# Structural, Thermal and Mechanical Assessment of Green Compounds with Natural Rubber
## 3 Results
### Mechanical Properties
#### 3.1.2 Compounds including Devulcanized GTR  
The mechanical properties of the rubber composites obtained with GTR previously devulcanized are shown in Figures 3 and 4. The tensile properties of compounds of GTR devulcanized with the combined process were much better than those of non-devulcanized GTR. In rubber composites with 50 phr of GTRcar and GTRtruck treated by microwave for \(10^{\prime}\), the elongation at break increased from 695% to 728% and 675% to 779%. This improvement of tensile properties, of 5% and 13.5%, respectively, is due to the devulcanization of GTR, which provides better bonding between GTR and NR and an increase in the roughness and specific surface of the devulcanized particles [27, 28]. As Figures 3 and 4 show, the mechanical properties of the GTRcar and GTRtruck compounds are very similar, regardless of the type of devulcanization (time of mW) that has been carried out on each of the samples. However, the tensile strength obtained by the GTRtruck samples is slightly higher than that of the GTRcar. The elongation  
Figure 3: Tensile properties of NR/GTRcar compounds. Figure 2: Stress–strain curves for NR vulcanizates with different amounts (0,10, 30 and 50 phr) of non-devulcanized GTRtruck (GTRt). at break and the modulus of Young at 300% present very similar values regardless of the amount of GTR used (phr) and the type (car or truck), but in any case they are higher than the samples with non-devulcanized GTR. The differences in the results of GTRcar and GTRtruck are related to the composition of each type of GTR. GTRtruck has a higher content of NR than GTRcar (27% and 14%) [29] and SiO2 (9.8% and 7.3%, respectively). As reported in a previous studies [14], GTR samples with a high content of SiO2 are easier to devulcanize than samples with low content of SiO2. This means that SiO2 acts as a catalyzer in the devulcanization process by microwaves. Higher compatibility between GTRtruck and NR is then achieved due to the better devulcanization and the similar composition of both components, which explain the tensile results. Figure 5 shows that the hardness values for the different samples of each type of GTR are analogous. Likewise, it is also worth noting that the values of the GTRtruck show greater dispersion and are lower than those of the GTRcar. This is mainly due to the components of each type of GTR; the GTRcar has a higher content of synthetic elastomers than the GTRtruck, these being more rigid than natural rubber. The type of devulcanization treatment does not have a significant influence, although the hardness decreases with the GTRd content, significantly for the GTRtruck. Figure 4: Tensile properties of NR/GTRtruck compounds. Figure 5: Hardness average values of different samples. The results of the crosslink density and percentage of sulfidic bridges for GTRcar and GTRtruck rubber composites are shown in Figures 6 and 7. The maximum value for crosslinking has been obtained for fresh natural rubber (reference sample) at 130 umol/cm\({}^{-3}\). The crosslink density was found independent of the type of GTR used (car or truck). For all quantities of GTR present in the rubber compounds, the crosslink density increased as a function of the time of treatment, with a maximum value obtained at TMmW5 and a subsequent decrease at TMmW10 treatment more obvious when using GTR from cars than GTRtruck. As can be observed in Figures 6b and 7b, the percentage of polysulfide bonds is much higher than that of mono- and disulfide bonds, completely in accordance with what is known as conventional vulcanization, when A/S (accelerator/sulphur) ratios are lower than 0.7. The TMmW devulcanization process breaks mainly the S-S bridges and, randomly, some parts of the main elastomer chain, as demonstrated by the Horikx plot obtained in several studies [30, 31].