Abstract
The classification of various mixtures of natural rubber and of synthetic rubber and of synthetic rubbers with respect to heat build-up under dynamic stress depends on the experimental conditions under which the vulcanizates are tested. In many cases a reversal of the order of the mixtures is found when the latter are compared at constant deformation instead of at constant periodic stress. This is true also of flexing to the point of destruction of various types of vulcanizates. The dependence of the dynamic properties on the temperature and on the stress is characteristic of the type of vulcanizate. Hence, any classification of various mixtures depends not only on the character, but also on the magnitude of the stress. These differences were found within the temperature range encountered in practice, as in tires. Consequently, no definite conclusions about the temperature rise in the range of heat build-up extending to the point of destruction can be drawn from measurements of the dynamic properties at room temperature. Changes of the dynamic properties can also take place during dynamic stressing without any considerable rise of temperature. In general, the temperature rise in the static state and the flex life are related in the sense that the higher the temperature, the shorter is the flex life, if the preceding facts are taken into consideration. However, the experiments on the temperatures of destruction show that the flex life of different vulcanizates differs even when the rise of temperature is the same. For a flex life of 100 minutes, natural-rubber vulcanizates containing active carbon blacks were destroyed at higher temperatures (170°–185° C) than was the base mixture containing no filler (about 160° C). The synthetic-rubber mixtures showed greater heat build-up (190°–220° C). These differences are found with different, as well as with equal, mechanical stress on the vulcanizate. The nature of the destruction for a given stress depends to a considerable degree on the structure of the vulcanizate. When flexed to the point of destruction, natural-rubber vulcanizates containing zinc oxide or SRF carbon blacks, and all synthetic-rubber vulcanizates, crack from the core outward, without showing any extensive thermal decomposition. On the other hand, natural-rubber vulcanizates containing active carbon blacks or Aerosil showed definite evidence of heat decomposition in the center, i.e., stickiness and porosity. This distinction in the destruction patterns gives the impression that the cause of destruction in the first case is chiefly a mechanical attack, whereas in the latter, it is chiefly thermal decomposition. It must be concluded from all the experimental results that several chemical reactions of destruction, having different temperature coefficients, proceed simultaneously, and that direct mechanical attack by the external forces is, to a greater or less extent, added to the heat effect.
Mullins Effect and Crack Growth in Natural Rubber Vulcanizates during Heat Aging and Cyclic Loading