Key Reaction Mechanisms on ZnO during Vulcanization: A Study Based on Density Functional Theory Calculations and Supporting Experiments

Abstract This study investigates the interfacial reaction mechanisms on zinc oxide (ZnO) as an activator in the N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS)-accelerated sulfur cross-linking (vulcanization) of isoprene rubber. Density functional theory calculations using the CASTEP and Gaussian programs elucidate plausible reaction pathways, combining experimental information from in situ Fourier transform infrared spectroscopy, rheometer measurements, and atomic force microscopy. The vulcanization reaction initiates via the decomposition and/or hydrolysis of CBS on a zinc-exposed ZnO (0001) surface, generating the N-(1,3-benzothiazol-2-ylsulfanil) group (BtS group). This group, after standing on the ZnO surface, transforms into an active intermediate with a thiyl radical through a ring-opening insertion reaction of sulfur (S8) on ZnO. This intermediate on ZnO initiates sulfur cross-linking to the isoprene unit of rubber through a thiyl-radical reaction. The carbon radical generated on the isoprene unit then promotes additional radical cross-linking reactions on both ZnO and the surrounding rubber matrix. Concerted reactions by the BtS site also proceed as part of the rubber cross-linking reactions. These diverse and complex cross-linking reactions are highly rational mechanisms for the formation of network domains and their continuous phases, which provide the reinforcing effect in vulcanizates. The mechanistic insights obtained in this study are expected to contribute significantly to the advancement of rubber science and technology.