A Hydrophilic Polydopamine / Ionic Liquid Coating Resistant to Acids and Bases and Its Preparation Method

Abstract

The present invention relates to a hydrophilic polydopamine / ionic liquid coating resistant to acids and alkalis, which is prepared by depositing on the surface of a substrate material through the Michael addition reaction or Schiff base reaction of polydopamine (PDA) and ionic liquid (IL). The PDA / IL coating has a certain universality and can be deposited on the surfaces of different polymers or metal sheets. The PDA / IL coating can improve the hydrophilicity of the substrate material surface, and while enhancing the acid and alkali resistance of the coating, it also enhances the antibacterial and antifouling capabilities of the coating. The preparation method of the present invention has a simple process, is easy to operate, has mild conditions and strong universality.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation and application of coatings, and particularly relates to a hydrophilic polydopamine / ionic liquid coating resistant to acids and alkalis and a preparation method thereof. Background Art

[0002] Pollutants come from the environment, such as macromolecules, microorganisms or suspended particles, which can adhere to the surface reversibly or irreversibly. This process can easily cause serious problems in industrial applications, the marine and medical fields. Therefore, anti-pollution performance is one of the most concerned issues when preparing coatings. Generally speaking, the rejection of pollutants is based on three effects: the rejection effect of the hydration layer, the electrostatic rejection effect, and the steric rejection effect.

[0003] Antibacterial is one type of anti-pollution. Bacterial contamination can bring many hazards. In food processing and water treatment, bacterial contamination is a major challenge to maintain safety, quality and security. In the marine industry, bacterial enhanced corrosion and biofouling have serious adverse effects on ships and other equipment, which can lead to shortened service life and increased maintenance costs. Especially in biomedical applications, the attachment and settlement of pathogenic bacteria on the surface of implanted devices may cause serious infections or even death of the recipient. Therefore, the development of coatings with antibacterial properties has become a major research hotspot. Currently, the two major categories of antibacterial coatings that have been developed and are being studied are antibacterial coatings and bactericidal coatings.

[0004] The mussel-inspired co-deposition method is a newly emerging type of surface modification method in recent years. Dopamine (DA) is a catechol derivative with both a catechol structure and a primary amine functional group. It can undergo oxidative self-polymerization in a weak alkaline environment and can form a polydopamine (PDA) coating on the surface of different substrates, which has strong adhesion and hydrophilicity. However, the polydopamine coating has the disadvantages of being uneven, unstable and easy to fall off.

[0005] Patent CN202210386191.4 discloses that polyethyleneimine (PEI) is grafted onto the surface of a reverse osmosis membrane by using the active groups on the polydopamine coating to obtain a stable and durable PDA / PEI modified reverse osmosis membrane. The PDA / PEI modification increases the hydrophilicity of the membrane surface, improves the anti-pollution ability of the reverse osmosis membrane, and endows it with certain antibacterial ability. However, the antibacterial property of the PDA / PEI coating only plays an antibacterial role by the electrostatic action of a large number of quaternary ammonium cations on the PEI, and the antibacterial rate can only reach about 40%. Its acid-base stability is poor, and the stability is limited to the acid-base environment of pH = 1 and pH = 13.

[0006] The poor acid-base stability of the PDA coating is based on three reasons: First, PDA is prone to agglomeration, and there are relatively large particles in the PDA coating. The intermolecular forces between dopamine / PDA molecules are mainly non-covalent forces, which are easily destroyed by polar solutions such as strong acids and strong bases. Second, in strong acid solutions, the amino groups on polydopamine will be protonated, and each structural unit will carry a positive charge. In strong base solutions, the hydroxyl groups on polydopamine will be deprotonated, and each structural unit will carry two negative charges. Therefore, there is electrostatic repulsion in polydopamine in strong acid and strong base solutions. When the electrostatic repulsion is strong enough to overcome the non-covalent forces that bind the polymer chains together, the PDA coating will detach from the substrate surface. Third, there are a large number of uncyclized protonated amino groups in the PDA coating in a mild alkaline environment, which interact with the hydrophobic substrate surface mainly through cation-π interactions. Increasing the pH value of the environment will cause PDA to be deprotonated and weaken the cation-π interaction force. Therefore, the PDA coating has poor stability in alkaline solutions.

[0007] Ionic liquids (ILs) are mainly composed of large organic cations and inorganic or organic anions, and have relatively low melting points, usually below 100 °C. According to the appropriate combination of cations and anions, ionic liquids with specific properties can be designed and applied in a wide range of fields. In recent years, the research on the toxicity and antibacterial properties of ionic liquids has become an emerging research field. Imidazole-based and quaternary ammonium salt-based ionic liquids have excellent antibacterial properties, which are caused by the electrostatic interaction between the cations in the ionic liquids and the anions in the bacterial cell membrane, as well as the hydrophilic and hydrophobic interactions between the ionic liquids. Summary of the Invention

[0008] The first object of the present invention is to provide a preparation method of an acid- and alkali-resistant hydrophilic polydopamine / ionic liquid coating (PDA / IL coating) in view of the deficiencies of the prior art, and the following technical solutions are adopted:

[0009] Step (1): Weigh tris(hydroxymethyl)aminomethane and concentrated hydrochloric acid at room temperature, mix them evenly, and then dilute with deionized water to pH = 8.5 to obtain a Tris-HCl buffer solution;

[0010] Step (2): Clean and remove impurities from the substrate with an organic solution, and then dry to remove the organic solution; the organic solution is one of ethanol and acetone; the substrate is one of a polymer, a metal, or glass; the polymer uses a porous polymer membrane or a dense polymer sheet; the membrane morphology of the polymer membrane includes a flat membrane and a hollow fiber membrane, which can be a microfiltration membrane or an ultrafiltration membrane;

[0011] Step (3): Place dopamine and a polyamino ionic liquid in a reaction vessel, and dropwise add the Tris-HCl buffer solution prepared in step (1) to completely dissolve dopamine to obtain a deposition solution;

[0012] Preferably, the polyamino ionic liquid is one of 1-aminoethyl-3-methylimidazolium glycinate, 1-aminopropyl-3-methylimidazolium glycinate, diaminomethylimidazolium bis(trifluoromethanesulfonyl)imide, diaminopropylimidazolium bis(trifluoromethanesulfonyl)imide, and 1,1,1-trimethylhydrazinium glycinate;

[0013] Step (4): At room temperature, the substrate treated in step (2) and the deposition solution in step (3) are fully oscillated to obtain a modified material;

[0014] Preferably, if the substrate is a polymer film or polymer sheet, the substrate is placed to float on the liquid surface of the deposition solution; if the substrate is metal or glass, the substrate is placed in the deposition solution;

[0015] Step (5): The modified material in step (4) is rinsed with deionized water more than 3 times to remove the residual solution on the material surface, then the modified material is immersed in deionized water overnight, and then dried under vacuum at room temperature to obtain a hydrophilic PDA / IL coating.

[0016] Preferably, in step (2), the drying time of the polymer film and polymer sheet is 3 hours, and the drying time of the glass is 6 - 8 hours.

[0017] Preferably, in step (3), the mass ratio of dopamine to the polyamino ionic liquid is 1:0.25 - 4.

[0018] Preferably, in step (4), the room temperature is 25°C.

[0019] Preferably, when the substrate is a polymer film or polymer sheet, the oscillation time is 1 - 4 hours; when the substrate is metal or glass, the oscillation time is 10 - 12 hours.

[0020] Preferably, in step (5), the drying time is 8 - 10 hours.

[0021] The second object of the present invention is to provide an acid- and alkali-resistant hydrophilic polydopamine / ionic liquid coating (PDA / IL coating), which is prepared by the above method, and the PDA / IL coating has good acid and alkali resistance and antibacterial and antifouling capabilities.

[0022] The technical solution of the present invention has the following advantages:

[0023] 1. The present invention provides a mussel - inspired PDA / IL coating with acid - alkali resistance, antibacterial and antifouling properties. The dopamine coating is used as a medium layer to introduce other functional components. The mussel - inspired coating with polyamino ionic liquid as a comonomer can make up for the deficiencies of the polydopamine coating, and the preparation operation is simple. Dopamine can undergo oxidative self - polymerization on the surface of the substrate material in an environment with pH = 8.5 and adhere to the surface of the substrate material through non - covalent forces. The carbonyl groups on polydopamine will undergo Schiff base reactions or Michael reactions with multiple amino groups on the ionic liquid. Compared with the PDA coating, the PDA / IL coating forms at a faster rate and has a higher hydrophilic modification efficiency. Due to the introduction of a large number of hydroxyl and amino groups on the substrate surface, the hydrophilic property of the substrate surface is significantly improved, and the water contact angle can reach about 30° after 4 hours of deposition modification.

[0024] 2. The PDA / IL coating improves the hydrophilicity of the substrate surface. A hydration layer will form on the hydrophilic surface, serving as a physical barrier for fouling organisms to adhere to the substrate surface. Fouling organisms require a large amount of energy to break through the hydration layer. There is an electrostatic repulsion between the negatively charged PDA / IL coating and negatively charged biological cells. In addition, the imidazole and quaternary ammonium cations on the ionic liquid have certain antibacterial properties. Taking the ionic liquid as a functional component, it has a higher antibacterial rate compared with polyethyleneimine. The PDA / IL coating can effectively inhibit biological fouling during long - term use. The preparation of the PDA / IL coating is not only simple in operation and mild in conditions, but also the hydrophilic modification degree of the substrate surface and the antibacterial property of the coating can be regulated by controlling the deposition time and the mass ratio of dopamine to ionic liquid.

[0025] 3. The polyamino ionic liquid will undergo Michael addition reactions or Schiff base reactions with polydopamine. Chemical covalent bonds replace the non - covalent forces of polydopamine, and the presence of quaternary ammonium cations can restore cation - π interactions. Therefore, the PDA / IL coating has good acid - alkali resistance, which is beneficial to increasing the stability of the coating and prolonging the service life of the coating.

[0026] 4. By introducing polyamino ionic liquid, the present invention can change the charge property of the coating, and the charge property and charge density of the coating can be regulated by adjusting the concentration of the ionic liquid in the deposition solution.

[0027] 5. The preparation method of the present invention has a simple process, is easy to operate, has mild conditions and strong universality. Description of the Drawings

[0028] Figure 1 It is a water contact angle diagram of the PDA / IL coating deposited on the surface of a polyvinylidene fluoride microfiltration membrane for 1 hour;

[0029] Figure 2Comparison diagram of water contact angles of PDA coating, PDA / PEI coating, and PDA / IL coating deposited on the surface of polyvinylidene fluoride microfiltration membrane for different times;

[0030] Figure 3 Comparison diagram of water contact angles of PDA / IL coating prepared with different mass ratios of dopamine and ionic liquid deposited on the surface of polyvinylidene fluoride microfiltration membrane for 4 hours;

[0031] Figure 4 Comparison diagram of water contact angles of PDA coating, PDA / PEI coating, and PDA / IL coating deposited on different polymer substrates for 4 hours. Among them, a is the surface of polyvinylidene fluoride (PVDF) microfiltration membrane (MM), b is the surface of polypropylene (PP) microfiltration membrane (MM), c is the surface of polytetrafluoroethylene (PTFE) microfiltration membrane (MM), and d is the surface of polyvinylidene fluoride (PVDF) sheet;

[0032] Figure 5 Comparison diagram of water contact angles of PDA coating, PDA / PEI coating, and PDA / IL coating deposited on different metals and glass for 4 hours. Among them, a is the surface of aluminum sheet, b is the surface of copper sheet, and c is the surface of glass;

[0033] Figure 6 XPS diagram of PDA / IL coating;

[0034] Figure 7 Comparison diagram of acid resistance of PDA coating, PDA / PEI coating, and PDA / IL coating;

[0035] Figure 8 Comparison diagram of alkali resistance of PDA coating, PDA / PEI coating, and PDA / IL coating;

[0036] Figure 9 Comparison diagram of antibacterial rates of PDA coating, PDA / PEI coating, and PDA / IL coating against Escherichia coli;

[0037] Figure 10 Comparison diagram of water contact angles of PDA / IL coating prepared with different ionic liquids deposited on the surface of polyvinylidene fluoride microfiltration membrane for 4 hours. Detailed implementation mode

[0038] As mentioned above, in view of the deficiencies of the prior art, the inventors of this case have proposed the technical solution of the present invention through long-term research and a large number of practices. The main bases at least include:

[0039] In the process of forming a hydrophilic coating on the surface of a substrate material through the co-deposition of polydopamine and ionic liquid, the adhesion of PDA is utilized, and an ionic liquid with an amino group is used as a functional component. PDA and the ionic liquid form a multifunctional coating on the surface of the substrate material through Michael addition reaction or Schiff base reaction. The water contact angle of the PDA / IL coating can reach about 30°. The above-mentioned PDA / IL coating has acid and alkali resistance, antibacterial and antifouling properties, and has broad application prospects in the fields of marine antifouling coatings, medical antibacterial materials, etc. The raw materials selected in the present invention are conventional, easy to obtain, and low in cost, and the provided preparation method has a simple process.

[0040] The present invention provides a method for preparing an acid- and alkali-resistant hydrophilic PDA / IL coating, and the specific implementation scheme is as follows:

[0041] Step (1): At room temperature, weigh 6.057 g of tris(hydroxymethyl)aminomethane, mix it with 15 mL of concentrated hydrochloric acid, and then dilute it with deionized water to 1000 mL to obtain a Tris-HCl buffer solution with pH = 8.5.

[0042] Step (2): When the substrate material is a polymer film and a polymer sheet, rinse the polymer film and polymer sheet with ethanol to clean the impurities on the surface of the material, and place the wet polymer film and polymer sheet in a vacuum drying oven to dry at room temperature for 3 hours; when the substrate material is a metal sheet, soak the metal sheet in acetone overnight to remove the impurities on the surface of the metal sheet, blot the residual acetone on the surface of the metal sheet with filter paper, and dry the metal sheet with nitrogen; when the substrate material is glass, soak the glass in acetone overnight to remove the impurities on the surface of the glass, blot the residual acetone on the surface of the glass with filter paper, and place it in a vacuum oven to heat and dry at 60 °C for 6 - 8 hours.

[0043] Step (3): Weigh dopamine and polyamino ionic liquid with a mass ratio of 1:0.25 - 4, place them in a petri dish, measure 5 mL of the Tris-HCl buffer solution prepared in step (1), and drop it into the petri dish to completely dissolve the dopamine and polyamino ionic liquid to obtain a deposition solution.

[0044] Step (4): Place the substrate material cleaned and dried in step (2) on the liquid surface or in the deposition solution in the petri dish in step (3), and then place the petri dish in an oscillator and fully oscillate at room temperature of 25 °C; when the substrate material is a polymer film and a polymer sheet, the oscillation time is 1 - 4 hours, and when the substrate material is a metal sheet and glass, the oscillation time is 10 - 12 hours.

[0045] Step (5): Take out the modified material from the culture dish, rinse the residual solution on the surface of the material with deionized water more than three times, and then immerse the modified material in deionized water overnight; after the modified material is cleaned, place the material in a vacuum drying oven and dry it at room temperature for 8-10 hours to obtain the coating.

[0046] The technical solutions of the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention. In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0047] In the following embodiments, the specific detection method of the contact angle is as follows.

[0048] Contact angle test:

[0049] Drop a liquid droplet on a solid horizontal plane, and measure the angle formed by the two tangents of the gas-liquid interface and the solid-liquid interface to characterize the wettability of the liquid to the solid. First, adjust the position of the capillary needle and the platform so that the capillary needle appears at 1 / 3 of the lens. Fix the 2 cm * 2 cm coating surface upward on a glass slide. Use the capillary needle to drop 3.0 μL of water droplets on the coating surface. After waiting for the contact angle to stabilize, use the camera in the instrument to take a picture of the liquid droplet contact curve, and then measure the contact angle. Take the average value of three or more measurements as the water contact angle value of the coating surface.

[0050] Example 1

[0051] (1) At room temperature, weigh 6.057 g of tris(hydroxymethyl)aminomethane, mix it with 15 mL of concentrated hydrochloric acid, and dilute it to 1000 mL with deionized water to obtain a Tris-HCl buffer solution with pH = 8.5.

[0052] (2) Use a polyvinylidene fluoride (PVDF) microfiltration membrane as the substrate membrane. Pre-wet the polyvinylidene fluoride microfiltration membrane with ethanol to remove the air in the pores of the polymer membrane and clean the impurities on the membrane surface. Place the wet polymer membrane in a vacuum drying oven and dry it at room temperature for 3-4 hours.

[0053] (3) Weigh 0.01 g of dopamine and 0.01 g of 1-aminopropyl-3-methylimidazolium glycinate, place them in a culture dish, measure 5 mL of Tris-HCl buffer solution, and drop it into the culture dish to completely dissolve dopamine and the ionic liquid, that is, obtain the deposition solution.

[0054] (4) Float the polymer membrane on the surface of the deposition solution in the petri dish of step (3), and then place the petri dish in an oscillator and shake it vigorously at room temperature for 1 hour.

[0055] (5) Take out the deposited membrane from the petri dish, rinse the residual solution on the surface of the membrane with deionized water more than three times, and then immerse the deposited membrane in deionized water overnight.

[0056] (6) After the deposited membrane is cleaned, place the deposited membrane in a vacuum drying oven and dry it at room temperature for 8 - 10 hours to obtain the modified membrane.

[0057] The water contact angle diagram of the coating surface prepared in Example 1 is as shown in the appendix Figure 1 shown, and the XPS diagram is as shown in the appendix Figure 6 shown.

[0058] Examples 2 - 4

[0059] Change the oscillation time to 2 hours, 3 hours, and 4 hours respectively, and keep other conditions the same as in Example 1 to prepare different PDA / IL coatings.

[0060] The water contact angle results of the PDA / IL coatings prepared in the above examples deposited on the surface of the polyvinylidene fluoride microfiltration membrane for different times are referred to in the appendix Figure 2 As shown. The PDA / IL coating covers evenly on the surface of the PVDF microfiltration membrane. As the oscillation time prolongs, the water contact angle gradually decreases, the hydrophilic modification degree increases, and compared with the PDA coating and the PDA / PEI coating, the PDA / IL coating has better hydrophilic performance.

[0061] Example 5:

[0062] The mass of dopamine is 0.01 g, and the mass of the ionic liquid is 0.005 g, that is, the mass ratio of the two is 1:0.5; other conditions are the same as in Example 1.

[0063] Example 6:

[0064] The mass of dopamine is 0.01 g, and the mass of the ionic liquid is 0.02 g, that is, the mass ratio of the two is 1:2; other conditions are the same as in Example 1.

[0065] Example 7:

[0066] The mass of dopamine is 0.02 g, and the mass of the ionic liquid is 0.01 g, that is, the mass ratio of the two is 2:1; other conditions are the same as in Example 1.

[0067] Change the mass ratio of dopamine and ionic liquid to 1:0.5, 1:2, and 2:1 respectively, and keep other conditions of the above examples the same as in Example 1. The results are referred to in the appendix Figure 3 As shown. When the mass ratio of dopamine and ionic liquid is 1:1, the hydrophilic modification effect on the PVDF microfiltration membrane is the best.

[0068] Examples 8 - 10

[0069] The types of substrate materials were respectively changed to polypropylene (PP) microfiltration membrane (MM), polytetrafluoroethylene (PTFE) microfiltration membrane (MM), and polyvinylidene fluoride (PVDF) sheet. Other conditions were the same as in Example 1, and different PDA / IL coatings were prepared.

[0070] The water contact angle results of PDA coating, PDA / PEI coating, and PDA / IL coating deposited on the surfaces of different polymer substrates for 4 hours are referred to in the appendix Figure 4 , The PDA / IL coating can be evenly deposited on the surfaces of different polymer substrate materials, improving the hydrophilic properties of the substrates. And compared with the PDA coating and PDA / PEI coating, the PDA / IL coating has the best hydrophilicity.

[0071] Examples 11 - 13

[0072] The types of substrate materials were respectively changed to aluminum (Al) sheet, copper (Cu) sheet, and glass. Ethanol in step (2) was replaced with acetone, and the oscillation time was changed to 10 hours. Other conditions were the same as in Example 1, and different PDA / IL coatings were prepared.

[0073] The water contact angle results of PDA coating, PDA / PEI coating, and PDA / IL coating deposited on the surfaces of different metal sheets and glass substrates for 4 hours are referred to in the appendix Figure 5 , The PDA / IL coating can be evenly deposited on the surfaces of different metal and glass materials, improving the hydrophilic properties of the substrates. And compared with the PDA coating and PDA / PEI coating, the PDA / IL coating has the best hydrophilicity.

[0074] Examples 14 - 17

[0075] The polyamino ionic liquids were respectively changed to 1 - aminoethyl - 3 - methylimidazolium glycinate, diaminoethylimidazolium bis(trifluoromethanesulfonyl)imide, diaminopropylimidazolium bis(trifluoromethanesulfonyl)imide, and 1,1,1 - trimethylhydrazine glycinate. Preparation was carried out under the same conditions as in Example 1, and different PDA / IL coatings were prepared.

[0076] Contact angle tests were carried out on the PDA / IL coatings prepared in Examples 14 - 17. The results are referred to in the appendix Figure 10 , Using different ionic liquids as comonomers to prepare different coatings has an obvious hydrophilic modification effect on PVDF microfiltration membranes.

[0077] Comparative Example

[0078] Acid resistance, alkali resistance, and antibacterial property tests were respectively carried out on the PDA coating, PDA / PEI coating, and PDA / IL coating prepared in Example 1. The results are respectively asFigure 7 , 8 As shown in Figure 9. It can be seen from the figure that the PDA / IL coating is least affected by acidic and alkaline environments, and its antibacterial property is significantly improved compared with the PDA coating and the PDA / PEI coating.

[0079] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A hydrophilic polydopamine / ionic liquid coating resistant to acids and alkalis, characterized in that, It is prepared by the following method: Step (1): Weigh tris(hydroxymethyl)aminomethane and concentrated hydrochloric acid at room temperature, mix them evenly, and dilute with deionized water to pH = 8.5 to obtain a Tris-HCl buffer solution; Step (2): Clean and remove impurities from the substrate material with an organic solution, and then dry to remove the organic solution; the organic solution is one of ethanol and acetone; the substrate material is one of a polymer, a metal, or glass; the polymer uses a porous polymer membrane or a dense polymer sheet; Step (3): Mix dopamine and a polyamino ionic liquid with a mass ratio of 1:0.25 - 4, and dropwise add the Tris-HCl buffer solution prepared in step (1). Wait until the dopamine is completely dissolved to obtain a deposition solution; the polyamino ionic liquid is one of 1-aminoethyl-3-methylimidazolium glycinate, 1-aminopropyl-3-methylimidazolium glycinate, diaminethylimidazolium bis(trifluoromethanesulfonyl)imide, diaminopropylimidazolium bis(trifluoromethanesulfonyl)imide, and 1,1,1-trimethylhydrazinium glycinate; Step (4): At room temperature, fully oscillate the substrate treated in step (2) with the deposition solution in step (3) to obtain a modified material; if the substrate is a polymer, place the substrate on the liquid surface of the deposition solution and float and oscillate for 1 - 4 hours; if the substrate is a metal or glass, place the substrate in the deposition solution and oscillate for 10 - 12 hours; Step (5): Rinse the surface of the modified material obtained in step (4) with deionized water more than 3 times, then immerse the modified material in deionized water overnight, and then dry it under vacuum at room temperature to obtain a hydrophilic polydopamine / ionic liquid coating.

2. The acid- and alkali-resistant hydrophilic polydopamine / ionic liquid coating according to claim 1, wherein The porous polymer membrane in step (2) includes a microfiltration membrane and an ultrafiltration membrane, and the membrane morphology of the porous polymer membrane includes a flat membrane and a hollow fiber membrane.

3. The acid- and alkali-resistant hydrophilic polydopamine / ionic liquid coating according to claim 1, wherein In step (2), if the substrate is a polymer, the drying time is 3 hours; if the substrate is glass, the drying time is 6 - 8 hours.

4. The acid- and alkali-resistant hydrophilic polydopamine / ionic liquid coating according to claim 1, wherein In step (4), the room temperature is 25°C.

5. The acid- and alkali-resistant hydrophilic polydopamine / ionic liquid coating according to claim 1, wherein In step (5), the drying time is 8 - 10 hours.

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