Additives for Water-borne Coatings E-Book

Wernfried Heilen

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 398

Veröffentlichungsjahr: 2021

Beschreibung

This book offers an overview of the most important aspects and applications of additives for waterborne systems in diverse market segments. Wernfried Heilen helps to understand how additives work and elucidates all kinds of mechanisms in great detail. Furthermore he dispels a lot of myths surrounding paint additives with an excellent combination of theory and practice. This enables a deep insight into all the different application areas for additives in waterborne paint systems.

Leseprobe

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

Cover: vartzbed, Adobe Stock

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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 9783748604877

© 2021 Vincentz Network GmbH & Co. KG, Hanover

Vincentz Network GmbH & Co. KG, Plathnerstr. 4c, 30175 Hanover, Germany

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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] Farbe & Lacke 12/2019, Olga Menukhin – Wässrige Lacksysteme//Marktübersicht

Foreword

1 Introduction

1.1 Literature

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

3 Defoaming of coating systems

3.1 Defoaming mechanisms

3.1.1 Foam

3.2 Defoamers

3.2.1 Composition of defoamers

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

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

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

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

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

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

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

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

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

10 Light stabilisers

10.1 Introduction

10.2 Light and photo-oxidative degradation

10.3 Stabilisation options for polymers

10.3.1 UV absorbers

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

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

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

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 (s-value)

12.5.4 Simulated dirt pick-up

12.5.5 Pigment-volume concentration (PVC):

12.6 Literature

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

Authors

Stichwortverzeichnis

1Introduction

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, 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.

1.1Literature

[1]

Kittel, “Lehrbuch der Lacke und Beschichtungen”, Volume 3, Hirzel Verlag, Stuttgart-Leipzig 2001

Wernfried Heilen et al.: Additives for Water-borne Coatings

© Copyright 2021 by Vincentz Network, Hanover, Germany

2Wetting 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.

2.1Modes 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

2.1.1Pigment wetting

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

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.

Figure 2.1: Equilibrium of forces according to Young

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

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].

2.1.2Grinding

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

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].

2.1.3Stabilisation

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

2.1.3.1Electrostatic 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.

Figure 2.2a: Electrostatic stabilisation

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 potentialζ 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 be taken into consideration.

2.1.3.2Steric 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

where

ΔH:

change of enthalpy

ΔS:

change of entropy

T:

absolute temperature

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].

Figure 2.2b: Steric stabilisation

2.1.3.3Electrosteric stabilisation

The complex demands made on wetting and dispersing additives in water-borne 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).

Figure 2.2c: Electrosteric stabilisation

2.1.4Influences 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.

2.1.4.1Viscosity

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.

Figure 2.3: Viscosity behaviour and additive dosage in direct grind

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 also interact with the polymer shells of other pigment particles. This bridging leads to reduced mobility of the pigment particles and, in consequence, to higher viscosity.

Figure 2.4: Viscosity behaviour and additive dosage in pigment concentrates

2.1.4.2Colour 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 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.

Figure 2.5: Colour strength and additive dosage (schematic)

2.1.4.3Compatibility

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.

2.1.4.4Stability

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.

2.2Chemical 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-der-Waals 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.

2.2.1Polyacrylate 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.

Figure 2.6a: Structural example of a polyacrylate salt

2.2.2Polyphosphates

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.

2.2.3Fatty 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.

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

2.2.4Acrylic copolymers

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.

Figure 2.6c: Structural example of a methacrylic-polyether copolymer

2.2.5Maleic 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.

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

2.2.6Alkyl 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.

Figure 2.6e: Structural example of a non-ionic APE

2.2.7Alkyl 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 2.6f: Guerbet derivative (R= fatty acid or phosphate)

Figure 2.6g: Modified polyether

2.3Wetting 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.

2.3.1Architectural coatings

2.3.1.1Direct 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.

2.3.1.2Pigment 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.

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.

2.3.2Wood and furniture coatings

2.3.2.1Direct 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.

2.3.2.2Pigment 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.

2.3.3Industrial coatings

2.3.3.1Direct 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.

2.3.3.2Pigment 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.

2.3.4Printing inks

2.3.4.1Direct 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.

2.3.4.2Pigment 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.

2.4Tips 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.

2.5Test methods

2.5.1Particle 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.

2.5.2Colour 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 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

2.5.3Rub-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.

2.5.4Viscosity

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 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.

2.5.5Zeta 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 2.7: CVI principle by using high frequency sound wave

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

Figure 2.8 shows a diagram of a measuring sensor. After applying a radio-frequency pulse, a cylindrical piezo element generates an acoustic pulse which 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

Ks: conductivity of dispersion

εm: dielectric constant of medium

Km: conductivity of medium

ζ : zeta potential

ρp: density of particle

φ: parts by weight

ρs: density of system

η: dynamic viscosity

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.

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