Mooney Viscosity Changes in Freshly Prepared Raw Natural Rubber

Abstract

Abstract In the course of investigations carried out in connection with the technical classification of natural rubber, large permanent increases in Mooney viscosity of the order of 30 to 40 units have been observed when freshly prepared raw or masticated rubber is stored under conditions of low relative humidity. The rate of change and the extent of the increase are both affected by the relative humidity of storage, the effects being most marked in the region below about 30 per cent relative humidity. Rate of change in dry air increases with decreasing sample thickness and with increasing temperature, although the effects are masked somewhat by oxidative breakdown on prolonged storage at temperatures higher than about 30° C. Mooney values continue to increase long after the rubber has reached constant weight on drying; the removal of acetone soluble and nitrogenous non-rubber components and of oxygen does not prevent the changes taking place. The effects are only partially reversed by subsequent storage at high relative humidity, and there is evidence to suggest that the hardening is associated with the development of a cross-linked benzene-insoluble gel structure throughout the rubber. The rate of mill-breakdown of dry stored rubber is greater than for fresh rubber, but in comparisons using a fixed breakdown procedure, the hardened rubber still has a higher Mooney value after mastication. Rubbers hardened by dry air storage exhibit slightly improved vulcanized properties in a pure-gum mix even after the additional breakdown required to reach normal stock viscosity levels. Fluctuation of temperature and humidity in commercial smokehouses and drying sheds may well account for part of the variability of natural rubber as measured by Mooney viscosity determinations on the freshly prepared product, and different conditions of storage will add a further contribution to the total hardness variation experienced by the consumer.