Aging Failure And Life Prediction Of Polymer Materials

Aging failure and life prediction of polymer materials
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, media, etc.) Photooxygen degradation, thermal degradation, chemical degradation, biological degradation, etc., lead to the gradual decline of various properties until destruction. 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 of it, such as gaskets, O-rings, cups, oil seals, valves, etc., are often in key positions in mechanical equipment, and at the same time are often the weak links of components or assemblies. If it loses its sealing ability, it must be disassembled and replaced, otherwise the entire product may be scrapped.
The essence of rubber aging is the cross-linking or breakage of rubber molecular chains, which is mostly an autocatalytic oxidation mechanism. The type and composition of rubber raw rubber determines the aging stability of the product to a large extent. For example, the heat resistance of silicone rubber and fluorine rubber is better than that of nitrile butadiene rubber (NBR); the heat resistance of hydrogenated nitrile butadiene rubber (HNBR) The higher the saturation, the better the thermal stability; as the acrylonitrile (AN) content increases, the oil resistance and aging resistance of NBR increase, but at the same time its sealing performance and low temperature resistance decrease. The rubber vulcanization system, stabilizing system, fillers and plasticizers will all affect the aging properties of the matrix. For silicone rubber or polyurethane rubber that is easily hydrolyzed or has certain hydrophilicity, humidity will accelerate its aging. During use, rubber sealing materials often have to withstand a certain amount of deformation and come into contact with oil media. This makes the aging process of the material not only a thermo-oxidative degradation process, but also the influence of oil media and stress.
The life of rubber is usually evaluated through an accelerated thermal oxygen aging test, that is, an accelerated aging test is performed at a higher temperature, and the life is predicted by extrapolating the measurement results to the use (service) temperature using the Arrhenius formula. This requires that the mechanism leading to degradation does not change within the temperature range under investigation. In most cases, the Arrhenius method has been proven to be applicable, but many researchers have reported that the Non-Arrhenius behavior of rubber aging is not completely applicable. For example, when Bernstein et al. studied the accelerated aging of fluorosilicone, they found that the Arrhenius curve of its compressive stress relaxation behavior deviated at 80°C, causing the high-temperature and low-temperature segments to show two activation energies (73kJ·mol-1 and 29kJ ·mol-1). Calculated from the low-temperature section activation energy, the life corresponding to 50% performance loss is 17 years, while the life span directly extrapolated from the high-temperature section activation energy is as long as 900 years. Editing, editing and reprinting by Jiayu Testing Network must indicate the source. Such a huge difference indicates that the actual aging conditions are different from accelerated aging, resulting in changes in the aging mechanism, or changes in the aging mechanism in different temperature ranges, which will make simple extrapolation results unreliable. However, current research work mostly starts from the actual needs of engineering applications, focusing on mechanical properties (such as strength, hardness, compression permanent deformation, stress relaxation, elastic recovery rate, etc.), regarding the aging mechanism of rubber under different conditions. Research is rarely involved, which means life prediction still uses the accelerated thermal oxygen aging method. There are considerable research gaps in the impact of complex temperature and humidity conditions, stress effects, medium effects, etc. in the rubber environment.
During the thermal oxidation process, rubber will generate various oxidation products, which are obviously distributed in the thickness direction of the product, and its cross-linking density will also change. After conducting in-depth research on the thermal oxygen 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, oxidation reaction and cross-linking reaction dominate, manifested by the increase in cross-linking degree and hardness, while the elastic recovery rate decreases. In the third stage of late thermal oxidation aging, severe oxidation may even cause molecular chains to break. However, at this time, the elasticity of NBR has almost completely been lost and it cannot be used as a sealing material. In this process, the change in antioxidant content is a very important indicator. When its content drops to a critical value, the elastic recovery rate will drop sharply, and the hardness will rise sharply, causing it to lose its performance. 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 properties for a long time. Second, although lubricating oil hinders the diffusion of oxygen to a certain extent, the degree of oxidation in the oil is higher due to the increased mobility of the rubber molecular chains. If the same type of oil is of different viscosities, the degree of oxidation in the low-viscosity oil will be higher than in the high-viscosity oil. Third, the extraction effect of lubricating oil on additives causes the migration speed of additives in rubber to be faster.
When used as a sealing material, rubber is subject to stress and relaxes over time. Gillen et al. from Sandia National Laboratory studied the stress relaxation behavior of butyl rubber with a certain strain at different temperatures and found that the stress relaxation rate was significantly accelerated under strained conditions.
When rubber sealing materials are used in dynamic sealing and lubrication situations, the friction and wear properties of the rubber must be considered. The friction coefficient of rubber is the joint contribution of liquid, adhesion and deformation. Adhesion is connection and destruction at the molecular level and decreases with elastic modulus, a function of viscoelasticity. The hysteretic friction of rubber is an energy-consuming process, accompanied by internal damping, but increases as the elastic modulus decreases. Wear is localized damage, the result of the breakdown of the cross-linked network into smaller molecules. If it is a sharp surface, wear will lead to tensile failure; if it is a blunt surface, it will lead to fatigue failure. Different oil media have different effects on the friction and wear properties of rubber. For example, ester base oil degrades the mechanical properties of NBR more seriously than mineral oil and polyolefin synthetic oil (PAO).