O2. rubber, synthetic 4 - Synthetic rubbers, often referred to as elastomers , are plastic materials, similar in structure to other plastic materials. However, they have the ability to stretch to at least double their length and to return almost immediately to the original length or close to it. 2 Much more synthetic rubber is produced in the world at this time than natural rubber. Styrene-butadiene rubber (SBR) is, by far, the most common synthetic rubber. It is also referred to as Buna-S and GR-S. 2 Other synthetics are: butadiene, ethylene-propylene, butyl, neoprene, nitrile and polyisoprene rubbers.

The manufacturing processes for these rubbers have much in common. The monomers that are reacted all can be handled similarly and the process equipment for one particular rubber is usable for several others. The process illustrated in Fig. 4O2 for making styrene-butadiene copolymer rubber is therefore quite typical for all the synthetic rubbers. The sequence starts with two separate sub-processes, one which produces styrene monomer and the other, butadiene monomer. Butadiene monomer is shown being made with the one-step Howdry process from n -butane with aluminum and chromium oxide catalysts. The butadiene thus produced is then purified by adsorption with cuprous ammonium acetate. The styrene is made from ethylbenzene which results from alkylating benzene with the butane raw material. It is then dehydrogenated to styrene over an aluminum chloride, solid phosphoric acid, or silica-alumina catalyst. Then the monomers are fed into a polymerization reactor along with some additives and a catalyst. The proportion of each monomer varies somewhat, depending on the planned application, but 70 to 75 parts butadiene and 25 to 30 parts of styrene is typical. The emulsion polymerization reaction takes place for 8 to 12 hours at a temperature of 41 ° F (5 ° C). The heat of polymerization is removed from the reactors with cooling coils. Following polymerization, which is partial, unpolymerized monomers are returned to the reactor. Polymerized material proceeds through several physical steps to put it into usable form.

Neoprene is a product of coal, limestone, salt and water. Calcium carbide, from coal and limestone is reacted with water, forming acetylene gas (C 2 H 2 ). The gas is reacted with hydrogen chloride to form chloroprene which is then polymerized to make neoprene.

Urethane, polysulfide, chlorinated polyethylene and silicone elastomers have superior properties for some applications where rubber-like material is needed. Urethane is used for forming pads for press forming of metal, solid tires, rollers and shock absorbing pads and bumpers. SBR rubbers are used extensively for tires and also for shoe soles, floor tile, in mechanical applications and as latex which becomes adhesives and coatings. Nitrile rubbers have particular resistance to oils, water, salts, soaps and most foods and are used in equipment where such resistance is important. Neoprene is used for automotive parts, adhesives, sealants, shoe soles, o-rings, bellows, conveyor belts, printing rolls and coatings. Butyl rubber is used for linings of tubeless tires, for innertubes, steam hose, tank lining and weather-stripping. Silicone rubbers are used for o-rings and seals for high temperature and corrosive conditions.

Fig. 4O2 The manufacturing sequence for styrene-butadiene rubber (SBR) showed in a simplified flow chart.

O3. rubber compounding 4 - Both natural and synthetic rubbers are seldom used without additives as part of their formulation. The additives are needed to impart the necessary strength, elasticity, toughness, degree of hardness and abrasion resistance. Additionally, all natural rubbers require vulcanization, usually with sulfur compounds and many synthetic rubbers are similarly processed. Accelerators speed up the vulcanization. Antioxidants are added to improve the life of the rubber product. Fine powder fillers are added to reduce overall cost and improve hardness and shape retention. Carbon black and silica fillers, however, actually provide greater strength and improved abrasion resistance and resilience. Pigments may be included to provide the desired color of the rubber product.

The first step in compounding is mastication . The operation is normally performed in a Banbury-type mixer (Fig. 4A4c). The rubber is sheared repeatedly, breaking down molecules, and providing easier flow. Mixing of the additives and rubber then follows.

O4. rubber fabrication methods - are very similar to those used with plastics, particularly thermosetting plastics. Molding is usually by compression or transfer molding techniques. Injection molding is also used, particularly for thermoplastic elastomers. Calendering is used to provide rubber coatings on fabrics. Extruding is used extensively in the manufacture of weather-stripping, hose, innertubes and tire components. Another processing method used with rubbers is dipping wherein a master form is immersed in a liquid formulation of rubber or elastomer (natural rubber latex, neoprene, silicone or vinyl plastisol). The form is removed and the liquid that adheres is permitted to dry. Dipping and drying are repeated to build the coating to the needed thickness. The operation may be aided by using electrostatic charges, speeding the attraction of the liquid and providing thicker coatings with each dip. The dipping method produces uniform wall thicknesses and is used for boots, gloves and fairings 5 . Some flat rubber parts for seals and pads are made by die cutting sheet rubber material with steel rule or other simple dies. (See sections above for descriptions of these processes. Also see tires, rubber .)