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EPA Self-Audit and Inspection Guide Organic Finishing of Metals

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COATING MATERIALS

Coating materials are applied in a thin film to provide protection or decoration to a surface. Most films are thin in comparison to the workpiece. In order to achieve the desired characteristics from the thin film, the coating material fon-nulation must be carefully considered in relation to the part characteristics, surface preparation, application technique and curing method. The correct combination of components and process steps can lead to a film that provides long-lasting beauty and defense against the elements.

Coatings can be formulated from a wide variety of chemicals and materials or a combination of different chemicals. Each component in the formulation serves a specific fiinction. Four common components, shown in Table 2-1, are pigments, additives, binders and the carrier fluid or solvent.

Table 2-1: Common Components of Coating Materials

COMPONENT CHEMISTRY FUNCTION
Pigments Insoluble solids Commonly a colorant, used for aesthetic quality
Binders Polymers, Resins Adhesive between solids and surface, create the coating film
Additives Varies Varies, can include stabilizers, curing agents, flow agents
Carrier Fluid Organic solvent, water Liquid portion, means by which to apply paint

Pigments
Pigments are defmed as any insoluble solid in coating materials. Pigments are typically the colorant portion of a coating material, but can also perform other functions. Some pigments provide corrosion protection, stability in ultraviolet (UV) light, or protection from mold, mildew or bacteria. Others can be used for their conductive ability, texture, or metallic or pearlescent appearance.

Binders
Binders primarily function as an adhesive to the substrate. Binders are polymer resin systems with varying molecular weights. The molecules in the binder crosslink during the curing stage to improve strength and create the thin film. The type of binder usually gives the paint formulation its name. Common binders are ,acrylics, epoxies, polyesters, and urethanes. The viscosity of the paint is often attributed to the binders contained in the coating formulation. Coating viscosity must be considered when choosing certain application techniques.

Additives
Additives are usually low molecular weight chemicals in coating formulations that allow coatings to perform specific functions but do not contribute to color. Non-pigment additives include stabilizers to block attacks of ultraviolet light or heat, curing additives to speed up the crosslinking reaction, co-solvents to increase viscosity, or plasticizers to improve uniform coating.

Carrier Fluid
The carrier fluid is typically a liquid such as an organic solvent or water. The carrier fluid allows the coating material to flow and be applied by methods such as spraying and dipping. This component may be in the coating formulation before application, but evaporates afterwards to allow the solid materials to immobilize and form the thin protective film. Despite its temporary presence in the coating material, the solvent plays a major role in how well the film will perform. Powder coatings have no carrier fluid; they consist only of the other three components.

While the solids portion adheres to the workpiece, the solvent component of coating materials evaporates and causes the most environmental concern. The solvent materials are mostly volatile organic compounds (VOCS) that contribute to the creation of ozone (smog) in the lower atmosphere and are toxic to human health. Some solvents may also be classified as hazardous air pollutants (HAPs). Federal environmental statutes now regulate these VOCs and HAPS. One way organic finishing facilities have responded to these regulations is by creating coatings with lower solvent content.

Coating formulations vary widely, with different types and amounts of pigments, binders, additives, and carrier fluids. The differences in coating formulations provide film characteristics specifically set for the part and its end-use. Often, one type of coating cannot be formulated to provide all of the desired properties. Several layers of different coating material may be applied to a surface to form a coating fihn that will thoroughly protect the part. The first coat is typically called the primer, or undercoat, and the final layers are called topcoats. Regardless of the coating formulation or number of layers applied, proper part preparation, application techniques, and curing processes are necessary for the desired coating characteristics to be achieved.

Four common types of coating materials are solvent-based coatings, high-solids coatings, waterbome coatings, and powder coatings. The names are descriptive of the main type of carrier fluid present in the coating. The chemical nature, coating characteristics and environmental issues of these four coating materials are described in this chapter.


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