Analysis of Toughening Mechanism of Toughener

Therefore, it is necessary to try to reduce brittleness, increase toughness, and increase bearing strength. Any substance that reduces brittleness and increases toughness without affecting other major properties of the adhesive is called a toughening agent. Tougheners generally contain reactive groups, which can react with the resin, are not completely compatible after curing, and sometimes have to be phase separated, which will obtain a better toughening effect, so that the heat distortion temperature is constant or decreases little. The impact resistance is also significantly improved. Some low molecular liquids or plasticizers are added to the resin. Although they can reduce brittleness, the rigidity, strength and heat distortion temperature are greatly reduced, which cannot meet the structural bonding requirements. Therefore, plasticizers and Tougheners are completely different.

Some linear polymer compounds, which are miscible with the resin, contain reactive groups, can participate in the curing reaction of the resin, increase the elongation at break and impact strength, but the heat distortion temperature decreases. This substance is called softening. (flexibizer), commonly used liquid polysulfide rubber, liquid nitrile rubber, because they are combined with the resin, can be made into a structural adhesive, so the softener is also classified as a toughener. Although softening and toughening are interrelated and different concepts, they are actually difficult to distinguish strictly. Theoretically speaking, toughening and softening are different. Toughening does not soften the material as a whole, but instead transforms the epoxy resin cured homogeneous system into a multi-phase system, ie the toughening agent is aggregated into spherical particles in the epoxy. The dispersed phase formed in the continuous phase composed of the crosslinked network of the resin has a sudden change in crack resistance, and the fracture toughness is remarkably improved, but the mechanical properties and heat resistance are less lost.

Toughening mechanism

Different types of toughening agents have different toughening mechanisms. The liquid polysulfide rubber reacts with the epoxy resin to introduce a portion of the soft segment, which reduces the modulus of the epoxy resin and improves the toughness, while sacrificing heat resistance. As a toughening agent for epoxy resin, liquid nitrile rubber has almost no toughening effect at room temperature, and the bonding strength decreases. Only the medium-high temperature curing system has obvious toughening and bonding effects. The carboxyl-terminated liquid nitrile rubber toughened epoxy resin is compatible before curing, and is phase-separated after solidification to form an "island structure", which can absorb impact energy without substantially reducing heat resistance. T-99 multi-functional epoxy curing agent curing epoxy resin introduces a soft segment into the cross-linked structure, does not produce a phase-separated structure, and does not reduce heat resistance substantially while improving toughness.

The thermoplastic resin continuously penetrates into the epoxy resin network to form a semi-interpenetrating network type polymer, resulting in an increase in toughness of the cured epoxy resin.

The size of the nanoparticles is 1-100 nm, the surface area is extremely large, and the surface atoms have extremely high unsaturation, so the surface activity is very large. The epoxy group forms a much larger interaction with the nanoparticles at the interface than the van der Waals force, which can induce microcracks and absorb energy. Nano-silica and nano-clay can both initiate silver streaks and terminate cracks. At the same time, the nanoparticles have a strong rigidity, and when the cracks expand, they encounter the orientation or deflection of the nanoparticles, and absorb energy to achieve the purpose of toughening. In addition, the nanoparticles have good compatibility with the resin, and the dispersion ability and absorption capacity of the matrix for impact energy are increased, resulting in an increase in toughness.