_Copolymers of vinyl acetate and vinyl chloride._—The simultaneous polymerization of mixtures of vinyl acetate and vinyl chloride yields resins with the desirable properties of the two reactants. The extent of plasticity is largely controlled by varying the ratio of the vinyl derivatives. Resins high in vinyl chloride content are better suited to molding, and those high in vinyl acetate are better lacquer ingredients. These resins are marketed as Vinylites by the Carbide and Carbon Chemicals Co., New York. They are thermoplastic, odorless, tasteless, and practically nonflammable. Their outstanding properties are resistance to water, soap, acids, alkalies, and alcohol, and their strength and good dielectric properties. Their stability to light is improved by the addition of ultraviolet absorbing compounds and their stability to heat by the addition of lead oleate, calcium stearate, or other bases. Water absorption and compatibility with other resins is increased as the chloride content increases. The principal types of copolymers are: Vinylite VYN, high molecular weight. This resin is used in dentures where good fatigue resistance, impact strength, and tensile strength are required. It contains 85 to 88 percent vinyl chloride. Vinylite VYN, medium molecular weight. This resin is used in general molding and extending applications including sheets, rods, and tubes. Its vinyl chloride content averages 85 to 88 percent. Vinylite VYN, low molecular weight. This resin is used in moldings, coated paper, lacquers, floor tile, phonograph records, and felt impregnation. It contains 85 to 88 percent vinyl chloride. Vinylite VYC. This resin of low molecular weight is compatible with nitrocellulose and is used in lacquers and finishes for industrial applications. Lacquers from the Vinylites are called Vinyloids. The Vinylites for molding are thermoplastic and shrink very little, making them applicable to large moldings. They may be used in extension processes such as tooth-brush preforms, pipe lining, and wall trim. Fillers and pigments may be added, although pigments containing iron and zinc have harmful effects on the stability of the resin. The fillers used are wood flour, mica, talc, and alpha cellulose. Fillers reduce the mechanical strength of the resin and lessen its resistance to water. Plasticizers, such as dibutyl phthalate or tricresyl phosphate, give a softer, more flexible resin. Resins from the copolymers resemble the cellulose derivatives in their molding characteristics, mechanical strength, and appearance.