Waterborne polyurethane emulsion with high bio-based content and preparation method thereof

Abstract

The invention discloses a waterborne polyurethane emulsion with high bio-based content and a preparation method thereof, and belongs to the field of polyurethane materials. The preparation method comprises the following steps: mixing aliphatic diisocyanate, castor oil and an internal emulsifier 2, 2-dimethylolbutyric acid, and stirring for 15-45 minutes at the temperature of 45-55 DEG C and the rotating speed of 100-250 r / min in a nitrogen environment; adding a catalyst dibutyltin dilaurate, heating to 75-85 DEG C, and stirring for 2-3 hours at the rotating speed of 400-750 r / min; plant polyphenol is added, and stirring continues to be conducted for 1.5-2 h at the rotating speed of 400-750 r / min under the condition of 75-85 DEG C; the waterborne polyurethane emulsion is prepared. The preparation method is simple, and the raw materials are renewable; the obtained waterborne polyurethane emulsion has relatively high bio-based content, and the bio-based content reaches 50% or above; the obtained waterborne polyurethane coating also has application performances such as good elasticity, relatively high mechanical strength and friction resistance, high hardness and the like.

Description

Technical Field

[0001] The invention belongs to the field of polyurethane materials, and in particular relates to an aqueous polyurethane emulsion with a high bio-based content and a preparation method thereof. Background Art

[0002] Polyurethane is a kind of high molecular polymer containing carbamate groups synthesized from polyisocyanates, polyols and small molecule diols or diamines. There are hard segments (carbamate segments) and soft segments (polyol segments) in the structure of its molecular chain. The soft and hard alternating structure makes it show good elasticity, high mechanical strength and friction resistance, high hardness and other properties. Therefore, it has been widely used in the fields of coatings, leather, fibers, foams, inks, etc. However, most of the polyurethane products currently used are solvent-based systems. The large amount of volatile organic compounds and harmful air pollutants released during production and use limits their application. Therefore, it is imperative to develop low-viscosity, low-cost, safe and environmentally friendly water-based polyurethanes that use water instead of organic solvents as a dispersion medium.

[0003] Biobased content usually refers to the proportion of ingredients in a material that come from renewable biological resources. The advantages of high biobased content are as follows: high biobased content means that the product uses more renewable resources, thereby reducing dependence on non-renewable resources such as oil and reducing environmental burden; biobased raw materials usually come from crops or other renewable resources, and their growth and regeneration cycles are relatively short, which helps to achieve sustainable development; compared with petroleum-based materials, biobased materials tend to have better biodegradability, which helps to reduce the impact of waste on the environment; biobased materials are generally less toxic and more friendly to humans and the environment.

[0004] However, the existing preparation technology of highly bio-based waterborne polyurethane still has some defects and challenges that need to be solved. First, bio-based monomers usually contain polar functional groups, which lead to mismatch of reactivity with isocyanate and poor compatibility between the two; secondly, highly bio-based waterborne polyurethane emulsions are prone to uneven particle size, separation and demulsification; thirdly, bio-based monomers usually contain hydrophilic groups, which leads to poor water resistance, durability and weather resistance of the prepared waterborne polyurethane coating, and its soft molecular chain structure will affect the tensile strength and wear resistance of the coating; finally, the preparation cost of bio-based monomers is high, and the supply of raw materials is unstable. These factors limit the large-scale application of bio-based waterborne polyurethane.

[0005] Therefore, it is of great significance to develop a high-biobased content and high-strength polyurethane. Summary of the invention

[0006] The object of the present invention is to overcome at least one disadvantage of the prior art and to provide an aqueous polyurethane emulsion and a preparation method thereof.

[0007] The technical solution adopted in this application is:

[0008] A method for preparing an aqueous polyurethane emulsion comprises the following steps:

[0009] (1) mixing aliphatic diisocyanate, castor oil and internal emulsifier 2,2-dimethylolbutyric acid, and stirring at 45° C. to 55° C. and 100 to 250 r / min for 15 to 45 minutes under a nitrogen environment;

[0010] (2) adding a catalyst, dibutyltin dilaurate, heating to 75°C to 85°C, and stirring at a speed of 400 to 750 r / min for 2 to 3 hours;

[0011] (3) adding plant polyphenols and continuing stirring at a speed of 400 to 750 r / min at 75° C. to 85° C. for 1.5 to 2 h;

[0012] (4) Cooling to room temperature, then adding triethylamine and stirring at 100-250 r / min for 25-40 min;

[0013] (5) Add an appropriate amount of water and stir at a speed of 1200 to 1800 r / min for 1 to 2 hours to prepare an aqueous polyurethane emulsion.

[0014] In some embodiments, the aliphatic diisocyanate in step (1) is one of hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexane diisocyanate, xylylene diisocyanate, trimethylhexamethylene diisocyanate and tetramethyl-m-xylylene diisocyanate.

[0015] In some embodiments, the molar ratio of the castor oil to the hydroxyl group of the 2,2-dihydroxymethylbutyric acid in step (1) is 1:(0.6-0.8).

[0016] In some of the embodiments, the amount of dibutyltin dilaurate added in step (2) is 0.1% to 0.3% of the total mass of the reaction materials in step (1).

[0017] In some embodiments, the plant polyphenol in step (3) is one of arbutin, catechin, ellagic acid and curcumin.

[0018] In some embodiments, the molar ratio of the plant polyphenols to the hydroxyl group of 2,2-dihydroxymethylbutyric acid in step (3) is (0.2-0.5):1.

[0019] In some of the embodiments, the molar ratio of the total amount of isocyanate groups in the aliphatic diisocyanate to the total amount of hydroxyl groups in castor oil, internal emulsifier 2,2-dihydroxymethylbutyric acid and plant polyphenols is 1:(0.83-1.10).

[0020] In some of the embodiments, the molar ratio of triethylamine added in step (4) to 2,2-dihydroxymethylbutyric acid added in step (1) is 1:1.

[0021] In some of the embodiments, the amount of water added in step (5) is 2 to 4 times the total mass of all substances added in steps (1) to (4).

[0022] In some of the embodiments, in step (5), a proper amount of water is added and the stirring is maintained at 1200-1800 r / min for 1-2 hours to obtain a water-based polyurethane emulsion with a uniform texture and a translucent blue luster.

[0023] A waterborne polyurethane emulsion is obtained by any of the above preparation methods.

[0024] The use of the above-mentioned aqueous polyurethane emulsion in the preparation of synthetic leather.

[0025] In some embodiments, the synthetic leather is microfiber synthetic leather.

[0026] The beneficial effects of this application are:

[0027] (1) The preparation method is simple and the raw materials are renewable;

[0028] (2) The aqueous polyurethane emulsion has a high bio-based content of more than 50%;

[0029] (3) The obtained aqueous polyurethane emulsion is applied in synthetic leather. The obtained aqueous polyurethane coating has application properties such as good elasticity, high mechanical strength and friction resistance, and high hardness. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The aqueous polyurethane emulsion obtained by the preparation method of the present application is translucent and has a bluish appearance. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in the field or the product instructions are used. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be purchased through regular channels.

[0033] Plant polyphenol modified waterborne polyurethane emulsion is a new type of material that improves the performance of waterborne polyurethane by introducing plant polyphenols. Plant polyphenols have abundant phenolic hydroxyl groups and hydrophobic aromatic ring structures, which can form hydrogen bonds, metal coordination or π-π interactions with polyurethane chain segments, thereby improving the mechanical properties, water resistance and thermal stability of the material.

[0034] A method for preparing an aqueous polyurethane emulsion comprises the following steps:

[0035] (1) mixing aliphatic diisocyanate, castor oil and internal emulsifier 2,2-dimethylolbutyric acid, and stirring at 45° C. to 55° C. and 100 to 250 r / min for 15 to 45 minutes under a nitrogen environment;

[0036] (2) adding a catalyst, dibutyltin dilaurate, heating to 75°C to 85°C, and stirring at a speed of 400 to 750 r / min for 2 to 3 hours;

[0037] (3) adding plant polyphenols and continuing stirring at a speed of 400 to 750 r / min at 75° C. to 85° C. for 1.5 to 2 h;

[0038] (4) Cooling to room temperature, then adding triethylamine and stirring at 100-250 r / min for 25-40 min;

[0039] (5) adding deionized water and stirring at a speed of 1200 to 1800 r / min for 1 to 2 hours to prepare an aqueous polyurethane emulsion.

[0040] In some embodiments, the aliphatic diisocyanate in step (1) is one of hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexane diisocyanate, xylylene diisocyanate, trimethylhexamethylene diisocyanate and tetramethyl-m-xylylene diisocyanate.

[0041] More preferably, it is at least one of hexamethylene diisocyanate and isophorone diisocyanate. The polyurethane prepared from hexamethylene diisocyanate has high tensile strength and elastic modulus, while the polyurethane prepared from isophorone diisocyanate has excellent flexibility and impact resistance.

[0042] In some embodiments, the molar ratio of the castor oil to the hydroxyl group of the 2,2-dihydroxymethylbutyric acid in step (1) is 1:(0.6-0.8).

[0043] Castor oil is a natural oil extracted from castor seeds. It has a short growth cycle, is easy to grow on a large scale, and has low cost. In addition, castor oil is one of the few natural plant oils with multiple hydroxyls. Its unique chemical structure makes it perform well in reactions such as polyesterification and polycondensation.

[0044] The selection and proportion of the internal emulsifier 2,2-dimethylolbutyric acid is crucial, and it has great significance for the particle size and stability of the emulsion. The present application can obtain a waterborne polyurethane emulsion with uniform particles and good stability by limiting the amount of castor oil and 2,2-dimethylolbutyric acid.

[0045] In some embodiments, the amount of dibutyltin dilaurate added in step (2) is 0.1% to 0.3% of the total mass of the reaction substances in step (1). The addition of dibutyltin dilaurate can improve the reaction activity.

[0046] In some embodiments, the plant polyphenol in step (3) is one of arbutin, catechin, ellagic acid and curcumin.

[0047] Plant polyphenols are a class of important and abundant polyphenolic secondary metabolites that are widely present in plants. They have multifunctional properties such as antioxidant, antibacterial, and UV shielding. Introducing them into waterborne polyurethane can not only enhance the functionality of the material, but also make it more environmentally friendly and sustainable.

[0048] Arbutin, foreign name: hydroquinone O-β-D-glucopyranoside, alias: p-hydroxyphenyl-β-D-pyranose glucoside, arbutin, chemical formula is C 12 H 16 O 7 , the structural formula is: Arbutin is an ingredient extracted from the bearberry leaves of the Ericaceae plant. It can inhibit the activity of tyrosinase in the body and prevent the production of melanin, thereby reducing skin pigmentation, removing spots and freckles, and also has bactericidal and anti-inflammatory effects.

[0049] Catechin, foreign name: catechin, Cianidanol, alias: catechin, catechin acid, tea tannin; chemical formula is C 15 H 14 O 6 , the structural formula is Catechins are a class of phenolic active substances extracted from natural plants such as tea. They have multiple pharmacological effects such as anti-tumor, antioxidant, antibacterial, and protection of heart and brain organs. These pharmacological activities are mainly determined by their polyhydroxy structure. However, the polyhydroxy structure also makes catechins structurally unstable under neutral and alkaline conditions.

[0050] Ellagic acid, foreign name: Ellagic acid, alias: di-(trihydroxy formic acid), bis-(trihydroxy formic acid), elaiodic acid; chemical formula is C 14 H 6 O 8 , the structural formula is Ellagic acid is a polyphenol dilactone and a dimer derivative of gallic acid. Ellagic acid is widely present in various plant tissues such as soft fruits and nuts. Ellagic acid has a variety of biologically active functions, such as antioxidant function, anti-cancer, anti-mutagenic properties, and inhibitory effects on human immunodeficiency virus. In addition, ellagic acid is also an effective coagulant, which has a good inhibitory effect on a variety of bacteria and viruses, can protect the wound from bacterial invasion, prevent infection, and inhibit ulcers. At the same time, studies have found that ellagic acid also has antihypertensive and sedative effects.

[0051] Curcumin, foreign name: Curcumin, alias: acid yellow, 1,7-bis (4-hydroxy-3-methoxyphenyl) -1,6-diene-3,5-heptanedione, molecular formula is C 21 H 20 O 6 , the structural formula is Curcumin is a natural phenolic antioxidant extracted from the rhizomes of turmeric (containing 3-6% curcumin), zedoaryl, mustard, curry, turmeric, etc., which are plants of the ginger family. The main chain is unsaturated aliphatic and aromatic groups, and the structure is diarylheptane. It is not only a diketone compound, but also a very rare pigment with a diketone structure in the plant kingdom. Because curcumin has anti-tumor, antioxidant, antibacterial, liver protection, and lipid-lowering effects, it is widely used in medicine, food, breeding (feed), daily chemical products and other fields.

[0052] The phenolic hydroxyl group and aromatic ring structure in the plant polyphenols in the present application have excellent free radical capture capabilities, and can effectively inhibit the oxidation reaction of unsaturated double bonds in castor oil-based polyurethane, delay the aging process of the material, and enhance the durability of the material; secondly, the plant polyphenol compound in the present application can inhibit bacterial growth by binding to bacterial cell membranes, interfering with enzyme activity or destroying cell structures, thereby enhancing the antibacterial properties of the material; furthermore, the conjugated structure in the plant polyphenols in the present application can absorb ultraviolet rays to reduce the degradation effect of ultraviolet rays on polyurethane materials, thereby improving the weather resistance of the material; finally, the plant polyphenols interact with the covalent bonds between the polyurethane molecular chains to increase the cross-linking density, thereby improving the wear resistance and tear resistance of the coating.

[0053] In some embodiments, the molar ratio of the plant polyphenols to the hydroxyl group of 2,2-dihydroxymethylbutyric acid in step (3) is (0.2-0.5):1.

[0054] In some of the embodiments, the molar ratio of the total amount of isocyanate groups in the aliphatic diisocyanate to the total amount of hydroxyl groups in castor oil, internal emulsifier 2,2-dihydroxymethylbutyric acid and plant polyphenols is 1:(0.83-1.10).

[0055] The present application can ensure the full completion of the reaction and obtain a water-based polyurethane emulsion with a high bio-based content by selecting the above components and limiting their contents.

[0056] In some of the embodiments, the molar ratio of triethylamine added in step (4) to 2,2-dihydroxymethylbutyric acid added in step (1) is 1:1.

[0057] In some of the embodiments, the amount of water added in step (5) is 2 to 4 times the total mass of all substances added in steps (1) to (4).

[0058] A waterborne polyurethane emulsion is obtained by any of the above preparation methods.

[0059] The aqueous polyurethane emulsion obtained by the preparation method of the present application is translucent and has a bluish appearance. Figure 1 .

[0060] The use of the above-mentioned aqueous polyurethane emulsion in the preparation of synthetic leather.

[0061] A method for preparing waterborne polyurethane ultra-fiber synthetic leather comprises the following steps:

[0062] First, the base fabric is pretreated to improve its bonding strength with the polyurethane coating; then the waterborne polyurethane dispersion is evenly coated on the surface of the substrate and pre-baked to form a polyurethane (PU) coating; then the coating and the base fabric are bonded by hot pressing; finally, the material is given specific functional and appearance characteristics through post-finishing processes such as stretching and embossing to obtain waterborne polyurethane microfiber synthetic leather.

[0063] The base fabric comprises one of woven fabric, woven fabric, spunbonded nonwoven fabric, spunlace nonwoven fabric and meltblown nonwoven fabric, and the pretreatment process comprises cleaning, surface activation and primer coating.

[0064] The pre-baking temperature is 60-120° C., the pre-baking time is 2-10 minutes, and the thickness of the polyurethane coating is 0.3-1 mm.

[0065] The temperature of the hot pressing treatment is 120-150° C., and the time of the hot pressing treatment is 1-3 minutes.

[0066] The obtained waterborne polyurethane coating on the waterborne polyurethane microfiber synthetic leather has application properties such as good elasticity, high mechanical strength and friction resistance, high hardness and the like.

[0067] The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0068] Example 1

[0069] A method for preparing a high bio-based content water-based polyurethane emulsion comprises the following steps:

[0070] (1) First, 4.23 g of isophorone diisocyanate, 7.80 g of castor oil and 1.01 g of 2,2-dimethylolbutyric acid were mixed, and stirred at 45° C. and 250 r / min for 30 min under a nitrogen environment;

[0071] (2) adding 40 uL of catalyst dibutyltin dilaurate to the obtained mixture, heating to 80° C., and stirring at 500 r / min for 2 h;

[0072] (3) Add 0.074 g of arbutin to the reaction system and stir at 500 r / min for 2 h;

[0073] (4) The temperature of the reaction system was lowered to room temperature, and then 0.69 g of triethylamine was added, and the mixture was stirred at 200 r / min for 30 min;

[0074] (4) 55.22 mL of deionized water was added to the reaction system, and the mixture was stirred at 1200 r / min for 1 h to obtain a plant polyphenol-modified waterborne polyurethane emulsion with a high bio-based content of 57.04% by mass.

[0075] Example 2

[0076] First, 5.61g of isophorone diisocyanate, 7.80g of castor oil and 1.35g of 2,2-dihydroxymethylbutyric acid were mixed, and stirred at 250r / min for 30min at a temperature of 55°C under a nitrogen environment; 40ul of catalyst dibutyltin dilaurate was added to the obtained mixture, the temperature was raised to 80°C, and the mixture was stirred at 500r / min for 3h; 0.50g of arbutin was added to the reaction system, and the mixture was stirred at 500r / min for 2h; the temperature of the reaction system was lowered to room temperature, and then 0.92g of triethylamine was added, and the mixture was stirred at 200r / min for 30min; finally, 64.72mL of deionized water was added to the reaction system and the mixture was stirred at 1200r / min for 1h to obtain a plant polyphenol-modified waterborne polyurethane emulsion with a mass fraction of 51.3% and a high bio-based content.

[0077] Example 3

[0078] First, 4.91g of isophorone diisocyanate, 7.80g of castor oil and 1.24g of 2,2-dihydroxymethylbutyric acid were mixed, and stirred at 250r / min for 30min at a temperature of 50°C under a nitrogen environment; 40ul of catalyst dibutyltin dilaurate was added to the obtained mixture, the temperature was raised to 80°C, and the mixture was stirred at 500r / min for 2.5h; 0.23g of arbutin was added to the reaction system, and the mixture was stirred at 500r / min for 2h; the temperature of the reaction system was lowered to room temperature, and then 0.85g of triethylamine was added, and the mixture was stirred at 200r / min for 30min; finally, 60.12mL of deionized water was added to the reaction system and the mixture was stirred at 1200r / min for 1h to obtain a plant polyphenol-modified waterborne polyurethane emulsion with a mass fraction of 53.43% and a high bio-based content.

[0079] Example 4

[0080] First, 4.91g of isophorone diisocyanate, 7.80g of castor oil and 1.24g of 2,2-dihydroxymethylbutyric acid were mixed, and stirred at 250r / min for 30min at a temperature of 50°C under a nitrogen environment; 40ul of catalyst dibutyltin dilaurate was added to the obtained mixture, the temperature was raised to 80°C, and the mixture was stirred at 500r / min for 2.5h; 0.24g of catechin was added to the reaction system, and the mixture was stirred at 500r / min for 2h; the temperature of the reaction system was lowered to room temperature, and then 0.85g of triethylamine was added, and the mixture was stirred at 200r / min for 30min; finally, 60.16mL of deionized water was added to the reaction system and the mixture was stirred at 1200r / min for 1h to obtain a plant polyphenol-modified waterborne polyurethane emulsion with a mass fraction of 53.46% and a high bio-based content.

[0081] Example 5

[0082] First, 4.91g of isophorone diisocyanate, 7.80g of castor oil and 1.24g of 2,2-dihydroxymethylbutyric acid were mixed, and stirred at 250r / min for 30min at a temperature of 50°C under a nitrogen environment; 40ul of catalyst dibutyltin dilaurate was added to the obtained mixture, the temperature was raised to 80°C, and the mixture was stirred at 500r / min for 2.5h; 0.32g of ellagic acid was added to the reaction system, and the mixture was stirred at 500r / min for 2h; the temperature of the reaction system was lowered to room temperature, and then 0.85g of triethylamine was added, and the mixture was stirred at 200r / min for 30min; finally, 60.48mL of deionized water was added to the reaction system and the mixture was stirred at 1200r / min for 1h to obtain a plant polyphenol-modified waterborne polyurethane emulsion with a mass fraction of 53.7% and a high bio-based content.

[0083] Example 6

[0084] First, 4.91g of isophorone diisocyanate, 7.80g of castor oil and 1.24g of 2,2-dihydroxymethylbutyric acid were mixed, and stirred at 250r / min for 30min at a temperature of 50°C under a nitrogen environment; 40ul of catalyst dibutyltin dilaurate was added to the obtained mixture, the temperature was raised to 80°C, and the mixture was stirred at 500r / min for 2.5h; 0.77g of curcumin was added to the reaction system, and the mixture was stirred at 500r / min for 2h; the temperature of the reaction system was lowered to room temperature, and then 0.85g of triethylamine was added, and the mixture was stirred at 200r / min for 30min; finally, 62.28mL of deionized water was added to the reaction system and the mixture was stirred at 1200r / min for 1h to obtain a plant polyphenol-modified waterborne polyurethane emulsion with a mass fraction of 55.04% and a high bio-based content.

[0085] Test Example 1

[0086] The total mass of the components and the total mass of the bio-based components of the aqueous polyurethane emulsions obtained in Examples 1-6 were calculated.

[0087] The total mass of the bio-based components is the mass of castor oil + plant polyphenols. The results are shown in Table 1.

[0088] Table 1

[0089]

[0090] Application Example 1

[0091] The application of waterborne polyurethane emulsion in preparing synthetic leather comprises the following steps:

[0092] First, the base fabric is pretreated to improve its bonding strength with the polyurethane coating; then the waterborne polyurethane dispersion is evenly coated on the surface of the substrate and pre-baked to form a polyurethane (PU) coating; then the coating and the base fabric are bonded by hot pressing; finally, the material is given specific functional and appearance characteristics through post-finishing processes such as stretching and embossing to obtain waterborne polyurethane microfiber synthetic leather.

[0093] The base fabric comprises one of woven fabric, woven fabric, spunbonded nonwoven fabric, spunlace nonwoven fabric and meltblown nonwoven fabric, and the pretreatment process comprises cleaning, surface activation and primer coating.

[0094] The pre-baking temperature is 60-120° C., the pre-baking time is 2-10 minutes, and the thickness of the polyurethane coating is 0.3-1 mm.

[0095] The temperature of the hot pressing treatment is 120-150° C., and the time of the hot pressing treatment is 1-3 minutes.

[0096] The obtained waterborne polyurethane coating on the waterborne polyurethane microfiber synthetic leather has application properties such as good elasticity, high mechanical strength and friction resistance, high hardness and the like.

[0097] The above is a further detailed description of the present invention, which should not be regarded as a limitation on the specific implementation of the present invention. For ordinary technicians in the technical field to which the present invention belongs, simple deduction or replacement without departing from the concept of the present invention is within the protection scope of the present invention.

Claims

1. A method for preparing an aqueous polyurethane emulsion, characterized in that: The following steps are involved: (1) mixing aliphatic diisocyanate, castor oil and internal emulsifier 2,2-dimethylolbutyric acid, and stirring at 45° C. to 55° C. and 100 to 250 r / min for 15 to 45 minutes under a nitrogen environment; (2) adding a catalyst, dibutyltin dilaurate, heating to 75°C to 85°C, and stirring at a speed of 400 to 750 r / min for 2 to 3 hours; (3) adding plant polyphenols and continuing stirring at a speed of 400 to 750 r / min at 75° C. to 85° C. for 1.5 to 2 h; (4) Cooling to room temperature, then adding triethylamine and stirring at 100-250 r / min for 25-40 min; (5) Add an appropriate amount of water and stir at a speed of 1200 to 1800 r / min for 1 to 2 hours to prepare an aqueous polyurethane emulsion.

2. The preparation method according to claim 1, characterized in that: The aliphatic diisocyanate in step (1) is one of hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexane diisocyanate, xylylene diisocyanate, trimethylhexamethylene diisocyanate and tetramethyl-m-xylylene diisocyanate.

3. The preparation method according to claim 1, characterized in that: The molar ratio of the castor oil to the hydroxyl group of the 2,2-dihydroxymethylbutyric acid in step (1) is 1:(0.6-0.8).

4. The preparation method according to claim 1, characterized in that: The amount of dibutyltin dilaurate added in step (2) is 0.1% to 0.3% of the total mass of the reaction substances in step (1).

5. The preparation method according to claim 1, characterized in that: The plant polyphenol in step (3) is one of arbutin, catechin, ellagic acid and curcumin; and / or; the molar ratio of the plant polyphenol to the hydroxyl group of 2,2-dihydroxymethylbutyric acid in step (3) is (0.2-0.5):

1.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The molar ratio of the total amount of isocyanate groups in the aliphatic diisocyanate to the total amount of hydroxyl groups in castor oil, internal emulsifier 2,2-dihydroxymethylbutyric acid and plant polyphenols is 1:(0.83-1.10).

7. The preparation method according to claim 1, characterized in that: The molar ratio of triethylamine added in step (4) to 2,2-dihydroxymethylbutyric acid added in step (1) is 1:

1.

8. An aqueous polyurethane emulsion, characterized in that The aqueous polyurethane emulsion is obtained by the preparation method according to any one of claims 1 to 7.

9. Use of the aqueous polyurethane emulsion according to claim 8 in the preparation of synthetic leather.

10. The use according to claim 9, characterized in that: The synthetic leather is ultra-fine synthetic leather.

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