Rubber aging
First, what is rubber aging? How does this appear on the surface?
Rubber and its products in the process of processing, storage and use, due to the combined effect of internal and external factors caused by the gradual deterioration of rubber physical and chemical properties and mechanical properties, and finally lose the use value, this change is called rubber aging. On the surface, it is cracked, sticky, hardened, softened, powdered, discolored, and mildew.
Second, what are the factors that affect the aging of rubber?
Factors that cause rubber aging are:
(a) Oxygen, oxygen in rubber with rubber molecules free group chain reaction, molecular chain fracture or excessive crosslinking, causing changes in rubber properties. Oxidation is one of the important reasons for rubber aging.
(b) Ozone, ozone chemical activity is much higher than oxygen, more destructive, it is also to break the molecular chain, but the effect of ozone on rubber varies with the rubber deformation or not. When used as a rubber for deformation (mainly unsaturated rubber), there is a crack perpendicular to the direction of stress action, that is, the so-called "ozone cracking "; When acting on the deformed rubber, only the surface of the oxide film is formed without cracking.
(c) Heat: Increasing the temperature can cause thermal cracking or thermal crosslinking of rubber. But the basic function of heat is activation. Increasing the oxygen diffusion rate and activation oxidation reaction, thereby accelerating the rubber oxidation reaction rate, which is a universal aging phenomenon - hot oxygen aging.
(d) Light: The shorter the light wave, the more energy. The damaging effect on rubber is the high energy ultraviolet light. In addition to directly causing the break and crosslinking of rubber molecular chains, ultraviolet rays can produce free groups due to the absorption of light energy, triggering and accelerating the oxidation chain reaction process. Ultraviolet light acts as a heating agent. Another characteristic of light action (different from heat action) is that it is mainly produced on the surface of the rubber. The sample with high rubber content will have a network crack on both sides, that is, the so-called "optical outer crack ".
(e) mechanical stress: Under the repeated action of mechanical stress, the rubber molecular chain will be broken to form a free group, which will trigger the oxidation chain reaction and form a mechanochemical process. Mechanical breakage of molecular chains and mechanical activation and oxidation processes. Which will prevail depends on the conditions in which it is situated. In addition, it is easy to cause ozone cracking under the action of stress.
(f) Moisture: The role of moisture has two aspects: When rubber is rained in wet air or soaked in water, it is easy to destroy, which is because the water-soluble substances and water groups in rubber are dissolved by water extraction. Caused by hydrolysis or absorption. Especially under the alternating 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) Other: The action factors on rubber are chemical media, variable metal ions, high-energy radiation, electricity and biology.
Third, rubber aging test methods can be divided into what categories?
It can be divided into two main categories:
(a) Natural aging test method. It is also divided into atmospheric aging test, atmospheric accelerated aging test, natural storage aging test, natural media (including buried ground, etc.) and biological aging test.
(b) Artificial accelerated ageing test method. For thermal aging, ozone aging, light aging, artificial climate aging, photoozone aging, biological aging, high-energy radiation and electrical aging and chemical media aging.
Fourth, what temperature level should be selected for the hot air aging test for various rubber materials?
For natural rubber, the test temperature is usually 50~100 ° C, synthetic rubber is usually 50~150 ° C, and some special rubber test temperatures are higher. Such as nitrile rubber with 70~150 ° C, silicone rubber is generally used 200~300 ° C. In short, it should be determined according to the test.