A phosphorus-free coating pretreatment agent for low-temperature curing coatings

Abstract

The present invention discloses a phosphorus-free coating pretreatment agent for low-temperature curing coating, including agent A and agent B for use in combination; wherein the agent A is composed of the following components in mass percentage: 2-3% strong acid, 2-4% fluorozirconic acid, 1-2% fluorotitanic acid, 1-10% nitrate and the balance of deionized water; the agent B is composed of the following components in mass percentage: 2-3% strong acid, 2-3% modified silane coupling agent and the balance of deionized water. The phosphorus-free coating pretreatment agent of the present invention can effectively enhance the interfacial bonding force between the zirconium-based film and the electrophoretic paint film, thereby greatly improving the adhesion and corrosion resistance of the low-temperature curing paint film.

Description

Technical Field

[0001] The invention relates to a phosphorus-free coating pre-treatment agent matched with a low-temperature curing coating. Background Art

[0002] Pre-treatment is essential for coating metal parts, including but not limited to multiple degreasing, multiple water washing, and chemical conversion. Chemical conversion involves forming a nano- or micron-scale film on the surface of a cleaned metal part through a chemical reaction. This film significantly improves the adhesion and corrosion resistance of the finished part.

[0003] Currently, industrial coatings are gradually developing towards low-temperature coatings. When used with low-temperature curing coatings, commonly used pre-treatment chemical conversion methods cannot meet post-coating performance requirements, including but not limited to neutral salt spray resistance, cyclic corrosion resistance, and coating adhesion. This is because existing pre-treatment coatings are developed for traditional conventional curing coatings. Due to the high curing temperature and long curing time of conventional curing coatings, the chemical modification on the surface of the pre-treatment conversion film can fully cross-link with the reactive groups in the coating to form a strong covalent bond. However, low-temperature curing coatings have a low curing temperature and short curing time. Existing pre-treatment conversion films cannot fully cross-link with the paint film during curing, resulting in poor paint film adhesion and poor neutral salt spray resistance. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a phosphorus-free coating pretreatment agent for matching low-temperature curing coatings. The phosphorus-free coating pretreatment agent can effectively enhance the interfacial bonding strength between the zirconium-based film and the electrophoretic paint film, thereby greatly improving the adhesion and corrosion resistance of the low-temperature curing coating film.

[0005] Technical solution: The phosphorus-free coating pretreatment agent described in the present invention includes agent A and agent B used in combination; wherein, agent A is composed of the following components in mass percentage: 2-3% strong acid, 2-4% fluorozirconic acid, 1-2% fluorotitanic acid, 1-10% nitrate and the balance deionized water; the agent B is composed of the following components in mass percentage: 2-3% strong acid, 2-3% modified silane coupling agent and the balance deionized water.

[0006] In the above-mentioned Agent A and Agent B, the strong acid is nitric acid or sulfuric acid. If the strong acid content in Agent A is low, nitrate precipitation will occur, affecting the stability of the product, while if the content is high, it will affect the growth of the conversion film.

[0007] Wherein, in the agent A, the nitrate is one of manganese nitrate, zinc nitrate, magnesium nitrate or nitrone; a low nitrate content will lead to a slow film forming speed, while a high content will lead to an excessively fast film forming speed and a loose film layer.

[0008] Among them, in the B agent, the modified silane coupling agent is obtained by modifying 3-isocyanatepropyltriethoxysilane or 3-isocyanatepropyltrimethoxysilane; if the content of the modified silane coupling agent is too low, it will lead to poor compatibility with the low-temperature curing coating, and if the content is too high, it will cause precipitation of the product and poor stability.

[0009] The modified silane coupling agent is prepared by mixing an isocyanate silane with an organic solvent, then adding a modifier and a catalyst, and allowing the mixture to react fully to obtain the modified silane coupling agent. The present invention uses an inorganic salt to block and modify the isocyanate silane, resulting in a product with high water solubility and strong compatibility with other components in the formulation, thereby improving the stability of the system.

[0010] Wherein, the isocyanate silane is 3-isocyanate propyl triethoxy silane or 3-isocyanate propyl trimethoxy silane.

[0011] Wherein, the organic solvent is acetone; the modifier is sodium bisulfite; and the catalyst is tetrabutylammonium bromide or hexadecyltrimethylammonium bromide.

[0012] The mixing volume ratio of the isocyanate silane and the organic solvent is 3 to 3.5:1; the added amount of the modifier is 50 to 55% of the total mass of the isocyanate silane and the organic solvent; and the added amount of the modifier is 1 to 1.5% of the total mass of the isocyanate silane and the organic solvent.

[0013] The reaction temperature is 30°C ± 5°C; the stirring speed is maintained above 700 rpm during the reaction, and the reaction time is not less than 24 hours.

[0014] The present invention uses a modified silane coupling agent with a specific structure. On the one hand, it can dehydrate and condense with Zr-OH and Ti-OH deposited on the surface of the substrate during the film formation process to form a bond, thereby increasing the density of the pre-treatment conversion film. On the other hand, after being deblocked at low temperature, it can form a covalent bond with the -OH group in the low-temperature curing coating during the curing process, thereby greatly improving the adhesion and corrosion resistance of the low-temperature curing coating film.

[0015]

[0016] Beneficial effects: Compared with the existing technology, the present invention has the following significant advantages: the phosphorus-free coating pretreatment agent of the present invention can effectively enhance the interfacial bonding force between the zirconium-based film and the electrophoretic paint film, thereby greatly improving the adhesion and corrosion resistance of the low-temperature curing paint film. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1These are digital photos of cold-rolled steel plate electrophoresis samples treated with Examples 1 and 2 and a commercially available phosphorus-free coating pretreatment agent after 1000 hours of neutral salt spray resistance testing;

[0018] Figure 2 These are digital photos of the galvanized steel plate electrophoresis samples treated with Examples 1 and 2 and a commercially available phosphorus-free coating pretreatment agent after 30 cycles of the cyclic corrosion resistance test. DETAILED DESCRIPTION

[0019] Example 1

[0020] The present invention provides a phosphorus-free coating pretreatment agent, comprising an agent A and an agent B for use together; wherein the agent A is composed of the following components in percentage by mass: 3% nitric acid, 4% fluorozirconic acid, 1% fluorotitanic acid, 1% copper nitrate, and the balance deionized water; and the agent B is composed of the following components in percentage by mass: 2% nitric acid, 3% modified silane coupling agent, and the balance deionized water.

[0021] The modified silane coupling agent is prepared by the following method: adding 1 / 2 of the volume of 3-isocyanatepropyltrimethoxysilane to a kettle, then starting stirring and adding 1 / 6 of the volume of acetone as a solvent, and slowly adding 50% of the total mass of the liquid (the total mass of 3-isocyanatepropyltrimethoxysilane and acetone) of sodium bisulfite powder to the kettle after stirring, and then slowly adding 1% of the total mass of the liquid (the total mass of 3-isocyanatepropyltrimethoxysilane and acetone) of tetrabutylammonium bromide powder to the kettle after stirring, and fully reacting; during the reaction, the reaction system temperature is maintained at 30°C ± 5°C, the stirring rate is maintained at above 700 rpm, and stirring is performed for 24 hours to obtain a modified silane coupling agent;

[0022] The reaction equation of the modification process of the modified silane coupling agent of the present invention is:

[0023]

[0024] Prepare the working solution: add agent A and agent B into water in sequence by adding agent A first and then agent B, and dilute to obtain a working solution with a mass concentration of 0.5% for agent A and 0.5% for agent B; that is, add 0.5g of agent A and 0.5g of agent B into 99g of water in sequence to obtain a working solution with a mass concentration of 0.5% for agent A and 0.5% for agent B.

[0025] Cold-rolled steel plate: Standard cold-rolled plate (70mm150mm×0.8mm) purchased from Guangdong Honghong Industrial Co., Ltd., made of Baosteel DC03.

[0026] Multi-metal degreaser: Prepare a 3wt% bath using KR-SC6830 or KR-SC64T multi-metal degreasers. Heat the bath to 50°C and soak standard cold-rolled steel sheets in the bath for 6 minutes. Remove and immediately rinse with pure water to remove any remaining degreaser.

[0027] Coating pretreatment agent: The phosphorus-free coating pretreatment agent of Example 1 is prepared into a working solution; in the working solution, the mass concentration of agent A is 0.5%, and the mass concentration of agent B is 0.5%; the method of use is immersion treatment, immersing the cold-rolled steel plate in the working solution for 3 to 5 minutes and then taking it out; the treatment temperature is room temperature, the treatment time is 3 to 5 minutes, and then the residual tank liquid on the surface is immediately rinsed with pure water.

[0028] Low-temperature curing electrophoretic paint: ACECRON 1400TF low-temperature curing electrophoretic paint from a certain OEM was used. The electrophoresis adopted two voltage stages: the first stage was 0-60V, stable for 1 minute, the second stage was 60-210V, stable for 2 minutes, and the paint was cured at 140°C for 20 minutes.

[0029] Electrophoretic sample preparation process: degreasing - water washing - pre-treatment for painting - water washing - electrophoretic coating - curing

[0030] Perform performance test on electrophoresis sample:

[0031] Cross-cut adhesion test (GB / T9286-2021)

[0032] Coating impact resistance test (GB / T1732-2020)

[0033] Neutral salt spray resistance test (GB / T10125-2021)

[0034] Adhesion grade is 0, impact resistance is 50kg·cm without cracking, neutral salt spray resistance is 1000h, maximum unilateral corrosion expansion is 1.90mm, and average unilateral corrosion expansion is 1.73mm.

[0035] Example 2

[0036] The present invention provides a phosphorus-free coating pretreatment agent comprising an agent A and an agent B for use together. Agent A comprises the following components in percentage by weight: 2% nitric acid, 2% fluorozirconic acid, 2% fluorotitanic acid, 10% manganese nitrate, and the balance deionized water; and agent B comprises the following components in percentage by weight: 2% nitric acid, 2% modified silane coupling agent, and the balance deionized water. The modified silane coupling agent in Example 2 is the same as that in Example 1.

[0037] Prepare the working solution: add agent A and agent B into water in sequence by adding agent A first and then agent B, and dilute to obtain a working solution with a mass concentration of 0.5% for agent A and 0.5% for agent B; that is, add 0.5g of agent A and 0.5g of agent B into 99g of water in sequence to obtain a working solution with a mass concentration of 0.5% for agent A and 0.5% for agent B.

[0038] Galvanized steel plate: hot-dip galvanized steel plate from a certain main engine factory, with specifications of (70mm150mmX0.8mm), and material is Baosteel DC56D+Z.

[0039] Multi-metal degreasing agent: Kerun multi-metal degreasing agent KR-SC6830 and KR-SC64T were used to prepare a 3wt% bath solution. The degreasing and water washing methods were the same as in Example 1.

[0040] Coating pretreatment agent: The phosphorus-free coating pretreatment agent of Example 2 is prepared into a working solution; in the working solution, the mass concentration of agent A is 0.5%, and the mass concentration of agent B is 0.5%; the method of use is immersion treatment, immersing the cold-rolled steel plate in the working solution for 3 to 5 minutes and then taking it out; the treatment temperature is room temperature, the treatment time is 3 to 5 minutes, and then take it out and immediately rinse the residual tank liquid on the surface with pure water.

[0041] Low-temperature curing electrophoretic paint: ACECRON 1400TF low-temperature curing electrophoretic paint from a certain OEM was used. The electrophoresis adopted two voltage stages: the first stage was 0-60V, stable for 1 minute, the second stage was 60-210V, stable for 2 minutes, and the paint was cured at 140°C for 20 minutes.

[0042] Electrophoretic sample preparation process: degreasing - water washing - pre-treatment for painting - water washing - electrophoretic coating - curing

[0043] Perform performance test on electrophoresis sample:

[0044] Cross-cut adhesion test (GB / T9286-2021)

[0045] Coating impact resistance test (GB / T1732-2020)

[0046] Cyclic corrosion resistance test (GMW14872-2021)

[0047] Adhesion grade is 0, impact resistance is 50kg·cm without cracking, and the maximum corrosion expansion on one side after 30 cycles of cyclic corrosion test is 1.32mm, and the average corrosion expansion on one side is 1.06mm.

[0048] Comparative Example 1

[0049] Comparative Example 1 differs from Example 1 only in that the silane coupling agent is not modified. This is a phosphorus-free coating pretreatment agent comprising Agent A and Agent B. Agent A is composed of the following components in percentage by weight: 3% nitric acid, 4% fluorozirconic acid, 1% fluorotitanic acid, 1% copper nitrate, and the balance deionized water; Agent B is composed of the following components in percentage by weight: 2% nitric acid, 3% silane coupling agent, and the balance deionized water.

[0050] Prepare the working solution: add agent A and agent B into water in sequence, adding agent A first and then agent B, and dilute to obtain a working solution with a mass concentration of 0.75% for agent A and 0.75% for agent B.

[0051] Comparative Example 2

[0052] The only difference between Comparative Example 2 and Example 1 is that when preparing the working solution, Agent B and Agent A are added to water in sequence by adding Agent B first and then Agent A to form a working solution with a mass concentration of Agent A of 0.75% and a mass concentration of Agent B of 0.75%.

[0053] Cold-rolled steel plate: Standard cold-rolled plate (70mm150mmX0.8mm) purchased from Guangdong Honghong Industrial Co., Ltd., made of Baosteel DC03.

[0054] Galvanized steel plate: hot-dip galvanized steel plate from a certain main engine factory, with specifications of (70mm150mmX0.8mm), and material is Baosteel DC56D+Z.

[0055] Multi-metal degreasing agent: Kerun multi-metal degreasing agent KR-SC6830 and KR-SC64T were used to prepare a 3wt% bath solution. The degreasing and water washing methods were the same as in Example 1.

[0056] Coating pretreatment agent: The phosphorus-free coating pretreatment agent of Example 1 is prepared into a working solution; in the working solution, the mass concentration of agent A is 0.75%, and the mass concentration of agent B is 0.75%; the method of use is immersion treatment, and the degreased and washed cold-rolled steel plates and galvanized steel plates are immersed in the working solution respectively, soak for 3 to 5 minutes, take out and immediately rinse the residual tank liquid on the surface with pure water.

[0057] The phosphorus-free coating pretreatment agent of Example 2 is prepared into a working solution; in the working solution, the mass concentration of agent A is 0.75%, and the mass concentration of agent B is 0.75%; the method of use is immersion treatment, and the degreased and washed cold-rolled steel plates and galvanized steel plates are immersed in the working solution respectively, soaked for 3 to 5 minutes, taken out and immediately rinsed with pure water to remove the residual tank liquid on the surface.

[0058] The phosphorus-free coating pretreatment agent of Comparative Example 1 was prepared into a working solution; in the working solution, the mass concentration of agent A was 0.75%, and the mass concentration of agent B was 0.75%; the method of use was immersion treatment, and the degreased and washed cold-rolled steel plates and galvanized steel plates were immersed in the working solution respectively, soaked for 3 to 5 minutes, and then taken out and the residual tank liquid on the surface was immediately rinsed with pure water.

[0059] The phosphorus-free coating pretreatment agent of Comparative Example 2 was prepared into a working solution; in the working solution, the mass concentration of agent A was 0.75%, and the mass concentration of agent B was 0.75%; the method of use was immersion treatment, and the degreased and washed cold-rolled steel plates and galvanized steel plates were immersed in the working solution respectively, soaked for 3 to 5 minutes, and then taken out and the residual tank liquid on the surface was immediately rinsed with pure water.

[0060] A commercially available phosphorus-free coating pretreatment agent was prepared into a working solution; the mass concentration of the commercially available phosphorus-free coating pretreatment agent in the working solution was 1%; the method of use was immersion treatment, and the degreased and washed cold-rolled steel sheets and galvanized steel sheets were immersed in the working solution respectively, soaked for 3 to 5 minutes, and then taken out and the residual tank liquid on the surface was immediately rinsed with pure water.

[0061] Low-temperature curing electrophoretic paint: ACECRON 1400TF low-temperature curing electrophoretic paint from a certain OEM was used. The electrophoresis adopted two voltage stages: the first stage was 0-60V, stable for 1 minute, the second stage was 60-210V, stable for 2 minutes, and the paint was cured at 140°C for 20 minutes.

[0062] Electrophoretic sample preparation process: degreasing - water washing - pre-treatment for painting - water washing - electrophoretic coating - curing

[0063] Conduct electrophoretic coating adhesion test, impact resistance test, neutral salt spray resistance test and cyclic corrosion resistance test on the electrophoretic sample:

[0064] Cross-cut adhesion test (GB / T9286-2021)

[0065] Coating impact resistance test (GB / T1732-2020)

[0066] Cyclic corrosion resistance test (GMW14872-2021)

[0067] Neutral salt spray resistance test (GB / T10125-2021)

[0068] Table 1 is a comparison of the performance of low-temperature curing electrophoretic paint films with various coating pretreatment agents

[0069]

[0070] As shown in Table 1, the paint film performance of the low-temperature curing coating matched with the coating pretreatment agent of the present invention can meet the paint film performance requirements of most OEMs (100-grid adhesion level 0, paint film impact resistance 50kg·cm qualified, neutral salt spray resistance 1000h single-side maximum corrosion expansion <1.55mm, cyclic corrosion resistance 30 cycles ...

[0071] <1.25mm).

[0072] Figure 1 and Figure 2 Digital photographs of Example 1 and Example 2 of the present invention and a commercially available phosphorus-free coating pretreatment agent after being matched with a low-temperature curing electrophoretic paint, respectively, after undergoing a 1000-hour neutral salt spray test and a 30-cycle cyclic corrosion test. As can be seen from the figures, the coating pretreatment agent prepared according to the present invention, when matched with a low-temperature curing electrophoretic paint, produces a paint film with excellent corrosion resistance.

Claims

1. A phosphorus-free coating pretreatment agent for low-temperature curing coatings, characterized in that: It comprises an agent A and an agent B for use together; wherein the agent A is composed of the following components in mass percentage: 2-3% strong acid, 2-4% fluorozirconic acid, 1-2% fluorotitanic acid, 1-10% nitrate and the balance deionized water; and the agent B is composed of the following components in mass percentage: 2-3% strong acid, 2-3% modified silane coupling agent and the balance deionized water.

2. The phosphorus-free coating pretreatment agent according to claim 1, characterized in that: In the agent A and the agent B, the strong acid is nitric acid or sulfuric acid.

3. The phosphorus-free coating pretreatment agent according to claim 1, characterized in that: In the agent A, the nitrate is one of manganese nitrate, zinc nitrate, magnesium nitrate or nitrate ketone.

4. The phosphorus-free coating pretreatment agent according to claim 1, characterized in that: In the agent B, the modified silane coupling agent is prepared by the following method, specifically: mixing isocyanate silane with an organic solvent, and then adding a modifier and a catalyst thereto in sequence, and after sufficient reaction, obtaining the modified silane coupling agent.

5. The phosphorus-free coating pretreatment agent according to claim 4, characterized in that: The isocyanate silane is 3-isocyanate propyl triethoxy silane or 3-isocyanate propyl trimethoxy silane.

6. The phosphorus-free coating pretreatment agent according to claim 4, characterized in that: The organic solvent is acetone; the modifier is sodium bisulfite; and the catalyst is tetrabutylammonium bromide or hexadecyltrimethylammonium bromide.

7. The phosphorus-free coating pretreatment agent according to claim 4, characterized in that: The mixing volume ratio of the isocyanate silane to the organic solvent is 3 to 3.5:

1.

8. The phosphorus-free coating pretreatment agent according to claim 4, characterized in that: The added amount of the modifier is 50-55% of the total mass of the isocyanate silane and the organic solvent.

9. The phosphorus-free coating pretreatment agent according to claim 4, characterized in that: The added amount of the catalyst is 1-1.5% of the total mass of the isocyanate silane and the organic solvent.

10. The phosphorus-free coating pretreatment agent according to claim 4, characterized in that: The reaction temperature is 30°C ± 5°C; the stirring speed is maintained above 700 rpm during the reaction, and the reaction time is not less than 24 hours.

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