A nanocellulose-based superhydrophobic coating and its preparation method

Abstract

This invention discloses a nanocellulose-based superhydrophobic coating and its preparation method, belonging to the field of superhydrophobic coating technology. The preparation method of the superhydrophobic coating of this invention includes the following steps: adding a precipitant to a nanocellulose aqueous suspension, mixing evenly, and centrifuging to obtain a nanocellulose precipitate with residual precipitant; dispersing the nanocellulose precipitate with residual precipitant in a displacement solvent, centrifuging, and repeating the dispersion-centrifugation process 1-3 times to obtain anhydrous nanocellulose precipitate; adding polydimethylsiloxane, a curing agent, and a solvent to the anhydrous nanocellulose precipitate, mixing evenly, and spraying onto the surface of a substrate to obtain the final product. The preparation method of this invention significantly reduces the use of chemical reagents, simplifies the preparation process of the superhydrophobic coating, reduces the production cost of the superhydrophobic coating, and the obtained superhydrophobic coating has good environmental protection and abrasion resistance properties.

Description

Technical Field

[0001] This invention relates to the field of superhydrophobic coating technology, and in particular to a nanocellulose-based superhydrophobic coating and its preparation method. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Superhydrophobic coatings have extremely strong hydrophobic properties. When water droplets come into contact with their surface, they form spheres and quickly roll off without remaining on the surface. At the same time, because water droplets cannot stay on the surface of superhydrophobic coatings, they can carry away dust, dirt and other impurities, achieving a self-cleaning effect. Therefore, they can be used in various occasions where surface cleanliness and waterproofing are required, and have wide applications in architectural coatings, automotive coatings, marine coatings and other fields.

[0004] Currently, commonly used superhydrophobic coatings are mainly fluorinated superhydrophobic coatings. However, fluorinated substances have poor environmental performance, complex production processes, and significant toxicity during production and use, posing considerable harm to the environment and human health. Therefore, increasing research is focused on the development of fluorine-free superhydrophobic coatings.

[0005] Patent CN 116856194 A (publication date: October 10, 2023) discloses a nanocellulose-based superhydrophobic coating and its preparation method. The method involves first grafting cellulose with chloroacetic acid, then preparing nanocellulose of different particle sizes by controlling various adjustments. After dehydration and drying to obtain powder, the powder is modified with methyltrimethoxysilane for hydrophobicity. Finally, it is dispersed in a tetrahydrofuran solution containing polydimethylsiloxane and a curing agent to obtain the superhydrophobic coating. Although this patent does not use a fluorine-containing modifier and is relatively environmentally friendly, the preparation process of its superhydrophobic coating is complex and involves multiple reactions.

[0006] Therefore, how to provide a simple method for preparing environmentally friendly superhydrophobic coatings is an urgent problem to be solved. Summary of the Invention

[0007] In view of this, the present invention provides a nanocellulose-based superhydrophobic coating and its preparation method. The preparation process of the superhydrophobic coating of the present invention is very simple and does not involve the use of fluorine-containing modifiers, and has good application prospects.

[0008] In a first aspect, the present invention provides a method for preparing a nano-cellulose-based superhydrophobic coating, comprising the following steps:

[0009] A) Add a precipitant to the aqueous suspension of nanocellulose, mix well, and then centrifuge to obtain nanocellulose precipitate with residual precipitant.

[0010] B) Disperse the nanocellulose precipitate containing residual precipitant in a replacement solvent, centrifuge, and repeat the dispersion-centrifugation process 1 to 3 times to obtain anhydrous nanocellulose precipitate;

[0011] C) Add polydimethylsiloxane, curing agent and solvent to the anhydrous nanocellulose precipitate, mix evenly and spray onto the substrate surface to obtain the final product.

[0012] Preferably, the nanocellulose is selected from one or more of cellulose nanofibers (CNF), nanocellulose crystals (CNC), or spherical nanocellulose (SNC).

[0013] Preferably, the nanocellulose aqueous suspension has a nanocellulose concentration of 0.1–10 wt% and the precipitant accounts for 0.5–5 wt% of the nanocellulose aqueous suspension.

[0014] Preferably, the precipitant is selected from potassium acetate, lithium chloride, or potassium propionate; and the displacement solvent is selected from ethanol, methanol, acetone, tetrahydrofuran, or ethylene glycol.

[0015] Furthermore, when the precipitant is potassium acetate, the displacement solvent is selected from any one of ethanol, methanol, tetrahydrofuran, and ethylene glycol; when the precipitant is lithium chloride, the displacement solvent is selected from any one of ethanol or acetone; when the precipitant is potassium propionate, the displacement solvent is selected from ethanol.

[0016] Preferably, the mass of the replacement solvent is 50 to 100 times the oven-dry mass of the nanocellulose.

[0017] Preferably, the centrifugal speed in steps A) and B) is 5000 to 12000 rpm.

[0018] Preferably, the mass ratio of the anhydrous nanocellulose precipitate to polydimethylsiloxane is 1:0.5 to 5.

[0019] Preferably, in step C), the solvent is selected from cyclohexane or tetrahydrofuran; the mass of the solvent is 10 to 100 times the mass of polydimethylsiloxane.

[0020] Secondly, the present invention provides a nano-cellulose-based superhydrophobic coating obtained by the above preparation method.

[0021] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0022] (1) The preparation method of the nanocellulose-based superhydrophobic coating provided by the present invention is very simple. It does not require heating of the reaction system or pre-hydrophobic modification of the nanocellulose. Instead, the nanocellulose is coated by PDMS during the mixing process with polydimethylsiloxane (PDMS), curing agent and solvent. Then, it is sprayed onto the surface of the substrate by spraying to obtain the superhydrophobic coating. The preparation method of the present invention greatly reduces the use of chemical reagents, simplifies the preparation process of superhydrophobic coatings, and reduces the production cost of superhydrophobic coatings.

[0023] (2) In the superhydrophobic coating prepared by the present invention, the coated nanocellulose provides the micro-nano structure, PDMS provides the hydrophobicity, does not contain highly toxic chemicals, and has good environmental protection properties; at the same time, the superhydrophobic coating prepared has good wear resistance. Attached Figure Description

[0024] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0025] Figure 1 This is a scanning electron microscope image of the superhydrophobic coating of Embodiment 1 of the present invention;

[0026] Figure 2 These are images of the water contact angle of the superhydrophobic coating in Embodiment 1 of the present invention;

[0027] Figure 3 This is an image showing the water contact angle of the superhydrophobic coating of Embodiment 1 of the present invention after being rubbed 30 times with a 200g weight;

[0028] Figure 4 This is a scanning electron microscope image of the superhydrophobic coating of Embodiment 2 of the present invention;

[0029] Figure 5 These are images of the water contact angle of the superhydrophobic coating in Embodiment 2 of the present invention;

[0030] Figure 6 This is an image showing the water contact angle of the superhydrophobic coating of Embodiment 2 of the present invention after being rubbed 30 times with a 200g weight. Detailed Implementation

[0031] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0032] This invention provides a method for preparing a nano-cellulose-based superhydrophobic coating, comprising the following steps:

[0033] A) Add a precipitant to the aqueous suspension of nanocellulose, mix well, and then centrifuge to obtain nanocellulose precipitate with residual precipitant.

[0034] B) Disperse the nanocellulose precipitate containing residual precipitant in a replacement solvent, centrifuge, and repeat the dispersion-centrifugation process 1 to 3 times to obtain anhydrous nanocellulose precipitate;

[0035] C) Add polydimethylsiloxane, curing agent and solvent to the anhydrous nanocellulose precipitate, mix evenly and spray onto the substrate surface to obtain the final product.

[0036] This invention reveals that directly mixing untreated nanocellulose aqueous suspension with PDMS, a curing agent, and a solvent and then spraying it onto a substrate surface fails to form a superhydrophobic surface. This is because nanocellulose is nanoscale, and the solid content of nanocellulose in the aqueous suspension is too low to meet the requirements for forming micro / nanostructured surfaces. Furthermore, the nanocellulose aqueous suspension has high surface tension, and drying it results in a smooth film or fragments, preventing the formation of micro / nanostructures. Therefore, this invention first anhydrousizes the nanocellulose. PDMS, a curing agent, and a solvent are added to the anhydrous nanocellulose. During this process, the nanocellulose is encapsulated by the PDMS adhesive, which possesses a certain degree of hydrophobicity. After mixing the substances, the mixture is sprayed. During spraying, the rapid evaporation of the solvent from the formed droplets and the spontaneous hydrogen bond aggregation tendency of the hydroxyl groups on the nanocellulose surface lead to natural assembly into micro / nanostructures coated with PDMS adhesive. These structures adhere to the substrate, and after drying, a superhydrophobic surface is obtained.

[0037] This invention does not impose any special restrictions on the type of nanocellulose, which can be selected from one or more of cellulose nanofibers (CNF), cellulose nanocrystals (CNC), or spherical cellulose nanocrystals (SNC). Further, the cellulose nanofibers include, but are not limited to, CNF prepared by TEMPO oxidation; the cellulose nanocrystals include, but are not limited to, CNC prepared by hydrolysis with sulfuric acid or hydrochloric acid; and the spherical cellulose includes, but is not limited to, SNC prepared by hydrolysis with a mixed acid composed of sulfuric acid, hydrochloric acid, and nitric acid.

[0038] In the nanocellulose aqueous suspension of the present invention, the mass concentration of nanocellulose is 0.1% to 10 wt%, more preferably 1% to 5 wt%; the precipitant accounts for 0.5% to 5 wt% of the mass fraction of the nanocellulose aqueous suspension. If the amount of precipitant is too large, the residual precipitant content in the nanocellulose precipitate after centrifugation will be large, which is not conducive to the subsequent dissolution and removal of the residual precipitant by the replacement solvent; if the amount of precipitant is too small, the nanocellulose cannot be completely settled, the yield will be reduced, and the binding between nanocellulose and water cannot be effectively broken, which is not conducive to the replacement and removal of residual water by the replacement solvent.

[0039] The precipitant of this invention is selected from any one of potassium acetate, lithium chloride, or potassium propionate. Potassium acetate, lithium chloride, and potassium propionate are all strong electrolyte salts. When added to an aqueous suspension of nanocellulose, they can disrupt the stable double-layer structure on the surface of the nanocellulose, significantly compressing the Stern layer and reducing the zeta sites. This prevents the negative charges on the surface of the nanocellulose, such as carboxyl and sulfonic acid groups, from repelling each other and stabilizing the suspension of the nanocellulose, causing the nanocellulose to aggregate and settle.

[0040] In this invention, the displacement solvent is selected from any one of ethanol, methanol, acetone, tetrahydrofuran, or ethylene glycol. The displacement solvent is used to dissolve and remove the precipitate and water from the nanocellulose precipitate containing residual precipitate obtained in step A). ​​Therefore, it needs to be miscible with water and have good dissolving effect on the precipitate. Due to differences in solubility properties, different precipitates correspond to different displacement solvents. Specifically, when the precipitate is potassium acetate, the displacement solvent is selected from any one of ethanol, methanol, tetrahydrofuran, or ethylene glycol; when the precipitate is lithium chloride, the displacement solvent is selected from any one of ethanol or acetone; when the precipitate is potassium propionate, the displacement solvent is selected from ethanol. The dispersion-centrifugation process in step B) is repeated multiple times to fully remove residual water and precipitate from the nanocellulose.

[0041] In order to ensure sufficient dispersion of nanocellulose precipitate while controlling costs, the mass of the replacement solvent in this invention is 50 to 100 times the oven-dry mass of nanocellulose.

[0042] In this invention, the centrifugal speed in steps A) and B) is 5000 to 12000 rpm.

[0043] In this invention, the mass ratio of the anhydrous nanocellulose precipitate to polydimethylsiloxane is 1:0.5-5. In this invention, polydimethylsiloxane (PDMS) is used to adhere nanocellulose particles to the substrate surface, directly affecting the adhesion and superhydrophobicity of the superhydrophobic coating. If the amount of PDMS is too small, the nanocellulose particles cannot be adhered to the material surface, resulting in a weak superhydrophobic coating after spraying. If the amount of PDMS is too large, although the nanocellulose particles can be firmly adhered to the material surface, the excessive coverage of the nanocellulose particles by PDMS weakens the micro-nano structure of the surface, causing a significant reduction in the superhydrophobicity of the coating, or even failing to meet the superhydrophobic requirements.

[0044] In step C) of this invention, the solvent is selected from cyclohexane or tetrahydrofuran; the mass of the solvent is 10 to 100 times the mass of polydimethylsiloxane. Cyclohexane and tetrahydrofuran can dissolve PDMS, thereby allowing PDMS to be uniformly distributed in the solution, which is beneficial for the full coating of anhydrous cellulose nanoparticles.

[0045] In this invention, polydimethylsiloxane (PDMS) has active silicon groups, and the curing agent can react with the active silicon groups of PDMS to form a cross-linked network, ensuring the stability of the coating. The mass ratio of anhydrous nanocellulose precipitate to curing agent is 1:0.5-5.

[0046] The present invention does not impose any special restrictions on the type of substrate. In the embodiments of the present invention, glass is used as an example for illustration. However, the scope of protection of the present invention is not limited to the application on the glass surface, but can also be applied to the surface of substrates such as silicon wafers, plastics, wood, fiber cloth and metals.

[0047] This invention does not impose special limitations on the spraying process; commonly used spraying methods in the field can be employed. The nozzle diameter of the spray gun is preferably 0.2-0.8 mm. If the nozzle diameter is too small, the nanocellulose agglomerates are more likely to clog the spray gun during the spraying process, causing spraying difficulties. If the nozzle diameter is too large, the spraying effect will decrease, and the droplet drying speed will slow down, which is not conducive to the formation of micro / nano structure surfaces.

[0048] This invention involves curing the coating after spraying to obtain a nano-cellulose-based superhydrophobic coating with a micro / nano structure. The curing temperature is 30–150°C, and the curing time is 0.5–5 hours.

[0049] The preparation process of the above-mentioned nanocellulose-based superhydrophobic coating of the present invention does not require complicated nanocellulose surface modification steps, consumes little energy, and is simple to operate, and has good application prospects.

[0050] This invention also provides a nanocellulose-based superhydrophobic coating obtained by the above preparation method. The main materials of the nanocellulose-based superhydrophobic coating obtained by this invention are nanocellulose and PDMS. Nanocellulose is extracted from common renewable resources such as plant fibers, exhibiting good environmental friendliness and being safe and non-toxic. Meanwhile, PDMS is non-toxic and odorless, highly safe, and possesses good adhesion, waterproof performance, and chemical stability. Therefore, the nanocellulose-based superhydrophobic coating provided by this invention exhibits excellent waterproof, dustproof, and stability properties.

[0051] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0052] Example 1

[0053] In a 50 mL aqueous suspension of 1 wt% cellulose nanofibers (CNF), 1 g of potassium acetate was added and mixed thoroughly. The mixture was then centrifuged at 7000 rpm to precipitate the precipitate. 50 mL of ethanol was added to the precipitate, and after thorough dispersion, the mixture was centrifuged again at 7000 rpm to dissolve and remove residual potassium acetate and water from the CNF. This purification step was repeated twice to obtain a pure, anhydrous CNF precipitate. 30 mL of cyclohexane, 1 g of polydimethylsiloxane, and 0.1 g of PDMS curing agent were added to this precipitate, and the mixture was mixed to obtain a homogeneous emulsion. This emulsion was then sprayed onto a glass surface using a 0.3 mm nozzle and dried at 30°C to form a superhydrophobic coating.

[0054] The scanning electron microscope image of the superhydrophobic coating obtained in this embodiment is as follows: Figure 1 As shown, the water contact angle is 158°, as Figure 2 As shown, it exhibits excellent superhydrophobic properties. After being rubbed 30 times with a 200g weight, the contact angle decreased to 151°, as... Figure 3 As shown, it still exhibits good superhydrophobicity, therefore this coating also has good abrasion resistance.

[0055] Example 2

[0056] 2.5 g of lithium chloride was added to 50 mL of a 2 wt% aqueous suspension of nanocellulose crystals (CNC), and after thorough mixing, the mixture was centrifuged at 5000 rpm to precipitate. 80 mL of acetone was added to the precipitate, and after thorough dispersion, the mixture was centrifuged again to dissolve and remove residual lithium chloride and water from the CNC. This purification step was repeated twice to obtain a pure, anhydrous CNC precipitate. 50 mL of tetrahydrofuran, 1.5 g of polydimethylsiloxane, and 0.15 g of curing agent were added to this precipitate, and the mixture was mixed to obtain a homogeneous emulsion. This emulsion was then sprayed onto a glass surface using a 0.5 mm nozzle and dried at 30°C to form a superhydrophobic coating.

[0057] The scanning electron microscope image of the superhydrophobic coating obtained in this embodiment is as follows: Figure 4 As shown, the water contact angle is 167°, as Figure 5 As shown, it exhibits excellent superhydrophobic properties. After being rubbed 30 times with a 200g weight, the contact angle decreased to 160°, as... Figure 6 As shown, it still exhibits good superhydrophobicity, therefore this coating also has good abrasion resistance.

[0058] Comparative Example 1

[0059] Compared with Example 1, this comparative example does not involve precipitation or displacement steps. The specific steps are as follows:

[0060] Adding 30 mL of cyclohexane, 1 g of polydimethylsiloxane, and 0.1 g of curing agent to 2 mL of an untreated 1 wt% aqueous suspension of cellulose nanofibers (CNF), and mixing resulted in a non-uniform emulsion suitable for spraying due to the hydrolysis of polydimethylsiloxane. When coated onto a glass surface and dried, it also lacked hydrophobicity.

[0061] Comparative Example 2

[0062] Compared to Example 2, this comparative example does not involve precipitation or displacement steps. The specific steps are as follows:

[0063] Adding 50 mL of tetrahydrofuran, 1.5 g of polydimethylsiloxane, and 0.15 g of curing agent to 2 mL of an untreated 2 wt% aqueous suspension of nanocellulose crystals (CNC), and mixing, resulted in a non-uniform emulsion suitable for spraying due to the hydrolysis of polydimethylsiloxane. When coated onto a glass surface and dried, it also lacked hydrophobicity.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for the preparation of a nanocellulose-based superhydrophobic coating, characterized by, comprising the steps of: A) adding a precipitant to a nanocellulose aqueous suspension, mixing uniformly, and centrifuging to obtain a nanocellulose precipitate with the precipitant remaining; the precipitant is selected from any one of potassium acetate, lithium chloride or potassium propionate; the mass concentration of the nanocellulose in the nanocellulose aqueous suspension is 0.1-10 wt%; the mass fraction of the precipitant in the nanocellulose aqueous suspension is 0.5-5 wt%; B) dispersing the nanocellulose precipitate with the precipitant remaining into a displacement solvent, centrifuging, and repeating the dispersing-centrifuging process 1-3 times to obtain a nanocellulose precipitate without water, the displacement solvent being selected from any one of ethanol, methanol, acetone, tetrahydrofuran or ethylene glycol; when the precipitant is potassium acetate, the displacement solvent is selected from any one of ethanol, methanol, tetrahydrofuran or ethylene glycol; when the precipitant is lithium chloride, the displacement solvent is selected from any one of ethanol or acetone; when the precipitant is potassium propionate, the displacement solvent is ethanol; C) adding polydimethylsiloxane, a curing agent and a solvent to the nanocellulose precipitate without water, mixing uniformly, and spraying onto a substrate surface to obtain the nanocellulose-based superhydrophobic coating.

2. The production method according to claim 1, wherein the nanocellulose is selected from one or more of cellulose nanofibril, nanocrystalline cellulose or spherical nanocellulose.

3. The production method according to claim 1, wherein the centrifugal speed in step A) and step B) is 5000-12000 rpm.

4. The production method according to claim 1, wherein the mass ratio of the nanocellulose precipitate without water to polydimethylsiloxane is 1:0.5-5.

5. The production method according to claim 1, wherein in step C), the solvent is selected from cyclohexane or tetrahydrofuran; the mass of the solvent is 10-100 times the mass of polydimethylsiloxane.

6. The nanocellulose-based superhydrophobic coating obtained by the preparation method according to any one of claims 1-5.

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