Thermoplastics are polymers that become liquid when heated and return to the solid state when cooled. This cycle of melting and freezing can be repeated, so that the plastic can be reshaped by heating it. There are many types of thermoplastics, some of which are presented below. They are useful for a variety of applications, including consumer goods, machine parts, medical equipment and packaging and storage materials.
Acrylic, a polymer called poly(methyl methacrylate) (PMMA), is also known by trade names such as Lucite, Perspex and Plexiglas. It serves as a sturdy substitute for glass for such items as aquariums, motorcycle helmet visors, aircraft windows, viewing ports of submersibles, and lenses of exterior lights of automobiles. It is extensively used to make signs, including lettering and logos. In medicine, it is used in bone cement and to replace eye lenses. Acrylic paint consists of PMMA particles suspended in water.
Nylon, belonging to a class of polymers called polyamides, has served as a substitute for silk in products such as parachutes, flak vests and women's stockings. Nylon fibers are useful in making fabrics, rope, carpets and strings for musical instruments. In bulk form, nylon is used for mechanical parts, including machine screws, gear wheels and power tool casings. In addition, nylon is used in the manufacture of heat-resistant composite materials.
Polyethylene (or polyethene, polythene, PE) is a family of materials categorized according to their density and molecular structure. For example, ultra-high molecular weight polyethylene (UHMWPE) is tough and resistant to chemicals, and it is used to manufacture moving machine parts, bearings, gears, artificial joints and some bulletproof vests. High-density polyethylene (HDPE) is used to make milk jugs, margarine tubs and water pipes. Medium-density polyethylene (MDPE) is used for packaging film, sacks and gas pipes and fittings. Low-density polyethylene (LDPE) is soft and flexible and is used in the manufacture of squeeze bottles, sacks and sheets.
Polypropylene (PP) is useful for such diverse products as reusable plastic containers, diapers, sanitary pads, ropes, carpets, plastic moldings, piping systems, car batteries, insulation for electrical cables and filters for gases and liquids. In medicine, it is used to repair hernias and to make heat-resistant medical equipment. Polypropylene sheets are used for stationery folders and packaging and storage boxes.
Polystyrene is manufactured in various forms that have differing applications. Extruded polystyrene (PS) is used in the manufacture of disposable cutlery, CD and DVD cases, plastic models of cars and boats, and smoke detector housings. Expanded polystyrene foam (EPS) is used in making insulation and packaging materials, such as the "peanuts" and molded foam used to cushion fragile products. Extruded polystyrene foam (XPS), known by the trade name Styrofoam, is used to make architectural models and drinking cups for heated beverages. Polystyrene copolymers are used in the manufacture of toys and product casings.
Polyvinyl chloride (PVC) is a tough, lightweight material that is resistant to acids and bases. Much of it is used by the construction industry, such as for vinyl siding, drainpipes, gutters and roofing sheets. It is also converted to flexible forms with the addition of plasticizers, thereby making it useful for items such as:
- electrical insulation
Flexible PVC is also used in inflatable products, such as water beds and pool toys.
Teflon is the brand name given by DuPont Corp. for a polymer called polytetrafluoroethylene (PTFE), which belongs to a class of thermoplastics known as fluoropolymers. It is famous as a coating for non-stick cookware. Being chemically inert, it is used in making containers and pipes that come in contact with reactive chemicals. It is also used as a lubricant to reduce wear from friction between sliding parts, such as gears, bearings and bushings.
About the Author
Dinshaw Dadachanji has been a science editor and writer for more than 14 years. He has a Ph.D. in molecular biology from Yale University. His articles have been published by "The World & I" magazine and the Institute of Human Virology, among others.