Chromium-free pretreatment liquid, chromium-free color-coated sheet and preparation method thereof

Abstract

The invention discloses a chromium-free pretreatment solution, a chromium-free color-coated sheet and a preparation method of the chromium-free color-coated sheet, and belongs to the technical field of metal material surface treatment and protective coatings. The pretreatment liquid comprises a water-soluble carboxyl-containing organic matter, phosphate, a strong oxidant and inorganic acid, and is prepared through step-by-step mixing, and a compact pretreatment layer is formed on the surface of the hot-dip galvanized sheet by utilizing the synergistic complexing effect among the components. The pretreatment liquid is applied to a base plate in a roller coating mode, after curing is conducted at the temperature of 70-150 DEG C, chromium-free primer, finish paint and back paint are sequentially applied, and the chromium-free color-coated plate is obtained after baking. The obtained color-coated sheet has excellent corrosion resistance and adhesive force, the color-coated sheet is chromate-free in the whole process, and the dual requirements for environmental protection and performance in the fields of household appliances and the like are met.

Description

Technical Field

[0001] This invention belongs to the field of metal material surface treatment and protective coating technology, and relates to a chromium-free pretreatment liquid, a chromium-free color-coated plate and its preparation method. Background Technology

[0002] Color-coated steel sheets are composite sheets made from hot-dip galvanized steel sheets as the base material. After surface pretreatment (including degreasing, cleaning, and chemical conversion), an organic coating is applied, followed by baking and cooling. Color-coated sheets combine the excellent decorative and protective properties of an outer coating with the high strength, ease of processing, and corrosion resistance of hot-dip galvanized steel sheets. Furthermore, with increasingly stringent environmental regulations and growing restrictions on painting lines across China, they have found widespread application in various industries, including construction, home appliances, and automobiles.

[0003] In 2022, new national standards for coatings were released, reflecting increasingly stringent national environmental protection requirements. Testing and analysis methods are gradually shifting from holistic to layer-by-layer analysis, leading to the gradual phasing out of existing chromium-containing technologies. Environmentally friendly color-coated steel sheets are the inevitable direction for future development. Currently, most companies in China have achieved chromium-free primers, but very few have achieved chromium-free pretreatment.

[0004] CN116694199B discloses a chromium-free thick-film primer, a pre-coated thick-film plate, and a preparation method. The primer comprises a film-forming resin, a curing agent, inorganic pigments, rust-inhibiting pigments, additives, and a solvent. The film-forming resin is a mixture of polyester and epoxy resin, the curing agent is a compound system of amino resin and terminal isocyanate, and the rust-inhibiting pigment is a combination of ion-exchange rust-inhibiting pigments and phosphates. While this method achieves a chromium-free primer with good processability, its focus remains on the primer system and does not address the chromium-free pretreatment process.

[0005] CN102121104B provides a water-based chromium-free surface treatment liquid that combines pre-treatment and primer coating for galvanized steel sheets. Its components include a water-based resin, organosilane, inorganic corrosion inhibitor, sealant, stabilizer, and water. This treatment liquid can form a passivation film on the surface of galvanized steel sheets, with performance approaching that of chromate passivation, and exhibits good adhesion. It can be used directly as a primer, simplifying the coating process. However, this technology combines pre-treatment and primer functions into one, resulting in a relatively complex system. Furthermore, it does not focus on optimizing the compatibility with traditional multi-layer structures of color-coated steel sheets or addressing the specific functions of pre-treatment.

[0006] In summary, current environmentally friendly color-coated steel sheet technologies mainly focus on the research and development of chromium-free primers, while chromium-free solutions for the pretreatment stage are still rarely reported. Therefore, to promote the chromium-free process of color-coated steel sheets and meet the requirements of industries such as home appliances for material flexibility and comprehensive performance, developing a dedicated chromium-free pretreatment solution and its supporting color-coated steel sheet preparation method is of significant research importance and application value. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a chromium-free pretreatment liquid to fill the technological gap in the chromium-free pretreatment process of color-coated steel sheets.

[0008] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows.

[0009] In a first aspect, the present invention provides a chromium-free pretreatment solution, based on 100 parts by weight of water, comprising: 10-30 parts by weight of water-soluble carboxyl-containing organic matter, 2-10 parts by weight of phosphate, 0.5-5 parts by weight of strong oxidant, and 5-15 parts by weight of inorganic acid.

[0010] Furthermore, the above-mentioned chromium-free pretreatment solution, based on 100 parts by weight of water, comprises: 15-25 parts by weight of water-soluble carboxyl-containing organic matter, 5-10 parts by weight of phosphate, 2-5 parts by weight of strong oxidant, and 5-10 parts by weight of inorganic acid.

[0011] Among them, the above-mentioned water-soluble carboxyl-containing organic compounds are any one of maleic acid, glucose, tartaric acid, oxalic acid, malic acid, citric acid, ascorbic acid, phytic acid, lactic acid, and acrylic acid.

[0012] The aforementioned phosphate is any one of aluminum dihydrogen phosphate, magnesium dihydrogen phosphate, calcium dihydrogen phosphate, manganese dihydrogen phosphate, zinc dihydrogen phosphate, nickel dihydrogen phosphate, sodium monohydrogen phosphate, potassium monohydrogen phosphate, and rubidium monohydrogen phosphate.

[0013] The aforementioned strong oxidizing agent is any one of sodium nitrite, potassium nitrite, rubidium nitrite, sodium nitrate, potassium nitrate, zinc nitrate, magnesium nitrate, potassium permanganate, sodium permanganate, potassium metavanadate, sodium metavanadate, ammonium metavanadate, cerium nitrate, lanthanum nitrate, sodium molybdate, and potassium molybdate.

[0014] The inorganic acid mentioned above is any one of phosphoric acid, nitrous acid, hydrofluoric acid, fluorosilicic acid, fluorotitanic acid, iodic acid, boric acid, and nitric acid.

[0015] Secondly, the present invention provides a method for preparing the above-mentioned chromium-free pretreatment solution, comprising the following steps: S1. Dissolve phosphate in water to obtain a phosphate solution; S2. Add inorganic acid to the phosphate solution and stir to mix; S3. Add water-soluble carboxyl-containing organic compounds to the solution obtained in step S2, stir and mix to carry out a complexation reaction; S4. Add a strong oxidant to the solution obtained in step S3, stir and mix evenly to obtain a chromium-free pretreatment solution.

[0016] In step S2 above, the stirring time is 5 to 10 minutes.

[0017] In step S3 above, the complexation reaction takes 0.5 to 5 hours.

[0018] In step S4 above, the mixing time is 10~30 minutes.

[0019] Thirdly, the present invention provides a method for preparing a chromium-free color-coated steel sheet: the above-mentioned chromium-free pretreatment liquid is applied to the surface of a hot-dip galvanized steel sheet by roller coating to form a pretreatment layer; subsequently, the hot-dip galvanized steel sheet with the pretreatment layer is dried and cured at a temperature of 70~150°C for a curing time of not less than 12 seconds; finally, a chromium-free primer, a topcoat, and a backcoat are sequentially coated on the pretreatment layer and baked and cured to obtain a chromium-free color-coated steel sheet.

[0020] The coating thickness of the chromium-free color-coated steel sheet is 0.3~1.0µm.

[0021] Fourthly, the present invention provides a chromium-free color-coated sheet prepared by the above-described preparation method.

[0022] The beneficial effects of this invention: The chromium-free pretreatment solution provided by this invention uses water-soluble carboxyl-containing organic compounds, phosphates, strong oxidants, and inorganic acids as key components. Through the complexation and synergistic effect of these components, a dense and strongly adherent chromium-free pretreatment layer is formed on the surface of hot-dip galvanized steel sheets. The preparation method of this treatment solution, through stepwise mixing and sequential reaction, ensures sufficient complexation and system stability of the carboxyl-containing organic compounds and phosphates. Based on this treatment solution, the pretreatment layer can be efficiently prepared through roller coating and a low-temperature curing process of 70~150℃. The chromium-free color-coated steel sheets obtained by subsequently coating with chromium-free primer, topcoat, and backcoat possess excellent corrosion resistance and adhesion. Its salt spray corrosion resistance meets the requirements of ≤2mm corrosion spread after 240h and ≤6mm after 500h. The T-bend and cross-cut adhesion tests both reach excellent levels, thus fully meeting the comprehensive requirements of environmental protection, processability, and durability of color-coated steel sheets in the home appliance and other fields. Detailed Implementation

[0023] To make the technical problems, solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with the embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as understood by one of ordinary skill in the art.

[0024] In one specific embodiment of the present invention, a chromium-free pretreatment solution is provided, which is based on 100 parts by weight of water and comprises: 10-30 parts by weight of water-soluble carboxyl-containing organic matter, 2-10 parts by weight of phosphate, 0.5-5 parts by weight of strong oxidant, and 5-15 parts by weight of inorganic acid.

[0025] Preferably, based on 100 parts by weight of water, the composition includes: 15-25 parts by weight of water-soluble carboxyl-containing organic matter, 5-10 parts by weight of phosphate, 2-5 parts by weight of strong oxidant, and 5-10 parts by weight of inorganic acid.

[0026] Among them, the water-soluble carboxyl-containing organic compounds are any one of maleic acid, glucose, tartaric acid, oxalic acid, malic acid, citric acid, ascorbic acid, phytic acid, lactic acid, and acrylic acid; the phosphates are any one of aluminum dihydrogen phosphate, magnesium dihydrogen phosphate, calcium dihydrogen phosphate, manganese dihydrogen phosphate, zinc dihydrogen phosphate, nickel dihydrogen phosphate, sodium monohydrogen phosphate, potassium monohydrogen phosphate, and rubidium monohydrogen phosphate; the strong oxidizing agents are any one of sodium nitrite, potassium nitrite, rubidium nitrite, sodium nitrate, potassium nitrate, zinc nitrate, magnesium nitrate, potassium permanganate, sodium permanganate, potassium metavanadate, sodium metavanadate, ammonium metavanadate, cerium nitrate, lanthanum nitrate, sodium molybdate, and potassium molybdate; and the inorganic acids are any one of phosphoric acid, nitrous acid, hydrofluoric acid, fluorosilicic acid, fluorotitanic acid, iodic acid, boric acid, and nitric acid.

[0027] The chromium-free pretreatment solution of this invention forms a stable complex structure and reaction network in the system through the synergistic effect of four components: water-soluble carboxyl-containing organic matter, phosphate, strong oxidant, and inorganic acid. The inorganic acid provides a suitable acidic environment and activates the metal surface; the carboxyl groups in the water-soluble carboxyl-containing organic matter complex with phosphate ions and metal ions in the solution to form a soluble pre-complexed precursor; and the strong oxidant further promotes uniform activation and micro-roughening of the substrate surface. When the treatment solution is coated on the surface of a hot-dip galvanized sheet and cured, the pre-complexed structure decomposes and deposits at the interface, and the phosphate is converted into an insoluble phosphate film layer, which, together with the organic matter, constitutes a dense and strongly adhesive pretreatment layer. This layer not only firmly bonds to the substrate through chemical bonding and physical anchoring, but its surface organic functional groups can also undergo secondary cross-linking with the subsequent chromium-free primer resin. Simultaneously, the suitable surface roughness and porous structure enhance the physical integration with the primer. Through the aforementioned synergistic chemical and physical mechanism, a tight bond between the layers of the color-coated steel sheet is ensured, thereby significantly improving the overall corrosion resistance and adhesion of the color-coated steel sheet.

[0028] In one specific embodiment of the present invention, the preparation method of the above-mentioned chromium-free pretreatment solution is as follows: phosphate is dissolved in water to provide a phosphate ion source; then an inorganic acid is added and stirred for 5-10 minutes to construct a stable acidic reaction environment; next, a water-soluble carboxyl-containing organic compound is added, and a complexation reaction is carried out under stirring for 0.5-5 hours to form a stable pre-complexed intermediate with phosphate and metal ions; finally, a strong oxidant is added, and stirring is continued for 10-30 minutes until the system is homogeneous, avoiding premature interference of the oxidant with the complexation process and imparting the final oxidative activation ability to the system. This step-by-step addition sequence is designed based on the action mechanism of each component. By sequentially controlling the reaction process, sufficient synergy and transformation between the components are ensured, which is the key to obtaining a chromium-free pretreatment solution with stable performance, dense film formation, and excellent adhesion.

[0029] In one specific embodiment of the present invention, a method for preparing chromium-free color-coated steel sheet using the above-mentioned chromium-free pretreatment liquid is further provided: the chromium-free pretreatment liquid is uniformly applied to the surface of hot-dip galvanized steel sheet by roller coating to form a pretreatment layer with a thickness of 0.3~1.0 μm; then, the hot-dip galvanized steel sheet with the pretreatment layer is dried and cured at 70~150°C for a curing time of not less than 12 seconds; after that, a chromium-free primer, a topcoat, and a backcoat are sequentially coated on the pretreatment layer, and cured by baking to finally obtain the chromium-free color-coated steel sheet.

[0030] The following specific embodiments will be provided to explain the solution of the present invention. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0031] Example 1: Preparation of Chromium-Free Pretreatment Solution 1. To verify the effectiveness of the technical solution of the present invention, nine sets of examples (Examples 1-9) and four sets of comparative examples (Comparative Examples 1-4) were set up. The specific composition and content of the treatment solution in each set (based on 100 parts by weight of water) are shown in Table 1.

[0032] Table 1. Composition of pretreatment solutions in the examples and comparative examples In Examples 1-9 and Comparative Examples 1-4, the specific selection of each component is as follows: Examples 1-5 and Comparative Examples 1-4: The water-soluble carboxyl-containing organic compound was tartaric acid, the phosphate was aluminum dihydrogen phosphate, the strong oxidant was ammonium metavanadate, and the inorganic acid was boric acid.

[0033] Example 6: Compared with Example 1, the water-soluble carboxyl-containing organic compound was replaced with phytic acid, while the other components remained unchanged.

[0034] Example 7: Compared with Example 1, the phosphate was replaced with manganese dihydrogen phosphate.

[0035] Example 8: Compared with Example 1, the strong oxidant was replaced with sodium nitrite.

[0036] Example 9: Compared with Example 1, the inorganic acid was replaced with fluorotitanic acid, while the other components remained unchanged.

[0037] 2. All the above treatment solutions were prepared according to the preparation method described in this invention, and the specific steps are as follows: (1) Weigh each component according to the formula in Table 1; (2) Dissolve the phosphate in water to obtain a phosphate solution; (3) Add inorganic acid to the phosphate solution and stir for 5-10 minutes to mix it thoroughly; (4) Add a water-soluble carboxyl-containing organic compound to the above mixture and stir to carry out a complexation reaction for 0.5 to 5 hours; (5) Finally, add a strong oxidant and continue stirring for 10 to 30 minutes until the mixture is uniform, and obtain the chromium-free pretreatment solution for each example.

[0038] Example 2: Preparation and performance testing of chromium-free color-coated steel sheets 1. Preparation of coated plates The chromium-free pretreatment solutions prepared in Examples 1-9 and Comparative Examples 1-4 were uniformly coated onto the surface of hot-dip galvanized sheets using a roller coating method. Subsequently, the sheets were cured at 100°C for 12 seconds to form a pretreatment layer. Next, a chromium-free primer, a polyester topcoat, and a back coat were sequentially applied to both sides of the sheet with the pretreatment layer. The dry film thickness of the primer was controlled to be 5±1 μm, the topcoat to be 15±1 μm, and the back coat to be 5±1 μm. Finally, the sheets were baked and cured at 240°C to obtain chromium-free color-coated sheet samples, which were labeled as Example 1-9 and Comparative Example 1-4.

[0039] 2. Performance Testing Methods and Standards The above-mentioned coated panel samples underwent comprehensive performance testing. The standards and evaluation methods used for each test are as follows: Corrosion resistance: Refer to the "Neutral salt spray test" in GB / T 13448-2019 "Test Methods for Color-Coated Steel Sheets and Strips". The sample surface is scratched with an X-shaped pattern. A 50 g / L NaCl solution is used, the test temperature is 35±2℃, and the salt spray deposition is 1~2 mL / (h·80 cm⁻¹). 2The samples were placed at an angle of 15-25° to the vertical. The corrosion spread distance was tested after 240 h and 500 h, respectively. The conventional requirements are ≤2 mm spread after 240 h and ≤6 mm spread after 500 h.

[0040] Adhesion: Evaluation is conducted using the cross-cut adhesion test and T-bend test specified in GB / T 13448-2019. For standard color-coated steel sheets, the cross-cut adhesion rating is 0, and the T-bend adhesion rating is ≤3.

[0041] 3. Test results: The performance test results of all samples are shown in Table 2.

[0042] Table 2 Performance Test Results of Chromium-Free Color Coated Steel Sheets Analysis of the formulation in Table 1 and the performance data in Table 2 shows that the chromium-free pretreatment solutions prepared in Examples 1-9 of this invention can all form a high-performance pretreatment layer on the surface of hot-dip galvanized steel sheets, ultimately resulting in chromium-free color-coated steel sheets with excellent overall performance. The corrosion resistance of the color-coated steel sheets in all examples meets the requirements of ≤2 mm corrosion spread after 240 h and ≤6 mm corrosion spread after 500 h. Furthermore, the T-bend and cross-cut adhesion test results all exceed the standards for conventional color-coated steel sheets, indicating that they possess good processability and interlayer bonding strength.

[0043] In contrast, comparative examples L1-L4, lacking any one of the components among water-soluble carboxyl-containing organic compounds, phosphates, strong oxidants, and inorganic acids, respectively, resulted in a significant decrease in the corrosion resistance and adhesion of their color-coated steel sheets, even failing to meet basic usage requirements. This result indicates that the four components—water-soluble carboxyl-containing organic compounds, phosphates, strong oxidants, and inorganic acids—in the chromium-free pretreatment solution described in this invention constitute a synergistic and indispensable organic system. The absence of any single component will disrupt the integrity of the system and the overall performance of the final color-coated steel sheet. The components of the chromium-free pretreatment solution described in this invention collectively achieve high corrosion resistance, strong adhesion, and excellent processing stability of the color-coated steel sheet under chromium-free conditions.

Claims

1. A chromium-free pretreatment liquor, characterized in that, The composition includes, based on 100 parts by weight of water: 10 to 30 parts by weight of a water-soluble carboxyl-containing organic substance, 2 to 10 parts by weight of a phosphate, 0.5 to 5 parts by weight of a strong oxidizing agent, and 5 to 15 parts by weight of an inorganic acid.

2. The chromium-free pretreatment solution of claim 1, wherein, The composition includes, based on 100 parts by weight of water: 15 to 25 parts by weight of a water-soluble carboxyl-containing organic substance, 5 to 10 parts by weight of a phosphate, 2 to 5 parts by weight of a strong oxidizing agent, and 5 to 10 parts by weight of an inorganic acid.

3. The chromium-free pretreatment solution according to claim 1 or 2, characterized in that, At least one of the following is satisfied: The water-soluble carboxyl-containing organic substance is any one of maleic acid, glucose, tartaric acid, oxalic acid, malic acid, citric acid, ascorbic acid, phytic acid, lactic acid, and acrylic acid; The phosphate is any one of aluminum dihydrogen phosphate, magnesium dihydrogen phosphate, calcium dihydrogen phosphate, manganese dihydrogen phosphate, zinc dihydrogen phosphate, nickel dihydrogen phosphate, sodium monohydrogen phosphate, potassium monohydrogen phosphate, and rubidium monohydrogen phosphate; The strong oxidizing agent is any one of sodium nitrite, potassium nitrite, rubidium nitrite, sodium nitrate, potassium nitrate, zinc nitrate, magnesium nitrate, potassium permanganate, sodium permanganate, potassium metavanadate, sodium metavanadate, ammonium metavanadate, cerium nitrate, lanthanum nitrate, sodium molybdate, and potassium molybdate; The inorganic acid is any one of phosphoric acid, nitrous acid, hydrofluoric acid, fluorosilicic acid, fluorotitanic acid, iodic acid, boric acid, and nitric acid.

4. A method for producing the chromium-free pretreatment liquid according to any one of claims 1 to 3, characterized in that, The method includes the following steps: S1. Dissolving the phosphate in water to obtain a phosphate solution; S2. Adding the inorganic acid to the phosphate solution and stirring to mix; S3. Adding the water-soluble carboxyl-containing organic substance to the solution obtained in step S2, stirring to mix, and performing a complexation reaction; S4. Adding the strong oxidizing agent to the solution obtained in step S3, stirring to mix, and obtaining a chromium-free pretreatment solution.

5. The method of claim 4, wherein the chromium-free pretreatment solution is prepared by the steps of: In step S2, the stirring time is 5 to 10 minutes.

6. The method of claim 4, wherein the chromium-free pretreatment solution is prepared by the steps of: In step S3, the complexation reaction time is 0.5 to 5 hours.

7. The method for preparing the chromium-free pretreatment solution according to claim 4, characterized in that: In step S4, the stirring time is 10 to 30 minutes.

8. A method for producing a chromium-free color coated steel sheet, characterized by: The chromium-free pretreatment solution of any one of claims 1 to 3 is applied to the surface of a hot-dip galvanized sheet by roll coating to form a pretreatment layer; Subsequently, the hot-dip galvanized sheet with the pretreatment layer is dried and cured at a temperature of 70 to 150°C for a curing time of no less than 12 seconds; Finally, a chromium-free color-coated sheet is prepared by sequentially applying a chromium-free primer, a topcoat, and a backcoat to the pretreatment layer and baking and curing the coatings.

9. The method of producing a chromium-free color coated steel sheet according to claim 8, characterized in that: The chromium-free color-coated sheet has a coating thickness of 0.3 to 1.0 µm.

10. A chromium-free color-coated sheet prepared by the method of claim 8 or 9.

Images