Published: 2020-08-12 16:29:39 Source: www.epoxy-e.cn Author: office
Cyanate ester (CE) resin usually refers to a dihydric phenol derivative containing two or more cyanate functional groups. Under the action of heat or a catalyst, a cyclization trimerization reaction occurs to form a highly cross-linked triazine ring. Macromolecules with connected network structure. After curing, CE resin has low shrinkage, low moisture absorption, good electrical properties, excellent adhesion, good flame retardancy, low smoke and ablation resistance, and high glass transition temperature. CE resin also has excellent mechanical properties, its flexural strength is higher than bifunctional epoxy resin (EP), and its flexural modulus is between bismaleimide (BMI) resin and multifunctional EP. CE resin is mostly solid or semi-solid at room temperature, soluble in common solvents, and has good wettability with reinforcing fibers, showing excellent adhesion, coating and rheological properties, not only can use traditional Injection molding, compression molding and other processes are also suitable for advanced aerospace composite material molding processes, such as winding, autoclave, vacuum bag, and resin transfer molding (RTM). Although CE resin has many excellent properties, its thermal curing reaction temperature is high, curing time is long, and the manufacturing cost is high; the crosslink density after monomer polymerization is too large, and the triazine ring structure in the molecule is highly symmetrical and crystalline. The high temperature causes the CE resin to be more brittle after curing. In addition, it has poor hydrolysis resistance and is easy to crystallize from the composite prepreg. Therefore, in order to reduce the application cost of CE resin and further improve its performance, research on the modification of CE resin has attracted more and more attention. Among them, thermosetting resins mainly include EP and BMI resins. Experts from China Epoxy Resin Industry Association recently introduced the process of epoxy resin modified cyanate ester.
Copolymerization reaction mechanism: CE/EP copolymers have very complicated reactions. In addition to their respective homopolymerization products, copolymers are also formed through copolymerization structural units such as isocyanurate and oxazolidinone. It is generally believed that there are three stages of copolymerization: CE homopolymerization into a triazine ring cross-linked structure; CE functional group reacts with epoxy functional group to form oxazolidinone and other structures; epoxy undergoes polyetherification reaction. It first forms the triazine ring, including the system without catalyst: the impurities in CE are mainly moisture and residual phenol and metal ions in resin synthesis. Under heating conditions, impurities containing active hydrogen react with monomers to produce The intermediates and products have a catalytic effect on the curing of CE; with the addition of a catalyst system, the main catalysts used in epoxy modified CE resin are: 4,4,-diaminodiphenyl sulfone, butyl tin dilaurate, cobalt naphthenate, acetyl Copper acetate (II), copper acetylacetonate (II), chromium acetylacetonate, imidazoles, acetylacetonate lanthanide transition metal complex, titanate, zinc octoate, N,N-dimethylaniline and N,N- Dimethylbenzylamine, etc. also used transition metal salts and nonylphenol systems as catalysts for EP modified CE resins in early research, but the effect was not very good. Experts from China Epoxy Resin Industry Association (www.epoxy-e.cn) said that regarding the copolymerization and crosslinking of EP and CE, the currently widely recognized EP/CE resin copolymerization mechanism is: CE trimerization and shrinkage. Glyceryl ether is inserted into the cyanurate ring, alkyl cyanurate is isomerized to alkyl isocyanurate, alkyl isocyanurate and glycidyl ether form oxazolidinone, and the alkyl group is eliminated to form phenol. Glycidyl ether is added with phenol.
Regarding the above-mentioned addition of glycidyl ether and phenol, according to experts from China Epoxy Resin Industry Association, the copolymerization of EP and CE also has a mutual catalytic effect. A small amount of EP can promote the curing reaction of CE, and a small amount of CE can also Promote the curing reaction of EP. The small amount of hydroxyl in epoxy resin can catalyze the trimerization reaction of cyanate ester; the triazine ring formed after cyanate ester homopolymerization can catalyze the polyetherification of epoxy resin and react with cyanate ester to form oxazolidinone . During the curing reaction process, some people think that CE self-polymerizes to form a triazine ring cross-linked network, and then EP homopolymerizes or EP reacts with residual CE. It is also believed that the oxazolidinone in the CE/EP reaction system is obtained by the reaction of EP and triazine ring to form oxazoline, and then the oxazoline is then isomerized. Using traditional CE curing catalysts (such as transition metal salts and nonylphenol systems) to catalyze the curing of CE/EP systems, it was found that these catalysts only act on the trimerization reaction of CE, but have no activity on EP. MDMartin et al. studied the blends of CE resin and tetrafunctional EP and found that the main reaction of the curing process is the trimerization of CE and the formation of alkyl isocyanurate, and oxazolidinone is only waiting It is formed when stoichiometric ratio and excessive EP are contained. The use of EP modified CE resin is a good way to reduce the application cost of CE trees and further improve its performance. The copolymerization reaction mechanism of CE and EP is very complicated. Most of the current research and future work are still focused on the in-depth study of the curing reaction mechanism. EP modified CE resin will also be one of the hot spots in the research of advanced resin-based composite materials in the recent period.