A Review of Sulfur Crosslinking Fundamentals for Accelerated and Unaccelerated Vulcanization

Abstract

This work has reviewed and summarized the present understanding of the fundamental science and chemistry of rubber vulcanization. The future of this area holds much potential. Recent studies have begun to fully elucidate the accelerator and accelerator complex chemistry, and definitively answer questions that have been open for debate for many years. Additionally, the understanding of the network structure is rapidly progressing through advances in analytical techniques. The largest area of future research will be the use of multiple experimental techniques to fully elucidate and understand the relationship between accelerator chemistry and the formation of specific network structures. With this knowledge, it will then be possible to molecularly engineer vulcanizates with specific properties.

Sulfur Vulcanization of Natural Rubber for Benzothiazole Accelerated Formulations: From Reaction Mechanisms to a Rational Kinetic Model

“…These accelerator polysulfides are formed by the interaction of the accelerator molecule with molecular sulfur. 2,[20][21][22][23][30][31][32] In the absence of an activator such as ZnO, these polysulfides are of Type I:…”

“…In the 1980's, Morita 9 highlighted the importance of S-N compounds as accelerators as well as retarders; while McCleverty [10][11][12][13][14] pioneered critical studies of the inorganic chemistry of zinc-accelerator complexes. More recently, Kresja and Koenig 15 summarized additional details of the chemistry of accelerated and unaccelerated vulcanization, while Nieuwenhuizen et al 3 exhaustively reviewed the mechanistic details of the chemistry of thiurams and dithiocarbamates. We hope that this paper may in some small way add to this distinguished collection of publications.…”

Vulcanization Kinetics of Natural Rubber Based On Free Sulfur Determination

“…These active accelerator complex and rubber backbones have reacted to form active sulfurating agent and rubber bound intermediate, then network formation with initial polysulfide crosslinks was followed by network maturation process. In the last stage, crosslink shortening with additional crosslinking, crosslink destruction with main chain modification (cyclic sulfide) and s-s bond interchange (pendent sulfidic group) are included [1].…”

ESR investigation of NR and IR rubber vulcanized with different cross-link agents

“…Post-vulcanization chemistry where the shortening of sulfur bridges in addition to other degradation process takes place. According to this mechanism, sulfur vulcanization is a complex process where radical route is considered as one of the main pathways which could explain the network structures reported for NR [2][3][4][5]. In this sense, the evaluation of model compounds with solid-state 13 C-NMR reveals that the main structures generated during sulfur vulcanization correspond to allylic attacks in both the secondary and primary carbons [31][32][33][34][35][36][37].…”

“…ionic or free radical pathways, is still ongoing [1][2][3][4]. Several works conclude that radical mechanisms are required to justify the crosslink structures formed on NR after the vulcanization with sulfur [2][3][4][5][6]: sulfur cross-links of different lengths, mainly polysulfidic bonds, pendant groups, cyclic sulfides and cis/trans isomerization of the double bonds and/or conjugated sequences in the polymer backbone [7]. In addition, the low reaction efficiency and the long time required to complete vulcanization process do not recommend the use of sulfur without accelerators for commercial purposes [6].…”