## 3 Results and Discussion  
### Analysis of NR Vulcanization Process  
To analyze the influence of the organic peroxides and fillers such SWCNTs and CB on the vulcanization of the NR composites, curing curves for each NR formulation were obtained. In order to illustrate this effect, the viscosity vs. time curves obtained for NR-SWCNTs composites using DCP at 160 \({}^{\circ}\)C are depicted in Figure 2. As observed, three zones can be distinguished in a typical vulcanization curve. The first one, at low viscosity values, is defined as an induction zone where the free radicals are forming, and the earlier reactions take place. The second zone corresponds to the curing stage, which is characterized by an increase in the viscosity of the elastomer; during this time, most of the cross-linking reactions occur. The vulcanization time (_t.v._) is the time at which the highest viscosity value is reached on the vulcanization curve. The third zone is defined as an over-curing or maturation stage due to an excess of free radicals; in this region, a decrease in viscosity can be observed since the applied torque, which can lead to a certain degree of degradation of the NR. As shown, the curing curve of neat NR-DCP composite exhibits a relatively extended induction zone maintaining viscosity values almost constant by around 200 s; after, it suddenly reaches its highest viscosity value at 252 s. On the other hand, when SWCNTs are added to the formulation, a significant decrease in induction time is observed, producing that the curing peak of NR occurs at lower vulcanization time and at lower viscosity values compared to the NR composites without fillers. As for the NR-DCP-CB composite, the vulcanization curve shows a much more gradual tendency toward the maximum point of curing, which is recorded at a higher _t.v._ value than the NR-DCP-SWCNTs composite and at a lower viscosity value compared to the neat NR-DCP and NR-DCP-SWCNT composites. Figure 2: Curing curves of the NR composites using DCP at 160 \({}^{\circ}\)C.  
The experimental vulcanization times (_t.v._) were obtained from the analysis of the curing curves of each NR formulation; results are listed in Table 4. As shown in this table, good agreement is obtained between the calculated and the experimental _t.v._ values of the neat NR formulations. When NR was filled with SWCNTs or CB, a reduction in the _t.v._ value is observed in all cases. In this set of composites, a remarkable 50% reduction in _t.v._ (from 600 to 300 s) was found when sulfur was used as the vulcanization promoter. Similar results are found for the NR filled with SWCNT composites using organic peroxides as cross-linking agents, recording a notable reduction in _t.v._ of up to 31.7% (for T29) and 22.4% (for DCP) compared to the neat NR composites; such a reduction in _t.v._ occurs in the following order T29 > DCP > T145. We suggest that whenever SWCNTs are used as filler for NR, it is necessary to consider the decrement in _t.v._ to avoid an over-cure when the NR is processed. The reduction in _t.v._ could be considered as an acceleration of the vulcanization reaction and be attributed to two possible effects: (i) the generation of free radicals due to the thermal decomposition of the organic peroxide or accelerator (when sulfur is used--i.e., MTBS); here, the heat transfer can drive the extent of the radical generation.