Elucidation of the reactive process involving ZnO crystal facets and stearic acid during rubber vulcanization

Zinc oxide (ZnO) is a widely used activator in rubber vulcanization, typically assumed to first react with stearic acid (StAc) to form zinc stearate, although the detailed reaction mechanism remains unclear. In this study, we first investigated the morphological evolution of ZnO and the diffusion of zinc ions after reacting with StAc. The results showed that ZnO changed from coarse to fine hexagonal prisms, with the aspect ratio increasing by about 1.04. This change was attributed to the higher reaction rate of the (1 0 0) facets compared to other crystal surfaces. The reaction process between ZnO and StAc during rubber vulcanization at different curing times was then analyzed using X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). It was confirmed that ZnO reacted with StAc to form zinc stearate, and the proportion of (1 0 0) facets increased during vulcanization. Density Functional Theory (DFT) calculations revealed that protons released from StAc preferentially reacted with (1 0 0) facets, altering ZnO morphology. Activation energy calculations showed values of 118.9 kJ/mol and 48.9 kJ/mol for (1 0 0) H and L-facets respectively, and 158 kJ/mol for (0 0 1) facets, indicating that (1 0 0) facets exhibit the highest reactivity with StAc.