The dependence of the physical properties of sulfur vulcanizates of diene rubbers on network structure is reviewed and the influence of degree of crosslinking, crosslink structure, and main-chain modification are discussed. In polyisoprenes, these are determined, in practice, by the balance between three competing types of reaction: the conversion of polysulfidic pendent groups into polysulfidic crosslinks; the desulfuration of polysulfidic pendent groups and crosslinks, eventually to the corresponding monosulfides, with recirculation of the removed sulfur into the crosslinking pathway; thermal decomposition of di- and polysulfidic pendent groups and crosslinks with the formation of cyclic sulfide, conjugated diene and triene, and cis, trans-isomerized groups in the rubber chains. At temperatures above about 160°C, the thermal breakdown of monosulfide crosslinks and pendent groups has to be considered. Zinc accelerator-thiolate complexes play a central role in controlling the balance between the various reactions because they promote the primary sulfuration of the rubber to form polysulfidic pendent groups and the conversion of these to crosslinks; they are the agents which desulfurate both pendent groups and crosslinks; they catalyze polysulfide exchange reactions; and in some cases they promote the decomposition of crosslinks. Other factors which affect the balance between these reactions are the temperature and the structure of the main rubber chain in the immediate vicinity of the crosslink. The latter is, in turn, at least partly controlled by the structure and concentration of zinc accelerator-thiolate complexes. The vulcanization of polybutadiene rubbers deviates substantially from this reaction pattern, evidently because the bulk of the accelerator becomes irreversibly bound to the rubber at an early stage. This denudes the vulcanizing system of zinc accelerator-thiolate complexes and, therefore, prevents desulfuration from occurring. The crosslinks thus remain di- and polysulfidic and are apparently less prone to decomposition than in the case of the polyisoprene rubbers, since cyclic sulfides and conjugated hydrocarbon groupings seem not to be prominent products. The removal of accelerator leaves the system unresponsive to zinc and gives it the characteristics of unaccelerated vulcanization with the result that vicinal crosslinking and crosslinks with saturated chain junctions become important products.
The degree to which HAF black restricts the swelling of natural rubber vulcanizates in n-decane has been determined using a vulcanizing system in which the stoichiometry of crosslinking is unaffected by the carbon black. The dependence of the degree of restriction, as measured by the ratio of the volume fractions of rubber in the filled and unfilled vulcanizates swollen to equilibrium, on the concentration of carbon black follows an exponential relationship previously proposed by Lorenz and Parks. This is found to be equivalent to a simple linear relationship between the apparent and actual crosslink concentrations: napparent/nactual=1+Kϕ, where K is a constant characteristic of the filler and φ is its volume fraction in the vulcanizate. The relation has been used to determine actual crosslink concentrations in filled natural rubber vulcanizates. HAF black is found to cause increases of up to 25 per cent in the yield of polymer to polymer crosslinks in conventional sulfur vulcanizing systems, accompanied by changes in rate of cure and of crosslink reversion. All these are small compared with the effect of the filler on many physical properties.
Earlier Parts of this series reported the partial separation and identification of the complex products of certain sulfur-olefin and related reactions chosen as models for examining the mechanism of vulcanization of natural and synthetic rubbers by sulfur. The main conclusion was that sulfur-olefin interaction proceeds by free-radical chain processes, and this has been quoted elsewhere and widely accepted. Results are now presented which extend and correct the earlier findings and lead to the different mechanistic conclusion that the reactions are polar in type, involving heterolysis of S—S and C—H bonds. This paper and Part XI deal with the products of the reactions of sulfur with mono-olefins and a 1,5-diolefin, respectively, and Part X presents a complementary kinetic investigation, the first for such reactions. All the experimental evidence is collated in Part XI in a general theory of sulfur-olefin interaction.
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