What is the Epoxy ?

Epoxy resin is a viscous liquid resin classified in the group of synthetic resin which contains an epoxide group in its chemical structure. It is characterized by its high adhesive properties (good adhesion with reinforcement), high chemical resistance, better moisture resistance, excellent mechanical properties, good fatigue resistance, low shrinkage, strong durability at low and high temperatures, and high electrical resistance, which justifies its numerous uses in modern industries. The epoxy resin having a transparent color is mixed with its suitable hardener (darker color) to polymerize and get an irreversible material.

Epoxy resin has a good adhesion, curing shrinkage is small, curing does not produce gas, has good heat resistance, excellent solvent resistance, and creep to obtain a wide range of applications in the adhesive, paint, and flux fields. It is also commonly used as fiber reinforced composite resin matrix. When lignin is added into the epoxy resin system, it can react with a variety of types of curing agent crosslinking reactions. Insoluble nonmelting three-dimensional networks then can form in such a lignin-modified epoxy-resin material. The preparation of lignin-modified epoxy resin has three methods:

① Ligin is directly mixed with the common epoxy resin blends, which can form interpenetrating polymer network structures.

② Lignin is oxidized before the first modification, and then used as a raw material to prepare epoxy resins.

③ Functional groups that can react with epoxy resin are introduced into lignin, and the modified lignin is then used to modify epoxy resin.

The epoxy resin film modified by sulphate lignin is prepared by a solution casting method as follows. The bisphenol-A epoxy resin is first mixed with sulphate lignin, which has a mass fraction of 10%–40% (by mass of the blend) by stirring. An appropriate amount of a curing agent (high activity of aliphatic polyamine) also is added. They cure at 100°C for 2 h or at room temperature for 24 h for molding. Studies show that lignin can be connected to the epoxy resin network through unreacted amine group of the curing agent. This reaction takes place only when the curing temperature is high. By measuring the reactivity of lignin with amine-based curing agents, the results show that the reaction of lignin and polyamines does not have the ability to react with epoxy groups or primary and secondary amines [25]. Bottom lignin (BL) extracted from wood can be modified by epichlorohydrin to obtain epoxy groups. Two lignin-modified epoxy resin materials are prepared with 1-(2-cyanoethyl)-2-ethyl-4-methyl imidazole (2E4MZ-CN) and lignin as curing agents, respectively. The mechanism of the reaction between lignin and epichlorohydrin is: With the phase transfer catalyst tetrabutyl bromide (TBAB), the lignin and epichlorohydrin react as a ring-open addition reaction. Then, with sodium hydroxide, the hydrogen chloride is removed and the epoxy groups reform [26]. Based on the idea that lignin can act as an epoxy resin curing agent, lignin can improve the quality stability of the modified epoxy resin by adjusting the amount of the group and the number of groups. These groups can react with the epoxy group by chemical reaction. The hydroxyl groups of enzymolyzed lignin or its derivatives can react with diacid anhydride and catalyst to produce prepolymers. They then can react with glycidyl ether to prepare polyester-type epoxy resin modified by enzymolyzed lignin. The results show that when the content of prepolymer is small, although the epoxy value is higher, the crosslinking density of the system will decrease because of the existence of more unreacted ethylene glycol diglycidyl ether monomers. The addition of epoxy groups of ethylene glycol diglycidyl ether to the carboxyl groups on the prepolymer is more complete. Moreover, the free ethylene glycol diglycidyl ether monomer will gradually decrease. The crosslinking density of epoxy resin will increase significantly because of the synergistic effect of the lignin component, thereby enhancing the bond shear strength. When the prepolymer content (mass fraction) is 50%, the adhesive shear strength of the modified epoxy resin reaches its maximum value [27]. So far, the research on lignin alone as a raw material to prepare epoxy resin is relatively small, and most of the research is on introduction of the lignin to epoxy resin through the blending method. Most of the lignin-modified epoxy resin materials are organic solvents and poor processing performance are among their shortcomings. Researchers are currently looking for a positive and effective way to solve the problems.