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In order to perform in a practical environment, a coating must convert, after its application, into a dense, solid, and adherent membrane with all the properties discussed previously. The binder is the material which makes this possible. It provides uniformity and coherence to the coating system. Not all binders are corrosion resistant so that only a few serve in the formulation of protective coatings. The binder ability to form a dense, tight film is directly related to its molecular size and complexity. Binders that have the highest molecular weight will form films by the evaporation of the vehicle while binders with smaller molecular weight will generally be reacted in situ. Binders cam be classified according to their essential chemical reactions: (reference)
Pigments are essentially dry powders that are insoluble in the paint medium and that consequently need to be mixed in it by a dispersion technique. They range from naturally occurring minerals to man made organic compounds. Pigments contribute several properties essential to the effective use of protective coatings. Several different pigments may be used within the same coating, all of them contributing to the coating general characteristics to perform important functions such as providing: (reference)
Zinc phosphates are now probably the most important pigments in anti-corrosive paints. The selection of the correct binder for use with these pigments is very important and can dramatically affect their performance. Red lead is likely to accelerate the corrosion of non-ferrous metals, but calcium plumbate is unique in providing adhesion to newly galvanized surfaces in the absence of any pretreatment, and is claimed to behave similarly on other metals.
Most coatings are made with multiple solvents and rarely with a single solvent. The choice of solvents influences viscosity, flow properties, drying speed, spraying or brushing characteristics, and gloss. There is no universal solvent for protective coatings, the best solvent in one system being often impractical for another. Asphalts, for example, can be readily dissolved by hydrocarbons but are insoluble in alcohols. One of the most serious problems associated with coatings is the wrong choice of solvent since it can severely affect the curing and adhesion characteristics of the final coating. One convenient way to describe solvents is to regroup them into the following categories:(reference)
