Effect of Gallic Acid and Its Ester Derivatives on Thermo-oxidative Aging Resistance of Natural Rubber

Abstract

The aging degradation process is significantly accelerated in unsaturated natural rubbers (NR) due to their susceptibility to α-H bond dissociation caused by exposure to heat and oxygen. Recently, the European Union has listed widely used antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) as the restricted category of compounds, sparking a growing desire to find environmentally friendly alternatives. Through a combination of molecular simulation and experimental methods, this study aims to investigate the protection mechanism and conformational relationship of the natural antioxidant gallic acid (GA) and its ester derivatives within the NR matrix. The ultimate objective is to identify natural antioxidants that possess the capability to effectively substitute 6PPD. Simulation results of the chemical aging factor suggest that the phenolic hydroxyl hydrogen dissociation energy of GA is lower than that of the α-H bond of NR, thereby delaying the aging process. Based on the influence of physical aging factors, the polarity of GA derivatives decreases as the modified alkyl chain lengthens, improving the compatibility and migration resistance with NR, strengthening interaction force, and lowering oxygen permeability. Additionally, comprehensive simulation results indicate that octyl gallate derivatives exhibit notable antioxidant properties. Subsequent DPPH radical scavenging experiments, microstructural characterizations, and macroscopic mechanical property tests have confirmed the optimal antiaging properties of octyl gallate in NR, which these ester derivatives are expected to be a nontoxic, nonhazardous, and safe natural antioxidant alternative to 6PPD.