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Composition of the Anticorrosion Coating 23 terephthalic acid, fumaric acid, and maleic acid. Alcohol reactants used in condensation include bisphenol A, neopentyl glycol, and propylene glycol [17]. The combinations of alcohol and organic acids used determine the mechanical and chemical properties, thermal stability, and other characteristics of polyesters. 2.2.4.2 Saponification In an alkali environment, the ester links in a polyester can undergo hydrolysis — that is, the bond breaks and reforms into alcohol and acid. This reaction is not favored in acidic or neutral environments but is favored in alkali environments because the alkali forms a salt with the acid component of the ester. These fatty acid salts are called soaps, and hence this form of polymer degradation is known as saponification. The extent to which a particular polyester is vulnerable to alkali attack depends on the combination of reactants used to form the polyester prepolymer and the unsaturated monomer with which it is crosslinked. 2.2.4.3 Fillers Fillers are very important in polyester coatings because these resins are unusually prone to build up of internal stresses. The stresses in cured paint films arise for two reasons: shrinkage during cure and a high coefficient of thermal expansion. During cure, polyester resins typically shrink a relatively high amount, 8 to 10 volume percent [17]. Once the curing film has formed multiple bonds to the substrate, however, shrinkage can freely occur only in the direction perpendicular to the substrate. Shrinkage is hindered in the other two directions (parallel to the surface of the substrate), thus creating internal stress in the curing film. Fillers and reinforcements are used to help avoid brittleness in the cured polyester film. Stresses also arise in polyesters due to their high coefficients of thermal expansion. Values for polyesters are in the range of 36 to 72 × 10 –6 mm/mm/°C, whereas those for steel are typically only 11 × 10 –6 mm/mm/°C [17]. Fillers and reinforcements are important for minimizing the stresses caused by temperature changes. 2.2.5 ALKYDS In commercial use since 1927 [18], alkyd resins are among the most widely used anticorrosion coatings. They are one-component air-curing paints and, therefore, are fairly easy to use. Alkyds are relatively inexpensive. Alkyds can be formulated into both solvent-borne and waterborne coatings. Alkyd paints are not without disadvantages: • After cure, they continue to react with oxygen in the atmosphere, creating additional crosslinking and then brittleness as the coating ages [18]. • Alkyds cannot tolerate alkali conditions; therefore, they are unsuitable for zinc surfaces or any surfaces where an alkali condition can be expected to occur, such as concrete. © 2006 by Taylor & Francis Group, LLC
24 Corrosion Control Through Organic Coatings • They are somewhat susceptible to UV radiation, depending on the specific resin composition [18]. • They are not suitable for immersion service because they lose adhesion to the substrate during immersion in water [18]. In addition, it should be noted that alkyd resins generally exhibit poor barrier properties against moisture vapor. Choosing an effective anticorrosion pigment is therefore important for this class of coating [1]. 2.2.5.1 Chemistry Alkyds are a form of polyester. The main acid ingredient in an alkyd is phthalic acid or its anhydride, and the main alcohol is usually glycerol [18]. Through a condensation reaction, the organic acid and the alcohol form an ester. When the reactants contain multiple alcohol and acid groups, a crosslinked polymer results from the condensation reactions [18]. 2.2.5.2 Saponification In an alkali environment, the ester links in an alkyd break down and reform into alcohol and acid, (see 2.2.4.2). The known propensity of alkyd coatings to saponify makes them unsuitable for use in alkaline environments or over alkaline surfaces. Concrete, for example, is initially highly alkaline, whereas certain metals, such as zinc, become alkaline over time due to their corrosion products. This property of alkyds should also be taken into account when choosing pigments for the coating. Alkaline pigments such as red lead or zinc oxide can usefully react with unreacted acid groups in the alkyd, strengthening the film; however, this can also create shelf-life problems, if the coating gels before it can be applied. 2.2.5.3 Immersion Behavior In making an alkyd resin, an excess of the alcohol reagent is commonly used, for reasons of viscosity control. Because alcohols are water-soluble, this excess alcohol means that the coating contains water-soluble material and therefore tends to absorb water and swell [18]. Therefore, alkyd coatings tend to lose chemical adhesion to the substrates when immersed in water. This process is usually reversible. As Byrnes describes it, “They behave as if they were attached to the substrate by water-soluble glue [18]”. Alkyd coatings are therefore not suitable for immersion service. 2.2.5.4 Brittleness Alkyds cure through a reaction of the unsaturated fatty acid component with oxygen in the atmosphere. Once the coating has dried, the reaction does not stop but continues to crosslink. Eventually, this leads to undesirable brittleness as the coating ages, leaving the coating more vulnerable to, for example, freeze-thaw stresses. © 2006 by Taylor & Francis Group, LLC
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Page 3 and 4: Corrosion of Ceramic and Composite
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Page 7 and 8: Preface This book has been written
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