增韧剂是指能增加胶黏剂膜层柔韧性的物质。某些热固性树脂胶黏剂，如环氧树脂、酚醛树脂和不饱和聚酯树脂胶黏剂固化后伸长率低，脆性较大，当粘接部位承受外力时很容易产生裂纹，并迅速扩展，导致胶层开裂，不耐疲劳，不能作为结构粘接之用。因此，必须设法降低脆性，增大韧性，提高承载强度。凡能减低脆性，增加韧性，而又不影响胶黏剂其他主要性能的物质称为增韧剂。

Toughening agent is the substance that can increase the flexibility of the film of the adhesive. Some thermosetting resin, such as epoxy resin, phenolic resin and unsaturated polyester resin adhesive after curing, low elongation, high brittleness, when bonding parts under external force is easy to crack, and the rapid expansion in layer cracking, fatigue, can not be used as adhesive for structure. Therefore, it is necessary to reduce the brittleness, increase the toughness and improve the bearing strength. Any substance that can reduce brittleness and increase toughness without affecting the other main properties of the adhesive is called toughening agent.