A preparation method of a modified mercaptan curing agent and its application in epoxy resin

Abstract

The invention discloses a preparation method of a modified mercaptan curing agent and its application in epoxy resin, wherein indole-5-formaldehyde and trimesoyl chloride are subjected to amidation reaction, then Schiff base condensation and reduction reaction are carried out with DL-cysteine, and finally esterification reaction is carried out with polyethylene glycol monomethyl ether to obtain a novel modified mercaptan curing agent. It contains multiple sulfhydryl groups and multiple imino groups, contains a large amount of active hydrogen, and can form a stable curing cross-linked network with epoxy resin, and the curing system temperature is low and the curing speed is fast. And the modified mercaptan curing agent contains multiple flexible polyether molecular chains, has a good toughening effect on epoxy resin, significantly improves the elongation at break of epoxy resin, and has good tensile strength and heat resistance.

Description

Technical Field

[0001] The invention relates to the technical field of curing agents, in particular to a preparation method of a modified thiol curing agent and application of the same in epoxy resin. Background Art

[0002] Epoxy resin can disperse and crosslink with active hydrogen curing agent to form a curing crosslinking system, which makes epoxy resin have good mechanical strength, insulation, water resistance and solvent resistance. At present, the methods for toughening and modifying epoxy resin mainly include blending modification, active diluent modification, curing agent modification, etc. Among them, epoxy resin curing agents mainly include amine curing agents, acid anhydride curing agents, thiol curing agents, etc.

[0003] Under the action of tertiary amine accelerator, mercaptan curing agent can carry out curing reaction with epoxy resin at room temperature or low temperature, and curing speed is fast. Developing high-performance mercaptan curing agent is a research trend. A Chinese patent for a preparation method of high-quality polythiol curing agent for epoxy resin (Announcement No. CN110527076B) discloses that NaSH and halogenated polyether polyol are reacted, and the obtained high-quality polythiol curing agent for epoxy resin has the advantages of colorless transparency and low halogen residue. However, the polythiol does not improve the toughness and heat resistance of epoxy resin. Summary of the invention

[0004] 1. Technical problems solved:

[0005] In view of the deficiencies of the prior art, the present invention provides a method for preparing a modified thiol curing agent that can be cured at low temperature, thereby improving the toughness and heat resistance of epoxy resin.

[0006] (II) Technical solution:

[0007] A preparation method of a modified mercaptan curing agent: add N,N-dimethylformamide, trimesoyl (indole-5-imidothiopropionic acid), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and 4-dimethylaminopyridine to a reaction container, stir and activate, add polyethylene glycol monomethyl ether, stir to react, add water and stir to mix, then extract with dichloromethane for 3 times, combine the organic phases, concentrate to remove dichloromethane, wash with petroleum ether, and dry to obtain a modified mercaptan curing agent. The reaction formula is:

[0008]

[0009] Preferably, trimesoyl (indole-5-imidothiopropionic acid), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and 4-dimethylaminopyridine are stirred and activated and then added with polyethylene glycol monomethyl ether in a ratio of 1 mol:(3-3.3):(0.72-0.9):(3-3.6).

[0010] Preferably, the temperature of the stirring activation is 20-35°C and the time is 3-4h; the temperature of the stirring reaction is 20-35°C and the time is 18-24h.

[0011] Preferably, the molecular weight of polyethylene glycol monomethyl ether is 200-800.

[0012] Preferably, the preparation method of trimesoyl (indole-5-imidothiopropionic acid) is:

[0013] Step (1), add dichloromethane, indole-5-carboxaldehyde, triethylamine, 4-dimethylaminopyridine to a reaction vessel, add trimesoyl chloride dropwise, react at 15-25°C for 7-10h, concentrate to remove dichloromethane, separate by column chromatography, eluent is a solution of ethyl acetate and petroleum ether, gradient elution, to obtain trimesoyl (indole-5-carboxaldehyde). The preparation reaction formula is:

[0014]

[0015] Step (2), add ethanol, trimesoyl (indole-5-carboxaldehyde) in a ratio of 1 mol: (3-3.9) mol, and DL-cysteine ​​to a reaction vessel, carry out condensation reaction at 50-65°C for 12-18h, then add sodium borohydride, carry out reduction reaction at 20-30°C for 3-4h, concentrate to remove ethanol, wash with water and dry, and separate by column chromatography, using a solution of ethyl acetate and petroleum ether as the eluent, and gradient elution to obtain trimesoyl (indole-5-imidothiopropionic acid). The preparation reaction formula is:

[0016]

[0017] Preferably, in step (1), the molar ratio of indole-5-carboxaldehyde, triethylamine, 4-dimethylaminopyridine and trimesoyl chloride is (3.6-4.2) mol:(4.8-5.4) mol:(0.33-0.42) mol:1 mol.

[0018] Preferably, when the modified thiol curing agent is used in the epoxy resin, the ratio of the modified thiol curing agent to the epoxy resin is (140-180) g:100 g.

[0019] Technical effects of the present invention:

[0020] (1) The present invention conducts an amidation reaction of indole-5-carboxaldehyde and trimesoyl chloride, then conducts a Schiff base condensation and reduction reaction with DL-cysteine, and finally conducts an esterification reaction with polyethylene glycol monomethyl ether to obtain a novel modified thiol curing agent.

[0021] (2) The modified mercaptan curing agent of the present invention contains multiple thiol groups and multiple imino groups, contains a large amount of active hydrogen, can form a stable curing cross-linking network with epoxy resin, the curing system temperature is low, and the curing speed is fast. In addition, the modified mercaptan curing agent contains multiple flexible polyether molecular chains, which has a good toughening effect on epoxy resin, significantly improves the elongation at break of epoxy resin, and has good tensile strength.

[0022] (3) The modified thiol curing agent of the present invention contains a polyaromatic nitrogen heterocyclic structure and has strong heat resistance. When introduced into the curing network of the epoxy resin molecular chain, it significantly increases the thermal decomposition temperature of the epoxy resin and exhibits good heat resistance. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some representative embodiments of the present invention, rather than all embodiments. All other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0024] Example 1

[0025] Step (1), add 180mL of dichloromethane, 84mmol of indole-5-carboxaldehyde, 96mmol of triethylamine, and 8.4mmol of 4-dimethylaminopyridine to a reaction vessel, add 20mmol of trimesoyl chloride dropwise, react at 25°C for 8h, concentrate to remove dichloromethane, separate by column chromatography, use a solution of ethyl acetate and petroleum ether as the eluent, and perform gradient elution to obtain trimesoyl (indole-5-carboxaldehyde).

[0026] Step (2), add 600 mL of ethanol, 40 mmol of trimesoyl (indole-5-carboxaldehyde), and 156 mmol of DL-cysteine ​​into a reaction container, carry out condensation reaction at 65° C. for 12 h, then add 140 mmol of sodium borohydride, carry out reduction reaction at 30° C. for 3 h, concentrate to remove ethanol, wash with water and then dry, and separate by column chromatography, using a solution of ethyl acetate and petroleum ether as the eluent, and gradient elution to obtain trimesoyl (indole-5-imidothiopropionic acid).

[0027] Step (3), add 400mL N,N-dimethylformamide, 40mmol trimesoyl (indole-5-imidothiopropionic acid), 124mmol 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 28.8mmol 4-dimethylaminopyridine to the reaction container, stir and activate at 25°C for 4h, add 144mmol polyethylene glycol monomethyl ether 200 (tetraethylene glycol monomethyl ether, CAS No. 23783-42-8), stir and react for 18h, add water and stir to mix, then extract with dichloromethane 3 times, combine the organic phases, concentrate to remove dichloromethane, wash with petroleum ether, and dry to obtain a modified thiol curing agent.

[0028] Example 2

[0029] Step (1), add 180mL of dichloromethane, 80mmol of indole-5-carboxaldehyde, 96mmol of triethylamine, and 6.6mmol of 4-dimethylaminopyridine to a reaction vessel, add 20mmol of trimesoyl chloride dropwise, react at 15°C for 10h, concentrate to remove dichloromethane, separate by column chromatography, use a solution of ethyl acetate and petroleum ether as the eluent, and perform gradient elution to obtain trimesoyl (indole-5-carboxaldehyde).

[0030] Step (2), add 500mL of ethanol, 40mmol of trimesoyl (indole-5-carboxaldehyde), and 120mmol of DL-cysteine ​​into a reaction container, carry out condensation reaction at 50°C for 18h, then add 150mmol of sodium borohydride, carry out reduction reaction at 20°C for 4h, concentrate to remove ethanol, wash with water and then dry, and separate by column chromatography, using a solution of ethyl acetate and petroleum ether as the eluent, and gradient elution to obtain trimesoyl (indole-5-imidothiopropionic acid).

[0031] Step (3), add 500mL N,N-dimethylformamide, 40mmol trimesoyl (indole-5-imidothiopropionic acid), 132mmol 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 36mmol 4-dimethylaminopyridine into the reaction vessel, stir and activate at 20°C for 4h, add 120mmol polyethylene glycol monomethyl ether 200 (tetraethylene glycol monomethyl ether, CAS No. 23783-42-8), stir and react for 24h, add water and stir to mix, then extract with dichloromethane 3 times, combine the organic phases, concentrate to remove dichloromethane, wash with petroleum ether, and dry to obtain a modified thiol curing agent.

[0032] Example 3

[0033] Step (1), add 150mL of dichloromethane, 72mmol of indole-5-carboxaldehyde, 108mmol of triethylamine, and 7.2mmol of 4-dimethylaminopyridine into a reaction vessel, add 20mmol of trimesoyl chloride dropwise, react at 25°C for 7h, concentrate to remove dichloromethane, separate by column chromatography, use a solution of ethyl acetate and petroleum ether as the eluent, and perform gradient elution to obtain trimesoyl (indole-5-carboxaldehyde).

[0034] Step (2), add 500 mL of ethanol, 40 mmol of trimesoyl (indole-5-carboxaldehyde), and 120 mmol of DL-cysteine ​​into a reaction container, carry out condensation reaction at 60° C. for 12 h, then add 145 mmol of sodium borohydride, carry out reduction reaction at 20° C. for 4 h, concentrate to remove ethanol, wash with water and then dry, and separate by column chromatography, using a solution of ethyl acetate and petroleum ether as the eluent, and gradient elution to obtain trimesoyl (indole-5-imidothiopropionic acid).

[0035] Step (3), add 400mL N,N-dimethylformamide, 40mmol trimesoyl (indole-5-imidothiopropionic acid), 120mmol 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 32mmol 4-dimethylaminopyridine to the reaction container, stir and activate at 35°C for 3h, add 132mmol polyethylene glycol monomethyl ether 200 (tetraethylene glycol monomethyl ether, CAS No. 23783-42-8), stir and react for 18h, add water and stir to mix, then extract with dichloromethane 3 times, combine the organic phases, concentrate to remove dichloromethane, wash with petroleum ether, and dry to obtain a modified thiol curing agent.

[0036] Comparative Example 1

[0037] The trimesoyl (indole-5-imidothiopropionic acid) prepared in Example 2 was used as the thiol curing agent.

[0038] Comparative Example 2

[0039] Step (1), add 500mL of ethanol, 40mmol of trimesoyl (indole-5-carboxaldehyde), and 120mmol of DL-cysteine ​​into a reaction vessel, carry out condensation reaction at 50°C for 18h, concentrate to remove ethanol, wash with water and dry, perform column chromatography separation, eluent is a solution of ethyl acetate and petroleum ether, gradient elution, and obtain trimesoyl (indole-5-Schiff base mercaptopropionic acid). The structural formula is

[0040] Step (2), add 500mL N,N-dimethylformamide, 40mmol trimesoyl (indole-5-Schiff base mercaptopropionic acid), 132mmol 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 36mmol 4-dimethylaminopyridine to the reaction container, stir and activate at 20℃ for 4h, add 120mmol polyethylene glycol monomethyl ether 200 (tetraethylene glycol monomethyl ether, CAS No. 23783-42-8), stir and react for 24h, add water and stir to mix, then extract with dichloromethane 3 times, combine the organic phases, concentrate to remove dichloromethane, wash with petroleum ether, and dry to obtain a modified thiol curing agent. The structural formula is as follows:

[0041]

[0042] Comparative Example 3

[0043] A commercially available polythiol curing agent (brand number QE-340M) was used as the epoxy resin curing agent.

[0044] The thiol curing agent prepared in Examples 1-3 and Comparative Examples 1-2, the polythiol curing agent (140 g) of Comparative Example 3, and 3.2 g of curing accelerator 2,4,6-tris(dimethylaminomethyl)phenol were added to the epoxy resin E44, stirred and mixed, cast into a mold, cured at 25° C. for 30 min, then cured at 50° C. for 6 h, and demolded. The mechanical properties were tested according to GB / T 2567-2021 standard.

[0045] Table 1 Epoxy resin casting performance test

[0046] Tensile strength(MPa) Elongation at break (%) Example 1 40.6 18.0 Example 2 42.3 18.4 Example 3 43.0 16.7 Comparative Example 1 48.8 3.6 Comparative Example 2 17.7 4.1 Comparative Example 3 38.8 6.7

[0047] The cured epoxy resin was tested for thermal properties in a thermogravimetric analyzer, with the temperature rising from room temperature to 700°C in a nitrogen atmosphere. 5％ It is the temperature at which the thermal weight loss is 5%. max is the maximum thermal weight loss temperature.

[0048] Table 2 Epoxy resin thermal performance test

[0049] <![CDATA[T 5％ (℃)]]> <![CDATA[T max (℃)]]> Example 1 352.8 406.5 Example 2 354.7 407.4 Example 3 354.1 407.7 Comparative Example 1 357.2 409.8 Comparative Example 2 282.7 330.3 Comparative Example 3 322.4 387.2

[0050] As shown in Table 1 and Table 2, the elongation at break of the epoxy resin cured casting in Examples 1-3 reached 16.7-18.4 MPa, showing good toughness. And the tensile strength also reached 40.6-43.0 MPa. This is attributed to the fact that the modified mercaptan curing agent contains multiple thiol groups and multiple imino groups, contains a large amount of active hydrogen, can form a stable cured cross-linked network with the epoxy resin, and the modified mercaptan curing agent contains multiple flexible polyether molecular chains, which has a good toughening effect on the epoxy resin, significantly improves the elongation at break of the epoxy resin, and has good tensile strength.

[0051] T of the epoxy resin cured castings of Examples 1-3 5％ Reach 352.8-354.7℃, T max Reaching 406.5-407.7℃, it shows good heat resistance. This is attributed to the polyaromatic nitrogen heterocyclic structure contained in the modified thiol curing agent. It has strong heat resistance and is introduced into the curing network of epoxy resin molecular chains, which significantly increases the thermal decomposition temperature of epoxy resin and exhibits good heat resistance.

[0052] Comparative Example 1: Trimesoyl (indole-5-imidothiopropionic acid) As a thiol curing agent, it contains multiple active hydrogens such as mercapto groups and imino groups, and can undergo curing and cross-linking reactions with epoxy resins. However, the curing agent has many rigid aromatic rings and does not contain flexible polyether molecular chains. It has no toughening effect on epoxy resins, resulting in an elongation at break of epoxy resins of only 3.6%, which has very poor toughness.

[0053] The modified thiol curing agent of Comparative Example 2 contains only mercapto groups, no imino groups, and a low active hydrogen content. It does not form a stable cross-linked network with the epoxy resin and has poor curing performance, resulting in the epoxy resin not forming a stable cured product, and the tensile strength and elongation at break are much lower than those of the embodiments.

[0054] Comparative Example 3: A commercially available polythiol curing agent is used as the curing agent for epoxy resin. The polythiol curing agent does not contain a flexible polyether molecular chain, has a poor toughening effect on epoxy resin, and does not contain a heat-resistant polyaryl nitrogen heterocyclic structure. The T 5％ and T max The temperature is low and the heat resistance is poor.

[0055] 140 g, 150 g, 160 g, 170 g, and 180 g of the modified thiol curing agent (prepared in Example 2) and 3.2 g of curing accelerator 2,4,6-tris(dimethylaminomethyl)phenol were weighed respectively and added to the epoxy resin E44, stirred and mixed, cast into a mold, cured at 25° C. for 30 min, then cured at 50° C. for 6 h, and demolded. Mechanical and thermal properties tests were performed.

[0056] Table 3 Performance test of epoxy resin with different dosage of modified mercaptan curing agent

[0057]

[0058] As shown in Table 3, when the dosage of the modified mercaptan curing agent is 140-180 g, the tensile strength of the epoxy resin reaches 38.8-46.0 MPa, and the elongation at break reaches 18.4-27.7%. 5％ Reach 354.7-363.0℃, T max Reaching 407.3-416.0℃, it has excellent mechanical properties and heat resistance.

[0059] Those skilled in the art may make various modifications to the above content without departing from the spirit and scope of the present invention as defined in the claims. Therefore, the scope of the present invention is not limited to the above description, but is determined by the scope of the claims.

Claims

1. A method for preparing a modified mercaptan curing agent, characterized in that: The preparation method comprises: adding N,N-dimethylformamide, trimesoyl (indole-5-imidothiopropionic acid), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and 4-dimethylaminopyridine into a reaction container, stirring and activating, adding polyethylene glycol monomethyl ether, stirring for reaction, adding water, stirring and mixing, then extracting with dichloromethane, combining organic phases, concentrating, washing, and drying to obtain a modified thiol curing agent; The modified mercaptan curing agent has the structural formula (I) as follows:

2. The method for preparing the modified mercaptan curing agent according to claim 1, characterized in that: The ratio of trimesoyl (indole-5-imidothiopropionic acid), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 4-dimethylaminopyridine and polyethylene glycol monomethyl ether is 1 mol: (3-3.3) mol: (0.72-0.9) mol: (3-3.6) mol.

3. The method for preparing the modified mercaptan curing agent according to claim 1, characterized in that: The stirring activation temperature is 20-35°C and the time is 3-4h; the stirring reaction temperature is 20-35°C and the time is 18-24h.

4. The method for preparing the modified mercaptan curing agent according to claim 2, characterized in that: The molecular weight of the polyethylene glycol monomethyl ether is 200-800.

5. The method for preparing the modified mercaptan curing agent according to claim 2, characterized in that: The preparation method of the trimesoyl (indole-5-imidothiopropionic acid) is: Step (1), adding dichloromethane, indole-5-carboxaldehyde, triethylamine, and 4-dimethylaminopyridine to a reaction vessel, adding trimesoyl chloride dropwise, reacting at 15-25° C. for 7-10 hours, concentrating, and separating by column chromatography to obtain trimesoyl (indole-5-carboxaldehyde); Step (2), adding ethanol, trimesoyl (indole-5-carboxaldehyde) and DL-cysteine ​​into a reaction container to carry out a condensation reaction, then adding sodium borohydride to carry out a reduction reaction, concentrating, washing and drying, and performing column chromatography separation, using an eluent of a solution of ethyl acetate and petroleum ether, and gradient elution to obtain trimesoyl (indole-5-imidothiopropionic acid).

6. The method for preparing the modified mercaptan curing agent according to claim 5, characterized in that: In the step (1), the molar ratio of indole-5-carboxaldehyde, triethylamine, 4-dimethylaminopyridine and trimesoyl chloride is (3.6-4.2) mol: (4.8-5.4) mol: (0.33-0.42) mol: 1 mol.

7. The method for preparing the modified mercaptan curing agent according to claim 5, characterized in that: In the step (2), the ratio of trimesoyl (indole-5-carboxaldehyde) to DL-cysteine ​​is 1 mol:(3-3.9) mol.

8. The method for preparing the modified mercaptan curing agent according to claim 5, characterized in that: In the step (2), the temperature of the condensation reaction is 50-65°C and the time is 12-18 hours; the temperature of the reduction reaction is 20-30°C and the time is 3-4 hours.

9. Use of the modified thiol curing agent obtained by the preparation method according to any one of claims 1 to 8 in epoxy resin.

10. Use of the modified mercaptan curing agent in epoxy resin according to claim 9, characterized in that: The ratio of modified thiol curing agent to epoxy resin is (140-180) g:100 g.

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