A method for stripping a cathodically electrodeposited paint film from a steel sheet, a swelling liquid and a stripping liquid

Abstract

The application belongs to the technical field of metal surface treatment, and particularly relates to a method for stripping a cathodic electrophoretic paint film on a steel plate, a swelling liquid and a stripping liquid. The method comprises the following steps: immersing the steel plate in a swelling liquid to cause volume swelling of the cathodic electrophoretic paint film, wherein the swelling liquid is composed of 5-15 wt% N-methyl pyrrolidone and the balance of water, and the immersion is performed at a temperature of 40-50 DEG C for 10-20 min; moving the steel plate after the swelling treatment into a stripping liquid to separate the swelled cathodic electrophoretic paint film from the surface of the steel plate, wherein the stripping liquid is composed of 10-20 wt% formic acid, 2-5 wt% thiourea, the balance of water and NaOH adjusted to pH 10-12, and the stripping is performed at a temperature of 50-70 DEG C for 10-20 min; and rinsing and drying the steel plate after the stripping treatment with deionized water to remove residual stripping liquid and fallen cathodic electrophoretic paint film fragments. The paint film is selectively removed by "swelling strength reduction-interfacial debonding-physical peeling", and the chemical conversion film is quickly completed at low temperature and completely reserved.

Description

Technical Field

[0001] This application belongs to the field of metal surface treatment technology, and particularly relates to a method for peeling off cathodic electrophoretic paint film on the surface of steel plate, a swelling solution and a peeling solution. Background Technology

[0002] Cathodic electrophoretic coating has become the preferred process for steel plate corrosion protection in industries such as automotive, home appliances, and building materials due to its excellent film thickness uniformity, edge coverage, and corrosion resistance. However, during steel plate forming, welding, or rework of surface defects, it is often necessary to partially or completely remove the cured cathodic electrophoretic paint film while retaining chemical conversion films such as phosphating and zirconium phosphate films to ensure the adhesion and corrosion resistance of subsequent recoating.

[0003] Existing technologies mainly employ three types of paint stripping methods: High-temperature strong alkali molten salt or sodium hydroxide solution treatment, the treatment temperature is usually above 80℃, and the treatment time is several hours to tens of hours; this method is not only energy-intensive, but also easily causes over-corrosion of the steel substrate, and the phosphating film or zirconium film is destroyed at the same time, requiring re-surface conditioning and phosphating, increasing the number of processes and wastewater discharge.

[0004] Organic paint removers containing halogenated hydrocarbons (chloroform, dichloromethane) or chloroacetic acid can be used at room temperature or medium temperature and remove paint quickly; however, halogenated hydrocarbons are persistent organic pollutants controlled under the Stockholm Convention, and chloroacetic acid has acute toxicity and skin corrosivity. The working environment has high VOCs, and environmental regulations are becoming increasingly strict.

[0005] High-boiling-point polar paint stripping systems containing ethylene glycol ethers (ethylene glycol butyl ether, ethylene glycol ethyl ether) can swell cathodic electrophoresis resins, but ethylene glycol ethers have been listed as "priority pollutants" by the EU REACH, and long-term exposure can lead to damage to the blood and reproductive systems. In addition, the system needs to be heated to 90–100°C, resulting in a large amount of volatile organic components and posing a risk of combustion and explosion.

[0006] In summary, existing technologies generally suffer from the contradiction of "high energy consumption, high pollution, high toxicity, high cost, and low selectivity," making it impossible to achieve efficient stripping of cathodic electrophoretic coatings under low temperature, water-based, non-toxic, and low-cost conditions while retaining phosphating or zirconium films. Summary of the Invention

[0007] This application provides a method for peeling off cathodic electrophoretic paint film from the surface of a steel plate, a swelling solution, and a peeling solution to solve the following technical problem: how to rapidly and halogen-free peel off electrophoretic paint at low temperature while retaining the chemical conversion film.

[0008] In a first aspect, embodiments of this application provide a method for peeling off a cathodic electrophoretic coating film from the surface of a steel plate, comprising: The steel plate is immersed in a swelling solution to cause the cathodic electrophoretic paint film to swell in volume. The swelling solution consists of 5–15 wt% N-methylpyrrolidone and the balance water. The immersion is carried out for 10–20 min at a temperature range of 40–50°C. The swollen steel plate is transferred into a stripping solution and the swollen cathodic electrophoretic paint film is separated from the surface of the steel plate. The stripping solution is composed of 10–20 wt% formic acid, 2–5 wt% thiourea, the balance water and NaOH adjusted to pH 10–12. The stripping is carried out for 10–20 min at a temperature range of 50–70°C. The stripped steel plate was rinsed with deionized water and dried to remove residual stripping liquid and detached cathodic electrophoretic paint film fragments.

[0009] Optionally, the swelling solution is composed of 10 wt% N-methylpyrrolidone and 90 wt% water, the immersion temperature is 45°C, and the immersion time is 15 min.

[0010] Optionally, the stripping solution consists of 15 wt% formic acid, 3 wt% thiourea and the balance water, and the pH of the stripping solution is adjusted to 11 with NaOH. The stripping temperature is 60°C and the stripping time is 15 min.

[0011] Optionally, the drying is air drying at room temperature.

[0012] Optionally, the steel plate retains a phosphate coating or a zirconium coating before impregnation, and the phosphate coating or the zirconium coating remains intact after peeling is completed.

[0013] Secondly, embodiments of this application provide a swelling solution for peeling off cathodic electrophoretic paint film from the surface of a steel plate, characterized in that the swelling solution is composed of 5–15 wt% N-methylpyrrolidone and the balance water, and is used to swell the cathodic electrophoretic paint film in a temperature range of 40–50°C for 10–20 min.

[0014] Optionally, the content of N-methylpyrrolidone is 10 wt%; the operating temperature of the swelling solution is 45°C, and the operating time is 15 min.

[0015] Thirdly, embodiments of this application provide a stripping solution for peeling off cathodic electrophoretic paint film on the surface of a steel plate. The stripping solution is characterized by being composed of 10–20 wt% formic acid, 2–5 wt% thiourea, the balance being water and NaOH adjusted to pH 10–12, and is used to chemically peel off the swollen cathodic electrophoretic paint film for 10–20 minutes within a temperature range of 50–70°C.

[0016] Optionally, the formic acid content is 15 wt%, the thiourea content is 3 wt%, and the pH is 11; the stripping solution is used at a temperature of 60°C for 15 minutes.

[0017] Optionally, the stripping solution is free of halogenated hydrocarbons, glycol ethers, chloroform, and chloroacetic acid.

[0018] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for peeling off cathodic electrophoretic paint films from steel plate surfaces. This method solves the three major contradictions of low temperature, halogen-free operation, and film preservation in one step through "two-step synergy and targeted destruction." First, a 5–15 wt% NMP aqueous solution is used at 40–50°C to cause the cathodic electrophoretic paint to swell. The polar NMP molecules penetrate the cross-linking network, destroying only van der Waals forces and hydrogen bonds without touching the inorganic conversion film, thus achieving low-temperature softening and reducing the peeling energy barrier. Subsequently, a formic acid-thiourea buffer system with a pH of 10–12 is introduced: formic acid saponifies and breaks ester bonds, and the S and N atoms of thiourea compete for coordination with the metal interface, weakening the paint film adhesion and promoting whole-plate peeling. The weakly alkaline + thiourea adsorption layer has almost no corrosion to the phosphating or zirconium film, and the peeling time is only 10–20 minutes. The entire process is halogen-free and free of strong acids and strong oxidants. It relies on selective paint removal through "swelling and strengthening reduction - interface debonding - physical peeling," completing the process quickly at low temperature while completely preserving the chemical conversion film. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0021] In a first aspect, embodiments of this application provide a method for peeling off a cathodic electrophoretic coating film from the surface of a steel plate, comprising: The steel plate is immersed in a swelling solution to cause the cathodic electrophoretic paint film to swell in volume. The swelling solution consists of 5–15 wt% N-methylpyrrolidone and the balance water. The immersion is carried out for 10–20 min at a temperature range of 40–50°C. The swollen steel plate is transferred into a stripping solution and the swollen cathodic electrophoretic paint film is separated from the surface of the steel plate. The stripping solution is composed of 10–20 wt% formic acid, 2–5 wt% thiourea, the balance water and NaOH adjusted to pH 10–12. The stripping is carried out for 10–20 min at a temperature range of 50–70°C. The stripped steel plate was rinsed with deionized water and dried to remove residual stripping liquid and detached cathodic electrophoretic paint film fragments.

[0022] S1. The steel plate is immersed in a swelling solution to induce volume swelling of the cathodic electrophoretic paint film. The swelling solution consists of 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, and 15wt% N-methylpyrrolidone and the balance water. The immersion temperature is maintained at 40℃, 41℃, 42℃, 43℃, 44℃, 45℃, 46℃, 47℃, 48℃, 49℃, and 50℃ for 10min, 11min, 12min, 13min, 14min, 15min, 16min, 17min, 18min, 19min, and 20min, respectively. S2. The swollen steel plate is transferred into the stripping solution, and the swollen cathodic electrophoretic paint film is separated from the surface of the steel plate. The stripping solution is prepared by adjusting the pH to 10, 10.2, 10.4, 10.6, and 10 by mixing 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, and 20wt% formic acid with 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, and 5wt% thiourea, the balance being water and NaOH. 8, 11, 11.2, 11.4, 11.6, 11.8, 12, with peeling temperatures within the range of 50℃, 51℃, 52℃, 53℃, 54℃, 55℃, 56℃, 57℃, 58℃, 59℃, 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, 66℃, 67℃, 68℃, 69℃, 70℃ for 10min, 11min, 12min, 13min, 14min, 15min, 16min, 17min, 18min, 19min, 20min respectively; S3. Rinse the stripped steel plate with deionized water and dry it to remove residual stripping liquid and detached cathodic electrophoretic paint film fragments.

[0023] Mechanism of action: N-methylpyrrolidone molecules, at 40-50℃ and a concentration of 5-15wt%, penetrate the cross-linking network of the cathodic electrophoretic coating film → disrupt the intermolecular hydrogen bonds and van der Waals forces within the cathodic electrophoretic coating film → increase the volume swelling rate of the cathodic electrophoretic coating film by 20-60% → stress concentration at the interface between the cathodic electrophoretic coating film and the phosphate film or zirconium film → decrease the interfacial adhesion by 30-70%; formic acid, at pH 10-12, forms a formic acid-sodium formate buffer system with NaOH → the formate concentration is maintained at 0.8-1.5mol / L → saponifies the ester bonds in the cathodic electrophoretic coating film → decreases the cross-linking density of the cathodic electrophoretic coating film by 20-50%; thiourea, at a concentration of 2-5wt%, reacts with Fe or... Zn ions form coordination bonds → thiourea adsorption layer covers phosphating or zirconium film → competitive occupation of cathodic electrophoretic coating adhesion sites → further reduction of 20-40% in interfacial adhesion between cathodic electrophoretic coating and phosphating or zirconium film; peeling temperature of 50-70℃ and peeling time of 10-20min together ensure a saponification rate ≥0.05μm / min → complete peeling of the entire cathodic electrophoretic coating ≤20min; deionized water rinsing dilutes the mass fraction of formic acid, thiourea, NaOH, and cathodic electrophoretic coating fragments in the peeling solution to <0.01wt% → avoids continued action of formic acid, thiourea, and NaOH residues on the phosphating or zirconium film → weight loss of phosphating or zirconium film <0.1g / m 2 The drying process removes moisture from the steel plate surface, preventing moisture from forming localized micro-batteries with residual formic acid, and maintaining the corrosion potential of the phosphating or zirconium film at >-400mV / SCE. These continuous steps simultaneously achieve low-temperature, rapid, halogen-free stripping of the cathodic electrophoretic coating while preserving the integrity of the phosphating or zirconium film.

[0024] In some embodiments, the swelling solution consists of 10 wt% N-methylpyrrolidone and 90 wt% water, the immersion temperature is 45°C, and the immersion time is 15 min.

[0025] 10wt% N-methylpyrrolidone at 45℃ causes the cathodic electrophoretic coating film to reach a maximum swelling rate of 55% → the peak interfacial stress occurs at 15 min → the density of crack initiation at the interface between the cathodic electrophoretic coating film and the phosphate film or zirconium film is ≥100 cracks / cm². 2 →The peeling force required for subsequent peeling steps decreased by 35%; the combination of 45℃ and 15min resulted in a diffusion coefficient of N-methylpyrrolidone of 2.1×10⁻⁶. -11 m 2 / s→Cathode electrophoretic coating swelling depth ≥12μm→Cathode electrophoretic coating cohesive strength decreases by 40%; The above quantitative parameters thus compress the total peeling time to ≤30min and preserve the integrity of the phosphating or zirconium film.

[0026] In some embodiments, the stripping solution consists of 15 wt% formic acid, 3 wt% thiourea and the balance water, and the pH of the stripping solution is adjusted to 11 with NaOH. The stripping temperature is 60°C and the stripping time is 15 min.

[0027] 15 wt% formic acid at pH 11 provides a formate concentration of 1.2 mol / L, resulting in a saponification reaction rate constant of 0.08 min. -1 →Cathode electrophoretic coating crosslinking fracture degree ≥60%; 3wt% thiourea forms [Fe(thiourea)2] on the steel plate surface at 60℃. 2+ Coordination layer → Coordination layer thickness 1.5nm → Reduces interfacial energy between cathodic electrophoretic coating film and phosphating film or zirconium film by 25mJ / m 2 The combined effect of 60℃ and 15min ensures that the peeling rate of the entire cathodic electrophoretic coating is ≥99%, and the dissolution rate of the phosphating or zirconium film is <0.005μm / min. These parameters enable rapid peeling within 15min while maintaining the integrity of the phosphating or zirconium film, while ensuring halogen-free operation.

[0028] In some embodiments, the drying is air drying at room temperature.

[0029] Room temperature air drying removes surface moisture from steel plates using 20-25℃ air convection → air drying time 10-15 minutes → avoids hot air >50℃ causing water loss between phosphate or zirconium films → phosphate or zirconium films maintain their original porosity <5%; room temperature air drying consumes no additional energy → process energy consumption decreases by 30%; the above scheme thus maintains the structure of phosphate or zirconium films and reduces energy consumption while completing the peeling.

[0030] In some embodiments, the steel plate retains a phosphate coating or a zirconium coating before impregnation, and the phosphate coating or the zirconium coating remains intact after peeling.

[0031] The dissolution rate of phosphating or zirconium oxide films in the swelling solution is <0.001 g / (m²). 2 During the swelling stage (15 min), the weight loss of the phosphating or zirconium film is <0.015 g / m². 2 The dissolution rate of phosphating or zirconium-plated films in a pH 11 stripping solution is <0.005 g / (m³). 2 During the 15-minute peeling stage (min) of phosphating or zirconium film, the weight loss is <0.075 g / m². 2 The cumulative weight loss over the two steps is <0.1g / m³. 2 →Surface roughness increment of phosphating or zirconium film <0.05μm →Phosphating or zirconium film remains intact; the above quantitative control directly solves the technical problem of "retaining chemical conversion film".

[0032] Secondly, embodiments of this application provide a swelling solution for peeling off cathodic electrophoretic paint film from the surface of a steel plate, characterized in that the swelling solution is composed of 5–15 wt% N-methylpyrrolidone and the balance water, and is used to swell the cathodic electrophoretic paint film in a temperature range of 40–50°C for 10–20 min.

[0033] The swelling solution for peeling off the cathodic electrophoretic paint film on the surface of the steel plate is composed of 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, and 15wt% N-methylpyrrolidone and the balance water. It is used to swell the cathodic electrophoretic paint film for 10min, 11min, 12min, 13min, 14min, 15min, 16min, 17min, 18min, 19min, and 20min at temperatures ranging from 40℃ to 50℃. A N-methylpyrrolidone concentration of 5-15 wt% provides sufficient polarity, resulting in a 20-60% swelling rate in the cathodic electrophoretic coating film. This leads to stress concentration at the interface between the cathodic electrophoretic coating film and the phosphating or zirconium coating film, resulting in a 30-70% decrease in interfacial adhesion. A temperature of 40-50℃ increases the diffusion coefficient of N-methylpyrrolidone to 1.5-2.5 × 10⁻⁶. -11 m 2 / s→swelling depth ≥10μm; time 10-20min to ensure swelling balance ≥90%; the above parameters create conditions for subsequent halogen-free rapid stripping of cathodic electrophoretic coating and avoid damage to phosphating or zirconium coating.

[0034] In some embodiments, the content of N-methylpyrrolidone is 10 wt%; the operating temperature of the swelling solution is 45°C, and the operating time is 15 min.

[0035] 10wt% N-methylpyrrolidone at 45℃ resulted in a 55% swelling rate in the cathodic electrophoretic coating film, leading to an interface crack density ≥100 cracks / cm². 2 →The subsequent peeling step reduces the peeling force by 35%; the swelling depth reaches 12μm in 15 minutes →the cohesive strength of the cathodic electrophoretic coating film decreases by 40%; the above parameters thus compress the total peeling time to ≤30 minutes while preserving the integrity of the phosphating or zirconium film.

[0036] Thirdly, embodiments of this application provide a stripping solution for peeling off cathodic electrophoretic paint film on the surface of a steel plate. The stripping solution is characterized by being composed of 10–20 wt% formic acid, 2–5 wt% thiourea, the balance being water and NaOH adjusted to pH 10–12, and is used to chemically peel off the swollen cathodic electrophoretic paint film for 10–20 minutes within a temperature range of 50–70°C.

[0037] The stripping solution for removing the cathodic electrophoretic paint film from the surface of the steel plate is prepared by adjusting the pH values ​​of 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, and 20 wt% formic acid, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, and 5 wt% thiourea, the balance being water and NaOH, to pH values ​​of 10, 10.2, 10.4, 10.6, 10.8, 11, 11.2, 11.4, and 11.6. Compositions 11.8 and 12 are used for chemically stripping swollen cathodic electrophoretic coatings for 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, 16 min, 17 min, 18 min, 19 min, and 20 min at temperatures ranging from 50℃ to 70℃. Formic acid (10-20 wt%) provides 0.8-1.5 mol / L formate ions at pH 10-12, which saponifies the ester bonds in the cathodic electrophoretic coating, reducing the crosslinking density by 20-60%. Thiourea (2-5 wt%) forms coordination bonds with Fe or Zn ions on the steel surface, reducing the interfacial energy by 20-30 mJ / m². 2 → The adhesion of the cathodic electrophoretic coating film decreases by 20-40%; the temperature is 50-70℃ to make the saponification rate ≥0.05μm / min → the peeling time is ≤20min; the pH is 10-12 weakly alkaline → the dissolution rate of the phosphating film or zirconium film is <0.005μm / min; the above parameters enable rapid peeling under halogen-free conditions while preserving the integrity of the phosphating film or zirconium film.

[0038] In some embodiments, the formic acid content is 15 wt%, the thiourea content is 3 wt%, and the pH is 11; the stripping solution is used at a temperature of 60°C for 15 minutes.

[0039] 15wt% formic acid at pH 11 with a formate concentration of 1.2 mol / L → saponification reaction rate of 0.08 min⁻¹ → cathodic electrophoretic coating film crosslinking fracture degree ≥60%; 3wt% thiourea forms [Fe(thiourea)₂]. 2+ Coordination layer → interface can reduce 25mJ / m 2 The combination of 60℃ and 15min ensures a peeling rate of ≥99% and a dissolution rate of <0.005μm / min for the phosphating or zirconium film. These parameters enable halogen-free peeling to be completed within 15min while preserving the integrity of the phosphating or zirconium film.

[0040] In some embodiments, the stripping fluid is free of halogenated hydrocarbons, glycol ethers, chloroform, and chloroacetic acid.

[0041] The stripping solution contains no halogenated hydrocarbons, ethylene glycol ethers, chloroform, or chloroacetic acid in its formic acid, thiourea, water, and NaOH components, resulting in a 90% reduction in volatile organic compound emissions. The absence of halogenated hydrocarbons prevents halogen ion adsorption on the phosphating or zirconium film surface, maintaining a corrosion potential >-400mV / SCE for the phosphating or zirconium film. The absence of ethylene glycol ethers prevents ethylene glycol ether penetration into the grain boundaries of the phosphating or zirconium film, resulting in a porosity increase of <0.5%. These parameters simultaneously meet the halogen-free environmental protection requirements and protect the integrity of the phosphating or zirconium film.

[0042] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0043] I. Implementation Examples Example 1 Step 1) Preparation of swelling solution: Place 100g of deionized water in a 250mL beaker, add 10g of N-methylpyrrolidone, and stir until N-methylpyrrolidone is completely dissolved to obtain 100g of 10wt% N-methylpyrrolidone swelling solution; Step 2) Swelling: Immerse the steel plate with the cathodic electrophoretic paint film and the phosphating film in the swelling solution. The swelling solution temperature is 45℃ and the swelling time is 15min. Step 3) Preparation of stripping solution: Place 82g of deionized water in a 250mL beaker, add 15g of formic acid and 3g of thiourea, stir until the thiourea is completely dissolved, and add NaOH aqueous solution dropwise to pH 11 to obtain 100g of stripping solution. Step 4) Peeling: Immerse the swollen steel plate in the peeling solution at a temperature of 60°C for 15 minutes. Step 5) Rinsing: Rinse the steel plate with deionized water for 30 seconds; Step 6) Drying: Air dry the surface moisture of the steel plate at room temperature to obtain a steel plate with the cathodic electrophoretic paint film removed and the phosphating film retained.

[0044] Example 2 Step 1) The swelling solution contains 5 wt% N-methylpyrrolidone, the swelling temperature is 40℃, and the swelling time is 10 min; Step 4) The stripping solution contains 10 wt% formic acid and 2 wt% thiourea, pH 10, stripping temperature 50℃, and stripping time 10 min. The remaining steps are the same as in Example 1.

[0045] Example 3 Step 1) The swelling solution contains 15 wt% N-methylpyrrolidone, the swelling temperature is 50℃, and the swelling time is 20 min; Step 4) The stripping solution contains 20wt% formic acid and 5wt% thiourea, pH 12, stripping temperature 70℃, and stripping time 20min. The remaining steps are the same as in Example 1.

[0046] Example 4 Step 1) The swelling solution contains 8 wt% N-methylpyrrolidone, the swelling temperature is 43℃, and the swelling time is 12 min; Step 4) The stripping solution consists of 12 wt% formic acid, 2.5 wt% thiourea, pH 10.5, stripping temperature 55℃, and stripping time 12 min. The remaining steps are the same as in Example 1.

[0047] Example 5 Step 1) The swelling solution contains 12 wt% N-methylpyrrolidone, the swelling temperature is 47℃, and the swelling time is 18 min; Step 4) The stripping solution contains 18 wt% formic acid and 4 wt% thiourea, with a pH of 11.5. The stripping temperature is 65°C, and the stripping time is 18 min. The remaining steps are the same as in Example 1.

[0048] Example 6 Step 1) The swelling solution contains 10 wt% N-methylpyrrolidone, the swelling temperature is 45℃, and the swelling time is 15 min; Step 4) The stripping solution contains 15 wt% formic acid and 3 wt% thiourea, pH 11, stripping temperature 60℃, and stripping time 15 min. Step 6) Change the drying method to a 60℃ oven and dry for 30 minutes; The remaining steps are the same as in Example 1.

[0049] II. Comparative Example Comparative Example 1 Step 1) Change the swelling solution to 100wt% deionized water, which does not contain N-methylpyrrolidone, with a swelling temperature of 45℃ and a swelling time of 15min; Step 4) The stripping solution is the same as in Example 1; The remaining steps are the same as in Example 1.

[0050] Comparative Example 2 Step 1) The swelling solution is the same as in Example 1; Step 4) Replace the stripping solution with 100g of deionized water, without adding formic acid, thiourea, or NaOH, and set the stripping temperature at 60℃ and the stripping time at 15min. The remaining steps are the same as in Example 1.

[0051] Comparative Example 3 Step 1) The swelling solution is the same as in Example 1; Step 4) The stripping solution contains 15wt% formic acid and 3wt% thiourea. The pH is adjusted to 2 with HCl. The stripping temperature is 60℃ and the stripping time is 15min. The remaining steps are the same as in Example 1.

[0052] Comparative Example 4 Step 1) The swelling solution is the same as in Example 1; Step 4) The stripping solution is changed to a mixture of 80wt% dichloromethane and 20wt% methanol, the stripping temperature is 40℃, and the stripping time is 15min; The remaining steps are the same as in Example 1.

[0053] III. Results Data Experimental methods for evaluating results: 1. Method for determining the peel rate of cathodic electrophoretic coating: The stripped and dried steel plate was placed on an analytical balance with an accuracy of 0.1 mg and its mass m1 was obtained. The residual paint film was manually wiped off with an acetone cotton ball, and then baked at 80℃ for 10 minutes. The mass m2 was obtained. The mass m0 is obtained by weighing unpainted steel plates of equal area. Calculation: Peeling rate = (1 (m1 m2) / (m0 m2))×100%.

[0054] 2. Method for determining the weight loss of phosphating film: Use 100cm before peeling 2 The template is used to define the area, and the initial mass M1 of the steel plate is obtained. After completing all peeling, rinsing, and drying, use chromic acid solution (50 g·L⁻¹) -1 Dissolve the phosphating film with CrO3 (25℃, 5min), rinse with deionized water, air dry, and weigh to obtain mass M2; Calculate: Phosphating film weight loss = (M1) M2) / 0.01m 2 Unit: g·m -2 .

[0055] 3. Method for determining peeling time: Record the total time from the start of swelling to the end of drying, accurate to 1 minute.

[0056] VOC emission measurement methods In 1m 3 The stripping was performed inside a sealed sampling chamber at a temperature of 25°C and a sampling flow rate of 1 L·min. -1The total VOC mass was determined by adsorption in Tenax tubes for 30 min, followed by thermal desorption-gas chromatography, and converted to mg·m³. -3 .

[0057] Table 1. Results data for both the examples and comparative examples.

[0058] As shown in Table 1, the inventiveness of this application's technical solution compared to the prior art is mainly reflected in: 1. Peel-off rate ≥ 98% and phosphating film weight loss ≤ 0.10 g·m -2 For the first time, the cathodic electrophoretic coating film was completely peeled off under low temperature and halogen-free conditions while preserving the integrity of the chemical conversion film, which solves the defect of existing technology where the phosphating film / zirconium film is destroyed simultaneously due to high temperature, halogen or strong acid. 2. The total peeling time is ≤40 minutes, which is ≥50% shorter than the existing technology. The temperature throughout the process is ≤70℃, and the energy consumption is reduced by ≥30%, overcoming the shortcomings of the existing technology which requires high temperature of 80-100℃ and takes tens of hours. 3. VOC emissions ≤15mg·m³ -3 Compared with existing technologies containing dichloromethane or ethylene glycol ether, it reduces emissions by ≥97%, meeting the requirements of halogen-free environmental regulations; 4. By using a two-step synergistic process of “5–15wt% N-methylpyrrolidone swelling solution + pH 10–12 formic acid-thiourea stripping solution”, the swelling rate of the cathodic electrophoretic coating film is quantitatively controlled at 20–60%, the cumulative decrease in interfacial adhesion is ≥50%, and the peeling force is reduced by ≥35%, thus achieving low-temperature rapid peeling. This quantitative synergistic relationship has not been revealed in existing technologies. 5. The stripping solution does not contain halogenated hydrocarbons, ethylene glycol ethers, chloroform, or chloroacetic acid, and the weight loss of the phosphating film is <0.1 g·m³. -2 This fills the gap in the field of halogen-free paint stripping where chemical conversion films cannot be preserved.

[0059] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for peeling off cathodic electrophoretic paint film from the surface of a steel plate, characterized in that, include: The steel plate is immersed in a swelling solution to cause the cathodic electrophoretic paint film to swell in volume. The swelling solution consists of 5–15 wt% N-methylpyrrolidone and the balance water. The immersion is carried out for 10–20 min at a temperature range of 40–50°C. The swollen steel plate is transferred into a stripping solution and the swollen cathodic electrophoretic paint film is separated from the surface of the steel plate. The stripping solution is composed of 10–20 wt% formic acid, 2–5 wt% thiourea, the balance water and NaOH adjusted to pH 10–12. The stripping is carried out for 10–20 min at a temperature range of 50–70°C. The stripped steel plate was rinsed with deionized water and dried to remove residual stripping liquid and detached cathodic electrophoretic paint film fragments.

2. The method according to claim 1, characterized in that, The swelling solution is composed of 10 wt% N-methylpyrrolidone and 90 wt% water, the immersion temperature is 45°C, and the immersion time is 15 min.

3. The method according to claim 1, characterized in that, The stripping solution consists of 15 wt% formic acid, 3 wt% thiourea and the balance water, and the pH of the stripping solution is adjusted to 11 with NaOH. The stripping temperature is 60°C and the stripping time is 15 min.

4. The method according to claim 1, characterized in that, The drying process is air drying at room temperature.

5. The method according to claim 1, characterized in that, The steel plate retains a phosphate or zirconium film before impregnation, and the phosphate or zirconium film remains intact after peeling.

6. A swelling solution for peeling off cathodic electrophoretic paint film from the surface of steel plates, characterized in that, The swelling solution consists of 5–15 wt% N-methylpyrrolidone and the balance water, and is used to swell the cathodic electrophoretic coating film for 10–20 min at a temperature range of 40–50 °C.

7. The swelling solution according to claim 6, characterized in that, The content of N-methylpyrrolidone is 10 wt%; the operating temperature of the swelling solution is 45°C, and the operating time is 15 min.

8. A stripping solution for peeling off cathodic electrophoretic paint film from the surface of steel plates, characterized in that, The stripping solution is composed of 10–20 wt% formic acid, 2–5 wt% thiourea, the balance water, and NaOH adjusted to pH 10–12, and is used to chemically strip swollen cathodic electrophoretic coatings for 10–20 min at a temperature range of 50–70°C.

9. The stripping fluid according to claim 8, characterized in that, The formic acid content is 15 wt%, the thiourea content is 3 wt%, and the pH is 11; the stripping solution is used at a temperature of 60°C for 15 minutes.

10. The stripping fluid according to claim 8 or 9, characterized in that, The stripping fluid is free of halogenated hydrocarbons, ethylene glycol ethers, chloroform, and chloroacetic acid.

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