Storage-stable polyisocyanate curing agent, preparation method and application

Abstract

The present invention relates to a polyisocyanate curing agent containing carbamate and stably stored at low temperature, prepared by reacting toluene diisocyanate and polyhydroxy compounds, wherein the toluene diisocyanate dimer content in the polyisocyanate curing agent is controlled to be 0.01-4.0wt%. The polyisocyanate curing agent prepared by the method of the present invention has the characteristics of low free monomer, low color value, good storage stability and excellent application performance.

Description

Technical Field

[0001] The invention relates to a polyisocyanate curing agent, in particular to a polyisocyanate curing agent with storage stability and a preparation method and application thereof. Background Art

[0002] TDI curing agent is an important member of the polyurethane coating industry. It has become the mainstream of the coating market because of its excellent properties such as good film gloss, good elasticity and good toughness. It is widely used in wood paint and adhesives. Common curing agents for polyurethane coatings can be divided into: TDI and TMP adducts, TDI and hydroxyl-containing substances adducts and TDI trimers. TMP-TDI adducts are an important member of the polyurethane coating industry. It has become the mainstream of the coating market because of its excellent properties such as good film gloss, good elasticity and good toughness. Therefore, domestic and foreign countries are competing to carry out this research.

[0003] There will be a certain amount of isocyanate monomer residue in the preparation process of polyurethane curing agent. There are many patent documents on the preparation of polyurethane curing agent with high solid content and low monomer content. The methods for reducing the unreacted isocyanate monomer in the curing agent include chemical synthesis, solvent extraction, and thin film evaporation. The main equipment of the thin film evaporation method is a thin film evaporator and a high vacuum system. Under high temperature and negative pressure, the prepolymer is quickly passed through the tower plate in a thin film state, and the free isocyanate monomer is evaporated.

[0004] China's public patent CN200510101596 separates free isocyanate in a curing agent by single-stage thin film evaporation, and the free TDI content in the final product is less than 0.5wt%. At present, the application of thin film evaporation technology in the separation of free isocyanate is relatively mature.

[0005] Low-free polyisocyanate curing agent can be obtained by thin film evaporation method. However, due to the heat sensitivity of isocyanate, toluene diisocyanate dimer will be produced during the reaction and separation process, and will eventually enter the product. If stored at zero degrees Celsius, flocculants or solid precipitates will be produced, affecting the transparency and performance of the product, thereby reducing product quality.

[0006] Chinese patent publication CN112341595 A proposes a polyisocyanate and a method for preparing the same, which solves the problem that floccules or even solid precipitation sometimes appear in the product solution at a lower storage temperature by controlling the ratio of the integral area of ​​the component peak with a weight average molecular weight of 800±50 to the integral area of ​​the shoulder peak with a weight average molecular weight of 950±50 to be 2-14, but the problem of poor storage stability still occurs at a lower storage temperature. Summary of the invention

[0007] The technical problem to be solved by the present invention is how to improve the storage stability of the polyisocyanate curing agent without affecting the performance of the product.

[0008] In order to solve the above technical problems, the present invention provides a polyisocyanate curing agent with storage stability, a preparation method and an application thereof. The curing agent of the present invention has the characteristics of stable color value, low free monomer content and excellent storage stability, and has wide applicability.

[0009] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0010] In one aspect, the present invention provides a polyisocyanate curing agent containing carbamate prepared by reacting toluene diisocyanate with a polyhydroxy compound, wherein the content of toluene diisocyanate dimer in the polyisocyanate curing agent based on solids is 0.01-4.0wt%, preferably 0.1%-3.0wt%.

[0011] The solids refer to the components in the polyisocyanate curing agent except the organic solvent, that is, the components after the unreacted polyisocyanate is removed after the toluene diisocyanate reacts with the polyhydroxy compound.

[0012] The molar ratio of the isocyanate groups in the toluene diisocyanate to the hydroxyl groups in the hydroxyl compound is (2-10):1, preferably (3-5):1.

[0013] The toluene diisocyanate dimer content of the present invention is determined by gel permeation chromatography using polystyrene as a standard sample and tetrahydrofuran as an eluent according to GB / T27810-2011. The toluene diisocyanate dimer is produced by the polymerization of two toluene diisocyanates.

[0014] The polyisocyanate curing agent prepared in the present invention has a color value of ≤70 Hazen, preferably ≤50 Hazen; and a free toluene diisocyanate content of ≤1.0 wt%, preferably ≤0.5 wt%, based on the total mass of the curing agent.

[0015] In the present invention, the solid content is 30-80 wt %, preferably 60-80 wt %, most preferably 74-76 wt %, based on the total mass of the curing agent.

[0016] On the other hand, the present invention also provides a method for preparing a polyisocyanate curing agent, the method comprising the following steps:

[0017] a. toluene diisocyanate and a polyol are subjected to an amination reaction to prepare a prepolymer reaction liquid;

[0018] b. The reaction solution is evaporated to remove unreacted toluene diisocyanate, and the unreacted polyisocyanate components are removed by dissolving with an organic solvent to obtain a polyisocyanate composition containing carbamate;

[0019] c. The unreacted toluene diisocyanate in step b is recycled in step a, and the content of toluene diisocyanate dimer in the unreacted toluene diisocyanate recycled in step a is controlled to be ≤8.0wt%.

[0020] The polyhydroxy compound in step a comprises trihydroxypropane and a polyol with a molecular weight of 60-200; wherein the molar ratio of the isocyanate group in toluene diisocyanate to the hydroxyl group in the hydroxyl compound is (2-10):1, preferably (3-5):1.

[0021] In the step a, the polyhydroxy compound comprises trimethylolpropane and polyol, and the molar ratio of the hydroxyl groups of trimethylolpropane to the polyol is (1-10):1, preferably (2-5):1.

[0022] In step a, the polyol is selected from one or more of ethylene glycol, propylene glycol, methylpropylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, diethylene glycol, dipropylene glycol, 2-methyl-1,3-propanediol, glycerol, 1,2,6-hexanetriol, trimethylolethane and pentaerythritol.

[0023] The reaction temperature of the amination reaction in step a is 40-110° C., preferably 50-80° C.; the reaction time is 0.5 h-12 h, preferably 1-5 h, most preferably 1-2 h.

[0024] In the toluene diisocyanate in step a, the content of 2,4-toluene diisocyanate is 40-100 wt %, preferably 60-80 wt %, based on the total mass of toluene diisocyanate.

[0025] The evaporation in step b is preferably thin film evaporation or short path evaporation, wherein the first stage separation temperature of thin film evaporation is 120-180°C and the pressure is 100-1000Pa, the second stage separation temperature is 150°C-210°C and the vacuum degree is 0-100Pa; the short path evaporator separation temperature is 140-200°C and the pressure is 1-50Pa.

[0026] In the step b, the organic solvent is one or more of ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, propylene glycol methyl ether acetate, acetone amyl acetate, hexyl acetate, methoxypropyl acetate, tetrahydrofuran, dioxane, acetone, N-methylpyrrolidone, methyl ethyl ketone, and white spirit, preferably ethyl acetate; the solid content of the polyisocyanate composition containing carbamate obtained by dissolving with the organic solvent is 30-80wt%, preferably 73-77wt%.

[0027] In the step c, the dimer in the unreacted toluene diisocyanate that is recycled is controlled by extraction with an organic solvent, and the organic solvent used for extraction is one or more of n-hexane, cyclohexane, n-pentane, cyclopentane, heptane, cycloheptane, octane, and cyclooctane, preferably n-hexane and / or cyclohexane; and the mixing ratio with the unreacted toluene diisocyanate is 1:10-1:1, preferably 1:5-1:2, and most preferably 1:4.

[0028] A use of the storage-stable polyisocyanate curing agent as described above is used for preparing polyurethane paint or polyurethane adhesive.

[0029] In the present invention, researchers have found that when the content of dimers in the prepared polyisocyanate curing agent is ≥3%, the storage stability of the product at low temperature will be greatly reduced. The introduction of dimers is mainly in the process of preparing the prepolymer reaction liquid and when the reaction liquid passes through the evaporator to remove the unreacted polyisocyanate. Due to the high temperature, a certain amount of toluene diisocyanate dimers will be produced in the recovered unreacted toluene diisocyanate. When the content of toluene diisocyanate dimers in the recovered unreacted toluene diisocyanate is greater than 8wt%, the content of toluene diisocyanate dimers in the product after it participates in the cycle will exceed 4wt%, thereby affecting the storage stability of the product. The present invention can effectively control the toluene diisocyanate dimers in the recycled unreacted polyisocyanate by extracting with an organic solvent, that is, after mixing the recovered unreacted toluene diisocyanate with an organic solvent, filtering the insoluble dimers, and removing the organic solvent by distillation, rectification or thin film evaporation, thereby controlling the toluene diisocyanate dimers in the recycled unreacted polyisocyanate to be below 8wt%.

[0030] The organic solvent extraction method can effectively control the toluene diisocyanate dimer in the recycled unreacted polyisocyanate, that is, after mixing the recovered unreacted toluene diisocyanate with the organic solvent, filtering the insoluble toluene diisocyanate dimer, and then removing the organic solvent by distillation, rectification or thin film evaporation.

[0031] The invention improves the low-temperature storage stability of the product by controlling the toluene diisocyanate dimer content of the polyisocyanate curing agent, and also has excellent application performance. DETAILED DESCRIPTION

[0032] The present invention is further described below by means of specific examples. The examples described in the present invention are only used to illustrate the present invention and do not limit the scope of the present invention.

[0033] The present invention determines the NCO content according to the method of GB / T 12009.4-1989;

[0034] The present invention is based on the method of GB / T18583-2008, and the residual monomer content in the reaction system is determined by gas chromatography;

[0035] The present invention is based on the method of GB / T1725-1979, and measures the solid content under the test conditions described for isocyanate;

[0036] The Hazen color value was determined using a color value measuring device (BYK LCS IV).

[0037] The components and number average molecular weight Mn of the polyisocyanate composition of the present invention can be obtained by GPC (gel chromatography) testing. The specific test conditions are as follows:

[0038] GPC equipment: Agilent1260

[0039] GPC columns: Pl1113-6520 and Pl113-6325 (Agilent)

[0040] Sample concentration: 3wt%

[0041] Mobile phase: tetrahydrofuran

[0042] Detection method: differential detector

[0043] Flow rate: 1ml / min

[0044] Column temperature: 35°C

[0045] The standard curve used polystyrene with a molecular weight range of 162-17900.

[0046] Materials and reagents:

[0047] T80: toluene diisocyanate, containing about 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate, purchased from Wanhua Chemical Group Co., Ltd.

[0048] T65: toluene diisocyanate, containing about 65% 2,4-toluene diisocyanate and 35% 2,6-toluene diisocyanate, purchased from Wanhua Chemical Group Co., Ltd.

[0049] Trimethylolpropane: purchased from Weifang Pasto Co., Ltd.

[0050] Diethylene glycol: purchased from Yanshan Petrochemical Co., Ltd.

[0051] Ethylene glycol: purchased from Sinopharm Chemical Reagent Co., Ltd.;

[0052] Glycerol: purchased from Sinopharm Chemical Reagent Co., Ltd.

[0053] Example 1

[0054] 10 kg of T80 was added to a reaction kettle, heated and stirred at 40° C., and then a mixed alcohol consisting of 1204 g of diethylene glycol and 1319 g of trimethylolpropane was dripped into the reaction kettle. After the dripping was completed, the reaction was continued to be stirred at 40° C. for 10 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 150° C., pressure 12 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A1; the separated unreacted TDI monomer was mixed with cyclohexane at a mass ratio of 1:4, filtered, and then the cyclohexane was removed by reduced pressure distillation to obtain a TDI monomer;

[0055] The recovered TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B1.

[0056] The obtained curing agent has the following parameters after measurement:

[0057] Solid content 75.1%

[0058] Free monomer content 0.28%

[0059] Color value 21Hazen

[0060] The content of toluene diisocyanate dimer in the recovered TDI is 0.5%

[0061] The toluene diisocyanate dimer content in the curing agent is 0.1% based on the solid content.

[0062] Example 2

[0063] 10 kg of T80 was added to a reactor, heated and stirred at 60° C., and then a mixed alcohol consisting of 677 g of diethylene glycol and 1141 g of trimethylolpropane was added dropwise to the reactor. After the addition was completed, the mixture was stirred and reacted at 60° C. for 4 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 165° C., pressure 10 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A2; the separated unreacted TDI monomer was mixed with cyclohexane at a mass ratio of 1:6, filtered, and then the cyclohexane was removed by reduced pressure distillation to obtain a TDI monomer;

[0064] The recovered TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B2.

[0065] The obtained curing agent has the following parameters after measurement:

[0066] Solid content 74.6%

[0067] Free monomer content 0.18%

[0068] Color value 25Hazen

[0069] The content of toluene diisocyanate dimer in the recovered TDI is 5.2%

[0070] The toluene diisocyanate dimer content in the curing agent is 2.6% based on the solid content.

[0071] Example 3

[0072] 10 kg of T80 was added to a reactor, heated and stirred at 70° C., and then a mixed alcohol consisting of 536 g of diethylene glycol and 1128 g of trimethylolpropane was added dropwise to the reactor. After the addition was completed, the mixture was stirred and reacted at 70° C. for 6 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 170° C., pressure 15 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A3; the separated unreacted TDI monomer was mixed with cyclohexane at a mass ratio of 1:2, filtered, and then the cyclohexane was removed by reduced pressure distillation to obtain a TDI monomer;

[0073] The recovered TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B3.

[0074] The obtained curing agent has the following parameters after measurement:

[0075] Solid content 75.9%

[0076] Free monomer content 0.38%

[0077] Color value 33Hazen

[0078] The content of toluene diisocyanate dimer in the recovered TDI is 6.1%

[0079] The toluene diisocyanate dimer content in the curing agent is 3.1% based on the solid content.

[0080] Example 4

[0081] 10 kg of T65 was added to a reactor, heated and stirred at 80° C., and then a mixed alcohol consisting of 362 g of diethylene glycol and 947 g of trimethylolpropane was added dropwise to the reactor. After the addition was completed, the mixture was stirred and reacted at 80° C. for 7 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 150° C., pressure 12 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A4; the separated unreacted TDI monomer was mixed with n-hexane at a mass ratio of 1:8, filtered, and then the n-hexane was removed by reduced pressure distillation to obtain a TDI monomer;

[0082] The recovered TDI monomer was mixed with fresh TDI-65 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B4.

[0083] The obtained curing agent has the following parameters after measurement:

[0084] Solid content 75.2%

[0085] Free monomer content 0.40%

[0086] Color value 50Hazen

[0087] The content of toluene diisocyanate dimer in the recovered TDI is 2.3%

[0088] The toluene diisocyanate dimer content in the curing agent is 0.9% based on the solid content.

[0089] Example 5

[0090] 10 kg of T80 was added to a reactor, heated and stirred at 50° C., and then a mixed alcohol consisting of 134 g of ethylene glycol and 833 g of trimethylolpropane was added dropwise to the reactor. After the addition was completed, the mixture was stirred and reacted at 50° C. for 5 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 140° C., pressure 5 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A5; the separated unreacted TDI monomer was mixed with cyclohexane at a mass ratio of 1:3, filtered, and then the cyclohexane was removed by reduced pressure distillation to obtain TDI monomer, and the obtained TDI was extracted again and recovered by reduced pressure distillation to obtain TDI monomer;

[0091] The recovered TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B5.

[0092] The obtained curing agent has the following parameters after measurement:

[0093] Solid content 75.5%

[0094] Free monomer content 0.25%

[0095] Color value 15Hazen

[0096] The content of toluene diisocyanate dimer in the recovered TDI is 0.05%

[0097] The toluene diisocyanate dimer content in the curing agent is 0.01% based on the solid content.

[0098] Example 6

[0099] 10 kg of T80 was added to a reactor, heated and stirred at 100° C., and then a mixed alcohol consisting of 119 g of diethylene glycol and 503 g of trimethylolpropane was added dropwise to the reactor. After the addition was completed, the mixture was stirred and reacted at 100° C. for 1 hour to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 200° C., pressure 10 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A6; the separated unreacted TDI monomer was mixed with cyclohexane at a mass ratio of 1:4, filtered, and then the cyclohexane was removed by reduced pressure distillation to obtain a toluene diisocyanate monomer;

[0100] The recovered TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B6.

[0101] The obtained curing agent has the following parameters after measurement:

[0102] Solid content 75.5%

[0103] Free monomer content 0.18%

[0104] Color value 42Hazen

[0105] The content of toluene diisocyanate dimer in the recovered TDI is 8.0%

[0106] The toluene diisocyanate dimer content in the curing agent is 3.9% based on the solid content.

[0107] Comparative Example 1

[0108] 10 kg of T80 was added to a reaction kettle, heated and stirred at 70° C., and then a mixed alcohol consisting of 536 g of diethylene glycol and 1128 g of trimethylolpropane was added dropwise to the reaction kettle. After the addition was completed, the mixture was stirred and reacted at 70° C. for 6 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 170° C., pressure 15 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A3; the separated unreacted TDI monomer

[0109] The separated TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B7.

[0110] The obtained curing agent has the following parameters after measurement:

[0111] Solid content 75.7%

[0112] Free monomer content 0.36%

[0113] Color value 30Hazen

[0114] The content of toluene diisocyanate dimer in the recovered TDI is 10.1%

[0115] The toluene diisocyanate dimer content in the curing agent is 5.6% based on the solid content.

[0116] Comparative Example 2

[0117] 10 kg of T80 was added to a reactor, heated and stirred at 70° C., and then a mixed alcohol consisting of 536 g of diethylene glycol and 1128 g of trimethylolpropane was added dropwise to the reactor. After the addition was completed, the mixture was stirred and reacted at 70° C. for 6 hours to obtain a prepolymer reaction liquid; the reaction liquid was separated by a short-path evaporator (separation temperature 170° C., pressure 15 Pa), and the separated carbamate-containing component was diluted with ethyl acetate to obtain a polyisocyanate curing agent A3; the unreacted TDI monomer was separated, and the unreacted TDI monomer was mixed with cyclohexane at a mass ratio of 1:3, filtered, and then the cyclohexane was removed by reduced pressure distillation to obtain TDI monomer, and the obtained TDI was repeatedly extracted until the dimer content was 0.01%;

[0118] The separated TDI monomer was mixed with fresh TDI-80 to 10 kg, and the above process was repeated to obtain polyisocyanate curing agent B8.

[0119] The obtained curing agent has the following parameters after measurement:

[0120] Solid content 75.7%

[0121] Free monomer content 0.33%

[0122] Color value 32Hazen

[0123] The content of toluene diisocyanate dimer in the recovered TDI is 0.01%

[0124] The toluene diisocyanate dimer content in the curing agent is 0.04% based on the solid content.

[0125] In order to test the storage stability of the polyisocyanate curing agents prepared in the examples and comparative examples, the obtained polyisocyanate curing agents were stored at -15°C for 1 month to observe the storage stability. The results are listed in Table 1.

[0126] Table 1. Storage stability test of polyisocyanate curing agent

[0127]

[0128]

[0129] It can be seen from the above test results that compared with Comparative Example 1, the polyisocyanate curing agent product prepared in the embodiment of the present invention still has a clear appearance after being stored for 6 months, and the product performance is stable, which has significant technical advantages.

[0130] The embodiments and comparative examples were compared with Wanhua TT-350B was mixed at a mass ratio of 2:1 to prepare a curing agent mixture, and the obtained mixture was then mixed with a common commercial polyol (matte varnish, HS-129) at a NCO / OH molar ratio of 1:1, and then a mixed solvent of butyl acetate / xylene (the two were mixed in equal masses) was added for dilution (the content of the curing agent in the solution formed by the curing agent and butyl acetate / xylene was 40wt%), and finally a paint was prepared. The prepared paint was tested for adhesion and other properties according to the following methods: (1) Drying (surface drying) test: GB / T 1728; (2) Matt gloss test: GB / T9754; (3) Leveling test: Prepare a paint film on the tinplate surface, place the sample under constant temperature and humidity (30°C, 35%-40% relative humidity range), and observe the time required for the painted surface to reach a uniform, smooth, wrinkle-free (no orange peel or goose skin) state; whether the uniform, smooth, wrinkle-free (no orange peel or goose skin) state of the painted surface is qualified depends on the user's standard requirements for different products; (4) Hardness test: GB / T1730; (5) Adhesion grade test: GB / T9286. The test results are shown in Table 2.

[0131] Table 2 Test results of paint properties

[0132]

[0133] The performance test results in Table 1 and Table 2 show that the embodiments have good storage stability and application performance. The storage stability of Comparative Example 1 is poor, and although Comparative Example 2 has good storage stability, the drying time, hardness and adhesion are poor. It can be seen that the toluene diisocyanate dimer content of the curing agent in the preparation of the polyisocyanate curing agent is controlled to be 0.01-4.0%, which has good storage stability and also has good application performance.

[0134] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a polyisocyanate curing agent containing carbamate prepared by reacting toluene diisocyanate with a polyhydroxy compound, characterized in that: The method comprises the following steps: a. toluene diisocyanate and a polyhydroxy compound undergo an amination reaction to prepare a prepolymer reaction liquid; b. The reaction solution is evaporated to remove unreacted toluene diisocyanate, and the unreacted polyisocyanate components are removed by dissolving with an organic solvent to obtain a polyisocyanate composition containing carbamate; c. The unreacted toluene diisocyanate in step b is recycled in step a, and the content of toluene diisocyanate dimer in the unreacted toluene diisocyanate recycled in step a is controlled to be ≤8.0%; the content of toluene diisocyanate dimer in the polyisocyanate curing agent is 0.01-4.0wt%.

2. The preparation method according to claim 1, characterized in that The content of toluene diisocyanate dimer in the polyisocyanate curing agent is 0.1%-3.0wt%.

3. The preparation method according to claim 1, characterized in that: Based on the total mass of the curing agent, the free toluene diisocyanate content is ≤1.0wt%; and / or the solid content is 30-80wt%, based on the total mass of the curing agent.

4. The preparation method according to claim 1, characterized in that: Based on the total mass of the curing agent, the free toluene diisocyanate content is ≤0.5wt%; and / or the solid content is 60-80wt%, based on the total mass of the curing agent.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The polyhydroxy compound in step a comprises trihydroxypropane and a polyol with a molecular weight of 60-200; and / or a hydroxyl molar ratio of trimethylolpropane to the polyol is (1-10):

1.

6. The preparation method according to claim 5, characterized in that: In the step a, the hydroxyl molar ratio of trimethylolpropane to polyol is (2-5):

1.

7. The preparation method according to any one of claims 1 to 4, characterized in that: The molar ratio of the isocyanate group in toluene diisocyanate to the hydroxyl group in the hydroxyl compound is (2-10):

1.

8. The preparation method according to claim 7, characterized in that: The molar ratio of the isocyanate group in toluene diisocyanate to the hydroxyl group in the hydroxyl compound is (3-5):

1.

9. The preparation method according to claim 5, characterized in that: The polyol is selected from one or more of ethylene glycol, propylene glycol, methylpropylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, diethylene glycol, dipropylene glycol, 2-methyl-1,3-propanediol, glycerol, 1,2,6-hexanetriol, trimethylolethane and pentaerythritol.

10. The preparation method according to any one of claims 1 to 4, characterized in that: The reaction temperature of the amination reaction in step a is 40-110° C. and the reaction time is 0.5 h-12 h.

11. The preparation method according to claim 10, characterized in that: The reaction temperature of the amination reaction in step a is 50-80° C. and the reaction time is 1-5 h.

12. The preparation method according to any one of claims 1 to 4, characterized in that: The evaporation in step b is selected from thin film evaporation or short path evaporation, wherein the first stage separation temperature of thin film evaporation is 120-180°C and the pressure is 100-1000Pa, the second stage separation temperature is 150°C-210°C and the vacuum degree is 0-100Pa; the short path evaporator separation temperature is 140-200°C and the pressure is 1-50Pa.

13. The preparation method according to any one of claims 1 to 4, characterized in that: In the step c, the dimer in the unreacted toluene diisocyanate that is recycled is controlled by extraction with an organic solvent, and the organic solvent used for extraction is one or more of n-hexane, cyclohexane, n-pentane, cyclopentane, heptane, cycloheptane, octane, and cyclooctane; and the mixing ratio with the unreacted toluene diisocyanate is 1:10-1:

1.

14. The preparation method according to any one of claims 1 to 4, characterized in that: In the step c, the dimer in the unreacted toluene diisocyanate that is recycled is controlled by extraction with an organic solvent, and the mixing ratio of the organic solvent used for extraction to the unreacted toluene diisocyanate is 1:5-1:2.

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