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Composition of the Anticorrosion Coating 21 hydrolysis resistance than acrylic- or polyester-based polyurethanes [10]. It should be emphasized that these are very broad generalizations; the performance of any specific coating depends on the particular formulation. It is entirely possible, for example, to formulate polyester polyurethanes that have excellent weathering characteristics. The stoichiometric balance of the two reactants affects the final coating performance. Too little isocyanate can result in a soft film, with diminished chemical and weathering resistance. A slight excess of isocyanate is not generally a problem, because extra isocyanate can react with the trace amounts of moisture usually present in other components, such as pigments and solvents, or can react over time with ambient moisture. This reaction of excess isocyanate forms additional urea groups, which tend to improve film hardness. Too much excess isocyanate, however, can make the coating harder than desired, with a decrease in impact resistance. Bassner and Hegedus report that isocyanate/polyol ratios (NCO/OH) of 1.05 to 1.2 are commonly used in coating formulations to ensure that all polyol is reacted [11]. Unreacted polyol can plasticize the film, reducing hardness and chemical resistance. 2.2.3.3 Blocked Polyisocyanates An interesting variation of urethane technology is that of the blocked polyisocyanates. These are used when chemical-cure urethane chemistry is desired but, for technical or economical reasons, a two-pack coating is not an option. Heat is needed for deblocking the isocyanate, so these coatings are suitable for use in workshops and plants, rather than in the field. Creation of the general chemical composition consists of two steps: 1. Heat is used to deblock the isocyanate. 2. The isocyanate crosslinks with the hydrogen-containing coreactant (see Figure 2.7). An example of the application of blocked polyisocyanate technology is polyurethane powder coatings. These coatings typically consist of a solid, blocked isocyanate and a solid polyester resin, melt blended with pigments and additives, extruded and then pulverized. The block polyisocyanate technique can also be used to formulate waterborne polyurethane coatings [8]. Additional information on the chemistry of blocked polyisocyanates is available in reviews by Potter et al. and Wicks [13-15]. 2.2.3.4 Health Issues Overexposure to polyisocyanates can irritate the eyes, nose, throat, skin, and lungs. It can cause lung damage and a reduction in lung function. Skin and respiratory RNHCBL O RNCO + R′OH RNCO + BLH RNHCOR′ O FIGURE 2.7 General reaction for blocked isocyanates. © 2006 by Taylor & Francis Group, LLC ∆
22 Corrosion Control Through Organic Coatings sensitization resulting from overexposure can result in asthmatic symptoms that may be permanent. Workers must be properly protected when mixing and applying polyurethanes as well as when cleaning up after paint application. Inhalation, skin contact, and eye contact must be avoided. The polyurethane coating supplier should be consulted about appropriate personal protective equipment for the formulation. 2.2.4.5 Waterborne Polyurethanes For many years, it was thought that urethane technology could not effectively be used for waterborne systems because isocyanates react with water. In the past twenty years, however, waterborne polyurethane technology has evolved tremendously, and in the past few years, two-component waterborne polyurethane systems have achieved some commercial significance. For information on the chemistry of two-component waterborne polyurethane technology, the reader should see the review of Wicks et al. [16]. A very good review of the effects of two-component waterborne polyurethane formulation on coating properties and application is available from Bassner and Hegedus [11]. 2.2.4 POLYESTERS Polyester and vinyl ester coatings have been used since the 1960s. Their characteristics include: • Good solvent and chemical resistance, especially acid resistance (polyesters often maintain good chemical resistance at elevated temperatures [17]) • Vulnerability to attack of the ester linkage under strongly alkaline conditions Because polyesters can be formulated to tolerate very thick film builds, they are popular for lining applications. As thin coatings, they are commonly used for coilcoated products. 2.2.4.1 Chemistry “Polyester” is a very broad term that encompasses both thermoplastic and thermosetting polymers. In paint formulations, only thermosetting polyesters are used. Polyesters used in coatings are formed through: 1. Condensation of an alcohol and an organic acid, forming an ester — This is the unsaturated polyester prepolymer. It is dissolved in an unsaturated monomer (usually styrene or a similar vinyl-type monomer) to form a resin. 2. Crosslinking of the polyester prepolymer using the unsaturated monomer — A peroxide catalyst is added to the resin so that a free radical addition reaction can occur, transforming the liquid resin into a solid film [17]. A wide variety of polyesters are possible, depending on the reactants chosen. The most commonly used organic acids are isophthalic acid, orthophthalic anhydride, © 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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