Chapter 10 Making Paint from Alkyd Emulsions A. Hofland
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DSM Resins bv, P.O. Box 615, 8000 AP Zwolle, Netherlands
Every producer and user of paints is familiar with binders that are dissolved in organic solvents. When changing from those 'conventional' systems towards water based alkyd emulsions and/or physically drying acrylic dispersions, certain difficulties concerning paint-formulation, application, open-time, and film formation can arise. The properties of coatings strongly depend on film formation. It turns out that in this respect, paints based on binder dispersions are more critical than solvent-based paints. This is because coalescence of the binder particles is a critical and property-detemining step in the case of dispersions. Film formation, and hence protective properties, are mainly governed by viscosity and miscibility of the resins that constitute the binder particles. By means of a comparison of these two parameters for acrylic and alkyd resins, the differences in film formation between acrylic dispersions and alkyd emulsions are highlighted. It is shown that during film formation an alkyd can spread completely because it inverts from an o/w emulsion to a w/o emulsion. The particles that form a physically drying acrylic dispersion, on the contrary, only coalesce to a certain degree during film formation, even when coalescing agents are used. Complete spreading of the alkyd is very favourable in obtaining high gloss, barrier properties, adhesion/penetration etc. Favourable properties of both acrylic dispersions and alkyd emulsions can be combined to obtain solvent-free paints with good properties.
The System It is hard to imagine that there are more complex systems than solvent based paints, yet there are: water based paints. In water based paints, one not only combines the non-miscible components, such as binder and pigment, but also suspends them in a medium they are not compatible with: water. On all relevant
© 1997 American Chemical Society
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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interfaces, surface active agents should be present. This is already the situation for a simple, model-like paint, without additives like anti-skin, anti-foam and anti-wrinkling agents (the latter two of which are also very surface active), leaving water based paints little less than small miracles. Using the proper combination of binder and pigment as well as the right (and not too many) additives, it is possible. One precondition is that the surfactant will have to stay on the interface it was meant for. This sounds logical, but examples of surfactant migration from water / resin interface to water / pigment interface are numerous. These alterations invariably result in flocculation of at least one of the two dispersed phases. In this context, a possible cosolvent can also be regarded as a surfactant. For example, butyl diglycol has an affinity (hydrophilic/lipophilic balance, HLB) towards water / resin interfaces, almost equal to some widely used nonionic surfactants. Unfortunately, it does not have the accompanying stabilizing properties. Although the effect of such a replacement (surfactant —> solvent) is not great, it will influence the time of flocculation / coagulation of the binder, resulting in poor gloss properties. To ensure that during the paint preparation no problems arise, it is wise to be acquainted with some effects that introduce instability: *The temperature can change the stabilizer's affinity towards the surface (temperature instability). For most emulsions this limit is 60 °C. This problem presents itself predominantly with nonionic surfactant stabilized emulsions. * Addition of metal salts will change the charge stabilization (minus repels minus), and so will lowing the pH below 3. Small amounts of salts like the driers will not present a problem. This is called charge instability and will present itself with ionically stabilized systems. The higher the valency of the cations, the more the electric double layer will be compressed. This will dramatically reduce electrostatic stabilization. Most commercial alkyd emulsions are stabilized in a mixed fashion: both non-ionically and by charge. This means that as long as either the temperature critérium or the salt critérium are met, there is no problem. * Applying high shear to an emulsion may mechanically remove the stabilizer from the surface. This means that grinding pigment in the emulsion should be avoided. Gear pump stability differs per product, and care should always be taken with grinding alkyd emulsions. * Finally, during freezing of an emulsion, the remaining impurities like calcium and magnesium salts in the water are concentrated. Because of the pure nature of the ice crystals, these impurities are driven out. This means that cosolvent destabilization or charge destabilization can occur (freeze-thaw stability), unless this is prevented by cosolvents that cause freezing point depression.
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
10. HOFLAND
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Making Paint from Alkyd Emulsions
The Film The process of film formation of water based paints is governed by three physicochemical principles:
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1. Two liquid phases having a large difference in viscosity are difficult to mix. 2. The range of miscibility of polymers becomes narrower when the molecular weight is increasing (7). 3. Structural similarity favours miscibility (2). Film Formation: Alkyd Emulsion vs. Acrylic Dispersion The difference in behaviour of physically drying acrylic dispersions compared to alkyd emulsions can be explained by the properties of the two systems (Table I) together with the guiding principles discussed above.
Table I. Important Parameters of the Two Binder Systems Acrylic Dispersion M , (g/Mol) binder-viscosity T (°C) g
5
6
10 - 10 'high' -37 =» +104
Alkyd Emulsion 2000 - 8000 'low' .90 => -30 >
As soon as an acrylic dispersion has been applied, water begins to evaporate from the film. At room temperature, the rate determining factor in the evaporation is the humidity of the air above the paint film. As soon as the polymer particles begin to touch each other, partial coalescence takes place. At the same time water is transported from the lower part of the film to the upper part. The driving force for both the sintering and the water flow is capillary pressure caused by the high surface tension of the water together with the curvature of the interfacial boundaries (3,4). This capillary flow proceeds very fast compared to the evaporation of water at the film-air interface. Hence, there is only a small gradient in the concentration of water in the film in the vertical direction. This is the reason, fortunately, that complete coalescence in the upper part of the film does not take place in this first stage of drying. At a certain moment, the polymer particles reach a distance from each other where the Critical Interparticle Distance (CID) is attained in a vertical direction in the film (5). From this moment on, deformation (coalescence) of the polymer particles can take place. The driving force for this process, called wet sintering, is again the capillary pressure. Of course, the deformation is opposed by the viscosity of the polymer. It should be noted that the shape in which the water between the particles is forced is unfavourable from an enthalpy point of view:
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Downloaded by STANFORD UNIV GREEN LIBR on August 18, 2012 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0663.ch010
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TECHNOLOGY F O RWATERBORNE COATINGS
because of its high surface tension, water is trying to adopt a form in which the surface-to-volume ratio is as small as possible. The high viscosity of the polymer at the interface is preventing this. When all the water has evaporated out of the film, the well known polyhedric structure of the polymeric particles remains. From this moment on autohesion has to take place: interfacial diffusion of polymeric chains occurs to obtain a continuous and homogeneous film. The interdiffusion process is only driven by cohesive energy between the diffusing polymeric chains. Capillary forces do not play a role any more. In describing polymeric particles that are dispersed in water, two phases are very often distinguished (
