# Influence of Eco-Friendly Processing Aids on Silica-Based Rubber Composites
## 3 Results and Discussion  
Rubber composites of silica and SBR matrix were prepared with a variety of processing aids via a latex mixing method. Most processing aids contain benzene or other ring structures, such as phenolic resin, a nonpolar hydrocarbon, and polar hydroxyl functional groups from Figure 1. Thus, while HCR and AMR with a hydrocarbon and a benzene ring have hydrophobic properties, the APR and TPR with a hydrocarbon, a benzene ring, and hydroxyl (-OH) groups have amphiphilic properties and this structure serves to enhance the dispersion of silica in the rubber matrix. Most resins with a benzene ring are more restricted to being a rubber chain than other resins with hydrocarbon groups [32]. The processability, which is important during the formulation, was determined by evaluating the curing properties, such as the cure time, minimum torque, and maximum torque. Figure 2 shows the curing properties, based on the cure time (t\({}_{0}\) and t\({}_{90}\)), of rubber composites filled with various processing aids. The times required for 40% and 90% cure completion were designated as the t\({}_{40}\) and t\({}_{90}\) times, respectively. The amount of added processing aids was small; consequently, the thermal conductivity did not change and there was no distinct difference in the curing time of rubber composites with various processing aids, as shown in Figure 2a,b. However, as the content of the processing aids with very low thermal conductivity increased, t\({}_{40}\) and t\({}_{90}\) tended to increase. Additionally, a rheometer is commonly used as an indicator of the processing safety of rubber composites. The SBR composite with TPR exhibited a reduced Mooney viscosity, which facilitated the preparation process of the rubber composites, as shown in Figure 2c. Figure 3 shows a comparison of the mechanical properties of the SBR composites with different added processing aids. A comparison of the torque in the SBR composites was investigated as a function of the increasing, diverse processing aids. The M\({}_{\rm H}\) (torque maximum) - M\({}_{\rm L}\) (torque minimum) (\(\Delta\) torque) value, which is the cross-link density of vulcanization, of the SBR composite decreased as the processing aid contents increased (Figure 3a-c); furthermore, the \(\Delta\) torque value of the SBR composite with TPR (27.3 N\(\cdot\)m) at 1.5 phr was higher than that of other composites. This result means that the silica dispersion and stronger cross-linking with the SBR during vulcanization were enhanced by the TPR with hydrophilic and hydrophobic groups because the interaction between hydrophilic silica and hydrophobic rubber increased. Figure 3d,e shows the modulus and tensile strength of the SBR composites with different processing aids as the processing aid contents increase. The modulus  
Figure 1: Molecular structure diagram of (**a**) hydrocarbon resin, (**b**) alpha methyl styrene resin, (**c**) alkyl phenol resin, and (**d**) terpene phenol resin. and tensile strength (T/S) of the composites with TPR and AMR at 3.0 phr increased by as much as 36% and 3%, respectively. This improvement was attributed to the enhanced dispersion and strong interfacial bonding of silica in the rubber matrix by TPR with amphiphilic functional groups. Moreover, the modulus and tensile strength of the TPR/SBR composite were higher than those of the APR/SBR composites due to the increased interaction between the benzene functional groups of TPR and SBR. The elongation at break of the SBR composite decreased with increasing amounts of processing aids, while the elongation increased with increasing viscosity. The elongation at break of the TPR/SBR composite (473%) was lower than that of the HCR/SBR composite (487%) due to stronger interfacial bonding and the restriction of the segmental elastomer chain motion [33].