The end groups of epoxy resin macromolecules formed by the polymerization of epichlorohydrin and active hydrogen compounds are not only epoxy groups, but also a small amount of heterogeneous end groups. Although these heterogeneous end groups are few in number, they not only affect the properties and curing reactivity of the epoxy resin, but also affect the properties of the cured epoxy resin, especially the electrical properties, so they cannot be ignored. Heterogeneous end groups have different structures and have different effects. However, their common effect is to increase the epoxy equivalent of the resin, resulting in a decrease in the crosslink density of the cured resin. In general, resins with a small epoxy equivalent have little effect on hetero-end groups. For resins with a large epoxy equivalent, the effect of heterogeneous end qualities cannot be ignored.
The heterogeneous end groups usually formed are as follows. The a-end group is formed by the epichlorohydrin normally added during the synthesis of epoxy resin due to the incomplete ring-closing reaction of HCl removal. In the form of 1,2-chlorinated hydroxyl groups in the resin. It is easily hydrolyzed, so it is called dechlorination solution, or easily saponified chlorine, which is active chlorine. The b-end group and the c-end group in Figure c are formed by the side reaction of abnormal addition of epichlorohydrin. Their tendency to hydrolyze to form Cl- is much smaller than that of easily saponifiable chlorine, which is combined chlorine and inactive chlorine. The b-end group is the 1,3-chlorinated hydroxyl group formed by the reaction of the bisphenol A anion with the middle carbon atom of epichlorohydrin. The c-terminal group is the 1-chloro-2-glycidyl ether group formed by the reaction of the hydroxyl group of 1,2-chlorinated alcohol or the corresponding alkoxy ion with epichlorohydrin.
The above-mentioned three kinds of organic chlorine-containing hetero end groups reflect the resin synthesis reaction. Although the content is small, it has a great influence on the epoxy resin performance. It not only has negative effects such as reducing the electrical properties and corrosion of cured products; but also has positive effects such as preventing liquid epoxy resin from crystallizing, shortening gel time, and promoting curing reaction; It is formed by mixing glycidol into the reaction. It is traditionally called the α-diethanol end group. The bisphenol A epoxy resin usually contains an α-diethanol structure of about 70 mmol/kg. The positive effect is to promote the curing reaction of amine or anhydride and epoxy resin, while the negative effect is to reduce the water resistance of the cured product. The end group in Figure e is the phenol structure remaining after the addition of epichlorohydrin. The concentration in epoxy resin is usually very small, and its effect is negligible.