Aryl-phosphine-oxide-containing materials show considerable promise for producing advanced organic materials with significantly improved flammability resistance. A new development is the possibility of bridging organic and inorganic materials to produce organic-inorganic composite networks.

Elastomers

Elastomers, or rubbers, are soft and compliant polymers that are able to experience large, reversible deformations. Only long-chain polymers are capable of this type of elasticity. Elastomers are typically amorphous, network polymers with lower cross-link density than thermoset plastics. Most thermosets can be made to function as elastomers above their glass transition temperatures.

Historically, elastomers have played an important role in the industrialization, prosperity, and security of the United States. Synthetic elastomers were born of necessity during World War II, when the United States was cut off from most of its supplies of natural rubber in Southeast Asia. Low-temperature emulsion polymerizations were developed to produce highly successful synthetic rubbers, particularly styrene-butadiene copolymers. In one of the most remarkable success stories in modern industry, a production capacity of 1.5 billion pounds per year was reached in 1945. This industry continues in the United States (see Table 3.3 for production figures for various types of synthetic rubber) and in other industrialized countries. Annual production figures have declined in recent years owing to a number of factors, including advances in the use-life of tires.

Originally, all elastomers were thermosets or chemically cross-linked materials, and so their flexibility in processing, especially reprocessing or recycling, was severely limited. Thermoplastic elastomers represent a current major growth area that comprises a growing number of chemical concepts. The first materials were styrene-based block copolymers that phase separate at the molecular level to produce relatively hard polystyrene domains, which act as temporary, physical

TABLE 3.3 U.S. Production of Synthetic Rubber, 1992

 

Pounds (billions)

Ethylene-propylene

0.45

Nitrile

0.16

Polybutadiene

1.02

Styrene-butadiene

1.75

Othera

0.86

SOURCE: Date from Chemical & Engineering News (1993).

a Includes, for example, butyl, polychloroprene, polyisoprene, silicone, and other synthetic elastomers.



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