The previous paper in this series on the tensile failure properties of rubber vulcanizates, dealt with styrene-butadiene copolymer. In this paper, the tensile failure properties of isoprene-butadiene, styreneisoprene and methyl methacrylate-isoprene copolymer vulcanizates were studied. As the result, the following conclusions were obtained.
1) The fracture of the vulcanizates of these copolymers compounded with carbon black is attributed to a scission of rubber main chains, but not to a scission of the bonds between the rubber molecules and the surface of carbon black.
2) The temperature which affords the ultimate maximum elongation depends on the transition temperature Tg, even if the rubbers used are not exactly the same in the number average molecular weights, the network chain densities and in the combined sulfur structures.
3) The ultimate elongation at the reduced temperature is little affected by the copolymer composition in such a case where the two homopolymers do not differ markedly in Tg, as isoprene-butadiene system. On the other hand, the elongation decreases with an increase in the component in when one component in homopolymer is markedly higher than the other in Tg. For example, the elongation of styrene-isoprene copolymer decreases with an increasing content of styrene.

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The effects of the continuous phases of general purpose polystyrene resins upon the physical properties of the vulcanizates were studied using the polybutadiene rubber-general purpose polystyrene resin blend. The unvulcanized samples were prepared according to the method already mentioned in our former report, and the radiation cure was adopted so that the phase change during cure might be avoided.
Some physical properties, hardness, modulus, elongation, permanent set and resilience, are much influenced by the existence of the continuous phases even if the polymer blend ratios are kept constant, whereas others, specific gravity and tensile strength are mainly determined by the polymer blend ratios.
The physical properties which belong to the former group are found to be controlled more strongly by the polymer which forms the continuous phase.

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Various cured ethylene propylene rubbers having different chemical structures of crosslinking point were prepared using various curing agents and the relationships between the physical properties (tensile strength, elongation, tear resistance, modulus, and hardness) and the chemical structure of crosslinking point were studied.
1) Modulus and hardness depended on the crosslinking density only and did not depend on the chemical structure of crosslinking point 2. Tensile strength, elongation and tear resistance depended not only on the crosslinking density but also on the chemical structure of crosslinking point.

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The relationships between various chemical structures of crosslinking point and physical properties (tension set, compression set and dynamic properties) were studied on ethylene propylene rubber.
The compared chemical structures of crosslinking point were C-C linkage, C-Sx-C linkge, C-R-C linkage etc.
From the results, it was found that :
1. Tension set and compression set were influenced not only by crosslinking density but also the chemical structure of crosslinking point under the condition of determination which changed the chemical structure of crosslinking point.
2. Dynamic viscoelasticity and impact resilience seemed to be influenced by crosslinking density and the chemical structure of crosslinking point, but the effects of chemical structure of crosslinking point were recognized only in impact resilience test.

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[in Japanese]

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