© 2006 by Taylor & Francis Group, LLC

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3 Waterborne Coatings Most of the important types of modern solvent-borne coatings — epoxies, alkyds, acrylics — are also available in waterborne formulations. In recent years, even urethane polymer technology has been adapted for use in waterborne coatings [1]. However, waterborne paints are not simply solvent-borne paints in which the organic solvent has been replaced with water; the paint chemist must design an entirely new system from the ground up. In this chapter, we discuss how waterborne paints differ from their solvent-borne counterparts. Waterborne paints are by nature more complex and more difficult to formulate than solvent-borne coatings. The extremely small group of polymers that are soluble in water does not, with a few exceptions, include any that can be usefully used in paint. In broad terms, a one-component, solvent-borne coating consists of a polymer dissolved in a suitable solvent. Film formation consists of merely applying the film and waiting for the solvent to evaporate. In a waterborne latex coating, the polymer particles are not at all dissolved; instead they exist as solid polymer particles dispersed in the water. Film formation is more complex when wetting, thermodynamics, and surface energy theory come into play. Among other challenges, the waterborne paint chemist must: • Design a polymer reaction to take place in water so that monomer building blocks polymerize into solid polymer particles • Find additives that can keep the solid polymer particles in a stable, even dispersion, rather than in clumps at the bottom of the paint can • Find more additives that can somewhat soften the outer part of the solid particles, so that they flatten easier during film formation. And all of this was just for the binder. Additional specialized additives are needed, for example, to keep the pigment from clumping; these are usually different for dispersion in a polar liquid, such as water, than in a nonpolar organic solvent. The same can be said for the chemicals added to make the pigments integrate well with the binder, so that gaps do not occur between binder and pigment particles. And, of course, more additives unique to waterborne formulations may be used to prevent flash rusting of the steel before the water has evaporated. (It should perhaps be noted that the need for flash rusting additives is somewhat questionable.) © 2006 by Taylor & Francis Group, LLC 55
56 Corrosion Control Through Organic Coatings 3.1 TECHNOLOGIES FOR POLYMERS IN WATER Most polymer chains are not polar; water, being highly polar, cannot dissolve them. Chemistry, however, has provided ways to get around this problem. Paint technology has taken several approaches to suspending or dissolving polymers in water. All of them require some modification of the polymer to make it stable in a water dispersion or solution. The concentration of the polar functional groups plays a role in deciding the form of the waterborne paint: a high concentration confers water-solubility, whereas a low concentration leads to dispersion [2]. Much research has been ongoing to see where and how polar groups can be introduced to disrupt the parent polymer as little as possible. 3.1.1 WATER-REDUCIBLE COATINGS AND WATER-SOLUBLE POLYMERS In both water-reducible coatings and water-soluble polymers, the polymer chain, which is naturally hydrophobic, is altered; hydrophilic segments such as carboxylic acid groups, sulphonic acid groups, and tertiary amines are grafted onto the chain to confer a degree of water solubility. In water-reducible coatings, the polymer starts out as a solution in an organic solvent that is miscible with water. Water is then added. The hydrophobic polymer separates into colloid particles, and the hydrophilic segments stabilize the colloids [3]. Water-reducible coatings, by their nature, always contain a certain fraction of organic solvent. Water-soluble polymers do not begin in organic solvent. These polymers are designed to be dissolved directly in water. An advantage to this approach is that drying becomes a much simpler process because the coating is neither dispersion nor emulsion. In addition, temperature is not as important for the formation of a film with good integrity. The polymers that lend themselves to this technique, however, are of lower molecular weight (10 3 to 10 4 ) than the polymers used in dispersions (10 5 to 10 6 ) [4]. 3.1.2 AQUEOUS EMULSION COATINGS An emulsion is a dispersion of one liquid in another; the best-known example is milk, in which fat droplets are emulsified in water. In an emulsion coating, a liquid polymer is dispersed in water. Many alkyd and epoxy paints are examples of this type of coating. 3.1.3 AQUEOUS DISPERSION COATINGS In a aqueous dispersion coatings, the polymer is not water–soluble at all. Rather, it exists as a dispersion or latex of very fine (50 to 500 nm diameter) solid particles in water. It should be noted that merely creating solid polymer particles in organic solvent, removing the solvent, and then adding the particles to water does not produce aqueous dispersion coatings. For these coatings, the polymers must be produced in water from the start. Most forms of latex begin as emulsions of the polymer building © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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© 2006 by Taylor & Francis Group, LLC

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