Performance enhancement of rubber composites using VOC-Free interfacial silica coupling agent

Abstract

Nano-silica is an important component for producing elastomer composites used for fabricating “green tires.” However, the poor dispersion of silica particles in the rubber matrix and the emission of volatile organic compounds (VOCs) during the silica modification limit the applications of the modifiers. Here, bis-epoxypropyl polysulfide (BEP), a novel epoxy-type coupling agent, was designed and synthesized for nano-silica modification to cause an interfacial interaction between nano-silica and the rubber matrix and avoid VOC emission. The thermogravimetric analysis result and the bound rubber content show that BEP effectively built a bridge between the nano-silica and the rubber, which led to a strong interfacial effect and promising mechanical performance characteristics. The silica dispersion in solution-polymerized styrene-butadiene rubber (SSBR) was studied using a transmission electron microscope and a rubber process analyzer, and the results demonstrate that BEP could significantly improve silica dispersion. The static and dynamic mechanical performance results indicate that BEP is a valid coupling agent that can achieve silica/SSBR composites with high moduli and reinforcement indices. Moreover, the combination of BEP and bis-(γ-triethoxysilylpropyl)-tetrasulfide (TESPT) was also found to demonstrate a synergistic effect, which resulted in excellent static and dynamic performances of silica/SSBR composites for preparing higher-energy-efficient “green tires.”

Introduction

In the tire industry, nano-silica is widely used as fillers in the rubber matrix of the treads of “green tires” [[1], [2], [3]]; this is because it can enhance mechanical properties [[4], [5], [6]], reduce the rolling resistance to improve the energy efficiency [[7], [8], [9]], and improve the wet skid resistance [[10], [11], [12]]. More than two million tons of nano-silica were used in tires in 2018. However, silica particles tend to aggregate in the rubber matrix due to the large amounts of silanol groups on the silica surface. This leads to poor silica dispersion in rubber and weak performances of the tire [[13], [14], [15]].

To solve this aggregation issue, researchers did a lot of works for modifying silica [[16], [17], [18]]. Silane coupling agents have been found to be the most important type of modifier. They can not only reduce the polarity of silica to improve silica dispersion in rubber [[19], [20], [21], [22], [23], [24]] but also enhance the interfacial effect between silica and rubber [[25], [26], [27], [28], [29], [30], [31], [32]]. Rubber composites filled with silane coupling agent–modified silica show better static and dynamic mechanical properties compared with those filled with coupling agent–free silica [[33], [34], [35]]. Nowadays, nearly all of the silica-reinforced tires contain coupling agents [[36], [37], [38], [39], [40]]. However, according to the reaction mechanism between silane and silica, ethanol is released, and recycling this ethanol in the actual production is difficult. Ethanol vapor in plants is inflammable and harmful to the human nervous system and blood system [41]. Presently, many countries have regulations governing volatile organic compound (VOC) emissions, such as the Clean Air Act (USA) and the Integrated Pollution Prevention and Control Directive (EU). In the large-scale tire industry, around 5–6 mL/kg VOCs (VOC/rubber composites) can be generated from coupling agents during the rubber processing procedures, about 130,000 m³ per year. Thus, discovering VOC-free coupling agents is essential.

In this study, we aim to design a new VOC-free coupling agent containing epoxy groups to react with the silanol groups of silica rather than silane. Meanwhile, we introduce polysulfide bonds to this coupling agent for further reaction with rubber. For this purpose, bis-epoxypropyl polysulfide (BEP) was synthesized via a substitution reaction between epichlorohydrin and sodium polysulfide. The coupling mechanism of BEP used in silica-filled rubber composites is shown in Scheme 1. The BEP builds a chemical bridge between the silica and rubber by epoxy groups and polysulfide bonds. The performances of silica-filled composites containing different substitution amounts of BEP were studied and compared to evaluate the potential of this new agent, aiming to design and produce “green tires.”