| Rubber materials are widely applied in industrial, technological and military fields under complicated environments because of their unique high elasticity and excellent comprehensive properties. However, under the influences of environmental factors such as temperature, oxygen or ozone, mechanical stress, light and salt-flog, rubbers are easy to go through thermo-oxidative aging, which will cause deterioration of their properties, and even lead to safety accidents. Thus, it is of vital practical significance to study the aging process, mechanisms of rubbers, and to improve their aging resistances by appropriate measures. Two kinds of widely used rubbers, butadiene rubber (BR) and natural rubber (NR),were chosen in this research. Based on experimental studies, this research combined with theoretical derivation and introduced multiscale molecular simulation methods to study aging behaviors, aging prediction models,aging mechanisms, rare earth antioxidant systems, and antioxidant selection criteria. Specific research contents are divided into the following four parts.(1) The thermo-oxidative aging behaviors of BR under compression loading were measured by compressive stress relaxation tests, and then predicted by constructing models. Firstly, based on the lifetime prediction results of BR by WLF equation and Arrhenius equation, we inferred that the stress relaxation properties of BR were dominated by chemical relaxation. Besides, from room temperature to accelerated aging maximum temperature, the reaction mechanisms inducing chemical relaxations were the same. After testing by magnetic resonance-cross-link density (MR-XLD) and fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), we identified that chemical relaxation was caused by three kinds of reactions, i.e., cross-linking reactions, chain scission reactions, and oxidation reactions. Then,combining with rheology theory and mechanisms of the three kinds of reactions, we derived an amendatory standard linear solid (SLS) model.Applying this model, we predicted the thermo-oxidative aging behaviors of BR under compression loading and revealed its aging mechanism. By comparing with results predicted by the empirical model, the reliability of this model was verified. Results showed that the two models fitted well.But the empirical model had a disadvantage which was contradictory to the fact of cross-linked rubber network. The amendatory SLS model made up this disadvantage, and showed that the cross-linking reactions and the oxidation reactions were the main reactions which dominated the thermo-oxidative aging of BR. This conclusion was consistent with the experimental results.(2) This part attempted to convert the complex themo-oxidative aging process into quantitative molecular simulation data to explore the causes of themo-oxidative aging. Thermo-oxidative aging process consists of two inseparable steps: one is the physical permeating process of 02 entering into the rubber network, the other is the complex chemical process of 02 reacting with the rubber network. The permeation coefficient of 02, which is equal to the product of diffusion coefficient (D)and solubility coefficient (S), is the key factor to evaluate step one. Thus,by molecular dynamics (MD) and Monte Carlo (MC) simulations, we studied the variations of D and S with temperature and explored the mechanisms of diffusion and solution processes. In addition, based on the experimental results and the calculated thermodynamic parameters, we identified that the rate determining step in the thermo-oxidative aging process of BR was the chain initiation reaction. By quantum mechanical(QM) simulation, we calculated the free energy of the chain initiation reaction in the temperature usage range of BR. Results showed that P reached a maximum at approximately 300 K and then tended to be stable.The free energy of the chain initiation reaction increased very little, but had large base value. Comprehensively considered, the occurrence of themo-oxidative aging of BR can be attributed to the high energy provided by high temperature.(3) This part systematically studied the protective effects of two new rare earth antioxidants, i.e., dithio-aminomethyl-glutamic acid lanthanum(DAGLa) and p-aminobenzene sulfonic acid lanthanum (p-ASALa), on the thermo-oxidative aging properties of NR. The thermo-oxidative aging protective effects of rare earth antioxidants and commonly used amine antioxidant antioxidant N-isopropyl-N'-phenylenediamine (IPPD) were compared by macroscopic properties, microscopic structures, and thermodynamic parameters. Results showed that protective effect of each rare earth antioxidant was better than that of antioxidant IPPD, and in the two rare earth antioxidants, p-ASALa was the better one. Furthermore,based on the thermo-oxidative aging mechanism of NR, we used QM simulation to explore the function mechanisms of three antioxidants.Results showed that by dissociating hydrogen radical earlier than NR,antioxidant IPPD could compete effectively with NR chains for reactions with propagating peroxy radicals to inhibite the thermo-oxidative aging process. In addition to the strong coordination abilities and large coordination numbers, rare earth antioxidants also owned multiple functional groups. When a small quantity of rare earth antioxidants were mixed into antioxidant IPPD, these functional groups could produce joint-,mixed-, and self-synergistic effects with IPPD, resulting in excellent long-term protective effect on the thermo-oxidative aging of NR. Besides,due to the large number of primary antioxidants functional groups with good protective effect in p-ASALa, it showed better thermo-oxidative aging protective effect than DAGLa.(4) Based on the research of part (2) and (3), we explored the relative importances of factors influencing the selection of antioxidants.In this study, these factors were subdivided into five items: the barrier ability of antioxidant to O2, the free energy for the dissociation reaction of antioxidant, the mass ratio of active radicals or hydroperoxides that could react with the same mass of antioxidant, the mobility of antioxidant, and the compatibility of antioxidant and rubber. The reason why these factors can influence the effect of an antioxidant is that they can lower at least one rate of the physical or chemical steps in the thermo-oxidative aging process. Three antioxidants with different function groups and mechanisms were chosen in this part. By multiscale molecular simulations, the five factors of each antioxidant were quantified over the entire temperature usage range of BR. After identifying the tensile strength and elongation at break in the thermo-oxidative aging process as the responses for the first and the second grey relational analysis, the grey relational grades of five factors were calculated to clarify their relative importances. Results showed that in the selection of antioxidants, we should give priority to the free energy for the dissociation reaction of antioxidant, then to the barrier ability of antioxidant to O2. This conclusion also showed that in the study of thermo-oxidative aging, the chemical process of 02 reacting with the rubber network and the physical permeating process of 02 entering into the rubber network should be studied in paralle. Neither of them can be omitted. |
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