Alvars are made by replacing part or all of the acetate groups in Gelva with acetaldehyde. Their viscosity varies with the degree of polymerization and their properties vary according to the extent of replacement of the acetate groups. The Alvar types do not cold flow when molded, are tougher, harder, and have better adhesion but are less resistant to weathering than the Gelva types. Other properties are about the same as those of the Gelvas. Alvars having 70 to 80 percent acetate group replacement are used chiefly in spirit type varnishes, lacquers, and enamels that must stand exposure to weather. Another Alvar type is used in injection and press molding. The high binding power of the resin permits the use of large percentages of filler without loss of desirable properties. Such moldings may be machined and polished, and take inserts, such as the wood core in shoe heels. Flexible phonograph and transcription records made from the Alvars have gained wide approval. An 85 percent (acetate replacement) type has better impact strength and is used in toilet articles. Sheets, rods, and tubes of this resin may be machined in much the same way as nitrocellulose plastic and used where noninflammability is an asset.
Formvars are made by replacing part or all of the acetate groups in Gelva with formaldehyde. These resins are colorless, odorless, tasteless, and thermoplastic. They have higher softening points and greater tensile and impact strength than the Alvars. They are resistant to alcohols, coal-tar solvents, fats, oils, or water. Moisture transmission rate through a film of this resin is about one-tenth that through regenerated cellulose and one-fourth that through cellulose acetate.
The grades of the Formvars available are designated by the extent of replacement of the acetate group. The 75-percent replacement type has excellent mechanical strength and flexibility and is unaffected by sunlight. Formvars of 95 percent acetate displacement have a tensile strength as high as 10,000 pounds per square inch and offer possibilities in the manufacture of artificial silk and photographic film.
The vinyl resins have made possible a new type of safety glass superior to any heretofore marketed. By condensing butylaldehyde with vinyl acetate, a polymer is obtained which is used as the inner layer between two sheets of glass. Heat and pressure secure complete adhesion and yield a sheet with greater resistance to breakage at low temperatures than the types now in general use.
Although safety glass was invented in 1905, and many substitutes for the original nitrocellulose inner layer have been proposed, only two reached commercial importance before the development of the vinyl resins. These are cellulose acetate and the acrylate resins. Safety glass used in automobile windshields up to about 1930 discolored after a year or two of service. This discoloration was due to the action of the actinic rays of the sun on the nitrocellulose layer. Since 1930 this difficulty has been largely overcome by using an actinic ray filter glass (a special glass with a high iron content) in front of the nitrocellulose sheet, or by using cellulose acetate, which is not discolored to the same extent by light, as a substitute for nitrocellulose. Both cellulose nitrate and cellulose acetate, however, have a tendency to lose toughness and strength at low temperatures, to absorb moisture, and to separate from the glass around the edge unless sealed, and to lose their plasticizer and shrink.
Although a vast improvement over ordinary plate glass, laminated glass made with cellulose nitrate or acetate has the serious defect of being brittle at low temperatures, such as prevail in the winters of northern States. It is easily shattered at zero Fahrenheit, while at 60° F. and above it is quite strong. This shortcoming led to the development of the vinyl resin sheet for safety glass with a remarkable degree of toughness. At normal temperatures it has rubberlike toughness which, although decreased at low temperatures, is not punctured by the impact of a half-pound steel ball falling from a 30-foot height at minus 10° F., whereas nitrocellulose or acetate laminated glass withstands the impact of a fall from not greater than one-tenth this height. A further advantage of the vinyl sheet is that it is water resistant, making the sealing of the edges of the glass unnecessary and thus reducing costs. Exposure to ultraviolet light in Florida sunlight for more than 2 years did not discolor it.
The many desirable properties of the vinyl resins, as outlined above, indicate their widespread use in laminated safety glass when it is available in sufficient quantities. It is estimated that our annual output of safety glass interlayer sheets exceeds 17,000,000 pounds, of which 25 to 30 percent are for windshields, and 70 to 75 percent for side and back windows of automobiles.
At least one of the series of Mowiliths made in Germany is polymerized vinyl acetate. It is recommended as an ingredient of water-white lacquers. It is compatible with nitrocellulose and is extremely durable and not disintegrated or discolored on exposure to weather.
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: