Natural rubber in tires

The emulsion SBR (styrene-butadiene rubber) developed in the 1940s, utilizing free-radical initiation, was of enormous benefit to tire technology. The improvement in performance and cost of SBR over natural rubber in tire treads firmly established the utility of synthetic rubber. The success of emulsion SBR led to further research. As a result, the percentage of natural rubber in rubber produced since that time has been steadily declining, and extensive research efforts have been carried along on a continuing basis to develop even better synthetic elastomers. 

Ethylene-Propylene Rubber n (E/P, EPDM, SPM, EPR) Any of a group of elastomers obtained by the stereospecific copolymerization of ethylene and propylene (EOM), or of these two monomers and a third monomer such as an unconjugated diene (EPDM). Their properties are similar to those of natural rubber in many respects, and they have been proposed as potential substitutes for natural rubber in tires. 

---