Preparation method of cationic polyurethane aqueous dispersion and application of cationic polyurethane aqueous dispersion in chromium-free passivator

Abstract

The invention belongs to the technical field of metal corrosion prevention, and particularly relates to a preparation method of a cationic polyurethane aqueous dispersion and application of the cationic polyurethane aqueous dispersion in a chromium-free passivator. The preparation method of the cationic polyurethane aqueous dispersion comprises the following steps: 1, synthesizing a prepolymer by taking diisocyanate, oligomer polyol, a micromolecular chain extender, a cross-linking agent and a cationic hydrophilic monomer as raw materials; and 2, neutralizing the prepolymer obtained in the step 1, adding water for emulsification and dispersion, carrying out post chain extension by using a post chain extender diamino silane, and finally, carrying out end capping by using amino-containing phosphate, thereby obtaining the cationic polyurethane aqueous dispersion with excellent performance. The cationic polyurethane aqueous dispersion can be used for preparing a chromium-free passivator, and has good compatibility with other components of the passivator. The salt spray resistance of the passivated plate and the coating adhesive force of the passivated plate with the powder coating are remarkably improved by utilizing the dual effects of the silane and the phosphate ester.

Description

Technical Field

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

[0002] Passivation treatment of metal surfaces is a key process to improve the corrosion resistance of materials and extend their service life. Traditional passivation technologies mainly use chromium-containing passivating agents (such as hexavalent chromium compounds), which can form a dense passivation film on the metal surface and have excellent corrosion resistance. However, hexavalent chromium is extremely harmful to the environment and human health and has been strictly restricted by international environmental protection regulations (such as RoHS, REACH, etc.). Therefore, the development of environmentally friendly chromium-free passivation technologies has become a research hotspot in recent years.

[0003] Currently, chromium-free passivation technologies mainly include zirconium-based, titanium-based, molybdenum-based, silane coupling agents, and organic polymer systems. The organic polymer system, especially the polymer polyurethane aqueous dispersion, shows broad application prospects in the field of chromium-free passivation due to its strong molecular structure designability, good adhesion, excellent film-forming properties, and environmental friendliness and non-toxicity. However, the following problems still exist in the existing technologies: 1. Insufficient salt spray resistance: The molecular structure of traditional polyurethane aqueous dispersions lacks groups with strong interactions with the metal surface, resulting in the corrosion resistance of the passivation film being difficult to meet the requirements of high-end applications, and generally can only meet the salt spray resistance requirements of 72 or 96 hours.

[0004] 2. Limited painting adhesion: The adhesion of existing passivating agents mainly relies on physical adsorption, and the chemical bonding with the metal substrate is weak. After powder spraying, the coating is prone to peeling off during cross-cut tape test and conical mandrel test. For passivating agents with poor performance, peeling off may also occur during cross-cut tape test without conical mandrel test.

[0005] In addition, the current research on polyurethane aqueous dispersions in China mainly focuses on the anionic system, and there is relatively little research on cationic polyurethane aqueous dispersions. Based on the unique charge properties and molecular structure of cationic polyurethane aqueous dispersions, it is urgent to develop cationic polyurethane aqueous dispersions with high salt spray resistance and excellent painting adhesion performance. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the present invention discloses a novel cationic polyurethane aqueous dispersion through molecular structure design, aiming to significantly improve its salt spray resistance and painting adhesion performance, while meeting environmental protection requirements, and providing a new solution for the development of chromium-free passivation technologies.

[0007] One object of the present invention is achieved through the following technical solutions: A preparation method of a cationic polyurethane aqueous dispersion, comprising the following steps: Step 1: Vacuum dehydrate 35 - 50 wt% of oligomeric polyol, 0.7 - 1.7 wt% of small molecule chain extender, and 0.7 - 1 wt% of crosslinking agent under stirring at a temperature range of 110°C - 120°C for 0.5 - 1 hour. Then, dropwise add 38 - 50 wt% of diisocyanate at 85°C - 90°C and react for 2 - 3 hours. Next, add 8 - 14 wt% of cationic hydrophilic monomer and solvent, and continue to react for 2 - 3 hours to obtain a prepolymer, where the wt% is based on the total weight of the above reactants excluding the weight of the solvent. Step 2: Cool the prepolymer obtained in Step 1 to below 30°C, and successively perform acid neutralization, water dispersion, post - chain extension, and capping treatments. The post - chain extension uses bis - amino silane as the post - chain extender, and the capping treatment uses amino - containing phosphate as the capping agent.

[0008] In the above - mentioned method for preparing the cationic polyurethane aqueous dispersion, the oligomeric polyol is one or more of poly(propylene oxide) polyol with a molecular weight of 1000 - 2000, poly(tetrahydrofuran) polyol with a molecular weight of 1000 - 2000, poly(caprolactone) polyol with a molecular weight of 1000 - 2000, and polycarbonate polyol with a molecular weight of 1000 - 2000.

[0009] In the above - mentioned method for preparing the cationic polyurethane aqueous dispersion, the small molecule chain extender is one or more of ethylene glycol, 1,4 - butanediol, 2,3 - butanediol, and dimethylol cyclohexane; the crosslinking agent is one or more of trimethylolpropane and pentaerythritol; the diisocyanate is one or more of toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and isophorone diisocyanate.

[0010] 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; the solvent is one or more of acetone and N - methylpyrrolidone.

[0011] 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.

[0012] In the above - mentioned method for preparing the cationic polyurethane aqueous dispersion, the bis - amino silane in Step 2 includes one or more of N-(2 - aminoethyl)-3 - aminopropyltrimethoxysilane, N-(2 - aminoethyl)-3 - aminopropyltriethoxysilane, and N-(2 - aminoethyl)-3 - aminopropylmethyldiethoxysilane.

[0013] In the above - mentioned method for preparing the cationic polyurethane aqueous dispersion, the post - chain extension coefficient in Step 2 is 65% - 85%.

[0014] The preparation method of the above-mentioned cationic polyurethane aqueous dispersion, the amino-containing phosphate ester described in step two includes: one or more of 3-aminopropyl monophosphate ester and ethanolamine phosphate ester.

[0015] Another object of the present invention is the application of the cationic polyurethane aqueous dispersion prepared by the above preparation method in the preparation of a chromium-free passivating agent.

[0016] Another object of the present invention is to provide a chromium-free passivating solution, which includes 60-70% water by weight percentage, 8-15% of the cationic polyurethane aqueous dispersion prepared by the above preparation method, 1-2% of an auxiliary agent, 20-30% of a sodium salt, and 1-3% of a silane.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The preparation method of the cationic polyurethane aqueous dispersion of the present invention selects bis(amino)silane as a post-chain extender. By using the reaction between the amino group and the isocyanate group, the silyl group is incorporated into the polyurethane main chain. The siloxane (Si-OR) structure in the silyl group can generate silanol (Si-OH) after hydrolysis, which undergoes a condensation reaction with the silane that needs to be added in the preparation of the chromium-free passivating solution to form a Si-O-Si bond, reducing the internal porosity of the coating and improving the denseness. In addition, the silanol undergoes a condensation reaction with the hydroxyl group (-OH) on the metal surface to form a strong Si-O-Metal covalent bond, which significantly improves the adhesion between the polymer and the metal substrate and avoids the peeling problem caused by insufficient physical adsorption force of the traditional passivation coating. In addition, the hydrophobic siloxane structure can greatly improve the water resistance and moisture resistance of the passivation film and improve the film-forming performance of the material.

[0018] 2. The preparation method of the cationic polyurethane aqueous dispersion of the present invention selects an amino-containing phosphate ester for end-capping, and the phosphate ester group is successfully introduced to both ends of the cationic polyurethane chain segment by using the reaction between the amino group and the isocyanate group. The phosphate ester group can further bond with the metal substrate to form a phosphate protection layer, effectively blocking the penetration of corrosive media (such as oxygen, water, and chlorine particles) and improving the anti-corrosion ability.

[0019] 3. Based on the properties of the polyurethane aqueous dispersion itself, the present invention further improves the comprehensive performance of the passivation film by the dual action of the post-chain extender bis(amino)silane and the end-capping agent amino-containing phosphate ester, making the synthesized cationic polyurethane aqueous dispersion simultaneously meet the characteristics of high salt spray resistance, excellent adhesion, environmental protection, and non-toxicity, meeting the performance requirements of the chromium-free passivating solution and having broad application prospects. Specific embodiments

[0020] A preparation method of a cationic polyurethane aqueous dispersion includes the following steps: Step 1: Vacuum dehydrate 35 - 50 wt% of oligomeric polyol, 0.7 - 1.7 wt% of small molecule chain extender, and 0.7 - 1 wt% of crosslinking agent under stirring at a temperature range of 110°C - 120°C for 0.5 - 1 hour. Then, dropwise add 38 - 50 wt% of diisocyanate at 85°C - 90°C and react for 2 - 3 hours. Next, add 8 - 14 wt% of cationic hydrophilic monomer and solvent, and continue to react for 2 - 3 hours to obtain a prepolymer, where the wt% is based on the total weight of the above reactants excluding the weight of the solvent.

[0021] The oligomeric polyol is one or more of poly(propylene oxide) polyol with a molecular weight of 1000 - 2000, poly(tetrahydrofuran) polyol with a molecular weight of 1000 - 2000, poly(caprolactone) polyol with a molecular weight of 1000 - 2000, and polycarbonate polyol with a molecular weight of 1000 - 2000.

[0022] The small molecule chain extender is one or more of ethylene glycol, 1,4 - butanediol, 2,3 - butanediol, and dimethylolcyclohexane; the crosslinking agent is trimethylolpropane or pentaerythritol; the diisocyanate is one or more of toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and isophorone diisocyanate.

[0023] The cationic hydrophilic monomer is one or more of N - methyldiethanolamine, N - ethyldiethanolamine, and triethanolamine; the solvent is acetone or N - methylpyrrolidone.

[0024] Step 2: Cool the prepolymer obtained in Step 1 to below 30°C, and successively perform acid neutralization, water dispersion, post - chain extension, and capping treatments. The post - chain extension uses bis - amino silane as the post - chain extender, and the capping treatment uses amino - containing phosphate as the capping agent.

[0025] The neutralizing agent used for acid neutralization is one or more of acetic acid or dilute hydrochloric acid.

[0026] The bis - amino silane includes one or more of N-(2 - aminoethyl)-3 - aminopropyltrimethoxysilane, N-(2 - aminoethyl)-3 - aminopropyltriethoxysilane, and N-(2 - aminoethyl)-3 - aminopropylmethyldiethoxysilane.

[0027] The post - chain extension coefficient is 65% - 85%.

[0028] The amino - containing phosphate includes one or more of 3 - aminopropyl monophosphate and ethanolamine phosphate.

[0029] In the present invention, bis - amino silane is selected as the post - chain extender. By utilizing the reaction between amino groups and isocyanate groups, the silyl group is incorporated into the polyurethane main chain. The siloxane (Si - OR) structure in the silyl group can generate silanol (Si - OH) after hydrolysis. It undergoes a condensation reaction with the silane that needs to be added in the preparation of the chromium - free passivation solution to form a Si - O - Si bond, reducing the porosity inside the coating and enhancing the denseness. Additionally, the silanol undergoes a condensation reaction with the hydroxyl groups (-OH) on the metal surface to form a strong Si - O - Metal covalent bond. This chemical bond significantly improves the adhesion between the emulsion and the metal substrate, avoiding the peeling problem caused by insufficient physical adsorption force in traditional passivation coatings. Moreover, the hydrophobic siloxane structure can greatly enhance the water resistance and moisture resistance of the passivation film and improve the film - forming performance of the material. In the present invention, amino - containing phosphate ester is selected for end - capping. By utilizing the reaction between amino groups and isocyanate groups, the phosphate ester group is successfully introduced to both ends of the cationic polyurethane chain segment. The phosphate ester group can further bond with the metal substrate to form a phosphate protection layer, effectively blocking the penetration of corrosive media (such as oxygen, water, and chloride particles) and improving the anti - corrosion ability.

[0030] The present invention will be described in detail below with reference to the embodiments.

[0031] Example 1 Cationic polyurethane aqueous dispersion Step 1: Preparation of the prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum - deaerating conduit, 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylol cyclohexane, and 0.4 g of trimethylol propane are added. The temperature is raised to 110 °C for dehydration and maintained for 60 minutes, then cooled to 85 °C. 22 g of isophorone diisocyanate is added dropwise and reacted for 3 hours. Then, 4.8 g of N - methyldiethanolamine and the solvent acetone are added dropwise and the reaction continues for 3 hours to obtain the prepolymer.

[0032] Step 2: The prepolymer is cooled to 30 °C, neutralized with acetic acid, and then emulsified and dispersed with ice - water to form an emulsion. Then, 4.7 g of the chain extender N - (2 - aminoethyl) - 3 - aminopropyltrimethoxysilane is added. After 1 hour, 2.2 g of 3 - aminopropyl monophosphate ester is added for end - capping, and the reaction continues for 30 minutes. The solvent is removed by vacuum, and the product is discharged to obtain the cationic polyurethane aqueous dispersion that can be applied to the chromium - free passivation agent.

[0033] Example 2 Cationic polyurethane aqueous dispersion Step 1: Preparation of prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane. Heat up to 120 °C for dehydration and maintain for 30 minutes, then cool down to 85 °C, dropwise add 22 g of isophorone diisocyanate and react for 3 hours, then dropwise add 5.6 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0034] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Then add 5.6 g of chain extender N-(2-aminoethyl)-3-aminopropyltriethoxysilane, and after 1 hour, add 2.2 g of 3-aminopropyl monophosphate for end-capping and continue to react for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to chromium-free passivators.

[0035] Example 3 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane. Heat up to 120 °C for dehydration and maintain for 30 minutes, then cool down to 90 °C, dropwise add 22 g of isophorone diisocyanate and react for 2.5 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0036] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Then add 4.96 g of chain extender N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane, and after 1 hour, add 2.2 g of 3-aminopropyl monophosphate for end-capping and continue to react for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to chromium-free passivation.

[0037] . Example 4 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polypropylene oxide diol (M = 1000), 0.34 g of ethylene glycol, and 0.4 g of trimethylolpropane. Heat up to 120 °C for dehydration and maintain for 30 minutes, then cool down to 90 °C, dropwise add 22 g of isophorone diisocyanate and react for 2.5 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0038] Step 2: Cool the prepolymer to 25°C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Then add 4.01 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 3.01 g of 3-aminopropyl monophosphate for end-capping, and continue the reaction for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to a chromium-free passivator.

[0039] Example 5 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, add 21 g of polycarbonate diol (M = 1000), 0.46 g of 1,4-butanediol, and 0.41 g of pentaerythritol to a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation conduit. Heat to 110°C for dehydration and hold for 60 minutes, then cool to 85°C, add dropwise 24.8 g of 4,4-diphenylmethane diisocyanate and react for 3 hours, then add dropwise 5.4 g of N-ethyldiethanolamine and solvent acetone, and continue the reaction for 3 hours to obtain a prepolymer.

[0040] Step 2: Cool the prepolymer to 30°C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Then add 5.27 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 1.3 g of 3-aminopropyl monophosphate for end-capping, and continue the reaction for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to a chromium-free passivator.

[0041] .Example 6 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, add 21 g of polytetrahydrofuran diol (M = 1000), 0.46 g of 2,3-butanediol, and 0.42 g of pentaerythritol to a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation conduit. Heat to 120°C for dehydration and hold for 40 minutes, then cool to 90°C, add dropwise 17.25 g of toluene diisocyanate and react for 2 hours, then add dropwise 6 g of triethanolamine and solvent N-methylpyrrolidone, and continue the reaction for 2 hours to obtain a prepolymer.

[0042] Step 2: Cool the prepolymer to 30°C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Then add 4.68 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 1.98 g of ethanolamine phosphate for end-capping, and continue the reaction for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to a chromium-free passivator.

[0043] Example 7 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, add 21 g of polycaprolactone diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane into a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation conduit. Heat up to 110 °C to dehydrate for 60 minutes, then cool down to 85 °C, dropwise add 19 g of isophorone diisocyanate and react for 3 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0044] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water to emulsify and disperse to form an emulsion. Then add 2.46 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 1.14 g of 3-aminopropylmonophosphate to cap, and continue to react for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to a chromium-free passivator.

[0045] Example 8 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane into a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation conduit. Heat up to 110 °C to dehydrate for 60 minutes, then cool down to 85 °C, dropwise add 26 g of isophorone diisocyanate and react for 3 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0046] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water to emulsify and disperse to form an emulsion. Then add 8.66 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 3.59 g of 3-aminopropylmonophosphate to cap, and continue to react for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to a chromium-free passivator.

[0047] Example 9 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of prepolymer. First, add 16 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane into a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation conduit. Heat up to 110 °C to dehydrate for 60 minutes, then cool down to 85 °C, dropwise add 22 g of isophorone diisocyanate and react for 3 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0048] Step 2: Cool the prepolymer to 30°C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Add 5.5 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 2.58 g of 3-aminopropyl monophosphate for end-capping, and continue the reaction for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion for use in a chromium-free passivator.

[0049] Example 10 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of the prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 27 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, 0.4 g of trimethylolpropane, heat up to 110°C for dehydration and hold for 60 minutes, then cool to 85°C, dropwise add 22 g of isophorone diisocyanate and react for 3 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue the reaction for 3 hours to obtain the prepolymer.

[0050] Step 2: Cool the prepolymer to 30°C, neutralize it with acetic acid, and then add ice water for emulsification and dispersion to form an emulsion. Add 3.96 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 1.72 g of 3-aminopropyl monophosphate for end-capping, and continue the reaction for 30 minutes. Remove the solvent under vacuum and discharge to obtain a cationic polyurethane aqueous dispersion that can be used in a chromium-free passivator.

[0051] Example 11 Cationic Polyurethane Aqueous Dispersion Step 1: Preparation of the prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, 0.4 g of trimethylolpropane, heat up to 110°C for dehydration and hold for 60 minutes, then cool to 85°C, dropwise add 22 g of isophorone diisocyanate and react for 3 hours, then dropwise add 4.8 g of N-methyldiethanolamine and the solvent N-methylpyrrolidone, and continue the reaction for 3 hours to obtain the prepolymer.

[0052] Step 2: Cool the prepolymer to 30°C, neutralize it with hydrochloric acid, and then add ice water for emulsification and dispersion to form an emulsion. Then add 4.7 g of chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after 1 hour, add 2.2 g of 3-aminopropyl monophosphate for end-capping, and continue the reaction for 30 minutes. Remove the solvent and discharge to obtain a cationic polyurethane aqueous dispersion that can be used in a chromium-free passivator.

[0053] Comparative Example 1 Step 1: Preparation of prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane. Heat up to 110 °C to dehydrate for 60 minutes, then cool down to 85 °C, add dropwise 22 g of isophorone diisocyanate and react for 2 hours, then add dropwise 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0054] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water to emulsify and disperse to form an emulsion. Then add 1.27 g of ethylenediamine for chain extension, and after 1 hour, add 2.19 g of 3-aminopropyl monophosphate for end-capping and continue to react for 30 minutes. Remove the solvent and discharge to obtain a cationic polyurethane aqueous dispersion that can be applied to a chromium-free passivator.

[0055] Comparative Example 2 Step 1: Preparation of prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane. Heat up to 110 °C to dehydrate for 60 minutes, then cool down to 85 °C, add dropwise 22 g of isophorone diisocyanate and react for 2 hours, then add dropwise 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0056] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water to emulsify and disperse to form an emulsion. Then add 8.77 g of 3-aminopropyl monophosphate for end-capping and continue to react for 30 minutes. Remove the solvent and discharge to obtain a cationic polyurethane aqueous dispersion applied to a chromium-free passivator.

[0057] Comparative Example 3 Step 1: Preparation of prepolymer. First, in a container equipped with a stirrer, thermometer, condenser, and vacuum evacuation duct, add 21 g of polycarbonate diol (M = 1000), 0.72 g of dimethylolcyclohexane, and 0.4 g of trimethylolpropane. Heat up to 110 °C to dehydrate for 60 minutes, then cool down to 85 °C, add dropwise 22 g of isophorone diisocyanate and react for 2 hours, then add dropwise 4.8 g of N-methyldiethanolamine and the solvent acetone, and continue to react for 3 hours to obtain the prepolymer.

[0058] Step 2: Cool the prepolymer to 30 °C, neutralize it with acetic acid, and then add ice water to emulsify and disperse to form an emulsion. Then add 4.7 g of the chain extender N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and after reacting for 1 hour, remove the solvent and discharge to obtain a cationic polyurethane aqueous dispersion applied to a chromium-free passivator.

[0059] Comparative Example 4 A commercially available chromium-free passivating agent product of a certain brand.

[0060] Example 12 Chromium-free Passivating Agent The cationic polyurethane aqueous dispersion prepared by the above preparation method can be applied to the preparation of a chromium-free passivating agent. The present invention provides a chromium-free passivating solution, which is made of the cationic polyurethane aqueous dispersion prepared by the above preparation method. Its components and specific preparation methods can adopt the contents and steps commonly used in the prior art. In this example, the chromium-free passivating solution includes 60-70% water by weight, 8-15% of the cationic polyurethane aqueous dispersion prepared by the above preparation method, 1-2% auxiliary agent, 20-30% sodium salt, and 1-3% silane.

[0061] Prepare the chromium-free passivating solution according to the formula shown in Table 1.

[0062] Put the formulated water and cationic polyurethane emulsion into a container and start stirring. Add acetic acid and disperse for 5 minutes, then add the following materials in sequence, with a 15-minute interval between each material.

[0063] Table 1 Chromium-free Passivating Agent Formula

[0064] Salt spray resistance performance test: Take the above chromium-free passivating solution and coat it on a degreased galvanized steel sheet with a film thickness of 0.6-1 μm. Dry it at 150 °C, cure it at room temperature for 24 hours, place it in a salt spray chamber, and observe the corrosion conditions at 72 hours, 96 hours, 120 hours, 144 hours, and 168 hours.

[0065] Coating adhesion performance test: First step: Take the above chromium-free passivating solution and coat it on a degreased galvanized steel sheet with a film thickness of 0.6-1 μm. Dry it at 150 °C, cure it at room temperature for 24 hours. Second step: Degrease and phosphating-free treat the sample plate, bake it at 200 °C for 10 minutes after treatment, take out the sample plate and cool it to room temperature. Third step: Spray powder coating treatment, cure it at 200 °C for 10 minutes after spraying the powder coating, and the thickness of the dried powder coating is 60-100 μm. Fourth step: Draw a hundred grids, then cup convex 5 cm deep, use 3M tape to stick to the hundred-grid area and then quickly pull off the tape, and observe the remaining situation of the powder coating in the hundred-grid area.

[0066] Table 2 Compatibility of Cationic Polyurethane Aqueous Dispersion with Other Components of Passivating Solution and Salt Spray Resistance and Coating Adhesion of Passivating Solution

[0067] As shown in the results of Table 2, for the cationic polyurethane aqueous dispersion prepared by the method of the present invention, the synergistic effect of using a bis-aminosilane as a post-chain extender and an amino-containing phosphate ester as a capping agent greatly improves the salt spray resistance and painting adhesion performance of the chromium-free passivator formulated with the polyurethane aqueous dispersion of the present invention.

[0068] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any modifications, substitutions, and improvements 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, It includes the following steps: Step 1: Vacuum dehydrate 35 - 50 wt% of oligomeric polyol, 0.7 - 1.7 wt% of small molecule chain extender, and 0.7 - 1 wt% of crosslinking agent under stirring at a temperature range of 110°C - 120°C for 0.5 - 1 hour. Then, dropwise add 38 - 50 wt% of diisocyanate at 85°C - 90°C, react for 2 - 3 hours, add 8 - 14 wt% of cationic hydrophilic monomer and solvent, and continue to react for 2 - 3 hours to obtain a prepolymer. The wt% is based on the total weight of the above reactants excluding the weight of the solvent. Step 2: Cool the prepolymer obtained in Step 1 to below 30°C, and successively carry out acid neutralization, water dispersion, post - chain extension, and capping treatments. The post - chain extension uses bis - amino silane as the post - chain extender, and the capping treatment uses amino - containing phosphate ester as the capping agent.

2. The preparation method of the cationic polyurethane aqueous dispersion according to claim 1, characterized in that, The oligomeric polyol is one or more of poly(propylene oxide) polyol with a molecular weight of 1000 - 2000, poly(tetrahydrofuran) polyol with a molecular weight of 1000 - 2000, poly(caprolactone) polyol with a molecular weight of 1000 - 2000, and poly(carbonate) polyol with a molecular weight of 1000 - 2000.

3. The preparation method of the cationic polyurethane aqueous dispersion according to claim 1, characterized in that, The small molecule chain extender is one or more of ethylene glycol, 1,4 - butanediol, 2,3 - butanediol, and dimethylolcyclohexane; the crosslinking agent is trimethylolpropane or pentaerythritol; the diisocyanate is one or more of toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and isophorone diisocyanate.

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

5. The preparation method of the cationic polyurethane aqueous dispersion according to claim 1, characterized in that, The neutralizing agent used for acid neutralization in Step 2 is one or more of acetic acid or dilute hydrochloric acid.

6. The preparation method of the cationic polyurethane aqueous dispersion according to claim 1, characterized in that, The bis - amino silane in Step 2 includes one or more of N-(2 - aminoethyl)-3 - aminopropyltrimethoxysilane, N-(2 - aminoethyl)-3 - aminopropyltriethoxysilane, and N-(2 - aminoethyl)-3 - aminopropylmethyldiethoxysilane.

7. The preparation method of the cationic polyurethane aqueous dispersion according to claim 1, characterized in that, The post - chain extension coefficient in Step 2 is 65% - 85%.

8. The preparation method of the cationic polyurethane aqueous dispersion according to claim 1, characterized in that, The amino - containing phosphate ester in Step 2 includes one or more of 3 - aminopropyl monophosphate and ethanolamine phosphate.

9. Use of the cationic polyurethane aqueous dispersion prepared by the preparation method according to any one of claims 1 - 8 in the preparation of a chromium - free passivator.

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

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