4.2.3. FTIR Analysis

In the next part of the study, the composites were subjected to FTIR analysis. Figures 5 and 6 show the sample spectra of SBR composites containing various amounts of grinding dust, as well as the reference spectrum. Based on the BDC spectra of both SBR composites, a small band can be seen in the 3400 to 3250 cm<sup>−</sup><sup>1</sup> range (slightly larger for SBR20), which is associated with vibrations of the -OH group (Figure 5). In the range of 3100 to 3000 cm<sup>−</sup>1, a peak is visible from the stretching vibrations of the C–H bonds of the aromatic ring that form part of the SBR vulcanizate side groups [29]. In the 2950 to 2800 cm<sup>−</sup><sup>1</sup> range, intense peaks are visible from the vibrations of the C–H groups present in the

material, both from the styrene aromatic ring and from the protein chain. The peak in the range of 1700 to 1630 cm<sup>−</sup><sup>1</sup> is caused by the presence of C=C diene groups. The visible change and shifts in absorption bands at 1750 to 1730 cm<sup>−</sup><sup>1</sup> can also be associated with the chromium complex and the free carboxyl group of oligomers that is generated by the fragmentation of peptide chains. The bands in the 1665 to 1550 cm<sup>−</sup><sup>1</sup> range are derived from the vibrations of the C=O peptide bond groups [13,14]. The deviation and significant decrease in the intensity of the absorption band at 1665 cm<sup>−</sup>1, as well as its shift to 1630 cm<sup>−</sup><sup>1</sup> in the SBR or SBR20 spectra relative to the spectrum for BDC, is caused by chromium, which can be coordinated with active collagen centers (amides I and II), thus forming a chromium complex [6]. Together with the shift, this significant reduction in the band (1730 cm<sup>−</sup>1) for the BDC spectrum compared to that for SBR20 supports our supposition that the probable mechanism of interaction between the elastomer macromolecule and BDC structure is as shown in Figure 4 [6].

The intensity of the BDC bands also changes depending on the amount of dust. In comparison with the spectrum for BDC alone (Figures 1 and 6), the intense band at 1665 cm<sup>−</sup><sup>1</sup> derived from primary and secondary amides –(H2N)CO; –R(NH)CO significantly reduces its intensity, which may indicate the possibility of interactions with the C-C groups in the SBR aromatic rubber ring. The absorption band in the 1600 to 1550 cm<sup>−</sup><sup>1</sup> range is also caused by the presence of C–C bonds in the aromatic ring. The intense bands at 1546 cm<sup>−</sup><sup>1</sup> are derived from the stretching vibrations of the –CH2 and –CH3 groups. The small band around 1300 cm<sup>−</sup><sup>1</sup> is from H–O–H groups. It is associated with the presence of the small amount of water in zinc oxide. The band between 1018 and 1080 cm<sup>−</sup><sup>1</sup> is related to the absorption of stretching vibrations originating from C–C moieties in the aromatic ring, which are much more intense when BDC is applied to the SBR structure. This may indicate some interaction between the filler and the elastomeric matrix. The clear peak around 831 cm<sup>−</sup><sup>1</sup> is responsible for the presence of disubstituted alkenes. Intense vibrations are visible at 746 cm<sup>−</sup>1, which may be due to vibrations by hydroxylysine N–H groups present in the BDC or at 581 cm<sup>−</sup><sup>1</sup> from sulfur groups S–H or S–S [13,30].
