Ozone Aging And Protection Of Rubber Products
Ozone is an important factor in the aging of rubber products in the atmosphere. Ozone is more active than oxygen, so its attack on rubber, especially unsaturated rubber, is much more severe than oxygen.
The ozone (o3) in the atmosphere is decomposed by the absorption of short-wave ultraviolet light in sunlight by oxygen molecules.
The oxygen atom is recombined with oxygen molecules. There is a layer of ozone with a concentration of about 5×10—in the shoal of 20~30km from the surface of the earth. With the vertical flow of air, ozone is brought to the surface of the earth, and the concentration of ozone gradually decreases from high to high. In addition, ozone is generated in places where ultraviolet light is concentrated, in discharge places, and in electric motors, especially where electric sparks are generated. Usually the concentration of ozone in the atmosphere is 0~5X10-8. The concentration of ozone varies from region to region; the concentration of ozone varies from season to season. Although the concentration of ozone near the ground is very low, the damage to rubber cannot be ignored.
Unsaturated rubber is prone to ozonation and its appearance after ozonation. Unlike thermo-oxidative aging, the ozonation of rubber products is only carried out on the surface layer contacted by ozone. The whole ozonation process is carried out by the surface. The second is that the rubber reacts with ozone to form a silver-white hard film (about lOnm thick). Under static conditions, the film can prevent deep contact between ozone and rubber, but under dynamic strain conditions or under static tension, when rubber When the elongation or tensile stress exceeds its critical elongation or critical stress, the film will crack, allowing the ozone to contact the new rubber surface, continue to ozonize and cause the crack to grow, and after the crack appears Since the base has stress concentration, it is easier to deepen the crack and form a crack. The direction of the crack is perpendicular to the direction of the stress. Generally, only a small amount of cracks appear under small strain (such as 5%), and the crack direction is clearly discernible. When the rubber is subjected to multiple directions, it is difficult to distinguish the crack direction.
First, the mechanism of rubber ozonation reaction
The reaction mechanism of ozone and unsaturated rubber can be explained by referring to the following formula.
When ozone contacts the rubber product, ozone first reacts with the active double bond to form molecular ozonide 1. The molecular ozonide is unstable and quickly decomposes to form carbonyl 2 and zwitterion 3. In most cases, the zwitterion and carbonyl compound will recombine into an odorous oxide©, and the zwitterion can also be polymerized to form a diperoxide© or a high peroxide©. In addition, when an active solvent such as methanol is present, the zwitterion will also It reacts with it to form methoxyhydrogen peroxide 7.
The activation energy of ozone and unsaturated rubber is very low, and the reaction is very easy to carry out. The reaction is completed until the double bond of the rubber is consumed. At this time, a silver-white loss-elastic film is formed on the surface of the rubber, as long as there is no external force to make the film turtle Crack, rubber will no longer continue to ozonate. If the ozonized rubber is stretched or dynamically deformed, the resulting ozonized film will crack, revealing a new rubber surface and reacting with ozone, which causes the crack to continue to grow.
Saturated rubber does not contain double bonds, although it can react with ozone, but the reaction proceeds very slowly and is not prone to cracking.
Many people have studied the production and growth of unsaturated rubber ozonation cracks. Based on their experimental data, these researchers proposed the mechanism of crack growth and growth. For example, it is believed that the occurrence of cracks is due to the tendency of the broken molecular chains generated by the decomposition of the ozonide under stress to separate from each other, which is greater than the recombination tendency. The growth of cracks is related to the concentration of ozone and the mobility of rubber molecular chains. When the concentration of ozone is constant, the greater the mobility of molecular chains, the faster the crack growth. It is also believed that the generation and growth of ozone cracking and the physical properties of the thin layer of ozonide formed by the ozonation of rubber and the surface layer of the original rubber
The physical properties are different. For example, Murray believes that the ozonation process of rubber is a process in which physical processes and chemical processes occur together. When the rubber is in contact with ozone, the double bonds on the surface react rapidly with ozone, and most of them form an ozonous oxide, which rapidly transforms the originally soft rubber chain into a rigid chain containing many odorous oxide rings. When stress is applied to the rubber, the stress stretches the rubber chain, causing more double bonds to contact the ozone, making the rubber chain contain more odorous oxide rings and becoming more brittle. The embrittled surface is prone to cracking under stress or dynamic stress.
Haining Qianlang Rubber Products Factory
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