1.1 Modifying aliphatic amines
Aliphatic amine curing agents such as ethylenediamine, hexamethylene diamine, diethylenetriamine and triethylene tetramine are commonly used room temperature curing agents for two-component epoxy resin. Through chemical modification, they undergo nucleophilic addition reaction with organic ketone compounds, and dehydration is an effective way to close, reduce their curing activity and improve their storage stability.
This one-component system composed of ketoimine curing agent and epoxy resin decomposes ketoimine into amine under the action of moisture and moisture, so the epoxy resin can be cured at room temperature. But generally, the curing speed is not fast and the service life is short, because the lone pair on the imine nitrogen atom still has a certain ring-opening activity. In order to solve this problem, the ketoimine obtained by the reaction of keto 3- methyl -2-butanone with steric hindrance groups at both ends of carbonyl group with highly active diamine 1,3-diamino methylcyclohexane not only has high curing reaction activity, but also has obviously improved storage stability. In addition, Japanese patent reports that the ketoimine obtained by the reaction of polyether-modified aliphatic amine compounds with methyl isobutyl ketone is also a latent curing agent for epoxy resin with good performance. Aliphatic amine curing agents can also reduce their curing reactivity by reacting with acrylonitrile, organophosphine compounds and transition metal complexes, thus having certain potential.
1.2 aromatic diamines
aromatic amines have been paid attention to because of their high Tg, but their application is limited because of their high toxicity. The modified aromatic diamine curing agent has the advantages of high Tg, low toxicity, low water absorption and good comprehensive performance. In recent years, the curing agents of aromatic diamines are diaminodiphenyl sulfone (DDS), diaminodiphenyl methane (DDM), m-phenylenediamine (m PDA), etc. Among them, DDS is the most studied and the most mature, and it has become a common curing agent in high-performance epoxy resin. When DDS is used as a latent curing agent for epoxy resin, compared with aromatic diamines such as MP DA and DDM, its reaction activity is greatly reduced and its service life is prolonged because of its sulfone group with strong electron absorption in its molecule. In the absence of accelerant, the pot life of 1 grams of epoxy resin complex can reach one year, and the curing temperature generally reaches 2℃. In order to reduce its curing temperature, accelerators are often added to realize medium-temperature curing. In recent years, in order to improve the wet-heat performance and toughness of the system, DDS has been modified, and a variety of polyether diamine curing agents have been developed, which reduces their heat resistance during drying. These diamines have a long distance between amino groups at both ends, resulting in a decrease in water absorption point amino groups, and have excellent impact resistance.
1.3 dicyandiamide
dicyandiamide, also known as dicyandiamide, has long been used as a latent curing agent in powder coatings, adhesives and other fields. The storage period of dicyandiamide mixed with epoxy resin can reach half a year at room temperature. The curing mechanism of dicyandiamide is complicated. Besides the four hydrogens on dicyandiamide, cyano group also has certain reactivity. When dicyandiamide is used as curing agent for epoxy resin alone, the curing temperature is very high, generally between 15℃ and 17℃. At this temperature, many devices and materials cannot be used because they can't withstand such temperature, or the curing temperature of one-component epoxy resin must be reduced because of the requirements of production process. There are two ways to solve this problem. One is to add accelerant to reduce the curing temperature of dicyandiamide without excessively damaging its storage life and service performance. There are many accelerators, mainly imidazole compounds and their derivatives and salts, urea derivatives, organic guanidine derivatives, phosphorus-containing compounds, transition metal complexes and composite accelerators, etc. These accelerators can obviously reduce the curing temperature of dicyandiamide, and the ideal curing temperature can be reduced to about 12℃, but at the same time, the storage period will be shortened and the water resistance will be affected to some extent.
another effective method to reduce the curing temperature of one-component epoxy resin is to chemically modify dicyandiamide by molecular design. Amines, especially aromatic amines, are introduced into dicyandiamide molecules to prepare dicyandiamide derivatives, such as HT 2833,HT 2844, which are modified by 3,5-disubstituted aniline. The chemical structural formula is as follows: < P > It is reported that this kind of curing agent has good compatibility with epoxy 3min, long storage period and fast curing speed, and can be cured at 1℃ for 1 hour. AEHD-61 and AEHD-21, special curing agents for powder coatings developed by Asahi Kasei Industrial Co., Ltd., Japan, are also modified dicyandiamide derivatives. In addition, there are reports in Japan that aromatic diamines such as 4,4' diaminodiphenyl methane (DDM),4,4' diaminodiphenyl ether (DDE),4,4' diaminodiphenyl sulfone (DDS) and p-xylene amine (DMB) react with dicyandiamide to prepare their derivatives. Compared with dicyandiamide, the compatibility of dicyandiamide derivatives with bisphenol A epoxy resin after benzene ring introduction is obviously increased. The one-component system composed of dicyandiamide and E 44 epoxy resin can be stored at room temperature for half a year, and the curing temperature is lower than dicyandiamide.
There are few reports about chemical modification of dicyandiamide to obtain dicyandiamide derivatives in China. In Wenzhou Qingming Chemical Industry, dicyandiamide MD 2 was prepared by the reaction of propylene oxide and dicyandiamide, and its melting point was 154 ~ 162℃, which was about 45℃ lower than that of dicyandiamide (27 ~ 21℃). The formula consisted of 1 parts of E 44 epoxy resin, 15 parts of MD 2 and .5 parts of 2-methylimidazole. The derivatives obtained by modifying dicyandiamide with aniline formaldehyde have increased miscibility with bisphenol A epoxy resin, good solubility in the mixed solution of acetone and alcohol, increased reactivity and longer storage.
1.4 imidazoles
imidazole curing agents such as imidazole, 2-methylimidazole, 2- ethyl -4- methylimidazole and 2- phenylimidazole are a kind of high-activity curing agents, which can cure epoxy resin in a short time at medium temperature, so the storage life of the one-component system composed of them and epoxy resin is short, so it is necessary to chemically modify them, and introduce large substituents into their molecules to form sterically hindered imidazole derivatives or transition metals. There are many methods for chemical modification of imidazole curing agents. From the reaction mechanism, there are two main methods: one is to modify imidazole curing agents with active hydrogen on the nitrogen atom of the first secondary amino group on the imidazole ring. Such modifiers include isocyanate, cyanate ester, lactone, etc. The modified imidazole derivatives have long storage life and good mechanical properties. Another method is to modify imidazole by the basicity of N atom at the 3-position on the imidazole ring, so that it can be compounded with compounds with empty orbitals, such as organic acids, metal inorganic salts, anhydrides, TCNQ, boric acid and so on. Among them, metal inorganic salts generally contain transition metal ions with empty orbitals, such as Cu2+, Ni2+, Zn2+, Cd2+, Co2+, etc. They form coordination complexes with imidazole, which have good storability and rapidly cure at 15 ~ 17℃. However, the introduction of inorganic salts, organic acids and their salts will destroy the hydrolysis resistance and wet heat resistance of the cured products of original imidazole.
there are few researches on imidazole latent curing agents in China, but there are relatively many in foreign markets. Japan's First Industrial Pharmaceutical Co., Ltd. reacted various imidazoles with toluene diisocyanate (TDI), isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI) to make blocked products, which weakened the activity of amino groups on imidazole ring and had a long service life. When the temperature rose above 1℃, the blocking effect was released, imidazole resumed its activity and epoxy resin was cured.
1.5 organic anhydrides
Organic anhydrides curing agents are similar to dicyandiamide and have good storage stability. Although the curing temperature is high, the mechanical properties, dielectric properties and heat resistance of the cured products are good. However, this kind of curing agent has poor moisture resistance because of the easy hydrolysis of anhydride bonds, and it is not easy to be chemically modified. Therefore, the curing temperature of organic anhydride curing agents is generally reduced by adding accelerators. Curing accelerators commonly used in organic anhydride curing agents include tertiary amines and tertiary amine salts, quaternary phosphonium salts, Lewis acid-amine complexes, acetylacetone transition metal complexes and so on.
1.6 organic hydrazides like dicyandiamide, organic hydrazides are also a high melting point solid, but their curing temperature is lower than dicyandiamide. The storage life of one-component epoxy resin adhesive system composed of organic hydrazide and epoxy resin can reach more than 4 months. The commonly used organic hydrazide compounds are: succinic hydrazide, adipic dihydrazide, sebacic hydrazide, isophthalic hydrazide and p-hydroxybenzoic acid hydrazide (POBH). The curing temperature of different kinds of organic hydrazides is different. Because of its high curing temperature, accelerators are often added to reduce the curing temperature, and the accelerators used are basically the same as dicyandiamide.
1.7 Lewis acid
amine complex Lewis acid amine complex is an effective latent curing agent for epoxy resin, which is formed by the complex of Lewis acids such as BF3, AlCl3, ZnCl2 and PF5 with primary or secondary amines. As the curing agent of epoxy resin, this kind of complex is quite stable at room temperature, but it cures epoxy resin rapidly at 12℃, among which boron trifluoride-amine complex is the most studied. It is reported that a new boron trifluoride-amine complex BPEA-2 has good latent property, adhesive property and toughness. Lewis acid amine complex is also a common accelerator for latent curing agents of anhydrides and aromatic amines.
1.8 Microcapsules
Microencapsulated epoxy resin latent curing agent actually uses physical methods to wrap a room temperature two-component curing agent with a fine oil drop film to form microcapsules, which are temporarily sealed after being added to epoxy resin, and the curing reactivity of the curing agent is temporarily sealed, and the capsules are broken by heating, pressurizing and other conditions to release the curing agent, thus curing the epoxy resin. Film-forming agents of microencapsulated latent curing agent for epoxy resin include cellulose, gelatin, polyvinyl alcohol, polyester, polysulfone, etc. Due to strict preparation process requirements, the thickness of capsule film will have different degrees of influence on storage, transportation and use.
2 Conclusion
Although there are many kinds of latent curing agents for epoxy resin, each type of curing agent has certain advantages and disadvantages. So far, a latent curing agent with excellent performance and ideal performance has not been found. At present, the research on latent curing agents of epoxy resin mainly focuses on dicyandiamide, imidazole and aromatic diamine curing agents. At the same time, on the basis of meeting the requirements of reducing curing temperature, shortening curing time and extending application period in the use of latent curing agent, further solving the problems of water resistance, heat resistance and improving toughness of cured products of epoxy resin is also the focus of future research on latent curing agent of epoxy resin. Moreover, with the improvement of people's awareness of environmental protection, it is an inevitable trend to study the low-toxicity and non-toxic environmental protection epoxy resin latent curing agent.