Environment-friendly low-free-amine modified amine epoxy curing agent and preparation method thereof

Abstract

The invention relates to the technical field of epoxy hardeners, in particular to an environment-friendly low-free-amine modified amine epoxy hardener and a preparation method thereof.The preparation method comprises the following steps that epoxy resin and a solvent are mixed, PEG and a catalyst are added, the temperature is increased to 75-85 DEG C, constant-temperature stirring is conducted for 2-3 h, then a chain extender is added into the mixture, constant-temperature stirring continues to be conducted for 4-6 h, and a mixture is obtained; the modified amine epoxy curing agent is obtained. The modified amine epoxy hardener provided by the invention can improve the flame retardant property and self-repairing property of epoxy resin.

Description

Technical Field

[0001] This invention relates to the field of epoxy curing agent technology, and in particular to an environmentally friendly modified amine epoxy curing agent with low free amine content and its preparation method. Background Technology

[0002] As a core component in epoxy resin application systems, the development of epoxy curing agents is closely intertwined with the industrialization of epoxy resins, jointly driving the innovation of modern materials science. Since the large-scale production of epoxy resins in the mid-20th century, the research on curing agents has become a key area determining the performance of epoxy materials. These substances, which can chemically react with epoxy resins to form three-dimensional network polymers, endow epoxy materials with diverse properties, ranging from flexible adhesives to high-strength composite materials, by controlling the crosslinking density and molecular structure, making them irreplaceable in industries such as electronics, aerospace, and architectural decoration.

[0003] From a chemical perspective, epoxy curing agents, through ring-opening addition or catalytic polymerization reactions between their active groups (such as amino, hydroxyl, and anhydride groups) and the epoxy groups of epoxy resins, construct a thermosetting three-dimensional network structure. This process not only determines the final form of the material but also directly affects its core properties such as mechanical strength, chemical resistance, and electrical insulation. For example, in the field of electronic packaging, a single-component system composed of dicyandiamide-based latent curing agents and epoxy resins, which achieves rapid curing triggered by heating, simplifies the production process and ensures the high reliability of integrated circuit packaging. In the aerospace field, the combination of aromatic polyamine curing agents and high-performance epoxy resins enables the structural stability of composite materials under extreme environments.

[0004] The classification system of curing agents reflects the diversity of their chemical structures and functional properties. Based on chemical structure, alkaline curing agents (such as aliphatic amines and polyamides) are the mainstream for room temperature curing systems due to their high reactivity, while acidic curing agents (such as acid anhydrides and organothiols) have an advantage in processes requiring precise control due to their low volatility and long pot life. Based on curing temperature, low-temperature curing agents (such as polythiols) can polymerize in sub-zero environments, meeting the needs of polar engineering; high-temperature curing agents (such as methylphenolic resins) initiate deep cross-linking at high temperatures, improving the heat resistance of materials. This classification logic not only provides a theoretical framework for material design but also promotes the specialization and functionalization of curing agents.

[0005] However, there are still some problems with epoxy resin curing agents that need to be solved. For example, patent number CN113736047B discloses "a phenolic modified amine curing agent and its preparation method using industrial waste as raw materials". It uses mixed phenols, paraformaldehyde, and compounded amines as raw materials. The compounded amines are a mixture of diamines, polyethylene polyamines, and aminothiourea. This curing agent has low viscosity, low exothermic peak during curing, is not prone to explosive polymerization, and has good chemical resistance of the cured product.

[0006] For example, patent number CN121005866A discloses "A free amine-free cationic waterborne epoxy curing agent and its preparation method," which includes the following steps: 1) mixing small molecule aldehydes and hydroxyethyl diethylenetriamine to react and obtain oxazolidine and aldolimide-terminated modified amines; 2) reacting the modified amines and multifunctional epoxy resins to obtain modified oxazolidine and aldolimide-terminated modified resin amines; 3) mixing the modified oxazolidine and aldolimide-terminated modified resin amines, monofunctional glycidyl ethers, and water to react and obtain the terminated modified resin amines; 4) adding small molecule acids to neutralize the terminated modified amines, and then converting to water to obtain a free amine-free cationic waterborne epoxy curing agent. This curing agent imparts excellent properties to the paint film, such as resistance to boiling water, acid and alkali, and neutral salt spray, and the preparation process is simple and the production cost is low.

[0007] However, conventional epoxy resins are flammable substances, posing a fire hazard to users during daily use and endangering their lives. Furthermore, epoxy resins are prone to damage over long-term use. The aforementioned patents do not address these issues, and the performance of the curing agent still needs improvement. Therefore, there is an urgent need to develop a curing agent that can enhance the flame retardancy and self-healing capabilities of epoxy resins. Summary of the Invention

[0008] The purpose of this invention is to provide an environmentally friendly modified amine epoxy curing agent with low free amine content and its preparation method, so as to solve the problems of poor flame retardant properties and difficulty in repairing damaged epoxy resins after curing.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this invention provides a method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content, comprising the following steps: mixing epoxy resin with a solvent, adding PEG and a catalyst, heating to 75~85℃ and stirring at a constant temperature for 2~3h, then adding a chain extender, and continuing to stir at a constant temperature for 4~6h to obtain the modified amine epoxy curing agent.

[0010] This application uses epoxy resin, PEG and a self-made chain extender as the main raw materials to prepare a modified amine epoxy curing agent. When used in epoxy resin, this curing agent can improve the flame retardant properties of epoxy resin and also endow epoxy resin with self-healing ability, thus extending its service life.

[0011] In some embodiments, the epoxy resin is any one or more of epoxy resin E51, epoxy resin E44, and epoxy resin E128.

[0012] In some embodiments, the solvent is propylene glycol methyl ether.

[0013] In some embodiments, the catalyst is boron trifluoride diethyl ether.

[0014] In some embodiments, the chain extender has the structure shown in Formula I. (I).

[0015] In some embodiments, the method for preparing the chain extender includes the following steps: S1. N-BOC-serine alcohol was mixed with DMF and cooled to -5~0℃. CBr4 and diphenyl-2-pyridylphosphine were added, followed by stirring at a constant temperature for 10~20 min. After returning to room temperature, stirring was continued for 1~2 h. The mixture was concentrated under reduced pressure and extracted. The organic phases were combined and subjected to column chromatography to obtain the compound shown in Formula II. (II); S2. Sodium phenylthiosulfonate and DMF were mixed and added to the compound represented by formula II in step S1. The mixture was heated to 50-70°C and stirred for 4-5 hours. Then, it was cooled to room temperature, concentrated under reduced pressure, and ethyl acetate and deionized water were added to adjust the pH of the system to 6-6.8. The organic phase was separated, concentrated, and subjected to column chromatography to obtain the compound represented by formula III. (III); S3. Mix the compound represented by formula III from step S2 with ethyl acetate, add a saturated ethyl hydrochloride solution, stir at room temperature for 6-8 hours, concentrate under reduced pressure, extract, combine the organic phases, concentrate under reduced pressure, dry, and perform column chromatography to obtain the compound represented by formula IV. (Ⅳ); S4. 2-Chloro-4,6-diamino-1,3,5-triazine and the compound shown in Formula IV in step S3 are mixed and added to DMF, followed by the addition of triethylamine. The mixture is heated to 70-80°C and stirred at a constant temperature for 20-28 hours. 1,3-dibromopropane is then added and stirred at a constant temperature for 18-20 hours. TETA is then added and stirred for 20-24 hours. After the mixture is finished, it is filtered, concentrated under reduced pressure, and dried to obtain the chain extender.

[0016] The chain extender of this application can improve the flame retardant and self-healing properties of cured epoxy resin. The reasons may be as follows: First, the chain extender contains a large amount of N and S elements. Under high temperature conditions, the cured epoxy resin can decompose to generate non-flammable gases such as nitrogen and ammonia. The non-flammable gases mix with oxygen in the air and flammable gases generated by the thermal decomposition of the material, reducing their concentration and slowing down or terminating the combustion reaction due to the lack of necessary conditions. Second, the chain extender contains dynamically reversible disulfide bonds. When epoxy resin is damaged, the disulfide bonds at the crack break under stress or heat to generate sulfur free radicals. The sulfur free radicals exchange with adjacent thiol groups or disulfide bonds to form new disulfide bonds and rebuild the crosslinking network. As the dynamic crosslinking network is reformed, the cracks gradually close, and the mechanical properties of the material are restored.

[0017] In some embodiments, in step S1, the molar ratio of N-BOC-serine alcohol to CBr4 is 1:(2.5~3).

[0018] Preferably, in step S1, the molar ratio of N-BOC-serine alcohol to CBr4 is 1:2.8.

[0019] In some embodiments, in step S1, the molar ratio of N-BOC-serine alcohol to diphenyl-2-pyridylphosphine is 1:(1.3~1.6).

[0020] In some embodiments, in step S2, the molar ratio of sodium phenylthiosulfonate to the compound represented by formula II is (2.3~2.6):1.

[0021] Preferably, in step S2, the molar ratio of sodium phenylthiosulfonate to the compound represented by formula II is 2.5:1.

[0022] In some embodiments, in step S3, the mass-to-volume ratio of the compound represented by Formula III to the saturated ethyl hydrogen chloride solution is 1 g:(10~15) ml.

[0023] In some embodiments, in step S4, the molar ratio of the compound represented by Formula IV to 2-chloro-4,6-diamino-1,3,5-triazine is 1:(1.1~1.3).

[0024] Preferably, in step S4, the molar ratio of the compound represented by Formula IV to 2-chloro-4,6-diamino-1,3,5-triazine is 1:1.2.

[0025] In some embodiments, in step S4, the molar ratio of the compound represented by Formula IV to triethylamine is 1:(4~6).

[0026] In some embodiments, in step S4, the molar ratio of 2-chloro-4,6-diamino-1,3,5-triazine to 1,3-dibromopropane is 1:(2.2~2.4).

[0027] In some embodiments, in step S4, the molar ratio of 1,3-dibromopropane to TETA is 1:(1~1.2).

[0028] In another aspect, the present invention provides a modified amine epoxy curing agent prepared by the above preparation method.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses epoxy resin, PEG and self-made chain extender as main raw materials to prepare a modified amine epoxy curing agent. The curing agent can improve the flame retardant performance of epoxy resin and also give epoxy resin self-healing ability and extend service life.

[0030] (2) The chain extender structure of the present invention contains a large number of N and S elements. The cured epoxy resin can decompose under high temperature conditions to generate non-flammable gases such as nitrogen and ammonia. The non-flammable gases mix with oxygen in the air and flammable gases generated by the thermal decomposition of the material, reducing their concentration and slowing down or terminating the combustion reaction due to lack of necessary conditions. In addition, the chain extender structure contains dynamic reversible disulfide bonds. When the epoxy resin is damaged, the disulfide bonds at the crack break under stress or heat to generate sulfur free radicals. The sulfur free radicals exchange with the adjacent mercapto groups or disulfide bonds to form new disulfide bonds and rebuild the crosslinking network. As the dynamic crosslinking network is reformed, the cracks gradually close and the mechanical properties of the material are restored. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0032] Unless otherwise specified, the post-processing operations described below, such as "stirring", "mixing", "heating", "concentrating under reduced pressure", "concentrating organic phase", "separation", "extraction", "drying", and "combining organic phases", can be selected by those skilled in the art based on actual conditions, and are not further limited.

[0033] Preparation Example 1 The preparation method of the chain extender includes the following steps: S1. Mix 0.1 mol N-BOC-serine alcohol with 300 ml DMF and cool to -3 °C. Add 0.28 mol CBr4 and 0.15 mol diphenyl-2-pyridylphosphine, then stir at a constant temperature for 15 min. After returning to room temperature, continue stirring for 2 h. Concentrate under reduced pressure and extract (dichloromethane / water). Combine the organic phases and perform column chromatography (ethyl acetate / petroleum ether) to obtain the compound shown in Formula II. (II); 1 H NMR (400 MHz, DMSO-d6) δ 6.21 (d, J = 6.6 Hz, 1H), 4.19 (dp, J =6.6, 2.7 Hz, 1H), 3.69 – 3.53 (m, 4H), 1.40 (s, 9H); S2. 0.25 mol sodium phenylthiosulfate was mixed with 400 ml DMF and 0.1 mol of the compound represented by formula II from step S1 was added. The mixture was heated to 60 °C and stirred at that temperature for 4 h. Then, it was cooled to room temperature, concentrated under reduced pressure, and 200 ml ethyl acetate and 100 ml deionized water were added. The pH of the system was adjusted to 6.3 with sodium bicarbonate. The organic phase was separated, concentrated, and subjected to column chromatography (ethyl acetate / petroleum ether) to obtain the compound represented by formula III. (III); 1 H NMR (300 MHz, DMSO-d6) δ 8.12 – 7.40 (m, 10H), 5.72 (d, J = 7.0 Hz, 1H), 4.07 (dt, J = 7.0, 5.0 Hz, 1H), 3.11 (dd, J = 5.0, 2.0 Hz, 4H), 1.40 (s,9H); S3. Mix 5g of the compound represented by Formula III from step S2 with 50ml of ethyl acetate, add 50ml of saturated ethyl hydrochloride solution, stir at room temperature for 7h, concentrate under reduced pressure, extract (dichloromethane / water), combine the organic phases, concentrate under reduced pressure, dry, and perform column chromatography (ethyl acetate / petroleum ether) to obtain the compound represented by Formula IV. (Ⅳ); 1H NMR (300 MHz, DMSO-d6) δ 8.03 – 7.44 (m, 10H), 3.58 – 3.29 (m, 1H), 3.04 (d, J = 4.4 Hz, 4H), 2.58 (dd, J = 7.2, 5.3 Hz, 1H), 2.30 (dd, J = 7.2,5.3 Hz, 1H); S4. Mix 0.12 mol of 2-chloro-4,6-diamino-1,3,5-triazine with 0.1 mol of the compound shown in Formula IV from step S3 and add to 500 ml of DMF. Then add 0.5 mol of triethylamine, heat to 75 °C, and stir at this temperature for 24 h. Next, add 0.23 mol of 1,3-dibromopropane and stir at this temperature for 19 h. Then add 0.23 mol of TETA and stir for 22 h. After the reaction is complete, filter, concentrate under reduced pressure and dry to obtain the chain extender, as shown in Formula I. (I); 1 H NMR (400 MHz, Deuterium Oxide): δ 1.77 (4H, tt, J = 6.61, 4.84Hz), 2.50-2.82 (28H, 2.56 (t, J = 4.84 Hz), 2.66 (t, J = 3.80 Hz), 2.68 (t, J= 3.80 Hz), 2.72 (t, J = 2.67 Hz), 2.73 (t, J = 2.67 Hz), 2.75 (t, J = 2.67Hz), 2.76 (t, J = 2.67 Hz)), 3.20 (4H, t, J = 6.61 Hz), 4.05-4.24 (5H, 4.11(d, J = 4.60 Hz), 4.11 (d, J = 4.60 Hz), 4.16 (tt, J = 4.60, 4.60 Hz)), 7.48-7.76 (6H, 7.55 (dddd, J = 8.03, 7.54, 1.52, 0.45 Hz), 7.69 (tt, J = 7.54,1.47 Hz)), 7.89 (4H, dddd, J = 8.03, 1.48, 1.47, 0.45 Hz).

[0034] Preparation Example 2 The preparation method of the chain extender includes the following steps: 0.12 mol of 2-chloro-4,6-diamino-1,3,5-triazine and 0.1 mol of benzylamine were mixed and added to 400 mL of DMF, followed by 0.5 mol of triethylamine. The mixture was heated to 75 °C and stirred at this temperature for 24 h. Then, 0.23 mol of 1,3-dibromopropane was added and stirred at this temperature for 19 h. Finally, 0.23 mol of TETA was added and stirred for 22 h. After the reaction was completed, the mixture was filtered, concentrated under reduced pressure, and dried to obtain the chain extender, as shown in Formula V. (V); 1 H NMR (400 MHz, Deuterium Oxide): δ 1.77 (4H, tt, J = 6.61, 4.83Hz), 2.49-2.82 (28H, 2.56 (t, J = 4.83 Hz), 2.66 (t, J = 3.80 Hz), 2.68 (t, J= 3.80 Hz), 2.72 (t, J = 2.67 Hz), 2.73 (t, J = 2.67 Hz), 2.75 (t, J = 2.67Hz), 2.76 (t, J = 2.67 Hz)), 3.24 (4H, t, J = 6.61 Hz), 4.52 (2H, s), 7.24-7.41 (5H, 7.32 (tt, J = 7.71, 1.29 Hz), 7.33 (dddd, J = 7.72, 7.71, 1.84, 0.54 Hz), 7.34 (dddd, J = 7.72, 1.29, 0.97, 0.54 Hz)).

[0035] Preparation Example 3 The preparation method of the chain extender is the same as that in Preparation Example 1, except that step S4 is as follows: 0.12 mol of 1,2,3-tribromopropane and 0.1 mol of the compound shown in Formula IV from step S3 were mixed and added to 500 mL of DMF. Then, 0.5 mol of triethylamine was added, the mixture was heated to 75 °C, and stirred at this temperature for 24 h. Next, 0.23 mol of TETA was added, and the mixture was stirred for 22 h. After the reaction was complete, the mixture was filtered, concentrated under reduced pressure, and dried to obtain the chain extender, as shown in Formula VI. (VI); 1H NMR (400 MHz, Deuterium Oxide): δ 2.60-3.03 (27H, 2.66 (dd, J =3.80, 3.80 Hz), 2.66 (dd, J = 3.80, 3.80 Hz), 2.66 (dd, J = 3.80, 3.80 Hz),2.66 (dd, J = 3.80, 3.80 Hz), 2.68 (dd, J = 3.80, 3.80 Hz), 2.68 (dd, J =3.80, 3.80 Hz), 2.68 (dd, J = 3.80, 3.80 Hz), 2.68 (dd, J = 3.80, 3.80 Hz),2.73 (dd, J = 2.67, 2.67 Hz), 2.73 (dd, J = 2.67, 2.67 Hz), 2.73 (dd, J =2.67, 2.67 Hz), 2.73 (dd, J = 2.67, 2.67 Hz), 2.74 (dd, J = 2.67, 2.67 Hz),2.74 (dd, J = 2.67, 2.67 Hz), 2.74 (dd, J = 2.67, 2.67 Hz), 2.74 (dd, J =2.67, 2.67 Hz), 2.82 (dd, J = 2.67, 2.67 Hz), 2.82 (dd, J = 2.67, 2.67 Hz),2.83 (dd, J = 2.67, 2.67 Hz), 2.83 (dd, J = 2.67, 2.67 Hz), 2.84 (dd, J =2.67, 2.67 Hz), 2.84 (dd, J = 2.67, 2.67 Hz), 2.88 (dd, J = 2.67, 2.67 Hz),2.88 (dd, J = 2.67, 2.67 Hz), 2.96 (dddd, J = 2.67, 2.67, 2.67, 2.67 Hz),2.97 (d, J = 2.67 Hz), 2.97 (d, J = 2.67 Hz)), 3.03-3.25 (3H, 3.09 (d, J =2.67 Hz), 3.09 (d, J = 2.67 Hz), 3.17 (dddd, J = 6.73, 6.73, 6.73, 6.73 Hz)),4.07-4.19 (4H, 4.13 (d, J = 6.73 Hz), 4.13 (d, J = 6.73 Hz), 4.13 (d, J =6.73 Hz), 4.13 (d, J = 6.73 Hz)), 7.48-7.76 (6H, 7.55 (dddd, J = 8.03, 7.54,1.52, 0.45 Hz), 7.55 (dddd, J = 8.03, 7.54, 1.52, 0.45 Hz), 7.69 (tt, J =7.54, 1.47 Hz), 7.69 (tt, J = 7.54, 1.47 Hz)), 7.82-7.95 (4H, 7.89 (dddd, J =8.03, 1.48, 1.47, 0.45 Hz), 7.89 (dddd, J = 8.03, 1.48, 1.47, 0.45 Hz)). .

[0036] Example 1 A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content includes the following steps: mixing 100g of epoxy resin E51 with 200g of propylene glycol methyl ether, adding 66g of PEG400 and 0.3g of boron trifluoride ethyl ether, heating to 80℃ and stirring at a constant temperature for 3h, then adding a chain extender (the molar ratio of active amine to epoxy group is 3:1), and continuing to stir at a constant temperature for 5h to obtain the modified amine epoxy curing agent.

[0037] The chain extender was prepared in Preparation Example 1.

[0038] Example 2 A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content includes the following steps: mixing 100g of epoxy resin E51 with 200g of propylene glycol methyl ether, adding 66g of PEG400 and 0.3g of boron trifluoride ethyl ether, heating to 75℃ and stirring at a constant temperature for 3h, then adding a chain extender (the molar ratio of active amine to epoxy group is 3:1), and continuing to stir at a constant temperature for 6h to obtain the modified amine epoxy curing agent.

[0039] The chain extender was prepared in Preparation Example 1.

[0040] Example 3 A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content includes the following steps: mixing 100g of epoxy resin E51 with 200g of propylene glycol methyl ether, adding 66g of PEG400 and 0.3g of boron trifluoride ethyl ether, heating to 85℃ and stirring at a constant temperature for 2h, then adding a chain extender (the molar ratio of active amine to epoxy group is 3:1), and continuing to stir at a constant temperature for 4h to obtain the modified amine epoxy curing agent.

[0041] The chain extender was prepared in Preparation Example 1.

[0042] Example 4 A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content is described. The specific implementation method is the same as in Example 1, except that the chain extender is prepared in Example 2.

[0043] Example 5 A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content is described. The specific implementation method is the same as in Example 1, except that the chain extender is prepared in Example 3.

[0044] Example 6 A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content is described. The specific implementation method is the same as in Example 1, except that the chain extender is TETA.

[0045] Performance testing; 100g of epoxy resin E51 was added to 10g of allyl glycidyl ether (AGE), heated to 80°C, and stirred until well mixed. 20g of the modified amine epoxy curing agent of each example was added, and the mixture was heated to 110°C and stirred vigorously for 30min. The mixture was placed in an oven, heated to 90°C, and degassed for 50min. The mixture was poured into a polytetrafluoroethylene mold, heated to 110°C, and pre-cured for 3h. The mixture was then heated to 160°C and cured for 7h to obtain the sample.

[0046] (1) The limiting oxygen index (LOI) was tested. The combustion behavior index of the sample (130×6.5×3.2mm) was evaluated using an HC-2CZ oxygen index meter (Nanjing, China). (2) Self-healing: Referring to ASTM D7027-05 "Test Method for Scratch Recovery of Plastics", the above-mentioned coatings were scratched using a multi-finger scratch tester. The scratch tip was made of stainless steel with a diameter of 1 mm. The applied load was 15 N, the scratch speed was 100 mm / s, and the scratch length was 100 mm. The surface scratch width was observed using a laser confocal microscope. Afterward, the surface was left to stand in a 25℃ 80% RH environment for 24 h, and the surface scratch width was observed again to calculate the self-healing rate. Subsequently, four more scratch-self-healing cycles were performed at the same location. After the fourth scratch-self-healing cycle, the self-healing rate was calculated: Self-healing rate = width of surface scratch after standing at room temperature ÷ original width of surface scratch × 100%.

[0047] Table 1 According to the data in Table 1, the modified amine epoxy curing agents prepared in Examples 1-3 can improve the flame retardancy and self-healing ability of epoxy resin; in Example 4, the structure of the chain extender was changed, and the chain extender structure does not contain disulfide bonds, resulting in the epoxy resin not having self-healing ability; in Example 5, the structure of the chain extender was changed, resulting in a decrease in the flame retardant performance of epoxy resin; in Example 6, the use of TETA as a chain extender resulted in a decrease in the flame retardant performance and self-healing performance of epoxy resin.

[0048] (2) Free amine content: tested by titration; The epoxy curing agent and epoxy resin curing agent 593 prepared in Examples 1-3 were tested according to method (2), and the results are shown in Table 2.

[0049] Table 2 The data in Table 2 show that the free amine content of the epoxy curing agent prepared in this application is relatively low.

[0050] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing an environmentally friendly modified amine epoxy curing agent with low free amine content, characterized in that, The process includes the following steps: mixing epoxy resin with solvent, adding PEG and catalyst, heating to 75~85℃ and stirring at a constant temperature for 2~3 hours, then adding chain extender and continuing to stir at a constant temperature for 4~6 hours to obtain modified amine epoxy curing agent.

2. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 1, characterized in that, The epoxy resin is any one or more of epoxy resin E51, epoxy resin E44, and epoxy resin E128.

3. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 1, characterized in that, The solvent is propylene glycol methyl ether.

4. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 1, characterized in that, The catalyst is boron trifluoride diethyl ether.

5. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 1, characterized in that, The chain extender has the structure shown in Formula I. （Ⅰ）。 6. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 5, characterized in that, The method for preparing the chain extender includes the following steps: S1. N-BOC-serine alcohol was mixed with DMF and cooled to -5~0℃. CBr4 and diphenyl-2-pyridylphosphine were added, followed by stirring at a constant temperature for 10~20 min. After returning to room temperature, stirring was continued for 1~2 h. The mixture was concentrated under reduced pressure and extracted. The organic phases were combined and subjected to column chromatography to obtain the compound shown in Formula II. （Ⅱ）； S2. Sodium phenylthiosulfonate and DMF were mixed and added to the compound represented by formula II in step S1. The mixture was heated to 50-70°C and stirred for 4-5 hours. Then, it was cooled to room temperature, concentrated under reduced pressure, and ethyl acetate and deionized water were added to adjust the pH of the system to 6-6.

8. The organic phase was separated, concentrated, and subjected to column chromatography to obtain the compound represented by formula III. （Ⅲ）； S3. Mix the compound represented by formula III from step S2 with ethyl acetate, add a saturated ethyl hydrochloride solution, stir at room temperature for 6-8 hours, concentrate under reduced pressure, extract, combine the organic phases, concentrate under reduced pressure, dry, and perform column chromatography to obtain the compound represented by formula IV. （Ⅳ）； S4. 2-Chloro-4,6-diamino-1,3,5-triazine and the compound shown in Formula IV in step S3 are mixed and added to DMF, followed by the addition of triethylamine. The mixture is heated to 70-80°C and stirred at a constant temperature for 20-28 hours. 1,3-dibromopropane is then added and stirred at a constant temperature for 18-20 hours. TETA is then added and stirred for 20-24 hours. After the mixture is finished, it is filtered, concentrated under reduced pressure, and dried to obtain the chain extender.

7. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 6, characterized in that, In step S1, the molar ratio of N-BOC-serine alcohol to CBr4 is 1:(2.5~3).

8. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 6, characterized in that, In step S2, the molar ratio of sodium benzyl thiosulfonate to the compound represented by formula II is (2.3~2.6):

1.

9. The preparation method of the environmentally friendly, low-free-amine modified amine epoxy curing agent according to claim 6, characterized in that, In step S4, the molar ratio of the compound represented by Formula IV to 2-chloro-4,6-diamino-1,3,5-triazine is 1:(1.1~1.3).

10. A modified amine epoxy curing agent prepared by the preparation method according to any one of claims 1 to 9.

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