Abstract

The data presented here support the contention that a polyisoprene rubber of high cis-1,4-content (Coral rubber) has been developed, which is strikingly similar in polymer structure to Hevea rubber, as indicated by infrared structure, x-ray crystalline pattern, perbenzoic acid determination of 1,4-addition, ozonolysis, unsaturation, carbon-hydrogen ratio, molecular-weight distribution, inherent viscosity, and specific gravity. It is also shown that those polymer structural features and other characteristics of Coral rubber provide evidence that these same structural features found in natural rubber account for its low hysteresis, high gum tensile strength, and good retention of physical properties at elevated temperatures. Coral rubber differs from Hevea rubber in that is has slightly less cis-1,4-structure and slightly more 3,4-structure; it does not absorb oxygen as readily as natural rubber, has higher thermal stability, and is more resistant to cracking in tire treads. Both Hevea rubber and balata (both being polyisoprenes) have small amounts of 3,4-structure in the polymer chain rather than exclusively 1,4-structure as heretofore generally accepted. In truck tires containing 100 per cent Coral rubber, the heat build-up during service (50 m.p.h.) was identical to tires built from natural rubber. Truck tires having Coral body stocks gave satisfactory performance in normal commercial service. Tread wear road tests on truck and passenger tires show that Coral rubber treads, containing similar amounts and type of carbon black as Hevea treads, give 95 per cent of the wear obtained with Hevea treads.