Epoxy resin is a type of thermosetting polymer material with good adhesion, electrical insulation, and chemical stability. As a resin matrix for adhesives, coatings, and composite materials, it is widely used in construction, Machinery, electronics and electrical, aerospace and other fields. When using epoxy resin, a curing agent must be added, and the curing reaction must be carried out under certain conditions to generate a three-dimensional network structure product. Only then will it show various excellent properties and become an epoxy material with real use value. Therefore, curing agents are indispensable in the application of epoxy resin, and even play a decisive role to some extent. Epoxy resin latent curing agents have been a hot topic in epoxy resin curing agent research at home and abroad in recent years. The so-called latent curing agent refers to a one-component system added to epoxy resin that has certain storage stability at room temperature, and can quickly undergo a curing reaction under conditions such as heating, light, moisture, and pressure. Curing agent, compared with the currently commonly used two-component epoxy resin system, the one-component epoxy resin system prepared by mixing latent curing agent and epoxy resin has simplified production operation process and prevents environmental pollution. Improve product quality and adapt to modern large-scale industrial production and other advantages.

Research on epoxy resin latent curing agents generally uses physical and chemical means to improve the curing activity of commonly used low-temperature and high-temperature curing agents. The following two improvement methods are mainly adopted: First, some The reactivity of curing agents with high reactivity and poor storage stability is blocked and passivated; the second is to increase and stimulate the reactivity of some curing agents with good storage stability and low reactivity. Ultimately, the curing agent has a certain storage stability when added to the epoxy resin at room temperature, and the reactivity of the curing agent is released through external conditions such as light and heat during use, thereby achieving rapid curing of the epoxy resin. purpose. This article provides a basic overview of the research progress of latent curing agents for epoxy resin at home and abroad.

1 Epoxy resin latent curing agent

1.1 Modified aliphatic amines

Aliphatic amine curing agents such as ethylenediamine and hexamethylenediamine , diethylenetriamine, triethylenetetramine, etc. are commonly used two-component epoxy resin room temperature curing agents. Through chemical modification methods, they undergo nucleophilic addition reactions with organic ketone compounds, and dehydration generates imines. An effective way to seal, reduce its curing activity and improve its storage stability.

This one-component system consisting of a ketimine-type curing agent and an epoxy resin decomposes the ketimine into an amine through the action of moisture and moisture, so the epoxy resin can be cured at room temperature. solidify. However, the curing speed is generally not fast and the service life is short. The reason is that the lone pair of electrons on the imine nitrogen atom still has a certain ring-opening activity. To solve this problem, Toshiyuki Takeda used ketimine obtained by reacting 3-methyl-2-butanone, a ketone with sterically hindered groups at both ends of the carbonyl group, and 1,3-diaminomethylcyclohexane, a highly reactive diamine. It not only has higher curing reaction activity, but also has significantly improved storage stability. In addition, the Japanese patent reports that the ketimine obtained by reacting a polyether-modified aliphatic amine compound 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 reaction activity by reacting with acrylonitrile, organic phosphine compounds, and transition metal complexes, thus having a certain degree of latent potential.

1.2 Aromatic diamines

Aromatic amines have attracted attention due to their high Tg, but their applications are limited due to their acute toxicity. The modified aromatic diamine curing agent has the advantages of high Tg, low toxicity, low water absorption, and good overall performance. Aromatic diamine curing agents that have been studied more in recent years include diaminodiphenyl sulfone (DDS), diaminodiphenylmethane (DDM), m-phenylenediamine (mPDA), etc. Among them, DDS has been studied the most. The most mature, becoming a commonly used curing agent in high-performance epoxy resins. When DDS is used as a latent curing agent for epoxy resin, compared with MP DA, DDM and other aromatic diamines, due to the strong electron-attracting sulfone group in its molecule, the reactivity is greatly reduced and its pot life is also extended.

Without accelerator, the pot life of 100 grams of epoxy resin complex can reach 1 year, and the curing temperature generally reaches 200°C. In order to lower the curing temperature, accelerators are often added to achieve medium temperature curing. In recent years, in order to improve the wet and hot properties 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 amine groups at both ends. The distance between them is longer, resulting in fewer amino groups at the water absorption point, and it 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. The storage period of dicyandiamide and epoxy resin at room temperature can be up to half a year. The curing mechanism of dicyandiamide is relatively complex. In addition to the four hydrogens on dicyandiamide that can participate in the reaction, the cyano group also has certain reactivity. When dicyandiamide is used alone as an epoxy resin curing agent, the curing temperature is very high, generally between 150 and 170°C. At this temperature, many devices and materials cannot be used because they cannot withstand such temperatures, or due to the requirements of the production process. Instead, the curing temperature of the one-component epoxy resin must be lowered. There are two ways to solve this problem. One is to add an accelerator to lower the curing temperature of dicyandiamide without excessively damaging its storage life and performance. There are many such accelerators, mainly including 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 make double The curing temperature of cyanamide is significantly lowered. The ideal curing temperature can be reduced to about 120°C, but at the same time, the storage period will be shortened, and the water resistance will also be affected to a certain extent.

Another effective method to reduce the curing temperature of one-component epoxy resin is to chemically modify dicyandiamide through molecular design. Introduce amines, especially aromatic amine structures, into the dicyandiamide molecule to prepare dicyandiamide derivatives, such as HT 2833 developed by Swiss Ciba Geigy Company. HT 2844 is a 3,5-disubstituted aniline modified The chemical structure of dicyandiamide derivatives is as follows:

According to reports, this type of curing agent has good compatibility with epoxy resin, long storage period, and fast curing speed. It cures for 1 hour at 100°C. The shear strength can reach 25MPa. After curing at 150℃ for 30 minutes, the shear strength can reach 27MPa. AEHD-610, a special curing agent for powder coatings developed by Japan's Asahi Kasei Industrial Co., Ltd., is also a modified dicyandiamide derivative. In addition, aromatic diamines such as 4,4' diaminodiphenylmethane (DDM), 4,4' diaminodiphenyl ether (DDE), 4,4' diaminodiphenyl sulfone (DDS) are used in Japan. Reports on the reaction of dimethylaniline (DMB) with dicyandiamide to prepare its derivatives. The compatibility between the above-mentioned dicyandiamide derivatives and bisphenol A-type epoxy resin after introducing the benzene ring is significantly increased compared with dicyandiamide. The one-component system composed of E 44 epoxy resin can be stored at room temperature for up to half a year. For a long time, the curing temperature is lower than that of dicyandiamide.

There are few domestic reports on the chemical modification of dicyandiamide to obtain dicyandiamide derivatives. Wenzhou Qingming Chemical used propylene oxide and dicyandiamide to react to produce dicyandiamide MD 02. Its melting point 154~162℃, about 45℃ lower than the melting point of dicyandiamide (207~210℃), using a formula consisting of 100 parts of E 44 epoxy resin, 15 parts of MD 02 and 0.5 parts of 2 methylimidazole, 150℃ The gel time is 4 minutes. The derivatives obtained by modifying dicyandiamide with aniline formaldehyde have increased miscibility with bisphenol A-type epoxy resin, have good solubility in a mixed solution of acetone and alcohol, have increased reactivity, and have longer storage properties.

1.4 Imidazoles

Imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole and other imidazole curing agents are a type of high-quality Reactive curing agent can cure epoxy resin in a short time at medium temperature. Therefore, the one-component system composed of it and epoxy resin has a short storage period. It must be chemically modified to introduce larger molecules into its molecules. The substituents form sterically hindered imidazole derivatives, or react with inorganic salts of transition metals Cu, Ni, Co, Zn, etc. to form corresponding imidazole salt complexes, which can become latent products with a certain storage period at room temperature. Curing agent. There are many ways to chemically modify imidazole curing agents. From the perspective of reaction mechanism, there are two main types: one is to use the active hydrogen on the secondary amino nitrogen atom at position 1 of the imidazole ring to modify it. Modifiers include isocyanate, cyanate, lactone, etc. The imidazole derivatives obtained after modification have a long storage period and good mechanical properties. Another method is to use the basicity of the N atom at position 3 of the imidazole ring to modify it so that it can be combined with compounds with empty orbitals. Such substances include organic acids, metal inorganic salts, acid anhydrides, TCNQ, boric acid, etc. 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 and have good storage properties. ℃ rapid solidification, but the introduction of inorganic salts, organic acids and their salts, etc., will destroy the hydrolysis resistance and moisture and heat resistance of the original imidazole cured product.

There is less research on imidazole latent curing agents in China, but there are relatively many in foreign markets. Daiichi Industrial Pharmaceutical Co., Ltd. reacts various imidazoles with toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI) to form blocked products, which weaken the amine on the imidazole ring. The activity of the base has a long service life. When the temperature rises above 100°C, the blocking effect is released, the imidazole regains its activity, and the epoxy resin solidifies.

1.5 Organic acid anhydrides

Organic acid anhydride curing agents are similar to dicyandiamide and have good storage stability. Although the curing temperature is high, the mechanical properties of the cured product, The dielectric properties and heat resistance are both good. However, this type of curing agent has poor moisture resistance because the acid anhydride bond is easily hydrolyzed, and it is not easy to undergo chemical modification. Therefore, the method of adding accelerators is generally used to reduce the curing temperature of organic acid anhydride curing agents. Commonly used curing accelerators for organic acid anhydride curing agents include tertiary amines and tertiary amine salts, quaternary phosphonium salts, Lewis acid-amine complexes, acetylacetone transition metal complexes, etc.

1.6 Organic hydrazides Like dicyandiamide, organic hydrazides are also high melting point solids, but their solidification temperature is lower than that of dicyandiamide. The storage period of a one-component epoxy resin glue system composed of organic hydrazide and epoxy resin can reach more than 4 months. Commonly used organic hydrazide compounds are: succinic acid hydrazide, adipic acid dihydrazide, and sebacic acid Hydrazide, isophthalic acid hydrazide and p-hydroxybenzoic acid hydrazide (POBH), etc. Different types of organic hydrazides have different curing temperatures. Since their curing temperatures are relatively high, accelerators are often added to lower the curing temperature. 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, consisting of BF3, AlCl3, ZnCl2, PF5, etc. Lewis acids form complexes with primary or secondary amines. As a curing agent for epoxy resin, this type of complex is quite stable at room temperature, but rapidly solidifies epoxy resin at 120°C. Among them, the most studied one is boron trifluoride-amine complex. It is reported that a new synthesized boron trifluoride-amine complex BPEA-2 has good latent properties, adhesive properties and toughness. Lewis acid amine complexes are also commonly used accelerators for acid anhydrides and aromatic amine latent curing agents.

1.8 Microcapsules

Microcapsule epoxy resin latent curing agents actually use physical methods to wrap room temperature two-component curing agents with fine oil droplets. Microcapsules are formed, and after being added to the epoxy resin, the curing reaction activity of the curing agent is temporarily blocked, and the capsules are ruptured through conditions such as heating and pressure to release the curing agent, thereby curing the epoxy resin. The film-forming agents of microcapsule epoxy resin latent curing agents include cellulose, gelatin, polyvinyl alcohol, polyester, polysulfone, etc. Due to strict preparation process requirements, the thickness of the capsule film will cause problems in storage, transportation and use. Different degrees of influence.

2 Conclusion

Although there are many types of epoxy resin latent curing agents, each type of curing agent has certain advantages and disadvantages. So far, there are still no discoveries. An ideal latent curing agent with particularly excellent properties. At present, the research on latent curing agents for epoxy resin mainly focuses on dicyandiamide, imidazole and aromatic diamine curing agents. At the same time, on the basis of meeting the requirements of lowering the curing temperature, shortening the curing time, and extending the pot life when using latent curing agents, further solving the problems of water resistance, heat resistance, and improving toughness of the epoxy resin cured product will also be the future epoxy resin latent curing agent. The focus of sexual curing agent research. Not only that, as people's awareness of environmental protection increases, the research on low-toxic and non-toxic environmentally friendly epoxy resin latent curing agents is also an inevitable trend.