Highly Efficient MeO Nanoparticles as Curing Activator for Rubber Composites

The vulcanization of rubber by sulfur is a large-scale industrial process that improves the mechanical properties of unsaturated rubber by the cross-linking of polymer chains. Microcrystalline ZnO particles are currently used as activator to enhance the efficiency of the crosslinking. Due to their not easy dispersion in the rubber matrix and to the relatively small fraction of ZnO which actually reacts with the other curative compounds, a large amount of oxide is required for the process. This leads to a not homogeneous vulcanization with inefficient polymer network structure and a possible release of the metal in the environment with potentially negative effects. Thus, a possible improvement of ZnO efficiency as activator requires an enhancement of its availability, distribution and capability to release Zn2+ ions in the rubber matrix. Aim of this thesis, is the enhancement of the curing efficiency by employing ZnO nanoparticles (NPs) grown on silica filler particles. A simple low-temperature sol-gel procedure was exploited to grow amorphous spherical ZnO NPs directly on silica surface and linked by Si-O-Zn covalent bonds (ZnO/SiO2) which simultaneously behave as curing agent and reinforcing filler. The morphology and crystal structure of ZnO/SiO2 NPs were investigated by XRD and TEM. The nanometric size was assessed by UV reflectance measure, while the Si-O-Zn bond interaction was studied by NMR, ATR-FTIR and XPS and discussed in relation to the NPs dimensions. The ZnO/SiO2 NPs were used to prepare cured polyisoprene nanocomposites loaded with silica. Moreover, the curing efficiency and dynamic mechanical properties of vulcanized ZnO/SiO2 nanocomposites were compared to those obtained by using microcrystalline ZnO as activator in conventional curing processes. In the case of nanocomposite cured with ZnO/SiO2 NPs showed better curing efficiency, higher cross-linking density and improved mechanical properties than the same composites treated by conventional ZnO particles. In order to explain the better efficiency of ZnO/SiO2 NPs in the vulcanization, the pseudo activation energies of the different stages of the process were evaluated by performing DSC measurements on the nanocomposites. The presence of ZnO/SiO2 NPs induces lower activation energies and, therefore, faster kinetics compared to microcrystalline ZnO, particularly in the first steps of the reactions. This is in agreement with the highest ability of zinc in ZnO/SiO2 to react with curatives, thus improving the cross-linking density. The role of ZnO/SiO2 NPs in the crosslinking reaction was further studied by a model compound vulcanization (MCV) approach, using tetramethylethylene as model monomer and analyzing the cured products by LC-MS chromatography and 1H-NMR spectroscopy. The results evidence a higher crosslinking for ZnO/SiO2 due to the larger amount of more stable mono- and di-sulphide crosslinking chains than for conventional ZnO. The comparison between the two catalysts have been also discussed on the basis of FTIR investigation which evidence in ZnO/SiO2 the formation of highly reactive stearate bridged bidentate zinc complex as possible intermediate in accelerating the cross-linking reaction. These results suggest that the proposed material could be considered as promising system for curing industrial application with reduce zinc loading. Finally, similar activator/filler NPs loaded with CaO and MgO were also prepared and tested to replace the Zn in curing process. Nanosized dimension and high dispersion of the oxides improve the efficiency of this activators with respect to system with unsupported crystalline oxides even if their catalityc activity is confirmed to be much lower than ZnO.