Blocked polyisocyanate composition as well as preparation method and application thereof

Abstract

The invention provides a blocked polyisocyanate composition as well as a preparation method and application thereof, and the composition is a blocked polyisocyanate composition obtained by blocking an isophorone diisocyanate homopolymer with caprolactam, the composition has the following characteristics: a) the integral area of a component peak with the weight-average molecular weight of 350 + / -50 accounts for 0.1-5% of the overall peak area; and b) the integral area of the peak of the component with the weight-average molecular weight of 650 + / -50 accounts for 25-40% of the overall peak area, preferably 30-40%. The blocked polyisocyanate composition provided by the invention has a higher glass transition temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of blocked polyisocyanates, and specifically relates to a blocked polyisocyanate composition with a high glass transition temperature, a preparation method thereof, and an application thereof. Background Art

[0002] Blocked isocyanates are formed by the reaction of isocyanates with blocking agents. The chemical bond formed between the blocking agent and NCO is relatively weak and can release free NCO under certain conditions. Blocked isocyanates are widely used in many aspects such as adhesives, coatings, elastomers, and sealants. Commonly used isocyanate blocking agents include caprolactam, phenols, acetylacetone, sodium bisulfate, methyl ethyl ketoxime, etc. Among them, caprolactam is a commonly used raw material for isocyanate blocking agents due to its cheap and easily available raw materials. Moreover, caprolactam-capped isocyanates are also widely used as powder coating curing agents.

[0003] The glass transition temperature of coating components is a very important parameter of concern to downstream application workers of coatings. It will directly or indirectly affect the physical and chemical stability of coating components during storage, and the rheological behavior during production and film formation.

[0004] Prior arts such as patents US4302351, US4246380, and US5331078 have all disclosed polyisocyanate products blocked with caprolactam and isophorone diisocyanate (IPDI) homopolymers. However, the caprolactam-blocked polyisocyanates described in these patents have the problem of relatively low glass transition temperature during downstream use.

[0005] Patent CN114276520A describes that the glass transition temperature of the product is increased by controlling the ratio of blocked trimers to blocked monomers in the product within a certain range. However, this method cannot increase the glass transition temperature to a higher temperature, cannot be used in fields with higher requirements for the glass transition temperature, and at the same time, the viscosity of the product increases, affecting the downstream use range.

[0006] Therefore, it is necessary to develop a blocked polyisocyanate composition with a high glass transition temperature. Summary of the Invention

[0007] Aiming at the above problems existing in the prior art, the purpose of the present invention is to provide a blocked polyisocyanate composition with a high glass transition temperature.

[0008] The purpose of the present invention is also to provide a preparation method of the above-mentioned blocked polyisocyanate composition. This method is to cap the isophorone diisocyanate homopolymer with caprolactam, and through regulating the reaction conditions, obtain the blocked polyisocyanate composition. The method is simple and efficient.

[0009] The present invention also aims to provide a use of the blocked polyisocyanate composition for polyurethane powder coatings.

[0010] To achieve the above-mentioned invention objectives, the present invention adopts the following technical solutions:

[0011] In a first aspect, the present invention provides a blocked polyisocyanate composition, which is a blocked polyisocyanate composition obtained by end-capping isophorone diisocyanate homopolymer with caprolactam. The composition has the following characteristics:

[0012] a) The integral area of the component peak with a weight-average molecular weight of 350 ± 50 accounts for 0.1 - 5% of the overall peak area, such as 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%;

[0013] b) The integral area of the component peak with a weight-average molecular weight of 650 ± 50 accounts for 25 - 40% of the overall peak area, such as 25%, 28%, 30%, 33%, 35%, 37%, 40%, preferably 30 - 40%.

[0014] In the present invention, the integral area of the component peak is obtained by characterization using gel permeation chromatography (GPC); the overall peak area refers to the sum of the integral areas of the component peaks with different weight-average molecular weights in the gel chromatogram measured by a gel chromatograph, which is counted as 100%.

[0015] Preferably, when testing the components and their weight-average molecular weights of the blocked polyisocyanate composition of the present invention by gel permeation chromatography, a conventional type of gel chromatograph equipped with a high-performance universal chromatographic column and a differential refractive index detector can be used for testing. Preferably, polystyrene is used as the standard sample, and the eluent is selected from tetrahydrofuran. Those skilled in the art can refer to the content disclosed in the prior art and determine according to actual needs. For example, the testing method mentioned in the examples of the present invention can be adopted.

[0016] When the inventors of the present application analyzed the component structure of the reaction solution of the blocked polyisocyanate composition by means of a high-performance gel chromatography instrument, they surprisingly found that the proportion of the integral area of the component peak with a weight-average molecular weight of 350±50 and the integral area of the component peak with a weight-average molecular weight of 650±50 would significantly affect the glass transition temperature of the product. Further research found that when the integral area of the component peak with a weight-average molecular weight of 350±50 accounted for 0.1-5% of the total peak area, and the integral area of the component peak with a weight-average molecular weight of 650±50 accounted for 25-40% of the total peak area, the glass transition temperature of the product could be significantly increased. When the proportion of the integral area of the component peak with a weight-average molecular weight of 350±50 was less than 0.1%, the increase in the glass transition temperature was not obvious. After the ratio was greater than 5%, the viscosity of the product increased significantly, affecting the downstream application of the product. Similarly, when the proportion of the area of the component peak with a weight-average molecular weight of 650±50 was less than 25%, the glass transition temperature of the product was relatively low. When the ratio was greater than 40%, the viscosity of the product increased rapidly, affecting the downstream usage scenario.

[0017] In a second aspect, the present invention provides a method for preparing the above-mentioned blocked polyisocyanate composition. It is prepared by the capping reaction of isophorone diisocyanate homopolymer with caprolactam. Those skilled in the art should understand that this preparation method is only an exemplary illustration of the product source mode of the blocked polyisocyanate composition with the above characteristics of the present invention, but does not constitute any limitation.

[0018] In a preferred embodiment, the present invention provides a method for preparing the blocked polyisocyanate composition, and the steps include:

[0019] 1) Polymerization reaction: In the presence of a trimerization catalyst, isophorone diisocyanate undergoes a polymerization reaction. After the NCO content in the reaction solution is reduced to a set value, a terminator is added to terminate the reaction;

[0020] 2) Capping reaction:

[0021] Method A) The product of step 1) is subjected to high-temperature heat treatment, and then caprolactam is quickly added for capping reaction. When the free NCO in the system is lower than 1 wt%, a blocked polyisocyanate composition is obtained; or

[0022] Method B) Caprolactam is slowly added to the product of step 1) for capping reaction. When the free NCO in the system is lower than 1 wt%, a blocked polyisocyanate composition is obtained.

[0023] In the present invention, the dosage of the trimerization catalyst in step 1) is 5-100 ppm of the mass of the isophorone diisocyanate monomer, such as 5 ppm, 10 ppm, 30 ppm, 50 ppm, 70 ppm, 90 ppm, 100 ppm, preferably 10-90 ppm;

[0024] The trimerization catalyst is selected from one or more of hydroxides of tetraalkylammonium and organic acid salts of tetraalkylammonium;

[0025] Preferably, the hydroxide of tetraalkylammonium is selected from one or more of tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrabutylammonium hydroxide, and the organic acid salt of tetraalkylammonium is selected from one or more of acetate, butyrate, and caprate of tetramethylammonium and tetraethylammonium.

[0026] The trimerization catalyst can be used without a solvent or dissolved in a solvent, and the solvent is selected from straight-chain or branched-chain monohydric alcohols or dihydric alcohols having 1 to 20 carbons, preferably one or more of methanol, ethanol, n-butanol, isobutanol, tert-butanol, n-octanol, isooctanol, and heptanol, and more preferably one or more of methanol, ethanol, n-butanol, and heptanol.

[0027] When the trimerization catalyst is used in the form of a solution, the concentration of the trimerization catalyst based on the catalyst solution is 5-50 wt%, such as 5 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, preferably 10-30 wt%.

[0028] In the present invention, the molar ratio of the amount of the terminator used in step 1) to the trimerization catalyst is 0.8-1.2:1, such as 0.8:1, 0.9:1, 1.0:1, 1.1:1, 1.2:1;

[0029] The terminator is selected from one or more of terminators such as phosphate esters and acyl chlorides, preferably one or more of dimethyl phosphate, diethyl phosphate, dibutyl phosphate, dioctyl phosphate, 2-ethylhexyl phosphate, and acyl chloride, and more preferably one or more of benzoyl chloride and acetyl chloride.

[0030] In the present invention, for the polymerization reaction in step 1), the reaction temperature is 10-100 °C, such as 10 °C, 20 °C, 40 °C, 60 °C, 80 °C, 100 °C, preferably 70-90 °C; preferably, at the end of the polymerization reaction, the NCO content in the reaction solution is reduced to a set value of 25-30 wt%, such as 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, 30 wt%; the temperature for terminating the reaction using the terminator is 90-150 °C, such as 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, preferably 100-120 °C, and the residence time is 30-60 min, such as 30 min, 40 min, 50 min, 60 min.

[0031] In the present invention, for the high-temperature heat treatment in Method A) of the capping reaction in Step 2), the temperature is 140 - 160 °C, such as 140 °C, 145 °C, 150 °C, 160 °C, the time is 1 - 8 h, such as 1 h, 2 h, 4 h, 6 h, 8 h, and preferably 4 - 6 h.

[0032] In the present invention, the rapid addition of caprolactam in Method A) of the capping reaction in Step 2) means that caprolactam is added within 30 min, such as 30 min, 25 min, 20 min, 15 min, 10 min, 5 min, 1 min, and preferably within 20 min. The feeding time is not included in the reaction time.

[0033] In the present invention, for the slow addition of caprolactam in Method B) of the capping reaction in Step 2), the dropping feeding method is preferably adopted, the dropping time is 4 - 12 h, such as 4 h, 5 h, 6 h, 10 h, 12 h, preferably 8 - 10 h, and the feeding time is not included in the reaction time.

[0034] In the present invention, for the capping reaction in Method A) of Step 2), the reaction temperature is 80 - 120 °C, such as 80 °C, 90 °C, 100 °C, 110 °C, 120 °C, and the reaction time is 1 - 6 h, such as 1 h, 2 h, 4 h, 6 h; for the capping reaction in Method B), the reaction temperature is 100 - 160 °C and the reaction time is 1 - 6 h.

[0035] In the present invention, the feeding of caprolactam in Step 2) is calculated by molar amount, and the ratio to NCO in the system is 0.95 - 1.1:1, such as 0.95:1, 1:1, 1.05:1, 1.1:1, and preferably 1 - 1.05:1.

[0036] In the present invention, each operation process and the said reaction are carried out under nitrogen protection.

[0037] In the preparation method of the present invention, the content of the component peak with a weight average molecular weight of 350 ± 50 is mainly determined by factors such as the heating time of the polymerization reaction solution at high temperature and the addition time of caprolactam. The area of the peak of the component with a weight average molecular weight of 650 ± 50 can be adjusted by adjusting the NCO content of the reaction solution in Step 1). When the NCO content of the reaction solution is reduced to 25 - 30 wt% based on the content of the reaction solution and a terminator is added to terminate the reaction, the proportion of this component peak can be controlled within 25 - 40%, preferably 30 - 40%.

[0038] In the third aspect, the present invention provides the use of the blocked polyisocyanate composition described above, which is mainly applied to polyurethane powder coatings.

[0039] Compared with the prior art, the technical solution of the present invention has the following technical effects:

[0040] The glass transition temperature of the product can be greatly increased by controlling the integral areas of the peaks of the component with a weight-average molecular weight of 350 ± 50 and the peak of the component with a weight-average molecular weight of 650 ± 50 accounting for the total peak area. Description of the Drawings

[0041] Figure 1 It is the gel chromatography spectrogram of Example 3. Peak 4 is the peak with a weight-average molecular weight of 650 ± 50, and Peak 5 is the peak with a weight-average molecular weight of 350 ± 50. Detailed Embodiments

[0042] The following examples will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed examples and should also include any other well-known changes within the scope of the claims of the present invention. The specific application of the present invention is not limited to the applications mentioned in the examples. Simple changes made to the present invention by those familiar with the art using the concept of the present invention are within the scope of protection of the present invention.

[0043] The following test methods are adopted in the examples of the present invention:

[0044] Determination of NCO content: Adopt the national standard GB / T12009.4.

[0045] For the determination of the integral area of the peak of the component with a weight-average molecular weight of 350 ± 50 accounting for the total peak area and the integral area of the peak of the component with a weight-average molecular weight of 650 ± 50 accounting for the total peak area, gel chromatography technology (LC-20AD / RID-10A, the chromatographic column is MZ-Gel SD plus10E3A, 5μm (8.0 * 300mm), MZ-Gel SDplus 500A 5μm (8.0 * 300mm), MZ-Gel SDplus 100A5μm (8.0 * 300mm) in series, Shimadzu, using polystyrene as the standard sample, mobile phase: tetrahydrofuran; flow rate: 1.0 mL / min; analysis time: 40 min, chromatographic column temperature: 35 °C) is used. A differential detector is used for determination. For the quantitative determination of isocyanate raw materials, the areas of the blocked polymers and blocked monomers in the system are determined by the area normalization method. Reaction conversion rate (%) = S (monomer peak area) / S (sum of the areas of each component peak) * 100%.

[0046] Glass transition temperature (Tg): Use a TA-Q25 thermal analyzer to test the glass transition temperature of the sample. Under a nitrogen atmosphere (50 mL / min -1 ) with a heating rate of 1.5 °C / min -1

[0047] Viscosity test: Brookfield CAP2000+ rotational viscometer, test temperature 130°C, using rotor No. 3, speed set to 20 rpm;

[0048] In the following examples, the raw materials used are as follows. Other materials were obtained from common commercial channels unless otherwise specified:

[0049] Isophorone diisocyanate (IPDI): Wanhua Chemical, purity > 99%;

[0050] Caprolactam: Baling Petrochemical;

[0051] Tetramethylammonium hydroxide: Sigma reagent, purity ≥95%, crystalline;

[0052] Tetramethylammonium acetate: Aladdin reagent, purity ≥95%, solution;

[0053] Dibutyl phosphate: Aladdin reagent, purity >95%.

[0054] Benzoyl chloride: Aladdin reagent, purity >99%.

[0055] Unless otherwise specified in the following examples and comparative examples, the reaction solution was kept under dry nitrogen protection from before the reaction to the addition of the catalyst and during the entire reaction process.

[0056] Preparation Example 1

[0057] 1000 g of IPDI was placed in a round-bottom flask equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen inlet; the reaction system was heated to 80° C., and then 0.133 g of a methanol solution of tetrabutylammonium hydroxide (30 wt%) was added dropwise to the reaction system under stirring, and the reaction temperature was controlled between 80 and 90° C. to carry out a polymerization reaction. When the NCO content of the reaction solution system was 30 wt%, dibutyl phosphate in an amount equimolar to that of tetrabutylammonium hydroxide was added and the reaction was stopped at 120° C. for 30 min to obtain a polymerization reaction solution 1.

[0058] Preparation Example 2

[0059] 1000 g of IPDI was placed in a round-bottom flask equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen inlet; the reaction system was heated to 70° C., and then 0.45 g of an isopropanol solution of tetramethylammonium acetate (20 wt %) was added dropwise to the reaction system under stirring, and the reaction temperature was controlled between 70 and 80° C. to carry out a polymerization reaction. When the NCO content of the reaction solution system was 25 wt %, benzoyl chloride in an amount equal to that of tetramethylammonium acetate was added and the reaction was stopped at 90° C. for 60 min to obtain a polymerization reaction solution 2.

[0060] Preparation Example 3

[0061] Prepare the polymerization reaction solution by referring to the method of Preparation Example 1, with the only difference being that when the NCO content of the reaction solution system is 27 wt%, terminate the reaction, and keep other operations and conditions unchanged to obtain the polymerization reaction solution 3.

[0062] Preparation Example 4 (for comparative example)

[0063] Prepare the polymerization reaction solution by referring to the method of Preparation Example 1, with the only difference being that when the NCO content of the reaction solution system is 33 wt%, terminate the reaction, and keep other operations and conditions unchanged to obtain the polymerization reaction solution 4.

[0064] Preparation Example 5 (for comparative example)

[0065] Prepare the polymerization reaction solution by referring to the method of Preparation Example 1, with the only difference being that when the NCO content of the reaction solution system is 22 wt%, terminate the reaction, and keep other operations and conditions unchanged to obtain the polymerization reaction solution 5.

[0066] Example 1

[0067] Place 200 g of the polymerization reaction solution 1 in a round-bottom flask equipped with a reflux condenser, a stirrer, a thermometer, and a nitrogen inlet. Heat it to 140 °C for heat treatment for 4 h, then lower the temperature of the above system to 80 °C. Rapidly add 162 g of caprolactam (the molar ratio to NCO in the system is 1:1) to the system under stirring conditions. The addition of caprolactam ends in 10 min, and continue the reaction for 6 h. Test that the free NCO content is lower than 1 wt%, then stop the reaction to obtain the blocked polyisocyanate combination 1.

[0068] Example 2

[0069] Place 200 g of the polymerization reaction solution 1 in a round-bottom flask equipped with a reflux condenser, a stirrer, a thermometer, and a nitrogen inlet. Heat the above system to 100 °C, and slowly dropwise add 154 g of caprolactam (the molar ratio to NCO in the system is 0.95:1) to the system under stirring conditions. The dropping ends in 8 h, and continue the reaction for 3 h. Test that the free NCO content is lower than 1 wt%, then stop the reaction to obtain the blocked polyisocyanate combination 2.

[0070] Example 3

[0071] Place 200 g of the polymerization reaction solution 3 in a round-bottom flask equipped with a reflux condenser, a stirrer, a thermometer, and a nitrogen inlet. Heat it to 160 °C for heat treatment for 3 h, then lower the temperature of the above system to 120 °C. Rapidly add 153 g of caprolactam (the molar ratio to NCO in the system is 1.05:1) to the system under stirring conditions. The addition ends in 20 min, and continue the reaction for 1 h. Test that the free NCO content is lower than 1 wt%, then stop the reaction to obtain the blocked polyisocyanate combination 3.

[0072] Example 4

[0073] Put 200 g of polymerization reaction solution 2 into a round-bottomed flask equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen inlet, and then heat the above system to 120 °C. Slowly add 142 g of caprolactam (molar ratio of NCO in the system is 1.05:1) dropwise to the system under stirring conditions. After 6 h of dropping, continue the reaction for 2 h. Test that the free NCO content is lower than 1%, stop the reaction, and obtain the blocked polyisocyanate combination 4.

[0074] Comparative Example 1

[0075] Prepare the blocked polyisocyanate combination by referring to the method of Example 1, with the only difference being that the reaction system is not heat-treated at 140 °C for 4 h, and other operations and conditions remain unchanged, to obtain the blocked polyisocyanate combination 5.

[0076] Comparative Example 2

[0077] Prepare the blocked polyisocyanate combination by referring to the method of Example 2, with the only difference being that caprolactam is added slowly and the dropping time is 60 min, and other operations and conditions remain unchanged, to obtain the blocked polyisocyanate combination 6.

[0078] Comparative Example 3

[0079] Prepare the blocked polyisocyanate combination by referring to the method of Example 1, with the only difference being that polymerization reaction solution 4 is selected for the initial reaction, and other operations and conditions remain unchanged, to obtain the blocked polyisocyanate combination 7.

[0080] Comparative Example 4

[0081] Prepare the blocked polyisocyanate combination by referring to the method of Example 1, with the only difference being that polymerization reaction solution 5 is selected for the initial reaction, and other operations and conditions remain unchanged, to obtain the blocked polyisocyanate combination 8.

[0082] Comparative Example 5

[0083] Prepare the blocked polyisocyanate combination by referring to the method of Example 1, with the only difference being that the reaction system is heat-treated at 170 °C for 6 h, and other operations and conditions remain unchanged, to obtain the blocked polyisocyanate combination 9.

[0084] Test the composition and performance of the blocked polyisocyanate compositions 1-9 prepared in the above examples and comparative examples, and the results are shown in Table 1 below:

[0085] Table 1 Test results of blocked polyisocyanate compositions 1-9 prepared in examples and comparative examples

[0086]

[0087] It can be found from Examples 1-4 that the blocked polyisocyanate of the present invention not only has a high glass transition temperature but also has a low viscosity. It can be seen from Comparative Examples 1-5 that when the peak area of the component with a weight average molecular weight of 350±50 accounts for <0.1% of the total peak area, the glass transition temperature of the product is low. When the peak area of the component with a weight average molecular weight of 350±50 accounts for more than 5% of the total peak area, the viscosity of the product increases significantly, affecting downstream applications; when the peak area of the component with a weight average molecular weight of 650±50 accounts for <25% of the total peak area, the glass transition temperature of the product is low, while when the peak area of the component with a weight average molecular weight of 650±50 accounts for >40% of the total peak area, the viscosity of the product is high.

[0088] The above experiments show that when the peak area of the component with a weight average molecular weight of 350±50 accounts for 0.1%-5% of the total peak area and the peak area of the component with a weight average molecular weight of 650±50 accounts for 25-40% of the total peak area, the product has a high glass transition temperature and a low viscosity, and is popular in downstream applications.

Claims

1. A blocked polyisocyanate composition, characterized in that, the composition is a blocked polyisocyanate composition obtained by blocking isophorone diisocyanate homopolymer with caprolactam, and the composition has the following characteristics: a) The integral area of the component peak with a weight-average molecular weight of 350 ± 50 accounts for 0.1-5% of the total peak area; b) The integral area of the component peak with a weight-average molecular weight of 650 ± 50 accounts for 25-40% of the total peak area, preferably 30-40%.

2. The blocked polyisocyanate composition according to claim 1, characterized in that, the integral area of the component peak is obtained by characterization using gel chromatography.

3. The preparation method of the blocked polyisocyanate composition according to claim 1 or 2, characterized in that the steps include: 1) Polymerization reaction: In the presence of a trimerization catalyst, isophorone diisocyanate undergoes a polymerization reaction. After the NCO content in the reaction solution is reduced to a set value, a terminator is added to terminate the reaction; 2) Blocking reaction: Method A) The product of step 1) is subjected to high-temperature heat treatment, and then caprolactam is quickly added for blocking reaction. When the free NCO in the system is lower than 1 wt%, a blocked polyisocyanate composition is obtained; or Method B) Caprolactam is slowly added to the product of step 1) for blocking reaction. When the free NCO in the system is lower than 1 wt%, a blocked polyisocyanate composition is obtained.

4. The preparation method according to claim 3, characterized in that, the dosage of the trimerization catalyst in step 1) is 5-100 ppm, preferably 10-90 ppm, based on the mass of isophorone diisocyanate monomer; and / or the trimerization catalyst is selected from one or more of hydroxides of tetraalkylammonium and organic acid salts of tetraalkylammonium; Preferably, the hydroxide of tetraalkylammonium is selected from one or more of tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrabutylammonium hydroxide, and the organic acid salt of tetraalkylammonium is selected from one or more of acetate, butyrate, and caprate of tetramethylammonium and tetraethylammonium.

5. The preparation method according to claim 3, characterized in that, the trimerization catalyst in step 1) is used under the condition of being dissolved in a solvent, and the solvent is selected from straight-chain or branched-chain monohydric alcohols or dihydric alcohols with 1-20 carbons, preferably one or more of methanol, ethanol, n-butanol, isobutanol, tert-butanol, n-octanol, isooctanol, and heptanol, more preferably one or more of methanol, ethanol, n-butanol, and heptanol; Preferably, when the trimerization catalyst is used in the form of a solution, the concentration of the trimerization catalyst based on the catalyst solution is 5-50 wt%, preferably 10-30 wt%.

6. The preparation method according to claim 3, characterized in that, the molar ratio of the dosage of the terminator to the trimerization catalyst in step 1) is 0.8-1.2:1; the terminator is selected from one or more of phosphate esters and acyl chloride terminators, preferably one or more of dimethyl phosphate, diethyl phosphate, dibutyl phosphate, dioctyl phosphate, 2-ethylhexyl phosphate, and acyl chloride, more preferably one or more of benzoyl chloride and acetyl chloride.

7. The preparation method according to claim 3, characterized in that for the polymerization reaction in step 1), the reaction temperature is 10 - 100 °C, preferably 70 - 90 °C, and the reaction time is 4 - 12 h, preferably 6 - 9 h; and / or preferably, at the end point of the polymerization reaction, the NCO content in the reaction solution is reduced to a set value of 25 - 30 wt%, the temperature for terminating the reaction using a terminator is 90 - 150 °C, preferably 100 - 120 °C, and the residence time is 30 - 60 min.

8. The preparation method according to claim 3, characterized in that for the high-temperature heat treatment in method A) of the capping reaction in step 2), the temperature is 140 - 160 °C, and the time is 1 - 8 h, preferably 4 - 6 h; and / or the rapid addition of caprolactam in method A) of the capping reaction in step 2) means that caprolactam is added within 30 min, preferably within 20 min, and the feeding time is not included in the reaction time; and / or the slow addition of caprolactam in method B) of the capping reaction in step 2) preferably adopts a dropping feeding method, the dropping time is 4 - 12 h, preferably 8 - 10 h, and the feeding time is not included in the reaction time.

9. The preparation method according to claim 3, characterized in that for the capping reaction in step 2), in method A), the reaction temperature is 80 - 120 °C, and the reaction time is 1 - 6 h; in method B), the reaction temperature is 100 - 160 °C, and the reaction time is 1 - 6 h; and / or the feed amount of caprolactam in step 2) is calculated by molar amount, and the ratio to NCO in the system is 0.95 - 1.1:1, preferably 1 - 1.05:

1.

10. The application of the blocked polyisocyanate composition according to claim 1 or 2 or the blocked polyisocyanate composition prepared by the method according to any one of claims 3 - 9 in a polyurethane powder coating.

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