The polyacrylonitrile resins are hard, horny, relatively insoluble, and high-melting materials. Polyacrylonitrile (polyvinyl cyanide) is used almost entirely in copolymers. The copolymers fall into three groups: fibers, plastics, and rubbers. The presence of acrylonitrile in a polymeric composition tends to increase its resistance to temperature, chemicals, impact, and flexing.
Acrylonitrile is generally prepared by several methods, including the catalyzed addition of hydrogen cyanide to acetylene. The polymerization of acrylonitrile can be readily initiated by means of the conventional free-radical catalysts such as peroxides, by irradiation, or by the use of alkali metal catalysts. Although polymerization in bulk proceeds too rapidly to be commercially feasible, satisfactory control of a polymerization or copolymerization may be achieved in suspension and in emulsion, and in aqueous solutions from which the polymer precipitates. Copolymers containing acrylonitrile may be fabricated in the manner of thermoplastic resins.
The major use of acrylonitrile is in the form of fibers. By definition an acrylic fiber must contain at least 85% acrylonitrile: a modacrylic fiber may contain less than 35 to 85% acryloni-trile. The high strength; high softening temperature; resistance to aging, chemicals, water, and cleaning solvents; and the soft wool-like feel of fabrics have made the product popular for many uses such as sails, cordage, blankets, and various types of clothing. Commercial forms of the fiber probably are copolymers containing minor amounts of other vinyl derivatives, such as vinyl pyrrolidone, vinyl acetate, maleic anhydride, or acrylamide. The comonomers are included to produce specific effects, such as improvement of dyeing qualities.
Copolymers of vinylidene chloride with small proportions of acrylonitrile are useful as tough, impermeable, and heat-sealable packaging films.
Extensive use is made of copolymers of acrylonitrile with butadiene, often called NBR (formerly Buna N) rubbers, which contain 15% acrylonitrile. Minor amounts of other unsatur-ated esters, such as ethyl acrylate, which yield carboxyl groups on hydrolysis, may be incorporated to improve the curing properties. The NBR rubbers resist hydrocarbon solvents such as gasoline and abrasion, and in some cases show high flexibility at low temperatures.
In the 1960s development of blends and interpolymers of acrylonitrile-containing resins and rubbers represented a significant advance in polymer technology. The products, usually called ABS resins, typically are made by blending acrylonitrile-styrene copolymers with a butadiene-acrylonitrile rubber, or by interpoly-merizing polybutadiene with styrene and acry-lonitrile. Specific properties depend on the proportions of the comonomer, on the degree of grattings, and on molecular weight. In general, the ABS resins combine the advantages of hardness and strength of the vinyl resin component with toughness and impact resistance of the rubbery component. Certain grades of the ABS resin are used for blending with brittle thermoplastic resins such as polyvinyl chloride to improve impact strength.
The combination of low cost, good mechanical properties, and ease of fabrication by a variety of methods, including typical metal-working methods such as cold stamping, led to the rapid development of new uses for ABS resins. Applications include products requiring high impact strength, such as pipe, and sheets for structural uses, such as industrial duct work and components of automobile bodies. ABS resins are also used for housewares and appliances, because of their ability to be electroplated for decorative items in general.
