Preparation method of transition metal coordination heterocyclic latent curing agent capable of being rapidly cured at medium and normal temperature

Abstract

The invention provides a transition metal coordination heterocyclic latent curing agent capable of rapidly curing at medium and normal temperature and a preparation method of the latent curing agent. The latent curing agent is formed by self-assembly of coordination bonds formed by empty orbits of transition metal ions and lone pair electrons on nitrogen atoms on heterocyclic rings. As the catalytic activity of tertiary amine on a heterocyclic ring is inhibited, the curing agent has a relatively long storage period at room temperature. Coordination bonds are broken at high temperature, tertiary amine released by the complex can continue to catalyze an oligomer monomer to carry out a ring-opening etherification polymerization reaction, and the curing agent has the capacity of rapid curing at medium and normal temperature. The preparation process provided by the invention is simple, convenient, efficient, high in yield and suitable for industrial production, and raw materials are easy to obtain.

Description

Technical Field

[0001] The invention relates to the technical field of resin curing agents, and in particular to a method for preparing a transition metal coordinated heterocyclic latent curing agent that can be rapidly cured at room temperature. Background Art

[0002] Resin-based composite materials have been widely used in many fields such as aerospace, automobile manufacturing, electronics and construction due to their excellent mechanical properties, temperature resistance and chemical resistance. Type IV hydrogen storage cylinders are the mainstream equipment in the current hydrogen storage field as an important carrier for green and low-carbon transformation due to their high hydrogen storage density, high fatigue life and high structural efficiency. Dry winding molding is the cutting-edge technology for the rapid manufacturing of type IV hydrogen storage cylinders by winding the fiber prepreg impregnated with resin onto the surface of the thermoplastic liner in a certain linear shape. However, the existing dry winding prepreg special resin system has a short storage period at room temperature and it is difficult to meet the winding process's demand for a longer process window time. In addition, common latent curing agents on the market, such as dicyandiamide and hydrazide compounds, usually require a high curing temperature of >150°C, the curing process consumes a lot of energy and the molding speed is slow, and the melting point of the plastic liner of the type IV hydrogen storage cylinder is low, which cannot withstand a high curing temperature. The design and preparation of a transition metal coordinated heterocyclic latent curing agent that can be quickly cured at room temperature is still full of challenges.

[0003] As a commonly used medium-temperature curing agent, heterocyclic curing agent has a curing temperature usually below 120°C. The cured resin-based composite materials show good mechanical properties, electrical insulation resistance and heat resistance. However, due to its high curing activity, heterocyclic curing agent has a short room temperature application period after mixing with resin, which limits its long storage capacity as a single-component system. Research on the latent modification of heterocyclic curing agents is of great significance to meet the needs of dry winding prepreg resin systems to achieve long room temperature storage period and long process window period. So far, many studies have focused on the latent modification of heterocyclic curing agents. For example, the patent "A benzothiadiazole-modified imidazole compound and its preparation method and application" uses a chemical modification method to introduce benzothiadiazole into the active site of the nitrogen atom at position 1 of the imidazole ring. Through the electron-withdrawing effect and steric hindrance effect of benzothiadiazole, the room-temperature curing activity of the imidazole curing agent is passivated, thereby improving the storage period of the epoxy resin single-component system. When the prepared benzothiadiazole-modified imidazole compound is used as an epoxy resin curing agent, the exothermic reaction temperature exceeds 140°C. Although this method can extend the storage time at room temperature to a certain extent, the higher curing temperature does not meet the heat resistance requirements of the existing IV type hydrogen storage cylinder plastic lining material. In addition, the preparation process of the compound is relatively complicated, requiring the use of ion catalysts and dangerous acid-binding agents such as sodium hydroxide and potassium hydroxide in a nitrogen atmosphere, and the reaction temperature is high and the cycle is long, which is not suitable for large-scale industrial production. In addition, there are also latent curing agents prepared using microcapsule technology, such as the patent "Preparation method of imidazole microcapsules based on mercapto-isocyanate click reaction and oil-in-oil interfacial polymerization", which uses a mixed solution of imidazole and thiol monomers as the dispersed phase and a mixed solution of isocyanate and non-polar solvent as the continuous phase to form an oil-in-oil emulsion; then, the core material imidazole catalyzes the mercapto-isocyanate click reaction at the interface of the two phases to obtain microcapsules with polythiourethane as the shell and imidazole as the core, which can be used as a latent curing agent for epoxy resin. However, due to the high polarity of imidazole substances, it is very difficult to microencapsulate them, making the preparation process of imidazole microcapsules complicated, the coating amount of imidazole is low, and the reaction is not easy to control. At the same time, during the curing process of epoxy resin, the microcapsule wall material remains in the epoxy resin system, which may have a negative impact on the physical and mechanical properties of the cured epoxy resin system. Overall, the current latent curing agent for resin has technical difficulties in meeting the requirements of rapid curing at medium and low temperatures and long storage period at room temperature, and it is difficult to prepare on a large scale and cannot be used in industry. In view of the above problems, the present invention designs and prepares a transition metal coordinated heterocyclic latent curing agent that cures rapidly at room temperature. The latent curing agent uses transition metal ions and heterocyclic curing agents for coordination self-assembly. The empty orbitals of the transition metal ions form coordination bonds with the lone pair of electrons on the nitrogen atom on the heterocyclic ring, thereby constructing a stable complex and effectively inhibiting the catalytic activity of tertiary amines. This can extend the room temperature applicability of the resin system and broaden the process window of the prepreg tape.Under heating conditions of 100-120°C, the complex is unblocked, releasing the tertiary amine to catalyze the resin for a ring-opening etherification polymerization reaction, thereby improving the latent property while ensuring its rapid curing ability at normal temperature, avoiding damage to the plastic lining caused by high curing temperature, and ensuring the overall performance of the gas cylinder. The synthetic preparation process of the present invention is simple, efficient, high in yield, and the raw materials are readily available, suitable for industrial production, and opens up a broader application prospect for resin-based composite materials. Summary of the invention

[0004] The present invention provides a method for preparing a transition metal coordinated heterocyclic latent curing agent that cures rapidly at medium and normal temperatures, and the agent is self-assembled by coordination of transition metal ions and heterocyclic curing agents. The heterocyclic thermal latent curing agent combines the empty orbit of the metal ion with the lone pair of electrons on the nitrogen atom on the heterocyclic ring by coordination bonds, constructs a stable complex, effectively inhibits the catalytic activity of the tertiary amine, and thus prepares a thermal latent curing agent with a long storage period at room temperature. Under heating conditions of 100-120°C, the complex is unblocked, and the re-released tertiary amine can quickly catalyze the polymerization reaction of the resin for ring-opening etherification, while improving the latency and ensuring its rapid curing ability at medium and normal temperatures. The transition metal coordinated heterocyclic thermal latent curing agent prepared by the present invention prolongs the room temperature applicability of the resin system, broadens the molding process window such as winding, and the preparation process is simple, efficient, and controllable, with low synthesis cost, and is suitable for large-scale production and preparation.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] A method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature, characterized in that it comprises the following preparation steps:

[0007] (1) Preparation of heterocyclic ligand precursor solution: Add the heterocyclic curing agent and solvent into a three-necked flask with a condenser, heat and stir under a nitrogen atmosphere to obtain a transparent and uniform heterocyclic ligand precursor solution;

[0008] (2) Preparation of a hydrated transition metal salt solution: Under a nitrogen atmosphere, place the hydrated transition metal salt in a drying oven at 60-80° C. for at least 4 h, then add the hydrated transition metal salt and the solvent into a three-necked flask with a condenser, and stir for 2-5 h to obtain a uniformly dissolved hydrated transition metal salt solution;

[0009] (3) Synthesis and preparation of transition metal coordinated heterocyclic latent curing agents:

[0010] The heterocyclic precursor solution prepared in step (1) is transferred into a three-necked round-bottom flask equipped with a mechanical stirring paddle and a reflux condenser, and then the hydrated transition metal salt solution prepared in step (2) is slowly added into the three-necked flask, and stirred at 30-60° C. for 3-5 hours under a nitrogen environment; after the reaction is completed, the experimental device is dismantled, the obtained mixture is naturally cooled to room temperature, and then washed with excess deionized water, filtered, and dried to obtain a crude product; the crude product is dissolved in dichloromethane, and then filtered, and the dichloromethane solvent is removed from the filtrate by vacuum distillation, and the product obtained by vacuum distillation is vacuum dried in a vacuum oven at 70° C. for two days to finally obtain a crude product of block powder; and by further nano-sizing it, a transition metal coordinated heterocyclic latent curing agent with a particle size of 50-200 nm is obtained;

[0011] (4) Compounding of the curing agent system: Weigh an appropriate amount of the transition metal coordinated heterocyclic latent curing agent prepared in step (3) and add it to the amine curing agent or the acid anhydride curing agent, stir for 5-10 minutes at 30-60°C and 200-500 r / min, and then perform ultrasonic dispersion treatment with a pulse cycle of 50s (on) / 25s (off), and the treatment time is 20-60 minutes. The setting of the reaction intermittent pulse cycle, on the one hand, effectively improves the dispersibility of the transition metal coordinated heterocyclic latent curing agent in the amine curing agent or the acid anhydride curing agent, and on the other hand, the ultrasonic off period can provide sufficient time for the heat in the system to dissipate, which helps to control the reaction temperature and slow down possible overheating reactions; in addition, the temperature during stirring and ultrasonic treatment is not higher than 80°C. Too high a stirring temperature will affect the complex structure of the transition metal coordinated heterocyclic heat latent curing agent and reduce the latent effect, and finally obtain a compounded curing agent system for the resin;

[0012] (5) Preparation of resin system: Weigh the curing agent system prepared in step (4) and add it to the resin matrix, stir it at 25-80° C. and 200-500 r / min for 10-20 min until the curing agent and the resin are fully mixed to obtain a resin / latent curing agent mixed system, which is then placed in an oven for curing according to a step-by-step curing system.

[0013] The molar ratio of the heterocyclic curing agent in step (1) to the hydrated transition metal salt in step (2) is 4-4.2:1-1.2.

[0014] The heterocyclic ligand in step (1) is one or more of a nitrogen-containing heterocyclic ligand, a CN-containing ligand, a carboxylic acid-containing ligand, a N- and O-containing multidentate ligand, and a N / S-containing ligand; wherein the nitrogen-containing heterocyclic ligand is preferably one of 4,4'-bipyridine, 2,4'-bipyridine, 4-ethylpyrimidine-2-amine, 2-methylimidazole, 2-ethylimidazole, 2-phenylimidazole, 2-ethyl-4-methylimidazole, 4-phenylimidazole, and 2-undecylimidazole. The ligands containing CN are preferably one or more of 2,4,6-tricyanobenzene and 2,4,6-tri(p-cyanophenylethynyl)benzene; the ligands containing carboxylic acid are preferably one or more of phthalic acid, terephthalic acid, isophthalic acid and tetrabenzoic acid; the ligands containing N and O multidentate are preferably one or more of 4,5-imidazoledicarboxylic acid and biphenyldicarboxylic acid; the ligands containing N / S are preferably one or more of 2-mercaptopyridine and 4-mercaptopyridine.

[0015] The hydrated transition metal salt in step (2) is a hydrate of a transition metal salt such as an iron salt, a cobalt salt, a nickel salt, a copper salt, etc., preferably ferric chloride hexahydrate (FeCl 3 6H 2 O), cobalt chloride hexahydrate (CoCl 2 6H 2 O), nickel chloride hexahydrate (NiCl 2 6H 2 O), cupric chloride dihydrate (CuCl 2 ·2H 2 O) one or more, accordingly, in step (3) can prepare a transition iron ion coordinated heterocyclic epoxy resin latent curing agent, a transition cobalt ion coordinated heterocyclic epoxy resin latent curing agent, a transition nickel ion coordinated heterocyclic epoxy resin latent curing agent, and a transition copper ion coordinated heterocyclic epoxy resin latent curing agent.

[0016] The solvent in step (1) and step (2) is a good solvent of a heterocyclic curing agent and a hydrated transition metal salt, and the solvent is a mixed solvent of an alcohol, ester or benzene organic solvent, including one or more of methanol, ethanol, n-butanol, ethyl acetate, butyl acetate, benzene, toluene and xylene; and the amount of the solvent accounts for 50-90% of the total mass of the corresponding prepared mixed solution system.

[0017] The transition metal coordinated heterocyclic latent curing agent in step (3) needs to undergo a two-stage nano-process. In the first stage, at least one of the pulverizing equipment such as a vibration mill, a high-pressure roller mill, a centrifugal mill, and a stirring mill can be selected; in the second stage, at least one of the pulverizing equipment such as a jet mill and a spherical mill can be selected; both stages are carried out using an intermittent pulverizing process, and the operation time of each pulverization is ≤200s and the temperature is ≤60°C.

[0018] The composite curing agent system for the resin in step (4) is composed of 5-20 parts by weight of a transition metal coordinated heterocyclic latent curing agent and 80-95 parts by weight of an amine curing agent or an acid anhydride curing agent, wherein the transition metal coordinated heterocyclic epoxy resin latent curing agent can be selected from transition iron ion coordinated heterocyclic epoxy resin latent curing agent, transition cobalt ion coordinated heterocyclic epoxy resin latent curing agent, transition nickel ion coordinated heterocyclic epoxy resin latent curing agent, At least one of the transition copper ion coordinated heterocyclic epoxy resin latent curing agents; the amine curing agent is liquid at 25-60°C, preferably one or more of dimethylthiotoluenediamine, diethyltoluenediamine, triethylenetetramine, isophoronediamine, etc.; the acid anhydride curing agent is liquid at 25-60°C, preferably one or more of phthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylnadic anhydride, etc.;

[0019] The resin matrix in step (5) is composed of a main resin and a diluent. The main resin can be selected from one or more of epoxy resin, vinyl resin, phenolic resin, bismaleimide resin, cyanate resin, polyimide resin and polyurethane; wherein the epoxy resin is preferably a mixture of one or more of glycidyl ether, glycidyl ester, glycidyl amine and hydantoin epoxy resin; the vinyl resin is preferably a mixture of one or more of bisphenol A acrylic vinyl ester resin, bisphenol A methacrylic vinyl ester resin and phenolic epoxy vinyl ester resin; the phenolic resin is preferably a mixture of one or more of thermoplastic phenolic resin, thermosetting phenolic resin and high-ortho-phenolic resin; the bismaleimide resin is preferably a mixture of one or more of bisphenol A diphenyl ether bismaleimide resin, alkyl diphenylmethane bismaleimide resin and polymerized polyamine bismaleimide resin. The cyanate resin is preferably a mixture of one or more of bisphenol A cyanate resin, bisphenol E cyanate resin and phenolic cyanate resin; the polyimide resin is preferably a mixture of one or more of monoether anhydride polyimide, bisether anhydride polyimide and ketone anhydride polyimide; the polyurethane is preferably a mixture of one or more of polyether polyurethane, polyester polyurethane, polyimide polyurethane and polyurea polyurethane; the diluent is preferably a mixture of one or more of butyl glycidyl ether, 1,4-butanediol diglycidyl ether, ethylene glycol diglycidyl ether, phenyl glycidyl ether, polypropylene glycol diglycidyl ether, alkyl glycidyl ether, glycerol glycidyl ether, benzyl glycidyl ether, 1,6-hexanediol diglycidyl ether and neopentyl glycol diglycidyl ether; wherein the mass ratio of the main resin to the diluent is 100:10-30;

[0020] The resin / latent curing agent mixed system prepared in step (5) is stored at room temperature, and the viscosity of the system is tested in accordance with "GB / T10247-2008 Viscosity Measurement Method". This test is intended to explore the performance sustainability of the resin system under storage conditions.

[0021] The heat curing in step (5) adopts the heating method of a blast oven, and the specific curing system is: a) first heating to 60-90°C and keeping warm for 0.5-1h; b) then heating to 100-120°C and keeping warm for 1-3h.

[0022] Effects of the Invention

[0023] Combining all the above technical solutions, the advantages and significant effects of the present invention are as follows:

[0024] (1) The transition metal coordinated heterocyclic latent curing agent prepared by the present invention is self-assembled by coordination of transition metal ions and heterocyclic curing agents. The heterocyclic latent curing agent combines the empty orbital of the transition metal ion with the lone pair of electrons on the nitrogen atom on the heterocyclic ring by coordination bonds, and forms a complex under certain conditions, which effectively inhibits the catalytic activity of the tertiary amine. The latent curing agent is used as a resin latent curing agent and has the advantages of long room temperature storage period, high stability and good dispersibility.

[0025] (2) The transition metal coordinated heterocyclic latent curing agent prepared by the present invention is used as a curing agent for the resin. Under heating conditions of 100-120° C., the coordination bonds in the latent curing agent are broken, and the heterocyclic curing agent is released to catalyze the resin to undergo a ring-opening etherification polymerization reaction. The reaction activity is high, and the resin matrix can be quickly cured under normal temperature conditions, ensuring that the resin-based composite material has excellent temperature resistance and mechanical properties;

[0026] (3) The synthesis process of the transition metal coordinated heterocyclic latent curing agent prepared by the present invention is simple, efficient, high in yield, and the raw materials are readily available, and it is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The isothermal DSC curves of epoxy resin / amine curing agent / imidazole, modified imidazole A, modified imidazole B, and modified imidazole C at 80°C for 240 min;

[0028] Figure 2 The isothermal DSC curves of epoxy resin / amine curing agent / imidazole, modified imidazole A, modified imidazole B, and modified imidazole C at 120°C for 60 minutes; DETAILED DESCRIPTION

[0029] The technical solution provided by the present invention is described in detail below in conjunction with embodiments, but the present invention is not limited to these embodiments.

[0030] Example 1

[0031] (1) Synthesis and preparation of transition metal coordinated imidazole epoxy resin latent curing agent: Weigh 110.16 g of 2-ethyl-4-methylimidazole into reaction bottle A, add 200 ml of anhydrous methanol, and continue stirring at 30°C and 200 r / min for 10 min to obtain a transparent and uniform imidazole precursor solution; then weigh 42.62 g of cupric chloride dihydrate into a jacketed reactor B, add 200 ml of anhydrous methanol, and continue stirring at 30°C and 200 r / min for 10 min to obtain a hydrated copper salt solution. Cooling water was introduced into the jacket, and the prepared hydrated copper salt solution was added to the reaction bottle A in batches under the stirring conditions of 30°C and 300r / min. After the addition was completed, nitrogen protective gas was introduced and the stirring reaction was continued for 2h. After the reaction was completed, the obtained product mixture was evaporated to remove the solvent by reduced pressure and fully dried to obtain a block solid product. The product was first coarsely crushed by a stirred mill, and then the product was ultra-finely treated by a spherical mill to obtain a powdered copper salt coordinated imidazole thermal latent curing agent.

[0032] (2) Preparation of epoxy resin / latent curing agent system: 10 parts by weight of copper salt coordinated imidazole type thermal latent curing agent was added to 100 parts by weight of dimethylthiotoluenediamine curing agent solution, stirred at 50°C and 300 r / min for 10 minutes, and then ultrasonically dispersed with a pulse cycle of 50s (on) / 25s (off) for 35 minutes to obtain a compound curing agent system for epoxy resin; 27 parts by weight of the compound curing agent system was added to 100 parts by weight of bisphenol A glycidyl ether type epoxy resin E51, stirred at 50°C and 300 r / min for 15 minutes until the curing agent and epoxy resin were fully mixed, to obtain an epoxy resin / latent curing agent system. The prepared resin system was stored at room temperature. The viscosity of the epoxy resin / latent curing agent system prepared in this example was tested according to the "GB / T10247 2008 Viscosity Measurement Method", and the results are recorded in Table 1. Test results: The applicable period of the resin system at room temperature is 30 days, and it has a long room temperature storage period.

[0033] (3) Preparation of resin casting and performance testing: The uniformly mixed epoxy resin / latent curing agent system was placed in a vacuum drying oven, degassed for 15 minutes at 50°C and -100KPa negative pressure, and then heated and cured in a blast oven. The specific curing system was as follows: a) heating to 90°C and keeping warm for 1h; b) heating to 120°C and keeping warm for 3h. After the curing was completed, the oven power was turned off, and when the oven temperature dropped below 40°C, the oven door was opened to take out the epoxy composite material obtained by curing. The temperature resistance of the resin casting was tested according to GB / T40396-2021, and the tensile and bending properties of the resin casting were tested according to GB / T 2567-2008, and the results were recorded in Table 1. Test results: The glass transition temperature of the resin system was 165°C, which had good temperature resistance; the tensile strength of the resin casting was 82MPa, and the bending strength was 135MPa, which had good mechanical properties.

[0034] The test results show that the epoxy resin / latent curing agent system of this embodiment has a long room temperature application period, and the resin system has high temperature resistance. The resin casting obtained by rapid curing at medium and low temperatures exhibits excellent tensile strength and flexural strength.

[0035] Comparative Example 1

[0036] (1) Preparation of epoxy resin / curing agent system: unmodified 2-ethyl-4-methylimidazole with the same imidazole content as in step 2 of Example 1 was added to 100 parts by mass of dimethylthiotoluenediamine curing agent solution, and stirred at 50°C and 300 r / min for 15 minutes to obtain a compounded curing agent system for epoxy resin; 27 parts by mass of the curing agent system was added to 100 parts by mass of bisphenol A glycidyl ether type epoxy resin E51, and stirred at 50°C and 300 r / min for 15 minutes until the curing agent and epoxy resin were fully mixed to obtain an epoxy resin / curing agent system. The prepared resin system was stored at room temperature. The viscosity of the epoxy resin / curing agent system prepared in this example was tested according to "GB / T10247 2008 Viscosity Measurement Method", and the results were recorded in Table 1. Test results: The applicable period of the resin system at room temperature is 3 days, and it has a short room temperature storage period.

[0037] (2) Preparation of resin casting and performance testing: The uniformly mixed epoxy resin / curing agent system was placed in a vacuum drying oven, degassed for 15 minutes at 50°C and -100KPa negative pressure, and then heated and cured in a blast oven. The specific curing system was as follows: a) heating to 90°C and keeping warm for 1 hour; b) heating to 120°C and keeping warm for 3 hours. After the curing was completed, the oven power was turned off, and when the oven temperature dropped below 40°C, the oven door was opened to take out the epoxy composite material obtained by curing. The heat resistance of the resin casting was tested according to GB / T 40396-2021, and the tensile and bending properties of the resin casting were tested according to GB / T 2567-2008, and the results were recorded in Table 1. Test results: The glass transition temperature of the resin system was 160°C, which had good heat resistance; the tensile strength of the resin casting was 80MPa, and the bending strength was 130MPa, which had good mechanical properties.

[0038] The test results show that when unmodified 2-ethyl-4-methylimidazole is added to the epoxy resin / curing agent system in this comparison, the room temperature storage period is shorter.

[0039] Example 2

[0040] (1) Synthesis and preparation of transition metal coordinated imidazole epoxy resin latent curing agent: 110.16 g of 2-ethyl-4-methylimidazole was weighed into reaction bottle A, and 200 ml of anhydrous methanol was added. The mixture was stirred at 30° C. and 300 r / min for 15 min to obtain a transparent and uniform imidazole precursor solution. Then 59.48 g of cobalt chloride hexahydrate was weighed into a jacketed reactor B, and 200 ml of anhydrous methanol was added. The mixture was stirred at 30° C. and 150 r / min for 15 min to obtain a hydrated cobalt salt solution. Cooling water was introduced into the jacket, and the prepared hydrated cobalt salt solution was added to the reaction bottle A in batches under the stirring conditions of 30°C and 250r / min. After the addition was completed, nitrogen protective gas was introduced and the stirring reaction was continued for 2h. After the reaction was completed, the obtained product mixture was evaporated to remove the solvent by reduced pressure and fully dried to obtain a block solid product. The product was first coarsely crushed by a stirred mill, and then the product was ultra-finely treated by a ball mill to obtain a powdered copper salt coordinated imidazole thermal latent curing agent.

[0041] (2) Preparation of epoxy resin / latent curing agent system: 10 parts by weight of cobalt salt coordinated imidazole type thermal latent curing agent was added to 100 parts by weight of methyl nadic anhydride curing agent solution, stirred at 45°C and 300 r / min for 10 minutes, and then ultrasonically dispersed with a pulse cycle of 50s (on) / 25s (off) for 40 minutes to obtain a compound curing agent system for epoxy resin; 70 parts by weight of the compound curing agent system was added to 100 parts by weight of bisphenol A glycidyl ether type epoxy resin E51, stirred at 50°C and 250 r / min for 20 minutes until the curing agent and epoxy resin were fully mixed, to obtain an epoxy resin / latent curing agent system. The prepared resin system was stored at room temperature. The viscosity of the epoxy resin / latent curing agent system prepared in this example was tested according to the "GB / T10247 2008 Viscosity Measurement Method", and the results are recorded in Table 1. Test results: The applicable period of the resin system at room temperature is 25 days, and it has a long room temperature storage period.

[0042] (3) Preparation of resin casting and performance testing: The uniformly mixed epoxy resin / latent curing agent system was placed in a vacuum drying oven, degassed for 15 minutes at 50°C and -100KPa negative pressure, and then heated and cured in a blast oven. The specific curing system was as follows: a) heating to 90°C and keeping warm for 1h; b) heating to 120°C and keeping warm for 3h. After the curing was completed, the oven power was turned off, and when the oven temperature dropped below 40°C, the oven door was opened to take out the epoxy composite material obtained by curing. The heat resistance of the resin casting was tested according to GB / T40396-2021, and the tensile and bending properties of the resin casting were tested according to GB / T 2567-2008, and the results were recorded in Table 1. Test results: The glass transition temperature of the resin system was 168°C, which had good heat resistance; the tensile strength of the resin casting was 83MPa, and the bending strength was 132MPa, which had good mechanical properties.

[0043] The test results show that the epoxy resin / latent curing agent system of this embodiment has a long room temperature application period, and the resin system has high temperature resistance. The resin casting obtained by rapid curing at medium and low temperatures exhibits excellent tensile strength and flexural strength.

[0044] Comparative Example 2

[0045] When preparing the epoxy resin / latent curing agent system in step (2), the stirring temperature of the cobalt salt coordinated imidazole latent epoxy resin curing agent and the methyl nadic anhydride curing agent is increased to 100°C, the stirring time is 30 minutes, the ultrasonic temperature of the ultrasonic dispersion treatment with a pulse cycle of 50s (on) / 25s (off) is adjusted to 80°C, and the ultrasonic time is adjusted to 60 minutes, and the rest is consistent with Example 2. The resin system prepared in this comparative example is stored at room temperature. The viscosity of the epoxy resin / latent curing agent system prepared in this example is tested according to the "GB / T10247 2008 Viscosity Measurement Method", and the results are recorded in Table 1. Test results: The applicable period of the resin system at room temperature is 11 days, and it has a short room temperature storage period. The temperature resistance of the resin casting is tested according to GB / T 40396-2021, and the tensile and bending properties of the resin casting are tested according to GB / T 2567-2008, and the results are recorded in Table 1. Test results: The glass transition temperature of the resin system is 158°C, which has good temperature resistance. The tensile strength of the resin casting is 80MPa, and the bending strength is 131MPa, which has good mechanical properties.

[0046] The test results show that excessively high stirring temperature and ultrasonic temperature will destroy the complex structure of the transition metal coordinated imidazole thermal latent curing agent, causing the complex to release the imidazole curing agent prematurely to catalyze the epoxy resin to undergo a ring-opening polymerization reaction to form an ether, thereby reducing the latent effect.

[0047] Example 3

[0048] (1) Synthesis and preparation of transition metal coordinated imidazole epoxy resin latent curing agent: Weigh 110.16 g of 2-ethyl-4-methylimidazole into reaction bottle A, add 200 ml of anhydrous methanol, and continue stirring for 15 min at 40°C and 200 r / min to obtain a transparent and uniform imidazole precursor solution; then weigh 42.62 g of cupric chloride dihydrate into jacketed reactor B, add 200 ml of anhydrous methanol, and continue stirring for 15 min at 40°C and 200 r / min to obtain a hydrated copper salt solution. Cooling water was introduced into the jacket, and the prepared hydrated copper salt solution was added to the reaction bottle A in batches under the stirring conditions of 40°C and 350r / min. After the addition was completed, nitrogen protective gas was introduced and the stirring reaction was continued for 2.5h. After the reaction was completed, the obtained product mixture was evaporated to remove the solvent by reduced pressure and fully dried to obtain a block solid product. The product was first coarsely crushed by a stirred mill, and then the product was ultra-finely treated by a spherical mill to obtain a powdered copper salt coordinated imidazole thermal latent curing agent.

[0049] (2) Preparation of epoxy resin / latent curing agent system: 10 parts by weight of copper salt coordinated imidazole type thermal latent curing agent was added to 100 parts by weight of dimethylthiotoluenediamine curing agent solution, stirred at 45°C and 300 r / min for 15 min, and then ultrasonically dispersed with a pulse cycle of 50 s (on) / 25 s (off) for 40 min to obtain a compound curing agent system for epoxy resin; 27 parts by weight of the compound curing agent system was added to 100 parts by weight of bisphenol A glycidyl ether type epoxy resin E51, stirred at 50°C and 300 r / min for 15 min until the curing agent and epoxy resin were fully mixed, to obtain an epoxy resin / latent curing agent system. The prepared resin system was stored at room temperature. The viscosity of the epoxy resin / latent curing agent system prepared in this example was tested according to the "GB / T10247 2008 Viscosity Measurement Method", and the results are recorded in Table 1. Test results: The applicable period of the resin system at room temperature is 29 days, and it has a long room temperature storage period.

[0050] (3) Preparation of resin casting and performance testing: The uniformly mixed epoxy resin / latent curing agent system was placed in a vacuum drying oven, degassed for 20 minutes at 45°C and -100KPa negative pressure, and then heated and cured in a blast oven. The specific curing system was as follows: a) heating to 90°C and keeping warm for 1 hour; b) heating to 120°C and keeping warm for 3 hours. After the curing was completed, the oven power was turned off, and when the oven temperature dropped below 40°C, the oven door was opened to take out the epoxy composite material obtained by curing. The temperature resistance of the resin casting was tested according to GB / T40396-2021, and the tensile and bending properties of the resin casting were tested according to GB / T 2567-2008, and the results were recorded in Table 1. Test results: The glass transition temperature of the resin system was 166°C, which had good temperature resistance; the tensile strength of the resin casting was 83MPa, and the bending strength was 135MPa, which had good mechanical properties.

[0051] The test results show that the epoxy resin / latent curing agent system of this embodiment has a long room temperature application period, and the resin system has high temperature resistance. The resin casting obtained by rapid curing at medium and low temperatures exhibits excellent tensile strength and flexural strength.

[0052] Comparative Example 3

[0053] The process of preparing the epoxy resin / latent curing agent system in step (2) is changed to: take 10 parts by mass of copper salt coordinated imidazole thermal latent curing agent and add it to 100 parts by mass of dimethylthiotoluene diamine curing agent solution, and stir it for 30 minutes at 45°C and 300r / min. Obtain a compounded curing agent system for epoxy resin; take 27 parts by mass of the compounded curing agent system and add it to 100 parts by mass of bisphenol A glycidyl ether type epoxy resin E51, and stir it for 20 minutes at 50°C and 300r / min until the curing agent and epoxy resin are fully mixed to obtain an epoxy resin / latent curing agent system. The prepared resin system is stored at room temperature. Others are consistent with Example 3. The resin system prepared in this comparative example is stored at room temperature. The viscosity of the epoxy resin / latent curing agent system prepared in this example is tested according to the "GB / T102472008 Viscosity Measurement Method", and the results are recorded in Table 1. Test results: The applicable period of the resin system at room temperature is 35 days, and it has a long room temperature storage period. The temperature resistance of the resin casting was tested according to GB / T 40396-2021, and the tensile and bending properties of the resin casting were tested according to GB / T 2567-2008, and the results are recorded in Table 1. Test results: The glass transition temperature of the resin system is 142°C, which has low temperature resistance; the tensile strength of the resin casting is 53MPa, the bending strength is 82MPa, and the mechanical properties are low.

[0054] The test results show that there are still solid particles with larger particle size in the compounded curing agent system without ultrasonic treatment. These particles exist as defect sources and affect the mechanical properties of the resin system.

[0055] In the example, the viscosity of the epoxy resin / latent curing agent system was tested according to GB / T10247-2008 Viscosity Measurement Method. This test aims to explore the performance sustainability of the resin system under storage conditions. Finally, we will use the number of days when the viscosity of the resin system doubles as the applicable period of this resin system at room temperature; test the temperature resistance of the resin casting according to GB / T 40396-2021; and test the tensile and bending properties of the resin casting according to GB / T 2567-2008. The above example data are collected, and the test results are shown in Table 1.

[0056] Table 1 Performance evaluation results of each instance

[0057]

[0058] As can be seen from Table 1, the epoxy resin / latent curing agent system prepared by adding the transition metal coordinated imidazole epoxy resin latent curing agent in Examples 1 to 3 has a room temperature applicability period of up to one month, and the resin system has good temperature resistance. The obtained resin-based composite material exhibits excellent tensile strength and flexural strength.

Claims

1. A method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature, characterized in that: The method comprises the following preparation steps: (1) Preparation of heterocyclic ligand precursor solution: Add the heterocyclic curing agent and solvent into a three-necked flask with a condenser, heat and stir under a nitrogen atmosphere to obtain a transparent and uniform heterocyclic ligand precursor solution; (2) Preparation of a hydrated transition metal salt solution: Under a nitrogen atmosphere, place the hydrated transition metal salt in a drying oven at 60-80° C. for at least 4 h, then add the hydrated transition metal salt and the solvent into a three-necked flask with a condenser, and stir for 2-5 h to obtain a uniformly dissolved hydrated transition metal salt solution; (3) Synthesis and preparation of transition metal coordinated heterocyclic latent curing agents: The heterocyclic precursor solution prepared in step (1) is transferred into a three-necked round-bottom flask equipped with a mechanical stirring paddle and a reflux condenser, and then the hydrated transition metal salt solution prepared in step (2) is slowly added into the three-necked flask, and stirred at 30-60° C. for 3-5 hours under a nitrogen environment; after the reaction is completed, the experimental device is dismantled, the obtained mixture is naturally cooled to room temperature, and then washed with excess deionized water, filtered, and dried to obtain a crude product; the crude product is dissolved in dichloromethane, and then filtered, and the dichloromethane solvent is removed from the filtrate by vacuum distillation, and the product obtained by vacuum distillation is vacuum dried in a vacuum oven at 60-70° C. for two days to finally obtain a crude product of block powder; and by further nano-sizing it, a transition metal coordinated heterocyclic latent curing agent with a particle size of 50-200 nm is obtained; (4) Compounding of the curing agent system: Weigh an appropriate amount of the transition metal coordinated heterocyclic latent curing agent prepared in step (3) and add it to the amine curing agent or the acid anhydride curing agent, stir for 5-10 minutes at 30-60° C. and 200-500 r / min, and then perform ultrasonic dispersion treatment with a pulse cycle of 50 s (on) / 25 s (off) for 20-60 minutes to obtain a compounded curing agent system; (5) Preparation of resin system: Weigh the curing agent system prepared in step (4) and add it to the resin matrix, stir it at 25-80° C. and 200-500 r / min for 10-20 min until the curing agent and the resin are fully mixed to obtain a resin / latent curing agent mixed system, which is then placed in an oven for curing according to a step-by-step curing system.

2. The method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature according to claim 1, characterized in that: The molar ratio of the heterocyclic curing agent in step (1) to the hydrated transition metal salt in step (2) is 4-4.2:1-1.

2.

3. The method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature according to claim 1, characterized in that: The heterocyclic ligand is one or more of a nitrogen-containing heterocyclic ligand, a CN-containing ligand, a carboxylic acid-containing ligand, a N- and O-containing multidentate ligand, and a N / S-containing ligand; wherein the nitrogen-containing heterocyclic ligand is preferably one or more of 4,4'-bipyridine, 2,4'-bipyridine, 4-ethylpyrimidine-2-amine, 2-methylimidazole, 2-ethylimidazole, 2-phenylimidazole, 2-ethyl-4-methylimidazole, 4-phenylimidazole, and 2-undecylimidazole. The CN-containing ligand is preferably one or both of 2,4,6-tricyanobenzene and 2,4,6-tri(p-cyanophenylethynyl)benzene; the carboxylic acid-containing ligand is preferably one or more of phthalic acid, terephthalic acid, isophthalic acid and tetrabenzoic acid; the N and O-containing multidentate ligand is preferably one or both of 4,5-imidazoledicarboxylic acid and biphenyldicarboxylic acid; the N / S-containing ligand is preferably one or both of 2-mercaptopyridine and 4-mercaptopyridine.

4. The method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature according to claim 1, characterized in that: The hydrated transition metal salt is a hydrate of a transition metal salt such as an iron salt, a cobalt salt, a nickel salt, or a copper salt, preferably one or more of ferric chloride hexahydrate (FeCl3·6H2O), cobalt chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), or cupric chloride dihydrate (CuCl2·2H2O).

5. The method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature according to claim 1, characterized in that: The solvent used in step (1) and step (2) is a good solvent of a heterocyclic curing agent and a hydrated transition metal salt, and the solvent is a mixed solvent of an alcohol, ester or benzene organic solvent, including one or more of methanol, ethanol, n-butanol, ethyl acetate, butyl acetate, benzene, toluene and xylene; and the amount of the solvent used accounts for 50-90% of the total mass of the corresponding prepared mixed solution system.

6. The method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature according to claim 1, characterized in that: The transition metal coordinated heterocyclic latent curing agent needs to undergo a two-stage nano-process. In the first stage, at least one of the pulverizing equipment such as a vibration mill, a high-pressure roller mill, a centrifugal mill, and a stirring mill can be selected; in the second stage, at least one of the pulverizing equipment such as a jet mill and a spherical mill can be selected; both stages are carried out using an intermittent pulverizing process, and the operation time of each pulverization is ≤200s and the temperature is ≤60°C.

7. The method for preparing a transition metal coordinated heterocyclic latent curing agent that cures rapidly at room temperature according to claim 1, characterized in that: The composite curing agent system for the resin is composed of 5-20 parts by weight of a transition metal coordinated heterocyclic latent curing agent and 80-95 parts by weight of an amine curing agent or an acid anhydride curing agent, wherein the amine curing agent is liquid at 25-60° C., and is preferably one or more of dimethylthiotoluenediamine, diethyltoluenediamine, triethylenetetramine, isophoronediamine, etc.; the acid anhydride curing agent is liquid at 25-60° C., and is preferably one or more of phthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylnadic anhydride, etc.

8. The method for preparing a transition metal coordinated heterocyclic latent curing agent that rapidly cures at room temperature according to claim 1, characterized in that: The resin matrix is ​​composed of a main resin and a diluent. The main resin can be selected from one or more of epoxy resin, vinyl resin, phenolic resin, bismaleimide resin, cyanate resin, polyimide resin and polyurethane. The mass ratio of the main resin to the diluent is 100:10-30.

9. The method for preparing a transition metal coordinated heterocyclic latent curing agent that cures rapidly at room temperature according to claim 1, characterized in that: The curing system is as follows: a) first heating to 60-90° C. and keeping the temperature for 0.5-1 h; b) then heating to 100-120° C. and keeping the temperature for 1-3 h.

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