optical properties
One of the main properties of a coating is optical performance. The primary optical properties of a coating are opacity, the ability to hide the substrate; color, the ability to reflect and absorb specific wavelengths of visible light; and gloss, the ability to act like a mirror when reflecting light directly. These three optical properties are key characteristics of topcoats and are carefully controlled by paint manufacturers due to the sensitivity of the human eye to optical properties. In automotive coatings, color and gloss are the main properties desired by consumers, while in housing coatings, opacity and color are the main concerns of users.
Optical properties are a well characterized and well controlled characteristic of a coating. Almost every major store that sells paint directly to consumers has a computer-controlled color measurement or color matching system.
Thermomechanical properties and adhesion
The way a coating responds to mechanical and thermal stress is an important part of the behavior of a coating film. Characteristics that are often considered in this area are hardness, modulus of elasticity, glass transition temperature, toughness and wear resistance. Adhesion to substrates is a property unique to organic coatings that often causes concern. There are many empirical tests for adhesion, such as peel and scribe tests, but the mechanism of adhesion is not fully understood or characterized.
Chemical and Corrosion Resistance
Many substrates, such as metals and composites, have unique mechanical properties, but they have no inherent chemical or corrosion resistance and require a protective coating to maintain their properties in environments containing chemicals or causing corrosion . Bridges, airplanes, pipes, washing machines, cars and many other items rely on coatings to provide protection from the environment. Coatings, for example, are expected to help protect cars from corrosion and corrosive chemicals within a decade; for refrigerators and dishwashers, the life expectancy is even longer. This protection is key to the sale and performance of many items.
Appearance Durability
Exterior durability, that is, the durability against exposure to the exterior of the substrate, is generally considered a special property of coatings. Durability includes many of the aspects of chemical and corrosion protection mentioned above, but is generally considered to primarily include resistance and protection from solar radiation. Many natural polymer substrates, such as wood, and many synthetic polymers are susceptible to damage and degradation from continued exposure to sunlight. Degradation occurs because radiation in the near ultraviolet and blue end of the visible spectrum contains enough energy to break chemical bonds within many types of polymers. The ability of a coating to provide chemical and corrosion protection is also reduced when radiation-induced degradation occurs, and this degradation can include chalking of the coating as well as delamination of the coating. Coatings designed to be used on the exterior surfaces of automobiles, road signs, houses, aircraft and commercial buildings are especially at risk of sunlight-induced degradation. In all these cases, the coating needs to have internal protection against solar radiation. The absorption and scattering of light by pigments, combined with the use of UV absorbers and free radical quenchers (much like the protection that sunscreens are designed to do in lotions), help provide this protection.
