The role of zinc in the vulcanisation of styrene-butadiene rubbers

There is pressure on the rubber industry to reduce, or even remove, the zinc oxide used in rubber compounds, because of concerns that zinc chemicals may have adverse effects on the aquatic environment. An EUsupported project, entitled Ecozinc, was set up to determine the scope for reducing, or eliminating, the zinc chemicals, predominantly zinc oxide, used in a wide range of rubber products. This presentation focuses on the project's findings that relate to the sulphur vulcanisation of different rubbers, in particular styrene-butadiene rubbers. Several studies within the project have indicated that the zinc oxide content in sulphur-vulcanised compounds of NR, EPDM and emulsion SBR can typically be reduced to about 2 phr. However, in the case of solution SBR and solution SBR/BR blends, much larger reductions of zinc oxide level, from 3 phrdownto 0.5 phrorless, appear to be feasible. Parallel studies indicated that the crosslinking chemistry in the high-vinyl solution SBR is quite different from that found in NR, with negligiblemonosulphidic crosslinking and much carbon-carbon crosslinking. It is postulated that, in vulcanisation of these SBRs, incursion of free-radical processes occurs with less involvement of zinc-accelerator species, and that the carbon-carbon crosslinks are forming between vinyl groups.