1. What is rubber aging? What does this show on the surface?
In the process of processing, storage and use of rubber and its products, due to the comprehensive action of internal and external factors, the physical and chemical properties and mechanical properties of rubber gradually deteriorate, and finally lose their use value. This change is called rubber aging. On the surface, it is manifested as cracks, stickiness, hardening, softening, chalking, discoloration, and mildew growth.
2. What are the factors that affect the aging of rubber?
The factors that cause rubber aging are:
(a) Oxygen and oxygen in the rubber undergo free radical chain reaction with rubber molecules, and the molecular chain is broken or excessively cross-linked, resulting in changes in rubber properties. Oxidation is one of the important reasons for rubber aging.
(b) The chemical activity of ozone and ozone is much higher than that of oxygen, and it is more destructive. It also breaks the molecular chain, but the effect of ozone on rubber varies with whether the rubber is deformed or not. When used on deformed rubber (mainly unsaturated rubber), cracks perpendicular to the direction of stress action appear, that is, the so-called “ozone crack”; when used on deformed rubber, only an oxide film is formed on the surface without cracking.
(c) Heat: Elevating the temperature can cause thermal cracking or thermal crosslinking of the rubber. But the basic effect of heat is activation. Improve the oxygen diffusion rate and activate the oxidation reaction, thereby accelerating 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. The damage to the rubber is the ultraviolet rays with higher energy. In addition to directly causing the rupture and cross-linking of the rubber molecular chain, ultraviolet rays generate free radicals due to the absorption of light energy, which initiates and accelerates the oxidation chain reaction process. Ultraviolet light acts as heating. Another characteristic of light action (different from heat action) is that it mainly occurs on the surface of the rubber. For samples with high glue content, there will be network cracks on both sides, that is, the so-called “optical outer layer cracks”.
(e) Mechanical stress: Under the repeated action of mechanical stress, the rubber molecular chain will be broken to generate free radicals, which will trigger an oxidation chain reaction and form a mechanochemical process. Mechanical scission of molecular chains and mechanical activation of oxidation processes. Which one has the upper hand depends on the conditions in which it is placed. In addition, it is easy to cause ozone cracking under the action of stress.
(f) Moisture: The effect of moisture has two aspects: rubber is easily damaged when exposed to rain in humid air or immersed in water. This is because the water-soluble substances and clear water groups in rubber are extracted and dissolved by water. Caused by hydrolysis or absorption. Especially under the alternating action of water immersion and atmospheric exposure, the destruction of rubber will be accelerated. But in some cases, moisture does not damage the rubber, and even has the effect of delaying aging.
(g) Others: There are chemical media, variable valence metal ions, high-energy radiation, electricity and biology, etc., which affect rubber.
3. What are the types of rubber aging test methods?
Can be divided into two categories:
(a) Natural aging test method. It is further divided into atmospheric aging test, atmospheric accelerated aging test, natural storage aging test, natural medium (including buried ground, etc.) and biological aging test.
(b) Artificial accelerated aging test method. For thermal aging, ozone aging, photoaging, artificial climate aging, photo-ozone aging, biological aging, high-energy radiation and electrical aging, and chemical media aging.
4. What temperature grade should be selected for the hot air aging test for various rubber compounds?
For natural rubber, the test temperature is usually 50~100℃, for synthetic rubber, it is usually 50~150℃, and the test temperature for some special rubbers is higher. For example, nitrile rubber is used at 70~150℃, and silicone fluorine rubber is generally used at 200~300℃. In short, it should be determined according to the test.
Post time: Feb-14-2022