Additives for Water-borne Coatings [vollständig überarbeitete Auflage] 9783748604884

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Wernfried Heilen Adalbert Braig | Anne Drewer | Patrick Glöckner Roman Grabbe | Juergen Kirchner | Frank Kleinsteinberg Benoît Magny | Thomas Matten | Ingrid Meier Kirstin Schulz | Heike Semmler | Jean-Marc Suau

Additives for Water-borne Coatings 2nd Revised Edition

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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.

Wernfried Heilen et al. Additives for Water-borne Coatings, 2nd Revised Edition Hanover: Vincentz Network, 2021 European Coatings Library ISBN 3-7486- 0486-6 ISBN 978-3-7486-0486-0 © 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: Qubus media GmbH, Hanover

European Coatings Library

Wernfried Heilen Adalbert Braig | Anne Drewer | Patrick Glöckner Roman Grabbe | Juergen Kirchner | Frank Kleinsteinberg Benoît Magny | Thomas Matten | Ingrid Meier Kirstin Schulz | Heike Semmler | Jean-Marc Suau

Additives for Water-borne Coatings 2nd Revised Edition

Foreword Since the publication of the 1st edition of this book almost ten years ago, some areas of paint applications have seen further technological developments that have driven significant advances in the coatings industry. In general, the use of water-borne coatings worldwide has increased by 13 % over the last six years. 92 % are used as architectural coatings, while 8 % find application in industrial coatings. The annual growth rate is expected to be 2.5 to 2.8 % for the next 3 years 1. Additives help to protect the environment by effectively reducing the use of organic solvents. Many of today’s water-borne coatings could not be formulated without them. Additives are often used in very small quantities, usually in proportions of less than one percent of the total formulation. When incorporated into water-borne paints, coatings and printing inks, they enhance both the production process and the performance of the applied inks and coating materials. The present book (2nd edition) is intended to provide an updated, state-of-the-art overview of the chemistry and technology of additives even unique solid products and co-binders for water-borne systems and their use from the industrial viewpoint. The information it contains will be useful for understanding the chemistry and the action of the newly developed additives in water-borne systems. Experienced coating specialists, too, will benefit from finding out about the latest developments in the industry. Ideally, this book will make paint chemists’ work easier and provide invaluable suggestions for meeting the challenges of formulating modern and environmentally-friendly coatings that will present themselves in the future. Many thanks to my colleagues for their assistance in bringing this book to fruition and to Evonik Operations GmbH for supplying literature and illustrative materials. Bad Grönenbach, Germany, September 2021, Wernfried Heilen

1

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Contents

Contents 1 Introduction 1.1 Literature

17 18

2 Wetting and dispersing additives 2.1 Modes of action 2.1.1 Pigment wetting 2.1.2 Grinding 2.1.3 Stabilisation 2.1.4 Influences on formulation 2.2 Chemical structures 2.2.1 Polyacrylate salts 2.2.2 Polyphosphates 2.2.3 Fatty acid and fatty alcohol derivatives 2.2.4 Acrylic copolymers 2.2.5 Maleic anhydride copolymers 2.2.6 Alkyl phenol ethoxylates 2.2.7 Alkyl phenol ethoxylate replacements 2.3 Wetting and dispersing additives in different market segments 2.3.1 Architectural coatings 2.3.2 Wood and furniture coatings 2.3.3 Industrial coatings 2.3.4 Printing inks 2.4 Tips and tricks 2.5 Test methods 2.5.1 Particle size 2.5.2 Colour strength 2.5.3 Rub-out 2.5.4 Viscosity 2.5.5 Zeta potential 2.6 Summary 2.7 Literature

19 19 19 21 21 24 27 27 27 28 28 29 29 29 30 30 31 31 32 32 33 33 33 34 34 35 37 38

3 Defoaming of coating systems 3.1 Defoaming mechanisms 3.1.1 Foam 3.2 Defoamers 3.2.1 Composition of defoamers

39 39 40 42 42 9

Contents 3.2.2 Defoaming mechanisms 3.3 Chemistry and formulation of defoamers 3.3.1 Active ingredients in defoamers 3.3.2 Defoamer formulations 3.3.3 Suppliers of defoamers 3.4 Product recommendations for different binders 3.4.1 Acrylic emulsions 3.4.2 Styrene acrylic emulsions 3.4.3 Vinyl acetate-based emulsions 3.4.4 Polyurethane dispersions 3.5 Product choice according to field of application 3.5.1 Influence of the pigment volume concentration (PVC) 3.5.2 Method of incorporating the defoamer 3.5.3 Application of shear forces during application 3.5.4 Surfactant content of the formulation 3.5.5 Recommended tests for evaluating defoamers 3.6 Tips and tricks 3.7 Summary 3.8 Literature

43 50 50 52 53 53 54 54 54 54 54 55 55 55 55 55 56 57 57

4 Synthetic rheology modifiers 4.1 General assessment of rheology modifiers 4.1.1 Market overview 4.1.2 Basic characteristics of rheology additives 4.1.3 Main rheology modifiers 4.1.4 ASE, HASE and HEUR chemistry 4.2 Requirements for rheology modifiers 4.2.1 Rheology 4.2.2 Case study 4.3 Ethoxylated and hydrophobic non-ionic thickeners 4.3.1 Associative properties of non-ionic additives 4.3.2 From self-association to purely associative behaviour 4.3.3 Associating mechanism for telechelic HEUR 4.3.4 Associating mechanism of water-soluble polymers 4.3.5 Associative behaviour of HEUR 4.3.6 Mechanism of latex associativity – associative thickeners 4.4 Alkali-swellable emulsions: ASEs and HASEs 4.4.1 Description 4.4.2 Associative properties of HASE copolymer solutions

59 59 60 61 62 64 67 67 70 71 71 72 75 75 77 77 79 79 81

10

Contents 4.4.3 ASEs 4.5 Influence of the latex’s characteristicson associative behaviour 4.5.1 Role played by the specific surface of latex particles 4.5.2 Influence of the nature of latex particle stabilisation 4.5.3 Influence of the density of acid groups on particles 4.5.4 Impact of particle surface energy 4.6 Influence of additives in the continuous phase 4.6.1 Effect of surfactants 4.6.2 Effect of co-solvents 4.6.3 Influence of variations in the constituents of the pigment phase 4.7 New trends in rheological profile requirement 4.8 Literature 5 Substrate wetting additives 5.1 Mechanism of action 5.1.1 Water as a solvent 5.1.2 Surface tension 5.1.3 Reason of the surface tension 5.1.4 Effect of the high surface tension of water 5.1.5 Substrate wetting additives are surfactants 5.1.6 Mode of action of substrate wetting additives 5.1.7 Further general properties of substrate wetting additives/side effects 5.2 Chemical structure of substrate wetting additives 5.2.1 Basic properties of substrate wetting additives 5.2.2 Chemical structure of substrate wetting additives important in coatings 5.3 Application of substrate wetting additives 5.3.1 Basic properties of various chemical classes 5.3.2 Reduction of static surface tension 5.3.3 Possible foam stabilisation 5.3.4 Effective reduction in static surface tension versus flow 5.3.5 Reduction of dynamic surface tension 5.3.6 Which property correlates with which practical application? 5.4 Use of substrate wetting additives in different market sectors 5.5 Tips and tricks 5.5.1 Successful use of substrate wetting additives in coatings 5.5.2 Metallic shades 5.6 Test methods for measuring surface tension 5.6.1 Static surface tension

83 86 86 86 87 88 89 89 91 91 94 96 103 103 103 104 104 105 107 107 108 108 108 108 112 112 113 113 114 114 114 117 118 118 119 119 119 11

Contents 5.6.2 Dynamic surface tension 5.6.3 Dynamic versus static 5.6.4 Further practical test methods 5.6.5 Analytical test methods 5.7 Literature

120 121 121 124 124

6 Improving performance with co-binders 6.1 Preparation of co-binders 6.1.1 Secondary dispersions 6.2 Applications of co-binders 6.2.1 Co-binders for better property profiles 6.2.2 Co-binders for pigment pastes 6.3 Summary 6.4 Literature

125 126 126 131 131 138 140 141

7 Deaerators 7.1 Mode of action of deaerators 7.1.1 Dissolution of micro-foam 7.1.2 Rise of micro-foam bubbles in the coating film 7.1.3 How to prevent micro-foam in coating films 7.1.4 How deaerators combat micro-foam 7.2 Chemical composition of deaerators 7.2.1 Silicone based products 7.2.2 Silicone-free products 7.3 Main applications according to binder systems 7.4 Main applications by market segment 7.5 Tips and tricks 7.6 Evaluating the effectiveness of deaerators 7.6.1 Test method for low to medium viscosity coating formulations 7.6.2 Test method for medium to high viscosity coating formulations 7.6.3 Further test methods for micro-foam 7.7 Conclusion 7.8 Literature

143 144 144 146 148 148 151 151 152 154 154 155 155 156 156 157 157 158

8 Flow and levelling additives 8.1 Mode of action 8.1.1 Mode of action in water-borne systems without co-solvents 8.1.2 Sagging 8.1.3 Total film flow

159 159 159 160 161

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Contents 8.1.4 Mode of action in water-borne systems with co-solvents 8.1.5 Mode of action in an example of a thermosetting water-borne system with co-solvents 8.1.6 Surface tension gradients 8.1.7 Summary 8.2 Chemistry of active ingredients 8.2.1 Polyether siloxanes 8.2.2 Polyacrylates 8.2.3 Side effects of polyether siloxanes 8.2.4 Slip 8.3 Film formation 8.4 Main applications by market segment 8.4.1 Industrial metal coating 8.4.2 Industrial coatings 8.4.3 Architectural coatings 8.5 Conclusion 8.6 Test methods 8.6.1 Measurement of flow 8.6.2 Measuring flow and sagging by DMA 8.6.3 Measuring the surface slip properties 8.6.4 Blocking resistance 8.7 Literature

162

9 Wax additives 9.1 Raw material wax 9.1.1 Natural waxes 9.1.2 Semi-synthetic and synthetic waxes 9.2 From wax to wax additives 9.2.1 Wax and water 9.2.2 Micronized wax additives 9.3 Wax additives for the coating industry 9.3.1 Mode of action 9.3.2 Coating properties 9.4 Summary

177 177 177 179 181 181 183 184 184 186 192

10 Light stabilisers 10.1 Introduction 10.2 Light and photo-oxidative degradation 10.3.1 UV absorbers

193 193 193 197

163 164 165 165 165 167 168 168 170 171 171 172 172 173 173 173 174 175 175 175

13

Contents 10.3.2 Radical scavengers 10.4 Light stabilisers 10.4.1 Market overview 10.4.2 Application fields and market segments 10.5 Conclusions 10.6 Test methods and analytical determination 10.6.1 UV absorbers 10.6.2 HALS 10.6.3 Weathering methods and evaluation criteria 10.7 Literature

201 204 207 208 209 210 210 210 210 212

11 In-can and dry film preservation 11.1 Sustainable and effective in-can and dry film preservation 11.2 In-can preservation 11.2.1 Types of active ingredients 11.2.2 Selection of active ingredients for the preservation system 11.2.3 Plant hygiene 11.3 Dry film preservation 11.3.1 Conventional dry film preservatives 11.3.2 New, “old” actives 11.3.3 Improvements in the ecotoxicological properties 11.4 External determining factors 11.5 Prospects 11.6 Literature

213 213 215 215 218 218 219 219 220 225 225 226 226

12 Hydrophobing agents 12.1 Mode of action 12.1.1 Capillary water absorption 12.1.2 Hydrophobicity 12.1.3 How hydrophobing agents work 12.2 Chemical structures 12.2.1 Linear polysiloxanes and organofunctional polysiloxanes 12.2.2 Silicone resins/silicone resin emulsions 12.2.3 Other hydrophobing agents 12.2.4 Production of linear polysiloxanes 12.2.5 Production of silicone resin emulsions 12.3 Water-borne architectural paints 12.3.1 Synthetic emulsion paints 12.3.2 Silicate emulsion paints

227 227 227 229 229 230 232 232 233 233 234 235 235 235

14

Contents 12.3.3 Emulsion paints with silicate character (SIL paints) 12.3.4 Siloxane architectural paints with strong water-beading effect 12.3.5 Silicone resin emulsion paints 12.4 Conclusions 12.5 Appendix 12.5.1 Façade protection theory according to Künzel 12.5.2 Measurement of capillary water absorption (w-value) 12.5.3 Water vapour diffusion (sd-value) 12.5.4 Simulated dirt pick-up 12.6 Literature

236 236 237 240 240 240 241 242 244 246

13 Functional silica with unique properties 13.1 Natural versus synthetic silica 13.1.1 Gas phase process: fumed silica 13.1.2 Conventional wet process: precipitated silica and silica gel 13.1.3 Continuous process technology for spherical precipitated silica 13.2 Particle characteristics 13.2.1 Particle size and particle size distribution 13.2.2 The significance of filler particle morphology in coatings 13.2.3 Spherical precipitated filler performance in architectural paints 13.3 Test methods 13.4 Results 13.5 Spherical precipitates and paint rheology 13.6 Conclusion 13.7 Literature

247 247 248 248 249 250 250 251 253 254 255 259 260 260

Authors Index

261 265



15

Introduction

1 Introduction Wernfried Heilen Water-borne coatings materials have very different properties from those of conventional solvent-based systems. The reason for this lies in the physical properties of water. The heat of evaporation of water is very high compared to that of many other solvents [1]. Consequently, air-drying systems dry more slowly at lower temperatures and/or higher relative humidity. Significantly more energy must be utilized in heat-curing systems. Numerous solvents with different heats of evaporation and boiling points can be used to optimize drying and film-forming of solvent-based systems. In contrast, formulators of water-borne coatings have only a limited choice of solvents which can be used as water-soluble co-solvents. As a strongly polar solvent, water has a comparatively high surface tension. Because of this and the make-up of the binder, which consists of incompletely dissociated polyelectrolytes or colloidal systems or emulsions based on various polymers, characteristic problems can occur during manufacture and application. This necessitates the development of specialist additives. Essential for the manufacture of water-borne- as well as for solvent-based coatings systems are: – wetting and dispersing agents Especially important in water-borne formulations are – defoamers as well as – rheology-modifying additives As is the case with solvent-borne coatings, polymeric wetting and dispersing additives are used nowadays in the production of water-borne automotive and industrial coatings, while polyphosphates and salts of polyacrylic acids are used in the production of emulsion paints. Aqueous pigment concentrates continue to be produced with the aid of fatty acid and fatty alcohol derivatives, and in addition of alkylphenol ethoxylates – although their ecotoxicity is controversial. Fortunately, polymeric wetting and dispersing agents also find use instead of alkylphenol ethoxylates. Foam-forming substances include emulsifiers used in the manufacture of water-based binders but also the wetting and dispersing agents mentioned above. Non-associative thickeners,

Wernfried Heilen et al.: Additives for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

16

Introduction such as those derived from cellulose, which have many hydrophilic segments in the molecules can also cause foam formation. Modern defoamers comprise a complex mixture of active substances, including mineral oils, polyether siloxanes, waxes, precipitated silicas, etc. Synthetic as well as inorganic thickeners are used to control viscosity in all shear conditions, as well as properties such as flow, sagging, settling and storage stability. The polyurethane thickeners described in this book belong to the class of associative thickeners. The thickening function of these products is dependent on the system and is strongly influenced by certain constituents in the formulation. The high surface tension of water can cause surface defects and inadequate adhesion on poorly-cleaned surfaces. Therefore, depending on the surface it is important to use – substrate wetting agents and – adhesion promoters as additives or as co-binders in water-borne coatings systems. Deaerators are also indispensible in many formulations and particularly useful during airless application. Flow and levelling additives based on polyether siloxanes or polyacrylates, which are also utilised in solvent-based coating systems, are only used in water-borne systems such as stoving enamels which contain large amounts of co-solvents. Such additives are essential in many cases where surface tension gradients occur. Polyether siloxanes and waxes are also used because of positive characteristics such as reduction of friction. Instead of rheological additives, low-solvent and solvent-free aqueous systems may contain gemini surfactants, which often also improve levelling thanks to effective substrate wetting. Film-formers have already been discussed extensively in the literature and will therefore not be covered in any detail in this book, although their importance in water-based emulsions is undisputed. To protect the applied water-borne coating from degradation the use of light absorbers, as well as of film preservatives, is absolutely essential. Hydrophobing agents, which are mainly used in facade protection as co-binders or additives, are described as well. The chapters of the book are also organised in the sequence set out above. Finally, the production and use of a newly developed synthetic solid for preventing burnishing of applied emulsion paints are described.

1.1 Literature [1] Kittel, “Lehrbuch der Lacke und Beschichtungen”, Volume 3, Hirzel Verlag, Stuttgart-Leipzig 2001

17

Modes of action

2 Wetting and dispersing additives Frank Kleinsteinberg Dispersing of pigments is indisputably one of the most demanding steps in the manufacture of coatings. Formulators therefore look for easy solutions and additives that fulfil a number of different demands. Already the first step in pigment dispersing – wetting of the pigment surface, which can have a very low energy – is highly problematic because the high surface tension of water needs to be reduced, without creating too many side effects. Even more problematic is finding the right stabilisation mode to match the water-borne binder. Finally, the pH also plays a key role in pigment wetting, stabilisation and compatibility. Meeting these complex demands calls for combinations of additives that have different functions. Where applications require outstanding performance by all components, modern, highly sophisticated wetting and dispersing additives are used. The mode of action of wetting and dispersing additives at the various stages of pigment grinding is explained below. Various chemical concepts are elucidated in terms of performance and regulatory constraints and their significance for specific market segments is examined.

2.1 Modes of action The function of wetting and dispersing additives can be considered under three headings: – pigment wetting – grinding of the pigment particles – stabilisation of the pigment particles

2.1.1 Pigment wetting The process of wetting a solid by a liquid is summarised by Young’s equation:

γs = γsl + γl • cosθ or γs - γsl/γl = cosθ

where γs: surface tension of the solid, γsl: interfacial tension solid/liquid, γl: surface tension of the liquid, θ: contact angle solid/liquid, see Figure 2.1. Wernfried Heilen et al.: Additives for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

19

Wetting and dispersing additives A contact angle of 0 indicates spontaneous wetting or spreading. The cosine of 0 is 1 and in this case the equation becomes:

γl = γs - γsl

To achieve wetting the surface tension of the liquid must be lower than the surface tension of the solid. A liquid with low surface tension wets a pigment surface better than a liquid with high surface tension. An additive which helps wetting must, as a first step, lower the surface tension. During wetting, the additive adsorbs on the surface of the pigment particles and forms an envelope around them. At this stage the pigment particles are still large. The interactions between these particles are lowered and the viscosity of the grind is reduced. A reduction in grinding viscosity is a first indication of pigment wetting and incipient stabilisation. Optimal grinding of pigments can only be achieved through very good wetting. In this context, optimal grinding means achieving the largest surface area possible. The larger the surface area, the more light that can be absorbed and the higher is the resulting colour strength. This means that achieving the low particle size needed for optimal colour strength calls for the best-possible wetting; i.e. to increase the colour strength, it is necessary to improve pigment wetting. The particle size also determines transparency and hiding power. While organic pigments show a higher transparency at lower particle size, inorganic pigments have a maximum hiding power at a particle size of λ/2 [1].

Figure 2.1:  Equilibrium of forces according to Young

20

Modes of action

2.1.2 

Grinding

During grinding the pigment agglomerates are broken down mechanically using a variety of equipment. The simplest device is the dissolver. Normal inorganic pigments such as titanium dioxide can be ground with good results using an appropriate blade. The dissolver can only be used for premixing when organic pigments, which are more difficult to disperse, must be ground. A bead mill is recommended for achieving the required fine grind. Because wetting and dispersing additives accelerate the wetting of the newly created surface, they improve the grinding process and reduce the dispersing time. During grinding, additive molecules adsorb on the new surfaces. They minimise the interaction between the increasingly smaller pigment particles and maintain a constant viscosity. At the same time the pigment particles are stabilised against flocculation. Without stabilisation the primary pigment particles would re-agglomerate and release the energy which was introduced into the system during the grinding process. The work needed to increase a surface area is given by the following equation: where

dW = γ • dA W: work to change the interface γ: surface tension A: surface area

Because the pigment grinding process increases the surface area, this equation can be used. It shows that the energy required to increase the surface area during dispersion, dW, is proportional to the surface tension γ. The lower the surface tension, the higher is the change of surface area for a given amount of dispersing energy. Wetting and dispersing additives reduce the surface tension. In other words, to achieve a certain change of surface area using a wetting and dispersing additive, a smaller amount of work is necessary. Wetting and dispersing additives thus perform some of the most important functions during the grinding process. They shorten the grinding time by reducing the surface tension, they reduce the amount of work necessary for dispersion and they prevent re-agglomeration of the pigment particles during the grinding process [2].

2.1.3 Stabilisation The basic requirement for stabilising the finely ground pigment particles is the adsorption of the additive molecules on the pigment surface. The additive molecules must have groups or segments that interact very strongly with the pigment surface. Possible interactions are

21

Wetting and dispersing additives ionic bonding, dipole-dipole forces and hydrogen bonding. Stabilisation is thought to involve several mechanisms, which will be discussed below.

2.1.3.1 Electrostatic stabilisation

Electrostatic repulsion is a very important mechanism for stabilising pigment particles in water-borne formulations. This makes use of the Coulombic interactions between similarly charged particles. These interactions can be described by the DLVO theory (named after Derjagin, Landau, Verwey and Overbeek). The wetting and dispersing additive, adsorbed on the pigment surface, dissociates into a polymeric segment, which is anionic, and cationic counter ions. These counter ions are not adsorbed and form a mobile diffuse cloud at the outer edge of the polymeric shell. An electrostatic double layer is created. This leads to repulsion and the particles are stabilised against flocculation. Electrostatic stabilisation induced by anionic polymeric segments is called anionic stabilisation. Cationic stabilisation can be induced by cationic polymers, in which case anions form a mobile cloud around the particle. Addition of electrolytes, especially multivalent cations, destabilises the electrostatic double layer, disrupting the balance between anionic polymer and cationic cloud or at least reducing the thickness of the cationic layer. Both lead to a weakening of stabilisation and increase the risk of flocculation. The zeta potential ζ describes the electrostatic interaction within the polymeric shell. The smaller the numerical value of ζ, the lower is the electrostatic stabilisation. The zeta potential gives no information about steric stabilisation because steric stabilisation does not involve the creation of ions and so no potential can be measured (Figure 2.2a). It is essential to know what type of stabilisation is employed in the binder system of the target application. If the binder system is cationically stabilised, as in the case of textile printing or leather coatings, the pigment particles should be stabilised in the same way. Otherwise, the pigment particles will flocculate with the binder droplets. Another important factor is the pH value. A high pH milieu greatly reduces the ability of the wetting and dispersing additive to dissociate and form a cationic cloud. The pKa value of the acid functionality needs to Figure  2.2a:  Electrostatic stabilisation be taken into consideration.

22

Modes of action

2.1.3.2 Steric stabilisation

In aqueous environments, steric stabilisation is another mechanism which frequently occurs. The adsorbed additive molecules form a polymeric shell around the pigment particle. This polymeric shell consists of the anchoring groups of the additives and a diffuse layer of polymeric chains. To achieve optimal stabilisation, the polymeric chains must be completely soluble in the surrounding water/binder mixture. They form an outer shell around the pigment particle. As particles come closer, the polymeric shells start to overlap, leading to steric hindrance. A simple model would be two wooden balls that carry wire springs. If the balls approach each other, the springs prevent contact between the wooden surfaces. In thermodynamic terms, the degree of freedom of movement of the polymeric chains is reduced when the chains overlap, leading to a reduction in entropy. To compensate for this reduction and to reinstate the mobility, the separation of the pigment particles must increase. The change in free energy is given by

ΔG = ΔH – TΔS

where

ΔH: change of enthalpy ΔS: change of entropy T: absolute temperature

Figure 2.2b:  Steric stabilisation

Important factors influencing the efficiency of stabilisation are the degree of adsorption of the polymers on the surface, the integrity of the polymeric shell and its thickness. The thickness of the polymeric shell and the degree of stabilisation are increased if the additive chains interact with binder molecules (Figure 2.2b) [3].

2.1.3.3 Electrosteric stabilisation The complex demands made on wetting and dispersing additives in water-borne

Figure 2.2c:  Electrosteric stabilisation

23

Wetting and dispersing additives coatings make it necessary to combine electrostatic repulsion and steric hindrance. This is called electrosteric stabilisation and modern wetting and dispersing additives for water-borne systems work on this principle. Only such additives can fulfil the high demands made on pigment stabilisation and long-term storage stability (Figure 2.2c).

2.1.4

Influences on formulation

The ability to wet a pigment particle and the various stabilisation mechanisms affect a number of properties which are very important in the development of formulations for grinds and pigment concentrates.

2.1.4.1 Viscosity

Stabilisation of the pigment particles reduces the interactions between them, leading to greater mobility and ultimately to lower viscosity. Electrostatic stabilisation utilises Coulomb forces, which are stronger than the forces arising from changes in entropy due to steric stabilisation. For this reason, the reduction in viscosity brought about by an anionic additive is greater than that of a non-ionic additive which employs only steric hindrance to stabilise the particles. If high pigment loadings are required, electrostatic stabilisation should be considered. Another important effect is the way in which viscosity is reduced when additives are used in different quantities. An additive which predominantly stabilises by electrostatic repulsion is adsorbed on the pigment surface when added to the grind and immediately decreases the interaction between the pigment particles, resulting in a strong reduction in viscosity. At higher addition levels the effect does not continue and, in fact, a small increase in viscosity can be observed arising from the higher concentration of polymer and the resulting higher solids content. High-polymer additives which stabilise by steric or electrosteric effects exhibit a different behaviour. At a particular level of addition there is a maximum reduction in viscosity. Amounts below this are not sufficient to stabilise the pigment particles which can interact with each other, leading to a high viscosity. However, amounts above the optimum also lead to high viscosity of the pigment concentrate. This increase cannot be explained by the higher amount of polymer in the formulation. Furthermore, the additive molecules in the outer polymer shell are not fully orientated and these can Figure 2.3:  Viscosity behaviour and additive also interact with the polymer shells of dosage in direct grind

24

Modes of action other pigment particles. This bridging leads to reduced mobility of the pigment particles and, in consequence, to higher viscosity.

2.1.4.2

Colour strength

Consideration of colour strength and amount of additive shows a different behaviour. As described in Chapter 2.1.2, wetting of pigment particles plays an important role in particle size, surface area and colour strength. The better the wetting, the smaller is the particle size, the larger is the surface area and the higher is the colour strength. Particle wetting can be achieved in a number of ways: The first and obvious option is to use wetting agents. These lower the surface tension of water and help the wetting of surfaces and particles. However, the dynamic nature of the grinding process and the risk of extensive foaming need to be taken into account. The wetting agent should not generate foam and should be highly dynamic. Experienced formulators know that the wettability of non-ionic additives can be considered greater than the wettability of anionic (cationic) additives. The second option for improving pigment wetting is therefore to use non-ionic dispersants. These wetting and dispersing additives possess higher colour strength than anionic additives used to create electrostatic repulsion. The dosage level of surface-active additives needs to be borne in mind. In contrast to the behaviour observed with regard to viscosity, namely that higher dosages lead to much higher viscosities, increasing the concentration of additive yields a higher colour strength but the curve (Figure 2.5) ends in a plateau, with additional amounts of additive producing

Figure 2.4:  Viscosity behaviour and additive dosage in pigment concentrates

25

Wetting and dispersing additives only small effects. Initially, increased addition of additive improves the pigment wetting and hence the colour strength, but very high concentrations lead to double layers on the pigment particle, at which stage the pigment wetting cannot be improved and the colour strength no longer increases.

2.1.4.3 Compatibility

At higher concentrations of wetting and dispersing additives, compatibility between pigment concentrate and base paint (pigment paste absorption) is also improved. The better colour acceptance leads to lower rub-out values. The additives used contain hydrophilic structures such as hydrophilic side chains or salt-like groups. These hydrophilic structures orientate to the aqueous medium. The more additive used, the more hydrophilic the pigment particle becomes, leading to increasingly greater compatibility of pigment dispersion and water-borne paint.

2.1.4.4 Stability

As already described, stabilisation depends on the thickness of the polymer shell around the pigment particle and is thus also related to additive concentration. A higher concentration of stabilising additive leads to a more stable dispersion.

Figure 2.5:  Colour strength and additive dosage (schematic)

26

Chemical structures

2.2

Chemical structures

To stabilise pigment particles two different molecule segments are necessary: anchor groups with an affinity for the pigment, which adsorb on the pigment surface, and water-soluble side chains which produce the steric hindrance. Because the groups with affinity for the pigment are mostly hydrophobic and the soluble side chains hydrophilic, wetting and dispersing additives are called amphiphilic structures. The simplest amphiphilic structure is a surfactant. On account of their low molecular weight, surfactants are not suitable for stabilising pigment particles. Commercially available products are mostly polymeric. The polymers can contain various functional groups with high pigment affinity (anchor groups). An aromatic ring forms a suitable anchor group for organic pigments with the adsorption being caused by van-derWaals forces. Adsorption on inorganic, oxidic pigment particles involves hydrogen bonding and induced dipoles; hydroxyl or carbonyl groups are suitable here. Additives containing both of these groups in the form of the carboxyl group, also show a strong affinity to inorganic pigments. Additives with nitrogen-containing groups (e.g. amines or imines) exhibit good adsorption on carbon black surfaces. Additives without nitrogen groups are of only limited suitability for carbon black pigments. In contrast to solvent-borne formulations, the use of primary amine groups as anchor groups for organic pigments and carbon blacks is very limited in water-borne formulations. Primary amine groups would create a cationic additive of limited binder compatibility.

2.2.1 Polyacrylate salts Polyacrylate salts are simple polymers which stabilise pigments in water-borne paints, mainly emulsion paints. They are characterised by their high acid value, which promotes good anchoring to inorganic pigments. The highly ionic character means that there is a strong tendency to dissociate in water and this allows good electrostatic repulsion, which is associated with outstanding viscosity reduction. Common counter cations are sodium and ammonium. Coatings containing ammonium-neutralised polyacrylates are more resistant to water than sodium-neutralised ones. In general, the water resistance of polyacrylate salts is rather weak.

2.2.2

Polyphosphates

Polyphosphates are salts of polymeric chains formed from tetrahedral PO4 structural units. They are good at wetting inorganic pigments and fillers and help to reduce the amount of multivalent cations that disrupt electrostatic stabilisation. In Europe, combinations of polyphosphates and polyacrylate salts are used in mill bases for emulsion paints.

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Wetting and dispersing additives

2.2.3 Fatty acid and fatty alcohol derivatives Polyacrylate salts are not suitable for stabilising organic pigments. Fatty acid derivatives are a group of simple compounds which stabilise pigments. Fatty acids have structures which will also anchor to organic and carbon black pigments. The hydrophilic part consists of a polyether chain. Fatty acid ethoxylates are excellent emulsifiers and allow the production of very compatible pigment grinds. As with polyacrylate salts, the water resistance of coatings containing fatty acid derivatives is limited. Fatty acid derivatives differ from polyacrylate salts in that they can be used to produce pigment concentrates. In many regions, fatty acid ethoxylates are combined with polyacrylate salts to make mill bases for emulsion paints. They also help to stabilise the binder systems.

2.2.4 Acrylic copolymers

Figure 2.6a:  Structural example of a polyacrylate salt

Figure 2.6b:  Structural example of a glycerol fatty acid ethoxylate

Figure 2.6c:  Structural example of a methacrylicpolyether copolymer

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Acrylic copolymers are also suitable for formulating pigment concentrates. The broad variety of monomers available allows the development of wetting and dispersing additives which are suitable for all kinds of pigments and compatible with many different binders. Acrylic copolymers can be modified, so that coatings containing them become very water resistant. Hydrophilic polyether chains also provide steric stabilisation of the pigment particles. A-B copolymers or comb-like structures are possible. CRP (controlled radical polymerisation) can be used to make A-B-A copolymers and a variety of different structural geometries. Additives of these species show outstanding wetting and viscosity reduction, especially in the case of very fine organic pigments.

Chemical structures

2.2.5

Maleic anhydride copolymers

These copolymers contain maleic anhydride instead of acrylic acid. The copolymers are mostly comb-like structures. Ethoxylate chains are also used here to sterically stabilise the pigments. Maleic anhydride chemistry does not allow the broad variety of structures available with acrylates but nevertheless enables development of additives for all kinds of pigments. These additives are very resistant to water.

2.2.6

Alkyl phenol ethoxylates

Alkyl phenol ethoxylates (APE) have very good pigment dispersing properties and are very low cost. Because of their broad compatibility and strong emulsifying performance, they can be used for the production of universal colorants suitable for tinting water-borne and solvent-based base paints. APEs can create nonyl phenol by hydrolysis. Nonyl phenol is very similar in structure to the female hormone oestrogen and can produce the same effects. If waters become polluted with this, aquatic animals will only bear female descendants and so die out within a few generations. The use of APEs is therefore controversial, yet they are still widely employed in the NAFTA region as a wetting component in mill bases for emulsion paints and in water-borne and universal colorants.

2.2.7

Alkyl phenol ethoxylate replacements

Among alternatives to APEs are the so called Guerbet derivatives (modified fatty acid ethoxylates) and modified polyethers. The modified polyethers contain only polyether bonds and are thus very stable against hydrolysis and are very suitable for exterior applications.

Figure 2.6d:  Structural example maleic anhydride-polyether-copolymer

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Wetting and dispersing additives

2.3 Wetting and dispersing additives in different market segments Wetting and dispersing additives are used in the grinding stage of paints and coatings as well as in pigment concentrates. The different applications make very distinct demands on the additives which must be taken into account when discussing market segments.

2.3.1

Architectural coatings

2.3.1.1 Direct grind

During production of water-borne emulsion paints, extenders such as calcium carbonate and titanium dioxide are used as a mill base. Grinding these materials is not particularly demanding and, because of their excellent viscosity reduction, polyacrylate salts are widely used. Electrostatic repulsion is sufficient in this case and the cost-performance ratio of these additives is appropriate to the application. Figure 2.6e:  Structural example of a non-ionic APE

Figure 2.6f:  Guerbet derivative (R= fatty acid or phosphate)

Figure 2.6g:  Modified polyether

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2.3.1.2 Pigment concentrates Pigment concentrates, which are used in architectural coatings, can be divided into several groups. Firstly, there are mass-tones and tinting colorants. These are coloured emulsion paints which can be used alone as a full shade or used to tint white base paints and have a relatively low pigment content. They contain binder and small amounts of fatty acid ethoxylates as wetting and dispersing additives. The second group comprises the binder-free tinters. They contain larger amounts of fatty acid derivatives and are used to tint white base paints or for colour corrections.

Wetting and dispersing additives in different market segments This market segment also includes high performance pigment concentrates which contain acrylic- or maleic-anhydride copolymers and which, because of their high price, are used only when essential, for example, in facade coatings with strong beading effects or silicate paints because they have a high pH. So called universal colorants are widely used in architectural coatings. These aqueous pigment concentrates can be used in water-borne emulsion paints as well as in solvent-based alkyd lacquers. Alkyl phenol ethoxylates can be used to formulate universal colorants. Because of environmental concerns, alternatives such as fatty acid derivatives (Guerbet derivatives) and modified polyethers are used.

2.3.2

Wood and furniture coatings

2.3.2.1 Direct grind

As with architectural coatings, polyacrylate-salts are used in the direct grind of titanium dioxide and iron oxide pigments in wood coatings. To stabilise transparent inorganic pigments, higher quality additives such as acrylic- and maleic-anhydride copolymers must be used.

2.3.2.2 Pigment concentrates

The use of very fine, transparent pigments and required resistance are the reason for using high performance acrylic- and maleic-anhydride copolymers with high chemical resistance. The manufacture of flatting agent pastes involves considerable dispersion input and thus requires the use of high-quality additives.

2.3.3

Industrial coatings

2.3.3.1 Direct grind

The high demands made on resistance and weatherability necessitate high quality additives such as acrylic- and maleic-anhydride copolymers. The use of very finely divided organic pigments also necessitates the use of additives with good performance.

2.3.3.2 Pigment concentrates

In industrial coatings the difference between direct grind and pigment concentrates is not significant as far as wetting and dispersing additives are concerned. Pigment concentrates in industrial coatings mostly contain binder or grinding resins. Because of this, compatibility between binder/grinding resin and wetting and dispersing additive is very important.

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Wetting and dispersing additives

2.3.4 Printing inks 2.3.4.1 Direct grind

The classical way of producing printing inks involves a resin solution based on an acidic styrene-acrylic or pure-acrylic resin neutralised with amines to make it water soluble. The resin solution is able to stabilise pigments very well, but pigment wetting is sometimes not acceptable. This is apparent particularly with carbon black pigments where the viscosity is very high and the colour strength quite poor so that use of suitable additives with good wetting is advantageous.

2.3.4.2 Pigment concentrates

Resin-free concentrates have also been developed. Avoiding the use of resin solution results in greatly improved water resistance. High levels of very fine, intensely coloured pigments pose special demands which can be met by the use of acrylic- and maleic-anhydride copolymers.

2.4 Tips and tricks When selecting a wetting and dispersing additive, the suitability of its chemical structure for the particular pigment and its compatibility with the surrounding binder are of prime importance. The suitability of a wetting and dispersing additive for a particular pigment is described in Chapter 2.3 “Chemical structures”. To summarise: an additive which contains acid groups is adequate for inorganic pigments; an additive with nitrogen groups is very effective on carbon black surfaces. The compatibility of wetting and dispersing additives for water-borne applications with the binder matrix can only be tested in conjunction with a pigment. The surfactant structure – hydrophobic anchor groups and hydrophilic side chains – of some additives makes it impossible for some surfactants to be water-soluble in pure form. As soon as pigment particles are present, the hydrophobic portions of the additive molecules orientate themselves to the pigment surface and the hydrophilic segment protrudes into the water phase. The pigment-particle/additive combination then becomes water-soluble. The pH is also of great importance. To avoid pigment shock, the pH of the pigment grind/pigment concentrate and the let-down resin or the base paint has to be the same. In many cases, and this is especially true for inorganic and carbon black pigments, the pH of the pigment concentrate needs to be adjusted. Amines and alkali hydroxides are among the suitable compounds for the neutralisation process. Neutralisation should be carried out after the pigments have been wetted by the wetting and dispersing additive. For many inorganic pigments a free acid group can be more conducive to adsorption of the additive than a neutralised acid group.

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Test methods

2.5

Test methods

2.5.1 Particle size The primary criterion for the quality of dispersion is the particle size. Monitoring the particle size allows a decision to be made as to when the grinding process can be terminated. The simplest method of measuring the particle size of inorganic pigments is the grindometer draw down. A sample of the mill base is poured into the deep end of a groove and scraped towards the shallow end with a flat metal scraper. At the point where the depth of the groove equals the largest particles in the suspension, irregularities (for example stripes in the draw down) will become visible. The depth of the groove is marked on a graduated scale next to it. With some practice, use of a grindometer allows the maximum particle size of the mill base to be determined quickly and simply but cannot be used to measure pigment particle size distribution. When grinding binder-free pigments, which dry very rapidly and have particle sizes smaller than 5 µm, the grindometer can easily give a false value. More sophisticated measurements such as laser diffraction or ultrasound give a more precise result in terms of particle size and particle size distribution. Due to their high cost, such methods are not suitable for routine use. Achievement of the desired particle size distribution can be detected by reliable secondary indications. Colour strength development of organic pigments is dependent on pigment particle size. Determining colour strength at different stages of the grinding process allows the final point of the grinding process to be detected.

2.5.2

Colour strength

For colour strength determination a mill base sample is let down in an appropriate coating formulation and applied. Evaluation is carried out by optical examination or by a spectrophotometer and compared with that of a standard grind using the same amount of the mill base sample. The amount of the mill base sample under evaluation is then adjusted. When both samples give the same optical appearance, the relative colour strength of the new sample in % of the standard can be calculated. Relative colour strength determination is very complex but provides meaningful comparative data. This test method is used mainly by pigment manufacturers. Determination of absolute values is based on Kubelka-Munck theory. This involves the relationship between reflection and transmission of light. Summation of the reflection over the entire spectrum gives a value for the colour strength. In practice, this method suffers from a systematic error since it is based on the assumption of an infinite film thickness

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Wetting and dispersing additives and a constant degree of reflection. It is therefore not suitable for pigment development and pigment manufacturers prefer the method of relative colour strength determination. Kubelka-Munck equation: where

2.5.3

CS = K/S = (1–R)²/2R CS = colour strength K = absorption coefficient S = scattering coefficient R = reflectance at infinite film thickness (hence no change in degree of reflection)

Rub-out

The rub-out test is used to check the stabilisation of pigment particles. It can be used to assess the compatibility of pigment concentrates, the tendency of pigment particles to flocculate or pigment flooding phenomena. An area of the moist but partially dry paint film is rubbed with a finger or a brush. If the pigments have separated or are strongly flocculated, this mechanical procedure of rubbing re-establishes a homogeneous pigment distribution. The viscosity of the dry film will already have increased strongly. The homogeneous distribution of pigment particles is stabilised this way. The colour difference relative to the unrubbed film is an indication of pigment separation or flocculation. The colour difference is usually quoted as the “separation” of the chromaticity ΔE (ΔE is dimensionless). For ΔE less than 0.5, no colour difference is visible. The automotive sector requires ΔE < 0.3. Between 0.5 and 1.0 the colour difference is only slightly visible. For architectural paints, ΔE of < 1.0 is still adequate but ΔE values greater than 1 are not acceptable.

2.5.4 Viscosity The viscosity of a mill base must be adjusted to suit the dispersing unit. If the viscosity of the mill base is excessive, the unit may be damaged. If it is too low, shear forces will be inadequate to break down the pigment agglomerates. The viscosity is also an important indicator of the stability of a pigment concentrate. Any change in rheology during storage indicates inadequate pigment stabilisation. An easy method to determine viscosity is by measuring the efflux time. For mill bases, however, the viscosity is usually simply too high to use a flow cup. A rotational viscometer is often used to determine the viscosity more precisely. The resultant complete flow curves provide information on flow characteristics of the particular material, from the manufacturing process through transport to the final application. During development of a pigment concentrate, its flow characteristics over the entire shear

34

Test methods rate spectrum are of great importance. For quality control purposes, measurement at two points, e.g. at low and medium shear rate, is usually sufficient.

2.5.5 Zeta potential Electrostatic stabilisation can be characterised by measuring the zeta (ζ) potential which assumes formation of an electrical double layer. In a solution of electrolyte, particles with a charged surface such as metal oxide pigments adsorb counter ions which form an immobile film known as a Stern layer. The diffuse cloud of ions, consisting of similarly charged ions and counter ions lies outside this layer. If a particle moves, part of the loosely-bound diffuse layer shears off. The potential at this shear plane is termed zeta (ζ) potential and is important in assessing the stabilisation of dispersion. The higher the ζ potential, the better a dispersion is protected against flocculation. Traditional, optical methods for determining ζ potential, which are based on electrophoretic mobility, can only be used for very dilute systems. However, strong dilution during investigation of coating of the pigment surface by additives leads, for example, to a change in the adsorption equilibrium and thus to measurements which do not correspond to reality. The ζ potential can be measured electro-acoustically. This method is also suitable for investigating concentrated dispersions with a pigment concentration of 50 % v/v. There are two different ways of determining the ζ potential electro-acoustically depending on the exciting force. If an alternating electrical field is applied to a dispersion of charged particles, the particles are excited to vibrate and emit sound waves. This method gives the value of the electric sonic amplitude. If the exciting force is an ultrasonic wave, an electrical signal can be detected and a colloid vibration current (CVI) measured. Figure 2.7 shows the CVI principle. A high frequency sound wave generated by a piezo crystal in the measuring sensor passes through the dispersion. The acoustic signal excites the particle to vibrate. The higher the inertia of the particles, the worse their ability to follow the sound wave and hence the larger the phase shift. The diffuse cloud of ions reacts without delay to the sound wave so that each particle with its ionic shell becomes a dipole which constantly changes its direction. At a particular point in time, the dipoles point in one direction so that an electric field arises, and the colloid vibration current can be measured with two electrodes. Figure 2.8 shows a diagram of a measuring sensor. After applying a radio-frequency pulse, a cylindrical piezo element gener- Figure 2.7:  CVI principle by using high frequency sound ates an acoustic pulse which wave

35

Wetting and dispersing additives passes through a quartz crystal for internal calibration. The quartz crystal is extended by a buffer section the acoustic impedance of which is more tailored to the dispersion than to the material of the quartz crystal. The end of the buffer rod is coated with gold and forms an electrode for measuring the electric signal. The second electrode required is provided by the steel casing. When the measurement sensor is immersed in a sample, a colloid vibration current can be detected between the gold electrode and the stainless-steel casing. The measured colloid vibration current is related to the ζ potential as follows:

where

ε0: dielectric constant of the vacuum εm: dielectric constant of medium ζ : zeta potential φ: parts by weight η: dynamic viscosity

Figure 2.8:  Principle of the use of an ultrasonic wave as exciting force

36

Ks: conductivity of dispersion Km: conductivity of medium ρp: density of particle ρs: density of system ω: circular or angular frequency The equation shows which parameters affect the ζ potential. A higher value of dielectric constant causes a lower ζ potential. Water with a dielectric constant of 80 and a very polar character weakens the dipole while, for example, in a non-polar solvent such as heptane, the dipole effects are more pronounced. A greater concentration leads to a lower ζ potential, because the individual particles move closer together and the electrical double layers overlap as the concentration increases. A high dynamic viscosity affects the inertia of the particle which causes the ion cloud to be more easily shifted against the particles in the medium. The difference in density between the particle and medi-

Summary um must be as great as possible so that the medium moves relative to the particle and dipoles are created which contribute to the measurement of the colloidal current. Using additive and pH titrations, one can deduce interactions between pigments and additives and thus characterise the electrostatic stabilisation of pigments. With iron oxide yellow pigments, it has been found that additives with a strongly ionic character promote electrostatic stabilisation while additives with polar anchor groups do not wet the surface [4]. If the pigment surface were wetted by the non-ionic additive, shielding of the partially charged pigment surface by the additive would lead to a measurable change in potential.

2.6

Summary

Wetting and dispersing additives fulfil different functions within a coating formulation. They help pigment wetting and stabilise the dispersed pigment particles. They reduce the grinding time and decrease the viscosity. The dosage of additive has different influences on properties of the grind such as: – viscosity – colour strength – compatibility – storage stability.

Figure  2.9:  ζ potential curves based on different wetting and dispersing additives with iron oxide yellow

37

Wetting and dispersing additives The surface tension has a major influence on pigment wetting. In water-borne formulations two stabilising mechanisms are used: – steric stabilisation – electrostatic stabilisation. The structure of wetting and dispersing additives is determined by the pigment wetting required, the anchoring to the pigment surface and the pigment stabilisation. The structures are mostly surfactant-like. Depending on requirements different chemical structures are available. The simplest are based on fatty acids. Higher demands can be fulfilled by acrylic- and maleic-anhydride copolymers.

2.7

Literature

[1] T. Brock, M. Groteklaes, P. Mischke, European Coatings Handbook, 2nd Edition, Vincentz Network 2010 [2] Tego Journal 2006

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[3] J. Bieleman, Additives for Coatings, Wiley/VCH 2001 [4] GdCH-Tagung 2008

Defoaming mechanisms

3 Defoaming of coating systems Juergen Kirchner * Foam is a colloidal system of gas surrounded by liquid or solid cell bridges. Foam reduces the quality of paints. Foam formation frequently occurs during the production or application of paints and coatings, resulting in impaired optical or technical performance of a coating. Foam also disrupts production processes, e.g. by reduced energy transfer during dispersion processes, reduced capacities of vessels or problems at filling stations. Effects on the optical performance of coatings include surface irregularities, reduced gloss and transparency. Defects caused by foam also significantly impair the coating’s ability to protect against penetrating media. There are various reasons for the formation of foam including: – mechanical introduction of air by mixing or application processes, – displacement of air from surfaces during wetting and dispersing processes, – generation of gas by chemical reactions, – bubbles caused by too rapid drying. The presence of surface active (amphiphilic) substances in paints and coatings often promotes the stabilisation of foam. To prevent its formation during manufacturing and processing of coatings and to destroy foam which has already built up, it is essential to use defoamers or deaerators [1].

3.1 Defoaming mechanisms To explain the mechanisms of defoaming it is necessary to understand the composition and stabilisation of foam. Defoaming always entails disrupting the mechanisms of foam stabilisation.

* revised by Wernfried Heilen

Wernfried Heilen et al.: Additives for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

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Defoaming of coating systems

3.1.1 Foam Foam is a dispersion of a gaseous material in a liquid or solid, in which the volumetric content of the gas predominates. The gas bubbles in the foam are separated by liquid or solid walls. Solid foams are often created from existing liquid foams.

3.1.1.1 Causes of foam

The simultaneous presence of a gas and a liquid does not necessarily result in the formation of foam. Intense mixing of both materials is required for a fine distribution of the gas in the liquid and this results in the creation of new gas/liquid interfaces. In most pure liquids these interfaces are unstable. Consequently, the gas bubbles rise very quickly and collapse on the surface of the liquid. Only the presence of surface active (amphiphilic) substances in the liquid stabilises the gas as foam. (Figure 3.1) [2, 3]. Stabilisation occurs through orientation of the surface-active substances at the newly formed liquid/gas interfaces. In practice, the presence of surface-active substances cannot be avoided in coating systems, as their use is essential to control properties such as wetting and dispersing, emulsification or substrate wetting [4, 5].

3.1.1.2 Types of foam

Classification of foams can be based on their state of aggregation or structure and whether they are liquid or solid.

Liquid foams

Liquid foams consist of a dispersion of gas in a liquid. Usually freshly formed liquid foams are unstable and, in time, undergo structural changes frequently to a more stable form. If a stable structure is not attained, the foam eventually collapses.

Solid foams

Solid foams are manufactured for use in specialist applications in the coatings industry such as in assembly. Solid foams also appear undesirably as entrapped air in dried paint films or as craters or pinholes on the surface. Foams can also be differentiated by structure, for example into micro- or macro-foam or dry or wet foam.

Micro-foam

Small gas bubbles trapped either in the liquid phase of a paint or in the solid bulk phase of a coating are termed micro-foam. Typically, the interface between the gas bubble and the surrounding media is stabilised by a single layer of surfactant. Stabilisation of micro-foam is promoted by factors such as a high coating viscosity or low temperatures [1, 5]. For combating micro foam special deaerators are used. These are described in Chapter 7.

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Defoaming mechanisms

Macro-foam

Macro- or surface foam is the type visible on the surface. In macro-foam the bubble is separated by a thin foam lamella from the surrounding atmosphere. The surfaces of the lamella orientated to the atmosphere and to the gas bubble are covered with a layer of surfactant. This double surfactant layer is frequently called a duplex film. Macro-foam can be further differentiated by the liquid content of the lamella. Freshly formed macro-foam created by rising bubbles consists of spherical bubbles, which are stabilised by thick lamellas with high water content. This foam is described as ball foam. Due to gravity-induced drainage, the water in the foam lamellas flows down until the bubbles are stabilised by very thin, but very stable lamellas. In this process the original spherical bubbles are transformed into a more stable polyhedral form. The resulting foam consists of a great deal of gas and little liquid and is also known as dry foam or polyhedral foam [1, 4–6].

Figure 3.1:  Formation and stabilisation of foam in a liquid. Micro-foam is stabilised by a single surfactant layer in the bulk phase of the liquid. Foam bubbles penetrating through the surface appear as macro foam and are stabilised by a double layer of surfactants, the so-called duplex film.

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Defoaming of coating systems Table 3.1:  Volume content of gas in foams [7] φ = Vg / (Vg + Vl) Gas dispersion Ball foam Polyhedral foam

φ = 0.52 0.52 < φ < 0.74 φ > 0.74

Vg = Volume of gas, Vl = Volume of liquid

Ball foam can be differentiated from polyhedral foam according to the volume content of gas as shown in Table 3.1 In gas dispersions the volume content of the gas is characteristically higher than the volume content of water.

Effect of time on foams/drainage

Most foams are thermodynamically unstable and undergo structural changes over time. The effect of gravity on foams causes drainage of the liquid from the lamellas. This results in a reduction of the thickness of the lamella. Small gas bubbles disappear by diffusion, which causes a reduction in the number of lamellas and the nodes between them. As drainage continues, ball foam converts into polyhedral foam, causing a deformation of the lamella and an increase in their radius. In some foams drainage creates a sufficiently high tension in the lamella for them and the foam to collapse spontaneously. Steric effects or electrostatic repulsion caused by surfactants can compensate the tension in the lamellas created by drainage. This counteracts further thinning of the lamella. Polyhedral foams stabilised in this manner are highly stable. To prevent the formation of stable polyhedral foams or to destroy them it is necessary to use defoamers [3, 6, 8].

3.2 Defoamers 3.2.1 Composition of defoamers Figure 3.2:  Structure of foam

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To understand how defoamers work it is necessary to know that defoamers consist basically of an insoluble

Defoamers or, at least in the application media, partly soluble active ingredient (commonly called oil), a hydrophobic particle or a mixture of both. A more detailed view on the composition of defoamers is given in Chapter 3.3.

3.2.2 Defoaming mechanisms Defoamers are used to destroy macrofoam. An effective defoamer must be able to break the stability of a foam lamella. Various defoaming mechanisms discussed in the technical literature are usually explained in terms of a sequence of individual steps. For clarity each step is first considered, and the interaction of the individual mechanisms is explained later in this chapter.

3.2.2.1 Defoaming by drainage/slow defoaming

To cause defoaming a defoamer must be able to penetrate from the inside of a foam lamella to the interface between the liquid and the foam-forming gas. The surfactants stabilising the lamella act as an entry barrier which impedes penetration of the defoamer droplet into the lamella interface. Some defoamers are unable to pass the entry barrier. These so-called slow defoamers only start to act once the foam lamella has been sufficiently thinned by continued drainage. The defoamer droplets (oil droplets) are trapped in the lamellas during this process or migrate to the nodes between the lamellas. If drainage continues, the narrowing lamellas exert a capillary pressure on the defoamer droplets. Once a critical point is reached, the capillary pressure of the upper bubbles in the foam is high enough for the oil droplets to penetrate the surface of the lamella causing its collapse [5, 8] and placing stress on adjacent lamellas. In many cases this stress is sufficient to make the neighbouring lamellas collapse as well. In foams highly thinned by drainage this often leads to a chain reaction until almost complete defoaming occurs. Remaining bubbles are often stable for a long period. The speed and efficiency of defoaming by drainage is inadequate for many processes in the coatings industry. For this reason, there is a demand for defoamers that provide faster, more efficient defoaming mechanisms [8].

3.2.2.2 Entry barrier/entry coefficient

The entry barrier describes the kinetic resistance which a defoamer droplet must overcome to break through the lamella surface. The first step of defoaming is always the entry of the defoamer droplet into the surface of the lamella, i.e. the interface to the surrounding air. To do so the defoamer must have a low surface tension [2, 3]. If a defoamer droplet comes into contact with the surface of a lamella as a result of motion in the lamella due, for example, to drainage-induced flow, the droplet needs to overcome the entry barrier to reach the lamella/air interface.

43

Defoaming of coating systems The entry barrier of the lamella surfaces is determined by the surfactants stabilising the foam. Systems with low surfactant content, below the critical micelle concentration have a low entry barrier. The entry barrier rises with increasing concentration of surfactant on the lamella interface to a maximum value at total saturation. Another parameter determining the entry barrier is the mobility of the surfactants. Dynamic processes create new interfaces or subject lamellas to stress due to expansion. To create a stable foam, the newly formed surfaces must be quickly covered with surfactant. In systems with less mobile surfactants, saturation of the newly created interfaces with surfactants is delayed, thus facilitating ingress of the defoamer droplet at these temporarily inadequately stabilised areas in the lamella [8]. Once a defoamer droplet passes the entry barrier and enters the interface of the lamella, the next step is determined by the entry coefficient. The entry coefficient E describes the thermodynamic equilibrium of the oil droplet in the lamella at the liquid/air. interface [4, 5].

E = σAW + σOW - σOA

If the entry coefficient is negative, complete wetting of the defoamer by the liquid phase is the most thermodynamically-stable condition. In this case, the droplet would leave the interface and migrate back into the liquid phase. Only if the entry coefficient is positive will the defoamer droplet remain on the surface of the lamella. A positive entry coefficient for the defoamer oil is an important prerequisite for many defoaming mechanisms. However, to be effective defoamers also need to fulfil other requirements [8].

Figure 3.3:  a) Distribution of defoamer droplets in a lamella, b) Defoamer droplet enters the lamella

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Defoamers

3.2.2.3 Bridging mechanism

The bridging mechanism requires that a defoamer droplet can pass the entry barrier and that it has a positive entry coefficient. Bridging occurs when a defoamer droplet penetrates on both sides of a lamella. The behaviour of a defoamer droplet in the lamella is determined by the bridging coefficient B. Only defoamers with a positive bridging coefficient are able to destabi- Figure 3.4:  Bridging of a foam lamella lise the foam lamella by subsequent defoaming mechanisms. If the bridging coefficient is negative, the oil remains as a stable bridge in the lamella.

B = σAW2 + σOW2 – σOA2

The value of the bridging coefficient is determined by the contact angles between the three phases, oil, liquid and air, and thus by the geometry of the droplet and the thickness of the lamella. If changes in the thickness of the lamella occur due to drainage, the absolute value of the bridging coefficient may change. The bridging defoamer drop may also grow by picking up spread-out oil. In both cases this may cause the bridging coefficient to change from negative to positive values, thus converting a stable bridge into an unstable bridge. A

Figure 3.5:  a) A contact angle αW lower than 90° results in a negative bridging coefficient, b) A contact angle αW higher than 90° results in a positive bridging coefficient.

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Defoaming of coating systems positive bridging coefficient is a prerequisite for the subsequent bridging-stretching or bridging-de-wetting mechanisms [6, 8].

3.2.2.4 Spreading mechanism

Defoaming via spreading is only possible with oils of low surface tension. Once a defoamer droplet enters the lamella it spreads on its surface. The ability of defoamer oil to spread is described by the spreading coefficient.. S= σAW - σOW - σOA Spreading of oil on the surface of a lamella causes a change in the surfactant distribution on the lamella’s surface [4, 5]. The newly created oil/water interface reduces the entry barrier for further defoamer droplets to penetrate into the lamella surface through the spreadout oil. Consequently, the area of the spread-out oil increases. Oil lenses caused by spreading may result in surface defects, such as craters or fish eyes, in coatings. A positive spreading coefficient is a prerequisite for defoaming by spread out liquids or by the spreading-wave-mechanism [8].

3.2.2.5 Bridging stretching mechanism

The prerequisite for the bridging stretching mechanism is a positive bridging coefficient. The mechanism also works in thick foam lamellas. A defoamer droplet penetrating the lamella causes bridging of the latter. In so doing the droplet becomes biconcave with the smallest thickness in its middle. Mechanical stress on the lamella, e.g. caused by uncompensated capillary forces, causes elongation and further thinning of the oil droplet. Once the droplet reaches a state when it can no longer compensate for the mechanical stress, it

Figure 3.6:  a) Defoamer droplet on the surface of a lamella, b) Spreading of a defoamer droplet on the surface of a lamella

46

Defoamers breaks, causing destabilisation of the lamella and consequent collapse of the foam bubble. The prerequisite for the bridging stretching mechanism is that the defoamer droplet is deformable. Due to their inflexible geometry hydrophobic particles cannot defoam via the bridging stretching mechanism [5, 6, 8].

3.2.2.6 Bridging de-wetting mechanism Analogous to the bridging stretching mechanism the first step of the stretching de-wetting mechanism is droplet entry and bridging. This is only possible if the entry barrier of the lamella can be overcome and the defoamer has a positive bridging coefficient. In systems with low surfactant content, spontaneous de-wetting of the defoamer droplet would be expected. In surfactant-rich systems, such as coatings, spontaneous de-wetting will not occur as the surfactants will wet the defoamer droplets. The ability to wet the defoamer droplet can be characterised by the contact angle between the defoamer droplet and the surrounding liquid. If the contact angle is less than 90° the defoamer droplet is wetted by the surrounding liquid. However, if the defoamer droplet is able to assume a lens shape, the contact angle at the edges of the lens becomes high enough to cause de-wetting of the defoamer droplet followed by collapse of the foam bubble [6, 8]. In cases where the lens undergoes further deformation without de-wetting, the bridg-

Figure 3.7:  a) Bridging of a defoamer droplet with a positive bridging coefficient, b) and c) Stretching of a defoamer bridge till collapse

47

Defoaming of coating systems ing de-wetting mechanism transforms into the bridging stretching mechanism. It is probable that the bridging de-wetting mechanism occurs preferentially with higher viscosity defoamer oils, which are able to form more stable oil lenses. For low viscosity defoamers the bridging stretching mechanism is more likely. Up to now, evidence has been found for the bridging de-wetting mechanism for hydrophobic particles only in systems free of strong surfactants. It is unlikely that the bridging de-wetting mechanism applies in real surfactant-rich coatings systems. Because of the very fast de-wetting process it has not been possible to definitely confirm the mechanism for defoamer oils yet [8].

3.2.2.7 Spreading fluid mechanism

Figure 3.8:  Bridging de-wetting mechanism

A possible mechanism for defoaming by spreading is the spreading-fluid mechanism in which the oil creates a flow of the liquid in the lamella in the direction of the spreading (Marangoni flow). This causes a thinning of the lamella, resulting in the lamella breaking and the foam collapsing [3, 5, 6, 8]. Spreading of defoamer oils in lamellas has been already detected experimentally. The destabilisation of the lamella by Marangoni flow seems likely but has not yet been proven experimentally [8].

3.2.2.8 Spreading wave mechanism

Figure 3.9:  Spreading fluid mechanism

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The spreading wave mechanism has been demonstrated only for slow-acting defoamers. The mechanism only works in very thin lamellas (approx. 1 micrometre). Spreading of a defoamer causes disruption and thinning of the surfactant film which stabilises the lamella. Resultant stresses in the lamella must be compensated by its deformation. The lamellas assume a wave like structure of varying thickness. This lamella is less resistant to mechanical stress than a defoamer free lamella. The spreading wave mechanism

Defoamers does not act locally at the point of entry of the defoamer droplet; rather the disruption created by the defoamer covers a larger area of the foam lamella. The mechanism has been proven only for slow defoamers in sufficiently large lamellas. Lamellas in the micrometre wavelength region and amplitudes in the nanometre range have been measured experimentally. The disruption caused by the defoamer extended over the whole foam lamella [8].

3.2.2.9 Effect of fillers on the performance of defoamers

Solid particles can also act as defoamers according to some of the mechanisms described (e.g. bridging de-wetting mechanism). The filling of defoamer oil with a solid particle can boost the defoaming characteristics to higher levels than that of the individual components [1, 8]. A solid content of only a few percent is sufficient to achieve this. Excessive solid content often causes excessive viscosity of the oil and impairs flow and deformation, resulting in reduced efficiency of the oil. For optimum performance it is necessary for the solid particles to cover the surface of the defoamer droplet. A strong hydrophobic particle will be too compatible with the oil and will be completely wetted by it. If the solid particle is too hydrophilic, there is a risk that it can be removed from the oil droplet into the aqueous phase of the coating. In both cases the efficiency of the defoamer will suffer. The filler has a major influence on the entry barrier and the bridging mechanism of defoamer oil.

Influence on the entry barrier

The solid covering of a defoamer droplet creates an irregular surface. The solid disturbs the symmetry between the air, oil and liquid phases and acts like a kind of pin, lowering the entry barrier into the lamella. Many defoamer oils can only overcome the entry barrier and penetrate the lamella surface if they are filled with solids. [3, 8].

Influence on bridging

Oils spreading at the surface only penetrate into the lamellas to a limited depth. For this reason, the lamellas need to be already widely thinned out and the drainage well advanced for the spread-out oil to bridge the lamella. A non-deformable filler enables the penetration depth to be significantly increased; bridging then occurs at higher lamella thicknesses [8].

3.2.2.10 Summary

To sum up, a defoamer must have – the capability to penetrate into foam lamellas, – a low surface tension to spread on the interface liquid/gas, – a de-wetting effect. The balance between compatibility and incompatibility (controlled incompatibility) must remain even when the paint is stored for a long time at elevated temperatures.

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Defoaming of coating systems

3.3 Chemistry and formulation of defoamers 3.3.1 Active ingredients in defoamers Active ingredients with various chemical compositions are suitable for formulating defoamers. The effectiveness of the defoamer is always dependent on its partial incompatibility in the application medium. This permits formation of defoamer droplets in the system, without causing film or surface defects due to excessive incompatibility. Defoaming is thus always a compromise between efficiency and compatibility. At the same time, a high defoaming power is required. The mostly commonly used defoamer active ingredients are described below. In practice, combinations of active ingredients are used to formulate very powerful defoamers.

3.3.1.1 Silicone oils (polysiloxanes)

The class of silicone oils covers pure silicone oils as well as organic modified polysiloxanes. In many applications, pure silicone oils are too incompatible and are thus mostly used in low dosages as a silicone tip in combination with other defoamer active species. Organic modification of polysiloxanes, for example with polyethers, enables the compatibility of the defoamer to be tailored to specific application systems. The chemistry of silicone defoamers provides a wide range of modifications for polysiloxanes, resulting in an extraordinary variety of possible polysiloxanes tailored to specific applications Thus silicone defoamers are the most widely used defoamers in the coatings industry [4].

3.3.1.2 Mineral oils

Mineral oils used as defoamers are mostly highly purified paraffin oils or white spirits. They offer the advantage of high stability against environmental influences and do not resinify or become rancid. White spirit is colourless, odourless and free of aromatic compounds. Because of their aliphatic structure, the active ingredients are highly hydrophobic and thus have only limited compatibility with coating systems. This limits the use of mineral oils in many coating systems. Mineral oils are mainly used in architectural coatings with medium to high PVC. In more sensitive applications mineral oils tend to result in flooding or surface defects such as reduced gloss, smearing or fogging which are well known in architectural coatings [9].

3.3.1.3 Vegetable oils

The requirement for environmentally compliant products and the use of renewable resources are increasingly important in the development of defoamer active ingredients.

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Chemistry and formulation of defoamers Vegetable oils consist of triglycerides of saturated or unsaturated fatty acids. The aliphatic backbone makes them very hydrophobic, which places limitations on the range of uses of vegetable oils. Like mineral oils, vegetable oils are used preferably as defoamer raw materials for medium to high PVC architectural paints. In more sensitive applications vegetable oils tend to produce flooding or impair the surface finish of the coating.

3.3.1.4 Polar oils

The numerous ways of varying synthetic polymers provide the ideal opportunity to tailor-active ingredients for specific coating systems. Commonly used active ingredients are fatty acids, alcohols based on fatty acids, polyethers, alkyl amines, alkyl amides, tributyl phosphate or thio ethers. The compatibility of oils in application systems can be easily controlled by varying their polarity. Polar oils are often used in combination with other defoamer active ingredients to control the compatibility of the formulation.

3.3.1.5 Molecular defoamers (gemini surfactants)

Molecular defoamers or gemini surfactants are a new class of defoamers. Like surfactants, molecular defoamers have pronounced surface active properties and the molecules orientate themselves at the surface of foam lamellas. The foam lamella is destabilised by the molecular defoamer and destroyed. In contrast to all other defoamer active ingredients, molecular defoamers do not act via their incompatibility in the system like an oil droplet. The defoaming mechanisms, described in the previous section, cannot therefore be applied to molecular defoamers. Molecular defoamers are also very effective at controlling micro-foam because they are able to migrate into bubbles and aid in deaeration. The use of a molecular defoamer as an antagonistic surfactant or polymer is one answer to today’s needs concerning the formulation of modern water-borne coatings and inks. This type of product boosts the efficiency of the defoaming process and can therefore be used at low dosages, as well as in combination with various classes of defoamers, such as mineral oil-based, silicone-based and water-borne emulsion types. Molecular defoamers can be used in UV coatings, furniture coatings, automotive coatings, and printing inks [10, 11].

3.3.1.6 Hydrophobic particles

It is possible to defoam water-borne systems using only hydrophobic particles, but it is more common to use them in combination with other defoamer actives. Commonly used hydrophobic particles are surface-modified silica, aluminium oxide, urea, waxes (e. g. magnesium stearate) or polymer particles (e.g. polyamides, polypropylene). Particles with an irregular amorphous surface are usually more effective than spherical or smooth particles [1, 3, 4]. The hydrophobicity of the particle needs to be such that the particle

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Defoaming of coating systems Table 3.2:  Comparison of supply forms of defoamers Concentrates

Emulsions

Solutions

Active content

100 %

10 – 25 %

≤ 60 %

Filled with hydrophobic particles

mostly

mostly

rarely

Solvent-free

yes

yes

no

Dilutability

good

limited

very good

Storage stability

long

limited

long

Freeze stability

yes

no

yes

Incorporation

requires high shear forces

easy

easy

Efficiency Compatibility

very high

high

moderate

in some systems critical

good

very good

orientates preferentially on the interface of the defoamer oil to the water phase. If the particle is too compatible with the oil, it will be completely wetted by the oil. If it is too hydrophilic it will migrate into the water phase. In both cases the efficiency of the defoamer will fall. In practice partially hydrophobic particles have proved particularly effective [4].

3.3.1.7 Emulsifiers

In the manufacture of water-borne defoamer emulsions emulsifiers are used to prevent the defoamer droplets coalescing or separating.

3.3.1.8 Solvents

Some defoamers are supplied as solutions of the active substances in solvents. The choice of solvent depends on the type of defoamer active ingredient(s) and the final application of the formulated defoamer.

3.3.2 Defoamer formulations Combinations of the active ingredients described above are frequently used in defoamers. This enables properties such as effectiveness against macro-foam and micro-foam or compatibility with the application system to be varied as required. Filling with a hydrophobic particle often boosts efficiency significantly [3]. Defoamers are supplied in different forms: concentrates, emulsions or solutions. The properties of the different supply forms are indicated in Table 3.2.

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Product recommendations for different binders Table 3.3:  Overview of defoamer suppliers Mineral Silicone Plant oil oil oil

Polymer

Molecular defoamer



 

 



 









 



 

Dow Corning Corp. DC…

 



 

 

 

Elementis plc “Dapro”

Supplier

Brand name

Ashland Inc.

“Drewplus”





BASF AG

“Foamaster”/ “Foam Star”



Byk Chemie GmbH

“Byk”…





 



 

Elken ASA

“Silcolapse”/ “Bluesil”

 



 

 

 

Evonik Operations GmbH

“Tego” “Foamex”/ “Surfynol”











Münzing GmbH

“Agitan”/ ”Dee Fo”









 

San Nopco Ltd.

“SN-Defoamer”/ “Noptam”





 

 

 

3.3.3 Suppliers of defoamers Table 3.3 gives an overview of major global suppliers of defoamers and of defoamer types. Because of the large number of suppliers, no claim is made to completeness. Suppliers only of regional importance or who do not supply the paint industry have not been included.

3.4 Product recommendations for different binders Defoamers are required in practically all water-borne coating formulations. Important criteria in choosing the right defoamer are the types of binder and cosolvent. The use of external emulsifiers is frequently necessary for stabilising water-borne binders. Depending on the binder chemistry, there are large differences between the type and amount of emulsifiers used. The hydrophobicity of the defoamer thus has to suit the binder chemistry and the emulsifiers to achieve efficient defoaming. Paint manufacturers only rarely have detailed knowledge of the composition of a binder, but general recommendations for choosing defoamers are possible based on experience.

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Defoaming of coating systems

3.4.1 Acrylic emulsions For defoaming and deaerating formulations based on acrylic emulsions, defoamers with low to moderately strong hydrophobicity are generally used. More strongly hydrophobic grades often cause incompatibility. Due to their good compatibility and good gloss retention, modified polysiloxanes or silicone free polymers are widely used defoamers. In architectural coatings, mineral oils and vegetable oils are also commonly used.

3.4.2 Styrene acrylic emulsions The main defoamers for styrene acrylic emulsions have moderate to very strong hydrophobicity. Grades suitable for acrylic emulsions are often less effective in defoaming styrene acrylic emulsions. Depending on the area of application, defoamers with different chemistries are used. Widely used defoamers for architectural coatings are mineral oils, natural or synthetic oils or polysiloxanes. For applications with low PVC more compatible, grades, such as modified siloxanes or silicone-free polymers, are preferred.

3.4.3 Vinyl acetate-based emulsions Moderately to strongly hydrophobic defoamers are preferred for formulations based on vinyl acetate emulsions. Depending on the area of application, defoamers with different chemistries are used. Mineral oils, natural or synthetic oils or polysiloxanes are frequently used in matt architectural coatings. In architectural coatings with lower PVC, where higher quality finishes and gloss are required, more compatible grades based on modified polysiloxanes or silicone-free polymers are mainly used.

3.4.4 Polyurethane dispersions Defoamers with comparatively low hydrophobicity are used in polyurethane dispersions. The best balance between efficiency and compatibility in such dispersions can be achieved with modified polysiloxanes or silicone-free polymers.

3.5 Product choice according to field of application Foam is practically unavoidable in water-borne coatings formulations. In addition to the binder, other criteria are important in choosing the right defoamer. These include: – pigment volume concentration (PVC), – method of incorporating the defoamer,

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Product choice according to field of application – application of shear forces during application, – surfactant content of the formulation.

3.5.1 Influence of the pigment volume concentration (PVC) Low PVC formulations or clear coats are generally more sensitive to film defects caused by defoamers than high PVC formulations. Low PVC formulations therefore require more compatible, less hydrophobic defoamers.

3.5.2 Method of incorporating the defoamer Defoamer concentrates with 100 % active content or strongly hydrophobic defoamers require high shear forces for incorporation to generate defoamer droplets of sufficiently fine particle size. If it is not possible to apply high shear forces it is better to use more compatible pre-emulsified defoamer emulsions or solutions of defoamer actives in solvents.

3.5.3 Application of shear forces during application More powerful, more hydrophobic defoamers can frequently be utilised in coatings, the application of which involves high shear forces, e.g. airless spraying. Due to the shear forces introduced during application the defoamer droplets may be further reduced in size and defoamer compatibility improved.

3.5.4 Surfactant content of the formulation High surfactant content, required for example to wet critical substrates, often reduces the efficiency of a defoamer. In such formulations the use of stronger, more hydrophobic defoamers is recommended. In formulations which are sensitive to strong defoamers, greater amounts of a milder defoamer are often the only solution.

3.5.5 Recommended tests for evaluating defoamers Since defoaming characteristics are influenced by the conditions during manufacturing and application of the paint, it is recommended that the test method replicate those conditions as closely as possible [12]. The evaluation should also include an assessment of surface defects that are caused by the defoamer under test.

55

Defoaming of coating systems The foaming behavior of dilute water-borne emulsions is tested by incorporating air into the system with a high-speed stirrer and then comparing the efficiency of each defoamer via the volume of foam created by the stirring process. This test is performed quickly but it must be considered a preliminary test only. – The stir-flow test is a more meaningful method for evaluating defoamer strengths and possible surface defects at the same time. It consists in pouring the freshly stirred sample onto an inclined glass plate or stretched polyester film and then visually assessing the dried film from the two aspects above. – The roller test, which simulates real-life application of paints applied by roller, is used for roller-applied water-borne emulsion paints, mainly in the architectural coatings market segment. Surface defects caused by foam, and incompatibilities or air occlusions in the coating, can be assessed on the dried paint film. The film is illuminated from behind to reveal even the smallest foam bubbles. The emulsion-based paint should be tested approx. 24 hours after production and again after storage for, e.g., 4 weeks at 50 °C, as defoamers can lose effectiveness over time.

3.6 Tips and tricks In many applications the use of defoamers cannot be avoided. To achieve optimum defoaming a few points must be considered. – It is important to ensure that the defoamer is sufficiently mixed in. Defoamers must have a limited compatibility with the system to be defoamed. Inadequate mixing can result in surface defects such as craters. – Defoamer concentrates need to be incorporated with high shear forces. Preferably they should be added to the mill base. – Foam formation is also related to application conditions. When testing defoamers, efficiency and compatibility should be checked under conditions close to those of the application. – The addition of a defoamer to the mill base increases the former’s compatibility with the system. – Slight defoamer incompatibility can be improved by the addition of, for example, a substrate wetting additive or a flow and glide additive. – In many cases optimum defoaming can be achieved with a combination of defoamers. Splitting the defoamer between the mill base and the let down often gives the best results.

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Summary

3.7 Summary The structure of foam is not homogeneous. Depending on the air content and composition of the liquid phase it can have very different properties. Moreover, many foams change their structure over time. There is no universal defoamer for combating all types of foam. Experiments on different model systems have demonstrated various defoaming mechanisms, some of which interact synergistically while others do not interact at all. The complexity of coating formulations does not allow defoaming mechanisms to be investigated directly in real systems. Even today the search for the best defoamer for a particular water-borne coating is largely dependent on empirical knowledge. Important selection criteria are the chemistry of the binder system, the application system, the pigment volume concentration, the application method and the incorporation conditions for the defoamer. Recommendations by suppliers of defoamers provide helpful advice for selecting the right product. However, in the end, testing under conditions close to practice is the only way of proving if the choice is correct.

3.8 Literature [1] H. F. Fink, W. Heilen, O. Klocker, G. Koerner, “Entschäumer und Entlüfter in wässrigen oder lösemittelarmen Lacksystemen“, Goldschmidt informiert, p. 9-21 [2] Roland Sucker, Katrin Lehmann, “Schaum-Killer Entschäumer für die Herstellung von Polymerdispersionen“, Farbe und Lack 4/2007, p. 100-104 [3] Ashland Deutschland GmbH, “ANTISPUMIN – Entschäumer und Entlüfter“, 12/2006 [4] Dr. Ralf R. Schnelle, Otto Klocker, “Keeping ahead of foam control”, Asia Pacific Coating Journal 6/2004, p. 14-17 [5] Dr. Ralf R. Schnelle, Otto Klocker, “Defoaming of PU Coatings – Preventing of fast escaping gas with new additives”, European Coatings Journal 12/2004, p. 26-31 [6] Carsten Penz, “Wirkungsmechanismen siloxanbasierter Schauminhibitoren in Mineralöl”, Essen 2005, Dissertation an der Universität Essen, Fachbereich Chemie

[7] Hans-W. Mindt und Wolfgang Ottow GbR, Literature and Internet research on foam [8] Nikolai D. Denkov, “Mechanisms of Foam Destruction by Oil-Based Antifoams”, Langmuir 2004, 20, 9463-9505 [9] Dr. Ulrich Zorll, Römpp Lexikon – “Lacke und Druckfarben“, Thieme-Verlag, p. 39 and 194 [10] Wim Stout, Christine Louis, “Molecular Defoamers – Resolving stability and compatibility problems”, European Coatings Journal, 4/2005, p. 132-137 [11] K. Lehmann, P. Hinrichs, S. Maslek, “Siloxane-based Gemini Surfactants – A novel class of low foaming substrate wetting agents”, European Coatings Journal, 5/2004 [12] The Big TEGO 2012 Technical Background on Defoamers [13] J. Mangano, P. Bene, S. Oestreich, APCJ, 12/2014

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General assessment of rheology modifiers

4 Synthetic rheology modifiers Jean-Marc Suau and Dr Benoît Magny

4.1 General assessment of rheology modifiers The last thirty years have seen a rapid evolution in formulations within the paint industry. This has mostly been to the benefit of water-borne latex paints, especially in the building and industrial finishing sector, for both economic and ecological reasons. The evolution in water-borne formulations is the most significant because water is cheap, natural, non-toxic, widely available and renewable. Hence, economics, regulations and long-term sustainability all favour the use of water in coatings systems. To perfectly meet the requirements of a given application, the viscosity of formulations has to be adjusted in order to obtain an accurate rheological profile. It has therefore proved essential to have, on one hand, viscometers that allow the behaviour of the mixtures constituting paints to be described as completely as possible and, on the other, additives capable of modifying and optimising them [1]. In this context, paint additives play a fundamental role, because they condition a large part of the coating properties, such as the stability of the stored paint, its applicability, the quality of the final paint as well as its cohesion [2]. A water-borne paint contains up to fifteen constituents, the most important of which are pigments (titanium dioxide, carbon black, natural-coloured pigments, such as iron dioxide, and organic pigments), fillers (natural calcium carbonate, precipitated calcium carbonate, clay, titanium dioxide…) and synthetic latex. These three main components do not have any rheological property that renders them directly usable in formulation, because they are not necessarily compatible (organic pigments could induce micro flocculation when introduced into a paint) and the viscosities of the mixtures are generally too low. The use of dispersing agents (generally acrylic homo- or copolymers) to disperse mineral pigments and wetting agents (such as surfactant types) to disperse organic pigments or carbon black makes it possible to obtain a mixture of satisfactory homogeneity, thereby

Wernfried Heilen et al.: Additives for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

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Synthetic rheology modifiers avoiding flocculation, allowing good incorporation of fillers as well as a sufficiently effective high content of non-volatile dry matter. When the main components reach the optimum state of dispersion, it is necessary to bestow on the formulation an ideal rheological profile for the sake of good paint applicability and paint storage stability. The use of thickening agents will provide the desired rheological profile by increasing the viscosities of the continuous phases. If, in the past, these products were essentially polysaccharides and/or synthetic water-soluble homopolymers of high molar mass, increasingly rigorous specifications currently prefer tri-dimensional acrylic thickeners (better known as ASEs or alkali-swellable emulsions or microgels) and new synthetic or semi-synthetic hydrophobically modified polymers, a better term for which is associative thickeners [3]. Depending on the constituents of the medium, they perform more efficiently in multiple applications. Associative thickeners can be anionic or non-ionic. Rheological additives not only are used to adjust the viscosity of paint, but also may solve issues linked to the state of dispersion of the particles in the formulation and some specific properties of paints after application and drying. In the present chapter, therefore the advantages are described that these particular structures can provide when they are formulated in aqueous formulations. The chemistry of these rheological additives is discussed and finally, the mechanisms that characterise their reactivity and make them more or less efficient depending on the components of the formulations are reviewed.

4.1.1 Market overview Thickeners are used in coating formulations to confer specific, required rheological properties. These properties are necessary during manufacture, storage and application. Synthetic thickeners offer these properties, including improved flow and levelling and can be adapted to new latexes. Several trends exist: – The reduction in emissions of organic solvents has led to an increased interest in high solid paint and, in this case, the ability of a thickener to increase viscosity at a medium or high-shear rate while keeping a very low impact on viscosity at low shear. – New environmentally friendly binders have recently been developed that use new surfactants: alkyl phenol ethoxylated (APE)-free binders are now an important part of the paints market. The reactivity of latex particles’ surfaces has changed, and this has led to the development of a new generation of thickeners. – The use of organic pigments that are dispersed without solvent and are APE-free also changes their behaviour when they are introduced into the formulation (shock effect, incompatibility with latex, loss of tint, decrease in storage stability).

60

General assessment of rheology modifiers

4.1.2 Basic characteristics of rheology additives The three most common thickeners are HEC (hydroxyl ethyl cellulose), HASE (hydrophobically alkali-swellable emulsion) and HEUR (hydrophobically ethoxylated urethane). The most cost-effective product, an alkali-thickenable emulsion, has some technical drawbacks with regard to gloss formulations (flow), water up-take, open-time and pH dependence. Polyurethanes are more costly but offer good flow, water resistance and open-time and they are ideal for gloss applications. Cellulose ethers are still considered to have good open-time. HEUR alkali-swellable emulsions (ASEs and HASEs) are the most diverse and most promising portfolio of products to evolve. There are many good books explaining the fundamentals of rheology; for example [4]. Rheology is the science of flow and deformation of materials. The scope of the present review is limited to the flow of water-borne coatings, which is characterised by the viscosity of the paint. Different types of rheology profile are characterised by a dependence of viscosity on shear: – the ideal newtonian profile is characterised by a constant viscosity upon shearing, – the non-newtonian profile is shear-rate dependent and most cases fall into one of two sub-groups: – the pseudo plastic profile undergoes a decrease in viscosity with shear (shear thinning). – the dilatant profile undergoes an increase in viscosity with shear (shear thickening). These profiles are important, because they model what happens to paint formulations when applied by brushing or spraying. Dilatant profiles are clearly to be avoided, with the result that all the work is directed at trying to harmonise Newtonian flow and pseudo plastic behaviour. Pseudo plasticity is essential for avoiding segregation issues at high shear viscosity. The rheology profile can also be time dependent. This is the case when a long time is required for a viscosity plateau to be reached at a constant shear rate. This phenomenon is called thixotropy and is often confused so with a pseudo plastic profile. It is beneficial when the viscosity of paint is required to increase slowly after application.

Measuring the rheology profile of a coating system

The rheology profile of coatings formulations is usually determined with the aid of a dynamic rheometer. Historically, paint formulators have limited themselves to four measurements to characterise the rheology: Brookfield viscosity at 10 rpm and 100 rpm (covering the 1 to 10 s-1 shear rate region), cone and plate viscosity (commonly known as ICI viscosity, measured at a high shear viscosity of about 104 s-1), and Stormer viscosity (medium

61

Synthetic rheology modifiers shear viscosity, 102 s-1). Some formulators rely on only a few measurements to define their required Process Shear rate [s-1] viscosity profile. Sedimentation 120°, water beading occurs PS 40 (see Chapter 12 “Hydrophobing Aluminium 40 agents”). The smaller the contact anPMMA 46 gle the better is the wetting behavSteel 50 iour. From the equation, the following rules for wetting can be deduced: – Substrate with high surface energy can be wetted relatively easily. – Liquid with low surface energy shows generally good wetting behaviour. In terms of paint this means that the surface tension of the coating must be less than the surface energy of the substrate for the paint to wet successfully and therein lies the difficulty for ordinary coatings when water is used as a solvent. The surface energies in Table 5.1 show that water with a value 73 mN/m has by far the highest surface tension [2]. For water as a universal solvent expected to wet all kinds of substrates, the rules given above are not satisfied and wetting of aqueous coatings on low energy substrates such as plastic is a challenge for the coatings formulator. There are two possible ways of remedying the imbalance and achieving successful wetting on a given substrate. Either the surface tension of the substrate must be increased, or the surface tension of the water phase must be decreased. Cleaning metal substrates leads to an increase in surface tension by removing grease and oil present from the production process. The same applies to corona treatment or flaming of plastic surfaces. This produces a high-energy surface by oxidation. These extra steps in coating are very time-consuming and costly. A good alternative for

106

Mechanism of action achieving good substrate wetting on low energy substrates is to reduce the surface tension of the water-borne coating. For the most difficult substrates, e.g. polyethylene or propylene, a combination of both procedures is necessary to ensure good wetting and sufficient adhesion of the coatings.

5.1.5 Substrate wetting additives are surfactants Surfactants are bifunctional compounds with at least one hydrophobic and one hydrophilic group in the molecule. Because of their structure, surfactants orient themselves at every interface. With water as a solvent, the hydrophobic part of the substrate wetting additive is pushed out of the aqueous phase, while the hydrophilic part has a high degree of compatibility with the medium. The incompatibility of at least one part of the wetting additive in the medium is the force driving surfactants to orientate at the interface. Many surfactants are used in detergents, cosmetics and coatings.

5.1.6 Mode of action of substrate wetting additives Substrate wetting additives accumulate in the aqueous phase at the liquid/air interface, with the non-polar part towards the air and the polar part towards the liquid. This enrichment affects the surface tension of the aqueous phase. In pure water, all the water molecules at the water/air interface are surrounded by water molecules in the liquid phase. Water, as a dipole has strong interacting forces, resulting in a high surface tension. The addition of substrate wetting additives to water causes a change in the force balance of molecules interacting in the surface area. The substrate wetting additives displace the water molecules and the interaction forces between the substrate wetting additive and water are much smaller than those between water molecules alone. The force diagram no longer represents interaction between water molecules alone but includes those between surfactant and water molecules. Within this interface area the surfactant molecules reduce the force directed into the aqueous solution resulting in significant reduction of surface tension of the aqueous phase. Consequently, addition of a substrate wetting additive allows a water-borne paint to wet substrates with a low surface tension. This creates a closed film, even at low film thicknesses, and the paint is less liable to cratering (see Figure 5.4:  Minimised force F2, resulting from adding substrate wetting additives crater test).

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Substrate wetting additives

5.1.7 Further general properties of substrate wetting additives/side effects The best-known side effect of substrate wetting additives is the relatively strong tendency to promote foam. If a mechanical process such as pumping, pouring or stirring causes air to be incorporated into the system, the substrate wetting additives will move to the newly formed air-bubble/paint interface and stabilise the air bubbles. Small bubbles will remain in the system and can lead to popping and pinholes. Large air bubbles will rise and form stable foam on the surface of the coating (see Chapter 3 “Defoamer” and Chapter 7 “Deaerator”). In general, very hydrophilic, water-compatible substrate wetting additives have the greatest tendency to stabilise foam. Another characteristic is that the cured coating remains to a greater or lesser degree hydrophilic and this can affect resistance to moisture.

5.2 Chemical structure of substrate wetting additives 5.2.1 Basic properties of substrate wetting additives Substrate wetting additives are characterised by the molecule having both hydrophobic and hydrophilic groups. These can be chemically very different. The hydrophobic part can be a hydrocarbon chain varying from 8 to 22 carbon atoms. In powerful substrate wetting additives this hydrophobic part is a siloxane-chain or perfluoridised hydrocarbon chain. The hydrophilic part can be either a negatively or positively charged or neutral head group. This molecular structure with one hydrophobic and one hydrophilic group is termed amphiphilic and leads to the basic properties of substrate wetting additives. They orientate themselves at every interface and form micelles at higher concentrations in the liquid phase. They are classified by their hydrophilic head groups as, e.g., ionic, non-ionic and amphoteric surfactants. The classic soaps and also the sulfosuccinates, which are important in paints, are anionic surfactants. Nonylphenylethoxylates are non-ionic surfactants. In coatings, polyethersiloxanes, fluoro surfactants, alkoxylates, allylphenylethoxylates, sulfosuccinates and gemini surfactants are used.

5.2.2 Chemical structure of substrate wetting additives important in coatings 5.2.2.1 Polyethersiloxanes

Polyethersiloxanes consist of a short siloxane chain and a polyether modification. The linear siloxane chain is hydrophobic, and the polyether modification is the polar part of the mole-

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Chemical structure of substrate wetting additives cule. In the coatings industry the term “silicone additives” is often used and many formulators associate this with cratering caused by silicone oil so that the use of this product group is sometimes avoided, even though polyethersiloxanes are markedly different from silicone oil. Silicones consist of several siloxane units. The silicon atoms are bonded to 1 to 4 oxygen atoms and when the number of oxygens is not sufficient to achieve the electron shell octet of the silicon; this is completed by bonding with organic groups. The siloxane unit is formed by having each oxygen atom as a bridging component between two silicon atoms. One siloxane unit can have one to four more substituents, depending on the number of oxygen atoms with unsatisfied free valencies. Thus, siloxane units can be mono-, di-, tri- or tetra-functional. Polydimethylsiloxanes (silicone oils) are hydrophobic, non-polar molecules, which are not water miscible. They have a very low surface tension around 22 mN/m and inter-molecular forces are so weak that they are classified by viscosity. Over a very large range the viscosity increases linearly with the molecular weight of the silicone oil but is almost independent of temperature. These basic features make silicone oils excellent wetting agents and lubricants. The disadvantage in coatings is the tendency to cratering which rises exponentially with increasing molecular weight. This is good for hammer effects but must be strongly avoided in ordinary coatings. Therefore, only modified short siloxane chains are used as substrate wetting additives. Modification is the key factor in excluding the tendency to cratering. The structure of a polyether-modified siloxane, used as substrate wetting additive is as follows. This structure retains the positive features of the silicone oil (low surface tension, high interface activity) but additionally improves compatibility in water-borne coatings thus removing the risk of cratering. The products are more or less foam stabilising depending on the polyether. The higher the relative Si content in the molecule, the more surface active is the substrate wetting additive. Substrate wetting additives have up to seven siloxane units; polyethersiloxanes with longer siloxane chains are classified as flow and levelling additives (see Chapter 8 “Flow additives”). Substrate wetting additives based on polyethersiloxanes are especially suitable for reducing the static surface tension, resulting in improvements in substrate wetting, atomisation in spray application and flow of water-borne coatings.

5.2.2.2 Gemini surfactants

Surfactants are essential during the production and application of paints and coatings. The term “gemini” for describing amphiphilic substances was coined by Menger  ­[7] in 1991 and refers to the fact that this type of surfactant has two polar centres or head groups in the polyether segment which are connected by a spacer segment.

Figure 5.5:  Schematic structure of a poly­ ethersiloxane

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Substrate wetting additives Today, siloxane-based gemini surfactants serve as substrate wetting additives. This group of products effects an outstanding reduction in the static surface tension in combination with a very low CMC. There is currently no known class of surfactants capable of providing extraordinary substrate wetting coupled with pronounced foam prevention and spontaneous defoaming. Acetylene diol-based gemini surfactants are used to reduce the dynamic surface tension of water-borne systems, without the need for foam stabilisation (see Chapter 5.2.2.4 “Acetylenediols and modifications”).

5.2.2.3 Fluorosurfactants

Perfluoro surfactants are generally called fluorosurfactants. Perfluoro organic compounds have all the hydrogen atoms attached to carbon replaced by fluorine atoms. The hydrophobic part of a fluorosurfactant is such a chain. The hydrophilic part can be an ethoxylate chain or a carboxylate group. Fluorosurfactants are generally characterised by high polarity, high thermal and chemical stability and high resistance to UV radiation. They repel dirt, oil, fat and water. Fluorosurfactants (PFT) can be divided into the following groups: – fluoromodified alkysulfonates, – fluoromodified carboxylic acids and – fluorotelomer alcohols. Structure of a carboxylate modified fluorosurfactant:

Rf-CH2CH2~CH2CH2COO-Li+, Rf = F(CF2CF2)3-8

The fluorocarbon chain (-CF2CF2-) is the reason for the stronger reduction in surface tension compared to that of other chemical classes. The fluorocarbon causes very strong orientation at the interface, an effect which can be traced back to the weak affinity of fluorocarbon to water and the weak interaction between fluorocarbon chains themselves. These relatively expensive additives are powerful wetting agents and so can be used for wetting surfaces with the lowest surface energy. Although fluorosurfactants improve the wetting behaviour, they can be difficult to overcoat because the dried film is itself difficult to wet. The fluorosurfactant orientates in such a way that a fluorocarbon-like surface is formed. The products are used where other substrate wetting additives would be chemically decomposed by, e.g., oxidising agents, acids or alkalis. However, fluorosurfactants have limited compatibility and a strong tendency to promote foam [4]. Discussions about toxicity are based on their frequent occurrence worldwide in oceans, lakes and human and animal tissue and blood. Ecological data are available in technical data sheets.

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Chemical structure of substrate wetting additives

Fluorinated polyacrylates

Fluorinated polyacrylates are a versatile group of inter-facially active additives. They combine good wetting behaviour with flow and levelling. Fluorinated polyacrylates with different fluorine contents are commercially available.

5.2.2.4 Acetylenediols and modifications

Table  5.3:  HLB value  

HLB-value

Defoamer

1.5 to 3.0

W/O-emulsions

3.0 to 8.0

Wetting additive

7.0 to 9.0

O/W-emulsions

8.0 to 18.0

Detergents

13.0 to 15.0

Co-solvents

12.0 to 18.0

The best-known substrate wetting additive in the coatings area is probably acetylenediol. It combines wetting and defoaming properties in one molecule and often also promotes flow. Because of its low molecular weight, it is very effective in the reduction of dynamic surface tension. It is characterised by good compatibility, easy handling and good moisture resistance. Polyether modifications increase the water solubility and ease of biological degradation [5]. It shows an average reduction of static surface tension.

5.2.2.5 Sulfosuccinates

In practice, these are divided into sulfosuccinic acid diesters and sulfosuccinic acid half esters. Sulfosuccinic acid esters with less than 8 carbon atoms per ester group are water soluble. Sulfosuccinates are very powerful surfactants which are used as wetting or emulsifying agents. They usually have a strong tendency to foam [6].

5.2.2.6 Alkoxylated fattyalcohols

Alkoxylated fatty alcohols are non-ionic surfactants, made by reacting ethylene oxide, propylene oxide or butylene oxide with long-chain primary fatty or alcohols. Alkoxylates based on fatty alcohols show inverse solubility behaviour, the solubility in water decreasing with increasing temperature. They show high cleaning and dispersing ability and the products, which are low foam, biodegrade easily. They do not tend to stabilise foam and show average reduction in dynamic and static surface tension. The HLB value is a measure of water and oil solubility mainly for non-ionic surfactants and the stability of emulsions. HLB is an abbreviation of hydrophilic/lipophilic balance. The HLB value of a mixture of surfactants or emulsifiers can be calculated in an additive manner from those of its components. This method cannot be used for polypropylene glycol ethers, polyethersiloxanes or anionic surfactants. The values range from 1 to 20. Substances with low HLB values are generally good water-in-oil emulsifiers, while hydrophobic surfactants with higher HLB values are effective as oil-in-water emulsifiers [1].

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Substrate wetting additives A special class of alkoxylated surfactants is that of star-shaped polymers. These non-ionic surfactants have a hyperbranched structure that gives them good wetting and defoaming properties.

5.2.2.7 Alkylphenol ethoxylates (APEO) Chemical constitution of alkylphenol ethoxylates:

These are made by addition of alkylene oxides (usually ethylene oxide) to alkylphenols. The most important are octyl- and nonyl- und tributyl-phenolpolyglycol ether. With an increasing length of ethylene oxide chain, alkylphenolpolyglycol ethers become water soluble but, for a given chain length, the solubility in water decreases with increasing temperature. The water soluble alkylphenol polyglycol ethers are good wetting and emulsifying additives. The ecological data can be found in the technical data sheets [1].

5.3 Application of substrate wetting additives 5.3.1 Basic properties of various chemical classes There is frequent criticism about the wetting behaviour of water-borne coatings during development and optimisation of formulations. Usually an additive which is easily available in the lab is added to the formulation and tested. In most cases wetting is not improved sufficiently, and additional undesired side effects are created. Further additives are tested often without properly considering the basic properties of the different groups of substrate wetting additives. This procedure is sometimes time consuming and expensive and the final result is still not satisfactory from the point of view of the formulator. Each additive class has typical properties but not all are desirable. It is very import before starting the testing procedure to decide on the most important criteria, so that the most suitable chemical class can be chosen. The investment in time and energy is dramatically reduced if the important factors underlying poor wetting are analysed in advance. The most important chemical classes used in coatings are described above. To clarify this, the following table shows the typical basic properties of the different substrate wetting classes.

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Application of substrate wetting additives Table 5.4: Properties of different substrate wetting additives

Additive

Reduction of static surface tension

Sulfo-succinate

Reduction of dynamic surface tension

Foam tendency

price

medium to good good

strong

low

Alcohol-alkoxylate

few

very good

low

medium

Polyether-modified siloxane

good

medium

medium

medium to high

Fluorosurfactant

very good

few

very strong

high

Acetylenediol and derivatives

medium to good very good

low

low

5.3.2 Reduction of static surface tension Generally, the fluorosurfactants show the strongest reduction in static surface tension (see Chapter 5.6 “Testing methods”) in water-borne coatings. This property stems from the chemical structure, as the fluoro modification, because of its lipophilic character, shows very little tendency to interact with water. Polyethersiloxanes constitute the second most efficient chemical group with regard to reduction of static surface tension: the siloxane part of the molecule is lipophilic. The ability of polyethersiloxanes to spread over the surface in aqueous solutions is excellent and this can also be traced back to the structure of the molecule. One small droplet placed on a substrate can easily spread out to an area with a diameter of 15 cm, but this only occurs in pure aqueous solutions.

5.3.3 Possible foam stabilisation The stronger the reduction of static surface tension in a water-borne coating, the greater is the tendency of the substrate wetting additive to stabilise foam. On one hand they stabilise the incorporated air because the substrate wetting additives move directly to newly formed surface areas and, on the other hand, the lower surface tension causes a smaller pressure difference between the air bubble and the surrounding medium (see Chapter 7 “Deaerators”). The foaming tendency of polyethersiloxanes can be reduced by the right polyether modification. A product which is only-just water-soluble is made and because of its limited compatibility in water it stabilises less foam, but it is still effective at reducing static surface tension. Such freedom to modify does not exist with fluorosurfactants, as they need a very polar modification to be compatible in water-borne coatings without any deleterious effects. To minimise the side effects of substrate wetting additives, the amount used should be as small as possible.

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Substrate wetting additives

5.3.4

 ffective reduction in static surface tension E versus flow

The formulator should always bear in mind how much the surface tension has to be reduced, because it is not advisable to immediately use, say, a fluorosurfactant for a small reduction in surface tension. The choice of chemical class should be predicated on the degree of difficulty in achieving the desired wetting. Sulfosuccinates and alkoxylates are not the most efficient reducers of static surface tension but they can be sufficient to overcome minor substrate wetting problems. A moderate reduction in surface tension also improves the final levelling.

5.3.5 Reduction of dynamic surface tension The sulfosuccinates and alkoxylates are more effective in reducing dynamic surface tension (see Chapter 5.6 “Testing methods”) than polyethersiloxanes or fluorosurfactants. The mobility of the molecule is one key factor for efficient dynamic substrate wetting additives and the mobility correlates directly with the molecular weight. As soon as a new surface is created the small, more mobile molecules orientate much faster to it. This property is very important where fast wetting processes occur, as in the printing industry. Further important factors are tendency to foam and price of the product. A foam promoting substrate wetting additive will necessitate choosing a more powerful defoamer with the inherently increased risk of surface defects. No substrate wetting additive is very poor or very good – it should be appropriate for the specific problem.

5.3.6 Which property correlates with which practical application? A formulator looks for a substrate wetting additive for various reasons. The most common defects which can be cured by substrate wetting additives are: – tendency to cratering, – defoamer craters, – insufficient substrate wetting on surfaces which have not been not perfectly cleaned, – difficulties in substrate wetting on plastics, – contamination by spray droplet – atomisation while spraying and – optimisation of flow.

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Application of substrate wetting additives

5.3.6.1 Craters

The issue of craters is strongly related to the static surface tension of the coating. If metal substrates are not completely cleaned, oil or grease may remain on the surface from the production process. Craters can also occur if particles and fibres in the air fall into the freshly applied coating. A carefully chosen defoamer can create craters in sensitive coatings. The basic reason for cratering is the relatively high surface tension of the applied coating against contamination such as fibres, particles or defoamer droplets. If the contaminant comes into contact with the wet paint, the coating material shrinks away from it and the crater is formed. The coating tries to minimise the contact area with the disturbing factor, and this can lead to complete de-wetting. Thus, only substrate wetting additives which are very effective in reducing static surface tension, i.e., fluorosurfactants and polyethersiloxanes, are helpful against craters. The strongest reduction in surface tension is achieved with fluorosurfactants and the second most effective class is the polyethersiloxanes. Both efficiently reduce static surface tension so that wetting and surrounding the fibre or particle by the coating is possible.

5.3.6.2 Wetting and atomisation of spray coatings

Substrate wetting when spray coating is optimised by improved atomisation and improved wetting of the spray droplets on the substrate. In spray application the surface area of the coating is enormously enlarged by the formation of many small spray droplets. This process is significantly improved by substrate wetting additives. The smaller the droplets the thinner is the film thickness at which a closed film is formed. One can picture this by imagining the stacking of large or small spheres. Substrate wetting additives enable water-borne coatings to be Figure 5.6:  Contamination crater – coating has a higher surface tension than the contaminant applied in thin layers without any difficulties. Here additives which reduce the static surface tension are the most helpful. The painting process is often assumed to be very dynamic but in practice the reduction of static surface tension is necessary to achieve good spraying characteristics and allow difficult substrates to Figure 5.7:  Crater in a clear coat be successfully wetted.

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Substrate wetting additives

5.3.6.3 Rewettability, reprintability, recoatability

Up to this point only the coating layer in which the additive is incorporated has been discussed but substrate wetting additives can affect the properties of subsequent coating layers. The use of highly effective polyethersiloxanes or fluorosurfactants can reduce the surface tension of the dried film. If this coating is a primer onto which a top coat is to be applied than recoatability should be tested. The fact that highly surface-active substrate wetting additives can impair wetting of the following coating layer must be considered. The higher the molecular weight of the substrate wetting additive or the higher the proportion of low energy molecules, the more difficult it is for additives in the next layer to initiate dissolution. However, dissolution of the additives from the dried first coating layer by the following layer is a key requirement for good substrate wetting characteristics of the following layer. This effect becomes increasingly important as the coating becomes more cross linked. In extreme cases not only is wetting poor but adhesion can be completely lost.

5.3.6.4 Flow

Basically, substrate wetting is a precondition for good flow. The compatibility of the substrate wetting additives is the main influence on gloss and flow of coatings. A small reduction in static surface tension is also very helpful in improving flow (see Chapter 8 “Flow and levelling additives”).

5.3.6.5 Spray mist uptake

The uptake of spray droplets is mainly influenced by two parameters: the ability of the coating to absorb the droplets and the surface tension of the partially dried coating surface. Both properties are influenced to a greater or lesser degree by substrate wetting ad-

Figure 5.8:  Loss of adhesion caused by insufficient dissolving power of the following layer

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Use of substrate wetting additives in different market sectors ditives. On one hand they interact with the evaporating water thus influencing drying characteristics and on the other hand they even out the differences in surface tension between fresh spray droplets and the partially dried coating surface, thus the spray droplets do not create craters in the dried coatings surface. Alkoxylated fatty alcohols and derivatives of acetylenediols are well known for extending open time so that absorption of spray droplets into the dried coating is improved. If the open time is increased, sagging must be tested. If, however, the spray creates craters, only additives such as polyethersiloxanes or fluorosurfactants which reduce static surface tension effectively, can help.

5.4 Use of substrate wetting additives in different market sectors The use of substrate wetting additives is largely dependent on the requirements of individual market sectors. For example, expensive additives are seldom used in the low-cost market sector. In printing inks, additives are used which do not stabilise foam and reduce the dynamic surface tension. Low foaming tendency is very important because during the printing process air must not be stabilised even though is incorporated by the fast-running presses. The substrate wetting additives must also move as quickly as possible to the newly formed interface areas. This is only achieved by small, mobile molecules such as acetylenediols and alkoxylated fatty alcohols. Sometimes sulfosuccinates are used but this also necessitates a very efficient defoamer. Increasingly powerful substrate wetting additives are being used in wood coatings because of the variable quality of wooden substrates. Furthermore, they allow the amount of co-solvent to be decreased almost to zero. Hydrophobic constituents in the wood can only be wetted successfully by coatings with appropriate surface tension. Sometimes polyethersiloxanes are used in addition to avoid craters caused by defoamers as coatings are often applied in ever increasing film thickness in a single spray pass. In water-borne dipping coatings it is very important that the substrate wetting additives are highly compatible in the coating system as, depending on the deposition mechanism, the additives can become Figure 5.9:  Smooth flow versus orange peel

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Substrate wetting additives more concentrated in the dipping bath. Furthermore, the dipping bath is in use for very extended periods and any risk of incompatibility must be excluded. This is why silicone- or acetylene diol-based gemini surfactants are often used. The additives also must not stabilise foam inside the bath. Foam can easily be created during the dipping process. The wrong selection of additives can result in foam stabilisation. In water-borne automotive and plastics coatings polyethersiloxanes are very popular. They allow the application of atomised spray coatings at a very low film thickness and reduce the tendency of the coatings to crater. In general, the choice of additives is determined by their properties and prices. Profound knowledge about the chemistry and the performance characteristics of the substrate wetting additives used helps to save time for the formulator to choose the most appropriate product.

5.5 Tips and tricks 5.5.1 Successful use of substrate wetting additives in coatings The correct use of substrate wetting additives is decisive for the success of a water-borne coating. Many formulators are of the opinion that only a coating without any additives is a good coating. This has elements of truth in it: only as much additive should be added as necessary. Over dosage leads to foam stabilisation and undesired side effects with other surface-active additives such as wetting and dispersing additives or emulsifiers. Storage stability can also be impaired by over dosage. It is not true that more is better! A basic formulation should be structured as follows: – water or resin – defoamer – wetting and dispersing additive – pigment/filler – resin – thickener if necessary – resin/water – substrate wetting additive – small amount of water to adjust Figure 5.10:  Poor and good flop effect of substrate wetting additives viscosity.

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Test methods for measuring surface tension The order is very important. It can be seen, that substrate wetting additives should be added at the lowest possible viscosity and as late as possible in the formulation. The reason that components should be added in the order shown above is that the tendency to foam is significantly reduced. If substrate wetting additives are added too early, they will stabilise foam during the manufacturing process. It is particularly important that no substrate wetting additive should be in the formulation before adding pigments because substrate wetting agents immediately orientate towards every new interface including the pigment surface. However, the pigment surface should be stabilised completely by wetting and dispersing additives. Substrate wetting additives do not have any pigment stabilising effects but promote the wetting process. Although they initially cover the pigment surface, as soon as a new interface in the coating occurs, the substrate wetting additives will migrate towards the new interface. As a result, the pigment surface is not stabilised, and flocculation of the pigments can result. Generally, it is helpful to select the appropriate defoamer first and then the right substrate wetting additive. The reason for choosing the substrate wetting additive must also be considered: What needs to be improved? Such basic considerations are essential for choosing the right chemical class to obtain this desired improvement. By following the processing rules above, deleterious side effects can be dramatically minimised.

5.5.2 Metallic shades Especially with high gloss metallic coatings, the formulator often finds that, despite many attempts to optimise the formulation, certain L values cannot be reached, or the flip-flop cannot be adjusted correctly. Here the choice of substrate wetting additive can have a decisive influence. It is advisable to check the effect of all additives which may have an influence on the orientation of effect pigments. Often silicone-free additives produce more brilliant effects and the colour is significantly easier to adjust.

5.6 Test methods for measuring surface tension 5.6.1 Static surface tension The best-known method for measuring surface tension of liquids is the ring method of Lecompte du Noüy. This method measures the static surface tension. A platinum-iridium ring is placed in the liquid so that the ring is completely covered by it. The ring is slowly withdrawn and a lamella forms below the ring. This lamella is an enlargement of the surface area of the liquid and requires energy for its production. The force is measured while the ring is withdrawn from the liquid and the maximum force which is needed is a direct

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Substrate wetting additives measure of the surface tension of the liquid. The measured force corresponds to the energy needed to enlarge the surface of the liquid in the form of a lamella [2]. This method is suitable for low viscosity and non-pigmented solutions, dispersions and clear coats. Solid ingredients may lead to incorrect readings because they can cause early rupture of the lamella which will lead to incorrect values of the static surface tension. However, measurement of non-pigmented binder dispersions is sufficient to give sufficient information about the surface tension of the final coatings.

Figure 5.11:  Measuring of static surface tension by forming a lamella

5.6.2 Dynamic surface tension

Figure 5.12:  Measurement of dynamic surface tension, formation of bubble

The second procedure for measuring dynamic surface tension is the bubble pressure measure method which is only suitable for low viscosity and non-pigmented solutions. The bubble pressure tensiometer is of simple construction. A very thin capillary is placed in the liquid. Using a connected air supply with a pressure gauge, an air bubble is produced in the liquid at the end of the capillary. The force which is needed to form the bubble is measured by monitoring the pressure and this correlates with the dynamic surface tension. In this method the newly formed air-bubble/liquid interface area is the enlargement of surface area for which force is needed. The dynamic aspect of this test method is introduced by varying the frequency of bubble formation. This usually starts at one bubble per second and is increased up to ten bubbles per second. At

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Test methods for measuring surface tension the high bubble rate, the air/liquid interface is formed very quickly. To reduce the dynamic surface tension efficiently, substrate wetting additives have to orientate very quickly to the newly formed interface area and this bubble method allows the mobility of substrate wetting additives to be evaluated [2].

5.6.3 Dynamic versus static The ring method is used to measure static surface tension. The enlargement of surface area occurs very slowly in a thermodynamic sense so that the liquid is in thermodynamic equilibrium. Because of their higher molecular weight, substrate wetting additives need comparatively more time than water molecules to orientate. Thus, the new surface area is formed very quickly (in milliseconds) as in a bubble pressure tensiometer and the dynamic surface tension will be a more appropriate measurement of surface tension during the orientation of substrate wetting additives. The system is not in thermodynamic equilibrium when measuring dynamic surface tension. The mobility of the molecules is crucial for achieving thermodynamic equilibrium. The surface tension of a solution with substrate wetting additives is a function of the time. The initially high surface tension decreases very rapidly and has already reached thermodynamic equilibrium after about 100 milliseconds [3].

5.6.4 Further practical test methods 5.6.4.1 Wedge spray application

In spray application, atomisation of the coating material, i.e., the formation of very small droplets, is a decisive factor for producing very thin films. The smaller the droplets, the thinner is the film thickness at which a closed film is achieved. Use of effective substrate wetting additives allows the demands of the coatings industry for ever thinner films to be achieved. One important factor besides viscosity, flow rate and pressure, which influences the atomisation (formation of a very large surface area) is the surface tension. The formation of the large surface area can be evaluated very successfully using wedge spray application (applying an increasing film thickness). Before using this method, it is essential Figure 5.13:  Principle of wedge spray that critical conditions are chosen. Usually application

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Substrate wetting additives these are low pressure, medium flow rate and slightly increased viscosity. The control should show very poor wetting behaviour so that improvement achieved by a substrate wetting additive can easily be observed. It is important that the same application conditions are rigidly observed, and this can be achieved by using a spray robot. The film thickness is measured at which a closed film is formed, that is, the limit of wetting is measured. If panels are always placed in the same position relative to the spray gun/Esta Bell application equipment and coated in the same way the position of the wetting limit on the panel this can also give information about wetting behaviour. If this wetting limit is higher vertically and has the same film thickness, then the position of the wetting limit indicates improved substrate wetting. The visual impression of the droplets can also be used for evaluation. If the application conditions are chosen to be as unfavourable as possible for atomisation and the droplets are comparatively small or the test sample is very homogeneously covered it indicates good substrate wetting behaviour. Looking at all the criteria together indicates which is the most efficient substrate wetting additive.

5.6.4.2 One spray path

This method also requires selection of critical application parameters. Furthermore, a substrate must be chosen which will easily show up differences in substrate wetting behaviour. A quick spray pass is performed over the substrate. After drying, the width of the spray dust/spray droplets on the substrate provides information about the substrate wetting behaviour. The more homogeneous the visual impression of the applied paint droplets, the better substrate wetting usually is.

5.6.4.3 Crater test

Figure 5.14:  Poor and optimised wetting limit determined by wedge spray application

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Before this test is performed, the component causing the problem, e.g. a defoamer or an insufficiently cleaned substrate, should be identified. This is because the results of crater testing are very specific. If an additive is effective against grease or oil on steel substrates it may not be of any use against craters which result from spray droplets or fibres falling into the freshly applied coating. To obtain reproducible results a

Test methods for measuring surface tension substrate which is able to absorb the contaminating solution should be chosen. This can be a primer but not a non-absorbing glass plate, because on the glass plate the contamination droplet moves in a very undefined manner over the substrate until the solvent has completely evaporated. The results obtained using glass are thus unreliable. For testing, appropriate solutions of the contaminant are made (e.g. 0.01 % 10,000 Si oil in xylene). Known amounts of solutions of various concentrations are placed at defined locations on the appropriate substrate. The solvent must then be allowed to evaporate, and the crater-prone coating is applied in the normal way. The control should show large craters where the contaminants were placed. Additives are then added to the crater-prone coating which is then applied in an identical manner on the contaminants. If the diameter of the crater is now reduced this indicates that the additive has made the coating material less crater prone.

5.6.4.4 Draw down

A simple but meaningful draw down test is often helpful in identifying components in the coating material which tend to promote cratering. For example, an efficient but incompatible defoamer creates visible craters. Either a more compatible defoamer can be chosen or, if the hydrophobic defoamer is needed because of specific application conditions which strongly incorporate air, the surface defects can be cured by substrate wetting additives. The reduction of static surface tension or the emulsifying effect of some alkoxylates can help cure surface defects.

Figure 5.15:  Poor and optimised substrate wetting indicated by draw down

Figure  5.16:  Spray mist

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Substrate wetting additives

5.6.4.5 Spray droplet uptake

The incorporation of spray droplets into the freshly applied coating is tested as follows. A first spray pass is performed. After drying at room temperature for the same time as used in practical application, a second thin spray pass is carried out. It can then be seen how well the spray droplets are incorporated into the first coating layer. With all test methods, pigmenting the coating helps to make small differences visible against the background and thus facilitates evaluation.

5.6.5 Analytical test methods In general, substrate wetting additives can be identified by the reduction of surface tension. Every product shows a typical behaviour in the reduction of static or dynamic surface tension in aqueous solutions. The most important task in the analysis of surfactants is their determination as raw materials and in formulations and, by trace analysis, in waste water, seas or lakes or as a result of biological degradation. Nowadays HPLC or ion chromatography, mass spectroscopy, thin layer chromatography and capillary electrophoresis are used to identify surfactants. Careful preparation and enrichment of the sample is necessary, especially for trace analysis.

5.7 Literature [1] RÖMPP Online Version 3.2 [2] tego journal, 3rd Edition 2007 [3] www.kruss.de, Theorie zur Blasedruckmethode [4] Technical Information, Zonyl fluortenside, DuPont, Zonyl-FSBR-d-0598 [5] Surfynol 400 Series Surfactants, Air Products and Chemicals 2004

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[6] Morell, Samuel P, Coatings Technology Handbook, Surfactants for Water-borne Coatings Applications, Marcel Dekker, Inc. [7] F. M. Menger, C. A. Littau, J. Am. Chem. Soc. 113

Improving performance with co-binders

6 Improving performance with co-binders Dr Patrick Glöckner* The other chapters of this book deal with different types of additives for water-borne coatings and printing inks. These products are used in small amounts of between 0.01 and 3 % of the total formulation to improve certain properties of the coating or the printing ink such as substrate wetting or flow and levelling. In contrast to additives, co-binders are used in higher levels of up to 30 % of the total formulation to achieve the required property profile of coatings and printing inks. Because of the large amounts used, co-binders actually influence several properties. By using co-binders, it is possible to adjust drying behaviour, hardness, flexibility, blocking resistance, gloss, haptic properties (soft touch) and adhesion to the most diverse substrates. Co-binders are used in formulations for all market segments starting from simple DIY coatings up to high quality industrial and automotive [1] OEM coatings. Often water-borne main binders are combined to obtain the required property profile so that it is not easy to distinguish between main and co-binders. Therefore, the difference between co-binders and main binders is not static and largely a matter of definition. As co-binders are not able to form films of high mechanical strength because of their tack or brittleness, they cannot be used as main binders. Apart from typical co-binders which are used to modify coating and ink properties there are many pigment paste resins with excellent pigment wetting, dispersing and stabilisation properties. Those resins are also often called grinding resins. Co-binders for water-borne formulations range from simple aqueous solutions of polyethers or polyesters, through emulsions of polyurethanes [2] or polyacrylics [3] to dispersions of polyesters [4] or polyurethanes [2, 5]. The procedure for manufacturing the products will be discussed in the following section and practical examples of their utilisation for improving coating properties and in pigment concentrates will be given.

*

revised by Dr Sascha Herrwerth

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Improving performance with co-binders

6.1 Preparation of co-binders Typical co-binders are based on dispersions of hydrophilic polyesters [4], polyurethanes [2] and polyacrylics [3]. Primary and secondary dispersions are differentiated from each other by the type of process by which they are produced [5]. In the case of primary dispersions the monomeric building blocks are emulsified in water and then polymerised in the aqueous phase. A typical example is the emulsion polymerisation of monomeric acrylates to obtain polyacrylate dispersions [1, 5]. By contrast, secondary dispersions are obtained by first polymerising the monomeric building blocks in a melt or in a solution of organic solvents and then transferring the polymer obtained into the aqueous phase by the addition of water. This type of manufacturing process is used if the monomers cannot be polymerised in the presence of water because they would react with it, e.g., in the case of polyurethanes from polyisocyanates, or if a certain property profile can only be achieved in this way, e.g., low molecular weight distribution of polyacrylates. Most of the co-binders available are secondary dispersions and so these will therefore be the focus of the following discussion.1

6.1.1 Secondary dispersions In order to transfer a more or less hydrophobic polymer to the aqueous phase emulsifiers that impart “water compatibility” are necessary. The emulsifier can be part of the polymer (internal hydrophilisation) or added physically (external hydrophilisation), see Figure 6.1. The advantage of internal hydrophilisation is that the emulsifiers are covalently bonded to the polymer and cannot then diffuse out of the coating and impair interlayer adhesion to following layers. Moreover, only a very small amount of internal emulsifier is required which may be beneficial for mechanical, chemical and water resistance of the resulting coating. Emulsifiers can also be classified as ionic or non-ionic [6]. Non-ionic emulsifiers such as polyethers result in final coating films which are permanently moisture-sensitive and subject to polymer degradation [7]. Their advantage lies in the absence of volatile components which would otherwise have to be included in the VOC. Ionic emulsifiers are preferred for high quality applications. As shown in Figure 6.2 ionic emulsifiers are divided into anionic and cationic types. The agents used to neutralise the “potentially hydrophilic” ionic groups of the ionic emulsifiers can be volatile and this 1

In physics, a dispersion is the generic term for two phases, one of which is distributed, but not dissolved, in the other [5]. An emulsion [6] is a mixture of two liquids that are not soluble in each other (e.g., an insoluble oligomer of low Tg in water). The coatings industry uses these terms to describe the film obtained after evaporation of water. Emulsions result in tacky films, whereas dispersions produce tack-free films prior to curing [7].

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Preparation of co-binders results in coatings having a high resistance to water and corrosion. Examples are amines such as dimethylethanolamine or carboxylic acids such as formic acid. The disadvantage is that highly volatile neutralising agents contribute to the VOC. In contrast, neutralising agents such as sodium hydroxide do not contribute to the VOC but result in lower water resistance since they remain in the resulting film. Secondary dispersions are obtained by first synthesising the polymer in melt or in solution in an organic solvent. After neutralisation the polymer is transferred to the aqueous phase. This transfer can be difficult, as the viscosity levels during the process vary greatly (Figure 6.2). In area A of Figure 6.2 the ionomeric polymer is in solution or in melt. The viscosity depends on the ability of the polymer chains to interact with each other by, e.g., entanglements, intermolecular hydrogen bonds, or association of the salt moieties [8]. The longer the chain the more pronounced are the entanglements. Thus, the viscosity depends strongly on the molecular weight of the polymers [8].

Figure 6.1:  Differentiation of emulsifiers for secondary dispersions

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Improving performance with co-binders After adding small amounts of water to the ionomeric polymer the ionic centres are hydrated. The viscosity decreases slightly (B). On adding further amounts of water, a waterin-oil emulsion is obtained [6]. The polymer chains are increasingly immobilised, resulting in a very strong increase in viscosity (C). The maximum is achieved immediately before the phase inversion since the mobility of the polymer chains is greatly restricted (D). After this maximum is exceeded, only small amounts of water are necessary to lower the viscosity strongly. The polymer chains are unable to form entanglements since they are no longer in contact with each other. The viscosity becomes independent of the molecular weight of the polymers (E).

6.1.1.1 Polyester dispersions

Polyesters [4] are obtained from the esterification and condensation of polyfunctional carboxylic acids and polyols (Figure 6.3). Properties of the polyesters such as reactivity, mechanical properties and chemical resistance can be varied simply by the choice of the monomer, the molecular weight and the degree of branching of the resulting polyester, as well as the nature of the functional groups. In order to prepare a polyester dispersion, the polyester itself has to contain some carboxylic groups. The carboxylic groups can either be at the end of the polymer chain or,

Figure 6.2:  Schematic explanation of the change in viscosity during phase transfer

128

Preparation of co-binders by using tri- or polyfunctional carboxylic acids, close to the polymer backbone. After neutralisation the resin can be transferred to the aqueous phase (Figure 6.4).

6.1.1.2 Polyurethane dispersions

Another important type of polymer dispersion for coatings and printing inks is based on polyurethanes [2]. The basic products are obtained by a polyaddition reaction of polyisocyanates and polyols. Here dimethylol propionic acid (DMPA) is often used for internal hydrophilisation (Figure 6.5).

Figure 6.3:  Synthesis of a linear polyester

Figure 6.4:  Transfer of a carboxy functional polyester into the aqueous phase

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Improving performance with co-binders The properties of the final dispersion are influenced strongly by the choice of reactants. By using aliphatic di- and polyisocyanates and aliphatic polyesters, polycarbonates or polyacrylates, products with very high weathering resistance and excellent mechanical properties can be obtained. Figure 6.6 shows the different steps in the synthesis of polyurethane dispersions based on isophorone diisocyanate (IPDI), which is frequently used in high quality polyurethanes. First dimethylol propionic acid (DMPA) is reacted with the isocyanate. This isocyanate functional adduct is then reacted with the polyol components to obtain an isocyanate functional prepolymer (Step 1, Figure 6.6). The acid groups are then neutralised, and the corresponding polyelectrolyte dispersed in water (Step 2). Chain extension is carried out by the addition of a diamine (Step 3). This last step is an elegant way of avoiding high viscosities associated with phase inversion (cf. Figure 6.2, see page 128) while still obtaining resins with a high molecular weight. Since high molecular weights of the binders are beneficial for mechanical and resistance properties, the last step is important especially for high quality binders. Moreover, the organic solvent content necessary for processing can be reduced to a minimum.

Figure 6.5:  Typical monomers for manufacturing polyurethane dispersions

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Applications of co-binders

6.2 Applications of co-binders 6.2.1

Co-binders for better property profiles

Since the number of co-binders for adjusting properties of water-borne coatings and printing inks is considerable and main binders are often combined to achieve the required property profile of the final formulation, only the effects of a few product types will be discussed in this section. Usually, co-binders are used if most of the required properties of the formulation are already adequate, but some small adjustments are necessary. In general, co-binders have to be as compatible as possible with the different classes of binders and not impair properties such as surface tension or foaming characteristics as this would necessitate the use of further additives.

Figure  6.6:  Preparation of polyurethane dispersions based on isophorone diisocyanate (IPDI) and dimethylol propionic acid (DMPA); U = urethane group, HO۸۸۸۸OH = saturated polymeric diol [7]

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Improving performance with co-binders

6.2.1.1 Drying time

During the film forming process, water present between the particles of the dispersions is “pressed” out of the film resulting in a continuous coating film [9]. The higher the minimum film forming temperature (MFT) and thus the glass transition temperature (Tg) of the resins, the faster the water is pressed out of the remaining film [10]. A high Tg results in a high hardness and this usually adversely affects the flexibility [11]. The use of special co-binders with hyperbranched structures and high glass transition temperatures enable hardness and drying speed to be increased significantly without greatly decreasing the flexibility. Figure 6.7 shows the drying speed of a water-borne wood coating (Table 6.1) as a function of the concentration of co-binder added. The co-binder used was an aqueous polyurethane dispersion with a Tg of approx. 120 °C. The higher the amount of co-binder present the higher is the drying speed. At the same time the hardness is increased while the flexibility remains at a similar level up to a concentration of 20 % co-binder.

Figure 6.7:  Reduction of drying time and increase of pendulum hardness of a water-borne wood coating based on a fatty acid modified PUD as a function of the concentration of co-binder (main binder replaced by co-binder solid to solid (w/w))

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Applications of co-binders Table 6.1:  Water-borne wood coating based on a fatty acid modified PUD Component

Amount [%]

Mill base Fatty acid modified PUD

32.4

Defoamer

0.9

Substrate wetting additive

0.4

Matting agent

2.4

Micronised wax

0.6

Dryers (25 % Co 6, 25 % Zn 8, 50 % water)

2.4

Let down Fatty acid modified PUD

33.0

Dipropylene glycol n-butyl ether

1.2

Water

23.6

PU thickener (1:1 in water)

1.6

PU thickener

1.5

Co-binder (replacement of main binder) Sum

0 to 30 % 100.0

6.2.1.2 Adhesion

The adhesion of a coating or a printing ink to a particular substrate is influenced by many different parameters [12]. Some of the most important are summarised in Table 6.2 [7]. Many of the commercially available main binders are tailor-made for certain substrates. This is the reason why it is often not possible to coat different kinds of substrates with just one formulation. Today there is a range of commercially available co-binders which optimise the adhesion of coatings or printing inks to different plastic substrates. They are often used in amounts of up to 20 % solid resin based on the main binder to improve poor adhesion of a coating (GT 5) to excellent (GT 0). The substrates can vary significantly, e.g., polycarbonate (PC), polymethyl methacrylate (PMMA), polyvinylchloride (PVC), polystyrene or noryl (PPO/PS). To give an example, the primer shown in Table 6.3 has good adhesion to polyamide, PMMA, PVC and noryl but adhesion to ABS and polystyrene is very poor (GT 5). The adhesion can be improved to excellent (GT 0) by using 5 to 10 % of a special polyester resin dispersion or 15 to 20 % of a polyurethane dispersion.

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Improving performance with co-binders Table 6.2:  Parameters influencing the adhesion of water-borne coatings and printing inks Parameter

Suggested action

Substrate pretreatment

cleaning of the substrate (dust, oily substances, etc.) pretreatment of metals, e.g., phosphating pretreatment of non-polar plastics, e.g., flame or corona treatment

Application conditions

formulation has to be adjusted to the climatic conditions (temperature, humidity)

Substrate wetting

surface tension of the formulation must be lower than that of the substrate use a substrate wetting agent if required (see Chapter 2)

Adsorption between film and substrate

increase the number and strength of contact points to the substrate: by using binders with hydrogen-bonding groups (urethane, OH, etc.) in the case of metals: acid groups (phosphates, carboxylic acids) lead to salt formation better orientation of functional groups of the co-binder due to low Tg substrate pretreatment often required

Diffusion into (absorbent) substrates

increase penetration by using a low-viscosity formulation body, flow and levelling and gloss can however be impaired.

Film forming

the minimum film forming temperature of the binder must be chosen so that a closed film is formed on application use coalescent agents if necessary

Adjusted flexibility

flexibility of the coating has to be adjusted to the substrate inflexible, hard film on a flexible substrate will not have optimal adhesion.

The investigated co-binders had markedly different Tg values (polyester approx. 30 °C and polyurethane polyol approx. 120 °C). This allows the formulator to adjust other coating properties such as hardness, flexibility, scratch and blocking resistance easily. In the case of metal substrates, excellent adhesion is a prerequisite for good corrosion protection [13]. Pretreated metals such as galvanised steel, but also aluminium, are a particular challenge for formulators. Figure 6.8 shows the degree of delamination (corrosion creep) of oxidatively drying acrylic coatings on steel after approx. 850 h exposure to salt spray as a function of the concentration of co-binder. The formulation is shown in Table 6.4. As can be seen from the diagram, 5 % of the co-binder (replacement of solid main binder) is sufficient to obtain a coating with excellent corrosion protection properties.

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Applications of co-binders Table 6.3:  Plastic primer (non-pigmented) based on an acrylic dispersion Component

Amount %

Acrylic dispersion of low Tg

70.5

Defoamer

0.5

Substrate wetting additive

0.5

Thickener (10 % in water, neutralised with AMP 90)

16.5

Water

6.0

Butyl glycol

3.0

Coalescent agent

3.0

Co-binder (replacement of main binder) Total

2.5 to 20.0 100.0

Figure 6.8:  Corrosion creep of an oxidatively drying acrylic coating on steel after approx. 850 h salt spraying as a function of the concentration of co-binder (main binder replaced to co-binder solid to solid (w/w).

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Improving performance with co-binders Table 6.4:  Oxidatively drying acrylic coating for metal

Table 6.5:  Water-borne metal coating based on an alkyd dispersion

Component

Component

Amount %

Oxidatively drying acrylic dispersion

55.32

Dryer

0.27

Defoamer

0.18

Silica

0.18

Corrosion protection pigment

9.71

Microtalc

9.71

Iron oxide red

9.62

Filler

4.67

Water

9.16

2-Butoxy ethanol

0.82

Mill base Water

7.60

Thickener

0.55

Defoamer

0.05

Wetting and dispersing additive

1.25

Substrate wetting additive

0.50

Titanium dioxide

24.00

Water

5.00

Let down Alkyd dispersion

53.00

Thickener

1.00

Thickener

2.90 0.45 1.25

Substrate wetting agent

0.18

Dryer

Thickener

0.18

Propylene glycol

Co-binder (replacement of main binder)1,2) Total 1

Water 2.5 to 20.0 100.0

 ater-borne, solvent-free dispersion of w a polyurethane polyol, Tg ~ 120 °C 2 water-borne, solvent-free dispersion of a polyester, Tg ~ 30 °C

6.2.1.3

Amount [%]

Co-binder (replacement of main binder)1,2 Total

2.45 2.5 to 20.0 100.0

1

 ater-borne, solvent-free dispersion of a w polyurethane polyol, Tg ~ 120 °C 2 water-borne, solvent-free dispersion of a polyester, T g ~ 30 °C

Hardness-flexibility balance

The higher the hardness of a coating film, the better are properties such as blocking and scratch resistance. The hardness is significantly influenced by the choice of the main components such as binders, crosslinking agents, and pigments as well as by the crosslink density [11]. The composition of the formulation is usually fixed by the property profile and thus the scope for variation is very limited. Increase of the hardness is limited because flexibility also has to be considered so it is up to the formulator to figure out the best compromise. An efficient tool here is the use of hyperbranched co-binders which allow the hardness to be increased within certain limits with-

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Applications of co-binders out greatly altering the flexibility as illustrated in Figure 6.9 which shows the performance of a water-borne alkyd-dispersion based metal coating. The formulation is given in Table 6.5. It can be clearly seen that the replacement of alkyd dispersion by the co-binder results in a significant increase in pendulum hardness from 20 to 90 while the flexibility (Erichsen cupping) remains at roughly 9.5 mm.

6.2.1.4 Gloss

In order to obtain glossy coating films, the components of the formula must be mutually compatible [14]. Co-binders are usually widely compatible and thus act within certain limits as compatibilisers. The gloss of pigmented systems is also significantly influenced by pigment wetting and stabilisation (see Chapter 2). Since many co-binders additionally function as grinding resins, they also improve the gloss of pigmented formulations (see Chapter 6.2.2). Gloss also depends on the surface texture, which can be improved by flow and levelling additives (see Chapter 8) and also by co-binders. Figure 6.10 shows that the gloss of a 1-pack acrylic coating is increased by adding a co-binder 2.

Figure 6.9:  Pendulum hardness and Erichsen cupping as a function of the concentration of co-binder (main binder replaced by co-binder solid to solid (w/w)) 2

water-borne, solvent-free dispersion of a polyurethane polyol, Tg ~ 120 °C

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Improving performance with co-binders

6.2.2 Co-binders for pigment pastes Pigment paste resins (grinding resins) are traditionally used in solvent-borne coatings. The four main reasons are: – lower price, – higher solids content, – greater hardness, – little effect on other coatings properties. It is very probable that the use of solvent-borne binder-free pigment concentrates will not become established for bulk applications. In contrast, binder-free pigment concentrates nowadays represent the state of the art in water-borne formulations, particularly in view of their universal compatibility (see Chapter 2). This technology is not limited solely to architectural coatings [15] but is also used in printing inks [16], industrial coatings and wood coatings [17]. Nevertheless, grinding resins are also used in water-borne coatings and printing inks. It is then relatively easy to obtain outstanding mechanical properties, such as hardness-flexibility-balance, and excellent chemical or water resistance and also reduced drying times.

Figure 6.10:  Increase in gloss of a 1-pack acrylic coating by adding a co-binder (main binder replaced by co-binder solid to solid (w/w))

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Applications of co-binders

Figure 6.11:  Comparison of gloss, haze, and pendulum hardness of binder-free and resin containing pigment concentrates

Figure 6.12:  Mass tone (4 % pigment) on glass (150 µm wet film thickness); the image shows the dried paint on a lamp

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Improving performance with co-binders Table 6.6:  Guide formulations for water-borne pigment concentrate Pigment type Water resin1

PBk 7

PR 101

PW 6

PB 15:4

34.2

20.4

11.7

30.7

33.4

10.0

16.7

33.9

Wetting and dispersing additive

8.4

8.0

5.3

4.3

Defoamer

1.0

1.0

1.0

1.0

AMP 90

0.3

-

-

-

Silica

0.6

0.5

0.2

-

Biocide

0.1

0.1

0.

0.1

Grinding

Pigment

22.0

60.0

65.0

30.0

Total

100.0

100.0

100.0

100.0

1

water-borne, solvent-free dispersion of a XE " polyurethane polyol” polyurethane polyol, Tg ~ 120 °C

Apart from many in-house products, a range of grinding resins is commercially available for pigment concentrates as well as direct grinds. These products contain functional groups with affinity for pigments and thus improve the interaction between resin and pigment surface. Hence the use of grinding resins in combination with small amounts of pigment wetting and dispersing additives results in excellent pigment stabilisation and colour strength development even at high pigment loadings. (see Chapter 2). Figure 6.11 shows a comparison of binder-free and resin-containing pigment concentrates based on a phthalo blue pigment (PB 15:4). The let down was the same in both cases. Colour strength, gloss and haze-gloss are very similar. The main difference is in the much higher hardness of the resin-containing pigment concentrate. Guide formulations for some water-borne pigment concentrates are shown in Table 6.6. After let down with compatible binders, it is possible to obtain high quality coatings. In order to emphasise this, the blue pigment concentrate (pigment PB 15:4) was applied after let down as mass tone on a glass plate. After drying the panel was attached to a lamp to demonstrate the excellent transparency (Figure 6.12).

6.3 Summary In contrast to additives, co-binders are used in higher amounts of up to 30 % of the total formulation. The formulator can conveniently adjust properties of coatings and printing inks simply by the use of co-binders. In the present chapter, the preparation of co-binders as well as their use in different applications has been discussed. Examples of properties that can be influenced are:

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Summary – – – – – –

drying speed adhesion and intercoat adhesion corrosion protection gloss hardness/flexibility balance blocking and scratch resistance.

Special co-binders are used as grinding resins to wet and stabilise different kinds of pigments. As well as high colour strength, gloss and transparency, very low viscosities can be achieved. At the same time, the use of grinding resins results in better drying speeds and better mechanical properties.

6.4 Literature [1] Ulrich Poth, Automotive Coatings Formulation, Vincentz Network, Hanover, Germany, 2008 [2] Hans-Ulrich Meier-Westhues, Karsten Danielmeier, Peter Kruppa, Edward Squiller, Polyurethanes, Vincentz Network, Hanover, Germany, 2019 [3] Ulrich Poth, Reinhold Schwalm, Manfred Schwartz, Acrylic Resins, Vincentz Network, Hanover, Germany, 2011 [4] Ulrich Poth, Polyester and Alkyd Resins, Vincentz Network, Hanover, Germany, 2020 [5] Dieter Urban, Koichi Takamura, Polymer Dispersions and Their Industrial Applications, Wiley-VCH, Weinheim, 2002 [6] Fernando Leal-Calderon, Véronique Schmitt, Jerôme Bibette, Emulsion Science - Basic Principles, 2nd edition, Springer-Verlag, New York, 2007 [7] Patrick Glöckner et al., Radiation Curing, Vincentz Network, Hanover, Germany, 2008 [8] Hans-Georg Elias, Macromolecules (volumes 1-4), Wiley-VCH, Weinheim, 2009 [9] Thomas Brock, Michael Groteklaes, Peter Mischke, European Coatings Handbook, Vincentz Network, Hanover, Germany, 2000

[10] Peter Mischke, Film Formation, Vincentz Network, Hanover, Germany, 2009 [11] Leslie Howard Sperling, Introduction to Physical Polymer Science, 2nd edition, John Wiley & Sons, Inc., Hoboken, New Jersey 2006 [12] Kashmiri Lal Mittal, Adhesion Aspects of Polymeric Coatings, Plenum Press, New York, 1983 [13] Joerg Sander, Anticorrosive Coatings, Vincentz Network, Hanover, Germany, 2010 [14] Artur Goldschmidt, Hans Joachim Streitberger, Basics of Coatings Technology, 2nd edition, Vincentz Network, Hanover, Germany, 2007 [15] Zeno W. Wicks, Frank Jones, S. Peter Pappas, Douglas Wicks, Architectural Coatings, In: Organic Coatings: Science and Technology, 3rd edition, pages 636-657, 2006 [16] Robert Leach, Ray Pierce, The Printing Ink Manual, 5th edition, Springer, Netherlands, 1993 [17] Jorge Prieto, Jürgen Kiene, Wood Coatings, Vincentz Network, Hanover, Germany, 2018

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Deaerators

7 Deaerators Heike Semmler Deaerators are an essential means of combating foam in coatings. There are two different types of foam, macro-foam and micro-foam, both of which often occur together in water-borne formulations. Because it is quite difficult to distinguish between the two types, the problem is generally just termed “foam”. Furthermore, magnifying glasses or microscopes are frequently necessary to detect that small bubbles are entrapped in the coating film: the so-called micro-foam. In principle it is possible to differentiate between macro- and micro-foam: – macro-foam is located on or in the coating surface whereas – micro-foam is located in the coating film. Both types can be distinguished in the liquid and in the dried or cross-linked coating film. As a rule, macro-foam is made up of air bubbles surrounded by a duplex film with two liquid/air interfaces. In contrast, micro-foam bubbles can be considered as air inclusions, characterised by a single liquid/air interface. Many coating problems, which do not at first sight appear directly associated with micro-foam, such as turbidity or a reduction in gloss of a coating film can, in fact, be caused by micro-foam. This is of course undesirable in, for example, high gloss formulations. Premature corrosion can be promoted by micro-foam because the effective film thickness is reduced. Furthermore, during drying of the coating film, micro-foam bubbles can be transformed into pinholes which are small channels reaching down to the metallic substrate. The protective function of the coating is impaired resulting in Figure 7.1:  Micro-foam and macro-foam, rapid corrosion. Source: Tego Journal 2007

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143

Deaerators Micro-foam bubbles are often not immediately apparent and in most cases, they appear during application of the coating so that this poses a particular challenge to formulators. Deaerators and some defoamers provide formulators with an effective means of preventing or eliminating micro-foam in coatings or printing inks without restricting the choice of raw material or method of application.

7.1 Mode of action of deaerators 7.1.1 Dissolution of micro-foam The problem of micro-foam is a frequent occurrence, especially in airless/airmix-applied coatings. If an airless-applied water-borne coating is observed through a micro-

Figure 7.2:  Dissolution of micro-foam bubbles as a function of time. Microscopic image of an airless-applied water-borne formulation on glass, 300 μm wet, 1; 5; 10; 15 minutes after application. For example, when the microbubbles are observed over the time indicated in the circle, the dissolution becomes obvious.

144

Mode of action of deaerators scope during the drying phase, the micro-foam bubbles can be seen changing, see Figure 7.2. Initially a mixture of micro-foam bubbles of various sizes is recognisable. As drying proceeds, relatively large micro-foam bubbles slowly increase in size while small micro-foam bubbles shrink significantly until they are no longer detectable. The driving force behind the shrinkage of small micro bubbles is the Laplace pressure. This is given by the Young-Laplace equation which describes the relationship between the internal pressure of the micro-foam bubble and the external pressure of the medium surrounding the micro-foam bubble: Pin= Paus + 2σ/r where Pin = internal air bubble pressure Paus = external air bubble pressure σ = surface tension r = radius of the air bubble The internal pressure is greater than the external pressure because the effect of the surface tension σ has to be overcome. 2σ/r, the ratio of surface tension to the radius of the air bubble, increases as the size of the air bubble decreases. Consequently, the internal pressure Pin will also rise (Figure 7.3). If the radius of the micro-foam bubble decreases further, the internal air bubble pressure will rise significantly compared to the external pressure. This difference in pressure will lead to a diffusion of the air in the micro-foam bubble into the surrounding media. The diffusion and with that shrinkage of the micro-foam bubble will accelerate as the micro-foam bubble becomes smaller. Small micro-foam bubbles thus literally dissolve. The phenomena that gases can be solubilised in gases is described by Henry`s law. Henry’s law states, that the solubility of a gas α in a liquid L is proportional to the partial pressure pα above the solution. For small and medium pressures (p ≤ 5 bar) the mole fraction xα of dissolved gas is given by: xα = pα / Hα,L (T) where Hα,L is Henry’s constant [2, 3]. In 1950 Epstein and Plesset described mathematically the dissolution of small gas

Figure  7.3:  Young-Laplace equation

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Deaerators bubbles in liquids [5]. A spray technique patented by Rohm and Haas makes use of the fact that the solubility of CO2 gas in water is up to 52 times higher than that of air. CO2 gas is thus used instead of air for spray application. CO2 filled micro-foam bubbles dissolve sufficiently fast, permitting the application of micro-foam-free films [6]. However, solubility decreases as the drying/crosslinking of the coating film progresses, because there is less liquid; eventually just a solid paint film remains. It is also conceivable that the air in small micro-foam bubbles could diffuse into larger micro-foam bubbles which have a significantly lower internal pressure. Larger micro-foam bubbles can thus grow further and rise to the liquid surface as long as the coating viscosity does not increase excessively as drying/crosslinking progresses. If these bubbles were in or on the coating film surface they would be described as macro-foam bubbles [4]. Finally, the dissolution of micro-foam bubbles is basically a physical process. It should be emphasised that this does not only apply to water-borne coatings and airless/airmix-applied coatings: small micro-foam bubbles also dissolve in solvent-borne formulations irrespective of the method of application.

7.1.2 Rise of micro-foam bubbles in the coating film As described in Chapter 7.1.1 small micro-foam bubbles literally dissolve. Larger micro-foam bubbles, however, rise very slowly to the coating surface1 where they form macro-foam bubbles, provided the air bubble is stabilised by a surfactant double layer (duplex film). If no surfactant or amphiphilic substance are present, the air bubble bursts on the coating film surface (see Chapter 3.2.1.1 “Defoamer”). The rise of the micro-foam bubble is dependent on its radius r and the coating viscosity η. The relationship of these two parameters is described by Stokes’ law and is immediately apparent from the simplified version 2 (see also Figure 7.4):

Figure 7.4:  Velocity of rise as a function of coating viscosity and size of air bubble

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v ~ r² /η where v = rising velocity of the foam bubble r = radius of the foam bubble η = viscosity of the coating

Mode of action of deaerators In its correct form, Stokes’ law describes the resistance Fw of a moving sphere in, for example, a laminar flowing liquid: Fw = 6πηvr where v = velocity of the moving sphere r = radius of the sphere The velocity v of, e.g., particles falling in a liquid can be calculated if the densities ρ1 of the particles and of the liquid are known. The force arising from the weight of a spherical particle is equal to the resistance force Fw so that:

v= 2(ρ1 – ρ2) gr2/9η [1]

Stokes’ law can be applied to air bubbles as they are spherical. Furthermore, it can be assumed that the same forces operate whether spherical particles are rising or falling and that Stokes’ law can be applied to rising air bubbles/micro-foam bubbles. If the coating viscosity η is relatively low, the velocity of rise of air bubbles/micro-foam bubbles is, according to Stokes’ law, relatively high. During the rise of larger micro-foam bubbles in an applied coating film to the surface, drying/crosslinking of the film proceeds further. The coating viscosity increases and consequently air bubbles/micro-foam bubbles persist in the coating film, resulting in the well-known problems. Stokes’ law indicates a further important relationship. Since the velocity of rise is dependent on the square of the radius of the bubble, it is affected far more by the size of the bubble than by the viscosity of the coating. Figure 7.4 shows the velocity of rise of a foam bubble at constant viscosity as a function of its radius, based on a calculation according to Stokes’ law. The velocity of rise of small foam bubbles is very low, even if the bubbles are increasing in size. Only when the bubbles are really large does the velocity increase exponentially.1 1 This statement is based on the assumption that the applied coating film is oriented upwards horizontally during drying/cross linking. However, it is still not clear how micro-foam bubbles behave or move if the film is oriented vertically or even horizontally facing downwards (e.g. during the application of window frame coatings) [9]

Figure 7.5:  Velocity of rise as a function of the radius of the air bubble at constant viscosity

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Deaerators

7.1.3 How to prevent micro-foam in coating films There are several possible methods for preventing micro-foam in water-borne coatings. – Increase coating drying time or open time of the coating film Small micro-foam bubbles have enough time to dissolve or larger bubbles can rise – Optimise application parameters Particularly relevant for spray or airless/airmix application: increasing the spray distance (substrate to spray gun), use of spray nozzles with small opening and low spray pressure can reduce micro-foam. – Low coating viscosity or Newtonian flow behaviour Micro-foam bubbles rise quickly to the surface of the liquid and do not remain in the coating film. – Create large foam bubbles Due to their size they rise quite quickly to the coating film surface – Reduce the amount of air incorporated during coating manufacture and application, especially on porous substrates (wood, stone, etc.). This avoids incorporating large amounts from air right from the start – Use low foaming raw materials Avoid surfactant structures able to stabilise micro-foam bubbles Some of the possibilities mentioned are not applicable or are not practical. Often the foam problem is just displaced (e.g. macro-foam is created on the coating surface). Many are simply not achievable because coating properties, application methods or substrates are predetermined. The coating formulator has little freedom. In contrast, addition of additives to the coating or printing ink formulation is the simplest way of preventing and effectively eliminating micro-foam. In general, these coating additives are called defoamers or deaerators. The term “deaerators” re-emphasises the effectiveness of such products against micro-foam. Of course, some defoamers are also effective against micro-foam. For simplification, the term “deaerators” is used to describe both defoamers and deaerators in the following. Ideally, by selecting the right deaerator or combination (e.g. different deaerators or deaerator and defoamer) the coating formulator does not have to accept any compromises on raw materials, production process, application parameters or substrates. Often deaerators or combinations are the only way of achieving a satisfactory finish.

7.1.4 How deaerators combat micro-foam In contrast to the mode of action of defoamers against macro-foam, that of deaerators in eliminating micro-foam bubbles still are not fully understood. This may be because macro-foams have been of interest to the industry for decades. It was only somewhat later that

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Mode of action of deaerators the problems of micro-foam were investigated more intensively by the coatings and raw materials sector, possibly due to the growing importance of water-borne and high solid coatings, which are more affected by them. Airless and airmix spray application methods, which are used extensively on cost grounds and are particularly susceptible to micro-foam problems, have attracted more interest too. Nowadays there is also increasing emphasis on cutting application steps and production time. Higher coating film thicknesses have to be applied in a single step and faster drying times achieved. Both cause an increase in micro-foam.

7.1.4.1 Deaerators promote the dissolution or formation of small micro-foam bubbles As described in Chapter 7.1.1, the dissolution of micro-foam bubbles is just a physical phenomenon, which occurs irrespective of the coating system or application method. The question thus arises of the role played by deaerators in eliminating micro-foam bubbles. Figures 7.6 and 7.7 are two micrographs of a water-borne coating taken immediately after airless application (substrate glass, 300 µm wet) while the coating film was still wet. A significant difference between the two images is immediately apparent. While the coating sample without deaerator (Figure 7.6) contains a mixture of numerous large and small micro-foam bubbles, only a few, quite small micro-foam bubbles are present in the treated sample (Figure 7.7). The images show that effective deaerators prevent the formation of large micro-foam bubbles immediately or during application of the coating and only few very small micro-foam bubbles occur. The latter dissolve rapidly due to the high Laplace internal pressure (see Chapter 7.1.1). Figure 7.6 (above) and 7.7: Airless applied high build coating on glass directly after apIn the ideal case, after drying or cross linkplication observed through a microscope; Figing, the coating film is free from micro-foam ure 7.6 without deaerator; Figure 7.7 with bubbles. (Figure 7.9). deaerator

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Deaerators

7.1.4.2 How deaerators promote the dissolution of micro-foam bubbles Up to now, there is no generally accepted theory of how deaerators act against micro-foam. However, the following points are discussed in the literature. a) The currently known active substances used as deaerators are partially insoluble in the medium to be deaerated. Consequently, they orientate themselves strongly at the liquid/air interfaces, i.e., the micro-foam bubble. It is assumed, that the active ingredients displace foam-stabilising surfactants and facilitate the diffusion of air into the medium surrounding the micro-foam bubble [4, 7]. b) According to C. Bell, micro-foam bubbles can occur if droplets of a liquid splash into the liquid media and thus entrap air. Small droplets of the liquid thus form small air bubbles while larger droplets create large air bubbles [8]. Bell’s explanation was also applied to airless-applied coatings and it was shown that effective deaerators are able to reduce the coating droplet size during application. Smaller coating droplets thus splash into the liquid coating film and only a few, very small micro-foam bubbles are generated. They dissolve correspondingly faster [7]. The formation and size of paint droplets, and especially the splashing of these paint droplets into the coating film, seem to be one factor in our understanding of the formation of micro-foam and its elimination. In recent years, close collaboration between Evonik and scientific institutes and universities has addressed the issue of micro-foam in water-borne coatings. One aspect of this collaborative work has been the use of high-speed cameras to observe and study micro-bubbles generated by airless application of water-borne coatings. Figure 7.8and 7.9: Airless-applied high build This has revealed that the use of suitable coating on glass after complete drying deaerators significantly prevents the forobserved through a microscope; Figure 7.8 mation of micro-bubbles even as paint without deaerator; Figure 7.9 with deaerator. droplets are splashing into the coating film Figure 7.9 shows complete dissolution of micro-foam bubbles (see Figure 7.10, Corona splash). In light of

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Chemical composition of deaerators this, a suitable deaerator can be described as one that acts more to prevent micro-foam than to eliminate it. c) Both the surface tension and the rheological profile of a coating formulation can be influenced by deaerators. Although there is, as yet, no proof, it can be assumed with a high degree of certainty that there is a relationship between the two properties and the effectiveness of defoamer/deaerators against micro-foam [9].

7.2 Chemical composition of deaerators 7.2.1 Silicone based products Deaerators based on organically modified polysiloxanes have proved effective against micro-foam. In most cases these polysiloxanes are also combined with small amounts (0 and ≤90°), sagging of the film may occur due to the effect of gravity. The rate of sagging can be calculated using Equations 8.2 to 8.4. Equation 8.2

Equation 8.3

Equation 8.4

where Figure 8.2:  Schematic illustration of sagging

Figure 8.1:  Surface profile of an applied water-borne coating

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vx = rate of sagging ρ = density h = film thickness dx = change in film thickness η = viscosity The rate of sagging is therefore proportional to the square of the coating thickness and inversely proportional to the viscosity. The sagging volume can be calculated from Equations 8.5 to 8.8.

Mode of action Equation 8.5

Equation 8.6

Equation 8.7

Equation 8.8

The flow of the coating surface and the sagging of the coating on non-horizontal surfaces are thus mutually conflicting properties when considered from a rheological standpoint. Although a low viscosity improves flow, this leads to sagging on non-horizontal surfaces. Therefore, rheological additives are incorporated in the coating material to ensure good flow and consequently low viscosity during and shortly after application while a little later creating a higher viscosity in order to reduce sagging. Products and test methods for rheological additives are discussed in Chapter 4.

8.1.3

Total film flow

Flow and the rate of sagging are mainly influenced by the coating thickness as well as by the viscosity: the coating thickness (h) occurs squared or cubed in Equations 8.4 and 8.8, respectively. The relationship between the applied film thickness (h) and the viscosity (η) (the total film flow (ψ∞)) is described by Equation 8.9, [3] Equation 8.9

where h(t) is the average applied film thickness and η(t) the viscosity at the time of measurement (t).

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Flow and levelling additives It follows that optimal flow is achieved by higher applied film thicknesses and lower viscosity. This formula is valid for a water-borne coating with Newtonian flow characteristics and no solvent content [3]. The “total film flow” (h3/η) is the film-fluidity as a function of time (t) and corresponds therefore to the area under the fluidity curve. This means that at a certain point after application the “fluidity” of a water-borne coating is very high. During evaporation of the solvent (water) the viscosity increases rapidly (with a simultaneous reduction in applied film thickness), leading to a smaller and smaller “total film flow” [3]. Corresponding model calculations can be found in the literature [4].

8.1.4 Mode of action in water-borne systems with co-solvents Film forming aids are frequently used in water-borne coatings, or co-solvents which are more volatile than water are incorporated in the binders during manufacture. This results in viscosity- and surface-tension gradients during film formation which are not taken into account in the equation postulated by Orchard (Equation 8.1). This is why the formula is only valid for solvent-free water-borne coatings and only then if the coating thickness (h) and the amplitude (a) are smaller than the wave length (λ). In cases where there are larger amounts of organic solvents in the coating material, the total film flow becomes a function of the 4th power of the film thickness, because volatilisation of solvent and simultaneous lowering of film thickness result in the viscosity increasing in inverse proportion to the film thickness [3]. The surface tension gradient in particular can either increase or decrease the flow process, even when coatings are applied to horizontal substrates. Overdiep et al. [6] expanded Orchard’s theory and included the surface tension gradients in their mathematical models. These models can describe very accurately the flow (as total film flow) but assume that the development of surface tension gradients during the drying process is Figure 8.3:  Total film flow as a function of time (schematic) known.

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Mode of action Equation 8.10

where

s = surface tension γ = shear rate δ = interfacial thickness = average coating thickness

8.1.5 Mode of action in an example of a thermosetting water-borne system with co-solvents Thys and Bosma have experimentally confirmed Overdiep’s mathematical model. A 2-component enamel based on polyacrylate-polyols (with a co-solvent) with isocyanate as a crosslinker was applied to a glass panel. The applied film had an amplitude of a = 4 µm and a wave length of λ = 4 mm which met the requirements postulated by Orchard. After application the film was conditioned at 21 °C and 55 % relative humidity. The glass panel was affixed to a measuring device at a specified angle. Changes in amplitude height were observed by light reflection and photographed and used to determine flow and sagging. The changes were calculated from the photos using specially developed software. In Figure 8.4 it is apparent that the relative amplitude initially decreases and then increases for a short period before decreasing again to reach its final level at 1300 s. The sagging characteristic can be calculated from the total film flow using Equation 8.9). Figure 8.5 shows the continuous reduction in fluidity as a result of the viscosity increasing as the solvent volatilises. In the subsequent drying process (thermal hardening) the fluidity increases because of the higher temperature and the total film flow thus also increases until the time (t= 600 s) at which re-orientation of the binder molecules occurs, and then approaches zero as cross-linking takes place. This means that the flow of water-borne coatings is definitely disrupted at the time when a discontinuous development of the surface Figure 8.4:  Relative amplitude as a function of drying time Source: Thys and Bosma, Allnex tension gradients occurs.

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Flow and levelling additives This phenomenon can be explained by a phase inversion during film formation. Prior to this phase inversion, the total film flow corresponds to that of a water-borne system. Subsequently, it behaves like that of a solvent-borne system. Figure 8.6 shows that the experimentally determined curve of the total film flows corresponds with the calculated curve thus confirming Overdiep’s mathematical model. These experiments demonstrate that the surface tension and the appearance of surface tension gradients strongly influence total film flow. This is confirmed by the work of Kojima et. al [8].

8.1.6 Surface tension gradients

Figure 8.5:  Decrease in fluidity as a function of drying time Source: Thys and Bosma, Allnex

The causes of surface tension gradients are as diverse as their effect on the flow and levelling of the applied coating films. One cause is, as already mentioned, the evaporation of solvents used in water-borne coatings. This leads on the one hand to a change in the total film flow and, on the other, to

Figure 8.6:  Total film flow and amplitude as a function of the surface tension gradients  Source: Thys and Bosma, Allnex

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Chemistry of active ingredients turbulent flow in the coating film. The coating flows from areas of low surface tension to areas of higher surface tension and can give rise to an uneven profile (orange peel effect). Besides the differences in surface tension occurring in water-borne coatings, defects can also be caused by surface tension differences between the coating and the substrate, or by contaminants. This can often result in localised flow and levelling problems such as craters and fish-eyes. Various authors agree [5, 9] that good flow also requires a uniform level of surface tension (surface flow = levelling) during the drying process. This will be discussed later.

8.1.7 Summary Good flow and levelling require perfect wetting of the substrate, good flow of the coating (total film flow) and uniform evaporation of the solvent (water and co-solvent or film formers). Surface active additives, substances which lower the surface tension of a liquid by concentrating at the surface, aid these effects. In water-borne systems with their relatively high surface tensions, many substances are surface-active. This will be the subject of the next section.

8.2 Chemistry of active ingredients 8.2.1 Polyether siloxanes The most important class of surface-active additives are modified siloxanes. They are derived from low molecular weight polydimethyl siloxanes by replacing individual methyl groups with diverse organic groups such as polyethers. Most of the products have a molecular weight range from 1000 to 15,000 g/mol. In order to be effective in water-borne systems, the right balance between water solubility and surface activity must be found. They must also be sufficiently compatible with the binders to ensure that hazing does not occur in liquid coatings or in dried films. Basically, the property profiles of modified siloxanes are dependent on their silicone

Figure 8.7:  Orange peel effect

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Flow and levelling additives content, the structure of the siloxane backbone and the organic side chains employed. This allows products to be developed which function preferentially at the substrate/coating interface (see Chapter 5 “Substrate wetting agents”) or are surface active. For water-borne coatings systems the polyethers are the most important modifiers. As a rule, the polyethers are derived from ethylene oxide units (EO) and propylene oxide units (PO). Polyethylene oxide is very hydrophilic (polar), polypropylene oxide, in contrast, is hydrophobic (non-polar). Polarity can be adjusted via the EO/PO ratio. Higher EO content improves compatibility in polar coatings systems and water solubility. Very good water solubility can often reduce the surface activity of polyether siloxanes but increase the foam-stabilising tendency. Higher PO content on the other hand lowers the water solubility and improves defoaming properties. Besides the EO/PO ratio of the polyether and the number of polyether chains in the polyether siloxane, the key determinant is whether the side chains are statistically distributed on the siloxane backbone (comb structure) or the molecule has a block structure or the polysiloxane chain is only end-modified. The way in which the polyether side chains are bound to the siloxane backbone is also important. It can be seen in Figure 8.8 that the polyether chain is not bonded directly, but via a (short) alkyl chain to increase resistance to hydrolysis. As a rule, a direct bond results in the polyether chains splitting off easily and to the polysiloxanes condensing to higher molecular weight structures (loss of compatibility and hence a risk of cratering). In general, in water-borne systems, products with Si-C bonds are used. Which active substances are used depends on the binders selected and on the resulting film forming process. Because of the complexity of water-borne coating materials, it is difficult to determine the relationship between the structure of the flow control agent and its activity spectrum. Since requirements may be very diverse in practice, only the general advice which follows can be given; universal solutions are, unfortunately, unlikely. However, the effectiveness of such additives is often insufficient in water-borne systems. Substrate wetting additives are related to surface active polyether siloxanes, the considerably lower molecular weight of which leads to a marked surfactant-like structure. Because of this structure, silicone Figure 8.8:  Structure of a polyether-modified polysiloxane surfactants very strongly lower the

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Chemistry of active ingredients surface tension of aqueous systems. The silicone surfactants have the important advantage over fluoro surfactants that they do not stabilise foam. Fundamental investigations into the use of modified siloxanes in water-borne coatings were published in 1996 [10]. – Polyether polysiloxane with side-substituted polyether, – Polyether polysiloxane with an end-substituted polyether, – Polyether polysiloxane with both ends-substituted polyether, – Polyether polysiloxane with more than two substituted polyethers, – fluoro-modified polysiloxane, – polysiloxane end-modified with carboxylic acid, and – acrylate-modified polysiloxane were tested in three different water-borne coatings systems based on polyester-polyurethane. In evaluating the flow, effects such as substrate wetting, edge flow and crater prevention were also considered. The results showed that polyether siloxanes with a side or end modification with an ether group/polysiloxane unit ratio of 1.6 to 2.5 gave the best results for the water-borne systems based on the binders listed above.

8.2.2 Polyacrylates In contrast to the polyether siloxanes, polyacrylates hardly affect the surface tension. They exhibit only limited solubility in water-borne coating formulations and therefore migrate to the coating/air interface during the drying process, forming a barrier on the surface. In this way they oppose the material flow which causes the difference in the surface tension [11] (see Chapter 4.2.5). Here, they hinder solvent evaporation because, with a molecular weight of 15,000 to 20,000 mol, the molecule is relatively immobile. The acrylate copolymers are made by radical polymerisation in a polar solvent. In order to transfer them into the aqueous phase the acid groups are neutralised with amines (e. g. DMEA) to make them water-soluble. In the case of physically drying coatings, the films are quite smooth immediately after application. As the surface energy of the dry film material is not reduced, wetting by a subsequent layer is not critical. Neutralised acrylate copolymers are used successfully in forced-drying acrylate, polyester and alkyd resin enamels. The gloss and flow (DOI) are improved (see Chapter 8.8 “Test methods”), without impairing the hardness or solvent resistance. However, in some applications and depending on the molecular weight, stoving temperature and time, there can be problems with intercoat adhesion in multi-coat systems. In addition to the commonly used neutralised acrylate-copolymers, products are therefore also available which contain cross-linkable sites and are modified with fluorine. Such

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Flow and levelling additives products produce good intercoat adhesion at high stoving temperatures. The polarity of the cross-linkable sites means that surface tension gradients are strongly reduced, leading to a corresponding improvement in the flow [9]. Furthermore, the surface-active additives (polyether siloxanes or polyacrylates) produce a uniform surface tension across the entire coating surface which remains relatively constant during solvent evaporation. Surface tension gradients on the coating surface are thus avoided resulting in the desired “smooth” surface. The surface activity of the polyacrylates is, however, not as good as that of the polyether siloxanes. Depending on the dosage, the total film flow can be negatively influenced by an increase in viscosity (η) or by too great a reduction in surface tension (σ). Newly developed polyacrylates with a fluorinated side chain potentially combine flow and levelling with anti-cratering and good substrate wetting behaviour.

8.2.3

Side effects of polyether siloxanes

Due to their interfacial activity and their ability to reduce surface-energy gradients (see Chapter 8.1.6) siloxane-based additives enable better surface levelling and ensure a flawless appearance. Depending on their chemical structure (see Chapter 8.2.1), certain siloxane-based additives can impact surface slip (see Chapter 8.2.4), affect haptic properties, act as anti-blocking agent and improve scratch resistance when they are used in combination with nano-silica particles. It is important to understand that effects such as surface flow and levelling, substrate wetting, slip, foam formation and anti-blocking are interdependent. The term “smooth” can be used in two ways in connection with a coating surface. Firstly, to describe a surface which is free of texture (waves), in other words, geometrically even. “Smooth” can also be used to describe the slip properties of the surface and is then a measure of the frictional resistance or resistance to slip of the surface [12].

8.2.4 Slip In order for two surfaces to slide past one another, friction, a force opposing movement, must be overcome. To start the motion, static friction must first be overcome. Sliding friction once motion is underway is less. The main cause of friction is the unevenness of the surfaces. Microscopic observation of even a carefully prepared surface reveals it to be rough and fissured. For friction between solids the frictional force (FR) is directly proportional to the load (FN). The proportionality factor between these forces is the dimensionless coefficient of friction (µG). The frictional force is independent of the contact area of the sliding bodies.

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Chemistry of active ingredients When a surface having a defined load slides over another surface, the resulting frictional force is dependent only on the coefficient of friction (µG) and hence on the surface characteristics: Equation 8.11

FR = µG * FN (at constant speed) High friction is the result of the successive collisions of many microscopically small surface irregularities. This causes abrasion or even penetration of these irregularities into the opposite surfaces and hence to scratching. Susceptibility to scratching can be minimised through the use of flow additives based on the previously-mentioned polyether siloxanes (slip effect). They function as a lubricating film. This means that the surface is well protected, especially after drying. These products are all surface active, i.e., they concentrate at the surface of the coating film. While the organic modifications orient themselves into the film, the dimethyl-siloxane units face outward and are therefore responsible for the slip effect. As a rule, the more dimethyl structures are present, the greater the reduction in the slip resistance by the additive. As with the surface tension of the coating, even the smallest amount of silicone additive (as little as approx. 0.01 % of the formulation) can markedly improve the smoothness of the surface. With increasing amounts, the effect increases until a plateau is reached. Increasing the silicone concentration above this does not lead to higher slip because the coating surface is entirely coated with silicone molecules. Determination of the correct dosage to reach the desired slip is of practical importance. Silicone additives with adjusted compatibility characteristics for water-borne coating systems are available commercially. Because of their low dimethyl group content, the silicone surfactants, which strongly reduce the surface tension of water-borne systems and are used as substrate wetting additives (see Chapter 5), have practically no effect on the surface smoothness of the coating. If higher surface slip is desired in water-borne coatings containing silicone surfactants, they must be used in combination Figure 8.9:  Measurement of slip effect with silicone polymers.

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Flow and levelling additives

8.3 Film formation As polymer emulsion are used in many market segments, film formation [1] is briefly discussed so as to give a better understanding of flow. After application of the water-borne emulsion, film formation is dominated by the evaporation of the water. With water as a solvent, the emulsion has a very high surface tension. The conditions fulfil the Orchard requirements for optimal flow. During this phase the polymer particles are able to move freely, but due to the shrinking volume their movement rapidly becomes restricted. Theoretically, after the drying coating reaches a solid content of ca. 74 %, the evaporation rate of the water slows down exponentially, as the water must then work its way through capillaries which have formed between the film particles. During this phase the polymer particles change shape and the hexagonal close packing of the polymer particles sinters into a rhombic dodecahedral structure. In the next phase the particle surfaces dissolve at places where the surfaces are in contact and the individual particles begin to coalesce. This can only occur as the capillary and surface tension forces become larger than the deformation resistance of the spheres, i.e. when the ambient temperature is higher than the minimum film forming temperature. Finally, the single particles fuse and the film forming process can take place through the interdiffusion of the polymer chain. This process can be improved through the use of film formers. Experimental results have shown that decreasing particle diameter and thus increasing particle surface area continually improve coalescence. In this wet-sintering, the decisive role is played by the polymer/water interface and not the polymer/air interface. Finally, it is not the capillary force alone, but rather the accompanying interdiffusion forces which are responsible for complete Figure 8.10:  Hypothetical model of film-formation of film-formation [1]. a polymer emulsion

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Main applications by market segment If a water-borne coating based on a polymeric emulsion is not coalesced properly, the coating will not deliver all the expected properties, and this leaves it susceptible to damage. The glass transition temperature, Tg, the temperature at which material becomes brittle like glass, can influence this property. A higher Tg means harder polymers, which are less prone to blocking. In most instances where softer polymers are used and improved blocking (see Chapter 8.8.4) is required, there are multiple formulation approaches that can be taken. A common approach in polymer-based water-borne coatings is the use of high-molecular polydimethylsiloxane emulsions. This type of product exerts a strong influence on the surface haptics and touch as well.

8.4 Main applications by market segment 8.4.1 Industrial metal coating 8.4.1.1 Electrophoretic coating

In cathodic electro-deposition epoxy adducts are used which are cross-linked by blocked isocyanates. The polymer, which is made water-soluble by the creation of salts, is deposited on the substrate as result of the application of an electrical potential on the substrate on which it forms a closed film. The neutraliser evaporates together with the water during the film forming process. As the water/neutraliser ratio has a considerable influence on the rheology, a set drying procedure must be followed. The rheology (total film flow) has a significant influence on the wetting and resistance properties as well as on the flow. Rheological additives are used to achieve the desired properties.

8.4.1.2 Water-borne coatings

Resins that are used for metal finishing applications are mostly polymers synthesised in the organic phase by polyaddition, polycondensation or polymerisation. They have hydrophilic emulsifying groups built into the polymer structure. The resins are either macro-molecular polyelectrolytes made water-soluble by salt formation, or secondary emulsions (see Chapter 6 “Co-binders”). Nowadays water-borne coatings for the automotive industry use polyester, polyacrylate, and polyurethane resins. The OH-functional polymers are frequently thermally cross-linked with melamine resins. Many water-thinnable polymers are not sufficiently hydrophilic to be considered truly water-thinnable. To achieve this, amphiphilic substances in the form of solubilisers (co-solvents) are used. These water-soluble solvents considerably influence the rheology, film forming process and drying properties.

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Flow and levelling additives As a consequence, the total film flow as well as the “surface flow” can be negatively affected. To improve the “surface flow”, fluoro-modified polyacrylates are often used as flow modifiers. As these water-borne coatings are often overcoated with solvent-based enamels, use is frequently made of polysiloxanes, which are able to migrate to the coating surface.

8.4.2 Industrial coatings Industrial coatings based on 1- and 2-pack polyurethane dispersions have almost totally replaced solvent-borne polyurethanes and alternative polymer emulsions. Polyurethane emulsions are often mixed with other emulsions, such as polyacrylates, to achieve properties which could not be obtained with other water-borne systems. To improve flow, polyurethane emulsions often contain slow-evaporating, high-boiling co-solvents. It is worth mentioning that manufacturers of polyurethane emulsions are striving to produce zero-VOC alternatives. The film-formation of air-drying 1- and 2-pack polyurethane dispersions follows principles analogous to those of other emulsions (see Chapter 8.3 “Film formation”). Exceptions to this are the 2-pack thermally-drying systems. Phase inversions can occur during the drying process as described by Thys and Bosma [7] (see Chapter 8.1.5 “Mode of action in an example of a thermosetting water-borne system with co-solvents”). In this process, surface tension gradients can occur which can negatively influence the flow. The main factor in achieving good flow in a water-borne coating based on a polymer emulsion is the choice of rheological additive which allows the control of total film flow. In thermally-drying systems surface flow additives can prevent the occurrence of surface tension gradients. Silicone surfactants have proved effective as surface flow additives in practice.

8.4.3 Architectural coatings 8.4.3.1 Flat and semi-gloss emulsion paints

Emulsion paints which are nowadays used in both interior and exterior applications are made with dispersions derived from homo- or co-polymers of vinyl acetate, vinyl chloride, vinyl propionate, acrylates, butadiene, as well as styrene. As the pigment volume concentration of these coatings lies between 40 and 80 %, the flow (total film flow) must be controlled by the proper selection of pigment wetting and dispersing agents and rheological additives.

8.4.3.2 High gloss emulsion paints

Emulsion paints today are mostly made based on pure acrylics. The emulsions used for this purpose have a minimum film forming temperature of 20 °C and the so-called “core-

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Test methods shell emulsions” from 0 °C. Although the use of film forming agents is not essential with “core-shell emulsions”, they are used to regulate the desired “open time”. Because the “open time” to some extent improves the “surface flow” it is not necessary to use surface-active additives as well. If prevention of the blocking effect (the tendency for coatings surfaces to stick together when in contact) is required, high molecular weight polyether siloxanes with a high concentration of methyl groups on the backbone are recommended.

8.5 Conclusion For optimal flow and levelling of a water-borne coating, the viscosity must remain low until a smooth surface is created by the action of the surface tension. At the same time, to avoid sagging from a surface having an angle >0° and 100 °C, such as HDPE waxes on their own or modified with e.g. amide waxes or PTFE, are the right choice for improving mechanical resistance.

Surface slip

The melting point of the wax is also decisive for increasing surface slip. In contrast to mechanical resistance, additives with a lower melting point, such as paraffin and carnauba waxes, are required here. Carnauba wax is frequently used in can and coil coating systems

Figure 9.12:  Influence of melting point on slip and anti-slip properties in a water-borne parquet laquer

187

Wax additives Table 9.2:  Water repellency in 100 % acrylic exterior coating Dosage wax emulsion, based on paraffin

Contact angle water droplet

0 %

76°

0.5 %

86°

1 %

103°

Solid wax on total formulation

for improving surface slip. High slip is, however, not required in all cases. A high slip is generally dangerous for sports flooring, parquet and floor coatings, and it can also negatively influence stacking when present in paper coatings. Anti-slip is more in demand for these applications. Polypropylene waxes provide anti-slip effects due to the high melting point. Figure 9.12 shows the influence of melting point on slip and anti-slip properties. The surface slip increase is measured in % in comparison to coating systems without wax additives. The force required to pull a weight over the surface is measured for this test.

Water repellence

One property of wax additives that has nothing to do with the melting point, but with the polarity of the base wax, is water repellence. Wax additives based on highly non-polar waxes, such as paraffin, create coating surfaces where water can no longer spread, the socalled duck's back effect. The water simply pearls off. This effect is used, for example for exterior coatings in the painting sector, for garden furniture and in addition for printing inks. The influence of the base wax can most easily be measured by measuring the contact angle of a water droplet. The higher the contact angle, the more the water droplet pulls together and the more non-polar the surface is (Figure 9.13). Table 9.2 shows the effect on an acrylic exterior wall coating. Non-polar surfaces do, however, come with the risk of adhesion loss (crawling). Adhesion must therefore always be checked when using non-polar wax additives. The influence

Figure 9.13:  Measurement contact angle

188

Wax additives for the coating industry on adhesion is relatively low for polar waxes. A second example shows the influence of paraffin on the water absorbency of paper. No moisture should penetrate through the paper used in foodstuff packaging and washing powder boxes. Paraffin waxes can be used here as a barrier (Figure 9.14).

Anti-blocking

Another effect of polarity is the influence on the blocking behaviour of a coating. This refers to the sticking of cured coating surfaces due to pressure, e.g. when stacked. The effect is also known in window paints. Non-polar coating surfaces with paraffin wax are less prone to sticking than polar surfaces (Figure 9.15). In a block test, two coated substrates are placed on top of each other after 24 hours room temperature drying and weighed down with a 1 kg weight. After 1 hour at 40 °C, the surfaces are separated again, and the blocking behaviour evaluated.

9.3.2.2 Gloss reduction

Coating surfaces appear matt when incidental light is reflected diffusely. This light scattering makes the surface appear matt. Small particles in the coating surface cause this micro-roughness. This effect can also be produced by wax additives. Normally silica would

Figure 9.14:  Water up-take measurement in overprint varnish

189

Wax additives be used for matting. The use of wax additives enables mechanical resistance and gloss reduction to be combined. Wax dispersions and micronized wax additives have, due to their particle size distribution, a more or less greater influence on the gloss grade in coating systems (Figure 9.16). Depending on the required gloss grade, the added volume of solid wax is between 2 and 10 %. Combination with silica is recommended for dull-matt systems. This combination ensures the most efficient matting with the best surface protection (Table 9.3). As already mentioned in Chapter 9.2.3 on micronized wax additives, it is difficult to find wax additives that are storage-stable in water-borne systems without emulsifiers and which do not cream up at the surface. There are specially modified wax additives for this

Figure 9.15:  Anti-blocking latex system with different waxes

190

Wax additives for the coating industry Table 9.3:  Matting and abrasion resistance test of a UV wood coating Gloss 60°

Steel wool test

without wax additive + 3 % silica

17

3 to 4

6 % wax additive* + 3 % silica

11

2

* Wax alloy based on mod. HDPE 1 = excellent 6 = worse

purpose which, due to their chemistry, are easily incorporated even at low shear forces, demonstrate excellent matting and in addition provide excellent surface protection because of their high melting points. So-called wax alloys are also new, offering a higher polarity and excellent mechanical resistance due to the alloying of different base waxes.

9.3.2.3 Texture and structure

Design and fashion are often the driving forces behind the modification of optical properties in coating surfaces. The surfaces of plastic coatings are particularly subject to this trend due to consumer demand. A trend towards textured surfaces is also apparent for plastic components as used, e.g. in entertainment electronics. Wax additives can also be used here. The wax particles protrude out of the coating film and thus create the structure. Various particle size distributions enable numerous different structures and texture ef-

Figure 9.16:  Matting of an aqueous acrylic parquet lacquer

191

Wax additives Table 9.4:  Influence of additive on orientation of effect pigment Flop index “BYK-mac” Rheology modifying wax emulsion based on mod. EVA

19.8

Arcylic thickener

16.8

fects. Very fine structures result in very beautiful even matting and a very delicate soft-feel effect. They are often used for the inner cladding of vehicles or airplanes. Very coarse structures can hide uneven backgrounds. Naturally, structured surfaces can also be found in furniture coatings or in the packaging coating sector, such as on mobile phone shells.

9.3.2.4 Rheology control

Effect coatings have taken a large part of the market, not only in the automotive industry, but also in the industrial coatings sector. This market in Europe is mainly based on environmentally-friendly water-borne coatings. In solvent-borne systems, ethylene vinyl acetate-based (EVA) wax additives have been used for years for the orientation of effect pigments and reduction of sedimentation. Due to the different rheological behaviour of water-borne systems, the use of conventional wax additives in these systems has so far not achieved the desired improvements. However, with the development of modified EVA wax emulsions, it is now possible to positively influence the rheology of aqueous effect coatings and therefore optimise the orientation of effect pigments (Table 9.4).

9.4 Summary The use of wax additives offers numerous possibilities of influencing coating properties in aqueous systems. It is important to find the right base wax/wax combination. Decisive factors for the properties aimed for are the melting points of the wax additives for optimal surface protection and polarity for water repellence and anti-blocking. In addition, the particle size of the wax additives is of decisive significance for gloss or matting and surface effects such as structure and texture. It is possible to optimise the orientation of effect pigments through novel rheology-modifying wax additives.

192

Light and photo-oxidative degradation

10 Light stabilisers Dr Adalbert Braig

10.1 Introduction Modern paint systems typically have to fulfil along with their decorative function also a protective function for the substrate underneath. Thereby they are themselves affected by a variety of environmental influences, e.g. the harmful effects of UV irradiation. As a result, the life time of such coatings is often dramatically reduced. This in turn can be significantly extended by the use of suitable light stabilisers. In most cases the UV stabilisation of coatings is achieved through the combined use of UV absorbers and radical scavengers. Through absorption of the incoming UV light the UV absorbers are predominantly responsible for colour retention and/or the protection of the substrate underneath. In addition, the radical scavengers prevent the photo-oxidative (light induced) degradation/cracking of the binder thereby ensuring gloss retention and the maintenance of the integrity of the coating. Following a short section covering „light and photo-oxidative degradation“, the stabilisation options for coatings, the mode of action and the chemical classes of light stabilisers, as well as the application fields and the criteria for proper stabiliser selection will be discussed in greater detail.

10.2 Light and photo-oxidative degradation The entire electromagnetic radiation spectrum surrounding our planet extends from cosmic radiation, highly energetic and short wavelengths to radio waves, low energy and long wavelengths. In this context the UV/VIS range as part of the entire spectrum is of particular importance and can be – according to DIN 5031 – subdivided as follows: UV-C: wavelength range 100 to 280 nm UV-B: wavelength range >280 to 315 nm UV-A: wavelength range >315 to 380 nm UV-VIS: wavelength range >380 to 720 nm (“visible light”)

Wernfried Heilen et al.: Additives for Water-borne Coatings © Copyright 2021 by Vincentz Network, Hanover, Germany

193

Light stabilisers The shorter the wavelengths of the incoming light, the higher its energy and the more harmful its effect on polymers (paints/binders). Fortunately, the short wavelengths (λ  T1 (“intersystem crossing”). Subsequent energy dissipation out of the T1 excited state can again take place in the form of a radiation-emitting process (phosphorescence) or through photochemical reactions. The above theoretical aspects are important both for the understanding of the photoChain initiation chemical degradation processes UV light of polymers, as well as for the Polymer (*) free radicals (P, PO, HO, HOO) mode of action of UV absorbers. Their mode of action will be disChain propagation cussed in greater detail in ChapPOO (1) P + O ter 10.3.1. However, it should be POOH + P (2) POO + PH mentioned at this point in time that the deactivation processes Chain branching in the case of UV absorbers have h  υ or ΔT to be largely different in comPOOH PO + OH (3) parison to the processes outh  υ or ΔT 2 POOH PO + POO + H2O (4) lined in the Jablonski diagram. In case of polymers, the abPOH + P (5) PO + PH sorption of the UV light results H O + P (6) HO + PH in the formation of free radicals, which can react with the Chain termination oxygen present, subsequently P + P P–P (7) leading to photochemical degraPOOP (8) P + POO dation processes. Such processPOP (9) P + PO es are characterised by chain POOP (10) PO + PO reactions, cleavage or branching of chains and can be subdi- Figure 10.2:  Photo-oxidation scheme of polymers P(*): vided in the following individu- these may contain: manufacturing-related agents, C=O, hydroperoxides, residual catalyst al reactions (Figure 10.2) [3]. 2

2

195

Light stabilisers

10.3 Stabilisation options for polymers As already discussed in Chapter 10.2, photo-oxidative processes are linked to the absorption of the incoming light by the polymer itself or by light-absorbing functional groups or moieties. Such groups are given the term chromophores Ch and can reach the previously described excited states Ch* through light absorption. Once the excited state is reached, a variety of subsequent reactions are possible: – Deactivation and return to the ground state through fluorescence or radiation-less deactivation (heat). Such a process is not harmful for the polymer and does not lead to subsequent degradation reactions. – Decomposition in radicals and subsequent reaction with the polymer and/or oxygen. – Radical formation through hydrogen abstraction from the polymer. – Energy transfer (e.g. to oxygen resulting in the formation of singlet oxygen 1O2). The reaction possibilities listed above must be considered harmful to the polymer, but they also indicate the principal stabilisation options which are feasible (Figure 10.3) [4–6]. The first possibility applies to the use of UV absorbers, which absorb the harmful UV light in competition to the UV-absorbing moieties contained in the polymer before Ch* can be formed. Prerequisites are fast absorption and conversion of the absorbed light into heat. Since UV absorbers can (by definition) not absorb at the very top surface of a polymeric coating (see Chapter 10.3.1), one cannot fully prevent the formation of Ch* through UV absorbers alone. This results in a second stabilisation option, i.e. the use of quenchers,

Figure 10.3: Photo-oxidative degradation of polymers and stabilising options (schematic) Ch: chromophore; P: polymer

196

Light and photo-oxidative degradation which can extinguish the excited state. Amongst these are, for example, nickel compounds, but such materials have not gained any widespread use in the stabilisation of coatings. This is mainly related to toxicity reasons and the inherent colour of such compounds. If radicals have already been formed, they can be deactivated by so-called radical scavengers, thereby preventing subsequent reactions which are damaging to the polymer. Another possibility exists in the use of peroxide decomposers. Such materials can destroy peroxides. Amongst these are in particular secondary antioxidants such as phosphites or thiosynergists. This option, however, also plays only a minor role (with a few exceptions) in the stabilisation of coatings. In conclusion, the combined use of UV absorbers and radical scavengers or for certain applications the use of radical scavengers alone became established.

10.3.1 UV absorbers In the case of UV absorbers, one has to differentiate between inorganic and organic products. Inorganic products are ideally nano-particulate materials comprised of, e.g. titanium dioxide, zinc oxide or cerium oxide, which predominantly absorb at short wavelengths. Organic UV absorbers are represented by a variety of different chemical classes, which are also characterised by significant differences in terms of their absorption characteristics. Amongst these are [7–14]: – 2-(2-hydroxyphenyl)-benzotriazoles – 2-hydroxy-benzophenones – 2-hydroxylphenyl-triazines – oxalic anilides – cyano acrylates – salicylic acid derivatives – hydroxyphenyl-pyrimidines All these classes have the ability to absorb the incoming UV light and to convert it into heat which is not harmful to the coating polymer matrix. For the stabilisation of coatings, predominantly organic UV absorbers (represented by the top four classes) have established themselves over the years. The general structures of the classes above are shown in Figure 10.4.

Figure 10.4:  General structures of the most important UV absorber classes for coatings

197

Light stabilisers One can assign characteristic UV absorption spectra (Figure 10.5) or UV transmission spectra respectively (Figure 10.6) to the different UV absorber classes. Independently in certain cases (e.g. hydroxy-phenyl-triazines), the absorption maxima can be shifted by suitable substitution patterns (Figure 10.7), either towards significantly higher absorbance (e.g. bis-biphenyl-triazines; HPT-3) or towards longer wavelengths (e.g. tris-hydroxy-phenyl-triazines (HPT-4), absorption maximum at 360 nm). This is of particular of interest for the protection of those substrates which are most sensitive in the 320 to 340 nm range (e.g. polycarbonate) or in the longer wavelength region (e.g. wood). As shown in Figure 10.5, the absorption characteristics of conventional UV absorbers can be described as follows: – Oxalic anilides show one absorption maximum in the 300 nm range. – In contrast hydroxy-benzophenones, hydroxy-phenyl-triazines and hydroxy-benzotriazoles each exhibit two absorption maxima (short wavelength range at around 300 nm and longer wavelength range at >320 nm). Of these, hydroxy-phenyl-triazines show the most pronounced absorption towards the short wavelengths. – Due to their second absorption maximum at wavelengths >340  nm hydroxy-phenyl-benzotriazoles exhibit the broadest spectral coverage. The exact position of the absorption maxima as well as the extent of extinction ultimately depend on the substitution patterns (see Figure 10.4) of the individual molecules.

Figure  10.5:  Absorption spectra of different conventional UV absorber classes (c = 10 mg/l in CHCl3; cell 1 cm), 1 = oxalic anilide; 2 = hydroxy-phenyl-benzotriazole; 3 = hydroxy-benzophenone; 4 = hydroxy-phenyl-triazine

198

Light and photo-oxidative degradation

Figure  10.6:  Transmission spectra of different conventional UV absorber classes (c = 50 mg/l in CHCl3; cell 1 cm) 1 = oxalic anilide; 2 = hydroxy- phenyl-benzotriazole; 3 = hydroxy-benzophenone; 4 = hydroxy-phenyl-triazine

Figure 10.7:  Influence of the substitution pattern on absorption in the case of hydroxyphenyltriazine UV absorbers (c = 20 mg/L in toluene, active substance each) 1= hydroxy-phenyltriazine (HPT-1); 2= hydroxy-phenyl-triazine (HPT-3); 3 = tris-hydroxy-phenyl-triazine (HPT-4)

199

Light stabilisers The extent of the spectral coverage can be even better explained with the aid of the transmission spectra (Figure 10.6), rather than the absorption spectra, of the neat material. The further the absorption is shifted towards the longer wavelength range the more damaging UV light can be filtered out. Besides the absorption characteristics, the efficacy of UV absorbers is predominantly defined by the Lambert-Beer law: E = Abs = ε . c . d = log I0/I where E: extinction Abs: absorbance intensity of the incident light I0: I: intensity of the emergent light ε: extinction coefficient (l/mol cm) c: concentration (mol/l) d: thickness (cm) of the paint film, substrate or cell The extinction depends on the wavelength and can be regarded as a measure of the filter effect of the UV absorber, i.e. the higher the extinction at a certain wavelength, the more light is being screened. As shown in the Lambert-Beer law, the extinction is directly proportional to the molecule specific extinction coefficient ε, the concentration c of the UV absorber and the film thickness d of the unpigmented polymer (paint film). Cutting the film thickness in half therefore means that the concentration c of the UV absorber needs to be doubled in order to achieve the same filter effect. Additionally, the Lambert-Beer law clearly indicates that the UV absorber can (by definition) not absorb at the very top surface (d = 0), see also Chapter 10.3.1.

Mode of action of UV absorbers

As already indicated in Chapter 10.2, the deactivation processes shown in the Jablonski diagram (Figure 10.1) have to be such that, in the case of UV absorbers, they convert the absorbed energy into something harmless before undesired side reactions can occur. This means that energy conversion has to take place in the singlet state S1. Furthermore, the crossing from S1 to T1 (intersystem crossing) resulting in phosphorescence must be excluded [7]. Concerning the mode of action of UV absorbers one has to distinguish between phenolic UV absorbers (e.g. hydroxy-benzophenones, hydroxy-phenyl-triazines, hydroxy-phenyl-benzotriazoles) and non-phenolic UV absorbers such as oxalic anilides. Figure 10.8 shows the energy conversion of phenolic UV absorbers using hydroxy-phenyl-benzotriazoles as an example. Once the UV absorber has absorbed the light a proton transfer from the oxygen to the nitrogen atom occurs in the excited singlet state thereby forming a photo tautomer (“keto

200

Light and photo-oxidative degradation form”). The original molecule subsequently re-forms (“enol form”) through radiation-less deactivation (release of heat) and returns to the ground state. Comparative investigations of oxalic anilides [13, 17] indicate an intramolecular proton transfer occurring during energy conversion.

10.3.2 Radical scavengers As mentioned in Chapter 10.3, excited chromophores Ch* can form radicals, subsequently leading to free-radical reactions (see Figure 10.3). In order to prevent such reactions, products are needed which have the ability to scavenge such radicals and thus prevent possible chain reactions from taking place. Such molecules are called radical scavengers. The most important representatives are antioxidants and sterically hindered amines.

10.3.2.1 Antioxidants

Depending on the mode of action one can divide antioxidants in two groups, i.e. primary antioxidants (chain terminating effect [18] via a radical mechanism) and secondary antiox-

Figure 10.8:  Energy conversion of phenolic UV absorbers (e.g. hydroxy-phenyl-benzotriazoles) according to Otterstedt [15]

201

Light stabilisers idants (decomposition of peroxides through an ionic mechanism). In this context, the secondary antioxidants will not be further discussed. Primary antioxidants are predominantly represented by sterically hindered phenols. The mode of action of this class is schematically shown in Figure 10.9. According to equation (1) in a first step a phenoxy radical is formed. The stability and thus the reactivity of this radical depends on the substituents R1 und R2, i.e. the possibility for resonance stabilisation (delocalisation of the electron), as shown in equation (2). The more stable the phenoxy radical [16] the lower the probability that further chain reactions can be initiated. This is typically achieved through rather bulky R1 and R2 substituents such as tertiary butyl groups. With respect to the stabilisation of coatings, one important drawback of phenolic antioxidants that exists is their non-cyclical mode of action, i.e. they are consumed after a certain period of time and are no longer capable of preventing further radical reactions. This in turn means that they are predominantly used as process stabilisers at high Figure 10.9:  Mode of action of sterically hindered processing temperatures (e.g. prophenols (schematic) cessing of plastics).

10.3.2.2 Sterically hindered amines

Figure 10.10:  Dependence of life time and reactivity of phenoxy radicals on substitution [16]

202

This class of compound is almost exclusively represented by derivatives of 2,2,6,6-tetramethylpiperidine (Figure 10.11). In the literature they are typically referred to as “HALS”, which stands for hindered amine light stabilisers. This term will be used in the remainder of this chapter.

Light and photo-oxidative degradation Of particular importance is the substituent R, since it directly influences both the basicity of the HALS molecule and the rate at which the nitroxyl radical, the actual active substance, is being formed. This will be discussed in greater detail in Chapter 10.3.2.

Basicity of HALS

Generally, HALS in which the nitrogen atom of the piperidine group is substituted by hydrogen or methyl display a high basicity and have a pKb value of approximately 5. In some cases, this can lead in amine neutralised water-borne coatings to a shift of the pH value, resulting in destabilisation of the formulation. HALS in which the nitrogen atom of the piperidine group is substituted by, e.g., COCH3 or O-alkyl are essentially non-basic in nature and have pKb values in the range of 10 to 12.

Mode of action of HALS

The mode of action of HALS has been extensively researched. This work was predominantly carried out in liquid or solid model systems. Nevertheless, it is unclear if the results obtained truly represent the processes occurring in polymers or paints. In contrast to the mode of action of primary antioxidants the HALS mechanism is cyclical in nature, an important prerequisite for their usefulness as longterm stabilisers for coatings. The bestknown mode of action is based on the research of Denisov and the eponymous cycle Figure 10.11:  General structure of is shown schematically in Figure 10.12 [19]. For the sterically hindered amines based on purpose of this chapter, the Denisov cycle is con- 2,2,6,6-tetramethylpiperidine sidered a good and sufficient model for describing the action of HALS in coatings. As shown in Figure 10.12, in a first step HALS (1) is transformed in the presence of oxygen and UV light into the nitroxyl radical (2), as the actual active species. The formation of the nitroxyl radical starting from a protonated HALS is quite difficult, if not impossible. In the next step the nitroxyl radicals Figure 10.12:  Mode of action of HALS (schematic) can scavenge the free radicals (Denisov cycle)

203

Light stabilisers (e.g. formed from reaction products of excited chromophores Ch*) by formation of the amino ether (3). Reaction of amino ether (3) with peroxide radicals leads to the formation of the intermediate (4), which can readily decompose thereby (re-)forming the nitroxyl radical (2) as well as the harmless bi-products R=O und R’OH. Thereafter the nitroxyl radical is available for a subsequent cycle.

10.4 Light stabilisers Generally speaking, all light stabilisers initially developed for use in solvent borne coatings can also be used in water-borne systems. The “only” difficulty is the ease of incorporation of such products in aqueous systems. Larger paint or resin manufacturers may have the principal option of incorporating such materials in the organic phase, i.e. during resin synthesis. For most paint suppliers, however, this is not considered a valid option. In the last years the industry has made considerable efforts in the development of water compatible product forms based on existing and performance proven light stabiliser technology, which can be easily incorporated or post-added to water-borne coating systems. From a concept point of view, the following product forms should be mentioned: – Hydrophilic light stabilisers such as the hydroxy-phenyl-benzotriazole I (BTZ-1) shown in Table 10.1. The polarity of the introduced side chain is sufficiently high in order to allow in most cases the incorporation of the compound without additional need for co-solvents [20, 22–23]. – Solid state dispersions: these products are solid UV absorbers and/or sterically hindered amines (Table 10.1), which have been dispersed in water in the presence of dis-

Figure  10.13:  Particle distribution and particle size of nano dispersions determined by ultracentrifuge (left) and TEM (right)

204

Light stabilisers Table 10.1:  Overview of the most important UV absorbers for water-borne coatings (by UV absorber class and technology) UV absorber

Technology

BTZ-1

hydrophilic

BTZ-2

nano dispersion or emulsion

BTZ-3

solid state dispersion

BTZ-4

solid state dispersion

BTZ-5(red shifted)

solid state dispersion

HPT-1

nano dispersionor emulsion

HPT-2

solid state dispersion

HPT-3

nano dispersion

HPT-4(red shifted)

nano dispersion

BP-1

solid state dispersion

Chemical composition

205

Light stabilisers Table 10.2:  Overview of the most important sterically hindered amines (HALS) for water-borne coatings (by technology) HALS

Technology

HALS-1(basic)

solid state dispersion

HALS-2

solid state dispersion

HALS-3

solid state dispersion

HALS-4(non-basic)

nano dispersion emulsion

HALS-5(non-basic)

nano dispersion

HALS-6(basic)

Emulsifying, may need some co-solvent

Chemical composition

oligomeric HALS

persants and small amounts of organic solvent (e.g. glycol ether). The solids content (active substance) is in most cases in the range of 50 to 55 %. Despite a certain sedimentation tendency these products can be readily re-dispersed whilst stirring [21, 22]. Their usefulness in ambient-curing systems is sometimes limited, due to insufficient solubility in the polymer matrix. – Emulsions: preparations in which the light stabilisers were converted into a water compatible form by using (non-ionic) emulsifiers [22, 23]. Due to the high levels of emulsifiers used, there is a certain risk that the water sensitivity of the paint film may be increased. – Neat (novel encapsulated additive technology): relatively new on the market, the light stabilisers are dissolved, rather than “encapsulated”, in an acrylic type polymer matrix characterised by an average particle size of 1000

5 – 10

5 – 10

0.3

0.1 – 0.5

4

0.5

0.5

0.5

5

1

2.5

0.5

5

5

Sclerophoma pithyophila

2.5

0.5

Sporobolomyces roseus

1

1–2

Trichoderma viride

0.5

1–2

Ulocladium atrum

2.5

500

stable fungicide with a low leach rate, it shows a gap in its spectrum of activity against Alternaria and Ulocladium species, this gap can be filled using OIT which is less stable with higher water solubility. In combination, however, these two actives provide good film protection against a wide range of mould fungi. The selection of the algicide, the third component, was strongly dependent – like today – on the individual demands of the coatings manufacturer. Besides those mentioned ahead combinations of other actives are used in practice. One thing they all have in common is that no single active ingredient can provide the answer for all situations. The important fungicides are OIT, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT); zinc bis-(2-pyridinthiol-1-oxide) (ZPT); 3-iodo-2-propynyl-butyl carbamate (IPBC) and carbendazim. For algicides, the choice is more limited, but includes triazines such as terbutryn and the phenylurea derivatives diuron and isoproturon (see Tables 11.3 and 11.4).

11.3.2 New, “old” actives Because of the high costs associated with the registration of new chemicals due to the need to generate large amounts of toxicological, ecotoxicological and environmental fate data,

220

Dry film preservation Table 11.3:  Fungicidal active ingredients for film preservation and their main features Fingicidal active ingredients

Main features

Carbendazim

efficacy gaps against Alternaria, Ulocladium stable against UV-light and alkalinity low leaching classified as CMR substance category 1B (mutagenic & reprotoxic)

IPBC

broadspectrum fungicide discolouration risk instable in presence of UV light, temperature, alkalinity sensitisation possible halogenic compound

OIT

broadspectrum fungicide stable, also at alkaline conditions highly water soluble leachable sensitizer

DCOIT

broadspectrum fungicide reduced water solubility, low leaching instable in presence of UV light, temperature, alkalinity sensitizer halogenic compound

ZPT

reduced water-soluble and leachable up to pH 90° the substance is said to be hydrophobic. Strongly hydrophobic surfaces have contact angles >140°. Façade-protecting coatings with very high contact angles will be discussed in Chapter 12.3.3. It should be noted that the size of a contact angle on a surface is no guarantee that the capillary hydrophobing effect of the coating will be good, rather it provides a measure of the water-beading effect of the coating surface [1].

12.1.3 How hydrophobing agents work As described in Chapter 12.1.1, water can rise in a capillary if the capillary can be wetted. For the protection of building materials wetting should not occur. This can be achieved by lining the capillaries with hydrophobing agents such as silicones and paraffin waxes. The low surface tension of the hydrophobing agents increases the surface energy between the capillary wall and the liquid (water). This means that the contact angle in the pore is increased. The relationship between the interfacial energy, Figure 12.4:  Contact angle of a water drop on a hydrosurface tension and contact an- phobic surface

Figure 12.5:  Alkoxy reaction of a silicone resins

229

Hydrophobing agents gle given by the Young equation will be discussed in the Chapter 5 “Substrate wetting additives”. The advantage of using silicones as hydrophobing agents is that their low surface tension results in a high degree of spread so that they build a molecular film on the capillary walls. In this way the silicones reduce the free surface energy of the substrate and thus produce their characteristic properties. Siliconising porous materials does not significantly change their permeability to air and water vapour. The silicones coat the pore walls without clogging them. Liquid water will be unable to enter the pores due to the increase in the interfacial surface tension [2]. In contrast, organic hydrophobing agents, such as paraffin waxes, are unable to spread and also do not provide a good anchor to the substrate. Pores in porous materials will be closed and this will influence the diffusion of water vapour. A further disadvantage of paraffin waxes is that they can lead to adhesion problems when attempting to apply a second coating. As there is no bond with the coating system, the hydrophobing effect is also reduced over time due to weathering effects such as rain. The use of silicone oils and resins results in a permanent hydrophobing effect as they are particularly strongly anchored to the capillary walls. This takes place in different ways, one of which involves the siloxane dipole, inducing the oxygen atoms to orient themselves to the hard surface (see Figure 12.6), and the methyl groups to pack themselves densely together. The packing of the methyl groups leads to an increase in the hydrophobing effect. Alternatively, bonds are formed between the reactive groups, such as the alkoxy groups in the silicone resins, and the filler pigments and/or substrate. They react by eliminating alcohol (methanol, ethanol), resulting in a long-lasting hydrophobing effect which cannot be impaired by weathering (see Figure 12.5). Figure 12.6:  Anchoring of polysiloxanes by an oxygen dipole

Figure 12.7:  The simplest linear silicone structure

230

12.2 Chemical structures Silicones can be categorised into oils, fats, resins and rubbers. Silicone oils and resins have become established as hydrophobing agents for water-borne coatings systems.

Chemical structures Table 12.1:  Mono, di-, tri- and tetra-functional siloxanes Structure

Summary

Functionality

Symbol

R3Si-O-

R3SiO

monofunktionell

M

R2SiO2/2

difunctional

D

RSiO3/2

trifunctional

T

SiO4/2

terafunctional

Q

The term silicone refers to organo-silicon-based polymers with Si-O-Si bonds. On the remaining free valences of the silicon atom are other organic groups, mostly methyl, connected by Si-C bonds. Due to their structure, silicones occupy a position between inorganic and organic polymers, in particular between silicates and organic polymers [2]. The very stable bonds result in silicones’ highly interesting characteristics including, for example, heat-resistance and weathering resistance. Furthermore, they are hydrophobic and surface active as well as possessing release and slip properties. The simplest linear silicone structure with organo groups is shown in Figure 12.7. Because of the bonding of the silicon atom to both oxygen (sil–oxane) and organic groups, the terms polyorganosiloxanes or organo-polysiloxanes are also used when discussing silicones. Owing to the numerous types of bonding and possibilities, introduction of simple silicone building blocks are established. According to this highly complex polymer structures of polysiloxanes can be easily and clear demonstrated. Thus, the siloxane units are categorised by their functionality, that is by the free valencies of the oxygen atoms attached to the silicone atom. These units are termed as mono, di, tri and tetrafunctional. In the manufacture of polymers, the units are abbreviated as M, D, T und Q units (see Table 12.1). Because of their monofunctionality, M-siloxane units can be combined with themselves to form a hexaorgano-disiloxane of the type M2. In combination with units of higher functionally the M-siloxanes are used as chain stoppers. R3Si-O-SiR3 (M2)

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Hydrophobing agents The difunctional D units can react with each other to produce ring-form siloxanes. The smallest cyclosiloxane is composed of three units (D3), but D4 and D5 siloxanes are also known. Combinations with M units yield chain-form linear siloxanes. Co-reacted T-siloxanes produce spatially cross-linked molecules and in combination with mono- and difunctional units result in resinous macromolecules [2].

12.2.1 Linear polysiloxanes and organofunctional polysiloxanes Linear polysiloxanes are generally referred to as silicone oils. Their molecular structure consists only of M and D units. M-D-D-D-D-D-M Linear organofunctional polysiloxanes are also composed primarily of M and D units but they additionally contain at least one functional group involving a silicon-carbon bond. Amino-functional polysiloxanes are important hydrophobing agents and are often referred to as silicone oils. Linear polysiloxanes are supplied either as liquid oil concentrates or dissolved in solvents. They are also available as emulsions. These products exhibit a good hydrophobing effect, reduce the water absorption in coatings and promote a high contact angle. High contact angles with the associated strong water-beading effect are desirable in architectural paints. To achieve strong water-beading, the contact angle on the coating surface must be more than 140°. Initially, emulsions give a weaker effect in terms of contact angle, as amphiphilic substances in the emulsion reduce the hydrophobing effect. However, when all hydrophilic emulsifiers have been washed away by rain, the contact angle will increase. In contrast, solutions of polysiloxanes immediately impart a higher contact angle to a coating as they do not contain emulsifiers. The assumption that the beading effect from raindrops will automatically lead to a cleaner façade cannot, however, be proved. Rather the opposite in fact – investigations show an increased affinity for dirt pick-up (see Table 12.3). Linear polysiloxanes are very resistant to UV radiation, heat and rain and are also very alkali-resistant, which is why they are frequently used in silicate emulsion paints.

12.2.2 Silicone resins/silicone resin emulsions Silicone resins differ from silicone oils in that, in addition to the chain-building dimethyl dichlorosilane, they incorporate the network-building trichlorosilane. A look at the possible combinations of the 4 building blocks M, D, T, and Q will clarify this. There are 15 possible combinations (see Table 12.2).

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Chemical structures Table 12.2: Combination possibilities

Source:according to Noll: Chemie und Technologie der Silicone [3]

Class

Siloxane units

Class

Siloxane units

1

QT

8

QTM

2

QD

9

QDM

3

QM

10

TDM

4

TD

11

QTDM

5

TM

12

TT

6

DM

13

DD

7

QTD

14

MM

15

QQ

The combinations DM, DD und MM are not branched and are therefore not used in resin production. Combinations with Q-units are of limited use in the hydrophobing of coatings. This leaves the possibilities TD, TM, TDM and TT, all of which are utilised in the production of technical resin products [3]. For the production of silicone resins for use in silicone-resin architectural paints, resins based on the building blocks TT, TD and TDM are currently used. TT silicone resins form films at room temperature ranging from clear through hard to brittle depending on the degree of condensation. TD or TDM resins dry to soft or hard films depending on the relative amounts of each building block. Water-borne silicone resin emulsions are the only ones used in architectural paints, especially silicone resin architectural paints.

12.2.3 Other hydrophobing agents In addition to polysiloxanes, paraffin waxes (unsaturated hydrocarbons with the general formula CnH2n+2 where n is in the range from 25 to 50), polyethylene and polypropylene waxes are also used. These polymers have a molecular weight range from 1000 to 4000. They are insoluble in water and organic solvents and are used in the form of a micronised emulsion (water-borne or solvent-based). Polyfluorocarbonates with molecular weights in the range 1000 to 5000 are also used [4].

12.2.4 Production of linear polysiloxanes The primary raw materials for the production of polysiloxanes are chlorosilanes, which can be hydrolysed into oligomers and/or cyclic dimethylsiloxanes. Dimethylsiloxane is pro-

233

Hydrophobing agents duced from dimethyl dichlorosilane in the usual manner. It comprises a mixture of cyclic and linear OH-containing oligomers. The ratio of cyclic to linear monomers and the chain length depend on the conditions of hydrolysis. Linear OH-functional siloxanes can be produced by the continuous hydrolysis of the chlorosilane. This occurs by breaking the siloxane bond using a catalyst. In this process the cyclosiloxane is continuously distilled off and fed back to the hydrolysis phase of the reaction [3]. The hydrochloric acid produced by both processes is reacted with methanol to produce methyl chloride and this is returned to the silane synthesis. The second step after hydrolysis is either polycondensation of the linear silicone oligomers or polymerisation of the cyclosiloxane to linear polysiloxanes. This procedure can be catalysed equally well by acid or alkali. In both cases this is an equilibrium reaction, which can be defined by adjusting the reaction conditions. Depending on the conditions, siloxane chains can be broken or formed. This procedure, known as equilibration [3] can be used to obtain silicone oil with the desired average chain length. Organofunctional polysiloxanes are mainly produced by a hydrosilylation reaction. In thes cases siloxanes are used which are Si-H functional. These siloxanes react by a radical addition reaction with the double bonds of vinyl or allyl groups. Platinum is the preferred catalyst. The equation below is a simplified representation of the addition of allyl amine to an aminofunctional polysiloxane.

12.2.5 Production of silicone resin emulsions There are several different processes for the production of silicone resins emulsions.

12.2.5.1 Secondary emulsification process

This classical method involves initial production of a solid silicone resin. A crosslinked but liquid oligomer is produced from the silane monomers by a catalytic polycondensation reaction, as described in the production of oils. This oligomer is further condensed to a solid resin that is soluble in aromatic as well as aliphatic solvents. In a second step, the solid resin is dissolved in suitable solvents Figure 12.9:  Addition reaction of allyl amine to an aminofunctional polysiloxane and then is emulsified in water

Figure 12.8:  Hydrolysis of dimethyl dichlorosilane

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Water-borne architectural paints in the third and final step. As a result of this process the silicone resin emulsions can contain up to 15 % solvent. Modern production methods now allow production of secondary emulsions with a lower solvent content. Silicone resin emulsions, which are based on a pre-polymer and then emulsified, are known as secondary emulsions.

12.2.5.2 Primary emulsification process

Another route for producing silicone resin emulsions, but with very little solvent, is the emulsion polycondensation process. In this primary emulsification process, liquid oligomer resin in the emulsion droplet is further condensed to solid resin. Alternatively, a silane/siloxane/ silicone resin mixture is used from the start. Solvents are not required for the production of primary emulsions and the emulsions can therefore be very low in solvent content.

12.3 Water-borne architectural paints 12.3.1 Synthetic emulsion paints There is a great variety of emulsion paints ranging from non-breathing, high-gloss to open, matt systems. The glossy coatings have higher binder content and are therefore less breathable, i.e. permeable to air/water vapour. Their closed films protect the façade very effectively from driving rain and damp, but they are not water vapour permeable. In contrast, over-critical formulated (see Chapter 12.5 “Appendix”) matt emulsion paints which are able to breathe require silicone resins or polysiloxanes to coat the capillaries. If silicone resins are used, the paints are again in the category of silicone resin emulsion paints (see Chapter 12.3.5).

12.3.2 Silicate emulsion paints The characteristic binder for silicate emulsion paints is waterglass. Potassium silicates whether as solutions or hydrates, also known as potash water glasses, are used in silicate emulsion paints. The potassium compounds show advantages over sodium water glass, such as better stability and better resistance to salt efflorescence. For stabilisation, organic binders such as styrene acrylic emulsions are used together with water glass. DIN 18363 specifies that the organic polymer content should not exceed 5 % of the total formulation. When other additives are incorporated, the silicate emulsion paints are more stable over many months than pure silicate paints and are ready-to-use. Pure silicate paints are two-component systems which must be mixed beforehand and allowed to stand before use. Silicate paints dry both physically through water evaporation, and Figure 12.10:  Reaction scheme for silicates

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Hydrophobing agents chemically with many reactions taking place simultaneously. In silicification the alkaline silicate reacts with atmospheric carbon dioxide forming carbonates and silica gels [5] (see Figure 12.10). Silicate paints react with mineral substrates to form potassium metasilicates and potassium hydroxide. Coatings based on water glass are very brittle and have a typical matt, mineral-like appearance. They also have very open pores and therefore exhibit good breathability: To reduce capillary water absorption, linear (functional) polysiloxanes are used. The use of silicone resins is quite limited, due to the high pH value of the water glass, which requires that only very alkali-stable emulsions, additives and pigments be used. Specific precautions are necessary such as: – eye protection, – avoidance of skin contact; the skin must be washed with much water immediately upon contact, – overspray on glass, aluminium or painted surfaces, must be avoided due to strong corrosion, – only alkaline-stable inorganic pigments, additives and emulsions may be used [6].

12.3.3 Emulsion paints with silicate character (SIL paints) Emulsion paints with silicate character have been used for more than 35 years. As their formulations contain quartz powder or other mineral fillers, they are particularly openpored and capillary-active. Because of this, silicone resins or functional polysiloxanes are used to reduce the water absorption in SIL-paints. These kinds of paints are especially useful in maintaining the mineral-like matt optical properties.

12.3.4 Siloxane architectural paints with strong water-beading effect A few years ago, paints which were noteworthy for their strongly hydrophobic effect became available commercially. The water-repellency on the coating surface is so great, that water drops actually form beads, rather like they do on a lotus leaf. For such a water-beading effect contact angles in excess of 140° are needed. Paints of this type have become known as lotus-effect paints. This beading effect is the result of the use of suitable hydrophobing agents and a special surface texture of the coating. The use of certain fillers produces a micro-peaked-structure which, in combination with aminofunctional polysiloxanes, results in a strong water-beading effect. All raw materials must have as low an emulsifier content as possible, otherwise the beading effect will be impaired.

236

Water-borne architectural paints Wetting agents and emulsifiers reduce the contact angle and destroy the water-beading effect. For this reason, polysiloxanes are used in a solvent solution and not in the form of an aqueous emulsion. These coatings are remarkable for their low water absorption and their high water vapour diffusion. A disadvantage of lotus effect paints is their strong tendency to pick up dirt (see Table 12.3).

12.3.5 Silicone resin emulsion paints Silicone resin emulsions play an important role in silicone resin architectural paints, where they function as co-binders or hydrophobing agents. The amount of silicone resin used in such coatings ranges from 2 to 12 %. Often combinations of silicone resins with modified silicone oils are used. Because of their breathability, the coatings are formulated so as to be “over-critical” (see PVC), meaning they contain a low level of binder and high amounts of pigments and fillers. The silicone resins react quickly through alkoxy-reaction with themselves and with the fillers to form a network and thereby provide both early- and long-term water-repellency. Because the silicone resin only coats the pores of the coating, there is little or no influence on the coating’s water vapour permeability. The degree of the breathability of the coating film is primarily dependent on the type and amount of organic resin present (for example styrene/acrylic emulsions) and not on the amount of the silicone resin. There is also no difference between the hard silicone resins, which consist mainly of T-units, or the more flexible TD silicone resins based on a higher amount of D units. Because of the low surface tension and the resulting spreading potential, the silicone resins distribute themselves along the walls of both large and small pores without changing the pore radius [2]. Because of this it is doubtful whether silicone resins should be considered as binders and used in the calculation of the PVC. Silicone resins have a lower dirt pick-up potential than linear, non-drying polysiloxanes (silicone oils). It can be argued that the oily character of the polysiloxane and its thermoplasticity would favour adhesion of particles to the paint surface and hence a higher affinity for dirt pick-up. The phenomenon of dirt pick-up affinity is still not completely understood. What is known is that the attraction of dirt is influenced by the glass transition temperature (Tg), surface roughness, type of contamination (hydrophobic, hydrophilic) the weather conditions (Sahara weather or industrial) and many other factors. Description of the processes by which the major influencing factors interact with one another is not yet possible. The basic tendency to pick up dirt cannot simply be explained by the thermoplasticity or the hardness of the silicone resins but is considerably more complex. This is outlined in Table 12.3 which provides a comparison of over-critical formulated coatings containing 8 % of an aminofunctional polysiloxane (emulsion paints 1&2), a silicone resin with a high T-unit content (hard) and a TD resin with low T-unit content (soft) (SREP 2&1). w24 water absorp-

237

Hydrophobing agents Table 12.3:  Comparison of different architectural paints Water absorption

Water vapour diffusion

w24-value

sd-value [m]

outdoor exposure (after 24 months)

simulated dirt pick-up

initial

after 3 minutes

SREP paint 1 TD resin (soft)

0.06

0.089

20.1

9.4

104

60

SREP paint 2 TD resin (hard)

0.11

0.099

19.9

17.0

107

88

Emulsion paint 1 aminosiloxane emulsion

0.09

0.088

21.4

14.6

120

62

Dispersions paint 2 aminosiloxane solution

0.09

0.083

22.2

16.5

135

121

Lotus effect paint

0.08

0.073

25.7

29.7

148

145

Silikate paint aminosiloxane emulsion

0.09

0.020

12.1

27.3

94

84

Architectural paint

Dirt pick up degree VG [%]

Contact angle [°]

tion, breathing characteristics, contact angle and dirt pick-up were tested under natural weathering conditions and simulated conditions in a dirt pick-up machine. In the artificial dirt pick-up machine, the softer TD-resin showed the lowest affinity for dirt. The harder silicone resin exhibited the greatest dirt pick-up in the machine, followed by both amino-dimethylpolysiloxanes. However, under real weathering conditions both silicone resins showed similar values. The measured sd-values were also similar, whether oil or silicone resins were used. This leads to the conclusion that the degree of crosslinking of a silicone resin neither serves to predict tendency to dirt pick-up nor indicates any other particular advantages. From experience [7] it is known that silicone resin-based architectural coatings have a low tendency to pick up dirt, especially when compared with emulsion-based coatings at low PVC. This probably stems from the different wetting behaviour of the resulting coating films. Dirt particles accumulate on the façade, especially in regions where rainwater contains concentrated matter such as soot. Inorganic dirt particles carried to the façade by the wind seem to be less of a problem than organic dirt. Long-term, south-facing open-air weathering shows that, for silicone resin coatings and renderings based on polymer emul-

238

Water-borne architectural paints Table 12.4:  Guiding formulation for a silicone resin coating based on nanohybrid emulsion Pos.

Components

1

Water

25.2

2

“Tylose” MH 30000 YG8

0.2

SE Tylose GmbH & Co. KG

3

“AMP-90”

0.3

The Dow Chemical Company (Dow) or an affiliated company of Dow

4

“Tego” Dispers 715 W

0.4

Evonik Industries AG or one of its subsidiary companies

5

“Agitan” 265

0.3

Münzing Chemie GmbH

6

“Acticide” MBS Methyl-/ Benzisothiazolinone 1:1

0.1

Thor GmbH

7

“Acticide” F (N)

0.1

Thor GmbH

8

“Kronos” 2056

15.0

Kronos Titan GmbH

9

“Finntalc” M 30

5.0

Mondo Minerals BV

10

“Dorkafill” H

10.0

Gebrüder Dorfner GmbH & Co. Kaolin- und Kristallquarzsand-Werke KG

11

“Omyacarb” 5 GU

10.0

Omya AG

12

“Omyacarb” 15 GU

10.0

Omya AG

13

Butyl diglycole acetate

1.2

14

“Tafigel” PUR 40

0.2

Münzing Chemie GmbH

15

“Tego” Phobe 1650

6.0

Evonik Industries AG or one of its subsidiary companies

16

“Mowilith” LDM 7717

8.0

Celanese Emulsions GmbH

17

“Mowilith” Nano 9451

8.0

Celanese Emulsions GmbH

Total mill base

p.b.w.

Trademark holder

100.0

sions with a glass transition temperature >10 °C, there is no significant difference in dirt pick-up or chalking properties between pure acrylates and acrylate/styrene copolymers (all other materials remaining the same). In the formulation of solvent-free silicone resin systems, the choice of raw materials influences the above film properties. Solvent-free silicone resin emulsions have a slightly lower tendency for dirt pick-up than solvent-containing systems. The use of newly developed colloidal silica-latex polymer nanocomposites as binder (hybrid polymers) further boosts the said tendency. Experience indicates that a triple combination of arcylate emulsion, nanohybrid emulsion and a silicone resin emulsion as shown in the formulation Table 12.4 is particularly effective.

239

Hydrophobing agents

12.4 Conclusions Because of the large number of silicone combinations, there is a wide variety of products available to the architectural paints sector. They allow the development and formulation of superior coatings for façade protection, which are both breathable and in addition, resistant to driving rain and other weathering effects. Due to their high efficiency, their longterm effectiveness and their recoatability, polysiloxanes and silicone resins have replaced other types of hydrophobing agents commercially.

12.5 Appendix 12.5.1 Façade protection theory according to Künzel In Künzel’s façade protection theory formulated in the 1960s, two properties play an important role: the water absorption capacity of the substrate and the water vapour permeability of the coating. Künzel noted the importance of a protective coating which is both moisture resistant and breathable in the sense of allowing water vapour to pass through. Two parameters are of crucial significance in this theory. The first is the water absorption coefficient (w-value). The second is the water vapour diffusion resistance (sd-value). The w-value is a measure of the time-dependent capillary water absorption of a porous building material in kg/m2√h. The water vapour diffusion resistance or sd-value of the coating is expressed in metres. The greater the value, the higher the resistance and the less the diffusion of the

Figure 12.11:  Architectural paints after artificial dirt exposure

240

Appendix paint film. For an optimal coating, both values should approach zero. Künzel proposed the ideal values for coatings at that time to be: – Capillary water up-take w-value ≤ 0.5 kg/m2√h – Water vapour diffusion sd-value ≤ 2 m

Table 12.5:  Classification of water-pickup to EN 1062-3 Class

Water absorption

w-value

III

low

0.5

Thus, the product w * sd should be ≤0.1 kg/m√h (shown as a striped area on the graph, Figure 12.12). Coatings whose values lie inside the yellow area offered the greatest protection at that time (c. 1960). Künzel’s work and the increasing quality demands on exterior coatings have resulted in the further optimisation of products in the past decade. Silicone resin architectural coatings can now attain w-values under 0.1 kg/m2√h and sd-values under 0.14 m.

12.5.2 Measurement of capillary water absorption (w-value) Capillary water absorption is measured according to EN 1062-3. The substrate used for this test is calcareous sandstone. DIN EN 1062-3 calls for the coated testing-blocks to be exposed to water three times. This allows the water-soluble components of the paint to wash out. A fourth exposure to water is then used for the determination of the w24-value. The w-value is defined as the amount of water absorbed in kg per surface area in m2 and the square root of the test time. Example: Water absorption 0.5 kg/m2 after 24 hours’ water exposure: The accuracy of the determined w-value becomes less with increasing film thickness. The w-value itself

Figure 12.12:  Façade Protection Theory according to Künzel

241

Hydrophobing agents is therefore not dependent on the film thickness, as the ability of the substrate to absorb decreases as long as the barrier-effect of the coating remains intact.

12.5.3 Water vapour diffusion (sd-value) According to Künzel, a quantitive relationship can be given between the water-repellency of a coating and its breathability, i.e. its water vapour diffusion characteristics. The water vapour diffusion rate (V-value) and the diffusion-equivalent air layer thickness (sd-value) for exterior coatings on mineral surfaces and concrete are determined according to DIN EN ISO 7783–2. The sd-value is given in metres [m] and describes an equivalent thickness of an air layer at rest which has the same water vapour diffusion rate as that of the coating. The diffusion capability is classified as shown in Table 12.6. The value depends on the coating thickness and the PVC. The higher the PVC (always greater than the CPVC), the more porous the film and therefore the more breathable the coating. Coatings which have water vapour permeable films similar to those of silicate paints give sd-values 6