# Structural, Thermal and Mechanical Assessment of Green Compounds with Natural Rubber
## 2 Materials and Methods
### Measurements  
Chemical structure of rubber composites (RCs) was determined using Fourier transform infrared spectroscopy (FTIR) analysis performed by means of a Spectrum Two spectrometer from Perkin Elmer (Waltham, MA, USA). The device had an ATR attachment with a diamond crystal. Measurements were performed with 2 cm\({}^{-1}\) resolution in the range 550-4000 cm\({}^{-1}\)Tensile strength and elongation at break of the obtained vulcanized rubber composites were tested according to the standard ISO 37 [19] using an Instron testing machine with pneumatic cell load capacity of 2 kN. Tensile tests were performed at a crosshead speed of 500 mm/min. Direct extension measurements were conducted periodically using an extensometer with sensor arms. Hardness was determined using a Zwick 3130 durometer Shore A (Ulm, Germany) in accordance with the standard ISO 7619-1 [20]. The reported results are the average of 5 measurements per sample. The crosslinking density (CLD) calculation is based on the equilibrium swelling theory, which is defined by [PERSON], and modified Flory-Rehner equation for filled rubber vulcanizate. The crosslink density was determined according to the ASTM D6814-02 [21] standard method, through swelling measurements of the rubber composites which were let to swell in cold toluene for 72 h, refreshing the solvent every 24 h, followed by drying to constant mass at 80 \({}^{\circ}\)C. The Flory-Rehner Equation (2) [22] was used to calculate the crosslink density. X is Flory-Huggins parameter interaction of the polymer-solvent set equal to 0.414 [23], considering NR as main polymer and toluene as solvent. \[v_{e}=\frac{-[\ln(1-V_{r})+V_{r}+\chi V_{r}^{2}]}{[V_{1}(V_{r}^{1/3}-V_{r}/2)]} \tag{1}\]  
where \(v_{e}\) = crosslink density, mol/cm\({}^{3}\); \(V_{r}\) = gel volume in the swollen sample; \(V_{1}\) = solvent molar volume (toluene = 106.2 cm\({}^{3}\)/mol);  
\[V_{r}=\frac{\frac{\mathrm{m_{p}}}{\mathrm{m_{p}}}}{\frac{\mathrm{m_{p}}}{ \mathrm{m_{p}}}+\frac{\mathrm{m_{s}}}{\mathrm{m_{p}}}} \tag{2}\]  
where \(\mathrm{m_{p}}\) is mass of the dry polymer, \(\mathrm{\rho_{s}}\) is density of the solvent (toluene 0.8662 g/cm\({}^{3}\)), \(\mathrm{m_{s}}\) is mass of the solvent absorbed by the swollen sample, \(\mathrm{\rho_{p}}\) is density of the polymer. The rubber density was 0.92 g/cm\({}^{3}\) (NR), 0.945 g/cm\({}^{3}\) (10 phrGTR), 0.985 g/cm\({}^{3}\) (30 phrGTR), 1.013 g/cm\({}^{3}\) (50 phrGTr). The Kraus correction model was applied due to the presence of carbon black as a filler. The carbon black density was chosen equal to 1.85 g/cm\({}^{3}\). The total apparent crosslink density (\(v_{total}\)) with Kraus correction was calculated according to Equations (3) and (4):  
\[v_{total}=\frac{v_{e}}{1+K+\Phi} \tag{3}\]  
\[\Phi=\frac{\varphi_{f}\times\rho_{r}\times m_{0}}{\rho_{f}\times m_{dry}} \tag{4}\]  
where \(v_{e}\) = measured chemical crosslink density, mol/cm\({}^{3}\); \(v_{total}\) = actual chemical crosslink density, mol/cm\({}^{3}\); \(K\) = constant characteristic of the filler, but which, independent of the solvent, was chosen to be 1.17 [24]; \(\varphi_{f}\) = volume fraction of filler in the sample which is calculated; \(\rho_{r}\) = density of studied compound, g/cm\({}^{3}\); \(m_{0}\) = weight of sample before extraction, g; \(\rho_{f}\) = density of filler, g/cm\({}^{3}\); \(m_{dry}\) = weight of sample after extraction, g.  
In addition to the total crosslink density values, the distribution of sulfur bonds (monosulfidic, disulfide and polysulfide) in the cured rubber was determined using a two-step thiol-amine reaction [25].