## 5 Results And Discussion
### Static Mechanical Properties  
Generally, vulcanization reaction reach a maximum and start to decrease, resulting in weaker mechanical properties for longer curing times. This _reversion_ phase is due to the breakdown of some  
Figure 2: Schematic illustration of the fabrication process involving a cross-linking reaction between leather collagen fibers and natural rubber (NR), in which carbon black serves as cross-linker \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Compounds**} & \(\mathbf{t_{02}}\) & \(\mathbf{f_{90}}\) & \(\Delta M\) & \(CRI\) & \(\rho\) & \(R_{E}\) \\  & **(s)** & **(s)** & **(Nm)** & **(s\({}^{-1}\))** & **kgm\({}^{-3}\)** & **(\%)** \\ \hline BC & \(131\pm 12\) & \(678\pm 65\) & 3.3 \(\pm\) 0.3 & 0.18 \(\pm\) 0.01 & 1260 \(\pm\) 25 & 80 \(\pm\) 4 \\ CMPTD & \(115\pm 11\) & \(504\pm 48\) & 3.7 \(\pm\) 0.4 & 0.26 \(\pm\) 0.02 & 1260 \(\pm\) 25 & 74 \(\pm\) 4 \\ CMCS & \(122\pm 12\) & \(534\pm 51\) & 3.8 \(\pm\) 0.2 & 0.24 \(\pm\) 0.01 & 1270 \(\pm\) 25 & 70 \(\pm\) 3 \\ COT & \(106\pm 10\) & \(492\pm 47\) & 2.8 \(\pm\) 0.2 & 0.27 \(\pm\) 0.01 & 1250 \(\pm\) 25 & 72 \(\pm\) 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristic parameters of vulcanization curves of rubber compounds obtained from leather waste. Experimental error for each value is calculated as standard deviation of repeated measurements. unstable polysulfide cross-links formed in the vulcanization step leading to deterioration of physical properties. In industrial practice, the vulcanization process lasts for long times, then, in order to understand the influence of the reversion on the mechanical properties of formulations based on NR with the incorporation of leather waste, the vulcanization effect was monitored for a time exceeding the optimum value. Figure 3a shows \(\sigma_{b}\) and \(\epsilon_{b}\) vs. the vulcanization time, for CMPTD, CMCS and COT compounds. For comparison, \(\sigma_{b}\) and \(\epsilon_{b}\) of the reference BC, measured under the same experimental conditions, have also been plotted. It is immediately seen that compounds, except COT, exhibit a \(\sigma_{b}\) lower than BC. As regards the elongation at the rupture point, all compounds display \(\epsilon_{b}\) lower than BC. Furthermore, COT formulation shows a \(\sigma_{b}\) value significantly larger than BC. Figure 3b shows \(\sigma_{300}\) and \(H\) (shore A) vs. the vulcanization time. All the formulations investigated have values of \(\sigma_{300}\) and \(H\) (shore A) greater than BC. In addition, \(\sigma_{300}\) decreases over time. This means that subjected to the same stress CMPTD, CMCS and COT compounds extend less than BC reference, therefore, in working conditions, these materials undergo less elongation. This is consistent with the literature [28], in which NR-based compounds with the incorporation of leather waste were found to have a significantly lower breaking load and a strong increase in hardness. Hence, it can be inferred that chrome-tanned leather waste like PTD and CS are suitable for manufacturing floors mats, soles and heels, while the COT compound can be used for making hose or conveyor belts where a greater tensile strength is required. The increased hardness of the compounds was further investigated by monitoring the vulcanization of CMPTD and BC compounds via SEM. Figure 4 illustrates the SEM micrographs of the flexural fractured surface of CMPTD and BC compounds, immediately after vulcanization. Inspection of Figure 4b shows that the polymeric matrix incorporates some partially melted and vitrified fibers. It is well-visible that a large fiber and many small fibers are emerging from the matrix, as highlighted by the yellow contour. Likely, the persistence of the CMPTD compound, for a long time, at high temperature leads to melting and vitrification of crystalline regions. Figure 3: Vulcanization rates for CMPTD, CMCS and COT compounds. (**a**) Breaking load and elongation-at-break vs. vulcanization time; (**b**) load at 300% and hardness \(H\) (shore A) vs. vulcanization time.