Traditional Types of Surface Finishes

Nearly all surface finishes and coatings, with the exception of ceramic types for high-temperature applications, are based on a polymer film of some sort They account for the use of a lot of polymer, but determining just how much and which polymers is not easy, because most formulations are proprietary, and production figures do not always separate polymer and nonpolymer components. The next section describes the five traditional types of surface finishes and the role played by polymers in each. Subsequent sections discuss more recent developments in the field. 

Lacquers. A lacquer consists of a polymer solution to which a pigment has been added. The film is formed simply by evaporation of the solvent, leaving the pigment trapped in the polymer film. Since no chemical change occurs in the polymer, it retains its original solubility characteristics. Hence, a major drawback of lacquers is their poor solvent resistance. 

A wide variety of polymers is used for lacquers. Newer systems favor acrylic polymers for their superior chemical stability. Acrylic lacquers may also incorporate a curing agent such as hexamethoxymelamine, which crosslinks the film 

The term spirit varnish is an old and imprecise name for a lacquer in which the solvent is alcohol. Shellac is an example. Since the polymers are soluble in a highly polar solvent, spirit varnishes have rather poor water resistance. 

Oil Varnish or Varnish. These coatings consist of a polymer, either natural or synthetic, dissolved in a drying oil, with perhaps an inert solvent to control viscosity, and a catalyst to promote the crosslinking reaction with oxygen. When cured, they produce a clear, tough, solvent-resistant film. 

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