A quick-drying waterborne polyurethane resin, a preparation method and application thereof

Abstract

This invention discloses a fast-drying waterborne polyurethane resin, its preparation method, and its application. By weight, it comprises the following components: 30-40 parts of self-made polyester polyol, 30-50 parts of polyether polyol, 2-5 parts of chain extender, 1-3 parts of hydrophilic monomer, 1-3 parts of trimethylolpropane, 40-60 parts of isocyanate, 1.1-1.5 parts of neutralizer, and 50-80 parts of solvent. This fast-drying waterborne polyurethane resin utilizes self-made branched polyester and polyether to construct flexible segments that combine rigidity and flexibility. Low hydrophilic monomer content enables self-emulsification and ensures water resistance. Trimethylolpropane and self-made polyester polyol form a double crosslinking network with built-in branching points. Isocyanate compounding balances hardness and pot life. The high R-value design allows residual NCO to naturally generate urea bonds during emulsification, compensating for the emulsification difficulties caused by low hydrophilicity. Ultimately, it achieves a balance between fast drying and high water resistance without the addition of external drying agents or amine-based chain extension. It combines fast drying, high water resistance, and excellent mechanical properties. The process is simple and low-cost, making it highly valuable for industrial promotion.

Description

Technical Field

[0001] This invention relates to the field of automotive coatings technology, specifically to a fast-drying waterborne polyurethane resin, its preparation method, and its application. Background Technology

[0002] In the automotive coatings industry, polyurethane resins have become one of the core film-forming substances in automotive coatings due to their excellent mechanical properties, chemical resistance, and adhesion. They are widely used in original equipment manufacturer (OEM) paints, refinish paints, and intermediate coats, base coats, and clear coats for commercial vehicles. While traditional solvent-based polyurethane coatings offer high gloss, high hardness, and good workability, their application scenarios are gradually being limited due to high VOC emissions and severe environmental pollution. With increasingly stringent global environmental regulations and the popularization of green manufacturing concepts, reducing volatile organic compound (VOC) emissions has become an irreversible trend in the automotive coatings industry. Waterborne polyurethane resins (WPU), using water as a dispersion medium, can significantly reduce VOC emissions and have significant advantages such as low VOCs, no pollution, safety, environmental friendliness, and convenient application, making them a key material for the green transformation of automotive coatings.

[0003] Currently, to improve efficiency and reduce VOC emissions and energy consumption, automotive painting production lines commonly employ a "wet-on-wet" process. This involves directly spraying the topcoat or clear coat onto the partially dried primer or intermediate coat, followed by simultaneous baking and curing. This reduces baking temperature and time, effectively shortening the painting process. However, the "wet-on-wet" process requires the underlying coating to have extremely fast physical drying speed (surface dry ≤ 20 minutes) and excellent resistance to re-solubility to prevent swelling or dissolution by solvents in the upper coating (such as alcohol ethers and ester co-solvents), which could lead to bleeding, loss of gloss, or decreased interlayer adhesion.

[0004] To maintain emulsion stability, conventional waterborne polyurethane requires the introduction of sufficient hydrophilic groups. These polar groups strongly bind water molecules, delaying moisture evaporation and resulting in slow coating drying. Even after drying, these groups remain permanently, acting as adsorption channels for water molecules and significantly reducing the coating's water resistance and resistance to damp heat aging. While adding driers or increasing baking temperatures can partially alleviate the drying speed problem, the former introduces metal salts that may affect the coating's water resistance, while the latter increases energy consumption, which is inconsistent with the automotive industry's low-carbon trend.

[0005] Chinese patent CN115651520B, "A High-Hardness, High-Abrasion-Resistant, Fast-Drying Waterborne Automotive Topcoat and Its Preparation Method," discloses a waterborne modified polyurethane resin. This method enhances the hardness and abrasion resistance of the main resin by modifying it with organosilicon. Further improvements are made by appropriately combining waterborne silica sol, nano-alumina, and graphene ceramic crystals. Additionally, the drying speed is further increased by adding driers and crosslinking agents, resulting in high hardness, high abrasion resistance, and fast drying. However, this polyurethane resin uses an excessive amount of hydrophilic monomers, leading to inherently insufficient water resistance in the coating. Its fast drying relies heavily on driers and film-forming aids, resulting in a narrow application window, demanding high operational skills, and complex and costly processes due to the addition of large amounts of functional monomers and inorganic fillers. Therefore, improvements are urgently needed.

[0006] Therefore, developing a fast-drying waterborne polyurethane resin that combines fast drying, high water resistance, excellent mechanical properties, simple processing, and low cost is of significant industrial value for promoting the waterborne upgrading of automotive coatings. Summary of the Invention

[0007] The purpose of this invention is to provide a fast-drying waterborne polyurethane resin, its preparation method, and its application, in order to solve the problems mentioned in the background art, such as the excessive amount of hydrophilic monomers in existing fast-drying waterborne polyurethane resin coatings, poor water resistance, reliance on driers and film-forming aids to achieve fast drying, and the addition of a large amount of functional monomers and inorganic fillers, resulting in a narrow application window, high operational requirements, complex processes, and high costs.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A fast-drying waterborne polyurethane resin, by weight, comprises the following components: 30-40 parts of self-made polyester polyol, 30-50 parts of polyether polyol, 2-5 parts of chain extender, 1-3 parts of hydrophilic monomer, 1-3 parts of trimethylolpropane, 40-60 parts of isocyanate, 1.1-1.5 parts of neutralizer, and 50-80 parts of solvent.

[0010] The fast-drying waterborne polyurethane resin of this invention constructs a flexible segment that combines rigidity and flexibility through self-made branched polyester and polyether. The hydrophilic monomer achieves self-emulsification and ensures water resistance with extremely low dosage. Trimethylolpropane and self-made polyester polyol form a double crosslinking network with built-in branching points. The isocyanate compound takes into account both hardness and pot life. The high R value design allows residual NCO to naturally generate urea bonds during emulsification to compensate for the emulsification difficulties caused by low hydrophilicity. Finally, it achieves a balance between fast drying and high water resistance without the addition of external drying agents and without amine post-chain extension.

[0011] Preferably, the self-made polyester polyol comprises the following raw material components by weight: 150-180 parts terephthalic acid, 50-70 parts isophthalic acid, 70-90 parts ethylene glycol, 10-15 parts glycerol, and 0.2-0.7 parts catalyst.

[0012] The self-made polyester polyol in the fast-drying waterborne polyurethane resin of this invention is formed by the condensation polymerization of terephthalic acid, isophthalic acid, ethylene glycol and glycerol. By providing a rigid benzene ring structure through terephthalic acid and isophthalic acid, the hardness and resistance to re-solution of the coating can be significantly improved. At the same time, the strong polarity of the ester bond gives the coating excellent adhesion to metal substrates. The trifunctionality of glycerol introduces branching points into the polyester backbone, so that the soft segment itself has crosslinking potential, forming a double crosslinking network of "soft segment internal branching plus hard segment external crosslinking" with the subsequent trimethylolpropane.

[0013] More preferably, the self-made polyester polyol is prepared by the following method:

[0014] Terephthalic acid, isophthalic acid, ethylene glycol, glycerol, and catalyst are added to a reaction vessel and slowly heated to 150°C. Once the materials begin to melt, stirring is started, and the temperature is gradually increased to 210-230°C and maintained for reaction. During this process, samples are taken periodically to test the acid value. Once the acid value is ≤5, the temperature is increased to 250-255°C, and a vacuum is slowly drawn to a negative pressure below 0.09MPa. The reaction continues until the acid value is ≤2. The product is then discharged to obtain the self-made polyester polyol.

[0015] The present invention preferably controls the final acid value of the self-made polyester polyol to ≤2, ensuring that the end groups of the polyester are mainly hydroxyl groups, which can effectively avoid the consumption of NCO or catalytic side reactions of free carboxylic acid when reacting with isocyanate in the subsequent process.

[0016] Preferably, the chain extender is one or more of 1,4-butanediol, ethylene glycol, neopentyl glycol, and 1,6-hexanediol.

[0017] More preferably, the chain extender is 1,4-butanediol.

[0018] Preferably, the hydrophilic monomer is one or both of dimethylolpropionic acid and dimethylolbutyric acid.

[0019] More preferably, the hydrophilic monomer is dimethylolpropionic acid.

[0020] Preferably, the isocyanate is two or more of isophorone diisocyanate, hexamethylene diisocyanate, and dicyclohexylmethane diisocyanate.

[0021] More preferably, the isocyanate is isophorone diisocyanate and hexamethylene diisocyanate.

[0022] This invention preferably uses a combination of isophorone diisocyanate and hexamethylene diisocyanate. Hexamethylene diisocyanate is an aliphatic linear diisocyanate whose symmetrical linear structure readily forms ordered hydrogen bonds, imparting high crystallinity and initial hardness to the hard segments while providing flexibility to balance overall mechanical properties. Isophorone diisocyanate is an alicyclic diisocyanate whose rigid cyclic structure significantly improves coating hardness and resistance to yellowing. Furthermore, one of its two NCO groups is sterically hindered, resulting in lower reactivity. This slow-reaction characteristic extends the pot life of the coating and effectively prevents gelation during wet-on-wet application. The combination of these two compounds achieves a synergistic balance between hardness, flexibility, reactivity, and pot life.

[0023] Preferably, the neutralizing agent is one or more of N,N-dimethylethanolamine, triethylamine, and 2-amino-2-methyl-1-propanol.

[0024] More preferably, the neutralizing agent is N,N-dimethylethanolamine.

[0025] Preferably, the solvent is one or more of acetone, butanone, and butyl acetate.

[0026] More preferably, the solvent is butanone.

[0027] Preferably, the R value of the fast-drying waterborne polyurethane resin is 1.7-2.0.

[0028] The present invention preferably uses a fast-drying waterborne polyurethane resin with an R value of 1.7-2.0. The high R value design allows residual NCO to naturally generate urea bonds during emulsification, compensating for the emulsification difficulties caused by low hydrophilicity. Ultimately, it achieves a balance between fast drying and high water resistance without the addition of external drying agents or amine-based chain extension.

[0029] In addition, the present invention also provides a method for preparing a fast-drying waterborne polyurethane resin.

[0030] A method for preparing a fast-drying waterborne polyurethane resin according to any one of the above claims includes the following steps:

[0031] S1. Weigh each component according to the required ratio, add the self-made polyester polyol, polyether polyol, chain extender, hydrophilic monomer and trimethylolpropane into the reactor, heat to 110-120℃ and dehydrate under vacuum for 1-2 hours, then cool to 70-80℃.

[0032] S2. Add isocyanate, heat to 85-90℃ and keep the temperature for 3-4 hours, taking samples to detect NCO value until the theoretical endpoint is reached;

[0033] S3. Cool to 50-60℃, add solvent to dilute, slowly add neutralizing agent to neutralize for 20-30 minutes, add deionized water at 5-15℃, emulsify at high speed, after the viscosity of the system drops sharply, continue stirring for 10 minutes, slowly add deionized water, vacuum desolventize at 60-70℃ for 1-2 hours, cool to below 40℃, filter, and the fast-drying waterborne polyurethane resin is obtained.

[0034] In the preparation method of the fast-drying waterborne polyurethane resin of this invention, the addition of deionized water is divided into two steps: First, a small amount of low-temperature water is rapidly added during initial dispersion to induce the system to invert from an oil-in-water phase to an oil-in-water emulsion under high shear, which is a key step in forming a stable emulsion. Then, the remaining water is slowly added to adjust the solid content. The use of low-temperature water suppresses the violent side reactions between residual NCO and water, avoids gelation, controls particle size, and improves preparation and storage stability.

[0035] Preferably, the vacuum degree of vacuum dehydration in S1 is below 0.09 MPa.

[0036] Preferably, the endpoint of vacuum dehydration in S1 is a moisture content of <0.05%.

[0037] Preferably, the vacuum dehydration process described in S1 further includes a step of purging with nitrogen gas to break up the air.

[0038] Preferably, step S2 is performed under nitrogen protection.

[0039] Preferably, the high-speed emulsification rate in S3 is 3000-5000 rpm.

[0040] Preferably, the slow addition of deionized water in step S3 requires adjusting the solid content to 40%-45%.

[0041] Preferably, the vacuum degree of vacuum desolvation in S3 is below negative pressure of 0.09 MPa.

[0042] Preferably, the filtration in S3 is filtration through a 200-mesh sieve.

[0043] In addition, the present invention also provides an application of a fast-drying waterborne polyurethane resin.

[0044] An application of the fast-drying waterborne polyurethane resin according to any one of the preceding claims, for use in automotive coatings.

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] The fast-drying waterborne polyurethane resin of this invention is prepared from components including a self-made polyester polyol, a polyether polyol, a chain extender, a hydrophilic monomer, trimethylolpropane, isocyanate, and a neutralizing agent. The self-made polyester polyol is synthesized by the condensation polymerization of an aromatic diacid and a polyol, introducing built-in branching points during the polyester synthesis stage. This gives the soft segments crosslinking potential, laying the foundation for the subsequent construction of a double crosslinking network. The benzene ring structure in the molecular chain improves hardness and resistance to re-solution, while the strong polarity of the ester bonds significantly enhances adhesion to substrates, making it suitable for automotive coatings. The flexible ether chain of the polyether polyol provides excellent low-temperature performance. The coating exhibits excellent flexibility and impact resistance. Furthermore, the hydrolysis resistance of the ether bonds effectively compensates for the susceptibility of polyester segments to hydrolysis. When combined with polyester, it achieves a synergistic balance between adhesion, hardness, and flexibility. The chain extender utilizes a small-molecule linear diol, whose primary hydroxyl groups react with isocyanates to form well-defined urethane hard segments. These hard segments form physical cross-linking points through hydrogen bonds, enabling rapid establishment of strength and anti-tack properties in the early stages of coating drying, thus improving fast-drying performance. The carboxyl groups on the hydrophilic monomer side chains are neutralized by a neutralizing agent to form ammonium carboxylate salts, imparting self-emulsifying ability to the resin. The low-dosage design significantly reduces the permanent hydrophilicity. The residual water groups, combined with the neutralizing agent, partially volatilize during the drying process, restoring the carboxyl groups to a hydrophobic state, thus achieving excellent water resistance. Trimethylolpropane, as a trifunctional crosslinking agent, can introduce branching and chemical crosslinking points in the prepolymer stage, synergistically constructing a high-density three-dimensional network structure with the branching points built into the polyester segments, directly improving surface drying speed, 24-hour hardness, and resistance to re-solution. Isocyanates are composed of two or more isocyanates, achieving a synergistic balance between hardness, flexibility, reactivity, and pot life. The neutralizing agent can convert the carboxyl groups in the hydrophilic monomers into ammonium carboxylate salts to achieve water dispersion. During the drying process, some of the carboxyl groups are volatilized, regenerating them into a hydrophobic state, which significantly enhances the water resistance of the fast-drying waterborne polyurethane resin. The high R-value design resulting from the coordinated proportions of each component in the overall formulation allows the residual NCO to naturally generate urea bonds during emulsification, compensating for the emulsification difficulties caused by low hydrophilicity. Ultimately, the fast-drying properties and high water resistance are achieved without the addition of external drying agents or amine-based chain extension, resulting in a fast-drying waterborne polyurethane resin that combines fast drying, high water resistance, and excellent mechanical properties. This fully meets the requirements of wet-on-wet processes, is simple in process, low in cost, and has good industrial promotion value. Detailed Implementation

[0047] To better illustrate the objectives, technical solutions, and advantages of this invention, the following embodiments are provided. Obviously, the following embodiments are only a part of the embodiments of this invention, and not all of them; these embodiments do not imply any limitation on this invention. Those skilled in the art should understand that these embodiments are only used to illustrate the technical effects of this invention, and not to limit the scope of protection of this invention.

[0048] Experimental methods in the following examples, unless otherwise specified, are generally performed under standard conditions or as recommended by the manufacturer. These examples are primarily intended to provide a better understanding of the analytical methods of this invention and do not exhaustively cover all possible procedures.

[0049] All raw materials used in the examples are commercially available; unless otherwise specified, the reagents, methods and equipment used in this invention are conventional reagents, methods and equipment in this technical field.

[0050] Example 1

[0051] A method for preparing a fast-drying waterborne polyurethane resin includes the following steps:

[0052] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0053] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 112°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 76°C.

[0054] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0055] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone (MEK) to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine (NMEK) and neutralize for 25 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 42%. Vacuum desolventize at 66℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no MEK odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the fast-drying waterborne polyurethane resin.

[0056] Example 2

[0057] A method for preparing a fast-drying waterborne polyurethane resin includes the following steps:

[0058] 175 parts of terephthalic acid, 66 parts of isophthalic acid, 87 parts of ethylene glycol, 13 parts of glycerol, and 0.6 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 212°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 252°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0059] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 41 parts of polyether polyol, 3 parts of 1,4-butanediol, 2 parts of dimethylolbutyric acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 115°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 78°C.

[0060] S2. Under nitrogen protection, add 20 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 88℃ and keep the reaction at this temperature for 4 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0061] S3. Cool to 57℃, add 78 parts of acetone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 25 minutes. Turn on the high-shear emulsifier head, add 80 parts of deionized water at 10℃, and emulsify at 4000 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water and control the water addition time to 28 minutes. Adjust the solid content to 44%. Desolventize under vacuum at 60℃ for 1.5 hours under a negative pressure of 0.09 MPa until there is no acetone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the fast-drying waterborne polyurethane resin.

[0062] Example 3

[0063] A method for preparing a fast-drying waterborne polyurethane resin includes the following steps:

[0064] 152 parts of terephthalic acid, 56 parts of isophthalic acid, 80 parts of ethylene glycol, 10 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 255°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0065] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 46 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 116℃, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 2 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 72℃.

[0066] S2. Under nitrogen protection, add 15 parts of hexamethylene diisocyanate and 27 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0067] S3. Cool to 55℃, add 75 parts of methyl ethyl ketone (MEK) to dilute and reduce viscosity. At 55℃, slowly add 1.2 parts of N,N-dimethylethanolamine (NMEK) and neutralize for 22 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 4000 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 23 minutes, and adjust the solid content to 40%. Vacuum desolventize at 61℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no MEK odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the fast-drying waterborne polyurethane resin.

[0068] Example 4

[0069] A method for preparing a fast-drying waterborne polyurethane resin includes the following steps:

[0070] 155 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0071] S1. Weigh each component according to the required ratio, and put 31 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 115°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to break the air and the temperature is lowered to 76°C.

[0072] S2. Under nitrogen protection, add 24 parts of hexamethylene diisocyanate and 24 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0073] S3. Cool to 55℃, add 70 parts of methyl ethyl ketone (MEK) to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine (NMEK) and neutralize for 25 minutes. Turn on the high-shear emulsifier head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 44%. Vacuum desolventize at 66℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no MEK odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the fast-drying waterborne polyurethane resin.

[0074] Example 5

[0075] A method for preparing a fast-drying waterborne polyurethane resin includes the following steps:

[0076] 160 parts of terephthalic acid, 63 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.4 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0077] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 117°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 76°C.

[0078] S2. Under nitrogen protection, add 30 parts of hexamethylene diisocyanate and 18 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use di-n-butylamine titration to detect the NCO value until the theoretical endpoint is reached.

[0079] S3. Cool to 55℃, add 69 parts of methyl ethyl ketone (MEK) to dilute and reduce viscosity. At 55℃, slowly add 1.2 parts of N,N-dimethylethanolamine (NMEK) and neutralize for 25 minutes. Turn on the high-shear emulsifier head, add 100 parts of deionized water at 10℃, and emulsify at 4000 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 42%. Vacuum desolventize at 66℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no MEK odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the fast-drying waterborne polyurethane resin.

[0080] Comparative Example 1

[0081] A method for preparing an aqueous polyurethane resin includes the following steps:

[0082] S1. Weigh each component according to the required ratio, and put 35 parts of commercially available polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid, and 2 parts of trimethylolpropane into the reactor. Heat to 112°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. Detect the moisture content using the Karl Fischer method and find it to be <0.05%. After dehydration, purge with nitrogen and cool to 76°C.

[0083] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0084] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 25 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 42%. Desolventize under vacuum at 66℃ for 1.5 hours under a negative pressure of 0.09 MPa until there is no methyl ethyl ketone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0085] The main difference between this comparative example and Example 1 is that this comparative example uses commercially available polyester polyols instead of self-made polyester polyols.

[0086] Comparative Example 2

[0087] A method for preparing an aqueous polyurethane resin includes the following steps:

[0088] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0089] S1. Weigh each component according to the required ratio, add 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol and 2 parts of trimethylolpropane into the reactor, heat to 115℃, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5h. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, purge with nitrogen and cool down to 76℃.

[0090] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0091] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 23 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 43%. Desolventize under vacuum at 60℃ for 2 hours under a negative pressure of 0.09 MPa until there is no methyl ethyl ketone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0092] The main difference between this comparative example and Example 1 is that no hydrophilic monomers are added in this comparative example.

[0093] Comparative Example 3

[0094] A method for preparing an aqueous polyurethane resin includes the following steps:

[0095] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0096] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 4 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 111°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5h. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 79°C.

[0097] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0098] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 25 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 4000 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 44%. Desolventize under vacuum at 68℃ for 2 hours under a negative pressure of 0.09 MPa until there is no methyl ethyl ketone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0099] The main difference between this comparative example and Example 1 is that the hydrophilic monomer in this comparative example is added in excess.

[0100] Comparative Example 4

[0101] A method for preparing an aqueous polyurethane resin includes the following steps:

[0102] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0103] S1. Weigh each component according to the required ratio, add 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol and 2 parts of dimethylolpropionic acid into the reaction vessel, heat to 118℃, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5h. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, purge with nitrogen and cool down to 75℃.

[0104] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0105] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone (MEK) to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine (NMEK) and neutralize for 25 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 42%. Vacuum desolventize at 61℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no MEK odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0106] The main difference between this comparative example and Example 1 is that this comparative example does not contain trimethylolpropane.

[0107] Comparative Example 5

[0108] A method for preparing an aqueous polyurethane resin includes the following steps:

[0109] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0110] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid, and 4 parts of trimethylolpropane into the reaction vessel. Heat to 119°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, purge with nitrogen and cool down to 74°C.

[0111] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0112] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 25 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 42%. Desolventize under vacuum at 68℃ for 1 hour under a negative pressure of 0.09 MPa until there is no methyl ethyl ketone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0113] The main difference between this comparative example and Example 1 is that trimethylolpropane was added in excess in this comparative example.

[0114] Comparative Example 6

[0115] A method for preparing an aqueous polyurethane resin includes the following steps:

[0116] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0117] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 116℃, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5h. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 76℃.

[0118] S2. Under nitrogen protection, add 13 parts of hexamethylene diisocyanate and 25 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0119] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 25 minutes. Turn on the high-shear emulsification head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 45%. Desolventize under vacuum at 66℃ for 1 hour under a negative pressure of 0.09 MPa until there is no methyl ethyl ketone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0120] The main difference between this comparative example and Example 1 is that this comparative example has insufficient isocyanate.

[0121] Comparative Example 7

[0122] A method for preparing an aqueous polyurethane resin includes the following steps:

[0123] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0124] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 110°C, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5 hours. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 76°C.

[0125] S2. Under nitrogen protection, add 22 parts of hexamethylene diisocyanate and 42 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During the reaction, take samples every 30 minutes and use di-n-butylamine titration to detect the NCO value until the theoretical endpoint is reached.

[0126] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine and neutralize for 20 minutes. Turn on the high-shear emulsifier head, add 100 parts of deionized water at 10℃, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 45%. Vacuum desolventize at 62℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no methyl ethyl ketone odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0127] The main difference between this comparative example and Example 1 is that the isocyanate is in excess in this comparative example.

[0128] Comparative Example 8

[0129] A method for preparing an aqueous polyurethane resin includes the following steps:

[0130] 160 parts of terephthalic acid, 60 parts of isophthalic acid, 80 parts of ethylene glycol, 12 parts of glycerol, and 0.5 parts of tetrabutyl titanate were added to a reaction vessel and slowly heated to 150°C. After the materials began to melt, stirring was started, and the temperature was gradually increased to 220°C and maintained for reaction. During the reaction, the acid value was sampled and tested at regular intervals. After the acid value was ≤5, the temperature was increased to 250°C, and a vacuum was slowly drawn to a negative pressure below 0.09MPa. The reaction continued until the acid value was ≤2. The product was then discharged to obtain the self-made polyester polyol.

[0131] S1. Weigh each component according to the required ratio, and put 35 parts of self-made polyester polyol, 35 parts of polyether polyol, 4 parts of 1,4-butanediol, 2 parts of dimethylolpropionic acid and 2 parts of trimethylolpropane into the reaction vessel, heat to 115℃, turn on the vacuum pump, and dehydrate under vacuum at a negative pressure of 0.09MPa for 1.5h. The moisture content is detected by Karl Fischer method as <0.05%. After dehydration, nitrogen gas is introduced to ventilate and the temperature is lowered to 76℃.

[0132] S2. Under nitrogen protection, add 18 parts of hexamethylene diisocyanate and 30 parts of isophorone diisocyanate in batches, control the exothermic reaction, raise the temperature to 85℃ and keep the reaction at that temperature for 3 hours. During this period, take samples every 30 minutes and use the di-n-butylamine titration method to detect the NCO value until the theoretical endpoint is reached.

[0133] S3. Cool to 55℃, add 60 parts of methyl ethyl ketone (MEK) to dilute and reduce viscosity. At 55℃, slowly add 1.3 parts of N,N-dimethylethanolamine (NMEK) and neutralize for 25 minutes. Turn on the high-shear emulsifier head, add 100 parts of deionized water, and emulsify at 3500 rpm. After the viscosity of the system drops sharply, continue stirring for 10 minutes. Slowly add deionized water, controlling the water addition time to 30 minutes, and adjust the solid content to 43%. Vacuum desolventize at 66℃ under a negative pressure of 0.09 MPa for 1.5 hours until there is no MEK odor. Cool to below 40℃ and filter through a 200-mesh sieve to obtain the waterborne polyurethane resin.

[0134] The main difference between this comparative example and Example 1 is that the initial dispersion of deionized water in this comparative example was at room temperature.

[0135] Performance testing:

[0136] Performance tests were conducted on Examples 1-5 and Comparative Examples 1-8, and the specific test methods are as follows:

[0137] Fast drying performance: Surface drying time was used as the core evaluation index, and the test was conducted according to the finger-touch method in GB / T 1728-2020 "Determination of Drying Time of Paint Film and Putty Film". The sample was scraped onto a tinplate or glass plate to prepare a wet film of approximately 40 μm thickness, resulting in a test plate. The test plate was placed in a constant temperature and humidity chamber and tested under conditions of 23±2℃ / 50±5%RH. The surface of the coating was gently touched with a finger; the time when the finger felt no adhesion to the coating surface and a continuous film layer had formed was recorded as the surface drying time.

[0138] Hardness: Determination of hardness of paint films by pencil method according to GB / T 6739-2006 "Paints and Varnishes - Determination of Hardness of Paint Films by Pencil Method". The sample was applied to a glass or tinplate plate and cured at 23±2℃ / 50±5%RH for 24 hours before testing. A set of drawing pencils of different hardnesses was used, starting with the hardest pencil and decreasing in hardness sequentially until the pencil could not scratch or break the coating. The pencil hardness grade at which the coating was just not scratched (not broken) was recorded. The pencil hardness was tested at two time points, 2 hours and 24 hours after the coating dried, to assess the development of early and final hardness.

[0139] Water resistance: Tested according to the immersion test method in GB / T 1733-1993 "Determination of Water Resistance of Coatings". The sample is applied to a tinplate or glass plate and cured under standard conditions for 7 days to allow complete curing. The test plate is vertically immersed in a glass tank containing deionized water at 23±2℃. The test plate is removed daily, and the surface moisture is blotted dry with filter paper before immediately observing changes in the coating surface. Evaluation indicators include: blistering level (refer to ASTM D714), degree of discoloration, gloss loss rate, and decrease in adhesion. The time when blistering, whitening, or peeling occurs is recorded.

[0140] Flexibility: Elongation at break was used as the evaluation index, and the test was conducted according to GB / T 528-2009 "Determination of Tensile Stress-Strain Properties of Vulcanized Rubber or Thermoplastic Rubber". The sample was coated onto a polytetrafluoroethylene (PTFE) plate, cured under standard conditions for 7 days, and then peeled off to form a film with a thickness controlled at 1-2 mm, yielding the specimen. The specimen was conditioned at 23±2℃ / 50±5%RH for 6 hours. A universal testing machine was used, with the tensile speed set to 50 mm / min. The pneumatic clamps held both ends of the specimen, ensuring vertical alignment. The testing machine was started, and the specimen was stretched until fracture. The gauge length at fracture was recorded. The formula for calculating elongation at break is:

[0141] Eb=(L b - L0) / L0×100%,

[0142] Where L b L is the gauge length at the time of fracture, and L0 is the initial gauge length.

[0143] Comparative Example 2, lacking hydrophilic groups, could not form a stable emulsion, making it impossible to measure relevant data.

[0144] The specific performance test results of Examples 1-5 and Comparative Examples 1 and 3-8 are shown in the table below:

[0145] Table 1. Specific performance test results of Examples 1-5 and Comparative Examples 1, 3-8

[0146]

[0147] As shown in Table 1, the fast-drying waterborne polyurethane resin prepared by this invention combines fast drying, high water resistance, and excellent mechanical properties. The surface drying time of Examples 1-5 is ≤20 minutes, the hardness is above H after 24 hours, meeting the scratch resistance requirements, the water resistance is maintained for about 10 days, and the elongation is above 160%, ensuring stone impact resistance. Among them, Examples 1-3 can achieve an elongation at break of ≥180% while maintaining a hardness above H after 24 hours, showing excellent balance of mechanical properties, fully meeting the requirements of wet-on-wet process, and does not rely on drying agents or film-forming aids. It can achieve optimal performance balance without a large amount of functional monomers and inorganic fillers. The process is simple and low-cost, and has good industrial promotion value.

[0148] Comparative Example 1 uses commercially available polyester polyol instead of the self-made polyester polyol. Lacking the built-in branching points provided by glycerol, it loses its two-stage branching process, resulting in lower crosslinking density and a significantly extended surface drying time of 30 minutes. Comparative Example 2, without added hydrophilic monomers, lacks self-emulsifying ability and is completely unusable. Comparative Example 3 has excessive hydrophilic monomers; the hydrophilic groups bind water molecules, increasing water absorption channels, slowing water evaporation, and extending the surface drying time to 35 minutes, resulting in poor fast-drying properties. Comparative Example 4, without added trimethylolpropane, lacks chemical crosslinking points, resulting in an incomplete crosslinking network, decreased crosslinking density, slow early strength development, extended surface drying time, decreased hardness, and poor water resistance. Comparative Example 5 has excessive trimethylolpropane, increasing crosslinking points and causing over-crosslinking, leading to a brittle coating. The first example had a large elongation at break of only 61%, indicating poor flexibility. The second example had insufficient isocyanate and total NCO, resulting in a low R value, reduced urea bonds, insufficient cross-linking, loss of fast drying properties, and a significantly slower surface drying time of 48 minutes, along with a marked decrease in water resistance. The third example had excessive isocyanate and a high R value, leading to a large amount of urea bond formation, resulting in high internal stress, microcracks in the coating, and a significant decrease in flexibility, with an elongation at break of only 27%. The fourth example used room temperature deionized water for initial dispersion. Compared to the other examples, the performance was not significantly reduced, although flexibility and stability were slightly lower. However, room temperature water accelerated the side reactions of NCO and water, leading to increased urea bond formation and higher system temperature during emulsification, which had a significant impact on emulsion particle size and storage stability, and a high risk of gelation during emulsification.

[0149] In summary, this invention prepares a fast-drying waterborne polyurethane resin by using self-made polyester polyol, polyether polyol, chain extender, hydrophilic monomer, trimethylolpropane, isocyanate, and neutralizer. The self-made branched polyester and polyether construct a flexible segment that combines rigidity and flexibility. The hydrophilic monomer achieves self-emulsification and ensures water resistance with extremely low dosage. Trimethylolpropane and the self-made polyester polyol form a double crosslinking network with built-in branching points. The isocyanate compound balances hardness and pot life. The high R-value design allows residual NCO to naturally generate urea bonds during emulsification, compensating for the emulsification difficulties caused by low hydrophilicity. Ultimately, it achieves a balance between fast drying and high water resistance without the addition of external drying agents or amine-based chain extension. It combines fast drying, high water resistance, and excellent mechanical properties, fully meeting the requirements of wet-on-wet processes. The process is simple, low-cost, and has good industrial application value.

[0150] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention. 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 fast-drying waterborne polyurethane resin, characterized in that: By weight, it includes the following components: 30-40 parts of self-made polyester polyol, 30-50 parts of polyether polyol, 2-5 parts of chain extender, 1-3 parts of hydrophilic monomer, 1-3 parts of trimethylolpropane, 40-60 parts of isocyanate, 1.1-1.5 parts of neutralizer, and 50-80 parts of solvent.

2. The fast-drying waterborne polyurethane resin according to claim 1, characterized in that: The self-made polyester polyol comprises the following raw material components by weight: 150-180 parts terephthalic acid, 50-70 parts isophthalic acid, 70-90 parts ethylene glycol, 10-15 parts glycerol, and 0.2-0.7 parts catalyst.

3. The fast-drying waterborne polyurethane resin according to claim 1, characterized in that: The chain extender is one or more of 1,4-butanediol, ethylene glycol, neopentyl glycol, and 1,6-hexanediol.

4. The fast-drying waterborne polyurethane resin according to claim 1, characterized in that: The hydrophilic monomer is one or both of dimethylolpropionic acid and dimethylolbutyric acid.

5. The fast-drying waterborne polyurethane resin according to claim 1, characterized in that: The isocyanate is two or more of isophorone diisocyanate, hexamethylene diisocyanate, and dicyclohexylmethane diisocyanate.

6. The fast-drying waterborne polyurethane resin according to claim 1, its preparation method, and its application, characterized in that: The neutralizing agent is one or more of N,N-dimethylethanolamine, triethylamine, and 2-amino-2-methyl-1-propanol.

7. The fast-drying waterborne polyurethane resin according to claim 1, its preparation method, and its application, characterized in that: The solvent is one or more of acetone, butanone, and butyl acetate.

8. A method for preparing a fast-drying waterborne polyurethane resin according to any one of claims 1-7, characterized in that, Includes the following steps: S1. Weigh each component according to the required ratio, add the self-made polyester polyol, polyether polyol, chain extender, hydrophilic monomer and trimethylolpropane into the reactor, heat to 110-120℃ and dehydrate under vacuum for 1-2 hours, then cool to 70-80℃. S2. Add isocyanate, heat to 85-90℃ and keep the temperature for 3-4 hours, taking samples to detect NCO value until the theoretical endpoint is reached; S3. Cool to 50-60℃, add solvent to dilute, slowly add neutralizing agent to neutralize for 20-30 minutes, add deionized water at 5-15℃, emulsify at high speed, after the viscosity of the system drops sharply, continue stirring for 10 minutes, slowly add deionized water, vacuum desolventize at 60-70℃ for 1-2 hours, cool to below 40℃, filter, and the fast-drying waterborne polyurethane resin is obtained.

9. The method for preparing a fast-drying waterborne polyurethane resin according to claim 8, characterized in that: Step S2 is carried out under nitrogen protection; the high-speed emulsification rate in step S3 is 3000-5000 rpm.

10. The application of a fast-drying waterborne polyurethane resin according to any one of claims 1-7, characterized in that, Used in automotive coatings.

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