# Influence of Eco-Friendly Processing Aids on Silica-Based Rubber Composites
## 1 Introduction  
Rubber can be employed in many applications that require viscoelastic polymers and in a wide range of industrial fields by adding fillers, especially in the tire industry [1]. Styrene butadiene rubber (SBR), which is a commercial rubber that has been used as a general tread compound in the tire industry, is a nonpolar and noncrystalline copolymer with a low tensile strength. Therefore, various reinforcing fillers are required in the matrix of SBR composites to improve their mechanical, thermal, and electrical properties [2; 3; 4; 5]. In general, carbon black (CB) is an important filler that is used for improving the mechanical properties of almost any industrial rubber, such as abrasion resistance, hardness, and tear strength. However, rubber composites using CB are limited by the lack of wet traction and rolling resistance. Furthermore, because CB is extracted from crude oil, its manufacturing process generates significant greenhouse gas emissions and waste. With the recent implementation of the European Tire Labelling Regulation [6], silica has been used as part of a \"green tires\" concept (a new breed of environmentally friendly tires with silica) in tread formulation, resulting in high wet grip and low rolling resistance [7; 8]. Furthermore, silica-filled rubber composites exhibited higher stiffness, modulus, tensile strength, lower heat build-up, and better resistance to wear and tear of the tire compounds [9; 10; 8]. However, silica, with silanol and siloxane functional groups, differs from CB in its chemical composition and conventional method of manufacturing the composites [11]. The silanol group is an acid [12] that interacts with basic accelerators to slow the curing rate and lower the cross-link density in sulfur curing systems [13]. The viscosity of the composite increases with the amount of silica, which tends tointeract strongly on their own due to their polar and hydrophilic properties [14]. Hence, manufacturing composites with silica is more difficult due to poor dispersibility caused by strong polar bonds between the filler particles when made from non-polar rubbers [15; 16; 17; 18]. Silica coupling agents, such as bis(triethoxysilypropyl)disulfide (TESPD) and bis(triethoxysilpropyl)tetrasulfide (TESPT), have been used to enhance filler-rubber interactions by modifying the silica surface. These act as chemical bonding agents between the rubber and silica in the vulcanization process [19; 20; 21]. Furthermore, processing agents are used in the manufacture of tire rubber composites. The processing aids include hydrocarbons, fatty acid derivatives, low-molecular-weight polymers, synthetic resins, and other organic compounds, which are used as lubricants and dispersion agents, according to their chemical structure [22]. Bi-functional silanes are used in the mixing process to improve the processability and the chemical bonding between the rubber and silica. In addition, various zinc soaps, including zinc naphthalene, zinc stearate, and zinc resinate, are used as intermolecular lubricants, owing to their ability to enhance the rubber solubility in rubber composites. However, because zinc consumption is an environmental problem, the major contributors are under pressure to reduce its use and develop a substitute. Thus, it is necessary to find environmentally friendly, alternative processing aids for use in rubber composites. Recently, rice bran oil and rice oil have been used for polychloroprene (CR) [22], acrylonitrile butadiene (NBR), and SBR [23] as processing aids. Furthermore, castor oil has been used in nitrocellulose, polystyrene rubber, and natural rubber [24; 25]. Moreover, soybean oil was used as a plasticizer in NR and ester gums [24]. [PERSON] et al. found that adding palm oil in an NBR matrix improved the aging resistance and plasticizing efficiency [26]. [PERSON] et al. reported that poly(methyl methacrylate) (PMMA) composites with palm oil exhibited enhanced mechanical properties [27].