Effect of rubber mastication and ageing on gel content, rheological properties and vulcanisation behavior of natural rubber

Natural rubber (NR) contains a soluble (sol) phase and an insoluble (gel) phase, the content of which affects processing and mechanical properties. This work describes the effect of mechanical and thermal ageing on the gel content and properties of natural rubber. The samples were subjected to ageing by mastication on a two-roll mill, in an internal mixer, and by heating in an air oven. The gel content was determined using ISO 17278 and a filtration method and related to analytical, rheological and mechanical data. The gel content increased with ageing time but declined with increasing mastication time due to greater disruption of polyisoprene chains. Two-roll milling caused greater degradation than internal mixing due to oxygen access, whereas ageing in an air oven initially promoted crosslinking followed by chain scission. The gel content correlated with Mooney viscosity and green strength, but had only a minor effect on the mechanical properties of the vulcanised rubber.