**5. Conclusions**

In this study, collagen grinding dust was applied as a biodegradable filler in the SBR polymer, to study its effects on the mechanical properties, biodegradation, and thermal stability of the SBR vulcanizates. Due to its particle size of less than 1 micrometer, BDC has potential to be used in SBR rubber as nanofillers. The elongated rod-like shape promotes more favorable dispersion of the filler in the elastomer medium, as shown by SEM photos. The presence of a scleroprotein additive results in a higher degree of cross-linking in the case of vulcanizates, which in turn increases the stiffness and hardness of the composites. Interactions may occur between the components of the elastomer matrix, as evidenced by the increased intensity of IR absorption bands in the range of 1000 to 1750 cm<sup>−</sup>1. A greater weight share of filler results in shift of glass transition temperatures towards lower values, and improved hardness. As a result of the biological or thermal aging with oxygen, greater water infiltration was observed in the structures of the material produced on the basis of SBR*30* rubber. Surface color stabilization was also visible, e.g., under the influence of ultraviolet radiation. The introduction of BDC dust had a stabilizing effect on thermo-oxidative aging processes, due to

the antioxidant properties of collagen dust itself. The obtained studies demonstrated that sifted and thermostabilized grinding dust can play a role as a biodegradable filler in polymer materials and, due to intense black color, can act as coloring additive. As expected, the thermal stability of the composites increased, most likely due to the incorporation of chromium ions in the polymer network. The results of this study expand the possibilities for managing hazardous tanning wastes in the form of dust from chrome tanned leathers, while reducing their environmental impact.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/13/7/1498/s1, Table S1: BDC Dust characteristic characteristics; Figure S1: BDC Oil number (the sample weight was 4.3 g at 125 rpm and the dosing frequency was 4.0 mL/mL).

**Author Contributions:** Conceptualization, investigation, writing—original draft preparation, formal analysis, M.P.; software, methodology, resources, M.P. and O.D.; visualization, project administration O.D.; supervision, validation A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflicts of interest.
