Hydrophilic low-temperature unblocking isocyanate curing agent, preparation method thereof and water-based coating composition containing curing agent

Abstract

The invention discloses a hydrophilic low-temperature unblocking isocyanate curing agent as well as a preparation method and application thereof in water-based paint, and belongs to the technical field of functional paint. The preparation method of the curing agent comprises the following steps: reacting organic polyisocyanate, polyol and carboxyl-containing polyol to prepare a-NCO-terminated prepolymer with a carboxyl-containing main chain; the-NCO group is blocked by using a chemical blocking agent; adding an organic alkali neutralizer and carboxyl to form salt; and finally, adding water for dispersion to obtain the water-based curing agent emulsion. A hydrophilic carboxyl group is copolymerized and built-in on a main chain of a polyurethane molecule and is neutralized to form a salt, so that the curing agent is endowed with internal water dilutability, and a small-molecule surfactant does not need to be additionally added. The curing agent prepared by the method has molecular-level compatibility with similar water-based matrix resin, the VOC content is reduced, the problems of film defects such as shrinkage cavities and needle holes caused by poor compatibility are solved, and the hardness, glossiness, adhesive force and water resistance and corrosion resistance of a film are improved.

Description

Technical Field

[0001] This invention belongs to the field of polymer chemistry and functional coatings technology, specifically relating to a method for preparing a waterborne low-temperature unblocking isocyanate curing agent that achieves intrinsic water dilution by introducing hydrophilic groups into the molecular backbone, and a high-performance two-component waterborne coating composition containing the curing agent. Background Technology

[0002] Blocked isocyanate curing agents are key components in two-component polyurethane coating systems. Their core mechanism of action lies in the fact that the isocyanate groups (-NCO) are temporarily protected by the blocking agent at room temperature, preventing them from reacting with active hydrogen groups (such as hydroxyl and amine groups) in the system, thus endowing the coating system with excellent storage stability (i.e., a long pot life). When the coating film is heated to a specific unblocking temperature, the blocking agent detaches, releasing highly active -NCO groups, which then undergo a cross-linking and curing reaction with the active groups of the host resin, forming a coating film with superior performance.

[0003] Existing blocked isocyanate curing agents are mainly divided into solvent-based and water-based types. Among them, water-based curing is the trend of the coating industry. However, the water-based blocked isocyanate curing agents on the market have obvious technical defects: their molecular structure is not fundamentally different from that of traditional solvent-based curing agents. Their water-based nature usually relies on the addition of a large amount of hydrophilic organic co-solvent (such as propylene glycol methyl ether acetate PMA, diethylene glycol butyl ether, and other amphoteric solvents) for dilution, and then relies on the emulsifying functional groups carried by the water-based main resin itself for forced and unstable emulsification and dispersion during use.

[0004] This water-based approach leads to the following problems: the curing agent molecules and the water-based main resin molecules have different hydrophilic mechanisms, essentially remaining an oil / water two-phase system with inherent limitations in compatibility. This easily causes the coating to separate and flocculate during storage or application, ultimately manifesting as defects such as pinholes, orange peel, and uneven gloss on the coating film. To improve these compatibility issues, various wetting, dispersing, and emulsifying agents must be added to the formulation, which not only increases the complexity and cost of formulation design but may also introduce new instabilities and even affect the water resistance of the final coating film. To achieve dilution and dispersion of the oil-based curing agent, the amount of hydrophilic organic co-solvent added is usually high, making it difficult for the product's volatile organic compound (VOC) content to meet increasingly stringent environmental regulations.

[0005] Therefore, developing a waterborne blocked isocyanate curing agent with inherent self-dispersing ability, excellent compatibility with waterborne main resin systems, and the ability to improve the overall performance of the final coating film is a technical challenge that needs to be solved in this field. Summary of the Invention

[0006] The primary objective of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing a low-temperature unblocking isocyanate curing agent that achieves water-based curing at the molecular structure level. The curing agent prepared by this method has inherent water dilution properties, requires no external surfactants, and exhibits excellent compatibility with conventional water-based resins, thereby reducing coating defects and improving coating performance.

[0007] A further object of the present invention is to provide an aqueous coating composition comprising the above-mentioned curing agent. This composition exhibits good storage stability, application performance, and comprehensive physicochemical properties of the cured coating film, such as gloss, adhesion, hardness, and water resistance, due to the compatibility between the curing agent and the main resin.

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

[0009] A method for preparing an aqueous low-temperature unblocking and sealing isocyanate curing agent, characterized by comprising the following steps:

[0010] (1) Preparation of NCO-terminated hydrophilic prepolymers:

[0011] In a reaction vessel, organic polyisocyanate monomers, oligomeric polyols, and polyol monomers containing carboxyl groups are reacted in the presence of a catalyst and an organic solvent. The total molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) is controlled to be 1.5:1 to 2.5:1, to generate a polyurethane prepolymer with -NCO end groups and carboxyl groups in the main molecular chain.

[0012] (2) Blocking of -NCO groups:

[0013] At a specific temperature, a chemical blocking agent is added dropwise to the prepolymer obtained in step (1) to react the -NCO groups at the end of the prepolymer with the blocking agent until the -NCO groups are completely blocked, thus obtaining a blocked isocyanate prepolymer.

[0014] (3) Neutralization of hydrophilic groups:

[0015] An organic base neutralizer is added to the closed isocyanate prepolymer obtained in step (2) to neutralize the carboxyl groups on the prepolymer molecular chain and generate a carboxylic acid amine salt with self-emulsifying ability.

[0016] (4) Aqueous phase dispersion:

[0017] Under high-speed shear (1000-2000 r / min), deionized water was slowly added dropwise to the product after neutralization in step (3) at a temperature below 40°C to disperse it and obtain an aqueous low-temperature unblocking type isocyanate curing agent emulsion.

[0018] The mass fraction ranges of each component in the above raw materials are as follows:

[0019] Organic polyisocyanates: 55-70 parts

[0020] Oligomeric polyols: 0-10 parts

[0021] Carboxyl-containing polyol monomers: 5-8 parts

[0022] Chemical sealant: 18-25 parts

[0023] Organic base neutralizer: 3.5-5 parts

[0024] Organic solvent: 15-25 parts

[0025] Deionized water: 110-140 parts;

[0026] Further, in step (1), the organic polyisocyanate is selected from one or more of aliphatic, alicyclic, or aromatic diisocyanates, preferably isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), and their trimers. The organic solvent is selected from one or more of divalent esters (DBE), N-methylpyrrolidone (NMP), or propylene glycol methyl ether acetate (PMA).

[0027] Further, in step (1), the carboxyl-containing polyol monomer is selected from dimethylolpropionic acid (DMPA) or dimethylolbutyric acid (DMBA); its amount accounts for 4% to 12% of the total solid mass. Within this range, the curing agent does not require the excellent water dispersion stability of an external emulsifier, and can form a coating film with extremely high water resistance after curing.

[0028] Further, in step (2), the chemical blocking agent is a compound that can react with -NCO at a low temperature and dissociate upon heating (preferably 80-120°C), preferably methyl ethyl ketone oxime, caprolactam, diethyl malonate, 3,5-dimethylpyrazole, or phenol. Methyl ethyl ketone oxime with a deblocking temperature between 80-110°C is particularly preferred.

[0029] Furthermore, in step (3), the organic base neutralizer is triethylamine (TEA) or N,N-dimethylethanolamine (DMEA), with a degree of neutralization of 80% to 110% of the molar number of carboxyl groups. After the prepolymer NCO is blocked, the degree of neutralization of the organic base must be controlled at 80% to 110%. If it is too low, it will not be able to self-emulsify, and if it is too high, it will cause the blocking agent to be partially unblocked prematurely during the water dispersion stage.

[0030] The organic solvent is an aprotic, high-boiling-point solvent used to adjust the viscosity of the reaction system. It is preferably one or more of DBE and propylene glycol methyl ether acetate (PMA), and its amount is strictly controlled.

[0031] To achieve the above-mentioned further objectives, the present invention also provides an aqueous coating composition, characterized in that it comprises the following components:

[0032] Component A: Aqueous host resin with hydroxyl groups (selected from aqueous acrylic dispersion or aqueous polyester resin);

[0033] Component B: A water-based low-temperature unblocking and sealing isocyanate curing agent prepared as described above.

[0034] The ratio of component B to component A is usually adjusted by a molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) of 0.8:1 to 1.5:1.

[0035] The aqueous host resin of component A is selected from one or more of aqueous acrylic resin, aqueous polyester resin, aqueous alkyd resin, or aqueous polyurethane dispersion (PUD). The active hydrogen groups mainly refer to hydroxyl groups.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) In this invention, hydrophilic carboxyl groups are incorporated into the molecular backbone of the curing agent through copolymerization, and then neutralized to form a salt, giving it water dilution properties. This carboxylate has the same hydrophilic mechanism as the mainstream water-based acrylics on the market (all of which are hydrophilic carboxylate). Therefore, the curing agent of this invention achieves water-based properties by incorporating hydrophilic groups, and has molecular-level compatibility with the main resin that is water-based on the same principle. The prepared coating is stable, and the cured film is smooth and free of defects such as pinholes and craters caused by poor compatibility, with high gloss.

[0038] (2) Due to the uniform mixing at the molecular level, the cross-linking reaction is more complete and the network structure is denser. Data shows that the coating film using the curing agent of this invention is superior to the coating film using the traditional forced emulsification water-based curing agent in terms of hardness, adhesion and especially water resistance.

[0039] (3) The inherent water dispersibility of this invention greatly reduces the dependence on organic cosolvents, resulting in lower VOC content in the coating formulation. At the same time, the excellent compatibility also reduces the need for additives such as wetting agents and emulsifiers, simplifying the formulation and improving stability. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of protection of the invention.

[0041] Example 1 (using IPDI and DMBA)

[0042] (1) Prepolymer preparation: In a four-necked flask equipped with a mechanical stirrer, dropping funnel, thermometer, and condenser, add 55.5 parts of isophorone diisocyanate (IPDI) and 15 parts of propylene glycol methyl ether acetate (PMA). Start stirring and heat to 60°C. Add a mixture of 6.5 parts of dimethylolbutyric acid (DMBA) and 5 parts of trimethylolpropane (TMP) in batches to the system. After complete dissolution, add 0.01 parts of dibutyltin dilaurate catalyst. Program the temperature to 80°C and maintain the reaction at this temperature for 2.5 hours. Take a sample and determine the content of free -NCO groups by di-n-butylamine titration. Stop the reaction when it reaches the theoretical calculated value (approximately 10.2%).

[0043] (2) Blocking reaction: Cool the above prepolymer solution to 70°C. Over 2 hours, add 21.5 parts of methyl ethyl ketone oxime (MEKO) dropwise at a uniform rate through a dropping funnel. After the addition is complete, continue to keep the reaction at 70°C for 2 hours until no -NCO characteristic peak is detected by infrared spectroscopy.

[0044] (3) Under stirring, slowly add 4.5 parts of the neutralizing agent triethylamine (TEA) to the sealed prepolymer, and continue stirring at the current temperature for 30 minutes to ensure complete neutralization of the carboxyl groups into salt. Subsequently, slowly add 120 parts of deionized water dropwise under high-speed dispersion shear at 1500 r / min. After the addition is complete, continue high-speed dispersion for 40 minutes to form a stable and uniform aqueous dispersion.

[0045] (4) Cool the obtained dispersion to room temperature and filter it with a 200-mesh filter to obtain the curing agent-A of the present invention.

[0046] Testing revealed that the solid content of curing agent-A was 38.5%, and its unsealing temperature was measured to be 90℃.

[0047] Example 2 (using HDI trimer and DMPA)

[0048] (1) Prepolymer preparation: Add 65 parts of hexamethylene diisocyanate (HDI) trimer (100% solid content), 10 parts of N-methylpyrrolidone (NMP), and 10 parts of propylene glycol methyl ether acetate (PMA) to the reactor. After heating to 65℃, add 5.5 parts of dimethylolpropionic acid (DMPA) and 0.01 parts of catalyst. Program the temperature to 85℃ and maintain the reaction for 2 hours until the -NCO content reaches the theoretical value.

[0049] (2) Blocking reaction: Cool to 75℃ and add 18.5 parts of 3,5-dimethylpyrazole (DMP) dropwise as a blocking agent. Keep the reaction at this temperature for 2 hours.

[0050] (3) Add 3.8 parts of dimethylethanolamine (DMEA) to the system for neutralization, and then add 130 parts of deionized water for high-speed dispersion at 1500 r / min.

[0051] (4) After cooling and filtration, the curing agent-B of the present invention is obtained. The solid content is 36.0%, and the unsealing temperature is 105°C.

[0052] Comparative Example 1 (Commercially available conventional external emulsion-based water-based curing agent)

[0053] Specifically, it is a commercially available water-based blocked isocyanate curing agent manufactured by Covestro, model number Bayhydur® BL 5335. Its solid content is approximately 39%, denoted as Curing Agent-C.

[0054] Comparative Example 2

[0055] (1) Preparation of hydrophobic blocked prepolymer:

[0056] In a four-necked flask equipped with a mechanical stirrer, dropping funnel, thermometer, and condenser, 59.8 parts of isophorone diisocyanate (IPDI) and 15 parts of propylene glycol methyl ether acetate (PMA) were added. Stirring was started and the temperature was raised to 60°C. 5.4 parts of trimethylolpropane (TMP) were added to the system in batches, and after complete dissolution, 0.01 parts of dibutyltin dilaurate catalyst were added. The temperature was programmed to 80°C and maintained at this temperature for 2.5 hours to obtain a hydrophobic NCO-terminated prepolymer. Subsequently, the prepolymer solution was cooled to 70°C, and 23.2 parts of butanone oxime (MEKO) were added dropwise at a uniform rate over 2 hours to initiate the blocking reaction. The reaction was maintained at this temperature for 2 hours until the -NCO groups were completely reacted.

[0057] (2) Forced emulsification and water dispersion:

[0058] To the hydrophobic, closed prepolymer obtained in step (1) that contains no inherent hydrophilic groups, fatty alcohol polyoxyethylene ether (AEO-9) was added as a nonionic emulsifier at an amount equal to 10% (8.8 parts) of the theoretical solid content of the prepolymer. The emulsifier and prepolymer were thoroughly mixed under shear force at 1500 r / min using a high-speed disperser. Subsequently, while maintaining high-speed dispersion, 125 parts of deionized water were slowly added dropwise for forced emulsification dispersion. After the addition was complete, high-speed dispersion was continued for 40 minutes.

[0059] (3) The obtained dispersion was cooled to room temperature and filtered through a 200-mesh filter to obtain curing agent-D. The product is an opaque white emulsion that is prone to separation after standing.

[0060] Comparative Example 3:

[0061] (1) Preparation of hydrophobic blocked prepolymer: Hydrophobic blocked prepolymer intermediate was prepared according to step (1) in Comparative Example 2.

[0062] (2) Keep the temperature at 70°C and slowly add 5 parts of 3-aminopropyltriethoxysilane (APTES) dropwise. After the addition is complete, continue stirring the reaction at this temperature for 2 hours.

[0063] (3) Cool the product obtained in step (2) to room temperature. Under the shear force of 1500 r / min in a high-speed disperser, try to slowly add 120 parts of deionized water dropwise.

[0064] In the initial stage of water addition, the viscosity of the system increases, and a large amount of white flocculent precipitate quickly appears, eventually forming a two-phase mixture of oil and water. This system cannot form a stable and uniform aqueous dispersion.

[0065] This demonstrates that the method of preparing the water-based curing agent using the post-grafting technique in Comparative Example 3 was unsuccessful. Therefore, the product of this comparative example, denoted as Curing Agent-E, was not available as a usable sample could not be obtained.

[0066] Comparative Example 4:

[0067] (1) Preparation of hydrophobic blocked prepolymer: Following the method of step (1) in Comparative Example 2, a hydrophobic blocked prepolymer intermediate without any intrinsic hydrophilic groups was obtained.

[0068] (2) Forced emulsification and water dispersion:

[0069] Sodium dodecyl sulfate (SDS) was added as an anionic emulsifier to the hydrophobic blocked prepolymer obtained in step (1). First, 4.4 parts of solid SDS (equivalent to 5% of the theoretical solid content of the prepolymer) were dissolved in 125 parts of deionized water to prepare an SDS aqueous solution. Then, under the shear force of 1500 r / min in a high-speed disperser, the SDS aqueous solution was slowly added dropwise to the hydrophobic blocked prepolymer for forced emulsification and dispersion. After the addition was complete, high-speed dispersion was continued for 40 minutes.

[0070] (3) Finished product: The obtained dispersion was cooled to room temperature and filtered through a 200-mesh filter to obtain curing agent-F. This product is a white emulsion in appearance. It has slightly better stability than curing agent-D, but still has a tendency to separate.

[0071] Comparative Example 5:

[0072] Comparative Example 5 differs from Example 1 in that the amount of dimethylolbutyric acid (DMBA) was reduced from 6.5 parts (approximately 8.3% of the solid content) to 1.5 parts (approximately 2% of the solid content). All other operations were the same as in Example 1.

[0073] The results showed that in the water dispersion step of step (3), even with high-speed shearing, it was difficult to form a stable and uniform emulsion. A large amount of co-solvent was required to barely disperse the product, and the product would separate and precipitate within a short time after standing.

[0074] Comparative Example 6:

[0075] The difference between Comparative Example 6 and Example 1 is that the amount of dimethylolbutyric acid (DMBA) was significantly increased from 6.5 parts to 12 parts (approximately 15% of the solid content). All other operations were the same as in Example 1.

[0076] Although a good water dispersion can be obtained, the water resistance decreases after curing with resin-based paints. In boiling water and salt spray tests, the coating film will turn white, blister, and peel off.

[0077] Comparative Example 7:

[0078] Compared with Example 1, Comparative Example 7 differs in that the amount of neutralizing agent triethylamine (TEA) added in step (3) is reduced to 2.7 parts (60% of the molar number of carboxyl groups on the prepolymer).

[0079] During the water dispersion in step (4), the viscosity of the system increased, making dispersion difficult. When the final product was filtered through a 200-mesh filter, undispersed polymer agglomerates remained on the filter screen. The collected emulsion showed obvious sedimentation and stratification after standing at room temperature for 24 hours.

[0080] Comparative Example 8

[0081] Compared with Example 1, Comparative Example 8 differs in that the amount of neutralizing agent triethylamine (TEA) added in step (3) is increased to 5.8 parts (corresponding to 130% of the molar number of carboxyl groups on the prepolymer).

[0082] Water dispersion was smooth, but when it was placed in a 50°C oven for accelerated storage stability testing, the emulsion viscosity began to increase significantly on day 5 and completely gelled on day 7.

[0083] Comparative Example 9

[0084] (1) Preparation of prepolymer: 55.5 parts of IPDI and 15 parts of PMA were added to a reactor and the temperature was raised to 60°C. 15 parts of polyethylene glycol monomethyl ether (molecular weight 1000) and 0.01 parts of catalyst were added. The reaction was carried out at 80°C for 2 hours. Then 5 parts of TMP were added and the reaction was continued until the theoretical NCO value was reached.

[0085] (2) Blocking reaction: Cool down to 70℃ and add 21.5 parts of butanone oxime for blocking.

[0086] (3) Dispersion: No neutralization is required; deionized water is added directly under high-speed shear to disperse the product. A water-based blocked isocyanate curing agent with a solid content of approximately 38% is obtained, denoted as curing agent-H.

[0087] Application performance testing:

[0088] The above-mentioned curing agents A, B, C, D, F, and H were respectively mixed with the water-based hydroxy acrylic dispersion (Bayhydrol® A 2601, hydroxyl content approximately 3.9%) at an NCO / OH molar ratio of 1.1:1. 0.5% leveling agent and 0.3% defoamer were added to the mixture, and after stirring evenly, the viscosity was adjusted to 35 seconds (25°C) with an appropriate amount of deionized water.

[0089] The prepared coatings were sprayed onto tinplate that had been sanded and degreased with acetone, and onto cleaned glass plates. The coatings were allowed to level at room temperature for 20 minutes, then baked in a 120°C oven for 30 minutes. After removal, the coatings were placed at 23±2°C and 50±5% relative humidity for 24 hours. The properties of the coatings were then tested according to the relevant national standards, and the results are shown in Table 1.

[0090] Table 1: Comparison Test Data of Overall Coating Performance

[0091]

[0092] Test standard description:

[0093] Gloss (60°): GB / T 9754-2007

[0094] Pencil hardness: GB / T 6739-2006

[0095] Adhesion (cross-cut test): GB / T 9286-1998

[0096] Storage stability: The prepared paint solution was placed in a 50℃ oven for accelerated storage for 14 days, and the appearance changes were observed and the viscosity growth rate was tested.

[0097] Salt spray resistance (NSS): After scribing the coating, place it in a neutral salt spray test chamber and test it according to GB / T 1771-2007 standard. Record the corrosion spread width at the scribing point after the specified time.

[0098] Coating gel rate: The cured coating was weighed (W1), placed in acetone for Soxhlet extraction for 24 hours, dried and weighed (W2). Gel rate = (W2 / W1) × 100%. Used to characterize the density of the cross-linked network.

[0099] As shown in Table 1, the curing agents (curing agents A and B) prepared using the method of this invention exhibit excellent compatibility with water-based hydroxyl resins, resulting in stable paint and a higher gloss level than all comparative examples. Particularly in terms of water resistance (resistance to boiling water), chemical resistance (resistance to methyl ethyl ketone wiping), hardness, and corrosion resistance (resistance to salt spray), this invention avoids the negative impacts of added small-molecule emulsifiers and simultaneously achieves a more uniformly distributed crosslinking network, thus demonstrating overwhelming performance advantages.

[0100] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a water-based low-temperature unblocking and sealing isocyanate curing agent, characterized in that, Includes the following steps: (1) In a reaction vessel, organic polyisocyanate monomers, polyols, and polyol monomers containing carboxyl groups are reacted in the presence of a catalyst and an organic solvent. The total molar ratio of NCO to OH is 1.5:1 ~ 2.5:1, to generate a polyurethane prepolymer with -NCO end groups and carboxyl groups in the main molecular chain; the polyol is a small molecule polyol or an oligomer polyol. (2) Add a chemical blocking agent to the prepolymer obtained in step (1) to react the -NCO groups at the end of the prepolymer with the blocking agent until the -NCO groups are completely blocked to obtain a blocked isocyanate prepolymer. (3) Add an organic base neutralizer to the closed isocyanate prepolymer obtained in step (2) to neutralize the carboxyl groups on the prepolymer molecular chain and generate a carboxylic acid amine salt with self-emulsifying ability. (4) Under high-speed shear (1000-2000 r / min), deionized water is slowly added dropwise to the product after neutralization in step (3) at a temperature below 40°C to disperse it and obtain the water-based low-temperature unblocking and sealing isocyanate curing agent emulsion.

2. The preparation method of the water-based low-temperature unblocking and sealing isocyanate curing agent according to claim 1, characterized in that: In step (1), the organic polyisocyanate monomer is selected from one or more of isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI) or their trimers; the organic solvent is selected from one or more of divalent ester (DBE), N-methylpyrrolidone (NMP) or propylene glycol methyl ether acetate (PMA).

3. The preparation method of the water-based low-temperature unblocking and sealing isocyanate curing agent according to claim 1, characterized in that: In step (1), the carboxyl-containing polyol monomer is selected from dimethylolpropionic acid (DMPA) or dimethylolbutyric acid (DMBA); its amount accounts for 4% to 12% of the total solid mass.

4. The preparation method of the water-based low-temperature unblocking and sealing isocyanate curing agent according to claim 1, characterized in that: In step (2), the desealing temperature of the chemical sealant is between 80 and 120°C, and the specific chemical sealant is selected from one or more of butanone oxime, caprolactam, diethyl malonate, 3,5-dimethylpyrazole or phenol.

5. The preparation method of the water-based low-temperature unblocking and sealing isocyanate curing agent according to claim 1, characterized in that: The mass fractions of each component are as follows: organic polyisocyanate: 55-70 parts; polyol: 0-10 parts; polyol monomer containing carboxyl group: 5-8 parts; chemical blocking agent: 18-25 parts; organic base neutralizer: 3.5-5 parts; organic solvent: 15-25 parts; deionized water: 110-140 parts.

6. The preparation method of the water-based low-temperature unblocking and sealing isocyanate curing agent according to claim 1, characterized in that: In step (3), the organic base neutralizer is triethylamine (TEA) or N,N-dimethylethanolamine (DMEA), and the amount added is 80% to 110% of the molar number of carboxyl groups.

7. An aqueous low-temperature unblocking and blocking isocyanate curing agent prepared by the preparation method according to any one of claims 1-6.

8. A water-based coating composition, characterized in that, It comprises the following components: Component A: an aqueous host resin containing carboxylate hydrophilic groups and hydroxyl groups; Component B: an aqueous low-temperature unblocking and blocking isocyanate curing agent as described in claim 7; wherein the carboxylate hydrophilic groups in Component A and the carboxylate amine salts on the main chain of Component B have the same anionic hydrophilic groups.

9. The water-based coating composition according to claim 8, characterized in that: The ratio of component B to component A, based on the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH), is 0.8:1 to 1.5:

1.

10. The waterborne coating composition according to claim 8, characterized in that: Component A is selected from one or more of anionic waterborne acrylic resin, anionic waterborne polyester resin, or anionic waterborne polyurethane dispersion (PUD).

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