Rubber in the process of thermal oxidative aging :

The types of structural changes of rubber in the process of thermal oxidative aging can be divided into:

• One is the thermal oxidative aging reaction (pyrolysis) which is based on molecular chain degradation.

• The second type is pyrolysis (structured), which is mainly cross-linked between molecular chains.

Natural rubber contains isoprene rubber, butyl rubber, diethylene propylene rubber, homopolymer chlorohydrin rubber and copolymer chlorohydrin rubber, etc. The appearance of this kind of rubber will become soft and sticky after thermal oxygen aging.

The main change for Butadiene-containing butadiene rubber in the thermal oxygen aging process is the cross-linking reaction. Such as NBR/SBR/CR/ERDM/FPM/CSM will also have similar changes. The appearance of this kind of rubber is hard and brittle after thermal aging.

The main factors which influences the rubber aging:

1. Oxygen: Oxygen has a free radical chain reaction with rubber molecules in rubber, and the molecular chain is broken or excessively cross-linking, which results to change in rubber properties. Oxidation is one of the important reasons for rubber aging.

2.Ozone: The chemical activity of 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 from whether the rubber is deformed or not. When it is used as a rubber for deformation (mainly unsaturated rubber), a crack which is straight in the direction of stress is generated, that is, "ozone cracking"; when it acts on a deformed rubber, only an oxide film will be formed on the surface and without cracking.

3. Heat: Increasing the temperature can cause the rubber thermal cracking or thermal cross-linking.But the basic function of heat is activation. Increasing oxygen diffusion rate and activation oxidation reaction in order to accelerate the rubber oxidation reaction rate, which is a common phenomenon of aging - thermal oxygen aging.

4.Mechanical stress: Under the repeated action of mechanical stress, the rubber molecular chain will be broken and to form a  free chain, and then trigger oxidation chain reaction and form a mechanochemical process. Between mechanical fracture molecular chain and mechanical activation oxidation process, which one takes advantage depending on the conditions.In addition, it is easy to cause ozone cracking under stress.

5. Water: There are two functions of water:  In the humid environment or long-term immersion in water, rubber is easy to damage. This is due to rubber water-soluble substances and hydrophilic groups and other components by water extraction solution, hydrolysis or absorption and other reasons.Especially in the alternate action of water immersion and atmospheric exposure, it will accelerate the destruction of rubber.However, in some cases, the water will not have destructive effect on the rubber, and may even delay the aging effect.

6. Oil: In the process of use, under long-term contact with the oil medium, the oil can penetrate into the rubber and cause it to swell, which reduces the strength and other mechanical properties of the rubber. Oil can cause rubber swelling, because the oil penetrated into rubber will make molecular diffusion so that the network structure of vulcanized rubber changes.

7. Other factors: chemical media, valence metal ions, high-energy radiation, electricity and biology,etc.

What are the protection methods for rubber aging?

Because aging of rubber is a complex and comprehensive chemical reaction process, and it's impossible to absolutely prevent rubber aging, appropriate measures can be taken to delay the aging rate of rubber so as to prolong the service life of rubber. The main anti-aging measures are physical protection and chemical protection.

Physical protection means avoiding the interaction between rubber and various aging factors as far as possible, such as using surface layer or treatment, adding light shielding agent, adding paraffin, etc.

Chemical protection means the addition of certain substances to prevent or delay the ageing of rubber, such as amine or phenolic chemical antioxidants.

What are the use simultaneously effect of antioxidants?

In order to improve the protection effect, two or more antioxidants with different mechanisms use simultaneously are often used in practical application,use antioxidants with the same protective mechanism,or choose antioxidants with different groups acting on the same molecule at the same time according to different mechanisms,there are three kinds of effects after using simultaneously, as follows:

1.Antagonistic effect means that when two or more antioxidants are used together, the effect is less than the sum of the effects when they are used alone,The results show that when acidic antioxidant and alkaline antioxidant are used together, they will produce a complex similar to salt, thus producing a Antagonistic effect.

Carbon black not only inhibits oxidation but also promotes oxidation in rubber. In the presence of chain-breaking antioxidant, the inhibition effect of carbon black decreases, or in the presence of carbon black, the protection effect of antioxidant decreases, which clearly shows that there is a confrontation effect between them.

2.Plus effect means that the protective effect of antioxidants is equal to the sum of their respective uses.It is a basic requirement to produce plus effect  when antioxidant is used together.The same type of antioxidant usually only produces the plus effect when used, but sometimes other benefits will be gained when used.For example, the combination of two phenolic antioxidants with different volatility can not only produce the plus effect , but also exert the inhibitory effect in a wider temperature range than the single antioxidant with the same amount.

3.Synergistic effect refers to the use simultaneously of antioxidants with greater effect than the sum of the effects of each antioxidant alone.