The invention relates to N, N, Napos; , Napos, Napos; 4-tetra (2-hydroxypropyl) adipamide curing agent as well as preparation method and application thereof

Abstract

The invention provides an N, N, N ', N'-tetra (2-hydroxypropyl) adipamide curing agent as well as a preparation method and application thereof, and belongs to the technical field of powder coating curing agents. The curing agent is prepared from dimethyl adipate, diisopropanolamine, a catalyst and a solvent through atomization feeding, normal-pressure staged reaction and staged crystallization. The preparation method has the advantages of simple process, no need of vacuumizing in the whole process, good synthesis reaction effect, high product yield and purity and few byproducts, and can be used in the high-temperature-resistant powder coating, and the cured coating film has excellent high-temperature yellowing resistance and good surface fineness, and can be used in the field with high requirements on high-temperature yellowing resistance and appearance.

Description

Technical Field

[0001] The invention belongs to the technical field of powder coating curing agents, and specifically relates to an N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent and a preparation method and application thereof. Background Art

[0002] At present, hydroxyalkylamide curing agents are the main curing agent varieties used in outdoor systems in powder coatings. The most important representative is N,N,N',N'-tetrakis(2-hydroxyethyl) adipamide (HAA for short). Hydroxyalkylamide curing agents have advantages such as non-toxicity and low cost, but they have obvious defects in high temperature yellowing resistance. The prepared coating film is easy to yellow at temperatures of 220℃ and above, and it is difficult to be used in fields with high requirements for high temperature yellowing. Another type of hydroxyalkylamide curing agent, N,N,N',N'-tetrakis(2-hydroxypropyl) adipamide, has obvious advantages in high temperature yellowing resistance due to the presence of ortho-methyl in its molecular structure.

[0003] At present, most of the preparation methods of N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide use a vacuum system for reaction, and in the process of crystallization, a one-pot method is used for direct cooling for crystallization. As a result, due to the rapid escape of the by-product methanol in the vacuum system, the proportion of ester exchange reaction in the system will be increased, resulting in a large amount of by-products, which affects the yield of the final product; and the increase in the production of by-products during the synthesis process also leads to greater difficulty in crystallization, and the one-pot method of direct crystallization cooling process used in the existing relevant materials has not been studied in detail on the crystallization section, resulting in low crystallization efficiency, which not only affects the yield of the product, but more importantly, affects the purity of the product. The crystallized product contains more by-products, resulting in application performance, and high temperature yellowing resistance is still defective. Summary of the invention

[0004] In view of the above problems, the present invention prepares N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide from dimethyl adipate, diisopropanolamine, a catalyst and a solvent through continuous atomization feeding, normal pressure staged reaction and staged crystallization. The product has high yield and purity and is used in high temperature resistant powder coatings. After curing, the coating film has good high temperature yellowing resistance and surface fineness.

[0005] One of the purposes of the present invention is to provide a method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent.

[0006] The second object of the present invention is to provide an N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent prepared by the preparation method.

[0007] The third object of the present invention is to provide an application of the N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent in the preparation of high temperature resistant yellowing type powder coating.

[0008] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted: In a first aspect, the present invention provides a method for preparing an N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, comprising the following steps: (1) Add ethylene glycol ethyl ether acetate and diisopropanolamine into a reaction kettle, start stirring and heating, and add a catalyst after they are completely dissolved into a homogeneous phase. Then, pump dimethyl adipate into an automatic atomizing feeder for continuous atomizing feeding. After the atomizing feeding is completed, cool down and continue to keep warm for reaction; (2) When the free dimethyl adipate content is less than 0.5%, the temperature is lowered, and then 4-methyl-2-pentanone is added as a crystallization solvent. After sufficient stirring and dissolution, the temperature is lowered to 30-35°C and crystallization is carried out by heat preservation; (3) When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add acetone as a crystallization solvent, stir evenly, and then perform step-by-step cooling and crystallization. First, cool to 12-15°C, keep warm and stir for crystallization for 1-1.5 hours. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring and let the crystallization stand for 20-30 minutes. Then start the stirring again, cool to 3-5°C, keep warm and stir for crystallization for 2-3 hours. When the amount of crystals precipitated no longer increases, separate the wet material and dry it to obtain the curing agent product.

[0009] In some embodiments, in step (1), the molar ratio of ethylene glycol ethyl ether acetate, diisopropanolamine and dimethyl adipate is 10-15:21-35:10-15.

[0010] In some embodiments, in step (1), the catalyst is one or more selected from sodium methoxide, sodium ethoxide, sodium hydroxide, and potassium hydroxide; and the amount of the catalyst used is 0.2-0.5% of the mass of dimethyl adipate.

[0011] In some embodiments, in step (1), heating is performed to 125-130°C. In some embodiments, in step (1), the atomization feed is controlled to have a diameter of 20-50 μm and a time of 30-35 min. During the atomization feed process, methanol, a byproduct of the reaction, is removed.

[0012] In some embodiments, in step (1), the temperature is lowered to 105-110° C. for heat preservation reaction.

[0013] The ethylene glycol ethyl ether acetate is used as a reaction solvent and also as a crystallization solvent during crystallization.

[0014] The crystallization-aiding solvents are 4-methyl-2-pentanone and acetone, which are added in stages.

[0015] In some embodiments, in steps (2) and (3), the total molar amount of the co-crystallization solvents 4-methyl-2-pentanone and acetone is 2.5-3 times the molar amount of dimethyl adipate, and the molar ratio of 4-methyl-2-pentanone to acetone is 1:0.5-0.7.

[0016] In some embodiments, in step (3), the temperature is lowered to 12-15°C at a cooling rate of 6-8°C / h; the temperature is lowered to 3-5°C at a cooling rate of 4-5°C / h.

[0017] In a specific embodiment, the preparation method of N,N,N',N'-tetrakis (2-hydroxypropyl) adipamide curing agent comprises the following steps: (1) Add ethylene glycol ethyl ether acetate and diisopropanolamine into the reactor, start stirring and heat to 125-130°C. After they are completely dissolved into a homogeneous phase, add the catalyst, and then pump dimethyl adipate into the automatic atomizing feeder. Control the diameter of the mist particles to 20-50 μm, and perform continuous atomizing feeding. The atomizing feeding time is controlled to 30-35 minutes. During the atomizing feeding process, the by-product methanol generated by the reaction is removed. After the atomizing feeding is completed, the temperature is lowered to 105-110°C, and the heat preservation reaction is continued; (2) Sampling and testing by gas chromatography. When the content of free dimethyl adipate is less than 0.5%, indicating that the basic reaction of dimethyl adipate is completed, the temperature is lowered to 90-95°C, and then a crystallization solvent 4-methyl-2-pentanone is added. After sufficient stirring to dissolve, the temperature is lowered to 30-35°C and kept warm. (3) Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add acetone as a crystallization solvent, stir evenly, reduce the stirring speed to 30-35 rpm, and then perform step-by-step cooling and crystallization. First, cool the temperature to 12-15°C at a cooling rate of 6-8°C / h, and then keep the temperature and stir for crystallization for 1-1.5 hours. Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring, and keep the crystallization at rest for 20-30 minutes to promote the formation and growth of grains. Then start the stirring again at a speed of 20-25 rpm, and cool the temperature to 3-5°C at a cooling rate of 4-5°C / h, and then continue to keep the temperature and stir for crystallization for 2-3 hours. When the amount of crystals precipitated no longer increases (the increase in crystals in 30 minutes is less than 0.5%), centrifuge to obtain wet material and filtrate. The wet material is dried to obtain the product. The filtrate is distilled and recycled.

[0018] In a second aspect, the present invention provides an N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent prepared by the above preparation method.

[0019] The obtained curing agent is white powder, with a hydroxyl equivalent of 94-100 g / mol, a yield of 94-96%, and a purity of 96-98%.

[0020] In a third aspect, the present invention provides a use of N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent in the preparation of high temperature yellowing resistant powder coatings.

[0021] Beneficial effects: The main raw materials of the product of the present invention are dimethyl adipate, diisopropanolamine, a catalyst and a solvent through atomization feeding, normal pressure staged reaction (atomization feeding and cooling after feeding) and staged crystallization. In the preparation method, a special solvent is used to increase the reaction temperature and reduce the viscosity of the reaction system during the synthesis process, and at the same time, the reaction efficiency is increased by continuous atomization feeding of dimethyl adipate. The use of special solvents reduces the viscosity of the system, and combined with the atomization continuous feeding method, sufficient and rapid mixing and reaction between the reaction raw materials are achieved. Then, low-temperature insulation reaction is adopted in the insulation synthesis process to reduce the occurrence of ester exchange side reactions during the insulation process. It has the advantages of no need for vacuuming throughout the process, good synthesis reaction effect, high product purity, and few by-products. With the help of the selected crystallization-aiding solvent combination, segmented crystallization and crystal growth methods are adopted to achieve efficient separation of by-products and products in the crystallization process. Finally, the product obtained after centrifugal drying has high yield and purity. The product is finally used in high-temperature resistant powder coatings. The cured coating film has excellent high-temperature yellowing resistance and good surface fineness. It can be used in fields with high requirements for high-temperature yellowing resistance and appearance.

[0022] The preparation method of the invention has simple process and is conducive to industrial application.

[0023] The present invention has been described in detail above, but the above embodiments are only illustrative in nature and are not intended to limit the present invention. In addition, this article is not limited by any theory described in the above prior art or invention content or the following examples. DETAILED DESCRIPTION

[0024] The present invention is further described below in conjunction with examples. It should be noted that the following examples are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0025] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0026] Hydroxyl equivalent: according to the corresponding method of GB T 27807-2011 "Curing agent for polyester powder coatings"; Yield: based on the amount of dimethyl adipate fed, actual product mass / theoretical product mass × 100%; Purity: Determined by HPLC.

[0027] Example 1 A method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent comprises the following steps: (1) 10 mol of ethylene glycol ethyl ether acetate and 21 mol of diisopropanolamine were added to the reactor, stirring was started and the mixture was heated to 125°C. After the mixture was completely dissolved into a homogeneous phase, sodium methoxide catalyst (the dosage was 0.2% of the mass of dimethyl adipate) was added. Then, 10 mol of dimethyl adipate was pumped into an automatic atomizing feeder (Shandong Xinhuiyuan Spray Technology Co., Ltd.), the diameter of the atomizing particles was controlled at 20-50 μm, and continuous atomizing feeding was performed. The atomizing feeding time was controlled at 30 min. During the atomizing feeding process, the byproduct methanol generated by the reaction was removed. After the atomizing feeding was completed, the mixture was cooled to 107°C and the reaction was continued at a temperature of 107°C. (2) Take a sample and test it by gas chromatography. When the content of free dimethyl adipate is less than 0.5%, it means that the basic reaction of dimethyl adipate is completed. Then cool it to 90°C, add 20 mol 4-methyl-2-pentanone, stir it thoroughly to dissolve, cool it to 30°C and keep it warm. (3) Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add 10 mol of acetone, stir evenly, reduce the stirring speed to 30 rpm, and then perform step-by-step cooling and crystallization. First, cool the temperature to 14°C at a cooling rate of 6°C / h, and then keep the temperature and stir for crystallization for 1 hour. Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring, and keep the crystallization at rest for 20 minutes to promote the formation and growth of the grains. Then start the stirring again at a speed of 20 rpm, and cool the temperature to 4°C at a cooling rate of 4°C / h, and then continue to keep the temperature and stir for crystallization for 2 hours. When the amount of crystals precipitated no longer increases (the increase in crystals in 30 minutes is less than 0.5%), centrifuge to obtain wet material and filtrate. The wet material is dried to obtain the product. The filtrate is distilled and recycled.

[0028] The obtained product is a white powder with a hydroxyl equivalent of 98.1 g / mol, a yield of 94.3% and a purity of 97.1%.

[0029] Example 2 A method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent comprises the following steps: (1) 12 mol of ethylene glycol ethyl ether acetate and 26 mol of diisopropanolamine were added to the reactor, stirring was started and the mixture was heated to 130°C. After the mixture was completely dissolved into a homogeneous phase, sodium methoxide catalyst (the dosage was 0.3% of the mass of dimethyl adipate) was added. Then, 12 mol of dimethyl adipate was pumped into the automatic atomizing feeder, the diameter of the atomizing particles was controlled to be 20-50 μm, and continuous atomizing feeding was performed. The atomizing feeding time was controlled to be 35 min. During the atomizing feeding process, the byproduct methanol generated by the reaction was removed. After the atomizing feeding was completed, the mixture was cooled to 110°C and the reaction was continued at a temperature of 100°C. (2) Take a sample and test it by gas chromatography. When the content of free dimethyl adipate is less than 0.5%, it means that the basic reaction of dimethyl adipate is completed. Then cool it to 90°C, add 20 mol of 4-methyl-2-pentanone, stir it thoroughly to dissolve, cool it to 35°C and keep it warm. (3) Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add 10 mol of acetone, stir evenly, reduce the stirring speed to 35 rpm, and then perform step-by-step cooling and crystallization. First, cool the temperature to 12°C at a cooling rate of 6°C / h, and then keep the temperature and stir for crystallization for 1 hour. Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring, and keep the crystallization at rest for 30 minutes to promote the formation and growth of the grains. Then start the stirring again at a speed of 25 rpm, and cool the temperature to 4°C at a cooling rate of 4°C / h, and then continue to keep the temperature and stir for crystallization for 2.5 hours. When the amount of crystals precipitated no longer increases (the increase in crystals in 30 minutes is less than 0.5%), centrifuge to obtain wet material and filtrate. The wet material is dried to obtain the product. The filtrate is distilled and recycled.

[0030] The obtained product is a white powder with a hydroxyl equivalent of 95.6 g / mol, a yield of 95.7% and a purity of 96.4%.

[0031] Example 3 A method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent comprises the following steps: (1) 11.2 mol of ethylene glycol ethyl ether acetate and 30 mol of diisopropanolamine were added to the reactor, stirring was started and the mixture was heated to 125°C. After the mixture was completely dissolved into a homogeneous phase, a catalyst of sodium hydroxide (the dosage was 0.4% of the mass of dimethyl adipate) was added. Then 14 mol of dimethyl adipate was pumped into the automatic atomizing feeder, the diameter of the atomizing particles was controlled to be 20-50 μm, and continuous atomizing feeding was performed. The atomizing feeding time was controlled to be 30 min. During the atomizing feeding process, the byproduct methanol generated by the reaction was removed. After the atomizing feeding was completed, the mixture was cooled to 105°C and the reaction was continued at a temperature of 105°C. (2) Take a sample and test it by gas chromatography. When the content of free dimethyl adipate is less than 0.5%, it means that the basic reaction of dimethyl adipate is completed. Then cool it to 95°C, add 23.3 mol 4-methyl-2-pentanone, stir it thoroughly to dissolve, cool it to 30°C and keep it warm. (3) Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add 11.7 mol of acetone, stir evenly, reduce the stirring speed to 30 rpm, and then perform step-by-step cooling and crystallization. First, cool the temperature to 12°C at a cooling rate of 6°C / h, and then keep the temperature and stir for crystallization for 1.5 hours. Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring, and keep the crystallization at rest for 20 minutes to promote the formation and growth of the grains. Then start the stirring again at a speed of 25 rpm, and cool the temperature to 4°C at a cooling rate of 4°C / h, and then continue to keep the temperature and stir for crystallization for 3 hours. When the amount of crystals precipitated no longer increases (the increase in crystals in 30 minutes is less than 0.5%), centrifuge to obtain wet material and filtrate. The wet material is dried to obtain the product. The filtrate is distilled and recycled.

[0032] The obtained product is a white powder with a hydroxyl equivalent of 97.5 g / mol, a yield of 95.2%, and a purity of 96.2%.

[0033] Example 4 A method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent comprises the following steps: (1) 11 mol of ethylene glycol ethyl ether acetate and 35 mol of diisopropanolamine were added to the reactor, stirring was started and the mixture was heated to 130°C. After the mixture was completely dissolved into a homogeneous phase, a catalyst of sodium hydroxide (the dosage was 0.5% of the mass of dimethyl adipate) was added. Then, 15 mol of dimethyl adipate was pumped into the automatic atomizing feeder, the diameter of the atomizing particles was controlled to be 20-50 μm, and continuous atomizing feeding was performed. The atomizing feeding time was controlled to be 35 min. During the atomizing feeding process, the byproduct methanol generated by the reaction was removed. After the atomizing feeding was completed, the mixture was cooled to 105°C and the reaction was continued at a temperature of 105°C. (2) Take a sample and test it by gas chromatography. When the content of free dimethyl adipate is less than 0.5%, it means that the basic reaction of dimethyl adipate is completed. Then cool it to 95°C, add 30 mol of 4-methyl-2-pentanone, stir it thoroughly to dissolve, cool it to 35°C and keep it warm. (3) Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add 15 mol of acetone, stir evenly, reduce the stirring speed to 35 rpm, and then perform step-by-step cooling and crystallization. First, cool the temperature to 12°C at a cooling rate of 6°C / h, and then keep the temperature and stir for crystallization for 1.5 hours. Detect the amount of crystals precipitated in the system. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring, and keep the crystallization at rest for 30 minutes to promote the formation and growth of the grains. Then start the stirring again at a speed of 20 rpm, and cool the temperature to 4°C at a cooling rate of 4°C / h, and then continue to keep the temperature and stir for crystallization for 3 hours. When the amount of crystals precipitated no longer increases (the increase in crystals in 30 minutes is less than 0.5%), centrifuge to obtain wet material and filtrate. The wet material is dried to obtain the product. The filtrate is distilled and recycled.

[0034] The obtained product is a white powder with a hydroxyl equivalent of 96.4 g / mol, a yield of 95.4% and a purity of 97.5%.

[0035] Comparative Example 1 The other steps are the same as in Example 1, except that the continuous atomization feeding of dimethyl adipate is changed to the continuous dripping feeding of dimethyl adipate.

[0036] The obtained product is a white powder with a hydroxyl equivalent of 101.4 g / mol, a yield of 90.3%, and a purity of 94.2%.

[0037] Comparative Example 2 The other steps are the same as those of Example 1, except that the heat preservation section of step (1) still uses a high temperature reaction (125-130°C).

[0038] The obtained product is a white powder with a hydroxyl equivalent of 99.7 g / mol, a yield of 91.8% and a purity of 95.3%.

[0039] Comparative Example 3 The other aspects are the same as those of Example 2, except that the formulated amount of the crystallization-aiding solvents 4-methyl-2-pentanone and acetone are directly added into the system, and the system is directly cooled to 4° C. and kept warm for crystallization. The obtained product is a white solid with obvious stickiness, a hydroxyl equivalent of 89 g / mol, a yield of 105%, and a purity of 91.5%.

[0040] Comparative Example 4 The product prepared by the method of Example 1 in CN201811463611.4 is used as Comparative Example 4.

[0041] Performance Testing The formula of high temperature yellowing resistant polyester resin powder coating is as follows in parts by weight: Polyester resin 550 Curing agent product 35 of embodiment and comparative example Titanium dioxide 200 Barium sulfate 185 Leveling agent 10 Brightener 10 Degassing agent 10 The polyester resin is a polyester resin product produced by Anhui Shenjian New Materials Co., Ltd., model SJ4D; the leveling agent model is HL968, the brightener model is HL701, and the degassing agent model is HA500, all of which are products produced by Huangshan Huahui Technology Co., Ltd., and other raw materials are common raw materials available on the market.

[0042] Coating film preparation: According to the above polyester powder coating formula, the materials are mixed evenly, extruded, tableted and crushed by a twin-screw extruder, and then the tablets are crushed and sieved (150-180 mesh) to make powder coating. The powder coating is sprayed on the surface treated tinplate substrate with an electrostatic spray gun, with a spraying thickness of 80-90μm, and cured at a high temperature of 200℃ / 10 min to obtain a powder coating film.

[0043] The application performance test of powder coating film is based on GB / T 21776-2008 "Guide to the Testing Standards of Powder Coatings and Their Coatings", and the adhesion grade is carried out in accordance with GB / T 9286-2021 "Scratch Test for Paints and Varnishes".

[0044] The results are shown in Table 1 below.

[0045] Table 1 Comparison of performance of products after coating in the embodiments and comparative examples

[0046] Note: The description of high temperature yellowing resistance in the table is defined as follows: (1) No yellowing when △b value ≤ 1; (2) Slight yellowing when △b value 1-2.5; (3) Obvious yellowing when △b value 2.5-3.5; (4) Severe yellowing when △b value ≥ 4; As can be seen from the above table, the product of the present invention adopts a specific synthesis and post-processing process without vacuum conditions. The prepared product not only has good synthesis effect and few by-products, but also the by-products are removed more thoroughly by means of a segmented crystallization process. The purity and yield of the final product are both high, with a purity of more than 96% and a yield of more than 94%. In terms of application performance testing, the coating film after high-temperature curing has a smooth and flat appearance, excellent fineness, good impact resistance, and passes 50cm positive and negative impacts. The coating film has a high gloss of more than 94%, and the coating film adhesion reaches level 0. In terms of high temperature resistance, no yellowing occurs in the coating film at 200°C or 230°C / 30min.

[0047] In Comparative Example 1, since the reaction was carried out by continuously dropping dimethyl adipate, the dimethyl adipate dropped into the kettle was difficult to be quickly and fully dispersed in the kettle, and the effect of the rapid dropwise reaction was not ideal. The content of by-products in the synthetic product was high, resulting in not only a significant decrease in the yield of the product after crystallization, but also a certain degree of decrease in the purity of the product, a significant increase in the hydroxyl equivalent, and a significant yellowing of the coating film finally prepared at 230°C / 30min; In Comparative Example 2, due to the continuous use of high temperature insulation reaction, the amount of by-products in the synthesis process increased significantly, the final product yield after crystallization decreased significantly to below 91.8%, and the purity also decreased slightly, resulting in slight yellowing of the finally prepared coating film under high temperature conditions of 230°C / 30min; Comparative Example 3 adopts a direct one-pot cooling crystallization method, resulting in faster product precipitation. During the crystallization process, the by-product and the excess diisopropanolamine in the raw material are wrapped to a certain extent. The impurity content of the product is high, resulting in a significant decrease in the hydroxyl equivalent of the product, and the purity is reduced to 91.5%. Due to the relatively large number of impurities, the yield reaches 105%, and the final product is not loose, and there is obvious stickiness. Due to the obvious deterioration of product quality, the impact resistance of the cured coating film has obvious defects, recoil cracking, and the coating film has low gloss and relatively poor high temperature resistance. Comparative Example 4 adopts the product of Example 1 in CN201811463611.4, which is a product prepared by vacuum synthesis. Due to the poor synthesis effect and many by-products, a direct cooling crystallization process is adopted, and dimethyl isopropanolamine with poor yellowing resistance is added in the later stage of crystallization, which affects the yellowing resistance of the product. The prepared product not only has a low yield of only about 60%, but also has a purity of only about 94%, and the prepared coating film shows slight yellowing after 200°C / 30min, and obvious yellowing at 230°C / 30min, which are significantly worse than the products of the present invention.

[0048] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and essence of the claims of the present invention; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, characterized in that: The following steps are involved: (1) Add ethylene glycol ethyl ether acetate and diisopropanolamine into a reaction kettle, start stirring and heating, and add a catalyst after they are completely dissolved into a homogeneous phase. Then, pump dimethyl adipate into an automatic atomizing feeder for continuous atomizing feeding. After the atomizing feeding is completed, cool down and continue to keep warm for reaction; (2) When the free dimethyl adipate content is less than 0.5%, the temperature is lowered, and then a crystallization solvent 4-methyl-2-pentanone is added. After sufficient stirring to dissolve, the temperature is lowered to 30-35°C and the crystallization is carried out by heat preservation; (3) When the amount of crystals precipitated reaches 25-30% of the theoretical mass of the product, add acetone as a crystallization solvent, stir evenly, and then perform step-by-step cooling and crystallization. First, cool to 12-15°C, keep warm and stir for crystallization for 1-1.5 hours. When the amount of crystals precipitated reaches 60-65% of the theoretical mass of the product, turn off the stirring and let the crystallization stand for 20-30 minutes. Then start the stirring again, cool to 3-5°C, keep warm and stir for crystallization for 2-3 hours. When the amount of crystals precipitated no longer increases, separate the wet material and dry it to obtain the curing agent product.

2. The preparation method according to claim 1, characterized in that: In step (1), the molar ratio of ethylene glycol ethyl ether acetate, diisopropanolamine and dimethyl adipate is 10-15:21-35:10-15.

3. The preparation method according to claim 1, characterized in that: In step (1), the catalyst is one or more selected from sodium methoxide, sodium ethoxide, sodium hydroxide and potassium hydroxide; the amount of the catalyst is 0.2-0.5% of the mass of dimethyl adipate.

4. The preparation method according to claim 1, characterized in that: In step (1), heating is performed to 125-130°C.

5. The preparation method according to claim 1, characterized in that: In step (1), the atomization feed is controlled to have a diameter of mist particles of 20-50 μm, and the atomization feed time is controlled to be 30-35 min.

6. The preparation method according to claim 1, characterized in that: In step (1), the temperature is lowered to 105-110°C for heat preservation reaction.

7. The preparation method according to claim 1, characterized in that: In steps (2) and (3), the total molar amount of the crystallization-aiding solvents 4-methyl-2-pentanone and acetone is 2.5-3 times the molar amount of dimethyl adipate, and the molar ratio of 4-methyl-2-pentanone to acetone is 1:0.5-0.

7.

8. The preparation method according to claim 1, characterized in that: In step (3), the temperature is lowered to 12-15°C at a cooling rate of 6-8°C / h; and the temperature is lowered to 3-5°C at a cooling rate of 4-5°C / h.

9. A N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent according to claim 9 in the preparation of high temperature resistant yellowing type powder coating.

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