# Influence of Eco-Friendly Processing Aids on Silica-Based Rubber Composites
## 3 Results and Discussion
Figure 3: Mechanical properties of the rubber composites: (**a**) torque of the rubber composites with different resins at 1.5 phr, (**b**) torque of the rubber composites with different resins at 3.0 phr, (**c**) torque of the rubber composites with different resins at 6.0 phr, (**d**) elastic modulus of the rubber composites with varying content fractions of different resins, (**e**) tensile strength of the rubber composites with varying content fractions of different resins, and (**f**) elongation of the rubber composites with varying content fractions of different resins. Figure 2: Curing properties of rubber composites: (**a**) curing properties (**t\({}_{40}\)**) of the rubber composites with varying content fractions of different resins, (**b**) curing properties (**t\({}_{90}\)**) of the rubber composites with varying content fractions of different resins, and (**c**) Mooney viscosity of the rubber composites with varying content fractions of different resins. To further investigate the mechanical properties, swelling tests of rubber composites with different processing aids were carried out, with the results shown in Table 2. The equilibrium swelling Q% was calculated as follows:  
\[\text{Q}=[(\text{Ws}-\text{Wd})/\text{Wd}]\times 100\%, \tag{1}\]  
where Ws is the weight of the swelled specimen, and Wd is the weight of the dried specimen. The cross-linking densities (V, mol/cm\({}^{3}\)) of the SBR composites were found via the solvent swelling measurements using the Flory-Rehner equation [34; 35], as follows:  
\[\text{V}=1/2\times\text{Mc}, \tag{2}\]  
where Mc is the molecular weight between the cross-links (g/mol) and:  
\[\text{Mc}=-\varrho\times\text{Vs}(\text{Vr}^{1/3}-2\text{Vr}/\text{f})/[ \text{ln}(1-\text{Vr})+\text{Vr}+\chi\times\text{Vr}^{2}], \tag{3}\]  
where Vr is the volume fraction of the swollen rubber, Vs is the molar volume of the toluene, \(\chi\) is the interaction parameter of rubber, f is the network functionality, and \(\varrho\) is the density of rubber. These results indicate that the functional groups of TPR could provide the chemical cross-link and physical entanglements for the cross-linking network between the silica and rubber matrix. Generally, the rubber chains were immobilized on the filler surface as a constrained area between the rubber matrix and the filler due to a processing agent, thereby preventing the enhanced reinforcement of fillers and phase separation in rubber composites [36; 37]. The weight fraction of rubber chains fixed on the filler surface (xim) was determined as follows [38]:  
\[\Delta\text{Cpn}=\Delta\text{Cp}/(1-\omega), \tag{4}\]  
\[\chi\text{im}=(\Delta\text{Cp}0-\Delta\text{Cpn})/\Delta\text{Cp}0, \tag{5}\]  
where \(\Delta\text{Cp}\) is the heat capacity jump at the glass transition temperature (Tg), \(\Delta\text{Cp}0\) is the heat capacity jump at the Tg of the neat rubber, \(\Delta\text{Cpn}\) is the heat capacity jump normalized to the rubber fraction, and \(\omega\) is the weight fraction of fillers in the rubber composites. These parameters were determined through a differential scanning calorimetry (DSC) experiment. The value of \(\chi\)im of the TPR/SBR composite was higher than that of other rubber composites, as shown in Figure 1:0 The higher the value of \(\chi\)im, the stronger the interfacial interaction between the filler and the rubber matrix. The improved reinforcement could be attributed to enhanced interfacial adhesion between the silica and the rubber, and the restriction of rubber segment chain movement was due to the improved dispersion of silica in the rubber with added TPR. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **HCR 3.0 phr** & **AMR 3.0 phr** & **APR 3.0 phr** & **TPR 3.0 phr** \\ \hline Equilibrium swelling (\%) & \(145.13\pm 0.3\) & \(152.12\pm 0.2\) & \(165.72\pm 0.4\) & \(173.14\pm 0.3\) \\ Crosslinking density (\(10^{4}\) mol/g) & \(2.35\pm 0.4\) & \(2.75\pm 0.5\) & \(2.87\pm 0.3\) & \(2.98\pm 0.5\) \\ \hline \hline \end{tabular}
\end{table}