Double-anchoring modified water-based closed isocyanate curing agent as well as preparation method and application thereof

Abstract

The invention discloses a double-anchoring modified water-based closed isocyanate curing agent as well as a preparation method and application thereof. The curing agent takes polyisocyanate as a raw material, firstly, the polyisocyanate reacts with soft segment dihydric alcohol to prepare an-NCO end group prepolymer, then, hydrophilic grafting and grafting of a silane anchoring unit and a phosphorus-containing anchoring unit are sequentially performed, and finally, residual-NCO is sealed by a sealing agent to prepare a stable aqueous dispersion. After the curing agent is compounded with water-based hydroxyl resin, the curing agent is unblocked to release-NCO and is cross-linked during baking, silane condensation and phosphorus-containing anchoring have synergistic interaction, the adhesive force, corrosion resistance and electrochemical impedance of a coating to a metal base material are remarkably improved, the dielectric constant is reduced, and the curing agent is suitable for the fields of metal protection, electronic packaging and medium-resistant protection.

Description

Technical Field

[0001] This invention relates to the field of waterborne coating curing agent technology, specifically to a dual-anchored modified waterborne blocked isocyanate curing agent, its preparation method, and its application. Background Technology

[0002] Waterborne hydroxyl resins (such as waterborne acrylic-modified epoxy dispersions) have advantages such as low VOCs, safe application, and environmental friendliness, and are widely used in metal protective coatings, electronic device encapsulation protection, and dielectric protection. To achieve excellent chemical resistance, water resistance, corrosion resistance, and mechanical strength in the coating, a cross-linking reaction between the curing agent and the hydroxyl groups in the resin is usually required to form a dense three-dimensional network structure. Isocyanate curing agents have high reactivity and high cross-linking efficiency, making them one of the commonly used curing systems for waterborne hydroxyl resins.

[0003] Compared to two-component isocyanate curing systems, waterborne blocked isocyanate curing agents can achieve single-component storage stability. Under baking conditions, they can deblock and release isocyanate groups (-NCO), which then crosslink and cure with the hydroxyl groups (-OH) in the resin, offering advantages such as easy application and stable storage. However, existing waterborne blocked isocyanate curing agents still have many shortcomings when applied to metal protection systems: to ensure water dispersion stability, a high content of hydrophilic structural units or the addition of external emulsifiers is usually required, which can easily lead to increased water absorption, intensified media penetration, and rapid decay of electrochemical impedance; in salt water immersion environments, the coating is prone to interfacial failure, insufficient adhesion to the metal substrate, and problems such as blistering and rust propagation under salt spray conditions; at the same time, the internal stress of the cured crosslinking network is relatively large, which can easily cause coating embrittlement and cracking, affecting the overall mechanical properties such as impact and bending.

[0004] Therefore, developing a modified waterborne blocked isocyanate curing agent that combines good water dispersion stability and low water absorption, can significantly improve the adhesion and corrosion resistance of coatings to metal substrates, optimize dielectric properties and improve electrochemical impedance, is of great significance for meeting the application requirements of high-performance waterborne protective coatings. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a dual-anchored modified waterborne blocked isocyanate curing agent, its preparation method, and its application. The aim is to simultaneously introduce silane-anchored structural units and phosphorus-containing anchored structural units into the curing agent's molecular structure, utilizing the addition reaction between the hydroxyl groups and isocyanate groups of these units to form urethane bonds, thereby grafting them onto the polyisocyanate backbone. This allows the curing agent to efficiently crosslink with waterborne hydroxyl resins, ultimately achieving the goals of improving coating adhesion and corrosion resistance, reducing dielectric constant, and enhancing electrochemical impedance.

[0006] To achieve its objectives, the present invention employs the following technical solution: This invention first provides a dual-anchored modified waterborne blocked isocyanate curing agent for waterborne hydroxyl resins. The preparation process of the curing agent is as follows: first, polyisocyanate and soft segment diol are reacted in a solvent to obtain a -NCO-terminated prepolymer; then, hydrophilic grafting, silane anchoring unit and phosphorus-containing anchoring unit are grafted sequentially, and urethane bonds are grafted onto the prepolymer backbone through the addition reaction of hydroxyl groups and -NCO; finally, the residual -NCO is blocked with a blocking agent to obtain a dual-anchored modified waterborne blocked isocyanate curing agent dispersion; wherein, the composition of each raw material by weight is as follows: 30 parts of polyisocyanate; 2-5 parts of soft segment diol; 0.8-1 part of hydrophilic structural unit; 0.2-0.5 parts of silane anchoring unit; 0.1-0.3 parts of phosphorus-containing anchoring unit; The amount of sealing agent used is 0.8-1.2 times the residual -NCO equivalent in the system; The solvent is measured with a final solids content of 25-30% in the curing agent dispersion.

[0007] In the above system: the polyisocyanate is one or more of HDI trimer, IPDI trimer, biuret-type polyisocyanate, and diurea-type polyisocyanate; the soft segment diol is one or more of polyether diol, polyester diol, polycarbonate diol, and polytetrahydrofuran diol; the hydrophilic structural unit is one or more of 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, and polyethylene glycol monomethyl ether; the silane anchoring unit is 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, and 2-(3-aminopropyltriethoxysilane). The phosphorus-containing anchoring unit is one or more of 2-hydroxyethyl methacrylate phosphate, 2-hydroxyethyl dimethyl phosphate, and 3-hydroxypropyl diethyl phosphate; the blocking agent is one or more of 3-methylpyrazole, 4-methylpyrazole, and 3,5-dimethylpyrazole; and the solvent is one or more of dipropylene glycol monomethyl ether, propylene glycol methyl ether, N,N-dimethylethanolamine, and N-methyldiethanolamine.

[0008] The specific preparation method of the dual-anchored modified waterborne blocked isocyanate curing agent of the present invention is as follows, including the following steps: (1) Preparation of prepolymer: Add polyisocyanate and solvent to a dry reactor, heat to 50-75℃ and stir for 10-20 min to make the system uniform; preheat soft segment diol to 40-60℃ and add it dropwise to polyisocyanate solution, react for 1-3 h to obtain -NCO end group prepolymer.

[0009] (2) Hydrophilic grafting: The temperature of the prepolymer is maintained at 55-70℃. The hydrophilic structural unit is dissolved in a solvent and added to the prepolymer in three batches, with an interval of 10 min between each batch. After the addition is complete, the reaction continues for 1-2 h to allow it to react with isocyanate for grafting.

[0010] (3) Dual anchoring grafting: The system temperature is lowered to 40-65℃, the silane anchoring unit and the phosphorus-containing anchoring unit are dissolved in solvent and slowly added to the system. The reaction is carried out for 60-180 min, so that -OH and -NCO add to form urethane bonds and graft them onto the prepolymer backbone. (4) Sealing reaction: Dissolve the sealing agent in a solvent and slowly add it to the system under stirring. React at 50-85℃ for 0.5-3 h to reduce the -NCO content and obtain the target curing agent dispersion.

[0011] This invention also provides the application of the curing agent in coatings: Aqueous hydroxyl resin is mixed and dispersed evenly with a solvent to obtain a coating base component; then, the curing agent dispersion is added to the coating base component and mixed evenly to obtain a coating; the coating is sprayed onto a substrate (such as a cleaned Q235 steel sheet) using an air spraying method, with a spraying pressure of 0.45~0.65MPa and a discharge rate of 2~3 mL / min; after spraying, it is cured at 80~180℃ for 4~8 h to form a metallic protective coating. The mass ratio of the coating base component to the curing agent dispersion is 8.5-9:1.

[0012] Compared with existing technologies, the beneficial effects of this invention are reflected in: 1. Improve wet adhesion retention on metal substrates. Phosphorus-containing structural units can adsorb / complex with the metal surface, enhancing interfacial bonding; silane structural units can hydrolyze and condense during curing or service to form a dense interfacial network. The synergistic effect of these two components effectively suppresses problems such as coating interface debonding and blistering under immersion / salt water conditions, thereby reducing the risk of interfacial failure.

[0013] 2. Enhanced corrosion resistance and reduced electrochemical decay. The dual-anchoring structure not only strengthens interface stability and inhibits interfacial corrosion and corrosion propagation at scratches, but also forms a highly dense coating structure through silane condensation and crosslinking with urethane, improving the coating's shielding performance and reducing the penetration of corrosive media such as water, oxygen, and chloride ions. The resulting coating exhibits excellent salt spray resistance, slow electrochemical impedance decay, and outstanding long-term protective capabilities.

[0014] 3. Balancing water dispersion stability with low water absorption and optimized dielectric properties. Compared with traditional systems that rely on a large number of hydrophilic groups for water dispersion, this invention, by rationally controlling the amount of hydrophilic units and combining silane condensation densification with the interfacial stabilizing effect of phosphorus-containing groups, significantly reduces the water absorption rate and ion migration rate of the coating film while ensuring the water dispersion stability of the curing agent, thus lowering the dielectric constant. This makes it more suitable for electronic packaging and insulation protection applications.

[0015] 4. Effectively regulates internal stress in the coating film, balancing hardness and toughness. The introduction of soft segment diols can alleviate the problems of internal stress concentration and coating embrittlement caused by high crosslinking density, enabling the coating to maintain high hardness, solvent resistance, and chemical resistance while significantly improving mechanical properties such as impact strength and flexural strength, and reducing the risk of coating cracking.

[0016] 5. Single-component, stable storage, and convenient application. The sealant blocks residual -NCO to below the threshold, and the curing agent is not prone to thickening, gelling, or dispersion instability in aqueous systems, enabling long-term stable storage of a single component. The product is an aqueous dispersion with good compatibility with aqueous hydroxyl resins, and can be adapted to conventional construction processes such as spraying and roller coating, facilitating transportation, preparation, and on-site use. Attached Figure Description

[0017] Figure 1 The FT-IR spectra of the modified isocyanate curing agent prepared in Example 1 are compared with those of the raw isocyanate.

[0018] Figure 2 The thermogravimetric analysis (TGA) curves of the coating samples obtained in various embodiments and comparative examples of the present invention are shown.

[0019] Figure 3 The dielectric constant curves of the coating samples obtained in various embodiments and comparative examples of the present invention are shown.

[0020] Figure 4 The electrochemical impedance spectroscopy (EIS) of the coating samples obtained in the various embodiments and comparative examples of this invention is shown. Detailed Implementation

[0021] The technical solution of the present invention will be described in detail below through specific embodiments. The following embodiments are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0022] Example 1 This embodiment first prepares a dual-anchored modified waterborne blocked isocyanate curing agent according to the following steps: (1) Preparation of prepolymer: 30g of polyisocyanate (HDI trimer) and 60g of dipropylene glycol monomethyl ether were added to a dry reactor and heated to 60℃ and stirred for 20min; 3.2g of soft segment diol (polyether diol) was preheated to 60℃ and then added dropwise to the polyisocyanate solution and reacted for 2h to obtain -NCO end-group prepolymer; (2) Hydrophilic grafting: The temperature of the prepolymer was maintained at 60℃. 0.92g of hydrophilic structural unit 2,2-dimethylolpropionic acid was dissolved in 5g of dipropylene glycol monomethyl ether and added to the prepolymer in three batches with an interval of 10 min between each batch. After the addition was completed, the reaction continued for 1 h. (3) Dual anchoring grafting: The system temperature was lowered to 60℃, and 0.46g of silane anchoring unit (3-aminopropyltriethoxysilane) and 0.12g of phosphorus-containing anchoring unit (2-hydroxyethyl methacrylate phosphate) were dissolved in 5g of dipropylene glycol monomethyl ether and slowly added to the system, so that -OH and -NCO added to form urethane bonds and grafted onto the prepolymer backbone; (4) Sealing reaction: Dissolve 0.9g of sealing agent (3,5-dimethylpyrazole) in 5g of dipropylene glycol monomethyl ether, slowly add it dropwise into the system, and react at 60℃ for 2 h to obtain the target curing agent dispersion.

[0023] Based on the obtained curing agent, this embodiment prepared a waterborne acrylic resin coating and a coating layer. The steps are as follows: 10g of waterborne hydroxyl resin (waterborne acrylic resin) was dissolved in 20g of dipropylene glycol monomethyl ether and dispersed for 3min using a disperser to ensure complete dispersion. 3.48g of the dual-anchored modified waterborne blocked isocyanate curing agent dispersion prepared in this embodiment was added to the dispersed resin solution. The mixture was thoroughly and uniformly mixed using a stirring paddle and ultrasound to obtain the target coating.

[0024] The coating was applied to the cleaned Q235 steel sheet using an air spraying method. The spraying pressure was 0.5 MPa and the output rate was 3 mL / min. After spraying, the steel sheet was placed in a forced-air drying oven and cured at 80℃ for 4 h, and then cured at 180℃ for 2 h to obtain the target coating.

[0025] Comparative Example 1 This comparative example prepared waterborne acrylic resin coatings and coatings using the same method as in Example 1, except that the added curing agent was 0.72 g of commercially available amino resin curing agent (amino resin 303).

[0026] Comparative Example 2 This comparative example prepared waterborne acrylic resin coatings and coatings using the same method as in Example 1, except that the added curing agent was 0.57 g of commercially available phenolic resin curing agent (BM897).

[0027] Table 1 compares the basic properties of the coatings obtained in Example 1 with those in Comparative Examples 1 and 2. As can be seen from the table, the coating adhesion and salt spray resistance of Example 1, using the modified isocyanate curing agent of this invention, are significantly improved, indicating that this curing system can effectively enhance the interfacial bonding and stability between the coating and the substrate. Meanwhile, the impact strength of Example 1 is slightly reduced, indicating that its cross-linked network is denser, more rigid, and relatively less tough.

[0028] Table 1. Basic properties of coatings obtained in each embodiment and comparative example Figure 1 The FT-IR spectra of the modified isocyanate curing agent prepared in Example 1 are compared with those of the raw isocyanate. The raw isocyanate spectrum is at approximately 2270 cm⁻¹. -1 The modified isocyanate curing agent exhibits a distinct -NCO stretching vibration characteristic peak, while this peak is significantly weakened or even virtually disappears, indicating that the free -NCO in the system has been fully consumed and the blocking transformation has been completed, and the modification and blocking reactions proceeded smoothly. The modified sample exhibits a peak at 1700-1730 cm⁻¹. -1 The presence of a distinct C=O (carbamate) absorption peak in the 1000-1130 cm⁻¹ range indicates that -NCO undergoes an addition reaction with the active hydrogen groups in the modified unit, forming a stable carbamate bond, thus achieving covalent grafting of the functional unit onto the isocyanate backbone. Simultaneously, in the 1000-1130 cm⁻¹ range... -1 Characteristic absorption of Si-O-Si can be observed in the region, at 1230-1280 cm⁻¹. -1 A strong P=O absorption peak appears in the region, and it is located at 1000-1100 cm⁻¹. -1 The accompanying POC-related absorption changes within the range indicate that silane and phosphorus-containing functional structural units have been successfully introduced into the curing agent molecule structure. In summary, the significant attenuation or disappearance of the -NCO characteristic peak, as well as the appearance and enhancement of characteristic peaks such as urethane, Si-O-Si, P=O, and POC, collectively demonstrate the successful synthesis of the modified isocyanate curing agent.

[0029] Figure 2 The thermogravimetric analysis (TGA) curves of the coating samples obtained in Example 1, Comparative Example 1, and Comparative Example 2 under a nitrogen atmosphere are shown. Comparative Example 2 shows an earlier onset of thermogravimetric loss, with its 5% thermogravimetric loss temperature (T0) being earlier. 5%The lower value of Example 1 compared to Comparative Example 1 can be attributed to the presence of more easily pyrolytic structural units or residual small molecules in the phenolic curing system, which decompose first in the initial stage of heating, thus triggering premature thermal decomposition of the resin. The TGA curve of Example 1 shifts overall towards the high-temperature region, with the main decomposition stage significantly delayed, indicating that the three-dimensional cross-linked network formed by the modified isocyanate curing can effectively improve the thermal stability of the coating. Simultaneously, Example 1 exhibits a significantly higher final char rate in the high-temperature region, indicating that this curing system is more conducive to promoting coating carbonization and forming a stable char layer. In summary, Example 1 demonstrates slower initial decomposition and a higher high-temperature char rate, confirming that the modified isocyanate curing agent of this invention can simultaneously improve the heat resistance stability and high-temperature carbonization protection capability of the coating.

[0030] Figure 3 The dielectric constant curves of the coating samples obtained in Example 1, Comparative Example 1, and Comparative Example 2, measured by a broadband dielectric impedance spectrometer, are shown. The dielectric constants of the coatings in Comparative Example 1 and Comparative Example 2 are generally at a high level, around 10. 6 -10 7 The dielectric constant of the coating in Example 1 is stable at approximately 1.67-1.70 within the Hz frequency band; while the dielectric constant of the coating in Example 1 is significantly reduced, stabilizing at approximately 0.98-1.00 within the same frequency band, which is about 40% lower than the two comparative samples. The low dielectric constant is crucial for modern high-frequency electronic devices: a lower dielectric constant can effectively reduce parasitic capacitance, reduce signal transmission delay, and significantly improve signal integrity and transmission rate, making the coating of this invention more suitable for the high-frequency, high-speed packaging insulation material requirements of 5G and next-generation communication technologies.

[0031] Figure 4 Electrochemical impedance spectroscopy (EIS Nyquist plots) of the coating samples obtained in Example 1, Comparative Examples 1 and 2 were measured using an electrochemical workstation. The tests were conducted with steel plates as the corrosive medium, and the samples were immersed at room temperature for 2 days before measurement. The impedance curve of Example 1 is located entirely on the far right, with the largest capacitive arc radius, indicating that after 2 days of immersion, its coating resistance and interfacial charge transfer resistance remain at a high level, the amount of corrosive medium penetration is significantly reduced, and the electrochemical reaction at the metal / coating interface is effectively suppressed, reflecting a denser coating structure and more stable interfacial bonding. The capacitive arc radii of Comparative Examples 1 and 2 are much smaller than those of Example 1, indicating that their coating shielding performance decays faster after immersion, the electrolyte penetrates more easily to the metal interface, and the coating's ability to block corrosive media is weaker, or the interface is more prone to failure.

[0032] The above are merely exemplary embodiments of the present invention and are not intended to limit the scope of protection 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 dual-anchored modified waterborne blocked isocyanate curing agent, characterized in that, The preparation process of the curing agent is as follows: First, polyisocyanate and soft segment diol are reacted in a solvent to obtain a -NCO-terminated prepolymer; then, hydrophilic grafting, silane anchoring unit and phosphorus-containing anchoring unit are grafted sequentially, and urethane bonds are grafted onto the prepolymer backbone through the addition reaction of hydroxyl groups and -NCO; finally, the residual -NCO is blocked with a blocking agent to obtain a dual-anchored modified aqueous blocked isocyanate curing agent dispersion; wherein, the composition of each raw material by weight is as follows: 30 parts of polyisocyanate; 2-5 parts of soft segment diol; 0.8-1 part of hydrophilic structural unit; 0.2-0.5 parts of silane anchoring unit; 0.1-0.3 parts of phosphorus-containing anchoring unit; The amount of sealing agent used is 0.8-1.2 times the residual -NCO equivalent in the system; The solvent is measured with a final solids content of 25-30% in the curing agent dispersion.

2. The curing agent according to claim 1, characterized in that, The polyisocyanate is one or more of HDI trimer, IPDI trimer, biuret-type polyisocyanate, and diurea-type polyisocyanate.

3. The curing agent according to claim 1, characterized in that, The soft segment diol is one or more of polyether diol, polyester diol, polycarbonate diol, and polytetrahydrofuran diol.

4. The curing agent according to claim 1, characterized in that, The hydrophilic structural unit is one or more of 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, and polyethylene glycol monomethyl ether.

5. The curing agent according to claim 1, characterized in that, The silane anchoring unit is one or more of 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

6. The curing agent according to claim 1, characterized in that, The phosphorus-containing anchoring unit is one or more of 2-hydroxyethyl methacrylate phosphate, 2-hydroxyethyl dimethyl phosphate, and 3-hydroxypropyl diethyl phosphate.

7. The curing agent according to claim 1, characterized in that, The sealing agent is one or more of 3-methylpyrazole, 4-methylpyrazole, and 3,5-dimethylpyrazole.

8. The curing agent according to claim 1, characterized in that, The solvent is one or more of dipropylene glycol monomethyl ether, propylene glycol methyl ether, N,N-dimethylethanolamine, and N-methyldiethanolamine.

9. A method for preparing the dual-anchored modified waterborne blocked isocyanate curing agent according to any one of claims 1-8, characterized in that, Includes the following steps: (1) Preparation of prepolymer: Add polyisocyanate and solvent to a dry reactor, heat to 50-75℃ and stir for 10-20 min; preheat soft segment diol to 40-60℃ and add it dropwise to polyisocyanate solution, react for 1-3 h to obtain -NCO end group prepolymer; (2) Hydrophilic grafting: The temperature of the prepolymer is maintained at 55-70℃. The hydrophilic structural unit is dissolved in a solvent and added to the prepolymer in three batches, with an interval of 10 min between each batch. After the addition is complete, the reaction continues for 1-2 h. (3) Dual anchoring grafting: The system temperature is lowered to 40-65℃, the silane anchoring unit and the phosphorus-containing anchoring unit are dissolved in solvent and slowly dripped into the system, so that -OH and -NCO add to form urethane bonds and graft onto the prepolymer skeleton; (4) Sealing reaction: Dissolve the sealing agent in a solvent and slowly add it to the system. React at 50-85℃ for 0.5-3 h to obtain the target curing agent dispersion.

10. The application of the curing agent according to any one of claims 1-8 in coatings, characterized in that, Aqueous hydroxyl resin is mixed and dispersed evenly with a solvent to obtain the coating base component; then the curing agent dispersion is added to the coating base component and mixed evenly to obtain the coating; the coating is sprayed onto the substrate using an air spraying method, with a spraying pressure of 0.45~0.65MPa and a discharge rate of 2~3 mL / min; after spraying, it is cured at 80~180℃ for 4~8 h to form a metallic protective coating; wherein, the mass ratio of the coating base component to the curing agent dispersion is 8.5-9:1.

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