Controlled degradation of natural rubber by microparticles containning titanium dioxide

Photooxidative degradation of natural rubber by of titanium dioxide (TiO2) under accelerated ultraviolet light and sunlight was investigated. Degradation of unvulcanized natural rubber both in solution phase and solid phase was determined by following number-average molecular weight using gel permeation chromatography. The degradation of vulcanized natural rubber was determined as a function of mechanical properties. Extent of molecular weight reduction of natural rubber in solution phase was increased when the amount of TiO2 was increased. In contrast, the degradation of natural rubber in solid phase (both as vulcanized and unvulcanized form), was inversely propotional to the amount of TiO2 incorporated indicating that TiO2 plays a major role as a stabilizer instead of a photocatalytic agent. The agglomeration of TiO2 is believed to account for its poor photocatalytic activity. As demonstrated by SEM analysis, TiO2 filled in solid natural rubber tended to aggregate into particles having the diameter of 1-2 µm which are large enough to reflect UV light. The appearance of fracture all over the surface of natural rubber sheets despite the presence of TiO2 after UV exposure suggested that the degradation of natural rubber in solid phase did not predominantly occur at the interface between TiO2 and natural rubber. According to viscosity-average molecular weight data, controlled degradation of unvulcanized natural rubber cannot be achieved by TiO2 microparticles. An incorporation of TiO2 microparticles insignificantly increase the extent of degradation instead of delaying the degradation as originally expected.