Synthetic Polymers—Olefin, Diene Elastomers, and Vinyl Halides

Abstract

Elastomers, also called rubber, can withstand considerably greater deformation than other materials and uniquely return essentially to their original shape even after substantial deformation. A familiar example is the behavior of a stretched rubber band after its release. All elastomers are composed of long macromolecular chains that assume a random coil conformation when undeformed. Deformation causes these coils to straighten out. Upon being allowed to relax, an elastomer returns essentially to its original shape because the chains reassume their random conformation. The first elastomer identified, natural rubber, was described by Columbus as a ball that bounced. The first specialty elastomers, polysulfides and polychloroprene, were commercialized in the 1930s, natural rubber was the major industry product until World War II, when styrene-butadiene rubber (SBR) and acrylonitrile-butadiene rubber (NBR) were established as important synthetic rubbers. From these early beginnings, the elastomer industry grew rapidly to a global elastomer demand of 15 million metric tons in 1990. The range and diversity of synthetic rubber becomes evident upon reviewing the Synthetic Rubber Manual that describes both thermosetting elastomers (TSE) and thermoplastic elastomers (TPE). TSE and TPE exhibit important similarities. The most useful properties are the result of their long molecular chains linking to one another to form a three-dimensional network. In TSE this network is linked together with essentially irreversible cross-links. Vulcanization is the process of forming these cross-links, most typically using sulfur as the cross-linking agent. TSE generally arrives at the rubber fabricators in bales. Ten or more ingredients might be added to the bale in heavy mixers before the compounded elastomer is shaped into a product and vulcanized. Schunk has characterized the health hazards of many of these ingredients, including carbon blacks, mineral fillers, plasticizers, protective and cross-linking agents, and accelerators. Broadly considered, these health hazards can be considered in terms of the following: monomers, solvents, and other materials used to prepare elastomers storage and handling of elastomer (bales, pellets, and powder) processing of elastomers, generally at high temperatures finished rubber product Health hazards in processing, and storage and handling elastomers are the dominant focus of this section; limited references will be made to the other two areas where appropriate. Certain portions of the material refer to monomer toxicology and epidemiology because some of the monomers used in manufacturing elastomers remain at low levels in the polymer. A full discussion of the toxicity of monomers is beyond the scope of this chapter. Typical basic properties of certain elastomers are latter. Properties within a given class of elastomers can vary significantly. For example, increasing acrylonitrile content in NBR reduces swelling of the NBR caused by some oils and solvents. Most rubber is sold raw or uncured as a solid or liquid latex. The basic steps in the manufacture of some types of dry synthetic rubber are polymerization, coagulation, washing, and drying. The basic steps in producing a latex are polymerization, stabilization, and usually, concentration. A latex is defined as a stable aqueous dispersion that contains discrete polymer particles about 0.05 to 5 mm in diameter. Emulsion polymerization systems contain water, monomer(s), initiator, and anionic or cationic surfactants. Solution polymerization with stereospecific catalysts involves reacting one or more monomers in an inert solvent; system conditions can be controlled to maximize a desired isomer arrangement in the polymer. Antioxidants are generally added for shelf, processing, and in-service stability. Vulcanization is usually done with sulfur, sulfur-containing compounds, or peroxides, but it may also be accomplished with other compounds that yield free radicals at curing temperature or by radiation. Various supplementary materials such as cure accelerators, cure retarders, or reinforcing agents are commonly part of the compounding recipe. Vulcanization ideally begins when the elastomer assumes its final shape in a mold. The elastomer type and its viscosity significantly affect molding behavior. Dry solid polymers usually contain less residual monomer (or solvent) than latex materials. The processing necessary to produce the dry product drives the residual monomer or solvent out of the resin, usually by heat. Several reports address worker health problems in the rubber fabrication industry. For example, one study suggests an association between the mortality risk of lung cancer and employment in operations involving reclaim, chemicals, and special products. Another study showed that processing workers had increased mortality from leukemia, emphysema, and cancers of the stomach, large intestine, biliary passages, and liver. Industrial dermatitis from finished rubber products due to the various chemicals added during polymerization, curing, and processing is not uncommon. Elastomers degraded at high temperatures around 800°C can yield more toxic products than elastomers degraded at smoldering temperatures or gradually rising temperatures. This is to be especially so with nitrile-butadiene. The commercial polymers in the vinyl halides group contain chlorine atoms, fluorine atoms, or both in a few cases. In very diverse ways these halogens can be used to produce vinyl polymers that have such characteristics as increased resistance to water, oils, and solvents, plus other distinctive properties. The prototypes are polyvinyl chloride and polyvinylidene chloride. Polyvinyl chloride and its copolymers rank first in production/consumption volume among polymers in the United States and abroad. Their key attribute is low-cost versatility. Polyvinylidene chloride resins have an extremely regular, closely packed molecular structure that results in outstanding impermeability to water, oils, and gases. Keywords: Elastomers; Olefin elastomers; Diene elastomers; Vinyl halides; PVC; Vinylidene chloride copolymers; Butyl rubber; Styrene—butadiene; EPR; Acrylic elastomers; Silicone; Polyurethanes; Polytetrafluoroethylene; Acrylonitrile-butadiene; Vinylidene fluoride coplolymers; Polyisoprene; Epichlorhydrin