The study of latent curing agent for epoxy resin generally adopts physical and chemical means to improve the curing activity of ordinary curing agents at low temperature and high temperature, and mainly adopts the following two improvement methods: one is to close and passivate the reactivity of some curing agents with high reactivity but poor storage stability; The second is to improve and stimulate the reactivity of some curing agents with good storage stability and low reactivity. Finally, the curing agent has certain storage stability when it is added into epoxy resin at room temperature, and the reactive activity of the curing agent is released through external conditions such as light and heat when it is used, so as to achieve the purpose of rapidly curing epoxy resin. This paper briefly summarizes the research progress of latent curing agent for epoxy resin at home and abroad.
1 epoxy latent curing agent
1. 1 modified fatty amine
Fatty amine curing agents, such as ethylenediamine, hexamethylenediamine, diethylenetriamine and triethylenetetramine, are commonly used two-component curing agents for epoxy resin at room temperature. Through chemical modification, they have nucleophilic addition reaction with organic ketones, and dehydration is an effective method to block, reduce their curing activity and improve their storage stability.
The 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, ketoimine obtained by methylcyclohexane reaction of ketone 3- methyl -2- butanone with sterically hindered groups at both ends of carbonyl group and diamine 1 3 with high activity not only has high curing reactivity, but also has obviously improved storage stability. In addition, Japanese patent reports that ketoimine obtained by the reaction of polyether modified aliphatic amine compound with methyl isobutyl ketone is also a latent curing agent for epoxy resin with good performance. Fatty amine curing agents can also reduce their curing reactivity by reacting with acrylonitrile, organic phosphine compounds and transition metal complexes, so they have certain potential.
1.2 aromatic diamine
Aromatic amines have attracted attention because of their high Tg, but their high toxicity limits their application. The modified aromatic diamine curing agent of the invention has high Tg, low toxicity, low water absorption and good comprehensive performance. In recent years, curing agents of aromatic diamines include diaminodiphenyl sulfone (DDS), diaminodiphenyl methane (DDM), m-phenylenediamine (m PDA) and so on. Among them, DDS is the most studied and the most mature, and it has become a commonly used curing agent in high-performance epoxy resin. When DDS is used as a latent curing agent for epoxy resin, its reaction activity is greatly reduced and its service life is prolonged compared with aromatic diamines such as MP DA and DDM, because its molecule contains sulfone groups with strong electron absorption. Without accelerator, the pot life of 100g epoxy resin composite can reach 1 year, and the curing temperature generally reaches 200℃. 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 was modified and a variety of polyether diamine curing agents were developed, which reduced its heat resistance during drying. The distance between amino groups at both ends of these diamines is very long, which leads to the decrease of amino groups at the water absorption point and has 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. Dicyandiamide mixed with epoxy resin can be stored for 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 150 ~ 170℃. At this temperature, many devices and materials can't be used because they can't stand this temperature, or because of the requirements of production process, the curing temperature of one-component epoxy resin must be reduced. 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 kinds of accelerators, mainly imidazole compounds and their derivatives and salts, urea derivatives, organic guanidine derivatives, phosphorus-containing compounds, transition metal complexes and composite accelerators. These accelerators can obviously reduce the curing temperature of dicyandiamide, and the ideal curing temperature can be reduced to about 65438 020℃, but at the same time, the storage period will be shortened and the water resistance will be affected to some extent.
Another effective way to reduce the curing temperature of one-component epoxy resin is to chemically modify dicyandiamide through molecular design. Amines, especially aromatic amines, are introduced into dicyandiamide molecules to prepare dicyandiamide derivatives, such as HT 2833 and HT 2844, which are dicyandiamide derivatives modified with 3,5 disubstituted aniline. The chemical structural formula is as follows:
It is reported that this curing agent has good compatibility with epoxy resin, long storage period and fast curing speed. When cured at 100℃ 1h, the shear strength can reach 25 MPa, and when cured at 150℃ for 30min, the shear strength can reach 27MPa. The curing agents AEHD-6 10 and AEHD-2 10 developed by Asahi Kasei Industrial Co., Ltd. in Japan are also modified dicyandiamide derivatives. In addition, it has been reported in Japan that aromatic diamines such as 4,4' diaminodiphenylmethane (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 is obviously improved after benzene ring is introduced. The one-component system composed of dicyandiamide and E 44 epoxy resin can be stored for half a year at room temperature, and its curing temperature is lower than dicyandiamide.
There are few reports on chemical modification of dicyandiamide to obtain dicyandiamide derivatives in China. Wenzhou Qingming Chemical Industry prepared dicyandiamide MD 02 by the reaction of propylene oxide and dicyandiamide. The melting point was 154 ~ 162℃, which was about 45℃ lower than the melting point of dicyandiamide (207 ~ 2 10℃), and 100 phr was used. The derivatives obtained by modifying dicyandiamide with aniline formaldehyde increase the miscibility with bisphenol A epoxy resin, and have good solubility in the mixed solution of acetone and alcohol, which increases the reactivity and storage time.
1.4 imidazole
Imidazole curing agents, such as imidazole, 2-methylimidazole, 2- ethyl -4- methylimidazole and 2- phenylimidazole, are highly active curing agents, which can cure epoxy resin in a short time at moderate temperature. Therefore, the one-component system composed of imidazole and epoxy resin has a short shelf life, and it needs to be chemically modified to introduce large substituents into its molecule to form sterically hindered imidazole derivatives or react with inorganic salts of transition metals such as copper, nickel, cobalt and zinc. There are many chemical modification methods of imidazole curing agent. According to the reaction mechanism, there are two main methods: one is to modify imidazole ring with active hydrogen on the nitrogen atom of the secondary amino group at 1, such as isocyanate, cyanate ester, lactone, etc. The modified imidazole derivatives have long storage life and good mechanical properties. Another method is to modify imidazole by using the basicity of N atom at position 3 of 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 empty orbital transition metal ions, such as Cu2+, Ni2+, Zn2+, Cd2+, Co2+ and so on. They form coordination complexes with imidazole, and have good storage performance. They can be cured rapidly at 150 ~ 170℃, but the introduction of inorganic salts, organic acids and their salts will destroy the original ones.
There is little research on imidazole latent curing agents in China, and there are relatively many 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 closed products, which weakened the activity of amino groups on imidazole rings and had a long service life. When the temperature rises above 100℃, the blocking effect is released, imidazole recovers its activity and the epoxy resin is cured.
1.5 organic anhydride
Organic anhydride curing agent is similar to dicyandiamide and has good storage stability. Although the curing temperature is high, the mechanical properties, dielectric properties and heat resistance of the cured product are good. However, this kind of curing agent is not easy to be chemically modified because of its easy hydrolysis of anhydride bond and poor moisture resistance. Therefore, the curing temperature of organic anhydride curing agent is generally reduced by adding accelerators. The 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 hydrazide Like dicyandiamide, organic hydrazide is a solid with high melting point, but its curing temperature is lower than dicyandiamide. The storage period of one-component epoxy resin adhesive system composed of organic hydrazide and epoxy resin can reach more than 4 months. Commonly used organic hydrazide compounds are succinic hydrazide, adipic dihydrazide, sebacic hydrazide, isophthalic hydrazide and p-hydroxybenzoic hydrazide (POBH). Different kinds of organic hydrazides have different curing temperatures. 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 complexing Lewis acids such as BF3, AlCl3, ZnCl2 and PF5 with primary or secondary amines. As curing agents of epoxy resin, these complexes are quite stable at room temperature, but they can cure epoxy resin rapidly at 120℃, among which boron trifluoride-amine complexes are the most studied. It is reported that a new boron trifluoride-amine complex BPEA-2 has good latency, adhesion and toughness. Lewis acid amine complex is also a common accelerator for anhydride and aromatic amine latent curing agent.
1.8 microcapsule
Microencapsulated latent curing agent for epoxy resin is actually a kind of physical method to wrap a room-temperature two-component curing agent with a fine oil drop film to form microcapsules, which are temporarily sealed after adding epoxy resin, and then the capsules are broken by heating and pressurizing to release the curing agent, thus curing the epoxy resin. The film-forming agent of latent curing agent for epoxy resin microcapsules includes cellulose, gelatin, polyvinyl alcohol, polyester, polysulfone and so on. Due to the strict requirements of preparation technology, 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 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 of latent curing agent for epoxy resin mainly focuses on dicyandiamide, imidazole and aromatic diamine curing agent. 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, it is also the focus of future research on epoxy resin latent curing agent to further solve the problems of water resistance, heat resistance and toughness improvement of cured products. 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 latent curing agent for epoxy resin.