4.2.6. DSC & TGA analysis

The results of the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of selected SBR and SBR10 composites are shown in Table 4, as well as in Figures 8 and 9.


**Table 4.** Thermal characteristics of SBR and SBR10.

T5%—decomposition temperature at 5% mass loss, Tp(DTG)—temperature of maximum conversion rate on the DTG curve, Δm total—total mass loss during thermal decomposition, Tg—glass transition temperature (standard deviations: T5, Tp (DTG) ± 2 ◦C; Δm total ± 0.6%; Tg ± 2◦C)

**Figure 8.** Thermogravimetric analysis (TGA) curves of SBR composites: reference sample SBR (black curve) and SBR10 sample (red curve).

**Figure 9.** Comparative analysis of DSC results: SBR—unfilled (black curie) and SBR10 (10 phr of BDC—redcurve).

The introduction of the BDC filler to SBR did not significantly affect the temperature of thermal decomposition. In thermogravimetric analysis, a 5% weight loss was observed for the SBR10 composite at a slightly lower temperature than the reference sample. As can be seen from the standard deviation, the maximum decomposition temperatures of all the composites are similar. The total weight loss of the tested elastomers during decomposition was 95% for the reference sample and 90–91% for the composites filled with BDC.

The glass transition temperature for pure styrene–butadiene rubber is reported in the literature data as 48/65 ◦C [31]. The DSC revealed that the glass transition temperature Tg begins for the SBR composite at onset, i.e., at −46.6 ◦C, whereas for SBR10, Tg begins at −49.60 ◦C (Table 4, Figure 9). The glass transition temperature is shifted relative to the reference composition (SBR, −42.08 ◦C) towards lower values for the composition containing BDC dust (−45.45 ◦C). This shift is clear in the case of the sample with 10 parts wt. BDC. It suggests that BDC filler acts as a plasticizer in SBR compounds. The heat capacity of the SBR composite was 0.450 Jgˆ-1Kˆ-1, whereas for SBR10, heat capacity decreased to 0.399 Jgˆ-1Kˆ-1.

4.2.7. Swelling Balance in Water, Color Research, Soil Test, and Thermo-Oxidative Aging Process

There is considerable interest in accelerated aging processes and the behavior of polymer materials in contact with various factors. The dust-based elastomer composites were therefore subjected to water infraction into the material structure (Qw, -), soil tests, color change, and accelerated thermo-oxidative aging ( ΔT, -). Figure 10 presents the results of equilibrium swelling in water. The addition of protein filler caused swelling extension relative to the unfilled sample. Due to the hydrophilic character of protein, higher water absorption was observed. The absorption of water facilitated the penetration of microorganisms, and thus the biodegradation process was more rapid.

**Figure 10.** Equilibrium of swelling in water results from SBR vulcanizates.

The next test was to determine the surface color change (dE × ab, -) of the vulcanizates exposed to elevated temperature and biological properties (soil tests). Table 5 shows photographs taken before and after biodegradation. As can be seen, topographic changes occurred in the composites containing the bionic biofiller. There were discolorations in the material where micro- and macro-cracks formed on the surface. The change in the appearance was influenced by the action of the biological agents found in soil, which, by breaking down the protein filler, contributed to the deterioration of the properties of the tested vulcanizates.


**Table 5.** Topography photos of surface changes of SBR composites before and after biodegradation tests.

Factor dL × ab was found to change the color of the sample after degradation relative to the sample before degradation. The highest values for dL × ab were found for the native composite, SBR, which proves that the other samples were protected against thermo-oxidative aging, especially in the case of the 10 or 20 phr biopolymer. Higher values for this indicator are associated with greater color deviation in the case of biodegradation. The action of soil organisms promotes the degradation of the filler, which leads to a loss of consistency in the material structure. This indicates much greater susceptibility to biodegradation in the case of vulcanizates containing the protein filler. The changes in the color of the composites were affected by the aging conditions, i.e., temperature, humidity, as well as microorganisms contained in soil in soil tests.

It was found that addition of protein filler resulted in acceleration of biodegradation in almost all of the samples. The mechanical deterioration of composites with BDC dust to the soli test can also be seen on the basis of the aging coefficient (ΔT, -), as the combination of tensile strength and elasticity before and after aging (Table 6).


**Table 6.** Mechanical properties after biodegradation.

On the basis of soil tests, it appears that biodegradation caused a decrease in tensile strength (Tables 5 and 6). This decrease is probably due to filler degradation, which causes a reduction in intermolecular bonding [32]. It was found that the addition of the protein filler resulted in accelerated biodegradation in almost all the samples. The hardness of samples was greater after the soil tests than before the tests. The greater hardness was a consequence of the biodegradation process. Increased hardness was noticed as a result of thermo-oxidative aging relative to the non-aged samples. The noticeable increase in hardness may be associated with greater cross-link density, secondary cross-linking, or the evaporation of water derived from the hydrophilic filler. Increasing the filler load resulted in higher hardness values.
