Cure Modification of Natural Rubber Containing Benzothiazole Accelerators by Some Modified Thiocarbamyl Sulfenamides R. N. Datta, S. C. Debnath, J. W. M. Noordermeer Rubber Technology, Faculty of Chemical Technology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands Received 26 June 2002; accepted 9 April 2003 ABSTRACT: The effects of N-cyclopentamethylene thio- carbamyl-N'-(cyclohexyl,thiocyclohexyl)sulfenamide (CPCTS) and N-oxydiethylene thiocarbamyl-N'-(cyclohexyl,thiocyclo- hexyl)sulfenamide (ODCTS) as cure modifiers on the vulcani- zation of natural rubber (NR) containing benzothiazole accel- erators were studied. CPCTS and ODCTS were used separately with 2-mercaptobenzothiazole and N-oxydiethylene benzo- thiazole sulfenamide to determine their effects on these accel- erators. CPCTS and ODCTS retarded the accelerators. En- hanced activity was found with respect to the torque, scorch, modulus, and tensile strength and was believed to be due to the formation of different intermediate components during the early stage of the vulcanization of NR. Reactions likely respon- sible for acceleration and retardation were explored. CPCTS and ODCTS provided better cure retardation than N-cyclo- hexyl thiophthalimide. The chemical characterization of the NR vulcanizates correlated with the physical properties ob- tained at 140°C. Stocks containing CPCTS or ODCTS produced mostly monosulfide linkages, which were affected when N- cyclohexyl thiophthalimide was used. This provided informa- tion concerning the heat-resistance properties of the vulcani- zates. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3835–3847, 2003 Key words: natural rubber; thiocarbamyl sulfenamide; high performance liquid chromatography; cure modification INTRODUCTION Scorch safety is a requirement for the processing of a rubber compound. The scorch behavior of 2-mercapto- benzothiazole (MBT) can be moderated by the chemical transformation of the compound into N-substituted 2-benzothiazole sulfenamide, which has been recog- nized as an accelerator with inhibitor activity. 1–3 In 1968, N-cyclohexyl thiophthalimide (CTP) emerged as an ef- fective retarder that could delay the process of the vul- canization of natural rubber (NR). 4 Other structurally related compounds containing bivalent sulfur linked to nitrogen have been reported to be effective retarders. 5–9 Benzothiazole sulfenamides activated by thiuram compounds usually result in a higher cure rate and modulus but less processing safety, which can be re- tained by the incorporation of CTP. 10 Unfortunately, CTP generates phthalimide, which sometimes causes blooming on the vulcanizates. In an earlier study by Das et al., 11 it was shown that phthalimide reduced the modulus and tensile strength of NR vulcanizates during aging. In this investigation, an attempt was made to formulate a compound that would produce thiuram accelerators during vulcanization and could scavenge MBT, thus delaying the process of vulcani- zation. In the structure of CTP, a cycloalkylthio moiety is attached to nitrogen to incorporate an SON bond that is reactive toward accelerators, especially MBT. 4 The SON bond can be introduced into a suitable thiocarbamyl sulfenamide, the curing efficiency of which is similar to that of thiuram ultra-accelerators, through the ONH functionality; as a result, the retard- ing and accelerating activity of the resultant com- pound can be observed. A binary combination of this compound and a benzothiazole accelerator gives an optimum balance of the scorch delay and cure rate and no undesirable side effects. N-Cyclopentamethylene thiocarbamyl-N'-cyclohexyl sulfenamide (CPCS) and N-oxydiethylene thiocarbamyl-N'-cyclohexyl sulfen- amide (ODCS) were selected as representative thiocar- bamyl sulfenamide accelerators that were chemically modified as shown in Scheme 1. To determine the dual activity of some other mod- ified thiocarbamyl sulfenamides accelerated by diben- zothiazyl disulfide, some similar studies were made previously. 12 In this investigation, the effects of N- cyclopentamethylene thiocarbamyl-N'-(cyclohexyl,- thiocyclohexyl)sulfenamide (CPCTS) and N-oxydieth- ylene thiocarbamyl-N'-(cyclohexyl,thiocyclohexyl)- sulfenamide (ODCTS) on thiazole accelerators were studied. The thermal and age-resistant properties of the vulcanizates obtained from these systems were included in this study. Also, the various reactions Correspondence to: R. N. Datta (r.n.datta@ct.utwente.nl). Journal of Applied Polymer Science, Vol. 90, 3835–3847 (2003) © 2003 Wiley Periodicals, Inc.