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Composition of the Anticorrosion Coating 25 2.2.5.5 Darkness Degradation Byrnes notes an interesting phenomenon in some alkyds: if left in the dark for a long time, they become soft and sticky. This reaction is most commonly seen in alkyds with high linseed oil content [18]. The reason why light is necessary for maintaining the cured film is not clear. 2.2.6 CHLORINATED RUBBER Chlorinated rubber is commonly used for its barrier properties. It has very low moisture vapor transmission rates and also performs well under immersion conditions. General characteristics of these coatings are: • Very good water and vapor barrier properties • Good chemical resistance but poor solvent resistance • Poor heat resistance • Comparatively high levels of VOCs [1,19] • Excellent adhesion to steel [19] Chlorinated rubber coatings have been more popular in Europe than in North America. In both markets, however, they are disappearing due to increasing pressure to eliminate VOCs. 2.2.6.1 Chemistry The chemistry of chlorinated rubber resin is simple: polyisoprene rubber is chlorinated to a very high content, approximately 65% [19]. It is then dissolved in solvents, typically a mixture of aromatics and aliphatics, such as xylene or VM&P naphtha [19]. Because of the high molecular weight of the polymers used, large amounts of solvent are needed. Chlorinated rubber coatings have low solids contents, in the 15% to 25% (vol/vol) range. Chlorinated rubber coatings are not crosslinked; the resin undergoes no chemical reaction during cure [1]; they are cured by solvent evaporation; in effect, the film is formed by precipitation. However, the chlorine on the rubber molecule undergoes hydrogen bonding. The tight bonding of these secondary forces gives the coating very low moisture and oxygen transmission properties. Because the film is formed by precipitation, chlorinated rubber coatings are very vulnerable to attack by the solvents used in their formulation and have poor resistance to nearly all other solvents. They are also vulnerable to attach by organic carboxylic acids, such as acetic and formic acids [19]. 2.2.6.2 Dehydrochlorination Chlorinated rubber resins tend to undergo dehydrochlorination; that is, a hydrogen atom on one segment of the polymer molecule joins with a chlorine atom on an adjacent segment to form hydrogen chloride. When they split off from the polymer molecule, a double bond forms in their place. In the presence of heat and light, this © 2006 by Taylor & Francis Group, LLC
26 Corrosion Control Through Organic Coatings double bond can crosslink, leading to film embrittlement. The hydrogen chloride also is a problem; in the presence of moisture, it is a source of chloride ions, which of course can initiate corrosion. The hydrogen chloride can also catalyze further breakdown of the resin [19]. Dehydrochlorination is increased by exposure to heat and light. Therefore, chlorinated rubber coatings are not suitable for use in high-temperature applications. Sensitivity to light, however, can be nullified by pigmentation. 2.2.7 OTHER BINDERS Other types of binders include epoxy esters and silicon-based inorganic zinc-rich coatings. 2.2.7.1 Epoxy esters Despite their name, epoxy esters are not really epoxies. Appleman, in fact, writes that epoxy esters “are best described as an epoxy-modified alkyd [20].” They are made by mixing an epoxy resin with either an oil (drying or vegetable) or a drying oil acid. The epoxy resin does not crosslink in the manner of conventional epoxies. Instead, the resin and oil or drying oil acid are subjected to high temperature, 240°C to 260°C and an inert atmosphere to induce an esterification reaction. The result is a binder that cures by oxidation and can therefore be formulated into one-component paints. Epoxy esters generally possess adhesion, chemical and UV resistance, and corrosion protection properties that are somewhere between those of alkyds and epoxies [21]. They also exhibit resistance to splashing of gasoline and other petroleum fuels and are therefore commonly used to paint machinery [18]. 2.2.7.2 Silicon-Based Inorganic Zinc-Rich Coatings Silicon-based inorganic zinc-rich coatings are almost entirely zinc pigment; zinc levels of 90% or higher are common. They contain only enough binder to keep the zinc particles in electrical contact with the substrate and each other. The binder in inorganic ZRPs is an inorganic silicate, which may be either a solvent-based, partly hydrolyzed alkyl silicate (typically ethyl silicate) or a water-based, highly-alkali silicate. General characteristics of these coatings are: • Ability to tolerate higher temperatures than organic coatings (inorganic ZRPs typically tolerate 700° to 750°F) • Excellent corrosion protection • Require topcoatings in high pH or low pH conditions • Require a very thorough abrasive cleaning of the steel substrate, typically near-white metal (SSPC grade SP10) For a more-detailed discussion of inorganic ZRPs, see Section 2.3.5, “Zinc Dust.” © 2006 by Taylor & Francis Group, LLC
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Page 7 and 8: Preface This book has been written
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