AGEING OF CARBON BLACK FILLED NATURAL RUBBERS AT MODERATE TEMPERATURE: fillerS’INFLUENCE and Tensile properties

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ABSTRACT Carbon Black filled Natural Rubber, with a “conventional” vulcanization, are aged by thermo-oxidation, at 77°C up to 21 days. With or without the presence of fillers, the evolution of the polymer matrix with ageing is predominantly driven by cross-linking mechanism. A rigorous study of the swelling ratio of aged materials in solvent, using various literature models, strongly suggests that the Elastically Active Chain (EAC) density is nevertheless slightly higher in the filled materials (50 phr). It is confirmed by the study of their secant moduli. This may correspond, for 50 phr filler content, to roughly a twofold acceleration of the ageing kinetics. Furthermore, the increase of the EAC density caused by ageing reduces the material's ability to crystallize at low temperature or under strain (both phenomena being strongy correlated). A plot of the local stretch at break λ break,local (corrected using Nielsen's formula) as a function of the EAC density (estimated from swelling) yields, for filled materials, a figure very similar to that obtained with unfilled material, with two regimes: for high EAC density, well above the one created by entanglements, and for which strain induced crystallization is not possible, λ break,local follows a power law in -0.72 of the EAC density. For lower EAC densities, a sufficient fraction of long EAC remains to allow strain induced crystallization, which ultimately improves elongation at break.