Coating

In binding and finishing, a finishing operation in which a printed substrate is covered with a clear film, such as a primer (usually added as a prelude to printing or other coating operations), a lacquer, a barrier coat, or an overprint varnish. Alternately, only a portion of a printed material may be coated, called spot coating or spot varnishing. Coatings applied after printing may either be aqueous (water-based) which dry by evaporation, or electron-beam or ultraviolet coatings which dry by polymerization when exposed to electron beams and ultraviolet light, respectively.

(See Binding and Finishing.)

In papermaking, the term coating refers to a paper finishing operation in which the surface of a paper is covered with a substance to impart a desired finish or texture to the paper and improve its printability. Coatings provide a smooth paper surface, and the amount and composition of a particular coating affects such properties as ink absorbency and ink holdout. Coatings also enhance the whiteness, opacity, and gloss of paper.

Coatings are typically made up of pigments and binders (also called adhesives). Pigments are usually made of refined clay (which enhances gloss and ink holdout), titanium dioxide (which enhances brightness and opacity), or calcium carbonate (which enhances ink absorbency, and brightness). Binders are added to increase the adhesion of the particles of pigment to each other and to the paper fibers. Binders are usually made from common natural sources such as starch, or synthetic sources, such as styrene-butadiene and vinyl acrylic latices. Natural binders are not water resistant, so synthetic binders are often used in addition to or in place of them. Synthetic binders also are more resistant to cracking when the paper is folded than are natural ones.

The coat weight required, or how much coating is added to a base stock of paper, is a function of the final basis weight of the paper and other end-use considerations. Coatings can either be added on the papermaking machine (called on-machine coating) or on a separate machine (called off-machine coating). There are a variety of methods used for applying coatings, such as the use of blade coaters, air knife coaters, and cast coaters. Coating can be applied to one side of a paper—called coated one side—such as is done with paper for labels and book jackets, or coated on both sides—called coated two sides. Coated papers are best for printing halftones, especially in letterpress printing. The removal of bits of a coating during printing and the effect of those coating particles on the printing system (for example, coating pile, or the collection of particles of coating on the blanket) is a consideration when determining a paper's runnability.

In screen printing, the term coating is also used to refer to any screen printed material applied to a substrate as a continuous film.

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