M. Shumane et al., Dimethylammonium dimethyldithiocarbamate-accelerated sulfur vulcanization.II. Vulcanization of rubbers and model compound 2,3-dimethyl-2-butene, J APPL POLY, 82(12), 2001, pp. 3074-3083
Rubber and model compound 2,3-dimethyl-2-butene were vulcanized for various
times with dimethylammonium. dimethyldithiocarbamate [(dma)dmte]-accelerat
ed sulfur formulations in the absence of ZnO. Model compound systems were a
nalyzed by HPLC, and no reaction intermediates containing pendent groups we
re found. Crosslinked sulfides, characterized by H-1-NMR, were found to be
essentially bis(alkenyl). Residual curatives were extracted from rubber com
pounds vulcanized for various times and analyzed by HPLC. Compounds, cured
to various crosslink densities, were found to crystallize readily in a dens
ity column at subambient temperatures. This supports evidence from model co
mpound systems that pendent groups are largely absent from vulcanizates. It
is suggested that a reaction mechanism, similar to that applicable to zinc
dimethyldithiocarbamate-accelerated sulfur vulcanization, may be applicabl
e with (dma)dmte accelerated formulations. Very limited crosslinking occurr
ed on heating compounds under vacuum, and this can be attributed largely to
the rapid loss of (dma)dmtc from rubber at elevated temperatures. However,
the slower rate of crystallization on cooling of the gels, compared to the
rate in press-cured vulcanizates of similar crosslink density, was interpr
eted as evidence that some pendent groups did form during heating with (dma
)dmtc/sulfur. Crosslinking of such pendent groups may be inhibited by the l
oss of (dma)dmtc, that, like zinc dimethyldithiocarbamate, may catalyze the
ir crosslinking, and/or to the loss under vacuum of dimethyldithiocarbamic
acid that would form thiol pendent groups that would rapidly crosslink with
thiuram pendent groups. (C) 2001 John Wiley & Sons, Inc.