Confocal Raman imaging, FTIR spectroscopy and kinetic modelling of the zinc oxide/stearic acid reaction in a vulcanizing rubber

The reaction of zinc oxide (ZnO) with stearic acid (StA) to form zinc stearate (ZnSt) has been investigated experimentally in a model matrix (unvulcanized styrene–butadiene rubber) by using confocal Raman microscopy and FTIR transmission spectroscopy. The heterogeneous nature of the reacting system has been confirmed. The Raman analysis has revealed the core–shell structure of the product, which is formed via the gradual shrinkage of the ZnO core and the concurrent formation of a surrounding ZnSt shell of increasing thickness. FTIR spectroscopy has provided information about the molecular state of aggregation of StA when dissolved in the rubber, as well as quantitative information on the reaction kinetics. The kinetic behaviour of the system has been interpreted using a semi-quantitative heterogeneous reaction model grounded on the Raman imaging results, which was able to catch the essential features of the phenomenon and to simulate reliably the experimental conversion vs time data at three different temperatures.