Polymers derived from hydrocarbon molecules that possess one or more alkenyl (or olefinic) groups. The term polyolefin typically is applied to polymers derived from ethylene, propylene, and other alpha-olefins, isobutylene, cyclic olefins, and butadiene, and other diolefins. See also Alkene.
Polyethylene
Polyethylene is any homopolymer or copolymer in which ethylene is the major component monomer. It is a semicrystalline polymer of low to moderate strength and high toughness; its stiffness, yield strength, and thermal and mechanical properties increase with crystallinity. Toughness and ultimate tensile strength increase with molecular weight. Polyethylene shows excellent toughness at low temperatures. Polyethylene is relatively inexpensive, extremely versatile, and adaptable to a large array of fabrication techniques. It is chemically inert; resistant to solvents, acids, and alkalis; and has good dielectric and barrier properties. It is used in many housewares, films, molded articles, and coatings.
Low-density polyethylene has densities ranging from 0.905 to 0.936 g/cm3. High-pressure low-density polyethylene is referred to simply as LDPE; linear low-density polyethylene (LLDPE) is a copolymer of polyethylene that is produced by a low-pressure polymerization process. High-density polyethylene (HDPE) covers the density range from 0.941 to 0.967 g/cm3. HDPE generally consists of a polymethylene (CH2)n chain with no, or very few, side chains to disrupt crystallization, while LLDPE contains side chains whose length depends on the comonomer used. See also
LLDPE finds wide application in plastic films such as garbage bags and stretch cling films. Sheathing and flexible pipe are applications that take advantage of the flexibility and low-temperature toughness of LLDPE. HDPE is used in food packaging, grocery bags, pickup truck bedliners, and large containers. Fibers have been produced that approach the strength of spider silk, and is used in fishing lines and in medical applications.
Rubbery ethylene copolymers are used in compounded mixtures and range in comonomer content from 25 to 60% by weight, with propylene being the most widely used comonomer to form ethylene-propylene rubber. In addition to propylene, small amounts of a diene are sometimes included, forming a terepolymer. Products containing ethylene-propylene rubber and terepolymer have many automotive uses such as in bumpers, facia, dashboard panels, steering wheels, and assorted interior trim. See also Rubber.
Ethylene copolymers are used to produce the polymer poly(ethylene-co-vinyl acetate) [EVA]. Applications include specialty film for heat sealing, adhesives, flexible hose and tubing, footwear components, bumper components, and gaskets. Foamed and cross-linked poly(ethylene-co-vinyl acetate) is used in energy-absorbing applications. Ionomers are ethylene copolymers that are produced from the copolymerization of ethylene with a comonomer containing a carboxylic group (COOH) such as methyl acrylic acid. Because of their toughness, ionomers are widely used in the covers for golf balls.
Polypropylene
Commercial polypropylene (PP) homopolymers are isotactic, high-molecular-weight, semicrystalline solids having melting points around 160–165°C (320–329°F), low density (0.90–0.91 g/cm3), and excellent stiffness and tensile strength. They have moderate impact strength (toughness), low density over a wide temperature range, excellent mechanical properties, and low electrical conductivity. Propylene, like ethylene, is produced in large quantities at low cost from the cracking of oil and other hydrocarbon feedstocks. Low-molecular-weight resins are used for melt spun and melt blown fibers and for injection-molding applications. Polypropylene resins are used in extrusion and blow-molding processes and to make cast, slit, and oriented films. Stabilizers are added to polypropylene to protect it from attack by oxygen, ultraviolet light, and thermal degradation; other additives improve resin clarity, flame retardancy, or radiation resistance.
Polypropylene homopolymers, random copolymers, and impact copolymers are used in such products as automotive parts, appliances, battery cases, carpeting, electrical insulation, fiber and fabrics, food packaging, and medical equipment.
Other poly(alpha-olefins)
Poly(1-butene) is a tough and flexible resin that has been used in the manufacture of film and pipe. Poly(4-methyl-1-pentene) is used in the manufacture of chemical and medical equipment. High-molecular-weight polyisobutylenes are rubbery solids that are used as sealants, inner tubes, and tubeless tire liners. Low-molecular-weight polyisobutylenes are used in formulations for caulking, sealants, and lubricants. Butadiene and isoprene can be polymerized to give a number of polymer structures. The commercially important forms of polybutadiene and polyisoprene are similar in structure to natural rubber. See also Polyacrylonitrile resins; Polymer; Polystyrene resin.
