Asymmetric double-closed isocyanate curing agent as well as preparation method and application thereof

Abstract

The invention relates to the technical field of cathode electrophoretic paint, and discloses an asymmetric double-blocked isocyanate curing agent as well as a preparation method and application thereof. Based on the reaction activity difference of primary carbon and secondary carbon NCO groups in isophorone diisocyanate molecules, a step-by-step asymmetric sealing strategy is adopted: in the first stage, a hydrophilic chain extender containing a tertiary amine structure is utilized to preferentially react with high-activity primary carbon NCO, and an internal catalytic center and a hydrophilic group are anchored in a molecular skeleton; and in the second stage, the active methylene compound and the bio-based long-chain phenol are used for carrying out hybrid dual-sealing on the residual secondary carbon NCO with relatively large steric hindrance. According to the preparation of the curing agent, the curing agent integrates internal catalysis, stepped curing and self-emulsifying functions, the deblocking energy barrier is effectively reduced, and 140-DEG C low-temperature curing or high-temperature high-performance curing can be realized under the tin-free catalysis condition.

Description

Technical Field

[0001] This invention relates to the field of cathodic electrophoretic coating technology, and in particular to a dual-blocking isocyanate curing agent based on the asymmetric structural design of isophorone diisocyanate, possessing tertiary amine internal catalytic function, and capable of achieving step-by-step deblocking and curing at low and high temperatures, as well as the application of this curing agent in high-performance cathodic electrophoretic coatings. Background Technology

[0002] Cathodic electrophoretic coatings have become an indispensable primer coating technology in industrial fields such as automobiles, home appliances, and hardware due to their excellent anti-corrosion performance, high penetration, and low environmental pollution. In this system, the blocked polyisocyanate curing agent is the core crosslinking component, and its performance directly determines the storage stability, curing energy consumption, and the mechanical strength, chemical resistance, and corrosion resistance of the final coating film.

[0003] Currently, the widely used blocked isocyanate curing agents in industry mainly employ methyl ethyl ketone oxime, caprolactam, or alcohol ether compounds as sealants. However, these traditional systems have significant drawbacks: firstly, their unsealing temperatures are generally high (usually above 160°C), resulting in high energy consumption and unsuitability for heat-sensitive substrates; secondly, these sealants are insufficient in terms of environmental friendliness and hydrophobicity. To balance performance, existing technologies have attempted to adjust the curing curve by physically blending curing agents with different unsealing temperatures. However, this method often leads to appearance defects in the coating film, such as orange peel and pinholes, due to poor compatibility between different curing agent molecules and mismatched curing kinetics.

[0004] To improve the hydrophobicity and corrosion resistance of coatings, some technologies have been explored. For example, Chinese patent CN118667113A discloses the use of hydroxyl polysiloxane modified with cashew phenol glycidyl ether followed by reaction with isocyanate. While this achieves a certain degree of hydrophobicity, the organosilicon component easily leads to pinhole defects in the electrophoresis bath, and once contaminated, it is extremely difficult to remove, severely affecting production stability and recoating properties. Another patent, CN106479338A, uses cationic fluorine-modified acrylic resin and physically doped with hydrophobic nano-silica to prepare a superhydrophobic coating. However, this approach faces numerous industrialization challenges, including the base resin's inferior salt spray resistance compared to epoxy systems, the tendency of nanoparticles to settle and cause bath instability, and the high cost and environmental risks of fluorinated monomers.

[0005] It is evident that existing technologies mostly employ physical modification (such as blending and doping) or simple molecular modification, failing to fundamentally resolve the contradictions in molecular structure design. Therefore, developing an innovative isocyanate curing crosslinking agent is of urgent need and significant importance for promoting the development of cathodic electrophoretic coating technology towards high performance, low energy consumption, and environmental friendliness. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides an asymmetric, dual-blocked isocyanate curing agent, its preparation method, and its applications. The isocyanate curing agent provided by this invention introduces an internal catalytic group through molecular structure design, effectively solving the technical difficulties of high deblocking temperature in traditional bio-based phenolic blockers and poor water resistance in active methylene blockers. Simultaneously, utilizing the special amphiphilic design of the molecular structure, stable dispersion of highly hydrophobic components in the aqueous phase is achieved, resulting in a coating that combines low-temperature curing characteristics with excellent hydrophobic and anti-corrosion properties. This curing agent can be catalytically cured using a tin-free drier, and the resulting cathodic electrophoretic coating can use general-purpose color pastes.

[0007] The specific technical solution of this invention is as follows:

[0008] In a first aspect, the present invention provides an asymmetric dual-blocked isocyanate curing agent, which is prepared from raw materials comprising the following parts by weight:

[0009] 100-120 parts isophorone diisocyanate, 70-80 parts dispersant, 0.3-0.5 parts catalyst, 10-20 parts chain extender, 50-120 parts blocking agent A and 80-110 parts blocking agent B;

[0010] The chain extender is selected from at least one of N-methyldiethanolamine and N-n-butyldiethanolamine;

[0011] The sealing agent A is selected from at least one of cashew phenol and dodecylphenol;

[0012] The sealing agent B is selected from at least one of diethyl malonate, ethyl acetoacetate, and dimethyl malonate.

[0013] The isocyanate curing agent provided by this invention employs asymmetric modification and blocking of isophorone diisocyanate using a hydrophilic chain extender containing a tertiary amine structure, an active methylene compound, and a bio-based long-chain phenol. Utilizing the difference in reactivity between the primary and secondary NCO groups in the isophorone diisocyanate molecule, the internal catalytic center (tertiary amine group) is anchored to the molecular framework through regioselectivity, forming a dual-blocking structure of active methylene / phenolic hydroxyl groups at sterically hindered secondary carbon sites. Therefore, the isocyanate curing agent provided by this invention possesses a unique structure with an intramolecular in-situ catalytic and step-release mechanism, which significantly reduces the energy barriers of transesterification and deblocking reactions, thereby improving the curing effect of the resulting curing agent.

[0014] The curing crosslinking agent of this invention allows for flexible adjustment of the curing temperature by adjusting the ratio of sealant A (bio-based phenol) to sealant B (active methylene). When pursuing ultimate anti-corrosion performance, the proportion of sealant A can be increased, and the curing temperature can be set to 150-160℃; when pursuing energy conservation and environmental protection or for use on heat-sensitive substrates, the proportion of sealant B can be increased to achieve low-temperature curing at 130-140℃. Furthermore, thanks to the surface shielding effect of the bio-based hydrophobic long chains, the coating film exhibits good resistance to neutral salt spray and possesses excellent anti-corrosion performance.

[0015] Preferably, the dispersant is composed of 25-30 parts of dispersant A and 45-50 parts of dispersant B; wherein dispersant A is selected from at least one of methyl isobutyl ketone (MIBK), acetone and butanone; and dispersant B is selected from at least one of ethylene glycol butyl ether (BCS) and ethylene glycol hexyl ether (HCS).

[0016] Preferably, the catalyst consists of 0.05-0.10 parts of bismuth isooctanoate and 0.25-0.45 parts of zinc acetylacetonate.

[0017] Preferably, the isocyanate curing agent described above is made from the following raw materials in parts by weight: 111 parts isophorone diisocyanate, 25 parts dispersant A, 50 parts dispersant B, 0.4 parts catalyst, 90-110 parts blocking agent A, and 70-80 parts blocking agent B.

[0018] Secondly, the present invention provides an asymmetric dual-blocked isocyanate curing agent, the chemical structure of which is shown in formula (I), formula (II) or formula (III):

[0019] (I);

[0020] (II);

[0021] (III).

[0022] Thirdly, the present invention provides a method for preparing an asymmetric dual-blocked isocyanate curing agent, comprising the following steps:

[0023] (1) According to the preset mass parts, add isophorone diisocyanate and dispersant A into the reaction vessel and mix well. Heat to 50~60℃, and then add chain extender dropwise. After the dropwise addition is completed, react at 50-60℃ for 1~2h. Monitor the NCO content in the system. When the NCO content drops to 60-80% of the initial value, the initial product is obtained.

[0024] (2) Add blocking agent A and bismuth isooctanoate catalyst to the initial product obtained in step (1), and keep it at 50~60℃ for 1~2h. Monitor the NCO content in the system. When the NCO content drops to 40~50% of the initial value, add blocking agent B and zinc acetylacetonate, and heat to 70~80℃ for 4~5h until the NCO content is less than 0.2% to obtain the blocked product.

[0025] (3) Add dispersant B to the closed product obtained in step (2) and mix evenly to obtain the isocyanate curing agent.

[0026] This invention utilizes the difference in reactivity between the NCO groups attached to primary and secondary carbons in isophorone diisocyanate molecules. Through a stepwise "chain extension followed by closure" process, in the first stage (catalyst-free and at a relatively low temperature (50-60°C), the reaction of a hydrophilic chain extender (MDEA) with the highly reactive primary carbon NCO is preferentially controlled, anchoring the internally catalytic tertiary amine group at the end of the molecular chain. Subsequently, in the second stage, a closure agent is used to close the remaining sterically hindered secondary carbon NCO. This specific "secondary-position closure" structure places the closure group in a highly sterically hindered environment, and combined with the internal catalytic effect of the ortho-tertiary amine, effectively reduces the unblocking energy barrier.

[0027] Fourthly, the present invention provides the application of an asymmetric dual-blocked isocyanate curing agent in the preparation of cathodic electrophoretic emulsions.

[0028] Fifthly, the present invention provides an application of an asymmetric dual-blocked isocyanate curing agent in the preparation of cathodic electrophoretic coatings.

[0029] In a sixth aspect, the present invention provides a cathodic electrophoretic emulsion, which, by weight, is made of the following raw materials: 100-200 parts of a main resin, 8-30 parts of a complex amine, 20-45 parts of an isocyanate curing crosslinking agent as described in any one of claims 1 to 4, 1-3 parts of an acid, and 100-200 parts of water.

[0030] Preferably, the complex amine is selected from at least one of diethanolamine, N-methylethanolamine, and ketoimine.

[0031] In a seventh aspect, the present invention provides a cathodic electrophoretic coating, which is formulated from the above-mentioned cathodic electrophoretic emulsion and a color paste. The present invention has no special requirements for the color paste; conventional tin-free color pastes in the art are sufficient.

[0032] Compared with the prior art, the present invention has the following technical effects:

[0033] (1) The isocyanate curing agent provided by the present invention uses a hydrophilic chain extender containing a tertiary amine structure, an active methylene compound, and a bio-based long-chain phenol to asymmetrically modify and block isophorone diisocyanate. Utilizing the difference in reactivity between the primary and secondary NCO groups in the isophorone diisocyanate molecule, the internal catalytic center (tertiary amine group) is anchored to the molecular framework through regioselectivity, and a dual-blocking structure of active methylene / phenolic hydroxyl groups is formed at the sterically hindered secondary carbon sites. Therefore, the isocyanate curing agent provided by the present invention has a special structure with an intramolecular in-situ catalytic and step-release mechanism, which can significantly reduce the energy barrier of transesterification and deblocking reactions.

[0034] (2) The isocyanate curing agent structure provided by the present invention effectively solves the technical bottleneck of the difficulty in emulsifying strong hydrophobic components, gives the coating film good salt spray resistance, and has excellent compatibility with general pigments and storage stability, which significantly improves the overall anti-corrosion quality and application convenience of cathodic electrophoretic coatings. Attached Figure Description

[0035] Figure 1 The infrared spectrum of the isocyanate curing agent prepared in Example 1 of this invention;

[0036] Figure 2 The results are DSC and TGA test results of the curing agent in Example 1 of this invention. Detailed Implementation

[0037] The present invention will be further described below with reference to embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0038] Example 1

[0039] An isocyanate curing agent is provided, which is prepared by the following method:

[0040] (1) 111 parts of isophorone diisocyanate and 25 parts of MIBK were placed in a reaction vessel and mixed and heated to 50°C. 12 parts of N-methyldiethanolamine were added dropwise. After reacting for 1.5 h, the NCO content in the system was detected. When the NCO content was 71% remaining, the initial product was obtained.

[0041] (2) Add 100 parts of cashew phenol and 0.11 parts of bismuth isooctanoate catalyst to the initial product, keep it at 60°C for 2 hours, and then check the remaining NCO content. When the remaining NCO content is 42%, add 80 parts of diethyl malonate and 0.3 parts of zinc acetylacetonate catalyst, keep it at 80°C for 5 hours, and then check the remaining NCO content. If it meets the requirement of <0.2%, the blocked product is obtained.

[0042] (3) Add 50 parts of BCS to the blocked product to adjust the solid content of the system to 80%, thus obtaining the isocyanate curing agent of this embodiment, the structure of which is shown in structural formula (I). Formula (I) is as follows:

[0043] (I).

[0044] (3) The isocyanate curing agent of the product was characterized by infrared and DSC tests, and the results are shown in the figure. Figure 1 , Figure 2 . Figure 1 This is an infrared spectrum; Figure 2 These are the DSC and TGA test results for the curing agent. Figure 2 As shown, the DSC curve exhibits a wide endothermic desealing range (accompanied by approximately 22% continuous weight loss in the TG curve). This range logically encompasses two reaction stages: the initial segment of the main endothermic peak is located in the 75-85℃ range, corresponding to the preferential desealing of the active methylene group (diethyl malonate); subsequently, the endothermic signal continues and tails to the 110-150℃ range, corresponding to the desealing of the thermally stable bio-based phenol (cashew phenol). This "wide-temperature-range relay desealing" characteristic confirms that the curing agent of this invention has step-curing properties, which can both utilize the desealing of diethyl malonate at relatively low temperatures to release stress and improve leveling, and utilize the desealing of cashew phenol at high temperatures to introduce hydrophobic long chains, thereby significantly improving the crosslinking density, flexibility, and water-resistant barrier properties of the final cured system.

[0045] Example 2

[0046] An isocyanate curing agent is provided, which is prepared by the following method:

[0047] (1) Mix 111 parts of isophorone diisocyanate and 25 parts of MIBK in a reaction vessel and heat to 50°C. Add 16 parts of N-n-butyldiethanolamine. After reacting for 1.5 h, check the NCO content in the system. If it meets the requirements (60~80%), the initial product is obtained.

[0048] (2) Add 88 parts of dodecylphenol and 0.11 parts of bismuth isooctanoate catalyst to the initial product, and after reacting at 60°C for 2 hours, the NCO content is detected and meets the requirements (40~50%). Continue to add 65 parts of ethyl acetoacetate and 0.3 parts of zinc acetylacetonate catalyst, and after reacting at 80°C for 5 hours, the NCO content is detected and meets the requirements (<0.2%), thus obtaining the blocked product.

[0049] (3) Add 50 parts of BCS to the blocked product to adjust the solid content of the system to 80%, thus obtaining the isocyanate curing crosslinking agent of this embodiment, the structure of which is shown in structural formula (II). Formula (II) is as follows:

[0050] (II).

[0051] Example 3

[0052] An isocyanate curing agent is provided, which is prepared by the following method:

[0053] (1) Mix 111 parts of isophorone diisocyanate and 25 parts of MIBK in a reaction vessel and heat to 50°C. Add 12 parts of N-methyldiethanolamine dropwise. After reacting for 1.5 h, check the NCO content in the system. If it meets the requirements (60-80%), the initial product is obtained.

[0054] (2) Add 100 parts of cashew phenol and 0.11 parts of bismuth isooctanoate catalyst to the initial product, and after reacting at 60°C for 2 hours, the NCO content is detected and meets the requirements (40-50%). Continue to add 66 parts of dimethyl malonate and 0.3 parts of zinc acetylacetonate catalyst, and after reacting at 80°C for 5 hours, the NCO content is detected and meets the requirements (<0.2%), thus obtaining the blocked product.

[0055] (3) Add 50 parts of BCS to the blocked product to adjust the solid content of the system to 80%, thus obtaining the isocyanate curing crosslinking agent of this embodiment, the structure of which is shown in structural formula (III). Formula (III) is as follows:

[0056] (III).

[0057] Comparative Example 1

[0058] An isocyanate curing agent is provided, the preparation method of which differs from that of Example 1 mainly in that: a single bio-based phenolic blocking agent (cashew nut shell powder) is used to completely block isophorone diisocyanate.

[0059] The preparation steps for this comparative example are as follows:

[0060] (1) 111 parts of isophorone diisocyanate and 25 parts of MIBK were placed in a reaction vessel, mixed, and heated to 50°C. 300 parts of cashew phenol and 0.3 parts of bismuth isooctanoate catalyst were added dropwise. After reacting at 60°C for 4 hours, the NCO content was measured to be <0.2%, which indicates the blocked product. This comparative example requires 300 parts of cashew phenol to completely block isophorone diisocyanate. Example 1 used 100 parts of cashew phenol and 80 parts of diethyl malonate, which was sufficient for complete blocking. This comparative example aims to investigate the difference between using a single bio-based phenolic blocking agent and using different blocking agents in Example 1 for completely blocked products.

[0061] (2) Add 50 parts of BCS to the closed product and adjust the solid content of the system to 80% to obtain an isocyanate curing agent.

[0062] Comparative Example 2

[0063] An isocyanate curing agent is provided, the preparation method of which differs from that of Example 1 mainly in that: a single active methylene blocking agent is used to completely block isophorone diisocyanate.

[0064] The preparation steps for this comparative example are as follows:

[0065] (1) 111 parts of isophorone diisocyanate and 25 parts of MIBK were placed in a reaction vessel and mixed and heated to 50°C. 160 parts of diethyl malonate and 0.5 parts of zinc acetylacetonate catalyst were added dropwise. After reacting at 60°C for 4 hours, the remaining NCO content was detected. If it met the requirement of <0.2%, the blocked product was obtained.

[0066] (2) Add 50 parts of BCS to the closed product and adjust the solid content of the system to 80% to obtain the isocyanate curing agent of this comparative example.

[0067] Comparative Example 3

[0068] An isocyanate curing agent is provided, the preparation method of which differs from that of Example 1 mainly in that N-methyldiethanolamine with a tertiary amine structure (with internal catalytic activity) is replaced with 1,4-butanediol, a common chain extender with no catalytic activity.

[0069] The preparation steps for this comparative example are as follows:

[0070] (1) 111 parts of isophorone diisocyanate and 25 parts of MIBK were placed in a reaction vessel, mixed and heated to 50°C. 9 parts of 1,4-butanediol were added dropwise. After reacting for 1.5 h, the initial product was obtained.

[0071] (2) Add 100 parts of cashew phenol and 0.11 parts of bismuth isooctanoate catalyst to the initial product, and react at 60°C for 2 hours. The remaining NCO content is then tested and found to be within the requirements (40-50%). 80 parts of diethyl malonate and 0.3 parts of zinc acetylacetonate catalyst are then added, and the reaction is carried out at 80°C for 5 hours. The remaining NCO content is then tested and found to be within the requirements of <0.2%, thus obtaining the blocked product.

[0072] (3) Add 50 parts of BCS to the closed product and adjust the solid content of the system to 80% to obtain the isocyanate curing agent of this comparative example.

[0073] Comparative Example 4

[0074] An isocyanate curing agent is provided, the main difference of which is that the preparation method is different from that of Example 1: the conventional industrial general-purpose sealing agent methyl ethyl ketone oxime (MEKO) is used for sealing.

[0075] The preparation steps for this comparative example are as follows:

[0076] (1) Mix 111 parts of isophorone diisocyanate and 25 parts of MIBK in a reaction vessel and heat to 50°C. Add 87 parts of methyl ethyl ketone oxime and keep the reaction at 60°C for 4 hours. Detect the remaining NCO content. If it meets the requirement of <0.2%, the blocked product is obtained.

[0077] (2) Add 50 parts of BCS to the closed product and adjust the solid content of the system to 80% to obtain the isocyanate curing crosslinking agent of this comparative example.

[0078] Performance Characterization

[0079] The viscosity and stability of the curing agents prepared in Examples 1-3 and Comparative Examples 1-4 were tested. The stability was tested after being sealed and placed at 25°C for 6 months. The results are shown in Table 1.

[0080] Table 1

[0081] Curing crosslinking agent Viscosity (cP) Storage stability (25°C, 6 months) Example 1 404 excellent Example 2 425 good Example 3 330 excellent Comparative Example 1 680 good Comparative Example 2 1600 Difference Comparative Example 3 2500 Range Comparative Example 4 4000 middle

[0082] As shown in Table 1, the curing agents prepared in Examples 1-3 exhibit excellent viscosity and stability. Comparative Example 1, which used only a single bio-based phenolic blocker to completely block IPDI, showed higher viscosity and decreased storage stability compared to Example 1. Comparative Example 2, which used only a single active methylene blocker to completely block IPDI, Comparative Example 3, which replaced N-methyldiethanolamine with a tertiary amine structure with the common chain extender 1,4-butanediol, and Comparative Example 4, which used the conventional industrial blocker methyl ethyl ketone oxime, all showed a significant decrease in storage stability.

[0083] The isocyanate curing agents prepared in Examples 1-3, Comparative Examples 1 and 4 were used to prepare cathodic electrophoretic emulsions. Then, they were mixed with Wuxi black paste 7968 (bismuth content < 0.8%) at a mass ratio of 4:1 and mechanically stirred evenly. Electrophoretic deposition was carried out at an electrophoretic temperature of 30℃, a voltage of 140V and an electrophoretic time of 100s. The performance of the emulsion coating film prepared with the above isocyanate curing agents was tested, and the results are shown in Table 2.

[0084] The preparation method of the cathodic electrophoretic emulsion is as follows: (a) Preparation of the main resin: 165 parts of epoxy resin NPES-906L, 8 parts of methylethanolamine and 13 parts of ketoimine are added to the reaction vessel and reacted at 120°C for 2 hours to obtain the main resin; (b) Preparation of the cathodic electrophoretic emulsion: 1.1 parts of acid are placed in the emulsification vessel, 50 parts of pure water are added, and the mixture is preheated to 40°C. 60 parts of the main resin are poured into the emulsification vessel. The temperature of the main resin is <95°C. The mixture is stirred for 30 minutes. The temperature is kept below 45°C. 22 parts of the above-mentioned isocyanate curing agent (prepared in Examples 1-3, Comparative Example 1 or Comparative Example 4) are added and stirred for 2.5 hours. 70 parts of pure water are added in two batches, and the mixture is stirred for 60 minutes each time to obtain the cathodic electrophoretic emulsion of this example.

[0085] Table 2

[0086] project standard Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 4 Baking temperature / 160℃, 30min 160℃, 30min 140℃, 30min 140℃, 30min 140℃, 30min Appearance / Smooth and free from yellowing Smooth and free from yellowing Smooth and free from yellowing Sticky surface, slightly wrinkled Smooth, noticeably yellowed Pencil hardness GB / T6739-2006 3H 2H 3H HB H Water contact angle / 90.8° 89.5° 91.2° 85.4° 77.6° Salt spray resistance GB / T17710-2007 No blistering or rust after 840 hours 780h no bubbling, no rust No blistering or rust after 720 hours <240h intensive bubbling, large-area peeling There are a few bubbles at the scratches after 480 hours.

[0087] As shown in Table 2, the present invention demonstrates significant advantages of its "internal catalytically driven step-curing asymmetric dual-sealing" technical concept. In stark contrast to the incomplete curing (sticky surface, hardness HB) and poor corrosion resistance (<240h) resulting from a single cashew phenol sealing system (Comparative Example 1), and the yellowing and hydrophilic defects (contact angle only 77.6°, salt spray resistance 480h) of traditional oxime sealing systems (Comparative Example 4), the present invention (Examples 1-3) successfully overcomes the bottleneck of the difficulty in achieving both low-temperature curing and high-performance corrosion protection through the synergistic effect of low-temperature preferential unsealing and stress release by active methylene groups and high-temperature hydrophobic shielding by bio-based phenols. It not only achieves complete curing at 140℃ (hardness up to 3H with no yellowing), but also increases the water contact angle to over 90° by introducing hydrophobic long chains, resulting in a significant leap in salt spray resistance to 720-840 hours. This perfectly demonstrates the superior comprehensive effect of this molecular structure design in improving coating appearance, mechanical properties, and corrosion resistance life.

[0088] Further comparison of Examples 1 and 3 reveals that the technical solution of this invention possesses high flexibility and process controllability. Example 1, cured at 160°C, fully leverages the hydrophobic shielding effect of the bio-based long chains, resulting in a coating with a salt spray resistance time of up to 840 hours, exhibiting excellent heavy-duty corrosion resistance and suitable for applications with extremely high corrosion resistance requirements. Example 3, cured at a low temperature of 140°C, effectively reduces curing energy consumption while maintaining a high hardness of 3H and excellent salt spray resistance for 720 hours.

[0089] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. An asymmetric, dual-blocked isocyanate curing agent, characterized in that: It is prepared from raw materials comprising the following parts by weight: 100-120 parts isophorone diisocyanate, 70-80 parts dispersant, 0.3-0.5 parts catalyst, 10-20 parts chain extender, 50-120 parts blocking agent A and 80-110 parts blocking agent B; The chain extender is selected from at least one of N-methyldiethanolamine and N-n-butyldiethanolamine; The sealing agent A is selected from at least one of cashew phenol and dodecylphenol; The sealing agent B is selected from at least one of diethyl malonate, ethyl acetoacetate, and dimethyl malonate.

2. The asymmetric dual-blocked isocyanate curing agent as described in claim 1, characterized in that: The dispersant is composed of 25-30 parts of dispersant A and 45-50 parts of dispersant B; dispersant A is selected from at least one of methyl isobutyl ketone, acetone and butanone; and dispersant B is selected from at least one of ethylene glycol butyl ether and ethylene glycol hexyl ether.

3. The asymmetric dual-blocked isocyanate curing agent as described in claim 1, characterized in that: The catalyst consists of 0.05-0.10 parts of bismuth isooctanoate and 0.25-0.45 parts of zinc acetylacetonate.

4. An asymmetric, dual-blocked isocyanate curing agent, characterized in that: The chemical structural formula of the curing agent is shown in formula (I), formula (II), or formula (III): （Ⅰ）； （Ⅱ）； （Ⅲ）。 5. The method for preparing the asymmetric dual-blocked isocyanate curing agent according to any one of claims 1 to 4, characterized in that: Includes the following steps: (1) According to the preset mass parts, add isophorone diisocyanate and dispersant A into the reaction vessel and mix well. Heat to 50~60℃, and then add chain extender dropwise. After the dropwise addition is completed, react at 50-60℃ for 1~2h. Monitor the NCO content in the system. When the NCO content drops to 60-80% of the initial value, the initial product is obtained. (2) Add blocking agent A and bismuth isooctanoate catalyst to the initial product obtained in step (1), and keep it at 50~60℃ for 1~2h. Monitor the NCO content in the system. When the NCO content drops to 40~50% of the initial value, add blocking agent B and zinc acetylacetonate, and heat to 70~80℃ for 4~5h until the NCO content is less than 0.2% to obtain the blocked product. (3) Add dispersant B to the closed product obtained in step (2) and mix evenly to obtain the isocyanate curing agent.

6. The application of the asymmetric dual-blocked isocyanate curing agent according to any one of claims 1 to 4 or the asymmetric dual-blocked isocyanate curing agent prepared by the preparation method according to claim 5 in the preparation of cathodic electrophoretic emulsion.

7. The application of the asymmetric dual-blocked isocyanate curing agent according to any one of claims 1 to 4 or the asymmetric dual-blocked isocyanate curing agent prepared by the preparation method according to claim 5 in the preparation of cathodic electrophoretic coatings.

8. A cathodic electrophoretic emulsion, characterized in that: It is made from the following raw materials by weight: 100-200 parts of main resin, 8-30 parts of compound amine, 20-45 parts of isocyanate curing crosslinking agent as described in any one of claims 1 to 4, 1-3 parts of acid and 100-200 parts of water.

9. The cathodic electrophoretic emulsion as described in claim 7, characterized in that: The complex amine is selected from at least one of diethanolamine, N-methylethanolamine, and ketoimine.

10. A cathodic electrophoretic coating, characterized in that: It is prepared by combining the cathodic electrophoretic emulsion as described in claim 8 or 9 with a color paste.

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