The most important synthetic rubbers and the most widely used rubbers in the entire world are the styrene butadiene rubbers (SBR). Formerly designated GR-S, SBRs are obtained by the emulsion polymerization of butadiene and styrene in varying ratios. When SBR is used in light-duty tires such as passenger car tires, the cold-rubber compounds have proved equal or superior to natural rubber treads. However, they are inferior to natural rubber for truck tires because of the greater heat buildup during flexing. The cold, oilextended type, prepared by the replacement of a portion of the polymer by a heavy-traction oil, accounts for more than 50% of the coldrubber production. Its advantage is primarily economic. Master batches, containing both oil and carbon black, have the advantage of process simplification.
