**3. Research Techniques**

The curing kinetics of the SBR compounds were studied in a moving die rheometer (MonTech MDR 300, Buchen, Germany) at 150 ◦C, according to the ISO 3417 standard. The rubber compounds were vulcanized according to the optimal curing time (τ90) in a standard electrically heated hydraulic press at a temperature of 150 ◦C, with a pressure of 15 MPa. The mechanical properties of the prepared composites were tested using a Zwick universal testing machine, model 1435 (according to PN-ISO 37:1998). The tensile strength (TSb) and percentage elongation at break (Eb) were determined. Hardness testing (H, ◦Sh) was carried out using a Shore electronic hardness tester, type A, with a force of 12.5 N, according to standard PN-80C-04238 (Zwick/Roell, Herefordshire, UK).

The polymer–solvent interaction parameter (0.378 for SBR rubber in toluene solvent) was determined based on the equilibrium swelling method (according to PN-ISO 1817:2001/ap1:2002). The cross-link density (<sup>ν</sup>, 10<sup>4</sup> mol/dm3) was calculated as the volume fraction of rubber in the swollen material, and VS = 106.3 mol/cm<sup>3</sup> for the molar volume of solvent (toluene) [13–17,27].

Cross-linking density, ν, was calculated on the basis of Flory–Rehner's Equation (1):

$$\mathbf{v} = \frac{\left[\ln(1 - \mathbf{V}\_{\mathbf{r}}) + \mathbf{V}\_{\mathbf{r}} + \mu \mathbf{V}\_{\mathbf{r}}^{3}\right]}{\mathbf{V}\_{\mathbf{O}} \left(\mathbf{V}\_{\mathbf{r}}^{\frac{1}{3}} - \frac{\mathbf{V}\_{\mathbf{r}}}{2}\right)} \tag{1}$$

where μ is the Huggins parameter for the uncross-linked polymer–solvent system and Vr is the molar volume of the swelling solvent.

The thermo-oxidative experiments were performed in a convection oven and a thermal chamber with air circulation (Binder GmbH, Tuttlingen, Germany). An unstressed sample was exposed to the action of circulating air at 70 ◦C for 168 h. To study the deformation energy of the vulcanizates as a result of biological aging, the aging factor (S) was calculated according to Equation (2):

$$\mathbf{S} = \frac{\mathbf{T} \mathbf{S}\_{\mathbf{b}1} \times \mathbf{E}\_{\mathbf{b}1}}{\mathbf{T} \mathbf{S}\_{\mathbf{b}2} \times \mathbf{E}\_{\mathbf{b}2}} \tag{2}$$

where TSb1 × Eb1 is the tensile strength (MPa) and elongation at break (%) after thermal-oxidative aging or soil test and TSb2 × Eb2 is the tensile strength (MPa) and elongation at break (%) before thermal-oxidative aging or the Soil Test.

A biodecomposition test was performed in soil with paddle-shaped samples with dimensions of 7.5 cm by 1.25 cm, and sampling of 0.4 cm. The samples were placed in an active universal soil (10 cm depth) and incubated at a temperature of 30 ◦C with 80% RH for 90 days in a climatic chamber (HPP 108 Memmert GmbH, Schwabach, Germany). Tests were carried out according to PN-EN ISO 846. The soil test was analyzed following the method described by Tadeusiak et al. [15]. Surface topography of the composites was conducted after the soil tests, using photos taken with a Canon CanoScan 4400F device. The morphology of the BDC powder and SBR composites were analyzed using a scanning electron microscope (SEM), Zeiss Ultra Plus (Bruker). Prior to the analysis, the samples were coated with a carbon target using a Cressington 208 HR system [13,16]. A Nicolet 6700 FT-IR spectroscope (Thermo Scientific, Waltham, MA, USA) with Fourier transformation and ATR snap was used to determine the characteristics of the composites. Analysis performed in the range of 4000 to 400 cm<sup>−</sup><sup>1</sup> [13]. Differential scanning calorimetry (DSC) was performed using a DSC1 analyzer (Mettler Toledo, Netzsch, Switzerland) at a heating rate of 10 ◦C/min. The SBR samples were heated from −150 ◦C to 350 ◦C under a nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed using a TGA/DSC1 analyzer (Mettler Toledo, Netzsch, Switzerland). The heating rate was 10 ◦C/min under a nitrogen atmosphere, across a temperature range of 25 to 900 ◦C. DSC was analyzed as described by Procho ´n et al. [28]. The changes in color of the samples after the thermo-oxidative aging process and soil test were studied using a UV–Vis CM-3600d spectrophotometer (Konica Minolta, London, UK). The difference in color was expressed as the color change parameter dE × Lab, where L is the level of lightness or darkness, a is the relationship between redness and greenness, and b is the relationship between blueness and yellowness [13].

Elemental analyses of the carbon, hydrogen, and nitrogen elements in the BDC powder were carried out using a Vario EL III analyzer equipped with special adsorption columns and a thermal conductivity detector (TCD). The absorption of dibuthylphtalate (DBP) by the BDC powder was measured using an Absorptometer C (Brabender mixer, Brabender GmbH & Co. KG, Duisburg, Germany). The sizes of the BDC particles were determined in water (filler concentration of 0.01 g/250 mL) using the dynamic light scattering (DLS) method on a Zetasizer Nano (Malvern Instruments, Malvern, Great Britain) analyzer [14].
