不少聚合物在室温下呈脆性，因而大大降低了它的使用价值、例如聚苯乙烯有良好的透明性、易加工性但需加入橡胶类的增加韧性才有较高的抗冲击强度。这种赋予塑料更好韧性的助剂称为增韧剂，也称为抗冲改性剂。当前开发增韧剂的主要目的是为了改善硬聚氯乙烯的脆性。世界上硬聚氯乙烯用量的不断增长与增韧剂、加工改性剂的日益开发有密切的关系。增韧剂是具有降低复合材料脆性和提高复合材料抗冲击性能的一类助剂。可分为活性增韧剂与非活性弹韧剂两类，活性增韧剂是指其分子链上含有能与基体树脂反应的活性基团，它能形成网络结构，增加一部分柔性链，从而提高复合材料的抗冲击性能。非活性增韧剂则是一类与基体树脂很好相溶、但不参与化学反应的增韧剂。根据需要增韧的材料的化学结构不同，有相应类型的增韧剂，塑料增韧剂的原理不同，是通过特殊的化学增聚和物理作用，在少量使用的条件下，能够有效增加各种塑料的韧性。

Many polymers are brittle at room temperature and thus greatly reduce their use value, such as polystyrene with good transparency and processability, but the increase in toughness of the rubber class is higher. The auxiliaries that give better toughness to plastics are called toughening agents, also known as impact modifiers. The main purpose of developing toughening agents is to improve the brittleness of rigid polyvinyl chloride. The continuous increase of the amount of PVC in the world is closely related to the increasingly development of toughening agents and processing modifiers. Toughening agent is a kind of additive which can reduce the brittleness of composites and improve the impact resistance of composites. It can be divided into two types: active toughening agent and non active elastic toughening agent. Active toughening agent refers to the active group on its molecular chain that can react with matrix resin. It can form a network structure and increase a part of the flexible chain, thus improving the impact resistance of the composite. The inactive toughening agent is a kind of toughening agent which is well soluble with the matrix resin but does not participate in the chemical reaction. According to the chemical structure of the material that needs toughening, there are corresponding types of toughening agent, and the principle of plastic toughening agent is different. It can increase the toughness of various kinds of plastics in a small amount of use by special chemical enrichment and physical action.