N,N '-pentamethylenethiuram disulfide- and N,N '-pentamethylenethiuram hexasulfide-accelerated sulfur vulcanization. I. Interaction of curatives in the absence of rubber

N,N'-pentamethylenethiuram disulfide (CPTD), CPTD/sulfur, and N,N'-pentamet hylenethiuram hexasulfide (CPTP6) were heated in a DSC at a programmed heat ing rate and isothermally at 140 degrees C. Residual reactants and reaction products were analyzed by HPLC at various temperatures or reaction times. CPTD rapidly formed N,N'-pentamethylenethiuram monosulfide (CPTM) and N,N'- pentamethylenethiuram polysulfides (CPTP) of different sulfur rank, CPTP of higher sulfur rank forming sequentially, as reported earlier for tetrameth ylthiuram disulfide (TMTD). As with TMTD, the high concentration of the acc elerator monosulfide that develops is attributed to an exchange between CPT D and sulfenyl radicals, produced on homolysis of CPTD. However, a differen t mechanism for CPTP formation to that suggested for TMTD is proposed. It i s suggested that disulfenyl radicals, resulting from CPTM formation, exchan ge with CPTD and/or CPTP already formed, to give CPTP of higher sulfur rank . CPTD/sulfur and CPTP6 very rapidly form a similar product spectrum with C PTP of sulfur rank 1-14 being detectable. Unlike with TMTD/sulfur, polysulf ides of high sulfur rank did not form sequentially when sulfur was present, CPTP of all sulfur rank being detected after 30 s. It is proposed that sul fur adds directly to thiuram sulfenyl radicals. Recombination with sulfenyl radicals, which would be the most plentiful in the system, would result in highly sulfurated unstable CPTP. CPTP of higher sulfur rank are less stabl e than are disulfides as persulfenyl radicals are stabilized by cyclization , and the rapid random dissociation of the highly sulfurated CPTP, followed by the rapid random recombination of the radicals, would result in the obs erved product spectrum. CPTP is thermally less stable than is TMTD and at 1 40 degrees C decomposed rapidly to N,N'-pentamethylenethiourea (CPTU), sulf ur, and CS2. At 120 degrees C, little degradation was observed. The zinc co mplex, zinc bis(pentamethylenedithiocarbamate), did not form at vulcanizati on temperatures, although limited formation was observed above 170 degrees C. ZnO inhibits degradation of CPTD to CPTU. (C) 2000 John Wiley & Sons, In c.