## 5 Results And Discussion
### Reinforced Materials  
Experimental results discussed above have revealed that the CMCS sample exhibits slightly more favorable mechanical performance than the CMPTD sample, such as the hardness which is a highly appreciated parameter for the products herein investigated. However, PTD waste is produced by leather industries in greater quantities. Furthermore, due to its chemical constitution PTD-waste must only be disposed of in landfills. Based on these considerations, formulations containing MPTD and OT waste were reinforced with carbon black. The reinforcing agent develops a high surface area, increases tensile strength and hardness (shore A) [29; 30] favoring the interactions between fiber-waste and polymer matrix. Thus, compounds were vulcanized in the presence of carbon black and hardness shore A monitored as the vulcanization proceeded. A direct comparison between reinforced and non-reinforced compounds reveals that both RBC and all reinforced samples undergo a hardness increase of about 10 \(H\)-unit and resilience RE around 60%. As expected, the carbon black compacts the collagen fibers and the hardness of the material grows. A careful inspection of Figure 5 reveals that mixtures containing a constant waste concentration, but a different fiber-length, namely RMPTD\({}_{10}\), RMPTD\({}_{10}\) undergo only a slight change in the mechanical properties, while RFPTD\({}_{10}\) (2-20 mm fibers) has a rather different behavior. In particular, the RFPTD\({}_{10}\) compound has lower \(\sigma\) and \(\sigma_{b}\) than RBC, furthermore \(\epsilon_{b}\) is significantly larger. Usually, the incorporation of fibers into NR causes an increase in \(\sigma\) and \(\sigma_{b}\) with a corresponding decrease in \(\epsilon_{b}\). To explain this unusual behavior, it must be borne in mind that both the texture of the leather and the three-dimensional structure of collagen-fibers contribute to the mechanical properties of these materials. In other words, texture-relaxation and collagen-stretching cooperate for the mechanical properties of the composite material. Generally, plastic relaxation of texture is the predominant effect, however if the sample is reduced to a sufficiently small size, the relaxation effect is negligibly small and collagen-fiber stretching prevails. Thus, results displayed in Figure 5 indicate that RFPTD\({}_{10}\) compound-sizes are such that the texture relaxation strongly influences the values of \(\sigma\) and \(\epsilon_{b}\). This aspect is particularly useful in the footwear industries, where shoe uppers must be manufactured to provide comfort by adapting to different shapes of feet. Table 4 reveals that for formulations with increasing fiber content (RMPTD\({}_{5}\), RMPTD\({}_{10}\), RMPTD\({}_{20}\)), several mechanical parameters vary very little for fiber concentrations higher than 10%. This implies that, where they can be used as materials, large quantities of leather waste will be disposable. Figure 4: SEM images of rubber compounds. (**a**) Sample BC immediately after vulcanization; (**b**) sample CMPTD immediately after vulcanization. Yellow lines highlight fibers emerging from the matrix. Furthermore, from Table 4 it is evident that ROT\({}_{10}\) compounds exhibit the lowest Mooney viscosity. This is because fish oil, present only in these cutouts, performs a lubricating function. Mechanical properties similar to those of CMPTD and RMPTD compounds were obtained by [PERSON] ([31]) through chemical pretreatments and use of compatibilizers. For the ROT compound, \(\sigma_{b}\) and \(\epsilon_{b}\) are greater than for RBC. Once again there is unusual behavior, however the OT fibers are now very small in size, so that the resulting effect cannot be explained by the texture-size. Indeed, the comparison between the various samples, as illustrated in Figure 2, suggests that carbon black activates and accelerates a strong collagen fiber-NR-tanning oil interaction. The OT sample still contains 3-5% of fish oil in the free form (removable by solvent).