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polypropylene

polypropylene (pŏl´ēprō´pəlēn), plastic noted for its light weight, being less dense than water; it is a polymer of propylene. It resists moisture, oils, and solvents. Since its melting point is 121°C (250°F), it is used in the manufacture of objects that are sterilized in the course of their use. Polypropylene is also used to make textiles, ropes that float, packaging material, and luggage.

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polypropylene

pol·y·pro·pyl·ene / ˌpäliˈprōpəˌlēn/ • n. a synthetic resin that is a polymer of propylene, used esp. for ropes, fabrics, and molded objects.

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Polypropylene

Polypropylene

OVERVIEW

Polypropylene (pol-ee-PRO-pih-leen) is a translucent white solid that is resistant to attack by heat, abrasion, inorganic acids and bases, most organic materials, and bacteria and fungi. It has high electrical resistance and tensile strength (its resistance to being pulled apart) and is very flexible. It takes on color well and can be coated with chrome. It can be prepared in a number of shapes and forms by extrusion and molding. Polypropylene is a thermoplastic polymer, meaning that it can be heated and cooled repeatedly, changing from a solid to a liquid and back again. Polypropylene is currently available under a number of trade names, including Amco®, Amerfil®, Azdel®, Beamette®, Clysar®, Daplen®, Dexon®, Epolene®, Gerfil®, Herculon®, Lambeth®, Lupareen®, Meraklon®, Mitsui Polypro®, Noblen®, Novolen®, Pellon®, Polypro®, Profax®, Propathene®, Propolin®, Propophane®, Shoallomer®, and Tuff-Lite®. Considerable dispute exists as to who should receive credit for inventing propylene. According to one history of the compound, it

KEY FACTS

OTHER NAMES:

Propylene polymer; 1-propene homopolymer

FORMULA:

-[-CH(CH3)CH2-]-n

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Organic polymer

STATE:

Solid

MOLECULAR WEIGHT:

Very large; 40,000 g/mol and up

MELTING POINT:

Varies: 165°C-170°C (330°F-340°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water and cold organic solvents; softens in warm organic solvents, but does not dissolve; soluble in hydrocarbons and halogenated hydrocarbons

was discovered independently about nine times. Patent disputes over the discovery lasted from the 1950s to 1989, when official credit was finally given to two researchers at the Phillips Petroleum Company, J. Paul Hogan (1919–) and Robert Banks (1921–1989). Phillips began selling polypropylene in 1951 under the trade name of Marlex®.

Much of the early research on polypropylene was conducted by the Italian chemist Giulio Natta (1903–1979), then an employee at the Italian chemical firm of Montecatini. While working earlier on the development of a related compound, polyethylene, Natta discovered some of the fundamental principles that governed the successful commercial production of polymers. In 1957, Montecatini began producing its own version of polypropylene. Six years later, Natta and his colleague, German chemist Karl Ziegler (1898–1973) shared the Nobel Prize in chemistry for their research on polymers.

HOW IT IS MADE

Polypropylene is made by the polymerization of propylene (propene; CH3CH=CH2). Polymerization is the process by which a single monomer unit (propylene in this case) is added to a second monomer of the same kind. The procedure is then repeated over and over again. Each time another monomer is added to the growing chain, the molecule gets larger and larger. Normally, polymerization is initiated by any of a number of agents, including radiation, light, or heat.

Polymerization of propylene presents a somewhat different problem, however, because of the presence of methyl (-CH3) groups extending off the main chain of the molecule (-[-CH(CH3)CH2-]-n). If polymerization is allowed to proceed on its own, some methyl groups will extend in one direction from the main chain, and others in a different direction. The product of this reaction is an amorphous product, one without crystalline shape, that has only a few very limited uses. To produce crystalline polypropylene, with all the desirable properties noted above, polymerization must be controlled to make sure that all methyl groups are on the same side of the main chain. One of Natta and Ziegler's great contributions was the discovery of catalysts capable of achieving the correct orientation of methyl groups. They found that metal halides, such as titanium chloride, could produce this effect. More recently, the German chemist Walter Kaminsky (1941–) and his colleagues found another group of catalysts that could polymerize crystalline polypropylene even more efficiently, a group of compounds called metallocenes. The manufacture of polypropylene today depends heavily on the use of such catalysts.

Interesting Facts

  • Natta applied for a patent for polypropylene before telling Ziegler of his success in making the compound. Ziegler was so angry that the two men did not speak for many years. They reconciled only when they were both awarded the Nobel Prize at the same time.
  • Australian bank notes (paper money) are made from polypropylene, which makes them more durable than paper bills.

COMMON USES AND POTENTIAL HAZARDS

About one-third of all the polypropylene consumed in the United States is used to make fibers, for the manufacture of products such as blankets, fabrics, carpets, yarns, fish nets, protective clothing, laundry bags, and ropes. The next largest uses are in the production of rigid packaging materials, such as crates, food containers, and bottles; in household products, such as dishes, bowls, outdoor carpeting, and outdoor furniture; and in packaging film. Hospitals use many surgical objects made out of polypropylene, taking advantage of its low cost and ability to be sterilized. Automobile manufacturers use the compound almost everywhere on the body of their cars. About 20 percent of all the polypropylene produced is used to make a large variety of products, including wire and cable insulation, medical tubing, pipe fittings, battery cases, drinking straws, and packaging foam.

No human health hazards have been identified for polypropylene in the form in which most people come into contact with the compound.

Words to Know

CATALYST
a material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.
POLYMER
a compound consisting of very large molecules made of one or two small repeated units called monomers.

FOR FURTHER INFORMATION

Karger-Kocsis, J. Polypropylene—An A-Z Reference. New York: Springer-Verlag, 1998.

Meikle, Jeffrey L. American Plastic: A Cultural History. Piscataway, N.J.: Rutgers University Press, 1997.

"Polypropylene." The Macrogalleria. Polymer Learning Center, University of Southern Mississippi. http://www.pslc.ws/macrogcss/pp.html (accessed on October 24, 2005).

"Polypropylene." World of Chemistry. Edited by Robyn V. Young. Detroit, Mich.: Gale, 1999.

See AlsoPolyethylene

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