Temporary protective coating capable of realizing irreversible color change through laser induction as well as preparation method and use method of temporary protective coating

Abstract

The invention provides a laser-induced irreversible color-changing temporary protective coating as well as a preparation method and a use method thereof. The preparation process comprises the following steps: preparing a melamine-formaldehyde resin prepolymer; preparing a crystal violet lactone emulsion; preparing a crystal violet lactone microcapsule; performing graphene oxide modification to obtain photothermal conversion microcapsule powder; the photothermal conversion microcapsule powder, the waterborne polyurethane-waterborne acrylic acid composite emulsion, bisphenol A and other components are used for preparing the photosensitive functional coating. The prepared photosensitive functional coating can realize irreversible color change under laser irradiation, and can be matched with laser projection equipment to be used in operation processes such as positioning and hole making.

Description

Technical Field

[0001] This invention belongs to the field of surface treatment and protection, and relates to a laser-induced irreversible color-changing temporary protective coating, its preparation and application method. Background Technology

[0002] The positioning accuracy of holes in aerospace parts is closely related to the reliability of fastener connections, induced stress, and the accuracy of aircraft shape, making it crucial for aircraft assembly and product quality. Currently, hole positioning in aerospace parts still relies on traditional manual scribing, a method that suffers from complexity, time-consuming nature, and low accuracy. For example, with large skin panels requiring hundreds of holes, the positioning accuracy of the hole distance must be controlled to within millimeters, considering the impact on part strength. Therefore, new positioning methods need to be developed to improve positioning efficiency and quality. Furthermore, temporary protection during part manufacturing and damage protection during part transport often involves covering with tape or kraft paper. However, this method suffers from problems such as loose application of protective media at high curvature areas, material residue contamination, and damage to the protective paper during impacts and scratches. For instance, when using non-painted areas with less flexible paper tape for paint protection, loose tape application often causes paint to seep into unpainted areas, while overly tight application causes residual adhesive transfer. Therefore, new protection methods are urgently needed to improve the current state of temporary surface protection.

[0003] Functional peelable coatings possess both functionality and peelability. Functionality refers to the property of acquiring special functions through the addition of specific functional fillers. For example, photosensitive or thermochromic coatings introduce photosensitive or thermosensitive functional materials, and the resulting coating undergoes localized color changes at specific wavelengths or temperatures. This phenomenon can be used to induce color changes at specific points under specific conditions for targeted positioning. Peelability refers to the fact that after the coating is applied to the surface of the protected object, it forms a uniform and dense film, isolating the protected object from external contaminants. After the protection is complete, it can be peeled off without affecting the substrate itself. This characteristic can be used to temporarily protect parts and improve surface quality. However, most commercially available photosensitive color-changing materials rely on ultraviolet light (320–380 nm) to trigger reversible color changes, while thermochromic materials have a wide trigger temperature range (-15℃ to 80℃). Both have short color-changing retention times and unstable color changes, making them unsuitable for targeted positioning. By developing a functional, peelable temporary coating that triggers color change under specific conditions, exhibits stable color change and long-lasting color change, it can simultaneously achieve rapid positioning and temporary protection, and has enormous application potential in the aerospace manufacturing field. Summary of the Invention

[0004] This invention provides a laser-sensing irreversible color-changing temporary protective coating, its preparation and application method. It uses crystal violet lactone microcapsules modified with graphene oxide as a photothermal conversion functional material to achieve irreversible color change of the protective film, and also provides an application method for scenarios such as hole positioning and temporary protection of aerospace parts.

[0005] The present invention adopts the following technical solutions:

[0006] A method for preparing a laser-induced irreversible color-changing temporary protective coating includes the following steps:

[0007] Step 1, Preparation of melamine-formaldehyde resin prepolymer:

[0008] A certain amount of melamine powder and formaldehyde solution were added to deionized water, wherein the mass ratio of melamine, formaldehyde and deionized water was 1:3:4 to 1.3:3.8:5. Sodium hydroxide was added to adjust the pH of the solution to 8 to 9. The solution was heated in a water bath at 65 to 75°C and stirred for 30 to 40 minutes using an electric stirrer at a speed of 300 to 600 r / min to obtain a transparent and clear melamine-formaldehyde resin prepolymer solution.

[0009] Step 2, Preparation of crystal violet lactone emulsion:

[0010] Crystal violet lactone and diarylethane were mixed at a mass ratio of 1:15 to 1.5:19 and heated in a water bath at 60 to 70°C. An electric stirrer was used to stir the mixture at a speed of 300 to 500 r / min until the crystal violet lactone was completely dissolved, resulting in a pale yellow transparent emulsion.

[0011] Step 3, prepare crystal violet lactone emulsion:

[0012] The emulsifier is dissolved in deionized water at a mass ratio of 3:50 to 4:55. The solution is heated in a water bath at 60 to 70°C and stirred for 20 to 40 minutes using an electric stirrer at a speed of 2000 to 4000 r / min to obtain a uniformly dispersed solution A. The crystal violet lactone emulsion is mixed with solution A at a volume ratio of 0.5:1.2 to 1:1.7 and heated in a water bath at 60 to 70°C until homogeneous to obtain an oil-in-water emulsion. The emulsifier is one of styrene-maleic anhydride copolymer (SMA type), sodium dodecyl sulfate (SDS), or sodium dodecylbenzene sulfonate (SDBS).

[0013] Step 4, Preparation of crystal violet lactone microcapsules:

[0014] Melamine-formaldehyde prepolymer solution and emulsion were mixed at a volume ratio of 1:3 to 1:5. The mixed emulsion was heated to 70 to 80°C and stirred for 1.5 to 2 hours using an electric stirrer at a speed of 600 to 1000 r / min to ensure a complete reaction, thus obtaining crystal violet lactone microcapsule emulsion. Citric acid was added to the crystal violet lactone microcapsule emulsion to adjust the pH of the solution to 4 to 4.5, making the microcapsule walls more dense and robust while protonating the surface amino groups to enhance the positive charge on the surface of the crystal violet lactone capsules.

[0015] Step 5, Graphene oxide modification:

[0016] Graphene oxide was added to deionized water and ultrasonically dispersed for 1–2 hours to ensure uniform dispersion. The concentration of graphene oxide in the mixed solution was 2–5 g / L. Citric acid was added to adjust the pH of the solution to 5–6 to enhance the ionization of carboxyl groups and increase the negative charge of the mixed solution. The crystal violet lactone microcapsule emulsion and the graphene oxide dispersion were mixed at a volume ratio of 1:0.3–1:0.7 and stirred at room temperature for 0.5–1.5 hours. The negatively charged carboxyl groups and positively charged amino groups were combined through electrostatic adsorption to obtain a graphene oxide-modified crystal violet lactone microcapsule emulsion. The obtained emulsion was filtered and washed repeatedly with deionized water. The crystal violet lactone core material that was not encapsulated in microcapsules was washed with isopropanol solution. Finally, the filtered product was vacuum dried at 45–60°C for 24–36 hours to obtain photothermal conversion microcapsule powder.

[0017] Step 6, Prepare the photothermal conversion functional coating:

[0018] The photosensitive functional coating comprises the following components by weight percentage:

[0019] Waterborne polyurethane-waterborne acrylic composite emulsion: 45-55%;

[0020] Bisphenol A: 8-12%;

[0021] Photothermal conversion microcapsules: 7-8%;

[0022] Mixed additives: 2-3.4%, including: dispersant 0.7-1.2%, pH adjuster 0.5-0.8%, film-forming aid 0.5-1%, and thickener 0.3-0.4%;

[0023] Pigment filler: 1.5-2%;

[0024] Defoamer: 0.3-0.5%;

[0025] Deionized water: to fill any remaining gaps;

[0026] Photothermal conversion microcapsules and bisphenol A were added to an aqueous polyurethane-aqueous acrylic composite emulsion and stirred until homogeneous to obtain solution B. The pigment was ground to a particle size of 10-20 μm using a grinder and added to the mixed additives and stirred until homogeneous to obtain solution C. Solution B and solution C were mixed and then defoamer was added and stirred for 45-55 minutes to finally form a laser-induced irreversible color-changing temporary protective coating.

[0027] A laser-induced irreversible color-changing temporary protective coating, prepared by the above method, is a photothermal conversion functional coating obtained by modifying crystal violet lactone with graphene oxide. The temporary protective film formed by this photothermal conversion functional coating can achieve irreversible color change under laser irradiation. The laser used is a near-infrared laser with an irradiation wavelength range of 800–1000 nm and a power density of 1.2–1.5 W / cm². 2 .

[0028] A method for applying a laser-sensored irreversible color-changing temporary protective coating includes the following steps:

[0029] Step 1, Part pretreatment:

[0030] Use compressed air to clean the surface of the parts and remove any adhering substances. Then, use a clean, dry cloth dampened with anhydrous ethanol to wipe the surface of the parts and remove any dirt.

[0031] Step 2, applying the protective film:

[0032] Select a spray gun with a 150-200 mesh filter, adjust the nozzle diameter to 1.2-1.5mm, and the pressure to 0.3-0.5MPa. Place the part on the screen plate, with the spray gun perpendicular to the part's surface and 150-250mm away. Use a cross-hatching method to spray the temporary protective coating onto the areas of the part's surface that require temporary protection. Control the spray gun's movement speed to 30-60cm / s. After spraying, let the part stand at room temperature for 10-20 minutes to ensure the coating is fully leveled. If painting is required on other areas of the part's surface, wait until the protective film is completely dry before painting as required. If painting is not required, proceed directly to step three.

[0033] Step 3, heating and drying:

[0034] Place the parts in a heating chamber and let them stand at 40-50℃ for 5-8 minutes. Then remove them and observe whether the protective film on the surface of the parts completely covers the corresponding area of ​​the parts. If there are any uncovered areas, use a brush to apply temporary protective coating to the surface of the parts and let them dry at room temperature.

[0035] Step 4, Positioning and Hole Making:

[0036] The part is placed horizontally on the worktable, and a laser projection device is fixed vertically above it to project the location of the hole to be drilled onto the part surface. Near-infrared laser irradiation is used with a wavelength range of 800–1000 nm and a power density of 1.2–1.5 W / cm². 2 Irradiate for 8-10 minutes, and the surface of the protective film will undergo irreversible discoloration; perform subsequent hole-making operations on the discolored areas.

[0037] Step 5, Post-processing:

[0038] After drilling, use compressed air to spray the edge of the protective film on the part to make it curl up, and then peel off the protective film. Use dry ice cleaning to remove residual adhesive on the edge of the part, threads and grooves.

[0039] The beneficial effects of this invention are:

[0040] 1) Using a suspended laser projection positioning device can achieve rapid and accurate marking of positioning holes, and the markings are permanent and do not fade, significantly improving assembly drilling efficiency and reducing damage such as bumps and scratches to parts during assembly.

[0041] 2) The laser-sensing irreversible color-changing temporary protective coating, combined with the spray painting protection positioning fixture, greatly improves the protection efficiency of complex local spray painting parts.

[0042] 3) Using temporary protective coatings to protect locally painted parts is more efficient than traditional tape protection, and the protection accuracy is high and it is less likely to affect other parts of the parts.

[0043] 4) The dry ice cleaning process can quickly remove the temporary protective film remaining on the edges and overlapping parts of the parts, ensuring that there are no other impurities on the surface of the parts. Attached Figure Description

[0044] Figure 1 The color rendering curve of a laser-sensored irreversible color-changing temporary protective coating;

[0045] Figure 2 Flowchart of the application method for laser-induced irreversible color-changing temporary protective coating. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] Example 1

[0048] A method for preparing a laser-induced irreversible color-changing temporary protective coating includes the following steps:

[0049] Step 1, Preparation of melamine-formaldehyde resin prepolymer:

[0050] A certain amount of melamine powder and formaldehyde solution were added to deionized water, with the mass ratio of melamine, formaldehyde and deionized water being 1.2:3.5:4.5. Sodium hydroxide was added to adjust the pH of the solution to 8.5, and the solution was heated in a 70°C water bath. An electric stirrer was used to stir the solution at a speed of 500 r / min for 35 min to obtain a transparent and clear melamine-formaldehyde resin prepolymer solution.

[0051] Step 2, Preparation of crystal violet lactone emulsion:

[0052] Crystal violet lactone and diarylethane were mixed at a mass ratio of 1:14, heated in a water bath at 65°C, and stirred with an electric stirrer at a speed of 400 r / min until the crystal violet lactone was completely dissolved, resulting in a pale yellow transparent emulsion.

[0053] Step 3, prepare the emulsion:

[0054] The emulsifier was dissolved in deionized water at a mass ratio of 3:45. The solution was heated in a 65°C water bath and stirred for 30 minutes using an electric stirrer at a speed of 3000 r / min to obtain a uniformly dispersed solution A. The crystal violet lactone emulsion was mixed with solution A at a volume ratio of 1:2 and heated in a 65°C water bath until homogeneous to obtain an oil-in-water emulsion. The emulsifier was a styrene-maleic anhydride copolymer.

[0055] Step 4, Preparation of crystal violet lactone microcapsules:

[0056] The melamine-formaldehyde prepolymer solution and the emulsion were mixed at a volume ratio of 1:4. The mixed emulsion was heated to 75°C and stirred for 1.8 hours using an electric stirrer at a speed of 800 r / min to ensure a complete reaction, thus obtaining a crystal violet lactone microcapsule emulsion. Citric acid was added to the crystal violet lactone microcapsule emulsion to adjust the pH of the solution to 4.5, making the microcapsule walls more dense and robust, while also protonating the surface amino groups to enhance the positive charge on the surface of the crystal violet lactone capsules.

[0057] Step 5, Graphene oxide modification:

[0058] Graphene oxide was added to deionized water and ultrasonically dispersed for 1.5 h to ensure uniform dispersion. The concentration of graphene oxide in the mixed solution was 3 g / L. Citric acid was added to adjust the pH of the solution to 5.5 to enhance the ionization of carboxyl groups and increase the negative charge of the mixed solution. The crystal violet lactone microcapsule emulsion and the graphene oxide dispersion were mixed at a volume ratio of 1:0.5 and stirred at room temperature for 1 h. The negatively charged carboxyl groups and positively charged amino groups were combined through electrostatic adsorption to obtain the graphene oxide modified crystal violet lactone microcapsule emulsion. The obtained emulsion was filtered and washed repeatedly with deionized water. The crystal violet lactone core material that was not encapsulated in microcapsules was washed with isopropanol solution. Finally, the filtered product was vacuum dried at 50 °C for 30 h to obtain photothermal conversion microcapsule powder.

[0059] Step 6, Prepare the photothermal conversion functional coating:

[0060] The photosensitive functional coating comprises the following components by weight percentage:

[0061] Waterborne polyurethane-waterborne acrylic composite emulsion: 50%;

[0062] Bisphenol A: 10%;

[0063] Photothermal conversion microcapsules: 7.5%;

[0064] Mixed additives: 3%, including: dispersant BYK-191 1.1%, pH adjuster triethanolamine 0.7%, film-forming aid ethylene glycol butyl ether 0.85%, and thickener hydroxyethyl cellulose 0.35%;

[0065] Titanium dioxide as pigment: 1.8%;

[0066] Defoamer BYK-333: 0.4%;

[0067] Deionized water: to fill any remaining gaps;

[0068] Photothermal conversion microcapsules and bisphenol A were added to an aqueous polyurethane-aqueous acrylic composite emulsion and stirred until homogeneous to obtain solution B. The pigment was ground to a particle size of 10-20 μm using a grinder and added to the mixed additives and stirred until homogeneous to obtain solution C. Solution B and solution C were mixed and an antifoaming agent was added and stirred for 50 minutes to finally form a laser-induced irreversible color-changing temporary protective coating.

[0069] Example 2

[0070] A method for preparing a laser-induced irreversible color-changing temporary protective coating includes the following steps:

[0071] Step 1, Preparation of melamine-formaldehyde resin prepolymer:

[0072] A certain amount of melamine powder and formaldehyde solution were poured into deionized water, with the mass ratio of melamine, formaldehyde and deionized water being 1:3:4. Sodium hydroxide was added to adjust the pH of the solution to 8. The solution was heated in a water bath at 75°C and stirred for 40 minutes at a speed of 300 r / min using an electric stirrer to obtain a transparent and clear melamine-formaldehyde resin prepolymer solution.

[0073] Step 2, Preparation of crystal violet lactone emulsion:

[0074] Crystal violet lactone and diarylethane were mixed at a mass ratio of 1:15, heated in a water bath at 70°C, and stirred with an electric stirrer at a speed of 300 r / min until the crystal violet lactone was completely dissolved, resulting in a pale yellow transparent emulsion.

[0075] Step 3, prepare the emulsion:

[0076] The emulsifier was dissolved in deionized water at a mass ratio of 3:50. The solution was heated in a 70°C water bath and stirred for 40 minutes at 2000 rpm using an electric stirrer to obtain a uniformly dispersed solution A. The crystal violet lactone emulsion was mixed with solution A at a volume ratio of 1:1.7 and heated in a 60°C water bath until homogeneous to obtain an oil-in-water emulsion. The emulsifier was sodium dodecyl sulfate.

[0077] Step 4, Preparation of crystal violet lactone microcapsules:

[0078] Melamine-formaldehyde prepolymer solution and emulsion were mixed at a volume ratio of 1:5. The mixed emulsion was heated to 70°C and stirred for 1.5 hours using an electric stirrer at a speed of 1000 r / min to ensure a complete reaction, resulting in crystal violet lactone microcapsule emulsion. Citric acid was added to the crystal violet lactone microcapsule emulsion to adjust the pH of the solution to 4.5, making the microcapsule walls more dense and robust while protonating the surface amino groups to enhance the positive charge on the surface of the crystal violet lactone capsules.

[0079] Step 5, Graphene oxide modification:

[0080] Graphene oxide was added to deionized water and ultrasonically dispersed for 1 hour to ensure uniform dispersion. The concentration of graphene oxide in the mixed solution was 2 g / L. Citric acid was added to adjust the pH of the solution to 6 to enhance the ionization of carboxyl groups and increase the negative charge of the mixed solution. The crystal violet lactone microcapsule emulsion and the graphene oxide dispersion were mixed at a volume ratio of 1:0.7 and stirred at room temperature for 1.5 hours. The negatively charged carboxyl groups and positively charged amino groups were combined through electrostatic adsorption to obtain the graphene oxide modified crystal violet lactone microcapsule emulsion. The obtained emulsion was filtered and washed repeatedly with deionized water. The crystal violet lactone core material that was not encapsulated in microcapsules was washed with isopropanol solution. Finally, the filtered product was vacuum dried at 45°C for 36 hours to obtain photothermal conversion microcapsule powder.

[0081] Step 6, Prepare the photothermal conversion functional coating:

[0082] The photosensitive functional coating comprises the following components by weight percentage:

[0083] Waterborne polyurethane-waterborne acrylic composite emulsion: 45%;

[0084] Bisphenol A: 12%;

[0085] Photothermal conversion microcapsules: 7%;

[0086] Mixed additives: 2%, including: dispersant BYK-180 0.7%, pH adjuster citric acid 0.5%, film-forming aid dipropylene glycol methyl ether 0.5%, and thickener hydroxypropyl methylcellulose 0.3%;

[0087] Pigment Calcium carbonate: 1.5%;

[0088] Defoamer BYK-066N: 0.5%;

[0089] Deionized water: to fill any remaining gaps;

[0090] Photothermal conversion microcapsules and bisphenol A were added to an aqueous polyurethane-aqueous acrylic composite emulsion and stirred until homogeneous to obtain solution B. The pigment was ground to a particle size of 10-20 μm using a grinder and added to the mixed additives and stirred until homogeneous to obtain solution C. Solution B and solution C were mixed and an antifoaming agent was added and stirred for 55 minutes to finally form a laser-induced irreversible color-changing temporary protective coating.

[0091] Example 3

[0092] A method for preparing a laser-induced irreversible color-changing temporary protective coating includes the following steps:

[0093] Step 1, Preparation of melamine-formaldehyde resin prepolymer:

[0094] A certain amount of melamine powder and formaldehyde solution were poured into deionized water, with the mass ratio of melamine, formaldehyde and deionized water being 1.3:3.8:5. Sodium hydroxide was added to adjust the pH of the solution to 9. The solution was heated in a water bath at 65°C and stirred for 30 minutes using an electric stirrer at a speed of 600 r / min to obtain a transparent and clear melamine-formaldehyde resin prepolymer solution.

[0095] Step 2, Preparation of crystal violet lactone emulsion:

[0096] Crystal violet lactone and diarylethane were mixed at a mass ratio of 1.5:19, heated in a water bath at 60°C, and stirred with an electric stirrer at a speed of 500 r / min until the crystal violet lactone was completely dissolved, resulting in a pale yellow transparent emulsion.

[0097] Step 3, prepare the emulsion:

[0098] The emulsifier was dissolved in deionized water at a mass ratio of 4:55. The solution was heated in a 60°C water bath and stirred for 20 minutes using an electric stirrer at a speed of 4000 rpm to obtain a uniformly dispersed solution A. The crystal violet lactone emulsion was mixed with solution A at a volume ratio of 0.5:1.2 and heated in a 70°C water bath until homogeneous to obtain an oil-in-water emulsion. The emulsifier was sodium dodecylbenzenesulfonate.

[0099] Step 4, Preparation of crystal violet lactone microcapsules:

[0100] Melamine-formaldehyde prepolymer solution and emulsion were mixed at a volume ratio of 1:3. The mixed emulsion was heated to 80°C and stirred for 2 hours using an electric stirrer at a speed of 600 r / min to ensure a complete reaction, thus obtaining crystal violet lactone microcapsule emulsion. Citric acid was added to the crystal violet lactone microcapsule emulsion to adjust the pH of the solution to 4, making the microcapsule walls more dense and robust, while protonating the surface amino groups to enhance the positive charge on the surface of the crystal violet lactone capsules.

[0101] Step 5, Graphene oxide modification:

[0102] Graphene oxide was added to deionized water and ultrasonically dispersed for 2 hours to ensure uniform dispersion. The concentration of graphene oxide in the mixed solution was 5 g / L. Citric acid was added to adjust the pH of the solution to 5 to enhance the ionization of carboxyl groups and increase the negative charge of the mixed solution. The crystal violet lactone microcapsule emulsion and the graphene oxide dispersion were mixed at a volume ratio of 1:0.3 and stirred at room temperature for 0.5 hours. The negatively charged carboxyl groups and positively charged amino groups were combined through electrostatic adsorption to obtain the graphene oxide modified crystal violet lactone microcapsule emulsion. The obtained emulsion was filtered and washed repeatedly with deionized water. The crystal violet lactone core material that was not encapsulated in microcapsules was washed with isopropanol solution. Finally, the filtered product was vacuum dried at 60°C for 24 hours to obtain photothermal conversion microcapsule powder.

[0103] Step 6, Prepare the photothermal conversion functional coating:

[0104] The photosensitive functional coating comprises the following components by weight percentage:

[0105] Waterborne polyurethane-waterborne acrylic composite emulsion: 55%;

[0106] Bisphenol A: 8%;

[0107] Photothermal conversion microcapsules: 8%;

[0108] Mixed additives: 3.4%, including: dispersant EFKA-4580 1.2%, pH adjuster AMP-95 0.8%, film-forming aid propylene glycol phenyl ether 1%, and acrylic acid alkali-swelling thickener 0.4%;

[0109] Filler pigment talc: 2%;

[0110] Defoamer BYK-028: 0.3%;

[0111] Deionized water: to fill any remaining gaps;

[0112] Photothermal conversion microcapsules and bisphenol A were added to an aqueous polyurethane-aqueous acrylic composite emulsion and stirred until homogeneous to obtain solution B. The pigment was ground to a particle size of 10-20 μm using a grinder and added to the mixed additives and stirred until homogeneous to obtain solution C. Solution B and solution C were mixed and then an antifoaming agent was added and stirred for 45 minutes to finally form a laser-induced irreversible color-changing temporary protective coating.

[0113] Example 4

[0114] This embodiment uses the positioning and drilling process of a large aircraft panel with an area ≥2000*1000mm to further illustrate the application method of the laser-induced irreversible color-changing temporary protective coating configured in Embodiment 1. The process is as follows: Figure 2 As shown, it includes the following steps:

[0115] Step 1, Pre-treatment of large wall panels:

[0116] Use compressed air to clean the surface of large wall panels to remove adhesive substances. Use a clean, dry cloth dampened with anhydrous ethanol to wipe the surface of the large wall panel film to remove dirt.

[0117] Step 2, applying the protective film:

[0118] Select a spray gun with a 180-mesh filter, adjust the nozzle diameter to 1.3mm, and the pressure to 0.4MPa. Place the large wall panel on the screen plate, with the spray gun perpendicular to the surface of the large wall panel and 200mm away from it. Use a cross-hatching method to spray the temporary protective coating onto the area of ​​the large wall panel that needs temporary protection. Control the spray gun movement speed at 40cm / s. After spraying, let the large wall panel stand at room temperature for 15 minutes to ensure that the coating is fully leveled.

[0119] Step 3, heating and drying:

[0120] Place the large wall panel in a heating chamber and let it stand at 45°C for 6 minutes. Then remove it and observe whether the protective film on the surface of the large wall panel completely covers the corresponding area of ​​the large wall panel. If there are any uncovered areas, use a brush to apply temporary protective coating to the surface of the large wall panel and let it dry at room temperature.

[0121] Step 4, Positioning and Hole Making:

[0122] A large panel is placed horizontally on a workbench. A laser projection device is fixed vertically above the panel to project the hole-making information onto its surface. Near-infrared laser with a wavelength of 900nm and a power density of 1.3W / cm² is used for irradiation. 2 After irradiation for 9 minutes, irreversible discoloration occurred on the surface of the protective film. The color development curve of the coating is shown below. Figure 1 ; Perform subsequent hole-making operations on the discolored points.

[0123] Step 5, Post-processing:

[0124] After drilling, use compressed air to pry open the edges of the protective film on the large wall panel, then peel off the film. Remove any residual adhesive from the edges, threads, and grooves of the large wall panel using dry ice cleaning. The specific procedure is as follows:

[0125] Preparation for startup: Turn on the compressed air (the pressure gauge reading rises and stabilizes), turn on the power, adjust the ice output to the lowest setting, start the spray gun, spray for 2 minutes, and then drain any remaining material from the pipes.

[0126] Add dry ice: Open the dry ice chamber and add 3mm diameter dry ice pellets (no more than 2 / 3 of the chamber's volume). Adjust to the appropriate ice output, aim at the crevices to be cleaned, start the spray gun, and begin cleaning.

[0127] Example 5

[0128] This embodiment uses a long rod with a length ≥1000mm to spray paint sections on the same surface, further illustrating the application method of the laser-induced irreversible color-changing temporary protective coating configured in Embodiment 1, including the following steps:

[0129] Step 1, Pre-treatment of the long pole:

[0130] Use compressed air to clean the surface of the long rod to remove any adhering substances. Then, use a clean, dry cloth dampened with anhydrous ethanol to wipe the surface of the long rod to remove dirt.

[0131] Step 2, applying the protective film:

[0132] Select a spray gun with a 150-mesh filter, adjust the nozzle diameter to 1.5mm, and the pressure to 0.3MPa. Place the long rod on the screen plate, with the spray gun perpendicular to the surface to be sprayed on the long rod and 250mm away from the long rod. Use a cross-hatching method to spray the temporary protective coating onto the area on the long rod surface that needs temporary protection. Control the spray gun movement speed at 30cm / s. After spraying, let the long rod stand at room temperature for 10 minutes to ensure that the coating is fully leveled.

[0133] Step 3, heating and drying:

[0134] Place the long rod in a heating chamber and let it stand at 40°C for 8 minutes. Then take it out and observe whether the protective film on the surface of the long rod completely covers the corresponding area. If there are any uncovered areas, use a brush to apply temporary protective coating to the surface of the long rod and let it dry at room temperature.

[0135] Step 4, Positioning and Hole Making:

[0136] Secure the long rod to the marking fixture, adjust the laser emitter to the required protection height, and use near-infrared laser irradiation with a wavelength range of 1000nm and a power density of 1.2W / cm². 2 The edge of the marking on the surface of the long rod is irradiated repeatedly for 10 minutes, and a dark blue marking positioning line is formed on the surface of the temporary protective film; the temporary protective film is removed from the parts that need to be painted and the subsequent painting operation is carried out as required.

[0137] Step 5, Post-processing:

[0138] After the painting is finished, use compressed air to spray the edges of the protective film on the long rod to make the edges peel off. Then peel off the protective film. Use dry ice cleaning to remove residual adhesive from the edges, threads, and grooves of the long rod.

[0139] Example 6

[0140] This embodiment uses the positioning and drilling process of a large aircraft skin to further illustrate the application method of the laser-inductive irreversible color-changing temporary protective coating configured in Embodiment 1, including the following steps:

[0141] Step 1, Skin pretreatment:

[0142] Use compressed air to clean the skin surface and remove any adhering substances. Then, use a clean, dry cloth dampened with anhydrous ethanol to wipe the skin surface and remove dirt.

[0143] Step 2, applying the protective film:

[0144] Select a spray gun with a 200-mesh filter, adjust the nozzle diameter to 1.2mm, and the pressure to 0.5MPa. Place the skin on the screen plate, with the spray gun perpendicular to the skin surface and 150mm away. Use a cross-hatching method to spray the temporary protective coating onto the areas of the skin surface that require temporary protection. Control the spray gun movement speed at 60cm / s. After spraying, let the skin stand at room temperature for 20 minutes to ensure the coating is fully leveled. If painting is required on other areas of the skin surface, wait until the protective film is completely dry before painting as required. If painting is not required, proceed directly to step three.

[0145] Step 3, heating and drying:

[0146] Place the skin in a heating chamber and let it stand at 50°C for 5 minutes. Then remove it and observe whether the protective film on the skin surface completely covers the corresponding area of ​​the skin surface. If there are any uncovered areas, use a brush to apply temporary protective coating to the skin surface and let it dry at room temperature.

[0147] Step 4, Positioning and Hole Making:

[0148] The skin is placed vertically on the clamping fixture, and a laser projection device is fixed vertically above the skin to project the positions of the holes to be made onto the skin surface. Near-infrared laser irradiation with a wavelength range of 800nm ​​and a power density of 1.5W / cm² is used. 2 After irradiation for 8 minutes, the surface of the protective film underwent irreversible discoloration; subsequent hole-making operations were performed on the discolored areas.

[0149] Step 5, Post-processing:

[0150] After drilling, use compressed air to pry open the edges of the protective film on the skin, then peel off the protective film. Remove any residual adhesive from the skin edges, threads, and grooves using dry ice cleaning.

[0151] The above description is only a partial embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a laser-induced irreversible color-changing temporary protective coating, characterized in that, Includes the following steps: Step 1: Prepare melamine-formaldehyde resin prepolymer; Step 2: Prepare crystal violet lactone emulsion; Step 3: Prepare crystal violet lactone emulsion; Step 4: Prepare crystal violet lactone microcapsules; Step 5, graphene oxide modification; Step 6: Prepare the photothermal conversion functional coating.

2. The method for preparing a laser-induced irreversible color-changing temporary protective coating according to claim 1, characterized in that, The specific steps include: Step 1, Preparation of melamine-formaldehyde resin prepolymer: Melamine powder and formaldehyde solution were added to deionized water, with the mass ratio of melamine, formaldehyde and deionized water being 1:3:4 to 1.3:3.8:

5. Sodium hydroxide was added to adjust the pH of the solution to 8 to 9. The solution was heated in a water bath at 65 to 75°C and stirred at 300 to 600 r / min for 30 to 40 min to obtain a transparent and clear melamine-formaldehyde resin prepolymer solution. Step 2, Preparation of crystal violet lactone emulsion: Crystal violet lactone and diarylethane were mixed at a mass ratio of 1:15 to 1.5:19, heated in a water bath at 60 to 70°C, and stirred with an electric stirrer at a speed of 300 to 500 r / min until the crystal violet lactone was completely dissolved. Step 3, prepare crystal violet lactone emulsion: The emulsifier is dissolved in deionized water at a mass ratio of 3:50 to 4:

55. The solution is heated in a water bath at 60 to 70°C and stirred at 2000 to 4000 r / min for 20 to 40 min to obtain a uniformly dispersed solution A. The crystal violet lactone emulsion is mixed with solution A at a volume ratio of 0.5:1.2 to 1:1.

7. The mixture is heated in a water bath at 60 to 70°C and stirred until homogeneous to obtain an oil-in-water emulsion. Step 4, Preparation of crystal violet lactone microcapsules: The melamine-formaldehyde prepolymer solution and the emulsion were mixed at a volume ratio of 1:3 to 1:

5. The mixed emulsion was heated to 70 to 80°C and stirred at 600 to 1000 r / min for 1.5 to 2 hours to obtain crystal violet lactone microcapsule emulsion. Citric acid was added to the crystal violet lactone microcapsule emulsion to adjust the pH of the solution to 4 to 4.

5. Step 5, Graphene oxide modification: Graphene oxide was added to deionized water and ultrasonically dispersed for 1–2 h, with a graphene oxide concentration of 2–5 g / L in the mixed solution. Citric acid was added to adjust the pH of the solution to 5–6. The crystal violet lactone microcapsule emulsion and the graphene oxide dispersion were mixed at a volume ratio of 1:0.3–1:0.7 and stirred at room temperature for 0.5–1.5 h to obtain a graphene oxide-modified crystal violet lactone microcapsule emulsion. The obtained emulsion was filtered and washed repeatedly with deionized water. The crystal violet lactone core material that was not encapsulated in microcapsules was washed with isopropanol solution. Finally, the filtered product was vacuum dried at 45–60 °C for 24–36 h to obtain photothermal conversion microcapsule powder. Step 6, Prepare the photothermal conversion functional coating: Photothermal conversion microcapsules and bisphenol A were added to an aqueous polyurethane-aqueous acrylic composite emulsion and stirred until homogeneous to obtain solution B. The pigment was ground to a particle size of 10-20 μm and added to the mixed additives and stirred until homogeneous to obtain solution C. Solution B and solution C were mixed and then an antifoaming agent was added and stirred for 45-55 minutes to finally form a laser-induced irreversible color-changing temporary protective coating.

3. The method for preparing a laser-induced irreversible color-changing temporary protective coating according to claim 2, characterized in that, In step 3, the emulsifier is one of styrene-maleic anhydride copolymer, sodium dodecyl sulfate, or sodium dodecylbenzene sulfonate.

4. The method for preparing a laser-induced irreversible color-changing temporary protective coating according to claim 2, characterized in that, In step 6, the photosensitive functional coating comprises the following components by mass percentage: waterborne polyurethane-waterborne acrylic composite emulsion: 45-55%; Bisphenol A: 8-12%; Photothermal conversion microcapsules: 7-8%; Mixed additives: 2–3.4%; Pigment filler: 1.5-2%; Defoamer: 0.3-0.5%; Deionized water: to fill any remaining gaps.

5. The method for preparing a laser-induced irreversible color-changing temporary protective coating according to claim 4, characterized in that, The mixed additives include: 0.7-1.2% dispersant, 0.5-0.8% pH adjuster, 0.5-1% film-forming aid, and 0.3-0.4% thickener.

6. A laser-induced irreversible color-changing temporary protective coating, prepared by the preparation method according to any one of claims 1-5, characterized in that, This is a photothermal conversion functional coating obtained by modifying crystal violet lactone with graphene oxide. The temporary protective film formed by this photothermal conversion functional coating can achieve irreversible color change under laser irradiation.

7. The laser-sensing irreversible color-changing temporary protective coating according to claim 5, characterized in that, The laser used is near-infrared laser, with an irradiation wavelength range of 800–1000 nm and a power density of 1.2–1.5 W / cm². 2 .

8. A method for using a laser-induced irreversible color-changing temporary protective coating according to claim 6 or 7, characterized in that, Includes the following steps: Step 1, Part pretreatment: Remove dirt and grime from the surface of the parts; Step 2, applying the protective film: The spray gun should be perpendicular to the surface of the part, 150-250mm away. Use a cross-hatching method to spray the temporary protective coating onto the areas of the part surface that need temporary protection. The spray gun movement speed should be controlled at 30-60cm / s. After spraying, let the part stand at room temperature for 10-20 minutes to ensure that the coating is fully leveled. If other areas of the part surface need to be painted, wait until the protective film is completely dry before painting as required. If painting is not required, proceed directly to step three. Step 3, heating and drying: Place the parts in a heating chamber and let them stand at 40-50°C for 5-8 minutes before removing them. For uncovered areas, apply the coating and let them dry at room temperature. Step 4, Positioning and Hole Making: The location of the hole to be made is projected onto the surface of the part using a laser projection device. Near-infrared laser irradiation is used with a wavelength range of 800–1000 nm and a power density of 1.2–1.5 W / cm². 2 After irradiation for 8–10 minutes, irreversible discoloration occurs on the surface of the protective film; subsequent hole-making operations are then performed on the discolored areas. Step 5, Post-processing: After drilling, use compressed air to spray air onto the edge of the protective film on the part to make it curl up, and then peel off the protective film.

9. The method of using a laser-sensored irreversible color-changing temporary protective coating according to claim 8, characterized in that, In step two, a spray gun with a 150-200 mesh filter is selected, and the nozzle diameter is adjusted to 1.2-1.5 mm, with a pressure of 0.3-0.5 MPa.

10. The method of using a laser-sensored irreversible color-changing temporary protective coating according to claim 8, characterized in that, In step five, the residual adhesive on the edges, threads, and grooves of the parts is removed using a dry ice cleaning method.

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