Influence of the coordination features of zinc single sites on the reactivity of zinc-based curing activators

The efficiency of the sulfur cross-linking in rubber is enhanced by zinc-based activators. Zn(II) centers are formed by the interaction of microcrystalline ZnO with fatty acids and generate reactive zinc-based organic complexes through the reaction with the accelerator and sulfur. Despite ZnO is the most used industrial activator, several efforts have been made to find more sustainable alternatives, by reducing the zinc content and the zinc leaching during the lifecycle of rubber products. Among them, a promising candidate recently proposed is composed of Zn(II) single sites anchored onto the surface of silica nanoparticles. This material has proven good curing efficiency, thanks to a good zinc reactivity towards the curing agents and high availability to form active Zn(II) complexes. In this context, the present study deeply investigates the reactivity of the Zn(II) single sites by modifying the zinc coordination onto the silica surface, through the introduction of five surface ligands (L) producing Zn-L-SiO2 curing activators. These were synthesized by using amino, carboxyl and sulfur-based terminated ligands, to promote different coordination geometry in the Zn(II) coordination sphere. This strongly impacts on the reactivity of Zn(II)-based activators tested during the sulfur curing process, paving the way for their fine tailoring.