Solvent-free polyurethane curing agent as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of coatings, and provides a solvent-free polyurethane curing agent and a preparation method and application thereof.The preparation method includes the steps that bio-based oil, linear alcohol and a first catalyst are subjected to an alcoholysis reaction, then linear acid and a second catalyst are added for an esterification reaction, and a bio-based polyol intermediate is prepared; the bio-based polyol intermediate and polymethylene polyphenyl polyisocyanate are subjected to a nucleophilic addition reaction to prepare the solvent-free polyurethane curing agent. The preparation method provided by the invention does not need any post-treatment, and is low in equipment cost, simple to operate and low in energy consumption; the solvent-free polyurethane curing agent prepared by the invention does not contain a solvent, has low viscosity (25 DEG C, 800-1400 mPa.s) and low NCO value (21.1-23.2%), is not easy to generate bubbles, has long operable time (not less than 1 hour), and has flexibility of not more than phi 2mm.

Description

[0001] This case is a divisional application of the patent application with application number 2022116076838, application date December 14, 2022, and titled "A solvent-free polyurethane curing agent, its preparation method and application". Technical Field

[0002] The present invention relates to the technical field of coatings, and more particularly to a solvent-free polyurethane curing agent, a preparation method thereof, and applications thereof. Background Art

[0003] Solvent-free polyurethanes are solvent-free, odorless, pollution-free, non-flammable, and non-explosive. They are widely used in the preparation of adhesives, decorative coatings, paper coatings, and leather finishing agents. Solvent-free polyurethanes are environmentally friendly, and therefore, their replacement by traditional solvent-based polyurethanes is inevitable.

[0004] Solvent-based polyurethane curing agents generally cannot be used alone and need to be mixed with another component containing polyhydroxy groups to produce two-component polyurethane coatings or adhesives. The conventional modification method of solvent-based polyurethane curing agents is to use small molecule alcohols such as trimethylolpropane, propylene glycol, glycerin, and methylpropylene glycol to react with substances containing isocyanate groups. The resulting polyurethane curing agent has strong rigidity, high viscosity (25°C, 40,000-50,000 mPa.s), and high activity. The high-activity curing agent is also more sensitive to humidity, which can easily cause bubble problems and has a short operating time (20-30 minutes).

[0005] Therefore, there is an urgent need to develop a solvent-free polyurethane curing agent with low viscosity, long operating time and good flexibility. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a solvent-free polyurethane curing agent, a preparation method, and an application thereof. The solvent-free polyurethane curing agent prepared by the present invention contains no solvent, has a low viscosity (800-1400 mPa·s at 25°C), and a low isocyanate group value (NCO value) (21.1-23.2%). Due to the low NCO value, it is not easy to react with water, thus not easily causing bubble problems, has a long working time (not less than 1 hour), and has a flexibility of less than Φ2 mm.

[0007] A first aspect of the present invention provides a method for preparing a solvent-free polyurethane curing agent.

[0008] Specifically, a method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0009] (1) mixing a bio-based oil, a linear alcohol, and a first catalyst to perform an alcoholysis reaction, and then adding a linear acid and a second catalyst to perform an esterification reaction to obtain a bio-based polyol intermediate;

[0010] (2) mixing the bio-based polyol intermediate prepared in step (1) with polymethylene polyphenyl polyisocyanate, and preparing the solvent-free polyurethane curing agent through a nucleophilic addition reaction;

[0011] In step (2), the polymethylene polyphenyl polyisocyanate accounts for 50-80% by mass; and the bio-based polyol intermediate accounts for 20-50% by mass.

[0012] The present invention first uses bio-based oils, linear alcohols, and a first catalyst to undergo an alcoholysis reaction. A linear acid and a second catalyst are then added to carry out an esterification reaction to produce a bio-based polyol intermediate. The bio-based polyol intermediate is then reacted with polymethylene polyphenyl polyisocyanate (PAPI) via a nucleophilic addition reaction to produce a solvent-free polyurethane curing agent. The present invention introduces a bulky bio-based polyol intermediate into the PAPI to create steric hindrance, reducing the activity of the isocyanate group while simultaneously improving the flexibility of the resulting solvent-free polyurethane curing agent. The use of bio-based oils, linear alcohols, and linear acids as raw materials results in a solvent-free polyurethane curing agent with low viscosity, excellent flexibility, and strong adhesion.

[0013] The isocyanate group value (NCO value) of the present invention refers to: the mass of isocyanate (-NCO) groups contained in 100 g of sample / the mass of 100 g of sample×100%.

[0014] Preferably, in step (2), the polymethylene polyphenyl polyisocyanate is 70-80% by mass; and the bio-based polyol intermediate is 20-30% by mass.

[0015] Preferably, the raw material components for preparing the bio-based polyol intermediate include, by mass percentage:

[0016]

[0017] More preferably, the raw material components for preparing the bio-based polyol intermediate include, by mass percentage:

[0018]

[0019] Preferably, in step (2), the isocyanate value of the polymethylene polyphenyl polyisocyanate is 25-35%.

[0020] More preferably, in step (2), the isocyanate value of the polymethylene polyphenyl polyisocyanate is 29-31%.

[0021] Preferably, in step (2), the viscosity of the polymethylene polyphenyl polyisocyanate is 200-1000 mPa.s at 25°C.

[0022] Preferably, the bio-based polyol intermediate has a hydroxyl value of 80-100 mgKOH / g and an acid value of ≤3 mgKOH / g.

[0023] More preferably, the bio-based polyol intermediate has a hydroxyl value of 90-100 mgKOH / g and an acid value of ≤1 mgKOH / g.

[0024] Preferably, the bio-based polyol intermediate has a viscosity of 500-800 mPa·s at 25°C.

[0025] Preferably, in step (1), the alcoholysis reaction temperature is 180-230° C., and the time is 2-3 h.

[0026] Preferably, in step (1), when the alcoholysis reaction reaches a hydroxyl value of 80-100 mgKOH / g, the temperature is lowered to 110-120°C.

[0027] Preferably, in step (1), during the alcoholysis reaction, the stirring speed is 150-250 rpm.

[0028] Preferably, in step (1), the temperature of the esterification reaction is 100-120° C., and the time is 1-2 h.

[0029] Preferably, in step (1), when the esterification reaction reaches an acid value of less than 2 mgKOH / g, heating is stopped, the temperature is lowered to 55-60° C., and the product is filtered, and the filter residue is the bio-based polyol intermediate.

[0030] Preferably, in step (2), the temperature of the nucleophilic addition reaction is 75-85° C., and the time is 4-6 h.

[0031] Preferably, in step (2), when the nucleophilic addition reaction reaches an isocyanate group value of 21-24%, heating is stopped, the temperature is lowered to 55-60° C., and the mixture is filtered, and the filter residue is the solvent-free polyurethane curing agent.

[0032] Preferably, in step (1), the bio-based oil is one or more of soybean oil, epoxidized soybean oil, castor oil, coconut oil, and rosin.

[0033] Preferably, in step (1), the linear alcohol is one or more of ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, methyl propylene glycol, hexylene glycol, polyether polyol 400, polyether polyol 1000, polyether polyol 2000 and polyether polyol 3000.

[0034] Preferably, in step (1), the linear acid is one or more of oleic acid, linoleic acid, and adipic acid.

[0035] Preferably, in step (1), the first catalyst is one or more of lithium hydroxide, calcium oxide, sodium methoxide, and zinc acetate.

[0036] Preferably, in step (1), the second catalyst is one or more of dodecylsulfuric acid, sulfonic acid, p-toluenesulfonic acid, and concentrated sulfuric acid.

[0037] Preferably, in step (2), the polymethylene polyphenyl polyisocyanate is one or more of Wanhua PM-200, Huntsman 5005, BASF M20s, and Covestro 44V20.

[0038] Preferably, in step (1), the bio-based oil, the linear alcohol and the first catalyst are mixed in a protective atmosphere.

[0039] Preferably, the protective atmosphere is a nitrogen atmosphere.

[0040] Preferably, in step (1), the alcoholysis reaction is carried out in vacuum.

[0041] A second aspect of the present invention provides a solvent-free polyurethane curing agent.

[0042] A solvent-free polyurethane curing agent has a viscosity of 800-1400 mPa·s at 25°C.

[0043] Preferably, the isocyanate group value (NCO value) of the solvent-free polyurethane curing agent is 21-24%.

[0044] A second aspect of the present invention provides an application of a solvent-free polyurethane curing agent.

[0045] The invention discloses an application of a solvent-free polyurethane curing agent in the preparation of adhesives, decorative coatings, paper coatings or leather finishing agents.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] (1) The present invention utilizes bio-based oils, linear alcohols, and a first catalyst to first carry out an alcoholysis reaction, and then adds a linear acid and a second catalyst to carry out an esterification reaction to prepare a bio-based polyol intermediate, which is a macromolecular substance. The bio-based polyol intermediate and polymethylene polyphenyl polyisocyanate are reacted by a nucleophilic addition reaction to synthesize a solvent-free polyurethane curing agent. The preparation method of the present invention does not require any post-processing and can be produced using general prepolymer production equipment without adding new equipment. It has the characteristics of low equipment cost and simple operation, and the treatment process has low energy consumption;

[0048] (2) The solvent-free polyurethane curing agent prepared by the present invention does not contain solvent, is environmentally friendly and non-toxic, has low viscosity (25°C, 800-1400mPa·s), and has a low NCO value (21.1-23.2%). Due to the low NCO value, it is not easy to react with water, so it is not easy to cause bubble problems, has a long operating time (not less than 1 hour), and has good flexibility <Φ2mm. DETAILED DESCRIPTION

[0049] In order to make the technical solution of the present invention more clearly understood by those skilled in the art, the following examples are given for illustration. It should be noted that the following examples do not limit the scope of protection claimed by the present invention.

[0050] Unless otherwise specified, the raw materials, reagents, or devices used in the following examples can be obtained from conventional commercial sources or by existing known methods.

[0051] Example 1

[0052] A method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0053] 1. Preparation of bio-based polyol intermediates:

[0054] (1) In a 1000 mL four-necked flask equipped with a stirring device, a thermometer, and nitrogen protection, 461.5 g of soybean oil, 129.6 g of hexanediol, and 2.4 g of lithium hydroxide were added in sequence and placed in a reactor. The reactor was heated at 220° C. to carry out alcoholysis reaction. During the heating process, the materials in the reactor were stirred at a stirring speed of 200 rpm. The reactor was vacuumed and the materials in the reactor were allowed to fully react for 2 h. When the hydroxyl value of the sample was detected to be 90 mgKOH / g, the temperature was lowered to 100° C. and the next step was carried out.

[0055] (2) Add 206.7 g of oleic acid and 15 g of p-toluenesulfonic acid according to the formula, raise the temperature to 110° C., and carry out esterification reaction. Stir the materials in the reactor during heating to allow the materials in the reactor to fully react for 4 hours. When the acid value of the sample is 1 mgKOH / g, proceed to the next step;

[0056] (3) stopping heating, cooling to 60° C., filtering the material to obtain a bio-based polyol intermediate (or bio-based resin intermediate); the hydroxyl value is 90 mgKOH / g, the acid value is 1 mgKOH / g, and the viscosity is 800 mPa.s (25° C.);

[0057] 2. Preparation of solvent-free polyurethane curing agent:

[0058] 780 g of Wanhua PM-200PAPI and 220 g of bio-based polyol intermediate were added and kept at 80° C. for 5 h to carry out nucleophilic addition reaction. When the NCO value was 22.2%, heating was stopped, the temperature was cooled to 60° C., and the material was filtered to obtain a solvent-free polyurethane curing agent.

[0059] Example 2

[0060] A method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0061] 1. Preparation of bio-based polyol intermediates:

[0062] (1) 545.7 g of coconut oil, 73 g of propylene glycol, and 2.4 g of sodium methoxide were added to a 1000 mL four-necked flask equipped with a stirring device, a thermometer, and nitrogen protection. The mixture was placed in a reactor, and the reactor was heated at 180° C. to carry out alcoholysis. During the heating process, the contents of the reactor were stirred at a stirring speed of 250 rpm. The reactor was vacuumed and the contents were allowed to fully react for 2 h. When the hydroxyl value of the sample reached 95 mg KOH / g, the temperature was lowered to 100° C. to proceed to the next step.

[0063] (2) Add 181 g of oleic acid and 16 g of p-toluenesulfonic acid according to the formula, raise the temperature to 110°C, stir the contents of the reactor during heating, and allow the contents of the reactor to fully undergo esterification reaction for 5 hours. When the acid value of the sample is 1.5 mgKOH / g, proceed to the next step;

[0064] (3) Stop heating, cool to 55° C., and filter the material to obtain a bio-based polyol intermediate; the hydroxyl value is 95 mgKOH / g, the acid value is 1.5 mgKOH / g, and the viscosity is 1050 mPa.s (25° C.);

[0065] 2. Preparation of solvent-free polyurethane curing agent:

[0066] 800 g of BASF M20s PAPI and 200 g of bio-based polyol intermediate were added and kept at 75° C. for 4 hours to carry out a nucleophilic addition reaction. When the NCO content reached 23.2%, heating was stopped, the mixture was cooled to 55° C., and the material was filtered to obtain a solvent-free polyurethane curing agent.

[0067] Example 3

[0068] A method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0069] 1. Preparation of bio-based polyol intermediates:

[0070] (1) In a 1000 mL four-necked flask equipped with a stirring device, a thermometer and nitrogen protection, 307 g of castor oil, 119 g of diethylene glycol and 2.4 g of lithium hydroxide were added in sequence, and the mixture was placed in a reactor. The reactor was heated at 230° C. to carry out alcoholysis reaction. During the heating process, the contents of the reactor were stirred at a stirring speed of 150 rpm. The reactor was vacuumed and the contents were allowed to fully react for 2 h. When the hydroxyl value of the sample reached 100 mgKOH / g, the temperature was lowered to 100° C. and the next step was entered.

[0071] (2) Add 374 g of linoleic acid and 18 g of p-toluenesulfonic acid according to the formula, raise the temperature to 120° C., stir the contents of the reactor during the heating process, and allow the contents of the reactor to fully undergo esterification reaction for 4.5 hours. When the acid value of the sample is ≤0.5 mgKOH / g, proceed to the next step;

[0072] (3) Stop heating, cool to 55° C., filter the material to obtain a bio-based polyol intermediate; the hydroxyl value is 100 mgKOH / g, the acid value is 0.5 mgKOH / g, and the viscosity is 950 mPa.s (25° C.);

[0073] 2. Preparation of solvent-free polyurethane curing agent:

[0074] 750 g of Huntsman 5005PAPI and 250 g of bio-based polyol intermediate were added and kept at 85° C. for 6 hours to carry out affinity addition reaction. When the NCO content reached 21.3%, heating was stopped, the temperature was cooled to 60° C., and the material was filtered to obtain a solvent-free polyurethane curing agent.

[0075] Example 4

[0076] A method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0077] 1. Preparation of bio-based polyol intermediates:

[0078] (1) In a 1000 mL four-necked flask equipped with a stirring device, a thermometer and nitrogen protection, 512.6 g of castor oil, 185.7 g of polyether 1000 and 6 g of calcium oxide were added in sequence, and the mixture was placed in a reactor. The reactor was heated at 180° C. to carry out alcoholysis reaction. During the heating process, the contents of the reactor were stirred at a stirring speed of 200 rpm. The reactor was vacuumed and the contents were allowed to fully react for 2.5 h. When the hydroxyl value of the sample was 80 mgKOH / g, the temperature was lowered to 100° C. to proceed to the next step.

[0079] (2) Add 101.7 g of adipic acid and 22 g of dodecyl sulfuric acid according to the formula, raise the temperature to 110° C., stir the contents of the reactor during the heating process, and allow the contents of the reactor to fully undergo esterification reaction for 4.5 hours. When the acid value of the sample is 0.1 mg KOH / g, proceed to the next step;

[0080] (3) Stop heating, cool to 55° C., and filter the material to obtain a bio-based polyol intermediate; the hydroxyl value is 90 mgKOH / g, the acid value is 0.1 mgKOH / g, and the viscosity is 900 mPa.s (25° C.);

[0081] 2. Preparation of solvent-free polyurethane curing agent:

[0082] 750 g of Covestro 44V20 PAPI and 250 g of bio-based polyol intermediate were added and kept at 75° C. for 4 hours for affinity addition reaction. When the NCO content reached 21.5%, heating was stopped, the temperature was cooled to 55° C., and the material was filtered to obtain a solvent-free polyurethane curing agent.

[0083] Example 5

[0084] A method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0085] 1. Preparation of bio-based polyol intermediates:

[0086] (1) 550 g of rosin, 161.8 g of methyl propanediol, and 2.4 g of lithium hydroxide were added to a 1000 mL four-necked flask equipped with a stirring device, a thermometer, and nitrogen protection, and the mixture was placed in a reactor. The reactor was heated at 180° C. to carry out alcoholysis. During the heating process, the contents of the reactor were stirred at a stirring speed of 150 rpm. The reactor was vacuumed and the contents were allowed to fully react for 2 h. When the hydroxyl value of the sample was 90 mg KOH / g and the mixture was sampled and tested, the temperature was lowered to 100° C. to proceed to the next step.

[0087] (2) Add 87.4 g of adipic acid and 10 g of concentrated sulfuric acid according to the formula, raise the temperature to 100° C., stir the contents of the reactor during the heating process, and allow the contents of the reactor to fully undergo esterification reaction for 4.5 hours. When the acid value of the sample is 1 mg KOH / g, proceed to the next step;

[0088] (3) Stop heating, cool to 55° C., and filter the material to obtain a bio-based polyol intermediate; the hydroxyl value is 90 mgKOH / g, the acid value is 1 mgKOH / g, and the viscosity is 1200 mPa.s (25° C.);

[0089] 2. Preparation of solvent-free polyurethane curing agent:

[0090] 740 g of Wanhua PM-200PAPI and 260 g of bio-based polyol intermediate were added and kept at 75° C. for 4 h to carry out affinity addition reaction. When the NCO content reached 21.1%, heating was stopped, the temperature was cooled to 55° C., and the material was filtered to obtain a solvent-free polyurethane curing agent.

[0091] Example 6

[0092] A method for preparing a solvent-free polyurethane curing agent comprises the following steps:

[0093] 1. Preparation of bio-based polyol intermediates:

[0094] (1) In a 1000 mL four-necked flask equipped with a stirring device, a thermometer and nitrogen protection, 639.2 g of rosin, 62.5 g of ethylene glycol and 5 g of zinc acetate were added in sequence, and the mixture was placed in a reactor. The reactor was heated at 180° C. to carry out alcoholysis reaction. During the heating process, the contents of the reactor were stirred at a stirring speed of 150 rpm. The reactor was vacuumed and the contents were allowed to fully react for 2 h. When the hydroxyl value of the sample was 90 mg KOH / g, the temperature was lowered to 100° C. to proceed to the next step.

[0095] (2) Add 98.2 g of adipic acid and 20 g of sulfonic acid according to the formula, raise the temperature to 100° C. Stir the contents of the reactor during the heating process to allow the contents of the reactor to fully undergo esterification reaction for 5 h. When the acid value of the sample is 1.5 mg KOH / g, proceed to the next step;

[0096] (3) Stop heating, cool to 60° C., and filter the material to obtain a bio-based polyol intermediate; the hydroxyl value is 90 mgKOH / g, the acid value is 1.5 mgKOH / g, and the viscosity is 1400 mPa.s (25° C.);

[0097] 2. Preparation of solvent-free polyurethane curing agent:

[0098] 800 g of BASF M20s PAPI and 200 g of bio-based polyol intermediate were added and kept at 85° C. for 4 hours to carry out affinity addition reaction. When the NCO content reached 23.1%, heating was stopped, the temperature was cooled to 60° C., and the material was filtered to obtain a solvent-free polyurethane curing agent.

[0099] Comparative Example 1

[0100] The difference from Example 1 is that the bio-based polyol intermediate is replaced by polyester polyol, with a hydroxyl value of 95 mgKOH / g, an acid value of 0.3 mgKOH / g, and a viscosity of 5500 mPa.s (25° C.).

[0101] Comparative Example 2

[0102] The difference from Example 1 is that toluene diisocyanate (TDI) is replaced by polymethylene polyphenyl polyisocyanate.

[0103] Comparative Example 3

[0104] The difference from Example 1 is that the mass percentage of polymethylene polyphenyl polyisocyanate is 45%, and the mass percentage of the bio-based polyol intermediate is 55%.

[0105] Comparative Example 4

[0106] The difference from Example 1 is that the mass percentage of polymethylene polyphenyl polyisocyanate is 85%, and the mass percentage of the bio-based polyol intermediate is 15%.

[0107] Application Examples

[0108] A solvent-free polyurethane coating comprising the following components in percentage by mass:

[0109]

[0110] The above-mentioned method for preparing the solvent-free polyurethane coating comprises the following steps:

[0111] Alkyd resin 012, 3A activated powder, titanium dioxide, barium sulfate, and additives (dispersant, leveling agent, defoamer, wetting dispersant, and silica M-5) are mixed and dispersed at a speed of 2500-4000 rpm for 20-30 minutes to obtain a solvent-free polyurethane coating. The solvent-free polyurethane coating can be used for two-component floor paint, waterproof coating, architectural coating, etc.

[0112] When in use, the solvent-free polyurethane coating and the solvent-free polyurethane curing agent are mixed in a mass ratio of 2:1, and the solvent-free polyurethane curing agent is any one of Examples 1-6. After the paint liquid is prepared according to the ratio, it is allowed to stand for 10-15 minutes. After defoaming, it is directly applied by brushing, rolling, or spraying. The construction time is greater than 60 minutes. The construction conditions are: humidity 30-100%, temperature 10-40°C, and construction viscosity: 2000-4000mPa.s (25°C).

[0113] Product effect testing

[0114] Table 1 Performance test results of polyurethane curing agents prepared in various embodiments

[0115]

[0116]

[0117] Table 2 Performance test results of polyurethane curing agents prepared in each comparative example

[0118]

[0119]

[0120] As shown in Table 1 above, the solvent-free polyurethane curing agent prepared by the present invention has a low NCO value within the range of 21.1-23.2%. Due to the low NCO value, it is not easy to react with water, and thus is not likely to cause bubble problems. The viscosity is low, within the range of 800-1400 mPa·s (25° C.); the operability time is long, not less than 1 hour, and can be up to 75 minutes; the flexibility is ≤ Φ2 mm, and the flexibility is good; the volatile organic compound content is not higher than 0.4 g / L, and the contents of benzene, toluene, xylene, and free formaldehyde all meet the requirements. No heavy metals are detected. This shows that the solvent-free polyurethane curing agent prepared by the present invention is green and environmentally friendly, with little pollution to the environment.

[0121] As shown in Table 2, Comparative Example 1 uses polyester polyol instead of the bio-based polyol intermediate of the present invention. However, due to the high viscosity, high activity, and high functional group content of polyester polyol, the obtained solvent-free polyurethane curing agent has high viscosity, poor flexibility, and short operating time.

[0122] Comparative Example 2 uses TDI instead of the polymethylene polyphenyl polyisocyanate of the present invention. Since TDI, IPDI (isophorone diisocyanate), HDI (hexamethylene diisocyanate) and other substances have high content, high toxicity and high cost, they do not meet the requirements of green environmental protection.

[0123] Comparative Example 3 reduces the amount of polymethylene polyphenyl polyisocyanate and increases the amount of bio-based polyol intermediate. Due to the large amount of bio-based polyol intermediate and the high degree of modification, the obtained solvent-free polyurethane curing agent has high viscosity and is difficult to construct.

[0124] Comparative Example 4 increases the amount of polymethylene polyphenyl polyisocyanate and reduces the amount of bio-based polyol intermediate. Due to the large amount of polymethylene polyphenyl polyisocyanate and the high NCO content, the resulting solvent-free polyurethane curing agent has poor flexibility, high activity, and short operating time.

Claims

1. A method for preparing a solvent-free polyurethane curing agent, characterized in that: The steps include: (1) mixing a bio-based oil, a linear alcohol, and a first catalyst to perform an alcoholysis reaction, and then adding a linear acid and a second catalyst to perform an esterification reaction to obtain a bio-based polyol intermediate; (2) mixing the bio-based polyol intermediate prepared in step (1) with polymethylene polyphenyl polyisocyanate, and preparing the solvent-free polyurethane curing agent through a nucleophilic addition reaction; In step (2), the polymethylene polyphenyl polyisocyanate accounts for 50-80% by mass; and the bio-based polyol intermediate accounts for 20-50% by mass.

2. The preparation method according to claim 1, characterized in that In step (2), the polymethylene polyphenyl polyisocyanate accounts for 70-80% by mass, and the bio-based polyol intermediate accounts for 20-30% by mass.

3. The preparation method according to claim 1, characterized in that The raw material components for preparing bio-based polyol intermediates include, by mass percentage:

4. The preparation method according to claim 1, characterized in that In step (1), the alcoholysis reaction temperature is 180-230° C., and the time is 2-3 hours.

5. The preparation method according to claim 1, characterized in that In step (1), the temperature of the esterification reaction is 100-120° C., and the time is 1-2 hours.

6. The preparation method according to claim 1, characterized in that In step (2), the temperature of the nucleophilic addition reaction is 75-85° C. and the time is 4-6 h.

7. The preparation method according to claim 1, characterized in that In step (2), the isocyanate value of the polymethylene polyphenyl polyisocyanate is 25-35%.

8. The preparation method according to claim 1, characterized in that In step (2), the hydroxyl value of the bio-based polyol intermediate is 80-100 mgKOH / g, and the acid value is ≤3 mgKOH / g.

9. The solvent-free polyurethane curing agent prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The viscosity of the solvent-free polyurethane curing agent is 800-1400 mPa·s at 25°C.

10. Use of the solvent-free polyurethane curing agent according to any one of claims 1 to 8 in the preparation of adhesives, decorative coatings, paper coatings or leather finishing agents.

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