The Migration of Extender Oil in Natural and Synthetic Rubber. IV. Effect of Saturates Geometry and Carbon Black Type on Diffusion Rates

Migration of oils, curatives, antioxidants, and other compounding materials in a vulcanized rubber matrix is a general phenomenon. A continuing, long range program has been undertaken in these laboratories to understand better this effect, in order that ultimately the compounder can predict, from a knowledge of the molecular nature of the penetrant and the physico-chemical properties of the cured matrix, the distribution of the various components during the service life of the finished rubber product. A sensitive radiotracer approach with earbon—14 is being used to study this system. Earlier work in this program has established the general value of the diffusion coefficient for whole paraffinic oils and for aromatic oils and their fractions. In general, this has shown that moderate variations in the molecular composition of the aromatic portions of the oils have only minor effects on these migration rates. Using similar computer derived diffusion coefficients, the current data indicate that naphthenic molecules migrate at equal to slightly higher rates than aromatic molecules of similar boiling points. Thermal diffusion as a mode of separation of the oil gives fractions showing more selectivity (larger differences in migration rates) than the formerly used silica gel procedures. Variations in carbon black type and loading levels have no detectable effect on migration. The most important factor in diffusion is the polymer matrix, which for the oils studied is in the order : polybutadiene (D≃6.4×10−7 cm2 sec−1 at 100° C), natural rubber (D≃3.5×10−7 cm2 sec−1), ethylene-propylene-diene rubber (D≃2.6×10−7 cm2 sec−1) and styrene—butadiene rubber (D≃1.9×10−7 cm2 sec−1). Activation energies for the diffusion process were PBR, 3.1 ; NR, 7.8; EPDM, 10.3; and SBR, 9.9 (energies in kilocalories per mole).