Ozone is an important factor in the aging of rubber products in the atmosphere. Ozone is more reactive than oxygen, and thus its attack on rubber, especially unsaturated rubber, is much more severe than that of oxygen.
　　Atmospheric ozone (o3) is decomposed by oxygen molecules after absorbing short-wave ultraviolet light from sunlight
　　Oxygen atoms are recombined with oxygen molecules. There is a layer of ozone with a concentration of about 5X10- in the high altitude of 20-30km from the earth's surface, and as the air flows vertically, ozone is brought to the earth's surface, and the concentration of ozone gradually decreases from high altitude to the ground. In addition, ozone is generated in places where ultraviolet light is concentrated, in places where electrical discharges occur, and near electric motors, especially in places where electric sparks are generated. The usual atmospheric concentration of ozone is 0?5X10-8. The concentration of ozone varies from region to region and from season to season. Although the concentration of ozone near the ground is very low, but the harm caused to rubber is not negligible.
　　Unsaturated rubber susceptible to ozonation and its ozonation after the appearance of characteristics, and thermal oxygen aging is different, one is the ozonation of rubber products only in the surface layer contacted by ozone, the whole ozonation process is the process from the surface to the inside; second is the reaction between rubber and ozone to generate a silver-white hard film (about lOnm thick), in static conditions this film can prevent ozone and rubber deep contact, but in dynamic strain conditions or in When the elongation or tensile stress of the rubber exceeds its critical elongation or critical stress, the film will crack, allowing the ozone to come into contact with the new rubber surface and continue the ozonation reaction and crack growth, in addition to cracks appear due to stress concentration at the base, so it is easier to deepen the cracks and form cracks. The direction of cracks is perpendicular to the direction of stress, and generally only a small number of cracks appear under small strain (e.g. 5%), and the direction of cracks is clearly identifiable, while it is difficult to identify the direction of cracks when the rubber is stressed in multiple directions.
　　I. Mechanism of rubber ozonation reaction
　　The reaction mechanism of ozone and unsaturated rubber, can refer to the following formula.
　　When the ozone contact with rubber products, ozone and active double bond addition reaction occurs first, generating molecular ozone ①, molecular ozone is very unstable, and soon decompose to generate carbonyl compounds ② and amphoteric ions ③. In most cases, amphoteric ions and carbonylates will recombine to form iso-odor oxides? The amphoteric ions can also polymerize to form di-peroxides? or higher peroxides? In addition, amphoteric ions can react with reactive solvents such as methanol to form methoxyhydroperoxides (7).
　　The reaction between ozone and unsaturated rubber has a low activation energy, and the reaction is easy to carry out until the double bond of the rubber is consumed, at which time a silvery-white film is formed on the surface of the rubber, which loses its elasticity. If the ozonated rubber is stretched or dynamically deformed, the resulting ozonated film will crack, exposing the new rubber surface and reacting with ozone, which makes the crack continue to grow.
　　Saturated rubber does not contain double bonds, although it can also react with ozone but the reaction proceeds very slowly, not easy to produce cracking.
　　Several studies have been done on the generation and growth of ozonation cracking in unsaturated rubber. Based on their experimental data, these researchers have proposed the mechanism of crack generation and growth respectively. For example, it has been suggested that cracking occurs as a result of the breakage of molecular chains due to the decomposition of the ozone oxide under stress, and the tendency to separate from each other is greater than the tendency to recombine. The growth of cracking is related to the concentration of ozone and the motility of rubber molecular chains, and when the concentration of ozone is certain, the greater the motility of molecular chains, the faster the crack growth. It is also believed that the generation and growth of ozone cracking is related to the physical properties of the thin layer of ozone oxide formed by rubber ozonation and the physical properties of the original rubber surface layer.
　　of the original rubber surface layer. For example, Murray suggests that the process of rubber ozonation is a combination of physical and chemical processes. When rubber comes into contact with ozone, the double bonds on the surface react rapidly with the ozone to form mostly ozonates, which rapidly transform the originally flexible rubber chains into stiff chains with many ozonate rings. When stress is applied to the rubber, the stress stretches and expands the rubber chains, causing more double bonds to come into contact with ozone, making the rubber chains more brittle with more ozone rings. The brittle surface is then susceptible to cracking under stress or dynamic stress.