Pyrolysis of vulcanized styrene-butadiene rubber via ReaxFF molecular dynamics simulation

Highlights

• Cross-linked Styrene-butadiene rubber is modeled in molecular simulation.

• Sulfur as an agent would promote the crack of polymer chain.

• High temperature promotes sulfur transformation into H₂S as products.

• At low sulfur content system, hydrocarbon sulfide is the main sulfur products.

• The elemental sulfur intermediate is observed at high sulfur content systems.

Abstract

Styrene-butadiene rubber (SBR) is widely used in tires in the automotive segment and vulcanization using sulfur is a common process to enhance its mechanical properties. However, the addition of sulfur as the cross-linking agent usually results in impurities in pyrolysis products during rubber recycling, and thus the desulfurization during tire pyrolysis attracts much attention. In this work, the pyrolysis of vulcanized SBR is studied in detail with the help of ReaxFF molecular dynamics simulation. A series of cross-linked SBR models were built with different sulfur contents and densities. The following ReaxFF MD simulations were performed to show products distributions at different pyrolysis conditions. The simulation results show that sulfur products distribution is mainly controlled by sulfur contents and temperatures. The reaction mechanism is proposed based on the analysis of sulfur products conversion pathway, where most sulfur atoms are bonded with hydrocarbon radicals and the rest transfer to H₂S. High sulfur contents tend to the formation of elemental sulfur intermediate, and temperature increase facilitates the release of H₂S.

Introduction

Styrene-Butadiene Rubber (SBR) is a general-purpose synthetic rubber and a main component in passenger car tires. The production of scrap tires increases with the development of automotive industry. However, only a small portion of scrap tires was recycled or reused, and the large part was treated unprofessionally and caused “black pollution”. Pyrolysis of scrap tires is an efficient and eco-friendly process that allows producing fuels and chemicals from scrap tires [1], [2], and also provides heat and electrical energy due to the high calorific value [1], [2], [3], [4], [5]. The products from thermal decomposition of scrap tires include gas and liquid products which can be used as energy sources, and char that can be transformed into carbon black after treatment. Additionally, sulfur is widely used in tires manufacture as an additive [1], [6]. The addition of sulfur forms a network structure by bringing in poly-, di-, and mono-sulfide bonds [7] to enhance the mechanical properties of rubber. In the pyrolysis process, the sulfur which is originally in organic forms releases into pyrolysis products such as oil, tar and gases. The removal of sulfur during rapid pyrolysis is of high importance for industrialization of scrap tires.

The sulfur usually remains in the oil after pyrolysis process resulting in low quality products [8], [9]. Thus, the desulfurization during pyrolysis is necessary to recycle vulcanized rubber effectively [10]. Previous studies indicate that sulfur is involved in a serial of reactions in the pyrolysis process [9], and sulfur removal is accomplished by H₂S release [11]. The pyrolysis product is strongly dependent on the temperature [12], heating rate, residence time and mass transfer, which are related with the reactor configurations [9], [13]. Among these factors, the temperature is a common factor discussed in their works. However, it is still a challenge to examine the effect of a factor (independent variable) in the pyrolysis experiment. During the last decade, rapid development of computer power has made molecular simulation indispensable in the study on many chemical processes such as the thermal decomposition of organic materials.

Reactive force field [14], [15] molecular dynamics (ReaxFF MD) simulation as an efficient computation tool has been used in simulating the oxidation of hydrocarbons. Importantly, single factor experiment is able to be performed with the help of this theoretical method. The bond order is employed in this force field to determine the bond formation and breakage. Unlike other simulation methods, ReaxFF MD can simulate the formation of radicals, the bonding and breaking dynamically. The accuracy of ReaxFF force field was insured by quantum mechanics (QM) data, and ReaxFF MD can simulate large-scale systems with lower computational expense than QM methods [16]. The application of ReaxFF has been expanded constantly, which has been used in simulating the pyrolysis of complex compound such as coal [17], [18], [19] and biomass [20]. Lately, the natural rubber and its pyrolysis mechanism was studied through ReaxFF MD combined with thermogravimetry-infrared spectroscopy experiments [21]. The simulation results are in good agreement of experimental pyrolysis data. Therefore, ReaxFF MD simulation is practicable to explain the pyrolysis mechanism of vulcanized SBR at various reaction conditions.

Our previous work [22] used density function theory (DFT) and ReaxFF MD simulation to study bond dissociation energies of SBR polymer chain and the pyrolysis process of pure SBR. In this work, we build the SBR models with cross-linking structures firstly. And the ReaxFF MD simulation is used to investigate the pyrolysis mechanism of vulcanized SBR at different temperatures and densities. Especially, we focus on the transfer of sulfur during the pyrolysis which is key to the devulcanization [7], and the sulfur products distribution are well concluded. We intend to provide a detailed theoretic understanding and yield guidelines on the devulcanization during pyrolysis of vulcanized SBR.