After the rubber is used for a long time, some will become sticky and soft, and some will harden and become brittle. In daily life, we can often see cracks on the sides of some old tires, which are professionally called cracks. In this case, the original elasticity and strength of the rubber are significantly reduced, and these phenomena are called aging.
Aging is the result of changes in rubber structure under the influence of various external factors, the most common of which is thermo-oxidative aging under the action of heat and oxygen.
Rubber will degrade when heated to a certain temperature. For example, natural rubber begins to decompose low-molecular substances when it exceeds 200 degrees Celsius. The thermal stability of rubber mainly depends on its chemical composition and structure. The thermal stability of silicone rubber and fluororubber is better than general rubber. Among general-purpose rubbers, butadiene rubber has the best thermal stability, followed by styrene-butadiene rubber.
Under the participation of oxygen, the aging results of various rubbers are different. Natural rubber, isoprene rubber, and butyl rubber are mainly broken in the macromolecular chain during the thermal oxygen aging process, and become soft and sticky; while styrene-butadiene rubber, butadiene rubber, nitrile rubber, and EPDM rubber In the aging process, the products after the broken chain are mainly intertwined, destroying the elastic structure of the rubber, so it becomes hard and brittle.
The bane of thermo-oxidative aging of rubber lies in its unsaturated bonds. Natural rubber and some general-purpose synthetic rubbers contain 150 to 250 double bonds per 1000 carbon atoms. These parts are most likely to undergo chemical reactions, so their thermo-oxidative aging properties are not good, especially natural rubber. Silicone rubber, fluororubber, butyl rubber, ethylene-propylene rubber, polyurethane rubber, acrylate rubber, etc., which tend to be saturated in structure, are heat-resistant and aging-resistant elastomers.
Although the aging of rubber cannot be completely avoided, it can be delayed. The most convenient and effective measure is to add anti-aging agents. The anti-aging agent can capture the active substances produced in the thermal-oxidative aging process of rubber, thereby protecting the rubber. There are many kinds of anti-aging agents, and the two most commonly used series are amines and phenols. The amount added is generally 1% to 1.5% of the weight of the rubber.