Preparation method of cationic polyurethane aqueous dispersion and its application in chromium-free passivating agent

Abstract

The present invention belongs to the field of metal corrosion protection technology, and specifically relates to a method for preparing a cationic polyurethane aqueous dispersion and its use in chromium-free passivators. The method for preparing the cationic polyurethane aqueous dispersion comprises step 1: synthesizing a prepolymer using diisocyanate, oligomeric polyol, small molecule chain extender, crosslinker, and cationic hydrophilic monomer as raw materials; step 2: neutralizing the prepolymer obtained in step 1, then emulsifying and dispersing it in water, then post-extending the chain using a post-extender, bisaminosilane, and finally capping it with an amino-containing phosphate ester, thereby obtaining a cationic polyurethane aqueous dispersion with excellent performance. The cationic polyurethane aqueous dispersion can be used to prepare a chromium-free passivator and has good compatibility with the other components of the passivator. The present invention utilizes the dual effects of silane and phosphate ester to significantly improve the salt spray resistance and coating adhesion of the passivated sheet material.

Description

Technical Field

[0001] The invention relates to the technical field of metal anticorrosion, and specifically discloses a preparation method of a cationic polyurethane aqueous dispersion and application of the same in a chromium-free passivating agent. Background Art

[0002] Metal surface passivation is a key process for improving material corrosion resistance and extending service life. Traditional passivation technologies primarily utilize chromium-containing passivants (such as hexavalent chromium compounds), which form dense passive films on metal surfaces, resulting in excellent corrosion resistance. However, hexavalent chromium poses significant risks to the environment and human health, and its use is strictly restricted by international environmental regulations (such as RoHS and REACH). Consequently, the development of environmentally friendly chromium-free passivation technologies has become a research hotspot in recent years.

[0003] Currently, chromium-free passivation technologies primarily include zirconium-based, titanium-based, molybdenum-based, silane coupling agents, and organic polymer systems. Organic polymer systems, particularly high-molecular-weight polyurethane aqueous dispersions, offer broad application prospects in the chromium-free passivation field due to their highly designable molecular structures, excellent adhesion, superior film-forming properties, and environmental and non-toxic properties. However, existing technologies still face the following challenges:

[0004] 1. Insufficient salt spray resistance: The molecular structure of traditional polyurethane aqueous dispersions lacks groups that strongly interact with metal surfaces, resulting in the corrosion resistance of the passivation film being unable to meet the requirements of high-end applications. Generally, it can only meet the salt spray resistance requirements of 72 or 96 hours.

[0005] 2. Limited coating adhesion: The adhesion of existing passivators mainly relies on physical adsorption, and the chemical bond with the metal substrate is weak. After powder coating, scratching and cupping are likely to cause the coating to fall off. Passivators with poor performance will also fall off if they are only scratched without cupping.

[0006] Furthermore, current research on aqueous polyurethane dispersions in my country focuses primarily on anionic systems, with relatively little research on cationic polyurethane dispersions. Due to the unique charge properties and molecular structure of cationic polyurethane dispersions, there is an urgent need to develop cationic polyurethane dispersions with high salt spray resistance and excellent coating adhesion. Summary of the Invention

[0007] In response to the problems existing in the prior art, the present invention discloses a new type of cationic polyurethane aqueous dispersion through molecular structure design, aiming to significantly improve its salt spray resistance and coating adhesion performance while meeting environmental protection requirements, providing a new solution for the development of chromium-free passivation technology.

[0008] One object of the present invention is to be achieved through the following technical solutions:

[0009] A method for preparing a cationic polyurethane aqueous dispersion comprises the following steps:

[0010] Step 1: 35-50 weight percent of oligomer polyol, 0.7-1.7 weight percent of small molecule chain extender, and 0.7-1 weight percent of cross-linking agent are vacuum dehydrated at a temperature range of 110° C. to 120° C. for 0.5-1 hour under stirring, and then 38-50 weight percent of diisocyanate is dropwise added at 85° C. to 90° C., reacted for 2-3 hours, and then 8-14 weight percent of cationic hydrophilic monomer and solvent are added, and the reaction is continued for 2-3 hours to obtain a prepolymer, wherein the weight percentages are based on the total weight of the reactants excluding the weight of the solvent;

[0011] Step 2: Cooling the prepolymer obtained in step 1 to below 30° C., sequentially performing acid neutralization, water dispersion, post-chain extension and end-capping treatment, wherein the post-chain extension uses bisaminosilane as a post-chain extender, and the end-capping treatment uses amino-containing phosphate as an end-capping agent.

[0012] In the above-mentioned preparation method of the cationic polyurethane aqueous dispersion, the oligomer polyol is one or more of polypropylene oxide polyol with a molecular weight of 1000-2000, polytetramethylene glycol polyol with a molecular weight of 1000-2000, polycaprolactone polyol with a molecular weight of 1000-2000, and polycarbonate polyol with a molecular weight of 1000-2000.

[0013] In the above-mentioned preparation method of the cationic polyurethane aqueous dispersion, the small molecule chain extender is one or more of ethylene glycol, 1,4-butanediol, 2,3-butanediol, and dihydroxymethylcyclohexane; the crosslinking agent is one or more of trimethylolpropane or pentaerythritol; and the diisocyanate is one or more of toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and isophorone diisocyanate.

[0014] In the above-mentioned method for preparing the cationic polyurethane aqueous dispersion, the cationic hydrophilic monomer is one or more of N-methyldiethanolamine, N-ethyldiethanolamine, and triethanolamine; and the solvent is one or more of acetone and N-methylpyrrolidone.

[0015] In the above-mentioned method for preparing the cationic polyurethane aqueous dispersion, the neutralizing agent in step 2 is one or more of acetic acid and dilute hydrochloric acid.

[0016] In the above-mentioned method for preparing the cationic polyurethane aqueous dispersion, the bisaminosilane described in step 2 includes one or more of: N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, and N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane.

[0017] In the above-mentioned method for preparing the cationic polyurethane aqueous dispersion, the post-chain extension coefficient in step 2 is 65% to 85%.

[0018] In the above-mentioned method for preparing the cationic polyurethane aqueous dispersion, the amino-containing phosphate ester in step 2 includes one or more of 3-aminopropyl monophosphate and ethanolamine phosphate.

[0019] Another object of the present invention is the use of the cationic polyurethane aqueous dispersion prepared by the above preparation method in the preparation of a chromium-free passivating agent.

[0020] Another object of the present invention is to provide a chromium-free passivation solution, which comprises 60-70% by weight of water, 8-15% by weight of a cationic polyurethane aqueous dispersion prepared by the above-mentioned preparation method, 1-2% of an additive, 20-30% of a sodium salt, and 1-3% of a silane.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The method for preparing a cationic polyurethane aqueous dispersion of the present invention uses bisaminosilane as a post-chain extender. Silane groups are incorporated into the polyurethane backbone through the reaction between amino groups and isocyanate groups. The siloxane (Si-OR) structure in the silane group generates silanol (Si-OH) upon hydrolysis. This silanol reacts with the silane required for the preparation of the chromium-free passivation solution to form Si-O-Si bonds, reducing the coating's internal porosity and improving its density. Furthermore, the silanol condenses with hydroxyl groups (-OH) on the metal surface to form a strong Si-O-Metal covalent bond. This chemical bond significantly enhances the adhesion of the polymer to the metal substrate, preventing the shedding problem associated with conventional passivation coatings due to insufficient physical adsorption. Furthermore, the hydrophobic siloxane structure significantly improves the water and moisture resistance of the passivation film, enhancing the film-forming properties of the material.

[0023] 2. The method for preparing the cationic polyurethane aqueous dispersion of the present invention uses amino-containing phosphate esters for end-capping. The amino groups react with isocyanate groups to successfully introduce phosphate groups onto both ends of the cationic polyurethane chain segments. The phosphate groups can further bond with the metal substrate to form a phosphate protective layer, effectively blocking the penetration of corrosive media (such as oxygen, water, and chlorine particles), thereby enhancing corrosion resistance.

[0024] 3. Based on the performance of the polyurethane aqueous dispersion itself, the present invention utilizes the dual effects of the post-chain extender bisaminosilane and the end-capping agent amino-containing phosphate to further improve the comprehensive performance of the passivation film, so that the synthesized cationic polyurethane aqueous dispersion can simultaneously meet the characteristics of high salt spray resistance, excellent adhesion, environmental protection and non-toxicity, meet the performance requirements of chromium-free passivation liquid, and have broad application prospects. DETAILED DESCRIPTION

[0025] A method for preparing a cationic polyurethane aqueous dispersion comprises the following steps:

[0026] Step 1: 35-50 weight percent of oligomer polyol, 0.7-1.7 weight percent of small molecule chain extender and 0.7-1 weight percent of cross-linking agent are vacuum dehydrated in a stirring state at a temperature range of 110°C to 120°C for 0.5-1 hour, and then 38-50 weight percent of diisocyanate is added dropwise at 85°C to 90°C, and the reaction is carried out for 2-3 hours. Then, 8-14 weight percent of cationic hydrophilic monomer and solvent are added, and the reaction is continued for 2-3 hours to obtain a prepolymer. The weight percentages are based on the total weight of the above reactants excluding the weight of the solvent.

[0027] The oligomer polyol is one or more of polypropylene oxide polyol with a molecular weight of 1000-2000, polytetramethylene glycol polyol with a molecular weight of 1000-2000, polycaprolactone polyol with a molecular weight of 1000-2000, and polycarbonate polyol with a molecular weight of 1000-2000.

[0028] The small molecule chain extender is one or more of ethylene glycol, 1,4-butanediol, 2,3-butanediol, and dimethylolcyclohexane; the crosslinker is trimethylolpropane or pentaerythritol; and the diisocyanate is one or more of toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and isophorone diisocyanate.

[0029] The cationic hydrophilic monomer is one or more of N-methyldiethanolamine, N-ethyldiethanolamine, and triethanolamine; and the solvent is acetone or N-methylpyrrolidone.

[0030] Step 2: Cooling the prepolymer obtained in step 1 to below 30° C., sequentially performing acid neutralization, water dispersion, post-chain extension and end-capping treatment, wherein the post-chain extension uses bisaminosilane as a post-chain extender, and the end-capping treatment uses amino-containing phosphate as an end-capping agent.

[0031] The neutralizing agent used in the acid neutralization is one or more of acetic acid and dilute hydrochloric acid.

[0032] The bisaminosilane includes one or more of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, and N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane.

[0033] The post-chain extension coefficient is 65%~85%.

[0034] The amino group-containing phosphate includes one or more of 3-aminopropyl monophosphate and ethanolamine phosphate.

[0035] The present invention selects bisaminosilane as a post-chain extender by utilizing the reaction between amino groups and isocyanate groups to incorporate silane groups into the polyurethane backbone. The siloxane (Si-OR) structure in the silane group can generate silanol (Si-OH) after hydrolysis. This silanol undergoes a condensation reaction with the silane required to prepare the chromium-free passivation solution to form a Si-O-Si bond, reducing the internal porosity of the coating and improving its density. In addition, the silanol condenses with the hydroxyl (-OH) groups on the metal surface to form a strong Si-O-Metal covalent bond. This chemical bond significantly improves the adhesion of the emulsion to the metal substrate, avoiding the problem of shedding caused by insufficient physical adsorption of traditional passivation coatings. In addition, the hydrophobic siloxane structure can significantly enhance the water and moisture resistance of the passivation film, improving the film-forming properties of the material. The present invention selects amino-containing phosphate esters for end-capping, and utilizes the reaction between amino groups and isocyanate groups to successfully introduce phosphate ester groups into both ends of the cationic polyurethane chain segment. The phosphate group can further bond with the metal substrate to form a phosphate protective layer, which effectively blocks the penetration of corrosive media (such as oxygen, water, and chlorine particles) and improves the corrosion resistance.

[0036] The present invention is described in detail below with reference to the embodiments.

[0037] Example 1 Cationic polyurethane aqueous dispersion

[0038] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration for 60 minutes, then cool to 85°C. Add 22g of isophorone diisocyanate dropwise and allow to react for 3 hours. Then, add 4.8g of N-methyldiethanolamine and acetone dropwise and continue to react for 3 hours to obtain a prepolymer.

[0039] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 4.7g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. After 1 hour, 2.2g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for chromium-free passivation.

[0040] Example 2 Cationic polyurethane aqueous dispersion

[0041] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 120°C for dehydration and hold for 30 minutes. Then cool to 85°C and dropwise add 22g of isophorone diisocyanate. Allow to react for 3 hours. Then, add 5.6g of N-methyldiethanolamine and acetone as solvent. Continue to react for 3 hours to obtain a prepolymer.

[0042] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 5.6g of chain extender N-(2-aminoethyl)-3-aminopropyltriethoxysilane was then added. After 1 hour, 2.2g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for chromium-free passivation.

[0043] Example 3 Cationic polyurethane aqueous dispersion

[0044] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 120°C for dehydration and hold for 30 minutes. Then cool to 90°C and dropwise add 22g of isophorone diisocyanate. Allow to react for 2.5 hours. Then, dropwise add 4.8g of N-methyldiethanolamine and acetone as solvent and continue the reaction for 3 hours to obtain a prepolymer.

[0045] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 4.96g of chain extender N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane was then added. After 1 hour, 2.2g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for chromium-free passivation.

[0046] Example 4 Cationic polyurethane aqueous dispersion

[0047] Step 1: Preparation of the prepolymer. First, add 21g of polyoxypropylene glycol (M=1000), 0.34g of ethylene glycol, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 120°C for dehydration and hold for 30 minutes. Then cool to 90°C and dropwise add 22g of isophorone diisocyanate. Allow to react for 2.5 hours. Then, dropwise add 4.8g of N-methyldiethanolamine and acetone as solvent and continue the reaction for 3 hours to obtain a prepolymer.

[0048] Step 2: The prepolymer was cooled to 25°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 4.01g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. After 1 hour, 3.01g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0049] Example 5 Cationic polyurethane aqueous dispersion

[0050] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.46g of 1,4-butanediol, and 0.41g of pentaerythritol to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration and hold for 60 minutes. Then cool to 85°C and dropwise add 24.8g of 4,4-diphenylmethane diisocyanate. Allow to react for 3 hours. Then, dropwise add 5.4g of N-ethyldiethanolamine and acetone as solvent and continue to react for 3 hours to obtain a prepolymer.

[0051] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 5.27g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. After 1 hour, 1.3g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0052] Example 6 Cationic polyurethane aqueous dispersion

[0053] Step 1: Preparation of the prepolymer. First, add 21g of polytetrahydrofuran diol (M=1000), 0.46g of 2,3-butanediol, and 0.42g of pentaerythritol to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 120°C for dehydration and hold for 40 minutes. Then cool to 90°C and dropwise add 17.25g of toluene diisocyanate. Allow to react for 2 hours. Then, add 6g of triethanolamine and N-methylpyrrolidone as solvent and continue to react for 2 hours to obtain a prepolymer.

[0054] Step 2: Cool the prepolymer to 30°C, neutralize with acetic acid, and then emulsify and disperse it in ice water to form an emulsion. Then, add 4.68g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane. After 1 hour, add 1.98g of ethanolamine phosphate to cap the end products. Continue the reaction for 30 minutes. Vacuum desolventization is performed to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0055] Example 7 Cationic polyurethane aqueous dispersion

[0056] Step 1: Preparation of the prepolymer. First, add 21g of polycaprolactone diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration for 60 minutes, then cool to 85°C. Add 19g of isophorone diisocyanate dropwise and allow to react for 3 hours. Then, add 4.8g of N-methyldiethanolamine and acetone dropwise and continue the reaction for another 3 hours to obtain a prepolymer.

[0057] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 2.46g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. One hour later, 1.14g of 3-aminopropyl monophosphate was added for end capping, and the reaction was continued for 30 minutes. The solvent was removed under vacuum to yield a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0058] Example 8 Cationic polyurethane aqueous dispersion

[0059] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration and maintain for 60 minutes. Then cool to 85°C and dropwise add 26g of isophorone diisocyanate. Allow to react for 3 hours. Then, add 4.8g of N-methyldiethanolamine and acetone as solvent. Continue to react for 3 hours to obtain a prepolymer.

[0060] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 8.66g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. After 1 hour, 3.59g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0061] Example 9 Cationic polyurethane aqueous dispersion

[0062] Step 1: Preparation of the prepolymer. First, add 16g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration and hold for 60 minutes. Then cool to 85°C and dropwise add 22g of isophorone diisocyanate. Allow to react for 3 hours. Then, add 4.8g of N-methyldiethanolamine and acetone as solvent. Continue to react for 3 hours to obtain a prepolymer.

[0063] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 5.5g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was added. After 1 hour, 2.58g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivator.

[0064] Example 10 Cationic polyurethane aqueous dispersion

[0065] Step 1: Preparation of the prepolymer. First, add 27g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration for 60 minutes, then cool to 85°C. Add 22g of isophorone diisocyanate dropwise and allow to react for 3 hours. Then, add 4.8g of N-methyldiethanolamine and acetone dropwise and continue to react for 3 hours to obtain a prepolymer.

[0066] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 3.96g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was added. After 1 hour, 1.72g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed under vacuum to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0067] Example 11 Cationic polyurethane aqueous dispersion

[0068] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration and maintain for 60 minutes. Then cool to 85°C and dropwise add 22g of isophorone diisocyanate. Allow to react for 3 hours. Then, dropwise add 4.8g of N-methyldiethanolamine and N-methylpyrrolidone as solvent. Continue the reaction for another 3 hours to obtain a prepolymer.

[0069] Step 2: The prepolymer was cooled to 30°C, neutralized with hydrochloric acid, and then emulsified and dispersed in ice water to form an emulsion. 4.7g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. After 1 hour, 2.2g of 3-aminopropyl monophosphate was added for end capping. The reaction was continued for 30 minutes. The solvent was removed to obtain a cationic polyurethane aqueous dispersion suitable for chromium-free passivation.

[0070] Comparative Example 1

[0071] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration for 60 minutes, then cool to 85°C. Add 22g of isophorone diisocyanate dropwise and react for 2 hours. Then, add 4.8g of N-methyldiethanolamine and acetone dropwise and continue the reaction for 3 hours to obtain a prepolymer.

[0072] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 1.27g of ethylenediamine was then added for chain extension. After one hour, 2.19g of 3-aminopropyl monophosphate was added for end-capping. The reaction was continued for 30 minutes. The solvent was removed to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivation agent.

[0073] Comparative Example 2

[0074] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration for 60 minutes, then cool to 85°C. Add 22g of isophorone diisocyanate dropwise and react for 2 hours. Then, add 4.8g of N-methyldiethanolamine and acetone dropwise and continue the reaction for 3 hours to obtain a prepolymer.

[0075] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 8.77g of 3-aminopropyl monophosphate was added for end-capping, and the reaction was continued for 30 minutes. The solvent was removed to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivator.

[0076] Comparative Example 3

[0077] Step 1: Preparation of the prepolymer. First, add 21g of polycarbonate diol (M=1000), 0.72g of dimethylolcyclohexane, and 0.4g of trimethylolpropane to a container equipped with a stirrer, thermometer, condenser, and vacuum tube. Heat to 110°C for dehydration for 60 minutes, then cool to 85°C. Add 22g of isophorone diisocyanate dropwise and react for 2 hours. Then, add 4.8g of N-methyldiethanolamine and acetone dropwise and continue the reaction for 3 hours to obtain a prepolymer.

[0078] Step 2: The prepolymer was cooled to 30°C, neutralized with acetic acid, and then emulsified and dispersed in ice water to form an emulsion. 4.7g of the chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was then added. After reacting for 1 hour, the solvent was removed to obtain a cationic polyurethane aqueous dispersion suitable for use as a chromium-free passivator.

[0079] Comparative Example 4

[0080] A certain brand of chromium-free passivation agent product is available on the market.

[0081] Example 12 Chromium-free passivating agent

[0082] The cationic polyurethane aqueous dispersion produced by the above preparation method can be used in the preparation of a chromium-free passivating agent. The present invention provides a chromium-free passivating solution, which is prepared using the cationic polyurethane aqueous dispersion produced by the above preparation method. Its components and specific preparation method can adopt the contents and steps commonly used in the prior art. In this embodiment, the chromium-free passivating solution comprises, by weight, 60-70% water, 8-15% of the cationic polyurethane aqueous dispersion produced by the above preparation method, 1-2% of an additive, 20-30% of a sodium salt, and 1-3% of a silane.

[0083] The chromium-free passivation solution was prepared according to the formula shown in Table 1.

[0084] Place the formulated water and cationic polyurethane emulsion in a container and start stirring. Add acetic acid and disperse for 5 minutes. Then add the following materials in sequence, with an interval of 15 minutes between each material.

[0085] Table 1 Formula of chromium-free passivating agent

[0086]

[0087] Salt spray resistance test: Apply the above-mentioned chromium-free passivation solution on the degreased galvanized sheet with a coating thickness of 0.6~1μm, dry it at 150℃, cure it at room temperature for 24 hours, place it in a salt spray cabinet, and observe the corrosion conditions after 72 hours, 96 hours, 120 hours, 144 hours, and 168 hours.

[0088] Coating adhesion performance test: Step 1: Apply the above-mentioned chromium-free passivation solution to the degreased galvanized sheet with a coating thickness of 0.6~1μm, dry at 150℃, and cure at room temperature for 24 hours. Step 2: Degrease and perform a phosphate-free chemical treatment on the sample. After treatment, bake at 200℃ for 10 minutes, take out the sample and cool it to room temperature. Step 3: Spray powder coating. After spraying the powder coating, cure it at 200℃ for 10 minutes. The thickness of the powder coating after drying is 60~100μm. Step 4: Draw a grid, then cup it to a depth of 5cm. Use 3M tape to stick to the grid area and quickly pull off the tape to observe the remaining powder coating in the grid area.

[0089] Table 2 Compatibility of cationic polyurethane aqueous dispersion with other components of passivation solution, salt spray resistance of passivation solution, and coating adhesion

[0090]

[0091] The results in Table 2 show that the cationic polyurethane aqueous dispersion prepared by the method of the present invention uses bisaminosilane as a post-chain extender and aminophosphoric acid ester as a capping agent to synergistically improve the salt spray resistance and coating adhesion performance of the chromium-free passivator prepared using the polyurethane aqueous dispersion of the present invention.

[0092] The above embodiments are preferred implementations of the present invention, but the implementation of the present invention is not limited to the above embodiments. Any modification, replacement, and improvement based on the present invention are within the scope of protection of this patent.

Claims

1. A method for preparing a cationic polyurethane aqueous dispersion, characterized in that: The following steps are involved: Step 1: 35-50 weight percent of oligomer polyol, 0.7-1.7 weight percent of small molecule chain extender, and 0.7-1 weight percent of cross-linking agent are vacuum dehydrated at a temperature range of 110° C. to 120° C. for 0.5-1 hour under stirring, and then 38-50 weight percent of diisocyanate is dropwise added at 85° C. to 90° C., reacted for 2-3 hours, and then 8-14 weight percent of cationic hydrophilic monomer and solvent are added, and the reaction is continued for 2-3 hours to obtain a prepolymer, wherein the weight percentages are based on the total weight of the reactants excluding the weight of the solvent; Step 2: Cooling the prepolymer obtained in step 1 to below 30° C., and sequentially performing acid neutralization, water dispersion, post-chain extension, and end-capping treatment, wherein the post-chain extension uses bisaminosilane as a post-chain extender, and the end-capping treatment uses amino-containing phosphate as an end-capping agent; The bisaminosilane includes one or more of: N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, and N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane; The amino group-containing phosphate includes one or more of 3-aminopropyl monophosphate and ethanolamine phosphate.

2. The method for preparing a cationic polyurethane aqueous dispersion according to claim 1, wherein The oligomer polyol is one or more of polypropylene oxide polyol with a molecular weight of 1000-2000, polytetramethylene glycol polyol with a molecular weight of 1000-2000, polycaprolactone polyol with a molecular weight of 1000-2000, and polycarbonate polyol with a molecular weight of 1000-2000.

3. The method for preparing a cationic polyurethane aqueous dispersion according to claim 1, wherein The small molecule chain extender is one or more of ethylene glycol, 1,4-butanediol, 2,3-butanediol, and dimethylolcyclohexane; the crosslinker is trimethylolpropane or pentaerythritol; and the diisocyanate is one or more of toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and isophorone diisocyanate.

4. The method for preparing a cationic polyurethane aqueous dispersion according to claim 1, wherein The cationic hydrophilic monomer is one or more of N-methyldiethanolamine, N-ethyldiethanolamine, and triethanolamine; and the solvent is acetone or N-methylpyrrolidone.

5. The method for preparing a cationic polyurethane aqueous dispersion according to claim 1, wherein: The neutralizing agent used in the acid neutralization in step 2 is one or more of acetic acid and dilute hydrochloric acid.

6. The method for preparing a cationic polyurethane aqueous dispersion according to claim 1, wherein: The post-chain extension coefficient in step 2 is 65%~85%.

7. Use of the cationic polyurethane aqueous dispersion prepared by the preparation method according to any one of claims 1 to 6 in the preparation of a chromium-free passivating agent.

8. A chromium-free passivation solution, characterized in that: The invention comprises 60-70% by weight of water, 8-15% of a cationic polyurethane aqueous dispersion prepared by the preparation method according to any one of claims 1 to 6, 1-2% of an auxiliary agent, 20-30% of a sodium salt and 1-3% of a silane.

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