Toughener is a kind of material that can increase the flexibility of adhesive film. Some thermosetting resin adhesives, such as epoxy resin, phenolic resin and unsaturated polyester resin, have low elongation and high brittleness after curing. When the bonding part bears external force, it is easy to produce cracks and expand rapidly, resulting in cracking of the adhesive layer and fatigue resistance, which can not be used for structural bonding. Therefore, it is necessary to reduce brittleness, increase toughness and increase bearing strength. Any material that can reduce brittleness and increase toughness without affecting other main properties of adhesive is called toughener.

　　Generally speaking, toughening agents contain active groups, which can react with resin chemically. After curing, they are not completely compatible. Sometimes, they need to separate phases, which can obtain ideal toughening effect, keep the temperature of thermal deformation unchanged or decrease slightly, and improve the impact resistance. Some low molecular liquids or plasticizers can also reduce the brittleness of the resin, but the rigidity, strength and thermal deformation temperature are greatly reduced, which can not meet the requirements of structural bonding. Therefore, plasticizers and tougheners are totally different.

　　Some linear polymer compounds, which can be miscible with resin and contain active groups, can participate in the curing reaction of resin, improve the elongation at break and impact strength, but the temperature of thermal deformation has decreased. This kind of material is called flexilizer. The commonly used ones are liquid polysulfide rubber and liquid nitrile rubber. Because they are properly combined with resin, they can be made into structural adhesive, Therefore, the softener is also classified as toughener. Although softening and toughening are interrelated and different concepts, they are difficult to distinguish strictly. In theory, toughening is different from toughening. Toughening does not soften the material as a whole, but changes the homogeneous system of epoxy resin curing compound into a multiphase system, i.e. the toughening agent gathers the spherical particles to form the dispersed phase in the continuous phase formed by the cross-linking network of epoxy resin. The anti cracking property changes abruptly, and the fracture toughness is significantly improved, but the loss of mechanical property and heat resistance is small.