A chromium-free fingerprint-resistant coating agent for galvanized steel sheets and its preparation method

Abstract

This invention discloses a chromium-free fingerprint-resistant coating agent for galvanized steel sheets and its preparation method, comprising the following elements by mass fraction: sodium molybdate and / or sodium tungstate: 1%–5%, phytic acid-chitosan complex: 0.5%–2%, chelated titanate coupling agent: 0.3%–1.0%, fluorosilane-modified nano-SiO2: 2%–5%, waterborne hyperbranched polyurethane resin: 30%–55%, oxidized polyethylene wax: 0.5%–2%, and the balance being deionized water. The chromium-free fingerprint-resistant coating agent of this invention exhibits excellent fingerprint resistance, high adhesion, and environmental friendliness. It shows no white rust after 150 hours of neutral salt spray and no red rust after 360 hours, with the white rust area being <5%, meeting the industrial requirements for surface treatment of galvanized steel sheets. Furthermore, this invention employs a combination of bio-based corrosion inhibitors, avoiding the environmental risks present in existing technologies.

Description

Technical Field

[0001] This invention belongs to the field of material surface treatment and metal protection technology, specifically relating to a chromium-free fingerprint-resistant coating agent for galvanized steel sheets and its preparation method. Background Technology

[0002] Galvanized steel sheets are widely used, but in humid environments, the galvanized layer is prone to corrosion, forming a dark gray or white porous corrosion product—white rust—on the surface. Over time, red rust will appear, thus losing its anti-corrosion effect. To improve the corrosion resistance of the coating, after electro-galvanizing or hot-dip galvanizing the steel material, the coating must undergo passivation treatment to form a dense passivation film on the surface, improving corrosion resistance and extending the service life of the workpiece.

[0003] Fingerprint-resistant treatment refers to directly coating the surface of galvanized steel sheet with a fingerprint-resistant coating agent using a chemical roller coating machine. This agent contains substances with optical properties similar to those of human fingerprints. After drying, the resulting fingerprint-resistant film is only 1-2 μm thick. This means that even if fingerprints adhere to the surface, the difference in optical reflection between the fingerprint-free areas and the non-fingerprint-affected areas is minimal, thus creating fingerprint resistance. This thin film not only resists fingerprints but also provides good corrosion resistance. Obtaining galvanized fingerprint-resistant steel sheets with excellent overall performance has become a hot research topic for major steel mills.

[0004] The base film-forming materials of existing fingerprint-resistant coating agents are all water-based organic resins, and the polymerization method is emulsion polymerization. These include polyethylene resins, silane resins, acrylic resins, epoxy resins, polyurethane resins, etc. Generally, polyurethane resins have relatively better performance.

[0005] In fingerprint-resistant coatings, hexavalent chromates are typically added as passivating agents. These form a film on the steel surface, providing passivation and further enhancing corrosion resistance. Chromate passivation films also possess self-healing properties. Metals such as Zr, Ti, V, Mo, W, Mn, and Ce exhibit similar effects but lack self-healing capabilities. However, hexavalent chromium poses significant health risks and is completely banned under the EU RoHS directive. Since trivalent chromium is only one percent as toxic as hexavalent chromium, passivation solutions with trivalent chromium as the main component are gradually replacing traditional hexavalent chromium passivation solutions. However, trivalent chromium passivation solutions are unstable; the trivalent chromium is easily oxidized to hexavalent chromium, and the corrosion resistance of trivalent passivation solutions does not reach the level of hexavalent chromium.

[0006] Patent document CN118271948A discloses a composite system using zirconium salt (ammonium zirconium carbonate), silane coupling agent, ammonium vanadate / sodium periodate, and waterborne polyurethane / acrylic resin. This system enhances film density through an interpenetrating network of linear and mesh structures, forms a stable passivation film through a redox reaction, improves physical barrier properties by filling the resin network with nanoparticles, and enhances interfacial adhesion and reduces film defects through silane coupling agents (such as KH550 / KH560). Patent document CN118389039A uses phosphoric acid instead of ammonium vanadate and adds nano-silica. However, the chromium-free anti-fingerprint agent in the above documents needs to be applied to the surface of hot-dip galvanized steel sheet and baked at 280℃ for 5-30 seconds to form a conversion film.

[0007] Patent document CN118389981A discloses that the composition of the chromium-free passivating agent includes zinc oxide, aluminum oxide and magnesium oxide, which are baked and cured in an oven at 200-300℃ to form a chromium-free passivating film of 0.3-0.6μm; the chromium-free fingerprint-resistant treatment solution uses 9%-13% lithium silicate, 4%-7% glycidyl etheroxypropyltrimethoxysilane and 3%-6% 1,2-bis-(trimethoxy)ethane as raw materials, which are baked and cured in an oven at 400-500℃ to form a chromium-free fingerprint-resistant film of 1-2μm.

[0008] Patent document CN113527994A proposes an immersion-type, low-temperature drying, chromium-free anti-fingerprint agent. The raw materials include anionic polyurethane emulsion, crosslinking curing agent, siloxane, phosphate, fluorosilicic acid, and ammonium citrate, with a pH value between 3.8 and 5.5. It cures at low temperature (2-7 minutes) under room temperature or heating, overcoming the limitations of traditional high-temperature curing (>120℃). However, this document lacks testing data, making it impossible to determine its performance. Summary of the Invention

[0009] The purpose of this invention is to provide a chromium-free fingerprint-resistant coating agent for galvanized steel sheets and its preparation method, so as to solve the problems of high toxicity, high curing temperature and poor corrosion resistance in the prior art. By utilizing the passivation effect of molybdate / tungstate, the chelating ability of phytic acid and the interface enhancement function of titanium compounds, a multi-layer anti-corrosion barrier is formed, which takes into account environmental protection, corrosion resistance and process economy.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] The present invention discloses a chromium-free fingerprint-resistant coating agent for galvanized steel sheets, comprising the following elements by mass fraction: sodium molybdate and / or sodium tungstate: 1%–5%, phytic acid-chitosan complex: 0.5%–2%, chelated titanate coupling agent: 0.3%–1.0%, fluorosilane-modified nano-SiO2: 2%–5%, waterborne hyperbranched polyurethane resin: 30%–55%, oxidized polyethylene wax: 0.5%–2%, with the balance being deionized water; and further comprising additives comprising 0.1%–3% of the total mass of the above raw materials.

[0012] Furthermore, the sodium molybdate and / or sodium tungstate are used as the main corrosion inhibitors to form a passivation film on the metal surface through oxidation. Sodium molybdate is preferred because it has lower toxicity and higher passivation efficiency.

[0013] Furthermore, the phytic acid-chitosan complex is a bio-based corrosion inhibitor that chelates metal ions and inhibits localized corrosion. Its preparation method is as follows: (1) Chitosan is dissolved in citric acid solution to prepare a chitosan solution with a concentration of 0.005-0.05 g / L. The solution is stirred until completely dissolved, and the pH value of the chitosan solution is adjusted to 4-6. (2) A phytic acid solution with a concentration of 0.005-0.01 mol / L is added dropwise to the chitosan solution according to the molar ratio of phytic acid to chitosan = (2-2.15):1. An ionic crosslinking reaction is carried out at 45-60℃ for 3-6 h to obtain the chitosan-phytic acid complex.

[0014] Preferably, the citric acid solution has a mass concentration of 0.5% to 3%; the pH value of the chitosan solution is adjusted using citric acid and / or ammonia.

[0015] Furthermore, the chelating titanate coupling agent is an aqueous chelating titanate coupling agent of alcohol amine (e.g., a chelate solution of bis(dioctyloxypyrophosphate) ethylene titanate and triethanolamine) or / and a quaternary ammonium salt of chelating phosphate titanate coupling agent, which can improve the adhesion between the coating and the substrate and promote the dispersion of nanoparticles, such as Kenrich KR-238T aqueous titanate coupling agent (domestic corresponding grade HY311W).

[0016] Furthermore, the fluorosilane-modified nano-SiO2 is nano-silica with a surface modified by fluorosilane, which improves hydrophobicity and dispersibility, enhances the physical barrier of the coating, and fills micro-defects in the coating. It can be prepared using the method disclosed in document CN104445218A, or commercially available products can be used directly, such as hydrophobic fumed silica of models HB-132, HB-151, and HB-612 from Hubei Huifu Nanomaterials Co., Ltd.

[0017] Furthermore, the waterborne hyperbranched polyurethane resin is a hydroxyl-terminated waterborne hyperbranched polyurethane resin prepared from dimethylolpropionic acid or dimethylolbutyric acid, diisocyanate and diethanolamine as raw materials, with a number average molecular weight of 3000-12000 and a solid content of about 40%. The preparation method can be found in the literature [Mei Jingang, Yang Jianjun, et al. Synthesis and characterization of hyperbranched waterborne polyurethane. Polymer Materials Science and Engineering, 2012, 28(6): 16-19.], or commercially available similar products can be used.

[0018] Furthermore, the additive is an auxiliary processing agent or an agent that adds auxiliary functions, such as one or more of the following: silicone leveling agent (preferably BYK-333), silicone defoamer (such as BYK-024), and UV oxidant, which can improve the surface smoothness of the coating, fingerprint wiping, and anti-yellowing properties.

[0019] The present invention discloses a method for preparing a chromium-free fingerprint-resistant coating agent for galvanized steel sheets, comprising the following steps:

[0020] S1: Add the waterborne hyperbranched polyurethane resin to the reactor, add deionized water for forming the premixed slurry, and stir at low speed to pre-dilute;

[0021] S2: In another container, add the fluorosilane-modified nano-SiO2, chelated titanate coupling agent, and deionized water for adjusting the viscosity of the mixture in sequence, and pre-disperse under high shear for 20-50 minutes until the nanoparticles D 50 ≤100nm, to obtain nano-slurry;

[0022] S3: Mix the sodium molybdate and / or sodium tungstate, phytic acid-chitosan complex and oxidized polyethylene wax, add deionized water for dissolving and dispersing the mixture, and disperse until completely dissolved to obtain a corrosion inhibitor mixture;

[0023] S4: Slowly add the nano slurry from step S2 to the resin system from step S1, stir at medium speed for 20-50 min while maintaining a temperature of 20-50℃, add the corrosion inhibitor mixture from step S3, and continue stirring for 10-30 min; adjust the pH of the system to 4.5-6.0.

[0024] S5: Add the remaining deionized water and stir for 10-30 minutes; add the additive and stir at low speed until uniform, degas, and sieve to obtain the chromium-free fingerprint-resistant coating agent for galvanized steel sheets.

[0025] The deionized water added in step S1 accounts for 20% to 40% of the total volume of deionized water; the deionized water added in step S2 accounts for 20% to 40% of the total volume of deionized water; and the deionized water added in step S3 accounts for 10% to 25% of the total volume of deionized water.

[0026] The high-shear pre-dispersion rate described in this invention is 3000-6000 rpm, the low-speed stirring rate is 100-700 rpm, and the medium-speed stirring rate is 800-1500 rpm.

[0027] In step S4, citric acid and / or ammonia are used to adjust the pH of the system, and the pH value of the system is controlled between 4.5 and 6.0 to ensure the phytic acid chelation activity and avoid the hydrolysis and precipitation of molybdate.

[0028] The present invention describes the application of a chromium-free fingerprint-resistant coating agent for galvanized steel sheets. The agent is applied to the surface of the galvanized steel sheet and cured under the following conditions: 120-150°C for 25-50 seconds (preferably 30-40 seconds).

[0029] Compared with the prior art, the positive effects of the present invention are:

[0030] This invention utilizes molybdate to generate MoO4 on a metal surface. 2- The passivation film utilizes phytic acid to chelate free metal ions of molybdate / tungstate via phosphate groups, inhibiting micro-area corrosion. Chitosan adsorbs onto the metal surface via amino groups, forming a dense protective layer. A chelating titanate coupling agent bridges the hyperbranched polyurethane resin and fluorosilane-modified nano-SiO2, enhancing interfacial bonding and reducing porosity. Fluorosilane-modified nano-SiO2 and oxidized polyethylene wax provide superior water resistance and water repellency. Through the above process optimization, the chromium-free fingerprint-resistant coating agent of this invention can be ensured to have excellent fingerprint resistance, salt spray resistance, high adhesion, and environmental friendliness, meeting the industrial requirements for surface treatment of galvanized steel sheets. Furthermore, this invention uses a combination of bio-based corrosion inhibitors, avoiding the environmental risks present in existing technologies. Detailed Implementation

[0031] Those skilled in the art should recognize that this embodiment is only used to illustrate the present invention and is not intended to limit the present invention. Any changes or modifications to the embodiment within the scope of the present invention are within the scope of the claims of the present invention.

[0032] Examples 1-5 and Comparative Examples 1-3

[0033] The preparation methods of the chromium-free fingerprint-resistant coating agent for galvanized steel sheets in Examples 1-5 and Comparative Examples 1-3 include the following steps:

[0034] S1: Add waterborne hyperbranched polyurethane resin (molecular weight 5100, solid content 40%, average particle size of latex particles 36μm) to the reactor and control the temperature at 25℃; add 30% deionized water by volume and stir at low speed of 200rpm for 10min for pre-dilution.

[0035] S2: In another container, add the fluorosilane-modified nano-SiO2 (HB-132 from Huifu Nanomaterials), chelated titanate coupling agent (Kenrich KR-238T aqueous titanate coupling agent), and 30% by volume of deionized water in sequence. Pre-disperse at 5000 rpm under high shear for 20–50 min until the nanoparticles reach D… 50 <100nm, to obtain nano-slurry;

[0036] S3: Mix sodium molybdate and / or sodium tungstate, phytic acid-chitosan complex and oxidized polyethylene wax (particle size <5μm), add 20% of deionized water by volume, and disperse under ultrasonic assistance at 40kHz for 15min until completely dissolved to obtain corrosion inhibitor mixture;

[0037] S4: Slowly add the nano slurry from step S2 to the resin system from step S1, stir at medium speed (800 rpm × 20 min) at 35℃, add the corrosion inhibitor mixture from step S3, and continue stirring for 15 min; adjust the pH of the system to 5.0 ± 0.2 with citric acid / ammonia.

[0038] S5: Add the remaining deionized water and stir for 10 minutes; add 0.1% of the total amount of the above raw materials as silicone defoamer (such as BYK-024), stir at low speed (300 rpm) to defoam for 5 minutes; pass through a 400-mesh sieve to remove undispersed particles and obtain a uniform chromium-free fingerprint-resistant coating agent for galvanized steel sheets.

[0039] The preparation method of phytic acid-chitosan complex is as follows: (1) Chitosan is dissolved in citric acid solution to prepare a chitosan solution with a concentration of 0.01 g / L. The solution is stirred at 40°C for 2 h until completely dissolved. The pH value of the chitosan solution is adjusted to 5.0 ± 0.2 with citric acid / ammonia. (2) Phytic acid solution with a concentration of 0.008 mol / L is added dropwise to the chitosan solution according to the molar ratio of phytic acid to chitosan = 2:1. The ionic crosslinking reaction is carried out at 50°C for 4.3 h to obtain chitosan-phytic acid complex.

[0040] In Comparative Example 3, all phytic acid was used to replace the phytic acid-chitosan complex, and the rest was the same as in Example 3.

[0041] Table 1. Raw material ratios for Examples 1-5 and Comparative Examples 1-3, in wt%.

[0042]

[0043] Test Experiment Example

[0044] The fingerprint-resistant coatings of Examples 1-5 and Comparative Examples 1-3 were respectively applied to galvanized steel sheets and cured at 130°C for 30 seconds. The fingerprint resistance, adhesion, water contact angle, salt spray resistance and damp heat resistance were then measured. The results are listed in Table 2.

[0045] Fingerprint resistance: After 50 rubs with artificial sweat (formulation reference ISO 3160-2:2015, containing 0.5% sodium lauryl), there should be no residue, no corrosion, and the gloss change should be <5%.

[0046] Adhesion: Tested according to GB / T 9286-2021 cross-cut test, no adhesion was observed after cutting with a cross-cut tester.

[0047] Contact angle: according to ASTM D7334 static water droplet method.

[0048] Salt spray resistance (NSS): According to GB / T 1771-2007, six samples were prepared for each type of test. The prepared samples were placed in a salt spray chamber at 35℃ and subjected to a 47℃ pressure chamber temperature, sprayed with a 5wt% sodium chloride saline solution (pH 7.0) at a rate of 1.0 kgf / cm³. 2 Spraying was performed using air pressure; after spraying the samples for 150 hours and 360 hours, three samples were taken out each time, washed with water and dried, and the surface of the samples was observed for white or red rust, and the percentage of the corrosion area to the total area of ​​the product was observed.

[0049] Moisture and heat resistance test: The passivation effect of moisture and heat resistance is judged based on the color difference ΔE value of the sample surface before and after moisture and heat resistance. Heat yellowing resistance test is performed separately. The passivation effect of heat resistance is judged based on the color difference ΔE value of the sample surface before and after heat resistance. The prepared sample is placed in a test chamber at 50℃ and placed in a 95% humidity environment for 120 hours. After the sample is taken out, washed with water and dried, the color difference ΔE value of the sample surface before and after moisture and heat resistance is measured.

[0050] Table 2. Coating performance verification of Examples 1-5 and Comparative Examples 1-3

[0051]

[0052] As shown in Table 2, the chromium-free fingerprint-resistant coating agents of Examples 1 to 5 of the present invention have excellent fingerprint resistance, high adhesion and environmental friendliness. They show no white rust after 150 hours of neutral salt spray and no red rust after 360 hours, with the white rust area being less than 5%, thus meeting the industrial requirements for surface treatment of galvanized steel sheets.

Claims

1. A chromium-free fingerprint-resistant coating agent for galvanized steel sheets, characterized in that, Raw materials including the following mass fractions: sodium molybdate and / or sodium tungstate: 1%–5%, phytic acid-chitosan complex: 0.5%–2%, chelated titanate coupling agent: 0.3%–1.0%, fluorosilane-modified nano-SiO2: 2%–5%, waterborne hyperbranched polyurethane resin: 30%–55%, oxidized polyethylene wax: 0.5%–2%, the balance being deionized water; also includes additives accounting for 0.1%–3% of the total mass of the above raw materials; the chelating titanate coupling agent is an alcoholic amine waterborne chelating titanate coupling agent or / and a chelating phosphate titanium coupling agent quaternary ammonium salt; The fluorosilane-modified nano-SiO2 is nano-silica with a surface modified by fluorosilane; The waterborne hyperbranched polyurethane resin is a hydroxyl-terminated waterborne hyperbranched polyurethane resin prepared from dimethylolpropionic acid or dimethylolbutyric acid, diisocyanate and diethanolamine, with a number average molecular weight of 3000-12000 and a solid content of 40%±3%. The preparation method of the phytic acid-chitosan complex is as follows: (1) Chitosan is dissolved in citric acid solution to prepare a chitosan solution with a concentration of 0.005-0.05 g / L. The solution is stirred until completely dissolved, and the pH value of the chitosan solution is adjusted to 4-6; (2) Phytic acid solution with a concentration of 0.005-0.01 mol / L is added dropwise to the chitosan solution according to the molar ratio of phytic acid to chitosan = (2-2.15):

1. The ionic crosslinking reaction is carried out at 45-60℃ for 3-6 h to obtain the chitosan-phytic acid complex.

2. The chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 1, characterized in that, The citric acid solution has a mass concentration of 0.5% to 3%; the pH value of the chitosan solution is adjusted using citric acid and / or ammonia.

3. The chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 1, characterized in that, The additive is one or more of the following: silicone leveling agent, silicone defoamer, and UV oxidant.

4. The method for preparing the chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1: Add the waterborne hyperbranched polyurethane resin to the reactor, add deionized water for forming the premixed slurry, stir at low speed to pre-dilute, and obtain the resin system. S2: In another container, add the fluorosilane-modified nano-SiO2, chelated titanate coupling agent, and deionized water for adjusting the viscosity of the mixture in sequence, and pre-disperse under high shear for 20-50 minutes until the nanoparticles D 50 ≤100nm, to obtain nano-slurry; S3: Mix the sodium molybdate and / or sodium tungstate, phytic acid-chitosan complex and oxidized polyethylene wax, add deionized water for dissolving and dispersing the mixture, and disperse until completely dissolved to obtain a corrosion inhibitor mixture; S4: Slowly add the nano slurry from step S2 to the resin system from step S1, stir at medium speed for 20-50 min while maintaining a temperature of 20-50℃, add the corrosion inhibitor mixture from step S3, and continue stirring for 10-30 min; adjust the pH of the system to 4.5-6.

0. S5: Add the remaining deionized water and stir for 10-30 minutes; add the additive and stir at low speed until uniform, degas, and sieve to obtain the chromium-free fingerprint-resistant coating agent for galvanized steel sheets.

5. The method for preparing the chromium-free fingerprint-resistant coating agent for galvanized steel sheet according to claim 4, characterized in that, In step S4, citric acid and / or ammonia are used to adjust the pH of the system, and the pH value of the system is controlled between 4.5 and 6.

0.

6. The application of the chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to any one of claims 1 to 3, characterized in that, The coating is applied to the surface of galvanized steel sheet, and the curing conditions are: 120-150℃ for 25-50 seconds.

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