Chromium-free fingerprint-resistant coating agent for galvanized steel sheet and preparation method of chromium-free fingerprint-resistant coating agent

Abstract

The invention discloses a chromium-free fingerprint-resistant coating agent for a galvanized steel sheet and a preparation method of the chromium-free fingerprint-resistant coating agent. The chromium-free fingerprint-resistant coating agent is prepared from the following elements in percentage by mass: 1%-5% of sodium molybdate or / and sodium tungstate, 0.5%-2% of a phytic acid-chitosan compound, 0.3%-1.0% of a chelating titanate coupling agent, 2%-5% of fluorosilane modified nano SiO2, 30%-55% of waterborne hyperbranched polyurethane resin, 0.5%-2% of oxidized polyethlene wax and the balance of deionized water. The chromium-free fingerprint-resistant coating agent disclosed by the invention has excellent fingerprint resistance, high adhesive force and environmental protection property, is free of white rust in neutral salt mist for 150 hours and red rust in neutral salt mist for 360 hours, the white rust area is less than 5%, the industrial requirements of galvanized steel sheet surface treatment are met, and the problem of environmental risk in the prior art is avoided by adopting the combination of bio-based corrosion inhibitors.

Description

Technical Field

[0001] The present invention belongs to the technical field of material surface treatment and metal protection, and particularly relates to a chromium-free fingerprint-resistant coating agent for galvanized steel sheets and a preparation method thereof. Background Art

[0002] Galvanized steel sheets are widely used. However, in a humid environment, the galvanized layer is prone to corrosion, forming dark gray or white loose corrosion products - white rust on the surface of the galvanized layer. Over time, red rust will appear, thus losing the anti-corrosion effect. In order to improve the corrosion resistance of the coating, after electro-galvanizing or hot-galvanizing the steel material surface, the coating needs to be passivated to form a dense passivation film on the surface of the coating, improve the corrosion resistance of the coating, and extend the service life of the workpiece.

[0003] Fingerprint-resistant treatment refers to directly coating a fingerprint-resistant coating agent on the surface of a galvanized steel sheet through a chemical roll coater. The coating agent contains substances with optical properties similar to those of human fingerprints. After drying, the formed fingerprint-resistant film has a thickness of only 1 - 2 μm. In this way, even if fingerprints are attached, the optical reflection difference between the fingerprint-attached part and the non-fingerprint-attached part is very small, resulting in fingerprint resistance. This thin film not only is not prone to generating fingerprints but also provides good corrosion resistance. How to obtain a galvanized fingerprint-resistant plate with good comprehensive performance has become a research hotspot for major steel mills.

[0004] The basic film-forming substances of existing fingerprint-resistant coating agents are all water-based organic resins, and the polymerization method is emulsion polymerization, including polyethylene resins, silane resins, acrylic resins, epoxy resins, polyurethane resins, etc. Generally, the performance of polyurethane resins is relatively good.

[0005] In the fingerprint-resistant coating agent, hexavalent chromate is usually added as a passivating agent, which can form a film on the steel sheet surface to play a passivating role, thereby further improving the corrosion resistance. At the same time, the chromate passivation film has a self-healing function. Metal elements such as Zr, Ti, V, Mo, W, Mn, and Ce have a similar effect but do not have a self-healing function. However, hexavalent chromium causes great harm to the human body and the environment and is completely prohibited by the EU ROHS directive. Since the toxicity of trivalent chromium is only one percent of that of hexavalent chromium, the passivation solution mainly composed of trivalent chromium has gradually replaced traditional hexavalent chromium and been widely used. However, the trivalent chromium passivation solution is unstable, and the trivalent chromium in it is extremely easy to be oxidized to form hexavalent chromium, and the corrosion resistance of the trivalent chromium passivation solution 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. The compactness of the film layer is improved through the interpenetrating network of linear and network structures, and a stable passivation film is formed through redox reaction; the physical barrier property is enhanced by filling the resin network with nanoparticles; the silane coupling agent (such as KH550 / KH560) enhances the interfacial adhesion and reduces film layer defects. In patent document CN118389039A, phosphoric acid is used instead of ammonium vanadate, and nano-silica is added. However, the chromium-free fingerprint-resistant agent in the above documents needs to be coated on the surface of hot-dip galvanized steel sheets, and a conversion film is formed after baking at 280 °C for 5 - 30 s.

[0007] Patent document CN118389981A discloses that the components of the chromium-free passivator include zinc oxide, aluminum oxide, and magnesium oxide. After baking and curing in an oven at 200 - 300 °C, a chromium-free passivation film with a thickness of 0.3 - 0.6 μm is formed; the chromium-free fingerprint-resistant treatment solution uses lithium silicate with a mass ratio of 9% - 13%, glycidoxypropyltrimethoxysilane with a mass ratio of 4% - 7%, and 1,2-bis-(trimethoxy)ethane with a mass ratio of 3% - 6% as raw materials. After baking and curing in an oven at 400 - 500 °C, a chromium-free fingerprint-resistant film with a thickness of 1 - 2 μm is formed.

[0008] Patent document CN113527994A proposes an immersion-type low-temperature drying chromium-free fingerprint-resistant agent, whose raw materials include anionic polyurethane emulsion, crosslinking curing agent, siloxane, phosphate, fluorosilicic acid, ammonium citrate, etc., and the pH value is 3.8 - 5.5; it is cured at low temperature by soaking for 2 - 7 min at normal temperature or heating, breaking through the limitation of traditional high-temperature curing (>120 °C). However, there are no test data in this document, and its performance cannot be judged. Summary of the Invention

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

[0010] In order to achieve the above purpose, the technical solution of the present invention is: A chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to the present invention comprises the following elements in mass fractions: sodium molybdate and / or sodium tungstate: 1% - 5%, phytic acid-chitosan complex: 0.5% - 2%, chelating 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 is deionized water; it also comprises an auxiliary agent accounting for 0.1% - 3% of the total mass of the above raw materials; Further, the sodium molybdate and / or sodium tungstate is the main corrosion inhibitor, which forms a passivation film by oxidizing the metal surface. Sodium molybdate is preferably selected as it has lower toxicity and higher passivation efficiency.

[0011] Further, the phytic acid-chitosan complex is a bio-based corrosion inhibitor that chelates metal ions and inhibits local corrosion. Its preparation method is as follows: (1) Dissolve chitosan in a citric acid solution to make a chitosan solution with a concentration of 0.005 - 0.05 g / L, stir until completely dissolved, and adjust the pH value of the chitosan solution to 4 - 6; (2) Drop a phytic acid solution with a concentration of 0.005 - 0.01 mol / L into the chitosan solution according to the molar ratio of phytic acid:chitosan = (2 - 2.15):1, and carry out an ion cross-linking reaction at 45 - 60 °C for 3 - 6 h to obtain a chitosan-phytic acid complex.

[0012] Preferably, the mass concentration of the citric acid solution is 0.5% - 3%; the pH value of the chitosan solution is adjusted using citric acid and / or ammonia water.

[0013] Further, the chelating titanate coupling agent is an alcoholamine waterborne chelating titanate coupling agent (such as a chelate solution of bis(dioctyloxyphosphate)ethylene titanate and triethanolamine) and / or a quaternary ammonium salt of a chelating phosphate titanate coupling agent, which can improve the adhesion between the coating and the substrate and promote the dispersion of nanoparticles. For example, Kenrich KR-238T waterborne titanate coupling agent (domestic corresponding brand HY311W).

[0014] Further, the fluorosilane-modified nano-SiO₂ is nano-silica whose surface is modified by fluorosilane, which improves the hydrophobicity and dispersibility, can enhance the physical barrier of the coating, and fill the micro-defects of the coating. It can be prepared by the method disclosed in Document CN104445218A, or directly use commercially available products, such as hydrophobic fumed silica of models HB-132, HB-151, HB-612, etc. from Hubei Huifu Nanomaterials Co., Ltd.

[0015] Further, the aqueous hyperbranched polyurethane resin is a hydroxyl-terminated aqueous hyperbranched polyurethane resin prepared from dimethylolpropionic acid or dimethylolbutyric acid, diisocyanate, and diethanolamine, with a number average molecular weight of 3,000 to 12,000 and a solid content of about 40%. The preparation method can refer to the literature [Mei Jingang, Yang Jianjun, etc. Synthesis and Characterization of Hyperbranched Waterborne Polyurethane. Polymer Materials Science and Engineering, 2012, 28(6): 16-19.], or commercially available similar products can also be used.

[0016] Further, the auxiliary agent is an auxiliary agent for assisting processing or increasing auxiliary functions, such as one or more of an organosilicon leveling agent (preferably BYK-333 of BYK), an organosilicon defoaming agent (such as BYK-024 of BYK), an anti-ultraviolet oxidant, etc., which can improve the surface smoothness of the coating, easy wiping of fingerprints, yellowing resistance, etc.

[0017] The preparation method of a chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to the present invention includes the following steps: S1: Add the aqueous hyperbranched polyurethane resin to a reaction kettle, add deionized water for forming a premixed slurry, and stir at a low speed for pre-dilution; S2: In another container, sequentially add the fluorosilane-modified nano-SiO2, chelating titanate coupling agent, and deionized water for adjusting the viscosity of the mixture, and pre-disperse at a high shear for 20 to 50 min until the nanoparticle D 50 ≤100 nm to obtain a nano-slurry; S3: Mix the sodium molybdate or / and 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 in step S2 to the resin system in step S1, stir at a medium speed for 20 to 50 min at a controlled temperature of 20 to 50 °C, add the corrosion inhibitor mixture in step S3, and continue to stir for 10 to 30 min; adjust the pH value of the system to 4.5 to 6.0; S5: Add the remaining deionized water, stir for 10 to 30 min; add the auxiliary agent and stir evenly at a low speed, defoam, and screen to obtain the chromium-free fingerprint-resistant coating agent for galvanized steel sheets.

[0018] 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; the deionized water added in step S3 accounts for 10% to 25% of the total volume of deionized water.

[0019] The high-shear predispersion rate of the present invention is 3000 - 6000 rpm, the low-speed stirring rate is 100 - 700 rpm, and the medium-speed stirring rate is 800 - 1500 rpm.

[0020] In step S4, citric acid and / or ammonia water are used to adjust the pH of the system, and the pH value of the system is controlled at 4.5 - 6.0 to ensure the chelating activity of phytic acid and avoid the hydrolysis and precipitation of molybdate.

[0021] The application of a chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to the present invention is coated on the surface of galvanized steel sheets, and the curing conditions are: curing at 120 - 150 °C for 25 - 50 s (preferably 30 - 40 s).

[0022] Compared with the prior art, the positive effects of the present invention are: The present invention uses molybdate to generate MoO4 2- passivation film on the metal surface. Phytic acid chelates free metal ions of molybdate / tungstate through phosphate groups to inhibit micro-area corrosion; chitosan adsorbs on the metal surface through amino groups to form a dense protective layer; the chelating titanate coupling agent bridges hyperbranched polyurethane resin and fluorosilane-modified nano-SiO2 to enhance the interfacial bonding force and reduce the porosity; fluorosilane-modified nano-SiO2 and oxidized polyethylene wax provide better water resistance and water repellency; through the above process optimization, it can be ensured that the chromium-free fingerprint-resistant coating agent of the present invention has excellent fingerprint resistance, salt spray resistance, high adhesion and environmental friendliness, meeting the industrial requirements of the surface treatment of galvanized steel sheets, and the present invention uses a combination of bio-based corrosion inhibitors to avoid the environmental risk problems existing in the prior art. Detailed embodiments

[0023] Those of ordinary skill in the art in this technical field should recognize that this embodiment is only used to illustrate the present invention and is not used as a limitation to the present invention. As long as changes and modifications are made to the embodiment within the scope of the present invention, they can be within the scope of the claims of the present invention.

[0024] Examples 1 - 5 and Comparative Examples 1 - 3 The preparation methods of the chromium-free fingerprint-resistant coating agents for galvanized steel sheets in Examples 1 - 5 and Comparative Examples 1 - 3 include the following steps: S1: Add aqueous hyperbranched polyurethane resin (molecular weight 5100, solid content 40%, average particle size of latex particles 36 μm) to the reaction kettle, control the temperature at 25 °C; add deionized water accounting for 30% of the total volume, and stir at a low speed of 200 rpm for 10 min for pre-dilution; S2: In another container, sequentially add the fluorosilane-modified nano-SiO₂ (HB-132 of Huifu Nano Materials), chelating titanate coupling agent (Kenrich KR-238T water-based titanate coupling agent), and deionized water accounting for 30% of the total volume, and perform high-shear predispersion at 5000 rpm for 20 - 50 min until the nanoparticle D 50 <100 nm, to obtain a nano-slurry; S3: Mix sodium molybdate and / or sodium tungstate, phytic acid-chitosan complex, and oxidized polyethylene wax (particle size <5 μm), add deionized water accounting for 20% of the total volume, and perform ultrasonic-assisted dispersion at 40 kHz for 15 min until completely dissolved to obtain a corrosion inhibitor mixture; S4: Slowly add the nano-slurry from step S2 into the resin system of step S1, control the temperature at 35°C and stir at medium speed (800 rpm × 20 min), 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; S5: Add the remaining deionized water and stir for 10 min; add 0.1% organosilicon defoamer (such as BYK-024) based on the total amount of all the above raw materials, stir at low speed (300 rpm) for 5 min to remove bubbles; sieve through a 400-mesh sieve to remove undispersed particles to obtain a chromium-free fingerprint-resistant coating agent for galvanized steel sheets.

[0025] Among them, the preparation method of the phytic acid-chitosan complex is as follows: (1) Dissolve chitosan in a citric acid solution to make a chitosan solution with a concentration of 0.01 g / L, stir at 40°C for 2 h until completely dissolved, and adjust the pH value of the chitosan solution to 5.0 ± 0.2 with citric acid / ammonia; (2) Drop a phytic acid solution with a concentration of 0.008 mol / L into the chitosan solution according to the molar ratio of phytic acid:chitosan = 2:1, and perform an ion crosslinking reaction at 50°C for 4.3 h to obtain a chitosan-phytic acid complex.

[0026] 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.

[0027] Table 1 Raw material ratios of Examples 1 - 5 and Comparative Examples 1 - 3, unit wt% Test Experimental Example Scrape the fingerprint-resistant coating agents of Examples 1 - 5 and Comparative Examples 1 - 3 onto galvanized steel sheets respectively, and cure at 130°C for 30 s; then measure the fingerprint resistance, adhesion, water contact angle, salt spray resistance, and damp heat resistance respectively, and the results are listed in Table 2.

[0028] Fingerprint resistance: Wipe 50 times with artificial sweat (formula reference ISO 3160-2:2015, containing 0.5% sodium laurate), and it is required that there is no residue, no corrosion, and the gloss change < 5%.

[0029] Adhesion: Determined by the cross-cut method according to GB / T 9286-2021, and there is no peeling after cutting with a cross-cut knife.

[0030] Contact angle: According to ASTM D7334 static water droplet method.

[0031] Salt spray resistance (NSS): According to GB / T 1771-2007, there are 6 specimens for each sample. In the test chamber at a temperature of 35°C, place the prepared sample panel in the salt spray chamber. At a pressure barrel temperature of 47°C, spray with 5wt% sodium chloride brine with a pH of 7.0 at an air pressure of 1.0 kgf / cm 2 ; After spraying the sample panel for 150h and 360h, take out 3 specimens each, wash with water and dry, and observe whether there is white rust or red rust on the surface of the sample panel and the percentage of the corrosion area in the total area of the product.

[0032] Damp heat resistance test: Judge the damp heat resistance effect of its passivation according to the magnitude of the color difference △E value of the sample panel before and after damp heat; conduct heat-resistant yellowing test respectively, and judge the heat resistance effect of its passivation according to the magnitude of the color difference △E value of the sample panel before and after heat resistance: In the test chamber at 50°C, place the prepared sample panel in the test chamber, and place it in an environment with a humidity of 95% for 120h. Then take out the sample panel, wash with water and dry, and test the color difference △E value of the sample panel before and after damp heat.

[0033] Table 2 Coating performance verification of Examples 1-5 and Comparative Examples 1-3 As can be seen from Table 2, the chromium-free fingerprint-resistant coating agent of Examples 1-5 of the present invention has excellent fingerprint resistance, high adhesion and environmental protection. There is no white rust under neutral salt spray for 150h, no red rust under 360h, and the white rust area < 5%, 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, The raw materials include the following mass fractions: sodium molybdate and / or sodium tungstate: 1% - 5%, phytic acid-chitosan complex: 0.5% - 2%, chelating 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 is deionized water; an auxiliary agent accounting for 0.1% - 3% of the total mass of the above raw materials is also included.

2. The chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 1, wherein The preparation method of the phytic acid-chitosan complex is as follows: (1) Dissolve chitosan in a citric acid solution to make a chitosan solution with a concentration of 0.005 - 0.05 g / L, stir until completely dissolved, and adjust the pH value of the chitosan solution to 4 - 6; (2) Drop a phytic acid solution with a concentration of 0.005 - 0.01 mol / L into the chitosan solution according to the molar ratio of phytic acid:chitosan = (2 - 2.15):1, and carry out an ion cross-linking reaction at 45 - 60 °C for 3 - 6 h to obtain a chitosan-phytic acid complex.

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

4. The chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 1, characterized in that, The chelating titanate coupling agent is an alcoholamine waterborne chelating titanate coupling agent and / or a quaternary ammonium salt of chelating phosphate titanate coupling agent; the fluorosilane-modified nano-SiO₂ is nano-silica whose surface is modified by fluorosilane.

5. The chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 1, characterized in that, 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%.

6. The chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 1, wherein The auxiliary agent is one or more of an organosilicon leveling agent, an organosilicon defoaming agent, and an anti-ultraviolet oxidant.

7. The preparation method of the chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to any one of claims 1 to 6, characterized in that, It includes the following steps: S1: Add the waterborne hyperbranched polyurethane resin to a reaction kettle, add deionized water for forming a premixed slurry, and stir at a low speed for pre-dilution to obtain a resin system; S2: In another container, sequentially add the fluorosilane-modified nano-SiO2, chelating titanate coupling agent, and deionized water for adjusting the viscosity of the mixture, and perform high-shear predispersion for 20 to 50 min until the nanoparticle D 50 ≤ 100 nm to obtain a 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 in step S2 to the resin system in step S1, stir at a medium speed for 20 - 50 min at a controlled temperature of 20 - 50 °C, add the corrosion inhibitor mixture in step S3, and continue stirring for 10 - 30 min; adjust the pH value of the system to 4.5 - 6.0; S5: Add the remaining deionized water, stir for 10 - 30 min; add the auxiliary agent and stir evenly at a low speed, defoam, and screen to obtain the chromium-free fingerprint-resistant coating agent for galvanized steel sheets.

8. The preparation method of the chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to claim 7, characterized in that, In step S4, the pH of the system is adjusted using citric acid and / or ammonia water, and the pH value of the system is controlled at 4.5 - 6.

0.

9. Use of the chromium-free fingerprint-resistant coating agent for galvanized steel sheets according to any one of claims 1 to 6, characterized in that, Coated on the surface of the galvanized steel sheet, the curing conditions are: curing at 120 - 150 °C for 25 - 50 s.

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