Content area
The ability to delay the onset of vulcanization to allow sufficient time for processing is an important factor in formulations. The most common compounds used for improving processing safety are the prevulcanization inhibitors (PVI). A PVI is used to increase the induction time before crosslinking, while still retaining the good rate of cure of the accelerator system. A number of PVIs are commercially available, the most commonly used one being N-(cyclohexylthio)phthalimide (CTP).
Full text
ABSTRACT
Prevulcanization inhibitors (PVI) provide an important delay in the curing reactions that occur in accelerated sulfur vulcanization. In this study, the reactions of N-(cyclohexylthio)phthalimide (CTP) with other curatives (excluding zinc oxide) were investigated in the presence and absence of rubber. The CTP has been shown to react very readily with 2mercaptobenzothiazole (MBT) and this reaction has previously been credited with the delay observed. However, recent studies have shown that MBT is not formed until crosslinking has started. In a 2-bisbenzothiazole-2,2'-disulfide (MBTS) accelerated sulfur cure, CTP was shown not to delay the onset of crosslinking, but once crosslinking began, with the concurrent formation of MBT, the CTP reacted with the MBT inhibiting further polysulfide formation, pendent group formation and thus crosslinking. Crosslinking continues after the CTP has been consumed.
INTRODUCTION
The ability to delay the onset of vulcanization to allow sufficient time for processing is an important factor in formulations. The most common compounds used for improving processing safety are the prevulcanization inhibitors (PVI). A PVI is used to increase the induction time before crosslinking, while still retaining the good rate of cure of the accelerator system. A number of PVIs are commercially available, the most commonly used one being N-(cyclohexylthio)phthalimide (CTP). The importance of the S-N bond in the inhibitor molecule has been described in detail by Sullivan, Morita and Leib.1 They found that they could increase the reactivity of the prevulcanization inhibitor by increasing the number of leaving groups, i.e. having more S-N groups in the molecule. Thus hexa(cyclohexylthio)melamine (6 S-N groups) gave a higher reactivity per phr (parts per hundred rubber) than CTP (1 S-N group). This was ascribed to the lower equivalent weight (mass per S-N bond) of these polyfunctional inhibitors compared to CTP. However, CTP remains the most favored commercial PVI.
Leib, Sullivan and Trivette2 studied the effect of CTP in natural rubber (NR) vulcanized with 2(4-morpholinothio)benzothiazole sulfenamide (MOR) and N-t-butyl-2-benzothiazole sulfenamide (TBBS). They found that addition of CTP to a TBBS accelerated NR cure caused an increase in the induction time, increasing proportionally with the loading of CTP. A proportional increase in induction time with increased CTP loadings was also demonstrated by Trivette, Morita and Maender.3 The induction time of compounds was shown to be sensitive to loadings of between 0. 1-0.4 phr of CTP, this without changing the vulcanization characteristics or the properties of the vulcanizates to any significant degree. Numerous authors have found that higher retarder levels, 0.5 phr and up, cause a decrease in crosslink density and affect physical properties of vulcanizates, which could be corrected by adding more sulfur.4-6
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial support of Continental Tyre SA, Karbochem, the National Research Foundation and the University of Port Elizabeth Research Committee.
[Paper 21, presented at the Fall ACS Rubber Division Meeting (Orlando), September 21-23, 1999; revised February, 1, 2000]
REFERENCES
1A. B. Sullivan, E. Morita, and R. I. Leib, Rubber World 195(1), 21 (1986).
2R. I. Leib, A. B. Sullivan, and C. D. Trivette, Jr., RUBBER CHEM. TECHNOL. 43, 1188 (1970).
3C. D. Trivette, Jr., E. Morita, and 0. W. Maender, RUBBER CHEM. TECHNOL. 50, 570 (1977).
4R. Mukhopadhyay and S. K. De, Polymer 20, 1527 (1979).
5R. N. Datta, P. K. Das, and D. K. Basu, J. Polym. Sci. 32, 5849 (1986).
6R. J. Hopper, RUBBER CHEM. TECHNOL. 53,1106 (1980).
7R. H. Campbell and R. W. Wise, RUBBER CHEM. TECHNOL. 37,635 (1964).
8W. Scheele and J. Helberg, RUBBER CHEM. TECHNOL. 38, 189 (1965).
9P. N. Son, K. E. Andrews, and A. T. Schooley, RUBBER CHEM. TECHNOL. 45, 1513 (1972).
10P. N. Son, RUBBER CHEM. TECHNOL. 46, 999 (1973).
11 M. H. S. Gradwell and W. J. McGill, J. Appl. Polym. Sci. 61, 1515 (1996).
12M. H. S. Gradwell and M. J. van der Merwe, RUBBER CHEM. TECHNOL. 72, 55 (1999).
13A. B. Sullivan, L. H. Davis, and 0. W. Maender, RUBBER CHEM. ,TCHNOL. 56,1061 (1983).
14j. J. D'Amico, K. T. Potts, J. Kane, D. McKeough, and K. F Moschner, RUBBER CHEM. TECHNOL. 48, 790 (1975).
15M. H. S. Gradwell and W. J. McGill, J. Appl. Polym Sci. 58, 2185 (1995).
1613. Banerjee, Kautsch. Gummi Kunstst. 42, 217 (1989).
17M. H. S. Gradwell, B. Morgan, and W. J. McGill, J. AppL Polym. Sci. 56, 1581 (1995).
tBR. S. Kapur, J. L. Koenig, and J. R. Shelton, RUBBER CHEM. TECHNOL. 47, 911 (1974).
19B. V. M. K. Giulliani and W. J. McGill, J. AppL Polym. Sci. 57, 1391 (1995).
20M. H. S. Gradwell and W. J. McGill, J. Appl. Polym. Sci. 61, 1131 (1996).
218. Morgan and W. J. McGill, J. AppL Polym. Sci., 76, 1395 (2000).
228. Dogadkin, V. Selyukova, Z. Tarasova, A. Dobromyslova, M. Feldstein, and M. Kaplunov, RUBBER CHEM. TECHNOL. 29,917 (1956).
23R. H. Campbell and R. W. Wise, RUBBER CHEM. TECHNOL. 37,650 (1964). 24j. Tsurugi and H. Fukuda, RUBBER CHEM. TECHNOL. 31, 788 (1958).
25C. G. Moore, J. Chem. Soc. 4232 (1952).
MICHAEL H. S. GRADWELL, * NIGEL R. STEPHENSON
POLYMER CHEMISTRY, UNIVERSTIY OF PORT ELIZABETH, PO Box 1600, PORT ELIZABETH, 6000, SOUTH AFRICA
*Corresponding author.
Copyright Rubber Division Mar/Apr 2001