Study On The Anti-oxidative Mechanisms Of Two Amine Antioxidants In Natural Rubber By Experiments And Molecular Simulations

As one of the most important elastomers,natural rubber(NR)has excellent comprehensive properties such as high strength,good elasticity etc.,so it is widely employed in energy,aircraft,industry,and automobile fields.However,due to the existence of considerable unsaturated double bonds and allyl hydrogens,NR is easily attacked by heat and oxygen which makes it an irreversible aging degradation and crosslinking.Such reaction may lead to the loss of using performance and even cause catastrophic accidents.Adding antioxidants is recognized as one of the most efficient methods to improve the aging resistance of NR.Because of the diversity of individual structure and rubber matrix,different antioxidants may show different anti-oxidative effect in rubber.Therefore,a deeper study on the anti-oxidative mechanisms and factors would better for us to select and design new antioxidants.In my paper,two common and excellent antioxidants 4020 and A were selected.Based on physical and chemical process of rubber aging,the multi-scale molecular simulations were introduced to explain the experiments and quantitatively analyze the chemical and physical protective mechanisms.The specific works are divided into the following two parts:(1)The influences of two antioxidant(4020 and A)to the thermo-oxidative aging of NR composites were investigated by experiments.Macroscopic properties,microstructure,crosslink density and degradation temperature results show that comparable to antioxidant A,NR with antioxidant 4020 has better retained rate of mechanical properties,lower carbonyl(C=O)and oxygen/carbon(O/C)molar ratio growth rates,lower crosslinking density change rate,and higher degradation temperature.Combined two non-isothermal thermal-oxidative degradation dynamics equation Kissinger and Fly nn-W all-Ozawa(FWO),we calculated the thermal-oxidative degradation activation energy(Ea)of NR.The results show that antioxidant 4020 can more effectively increase the Ea.All the results demonstrate that antioxidant 4020 has better anti-oxidative effect than antioxidant A in NR.(2)Rubber aging consists of two processes.One is the physical process of oxygen entering into rubber,two is the chemical process of oxygen reacting with rubber chains.Therefore,analysis of anti-oxidative activity can be started from the impact of antioxidants on the above two processes.Based on the literature,we summarize the main factors affecting the above two processes as four:the dissociation ability of antioxidants N-H bond,the inhibit ability of antioxidants to oxygen,the compatibility between antioxidants and rubber matrix,and the mobility of antioxidants.However,due to the multiplicity of influencing factors,the limitations of instrumental analysis,and the weakness of theoretical basis,the exact value of each parameter cannot be accurately calculated using experimental methods.At the same time,the pure effect of each factor on rubber aging cannot be studied separately.The thermodynamic behavior and the motion laws of any macroscopic matter are determined by the nature of the electrons,atoms,and molecules that make up it.Multi-scale molecular simulations can obtain data that can reflect macroscopic material behaviors by quantitatively calculating the microscopic particle motion trajectories and thermodynamic changes.In our study,using multi-scale molecular simulations,changes of above four factors in antioxidants and NR systems were analyzed,and mechanisms for better protective effect were explained quantitatively.Results show that antioxidant 4020 has lower dissociation energy,better inhibition to oxygen permeation,and better compatibility with NR.So,in terms of physical protection,addition of antioxidant could effectively hinder the permeation of oxygen and in terms of chemical protection,antioxidant could capture free radicals by breaking its N-H bonds and consequently delay the NR aging.However,when we select antioxidants,not only anti-oxidative activity but also long-term effects should be considered.The calculated results also show that antioxidant 4020 has larger mobility than antioxidant A.So we should also consider how to reduce mobility of antioxidant 4020.Molecular simulation results verify the experimental results and are consistent with the experimental results.It shows that molecular simulation can be used as a calculation method to predict and guide the development and selection of antioxidants.