The influence of cure and carbon black on the high temperature oxidation of natural rubber - I. Correlation of physico-chemical changes

The thermal oxidative stability of natural rubber (NR) has been examined at 150 degrees C, as a function of cure and carbon black (CB) properties. The progress of oxidation was followed by iodometric analysis of hydroperoxide levels, Fourier-transform infra-red spectroscopy, and thermal analysis. As expected the nature of the cure system had an influence on the degradation profile, but more significantly the surface reactivity and structure of the CB played a controlling role within each system. Changes in infra-red absorptions at 830 cm(-1) suggested that the cis-1,4-vinyl content of the polymer was reduced during degradation, in-line with the formation/destruction of hydroperoxides. An infra-red absorption at 1090 cm(-1) was assigned to cyclic/aliphatic ethers. For the majority of samples this band decreased during degradation, at a rate controlled by the CB (faster for samples of high surface area and structure). Differential thermal analysis further supported this premise, with samples containing carbon blacks of high surface reactivity and larger particle size showing a much-reduced mass loss in comparison to their smaller particle size, higher structure counterparts. In these higher surface area fillers there was evidence that reduction of the band at 1090 cm(-1) was accompanied by the development of a band at 1020 cm(-1), due to aromatic ether, i.e. scission and reformation of links between the rubber and filler were taking place during degradation. (C) 1999 Elsevier Science Ltd. All rights reserved.