Many types of synthetic rubber have been developed during the 20th century. Some are used as substitutes for natural rubber. Others have properties that make them superior to natural rubber for certain uses. Among the synthetic rubbers most used in the United States are SBR (styrene-butadiene rubber) and polybutadiene, neoprene, and butyl rubber. Synthetic rubber represents about two-thirds of the total worldwide production of new rubber.

Synthetic rubber is made by a process called polymerization, the process of changing monomers (compounds of relatively low molecular weight and simple structure, such as isoprene) into polymers.

(formerly called GR-S [government rubber—styrene]) is durable and resistant to abrasion, heat, and cold. It is used mainly in tires, shock absorbers, gaskets, hoses, heels, and adhesives.

SBR is produced from two monomers—butadiene and styrene. Butadiene is obtained from petroleum, butane, or ethyl alcohol. Styrene is obtained from petroleum or benzene. These monomers are converted to SBR by the emulsion polymerization process. The monomers are mixed with water and soap to form an emulsion and with a catalyst to begin the process. Other substances are added to control the molecular weight of SBR. The monomers are usually polymerized at about 40° F. (4° C). (SBR formed at this temperature is known as cold rubber.) The process produces a latex.

The latex is used as is (for example, in paints and in adhesives), or it is coagulated with an acidified brine solution or an aluminum sulfate solution to concentrate the rubber. An antioxidant is added to the rubber before coagulation to prevent oxidation during drying and storage. The coagulated rubber forms fine crumbs, which are washed, dried by heat, and pressed into bales for shipment.

closely resembles natural rubber in many properties. It is highly elastic and resistant to abrasion, heat, and cold. It is mainly blended with natural rubber or with SBR in tires.

Polybutadiene is produced from butadiene by the solvent polymerization process. The monomer is dissolved in a hydrocarbon solvent (such as kerosine), then polymerized at about 120° F. (49° C), using an alkylaluminum catalyst. This process does not produce a latex. Instead, it forms a thick, pastelike solution. An antioxidant is added to the rubber. The rubber is dried by heat to remove the solvent, then baled.

is resistant to heat, weathering, abrasion, oils, and solvents. It is used in adhesives, paints, hoses, linings for chemical containers, and surgical and other rubber gloves.

Neoprene is produced from chloroprene, which is obtained from petroleum or acetylene. The monomer is polymerized in an emulsion, using potassium persulfate as a catalyst. Neoprene is processed similarly to SBR, except that neoprene latex is usually coagulated by freezing.

is resistant to acids, oxygen, and heat. Because it is highly impermeable to gases, butyl rubber is used mainly in tire linings and other gas-retaining applications. It is also used in electrical insulation, hoses, and adhesives.

Butyl rubber is produced from isobutylene and isoprene, which are obtained from petroleum. The monomers are dissolved in a chlorinated solvent and polymerized at about -150° F. (-101° C.), using aluminum chloride as a catalyst. The process does not produce a latex. The rubber forms fine particles. An antioxidant is added to the rubber. The rubber is then washed to remove the solvent, dried by heat, and baled.