When ozone comes into contact with rubber products, it first undergoes an addition reaction with active double bonds, generating molecular ozonates. 1 molecular ozonates are very unstable and quickly decompose to form carbonyls 2 and zwitterions. In most cases, zwitterions and carbonyls will recombine to form odorous oxides ©， Amphoteric ions can also polymerize to form peroxides © Or high peroxide ©， In addition, when active solvents such as methanol are present, zwitterions will react with them to form methoxy hydrogen peroxide. The activation energy of the reaction between ozone and unsaturated rubber is very low, and the reaction is very easy to proceed. The reaction is until the double bonds of the rubber are completely consumed. At this time, a silver white film that loses elasticity is formed on the surface of the rubber. As long as there is no external force to crack the film, the rubber will no longer continue to undergo ozone oxidation. If the rubber that has already undergone ozone oxidation is stretched or subjected to dynamic deformation, the generated ozone oxidation film will crack, exposing the new rubber surface and reacting with ozone, causing the cracks to continue to grow. Saturated rubber does not contain double bonds, and although it can react with ozone, the reaction proceeds slowly and is not prone to cracking.
Ozone (O3) in the atmosphere is formed by oxygen molecules absorbing shortwave ultraviolet light from the sun, decomposing oxygen atoms and recombining with oxygen molecules. There is an ozone layer with a concentration of approximately 5X10- in the air at a distance of 20-30km from the Earth's surface. With the vertical flow of air, ozone is brought to the Earth's surface, and the concentration of ozone gradually decreases from high altitude to the ground. In addition, ozone can be generated in areas with concentrated ultraviolet light, discharge areas, and near electric motors, especially in areas where electric sparks are generated. The usual concentration of ozone in the atmosphere is 0-5X10-8. Different regions have different concentrations of ozone; The concentration of ozone varies with different seasons. Although the ozone concentration near the ground is very low, the harm to rubber cannot be ignored.
Unsaturated rubber is prone to ozone oxidation and its appearance characteristics after ozone oxidation, which is different from thermal oxygen aging. Firstly, the ozone oxidation of rubber products only occurs on the surface layer in contact with ozone, and the entire ozone oxidation process is from the surface to the inside; The second is that rubber reacts with ozone to form a silver white hard film (about lOnm thick), which can prevent deep contact between ozone and rubber under static conditions. However, under dynamic strain conditions or static tensile conditions, when the elongation or tensile stress of the rubber exceeds its critical elongation or critical stress, this film will crack, allowing ozone to come into contact with the new rubber surface, continuing to undergo ozone oxidation reaction and causing crack growth, In addition, after the crack appears, due to stress concentration at the base, it is easier to deepen the crack and form a crack. The direction of the crack is perpendicular to the direction of stress, and generally only a small number of cracks appear under small strain (such as 5%). The crack direction is clearly distinguishable, but it is difficult to distinguish the crack direction when the rubber is subjected to multiple directions of force.
The activation energy of the reaction between ozone and unsaturated rubber is very low, and the reaction is extremely easy to carry out. The reaction occurs until the double bonds of the rubber have been completely consumed, and a silver white film that loses elasticity is formed on the surface of the rubber. As long as there is no external force causing the film to crack, the rubber will no longer continue to undergo ozonation. If the rubber that has already undergone ozone oxidation is stretched or subjected to dynamic deformation, the generated ozone oxidation film will crack, exposing the new rubber surface and reacting with ozone, causing the cracks to continue to grow.
Saturated rubber does not contain double bonds, and although it can react with ozone, the reaction proceeds slowly and is not prone to cracking. Many people have conducted research on the generation and growth of odor oxidation cracking in unsaturated rubber. These researchers have proposed the mechanisms of crack formation and growth based on their own experimental data. For example, some people believe that the occurrence of cracking is due to the tendency of broken molecular chains generated by the decomposition of ozone oxides under stress, resulting in a greater tendency to separate from each other than to recombine. The growth of cracking is related to the concentration of ozone and the mobility of rubber molecular chains. When the ozone concentration is constant, the greater the mobility of the molecular chains, the faster the crack growth. Some people also believe that the generation and growth of ozone cracking are related to the physical properties of the ozone oxide thin layer formed by rubber ozone oxidation and the difference in physical properties from the original rubber surface layer. For example, experts believe that the ozonation process of rubber is a process that occurs both physically and chemically. When rubber comes into contact with ozone, the double bonds on the surface quickly react with ozone, mostly generating ozone oxides, causing the originally smooth rubber chain to quickly transform into a rigid chain containing many ozone oxide rings. When stress is applied to the rubber, it stretches and unfolds the rubber chain, causing more double bonds to come into contact with ozone, making the rubber chain more brittle and containing more ozone rings. Brittle surfaces are prone to cracking under stress or dynamic stress.
Many people have conducted research on the generation and growth of odor oxidation cracking in unsaturated rubber. These researchers have proposed the mechanisms of crack formation and growth based on their own experimental data. For example, some people believe that the occurrence of cracking is due to the tendency of broken molecular chains generated by the decomposition of ozone oxides under stress, resulting in a greater tendency to separate from each other than to recombine. The growth of cracking is related to the concentration of ozone and the mobility of rubber molecular chains. When the ozone concentration is constant, the greater the mobility of the molecular chains, the faster the crack growth. Some people also believe that the generation and growth of ozone cracking are related to the physical properties of the ozone oxide thin layer formed by rubber ozone oxidation and the difference in physical properties from the original rubber surface layer.