A bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent and its application

Abstract

This invention discloses a bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent, comprising the following components by mass fraction: castor oil-based polyurethane acrylate: 35%–42%, waterborne epoxy acrylate: 18%–22%, itaconic acid dialkyl ester: 12%–15%, diethanolamine: 0.8%–1.2%, modified chitosan-cerium complex: 2%–3.5%, fluorosilane-modified nano-SiO2: 2%–4%, vinyl silane: 4%–6%, photoinitiator: 3.5%–5%, dispersant: 1.5%–2.5%, organosilicon leveling agent: 1%–1.5%, defoamer: 0.3%–0.5%, preservative: 0.2%–0.5%, with the balance being deionized water. The coating agent of this invention exhibits excellent impact resistance, fingerprint resistance, salt spray resistance, high adhesion, and environmental friendliness, meeting the industrial requirements for surface treatment of galvanized steel sheets.

Description

Technical Field

[0001] This invention belongs to the field of material surface treatment and metal protection technology, specifically relating to a bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent and its application. 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 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 cannot match that 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 bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent and its application, in order to solve the problems of high toxicity, high curing temperature, and poor corrosion resistance in the existing technology, forming a multi-layered anti-corrosion barrier, and taking 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] This invention discloses a bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent, comprising the following components by mass fraction: castor oil-based polyurethane acrylate: 35%–42%, waterborne epoxy acrylate: 18%–22%, dialkyl itaconic acid ester: 12%–15%, diethanolamine: 0.8%–1.5%, modified chitosan-cerium complex: 2%–3.5%, fluorosilane-modified nano-SiO2: 2%–4%, vinyl silane: 4%–6%, photoinitiator: 3.5%–5%, dispersant: 1.5%–2.5%, organosilicon leveling agent: 1%–1.5%, defoamer: 0.3%–0.5%, preservative: 0.2%–0.5%, with the balance being deionized water; the modified chitosan-cerium complex is obtained by modifying chitosan with citric acid to produce carboxylated chitosan, and then reacting it with cerium... 3+ The resulting complex;

[0012] The castor oil-based polyurethane acrylate (CO-PUA) is a bio-based flexible resin that enhances impact resistance and low-temperature toughness. It has a number-average molecular weight of 10,000–13,000 and a solid content of 50%–60%. It can be prepared using the method described in reference 1 [Rao Ximei, Xie Wei, et al. Preparation and performance study of castor oil-based polyurethane polyacrylate composite emulsion [J]. Polyurethane Industry, 2006, 21(6): 22-25.], or it can be directly purchased from commercially available products of the same type.

[0013] The waterborne epoxy acrylate has an acid value of 100-150 mgKOH / g, a viscosity of 12000-16000 mPa·S at 25°C, and a solid content of 68%-72%. For example, S-953 waterborne epoxy modified acrylate produced by Xuzhou Peize New Material Co., Ltd.

[0014] The itaconic acid dialkyl ester is at least one of itaconic acid diethyl ester, itaconic acid methyl ethyl ester, and itaconic acid dibutyl ester, preferably itaconic acid diethyl ester; it is a bio-based low-viscosity monomer containing double bonds and carboxyl groups, which participates in cross-linking and enhances metal chelation ability.

[0015] The modified chitosan-cerium complex is prepared as follows: (1) Chitosan is dissolved in 1% acetic acid solution and stirred until completely dissolved; citric acid and EDC / NHS catalyst are added, wherein: the molar ratio of citric acid to chitosan is (1.95~2.2):1, and the molar ratio of citric acid to EDC and NHS is (0.95~1.05):1:(0.45~0.55), and the reaction is carried out at 40~55℃ for 18~24h; unreacted citric acid is removed, and carboxylated chitosan is obtained by freeze drying; (2) Carboxylated chitosan is dispersed in deionized water, and Ce(NO3)3 solution is added. 3+The molar ratio of chitosan to carboxylated chitosan is 1:(1.9–2.2); the pH is adjusted to 5–6, and the reaction is stirred at 55–65°C for 5–8 hours; the solid is separated by centrifugation and freeze-dried to obtain a pale yellow powder. The EDC / NHS catalyst is a mixture of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS). In this process, chitosan is modified with citric acid to form carboxylated chitosan, whose carboxyl groups react with Ce... 3+ Chitosan forms coordination bonds via -NH2 / -COO - Adsorption of corrosive media, Ce 3+ The metal surface is oxidized to form a CeO2 / Ce(OH)3 passivation film, which gives the Cs-Ce complex both organic adsorption and inorganic passivation corrosion inhibition functions.

[0016] 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 by the method disclosed in literature 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] The vinyl silane is at least one of vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, and vinyldimethylethoxysilane; preferably vinyltrimethoxysilane, which has both dilution and silane coupling functions, and forms -Si-O-Zn bonds in situ, thereby increasing the wear resistance and corrosion resistance of the galvanized steel sheet.

[0018] The photoinitiator is a compound of 3%–4% bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) and 0.5%–1% ketone photoinitiator, wherein the ketone photoinitiator is photoinitiator 1173 (2-phenyl-2-hydroxymethyl ketone) and / or photoinitiator ITX (2-isopropylthioxanthraquinone). BAPO, when matched with 395 nm UV-LED light, can promote efficient deep curing of the film, while the ketone photoinitiator acts as a surface curing accelerator, synergistically expanding the absorption range with BAPO, improving the speed and strength of film curing, and reducing initiator residue.

[0019] The dispersant is a high molecular weight block copolymer solution containing pigment affinity groups, such as TEGO Dispers 755W or / and BYK-190; the silicone leveling agent is, for example, BYK-349 or / and BYK-346; the defoamer is, for example, Surfynol MD-20 or TEGO Foamex 810; and the preservative is, for example, BIT-20 produced by Foshan Liyuan Chemical Co., Ltd.

[0020] The preparation method of the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent of the present invention includes the following steps: D1, adding castor oil-based polyurethane acrylate and slowly adding water-based epoxy acrylate in a vacuum disperser, premixing for 20-30 min at a stirring paddle linear speed of 6-8 m / s; D2, adding itaconic acid dialkyl ester in batches, with an interval of 3-5 min between each addition; simultaneously adding fluorosilane-modified nano-SiO2; D3, adding a mixture of modified chitosan-cerium complex and dispersant, and stirring evenly; sequentially adding vinyl silane and diethanolamine, and stirring evenly; adding organosilicon leveling agent, defoamer and preservative; D4, adding photoinitiator under a red safety lamp, with the system temperature ≤30℃; D5, adjusting to a Ford cup 4 viscosity of 25-30 s / 25℃ with deionized water; D6, after vacuum degassing, allowing to stand and mature at 15-28℃ for 20-30 h.

[0021] The vacuum degree of step D1 is -0.08 to -0.065 MPa, and the temperature is 15 to 35°C.

[0022] Preferably, the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent of the present invention further includes a pretreatment agent, which is composed of the following components in the indicated mass fractions: 1.5%–2% phytic acid, 0.5%–1% γ-aminopropyltriethoxysilane (KH-550), 0.1%–0.3% zinc gluconate, 0.3%–0.8% nonionic surfactant (e.g., Triton X-100), and the balance deionized water. After mixing evenly, the pH is adjusted to 5.0–6.0 using diethanolamine.

[0023] The preparation method of the pretreatment agent includes the following steps: adding 80% of the formula amount of deionized water to a reaction vessel, slowly adding phytic acid and stirring; adding γ-aminopropyltriethoxysilane and zinc gluconate sequentially under stirring; adding nonionic surfactant dropwise to the reaction vessel; adding the remaining deionized water to the total amount; adjusting the pH to 5.2-5.5 with diethanolamine; and filtering.

[0024] The application of the bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent described in this invention includes the following steps:

[0025] S1, Clean and dry the galvanized steel sheet substrate;

[0026] S2, immerse the substrate in the pretreatment agent for 80-100s, control the surface passivation film thickness to 0.8-1.2μm; bake at 78-85℃ for 2-3min, then bake at 105-110℃ for 1-2min, and air cool to room temperature;

[0027] S3, the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent is roller-coated onto the substrate treated in S2, with a coating thickness of 13-17 μm and a coating line speed of 12-15 m / min.

[0028] S4 is pre-cured by irradiation with a 395nm LED lamp at a temperature ≤50℃; then cured by irradiation with a mercury lamp at a peak temperature ≤75℃.

[0029] S5. Place the galvanized steel sheet in a constant temperature chamber at 65-72℃ for 2-5 hours to cure.

[0030] Compared with the prior art, the positive effects of the present invention are:

[0031] This invention uses castor oil-based polyurethane acrylate as the main film-forming agent. Its bio-based long-chain structure endows the coating with excellent impact resistance. The hydrophobic castor oil segments can reduce water vapor permeability. Combined with the high rigidity of waterborne epoxy acrylate, the two cross-link to ensure the optimal balance between the coating's toughness and hardness. Itaconic acid dialkyl ester is used as an active diluent, with high double bond density, which can increase the photocuring rate by 20%. The modified chitosan-cerium complex's chitosan molecular chains fill the coating's micropores, enhancing the coating's physical barrier. 3+ Can be combined with Zn in pretreatment agents 2+ A [Zn(OH)2·CeOx] chemical passivation film is formed; vinyltriethoxysilane is hydrolyzed, and the silanol group can condense with the -OH group of the zinc plating layer, while the vinyl group participates in the curing process; all of these further enhance the corrosion resistance of the coating. Through the above process optimization, the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent of the present invention can be ensured to have excellent impact resistance, fingerprint resistance, salt spray resistance, high adhesion, and environmental friendliness, meeting the industrial requirements for surface treatment of galvanized steel sheets. Detailed Implementation

[0032] 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.

[0033] Examples 1-5 and Comparative Examples 1-4

[0034] The components (mass fraction) of the pretreatment agents of Examples 1-5 and Comparative Examples 1-3 are listed in Table 1, and the components (mass fraction) of the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent are listed in Table 2.

[0035] The preparation method of modified chitosan-cerium complex is as follows: (1) Dissolve 20g of chitosan in 1% acetic acid solution (2000 mL) and stir until completely dissolved; add citric acid and EDC / NHS catalyst, wherein: the molar ratio of citric acid to chitosan is 2:1, and the molar ratio of citric acid to EDC and NHS is 1:1:0.5, and react at 50℃ for 22h; remove unreacted citric acid by dialysis with deionized water, and freeze-dry to obtain carboxylated chitosan; (2) Disperse 10g of carboxylated chitosan in 500mL of deionized water, add 0.1mol / L Ce(NO3)3 solution, Ce 3+ The molar ratio of the carboxylated chitosan to the solid was 1:2; the pH was adjusted to 5.5, and the mixture was stirred at 60°C for 6 hours; the solid was separated by centrifugation and freeze-dried to obtain a pale yellow powder.

[0036] The pretreatment agent is prepared as follows: 80% of the formula amount of deionized water is added to the reactor, phytic acid is slowly added and stirred, and the feeding time is ≥15min; γ-aminopropyltriethoxysilane and zinc gluconate are added sequentially under stirring; the nonionic surfactant is diluted 3 times with 10% ethanol solution and then added dropwise to the reactor; the remaining deionized water is added to the total amount; the pH is adjusted to 5.4 with diethanolamine, and then homogenized by ultrasonic treatment at 40kHz for 30min; and finally filtered through a 500-mesh nylon screen.

[0037] The preparation method of the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent is as follows: D1, castor oil-based polyurethane acrylate (prepared according to the method in Reference 1, with a number average molecular weight of 12300 and a solid content of 54%) is added to a vacuum disperser, and water-based epoxy acrylate (S-953, viscosity of 13000 mPa·s at 25℃, acid value of 125 mg KOH / g, and solid content of 70%) is slowly added. Premixing is performed for 25 min at a paddle speed of 7 m / s, with a vacuum degree of -0.08 and a temperature of 25℃ in the vacuum disperser; D2, itaconic acid dialkyl ester is added in three batches, with an interval of 3 min between each addition. In; simultaneously add fluorosilane-modified nano-SiO2; D3, pre-grind the modified chitosan-cerium complex and dispersant mixture using a three-roll mill with a gap of 20μm, then add it to the system and stir evenly; then add vinylsilane and diethanolamine step by step and stir evenly; add organosilicon leveling agent, defoamer and preservative; stir for 10 minutes between each addition; D4, add photoinitiator under a red safety lamp, system temperature ≤30℃; D5, adjust to a Ford cup 4 viscosity of 28s / 25℃ with deionized water; D6, after vacuum degassing, let it stand and mature for 25 hours at 15~28℃.

[0038] The application of bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agents includes the following steps:

[0039] S1, Substrate cleaning: Immerse the galvanized steel sheet substrate in a 5% NaOH solution at 50°C for 3 minutes with a solution flow rate of 0.5 m / s; perform a three-stage countercurrent water rinse, using water with a conductivity of <50 μS / cm; and then dry it in hot air at 120°C for 3 minutes.

[0040] S2, Substrate Pretreatment: After the raw materials of the pretreatment agent are mixed evenly, the pH is adjusted to 5.5 using diethanolamine; the substrate is immersed in the pretreatment agent at 40°C for 90s, and the surface passivation film thickness is controlled to be 0.8~1.2μm; then it is baked at 80°C for 2min, and then baked at 110°C for 1.5min, and then air-cooled to room temperature;

[0041] S3, a bio-based environmentally friendly chromium-free fingerprint-resistant coating agent is roller-coated onto the substrate treated by S2, with a coating thickness of 15μm and a coating line speed of 13m / min;

[0042] S4 is pre-cured using 395nm LED light irradiation, with a radiation energy of 150mJ / cm². 2 Temperature ≤ 50℃; then cured by mercury lamp irradiation with a radiation energy of 850 mJ / cm². 2 Peak temperature ≤75℃;

[0043] S5. Place the galvanized steel sheet in a constant temperature chamber at 70℃ for 2 hours to cure.

[0044] Comparative Examples 1-4 are comparative examples of Example 4. In Comparative Example 1, no modified chitosan-cerium complex was added; instead, all nano-SiO2 was modified with fluorosilane. In Comparative Example 2, the amount of castor oil-based polyurethane acrylate added was relatively low, while the amount of waterborne epoxy acrylate added was relatively high. In Comparative Example 3, the amounts of itaconic acid dialkyl ester and vinyltrimethoxysilane added were relatively low. Comparative Example 4 did not include a pretreatment agent and had no substrate pretreatment step.

[0045] Table 1. Pretreatment agent ratios for Examples 1-5 and Comparative Examples 1-3, in wt%.

[0046]

[0047] Table 2. Fingerprint-resistant coating agent formulations for Examples 1-5 and Comparative Examples 1-3, in wt%.

[0048]

[0049] Test Experiment Example

[0050] The pretreatment agents and fingerprint-resistant coating agents of Examples 1-5 and Comparative Examples 1-4 were used to treat galvanized steel sheets, and the pencil hardness, reverse impact strength, fingerprint resistance, adhesion, water contact angle, salt spray resistance and damp heat resistance were measured respectively. The results are listed in Table 3.

[0051] Pencil hardness: determined according to ASTM D3363:2005.

[0052] Reverse impact strength: determined in accordance with GB / T 1732-2020.

[0053] 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%.

[0054] Adhesion: Tested according to GB / T 9286-2021 cross-cut test, no adhesion was observed after cutting with a cross-cut tester.

[0055] Contact angle: according to ASTM D7334-08:2013 static water droplet method.

[0056] Salt spray resistance (NSS): According to GB / T 1771-2007, nine 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 240h, 360h and 480h, three samples were taken out each time, washed with water and dried, and the surface of the samples was observed for white rust or red rust, and the percentage of corrosion area to the total area of ​​the product was observed.

[0057] Table 3. Coating performance verification of Examples 1-5 and Comparative Examples 1-4

[0058]

[0059] As shown in Table 3, the chromium-free fingerprint-resistant coating agents of Examples 1 to 5 of the present invention have excellent reverse impact resistance, fingerprint resistance, high adhesion and environmental friendliness. They show no white rust after 360 hours of neutral salt spray and no red rust after 480 hours, with the white rust area being <5%, thus meeting the industrial requirements for surface treatment of galvanized steel sheets.

[0060] Comparative Example 1 did not add modified chitosan-cerium complex, but instead used fluorosilane modified nano-SiO2. Although the hydrophobic effect was excellent, the hardness increased and the composite passivation film could not be formed, resulting in a decrease in reverse impact resistance and salt spray resistance.

[0061] In Comparative Example 2, the amount of castor oil-based polyurethane acrylate added was too low, while the amount of waterborne epoxy acrylate added was too high. Although the fingerprint resistance was good, the adhesion to the galvanized steel plate substrate decreased, resulting in a decrease in reverse impact resistance and salt spray resistance.

[0062] Dialkyl itaconic acid, as an reactive diluent, not only reduces the system viscosity but also enhances the photocuring rate due to its double bonds. In Comparative Example 3, the low level of dialkyl itaconic acid leads to increased viscosity of the coating system, poor leveling, and defects such as dark streaks and uneven coating. It also reduces the photocuring rate, ultimately resulting in substandard fingerprint resistance. Vinyltrimethoxysilane can improve the coating's hydrolysis resistance, and its vinyl groups also participate in film formation during curing. Low levels of vinyltrimethoxysilane decrease the coating's fingerprint and salt spray resistance.

[0063] Comparative Example 4 did not include a pretreatment agent and had no substrate pretreatment, so it could not form an inner passivation film, resulting in a sharp decline in corrosion resistance. White rust appeared within 240 hours of neutral salt spray and red rust appeared within 360 hours.

Claims

1. A bio-based, environmentally friendly, chromium-free, fingerprint-resistant composite coating agent, characterized in that, It is composed of a pretreatment agent and a bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent; the bio-based, environmentally friendly, chromium-free, fingerprint-resistant coating agent comprises the following components by mass fraction: castor oil-based polyurethane acrylate: 35%–42%, waterborne epoxy acrylate: 18%–22%, itaconic acid dialkyl ester: 12%–15%, diethanolamine: 0.8%–1.5%, modified chitosan-cerium complex: 2%–3.5%, fluorosilane-modified nano-SiO2: 2%–4%, vinyl silane: 4%–6%, photoinitiator: 3.5%–5%, dispersant: 1.5%–2.5%, organosilicon leveling agent: 1%–1.5%, defoamer: 0.3%–0.5%, preservative: 0.2%–0.5%, with the balance being deionized water; The modified chitosan-cerium complex is produced by modifying chitosan with citric acid to generate carboxylated chitosan, which is then combined with Ce. 3+ The preparation method of the complex is as follows: (1) Dissolve chitosan in 1% acetic acid solution and stir until completely dissolved; add citric acid and EDC / NHS catalyst, wherein: the molar ratio of citric acid to chitosan is (1.95~2.2):1, and the molar ratio of citric acid to EDC and NHS is (0.95~1.05):1:(0.45~0.55), and react at 40~55℃ for 18~24h; remove unreacted citric acid and freeze-dry to obtain carboxylated chitosan; (2) Disperse carboxylated chitosan in deionized water, add Ce(NO3)3 solution, Ce 3+ The molar ratio of the carboxylated chitosan to the carboxylated chitosan was 1:(1.9-2.2); the pH was adjusted to 5-6, and the mixture was stirred at 55-65℃ for 5-8 hours; the solid was separated by centrifugation and freeze-dried to obtain a light yellow powder; The pretreatment agent consists of the following components in the indicated mass fractions: 1.5%–2% phytic acid, 0.5%–1% γ-aminopropyltriethoxysilane, 0.1%–0.3% zinc gluconate, 0.3%–0.8% nonionic surfactant, and the balance deionized water; after mixing evenly, the pH is adjusted to 5.0–6.0 using diethanolamine.

2. The bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to claim 1, characterized in that, The castor oil-based polyurethane acrylate has a number average molecular weight of 10,000–13,000 and a solid content of 50%–60%; the waterborne epoxy acrylate has an acid value of 100–150 mg KOH / g, a viscosity of 12,000–16,000 mPa·s at 25°C, and a solid content of 68%–72%; the itaconic acid dialkyl ester is at least one of diethyl itaconic acid, methyl ethyl itaconic acid, and dibutyl itaconic acid.

3. The bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to claim 1, characterized in that, The fluorosilane-modified nano-SiO2 is nano-silica with a surface modified by fluorosilane; the vinylsilane is at least one of vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, and vinyldimethylethoxysilane.

4. The bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to claim 1, characterized in that, The photoinitiator is a compound of 3% to 4% bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide and 0.5% to 1% ketone photoinitiator; the ketone photoinitiator is photoinitiator 1173 and / or photoinitiator ITX.

5. The bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to any one of claims 1 to 4, characterized in that, The preparation method of the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent includes the following steps: D1, adding castor oil-based polyurethane acrylate and slowly adding water-based epoxy acrylate in a vacuum disperser, premixing for 20-30 min at a stirring paddle speed of 6-8 m / s; D2, adding itaconic acid dialkyl ester in batches, with an interval of 3-5 min between each addition; simultaneously adding fluorosilane-modified nano-SiO2; D3, adding a mixture of modified chitosan-cerium complex and dispersant, stirring until homogeneous; sequentially adding vinyl silane and diethanolamine, stirring until homogeneous; adding organosilicon leveling agent, defoamer, and preservative; D4, adding a photoinitiator under a red safety lamp, with the system temperature ≤30℃; D5, adjusting to a Forte 4 cup viscosity of 25-30 s / 25℃ with deionized water; D6, after vacuum degassing, allowing to stand and mature at 15-28℃ for 20-30 h.

6. The bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to claim 5, characterized in that, The vacuum level in step D1 is -0.08 to -0.065 MPa, and the temperature is 15 to 35°C.

7. The bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to claim 1, characterized in that, The preparation method of the pretreatment agent includes the following steps: adding 80% of the formula amount of deionized water to a reaction vessel, slowly adding phytic acid and stirring; adding γ-aminopropyltriethoxysilane and zinc gluconate sequentially under stirring; adding nonionic surfactant dropwise to the reaction vessel; adding the remaining deionized water to the total amount; adjusting the pH to 5.2-5.5 with diethanolamine; and filtering.

8. The application of the bio-based environmentally friendly chromium-free fingerprint-resistant composite coating agent according to any one of claims 1 to 7, characterized in that, Includes the following steps: S1, Clean and dry the galvanized steel sheet substrate; S2, immerse the substrate in the pretreatment agent for 80-100s, controlling the surface passivation film thickness to 0.8-1.2μm; then bake at 78-85℃ for 2-3min, then at 105-110℃ for 1-2min, and air-cool to room temperature; S3, the bio-based environmentally friendly chromium-free fingerprint-resistant coating agent is roller-coated onto the substrate treated in S2, with a coating thickness of 13-17 μm and a coating line speed of 12-15 m / min. S4 is pre-cured by irradiation with a 395nm LED lamp at a temperature ≤50℃; then cured by irradiation with a mercury lamp at a peak temperature ≤75℃. S5. Place the galvanized steel sheet in a constant temperature chamber at 65-72℃ for 2-5 hours to cure.

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