Surface treating agent for improving cohesion of conversion film on metal surface

Abstract

The invention discloses a surface treating agent for improving cohesion of a conversion film on a metal surface, and each liter of the surface treating agent comprises the following components by mass: 75-200mg of hexafluorozirconic acid or hexafluorotitanic acid, 10-100mg of nitrate, 10-50mg of an organic coupling agent, 20-50mg of a dispersing agent, 50-200mg of dopamine coated nano oxide particles and the balance of water. The obtained conversion film layer is high in cohesion strength, the compactness of the conversion film layer can be maintained under the high-temperature condition, stress release during dehydration is promoted, and therefore cracking and falling of the conversion film layer after high temperature are resisted; and after the conversion film and the organic coating are jointly heated and cured, the adhesive force of the organic coating is obviously improved, so that the long-term durability of the whole coating is ensured.

Description

Technical Field

[0001] The invention relates to a surface treatment agent for improving the cohesive force of a metal surface conversion film. Background Art

[0002] Zirconium / titanium chemical conversion coatings are located between the metal substrate and the organic coating. Their excellent interfacial compatibility allows for a strong bond between the organic coating and the metal substrate, significantly improving the overall coating's corrosion resistance. In recent years, zirconium / titanium chemical conversion coatings have been widely used in industries such as aerospace, automotive manufacturing, and building materials.

[0003] As the base layer of the organic coating, the zirconium / titanium chemical conversion film inevitably needs to be thermally cured together with the organic coating. For example, in the automotive coating industry, the zirconium conversion film undergoes two thermal curing processes at 180°C, along with the electrophoretic coating, color paint coating, and clear coat. After heating, the existing zirconium / titanium conversion coating will crack and peel. Cracks and delamination on the surface of the conversion film can lead to the penetration of corrosive ions, which greatly reduces the anti-corrosion effect of the coating and weakens the adhesion between the organic coating and the metal substrate. In addition, during the formation process of the existing zirconium / titanium chemical conversion film, if the metal substrate is immersed in the conversion solution for too long (300-600 seconds), the conversion film formed on the metal substrate surface will also crack and peel. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a surface treatment agent that can effectively improve the cohesion of the metal surface conversion film, so that the conversion film surface will not crack or peel off even if the heating or immersion time is too long (300 to 600 seconds).

[0005] Technical solution: The surface treatment agent for improving the cohesion of the metal surface conversion film described in the present invention includes the following components by mass per liter of the surface treatment agent: 75-200 mg of fluorozirconic acid or fluorotitanic acid, 10-100 mg of nitrate, 10-50 mg of organic coupling agent, 20-50 mg of dispersant, 50-200 mg of dopamine-coated nano-oxide particles, and the balance is water.

[0006] The present invention utilizes the adhesion effect of dopamine to coat the surface of zirconium, titanium or silicon oxide particles, so that they are evenly dispersed in the conversion liquid and participate in the formation of a composite zirconium / titanium chemical conversion film by co-deposition with the film-forming material.

[0007] Wherein, the nano-oxide particles are at least one of silicon dioxide, titanium dioxide or zirconium dioxide nano-particles; the particle size of the nano-oxide particles is 5 to 15 nm. When the particle size of the nano-oxide particles is too small, they are prone to agglomeration and failure, resulting in instability of the surface treatment agent. When the particle size is too large, it is difficult to disperse and the surface energy is low, which is not conducive to participating in the conversion film deposition process.

[0008] The dopamine-coated nano-oxide particles are prepared by the following method: nano-oxide particles are mixed with a dopamine solution at a mass-to-volume ratio of 1 mg:10 mL, and after thorough stirring (mixing and stirring for 60 minutes), the mixture is washed, filtered, and dried to obtain the dopamine-coated nano-oxide particles. The dopamine solution has a concentration of 10 to 50 mmol / L and is weakly alkaline (pH adjusted to 8.5; dopamine molecules contain catechol groups and can undergo oxidative polymerization under neutral or weakly alkaline conditions). The dopamine concentration is controlled to control the amount of polydopamine coating on the surface of the nano-oxide particles. A low dopamine coating level results in poor results, while a high dopamine coating level easily increases the particle size of the oxide nanoparticles, making dispersion unfavorable.

[0009] The nitrate is one of copper nitrate, manganese nitrate, zinc nitrate, magnesium nitrate, aluminum nitrate, or zinc nitrate. Nitrate compounds can participate in the formation of the conversion coating, further improving the density of the conversion coating structure. The organic coupling agent is one of an organosilane coupling agent, an organophosphonate coupling agent, or an organotitanate coupling agent; further, it is one of an aminosilane coupling agent, an epoxy silane coupling agent, or a monoalkoxy organophosphate coupling agent. The organic coupling agent can synergistically enhance the interaction between different species within the conversion coating, further improving the cohesive strength of the conversion coating. The dispersant is one of ethylenediaminetetraacetic acid, nitrilotriacetic acid, tartaric acid, or citric acid. Soluble metal salts of the above compounds can also achieve the same effect. The dispersant can improve the stability of the nano-oxide particles in the surface treatment agent.

[0010] The formation mechanism of zirconium / titanium conversion films is due to hydrogen evolution or oxygen absorption reactions in the microcathode region of the metal substrate surface. The local increase in pH promotes the hydrolysis of zirconium / titanium ions, which in turn deposits a zirconium or titanium hydroxide conversion film on the metal surface. This conversion film is primarily composed of amorphous zirconium / titanium hydroxide, with intermolecular forces primarily hydrogen bonding and van der Waals forces. The film has weak cohesion and contains a large amount of bound water, including physically adsorbed water and chemically bound water. When the conversion film is exposed to air for a long time or is exposed to high temperatures, it gradually dehydrates, inducing internal stress. This stress can easily lead to cracking of the conversion film and even peeling off from the metal substrate. The present invention uses dopamine-coated nano-oxide particles to participate in film formation to obtain an amorphous conversion film with a skeleton structure. Taking a zirconium-based conversion film as an example, in the conversion film, dopamine molecules can simultaneously combine with the nano-oxide and zirconium hydroxide film-forming materials in the form of covalent bonds, exerting a coupling effect between different species, and enhancing the interaction between the amorphous zirconium hydroxide and the crystalline nano-oxide particles in the conversion film. Since the cohesive force of the conversion film is enhanced, the occurrence of thermal cracking of the conversion film can be effectively suppressed.

[0011] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the conversion film layer obtained after being treated with the surface treatment agent of the present invention has high cohesive strength, which helps to maintain the density of the conversion film under high temperature conditions and promotes stress release during dehydration, thereby resisting cracking and falling off of the conversion film layer after high temperature; and after the conversion film of the present invention and the organic coating are heated and cured together, the adhesion of the organic coating is significantly improved, thereby ensuring the long-term durability of the overall coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a schematic diagram of the principle of using dopamine-modified oxide as the skeleton of the conversion film of the present invention to enhance the cohesion of the zirconium / titanium conversion film;

[0013] Figure 2 Surface morphologies of the conversion films of Example 1(b) and Comparative Example 1(a) after drying at 180°C;

[0014] Figure 3 Schematic diagram comparing the adhesion performance of the organic coatings of Examples 1, 2 and 3 with Comparative Examples 1 to 3.

[0015] Table 1 is a comparison table of the electrochemical impedance performance of Examples 1, 2 and 3 and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0016] Example 1

[0017] The surface treatment agent for improving the cohesion of a metal surface conversion film comprises, per liter of the surface treatment agent, the following components by mass: 200 mg of fluorozirconic acid, 100 mg of zinc nitrate, 20 mg of γ-aminopropyltriethoxysilane, 50 mg of disodium ethylenediaminetetraacetic acid, 50 mg of dopamine-coated zirconium oxide nanoparticles, and the balance water; the pH of the surface treatment agent is 4.0.

[0018] Dopamine-coated zirconia nanoparticles were prepared by the following method: preparing a dopamine solution with a concentration of 20 mmol / L and adjusting the pH of the dopamine solution to 8.5; mixing the zirconia nanoparticles with the dopamine solution at a mass volume ratio of 1 mg:10 mL, stirring for 60 minutes, and then washing, filtering, and drying to obtain dopamine-coated zirconia nanoparticles; wherein the particle size of the zirconia nanoparticles is 10 to 12 nm.

[0019] Example 2

[0020] The surface treatment agent for improving the cohesive force of a metal surface conversion film comprises, per liter of the surface treatment agent, the following components by mass: 100 mg of fluorotitanic acid, 10 mg of copper nitrate, 50 mg of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 20 mg of nitrilotriacetic acid, 100 mg of dopamine-coated silicon oxide nanoparticles, and the remainder being water; the pH of the surface treatment agent is 4.5.

[0021] Dopamine-coated zirconium oxide nanoparticles were prepared by the following method: preparing a dopamine solution with a concentration of 50 mmol / L and adjusting the pH of the dopamine solution to 8.5; mixing silicon oxide nanoparticles with the dopamine solution at a mass volume ratio of 1 mg:10 mL, stirring for 60 minutes, and then washing, filtering, and drying to obtain dopamine-coated silicon oxide nanoparticles; wherein the particle size of the silicon oxide nanoparticles is 5 to 8 nm.

[0022] Example 3

[0023] The surface treatment agent for improving the cohesion of a metal surface conversion film comprises the following components per liter of the surface treatment agent: 75 mg of fluorozirconic acid, 200 mg of manganese nitrate, 10 mg of a monoalkoxy organic phosphonate coupling agent, 30 mg of sodium tartrate, 200 mg of dopamine-coated titanium oxide nanoparticles, and the remainder of water; the pH of the surface treatment agent is 4.2.

[0024] Dopamine-coated zirconium oxide nanoparticles were prepared by the following method: a dopamine solution with a concentration of 10 mmol / L was prepared, and the pH of the dopamine solution was adjusted to 8.5; titanium oxide nanoparticles and the dopamine solution were mixed at a mass volume ratio of 1 mg:10 mL, and the mixture was stirred for 60 minutes, followed by washing, filtering, and drying to obtain dopamine-coated titanium oxide nanoparticles; wherein the particle size of the titanium oxide nanoparticles was 10 to 14 nm.

[0025] Comparative Example 1

[0026] A surface treatment agent comprises the following components by mass per liter of the surface treatment agent: 200 mg of fluorozirconic acid, 100 mg of zinc nitrate, 20 mg of γ-aminopropyltriethoxysilane, 50 mg of disodium ethylenediaminetetraacetate, and the balance of water; the pH value of the surface treatment agent is 4.0.

[0027] Comparative Example 2

[0028] A surface treatment agent comprises the following components by mass per liter of the surface treatment agent: 200 mg of fluorozirconic acid, 100 mg of zinc nitrate, 20 mg of γ-aminopropyltriethoxysilane, 50 mg of disodium ethylenediaminetetraacetic acid, 50 mg of zirconium oxide nanoparticles, and the balance of water; the pH value of the surface treatment agent is 4.0.

[0029] Comparative Example 3

[0030] A surface treatment agent comprises, per liter of the surface treatment agent, the following components by mass: 200 mg of fluorozirconic acid, 100 mg of zinc nitrate, 20 mg of γ-aminopropyltriethoxysilane, 50 mg of disodium ethylenediaminetetraacetic acid, 50 mg of polyamidoamine (hyperbranched polymer)-coated zirconium oxide nanoparticles, and the balance water; the pH of the surface treatment agent is 4.0.

[0031] Polyamidoamine-coated zirconium oxide nanoparticles were prepared by the following method: a polyamide solution with a concentration of 20 mmol / L was prepared, and the pH of the solution was adjusted to 8.5; the zirconium oxide nanoparticles were mixed with the polyamidoamine solution at a mass volume ratio of 1 mg:10 mL, and the mixture was stirred for 120 minutes, followed by washing, filtering, and drying to obtain polyamidoamine-coated zirconium oxide nanoparticles; wherein the particle size of the zirconium oxide nanoparticles was 10 to 12 nm.

[0032] DC06 cold-rolled steel plate (produced by Shanghai Baosteel) with specifications (170×100×0.7mm) was used as the metal sample.

[0033] Step 1 (conversion coating treatment): a clean metal sample was placed in the surface treatment agent working solution of Comparative Examples 1 to 3 and Examples 1 to 3 and immersed for 10 minutes, then rinsed with deionized water and dried with compressed air.

[0034] Heat aging treatment: The metal sample after conversion coating treatment (step 1) was placed in an oven at 180°C and dried for 20 minutes. After being taken out, it was observed by scanning electron microscopy. The results were as follows: Figure 2 As shown, through Figure 2 It can be seen that the conversion film on the surface of the substrate in Comparative Example 1 cracked and even fell off, while the conversion film on the surface of the substrate in Example 1 had a complete morphology and uniform particle distribution.

[0035] Electrochemical impedance spectroscopy (EIS) was performed on the metal samples after heat aging treatment. The fitting results are shown in Table 1. As can be seen from Table 1, the impedance of the conversion film on the substrate surface in Examples 1 to 3 is significantly higher than that in Comparative Examples 1 to 3. This is mainly because as the conversion film cracks, a channel is provided for the infiltration of the corrosive medium, thereby reducing the corrosion resistance of the substrate.

[0036] Step 2 (electrophoretic coating): The metal sample treated with the conversion film (step 1) was subjected to cathodic electrophoretic coating using cathodic electrophoretic coating Enviro-Prime 8000 (manufactured by PPG). The coated metal sample was rinsed with water and dried at 180°C for 20 minutes.

[0037] Adhesion test: Use a grid knife to divide the surface of the metal sample after electrophoretic coating (step 2) into 10×10 squares at 1mm intervals, and then use a paint film impact tester to test the impact height at 50cm. The results are as follows: Figure 3 As shown, through Figure 3 It can be seen that the electrophoretic coatings in Comparative Examples 1 to 3 partially peeled off and their adhesion properties decreased, while the coatings in Examples 1 to 3 had complete morphology and excellent adhesion properties after electrophoretic coating.

[0038] Table 1

[0039]

Claims

1. A surface treatment agent for improving the cohesion of a metal surface conversion film, characterized in that: Each liter of the surface treatment agent includes the following components by mass: 75-200 mg of fluorozirconic acid or fluorotitanic acid, 10-100 mg of nitrate, 10-50 mg of organic coupling agent, 20-50 mg of dispersant, 50-200 mg of dopamine-coated nano-oxide particles, and the balance is water.

2. The surface treatment agent according to claim 1, characterized in that: The nano-oxide particles are at least one of silicon dioxide, titanium dioxide or zirconium dioxide nano-particles.

3. The surface treatment agent according to claim 2, characterized in that: The particle size of the nano-oxide particles is 5 to 15 nm.

4. The surface treatment agent according to claim 1, characterized in that: The dopamine-coated nano-oxide particles are prepared by the following method: nano-oxide particles are mixed with a dopamine solution at a mass volume ratio of 1 mg:10 mL, stirred thoroughly, washed, filtered and dried to obtain dopamine-coated nano-oxide particles.

5. The surface treatment agent according to claim 4, characterized in that: The concentration of the dopamine solution is 10-50 mmol / L, and the dopamine solution is weakly alkaline.

6. The surface treatment agent according to claim 4, characterized in that: The mixing time is not less than 60 minutes.

7. The surface treatment agent according to claim 1, characterized in that: The nitrate is one of copper nitrate, manganese nitrate, zinc nitrate, magnesium nitrate, aluminum nitrate or zinc nitrate.

8. The surface treatment agent according to claim 1, characterized in that: The organic coupling agent is one of an organic silane coupling agent, an organic phosphonate coupling agent or an organic titanate coupling agent.

9. The surface treatment agent according to claim 8, characterized in that: The organic coupling agent is one of an amino silane coupling agent, an epoxy silane coupling agent or a monoalkoxy organic phosphate coupling agent.

10. The surface treatment agent according to claim 1, characterized in that: The dispersant is one of ethylenediaminetetraacetic acid, nitrilotriacetic acid, tartaric acid or citric acid, or a soluble metal salt of the above compounds.

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