With the development of rubber industry, rubber materials are widely used in construction, automobile, electronic and electrical, aerospace and other fields. However, due to the easy aging of the material itself, its service life is restricted, and it is inconvenient for practical application. The so-called aging of rubber means that in the process of storage, transportation, processing and use of raw rubber and vulcanized rubber (including rubber products), due to the comprehensive effect and influence of internal and external factors, the composition and structure of rubber are damaged, so that it gradually loses its original excellent performance and even use value. "Aging" is an irreversible chemical reaction, which is the property of all polymer materials. Its appearance is the deterioration of properties.
The factors that cause rubber aging are quite complex. Under the action of different factors, the aging mechanism is also different. Among them, thermal (photo) oxygen aging is the most common and basic form of rubber aging.
1. Rubber thermal oxygen aging
Natural rubber (NR), ethylene propylene rubber (EPM, EPDM), styrene butadiene rubber (SBR), butyl rubber (IIR), silicon rubber (NVQ) and so on, which are used for capacitor sealing, are all in this form. Because NR contains a large number of unsaturated double bonds, under the joint action of heat and oxygen, the vulcanizate will produce degradation reaction, the cracking and fracture of molecular chain and cross-linking bond, resulting in aging.
Thermal oxygen aging is free radical reaction and oxidation reaction; under the condition of no oxygen thermal aging, free radical decomposition will take place, and the lower the bond cracking energy is, the faster the decomposition will take place; the polymer will be excited at the same time of ionization due to radiation lines, at this time, the molecule will split and generate free radicals, and then the main chain will break and form crosslinking, and finally all kinds of gases will be produced; during ultraviolet aging After absorbing UV light, the molecule excites specific functional groups, decomposes or decomposes through energy transfer in other functional groups, and reacts by generating active groups.
In the high temperature open environment with oxygen, the oxidation and decomposition reaction of side chain organic group mainly takes place, which leads to the hardening of silicone rubber; in the high temperature closed environment without oxygen, the main chain fracture reaction mainly takes place, which produces volatile cyclic polysiloxane, which leads to the softening of silicone rubber. The oxidation reaction of side chain methyl and the degradation and fracture reaction of main chain occur in silicone rubber at high temperature.
2. Ozone aging
The content of ozone in the atmosphere is very low. During the aging process of rubber, ozone attacks rubber molecules and makes rubber expand, resulting in cracks on its surface. Ozone reacts with double bonds in rubber molecules to form moles of ozono and peroxides, which are then regenerated into ozono. Under the action of light and heat, the ozono decomposes into free radicals, leading to chain growth reaction. In addition, the rubber will produce molecular fracture under the action of stress, and no odor oxide will be generated, resulting in cracking and aging.
Through the study of different kinds of tread rubber under the action of ozone, including natural rubber, oil filled methyl styrene butadiene rubber, polyisoprene rubber and cis-1,4-polybutadiene rubber, the results showed that the service life of rubber decreased with the prolongation of ozone action time, and decreased rapidly within a short period of ozone action, which proved that rubber had obvious degradation at the early stage of ozone action. With the extension of ozone action time, the life-span decreased slowly. This may be due to the slow degradation process in the deepening stage of polymer decomposition.
3. Fatigue aging
The fatigue aging of rubber refers to the aging phenomenon caused by the change of molecular structure of rubber material under the action of certain frequency and periodic stress. It is the result of the activation of two main factors, that is, the effect of force and heat (due to the lagging phenomenon of rubber during multiple deformation, the internal heat of rubber is generated).
The anti fatigue performance of natural rubber is a very important performance for the vibration damping elastic elements which are subjected to repeated stress. It is directly related to the failure of vibration damping function and the safe operation of locomotives and vehicles.
4. Aging of contact medium
Rubber materials exposed to natural environment will be affected by moisture in the air, or other conditions, such as external insulation materials of power system, in salt polluted areas, it is very easy to be affected by salt fog, so it is necessary to carry out aging research and analysis of aging performance under such conditions. In addition, some rubber products that contact with oil are tested for oil resistance, and the improvement of oil resistance is also the research focus of practical application.
The aging behavior of rubber in different kinds of oil, on the one hand, is due to the aging mechanical damage, but also due to the degradation of material cross-linking system, resulting in low molecular weight substances. The existence of fluid can reduce the friction coefficient, but also accelerate the chemical degradation of rubber, thus accelerating wear.
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