Superhydrophilic easy-to-clean coatings, methods of making and coating methods

Abstract

The application discloses a super-hydrophilic easy-to-clean coating, characterized in that the coating comprises the following components: 8.5-11 parts of potassium hydroxyethyl sulfonate, gamma-aminopropyl triethoxysilane, perfluoroalkyl betaine, an alcohol solvent, water and a mixed solution; and the application further discloses a preparation method of the coating and a preparation method of a coating layer. In the application, a Si-O-Si silane network structure is formed between amphoteric silane molecules through a hydrolytic condensation reaction of siloxane groups, so that the coating layer forms a three-dimensional crosslinked network, and the mechanical strength and weather resistance of the coating layer are further enhanced.

Description

Technical Field

[0001] This invention relates to a coating, and more particularly to a coating for application on the surface of kitchen appliances. The invention also relates to methods for preparing the coating and methods for preparing the coating layer. Background Technology

[0002] In the fields of kitchen and bathroom appliances, home appliances, and industrial equipment surface protection, hydrophilic coatings are the core functional materials for achieving easy cleaning, anti-fogging, and oil resistance of substrates. Their long-lasting hydrophilicity, resistance to complex working conditions, and compatibility with multiple substrates directly determine the user experience and maintenance costs of the product. Currently, the main types of hydrophilic coatings available for range hoods are as follows:

[0003] The first type is the TiO2 photocatalytic superhydrophilic coating. Its technical principle is that ultraviolet light excites TiO2 to generate photogenerated holes, oxidizing and decomposing adsorbed organic pollutants on the surface. Simultaneously, the micro-nano rough structure formed by nano-TiO2 particles enhances water spreading through capillary action. The advantage of this type of coating is its photocatalytic ability to decompose organic dirt and its bactericidal effect; however, its disadvantages include heavy reliance on ultraviolet light. In indoor kitchen environments, the natural ultraviolet light content is extremely low, and its performance drops significantly or even fails under cloudy or nighttime conditions, failing to meet the actual usage requirements of range hoods. Furthermore, the surface temperature of the range hood panel can reach 60℃ to 80℃ during use, at which point organic pollutants easily form a difficult-to-remove coked layer on the coating surface.

[0004] The second type is silicate-based superhydrophilic coatings. Based on the sol-gel method, this involves forming a SiO2 nanoparticle coating containing a large number of silanol groups on the glass surface, utilizing the hydrogen bonding between the silanol groups and water molecules to achieve surface wetting. This type of coating is low-cost and chemically stable, but its hydrophilicity relies entirely on the capillary effect of physical adsorption. In the high-temperature, dry environment of a kitchen, adsorbed water easily desorbs, and the superhydrophilic properties are rapidly lost. Furthermore, the significant difference in thermal expansion coefficients between the pure inorganic coating and the glass substrate makes it prone to microcracks in alternating hot and cold environments, affecting adhesion durability. Traditional silicate coatings are prone to microcracks at high temperatures, and polymer coatings are prone to softening and swelling under the action of cleaning agents, neither of which can meet the performance stability requirements of range hood panels throughout their product lifecycle.

[0005] The third type is polymer-grafted superhydrophilic coatings. These coatings utilize the water-absorbing and swelling properties of polymer chains, such as polyacrylic acid and polyvinyl alcohol, grafted onto the substrate surface to maintain surface moisture. This method is applicable to various substrates, including glass, metal, and plastics. However, the grafting reaction typically requires complex pretreatment processes such as plasma treatment or ultraviolet pretreatment, making engineering implementation difficult. Furthermore, the polymer coatings have low hardness and poor scrub resistance, making them prone to damage and failure under friction in environments like kitchens where frequent contact with oil and cleaning agents is common.

[0006] The fourth category, zwitterionic superhydrophilic coatings, has attracted attention in recent years. Zwitterionic materials (such as sulfonate lactones and carboxylate lactones) contain both cationic and anionic groups in their molecular structure, exhibiting overall electroneutrality and extremely strong hydration capabilities. They can achieve stable and durable superhydrophilic properties without external energy activation. However, existing zwitterionic superhydrophilic technologies are mostly concentrated in organic polymer systems (such as polysulfonate lactones and polycarboxylate lactones), with complex synthesis processes and poor compatibility with silane systems, making them difficult to directly apply to large-area coating processes in building decoration and kitchen appliances.

[0007] In summary, the key technical problem that urgently needs to be solved in the field of kitchen appliance coating technology is how to easily synthesize amphoteric silane hydrophilic coating materials that are fully compatible with silane systems at room temperature, achieve stable and long-lasting superhydrophilic properties without any external energy activation, and have simple industrial coating process adaptability. Summary of the Invention

[0008] The first technical problem to be solved by the present invention is to provide a superhydrophilic and easy-to-clean coating that is easy to process and has strong adhesion to the substrate, in view of the above-mentioned technical status.

[0009] The second technical problem to be solved by the present invention is to provide a method for preparing a superhydrophilic and easy-to-clean coating that is easy to implement and has strong adhesion to the substrate, in view of the above-mentioned technical status.

[0010] The third technical problem to be solved by the present invention is to provide a method for preparing a superhydrophilic and easy-to-clean coating that is easy to implement and has strong adhesion to the substrate, in view of the above-mentioned technical status.

[0011] The technical solution adopted by the present invention to solve the first technical problem mentioned above is: a super-hydrophilic and easy-to-clean coating, characterized in that it comprises the following components and their weight ratios:

[0012] Potassium hydroxyethyl sulfonate 8.5-11 parts;

[0013] 8-12 parts of γ-aminopropyltriethoxysilane;

[0014] Perfluoroalkyl betaine, 0.1-0.5 parts;

[0015] 70-110 parts alcohol solvent;

[0016] 70-110 parts water;

[0017] Mixed solution 1200~2400 parts;

[0018] The aforementioned mixed solution is a mixture of 600-1200 parts water and 1200-600 parts anhydrous ethanol.

[0019] Preferably, the alcohol solvent is anhydrous ethanol or isopropanol.

[0020] The technical solution adopted by the present invention to solve the second technical problem mentioned above is: a method for preparing a super-hydrophilic and easy-to-clean coating, characterized by comprising the following steps:

[0021] Step 1: Pour the alcohol solvent into the reaction vessel and stir at a speed of 200 to 400 rpm;

[0022] Step 2: Add potassium hydroxyethyl sulfonate to the alcohol solvent while stirring, so that the potassium hydroxyethyl sulfonate is fully dissolved to obtain a potassium hydroxyethyl sulfonate alcohol solution;

[0023] Step 3: Under continuous stirring, add γ-aminopropyltriethoxysilane to a potassium hydroxyethyl sulfonate solution; after the reaction is complete, the system is a pale yellow transparent or slightly turbid liquid, which is the reaction solution.

[0024] Step 4: Filter the reaction solution to remove insoluble impurities, and obtain a pale yellow transparent amphoteric silane solution;

[0025] Step 5: While stirring, add water to the amphoteric silane solution, then adjust the pH to 2.2-3.5 using hydrochloric acid; stir until fully hydrolyzed, then add the mixed solution; stir thoroughly to obtain the hydrolysate.

[0026] Step 6: Add perfluoroalkyl betaine to the hydrolysate, stir well, and you will get a super-hydrophilic and easy-to-clean coating.

[0027] The technical solution adopted by the present invention to solve the third technical problem mentioned above is: a method for preparing a coating using a superhydrophilic and easy-to-clean coating, characterized by comprising the following steps:

[0028] Step 1: Wipe the substrate clean with anhydrous ethanol, then immerse it in pure water for ultrasonic cleaning;

[0029] Step 2: Rinse the ultrasonically treated glass with pure water and dry it.

[0030] Step 3: After drying, clean with a plasma agent;

[0031] Step 4: After cleaning with the plasma machine, the paint is sprayed onto the surface using a spray gun;

[0032] Step 5: Remove after drying.

[0033] Compared with the prior art, the advantages of the present invention are as follows:

[0034] First, this invention uses potassium hydroxyethyl sulfonate and KH-550 (γ-aminopropyltriethoxysilane) as starting materials to directly generate amphoteric silane products containing sulfonic acid groups and siloxane groups through a one-step condensation reaction at room temperature. This breaks through the limitation of traditional zwitterionic hydrophilic coatings requiring complex synthesis processes. The sulfonic acid groups are permanently embedded into the silane skeleton by covalent bonds, so that the superhydrophilic function exists stably in the coating system by chemical bonds, realizing one-step synthesis at room temperature.

[0035] Secondly, this invention uses the bridging effect of hydroxyethyl fragments to fix sulfonic acid groups on the silane network framework in a covalent manner, so that the sulfonic acid groups can exert strong hydrophilic function and will not be lost during use, thus realizing the construction of a permanent superhydrophilic surface.

[0036] Third, the room temperature synthesis, purification-free, and catalyst-free formulation characteristics of this invention make it directly compatible with existing kitchen appliance coating processes (such as dip coating, spray coating, and brush coating), without requiring equipment modification to the production line, and it has good prospects for industrial implementation.

[0037] The technical implementation system of the coating of this invention includes the following three core functional levels:

[0038] The first layer is a superhydrophilic surface layer with sulfonic acid groups: the sulfonic acid groups (-SO3-) at the ends of the amphoteric silane molecules are enriched on the outermost surface of the coating. These groups form a stable hydrogen bond network with water molecules in the air without requiring any external energy activation, reducing the water contact angle to below 5 degrees within seconds. This hydrophilic layer maintains its hydration structure even in a dry environment with a high temperature of 80°C and a relative humidity of 30%, ensuring the coating retains its superhydrophilic properties under various kitchen usage conditions.

[0039] The second layer is the silane covalent anchoring layer: During the curing process, the siloxane groups in the amphoteric silane molecules undergo hydrolysis and condensation reactions with the silanol groups on the surface of the glass substrate to form a strong Si-O-Si covalent network, which permanently anchors the sulfonic acid functional layer to the surface of the glass substrate, ensuring the performance stability of the coating throughout the product's life cycle.

[0040] The third layer is the silane network reinforcement layer: A three-dimensional Si-O-Si cross-linked network is formed between amphoteric silane molecules through the hydrolysis and condensation reaction of siloxane groups, which gives the coating good mechanical strength, scrub resistance and weather resistance, enabling the coating to withstand physical friction and chemical cleaning agent erosion during kitchen cleaning.

[0041] This invention creatively skips the complex pretreatment steps of traditional silane modification technology and directly utilizes the condensation reaction of potassium hydroxyethyl sulfonate with KH-550 at room temperature to generate amphoteric silane products containing sulfonic acid groups and siloxane groups in one step.

[0042] In the reaction pathway design, this invention chose potassium hydroxyethyl sulfonate instead of other sulfonates as the sulfonic acid group donor. The key consideration is that the primary alcohol hydroxyl group on the hydroxyethyl fragment (-CH2CH2OH) is active in transesterification / condensation reactions with the siloxane group of KH-550, forming a stable Si-OC covalent bond, thus fixing the sulfonic acid group to the silane backbone by chemical bonds. This design ensures that the sulfonic acid group is no longer a free small molecule, but is permanently embedded in the silane network through covalent bonds, avoiding the loss of sulfonic acid groups and the degradation of the coating's hydrophilicity during use.

[0043] During the coating drying and curing process (heating at 80℃ to 120℃ for 10 to 30 minutes), the siloxane groups undergo the following reaction process: First, the siloxane groups hydrolyze with trace amounts of water and alcohol (Si-OEt + H2O -> Si-OH + EtOH); subsequently, the generated Si-OH undergoes a condensation reaction with the silanol groups (HO-Glass) on the glass substrate surface (Si-OH + HO-Glass -> Si-O-Glass + H2O), establishing a strong Si-O-Si covalent bond between the coating and the substrate. The strength of this covalent bond is much higher than that of physical adsorption or electrostatic adsorption, enabling the coating to maintain stable superhydrophilic properties throughout its product lifespan (over 10 years). Furthermore, amphoteric silane molecules can also form a Si-O-Si silane network structure through the hydrolysis and condensation reaction of the siloxane groups, creating a three-dimensional cross-linked network in the coating, further enhancing its mechanical strength and weather resistance. Detailed Implementation

[0044] The present invention will be further described in detail below with reference to the embodiments.

[0045] Example 1

[0046] Step 1: Raw material pretreatment and dissolution. Pour 90 parts of anhydrous ethanol or isopropanol solvent into a reaction vessel (a three-necked flask or a beaker with a stirrer is acceptable), and start magnetic stirring or mechanical stirring, controlling the stirring speed at 200 to 400 rpm. Solvent 1 is obtained.

[0047] Step Two: Slowly add 9.25 parts of potassium hydroxyethyl sulfonate solid powder to solvent one, and stir continuously at room temperature for 30 to 60 minutes to ensure complete dissolution of the potassium hydroxyethyl sulfonate. This will yield solution two. (Since potassium hydroxyethyl sulfonate is more soluble in water than in alcohol, some turbidity may occur during the dissolution process; this is normal and does not affect subsequent reactions.)

[0048] Step 3: Under continuous stirring, slowly add 10 parts of KH-550 to the alcoholic solution of potassium hydroxyethyl sulfonate. The addition rate should be controlled to ensure that KH-550 is added completely within 30 to 60 minutes. After addition, continue stirring at room temperature for 60 to 120 minutes to ensure the condensation reaction between KH-550 and potassium hydroxyethyl sulfonate is complete. During the reaction, the -Si(OEt)3 group of KH-550 undergoes transesterification / condensation with the terminal -OH group of potassium hydroxyethyl sulfonate, while the -NH2 group of KH-550 is protonated in the alcoholic solvent (-NH3). + The ions form ion pairs with the sulfonic acid group (-SO3-), which together promote the stability of the product. After the reaction is complete, the system is a pale yellow transparent or slightly turbid liquid, yielding liquid trioxide.

[0049] Step 4: Filter liquid 3 through conventional filter paper or a sand core funnel to remove any insoluble impurities, resulting in a pale yellow, transparent amphoteric silane solution, which is liquid 4.

[0050] Step 5: While stirring at 200-400 rpm, slowly add 90 parts of water dropwise to Liquid 4, then adjust the pH to 3 using hydrochloric acid. After stirring for 12 hours to ensure complete hydrolysis, simultaneously add a mixed solution of 900 parts of water and 900 parts of anhydrous ethanol. Stir thoroughly to obtain Liquid 5.

[0051] Step 6: Add 0.5 parts of perfluoroalkyl betaine to liquid 5, stir well to obtain the coating.

[0052] Example 2

[0053] Step 1: Raw material pretreatment and dissolution. Pour 70 parts of anhydrous ethanol or isopropanol solvent into a reaction vessel (a three-necked flask or a beaker with a stirrer is acceptable), and start magnetic stirring or mechanical stirring, controlling the stirring speed at 200 to 400 rpm. Solvent 1 is obtained.

[0054] Step 2: Slowly add 8.5 parts of potassium hydroxyethyl sulfonate solid powder to solvent one, and stir continuously at room temperature for 30 to 60 minutes to ensure complete dissolution of the potassium hydroxyethyl sulfonate. This will yield solution two. (Since potassium hydroxyethyl sulfonate is more soluble in water than in alcohol, some turbidity may occur during the dissolution process. This is normal and does not affect subsequent reactions.)

[0055] Step 3: Under continuous stirring, slowly add 8 parts of KH-550 to the alcoholic solution of potassium hydroxyethyl sulfonate. The addition rate should be controlled to ensure that KH-550 is added completely within 30 to 60 minutes. After addition, continue stirring at room temperature for 60 to 120 minutes to ensure the condensation reaction between KH-550 and potassium hydroxyethyl sulfonate is complete. During the reaction, the -Si(OEt)3 group of KH-550 undergoes transesterification / condensation with the terminal -OH group of potassium hydroxyethyl sulfonate, while the -NH2 group of KH-550 is protonated in the alcoholic solvent (-NH3). + The ions form ion pairs with the sulfonic acid group (-SO3-), which together promote the stability of the product. After the reaction is complete, the system is a pale yellow transparent or slightly turbid liquid, yielding liquid trioxide.

[0056] Step 4: Filter liquid 3 through conventional filter paper or a sand core funnel to remove any insoluble impurities, resulting in a pale yellow, transparent amphoteric silane solution, which is liquid 4.

[0057] Step 5: While stirring at 200-400 rpm, slowly add 110 parts of water dropwise to Liquid 4, then adjust the pH to 3 using hydrochloric acid. After stirring for 12 hours to ensure complete hydrolysis, simultaneously add a mixed solution of 600 parts of water and 1200 parts of anhydrous ethanol. Stir thoroughly to obtain Liquid 5.

[0058] Step 6: Add 0.5 parts of perfluoroalkyl betaine to liquid 5, stir well to obtain the coating.

[0059] Example 3:

[0060] Step 1: Raw material pretreatment and dissolution. Pour 110 parts of anhydrous ethanol or isopropanol solvent into a reaction vessel (a three-necked flask or a beaker with a stirrer is acceptable), and start magnetic stirring or mechanical stirring, controlling the stirring speed at 200 to 400 rpm. Solvent 1 is obtained.

[0061] Step 2: Slowly add 11 parts of potassium hydroxyethyl sulfonate solid powder to solvent one, and stir continuously at room temperature for 30 to 60 minutes to ensure complete dissolution of the potassium hydroxyethyl sulfonate. This will yield solution two. (Since potassium hydroxyethyl sulfonate is more soluble in water than in alcohol, some turbidity may occur during the dissolution process; this is normal and does not affect subsequent reactions.)

[0062] Step 3: Under continuous stirring, slowly add 12 parts of KH-550 to the alcoholic solution of potassium hydroxyethyl sulfonate. The addition rate should be controlled to ensure that KH-550 is added completely within 30 to 60 minutes. After addition, continue stirring at room temperature for 60 to 120 minutes to ensure the condensation reaction between KH-550 and potassium hydroxyethyl sulfonate is complete. During the reaction, the -Si(OEt)3 group of KH-550 undergoes transesterification / condensation with the terminal -OH group of potassium hydroxyethyl sulfonate, while the -NH2 group of KH-550 is protonated in the alcoholic solvent (-NH3). + The ions form ion pairs with the sulfonic acid group (-SO3-), which together promote the stability of the product. After the reaction is complete, the system is a pale yellow transparent or slightly turbid liquid, yielding liquid trioxide.

[0063] Step 4: Filter liquid 3 through conventional filter paper or a sand core funnel to remove any insoluble impurities, resulting in a pale yellow, transparent amphoteric silane solution, which is liquid 4.

[0064] Step 5: While stirring at 200-400 rpm, slowly add 70 parts of water dropwise to Liquid 4, then adjust the pH to 3 using hydrochloric acid. After stirring for 12 hours to ensure complete hydrolysis, simultaneously add a mixed solution of 1200 parts of water and 600 parts of anhydrous ethanol. Stir thoroughly to obtain Liquid 5.

[0065] Step 6: Add 0.5 parts of perfluoroalkyl betaine to liquid 5, stir well to obtain the coating.

[0066] Comparative Example 1:

[0067] Step 1: Raw material pretreatment and dissolution. Pour 90 parts of anhydrous ethanol or isopropanol solvent into a reaction vessel (a three-necked flask or a beaker with a stirrer is acceptable), and start magnetic stirring or mechanical stirring, controlling the stirring speed at 200 to 400 rpm. Solvent 1 is obtained.

[0068] Step Two: Under continuous stirring, slowly add 10 parts of KH-550 to Solvent One. The adding rate should be controlled to ensure the addition of KH-550 is completed within 30 to 60 minutes. After addition, continue stirring at room temperature for 60 to 120 minutes to ensure the condensation reaction between KH-550 and potassium hydroxyethyl sulfonate is complete. During the reaction, the -Si(OEt)3 group of KH-550 undergoes transesterification / condensation with the terminal -OH group of potassium hydroxyethyl sulfonate, while the -NH2 group of KH-550 is protonated in the alcohol solvent (-NH3). +The ions form ion pairs with the sulfonic acid group (-SO3-), which together promote the stability of the product. After the reaction is complete, the system is a pale yellow transparent or slightly turbid liquid, yielding liquid trioxide.

[0069] Step 3: Filter liquid 3 through conventional filter paper or a sand core funnel to remove any insoluble impurities, resulting in a pale yellow, transparent amphoteric silane solution, which is liquid 4.

[0070] Step 4: While stirring at 200-400 rpm, slowly add 90 parts of water dropwise to Liquid 4, then adjust the pH to 3 using hydrochloric acid. After stirring for 12 hours to ensure complete hydrolysis, simultaneously add a mixed solution of 900 parts of water and 900 parts of anhydrous ethanol. Stir thoroughly to obtain Liquid 5.

[0071] Step 5: Add 0.5 parts of perfluoroalkyl betaine to liquid 5, stir well to obtain the coating.

[0072]

[0073] Test section:

[0074] The coating is sprayed onto the glass surface, as follows:

[0075] Step 1: Wipe the glass clean with anhydrous ethanol, then immerse it in pure water for ultrasonic cleaning for 10 minutes.

[0076] Step 2: Rinse the ultrasonically treated glass with pure water and dry it in a clean oven at 80°C for 20 minutes.

[0077] Step 3: After drying, clean with a plasma agent.

[0078] Step 4: After cleaning with the plasma machine, manually spray the coating onto the surface using an air spray gun.

[0079] Step 5: Place in an oven at 110℃ for 25 minutes to dry, then remove. Test Method

[0080] The testing method is as follows:

[0081] The initial hydrophilic angle was determined using a Shengding SC-200 contact angle tester. 2 μL was added each time, and the water contact angle was measured 5 seconds after each drop.

[0082] For the 10,000-cycle abrasion-resistant hydrophilic angle test, a diluted White Cat dish soap (volume ratio of dish soap:water = 1:2) was applied to the rough side (green side) of a Miaojie scouring pad. A force of 1 kg was then applied above a 70mm (length) × 30mm (width) grinding head to wipe the glass surface 10,000 times (the applied force can be adjusted according to the different sizes of the grinding head to ensure consistent pressure). After the experiment, the test area was wiped clean with alcohol (purity not less than 90%) (it is necessary to wait for the alcohol to completely evaporate) to ensure that the glass surface is dry and clean, and then the contact angle test was performed.

[0083] 140℃ for 5 hours: Place the product in an oven at 140℃ for 5 hours, remove it and cool it to room temperature, rinse it once with pure water, dry it, and then test the contact angle.

[0084] After cleaning 200 times, apply 2ml of rapeseed oil evenly to the 50*50mm glass surface, leave for 1 minute, then rinse with tap water. Repeat this process 200 times, then clean with Mr. Muscle once.

[0085] Test results:

[0086]

Claims

1. A super-hydrophilic and easy-to-clean coating, characterized in that... It includes the following components and their weight ratios: Potassium hydroxyethyl sulfonate 8.5-11 parts; 8-12 parts of γ-aminopropyltriethoxysilane; Perfluoroalkyl betaine, 0.1-0.5 parts; 70-110 parts alcohol solvent; 70-110 parts water; Mixed solution 1200~2400 parts; The aforementioned mixed solution is a mixture of 600-1200 parts water and 1200-600 parts anhydrous ethanol.

2. The super-hydrophilic and easy-to-clean coating according to claim 1, characterized in that... The alcohol solvent is anhydrous ethanol or isopropanol.

3. A method for preparing the superhydrophilic and easy-to-clean coating according to claim 1 or 2, characterized in that... Includes the following steps: Step 1: Pour the alcohol solvent into the reaction vessel and stir at a speed of 200 to 400 rpm; Step 2: Add potassium hydroxyethyl sulfonate to the alcohol solvent while stirring, so that the potassium hydroxyethyl sulfonate is fully dissolved to obtain a potassium hydroxyethyl sulfonate alcohol solution; Step 3: Under continuous stirring, add γ-aminopropyltriethoxysilane to a potassium hydroxyethyl sulfonate solution; after the reaction is complete, the system is a pale yellow transparent or slightly turbid liquid, which is the reaction solution. Step 4: Filter the reaction solution to remove insoluble impurities, and obtain a pale yellow transparent amphoteric silane solution; Step 5: While stirring, add water to the amphoteric silane solution, then adjust the pH to 2.2-3.5 using hydrochloric acid; stir until fully hydrolyzed, then add the mixed solution; stir thoroughly to obtain the hydrolysate. Step 6: Add perfluoroalkyl betaine to the hydrolysate, stir well, and you will get a super-hydrophilic and easy-to-clean coating.

4. A method for preparing a coating using the superhydrophilic and easy-to-clean coating according to claim 1 or 2, characterized in that... Includes the following steps: Step 1: Wipe the substrate clean with anhydrous ethanol, then immerse it in pure water for ultrasonic cleaning; Step 2: Rinse the ultrasonically treated glass with pure water and dry it. Step 3: After drying, clean with a plasma agent; Step 4: After cleaning with the plasma machine, the paint is sprayed onto the surface using a spray gun; Step 5: Remove after drying.

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