Oven and Bomb Aging of GR-S at Corresponding Temperatures

The relations observed in this comparison of air oven and oxygen-bomb aging of GR-S at corresponding temperatures and the interpretations suggested by these relations may be summarized as follows: 1. Comparison of the results of tests in which some samples were aged in the form of tensile test strips while others were cut from rectangular strips after aging indicates that edge effects during aging are negligible and that the former practice is satisfactory. 2. Increased temperature increases the severity of both types of aging, but the effect is greater at the higher oxygen concentration of the bomb. 3. The hardening reaction, as measured by the increase in modulus, appears to be little affected by increased oxygen concentration above that in air and is the predominant reaction in the air oven. 4. Chain scission, as measured by decrease in tensile strength, increases with oxygen concentration and is the predominant reaction in the oxygen bomb. 5. Chain scission is also reflected in the smaller modulus increase observed with oxygen bomb aging compared to the air oven, and is responsible for the reversion of the modulus observed at 100° C and 300 pounds per square inch oxygen pressure. 6. Changes in temperature and in oxygen concentration, therefore, alter the ratio of the fundamental aging reactions of chain scission and cross-linking. Since the effect of these two reactions on physical properties is different, no direct correlation can be expected between methods of aging which differ in both temperature and oxygen concentration. 7. Activation energies calculated from the temperature coefficients observed for tensile breakdown in the oxygen bomb (29.7 kg.-cal.) and for modulus increase in the air oven (18.2 kg.-cal.) may be regarded as approximations of the activation energies required for chain scission and cross-linking, respectively. The observation has been made that the latter reaction is virtually independent of oxygen concentration above that in air, whereas the former increases with oxygen concentration. This may be interpreted as indicating that, at the temperatures involved, a relatively large number of oxygen molecules are sufficiently activated to maintain the lower energy reaction, even in air, while a much smaller number have the energy required for the scission reaction; consequently, increased oxygen concentration will be more effective in making a larger number of active molecules available for reaction in this case. 8. The greater dependence of chain scission on oxygen concentration may also be regarded as an indication that, although both reactions are apparently initiated by oxygen, the subsequent reactions leading to chain scission may require additional oxygen while the reactions resulting in cross-linking may not.