SBR Rubber: Styrene and Butadiene

SBR Rubbers

Styrene Monomer

A key component of styrene-butadiene rubber (SBR) is the monomer styrene. Styrene, chemically known as phenylethylene, is a colorless liquid with a distinctive odor. It is produced through the dehydrogenation of ethylbenzene, a process that also generates ethylene as a byproduct. Styrene is used not only in the production of SBR but also in the synthesis of polystyrene, a thermoplastic polymer known for its ability to be easily molded into various shapes.

Butadiene Monomer

The other primary monomer in SBR is butadiene. This monomer is a colorless gas at room temperature and pressure. In the production of SBR, butadiene is usually polymerized in an emulsion process, where a soap-like surface-acting agent disperses and emulsifies the materials in a water solution. The microstructure of SBR, which includes 1,4-cis, 1,4-trans, and 1,2-vinyl, is determined by the styrene-to-butadiene ratio and the microstructure of the copolymer.

Polymerization Process

The polymerization process for SBR typically involves copolymerizing styrene and butadiene in an emulsion process. This process involves the use of a soap-like surface-acting agent, which disperses and emulsifies the materials in a water solution. Free-radical initiators are also added to begin the polymerization process, and stabilizers are included to prevent deterioration of the final product. Upon polymerization, the styrene and butadiene repeating units are arranged in a random manner along the polymer chain, which is then cross-linked in the vulcanization process.

Applications

SBR is widely used in various applications due to its excellent abrasion resistance, crack resistance, and generally better aging characteristics. It is most commonly used in automobile and truck tires as an abrasion-resistant replacement for natural rubber. In addition to tires, SBR is also used in belting, flooring, wire and cable insulation, and footwear. It can also be produced in latex form as a rubbery adhesive for carpet backing and other applications.

Chemical Properties

SBR is a blend of approximately 75% butadiene and 25% styrene. The composition and microstructure of SBR, which includes 1,4-cis, 1,4-trans, and 1,2-vinyl, play important roles in determining the final properties of the rubber. SBR has a glass transition temperature, Tg, of around -52°C, which indicates the synthesis of a homogeneous statistical copolymer. It tends to harden with age instead of softening, unlike natural rubber, due to increased interlinking of the polymer chains. However, SBR has some limitations, such as poor strength without reinforcement by fillers, low resilience, low tear strength (particularly at high temperatures), and poor tack. These characteristics determine the use of the rubber in tire treads, with the proportions of SBR decreasing as the need for heat resistance increases until 100% natural rubber is reached in the heaviest and most severe uses.