Epoxy Resin
Epoxy resin is a high-molecular polymer with the molecular formula (C₁₁H₁₂O₃)ₙ. It refers to a general term for a class of polymers containing more than two epoxy groups in their molecules. It is a condensation product of epichlorohydrin and bisphenol A or polyols. Due to the chemical reactivity of the epoxy group, it can be ring-opened by various compounds containing active hydrogen, and then cured and cross-linked to form a network structure, so it is a thermosetting resin. Bisphenol A epoxy resin not only has the largest output and the most complete varieties, but also new modified varieties are constantly increasing, and its quality is continuously improving.
Material Properties
Epoxy resin has secondary hydroxyl groups and epoxy groups, and the secondary hydroxyl groups can react with isocyanates. When epoxy resin is directly added as a polyol to the hydroxyl-containing component of polyurethane adhesives, only the hydroxyl groups participate in the reaction in this method, while the epoxy groups fail to react.
The carboxyl groups of acidic resins can open the epoxy rings, which then react with the isocyanates in polyurethane adhesives. Epoxy resin can also be dissolved in ethyl acetate, and phosphoric acid is added for heating reaction. The adduct added to polyurethane adhesives can improve the initial adhesion, heat resistance and hydrolysis stability of the adhesive. In addition, polyols can be generated by reacting with alkanolamines or amines, and the presence of tertiary nitrogen atoms in the adduct can accelerate the NCO reaction.
Using epoxy resin as a polyhydroxyl component combines the advantages of polyurethane and epoxy resin, having good bonding strength and chemical resistance. The epoxy resins generally used in the manufacture of polyurethane adhesives are varieties such as EP-12, EP-13, EP-16 and EP-20.
Modification Methods
1. Selecting curing agents;
2. Adding reactive diluents;
3. Adding fillers;
4. Adding special thermosetting or thermoplastic resins;
5. Improving the epoxy resin itself.
Application Fields
Epoxy resin has excellent physical, mechanical and electrical insulation properties, bonding properties with various materials, and the flexibility of its application process, which are not possessed by other thermosetting plastics. Therefore, it can be made into coatings, composite materials, casting materials, adhesives, molding materials and injection molding materials, and is widely used in various fields of the national economy.
Coating Applications
Epoxy resin accounts for a large proportion in coating applications, and it can be made into varieties with distinct characteristics and different uses. Their common features are:
- 1. Excellent chemical resistance, especially alkali resistance.
- 2. Strong paint film adhesion, especially to metals.
- 3. Good heat resistance and electrical insulation.
- 4. Good color retention of the paint film.
However, the weather resistance of bisphenol A epoxy resin coatings is poor, and the paint film is prone to chalking and loss of gloss outdoors and lacks fullness, so it is not suitable for outdoor coatings and high decorative coatings. Therefore, epoxy resin coatings are mainly used as anti-corrosion paints, metal primers and insulating paints, but coatings made of heterocyclic and alicyclic epoxy resins can be used outdoors.
Adhesive Applications
Except for poor adhesion to non-polar plastics such as polyolefins, epoxy resin has excellent bonding properties to various metal materials such as aluminum, iron, copper; non-metallic materials such as glass, wood, concrete; and thermosetting plastics such as phenol formaldehyde, amino, and unsaturated polyester. Therefore, it is known as the "universal adhesive". Epoxy adhesive is an important variety of structural adhesives.