Aqueous polyurethane resin and process for its preparation

Abstract

The present application relates to the technical field of high polymer, and in particular to a waterborne polyurethane resin and a preparation method thereof.The preparation method comprises the following steps: mixing polyol, polyisocyanate and a catalyst to perform a segmented temperature control prepolymerization reaction to form a prepolymer; mixing the prepolymer with a hydrophilic chain extender, a small molecule chain extender and a crosslinking agent to perform a gradient chain extension reaction to form a chain extension product; and then, under the conditions of ultrasonic and shearing, the chain extension product is emulsified.The preparation method can realize the preparation of a waterborne polyurethane resin with high molecular weight and high stability.

Description

Technical Field

[0001] This invention relates to the field of polymer technology, and more specifically, to waterborne polyurethane resins and their preparation methods. Background Technology

[0002] Waterborne polyurethane (WPU) is a polyurethane system that uses water as a dispersion medium. It has advantages such as being non-toxic, non-flammable, and environmentally friendly. High molecular weight waterborne polyurethane resins have a much larger molecular weight than conventional molecular weight waterborne polyurethane resins. The higher the molecular weight, the stronger the overall performance, with comprehensive improvements in properties such as feel, strength, water resistance, and resistance to re-tack. It is widely used in coatings, adhesives, leather finishing, inkjet printing, and other fields.

[0003] In the existing technology, the preparation of waterborne polyurethane mainly suffers from the following technical defects: 1. Low molecular weight: Traditional processes use one-time feeding and constant temperature reaction, which makes it difficult to obtain high molecular weight products, resulting in insufficient film strength and mechanical properties; 2. Poor emulsification effect: Conventional mechanical shear emulsification has high energy consumption, uneven dispersion, poor emulsion stability, and is prone to sedimentation and aggregation. 3. Low stability of post-chain extension process: The post-chain extension process generally uses diamine chain extenders, which react very quickly with isocyanate-terminated polyurethane prepolymers, making it very easy for local agglomeration to occur, affecting the quality and stability of the product.

[0004] Therefore, developing an integrated process for preparing high molecular weight, high-stability waterborne polyurethane has significant industrial application value.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a waterborne polyurethane resin and a method for preparing the same. This invention provides a method for preparing a waterborne polyurethane resin that enables the preparation of high molecular weight, high-stability waterborne polyurethane resin.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing an aqueous polyurethane resin, comprising: mixing a polyol, a polyisocyanate and a catalyst to carry out a segmented temperature-controlled prepolymerization reaction to form a prepolymer; The prepolymer is mixed with a hydrophilic chain extender, a small molecule chain extender and a crosslinking agent to carry out a gradient chain extension reaction to form a chain extension product; Next, the chain-extended product is emulsified under ultrasonic and shear conditions.

[0008] In an optional embodiment, the segmented temperature-controlled prepolymerization reaction includes: mixing a polyol, a first polyisocyanate, and a catalyst at 65-75°C for initial prepolymerization; then adding a second polyisocyanate and performing secondary prepolymerization at 78-85°C; wherein the mass of the first polyisocyanate accounts for 70-80% of the total mass of the polyisocyanate; and the mass of the second polyisocyanate accounts for 30-20% of the total mass of the polyisocyanate.

[0009] In an optional implementation, the conditions for the segmented temperature-controlled prepolymerization reaction satisfy at least one of the following requirements: (1) The reaction time for the initial prepolymerization is 1.0-2.0 hours; (2) The reaction time for secondary prepolymerization is 1.5-2.5 hours; (3) The catalyst includes bismuth-based substances, preferably organic bismuth; (4) The amount of the catalyst used is two to five ten-thousandths of the total mass of all the reactants that form the waterborne polyurethane resin; (5) The molar ratio of the polyisocyanate to the polyol is 1:(0.25-0.4); (6) The polyol is selected from one or more of polyethylene oxide diol, polypropylene oxide diol, polytetrahydrofuran ether diol and polycarbonate diol; (7) The polyisocyanate is selected from diisocyanates; preferably one or more of isophorone diisocyanate, hexamethylene diisocyanate, 4,4-dicyclohexylmethane diisocyanate and toluene diisocyanate.

[0010] In an optional embodiment, the gradient chain extension reaction includes: after the segmented temperature-controlled prepolymerization reaction is completed, the temperature of the reaction system is reduced to 50-60°C, and then the hydrophilic chain extender and solvent are added, and the first gradient chain extension reaction is carried out at 70-80°C. Then, the first small molecule chain extender is added and a second gradient chain extension reaction is carried out at 75-85℃. Next, a second small molecule chain extender and a crosslinking agent are added, and a third gradient chain extension reaction is carried out at 80-85°C. The mass of the first small molecule chain extender accounts for 70-80% of the mass of the small molecule chain extender, and the mass of the second small molecule chain extender accounts for 30-20% of the mass of the small molecule chain extender.

[0011] In an optional implementation, the conditions for the gradient chain extension reaction satisfy at least one of the following requirements: (1) The reaction time for the first gradient chain extension reaction is 1-1.5 hours; (2) The reaction time for the second gradient chain extension reaction is 1-2 hours; (3) The reaction time for the third gradient chain extension reaction is 1.5-2 hours; (4) The molar ratio of the polyisocyanate to the hydrophilic chain extender is 1:(0.2-0.35); (5) The amount of solvent used is 15-25% of the total mass of all reactants used to form the waterborne polyurethane resin; (6) The molar ratio of the polyisocyanate to the small molecule chain extender is 1: (0.2-0.42); (7) The molar ratio of the polyisocyanate to the crosslinking agent is 1: (0.05-0.09).

[0012] In an optional embodiment, the starting materials for the gradient chain extension reaction meet at least one of the following requirements: (1) The hydrophilic chain extender is selected from hydrophilic diols, preferably one or more of dimethylolpropionic acid, dimethylolbutyric acid and dihydroxysulfonate; (2) The solvent is a ketone solvent, preferably acetone; (3) The small molecule chain extender is a small molecule diol, preferably one or more of ethylene glycol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, neopentanediol and 1,6-hexanediol; (4) The crosslinking agent is one or more of glycerol and trimethylolpropane.

[0013] In an optional embodiment, the method further includes: mixing and shearing the chain extender and neutralizer before emulsification; Or it may also include: removing the solvent after emulsification.

[0014] In an optional embodiment, the shearing conditions are as follows: shearing speed is 1000-2000 rpm; shearing and stirring time is 20-30 min; shearing temperature is 30-40℃. Preferably, the neutralizing agent includes an amine, and more preferably triethylamine; Preferably, the neutralizing agent has a degree of neutralization of 90-100%; Preferably, the ultrasonic conditions are as follows: ultrasonic power is 1800-2100W; ultrasonic frequency is 18-22kHz; and ultrasonic time is 20-30min.

[0015] Secondly, the present invention provides an aqueous polyurethane resin, which is prepared by the method for preparing aqueous polyurethane resin described in any of the foregoing embodiments.

[0016] In an optional embodiment, the waterborne polyurethane resin has a molecular weight ≥180,000.

[0017] The present invention has the following beneficial effects: The embodiments of the present invention adopt a three-step integrated synergistic process of "precisely temperature-controlled segmented temperature-controlled prepolymerization reaction - multi-stage gradient chain extension reaction - ultrasonic shear synergistic emulsification" to achieve the preparation of high molecular weight, high stability waterborne polyurethane resin. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0019] This invention provides a method for preparing waterborne polyurethane resin. This method is a "three-step integrated" preparation method, specifically referring to an integrated process of "segmented temperature-controlled prepolymerization - gradient chain extension - ultrasonic shear synergistic emulsification." The resulting waterborne polyurethane resin has the advantages of high molecular weight, high stability, and strong mechanical properties. The specific process is as follows: S1. Segmented temperature-controlled prepolymerization; A prepolymer is formed by mixing polyol, polyisocyanate, and catalyst in a segmented, temperature-controlled prepolymerization reaction. Specifically, the vacuum-dehydrated polyol is added to a reactor, and under the protection of an inert gas (e.g., nitrogen), the first polyisocyanate and catalyst are added for preliminary prepolymerization at 65-75°C for 1.0-2.0 hours. The purpose is to form a stable NCO-terminated prepolymer and suppress side reactions.

[0020] The catalyst includes bismuth-based substances (e.g., including but not limited to organic bismuth). The amount of catalyst used is 0.02% to 0.05% of the total mass of all reactants involved in forming the waterborne polyurethane resin.

[0021] It should be noted that all the reactants involved in forming the waterborne polyurethane resin include polyisocyanate, polyol, hydrophilic chain extender, first small molecule chain extender, and crosslinking agent.

[0022] The molar ratio of polyisocyanate to polyol is 1:(0.25-0.4); the mass of the first polyisocyanate accounts for 70-80% of the total mass of the polyisocyanate; in the embodiments of the present invention, the polyisocyanate is added in batches, namely the first polyisocyanate and the second polyisocyanate.

[0023] Furthermore, the polyol is selected from one or more of polyethylene oxide diol, polypropylene oxide diol, polytetrahydrofuran ether diol, and polycarbonate diol.

[0024] The polyisocyanate is selected from diisocyanates; preferably, it is one or more of isophorone diisocyanate, hexamethylene diisocyanate, 4,4-dicyclohexylmethane diisocyanate and toluene diisocyanate.

[0025] Then, a second polyisocyanate (i.e., the remaining polyisocyanate) is added and subjected to secondary prepolymerization at 78-85℃ for 1.5-2.5 hours; the second polyisocyanate accounts for 30-20% of the total mass of the polyisocyanate. The reaction viscosity is controlled by gradually increasing the temperature to promote molecular chain growth.

[0026] S2, gradient chain extension; The prepolymer is mixed with a hydrophilic chain extender, a small molecule chain extender and a crosslinking agent to carry out a gradient chain extension reaction to form a chain extension product.

[0027] Specifically, after the segmented temperature-controlled prepolymerization reaction is completed, the temperature of the reaction system is reduced to 50-60℃, and then the hydrophilic chain extender and solvent are added. The first gradient chain extension reaction is carried out at 70-80℃ for 1-1.5 hours, so that the hydrophilic groups are arranged in an orderly manner on the molecular backbone.

[0028] The hydrophilic chain extender is selected from hydrophilic diols, preferably one or more of dimethylolpropionic acid, dimethylolbutyric acid, and dihydroxysulfonate. The molar ratio of the polyisocyanate to the hydrophilic chain extender is 1:(0.2-0.35). The solvent is a ketone solvent (e.g., including but not limited to acetone); the amount of solvent used is 15-25% of the total mass of all reactants forming the waterborne polyurethane resin.

[0029] Then, a first small molecule chain extender is added, and a second gradient chain extension reaction is carried out at 75-85°C for 1-2 hours to achieve preliminary chain extension. The first small molecule chain extender accounts for 70-80% of the total mass of the small molecule chain extender, and is added in batches, forming the first and second small molecule chain extenders. The small molecule chain extender is a small molecule diol, preferably one or more of ethylene glycol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, neopentanediol, and 1,6-hexanediol; the molar ratio of polyisocyanate to the small molecule chain extender is 1:(0.2-0.42).

[0030] Next, the remaining small molecule chain extender (i.e., the second small molecule chain extender) and crosslinking agent are added and a third gradient chain extension reaction is carried out at 80-85°C for 1.5-2 hours to complete the final chain extension and further improve the mechanical properties of the polyurethane resin by introducing a crosslinking structure. During the gradient chain extension process, the reaction progress is monitored in real time by online viscosity monitoring and NCO titration.

[0031] The second small molecule chain extender accounts for 30-20% of the total mass of the small molecule chain extender. The molar ratio of polyisocyanate to crosslinking agent is 1:(0.05-0.09). The crosslinking agent is one or more of glycerol and trimethylolpropane.

[0032] S3, ultrasonic shearing synergistic emulsification; The chain-extended product is emulsified under ultrasonic and shear conditions. Specifically, the chain-extended product obtained in S2 is transferred to an ultrasonic emulsification apparatus equipped with shear stirring: a neutralizing agent is added, and the mixture is stirred at a high-speed shearing speed of 1000-2000 rpm for 20-30 minutes, with the temperature controlled at 30-40°C. The neutralizing agent includes amines (e.g., including but not limited to triethylamine); the degree of neutralization of the neutralizing agent is 90-100%.

[0033] It should be noted that the degree of neutralization refers to: (actual moles of neutralizing agent added ÷ moles of carboxyl groups in dimethylolpropionic acid or dimethylolbutyric acid) × 100%. Next, the ultrasound was turned on using a pulsed ultrasound mode. Under the combined action of ultrasound and shearing, deionized water was added, and emulsification was achieved by shearing, stirring, and ultrasonic dispersion. The rotation speed was 1000-2000 rpm, the ultrasonic power was 1800-2100W, the ultrasonic frequency was 18-22kHz, and the dispersion time was 20-30 minutes.

[0034] Then, the solvent is removed. Specifically, the temperature is raised to 55-60℃ and the solvent is removed under a vacuum of ≤-0.09 MPa to obtain the high molecular weight waterborne polyurethane resin product.

[0035] Secondly, the present invention provides an aqueous polyurethane resin, which is prepared by the method for preparing aqueous polyurethane resin according to any one of the foregoing embodiments. The molecular weight of the aqueous polyurethane resin is ≥180,000.

[0036] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0037] Example 1 This invention provides a method for preparing an aqueous polyurethane resin, wherein the raw materials used are as follows: Polytetrahydrofuran ether diol (Mn=2000) (purchased from Hyosung Chemical, South Korea): 145.6 kg; Polycarbonate diol (Mn=2000) (purchased from Ube Chemical, Japan): 36.4 kg; Isophorone diisocyanate (IPDI) (purchased from Wanhua Chemical): 60.8 kg; Dimethylolpropionic acid (DMPA) (purchased from Pastor): 12.0 kg; 1,4-Butanediol (purchased from Xinjiang Meike): 6.2 kg; Trimethylolpropane (TMP) (purchased from Jiangsu Baichuan): 1.32 kg; Triethylamine (purchased from Shanghai Titan Technology): 8.87 kg; Deionized water: 612 kg; Organic bismuth (purchased from Shepherd Chemical Company): 0.079 kg; Acetone (AC): 61.2 kg.

[0038] The specific preparation process is as follows: 145.6 kg of dehydrated polytetrahydrofuran ether diol (Mn=2000) and 36.4 kg of polycarbonate diol (Mn=2000) were added to the reactor.

[0039] (1) Segmented temperature-controlled prepolymerization: First stage: Add 48.64 kg IPDI and 0.079 kg organic bismuth at 50 °C, and react at 70 °C for 2.0 h.

[0040] Second stage: Heat to 78℃, add 12.16Kg IPDI, and react for 2.5h.

[0041] (2) Gradient chain extension: First gradient: Add 12.0 kg DMPA at 50℃ and react at 70℃ for 1.5 h.

[0042] Second gradient: Add 4.65 kg 1,4-butanediol and 61.2 kg AC, and react at 80 °C for 1.5 h.

[0043] Third gradient: Add 1.55 kg of 1,4-butanediol and 1.32 kg of TMP, and react at 80 °C for 2 h.

[0044] (3) Ultrasonic shearing synergistic emulsification: Add triethylamine at 40°C and stir at 2000 rpm for 20 minutes.

[0045] Turn on the ultrasound, power 2000 W, frequency 20 kHz, pulse mode, and continue stirring at the above stirring speed.

[0046] Add deionized water dropwise at a rate of 100 L / min, and continue to treat under the combined action of ultrasound and shearing for 30 min after the dropwise addition is complete.

[0047] Solvent removal at 55℃ and vacuum degree ≤-0.09MPa for 1 hour.

[0048] Example 2 This invention provides a method for preparing an aqueous polyurethane resin, wherein the raw materials used are as follows: Polytetrahydrofuran ether glycol (Mn=1000): 39.0 kg; Polypropylene glycol (Mn=2000): 78.0 kg; Dicyclohexylmethane diisocyanate (HMDI): 57.35 kg; Dimethylolbutyric acid (DMBA): 9.4 kg; 1,4-Butanediol: 4.7 kg; Glycerin: 1.2 kg; Triethylamine: 6.10 kg; Deionized water: 442.5 kg; Organic bismuth: 0.038 kg; AC: 35.4 kg.

[0049] The specific preparation process is as follows: Add 39.0 kg of dehydrated polytetrahydrofuran ether glycol (Mn=1000) and 78.0 kg of polyoxypropylene glycol (Mn=2000) to the reactor.

[0050] (1) Segmented temperature-controlled prepolymerization: First stage: Add 43.0 kg HMDI and 0.038 kg organic bismuth at 50℃, and react at 75℃ for 1.0 h.

[0051] Second stage: Heat to 85℃, add 14.35kg HMDI, and react for 1.5h.

[0052] (2) Gradient chain extension: First gradient: Add 9.4 kg of DMBA at 60℃ and react at 78℃ for 1.0 h.

[0053] Second gradient: Add 3.76 kg of 1,4-butanediol and 35.4 kg of AC, and react at 85 °C for 1.0 h.

[0054] Third gradient: Add 0.94 kg of 1,4-butanediol and 1.2 kg of glycerol, and react at 85 °C for 1.5 h.

[0055] (3) Ultrasonic shearing synergistic emulsification: Add triethylamine at 40°C and stir at 2000 rpm for 20 minutes.

[0056] Turn on the ultrasound, power 2000 W, frequency 20 kHz, pulse mode; at the same time, continue stirring at the above stirring speed.

[0057] Add deionized water dropwise at a rate of 100 L / min, and continue to treat under the combined action of ultrasound and shearing for 30 min after the dropwise addition is complete.

[0058] Solvent removal at 55℃ and vacuum degree ≤-0.09MPa for 1 hour.

[0059] Example 3 This invention provides a method for preparing an aqueous polyurethane resin, wherein the raw materials used are as follows: Polytetrahydrofuran ether diol (Mn=2000): 130 kg; Isophorone diisocyanate (IPDI): 37.4 kg; Hexamethylene diisocyanate (HDI): 10.0 kg; Dihydroxysulfonate: 20.0 kg; Ethylene glycol: 5.8 kg; Trimethylolpropane (TMP): 1.5 kg; Deionized water: 480.0 kg; Organic bismuth: 0.082 kg; AC: 41.14 kg.

[0060] The specific preparation process is as follows: Add 130.0 kg of dehydrated polytetrahydrofuran ether diol (Mn=2000) to the reactor.

[0061] (1) Segmented temperature-controlled prepolymerization: First stage: Add 26.18 kg IPDI, 7.0 kg HDI and 0.082 kg organic bismuth at 50 °C, and react at 72 °C for 1.5 h.

[0062] Second stage: Add 11.22 kg IPDI and 3.0 kg HDI, heat to 82°C, and react for 2.0 h.

[0063] (2) Gradient chain extension: First gradient: 20.0 kg of dihydroxysulfonate was added at 55℃ and reacted at 75℃ for 1.0 h.

[0064] Second gradient: Add 4.06 kg of ethylene glycol and 41.14 kg of AC, and react at 82 °C for 1.5 h.

[0065] Third gradient: Add 1.74 kg of ethylene glycol and 1.5 kg of TMP, and react at 82 °C for 1.8 h.

[0066] (3) Ultrasonic shearing synergistic emulsification: Turn on the ultrasound, power 2000 W, frequency 20 kHz, pulse mode. At the same time, turn on the stirring speed to 2000 rpm and continue stirring.

[0067] Add deionized water at a rate of 100 L / min. After the addition is complete, continue the process under the combined action of ultrasound and shearing for 30 minutes. Since the dihydroxysulfonate has already formed a salt, it does not need to be neutralized to form a salt.

[0068] Solvent removal at 55℃ and a vacuum of -0.09MPa for 1 hour.

[0069] Comparative Example 1 This comparative example provides a method for preparing waterborne polyurethane resin, with the same raw materials and proportions as in Example 1. The only difference is that a one-pot method for prepolymerization and chain extension is used, with all other operations consistent with Example 1. Specifically, (1) One-pot method for prepolymerization and chain extension: Add 12.0 kg DMPA, 61.2 kg AC, 1.32 kg TMP and 6.2 kg 1,4-butanediol at 50°C and react at 80°C for 5 hours.

[0070] (2) Ultrasonic shearing synergistic emulsification: consistent with Example 1.

[0071] Comparative Example 2 This comparative example provides a method for preparing waterborne polyurethane resin, with the same raw materials and proportions as in Example 1, except that the prepolymerization temperature is 78°C and the material is added in one go; all other operations are consistent with Example 1. Specifically, (1) Prepolymerization: 60.8 kg of isophorone diisocyanate was added at 50°C and reacted at 78°C for 4.5 hours.

[0072] (2) Gradient chain extension and (3) Ultrasonic shearing synergistic emulsification: consistent with Example 1.

[0073] Comparative Example 3 This comparative example provides a method for preparing an aqueous polyurethane resin, using the same raw materials and proportions as in Example 1, except that the chain extension temperature is 80°C and the material is added in a single batch; all other operations are consistent with Example 1. Specifically, (1) Segmented temperature-controlled prepolymerization: Same as in Example 1.

[0074] (2) Chain extension: Add 12.0 kg DMPA, 61.2 kg AC, 1.32 kg TMP and 6.2 kg 1,4-butanediol at 50℃ and react at 80℃ for 5 hours.

[0075] (3) Ultrasonic shearing synergistic emulsification: consistent with Example 1.

[0076] Comparative Example 4 This comparative example provides a method for preparing an aqueous polyurethane resin, with the same raw materials and proportions as in Example 1. The only difference is that mechanical shear emulsification is used only; all other operations are consistent with Example 1. Specifically, (1) Segmented temperature-controlled prepolymerization and (2) Gradient chain extension: consistent with Example 1.

[0077] (3) Shear emulsification: Triethylamine was added at 30°C and stirred at 2000 rpm for 20 min. Then, deionized water was added dropwise at a rate of 100 L / min while stirring. After the addition was complete, the mixture was mechanically stirred for 20 min and then desolventized at 55°C and a vacuum of ≤-0.09 MPa for 1 h.

[0078] Performance testing The waterborne polyurethane resins prepared in Examples 1-3 and Comparative Examples 1-4 were tested for properties such as storage stability, weight-average molecular weight, average particle size, and tensile strength. The test results are shown in Table 1.

[0079] Storage stability: The sample is placed in a test bottle and tested in a 50℃ oven. The condition of the bottle is observed at regular intervals.

[0080] Weight-average molecular weight: The samples were tested using an Agilent 1260 Infinity II GPC instrument.

[0081] Average particle size: The sample was tested using a Winner 802 nanoparticle size analyzer.

[0082] Tensile strength: The samples were tested using the new GoTech AI-7000SU1 servo-controlled tensile testing machine.

[0083] Table 1 Test Results

[0084] As shown in Table 1, the high molecular weight waterborne polyurethane resin prepared by this invention exhibits excellent performance. Its uniform dispersion ensures excellent storage stability, its smaller particle size results in better film appearance, feel, and gloss, and its larger molecular weight and stronger strength make its mechanical properties more prominent. It can effectively solve the physical property problems that conventional resins cannot achieve in fields such as coatings, adhesives, synthetic leather, and inkjet printing, and has broad industrial application prospects and market value.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an aqueous polyurethane resin, characterized in that, include: A prepolymer is formed by mixing polyols, polyisocyanates and catalysts and carrying out a segmented temperature-controlled prepolymerization reaction. The prepolymer is mixed with a hydrophilic chain extender, a small molecule chain extender and a crosslinking agent to carry out a gradient chain extension reaction to form a chain extension product; Next, the chain-extended product is emulsified under ultrasonic and shear conditions.

2. The preparation method according to claim 1, characterized in that, The segmented temperature-controlled prepolymerization reaction includes: mixing a polyol, a first polyisocyanate, and a catalyst at 65-75°C for initial prepolymerization; then adding a second polyisocyanate and performing secondary prepolymerization at 78-85°C; wherein the mass of the first polyisocyanate accounts for 70-80% of the total mass of the polyisocyanates; and the mass of the second polyisocyanate accounts for 30-20% of the total mass of the polyisocyanates.

3. The preparation method according to claim 2, characterized in that, The conditions for the segmented temperature-controlled prepolymerization reaction must meet at least one of the following requirements: (1) The reaction time for the initial prepolymerization is 1.0-2.0 hours; (2) The reaction time for secondary prepolymerization is 1.5-2.5 hours; (3) The catalyst includes bismuth-based substances, preferably organic bismuth; (4) The amount of the catalyst used is two to five ten-thousandths of the total mass of all the reactants that form the waterborne polyurethane resin; (5) The molar ratio of the polyisocyanate to the polyol is 1:(0.25-0.4); (6) The polyol is selected from one or more of polyethylene oxide diol, polypropylene oxide diol, polytetrahydrofuran ether diol and polycarbonate diol; (7) The polyisocyanate is selected from diisocyanates; preferably one or more of isophorone diisocyanate, hexamethylene diisocyanate, 4,4-dicyclohexylmethane diisocyanate and toluene diisocyanate.

4. The preparation method according to claim 1, characterized in that, The gradient chain extension reaction includes: after the segmented temperature-controlled prepolymerization reaction is completed, the temperature of the reaction system is reduced to 50-60℃, and then the hydrophilic chain extender and solvent are added, and the first gradient chain extension reaction is carried out at 70-80℃. Then, the first small molecule chain extender is added and a second gradient chain extension reaction is carried out at 75-85℃. Next, a second small molecule chain extender and a crosslinking agent are added, and a third gradient chain extension reaction is carried out at 80-85°C. The mass of the first small molecule chain extender accounts for 70-80% of the mass of the small molecule chain extender, and the mass of the second small molecule chain extender accounts for 30-20% of the mass of the small molecule chain extender.

5. The preparation method according to claim 4, characterized in that, The gradient chain extension reaction must meet at least one of the following conditions: (1) The reaction time for the first gradient chain extension reaction is 1-1.5 hours; (2) The reaction time for the second gradient chain extension reaction is 1-2 hours; (3) The reaction time for the third gradient chain extension reaction is 1.5-2 hours; (4) The molar ratio of the polyisocyanate to the hydrophilic chain extender is 1:(0.2-0.35); (5) The amount of solvent used is 15-25% of the total mass of all reactants used to form the waterborne polyurethane resin; (6) The molar ratio of the polyisocyanate to the small molecule chain extender is 1: (0.2-0.42); (7) The molar ratio of the polyisocyanate to the crosslinking agent is 1: (0.05-0.09).

6. The preparation method according to claim 4 or 5, characterized in that, The starting materials for the gradient chain extension reaction must meet at least one of the following requirements: (1) The hydrophilic chain extender is selected from hydrophilic diols, preferably one or more of dimethylolpropionic acid, dimethylolbutyric acid and dihydroxysulfonate; (2) The solvent is a ketone solvent, preferably acetone; (3) The small molecule chain extender is a small molecule diol, preferably one or more of ethylene glycol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, neopentanediol and 1,6-hexanediol; (4) The crosslinking agent is one or more of glycerol and trimethylolpropane.

7. The preparation method according to claim 1, characterized in that, Also includes: Before emulsification, the chain extender and neutralizer are mixed and sheared. Or it may also include: removing the solvent after emulsification.

8. The preparation method according to claim 7, characterized in that, The shearing conditions are as follows: shearing speed is 1000-2000 rpm; shearing stirring time is 20-30 min; shearing temperature is 30-40℃. Preferably, the neutralizing agent includes an amine, and more preferably triethylamine; Preferably, the neutralizing agent has a degree of neutralization of 90-100%; Preferably, the ultrasonic conditions are as follows: ultrasonic power is 1800-2100W; ultrasonic frequency is 18-22kHz; and ultrasonic time is 20-30min.

9. A waterborne polyurethane resin, characterized in that, It is prepared by the method of preparing waterborne polyurethane resin according to any one of claims 1-8.

10. The waterborne polyurethane resin according to claim 9, characterized in that, The molecular weight of the waterborne polyurethane resin is ≥180,000.

Images