CN111393886B - Hydroxyalkyl amide curing agent and preparation method and application thereof - Google Patents

Abstract

The invention belongs to the technical field of curing agent preparation, and particularly relates to a hydroxyalkyl amide curing agent for low-cost, low-temperature curing and high-hardness powder coating, and a preparation method and application of the hydroxyalkyl amide curing agent. The hydroxyalkyl amide curing agent provided by the invention is characterized in that the main raw materials of the curing agent are obtained by jointly polymerizing dimethyl nylon acid, dimethyl terephthalate, diethanol amine, a reaction solvent and a crystallization solvent. The curing agent and carboxyl polyester can be used in powder coating. The curing agent of the invention has the following advantages: the low-temperature melt fluidity is excellent, the activity is higher, and the low-temperature melt fluidity-modified epoxy resin is suitable for low-temperature solidification (150 ℃/15 min); and the dimethyl terephthalate is used, so that the hardness of the coating film is obviously improved, and the coating film can be used for preparing a high-hardness powder coating film while the impact toughness is kept.

Description

Hydroxyalkyl amide curing agent and preparation method and application thereof

Technical Field

The invention belongs to the technical field of curing agent preparation, and particularly relates to a hydroxyalkyl amide curing agent for low-cost, low-temperature curing and high-hardness powder coating, and a preparation method and application of the hydroxyalkyl amide curing agent.

Background

The traditional hydroxyalkyl amide curing agent is mainly prepared by taking dimethyl adipate as a raw material, and the melting range of the hydroxyalkyl amide curing agent (HAA) prepared by taking dimethyl adipate as the raw material is generally 120-130 ℃, and the melting range is higher.

Zhao Liangyan et al published synthesis and characterization of a novel curing agent, beta-hydroxyalkylamide, N1, N4-bis (2-hydroxyethyl) -terephthalamide, which was prepared from terephthalic acid and thionyl chloride as raw materials by synthesis of terephthaloyl chloride in the presence of a phase transfer catalyst, and then reaction of terephthaloyl chloride with ethanolamine. The result shows that the melting double peak appears in the BAET melting process, the thermal decomposition temperature is 299 ℃, and the heat resistance is excellent. However, the method is complicated in process and strict in requirements (the acyl chloride is required to be prepared), so that the preparation cost is high, and the method is not disclosed in the aspects of reducing the cost, reducing the melting range, being capable of curing at low temperature and improving the hardness of a coating.

Therefore, there is a need to improve upon the problems of the prior art by inventing a hydroxyalkyl amide curing agent for powder coating materials with low cost, low temperature curing, and high hardness.

Disclosure of Invention

The hydroxyalkyl amide curing agent provided by the invention is prepared by jointly polymerizing dimethyl nylon acid, dimethyl terephthalate, diethanol amine, a reaction solvent and a crystallization solvent.

The curing agent also comprises a catalyst, wherein the catalyst is one of sodium methoxide or potassium hydroxide;

preferably, the crystallization solvent is a mixed solvent of methanol and ethyl acetate, and the mass ratio of the methanol to the ethyl acetate is 1-3: 1; the mass of the crystallization solvent is 2-4 times of that of diethanol amine;

preferably, the mass ratio of the methanol to the ethyl acetate is 1-3: 1;

preferably, the mass of the crystallization solvent is 2-4 times of that of the diethanolamine;

preferably, the reaction solvent is ethylene glycol diethyl ether or diethylene glycol dimethyl ether.

The weight parts of the main raw materials are as follows:

200 portions of dimethyl nylon acid; 50-90 parts of dimethyl terephthalate; 100 portions and 150 portions of reaction solvent; 280 and 350 parts of diethanolamine; 700 portions of crystallization solvent and 1000 portions of crystallization solvent; 5-10 parts of a catalyst.

The hydroxyl equivalent weight of the curing agent is 78-85g/mol, and the melting range is 95-105 ℃.

The preparation method of the hydroxyalkyl amide curing agent comprises the following steps:

(1) adding dimethyl terephthalate and a reaction solvent into a first reactor, heating, stirring and dissolving the dimethyl terephthalate and the reaction solvent into a uniform solution, adding a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and conveying the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding diethanolamine into a second reactor, heating, then uniformly dropwise adding a dimethyl terephthalate mixed solution into the second reactor from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out an amidation reaction by heat preservation;

(3) dropwise adding dimethyl nylon acid into the second reactor, and continuing the heat preservation reaction;

(4) starting a vacuum system, gradually heating, and carrying out reduced pressure distillation to remove a solvent and a reaction byproduct methanol;

(5) after no obvious distillate is evaporated, cooling, placing the obtained material in a crystallization kettle in which a crystallization solvent is added, uniformly stirring, cooling, crystallizing, centrifuging, and drying to obtain the hydroxyalkyl amide curing agent.

Preferably, in (1), the mixture is heated to 60 to 70 ℃.

Preferably, in the step (2), the temperature is raised to 90-105 ℃;

preferably, (2), keeping the temperature at 95-100 ℃;

preferably, in (2), amidation reaction is carried out for 1-2 h;

preferably, (2) adding diethanol amine into the second reactor, heating to 90-105 ℃, then uniformly dropwise adding the dimethyl terephthalate mixed solution into the second reactor from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 1-2 hours at 95-100 ℃.

Preferably, in the step (3), when the collected methanol reaches 70% of the theoretical amount, the dimethyl nylon acid is dripped into the reactor;

preferably, in the step (3), the nylon acid dimethyl ester is dropwise added within 1.5-2 h;

preferably, (3) the reaction is carried out at the temperature of 95-100 ℃;

preferably, in the step (3), the reaction time is kept for 2-4 h;

preferably, when the methanol collected in the step (3) reaches 70% of the theoretical amount, the dimethyl nylon acid is dripped into the second reactor, the dripping is controlled to be finished within 1.5-2h, and the reaction is continuously carried out for 2-4h at the temperature of 95-100 ℃;

preferably, in (4), when the collected methanol reaches 90% of the theoretical amount, the vacuum system is started;

preferably, the vacuum degree in the step (4) is controlled between-0.095 MPa and-0.098 MPa;

preferably, the temperature in the step (4) is raised to 120-130 ℃;

preferably, the temperature in the step (5) is reduced to 80-90 ℃;

preferably, in the step (4), when the collected methanol reaches 90% of the theoretical amount, starting a vacuum system, controlling the vacuum degree between-0.095 and-0.098 MPa, gradually raising the temperature to 120-;

preferably, the crystallization is carried out in the step (5) by cooling to 0-5 ℃;

preferably, the crystal is crystallized for 1 to 2 hours in the step (5);

preferably, after no obvious distillate is evaporated in the step (5), the temperature is reduced to 80-90 ℃, the obtained material is placed in a crystallization kettle added with a crystallization solvent, the mixture is uniformly stirred and cooled to 0-5 ℃ for crystallization for 1-2h, and then the hydroxyalkyl amide curing agent product is obtained after centrifugation and drying.

The preparation method of the hydroxyalkyl amide curing agent comprises the following steps:

(1) adding dimethyl terephthalate and a reaction solvent into a first reactor, heating to 60-70 ℃, stirring and dissolving into a uniform solution, adding a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and adding the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding diethanolamine into a second reactor, heating to 90-105 ℃, then uniformly dropwise adding a dimethyl terephthalate mixed solution into the second reactor from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 1-2h at 95-100 ℃;

(3) when the collected methanol reaches 70% of the theoretical amount, beginning to dropwise add dimethyl nylon acid into the second reactor, controlling the dropwise addition to be finished within 1.5-2h, and continuously keeping the temperature of 95-100 ℃ for reaction for 2-4 h;

(4) when the collected methanol reaches 90% of the theoretical amount, starting a vacuum system, controlling the vacuum degree between-0.095 MPa and-0.098 MPa, gradually heating to 120 ℃ and 130 ℃, and removing the solvent and the reaction byproduct methanol by reduced pressure distillation;

(5) cooling to 80-90 deg.c after no obvious distillate is evaporated, setting the obtained material in a crystallizing kettle with added crystallizing solvent, stirring, cooling to 0-5 deg.c for crystallization for 1-2 hr, centrifuging and drying to obtain the hydroxyalkyl amide curing agent product.

The curing agent obtained by the above process has the appearance: the hydroxyl equivalent weight of the white powder is 78-85g/mol, and the melting range is 95-105 ℃.

The application of the curing agent prepared by the preparation method in powder coating is also the content to be protected by the invention.

The dimethyl nylon acid (DBE) is obtained by carrying out methyl esterification by using mixed dibasic acid (DBA) which is a byproduct in an adipic acid production process as a raw material, wherein the DBA contains 15-25% of adipic acid, 55-65% of glutaric acid and 15-25% of succinic acid, so that the price (generally about 7000 yuan/ton) of the DBE is far lower than that of commercial dimethyl adipate (about 12000 yuan/ton), and the cost is low. The other raw material of dimethyl terephthalate is obtained by methyl esterification of terephthalic acid, the price of the terephthalic acid (6000 yuan/ton) is also far lower than that of adipic acid (18000 yuan/ton), and compared with HAA, the product of the invention has the advantages of remarkably reduced cost and obvious improvement.

The hydroxyalkyl amide curing agent prepared by dimethyl nylon acid (DBE) and dimethyl terephthalate contains different amide substances (including adipic acid diamide, glutaric acid diamide, succinic acid diamide and terephthalic acid diamide) at the same time, so that the melting range of the product is obviously reduced, and the melting range is 95-105 ℃, which is far lower than that of the hydroxyalkyl amide curing agent (HAA) prepared by dimethyl adipate (generally 120-130 ℃).

In addition, the curing agent has excellent melt fluidity at low temperature and higher activity, and is suitable for low-temperature curing (150 ℃/15 min); and the dimethyl terephthalate is used, so that the hardness of the coating film is obviously improved, and the coating film can be used for preparing a high-hardness powder coating film while the impact toughness is kept.

Detailed Description

In order that those skilled in the art will better understand the present invention, the inventors will further describe and illustrate the present invention by the following specific examples, but do not limit the present invention.

Example 1

A method for preparing a hydroxyalkyl amide curing agent, comprising the steps of:

(1) adding 70 parts by weight of dimethyl terephthalate and 120 parts by weight of reaction solvent ethylene glycol diethyl ether into a first reactor, heating to about 65 ℃, stirring and dissolving to form a uniform solution, adding 8 parts by weight of sodium methoxide serving as a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and adding the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding 300 parts by weight of diethanolamine into a second reactor, heating to about 90 ℃, then uniformly dropwise adding the dimethyl terephthalate mixed solution obtained in the step (1) into the second reactor from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 1.5 hours at the temperature of about 95 ℃;

(3) when the methanol collected in the step (2) reaches 70% of the theoretical amount, beginning to dropwise add 180 parts by weight of dimethyl nylon acid into the second reactor, controlling the dropwise adding to be finished within 1.8h, and continuously keeping at 95 ℃ for reaction for 3 h;

(4) when the collected methanol reaches 90% of the theoretical amount, starting a vacuum system, controlling the vacuum degree to be about-0.095 Mpa, gradually heating to 125 ℃, and removing the solvent and the reaction byproduct methanol by reduced pressure distillation;

(5) when no obvious distillate is evaporated, the temperature is reduced to 85 ℃, the materials are put into a crystallization kettle added with a crystallization solvent (the mass of the crystallization solvent is 3 times of that of diethanol amine) and are uniformly stirred, the materials are crystallized for 1.5 hours when the temperature is cooled to about 2 ℃, and then the hydroxyalkylamide curing agent product is obtained after centrifugation and drying.

The curing agent obtained by the above process has the appearance: the hydroxyl equivalent weight of the white powder is 84g/mol, and the melting range is 98-101 ℃.

Example 2

A method of preparing a hydroxyalkyl amide curing agent comprising the steps of:

(1) adding 80 parts by weight of dimethyl terephthalate and 100 parts by weight of reaction solvent ethylene glycol diethyl ether into a first reactor, heating to about 60 ℃, stirring and dissolving to form a uniform solution, adding 10 parts by weight of sodium methoxide serving as a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and adding the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding 320 parts by weight of diethanolamine into a second reactor, heating to about 95 ℃, then uniformly dropwise adding the dimethyl terephthalate mixed solution obtained in the step (1) from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 2 hours at the temperature of about 98 ℃;

(3) when the methanol collected in the step (2) reaches 70% of the theoretical amount, 160 parts by weight of dimethyl nylon acid are dripped into the second reactor, the dripping is controlled to be finished within 1.5h, and the reaction is continuously carried out for 2h at the temperature of 95 ℃;

(4) when the collected methanol reaches 90% of the theoretical amount, starting a vacuum system, controlling the vacuum degree to be about-0.095 Mpa, gradually heating to 130 ℃, and removing the solvent and the reaction byproduct methanol by reduced pressure distillation;

(5) when no obvious distillate is evaporated, cooling to 90 ℃, placing the materials in a crystallization kettle added with a crystallization solvent (the mass of the crystallization solvent is 3 times of that of diethanol amine), uniformly stirring, cooling to about 2 ℃, crystallizing for 2 hours, centrifuging, and drying to obtain the hydroxyalkylamide curing agent product.

The curing agent obtained by the above process has the appearance: hydroxyl equivalent of white powder is 81g/mol, melting range is 101-103 ℃.

Example 3

A method for preparing a hydroxyalkyl amide curing agent, comprising the steps of:

(1) adding 60 parts by weight of dimethyl terephthalate and 130 parts by weight of reaction solvent ethylene glycol diethyl ether into a first reactor, heating to about 65 ℃, stirring and dissolving to obtain a uniform solution, adding 8 parts of sodium methoxide serving as a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and adding the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding 300 parts by weight of diethanolamine into a second reactor, heating to about 90 ℃, then uniformly dropwise adding the dimethyl terephthalate mixed solution obtained in the step (1) from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 1.5 hours at the temperature of about 90 ℃;

(3) when the methanol collected in the step (2) reaches 70% of the theoretical amount, 160 parts by weight of dimethyl nylon acid are dripped into the second reactor, the dripping is controlled to be finished within 1.8h, and the reaction is continuously carried out for 3h at 100 ℃;

(4) when the collected methanol reaches 90% of the theoretical amount, starting a vacuum system, controlling the vacuum degree to be about-0.095 Mpa, gradually heating to 125 ℃, and removing the solvent and the reaction byproduct methanol by reduced pressure distillation;

(5) when no obvious distillate is evaporated, cooling to 85 ℃, placing the materials in a crystallization kettle added with a crystallization solvent (the mass of the crystallization solvent is 3 times of that of diethanol amine), uniformly stirring, crystallizing for 1.5h when cooling to about 2 ℃, and then centrifuging and drying to obtain the hydroxyalkylamide curing agent product.

The curing agent obtained by the above process has the appearance: the hydroxyl equivalent weight of white powder is 82g/mol, and the melting range is 96-99 ℃.

Example 4

A method for preparing a hydroxyalkyl amide curing agent, comprising the steps of:

(1) adding 85 parts by weight of dimethyl terephthalate and 140 parts by weight of reaction solvent ethylene glycol diethyl ether into a first reactor, heating to about 65 ℃, stirring and dissolving to obtain a uniform solution, adding 10 parts by weight of sodium methoxide serving as a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and adding the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding 350 parts by weight of diethanolamine into a second reactor, heating to about 90 ℃, then uniformly dropwise adding the dimethyl terephthalate mixed solution obtained in the step (1) from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 1.5h at about 95 ℃;

(3) when the methanol collected in the step (2) reaches 70% of the theoretical amount, 200 parts by weight of dimethyl nylon acid are dripped into the second reactor, the dripping is controlled to be finished within 2 hours, and the reaction is continuously carried out for 4 hours at 95 ℃;

(4) when the collected methanol reaches 90% of the theoretical amount, starting a vacuum system, controlling the vacuum degree to be about-0.098 Mpa, gradually heating to 130 ℃, and removing the solvent and the reaction byproduct methanol by reduced pressure distillation;

(5) when no obvious distillate is evaporated, cooling to 90 ℃, putting the materials into a crystallization kettle added with a formula amount of crystallization solvent (the mass of the crystallization solvent is 3 times of that of diethanol amine), uniformly stirring, cooling to about 2 ℃, crystallizing for 1.5h, centrifuging, and drying to obtain the hydroxyalkylamide curing agent product.

The curing agent obtained by the above process has the appearance: the hydroxyl equivalent weight of the white powder is 79g/mol, and the melting range is 102-105 ℃.

Comparative example 1

Otherwise, just as in example 1, the nylon acid dimethyl ester was changed to adipic acid dimethyl ester of equal mass;

the hydroxyl equivalent weight of the obtained curing agent is 87g/mol, and the melting range is 130-133 ℃.

Comparative example 2

The other steps are the same as example 1, except that dimethyl nylon acid is changed into dimethyl terephthalate with equal mass;

the hydroxyl equivalent weight of the obtained curing agent is 87g/mol, and the melting range is 139-143 ℃.

Comparative example 3

The other steps are the same as example 1, except that dimethyl terephthalate is replaced by dimethyl nylon acid of equal mass;

the curing agent obtained has a hydroxyl equivalent weight of 79g/mol and a melting range of 85-90 ℃.

Comparative example 4

Otherwise, the same as example 1 except that no reaction solvent was used;

the obtained curing agent had a hydroxyl equivalent of 114g/mol, a melting range of 70 to 82 ℃ and was sticky and difficult to use.

Example 5

The low-temperature curing pure polyester powder coating is generally prepared by the following formula in parts by weight:

a commercially available HAA curing agent (HH 8080, a commercially available curing agent from wayowa-hui technologies ltd.) was used as comparative example 5 in place of the hydroxyalkyl amide curing agent in the above powder formulation;

the polyester resin is hydroxyalkyl amide system low-temperature curing special polyester resin SJ5701 produced by Anhuishen sword new material GmbH.

Preparation of paint coatings

The materials are uniformly mixed according to the formula of the polyester powder coating, extruded, tabletted and crushed by a double-screw extruder, and then the tablet materials are crushed and sieved (150 meshes and 180 meshes) to prepare the powder coating. The powder coating is sprayed on the galvanized iron substrate after surface treatment by adopting an electrostatic spray gun, the spraying thickness is 90-100 mu m, and the powder coating is solidified at 150 ℃/15min to obtain the powder coating film.

The coating index detection is carried out according to GB/T21776-2008 'Standard guide for powder coating and coating thereof', the adhesion grade is carried out according to GB/T9286-1998 'test for drawing grids of paint and varnish paint film', and the hardness test is carried out according to GB/T6379-2006 'test for paint film hardness by color paint and varnish pencil method'.

TABLE 1 comparison of the properties of the products of the examples and comparative examples after film coating

From the data comparison in table 1 above, it can be seen that: the product of the invention has excellent comprehensive properties such as coating film appearance, shock resistance, gloss, adhesive force and the like, and simultaneously has the capability of realizing low-temperature curing (150 ℃/15min) and the characteristic of high hardness (more than or equal to 2H).

The curing agent has the greatest advantage of low cost, for example, the curing agent is prepared by selecting dimethyl nylon acid as a raw material, wherein the dimethyl nylon acid is prepared by performing methyl esterification by taking a byproduct mixed dibasic acid (DBA) of an adipic acid production process as a raw material, and the DBA contains 15-25% of adipic acid, 55-65% of glutaric acid and 15-25% of succinic acid, so that the price of DBE (generally about 7000 yuan/ton) is far lower than that of commercial dimethyl adipate (about 12000 yuan/ton), and the cost is low;

the other raw material, dimethyl terephthalate, is obtained by methyl esterification of terephthalic acid, and the price of terephthalic acid (6000 yuan/ton) is also far lower than that of adipic acid (18000 yuan/ton). Therefore, compared with HAA, the product of the invention has obvious cost advantage in terms of raw material selection.

As analyzed from the aspect of product performance, the hydroxyalkyl amide curing agent prepared by dimethyl nylon acid (DBE) and dimethyl terephthalate contains different amide substances (including adipic acid diamide, glutaric acid diamide, succinic acid diamide and terephthalic acid diamide) at the same time, and the melting range of the product is obviously reduced due to the mixed existence of a plurality of amide compounds, wherein the melting range is 95-105 ℃, and is far lower than that of the hydroxyalkyl amide curing agent (HAA) prepared by dimethyl adipate (generally 120-130 ℃). Therefore, the invention has excellent melt fluidity at low temperature and higher activity, and is suitable for low-temperature curing (150 ℃/15 min).

In addition, the invention also adopts dimethyl terephthalate, obviously improves the hardness of the coating film, and can be used for preparing a high-hardness powder coating film while keeping the impact toughness.

The results of comparative examples 1 to 3 show that the products synthesized by using dimethyl terephthalate alone or DBE alone as a raw material have certain defects, such as that the melting process of the products is too high and low-temperature (150 ℃/15min) curing cannot be realized, so that the performance of the coating film is poor in the results of comparative examples 1 and 2; the amide product synthesized by DBE alone can realize low-temperature curing, but the main defect is that the hardness is insufficient, and a coating film with high hardness cannot be obtained; comparative example 4 this experiment could not be sufficiently performed because no reaction solvent was used, and the synthesis process was not sufficiently performed because the viscosity was too high, and the by-products were too much to be used. The raw materials, the proportion and the process provided by the invention enable the product to have the characteristics of low cost, low-temperature curing and high hardness after being applied to powder coating, if any one of the raw materials is replaced or some one of the raw materials is reduced, the product cannot achieve the effect of the invention, even an ideal product cannot be obtained, and even if the product is obtained, the by-product is too much to use.

Claims (4)

1. A hydroxyalkyl amide curing agent is characterized in that the curing agent is obtained by jointly polymerizing main raw materials of dimethyl nylon acid, dimethyl terephthalate, diethanol amine, a reaction solvent, a crystallization solvent and a catalyst; the weight portions of the main raw materials are as follows:

200 portions of dimethyl nylon acid; 50-90 parts of dimethyl terephthalate; 100 portions of reaction solvent and 150 portions; 280 and 350 parts of diethanolamine; 700 portions of crystallization solvent and 1000 portions of crystallization solvent; 5-10 parts of a catalyst; the catalyst is one of sodium methoxide or potassium hydroxide; the crystallization solvent is a mixed solvent of methanol and ethyl acetate, and the mass ratio of the methanol to the ethyl acetate is 1-3: 1; the mass of the crystallization solvent is 2-4 times of that of diethanol amine; the reaction solvent is ethylene glycol diethyl ether or diethylene glycol dimethyl ether;

the preparation method of the hydroxyalkyl amide curing agent comprises the following steps:

(1) adding dimethyl terephthalate and a reaction solvent into a first reactor, heating to 60-70 ℃, stirring and dissolving to obtain a uniform solution, adding a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and adding the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding diethanolamine into a second reactor, heating to 90-105 ℃, then uniformly dropwise adding a dimethyl terephthalate mixed solution into the second reactor from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out heat preservation and amidation reaction for 1-2h at 95-100 ℃;

(3) when the collected methanol reaches 70% of the theoretical amount, beginning to dropwise add the dimethyl nylon acid into the second reactor, controlling the dropwise adding to be 1.5-2h, and continuously keeping the temperature of 95-100 ℃ for reaction for 2-4 h;

(4) when the collected methanol reaches 90 percent of the theoretical amount, starting a vacuum system, controlling the vacuum degree between-0.095 MPa and-0.098 MPa, gradually heating to 120-;

(5) cooling to 80-90 deg.c after no obvious distillate is evaporated, setting the obtained material in a crystallizing kettle with added crystallizing solvent, stirring, cooling to 0-5 deg.c for crystallization for 1-2 hr, centrifuging and drying to obtain the hydroxyalkyl amide curing agent product.

2. A hydroxyalkyl amide curing agent as claimed in claim 1, wherein the curing agent has a hydroxyl equivalent weight of 80 to 85g/mol and a melting range of 100-105 ℃.

3. A method of preparing a hydroxyalkyl amide curing agent as claimed in claim 1 or 2, comprising the steps of:

(1) adding dimethyl terephthalate and a reaction solvent into a first reactor, heating, stirring and dissolving the dimethyl terephthalate and the reaction solvent into a uniform solution, adding a catalyst, stirring, uniformly mixing to obtain a dimethyl terephthalate mixed solution, and conveying the dimethyl terephthalate mixed solution into a dropwise adding tank for later use;

(2) adding diethanolamine into a second reactor, heating, then uniformly dropwise adding a dimethyl terephthalate mixed solution into the second reactor from a dropwise adding tank, simultaneously collecting methanol distilled from the second reactor, and then carrying out amidation reaction by heat preservation;

(3) dropwise adding dimethyl nylon acid into the second reactor, and continuing the heat preservation reaction;

(4) starting a vacuum system, gradually heating, and removing the solvent and a reaction byproduct methanol by reduced pressure distillation;

(5) after no obvious distillate is evaporated, cooling, placing the obtained material in a crystallization kettle in which a crystallization solvent is added, uniformly stirring, cooling, crystallizing, centrifuging, and drying to obtain the hydroxyalkyl amide curing agent.

4. Use of a hydroxyalkyl amide curing agent according to claim 1 or 2 in powder coatings.