Development of property correlations for nitrosamine safe binary accelerator systems in sulfur vulcanized natural rubber

Abstract

In sulfur vulcanization of natural rubber (NR), organic accelerators play an important role by increasing the rate of vulcanization and enhancing the properties of the end products. Most of the traditional accelerators are derivatives of dialkyl amines (tetra alkyl thiuram disulfides, zinc dialkyl dithiocarbamates) and are thus capable of generating regulated nitrosamines, which are also known as carcinogenic in human. Therefore, there is a requirement for alternative accelerators, which can be used as substituents for traditional accelerators. As a result, variety of novel accelerators had been developed in the past in respect to changes to the basic structure of traditional accelerators. Nitrosamine-safe, dibenzyl amine containing thiurams (tetrabenzylthiuram disulfide - TBzTD) and dithiocarbamate (dibenzyl dithiocarbamate - ZBeC), are two commercially available such accelerators. In addition, nitrosamine-free diisopropyl xanthogen polysulfide (DIXP) accelerator is another possible al- ternative accelerator. Although application of DIXP in NR latex and other synthetic rubbers (bromobutyl elastomers and synthetic polyisoprene latex compounds) has been found, its use as a single accelerator, or in binary accelerator system, for NR compounding has not been found and the available information on use of DIXP in NR is also very scanty. Further, no studies were reported specifically on the use of DIXP combined with other nitrosamine-safe accelerators in efficient sulfur vulcanizing system to produce vulcanizates from dry rubber-based NR. Thus, the present research aims to study the behavior of DIXP as a single accelerator and in combination with TBzTD, ZBeC and N-tert-butyl-2- benzothiazole sulfenamide (TBBS) which is commonly used nitrosamine-safe sulfenamide accelerator to establish nitrosamine-safe binary accelerator system/s for sulfur vulcanized NR to overcome the issues associated with the traditional accelerator/s. In that context, cure characteristics of NR compounds, physico- mechanical properties of NR vulcanizates accelerated with DIXP, TBzTD and ZBeC were evaluated and compared with compounds/vulcanizates accelerated with TBBS accelerator. Effect of three different combinations of DIXP/TBBS, DIXP/TBzTD and DIXP/ZBeC binary accelerator systems on cure, physico-mechanical, crosslink density, ageing, dynamic mechanical and thermal properties of NR vulcanizates prepared with efficient sulfur vulcanizing system were evaluated and compared with those of the vulcanizates prepared with single accelerators. Correlation between crosslink density and properties of DIXP accelerated vulcanizates were evaluated for the three binary accelerator systems as well as for overall DIXP accelerated vulcanization. Intermediates generated during vulcanization of DIXP-accelerated NR was analyzed by evolved gases using coupled thermogravimetric infrared (TGA-IR) spectroscopy and gas chromatographic-mass spectrometry (GC-MS) and cure kinetics. Results indicate that in comparison to the vulcanizates prepared with nitrosamine-safe TBBS, TBzTD and ZBeC accelerators, most of the cure properties and physico-mechanical properties of vulcanizates prepared with DIXP accelerator are inferior. Nevertheless, DIXP accelerator provides satisfactory results regarding cure time compared to the TBBS accelerator. However, a significant enhancement in cure and physco-mechanical properties of vulcanizates prepared with the three binary accelerator systems was noticed and vulcanizates accelerated with DIXP/TBBS (0.6/1.4), DIXP/TBzTD (1.4/0.6) and DIXP/ZBeC (1.4/0.6) showed optimum mechanical properties. Further, the trend in the variation of properties of vulcanizates prepared with DIXP accelerated systems against the crosslink density follows the same pattern of vari- ation of traditional accelerators irrespective of the accelerator system used. Finally, a cross- linking reaction mechanism was proposed based on the evidence of the evolved gases, cure kinetics and considering the commonly accepted mechanism of accelerated sulfur vulcaniza- tion for elastomers.