical measure for assessing the average degree of polymerization, in particular for process control, is Fikentscher’s K-value, which is obtained by measuring the relative viscosity of a dilute polymer solution [32]. This characteristic constant, which can be determined quickly and simply at a single concentration, enables a rough estimate of the intrinsic viscosity and molecular weight [27]. The following measures may be used for varying degree of polymerization: addition of a chain transfer agent, crosslinking additives, mixing high-molecularweight with low-molecular-weight components, and variation of the polymerization process [27]. The following example is intended to clarify the effect of molecular weight. Acrylic dispersions of identical composition, but of different K-value, varied through different amounts of a chain-transfer agent, were investigated (Fig. 8-3) [27]. The peel strength is low at very low K-values and high at a K-value of 50 (cohesive fracture). At K-values above 50, the failure mechanism changes from cohesive into adhesive, and the peel strength drops to a low value. If the K-value is increased further, the peel strength drops slightly. Substrate wetting and polymer cohesion strength are responsible for these results. Although wetting is good at low K-values, the cohesion of the film is very low. The higher the K-value, the higher the internal strength of the film and the worse the wetting, i.e. the lower the tack or adhesion.

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8 Applications in the Adhesives and Construction Industries

peel strength in N / 2 cm

peel rate 300 mm / min

20 cohesion failure adhesion failure

15

10

5

0 20

50

80

K-value

Fig. 8-3 Peel strength as a function of molecular weight (K-value), according to W. Druschke [27].

However, adhesion is not affected by the average molecular weight alone, but also by the molecular weight distribution. This can be seen in Fig. 8-4, which shows the

peel strength in N / 2 cm

peel rate 300 mm / min

20

cohesion failure adhesion failure

15

10

5

0 10:0

8:2

6:4

4:6

2:8

0:10

lower/higher molecular weight component Peel strength as a function of the content of high-molecularweight component, according to W. Druschke [27].

Fig. 8-4

8.2 Pressure-sensitive Adhesives

peel strength of various mixtures of relatively high molecular weight acrylic polymer (K-value ca. 90) with low molecular weight material having the same composition (K-value ca. 55). With increasing content of high-molecular-weight component, i.e. an increase in the average degree of polymerization, the transition from cohesive to adhesive fracture occurs again with a sudden drop in the measured peel values. The maximum peel strength is obtained at a high molecular weight component content of 80 % [27]. When an acrylic dispersion with good low temperature peel strength (A) is mixed with one having high cohesive strength (B), the actual performance of the mixtures can deviate strongly from a linear mixing relationship. As Fig. 8-5 shows, the shear values are significantly reduced by small amounts of dispersion A and low temperature peel strength drops to zero at only 20 % of component B.

Shear values at 20 °C and low temperature peel strength at –23 °C depending on the mixing ratio of a high peel dispersion A and a high shear dispersion B [33]

Fig. 8-5

Interestingly, copolymerized functional monomers like acrylic acid, for example, results in an increase in cohesion (Fig. 8-6) and produces a significant increase in the shear strength. The two contradictory requirements for high tack and needed internal strength can be best achieved in polyacrylate dispersions when adhesive and cohesive components are produced stepwise during the emulsion polymerization. The transfer of free radicals to pre-formed polymer chains results in the formation of branches, which crosslink on recombination. Crosslinking affects the viscoelastic behavior in two ways: by slightly raising the glass transition temperature and by increasing the modulus level above the glass transition region. It also reduces the mobility of the crosslinked polymer chains. This behavior is pronounced in polyacrylates and can be

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8 Applications in the Adhesives and Construction Industries

shear in min

1/2 inch x 1/2 inch, 500g

1 500

Shear strength as a function of copolymerized acrylic acid. Fig. 8-6

1 000

500

0 0

1

2 acrylic acid in pphm

used to improve cohesion. Adhesion can be improved using a chain transfer agent, which suppresses crosslinking and produces lower molecular weights. Figure 8-7 shows that even small amounts of chain transfer agent cause a significant reduction in cohesion. The maximum peel strength is achieved at 0.2 pphm and the maximum quick-stick (or loop tack) at 0.3 pphm of chain transfer agent. An optipeel (Cr) [N/inch] 300 mm/min

quick-stick (Cr) [N/inch] 300 mm/min

cohesion [min] 1/2 x 1/2 inch, 500 g

peel strength in N / inch 20

time in min 100 80

15

60 10 40 5

20

0

0 0

0,1

0,2 0,3 0,4 chain transfer agent in pphm

0,5

Effect of the chain transfer agent on the adhesive properties of an acrylic dispersion.

Fig. 8-7

8.2 Pressure-sensitive Adhesives

mum balance between plasticity and bond strength is clearly achieved at a certain degree of crosslinking. The polyacrylate dispersions optimized in this way for pressure sensitive adhesives are produced and supplied with solids contents between 50 % and 70 %. Average particle sizes are typically between 100 nm and 1000 nm. However, dispersions with a high solids content (> 60 %) and low viscosity (<500 mPa s) need to have a bimodal or multimodal particle size distributions. The viscosities of most commercial products are generally between 10 and 1000 mPa s but high-viscosity dispersions – greater than 10 000 mPa s – are also used in certain packaging applications. Polymers used in pressure sensitive applications have glass transition temperatures of between –60 and –20 °C. Formulation modifications Acrylate dispersions can be used directly in only a small number of applications. Technical or economic considerations often make modification necessary [34]: – optimization and refinement of the adhesion properties; – production of a large number of self-adhesive products using a small number of starting materials; – optimization of the formulation from a commercial point of view; and – matching of the viscosity, rheology and wetting behavior of the adhesive to the proposed coating method and substrate. This is generally done by mixing acrylic dispersions with one another, and adding tackifying resins and plasticizers. Wetting agents, defoamers and thickeners are added to adapt the adhesive to the prevailing coating conditions. Fillers or pigments are used to color the adhesive material (e.g. brown adhesive tapes for packaging) or to achieve special effects (e.g. zinc oxide in medical tapes). Dispersion mixtures In some self-adhesive products (for example, protective films), the goal is a weak tack which does not increase after extended bonding times. These products are based on special self-crosslinking acrylics. In order to improve the tack, cohesive products with good tack are admixed. In certain applications requiring chemical resistance (e.g. against plasticizers), a dispersion with superior resistance can be mixed in certain proportions to increase the overall formulation performance. In wet labeling applications (e.g. bottles), which previously used “natural adhesives” such as casein, a blend of acrylic dispersions, natural latex, and wax dispersion have been employed. Note that blends of acrylics and styrene-butadiene latex (SBR), for example, produce dry films which are opaque – because of differences in refractive index. The formulator can take advantage of this effect to impart opacity to a substrate. But it is important to note that when blending acrylics and SBR, water resistance of the dried films can be affected in unexpected ways (Fig. 8-8).

199

8 Applications in the Adhesives and Construction Industries 70 60 Water Update (%, after 24hrs)

200

50 40 30 20 10 0 100/0

80/20

50/50

20/80

0/100

PSA Blend Ratio (Acrylate / SBR)

Water resistance of dried PSA films depending on the blend ratio acrylic/styrene-butadiene latex.

Fig. 8-8

Addition of tackifying resins Resins are used to improve the tack of pressure sensitive adhesives. They must be compatible with the polymer (i.e. mixture has a single Tg) and modify its viscoelastic properties resulting in improved polymer flow characteristics, substrate wetting, and adhesive bond formation. The most suitable products for improving the tack of polyacrylates are modified natural resins, such as dimerized or hydrogenated gum rosins and esterified abietic acids. With resins whose softening points are about 70 °C, pressure sensitive adhesives with high shear strength but relatively low tack are obtained. If these resins are combined with softer resins, the tack is significantly increased. Besides the general increase in tack, addition of resin also specifically increases the adhesion to polyolefin surfaces. However, the addition of resin may impair the aging resistance of the adhesive. Preferred applications for acrylate–resin combinations are paper labels and double-sided adhesive tapes (for example, carpet laying tapes). Hydrocarbon resins based on petroleum oil derivatives are preferred for tackifying styrene–butadiene dispersions [24, 35]. To retain the advantage of the “solvent-free” feature offered by polymer dispersions, resin dispersions should be used instead of resin solutions. Resin dispersions are available, for example, from Akzo Nobel/EKA Chemicals (Netherlands, USA), DRT/ND Dispersions (France, USA), and Hercules (Germany, USA). However, it should be noted that a resin dispersion generally reduces cohesion to a greater extent than does a resin solution – because of the surfactants present. In addition, the water resistance, evident from blushing, is often reduced. Polymeric tackifier has been reported to increase the elastic properties of a pressure sensitive adhesive resulting in improved peel strengths, especially at high peel rates [36].

8.2 Pressure-sensitive Adhesives

Addition of plasticizers Plasticizers increase the flow properties of the adhesive film. This results in faster wetting of the substrate surface to be bonded and consequently increases the initial peel strength. This naturally occurs at the expense of cohesion and heat resistance. In contrast to resins, which normally increase the Tg of the adhesive polymer, plasticizers decrease the Tg and make the polymer softer. The tack-increasing effect of resins can be augmented by addition of small amounts of plasticizers. Particularly in the case of resins with high softening point that have a deadening action on the adhesive, addition of a small amount of plasticizer results in a lowering of the softening point. A resin-plasticizer combination consequently increases the tack very effectively. Small amounts of plasticizer (2–5 %) are frequently added to the acrylic dispersions in order to obtain gentler removal. The main plasticizers used are the classical plasticizers (i.e. phthalates such as DOP, DBP and DIDP). Adipates are also suitable as polymeric plasticizers, but are rarely used owing to price reasons and their somewhat lower compatibility. A special polymeric plasticizer polypropylene glycol alkyl phenyl ether (Plastilit 3060) from BASF, has extremely good compatibility with acrylate polymers and does not migrate. This product has now proven highly successful in a number of applications (including pressure sensitive adhesives, acrylic sealants and paints). Thickening of pressure sensitive adhesive dispersions Suitable protective colloids and thickeners include animal glues, gelatin, casein, vegetable gum, dextrins, enzymatically digested starches, alginates, cellulose derivatives (methyl-, carboxymethyl- and hydroxyethylcellulose), polyvinyl alcohols, polyacrylic acids, polyacrylamides and polyvinylpyrrolidones, amongst others. They increase the viscosity and water retention, modify the rheology for optimum coatability and speed, and in addition generally also improve the mechanical stability and compatibility of the dispersions with electrolytes and fillers. In order to achieve optimum results, precise metering and suitable selection and combination of the protective colloids and thickeners are necessary. Since the water resistance of polymer films from dispersion decreases with increasing addition of thickener, the proportion of thickener should always be minimized. Addition of wetting agents Thickened dispersions generally have little problem wetting various surfaces. Even surfaces with extremely low surface tension, such as silicone paper and corona treated polyolefin film, can be wetted under the uniform pressure imparted by the liquid adhesive on to the substrate during the coating operation. A high resistance to flow (i.e. high viscosity) prevents subsequent dewetting and minimizes the occurrence of coating flaws (e.g. fish eyes). The situation is quite different, however, in high speed, low viscosity coating systems such as modern vario-gravure where the dispersion film, which is initially uniformly distributed on the surface, re-coalesces relatively quickly if its surface tension is excessively high. In this case, the surface tension of the pressure sensitive adhesive dispersion must be matched to that of the substrate to be coated (film or silicone

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paper). This is achieved by adding corresponding surfactants, so-called wetting agents. These surfactants naturally increase the risk of foaming and in some cases increase the water sensitivity of the adhesive film. The amount added should again be kept as low as possible. The sodium salt of a sulfosuccinic acid ester (Lumiten I-SC, BASF, Aerosol OT70-PG, Cytec) is a relatively low-foaming surfactant whose particular effectiveness as wetting agent arises due to its tendency to migrate to newly formed surfaces very quickly. Addition of antifoams Surfactants (emulsifiers and wetting agents) often cause foaming, which results in coating defects particularly when operating at high speeds. This is prevented by addition of antifoams. The effectiveness of antifoams, for example higher alcohols, non-ionogenic acetylenic compounds, or aliphatic hydrocarbons with non-ionogenic constituents, varies depending on the other auxiliaries present in the formulation and in addition can drop with increasing storage time. The antifoam may float, settle or diffuse into the polymer and thus, is no longer available at the liquid–air interface where it is needed. It is therefore recommended that the dilute antifoam only be added immediately before processing or, even better, sprayed directly on to the foam surface by means of an atomizer if needed. It should be noted that excessive amounts can impair flow of the dispersion, which rapidly results in flaws (pinholes, fisheyes) in the coating. A general rule of thumb is that the amount of antifoam in initial experiments is 10 % of the amount of wetting agent, i.e., for example, one part of wetting agent and 0.1 part of antifoam per 100 parts of dispersion. Owing to interaction with the auxiliaries in the dispersion, the most effective antifoam for each adhesive must be determined experimentally. Companies which offer suitable antifoams include: Air Products and Chemicals Inc., Allentown PA (USA); BASF AG/Corporation, Ludwigshafen/Mount Olive (Germany/USA); Ashland Chemical Co., Drew Industrial Division, Boonton, NJ (USA), or Drew Ameroid Europe, Ultra Additives Inc, Patterson, NJ (USA); Henkel AG, Düsseldorf (Germany); ICI Surfactants, Middlesbrough (England), Muenzig Chemie, Heibronn (Germany). Addition of fillers or pigments Fillers cause significant deadening of the adhesive, which can be compensated again by a slightly increased amount of tackifier. In labels made from relatively thin paper, small amounts of fillers or pigments can be added in order to increase the hiding power. Zinc oxide has proven successful as an antiseptic, moisture-absorbent filler in rubber-based medical adhesive tapes. In contrast, modern medical adhesive tapes with non-woven or woven fabric as support material use polyacrylates owing to their lower skin irritation action. These contain virtually no filler. If necessary, white coloration is produced using TiO2. Fine sand (SiO2) also has a very strong deadening action. Even on addition of only 10 %, the tack can no longer be measured. Calcium carbonate filler exhibits much

8.2 Pressure-sensitive Adhesives

better behavior. The smallest reduction in tack is caused by titanium dioxide, but this is relatively expensive. For homogeneous incorporation of fillers or pigments, it is advantageous to prepare a paste with a little water. Additives to the water, such as ammonia, polyacrylic acid salts and polyphosphates, aid in dispersing and also prevent the filler particles from subsequently re-agglomerating. A “wetting fluid” comprising one part of ammonia, one part of Pigment Dispersant A (BASF), one part of Calgon N (sodium polyphosphate, Benkiser, Ladenburg) and 97 parts of water, to which the pigments are mixed to yield a paste, has proven successful. Protection against microorganisms The risk of bacterial, yeast or fungal contamination is naturally the greatest in the neutral or weakly basic pH range. This pH range is in most cases established during modification as it also provides the best compatibility of the dispersion with formulation additives. Preservatives used include formols, benzisothiazolinones (Proxel, Avecia, Wilmington, DE, USA and Manchester, UK), isothiazolones (Kathon , Rohm and Haas, Philadelphia, PA, USA, Rohm and Haas France SA, Valboonne-Cedex, France). Other preservative suppliers include Riedel-de Haën AG (Germany), Troy Corporation, Florham Park, NJ (USA) and Arch Chemicals (Paris, France and Cheshire, Connecticut, USA). General notes on modification The following is a suitable compounding procedure: the pH of the starting dispersion is firstly increased in order to improve compatibility. Secondary acrylate dispersions are then added, followed by resin solution or resin dispersion, other tackifiers, plasticizers and, if used, a pigment and/or filler paste. The final step is adjustment to the processing solids content and viscosity by thickening, dilution and addition of wetting agent. Guiding formulations 1. Acrylic permanent paper label (resin tackified)

Polymer dispersion Tackifier dispersion Defoamer Wetting agent Neutralizing agent Water Rheology modifier Application rate: 20 g solid m–2

Acrylic dispersion, 60–70 % Tacolyn 1070, 55 %* or Snowtack 880G, 57 %† Drewplus L-108‡ Lumiten I-SC§ Ammonia (10 %) to pH 7.5–8.5 As required to max. 65 % solids As needed for coating head

Suppliers: *Hercules, Wilmington, DE, USA; †Akzo Nobel, Toronto, Ontario, Canada; ‡ Ashland/Drew Chemical; §BASF, Charlotte, NC, USA

Wet wt. 116 36.4 0.1–0.5 0.5–1.5 0–2 0–2

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8 Applications in the Adhesives and Construction Industries

2. All-temperature paper label (non-tackified) Polymer dispersion Acrylic dispersion Neutralizing agent Ammonia (10 %) to pH 7.5–8.5 Wetting agent Aerosol OT70-PG# Rheology modifier As needed for coating head Water To maximum 65 % solids Application rate: 20 g solid m–2

100 3–5 0.5–1 0–2

Suppliers: #Cytec

Styrene-butadiene dispersions In North America approximately 5–10 % of the dispersions used for labels are based on styrene-butadiene rubber (SBR) [24]. SBR used in pressure sensitive adhesives are produced by emulsion polymerization with butadiene contents typically between 25 and 45 %. With careful control of process conditions (temperature, styrene and butadiene feed rates) and ingredient feed levels (chain transfer agent, initiator, monomers), intermediate molecular weight, lightly cross-linked elastomers having an excellent balance of cohesive and adhesive properties can be obtained. SBR based systems are primarily used in cost-sensitive paper label applications; they are not used, when clear films with long-term UV or heat aging resistance are required. While the hydrophobic nature of SBR promotes superior initial tack and adhesion to low energy substrates, this feature also makes them susceptible to plasticizer attack. Compared to tackified acrylic PSA, styrene-butadiene based systems normally require significantly increased tackifier contents (up to 2 times) to achieve desired tack and peels levels. SBR are compatible with both commercial rosin and hydrocarbon based tackifying resins. Differences in chemical composition, softening point, and surfactant stabilizers in tackifier dispersions can have a significant impact on the peel-shear balance of the formulated product [26]. Since commercial SBR are available at relatively low solids contents (ca. 50–55 %), the formulated adhesives have higher water contents compared to those based on acrylics. This subsequently translates into slower line speeds and/or higher energy demand during the drying operation. An antioxidant would be recommended for SBR applications requiring extended stability against heat exposure or oxygen attack. Guiding formulation SBR permanent paper label (resin tackified)

Polymer dispersion Butonal NS 166, 51 %* Tackifier dispersion Aquatack 6085, 60 % Defoamer, wetting agent, and rheology modifiers added as needed. Application rate: 20 g solid m–2

Wet wt. 63.8 36.2

Suppliers: *BASF Corporation, Charlotte, NC, USA; Arizona Chemical, Panama City, FL, USA

8.2 Pressure-sensitive Adhesives

Coating Self-adhesive labels and films are produced by coating support materials such as silicone release liner, paper stock, and film webs with pressure sensitive adhesives (Fig. 8-9). Various coating methods are used to ensure the correct amount of adhesive is applied per unit area of substrate [37–40]. High viscosity adhesives can be applied using a knife-over-roll coater. In reverse-roll coating, the adhesive is transferred to the substrate web after being taken up by an application roll rotating in a direction opposite to that of the web. Knife coaters and reverse roll coaters are traditional systems originally developed for coating solvent-based adhesives. But with aqueous dispersions, coating speeds of only 100–120 m min–1 are possible with these coating methods – at application rates of about 20 g m–2. For higher production speeds, Meyer rod (150–250 m min–1), reverse gravure (300 m min–1), vario gravure (600+ m min–1), and slot die technologies are available. These systems require lowviscosity dispersions and their development in the 1960s paved the way for a major breakthrough by acrylate dispersions for mass-produced pressure sensitive articles in Europe. Improvements in coating speeds, reliability, coating consistency, and product quality led to lower production costs and continuation of the trend to waterbased emulsion coating technologies. Coating head

Steam Dryer Laminating station Release liner

Schematic representation of PSA label coater. Fig. 8-9

Unwind

Backing Rewind

Reverse gravure was introduced by BASF at the beginning of the 1980s for pressure sensitive adhesive processing and basically consists of a blade pressed on to a gravure cylinder rotating in a pan of wet adhesive in a direction opposite to that of the web. Adhesive is transferred from the pan into the recesses of the gravure roll and then on to the web. The blade and roll assembly are primarily responsible for metering on the correct quantity of wet adhesive and establishing a consistent, defect free adhesive coating. Gravure rolls (Fig. 8-10) with 14 to 18 lines cm–1 give a dry coating weight of about 20 g m–2. A gravure roll with 36 lines cm–1 gives, for example, about 10 g m–2 (in each case with an approximately 50 % solids adhesive). If desired, the coating weight can be varied slightly by adjusting the blade position and by modifying the viscosity of the adhesive. For significant changes, a roll with a different grid must be used. When high reverse-gravure coating speeds (600+ m min–1) are attempted, coating weight is found to drop off drastically above about 300 m min–1. This behavior occurs at high speeds because of the shorter residence time of the gravure roll in the dispersion reservoir – i.e., so short that the gravure line cells are no longer completely

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8 Applications in the Adhesives and Construction Industries

Fig. 8-10 Portion of a gravure roll (left), cross-sectional schematic (right). (a) 10–20 µm chrome layer, (b) 10–15 µm nickel layer, (c) 70–250 µm copper layer, (d) steel base roll.

filled and too little dispersion is applied to the web. However, if the dispersion is forced into the engraving under pressure, it is possible to vary coating weight over a broader range, even with a constant number of lines. For example, application rates from 15 to 30 g m–2 can be achieved at 600 m min–1 with an 18 lines cm–1 gravure roll and from 20 to 40 g m–2 with a 14 lines cm–1 gravure roll. Vario gravure is a substantial refinement of the standard reverse gravure method (Fig. 8-11). The side seal consists of two polyethylene “margin wipes” pressed against the polished ends of the gravure roll. Two grooves, providing pressure release and lubrication, are incorporated into each margin wipe. Rubber parts are also built into the margin wipes to seal the side edges of the coating blade – the entire “casting box” assembly is sealed by a lateral force applied on to the sides of two margin wipes. The upper blade is additionally pressed against the gravure roll surface, which simultaneously prevents air being drawn in and results in very low air entrainment (i.e. low foaming).

substrate

adhesive

blade adjustment margin wipe

out blade adjustment Fig. 8-11

Vario gravure coating head.

in

PSA dispersion

8.2 Pressure-sensitive Adhesives

Coating weight is controlled mainly via the pump pressure, designed to operate without pulsation (Mohno pump). Due to the excess pressure prevailing in the casting box (0.2–0.6 bar), not only are the recesses of the gravure roll surface filled, but a film, i.e. an excess, is also applied on to the roll. This is the only way that a coating weight of 20 g m–2 can be maintained at higher speeds (400–600+ m min–1). The flow rate of wet adhesive through the pump can also be coupled to the web speed in order to keep coating weight constant during speed changes. Water based pessure sensitive adhesives can also be directly applied on to the substrate web using a slot die coater (Fig. 8-12). Die coating is widespread in the USA (est. 60–70 % of total label production), but is only used occasionally in Europe.

adhesive substrate width adjustment of slot die opening die

die lip offset adjustment

Fig. 8-12

Slot-die coating head.

PSA dispersion feed line

Coating weight can be easily varied over a broad range at different web speeds. Uniform distribution of medium-viscosity adhesive over the web width is achieved with a special die geometry where the outlet aperture is larger at the web edges than in the center. This method provides an impressive final coating – characterized by unusual levelness. 8.2.2

Self-adhesive Tapes

Self-adhesive tapes are flexible substrates coated with pressure sensitive adhesives which are wound up in roll form and cut to different widths. About 4.3 billion m2 of adhesive tapes were produced in Europe in 1996 [41]. Seventy percent of these are packaging tapes, while the remainder are double-sided adhesive tapes, masking tapes and other adhesive tapes (medical tapes, electrical insulation tapes, household tapes, office tapes and protective films). Solvent-based, resinmodified rubber solutions, used mainly for packaging adhesive tapes, are still dominant in this segment. The stringent requirements on tack and cohesion for tapes

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8 Applications in the Adhesives and Construction Industries

have still not been achieved by any other adhesive system in such a balanced way. The proportion of water-borne adhesives used in the European tape market is about 14 % [19]. Water-borne adhesives are mainly used for double-sided adhesive tapes and electrical insulation tapes. In North America, about 4.5 billion m2 of adhesive tapes were produced in 1999. Assuming an average coating weight of 40 g m–2, this corresponds to a polymer demand of 180 000 tons year–1. About 70 % are used for industrial and packaging tapes, the rest is used for consumer, surgical, electrical and masking tapes [18]. Waterborne systems are used mainly to produce carton-sealing tapes. Packaging tapes Packaging tapes are required to form the contact points necessary for box or carton closure (typically corrugated), even on gentle pressure. This means that the adhesive must have high tack and good adhesion. In Europe, natural rubber is predominately used to meet these requirements. Although acrylate dispersions have better aging resistance, they have lower tack at the high cohesion levels needed. Experiments have shown that the lower tack values can be fully compensated through increased positioning or “application” pressure. In North America, carton sealing tapes made from oriented polypropylene and water-borne acrylic PSA are also produced. Anchoring of the dispersion adhesive to the polypropylene film is achieved by corona pretreatment. A release coating is not necessary. The main requirements for this application are withstanding the continuous shear and low-angle peel forces transmitted to the tape at the carton closure. Coating weights of about 15–35 g dry adhesive m–2 are typically used. Paper backed tapes, such as masking tapes, require coating weights in the 30–60 g m–2 range. Foil duct tapes Foil duct tapes, used in North America for heating, ventilation and air conditioning systems, are constructed of 50 µm aluminum foil coated with 50–100 µm of an acrylic dispersion PSA to provide high adhesion and functionality at extreme “use” temperatures. Once coated, the adhesive is covered with a release liner to prevent adhesion to the top side of the foil and subsequent blocking of the tape roll. Some foil tapes use strands of fiber reinforcement laminated between foil and kraft paper layers to improve tensile strength [24]. Electrical tapes Flexible PVC tapes of various types are predominately used for electrical insulation purposes and to a lesser extent, for pipe insulation. In Europe, acrylate dispersions have been used in the manufacture of electrical tapes for more than four decades. Coating weights of ca. 25 g dry adhesive m–2 are typically used. Their advantage is that they can be coated on to flexible PVC films without a primer. However, the use of dispersions requires a high level of knowledge of the compositions and interactions between particular flexible PVC substrates and acrylic adhesives. As a result, suitable dispersions must be carefully selected and in many cases, it is necessary to use more than one dispersion type to achieve required performance levels. Of prime

8.2 Pressure-sensitive Adhesives

importance are plasticizer resistance screening tests on the adhesive, carried out in order to examine for undesired changes in adhesive properties due to plasticizer migration from the PVC film. Masking tapes This term covers tapes for protecting surfaces during painting, sand blasting, etc. The support material used is crepe paper of various quality and extensibility with natural rubber predominately used in the adhesive. Nonetheless, adhesives made from mixtures of certain acrylate dispersions are highly suitable for “painters-grade” masking tapes. Key requirements of masking tapes include adequate adhesion and then easy removability without leaving a residue – even after extended storage times or after exposure at high ambient temperatures. Coating weights of ca. 45 g m–2 dry adhesive are used. Double-sided adhesive tapes Double-sided adhesive tapes or “mounting” tapes have been used to replace conventional attachment methods in a range of different end-use applications. They are used on a wide variety of surfaces, for example, as an assembly aid in the automotive industry, in graphic arts for plate mounting and in home and office uses that previously required mechanical fasteners (e.g. nails and screws). The central support substrate used includes non-wovens, textile fabrics, foams and other materials and in some cases, support-free mounting tapes are also available. Foam mounting tapes are especially useful because the foam provides stress distribution for increased shear strengths. While acrylate dispersions are often used, the most demanding applications employ solvent based acrylates that are subsequently crosslinked chemically or thermally to achieve performance requirements. In applications requiring exceptionally high adhesive coating weights (up to 100 g dry adhesive m–2), dispersions having high solids contents have been advantageous to increase drying rates during the coating operation. Protective films Protective films are used to protect high-value items like painted or polished metal surfaces (e.g. automobile paint finish), anodized aluminum, acrylic sheets, lacquered furniture surfaces and automotive carpets from scratching, soiling and marring during manufacture, shipping and installation. A range of support substrates are used, including paper, flexible PVC, PE-, PP- and polyester films [24]. Here too, acrylate dispersions are being used instead of the cross-linkable acrylate solutions used in the past. In particular, self-crosslinking emulsion adhesives providing crucial cohesion and anchoring to PE films have been developed especially for protective films [24, 42]. Coating weights of about 5 g dry adhesive m–2 are employed.

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8.2.3

Test Methods

Tack, adhesion and cohesion are the three main properties required of a pressure sensitive adhesive. The tack is the ability of a pressure sensitive adhesive to adhere immediately to a surface. A pressure sensitive adhesive with good tack forms the contact points necessary for adhesion of the tape or label after only brief contact with a substrate. Solvent based rubber adhesives can have very good tack, depending on the formulation. Acrylate solutions and dispersions, as well as hot-melt contact adhesives, meet the usual demands. The peel strength (adhesion) is a measure of the separating force necessary to peel a label or tape off from the surface to which it was applied. The term describes the strength of adhesion or “grab” to a surface. Acrylate compositions require a few hours before achieving full adhesion. Rubber adhesives and hot-melt contact adhesives exhibit high peel values after only a relatively short contact time. The shear strength (cohesion) is the ability of a pressure sensitive adhesive to withstand applied forces or loads. Good shear strength is required when labels are stuck to curved surfaces and for processing purposes, i.e. die-cuttability, slitting and to minimize edge-ooze of roll products. When assessing shear strength, a distinction must be made between the performance at room temperature and at elevated temperatures. Pressure sensitive adhesives designed having adequate room temperature shear strengths should also be formulated to resist failure at higher service temperatures. While acrylic dispersions exhibit only a slight change in shear strength at elevated temperatures (due to internal gel structure), hot-melt adhesives tend to soften and the shear strength drops significantly. Depending on the use, different requirements are made of pressure sensitive adhesives with respect to adhesion and cohesion (Fig. 8-13). adhesion

permanent labels

packaging tapes

removable labels

protective films

cohesion Fig. 8-13

Adhesion level of self-adhesive articles.

8.2 Pressure-sensitive Adhesives

A protective film must be removable without leaving a residue, i.e. must have very low adhesion, even after long bonding times, in conjunction with high cohesive strength. By contrast, a packaging tape must stick immediately and durably, i.e. must have both very high adhesion, even after brief contact, and high cohesion. In paper labels for permanent bonding, high adhesion is needed in order to ensure rapid bonding to the various surfaces, while the cohesion need only be sufficiently high to avoid formation of adhesive filaments during label stamping and stripping operations. Removable labels have low adhesion and sufficient cohesion for removal. Numerous methods are available for quantifying these different property profiles. For better comparison of the properties of pressure sensitive adhesives, standardized test methods have been developed by various organizations, including: – FINAT – Fédération Internationale des Fabricants et Transformateurs d’Adhésifs et Thermocollants sur Papiers et autres Supports – PSTC – Pressure Sensitive Tape Council. These and other PSA test methods were compared in a review article by R.P. Muny [43]. One common feature of these established test methods is that they are all destructive measurements. In the first phase of the test, a bond is formed on contact of pressure sensitive adhesive with the substrate. Initially, depending on the applied pressure, only individual, small points of adhesion form, whose number and size increase during the contact phase due to elastic deformation, viscous flow and wetting of the substrate with the adhesive. Contact formation is, therefore, determined by mechanical behavior and surface properties, such as surface tension, roughness, and adsorbate layers. Other important influencing factors are contact time, contact pressure and temperature. During the second phase, the bond is separated under the action of a tensile force, with the bond being deformed. Both processes, i.e. contact formation and separation, are influenced by the test conditions, which are different in each measurement method [27, 30, 44]. It is conceivable to carry out nondestructive measurement of an adhesive joint using nuclear magnetic resonance methods (NMR imaging). However, such methods are still under development [45]. Peel strength The most common adhesion test is peel strength testing [27], in which the force which occurs on peeling the adhesive layer off from a substrate is measured. Conditioned test strips with a certain width are rolled on to test panels using a roller with a defined weight. Stainless steel with a surface of defined roughness is used in method PSTC-1 while glass is employed in the FINAT method. But in both cases, after a certain dwell or bonding time, the peel measurement is carried out in a mechanical testing machine at constant peel rate and at a peel angle of either 180° as in Fig. 8-14, or alternatively at 90°. There are various standards for the peel strength test which differ essentially through the type of cleaning of the test panels and the bonding times. For example, the bonding time is a maximum of 1 min in the PSTC test, and 20 min and 24 h in the FINAT method. Since the dwell time has a significant effect on the level of adhesion, different values are obtained by the two methods mentioned [27].

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clamp support substrate (paper, film) test substrate (steel, glass, polyolefin)

adhesive

method A.F.E.R.A. 4001 PSTC-1

dwell time max. 10 min max. 1 min

FINAT

20 min 24 h

clamp Fig. 8-14

Peel strength at 180° [27].

Even after a bonding time of 1 h, the wetting process is not complete in all cases (Fig. 8-15). Besides the immediate value, a measurement is therefore often taken after, for example, a bonding time of 24 h, corresponding to FINAT test method 1 [27]. Besides numerous other authors [46–49], the main contributor to understanding of the very complex peel mechanism in self-adhesive tapes is Kaelble [50, 51]. He has shown that the peel strength is dependent on the moduli of elasticity and the thicknesses of both the adhesive and the support, on the peel angle, and the interaction forces at the adhesive-substrate interface [27]. Peel strength also depends on temperature and peel rate. At a given temperature, the peel strength increases with increasing peel rate. At low peel rates the viscous properties are dominant, polymer moledwell time

peel strength in N / 2 cm

10 min

4,0

30 min

4,6

1h

5,3

3h

5,5

24 h

8,0

Fig. 8-15 Peel strength of an acrylic pressure sensitive adhesive depending on the dwell time, according to W. Druschke [27].

8.2 Pressure-sensitive Adhesives

cules have time to slide past one another, to disentangle and to dissipate energy. At high peel rates the elastic properties of the polymer network predominate, the polymer molecules are not able to disentangle, and so the polymer modulus or “stiffness” increases. Since the mobility of polymer chains increase with increasing temperature, the peel strength will decrease as well – at a constant peel rate [52]. Tack Tack is defined as the limiting value of the adhesion as the contact time approaches zero. Targets for tack measurements are shortest possible contact time and lowest possible contact pressure. With this aim, a number of methods have been developed [53, 54]. The best known tack measurement methods are quick-stick, probe tack, Zosel tack and rolling ball [27]. All these methods are ultimately a refinement of the subjective finger test, which still plays significant role in forming a qualitative practical opinion [27]. In the quick-stick method corresponding to FINAT test method No. 9, a test strip is formed into a loop, brought into contact with a glass plate and then immediately peeled off again, as shown in Fig. 8-16.

clamp

support substrate

adhesive

clamp

support substrate

adhesive test plate (steel, glass, polyolefin) Fig. 8-16

Quick-stick tack measurement [27].

This FINAT method differs from the PSTC (PSTC-5) quick-stick method, in which peeling is carried out at an angle of 90° without formation of a loop [55]. The Tag and Label Manufacturers Institute (TLMI, Iowa City, IA 319-337-8247) specifies a loop tack test and within their manual, includes a host of useful TAPPI and ASTM methods for testing paper and plastic film substrates used in pressure sensitive labels, respectively.

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The advantage of the quick-stick methods compared with other tack measurement methods is that the test can be carried out in any mechanical test machine with only minimal contact pressures. Disadvantages of the method include a relatively long contact time, different contact times within a test area and different contact areas. Moreover, the peel angle is not constant in the FINAT method. A widespread tack measurement method is the probe tack method proposed by Wetzel [56] and refined by Hammond [57]. In this method, known as the Polyken probe tack method (Fig. 8-17), a cylindrical ram with a diameter of 0.5 cm is pressed from below against the adhesive layer at a defined pressure and speed and removed again at a defined speed after a certain contact time (see ASTM D2979-71). substrate adhesive

weight

weight

support

support

piston

Fig. 8-17

Polyken Probe Tack method [27].

Contact times in the region of 0.1 s are possible using this method. The fact that the measurements can be carried out simply and quickly and the conditions varied easily and widely is advantageous. However, a very complex instrument is necessary. In addition, the very small ram contact area of only 0.2 cm2 means that only small areas of the adhesive layer are measured. Air inclusions in the adhesive layer can result in incomplete wetting of the piston surface [27]. An instrument developed at BASF by A. Zosel [30–31] for fundamental studies [58–63] on the theory of adhesion operates on a similar principle to the probe tack method (Fig. 8-18). The polymer to be tested is applied to a flat steel plate in a defined layer thickness and dried. With the aid of an electric motor, the sample platform within the test chamber is moved against the piston and then away again in the opposite direction after contact. The shortest contact time that can be set is 0.01 s. The piston is connected to a piezoelectric force transducer. Variable parameters are the piston area, contact force, contact time and approach speed of the tack experiment. The instrument allows measurements from –50 °C to 200 °C. This method also allows basic studies with variation of other key parameters, such as separation speed, surface tension of the test piston, and composition of the adhesive layer [64, 65].

8.2 Pressure-sensitive Adhesives

force transducer rod test chamber sample steel support

electrical motor raises and lowers test chamber

Fig. 8-18

Zosel tack measurement.

For adhesive layers whose tack is not too low, the rolling ball tack method may be used [66]. In accordance with PSTC-6, a steel ball of defined diameter is rolled down an inclined plane at a certain tilt angle on to the adhesive test strip, as shown in Fig. 8-19. The distance traveled before the ball stops is a measure of the tack.

Fig. 8-19 Rolling ball method [27].

In contrast to the other tack measurement methods mentioned above, the rolling ball test requires simple equipment and is easy to carry out. The main difference to the other tack measurement methods is the fact that the rolling ball method does not measure force. The meaningfulness of the method is also impaired by the following characteristics: The surface of the ball can change its nature even during the first rotation, because of transfer of traces of adhesive – ball contamination. The adhesive values are relatively dependent on the viscosity and on the thickness of the adhesive layer [27].

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Reproducibility of adhesion measurements Figure 8-20 shows a statistical evaluation of tests on a commercially available adhesive tape. Test

Number of samples

Mean

Standard deviation

Coefficient of variation (%)

Peel strength after 10 min Peel strength after 24 h Quick-stick Sample tack Zosel tack Rolling ball tack

25 42 50 43 25 50

3.8 N/2 cm 9.3 N/2 cm 10.2 N/2 cm 11.3 N cm–12 13.9 J m–2 3.9 cm

0.31 N/2 cm 0.54 N/2 cm 0.34 N/2 cm 1.24 N cm–12 4.67 J m–2 1.46 cm

8 5 3 11 34 38

Fig. 8-20

Statistical evaluation of the test results [27].

The scatter in the values after exclusion of outliers is 3–38 % of the mean, depending on the test. Apart from the very low scatter in the quick-stick method, the scatter is lower for methods with longer contact times. Prerequisites for such results are very uniformly defined test specimens and exact compliance with defined test conditions [27]. Shear strength The cohesive properties of a pressure sensitive adhesive are generally determined by measuring the shear strength. Corresponding to FINAT test method No. 8, the shear strength is the time required for a certain area (25 mm × 25 mm) of a self-adhesive material to slide off a standard surface in the parallel direction to the surface with a load of 1 kg (Fig. 8-21). The standard surface used is glass. In PSTC-7, a 12.5 × 12.5 mm adhesive contact area on corrugated or stainless steel is subjected to a 1 kg load and the time to adhesive failure is recorded.

Fig. 8-21 Shear strength measurement.

8.3 Laminating Adhesives

8.3

Laminating Adhesives

Laminating adhesives are used to permanently bond various types of substrate webs together in industrial manufacturing processes. The term lamination has gained general acceptance for this industrial process. These multi-layer laminated products are generally known as laminates and can consist of three, and in many cases more, distinct layers in the total construction. Depending on the sector of industry and product class, a distinction is made below between film-to-film lamination for flexible packaging, glossy film lamination and technical lamination applications such as furniture assembly. 8.3.1

Flexible Packaging

The process of laminating single-layer web materials to give flexible multilayer film structures has been an established method for many years. Alternate production techniques include extrusion coating and co-extrusion, where plastics are melted and extruded in thin layers through an extrusion die. A wide variety of flexible laminates can be produced on fast, high-performance laminating machines using suitable adhesives. Materials with specific properties are utilized in each layer which altogether impart the performance attributes needed for the particular application. The multilayer film for vacuum-packed coffee, for example, consists of polyethylene so that the pack is heat-sealable, an aluminum foil layer for aroma retention and light barrier, and a polyester film for mechanical strength and good printability. Multilayer structures are widely used in packaging of foods, such as cheeses, snack foods (potato chips), bacon, juice pouches and boil-in-bag meals where oxygen barrier, water and heat resistance, and oil or fat barrier characteristics are required. Common laminating adhesives include solvent containing and solvent free, crosslinking polyurethanes, and two-component, water-based polyurethanes – the latter have been increasing in importance in recent years because of environmental pressures. The choice of adhesive depends on the type of film to be bonded and on the end use application. In food packaging, for example, food regulations and the resistance of the adhesive (e.g. to boiling water) are also important. Polymer dispersions The prime advantage of aqueous polyurethane and polyacrylate dispersions over solvent-containing systems is that recovery or disposal of significant amounts of solvent is unnecessary. In the packaging industry, potential for residual solvent traces in the adhesive and subsequent migration into food are also major concerns. Therefore, solvent-containing adhesives have already been replaced by environmentally friendly water-borne adhesives in a number of applications. Film laminates are produced by coating adhesive on to one side of the primary film, drying, and then laminating a second film on to the dried adhesive layer under

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heat and pressure. Multiple layer structures are typically built up by applying further adhesive and film layers at subsequent coating and laminating stations. Application rates between 1 and 3.5 g dry adhesive m–2 are typical, depending on the film or web type the adhesive used. Adhesive purchases into flexible packaging applications (including paper and plastic laminates) totaled in USA approximately 150 million dollars in 1997 [67]. In low-performance laminates, dispersions are employed as the only adhesive component. If additional boiling resistance or sterilization capability is required, from 3 to 5 % of a suitable curing agent should be added – e.g., water-dispersible polyisocyanates. The water-dispersible polyisocyanate does not just act as crosslinking agent yielding increased heat resistance, but also significantly increases adhesion to most films [68]. Figure 8-22 shows the reaction of a trifunctional polyisocyanate with an acrylate polymer and the OH groups of a corona-pretreated film.

Fig. 8-22 Reaction of a two-component aqueous lamination adhesive (acrylate dispersion + water-dispersible triisocyanate) with corona-pretreated films.

Addition of curative of this type results in a significant increase in adhesive strength; however, covalent reaction occurs to a small extent owing to steric factors. The majority of the adhesion increase is attributable to the formation of hydrogen bonds predominantly formed between OH and NH groups and polar groups of the individual substrate. A mixture of polymer dispersion and polyisocyanate has a maximum processing time of about 5–7 h. If this time is exceeded, the laminate adhesion and hence peel strength drops. Additionally, pH must be maintained between 3 and 4, higher pH result in shorter pot-lives. Polyurethane dispersions adhere strongly to a broad range of corona-pretreated plastic films. Achievable adhesion levels with PU dispersions are in many cases higher than with acrylates. As mentioned above, PU can be crosslinked for use in

8.3 Laminating Adhesives

higher-end applications, or used alone to produce medium-performance laminates. In either case, low initial “green-strengths” after forming the laminate bond are common. Moreover, it can take as many as seven days to achieve final laminate peel strengths (by chemical reaction of isocyanates with hydroxyl groups), making it necessary to store laminate rolls temporarily prior to downstream converting. A specially designed high performance acrylic dispersion that does not require addition of crosslinking agent and which yields excellent green strengths has been developed [69]. Due to the rapid development of peel strengths, laminates can be used immediately after production – thus, eliminating the need for inventory storage. Moreover, adhesive pot-life issues are eliminated because of the absence of a reactive second component. Guiding formulation Two-component polyurethane laminating adhesive

Wet parts Dispersion Crosslinking agent

Polyurethane dispersion (40 %) anionically stabilized Water-dispersible, aliphatic polyisocyanate, NCO content approximately 18 %

100 3–5

Test methods With film laminates made by adhesive bonding, the aim is to form the strongest possible laminate, thus, peel strength (i.e. laminate adhesion) is typically measured. The test is usually carried out with 15 mm wide test strips where the film layers are peeled apart using a tensile tester. The peel strength is specified in N/15 mm. The test report should furthermore indicate the failure mode; possibilities include film tear, cohesive failure, and adhesive failure to either of the film surfaces involved (including printed layers). The peel strength in the region of a heat-sealed seam is known as the seal seam strength. 8.3.2

Glossy Film Lamination

Glossy film lamination involves the covering of printed paper or board products with an optically clear, high-gloss plastic film. The process improves the brightness of the printing inks and protects the printed material from external influences (e.g. scratching, bleaching and moisture). Examples include book covers, advertising and packaging materials. Other methods besides film lamination are used to “finish” print products, including both physically and chemically cured coating systems. Glossy film lamination is used to a large extent in Europe but in North America, a special form of film lamination, predominates, called “thermo-lamination” [70]. In thermolamination, oriented polypropylene (OPP) films with a pre-applied heat-sealable adhesive layer are thermally bonded to the substrate, therefore, eliminating the need for additional adhesive.

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8 Applications in the Adhesives and Construction Industries

High-gloss film laminates have been available in Europe since the 1960s. Lamination is frequently still carried out using solvent-containing adhesives. Embossable paper board laminates were only made possible by high adhesion, two-part solventcontaining polyurethanes with crosslinking agents. However, the limited pot lives of two-component systems require increased care from the processor. Efforts to eliminate solvents to comply with more stringent emission regulations likewise here resulted in the use of aqueous polymer dispersions. One-component, self-crosslinking dispersions with shelf lives equal to those of standard polymer dispersions were developed, solving the pot life problem. Wetting and flow of the adhesive on the film are the main prerequisites for high clarity lamination. This requires the polymer dispersion adhesive layer to be as plastic and film-forming as possible during the lamination process. However, such adhesive films would then be too soft and would result in partial separation between film and board during subsequent bending and embossing operations. A considerable advance was made, however, with the development of aqueous acrylate-based polymer dispersions which crosslink after evaporation of the water, after the film has formed. Crosslinking takes place at room temperature using a reactive ketone-dihydrazide chemistry designed into the polymer dispersion (Fig. 8-23).

Fig. 8-23

Chemical crosslinking reaction of a one-component acrylic adhesive.

The polymer particles contain co-polymerized carbonyl groups which, on film formation, react with hydrazide groups of the water-soluble acid dihydrazide to form hydrazone. Increased cohesion strength results due to both inter-particle and intraparticle crosslinking reactions. A similar process also occurs between corona-pretreated polypropylene film and the emulsion adhesive thereby significantly increas-

8.3 Laminating Adhesives Fig. 8-24 Infrared spectrum of polyethylene films, according to W.-D. Domke and H. Steinke [71].

absorption 0,8 C=O COOH

0,6

0,4

corona treated PE film unteated PE film (according to W.-D. Domke u. H. Steinke)

0,2 1900

1800

1700

1600 1500 wave number in cm-1

ing the adhesion strength of the laminate. As shown in the IR spectra for the PE films in Fig. 8-24, additional carbonyl and carboxyl groups are observed on the film surface after pretreatment [71], which can then react with the dihydrazide crosslinking agent. The measurement of peel strengths after drying shows that significant crosslinking occurs after only approximately 2 h and is complete after about 48 h. In practice, this means that bending and embossing of freshly produced board laminates should only be carried out after this time period, in order to maintain film-to-film bonding [72]. Test methods Drying The progress of crosslinking over time can be recorded by measuring the surface tack. This can be carried out using the Zosel tack measurement tester described in Sect. 8.2.3 (Fig. 8-18). This enables measurement of the separation work of adhesive layers throughout the course of drying. Resistance to Delamination After allowing sufficient crosslinking time, the lamination is formed, embossed and evaluated after certain time intervals. Formed and embossed laminates are classified as “failures” if the film is observed to have delaminated in the high-stress zones, as evidenced by pale strips or spots in the otherwise high-gloss, if possible dark, lamination.

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Yellowing High-quality, durable laminates are expected to maintain print color and gloss levels even after prolonged light exposure. In addition to adhesive, the behavior of the top film layer and underlying paper and printing should also be evaluated by including appropriate control samples light exposure tests. Accelerated tests can be carried out, for example, using a Q-UV type exposure instrument with a radiation spectrum and intensity matched to that of natural sunlight. One-component acrylic adhesives perform very well in this respect, while conventional solvent-containing two-component polyurethane adhesives yellow after a relatively short exposure time. Gloss No reliable test methods for measuring the surface gloss of film laminates have, so far, been established. As such, the assessment of surface film gloss is best carried out visually. Evident “graying” is an indication of tiny air bubbles between board and film. These may be caused by inadequate application of adhesives, insufficient drying, or coalescence of the adhesive during lamination. Optical microscopy has also proven useful in confirming defect types in laminates. 8.3.3

Furniture and Automotive

Solvent based, hot-melt, and water-based dispersion adhesives are typically used in producing technical laminates for the furniture and automotive industries. The market share for aqueous dispersions is about 40 %. The main products are “heat-activatable” polyurethane dispersions, which provide excellent adhesion and extremely high bond strengths to a range of substrates. Important applications are furniture lamination (lamination of medium density fiberboard to decorative sheeting) and the lamination of moldings for interior automotive parts (e.g. dashboards, door interior panels) [73]. In North America, the total automotive adhesive market is estimated to be roughly $ 200 million (in 1997) [74]. Polyurethane dispersions are secondary dispersions typically produced by polymerization of isocyanates and diols in organic solvent. After polymerization water is added followed by solvent removal. The polyester–polyol component can be designed to form crystalline structures (Fig. 8-25), which make a significant contribution to the internal strength. The cohesive polyester-polyol crystals of commercially available PU adhesives melt at about 50 °C – the lamination adhesive is thermally activated and becomes soft and capable of heat-sealing (Fig. 8-26). During cooling, the polyester–polyol segments recrystallize, resulting in a rapid increase in the internal strength of the adhesive film. This effect is utilized in furniture lamination, in which the polyurethane dispersion is usually mixed with a reactive crosslinking agent. The polyurethane adhesive is applied using a spray gun. After the adhesive has dried, a blocking-resistant film forms, and the fiberboard elements can be stacked. They are then pressed with the decorative sheet at 60–80 °C, for example in a membrane press. On cooling, recrystallization produces a rapid increase in strength. This is necessary to counter the re-

8.3 Laminating Adhesives

Fig. 8-25

Crystalline structures of a polyurethane adhesive.

Fig. 8-26

Differential heat flow measurement of a water-borne PU adhesive.

covery forces in the thermoformed decorative sheet which are effective in the first minutes after pressing. The increase in cohesion due to the crosslinking reaction of water-dispersible isocyanate is, by contrast, a slower process, with the final strength only being achieved after days. The reversible melting of the polyester-polyol segments is the physical basis for the thermal activation ability. The possibility of heat activation is a very important ad-

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vantage of polyurethane adhesives. It is also an example of switchable properties of polymers, where heat is the switch. Formulation modifications The addition of suitable resin dispersions and small amounts of plasticizer enables the thermal activation temperature to be reduced, although this also results in a reduction in heat resistance. In contrast, the addition of crosslinking agents improves adhesion and water resistance and increases the heat resistance. The crosslinking agents used are water-dispersible triisocyanates, carbodiimides and polyaziridines. The reactivity of the crosslinking agents is reduced by various methods so that an adequate processing time is available. Mixing with other acrylic dispersions allows the properties to be modified and the costs of the adhesive to be reduced. Guiding formulation Two-component polyurethane dispersion for furniture lamination

Dispersion Crosslinking agent

Luphen D 200 A, 40 %,* or Dispercoll U 53§ Water-dispersible polyisocyanate

Wet parts 100 5

Suppliers: *BASF AG, Ludwigshafen, Germany; §Bayer AG, Leverkusen, Germany

Test methods The static peel strength of the laminate made from PVC furniture sheet and MDF is assessed visually for delamination after storage at elevated temperature.

8.4

Construction adhesives 8.4.1

Floor-covering Adhesives

The term floor-covering adhesives denotes all materials for laying flexible floor-coverings. These include secondary and unitary backed carpets, felt backed vinyl, vinyl composition tile (VCT), homogeneous vinyl sheet, rubber, and various vinyl and polyurethane backed carpet tile products. Rigid coverings, such as natural stone, ceramic and parquet, are not included here. The main driving force for the development of modern floor-covering adhesives is the need for reduced volatile organic content (VOC). Though there are regional differences, reducing emissions of organic solvents is a worldwide consumer driven trend based on environmental and health concerns (sick building syndrome). In Europe at the beginning of the 1960s, flexible floor-coverings (such as linoleum) were bonded using alcohol-soluble resin adhesives. However, these were unsuitable for the new PVC floor-coverings on the market, for which solvent-containing polychloroprene adhesives consisting of 75 % of organic solvents and 25 % of polychloroprene and resins were used. These contact adhesives had to be applied to both

8.4 Construction adhesives

sides, i.e. to the back of the floor-covering and to the floor. A precise laying technique was vital as the floor-coverings, which were laid after the solvent had evaporated, could not be corrected once laid. This technique was soon also used for bonding high-quality carpeting. A severe disadvantage of contact adhesives with a solvent content of 50 to 70 % was the emission of large amounts of solvents. Poor ventilation and the presence of an ignition source resulted in explosions, burns and even fatalities. The first polyacrylate dispersion for the production of aqueous floor-covering adhesives became available in Europe in the mid 1960s. The adhesive consisted of 40 parts of Acronal 80 D (50 %), 40 parts of chalk and – to improve the wet tack – 20 parts of balsam resin solution (70 % in toluene). The solvent content was only 6 %, and the bond strengths which could be achieved for PVC floor-coverings corresponded to the level of polychloroprene contact adhesives. In addition to significantly reducing the risk of accident, further advantages of the new type of adhesive were: – application of adhesive to one side only – the floor-coverings could be corrected – good aging resistance – fresh adhesive residues could be removed from the floor-covering and tools using water Solvent based polychloroprene adhesives have a different setting mechanism compared to water based flooring adhesives. Polychloroprene adhesives develop their bond strength through recrystallization of the elastomer from solution. In the case of emulsion adhesives, the polymer particles are initially swollen by the resin solution. During evaporation of the water, a film-formation phase occurs in which the polymer film, due to the residual solvent, has particularly high tack and low cohesion. The final strength is then achieved by two processes occurring in parallel: Firstly, post-flow of the polymers on to the substrate surfaces results in an increased contact area and consequently in an increase in adhesive strength; secondly, evaporation of the residual solvent and recrystallization of the small and rigid abietic acid molecules increase the cohesion of the polymer film to its final strength. At the beginning of the 1970s, rapid growth commenced for back-coated textile floor-coverings, which were also bonded using one-side adhesives. At the end of the 1980s, solvent-free floor-covering adhesives were produced for the first time with Acronal A 323 in combination with a plasticizer. In these, the tackifying resin solution was replaced by a resin melt. Adhesives formulated in this way have proven adhesive properties and are solvent-free substitutes as defined in the Technical Rules for Hazardous Materials, TRGS 610 [75]. The next generation of floor-covering adhesives was developed in 1994. In these, solvents, plasticizers and resins were deliberately omitted. Although plasticizers are not solvents in the sense of TRGS 610, they are nevertheless low-molecular-weight substances with a certain vapor pressure and are consequently a source of emissions. Resins were omitted because the usual abietic acid derivatives are odor carriers. The absence of the action of these two proven starting materials was compensated by a larger proportion of the softer and tackier acrylate dispersion Acronal A 200 or Primal CA-187 (see guide formulation 2) [76].

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The adhesive formulated in this way has very low emissions as defined in the requirements published by the Association of Emission-Controlled Laying Materials (Gemeinschaft Emissionskontrollierter Verlegewerkstoffe, founded in Germany) at the beginning of 1997. These requirements have no legal foundation, but are based on a voluntary self-commitment by the member companies for processor and consumer protection. The emissions are measured using a chamber test method (see Test Methods), which enables firstly, volatile carcinogenic (suspected or proven) constituents to be identified and measured after 24 h (processor protection) and secondly, the long-term total emissions of volatile organic compounds (TVOC) to be determined after 240 h (consumer protection). Corresponding to the TVOC after 240 h, three emission classes are defined: EC 1 very low emissions (TVOC <500 µg m–3), EC 2 low emissions (500 to 1500 µg m–3) and EC 3 not low emissions (TVOC >1500 µg m–3). For all classes the maximum emission of carcinogenic compounds after 24 hours has to be less than 10 µg m–3. In North America reductions in solvent levels in floor-covering adhesives have been essentially driven by environmental and VOC concerns. In March 2001, the California South Coast Air Quality Management District (SCAQMD) approved a proposal, Rule 1168, to reduce solvent levels, specifically non-exempt VOC, in floor-covering adhesives from approximately 150–200 g L–1 to approximately 50–70 g L–1. Other state agencies are expected to implement similar solvent and VOC criteria in years to come. These regulatory changes necessitate a further shift from solvent-rich adhesive systems towards water-based adhesive technology. Total North American carpet consumption in 1999 was approximately 1.6 billion square meters, split between two main sectors, residential (75 %) and commercial (25 %). Residential carpets are typically installed using glue-less installation techniques (e.g. tack-strip). However, carpets in commercial installations including schools, retail establishments, hospitals, workplaces and hospitality facilities are installed employing specially designed or multipurpose adhesive systems. These adhesives are typically formulated employing non-carboxylated, high-solids, styrene-butadiene lattices (SBR HSL), hydrocarbon resin-oil blends, and fillers as the primary components. In the past, such adhesives were formulated with resin solutions based on hydrocarbon solvents (e.g. mineral spirits) and/or plasticizers. Environmental pressures led to near elimination of solvents and plasticizers in the 1990s and introduction of increasingly lower VOC adhesives made with increasingly higher viscosity “naphthenic” oils. Flooring mastics based on non-carboxylated SBR HSL are employed primarily in carpet and mineral fiber or felt-backed vinyl glue-down applications over most common sub-floor surfaces. Such SBR based adhesives are not recommended for “unbacked” vinyl (PVC) applications due to plasticizer migration from PVC to the adhesive and bond loss issues. Conventional “felt” backings provide an effective barrier to plasticizer migration. In contrast, acrylic copolymer based floor-covering adhesives are employed in direct vinyl contact applications where plasticizer resistance is required. The main constituents of the SBR HSL based flooring adhesive are the resin–oil blend, surfactant, latex and filler (see guiding formulation 1). Carefully selected resin–oil systems are employed for both cost and property reasons (early wet tack

8.4 Construction adhesives

development, legging or webbing, initial or “green” strength, final bond strength, aging resistance and low VOC). Webbing and bond strength are also sensitive to formulation latex content, typically in the order of 10–15 %. The rosin acid and non-ionic surfactants in this formulation serve to stabilize the oil in water emulsion during compounding and to provide end-product in-can stability. Urea can be employed to achieve freeze-thaw resistance (low molecular weight alcohols and/or glycols can also be considered). Caustic solution (e.g. 20 % KOH) is added to neutralize the rosin acid and to achieve sufficiently high pH so that when the non-carboxylated HSL is added, pH shocking effects with coagulation are avoided. Clay is added for reinforcement purposes and for cost–performance optimization. Alkali sensitive emulsions (e.g. Latekoll D) are added for thickening purposes. The key parameters to control are formulation water content, clay/latex ratio and resin/latex ratio, depending on the desired adhesive cost structure. Not surprisingly, too much filler or too little resin and/or oil will result in inferior properties. Excess water will result in slower drying, and probably the need for more “water sensitive” thickeners. Like many other SBR based adhesives, HSL based flooring mastics include in-can and dry-film preservatives as well as an antioxidant package to ensure long-term performance. The total volatile organic content (TVOC) is a key property of floor-covering adhesives. In the mid 1990s, the Carpet and Rug Institute (CRI) introduced a voluntary TVOC specification for floor-covering adhesives, defined as the Green Label Program in North America. The method employs a small chamber test apparatus described in ASTM D-5116. For CRI Green Label certification, 24-h emission rates from adhesives must be <10 mg m–2 h–1 for TVOC, <3 mg m–2 h–1 of 2-ethylhexyl alcohol, and <0.05 mg m–2 h–1 for formaldehyde. TVOC and formaldehyde are quantitated employing thermal desorption GC–MS and HPLC techniques, respectively. In North America, formulation technology based on high solids content styrene butadiene lattices and naphthenic oils are widely used in the market place. Low TVOC products have evolved to meet the CRI Green Label requirements and to provide cost-effective, high performance adhesives for carpet and felt-back vinyl floorcoverings. Floor-covering adhesives made from water-based polymer emulsions contain approximately 10–20 % dry polymer in both European and North American systems. Floor-coverings are bonded using an application rate of about 250–500 g m–2. Guiding formulations 1. High solids content styrene butadiene (HSL) and naphthenic oil based flooring adhesive for carpet and felt backed vinyl floor-coverings with emissions satisfying CRI Green Label TVOC requirements. Such adhesives are not recommended for homogeneous or solid vinyl sheet goods where plasticizer migration is a concern.

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Wet parts 9.7 Hydrocarbon resin Neville LX 1200* Rosin acid Melhi 2.2 12.0 Naphthenic oil Tufflo 1200‡ 0.2 Surfactant Igepal CO-897 (70 %)§ Surfactant Igepal CO-530 0.1 Anti freeze Urea (50 %) 1.4 Neutralizing agent Potassium hydroxide (2.5 %) 8.7 41.4 Filler Huber 95 (70 % clay slurry)# 20.4 HSL dispersion Butonal NS 104 (71 % s.c.)** 3.9 Thickener Latekoll D (pH adjusted 8 % solution)** Total 100 Procedure: Add resins to oil at 140–150 °C, stir until homogeneous, then cool to 95–98 °C. Surfactant, urea and KOH solution are slowly added under mild agitation to form an emulsion. The clay, SBR, and thickener are then added to the resin emulsion in the order indicated above. 2. Very low emissions corresponding to the requirements of the German association of Emission-Controlled Laying Materials (GEV) Wet parts Dispersion Acrylic dispersion 24.4 2.0 Plastiziser Plastilit 3431** 0.2 Antifoam Agitan 282†† 0.5 Dispersant Pigment dispersant NL** 42.0 Chalk Ulmer white XM‡‡ 10.9 Thickener Latekoll D 2 %** Resin melt Gum resin WW: Plastilit 3431 = 8:2 16.0 4.0 Resin Poli melt 15§§ Total 100 Suppliers: *Neville Chemical Company, Pittsburgh, PA, USA; †Hercules Incorporated, Wilmington, DE, USA; ‡Lyondell Lubricants, Houston, TX, USA; §Rhone-Poulenc, Cranbury, NJ, USA; # J.M. Huber Corp., Wrens, GA, USA; **BASF, Charlotte, NC, USA; Ludwigshafen, Germany; ††Münzing Chemie, Heilbronn, Germany; ‡‡Omya, Cologne, Germany; §§ Erbsloeh, Krefeld, Germany

Test methods North American industry standard test methods for floor-covering adhesives are currently not available. However, methods are under development by ASTM committee D14.70.12, Carpet Adhesives. As a guiding method for evaluating carpet to plywood peel strength the adhesive is troweled on to plywood (application weight ca. 300 g m–2), after 20 min the carpet or vinyl is laid and pressed down at room temperature, and, after a certain curing time at room temperature or 60 °C, the floor-covering is peeled off at 90° angle using a tensile tester. Drafts of European test standards for measuring peel and shear strengths of floor and wall coverings were submitted for approvals in early 1999 by Technical Committee CEN/TC193. These methods, referred to as prEN 1372 and prEN 1373 respectively, are fundamentally quite similar to methods developed in Germany through a

8.4 Construction adhesives

collaboration between floor-covering manufacturers, official test institutes, and adhesive and raw material producers. German test standards and specifications for peel and shear strength discussed below are summarized in DIN 16860 for PVC floor-coverings and in DIN 53269 for textile floor-coverings. Peel resistance The peel resistance is the force, per unit width of floor-covering, which results when peeling-off a bonded sample perpendicular to the original adhesive bond line. According to regional requirements, the adhesive is applied to plywood, cement board or other substrate using a trowel spreader and, after a certain “open” or evaporation time, a 5 cm × 30 cm floor-covering strip is laid on to the adhesive bed and pressed down, preferably using a fixed weight roller (e.g. 5 kg). After a storage time under defined standard laboratory conditions, the floor-covering strip is peeled perpendicular to the adhesive join at a certain speed using a tensile testing machine (Fig. 8.27). The peel forces which occur during this operation are measured and specified in N mm–1. According to DIN 16860, the average peel strength must have a certain minimum value (e.g. 1 N mm–1). Other factors such as early bond strength development, water resistance, and accelerated oven aging (e.g. 50 °C) can also be evaluated using variations of this general method.

Fig. 8-27

Measurement of peel resistance.

Shear strength The shear strength is the force per unit area which results in fracture of bonded samples parallel to the bond joint. Adhesive areas of 1000 mm2 (ca. 2.5 cm × 5 cm) are produced on plywood or other cementitious substrates using a template and trowel, then the floor-covering is laid on to the adhesive area and pressed down. After a storage time under standard laboratory conditions, the floor-covering is removed parallel to the adhesive join at a defined speed using a tensile testing machine. The shear

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forces which occur during this operation are measured and specified in N mm–2 . According to DIN 16860, the mean of the shear strength must have a certain minimum value (e.g. 0.3 N mm–2) to be considered “passing”. The curing of water-based adhesives is typically dependent on temperature and humidity conditions. For this reason, it has proven useful in the development of water-based adhesives for carpets and other floor-coverings to also test peel strength as a function of time. There are two additional test methods for this: Green strength development The 5 cm wide floor-covering strip is laid in the wet emulsion adhesive after a 10–20 min drying time, and then pressed down. Floor-covering specimens are peeled off at a certain speed perpendicular to the adhesive join after a further 10, 20, 30, and 60 min. The peel forces, which occur during this operation are measured and specified in N/5 cm strip. Depending on drying conditions and adhesive composition, peel strengths on the order of 10–15N/5 cm strip should be expected within 15–30 min after applying the floor-covering strip. When peel testing SBR HSL based mastics in particular, it is equally important to also report the degree of web or leg development as a function of time. While not necessarily correlated with final adhesive bond strength, installers nevertheless commonly look for early web development as an indication of adhesive quality. Open time Open time represents the maximum recommended time after troweling that the floor-covering should be laid into the adhesive. To quantify this parameter, a 5 cm wide floor-covering specimen is placed in the partially dried emulsion adhesive after 30, 45, 60 and 90 min airing times and pressed down. The floor-covering is immediately peeled off at a certain speed perpendicular to the adhesive join. The peel forces, which occur during this operation are measured and specified in N/5 cm. After exceeding the open time, unacceptably low peel strengths and poor adhesive grab on to the floor-covering substrate are observed. As with green strength development, temperature and humidity conditions also have a significant impact on the open time actually found under installation conditions. Typically, longer open times and delayed green strength development are expected at higher humidity and lower installation temperatures (i.e. because of slower water evaporation from the adhesive itself). These two methods have a reproducibility of ±20 %. Chamber method for emission measurement North American “CRI Green Label” and European chamber TVOC methods for flooring adhesives are fundamentally similar. In both cases, the adhesive sample is applied to a stainless steel or glass plate and immediately sealed in a stainless-steel test chamber carefully maintained under well defined temperature and humidity conditions (e.g. 23 °C, 50 % rel. humidity). VOC are collected onto adsorption tubes containing suitable adsorbents after flowing purified air through the chamber at a controlled rate. The CRI Green Label method requires volatile and formaldehyde samples be collected at the 24 h point. However, the German Association of Emis-

8.4 Construction adhesives

sion-Controlled Laying Materials specifies that substances known or suspected of being carcinogenic must be determined at the 24 h point (in accordance with Hazardous Materials Regulation/TRGS 905), but also that long-term emissions be determined after 10 days. After desorption, the emitted substances are determined by gas chromatography (GC-MS coupling) or liquid chromatography. The long term emissions are quantified using toluene as standard substance for volatile substances in low concentration. Loading of the chamber should be at 0.4 m2 m–3 and the air exchange rate at 0.5 chamber volumes per hour (1.0 air exchanges per hour in the ASTM protocol). 8.4.2

Sub-floor and Wall Mastics

Sub-floor and wall mastics are employed in a host of building and home construction applications. Sub-floor type mastics are typically used for fixing plywood subflooring to lumber floor joists and more recently applied to secure plywood roof sheathing to roof rafters and trusses of existing roofs. In both of these applications, nails are also used to hold the plywood in place, but the adhesive greatly augments the structural strength. For example, in wind exposure tests [77], adhesives were found to increase resistance to wind uplift forces by factors of 2–3 compared to traditional nail down methods, important for homes and buildings in areas prone to tornadoes and hurricanes. Plywood-lumber sub-floor mastics are employed in quality construction systems to eliminate nail pops and floor squeaking. Improved mastic systems are required which are compatible with commercial oriented strand boards (OSB) used increasingly in place of standard plywood in both sub-floor and roofing constructions. Sub-floor mastics are generally supplied in cartridge form (310 cm3 and 860 cm3 tubes) and applied as a 6 mm bead using standard caulk gun applicators. Overall market in North America for this class of adhesive is approximately $35 million year–1 with 3 % growth per annum. The market consists predominately of formulated solvent based styrene–butadiene polymers, moisture-cure polyurethanes, and high performance water based acrylic systems. Increasing VOC concerns coupled with lower solvent threshold limits in California [78] are driving the eventual movement to environmentally friendly, low VOC water-based mastics. An estimated 75 % of the mastic products sold in North America indicate compliance to stringent AFG-01 (i.e. “Adhesives for Field Gluing”) requirements. ASTM D 3498 is quite similar to the AFG-01 standard originally written by the American Plywood Association. These methods define a series of six test specifications (Fig. 8-28), five of which involve shearing plywood to lumber wood block specimens in compression mode after various carefully controlled material conditioning and specimen curing protocols (Fig. 8-29). The sixth test, called oxidation resistance, is a mandrel flexibility check on films of dried mastic after exposure to oxygen at high temperature in an oxygen bomb apparatus. Water based mastics formulated with acrylics or styrene-acrylic polymer dispersions typically pass oxidation resistance requirements. The most challenging pa-

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8 Applications in the Adhesives and Construction Industries Fig. 8-28

Force

Plywood on lumber shear test

(AFG-01).

Adhesive Bond Line Plywood Lumber

Conditioning of materials

Assembly curing

Test specifications

Dry lumber Wet lumber

48 h, 37 °C/30 % rh 48 h soak lumber; 48 h, 37 °C/90 rh for Plywood

28 days, 37 °C/30 % rh 28 days, 37 °C/90 % rh

Shear >1000 N Shear >1000 N

Frozen lumber

48 h soak lumber 48 h; –14 °C for lumber and plywood

5 days, –14 °C 21 days, 4 °C/50 % rh 7 days recovery

Shear >667 N

Moisture resistance

48 h, standard conditions

28 days, 22 °C/50 % rh 3 cycles of 4 h H2O/o.n. dry 37 °C 7 day recovery

Shear >1000 N; <10 % bond failure

Gap filling

48 h, standard conditions

28 days, 22 °C/50 % rh (16 gauge wire)

Shear >667 N

3 days, 22 °C/50 % rh; 2 days, 48 °C/50 % rh; 500 h, 70 °C/20bar O2

bend over 6 mm Mandrel: no cracking

Oxidation resistance

Fig. 8-29 Performance specifications for Adhesives for Field Gluing Plywood to wood framing (AFG-01) rh relative humidity, o.v. over night, 1 N = 0.2248 lbf.

rameters to balance, however, involve moisture resistance and frozen lumber. Frozen lumber adhesion is promoted through the use a combination of coalescents and freeze-thaw aids (e.g. glycols, alcohols). Levels of such additives are kept to a minimum in order maintain maximum resistance to moisture. The wall mastic market in North America is roughly twice the size of the sub-floor market mentioned above. Typical wall mastic applications include: – wood bonding to concrete walls and floors – bonding drywall or gypsum board and paneling to studs and framing structures

8.4 Construction adhesives

In concrete bonding applications, use of adhesives eliminate the need for time consuming drilling operations and prevents damage to the concrete substrate. Drywall adhesives are used increasingly as builders look for ways to replace time consuming hammer and nail approaches [79]. Test specifications for drywall adhesives are described in ASTM C-557, Adhesives for Fastening Gypsum Wallboard to Wood Framing. Like sub-floor mastics, wall mastics are also manufactured in solvent based, polyurethane and water based formulations. Due to reduced strength requirements (i.e. relatively low inherent strength of drywall), drywall mastics are typically filled to a higher degree and thus, are less costly compared to AFG-01 sub-floor mastics. Guiding formulation Solvent-free plywood-lumber sub-floor adhesive

Wet parts 56.7 Dispersion Acronal DS 2159* 0.1 Dispersant Pigment disperser N* Dispersant Sodium tripolyphosphate 0.1 0.05 Defoamer Nopco NXZ† 3.6 Tackifier Snowtack 301 A‡ Anti freeze Ethylene glycol 3.0 2.3 Coalescent Eastman DBA§ 11.4 Filler Duramite†† 22.7 Filler Clay†† 0.3 Thickener Latekoll D* Total 100 Ingredients are combined in the order indicated above at room temperature with high-speed agitation. Adhesive properties: 70 % solids content, pH 9, 40 % polymer on dry AFG-01 testing: Moisture resistance: 3100 N (>1000 N required) Frozen lumber: 880 N (>667 N required) Bond failure: None (<10 % required) Suppliers: *BASF Corporation, Charlotte, NC, USA; †Henkel, Ambler, PA, USA; ‡Akzo Nobel, Woodstock, CT, USA; §Eastman Chemical Company, Kingsport, TN, USA; ††ECC International, Atlanta, GA, USA

8.4.3

Sealants

Sealants or caulks in accordance with ISO 6927 are materials which remain plastic or elastic and are used for sealing a joint between two separate construction parts, thus, eliminating passage of the “elements” through the joint (i.e. hot or cold air, moisture, insects). While a caulk needs only fulfill the above general purpose, sealants are considered higher end products that must additionally perform after repeated extension-compression cycles originating from material temperature and humidity fluctuations. Historically, the starting point for sealants was the natural raw material linseed oil, mixed with chalk to give window putty.

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Synthetic polymers suitable as binders for the production of caulks and sealants include silicones, polyurethanes, polysulfides, and aqueous polymer emulsions. The importance of these sealants in the construction industry has also increased considerably through the increased use of prefabricated elements. The movement of components must be absorbed and compensated by the joint sealant. These movements can be expansion, contraction, or shear. While silicones, polyurethanes and polysulfides set through a chemical reaction, water based emulsion sealants achieve their functional end state by simple physical drying, i.e. evaporation of the water. The majority of emulsion sealants are composed of acrylic emulsions and to a lesser extent vinylacrylic and other copolymers. Polymer emulsion sealants can be used on all sorptive substrates, such as concrete, aerated concrete, cement panels, plaster and wood. They are used for sealing all types of internal joints, connecting joints (internal and external) and expansion joints (internal and external) with a movement capability of 10–15 %. In Europe, the joint design is stipulated in the relevant standards, for example DIN 18540. The total caulk and sealant market in North America is estimated 500 000 tons of formulated sealants [80], with acrylic emulsion sealants comprising approximately 15 % of the total market, and 23 000 tons dry acrylic resin. Water based sealants, sold primarily in cartridge tubes, are used predominately in construction applications whereas reactive urethanes and silicones are used in more demanding construction and automotive applications. Formulation ingredients The properties of an emulsion caulk or sealant are affected by the type of emulsion, and by the type and amount of the fillers and/or pigments, plasticizers and thickeners [81]. Fillers reinforce and increase the volume of the sealant. Fillers also reduce formulation costs and affect the technical properties of the sealant itself. Common fillers for sealants are calcium carbonate (chalk), aluminum silicate (clay), barium sulfate and silicic acids. Finely divided fillers, such as talc and Microdol 1, reduce the surface tack while simultaneously stiffening the film. Thixotropic fillers, for example fumed silica or SiO2, improve the gunnability and reduce the sag of the compositions. Pigments are used to color sealants; the white pigment used is usually titanium dioxide. Depending on the degree of compatibility, plasticizers and coalescents reduce the glass transition temperature of the polymeric component and thereby enhance low-temperature flexibility and film elongation. While improving formulation cost, excessive plasticizer detrimentally affects film strength, tack and therefore, dirt pickup resistance. For the production of polymer emulsion sealants, phthalates, dibenzoates, polyisobutenes and Plastilit 3060 have proven successful. With Plastilit 3060 (propylene glycol alkylphenyl ether) as plasticizer, sealants with faster skin formation after application and lower Shore hardness which are particularly elastic at low temperatures are obtained. Experimental testing to ensure long-term polymer-plasticizer compatibility and minimum tendency to volatilize or migrate to the exposed surface is always recommended.

8.4 Construction adhesives

Dispersing aids and surfactants improve the incorporation of fillers and pigments and improve the sealants’ storage stability. In general, low molecular weight polycarboxylic acid salts are used. Silane-based coupling agents can be employed to improve adhesion to difficult substrates (e.g. glass, aluminum). A new class of hydrolysis resistant silanes are now available [82] which minimize self-crosslinking reactions, improve storage stability and provide desired enhanced adhesion performance even after extended package aging. Sealants containing small amounts of silane adhesion promoter are referred to as “siliconized”. Anionic thickeners based on polycarboxylic acids, associative thickeners and fumed silica are used to adjust the rheological behavior of sealants. Highly disperse fumed silica with a average particle diameter of from 10 to 30 µm is used as thixotropic agent to ensure that the sealant flows out of the cartridge well even under gentle pressure, but has low sag after removal of the shear stress. Addition of suitable antifreeze agents, for example ethylene glycol, protects the sealants against freezing during storage and transport. Both “wet phase” biocides and “dry film” preservatives should also be added to the sealants produced using polymer dispersions in order to achieve adequate protection against microbiological attack. The suitability of these preservatives must be established and monitored experimentally. The mechanical properties of water-based sealants are essentially determined by the ambient temperature and atmospheric humidity. Figure 8-30 shows the tensile stress of a sealant at 50 % elongation as a function of time under various climatic drying conditions. Sealant types Three types of water-based sealant are commercially available; clear, translucent, and filled sealants. The majority of the market (>75 %) consists of the filled variety. Clear and translucent sealants are used when a clear or translucent “look” is desired and in applications requiring higher adhesion performance, elongation, and dried film strength. While fillers serve to reduce formulation cost and surface tack, thus, dirt pickup, they reduce adhesion performance and film elongation. Clear and translucent sealants consist primarily of polymer dispersion (ca. 75–95 % by weight) and various formulation auxiliaries such as plasticizers, defoamers, preservatives, thickeners, and freeze-thaw agents. Since fillers are not used, formulation solids content is essentially defined by the solids content of the polymer dispersion used, which for currently available materials, is typically <65 %. Higher water contents promote slower sealant drying rates. Relatively hard acrylic copolymers (Tg = –10 to +10 °C) are needed in clear sealants to minimize surface tack and subsequent dirt pick-up. Translucent sealants are formulated similarly but with small amounts of fumed silica thickener to adjust flow properties. Filled sealants are typically formulated with approximately 25–35 wet parts polymer dispersion per 100 parts total formula, with filler to binder ratios in the range from 2 to 4. With high filler loads, formulation solids contents in the vicinity of 90 % are possible. Because of the “stiffening” effect of most fillers, filled sealants are pro-

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8 Applications in the Adhesives and Construction Industries

Fig. 8-30 Tensile stress values of a dispersion sealant at 50 % elongation and various climatic influences, r.h. relative humidity.

duced either with lower Tg emulsion polymers (–40 to –30 °C) and low plasticizer levels or with higher Tg copolymers (<0 °C) and higher levels plasticizers which “soften” the dried sealant film. Guiding formulations 1. Filled sealant with good elasticity (even at low temperatures) and a broad adhesion spectrum Wet parts Dispersion Acrylic dispersion, pH 8 with NaOH 20 % 31.5 2 Plasticizer Plastilit 3060* 10 Pigment paste Plastilit 3060/Kronos 2056 TiO2† (1:1) 0.2 Emulsifier Lumiten N-OG* 0.1 Dispersant Pigment dispersant N* Omya BLP 3† 55.5 Filler, CaCO3 0.7 Thixotropic agent Silicic acid HDK H 20‡ Total 100 Sealant properties: filler/binder ratio = 2.71, solids content = 89 % Polymer dispersion properties: 65 % solids, Tg = –30 °C

8.4 Construction adhesives

2. High solids, rapid drying, low cost formulation for common gap filling applications; paintable Wet parts Dispersion Acrylic dispersion 23.8 10 Plasticizer Palatinol N, C-9 phthalate* 0.3 Surfactant Emulphor OPS 25, non-ionic* 0.6 Dispersant Pigment disperser N* Filler Mikrodol 1, CaCO3 61 Sealant properties: filler/binder ratio = 4.3, solids content = 90 % Polymer dispersion properties: 60 % solids, Tg = –10 °C Suppliers: *BASF Corporation, Charlotte, NC, USA; †Omya GmbH, 50968 Cologne, Germany; Tipure R901-01 TiO2 also available from Dupont Chemical, Wilmington, DE, USA; ‡ Wacker-Chemie, 81737 Munich, Germany; Aerosil 200 fumed silica also available from Degussa Corporation, Ridgefield Park, NJ, USA.

Production of sealants Vacuum planetary mixers have proven particularly successful for the production of water-based polymer emulsion sealants. The dispersion is adjusted to pH 8 using 20 % sodium hydroxide solution. Thickener and dispersant are then added, and the mixture is stirred briefly at low speed. The plasticizer is then added directly. It has proven favorable in experiments to grind finely divided pigments (for example titanium white and iron oxide black) with the same amount of plasticizer in a roll mill and to incorporate the resultant pigment paste into the dispersion before the fillers. This suppresses the formation of pigment particle agglomerates, which can otherwise easily occur. After a stirring time of about 8 min at 30–40 rpm, the fillers are added in 3 or 4 portions; after each addition, the mixture is stirred for about 5 min until smooth. The speed is then gradually increased to about 80 rpm. After the final addition of filler, the stirring arms are scraped and stirring is continued for a further 5 min. The homogeneous mixture is then deaerated for about 5 min under a vacuum of 900 mbar with stirring at 20 rpm and then packed into polyethylene or foil carton cartridge tubes. A minimum shelf life of 6 months can be assumed in properly sealed cartridges. Test methods Resistance to flow The resistance to flow is the property of a sealant to remain in the specified shape after processing. For testing, a U-profile is filled with sealant (EN 27390, DIN 52454, ISO 7390, ASTM D-2202). Elastic recovery Elastic recovery is the magnitude of the recovery of a sealant after prior elongation followed by release. For the measurement, two concrete test specimens are joined together, the joint is stretched by 50 %, and after 24 h the separation is measured after release (EN 27389, DIN 52458, ISO 7389, ASTM C-736).

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Adhesion-elongation test Two concrete test specimens are joined together and pulled apart in a tensile testing machine at 6 mm min–1 until the breaking point is reached (DIN 52455, EN 28340, ISO 8339, ASTM C-735). The mechanical properties of a good emulsion-based sealant should be an elastic recovery of 60–70 %, a tensile stress of 0.1–0.15 N mm–2 and an elongation at break of 200–300 %. In North America ASTM C-834 and C-920 are used. These are actually umbrella specifications constructed from a host of individual ASTM test methods [83]. In general, latex sealants are typically unsuitable for sealing joints which are constantly exposed to water or subjected to strong expansion movements. However, sealants can be developed which satisfy the specifications defined in ASTM C-834 for “latex sealing compounds”. Higher performance “elastomeric joint sealants” based on silicone, polyurethane and advanced acrylic emulsion technologies are typically designed to satisfy the ASTM C-920 standard. ASTM C-920 class A sealants are those that can withstand deformations as high as 50 % while class B sealants tolerate deformations as high as 25 %. Notes for use Water-based sealants generally adhere sufficiently well to sorptive substrates without pre-coating. However, in order to achieve greater reliability, in particular to bind dust particles, pre-coating of the joint edges with dilute sealant (for example 1 part of sealant with 3 parts of water) has proven successful in practice. The sealant is introduced into the joint using a manual or compressed-air caulking gun. The surface is then smoothed using a wet flat brush. Jointing should not be carried outside in the rain or at temperatures below +5 °C since the dispersion is still water-sensitive after application. Depending on the temperature and relative atmospheric humidity, it requires 30–60 min to form a sufficiently thick skin on the surface. Tools should be cleaned with water immediately after use as the residues can only be removed mechanically once they have dried. 8.4.4

Ceramic Tile Adhesives

In contrast to the traditional thick-bed method in which tiles are laid in thick layers of mortar, the thin-bed method involves adhesive bonding. This means that the tiles are laid into a wet adhesive bed trowel applied on to a substrate. The thickness of the adhesive bed is variable and depends both on the size of the tiles and on the nature of the tile undersurface. The greatest advantages of the thin-bed method are the high application speeds and lower mortar coat or application weights (i.e. cost). However, the thin-bed method can only be used if the substrate surface is relatively flat. This prerequisite is achieved in most cases by using appropriate substrate preparation techniques (e.g. cementitious self-leveling repair underlayments and floor patching compounds) and industry proven construction materials, such as cementitious backerboard, gypsum wallboard, underlayment grade plywood or prefabricated con-

8.4 Construction adhesives

crete panels. In thin-bed ceramic tile applications, both cementitious adhesives (mortar) and non cementitious systems (mastics) are applied. Ceramic tile mortar adhesives (thinsets) Thinset mortars are employed in demanding interior and exterior floor and wall applications where there may be standing water or high moisture exposure. Both one component polymer modified thinsets and two component cementitious adhesives are used. Ethylene vinyl acetate copolymers (EVA) are the predominant powder polymer base used in one component polymer modified thinsets, they are described in Chapter 13. Recently styrene acrylics or straight acrylics and styrene butadiene copolymer powders gain an increasing market share. Two component thinsets systems combine a cementitious powder mix and a separate polymer dispersion admixture. Polymer modification imparts a wide range of performance improvements to ceramic tile mortar adhesives, including improved bond strength, water resistance, flexibility, impact strength, freeze-thaw resistance, improved mix workability. Polymers also aid in promoting adhesion to difficult substrates such as plywood and vitreous tiles (porcelain). Two component thinset adhesives employ acrylic, styrene-acrylic and SBR polymer emulsions in the admix component. The benefit of a styrene-acrylic polymer compared to a straight acrylic backbone involves increased hydrophobicity, thus, improved moisture and alkali resistance. Ceramic tile adhesive mastics One component, ready to use ceramic tile mastics are used in interior residential and light commercial applications where only intermittent water exposure is expected. They are used both for professional tile setting and – owing to their simple processing properties – in the “do it yourself” (DIY) market. They have a broad adhesion spectrum, a long working time and form a flexible adhesive film. Mastics are generally water based and are supplied as a smooth trowelable paste for setting smaller sized tiles (<15 cm × 15 cm). Guiding formulation Low emission, ammonia-free, water resistant ceramic tile mastic without film forming aids Wet parts Diluent Water 6.4 0.1 Defoamer Nopco NXZ* 0.6 Thickener Natrosol CG 450 † Dispersion Acrylic dispersion 20 Filler Silica sand (0.16 mm) 39 Filler Calcium carbonate (23 µm) 17.1 Filler Calcium carbonate (7 µm) 17 Total 100 Suppliers: *Henkel Corporation, Ambler, PA, USA; †US Silica, Ottawa, IL, USA

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Action of the additives Cellulosic thickeners (Natrosol) are used to adjust the viscosity and regulate mix consistency, working time and spreadability. Special thickeners (Attagel 50, an attapulgite clay) are used to generate a flow barrier, so that the tiles do not slip under their own weight. In cementitious systems, cellulosics are used to provide water for cement hydration over an extended time period due to their natural tendency to retain water. Surfactants improve the homogeneity and shelf life of adhesives with high filler content and also have a tendency to extend open time. Preparation of polymer-emulsion-based CTA mastics Additives are incorporated into the polymer emulsion in the stated sequence. The additives are added in portions, and the mixture is stirred after each addition until it is smooth again. These high-viscosity mastic adhesives (ca. 500 000 mPa s) can in principle be prepared using any mixing equipment with an appropriate stirrer geometry. Planetary mixers and turbulent mixers are particularly recommended. Preservatives are added to the adhesives to protect them against microbiological attack. Test methods for assessing tile adhesives Well defined test methods are available in North America and Europe for evaluating ceramic tile adhesives. Test methods used in Germany for emulsion-based mastics are described in DIN EN 1324 (adhesive shear strength) and DIN EN 1346 (correction or adjustment time). Cementbased CTA are tested by DIN EN 1348 (pull of strength) and again DIN EN 1346 (open time). Classification is determined for both types of adhesives by prDIN EN 12004. North American test methods and specifications for tile adhesives are described in the following American National Standards Institute (ANSI) standards: One-component CTA mastics with glazed wall tiles ANSI 136.1 Latex-Portland cement mortar with glazed wall tile, quarry tile and porcelain and/or mosaic tile ANSI 118.4 Latex-Portland cement mortar with quarry tile on plywood ANSI 118.11 DIN Methods In the pull-off strength test performed with cementions CTA, the tile adhesive is first applied to concrete. After 20 min, earthenware tiles (50 mm × 50 mm) are pressed into the adhesive bed. After a storage time under standard atmospheric conditions, tension anchors (50 mm × 50 mm) are attached to the smooth tile surface using a two-component adhesive (epoxy). A tensile tester is used to measure the force needed to detach the tile from the concrete; it must be greater than 0.5 N mm–2. To estimate adjustment time, earthenware tiles are laid in the adhesive bed as described above, rotated by 90° after 10 min and then rotated back into the original position. The adhesion pull strength after storage must still be at least 0.5 N mm–2. Also described in DIN 18156 Part 3 is a method for evaluating vertical slip of tiles under their own weight.

8.4 Construction adhesives

ANSI Methods The key mechanical tests defined in ANSI 136.1 involves tile-to-tile shear strength after dry conditioning and after water immersion. In this method, adhesive mastic is applied to the unglazed back of a 108 mm × 108 mm test tile using a specified template to produce a pattern of equally spaced circles of adhesive on the back of the tile. After 2 min airing time, the unglazed back of a second tile is carefully oriented on to the adhesive bed – tile to tile separation is controlled with the use of spacer rods. Test assemblies are then subjected to compression under a 6.8 kg load for a period of 3 min. Bonded tile assemblies are then dried for 72 h at 50 % rel. humidity and 22 °C and then further conditioned for 21 days in an air circulating oven set at 49 °C. Wet, type 1 shear strengths are determined by shearing wet test specimens at a defined rate using a tensile tester, immediately after immersing the bonded test assemblies in a water bath for 7 days. The minimum ANSI wet, Type 1 shear strength specification for CTA mastics is 3.5 kg cm–2 (50 psig). ANSI 118.4 includes a complete series of methods to evaluate application properties (initial and final set, open time, adjustability, vertical sag) of latex–Portland cement mortars and shear strength of bonded tile assemblies. 8.4.5

Polymer-modified Mortars

Polymer modified Portland cement mortars are used in a range of primary construction and concrete and mortar repair applications (Fig. 8-31) [86, 90].

Damaged Concrete Fig. 8-31

Waterproof Membrane Fine screed Repair Mortar

Application of repair mortar.

They are applied in bridge decking and airport runway applications, in repair applications, where the mortar layer can be as much as 5–8 cm thick, in industrial floor screeds, where up to a 5 cm layer is applied over standard concrete, in fine screeds or patching compounds, where up to 1.5 cm of repair mortar is applied on floors, in self-leveling underlayments, where thickness from 2–3 cm to “feather edging” are common. Fundamentals of concrete and Portland cements, hydration chemistry and classification of admix agents were reviewed by Kosamatka, Panarese and Soroka [84, 85]. Polymers are added to Portland cement based mortar systems for a number of reasons. Firstly, polymers improve the key properties of the fresh, non hardened mortar, i.e. adhesion, workability and open time. Polymer additives also tend to have a plasticizing effect on cementitions mortars, there by reducing the amount of added water to achieve needed workability and mortar flow properties. Minimizing added water results in fewer capillary pores, lower porosity and stronger cements. Secondary, the properties of a hardened cement mortar are improved. Properly selected

241

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8 Applications in the Adhesives and Construction Industries

polymers, i.e., those with Tg lower than 15 °C, form a film within the mortar matrix [86] thereby filling voids, pores and reducing the potential for ingress of water and dissolved salts. This reduced permeability to salts (e.g. chloride) provides protection against corrosion of underlying steel reinforcing elements. With reduced water ingress, polymer modification also promotes improved freeze-thaw resistance of the mortar – a feature which is especially important in exterior or cold climate applications. Improved tensile, compressive and flexural strength are generally realized in polymer modified cement mortars, provided sufficient levels of defoamer are added to counteract the tendency of emulsion polymer additives to induce excessive foam generation. Similarly, concrete used, e.g. in critical bridge decking applications is typically polymer modified in North America to extend service life and reduce repair costs by minimizing deterioration caused by exposure to the many forces of nature (Fig. 8-32). Deterioration of concrete concrete surface H 2O

Filler particle Crack Steel corrosion - Rust

CaCl

Reinforcing steel

SO2 Pores

Concrete damage

pH = 12 - 13 CO2

Carbonation zone

Fig. 8-32

Deterioration of concrete.

As with ceramic tile thinset mortars, styrene-acrylics, acrylics and SBR polymer emulsions and their dry polymer analogs, including EVA powders, are employed across the range of polymer modified cementitious applications. While polymermodified mortars are used widely, for cost reasons, polymer modified concretes are seldom used – with the exception of bridge decks where SBR are almost exclusively employed. However, in Europe the use of additives in construction concrete is regulated by technical guide lines. When developing new polymer modified mortars these days, formulators should also consider the impact of alkali exposure on long-term hydrolytic stability of the polymer and the subsequent impact of any degradation on application performance

8.4 Construction adhesives

and service life. By their very nature, SBR are inherently resistant to alkali induced hydrolysis compared to some vinyl acetate containing polymers which, by virture of the vinyl ester linkage, are more prone to hydrolysis. Guiding Formulations Repair mortar Part A: Liquid Component

Dispersion Diluent Part B: Dry Component Filler Cement Filler

Carboxylated SB Hydration water

Wet parts 54 83

Silica sand Portland cement Microsilica

721 273 6

Polymer/cement ratio Water/cement ratio Performance after Flexural strength (bar) Compressive strength (bar)

0.1 0.4 1 day 60 270

28 days 110 560

Test methods The test methods are summarized in Chapter 13. 8.4.6

Waterproofing Membranes

Concrete is the most prevalent building material used in the world today [84, 86]. While concrete typically exhibits tremendously high strengths, the fact remains that it is nevertheless porous in nature and thus, is susceptible to direct moisture penetration and water vapor ingress. Additionally, acids (i.e. CO2, SO2) may penetrate into the concrete and lower the pH of cementitious materials. Along with migration of chloride and other salts dissolved in water deep within the concrete, this may cause severe irreversible damage to steel reinforcements present (i.e. corrosion). Certain additives can be introduced into the concrete mix to minimize water and salt penetration (e.g. latex admixture, calcium stearate), but due to the costs involved, these approaches are employed in only the most demanding applications, such as, bridge decks and parking garages. Moreover, concrete admixtures do not protect the concrete surface layer nor are they able to eliminate moisture penetration in cases where direct hydrostatic pressure forces exist – protective waterproofing membranes are needed. A host of different commercial waterproofing systems have been developed to protect against the harmful effects associated with water intrusion into concrete. These can be broken down into pre-formed membrane sheets and applied coating systems.

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8 Applications in the Adhesives and Construction Industries

Pre-formed membrane sheets typically consist of rubber modified bitumen supported on a polyethylene web with a release liner to protect the highly adhesive bitumen layer. These products are available in rolls with total membrane thickness of approximately 1–2 mm and can be fiberglass-reinforced to provide improved membrane dimensional stability. The adhesive nature of the rubber-modified bitumen assures excellent sealing at the roll overlaps. A host of applied coating systems exist based on rubberized bitumen emulsions, solvent borne synthetic rubber and asphalt solutions, two-part epoxies, two-part self-curing bitumen-free liquid applied membranes [91], one-component water-based elastic coatings, and flexible one or twocomponent cementitious waterproofing slurries. Water based systems employing polymer emulsion binders will be discussed further below. Waterproofing products are used in a wide range of construction and repair applications, applied directly on to pre-formed concrete, concrete blocks, bricks, and stone products. The most important applications include foundation building walls, bridges, balconies and terraces, tunnels, basements, planters, silos, and parking decks. Coating systems are typically applied to uniform thickness (1–3 mm) using a brush, roller, trowel or spray system. In all cases, strong adhesion and intimate contact of the waterproofing layer to the underlying substrate, as well as proper waterproof system design, are required to eliminate water seepage through or around the membrane. Depending on the application, waterproofing membranes should also exhibit chemical resistance (e.g. to oils and acids), should remain flexible over all potential use temperatures and be capable of resisting hydrostatic pressures, even over cracks. Both, one-component, water-based elastic coatings and one or two-component flexible cementitious waterproofing slurries have the advantage that, by virtue of the water carrier, they are environmentally friendly and easy to clean after application. They both yield monolithic, seamless, puncture resistant and watertight layers after curing but which nevertheless allow water vapor to escape from the inside to the outside. Cementitious systems are particularly ideal on damp substrates – because of hydration reactions which occur in the freshly applied membrane. As they consist of polymeric binder, sand and Portland cement, through hydration reaction chemistry they become integrally bonded to and thereby become a part of the underlying concrete. Minor cracks can be bridged at low temperatures, if a sufficiently low Tg, flexible polymer dispersion binder component is selected [87, 93]. Elastic waterproofing membranes used on the exterior of concrete structures should also exhibit resistance to the damaging effects of UV radiation, sulfur dioxide and acid rain, carbon dioxide (carbonation), and repeated freeze-thaw cycles. Rubber based waterproofing systems are typically based on styrene-butadiene copolymers and SBS resins while acrylic and styrene-acrylic dispersions are employed in most flexible one-component and two-component cementitious membrane systems. In Europe, new regulations defined in ZTV-SIB (Additional Technical Contract Conditions and Regulations for the Protection and Restoration of Concrete Building Components) specify crack-bridging at –20 °C [88]. As a result, lower Tg polymer systems have been developed, which yield membrane flexibility and hairline crack

8.4 Construction adhesives

bridging at lower use temperatures. The industry has moreover shifted to systems containing, as a rule, higher proportions of polymer binder (0.8 < polymer/cement ratio < 1) compared with earlier membrane systems. Guiding formulations 1. One-component flexible waterproofing membrane (500 µm dry film) Wet parts Diluent Water 123.0 5.5 Defoamer BYK 035* Freeze–thaw additive Propylene glycol 27.1 5.5 Pigment disperser Pigment disperser NL† 1.6 Surfactant Triton X-405‡ 0.8 Thickener Natrosol 250 MXR§ 134.5 White pigment Kronos 2101 TiO2# 318.2 Filler Duramite** 16.4 Filler Atomite** 99.5 Filler Microtalc MP 10-52†† 3.3 Biocide Proxel GXL‡‡

Grind until smooth, then add Polymer binder Acrylic dispersion Defoamer BYK 035* Thickener Natrosol 250 MXR Neutralizing agent Ammonium hydroxide Total Pigment volume concentration: 42.3 %; solids content: 72.8 % 2. Two-component flexible cementitious waterproofing membrane Dry component 28.0 F-110 Silica sand§§ 27.3 F-95 Silica sand§§ 19.6 Portland cement type I/II## 0.2 Pigment disperser N† Lumiten E-P3108 defoamer 1.6 Styrene acrylic dispersion – Antifoam – Water (to desired flow) –

475.7 7.4 2.8 1.8 1223

Wet component – – – – – 20.6 0.2 0.5

Mix dry and wet components separately, then blend dry mix into the wet phase. Apply two or three layers of 400–600 µm each. Water/cement ratio: 0.45; sand/cement ratio: 2.82; dry polymer/cement ratio: 0.60

245

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8 Applications in the Adhesives and Construction Industries

3. Waterproofing membrane Polymer binder Defoamer Thickener Total

Styrofan D 422† BYK 035* Rheolate 300***

Wet parts 98.4 0.1 1.5 100

Formulation 3 is intended for sealing interior walls and floors in bathrooms and other damp areas – preventing underlying moisture damage. Compounds based on Styrofan D 422 can also serve as “wet” concrete curing compound (by spray application), preventing premature concrete drying during the critical hydration stage. Suppliers: *BYK Chemie, Wallingford, CT, USA: †BASF Corporation, Charlotte, NC, USA; ‡ Union Carbide, Danbury, CT, USA; §Aqualon, Wilmington, DE, USA; #Kronos, Houston, TX, USA; **ECC International, Atlanta, GA, USA; ††Pfizer, Easton, USA; ‡‡Zeneca Biocides, Wilmington , DE, USA; §§US Silica, Ottawa, IL, USA; ##Leghigh Portland Cement Co., Allentown, PA, USA; ***Rheox, Hightstown, NJ, USA

Test methods The ANSI 118.10 test specification describes key test requirements for load bearing, bonded, waterproof membranes for thinset ceramic tile and dimension stone installation. The standard applies to trowel applied, liquid, and sheet membranes. Requirements include a seam strength evaluation, membrane tensile or breaking strength, shrinkage or dimensional stability, “waterproofness” in accordance with ASTM D 4068, and shear strength of ceramic tile and cement mortar applied on the waterproofing membrane. Membrane water vapor transmission is typically determined according to ASTM E 96, employing a permeation cup apparatus (Fig. 8-33). Waterproof membranes in Europe are tested according to [92]. Waterproofness is an indication of a particular membrane material’s ability to withstand a 60 cm hydrostatic pressure head. The apparatus employed in this test is shown in Fig. 8-34. The membrane film is affixed at the bottom end of the J-tube and water is then carefully introduced to an overall height of 60 cm above the level of the membrane.

Fig. 8-33

assembly.

Moisture vapor transmission cup test

8.4 Construction adhesives Fig. 8-34 J-Tube apparatus for measuring the hydrostatic pressure resistance of water-proofing membranes.

Fracture of the membrane or evidence of wetness on top of the material (even the formation of a single droplet) within the first 48 h exposure, are considered as visible signs of water penetration and require rejection of the material. The performance of a concrete curing compound is assessed by measuring the water loss of green concrete according to ASTM C 156-94, the water loss after three days may not be higher than 0.7 kg m–2, acievable with SB coatings (Fig. 8-35) Water loss (kg/m2)

4

3 without coating

2

1

ASTM Spec. with SB coating

0 Water loss of green concrete according to ASTM C 156-94. Fig. 8-35

4

24

48 72 Time (hours)

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8 Applications in the Adhesives and Construction Industries

8.4.7

Elastomeric Roof Coatings

Water based elastomeric roof coatings can be described as formulated liquid products, that are applied by spray or roller coating to a sloped roof surface, which have the ability to form a continuous protective polymer film over the substrate upon evaporation of the water. Elastomeric roof coatings are used in repair applications to seal existing roof structures and also in new building construction applications, particularly for protecting polyurethane insulating foam roofs. Elastomeric polymer films require a proper balance of properties for the film to expand and contract and return to its original state every time external stress-strain forces have been applied and removed. Acrylic polymer dispersions are ideal for the manufacture of water based liquid elastomeric roof coatings. In particular, white pigmented roof coating membranes are becoming increasingly prevalent as a result of a new EPA program called “Energy Star”. This program is aimed at promoting energy efficient buildings and in doing so, reduce building heating and/or cooling costs [89]. The program requires the roof membrane to demonstrate both a minimum initial solar reflectivity and maintenance of that solar reflectance after three years field exposure (by ASTM E 903). Buildings covered with white coatings, compared to black asphalt type roofs, reflect rather than absorb light radiation resulting in cooler surface temperatures and reduced cooling demands for buildings located in hot climates. While field exposure conditions can vary, the Tg for an elastomeric copolymer should be lower than the minimum low temperature for a given geographic region where the roof coating is to be applied (i.e. <20 °C). This helps assure that during cold weather, the membrane will remain flexible and thus, prevent the roof membrane from cracking under contraction stresses. A typical “white” roof coating formulation consists of polymer dispersion mixed with various fillers and pigments and small amounts of additives to provide stability and to build viscosity to the roof coating mixture. The mixture is applied to a clean roofing substrate, in two or more coats, and upon drying forms a continuous film or coating membrane. The polymeric component binds the materials in a monolithic state and forms the film. This coating membrane must also be sufficiently flexible to withstand the movement of the substrate due to the diurnal cycle. It also has to provide resistance to water intrusion, cracking, and weathering while maintaining adhesion to the substrate under all exposure conditions. Low-Tg (–20 °C) pure acrylic dispersions provide excellent adhesion to polyurethane roofing foam and many other construction substrates. They may contain internal crosslinking agents that crosslink the polymer film after the water has evaporated. Crosslinking occurs both on the membrane surface and throughout the coating providing required elastomeric film properties with only very slight residual surface tack – thereby maximizing dirt pick up resistance and long-term reflectivity. The demand for dispersions in this market is estimated to be on the order of 20 000 tons wet per annum.

8.4 Construction adhesives

Guiding formulation Water based elastomeric roof coating

Diluent Water Freeze Thaw Propylene glycol Dispersing aid 30 % Pigment Disperser NL* Binder Acrylic dispersion Defoamer BYK 035† Kronos 2101‡ TiO2 pigment Filler Duramite§ Filler Atomite§ Filler Microtalc MP 10-52# Biocide Proxel GXL** Grind, then add Binder Acrylic dispersion Thickener Natrosol 250 MXR†† Neutralizing agent Ammonia Solution Defoamer BYK 035† Weight % solids = 72; volume % solids = ca. 59 % Pigment volume concentration = ca. 42; viscosity (Krebs) = ca. 105

Wet parts (g) 6.88 2.23 0.45 28.45 0.45 11.17 26.36 1.34 18.27 0.22 12.78 0.34 0.23 0.91

Suppliers: *BASF, Charlotte, NC, USA; †BYK-Chemie USA, Wallingford, CT, USA; ‡Kronos, Houston, TX, USA; §ECC International, Roswell, GA, USA; #Pfizer, Easton, PA, USA; **Zeneca Biocides, Wilmington, DE, USA; ††Aqualon, A Div. of Hercules, Wilmington, DE, USA

Test methods In 1997 an American National Standard, ASTM D 6083-97a (Fig. 8-36), was issued for determining the acceptable performance of liquid acrylic flexible roof coating mixtures based on laboratory testing. These specifications are based on a somewhat

Film physical property After 14 days drying at room temperature Tensile strength D2370 % Elongation at break After 1000 h aging in a xenon arc weatherometer % Elongation at break Accelerated weathering checking Low-temperature mandrel flexibility Adhesion (wet) C 794 Water swelling (%) Permeance – inverted (perms) Tear resistance (lbf in–1) Fungi resistance (after 28 days) Fig. 8-36

roofing.

ASTM test

Requirement

≥200 psig D2370

≥100 %

D2370 D4798

≥100 % No cracking and

D522

Pass (1/2 in at –15 °F)

>2 lb in–1 D471 D1653 D624 G21

<20 % <50 >60 (die C) Zero observed

Standard specification for liquid applied acrylic coating used in

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broader set of test requirements described in Dade County Florida, Protocol PA 12995 and Protocol PA 143-95. The standard has minimum specifications, when testing the free film for tensile strength, elongation at break, water swelling, permeability and tear resistance. These tests are carried out after a minimum of 14 days drying at standard lab conditions (22 °C and 50 % relative humidity). The coated film must also meet standards for, adhesion to various substrates, tensile strength, elongation at break and mandrel flexibility. These tests are carried out at various specified temperatures, relative humidity and aging conditions. Acknowledgments The authors would like to express their sincere thanks to the following colleagues for friendly assistance in writing the “Applications for Adhesive and Construction Industries” chapter and for critical checking of the manuscript: H. Anders, G. Auchter, O. Aydin, M. Drewery, W. Druschke, P. Fickeisen, P. Fitzgerald, H. J. Fricke, R. Füßl, H. Jäger, J. Krobb, U. Licht, W. Mächtle, L. Maempel, H.W.J. Müller, J. Neumann, J. Pakusch, H. Seibert, B. Schuler, K.-H. Schumacher, F. Schwarz, J. TorresLosa, J. Türk, A. Zettl, A. Zosel

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91 92

93

Management District (SCAQMD)Rule 1168. Adhes. Age March1999, 4. Adhes. Age May 2000, 44. J. Krobb, E. Wistuba, Adhäsion, 1992, 5, 14–20. OSI Witco, Technical brochure. V. F. Foster, Caulks and Sealants – Overview, Caulks and Sealants Short Course, The Adhesives and Sealants Council, Pittsburgh, March 1997. S. H. Kosamatka, W. C. Panarese, Design and Control of Concrete Mixtures, 13th edn, Portland Cement Association, Skokie, Illinois, 1988. I. Soroka, Portland Cement Paste and Concrete, Chemical Publishing Co., New York, NY, 1979. S. Chandra, Y. Ohama, Polymers in Concrete, CRC Press, Boca Raton, 1994. M. Angel, H-J. Denu, Waterproof Membranes for Concrete Surfaces Protection, Farbe and Lack, Vol. 103(8), 1997. http//www.bast.de/htdocs/qualitaet/ dokument/doku.htm http//www.energystar.gov/ Y. Ohama, Recent progress in Concrete–Polymer Composites, Elsevier, 1997, pp. 31–40. M. T. Pickett, W. R. Grace and Co. Conn., US Patent 5763014, 1998. Mineralische Dichtungsschlämme für Bauwerksabdichtungen (Prüfgrundsätze zur Erteilung von allg. neuaufsichtlichen Prüfgrundsätzen) 02/2001. J. Pakusch, H.-J. Denn, B. Reck, Polymer Powders with elastic properties, Farbe + Lack, Vol. 105(12), 1999.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

9

Applications in the Carpet Industry Peter R. J. Blanpain, Richard L. Scott, Onno Graalmann, and J. Arthur Smith

9.1

Introduction

This chapter covers the use of synthetic polymer dispersions in the carpet industry. In 1999, carpet accounted for approximately 60 % [1] of the volume of all floor coverings (soft and hard) sold in the USA, with an estimated [2] sales volume and value of 1.6 billion m2 and $11.7 billion respectively. Of this volume, the tufted carpet and rug segment, the largest user of polymer dispersions, is dominant with an estimated 1.4 billion m2, with broadloom’s share accounting for approximately 1.3 billion m2. Europe and the Asia-Pacific countries produced during the same period an estimated 1.13 billion m2 [3] and 284 million m2 soft floor covering respectively. Synthetic polymer dispersions have been used as binders for the backing of carpet since the late nineteen-forties. The function of the polymeric binder in carpet backings is primarily to anchor the pile fibers in place, give improved dimensional stability, hand, and resistance to fraying or tuft loss at cut edges of the carpet. Carpet backing is the fourth largest user of synthetic dispersions in North America (NA) after paints and coatings, paper, and adhesive applications. During 1999, dispersion consumption in carpet backings was around 490 kt (wet) [4], which represents around 9 % of the estimated 5,300 kt total dispersions (wet) produced in the USA. In Western Europe and the Asia-Pacific regions, dispersion usage during 1999 has been estimated at respectively 400 kt [3] and 100 kt wet.

9.2

History of Carpet

The history of the manufacturing of rugs and carpets began with weaving. Evidence obtained from excavations near the Caspian Sea indicates that the spinning and weaving of sheep and goat wool was practiced as early as 6000 BC. It is known that the Egyptians of 3000 BC wove linen carpets ornamented by sewn on pieces of colored woolen cloth. A Turkish knotted pile rug, dated back to 500 BC, was found in

253

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Siberia in the nineteen-fifties. The weaving of hand-knotted rugs spread throughout the Orient, and Persia (Iran) became the predominant manufacturer. Oriental rugs were carried to Europe by the Saracen conquerors of Spain, by returning Crusaders, and later Italian merchants. The Spanish in the 13th century, were the first Europeans to make hand-pile rugs. Moorish weavers were probably taken from Spain in the 13th century to start the early French carpet weaving industry at Aubusson. Deep-pile rugs, called Savonneries, were first produced in Paris during the early 17th century. The revocation of the Edict of Nantes in the late 17th century, that had guaranteed religious and civil freedom to French Protestants, drove French and Walloon Protestants (the Huguenots) into England, The Netherlands and Germany, where they made significant contributions to the early development of the spinning and weaving industries in these countries. The chartering of carpet weavers in Wilton and Axminster in 1701, and the introduction of carpet production in Kidderminster around the 1740s, was the beginning of the establishment of England as the world’s major woven carpet producer. This situation continued until the 1960s, when technology developed in America for tufted carpet production was introduced into Europe. This resulted in woven carpet production declining in England by about 70 % by the1970s, and the establishment of Belgium, The Netherlands, Germany, and Great Britain as Europe’s major tufted carpet manufacturers. Before the 1790s, the carpet business in the USA was monopolized by expensive woven imports from The United Kingdom. The US carpet industry had its modest beginning in 1791 when William Sprague founded the first woven carpet mill in Philadelphia. During the early 1800s, as carpet became more popular, other factories were established in New England, New York, and Pennsylvania. Continued domination by British imports, stimulated efforts to improve methods of production, and in 1839 Erastus Bigelow’s invention of the power loom, which made the mass production of woven carpet possible, reshaped the industry. This, together with the invention of the Axminster loom in 1876 to produce woven carpets with a wide range of designs and colors, increases in loom widths, and other advances in technology, further stimulated the expansion of the US woven carpet industry. The woven carpet industry continued to thrive until the end of the nineteen-forties and the advent of tufted carpets. The tufted carpet industry had its beginning in the late 19th century, when a Dalton (Georgia, USA) woman, Catherine Evans Whitener, produced bedspreads by sewing thick cotton yarns with a running stitch, into an unbleached muslin base cloth, and cutting the surface loops of the yarn so they would fluff out. After tufting, the material was washed to cause the muslin to shrink around the tufts to mechanically hold them in place. She sold the first bedspread in 1900, and generated so much interest that a thriving cottage industry started. Bedspreads led to other small tufted goods such as toilet covers, robes, and small “scatter rugs,” and by the 1930s there were around 10 000 “Tufters” in the Dalton area. During the late 1920s and early 1930s, increased costs, and falling prices due to increased competition, resulted in the development of multi-needle tufting machines, the mechanization of looms, and building of looms of greater width in order to meet demands for more bedspreads. Tufting machines for producing carpet appeared in the late nineteen-forties, and by the late nineteen-fifties, carpet affordable to virtually

9.3 Present Day Carpet Business

every home owner in the USA, was being produced in twelve foot widths, using nylon fiber, and jute as the secondary backing cloth. The evolution of the US carpet industry since this time is depicted below in Tab. 9-1. Tab. 9-1

Evolution of the US carpet industry (in million m2).

Year

Tufted carpet

Woven carpet

Total

1950 [5] 1974 [5] 1992 [6] 1998 [6] 1999 [2]

16 724 1094 1404 1432

65 61 19 30 34

81 785 1113 1434 1466

9.3

Present Day Carpet Business

In 1999 [2], the US carpet industry produced an estimated 1.6 billion square meters of carpet floor coverings, with a value of $11.7 billion. This represents 45 % of the total world carpet production. The state of Georgia accounts for 74 % of US production nearly all of which is concentrated in the northwest corner of the state around the city of Dalton [7]. As shown in Tab. 9-2, tufted carpet is by far the largest carpet type produced with approximately 90 % of the total carpet volume, with broadloom carpet representing around 90 % of the tufted volume. Second largest are the carpets grouped under others (knitted, knotted, needlepunched, etc.) with around 8 % of the total. Woven carpet has almost disappeared and today only represents around 2 % of the total carpet manufactured. Tab. 9-2

1999 US carpet production (in million).

Total Tufted Woven Other Total

m2

US $

1432 34 129 1595

10692 450 548 11690

The situation in Europe, as indicated in Tab. 9-3, is somewhat different in that woven (13 %) and needlepunched (26 %) carpet floor coverings still have an appreciable share of the total. In the USA tufted broadloom carpet, the most important segment of the tufted carpet business with an estimated 1.3 billion m2 in 1999, is constructed for use in three markets: – consumer residential: purchases for use in a home by members of a family. – contract residential: purchases by persons other than the home owner for new homes, apartments, condominiums, multifamily units, recreational vehicles, manufactured housing, etc.

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9 Applications in the Carpet Industry Tab. 9-3

1999 European carpet production (in million). m2

Tufted Woven Needlepunched Total

682 143 296 1121

I* 6656 1746 441 8843

*Courtesy: GUT e.V., Aachen, Germany

– contract commercial:

purchases other than by a home owner for hotels, motels, college dormitories, school rooms, business offices, banks, institutional buildings and industrial buildings. In terms of square meters and market share percent, 1999’s production forecast (USA) [1] for the three types of carpet is summarized in Tab. 9-4. Tab. 9-4

Broadloom carpet by market type (in million).

Type

m2

%

Consumer residential Contract residential Contract commercial Total broadloom

707 262 339 1308

54 20 26 100

In all three, whilst the binder used for construction may be the same, the type and formulation of the backing adhesives differ significantly, as each category of carpet has different performance specifications. The formulations employed, and methods of adhesive application, for producing the three carpet types will be detailed in sections following, together with insight into their end use property requirements.

9.4

Carpet Backing Binders

During the late nineteen-forties, the starches and natural gums initially used as binders for improved tuft bind, were largely replaced by rubber dispersions. In the nineteen-fifties, the major binders were natural latex, cold SB (styrene-butadiene) and hot SB dispersions, the polymers usually being vulcanized to obtain good strength. The mid nineteen-fifties saw the introduction of non-cure hot SB dispersions, which began to replace the sulfur cure SB. The late nineteen-fifties saw the development of carboxylated styrene-butadiene dispersions (XSB) and their introduction as binders for carpet backings. These XSB dispersions, as a result of the easier and less costly compounding, faster drying rate, and improved performance in terms of specific adhesion to the fabric substrates, have since become the workhorse of the carpet backing industry. The 70s through to the early nineteen-eighties was the era of the attached SBR foam cushion backings. Such carpets were con-

9.4 Carpet Backing Binders

structed using a compounded XSB dispersion pre-coat or tie-coat to bind the tufts in place, followed by the application of a foamed compound of a high solids styrenebutadiene latex (HSL), to provide under foot comfort, and prolong the useful life of the carpet. Foam backing is a process in which a high solids SB dispersion, a vulcanizing agent (suspension of sulfur, zinc oxide, accelerators), chalk and emulsifier together with air is frothed. The wet foam structure needs to be maintained until vulcanization takes place. This can be achieved by gelling agents which destabilize the polymer particles by smoothly decreasing pH and coagulating the latex within the membranes of the wet foam. Non-gel foam is stabilized with emulsifiers which maintain the foam structure during the evaporation of water until vulcanization takes place. Vulcanization is carried out at about 100 °C resulting in an elastic polymer network. The polymer/filler ratio varies from 1:0.5 to 1:2. In 1975 it was estimated that one third of all the broadloom carpet produced in the USA had an attached SBR cushion. Unfortunately, the highly competitive situation prevailing at the time, compelled manufacturers to reduce the cost of the attached foam by either increasing the level of the cheap calcium carbonate filler and/or reducing foam application weights and density. This inevitably adversely affected the durability of the foam to such an extent that failure occurred resulting in premature wear of the carpet. As consequence of the bad name that attached SBR cushions obtained in the eyes of consumers, today HSL foam backed carpet has virtually disappeared in the USA, with the exception of a few specialty floor coverings such as bath mats. The situation in Europe is somewhat different to that of the United States of America. In general the styles of carpet produced in Europe have a much lower face fiber content, the pile height being much lower and pile density higher. The foam backing was also generally of higher density therefor thinner for an equivalent weight. The transfer of heat into the foam product is much easier, and enabled higher production speeds to be achieved than were possible in the USA. A similar situation to that observed in the USA has been experienced for similar reasons, but at a much slower speed. The consumption of HSL for foam backing remained fairly static for many years, the growth in the market being almost entirely restricted to secondary backed products, and the market share of foam backed carpet being gradually eroded. In the late nineteen-nineties environmental pressure, particularly in Germany, resulted in a very steep decline in the consumption of HSL for floor coverings. Several of the ingredients used in the formulations for HSL foam in the floor coverings industry were brought into ecological question. Since 1998, when foam backed carpets made up approximately 45 % of textile floor coverings in Europe, this has decreased to approximately 20 % in 2000. An alternative more ecologically friendly product was required. In Europe this was achieved by replacing the HSL foam by a needlefelt product which is adhered to the carpet by means of an XSB latex. In North America carpet underlays made of polyurethane foam are commonly used to provide the soft comfort of residential carpets.

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Over the years, it is fair to say that virtually every type of polymer available in dispersion form has been tried for use in the backing compound for tufted carpet. However, because of its versatility and cost-effectiveness, it is the carboxylated styrene-butadiene (XSB) polymer dispersions that hold the major share of this business today with an estimated 95 % of the volume sold in 1999, the remaining volume being shared by ethylene-vinyl acetate, polyvinyl chloride and polyurethane dispersions. During 1999, the US carpet industry consumed approximately 490 kt wet dispersion, of which 463 kt were XSB [4]. The majority of the XSB is supplied direct to the carpet mills by the three major dispersion producers BASF, Dow Chemical, Omnova, with a minor proportion being supplied by so-called re-sellers or compounders such as General Latex, Polymer Products, Southeastern Latex and Textile Rubber. The XSB binders of today are considerably different from those at the time of their introduction into carpet backings. Ongoing advances in polymerization technology have enabled tailoring of the physical and polymeric properties of dispersions to better meet the evolving demands of present day backing machines and performance requirements. Whilst the dispersion formulations are the proprietary information of the producers, some typical characteristics of the dispersions commercially available today are given below. Polymer type: Bound styrene: Carboxylation:

Carboxylated styrene butadiene dispersion (XSB) Typically in the range 60–67 % Typically less than 3 % by weight of the polymer. The carboxylic acid may be itaconic acid alone or blends of itaconic acid with either acrylic acid or methacrylic acid. The actual type/level of carboxylation is proprietary information to each producer. Solids content: 51–53 % dry weight pH: 7.5–9.0 Particle size: Up to 10 years ago, 170–200 nm used to be the norm. Today, as consequence of the need to reduce 4-PCH content, particle sizes are generally within the 140–155 nm range. Volatile organic Over the last 10–12 years, and the advent of “Sick Building components (VOC): Syndrome”, the dispersion producers have radically improved their XSB manufacturing processes to minimize the level of residual organic compounds. Today, the maximum target values, in ppm on wet dispersion, for the four major VOC are given in Tab. 9-5. Surfactants: Type and level is the proprietary information of the individual producers. They are usually selected not only for good stability and low foaming during the dispersion manufacturing process, but also to impart the required foaming properties to the backing binder formulations during processing in the customer’s plant.

9.5 Carpet Laminating Tab. 9-5

Maximum limits (ppm) of volatile organic components.

Styrene Ethylbenzene 4-Vinylcyclohexene 4-Phenylcyclohexene Total

North America

Europe*

Denmark

35 10 15 60

200 50 50 200 <400

40 20 10 50

*Defined

by EPDLA (European Polymer Dispersion and Latex Association) and GuT (Association for environmental friendly carpet)

9.5

Carpet Laminating 9.5.1

Background

Several types of carpets including woven, needlepunch, knitted, and tufted are subjected to the latex laminating procedure [8]. By far the most prevalent carpet construction method is tufting. The first tufting machines were very similar to a giant sewing machine that uses thousands of threaded needles in a row across the width of the machine. Today’s machines are far more complex and sophisticated, although they still work in the same basic way. The creel, or rack of yarn cones, are located in front of the tufter. From the creel, the yarns are passed overhead through guide tubes to puller rolls. The speed of the puller rolls controls the amount of yarn that is supplied to the tufter and, with other factors, determines the carpet’s pile height. The needles, which number up to 2000 for very fine gauge machines, insert the yarn into a primary backing supplied from a roll of material located in front of the machine. Spiked rolls on the front of the tufting machines feed the backing through the machine. Below the needle plate are loopers, devices shaped like inverted hockey sticks, timed with the needles to catch the yarn and hold it to form loops. If a cut pile is called for, a looper and knife combination is used to cut the loops. For cut-loop combinations, a special looper and conventional cutting knife are used. Tufting has reached a high degree of specialization utilizing a variety of patterning devices, many of which are computer-controlled. Stepping, or zigzag moving, needle bars and individually controlled needles greatly expand patterning possibilities. Such patterned carpet is frequently referred to as a graphic pattern. Other advanced tufting techniques are loop over loop and loop over cut (LOC) machines. After completion of tufting, the unfinished tufted carpet will be dyed, if precolored yarns are not used, followed by a finishing step to add a compound and usually a secondary backing material. Carpet laminating or backcoating is an essential step in the carpet manufacturing process. Since the backcoating is hidden from view, the attributes it imparts to the finished carpet are rarely appreciated. Both the long term performance and the aes-

259

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9 Applications in the Carpet Industry

thetic value of the installed carpet are vitally dependent upon a correctly formulated and a properly applied backcoating. Among the most important performance requirements of a backcoating are: – high tuftlock – minimum pilling and fuzzing – adhesion to secondary backing – dimensional stability – bundlewrap – proper hand – durability – water resistance – resistance to heat, light, and atmospheric contaminants – flammability-pill, tunnel, radiant panel, smoke, vertical burn – resistance to edge fraying – odor 9.5.2

Carpet Terminology

Before proceeding further with carpet laminating, a review of carpet terminology would be useful, since physical properties imparted by the SB latex backcoating relate directly to its construction. Figure 9-1 illustrates a typical cut pile carpet and the physicals associated with it. A level loop carpet would appear the same but the tops of the tufts would not be cut. Physicals and terminology for a level loop carpet would be the same as for cut pile.

Fig. 9-1

Carpet terminology.

9.5 Carpet Laminating

Tuft: Bundle: Filament: Primary:

One cut or uncut loop of a pile fabric A continuous tufted collection of fibers or filaments A single continuous strand of fiber Woven or non-woven fabric into which the pile yarn is inserted by tufting Secondary: A woven or non-woven fabric laminated to the tufted primary to provide dimensional stability Delamination: The force expressed in lb/inch required to remove the secondary backing from the primary carpet (ASTM D 3936, ISO 11857). Tuftbind: The force expressed in pounds required to remove a single tuft from its primary backing (ASTM D 1335) Pill and fuzz: Hairy effect on the carpet surface caused by slippage of individual filaments or fibers Bundlewrap: A subjective rating, usually expressed as a percentage, to indicate the degree of latex encapsulating the yarn Bundle penetration: A subjective rating, usually expressed as a percentage, to indicate the degree of penetration into the yarn Hand: A subjective rating to indicate the stiffness of the finished carpet 9.5.3

Back-coating Applications

Over the last few years, direct coating with scrim lock has replaced pan application as the preferred method for back-coating carpet. The major advantages associated with direct coating are: – less waste (mill and disposal savings) – easier clean up – overall ease of operation – uncoated selvages (cost and clean up savings) – uniform weight control side to side – uniform coating side to side – fresh compound always available – less problems with filler fallout – higher compound solids – immediate response to cup weight changes – short lag time for compound changes – elimination of density variations (airing up or collapsing in pans) – thixotropic effects eliminated (troughing, poor pickup) – better visual weight control (puddle gain or loss seen immediately) – faster drying due to lighter densities Figure 9-2 depicts a typical direct coating unit with scrim coat. This schematic shows a set up utilizing a bed-plate for the application of the pre-coat and a pan for the application of the adhesive scrim coat. Variations of this include a roll over roll for the pre-coat application and a roll over roll or Tillitson application for the adhesive.

261

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9 Applications in the Carpet Industry

Direct coating with scrim coat.

Fig. 9-2

9.5.4

Back-coating Formulations and Ingredients

Direct coating is used to back-coat both residential and commercial carpets. Backcoating residential carpets involves two latex compounds. One is highly loaded with filler and is deposited directly onto the tufted primary after having passed through a mechanical froth machine. The froth machine lowers the density thus allowing for proper placement and weight control. This compound is usually referred to as the pre-coat or undercoat. Its primary purpose is to securely lock the tufts into the primary backing. Other properties affected by the pre-coat are pill and fuzzing, hand, delamination, and flammability. Along with latex, the pre-coat compound would typically contain water, filler, surfactant, and thickener. Water is used to adjust the solid content of the compound, aid in the dispersion of the filler, and extend shelf life. The filler is almost always calcium carbonate due to its universal availability and economical price. Its grind and purity are critical for compound stability and runability. Typical pre-coat loadings are between 400 and 600 parts per 100 parts dry latex. To enhance flame retardant properties, aluminum trihydrate can be substituted for all or part of the filler in the pre-coat. Surfactants are used to increase stability and frothability of the compound. Sodium lauryl sulfate (SLS) and ammonium lauryl sulfate (ALS), sodium sulfosuccinamate, and combinations of ALS and long chain alcohol are commonly used. Thickeners are almost always sodium polyacrylates. They impart the proper viscosity and rheology to allow proper placement of the compound. They also help to suspend the filler in the compound. Miscellaneous ingredients occasionally will be used, if a back-coating needs a specific appearance or specialized performance property. Miscellaneous ingredients include pigment, penetrant, defoamer, dispersant, chelating agent, anti-blistering

9.5 Carpet Laminating

agent, antistatic agent, and stabilizer. A typical pre-coat formulation used in North America is represented in Tab. 9-6. Tab. 9-6

Pre-coat formulation (US).

Water XSB latex Calcium carbonate Surfactant Polyacrylate thickener Total

Solids content (%)

Dry parts

Wet parts

– 53 100 35 13 83

– 100 550 2 1 653

35 188 550 5.7 7.7 786

Solids content of the formulation 83 %; viscosity 17–18 Pa s

In Europe a typical pre-coat formulation does not exist due to widely differing styles and quality requirements. In general they contain more filler (600–1000 dry parts of calcium carbonate), less surfactant (0.2–0.5 dry parts), and less thickener (0.4 dry parts), and viscosity of 3–5 Pa s. The second compound used in residential carpet coating is referred to as the adhesive scrim or skip coat. It is applied by means of a pan and lick roll directly to the secondary backing. This coating provides the strength necessary to sufficiently adhere the secondary backing to the primary backing, which in turn imparts dimensional stability to the carpet. The adhesive scrim coat formulation is similar to the pre-coat with two exceptions. First, since the compound is not frothed, surfactant is eliminated from the formulation. Secondly, since a stronger compound is required for the adhesive coating, filler loading is reduced. Loadings between 350 and 400 parts of filler per 100 parts of dry latex are typical. Table 9-7 is representative of a typical adhesive formulation. Tab. 9-7

Adhesive scrim coat formulation (US).

Water XSB latex Calcium carbonate Polyacrylate thickener Total

Solids content (%)

Dry parts

Wet parts

– 53 100 13 82

– 100 375 0.6 475.6

12 188 375 4.6 579.6

Solids content of the formulation 82 %; viscosity 9–10 Pa s

In Europe the ingredients for secondary backing compounds are similar but the filler loads can vary from 0–450 parts per hundred part of dry latex. The configuration of the coating machines determines the compound to be used. On average the viscosity is 5 Pa s. Since most commercial contract carpets are glued directly to the substrate, the need for a secondary backing is eliminated. Thus most commercial contract carpets

263

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9 Applications in the Carpet Industry

are coated with a single high strength compound referred to as a unitary coating. To meet the enhanced performance requirements of commercial contract carpets a high density, low filled compound is used. High density is achieved by using low surfactant levels and lightly frothing the unitary compound. Filler levels are typically in the range of 150–200 parts of filler per 100 dry parts of latex. The two most important properties of a unitary coating are high tuft-bind requirements (11–20 lb, 50–90 N) and pill and fuzzing. Table 9-8 illustrates a typical North American unitary formulation. Tab. 9-8

Unitary backing formulation (US).

XSB latex Calcium carbonate Surfactant Polyacrylate thickener Total

Solids content (%)

Dry parts

Wet parts

53 100 35 13 73

100 150 0.5 0.4 250.9

188.6 150 1.4 3.1 343.1

Solids content of the formulation 73 %; viscosity 9–10 Pa s

European unitary formulations would be very similar both in ingredients and filler loads to the above North American formulation. Typical viscosity is 5 to 7 Pa s. 9.5.5

Industry Issues

Although direct coat has been universally accepted as the latex application technique of choice, there still exist large variations in processing speeds due to a wide range of dryer configurations and their efficiencies. Current processing speeds for a light weight carpet can range between 10 to 60 m min–1. As the industry becomes even more competitive, manufacturers will be forced to continue to reduce fixed and variable costs. More and more high speed dryers will replace slow inefficient ones. Computerized froth machines will become more common as mills focus on reducing variable costs. Utilizing computerized frothing machines eliminates the need to sample for density control and allows for a more consistent latex application due to the consistent froth produced. Feed forward application systems are beginning to be used by some manufacturers. This system produces a more consistent compound application resulting in enhanced performance. Electronic monitoring from the compounding area to final inspection is reducing manpower requirements and increasing dryer efficiency dramatically. Following is a list of process improvements currently being implemented or anticipated for implementation in the future: – movement toward high efficiency, high speed finishing ovens – increased use of computerized froth machines

9.5 Carpet Laminating

– feed forward application systems – some carpet producers beginning to question delamination testing as best indicator of “fit for use” – increased use of polypropylene fibers for commercial carpet resulting in greater need for latex that has an affinity for polypropylene – greater need for blister resistant latex due to higher heat and more efficient ovens – increased commitments through quality partnerships by both latex producers and carpet manufacturers in the use of statistical process control – movement towards low/zero ammonia systems to reduce emissions into the workplace – quality issues have replaced VOC issues as the primary concern of carpet manufacturers. Since 1988 there has been a heightened awareness of volatile organic compounds (VOC) emitted from carpet. In the early nineteen-nineties allegations stemming from flawed scientific work arose connecting a chemical (4-PCH) emitted from carpeting to adverse health effects. 4-phenyl cyclohexene or 4-PCH is a byproduct of the SB latex manufacturing process and has a low odor threshold. It is formed by a DielsAlder reaction of styrene and 1,3-butadiene and is responsible for “new carpet odor”. As a result of the allegations the Styrene Butadiene Latex Council (SBLC), the trade association of US latex producers and the EPA (Environmental Protection Agency) undertook extensive animal toxicological testing to investigate whether there was a link between 4-PCH and adverse health effects. After exhaustive testing and numerous reviews, the EPA declared 4-PCH to be an “unremarkable chemical” [9]. EPA has repeatedly concluded that valid scientific data showed no link between 4-PCH or any other carpet VOC emission, and adverse health effects. Even though carpet emissions have been declared to produce no adverse health effects, the issue of new carpet odor had to be addressed. As a result, the SBLC member companies have reduced VOC emissions by 95 % since 1988. With present low VOC latex and proper drying technique, carpet manufacturers today can produce odor free carpet. As well as creating an odor free environment for the carpet consumer by reducing VOCs, the trend toward ammonia free latex is also creating a more worker friendly environment in the manufacturing site. Even though alternative backing systems are available, SB latex still accounts for over 90 % of the carpet back-coating market. Due to its performance, versatility, and economics, SB latex continues to afford the carpet manufacturer the best value in back-coating systems today and for the foreseeable future.

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References 1 Floor Covering Weekly, Statistical Report 2

3 4 5

’99, 49(19), July 17/24, 2000. US Department of Commerce, Bureau of Census, Current Industrial Reports, Carpet and Rugs, 2000. Intercontuft, 2000. American Plastics Council Monthly Statistical Report, December, 1999. L. D. Martino, Carpet Backfinishing Review, Brunswick Corporation, 1975.

6 Carpet and Rug Institute, 1999 Industry

Review, 1999. 7 CRI The Tufted Carpet Industry, The

Pride of Georgia, History and Current Statistics, 2000. 8 Carpet and Rug Institute, Carpet Primer, 1997. 9 55 Federal Register 17404, April 24, 1990.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

10

Non-wovens Application Koichi Takamura, Marilyn Wolf, and Jim Tanger

10.1

Introduction

Non-wovens are described as flat, porous sheet or web structures produced by binding and interlocking fibers, yarns, or filaments by mechanical, thermal, chemical, or solvent means. They are porous sheets that are made directly from separate fibers, molten plastic or plastic film. They are not made by weaving or knitting and do not require converting the fibers to yarn. Non-woven fabrics are engineered fabrics that may be of single-use, limited life, or very durable. They provide specific functions such as absorbency, liquid repellency, resilience, stretch, softness, strength, flame retardancy, washability, cushioning, filtering, bacterial barrier and sterility. These properties are often combined to create fabrics suited for specific jobs, while achieving a good balance between product uselife and cost. In combination with other materials, non-woven fabrics provide a spectrum of products with diverse properties, and are used alone or as components of apparel, home furnishing, health care, engineering, industrial, and consumer goods. They are categorized according to application as either disposables or durables. Some familiar products made with non-wovens are listed in Tab. 10-1 [1–3]. The worldwide production of non-wovens is estimated at 2.7 million tons (or 6.0 billion pounds) in 1999 [4–6]. Western Europe accounts for approximately 35 % of the world production of non-woven products, followed by North America (30 %), China (12 %), Japan (12 %) and others, as shown in Fig. 10-1. The value of nonwoven product shipment in North America reached an all-time high in 1997 with an approximate value of $2.8 billion [7] and disposables account for nearly 70 % in value. The non-woven industry in North America is expected to grow at 2–3 % annually and to be $3.2 billion in 2002. China and Japan dominate non-wovens production in Asia followed by Taiwan and Korea as shown in Fig. 10-2. Double-digit growth is expected in these countries, and production in China and Japan reached to 350 000 tons each in 2000 with an an-

267

268

10 Non-wovens Application Tab. 10-1

Some familiar products made with non-wovens.

Disposable products Diapers, sanitary napkins and tampons Sterile wraps, caps, gowns, masks Drapings used in the medical field Household and personal wipes Filtration media Laundry aids (fabric-dryer sheets) Embroidery backing Durable products Apparel interlining Carpet backing and upholstery fabrics, high loft padding and backing Wall coverings Agricultural coverings and seed strips Electronic components (i.e. battery separations, disk liners, insulation liners, polishing cloth, etc.) Envelopes, tags and labels Insulation and house wraps Roofing products Civil engineering fabrics/geotextiles

Global Production of Nonwovens W. Europe

Asia

Global non-woven production reached to 2.7 million tons in 1999. Western Europe and Asia produced approximately 35 % each.

Fig. 10-1

North America

nual growth rate of 11 % [5, 6]. China non-woven production increased 87 times during the last two decades from 4000 tons in 1980. The North American market for disposable applications was approximately $1.9 billion in 1997, and represent roughly 15.5 billion yards (13 billion m2) and 900 million pounds (400 000 tons) of material. As seen in Fig. 10-3, filtration and medical applications represent nearly 60 % of the dollar value, but fewer than 20 % Nonwoven Production in Asia Taiwan 100 kton Japan 315 kton China 315 kton

China and Japan are two major non-woven producers in Asia. They reported 11 % annual growth in 2000.

Fig. 10-2 Korea 100 kton

10.1 Introduction

Total $1.9 billion

2

2

Total 15.5 billion yard (13 billion m )

Dryer Sheets

Wipes

Wipes

Dryer Sheets Filtration

Filtration Medical

Cover Stock Cover Stock

Medical

Fig. 10-3 1997 figures show disposable products account for nearly 70 % in the dollar value and 85 % in volume in North America.

of disposable square yard volume. Cover stock applications dominate disposable volume, and approximately 80 % of this volume is used for diaper production [7]. The volume for durable non-wovens exceeds that used for disposables in Europe. Europe has a longer history in the use of durable non-wovens, in particular geotextiles and roofing substrates. This is in contrast to the North American market, where disposable non-wovens play a stronger role as discussed above. Electronic, Interlining, and Furniture/Bedding/Home applications account for approximately 55 % of $0.9 billion non-woven materials in durable applications in the North American market, but furniture, bedding and home furnishing applications are the largest volume (42 %) of non-wovens in this durable category as shown in Fig. 10-4. Total $ 0.9 billion

Total 3.1 billion yard 2 (2.6 billion m2 )

Automotive Electronic Construction Materials

Construction Materials

Automotive

Electronic Interlining

Coating/ Laminates

Coating/ Laminates

Interlining

Geotextiles

Geotextiles Furniture/ Bedding/Home

Furniture/ Bedding/Home

Fig. 10-4 Electronic, interlining and furniture related applications account for 55 % of $ 0.9 billion in durable applications in North America, but furniture related applications are the largest in volume.

North American, European and Japanese markets are shown in Fig. 10-5 comparing total weight of products in 1999. Hygiene (cover stock in North America and consumer goods in Japan) medical products and wipes account for greater than 50 % of

269

270

10 Non-wovens Application North American Production (800 kton)

European Production (910 kton)

Coaging/ Laminates

Civil Engineering

Geotextitle

Coating Substrates

Filtration

Automotive Carpet Backing

Building Medical

Furniture/ Bedding/ Home

Hygiene

Construction Materials

Others

Interlining Electronic

CoverStock

Garments

Wipes

Dryer Sheets

Footwear/ Leather Goods

Floor Coverings

Wipes

Upholstery/ Bed Linen

Japanese Production (315 kton) Others

Medical Interlinings Filtration

Consumer Goods

Industrial Products

Construction

Medical

Fig. 10-5 Non-woven applications in North America, Europe and Japan are compared by weight of products in 1999. Hygiene, medical and wipes account for greater than 50 % in all these continents.

total production in these continents, though consumer goods in Japan appear to include some durable products.

10.2

Manufacturing Systems

Early thrust in non-woven usage emphasized replacing traditional knits and woven fabrics in low-end applications. During this initial phase, proprietary technology was used to produce fabric structures that performed not only better than items they were designed to replace, but often when traditional fabrics could not. As a result, new applications and markets were established and the industry expanded. Many non-woven products listed in Fig 10-5 were virtually non-existent a generation ago. The basic non-woven manufacturing systems have four principal elements or phases: fiber selection and preparation, web formation, web consolidation, and finishing treatments.

10.2 Manufacturing Systems

10.2.1

Web Formation

Four basic methods are used to form a web, and non-wovens are usually referred to by one of these methods: dry-laid, spun-laid, wet-laid and other techniques. Carding, garnetting and air-laying are examples of the dry-laid processes. The dry-laid processes provide maximum product versatility, since most textile fibers and bonding systems can be utilized and conventional textile fiber processing equipment can be readily adapted with minimum additional investment. The wet-laid process is similar to paper making, where a dilute slurry of water and fibers are deposited on a moving wire screen and drained to form a web. A wide range of natural, mineral, and synthetic fibers of varying length can be used. In the spun-laid process, polymer granules are melted and molten polymer is extruded through spinnerets. The continuous filaments are cooled and deposited on to a conveyor to form a uniform web. Even though filaments adhere to one another during this cooling process, this cannot be regarded as the principal method of bonding. The spun-laid process (also known as spun-bonded) has the advantage of giving non-wovens greater strength, but raw material flexibility is more restricted. Other techniques include a group of specialized technologies in which the fiber production, web structure and bonding usually occur at the same time and in the same place, as melt-blown and flash spun web formation methods. Detailed description of these technologies can be found elsewhere [1, 4]. As seen in Fig. 10-6, spun-laid and dry-laid are two preferred processes both in North America and Europe. The spun-laid process showed the strongest growth during a last decade [1, 7]. Products produced by the spun-laid technique account for 20–25 % of the market in Japan and China [5, 6]. North American Market by Process

European Market by Process Wetlaid

Wetlaid

Spunlaid Spunlaid

Drylaid

Drylaid

Other

Fig. 10-6 Comparison of web formation technologies in North America and Europe. Spun-laid and dry-laid are two preferred processes both in North America and Europe.

Other

271

272

10 Non-wovens Application

10.2.2

Web Consolidation

Webs produced with the above described processed have limited strength in their unbonded form and need to be consolidated. There are three basic types of bonding; thermal, mechanical and chemical. The thermal bonding uses the thermoplastic properties of certain synthetic fibers to form bonds under controlled heating. In some cases the web fiber itself can be used, but often a low melt fiber or bi-component fiber is introduced at the web formation stage to perform the binding function later in the process. In mechanical bonding the strength of the web is obtained through the physical entanglement of the fibers. Needle punching and hydro-entanglement (also known as spunlace) are two main mechanical bonding processes. Needle-punching can be used on most fiber types, whereas hydro-entanglement is mainly applied to carded or wet-laid webs. Chemical bonding mainly refers to the application of a latex dispersion based bonding agent to the web. The major binder application methods include saturation, printing, spraying, and foaming. Factors to consider when consolidating a web with binder are the end-use characteristics, the type of web substrate that is used, process compatibility, line speed, drying capacity and cost [1]. Carded thermally bonded technology is losing significant share in the cover stock market in North America and spun-bonded non-wovens accounted for 75 % of cover stock in 1999 (Fig. 10-7). In contrast widely different technologies are utilized to produce non-wovens for wipes. Hydro-entangled products are expected to grow due to superior strength and softness [7]. Cover Stock

Wipes Melt Blown

Carded Thermal Bounded

Unbounded Carded Web

Other Carded Thermal Bounded Hydroentangled

Unbounded Carded Web

Carded Chemical Bounded Wet Laid

Spunbounded

Fig. 10-7 Spun-bonded products dominate the cover stock market in North America due to better performance and lower cost. Hydro-entangled products are expected to grow in wipes.

Air Laid

10.3 Polymer Dispersions for Chemical Bonding

10.3

Polymer Dispersions for Chemical Bonding

Most non-wovens use 5 % to 50 % of polymer binder to provide one or more of following characteristics: softness, non-linting, smoothness, stiffness, dry and wet tensile strength, tear resistance, resiliency, flame retardancy, heat sealability, water repellency, absorbency, durability, dry cleanability, abrasion resistance, pilling resistance, color fastness, and bulkiness [1]. An article by Wiaczek [3] estimates that total latex consumption in the US non-wovens market will reach 160 000 dry tons in 2001. Latex binders fall into following two categories: those that provide rigidity to a product and those that render a web soft and drapeable. Acrylics are the predominant binders used in non-wovens as shown in Fig. 10-8 [1, 8]. They are versatile and offer the ultimate in durability, color stability and dry/wet performance. Ethylene vinyl acetate binders provide high tensile strength and excellent absorbency. They are less costly than acrylics. Styrene-butadiene latex offers an excellent combination of flexibility and toughness. It also provides hydrophobicity and durability to products. Vinyl acetate binders offer good dry strength and toughness but tend to be hydrophilic. Vinyl acrylics are more hydrophobic than vinyl acetate binders, and maintain excellent toughness, flexibility and better color stability. Chlorinated polymers such as poly(vinyl chloride) and ethylene vinyl polymers promote flame retardancy, especially together with antimony oxide [11]. Characteristics of these latex binders and typical applications of non-woven products are summarized in Tab. 10-2.

Vinyl Acrylic

Vinyl Chloride/ Others

Vinyl Acetate

Acrylic

Ethylene Vinyl Acetate Fig. 10-8 Acrylics are the predominant binders used in non-wovens, but the vinyl acetate ethylene latex showed strongest growth during the last decade

StyreneButadiene

For all these latices, monomer compositions are optimized to obtain desired physicochemical properties, such as glass transition temperature, Tg, molecular mass, cross-linking density, colloidal stability and specific surface functionality for post chemical reactions.

273

274

10 Non-wovens Application Tab. 10-2

Basic characteristics and typical applications of latex binders used for non-wovens pro-

duction. Latex binder type

Characteristics

Typical applications

Polyvinyl acetate

Resilient, somewhat stiff, moderate durability, limited washability and dry-cleanability

Highloft webs, filter media, industrial, home furnishings

Acrylic

Excellent adhesion, stiff to soft, excellent durability, launderability, dry-cleanability, good cross-linking

Coverstock, interlinings medical/ health care, fabric softener, carder, wet wipes

Vinyl acetateacrylic copolymer

Flexibility, good adhesion, good solvent resistance

Medical and/or surgical, wipes, coverstock

Ethylene-vinyl acetate

Good softness, durability and adhesion

Coverstock, wipes, air-laid pulp

Styrene-butadiene

Good tear and tensile

Filters, wipes, home furnishings

Poly(vinyl chloride)

Stiff to soft, can be plasticized, heat sealable, low temperature cure

Scouring pads, filter media, wall covering

Ethylene-vinyl chloride

Excellent mechanical stability

Underpads, highloft webs

Poly(vinyl alcohol)

Resilient, water absorbent

Filter media, medical

Acrylonitrile-butadiene copolymer

Resilience, heat sealable, launderability, dry-cleanability

Synthetic leather

As shown in Fig. 10-7, widely different technologies are utilized to produce wipes. This is also applicable to the binder application method. The acrylic latex can be applied through saturation, foam impregnation or print bonding as shown in Tab. 10-3. Tab. 10-3

Typical latex-based formulations for wipes with saturation, foam and print bonding methods. Saturation

Foam Impregnation

Print Bonding

Materials

Weight, g

Solids, %

Materials

Weight, g

Solids, %

Materials

Weight, g

Solids, %

Acrylic latex Water Defoamer Surfactant

100 120–450 0.1 0–1

55

Acrylic latex Water* Ammonia* Surfactants

100 80–150

55

100 150–450

55

0–5

50

Acrylic latex Water1 Dye2 Thickener1

*Adjust to 10-25% solids

100 50

*Adjust to 15-30% solids and pH=8 Typical foam weight of 70–150 g/liter

1

2

Adjust to 10–15% solids and 10Pas viscosity Add to the desired color

10.4 Application Test Methods

10.4

Application Test Methods

A series of well-defined standardized test methods have been established through various trade organizations, government and university research institutes. INDA (Association of the Non-woven Fabrics Industry) Standard Test [2] and EDANA (European Disposables and Non-wovens Association) Recommended Test Method [4] are two major standards for non-wovens. International Standardization Organization (ISO) and the American Society for Testing and Materials (ASTM) refine and approve a wide range of test methods developed by these trade organizations. INDA Standard Test Methods, and some corresponding EDANA, ASTM and ISO test methods are listed in Tab. 10-4. The crosslinking resins such as melamine can be used to enhance wet and dry tensile strength, moisture resistance, heat resistance and solvent resistance (dry-cleanability) of non-woven fabrics. Figure 10-9 illustrates examples of the stiffness improvement of an acrylic latex bonded non-woven sheet as a function of the latex level in the sheet at two different levels of the melamine resin [12]. Here, the stiffness was measured by the Handle-O-Meter (IST 90.3), where a fabric specimen is pushed through a slot with a blade on an arm at a constant rate and the resultant force on the center point of the fabric measured. Dent [13] recently reported through theoretical analysis that the initial slope of the load-deflection curve

Handle-O-Meter Reading, mN

250

with 10% Melamine

200

150

100

with 5% Melamine

50

without Melamine 0 0

20

40

% Acrylic in Sheet Fig. 10-9 Improvement in the stiffness of an acrylic latex bonded non-woven sheet as a function of the latex level.

60

275

IST GL non-wovens IST GL felts IST 1 IST 10.1 IST 10.2 IST 10.3 IST 20.1 IST 20.2 IST 20.3 IST 20.4 IST 20.5 IST 20.6 IST 30.1 IST 30.2 IST 40.1 IST 40.2

Absorption Non-woven absorption Rate of sorption of wiping materials Demond absorbency

Abrasion resistance Inflated diaphragm Flexing and abrasion Oscillatory cylinder Rotary platform Martindale Uniform abrasion method

Bursting strength Diaphragm Non-woven burst

Electrostatic properties Surface resistivity Decay

INDA standard test method

Guideline test methods for evaluating non-woven felt Non-woven vocabulary

Guideline test methods for non-woven fabrics

Description

Tab. 10-4 INDA Standard Test Methods for non-wovens and the corresponding EDANA, ASTM and ISO tests.

ERT 80.3-99

ERT 230.0-99

ERT 10.3-99

ERT 1.3-99

EDANA recommended test method

ASTM D3786-87

ASTM D3886-92 ASTM D3885-99 ASTM D4157-92 ASTM D3884-92 ASTM D4966-98 ASTM D4158-92

ASTM D1117-99

ASTM method

ISO 9073-6

ISO 13938-1

ISO 12947-3

ISO 9073.6

ISO

276

10 Non-wovens Application

IST 50.1 IST 50.2 IST 60.1 IST 60.2 IST 70.1 IST 70.2 IST 70.3 IST 70.4 IST 80.1 IST 80.2 IST 80.3 IST 80.4 IST 80.5 IST 80.6 IST 80.7 IST 80.8 IST 80.9 IST 90.1 IST 90.2 IST 90.3 IST 90.4

Optical properties Opacity Brightness

Permeability Air permeability Water vapor transmission (multiple tests) Liquid strike-through time Water vapor transmission (Mocon)

Repellence Surface wetting spray test Penetration by water (rain test) Penetration by water (spray impact test) Penetration by water (hydrostatic pressure test) Penetration by saline solution (automated mason jar test) Water resistance hydrostatic pressure test) Penetration by oil (hydrocarbon resistance) Alcohol repellence of non-woven fabrics Non-wovens run-off

Stiffness Cantilever Gurley Handle-O-meter Drape

INDA standard test method

Binder properties Resin binder distribution and penetration Appearance and integrity of highloft batting

Description

ERT 90.4-99

ERT50.5-99

ERT 152.0-99

ERT 120.1-80 ERT 170.0-89 ERT 120.1-80

ERT 150.4-99

ERT 140.1-99

ERT 110.1-78 ERT 100.1-78

EDANA recommended test method

ASTM D5732-95

ASTM D737-96

ASTM D5908-96 ASTM D4770-00

ASTM method

IS09073-7

ISO 811-1981

ISO 811-1981

ISO 4920-1981 (E)

ISO 9073 8:1995

ISO 2471-198 ISO 2470-1997

ISO

10.4 Application Test Methods 277

Continue.

IST 120.5 IST 130.5 IST 140.1 IST 150.1 IST 150.2 IST 160.1 IST 160.2 IST 160.3

Weight Non-wovens mass per unit area

Friction Static arid kinetic

Dry-cleaning Resistance Appearance and integrity of highloft batting

Linting Particulate shedding (dry) Particulate shedding (wet) Fibrous debris from non-woven fabrics

IST 120.4

IST 120.1 IST 120.2 IST 120.3

IST 110.1 IST 110.2 IST 110.3 IST 110.4

Tensile Grab Seam strength Internal bond strength Strip

Thickness Thickness of non-woven fabrics Highloft non-wovens Highloft compression and recovery (measurematic) Highloft compression and recovery (plates and weights, room temperature) Highloft Compression and Recovery (plates and weights, high temperature, high humidity)

IST 100.1 IST 100.2 IST 100.3

INDA standard test method

Tear strength Elmendorf Trapezoid Tongue

Description

Tab. 10-4

ERT 220-0-96 & 300-84

ERT 40.3-90

ERT 30.5-99

ERT 20.2-89

ERT 70.4-99

EDANA recommended test method

ASTM D2724-87

ASTM D5729-97 ASTM D5736-95

ASTM D5035-95

ASTM D5034-95 ASTM D1683-90A

ASTM D5734-95 ASTM D5733-95

ASTM method

ISO 9073-2:1995(E)

ISO 9073-3

ISO 13934-2:1999

ISO 1974-1974(E) ISO 9073-1997(E)

ISO

278

10 Non-wovens Application

INDA IST 10.1-95

According to ASTM D 1776

In MD direction cut 75 mm and a length sufficient so the strip weight is 5 ± 0.1 g

Wire basket, height 8 cm, diameter 5 cm, weight 3 to 8 g, number 20 to 26 gage B&S copper wire, 2 cm mesh; liquid container; stopwatch

Drop basket from height of 25 mm into liquid. Time for specimen to become completely wet is measured

5

Absorbency time in s

Properties

Sample conditioning

Test specimen size

Equipment used

Procedure

Number of tests

Properties reported

Liquid absorbency time in s

5

Drop basket from height of 25 ± 1 mm into room temp. liquid. Record time for the basket to sink completely below the surface of the liquid

Wire basket, height 8 cm, diameter 5 cm, weight 3 ± 1 g, 2 cm mesh; 0.5 mm diameter stainless steel wire; liquid container; stopwatch

In MD direction cut 76 ± 1 mm wide and a length so the strip weight is 5 ± 0.1 g

Condition test specimens according to ERT 60.2-99

EDANA ERT 10.3-99

Excerpt from 2000 Global Comparison of Test Methods for non-woven absorption [10]. (Absorption – Liquid Absorbency Time)

Tab. 10-5

ASTM

Average, max., and minimum absorption time in s, the volume used, and type of paper used

10

Drop liquid from height of at least 10 mm on to the specimen. 1.0 mL 0.1 mL and 0.01 mL are the amounts used.

Drop measuring device. A specimen support consisting of a non-absorbent material 100 × 100 mm with a central hole of approx. 40 mm diameter

Approximately 100 mm × 100 mm

According to TAPPI T 402

TAPPI T432 OM-94

ISO ISO9073-6

Absorbency in s (of bleached textiles)

5

Deliver one drop of water (21 ± 3 °C) 1.0 ± 0.1 cm from hoop. It is important to condition the fabric.

Embroidery hoop 15 cm dia. or more. Burette, delivering 15–25 drops per mL; stopwatch; burette stand; light source

Swatch or skein to fit tightly over embroidery hoop

As directed in EDANA 10.3-99

5

As directed in EDANA 10.3-99

As directed in EDANA 10.3-99

As directed in EDANA 10.3-99

At moisture According to equilibrium ISO 139 65 ± 2 % RH, 21 ± 1 °C

AATCC 79-1995

10.4 Application Test Methods 279

280

10 Non-wovens Application

gives the fabric stiffness or flexural rigidity, while the ratio of maximum load to initial slope gives the fabric friction or smoothness. Thus, a single measurement can measure two basic parameters governing the fabric “hand” or “feel”. In addition, TAPPI (Technical Association of the Pulp and Paper Industry) is active in the wet-form non-woven segment of the industry [9]. Some of the standard test methods established by AATCC (American Association of Textile Chemists and Colorists) are also applicable to non-wovens. INDA recently published “2000 Global Comparison of Test Methods” [10], which conveniently compares standard test methods by the above listed organizations. Table 10-5 is an excerpt from the Absorption – Liquid Absorbency Time, which demonstrates two different principles used to quantify similar properties. Oathout [14] has discussed the water-absorption characteristics of eleven wiping materials including one 100 % wood pulp with binder, paper making process, six hydro-entangled, two knitted polyester and one woven cotton. In addition to the static absorption measurements specified by IST 10.1 and 10.2, he describes results of the dynamic wiping efficiency, or “wipe-dry” test. In this test, a wiper is affixed to the bottom side of a 1 kg sled, which is placed on a stainless steel pan. A known amount of the liquid challenge was placed in front of the sled pulled into and through the pool at a wiping speed of 25 cm s–1. The test tries to simulate manual wiping operations. His results demonstrate that fabrics with bulky character, imparted through creping or stitch-bonding exhibited superior “wipe-dry”.

281

References 1 E. A. Vaughn, Non-wovens World

2

3

4

5 6 7

8

Factbook 1991, ISBN 0-87930-227-5, Miller Freeman Publications, 1991. Association for the Non-woven Fabrics Industry, Cary, North Carolina, USA; www.INDA.org P. Wiaczek, Comparison of Trends in Latex Emulsions for Non-wovens and Textiles: China and the United States, International Non-wovens Journal, 1999. EDANA – European Disposables and Non-wovens Association, Brussels, Belgium, www.edana.org All Nippon Non-wovens Association; www.anna.gr.jp China Non-woven Technical Association; www.chinanonwovens.com Association of the Non-woven Fabrics Industry, Analysis – The Non-woven Industry in North America, Cary, North Carolina, USA. B. M. Koltisko, Vinyl Copolymer Materials, Principles of Non-wovens, 221–248, 1992, Association of the Non-woven Fabrics Industry.

9 1998-1999 TAPPI Test Methods, 1998,

10

11

12

13

14

TAPPI Press, Atlanta, GA; www.tappi.org 2000 Global Comparison of Test Methods, Association of the Non-woven Fabrics Industry, 2000. E. D. Weil, Flame Retardant Nonwovens, Non-wovens Binders and Additives Seminar, 53-61, TAPPI PRESS, 1988. P. D. Wallace, Crosslinker resins in non-woven binder systems, Non-wovens Binders and Additives Seminar, TAPPI Press, 1988. R. W. Dent, An analysis of fabric ‘Hand’ and ‘Feel’, International Non-wovens Journal, Vol. 9, 2000. J. M. Oathout, Determining the Dynamic Efficiency with which Wiping Materials Remove Liquids from Surface, International Non-wovens Journal, Vol. 9, 2000.

Polymer Dispersions and Their Industrial Applications. Edited by Dieter Urban and Koichi Takamura Copyright © 2002 Wiley-VCH Verlag GmbH & Co. KGaA ISBNs: 3-527-30286-7 (Hardback); 3-527-60058-2 (Electronic)

11

Applications in the Leather Industry Johannes P. Dix and Werner Kirchner

11.1

Introduction

Leather making is an ancient art. Methods for converting the fresh animal hide into leather (Fig. 11-1) have been known for approximately 100 000 years [1].

Fig. 11-1

Structure

of leather.

Stone Age man, for example, used smoke or fat for preserving the hides. Tanning with vegetable tannins (vegetable tanning) and with alum, a naturally occurring aluminum sulfate (mineral tanning), then became established in the Middle Ages. It was only about 100 years ago that the development of chrome tanning (tanning with chromium salts) produced the decisive breakthrough which has made it possible to produce leather in an efficient, economical manner.

283

284

11 Applications in the Leather Industry

The excellent properties of chrome-tanned leather opened up new fields of use and made possible the mass production of leather goods, for example in the shoe or apparel leather sector. This created the need for fashionable styling of these leather goods and for making them resistant to soiling and damage. It is this function that is performed by leather finishing [1–7].

11.2

Market Situation

To describe the economic importance of polymer dispersions, a brief look at the structure of the leather industry is helpful. The leather industry is one of the oldest and most complex industries world-wide. It is closely coupled to raw hide production and hence to meat consumption. Livestock is bred world-wide. Accordingly, tanneries are located all around the globe. The articles made from leather differ greatly, ranging from shoe upper leather to apparel leather and to automotive leather. The leather industry has undergone global changes in the last decades. This structural change is still not complete. It is driven by the high proportion of leather processing and leather production costs attributable to wages (Fig. 11-2). This has led and is still leading to a relocation of leather manufacture from the traditional industrialized countries of Europe and from the U.S. to low wage countries, especially in the Far East (Fig. 11-3). At the same time, the industry, which is predominantly based on small and medium size companies, is undergoing a process of consolidation. Costs of leather production in Europe: ca. 7.5 e/m2

Breakdown of leather production costs (without raw hide costs) in Europe.

Fig. 11-2

Regional distribution of finished leather production.

Fig. 11-3

11.2 Market Situation

Since finished leather production is determined by raw hide production (Fig.11-4), the annual growth rate of leather produced is only small (ca. 0.8 % year–1). The predominant portion of leather produced is cattle hide (ca. 60 %), followed by small-animal skins (sheep, goat) at about 30 % and pork leather at ca. 10 %.

Fig. 11-4 World-wide raw hide production in 1990 to 1995.

The market volume of chemicals used in leather manufacture is about Euro 2.5 billion world-wide. The largest segment by far in terms of value is the product range for tanning (ca. 50 %), followed by finishing (ca. 30 %). The finishing segment subdivides into the categories of binders/top coats, finishing dyes and finishing auxiliaries (Fig. 11-5). Finishing market volume: ca. e 0.75 billion

Fig. 11-5 Distribution of market volume in leather finishing between individual product ranges.

Base coats and pigmented coats are today already commonly applied in low-solvent and solvent-free processes. Polyacrylate or polybutadiene dispersions are preferred here. Polyurethane dispersions and casein formulations are used as well, depending on the end use. In the case of top coats, aqueous systems based on polyurethane dispersions are used as well as, still, solvent-containing lacquers or emulsions. However, here too increasingly tougher environmental regulations are driving a changeover to solvent-reduced and waterborne systems. About 180 000 tons annum–1 of binders are estimated to be used in leather finishing world-wide. Of that, about 60 % are polyacrylate and polybutadiene dispersions and about 12 % polyurethane dispersions (Fig. 11-6). At about 20 %, the solvent-containing lacquers still account for a relatively large share today. However, this share will in future decrease further in favor of polyurethane dispersions.

285

286

11 Applications in the Leather Industry World-wide use of binders in leather finishing.

Fig. 11-6

11.3

Leather Finishing

The tanned animal hide, i.e., the leather, usually becomes a sailable article with an upgrading post-treatment. The further processing takes place in two stages: In wet finishing, the character of the leather article (softness, strength, water repellency) is substantially determined by the retanning and the fat-liquoring operations. The leather is also dyed. This is done using soluble dyes. This is followed by the further processing of the dried leather (crust). It is this operation which is commonly referred to as finishing. It makes a significant contribution to enhancing the performance characteristics. A surface coating provides better protection against wetness, soiling and mechanical action. Surface properties such as hue, luster or feel and also light- or rub-fastness are imparted or improved. At the same time, leather damages or unlevelness (grain defects, scratches) are covered up. The finishing of buffed leather or split leather permits the use and upgrading of otherwise less suitable leather qualities. Similarly, many fashion effects are not possible without finishing. Interestingly, leather finishing was one of the first industrial applications for polymer dispersions. In Corialgrund E, the then I.G. Farben commercialized a dispersion based on poly(methyl acrylate) in 1931. This was the first polymer dispersion on the market and the starting point for the immense development of polymer dispersions for other applications too. While Corialgrund E was primarily used as a barrier to avoid migration of the plasticizer out of the nitrocellulose lacquers then used and thus to counteract finish embrittlement, the immense potential of polymer dispersions as binders for finishing was soon recognized. They make it possible to apply thicker finish coats on the leather. The thermoplasticity of polymer dispersions makes it possible to emboss the leather surface and so create any desired surface texture. The use of polymer dispersions provides not only better adhesion of the finish to the leather, but also highly flexible finishes that are stable to light and aging. It is accordingly no surprise that polymer dispersions have become widely established in leather finishing.

11.3 Leather Finishing

11.3.1

Modern Finishing

The various leather articles with their specific requirements each require a specific optimized process in the tannery. Classifying leather finishes according to, say, the binders used or the method of application, the appearance or the ready-produced leather article is thus possible only to a limited extent. Frequently there are many different ways of producing the desired article. Leather finishing is therefore regarded as being more an art than a science. 11.3.2

General Construction of Finishing Coats

The finish is generally made up of a number of coats. Each coat has a certain purpose. The coating technique can be the same for each coat, but need not be. It depends primarily on the type of leather used and the effect desired. 11.3.3

Spray Dyeing

Metallized dyes, for example, are used to dye, or correct the hue of the surface of leathers which have not been drum dyed and to match it to the hue of the finish. As a result, damages to the finish in the course of the use of the leather article are less pronounced. 11.3.4

Grain Impregnation

Grain impregnation is used to improve the firmness of the buffed grain layer. The impregnating float or liquor has to absorb into the leather and must not become deposited at the surface. To this end, very dilute, finely divided acrylate dispersions (solids content about 10 %) are applied in combination with capillary-active substances known as penetrators. 11.3.5

Base Coat

Depending on the crust leather used, the adhesion of the finish layer to the leather has to be improved in some cases by means of a separate base coat. Adequate adhesion is the precondition for many application properties and important to achieve the required physical fastnesses for the ready-produced leather articles. Soft, finely divided polyurethane dispersions have won out in this sector over polyacrylate dispersions.

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11.3.6

Pigment Coat

Its components are pigments, binders and auxiliaries such as waxes and fillers. The pigment coat imparts the desired appearance to the leather and levels out the leather surface. The choice of binders is made according to the finishing effect and fastness profile desired. Generally, polyacrylate dispersions are used in the pigment coat. A distinction is made between finishes which preserve the natural character of the leather (e.g., semi-aniline finish) and high hiding finishes which receive the desired grain structure through embossment. Finely divided acrylate dispersions (<100 nm) are useful for less hiding finishes. The butadiene-based dispersions used in leather finishing have high filling effect and can enhance the hiding power of the finish. As well as the degree of hiding, the hardness/softness balance of the pigment coat has a significant influence on application and processing properties. Especially on thin, soft leather types (e.g., nappa), hard pigment coats lead to an unwanted doubleskin appearance. Soft binders, however, tend to be tacky and cause processing problems in leather production. Auxiliaries (waxes and fillers) can be used to reduce the tackiness within certain limits. The glass transition temperatures of the polyacrylate dispersions used are typically between –10 and +10 °C. Polyurethane dispersions are used in the pigment coat in particular when very high fastness properties are required. 11.3.7

Top Coat

The top coat determines the ultimate appearance and the feel of the leather surface. It further substantially influences the fastness properties of the finish. Instead of organic, solvent-containing lacquers and top coats (e.g., nitrocellulose emulsions), today there is an increasing trend towards waterborne top coat systems. These are usually polyurethane dispersions which, owing to their specific properties (no emulsifier, good film formation despite relatively high hardness), are superior to polyacrylate dispersions in application terms. The degree of luster of the finish is controlled using matting agents (e.g., silica derivatives).

11.4

Application Methods

The application method used depends not only on the processing step but also on the type of leather and the desired finishing effect. Currently applied methods of coating will now be briefly described.

11.4 Application Methods

11.4.1

Spraying

Spraying is the most widely used method of application in leather finishing. Rotating spray guns inside spray machines (Fig. 11-7) apply the low-viscosity finish liquor to the horizontal leather. Even soft leathers can be processed by this technique.

Fig. 11-7

Spray machine.

Modern spray units are equipped with computer-controlled spray guns which recognize the outlines of the leather and so minimize overspray. They further operate under high volume, low pressure conditions to reduce spray drift. As well as the HVLP process there is the airless process whereby the spray jet is not mixed with compressed air, but is generated by very high pressure in the spray nozzle. This method is suitable for high amounts applied. 11.4.2

Roll Coating

Roll coating (Fig. 11-8) is the second most important method for application after spraying. The leather passes between two rolls (color print roll and transportation roll) by means of a transportation belt. The top roll transfers the relatively viscous color to the leather. The texture and the direction of rotation (synchronous or reverse) of the color print roll determine the amounts applied. Soft leathers can be processed on this machine only if the rolls turn in the same direction. Consequently, these types of leather can be roll coated only with finishes that do not require high amounts. 11.4.3

Curtain Coater

The casting process comes from the surface coating of wood. In this process (Fig. 11-9) a casting head creates a curtain of liquid. The horizontal leather passes through this vertically descending curtain. Casting finds application in particular in

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Roll coater.

grain impregnation and in pigment finish applications for patent leather and buffed leather. This technique is not suitable for processing very soft leathers, since they buckle as they pass through the curtain of liquid. This application technology places particularly high demands with regard to shear stability and foam control on the dispersions used. The use of antifoams is limited by wetting and flow-out requirements.

Fig. 11-9

coater.

Curtain

11.5 Binders

11.5

Binders

Binders are among the most important components of a finish system, whether it is a pigment finish or a top coat. They bind the color-conferring pigments, which have no inherent affinity for leather, and protect the leather surface through their filmforming property. Aqueous finishes generally utilize aqueous polymer dispersions. As well as from binders and pigment preparations, finishes are prepared from rheological additives (thickeners, flow control agents, solvents in appropriate cases), matting agent, crosslinker and handle modifier. The following product classes are available as polymer dispersions: – polyacrylate (copolymers) – polybutadiene (copolymers) – polyurethanes The multiplicity of possible monomer combinations and their different blend ratios, the various molecular weight distributions and degrees of crosslinking of the polymers, the effect of process conditions and the colloid-chemical properties of the dispersions make for an immense range of binder properties that can be obtained. These include, for example, hardness and softness, elasticity, water resistance, coldflex stability and hiding power. In addition, film formation in the course of drying has a decisive effect on many fastnesses of the finish. Owing to their different chemistries, the three types of polymer dispersion differ in their application properties: 11.5.1

Polyacrylate Dispersions

Typical leather-finishing polyacrylate dispersions are based on ethyl acrylate or copolymers of butyl acrylate with acrylonitrile or methyl methacrylate. Glass transition temperatures range from –10 °C to +10 °C. The polyacrylate dispersions used are lightfast and compatible in the color batch. Owing to the monomers on which they are based, polyacrylate dispersions are relatively inexpensive. They provide finishes having good application properties. However, they do not (as yet) meet the highest requirements, as required for automotive leather for example. 11.5.2

Polybutadiene Dispersions

The polybutadiene dispersions used are customarily copolymers based on butadiene, styrene and acrylonitrile. Their advantage is the substantial flexibility in thick layers, as required in the finishing of split leathers for example. These systems are typically crosslinked using zinc oxide. As the hiding component, polybutadiene dispersions are also used in combination with polyacrylate dispersions. The double bonds in the polymer make polybutadiene dispersions susceptible to oxidative aging (light, heat) and sensitive to heavy metals.

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11.5.3

Polyurethane Dispersions

Polyurethanes are polyaddition compounds of isocyanates with polyols and/or NH-functional compounds. Owing to the incorporation of hydrophilic groups in the polymer, polyurethanes form stable – usually anionically stabilized – dispersions in water. Unlike the systems described above, polyurethane dispersions can be made without an emulsifier. Finishes with polyurethane dispersions are notable for a very high fastness level. Flexing endurance, even at low temperatures, and rub-fastness (after crosslinking) meet the highest standards. Owing to the hydrophilic groups, polyurethane dispersions possess very good adhesion to leather. Their chemistry makes carboxylate-stabilized systems pH-sensitive. Since monomer costs are distinctly above those of the acrylates, the use of polyurethane dispersions is mainly restricted to applications where the special properties of polyurethane dispersions are essential. Depending on the polyol component used, there are polyetherurethanes and polyesterurethanes. With regard to the isocyanate component, a distinction is made between aromatic and aliphatic monomers. The somewhat less costly aromatic systems, however, do not meet the extreme aging resistance requirements of automotive leathers, for example.

11.6

Production of Selected Leather Articles

Guideline recipes for finishes for some selected leather articles will now be used by way of example to discuss the particular requirements that have to be met by the polymer dispersions used. The finisher has to adapt these guideline recipes to the leather to be finished and to the final properties demanded (feel, appearance, fastness). 11.6.1

Shoe Upper Leather

In terms of area, about 60 % of all leather produced is further processed as shoe upper leather. Shoe upper leather is the largest sector by far. Finish requirements are dictated not only by the performance characteristics but also greatly by the particular processing methods in the footwear industry. In shoe making, the previously moistened leather is wiped (pulled) over the last by heated irons. The folds appearing at the round edges of the shoe are smoothed away by heat treatment with a hot air blower or a smoothing iron (the leather shrinks at these high temperatures). So the finish has to be heat resistant. In addition, the finish must not scratch under the rubbing by the irons. The shoe sole is injection molded on in a further operation. The finish coat therefore has to be solvent-fast and the dyes may not migrate into the shoe sole.

11.6 Production of Selected Leather Articles

Shoe upper leather requires good flexing endurance and good adhesion of the finish. Rub-fastness is of minor importance. An example of a finish recipe for shoe upper leather is: Leather type: Cattle leather box-type Base coat: (depending on crust) Pigment finish: Pigments 100 parts Polyacrylate dispersion (40 %) 200 parts Waxes (40 %) 50 parts Casein binder (20 %) 100 parts Water 350 parts Top coat: Polyurethane dispersion (35 %) 400 parts Waxes (40 %) 20 parts Crosslinker (50 %) 30 parts Water 550 parts Thickener to a 4 mm Ford cup viscosity of approximately 24 s Processing: Pigment coat: 2 × spraying (each ca. 80 g m–2); dry Plating: 2 s at 80 °C and 150 bar 1 × spraying (ca. 50 g m–2); dry Top coat: 2 × spraying (each ca. 50 g m–2); dry The non-thermoplastic casein binder ensures processability in the wiping process by reducing sensitivity to heat and improving hot rub resistance. Useful top coats include a nitrocellulose lacquer or an aqueous system with a very hard and hence plating-fast polyurethane dispersion. If necessary, the latter can be crosslinked to improve the rubfastness. 11.6.2

Apparel Leather

Apparel leather is the second largest sector after shoe upper leather. It accounts for about 20 % of leather production in terms of area. Apparel leathers preferably utilize sheep and goat leathers, but in some cases also calf leather. Inevitably, fashion aspects are of primary importance with apparel leathers. Other decisive aspects are wear properties such as softness and feel. With regard to processing, the finish does not have to meet special requirements. One important performance characteristic is the light-fastness of the finish. Flexing endurance and rub-fastness are of lesser importance.

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An example of a finish recipe for apparel leather is: Leather type: Sheepskin Pigment coat: Pigment 50 parts Spray dye 50 parts Polyacrylate dispersion (40 %) 200 parts Wax (40 %) 100 parts Water 600 parts Top coat: Nitrocellulose emulsion (15 %) 500 parts Wax (40 %) 20 parts Water 480 parts Processing: Pigment coat: 2 × spraying (each ca. 50 g m–2); dry Plating: 2 seconds at 80 °C and 30 bar 5 × spraying (each ca. 40 g m–2); dry Top coat: 2 × spraying (each ca. 50 g m–2); dry Hydraulic ironing: 0.5 s at 120 °C and 30 bar Apparel leathers generally are not provided with a base coat. The pigment content in the pigment coat has been reduced in favor of the spray dyes in order that a more transparent, less coated appearance may be obtained for the finish. This is intensified by means of low concentration of the pigment finish and the large number of spray applications. At present, solvent-containing top coats are still customary for apparel leathers, but, as with shoe upper leather, there is an increasing trend toward the use of aqueous top coats. 11.6.3

Automotive Leather

Although the amount of leather processed in the automotive sector, as leather seats or steering wheel leather, amounts to only about 2 % in terms of area of total leather production (and rising), finishing chemical demand greatly outweighs those for shoe upper leather or apparel leather, for example, because of the high fastness requirements. These high fastness requirements in the automotive leather sector include – depending on the specific requirements of the automotive manufacturer – adhesions of greater than 4 N cm–1, flexing endurance’s (dry, 23 °C) of 100 000 cycles and 30 000 cycles at –10 °C. Similarly, rub-fastness has to meet extreme requirements: >1000 rubs wet and a swelling resistance of >2000 rubs. In addition, these leathers have to be aging resistant, i.e., they have to have adequate flexing endurance and rubfastness even after simultaneous exposure to heat, UV light and moisture. Nor may any color shifts occur. To achieve these very high fastnesses it is predominantly necessary to use polyurethane dispersions.

11.6 Production of Selected Leather Articles

A guideline recipe for automotive leather is: Leather type: Cattle hide Base coat: Polyurethane dispersion (20 %) 200 parts Water 600 parts Pigment coat: Pigment 100 parts Polyurethane dispersion (35 %) 250 parts Polyacrylate dispersion (40 %) 100 parts Waxes (40 %) 80 parts Matting agent 80 parts Water 290 parts Thickener to a 4 mm Ford cup viscosity of 16–18 s Top coat: Polyurethane dispersion (35 %) 500 parts Waxes (40 %) 20 parts Crosslinker (50 %) 60 parts Water 420 parts Thickener to a 4 mm Ford cup viscosity of approximately 24 s Processing: Base coat: 1 × spraying (ca. 100 g m–2); dry Hydraulic ironing: 2 s at 80 °C and 80 bar Pigment coat: 1 × spraying (70–100 g m–2); dry Press embossing: 5 s at 80 °C and 250 bar 1 × spraying (50–70 g m–2); dry Top coat: 2 × spraying (each ca. 50 g m–2); dry The polyurethane dispersions used in the base coat are soft and very finely divided. In contrast, the polyurethane dispersions used in the pigment coat are of medium hardness. A portion of the polyurethane dispersions may also be replaced by polyacrylate dispersions. These must not have an adverse effect on the cold flexing endurance and so the polyacrylate dispersions used must have a low glass transition temperature. The high rub-fastnesses are primarily achieved by the crosslinking of the top coat. Useful crosslinkers include, for example, modified aliphatic polyisocyanates. The leathers are strongly embossed to conform the surface structure of the leathers to the interior styling of the car. As an alternative to the application method described, the base and pigment coats can also be applied by synchronous roll coating. However, for this the recipe needs to be adjusted to a smaller water quantity and a higher color batch viscosity (about 50 s in 6 mm Ford cup). 11.6.4

Furniture Leathers

Furniture leathers, unlike automotive leathers, need less high fastnesses. Light-fastness is an exception. Primary furniture leather criteria are the feel properties and the visual appearance of the leather. The use of soft polyacrylate dispersions has proved

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advantageous here. The greater use of inferior leather grades increasingly forces the use of binders that provide high covering. An example of a recipe for furniture leather is: Leather type: Cattle hide, buffed Base coat: (depending on crust leather used) Pigment coat: Pigment 100 parts Mattin