Resins for Water-borne Coatings 9783748605249

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Jaap Akkerman | Toine Biemans | Cathrin Corten Rudolf Hager | Ingrid Heußen | Claas Hövelmann Joachim Krakehl | Martin Leute | Dirk Mestach Oliver Seewald | Adrian Thomas | Jacques Warnon

Resins for Water-borne Coatings

Cover: Vladyslav Bashutskyy, Adobe Stock

Bibliographische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliographie; detaillierte bibliographische Daten sind im Internet über http://dnb.ddb.de abrufbar.

Jaap Akkerman, Dirk Mestach et al. Resins for Water-borne Coatings Hanover: Vincentz Network, 2021 European Coatings Library ISBN 3-7486-0524-2 ISBN 978-3-7486-0524-9 © 2021 Vincentz Network GmbH & Co. KG, Hanover Vincentz Network GmbH & Co. KG, Plathnerstr. 4c, 30175 Hanover, Germany This work is copyrighted, including the individual contributions and figures. Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punishable by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems. The information on formulations is based on testing performed to the best of our knowledge. Please ask for our book catalogue Vincentz Network, Plathnerstr. 4c, 30175 Hanover, Germany T +49 511 9910-033, F +49 511 9910-029 [email protected], www.european-coatings.com Layout: Vincentz Network, Hanover, Germany Printed by: Gutenberg Beuys Feindruckerei GmbH, Hanover, Germany

European Coatings Library

Jaap Akkerman | Toine Biemans | Cathrin Corten Rudolf Hager | Ingrid Heußen | Claas Hövelmann Joachim Krakehl | Martin Leute | Dirk Mestach Oliver Seewald | Adrian Thomas | Jacques Warnon

Resins for Water-borne Coatings

Foreword

Foreword There is no doubt that water-borne resins for coatings have a bright future. The environmental and health related issues have caused a slow but unstoppable change from solvent-borne resins and coatings to water-borne. It started slowly with some pioneering countries in Europe in the 1980s and became more imbedded in legislation when the solvent emission directives were introduced in 2007 and 2010. In the last decade other stricter regulations appeared that in general led to the increasing preference for water-borne resins and coatings. In fact, the change can be called a “solvent-to-water-transition” similar to the “energy transition” that we are facing to reduce CO2 emission. Simultaneously the legislation on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) highly influenced the resins and coatings market, not only for water-borne but for all resins and coatings. One could even say that the water-borne resins and coatings technologies even suffer more from REACH than solvent-borne for two simple reasons: water-borne resins and coatings use far more additives and these additives cannot easily replaced by alternatives that are not phased out by REACH. What are the largest problems to overcome during the “solvent-to-water-transition” in the coming decennia? We mention a few but not by necessarily in order of priority. First there is the overall performance of the water-borne alternatives for solvent-borne paints. There are more obvious and more well-developed transitions in, for instance, joinery and furniture coatings where spraying, heating, radiation curing, or two-component systems are currently used with much success. The transition in the automotive OEM industries is happening as well, but strongly depends on investments into new production lines suitable for the use of water-borne coatings. In this market segment, the conditions are favourable for water-borne coatings as the application can be controlled easily and higher temperature curing is often used. In both examples the performance of water-borne coatings is up to par or even better. In other markets such as architectural or vehicle-refinish coatings, the transition is also happening but is not so obvious since water-borne coatings have different performance and not always up to par, when it comes to the performance. The second problem is the presence of water in coatings where conditions such as relative humidity and temperature affect drying. When it rains outside, coating walls or wood is not possible and can only be overcome by excluding outdoor conditions, such as scaffolding with rain cover and a heater. The third problem is the pseudoplastic rheology of water-borne paints especially in architectural paints. This is not only the case in the can, but also during application, resulting in poor open time and in the final film formation leading to limited flow. Water-borne paints often still suffer from limited flow and gloss and this may lead to inferior appearance. “Telegraphing” is an often-occurring problem as this copying the surface roughness

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Foreword is caused by a lack of change in surface tension as described by Filip Oosterlinck et al. in Surface Coatings International in 6/2009. For these reasons, professional painters often still use the alternative high solids solvent-borne alkyd paints for application and aesthetic reasons. The fourth problem is the sustainability of water-borne resins and paints when compared to solvent-borne types. Especially, in the choice between water-borne acrylics and alkyds. The use of renewable raw materials and life cycle analysis have not clearly steered the resin development in one clear direction. The fifth problem with water-borne resins and coatings is the relatively low solid content leading to the need to apply more layers. As a consequence, this requires more application work and also road transport of more paint. And therefore, this can have a negative influence on the life cycle analysis. All these issues will be discussed with the best of our know-how, however, not always supported by hard evidence. Resin producers and paint makers know the stodgy reality where some phenomenon or problems cannot be understood and only be solved by hard trial and error efforts with often unexplained solutions (“don’t ask me why but it works”). The so-called “tacit” know-how is often our day to day reality. Because of this issue the writing of this book was a challenge and may lead to disagreement with other specialists. We absolutely liked this challenge. If one overlooks the last 40 to 50 years, the “solvent-to-water-transition” had a large impact of the research and development budgets of resin and paint companies. A lot, if not most developments have been done to keep-up with the evolving legislation. We wish you a pleasant and instructive reading on behalf of all co-authors. Jaap Akkerman and Dirk Mestach Goes and Bergen op Zoom, The Netherlands, January 2021

 

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Contents

Contents 1 A brief introduction 

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2 Water-borne resins and coatings: history, markets and definitions 2.1  History of water-borne coatings and resins 2.1.1  The past 2.1.2  The present: from polymer science to resins 2.1.3  The present: water-borne resins and coatings 2.1.4 The future 2.2 Markets and applications 2.2.1 The market for water-borne coating resins 2.2.2  Coatings market definitions 2.3 Definitions 2.3.1  Definitions of water-borne paints 2.3.2  Definitions of water-borne resins 2.3.3  Volatile Organic Compounds 2.3.4  Polymer dispersion terminologies 2.4 References

19 19 19 21 23 25 25 25 31 35 35 37 37 39 39

3 Polymer dispersions and emulsions  3.1 General introduction 3.2  Preparation of polymer dispersions 3.2.1  Stability of polymer dispersions 3.2.2  Free radical (co)polymerization mechanism 3.2.3  Free radical emulsion (co)polymerization  3.2.4  Raw materials: emulsion polymerization 3.2.5  Process variation and morphology control 3.2.6 Crosslinking of polymer dispersions  3.2.7  Polymer dispersions made by other processes 3.3  Parameters and mechanisms of polymer dispersions 3.3.1  Particle size and particle morphology 3.3.2  Glass transition temperature 3.3.3  Film formation – coalescence 3.3.4  Minimum film formation temperature (MFFT) 3.3.5  Coalescing aids, cosolvents and plasticizers 3.3.6  Stabilization mechanism 3.3.7  Rheology of polymer dispersions and paints  3.3.8  Guideline formulations and performance 3.4 Coating applications and formulations of polymer dispersions

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41 41 42 42 46 49 59 65 68 75 89 89 89 93 94 95 99 99 99 100

Contents 3.4.1 Paint formulation with polymer dispersion resins 3.4.2  Styrene copolymer dispersions 3.4.3  Styrene acrylic dispersions 3.4.4  Pure acrylic dispersions 3.4.5  Poly vinyl acetate and vinyl acetate copolymer dispersions 3.4.6  Poly butadiene dispersions 3.4.7  Cationic polymer dispersions 3.5 References

100 106 110 115 152 159 161 165

4 Alkyd resins 4.1  Water-soluble alkyds 4.1.1  Molecular structure of the alkyd resins 4.1.2  Cosolvents 4.1.3  Applications 4.1.4  Examples 4.2  Externally emulsified alkyds 4.2.1  Molecular structure of the alkyd resins 4.2.2  Anionic and non-ionic modification 4.2.3  Surfactants 4.2.4  Technology 4.2.5  Emulsification in theory and practice 4.2.6  Applications 4.2.7  Examples 4.3  Internally emulsified alkyds 4.3.1  Anionic and non-ionic modification 4.3.2 Molecular structure 4.3.3  Applications 4.3.4  Examples 4.4 References

171 171 171 176 176 177 177 177 180 180 186 187 189 191 191 192 192 195 196 196

5 Epoxy resins 5.1 Historical background 5.2  Basics 5.3  Reaction and crosslinking 5.4  Epoxy dispersions 5.5  Application 5.6  Trends 5.6.1 Epoxy hybrids 5.6.2  BPA substitutes, biobased epoxy resins 5.7  References

197 197 198 200 203 207 209 209 210 212

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Contents

6 Polyurethane resins and coatings 215 6.1  One-component polyurethane coatings 216 6.1.1  Synthesis of polyurethane dispersions 216 6.1.2  Raw materials for the production of polyurethane dispersions 218 6.1.3  Production process 223 6.1.4  PUD producers 228 6.1.5  Applications and formulations of polyurethane dispersions 228 6.1.6  Automotive OEM applications/ formulations 230 6.1.7  Industrial wood applications 234 6.1.8  Architectural: deco/DIY applications/formulations 241 6.1.9  Automotive plastics applications/formulations 245 6.1.10 Teletronics – industrial plastics 248 6.1.11 Vehicle refinishing 248 6.1.12 Metal applications/ formulations 248 6.2  Two-component polyurethane coatings 251 6.2.1  Polyols and polyisocyanates 251 6.2.2  Coating applications of two-component polyurethanes 257 6.2.3  Coatings formulation with acrylic emulsion resins 260 6.2.4  Coatings formulation with polyester emulsions 285 6.3 References 295

PAINTS AND LACQUER RAW MATERIALS APPLICATION ENGINEERING FOR RAW INGREDIENTS FOR DYES, PAINTS AND COATINGS OF THE HIGHEST QUALITY Going far beyond the fast delivery of products, CSC JÄKLECHEMIE can also offer comprehensive support with application engineering - from sample service and laboratory support for your development to commercial support and powerful logistics services. In the process, our highly-qualified employees will also be happy to bring their solution-focused know-how about the entire product cycle to help you on-site at your own premises. MAIN PRODUCTS: hardeners aliphatic, aromatic isocyanates and blocked isocyanates binder polyurethanes, polyacrylates, polyols, polychloroprens, polyamines/ STP; alkyds rheological modifiers carboxymethyl cellulose, silica, moisture scavengers, flexibiliser pigments inorganic, organic, anticorrosion pastes, slurries, tinting pastes additives anticorrosion, matting agent, free flow etc. biocides, catalysts

CSC JÄKLECHEMIE GmbH & Co. KG Matthiasstrasse 10-12 D-90431 Nuremberg T +49 (0) 911 32 646 - 0 F +49 (0) 911 32 646 - 111 www.csc-jaekle.de

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Rubbertstraße 44 D-21109 Hamburg T +49 (0) 229 457 - 0 F +49 (0) 229 457 - 99 M [email protected]

Contents 7 Silicone resins 7.1  Silicone chemistry 7.1.1  Structure of silicones 7.1.2  Silicone resins 7.1.3  Silanes, siloxanes and siliconates 7.2  Silicone resin-based binders for coatings 7.2.1  Silicone resin emulsions 7.2.2  Performance profile of a binder 7.3  Other silicone ingredients for water-borne coatings 7.3.1  Hydrophobic primers 7.3.2  Hydrophobic additives 7.3.3  Special effect additives 7.3.4  Silicone-based pH adjuster 7.4  Silicone resin emulsion paints and plasters  7.4.1  Definition of silicone resin emulsion paints and plasters 7.4.2  Properties of silicone resin paints and plasters 7.4.3 Ingredients and formulation principles 7.4.4  Selected formulations and properties 7.5  Summary and outlook 7.6  References

299 299 300 300 301 302 302 304 306 306 307 309 310 311 311 312 315 316 319 320

8 Alkali silicates 8.1  Historical background 8.2  Chemical compositions 8.3  Molecular structure of water-borne alkali silicates 8.4  Production of alkali silicates 8.5  Water-borne silicates 8.5.1  Classification according to EU Regulations 8.5.2  Sodium silicates 8.5.3  Potassium silicates 8.5.4  Lithium silicates 8.6 Curing or hardening processes of alkali silicates 8.6.1  Dehydration 8.6.2  Reaction with alkali taking substances 8.6.3  Reaction with mineral acids and acidic salts 8.6.4  Reaction by increasing molar ratio 8.6.5  Reaction with CO2 8.6.6  Reaction with esters

321 321 322 323 324 326 326 327 328 328 329 329 330 330 330 330 331

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Contents 8.6.7  Gel formation by reaction with polyvalent metal ion containing compounds or base metals 331 8.6.8  Reaction with base metal powders 331 8.6.9  Reaction with the constituents of the substrate 332 8.6.10 Reaction with water soluble polyvalent metal salts 332 8.6.11 Reactions with alkaline solutions of salts from polyvalent amphoteric metals 333 8.7 Water-borne alkali silicate containing surface coatings 333 8.7.1  Silicate emulsion paint 335 8.8 Organo-silicate paint 340 8.9  Various applications of soluble silicates 340 8.10  References  346 9  Amino resins as hardeners  347 9.1  Structure of amino resins 347 9.2  Types and properties of amino resins 350 9.2.1  Melamine resins 350 9.3  Combination partners for amino resins 353 9.3.1  Crosslinking reactions 354 9.4  Water-borne stoving enamels based on amino resins 356 9.4.1  Selecting amino resins 356 9.4.2  Combination resins for waterborne stoving enamels 357 9.4.3  Hybrid systems 358 9.4.4  Neutralization agents 358 9.4.5  Cosolvents for water-borne stoving enamels 359

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Contents 9.4.6  Mixing ratios and crosslinking 9.4.7 Acid catalysts 9.4.8  Film properties 9.4.9  Pigmentation 9.4.10 Additives 9.5  Formaldehyde free melamine-based resins 9.6  Formulation examples 9.7  Conclusion and comparison water-borne to solvent-based stoving enamels 9.8 References

372 373

10 REACH and other regulations 10.1  Legislation on volatile organic compounds 10.2 Legislation on chemical substances ‘REACH’ and ‘CLP’ 10.3 Conclusions

375 375 376 379

11 Outlook Authors

381

Index

388

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360 360 360 361 361 362 363

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A brief introduction

1 A brief introduction The discussion on water-borne resins and their application in coatings immediately leads to a division in chapters on resin technologies. This is also the approach in the present book. Every resin technology is very often specifically used in a specific application area. Per resin technology the important application areas are discussed and wherever possible guidelines for formulation of the paint including starting point formulations are supplied. Comparison of performances to the alternative solvent-borne are made and shortcomings as well as better properties are discussed. Important for any formulator of paints is to understand the resin technology he or she is using. For that reason, it is even important to know how the resins are made including all chemistry that is used and the physics transforming a hydrophobic resin into a water-borne resin. If this is not known, the formulator cannot ask the important and correct questions to the resin company's technical support staff. Then the wrong resins or the wrong raw material components may be used in the paint. Attention is paid to the musthave information that a paint formulator needs to receive from the resin technical service people. In Chapter 2 the history of water-borne resins and coatings is discussed. The various businesses and their importance in the market are shown. Next to that the most important definitions are reviewed critically. In Chapter 3 water-borne dispersions are discussed. Important is the definition of dispersion polymers in comparison with emulsion polymers (discussed in Chapter 6). They use completely different technologies, production processes and even monomers. The process will influence for instance the morphology or surface of the dispersed particle. It is also important to understand that the dispersed particles, whether in the form of a dispersion or emulsion, will only lead in a perfect film if coalescence is possible during the application and drying of the film. The various chemically based classes of dispersion polymers are discussed: their chemical composition almost directly will lead to the application area. It can happen that a “dispersion” polymer actually proves to be an emulsion polymer. It is also important to realize that not all resin suppliers use the same definition. This may lead to incorrect choice of the resin that you will use to start the development of a paint formulation.

Akkerman, Mestach et al.: Resins for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

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A brief introduction Chapter 4 focuses on water-borne alkyds. They are divided into three categories: dissolved, externally emulsified and internally emulsified alkyds. Each of these will be discussed in detail, including the demand on the resin properties, the different requirements and possibilities regarding resin functionalization and processing technology. Also, recipes and applications are presented. In Chapter 5 water-borne epoxies will be reviewed. An overview of properties and application areas and their relationship to the structure and crosslinking of epoxy resins is followed by a summary of emulsification methods for the preparation of anionic-, cationicand non-ionic epoxy resin dispersions. The use of electrophoretic dip coating as the sole application method for water-borne epoxy resins in the automotive industry is described. The chapter ends with a discussion of future trends in the area of water-borne epoxy resins with an emphasis on the benign replacement of bisphenol A (BPA) and examples of biobased epoxy resins. In Chapter 6 water-borne polyurethane (PU) paint formulations are discussed that use PU dispersions (one and two-component) or hydroxyl functional emulsion resins that are cured with polyisocyanate crosslinkers (two component). PU dispersion resins often have very special applications and in most cases are used in combination with other resins, as will be shown in application examples. Attention will be paid to their various and complex production processes. The well and broadly used two component hydroxyl functional emulsion resins cured with polyisocyanates are subsequently discussed and illustrated with several guideline formulations in a variety of applications. In Chapter 7 silicone resins and their use in silicone resin emulsion paints and plasters are described. The key features of silicone resin-based coatings are low water absorption and at the same time high water vapour permeability. With these physical principles the coatings are extremely durable and last for decades. Guide formulations for paints and plasters are presented and the performance is discussed. Aside of the silicone resins other silicone components are presented which are used in coatings for special effects e.g. anti-scratch or easy-clean. In Chapter 8 soluble silicates and their properties are discussed. Soluble silicates, also commonly known as water glass or alkali silicates, are the only type of glasses, which are soluble in water again. The different production routes as well as the different structures in solution and types of water glass are shown, which have an influence on the chemical properties of the products. By curing, they form very stable inorganic covalent bonds, which match especially with inorganic fillers, pigments and substrates. Different curing methods are presented, as well as the broad range of different applications where soluble silicates are the means of choice. Especially in combination with organic emulsions they show a very high potential for versatile hybrid coatings. In Chapter 9 melamine resins as the most important type of amino resins are presented. Formulations of water-borne stoving enamels with melamine resins in combination

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A brief introduction with alkyds, polyesters and acrylics are discussed and formulation examples for different applications are given. In Chapter 10 the REACH situation and other regulations will be discussed. During the last twenty years several important regulations have been implemented, which have had and continue to have a direct impact on the coating sector in Europe. Among these we may indicate the European Directives to reduce the emissions of volatile organic compounds (VOC), the ‘REACH’ regulation for the control of chemical substances and the European Regulation on biocidal products. Other topics are currently on European or national authorities’ agendas, like the nanomaterials, the emissions of dangerous substances into the indoor air in buildings and sustainable development. The closing Chapter 11 will take you through an outlook in the future. What problems – as described in the foreword – need to be solved, are there still solvent-borne resins and what new resin technologies or chemistries will appear on the horizon. This book does not cover in detail all the individual coating raw materials required to produce a good coating. Only if the coating raw material is essential and specifically critical in interaction with the resin, it is discussed.

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History of water-borne coatings and resins

2 Water-borne resins and coatings: history, markets and definitions Jaap Akkerman and Dirk Mestach This chapter deals with the history of water-borne resins and its coatings. In order to understand water-borne resins that are used in the water-borne coating formulations, not only the history is important, but also the markets and application areas in these markets. In order to use the same language, the definitions used in Europe and the deviating ones used elsewhere are discussed.

2.1 History of water-borne coatings and resins 2.1.1 The past Paint made its earliest appearance about 40,000 years ago. It was one of the earliest inventions of mankind as it truly can be traced to the dawn of history, to artefacts from pre-historic humans, and all cultures. Tribes in Europe, Australia, and Indonesia painted images of hunters and herders on cave walls and had expanded their colour palette to include many colours. Some cave paintings drawn with red or yellow ochre, hematite, manganese oxide, and charcoal were made by early Homo sapiens as long as 40,000 years ago [1]. They used eggs, blood, milk or tree sap as “resins” in order to improve the adhesion and durability. Water was the only solvent available at that time. They then applied the paint with fingers, brushes, or by blowing them through hollow bones, very much like today’s airbrushes. Ancient coloured walls at Dendera, one of the best-preserved temple complexes in Egypt, which were exposed for years to the elements, still possess their brilliant colour, as vivid as when they were painted more than 2,300 years ago. The Egyptians mixed their colours with a binding substance, such as casein, egg whites, bees wax and vegetable gums such as Gum Arabic, the hardened sap of various species of the acacia tree and applied

Akkerman, Mestach et al.: Resins for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

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Water-borne resins and coatings: history, markets and definitions them separately from each other without any blending or mixing them. They appear to have used only six basic colours: white, black, blue, red, yellow, and green. First the area was covered entirely with white, then the design was traced in black, leaving out the lights of the ground colour. The Egyptians continued the advancements and began painting on fresh lime plaster, where the pigments became fixed by carbonization and drying of the lime. Greeks and Romans expanded upon these techniques, to create a painting style not matched till the Renaissance – when Italian artists made paint with plant oils to create works of astonishing colour and depth that still captivate viewers today. The Greeks also developed lead white paint, which was the most popular white paint in use until titanium dioxide replaced it in the nineteenth century. White lead is the basic lead carbonate, 2PbCO3·Pb(OH)2 [2] and occurs naturally as a mineral. Lead-based paint has been the source of health issues for painters and others for centuries. Before the industrial revolution “whitewash” also known as calcimine, kalsomine, calsomine, or just lime paint, a type of paint made from slaked lime (calcium hydroxide, Ca(OH)2 or chalk calcium carbonate, (CaCO3), was one of the main paint types used. Whitewash cures through a reaction with carbon dioxide in the atmosphere to form calcium carbonate in the form of calcite, a reaction known as carbonization. It was usually used for exterior applications, however, it has been also applied for interior applications, for example in kitchens. Whitewash could be tinted for decorative use however it can rub off to a small degree [3]. Various materials were added to the lime to improve the quality of the coating. Portland cement was added to improve the durability in harsh environments. Casein, a protein extracted from milk, was added to improve adhesion and durability. Actually, the origins of modern water-borne paints begin with these casein paints. At the onset of the industrial revolution, in the mid-18th century, paint was being ground in steam-powered mills, and an alternative to lead-based pigments had been found in a white derivative of zinc oxide. Interior house painting increasingly became the norm as the 19th century progressed, both for decorative reasons and because the paint was effective in preventing the walls being affected by moisture. Linseed oil was also increasingly used as an inexpensive “resin”. The paint and coatings industry, however, had to wait for the industrial revolution before it became a recognized element of the economy. The first US paint patent dating from 1865 to Flinn (US 50,068), covers a composition based on zinc oxide, potassium hydroxide, resin, milk and linseed oil. In 1867, Averill of Ohio patented the first prepared or “ready mixed” paints in the United States. In the mid1880s, paint factories began springing up in populated and industrial centres across the nation. In 1866, Sherwin-Williams in the United States started as a large paint-maker and invented a paint that could be used directly from the tin without additional preparation. The casein paints were continually improved. By the 1930s they contained pigments with a high refractive index (high covering power), similar to those used in oil paints. The

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History of water-borne coatings and resins casein paints were traded as a powder or in the form of a paste. In order to trade the paste form, it was necessary to protect the paint against hydrolysis and infestation by micro-organisms. A major advantage of casein binders was their low cost. The paints were easy to apply and provided covering in one layer. A disadvantage of the casein paints was, that they are porous and therefore easily fouled. Adding drying oils resulted in a more compact film and improved durability as well. A drawback was that the drying process became slower. When the amount of oil in the paint was increased, the casein paint evolved into an “emulsion paint”. In the 1920s, however, alkyd resins were first developed [4,5]. In many cases, the drying oil was partially or completely replaced by an alkyd to accelerate the drying of the paint film. The role of the casein in the paint shifted from “resin” to emulsion stabilizer and thickener. The drying oil and the alkyd became the real binders.

2.1.2 The present: from polymer science to resins The development of modern paint is closely associated with the event of polymer science. However, as early as the 1830s people like Braconnot and Schönbein developed derivatives of the natural polymer cellulose such as celluloid and cellulose acetate. In 1844, Goodyear, amongst others, discovered that adding sulphur to natural rubber, polyisoprene, transformed the material from a flexible, non-sticky material to a hard solid (“ebonite”) [6], depending on the amount of sulphur used. In 1907 Baekeland invented the first synthetic polymers: phenol-formaldehyde condensation resins called Bakelite[7] and Novolac [8]. Despite advances made, the molecular nature of polymers was not well understood until the work of Staudinger in 1922[9]. He was the first to propose that polymers consisted of long chains of atoms held together by covalent linkages. He presented this at a meeting of the Swiss Chemical Society, coining the term “macro-molecules” [10]. He was awarded the Nobel Prize for this in 1953. Meanwhile, also the very first inventions were made that would eventually lead to the development of water-borne paints. In 1912 Gottlob patented the dispersion polymerization of isoprene (German patents 254 & 255), using egg albumin or starch as emulsifier. Polyvinyl acetate was patented by Klatte and Rollet in Germany in 1914. Rather than using naturally occurring stabilizers, the first synthetic surfactants developed in Germany during World War I(WWI) were used. These were short-chain alkyl naphthalene sulfonates, similar to the materials are still used today. It was not until the end of the World War I, when a lot of paint raw materials such as linseed oil were in short supply, that artificial resins became available on the market. In 1920 Kienle of General Electric develops unsaturated alkyds [11] that were commercialized under the name “Glyptal” resins (from glycerol phthalate). A patent is applied for, but the patent is ruled invalid in 1935 due to existing prior art, which enables other companies to produce and sell alkyds (after 1935). Kienle was probably responsible for the combination

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Water-borne resins and coatings: history, markets and definitions of the word alkyd – from the condensation of alcohols and acids. Alkyd modified oil paints and then alkyd paints were eventually used on cars and household appliances. In 1929 Carothers (DuPont) publishes papers on linear polyesters [12], He is generally credited with formalizing the concept of functionality, although Kienle had almost certainly been thinking along the same lines. By 1930 Flory [13] starts work on molecular weight distributions (experimentally and theoretically) and shows that step-growth polymerizations follow the Gaussian distribution for molecular weight. In the years leading up to the second World War, a range of additional synthetic resins was appearing. In 1933 Schlak patents the first epoxy resins: diglycidyl ethers made from epichlorhydrin and bisphenol A [14]. Also, Castan (Switzerland) and Greenlee (USA) file patents on epoxy resins. Epoxies resins only became commercially available in about 1947. They impart outstanding resistance properties and find many applications, including one- and two pack air drying systems. The latter includes a polyamine, polyamide or polyamino-amide as a crosslinker. This results in durable and resistant films used in heavy duty coatings. Urea formaldehyde resins started to be combined with alkyd resins. Several US automobile producers, such as Ford and Chrysler, started using alkyd enamel topcoats. Urethane resins produced by Bayer for elastomers and foams are first being introduced in 1937. Later on, polyurethane paints were produced by Bayer based on solvent-borne polyols (“Desmophen”) and polyisocyanates (“Desmodur”), often referred to as “D-D” coatings, for example on German military planes. In 1939 DuPont introduced thermosetting acrylics developed by Strain [15]. During the years of the second World War, there were a couple of important developments in the field of water-borne paints. In the US, the classic cheap interior wall paints based on casein were improved by combining them with both drying oil and alkyd resin. The acid number of the resin was high enough to be neutralized with ammonia in order to make them dispersible in water. In order to obtain fast drying it was necessary that the binder had a high molecular weight (degree of polymerization). The first drying was caused by the evaporation of the water in the paint. The through-drying was due to the oxidative crosslinking of the alkyd resin and the drying oil. A disadvantage of this type of paint was the fact that only low gloss coatings could be made, limiting the use to interior wall paints. In Germany in the meantime, due to the shortage of linseed oil, polyvinyl acetate and polyvinyl propionate latexes were used for the preparation of wall paints. One of the most important developments was the emulsion polymerization process for the synthesis of styrene-butadiene latexes. Even though in Germany, Luther and Hück patented a method for making a styrene-butadiene dispersion already in 1932[16], the biggest progress was made in the US. Being cut off from their supplies of natural latex by the occupation by Japan of big parts of south-east Asia, a project was set-up that was the second largest in the US during WW2, after the Manhattan project aimed at developing

22

History of water-borne coatings and resins the atomic bomb. The so-called GR-S (government rubber styrene), was developed originally for the production of car tires and tracks used for military vehicles. GR-S according to the “Mutual Recipe” consisted of 75 % butadiene, 25 % styrene with a rosin soap as emulsifier and a small amount of mercaptan. At the end of WWII, it was found that the production capacity that was installed could also be used for paint applications, mainly in the US. Later, polyvinyl acetate dispersions became available which were also increasingly used. In 1953 the first all-acrylic dispersion was introduced commercially under the tradename “Rhoplex” AC-33 in the USA or “Primal” AC-33 in Europe. In the 1970s, satin gloss latex paints were introduced. For these paints, new types of fine-particle size polymer dispersions were developed. These were combined with new coalescing agents. In recent years, a large variety of high-quality acrylic dispersions have been introduced on the market, including types with a controlled particle morphology or types that contained functional groups for (self) crosslinking. At the end of the 1960s, the first polyurethane dispersions were patented[17] by Bayer although it would at least take until the 1980s before they saw widespread use. For several years, alkyd emulsions have also made a “come-back”. In the 1950s, water-reducible alkyd paints were introduced for industrial applications[18]. These alkyd paints contained both alkyd resins in dispersed and dissolved form and were combined with amino-formaldehyde resins for thermo-setting applications. Vianova, a heritage company of the current Allnex, pioneered the use of “core-shell” alkyd emulsions under the trade-name “Resydrol”, where the alkyd core was stabilized by an acrylic carboxylic acid functional shell. Also, other internally stabilized types of alkyd emulsions appeared on the market. One particular driver was that alkyds are easily produced from renewable oils or fatty acids. Also, other alkyd building blocks, such as the polyols or polacids can be derived from sustainable building blocks. The following chapters will discuss the different aspects of modern resins for water-borne coatings.

2.1.3 The present: water-borne resins and coatings The modern development of water-borne resins and coatings may be roughly divided in the years before 1970, the years between 1970 and 1990 and the years after 1990. This choice is maybe arbitrary, since this differs depending on the end use application and region. Before 1970, water-borne coatings and resins were only applied in those areas and applications where they performed better than solvent-borne paints, be it on performance or on cost. Performance wise there are not many examples except for the widely used interior and exterior wall paints. Here classical acrylic, styrene or styrene-butadiene dispersions were used in formulations with a low binder content. Chalk and water-glass binders

23

Water-borne resins and coatings: history, markets and definitions and coatings are other well-known examples used in both interior and exterior wall paints. Other main application of early water-borne coatings is the electrophoresis coatings used in industry: both in automotive OEM as well as industrial metal coatings. In the years between 1970 and 1990, environmental awareness resulted in a slow but steady reducing of the amounts of organic solvents. Solvent emissions could be avoided either by lowering the content of volatile organic solvents (VOC) such as high solids coatings or using water-borne or solvent-free solutions such as radiation cured or powder coatings. Legislation initiatives started in Europe around 2000, amongst others in the Netherlands. The KWS2000 (“KoolWaterStoffen” or hydrocarbon) decree was an initiative to reach a reduction of 100 KT/year solvent emissions, by transfer from low or medium solid solvent-borne coatings to water-borne or high solid coatings in the architectural paint markets [19]. In spite of these initiatives, it took until 2007 to 2010 before a VOC legislation was implemented in the European Union [20]. During this period frantic research efforts were initiated in most resin and paint companies as well as in Universities and Research Institutes. The many disadvantages of the older generation water-borne resins and paints still had to be overcome. This will be discussed in Chapter 3. The results of these efforts can be observed in the next period starting from 1990. If we focus on architectural paints, a continuous stream of new water-borne resins and paints appeared on the market with for instance for water-borne trim paints VOC’s of 120 to 150 g/l (incl. water) as from 1990 till recently VOCs of 30 to 50 g/l (incl. water). This was also the case for high solids resins and paints and has led to low VOC alkyd paints for instance with VOCs of 250 to 300 g/l (a 300 g/l VOC became the limit for trim paints in 2010). For wall paints, the VOCs can be as low as close to 0 g/l for interior application and up to 5 to 30 g/l (incl. water) for high resistance or outdoor wall paints where the limit is 30 g/l (incl. water). For automotive coatings – from primer to topcoat – the choice between water-borne and high solid resin technology strongly depends on the production lines that are able to use water-borne coatings. The market is still in transition depending on the characteristics of these production lines. On most automotive OEM production lines, a water-borne cataphoretic electro deposition (CED) coating is applied to new car bodies. Use of water-borne base coat application is state of the art though the VOCs of these coatings can still be quite high (420 g/l excl. water). Solid colour topcoats are water-borne in a number of automotive OEM factories. Clear coats are less commonly using water-borne technology, though some car producers have water-borne production lines. Clear coats are in most cases still solvent-borne (ultra) high solids ( 7h

Ratio NCO/OH

0.66

6.1.8 Architectural: deco/DIY applications/formulations Architectural coatings follow more or less above described formulations, however, a full PUD-based water-borne wood coating will be sold only as top-performing coating. Most formulations are based on self-crosslinkable acrylic dispersions and are often upgraded with a limited amount of thermoplastic PUD to improve for instance scratch and or wine/48 % ethanol resistance. Besides thermoplastic PUDs, oxidatively curing or self-crosslinking PUDs can be used. An example of a high gloss white topcoat based on oxidative curing PUD, is shown in Table 6.14. A 50 g/l VOC (incl. water) paint is obtained using a cobalt-free drier [29]. The formulation is intended for spray application. For brush application the recipe is almost identical except for the choice of a newtonian HEUR thickener that should be used. The oxidatively curing PUDs are also very suitable for direct to metal application, mainly because by their relatively high hydrophobicity.

241

Polyurethane resins and coatings Table 6.14:  The formulation of a white, high gloss topcoat for wood finishes on oxidative curing polyester PUD Raw material

Component

Technology

Parts by Parts by weight [g] volume [ml]

Part 1 mill base Resin

“Bayhydrol” UH 2592

oxidative curing polyester PUD

21.81

26.11

Solvent

demi water

solvent

0.98

1.25

Solvent

butyl glycol

solvent

1.99

2.80

Solvent

“Dowanol” DPnB

di propylene n-butylether

1.99

2.77

Antiskinning

“Ascinin” Anti Skin 0444

anti-skinning agent

0.07

0.10

Solvent

solvent mix

N-ethyl diisopropylam.50 % in ethanol

0.00

0.00

Solvent

demi water

solvent

1.57

2.00

Pigment

“Sachtleben” R-KB-6

Al-Zr-titanium oxide

23.62

7.31

Solvent

demi water

solvent

0.87

1.11

Dispersing agent

“Disperbyk” 2080

acrylic dispersing agent

1.49

1.82

Earlier ingredients have to be added one after another to the mill-base Part 2 let-down Resin

“Bayhydrol” UH 2592

oxidative curing polyester PUD

43.71

52.35

Drier

“Borchi” OXY-coat 1101

Fe complex in water

1.08

1.38

Surfactant

“Byk” 346

surfactant silicone

0.07

0.09

Thickener

“Borchi” Gel PW 25 (10 % in demi water)

rheology modifier WB

0.49

0.62

0.25

0.32

100.00

100.02

Part 3 to be added after 16 hrs ripening Thickener

“Borchi” Gel PW 25 (10 % in demi water)

rheology modifier WB

End viscosity 80–90 s DIN cup 4 mm, 23 °C Sum

242

One-component polyurethane coatings Table  6.14 (continue)

Characteristics

Unit

Value

Density

kg/l

1.28

Weight solid

%

53.7

Volume solid

%

39.0

PVC

%

19.2

% Cosolvent on formulation

%

4.0

VOC excl. H2O

g/l

115.0

VOC incl. H2O

g/l

52.0

CPVC

%

55.8

Alpha (ratio PVC/CPVC)

0.34

The use of oxidatively drying PUDs is also suitable for the formulation of brushable paints with improved good open time when combined with a hydrophilic acrylic dispersion [30]. Applied on-site parquet coatings also use PUDs – in some cases a self-crosslinkable fatty acid-modified PUDs – either as one-component or in this example a two-component clear coat [31, 32] (Table 6.15). The crosslinker in this example is a bio-based polyisocyanate based on pentamethylene diisocyanate. It can be seen in the paint properties that this formulation still has a reasonable VOC for a hard parquet coating. The resin has a large difference in Tg and MFT (Table 6.16), suggesting easy film formation. The formulation is obviously an industrial application and needs to be adjusted for do-it-yourself use. Some of the characteristic properties of the wet and dry coating are shown in Table 6.17. A fast-drying hard film is obtained with good gloss and nice pot-life. Typical parquet performances – like wear-resistance ("Taber" abrader) – are not supplied.

Summary of performances

Excellent properties when compared to high solids solvent-borne coatings: fast drying, high flexibility and excellent chemical resistance, suitable as co-binder in water-borne dispersion coatings. May lower the VOC from 150-200 g/l (excl. water) to 100 to 150 g/l (excl. water). More critical properties when compared with high solids solvent-borne: brush application with short open time, appearance both in gloss and flow. Coatings are still having a substantial VOC when excluding water (100 to 150 g/l excl. water, 50 g/l incl. water).

243

Polyurethane resins and coatings Table 6.15:  Formulation of a two-component water-borne glossy parquet or wood clear coat based on self-crosslinking PUD Raw material Component

Technology

Parts by Parts by weight [g] volume [ml]

Part 1 Resin

“Bayhydrol” UH 2593/1

Self-crosslinking fatty acid modified PUD

70.90

70.74

Defoamer

“Byk” 028

defoamer silicone

1.00

0.99

Surfactant

“Byk” 346

surfactant silicone

0.20

0.21

Solvent

butyl diglycol

solvent

2.20

2.54

Solvent

demi water

solvent

16.30

16.75

Thickener

solvent mix

rheology modifier WB

0.40

0.39

Part 2: Manual mixing possible Resin

“Bayhydur” eco 701-90 Hydrophobic bio-based aliphatic isocyanate

7.00

6.10

Solvent

“Dowanol” PGDA

2.00

2.28

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

1.03

Weight solid

%

31.7

Volume solid

%

29.9

% solid binder

%

31.1

% Solvent

%

5.1

PVC

%

0.0

% Cosolvent on formulation

%

5.1

VOC excl. H2O

g/l

149.2

VOC incl. H2O

g/l

52.2

Sum

NCO/OH

244

Propylene glycol diacetate

not known

One-component polyurethane coatings Table 6.16:  Some characteristics of the resin and coating Characteristics

Unit

Value

Resin: “Bayhydrol” UH 2593/1, oxidative curing PUD Solids

%

35

Tg

°C

103

MFT

°C

60

Equivalent weight OH

not known

Hardener: eco friendly hydrophylic isocyanate based on pentamethylene diisocyanate (PDI) Solids (in PGDA)

%

90

Equivalent weight NCO

230

NCO functionality

3.6

Data of parquet clear coat Gloss 20°/60° on black plexiglass

GU

75/85

s

90/150

Dust free

min.

40

Total dry

h

1

Pot-life

h

>8

Pendulum hardness (1d/7d @ RT) König, 120 μm wet on glass) Drying time (120 μm wet on glass)

6.1.9 Automotive plastics applications/formulations Plastic components are being used increasingly in Western European automobile manufacturing for both interior and exterior applications. For exterior plastics the earlier described metallic base coats can be used (Table 6.4 to 6.6) and will be finished with a two-component clear coat based on an acrylic polyol emulsion cured with a polyisocyanate crosslinker. (Table 6.17 to 6.18) or with an acrylic hydroxyl-functional PUD that is also cured with a polyisocyanate crosslinker. Critical properties are adhesion on the plastic substrate. This is, however not realized by above mentioned systems, but with special, most of the time still solvent-borne, primers and special surface treatments like plasma treatment to obtain a more polar anchor on the extremely hydrophobic plastic (polypropylene/polyethylene). The expected performance of the topcoats are stone chip resistance, adhesion and outdoor durability. Special polycarbonate-based PUDs are marketed for automotive plastics are for example “Daotan” TW 6450/30WA from Allnex. It is a NMP/NEP-free polycarbonate-based

245

Polyurethane resins and coatings Table 6.17:  Formulation of a PUD polyester polyol-based metallic base coat for exterior substrates Raw Material

Component

Technology

Parts by Parts by weight [g] volume [ml]

Part 1 Resin

“Daotan” TW 6466/36WA

polyester-based acrylic modified PUD dispersion

25.00

24.40

Resin

“Cymel” 327

melamine

1.10

0.96

Solvent

DMEA 10 % in water

dimethyl ethanol amine

1.00

1.05

Solvent

demi water

solvent

8.20

8.40

Thickener

“Rheovis” AE 1130 alkali swellable thickener 10 % in water

6.20

6.23

Solvent

demi water

13.30

13.63

solvent

Part 2: 30 min. stirring Pigment paste

“Stapa” IL Hydrolan 2154

aluminium paste non-leaving

6.70

4.91

Dispersant

“Additol” XL 250

dispersant

0.40

0.45

butyl glycol

solvent

4.90

5.58

Solvent

Part 3: 5 min. stirring Wax

“Ultralube” E 500 V HDPE wax

5.40

5.53

Solvent

demi water

8.00

8.20

solvent

Part 4: adjust to pH 8.0 after 12 hours Solvent

iso-butanol

2-methylpropan-1-ol

1.40

1.79

Solvent

DMEA 10 % in water

dimethyl ethanol amine

0.40

0.42

Solvent

demi water

solvent

18.00

18.45

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

1.02

Weight solid

%

16.7

Sum

Volume solid

%

11.7

PVC

%

9.4

% Cosolvent on formulation

%

9.4

246

One-component polyurethane coatings Table  6.17 (continue)

Characteristics

Unit

Value

% Metallics

%

6.2

% Solid binder

%

9.0

% Melamine on solids

%

1.0

VOC excl. H2O

g/l

398.2

VOC incl. H2O

g/l

96.8

CPVC

%

100.0

Table 6.18:  Some resin and coating characteristics of a PUD-based exterior base coat for plastics Characteristics

Unit

Value

%

36

Resin: “Daotan” TW 6466, polyester-based acrylic-modified PUD dispersion resin) Solids in water Neutralizing agent Data base coat, 5 min. flash off, 8 min. 60 to 80 °C

DMEA salt  

 

mPas

400

Can be overcoated with 1/2K clear, SB as well as WB Viscosity pH

8.1

aqueous aliphatic polyurethane dispersion. Another PUD that can be used for plastic coatings is “Daotan” TW 6466/36WA. It is an aqueous dispersion of an aliphatic urethane-acryl-hybrid, solvent and emulsifier free. It is a polyester-based, acrylic modified, high molecular weight polyurethane dispersion. In the interior of cars most plastic parts are being more upgraded with a water-borne coating for better appearance and especially for better haptics such as “soft touch”. This soft touch can also be obtained in the plastic part, but this is more costly than coating a hard-plastic part. An interesting view inside the haptic criteria on soft touch is given by the American Coatings Association [33]. The essence of soft touch coatings is to build in softness over the resins using special PUDs if needed crosslinked with a polyisocyanate

247

Polyurethane resins and coatings crosslinker or using soft polymer particles in a harder crosslinked resin or use special additives to obtain the soft touch. Another interior trend for cars is high gloss plastic or high gloss finished wood or imitated wood structures. In this case are the topcoat needs to have excellent gloss and flow: the so called “piano-lacquer-look”. This is can be realized with 2-component water-borne polyol emulsions cured with hydrophilic isocyanates (see Chapter 6.2.3).

Summary of performances

Excellent properties when compared to solvent-borne coatings for exterior use: fast drying, high flexibility and stone chip resistance, single resin use with substantial VOCs. Interior use to realized soft touch coatings with similar good performances especially when two-component curing is applied. Critical performance is the resistance against human sweat, sun-tan cream and finger prints. Solvent-borne options perform slightly better, but since this use is quite new, water-borne was selected from the start.

6.1.10 Teletronics – industrial plastics Water-borne polyurethane lacquers are used to coat computer housing, instrumentation, and audio and video equipment. In most cases self-crosslinking acrylic dispersions are used that in some cases are further crosslinked with a limited amount of isocyanate or with for example trifunctional aziridine crosslinkers. Addition of PUDs as second binder lead to improved flexibility and abrasion resistance.

6.1.11 Vehicle refinishing Water-borne vehicle refinish coatings quickly conquered the market in recent years together with ultra-high solid coatings. The topcoats – both clear and solid colour – are almost all based on water-borne polyol emulsions cured with an aliphatic isocyanate crosslinker (discussed in Chapter 6.2). Since present options for water-borne and high solids are having the same VOC limits of 420 g/l excl. water, both options are present in the market. Water-borne polyurethane dispersions are also being developed for this market. For metallic base coats formulations as discussed in Table 6.3 to 6.6, special PUDs are used.

6.1.12 Metal applications/formulations Acceptance of polyurethane dispersions, as all other water-borne coating systems in the metal market has been slow. Especially, direct to metal applications – application without a special anti-corrosion primer – is limited to only a few reasonable options. One of them is the use of oxidative crosslinking PUDs. For example, an aliphatic, oil-modified, polyurethane

248

One-component polyurethane coatings Table 6.19:  Formulation of air-drying 1 component polyurethane primer Parts by Raw Material Technology weight [g] Mill base

Parts by volume [ml]

Solvent

demi water

10.47

14.44

Dispersing agent

acrylic based dispersant

0.64

0.85

Defoamer

defoamer

0.43

0.57

Solvent

butyl glycol

3.42

5.24

Pigment/filler

titanium dioxide

5.88

1.98

Filler

dolomite

5.24

2.49

Filler

zinc oxide

1.60

0.39

Filler

zinc phosphate

9.19

3.84

Filler

talc

9.19

4.61

Filler

mica

5.34

2.83

Pigment paste

transparent iron oxide

1.18

1.23

fumed silica

0.32

0.20

Filler

Letdown Resin

“Setaqua” oxidatively drying PUD

44.02

58.94

Drier

calcium drier

0.21

0.32

Filler

corrosion inhibitor

0.85

0.45

Drier

cobalt-free drier

1.18

1.62

Thickener

polyurethane thickener

0.85

1.09

100.00

101.09

Characteristics

Sum

Unit

Value

Density

kg/l

1.38

Weight solid

%

59.4

Volume solid

%

44.5

PVC

%

38.6

% Cosolvent on formulation

%

4.1

% Solid binder

%

19.8

VOC excl. H2O

g/l

113.0

VOC incl. H2O

g/l

56.1

CPVC

%

49.1

Lambda λ

0.79

249

Polyurethane resins and coatings Table 6.20:  Adhesion of primer based on an oxidatively drying PUD (solids in water: 45 %)

Characteristic primer

Adhesion (Gutterschnitt ASTM D 3359B)

Cold rolled steel

Water spot test Water spot test 15 min. 1h

1

0

0

Zinc plated steel (thermal)

1

0

0

Aluminium (Alu S2)

0

0

0

Bonder panel

0

0

0

Aluminium Q-panel

0

0

0

Zinc plated steel (electrolytic)

0

0

0

* 0 is good, 5 is bad

Table 6.21:  Salt-spray results: primer based on an oxidatively drying PUD Salt-spray test Topcoat

Characteristics 100 h 250 h 500 h

750 h

1000 h

Primer as such 

blistering

-

-

f 4–2

f 4–2

f 6–4

 

under rusting [%]

-

-

-

-

none

 

adhesion* [mm]

-

-

-

-

2–4

 

discolouration

none

none

none

none

none

Water-borne 2-component urethane coating

blistering

none

none

none

m 4–2

f 8–6

 

under rusting [%[

-

-

-

-

none

 

adhesion [mm]

-

-

-

4

0

 

discolouration

none

none

none

none

slight

Water-borne one-component

blistering

-

-

-

-

none

 

under rusting [%]

-

-

-

-

none

 

adhesion [mm]

-

-

-

5

2–3

 

discolouration

none

none

none

moderate

slight

Solvent-borne one-component

blistering

-

-

-

-

f 8–6

 

under rusting [%]

-

-

-

-

none

 

adhesion [mm]

-

-

-

3

0

 

discolouration

none

none

none

none

slight

* around scratch

250

Two-component polyurethane coatings dispersion specially designed for high quality metal applications is offered to the market (for example “Daotan” VTW 1252w/42WA). It is claimed that the performance exceeds that of solvent-borne, oil-modified urethanes. Applications are in high gloss enamel and primer formulations. A basic formulation with an oxidative PUD is shown in Table 6.19: the use of a large range of pigments/fillers in combination with the hydrophobic PUD leads to an interesting water-borne one-component primer. The formulation has a good lambda of 0.79 and apparently close enough to 1.0 in order to form a closed film in combination with the low Tg of the oxidative PUD. Having good adhesion is crucial for a primer, tests were performed on a number of different metal substrates. Very positive results are given in Table 6.20 and 6.21. Primers based on this oxidatively curing PUD can be easily over-coated with both oneand two-component water-borne and solvent-borne topcoats. Even the primer as such shows very good salt-spray resistance. Test results are given in Table 6.21.

6.2 Two-component polyurethane coatings In the previous paragraph, examples have been given of coating formulations based on polyurethane dispersions that in several cases were “up-graded” by the use of relatively small amounts of polyisocyanate crosslinkers. However, when one wants to mimic the performance properties of solvent-borne 2K coatings, the polyols used should be different from the acrylic dispersions or PUDs that were designed primarily for use as one-component systems. In most cases, the polyols are synthesized in the same way as their solvent-borne counterparts, with one major difference that they are rather emulsified in water than in an organic solvent.

6.2.1 Polyols and polyisocyanates Two-component water-borne polyurethane systems use a hydroxyl-functional resin that is supplied as a stable dispersion in water (component I) and a polyisocyanate (component II). These components are kept separate until just prior to application, when the polyisocyanate is mixed into the water-borne polyol to form a stable emulsion. After application to the substrate the isocyanate/hydroxyl reaction proceeds to form a crosslinked polyurethane film (Figure 6.20). There is obviously the competing reaction between water and the polyisocyanate component. The polyisocyanate reacts with water to give CO2 and a primary amine. This highly reactive amine then can react rapidly with another isocyanate group to form an urea group. This water reaction is competing with the urethane formation that is responsible for the molecular weight build-up and network properties.

251

Polyurethane resins and coatings The tertiary amine neutralizing agents used in the polyol dispersion are well known catalysts for both the isocyanate/polyol and the isocyanate/water reactions and play a role in the pot-life of the system. The polyols can be modified with hydrophilic monomers as to become self-emulsifying. For acrylic polymers these can be acrylic of methacrylic acid or other acid-functionality bearing monomers (see Chapter 3). Also, non-ionic hydrophilic monomers can be used such as methoxypolyethylene oxide (meth)acrylates (Figure 6.21).

Figure 6.20:  Curing chemistry in water-borne 2-component urethane system

Figure  6.21:  Methoxypolyethyleneoxide methacrylate

Figure 6.22:  Secondary or direct emulsification process

252

Production of acrylic polyol emulsions Most of the time the acrylic polymers are polymerized in an organic solvent such as butyl glycol that is – in most cases – distilled-off after the synthesis. The process of radical copolymerization is extensively described in Chapter 3.2.2, and in the

Two-component polyurethane coatings case of acrylic emulsions it is not different. The same is true for the selection of the ethylenically unsaturated monomers as described in Chapter 3.2.4 as is true for additional other reactive groups if needed (Chapter 3.2.6) and with the restriction that they must be stable against hydrolysis. The traditional solvents are replaced by a water-miscible solvent such as butyl glycol or other glycol ethers to reduce viscosity. The acid groups in the polyol are then neutralized using a tertiary amine such as 2-dimethylamino-ethanol (DMAE) and the resin is emulsified in water. This process is called secondary or direct emulsification: the hot resin is prepared in a separate reactor and is directly emulsified in water by dosing (see Figure 6.22). A significant difference between these emulsified acrylic resins and the polymer dispersions discussed in Chapter 3 is the fact that the molecular weight of the latter can be much higher as in emulsified polymers. In order to make the emulsion successful the viscosity of the acrylic polyol needs to be acceptable to make dosing possible and the emulsification successful. An advantage of emulsified polyols is the fact that they do not contain auxilia-

Figure 6.23:  Microscopic pictures of the phase inversion

Source: Akzo Nobel Resins

253

Polyurethane resins and coatings ry substances such as surfactants or salts and that they have good to excellent hydrolytic stability. The alternative process is to make use of primary or indirect emulsification: in this case the resin is transferred to the thinning tank and water and emulsifiers are dosed to the warm resin. In this case the resin undergoes a phase inversion from water in oil to oil in water (Figure 6.23). It is difficult to control the process since the phase transfer stage is accompanied by a huge viscosity increase, the so called “water mountain” (Figure 6.24) with an implicit inhomogeneous high viscosity phase. A basic example is the preparation of a short oil (polyol) alkyd (50 %): neutralization of the acid present, addition of 5 % of emulsifier and then dosing 45 % of water. To at least control the viscosity increase, up to 5 to 10 % of co-solvent is added.

Production of polyester polyols

For polyester polyols, no solvent is used during the polymerization, except for an azeotropic entraining solvent such as xylene to remove water from the reaction mixture. Also, in this case hydrophilic groups are being introduced in the polymer back-bone using, forexample, acid anhydrides such as trimellitic anhydride (Figure 6.25) or polyethyleneglycol.

Figure 6.24:  Example of viscosity increase and decrease during dosing of water Source: Akzo Nobel Resins

254

Two-component polyurethane coatings It is important to realize that in the case of polyester resins, the formed bonds may be suspectable to hydrolysis. Hence the broad selection of monomers as is the case in solvent-borne ester polyols is limited to hydrolytically stable esters either by steric hindrance at the carboxyl or hydroxyl side or preferably both. Examples are dimethylol cyclohexane, trimethylol propane (TMP), dimethylol propanoic acid (DMPA) and neodecanoic acid. (Figure 6.26). Another often neglected reaction is the transamidation of an ester into an amide when amines – worst are the primary amines – are present as neutralizing agents in the resin. The process is slow but upon storage and in long term stability a polyester can be severely broken down to lower molecular weight by for instance ethanolamine. Both acrylic polyols and polyester polyol emulsions are made with both processes, though polyester have both processes widely present, whereas acrylics are made mostly by the secondary emulsification.

Mixing of emulsion resin with isocyanate Before application, the polyol component is mixed with a polyisocyanate hardener. This polyisocyanate can be a conventional hydrophobic type such as a hexamethylene diisocyanate (HDI) cyclotrimer. However, studies have indicated that water-borne acrylic polyols could not emulsify conventional, hydrophobic isocyanates sufficiently to allow for good film formation and performance. In this case high shear mixing of the polyol and the polyisocyanate is required to obtain intimate contact between both reactants. Melchoir [34] concluded that water-miscible isocyanates must preferably be used in order to produce stable water-borne two-component urethane coatings without high shear mixing. Water dispersible polyisocyanates can be prepared by reacting isocyanate trimers with mono-functional polyethers [35] as shown in Figure 6.27. The level of

Figure  6.25:  Trimellitic anhydride

Figure 6.26:  Dimethylol cyclohexane, trimethylol propane (TMP), dimethylol propanoic acid (DMPA) and neodecanoic acid

255

Polyurethane resins and coatings this polyether affects the dispersibility of the isocyanate as well as the application characteristics and final film properties. The polyisocyanates most commonly used are the isocyanurate trimer of HDI, although other materials such as IPDI-based polyisocyanates have been used as well. The first reaction path, as shown in Figure 6.20, is the preferred route for the development of a two-component polyurethane system. The polyisocyanate group will react with the active hydrogen of the hydroxyl group on the co-reactant to form the polyurethane. This is the dominant reaction step in solvent-borne systems. However, in water-borne

Figure 6.27:  Hydrophilically modified polyisocyanate

Figure 6.28:  Mixing in water-borne 2-component urethane systems

256

Two-component polyurethane coatings systems some of the polyisocyanate groups are sacrificed to the water reaction shown in the second reaction path. The polyisocyanate and water react to form the unstable carbamic acid which immediately decomposes and releases CO2 gas and an amine. The amine then reacts with another polyisocyanate forming polyurea. Because of this, the NCO : OH ratio will have a significant impact on the coating properties: Drying times, pot-life, hardness development and chemical resistance. The competing kinetics of the urethane versus urea forming reactions is determined by the surface area/particle size of the polyisocyanate emulsion, the mechanism of emulsion stabilization, the emulsion stability, the chemical make-up of the polyisocyanate, and how long after mixing the coating is applied. The tertiary amine neutralizing agents used in the polyol dispersion are well known catalysts for both the isocyanate/polyol and the isocyanate/water reactions and have been shown to play a role in the pot-life of the system. Other important factors are the reactivity of the hydroxyl group, the type of catalyst and the stabilizing group of the polymer (e.g. carboxylic acid versus sulfonic acid or (poly)ethylene oxide). Hence, primary hydroxyl groups promote urethane formation since they are more reactive towards the isocyanate group. An example of the influence of the reaction to urethane versus urethane is discussed in Chapter 6.2.3 where too much urea formation leads to CO2 formation and subsequent popping in a clear coat formulation. Figure 6.28 finally shows the possible effect on the final film of improper mixing. It may lead to heterogeneous film when the isocyanate is not properly mixed and most probably is not penetrated in the resin particle during the mixing. It may be present as separate particles when arriving at the substrate and form a heterogeneous film. It may lead to several problems such as loss of gloss, lack of chemical resistance and surface defects such as popping. The best mixing is obtained by use of hydrophilic isocyanates or mixtures of hydrophobic and hydrophilic isocyanates or high-speed mixing and intensive control of foam formation.

6.2.2 Coating applications of two-component polyurethanes All critical parameters and other data needed as described in Chapter 3.4.1 are as important for water-borne two-component polyurethane coatings as they are for formulating with polymer dispersions. So, in order to prepare for formulation work the authors refer to Chapter 3.4.1. There are, however, some additional critical issues to consider before starting selection of the polyol emulsion.

Polyisocyanate

The choice between a hydrophilic or a hydrophobic polyisocyanate is quite crucial. Hydrophobic water miscible or insoluble isocyanates need high speed mixing equipment in order

257

Polyurethane resins and coatings to be properly combined with the polyol emulsion. On the other hand side, hydrophilic water miscible polyisocyanates can be mixed by hand which is required in many applications, such as body shops for vehicle refinish or smaller scale industrial operations. An extensive overview of available polyisocyanates is available from their suppliers [36]. Important is to realize that the main di- and trimer isocyanates are often presented as one ideal molecule. When forming di- or trimers one will always form a statistical mixture of several molecules: it is better to consider di- or trimers as a combination of oligomer structures. (Figure 6.29). This also means that it is important to know the average isocyanate functionality (fNCO) that preferably is higher than 3, in fact several suppliers do supply the fNCO in their technical data sheets. Of course, the actual crosslink efficiency or density also depends on the theoretical hydroxyl functionality of the polyol. The equivalent weight is the molecular weight (number average molecular weight, Mn) of the polymer divided by its functionality. In fact, it is the molecular weight of one reactive group. For emulsified resins (lower molecular weight) the % OH should be in the range of 2.5 to 4.5 %. Next consideration is the choice between the use of hydrophobic or hydrophilic isocyanates. The hydrophobic isocyanates, as described above, are either 100 % solids or dissolved in co-solvents suitable for the water-borne coating. They are difficult to mix and need high speed stirring to realize complete mix and good films. As a consequence, the users should have suitable equipment to mix properly. Advantage is a better water-resistant film, disadvantage next to the mixing may be the inclusion of foam in the coating. The hydrophilic isocyanates are made by reacting one of the isocyanate groups of a tri- or oligomer isocyanate with or amino ethylene sulphonic acid or a non-ionic ethoxylated amine (see Figure 6.27). Note that the functionality drops from 3 to 2 with a statistical mix of functionalities of 1, 2 and 3, consequently often a lower crosslink density is obtained with hydrophilic isocyanates. A final consideration is the choice of the actual type of isocyanate in order to tune the most important coating properties such as drying-speed, hardness, flexibility or exterior durability of the coating (Table 6.22).

Figure 6.29:  Oligomer formation of an isocyanurate or trimer isocyanurate (left) and the structure of a dimer isocyanurate or uretdione/biuret (right) [37]

258

Two-component polyurethane coatings Table 6.22:  Some typical characteristics of IPDI and HDI/PDI based crosslinkers2 IPDI types

HDI/PDI types

Harder chemical structure

softer chemical structure

Hard and ductile coatings

tough and elastic coatings

Slower curing reaction

fast crosslinking reaction

Cures at higher temperature

cure at low temperature

When cured at higher temperature

When cured at lower temperature



higher early strenght



better strength



excellent chemical resistance



better chemical resistance

Longer pot-life

shorter pot-life

High popping/pinhole limit

excellent scratch resistance

High weather resistant

lightfast

Fast physical drying

bio-based for PDI

Metal applications

wood applications

Less cost effective

more cost effective

Polyol selection

After having selected the isocyanate, the choice of the polyol, either an acrylic polyol emulsion or a polyolpolyester emulsion is very important. It is significant to know all typical properties for all applications such as solids content, Tg, hydroxyl equivalent weight or OH %, type of hydroxyl group, durability potential (outdoor, scratch resistance, etc.), presence of small amounts of solvents, neutralization type, stabilization type (carboxylic acid, non-ionic or other acids) and if possible the chemical raw materials that influence for instance the outdoor durability such as the absence of aromatics (styrene in acrylics or phthalic acid in polyester emulsions). Additional to these information it is important to know how the emulsified resin is made: secondary or primary emulsion, how stabilization is realized and the presence of solvents. The application areas as discussed in the next chapter, determine the type of emulsion that is used. Acrylic polyols are mostly made by secondary emulsification and contain dilution solvent that may or may not be removed by distillation. Polyester polyols are made by both processes. In general, one can say that secondary emulsions are less stable to sedimentation, creaming and flocculation. Their shear stability and reproducibility in production may be an issue. In case of both processes it is important to know details about levels, type and fixation of the emulsifying monomers. Classical primary emulsions may contain large amounts of water-soluble material (anchored or not anchored to the polymer).

259

Polyurethane resins and coatings

Other components in the formulation

Formulation advise can also be extended to proper and sufficient pot-life results. More precisely what film parameters are failing at what time. Information with regard to the catalyst that is needed and the reactivity of the hydroxyl group in relation with the catalyst. Resin stability is also essential, such as information with regard to instability related to certain co-solvents, additives but also shear resistance (pigment grinding in the presence of the resin) and freeze/thaw resistance.

6.2.3 Coatings formulation with acrylic emulsion resins Application areas

Most acrylic emulsion resins for curing with polyisocyanates are secondary emulsions. The polymer is made in solvent and emulsified or dosed into water. Acrylic emulsion resins are applied in two completely different areas. First there are the uses as two-component ambient curing coatings. Ambient meaning under critical often primitive ambient conditions (out and indoors: 5 to 45 °C, higher relative humidity) or under better controlled conditions in for instance in a spray booth (indoors: 15 to 80 °C and better controlled relative humidity). Most paints are applied by spraying though brushing, rolling, dipping or curtain application is also frequently used. The formulations differ, however, substantially depending on the application as well as in some cases the emulsion resin. The main areas of application for acrylic emulsion resins are industrial metal and industrial wood. More specifically for metal these are agricultural, construction and earth-moving equipment (ACE), trains, trucks and busses, large metal and small (garden furniture) constructions. The use in vehicle refinish coatings is limited since it has to compete with solvent-borne ultra-high solids options (VOC in the range of 125 to 250 g/l) that are performing more similar to the conventional solvent-borne high solid coatings (VOC just 250 g/l). More specific uses in industrial wood are furniture, parquet (with or without UV-curing or additional use of PUDs) and joinery coatings. In some specific cases there are professional painters using roll or brush application (e.g. kitchen cabinets or tables or kitchen counters). Secondary acrylic emulsion resins may still contain substantial amounts of solvents (4 to 10 %), often mixes of two solvents suitable to aid as co-solvent in the film formation (see Table 3.12, Chapter 3.3.5). Some suppliers remove the solvents by distillation in order to have more freedom in the choice of solvents during formulation. These process solvents are necessary to obtain a low enough resin viscosity at elevated temperatures (90 to 100 °C) in order to be able to emulsify the resin into the water. Remarkable is that some resins contain butyl glycol or other hydroxyl group containing solvents, of which it would be expected that they will react with the isocyanate, disturbing the crosslinking reaction. Nevertheless, it is a common practice to use butyl (di)glycol, the same is true for some

260

Two-component polyurethane coatings neutralizing agents like tri-ethanolamine (TEA) or dimethyl-ethanolamine (DMEA). It may be clear that they may substantially influence the cross-density of the final coating. Most important producers of acrylic polyol emulsions are Allnex (“Setaqua”, “Macrynal”), BASF (“Joncryl”), Covestro (“Bayhydrol”, “Neocryl”) and Synthopol (“Lyocryl”).

Guideline formulations and characteristic properties

In this chapter several guideline formulations are discussed, from high gloss enamels for metal to furniture coatings. Several differences can be observed when comparing the polymer emulsion formulations to the polymer dispersion formulations discussed in Chapter 3.4. This comparison is superficial since the application areas are often different and so the required performances. However, the following major differences can be observed: – Polymer emulsion formulations require a substantial amount of crosslinker: this leads to a limited pot-life and application window. – The application of two-component polymer emulsion paints requires special safety precautions (as is true for the solvent-borne alternatives) during spraying. – In general, the PVC of polymer emulsion paints are equal or higher than these based on polymer dispersions. More pigment can be used when needed. Also, the CPVC will be higher. – The volume solids, important for the resulting layer thickness, for an emulsion-based paint are between 5 to 10 % higher. In fact, the dry layer thickness when applied at similar wet layer thickness can increase with 10 to 30 %. – In general, the polymer dispersion formulations have a lower VOC – especially excluding water – if compared for similar applications. Zero to low VOC formulations (0 to 50 g/l) are hardly possible for formulations with polymer emulsions. – With the selection of the crosslinker an additional tool is present to largely influence the important coating properties. As discussed earlier – see also Table 6.23 – one can influence the durability/toughness/hardness by the use of durable (HDI or IPDI-based) or hard (like TDI or MDI-based) isocyanates. – One superior property that is possible with two-component emulsions and difficult to obtain with polymer dispersions is appearance (gloss at 20° or wave scan or clarity of a clear coat). This is also due to the fact that the lower molecular weight resin emulsions are able to mix/coalesce much better than the very high molecular weight dispersions. – When two-component polymer emulsion can be used and are affordable, it is preferred over one-component polymer dispersions. In general, two-component polymer emulsions are used in high-end applications where better appearance/gloss and film properties such as corrosion protection, balance of hardness/elasticity and exterior durability are mandatory.

261

Polyurethane resins and coatings Table 6.23:  General purpose two-component polyurethane coating for application on wood and metal (example 1) Raw Material

Component

Technology

Parts by weight [g]

Parts by volume [ml]

Mill base/letdown Resin

“Bayhydrol” A 145

WB hydroxy acrylic sec. emulsion

45.61

57.59

Surfactant

“Surfynol” 104 BC (50% in butoxyethanol)

gemini surfactant

0.69

1.02

Dispersant

“Borhi” Gen SN 95

non-ionic dispersing agent

2.21

2.82

Anti-slip

“Borhi” Gol LA200 premix

polysiloxane mix

0.78

1.03

Solvent

butyl glycol premix

solvent

0.09

0.13

Thickener

rheology modifier “Borchi” Gel PW 25 WB

0.13

0.16

Pigment

“Sachtleben” R-KB-4

rutile TiO2 Al/Zr treated

27.58

9.00

Solvent

demi water

solvent

5.48

7.33

Solvent

methoxypropyl acetate MPA

solvent

3.49

4.82

Resin

“Bayhydrol” A 145

WB hydroxy acrylic sec. emulsion

45.61

57.59

100.00

100.00

Component 2

Sum Characteristics

Unit

Value

Density

kg/l

1.34

Weight solid

%

63.8

Volume solid

%

50.7

PVC

%

17.8

% Cosolvent on formulation

%

7.6

% TiO2

%

27.6

% Solid binder

%

20.5

% Isocyanate

%

13.9

262

Two-component polyurethane coatings Table  6.23 (continue) Unit

Value

VOC excl. H2O

Characteristics

g/l

165.6

VOC incl. H2O

g/l

102.2

CPVC

%

51.9

NCO/OH

1.52

Lambda λ (ratio PVC/CPVC)

0.34

The first example is referring to a formulation (Table 6.23) with a frequently used resin in the European business for a long time: “Bayhydrol” A 145. It is a general-purpose hydroxyl acrylic secondary emulsion [38], in this case in a formulation for metal white topcoats having a high gloss and a relatively low VOC (165 g/l excl. water). The resin contains 8 % of co-solvents: normal for secondary acrylic emulsions. Hardly any other co-solvents are necessary. The isocyanate used is a hydrophilic HDI-based hardener that is easily miscible by hand and leads to very good exterior durability (see Table 6.24). The ratio NCO/OH is 1.5, a usual ratio to counter-balance the side-reactions with water, OH-functional co-solvent and neutralizing amines. The TiO2 is milled in the presence of the resin meaning that this resin is shear stable, however, still a dispersing agent is necessary. Milling in the presence of the resin has the advantage of a simpler production process. It concerns a one-step paint production instead of two or more. The mill base volume in the reactor is also larger and thus more accurate (when no extensive, separate milling is needed). This is for many resins not possible. The formulation leads to very durable coatings with good gloss and reasonable hardness and dry speed. Pot-life nor impact and adhesion information is supplied. The second example [39] involves a high gloss clear coat for metal and vehicle refinish applications. When properly formulated, high gloss values are obtained making the formulation suitable for refinish clear coats (Tables 6.25 and 6.26). The resin is a secondary acrylic polyol emulsion. Noticeable is the substantially higher VOC of 249.4 g/l (excl. water) when compared to the first guideline formulation of Table 6.24 and 6.25. This is essential to obtain the better appearance required for vehicle refinish clear coats. With the same resin and additives, a very high gloss white topcoat can be made. One of the main problems observed with these high gloss clear coats is pin-holing (Figure 6.30) in the fast-drying film, when the applied at high wet layer thickness. It is caused by inclusion of carbon dioxide coming from the polyisocyanate/water reaction. Pin-holing is not the same as cratering: where craters are present as deep as to the sur-

263

Polyurethane resins and coatings Table 6.24:  Characteristics of general purpose two-component polyurethane resin and coating Characteristics

Unit

Value

Resin: acrylic secondary emulsion (“Bayhydrol” A 145) Solids

%

45

% OH

%

3.3

Tg

°C

36

Solvents (1:1 mix SN100 : BG)

%

Neutralizing agent

8 DMEA

Hardener: polyether mod. hydrophylic HDI isocyanate % NCO

%

Equivalent weight Functionality Free HDI

18.2 230

NCO/molecule

3.8

%

< 0.1

Data of the paint and topcoat Viscosity DIN A4

s

30

Drying time T1/T2/T3

h

1/7/8.5

20/60° GU

76/86

Gloss on steel Pendulum hardness (1d/14d room temp.) Loss of gloss (Florida 0/6/12 month)

s

70/90

60° GU

92/92/87

face of the primer or metallic base coat, pin-holes are in most cases not. Cratering can be solved with special anti-cratering additives. They are also not comparable to foam defects: foam is present from the moment when the film is formed during spraying and are present as bubbles on the surface but also inside the film. Pin-holes are formed during curing and swell as CO2 continues to develop. Foam can be avoided by defoamers, pin-holes not. Hence the application freedom or layer thickness window is an important characteristic. This window can be highly influenced by the following parameters or technologies: – Selection of the proper co-solvent mix: slower evaporating solvents decrease the pin-holing but lower the hardness development. – Use resins with more reactive primary or less sterically hindered hydroxyl groups. For example, hydroxy ethyl methacrylate (HEMA) or hydroxy ethyl acrylate (HEA). – Optimize the catalyst to the lowest acceptable level with regard to dry/curing speed. Some catalysts inhibit the water reaction in favour of the hydroxyl reaction.

264

Two-component polyurethane coatings Table 6.25:  High performance two-component clear coat with secondary acrylic polyol emulsion (example 2) Raw Parts by Parts by Material Component Technology weight [g] volume [ml] Component 1 Resin

“Setaqua” 6515

WB hydroxy acrylic secondary emulsion

68.90

68.98

Surfactant

“Byk” 348

surfactant silicone

0.11

0.11

Surfactant

“Tego” Wet 270

substrate wetting

0.11

0.12

UV

“Tinuvin” 1130

UV absorber

0.73

0.66

HALS

“Tinuvin” 292

hindered amine stabilizer

0.35

0.37

demi water

solvent

5.00

5.26

“Desmodur” N 3900

hydrophobic HDI trimer

15.80

14.44

Butyl glycol acetate

solvent

Solvent

Component 2 Isocyanate Solvent

Sum Characteristics Density

9.00

10.07

100.00

100.00

Unit kg/l

Value 1.05

Weight solid

%

48.1

Volume solid

%

44.1

PVC

%

0.0

% Cosolvent on formulation

%

14.4

% TiO2

%

0.0

% Solid binder

%

31.0

% Isocyanate

%

15.8

VOC excl. H2O

g/l

249.5

VOC incl. H2O

g/l

151.1

NCO/OH

1.46

– Pin-holing is also observed to be very dependent on the stabilization mechanism: the use of carboxylic acid or sulfonic acid stabilizers or both, are reported to decrease pin-holing [40]. – Pin-holing should not be confused with foam or micro-foam. Pin-holing is formation of voids by CO2, whereas foam and micro-foam are voids by inclusion of air.

265

Polyurethane resins and coatings Table 6.26:  Characteristics of the high performance clear coat formulation Characteristics

Unit

Value

Resin: acrylic secondary emulsion (“Setaqua 6515”) Solids

%

45

% OH on solid material

%

3.3

Tg

°C

Solvents (6.6/7.6 mix SN100 : BG)

%

31

Solids

%

100

% NCO

%

23,5

Neutralizing agent Hardener: HDI trimer, alphatic and hydrophobic

Equivalent weight Functionality Free HDI

230 NCO/molecule

3,2

%

< 0.251

Data of clear coat 30 min. 60 °C and 7 days 23 °C (film thickness 55 µm) Viscosity DIN A4 Impact resistance direct/reverse ASTM D 2794 Adhesion cross cut DIN 53151 Gloss on gardobond Persoz hardness 30 Chemical resistance xylene/ethanol ASTM D-1308

s

23

kg.cm

>105/>105

Gt

0

20/60° GU

78/98

s

203

1 = ok, 5 = bad

1/1

The vehicle refinish clear coating has very good impact resistance, gloss, chemical resistance and hardness (Table 6.27). Durability data nor pot-life is supplied. The third guideline formulation [42], see Table 6.27, is an industrial clear coat wood finish with excellent gloss and “anfeuerung” (grain enhancement, Figure 6.31). It uses a 61 % bio-based PDI-based oligomeric polyisocyanate and a secondary acrylic polyol emulsion with a limited amount of co-solvent (3.5 %). In this case, a very low VOC content is possible (34.8 g/l excl. water) with still a very high gloss result. The curing conditions are at elevated temperatures, typical for industrial wood finishes: curing at 30 minutes at 60 °C (Table 6.28). The film has acceptable hardness, , excellent gloss and reasonable pot-life based on doubling of viscosity (the last conclusion may be incorrect since the other properties of the film may worsen earlier).

266

Two-component polyurethane coatings Table 6.27: Guideline formulation 3, high performance tow-component clear coat for industrial wood, with very low VOC Raw Material

Component

Parts by weight [g]

Parts by volume [ml]

WB hydroxy acrylic sec. emulsion

72.09

72.75

Technology

Component 1 Resin

“Bayhydrol” A 2651

Surfactant

“Surfynol” AD01

gemini surfactant

1.88

2.22

Defoamer

“Tego” Flow 425

polyether siloxane polymer

0.09

0.10

Thickener

“Optiflo” TVS VF

non-ionic. hydrophobe thickener

0.09

0.10

Component 2 Resin

“Bayhydur” eco 701-90

hydrophylic PDI aliphatic isocyanate

17.67

16.02

Solvent

methoxypropyl acetate MPA

solvent

2.16

2.38

Thinning to spray viscosity Solvent

demi water

6.02

6.43

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

1.07

Weight solid

%

47.5

Volume solid

%

44.2

PVC

%

0.0

% Cosolvent on formulation

%

1.5

Sum

solvent

% TiO2

%

0.0

% Solid binder

%

29.6

% Isocyanate

%

17.7

VOC excl. H2O

g/l

34.8

VOC incl. H2O

g/l

15.8

NCO/OH

1.53

267

Polyurethane resins and coatings Table 6.28: Characteristics of the coating formulation 3: industrial wood clear coat Characteristics

Unit

Value

Solids

%

41

% OH on solid material

%

3

Tg

°C

54

Solvent PnB

%

3.4

Resin: acrylic secondary emulsion (“Bayhydrol” A 2651)

Neutralizing agent

DMEA

Hardener: isocyanate (“Bayhydur” eco 701-90) PDI oligomer, alphatic and hydrophilic Solids (90 % in PGDA)

%

90

% NCO

%

17.9

Equivalent weight Functionality

230 NCO/molecule

3.6

Data of clear coat 30 min. 60 °C and 7 days 23 °C (film thickness 55 µm) Viscosity DIN A4

s

35

Pendulum hardness (1 day RT/15 h 50 °C)

kg.cm

60/110

Drying time (DIN 53150, 120 µm on glass)

T1/T4

35 min./5 h30 min.

20°/60° GU

85/90

h

4–5

Gloss on black plexiglass Pot-life (viscosity) “Anfeuerung”

excellent

The fourth formulation is again a clear coat for application on wood, but this time optimized for brush application by professional painters [44, 45]. By careful optimization with a selection of isocyanate crosslinkers, additives and co-solvents a brushable formulation has been developed that has no foaming issues, excellent appearance and very good hardness as well as chemical resistance. The optimized formulation – Table 6.29 shows that a combination of two different hydrophilic isocyanate crosslinkers is essential for this application as well as up to three different co-solvents. Due to the high VOC (195 g/l excl. water) compared to the third formulation, the clear coats not only provide an excellent appearance in terms of flow after brush or roll application, but also excellent “anfeuerung”. A lot of attention was paid to the brush and roll application being tested with the foam and flow test [46]. The resin has 4 % butyl glycol, over 5% of the additional solvent mix is needed to come to good brush properties: proper high shear viscosity, excellent flow, gloss and no foam or other defects. The wet paint and film properties are depicted in Table 6.30.

268

Two-component polyurethane coatings The same approach was done for a white topcoat based on the same acrylic emulsion resin, the same combination of crosslinkers and the same mix of co-solvents with one modification in the type of co-solvent [47] (Tables 6.32 and 6.33). Both formulations offered almost “piano-lacquer” appearance (Figure 6.32). The gloss values (20/60°/haze) for the clear are 85/92 GU/15 HU and for the with topcoat 79/90 GU/22 HU. The clear coat has a distinctness of image (DOI) of 84 both when applied by rolling as well as brush applied. For both coatings the chemical resistance is good except for some aggressive household chemicals in the case of the white topcoat.

Figure 6.30:  Formation of pinholes (ranges around 20 µm) when too thick layers are applied (> 150 µm wet, equals 65 µm ) [41]

Figure 6.31:  Two examples of good versus bad “anfeuerung” or wood grain enhancement on oak flooring (left) and nutwood veneer [43]

269

Polyurethane resins and coatings Table 6.29: Formulation 4: brush- and roller applied clear coat formulation for decorative coatings Raw material

Component

Technology

Parts by weight [g]

Parts by volume [ml]

Component 1 Resin

“Setaqua” 6511

WB hydroxy acrylic

51.32

50.79

Surfactant

“Byk” 348

surfactant silicon

0.26

0.25

Surfactant

“Tego” Wet 270

Anti-crater polyether siloxane copolymer

0.26

0.27

Defoamer

“Tego” Foamex 860

silicon emulsion defoamer

0.17

0.19

Solvent

demi water

solvent

13.70

14.24

Thickener

“Acrysol” RM2020

rheology modifier WB

2.70

2.67

Component 2 Resin

“Vencorex Easaqua” XD 401

hydrophilic isocyanate HDI/IPDI

9.83

9.37

Resin

“Vencorex Easaqua” XM 502

hydrophilic isocyanate HDI

8.64

7.95

Solvent

butyl acetate

solvent

0.76

0.90

Solvent

“Proglyde” DMM

di-propylene glycol dimethylether

4.30

4.97

Solvent

butyl diglycol acetate

solvent

0.76

0.81

solvent

7.30

7.59

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

1.04

Dilution Solvent

demi water Sum

Weight solid

%

42.3

Volume solid

%

38.9

PVC

%

0.0

% Cosolvent on formulation

%

9.3

% TiO2

%

21.0

% Solid binder

%

24.1

270

Two-component polyurethane coatings Table 6.29 (continue)

Characteristics

Unit

Value

% Isocyanate

%

18.5

VOC excl. H2O

g/l

195.0

VOC incl. H2O

g/l

97.1

CPVC

%

NCO/OH

1.53

Table 6.30 shows some more properties of the clear coat. An important conclusion with regard to the best stabilization mechanism of the secondary emulsions was the use of a combined stabilization mechanism of carboxylic as well as sulfonic acid polymer stabilizers. The earlier mentioned avoidance of pin-holes, popping in industrial applications proved also to be true for brush and roller application. In this case it is about the disappearance of foam caused by brushing or rolling. Table 6.31 shows one example of the optimization work. It uses besides a well know brush/foaming test [47], automotive measuring tools – haze and wave scan – to obtain an excellent appearance. The guideline formulations proved – after further optimization with the chosen wetting agents and co-solvents –-to be applicable in high appearance decorative as well as industrial coatings such as topcoats for yachts. Excellent brush/rolling properties along with excellent gloss and flow and excellent outdoor weathering were obtained. The fifth example is a formulation for industrial furniture based on an acrylic emulsion resin cured with a hydrophilic polyisocyanate for maximum performance (Table 6.34) [48]. The paint can be mixed manually and is suitable for typical furniture spraying conditions. It uses the same resin as in the third example, however this paint is opaque white and uses a different additive mix and different polyisocy- Figure 6.32:  Appearance of both clear coat as white anate crosslinker (Table 6.35) topcoat paints on oak veneered panels

271

Polyurethane resins and coatings Table 6.30: Characteristics of Formulation 4 a two-component brush and roller applied clear coat for wood Characteristics

Unit

Value

Solids

%

47

% OH on solid material

%

Resin: secondary acrylic emulsion (“Setaqua” 6511)

Equivalent weight

4.2 861

Tg

°C

Solvent: butyl glycol

%

Neutralizing agent

12 DMEA

Hardener: isocyanate (“Vencorex Easaqua” XD 401), a hydrophilic isocyanate HDI/IPDI Solids (85 % in butyl acetate)

%

85

% NCO

%

15.8

Equivalent weight

260

Hardener: isocyanate (“Vencorex Easaqua” XM 502), a hydrophilic isocyanate HDI Solids

%

100

% NCO

%

18.3

Equivalent weight Data of a clear coat 30 min. 60 °C and 7 days 23 °C (film thickness 55 μm) Viscosity ICI (at 10.000 s-1, RT)

214  

 

Poise

1.5

s

107/284

BK drying time (45 μm on glass)

1/2/3/4 (min.)

15/195/360/570

Gloss/haze brush applied on black Leneta

20/60° GU/HU

85/95/15

brush/roll

84/84

h

excellent

Chemical resistance

(1 = NOK, 5 = OK)

all 5

Foam test (BYK 2813 charts cross brushing and stipulating)

(1 = NOK, 5 = OK)

5

Persoz hardness (1 day RT/7 days)

DOI “Anfeuerung”

272

Two-component polyurethane coatings Table 6.31:  An example of the optimization of the clear coat formulation using the foam test, wave-scan, haze and gloss measurements. In this case the variation was in different combinations and levels of defoamers. Out of samples 4 and 6 the final formulation was further optimized (Table 6. 29)

Sample Defoamer

Amount on total paint [%]

Foam test* Before

After

Appearance

Clarity on glass panel*

1

None

0.00

2.0

1.0

5.0

5.0

2

Mix of hydro­ phobic solids and polysiloxanes

0.17

5.0

5.0

1.0

1.0

3

Mix of hydro­ phobic solids and polysiloxanes

0.34

5.0

5.0

1.0

1.0

4

Mix of polymers and hydrophobic solids

0.17

4.0

4.0

5.0

5.0

5

Mix of polymers and hydrophobic solids

0.34

5.0

4.5

5.0

5.0

6

Polyether siloxane copolymer

0.17

5.0

5.0

3.0

5.0

7

Polyether siloxane copolymer

0.34

5.0

5.0

1.0

5.0

* 1 = bad, 5 = good Amount on Sample total paint (%)  

Brush application

Roller application

20° 20° Long Short Long Short Haze Haze Gloss Gloss wave wave wave wave

  1

0,00

39

34

24

84

54

44

64

80

2

0,17

44

47

93

76

43

50

151

70

3

0,34

44

44

39

82

56

70

54

80

4

0,17

26

28

20

84

42

48

31

83

5

0,34

38

46

28

83

56

64

54

80

6

0,17

31

32

22

84

45

49

26

83

7

0,34

44

40

24

84

51

53

41

82

273

Polyurethane resins and coatings Table 6.32: Formulation no. 4 adjusted to a white topcoat with brush- and roller application for decorative coatings based on sec. emulsion Raw Parts by Parts by Material Component Technology weight [g] volume [ml] Mill base Solvent

demi water

solvent

5.63

6.95

Dispersant

“Tego” Dispers 755 W

pigment wetting agent

1.01

1.16

“Kronos” 2160

pigment white

21.66

6.85

0.16

0.20

WB hydroxy acrylic

35.60

41.85

Pigment Thickener

“Tafigel” PUR 60 rheology modifier (62.5% in Dow PnB) WB Component 1

Resin

“Setaqua” 6511

Surfactant

“Byk” 348

surfactant silicone

0.18

0.21

Surfactant

“Tego” Wet 270

anti-crater polyether siloxane copolymer

0.18

0.22

Defoamer

“Tego” Foamex 860

silicone emulsion defoamer

0.17

0.22

Solvent

demi water

Solvent

11.61

14.33

“Acrysol” RM2020

rheology modifier WB

1.87

2.20

Thickener

Add mill base Hardener solution, add as mix Resin

“Vencorex Easaqua” XD 401

hydrophilic isocyanate HDI/IPDI

6.82

7.73

Resin

“Vencorex Easaqua” XM 502

hydrophilic isocyanate HDI

5.99

6.54

Solvent

butyl acetate

solvent

0.53

0.75

Solvent

“Dowanol TPM

tri-propylene glycol methyl ether

2.98

3.83

Solvent

butyl diglycol acetate

solvent

0.53

0.67

solvent

5.10

6.29

100.00

100.00

Dilution Solvent

demi water Sum

274

Two-component polyurethane coatings Table  6.32 (continue) Characteristics

Unit

Value

Density

kg/l

1.23

Weight solid

%

51.5

Volume solid

%

39.4

PVC

%

17.4

% Cosolvent on formulation

%

6.6

% TiO2

%

21.7

% Solid binder

%

16.7

% Isocyanate

%

12.8

VOC excl. H2O

g/l

167.9

VOC incl. H2O

g/l

81.0

CPVC

%

58.4

NCO/OH

1.53

Lambda λ (ratio PVC/CPVC)

0.30

Table 6.33: Characteristics of two-component brush and roller applied white topcoat for wood with 30 min. 60 °C and 7 days 23 °C (film thickness 45 μm) Characteristics

Unit

Value

Viscosity ICI (at 10.000 s-1, RT)

Poise

1,5

Persoz hardness (1 day RT/7 days RT)

s

105/234

BK drying time (45 μm on glass, RT)

1/2/3/4 (min.)

15/210/270/810

Gloss/haze brush applied on black Leneta

20/60° GU/HU

79/90/22

Chemical resistance (2 layers on oak veneer) Acetic acid, ammonia, ethanol/water, acetone, olive oil 

all 5

Warm coffee, mustard, black ink, cleaning solution, red wine 

all 3

Foam test (BYK 2813 charts crossbrushing and stipulating)

5

Foam defects after foam test and dry

 

none

MEK double rubs

 

57

1 = NOK, 5 = OK

275

Polyurethane resins and coatings Table 6.34: Formulation no 5, a white furniture coating based an acrylic emulsion resin Raw Material Component

Technology

Parts by Parts by weight [g] volume [ml]

Component 1 Resin

“Bayhydrol” A 2651

WB hydroxy acrylic sec. emulsion

49.88

56.95

Defoamer

“Surfynol” AD01

defoamer silicone-free

1.04

1.40

Pigment

aquis” White 0062

pigment in-plant colourant

19.07

7.21

Solvent

demi water

solvent

14.85

17.98

Thickener

“DSX” 1514 (Cognis) non-ionic HEUR thickener

0.30

0.35

Component 2 Crosslinker

“Bayhydur” XP 2655

hydrophylic aliphatic (HDI) isocyanate

11.84

12.36

Solvent

methoxypropyl acetate MPA

solvent

3.01

3.76

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

1.21

Weight solid

%

47.3

Volume solid

%

35.8

PVC

%

15.5

% Cosolvent on formulation

%

4.8

% TiO2

%

13.9

% Solid binder

%

20.5

% Isocyanate

%

11.8

VOC excl. H2O

g/l

137.6

VOC incl. H2O

g/l

57.7

CPVC

%

52.8

Sum

NCO/OH

1.51

Lambda λ (ratio PVC/CPVC)

0.29

276

Two-component polyurethane coatings Table 6.35: Characteristics of the formulation example number 5 based on an acrylic secondary emulsion Characteristics

Unit

Value

Resin: acrylic secondary emulsion (“Bayhydrol” A 2651) Solids

%

41

% OH on solid material

%

3

Tg

°C

54

Solvent PnB

%

3,4

Neutralizing agent

DMEA

Hardener: isocyanate HDI oligomer, aliphatic and hydrophilic (“Bayhydur” XP 2655) Solids (90 % in PGDA)

%

90

% NCO

%

17,9

Equivalent weight

230

Functionality

NCO/molecule

3,6

Data of a clear coat 30 min. 60 °C and 7 days 23 °C (film thickness 55 μm) Viscosity DIN A4 (mix)

s

40

König pendulum hardness (4 h, 1 day RT/7 days RT)

s

30/125/130

Drying time (DIN 53150, 120 µm on glass)

T1/T4

35 min./5 h 45 min.

20/60° GU

83/90

Gloss on black plexiglass

Table 6.36: Some properties of the white furniture coating cured with two different polyisocyanates: the left column shows the use of the bio-based polyisocyanate, the right column shows the results for the above mentioned formulation [49] Hardener used Hardener solution

 

Bio-based hydroConventional hydrophilic polyisocyanate philic polyisocyanate

 

diluted to 80 % in propylene glycol diacetate

diluted to 80 % in propylene glycol diacetate

Mixing

 

manual

manual

Final VOC

 

52 g/l

53 g/l

Bio-based content

on-solids

0.24

0.04

Drying time (120 μm wet)

tack-free

55 m

45 m

 

through dry

7.2 h

8.2 h

277

Polyurethane resins and coatings Table 6.36 (continue) Bio-based hydroConventional hydrophilic polyisocyanate philic polyisocyanate

Hardener used

 

Hardness (Konig, s)

1 d, RT

60

70

120 μm wet on glass

16 h, 50 °C

95

100

Gloss (GU)

20°

80

84

120 μm wet, after 7 d 60°

89

91

 

haze

12

3

water

16 h

5

5

ethanol (48 %)

1h

5

5

coffee

16 h

5

5

red wine

6h

5

5

Chemical resistance

Table 6.37: The comparison of two hydrophilic polyisocyanates that equally perform in for instance above discussed furniture coating. Figure 6.18 shows the simplified chemical structure [49] Hydrophilic polyisocyanate type

Based Hydro on philization

Solids content [%]

NCO [%]

Viscosity [mPa.s]

Biobased content

Bio-based PNCO

PDI

anionic

90 % in PGDA

18

± 5000

61 %

Conventional PNCO

HDI

anionic

100

20.8

± 3500

0%

The main requirements in furniture coatings are hardness, chemical, mechanical resistance and fast drying. In this case the white formulation looks less complicated with less raw materials. This is due to the fact, that a commercial white pigment in-plant colourant is used instead of solid TiO2 that needs to be dispersed with the necessary additives and milling. Some more characteristics of the discussed formulation are shown in Table 6.3, but now compared to an almost identical formulation using a biobased polyisocyanate (Table 6.37). It leads to almost identical results. The eco 701 chemical structure is the theoretical IPDI/PDI structure since the specified functionality is that of an oligomer: the fNCO = 3.6. Example six is a blending experiment for furniture clear and opaque coatings. Formulations as mentioned in example 3 and 4, can be made more cost effective with limited loss of properties, by blending the high OH % emulsion resin with a low hydroxyl level polymeric dispersion and consequently consuming lower amounts of polyisocyanate crosslink-

278

Two-component polyurethane coatings Table 6.38: Furniture clear coat formulation using acrylic emulsion, acrylic dispersion and a 60/40 blend (example 6-1) Supply Viscosity Clear coat Type form 23 ° NCO  Water-dispersable isocyanate

%

mPas

%

Type 1

polyether modified HDI trimer

1

3300

17

Type 2

IPDI based

70 % *

800

9

 Clear coat formulation Polyol Setaqua 6516 (OH-functional component acrylic emulsion)

Emulsion

60/40 mix

Dispersion

66.5

41.5

 

 

Setaqua 6522 (OH-functional acrylic dispersion)

 

butyl glycol

Hardener   Properties

 

27.1

72.3

2.5

2.8

3.3

mix type 1/2 (eq. 1/1)

31

28.6

24.4

Sum

100

100

100

VOC (incl. water)

200 (117)

200 (117)

200 (116)

 

weight solids [%]

49.5

49.4

49.1

 

Relative costs [%]

100

95

86

* in MPA/Xylene 1 : 1

er as well as in most cases using a more cost-effective resin. The examples of blending discussed here, involve a low molecular weight polyol emulsion (25,000 Dalton) with 3.5 % OH and a high molecular weight (> 500,000 Dalton) polyol dispersion with 2.4 % OH [50]. Both resins have a T around 310 K. g In Table 6.38 simple furniture clear coat formulations are shown for the high performance polyol emulsion, the polyol dispersion and a 60/40 blend (on solids) of both resins. The relative cost compared to the full use of the emulsion resins, is 95 % for the blend and 86 % for the pure dispersion. Some coating properties are shown in Figures 6.33 and 6.34. Both for hardness and drying as well as for gloss and haze, the 60/40 mix performs close to “as good as” the pure polyol emulsion with a 5 % cost effectiveness. There was no loss of chemical resistance in all three formulations for (14 h), xylene (5 min.), ethanol (2 min.) and MEK (1 min.). All resistance properties have a score of 1 (1 = excellent, 5 = bad). The formulations are based on hydroxyl-functional polyol dispersions and plasticized with a low Tg hydroxyl-functional emulsion. Similar results were reported for the white topcoat using the same formulation but adding a pigment paste. The formulation characteristics are mentioned in Table 6.40. The VOC and ratio NCO/OH were kept constant are

279

Polyurethane resins and coatings

Figure 6.33:  WB clear coat: hardness and drying (on metal, 23 °C, 45 % RH)

Figure 6.34:  WB glossy clear coat: gloss and haze

280

Two-component polyurethane coatings levels of 250 g/l. In Figure 6.35 it is shown that addition of limited amounts of the dispersion leads to substantial improvement of drying (essential for furniture coatings). The addition of too much dispersion polymer leads to the loss of gloss when going to a 50/50 mix (Figure 6.36). Hardness and adhesion to metal were comparable in all mixes, so no big influence was observed. The chemical resistance after recovery was measured for MEK, xylene and ethanol. No major differences were found in all mixes, proving that a good crosslinking was achieved. Good chemical resistance was achieved with an NCO/OH ratio of 1.2. The next step was to optimize the 50/50 mix with different additives and co-solvents to upgrade amongst others the gloss and haze to an acceptable level when compared to the performance of the OH emulsion on its own. The results are shown in Figure 6.37. It shows that the 50/50 mix could be reformulated with almost similar performance as the pure OH emulsion except for the gloss values at 20° (68 GU versus 59 GU). It should be noted that for non-high gloss furniture coatings the blending ratios could even be more in favour of the polyol dispersion and thus more cost effective. Other applications for acrylic polyol emulsions are in coatings for plastics, both OEM (bumpers and car rims), car refinish and industrial plastics (televisions etc.). Example 6 shows a simple formulation for a general purpose plastic clear coat (Table 6.40 and 6.41) with VOC of 139 g/l excl. water. A very low Tg acrylic emulsion resin is used (-27 °C) with

Figure 6.35:  WB glossy white topcoat: The drying properties, when mixing with limited amounts of polyol dispersions (on glass, 23 °C, 45 % RH)

281

Polyurethane resins and coatings Table 6.39: The blending of a OH acrylic emulsion with a polyol dispersion to reduce cost without losing performance (example 6-2) White topcoat

Emulsion

80/20 mix

60/40 mix

50/50 mix

Disper­­sion A

Dispersion B (competitor)

Setaqua 6516 (OH-functional acrylic emulsion)

40.4

32.3

24.3

20.3

 

 

Setaqua 6522 (OH-functional acrylic dispersion)

 

8.1

16.2

20.3

40.7

 

 

 

 

 

39

0.2

0.2

0.2

0.2

0.2

Acrylic B (competitor)  Silicone wetting agent

0.2

Surface wetting agent

0.2

0.2

0.2

0.2

0.2

0.2

Piment dispersion

25.3

25.6

25.8

25.9

26.6

28.8

Hardener type 3

5.2

4.9

4.7

4.5

3.8

5.4

Hardener type 4

4.5

4.3

4

3.9

3.3

4.7

Butyl glycol acetate

7.4

7.5

7.6

7.6

8.1

5.4

demi water

16.8

16.9

16.9

16.9

17

16.3

Sum

100.0

100.0

99.9

99.8

99.9

100

VOC (incl. water ) [g/l]

250 (113)

250 (113)

250 (113)

250 (114)

250 (114)

250 (125)

Weight solids [%]

45.0

45.0

45.0

45.0

45.0

45.0

Relative costs [%]

100.0

97.0

94.0

93.0

85.0

100.0

1.2

1.2

1.2

1.2

1.2

1.2

Characteristics 

NCO/OH Pigment dispersion

18.6

 

 

 

 

 

Dispersing agent

Demi water

3.8

 

 

 

 

 

TiO2

74.6

 

 

 

 

 

Thickener

3.0

 

 

 

 

 

100.0

 

 

 

 

 

Sum

282

Two-component polyurethane coatings

Figure 6.36:  WB glossy white topcoat: Gloss development upon mixing of the polyol dispersion (on glass, 24 hrs, 23 °C, 45 % RH)

Figure 6.37:  WB glossy white topcoat: optimization of a cost effective 50/50 mix of OH emulsion with OH dispersion. The gloss values are measured on Bonder metal. Unfortunately, the pure OH emulsion formulation was not reported on metal. The values on glass were resp. 68 and 91 GU (Figure 6.35)

283

Polyurethane resins and coatings Table 6.40: Industrial and refinish plastic clear coat formulation based on acrylic emulsion polyol Raw Material

Component

Technology

Parts by weight [g]

Parts by volume [ml]

Component 1 Resin

“Bayhydrol” A 2861 XP

WB hydroxy acrylic sec. emulsion

47.55

47.27

Solvent

demi water

solvent

30.47

32.10

Surfactant

“Byk” 333 mix

surfactant silicone

0.30

0.11

Solvent

demi water mix

solvent

0.10

0.11

Surfactant

“Byk” 347

surfactant silicone

0.40

0.41

Component 2 Resin

“Bayhydur” XP 2655 mix

hydrophylic PDI aliphatic isocyanate

15.81

14.12

Solvent

“Dowanol” PMA mix

methoxy propyl acetate

5.37

5.89

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

Sum

1.05

Weight solid

%

41.7

Volume solid

%

38.3

PVC

%

0.0

% solid resin

%

25.2

% Cosolvent on formulation

%

5.9

VOC excl. H2O

g/l

139.6

VOC incl. H2O

g/l

62.6

NCO/OH

1.46

a proper high level of hydroxyl groups (3.5 %) [51]. A similar formulation for soft touch plastic is given [52]. In most cases elevated temperatures are used to obtain sufficient hardness and fast drying. In the case of ambient curing conditions or non-relative humidity-controlled conditions, the pure emulsion formulations may not dry fast enough. In those circumstances blends with OH-functional dispersions are used and cured with isocyanate or even single use of OH-functional dispersions cured with isocyanates are used as discussed in Chapter 3.4. When superior gloss and haze conditions are needed the use of OH emulsions are a must, together with the use of controlled curing conditions.

284

Two-component polyurethane coatings Table 6.41: Characteristics of plastic clear coats based on an acrylic polyol emulsion Characteristics

Unit

Value 53

Resin: acrylic polyol emulsion (“Bayhydrol” A 2861 XP) Solids in water (1.2 % PnB)

%

Mean particle size

nm

% OH on solid material

%

Equivalent weight as such Tg

3.5 900

°C

Neutralizing agent

-27 DMEA

Hardener: HDI-based hydrophyilic isocyanate (“Bayhydur” XP 2655) Solids

%

100

% NCO

%

20.5

Equivalent weight

205

Functionality NCO

3.2

Characteristics of curing: 32 µm dry,cure 10 min. flash off, 30 min. 80 °C + 15 h 60 °C aging

Summary of performances

Acrylic hydroxyl-functional emulsions are used in two-component polyurethane coatings mainly in industrial applications such ACE, OEM (except automotive), trains and busses. The appearance is sufficient in this case. For high appearance applications such as vehicle refinish or automotive topcoats there is still competition with high solids solvent-borne acrylic polyol. Especially due to the fact that the water-borne alternatives still have high VOC values to achieve a similar appearance. All other properties for the water-borne systems are on par or even better (e.g. VOC and outdoor durability).

6.2.4 Coatings formulation with polyester emulsions Polyester emulsion polymers are less used in two-component polyurethane coatings. The first and most important reason is often their limited resistance against hydrolysis. All polyester emulsions may have this problem unless special measures are taken to prevent hydrolysis like using monomers that lead to sterically hindered ester groups. Examples of hydrolysis stable monomers are dimethylol cyclohexane, trimethylol propane (TMP), dimethylol propionic acid (DMPA) and neodecanoic acid. Examples of very unstable monomers are fatty acids, linear dicarboxylic acids like 1,4-butane dicarboxylic acid or 1,6-hexane diol. The issue of hydrolytic instability is sometimes circumvented by supplying the resin in a co-solvent with no or only limited amounts of water. In that case the paint for-

285

Polyurethane resins and coatings mulator should take into account the (in)stability of the final paint and measure shelf life more intensively. In this case the resin is not a water-borne emulsion but a water dilutable resin with the limitation of having a short shelf life once it is formulated in a water-borne paint. For some industrial applications where the turn-over time is limited, this is not a problem. A well-known application of polyester polyol emulsions are industrial dip coatings for the coating of metal objects or for OEM frames for trucks. These coatings can be cured with water-miscible amino formaldehyde resins (see Chapter 9) or with blocked polyisocyanates. Next to the above mentioned resins there is a wide range of emulsified/dissolved/diluted resins. Some variations are shown in the next paragraph. It should be noted that most resins are supplied in a co-solvent (solids 50 to 70 %) and are emulsified or diluted in water and used not too long before being used. Known suppliers of emulsified polyesters are Allnex (“Setaqua” B E, “Resydrol”), Worlée (“Worléesol”), Synthopol (“Synthoester”) and Arkema (“Envia”).

Guideline formulations and characteristic properties

In Table 6.42 a formulation is shown of a general purpose bake white topcoat formulation. As shown in Table 6.42 and 6.43 the resin is dissolved in butyl glycol (20 %) and emulsified in-situ. This example does not use isocyanates in the formulation and is mentioned only here to pay attention to a sometimes overlooked fact. Since a lot of polyester polyols are containing substantial amounts of butyl glycol or other alcohols as solvents to make in-situ emulsification in water possible, they are less or not at all suitable to crosslink with isocyanates. In fact, the isocyanate may react with the excess of primary alcohol groups and yields only limited crosslinking. Nevertheless, these resins are often cured with isocyanates as the next examples will show. Often combination of isocyanate with a melamine is observed, indicating higher curing temperatures. It means that above guideline formulation can also be performed with isocyanates, if one takes the “side” curing into account. In the guideline formulation no more solvent is added during the formulation, hence resulting in a VOC of 172 g/l excl. water, white topcoat. The formulation is crosslinked with a methylated melamine. In the final VOC calculation, the methanol emission is not included. This is estimated to be substantial. In Table 6.44 a formulation is given for a soft touch coating using a water emulsifyable polyester polyol cured with an hydrophobic isocyanate (HDI trimer) [52]. Also, in this case large amounts of solvent alcohol groups are available in the form of dipropylene glycol monomethyl ether. In this case the free alcohol is a secondary alcohol group and less reactive. Nevertheless, some isocyanate may be inactive for crosslinking, certainly when application is done at the end of its pot-life. In addition, the main characteristics that differentiate from all previously discussed formulations are given.

286

Two-component polyurethane coatings First, the use of a fumed silica is necessary to realize a soft touch together with sufficient damage resistance like scratch resistance with a soft resin (Tg is expected to be low). The fumed silica also leads to a very low gloss, as can be seen in Table 6.45: 1.1 GU at 60°. In fact, soft touch can only be reached with this combination of resin and filler: high gloss soft touch is not possible with this resin and filler chemistry. Second, observation is the use of a soft HDI trimer isocyanate, it adds to the soft touch. Unfortunately, the Tg of the resin is not mentioned in the technical data-sheet of the resin: it is expected to be low in order to realize soft touch. It is a delicate balance of “soft” raw materials and can in some cases lead to sticky coatings upon aging of the coating. Last remarkable fact is the high VOC of 335 g/l excl. water, if one considers the low Tg resin and softer isocyanate. Due to the good adhesion on a wide range of plastics (Table 6.45) the resin can also be used as a clear coat on plastic (but in this case without silica and softer isocyanate) or as modifier resin to improve adhesion properties of for example water-borne based on acrylic resins. Adhesion to plastics is normally – when performed industrially – improved by pretreatment of the plastic. Techniques like flame-plasma treatment are often used especially on polyethylene or polypropylene or mixtures thereof. Additives like t-butanol or other softeners like Eastman chlorinated polyolefins (CPOs) [53] are often added in the last phase of a paint development if the intrinsic adhesion is not sufficient or the polyester polyol is too brittle. The next application example (Table 6.46) is a primer-surfacer paint, using a water-thinnable oil-free saturated polyester. It leads to coatings having normal PVC, very good gloss and a reasonable VOC of 174 g/l (excl. water). The mill base contains besides titanium dioxides a mix of fillers based on barium sulphate and talcum. The rheology for spray application is controlled by a fumed silica. The characteristics are shown in Table 6.47 and show the typical high-performance parameters for use as a primer-surfacer. This formulation contains substantial amounts of both primary (DMEA) and secondary alcohol (DPM) groups from the co-solvent. For that reason, obviously a blocked isocyanate is used, together with a melamine crosslinker. In the calculation of this formulation the VOC contribution of the melamine and the blocked isocyanate is not calculated, since details are not known to the authors: it is, however, expected to be substantial. The formulation can be completely copied – with equal amounts of all components – using conventional isocyanates with the note that application should be done directly after mixing of the two-components. Table 6.48 shows examples of well-known blocking agents for polyisocyanates and their deblocking temperatures.

287

Polyurethane resins and coatings Table 6.42: Guideline formulation of a stoving enamel based on a emulsify able polyester polyol (water dilutable) Raw material

Component

Technology

Parts by weight [g]

Parts by volume [ml]

Mill base Resin

“WorléePol” 191

polyester polyol emulsifiable

10.00

11.96

Amine

DMEA

N,N-dimethyl ethanol amine

0.70

1.03

Solvent

demi water

solvent

10.00

13.16

Pigment/Filler

“Tioxide” TR92

Al/Zr/titanium oxide

28.00

8.99

Dispersant

“Hydropalat” 535 N

dispersant

0.50

0.71

Solvent

demi water

solvent

4.00

5.26

Letdown Resin

“WorléePol” 191

polyester polyol emulsifiable

15.50

18.54

Amine

DMEA

N,N-dimethyl ethanol amine

1.10

1.63

Melamine

“Cymel” 303

methylated monomeric melamine

4.60

5.04

Solvent

demi water

solvent

25.60

33.68

100.00

100.00

Characteristics

Unit

Value

Density

kg/l

1.32

Weight solid

%

54.2

Sum

Volume solid

%

33.7

PVC

%

26.7

% Cosolvent on formulation

%

7.8

% TiO2

%

28.0

% Solid binder

%

23.4

% Melamine

%

4.6

VOC excl. H2O

g/l

213.7

VOC incl H2O *

g/l

102.4

288

Two-component polyurethane coatings Table  6.42 (continue) Characteristics

Unit

Value

%

55.8

CPVC OCH3/OH

n.a.

Lambda λ (ratio PVC/CPVC)

0.48

Estimated contribution of melamine to solids is 2/3 of the melamine and 1/3 of methanol

Table 6.43: Characteristics of the stoving enamel coating based on a emulsify able polyester polyol (water dilutable) Characteristics

Unit

Value

Resin: polyester polyol for baking enamels, emulsifiable in water (“WorléePol” 191) Solids in butyl glycol

%

80

% OH on solid material

%

4,3

Equivalent weight

881

Tg

°C

Solvent butyl glycol

%

Neutralizing agent

20 DMEA

Hardener: methylated monomeric melamin (“Cymel” 303) Solids

%

100

-CH2-O-Me/ molecule

< 6.0

Free formaldehyde

%

< 0.1

Data of enamel coating (10 min. 160 °C)

 

 

Viscosity DIN 53211

s

120

Functionality

Pencil hardness Gloss

2H 20° GU

Adhesion MEK resistance

79 GT0

rubs

100

Impact direct

cm/kg

< 100

Impact reverse

cm/kg

< 100

mm

8.3

Erichsen indentation

289

Polyurethane resins and coatings Table 6.44: Guideline formulation for soft touch coating based on water emulsifyable polyester polyol Parts by weight [g]

Parts by volume [ml]

 

 

Raw Material

Component

Technology

 

Mill base

 

Resin

“Resydrol” AN 6617w/65MPP

emulsify-able polyester polyol

32.79

32.15

Solvent

demi water

solvent

19.67

21.22

Defoamer

polyether modi“Additol” VXW 6503 N fied polysiloxane 

0.62

0.67

Dispersant

“Additol” XL 250

dispersant

0.10

0.12

Filler

“Acematt” TS 100

fumed silica matting agent

4.69

2.30

Solvent

demi water

solvent

19.67

21.22

 

Crosslinker

 

 

 

Resin

“Desmodur” N 3390

isocyanaat HDI trimer

17.48

16.65

Solvent

MPP (DPM)

methoxypropoxy propanol

4.98

5.66

 

Dilution to spray viscosity

 

 

 

Solvent

demi water

solvent

0.00

0.00

Sum

 

100.00

100.00

 

Unit 

Value

  Characteristics Density Weight solid

kg/l

1.08

 

 

%

42.1

%

45.6

 

 

%

5.0

%

17.6

Volume solid PVC

% Cosolvent on formulation % Matting agent

 

 

% Solid binder % Isocyanate VOC excl. H2O

290

 

 

%

4.7

%

21.3

%

17.5

g/l

335.8

Two-component polyurethane coatings Table 6.44 (continue)

Characteristics

 

Unit 

Value

g/l

189.7

 

%

10.5

 

1.43

 

 

0.48

VOC incl. H2O CPVC

 

NCO/OH Lambda λ (ratio PVC/CPVC)

Table 6.45: Characteristics of the soft touch resin and coating Characteristics

Unit

Value

Resin: polyester polyol (“Resydrol” AN 6617w) for soft touch plastic coatings Solids in DPM

%

65

% OH on solid material

%

4.5

Equivalent weight

575

Tg

°C

Solvent (methoxy propoxy propanol, DPM)

%

Neutralizing agent

n.a. DMEA

Hardener HDI trimer isocyanate “Desmodur” N3390 Solids in butyl acetate

%

90

% NCO

%

19.3

Equiv. weight Functionality

214 NCO/ molecule

3.2

%

< 0.1

Free monomer Data of soft touch coating (30 min. 80 °C plus15 h 70 °C post cure)  Viscosity Pendulum hardness (König) Gloss

 

mPas

250–300

s

55

60° GU

1.1

 

full

Acetone resistance spot test

min.

2

Xylene resistance spot test

min.

>10

Adhesion (ANS, PC, ABS/PC, SMC, glass and carbon fibre)

291

Polyurethane resins and coatings Table 6.46: Guideline formulation of a primer-surfacer for metal-OEM application using a polyol emulsion and a blocked isocyanate along with a melamine resin as crosslinker Raw Material

Component/ technology

Technology

Parts by weight [g]

Parts by volume [ml]

Mill base Resin

“Setaqua” B E 270

oil-free water-thinnable polyester polyol

3.68

4.08

Solvent

demi water

solvent

7.74

9.59

Solvent

DMEA 10 % in water

dimethyl ethanol amine solution in water

0.52

0.66

Dispersant

“Disperbyk” 180

dispersant

0.46

0.57

Defoamer

“Byk” 011

defoamer silicone-free

0.84

1.32

Pigment

“Kronos” 2190

pigment white

9.16

2.77

Filler

“Blanc Fix Micro” barium sulphate

9.16

2.58

Filler

“Microtalc” IT extra

talcum (Mg silicate)

2.32

1.04

Filler

“Aerosil” R972

hydrophilic fumed silica

0.31

0.18

Letdown Resin

“Setaqua” B E 270

oil-free water-thinnable polyester polyol

21.65

23.95

Crosslinker

“Trixene” Aqua BI201

anionic aqua blocked isocyanate dispersion

10.48

12.37

Crosslinker

“Cymel” 327

melamine

4.30

4.52

Surfactant

“Byk” 346

surfactant silicone

0.49

0.61

28.88

35.78

100.00

100.00

Thin to spray viscosity 35 s, DIN 4 Solvent

demi water Sum

Solvent

Characteristics

Unit 

Density

kg/l

Value 1.24

Weight solid

%

47.7

Volume solid

%

40.4

PVC

%

16.3

% Cosolvent on formulation

%

6.0

292

Two-component polyurethane coatings Table 6.46 (continue) Characteristics

Unit 

Value

% Pigment

%

9.2

% Matting agent

%

11.8

% Solid binder

%

17.7

% Isocyanate

%

10.5

VOC excl. H2O*

g/l

174.5

VOC incl. H2O*

g/l

74.4

CPVC

%

53.4

NCO/OH

0.30

Lambda λ (ratio PVC/CPVC)

0.30

* Trixene hardener contains unknown level of DPGDME (dipropylene glycol dimethyl ether). Also unknown emission blocking agent

Summary of performances

Polyester polyol emulsions are applied on metal and plastics by curing at ambient or elevated temperatures, as above guideline formulations show. The resins are often of low molecular weight and supplied in a co-solvent or a mix of water and co-solvent. They can be even solutions or close to solutions and emulsify upon use in the formulation (in-situ emulsification). Since hardly any other solvent is added apart from the resin, the VOCs of the formulations can be reasonable low. Although, when using amino formaldehyde or blocked polyisocyanates, a small to substantial increase of the VOC is present due to the alcohol and/or blocking agent. They are mainly applied in industrial metal primer surfacers, ACE coatings and coatings for trains and busses. Compared to the solvent-borne alternatives they have substantially lower VOC, except for possibly soft touch plastic coatings.. The appearance is in this case sufficient though even high gloss coatings can be obtained (Table 6.43). However, for high appearance topcoat applications such as car refinish, or automotive topcoats acrylic polyol emulsions are favoured. When compared to high solid solvent-borne coatings: appearance may still be an issue, though in metal primers and surfacers the switch from high solids to water-borne has been substantial, whereas this is not the case in high application performance topcoats made with acrylic polyol emulsions. All other properties are up to par or even better (e.g. VOC is substantially lower), though main issues are the limited stability of the coatings due to hydrolysis leading to limited shelf-life and the risk of co-reaction of the isocyanate with the alcohol groups of the co-solvent.

293

Polyurethane resins and coatings Table 6.47: Characteristics of the primer-surfacer for metal and OEM Characteristics

Unit

Value

Resin: polyester polyol (“Setaqua” B E 270) for metal coatings Solids in water, BDG and DMEA

%

70

Mean particle size

nm

< 20

% OH on solid material

%

Equivalent weight

600

Tg

°C

n.a.

Solvents water/BDG/DMEA

%

11.5/13.3/5.2

Neutralizing agent

DMEA

Hardener: anionic blocked isocyanate dispersion (“Trixene” Aqua BI201, Lanxess) Solids in butyl acetate

%

% NCO

%

Equivalent weight

40 5 840

Blocking agent

no information

Hardener: high imino-methylated melamine (“Cymel” 327) Solids in butanol

%

90

Free formaldehyce

%

< 0.8

s

35

s

210/101

mm

7.4

Data of metal primer/surfacer h coating (32 µm dry) Viscosity DIN 4 (at spray) Cure 10 min. 80 °C plus 24 min. 145 °C post cure Pendulum hardness (Persoz/König) Erichsen Adhesion cross cut* Conical mandrel ISO 6860

0–1 mm

Xylene resistance spot test 3 min. ASTM D-1308* Gloss over cataphoresis and primer (20/60 °C) *1 =ok, 5 =bad

294

0 S1, L1

GU

59/92

References Table 6.48: Curing temperatures of standard blocking agents (examples) for polyisocyanates Substance

Stucture

Crosslinking temperature [°C]

Di iso-propyl amine

Dimethyl pyrazole

Caprolactam

Methyl ethyl ketoxime

Catalyst

103

DBTL

93

2-ethylhexanoate

134

no catalyst

109

DBTL

97

2-ethylhexanoate

138

no catalyst

144

DBTL

142

2-ethylhexanoate

153

no catalyst

114

DBTL

114

2-ethylhexanoate

Remark strong tendency towards thermal and overbake yellowing

for high quality applications

high deblocking temperature

 

134 no catalyst

6.3 References [1] Polyurethanes: Coatings, adhesives and sealants, 2nd Revised Edition, Vincentz, 2019, p 61–70 [2] D. Dieterich, “Aqueous emulsions, dispersions and solutions of polyurethanes; synthesis and properties”, Progress in Organic Coatings, 9 (3) 281–340, 1981 [3] US Patent 3,491,050, 1970, W, Keberle et. al., Bayer Aktiengesellschaft [4] “Polyurethan dispersionen” Bas Tuijtelaars, Cor Koning, Bauke de Vries, Bert Hofkamp, Roel Swaans, Diane de Bruijne und Derrick Twene, DSM Coating Resins, FARBE UND LACK, 01/2018, p 54 [5] New approaches for solvent-free waterborne polyurethanes, D. Mestach, J. Goossen and D. Twene, European Coatings Congress, Polyurethanes 2006

[6] US Patent 3,920,598, 1975, H. Reiff et. al., Bayer Aktiengesellschaft [7] EP 0 622 378 A1, E. Esselborn, Th. Goldschmidt AG [8] EP2559718A1, Haruhiko Kusaka et al., Mitsubishi Chemical Corp. [9] www.perstorp.com/en/products/resins_and_ coatings/polyurethane_dispersions_pud [10] C. Y. Li, W. Y. Chiu, and T. M. Don, J. Polym. Sci. Pol. Chem., 43, 4870 (2005) [11] R. Arnoldus in “Waterborne Coating, Surface Coating” (A. D. Wilson, J. W. Nicholson, and H. J. Prosser Eds.), Vol. 3, Chap. 5, pp.186–188, Elsevier Applied Science, London, 1990 [12] A. K. Nanda, D. A. Wicks, S. A. Madbouly, and J. U. Otaigbe, J. Appl. Polym. Sci., 98, 2514 (2005)

295

Polyurethane resins and coatings [13] A. K. Nanda and D. A. Wicks, Polymer, 47, 1805 (2006) [14] A. Barni and M. Levi, J. Appl. Polym. Sci., 88, 716 (2003) [15] Review autoxidizable urethane resins, D. Wicks and Z. Wicks, Progress in Organic Coatings 54, 3, 2005, p 146 [16] EP2024412B1, Bakx F., Mestach D., Nuplex Resins BV [17] J. Pytela and M. Sufcak, “New Anionic Polybutadiene Diols for Polyurethane Systems”, Proceedings of the Polyurethanes World Congress 1997, Amsterdam, The Netherlands, September 1997, p. 704 [18] C. J. Patel and V. Mannari, Progr. Org. Coatings, 77 (2014) p 997–1006 [19] www.research and markets.com/research/ db3vkj/global?w=12 [20] D. Mestach “Acrylic dispersions for waterborne metallic base-coats”, Fatipec 1996 [21] Covestro PUD/PAD formulation for automotive 1C, metallic basecoat formulation HEBE 4052-5, 2019 [22] J. Bohorquez and D. Mestach, “New waterborne urethane-acrylic technology for automotive basecoats”ECS Nuremberg 2017, Speaker session 15.1; www.allnex.com/cn/info-hub/news/ meet-the-new-allnex-at-the-2017-ecs [23] Private picture Akkerman [24] Covestro PUD guideline formulation UV curable clear coat for furniture TERO 0200, 2016 [25] Covestro PUD guideline formulation TERO UV 216, 2019 [26] Covestro CPP guideline formulation Bayhydur eco 701-90, September 2018 [27] Covestro OH-PUD guideline formulation for 2 component furniture clear coat, TERO 6099, 2019 [28] Covestro CPP guideline formulation Bahydur quix 306-70 [29] Covestro PUD formulation for wood: air drying high gloss white top-coat RR 6161, 2019 [30] J. M. Akkerman et al., New Developments on Open Time Resins for Waterborne Decorative Coatings, Fatipec Proceedings, Genoa 2010, p 589

296

[31] Covestro PUD formulation for wood and parquet high gloss clear-coat, TERO 5830, 2018 [32] Semigloss clear formulation for wood and parquet application using Bahydrol U2874, but semi-gloss TERO 5829, 2018 [33] www.paint.org/coatingstech-magazine/ articles/formulating-soft-touch-coatings-balancing-act/ [34] M. Melchoirs, C. Kobuswch, K. Noble, and M. Sonntag, ‘Aqueous Two-Component Polyurethane (2K-PUR) Coatings: An Evolving Technology’, Presented at the International Waterborne, High Solids, and Powder Coatings Symposium, New Orleans, LA, February 10, 1999 [35] R. Hombach, H. Reiff, M. Dollhausen, US4663377 to Bayer AG [36] Brochure Polyisocyanates and Prepolymers, Covestro, Edition: 2019; Polyurethanes, Coatings, adhesives and sealants, 2nd revised edition, European Coatings Library, Vincentz, 2019, p. 34, 38 and 48–51 [37] Structure–Property Relations in Oligomers of Linear Aliphatic Diisocyanates, Max Widemann, et al., ACS Sustainable Chemistry & Engineering 2018 6 (8), 9753–9759 [38] Guideline formulation Covestro, FGB 5518-3, 2016 [39] Guideline formulation Allnex, REC 10029, 2019, for opaque REC 11017, 2018 [40] Presented at several conferences by Allnex as early as 2010 [41] Picture supplied by courtesy of Allnex [42] Guideline formulation Covestro, TERO 5831, 2018 [43] Photo from private collection Akkerman, resp. New waterborne radiation curing technology for parquet coatings, J. Akkerman, Adriaan Sanderse, Dirk Mestach and Anneke van der Zande, Nuplex Resins BV, The Netherlands. Vincentz European Coatings Conference, UV coatings, Nov. 2008 [44] Guideline formulation Allnex, REC 10024 [45] A. Sanderse, J. Goossen and J. M. Akkerman, Proceedings Fatipec Congress,

References

[46] [47] [48] [49] [50]

Genova, Nov. 2010 and Proceedings Wood Coatings Congress Oct. 2010 Foaming test for brush application supplied by the courtesy of BYK Chemi and extended to roller application Guideline formulation Allnex REC 10025 for the white topcoat, Sept. 2010 Guideline formulation Covestro, RR 6700, 2015 Covestro documentation CPP – Bayhydur eco 701–90, September 2018 J. Akkerman, D. Mestach, R. Esser and J. Goossen, European Coatings Conference on Polyurethanes for High Performance Coatings, Berlin 14th February, 2008

[51] Guideline formulation Covestro, PCO-140-CC, 2015 [52] Guideline formulation Covestro, PCO-SF-0052-CC, 2017 [53] Resydrol AN6617w/65MPP, Allnex, 2020 [54] www.eastman.com/Markets/Coatings/ TechnologySolutions/Pages/Adhesion_ Promoters.aspx [55] Polyurethans, Coatings, adhesives and sealants, 2nd revised edition, European Coatings Library, Vincentz, 2019, p. 44–48

297

Silicone chemistry

7 Silicone resins Rudolf Hager Silicon is the most abundant element in nature after oxygen and makes up 25 % of the earth crust [1]. It is always bond to oxygen and occurs as silica, silicates and quartz. Organosilicon chemistry dates back to the early 20th century when Kipping worked on organochlorosilanes. In the 1940s Rochow invented the direct synthesis and made organosilicon compounds – now called silicones – economically viable [2]. Silicones comprise basically 3 product categories: elastomers, fluids and resins. This chapter mainly deals with silicone resins used as binders in water-borne coatings, plasters and other construction related applications. Silicone resin emulsion paints were first mentioned in a patent in 1963 [3]. This kind of coating represents an ideal combination of water protection and breathability. The resins react to form a three-dimensional siloxane network which is extremely resistant to environmental influences. Therefore, silicone resin-based paints and plasters are very durable and keep their aesthetic appearance for many years. Besides these coating applications, water-borne silicone resins or resin precursors are used as water repellent primers and penetrating sealers. In the following chapters silicone and especially silicone resin chemistry is explained in more detail before their use as binders in coatings as well as other applications is described.

7.1 Silicone chemistry Silicones have been commercialized since 1950 and rapidly found numerous applications in daily life. From the basic three categories, elastomers, fluids und resins, thousands of products have been created. They are used as sealants, rubbers, lubricants, release agents, antifoam agents, in cosmetics, plastics coatings, construction and many other applications. The characteristic properties of silicones are: – temperature resistance (from cold to hot) – UV resistance – electrical insulation – water repellency

Akkerman, Mestach et al.: Resins for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

299

Silicone resins

7.1.1 Structure of silicones Silicones consist of siloxane units in which two or more silicon atoms are bonded via oxygen (Si-O-Si). Dependent on the number of oxygen atoms per silicon atom so called M-, D-, T- or Q-units are obtained (see Figure 7.1). Silicone fluids and rubbers, which count for the majority of silicone products, are mainly based on D-units, many of them end capped with M-units. T- and Q-units form three-dimensionally crosslinked structures. T-units are the basis of silicone resins, Q-units of natural silicates and quartz.

7.1.2 Silicone resins Silicone resins [4] can be considered as “organo-modified quartz”. They form a crosslinked network of siloxane units (see Figure 7.2 ) and are brittle solid materials (see Figure 7.2). The number of siloxane units in such a polymer is typically between 30 and 80. The brittleness can be reduced by incorporation of a certain number of D-units. With more than 20 % D-units honey-like high viscosity liquids result.

Figure 7.1:  Basic siloxane units in silicone chemistry

Figure 7.2:  a) Molecular structure of silicone resins b) Brittle silicone resin

300

Silicone chemistry The organic groups R can be methyl, alkyl or phenyl. In construction applications methyl is dominating. Therefore, the resins are called methyl resins. Partial replacement of methyl by long chain alkyl, e.g. octyl, improves the alkaline stability of such resins which is important for use in high-pH coatings, e.g. silicate paints, or on highly alkaline substrates, e.g. concrete. The amount of alkoxy groups OR’ – mainly methoxy and ethoxy – determines viscosity and compatibility of the resins with organics. Methoxy is more reactive than ethoxy. However, for toxicity reasons methoxy is not tolerated in many formulations. After application, whether in coatings or directly on mineral substrates, the resins react with moisture to produce a completely crosslinked network which is chemically bonded to mineral surfaces (Figure 7.3). The properties of such crosslinked resins are: – excellent water repellency – high water vapor permeability – UV stability – long term durability – physiological inertness Silicone resins can be used as such or as low molecular weight precursors, either monomeric silanes or oligomeric siloxanes, as described in the following chapter.

7.1.3 Silanes, siloxanes and siliconates Silanes in the context of construction applications always means alkylalkoxysilanes as shown in Figure 7.4a. The smallest molecule is methyltrimethoxysilane. Low flash point and high volatility prevent the use of methylalkoxysilanes in most construction applications. These problems can be overcome by oligomerizing monomeric silanes thus leading to so-called siloxanes (Figure 7.4b). Linear and branched octyltriethoxysilanes are most common in construction use. Long alkyl chains are preferred not only because of higher flash point and lower volatility but also better resistance to alkalinity. Both, silanes and siloxanes, react with moisture or humidity to form crosslinked resins (Figure 7.2 and finally Figure 7.3). Under the influence of alkalinity, which Figure 7.3:  Crosslinked siloxane units (small is almost always present in mineral con- balls) with shielding methyl groups (large struction materials, siloxane bonds of the ellipses)

301

Silicone resins resins are attacked and siliconates formed (Figure 7.4c). Methylsiliconates are soluble in water and therefore leached out. In case of octyl groups, siliconate formation is on the one hand slowed down, and on the other hand such long chain siliconates are not soluble in water. They stay in the crosslinked network and therefore impart alkaline stability to the resin. Water-based solutions of potassium methylsiliconate (Figure 7.4c) are the oldest generation of water repellents. They react with CO2 to produce crosslinked silicone resin networks (Figure 7.3). Aside of their use for water repellent treatment of clay- and gypsum-based products, they are currently gaining a lot of attention as pH adjusters in paints in general and in biocide-free high-pH indoor paints especially.

7.2 Silicone resin-based binders for coatings 7.2.1 Silicone resin emulsions Silicone resins are well established as binder components in non-aqueous protective coatings. They impart heat resistance, UV stability and chemical resistance. In water-borne systems, silicone resin emulsions have been used for more than 50 years as binders in façade coatings, so-called silicone resin emulsion paint and plaster [5]. As described in detail in Chapter 7.4, such coatings combine perfectly water repellency and water vapor permeability and are for this reason extremely durable.

Active ingredients

The first generation of resin emulsions was based on solid silicone resins (see Figure 7.2) which were dissolved in organic solvents and then emulsified. Typical resins have an average molecular weight of approximately 5000 g/mol and a glass transition temperature of 40 to 50 °C. After drying of such an emulsion, a tack-free transparent film is formed. Already in the 1990s a strong trend towards solvent-free products was recognized. There are two pathways to solvent-free resin emulsions [6]:

Figure 7.4:  a) Monomeric silane b) Oligomeric siloxane c) Siliconate

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Silicone resin-based binders for coatings 1. The polycondensation reaction of silanes/siloxanes to silicone resins is stopped at a liquid stage, the liquid resin then emulsified and polycondensation within the emulsion droplets initiated by adding a catalyst. 2. A high molecular weight silicone resin is blended with silanes and low molecular weight siloxanes and then emulsified. Both methods lead to silicone resins which have different composition and molecular weight than the original sold resins, however, their properties as binders in silicone resin emulsion paints and plasters are similar.

Surfactants

The most common surfactants for silicone resin emulsions are non-ionic emulsifiers such as polyvinylalcohol (PVAL) and ethoxylates, mainly iso-tridecanolethoxylates. Emulsions with PVAL as “protective colloid” have generally a high viscosity and are therefore limited in actives content (< 55 %). The use of ethoxylates leads to smaller particles and lower viscosity. A resin content of 60 % is no issue with this type of emulsifier. The smaller particle size provides compatibility with a very broad range of pigments and improves the mechanical strength of paint films as demonstrated in wet scrub testing. On the other hand, with PVAL-based emulsions, paints show better early water resistance.

Figure 7.5 a and b

monomeric silane

oligom. siloxane (liquid resin)

polyconden -sation

emulsifying

liquid resin emul.

polyconden -sation

solid resin emulsion

monomeric silane

oligomeric siloxane

blending

emulsifying

liquid resin emul.

polyconden -sation

solid resin emulsion

solid resin

Figure 7.5:  Two pathways to solvent-free silicone resin emulsions

303 Title of the presentation Date

Silicone resins Due to these specific advantages and disadvantages, both emulsifying technologies are used in commercial products.

Biocides

As in all water-borne products silicone resin emulsions require preservatives to keep them stable and performing for the period between manufacturing and application at the formulator of paints and plasters, at minimum 12 months. Most common are nowadays so-called activated halogen compounds, e.g. bronopol, and isothiazolinones, e.g. BIT (1,2-benzisothiazolinone) and MIT/CMIT (2-methylisothiazolinone, 5-chlor-2-methylisothiazolinone in ratio 1:3). BIT is mainly useful in formulations with pH above 9, MIT/CMIT at pH below 9. In the past also MIT alone has been used in concentrations above 20 ppm until labelling changed in 2019: above 15 ppm products with MIT must be labelled with warning phrase H 317 which says, “may cause an allergic skin reaction”. Products with this labelling are not allowed to be sold to and used by unskilled applicators. Therefore, they cannot be handled through DIY. For the MIT/CMIT blend, which has an own CAS number, the same limit counts, however, this blend is already well protecting at concentrations < 15 ppm. Very common is nowadays a combination of < 15 ppm MIT/CMIT and < 500 ppm bronopol or < 500 ppm BIT which protect over a broad pH range. With the new restrictions and presumably even more stringent regulations coming in future, concepts to secure a high degree of plant hygiene are mandatory. This relates to the resin manufacturer, the forwarder and the formulator.

7.2.2 Performance profile of a binder In the first three decades after their invention, silicone resin emulsion paints have exclusively been formulated with solid methylsilicone resins (see Figure 7.2a, R = CH3). In terms of performance, these solid resin-based binders have been and are still the benchmark in terms of binder performance. Methyl resins have a strong pigment binding power, provide permanent water repellency and avoid dirt pickup. Nowadays, most silicone resin binders are solvent-free and produced according to the blending and polycondensation process in Figure 7.5. Properly done, these binders perform very similarly to those based on solid resin solutions. Remaining alkoxy groups OR’ (see Figure 7.2a) are preferably ethoxy groups. Methoxy is not desired for two reasons: After application of the coating, they release as toxic methanol and, since methoxy is more reactive than ethoxy, they react already in the paint formulation to finally cured resins. Such cured resins are no longer reactive to interact with fillers and pigments as binder but behave themselves like a non-reactive hydrophobic filler. In other words: they have no binding power.

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Silicone resin-based binders for coatings Frequently, silicone resin emulsions are offered in the market which are not pure resins but blends of silicone resins and oils, sometimes even pure silicone oils (only D-units, see Figure 7.1 ). Aside of the misleading naming, these products have much lower or even no pigment binding power and cause high dirt pickup of paints and plasters. Sometimes such silicone resin and/or silicone oil emulsions are premixed with acrylate or styrene-acrylate dispersions and offered as “silicone resin binders”. With modern analytical methods such as NMR or IR, it is easy to characterize the silicone resin binders. IR is the easiest way to identify silicone oil in so-called resin emulsions ([5], p. 106 ff). These analytical methods are mainly used for formulated or already cured coatings. To get a fast impression of the non-formulated binder emulsions, two very simple tests can be applied: film on glass and heat stressing. When applied to glass and subsequently aged at room temperature for one week, the remaining film consistency permits conclusions on the resin emulsion quality: A high quality resin-based binder forms a tack-free transparent film. A white appearance of the film indicates a mixture with polymer dispersions. If the film is tacky or oily, most likely silicone fluids, plasticizers or low volatile solvents are present. The method of graduated heat stressing up to 450 °C is used to thermally decompose the organic ingredients (test in accordance with DIN 18556, determination of the organic content of silicate emulsion paints). In this test pure silicone resin emulsions lead to a white residue of silicon dioxide according to the following reaction scheme (see Figure 7.6). Typical silicone resin-based binder emulsions lose approximately 25 % of the weight of their solids content during this heat treatment. The precise number is dependent on the nature of the organic groups R (methyl, phenyl, alkyl) and the amount and nature of alkoxy groups OR’ (methoxy, ethoxy). If blended with organic dispersions, the heat treatment loss is much higher (50 % or more) and the residue turns black.

Figure 7.6

R

O

Si

O

O

R

Si

OR‘

R

OR’

Si

Si

O

R

O

O

Si

R

R

O

R

Si

R

O

O

OR’

Si

R

O

O

O

O

Si

Si

R

450 °C, O2 - CO2, - H2O

Si

O O

O

O

Si O

Si

O O

Si O

O

O

O O

O

O

O

Si

Si

O

O

O

Si O

Si

O

O

O

Si

O

O

Figure 7.6:  Under heat silicone resins burn to silicon dioxide

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Silicone resins

7.3 Other silicone ingredients for water-borne coatings 7.3.1 Hydrophobic primers Although the primer is not visible on a façade, it is very important for long-lasting weather exposed coatings. Its main action is to stop moisture transportation from outside through defects of a coating to the substrate and from inside the substrate to the coating as shown in the scheme (see Figure 7.7).

Figure 7.7:  Action of a hydrophobic primer: a) coating without primer, b) coating with primer

Figure 7.8:  Properties of a hydrophobic primer a) scheme of hydrophobic impregnation, b) crack-deactivation, c) cohesive failure of adhesion test of silicone resin paint on primer treated lime sandstone

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Other silicone ingredients for water-borne coatings A hydrophobic primer is typically an impregnating agent (definition according to DIN 55 945) which penetrates the pores of an absorbing substrate and turns them water repellent (Figure 7.8a). Even small cracks are no longer able to absorb water (Figure 7.8b). The pores must remain open and allow subsequently water vapor diffusion, especially when vapor permeable coatings such as silicone resin paints are applied. Of course, such a primer must be easily paintable and provide good paint adhesion (Figure 7.8c). Another desired property of a hydrophobic primer is strengthening or reinforcement of the substrate. In the past, 10 % solutions of siloxanes (Figure 7.4b) or silicone resins (Figure 7.2a) in white spirit have been used as hydrophobic primers. Nowadays, water-borne products are dominating this market. The first generation of water-borne primer were silicone microemulsions [7]. The products were handled as so-called silicone microemulsion concentrates (e.g. Wacker “SMK” technology), chemically consisting of amino modified silanes and siloxanes. Just before use, the concentrates are poured into water forming spontaneously fine particle silicone microemulsions. Handling as a concentrate helps to save packaging, transportation and storage costs. The sole disadvantage of this technology is the short shelf life of the ready-to-use products. It should be applied within 24 hours after dilution. This was the reason why nowadays silane/siloxane macro emulsions [8] are mainly used. Emulsions are typically liquids, but thixotropic cremes [9] are gaining market share. To increase the strengthening properties, the emulsions can be blended with fine particle organic polymer dispersions, mainly acrylics or styrene acrylics.

7.3.2 Hydrophobic additives Porous and therefore breathable coatings such as silicone resin and silicate paints and plasters as well as dispersion paints with low binder content require hydrophobic additives in order to achieve early rainwater resistance and permanent low water uptake. In contrast to binders, which interact with fillers and pigments in the coating, a hydrophobic additive is required to migrate to the surface (Figure 7.9a). Therefore, small amounts of 1 to 2 % on the whole formulation are enough to give the desired effect. Hydrophobic additives are typically emulsions of reactive functional polydimethylsiloxanes (Figure 7.9b) with some T-functions (see Figure 7.1), reactive alkoxy groups OR’ and amino functional groups R* in Figure 7.9b. In contrast to non-silicones, e.g. hydrocarbon waxes, high-quality silicone additives are durable and provide long term performance. Especially when used in alkaline formulations, such as organo-silicate paints, the additive must be alkaline stable. This can be achieved by incorporation of some octyl groups R instead of methyl, an appropriate ratio of so-called D- and T-units and amino-functional side groups R* instead of end groups. If the additive is not stable in alkaline environment – this happens if a non-functional poly-dimethylsiloxane is used which is well-known to be decomposed by alkalinity, the

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Silicone resins water uptake of an alkaline paint increases significantly within 6 months after production (Figure 7.10). Hydrophobic additives are often designed to improve the water beading of a paint. A strong beading effect is desired as visible effect to repel water. However, much more important is the long-term reduction of the capillary water uptake. Sometimes even a hydrophilic surface is preferred rather than a strong beading effect. Various publications claim that a hydrophilic surface dries faster than a hydrophobic, leads to less algae formation and stays cleaner for longer [10]. Partial replacement of methyl groups in Figure 7.9b by phenyl groups lowers the beading effect without affecting the water uptake reduction [11].

Figure 7.9:  a) Function and b) molecular structure of hydrophobic silicone additives

308

Other silicone ingredients for water-borne coatings

7.3.3 Special effect additives Silicone additives attract more and more attention in interior paints as well. Here it is the typical silicone release effect which leads to various desired properties. These can be categorized as following: – Effective surface protection: resistance against stains and scratches – Perceptible properties: soft-feel and dry burnishing effect – Excellent workability: easy application and processing Chemically, these additives are similar to the hydrophobic additives (see Figure 7.9b). The molecular weight is usually higher and the functionality in terms of crosslinking and chemical reactivity lower. Main ingredients are long chain polydimethylsiloxanes. Similar to the case of hydrophobic additives, they are offered as emulsions with actives content of approximately 50 %. The products have a low surface tension and strong spreading behaviour. The performance of the silicone additives is best apparent in high PVC (pigment volume concentration) paints: dispersion paints with less than 20 % binder dispersion (acrylate, styrene acrylate, vinyl acetate-ethylene-copolymers) or silicate dispersion paints. Especially on matt intensive colour coatings, the so-called “writing effect” occurs: visible scratch marks at low mechanical impact, e.g. scratching with fingernails (Figure 7.11a and 7.11b). Silicone additives reduce the formation of such scratch marks and allow easy removal by slight wiping with a wet tissue or sponge (Figure 7.11c). Due to only low mechanical stress, burnishing effects, e.g. gloss increase by polishing, can be avoided.

Figure 7.10:  Capillary water absorption (w24 value) of a dispersion silicate paint with 2 % of two different silicone additives

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Silicone resins Another important characteristic of a paint is its workability. In matt high PVC dispersion paints and especially in dispersion silicate paints – these are becoming more and more popular for environment and health reasons (low VOC, biocide-free) – fast drying is an issue. Unskilled painters struggle with visible stripes at the overlap of first and second coats. With 1 % of an appropriate silicone additive the open-time, the time period in which a freshly applied coat can be worked over without any visible structures, is more than tripled, from 3 minutes up to 10 minutes. Workability is also improved by better pigment wetting and levelling of a silicone additive modified paint.

7.3.4 Silicone-based pH adjuster Volatile organic compounds (VOC) is an ongoing important issue for indoor paints. Target is zero VOC. Therefore, not only the resins in a paint but all ingredients are under scrutiny. One of the essential auxiliaries is the pH adjuster which is often either ammonia or an organic amine. Both fall into the VOC category and emit irritating odours. In recent years water-borne solutions of siliconates (molecular structure Figure 7.4c), particularly potassium methyl siliconate, have been very successfully investigated as pH adjusters. Commercial products contain approx. 50 % solids (mix of potassium methyl siliconate

Figure 7.11:  Improvement of scratch mark resistance: a) scratching with sanding paper, b) scratch marks, c) after cleaning with a wet sponge

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Silicone resin emulsion paints and plasters and potassium hydroxide) and are highly alkaline with pH > 13. For pH-adjustment in a paint, the concentrated product is typically diluted with 1 to 2 parts of water and then added to the paint until the target pH value, in standard dispersion paints around 9.5, is achieved. Here are the properties of siliconates as pH adjusters in a glance: – no odour – very low VOC – excellent pH stability and storage stability of the paint – improved mechanical and water resistance of the paint film due to crosslinking – effective in small dosage (0.1 to 0.2 % of total formulation), cost efficient – useful in all kinds of common water-borne paints – prolongation of open-time and improvement of smooth surface coverage In recent years siliconate pH adjusters attracted even more attention for use in very high-alkaline indoor paints with pH up to 11 or 12. At this high pH storage stable dispersion paints can be formulated without biocides and preservatives [12, 13] – another step towards a healthy room climate. Siliconates are the most preferred alkaline additives because of their excellent pH stability, hydrophobic properties and prolongation of open-time.

7.4 Silicone resin emulsion paints and plasters As described in the beginning of Chapter 7, silicone resin-based coatings were first described in 1963 [3]. As with many new technologies, it took almost 20 years until this technology had its commercial breakthrough. The success story started in Germany, was rolled-out over Europe in the 1980s and globally spread in the 1990s. The silicone resin coatings belong doubtless to the category of high-quality coatings with extraordinary properties which justify their higher prices. From the perspective of a project owner or a house owner, the main attributes of these coatings are aesthetics and durability. In the following chapter the background of these attributes is described in detail.

7.4.1 Definition of silicone resin emulsion paints and plasters The first definition of a silicone resin paint was already given in the basic patent [3] which claims, “coating materials based on water-borne dispersions of organo-polysiloxanes having an organic resin content, which however, does not exceed the portion of organo-polysiloxane”. In other words: a silicone resin emulsion is the main binder.

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Silicone resins DIN 18363 describes the ingredients of silicone resin emulsion paints as following: – silicone resin emulsion – polymer dispersion – pigments and fillers – auxiliaries such as wetting agents, thickener, pH adjuster, defoamer, preservatives. The DIN standard does not mention anything about quantities of the ingredients; however, it claims that the composition meets requirements in terms of low water uptake and high water vapor permeability. The definition of silicone resin emulsion paint applies also to silicone resin emulsion plaster: the main binder is a silicone resin emulsion which determines the final properties. The key feature is again the combination of strong water repellency and high water vapor permeability. Unfortunately, paint producer and associations never succeeded to create a common standard for silicon resin paint and plaster. Consequently, nowadays products are in the market which contain just a “drop of a silicone”, sometimes even not a silicone resin but a fluid, and benefit from the quality attribute of a silicone resin emulsion paint. Therefore, it is most important to deal with the physical properties of the formulation which correlate ultimately with quality and durability. The properties are described in the next chapter.

7.4.2 Properties of silicone resin paints and plasters Façade coatings have two basic purposes: colour design and protection. Protection refers to mainly moisture control. High and fluctuating moisture content leads to discoloration, efflorescence, algae formation, cracking and loss of heat insulation. The coating, independent whether it is paint or plaster, must prevent moisture penetration into the façade material. The development of silicone resin emulsion paints and plasters results from the intention to combine the favourable properties of typical mineral and modern polymer dispersion-based coatings. Mineral coatings such as lime paints and plasters or silicate paints are characterized by good adhesion, curing without tension and high permeability for gases, water vapor as well as carbon dioxide. Dispersion paints on the other side form a tight polymer film and protect effectively against water uptake. Already in the late 1960s the so-called façade protection theory was developed by Künzel at Fraunhofer-Institute Holzkirchen/Germany. It says in brief: For long durability moisture content of a façade must be kept low. Moisture which is absorbed during rain or by condensation must be able to evaporate rapidly during drying time [14]. In other words: If water absorption is high, vapor diffusion rate must be high as well and vice versa.

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Silicone resin emulsion paints and plasters Table 7.1: Classification of coatings in terms of water transmission and vapor permeability w [kg/m2h0.5]

Category

sd [m]

Category

>0.5

I (high)

3.2) to 13.0 (e.g. MR 1.0). Even though the soluble silicates with a molar ration above 3.2 and a concentration below 40 % are not classified as hazardous substances, they are alkaline chemicals. Therefore, the corresponding safety data sheets should be considered and contact with eyes, skin, and clothes should be avoided. Main properties of water glass: – Inorganic substances – Free of volatile organic compounds (no VOC) – Solvent-free – No crystalline SiO2-phases – Free from preservatives – Allergy-friendly properties – Neither combustible nor oxidizing – No chemical decomposition – even at high temperatures  no noxious gases Soluble silicates stand for a major class of chemicals finding a wide range of applications in numerous industry sectors. It can be seen, that soluble silicates are generally low-risk products based on their chemical and physical properties.

326

Water-borne silicates

8.5.2 Sodium silicates Sodium silicate represents a generic family of inorganic compounds composed of silicon dioxide and sodium oxide in various proportions. The molar ratio plays an important role in the chemical behaviour of sodium silicate and can vary between 1.7 and 4. So far, the alkali silicates in order of increasing atomic number (i.e. lithium, sodium, and potassium) have been shown. However, in terms of general commercial importance by far the most important alkali silicates which are used in a wide variety of applications are sodium silicates. Because they are generally the most economical in production cost and selling price they represent the greatest majority of alkali silicates manufactured worldwide and tend to be the most used wherever possible. Sodium silicates are used on a large scale to produce precipitated silica, detergents, and cleaning agents. They are also used for the synthesis of zeolites, silica gels, and silica sols as well as for adhesive purposes, for ore flotation, in the ceramic industry, cement, and foundry industries, for water purification by flocculating contaminants and chemical soil stabilization. Sodium silicate is mainly produced by the fusion of pure quartz sand and sodium carbonate in tank furnaces at temperatures of 1,300 to 1,500 °C. The following equation shows an example of the stoichiometry of the reactions taking place in the production of a sodium silicate with a molar ration of 2: 2 SiO2 + Na2CO3  Na2O ∙ 2 SiO2 + CO2 By using different proportions of sand and soda, different molar ratios can be obtained. The resulting products thus obtained have different properties that are used for different applications. A basic distinction is made between solid silicates and their aqueous solutions. Silicates are solid glasses that are formed by solidification from the melting of soda and sand. Liquid water glass results from dissolving glasses in water. The technically most important sodium silicates have a molar ratio between 1.7 and 4. The usual commercial products with the highest volume are concentrated water glass solutions with a molar ratio of 3.4. The hydrothermal production process is only efficient for sodium silicate solutions which have a low molar ratio.

Figure 8.6:  Labelling of liquid water glass according to the CLP classification

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Alkali silicates Table 8.2: Lithium silicates properties Degree of hydration

Molecular formula

Anhydrous

Li2CO3

Solubility in water (g/100 g) 1.30 @ 25 °C

Below molar ratio 1.7 sodium silicate solutions tend to form crystal seeds and are no longer stable. That is the reason why high alkaline sodium silicate solutions are not commercially available on the market.

8.5.3 Potassium silicates In addition to the different available liquid sodium silicates on the market, potassium silicates are the most important raw materials especially for the decorative coating and construction market as well as special applications. Depending on the molar ratio the main techniques to produce liquid potassium silicate are the furnace and the hydrothermal route. There are essential characteristics that distinguish potassium water glass from sodium water glass. The most important differences in the corresponding sodium silicates are the different viscosity behaviour when compared at the same molar ratio and same concentration, the reduced tendency to efflorescence, the stability even in the low molar ratio range, superior solubility, a higher compatibility with other ingredients like emulsions and fillers and the greater refractoriness at a higher temperature. The different tendency of efflorescence for sodium and potassium silicates can be explained by the final carbonate salts, which are formed by the reaction of the alkali content with carbon dioxide in the air. In the case of sodium, sodium carbonate is formed which can contain up to ten crystal water molecules and therefore develops bulk and builds long needles which have only low solubility in water. In the case of potassium, potassium carbonate is formed containing only up to two crystal water molecules and compact crystals which have good water solubility and are easily washed away during the next rain.

8.5.4 Lithium silicates Lithium silicates exhibit entirely different physical and chemical properties compared with any other alkali silicate-based systems. Lithium silicates with a molar ratio of up to 8.5 are generally used for coating systems. The carbonate only exists as the anhydrous form with relatively low water solubility (which decreases with rising temperature), as shown in Table 8.2. Because lithium silicates have high molar ratios (i.e. they have a low alkali content) as well as the arising lithium carbonate having a low water solubility, lithium silicate finds uses in applications where the production of water-soluble by-products or any form of

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Curing or hardening processes of alkali silicates efflorescence is undesirable, including 2K inorganic zinc silicate protective coatings as well as the strengthening or consolidation of masonries such as natural stone or concrete. It should be pointed out, that lithium silicates are stable at and around room temperature, but precipitate on heating to > 60 °C. Thus, they should not be used in heat accelerated systems.

8.6 Curing or hardening processes of alkali silicates The curing or hardening process of alkali silicates can involve several various basic reactions either with the constituents of the substrate or with the components of air. Three common reaction types are crucial for technical applications. – Dehydration – Reaction with alkali reactive substances – Gel formation by reaction with polyvalent metal ion containing compounds or base metals

8.6.1 Dehydration The easiest way to cure water glass is the drying process by heating, vacuum, absorption, or a combination of these procedures. If water is removed from a water glass solution, solid silicate precipitates, which is described in the following equation. The evaporation of water increases the concentration of the water glass solution, so the silicate anions, despite their negative charge, move much closer together until condensation reactions and gel formation takes place.

Through condensation processes and building more complex structures the viscosity is increased till the formation of solid gels occurs. In general heat-stable, but not water-resistant bonds are formed by this process. Water is present in three different forms: c) condensed water d) adsorbed water e) constituted water Depending on the bonding type, there are different stages of dehydration. Between 120 °C and 150 °C, the condensed water can easily disappear, while between 200 °C to 250 °C

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Alkali silicates adsorbed water will evaporate. The strongest bond is constituted water. It belongs to chemically bonded silanol groups which are removed from the chemical structure at temperatures above 730 °C, depending on the chemical composition of the water glass. Above the melting point, sodium water glass turns into a water insoluble amorphous glassy structure.

8.6.2 Reaction with alkali taking substances Exposure to acids and acid anhydrides including acid gases, e.g. CO2, causes the pH value to decrease, thus reducing the repulsion of the particles. As a consequence, the polymerization equilibrium shifts towards higher molecular groups: condensation and gel formation take place.

8.6.3 Reaction with mineral acids and acidic salts There are two effects when acid is added. Firstly, during the protolysis of the mono-silicate ions, the hydroxide ions are neutralized, thereby shifting the protolysis equilibrium in favour of the silanol groups. This also disturbs the reaction equilibrium between condensation and hydrolysis of the silica present in water glass solutions, resulting in condensation to poly-silicic acids. According to the example equation, the alkali metal is “removed” from the alkali silicate, producing a sulphate salt, thereby increasing the molar ratio of the water glass and thus disturbing the equilibrium, destabilizing the solution, and accelerating gelation.

8.6.4 Reaction by increasing molar ratio By raising the molar ratio of water glass with hydrated silica, the equilibrium of the solution is disturbed, which can cause gelation. This effect leads to viscosity increase resulting in gel-formation. Usually, bonds with limited water-resistance are formed.

8.6.5 Reaction with CO2 When exposed to air or to an excess of CO2, water glass precipitates, following the same processes as in gelation with acids. This process can increase the molar ratio of alkali silicate. Especially low alkaline water glass with a high molar ratio tends to form a thin layer of skin in direct contact to air.

330

Curing or hardening processes of alkali silicates

8.6.6 Reaction with esters When a carboxylic ester is added to the basic water glass solution, a saponification reaction takes place in which an alcohol and an organic acid, resp. acid anion is released. In combination with the same process as gelation with acids, the alcohol which is formed also has a negative influence on the stability of the water glass solution.

8.6.7 Gel formation by reaction with polyvalent metal ion containing compounds or base metals Polyvalent cations (e.g. Ca2+, Al3+) can cause gel formation due to their strong bonding effect with silicate ions. These cations can preferentially compensate for the charge of the negatively charged oxygen and thus enable bridges between the silicate anions, which then leads to network building. Using hardener systems based on e.g. aluminium phosphates, water glass solutions are obtained which can set very quickly. These can be summarised as below in the following sections.

8.6.8 Reaction with base metal powders Such metals include: – magnesium – zinc – aluminium Due to the alkaline character of alkali silicates, by mixing with base metal powders hydrogen gas is evolved which may lead to a foaming reaction of the mixture. Through this reaction, alkalinity is consumed, so that the molar ratio of silica to alkali oxide is raised and a gel is formed. Through interaction with the water-soluble metal or resp. metallate ions formed with the silicate chains additionally insoluble silicates of polyvalent metals are formed.

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Alkali silicates The example in this equation shows the formation of the alkali zincate which forms water-insoluble zinc silicate. Similar reactions may occur also to a small extent when alkali silicate solutions come into contact with the surface of such base metals and will lead to very strong bonds with these metals.

8.6.9 Reaction with the constituents of the substrate (e.g. in case of wall coatings) Reaction with burnt lime (calcium hydroxide), where M = sodium (Na), potassium (K) or lithium (Li), as follows:

The alkali hydroxide can further react with atmospheric carbon dioxide to produce alkali carbonate:

The curing reaction with quartz sand, particularly on the particle surface can be represented as follows:

8.6.10 Reaction with water soluble polyvalent metal salts In these reactions, several silicate chains are interconnected by polyvalent cations (ionic bonds) to form insoluble or barely water-soluble compounds as shown in the equation below. Examples include: – calcium chloride, CaCl2 – calcium hydroxide, Ca(OH)2 – magnesium chloride, MgCl2 – magnesium sulphate, MgSO4 – aluminium sulphate, Al2(SO4)3 – zinc sulphate, ZnSO4

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Water-borne alkali silicate containing surface coatings The main effect that causes gelation when salts are added is the reduction of the electrolytic bilayer, which reduces the repulsive forces between the individual colloidal silicate particles and enables or accelerates coagulation.

As an example, the last equation shows the reaction equation when calcium sulphate is added. It should be noted here that the greater the ion solubility and the charge of the metal ions of the salts, the more the electrolytic double layer is weakened. Consequently, gelation with salts of monovalent metal ions is more difficult than with salts of polyvalent metal ions.

8.6.11 Reactions with alkaline solutions of salts from polyvalent amphoteric metals An example of these salts is sodium aluminate. By insertion of hetero-polyvalent metals (e.g. aluminium) into the silicate chain structure, water-insoluble, three-dimensionally linked hetero-polysilicates, e.g. aluminosilicates “zeolite-like structures” are formed.

Since aluminium has an ionic radius similar to that of silicon, the aluminium is incorporated into the gel framework during gelation. During gelation, a homogenous gel is only formed as long as the viscosity is low enough and the ions can easily move, which enables a maximum entropic distribution. With increasing viscosity, the movement of the ions is hindered, resulting in a rising inhomogeneity in structure.

8.7 Water-borne alkali silicate containing surface coatings In the late 1870s in Europe, soluble silicate-based coatings were formulated as 2-component coating systems. Although still used now, they suffer from the disadvantages of requiring mixing and permitting to mature before application. Stable 1K silicate emulsion coating formulations came to prominence in the 1980s. With the experience of long-term durability of exterior coatings based on 2K silicate paints as well as the benefits of organic emulsion paints with their attractiveness, ease of production, and good storage stability, there arose a desire for a single pack coating system to exhibit the

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Alkali silicates properties of mineral paints together with ease of production, ease of application and good storage stability. Silicate emulsion paints and plasters when appropriately formulated offer a variety of features conferring benefits for the paint manufacturer, applicator, architect, building owner, and user. They are suitable for interior and exterior applications in new and old buildings, and dependent on the formulation, offer the benefits outlined below. – Ready-to-use, storage stable formulas – Good application properties on most substrates and easy to overpaint – Open-pored coating, hence high water vapor and carbon dioxide permeability – Enhanced colour stability and lightfastness – Harmonious, “naturally shaded”, matt surface appearance consistent over the coating’s long life – Reduced mold and fungal growth – Low tendency to soil, or dirt pick-up with good self-cleaning properties – High durability, hence very efficient – No biocide in-can preservation required to ensure storability – Silicate binders are odour-free and are desirable paint systems for allergy sufferers – Suitable for interior and exterior applications in new and old buildings – Solvent-free, volatile organic compounds (VOC)-free paints possible – Lower fire loads in comparison with standard emulsion paints, fire classification A2 according to EU standards possible – Good thermal insulation properties lead to good temperature regulation – Enhanced electrostatic properties compared with conventional emulsion paints – Enhanced thermoplastic properties with no “glue effect” – Photocatalytic materials can be incorporated in formulations – Harmless to the environment – Strengthening of friable, weathered, or sandy substrates through silicification The above properties were achieved using appropriately formulated systems based on potassium silicate binders, blended with alkali stable organic emulsion resins, with appropriately selected pigments and additives. Today, modern and eco-friendly, silicate emulsion-based coating systems are an indispensable tool for professionals in the architectural paints and plasters industry. They can meet the high requirements of both architects and building owners. Introduction to other markets including the United States of America, Asia, South Africa, and Australia continues to occur. The trend towards silicate paints can be explained by the keywords of naturalness, durability, ecology, and economically-friendly. In the same context, silicate emulsion renders, or plasters based on similarly selected binders and resins may be formulated using appropriately selected fillers such as those based on carbonates or silicates.

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Water-borne alkali silicate containing surface coatings According to DIN 18363 (work section 2.4.1.) there are two common types defined: a) pure silicate paints Pure silicate paint shall be made of potassium water glass solutions and pigments resistant to the potassium water glass and shall contain no organic components such as polymer dispersions. This type of products is based on a pigment powder in dry or water paste form and the liquid binder water glass (usually a potassium silicate product) which comprises about 18 wt.% of the total mixed silicate paint formulation resulting in a potassium silicate solids content of about 5 wt.%. As the authors understand, the processing and application of pure silicate paints require great experience and know-how. They are especially common for the heritage area and applied by specially trained workers. b) silicate emulsion paints All types of silicate emulsion paints and coating materials (for textured surfaces such as plaster) are to be made from potassium water glass with pigments and fillers resistant to its alkalinity, in combination with suitable organic emulsions and corresponding additives, with an organic content of no more than 5 wt.% in total. Silicate emulsion coatings for interior use shall meet the requirements for class 3 wet scrub resistance according to DIN EN 13300. The following names and types can be found under this group: – silicate emulsion paints (1K silicate paints) – sol silicate paints – organo-silicate paints

8.7.1 Silicate emulsion paint Since the silicate is the major binder component that gives the coating its essential character and the organic dispersion is the minor binder component the characterisation as “silicate emulsion paint” should be the favoured nomenclature. These coatings were developed especially in Europe and became particularly important from the 1980s onwards. Through the addition of up to 5 wt.% of organic components (e.g. acrylate binder, styrene acrylate binder, etc. as well as hydrophobic agents, thickeners, etc. which are tolerant of the high pH of the silicate binder) it became possible to produce a one component readyto-use paint. The liquid water glass normally used is a potassium silicate product that comprises about 16 to 18 wt.% of the dispersion silicate paint formulation resulting in a potassium silicate solids content of about 5 wt.%. The range of applications for such silicate paints is significantly broader than for 2K pure silicate paints because the presence of the emulsion in these paints permits coating on weakly friable substrates or some substrates already coated with organic dispersion coatings. Besides, because these are 1K products the handling and processing is simpler than for 2K pure silicate paints.

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Alkali silicates Table 8.3: Classification of silicate emulsion paints Class

Loss of thickness of the coat

Class 1

< 5 μm at 200 scrubs

Class 2

≥ 5 μm and < 20 μm at 200 scrubs

Class 3

≥ 20 μm and < 70 μm at 200 scrubs

Class 4

< 70 μm at 40 scrubs

Class 5

≥ 70 μm at 40 scrubs

Table 8.4: Interior silicate emulsion paint Pos. Raw material

Generic name/technology

Parts by weight [g]

01

Solvent

water

Rest to 100

02

Anti-settling agent

“Betolin” V 30

0.05 – 0.1

03

Cellulosic thickener

“Tylose” H 15 000 YP2, “Natrosol 2” HBR 250, “Bermocoll” EHM 300

0.1 – 0.3

04

Dispersing agent

“Sapetin” D 20, “Sapetin” D 27

0.2 – 0.4

05

Stabilizer

“Betolin” Q 40

0.3 – 0.5

06

Titanium dioxide

“Kronos” 2056/2190, “Tiona” R-KB2, “Tiona” 595, “Cinkarna” RC82

5.0 – 8.0

07

Defoamer

“Agitan” 260/265/350/351, “Byk” 014/037, “Tego” Foamex K3

0.1 – 0.3

08

Filler (carbonate type)

“Durcal” 5, “Omyacarb” 5 GU, 5 NP

25.0 – 30.0

09

Filler (silicate type)

“Dorkafill” H, “Plastorit” 000, “Finntalc” M30SL

5.0 – 8.0

10

Polymer emulsion (styrene acrylate copolymer)

“Acronal” S 559, “Acronal” 6292, “Alberdingk” AS2076, “Mowilith” LDM 6119, DM 765 A

6.0 – 8.0

11

Potassium silicate

“Betolin” K 28, “Betolin” P 35

14.0 – 17.0

12

Viscosity stabilizer

“Betolin” A 11

0.5 – 1.0

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Water-borne alkali silicate containing surface coatings

Silicate emulsion coatings for interior use

According to the standard DIN EN 13300:2001 “Water-borne coating materials and coating systems for interior walls and ceilings – Classification”, silicate emulsion coatings for interior use shall meet the requirements for class 3 wet scrub resistance where the wet scrub resistance signifies the resistance of the coating to repeated cleaning. It can only be measured on coatings of the largest grain size (granularity) smaller than 100 μm thickness applied to smooth, non-textured or coarse surfaces. The wet scrub resistance is determined by the procedure in ISO 11998 after a drying period of 28 days at (23 ± 2) °C and (50 ± 5) % relative humidity. Silicate emulsion paints are classified according to the loss of thickness of the coat, as shown in Table 8.3. All coatings according to this standard shall be recoatable with the same coating material. The liquid water glass normally used is a potassium silicate product that comprises about 14 to 17 wt.% of the silicate emulsion coating formulation resulting in a potassium silicate solids content of about 4 to 5 wt.%. It should be noted that some formulations exist where the potassium silicate product comprises a higher concentration of about 21 wt.% of the silicate emulsion coating formulation resulting in a potassium silicate solids content of about 6 wt.%.

Examples for interior and exterior use

Table 8.4 and 8.5 gives an example for an interior and an exterior silicate emulsion paint. Table 8.6 on page 339 shows the mixing procedure of the silicate emulsion paints.

Sol-silicate paint

A milestone in silicate technology was achieved with the invention of sol-silicate paint increased in 2002. These types kind of paints which satisfy the requirements of DIN 18363 for “silicate emulsion paints” are based on the combination of potassium silicate with modified colloidal silica to reinforce the structure by modifying the film surface. Physical properties of the dried film are enhanced such as film adhesion, and lower dirt pick-up. Silica sols are solutions of colloidal silica with typical particle sizes ranging from 8 to 125 nm. Silica sol colloidal solutions may appear nearly clear and almost colourless at the lowest particle size, and with increasing particle size and concentration may range in appearance from opalescent (around 40 nm particle size) to milky at larger particle sizes. The organic binder fraction is ≤ 5 wt.%, similar to those in silicate emulsion paints, permitting chemical setting and retaining the specific advantages of silicates. The sol-silicate paint permits use on non-mineral coated substrates. They can be applied easily and safely to nearly all common substrates including non-mineral plaster (e.g. gypsum render or gypsum wallboard) with substrate bonding occurring both chemically and physically. There are several types of colloidal silica on the market that can be used in paints. They are divided into untreated and organic modified types.

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Alkali silicates Table: 8.5: Exterior silicate emulsion paint Pos.

Raw material

Generic name/technology

Parts by weight [g]

01

Solvent

water

Rest to 100

02

Anti-settling agent

“Betolin” V 30

0.05 – 0.1

03"

Cellulosic thickener

“Tylose” H 15 000 YP2, “Natrosol” HBR 250, “Bermocoll” EHM 300

0.1 – 0.3

04

Dispersing agent

“Sapetin” D 20, “Sapetin” D 27

0.2 – 0.4

05

Stabilizer

“Betolin” Q 40

0.3 – 0.5

06

Titanium dioxide

“Kronos” 2056/2190, “Tiona” R-KB2, “Tiona” 595, “Cinkarna” RC82

7.5 – 10.5

07

Defoamer

“Agitan” 260/265/350/351, “Byk” 014/037, “Tego” Foamex K3

0.1 – 0.3

08

Filler (carbonate type)

“Durcal” 5, “Omyacarb” 5 GU, 5 NP

25.0 – 35.0

09

Filler (silicate type)

“Dorkafill” H, “Plastorit” 000, “Finntalc” M30SL

5.0 – 8.0

10

Polymer emulsion (styrene acrylate copolymer)

“Acronal” S 559, “Acronal” 6292, “Alberdingk” AS2076, “Mowilith” LDM 6119, DM 765 A

8.0 – 9.0

11

Potassium silicate

“Betolin” K 28, “Betolin” P 35

18.0 – 22.0

12

Viscosity stabilizer

“Betolin” A 11

0.5 – 1.0

13

Hydrophobic agent

“Betolin” AH 250

0.6 – 1.0

Key features and benefits of sol-silicate coatings are: – extremely long-life/durable – weathering protection/waterproof – breathable/highly moisture vapor permeable – economical and easy to apply – inherently incombustible – suitable for use on mineral and previously painted substrates

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Water-borne alkali silicate containing surface coatings Table 8.6:  Mixing procedure of silicate emulsion paints (interior and exterior) Raw material

Procedure

Water

incorporate first

Anti-settling agent and cellulosic thickener

mix in and let ripe for approx. 10 to 15 min.

Dispersing agent and stabilizer

mix and homogenize

Pigments (titanium dioxide)

mix and disperse vigorously

Defoamer

mix in

Fillers

mix and disperse vigorously starting with the finest grade

Polymer emulsion

mix in and stir to homogeneity

Potassium silicate

mix in, but not too fast

Viscosity stabilizer

mix in

Exterior paints: hydrophobic agent

mix in

Store at least 1 to 2 days Colour pigments

– – – –

mix in

low VOC and solvent-free lightfast/colourfast and UV-stable silicate matt surface appearance environmentally friendly

The replacement of part of the liquid silicate with surface-treated colloidal silica enhances several properties such as liquid water permeability, dirt pick-up resistance, and adhesion but induces a slight reduction in the paint hardness. Nevertheless,the dirt pick-up results are relatively good. The adhesion reinforcement is explained by lower developed internal stresses during drying of the mineral layer when surface-treated colloidal silica is present. Untreated colloidal silica products usually have a particle size of 7 to 20 nm and are stabilized with ammonium or sodium ions. They are sensitive to electrolytes especially in combination with alkali silicates. Organically modified products are often epoxy silane-modified and much more stable than sodium-stabilized silica sol. Colloidal silica helps to reduce the alkalinity by lowering the pH value. As a result, the weather resistance will improve, and coating of problematic substrates is much easier than with a pure silicate emulsion system. The anti-soiling effect of the sol-silicate paints is a result of colloidal silica making it more difficult for dirt to adhere to the painted stone or masonry. The reason behind this is given by the nature of silica particles which make the painted surface smooth and more “hydrophilic”. The modified surface of the paint also reduces the contact angle of the par-

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Alkali silicates ticles. As a result, water spreads evenly on the surface instead of forming drops. An additional benefit is that rainfall or a simple cleaning easily rinses the surface. Furthermore, colloidal silica is easy to use and not shear sensitive. Additionally, well-dispersed pigments and fillers give greater colour strength and intensity to paints, thereby improving their cost-efficiency. All this gives the silicate paints qualities that secure longer-lasting protection against weather and wind and pose a natural means of greater sustainability when caring for our buildings and monuments. A stabilized combination of silica sol and water glass as a binding agent shows completely new properties, with which also organic substrates can be painted directly, simply, and siliceously. The so-called third generation is based on a new siliceous binding-agent technology that also enables the universal application on old organic substrates.

8.8 Organo-silicate paint For a long time, the designation “organo-silicate paint” has been a synonym for “silicate emulsion paint”. Nowadays, it stands more or less for another kind of paint, where the silicate parts are much lower than the organic emulsion content. Therefore, the properties of such paints are similar to a standard emulsion paint and not to a silicate emulsion paint. Especially due to marketing aspects, the labelling of modern interior wall paints requires that they comply with the guidelines of the “Blue Angel” eco-label in Germany. Therefore, formulators are forced to use less and less preservatives. As the high pH of organo-silicate paints helps to keep the growth of microbes inside the paint very low” to minimize the microbial growth inside the paint, it is possible to avoid the use of preservatives and consequently these paints comply with the necessary requirements for eco labelling.

8.9 Various applications of soluble silicates Silicates have been used as binders and adhesives in the past because of their functionality and ease to use. These two characteristics, along with new environmental regulations, are encouraging surface coatings formulators to experiment with this inorganic, cost-effective binder. Silicates can be used in pure form as a binder or in conjunction with a setting agent, depending on the chemistry of the materials being formulated. As with all binder systems, optimization studies should be performed before production. Although water glass is one of the oldest known chemicals, it has properties based on the most current criteria. This has opened the door to widespread use in the industry. Sodium silicate is used in far greater quantities than silicates based on other alkali metals.

Chemical industry

Precipitated silica and silica gels are used widely as for example reinforcing fillers in rubber, as matting agents for surface coating, and as polishing and structuring agents in

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Various applications of soluble silicates toothpaste. They are produced by using acids to neutralize the sodium silicate under carefully controlled conditions. Zeolites, which are crystalline alumo-silicates, are synthesized from sodium silicate and sodium aluminates. Their basic structural unit is the SiO4 tetrahedral network, with partial replacement of silicon by aluminium. These materials, sometimes referred to as molecular sieves, are used for ion exchange, and as highly selective catalyst absorbents. The production of colloidal silica and silica gel plays an important role even in the coating industry. Liquid sodium silicate is the main silica source for this production process and undergoes a hydrogen-type acidic cation exchanger step. The resulting silica has a pH value of 2 to 4 and needs further treatment with alkali to be stabilized at a pH level of 8 to 10. Colloidal silica can be used in widespread applications such as processing aids in beverage clarification, in combination with potassium silicate in sol-silicate coatings, in batteries as a gelling aid for sulfuric acid, a component in catalyst manufacturing, a surface polishing aid for silicon wafers in electronic applications, as well as for metals and gems. Apart from its use as a source of silica, sodium silicate often assists in the production, or end-use enhancement, of several other important industrial chemicals. Titanium dioxide, for example, is widely used as the primary specified pigment in the paint, plastic, and paper industries. Its performance is enhanced by silicate treatment to modify its surface characteristics.

Pulp and paper

Liquid sodium silicates are widely used in the pulp and paper industry in a variety of functions. The main application is the flotation deinking process, as well as sizing, coating, and pulp bleaching. In the case of hydrogen peroxide bleaching, water glass has been shown to exhibit benefits compared with other bleaching agents. It can be shown that higher alkaline sodium silicate is efficient and economical in bleaching agents as a stabilizer, alkali carrier, and buffers the pH value at which the peroxide is most effective. Additionally, water glass can deactivate metals, such as iron, copper, and manganese, which can catalyse the decomposition of hydrogen peroxide. Due to its dispersing properties, sodium silicate is the key ingredient in the waste paper deinking process. Sodium silicate has an undisputed role in separating inks from paper fibres. In combination with surfactants, it can help to prevent the ink particles from redepositing on the paper pulp. So, sodium silicate solutions are strongly connected with the production of recycled paper.

Adhesive

Alkali silicates have a long history as cost-effective and dependable adhesives. They are particularly suitable for adhesive applications because of their favourable properties, e.g. high strength. The main application is the paper processing industry, especially in the sector of cardboard tubes. The production of cardboard products, particularly spiral

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Alkali silicates wound and parallel wound tubes, and drums, where strength is at a premium, provides an extensive field of application and there are many other industries where silicates are employed. Insulation and acoustic materials such as mineral-wool boards, wood-wool panels, foamed glass boards, gypsum plasterboards, vermiculite, and perlite boards benefit from these inorganic adhesives which are water-based liquids, which are non-combustible and do not contribute to the fire load. Therefore, the choice usually falls to water-soluble silicate-based products. The range of viscosity from fluid liquid to thick paste depends on the water glass ratio, special additives, and inorganic fillers. Beside sodium silicates, sometimes potassium silicate is preferred, especially for higher moisture-rich applications and where heat insulation and fire resistance are chief goals, as it has a slightly higher softening/flow point than its sodium counterpart. Sodium silicates when modified with special inorganic fillers and additives are well-adapted for bonding nonwoven glass or aluminium foil on substrates as mineral wool, paper, and mineral boards. When silicates are used as adhesives, they offer versatile advantages: – non-combustible – air-drying – dilutable to required viscosity – strong adhesion to the substrate – easy to apply – rapid hardening – cost-effective, especially compared with organic adhesives

Water treatment

Alkaline silicates are usually used in various ways to treat potable or industrial water. Partial neutralization of silicate with acids or acid salts produces an anionic sol which is an extremely efficient coagulation aid, for use with aluminium sulphate during the normal purification process. Many borehole water sources contain a high source of iron and manganese. Upon aeration or chlorination, these will precipitate as red/brown coloured hydroxides, either in main pipelines or during use, giving rise to a high level of consumer complaints. This problem can be overcome by adding a few parts per million of a sodium silicate solution during the treatment process before aeration or chlorination.

Corrosion prevention

Corrosion is a well-known problem in industries where water systems are involved. Many systems have been developed over the years to protect metal surfaces from the effects of corrosion. One such method which has proved extremely successful is the addition of small quantities of sodium silicate to the water in the system. Sodium silicate is effective in controlling the corrosion of many metals and will protect systems containing several different metals where electrolytic corrosion is often a significant problem. Sodium silicate

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Various applications of soluble silicates reacts with the corrosion products of the metal to form an almost insoluble protective film of metal silicate. Ferrous metals, lead, brass, copper, zinc, and aluminium are protected, and problems caused by such phenomena as plumbosolvency eliminated.

Silicate based injection grouting

Various techniques are employed to consolidate or seal the ground in excavation pits before further operations. Silicate-based injection grouting is a cost-efficient solution for temporary groundwater control in construction projects. It can be used in high ground-water table-bearing sand sediments. There are various methods for creating excavations that reach deep into the groundwater. The method to seal sand layers with chemical grouts can be done successfully with sodium silicate and reactive agents. This technology which is better known under the name “softgel injection grouts” has economic advantages compared with other systems. Silicate admixtures based on sodium silicate, hardener, and water are injected as a deep grout. Silicate admixture displaces water from the sand-bearing capillaries and precipitates after a certain period into a water-insoluble hydrated silicate gel. The method by which the gel is prepared varies. Most common are systems based on sodium silicate with sodium bicarbonate or sodium aluminate and water. There are many hardening or setting agents which vary in the application technique. Organic hardeners, i.e. dibasic ester, propylene carbonate, or triacetin will react with sodium silicate to form silica gels through pH modification. This type of process is better known under the name “hard gel grouting” which can be used to consolidate sand, oil-bearing wells and is quite versatile in its gelling response.

Heat resistant coating

Coatings based on soluble silicates are used for various purposes including sealing porous surfaces, heat insulation, binding loose fibres, and formulation of certain paints. Although sodium silicate can be used for these applications, it tends to form an unsightly white bloom by interacting with atmospheric carbon dioxide. Potassium silicate is less affected and therefore often preferred. Surfaces suitable for coating or sealing with silicates include vermiculite, perlite, asbestos, and other fibrous or loose-grained minerals and the inflammability of the silicate is often an important consideration. Alkaline silicates are employed in anti-corrosion paints, particularly those based on zinc, and in situations where heat resistance is vital and can be formulated with aqueous synthetic resin dispersions to reduce cost.

Ceramic and minerals

A clay slip, a suspension of clay in water is used to produce ceramic shapes for such applications as the production of for example bathroom ware. It is important to keep the slip mobile while minimizing water content, which is later removed in the process. To achieve

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Alkali silicates this, a small amount of sodium silicate, usually with a low molar ratio between 1 and 2, is added to act as deflocculant or "slip aid" as it is known in the industry. In this application, the water glass acts as a liquefier, as binder and as fluxing agent at the same time. A similar use occurs in the wet process for portland cement where sodium silicate is used to deflocculate aqueous slurries of chalk, milk of lime, and clay, again to maximize solids with minimum viscosity.

Concrete treatment

For several years an increasing focus has been occurring worldwide on the trend to treat concrete surfaces with liquid alkali silicates. The use of chemical concrete densifiers add value to industrial floors, providing a dustproof surface, improve durability, decreasing permeability, and increasing stain resistance as well as enabling diamond grinding or polishing to produce uniformly even floors in warehouses. The benefits of today´s chemistry have made silicate densifiers a welcome alternative to organic, film forming coatings as they create a non-membrane forming barrier that does not peel off or discolour meeting the requirements of the European fire protection Norm (DIN EN 4102) to be classified as A1. In the cement hydration process, calcium hydroxide dissolves in water and moves to the surface where silicate can react to form a hydrated calcium silicate hydrate gel (C-S-H), which gives the concrete the desired strength and hardness. This newly created C-S-H gel is deposited in the fine pores and capillaries near the concrete surface. The effect is not only confined to the surface, as the silicate can penetrate the concrete to reduce porosity depending on the age of concrete. By preserving more moisture at and near the concrete surface, it reduces the build-up of surface tension that is responsible for shrinkage cracks. The most common choice is diluted mixtures of pure or organically modified lithium silicates. The smaller lithium ions stabilize the silicate ions more efficiently resulting in better performance compared with sodium or potassium silicate formulations, while not contributing to higher pH values. Additionally, lithium silicate can penetrate deeper into porous concrete and has a higher silicification potential. Water glass is typically applied onto thoroughly cleaned concrete with simple hand spray equipment for small areas up to motorized spray units for large projects. Lithium and potassium silicates-based formulations have the advantage that they can be polished dry after application. Sodium-based formulation needs a wet polishing step (because of the presence of sodium carbonate efflorescence) and requires more complex processing. Benefits with silicate technology: – consolidation of cementitious substrates – excellent stain protection – increase of surface strength – added abrasion resistance to the surface – pore sealing

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Various applications of soluble silicates – – – – – – – –

eliminates dusting significant increase in chemical resistance fire resistance easy and quick application (wiping/rolling/brushing) environmentally friendly and odourless solvent- and VOC-free lasts for years without the need for reapplication easy to apply

Welding

A welding rod flux coating consists of a variety of mineral and metal powders, bound together with silicate binder, extruded onto a supporting wire. Apart from acting as a binder, the silicate provides a source of alkali for the arc process itself. The coating has several functions. It must improve the conductibility of the welding arc and is responsible to stabilize the arc during the welding process producing an inert gas atmosphere. The drop size and slag are also influenced by the coating. Welding producers use water glass to act as a plasticizer, lubricant, fluxing agent, slag creator, and arc stabilizer. In general, the welding process is very sensitive to moisture absorption and welding rods must be stored properly before use. Therefore, the correct choice of water glass can influence the whole complex process of welding.

Coloured sand

Roofing granules are mineral particles, coated with a colouring pigment before incorporating into tiles. The silicates serve to bind the colouring pigment to the granule, before firing at a high temperature. Sodium silicate is commonly employed but, under certain conditions, potassium silicate may offer advantages by helping to lower the "fixing" temperature. Coloured sand and slate are mainly used in particular to cover bitumen-based roofing felts and sanding concrete roofing materials. This gives them a rough, pleasant surface structure and a varied, colourful appearance. Roofing materials are required to be very durable and weather resistant. Organic binders do not satisfy tthese criteria and therefore inorganic binders are the right choice for this application. The most frequently used binder is sodium silicate in combination with an inorganic hardening agent. At temperatures around 450 °C to 650 °C, a mixture of inorganic pigments, quartz sand/slate, and binder is embedded and fixed onto the surface of the sand. The binders are transformed into a glass-like insoluble layer.

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Alkali silicates

8.10 References In general A. Thomas, Waterborne Silicates in Coatings and Construction Chemicals; Paper V7 201126, 2021 R. Iler, The Chemistry of Silica; John Wiley & Sons, New York 1979 E. Ebrecht, NMR Untersuchung zur Medium Range Order in binären Alkalisilicatgläsern, Thesis, Technische Universität Bergakademie Freiberg, 2003 G. Gettwert, Alkalisilikate; Woellner Silikat GmbH, Special reprint, Ludwigshafen, 1996 G. Gettwert, W. Rieber, J. Bonarius, One pack silicate binder systems for coatings; Surface Coatings International, 81, 596–603, 1998 G. Gettwert, Alkalisilikat Bindemittel in Dispersions Silikat Beschichtungen; ConChem Journal, 5. Jahrgang, 2/1997 G. Parashar, M. Bajpayee,P. K. Kamani, Water-borne non-toxic high-performance inorganic silicate coatings; Surface Coatings International Part B, Vol. 86, B3, 169–246, India

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W. Schultze and 14 co-authors, DispersionsSilikatsysteme, Kontakt & Studium, expert Verlag, D-71272 Renningen-Malmsheim, 1995 R. Engler, Lösliche Silikate; SÖFW, 100, No. 7–12 (Special reprint of Woellner-Werke, Ludwigshafen), 1974 A. Thorn, Lithium silicate consolidation of wet stone and plaster; 12th International Congress on the Deterioration and Conservation of Stone, Columbia University, New York, 2012 Centre Européen d´Etude des Silicates, Soluble Silicates; European Chemical Industry Council, 15, Belgium, 2005 J. N. Fuchs, Ueber ein neues nutzbares Produkt aus Kieselerde und Kali; Johann Leonhard Schrag Verlag, Nürnberg, 1825 C. H. Baehr, W. Koehl, Soluble Silicates – highly versatile and safe; SÖFW-Journal, 133, 2007 Institut für Umwelttechnik und Management an der Universität Witten/Herdecke GmbH, Large Volume Solid Inorganic Chemicals: Natriumsilikat; Dessau-Roßlau, 2001

Structure of amino resins

9 Amino resins as hardeners Oliver Seewald1 Amino resins are used as hardeners for hydroxy functional binders in both solvent- and water-borne stoving enamels. The reactive group of amino resins is the methylol group. For use in paint systems amino resins have to be modified. The oligomers which are varied by etherification with methanol are water-soluble or at least water-borne. The most important partners are alkyd resins, saturated polyesters, and acrylic resins all containing hydroxyl groups. Alkyd resins in water-borne paints confer optimum wetting of pigments and substrate surfaces, their films are highly glossy and have an excellent appearance. The resins are used in water-borne topcoats, one-coat paints, and primers. In water-borne paints, saturated polyesters confer optimum balance of hardness and flexibility. They are therefore preferred for primer surfacers and base coats for automotive OEM application, water-borne paints for industrial application, and can coating and coil coating systems. Acrylic resins are notable for having greater saponification stability than alkyd and polyester resins. Water-borne acrylic resins are used in topcoats, clear coats, and base coats. Co-crosslinking is the result of reactions between the methylol groups or methylol ether groups of the amino resins with hydroxyl groups of the binder. However, self-crosslinking by the amino resins also occurs in all cases. Melamine resins are the most important amino resins in the paint industry. Therefore, this chapter focuses mainly on this type.

9.1 Structure of amino resins The common definition of “amino resin” [1–5] is not chemically correct because the base raw materials for amino resins are amides not amines. These are urea (diamide of carbonic acid), carbamates (esters of carbaminic acid, the monoamide of carbonic acid), melamine (triamide of cyanuric acid, or 2,4,6-triaminotriazine), benzoguanamine (2-phenyl-4,6-diaminotriazine) and glycoluril (acetylene diurea). The chemical structures of these components are presented in Figure 9.1.

1 based on Ulrich Poth, Coatings Formulation, Vincentz Network, Hanover 2017, Part III, Chapter 4

Akkerman, Mestach et al.: Resins for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

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Amino resins as hardeners Due to the neighbouring double bond (C=O, C=N), the hydrogen atoms attached to an amide nitrogen atom are quite reactive. They are thus able to add formaldehyde to form methylol compounds. Figure 9.2 shows the reaction between melamine and 3 mole formaldehyde, for 6 reactive N-H-groups a maximum of 6 mole formaldehyde can be added.

Figure 9.1:  Chemical structure of components for amino resins

Figure 9.2:  Formation of methylol groups by adding formaldehyde to melamine

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Structure of amino resins Methylol compounds are reactive and will form methylene bridges with other amide hydrogen atoms, splitting off water. Methylol groups also react with each other to form dimethylene ether bridges and water. Figure 9.3 shows the methylol reactions that act as the source of molecular growth. The resulting molecules are so-called non-modified amino resins, which consist of several amide molecules connected via methylene bridges and dimethylene ether bridges and containing methylol end groups and residual hydrogen atoms on the nitrogen atoms. The compounds are branched, but there is also the possibility of forming cyclic structures. Non-modified amino resins are highly polar compounds. They are soluble in water and lower molecular alcohols. They therefore find little use in paint formulations. However, they are used in large quantities for adhesives, laminated sheets, and textile coatings.

Figure 9.3:  Methylol reactions for molecular growth: formation of bridging elements in the preparation of amino resins

349

Amino resins as hardeners Amino resins are modified for use in paint systems. The modification consists in etherifying the free methylol groups with monoalcohols. Figure 9.4 shows the reaction of methylol groups with monoalcohol to form ether groups. Suitable monoalcohols are methanol, isobutanol, and 1-butanol. Etherified amino resins are much more soluble in the usual solvents and compatible with other resins. Also, they have better storage stability. Amino resins which are modified by etherification with methanol are water-soluble or at least water-borne. Amino resins for paint systems are characterized by their building elements (amides), the average molecular mass and the number of different functional groups (residual amide hydrogen, methylol groups, and etherified methylol groups). These characteristic values are influenced by the ratios of components (molecular ratio of amide : formaldehyde : monoalcohol), the use and amounts of catalysts for the addition and condensation reactions (acids and bases), and the reaction conditions (temperature, reaction time, concentration of solution).

9.2 Types and properties of amino resins Amino resins are classified first by their amide component. There are urea resins, carbamate resins, melamine resins, benzoguanamine resins and glycoluril resins. In this chapter the focus is on melamine resins.

9.2.1 Melamine resins Melamine resins are the most important amino resins in the paint industry. They consist of melamine, formaldehyde and monoalcohols, e.g. methanol, 1-butanol and isobutanol. For water-dilutable or at least water-borne melamine resins methanol is used for etherification of free methylol groups. Classification by structure and properties yields three groups:

Figure 9.4:  Etherification to modify amino resins; methanol as alcohol is used for water-soluble or at least water-borne amino resins

350

Types and properties of amino resins a) partially etherified melamine resins, containing free methylol groups, with different molecular masses and different reactivities. b) low-molecular, highly methylolated, fully etherified melamine resins c) relatively low-molecular, highly etherified melamine resins, which still contain residual NH groups (imino grades)

a) Partially  etherified types, containing free methylol groups, with different molecular masses and different reactivities Partially etherified melamine resins containing free methylol groups are produced with an excess of formaldehyde but lower amounts of monoalcohol. The excess of formaldehyde and the reactions conditions influence the resultant molecular mass. The products mostly have comparatively high molecular masses; they differ in viscosity and reactivity. These melamine resins (see Figure 9.5) react preferentially via their methylol groups in self-crosslinking reactions or with resins containing OH groups (co-crosslinking). In addition, etherified methylol groups may react in the same manner. Under the usual stoving conditions of 120 and 160 °C, an acid catalyst is unnecessary, but may be used in solvent borne coatings. If coatings are formulated for stoving temperatures below 100 °C, an acid catalyst must be used; this also applies to formulations based on the more reactive types. Grades of such melamine resins which have higher molecular weight are responsible for the physical drying of paint systems. Melamine resins of this group containing higher amounts of free methylol groups can undergo more extensive self-crosslinking to form dimethylene ether groups. This may lead to less acid stability. The resins of this group have two disadvantages: first they contain higher amounts of free formaldehyde and – in addition – may form free formaldehyde during crosslinking. Second – due to their higher

Figure 9.5:  Partially with methanol etherified melamine resin containing free methylol groups

Figure 9.6:  Structure of hexamethoxymethyl melamine

351

Amino resins as hardeners solution viscosity – they are not qualified to formulate paints with higher solid contents. And partially etherified melamine resins are reactive systems whose viscosity increases with age. Consequently, these grades have been widely replaced by highly etherified melamine resins that contain residual NH groups (see group c).

b) Low-molecular, highly methylolated, fully etherified melamine resins The best known low-molecular, highly methylolated, fully etherified melamine resins are those which contain methanol as etherification monoalcohol. On account of their idealized structure, they are called hexamethoxymethyl melamine resins (HMMM resins). This structure is shown in Figure 9.6. The HMMM resins are delivered as 100 % products. They are either viscous liquids or waxy and are more or less water-soluble. As the trans-etherification reaction of etherified methylol groups is the slowest of all the crosslinking reactions, relatively high stoving temperatures (180 to 200 °C) or efficient catalysis is required in solvent borne coatings. The preferred catalysts are acids, particularly sulphonic acids, although acidic phosphoric esters or their amine salts may be used. In most cases in water-borne systems, there is no need to use catalysts for crosslinking melamine resins as the usually high content of carboxyl groups of the binder is adequate for effective crosslinking. Given an optimum balance of temperature, time, and catalyst, film formation of the paints effectively leads to co-crosslinked structures. These films are distinguished by comparatively good weatherability and good flexibility and intercoat adhesion. Analysis of the trans-etherification reaction of HMMM resins shows that up to three of the six etherified methylol groups of an idealized molecule participate effectively in the crosslinking reaction. HMMM resins are relatively low-molecular products, the molecular mass of the HMMM molecule being 390.4 g/mol. However, actual products additionally contain molecules which have not only one triazine unit. Nevertheless, due to their optimum solubility, the resultant low viscosity of HMMM resins offers latitude for formulating high-solid paints. But, of course, the viscosity of the combination partner has to be taken into consideration. Generally, HMMM resins are used for stoving enamels in industrial coatings, particularly for automotive OEM paints [6] and for coil coating and can coating as well.

c) R  elatively low-molecular, highly etherified melamine resins which still contain residual NH groups (imino types) Relatively low-molecular, highly etherified melamine resins that still contain residual NH groups (imino types) are significantly more reactive than the fully etherified types. The structure of the resins is presented in Figure 9.7. At usual stoving temperatures (120 to 160 °C), they do not need any catalysts. However, acid catalysts may be used in solvent borne coatings, too. The residual NH groups may

352

Combination partners for amino resins react with methylol groups to form methylene bridges, with cleavage of monoalcohol (self-crosslinking). On the other hand, the directional effects exerted by this NH group onto the neighbouring methylol ether group accelerates the trans-etherification, and so effective crosslinking may already take place at lower temperatures, in comparison to the low-molecular, fully etherified melamine resins. The different solubility of these grades results in higher viscosity compared to the HMMM resins, leading to somewhat lower solid contents at the application viscosity. These resins are usually delivered in alcohol, most commonly in butanols; solid contents are 60 to 90 wt.%. Due to the low content of methylol, the amount of free formaldehyde is lower than in other grades. Together with the fact that relatively low amounts of free formaldehyde are generated during stoving and curing, this is an advantage where harmful pollution needs to be avoided.

9.3 Combination partners for amino resins All combination partners for amino resins are binders which contain a significant amount of hydroxyl groups. The important partners for stoving enamels are alkyd resins, saturated polyesters, and acrylic resins (containing hydroxyl groups). Of minor importance are epoxy resins, epoxy esters and special vinyl resins.

Figure 9.7:  “Imino” melamine resin

353

Amino resins as hardeners

9.3.1 Crosslinking reactions In most cases, the aim is to boost the co-crosslinking reaction of hydroxyl groups of alkyd resins, polyester resins and acrylic resins with the functional groups of amino resins. However, self-crosslinking by the amino resins also occurs in all cases. Co-crosslinking is the result of reactions between hydroxyl groups of alkyds, polyesters, and acrylic resins with the methylol groups or methylol ether groups of the amino resins. Water or monoalcohols are the cleavage products. The reactions follow an etherifi-

Figure 9.8:  Possible co-crosslinking reactions

354

Combination partners for amino resins cation or a trans-etherification mechanism. The reaction rate is supported by the directional effect exerted by the amide nitrogen. The principles behind such reactions are presented in Figure 9.8. The reactions of self-crosslinking amino resins are shown in Figure 9.9. Self-crosslinking of amino resins is basically the continuation of the molecular-growing process. Free NH groups react with methylol groups, with cleavage of water, to form methylene groups. If they react with methylol ethers, monoalcohol is cleaved. Methylol groups react with themselves to form dimethylene ether bridges, with cleavage of water. If the stoving temperatures are high or a higher amount of acid catalysts is added, cleavage of formaldehyde from methylol and methylol ether groups may occur, followed by other reactions. The various reactions have different reaction rates. The order of reactivity for the various crosslinking mechanisms, starting with the fastest, is as follows: 1st NH-groups with methylol groups forming methylene bridges, splitting of water 2nd NH-groups with methylol ether groups forming methylene bridges, splitting off alcohol 3rd methylol groups with hydroxy groups, splitting off water 4th two methylol groups, forming dimethylene ether groups, splitting off monoalcohol 5th etherified methylol groups with hydroxy groups, splitting off water 6th etherified methylol groups with methylol groups, forming dimethylene ether groups, splitting off alcohol 7th splitting off formaldehyde of dimethylene ether groups and methylol groups, followed by further reactions

Figure 9.9:  Self-crosslinking reactions of melamine resins

355

Amino resins as hardeners Co-crosslinking and self-crosslinking of amino resins and resins containing hydroxyl groups produce different film properties. Co-crosslinking promotes flexibility, chemical resistance and weatherability. Self-crosslinking supports hardness and solvent resistance. The amount of co-crosslinking can be controlled relative to self-crosslinking to meet different application requirements. Co-crosslinking is favoured if the hydroxyl number of the co-resin is high, the hydroxyl groups are reactive, and the reactivity of the amino resin is relatively low. Lower stoving temperatures and lower amounts of catalysts tend to have an effect in the same direction. In contrast, self-crosslinking occurs if the hydroxyl numbers of co-resins are low, the hydroxyl groups are less reactive, the amino resins are highly reactive, the catalysts are strong acids and their addition levels are high, and if the stoving temperatures are also high. Given these different tendencies, there is no way of calculating the mixing ratios of co-resins and amino resin stoichiometrically. The blending ratios for polyesters or alkyds with melamine resins depend on the molecular weights and quantities of the functional groups, but range from 60 : 40 to 80 : 20 (in terms of solids).

9.4 Water-borne stoving enamels based on amino resins 9.4.1 Selecting amino resins As already mentioned above, amino resins [5] that are either not etherified or etherified with methanol are water-soluble as such, or at least water-dilutable. Since the non-etherified resins have no adequate storage stability and are not compatible with many other resins, the methanol-etherified types are preferred for water-borne paints. The most important types of amino resins belong to the group of melamine resins [4–9]. The highly etherified types, which consist of residue imino groups, are more water-soluble than the highly methylolated and fully etherified types (HMMM resins). However, the latter are easily brought into aqueous phase with cosolvents or with anionically stabilized resins. Melamine resins can hydrolyze in aqueous phase [10]. The hydrolysis of imino grades takes place under basic and acidic conditions as well. The free methylol groups generated by hydrolysis of imino grades are more reactive than the etherified methylol groups. Pre-reactions are possible and storage stability is limited. Other types of melamine resins can undergo hydrolysis as well. The self-condensation reaction starts with the hydrolysis of the methoxymethyl groups, followed by demethylolation and the subsequent release of formaldehyde. The hydrolysis reactions are presented in Figure 9.10. Combinations of anionically stabilized resins containing hydroxyl groups and HMMM resins are much more stable under basic conditions. Consequently, HMMM crosslinkers are widely used for water-borne one-component (1K) systems with pH values above 7.5. As the

356

Water-borne stoving enamels based on amino resins melamine resins for water-borne paints mainly have low molar masses, they mix easily with hydroxyl resins and diffuse into the colloidal particles very well. Due to the unusual pseudoplastic behaviour of the viscosity of the systems, more intensive mixing is required.

9.4.2 Combination resins for water-borne stoving enamels The most important partners for melamine crosslinkers in water-borne paints are alkyd resins, saturated polyester resins [11] (see Chapter 4), and acrylic resins [12] (see Chapter 3). These all contain hydroxyl and carboxyl groups. The carboxyl groups are neutralized by amines to become water-soluble or water-borne. The resins have relatively low molar masses, so there is the possibility of effective mixing with crosslinkers by diffusion in the aqueous phase, yielding optimum crosslinking reactions. As already mentioned in Chapter 4, alkyd resins and saturated polyester resins have to be modified such that they are optimally resistant to saponification. This property is achieved by selecting molecular structural elements that form esters which are relatively resistant to saponification. Other important parameters for stability are molar masses and the type and content of functional groups (acid number, hydroxyl number). Additionally, the choice of neutralization agent and the type and quantity of cosolvent can assist in

Figure 9.10:  Hydrolysis of amino resins

357

Amino resins as hardeners protecting the ester groups against saponification. All these influences can thus be exploited to enable all the positive properties of alkyd resins and polyesters in water-borne stoving enamels to be used. Alkyd resins [11] in water-borne paints therefore confer optimum wetting of pigments and substrate surfaces, their films are highly glossy and have an excellent appearance. The resins are used in water-borne topcoats, one-coat paints, and primers. In water-borne paints, saturated polyesters [11] confer optimum balance of hardness and flexibility. They are therefore preferred for primer surfacers and base coats for automotive OEM application, water-borne paints for industrial application, and can-coating and coil-coating systems. Acrylic resins [12] are notable for having greater saponification stability than alkyd and polyester resins. Due to their large, linear but coiled molecules, acrylic resins have relatively dense colloidal particles. The colloidal particles of acrylic resins in aqueous phase have more the appearance of dispersions than of solutions, especially if the molar masses of the acrylics are high. In such cases, the efficiency of mixing with crosslinkers is limited, and that leads to lower crosslinking efficiency. Water-borne acrylic resins are used in topcoats, clear coats, and base coats.

9.4.3 Hybrid systems Combinations of water-borne resins with primary dispersions that form films only by physical drying are suitable for special applications. Such resin combinations are called hybrid systems and are distinguished by rapid film formation. They are therefore used for paints that have to achieve rapid physical drying, e.g. water-borne base coats for the two-coat process, where the clear coat is applied over the base coat without separate stoving in between.

9.4.4 Neutralization agents Neutralization agents for carboxyl groups on water-borne binders for stoving enamels are mainly N,N-dimethylethanolamine (DMEA) and 2-amino-2-methylpropanol (AMP) (see Chapter 1.2.4). N,N-dimethyl ethanolamine has been classified as toxic since 2012 and must be declared if a specific amount is exceeded. Also, triethylamine (TEA) is classified in the German TA-Luft regulation as a class 1 air pollutant, the use of the amine is restricted. Amines containing hydroxyl groups (alkanolamines) have the advantage not only of acting as neutralization agents, but also of having the properties of cosolvents. Suitable pH values after neutralization are between 7.5 and 8.5. In this regard, the degree of neutralization is 0.75 to 1.00 moles of amine per carboxyl group. Although hydrolysis may

358

Water-borne stoving enamels based on amino resins take place at this low concentration of protons, there is the advantage that reactions of functional groups on melamine resins (start of co-crosslinking or self-crosslinking) are widely restricted. Therefore, amines for example in water-borne polyester/HMMM formulations are needed on the one hand to solubilize the saturated polyester in water and on the other hand to stabilize the HMMM crosslinker. Amines as neutralization agents have to evaporate effectively during stoving. Firstly, residual amines increase the polarity of the coating and following from this decrease its moisture resistance. And secondly amines can react with the formaldehyde by-product of melamine self-condensation resulting in discoloration such as yellowing. Their evaporation rate is influenced not only by the vapour pressure at the given temperature, but also by the basic strength of the amine and its interaction with the anion. Sometimes lower vapour pressures bring about advantages as regards application behaviour and film forming.

9.4.5 Cosolvents for water-borne stoving enamels Suitable cosolvents for water-borne stoving enamels are alcohols, glycol ethers, glycol ether esters and N-methylpyrrolidone. It must be remembered that the ester groups of cosolvents may be saponified at the given pH values. The use of N-methylpyrrolidone is restricted due to the reproductive toxicity of this solvent. In addition, the methyl and ethyl ethers of ethylene glycol and their acetates are classified as teratogenic and are no longer considered. Butyl glycol (monobutyl ether of ethylene glycol) is the most important cosolvent for this group of water-borne paints. Due to the molecular structure of butyl glycol (polar and nonpolar part of the molecule), the cosolvent supports the stability of colloidal solutions. Additionally, use of butyl glycol enhances gloss and levelling during film formation. In water-borne solutions of resins, the butyl glycol is distributed more in the colloidal particles and less in the mobile phase. This leads to lower amounts of water in the colloidal particles, which decreases the saponification reaction. At the end of the evaporation process, the colloidal particles with butyl glycol are in the state of an organic solution. This avoids blistering and confers optimum gloss and levelling. The monoethers of propylene glycol are classified as toxicologically harmless, but lag behind butyl glycol when it comes to application. Solvents that are more hydrophilic than butyl glycol are distributed mainly in the mobile phase (aqueous phase). They only support the film forming properties if their evaporation rate is relatively high. But non-water-soluble solvents, e.g. higher alcohols, esters and aromatic hydrocarbons, can also be added in small amounts to water-borne systems. These solvents are thought to diffuse almost completely into the colloidal phase where they are then borne by the ionically stabilized resins. They can benefit diffusion processes, help to avoid foam and blistering and support levelling.

359

Amino resins as hardeners

9.4.6 Mixing ratios and crosslinking The same rules apply to the mixing ratios of the resins containing hydroxyl and carboxyl groups and amino resins (particularly melamine resins) used in water-borne systems as apply to solvent-borne systems. The ratios can vary from 65 : 35 to 85 : 15. Since mainly the HMMM resins, which have relatively low molecular masses and a high content of functional groups, are preferred for water-borne paints, the preferred ratios are lower amounts of melamine resin, e.g. 75 : 25 to 85 : 15 (resin containing hydroxyl groups versus HMMM resin). If the stoving temperatures are below 160 °C, only the hydroxyl groups will react with functional groups of melamine resins. The carboxyl groups will remain in the film and, as they are less hydrophilic than carboxylic anions, they do not significantly influence sensitivity to humidity. Carboxyl groups act as catalysts for the reactions of functional groups of melamine resins. As the content of carboxyl groups in resins for water-borne paints is significantly higher than in resins for solvent-borne systems, the reactions are accelerated to a greater extent. Therefore, HMMM resins crosslink effectively in water-borne systems at much lower temperatures than in solvent-borne systems.

9.4.7 Acid catalysts In most cases, there is no need to use catalysts for crosslinking melamine resins as the usually high content of carboxyl groups is adequate for effective crosslinking. However, if the stoving temperature is low, it is possible to accelerate the reaction rate with catalysts. For this, it makes sense to use the water-soluble amine salts of compounds containing sulphonic, phosphoric or carboxylic groups. The products are mainly commercially available in alcohol solution [13]. The suppliers of such acid catalysts offer products that contain, in addition to strong acid groups, hydrophobic molecule parts, which confer excellent compatibility with resin molecules. Besides p-toluene sulphonic acid, dodecyl benzene sulphonic acid, naphthalene sulphonic acid and naphthalene disulphonic acid [14] are preferred. Neutralization agents for the acids are DMEA, TEA, AMP, morpholine and substituted oxazolidines. However, solutions of the free acids are also suitable; with these, it is necessary to add more neutralization agents to the whole paint formulation. As the pH values are sufficiently high, there is no restriction on storage stability.

9.4.8 Film properties As already mentioned, amino resins in water-borne paints react by self-crosslinking in addition to co-crosslinking. Depending on the type of melamine resin, the mass ratios of

360

Water-borne stoving enamels based on amino resins hydroxyl resin and the stoving conditions, the amount of self-crosslinking can be higher than in solvent-borne paints. The resultant film properties reflect this. The same trends described for solvent-borne systems apply. Self-crosslinking promotes hardness and solvent resistance, while co-crosslinking promotes flexibility, chemical resistance and weather resistance.

9.4.9 Pigmentation The same pigment types are suitable for water-borne stoving enamels as for solvent-borne paints. It is possible that selecting the same pigment mixture as used in solvent-borne paints with the same PVC will yield different colour shades. The reasons are the different wetting properties and different flocculation behaviours of solvent-borne and water-borne systems. For example, it is very difficult to disperse carbon black pigments in binders for water-borne stoving enamels to achieve a pure black, which has a bluish shade. Pigment suppliers therefore have to offer pigment grades which are specifically modified in such a way that they are efficiently wetted in water-borne systems. In addition, recommendations have been made concerning the use of pigment preparations for water-borne systems, as this eliminates the expensive dispersion processes. These pigment preparations mainly contain higher amounts of wetting agent (emulsifier), which may restrict the application area.

9.4.10 Additives Many of the additives used in solvent-borne paints are also suitable for water-borne systems. However, the suppliers of additives offer products that are developed and produced specifically for water-borne paints [15]. Due to their surfactant-like structures, anionically stabilized resins have excellent inherent wetting properties for pigments and substrates. Nevertheless, special wetting agents for water-borne paints are available. Primarily water-borne stoving enamels should contain antifoam additives in order that film forming defects during physical drying and crosslinking may be avoided. The above-mentioned solvents, which are not water-tolerable, already act as defoamer. Candidate antifoam additives here are special polydimethylsiloxanes or hydrophobically modified silica, or a combination of both. Polyether-modified silicone oils act as levelling agents. Other additives for water-borne systems are thickeners for avoiding settling of pigments and sagging after application on vertical surfaces. For water-borne stoving primers, corrosion protection additives may be used.

361

Amino resins as hardeners

9.5 Formaldehyde free melamine-based resins There is a specific product which may also be defined as a blocked polyisocyanate. Its high reactivity at lower temperatures depends not on the type of blocking agent or the reaction partner, but on the activity of the potential isocyanate group itself. This product, trisalkoxy carbamato triazine (TACT) [16–18], is formally a triurethane of 2,4,6-triisocanato-1,3,5-triazine. The monoalcohols bearing the urethane groups are methanol and butanol. The molecular structure of TACT is given in Figure 9.11. The product combines the positive properties of triazines, e.g. resistance to heat and chemicals and a high refractive index that leads to high gloss values, with the positive crosslinking behaviour of blocked polyisocyanates. The crosslinker reacts with hydroxyl groups of partner resins (polyesters, alkyd resins, acrylic resins), cleaving the monoalcohols of the carbamate group (see Figure 9.12). Although monoalcohols as blocking agents for “normal” isocyanates do not react at temperatures below 200 °C, clear coats containing TACT are crosslinked effectively at temperatures of 130 to 140 °C. The reason is the directing effect of the double bond system of the triazine ring. Nevertheless, the resultant networks are relatively stable. The product has a comparatively low equivalent mass, i.e. only small amounts need to be added for effective stoichiometric crosslinking. The films produced by these combinations are distinguished by excellent gloss, high hardness, very good yellowing resistance and – depending on the type of reaction partner – by outstanding weather resistance. TACT does not contain or emit formaldehyde during curing. In coating formulations, it can be used as stand-alone crosslinker, or it may be used at lower levels in combination with other crosslinkers, such as melamine resins or isocyanates obtaining a balance of properties. Hybrid-crosslinked clear coats exhibit optimum values for resistance to weathering, chemicals and solvents, and a balance of hardness and flexibility. It is recommended for high quality duFigure 9.11:  Molecular structure of trisalkyl carbamato triazine (TACT) rable finishes such as automotive topcoats,

362

Formulation examples exterior can coatings, and coil coatings and can not only be used in solvent-borne but also in water-borne coatings [19, 20]. Although it is water insoluble, it can be incorporated into a water-reducible system by blending with the hydroxy-functional resin prior to amine neutralization. These systems can be cured at ~125 °C to give clear, high gloss films with excellent resistance properties. When using the material in water-borne systems it is recommended that a tertiary amine be utilized as the neutralizing agent for the system in order to avoid the possibility of degrading the TACT resin [19].

9.6 Formulation examples a) Water-borne stoving primer for general industrial use The resins chosen for a water-borne stoving primer [21] are a combination of a water-borne epoxy ester (“Resydrol” AX 250w [Allnex], density (calculated on solids, in terms of the density of the components as delivered) 1.150 g/cm3) with an HMMM resin (“Cymel” 303 [Allnex], density 1.200 g/cm3) in the mass ratio of 85 : 15. The solution of the epoxy ester consists of 75 wt.% solids in 6.0 wt.% butyl glycol (butyl cellusolve), 12.5 wt.% ethoxypropanol, and 6.5 wt.% N,N-dimethylethanolamine as neutralizing agent. The melamine resin is free of solvent. The pigment composition for this grey primer consists of titanium dioxide (“Kronos” 2310 [Kronos], density 4.000 g/cm3), and barium sulphate (“Blancfixe” N [Sachtlaben], density 4.300 g/cm3) in the weight ratio of 1 : 1 (32 : 32 wt.%). Functional pigments for this formulation are 15 wt.% talc (“Micro Talc” AT 1 [Mondo], density: 3.100 g/cm3) as barrier pigment, 15.0 wt.% zinc phosphate to support corrosion resistance (“Heucophos” ZPA [Heubach], density: 3.100 g/cm3) and as colouring pigments 5.0 wt.% yellow iron oxide (“Bayferrox” 3910 LV [Lanxess], density: 3.900 g/cm3) and 1.0 wt.% black iron oxide (“Bayferrox” 318 M [Lanxess], density: 4.700 g/cm3).

Figure 9.12:  Crosslinking of TACT with hydroxy-functional binders

363

Amino resins as hardeners Table 9.1:  Water-borne, grey stoving primer for industrial application Solids Density [wt.] [g/cm3]

Formulation [wt.%]

Item Ratios

Content

Products

01

85.0

epoxy ester (75 %)

“Resydrol” AX 250w

18.37

1.150

24.49

02

15.0

melamine resin

“Cymel” 303

3.24

1.200

3.24

100.0

sub total

03

32.0

titanium dioxide

“Kronos” 2310

7.48

4.000

7.48

04

32.0

barium sulphate

“Blanc fixe” N

7.48

4.300

7.48

21.61

05

15.0

talc

“Micro Talc” AT 1

3.51

3.100

3.51

06

15.0

zinc phosphate

“Heucophos” ZPS

3.51

3.100

3.51

07

5.0

yellow iron oxide

“Bayferrox” 3920 LV

1.18

4.100

1.18

08

1.0

black iron oxide

“Bayferrox” 318 M

0.23

4.700

0.23

100.0

sub total

09

deionized water

10

wetting agent

23.39 42.78 “Borchi Gen” SN 95

2.00

11

thickener

“Borchi Gel” 0434

1.00

12

defoamer

“Surfynol” 104 E

0.30

13

levelling agent

“Additol” XW 395

0.20

14

cosolvent

butyl carbitol

2.60

Sum

45.00

100.00

The primer will have a solid content of 45 wt.% and the pigment volume concentration (PVC) of the film will be 25 vol.%. The primer contains as additive a polyacrylate bearing a group which has an affinity for pigments and provides the wetting properties (2.00 wt.% “BorchiGen” SN 95, 25 % in water), a polyurethane thickener to avoid settling of pigments (1.00 wt.% “BorchiGel”

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Formulation examples 0434, 20 % in water [Borchers]). The antifoam agent is a tetramethyl dodecinediol (0.30 wt.% “Surfynol” 104 E, 100 % [Air products]) and the levelling agent is a neutralized poly­ acrylate containing carboxyl groups (0.20 “Additol” XW 395, 58 % in water [Allnex]). Besides these additives, a high-boiling solvent (butyl diglycol [butyl carbitol, BASF]) is added to improve levelling. The formulation is presented in Table 9.1. The production of this primer starts with the preparation of an aqueous solution of the epoxy ester by adding deionized water to the already neutralized organic solution. The solution has a solid content of 40 wt.%. The pigments are mixed with a portion of this aqueous solution. The wetting agent and the thickener are added, along with some of the deionized water. The amount of water depends on the planned dispersing equipment. This batch is pre-dispersed in a dissolver, and then dispersed to the desired final extent in a stirrer mill. After the grinding process, the pH value needs to be checked and, if necessary, adjusted to 8.2 by adding N,N-dimethylethanolamine. The resulting mill base is completed by adding the remaining amount of aqueous epoxy ester solution, further amounts of deionized water and the melamine resin. After this, the cosolvent, the levelling agent and the defoamer are added. Depending on the manner of dispersing, it may make sense to add some of the defoamer into the grinding batch. Finally, the pH is adjusted again, if necessary. Regarding the amount of volatile organic compounds (8.72 wt.%), the amount of water (45.21 wt.%) and the average density of paint (1.246 g/cm3, calculated from the individual densities of the components as delivered), the VOC content is calculated as 249 g/l. The VOC value may be lower, depending on the amount of deionized water added to adjust the viscosity to the application state. The primer is applied on pretreated steel panels by spraying, and – after a flash-off time – stoved for 20 minutes at 160 °C. Due to the catalytic effect of the carboxyl groups of the epoxy ester, the HMMM resin crosslinks efficiently, i.e. without additional catalyst. The resulting dry film thickness should be about 30 μm. The film is hard and durable and is distinguished by sufficient flexibility and outstanding adhesion. The primer confers excellent corrosion protection properties. The primer is covered with a topcoat system.

b) Water-borne metallic base coat for automotive OEM The binders selected for a water-borne metallic base coat for an automotive OEM application [22] consist of a combination of a polyurethane dispersion based on aliphatic polyisocyanate (“Bayhydrol” UH 2621 [Covestro], density: 1.125 g/cm3 [calculated on solids, in terms of the density of the components as delivered), 40 wt.% diluted in water, and partially neutralized with 1 wt.% ethyl diisopropanolamine with a highly methylated amino resin containing residual imino groups (“Cymel” 328 [Allnex], density 1.200 g/cm3, 85 wt.% in water).

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Amino resins as hardeners Table 9.2:  Water-borne metallic base coat for an automotive OEM Solids [wt.]

Density [g/cm3]

Formulation [wt.%]

11.220

1.125

28.050

Item

Content

Products

01

PUR dispersion

“Bayhydrol” UH 2621 40% [Covestro]

02

deionized water

deionized water

17.900

03

cosolvent

butyl glycol [BASF]

3.080

04

neutralization agent

DMEA 10 % in water

4.240

10' stirring, 2000 RPM 05

thickener

“Viscalex” HV 30, 30 % w [BASF]

2.620

06

deionized water

deionized water

17.900

10' 2000 U/min. 07

melamine resin

“Cymel” 328 85 % w [Allnex]

08

cosolvent

butyl glycol [BASF]

3.080

deionized water

deionized water

8.790

09

2.805

1.200

3.300

10' stirring, 2000 RPM separate preparation, then added and well stirred in: 10

cosolvent

butyl glycol [BASF]

11

saturated polyester

“Setaqua” E 270w 70 % [Nuplex]

12

wetting agent

“Additol” XL 250 [Allnex]

13

aluminium pigment

“Hydrolan” 2156 [Eckart]

14

neutralization agent

DMEA 100 % [BASF]

Sum

4.624 0.223

3.355

1.120

0.319

1.015

0.481

2.700

5.591 0.025

17.603

100.000

In addition, the base coat contains a small amount of a water-soluble, saturated polyester (“Setaqua” E 270w [Nuplex], density: 1.120 g/cm3, diluted in water (11.4 wt.%) and butyl diglycol (butyl carbitol, 13.3 wt.%), containing N,N-dimethylethanolamine as neutralization agent (5.20 wt.%). The water-borne polyester solution is used for the preparation of metal pigment paste.

366

Formulation examples The aluminium pigment (“Hydrolan” 2156 [Altana/Eckardt], density: 2.700 g/cm3, 60 wt.% in isopropanol) consists of 56 wt.% aluminium of high purity, which is surface coated with a thin silica layer (4.0 wt.%), to protect the aluminium against reaction with water at high pH values. The base coat contains, apart from additional neutralization agent (N,N-dimethylethanolamine), an anionic stabilized acrylic thickener as additive (“Viscalex” HV 30 [BASF], density: 1.100 g/cm3, 30 wt.% in water), and a wetting agent, stabilized by anions and efficient at wetting metal pigments (“Additol” XL 250 [Allnex], density: 1.016 g/cm3, 55 wt.% in isopropanol). Production: The polyurethane dispersion, deionized water, butyl glycol and the additional neutralization agent (items 1 to 4) are mixed and stirred intensively (10 min at 2000 rpm). Then the thickener and further deionized water are added (items 5 and 6) and stirred intensively again. Then the melamine resin, cosolvent and deionized water are added, under intense stirring. The aluminium pigment paste is mixed separately with the aqueous polyester solution, the wetting agent and an amount of neutralization agent; and is then added to the batch described above. Finally the pH value of the base coat is checked, and, if necessary, adjusted to 8.2 with a solution of N,N-dimethylethanolamine (10 wt.% in water). Formulation analysis: The ratio of polyurethane to polyester and melamine resin is 80.7 : 19.3 (calculated on solids). The solid content (binder and pigment) is 17.6 wt.%. The pigment/binder ratio is 0.235 : 1.000. The pigment volume concentration is 9.2 vol.%. The VOC value, as calculated from the amount of solvent (13.28 wt.%), the water content (67.32 wt.%) and the average density of the base coat (1.035 g/cm3), is 453 g/l, which is a relatively high. For application, the base coat is thinned with deionized water to a viscosity of about 125 s. (DIN ISO 2431, flow cup, 4 mm jet, 23 °C). The base coat is sprayed onto panels with primer and primer surfacer, pre-dried for 10 minutes at 80 °C and then recoated with a clear coat for automotive OEM application. The film thickness (dry) of the base coat will be 15 μm and that of the clear coat, 42 μm. Both layers are stoved for 20 minutes at 140 °C. The two-layer system is distinguished by excellent flop and good clear coat hold-out; redissolving of the base coat layer does not occur.

c) Formulations for two water-borne stoving topcoats The following formulations for two water-borne topcoats are both based on a water-dilutable alkyd resin (“WorléeSol” 85 A, 43 wt.% in water [Worlée], density [of solids]: 1.145 g/cm3 [determined from density of delivery form], pre-neutralized) and a reactive melamine resin (“Maprenal” MF 920 [Ineos], density [of solids]: 1.350 g/cm3, 75 wt.% in water). The paints will have a solid content of 45 wt.%. The chosen pigments are titanium dioxide (titanium dioxide RKB-4 [Sachtleben], density: 4.100 g/cm3 and a phthalocyanine

367

Amino resins as hardeners Table 9.3:  Formulations of the two water-borne stoving topcoats Item

Content

Products

Solids

wt.%

70 : 30

Solids

wt.%

80 : 20

01

alkyd resin

“WorléeSol” 85 A (43 % w) [Worlée]

20.94

48.70

23.83

55.42

02

melamine resin

“Maprenal” MF 920 (75 % w) [Ineos]

8.98

11.67

5.96

7.95

03

titanium dioxide RKB-4 [Sachtleben]

10.87

10.87

10.99

10.99

04

PC-blue

“Heliogen” blue 7101 F [BASF]

1.21

1.21

1.22

1.22

05

wetting agent

“Tego Dispers” 750 W (40 % w) [Evonik]

1.90

1.92

06

PUR-thickener

“Tego Visco” Plus 3030 [Evonik]

0.80

0.81

07

defoamer

“Tego Foamex” 830 (100 %) [Evonik]

0.50

0.50

08

levelling agent

“Byk” 307 (100 %) [Altana/Byk]

0.03

0.03

09

neutralization agent

DMEA (10 % w)

1.00

1.00

10

cosolvent

butyl glycol

11

deionized water Sum

42.00

5.00

5.00

18.32

15.16

100.00

42.00

100.00

blue (“Heliogen” Blue 7101 F [BASF], density: 1.610 g/cm3) in the mass ratio of 9 : 1 (nearly identical with colour RAL 5012, light blue). The pigment volume concentration is 12 vol.%. The first topcoat has a mass ratio of alkyd to melamine resin of 70 : 30, while the mass ratio in the second topcoat is 80 : 20. The two formulations have the same mill base. The additives employed are an additive to support wetting and dispersion efficiency (“Tego Dispers” 750 W, 40 wt.% in water [Evonik]), a thickener (“Tego Visco” Plus 3030 [Evonik], to avoid settling of pigments, an antifoam additive (“Tego Foamex” 830, 100 % [Evonik] and a levelling agent (“Byk” 307, 100 % [Altana/Byk]). A small amount of butyl glycol is used as cosolvent; and a solution of N,N-dimethylethanolamine (10 % in water) serves as an additional neutralization additive. Production: The pigments are dispersed in a portion of the pre-neutralized alkyd resin solution, and a portion of the neutralization agent, the wetting agent, the thickener, a

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Formulation examples portion of the defoamer, and a quantity of deionized water are added. The dispersion process is carried out with the aid of a dissolver and a stirrer mill until the given particle size is achieved. This mill base is suitable for both topcoats. For completion, the appropriate amounts of water-borne alkyd and the different amounts of melamine resins are added. Then the levelling agent, the second portion of the defoamer, the cosolvent, the remaining deionized water and a portion of the neutralization agent (N,N-dimethylethanolamine, 10 % in deionized water) are added in the let-down. The neutralization agent is used to adjust the pH values of both topcoats to 8.2. The production recipes are listed in Table 9.4. The viscosities of both topcoats are adjusted to application viscosity with deionized water. The topcoats are then applied by pneumatic spray guns to panels coated with primer and primer surfacer. After a brief flash-off, the panels are stoved for 20 minutes at 140 °C. The resulting film thickness is 45 μm. The topcoat films are highly glossy. The topcoat film containing the higher amounts of melamine resin is harder, has higher gloss values and has good solvent resistance. The film topcoat containing less melamine resin is more flexible and offers somewhat better chemical resistance.

d) Formulation for water-borne primer surfacer [6] The chosen example of a water-borne primer surfacer [23] contains a water-borne polyester, polyurethane dispersion, and melamine resin. Pigments for water-borne primer surfacers are principally the same as for solvent-borne primer surfacers. In the example given, these are titanium dioxide, carbon black, barium sulphate, and talc. The water-borne polyester [24] contains isophthalic acid, a dimer fatty acid (trimer content 18 % by weight), trimellitic anhydride, 1,6-hexanediol, and a special modification comprising an adduct of two moles of epoxy resin (EEW 185 g/mol) and one mole of dimer fatty acid (trimer content less than 2 % by weight). Owing to the content of relatively hydrophobic building blocks, this polyester is relatively resistant to saponification. The characteristics are: average molecular weight Mn 2029 g/mole; acid value 40.9 mg KOH/g; OH-value 101.3 mg KOH/g, pH-value 7.8 to 8.0, viscosity 120“ (DIN 4/20 °C) or 460” (ISO cup 20 °C). The polyurethane dispersion [25] contains a soft segment, which is a polyester prepared by making two moles of 1,6-hexanediol and one mole of neopentyl glycol react with two moles of adipic acid (the acid value is less than 1 mg KOH /g, number average molecular weight is 555 g /mole). The diisocyanate is 4,4’-diisocyanato biscyclohexylmethane (H12 MDI). Water solubility is introduced via dimethylol propionic acid. Chain extension and doping with lateral hydroxyl groups are effected with trimethylol propane (TMP). The reactions are carried out in a ketone process solvent that is water-compatible. The resultant polymer is neutralised with N,N-dimethyl ethanolamine (DMEA) and transferred to aqueous phase. Then the process solvent is distilled off. The solid content is adjusted to 40 %

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Amino resins as hardeners Table 9.4:  Production recipes of the two topcoats containing different amounts of melamine resin Item

Products

Formulation

Pigment paste 01

“WorléeSol” 85 A (43 % w) [Worlée]

44.500

02

titanium dioxide RKB-4 [Sachtleben]

36.000

03

“Heliogen” Blue 7101 F [BASF]

4.000

04

dimethyl ethanolamine (10 % w)

1.800

05

“Tego Dispers” 750 W (40 % w) [Evonik]

6.290

06

“Tego Visco” Plus 3030 [Evonik]

2.640

07

“Tego Foamex” 830 (100 %) [Evonik]

0.800

08

deionized water

3.970

Sum Let down

100.000 (70 : 30)

(80 : 20)

01

pigment paste

30.19

30.53

02

“WorléeSol” 85 A (43 % w) [Worlée]

35.26

41.84

03

“Maprenal” MF 920 75 WA [Ineos]

11.67

7.95

04

“Byk” 307 (100 %) [Altana/Byk]

0.03

0.03

05

“Tego Foamex” 830 (100 %) [Evonik]

0.26

0.26

06

butyl glycol

5.00

5.00

07

deionized water

17.13

13.94

08

dimethyl ethanolamine (10 % w)

0.46

0.45

100.00

100.00

Sum

by weight with deionised water, and the pH value is adjusted to 7.2 by adding small quantities of DMEA. The melamine resin is a HMMM type [26]. Production of the primer surfacer consists in mixing the pigments and extenders (titanium dioxide, carbon black, barium sulphate, and talc) with some of the aforementioned polyester, and adding defoamer, some more DMEA and water. The products are premixed and then dispersed on a sand mill, until the particle size (as measured by the Hegmann method) is less than 12 μm. The let-down portion consists of the residual polyester, polyurethane dispersion, and melamine resin. Some deionised water is used to adjust the solid content. The composition of the water-borne primer surfacer is given in Table 9.5.

370

Formulation examples Table 9.5:  Composition of water-borne primer surfacer Pos.

Content

Products

01

polyester (35 %)

Polyester 1, example of EP 0339433

02

defoamer

“Surfynol” 50 % in ethylene glycol (Air Products)

03

deionised water

04

DMEA

05

titanium dioxide, rutil

nfA Formulation wt-% wt-‰ 6.3

18.0 0.2 4.3 0.1

“Kronos” 2310 (Kronos)

11.2

11.2

06

blanc fixe

“Blanc Fixe” F (Sachtleben)

11.0

11.0

07

talc

Talkum NT (GFR)

1.3

1.3

08

furnace black

Furnace black 101 (Degussa)

0.1

0.1

dispers on ≤12 μm, then add: 09

polyester (35 %)

Polyester 1, example of EP 0339433

2.1

6.0

10

polyurethane dispersion Polyurethane dispersion 1, (40 %) example of EP 0339433

16.0

40.0

11

HMMM resin (100 %)

4.0

12

deionised water Sum

“Cymel” 301 (Cytec)

4.0 4.0

52.0

1000.0

The water-borne primer surfacer has a solid content of 52 % by weight, it contains just 4.8 % by weight of solvent, the pH value is between 7.8 and 8.0, and the viscosity is 120 s (DIN 3511, Ø 4 mm, 20 °C equivalent to ISO cup 460 sec). For spray application, the material is thinned down to 25 s. The primer surfacer is sprayed electrostatically onto electro primed panels. The flash-off conditions are 10 min at 23 °C plus 10 min at 80 °C. The stoving conditions are 20 min at 160 °C. The resultant film thickness is 35 μm. The primer surfacer is then topcoated. Final assessment includes levelling, topcoat holdout, adhesion, and stone chip resistance. Since the primer surfacer formulation contains a sufficient quantity of plasticising building blocks, very good results are achieved in the stone chip tests (VDA method).

371

Amino resins as hardeners

9.7 Conclusion and comparison water-borne to solvent-based stoving enamels Melamine resins can undergo different hydrolysis reactions in aqueous systems depending on their reactivity and the pH value. Under basic conditions HMMM resins are the most stable types. Consequently, HMMM crosslinkers are widely used for water-borne one-component (1K) systems in combination with anionically stabilized resins containing hydroxyl groups. Amines as their neutralization agents must evaporate effectively during stoving. Residual amines decrease the moisture resistance of the coating and cause yellowing. Melamine resins in water-borne paints react by self-crosslinking in addition to co-crosslinking. Depending on the type of melamine resin, the mass ratios of hydroxyl resin and the stoving conditions, the amount of self-crosslinking can be higher than in solvent-borne paints. The resultant film properties reflect this. Just like solvent-borne coatings self-crosslinking promotes hardness and solvent resistance, while co-crosslinking promotes flexibility, chemical resistance and weather resistance. The same rules apply to the mixing ratios of the resins containing hydroxyl and carboxyl groups and melamine resins used in water-borne systems as apply to solvent-borne systems. The ratios can vary from 65 : 35 to 85 : 15. Since mainly the HMMM resins having relatively low molecular masses and high content of functional groups are preferred for water-borne paints, the preferred ratios are lower amounts of melamine resin, e.g. 75 : 25 to 85 : 15 (resin containing hydroxyl groups versus HMMM resin). If the stoving temperatures are below 160 °C, only the hydroxyl groups will react with functional groups of melamine resins. The carboxyl groups will remain in the film and, as they are less hydrophilic than carboxylic anions, they do not significantly influence sensitivity to humidity. Carboxyl groups act as catalysts for the reactions of functional groups of melamine resins. As the content of carboxyl groups in resins for water-borne paints is significantly higher than in resins for solvent-borne systems, the reactions are accelerated to a greater extent. Therefore, HMMM resins crosslink effectively in water-borne systems at much lower temperatures than in solvent-borne systems. In most cases, there is no need to use additional catalysts for crosslinking melamine resins as the usually high content of carboxyl groups is adequate for effective crosslinking. However, if the stoving temperature is low, it is possible to accelerate the reaction rate with catalysts like water-soluble amine salts of compounds containing sulphonic, phosphoric or carboxylic groups. Many additives used in solvent-borne paints are also suitable for water-borne systems. However, the suppliers of additives offer products that are developed and produced spe-

372

References cifically for water-borne paints. Due to their surfactant-like structures, anionically stabilized resins have excellent inherent wetting properties for pigments and substrates. Nevertheless, special wetting agents for water-borne paints are available. Primarily water-borne stoving enamels should contain antifoam additives in order that film forming defects during physical drying and crosslinking may be avoided. Polyether-modified silicone oils act as levelling agents. Other additives for water-borne systems are thickeners for avoiding settling of pigments and sagging after application on vertical surfaces. For water-borne stoving primers, corrosion protection additives may be used.

9.8 References [1] Hönel: Neue Wege in der Anstrichtechnik, Farbe & Lack 59, 174 (1953), Vincentz Network (Hanover) [2] Cooperation of Glidden (Cleveland, Ohio, USA) and Ford Motor Corp. (Detroit, Michigan, USA), A. Gilchrist: Electrodeposition, Metal Prod. Magazine 1964 (February) [3] USP 37 99 854 Method of Electro­ depositing Cationic Comitems. PPG Ind. Inc. (1974), USP 39 22 353 Self Crosslinking Cationic Electrodepositable Comitems. PPG Ind. Inc. (1975) [4] H. Wagner, H. F. Sarx: Lackkunstharze, Chapter II-3.1, C. Hanser München 1971 [5] U. Poth: Synthetische Bindemittel für Beschichtungssysteme, Vincentz Network, Hanover 2016 [6] U. Poth, Automotive Coatings Formulation, Vincentz Network, Hanover 2008 [7] H. van Dijk: The Chemistry and Application of Amino Crosslinking Agents or Aminoplasts, Publisher J. Wiley and Sons, London etc. 1998 [8] D. Stoye, W. Freitag (Editor): Lackharze, Chapter 6.2.3, C. Hanser, Munich; Vienna 1996 [9] H. van Dijk: The Chemistry and Application of Amino Crosslinking Agents or Aminoplasts, Chapter II-7, Publisher J. Wiley and Sons, London etc. 1998 [10] H. Kittel, Lehrbuch der Lacke und Beschichtungen, Volume 2, Chapter 3.1.2.3, Neutralisationsmittel, 2nd Edition Publisher S. Hirzel, 1998

[11] U. Poth, Polyester and Alkyd Resins, Vincentz Network, Hanover 2020] [12] U. Poth, R. Schwalm, M. Schwartz, Acrylic Resins, Vincentz Network, Hanover 2011 [13] O. Lückert: Karsten – Lackrohstofftabellen, Chapter 38.16, Catalysts, 10th Edition, Vincentz Network, Hanover, 1996 [14] Publications of King Industries [15] Technical data sheets of OMG Borchers [16] A. Essenfied, K.-J. Wu, “A New Formaldehyde-Free Etch Resistant Melamine Crosslinker,” in: Proceedings of the 24th Waterborne, High-solids, and Powder Coatings Symposium, New Orleans, LA, February 5–7, 1997, p. 246 [17] Cylink® 2000 [Cytec], CYMEL® NF 2000 [Allnex], Larotact® [BASF AG] [18] Larotact, description in patent WO 2008/022922 [19] Technical Datasheets CYMEL® NF 2000A, CYMEL® NF 2000 [Allnex] [20] K.-J. Wu, A. Essenfeld, F. M. Lee, P. Larkin, Prog. Org. Coat., 2001, 43(1), 167 [21] Formulation model following the suggestions of raw material supplier (Allnex) [22] Formulation model following the suggestions of raw material supplier (Altana-Eckart) [23] EP 0 339 433 (BASF-Coatings), example 1 [24] EP 0 339 433 (BASF-Coatings), polyester 1 [25] EP 0 339 433 (BASF-Coatings), polyurethane 1 [26] Cymel 301, Cytec Surface Specialties (Dyno-Cytec)

373

Legislation on volatile organic compounds

10 REACH and other regulations Jacques Warnon During the last twenty years several important regulations have been implemented, which have had and continue to have a direct impact on the coating sector in Europe. Among these we may indicate the European Directives to reduce the emissions of Volatile Organic Compounds (VOC), the ‘REACH’ Regulation for the control of chemical substances and the European Regulation on biocidal products. Other topics are currently on European or national authorities’ agendas, like the nanomaterials, the emissions of dangerous substances into the indoor air in buildings and sustainable development.

10.1 Legislation on volatile organic compounds The European Directive 1999/13/EC is covering volatile organic compound (VOC) emissions from industrial installations, among others coating application on cars, airplanes, ships, etc. It was incorporated as such into the Industrial Emissions Directive 2010/75/EU (IED) including all types of industrial emissions. A revision of the reference document BREF (best available technology reference) is currently under preparation. The European IPPC bureau has published the final draft, which has still to be endorsed by the European Commission. Although the coating industry was already looking for the reduction of solvent use in its products in the nineteen sixties Directive 1999/13/EC has accelerated the move to more environmentally friendly techniques, including the water-borne technology. For the coating of cars electrocoat primers were already widely used. The regulation has accelerated the use of water-borne primers and water-borne base coats on cars. For clear coats various technologies are still in use including high solid solvent-borne, water-borne and even powder coatings. In the furniture and wood coating industry there was some replacement of solvent-borne coatings by water-borne coatings. In this sector, solvent-free UV-curing technology is also widely used. On the contrary, the coil coating sector has preferred to continue using solvent-borne systems, but their installations are equipped with an incinerator connected to an oven and the solvent fumes are burnt.

Akkerman, Mestach et al.: Resins for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

375

REACH and other regulations The European Directive 2004/42/EC is covering VOC emissions during the application of architectural coatings and vehicle refinishes. The implementation of this legislation has asked major efforts from the coating industry. Resin suppliers have developed new technologies to reduce the VOC content in coatings. New coating generations have been invented in order to maintain optimal performances during the application of the liquid coating as well as decorative and protection properties of the coating film. These new developments represent a financial investment of several hundred million Euros, to cover the research and development costs, as well as the labeling costs with the mandatory mention of the VOC content of the product. Currently close to 90 % of architectural coatings are water-borne. On interior walls and ceilings only water-borne products may be applied. For other paint systems there is a wide replacement of solvent-borne products by their water-borne alternatives although there are still some solvent-borne products used on the most critical substrates or in case of adversary application conditions. This is e.g. the case for primers, for trim and cladding paints for wood and metal and for two pack reactive performance coatings. Directive 2004/42/EC has also severely impacted the vehicle refinishing sector. The primers are solvent-borne or water-borne, the base coats are almost 100 % water-borne and the clear coats are two pack water-borne or high solid solvent-borne.

10.2 Legislation on chemical substances ‘REACH’ and ‘CLP’ The European Regulation ‘REACH’ N° 1907/2006 is covering the registration, the evaluation, the authorization and the restriction of chemical substances as such, as well as in mixtures and in articles. This regulation has an enormous impact on the coating sector. The registration of substances produced or imported in Europe was completed in November 2010 for volumes above 1,000 tons per year, in May 2013 for volumes above 100 tons per year and in May 2018 for volumes above 1 ton per year. The evaluation of registration dossiers by the authorities may modify the conclusions regarding the hazards that these evaluated substances may have on human health and the environment. There is a serious threat that some substances will be either “reclassified” or placed on the lists of substances submitted to authorization or restriction. The Community rolling action plan (CoRAP) update for the years 2020 to 2022 lists 74 substances for evaluation by the Member State competent authorities. Under the raw materials used in coatings that will be evaluated we find carbon black and xylene. Carbon black, a pigment with a wide use in all coating types, is suspected to be a carcinogen and a reprotoxic substance. When xylene would be classified as CMR and sensitizer this would push professional and industrial users to prefer using water-borne or solvent-free technologies.

376

Legislation on chemical substances ‘REACH’ and ‘CLP’ On the list of substances that need to be authorized one finds already chromate pigments and several phthalates. Lead chromate classified as carcinogenic and toxic for reproduction had to be replaced by less hazardous anti-corrosive pigments. Lead sulpho chromate yellow and lead chromate molybdate sulphate red are classified as carcinogenic and toxic for reproduction; they have been both replaced by organic pigments. Strontium chromate well known as an anti-corrosion pigment used on aluminium in the aerospace industry was classified as carcinogenic. It was quite difficult to replace it by less hazardous technologies, thus several enterprises, including some coating manufacturers, have decided to apply for authorization for the further use of strontium chromate in aerospace primer coatings. Phthalates were widely used as plasticizing additives in the coating industry. They are classified as toxic for reproduction and endocrine disruptors, and therefore now non phthalate plasticizers are being used in coatings. The list of candidate substances proposed for authorization includes several critical substances for the coating industry. On this last list, near substances that are carcinogenic, mutagenic and toxic for the reproduction, other groups of substances have been added, including endocrine disruptors and sensitizing substances. The REACH Regulation includes also a restriction list with substances whereof the use is forbidden for some specific applications. For the coating sector it concerns among others cadmium pigments, organotin derivatives and some glycol ethers. Cadmium pigments have been replaced by their organic alternatives. Organotin derivatives have been replaced in antifouling paints mainly by copper compounds. Diethyleneglycol butyl ether (DEGBE) that may cause serious eye irritation is a constituent of architectural water-borne coatings. It shall not be placed on the market for supply to the general public as a constituent of spray paint in concentrations equal or greater than 3 % by weight. Diethyleneglycol monomethyl ether (DEGME) that is suspected of damaging the unborn child shall not be placed on the market for the supply to the general public as a constituent of paints in concentration equal or greater than 0.1 %. For these applications both DEGBE and DEGME have been replaced by other glycol ethers in water-borne coatings. Other substances see some of their coating uses now restricted e.g. polyisocyanate crosslinkers because these products still contain small concentrations of isocyanate monomers that are respiratory sensitizers. Their use is therefore restricted to professionals who will have to be duly trained to use these products safely. Training material is currently being prepared by both isocyanate manufacturers and their downstream users. Among the substances wherefore a new CLP classification has been agreed we find the pigment titanium dioxide suspected to be a carcinogen. Another example is the monomer vinyl acetate also suspected to be a carcinogen. Silicone products like octamethylcyclotetrasiloxane (D4) are used in a wide diversity of coatings as defoamers and flow control additives. This substance is classified as very toxic to aquatic life with long lasting effects and is suspected of damaging fertility. Consumer products and some

377

REACH and other regulations professional products containing D4 in concentrations above 0.1 % shall not be placed on the market.

Biocidal products regulation The European legislation on biocides was revised in 2012 when EU Regulation N° 528/2012 was published. Some products manufactured by the coating industry must obtain an authorization like antifouling products for the protection of ship hulls, wood protection products and façade protection products against algae. Necessary tests on biocidal substances and biocidal products, the preparation of dossiers and the administration of industry consortia are causing enormous costs, which are exceeding one million Euros per dossier. The new regulation on biocides includes also requirements for articles treated with biocides. This has important consequences for water-borne paints that contain in general a biocidal substance in order to protect the paint against bacteria during its storage. There is also a labelling requirement when one claims a biocidal effect of the paint or in case of risks caused by the presence of the biocide in the paint. The coating industry is concerned that most of the biocides that are used to protect paints and paint films against the growth of microorganisms will no more be available. For example, zinc pyrithione is currently used as film and in-can preservative due to the mandatory substitution of other biocidal substances. For antifouling coatings, it is one of the few remaining co-biocides. Now zinc pyrithione has been classified as reprotoxic, which means that it is no longer available for the coating industry. Currently there are only a handful of biocidal substances remaining offering similar fungicidal properties for the same applications, and these substances have their own limitations. Some biocidal substances that are widely used in architectural coatings are now under regulatory pressure. It is the case among others for formaldehyde releasers and for methylisothiazolinone. Formaldehyde releasers may emit formaldehyde, classified as a carcinogen and suspected to be mutagen. Methylisothiazolinone is classified as skin sensitizer. Alternatives will need to be found for their use in consumer paints. There is a concern now that the limited number of biocides left over may cause bacterial resistance and thus insufficient biocidal protection of the paint.

Nanomaterials Substances manufactured in the nanoparticular stage (nanomaterials) are currently registered under REACH together with their bulk countertype. However, there is more a more pressure from EU and national authorities to impose more severe requirements on nanomaterials. REACH annexes have been reviewed in order to include additional data requirements for nanomaterials. Moreover, a European Observatory for nanomaterials has been created by ECHA in order to collect all available data regarding nanomaterials.

378

Conclusions Some EU Member States, including Belgium, Denmark and France, have published regulations requiring manufacturers and users of nanomaterials to submit a registration dossier to the authorities in these countries. This is an additional administrative burden for enterprises placing nanomaterials on the market. These regulations have also an impact on resin and coating manufacturers who must register mixtures containing nanomaterials that they are placing on the market. Moreover, the data to be communicated in the various countries are not always the same. Finally, we do not currently know how the data collected from industry will be used and whether future new regulations on nanomaterials will be elaborated.

Indoor air quality A new emerging topic may impact the coating industry in the near future. Some EU Member States have started to publish national regulations to limit the emissions of volatile organic compounds (VOC) into the ambient air in buildings. These regulations include VOC emission limits for construction materials but also for coatings or adhesives used on interior surfaces in buildings. In France there is an obligation to indicate the level of emissions on the paint can. There are four categories going from A+ (lowest emissions) to C (highest emissions). In Belgium and Germany, it is even more difficult because paints with high VOC emissions may not be placed on the market. Surprisingly these regulations are impacting more the water-borne products than the solvent-borne products. Indeed, substances that are emitted in large quantities during a long period after the paint application are e.g. the coalescing agents used to improve the film formation in water-borne dispersion paints.

10.3 Conclusions The regulations have more and more important impact on the coating sector. To reduce the additional costs for the enterprises global or European regulations should be preferred instead of diverse national regulations. It is the role of European and national professional organizations to monitor these developments and to orientate them in directions that are less damaging for the industry. Enterprises from the coating sector and their suppliers should anticipate future regulations and adapt their formulations as soon as possible. On longer term “sustainable” formulations having a negligible impact on human health and the environment should not only get the preference from users but also be more profitable for the coating manufacturers.

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Outlook

11 Outlook Ingrid Heußen In our high-tech world and in all research work we know the moment of lurking doubt where the question occurs repeatedly: “Did we consider all options?” The experts who have investigated and searched for answers step by step have also experienced the long way of building up empirical knowledge by trial and error or using desktop studies or using open innovation: “What do we need and what did we look for – for input to create new ideas. We follow this idea and design trials, and we analyze and evaluate results and repeat the trials and we find answers and new questions.” This is one part of scientific work. Automation helped us to get more work done in a shorter time and in a more reproducible way. And how did we fill this won time? We read more, write more, meet more and have more administrative targets. This is another part of scientific work. How do we get our best ideas? In this solitary moment where a new thought hits our brain? Or in an inspiring discussion with colleagues or competitors or other experts at a conference? However – it happens and that is another evident for the growing of knowledge by sharing knowledge. Collecting information – summarize the current state of the art – description of the unsolved problems, we must investigate more scientific literature, to consider the current regulatory status. This is also an increasing part of scientific work. The authors bring us into a new dimension of approach and scientific work, because standing on a strong fundament they can dare to try an outlook. An intensive debate about scientific questions and the consistent research on these questions lead us to new results and mindsets. To challenge and to scrutinize a new mindset will let us try to get to the bottom of a problem or phenomenon. That all will strengthen the scientific fundament as the matrix or breeding ground for new mindsets and ideas. The dynamic in today’s development is critical and a paradigm shift in sciences in general. We cannot know it all. Physics and chemistry is not enough. We need to cooperate and collaborate more closely and that means international, intercultural and interdisciplinary. This may open a door to accomplish the transition from solvent to water with the best equipment, best method and finally the best team.

Akkerman, Mestach et al.: Resins for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

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 Overall performance of e.g., architectural paint or vehicle refinish coatings, also requires new measurements to better study the interface phenomenon in the liquid phase and of course during the drying and interdiffusion phase. Hansen solubility parameters can, for example, help to find a combination of coalescing agents to enhance the interdiffusion phase. Performance in “anfeuerung” (wood warmth/wet look) can be enhanced by having the right molecular weight fraction which can migrate into the wood filament. Does this molecular weight fraction need to be from the main polymer backbone? Perhaps it is enough to migrate into the wood filament? Trends like “soft touch” and other haptics, “nanoparticles” or “indoor air quality” are functional aspects of a coating we must fill with scientific live. “Soft touch” can be solved with special additives – what means it can be solved with money. “Nanoparticles” can bring surprisingly new functions into a coating. Properties like flow and leveling and the mechanical strength will be affected by using “nanoparticles”. Unfortunately comes an increasing administrative burden with it and that will make it difficult to place it in the market. Administration and rules can change but ideas are powerful leaders. Indoor air quality will have an impact on coatings, but we should also check the plausibility of such emerging topics. If we focus on functions, we need to consider that we sacrifice some other properties on behalf of the required function. We want it all, but do we need it all? What about “nature inspired” solutions – they are not always healthy as chemists know – but in a multi-disciplinary and cross-competence team this thought can bring us to more sustainable polymers. Albert Einstein said: “Fantasy is more important than knowledge, because knowledge is limited.” Let us dare to follow our fantasy. Looking into the additive portfolio, what do we really know about additives their site of action and their surface activity in the wet and dry film of a coating. Looking into the anti-fouling coatings, e.g., barnacles do not stick to a greasy surface. Let us think about an inbuild incompatibility of polymers that emerges during the drying stage of the applied coating. Looking into water based anti-corrosion coatings, they are up to par in performance, perhaps more trainings for decision-makers, experts and craftspeople can support the transition to water-based products in every market segment, which is seen as conservative. Other accompanying measures is e.g., to enforce more simple rules to use the material with the best environmental impact, like lowest or zero emission, most sustainable, easy to use, easy to produce and easy to recycle, will support the transition too. What can we expect from the single polymer groups in the future? And what do we expect e.g., from polyurethanes in the future? The best thing we can get from PUD is its flexibility, even if the coating is crosslinked. We can design a non-isocyanate polyurethane dispersion (NISO-PUD) with an inbuilt hy-

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 drolysis/polycondensation mechanism to crosslink it based on well-known alkoxysilanes. Price and availability may be an obstacle on the way to new products, and review of the raw material production process needs to be done. This very interesting transition stage of alkyd emulsions does not show a defined interface between the status of an aqueous dispersed polymer particle or a real solvent-borne solution. What can we learn from this transition stage? Is it useful for other compounds? The carbon footprint represents a part of a life cycle assessment as an impact of global warming potential. The concept goes back to the 1990s and is looking into a single resource and has less focus on supply chains. Today we have more mature concepts like an eco-balance/LCA, as a very complex concept for experts only and even the interpretation of an LCA is anything but trivial. And we have more to offer, more concepts like “sustainability” and “resource efficiency” or “energy efficiency” and not to forget the “green deal” in Europe. What will lead us, or guide us, or steer us in the right direction? Should we follow the trend with the highest political relevance, or the most elaborated standardized method? If it is ISO certified, we can measure and collect data. Perfect. But will it give us a new direction in product development? Strongly influenced the development and with this equation we start to classify processes of any kind – both the production of raw materials and our consumer behavior and express them in carbon consumption. This equation will be improved over the years and with new insights of stratospheric ozone depletion and the impact from carbon-NOx or other more and more elaborated interrelations, we will one day forget about this equation. We possibly can achieve this by independent and cooperative research. If there are more open questions than answers, curious scientists will go on in research and development. Either with smaller or huge steps, either with incremental improvements or with one giant step. We create knowledge and need to share it to get more input. In the polarity between scientific curiosity and open questions reclines the power to find answers for future challenges. Let’s go for it.

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Authors

Authors Dr Jaap Akkerman studied organic chemistry at the University of Amsterdam and obtained his PhD on the Asymmetric Synthesis of Vitamine E in 1982. He started in 1980 as a resin/ polymer chemist at Sikkens BV (later Akzo Nobel) in Sassenheim, the Netherlands. For five years he did innovative research on completely new two component systems. Subsequently, he became manager of the lab for Decorative Waterborne Coatings. Akkerman became R&D Manager of the Innovative Lab for Decorative Coatings and founded and managed the Technology Center for Pigments and Pigment Dispersing Agents. Simultaneously, he founded and managed a new Technology Center Effect Coatings that did research on new technology as well as new aesthetics in the Esthetic Research Center. In 1998 he changed to Akzo Nobel Resins (later Nuplex/Allnex Resins) in Bergen op Zoom, the Netherlands, to become R&D Manager Surface Coating Resins Europe to develop high solids polymers of low to high molecular weight, as well as new water-borne dispersion resins. After six years he became Technical Development Manager, taking care of implementation of new innovations & technologies, new business and new tools for management. Before retirement in 2016, Jaap was Principle Chemist, continuing special innovative projects on waterborne coatings, research co-operations and guiding young new chemists into the world and treasures of coating and resin chemistry. Since 2011 Jaap is board member of the NVVT (the Dutch Paint Technician Society) and took part in the organization of the ETCC conference in 2017 in Amsterdam. He published over 60 papers in coatings literature, presented a similar amount of papers and won three prestigious prizes for best presentation of congress. Dr.ir. Toine Biemans obtained his PhD (1997) in macromolecular and organic Chemistry with Prof. Bert Meijer from the University of Technology Eindhoven, the Netherlands, which was followed by a postdoctoral stay in Durham, UK with Prof. Jim Feast. He started his professional career when he joined DSM Coating Resins in Zwolle, the Netherlands. In 2004 he joined Worlée Chemie in Lauenburg, Germany where he is Head of R&D. At Worlée he focuses on innovations in a variety of resin chemistries with an emphasis on the use of renewable raw materials and the development of more environmentally friendly and sustainable binders. Dr Cathrin Corten studied Chemistry at the University of Dresden, Germany until 2005 and received her PhD in Polymer Science from the same in 2008. From 2008 to 2010 she worked as a Research Associate at the School of Polymer Science & Engineering, University of Southern Mississippi USA. Since 2010 she started at BASF Coatings GmbH, Germany in various positions dealing with synthesis for polymers for coatings applications. Since 2018 she is the Head of Development for Coatings Binder Systems for OEM and Refinish Coatings.

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Authors Dr Rudolf Hager, born in 1961, studied Chemistry at Technical University Munich where he graduated in 1987 and received the PhD in 1990 with a thesis about hydrogen-rich silanes. He started his professional career in 1990 at Wacker Chemie AG, Burghausen in Germany, where he had different management functions in silicone chemistry. In 2013 he became director of an international business team dealing with silicones in coatings and construction chemicals. Ingrid Heußen obtained her diploma in “Industrial Organic Chemistry” at the HS Niederrhein in Krefeld, Germany. In 1987 she joined Alberdingk Boley GmbH as Head of R&D of polyurethane dispersions and was responsible for product development in the field of water-based polymer dispersions for various market segments. Later positions at Ercros and Evonik brought her to various R&D-related management levels. After 5 years as a global Senior Vice President in the BONA management team in Sweden, she is now responsible for innovative networking within and outside BONA. She continues to be involved with the future challenges of water-based polymer dispersions. Dr Claas Hövelmann, born 1978 in Germany, studied chemistry at the universities of Duisburg and Bonn. In 2008 he finished his PhD Thesis “Oxidative Intramolecular Diamination of Alkenes” under the guidance of Prof. K. Muniz at the Université Louis Pasteur, Strasbourg, France. After a postdoctoral stay at Scripps Research he worked at the Helmholtz Research center in Jülich in polymers science before joining BASF Coatings in 2017 where he is currently developing polymeric binders for coating applications Dr Joachim Krakehl, born in 1971, studied organometallic chemistry at the University of Kaiserslautern, Germany. He started his professional career at Nanogate AG as Technical Product Manager for sol-gel-coatings based on silane-based nanocomposites. Since 2004 he is employed at Wöllner GmbH, Ludwigshafen in Germany, and is now responsible for the Technical Marketing of soluble silicate-based systems, mainly for applications like construction, paints and coatings, adhesives, industrial silicates, and specialties. Dr Martin Leute, studied mineralogy at the University of Vienna in Austria, and technical chemistry at the Technical University of Vienna. From 2002 to 2005 he was employed by Thermax Brandschutzbauteile GmbH in Amstetten, Germany as Head of Research and Development. Since 2005, he worked for Wöllner GmbH and Wöllner Austria GmbH as a sales manager for the business unit “Coating and Construction Chemicals (CCC)”. He brings broad experience from the chemical industry with a focus on mineral coatings, binders and adhesives. Dr Dirk Mestach, born in 1961, studied organic chemistry at the State University of Ghent (Belgium). After obtaining a Ph.D. in polymer chemistry, he started working for Akzo Coatings in Vilvoorde, Belgium as a project manager at the resin development laboratory. In

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Authors 1993 he moved to the Resins business unit of AkzoNobel in Bergen op Zoom, The Netherlands. He worked as product development manager mainly in the field of water-borne polymer dispersions for the coatings and the printing ink industry. In 2004 he assumed the role of Research and Development Manager for the Wood, Decorative and Construction Resins business (Europe, Middle East and Africa) with Nuplex Resins. After the merger of Nuplex with Allnex, in 2016, he became Synthesis Manager for the liquid resins research and development lab in the Netherlands. Dr Oliver Seewald, born in 1973: After his doctoral thesis in “Inorganic and Analytical Chemistry” in 2001 he became in 2005 a member of the working group “Coatings, Materials & Polymers” of Prof. Bremser in the Institute for Lightweight Design with Hybrid Systems at Paderborn University, Germany. As an academic senior council, he deals as an assistant professor. The bachelor students of chemistry and materials science are taught in “Chemistry and Technology of Coating Systems”. Research topics are synthesis of polymers for coatings, adhesives or as matrix systems for reinforced plastics. B.Sc. Adrian Thomas was born in England and graduated with B.Sc. (Hons) from the University of Southampton, England, in 1969 prior to transferring to Australia. There he has had extensive experience with a number of multinational companies (Unilever, Shell and Wacker Chemicals) in a range of manufacturing, sales and marketing positions at management levels. He is the Director and CEO of Chemicalia Pty Ltd, a company involved in consulting in areas including technical consulting and regulatory affairs as well as trading in a range of specialty raw material chemicals for the surface coating industry. Dr Jacques Warnon obtained his PhD in polymer chemistry with Prof. Georges Smets from the University of Louvain, Belgium. He started his professional career as research chemist in the coating industry. In 1988, he joined AkzoNobel as R&D Director of the Business Unit Resins in Bergen op Zoom, the Netherlands. In 1999, he became Technical Director at the European Council of the Paint, Printing Ink and Artist’s colours Industry (CEPE) in Brussels. Since 2013, he is managing director of Warnon Consultancy SCS, providing support to companies from the chemical sector in order to help them to comply with national and European regulations related to human health and environmental issues.

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Index

Index

Symbols [(2-oxiranylmethoxy)methyl]-furan (BOF)  211 2,2-bis(hydroxymethyl)propionic acid  216 2,5-bis 2,2-azobis 2-methyl-butyronitrile, initiator (AMBN)  79 29Si-NMR spectroscopy  323 2-dimethylamino-ethanol 253 2-ethyl hexyl acrylate (EHA)  106 2K silicate paints  333 3,5-dimethyl pyrazole  295

A absorbents 341 acetoacetyl crosslinking  72 acetone 223 acids 330 acid base equilibrium  324 acid catalysts, water-borne stoving enamels 360 acoustic 342 acrylic  48, 59, 115, 150 acrylic dispersion  115, 126, 230 acrylic emulsion resins, ambient curing  260 acrylic emulsion resins, two-component  260 acrylic polyol  259 acrylic resin  230 acrylic resins, with melamine resins  358 addition polymerization  46 additive 307 additives, for stoving enamels  361 adhesion  251, 281, 287 adhesive  197, 341 adsorbed water  329 alkali carrier  341 alkali-swellable dispersion  65 alkyd acrylic hybrid dispersions  77 alkyd resins, with melamine resins  358 alkyds, externally emulsified  177 alkyds, internally emulsified  191 alkyds, water-borne  171, 172, 173, 175, 176, 180, 181, 189 alkyds, water-soluble  171 aluminium phosphates  331 aluminium sulphate  332 amine  174, 193, 201, 257

amine, latent  203 amino ethylene sulphonic acid  258 amino formaldehyde  293 amino resins, combination partners  353 amino resins, crosslinking reactions  354 amino resins, hardeners  347 amino resins, hydrolysis  356 amino resins, for stoving enamels  356 anfeuerung  126, 131 anhydride 201 anhydrous 328 anionic  42, 172, 205, 208 anionic dispersion  206 anionic modification  173, 180, 192 anti-corrosion 147 anti-corrosion paints  343 anti-corrosion primer  248 anti-soiling effect  339 application window  261 application, hydrophobicity  241 atom transfer radical polymerization (ATRP)  86, 88 atomic force microscopy (AFM)  83 aziridine 70 aziridine, crosslinking  70 azo-compounds 47

B Bancroft 181 base coat  230 base metal  331 Bayer, Otto   215 beading 308 benefits  334 benzoguanamine 347 bi-liquid foam  188 binder 304 bio-based  60, 210, 227 bishydrazides 223 bisphenol A (BPA)  197, 198 bisphenol A replacement  210, 211 bisphenol F (BPF)  199, 211 bisphenol H (BPH)  199 bleeding  134, 162, 165 blocked isocyanates  209 blocked polyisocyanates  293 blocking resistance  235 Blue Angel, eco-label  340 boost  49, 57

389

Index brush drag  99 brushing  116, 126 butadiene 159 butanon oxime  295 butyl acrylate (n-BA)  106 butylene diamine  223

C calcium chloride  332 calcium hydroxide  332 capillary water up-take, W-value  148 caprolactam 295 carbamate 347 carbamic acid  257 carbodiimide  59, 62, 69 carbodiimide crosslinking  69 carboxylated oligomers  62 carboxylic ester  331 cardboard tubes  341 castor oil  227 catalyst  201, 257, 264 catalyst, latent  203 catastrophic phase inversion  182 cationic  42, 205 cationic deposition paints  208 cationic dispersion  161, 163, 206 ceramic tiles  113 chain extender  222, 225 chain extension  225 chain transfer agent  47 checklist, paint formulation with dispersion 100 chemical resistance  198, 257, 266, 269 clarity test  98 clear coat  131, 325, 270, 272, 273 click characteristic  203 coagulates 58 coagulation aid  342 coagulation test  98 coalescence, alkyd  186 coalescing aids, coalescing agents or co-solvents 95 coalescing aids, co-solvents  95 coatings 197 coating, direct to metal  241 co-binder, water-borne  243 coffee and hand-cream resistance  123, 126 colloidal dispersions  216 colloidal silica  341

390

colour  113, 132, 137 coloured sand  345 composition drift  49, 55 concrete coatings  106, 110, 159, 344 condensed water  329 constituted water  329 contrast ratio  114 controlled radical emulsion polymerization  85 copolymerization 252 core-shell dispersion  65, 210 corrosion application  177 corrosion protection  208 co-solvent  230, 260 co-solvents, alkyd resins  176 co-solvents, for stoving enamels  359 crack bridging  159 cratering 263 creep 198 critical micelle concentration (CMC)  51 critical parameters emulsion polymerization  58 critical pigment volume concentration (CPVC)  101, 102, 134 crosslink density  68, 92, 199, 206, 258 crosslinked 251 crosslinker  243, 261 crosslinking reactions, epoxy resins  200, 203 crystal seed  328 curing sequence  201 curing, epoxy resin  200 cyclohexyl dimethanol  221 cyclohexylene diamine  223

D Daniel’s flow point  103 daylight curing  139 deco/DIY  115, 126 decorative coating  328 deflocculant  344 dehydration 329 design of experiment (DoE)  119, 126 diacetone acrylamide (DAAM)  73 diamine 218 diethylene triamine  223 differential scanning calorimetry (DSC)  90 dihydrazide or bishydrazide, crosslinking  73 diisocyanate  173, 193, 224 diisopropyl amine  295 dimethyl butanoic acid  222 dimethyl ethanolamine  261

Index dimethylol cyclohexane  255, 285 dimethylol propionic acid (DMPA)  173, 192 DIN EN 18363  335 DIN EN 13300  335 dip coatings  286 dipropylene glycol methyl ether (DPM)  95 dipropylene glycol n-butyl ether (DPnB)  95 dirt pick-up  337 dispersing agent  263 dispersion, acrylic  171, 177 dispersion, polyurethane  192 disproportionation 47 dosing strategies  49 dual curing  203 DVLO theory  44 dynamic mechanical thermal analysis (DMTA)  68, 90

E eco-friendly 334 eco-labels 57 efflorescence  328 elastic modulus  92 electrical double layer  43 electrical insulation  198 electrolysis 209 electrolytic corrosion  342 electrophoretic dip coating  207 electrostatic repulsion  45 electrostatic surface potential  44 elongation at break  139 emulsification, epoxies  203 emulsification technology  186 emulsification, indirect  254 emulsifier  51, 60, 186 emulsifier, alkyds  180 emulsifier, grafted  42 emulsifier-free  247 emulsion polymerization mechanism  50 emulsion polymerization  41, 207 emulsion polymerization, critical parameters 58 emulsion, stable  251 epichlorohydrin  197, 198 epoxidized linseed oil  211 epoxidized natural rubber  211 epoxidized olefins  198 epoxidized soybean oil  211 epoxy acrylate hybrids  209

epoxy dispersions  203 epoxy resin  59, 197 epoxy resin, application systems  207 epoxy resin, bio-based alternatives  211 epoxy resin, bio-based  211 epoxy resin, catalysts  201 epoxy resin, curing mechanism  200 epoxy resin, curing  197 epoxy resin, deposition  209 epoxy resin, range of applications  197 epoxy resin, trends  209 epoxy urethane acrylate  210 epoxy-acrylate hybrid  206 epoxy-alkyd 210 equivalent weight  258, 259 ethanolamine 255 ethylene 139 ethylene diamine  223 ethylene vinyl acetate (EVA) copolymers  153 ethylenically unsaturated monomer  226 EU regulations  326 evaporation 329 exotherm reaction  55 experimental design  122 exterior  158, 245 exterior durable  139, 150, 158, 159 exterior wall paint  137

F fatty acid  211, 227, 243 fatty acid, crosslinking dispersion  74 film formation or coalescence  93, 173 flooring  234 flow  99 flow diagram  118 foam 265 formaldehyde 226 formulation composition maps  181 free radical (co)polymerization mechanism  46 freeze/thaw resistance  260 functional groups  206 functional monomers  59 furnace process  324 furniture coating  115, 119, 124, 139, 235, 278, 281

391

Index

G gas permeability  313 gel formation  329 gelation 330 glass transition temperature (Tg)  89, 315 global market  26 global value of paints and resins  26 glycidyl ether  198 glycoluril 347 glyptal 21 gradient morphology  65 graft copolymer dispersion  88 grafting 206 grain raising  151 grit or coagulates  58

H hard gel grouting  343 hardener 207 hardness 281 hardness development  257 health risks  210 heat of polymerization  54 hetero-polyvalent metals  333 HEUR thickener  136 hexamethoxymethyl melamine resin (HMMM resin)  352 hexamethylene diamine  223 hexamethylene diisocyanate  219 high shear  118 high solids  243 high speed mixing  257 high throughput experimentation (HiTE)  101, 119, 126 homo-polymers 47 hot melt process  225 hybridization 206 hydrated silica  330 hydrazine hydrate  222 hydrodynamic volume  230 hydrogen bonding  175, 229 hydrogen gas  331 hydrogen peroxide  341 hydrogenated p-methylene diphenyl diisocyanate 219 hydrolysis 285 hydrolytic stability  254 hydroperoxide 226

392

hydrophilic lipophilic balance (HLB)  180 hydrophilic lypophilic balance (HLB)-theory 180 hydrophobic 307 hydro-plasticization, dispersion  92 hydrothermal process  324 hydroxyl-functional monomers  59 hydroxyl-functional 251

I impact resistance  266 impregnation  148, 160 incompatibility test  98 industrial  115, 126, 139, 141 industrial coatings  34 industrial wood coatings  33, 234 initiation  47, 54 initiator  42, 63 injection grouting  343 interior 245 interior wall paint  106, 113 ionic group  206 isobornyl methacrylate (IBOMA)  60 isophorone diamine  223 isophorone diisocyanate (IPDI)  193, 219

J joinery  131, 151, 230, 234

K ketimine  209, 226 ketimine/ketazine process  223, 226 keto-hydrazide  115, 139 ketone crosslinking dispersion  73

L lambda (λ) 107 lapping 99 lauryl methacrylate  76 layer thickness  261 life cycle analysis  25 lignin 211 linolenyl acrylate  74 lithium silicates  322 loop-reactors 57 low VOC  177

Index

M magnesium chloride  332 magnesium sulphate  332 maleinate 159 Mannich 209 mar resistance  113, 235 masonry 116 melamine  286, 347 melamine crosslinker, combination resin  357 melamine resin  350 melamine resin, formaldehyde-free  362 melamine resin, highly etherified with residual NH groups  352 melamine resin, highly methylolated fully etherified  352 melamine resin, partially etherified containing free methylol groups  351 melamine, curing  177 metal  147, 331 metal ion crosslinking  71 metal ions  331 metal primer  105 metallic base coat  248 methacrylic acid, copolymer  59 methacrylic  59, 115 methoxypolyethylene oxide (meth)acrylate  252 methyl ethyl ketone  223 methyl methacrylate (MMA)  110 methylene diphenyl diisocyanate (MDI)  219 methylol compounds  349 micellar mechanism  50 micelles 51 micro-foam  134, 265 milk of lime  344 mineral board  342 mineral substrate  113 mini-emulsion polymerization  75, 79 minimum film formation temperature (MFFT or MFT)  94 molar ratio  322 molecular weight  172, 251 molecular weight, epoxy resin  198 monomer  47, 59 monomer addition  56 monomer limits  57 monomer starved conditions  56 monomer, aqueous phase  51 morphology control  49, 65

morphology  65, 89, 115 m-tetramethylxylene diisocyanate (m-TMXDI) 219

N N,N-diisopropyl-N-ethylamine 221 natural feedstocks  211 N-butyl pyrrolidone  (NBP) 218 neodecanoic acid  255 neopentyl glycol  221 neutralization agents  358 neutralizing of ammonium groups  209 newtonian viscosity  325 nitro-cellulose resins  234 nitroxide-mediated living radical, polymerization 86 N-methyl-2-pyrolidone  (NMP) 217 non-aqueous dispersion (NAD)  81 non-bleed 141 non-ionic alkyds  175, 177 non-ionic modification  180, 192 non-ionic  42, 205, 218 nucleation, micellar  52

O octyltriethoxysilane 301 OEM coatings  33 oil adsorption (OA)  102 oleum silicium  321 oligomer 53 oligomer structure  258 oligomers as surfactants  62 opaque 119 opaque primer  134 opaque wood trim  116 open time  99, 116, 151, 227, 243 organic initiators  64 organic solvent  252 organo-silicate paints  335 outdoor durability  99, 110, 126 oxidatively drying  227 oxirane 198

P packaging coatings  34 parquet 230 particle size  187 particle size, of a pigment dispersion  98

393

Index particle size, of dispersion  89 pentamethylene diisocyanate  235 peroxide 47 persulphate  47, 64, 226 persulphate, water-soluble initiator  51, 64 pH adjuster  310 phase inversion  187, 188, 203, 254 phase inversion temperature (PIT)  181 photosensitive 199 pigment 230 pigment volume concentration (PVC)  100, 102, 313 pigment wettability  110 pigmentation, for stoving enamels  361 pin-holing 263 plaster  155, 159 plastic  284, 287 plastic coatings  34 plastic components  245 plasticizer 95 polar monomer  131 poly vinyl acetetate (co)polymers (pVAc)  152 polyacrylic acid  42 polyamine 62 polybutadiene diol  227 polycarbonate  216, 245 polycondensation 172 polydimethylsiloxane 307 polyester 255 polyester emulsion polymer  285 polyester polyol  227, 254, 259 polyester-diol 216 polyether 216 polyethylene 287 polyethylene glycol (PEG)  175, 176, 177, 180, 185, 192, 254 polyethylene oxide  42 polyisocyanate  215, 251 polyisocyanate, blocked  293 polyisocyanate, crosslinkers  251 polyisocyanate, hydrophilic  257 polyisocyanate, hydrophobic  257 polyketimines 209 polymer dispersion  315 polymer property  199 polymer science  21 polymer stabilized dispersions  63 polyol  251, 259 polyoxyalkylene 218 polypropylene glycol (PPG)  175, 185

394

polypropylene 287 polysaccharide 176 polyurethane  229, 256 polyurethane dispersion  215 polyurethane, film formation  255 polyvinyl acetate  58 polyvinyl alcohol  42, 62 polyvinyl pyrrolidone  42, 62 popping 257 potassium silicates  322 pot-life  240, 252, 261 pot-life, epoxy resins  201 pour test  97 powder coating  199 precipitated silica  340 prepolymer 218 prepolymer mixing process  224 pre-treatment 208 primer  106, 139, 141, 148, 199, 306 primer for MDF  136 primer surfacer  287, 292, 293 primer, adhesion  250 primer, salt-spray results  250 propagation  47, 54 propylene diamine  223 protective coatings  34 protective colloid  42, 62 protolysis 330 pseudoplastic rheology  99 pseudoplastic  115, 151, 230 PUD 192 PUD, acrylic hydroxyl-functional  245 pulp and paper  341 PU-macro-monomer 84 pure acrylic dispersion  115 pure silicate paints  335 pVAE 153 PVC  101, 102 PVC ladder  104 PVC range  106

Q Q structure  323 quartz sand  327 quaternary ammonium  141 QUV-testing 131

Index

R rate constant  47 reactive emulsifier  61 reactivity ratio  47 reactors for emulsion polymerization  57 recoatability 151 recombination 46 redox system  64 refinish clear coat  263 regulations 375 relative humidity  151 residual monomers  49 resin, viscosity  172 resistance properties  279 resistance, chemical  198, 257, 266, 269 resistance, coffee  123, 126 resistance, hand-cream  123, 126 reversible addition fragmentation chain transfer (RAFT)  86, 87 reversible addition fragmentation chain transfer (RAFT), agents  87 reversible addition fragmentation chain transfer (RAFT), polymerization  86 rheology of polymer dispersions  99

S safety data sheet (SDS)  102 Salager  181, 187 salt-spray resistance  251 saponification  106, 110, 113, 172 saturated polyesters, water-borne paints with melamine  358 scrub 317 sd value  148, 313 sealer 139 secondary emulsion  42, 260 seeding  55, 207 self-crosslinkable oligomer  63 self-crosslinking  61, 63, 115, 116, 136, 151, 235 self-emulsifying resins  205 setting agent  340 shape-controlled dispersions  65 Shinoda 181 shrinkage, during curing  197 silane  113, 139, 148, 301 silane, crosslinking dispersion  74 silicate emulsion paints  334

silicate 159 siliconate 301 silicone  148, 299 silicone microemulsions  307 silicone resin emulsion paint  311 silicone resin emulsion  302 silicone resin  300 silicone, structural elements  323 siloxane  113, 148, 301 siloxane-modified  210 sodio sulpho isophthalic  222 sodium carbonate  327 sodium silicate, fusion  327 sodium silicates  322 soft touch coating  247, 286 softgel 343 sol silicate paints  335 soluble silicates  321 solvent 260 solvent-borne 260 spray application  115 spreading rate  114 stability test  98 stabilization mechanism, dispersion  99 stackability 151 stain blocking  162 statistical design  131 steric stabilization  45 stone chip  230 styrene 47 styrene butadiene  50 styrene butadiene rubber (SBR)  159 styrene copolymer dispersions  106 styrene for rubber  50 surface defect  257 surfactant affinity difference (SAD)  181, 187 surfactant  94, 177, 180, 182, 223, 254

T tacit know-how  100 tannin  113, 134, 141, 162 technical data-sheet (TDS)  101, 102 telegraphing  99, 141, 152 teletronic coatings  248 termination  46, 54 tertiary amine  252, 257 tetramethyl bisphenol F (TMBPF)  211 Texanol 95 Tg calculation  90

395

Index Tg glass transition temperature  67, 92 Tg values  90 thermoplastic 241 thiol epoxy  202, 203 thiol-ene 203 tile coating  106, 159 titanium dioxide (TiO2)  126, 341 toluene diisocyanate  (TDI) 219 topcoat 199 toxicity 211 transitional phase inversion  182 tri-ethanolamine 261 triethylamine  174, 193, 221 triethylene tetramine  223 trim paints  33 trimellitic anhydride  254 trimethylol propane  221, 255 trisalkoxy carbamato triazine (TACT)  362 Trommsdorff-effect  55 true glasses  322 two-component polyurethane coatings  251 two-component system  235, 261

U urea  257, 347 uretdione 258 urethane  251, 257, 357 urethane linkages  173 urethane-acrylic hybrid dispersions  82 UV-crosslinking 227 UV-curing 235

V vacuum 329 vehicle refinish  33, 248, 258, 260 vehicle refinish coatings  33 vermiculite 342 versatate esters  153 versatile plastics  197 vinyl acetate ethylene (VEA)  158 vinyl acetate  139, 152 vinyl-trimethyl silane  74 viscosity  172, 182, 189, 194 viscosity, alkyd emulsion  186 viscosity, alkyds  175, 176, 178, 181, 189 VOC limits  101 volatile organic compounds (VOC)  37, 171, 174, 203

396

volumes of deco/DIY paints, Europe  28 volumes of industrial and OEM paints, Europe 29

W w24 value  317 wall paints  31 water absorption coefficient  313 water absorption  313 water glass  321 water uptake, immersion  110 water vapor diffusion resistance  110 water vapour diffusion sd-value 148 water-borne 230 water-borne coating resins, market  25 water-borne metallic base coat for automotive OEM  365 water-borne paints, definitions  35 water-borne primer surfacers  369 water-borne resin, definition  19, 37 water-borne resin, development  23 water-borne resin, history  19 water-borne resin, market  19 water-borne stoving primer, general industrial use  363 water-borne stoving topcoat  367 water-borne, epoxies  203 water-miscible 253 water-soluble initiator  51 wed-edge 227 weight ratio  322 welding rod  345 wet scrub resistance  113 wood care oils  177 wood coating  113, 241 wood finish  266 wood-knots 162 W-value 148

Y yellowing 199

Z zero VOC  113, 155, 158 zinc oxide  141 zinc silicate  329 zinc sulphate  332 zirconium, crosslinking  71