This work has reviewed and summarized the present understanding of the fundamental science and chemistry of rubber vulcanization. The future of this area holds much potential. Recent studies have begun to fully elucidate the accelerator and accelerator complex chemistry, and definitively answer questions that have been open for debate for many years. Additionally, the understanding of the network structure is rapidly progressing through advances in analytical techniques. The largest area of future research will be the use of multiple experimental techniques to fully elucidate and understand the relationship between accelerator chemistry and the formation of specific network structures. With this knowledge, it will then be possible to molecularly engineer vulcanizates with specific properties.
Results of previously published work involving network analysis and accelerator intermediate analysis of cis-polyisoprene were compared to help correlate and rationalize network/chemistry relationships. Both classical chemical probe network analysis and further NMR measurements (DEPT analysis) were used as necessary to verify NMR peak assignments. Furthermore, samples of the conventional formulation were vulcanized in the absence of zinc oxide and stearic acid to help elucidate the role of zinc in the vulcanization process. Polysulfidic dibenzothiazole accelerator intermediates were proposed to produce allylic substituted cis-polysulfides both with and without double bond migration, while zinc polysulfidic dibenzothiazole accelerator intermediates were proposed to produce allylic cis and trans polysulfides substituted structures with no double bond migration. Polysulfidic substitution on the isoprene methyl carbon was shown to result from exchange reactions during network maturation. The network-chemistry relationships were compared with earlier mechanistic studies and several points of agreement were noted.
NMR imaging is a useful technique for studying the physical and spatial microstructure of cured elastomers. Different swelling agents can be used as chemical probes to detect varying amounts of microstructural differences. Imaging can be used to detect highly cured regions due to aging, poor mixing, and thermal gradients. NMRI is thus useful to study spatial distribution of crosslinks and is sensitive to changes in this distribution of crosslinks due to thermal gradients and the effects of aging and reversion processes. It can also be used to observed entrapped air in air-aged samples. Spin-lattice T1, relaxation times for solvent in cured elastomers have been shown to be shorter than the bulk solvent T1 values, providing a new method for determining the crosslink density. NMRI results have suggested that cure reversion and postcuring processes produce similar spatial results.
Spin-spin relaxation (T2) measurements were used to analyze the motional behavior of accelerated-sulfur vulcanized cis-polyisoprene. Biexponential decay behavior was found in all samples studied. This was modeled to consist of T2s (short) and T2l (long) components, with the T2s due to crosslinked regions and T2l due to regions distant from nodal points. T2 behavior was shown to depend strongly on crosslink density, but not to vary with concentration of cyclic structures. A simplified model was used for calculation of the average correlation time.
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