Antioxidants are a class of chemicals that, when present in only a small amount in a polymer system, can delay or inhibit the oxidation process of the polymer, thus Prevents the aging of polymers and extends their service life, also known as “anti-aging agents”. Substances that can delay or prevent oxidation or autoxidation are called antioxidants. The U.S. Food and Drug Administration (FDA) defines antioxidants as “antioxidants that retard deterioration, rancidity, or discoloration caused by oxidation.” substance”. Antioxidants must be added to greases and oily foods to prevent rancidity, petroleum products such as gasoline and lubricants to prevent viscosity changes and precipitation, and polymers such as plastics, rubber and synthetic fibers to prevent aging.
Antioxidants can slow down the auto-oxidation reaction speed of polymer materials. In addition to being used in plastics and rubber, antioxidants are also widely used in petroleum products, greases and In the food industry, it is used to prevent the acidity and viscosity of fuel oil and lubricating oil from rising and the rancidity of grease, meat and feed.
Antioxidant classification
Aromatic amine antioxidants
Aromatic amine antioxidants, also known as rubber antioxidants, are the most produced type and are mainly used in rubber products Among them, this type of antioxidant is cheap and has significant antioxidant effect, but it causes discoloration of the product, which limits its application in light-colored and white products. Important aromatic amine antioxidants include: diphenylamine, p-phenylenediamine, dihydroquinoline and other compounds and their derivatives or polymers. Representative products include N-isopropyl-N′-phenyl-p-phenylenediamine (see structural formula a). The product is gray to purple crystal with a melting point above 70°C. It can be used in products such as natural rubber, styrene-butadiene rubber, chloroprene rubber and isoprene rubber.
Hindered phenolic antioxidant
Hindered phenolic antioxidants are phenolic compounds with steric hindrance. They have significant thermal oxidation resistance, will not pollute products, and are developing rapidly. There are many varieties of this type of antioxidant. Important products include: 2,6-tertiary butyl-4-methylphenol (b), bis(3,5-tertiary butyl-4-hydroxyphenyl) sulfide Ether (c) and tetrakis [β-(3,5-tertiary butyl-4-hydroxyphenyl)propionate] pentaerythritol ester (d), etc. Pentaerythritol tetrakis[β-(3,5-tertiary butyl-4-hydroxyphenyl)propionate] is an excellent antioxidant for polyolefins. Hindered phenolic antioxidants are mainly used in plastics, synthetic fibers, latex, petroleum products, food, drugs and cosmetics.
Supplementary antioxidant
Thiodipropionic acid diester is a type of auxiliary antioxidant, which is often used together with hindered phenolic antioxidants and has remarkable effects. The main products are: didecyl alcohol ester, distetradecanol ester and disoctadecyl alcohol ester. Phosphite is also an auxiliary antioxidant. The main products are: trioctyl ester, tridecyl ester, tris(dodecanol) ester and tris(cetadecanol) ester. When used in conjunction with the main antioxidant, the effect is Very good too.
Mechanism of action
1. Stability of polymers using chain-breaking antioxidants.
The reaction mechanism of antioxidant intervention chain reaction active species, namely segmented chain donor mechanism (CB-D) and segmented chain receptor mechanism (CB-A)
The typical CB-D mechanism is the reaction between the peroxidation group and inhibitors such as phenols, followed by aromatic amines. The only radical generated from the inhibitor AH can destroy a peroxide group PO2 according to the reaction formula (1-43).
2. High polymer stability using pre-antioxidants
Preventative or co-antioxidants decompose hydroperoxides without forming free radical intermediates; therefore, they prevent the decomposition of hydroperoxides into Free radicals are caused by chain branching.
3. Synergy between antioxidants
A very famous example is biscinnamoyl thiodipropionate (DLTDP) or distearoyl thiodipropionate Lipid (DSTDP) and sterically hindered phenols are used in the thermal stability of certain polymers. Another very important example of synergy is the complex interaction between sterically hindered phenols and phosphites when improving the melt stability of polyolefins.
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