Maturation of natural rubber cup coagula: Characterization of industrial conditions

In factory practice, cup coagula are stored in pile for a variable period of time before being processed as Technically Specified Rubber (TSR). During this so-called "maturation " period, neither the microbiological phenomena occurring, nor their implications on properties of raw natural rubber are known. Before conducting experiments in controlled conditions (laboratory), the present work focused on the characterization of real maturation conditions (factory) in a pile of coagula. Physicochemical conditions [temperature, relative humidity (RH) and oxygen content] and rubber properties [total solid content (TSC), pH, initial plasticity (Po) and plasticity retention index (PRI)] were analyzed at different depths from the top of the piles. It was found that temperature and relative humidity of the air within the piles increased with the depth (34°C and 55% RH at the top, more than 40°C and 80% RH at 1 m deep). In contrast, oxygen content decreased as the depth increased (21% at top and less than 5% at lm deep). Rubber properties were found to depend not only on the maturation time but also on the position within the pile. During maturation, an increase was observed for the mean value of several parameters: TSC (from 78 to 82), pH (from 6 to 6.5), Po (from 35 to 45) and PRI (from 30 to 45). An influence of the location of cup coagula within the piles was also observed. At the end of the maturation period, the pH of cup coagula decreased significantly with the depth of the pile (from 7.2 at top to 6.3 at 2 m deep). An inverse pattern was observed for Po (from 28 at top to 45 at 2 m deep). TSC was found to be minimal at the heart of the pile. The results suggest that microbial activities may play an important role in the evolution of some physico-chemical properties of cup coagula and of their environment within storage piles during maturation. (Résumé d'auteur)