4.2.2. Cross-Linking Density

Figure 3 shows the e ffect of BDC on the cross-linking density of the SBR composites. Increasing the concentration of the protein filler caused a gradual increase in the degree of cross-link density in the SBR vulcanizates. These results are in agreemen<sup>t</sup> with the previous rheometer measurements.

**Figure 3.** Influence of BDC on the cross-linking density of SBR vulcanizates.

The higher degree of cross-link density in the SBR/BDC composites can be explained by the reactivity of groups originating in the BDC filler and the elastomeric matrix. The most probable interactions occur between the styryl group in rubber and the polar fragments of the collagen dust, as shown in Figure 4. The interactions between the BDC dust and the elastomer matrix were also confirmed by infrared analysis (Figures 5 and 6).

**Figure 4.** Possible mechanism of interaction between the elastomer macromolecule and BDC structure.

**Figure 5.** Fourier transform infrared spectroscopy (FTIR) spectrum of samples containing 0 and 20 phr BDC in SBR composites relative to the BDC spectrum.

**Figure 6.** Comparison of the FTIR spectra for the standard sample (SBR), 10, and 30 phr BDC in SBR composites.
