Towards the up-scaled production of highly dispersed ZnO nanoparticles on silica as novel catalyst for the industrial rubber vulcanization process

The mechanical properties of rubber can be highly increased thanks to the vulcanization process, leading to the formation of sulfur cross-linking between polymer chains. To improve the rate and efficiency of this process, several catalysts, including zinc oxide nanoparticles (ZnO NPs) are generally added to the matrix. However, the high hydrophilicity of ZnO hinders a fine distribution of ZnO that tends to agglomerate in the hydrophobic polymer, thus creating non-homogeneous islands of vulcanized rubber and requiring the addition of high zinc content to the matrix. Besides, an environmental issue has been recently connected to zinc release during the life cycle of rubber materials, especially in connection with tyres application. In this scenario, with the aim of reducing the amount of ZnO and improve the vulcanization efficiency, a new vulcanization catalyst has been developed [1], in which ZnO NPs are dispersed onto the surface of silica particles (ZnO/SiO2); as silica is a common reinforcing agent, the supported catalyst behaves as a “double function filler”. A laboratory scale synthetic procedure has been studied in detail, based on a sol-gel procedure, that allows the formation of amorphous ZnO NPs on silica surface and lead to an improved vulcanization efficiency, with half of the amount of zinc employed. Besides, the production of higher reinforced materials with reduced zinc content was achieved on a preindustrial scale (TRL5). Thus, ZnO/SiO2 has showed to be a good candidate to substitute the conventional catalyst in the vulcanization process. As a next step, the development of the large-scale production of the material is under examination, in order to get a low-cost production process, maintaining at the same time the initial catalytic properties and to allow its further commercialization.