An N,N,N',N'-tetrakis(2-hydroxypropyl) adipamide curing agent, its preparation method and applications

Abstract

The present invention provides an N,N,N',N'-tetrakis(2-hydroxypropyl) adipamide curing agent, a preparation method thereof and an application thereof, belonging to the technical field of powder coating curing agents. The curing agent is obtained by atomizing and feeding dimethyl adipate, diisopropanolamine, a catalyst and a solvent, and carrying out stepwise reaction under normal pressure and stepwise crystallization. There is no need to evacuate the vacuum throughout the process, the synthesis reaction effect is good, the product yield and purity are high, and the by-products are few. When used in high-temperature resistant powder coatings, the cured coating film has very excellent high-temperature yellowing resistance performance, and the surface fineness is good, and it can be used in fields with high requirements for high-temperature yellowing 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] A 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 a high-temperature resistant yellowing powder coating.

[0008] In order to achieve the above objects of the present invention, the following technical solutions are specifically adopted:

[0009] In a first aspect, the present invention provides a preparation method of an N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, comprising the following steps:

[0010] (1) Add ethylene glycol monoethyl ether acetate and diisopropanolamine into a reaction kettle, start stirring and heating. After complete dissolution into a homogeneous phase, add a catalyst, and then pump dimethyl adipate into an automatic atomizing feeder for continuous atomizing feeding. After the atomizing feeding is completed, lower the temperature and continue the heat preservation reaction;

[0011] (2) When the content of free dimethyl adipate is lower than 0.5%, lower the temperature, then add the co-crystallization solvent 4-methyl-2-pentanone. After fully stirring and dissolving, lower the temperature to 30-35°C and carry out heat preservation crystallization;

[0012] (3) When the amount of crystal precipitation reaches 25-30% of the theoretical mass of the product, add the co-crystallization solvent acetone, stir evenly, and then carry out stepwise cooling crystallization. First, lower the temperature to 12-15°C, keep stirring and crystallizing for 1-1.5 h. When the amount of crystal precipitation reaches 60-65% of the theoretical mass of the product, stop stirring and let it stand for crystallization for 20-30 min; then start stirring again and lower the temperature to 3-5°C, keep stirring and crystallizing for 2-3 h. When the amount of crystal precipitation no longer increases, separate to obtain the wet material, and obtain the curing agent product after drying.

[0013] In some embodiments, in step (1), the molar ratio of ethylene glycol monoethyl ether acetate, diisopropanolamine and dimethyl adipate is 10-15:21-35:10-15.

[0014] In some embodiments, in step (1), the catalyst is one or more selected from sodium methoxide, sodium ethoxide, sodium hydroxide, and potassium hydroxide; the catalyst dosage is 0.2-0.5% of the mass of dimethyl adipate.

[0015] In some embodiments, in step (1), heat to 125-130°C.

[0016] In some embodiments, in step (1), the atomizing feeding controls the droplet diameter within 20-50 μm and the atomizing feeding time within 30-35 min. During the atomizing feeding process, the by-product methanol generated by the reaction is removed simultaneously.

[0017] In some embodiments, in step (1), the temperature is lowered to 105 - 110 °C for holding reaction.

[0018] The ethylene glycol monoethyl ether acetate serves both as a reaction solvent and also participates as a crystallization solvent during crystallization.

[0019] The co-crystallization solvent is 4-methyl-2-pentanone and acetone, and the addition method is a staged addition method.

[0020] 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.

[0021] In some embodiments, in step (3), the temperature is lowered to 12 - 15 °C at a cooling rate of 6 - 8 °C / h; and then the temperature is lowered to 3 - 5 °C at a cooling rate of 4 - 5 °C / h.

[0022] In a specific embodiment, a method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent includes the following steps:

[0023] (1) Add ethylene glycol monoethyl ether acetate and diisopropanolamine into a reaction kettle, start stirring and heat to 125 - 130 °C. After completely dissolving into a homogeneous phase, add a catalyst, then pump dimethyl adipate into an automatic atomizing feeder, control the droplet diameter at 20 - 50 μm, carry out continuous atomizing feeding, control the atomizing feeding time within 30 - 35 min, and simultaneously remove the by-product methanol generated during the reaction during the atomizing feeding process; after the atomizing feeding is completed, lower the temperature to 105 - 110 °C and continue the holding reaction;

[0024] (2) Take a sample and detect it by gas chromatography. When the content of free dimethyl adipate is lower than 0.5%, it indicates that the reaction of dimethyl adipate is basically completed. Lower the temperature to 90 - 95 °C, then add the co-crystallization solvent 4-methyl-2-pentanone, fully stir and dissolve it, and then lower the temperature to 30 - 35 °C for holding;

[0025] (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.

[0026] 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.

[0027] 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%.

[0028] 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.

[0029] Beneficial effects:

[0030] 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.

[0031] The preparation method of the present invention has a simple process and is conducive to industrial application.

[0032] The present invention has been described in detail above. However, the above embodiments are essentially illustrative only and are not intended to limit the present invention. In addition, the present invention is not limited by any theory described in the foregoing prior art or the summary of the invention or the following examples. Detailed Description of the Invention

[0033] The present invention will be further described below in conjunction with embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0034] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0035] Hydroxyl equivalent: According to the corresponding method in GB / T 27807-2011 "Curing Agents for Polyester Powder Coatings";

[0036] Yield: Based on the feeding amount of dimethyl adipate, actual product quality / theoretical product quality × 100%;

[0037] Purity: Detected by high performance liquid chromatography.

[0038] Example 1

[0039] A preparation method of N,N,N',N'-tetrakis(2-hydroxypropyl) adipamide curing agent, comprising the following steps:

[0040] (1) Add 10 mol of ethylene glycol monoethyl ether acetate and 21 mol of diisopropanolamine into the reaction kettle, start stirring and heat to 125 °C. After completely dissolving into a homogeneous phase, add sodium methoxide as a catalyst (the dosage is 0.2% of the mass of dimethyl adipate), and then pump 10 mol of dimethyl adipate into an automatic atomizing feeder (Shandong Xinhuiyuan Spray Technology Co., Ltd.), control the droplet diameter at 20 - 50 μm, and carry out continuous atomizing feeding. The atomizing feeding time is controlled at 30 min, and the by-product methanol generated during the reaction is removed simultaneously during the atomizing feeding process; after the atomizing feeding is completed, cool down to 107 °C and continue the heat preservation reaction;

[0041] (2) Take a sample and detect it by gas chromatography. When the content of free dimethyl adipate is lower than 0.5%, it indicates that the reaction of dimethyl adipate is basically completed. Cool down to 90 °C, then add 20 mol of 4-methyl-2-pentanone, fully stir and dissolve, and then cool down to 30 °C and keep warm;

[0042] (3) Detect the amount of crystal precipitation in the system. When the amount of crystal precipitation reaches 25 - 30% of the theoretical mass of the product, add 10 mol of acetone, stir evenly, then reduce the stirring speed to 30 rpm, and then carry out stepwise cooling crystallization. First, cool down at a cooling rate of 6 °C / h to 14 °C, then keep stirring and crystallize for 1 h, and detect the amount of crystal precipitation in the system. When the amount of crystal precipitation reaches 60 - 65% of the theoretical mass of the product, turn off the stirring and let it stand and crystallize for 20 min to promote the formation and growth of crystal grains; then start stirring again at a speed of 20 rpm, and cool down at a cooling rate of 4 °C / h to 4 °C, then continue to keep stirring and crystallize for 2 h. When the amount of crystal precipitation no longer increases (the increase in crystal amount is less than 0.5% in 30 min), carry out centrifugal separation to obtain wet material and filtrate. The wet material is dried to obtain the product; the filtrate is recycled after rectification treatment.

[0043] 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%.

[0044] Example 2

[0045] A preparation method of N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, comprising the following steps:

[0046] (1) Add 12 mol of ethylene glycol monoethyl ether acetate and 26 mol of diisopropanolamine into the reaction kettle, start stirring and heat to 130 °C. After completely dissolving into a homogeneous phase, add sodium methoxide as a catalyst (the dosage is 0.3% of the mass of dimethyl adipate), then pump 12 mol of dimethyl adipate into the automatic atomizing feeder, control the droplet diameter within 20 - 50 μm, carry out continuous atomizing feeding, and control the atomizing feeding time within 35 min. During the atomizing feeding process, simultaneously remove the by-product methanol generated by the reaction; after the atomizing feeding is completed, cool down to 110 °C and continue the heat preservation reaction;

[0047] (2) Take a sample and detect it by gas chromatography. When the content of free dimethyl adipate is lower than 0.5%, it indicates that the reaction of dimethyl adipate is basically completed. Cool down to 90 °C, then add 20 mol of 4-methyl-2-pentanone, stir and dissolve it fully, then cool down to 35 °C and keep it warm;

[0048] (3)Detect the amount of crystal precipitation in the system. When the amount of crystal precipitation reaches 25 - 30% of the theoretical mass of the product, add 10 mol of acetone, stir evenly, then reduce the stirring speed to 35 rpm, and then carry out stepwise cooling crystallization. First, cool down at a cooling rate of 6 °C / h to 12 °C, then keep stirring and crystallize for 1 h, and detect the amount of crystal precipitation in the system. When the amount of crystal precipitation reaches 60 - 65% of the theoretical mass of the product, turn off the stirring and let it stand and crystallize for 30 min to promote the formation and growth of crystal grains; then start stirring again at a speed of 25 rpm, and cool down at a cooling rate of 4 °C / h to 4 °C, and then continue to keep stirring and crystallize for 2.5 h. When the amount of crystal precipitation no longer increases (the increase in crystal amount within 30 min is less than 0.5%), carry out centrifugal separation to obtain wet material and filtrate. The wet material is dried to obtain the product; the filtrate is recycled after rectification treatment.

[0049] 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%.

[0050] Example 3

[0051] A preparation method of N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, comprising the following steps:

[0052] (1)Add 11.2 mol of ethylene glycol monoethyl ether acetate and 30 mol of diisopropanolamine into the reaction kettle, start stirring and heat to 125 °C. After completely dissolving into a homogeneous phase, add the catalyst sodium hydroxide (the dosage is 0.4% of the mass of dimethyl adipate), and then pump 14 mol of dimethyl adipate into the automatic atomizing feeder, control the droplet diameter at 20 - 50 μm, and carry out continuous atomizing feeding. The atomizing feeding time is controlled within 30 min, and the by-product methanol generated during the reaction is removed simultaneously during the atomizing feeding process; after the atomizing feeding is completed, cool down to 105 °C and continue the heat preservation reaction;

[0053] (2)Take a sample and detect it by gas chromatography. When the content of free dimethyl adipate is lower than 0.5%, it indicates that the reaction of dimethyl adipate is basically completed. Cool down to 95 °C, then add 23.3 mol of 4-methyl-2-pentanone, stir and dissolve it fully, and then cool down to 30 °C and keep it warm;

[0054] (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.

[0055] 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%.

[0056] Example 4

[0057] A method for preparing N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent comprises the following steps:

[0058] (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.

[0059] (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.

[0060] (3) Detect the amount of crystal precipitation in the system. When the amount of crystal precipitation reaches 25 - 30% of the theoretical mass of the product, add 15 mol of acetone, stir evenly, then reduce the stirring speed to 35 rpm, and then carry out stepwise cooling crystallization. First, cool down at a cooling rate of 6 °C / h to 12 °C, then keep stirring and crystallize for 1.5 h, and detect the amount of crystal precipitation in the system. When the amount of crystal precipitation reaches 60 - 65% of the theoretical mass of the product, turn off the stirring and let it stand and crystallize for 30 min to promote the growth of crystal grains; then start stirring again at a speed of 20 rpm, and cool down at a cooling rate of 4 °C / h to 4 °C, then continue to keep stirring and crystallize for 3 h. When the amount of crystal precipitation no longer increases (the increase in crystal amount within 30 min is less than 0.5%), carry out centrifugal separation to obtain wet material and filtrate. The wet material is dried to obtain the product; the filtrate is recycled after rectification treatment.

[0061] 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%.

[0062] Comparative Example 1

[0063] Other conditions are the same as in Example 1, except that the continuous atomized feeding of dimethyl adipate is changed to continuous dropping feeding of dimethyl adipate.

[0064] 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%.

[0065] Comparative Example 2

[0066] Other conditions are the same as in Example 1, the difference is that the heat preservation section in step (1) still uses high-temperature reaction (125 - 130 °C).

[0067] 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%.

[0068] Comparative Example 3

[0069] Other conditions are the same as in Example 2, the difference is that the crystallization assistant solvents 4-methyl-2-pentanone and acetone in the formula amount are directly added to the system, and directly cooled to 4 °C and kept for crystallization.

[0070] The obtained product is a white solid, which is significantly sticky, with a hydroxyl equivalent of 89 g / mol, a yield of 105%, and a purity of 91.5%.

[0071] Comparative Example 4

[0072] The product prepared by the method of Example 1 in CN201811463611.4 is used as Comparative Example 4.

[0073] Performance Test

[0074] The formula of the high-temperature resistant yellowing polyester resin powder coating is as follows by weight:

[0075] Polyester resin 550

[0076] Curing agent product of the example and comparative example 35

[0077] Titanium dioxide 200

[0078] Barium sulfate 185

[0079] Leveling agent 10

[0080] Brightening agent 10

[0081] Degassing agent 10

[0082] Among them, the polyester resin adopts the polyester resin product produced by Anhui Shenjian New Materials Co., Ltd., model SJ4D; the leveling agent model is HL968, the brightening agent 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 commercially available raw materials.

[0083] Preparation of the coating film:

[0084] Mix the various materials according to the above polyester powder coating formula, extrude, press into tablets, and crush them with a twin-screw extruder, and then crush the tablet material and sieve it (150 - 180 meshes) to make a powder coating. The powder coating is sprayed on the surface-treated tinplate substrate with an electrostatic spray gun, the spraying thickness is 80 - 90 μm, and it is cured at a high temperature of 200 °C for 10 min to obtain the powder coating film.

[0085] The detection of the application performance of the powder coating film is based on GB / T 21776-2008 "Test Standard Guide for Powder Coatings and Their Coatings", and the adhesion grade is carried out according to GB / T 9286-2021 "Paints and Varnishes - Cross-Cut Test".

[0086] The results are shown in Table 1 below.

[0087] Table 1 Performance comparison of the products after film coating in the example and comparative example

[0088]

[0089] Note:

[0090] The description of the high-temperature resistant yellowing performance in the table is defined as: (1) When the △b value ≤ 1, there is no yellowing; (2) When the △b value is 1 - 2.5, it is slightly yellowing; (3) When the △b value is 2.5 - 3.5, it is obviously yellowing; (4) When the △b value ≥ 4, it is severely yellowing;

[0091] As can be seen from the above table, the product of the present invention adopts a specific synthesis and post-treatment process under non-vacuum conditions. The prepared product not only has good synthesis effect and few by-products, but also the by-products can be removed more thoroughly by means of fractional crystallization process. The purity and yield of the final product are both relatively high, with the purity basically above 96% and the yield basically above 94%. In terms of application performance testing, the film after high-temperature curing is apparently flat, smooth and has excellent fineness, good impact resistance, passing both front and back impacts at 50 cm, with a relatively high film gloss reaching above 94%, the film adhesion reaching grade 0, and in terms of high-temperature resistance, there is no yellowing of the film after either 200 °C or 230 °C / 30 min.

[0092] 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 disperse quickly and fully in the kettle, and the effect of the rapid dropping 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, and a significant increase in the hydroxyl equivalent. Finally, the prepared film showed obvious yellowing at 230 °C / 30 min.

[0093] In Comparative Example 2, due to continuously using high-temperature heat preservation reaction, the amount of by-products in the synthesis process increased significantly. Finally, the yield of the product after crystallization decreased significantly, dropping below 91.8%, and the purity also decreased slightly, resulting in slight yellowing of the finally prepared film under the high-temperature condition of 230 °C / 30 min.

[0094] In Comparative Example 3, due to adopting the direct one-pot cooling crystallization method, the product precipitated relatively quickly, and there was a certain degree of encapsulation of by-products and excessive diisopropanolamine in the raw materials during the crystallization process. The impurity content of the product was relatively high, resulting in a significant decrease in the hydroxyl equivalent of the product, and the purity decreased to 91.5%. Due to relatively more impurities, the yield reached 105%. The final product was not loose and showed obvious stickiness. Due to the obvious deterioration of the product quality, the impact resistance of the cured film showed obvious defects, with reverse impact cracking, and the film gloss was relatively low, and the high-temperature resistance performance was also relatively poor.

[0095] Comparative Example 4 adopted the product of Example 1 in CN201811463611.4, which was a product prepared by vacuum synthesis. Due to poor synthesis effect and many by-products, and adopting a direct cooling crystallization process, and adding dimethyl isopropanolamine with poor yellowing resistance in the later stage of crystallization, the yellowing resistance of the product was affected. The prepared product not only had a low yield, only reaching about 60%, but also the purity only reached about 94%. Moreover, the prepared film showed slight yellowing after 200 °C / 30 min and obvious yellowing at 230 °C / 30 min, all of which were significantly worse than the product of the present invention.

[0096] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: within the scope not departing from the spirit and essence defined by the claims of the present invention, the technical solutions described in the foregoing embodiments may be modified, or some or all of the technical features may be equivalently replaced; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A preparation method of N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide curing agent, characterized in that, It includes the following steps: (1) Add ethylene glycol monoethyl ether acetate and diisopropanolamine into the reaction kettle, start stirring and heat to 125 - 130 °C. After completely dissolving into a homogeneous phase, add a catalyst, then pump dimethyl adipate into the automatic atomizing feeder for continuous atomizing feeding. After the atomizing feeding is completed, cool down to 105 - 110 °C and continue the heat preservation reaction; (2) When the content of free dimethyl adipate is lower than 0.5%, cool down, then add the co-crystallization solvent 4-methyl-2-pentanone. After fully stirring and dissolving, cool down to 30 - 35 °C and carry out heat preservation crystallization; (3) When the amount of crystal precipitation reaches 25 - 30% of the theoretical mass of the product, add the co-crystallization solvent acetone, stir evenly, and then carry out stepwise cooling crystallization. First, cool down to 12 - 15 °C, keep stirring and crystallize for 1 - 1.5 h. When the amount of crystal precipitation reaches 60 - 65% of the theoretical mass of the product, stop stirring and let it stand for crystallization for 20 - 30 min; then start stirring again and cool down to 3 - 5 °C, keep stirring and crystallize for 2 - 3 h. When the amount of crystal precipitation no longer increases, separate to obtain the wet material, and obtain the curing agent product after drying.

2. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of ethylene glycol monoethyl 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 several selected from sodium methoxide, sodium ethoxide, sodium hydroxide, potassium hydroxide; the dosage 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), the atomizing feeding controls the fog particle diameter within 20 - 50 μm and controls the atomizing feeding time within 30 - 35 min.

5. The preparation method according to claim 1, characterized in that, 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.

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

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