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Antioxidant is a kind of chemical substance, which can delay or inhibit the process of polymer oxidation when it only exists in a small amount in the polymer system, thus preventing the aging of polymer and extending its service life. It is also called "antioxidant". For the processing of engineering plastics, antioxidants can prevent the thermal oxidative degradation of some polymers (such as ABS) in the processing process, so that the molding process can be carried out smoothly. How much antioxidant is added? Generally, only 0.1-0.5 portions.
Conditions for ideal antioxidant
The ideal antioxidant should have the following conditions: ① it should have high antioxidant capacity; ② it has good compatibility with resin and does not precipitate; ③ Good processing performance. It will not volatilize and decompose at the processing temperature of polymer; ④ Good extraction resistance, insoluble in water and oil; ⑤ Its color should be colorless or light, so as not to pollute the product; ⑥ Non-toxic or low-toxic; ⑦ Low price.
In fact, any kind of antioxidant can not fully meet these conditions. Therefore, in actual use, it is often based on the type, use and processing method of engineering plastics to make use of the advantages of various additives to produce synergy.
With the development of engineering plastics industry, the use of antioxidants is increasingly widespread and the demand is increasing. At present, the engineering plastics widely used with antioxidants include polyamide, polyformaldehyde, ABS, etc
Action mechanism of antioxidant
According to the thermal oxidation mechanism of polymers, unstable free radicals and hydroperoxides produced in the thermal oxidation process are the main factors that cause the deterioration of material properties. The action mechanism of antioxidants is precisely used to terminate the active free radicals and decompose hydroperoxides. The free radical inhibitors with the effect of inhibiting the chain reaction of free difference are called main antioxidants, which include two series of amines and phenols, Phenolic antioxidants are mainly used in engineering plastics. The hydroperoxide decomposing agents with the function of decomposing the hydroperoxide structure are called auxiliary antioxidants, mainly including thioesters and phosphite esters, which are usually used together with the main antioxidants
(1) Action mechanism of main antioxidant
What is contained in phenolic and amine antioxidants? OH and? NH can react with the free radicals formed in the autocatalytic oxidation reaction, and finally make the free radicals disappear, so that the chain reaction of oxidation is terminated:
RH’+ROO? One ROOH ten R '?
(R - resin molecular chain main body; R '- antioxidant structural main body)
R 'H bucket R? - RH+R '?
R’ ?+ ROO? One ROOR '
R’ ? Ten R '? - R 'R'
(2) Action mechanism of auxiliary antioxidant
Phosphite antioxidants can decompose the hydroxide formed in the oxidation reaction process into inactive products and make them inactive:
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