Types & Properties of Coatings
Coatings are used in one form or other since the beginning of civilization. From the first cave drawings to the Greek statues and Chinese artifacts, coatings were used for decorative and protective purposes. Today most engineered products are coated to be protected from usage and working environments.
There two classes of coatings: Organic and Inorganic. However, both may integrate in a multifunctional coatings system. Organic coatings contain carbon and used to be made from vegetable or animal oils. Nowadays most of them come from refined and modified petroleum products. Organic coatings include paints, resins, lacquers and varnishes. Inorganic coatings include porcelain enamels, glass linings and metallic coatings. Metallic coatings are produced using a variety of techniques such as hot dipping, electroplating, cladding, thermal spray and other methods.
Protective coatings are commonly referred to by their generic resin type, such as acrylic, alkyd, epoxy or polyurethane. In addition, they may be referred to by the type of resin and the curing agent used, such as epoxy amine, where an amine is used as the cure. Still other products contain more than one resin, such as an epoxy acrylic or a silicone alkyd. The same generic coating type from two different manufacturers may not necessary share the same properties. Each manufacturer has its own blend of resin, pigment, additives and solvent, resulting in major differences in the properties and serviceability of each one’s particular coating.
A coating must exhibit a variety of properties to fulfill its role as a protective coating. Desirable properties include:
- Chemical Resistance: The coating must resist breakdown from the chemicals to which it is exposed. Chemical resistance is primarily a function of the resin used.
- Water Resistance: Water affects virtually all coatings. Greater water resistance equates to more effective corrosion control.
- Ease of Application: Ease of application is a vital characteristic, especially with intricate structural details. The more difficult the application, the more opportunity for defects to be created, leading to premature failure.
- Adhesion to Substrate: Adhesion is based on physical and chemical interactions between the coating and the substrate. Poor adhesion equates to poor performance.
- Cohesive Strength: Coatings must be able to withstand the stresses of the curing process and changes in temperature and moisture content.
- Flexibility and Elongation: The ability to expand and contract with the substrate is critical in some coating applications.
- Impact Resistance: The coating may have to resist impact loads.
- Abrasion Resistance: Coatings in some areas may have to be abrasion-resistant.
- Temperature Resistance: The environment may expose the coating to extremes of temperature, usually elevated.
- Dielectric Strength: A key variable in the barrier coatings and when using coatings in conjunction with cathodic protection.
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