EFFECT OF LONG-TERM STORAGE ON RAW RUBBER PROPERTIES.

The effect of 7 and 13 years storage on the Mooney viscosity, Wallace plasticity and plasticity retention index (PRI) of samples of Standard Malaysian Rubber and other grades of raw rubber is discussed. All samples underwent storage hardening during the first 7 years of storage, but generally this was succeeded by a viscosity decrease during th 7-13 year period. Changes in Wallace plasticity show poor correlation with changes in viscosity and a different correlation between these two measurements is evident for storage-hardened rubbers as compared to fresh materials. A fall in PRI was observed generally during the first period of storage and this fall continued during the next period, but even after 13 years only one PRI value less than 50 was found.