*3.3. Abrasion Test*

Figure 6 shows the abrasion resistance index (*ARI*) of the unfilled vulcanizate, in-situ silica and precipitated silica-filled and their respective silane modified composites. The unfilled gum vulcanizate exhibits least *ARI* and this is due to the poor physical properties of the gum rubber, where the rubber surface is easily cut-out by the abrasion forces. Addition of silica as reinforcing filler into the SSBR-rubber improves the *ARI* values. In addition, increasing the amount of silica in rubber gradually increases the *ARI* values. However, the improvements in *ARI* are not significant without the presence of silanes. In the case of precipitated silica without silane, no improvements could be seen in the ARI values beyond the addition of 30 phr of silica. The incorporation of TESPT as a coupling agen<sup>t</sup> in the silica-rubber system improves the abrasion properties further in both in-situ and precipitated silica composites. Experimental observations indicate improved abrasion properties for the precipitated silica compounds in comparison to them with in-situ silica. The effective particle size and surface area of the commercial precipitated silica are higher than in-situ silica, which could be the reason for the improved abrasion characteristics.

To understand the abrasion behavior in detail, the topology of abraded surfaces is investigated by SEM spectroscopy and their respective micrographs are given in Figure 7. The abraded surfaces of SSBR samples filled with precipitated silica and in-situ silica compounds without the coupling agen<sup>t</sup> TESPT are found to be rough in comparison with the silane modified samples. Absence of silane as a coupling agen<sup>t</sup> easily allows the rubber particles to cut-off due to the rubbing force. This is because of the high inhomogeneity in the elastomer material and the poor filler-rubber interaction. Silane modification of both silica composites greatly improves the surface wear mechanism. Formation of ridges or waviness protects the rubber surface from the further abrasion process by the gradual reduction of the surface contact area [32,33]. Such ridge topologies are formed on the surface of the samples when they are modified with a silane coupling agent. This also shows the improved interaction between silica and polymer assisted by the silane coupling.

**Figure 6.** ARI of gum, in-situ, and precipitated silica compounds and its TESPT modified SSBR composites.

**Figure 7.** SEM and EDX images of abraded surfaces (after the DIN abrasion test) of 30 phr precipitated silica, in-situ silica, and their TESPT modified SSBR composites.

The lower ARI of the in-situ silica/SSBR composite could be attributed to the lower surface area and also due to the bigger particle size of the in-situ silica filler. In the EDX images in Figure 7, for samples without a silane coupling agent, it could be noticed that the silica filler particles are highly visible or exposed (noticeable as a blue color) after the abrasion experiment, since the silica particles are covered by a polymer layer. In in-situ silica-filled systems, the filler particles (noticeable as a blue color) are highly separated from the rubber matrix, when compared to the precipitated silica system. This is an outcome of the poor abrasion behavior of the in-situ silica. In our previous investigation, we observed that the specific surface area of in-situ silica is very low in comparison to a precipitated silica system [34].

### *3.4. Tear Fatigue Analyzer (TFA)—Crack Propagation Measurements*

A tear fatigue analysis was performed to understand the crack propagation behavior of di fferent silica-filled SSBR composites and the influence of silica surface modification by a silane coupling agent. A logarithmic plot of the crack propagation rate (*da*/*dn*) in relation to the tearing energy ( *T*) is known as a Paris plot and is given for all the samples in Figure 8. The rate of crack growth (*da*/*dn*) is directly proportional to the tearing energy *T* of the material. Therefore, all the samples are tested inside the stable crack growth regime. In Figure 8a, as expected, the unfilled gum sample shows a smaller tearing energy and a quicker crack formation compared to filled samples. It means that there is no active material to prevent the crack propagation in the gum vulcanizate. Incorporation of both in-situ and precipitated silica significantly improves the tear fatigue behavior. The main mechanism is the hindrance of filler particles that resist the crack tip growth [3]. Interestingly, the crack propagation behavior is found to be significantly di fferent for the in-situ and precipitated silica, possibly due to the di fferences in their morphologies. The silane modification improved the crack propagation behavior for both the silica fillers. It is plausible that the significantly better crack resistance in silane-containing systems is a result of the establishment covalent chemical bonds between the elastomer matrix and filler particles by means of the bi-podal sulfur containing silane molecules. However, the in-situ silica system shows a higher crack growth rate than the precipitated silica system. The results, therefore, show that the state of the silica dispersion is not only a critical parameter for crack propagation, but that particle size, particle morphology, cross-linking density, and filler-polymer interfaces also play an important role.

**Figure 8.** (**a**) Paris plots showing the crack propagation rate as a function of tearing energy for in-situ silica and precipitated silica-filled composites and (**b**) crack propagation of equally adjusted crosslink density of in-situ silica and precipitated silica compounds.

Our previous investigations of the SEM and TEM analysis distinctively states that particle size and morphology of precipitated and in-situ silica as well as the reinforcement characteristics are entirely di fferent [20]. Moreover, the quantity of assessed cross-linking density is higher for the in-situ silica composites in comparison to precipitated silica composites. To eliminate the e ffect of di fferent cross-linking density on the crack propagation behavior, the crosslink density of in-situ samples are reduced and matched with those of precipitated silica composites by modifying the state of cure from *TC*90 to *TC*50. The amount of cross-linking density was measured by means of the equilibrium swelling method and the Flory-Rehner equation, which is described in detail in our previous paper [20]. The values of the cross-linking densities from this measurement are given in Table 2. The crack propagation behavior is completely di fferent after adjusting the crosslink density of the in-situ silica composites. The plots in Figure 8b clearly indicate that the crack propagation behavior of in-situ silica composites is significantly improved. The in-situ silica compounds without silane modification yield better properties than the precipitated silica samples. However, the incorporation of silane in both of the silica systems reduces the crack propagation properties by a certain amount.


**Table 2.** Tear fatigue behavior of SSBR compounds filled with di fferent silica systems with a reference to crosslinking density.
