# Influence of Eco-Friendly Processing Aids on Silica-Based Rubber Composites
## 3 Results and Discussion

Table 2: Swelling and cross-linking density characteristics of rubber composites with varying content fractions of different resins. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & \(\omega\) & \(\Delta\text{Cp}\) & \(\Delta\text{Cpn}\) & \(\chi\text{im}\) \\ \hline HCR 3.0 phr & 3.0 & 0.485 & 0.5 & 20.0 \\ AMR 3.0 phr & 3.0 & 0.451 & 0.465 & 25.6 \\ APR 3.0 phr & 3.0 & 0.467 & 0.481 & 23.0 \\ TPR 3.0 phr & 3.0 & 0.423 & 0.436 & 30.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameter values of rubber composites with varying content fractions of different resins. Furthermore, transmission electron microscopy (TEM) provided direct evidence for evaluating the dispersion of fillers and the phase morphology in relation to the mechanical properties of rubber composites. The TEM images of the fractured rubber composites are exhibited in Figure S1. In the rubber composite with HCR, these silica fillers were observed as globular aggregates of various sizes. However, for the rubber composites with TPR, large aggregates were not observed anymore but the silica was well distributed throughout the rubber matrix in Figure S1. The dynamic mechanical properties of engineering rubber are generally important because many forms of rubbers are subjected to dynamic loads. One such property, the Tg, is shown as a function of resin contents in Figure 4a. With the addition of TPR in the SBR composite, the Tg was higher than that of HCR, i.e., the Tg was \(-\)29.6 \({}^{\circ}\)C and \(-\)35.8 \({}^{\circ}\)C at 3 phr, respectively, as shown in Figure 4a. Furthermore, the Tg of the TPR/SBR composite was higher than that of the APR/SBR composite. As the content of processing aids increased, Tg tended to decrease slightly. The dynamic mechanical properties (DMA) of SBR composites with different processing aids versus temperature are shown in Figure 4b,c. The DMA test to investigate tan \(\delta\) (the ratio of loss modulus to storage modulus) showed the following physical properties of SBR composites, which was used to predict both the wet grip and rolling resistance properties [39; 40]. The 0 \({}^{\circ}\)C tan \(\delta\) of TPR (3 phr)/SBR composites, which indicates the pneumatic grip tire performance, was about 3.5% higher than that of HCR in Figure 4b. The 60 \({}^{\circ}\)C tan \(\delta\) of SBR composites with TPR (3 phr), which indicates the pneumatic rolling resistance, was approximately 92% lower than that of the SBR composite with HCR in Figure 4c. Thus, the TPR/SBR composite had immense potential for improving wet grip and rolling resistance, and these results were consistent with the mechanical properties in Figure 2.  
The abrasion properties of an SBR composite with different processing aids with increasing contents are shown in Figure 5a-c. The abrasion properties of the rubber composites tended to decrease with the content of processing aids due to an increase in the viscosity and a decrease in mechanical properties. A large amount of HCR/SBR composite debris was found among all samples on the worn surfaces, corresponding to poor wear resistance, as shown in Figure 5a-c. Moreover, the abrasion properties of the TPR/SBR composites were much better than those of the HCR/SBR composites. The higher abrasion properties were attributed to the functional groups of the TPR and the siloxanes, as well as silanols on the silica, which provided chemical cross-links and physical entanglements within the cross-linked network [41]. The fatigue properties of the SBR composites (Figure 4c and Table 4) were measured after 2000, 6000, and 10,000 cycles. The fatigue crack growth (dc/dn) was calculated as follows [42]:  
\[\mathrm{dc/dn}=\mathrm{A}\times\mathrm{G}^{\alpha} \tag{6}\]  
where n is the number of loading cycles, c is the crack length, and G is the tear energy.