During storage and use, polymer materials will be affected by various environmental factors (such as ultraviolet light, heat, humidity, ozone, microorganisms, etc.) and working conditions (such as stress, electric field, magnetic field and medium). Photo-oxidative degradation, thermal degradation, chemical degradation and biological degradation processes may lead to a gradual decline in various properties until they are damaged. Therefore, it is of great significance to study the aging failure mechanism and life prediction of polymer materials. Taking rubber sealing materials as an example, the products made with it, such as gaskets, O-rings, leather cups, oil seals, valves, etc., are often in key parts of mechanical equipment, and at the same time are often the weak link of components or assemblies. If it loses its sealing ability, it has to be disassembled and replaced, otherwise the entire product may be scrapped.
The essence of rubber aging is the crosslinking or breaking of rubber molecular chains, which is mostly an autocatalytic oxidation mechanism. The type of raw rubber and its composition determine the aging stability of the product to a large extent. For example, the heat resistance of silicone rubber and fluororubber is better than that of nitrile rubber (NBR); hydrogenated nitrile rubber (HNBR) The higher the saturation, the better the thermal stability; as the content of acrylonitrile (AN) increases, the oil resistance and aging resistance of NBR improve, but at the same time its sealing performance and low temperature resistance decrease. The vulcanization system, stabilizing system, filler and plasticizer of rubber will all have an impact on the aging performance of the matrix. For silicone rubber or polyurethane rubber that is easily hydrolyzed or has a certain degree of hydrophilicity, humidity will accelerate its aging. During use, rubber sealing materials often have to bear a certain amount of deformation and come into contact with oil medium, which makes the aging process of materials not only thermal oxygen degradation process, but also consider the influence of oil medium and stress.
Usually, the life of rubber is evaluated by accelerated thermal oxygen aging test, that is, the accelerated aging test is carried out at a higher temperature, and the test results are extrapolated to the use (service) temperature using the Arrhenius formula to predict the life. This requires that the mechanism leading to degradation does not change over the temperature range. In most cases, the Arrhenius method has been proved to be applicable, but many researchers have reported that the Non-Arrhenius behavior of rubber aging is not completely applicable. For example, Bernstein found that the Arrhenius curve of the compressive stress relaxation behavior deviated at 80°C when studying the accelerated aging of fluorosilicone, so that the high temperature section and the low temperature section showed two activation energies (73kJ mol-1 and 29kJ mol-1). The lifespan corresponding to 50% performance loss calculated from the activation energy of the low-temperature section is 17 years, while the lifespan directly extrapolated from the activation energy of the high-temperature section is as long as 900 years. Such a large difference indicates that the actual aging conditions are different from the accelerated aging, resulting in changes in the aging mechanism, or the aging mechanism changes in different temperature ranges, which makes simple extrapolation results unreliable. However, the current research work mostly starts from the actual needs of engineering applications, focusing on the mechanical properties (such as strength, hardness, compression set, stress relaxation, elastic recovery rate, etc.), and the aging mechanism of rubber under different conditions. However, the research is rarely involved, which makes the life prediction still adopt the accelerated thermo-oxidative aging method, and there are quite a lot of research gaps on the influence of complex temperature and humidity conditions, stress effects, and medium effects in the rubber environment.
During the thermal oxidation process of rubber, various oxidation products will be generated, and there are obvious distributions in the thickness direction of the product, and the crosslinking density will also change. After an in-depth study on the thermo-oxidative aging behavior and mechanism of NBR in air and lubricating oil, the author found that the aging process of NBR in air can be divided into three stages. The first stage is mainly the migration of additives (plasticizers, antioxidants, etc.). In the second stage, the oxidation reaction and crosslinking reaction are dominant, which is manifested by the increase of the degree of crosslinking and the increase of hardness, while the elastic recovery rate decreases. In the third stage in the later stage of thermo-oxidative aging, severe oxidation can even lead to molecular chain breakage, but at this time, the elasticity of NBR is almost completely lost and cannot be used as a sealing material. In this process, the content change of the antioxidant is a very important indicator. When the content of the antioxidant drops to a critical value, the elastic recovery rate will drop sharply, and the hardness will rise sharply at the same time, making it lose its usability. When NBR is thermally aged in lubricating oil, first of all, due to the diffusion of lubricating oil into the rubber, the rubber can maintain good resilience performance for a long time. Second, although the lubricating oil hinders the diffusion of oxygen to a certain extent, the degree of oxidation in the oil is relatively high due to the enhanced mobility of the rubber molecular chain. If it is the same type of oil with different viscosities, the degree of oxidation is higher in low viscosity oils than in high viscosity oils. Third, the extraction of additives by lubricating oil makes the migration of additives in rubber faster.
When used as a sealing material, rubber is stressed and relaxes over time. Gillen of Sandia National Laboratory studied the stress relaxation behavior of butyl rubber with a certain amount of strain at different temperatures, and found that the stress relaxation rate was significantly accelerated under strained conditions.
When the rubber sealing material is used in the case of dynamic sealing and lubrication, the friction and wear properties of rubber must be considered. The coefficient of friction of rubber is the combined contribution of liquid, adhesion and deformation. Adhesion is the joining and breaking at the molecular level, which decreases with the modulus of elasticity and is a function of viscoelasticity. Hysteresis friction in rubber is an energy-dissipating process with internal damping, but increases with decreasing modulus of elasticity. Abrasion is localized damage and is the result of the breakdown of the cross-linked network into smaller molecules. Wear leads to tensile failure if it is a sharp surface and fatigue failure if it is a blunt surface. Different oil media have different effects on the friction and wear properties of rubber. For example, the degradation of the mechanical properties of NBR by ester base oil is more serious than that of mineral oil and polyolefin synthetic oil (PAO).
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