Studying stearic acid interaction with ZnO/SiO2 nanoparticles with tailored morphology and surface features: A benchmark for better designing efficient ZnO-based curing activators

The interaction between activators (ZnO) and co-activators (stearic acid, SA) represents a key step in the vulcanization process, to generate Zn(II) intermediate complexes that enhance the reaction kinetic and promote the shortening of sulfur bridges, leading to highly cross-linked materials. To understand the influence of the structural, morphological and surface properties of ZnO in the reactivity with SA, in this work, pure ZnO nanoparticles (NPs) and ZnO NPs anchored on SiO2 (ZnO/SiO2) were prepared through soft chemistry techniques. Tailoring of morphology and surface features was accomplished through a fine control of the synthetic parameters, as demonstrated by the careful characterization of the activators. Then, the interaction of pure ZnO and ZnO/SiO2 activators with SA in the absence of rubber was investigated by using Differential Scanning Calorimetry (DSC). The occurrence of Zn(II)-SA complexes with different thermal stability and structural properties was assessed by a comprehensive thermogravimetric and spectroscopic survey. The generation and the chemical structure of these specific vulcanization intermediates was related to the peculiar characteristics of the ZnO/SiO2 systems and, in turn, to their ability in imparting faster kinetics and higher curing efficiency to isoprene rubber nanocomposites, compared to bare ZnO.