How to evaluate the lifespan of rubber through aging analysis
Rubber life assessment is actually an analysis of the aging performance of rubber materials. Aging resistance tests often include natural aging, artificial accelerated aging, and other tests. Natural aging testing is the best method to evaluate the environmental lifespan of materials. However, the testing cycle is long, the environmental conditions are uncontrollable, and the environmental conditions in different regions are different, and various influencing factors cannot be separated. In the early stages of artificial accelerated aging testing, oxygen absorption was mainly used to characterize the aging rate and degree of rubber. Later, people began to pay attention to the study of changes in the physical properties of rubber and developed the oven accelerated aging test method. At the same time, there have been testing methods for oxygen bomb accelerated aging, air accelerated aging, and artificial climate accelerated aging, but most of the aging resistance tests are still based on oven accelerated aging testing. The accelerated aging of the oven is closest to the actual natural aging. Using the oven accelerated aging method can first shorten the testing cycle, then control the environmental conditions of the study, and obtain comparable results.
1、 Principles of Rubber Life Assessment
Rubber lifespan assessment utilizes the essence of rubber aging, where the main chain, side chain, and cross-linking bond fracture reactions of rubber molecular chains dominate. Aging manifests as rubber softening and surface adhesion, as the molecular chains break into small molecules and chain segments, such as NR, IR, IIR, PU, CHR, etc. Rubber molecular chains first undergo fracture reactions, while new cross-linking reactions dominate. Aging exhibits surface hardening, embrittlement, and cracking, as the molecular chains generate many new cross-links, such as BR, SBR, NBR, EPDM, etc. During the aging process, general rubber molecular chains complete all chemical reactions according to three basic mechanisms (heterocracking, homocracking, and cyclization reactions).
2、 Rubber Life Assessment Model
The conventional accelerated thermal oxygen aging method involves conducting hot air oxidation aging tests at several aging temperatures for hundreds to thousands of hours after making a sample of vulcanized rubber, until the physical properties of the sample decrease to the specified level. Critical value. So far, a mathematical model has been established to calculate the storage life of materials. The theoretical models for predicting rubber life using accelerated aging testing are the time-temperature equivalence principle and the diffusion limited oxidation model.
1. Time Temperature Superimposed Life Prediction Model
The method of accelerating the oxidative aging of rubber is to increase the rate of oxidation reaction by increasing the aging temperature. The oxidative degradation reaction of rubber is very complex and usually cannot predict specific results, but the oxidative aging process of rubber follows the thermal oxidative aging mechanism. The time temperature superposition life prediction model utilizes this theory.
2. Diffusion limited oxidation model
The diffusion limited oxidation model is a series of tests to determine the relationship between the oxygen concentration in rubber and the modulus of rubber, and then predict the lifespan of rubber by measuring the oxygen concentration in rubber. When a polymer material reaches a steady state in an air environment, the oxygen concentration dissolved in the polymer material can be obtained from the actual pressure of the surrounding environment during the production of the material and the dissolution parameters of oxygen in the material. These oxygen will participate in the reaction during oxidative aging. If the rate of oxygen consumption through oxidation reaction is greater than the rate of oxygen supplementation to the material through environmental diffusion effect, the oxygen concentration inside the material will decrease.
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