During the processing, storage and use of rubber and its products, the physical and chemical properties and mechanical properties of rubber gradually deteriorate due to the combined effect of internal and external factors, and finally lose their use value. This change is called rubber aging.
The surface is cracked, sticky, hardened, softened, powdery, discolored, mildewed, etc.
A) Oxygen: the oxygen reacts with the rubber molecules in the rubber by free radical chain reaction, and the molecular chains break or cross link excessively, which causes the change of rubber properties. Oxidation is one of the important reasons for rubber aging.
B) Ozone: The chemical activity of ozone is much higher than that of oxygen, and it is more destructive. It also breaks the molecular chain. However, the effect of ozone on rubber varies with rubber deformation. When used as rubber for deformation (mainly unsaturated rubber), cracks that are straight to the direction of stress action appear, which is called “ozone cracking”; When applied to deformed rubber, only oxide film is formed on the surface without cracking.
C) Heat: Increasing temperature can cause thermal cracking or thermal crosslinking of rubber. But the basic function of heat is activation. Increase the oxygen diffusion rate and activate the oxidation reaction, so as to accelerate the oxidation reaction rate of rubber, which is a common aging phenomenon – thermal oxygen aging.
D) Light: The shorter the light wave, the greater the energy. It is ultraviolet rays with high energy that can destroy rubber. In addition to the direct breakage and cross-linking of rubber molecular chains caused by ultraviolet radiation, rubber absorbs light energy to produce free radicals, which initiate and accelerate the oxidation chain reaction process. The external line light plays the role of heating. Another characteristic of light effect (different from heat effect) is that it mainly grows on the surface of rubber. The specimen with high glue content will have a network crack on both sides, which is called “light outer layer crack”.
E) Mechanical stress: under the repeated action of mechanical stress, the rubber molecular chain will break and generate free spring, which will lead to oxidation chain reaction and form a mechanochemical process. Mechanical breaking of molecular chains and mechanical activation of oxidation processes. Which one is superior depends on the conditions. In addition, it is easy to cause ozone cracking under stress.
F) Moisture: There are two functions of moisture: rubber is easy to be damaged when it is exposed to rain in humid air or immersed in water, which is caused by the water-soluble substances and hydrophilic groups in rubber being extracted, dissolved, hydrolyzed or absorbed by water. Especially under the alternate action of water immersion and atmospheric exposure, the destruction of rubber will be accelerated. However, in some cases, the moisture does not damage the rubber, and even has the effect of delaying aging.
G) Oil: If it contacts with oil medium for a long time during use, oil can penetrate into the rubber and cause it to swell, reducing the strength and other mechanical properties of the rubber. The reason why oil can cause rubber to swell is that after oil seeps into rubber, molecules diffuse each other, which changes the network structure of vulcanizate.
H) Others: Chemical media, variable metal ions, high-energy radiation, electricity and biology are also factors affecting rubber.