Pigments help protect the resin in the film from

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Pigments help protect the resin in the film from degradation of solar radiation. Hiding pigments do the best job of protecting the resin from the harmful portion of solar radiation by blocking its penetration into a film. Pigments in the film also inhibit penetration of corrosive elements, thus protecting the substrate. Pigments also can add mechanical reinforcement to a film, adding strength, flexibility, and abrasion resistance. Non-Volatile Vehicles (Binders) The binder or resin portion (polyurethane, epoxy, etc.) of the coating is the "glue" that holds the coating togeth- er and onto the substrate. The physical properties of the coating are mainly derived from the physical properties of the solid resin, but pigments and additives can affect the final properties. Coatings are generally named after the type of resin used as the coating binder. Resin binders change from the liquid to the solid state by several different dying curing mechanisms: ! Lacquer, dispersion and latex paints dry through the evaporation of solvent and/or water. ! Vegetable oil and alkyd paints harden through oxidative cure. ! Two-component chemically reactive paints harden through chemical cure, i.e., two components are mixed prior to application and polymerize on the substrate, e.g., epoxy or polyurethane. ! One-component chemically reactive paints harden through the reaction of a resin that has an active chemical group, with atmospheric moisture releasing a new chemical group that causes the resin to crosslink. The simplest drying mechanism is evaporation of the volatile vehicle. Solventborne lacquers generally have very high solvent content because very hard resins needed for good film protection require a lot of solvent to reduce the paint viscosity to application consistency. Vinyl and chlorinated rubber coatings are examples of resins rely- ing on solvent evaporation. Another type of paint that dries through simple evaporation of the volatile vehicle is waterborne paint. Here a major portion of the volatile vehicle is water which acts to lower the viscosity of the paint. Acrylic and vinyl latex- es, water-based epoxies and polyurethane dispersions are examples of this technology. Coatings based on natural oils or alkyd binders modified with drying oils develop their film properties principal- ly through oxidative curing. Atmospheric oxygen creates active crosslinking sites on vegetable oil or the drying oil portion of the synthetic resin. These sites connect to form a three dimensional, chemically bonded network. Linseed, alkyd and epoxy ester binders are examples of systems that cure by a combination of solvent evapora- tion and oxidation. Two-component chemically reactive paint is manufactured and sold in two separate containers. The two multi- functional reactive resinous materials are mixed together just prior to use. The two resins immediately begin to react together to form a polymeric matrix. During polymerization, the paint viscosity will increase. This means that the paint has a specific use life before the paint will gel. Polyurethane and epoxy are examples of these coatings.
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