Preparation of modified nanometer starch super-hydrophobic paper

Abstract

The application provides a preparation method and application of modified nano-starch coating super-hydrophobic paper, and belongs to the technical field of papermaking. The method comprises the following steps: nano-starch preparation, nano-starch modification and super-hydrophobic paper preparation. First, starch dispersion paste is gelatinized, and nano-starch is precipitated by using anhydrous ethanol to form a nano-starch colloidal suspension. Hydrophobic modification of the nano-starch is realized by using triethylamine as a catalyst, ethyl acetate as a solvent and stearoyl chloride as a modifier. Then, the modified starch ethanol dispersion is sprayed onto the surface of paper, and super-hydrophobic paper is obtained after solidification. The super-hydrophobic paper prepared by the application is a full bio-based material, is biodegradable, green and environmentally friendly, non-toxic, simple in process flow, low in cost, wide in application range, and has the performances of oil-water separation, self-cleaning and excellent dye compatibility.

Description

Technical Field

[0001] This invention belongs to the field of papermaking technology, specifically relating to a method for preparing and applying modified nano-starch coated superhydrophobic paper. Background Technology

[0002] The information disclosed in this background section is intended only to enhance 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] Many superhydrophobic materials exist in nature, such as lotus leaves, fish scales, water striders, butterfly wings, and rose petals. The lotus leaf, with its unique papillary waxy structure, allows water droplets to maintain a large contact angle, easily rolling off and carrying away mud and dust, thus exhibiting the characteristic of "emerging from the mud unsullied." In recent years, inspired by nature, superhydrophobic materials have received increasing attention due to their unique properties and have shown application potential in various scenarios, such as waterproof surfaces, anti-fog surfaces, anti-icing surfaces, self-cleaning surfaces, corrosion-resistant surfaces, packaging materials, and oil-water separation materials.

[0004] Superhydrophobicity typically refers to the phenomenon where, when a water droplet is applied to a sample surface, the contact angle can reach over 150° while the roll-off angle is less than 10°. Low surface energy materials and the roughness of micro / nano structures are two key factors in achieving superhydrophobic surfaces. Therefore, inspired by natural surfaces, creating superhydrophobicity requires introducing low surface energy groups, such as perfluorinated functional groups, long-chain alkanes, and silicon-based compounds, onto rough surfaces, or constructing micro / nano-scale surface structures using hydrophobic materials. A series of studies have revealed that the key to superhydrophobic fabrication technology depends on the construction of suitable surface structures. There are many methods for constructing micro / nano-scale surfaces, such as etching, template methods, electrospinning, phase separation, layer-by-layer self-assembly, immersion, spraying, and chemical deposition.

[0005] Bio-based materials, as environmentally friendly materials, have been applied in the development of superhydrophobic surfaces in recent years, greatly reducing the health hazards and environmental pollution caused by superhydrophobic coatings constructed with fluorinated compounds. As a low-cost, non-toxic, and biodegradable natural bio-based material, nano-starch possesses excellent biocompatibility, biosafety, and biodegradability, and has shown broad application prospects in the food industry and biomedicine.

[0006] Currently, some studies have explored improving the strength and durability of paper or paperboard by applying sizing starch to the surface, while reducing ink penetration and diffusion. Other studies have used stearoyl chloride as an esterifying agent to modify starch. However, there is currently no research on using modified nano-starch to prepare superhydrophobic paper. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing and applying a modified nano-starch coated superhydrophobic paper. The superhydrophobic paper prepared by this invention is a fully bio-based material, biodegradable, environmentally friendly, with a simple process, low cost, and wide applicability. This invention utilizes the unique small-size and surface effects of nano-starch, combined with stearoyl chloride long-chain alkyl hydrophobic modified nano-starch, to induce a chain fusion effect during the drying process, forming a high-strength superhydrophobic coating structure on the paper.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A first aspect of the present invention provides a method for preparing superhydrophobic paper based on modified nano-starch, comprising:

[0010] Preparation of nano starch;

[0011] Stearoyl chloride was used to modify nano starch, and the modified nanoparticles were collected, washed, and prepared into a modified starch ethanol dispersion.

[0012] The modified starch ethanol dispersion is sprayed onto the surface of a paper-based material and dried to obtain superhydrophobic paper.

[0013] This invention utilizes the advantages of nano-starch as a natural bio-based material, combined with the unique small size effect and surface effect of nanoparticles, to develop an economical, environmentally friendly and sustainable all-bio-based biodegradable superhydrophobic coating.

[0014] In some embodiments, the method for preparing the nano-starch includes:

[0015] Disperse starch in water, mix well, and prepare a starch dispersion of 0.9-1.2 wt%.

[0016] The starch dispersion was gelatinized, cooled, and ultrasonically treated to obtain a gelatinized liquid;

[0017] Under mechanical stirring, the gelatinized liquid is added dropwise to anhydrous ethanol to form a nano-starch colloidal suspension. The suspension is homogenized, centrifuged, and washed with ethanol to obtain nano-starch particles.

[0018] In some embodiments, the specific conditions for gelatinization are gelatinization in a boiling water bath for 27-32 minutes;

[0019] In some embodiments, the ultrasonic treatment time is 4-7 minutes.

[0020] In some embodiments, the volume ratio of the gelatinized liquid to anhydrous ethanol is 1:1-1.2;

[0021] In some implementations, the dropping rate is 3-6 ml / min, and the stirring speed for the nanoprecipitate is 500-700 rpm.

[0022] In some embodiments, the homogenization process is performed by homogenizing the colloidal suspension at a rotation speed of 7000-9000 rpm for 55-85 seconds.

[0023] In some embodiments, the specific steps of the modification include:

[0024] The nano starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate to obtain a nano starch solution with a concentration of 0.9-1.2 wt%.

[0025] Triethylamine and stearoyl chloride were added sequentially to the nano starch solution, mixed evenly, and reacted to obtain stearoyl chloride-modified nano starch particles.

[0026] In some embodiments, the amount of triethylamine used is 120%-160% of the starch mass;

[0027] In some embodiments, the triethylamine and starch are stirred at 78-82°C for 10-20 minutes to mix evenly.

[0028] Besides stearoyl chloride, this invention also uses common acetic anhydride and octenyl succinic anhydride to hydrophobically modify nano-starch. However, since the grafted side chain groups cannot provide sufficient low surface energy substances, acetic anhydride and octenyl succinic anhydride-modified nano-starch coatings cannot achieve superhydrophobic properties. Stearoyl chloride with long-chain alkyl groups is the optimal choice for the nano-starch modifier used in this invention to prepare superhydrophobic coatings. Therefore, in some embodiments, the molar amount of stearoyl chloride is 1.5-1.8 times that of triethylamine.

[0029] In some embodiments, the reaction time is 4.5-6.0 h.

[0030] In some embodiments, the modified starch ethanol dispersion concentration is 3-4 wt%, ultrasonication is performed for 5-7 min, and the smoothness of the base paper is 60-120 s.

[0031] In some implementations, the conditions for spraying the base paper are as follows: the distance between the spray gun and the paper surface is 8-12 cm, the spray gun pressure is 53-58 kPa, the flow rate is 6-8 mL / min, and the moving speed is 2.5-3.5 cm / s.

[0032] In some embodiments, the drying conditions for the sprayed paper are: temperature 50-70°C, curing time 25-35 minutes.

[0033] More specifically, including:

[0034] Nano starch was prepared using starch as raw material and modified with stearoyl chloride. The modified starch ethanol dispersion was sprayed onto the surface of a paper-based material and dried to obtain superhydrophobic paper.

[0035] Preparation of nano-starch: Starch was dispersed in deionized water and mixed evenly to prepare a starch dispersion of 0.9-1.2 wt%. The starch dispersion was gelatinized in a boiling water bath for 27-32 min, then cooled to room temperature and sonicated for 4-7 min. Anhydrous ethanol was used to precipitate the nano-starch. The gelatinized solution was added dropwise to an equal volume of anhydrous ethanol at a stirring speed of 500-700 rpm to form a stable nano-starch colloidal suspension. The suspension was then homogenized using a high-speed disperser at 7000-9000 rpm for 55-85 s. The suspension was centrifuged to collect the nano-precipitated particles, which were then washed with anhydrous ethanol.

[0036] Stearoyl chloride-modified nano-starch: Nano-starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate at a concentration of 0.9-1.2 wt%. 120-160% triethylamine catalyst (based on starch) was added, and the mixture was stirred at 78-82℃ for 10-20 min until homogeneous. Then, 1.5-1.8 times the molar amount of triethylamine stearoyl chloride was added, and the reaction continued for 4.5-6.0 h. After the reaction was completed, the mixture was cooled and centrifuged to obtain stearoyl chloride-modified nano-starch, which was then washed with ethanol.

[0037] Preparation of superhydrophobic paper: A 3-4 wt% modified starch-ethanol dispersion was ultrasonically dispersed for 5-7 minutes. This dispersion was then uniformly sprayed onto paper using a 0.5 mm nozzle spray gun. The base paper was unsizing and unfilled, achieving a smoothness of 60-120 seconds. The distance between the spray gun and the paper surface was 8-12 cm, the spray gun pressure was 53-58 kPa, the flow rate was 6-8 mL / min, and the moving speed was 2.5-3.5 cm / s. Subsequently, the sprayed paper was cured at 50-70℃ for 25-35 minutes to obtain the superhydrophobic paper.

[0038] In a second aspect, the present invention provides a superhydrophobic paper based on modified nano starch prepared by the above method.

[0039] A third aspect of the present invention provides the application of the above-described modified nano-starch superhydrophobic paper in the food and biopharmaceutical fields.

[0040] Beneficial effects of the present invention

[0041] (1) This invention develops a simple preparation strategy for a robust and durable superhydrophobic coating by controlling the balance between the degree of modification of amorphous nano-starch and the curing temperature of the coating. The grafting of long-chain alkyl groups promotes the "chain fusion" induced by the enhanced Brownian motion of molecular chains under heating conditions, thus achieving both high hydrophobicity and mechanical durability. Thanks to the good permeability and mechanical flexibility of the paper-based coating, the coating exhibits excellent oil removal, underwater oil absorption, and oil-water separation capabilities.

[0042] (2) The bonding strength between the coating and the substrate is also a hot topic of concern in the practical application of superhydrophobic coatings. At present, PDMS, epoxy resin and other materials are often used as adhesives to enhance the mechanical strength of the coating. However, this inevitably sacrifices the biodegradability of the bio-based coating. This invention successfully prepared a fully bio-based biodegradable superhydrophobic coating that takes into account both mechanical strength and superhydrophobicity by "chain fusion" of grafted long-chain alkyl groups during the drying process, which can eliminate the need for adhesives.

[0043] (3) The preparation method of the present invention is simple, practical and easy to promote. Attached Figure Description

[0044] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. Exemplary embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0045] Figure 1 Self-cleaning process of methylene blue powder on (a) base paper and (b) superhydrophobic paper; schematic diagram of self-cleaning process on (c) base paper and (d) modified nano starch coating surface;

[0046] Figure 2 (a) i) Modified starch ethanol dispersions stained with Brilliant Blue, Sunset Yellow, Carmine, and Tartrazine respectively, and ii) Color-modified nano-starch coated paper umbrellas. (b) Wetting properties of the original substrate. (c) Optical images of the modified nano-starch coated painted coating and its anti-fouling properties against various liquids;

[0047] Figure 3 (a) Biodegradation experiment of original filter paper, (b) Brilliant Blue and (c) Carmine-stained modified nano-starch coated filter paper in soil;

[0048] Figure 4 Confocal microscopy reveals fluorescence images of live (green) and dead (red) cells after culturing (a) 4T1 cells and (b) chondrocytes with different concentrations of HSNP (modified nano-starch). Cell viability of (c) 4T1 cells and (d) chondrocytes in the presence of HSNP was determined by the MTT assay. Detailed Implementation

[0049] 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 in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0050] As described in the background section, in order to construct green, environmentally friendly, and high-performance superhydrophobic paper, this invention provides a method for preparing a fully bio-based superhydrophobic paper. This technology relies on four core factors: nanoscale starch particles; hydrophobic modification of starch hydroxyl groups; tight bonding between the nano-starch and the paper; and the strength of the nano-coating.

[0051] Surface micro / nano structures and low surface energy can endow superhydrophobic surfaces. Existing technologies reduce the surface energy of nanoparticles by adding fluorine-containing compounds to organic or inorganic nanostructures. This invention aims to prepare a fully bio-based superhydrophobic paper, eliminating the use of fluorine-containing compounds, and requires the design of surface-modified hydrophobic starch nanoparticles. To ensure the strength and toughness of the superhydrophobic coating, and to achieve a dense bond between the coating and the paper surface, it is also necessary to ensure the amorphous starch structure, suitable modifying groups, and the degree of modification. Under the premise of suitable nanosize and hydrophobic groups, the starch molecular chains and hydrophobic groups on the particle surface possess fluidity, and a chain fusion effect occurs during the coating drying process, maintaining the nanoparticle morphology and forming a high-strength coating structure with a high degree of bonding between the coating and the paper.

[0052] Nano starch prepared by nanoprecipitation disrupts the crystalline structure of the original starch, resulting in amorphous nano starch that significantly reduces the crystallinity of the starch, exposes more hydroxyl groups, and provides more reaction sites for further modification. Moreover, the loose amorphous structure lays the foundation for the "chain fusion" effect of hydrophobic coatings.

[0053] The modification of nano starch particles faces the problem of easy particle aggregation. This invention converts the prepared nano starch from ethanol to ethyl acetate system by centrifugation, which promotes the full dispersion of starch and ensures that the nano starch maintains a nanoscale of 50-200 nm before and after modification without producing large micron-sized aggregates. At the same time, the drying step is eliminated, simplifying the operation steps.

[0054] Further preparation of superhydrophobic coatings using stearoyl chloride long-chain alkyl hydrophobic modification of nano-starch requires achieving a suitable degree of substitution while maintaining the final micro / nano-sized structure. The method of this invention utilizes triethylamine as a base catalyst, which neutralizes the hydrochloric acid generated during the stearoyl chloride reaction, forming a triethylamine salt. This avoids starch degradation due to excessively low or high pH, ​​thus making it easier for stearoyl chloride to react with the hydroxyl groups in the nano-starch, resulting in a high degree of substitution. The highly substituted hydrophobic groups, amorphous starch, and nano-sized dimensions ensure the preparation of high-performance superhydrophobic paper.

[0055] Compared to other methods that use catalysts such as pyridine and solvent systems such as chloroform and xylene, the triethylamine catalyst and ethyl acetate solvent system used in this invention are more environmentally friendly and safer, and can achieve a high degree of substitution. This high degree of substitution maintains the granular structure of starch, ensuring the full dispersion of nano-starch. The higher degree of substitution also reduces the tight packing between molecular chains, significantly improving the conformational flexibility of the polymer chains and promoting the formation of a robust and durable superhydrophobic coating with enhanced mechanical stability through "chain fusion." Other solvents and catalytic systems struggle to guarantee suitable nanoscale size, degree of substitution, and hydrophobic properties.

[0056] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are explanations of the present invention and not limitations thereof.

[0057] Example 1

[0058] Starch was dispersed in deionized water and mixed thoroughly to prepare a 1.0 wt% starch dispersion. The starch dispersion was gelatinized in a boiling water bath for 32 min, cooled to room temperature, and sonicated for 6 min. Anhydrous ethanol was used to precipitate the starch nanoparticles. The gelatinized solution was added dropwise to an equal volume of anhydrous ethanol at a stirring speed of 700 rpm to form a stable nano-starch colloidal suspension. The suspension was then homogenized using a high-speed disperser at 9000 rpm for 85 s. The suspension was centrifuged to collect the nano-precipitated particles and washed with anhydrous ethanol.

[0059] The nano-starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate at a concentration of 0.9 wt%. 140% triethylamine catalyst (based on starch) was added, and the mixture was stirred at 81 °C for 18 min until homogeneous. Then, 1.6 times the molar amount of triethylamine stearoyl chloride was added, and the reaction continued for 5.5 h. After the reaction was completed, the nano-starch was cooled and centrifuged to obtain stearoyl chloride-modified nano-starch, which was washed with ethanol to prepare a modified starch ethanol dispersion.

[0060] A 3.2 wt% modified starch-ethanol dispersion was ultrasonically dispersed for 5.4 min and then uniformly sprayed onto paper using a 0.5 mm nozzle. The base paper was unsizing and unfilled, achieving a smoothness of 75 seconds. The distance between the spray gun and the paper surface was 12 cm, the spray gun pressure was 56 kPa, the flow rate was 7.5 mL / min, and the moving speed was 2.5 cm / s. Subsequently, the sprayed paper was cured at 70 °C for 25 min to obtain superhydrophobic paper.

[0061] The superhydrophobic paper has a contact angle of 151.8°, a roll-off angle of 5.2°, and a roughening strength of 1.46 m / s.

[0062] The prepared superhydrophobic paper can be used for oil-water separation. When the superhydrophobic paper is immersed in a dichloromethane (oil phase) / water mixture (50%, volume ratio), the dichloromethane is completely absorbed by the superhydrophobic paper, leaving no residue. Furthermore, the superhydrophobic paper floats freely on a hexane (oil phase)-water mixture (50%, volume ratio) and adsorbs floating oil. The superhydrophobic paper can be reused after drying. The oil-water separation characteristics benefit from the hydrophobic and oleophilic properties of the superhydrophobic paper; water molecules are repelled, while hexane is effectively collected and removed from the paper surface, thus achieving the purpose of purification and oil removal. After 30 cycles of use (each time used for oil-water separation, the paper is dried to allow the oil to evaporate from the paper surface before reuse), the oil-water separation efficiency is still above 99.6%, demonstrating the excellent recyclability and reusability of the superhydrophobic paper. The superhydrophobic paper possesses selective wettability for oil and water, and based on its superhydrophobic and superoleophilic properties, it achieves highly efficient separation of oil-water mixtures.

[0063] Example 2

[0064] Starch was dispersed in deionized water and mixed thoroughly to prepare a 0.9 wt% starch dispersion. The starch dispersion was gelatinized in a boiling water bath for 31 min, cooled to room temperature, and sonicated for 7 min. Anhydrous ethanol was used to precipitate the starch nanoparticles. The gelatinized solution was added dropwise to an equal volume of anhydrous ethanol at a stirring speed of 550 rpm to form a stable nano-starch colloidal suspension. The suspension was then homogenized using a high-speed disperser at 7000 rpm for 55 s. The suspension was centrifuged to collect the nano-precipitated particles and washed with anhydrous ethanol.

[0065] The nano-starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate at a concentration of 1.1 wt%. 160% triethylamine catalyst (based on starch) was added, and the mixture was stirred at 82 °C for 20 min until homogeneous. Then, 1.7 times the molar amount of triethylamine stearoyl chloride was added, and the reaction continued for 4.5 h. After the reaction was completed, the nano-starch was cooled and centrifuged to obtain stearoyl chloride-modified nano-starch, which was washed with ethanol to prepare a modified starch ethanol dispersion.

[0066] A 3.0 wt% modified starch-ethanol dispersion was ultrasonically dispersed for 5.0 min and then uniformly sprayed onto paper using a 0.5 mm nozzle. The base paper was unsizing and unfilled, and the smoothness was achieved in 60 seconds. The distance between the spray gun and the paper surface was 11 cm, the spray gun pressure was 58 kPa, the flow rate was 6.0 mL / min, and the moving speed was 2.8 cm / s. Subsequently, the sprayed paper was cured at 55 °C for 32 min to obtain superhydrophobic paper.

[0067] The superhydrophobic paper has a contact angle of 152.3°, a roll-off angle of 5.6°, and a roughening strength of 1.44 m / s.

[0068] The self-cleaning properties of superhydrophobic paper are of great significance for removing solid contaminants from surfaces. The self-cleaning properties of the prepared superhydrophobic paper were verified using methylene blue powder as an example. Figure 1 As shown, the base paper has a porous structure and is hydrophilic, allowing water droplets to adhere strongly to its surface and be quickly wetted, accompanied by the dissolution and penetration of methylene blue contaminants. In contrast, the methylene blue powder contaminants on the superhydrophobic paper can be effectively captured and removed by water droplets, achieving self-cleaning and the effect of self-removing solid particulate contaminants. This phenomenon is attributed to the fact that the methylene blue powder contaminants only contact the top of the micro-nano structure of the superhydrophobic surface, and the interaction force between solid particles and the superhydrophobic surface is much lower than the binding force between water and solid particles, making it easy for water droplets to capture the methylene blue powder contaminants on the superhydrophobic surface and preventing them from wetting the coating surface. The superhydrophobic paper exhibits a Cassie superhydrophobic state, displaying weaker interaction between water droplets and the substrate surface and significantly more hydrophobic properties on the coating surface compared to the base paper.

[0069] Example 3

[0070] Starch was dispersed in deionized water and mixed thoroughly to prepare a 1.1 wt% starch dispersion. The starch dispersion was gelatinized in a boiling water bath for 30 min, cooled to room temperature, and sonicated for 5 min. Anhydrous ethanol was used to precipitate the starch nanoparticles. The gelatinized solution was added dropwise to an equal volume of anhydrous ethanol at a stirring speed of 600 rpm to form a stable nano-starch colloidal suspension. The suspension was then homogenized using a high-speed disperser at 8000 rpm for 75 s. The suspension was centrifuged to collect the nano-precipitated particles and washed with anhydrous ethanol.

[0071] The nano-starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate at a concentration of 1.0 wt%. 130% triethylamine catalyst (based on starch) was added, and the mixture was stirred at 78 °C for 16 min until homogeneous. Then, 1.5 times the molar amount of triethylamine stearoyl chloride was added, and the reaction continued for 5.5 h. After the reaction was completed, the nano-starch was cooled and centrifuged to obtain stearoyl chloride-modified nano-starch, which was washed with ethanol to prepare a modified starch ethanol dispersion.

[0072] A 3.6 wt% modified starch-ethanol dispersion was ultrasonically dispersed for 7.0 min and then uniformly sprayed onto paper using a 0.5 mm nozzle spray gun. The base paper was unsizing and unfilled, achieving a smoothness score of 112 s. The spray gun was positioned 10 cm above the paper, with a pressure of 53 kPa, a flow rate of 7.0 mL / min, and a travel speed of 3.0 cm / s. The coated paper was then cured at 60°C for 30 min to obtain superhydrophobic paper.

[0073] The superhydrophobic paper has a contact angle of 152.5°, a roll-off angle of 6.3°, and a roughening strength of 1.52 m / s.

[0074] Application of superhydrophobic paper in water-resistant paints. Superhydrophobic paper exhibits excellent dye compatibility; by mixing different colored dyes with a modified nano-starch-ethanol suspension (mixing ratio 1:100) and spraying, a variety of coating colors can be achieved. For example... Figure 2 As shown, the colored superhydrophobic paper umbrellas exhibit four distinct and uniform colors, offering excellent coverage and a vibrant appearance. The blue, orange, red, and yellow colors correspond to the addition of four different food-grade dyes: brilliant blue, sunset yellow, carmine, and lemon yellow. These food-grade dyes ensure the coating's suitability for applications in food packaging and other fields. Figure 2 The original substrate, as shown in Figure b, exhibits extremely strong hydrophilic properties, allowing it to be completely wetted by various types of water droplets, including water (treated with methylene blue), coffee, milk, and cola. In contrast, the modified nano-starch colored coating possesses superhydrophobicity, demonstrating excellent anti-fouling performance against a variety of everyday liquids. Figure 2 Image c shows the process of the painted coating and the liquid repellency of the colored coating to different types of water droplets. All test droplets formed complete beads on the coated substrate, demonstrating low adhesion between the droplets and the substrate and the coating's excellent superhydrophobicity. Therefore, by adding various colorants, the color and texture of the coating can be designed and improved, thereby controlling the appearance of the coating, enriching its colors, and catering to different aesthetic needs.

[0075] Example 4

[0076] Starch was dispersed in deionized water and mixed thoroughly to prepare a 1.2 wt% starch dispersion. The starch dispersion was gelatinized in a boiling water bath for 28 min, cooled to room temperature, and sonicated for 5 min. Anhydrous ethanol was used to precipitate the starch nanoparticles. The gelatinized solution was added dropwise at a rate of 5 ml / min to an equal volume of anhydrous ethanol while stirring at 650 rpm to form a stable nano-starch colloidal suspension. The suspension was then homogenized using a high-speed disperser at 8500 rpm for 65 s. The suspension was centrifuged to collect the nano-precipitated particles and washed with anhydrous ethanol.

[0077] Nano-starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate at a concentration of 1.0 wt%. 150% triethylamine catalyst (starch-based) was added, and the mixture was stirred at 80 °C for 13 min until homogeneous. Then, 1.8 times the molar amount of triethylamine stearoyl chloride was added, and the reaction continued for 5.0 h. After the reaction was completed, the mixture was cooled and centrifuged to obtain stearoyl chloride-modified nano-starch, which was washed with ethanol to prepare a modified starch ethanol dispersion.

[0078] A 3.8 wt% modified starch-ethanol dispersion was ultrasonically dispersed for 6.5 min and then uniformly sprayed onto paper using a 0.5 mm nozzle. The base paper was unsizing and unfilled, achieving a smoothness of 89 s. The distance between the spray gun and the paper surface was 9 cm, the spray gun pressure was 54 kPa, the flow rate was 6.5 mL / min, and the moving speed was 3.5 cm / s. Subsequently, the sprayed paper was cured at 50 °C for 28 min to obtain superhydrophobic paper.

[0079] The superhydrophobic paper has a contact angle of 154.1°, a roll-off angle of 5.6°, and a roughening strength of 1.41 m / s.

[0080] The biodegradability of the prepared superhydrophobic paper was evaluated. Figure 3 As shown, the base paper is a cellulose-rich biomaterial that completely degrades in the natural environment in just 15 days. The cellulose in the base paper contains abundant hydroxyl groups, making it easy for moisture in the environment to penetrate and disrupt the hydrogen bond network of the fibers, leading to rapid destruction of its network structure. On the other hand, soil contains organic matter, humic acid, and microbial communities, especially cellulolytic enzymes in the biological community, which readily break down the chemical bonds of lignocellulose, causing the cellulose structure to break down. After burying in soil for 15 days, the superhydrophobic paper cracked, showing varying degrees of degradation at the edges, while some of the dye on the coating surface was removed. At 30 days, the superhydrophobic paper developed obvious cracks and lost its original shape. After 45 days of burial, most of the superhydrophobic paper had degraded, leaving only a few fragments. Finally, the superhydrophobic paper completely degraded and disappeared after 60 days of burial. Therefore, the superhydrophobic paper can achieve natural biodegradation.

[0081] Example 5

[0082] Starch was dispersed in deionized water and mixed thoroughly to prepare a 1.0 wt% starch dispersion. The starch dispersion was gelatinized in a boiling water bath for 27 min, cooled to room temperature, and sonicated for 4 min. Anhydrous ethanol was used to precipitate the starch nanoparticles. The gelatinized solution was added dropwise at a rate of 5 ml / min to an equal volume of anhydrous ethanol while stirring at 500 rpm to form a stable nano-starch colloidal suspension. The suspension was then homogenized using a high-speed disperser at 7500 rpm for 60 s. The suspension was centrifuged to collect the nano-precipitated particles and washed with anhydrous ethanol.

[0083] Nano-starch was washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate at a concentration of 1.2 wt%. 120% triethylamine catalyst (starch-based) was added, and the mixture was stirred at 79 °C for 10 min until homogeneous. Then, 1.5 times the molar amount of triethylamine stearoyl chloride was added, and the reaction continued for 6.0 h. After the reaction was completed, the mixture was cooled and centrifuged to obtain stearoyl chloride-modified nano-starch, which was washed with ethanol to prepare a modified starch ethanol dispersion.

[0084] A 4.0 wt% modified starch-ethanol dispersion was ultrasonically dispersed for 5.8 min and then uniformly sprayed onto paper using a 0.5 mm nozzle. The base paper was unsizing and unfilled, with a smoothness rating of 120 s. The distance between the spray gun and the paper surface was 8 cm, the spray gun pressure was 55 kPa, the flow rate was 8.0 mL / min, and the moving speed was 3.3 cm / s. Subsequently, the sprayed paper was cured at 65 °C for 35 min to obtain superhydrophobic paper.

[0085] The superhydrophobic paper has a contact angle of 153.3°, a roll-off angle of 5.7°, and a roughening strength of 1.56 m / s.

[0086] The cytotoxicity of the prepared superhydrophobic paper was evaluated using live / dead staining experiments with 4T1 breast cancer cells and chondrocytes. Laser confocal microscopy revealed strong green fluorescence in both 4T1 and chondrocytes at different concentrations, with a significantly higher number of live cells than dead cells, indicating that the superhydrophobic paper provides a physiological environment for cell attachment and exhibits good biocompatibility. Further MTT assays were performed on 4T1 and chondrocytes to assess cell viability by evaluating their metabolic activity in the presence of four different concentrations of modified starch nanoparticles (HSNPs). Figure 4 As shown, even at high polymer concentrations, the viability of both cell types remained above 80%, confirming that the coating has good compatibility with both 4T1 and chondrocytes and does not have a toxic effect on the cells.

[0087] Comparative Example 1

[0088] The difference from Example 4 is that gelatinized starch was used instead of nano starch, and the coating was prepared after modification with stearoyl chloride. The superhydrophobic paper has a contact angle of 96.2°, a roll-off angle of 35.2°, and a roughening strength of 1.02 m / s.

[0089] Comparative Example 2

[0090] The difference from Example 4 is that stearoyl chloride-modified starch was not used; instead, nano-starch was used to directly prepare the coating. The superhydrophobic paper has a contact angle of 67.4°, a roll-off angle of 37.2°, and a napping strength of 1.12 m / s.

[0091] Comparative Example 3

[0092] The difference from Example 4 is that acetic anhydride was used instead of stearoyl chloride. The superhydrophobic paper has a contact angle of 112.4°, a roll-off angle of 28.5°, and a napping strength of 1.08 m / s.

[0093] Comparative Example 4

[0094] The difference from Example 4 is that octenyl succinic anhydride is used instead of stearoyl chloride. The superhydrophobic paper has a contact angle of 114.7°, a roll-off angle of 27.5°, and a napping strength of 1.10 m / s.

[0095] As can be seen from the comparison between Example 4 and Comparative Examples 1 and 2, a superhydrophobic coating cannot be prepared without the presence of nano-sized starch particles or stearoyl chloride modification.

[0096] As can be seen from the comparison between Example 4 and Comparative Examples 3 and 4, compared with acetic anhydride and octenyl succinic anhydride, the use of stearoyl chloride modified nano starch particles can effectively improve the hydrophobicity of the coating and obtain a superhydrophobic coating.

[0097] Therefore, it can be seen that the present invention utilizes the unique small size effect and surface effect of nano starch, combined with stearoyl chloride long-chain alkyl hydrophobic modified nano starch, so that the coating undergoes chain fusion effect during the drying process, forming a high-strength superhydrophobic coating structure on the paper.

[0098] 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 preparing superhydrophobic paper based on modified nano-starch, characterized in that, include: Preparation of nano starch; Stearoyl chloride was used to modify nano starch, and the modified nanoparticles were collected, washed, and prepared into a modified starch ethanol dispersion. The modified starch ethanol dispersion was sprayed onto the surface of a paper-based material and dried to obtain superhydrophobic paper. The specific steps for modifying nano-starch with stearoyl chloride are as follows: The nano-starch is washed with ethyl acetate to remove ethanol and dispersed in ethyl acetate to obtain a nano-starch solution with a concentration of 0.9-1.2 wt%; triethylamine and stearoyl chloride are added sequentially to the nano-starch solution, mixed evenly, and reacted to obtain stearoyl chloride-modified nano-starch; the amount of triethylamine used is 120%-160% of the starch mass; the molar amount of stearoyl chloride is 1.5-1.8 times that of triethylamine. The preparation method of the nano starch includes: Disperse starch in water, mix well, and prepare a starch dispersion of 0.9-1.2 wt%. The starch dispersion was gelatinized, cooled, and ultrasonically treated to obtain a gelatinized liquid; Under mechanical stirring, the gelatinized liquid is added dropwise to anhydrous ethanol to form a nano-starch colloidal suspension. The suspension is homogenized, centrifuged, and washed with ethanol to obtain nano-starch particles with a particle size of 50-200 nm.

2. The preparation method of superhydrophobic paper based on modified nano-starch as described in claim 1, characterized in that, The specific conditions for gelatinization are gelatinization in a boiling water bath for 27-32 minutes; Alternatively, the ultrasonic treatment time after gelatinization is 4-7 minutes.

3. The preparation method of superhydrophobic paper based on modified nano-starch as described in claim 1, characterized in that, The volume ratio of the gelatinized liquid to anhydrous ethanol is 1:1-1.2; Alternatively, under mechanical stirring conditions, the gelatinized liquid is added dropwise to anhydrous ethanol at a rate of 3-6 mL / min and a stirring speed of 500-700 rpm.

4. The preparation method of superhydrophobic paper based on modified nano-starch as described in claim 1, characterized in that, The homogenization process involves homogenizing the colloidal suspension at a rotation speed of 7000-9000 rpm for 55-85 seconds.

5. The preparation method of superhydrophobic paper based on modified nano-starch as described in claim 1, characterized in that, The triethylamine and starch were stirred at 78-82℃ for 10-20 min until they were evenly mixed. Alternatively, stearoyl chloride can be added and the reaction can continue for 4.5-6.0 h.

6. The method for preparing superhydrophobic paper based on modified nano-starch as described in claim 1, characterized in that, The modified starch ethanol dispersion concentration is 3-4 wt%, and the ultrasonic treatment is performed for 5-7 min before spraying. The smoothness of the base paper is 60-120 s. Alternatively, the conditions for spraying the base paper are: the distance between the spray gun and the paper surface is 8-12 cm, the spray gun pressure is 53-58 kPa, the flow rate is 6-8 mL / min, and the moving speed is 2.5-3.5 cm / s. Alternatively, the drying conditions for sprayed paper are: temperature 50-70 ℃, curing time 25-35 min.

7. The modified nano starch-based superhydrophobic paper prepared by the method according to any one of claims 1-6.

8. The application of the modified nano-starch-based superhydrophobic paper as described in claim 7 in the food and biomedical fields.

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