Some Major Rubber Products

There are no ISO, ASTM or British fire test method standards specifically for solid mbbers and there is no active fire test work being pursued in TC 45. There are, however, a number of published international test methods for cellular materials and plastics, the majority of which could be applied to rubbers. A comprehensive account of fire testing of plastics has been given by Paul in the Handbook of Polymer Testing81. There may be fire resistance requirements for particular rubber products and some examples were given by Schultz110. 

Thus, the chemical industry is a key component in the U.S. economy. It converts raw materials such as gas, oil, condensates, water, metals, and minerals into more than 70,000 products that are used in a variety of ways. In some fashion, this industry impacts the daily lives of everyone. Industrial customers for chemicals are many, but some of the major ones are apparel, plastic and rubber products, petroleum refining, textiles, pulp and paper, primary metal, and the like. 

Rubber tyres are by far the most visible of rubber products. Identification is trivial and collection is well organized. Recycling and disposal, however, are less evident. A major route for tyres is their use as a supplemental fuel in cement kilns. Major compounds in tyres are styrene-butadiene rubber (SBR), synthetic and natural polyisoprene rubber, steel cord, carbon black, zinc oxide, sulphur and vulcanization-controlling chemicals. Tyres can be retreaded, which is economic for large sizes (truck tyres), or ground to crumb or powder (cryogenic grinding). Such materials have some limited market potential as an additive in asphalt, and in surfaces for tennis courts or athletics. 

Crumb rubber used in hot mix asphalt normally has 100% of the particles finer than 4.75 mm. Although the majority of the particles used in the wet process are sized within the 0.42-1.2 to mm range, some crumb rubber particles maybe as fine as 0.075 mm. The specific gravity of crumb rubber is approximately 1.15, and the product must be free of fabric, wire, or other contaminants. 

In an emulsion polymerization process, there are numerous variables that may be changed. Blackley s recent book discusses emulsion rubber production and provides an outline of the mechanism of emulsion polymerization. Some of the major polymerization variables will be discussed here, as they affect the adhesive properties of the resulting polymers. 

The process of anionic polymerisation was first used some 60 or more years ago in the sodium-catalysed production of polybutadiene (Buna Rubbers). Typical catalysts include alkali metals, alkali metal alkyls and sodium naphthalene, and these may be used for opening either a double bond or a ring structure to bring about polymerisation. Although the process is not of major importance with the production of plastics materials, it is very important in the production of synthetic rubbers. In addition the method has certain special features that make it of particular interest. 

Current production of NR is about 5.2 X 10 tonnes. For some years it has enjoyed a premium price over SBR because of its desirable characteristics described above and, compared with other large tonnage polymers, a somewhat restricted supply. Clearly it is difficult to substantially increase the production of such a material in a short period of time and indeed the attractions of other crops such as palm oil as well as the desire to move away from a monoculture economy mitigate against this. The indications are that, unless there is undue intervention of political factors, the future of natural rubber as a major elastomer remains secure. 

The butadiene-acrylonitrile rubbers were first prepared about 1930 about five years after the initial development of free-radical-initiated emulsion polymerisation. Commercial production commenced in Germany in 1937, with the product being known as Buna N. By the late 1980s there were about 350 grades marketed by some 20 producers and by the early 1990s world production was of the order of 250000 tonnes per annum, thus classifying it as a major special purpose rubber. 

Most polystyrene products are not homopolystyrene since the latter is relatively brittle with low impact and solvent resistance (Secs. 3-14b, 6-la). Various combinations of copolymerization and blending are used to improve the properties of polystyrene [Moore, 1989]. Copolymerization of styrene with 1,3-butadiene imparts sufficient flexibility to yield elastomeric products [styrene-1,3-butadiene rubbers (SBR)]. Most SBR rubbers (trade names Buna, GR-S, Philprene) are about 25% styrene-75% 1,3-butadiene copolymer produced by emulsion polymerization some are produced by anionic polymerization. About 2 billion pounds per year are produced in the United States. SBR is similar to natural rubber in tensile strength, has somewhat better ozone resistance and weatherability but has poorer resilience and greater heat buildup. SBR can be blended with oil (referred to as oil-extended SBR) to lower raw material costs without excessive loss of physical properties. SBR is also blended with other polymers to combine properties. The major use for SBR is in tires. Other uses include belting, hose, molded and extruded goods, flooring, shoe soles, coated fabrics, and electrical insulation. 

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