Polyvinyl Acetate, Alcohol, and Derivatives, Polystyrene, and Acrylics

Polyvinyl acetate, the most widely used vinyl ester, is noted for its adhesion to substrates and high cold flow. Polyvinyl acetate serves as the precursor for polyvinyl alcohol and, directly or indirectly, the polyvinyl acetals. Both polyvinyl acetate and polyvinyl alcohol are insoluble in many organic solvents but water sensitive. Polyvinyl acetate absorbs from 1 to 3% water, up to 8% on prolonged immersion. Polyvinyl alcohol absorbs 6–9% water when humidity conditioned and can usually be dissolved completely in water above 90°C, but it can also be insolubilized by chemical treatment. U.S. manufacturers currently sell polyvinyl acetate in emulsion form and polyvinyl alcohol as granules. Polyvinyl alcohol is processed into films and formulated with other materials into emulsion intermediates. Both polymers are typically used in aqueous systems. Both polyvinyl acetate and polyvinyl alcohol meeting certain specifications are permitted in stated food contact applications such as packaging, coatings, and adhesives. Ethylene–vinyl acetate copolymers and ethylene–vinyl acetate–vinyl alcohol terpolymers are similarly permitted in certain food contact applications. Polyvinyl acetate with a minimum molecular weight of 2000 is permitted as a synthetic masticatory substance in chewing gum base. Monomer residue has not been considered a problem in end-use products. Latexes or solutions of polyvinyl acetate that are essentially intermediates may contain residual vinyl acetate, essential emulsifiers, or initiators. No detailed information is available on the amount of unreacted monomer in either polyvinyl acetate or polyvinyl alcohol resins. Local sarcomas have been produced in rats with polyvinyl alcohol sponges, but implants of both polyvinyl alcohol and polyvinyl acetate in powder form did not produce tumors. IARC considered that additional studies would be required prior to evaluation of carcinogenic potential. Inhalation and combustion toxicity have not been considered problems. This may be attributed to polymer structure and degradation characteristics as well as the nature of ordinary intermediate and end-use products. Since the 1700s when Newman first isolated styrene by stream distillation from liquid ambar, a solid resin obtained directly from a family of trees native to the Far East and California, a substantial industry has developed for styrene-based products. Today, “styrene-based” plastics most commonly are polystyrene, successfully commercialized in 1938, plus the derivatives containing butadiene, acrylonitrile, or both. The derivatives containing acrylonitrile are also called “acrylonitrile polymers” or “nitrile polymers.” Polystyrene is made in three different forms: crystal, impact, and expandable. Producers generally refer to the polystyrene market as including only crystal and impact grade. Expandable polystyrene—a foam product, with primary markets in construction and packaging—is a separate specialty product. Structurally the acrylic polymers include those containing repeating units of acrylonitrile, acrylic acid, acrylates, methacrylates, and all the various derivatives. “Acrylic plastics” may imply only polymers of acrylic or methacrylic acid ester, among which the prototype is polymethyl methacrylate. The demand for sheet polymethyl methacrylate dates from World War II, when it was used for aircraft glazing. Polyacrylonitrile is used primarily as fibers, commonly called “acrylic,”that have been formulated with varying amounts of comonomer. The main copolymer types derived from both styrene and acrylonitrile are (1) styrene–acrylonitrile (SAN) copolymer resin and (2) acrylonitrile–butadiene–styrene (ABS), in which discrete butandiene particles are dispersed in a SAN copolymer matrix and then sold as pellets or powder. Temperatures described for ABS processing are in the 190–275°C range. Acrylate and methacrylate esters are generally available from the manufacturer in granules or powder. Dyes, pigments, plasticizers, or ultraviolet absorbers may be added during processing. Commercial processing of polymethyl methacrylate per se uses three intermediate types of approach: the melt state for injection molding and extrusion; sheets, rods, and tubes that are mechined or welded; and monomer–polymer dough, primarily for dentures.