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C O AT I N G S & L I N I N G S FIguRE 1 • Esters: examples include ethyl acetate and n-butyl acetate • Alcohols: examples include isopropanol and n-butanol • Ether alcohols/glycols: examples include ethylene glycol monomethyl ether and propylene glycol monomethyl ether Solvent Categories These unique solvent types are typically placed into one of three solvent classifcations—primary (also known as true or active), latent (sometimes referred to as "tail" solvents), and diluent solvents. The function of a primary solvent is to dissolve the resinous binder and give the solution created the lowest viscosity at the highest solids level. Latent solvents do a poor job of dissolving a resin, but are useful in performing other desirable functions such as controlling the release of solvent out of the flm and improving the flm properties of a coating. Diluent solvents do not have the solvency power to dissolve a resin, but as the name suggests, have a dilution effect on a resin when used in combination with a primary solvent. These types of solvents may be used for many different reasons, such as reducing the volatile organic compound (VOC) level of the coating. Runs and curtains occurring from improper thinning. than not a pure, unique chemical rather than a blend of chemicals. These solvents tend to have very good solvency power for the majority of the common polymers used as resin binders in the protective coatings industry. The level of polar attraction a resin or solvent has to each other within a coating plays a key role in the release of solvents from the flm. Generally speaking, the higher the polarity of a solvent, the greater the tendency there will be for that solvent to associate more readily with a similar polar solvent or binder. A solvent that has a greater attraction to another Action of Solvents Hydrocarbon-based solvents are solvent or to the resin within a coating mostly non-polar solvents with low hy- tends to evaporate at a slower rate. drogen bonding values. These solvents have been used as both diluent solvents Stages of Solvents While the polarity of the solvent and/ as well as primary solvents. Because aliphatic hydrocarbon-based solvents have or resin plays a critical role in the release relatively low solvency power, their use is of the solvents from the flm, there are limited to paints such as long oil alkyds. several other key factors that should be Aromatic hydrocarbon-based solvents considered. These factors can be categohave a higher level of solvency power rized as frst- and second-stage solvent than their aliphatic counterparts and loss. It is important to understand that the have been used in coatings such as alkyds, majority of the solvents are released from a coating flm during the frst stage, with acrylics, and epoxies. Oxygenated solvents have a structure the remaining solvents leaving the flm at that contains oxygen and are more times a slower pace during the second stage. NACE International, Vol. 52, No. 5 First Stage The loss of solvent in the frst stage is a volatility controlled occurrence and is for the most part autonomous of the resin. Throughout the frst stage of solvent loss the release of solvents is primarily controlled by factors such as environmental conditions, vapor pressure at the surface of the coating, thickness of the coating flm, and the amount of air fowing across the coating. Second Stage Depending upon the coating's resin/ binder formulation, the rate of solvent evaporation signifcantly slows down as it nears 40 to 80% loss by weight of the total solvent in the flm. It is at this point that the second stage of solvent loss begins. As the second stage progresses, the release of solvent from the flm becomes increasingly more challenging. At this point in the process, molecules within the resin begin to connect with each other, the viscosity of the applied flm is rising, and the coating is evolving toward its fnal cured state. Solvent loss is now primarily controlled by the rate in which the solvent molecules can diffuse through to the surface of the flm. This is dictated in part May 2013 MATERIALS PERFORMANCE 45