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Composition of the Anticorrosion Coating 49 2.4.1.1 Thixotropic Agents Thixotropic agents are used to control the rheology of a coating — that is, how thick the coating is under various conditions, how much it spreads, and how quickly it does so. This group includes several very different types of additives: thickeners, antisagging compounds, antisettling and suspension agents, antigelling agents, leveling and coalescing aids, wet-edge extenders, anticratering agents, and plasticizers [128]. The rheology of a coating might need to be modified for a number of reasons. One is to prevent pigment sedimentation; the pigment must be able to remain in suspension after mixing, rather than settling in a solid mass at the bottom of the container before it can be applied. Rheology is also modified so that the coating can be applied in a particular method. Brush, roller, spray, curtain, and knife coating techniques all produce different amounts of shear in the paint at the moment it contacts the substrate. For a thixotropic coating, in which viscosity is inversely related to shear, this means that the coating will have very different viscosities at the moment of application and hence different wetting and spreading behaviors. Rheology modifiers are used to control the shear viscosity for the various application methods, so that the coating wets and spreads on the metal surface [3]. Examples of thixotropic agents include fumed silicas and treated clays. These inert pigments are sometimes added to aid in film build, to add body to a paint, or for antisettling characteristics [2]. 2.4.1.2 Surfactants Surfactants are used when the surface energy of a coating as a whole, or one or more of its components, must be controlled. This group of additives includes wetting agents, pigment dispersers, defoamers, and antifoaming agents. Wetting agents help lower the surface tension of the coating, so that it spreads out and wets the surface, forming a continuous film across it. Foaming problems can occur both in the manufacture of the coating and in its application. Defoamers are used to prevent such problems, especially in waterborne formulations [3]. “Microfoaming” is the term for the tiny bubbles that occasionally form on the surface of a wet film, affecting the film appearance. They are more commonly seen in waterborne coatings than in solvent-borne ones and can be prevented with antifoaming agents. 2.4.1.3 Dispersing Agents Pigments are generally manufactured to a specific particle size, or range of sizes, for optimal strength and opacity (if the pigment is a filler), color strength (if the pigment is a colorant), solubility rate (anticorrosion pigments), and other desired properties. However, during transportation and storage, pigment particles tend to agglomerate. In the process of making paint, these agglomerations must be broken up and the pigment or coating must be treated with an additive to ensure that the pigment particles stay dispersed. This additive is known as the dispersing agent. In solvent-borne paints, the dispersing agent is commonly a steric barrier, whereas in waterborne coatings, electrostatic repulsion is used [29]. © 2006 by Taylor & Francis Group, LLC
50 Corrosion Control Through Organic Coatings 2.4.2 REACTIVE REAGENTS Reactive reagents generally aid in film formation, forming bonds to the substrate, crosslinking, and curing. Examples of this class of additives include metallic driers, such as zinc or tin salts, to aid in crosslinking [10,18]; curing catalysts and accelerators; photoinitiators; and adhesion promoters. 2.4.3 CONTRA-ENVIRONMENTAL CHEMICALS As their name implies, contra-environmental chemicals are a group of additives that are intended to provide the coating with protection against its service environment. Examples of this type of additive include [128]: • Performance enhancers (antiskinning agents, antioxidants, light stabilizers, nonpigmental corrosion inhibitors) • Thermal controllers (freeze-thaw controllers, heat stabilizers) • Biological controllers (biocides, antifouling agents) Antioxidants and light stabilizers are used to provide topcoats with thermooxidative and UV stabilization, thus increasing service life in outdoor applications. For thermo-oxidative stabilization, phenolic antioxidants and aromatic amines are generally used [129]. Hindered amine light stabilizers (HALS; for example, Hostavin N30 TM , Goodrite 3150 TM , Chimassorb 944 TM ) or UV absorbers (for example, Cyasorb UV-531 TM ) [130] are added to the coating mostly for UV protection and, to some extent, for thermo-oxidative stabilization. A mixture of antioxidants and light stabilizers is frequently used; this must be carefully formulated because both positive and negative effects have been reported from combining these additives [131,132]. Barret and colleagues suggest that the phenol in the antioxidant prevents the conversion of HALS to a stabilizing nitroxide [133]. Another mechanism may be that the radicals of different stabilizers interact. The term corrosion inhibitors is not meant to include anticorrosion pigments in this section. These additives are completely soluble in order to provide the maximum possible corrosion protection immediately upon application of the paint. Pigments have a much more controlled solubility rate in order to have an effect over a long period. Corrosion inhibitors are commonly used for preventing spot or “flash” rusting. Sodium nitrate, for example, is sometimes added to waterborne coatings to prevent flash rusting [3]. These corrosive-inhibiting additives are used in addition to, rather than as a substitute for, anticorrosion pigments. Corrosion inhibitors and anticorrosion pigments must be chosen with care if used together, so as not to adversely affect the in-can stability of the formulation [3]. Biocides prevent microbial growth in coatings, both in-can and in the cured paint. They are more important in waterborne coatings than in solvent-borne coatings. Antifouling agents prevent the growth of mussels, sea urchins, and other marine life on marine coatings. They are used exclusively in topcoats, rather than in the primers that provide the corrosion protection to the metal substrate. © 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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