Photooxidative degradation of natural rubber by titanium dioxide

Photooxidative degradation of natural rubber by 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. While 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 titanium dioxide was increased. In contrast, the degradation of natural rubber in solid phase (both as vulcanized and unvulcanized form), was inversely proportional to the amount of TiO₂ incorporated. This indicates that TiO₂ plays a major role as a stabilizer instead of a photocatalytic agent. 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 degaradation of natural rubber in solid phase did not predominantly occur at the interface between TiO₂ and natural rubber.