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Epoxy resin refers to organic polymer compounds containing two or more epoxy groups in their molecules, and except for a few, their relative molecular weight is not high. The molecular structure of epoxy resin is characterized by the presence of active epoxy groups in the molecular chain, which can be located at the end, middle, or in a cyclic structure of the molecular chain. Due to the presence of active epoxy groups in their molecular structure, they can undergo cross-linking reactions with various types of curing agents to form insoluble and non melting polymers with a three-dimensional network structure.
★ Properties and characteristics of epoxy resin
1. There are various forms. Various resin, curing agent, and modifier systems can almost meet the form requirements of various applications, ranging from extremely low viscosity to high melting point solids.
2. Curing is convenient. By using various curing agents, the epoxy resin system can be cured almost within the temperature range of 0-180 ℃.
3. Strong adhesion. The inherent polar hydroxyl and ether bonds in the molecular chain of epoxy resin give it high adhesion to various substances. The shrinkage of epoxy resin during curing is low, and the generated internal stress is small, which also helps to improve the adhesion strength.
4. Low contractility. The reaction between epoxy resin and the curing agent used is carried out through direct addition reaction or ring opening polymerization reaction of epoxy groups in resin molecules, without the release of water or other volatile by-products. Compared with unsaturated polyester resin and phenolic resin, they exhibit very low shrinkage (less than 2%) during the curing process.
5. Mechanical properties. The cured epoxy resin system has excellent mechanical properties.
6. Electrical performance. The cured epoxy resin system is an excellent insulation material with high dielectric properties, surface leakage resistance, and arc resistance.
7. Chemical stability. Generally, cured epoxy resin systems have excellent alkali resistance, acid resistance, and solvent resistance. Like other properties of cured epoxy systems, chemical stability also depends on the resin and curing agent selected. Proper selection of epoxy resin and curing agent can give it special chemical stability properties.
8. Dimensional stability. The combination of many of the aforementioned properties gives the epoxy resin system outstanding dimensional stability and durability.
9. Resistant to mold. The cured epoxy resin system is resistant to most molds and can be used under harsh tropical conditions.
★ Classification
According to their molecular structure, epoxy resins can generally be divided into five categories:
1. Glycidyl ether epoxy resin
2. Glycidyl ester epoxy resin
3. Glycidyl amine epoxy resin
4. Linear aliphatic epoxy resin
5. Cycloaliphatic epoxy resin
The most widely used type of epoxy resin in the composite material industry is the first type of glycidyl ether epoxy resin mentioned above, among which bisphenol A epoxy resin (abbreviated as bisphenol A epoxy resin) is the main one. Next is glycidyl amine epoxy resin.
1. Glycidyl ether epoxy resin
Glycyrrhetinic ether epoxy resin is formed by the condensation of phenols or alcohols containing active hydrogen with epichlorohydrin.
(1) Diphenylpropane type epoxy resin is formed by the condensation of diphenylpropane and epichlorohydrin.
Industrial diphenylpropane epoxy resin is actually a mixture of molecules with different degrees of polymerization. Most of these molecules are linear structures with two epoxy end groups. A few molecules may be branched, and a very small number of molecules terminate with chlorohydrin groups instead of epoxy groups. Therefore, the epoxy group content and chlorine content of epoxy resin have a significant impact on the curing of the resin and the properties of the cured product. The control indicators for resins in industry are as follows:
① Epoxy value. Epoxy value is the most important indicator for identifying the properties of epoxy resin, and industrial epoxy resin models are distinguished based on different epoxy values. Epoxy value refers to the amount of epoxy based substances contained in 100g of resin. The reciprocal of the epoxy value multiplied by 100 is called the epoxy equivalent. The meaning of epoxy equivalent is the number of grams of epoxy resin containing 1 mol of epoxy group.
② Inorganic chlorine content. The chloride ions in the resin can interact with amine curing agents to affect the curing of the resin, as well as the electrical properties of the cured resin. Therefore, the chlorine content is also an important indicator of epoxy resin.
③ Organic chlorine content. The organic chlorine content in the resin indicates the amount of chlorohydrin groups in the molecule that have not undergone closed-loop reactions. Its content should be reduced as much as possible, otherwise it will also affect the curing of the resin and the properties of the cured product.
④ Volatile matter.
⑤ Viscosity or softening point.
(2) Phenolic multi epoxy resin includes phenol formaldehyde type and ortho cresol formaldehyde type multi epoxy resins. Compared with bisphenol A type epoxy resin, it contains two or more epoxy groups in the linear molecule, so the crosslinking density of the cured product is high, and it has excellent thermal stability, mechanical properties, electrical insulation, water resistance, and corrosion resistance. They are condensed from linear phenolic resin and epichlorohydrin.
(3) Other practical representatives of polyhydroxyphenolic glycidyl ether type epoxy resins include resorcinol type epoxy resin, resorcinol formaldehyde type epoxy resin, tetraphenylethane type epoxy resin, and trihydroxyphenylmethane type epoxy resin. These multifunctional glycidyl ether resins have high heat distortion temperature and rigidity after curing, and can be used alone
Alternatively, it can be blended with general-purpose E-type resin for use as high-performance composite materials (ACM), printed circuit boards, and other matrix materials.
(4) Aliphatic polyol glycidyl ether type epoxy resin contains two or more epoxy groups in its molecules, and the vast majority of these resins have low viscosity; Most of them are long-chain linear molecules, so they are flexible.
2. Other types of epoxy resins
(1) Compared with bisphenol A epoxy resin, glycidyl ester epoxy resin has low viscosity and good processability; High reactivity; The adhesive force is higher than that of general epoxy resin, and the mechanical properties of the cured material are better; Good electrical insulation; Good weather resistance and excellent ultra-low temperature resistance. Under ultra-low temperature conditions, it still has higher bonding strength than other types of epoxy resins. Has good surface glossiness, good transparency, and weather resistance.
(2) The advantages of glycidyl amine epoxy resin are multifunctionality, high epoxy equivalent, high crosslinking density, and significantly improved heat resistance. Both domestically and internationally, the superior adhesion and heat resistance of glycidyl amine epoxy resin have been utilized to manufacture carbon fiber reinforced composite materials (CFRP) for aircraft secondary structural materials.
(3) Alicyclic epoxy resin is a type of epoxy resin produced by epoxidation of the double bonds of cycloaliphatic olefins. Its molecular structure differs greatly from that of diphenolpropane type epoxy resin and other epoxy resins. The former has epoxy groups directly attached to the cyclic ring, while the latter has epoxy groups attached to the benzene nucleus or aliphatic hydrocarbon via epoxy propyl ether. The cured product of cycloaliphatic epoxy resin has the following characteristics: ① high compression and tensile strength; ② Long term exposure to high temperature conditions can still maintain good mechanical properties; ③ Good arc resistance, UV aging resistance, and weather resistance.
(4) Aliphatic epoxy resins have not only no benzene nucleus but also no cycloaliphatic structure in their molecular structure. Only the fatty chain is connected to the epoxy group. The strength, toughness, adhesion, and resistance to positive and negative temperatures of epoxidized polybutadiene resin after curing are all good.
