Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of synthetic rubbers derived from styrene and butadiene (the version developed by Goodyear is called Neolite). These materials have good abrasion resistance and good aging stability when protected by additives. In 2012, more than 5.4 million tonnes of SBR were processed worldwide. About 50% of car tires are made from various types of SBR. The styrene/butadiene ratio influences the properties of the polymer: with high styrene content, the rubbers are harder and less rubbery. SBR is not to be confused with the thermoplastic elastomer, styrene-butadiene block copolymer, although being derived from the same monomers. Types of SBR SBR is derived from two monomers, styrene and butadiene. The mixture of these two monomers is polymerized by two processes: from solution (S-SBR) or as an emulsion (E-SBR). E-SBR is more widely used. Emulsion polymerization E-SBR produced by emulsion polymerization is initiated by free radicals. Reaction vessels are typically charged with the two monomers, a free radical generator, and a chain transfer agent such as an alkyl mercaptan. Radical initiators include potassium persulfate and hydroperoxides in combination with ferrous salts. Emulsifying agents include various soaps. By "capping" the growing organic radicals, mercaptans (e.g. dodecylthiol), control the molecular weight, and hence the viscosity, of the product. Typically, polymerizations are allowed to proceed only to ca. 70%, a method called "short stopping". In this way, various additives can be removed from the polymer. Solution polymerization Solution-SBR is produced by an anionic polymerization process. Polymerization is initiated by alkyl lithium compounds. Water and oxygen are strictly excluded. The process is homogeneous (all components are dissolved), which provides greater control over the process, allowing tailoring of the polymer. The organolithium compound adds to one of the monomers , generating a carbanion that then adds to another monomer, and so on. For tire manufacture, S-SBR is increasingly favored because it offers improved wet grip and reduced rolling resistance, which translate to greater safety and better fuel economy, respectively. Buna S The material was initially marketed with the brand name Buna S. Its name derives Bu for butadiene and Na for sodium (natrium in several languages including Latin, German, and Dutch), and S for styrene. Buna S is an addition copolymer. Properties Applications It is a commodity material which competes with natural rubber. The elastomer is used widely in pneumatic tires. This application mainly calls for E-SBR, although S-SBR is growing in