Effect of thermal oxidative ageing on the morphology of natural rubber networks as viewed by ESR

The change of network morphology of unfilled sulfur crosslinked natural rub ber exposed to thermal oxidative ageing has been studied. Three model netwo rks with different sulfur contents (efficient network, semi-efficient and c onventional vulcanizate) have been analysed by applying spin probe ESR, equ ilibrium swelling measurements and mechanical measurements. By combining th e density distribution deduced from the complex ESR spectra and the equilib rium swelling density, it is shown that the major network changes occur in the networks with predominantly polysulfidic bonds (conventional network) a fter 96 h of ageing at 373 K. The effect of ageing on the network with pred ominantly monosulfidic bonds is much less pronounced. Both effects (main ch ain scission and additional local crosslinking) have been analysed in the c ourse of prolonged ageing up to 240 h. A correlation between network struct ure and its mechanical performance is established. (C) 2000 Society of Chem ical Industry.