抗氧剂是一类化学物质，当其在聚合物体系中仅少量存在时，就可延缓或抑制聚合物氧化过程的进行，从而阻止聚合物的老化并延长其使用寿命，又被称为“防老剂”。对工程塑料加工来说，抗氧剂可以防止某些聚合物(如ABS等)加工过程中的热氧化降解，使其成型加工能顺利进行。抗氧剂的添加量—般只有0.1-0.5份。
理想抗氧剂应具备以下条件：①应具有高的抗氧化能力：②与树脂的相容性好，不析出；③加工性能良好．在高聚物的加工温度下不挥发、不分解；④耐抽出性好，不溶于水和油中；⑤本身颜色最好为无色或浅色．以不污染制品；⑥无毒或低毒；⑦价格低廉。
事实上，任何一种抗氧剂都不能完全满足这些条件，因此，实际使用中常根据工程塑料的种类、用途和加工方法，利用各种助剂之长，配合使用，以生产协同效应。随着工程塑料工业的发展，抗氧剂的使用日益广泛，需求量不断增加，目前，广泛应用抗氧剂的工程塑料有聚酰胺，聚甲醛、ABS、PC、PU、PET、PP、PE、PS、PVC、PMMA、TPE等．
根据高分子的热氧化机理，在热氧化过程中产生的不稳定自由基和氢过氧化物是引起材料性能劣化的主要因素，抗氧剂的作用机理正是用来终止活性自由基和分解氢过氧化物的．具有抑制自由差连锁反应作用的自由基抑制剂称为主抗氧剂，它包含胺类和酚类两大系列，用于工程塑料的主要是酚类抗氧剂．具有分解氢过氧化构作用的氢过氧化物分解剂称为辅助抗氧剂，主要包括硫代酯和亚磷酸酯两大类，通常与主抗氧剂并用．
(1)主抗氧剂的作用机理
酚类、胺类抗氧剂中含有的·OH和·NH能与自动催化氧化反应中形成的自由基作用，最终使自由基消失，使氧化的链式反应终止：
RH’+ROO·一ROOH+R’ ·
(R一—树脂分子链主体；R’——抗氧剂结构主体) R’H+R·一RH+R’ · R’ ·+ROO·一ROOR’ R’ ·+R’ ·一R’R’
(2)辅助抗氧剂的作用机理
亚磷酸酯类抗氧剂能够将氧化反应过程中形成的氢氧化物分解为不活泼产物，使其失去活性：
(R’O)3P+ROOH→ROH+(R’O)3P＝O
抗氧剂种类繁多，作用机理各异，通过复配使用，能有效地发挥各抗氧剂的优势而将其劣势减小到最低程度，这是今后抗氧剂发展的大趋势。近年来我国聚烯烃工业迅速发展，必将提高对抗氧剂使用性能的要求，而在聚烯烃添加剂中居重要地位的受阻酚类抗氧剂将趋向于多用途化、复合化、环保型化。因此，必须认识和了解抗氧剂之间的相互作用，重视抗氧剂之间的协同效应。只有利用好抗氧剂之间的协同效应，才能做到事半功倍，并会获得性能优良的复合型抗氧剂。
Antioxidants are a type of chemical substance that, even when present in small amounts in polymer systems, can slow down or inhibit the oxidation process of polymers, thereby preventing the aging of polymers and extending their service life. They are also called "anti-aging agents." For engineering plastics processing, antioxidants can prevent thermal oxidative degradation of certain polymers (such as ABS) during processing, ensuring smooth molding. The amount of antioxidant added is usually only 0.1-0.5 parts. An ideal antioxidant should have the following conditions: ① High antioxidant capacity; ② Good compatibility with the resin and no precipitation; ③ Good processing performance, non-volatile and non-decomposing at polymer processing temperatures; ④ Good extraction resistance, insoluble in water and oil; ⑤ Preferably colorless or light-colored to avoid contaminating products; ⑥ Non-toxic or low toxicity; ⑦ Low cost. In fact, no single antioxidant can fully meet these conditions, so in practice, different antioxidants are used in combination based on the type, application, and processing method of the engineering plastic to produce a synergistic effect. With the development of the engineering plastics industry, the use of antioxidants has become increasingly widespread and demand continues to rise. Currently, antioxidants are widely used in polymers like polyamide, polyoxymethylene, ABS, PC, PU, PET, PP, PE, PS, PVC, PMMA, TPE, and others. According to the thermal oxidation mechanism of polymers, unstable free radicals and hydroperoxides formed during thermal oxidation are the main factors causing material property deterioration. The mechanism of action of antioxidants is to terminate active free radicals and decompose hydroperoxides. Free radical inhibitors that prevent chain reactions are called primary antioxidants. These include amine and phenolic types, with phenolic antioxidants being mainly used in engineering plastics. Hydroperoxide decomposers, which break down hydroperoxides, are called secondary antioxidants, mainly including thioesters and phosphites, usually used together with primary antioxidants. (1) Mechanism of primary antioxidants Phenolic and amine antioxidants contain ·OH and ·NH, which can react with free radicals formed in autocatalytic oxidation reactions, ultimately eliminating the free radicals and terminating the chain oxidation reaction: RH’ ROO· → ROOH R’ · (R — main resin chain; R’ — antioxidant structure) R’H R· → RH R’ · R’ · ROO· → ROOR’ R’ · R’ · → R’R’ (2) Mechanism of secondary antioxidants Phosphite antioxidants can decompose hydroperoxides formed during oxidation into inactive products, rendering them inert: (R’O)3P + ROOH → ROH + (R’O)3P=O There are many types of antioxidants with different mechanisms of action. By using them in combination, the advantages of each antioxidant can be maximized while minimizing their disadvantages, which is the future trend of antioxidant development. In recent years, with the rapid development of China’s polyolefin industry, the demand for antioxidant performance is expected to increase, and hindered phenolic antioxidants, which play an important role in polyolefin additives, will tend to become multi-functional.
