Aluminum phosphate / shellac composite super-hydrophobic coating and preparation method thereof
Through the preparation method of aluminum phosphate/shellac composite superhydrophobic coating, the problem of poor stability of the existing coating is solved, mechanical and chemical stability is improved, and interface adhesion and biofriendliness are enhanced.
Patent Information
- Application Number
- CN202510657034.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-21
- Publication Date
- 2025-07-22
AI Technical Summary
The existing superhydrophobic coatings in the fields of food packaging and biomedicine have loose coating structures and easy particles to fall off due to poor interfacial adhesion, which affects stability and usage performance. Commonly used adhesives have safety risks.
Using the preparation method of aluminum phosphate/shellac composite superhydrophobic coating, aluminum phosphate adhesive was prepared by dispersing Al(OH)3 at 100°C in H3PO4 solution, dissolved with shellac and starch and sprayed onto the matrix membrane, and heat curing to form a stable composite coating.
It enhances the mechanical and chemical stability of the coating, improves the interface adhesion with polar organic substrates, forms a solid superhydrophobic surface, and has excellent adhesive properties and biofriendliness.
Smart Images
Figure CN120349730A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the field of superhydrophobic materials, and specifically relates to an aluminum phosphate / shellac composite superhydrophobic coating and a preparation method thereof. Background Art
[0002] The stability of superhydrophobic coatings has always been a key factor restricting their practical applications. Common micro-nano particles (such as starch particles, protein particles, etc.) combined with low surface energy chemical agents (PDMS, biological wax) show application potential in the fields of food packaging and biomedicine. However, their poor interfacial adhesion leads to loose coating structures and easy particle detachment, seriously affecting the stability and performance of the coatings. And various adhesives may bring multiple safety hazards due to their chemical compositions and reaction characteristics. Summary of the Invention
[0003] Aiming at the problems existing in superhydrophobic coatings in the prior art, the present invention provides an aluminum phosphate / shellac composite superhydrophobic coating and a preparation method thereof. The aluminum phosphate / shellac composite enhances the mechanical and chemical stability of the superhydrophobic coating.
[0004] Technical Solution: A preparation method of an aluminum phosphate / shellac composite superhydrophobic coating includes the following steps:
[0005] Step 1: Prepare an aluminum phosphate adhesive: Disperse Al(OH)3 in an H3PO4 solution at 100 °C, stir for a period of time, and obtain an aluminum phosphate adhesive, then cool it to room temperature;
[0006] Step 2: Prepare a shellac-aluminum phosphate-starch solution;
[0007] First, dissolve shellac in ethanol to prepare a shellac solution, then add the aluminum phosphate adhesive to the shellac solution and stir evenly, and then add rice starch, and stir at room temperature for a period of time to generate a shellac-aluminum phosphate-starch solution;
[0008] Step 3: Spray the shellac-aluminum phosphate-starch solution onto a substrate film, and obtain a substrate film with an aluminum phosphate / shellac composite superhydrophobic coating after thermal curing.
[0009] Further, in Step 1:
[0010] The stirring method is magnetic stirring for 3 h; the mass ratio of Al(OH)3 to the H3PO4 solution is 15.6:98; the mass fraction of the H3PO4 solution is 60%.
[0011] Further, in the shellac-aluminum phosphate-starch solution prepared in Step 2:
[0012] The mass ratio of the shellac to ethanol is 1:9;
[0013] The mass of the aluminum phosphate adhesive is 10%-30% of the sum of the masses of shellac and ethanol;
[0014] The mass of the starch is 10%-30% of the sum of the masses of shellac, aluminum phosphate adhesive, and ethanol.
[0015] Furthermore, in step two, the starch is one of rice, corn, potato, quinoa, and cassava, and it is the starch after ball milling for 2 hours.
[0016] Furthermore, in step two, the stirring time at room temperature is 1h.
[0017] Furthermore, step three is specifically as follows: Use a spray gun with an inner diameter of 0.5mm, apply a pressure of 276Kpa, and spray for 20s - 30s at a distance of 10cm from the substrate film. Then thermally cure the coating at 105°C for 3h to obtain a substrate film with an aluminum phosphate / shellac composite superhydrophobic coating.
[0018] Furthermore, in step three, the substrate film is a hydroxypropyl methylcellulose film.
[0019] Beneficial effects:
[0020] 1) The present invention uses a composite of aluminum phosphate (AP) / shellac with high biosafety to enhance the interfacial adhesion between polar organic particles and between them and polar organic substrates. This enables the coating to have excellent adhesive properties, mechanical stability, and biocompatibility, etc., forming a strong superhydrophobic surface.
[0021] The shellac molecule contains various hydroxyl groups (–OH), carboxyl groups (–COOH), and ester groups (–COOR). These functional groups can attach to the surface of polar materials (such as cellulose substrates, starch substrates, ceramics, etc.) through hydrogen bonds, electrostatic adsorption, or van der Waals forces.
[0022] Aluminum phosphate is formed by the reaction of phosphoric acid or metaphosphoric acid with aluminum salts (such as aluminum oxide, aluminum hydroxide, etc.). When it is coated on the substrate surface, it will form a three-dimensional network structure with Al-O-P as the backbone through a polycondensation reaction. This structure has high-temperature stability, chemical inertness, and high strength. During the curing process, aluminum phosphate will form strong polar bonds (such as hydrogen bonds, coordination bonds, covalent bonds) with the surface of polar materials, thereby enhancing the interfacial bonding force. In addition, the aluminum phosphate colloid has good wettability and can well penetrate micropores and surface irregular regions, thereby enhancing the mechanical interlocking force.
[0023] 2) In the present invention, the aluminum phosphate / shellac composite adhesive enhances the adhesiveness through physical-chemical synergistic effects:
[0024] Hydrogen bonding: Shellac contains abundant hydroxyl groups (-OH) and carboxyl groups (-COOH), which can form hydrogen bonds or van der Waals forces with the residual hydroxyl groups or phosphate groups (P=O, Al–OH) in the aluminum phosphate system, contributing to the formation of interfacial adhesion.
[0025] Coordination: The carboxyl groups in shellac may form coordination bonds with aluminum ions (Al3+), which is a relatively strong chemical bonding and may improve the interfacial compatibility.
[0026] Physical entanglement: The hydroxyl groups in AP molecules can undergo condensation-polymerization reactions through intermolecular and intramolecular dehydration to form aluminum phosphate colloidal particles. The aluminum phosphate colloidal particles can be dispersed in the shellac matrix, serving as physical cross-linking points, increasing the mechanical interlocking between shellac molecular chains, forming a certain degree of entangled network, enhancing the cohesive strength, and strengthening the integrity of the composite structure. Description of the Drawings
[0027] Figure 1 Schematic diagram for the preparation of the shellac / AP / RS superhydrophobic film in Example 1.
[0028] Figure 2 SEM images of different coatings prepared in Example 1.
[0029] Figure 3 SEM-EDS spectra of the surfaces of different coatings in Example 1.
[0030] Figure 4 SEM-EDS spectra of the cross-sections of different coatings in Example 1.
[0031] Figure 5 Cross-cut test results of different coatings in Example 1.
[0032] Figure 6 Sandpaper abrasion test results of different coatings in Example 1.
[0033] Figure 7 Anti-sticking test results of high-viscosity food droplets on different coatings in Example 1.
[0034] Figure 8 Oil absorption test results of different coatings in Example 1.
[0035] Figure 9 Particle size distributions of rice starch (RS), corn starch (CS), and potato starch (PS) milled for 0, 2, 6, and 8 hours.
[0036] Figure 10 SEM images of rice starch after being milled for different times. Detailed Description of the Invention
[0037] The technical solution of the present invention will be described in detail below through embodiments, but the protection scope of the present invention is not limited to the described embodiments.
[0038] The main materials and reagents used in the following embodiments are shown in Table 1:
[0039] Table 1 Main materials and reagents
[0040]
[0041]
[0042] Example 1: Preparation of shellac / AP / RS superhydrophobic film
[0043] The preparation process of the shellac / AP / RS superhydrophobic film is intuitively shown as Figure 1 shown, including the following steps:
[0044] Step 1: Prepare an inorganic AP adhesive (aluminum phosphate adhesive):
[0045] Dilute the 85% H3PO4 solution with deionized water to a 60% H3PO4 solution (mass fraction 60%, that is, 100 g of the solution contains 60 g of H3PO4).
[0046] Place 98.0 g of the diluted H3PO4 solution in a water bath at 100 °C, then disperse 15.6 g of Al(OH)3 in the H3PO4 solution, and stir magnetically for 3 h. After 3 h, an inorganic AP adhesive is obtained. Cool the AP adhesive to room temperature and place it in a volumetric flask for later use.
[0047] Reacting at 100 °C can significantly increase the reaction rate, and the reaction between Al(OH)3 and H3PO4 reaches a relatively high conversion rate in a short time. Reacting at a temperature lower than 100 °C may not be complete, resulting in unreacted Al(OH)3 or free H3PO4 remaining in the adhesive, affecting the final performance. The performance of the aluminum phosphate adhesive depends on the degree of polymerization of aluminum phosphate (such as chain-like, cyclic or cross-linked structures). At 100 °C, H3PO4 will partially dehydrate and condense to form polyphosphoric acid (such as pyrophosphoric acid, polyphosphoric acid), and then react with Al 3+ to form a more complex network structure, enhancing the heat resistance and mechanical strength of the adhesive. At low temperatures, only simple phosphates (such as Al(H2PO4)3) may be formed, with insufficient colloidal stability or cross-linking density after curing.
[0048] The 60% H3PO4 solution can balance the reaction activity and safety.
[0049] Step 2: Use shellac, ethanol, rice starch, and the inorganic AP adhesive prepared in Step 1 to prepare a shellac-AP-RS solution, and spray the shellac-AP-RS solution onto the substrate film to obtain a superhydrophobic film;
[0050] Specific steps: According to the ratio in Table 2, first dissolve shellac in ethanol to prepare a shellac solution, then add AP glue to the shellac solution, stir evenly, and then add rice starch that has been ball-milled for 2 h, and stir at room temperature for 1 h to generate a shellac-AP-RS solution. Use a spray gun with an inner diameter of 0.5 mm, apply a pressure of 276 Kpa, and spray for 30 s at a distance of 10 cm from the HPMC Film, and thermally cure the coating at 105 °C for 3 h to obtain a superhydrophobic film.
[0051] The sequential addition order of glue, ethanol, and rice starch avoids the problem that the raw materials form lumps and cannot be dispersed. The ratios of shellac, ethanol, rice starch, and the inorganic AP adhesive prepared in Step 1 are shown in Table 2:
[0052] Table 2: Raw material ratios
[0053]
[0054]
[0055] In Table 2:
[0056] (1) AS represents AP + shellac, and 10%-AS represents the proportion of AP glue in ethanol + shellac.
[0057] (2) Preparation method of HPMC Film: Dissolve a certain amount of HPMC powder in hot water at 85 °C. Prepare a 1% (w / w) HPMC film-forming solution, stir and cool to room temperature. Place the 1% (w / w) HPMC solution in a vacuum oven at room temperature to defoam for 3 h. Subsequently, pour 35 g of this solution into a polystyrene petri dish (diameter 15 cm) and dry it in an oven at 37 °C for 24 h. After drying, take out the film from the petri dish and place it at 57% relative humidity (RH) for 7 days to reach equilibrium.
[0058] (3) Shellac Film is used for comparison to reflect the importance of the roughness of the starch granule structure. Preparation method: Dissolve a specific amount of shellac in 10 mL of ethanol solution to prepare a shellac solution. Use a spray gun with an inner diameter of 0.5 mm, apply a pressure of 276 Kpa, and spray for 15 s at a distance of 10 cm from the HPMC Film. Finally, prepare the Shellac Film by thermally curing the coating at 105 °C for 3 h.
[0059] Step 3: Analyze and test the prepared superhydrophobic film.
[0060] 1) Coating microscopic morphology analysis (SEM)
[0061] SEM images of different coatings are asFigure 2 As shown Figure 2 In it, a is the SEM image of 0%-AS, b is the SEM image of 10%-AS, c is the SEM image of 20%-AS, d is the SEM image of 30%-AS, and e is the SEM image of the 40%-AS coating.
[0062] As Figure 2 shown, the roughness required for the 0%-AS coating to achieve superhydrophobicity is reached. At the same time, the surface of the starch granules is relatively smooth, the stacking is relatively loose, and the cavitation structure is obvious. For the samples with added AP glue, obvious small particles (AP glue particle structure) are observed on the coating surface, and there is a strong adhesion between the coating particles.
[0063] A small number of dispersed small particles can be observed on the surface of the 10%-AS starch granules, and the cavitation structure decreases. On the surface of the 20%-AS starch granules, an increase in AP small particles can be observed, which are densely distributed and more tightly wrapped. However, the cavitation structure increases slightly, which may be attributed to the fact that the introduction of an appropriate amount of AP changes the stacking mode of the starch granules. Most of the small particles disappear on the 30%-AS surface and seem to fuse with the starch granules. With the further increase of the AP glue content, the small particles completely merge with the starch granules, and the formed cavitation structure decreases significantly. This may be because AP glue is a hydrophilic colloid. When AP is added in excess (40%-AS), it will lead to an increase in the hydrophilicity of the coating, making it easier to blend with starch.
[0064] 2) EDS energy spectrum analysis
[0065] Figure 3 And Figure 4 are the energy-dispersive spectrum (EDS) maps, showing the elemental distributions of C, Al, and P on the surface and cross-section of the coatings with different AP glue contents. The C element comes from shellac and rice starch; the Al and P elements come from AP and can be used as localization elements for AP. From Figure 3 it can be seen that the newly appeared Al element on the coating surface reduces the percentage of the C element, which confirms the fusion of shellac-starch with AP glue. By observing the EDS surface scan map, it can be found that the Al and C elements are evenly distributed on the coating surface. With the increase of the AP content, the C element gradually decreases and the Al element content gradually increases.
[0066] From Figure 4 the EDS map of the coating cross-section, it can be seen that with the increase of the AP content, the percentage of the P element also increases. It is clearly observed on the cross-sections of 30%-AS and 40%-AS that AP penetrates into the starch layer, which also confirms the fusion of shellac-starch with AP. This result is consistent with the surface EDS result. Combining with the FT-IR result, it can be speculated that AP forms covalent bonds and hydrogen bonds with the hydroxyl groups in shellac-starch, enhancing the adhesion between shellac-starch and between the coating and shellac-starch.
[0067] 3) Mechanical stability test - Cross-cut tape test:
[0068] The interfacial adhesion between the coating and the HPMC film matrix was evaluated by the cross-cut tape test. The cross-cut tape adhesion was measured using a Cross-Cut Kit from Precision Gauge and Tool Company according to ASTM D-3359.
[0069] The cross-cut tape test mainly consists of two steps: Before the tape adhesion test, the target area of the film needs to be scored first to form a hundred square grids, and then 3M tape is attached to the film at an angle of 45° to the cross-section. After waiting for 1 min, it is removed. The adhesion strength grade is evaluated by the percentage of the removed area after the tape is peeled off.
[0070] Micro / nanostructures and low surface energy are two key features contributing to superhydrophobic properties. However, these properties are extremely vulnerable to mechanical abrasion, which may reduce the waterproof performance. Therefore, the mechanical wear resistance of the prepared superhydrophobic coating was evaluated. The cross-cut tape test was carried out with a cross-cut knife to evaluate the stability of the coating according to ASTM grade, and the CA (contact angle) and SA (rolling angle) of each group of samples were measured, and SEM was taken. The results of the cross-cut tape test are as Figure 5 shown, where a is the CA value, SA value, and optical image of the water droplet; b is the digital photo and ASTM grade after the cross-cut tape test; c is the SEM image of the coating after the cross-cut tape test. Note: NAN indicates that the result is not measurable, and different letters indicate significant differences (p < 0.05).
[0071] As Figure 5 shown, the ASTM grade of 0%-AS is 0B, the coating peeling area is greater than 65%, and the changes in CA and SA are not obvious; the ASTM grade of 10%-AS is 3B, the coating peeling area is greater than 5% and less than 15%, and SA > 15°; the highest ASTM grade of 20%-AS is 5B, the peeling area is almost 0%, the maximum contact angle is 152.4 ± 1.6°, and SA = 10.0 ± 0.7°; the coating peeling area of 30%-AS is about 15%-35%, the ASTM grade is 2B, and the changes in the rolling angle and contact angle are not obvious; the coating peeling area of 40%-AS < 5%, the ASTM grade is 4B, and the coating is not superhydrophobic. The test results show that the introduction of AP can improve the stability of the coating, and the proportion of 20% has the best effect on enhancing the adhesion performance between the coating and the base film.
[0072] 4) Mechanical stability test - Sandpaper abrasion test
[0073] To further evaluate the mechanical stability of the superhydrophobic coating, a sandpaper abrasion test was conducted. The superhydrophobic film sample was placed on 400# sandpaper, and under the weight of a 200 g weight, the sample was pushed a distance of 20 cm and repeated 3 times. The results are as Figure 6 shown, where a is the experimental setup diagram of the sandpaper abrasion test adopted; b is the CA value, SA value, and optical image of the water droplet; c is the SEM image of the coating after sandpaper abrasion.
[0074] The sandpaper abrasion test examined the interfacial bonding between the coating and the HPMC-based film. As shown in b and c of Figure 6 , after the sample 0%-AS without added AP glue was cyclically abraded with sandpaper, there were obvious damages on the surface of the starch granules, and the CA decreased (CA = 150.8 ± 0.8°); there were slight cracks on the surface of the 10%-AS coating, but the granules remained intact, the contact angle remained basically unchanged (CA = 152.3 ± 1.4°), and the rolling angle increased significantly (SA = 10°); the cracks on the surface of the 20%-AS coating were not obvious and the small particles of AP glue were denser, the starch granule morphology was intact and tightly wrapped, the contact angle decreased (CA = 150.3 ± 0.8°), and the SA increased slightly (SA = 8°); there were obvious damages on the rice starch granules on the 30%-AS coating, and the small particles of AP glue tended to fuse with the starch granules, the CA decreased to 149.5 ± 0.8°, and the SA increased significantly to 13°; there were slight cracks on the surface of the 40%-AS coating, and the starch granules and AP glue were almost completely fused, the CA decreased to 142.2 ± 0.9°, and the SA > 15°. Generally speaking, for the samples with added AP glue, the starch granules were intact and the coating had a lower degree of abrasion, indicating that AP has a protective effect on the micro-nano structure of the surface.
[0075] Based on the results of the cross-cut test and sandpaper abrasion test, it can be seen that AP can enhance the mechanical stability of the coating. In addition, when the AP addition amount is 20%, the adhesion between the coating and the base film is the strongest, and the coating still remains superhydrophobic. This is because during the high-temperature curing process of the coating, the hydroxyl groups contained in AP will undergo a condensation-polymerization reaction through dehydration of intermolecular and intramolecular hydroxyl groups, thereby increasing the interfacial adhesion; on the other hand, the molecular attraction between AP and the substrate or the shellac-starch surface is determined by the adhesion performance of AP.
[0076] 5) Anti-adhesion analysis of high-viscosity food droplets
[0077] Superhydrophobic materials are widely used in preventing droplet adhesion. The anti-adhesion test results of high-viscosity food droplets are as Figure 7 shown, where a is the support situation of each coating for different droplets; b is the residual rate of honey and yogurt on each coating; c is the residual situation of honey and yogurt on each coating; Note: Different letters indicate significant differences (p < 0.05).
[0078] As Figure 7 shown in a, droplets of different properties, including methylene blue-stained water, milk, Coca-Cola, and honey, were dropped onto the membrane. The methylene blue-stained water and cola could maintain a spherical shape on each coating, but milk slightly collapsed on the 10%-AS, 30%-AS, and 40%-AS samples, and honey showed an ellipsoidal shape on the 0%-AS, 30%-AS, and 40%-AS samples and had better support on the 10%-AS and 20%-AS samples. Generally, all samples showed good support for all droplets.
[0079] As Figure 7 shown in b and c, for yogurt droplets, the 0%-AS, 10%-AS, and 20%-AS all had good anti-adhesion properties, and the residue rate was almost 0%, but there was obvious yogurt residue on the 30%-AS and 40%-AS surfaces, and the yogurt residue rate of the 40%-AS sample reached about 8.6%. For honey droplets, all samples had good anti-adhesion properties. Among them, the residue rate of the 20%-AS sample could reach 0.2%, and the residue rate of the 40%-AS was the highest, only about 2.5%. The poor anti-adhesion property of the 40%-AS may be related to the weakening of its hydrophobicity, which is consistent with the CA / SA results.
[0080] Generally, the 20%-AS sample had the best support and anti-adhesion properties for different liquids, which was attributed to its superhydrophobicity and good mechanical and chemical stabilities, which was consistent with the previous test results.
[0081] 6) Oil absorption analysis
[0082] To study the oil absorption of the shellac-AP-RS superhydrophobic membrane, it was immersed in naturally cooled hot pot oil for 30 min and then taken out and dried. The results are as Figure 8 shown, where a is the oil absorption rate of different coatings; b is the oil absorption situation of different coatings.
[0083] As Figure 8 shown in a, the 0%-AS had the highest oil absorption rate, about 82.5%, which was significantly lower than that of the HS-30%-2h sample. This may be because the 30 s spraying time caused the starch to collapse and the cavitation structure to decrease. The addition of AP reduced the oil absorption rate. Among them, the 40%-AS sample had the lowest oil absorption rate of 33.9%. Perhaps because AP is hydrophilic and adsorbs water while absorbing oil, resulting in less oil absorption. However, the 20%-AS sample had a relatively high oil absorption rate, about 80.4%. This may be related to its good physical and chemical stabilities. The stable cavitation structure enabled it to absorb more oil compared to other samples with added AP. The above results were consistent with the SEM and stability test results.
[0084] Preparation method of sample HS-30%-2h: 3 g of rice starch (ball milled for 2 hours) + 1 g of shellac + 10 g of ethanol were sprayed on the HPMC film, the spraying time was 15 s, and the oil absorption rate was 205.7%.
[0085] 7) Regarding the ball milling time of starch:
[0086] Specific ball milling parameters: The starch was placed in four nylon grinding cylinders (500 mL) at a filling rate of 30% (v / v), and agate balls with different diameters (5, 10, and 15 mm) were used and rotated at a speed of 400 rpm for several hours, and the rotation direction was changed every 30 min.
[0087] Figure 9 Particle size distributions of rice starch (RS), corn starch (CS), and potato starch (PS) ball milled for 0, 2, 6, and 8 hours. Figure 10 SEM images of rice starch after ball milling for different times.
[0088] The longer the ball milling time, the more energy consumption is required. After ball milling for 2 hours, the particle size becomes smaller and shows a bimodal distribution, and the microscopic morphology becomes rougher, which has met the requirements. Therefore, the present invention selects the ball milling time of 2 hours.
[0089] As described above, although the present invention has been shown and described with reference to specific preferred embodiments, it should not be construed as a limitation of the present invention itself. Various changes can be made in its form and details without departing from the spirit and scope of the present invention.
Claims
1. A preparation method of an aluminum phosphate / shellac composite superhydrophobic coating, characterized in that, It includes the following steps: Step 1, preparing an aluminum phosphate adhesive: dispersing Al(OH)3 in an H3PO4 solution at 100 °C, stirring for a period of time, obtaining the aluminum phosphate adhesive, and cooling it to room temperature; Step 2, preparing a shellac-aluminum phosphate-starch solution; First, dissolving shellac in ethanol to prepare a shellac solution, then adding the aluminum phosphate adhesive to the shellac solution and stirring evenly, and then adding rice starch, and stirring at room temperature for a period of time to generate a shellac-aluminum phosphate-starch solution; Step 3, spraying the shellac-aluminum phosphate-starch solution onto a substrate film, and obtaining a substrate film with an aluminum phosphate / shellac composite superhydrophobic coating after thermal curing.
2. The preparation method according to claim 1, characterized in that, In Step 1: The stirring method is magnetic stirring for 3 h; the mass ratio of Al(OH)3 to the H3PO4 solution is 15.6:98; The mass fraction of the H3PO4 solution is 60%.
3. The preparation method according to claim 1, characterized in that, In the shellac-aluminum phosphate-starch solution prepared in Step 2: The mass ratio of the shellac to ethanol is 1:9; The mass of the aluminum phosphate adhesive is 10%-30% of the sum of the masses of the shellac and ethanol; The mass of the starch is 10%-30% of the sum of the masses of the shellac, aluminum phosphate adhesive, and ethanol.
4. The preparation method according to claim 1, characterized in that, In Step 2, the starch is one of rice, corn, potato, quinoa, and cassava, and it is the starch after ball milling for 2 hours.
5. The preparation method according to claim 1, wherein In Step 2, the stirring time at room temperature is 1 h.
6. The preparation method according to claim 1, characterized in that, Step 3 is specifically: using a spray gun with an inner diameter of 0.5 mm, applying a pressure of 276 Kpa, and spraying for 20 s - 30 s at a distance of 10 cm from the substrate film, and thermally curing the coating at 105 °C for 3 h to obtain a substrate film with an aluminum phosphate / shellac composite superhydrophobic coating.
7. The preparation method according to claim 1, wherein In Step 3, the substrate film is a hypromellose film.
8. An aluminum phosphate / shellac composite superhydrophobic coating prepared by the preparation method according to any one of claims 1 - 7.