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 of 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, and the effects are most pronounced in the region below about 30 per cent relative humidity. The rate of change in dry air increases with decreasing thickness of the sample and with rise of temperature, although these 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 nitrogeneous nonrubber components and of oxygen does not prevent the changes from 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.