Polyolefins

Polyethylene, polypropylene, ethylenevinyl acetate, ionomer, polybutylene, polymethylpentene, polydicyclopentadiene) Polyolefin homopolymers are made from ethylene, propylene, butylene, and methyl pentene. Other olefin monomers such as pentene and hexene are used to make copolymers. Because the chemical and electrical properties of all olefins are similar, they often compete for the same applications. They differ from each other primarily in their crystalline structure. However, since strength properties vary with the type and degree of crystallinity, the tensile, flexural, and impact strength of each polyolefm may be quite different. Stress-crack resistance and useful temperature range also vary with crystalline structure. In addition to the solid polyolefin resins, these materials are also available as beads from which very low density (1.25 to 5.0 lb/ ft3) foam shapes and blocks are produced. Resilience and energy-absorption properties of these products are exceptional compared to those of conventional polystyrene foams. Polymers available as moldable beads include polyethylene, polypropylene, and a PE/polystyrene copolymer alloy. The bead forms can be processed by the same methods used for expandable polystyrene (EPS). After the beads are expanded (20 to 40 times that of the solid resin) nd conditioned, they are poured into a mold and heated, usually by direct injection of steam. This softens, expands further, nd fuses the particles together, forming a niform, void-free, closed-cell shape. After olding, the shapes are usually annealed usually stored at 120 to 160°F to stabilize shape and dimensions. Because they contain 80 to 95% air by volume, the foamed shapes are not nearly as strong as solid moldings. They are used priarily to cushion impact, insulate therally, and provide high stiffness-to-weight ore materials in composite components. important application for polypropylene am is in bumper cores for automobiles. A to 4-in.-thick section of foam at 2 to 4 b/ft3 can absorb the energy of a 5-mph imact. Package cushioning for fragile and aluable products such as electronic or auio components is another application for ither PE or PP foam. The toughness of E/PS alloy foams qualifies them for mateial-handling applications where repeated use is required. The principal resins of the polyolefin family are polyethylene and polypropylene. Other polyolefin polymers and copolymers described here are ethylene-vinyl acetate, ionomer, polybutylene, and polymethyl pentene. The largest volume thermoplastic polymers used today, polyethylene is available in a variety of grades that have an equally wide range of properties. Some are flexible, others are rigid; some have low impact strength, others are nearly unbreakable; some have good clarity, others are opaque. Service temperatures can range from -40 to 200°F. In general, however, polyethylenes are characterized