# Structural, Thermal and Mechanical Assessment of Green Compounds with Natural Rubber
## 3 Results
### Spectroscopy Characterization  
To obtain information on the microstructure of rubber composites, FTIR/ATR spectroscopic studies were conducted. Figures 8 and 9 show the spectral area comprised between 1800 and 1450 cm\({}^{-1}\). The spectra of natural rubber samples filled with 50 phr of GTR from cars and trucks devulcanized by microwaves are compared.  
Two bands appearing at 1736 cm\({}^{-1}\) and 1728 cm\({}^{-1}\) are present in the spectra. These bands are related to the stearic acid included in the sample, which migrates from the core to the surface as exudation. The band at 1736 cm\({}^{-1}\) is assigned to the free carbonyl group, and the band at 1728 cm\({}^{-1}\) is attributed to a carbonyl group bonded with water molecules from the environment or other polar groups. These bands are bigger in NR than in NR filled with GTR because the presence of GTR avoids the movement of the stearic acid chain through the rubber structure, producing more impediment to reach the surface. The band at 1538 cm\({}^{-1}\) assigned to zinc steater (ZnSt\({}_{2}\)) appears due to the reaction between ZnO and stearic acid during the manufacturing process [33], and it is related to the crosslink density and due to the amount of stearic acid that has been able to react. When the crosslinking decreases, there is an increase in the amount of ZnSt\({}_{2}\) on the surface that causes a high band at 1538 cm\({}^{-1}\). This is indicative of the facility of the molecule of ZnSt2 to pass through the elastomeric structure. TMmW5 treatment, and in minor measure TMmW10 treatment, improve the ability of the sample to create more polysulfide bridges and avoid the movement of ZnSt\({}_{2}\) (see Figure 9). After 10\({}^{\prime}\) min of treatment, as commented above, the degradation of the main elastomeric chains results in a loss of mechanical properties.  
Figure 8: Spectral area of 100 NR with 50 phr GTRcar devulcanized by microwaves for different periods of time: (**a**) NR, (**b**) 50 GTRc(NT)/NR, (**c**) 50 GTRc(TMmW3)/NR, (**d**) 50 GTRc(TMmW5)/NR and (**e**) 50 GTRc(TMmW10)/NR.  
Another interesting group of bands appear from 1596 to 1660 cm\({}^{-1}\) and are related to the double bonds C=C. NR spectra show lower absorbance values of C=C than samples with GTR. In samples with GTR devulcanized, these values increase with the treatment time. This means that, as mentioned previously, there is a scission of the main chain of the polymer produced by the devulcanization, and double bonds are generated with TMmW treatment, although not all C=C react subsequently with S to create sulfur bridges during the revulcanization. Some of them remain unreacted.  
The spectra of the samples of rubber composites with GTR from trucks show little difference compared to the samples of GTR from cars. The presence of ZnSt\({}_{2}\) on the surface of samples with GTR from trucks is lower than that on samples from cars. This means that, as found in the swelling results, the crosslink degree in samples with GTRtruck is higher, avoiding the diffusion of zinc stearate from the core to the surface [34]. Also, the amount of double bonds (C=C) is lower in samples with GTR from trucks because the TMmW devulcanization treatment is more efficient and allowed the formation of more sulfur bridges than in case of GTR from cars.  
Figure 10a shows the particles of zinc stearate on the surface of 0 GTRc (NT)/NR. Figure 10b shows a magnification of 8000X; the scanning electron micrograph, SEM, reveals the structural morphology displayed by the crystalline shape of ZnSt2. It is a random platelike crystal with a size that varies between 1 and 3 microns [35].  
Figure 9: Spectral area of 100 NR with 50 phr GTRtruck devulcanized by microwaves for different periods of time: (**a**) NR, (**b**) 50 GTR(NT)/NR, (**c**) 50 GTR(TMmW3)/NR, (**d**) 50 GTR(TMmW5)/NR and (**e**) 50 GTR(TMmW10)/NR.