Activators in Accelerated Sulfur Vulcanization

Abstract

This review provides relevant background information about the vulcanization process, as well as the chemistry of thiuram- and sulfenamide-accelerated sulfur vulcanization with emphasis on the role of activators, to lay a base for further research. It commences with an introduction of sulfur vulcanization and a summary of the reaction mechanisms as described in literature, followed by the role of activators, particularly ZnO. The various possibilities to reduce ZnO levels in rubber compounding, that have been proposed in literature, are reviewed. A totally different approach to reduce ZnO is described in the paragraphs about the various possible roles of multifunctional additives (MFA) in rubber vulcanization. Another paragraph is dedicated to the role of amines in rubber vulcanization, in order to provide some insight in the underlying chemical mechanisms of MFA systems. Furthermore, an overview of Model Compound Vulcanization (MCV) with respect to different models and activator/accelerator systems is given. In the last part of this review, the various functions of ZnO in rubber are summarized. It clearly reveals that the role of ZnO and zinc compounds is very complex and still deserves further clarification.

Three-Dimensional Imaging in Polymer Science: Its Application to Block Copolymer Morphologies and Rubber Composites

“…A complete fundamental understanding of this complex mechanism remains a significant scientific challenge. [7][8][9][10][11][12][13][14] THREE-DIMENSIONAL (3D) IMAGING In order to establish the structure-mechanical properties relationship, one needs proper methods to characterize the morphologies. Up to now, morphological studies of the polymeric materials have been carried out by various microscopic and scattering methods.…”

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].…”

“…In spite of sulfur vulcanization mechanism is still a controversial issue, there is a widespread agreement about the basic steps of this process, which can be divided into three stages [2,3]: 1. Accelerators chemistry where the active sulfurating agent is created.…”

“…rubber is the density of NR (0.92 g/cm 3 ); w ZnO is the ZnO weight, w compound is the recipe weight and ! ZnO is the density of ZnO (5.6 g/cm 3 ). Assuming an affine model to characterize the elastic behavior of swollen networks, cross-link density (") was obtained by using the Flory-Rehner equation (Equation (2)) [17]: (2) where V rubber is the volume fraction of polymer in the swollen mass, V 1 is the molar volume of the solvent and # is the Flory-Huggins polymer-solvent interaction parameter.…”

“…In the case of sulfur systems, the discussion concerning the nature of the reactions involved in the vulcanization process, i.e. 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].…”