Polyurethane automobile finish paint and preparation method thereof

Abstract

The invention discloses polyurethane automobile finish paint and a preparation method thereof, and relates to the field of automobile finish paint. When the polyurethane automobile finish paint is prepared, titanium dioxide, 3-butene-1, 2-diol and 6-bromine-3-(4-bromophenyl)-2, 4-dihydro-1, 3-benzoxazine are uniformly mixed, and modified titanium dioxide is prepared; carrying out formylation on microcrystalline cellulose, and grafting n-butyric acid, acetic anhydride and 2-aminopiperazine to obtain modified cellulose; the preparation method comprises the following steps: reacting 1, 3, 5, 7-tetramethylcyclotetrasiloxane with vinyl pentamethyldisiloxane to obtain single-ended vinyl hydrogen-containing silicone oil; acrylic acid, methyl acrylate, hydroxypropyl methacrylate and single-ended vinyl hydrogen-containing silicone oil are subjected to copolymerization, and modified acrylic resin is prepared; and uniformly mixing the modified acrylic resin, the modified cellulose, the modified titanium dioxide and various assistants to prepare the polyurethane automotive finish. The polyurethane automobile finish paint prepared by the invention has excellent quick-drying property, impact resistance and aging resistance.

Description

Technical Field

[0001] The invention relates to the field of automobile topcoat, in particular to a polyurethane automobile topcoat and a preparation method thereof. Background Art

[0002] Traditional automotive topcoats face numerous challenges in industrial production, the most prominent of which is inefficient drying. Due to the insufficient volatility of existing coatings' solvent systems and the limited curing capabilities of the cross-linked resins, topcoats require lengthy drying and baking times, severely impacting coating production efficiency. Furthermore, these topcoats' performance in terms of weather resistance, impact resistance, and chemical stability fails to meet the stringent requirements of the modern automotive industry for film durability and protection, directly impacting the vehicle's appearance and service life.

[0003] Polyurethane automotive topcoat has become a mainstream choice in the automotive coating industry due to its exceptional comprehensive properties, including excellent weather resistance, outstanding impact resistance, good chemical resistance, and outstanding decorative effects. However, currently available polyurethane topcoats still have significant process defects: slow solvent release rates make them difficult to adapt to modern high-speed baking processes; surface sealing occurs easily during film formation, hindering the volatilization of the inner layer solvent; and the curing time is too long to meet the requirements of efficient assembly line operations. To address these technical bottlenecks, the present invention has developed a polyurethane automotive topcoat with excellent quick-drying properties. By optimizing the cross-linking network, while maintaining the excellent properties of traditional polyurethane coatings, the drying efficiency is significantly improved, providing a more efficient solution for the automotive coating process. Summary of the Invention

[0004] The object of the present invention is to provide a polyurethane automotive topcoat and a preparation method thereof, so as to solve the problems existing in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a polyurethane automotive topcoat, which is prepared by mixing modified acrylic resin, modified cellulose, modified titanium dioxide and various additives.

[0006] As an optimization, the modified acrylic resin is prepared by copolymerization of acrylic acid, methyl acrylate, hydroxypropyl methacrylate, and single-end vinyl hydrogen-containing silicone oil.

[0007] As an optimization, the vinyl-terminated hydrogen-containing silicone oil is obtained by reacting 1,3,5,7-tetramethylcyclotetrasiloxane and vinylpentamethyldisiloxane.

[0008] As an optimization, the modified cellulose is obtained by aldehyde-modifying microcrystalline cellulose and then grafting n-butyric acid, acetic anhydride and 2-aminopiperazine.

[0009] As an optimization, the microcrystalline cellulose model is 200 mesh and is from Shandong Shouhua Chemical Co., Ltd.

[0010] As an optimization, the modified titanium dioxide is prepared by mixing titanium dioxide, 3-butene-1,2-diol and 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine.

[0011] As an optimization, the titanium dioxide model is THR-218, which comes from Hebei Kuoyou Chemical Technology Co., Ltd.

[0012] As an optimization, the various types of additives include dispersants, wetting agents, leveling agents, curing agents and diluents.

[0013] As an optimization, the dispersant model is BYK-110, which comes from Shanghai Kaiyin Chemical Co., Ltd.

[0014] As an optimization, the wetting agent model is BYK-220S, which comes from Jining Fangyu Chemical Co., Ltd.

[0015] As an optimization, the leveling agent model is EFKA-3600, which comes from Guangzhou Haoliangda International Trade Co., Ltd.

[0016] As an optimization, the curing agent model is N3390BA / SN, which comes from Dongguan Tisen New Materials Co., Ltd.

[0017] As an optimization, the diluent is obtained by mixing 40-50 parts of butyl acetate, 20-30 parts of xylene, 10-15 parts of propylene glycol methyl ether acetate and 3-5 parts of methyl nylonate in parts by mass.

[0018] A method for preparing a polyurethane automotive topcoat comprises the following steps: (1) titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide are mixed and reacted to obtain modified titanium dioxide; (2) reacting microcrystalline cellulose with sodium periodate to obtain aldehyde-modified cellulose; (3) mixing aldehyde-modified cellulose, n-butyric acid, acetic anhydride and sulfuric acid solution and reacting them to obtain pre-modified cellulose; reacting the pre-modified cellulose with 2-aminopiperazine to obtain modified cellulose; (4) reacting 1,3,5,7-tetramethylcyclotetrasiloxane with vinylpentamethyldisiloxane to obtain a single-end vinyl hydrogenated silicone oil; (5) mixing acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide, propylene glycol methyl ether acetate and N,N-dimethylethanolamine and reacting them to obtain a modified acrylic resin; (6) The modified acrylic resin, dispersant, wetting agent, modified cellulose, butyl acetate, propylene glycol methyl ether acetate, modified titanium dioxide, leveling agent, curing agent, diluent and chloroplatinic acid are mixed and reacted to obtain a polyurethane automotive topcoat.

[0019] As an optimization, the preparation process of the modified titanium dioxide in step (1) is as follows: titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide are mixed in a mass ratio of 1:(0.3~0.5):(1.6~1.8):(0.85~0.95):(20~30), and refluxed at 153~155°C and 100~300 rpm for 9~11 hours to obtain modified titanium dioxide.

[0020] As an optimization, the reaction equation of the modified titanium dioxide in step (1) is: .

[0021] As an optimization, the preparation process of the aldehyde-modified cellulose in step (2) is as follows: microcrystalline cellulose and sodium periodate are mixed in a mass ratio of 1:(0.9-1.1), and reacted in a hydrochloric acid aqueous solution with a pH of 2-4 at 25-35° C. in the dark for 7-9 hours to obtain aldehyde-modified cellulose.

[0022] As an optimization, the preparation process of the modified cellulose in step (3) is as follows: pre-modified cellulose and 2-aminopiperazine are mixed in a mass ratio of 1:(1.2~1.4), reacted in a phosphate buffer solution with a pH of 7.1~7.3 at 36~38°C for 3~5h, and modified cellulose is obtained.

[0023] As an optimization, the preparation process of the pre-modified cellulose is as follows: aldehyde-modified cellulose, n-butyric acid, and acetic anhydride are added to 200-400 mL of zirconia balls in a mass ratio of 1:(6.5-6.7):(1-3), and the mixture is ball-milled in a 97%-99% sulfuric acid solution at 65-75°C for 55-65 minutes to obtain pre-modified cellulose.

[0024] As an optimization, the reaction equation for the modified cellulose in step (3) is: ; Where R is or .

[0025] As an optimization, the preparation process of the single-end vinyl hydrogenated silicone oil in step (4) is as follows: 1,3,5,7-tetramethylcyclotetrasiloxane, vinylpentamethyldisiloxane and trifluoromethanesulfonic acid are mixed in a mass ratio of 1:(0.3~0.4):(0.01~0.02), reacted at 65~75°C for 5~7h, adjusted the pH to 6~7, and distilled at 105~115°C and 0.1~0.3MPa for 1~2h to obtain the single-end vinyl hydrogenated silicone oil.

[0026] As an optimization, the preparation process of the modified acrylic resin in step (5) is as follows: acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide and propylene glycol methyl ether acetate are mixed in a mass ratio of 1:(0.7~0.8):(1~3):(0.4~0.6):(0.1~0.2):(7~9), reacted at 85~95℃ for 4~8h, cooled to 55~65℃, and adjusted to pH 7~8 to obtain the modified acrylic resin.

[0027] As an optimization, the reaction equation of the modified acrylic resin in step (5) is: .

[0028] As an optimization, the preparation process of the polyurethane automotive topcoat in step (6) is as follows: by mass, 90-110 parts of modified acrylic resin, 3-6 parts of dispersant, 3-6 parts of wetting agent, 6-14 parts of modified cellulose, 14-28 parts of butyl acetate, 14-28 parts of propylene glycol methyl ether acetate and 80-90 parts of modified titanium dioxide are ground and sieved at 4-10 μm at 55-65°C and 15-17 L / min, 3-6 parts of leveling agent, 40-60 parts of curing agent, 5-15 parts of diluent and 1-3 parts of chloroplatinic acid are added and mixed, sieved at 100-200 mesh, allowed to stand for 10-20 min, sprayed to a thickness of 40-50 μm, flash-dried at 40-60°C for 3-5 min, and dried at 70-80°C for 55-65 min to obtain the polyurethane automotive topcoat.

[0029] As an optimization, the spraying process parameters are: spraying temperature 20~30℃, spraying pressure 2~2.5kg / cm 2 , spray gun caliber 1.3~1.5mm, spray gun distance 15~25cm.

[0030] Compared with the prior art, the present invention has the following beneficial effects: when preparing polyurethane automotive topcoat, titanium dioxide, 3-butene-1,2-diol and 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine are mixed to obtain modified titanium dioxide; microcrystalline cellulose is aldehyde-modified and then grafted with n-butyric acid, acetic anhydride and 2-aminopiperazine to obtain modified cellulose; 1,3,5,7-tetramethylcyclotetrasiloxane and vinylpentamethyldisiloxane are reacted to obtain single-end vinyl hydrogen-containing silicone oil; acrylic acid, methyl acrylate, hydroxypropyl methacrylate and single-end vinyl hydrogen-containing silicone oil are copolymerized to obtain modified acrylic resin; and the modified acrylic resin, modified cellulose, modified titanium dioxide and various additives are mixed to obtain the polyurethane automotive topcoat.

[0031] First, titanium dioxide, 3-butene-1,2-diol and 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine are mixed to prepare modified titanium dioxide; microcrystalline cellulose is formaldehyde-modified and then grafted with n-butyric acid, acetic anhydride and 2-aminopiperazine to obtain modified cellulose; benzoxazine rings and carbon-carbon double bonds are introduced on the surface of titanium dioxide through polymerization of 3-butene-1,2-diol and 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine. Benzoxazine can undergo thermal ring-opening polymerization to form a structure similar to phenolic resin, optimize the cross-linking network, accelerate the drying speed, and also capture free radicals, thereby improving the quick-drying and anti-corrosion properties of polyurethane automotive topcoat. Impact resistance and aging resistance; the carbon-carbon double bond can react with the silicon-hydrogen bond on the modified acrylic resin to optimize the cross-linking network, further improving the quick-drying and impact resistance of the polyurethane automotive topcoat; by forming cellulose into butyric acid and acetic anhydride and then grafting them, butyric acid and acetic acid groups are introduced to form hydrogen bonds with the solvent molecules in the paint film, promoting the release of solvent during the film formation process and improving the quick-drying properties of the polyurethane automotive topcoat; using the Schiff base reaction to graft 2-aminopiperazine, piperazine reacts with the isocyanate group in the curing agent to undergo an amidation reaction to generate a structure similar to a hindered amine, forming a cross-linking network, and can also remove oxidative free radicals, further improving the quick-drying and aging resistance of the polyurethane automotive topcoat.

[0032] Secondly, 1,3,5,7-tetramethylcyclotetrasiloxane and vinyl pentamethyldisiloxane are reacted to obtain single-end vinyl hydrogen-containing silicone oil; then, acrylic acid, methyl acrylate, hydroxypropyl methacrylate, and single-end vinyl hydrogen-containing silicone oil are copolymerized to obtain a modified acrylic resin; the modified acrylic resin, modified cellulose, modified titanium dioxide, and various additives are mixed to obtain a polyurethane automotive topcoat; through copolymerization, long silicone chains are introduced into the acrylic resin to reduce the surface tension of the resin, improve the defoaming properties, and reduce the amount of functional additives added; N,N-dimethylethanolamine is used to neutralize the carboxyl groups in the polyacrylic resin, reduce the charge repulsion between polymer chains, make the polymer chains more easily entangled with each other, and improve the impact resistance of the polyurethane automotive topcoat. DETAILED DESCRIPTION

[0033] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0034] The raw materials used in the following examples and comparative examples are all commercially available: the microcrystalline cellulose model is 200 mesh, from Shandong Shouhua Chemical Co., Ltd.; the titanium dioxide model is THR-218, from Hebei Kuoyou Chemical Technology Co., Ltd.; the dispersant model is BYK-110, from Shanghai Kaiyin Chemical Co., Ltd.; the wetting agent model is BYK-220S, from Jining Fangyu Chemical Co., Ltd.; the leveling agent model is EFKA-3600, from Guangzhou Haoliangda International Trade Co., Ltd.; the curing agent model is N3390BA / SN, from Dongguan Tissen New Materials Co., Ltd.

[0035] The diluents used in the following examples and comparative examples are all calculated by weight, and are obtained by mixing 45 parts of butyl acetate, 25 parts of xylene, 12.5 parts of propylene glycol methyl ether acetate and 4 parts of methyl nylonate.

[0036] Example 1: A method for preparing a polyurethane automotive topcoat, comprising the following steps: (1) Titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide were mixed in a mass ratio of 1:0.3:1.6:0.85:20, refluxed at 153°C and 100 rpm for 11 h, cooled naturally to room temperature and centrifuged, washed with anhydrous ethanol and deionized water three times respectively, and dried at 55°C for 25 h to obtain modified titanium dioxide; (2) Microcrystalline cellulose and deionized water were mixed in a mass ratio of 1:55, hydrochloric acid was added to adjust the pH to 2, and the mixture was stirred at 25°C and 100 rpm in the dark for 65 min. Sodium periodate (0.9 times the mass of the microcrystalline cellulose) was added and stirred for 9 h. After that, the mixture was filtered, washed with deionized water for 3 times, freeze-dried for 11 h, and ground and sieved through 200 mesh to obtain aldehyded cellulose. (3) Formaldehyde-modified cellulose, n-butyric acid, acetic anhydride and 97% sulfuric acid solution were mixed in a mass ratio of 1:6.5:1:0.01, added with 200 mL of zirconia balls, and ball-milled at 65 °C and 200 r / min for 65 min, separated, washed three times with deionized water, and dried at 50 °C for 13 h to obtain pre-modified cellulose; pre-modified cellulose, 2-aminopiperazine and phosphate buffer with a pH of 7.1 were mixed in a mass ratio of 1:1.2:120, stirred at 36 °C and 100 rpm for 5 h, centrifuged, washed three times with deionized water, and freeze-dried to obtain modified cellulose; (4) 1,3,5,7-tetramethylcyclotetrasiloxane and vinylpentamethyldisiloxane were mixed in a mass ratio of 1:0.3, stirred at 65°C and 100 rpm for 20 min, trifluoromethylsulfonic acid (0.01 times the mass of 1,3,5,7-tetramethylcyclotetrasiloxane) was added, and stirring was continued for 7 h. Anhydrous sodium carbonate was added to adjust the pH to 6, and then filtered. The mixture was distilled at 105°C and 0.1 MPa for 2 h to obtain single-end vinyl hydrogenated silicone oil; (5) Acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide and propylene glycol methyl ether acetate were mixed in a mass ratio of 1:0.7:1:0.45:0.1:7, stirred at 85°C and 100 rpm for 8 h, cooled to 55°C, and N,N-dimethylethanolamine was added to adjust the pH to 7 to obtain a modified acrylic resin; (6) Modified acrylic resin, dispersant, wetting agent, modified cellulose, butyl acetate and propylene glycol methyl ether acetate were mixed in a mass ratio of 1:0.03:0.03:0.06:0.14:0.14, stirred at 20 ° C and 500 rpm for 30 minutes, added modified titanium dioxide 0.8 times the mass of modified acrylic resin, continued stirring for 10 minutes, ground and sieved 4 μm at 55 ° C and 15~17 L / min, added leveling agent 0.03 times the mass of modified acrylic resin and curing agent 0.4 times the mass of modified acrylic resin, stirred at 450 rpm for 3 minutes, added diluent 0.05 times the mass of modified acrylic resin and chloroplatinic acid 0.01 times the mass of modified acrylic resin, continued stirring for 3 minutes, sieved 100 mesh, let it stand for 20 minutes, and sprayed at 20 ° C and spray pressure 2 kg / cm 2 , spray gun diameter 1.3mm, spray gun distance 15cm to 40μm thickness, flash dry at 40℃ for 5min, dry at 70℃ for 65min to obtain polyurethane automotive topcoat.

[0037] Example 2: A method for preparing a polyurethane automotive topcoat, comprising the following steps: (1) Titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide were mixed in a mass ratio of 1:0.4:1.7:0.9:25, refluxed at 154°C and 200 rpm for 10 h, cooled naturally to room temperature and centrifuged, washed with anhydrous ethanol and deionized water four times respectively, and dried at 60°C for 24 h to obtain modified titanium dioxide; (2) Mix microcrystalline cellulose and deionized water in a mass ratio of 1:60, add hydrochloric acid to adjust the pH to 3, stir at 30°C and 200 rpm in the dark for 60 min, add sodium periodate (1 times the mass of microcrystalline cellulose), continue stirring for 8 h, filter, wash with deionized water 4 times, freeze-dry for 12 h, grind and sieve 250 mesh to obtain aldehyded cellulose; (3) Formaldehyde-modified cellulose, n-butyric acid, acetic anhydride and 98% sulfuric acid solution were mixed in a mass ratio of 1:6.6:2:0.02, added with 300 mL of zirconia balls, and ball-milled at 70 °C and 300 r / min for 60 min. The mixture was separated, washed with deionized water four times, and dried at 55 °C for 12 h to obtain pre-modified cellulose; pre-modified cellulose, 2-aminopiperazine and phosphate buffer with a pH of 7.2 were mixed in a mass ratio of 1:1.3:125, stirred at 37 °C and 150 rpm for 4 h, centrifuged, washed with deionized water four times, and freeze-dried to obtain modified cellulose; (4) 1,3,5,7-tetramethylcyclotetrasiloxane and vinylpentamethyldisiloxane were mixed in a mass ratio of 1:0.35, stirred at 70°C and 150 rpm for 15 min, trifluoromethylsulfonic acid (0.015 times the mass of 1,3,5,7-tetramethylcyclotetrasiloxane) was added, and stirring was continued for 6 h. Anhydrous sodium carbonate was added to adjust the pH to 6.5, and then filtered. The mixture was distilled at 110°C and 0.2 MPa for 1.5 h to obtain single-end vinyl hydrogenated silicone oil; (5) Acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide and propylene glycol methyl ether acetate were mixed in a mass ratio of 1:0.75:2:0.5:0.15:8, stirred at 90 ° C and 150 rpm for 6 h, cooled to 60 ° C, and N, N-dimethylethanolamine was added to adjust the pH to 7.5 to obtain a modified acrylic resin; (6) Modified acrylic resin, dispersant, wetting agent, modified cellulose, butyl acetate and propylene glycol methyl ether acetate were mixed in a mass ratio of 1:0.045:0.045:0.1:0.21:0.21, stirred at 25 ° C and 550 rpm for 25 min, added modified titanium dioxide 0.85 times the mass of modified acrylic resin, continued stirring for 15 min, ground and sieved 7 μm at 60 ° C and 15~17 L / min, added leveling agent 0.045 times the mass of modified acrylic resin and curing agent 0.5 times the mass of modified acrylic resin, stirred at 500 rpm for 2 min, added diluent 0.1 times the mass of modified acrylic resin and chloroplatinic acid 0.02 times the mass of modified acrylic resin, continued stirring for 2 min, sieved through 150 mesh, let it stand for 15 min, and sprayed at 25 ° C and spray pressure 2.25 kg / cm 2 , spray gun diameter 1.4mm, spray gun distance 20cm to 45μm thickness, flash dry at 50℃ for 4min, dry at 75℃ for 60min to obtain polyurethane automotive topcoat.

[0038] Example 3: A method for preparing a polyurethane automotive topcoat, comprising the following steps: (1) Titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide were mixed in a mass ratio of 1:0.5:1.8:0.95:30, refluxed at 155°C and 300 rpm for 9 h, cooled naturally to room temperature and centrifuged, washed with anhydrous ethanol and deionized water for 5 times respectively, and dried at 65°C for 23 h to obtain modified titanium dioxide; (2) Microcrystalline cellulose and deionized water were mixed in a mass ratio of 1:65, hydrochloric acid was added to adjust the pH to 4, and the mixture was stirred at 35°C and 300 rpm in the dark for 65 min. Sodium periodate (1.1 times the mass of the microcrystalline cellulose) was added and stirred for 7 h. The mixture was filtered and washed with deionized water for 5 times, freeze-dried for 13 h, and ground and sieved through 300 mesh to obtain aldehyded cellulose. (3) Formaldehyde-modified cellulose, n-butyric acid, acetic anhydride and 99% sulfuric acid solution were mixed in a mass ratio of 1:6.7:3:0.03, added with 400 mL of zirconia balls, and ball-milled at 75 °C and 400 r / min for 55 min. The mixture was separated, washed with deionized water 5 times, and dried at 60 °C for 11 h to obtain pre-modified cellulose; pre-modified cellulose, 2-aminopiperazine and phosphate buffer with a pH of 7.3 were mixed in a mass ratio of 1:1.4:130, stirred at 38 °C and 200 rpm for 3 h, centrifuged, washed with deionized water 5 times, and freeze-dried to obtain modified cellulose; (4) 1,3,5,7-tetramethylcyclotetrasiloxane and vinylpentamethyldisiloxane were mixed in a mass ratio of 1:0.4, stirred at 75°C and 200 rpm for 10 min, trifluoromethylsulfonic acid (0.02 times the mass of 1,3,5,7-tetramethylcyclotetrasiloxane) was added, and stirring was continued for 5 h. Anhydrous sodium carbonate was added to adjust the pH to 7, and then filtered. The mixture was distilled at 115°C and 0.3 MPa for 1 h to obtain single-end vinyl hydrogenated silicone oil; (5) Acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide and propylene glycol methyl ether acetate were mixed in a mass ratio of 1:0.8:3:0.6:0.2:9, stirred at 95°C and 200 rpm for 8 h, cooled to 65°C, and N,N-dimethylethanolamine was added to adjust the pH to 8 to obtain a modified acrylic resin; (6) Modified acrylic resin, dispersant, wetting agent, modified cellulose, butyl acetate and propylene glycol methyl ether acetate were mixed in a mass ratio of 1:0.06:0.06:0.14:0.28:0.28, stirred at 30°C and 600 rpm for 20 min, added modified titanium dioxide at a mass of 0.9 times the mass of the modified acrylic resin, continued stirring for 10 min, ground and sieved 10 μm at 65°C and 15~17 L / min, added leveling agent at a mass of 0.06 times the mass of the modified acrylic resin and curing agent at a mass of 0.6 times the mass of the modified acrylic resin, stirred at 550 rpm for 1 min, added diluent at a mass of 0.15 times the mass of the modified acrylic resin and chloroplatinic acid at a mass of 0.03 times the mass of the modified acrylic resin, continued stirring for 1 min, sieved through 200 mesh, allowed to stand for 10 min, and sprayed at 30°C and a spray pressure of 2.5 kg / cm 2 , spray gun diameter 1.5mm, spray gun distance 25cm to 50μm thickness, flash dry at 60℃ for 3min, dry at 80℃ for 55min to obtain polyurethane automotive topcoat.

[0039] Comparative Example 1: The preparation method of the polyurethane automotive topcoat in Comparative Example 1 differs from that in Example 2 only in step (1), which is omitted. The modified titanium dioxide in step (6) is replaced with titanium dioxide. The remaining steps are the same as in Example 2.

[0040] Comparative Example 2: The preparation method of the polyurethane automotive topcoat of Comparative Example 2 differs from that of Example 2 only in step (3). Step (3) is modified as follows: formaldehyded cellulose, n-butyric acid, acetic anhydride, and a 98% sulfuric acid solution are mixed in a mass ratio of 1:6.6:2:0.02, 300 mL of zirconium oxide balls are added, and the mixture is ball-milled at 70°C and 300 rpm for 60 min. The mixture is then separated, washed four times with deionized water, and dried at 55°C for 12 h to obtain modified cellulose. The remaining steps are the same as those of Example 2.

[0041] Comparative Example 3: The preparation method of the polyurethane automotive topcoat in Comparative Example 3 differs from that in Example 2 only in step (3). Step (3) is modified as follows: aldehyde-modified cellulose, 2-aminopiperazine, and phosphate buffer solution with a pH of 7.2 are mixed in a mass ratio of 1:1.3:125, stirred at 37°C and 150 rpm for 4 h, centrifuged, washed four times with deionized water, and freeze-dried to obtain modified cellulose. The remaining steps are the same as in Example 2.

[0042] Test Case 1. Anti-aging Test method: The polyurethane automotive topcoat obtained in each embodiment and comparative example was tested for gloss loss after polishing according to GB / T1865.

[0043] 2. Quick-drying Test method: The polyurethane automotive topcoat obtained in each embodiment and comparative example was tested for surface drying time by finger touch method and through drying time by blade method according to GB / T1728.

[0044] 3. Impact resistance Test method: The polyurethane automotive topcoat obtained in each embodiment and comparative example was sprayed onto a 0.3 mm thick tinplate sample. In accordance with GB / T1732, a 1000 g weight was fixed at 50 cm from the slide using a control device. The weight was allowed to fall freely onto the punch. The test sample was removed and observed with a magnifying glass to determine whether the paint film had cracks, wrinkles, or peeling.

[0045] Table 1 below shows the analysis results of the quick-drying property, impact resistance and aging resistance of the polyurethane automotive topcoats of Examples 1 to 3 of the present invention and Comparative Examples 1 to 3.

[0046]

[0047] From the comparison of the experimental data of Examples 1 to 3 and Comparative Examples 1 to 3 in Table 1, it can be found that the polyurethane automotive topcoat prepared by the present invention has good quick-drying property, impact resistance and aging resistance.

[0048] Comparative Example 1 does not polymerize and coat 3-butene-1,2-diol and 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine on the surface of titanium dioxide. By comparison, Examples 1, 2, and 3 have lower gloss loss after polishing, shorter surface-drying time and actual-drying time than Comparative Example 1, and exhibit obvious cracking after impact. This indicates that by polymerizing 3-butene-1,2-diol and 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, benzoxazine rings and carbon-carbon double bonds are introduced on the surface of titanium dioxide. Benzoxazine can undergo thermal ring-opening polymerization to generate a structure similar to phenolic resin, optimize the cross-linking network, accelerate the drying speed, and capture free radicals, thereby improving the quick-drying, impact resistance, and aging resistance of the polyurethane automotive topcoat. The carbon-carbon double bonds can undergo addition reactions with the silicon-hydrogen bonds on the modified acrylic resin, optimizing the cross-linking network and further improving the quick-drying and impact resistance of the polyurethane automotive topcoat.

[0049] Comparative Example 2 does not graft 2-aminopiperazine onto cellulose; by comparison, Examples 1, 2, and 3 have lower gloss loss after polishing, shorter surface-drying time and through-drying time than Comparative Example 2, and slight cracking after impact. This indicates that by utilizing the Schiff base reaction to graft 2-aminopiperazine, piperazine undergoes an amidation reaction with the isocyanate group in the curing agent to form a hindered amine-like structure, forming a three-dimensional cross-linked network while also removing oxidative free radicals, thereby further improving the quick-drying, impact resistance, and aging resistance of the polyurethane automotive topcoat.

[0050] Comparative Example 3 does not graft n-butyric acid and acetic anhydride onto cellulose; by comparison, the surface-free time and actual drying time of Examples 1, 2, and 3 are shorter than those of Comparative Example 3, indicating that by grafting n-butyric acid and acetic anhydride onto cellulose after formaldehyde formation, butyric acid and acetic acid groups are introduced, forming hydrogen bonds with solvent molecules in the paint film, promoting the release of solvent during the paint film formation process, and improving the quick-drying properties of the polyurethane automotive topcoat.

[0051] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a polyurethane automotive topcoat, characterized in that: The method comprises the following preparation steps: (1) titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide are mixed and reacted to obtain modified titanium dioxide; (2) reacting microcrystalline cellulose with sodium periodate to obtain aldehyde-modified cellulose; (3) mixing aldehyde-modified cellulose, n-butyric acid, acetic anhydride and sulfuric acid solution and reacting them to obtain pre-modified cellulose; reacting the pre-modified cellulose with 2-aminopiperazine to obtain modified cellulose; (4) reacting 1,3,5,7-tetramethylcyclotetrasiloxane with vinylpentamethyldisiloxane to obtain a single-end vinyl hydrogenated silicone oil; (5) mixing acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide, propylene glycol methyl ether acetate and N,N-dimethylethanolamine and reacting them to obtain a modified acrylic resin; (6) The modified acrylic resin, dispersant, wetting agent, modified cellulose, butyl acetate, propylene glycol methyl ether acetate, modified titanium dioxide, leveling agent, curing agent, diluent and chloroplatinic acid are mixed and reacted to obtain a polyurethane automotive topcoat.

2. The method for preparing a polyurethane automotive topcoat according to claim 1, wherein: The preparation process of the modified titanium dioxide in step (1) is as follows: titanium dioxide, 3-butene-1,2-diol, 6-bromo-3-(4-bromophenyl)-2,4-dihydro-1,3-benzoxazine, anhydrous potassium carbonate and N,N-dimethylformamide are mixed in a mass ratio of 1:(0.3~0.5):(1.6~1.8):(0.85~0.95):(20~30), and refluxed at 153~155°C and 100~300rpm for 9~11h to obtain modified titanium dioxide.

3. The method for preparing a polyurethane automotive topcoat according to claim 1, wherein: The preparation process of the aldehyded cellulose in step (2) is as follows: microcrystalline cellulose and sodium periodate are mixed in a mass ratio of 1:(0.9~1.1), and the mixture is reacted in a hydrochloric acid aqueous solution with a pH of 2~4 at 25~35°C in the dark for 7~9 hours to obtain aldehyded cellulose.

4. The method for preparing a polyurethane automotive topcoat according to claim 1, wherein: The preparation process of the modified cellulose in step (3) is as follows: pre-modified cellulose and 2-aminopiperazine are mixed in a mass ratio of 1:(1.2-1.4), reacted in a phosphate buffer solution with a pH of 7.1-7.3 at 36-38° C. for 3-5 hours, to obtain modified cellulose; The preparation process of the pre-modified cellulose comprises: adding 200-400 mL of zirconia balls to formaldehyde-modified cellulose, n-butyric acid, and acetic anhydride in a mass ratio of 1:(6.5-6.7):(1-3), and subjecting the mixture to ball milling in a 97%-99% sulfuric acid solution at 65-75° C. for 55-65 minutes to obtain the pre-modified cellulose.

5. The method for preparing a polyurethane automotive topcoat according to claim 1, wherein: The preparation process of the single-end vinyl hydrogenated silicone oil in step (4) is as follows: 1,3,5,7-tetramethylcyclotetrasiloxane, vinylpentamethyldisiloxane and trifluoromethanesulfonic acid are mixed in a mass ratio of 1:(0.3~0.4):(0.01~0.02), reacted at 65~75°C for 5~7h, adjusted the pH to 6~7, and distilled at 105~115°C and 0.1~0.3MPa for 1~2h to obtain the single-end vinyl hydrogenated silicone oil.

6. The method for preparing a polyurethane automotive topcoat according to claim 1, wherein: The preparation process of the modified acrylic resin in step (5) is as follows: acrylic acid, methyl acrylate, hydroxypropyl methacrylate, single-end vinyl hydrogen silicone oil, azobisisobutyl cyanide and propylene glycol methyl ether acetate are mixed in a mass ratio of 1:(0.7~0.8):(1~3):(0.4~0.6):(0.1~0.2):(7~9), reacted at 85~95℃ for 4~8h, cooled to 55~65℃, and adjusted to pH 7~8 to obtain the modified acrylic resin.

7. The method for preparing a polyurethane automotive topcoat according to claim 1, wherein: The preparation process of the polyurethane automotive topcoat in step (6) is as follows: by weight, 90-110 parts of modified acrylic resin, 3-6 parts of dispersant, 3-6 parts of wetting agent, 6-14 parts of modified cellulose, 14-28 parts of butyl acetate, 14-28 parts of propylene glycol methyl ether acetate and 80-90 parts of modified titanium dioxide are ground and sieved to 4-10 μm at 55-65° C. and 15-17 L / min, 3-6 parts of leveling agent, 40-60 parts of curing agent, 5-15 parts of diluent and 1-3 parts of chloroplatinic acid are added and mixed, sieved to 100-200 mesh, allowed to stand for 10-20 minutes, sprayed to a thickness of 40-50 μm, flash-dried at 40-60° C. for 3-5 minutes, and dried at 70-80° C. for 55-65 minutes to obtain the polyurethane automotive topcoat.

8. The method for preparing a polyurethane automotive topcoat according to claim 7, wherein: The spraying process parameters are: spraying temperature 20~30℃, spraying pressure 2~2.5kg / cm 2 , spray gun caliber 1.3~1.5mm, spray gun distance 15~25cm.

9. The method for preparing a polyurethane automotive topcoat according to claim 7, wherein: The diluent is obtained by uniformly mixing 40 to 50 parts of butyl acetate, 20 to 30 parts of xylene, 10 to 15 parts of propylene glycol methyl ether acetate and 3 to 5 parts of methyl nylonate, in parts by mass.

10. A polyurethane automotive topcoat prepared according to the method for preparing a polyurethane automotive topcoat according to any one of claims 1 to 9.

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