A trifunctional modified imidazole curing agent and its preparation method and application

Abstract

The present invention relates to the technical field of epoxy resin curing agents, and provides a trifunctional modified imidazole curing agent, a preparation method thereof, and an application thereof. The present invention introduces a benzene ring group having good compatibility with epoxy resin into the structure of the modified imidazole curing agent, and at the same time increases the functionality of the curing agent, thereby improving the curing degree of the epoxy resin and improving the mechanical properties of the cured product. The trifunctional modified imidazole curing agent provided by the present invention has a low curing temperature and a high hardness of the cured product, and has very important application significance in the field of low-temperature curing of epoxy powder coatings. The present invention also provides a preparation method of the trifunctional modified imidazole curing agent described in the above scheme, which adopts the glycidyl ether of the triphenol methane structure and the common imidazole curing agent as the main raw materials to prepare the trifunctional modified imidazole curing agent, which has low cost, simple process, and is suitable for industrial production.

Description

Technical Field

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

[0002] Powder coating is a powder coating with solid resin as the main component, and the solid resin is usually epoxy resin. Epoxy resin contains highly active epoxy groups, which can react with substances containing more active hydrogen to cause ring-opening curing. The cured epoxy resin coating has good mechanical properties and can be used to protect the substrate. The curing temperature of traditional epoxy resin is usually 180-200℃. The high curing temperature makes it only applicable to heat-resistant substrates, which limits its use on heat-sensitive substrates.

[0003] Imidazole curing agents are a type of low-temperature curing agent for epoxy resins with high curing activity. They can usually be completely cured at a temperature of 90-120°C. However, the high activity of this type of curing agent also makes its mixed resin system prone to curing and deterioration at room temperature. At present, there are many studies on the modification of imidazole curing agents to reduce their activity, but they are all limited to use in liquid resins. Therefore, the development of modified imidazoles suitable for solid epoxy resin systems is of great significance and application value.

[0004] A good modification method is to introduce electron-withdrawing groups and macromolecular groups by ring-opening addition between the secondary amine on imidazole and the epoxy group on the epoxy resin. Such groups usually include benzene rings, cyano groups, etc. The introduction of the above groups reduces the activity of imidazole itself by reducing the electron cloud density on imidazole and producing a steric effect. However, the introduction of the above groups may cause the imidazole activity to be reduced too much and the curing agent and epoxy resin are not compatible, resulting in incomplete curing, and the mechanical properties of the obtained cured product are also reduced.

[0005] Therefore, there is an urgent need to provide a curing agent that is suitable for solid epoxy resin and can improve the mechanical properties of the cured product. Summary of the invention

[0006] In view of this, the present invention provides a trifunctional modified imidazole curing agent and a preparation method and application thereof. The trifunctional modified imidazole curing agent provided by the present invention is suitable for solid epoxy resin, has a low curing temperature, and has a high mechanical strength of the cured product.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] A trifunctional modified imidazole curing agent having a structure shown in Formula I:

[0009]

[0010] In formula I: R1 is methyl, ethyl or phenyl; R2 is H or methyl.

[0011] Preferably, the molecular weight of the trifunctional modified imidazole curing agent is 706.84-892.39 g / mol.

[0012] The present invention also provides a method for preparing the trifunctional modified imidazole curing agent described in the above scheme, comprising the following steps:

[0013] 4,4',4"-trihydroxytrimethylbenzene, tetrabutylammonium bromide and epichlorohydrin are mixed for substitution reaction, and the obtained substitution reaction solution is mixed with sodium hydroxide solution for ring closing reaction to obtain trifunctional glycidyl ether, the structure of which is shown in Formula II;

[0014]

[0015] The trifunctional glycidyl ether, imidazole compound and solvent are mixed to carry out a ring-opening reaction to obtain the trifunctional modified imidazole curing agent; the structure of the imidazole compound is shown in Formula III;

[0016]

[0017] Preferably, the mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to tetrabutylammonium bromide is 8-10:1-5;

[0018] The mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to epichlorohydrin is 8-10:85-92.

[0019] Preferably, the temperature of the substitution reaction is 85-120° C., and the time is 2-3.5 h.

[0020] Preferably, the mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to the sodium hydroxide solution is 7-7.5:10-11; and the concentration of the sodium hydroxide solution is 30wt%.

[0021] Preferably, the temperature of the ring-closing reaction is 40-44° C., and the time is 1.5-2 h.

[0022] Preferably, the mass ratio of the trifunctional glycidyl ether to the imidazole compound is 11-19:10-11; the mass ratio of the trifunctional glycidyl ether to the solvent is 14-27:10-11.

[0023] Preferably, the temperature of the ring-opening reaction is 100-120° C., and the time is 0.5-1 h.

[0024] The present invention also provides the use of the trifunctional modified imidazole curing agent described in the above scheme or the trifunctional modified imidazole curing agent prepared by the preparation method described in the above scheme in epoxy powder coatings.

[0025] The present invention provides a trifunctional modified imidazole curing agent having a structure shown in Formula I. The modified imidazole curing agent provided by the present invention contains a rigid benzene ring structure in its molecular structure, and has good compatibility with bisphenol A epoxy resin. At the same time, a large number of rigid structures also provide good mechanical properties for the cured sample. The present invention also increases the functionality of the modified imidazole curing agent. The trifunctional structure during the curing process helps to improve the crosslinking density of the system and the content of the rigid structure, thereby improving the degree of curing of the epoxy resin and the mechanical properties of the cured product. The curing temperature is low, which is of great significance in the field of low-temperature curing of epoxy powder coatings and has broad application prospects. The results of the embodiments show that the trifunctional modified imidazole curing agent provided by the present invention is used to cure solid epoxy resin, and the curing temperature is about 140°C, with good low-temperature curing performance. The surface pencil hardness of the cured product can reach 3-4H, and it has good mechanical properties.

[0026] The present invention also provides a method for preparing the trifunctional modified imidazole curing agent described in the above scheme. The present invention uses glycidyl ether of triphenol methane structure and common imidazole curing agent as main raw materials to prepare the trifunctional modified imidazole curing agent, which has low cost, simple process, and easy separation of products after the reaction is completed, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the H-NMR spectrum of the trifunctional modified imidazole curing agent prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The present invention provides a trifunctional modified imidazole curing agent having a structure shown in Formula I:

[0029]

[0030] In formula I: R1 is methyl, ethyl or phenyl; R2 is H or methyl.

[0031] In the present invention, when R1 is a methyl group or a phenyl group, R2 is preferably H, and when R1 is an ethyl group, R2 is preferably a methyl group.

[0032] The structure of the trifunctional modified imidazole curing agent provided by the present invention is similar to that of bisphenol A epoxy resin, and has good compatibility. The imidazole connected to the glycidyl ether end group can be cured and cross-linked with the epoxy resin to form a polymer network. The obtained cured product has the characteristics of high hardness and low curing temperature, and can be used in the surface coating of some heat-sensitive substrates.

[0033] In the present invention, the molecular weight of the trifunctional modified imidazole curing agent is 706.84-892.39 g / mol.

[0034] The present invention also provides a method for preparing the trifunctional modified imidazole curing agent described in the above scheme, comprising the following steps:

[0035] 4,4',4"-trihydroxytrimethylbenzene, tetrabutylammonium bromide and epichlorohydrin are mixed for substitution reaction, and the obtained substitution reaction solution is mixed with sodium hydroxide solution for ring closing reaction to obtain trifunctional glycidyl ether, the structure of which is shown in Formula II;

[0036]

[0037] The trifunctional glycidyl ether, imidazole compound and solvent are mixed to carry out a ring-opening reaction to obtain the trifunctional modified imidazole curing agent; the structure of the imidazole compound is shown in Formula III;

[0038]

[0039] In the present invention, the synthesis route of the trifunctional modified imidazole curing agent is shown in Formula IV

[0040]

[0041] The preparation method of the trifunctional modified imidazole curing agent of the present invention is described in detail below.

[0042] The present invention mixes 4,4',4"-trihydroxytrimethylbenzene, tetrabutylammonium bromide (TBAB) and epichlorohydrin for substitution reaction, mixes the obtained substitution reaction liquid with sodium hydroxide solution for ring-closing reaction, and obtains trifunctional glycidyl ether. In the present invention, the mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to tetrabutylammonium bromide is preferably 8-10:1-5, more preferably 8.5-9.5:2-4; the mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to epichlorohydrin is preferably 8-10:85-92, more preferably 8.5-9.5:88-90.

[0043] In the present invention, the temperature of the substitution reaction is preferably 85 to 120° C., more preferably 90 to 110° C., and the time of the substitution reaction is preferably 2 to 3.5 h, more preferably 2.5 to 3 h.

[0044] In the present invention, the mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to the sodium hydroxide in the sodium hydroxide solution is preferably 7-7.5:10-11; the concentration of the sodium hydroxide solution is preferably 30wt%; the temperature of the ring-closing reaction is preferably 40-44°C, and the time is preferably 1.5-2h. In the present invention, after the substitution reaction is completed, the obtained ring-opening reaction liquid is preferably cooled to 40-44°C, and then the sodium hydroxide solution is slowly added dropwise to the system; the dropping time of the sodium hydroxide solution is preferably 0.5-1h.

[0045] After the ring-closing reaction is completed, the present invention preferably cools the obtained product liquid and then filters it, washes the obtained filtrate and then performs vacuum distillation to obtain trifunctional glycidyl ether; the present invention removes the sodium chloride generated during the reaction by filtering, and removes the unreacted epichlorohydrin and residual detergent by vacuum distillation; the washing detergent is preferably hot water or acetone, and the temperature of the hot water is preferably 80-90°C.

[0046] After obtaining the trifunctional glycidyl ether, the present invention mixes the trifunctional glycidyl ether, the imidazole compound and the solvent for a ring-opening reaction to obtain a trifunctional modified imidazole curing agent having a structure shown in Formula I. In the present invention, the structure of the imidazole compound is shown in Formula III, and the types of R1 and R2 in Formula III are consistent with those in Formula I, which will not be repeated here; in a specific embodiment of the present invention, the imidazole compound is preferably one of 2-methylimidazole, 2-ethyl-4-methylimidazole and 2-phenylimidazole; the mass ratio of the trifunctional glycidyl ether to the imidazole compound is preferably 11-19:10-11, more preferably 12-18:10-11; the mass ratio of the trifunctional glycidyl ether to the solvent is preferably 14-27:10-11, more preferably 15-26:10-11; the solvent is preferably a benzene solvent, more preferably one or both of toluene and xylene. In the present invention, the temperature of the ring-opening reaction is preferably 100 to 120° C., more preferably 110 to 120° C., and the time of the ring-opening reaction is preferably 0.5 to 1 h, more preferably 0.6 to 0.8 h.

[0047] In a specific embodiment of the present invention, it is preferred to first mix the imidazole compound and the solvent, heat the mixture to 100-120°C and stir for 1 hour to completely dissolve the imidazole compound, and then drop the trifunctional glycidyl ether into the system; the dropwise addition time of the trifunctional glycidyl ether is preferably 0.5-1 hour; and the time of the ring-opening reaction is counted from the completion of the dropwise addition of the trifunctional glycidyl ether.

[0048] After the ring-opening reaction is completed, the present invention preferably pours out the upper liquid in the product liquid, and the remaining product is dried after rotary evaporation to obtain a trifunctional modified imidazole curing agent having a structure shown in Formula I; in the present invention, the drying temperature is preferably 80 to 90° C., and the drying time is preferably 5 to 8 hours; the drying is preferably carried out in a vacuum environment; the present invention removes the residual solvent on the surface of the product by drying.

[0049] The present invention also provides the use of the trifunctional modified imidazole curing agent described in the above scheme or the trifunctional modified imidazole curing agent prepared by the preparation method described in the above scheme in epoxy powder coatings. In the present invention, the trifunctional modified imidazole curing agent is specifically used as a curing agent for solid epoxy resin in epoxy powder coatings, and the solid epoxy resin is preferably bisphenol A epoxy resin, and more preferably E12 solid epoxy resin; the mass ratio of the trifunctional modified imidazole curing agent and the solid epoxy resin is preferably 0.15 to 5:100, and more preferably 1.5 to 5:100; in a specific embodiment of the present invention, the trifunctional modified imidazole curing agent and the solid epoxy resin are preferably mixed and melted and extruded, and the resulting mixture is sprayed on the surface of the substrate under melting conditions, and a solid coating is formed after curing; the curing temperature is preferably 138 to 142°C, and more preferably 140°C, and the curing time is preferably 15 to 20 minutes; the substrate is preferably a heat-sensitive substrate.

[0050] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] Example 1

[0052] Add 29.2g 4,4',4"-trihydroxytriphenylmethane, 3.4g tetrabutylammonium bromide and 294g epichlorohydrin to a three-necked flask equipped with a reflux condenser and a thermometer, and slowly raise the temperature until the liquid temperature in the flask reaches 85°C. Keep the temperature for 3h. When the liquid in the flask changes from rose red to yellow, lower the temperature to 44°C. Dissolve 13.2g NaOH in 30.8g deionized water and slowly add it dropwise to the above solution. After 0.5h of addition, keep the reaction temperature and continue to react for 1.5h. After the reaction is completed, cool and precipitate to remove NaCl generated in the system. Add 80-90°C deionized water to the yellow liquid product for washing 3-4 times, and remove the unreacted epichlorohydrin and excess water by vacuum distillation to obtain a yellow product, which is a trifunctional glycidyl ether.

[0053] Mix 16.524g of 2-ethyl-4-methylimidazole and 10.62g of xylene in a round-bottom flask equipped with a reflux condenser, heat to 120°C and stir for 1h until the imidazole is completely dissolved. Slowly add 23.02g of the above yellow product, and add dropwise within 0.5h. Keep the oil bath temperature at 120°C and continue the reaction for 0.5h. Pour off the upper layer of solvent, evaporate the lower layer of product to remove the solvent, and dry it in a vacuum oven at 85°C for 6h until it is completely dry to obtain an orange-yellow solid product, which is a trifunctional modified imidazole curing agent with the structure shown in Formula I, wherein R1 is ethyl and R2 is methyl. The H NMR spectrum of the product is as follows: Figure 1 shown.

[0054] 5.25g of the modified imidazole curing agent and 350g of E12 solid epoxy resin were mixed and melt extruded to obtain a mixture. The constant temperature curing temperature of the mixture was measured by DSC to be 140°C, which was relatively low. The mixture was sprayed on a metal substrate, and the coating surface leveling was good after melt curing for 15 minutes. The cured product passed through a forward impact height of 50cm, cracked at a recoil height of 50cm, and had a pencil hardness of 3H.

[0055] Example 2

[0056] Add 14.6g 4,4',4"-trihydroxytriphenylmethane, 1.7g tetrabutylammonium bromide and 147g epichlorohydrin to a three-necked flask equipped with a reflux condenser and a thermometer, slowly increase the temperature and heat the three-necked flask until the liquid temperature in the flask reaches 115°C, maintain the temperature and reflux for 2h, and when the liquid in the flask changes from rose red to orange-yellow, reduce the temperature to 44°C. Dissolve 6.6g NaOH in 15.4g deionized water and slowly add it dropwise to the above solution. After 0.5h of addition, maintain the reaction temperature and continue to react for 1.5h. After the reaction is completed, cool and precipitate to remove NaCl generated in the system, add 15mL acetone to wash the yellow liquid product 3 to 4 times, and remove the unreacted epichlorohydrin and residual acetone by vacuum distillation to obtain a yellow product, which is a trifunctional glycidyl ether.

[0057] 7.39g 2-methylimidazole and 5.31g xylene were mixed and added to a round-bottom flask equipped with a reflux condenser, and the temperature was raised to 120°C and stirred for 1h until the imidazole was completely dissolved. 13.82g of the above yellow product was slowly added, and the addition was completed within 0.5h. The oil bath temperature was maintained at 120°C and the reaction was continued for 0.5h. The upper layer of solvent was poured off, and the lower layer of product was ground after the solvent was removed by rotary evaporation. It was dried in a vacuum oven at 90°C for 3h until it was completely dry to obtain a brown solid product, which is a trifunctional modified imidazole curing agent having a structure shown in Formula I, wherein R1 is methyl and R2 is H. The characteristic displacements of the product's H NMR spectrum are 7.08, 6.91, 5.44, 3.75-4.22, and 2.54ppm.

[0058] 5g of the modified imidazole curing agent and 100g of E12 solid epoxy resin were mixed and melt extruded to obtain a mixture. The DSC measured that the constant temperature curing temperature of the mixture was 140°C, which was relatively low. The mixture was sprayed on a metal substrate, and the coating surface leveling was good after melt curing for 15 minutes. The cured product passed through a forward impact height of 45cm and a recoil impact height of 35cm, and the pencil hardness was 4H.

[0059] Example 3

[0060] Add 14.6g 4,4',4"-trihydroxytriphenylmethane, 1.7g tetrabutylammonium bromide and 147g epichlorohydrin to a three-necked flask equipped with a reflux condenser and a thermometer, slowly increase the temperature and heat the three-necked flask until the liquid temperature in the flask reaches 115°C, maintain the temperature and reflux for 2h, and when the liquid in the flask changes from rose red to orange-yellow, reduce the temperature to 44°C. Dissolve 6.6g NaOH in 15.4g deionized water and slowly add it dropwise to the above solution. After 0.5h of addition, maintain the reaction temperature and continue to react for 1.5h. After the reaction is completed, cool and precipitate to remove NaCl generated in the system, add 90°C deionized water to wash the yellow liquid product 3 to 4 times, and remove the unreacted epichlorohydrin and residual deionized water by vacuum distillation to obtain a yellow product, which is a trifunctional glycidyl ether.

[0061] 8.77g of 2-phenylimidazoline and 6.37g of xylene were mixed and added to a round-bottom flask equipped with a reflux condenser, and the temperature was raised to 120°C and stirred for 1h until the imidazole was completely dissolved. 9.22g of the above yellow product was slowly added, and the addition was completed within 0.5h. The oil bath temperature was maintained at 110°C and the reaction was continued for 0.25h. The upper layer of solvent was poured off, and the lower layer of product was ground after the solvent was removed by rotary evaporation. It was dried in a vacuum oven at 90°C for 3h until it was completely dry to obtain a brown solid product, which is a trifunctional modified imidazole curing agent having a structure shown in Formula I, wherein R1 is phenyl and R2 is H. The characteristic displacements of the product's H NMR spectrum are 7.49, 7.68, 6.95, 6.85, 3.75-4.25, 3.35ppm.

[0062] 7.5 g of the modified imidazole curing agent and 150 g of E12 solid epoxy resin were mixed and melt-extruded to obtain a mixture. The mixture was sprayed on a metal substrate. After melt curing at 140° C. for 15 minutes, the coating surface had good leveling. The cured product passed through a forward impact height of 35 cm and a reverse impact height of 20 cm, and the pencil hardness was 3H.

[0063] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A trifunctional modified imidazole curing agent, characterized in that: It has the structure shown in formula I: In formula I: R1 is methyl, ethyl or phenyl; R2 is H or methyl.

2. The method for preparing the trifunctional modified imidazole curing agent according to claim 1, characterized in that: The following steps are involved: 4,4',4"-trihydroxytrimethylbenzene, tetrabutylammonium bromide and epichlorohydrin are mixed for substitution reaction, and the obtained substitution reaction solution is mixed with sodium hydroxide solution for ring closing reaction to obtain trifunctional glycidyl ether, the structure of which is shown in Formula II; The trifunctional glycidyl ether, imidazole compound and solvent are mixed to carry out a ring-opening reaction to obtain the trifunctional modified imidazole curing agent; the structure of the imidazole compound is shown in Formula III; 3. The preparation method according to claim 2, characterized in that: The mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to tetrabutylammonium bromide is 8-10:1-5; The mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to epichlorohydrin is 8-10:85-92.

4. The preparation method according to claim 2 or 3, characterized in that: The temperature of the substitution reaction is 85-120° C. and the time is 2-3.5 hours.

5. The preparation method according to claim 2, characterized in that: The mass ratio of the 4,4',4"-trihydroxytrimethylbenzene to the sodium hydroxide solution is 7-7.5:10-11. The concentration of the sodium hydroxide solution is 30wt%.

6. The preparation method according to claim 2 or 5, characterized in that: The temperature of the ring-closing reaction is 40-44° C., and the time is 1.5-2 hours.

7. The preparation method according to claim 2, characterized in that: The mass ratio of the trifunctional glycidyl ether to the imidazole compound is 11-19:10-11; the mass ratio of the trifunctional glycidyl ether to the solvent is 14-27:10-11.

8. The preparation method according to claim 2, characterized in that: The ring-opening reaction is carried out at a temperature of 100 to 120° C. and for a time of 0.5 to 1 h.

9. Use of the trifunctional modified imidazole curing agent according to claim 1 or the trifunctional modified imidazole curing agent prepared by the preparation method according to any one of claims 2 to 8 in epoxy powder coatings.

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