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The tensile strength of test pieces made from natural rubber vulcanizates drops abruptly at a critical temperature which can vary from 40 to 130°C. This variation in critical temperature is shown here to be a result of the variation in critical cut length with temperature. When the naturally occurring flaws in the test piece are smaller than the critical cut length, high tensile strength values occur, but when the flaws are longer than the critical cut length, low tensile strength values occur. The critical cut length decreases as the temperature increases, and the abrupt drop in tensile strength occurs as the critical cut length reaches the natural flaw size in the test piece. The natural flaw size in tensile test pieces depends on the sharpness of the cutter, and for tensile strength measurements at elevated temperature, it is shown that even a slightly blunt cutter may give markedly different results from a sharp one.

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synopsisThe oxidation characteristics of protected and unprotected vulcanizates from natural rubber have been examined. With unpurified rubber (RSS1) in the presence or absenee of added antioxidants the rate of oxidation of the VulCSnizates is: rmaccelerakd sulfur > accelerated sulfur > TMTD sulfurless or EV > peroxide. This wdez is obtained because the sulfur compounds present in the sulfurated vulcanizates are antagonistic towards both natural and added antioxidants. The antagonism is greatest with polysulfides, and when they are present they dominate the aging behavior of vulcanizates.

synopsisThe effect of high strain on the oxidative stress relaxation of several natural rubber vulcanizates has been investigated. In the case of peroxide and CBS accelerated vulcanizates, the rate of stress relaxation increases with increasing strain, and this increase appears to be due to an increase in the rate of oxidation of the network. TMTD and MBT vulcanizates showed marked premature failure at high strains and no oxidative stress relaxation measurement could be made. It was found that the tensile strength of a TMTD vulcanizate was a t least 20% higher in vacuum than in oxygen, due, it is believed, to stress-induced oxidative degradation a t the tip of surface flaws in the rubber.When a piece of rubber containing a small cut is repeatedly strained, the cut will not grow unless the strain energy exceeds a certain value. It has been observed' that this critical strain energy is dependent on oxygen pressure, being higher at low pressures. This suggests that some of the bonds in the rubber molecule are weakened in the presence of oxygen, in spite of observations to the contrary2-'j and that the rate of oxidative scission of the rubber chains might be stress-dependent.Stress dependence of chemical reactions is well known in metal corrosion,' in ozone attack on rubber; and in mastication-induced polymerizations in rubber.s This investigation examines the stress dependence of the oxidative breakdown of vulcanized rubbers by stress relaxation, since this is a very sensitive measure of network destruction. EXPERIMENTALIn order to undertake stress-relaxation studies at comparatively high strains up to 500%, a completely new type of apparatus has been developed. This is to be described in detail elsewhere, but briefly the sample, in the form of a ring cut from 0.3-mm. thick sheet, is stretched between polytetrafluoroethylene pulleys by means of a small induction motor and the stress on the strip measured by a proof ring and transducer. The whole apparatus can be evacuated, and is immersed in a silicone oil bath thermostat. 3101