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.
Liu Yang,Xue Tang,Yongsheng Zhou,Shiniu Dai,Kexin Liu,Lina Shi,Xinli Wen
A common tire additive of N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) is frequently detected in freshwater. To date, the toxicity of 6PPD to aquatic organisms has mainly been based on partial life-cycle test results. However, it remains unclear whether this approach leads to an underestimation of the true environmental risk of 6PPD. Here, the effects of environmentally relevant or greater concentrations of 6PPD on the reproduction and growth of Daphnia magna were specifically compared between day 21 and the day all the maternal daphnia died, with the underlying toxic mechanisms of 6PPD also explored. 6PPD at environmental or near-natural water concentrations (5.68 and/or 9.20 µg/L) significantly decreased the total number of offspring and net reproductive rate after whole-life-stage exposure. However, such adverse effects were not observed on day 21, indicating that environmental levels of 6PPD exerted reproductive toxicity on D. magna , and short-term exposure may underestimate the environmental risk of 6PPD. Elevated malondialdehyde levels in the 5.68, 9.20 and 44.58 µg/L treatment groups, coupled with reactive oxygen species (ROS) emerging as the dominant contributors to the biomarker response index, indicated that ROS overproduction induced by 6PPD triggered lipid peroxidation despite antioxidant capacity was enhanced. Downregulation of the vitellogenin expression in D. magna exposed to 5.68, 9.2 and 44.58 µg/L 6PPD, likely resulting from competitive binding to ecdysteroid receptor based on molecular docking and molecular dynamics simulation, might compromise nutrient provisioning during embryogenesis. Coupled with oxidative stress, these dual mechanisms appeared to mediate 6PPD-induced reproductive toxicity.
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