Nano ZnO As Activator of Rubber Vulcanization. Is Only a Matter of Size?

During the last years, a great effort has been devoted to study alternatives to ZnO as activator of sulphur rubber vulcanization.

Among different strategies tested to reduce the amount of ZnO in rubber formulation, the use of nano-sized ZnO is one of the most popular. The rational behind this alternative is that the high surface/volum ratio of the ZnO nanoparticles would allow a higher efficacy of the oxide, obtaining the same activating potential with less amount of the nano-ZnO in the formulations. Although the large number of references, there is a lack of consensus about the exact influence of the particle size in the vulcanization efficiency.

This work proposes the use of Model Compound Vulcanization Approach, to give insight in the role of nano ZnO in rubber vulcanization. For this, using squalene as model molecule of Natural Rubber, the activating potential of a new nano-ZnO, developed at the University of Bari, has been tested and compared with three different ZnO, two commercial nano ZnO, and a standard micro-sized ZnO. Simultaneously, a complete characterization of the different ZnOs has been performed. This characterization includes, specific surface, particle size, and surface composition by ToF-SIMS and XPS.

The results obtained have shown that among the particle size, surface composition plays a more important role in the activating behaviour of ZnO. Moreover the nano-ZnO production method modifies completely the hability of ZnO to control the vulcanization reaction. As it could be expected, no direct influence in the vulcanization mechanism has been observed for any of the nano-ZnO studied