Optimization of Rubber Formulation for Silica-Reinforced Natural Rubber Compounds

Abstract

The rubber formulation plays a significant role in the properties of NR compounds filled with silica. In this work, the influences of various silicas, silane coupling agents, and diphenylguanidine (DPG) on the properties of compounds and vulcanizates—that is, cure characteristics, Mooney viscosity, flocculation kinetics, bound rubber content, Payne effect, tan δ at 60°C, tensile properties, and tear properties—are investigated. The results demonstrate that compound viscosity and curing behavior, as well as vulcanizate properties of the silica-filled NR, are much improved by incorporating silane coupling agents. Bis-triethoxysilylpropyltetrasulfide clearly gives better overall properties than the disulfide-based silane bis-triethoxysilylpropyldisulfide, except for scorch safety. DPG acts as a synergist to sulfenamide primary accelerators, as well as activator for the silanization reaction. Highly dispersible (HD) silicas can significantly enhance the degree of dispersion and so lead to higher filler–rubber interaction. As a consequence, the HD silicas provide better dynamic and mechanical properties for filled NR vulcanizates compared with conventional counterparts. The optimal quantities of both silane coupling agent and DPG required in the formulation are correlated to the cetyl trimethylammonium bromide specific surface area of the silicas. Furthermore, the results reveal that the silica structure as characterized by the dibutylphthalate adsorption also strongly influences the reinforcing efficiency.

Mechanistic aspects of silane coupling agents with different functionalities on reinforcement of silica‐filled natural rubber compounds

“…After the Mooney viscosities have reached the maximum value, the compounds show some sort of reversion which could be linked to a change within the polysulfide linkages. According to the conclusion given in our previous studies , this result is in good agreement with the fact that, apart from the silanization reaction, a certain amount of coupling and crosslinking reactions of active sulfur toward NR also takes place during mixing in the mixer, which results in a rise of the Mooney viscosity. Basically, all these reactions are temperature dependent.…”

“…These network contributions result in a better stability of the dispersed silica aggregates as the chemical linkages restrict the silica mobility in the rubber matrix, and hence suppress the reagglomeration of silica. The explanation was already elaborated in terms of the correlation between flocculation and network contributions in the previous studies . The sulfur moiety contained in the silanes provides additional aid for the improvement of the silica dispersion stability and Payne effect of silica‐filled rubber systems.…”

Silane grafted natural rubber and its compatibilization effect on silica-reinforced rubber tire compounds

“…The partially pre-grafted silane to NR may enhance the compatibility between rubber and silica and allow the ethoxygroups of NXT to react more effectively with the silica during the silanization reaction, compared to the straight non-grafted silane. However, the use of TESPT in the silica-filled NR compound still gives an outstanding chemically bound rubber content, which is due to its tetrasulfide structure that results in premature crosslinking with the NR molecules [38] and the bis-triethoxy-groups to react with the silanol groups during silanization. The filler-filler interaction or Payne effect of the silica-filled NR compounds with both non-grafted silane and silane-grafted-NR is remarkably decreased with increasing silane content to meet the same level of Payne effect of the compound with 8.6 wt% of TESPT, as shown in Figure 6b.…”

Influence of highly dispersible silica filler on the physical properties, tearing energy, and abrasion resistance of tire tread compound

“…The hysteresis of silica‐filled rubber increases with the temperature, which is due to stable covalent bond between silica and polymer network. Influence of silica on the properties of rubber compounds has been demonstrated in the literature . However, there is still scope to understand the behavior of rubber–silica compounds, especially the tear and wear properties.…”