1. The character of the dependence of the time to first cracking (τ0), of the rate of crack growth in the steady-rate portion (v) and of the time to rupture (τ1) upon deformation does not alter on altering the ozone concentration from 0.15 to 0.0002%. 2. The value of the critical deformation does not alter with alteration in the ozone concentration. 3. The process of destruction of deformed rubber in ozone proceeds kinetically, i.e., it does not depend upon the rate of supply of ozone to the surface of the specimen. The character of the dependences τ0 and τ1 and of the rate of crack growth (vgrowth) upon the ozone concentration is determined by equations of the type of the Freundlich adsorption equation. 4. In the determination of the relative resistance of various vulcanized rubbers to ozone cracking it is desirable to select as an index the value of the ozone concentration at which we get a time to rupture equal for all vulcanized rubbers. 5. The relative resistance of the different vulcanized rubbers to ozone cracking (in assessment by the time to rupture) depends upon the ozone concentration at which it is determined. It is advisable to determine the resistance to cracking at the critical deformation of the vulcanized rubber. 6. The order of resistance to ozone of specimens of one rubber deformed by differing amounts does not depend upon their concentration in the range of concentrations under investigation and usually employed. However, if in the range of high concentrations of ozone the times to rupture of the specimens have values close to each other, then in the range of low concentrations of ozone these values may differ markedly.
It is known from the literature that there exists a socalled critical elongation at which disruption of the structure of rubbers under the influence of ozone is most severe. However, the available data concerning this problem are fairly contradictory. According to a number of statements the critical elongation is observed in the case of vulcanizates of natural rubber, but its estimation by different authors varies from 5 to 50%. Some authors consider that a critical elongation exists in the case of synthetic rubbers susceptible to attack by ozone, while others consider that no such characteristic exists. It is said that polychloroprene and butyl rubber do not possess this characteristic. However, none of these data can be regarded as reliable since in most cases ozone cracking of the rubbers was characterized by arbitrary methods, as a rule by the “degree of cracking” expressed by the number of marks. We have carried out a detailed investigation of the effect of the degree of elongation on ozone cracking of rubbers, the rate of growth of cracks being determined by an objective method based on the effective depth of the cracks calculated from the decrease of stress in the relaxed rubber sample when exposed to the action of ozone. The following rubbers—NK, SKS, neurite, SKN, and SKB were investigated in standard formulas, at optimum true tensile strength. Gutta-percha (elastic vulcanizate) and butyl rubber compositions in phr were: gutta-percha 100, MBT 0.8, sulfur 5; butyl rubber 100, stearine 3, MBT 0.65, thiuram disulfide 1.3, zinc oxide 5, sulfur 2.
1. The rate of reaction of an undeformed rubber stock with a reactive medium is determined by the diffusion, while the rate of destruction of a deformed stock is determined by its chemical interaction with the medium. 2. The magnitude of the apparent activation energy (u) of the rupture of a stock in the presence of a reactive medium does not undergo perceptible alteration in the range of deformations 30 to 80%, while on transition to deformations of 500 to 700% the values of the apparent activation energy increase. 3. The temperature coefficient of rupture depends upon the nature of the bonds being destroyed and upon the capacity of the reactive medium for adsorption on the stock. On rupture in the gaseous phase the apparent activation energy has a lower value than on rupture in a solution of the same agent. 4. In a wide range of deformations the time to rupture of stocks obeys a complex system of laws, passing through a minimum value in the region of the critical deformation, εcrit. The position of εcrit depends upon the temperature, type of ‘aggressor’ (i.e. reactive agent) and physical state of the medium (gas, solution). 5. As a result of the shift of εcrit with alteration in temperature there is possible the phenomenon of anomalous dependences for certain deformations, i.e., the time to rupture at low temperatures are less than for higher temperatures with identical deformations and concentrations of the medium.
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