CN114230764B - Water-based closed polyisocyanate curing agent and preparation method and application thereof - Google Patents

Abstract

The invention belongs to the technical field of paint curing agents, and discloses a water-based closed polyisocyanate curing agent and a preparation method and application thereof. The preparation method comprises: mixing sulfonate, diisocyanate trimer and catalyst, performing the first reaction, adding bisulfite after the reaction, and performing the second reaction to obtain the water-based blocked poly Isocyanate curing agent. The water-based blocked polyisocyanate curing agent provided by the invention uses bisulfite as the blocking agent, has low unblocking temperature, excellent water dispersibility and storage stability, and can be applied to various baking coatings. The water-based polyurethane paint prepared by using the water-based blocked polyisocyanate curing agent provided by the invention has strong stability, little change in storage viscosity at high temperature, and the formed paint film has high gloss and high transparency.

Description

A kind of water-based blocked polyisocyanate curing agent and its preparation method and application

technical field

The invention belongs to the technical field of coating curing agents, and in particular relates to a water-based closed polyisocyanate curing agent and its preparation method and application.

Background technique

With people's emphasis on environmental protection and the continuous improvement of environmental protection requirements, water-based coatings have undoubtedly become one of the important development directions of today's coating technology. Among them, by using water-based polyurethane coatings with cross-linking effects of isocyanate compounds, the obtained films have excellent abrasion resistance, chemical resistance and stain resistance, and are widely used.

Water-based isocyanate curing agent has excellent performance. Due to the high reactivity of isocyanate groups, it can be cross-linked and cured with polymers with active hydrogen atoms such as alcohols and amines at room temperature, so it can basically only be used in double In component coatings, its application is limited. Moreover, the water-based isocyanate curing agent is extremely sensitive to water in the environment because of the introduction of a large number of hydrophilic groups, which greatly increases the storage difficulty of the water-based isocyanate curing agent. In order to solve this technical problem, a water-based blocked polyisocyanate curing agent emerged as the times require. The active isocyanate group is protected by a small molecule blocking agent, so that the crosslinking agent can coexist stably with other components in the coating at room temperature. It also greatly reduces the difficulty of storage.

At present, blocked polyisocyanate is mostly prepared by reacting HDI trimer (hexamethylene diisocyanate trimer) or IPDI trimer with 3,5-dimethylpyrazole or methyl ethyl ketone oxime as blocking agent. Then mix it with acrylic resin or amino resin in a certain proportion to prepare automotive coatings. The product has the advantages of good chemical resistance and yellowing resistance, but its unsealing temperature (baking temperature) is relatively high, generally 100°C-150°C ℃; and most of them contain a large amount of low boiling point solvents.

Therefore, there is an urgent need to provide a water-based blocked polyisocyanate curing agent, which can reduce the amount of low-boiling solvents, reduce the unblocking temperature, and the prepared polyurethane coating has good storage stability.

Contents of the invention

The present invention aims to solve at least one of the technical problems in the above-mentioned prior art. Therefore, the present invention proposes a water-based blocked polyisocyanate curing agent and its preparation method and application. The water-based blocked polyisocyanate curing agent provided by the invention can reduce the consumption of low-boiling solvents and lower the unblocking temperature, and the prepared polyurethane coating has good storage stability.

The first aspect of the present invention provides a method for preparing a water-based blocked polyisocyanate curing agent.

Concrete, a kind of preparation method of water-based closed polyisocyanate curing agent comprises the following steps:

The sulfonate, diisocyanate trimer and catalyst are mixed for the first reaction, and after the reaction is completed, bisulfite is added for the second reaction to obtain the water-based blocked polyisocyanate curing agent.

Preferably, the sulfonate is sulfamate; further preferably, the sulfamate is sodium 2-tert-butyl-4-(cyclohexylamino)-1-butanesulfonate, 2- At least one of sodium tert-butyl-3-(cyclohexylamino)-1-propanesulfonate or sodium 2-isoamyl-3-(cyclohexylamino)-1-propanesulfonate; by using amino Sulfonate and diisocyanate trimer synthesize polyisocyanate, which provides hydrophilic property for polyisocyanate, so as to ensure that water-based polyisocyanate curing agent has excellent dispersibility without diluting with solvent.

Preferably, the diisocyanate trimer is selected from isophorone diisocyanate trimer, hexamethylene diisocyanate trimer, toluene diisocyanate trimer or diphenylmethane diisocyanate trimer At least one; further preferably, the diisocyanate trimer is selected from isophorone diisocyanate trimer and/or hexamethylene diisocyanate trimer. The reaction of hexamethylene diisocyanate trimer and isophorone diisocyanate trimer is mild and easy to control during the hydrophilization process.

Preferably, the -NCO content in the diisocyanate trimer is 18%wt-28%wt; further preferably, the -NCO content in the diisocyanate trimer is 21%wt-24%wt.

Preferably, the temperature of the first reaction is 70-110°C, and the time of the first reaction is 1-15h; more preferably, the temperature of the first reaction is 80-100°C, and the time of the first reaction is 1-15h; The time for the first reaction is 3-10h.

Preferably, the bisulfite is at least one selected from sodium bisulfite, potassium bisulfite, magnesium bisulfite or calcium bisulfite.

Preferably, in the preparation method, by weight, the sulfonate is 1-8 parts, the diisocyanate trimer is 80-120 parts, and the bisulfite is 40-70 parts .

Further preferably, in the preparation method, by weight, the sulfonate is 1.5-5 parts, the diisocyanate trimer is 95-100 parts, and the bisulfite is 50-60 parts share.

Preferably, the catalyst is nitrogen nitrogen dimethylbenzylamine. In parts by weight, the catalyst is 0.15-0.5 parts.

Preferably, the temperature of the second reaction is 40-70°C; the time of the second reaction is 2-10h; more preferably, the temperature of the second reaction is 50-60°C; the The time for the second reaction is 3-8h.

Preferably, the second reaction ends when the mass content of -NCO in the reaction system is less than 0.05%. The end point of the second reaction is determined by the mass content of isocyanate falling below 0.05%, and the content of isocyanate is measured by a toluene-di-n-butylamine titration method. By controlling the content of -NCO in the reaction system, it is avoided that too much residual -NCO reacts with water to extend the chain when diluting with water in the subsequent preparation of coatings and other processes, making the system viscosity too high, which is not conducive to dispersion and use.

Preferably, the first reaction and the second reaction are performed under protective gas. Such as nitrogen.

Preferably, the preparation method of the water-based blocked polyisocyanate curing agent comprises the following steps:

Mix sulfonate, diisocyanate trimer and catalyst, react at 70-110°C for 1-15 hours, add bisulfite after the reaction, and react at 40-70°C until the mass content of -NCO is less than 0.05 % to end the reaction to obtain the water-based blocked polyisocyanate curing agent.

Further preferably, the preparation method of the water-based blocked polyisocyanate curing agent comprises the following steps:

Mix sulfonate, diisocyanate trimer and catalyst, react at 80-100°C for 3-10 hours, add bisulfite after the reaction, and react at 50-60°C until the content of -NCO is less than 0.05% The reaction is completed at wt to obtain the water-based blocked polyisocyanate curing agent.

The second aspect of the present invention provides a water-based blocked polyisocyanate curing agent.

Specifically, a water-based blocked polyisocyanate curing agent is prepared by the above preparation method, the solid content of the polyisocyanate curing agent is 99-100%, and the sealing rate is greater than 99.9%.

The third aspect of the present invention provides an application of a water-based blocked polyisocyanate curing agent.

Specifically, the application of the water-based blocked polyisocyanate curing agent in polyurethane coatings. Preferably, the polyurethane coating is water-based polyurethane coating.

A water-based polyurethane coating, comprising A component and B component, the A component includes a water-based polyurethane dispersion, and the B component includes the above-mentioned water-based blocked polyisocyanate curing agent.

Further preferably, the A component also includes a wetting agent, a thickener, a leveling agent and a solvent.

Preferably, the mass ratio of the A component to the B component is (3-8):1.

Preferably, the preparation method of the water-based polyurethane coating includes the following steps: mixing the A component and the B component to prepare. In fact, the A component and the B component in the waterborne polyurethane coating provided by the present invention can be mixed and stored, which has good storage stability; the A component and the B component can also be mixed Store separately and mix until ready to use. The specific method can be selected according to different environments and needs.

The invention uses hydrogen sulfite to seal the water-based polyisocyanate obtained by reacting sulfonate and diisocyanate trimer to prepare the water-based blocked polyisocyanate curing agent. Specifically, in the preparation, the catalyst existing in the reaction system is used as a phase transfer catalyst of bisulfite (sealing agent) to block the isocyanate group of the water-based polyisocyanate. The reaction conditions are mild and no additional catalyst is needed, which makes the system more stable during later storage; the prepared water-based blocked polyisocyanate curing agent has a low unblocking temperature (about 75°C), and can be applied to various baking coatings; The waterborne polyurethane paint prepared by using it has strong stability, little change in viscosity when stored at high temperature, and the formed paint film has high gloss.

Compared with the prior art, the beneficial effects of the present invention are as follows:

(1) The water-based blocked polyisocyanate curing agent provided by the present invention uses bisulfite as a blocking agent, which has excellent water dispersibility and storage stability, and has a low unblocking temperature (not exceeding 80°C), and can be applied to Various baking coatings.

(2) The water-based polyurethane coating prepared by the water-based blocked polyisocyanate curing agent provided by the invention has strong stability, little change in storage viscosity at high temperature, high gloss and high transparency of the formed paint film.

(3) The water-based closed polyisocyanate curing agent of the present invention can reduce the consumption of low-boiling point solvents, and does not need to use any high-boiling point solvents or ketone solvents to reduce viscosity, and VOC is effectively controlled in curing agents and coatings; and its The preparation process is simple and environmentally friendly.

Detailed ways

In order to make those skilled in the art understand the technical solution of the present invention more clearly, the following examples are listed for illustration. It should be pointed out that the following examples do not limit the protection scope of the present invention.

Twin 4100、/>

Twin 810购买于赢创迪高公司，增稠剂COAPUR XS71购买于法国高泰公司。其余原料、试剂或装置如无特殊说明，均可从常规商业途径得到，或者可以通过现有已知方法得到。In the following examples, comparative examples, application examples or comparative application examples, the waterborne polyurethane dispersion Bayhydrol@2470 was purchased from Covestro Polymers (China) Co., Ltd., wetting agent

Twin 4100, />

Twin 810 was purchased from Evonik Tego, and the thickener COAPUR XS71 was purchased from Coatex, France. The rest of the raw materials, reagents or devices, unless otherwise specified, can be obtained from conventional commercial channels, or can be obtained by existing known methods.

Example 1

A preparation method of a water-based closed polyisocyanate curing agent, comprising the following steps:

Prepare raw materials according to the recipe shown in Table 1. Put 97.1 parts by mass of hexamethylene diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 2.8 parts by mass of 2-tert-butyl-4-( Cyclohexylamino)-1-butanesulfonate sodium, 0.2 parts by mass of nitrogen nitrogen dimethylbenzylamine, after reacting at 80°C for 6 hours, take a sample to detect the isocyanate content (the determination of the isocyanate content uses HGT 2409 -Toluene-di-n-butylamine titration method in 1992), add sodium bisulfite when approaching the theoretical value (difference from the theoretical value of 0.05% mass content is not more than 0.5%), react at 55 ° C for 5 hours, and then take a sample The content of isocyanate is detected, and the reaction is terminated when the mass content of isocyanate is less than 0.05%, so as to obtain a water-based blocked polyisocyanate curing agent. The solid content of the prepared water-based blocked polyisocyanate curing agent was 99.7%, and the sealing rate was 99.97%. (By measuring the mass content of -NCO, calculate the sealing rate, sealing rate=(mass content of -NCO before sealing-mass content of -NCO after sealing)/mass content of -NCO before sealing.)

Table 1

Raw material type parts by mass Hexamethylene diisocyanate trimer 97.1 Sodium 2-tert-butyl-4-(cyclohexylamino)-1-butanesulfonate 2.8 Nitrogen dimethyl benzylamine 0.2 sodium bisulfite 60

Example 2

A preparation method of a water-based closed polyisocyanate curing agent, comprising the following steps:

Prepare raw materials according to the formula shown in Table 2. Put 96.9 parts by mass of hexamethylene diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 3.0 parts by mass of 2-tert-butyl-4- (Cyclohexylamino)-1-sodium butanesulfonate, 0.35 parts by mass of nitrogen nitrogen dimethyl benzylamine, after reacting at 90°C for 4.5 hours, take a sample to detect the isocyanate content (the determination of the isocyanate content uses HGT 2409 -Toluene-di-n-butylamine titration method in 1992), sodium bisulfite was added when approaching the theoretical value (not more than 0.5% difference from the theoretical value mass content of 0.05%), then reacted at 55°C for 5 hours, and waited When the mass content of isocyanate group is less than 0.05%, the reaction is terminated to obtain a water-based blocked polyisocyanate curing agent. The solid content of the prepared water-based blocked polyisocyanate curing agent was 99.5%, and the sealing rate was 99.5%.

Table 2

Raw material type parts by mass Hexamethylene diisocyanate trimer 96.9 Sodium 2-tert-butyl-4-(cyclohexylamino)-1-butanesulfonate 3.0 Nitrogen dimethyl benzylamine 0.35 sodium bisulfite 55

Example 3

A preparation method of a water-based closed polyisocyanate curing agent, comprising the following steps:

Prepare raw materials according to the formula shown in Table 3. Put 97.1 parts by mass of hexamethylene diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 2.8 parts by mass of 2-tert-butyl-4- (Cyclohexylamino)-1-sodium butanesulfonate, 0.5 parts by mass of nitrogen dimethylbenzylamine; after reacting at 80°C for 5.5 hours, take a sample to detect the isocyanate content (the determination of the isocyanate content uses HGT 2409 -Toluene-di-n-butylamine titration method in 1992), add sodium bisulfite when approaching the theoretical value (difference from the theoretical value of 0.05% mass content is no more than 0.5%); then react at 55°C for 5 hours, wait for When the mass content of isocyanate group is less than 0.05%, the reaction is terminated to obtain a water-based blocked polyisocyanate curing agent. The solid content of the prepared water-based blocked polyisocyanate curing agent was 99.5%, and the sealing rate was 99.96%.

table 3

Raw material type parts by mass Hexamethylene diisocyanate trimer 97.1 Sodium 2-tert-butyl-4-(cyclohexylamino)-1-butanesulfonate 2.8 Nitrogen dimethyl benzylamine 0.5 sodium bisulfite 58

Example 4

A preparation method of a water-based closed polyisocyanate curing agent, comprising the following steps:

Prepare raw materials according to the recipe shown in Table 4. Put 97.1 parts by mass of isophorone diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 2.8 parts by mass of 2-tert-butyl-3-( Cyclohexylamino)-1-propanesulfonate sodium, 0.2 parts by mass of nitrogen nitrogen dimethylbenzylamine, after reacting at 80°C for 6 hours, take a sample to detect the isocyanate content (the determination of the isocyanate content uses HGT 2409- Toluene-di-n-butylamine titration method in 1992), potassium bisulfite was added when approaching the theoretical value (not more than 0.5% difference from the theoretical value mass content of 0.05%), reacted at 55°C for 5 hours, and then sampled for detection The content of isocyanate radicals, the reaction is completed when the mass content of isocyanate radicals is less than 0.05%, and a water-based blocked polyisocyanate curing agent is obtained. The solid content of the prepared water-based blocked polyisocyanate curing agent was 99.8%, and the sealing rate was 99.97%.

Table 4

Comparative example 1

The difference between Comparative Example 1 and Example 3 is that an equal amount of methyl ethyl ketone oxime is used instead of sodium bisulfite as the blocking agent, and the rest of the raw materials and preparation methods are the same as in Example 3.

Concrete, a kind of preparation method of water-based closed polyisocyanate curing agent comprises the following steps:

Put 97.1 parts by mass of hexamethylene diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 2.8 parts by mass of 2-tert-butyl-4- (Cyclohexylamino)-1-sodium butanesulfonate, 0.5 parts by mass of nitrogen dimethylbenzylamine; after reacting at 80°C for 5.5 hours, take a sample to detect the isocyanate content (the determination of the isocyanate content uses HGT 2409 -Toluene-di-n-butylamine titration method in 1992), add 58 parts of methyl ethyl ketone oxime when approaching the theoretical value (difference from the theoretical value of 0.05% mass content is not more than 0.5%); then react at 55°C for 5 hours, wait for When the mass content of isocyanate group is less than 0.05%, the reaction is terminated to obtain a water-based blocked polyisocyanate curing agent. The solid content of the prepared water-based blocked polyisocyanate curing agent was 99.8%, and the sealing rate was 99.95%.

Comparative example 2

The difference between Comparative Example 2 and Example 3 is that sodium 2-tert-butyl-4-(cyclohexylamino)-1-butanesulfonate is not added, and the remaining raw materials and preparation methods are the same as in Example 3.

A preparation method of a blocked polyisocyanate curing agent, comprising the following steps:

Prepare raw materials according to the formula shown in Table 3. Put 97.1 parts by mass of hexamethylene diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 0.5 parts by mass of nitrogen nitrogen dimethylbenzylamine and 58 parts by mass of sodium bisulfite; react at 55° C. for 5 hours, and stop the reaction when the mass content of isocyanate is less than 0.05%, to obtain a blocked polyisocyanate curing agent. The solid content of the obtained blocked polyisocyanate curing agent was 99.9%, and the blocking rate was 99.96%.

Comparative example 3

The difference between Comparative Example 3 and Example 3 is that no blocking agent sodium bisulfite is added, and the rest of the raw materials and preparation methods are the same as in Example 3.

A preparation method of water-based polyisocyanate curing agent, comprising the following steps:

Put 97.1 parts by mass of hexamethylene diisocyanate trimer into a four-necked flask equipped with a stirrer, a thermometer and a nitrogen inlet, raise the temperature to 60°C, and add 2.8 parts by mass of 2-tert-butyl-4- (Cyclohexylamino)-1-sodium butanesulfonate, 0.5 parts by mass of nitrogen dimethylbenzylamine; after reacting at 80°C for 5.5 hours, take a sample to detect the isocyanate content (the determination of the isocyanate content uses HGT 2409 -1992 in the toluene-di-n-butylamine titration method), the reaction is completed when the mass content of isocyanate <0.05%, and the water-based polyisocyanate curing agent is obtained. The obtained aqueous polyisocyanate curing agent has a solid content of 99.9%, is not blocked, and has no blocking rate data.

Application example 1-4

The water-based blocked polyisocyanate curing agents prepared in Examples 1-4 were used to prepare water-based polyurethane coatings respectively.

Concrete, the preparation method of waterborne polyurethane coating comprises the following steps:

Twin 4100、0.15％润湿剂/>

Twin 810、2.5％二丙二醇丁醚、2.5％二丙二醇单甲醚、0.25％增稠剂COAPUR XS71、0.35％水性流平剂BYK-333和14％水，混合，制A组分；(1) By mass percentage, take 80% waterborne polyurethane dispersion Bayhydrol@2470, 0.25% wetting agent

Twin 4100, 0.15% wetting agent/>

Twin 810, 2.5% dipropylene glycol butyl ether, 2.5% dipropylene glycol monomethyl ether, 0.25% thickener COAPUR XS71, 0.35% water-based leveling agent BYK-333 and 14% water, mixed to prepare component A;

(2) According to the mass ratio of components A and B as 5:1, weigh component B (the water-based blocked polyisocyanate curing agent prepared in Example 1-4) and add it to component A, mix well , to prepare waterborne polyurethane coatings.

Comparative application examples 1-3

The polyisocyanate curing agent prepared in Comparative Examples 1-3 was used to prepare waterborne polyurethane coatings.

Concrete, the preparation method of waterborne polyurethane coating comprises the following steps:

Twin 4100、0.15％润湿剂/>

Twin 810、2.5％二丙二醇丁醚、2.5％二丙二醇单甲醚、0.25％增稠剂COAPUR XS71、0.35％水性流平剂BYK-333和14％水，混合，制A组分；(1) By mass percentage, take 80% waterborne polyurethane dispersion Bayhydrol@2470, 0.25% wetting agent

Twin 4100, 0.15% wetting agent/>

Twin 810, 2.5% dipropylene glycol butyl ether, 2.5% dipropylene glycol monomethyl ether, 0.25% thickener COAPUR XS71, 0.35% water-based leveling agent BYK-333 and 14% water, mixed to prepare component A;

(2) According to the mass ratio of A and B components being 5:1, respectively weigh B component (the polyisocyanate curing agent prepared in Comparative Example 1-3) and add it to A component, after mixing evenly, the obtained Waterborne polyurethane coating.

Product Effect Test

Use the water-based polyurethane coatings prepared in Application Examples 1-4 and Comparative Application Examples 1-3 respectively to prepare paint films with a 150 μm wet film preparer, and bake them at the unsealing temperature to obtain the paint film after drying and crosslinking . Conduct performance tests on water-based blocked polyisocyanate curing agents, water-based polyurethane coatings, and the prepared paint films. The test of water-based blocked polyisocyanate curing agent includes the test of water solubility and unblocking temperature, the test of water-based polyurethane coating includes the test of high temperature storage (50 ℃ for 6 months) stability test (viscosity, paint film gloss), Tests on paint films include a paint film gloss test. The basic test method is as follows:

(1) Measurement method of unblocking temperature: After dissolving the water-based blocked isocyanate curing agent with acetone, add a few drops of phenol, and heat up. The temperature at which turbidity occurs is the initial unblocking temperature of the sample.

(2) Viscosity measurement standard: GB/T 1723-1993.

(3) Measurement standard for paint film gloss: GBT 9754-2007.

The test results are shown in Table 5.

Table 5 Water-based blocked polyisocyanate curing agent technical indicators and water-based polyurethane coating test results

It can be seen from Table 5 that the water-based blocked polyisocyanate curing agent provided by the examples of the present invention has excellent water solubility, and often forms very fine particles after being dispersed in water (90% of the particle size of the system is less than 0.9 μm), thereby improving the water-based The transparency and gloss of polyurethane coatings; the unblocking temperature of the water-based blocked polyisocyanate curing agent is as low as 70 ° C; the water-based polyurethane coatings prepared by it have strong stability, and the viscosity changes little after being stored at 50 ° C for 6 months; The gloss of the paint film prepared by using it is high, reaching 90° and above, and the paint film prepared after the water-based polyurethane coating is stored at 50°C for 6 months is still high in gloss, basically unchanged, and the water-based polyurethane coating has good storage stability. And the unsealing temperature in comparative application example 1 is as high as 140 ℃; Although the curing agent also has lower unsealing temperature in comparative application example 2, its water solubility is poor, and the glossiness of its paint film is obviously not as good as application example; comparative application example 3 Poor storage stability. At the same time, according to tests, the haze of the paint film prepared by the application example of the present invention is lower than 20%, the light transmittance is higher than 85%, and has relatively high transparency.

Claims (10)

1. a preparation method of polyisocyanate curing agent, is characterized in that, comprises the following steps: sulfonate, diisocyanate trimer and catalyzer are mixed, carry out first reaction, after reaction finishes, add bisulfite , carrying out the second reaction to obtain the water-based blocked polyisocyanate curing agent; The catalyst is nitrogen nitrogen dimethyl benzylamine. 2. preparation method according to claim 1, is characterized in that, described sulfonate is sulfamic acid salt. 3. preparation method according to claim 1, is characterized in that, described diisocyanate trimer is selected from isophorone diisocyanate trimer, hexamethylene diisocyanate trimer, toluene diisocyanate trimer body or at least one of diphenylmethane diisocyanate trimer. 4. The preparation method according to claim 1, wherein the bisulfite is selected from at least one of sodium bisulfite, potassium bisulfite, magnesium bisulfite or calcium bisulfite. 5. according to the preparation method described in any one in claim 1-4, it is characterized in that, in described preparation method, by weight, described sulfonate is 1-8 part, and described diisocyanate three The polymer is 80-120 parts, and the bisulfite is 40-70 parts. 6. The preparation method according to claim 1, characterized in that, the temperature of the second reaction is 40-70°C. 7. The preparation method according to claim 6, characterized in that, the second reaction ends when the mass content of -NCO in the reaction system is less than 0.05%. 8. A polyisocyanate curing agent, characterized in that it is prepared by the preparation method according to any one of claims 1-7, the solid content of the polyisocyanate curing agent is 99-100%, and the sealing rate is greater than 99.9% %. 9. the application of polyisocyanate curing agent described in claim 8 in polyurethane coating. 10. A water-based polyurethane coating, characterized in that it comprises A component and B component, the A component comprises a water-based polyurethane dispersion, and the B component comprises the polyisocyanate curing agent according to claim 8.