Reinforcing effects of precipitated silicas on properties of some vulcanizates of styrene-butadiene rubber

The tear strengths of some conventional accelerated-sulphur vulcanizates of styrene-butadiene rubber (SBR) with a sulphur to accelerator ratio of similar to1.8, containing no filler, 10, 30, or 50 parts per hundred rubber by weight (pphr) precipitated amorphous white silica filler type VN3 were measured. The tear tests were performed at different rates ranging from 0.017 to 8.33 mm/s, at an angle of 180 degrees and at an ambient temperature (similar to 23 degreesC). The tear strengths of the vulcanizates improved progressively and quite substantially with silica; a trend which continued strongly when the amount of silica in the formulation was increased to 50 pphr, but remained mostly unaffected by changes in the rate of tear. For instance, the tearing energy rose by almost two orders of magnitude, from similar to1.5 to similar to 100 kJ/m(2) at a tear rate of 0.017 mm/s, when 50 pphr silica was loaded into the rubber. Likewise, the hardness of the vulcanizates, which was measured at 24.5 degreesC and a 15 s instant, was enhanced significantly from similar to 35 to similar to 65 Shore A, when silica was added and then increased progressively to its full amount in the rubbers.