A polyurethane primer for rapidly improving the adhesion of coil coating and its preparation method and application

Abstract

The invention discloses a polyurethane primer for quickly improving the adhesion of coil coating and its preparation method and application, and relates to the field of metal iron-based surface coating, and is prepared by the following components in percentage by mass: 40%-50% polyhydroxyl branched polyurethane resin, 5%-10% titanium dioxide, 2%-5% curing agent, 3%-7% adhesion promoter, 5%-9% amino resin, 7%-11% blocked isocyanate, 5%-9% auxiliary agent, 6%-12% antirust pigment, 7%-11% solvent. The present invention improves the adhesion between the primer and the substrate and the topcoat by synthesizing polyhydroxyl linear polyurethane primer to improve the wipe resistance of the color-coated plate, meet the existing technical standards of the existing color-coated plate, and improve the impact resistance grade of the color-coated plate. The color-coated plate coated with the polyurethane primer of the present invention has a wipe resistance of up to 160 times, achieving a good balance of flexibility, hardness and excellent adhesion.

Description

Technical Field

[0001] The invention relates to the field of metal iron-based surface coating, and in particular to a polyurethane primer for rapidly improving the adhesion of a coil coating, and a preparation method and application thereof. Background Art

[0002] Color coated sheet is abbreviated as color coated sheet. It is a product made of cold rolled steel sheet, galvanized steel sheet or aluminum sheet and other metal sheets as substrates. After surface pretreatment, the coating is applied in a continuous manner, and finally baked and cooled. Color coated sheet is light, beautiful and has good corrosion resistance. It is widely used in construction, shipbuilding, vehicle manufacturing, furniture, electrical, decoration and other industries. In order to meet the higher processing requirements of coil coating, coil coating should not only have good corrosion resistance, but also good adhesion and flexibility.

[0003] When processing color coated steel plates, not only the deformation of the steel plate coating should be considered, but also the scratches and plastic deformation resistance of the coating should be taken into account. When processing color coated steel plates, metal processing lubricants are generally not used, but the compactness and certain hardness of the coating are required for the forming machinery. From the longitudinal structure of the color coated plate, the color coated plate is divided into topcoat, primer, passivation layer and substrate; from the perspective of their respective functions, the primer should have good corrosion resistance and adhesion to the substrate and topcoat, and the topcoat should have good hiding power and decorative properties, and should also have good flexibility, resistance to organic solvent wiping and hardness to meet the overall processing and use requirements of the color coated plate. How to ensure the beautiful life of ordinary architectural color coated plates for at least ten years is a job that must be checked in advance for product quality inspection. The items that need to be quickly inspected for color coated plates off the line in the industry include film thickness, color difference, pencil hardness, gloss, impact, T-bend and wiping resistance. The wipe resistance is generally used to examine the solvent resistance of the topcoat, which indirectly reflects the baking process of the overall coating and the density of the topcoat, thus guiding the on-site process adjustment and the adjustment of the density of the coating system of the paint and topcoat. Therefore, if the wipe resistance of the offline coil is unqualified, the inertial technical idea is to adjust the curing process and the density of the topcoat, such as increasing the cross-linking density of the topcoat.

[0004] Chinese patent CN 117987839 A "A method for improving the adhesion of metal surfaces" treats the metal surface through low-temperature plasma. The invention mainly uses low-temperature plasma to roughen the metal surface, thereby increasing the roughness to improve the adhesion, and the object of action is aluminum alloy. Chinese patent CN 108301009 A "A method for improving the adhesion of color-coated steel plates" improves the adhesion by increasing the roughness through chemical etching in pre-treatment, thereby improving the adhesion level of the color-coated plate. Chinese patent CN104149452A "A VHP-resistant color-coated plate" also improves the paint film on the surface of the color-coated plate to achieve excellent chemical resistance and excellent adhesion. The above-mentioned invention patents or prior art ideas are to improve the adhesion and topcoat density by increasing the roughness of the substrate to improve the coating's wipe resistance, but none of them involve adjusting the adhesion of the primer to improve the wipe resistance of the color-coated plate.

[0005] Therefore, it is necessary to develop a polyurethane primer that can quickly improve the adhesion of the coil coating, thereby improving the wipe resistance of the color-coated plate by improving the adhesion between the primer and the substrate (i.e., the passivated plate) and the topcoat, and solving the problem of paint peeling due to poor wipe resistance of the coil coating. Summary of the invention

[0006] The purpose of the present invention is to solve the shortcomings of the above-mentioned background technology and provide a polyurethane primer that can quickly improve the adhesion of the steel coil coating. By improving the adhesion between the primer and the substrate and the topcoat, the wipe resistance of the color-coated plate is improved, and the problem of paint peeling due to poor wipe resistance of the steel coil coating is solved.

[0007] The technical solution of the present invention is: a polyurethane primer for quickly improving the adhesion of a coil coating, which is prepared from the following components in percentage by mass:

[0008] 40%-50% polyhydroxyl branched polyurethane resin, 5%-10% titanium dioxide, 2%-5% curing agent, 3%-7% adhesion promoter, 5%-9% amino resin, 7%-11% blocked isocyanate, 5%-9% additive, 6%-12% anti-rust pigment, 7%-11% solvent, the sum of the mass percentages of the above components is 100%;

[0009] The multi-terminal hydroxyl branched polyurethane resin is prepared from the following raw materials in percentage by mass: 8%-15% of phthalic anhydride, 10%-20% of terephthalic acid, 5%-10% of adipic acid, 15%-25% of 1,6-hexanediol, 8%-15% of neopentyl glycol, 10%-20% of meta-xylene, and 8%-12% of cyclohexanone, and the sum of the percentages by mass of the above raw materials is 100%.

[0010] In the above scheme, the dosage of multi-terminal hydroxyl branched polyurethane resin, amino resin and adhesion promoter is very critical:

[0011] The sum of the percentage of the polyhydroxyl branched polyurethane resin + amino resin is in the range of 45%-59%. On the one hand, if the sum of the two contents is too much (more than 59%), the rigid segments in the amino resin and the polyhydroxyl branched polyurethane resin will be cured, and the coating will have a large crosslinking density, high hardness, and excellent solvent resistance, but the processing performance will decrease; on the other hand, from the perspective of the corrosion resistance of the coating, if the two contents are too low (less than 45%), the corrosive medium will more easily penetrate the coating, and the water resistance will be poor. Therefore, the polyhydroxyl branched polyurethane resin + amino resin of the present invention needs to be 45%-59% to balance various properties.

[0012] The dosage ratio of multi-terminal hydroxyl branched polyurethane resin / adhesion promoter is in the range of 5.7-16.7:1. The purpose of such setting is that when the dosage ratio of multi-terminal hydroxyl branched polyurethane resin to adhesion promoter is less than 5.7:1, the hydroxyl groups in the multi-terminal hydroxyl branched polyurethane resin cannot be fully cured. The resin is the main bearer of various properties of the coating. If the content is too low, the cross-linking density is too low, which seriously affects the salt spray resistance, acid and alkali resistance and other properties of the coating. However, when the dosage ratio of multi-terminal hydroxyl branched polyurethane resin to adhesion promoter is greater than 16.7:1, the relatively insufficient content of adhesion promoter leads to insufficient cohesion of the primer, which can easily lead to self-decomposition of the primer during actual use, thereby causing the coating to lose its function of protecting the substrate.

[0013] Preferably, the polyhydroxyl-terminated branched polyurethane resin is prepared by a method comprising the following steps:

[0014] Phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and m-xylene are put into a reaction kettle and nitrogen is introduced. The temperature is raised to 200-220°C and the reaction is kept warm for 3-4 hours. The temperature is then raised to 250-280°C and the reaction is kept warm until the reaction liquid meets the acid value of less than 5mgKOH / g and the coating-4 cup viscosity is 15-25s. Vacuuming is carried out for 30-45 minutes. Cyclohexanone is added after the temperature is lowered to 100-120°C. The material is fully stirred and filtered to obtain a multi-terminal hydroxyl branched polyurethane resin.

[0015] Further, the temperature is increased to 200-220°C at a rate of 20-40°C / h, and then increased to 250-280°C at a rate of 20-40°C / h.

[0016] Preferably, the solvent comprises propylene glycol and ethylene glycol methyl ether acetate in a volume ratio of 1-1.2:2.

[0017] Preferably, the auxiliary agent includes a dispersant, a leveling agent, and a defoamer, all of which are BYK products, including dispersant BYK115, leveling agent BYK392, and defoamer BYK054.

[0018] Preferably, the adhesion promoter is FAT590.

[0019] Preferably, it is prepared by comprising the following components in percentage by mass:

[0020] 40%-50% multi-terminal hydroxyl branched polyurethane resin, 5%-10% titanium dioxide, 2%-4% curing agent, 3%-7% adhesion promoter, 6%-7% amino resin, 8%-11% blocked isocyanate, 8%-9% additives, 6%-9% anti-rust pigment, 7%-8% solvent, the sum of the mass percentages of the above components is 100%.

[0021] Preferably, the multi-terminal hydroxyl branched polyurethane resin is prepared from the following raw materials in percentage by mass: 13%-14% phthalic anhydride, 10%-20% terephthalic acid, 8%-10% adipic acid, 20%-23% 1,6-hexanediol, 10%-14% neopentyl glycol, 16%-20% m-xylene, and 10%-12% cyclohexanone, and the sum of the percentages by mass of the above raw materials is 100%.

[0022] The present invention also provides a method for preparing any of the above polyurethane primers for quickly improving the adhesion of coil coatings, comprising the following steps:

[0023] Preparation of polyhydroxyl-terminated branched polyurethane resin;

[0024] The prepared multi-terminal hydroxyl branched polyurethane resin, titanium dioxide, curing agent, adhesion promoter, amino resin, blocked isocyanate, additives, anti-rust pigment and solvent are uniformly stirred, ground and dispersed to obtain a multi-terminal hydroxyl branched polyurethane primer.

[0025] Preferably, the preparation of the multi-terminal hydroxyl branched polyurethane resin specifically includes: putting phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and m-xylene into a reaction kettle and introducing protective nitrogen, heating to 200-220°C and keeping the temperature for reaction for 3-4 hours, then heating to 250-280°C and keeping the temperature for reaction until the reaction liquid satisfies the acid value of <5mgKOH / g and the coating-4 cup viscosity is 15-25s, vacuuming for 30-45min, cooling to 100-120°C, adding cyclohexanone, fully stirring and filtering the material to obtain the multi-terminal hydroxyl branched polyurethane resin.

[0026] The present invention also provides the use of any of the above polyurethane primers for quickly improving the adhesion of coil coatings, which is used as primer for color-coated plates.

[0027] The design principle of each chemical component in the primer formula used for the color-coated plate with excellent wipe resistance of the present invention is: the position and number of the hydroxyl group of the polyurethane resin have a great influence on the performance of the primer. If the hydroxyl group is located in the middle of the molecular chain, through the condensation reaction between molecules, it can provide a higher cross-linking density of the coating, thereby ensuring the good hardness and wipe resistance of the coating, but it will lead to a corresponding decrease in the flexibility of the coating. For ordinary primers for coiled steel, a linear polyurethane resin with a higher relative molecular mass is generally selected, and its hydroxyl group is generally located at both ends of the molecular chain. The cross-linking of the hydroxyl groups at both ends of the molecular chain can improve the hardness, wipe resistance and good flexibility of the coating to a certain extent, but at the same time, the cross-linking density of the resin is relatively low, thereby affecting the wipe resistance of the entire color-coated plate. Increasing the number of terminal hydroxyl groups can also increase the cross-linking density accordingly, and the wipe resistance is improved accordingly, but the number of terminal hydroxyl groups is limited, resulting in average hardness and wipe performance. In addition, the fluctuation of the production line process window can easily lead to unqualified wipe resistance of the coating.

[0028] The principle of preparing the multi-terminal hydroxyl branched polyurethane resin by using phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and xylene is: monomers of acid, anhydride and polyol are gradually polymerized to generate a molecular chain, the main chain of the molecule contains carbamate groups, and also contains unreacted hydroxyl groups, ether groups, ester groups, amide groups and other groups, which are easy to form intermolecular van der Waals forces and hydrogen bonds, so the multi-terminal hydroxyl branched polyurethane resin has good adhesion and flexibility.

[0029] The beneficial effects of the present invention are:

[0030] 1. A branched polyurethane resin with multiple terminal hydroxyl groups was synthesized, and an amino resin and a blocked isocyanate were used as crosslinkers to prepare a new type of modified polyurethane primer with multiple terminal hydroxyl groups suitable for coiled steel. The purpose of synthesizing branched polyurethane resins is to improve the flexibility of polyurethane resins by introducing branched hydroxyl groups. The crosslinking density of branched polyurethane resins is relatively high, thereby improving the corrosion resistance of the product at the same time; multiple terminal hydroxyl groups make the molecular polarity of polyurethane resins stronger and can form more hydrogen bonds. Hydrogen bonds play a role in physical crosslinking. The more hydrogen bonds there are, the stronger the intermolecular forces are, and the higher the strength of polyurethane materials.

[0031] 2. The adhesion of the primer to the passivation plate and the topcoat is improved by synthesizing a polyhydroxyl-terminated linear polyurethane primer, thereby improving the wiping resistance of the color-coated plate, meeting the existing technical standards of the existing color-coated plate, and improving the impact resistance level of the color-coated plate. The color-coated plate coated with the polyurethane primer of the present invention has a wiping resistance of up to 160 times, achieving a good balance of flexibility, hardness and excellent adhesion. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1The original surface and the surface state after wiping of Example 1 of the present invention (the black part on the left is the original state, and the white part on the right is the state after being wiped 150 times with MEK);

[0033] Figure 2 The state of the color-coated plate of comparative example 2 after poor wipe resistance and microscopic scanning elements;

[0034] Figure 3 For Figure 2 SEM morphology of the coating peeling off the light board

[0035] Figure 4 for Figure 3 Select 4 elements and content diagrams

[0036] Figure 5 The state of the color-coated plate of comparative example 3 after the poor wiping resistance

[0037] Figure 6 For Figure 5 SEM morphology of the coating peeling off the light board

[0038] Figure 7 for Figure 6 Select 2 elements and content diagrams

[0039] Figure 8 The top of the cup of comparative example 4 is slightly off. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0041] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0042] The present invention intends to improve the wiping resistance of color-coated sheets. Ordinary primers for coiled steel generally select linear polyurethane resins with higher relative molecular weights, whose hydroxyl groups are generally located at both ends of the molecular chain, and the cross-linking density is relatively low, which in turn affects the wiping resistance of the entire color-coated sheet. The number of terminal hydroxyl groups is limited, resulting in average final hardness and wiping performance. If the hydroxyl group is located in the middle of the molecular chain, the condensation reaction between molecules can provide a higher cross-linking density of the coating, thereby ensuring better hardness and wiping resistance of the coating, but it will correspondingly lead to a decrease in the flexibility of the coating. Therefore, it is necessary to develop a polyurethane primer with excellent adhesion to improve the wiping resistance of the color-coated sheet, while having good flexibility and hardness.

[0043] To further describe the present invention, a method for rapidly improving the adhesion of a steel coil coating is described in more detail below in conjunction with an embodiment. The raw materials and models used are as follows:

[0044] Phthalic anhydride and terephthalic acid: purchased from Mitsubishi Chemical;

[0045] Adipic acid and 1,6-ethylene glycol: purchased from Anaiji;

[0046] Neopentyl glycol, m-xylene, propylene glycol, ethylene glycol methyl ether acetate: purchased from Sinopharm Group;

[0047] Titanium dioxide: model rutile titanium dioxide R-2009, produced by Maanshan Jinxing Chemical;

[0048] Polyurethane curing agent: Model BI301, a product of Tosoh Corporation of Japan;

[0049] Anti-rust pigment: domestically produced and commercially available;

[0050] Adhesion promoter: Model FAT590, produced by Zengcheng Boya Chemical Co., Ltd.

[0051] Amino resin: Model: methyl etherified amino resin YP5650W, produced by Kaiyin Chemical;

[0052] Blocked isocyanate: model WANNATE HT 100, purchased from Kain Chemical;

[0053] Polyester topcoat: Model DJ-216-98601, produced by Beko Industrial Coatings (Shanghai) Co., Ltd.

[0054] Additives: including dispersant BYK115, leveling agent BYK392, and defoamer BYK054, purchased from BYK Chemical.

[0055] The ingredients of the embodiments and comparative examples are shown in Table 1 below (wt %).

[0056] Table 1 Example and comparative example composition control

[0057]

[0058] Example 1

[0059] This embodiment provides a method for preparing a polyhydroxyl-terminated branched polyurethane resin, which is as follows:

[0060] Prepare raw materials according to mass percentage: 13% phthalic anhydride, 20% terephthalic acid, 8% adipic acid, 23% 1,6-hexanediol, 10% neopentyl glycol, 16% m-xylene, and 10% cyclohexanone;

[0061] Phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and xylene are added to a clean reactor in proportion, and inert protective gas N2 is introduced at the same time. The temperature is raised. After the reactants are melted, stirring is slowly started, and the heating rate is controlled at 20°C / h, until the temperature rises to 210°C and is kept warm for 4 hours. Then the temperature is slowly raised to 280°C at 30°C / h and kept warm for reaction, and the acid value and Tu-4 cup viscosity are measured intermittently. When the acid value is less than 5mgKOH / g and the viscosity is 15s-25s, vacuum is applied for 0.5h, the temperature is lowered to 100°C, cyclohexanone is added, the material is fully stirred and filtered to obtain a multi-terminal hydroxyl branched polyurethane resin.

[0062] This embodiment also provides a method for preparing a polyurethane primer using the above multi-terminal hydroxyl branched polyurethane resin, which is specifically as follows:

[0063] 40% of multi-terminal hydroxyl branched polyurethane resin, 10% of titanium dioxide, 4% of curing agent, 3% of adhesion promoter, 6% of amino resin, 11% of blocked isocyanate, 9% of additives (including 4% of BYK115 as dispersant, 3% of BYK392 as leveling agent, and 2% of BYK054 as defoamer), 9% of anti-rust pigment and 8% of solvent (obtained by propylene glycol and ethylene glycol methyl ether acetate in a volume ratio of 1:2) are mixed evenly by mass percentage, ground and dispersed to a fineness of less than 25 μm to obtain a multi-terminal hydroxyl branched polyurethane primer.

[0064] Example 2

[0065] This embodiment provides a method for preparing a polyhydroxyl-terminated branched polyurethane resin, which is as follows:

[0066] Prepare raw materials according to mass percentage: 14% phthalic anhydride, 10% terephthalic acid, 10% adipic acid, 20% 1,6-hexanediol, 14% neopentyl glycol, 20% m-xylene, and 12% cyclohexanone;

[0067] Phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and xylene are added to a clean reactor in proportion, and inert protective gas N2 is introduced at the same time. The temperature is raised. After the reactants are melted, stirring is slowly started, and the heating rate is controlled at 40°C / h, until the temperature rises to 220°C and is kept warm for 4 hours. Then the temperature is slowly raised to 250°C at 20°C / h for reaction, and the acid value and Tu-4 cup viscosity are measured intermittently. When the acid value is less than 5mgKOH / g and the viscosity is 15s-25s, vacuum is applied for 0.5h, the temperature is lowered to 100°C, cyclohexanone is added, the material is fully stirred and filtered to obtain a multi-terminal hydroxyl branched polyurethane resin.

[0068] This embodiment also provides a method for preparing a polyurethane primer using the above multi-terminal hydroxyl branched polyurethane resin as follows:

[0069] 50% of multi-terminal hydroxyl branched polyurethane resin, 5% of titanium dioxide, 2% of curing agent, 7% of adhesion promoter, 7% of amino resin, 8% of blocked isocyanate, 8% of additives (including 3% of BYK115 as dispersant, 3% of BYK392 as leveling agent, and 2% of BYK054 as defoamer), 6% of anti-rust pigment and 7% of solvent (obtained by propylene glycol and ethylene glycol methyl ether acetate in a volume ratio of 1:2) are mixed evenly by mass percentage, ground and dispersed to a fineness of less than 25 μm to obtain a multi-terminal hydroxyl branched polyurethane primer.

[0070] Example 3

[0071] This embodiment provides a method for preparing a polyhydroxyl-terminated branched polyurethane resin, which is as follows:

[0072] Prepare raw materials according to mass percentage: 15% phthalic anhydride, 14% terephthalic acid, 10% adipic acid, 19% 1,6-hexanediol, 13% neopentyl glycol, 20% m-xylene, and 9% cyclohexanone;

[0073] Phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and xylene are added to a clean reactor in proportion, and inert protective gas N2 is introduced at the same time. The temperature is raised. After the reactants are melted, stirring is slowly started, and the heating rate is controlled at 25°C / h, until the temperature rises to 220°C and is kept warm for 4 hours, and then the temperature is raised to 270°C at 25°C / h and kept warm for reaction, and the acid value and Tu-4 cup viscosity are measured intermittently. When the acid value is less than 5mgKOH / g and the viscosity is 15s-25s, evacuate for 0.5h, cool to 100°C, add cyclohexanone, stir well and filter the material to obtain a multi-terminal hydroxyl branched polyurethane resin.

[0074] This embodiment also provides a method for preparing a polyurethane primer using the above multi-terminal hydroxyl branched polyurethane resin as follows:

[0075] 42% of multi-terminal hydroxyl branched polyurethane resin, 8% of titanium dioxide, 5% of curing agent, 5% of adhesion promoter, 9% of amino resin, 7% of blocked isocyanate, 5% of additives (including 2% of BYK115 as dispersant, 2% of BYK392 as leveling agent, and 1% of BYK054 as defoamer), 10% of anti-rust pigment and 9% of solvent (obtained from propylene glycol and ethylene glycol methyl ether acetate in a volume ratio of 1:2) are mixed uniformly by mass percentage, ground and dispersed to a fineness of less than 25 μm to obtain a multi-terminal hydroxyl branched polyurethane primer.

[0076] Comparative Example 1

[0077] This comparative example provides a method for preparing a polyurethane primer, which is the same as Example 1 except that the multi-terminal hydroxyl branched polyurethane resin prepared in Example 1 is replaced with a single-terminal hydroxyl branched polyurethane resin (model and source: IROGRAN A 95D 6013DP, Shanghai Yunhe Material Technology Co., Ltd.).

[0078] Comparative Example 2

[0079] This comparative example provides a method for preparing a polyurethane primer, which is the same as Example 1 except that the multi-terminal hydroxyl branched polyurethane resin prepared in Example 1 is replaced with a multi-terminal hydroxyl linear polyurethane resin (model and source: WPU-106, Anhui Yuanchen New Materials Technology Co., Ltd.).

[0080] Comparative Example 3

[0081] This comparative example provides a method for preparing a polyurethane primer, which is the same as Example 1 except that the multi-terminal hydroxyl branched polyurethane resin prepared in Example 1 is replaced with a single-terminal hydroxyl linear polyurethane resin (model and source: ADM-F209, Shandong Aodemei Polymer Materials Co., Ltd.).

[0082] Comparative Example 4

[0083] This comparative example provides a method for preparing a polyurethane primer, except that the amount of amino resin is 11% which does not meet the specified range of the present invention, the amount of blocked isocyanate is 7%, and the amount of auxiliary agent is 5% (including 2% BYK115, 2% BYK392, and 1% defoaming agent BYK054), and the rest is the same as Example 2.

[0084] Comparative Example 5

[0085] This comparative example provides a method for preparing a polyurethane primer, except that the amount of adhesion promoter is 8% which does not meet the specified range of the present invention, the amount of titanium dioxide is 7%, and the amount of amino resin is 7%, and the rest is the same as Example 3.

[0086] Performance Testing

[0087] The above primers were applied on the substrate (the substrate was an aluminum-zinc passivated plate), the front surface paint was a polyester coating with a film thickness of 15 microns, the primer film thickness was 5 microns, and the same curing process (specifically primer PMT224°C, topcoat PMT241°C) was used to produce a color-coated plate.

[0088] The above color-coated sheets were subjected to the following tests: the test methods and conditions refer to GB / T 13448 "Test methods for color-coated steel sheets and strips".

[0089] MEK (methyl ethyl ketone) wiping resistance test, the test conditions are: Wrap the entire contact area of ​​the artificial finger with absorbent cotton, soak the artificial finger with the specified solvent, and then place the artificial fingertip on the surface of the sample, away from the edge of the sample to avoid wetting the edge. Move the fingertip according to the specified number of wipes, the speed is one back and forth per second, and the movement must be continuous. After the specified number of wipes, observe whether the coating is damaged. The coating damage at both ends of the wiped area is not counted.

[0090] The results are shown in Table 2 below.

[0091] Table 2 Color coated sheet properties

[0092]

[0093] As can be seen from Table 2 above, except for the different terminal hydroxyl content of the polyurethane resin, the other parts of Comparative Example 1 and Example 1 are the same, and the wipe resistance of Comparative Example 1 is significantly lower than that of Example 1. The reason is that the type of resin is one of the key factors affecting the wipe resistance of the color-coated plate coating, and the more terminal hydroxyl groups, the better the MEK wipe resistance. The increase in the number of hydroxyl groups increases the cross-linking density of the film-forming material, thereby improving the MEK wipe resistance of the color-coated plate. The number of terminal hydroxyl groups of resins with different hydroxyl contents has little effect on the flexibility of the primer. The reason may be that the relative molecular mass of the polyurethane resin used for the primer is high, and the increase in the number of terminal hydroxyl groups has a negligible effect on the flexibility of the color-coated plate. Hydroxyl is the main functional group of polyester resin. The position and number of hydroxyl groups affect the T-bend and wipe resistance of the color-coated plate coating. The more hydroxyl groups, the better the wipe resistance. The reason is that the increase in the number of hydroxyl groups increases the cross-linking density of the film-forming material, thereby improving the wipe resistance of the system. However, there is no obvious difference in the T-bend, which indicates that the number of terminal hydroxyl groups has little effect on the flexibility of the primer. The reason may be that the relative molecular mass of the polyurethane resin used in the primer is relatively high, and the increase in the number of terminal hydroxyl groups has little effect on the flexibility of the coating.

[0094] The difference between Comparative Example 2 and Example 1 is that the polyurethane resin is linear, and the conventional polyurethane resin uses a straight-chain type with a higher relative molecular weight, and the flexibility of the straight-chain type is weaker than that of the branched type.

[0095] Comparative Example 3 differs from Example 1 in the number of end groups and the degree of branching of the resin, so flexibility and scratch resistance are reduced.

[0096] Compared with Example 2, the amount of amino resin in Comparative Example 4 does not meet the specified range of the present invention. The reason is that the combination of different resins can make the new resin take into account the advantages of their respective monomer resins. The coating cross-linked with the amino resin has a good balance of flexibility, hardness and excellent adhesion. However, the amount of amino resin in Comparative Example 4 is too large, and the multi-terminal hydroxyl groups have been fully cured, and there are no excess hydroxyl groups to cross-link with the amino resin. Therefore, increasing the amount of amino resin has no significant effect on the MEK abrasion resistance of the coating.

[0097] Compared with Example 3, the amount of adhesion promoter in Comparative Example 5 does not meet the specified range of the present invention. The synergistic effect of the resin and the crosslinking agent, the amount and ratio of the resin and the adhesion promoter directly determine the crosslinking degree of the resin during the film forming process, thereby affecting the overall scratch resistance of the coating. If the amount of adhesion promoter is increased too much, the scratch resistance of the color-coated plate is already quite sufficient. If the scratch resistance is increased too much, it may be detrimental to the flexibility of the coating, so it is sufficient to master the appropriate ratio.

[0098] Figure 1 This is a state diagram of Example 1 before and after wiping, and the coating has no paint falling off. Figure 2 In comparative example 2, after being wiped with butanone for 30 times, the coating peeled off severely, and the coating at the damaged part was scraped off by hand, and the entire coating was completely uncovered. Figure 3 For Figure 2 SEM scanning was performed on the light plate where the coating fell off. Figure 4 for Figure 3 The elements and their contents at four locations were selected, and the results showed that the bare plate was mainly composed of Zn, indicating that the adhesion between the coating and the substrate cross section was poor.

[0099] Figure 5 This is the state of the color-coated plate in Example 3 after the wipe resistance is poor. The coating is severely peeled off. The coating at the damaged part is scratched off by hand, and the entire coating is completely peeled off. Figure 6 For Figure 5 SEM scanning was performed on the light plate where the coating fell off. Figure 7 for Figure 6 The elements and their contents at two locations were selected, and the results showed that the bare plate was mainly composed of Zn, indicating that the adhesion between the coating and the substrate cross section was poor.

[0100] Figure 8 For the comparative example 4, the top of the cup protrusion slightly dropped. Figure 8 It can be seen that the amount of resin used in Comparative Example 4 is excessive, the multi-terminal hydroxyl groups have been fully cured, and under the cupping condition, the processing ductility of the coating decreases, which is not conducive to the subsequent deformation processing of the color-coated plate.

Claims

1. A polyurethane primer for rapidly improving the adhesion of a coil coating, characterized in that: It is prepared by including the following components in percentage by mass: 40%-50% polyhydroxyl branched polyurethane resin, 5%-10% titanium dioxide, 2%-5% curing agent, 3%-7% adhesion promoter, 5%-9% amino resin, 7%-11% blocked isocyanate, 5%-9% additive, 6%-12% anti-rust pigment, 7%-11% solvent, the sum of the mass percentages of the above components is 100%; The multi-terminal hydroxyl branched polyurethane resin is prepared from the following raw materials in percentage by mass: 8%-15% of phthalic anhydride, 10%-20% of terephthalic acid, 5%-10% of adipic acid, 15%-25% of 1,6-hexanediol, 8%-15% of neopentyl glycol, 10%-20% of meta-xylene, and 8%-12% of cyclohexanone, and the sum of the percentages by mass of the above raw materials is 100%.

2. The polyurethane primer for rapidly improving the adhesion of coil coating according to claim 1, characterized in that: The polyhydroxyl-terminated branched polyurethane resin is prepared by a method comprising the following steps: Phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and m-xylene are put into a reaction kettle and nitrogen is introduced. The temperature is raised to 200-220°C and the reaction is kept warm for 3-4 hours. The temperature is then raised to 250-280°C and the reaction is kept warm until the reaction liquid meets the acid value of less than 5mgKOH / g and the coating-4 cup viscosity is 15-25s. Vacuuming is carried out for 30-45 minutes. Cyclohexanone is added after the temperature is lowered to 100-120°C. The material is fully stirred and filtered to obtain a multi-terminal hydroxyl branched polyurethane resin.

3. The polyurethane primer for rapidly improving the adhesion of coil coating according to claim 2, characterized in that: The temperature was raised to 200-220°C at a rate of 20-40°C / h, and then raised to 250-280°C at a rate of 20-40°C / h.

4. The polyurethane primer for rapidly improving the adhesion of coil coating according to claim 1, characterized in that: The solvent comprises propylene glycol and ethylene glycol methyl ether acetate in a volume ratio of 1-1.2:

2.

5. The polyurethane primer for rapidly improving the adhesion of coil coating according to claim 1, characterized in that: The auxiliary agents include dispersants, leveling agents and defoamers.

6. The polyurethane primer for rapidly improving the adhesion of coil coating according to claim 1, characterized in that: It is prepared by including the following components in percentage by mass: 40%-50% multi-terminal hydroxyl branched polyurethane resin, 5%-10% titanium dioxide, 2%-4% curing agent, 3%-7% adhesion promoter, 6%-7% amino resin, 8%-11% blocked isocyanate, 8%-9% additives, 6%-9% anti-rust pigment, 7%-8% solvent, the sum of the mass percentages of the above components is 100%.

7. The polyurethane primer for rapidly improving the adhesion of coil coating according to claim 1, characterized in that: The multi-terminal hydroxyl branched polyurethane resin is prepared from the following raw materials in percentage by mass: 13%-14% phthalic anhydride, 10%-20% terephthalic acid, 8%-10% adipic acid, 20%-23% 1,6-hexanediol, 10%-14% neopentyl glycol, 16%-20% m-xylene, and 10%-12% cyclohexanone, and the sum of the percentages by mass of the above raw materials is 100%.

8. A method for preparing a polyurethane primer for rapidly improving the adhesion of a coil coating as claimed in any one of claims 1 to 7, characterized in that: The following steps are involved: Preparation of polyhydroxyl-terminated branched polyurethane resin; The prepared multi-terminal hydroxyl branched polyurethane resin, titanium dioxide, curing agent, adhesion promoter, amino resin, blocked isocyanate, additives, anti-rust pigment and solvent are uniformly stirred, ground and dispersed to obtain a multi-terminal hydroxyl branched polyurethane primer.

9. The method for preparing a polyurethane primer for rapidly improving the adhesion of a coil coating as claimed in claim 8, characterized in that: The preparation of multi-terminal hydroxyl branched polyurethane resin specifically includes: putting phthalic anhydride, terephthalic acid, adipic acid, 1,6-hexanediol, neopentyl glycol and m-xylene into a reaction kettle and introducing protective nitrogen, heating to 200-220°C for heat preservation reaction for 3-4 hours, then heating to 250-280°C for heat preservation reaction until the reaction liquid meets the acid value of less than 5mgKOH / g and the coating-4 cup viscosity is 15-25s, vacuuming for 30-45 minutes, cooling to 100-120°C, adding cyclohexanone, fully stirring and filtering the material to obtain the multi-terminal hydroxyl branched polyurethane resin.

10. An application of a polyurethane primer for rapidly improving the adhesion of a steel coil coating as claimed in any one of claims 1 to 7, characterized in that: Used as primer for color coated panels.

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