Preparation method and application of waterborne polyurethane dispersion liquid

Abstract

The invention relates to a preparation method of a waterborne polyurethane dispersion liquid, which comprises the following steps: heating polyester polyol, a solvent and a catalyst in a container to 50 DEG C, stirring, adding isocyanate in an inert atmosphere, continuously stirring, heating to 85 DEG C, and carrying out heat preservation reaction; adding a hydrophilic agent, reacting until the hydroxyl value in the hydrophilic agent is completely consumed, and adding a micromolecular chain extender and a cross-linking agent; cooling to 50 DEG C, adding a neutralizer into the system, continuously stirring, and standing at room temperature to obtain a prepolymer; slowly transferring the prepolymer after standing into a reaction kettle in which quantitative water is added, stirring, dropwise adding a defoaming agent into a system after the prepolymer is added, stirring at a high speed, adding a chain extender, reacting for 30 minutes, reducing the speed, and continuously stirring for 5 minutes, so as to obtain the waterborne polyurethane dispersion liquid. According to the invention, polyester polyol prepared from polyacid containing a benzene ring structure is selected, and a TMP cross-linking agent is added into a prepolymer system, so that the prepared waterborne polyurethane dispersion liquid has high stability, good adhesive force with BOPET (Biaxially Oriented Polyester), no adhesion after high temperature and high humidity, and organic solvent wiping resistance.

Description

Technical Field

[0001] The invention relates to a method for preparing waterborne polyurethane, in particular to the synthesis of a waterborne polyurethane dispersion liquid which can be applied to BOPET film. Background Art

[0002] The dispersion of water-based polyurethane used in optical BOPET is generally imported from abroad and has poor quality and poor dispersion stability. The coated film surface has many defects, such as gel particles, poor surface leveling, and film adhesion after high temperature and high humidity.

[0003] At present, the methods for producing waterborne polyurethane mainly include external emulsification method and internal emulsification method. Among them, the external emulsification method is gradually eliminated due to its poor product quality and poor physical and mechanical properties of the adhesive layer; the internal emulsification method can be divided into acetone method, prepolymer mixing method, melt dispersion method, ketimine / ketone azine method, and protected end group emulsification method. The internal emulsification method refers to the method of introducing hydrophilic groups into the molecular structure of polyurethane, so that it can disperse itself into an emulsion without an emulsifier, and thus has become the main method currently used in the production and research of waterborne polyurethane. Among them, the prepolymer method avoids the large-scale use of organic solvents, and the process is simple, environmentally friendly, and convenient for industrial continuous production. However, when preparing waterborne polyurethane dispersions by conventional prepolymer method, polyester polyols prepared from linear polyacids are mostly used as soft monomers, such as common adipic acid polyester polyols, which are also the most common in polyurethane synthesis. Although it has the advantages of good flexibility and low price, it is poor in solvent resistance and weather resistance. Summary of the invention

[0004] The invention provides a method for preparing an aqueous polyurethane dispersion to overcome the drawbacks of the prior art. The aqueous polyurethane is prepared by a prepolymer method, polyester polyol prepared from a polyacid containing a benzene ring structure is selected, and a TMP cross-linking agent is added to the prepolymer system. The prepared aqueous polyurethane dispersion has high stability, good adhesion to BOPET, no adhesion after high temperature and high humidity, and resistance to wiping with organic solvents.

[0005] The technical solution adopted by the present invention to solve its technical problem is:

[0006] A method for preparing an aqueous polyurethane dispersion, the preparation method being a prepolymer method, including a prepolymer stage and an emulsification stage, specifically:

[0007] a. Prepolymer stage:

[0008] Add polyester polyol, solvent and catalyst into a container, slowly heat to 50°C, stir at 350rpm, add isocyanate in an inert atmosphere, continue stirring until the system temperature is stable, heat to 85°C, keep warm for reaction, and the reaction time is 2h;

[0009] Adding a hydrophilic agent to the above reaction system, reacting until the hydroxyl value in the hydrophilic agent is completely consumed, and then adding a small molecule chain extender and a cross-linking agent;

[0010] The reaction system was cooled to 50°C, a neutralizing agent was added to the system, and the mixture was stirred for 1 hour and then allowed to stand at room temperature for 12 hours to obtain a prepolymer.

[0011] b. Emulsification stage:

[0012] The stationary prepolymer was transferred to a reaction kettle to which a certain amount of water had been added, and stirred at 1000 rpm. After the prepolymer was added, a defoamer was added dropwise to the system, and the mixture was stirred at a high speed of 3500 rpm. A post-chain extender was added, and the mixture was reacted for 30 min. The mixture was slowed down to 1000 rpm, and stirring was continued for 5 min. The reaction was stopped to obtain an aqueous polyurethane dispersion.

[0013] In the preparation method of the above aqueous polyurethane dispersion, the hydroxyl value of the polyester polyol is one or two of 55 mgKOH / g, 110 mgKOH / g and 220 mgKOH / g.

[0014] The preparation method of the above-mentioned aqueous polyurethane dispersion, the hydroxyl value of the polyester polyol is 220 mgKOH / g, and it is synthesized from a polyacid containing a benzene ring structure and a diol; the isocyanate is one or a combination of two of MDI, HDI, HMDI, NDI, and XDI, preferably HMDI, and the molar ratio of the isocyanate to the polyester polyol is 5:1 to 4:1.

[0015] In the preparation method of the above-mentioned aqueous polyurethane dispersion, the polyacid containing a benzene ring structure is one of terephthalic acid, phthalic acid, phthalic anhydride, and terephthalic anhydride, preferably phthalic anhydride; and the diol is one of ethylene glycol, butanediol, or hexanediol.

[0016] The preparation method of the above-mentioned aqueous polyurethane dispersion, the small molecule chain extender is a diamine or a diol, preferably a diol, the diol is one or more of ethylene glycol, propylene glycol, 1,4-butanediol and 1,6-hexanediol, preferably 1,4-butanediol (BDO); ​​the crosslinking agent is TMP, the total molar amount of the small molecule chain extender and the crosslinking agent is 25% of the molar amount of the isocyanate, and the molar ratio of the small molecule chain extender to the crosslinking agent is 2:1.

[0017] In the preparation method of the above-mentioned aqueous polyurethane dispersion, the hydrophilic agent is one or a combination of DMPA and DMBA, preferably DMPA, and its addition amount is 5-8% of the molar amount of isocyanate; the solvent is nitrogen methyl pyrrolidone, and the ratio of its addition mass to the prepared prepolymer mass is 2:5.

[0018] The preparation method of the above-mentioned aqueous polyurethane dispersion, the catalyst is one of titanium, antimony or tin, preferably a titanium catalyst, tetrabutyl titanate, and its addition amount accounts for 200 to 400 ppm of the total mass of polyester polyol and isocyanate; the neutralizing agent is an amine compound, such as triethylamine, which is added in the same proportion as the hydrophilic agent.

[0019] In the preparation method of the above-mentioned aqueous polyurethane dispersion, the post-chain extender is a diamine chain extender, and its added amount is 90% of the unreacted NCO molar amount in the prepolymer; in the emulsification stage, the added amount of water is 1.5 times the total weight of the prepolymer and the post-chain extender; the added amount of the defoamer is 3g.

[0020] In the preparation method of the above aqueous polyurethane dispersion, the solid content of the aqueous polyurethane dispersion prepared in the emulsification stage is 30%, the particle size of the dispersion is 0.1 μm to 1.0 μm, and the number average molecular weight of the aqueous polyurethane is 30,000 to 40,000.

[0021] An application of an aqueous polyurethane dispersion, wherein the aqueous polyurethane dispersion prepared by the above method is used for online coating of BOPET film. The refractive index of the aqueous polyurethane dispersion after being coated on the BOPET film into a dry film is 1.50-1.55, and the Tg of the aqueous polyurethane after being made into a dry film is 100°C.

[0022] The beneficial effects of the present invention are:

[0023] The polyester polyol selected in the preparation process of the aqueous polyurethane dispersion of the present invention is synthesized from phthalic anhydride and diol, and contains a benzene ring structure to obtain a polyacid. Because it contains a benzene ring structure, the rigidity and stability of the polymer are increased, and the solvent resistance is better. Phthalic anhydride is further selected, and the ester group formed by the molecule is in the ortho position, and has good solubility when reacting with the diol for esterification. The prepared polyester polyol has low viscosity and excellent chemical resistance. TMP cross-linking agent is also added in the preparation process of the prepolymer, which forms a three-dimensional network structure in the prepolymer reaction, so that the prepared polyurethane is more solvent-resistant. DETAILED DESCRIPTION

[0024] The present invention adopts a prepolymer method to prepare an aqueous polyurethane dispersion, wherein a prepolymer containing a hydrophilic group is dispersed in water under high-speed stirring, and a post-chain extender is added while emulsifying in water to extend the chain. The prepared aqueous polyurethane dispersion has a refractive index of 1.50 to 1.55 after being coated on a BOPET film to form a dry film, a particle size of the aqueous polyurethane dispersion is 0.1 μm to 1.0 μm, a number average molecular weight of 30,000 to 40,000, and a Tg of 100° C. after the aqueous polyurethane is made into a dry film.

[0025] The synthesis of the aqueous polyurethane dispersion of the invention comprises two stages: a prepolymer stage and an emulsification stage. The synthetic reactants of the prepolymer comprise polyol, isocyanate, a small molecule chain extender, a hydrophilic agent, a catalyst, a solvent, a crosslinking agent and a neutralizing agent.

[0026] The polyol of the present invention is selected from one or two of polyester polyol, polycaprolactone polyol and polycarbonate diol, preferably polyester polyol, and its hydroxyl value is selected from one or two of 55, 110 and 220 mgKOH / g, preferably 220 mgKOH / g of polyester polyol synthesized from phthalic anhydride and diol. The polyacid used to synthesize polyester polyol can be selected from linear adipic acid and terephthalic acid, phthalic acid, phthalic anhydride and terephthalic anhydride containing benzene ring structure; wherein, linear polyacid molecular chain rotation angle is large, toughness is good, but solvent resistance and weather resistance are poor; and polyacid containing benzene ring structure, because it contains benzene ring structure itself, increases the rigidity and stability of polymer, so it has excellent solvent resistance. When synthesizing polyurethane in the present invention, a polyacid containing a benzene ring structure is preferably selected; the polyacid containing a benzene ring structure is preferably phthalic anhydride, because the intramolecular ester group formed by phthalic anhydride is in the ortho position, and has good solubility when reacting with polyols for esterification, and the prepared polyester polyol has low viscosity and excellent chemical resistance.

[0027] The isocyanate of the present invention is selected from one or a combination of two of MDI, HDI, HMDI, NDI and XDI. Considering the yellowing resistance and reaction rate control of the aqueous polyurethane dispersion, the isocyanate is preferably HMDI, because the isocyanate containing a benzene ring has a benzene ring structure in its structure, and the reaction rate is too fast when synthesizing the aqueous polyurethane system, and it is easy to produce gel.

[0028] The small molecule chain extender for synthesizing the aqueous polyurethane dispersion of the present invention is selected from diamines or diols, preferably diols. The diols are selected from one or more of ethylene glycol, propylene glycol, 1,4-butanediol and 1,6-hexanediol, and are preferably selected from 1,4-butanediol, hereinafter referred to as BDO, in consideration of molecular activity and molecular weight after chain extension.

[0029] The molar ratio of the small molecule chain extender BDO and the crosslinker TMP of the present invention is 2:1, and the total molar amount of the two accounts for 25% of the molar amount of isocyanate in the prepolymer. The total content of the chain extender and TPM accounts for a high proportion, resulting in a high molecular weight, a high degree of crosslinking, and easy gelation. If the molecular weight is too low, it is not enough and the chemical resistance is poor.

[0030] The neutralizing agent used for salt formation in the present invention is an amine compound, specifically triethylamine, and the salt formation amount is added in the same proportion as the hydrophilic agent DMPA. In the prepolymer, too little salt formation leads to poor dispersibility in water, while too high a salt formation ratio leads to good solubility in water, but poor water resistance.

[0031] The catalyst used for prepolymer synthesis of the present invention is selected from one of titanium, antimony or tin. Further, the catalyst in the prepolymer is preferably a titanium catalyst, tetrabutyl titanate, which is non-toxic and is preferred in the synthesis process. The addition amount accounts for 200-400ppm of the total mass of polyester polyol and isocyanate in the prepolymer. Too little addition leads to a very slow reaction rate, and too much addition leads to a rapid reaction and easy to produce implosion.

[0032] The solvent selected is nitrogen-methyl pyrrolidone; the mass ratio of the solvent used: prepolymer is about 2:5. Too little solvent leads to high solution viscosity, which is not conducive to molecular chain extension. Small amount of solvent leads to slow reaction rate.

[0033] The post-chain extender added in the emulsification stage is a diamine chain extender, preferably isophoronediamine, hereinafter referred to as IPDA.

[0034] Furthermore, the IPDA added in the reaction is 90% of the molar number of unreacted NCO in the prepolymer. The remaining 10% unreacted NCO in the final waterborne polyurethane system slowly reacts with water to produce urea. Urea can significantly improve the chemical resistance of waterborne polyurethane. If the urea content is small, the effect is not obvious. If the urea content is large, gel will be produced in the final waterborne polyurethane emulsion.

[0035] Furthermore, during the emulsification stage, the amount of water added is 1.5 times the total weight of the prepolymer and IPDA, which can ensure that the viscosity of the system is not too high and that the waterborne polyurethane can be well dispersed during emulsification.

[0036] Furthermore, the defoamer used in the emulsification stage of the waterborne polyurethane is selected from Evonik Foamex812, added amount is 3g.

[0037] Furthermore, the final solid content of the aqueous polyurethane dispersion is about 30%.

[0038] The present invention provides a method for preparing an aqueous polyurethane dispersion comprising the following steps:

[0039] 1. Prepolymer stage:

[0040] Add polyester polyol, solvent and catalyst into a 4-necked flask, slowly heat up to 50°C, start stirring, and maintain the stirring speed at 350rpm; maintain the synthesis system in an inert gas atmosphere, add isocyanate and continue stirring, and when the temperature of the reaction system stabilizes, heat up to 85°C and keep warm until the polyester polyol reacts completely.

[0041] The hydrophilic agent is added into the above reaction system, the hydrophilic agent is introduced, the reaction is continued until the hydroxyl value in the hydrophilic agent is completely consumed, and then a small molecule chain extender and a cross-linking agent are added.

[0042] In the neutralization and salt formation process, the reaction system was cooled to 50°C, triethylamine was added to neutralize the salt, the system was stirred for 1 hour, bottled and allowed to stand at room temperature overnight.

[0043] 2. Emulsification stage

[0044] Pour the prepolymer to a reactor that has been added with a certain amount of water, slowly add it, and start stirring, maintaining the stirring speed at 1000rpm. After the addition is completed, add the defoamer dropwise and adjust the stirring speed to 3500rpm, add a certain mole of post-chain extender, react for 30min, reduce the speed to 1000rpm, continue stirring for 5min, and then stop the reaction.

[0045] 3. Observe the appearance of the prepared aqueous polyurethane dispersion, make a film, and apply coating.

[0046] Some properties of the aqueous polyurethane dispersion of the present invention are tested according to the following methods:

[0047] Glass transition temperature Tg: The prepared aqueous polyurethane dispersion was coated on tin foil, dried in an oven at 75°C, and then sampled for testing. A DSC instrument was used for testing with a temperature setting range of room temperature to 200°C and a heating rate of 10°C / min. The glass transition temperature was recorded and marked.

[0048] Particle size test of waterborne polyurethane dispersion: laser particle size analyzer is used for testing, 3 sets of data are tested for each sample, the D50 value is recorded, and the average value is taken as the particle size value.

[0049] Molecular weight test of waterborne polyurethane dispersion: The prepared waterborne polyurethane dispersion is coated on tin foil, dried in an oven at 75°C, and then sampled for testing. A GPC instrument is used, and 3 samples are taken for each sample. The sample is dissolved in tetrahydrofuran for testing, and the average value of the number average molecular weight is recorded and calculated as the molecular weight of the sample. The polydispersity index PDI of the polymer is also recorded. Generally, the smaller the PDI, the more uniform the molecular weight of the polymer. The theoretical PDI is <2, and the molecular weight distribution of the polymer is within an acceptable range.

[0050] Refractive index: JAW2000 was used for testing. The aqueous polyurethane dispersion was coated on BOPET, dried, and tested. The refractive index of the coating was obtained by fitting the Cauchy model.

[0051] Adhesion: According to ISO 2409 standard, place the coated film on smooth marble, use a grating knife to cut a cross grid pattern on the coating, stick 3M-610 tape on the cut and pull it apart, place the film under a 3D laser microscope, observe the coating shedding at 200 times, and record the grade.

[0052] Resistance to ethyl acetate: Wet the dust-free cloth with ethyl acetate, wipe the coated film, and observe the film against the light. If the coating does not fall off, it is resistant to ethyl acetate. If the coating falls off, it is considered a defective sample.

[0053] Prepolymerization time determination: urea is produced by reacting the isocyanate group in the prepolymer with excess di-n-butylamine, and then the excess di-n-butylamine is titrated with hydrochloric acid to quantitatively calculate the content of isocyanate groups. Weigh a small amount of solution, add 10mL toluene, dissolve, and add 20mL di-n-butylamine toluene solution, shake well, and place at room temperature for 30 minutes. Add bromocresol green as an indicator and use 0.5mol / mL hydrochloric acid as an indicator. When the solution changes from blue to yellow and does not fade for 15 seconds, it is the end point of the titration, and record the volume used V 样 At the same time, a blank experiment was performed and the volume of hydrochloric acid used for the blank, V0, was recorded. According to the formula n = (V-V0)*C / 1000; where n is the amount of unreacted isocyanate and C is the molar concentration of hydrochloric acid; the time required for n to reach the theoretical value was recorded.

[0054] In order to make it easier to understand the features of the present invention, the technical solution provided by the present invention is described in detail below in conjunction with embodiments.

[0055] Example 1

[0056] a. Prepolymer stage:

[0057] 0.4 mol of polyester polyol (prepared from phthalic anhydride and butanediol) with a hydroxyl value of 220 mgKOH / g, nitrogen methyl pyrrolidone, and 200 ppm of tetrabutyl titanate were placed in a container, slowly heated to 50°C, stirred at 350 rpm, and 2 mol of HMDI was added in an inert atmosphere. The mixture was stirred until the system temperature was stable, and then heated to 85°C and kept warm for 2 hours.

[0058] Add 0.1 mol of DMPA to the above reaction system, react until the hydroxyl value in the hydrophilic agent is completely consumed, and then add 0.5 mol of a small molecule chain extender (BDO) and a cross-linking agent (TMP);

[0059] The reaction system was cooled to 50°C, 0.1 mol of a neutralizing agent, triethylamine, was added to the system, and the mixture was stirred for 1 hour and then allowed to stand at room temperature for 12 hours to obtain a prepolymer.

[0060] b. Emulsification stage:

[0061] The static prepolymer was slowly transferred to the reactor to which 1.5 times the total weight of the prepolymer and IPDA was added, and stirred at 1000 rpm. After the prepolymer was added, 3 g of defoamer (Evonik's Foamex812), high-speed stirring at 3500 rpm, adding 0.83 mol of post-chain extender (IPDA), reacting for 30 minutes, reducing the speed to 1000 rpm, continuing stirring for 5 minutes, stopping the reaction, and obtaining an aqueous polyurethane dispersion. The various properties of the prepared aqueous polyurethane dispersion are shown in Table 3.

[0062] The synthesis formulas of other embodiments 2-7 are shown in Table 1 and Table 2. The selection criteria of each component are the same as those of embodiment 1, the synthesis method is the same as that of embodiment 1, and the properties of the prepared aqueous polyurethane dispersion are shown in Table 3.

[0063] Table 1 Synthesis formula of Examples 1-7 and Comparative Example 1

[0064]

[0065] Table 2 Synthesis formula of Examples 1-7 and Comparative Example 1

[0066]

[0067] Table 3 Properties of waterborne polyurethane prepared in Examples and Comparative Examples

[0068] Example Tg℃ Particle size nm Molecular weight PDI Refractive Index Adhesion Resistant to double ethylene Prepolymerization time h Example 1 107.5 573 37590 1.52 1.52 OK No shedding 3.5 Example 2 106.4 486 37482 1.52 1.53 OK No shedding 3.5 Example 3 106.3 392 37491 1.53 1.53 OK No shedding 3.5 Example 4 105.8 301 37479 1.52 1.52 OK No shedding 3.5 Example 5 103.4 387 35891 1.63 1.52 OK No shedding 2.8 Example 6 102.4 395 33287 1.72 1.51 OK No shedding 1.9 Example 7 108.9 425 38791 1.64 1.54 OK No shedding 2.5 Comparative Example 1 62.8 289 17264 1.35 1.50 NG Shedding 2.8

[0069] As shown in Table 3, Examples 1-7 and Comparative Example 1 show that adding an appropriate amount of TMP to the prepolymer can improve the diethyltoluene adhesion performance of the waterborne polyurethane film after coating. Increasing the content of the catalyst in Examples 1-7 will shorten the polymerization time, but at the same time the polydispersity of the polymer will also increase; increasing the amount of the salt-forming neutralizer will reduce the particle size; comprehensive comparison, if the reaction time and molecular polydispersity are considered, Example 5 has the best performance. In Comparative Example 1, the total amount of small molecule chain extender and crosslinker added exceeds 25% of the molar amount of isocyanate, and the aqueous polyurethane dispersion synthesized therefrom has poor adhesion and poor solvent resistance after coating.

[0070] The above is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. All equivalent changes and modifications made according to the content of the present invention are included in the patent scope of the present invention.

Claims

1. A method for preparing an aqueous polyurethane dispersion, characterized in that: The preparation method is a prepolymer method, including a prepolymer stage and an emulsification stage, specifically: a. Prepolymer stage: Place polyester polyol, solvent and catalyst in a container, slowly heat to 50°C, stir at 350rpm, add isocyanate in an inert atmosphere, continue stirring until the system temperature is stable, heat to 85°C, keep warm and react for 2h; Adding a hydrophilic agent to the above reaction system, reacting until the hydroxyl value in the hydrophilic agent is completely consumed, and then adding a small molecule chain extender and a cross-linking agent; The reaction system was cooled to 50°C, a neutralizing agent was added to the system, and the mixture was stirred for 1 hour and then allowed to stand at room temperature for 12 hours to obtain a prepolymer. b. Emulsification stage: The stationary prepolymer was slowly transferred to a reaction kettle to which a certain amount of water had been added, and stirred at 1000 rpm. After the prepolymer was added, a defoamer was added dropwise to the system, and the mixture was stirred at 3500 rpm. A post-chain extender was added, and the mixture was reacted for 30 min. The mixture was slowed down to 1000 rpm, and stirring was continued for 5 min. The reaction was stopped to obtain an aqueous polyurethane dispersion.

2. The method for preparing an aqueous polyurethane dispersion according to claim 1, characterized in that: The hydroxyl value of the polyester polyol is one or two of 55 mgKOH / g, 110 mgKOH / g and 220 mgKOH / g.

3. The method for preparing an aqueous polyurethane dispersion according to claim 2, characterized in that: The polyester polyol has a hydroxyl value of 220 mgKOH / g and is synthesized from a polyacid containing a benzene ring structure and a diol; the isocyanate is one or a combination of two of MDI, HDI, HMDI, NDI, and XDI, and the molar ratio of the isocyanate to the polyester polyol is 5:1 to 4:

1.

4. The method for preparing an aqueous polyurethane dispersion according to claim 3, characterized in that: The polyacid containing a benzene ring structure is one of terephthalic acid, phthalic acid, phthalic anhydride and terephthalic anhydride; the diol is one of ethylene glycol, butanediol and hexanediol.

5. The method for preparing an aqueous polyurethane dispersion according to claim 1, characterized in that: The small molecule chain extender is a diamine or a diol, and the diol is one or more of ethylene glycol, propylene glycol, 1,4-butanediol and 1,6-hexanediol; the crosslinking agent is TMP, the total molar amount of the small molecule chain extender and the crosslinking agent is 25% of the molar amount of the isocyanate, and the molar ratio of the small molecule chain extender to the crosslinking agent is 2:

1.

6. The method for preparing an aqueous polyurethane dispersion according to claim 1, characterized in that: The hydrophilic agent is one or a combination of DMPA and DMBA, and the added amount is 5-8% of the molar amount of isocyanate; the solvent is nitrogen methyl pyrrolidone, and the added mass ratio of the added mass to the prepared prepolymer is 2:

5.

7. The method for preparing an aqueous polyurethane dispersion according to claim 6, characterized in that: The catalyst is one of titanium, antimony or tin, and its added amount accounts for 200-400 ppm of the total mass of polyester polyol and isocyanate; the neutralizer is an amine compound, which is added in the same proportion as the hydrophilic agent.

8. The method for preparing an aqueous polyurethane dispersion according to claim 1, characterized in that: The post-chain extender is a diamine chain extender, and its addition amount is 90% of the unreacted NCO molar amount in the prepolymer; in the emulsification stage, the addition amount of water is 1.5 times the total weight of the prepolymer and the post-chain extender; the addition amount of the defoamer is 3g.

9. The method for preparing an aqueous polyurethane dispersion according to claim 1, characterized in that: The solid content of the aqueous polyurethane dispersion prepared in the emulsification stage is 30%.

10. An application of an aqueous polyurethane dispersion, characterized in that: The aqueous polyurethane dispersion prepared according to any one of claims 1 to 9 is used for online coating of BOPET film.

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