Effect of some non-rubber constituents on discoloration and cure characteristic of natural rubber

Abstract

Procession of fresh field latex (Hevea brsiliensis) to solid rubber sometimes result in discoloration. This is an important problem in some industries which require pale color rubber as raw material. The object of this research is to investigate the factors present in field latex which should be involved in the darkening of rubber namely tocotrienols, carotenoid pigments, polyphenols and polyphenol oxidase in latex clone RRIM 600. GT 1 and PB 5/51 which are the recommended clones for growing in Thailand. The results show that 100 g of dry rubber from 3 clones tested contain 0.07-0.08 g tocotrienols, 0.11-0.13 g polyphenols and 140,800-267,770 units polyphenol oxidase. But carotenoid content vary with clone among which PB 5/51 has the highest content about 190 ug followed by GT 1 (119 ug) and RRIM 600 (60 ug) per 100 g dry wt. Comparative study on color index and physical properties of raw rubber depleted in total lipids plus polyphenols and addition of these factors individually with the control untreated rubber shows that removal of tatal lipids and polyphenols by chloroform-methanol have decreased the color index about 30-60%. The most important factor on discoloration of raw rubber is polyphenols which increase the color index ranging from 43-97%. When these raw rubber are mixed with vulcanizing ingredients to be compounded rubber and then vulcanized to be vulcanizate, the rubbers obtained exhibit higher degree of color than its original raw rubber. The processing of rubber in each step also has an effect on discoloration. However the factor which has the most remarkable effect on darkening of compound and vulcanized rubber is polyphenols which react with some substances such as proteins to give color products by non-enzymatic reaction. Mooney viscosity of raw rubber depleted in total lipids plus polyphenols decreased about 10-30% but the substances which affect this viscosity reduction are not discolorating substances tested in this research. Cure characteristics and some physical properties tested are not affected by these discolorating factors either depletion or addition.