*4.1. Characterization of BDC Powder*

The valence vibrations of bands derived from methyl and methylene groups in the tested BDC dust are visible in the wave number range of 3420 to 3479 cm<sup>−</sup><sup>1</sup> (Figure 1). Maxima of the bands appeared in the dust spectrum at a lower wave number intensity. Bands at 1750 cm<sup>−</sup><sup>1</sup> (–COO−) indicate the presence of fatty substances [13,14,28].

**Figure 1.** IR spectra of buffing dusts of collagen with ranges: (**a**) 3500–3200 cm<sup>−</sup>1; (**b**) 1650–1500 cm<sup>−</sup>1; (**c**) 590–510 cm<sup>−</sup>1.

In the area from 3200 to 3500 cm<sup>−</sup><sup>1</sup> there is a wide band corresponding to the valence vibrations ofhydroxyl (–OH) side chains and end groups, with higher intensity in the dust spectrum. In the range of 1650 to 1500 cm<sup>−</sup>1, a band of deformation vibrations of the first-order amide appears for (C=O) and second-order amide (NH). The effect of chromium on interactions with other components of the BDC powder, and thus other interactions, was visible through shifts in the absorption bands, reducing their intensity, etc. The characteristic absorption band at 1654 cm<sup>−</sup><sup>1</sup> may be attributed to the possible mechanism of interaction of Cr with the protein-like system –Cr–OOC– (Figure 1b) [6]. The band of COOH stretching vibrations derived from amide I is shifted from 1660 cm<sup>−</sup>1. There is also a characteristic wide absorption band for chromate samples at about 1000 cm<sup>−</sup>1, which formed as a possible result of the interaction of chromium with a carboxyl group. The presence of Cr–O–Cr bonds is indicated by the bands between 510 and 650 cm<sup>−</sup><sup>1</sup> [6,13].

The shape, particle size, and specific surface of the filler are known to have a decisive impact on the strength of rubber–filler joints. Dust morphology was assessed based on photos taken using SEM, as shown in Figure 2. Dust agglomerates are visible as primary particles with a regular structure: elongated, insulated fibers with a wide size distribution from several hundred nanometers to several micrometers.

One collagen macrofibrillary fiber is connected to several or even several dozen individual helical microfibers (Figure 2a), with diameters of ~3–4 micrometers and longitudinal segmentation.

The particle size distribution of the tested BDC was measured by dynamic light scattering (DLS) in an aqueous solution. The particle size distribution, which is a compilation of measuring the length and diameter of particles oriented in the laser light field, was in the range from 469 to 295 nm. The isoelectric Point (IEP) was at pH 5.9. There was an appropriately small area of 9 m<sup>2</sup>/g. However, based on the elemental analysis of BDC, the nitrogen conversion to protein substance, which determines the

nitrogen content in the collagen, was 7.92%. The Cr converted to Cr2O3 was at 4.48%. Dry matter was 89.49%, and ash was ~7% [13,14].

**Figure 2.** SEM images of buffing dust collagen (BDC): (**a**) 5000× magnification, (**b**) 50,000× magnification, and (**c**) 100,000× magnification.

Oil number is one of the important factors that measure the structure and surface of fillers. From a morphological point of view, fillers have the ability to form aggregates or agglomerates. To prevent the influence of physico-chemical interactions, fillers are often subjected to modifications aimed at changing their structural or surface characteristics. The structure of the BDC had been changed under the influence of the chemical modification processes during tanning.

As can be seen in the Annex (see Supplementary Materials Figure S1), after the addition of dibutyl phthalate (DEP), the BDC molecules begin to approach each other and form agglomerates. There is increasing resistance to mixing, due to the higher torque. At the moment of maximum saturation with PBT, the process ends, obtaining maximum torque. The maximum torque was 248.4 mNm. The filling volume efficiency was determined using the Medalia model, according to Equation (3):

$$\varphi\_{\rm eff} = 0.5 \mathcal{Q} \left[ 1 + \left( 1 + \frac{0.0213(\rm DBP)}{1.46} \right) \right] \tag{3}$$

where ϕeff is the actual volume of the filler. The efficiency of the BDC was 4.33 mL/g. The moisture content of the BDC particles was in the range of 10<sup>2</sup> to 10<sup>4</sup> nm. This important parameter classifies the BDC filler in the group of semi-reinforcing fillers. The filler shows a high degree of orderliness and a tendency to form aggregates.
