Enthalpic Analysis of Vulcanization by Calorimetry. Thiuram Monosulfide/Sulfur Vulcanization of NR, BR and SBR

Abstract

Abstract As demonstrated in the foregoing discussion, the chemical complexity of accelerated sulfur vulcanization reactions prevents detailed interpretation of the enthalpy-temperature and enthalpy-time relationship. Nevertheless, the following conclusions can be reached regarding the origin of the enthalpy terms observed under scanning and isothermal temperature regimes. (1) The total enthalpy observed under scanning conditions is the sum of all reactions occurring. This includes (among others) crosslink formation, formation of non-crosslinking sulfidic products and maturation or subsequent polysulfide reactions. (2) In the isothermal experiment, the major contribution to the observed enthalpy arises from reactions resulting in the formation of sulfidic products (although some maturation must be expected and its extent will depend upon the temperature and time of cure). (3) When a precured sample is scanned, the observation of an enthalpic term depends upon the original cure system used. It is small or unobserved, for example, when the sulfur/TMTM ratio is small or with sulfur donor cure systems. On this basis, the DSC scan of a vulcanizate is potentially useful for characterizing that vulcanizate, and in particular, for the rapid quality control assessment of the state of sulfidic products in the vulcanizate. (4) Evidence has been discussed which suggests that the origin of the enthalpy term obtained with vulcanizates which have been precured can be related to the polysulfidic products in the sample. In laboratory experiments, not reported here, the magnitude of this term is reduced as the vulcanizate is heat aged or over-cured, which again supports the conclusion that we are observing the maturation of polysulfidic products. (5) Finally, comparison of rheometer and calorimeter data indicates that, as expected, we are observing all, reactions accompanying cure in the calorimeter; whereas, the rheometer responds only to crosslinking and chain scission, factors having a large effect on viscosity. DSC does not detect chain scission reactions at normal cure temperatures because the rate of scission is too small.