Heilen Additives for Water-borne Coatings

vollständig überarbeitete Auflage
ISBN: 978-3-7486-0487-7
Verlag: Vincentz Network
Format: EPUB
Kopierschutz: 0 - No protection

E-Book, Englisch, 272 Seiten, Format (B × H): 155 mm x 225 mm, Gewicht: 666 g

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.

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Autoren/Hrsg.

Heilen, Wernfried

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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: ? s = ? sl + ? l • cos? or ? s - ? sl/? l = cos? where ? s: surface tension of the solid, ? sl: interfacial tension solid/liquid, ? l: surface tension of the liquid, ?: 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: ? l = ? s - ? sl 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 ?/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: dW = ? • dA where W: work to change the interface ?: 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 ?. 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. 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 potential ? describes the electrostatic interaction within the polymeric shell. The smaller the numerical value of ?, 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...