Mixed coating process of nitrolacquer and polyurethane paint

Abstract

The invention provides a nitrolacquer and polyurethane paint mixed coating process, which comprises the following steps: dissolving nitrocellulose in an alcohol ester mixed solution, heating to obtain nitrolacquer, reacting polyol with an isocyanate prepolymer under the protection of inert gas to obtain polyurethane paint, and drying to obtain the nitrolacquer and polyurethane paint mixed coating. Mixing nitrolacquer and polyurethane paint to obtain a mixed base material; the wood base material is ground, then the mixed base material is sprayed to form a bottom coating, low-temperature drying is carried out after natural volatilization, grinding is carried out after drying, and a primary plate with a paint layer formed on the surface is obtained; the damaged area of the primarily-manufactured plate is detected, first repairing liquid or second repairing liquid is sprayed to the damaged area, then the mixed base material is sprayed for drying and grinding, mixed coating is completed, the first repairing liquid is used for swelling nitrolacquer, and the second repairing liquid is used for swelling polyurethane paint. Local repair is achieved through the swelling effect, so that the overall film forming flatness and consistency can still be kept when the mixed coating faces local damage, and the final film forming quality is improved.

Description

Technical Field

[0001] This invention belongs to the field of coating technology, and specifically relates to a coating process for a mixture of nitrocellulose lacquer and polyurethane lacquer. Background Technology

[0002] Nitrocellulose lacquer and polyurethane lacquer are often used interchangeably in woodworking applications, primarily due to their complementary film-forming mechanisms, curing methods, and performance characteristics. Nitrocellulose lacquer cures quickly, dries rapidly in the initial stages, applies smoothly, and has good wetting properties, forming a smooth and fine initial film in a short time, providing a good adhesion base for subsequent coatings. However, nitrocellulose lacquer itself has relatively weak abrasion resistance, chemical resistance, and weather resistance, making it unsuitable as a long-term protective layer. Polyurethane lacquer, on the other hand, cures through isocyanate cross-linking reactions, resulting in a film with advantages such as high hardness, high abrasion resistance, and resistance to water and chemical media. However, when applied alone, it is sensitive to the substrate, dries slowly, and requires a relatively strict application environment. Therefore, in practical applications, nitrocellulose lacquer is often used as a base coat to quickly seal the substrate and improve initial smoothness, followed by a polyurethane lacquer as a top coat to establish a high-strength protective film. This combination of good workability and high durability improves the overall appearance quality and service life of the coating system.

[0003] However, nitrocellulose lacquer evaporates and cures quickly, and the film is relatively brittle, while polyurethane lacquer crosslinks and cures slowly, and is prone to microcracks when subjected to uneven stress. When the two are used together, micro-damage is more likely to form when the base coat is repeatedly ground or subjected to local stress, and this damage is further amplified in subsequent spraying, affecting the final film quality. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a mixed coating process of nitrocellulose lacquer and polyurethane lacquer, which aims to solve the problem that local paint layer damage is difficult to repair in mixed coating and affects the final film quality.

[0005] To address the aforementioned problems, a mixed coating process of nitrocellulose lacquer and polyurethane lacquer is provided for coating wood, characterized by the following steps: S1. Nitrocellulose is dissolved in an alcohol-ester mixture solution and heated to obtain nitrocellulose lacquer. Under inert gas protection, polyol and isocyanate prepolymer are reacted to obtain polyurethane lacquer. Nitrocellulose lacquer and polyurethane lacquer are mixed to obtain a mixed base material. S2. Grind the wood substrate, then spray the mixed base material to form a base coating, let it evaporate naturally, and then dry it at a low temperature. After drying, grind it to obtain a preliminary board with a paint layer on the surface. S3. Inspect the damaged areas of the initial board material, spray the first repair liquid or the second repair liquid on the damaged areas, then spray the mixed base material and dry and grind to obtain the board material with mixed coating. The first repair liquid is used to swell nitrocellulose lacquer, and the second repair liquid is used to swell polyurethane lacquer.

[0006] In some embodiments, step S1 includes: S1.1 The alcohol-ester mixture includes butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate. Butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate are added to a mixing tank according to the mass ratio. After stirring evenly at 20-25°C, nitrocellulose powder that has been dried in a vacuum oven at 60°C for 2-4 hours is slowly added. The mixture is heated to 40-50°C and stirred at a constant temperature of 200-300 rpm. Acrylic leveling agents and thixotropic agents are added, and the mixture is stirred evenly to obtain nitrocellulose lacquer. S1.2 Add the metered polyol and alcohol ester mixed solvent to the reactor, heat to 50-60℃ and stir under nitrogen protection, then add isocyanate prepolymer dropwise, controlling the dropwise addition time to 1-2 hours. During the reaction, maintain the NCO / OH molar ratio at 1.05-1.10 and control the reaction temperature at 60-75℃. Confirm the degree of prepolymerization by online viscosity monitoring or NCO content titration. When the system reaches the target viscosity and the residual NCO content is within the preset window, stop heating and cool down to below 40℃ to obtain polyurethane paint. S1.3 Add nitrocellulose lacquer to a mixing tank and simultaneously purge with nitrogen at 25-35°C for protection. Then add polyurethane lacquer in batches, controlling the total addition time to 30-60 minutes and the stirring speed to 150-250 rpm. The mass ratio of nitrocellulose lacquer to polyurethane lacquer is 40:60 to 60:40 to obtain a mixed base material.

[0007] In some embodiments, after step S1.3, step S1 further includes: Aqueous nano-alumina dispersion is added to the mixed base material and dispersed at a speed of 500-800 rpm to obtain a mixed base material containing nano-alumina, wherein the mass ratio of aqueous nano-alumina dispersion to mixed base material is (0.3-1.0):1.0.

[0008] In some embodiments, the acrylic leveling agent includes at least one of hydroxy acrylic resin, butyl acrylate-methyl acrylate copolymer, and hydroxyethyl polyacrylate; the thixotropic agent includes at least one of organobentonite, fumed silica, and polyamide wax; the polyol includes at least one of polypropylene glycol polyol, polyethylene glycol polyol, and polyadipate-neopentyl glycol copolyester polyol; the alcohol-ester mixed solvent includes butyl acetate, ethyl acetate, and propylene glycol ether esters; and the propylene glycol ether ester includes at least one of propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and dipropylene glycol monomethyl ether acetate.

[0009] In some embodiments, step S2 includes: S2.1 The wood substrate is ground sequentially by a combination of a wide belt sander and a handheld sander. First, a 180-240 grit sanding belt is used for longitudinal initial grinding, and then 320-400 grit sandpaper is used for cross-grit grinding along the fiber direction. The surface of the ground wood substrate is then cleaned by an electrostatic dust removal device to remove dust. S2.2 After the wood substrate pretreatment is completed, the mixed base material is evenly sprayed onto the substrate surface to form a wet film. The spraying pressure is controlled at 0.25-0.35MPa, the distance between the spray gun and the workpiece is kept at 15-25cm, and the wet film thickness is maintained at 30-40μm. S2.3 After spraying, let the wood substrate stand naturally for 5 to 15 minutes, then heat it to 40 to 55°C for drying. The drying time is 20 to 40 minutes to obtain a wood substrate with a dry film. S2.4. After cooling to room temperature, use 400-600 grit sandpaper with a handheld sander or a flat sander to grind the surface of the dry film to obtain a preliminary board with a paint layer on the surface.

[0010] In some embodiments, step S3 includes: S3.1. Send the pre-made board to the inspection station to identify and mark the damaged areas on the board surface; S3.2. Using a small-diameter spray gun with a nozzle diameter of 0.8 to 1.0 mm, spray the first or second repair liquid onto the damaged area of ​​the initial board material. Control the spraying pressure at 0.15 to 0.25 MPa and the distance between the nozzle and the board surface at 10 to 15 cm to form a repair film with a thickness of 10 to 20 μm. S3.3. Let stand for 3-8 minutes, then apply the mixed base material to the repair film, with each application thickness controlled at 15-25 μm. Heat to 40-55℃ and dry for 20-40 minutes to obtain a dry board. S3.4. Let the dried board stand at room temperature for at least 12 hours, and then use 800-1000 grit sandpaper to finely grind the board to obtain the board with the mixed coating completed.

[0011] In some embodiments, the preparation step of the first repair solution in step S3 includes: The main solvent is obtained by mixing butyl acetate, ethyl acetate and isopropanol in a mass ratio of 4:3:2. Heat the main solvent to 25-30°C, add 3-5 wt% nitrocellulose and 5-15 wt% hydroxyl acrylic resin, stir to dissolve, and then add 0.05-0.2 wt% dispersant, silicone leveling agent and surface tension modifier. Maintain a temperature of 25–30°C and stir for 20–30 minutes. Filter the solution using a 0.2–0.45 μm filter cartridge. After filtration, allow the solution to stand and age in a sealed environment for 12–24 hours to obtain the first repair solution.

[0012] In some embodiments, the dispersant includes at least one of polymeric block dispersants, polyester dispersants, and polyacrylate dispersants; the silicone leveling agent includes at least one of polydimethylsiloxanes, polyether-modified siloxanes, and amino-containing silicone oils; and the surface tension modifier includes at least one of fluorocarbon surfactants, organosilicon surfactants, and alkylamide surfactants.

[0013] In some embodiments, the preparation step of the second repair solution in step S3 includes: Add cyclohexanone, methyl isobutyl ketone and N-methylpyrrolidone, mix them in a mass ratio of 4:4:2, and stir at 25-30℃ for 15 min to obtain the initial mixture; Add 2-5 wt% of low-viscosity acrylic resin to the initial mixture, stir to dissolve, then add a surface wetting agent and a fluorine-modified surface conditioner. Filter the mixed repair solution through a 0.2-0.45 μm filter, then seal and let it stand for 12-24 hours to obtain the second repair solution.

[0014] In some embodiments, the low-viscosity acrylic resin includes at least one of hydroxyl acrylic resin, methyl methacrylate-butyl acrylate copolymer resin, and hydroxyethyl acrylate-butyl acrylate copolymer; the surface wetting agent includes at least one of polyether modified polysiloxane wetting agent, nonionic polyether polyol wetting agent, and organosilicon polyether copolymer wetting agent; and the fluorinated surface conditioner includes at least one of fluorinated alcohol polyoxyethylene ether, fluorinated polyether siloxane conditioner, and fluorinated alkylamide surfactant.

[0015] Compared with existing technologies, the mixed coating process of nitrocellulose lacquer and polyurethane lacquer in this invention has the following advantages: By first spraying a first or second repair liquid that can swell nitrocellulose lacquer or polyurethane lacquer onto the damaged area, the local paint film softens and expands at the microstructure level, creating a re-fusion effect with the surrounding intact paint layer. This effectively fills micro-cracks, sand marks, or local depressions formed during sanding or construction. Subsequently, the mixed base material is sprayed again and dried and ground, allowing a continuous and dense paint layer to be re-established on the softened and reorganized area. This completely covers the originally difficult-to-repair damage points, achieving local repair through swelling. This ensures that the mixed coating can maintain overall film smoothness and consistency even when facing local damage. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process for mixing nitrocellulose lacquer and polyurethane lacquer in one embodiment of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0018] Please refer to Figure 1 This invention proposes a mixed coating process of nitrocellulose lacquer and polyurethane lacquer for coating wood, the steps of which include: S1. Nitrocellulose is dissolved in an alcohol-ester mixture solution and heated to obtain nitrocellulose lacquer. Under inert gas protection, polyol and isocyanate prepolymer are reacted to obtain polyurethane lacquer. Nitrocellulose lacquer and polyurethane lacquer are mixed to obtain a mixed base material.

[0019] Step S1 includes: S1.1 The alcohol-ester mixture solution includes butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate. Butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate are added to a mixing tank according to the mass ratio. After stirring evenly at 20–25°C, nitrocellulose powder that has been pre-dried in a vacuum oven at 60°C for 2–4 hours is slowly added. The mixture is heated to 40–50°C and stirred at a constant temperature of 200–300 rpm. Acrylic leveling agents and thixotropic agents are added, and after stirring evenly, nitrocellulose lacquer is obtained. The acrylic leveling agent includes at least one of hydroxyl acrylic resin, butyl acrylate-methyl acrylate copolymer, and hydroxyethyl polyacrylate. The thixotropic agent includes at least one of organobentonite, fumed silica, and polyamide wax.

[0020] Step S1.1 involves dissolving nitrocellulose in a mixed solution of butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate, thereby achieving a solvent system with both strong solubility and a moderate evaporation rate. Ethyl acetate provides rapid initial solubilization, butyl acetate provides a moderate evaporation rate for a more uniform film formation process, isopropanol enhances the wetting and penetration capabilities of the system, and propylene glycol monomethyl ether acetate enhances the deep swelling of nitrocellulose, resulting in a fine and uniform matrix resin phase in the nitrocellulose varnish. The added hydroxyl acrylic resin, butyl acrylate-methyl acrylate copolymer, or hydroxyethyl polyacrylate can form a smooth and flexible flow interface in the nitrocellulose varnish, improving the leveling properties, film uniformity, and anti-cratering ability of the coating. Thixotropic agents such as organobentonite, fumed silica, and polyamide wax form a weak three-dimensional network structure, which improves the thixotropic behavior of the system. This gives the nitrocellulose lacquer good anti-sagging properties after spraying and film application, ensuring a smooth and uniform coating on the wood surface, thus laying a stable foundation for subsequent mixing with the polyurethane system.

[0021] S1.2. Add a metered amount of polyol and alcohol ester mixed solvent to the reactor, heat to 50-60℃ and stir under nitrogen protection, then add isocyanate prepolymer dropwise, controlling the dropwise addition time to 1-2 hours. During the reaction, maintain the NCO / OH molar ratio at 1.05-1.10 and control the reaction temperature at 60-75℃. Confirm the degree of prepolymerization by online viscosity monitoring or NCO content titration. When the system reaches the target viscosity and the residual NCO content is within the preset window, stop heating and cool down to below 40℃ to obtain polyurethane paint. The polyol includes at least one of polypropylene glycol polyol, polyethylene glycol polyol, and polyadipate-neopentyl glycol copolyester polyol. The alcohol ester mixed solvent includes butyl acetate, ethyl acetate, and propylene glycol ether esters. The propylene glycol ether esters include at least one of propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and dipropylene glycol monomethyl ether acetate.

[0022] Step S1.2 involves reacting polypropylene glycol polyol, polyethylene glycol polyol, or polyadipate-neopentyl glycol copolyester polyol with isocyanate prepolymer to generate a polyurethane resin with a regular structure and controllable chain segment flexibility. By using a mixed alcohol-ester solvent for pre-dissolution and adjusting the molecular dispersion of the polyol, the uniformity of the reaction can be improved, resulting in more stable chain growth in the prepolymer reaction. Maintaining the NCO / OH molar ratio within a range greater than 1 ensures that the polyurethane prepolymer has a suitable number of unreacted terminal isocyanate groups, allowing for weak cross-linking with active groups in acrylic resins or wood fibers during subsequent mixing with nitrocellulose lacquer, thus improving the adhesion strength of the final coating. Maintaining the reaction temperature in the medium temperature range avoids gelation caused by over-reaction, ensuring the polyurethane lacquer maintains suitable fluidity. Propylene glycol ether solvents, such as propylene glycol monomethyl ether acetate, can stabilize the swelling state between polyurethane segments, giving the polyurethane lacquer good flexibility and impact resistance. This step ensures that the resulting polyurethane coating has high wear resistance, high hardness, and strong adhesion, providing a key structural basis for the subsequent formation of a complementary hybrid system with nitrocellulose lacquer.

[0023] S1.3 Add nitrocellulose lacquer to a mixing tank and simultaneously purge with nitrogen at 25-35°C for protection. Then add polyurethane lacquer in batches, controlling the total addition time to 30-60 minutes and the stirring speed to 150-250 rpm. The mass ratio of nitrocellulose lacquer to polyurethane lacquer is 40:60 to 60:40 to obtain a mixed base material.

[0024] Step S1.3 involves mixing the nitrocellulose lacquer and polyurethane lacquer, which have already formed a stable structure, under nitrogen protection. This allows the two resin systems with different chemical properties to form a complementary composite structure under uniform dispersion. The nitrocellulose resin forms a hard, brittle, and fast-drying skeleton, providing excellent initial drying speed and sanding performance. The polyurethane resin provides high ductility and high abrasion resistance, resulting in improved flexibility, scratch resistance, and impact resistance after curing. Nitrogen protection prevents the isocyanate groups from reacting with moisture in the air, thus avoiding foaming or uncontrolled condensation and ensuring the stability of the mixture. By adding the mixture in batches and controlling the stirring speed, phase separation caused by excessive shear is avoided, allowing the nitrocellulose phase and polyurethane phase to form a stable micro-crosslinked interpenetrating network structure. This results in a final coating that combines fast drying, high hardness, and high abrasion resistance.

[0025] Following step S1.3, step S1 further includes: Aqueous nano-alumina dispersion is added to the mixed base material and dispersed at a speed of 500-800 rpm to obtain a mixed base material containing nano-alumina, wherein the mass ratio of aqueous nano-alumina dispersion to mixed base material is (0.3-1.0):1.0.

[0026] Aqueous nano-alumina dispersion is added after step S1.3, allowing the nano-alumina to enter the mixed resin system in a uniformly dispersed particulate state. Nano-alumina possesses a high specific surface area and a highly polar surface, enabling it to form a physical adsorption interface with nitrocellulose resin and polyurethane segments, thereby improving the density and hardness of the coating film. High-speed dispersion allows the nanoparticles to form a stable dispersion structure within the resin, significantly enhancing the coating's abrasion resistance, scratch resistance, and impact resistance. Simultaneously, nano-alumina fills the micropores in the resin matrix, resulting in a denser and smoother coating surface and enhanced water and alcohol resistance. This step further improves the mechanical strength of the mixed base material while retaining its original quick-drying properties and flexibility, making the coating system suitable for wood products subjected to high-frequency use or high-impact environments.

[0027] S2. Grind the wood substrate, then spray the mixed base material to form a base coating, let it evaporate naturally, and then dry it at a low temperature. After drying, grind it to obtain a preliminary board with a paint layer on the surface. Step S2 includes: S2.1 The wood substrate is ground sequentially using a combination of a wide belt sander and a handheld sander. First, a 180-240 grit sanding belt is used for longitudinal initial grinding, and then 320-400 grit sandpaper is used for cross-grit grinding along the fiber direction. The surface of the ground wood substrate is then cleaned by an electrostatic dust removal device to remove dust.

[0028] Step S2.1 involves multi-stage grinding of the wood substrate using a wide-band sander and a handheld sander, gradually transitioning the fibrous structure of the wood surface from rough to fine. Initial grinding with 180-240 grit sandpaper effectively removes burrs and coarse fibers, resulting in a smoother surface. Subsequent cross-grit sanding with 320-400 grit sandpaper along the fiber direction breaks down residual micropores, improving surface micro-uniformity and enhancing the wettability and penetration of the mixed base material during subsequent spraying. The grinding dust is removed by an electrostatic precipitator to prevent sawdust residue from causing adhesion defects. This step creates a uniform adsorption interface on the wood surface, allowing nitrocellulose and acrylic resins in the nitrocellulose lacquer to fully penetrate the wood pores. The polar structure of the polyurethane segments creates a stronger physical adsorption effect on the smooth surface, thereby improving coating adhesion and overall durability.

[0029] S2.2 After the wood substrate pretreatment is completed, the mixed base material is evenly sprayed onto the substrate surface to form a wet film. The spraying pressure is controlled at 0.25-0.35MPa, the distance between the spray gun and the workpiece is kept at 15-25cm, and the wet film thickness is maintained at 30-40μm.

[0030] Step S2.2 involves spraying the mixed base material evenly onto the wood surface, allowing the nitrocellulose resin, acrylic resin, polyurethane resin, and nano-alumina to form an initial wet film structure on the substrate surface. Maintaining a moderate spraying pressure ensures a fine and uniform atomization of the mixed base material, preventing overspray due to excessive pressure and granular coating due to insufficient pressure. Controlling the spray gun distance and wet film thickness allows the mixed resin system to spread rapidly on the wood surface, fully demonstrating the fast-drying and penetrating characteristics of nitrocellulose lacquer. During the wet film stage, the nitrocellulose resin rapidly forms a rigid framework, while the polyurethane segments provide flexibility and micro-elasticity, giving the wet film excellent anti-sagging properties. Nano-alumina particles are distributed within the resin matrix at this stage, contributing to improved hardness and abrasion resistance of the subsequent dry film. Through this step, the base coating forms a uniform and dense basic structure, providing a stable foundation for subsequent drying and curing.

[0031] S2.3 After spraying, let the wood substrate stand naturally for 5 to 15 minutes, then heat it to 40 to 55°C for drying. The drying time is 20 to 40 minutes to obtain a wood substrate with a dry film.

[0032] Step S2.3 involves first allowing the wet film to stand naturally for a short period, allowing the ethyl acetate, butyl acetate, and isopropanol in the alcohol ester solvents to evaporate rapidly, gradually solidifying the primary structure of the coating surface and preventing pinholes caused by solvent boiling during drying. Subsequently, a constant-temperature drying process is performed at 40–55°C, allowing the nitrocellulose resin to continue forming a film, while the polar segments in the polyurethane resin begin to align, forming a preliminary interpenetrating network structure. Propylene glycol monomethyl ether acetate evaporates slowly, delaying the release of solvent from the coating interior, allowing the dry film to form a dense internal and smooth external structure during curing. This stage facilitates physical cross-linking between the nitro phase and the polyurethane phase in a non-chemically reactive environment, significantly improving the hardness, adhesion, and solvent resistance of the primer, and ensuring a more uniform and smooth surface during subsequent sanding.

[0033] S2.4. After cooling to room temperature, use 400-600 grit sandpaper with a handheld sander or a flat sander to grind the surface of the dry film to obtain a preliminary board with a paint layer on the surface.

[0034] Step S2.4 involves fine grinding of the dried primer coating to effectively remove micro-wrinkles, particles, or slight orange peel texture that develops on the surface during drying. Using 400-600 grit sandpaper with a vibratory sander precisely enhances the micro-roughness of the coating surface, optimizing surface smoothness without compromising the primer thickness. This step allows for slight cutting of the hard layer formed by the nitrocellulose resin in the primer, while the flexibility provided by the polyurethane segments prevents cracking during grinding. The resulting uniform micro-concave structure on the primer surface enhances the mechanical interlocking between coating layers in subsequent coating processes, improving topcoat adhesion, film continuity, and the final decorative effect. The resulting pre-finished substrate exhibits high flatness, high adhesion, and excellent compatibility with subsequent coatings.

[0035] S3. Inspect the damaged areas of the initial board material, spray the first repair liquid or the second repair liquid on the damaged areas, then spray the mixed base material and dry and grind to obtain the board material with mixed coating. The first repair liquid is used to swell nitrocellulose lacquer, and the second repair liquid is used to swell polyurethane lacquer.

[0036] Step S3 includes: S3.1. Send the pre-made board to the inspection station to identify and mark the damaged areas on the board surface.

[0037] Step S3.1 involves sending the pre-fabricated substrate to the inspection station and identifying and marking damaged areas on the surface. This allows for precise recoating of scratches, indentations, pinholes, and localized depressions during subsequent repair processes. Precise marking avoids unnecessary repairs to intact areas, maintaining the overall uniformity and gloss of the coating. Since the dry film of the pre-fabricated substrate already contains nitrocellulose, acrylic resin, and polyurethane resin, damaged areas will visually differ. By detecting changes in local light reflection angles and surface flatness at the inspection station, the location of damage can be quickly identified, making the repair steps highly targeted and improving repair efficiency and the accuracy of material application.

[0038] S3.2. Using a small-diameter spray gun with a nozzle diameter of 0.8 to 1.0 mm, spray the first or second repair liquid onto the damaged area of ​​the initial board material. Control the spraying pressure at 0.15 to 0.25 MPa and the distance between the nozzle and the board surface at 10 to 15 cm to form a repair film with a thickness of 10 to 20 μm.

[0039] Step S3.2 involves uniformly spraying the repair solution onto the damaged area using a small-diameter spray gun. The selective swelling effect of the alcohol ester solvents (butyl acetate, ethyl acetate, and isopropanol) in the repair solution on the nitrocellulose lacquer causes localized softening of the nitrocellulose substrate around the damaged area. Nitrocellulose exhibits rapid swelling in these solvents, enabling it to form a re-film interface with the nitrocellulose in the repair solution, thus achieving coating reshaping. During the swelling process, the hydroxyl acrylic resin in the repair solution penetrates microcracks and fills depressions, gradually restoring the repair film to a smooth surface. Dispersants ensure uniform distribution of solid components, preventing agglomeration; silicone leveling agents improve the spreadability of the repair film; and surface tension modifiers ensure more thorough wetting of the repair solution in the damaged area, avoiding new pinholes or craters. By controlling the film thickness and spraying distance, a thin and uniform swelling interface can be formed, laying a stable foundation for subsequent touch-up coating.

[0040] S3.3. Let stand for 3-8 minutes, then apply the mixed base material to the repair film, with each application thickness controlled at 15-25 μm. Heat to 40-55℃ and dry for 20-40 minutes to obtain a dry board.

[0041] Step S3.3 involves allowing the repair film to undergo initial penetration and swelling through static setting, enabling the nitrocellulose resin segments to rearrange and forming an integrated structure between the repair film and the original primer. Subsequently, a mixed base material is sprayed, establishing a smooth transition between the swollen area and the new coating. This allows the nitrocellulose resin and polyurethane resin to form an interpenetrating network structure locally, improving the mechanical strength and adhesion of the repaired area. The thickness of each recoating is precisely controlled to prevent localized over-thickness and bulging in the repaired area. The heating and drying process promotes the uniform evaporation of the alcohol ester solvents, allowing the hydroxyl acrylic resin, nitrocellulose, and polyurethane resin to form a uniform and dense cured layer. Nano-alumina remains dispersed during this stage, enhancing the abrasion resistance and scratch resistance of the repair film, ensuring the final effect of the repaired area is consistent with the original coating.

[0042] S3.4. Let the dried board stand at room temperature for at least 12 hours, and then use 800-1000 grit sandpaper to finely grind the board to obtain the board with the mixed coating completed.

[0043] Step S3.4 involves further allowing the dried board to stand to ensure complete solvent evaporation and final stabilization of the resin segments. The repaired area is then finely ground with high-grit sandpaper to effectively eliminate minor height differences, haze, or flow marks caused by touch-up coating. Fine sanding improves the micro-roughness of the coating surface within a controlled range, ensuring the reflectivity of the repaired area matches the surrounding coating, making the repair marks virtually undetectable. Because the coating contains both hard nitrocellulose segments and flexible polyurethane segments, good machinability and anti-whitening properties are simultaneously achieved during grinding. Nano-alumina dispersed in the coating ensures the repaired area retains high wear resistance and surface density after grinding, resulting in a hybrid-coated board with consistent appearance and performance.

[0044] In step S3, the preparation steps of the first repair solution include: The main solvent is obtained by mixing butyl acetate, ethyl acetate and isopropanol in a mass ratio of 4:3:2. The main solvent is heated to 25-30°C, and 3-5 wt% nitrocellulose and 5-15 wt% hydroxyl acrylic resin are added. After stirring and dissolving, 0.05-0.2 wt% dispersant, silicone leveling agent and surface tension modifier are added. The dispersant includes at least one of polymeric block dispersant, polyester dispersant and polyacrylate dispersant. The silicone leveling agent includes at least one of polydimethylsiloxane, polyether-modified siloxane and amino silicone oil. The surface tension modifier includes at least one of fluorocarbon surfactant, organosilicon surfactant and alkylamide surfactant.

[0045] Maintain a temperature of 25–30°C and stir for 20–30 minutes. Filter the solution using a 0.2–0.45 μm filter cartridge. After filtration, allow the solution to stand and age in a sealed environment for 12–24 hours to obtain the first repair solution.

[0046] The preparation process of the first repair solution involves mixing butyl acetate, ethyl acetate, and isopropanol in a specific ratio as the main solvent to balance the system's solubility, evaporation rate, and permeability. Ethyl acetate provides rapid penetration, quickly softening the nitro resin in the damaged area; butyl acetate provides a medium-speed evaporation process, maintaining a certain fluidity in the early stages of repair film formation and preventing pinholes; isopropanol improves the wettability of the first repair solution to the wood grain, making it easier for the solution to penetrate micro-cracks. The addition of nitrocellulose forms the same matrix resin as the original coating, enhancing the integrity of the repair film; hydroxyl acrylic resin enhances the flexibility and adhesion of the repair film. Dispersants ensure sufficient dispersion of organic resin particles in the first repair solution, silicone leveling agents improve the spreadability of the repair film, and surface tension modifiers prevent pinholes and craters. Filtration and aging processes eliminate microparticles and bubbles, resulting in higher stability of the first repair solution during spraying, thus ensuring the formation of a uniform and continuous re-film structure in the damaged area. Although the solvent system of nitrocellulose lacquer before film formation also includes butyl acetate, ethyl acetate, and isopropanol, these solvents evaporate and lose their swelling capacity after the coating is fully cured. The first repair solution of this invention uses a specific ratio of alcohol-ester swelling system (especially by adding propylene glycol ether solvents not present in nitrocellulose lacquer), and introduces dispersants, silicone leveling agents, surface tension modifiers, and low-viscosity resin compensation components, thereby constructing a secondary swelling window with selective activation capability for the nitrocellulose phase. This system can achieve local swelling, automatic leveling, and compensated film formation without damaging the polyurethane crosslinking skeleton. Therefore, the first repair solution of this invention is fundamentally different from the original solvent system in nitrocellulose lacquer in both function and mechanism of action.

[0047] In step S3, the preparation steps of the second repair solution include: Add cyclohexanone, methyl isobutyl ketone and N-methylpyrrolidone, mix them in a mass ratio of 4:4:2, and stir at 25-30℃ for 15 min to obtain the initial mixture; Add 2-5 wt% of low-viscosity acrylic resin to the initial mixture, stir to dissolve, then add a surface wetting agent and a fluorine-modified surface conditioner. Filter the mixed repair solution through a 0.2-0.45 μm filter, then seal and let it stand for 12-24 hours to obtain the second repair solution.

[0048] In some embodiments, the low-viscosity acrylic resin includes at least one of hydroxyl acrylic resin, methyl methacrylate-butyl acrylate copolymer resin, and hydroxyethyl acrylate-butyl acrylate copolymer; the surface wetting agent includes at least one of polyether modified polysiloxane wetting agent, nonionic polyether polyol wetting agent, and organosilicon polyether copolymer wetting agent; and the fluorinated surface conditioner includes at least one of fluorinated alcohol polyoxyethylene ether, fluorinated polyether siloxane conditioner, and fluorinated alkylamide surfactant.

[0049] The second repair solution is a mixture of cyclohexanone, methyl isobutyl ketone (MIBK), and N-methylpyrrolidone (NMP) in a 4:4:2 ratio, forming a compound solvent system with strong dissolving power and a staggered volatility profile. The medium-to-high solubility parameters of cyclohexanone and MIBK effectively soften the surface functional group regions of the polyurethane coating, causing moderate chain relaxation. NMP, as a high-boiling-point polar solvent, further promotes the rearrangement of residual polar groups (such as hydrogen bond sites in urethane bonds), achieving a slow and controllable surface opening effect. This solvent structure allows the system to both promote slight remodeling of the polyurethane coating surface and avoid excessive erosion that produces pinholes or haze, providing a stable re-film formation window in the repaired area and creating an optimal substrate for subsequent touch-up coating.

[0050] The introduction of low-viscosity acrylic resin allows the repair solution to quickly penetrate shallow micropores and entangle with local polyurethane segments after the surface opens, repairing minor scratches or dents. Its low viscosity ensures high fluidity, enabling it to spontaneously spread and form a smooth, thin layer in micro-stretched or dented areas, avoiding build-up marks and edge shadows caused by high-viscosity resins. In particular, hydroxyl acrylic resins and hydroxyethyl ester copolymers can form auxiliary interactions with softened polyurethane segments through hydrogen bonds, resulting in a more consistent hardness and flexibility in the repaired area, reducing gloss breaks caused by material mismatch after touch-up.

[0051] The addition of surface wetting agents solves common problems in polyurethane film repair, such as edge shrinkage, fisheye, and localized rejection. Polyether-modified siloxane wetting agents significantly reduce the dynamic surface tension of the repair solution, allowing it to spread rapidly before curing and fully contact the scratched or recessed edges, thus forming a continuous, transitional surface. Nonionic polyether polyols possess excellent uniform spreading ability and are particularly stable on polyurethane surfaces containing polar bonds, significantly improving the micro-surface leveling properties of the repair area, resulting in a smooth, pinhole-free finish.

[0052] Fluorinated surface conditioners form an interface layer with extremely low surface energy, enabling the repair solution to exhibit highly consistent reflectivity in the repaired area, avoiding bright spots, dark spots, or repair boundary lines caused by differences in surface energy. The presence of fluorinated alcohol polyoxyethylene ethers and fluorinated modified siloxanes induces the formation of a nanoscale uniform arrangement on the surface of the repair film during film formation, ensuring consistent light reflection direction and thus significantly reducing the ΔG60 difference. Fluorinated alkyl amides further enhance chemical resistance, ensuring consistent performance in the repaired area when exposed to liquids such as detergents and sweat, preventing premature aging or whitening of the repaired area.

[0053] Example 1: First, nitrocellulose lacquer was prepared by adding 35 parts of butyl acetate, 25 parts of ethyl acetate, 20 parts of isopropanol, and 20 parts of propylene glycol monomethyl ether acetate to a mixing tank. After stirring evenly at 250 rpm at 20-25°C, 8 parts of nitrocellulose powder that had been dried in a vacuum oven at 60°C for 3 hours were slowly added. The mixture was heated to 45°C and stirred at a constant temperature of 250 rpm. At the same time, 4 parts of hydroxyl acrylic resin, 2 parts of butyl acrylate-methyl acrylate copolymer, and 1 part of hydroxyethyl polyacrylate were added as acrylic leveling agents. 0.3 parts of organobentonite, 0.2 parts of fumed silica, and 0.2 parts of polyamide wax were added as thixotropic agents. The mixture was stirred until the system was transparent and homogeneous to obtain nitrocellulose lacquer. Then, polyurethane paint was prepared by adding 10 parts of polypropylene glycol polyol, 6 parts of polyethylene glycol polyol, and 4 parts of poly(adipate-neopentyl glycol) copolyester polyol to a reaction vessel, along with 15 parts of butyl acetate, 10 parts of ethyl acetate, and 8 parts of propylene glycol monomethyl ether acetate as an alcohol-ester mixed solvent. The mixture was heated to 55°C under nitrogen protection and stirred for 20 min. Subsequently, 15 parts of isocyanate prepolymer were added dropwise over a period of 1.5 h. During the reaction, the NCO / OH molar ratio was maintained at 1.08, and the reaction temperature was controlled at 65°C. The degree of prepolymerization was confirmed by online viscosity monitoring and NCO content titration. When the system reached the target viscosity and the residual NCO content was within the preset range, heating was stopped and the temperature was lowered to below 40°C to obtain the polyurethane paint. Add 50 parts of nitrocellulose lacquer to a mixing tank and purge with nitrogen at 25-30°C. Then, add 50 parts of polyurethane lacquer in batches over 30 minutes. The total stirring time is 45 minutes, and the stirring speed is controlled at 200 rpm to obtain a mixed base material with a nitrocellulose lacquer:polyurethane lacquer mass ratio of 50:50. After step S1.3, add 0.5 parts of aqueous nano-alumina dispersion to the mixed base material to make the mass ratio of aqueous nano-alumina dispersion to mixed base material 0.5:1.0. Disperse at 700 rpm for 20 minutes to obtain a mixed base material containing nano-alumina.

[0054] The wood substrate was sequentially ground using a combination of a wide-band sander and a handheld sander. First, a 180-grit sanding belt was used for longitudinal initial grinding, followed by 360-grit sandpaper for cross-grit fine grinding along the fiber direction. The ground wood substrate surface was then thoroughly dust-free using an electrostatic dust removal device. After the wood substrate pretreatment, the aforementioned mixed base material containing nano-alumina was uniformly sprayed onto the substrate surface to form a wet film. The spraying pressure was controlled at 0.30 MPa, the distance between the spray gun and the workpiece was maintained at 20 cm, and the wet film thickness was maintained at approximately 35 μm. After spraying, the wood substrate was allowed to stand naturally for 10 minutes. n. To allow some solvent to evaporate, the substrate is heated to 50°C and dried for 30 minutes to obtain a wood substrate with a dry film. Further, after the low-temperature drying in step S2.3, a layer of aqueous ethanol / weak acid composite solution is uniformly wiped on the surface of the wood substrate with the dry film. The composite solution consists of an aqueous ethanol solution with a volume fraction of 30-40% and an organic acid. The wiping time is controlled at 0.5-2 minutes. Then, the solution is allowed to evaporate naturally and is dried again at 40-45°C for 5-10 minutes to form a dense interface layer that is less sensitive to liquids on the wood surface. After the low-temperature drying in step S2.3, a composite solution of aqueous ethanol and weak acid is uniformly wiped onto the surface of the dry film. A more uniform and dense interfacial layer is formed during a short-term secondary low-temperature drying process. This layer exhibits higher stability and insensitivity to liquids because the aqueous ethanol system promotes the reorientation of polar groups on the outermost layer of the paint film, bringing the previously uneven surface energy regions to equilibrium. The weak acid further regulates any remaining micro-alkaline points on the surface, resulting in a more regular and continuous polar distribution structure at the interface. After this rehoming treatment, the surface does not exhibit localized selective adsorption of liquids such as water, alcohol, or acidic beverages. The contact behavior between droplets and the surface is more consistent, thus reducing the likelihood of residual water rings, whitening, or liquid marks. Furthermore, short-duration low-temperature drying promotes the rearrangement of the surface microscale structure, causing the shallow micropores and capillary pathways to converge, thereby forming a denser interface layer that is difficult for liquids to penetrate. Combined with the nano-alumina introduced in the previous steps, these are further stably embedded in the surface microscale structure during this process, making the interface more effective at preventing liquid diffusion and enhancing the surface's scrub resistance and chemical resistance. Under the combined effect, this interface layer maintains stable gloss, is not easily contaminated, and is less prone to liquid spots during daily liquid contact, significantly improving the system's durability and surface quality. After cooling to room temperature, the dry film surface is lightly polished using 500-grit sandpaper and a handheld sander to obtain a preliminary board with a paint layer on the surface.

[0055] The pre-made boards are then sent to the inspection station to identify and mark damaged areas such as scratches and dents on the board surface. In step S3, a first repair solution that only swells nitrocellulose lacquer is prepared. 40 parts of butyl acetate, 30 parts of ethyl acetate, and 20 parts of isopropanol are mixed in a mass ratio of 4:3:2 to obtain the main solvent. The main solvent is heated to 25-30°C, and 4 wt% nitrocellulose and 10 wt% hydroxyl acrylic resin are added. After stirring and dissolving, 0.1 wt% polymer block dispersant, 0.1 wt% polydimethylsiloxane silicone leveling agent, and 0.1 wt% fluorocarbon surfactant are added. The temperature is maintained at 25-30°C and stirred for 25 minutes. The solution is filtered using a 0.2 μm filter cartridge and then aged in a sealed environment for 18 hours to obtain the first repair solution. During repair, a small-diameter spray gun with a nozzle diameter of 0.9 mm is used to spray the first repair liquid onto the damaged area. The spraying pressure is controlled at 0.20 MPa, and the distance between the nozzle and the board surface is controlled at 12 cm to form a repair film with a thickness of about 15 μm. After standing for 5 minutes, the above-mentioned mixed base material is used to recoat the repair film, with each recoat thickness controlled at about 20 μm. The film is then heated to 45°C and dried for 30 minutes to obtain a dry board. The dried board is then left to stand at room temperature for at least 12 hours, and then finely ground with 1000-grit sandpaper to obtain a board with a mixed coating.

[0056] Example 2: First, nitrocellulose lacquer was prepared by adding 40 parts of butyl acetate, 30 parts of ethyl acetate, 15 parts of isopropanol, and 15 parts of propylene glycol monomethyl ether acetate to a mixing tank. After stirring evenly at 230 rpm at 22°C, 10 parts of nitrocellulose powder that had been dried in a vacuum oven at 60°C for 3 hours were slowly added. The mixture was heated to 45-48°C and stirred at a constant temperature of 250 rpm. At the same time, 5 parts of hydroxyl acrylic resin and 2 parts of hydroxyethyl polyacrylate were added as acrylic leveling agents, and 0.3 parts of fumed silica and 0.4 parts of polyamide wax were added as thixotropic agents. The mixture was stirred until the lacquer was transparent and uniform, resulting in a nitrocellulose lacquer with higher solids content. To prepare the polyurethane paint, 8 parts of polypropylene glycol polyol, 4 parts of polyethylene glycol polyol, and 8 parts of poly(adipate-neopentyl glycol) copolyester polyol were added to a reaction vessel. 12 parts of butyl acetate, 8 parts of ethyl acetate, and 8 parts of propylene glycol monoethyl ether acetate were added as a mixed solvent. The mixture was heated to 55–58°C and stirred for 20 minutes under nitrogen protection. Then, 14 parts of isocyanate prepolymer were added dropwise over a time of approximately 1.2 hours. During the reaction, the NCO / OH molar ratio was maintained between 1.06 and 1.08, and the reaction temperature was controlled at 65–70°C. The reaction endpoint was confirmed by online viscosity and residual NCO content. The mixture was then cooled to below 40°C to obtain the polyurethane paint. Add 60 parts of nitrocellulose lacquer to a mixing tank and purge with nitrogen at 25-30°C. Then add 40 parts of polyurethane lacquer in batches over a total addition time of approximately 40 minutes, maintaining a stirring speed of 180-220 rpm to achieve a nitrocellulose lacquer:polyurethane lacquer mass ratio of 60:40, resulting in a mixed base material with a more dominant nitrocellulose phase. After step S1.3, add 1.0 part of aqueous nano-alumina dispersion to the mixed base material, achieving a mass ratio of aqueous nano-alumina dispersion:mixed base material of 1.0:1.0. Disperse at 750 rpm for 25 minutes to obtain a mixed base material with a higher nano-alumina content. In the wood pretreatment stage, the wood is first initially sanded longitudinally with 240-grit sandpaper, and then finely sanded crosswise along the fiber direction with 400-grit sandpaper. After completely removing dust through electrostatic dust removal, the above-mentioned mixed base material is sprayed at a spraying pressure of 0.28MPa and a spraying distance of 20cm, controlling the wet film thickness to about 38μm. After naturally standing for 10-12 minutes, it is dried at 48℃ for 25 minutes to obtain a wood substrate with a slightly higher dry film thickness. After cooling, it is lightly sanded once with 400-grit sandpaper, and then finely sanded with 600-grit sandpaper to obtain a pre-made board with higher flatness. The repair stage is the same as in Example 1, using the same swelling nitrocellulose lacquer repair liquid. The spraying conditions are adjusted to a nozzle diameter of 0.8 mm, a spraying pressure of 0.18 MPa, a spraying distance of 11 cm, and a repair film thickness of about 12 μm. After standing for 3 to 5 minutes, a mixed base material of about 18 μm is applied, and the mixture is dried at 45°C for 25 minutes. Finally, after standing at room temperature for 12 to 16 hours, the mixture is finely ground with 800-grit sandpaper to obtain the board with the mixed coating completed.

[0057] Example 3: The preparation of nitrocellulose lacquer is similar to that in Example 1. 30 parts of butyl acetate, 30 parts of ethyl acetate, 20 parts of isopropanol and 20 parts of propylene glycol monomethyl ether acetate are added to a mixing tank and stirred evenly at 20-25°C. Then, 6 parts of nitrocellulose powder that has been vacuum dried at 60°C for 3 hours are added. The mixture is heated to 45°C and stirred at a constant temperature. 3 parts of hydroxyl acrylic resin, 2 parts of butyl acrylate-methyl acrylate copolymer and 1 part of polyhydroxyethyl acrylate are added as acrylic leveling agents. At the same time, 0.2 parts of organic bentonite and 0.2 parts of fumed silica are added as thixotropic agents. The mixture is stirred until the system is uniform and transparent. The polyurethane paint was prepared by adding 12 parts of polypropylene glycol polyol, 8 parts of polyethylene glycol polyol, and 5 parts of poly(adipate-neopentyl glycol) copolyester polyol, along with a mixed solvent consisting of 15 parts of butyl acetate, 10 parts of ethyl acetate, and 10 parts of dipropylene glycol monomethyl ether acetate, to a reaction vessel. Under nitrogen protection, the mixture was heated to 60°C and stirred for 20 minutes. Subsequently, 18 parts of isocyanate prepolymer were added dropwise over a time controlled between 1.5 and 2.0 hours. During the reaction, the NCO / OH molar ratio was maintained at 1.10, and the reaction temperature was controlled between 70 and 75°C. The degree of prepolymerization was controlled by online viscosity and residual NCO content. The polyurethane paint with high crosslinking potential was obtained by cooling the temperature to below 40°C. Add 40 parts of nitrocellulose lacquer to a mixing tank and purge with nitrogen at 25-30°C. Add 60 parts of polyurethane lacquer in batches, controlling the total addition time to about 50 minutes and the stirring speed to about 200 rpm, so that the mass ratio of nitrocellulose lacquer to polyurethane lacquer is 40:60, resulting in a mixed base material with polyurethane phase dominance. After step S1.3, add 0.5 parts of aqueous nano-alumina dispersion to the mixed base material, with a mass ratio of 0.5:1.0, and disperse at 650 rpm for 20 minutes to obtain a mixed base material containing nano-alumina. The wood treatment is basically the same as in Example 1. Grind with a combination of 180-grit and 320-grit sandpaper. After electrostatic dust removal, spray the mixed base material with a spray pressure of 0.30 MPa and a spray distance of 20 cm, with a wet film thickness of about 35-38 μm. After standing naturally for 10 minutes, dry at 50°C for 30 minutes. After cooling, lightly sand with 400-grit sandpaper, and then finely sand with 600-grit sandpaper to obtain the preliminary board material.In step S3, the second repair solution of the swelling polyurethane paint is prepared by adding 40 parts of cyclohexanone, 40 parts of methyl isobutyl ketone, and 20 parts of N-methylpyrrolidone at a mass ratio of 4:4:2 and stirring at 25-30°C for 15 minutes to obtain a preliminary mixture. Then, 3 wt% of hydroxyl acrylic resin and 2 wt% of methyl methacrylate-butyl acrylate copolymer resin as low-viscosity acrylic resin are added as the preliminary mixture. After stirring and dissolving, 0.1 wt% of polyether-modified polysiloxane wetting agent, 0.1 wt% of organosilicon polyether copolymer wetting agent, 0.1 wt% of fluorine-modified polyether siloxane modifier, and 0.05 wt% of fluorinated alcohol polyoxyethylene ether are added. After mixing evenly, the second repair solution is filtered through a 0.2 μm filter and then sealed and allowed to stand for 18 hours to obtain the second repair solution of the swelling polyurethane paint. During the repair process, the initial board material is sent to the inspection station to locate the damaged area. A small-diameter spray gun with a nozzle diameter of 0.9 mm is used to spray the second repair liquid onto the damaged area. The spraying pressure is controlled at 0.18-0.20 MPa, and the distance between the nozzle and the board surface is controlled at 12 cm to form a repair film with a thickness of about 15 μm. After standing for 5-6 minutes, the cyclohexanone, methyl isobutyl ketone, and N-methylpyrrolidone in the second repair liquid selectively swell the polyurethane phase coating, while the swelling of the nitrocellulose base layer is weak. Then, a mixed base material is applied to cover about 20 μm, and the mixture is dried at 45-50℃ for 30 minutes. Finally, the mixture is left to stand at room temperature for at least 12 hours and then finely ground with 1000-grit sandpaper to obtain the board material with the mixed coating completed.

[0058] Comparative Example 1: The only difference between this comparative example and Example 1 is that the aqueous nano-alumina dispersion is not added in step S1. The formulations and process conditions for the remaining steps S1, S2, and S3 are the same as in Example 1. That is, after S1.3, the addition and dispersion of the aqueous nano-alumina dispersion is no longer performed; the mixed base material without nano-alumina is directly used for spraying and repair. The ratio of nitrocellulose lacquer to polyurethane lacquer, the formulation of the first repair solution that only swells the nitrocellulose lacquer, and the spraying conditions remain the same.

[0059] Comparative Example 2: The only difference between this comparative example and Example 1 is that the formulation of the first repair solution in step S3 deviates from the scope of claim 7. Nitrocellulose is not added to the first repair solution; only the main solvent and hydroxyl acrylic resin are used. Specifically, 40 parts of butyl acetate, 30 parts of ethyl acetate, and 20 parts of isopropanol are mixed in a mass ratio of 4:3:2. After heating to 25–30°C, only 8 wt% of the hydroxyl acrylic resin (based on the weight of the main solvent) is added, without adding 3–5 wt% of nitrocellulose. Then, 0.1 wt% of dispersant, 0.1 wt% of silicone leveling agent, and 0.1 wt% of surface tension modifier are added. After stirring, filtering, and aging, it is used as the first repair solution. The formulations and process conditions of S1 and S2 are completely consistent with Example 1. The spraying pressure, repair film thickness, and subsequent recoating, drying, and grinding steps in S3 are all the same as in Example 1.

[0060] All samples were prepared according to processes S1 to S3 of the embodiments and allowed to stand naturally for ≥24 hours. Experiments were conducted in a constant temperature and humidity laboratory (23±2℃, 50±5%RH). The wood substrates of the master samples were all from the same batch to eliminate substrate differences. Performance tests of this invention were all performed in a constant temperature and humidity laboratory. Coating hardness was tested according to GB / T6739, adhesion according to GB / T9286, abrasion resistance according to ASTM D4060 or GB / T1768, and gloss measurement according to GB / T9754, ensuring the repeatability of the test methods and the comparability of the results. Experimental results are shown in Table 1.

[0061] Table 1: As can be clearly seen from Table 1, Examples 1-3 of the present invention exhibit significant advantages over Comparative Examples 1-2 in terms of overall coating performance. First, regarding adhesion level, all examples achieved level 0, indicating that the compound system of nitrocellulose lacquer and polyurethane lacquer formed a continuous and firm interface on the wood surface, with no peeling, cracking, or interlayer delamination between the coatings; while the adhesion of the comparative examples decreased to level 1 or 2, reflecting insufficient interfacial bonding, proving that the systems of the comparative examples lacked the combined effect of synergistic film formation of the mixed base materials, matching swelling of the repair solution, and interfacial rearrangement in the process of the present invention.

[0062] Regarding wear resistance and mass loss, the wear amounts in Examples 1-3 were approximately 8 mg, 10 mg, and 12 mg, respectively, significantly better than the 20-22 mg and 18 mg in the comparative examples. This indicates that the system of the present invention can form a denser cross-linked structure, and through the rigid skeleton of nitrocellulose, the tough segments of polyurethane, and possibly the added nano-alumina, a wear-resistant reinforcing network is formed, making the coating film less susceptible to weakening and significant weight loss during friction. The comparative examples, lacking these synergistic structural reinforcement effects, exhibited greater wear loss.

[0063] Regarding the 60° gloss difference ΔG60 (repaired area – unrepaired area), Examples 1-3 have values ​​of 0.8, 1.2, and 1.5, respectively, significantly lower than the comparative examples' ≥3.0 and 5.0-6.0. The ΔG60 data reflects the gloss difference between the repaired area and the original coating. The embodiments of this invention enable the repaired area to have uniform gloss and consistent reflection after curing, with almost no visible repair marks. In particular, Example 1's ΔG60≈0.8 indicates that the repaired area is almost indistinguishable from the original area. In contrast, the comparative examples exhibited obvious bright spots, shadows, or hazy differences after repair, indicating poor compatibility between the repair solution and the original coating system, and uneven film formation leading to an increased gloss difference.

[0064] Based on the above data, it can be seen that the present invention has significant technical effects in terms of adhesion, abrasion resistance, gloss consistency, and repair visibility. Among them, the significant reduction in ΔG60 and the substantial decrease in abrasion loss are the most intuitive performance improvements reflected in the experimental data, proving that the hybrid coating and repair scheme proposed in this invention can form a more uniform, more durable, and more difficult-to-detect repair traces high-performance coating system.

[0065] The unique feature of Example 1 is that, after the low-temperature drying in step S2.3, it additionally introduces an interface conditioning step involving wiping with an aqueous ethanol / weak acid composite solution and a second short-time low-temperature drying. Because Example 1 employs this interface layer optimization step, it exhibits significantly better results than Examples 2 and 3 in terms of gloss difference ΔG60 at 60 degrees, abrasion loss, and liquid resistance. The gloss difference of Example 1 is approximately 0.8, significantly lower than that of Examples 2 (1.2) and 3 (1.5) without this step, indicating a more uniform surface energy between the repaired and unrepaired areas, more consistent liquid spread on the surface, and less likelihood of leaving marks. Similarly, the abrasion loss of Example 1 is approximately 8 mg, indicating that the dense layer formed after interface optimization remains more stable under mechanical friction. Since Examples 2 and 3 lack this homogenization and densification process, their interface layers rely solely on the cured structure of the original paint film, thus their liquid resistance and gloss consistency are slightly inferior to Example 1.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A coating process for mixing nitrocellulose lacquer and polyurethane lacquer, used for coating wood, characterized by the following steps: include: S1. Nitrocellulose is dissolved in an alcohol-ester mixture solution and heated to obtain nitrocellulose lacquer. Under inert gas protection, polyol and isocyanate prepolymer are reacted to obtain polyurethane lacquer. Nitrocellulose lacquer and polyurethane lacquer are mixed to obtain a mixed base material. S2. Grind the wood substrate, then spray the mixed base material to form a base coating, let it evaporate naturally, and then dry it at a low temperature. After drying, grind it to obtain a preliminary board with a paint layer on the surface. S3. Inspect the damaged areas of the initial board material, spray the first repair liquid or the second repair liquid on the damaged areas, then spray the mixed base material and dry and grind to obtain the board material with mixed coating. The first repair liquid is used to swell nitrocellulose lacquer, and the second repair liquid is used to swell polyurethane lacquer.

2. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 1, characterized in that, Step S1 includes: S1.1 The alcohol-ester mixture includes butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate. Butyl acetate, ethyl acetate, isopropanol, and propylene glycol monomethyl ether acetate are added to a mixing tank according to the mass ratio. After stirring evenly at 20-25°C, nitrocellulose powder that has been dried in a vacuum oven at 60°C for 2-4 hours is slowly added. The mixture is heated to 40-50°C and stirred at a constant temperature of 200-300 rpm. Acrylic leveling agents and thixotropic agents are added, and the mixture is stirred evenly to obtain nitrocellulose lacquer. S1.2 Add the metered polyol and alcohol ester mixed solvent to the reactor, heat to 50-60℃ and stir under nitrogen protection, then add isocyanate prepolymer dropwise, controlling the dropwise addition time to 1-2 hours. During the reaction, maintain the NCO / OH molar ratio at 1.05-1.10 and control the reaction temperature at 60-75℃. Confirm the degree of prepolymerization by online viscosity monitoring or NCO content titration. When the system reaches the target viscosity and the residual NCO content is within the preset window, stop heating and cool down to below 40℃ to obtain polyurethane paint. S1.3 Add nitrocellulose lacquer to a mixing tank and simultaneously purge with nitrogen at 25-35°C for protection. Then add polyurethane lacquer in batches, controlling the total addition time to 30-60 minutes and the stirring speed to 150-250 rpm. The mass ratio of nitrocellulose lacquer to polyurethane lacquer is 40:60 to 60:40 to obtain a mixed base material.

3. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 2, characterized in that, Following step S1.3, step S1 further includes: Aqueous nano-alumina dispersion is added to the mixed base material and dispersed at a speed of 500-800 rpm to obtain a mixed base material containing nano-alumina, wherein the mass ratio of aqueous nano-alumina dispersion to mixed base material is (0.3-1.0):1.

0.

4. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 2 or 3, characterized in that, Acrylic leveling agents include at least one of hydroxy acrylic resin, butyl acrylate-methyl acrylate copolymer, and hydroxyethyl polyacrylate; thixotropic agents include at least one of organobentonite, fumed silica, and polyamide wax; polyols include at least one of polypropylene glycol polyol, polyethylene glycol polyol, and poly(adipate-neopentyl glycol) copolyester polyol; alcohol-ester mixed solvents include butyl acetate, ethyl acetate, and propylene glycol ether esters; and propylene glycol ether esters include at least one of propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and dipropylene glycol monomethyl ether acetate.

5. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 1, characterized in that, Step S2 includes: S2.1 The wood substrate is ground sequentially by a combination of a wide belt sander and a handheld sander. First, a 180-240 grit sanding belt is used for longitudinal initial grinding, and then 320-400 grit sandpaper is used for cross-grit grinding along the fiber direction. The surface of the ground wood substrate is then cleaned by an electrostatic dust removal device to remove dust. S2.2 After the wood substrate pretreatment is completed, the mixed base material is evenly sprayed onto the substrate surface to form a wet film. The spraying pressure is controlled at 0.25-0.35MPa, the distance between the spray gun and the workpiece is kept at 15-25cm, and the wet film thickness is maintained at 30-40μm. S2.3 After spraying, let the wood substrate stand naturally for 5 to 15 minutes, then heat it to 40 to 55°C for drying. The drying time is 20 to 40 minutes to obtain a wood substrate with a dry film. S2.

4. After cooling to room temperature, use 400-600 grit sandpaper with a handheld sander or a flat sander to grind the surface of the dry film to obtain a preliminary board with a paint layer on the surface.

6. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 1, characterized in that, Step S3 includes: S3.

1. Send the pre-made board to the inspection station to identify and mark the damaged areas on the board surface; S3.

2. Using a small-diameter spray gun with a nozzle diameter of 0.8 to 1.0 mm, spray the first or second repair liquid onto the damaged area of ​​the initial board material. Control the spraying pressure at 0.15 to 0.25 MPa and the distance between the nozzle and the board surface at 10 to 15 cm to form a repair film with a thickness of 10 to 20 μm. S3.

3. Let stand for 3-8 minutes, then apply the mixed base material to the repair film, with each application thickness controlled at 15-25 μm. Heat to 40-55℃ and dry for 20-40 minutes to obtain a dry board. S3.

4. Let the dried board stand at room temperature for at least 12 hours, and then use 800-1000 grit sandpaper to finely grind the board to obtain the board with the mixed coating completed.

7. A coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 1 or 6, characterized in that, In step S3, the preparation steps of the first repair solution include: Butyl acetate, ethyl acetate and isopropanol were mixed in a mass ratio of 4:3:2 to obtain the main solvent; Heat the main solvent to 25-30°C, add 3-5 wt% nitrocellulose and 5-15 wt% hydroxyl acrylic resin, stir to dissolve, and then add 0.05-0.2 wt% dispersant, silicone leveling agent and surface tension modifier. Maintain a temperature of 25–30°C and stir for 20–30 minutes. Filter the solution using a 0.2–0.45 μm filter cartridge. After filtration, allow the solution to stand and age in a sealed environment for 12–24 hours to obtain the first repair solution.

8. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 7, characterized in that, The dispersant includes at least one of polymeric block dispersants, polyester dispersants, and polyacrylate dispersants; the silicone leveling agent includes at least one of polydimethylsiloxanes, polyether-modified siloxanes, and amino-containing silicone oils; and the surface tension modifier includes at least one of fluorocarbon surfactants, organosilicon surfactants, and alkylamide surfactants.

9. A coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 1 or 5, characterized in that, In step S3, the preparation steps of the second repair solution include: Add cyclohexanone, methyl isobutyl ketone and N-methylpyrrolidone, mix them in a mass ratio of 4:4:2, and stir at 25-30℃ for 15 min to obtain the initial mixture; Add 2-5 wt% of low-viscosity acrylic resin to the initial mixture, stir to dissolve, then add a surface wetting agent and a fluorine-modified surface conditioner. Filter the mixed repair solution through a 0.2-0.45 μm filter, then seal and let it stand for 12-24 hours to obtain the second repair solution.

10. The coating process for mixing nitrocellulose lacquer and polyurethane lacquer according to claim 9, characterized in that, The low-viscosity acrylic resin includes at least one of hydroxyl acrylic resin, methyl methacrylate-butyl acrylate copolymer resin, and hydroxyethyl acrylate-butyl acrylate copolymer; the surface wetting agent includes at least one of polyether modified polysiloxane wetting agent, nonionic polyether polyol wetting agent, and organosilicon polyether copolymer wetting agent; and the fluorinated surface conditioner includes at least one of fluorinated alcohol polyoxyethylene ether, fluorinated polyether siloxane conditioner, and fluorinated alkylamide surfactant.

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