Fresh-cut leaf fresh-keeping packaging material based on nanotechnology

Abstract

The invention provides a nanotechnology-based fresh-cut leaf fresh-keeping packaging material, and relates to the technical field of packaging materials. The fresh-cut leaf fresh-keeping packaging material based on the nanotechnology comprises the following raw materials in parts by weight: 100 parts of compound polysaccharide, 2-4 parts of core-shell type adsorption functional filler, 1-2 parts of nano silver, 3-4 parts of nano ZnO, 3-5 parts of nano cellulose and 20-30 parts of a plasticizer, the compound polysaccharide is a compound system of chitosan, starch, radix puerariae powder or sodium alginate, and the compounding mass ratio is 3: 1-5: 1; the core-shell type adsorption functional filler is composed of an adsorption functional core and a nano wrapping layer, wherein the adsorption functional core is ZIF-8 with the particle size of 50-200nm or a compound filler of ZIF-8 and montmorillonite (the proportion of montmorillonite is 10-20wt%); and the nano wrapping layer is made of a force-induced rupture type biodegradable material with the thickness of 10-20 nm. The adsorption starting is controllable, the adaptability is high, the preservation time is long, and the preservative is safe and degradable.

Description

A nanotechnology-based packaging material for preserving fresh-cut leaves Technical Field

[0001] This invention relates to the field of packaging materials technology, specifically to a fresh-cut leaf preservation packaging material based on nanotechnology. Background Technology

[0002] Fresh produce such as cut flowers and leaves continuously release ethylene gas after harvesting, along with moisture evaporation. The accumulation of ethylene accelerates yellowing and aging of the produce, while excessive humidity promotes microbial growth, leading to spoilage. Polysaccharide-based nanocomposite preservation films have gained widespread attention in the field of fresh produce preservation due to their biodegradability, good moisture permeability, and biocompatibility. By adding adsorbent nanofillers (such as ZIF-8 and montmorillonite) to a polysaccharide matrix, they can adsorb ethylene and moisture, extending the shelf life of fresh produce.

[0003] However, existing polysaccharide-based nanocomposite preservation films have significant technical defects: the adsorption-type nanofillers are directly exposed to the external environment during the film production, storage, and transportation processes, prematurely adsorbing ethylene, moisture, and impurities from the air. As a result, when packaging fresh products, the adsorption sites are largely occupied, significantly reducing the adsorption function and shortening the shelf life by 30% to 50%. In addition, different cut flowers and leaves have significantly different decay rates. The adsorption efficiency of existing preservation films is fixed and cannot adapt to the differentiated preservation needs, resulting in redundant preservation functions for slow-decaying products and insufficient adsorption capacity for fast-decaying products.

[0004] Patent CN120888152B discloses a high-barrier antibacterial food packaging film. It improves the dispersibility of montmorillonite by modifying polyvinyl alcohol, thereby enhancing the film's antibacterial and barrier properties. However, this patent does not address the controllable activation of the adsorption function, leaving the drawback of premature adsorption failure and the inability to adjust adsorption efficiency on demand, making it difficult to meet the differentiated preservation needs of fresh-cut flowers and leaves. Therefore, developing a polysaccharide-based nanocomposite preservation film that can precisely control the activation timing of the adsorption function and adjust adsorption efficiency on demand has become a pressing technical problem in this field. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a fresh-cut leaf preservation packaging material based on nanotechnology, which solves the problems of premature failure of adsorption function and inability of adsorption efficiency to meet differentiated needs in existing polysaccharide-based nanocomposite preservation films.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution: a fresh-cut leaf preservation packaging material based on nanotechnology, the raw materials of which include 100 parts by weight of compound polysaccharide, 2-4 parts by weight of core-shell type adsorption functional filler, 1-2 parts by weight of nano silver, 3-4 parts by weight of nano ZnO, 3-5 parts by weight of nano cellulose, and 20-30 parts by weight of plasticizer.

[0009] The compound polysaccharide is a compound system of chitosan with starch, kudzu root powder or sodium alginate, with a compound mass ratio of 3:1 to 5:1;

[0010] The core-shell type adsorption functional filler consists of an adsorption functional core and a nano-coating layer. The adsorption functional core is ZIF-8 with a particle size of 50-200 nm, or a composite filler of ZIF-8 and montmorillonite (montmorillonite content 10-20 wt%). The nano-coating layer is a force-induced rupture biodegradable material with a thickness of 10-20 nm, selected from gelatinized starch nanospheres, low molecular weight polylactic acid nanoshells, or gelatin-sodium alginate nanocomposite layers (gelatin to sodium alginate mass ratio 1:1). The rupture threshold of the nano-coating layer is 3-10 N.

[0011] The plasticizer is glycerin or sorbitol.

[0012] Preferably, the low molecular weight polylactic acid has a molecular weight of 1000 to 3000.

[0013] Preferably, the core-shell type adsorption functional filler has an overall particle size of 60-220 nm, and is uniformly dispersed in the compound polysaccharide matrix without agglomeration.

[0014] A method for preparing a fresh-cut leaf preservation packaging material based on nanotechnology includes the following steps:

[0015] S1. Dissolving the compound polysaccharide: Add the compound polysaccharide to a 1% to 2% dilute acetic acid solution in a certain proportion, stir in a water bath at 40 to 50°C for 30 to 60 minutes until completely dissolved, add the plasticizer, and continue stirring for 15 minutes to obtain the polysaccharide sol;

[0016] S2. Preparation of core-shell type adsorption functional filler: The adsorption functional core is dispersed in a nano-coating material solution, and ultrasonic emulsification is performed at 300-500W for 20-30 minutes to make the nano-coating layer uniformly coat the surface of the adsorption functional core. After filtration and drying, the core-shell type adsorption functional filler is obtained.

[0017] S3. Multi-filler co-dispersion: Mix core-shell type adsorption functional filler, nano silver, nano ZnO and nano cellulose in a certain proportion, add deionized water, and sonicate for 20-30 min to form a functional filler suspension.

[0018] S4. Blending and homogenization: The functional filler suspension is slowly added dropwise to the polysaccharide sol, and magnetic stirring (300-500 r / min) and ultrasonic homogenization are alternated for 15 min to form a composite sol;

[0019] S5. Casting film: The composite sol is placed in a vacuum degassing chamber and degassed for 10 to 20 minutes under conditions of -0.08 to -0.1 MPa. Then, it is cast on a polytetrafluoroethylene plate with a thickness of 0.1 to 0.2 mm and dried at 40 to 60°C for 12 to 24 hours. After peeling, a fresh-cut leaf preservation packaging material based on nanotechnology is obtained.

[0020] Preferably, the solid content of the nano-coating material solution in S2 is 8% to 12%.

[0021] Preferably, the alternation cycle of magnetic stirring and ultrasonic homogenization in S4 is 3 minutes of stirring + 2 minutes of ultrasonication.

[0022] Preferably, the moisture content of the dried plastic wrap in step S5 is controlled at 8% to 12%.

[0023] (III) Beneficial Effects

[0024] This invention provides a fresh-cut leaf preservation packaging material based on nanotechnology. It offers the following advantages: The nano-coating layer isolates the adsorption core from the external environment, keeping the adsorption function closed during production and storage, only passively activating during packaging and bundling. This completely solves the problem of premature adsorption failure in existing film materials, extending the shelf life of the film's adsorption function from one month to over six months. Through active reinforcement via manual rubbing, it adapts to the differentiated preservation needs of fresh-cut flowers / leaves with varying decay rates, broadening the film's applicability. Furthermore, the core-shell type adsorption filler has a simple preparation process, is fully compatible with existing blending and casting methods, requires no new core equipment, only an additional coating step, making industrialization difficult. The nano-coating layer uses biodegradable materials, the entire film is biodegradable, heavy metal ion migration meets standards, and it is safe, environmentally friendly, and produces no secondary pollution. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] This invention provides a fresh-cut leaf preservation packaging material based on nanotechnology, the raw materials of which include 100 parts by weight of compound polysaccharide, 2-4 parts by weight of core-shell type adsorption functional filler, 1-2 parts by weight of nano silver, 3-4 parts by weight of nano ZnO, 3-5 parts by weight of nano cellulose, and 20-30 parts by weight of plasticizer.

[0027] The compound polysaccharide is a compound system of chitosan with starch, kudzu root powder or sodium alginate, with a compound mass ratio of 3:1 to 5:1; the core-shell type adsorption functional filler consists of an adsorption functional core and a nano-coating layer. The adsorption functional core is ZIF-8 with a particle size of 50-200 nm, or a compound filler of ZIF-8 and montmorillonite (montmorillonite content 10-20 wt%); the nano-coating layer is a force-induced rupture biodegradable material with a thickness of 10-20 nm, selected from gelatinized starch nanospheres, low molecular weight polylactic acid nanoshells or gelatin-sodium alginate nanocomposite layers (gelatin to sodium alginate mass ratio 1:1), and the rupture threshold of the nano-coating layer is 3-10 N; the plasticizer is glycerol or sorbitol, the molecular weight of low molecular weight polylactic acid is 1000-3000, and the overall particle size of the core-shell type adsorption functional filler is 60-220 nm. It is uniformly dispersed in the compound polysaccharide matrix without agglomeration.

[0028] In addition, the preparation method of the fresh-cut leaf preservation packaging material based on nanotechnology in this invention includes the following steps:

[0029] S1. Dissolving the compound polysaccharide: Add the compound polysaccharide to a 1% to 2% dilute acetic acid solution in a certain proportion, stir in a water bath at 40 to 50°C for 30 to 60 minutes until completely dissolved, add the plasticizer, and continue stirring for 15 minutes to obtain the polysaccharide sol;

[0030] S2. Preparation of core-shell type adsorption functional filler: The adsorption functional core is dispersed in a nano-coating material solution, and ultrasonic emulsification is performed at 300-500W for 20-30 minutes to uniformly coat the surface of the adsorption functional core with the nano-coating layer. After filtration and drying, a core-shell type adsorption functional filler is obtained. The solid content of the nano-coating material solution is 8%-12%.

[0031] S3. Multi-filler co-dispersion: Mix core-shell type adsorption functional filler, nano silver, nano ZnO and nano cellulose in a certain proportion, add deionized water, and sonicate for 20-30 min to form a functional filler suspension.

[0032] S4. Blending and homogenization: The functional filler suspension is slowly added dropwise to the polysaccharide sol, and magnetic stirring (300-500 r / min) and ultrasonic homogenization are alternated for 15 min to form a composite sol. The alternation cycle of magnetic stirring and ultrasonic homogenization is 3 min stirring + 2 min ultrasonication.

[0033] S5. Casting film: The composite sol is placed in a vacuum degassing chamber and degassed for 10 to 20 minutes under conditions of -0.08 to -0.1 MPa. Then, it is cast on a polytetrafluoroethylene plate with a thickness of 0.1 to 0.2 mm and dried at 40 to 60°C for 12 to 24 hours. After peeling, a fresh-cut leaf preservation packaging material based on nanotechnology is obtained. The moisture content of the dried preservation film is controlled at 8% to 12%.

[0034] The specific raw materials are as follows:

[0035] Chitosan (degree of deacetylation ≥90%) was sourced from Qingdao Yuekang Biotechnology Co., Ltd.

[0036] Starch (corn starch), kudzu root powder, and sodium alginate are sourced from Henan Huayuyuan Biotechnology Co., Ltd.

[0037] ZIF-8 (particle size 50-200nm) and montmorillonite were sourced from Nanjing Xianfeng Nanomaterials Technology Co., Ltd.

[0038] Nano silver (particle size 1050nm), nano ZnO (particle size 2080nm), and nano cellulose (particle size 10-30nm) were sourced from Suzhou Haibo Nanomaterials Co., Ltd.

[0039] Low molecular weight polylactic acid (molecular weight 1000-3000), sourced from Zhejiang Hisun Biomaterials Co., Ltd.;

[0040] Gelatin, glycerin, and sorbitol are all food-grade and sourced from Shanghai Yuanye Biotechnology Co., Ltd.

[0041] Example 1

[0042] A method for preparing a fresh-cut leaf preservation packaging material based on nanotechnology includes the following steps:

[0043] S1 polysaccharide dissolution: Weigh 100g of chitosan and starch at a mass ratio of 4:1, add 1000mL of 1% dilute acetic acid solution, stir in a 45℃ water bath for 40min until completely dissolved, add 25g of glycerol, continue stirring for 15min to obtain polysaccharide sol.

[0044] Preparation of S2 core-shell adsorption functional filler: Weigh 2g ZIF-8 (particle size 50-100nm), disperse it in 100mL of gelatinized starch solution (solid content 10%), emulsify it by ultrasonication at 400W for 25min, filter it, and dry it at 60℃ for 2h to obtain core-shell adsorption functional filler (coating thickness 15nm); Co-dispersion of S3 multiple fillers: Weigh 2g core-shell adsorption functional filler, 1g nano silver, 3g nano ZnO, and 3g nano cellulose, add 200mL deionized water, and homogenize by ultrasonication for 25min to form a functional filler suspension;

[0045] S4 Blending and Homogenization: The functional filler suspension is slowly added dropwise to the polysaccharide sol, and magnetic stirring (400 r / min) and ultrasonic homogenization are alternately performed for 15 min (3 min stirring + 2 min ultrasonication) to form a composite sol; S5 Casting and Film Forming: The composite sol is placed in a vacuum degassing chamber and degassed at -0.09 MPa for 15 min. It is then cast onto a polytetrafluoroethylene plate with a thickness of 0.15 mm, dried at 50 °C for 18 h, and peeled off to obtain a polysaccharide-based nanocomposite food preservation film with mechanics-triggered adsorption function.

[0046] Example 2

[0047] A method for preparing a fresh-cut leaf preservation packaging material based on nanotechnology includes the following steps:

[0048] S1 polysaccharide dissolution: Weigh 100g of chitosan and kudzu root powder at a mass ratio of 5:1, add 1000mL of 2% dilute acetic acid solution, stir in a 50℃ water bath for 30min until completely dissolved, add 30g of sorbitol, and continue stirring for 15min to obtain polysaccharide sol.

[0049] Preparation of S2 core-shell type adsorption functional packing: Weigh 3g of ZIF-8 and montmorillonite compound packing (montmorillonite content 15wt%, ZIF-8 particle size 100-150nm), disperse in 150mL of low molecular weight polylactic acid solution (solid content 12%), ultrasonically emulsify at 500W for 20min, filter, and dry at 60℃ for 2h to obtain core-shell type adsorption functional packing (coating thickness 12nm).

[0050] S3 Multi-filler Co-dispersion: Weigh 3g of core-shell type adsorption functional filler, 1.5g of nano silver, 3.5g of nano ZnO, and 4g of nano cellulose, add 250mL of deionized water, and sonicate for 20min to form a functional filler suspension.

[0051] S4 Blending and Homogenization: The functional filler suspension is slowly added dropwise to the polysaccharide sol, and magnetic stirring (500 r / min) and ultrasonic homogenization are alternated for 15 min (3 min stirring + 2 min ultrasonication) to form a composite sol.

[0052] S5 Casting Film Forming: The composite sol is placed in a vacuum degassing chamber and degassed for 10 min under -0.1 MPa conditions. It is then cast onto a polytetrafluoroethylene plate with a thickness of 0.12 mm and dried at 55 °C for 15 h. After peeling, a polysaccharide-based nanocomposite preservation film with mechanics-triggered adsorption function is obtained.

[0053] Example 3

[0054] A method for preparing a fresh-cut leaf preservation packaging material based on nanotechnology includes the following steps:

[0055] S1 polysaccharide dissolution: Weigh 100g of chitosan and sodium alginate at a mass ratio of 3:1, add 1000mL of 1.5% dilute acetic acid solution, stir in a 40℃ water bath for 60min until completely dissolved, add 20g of glycerol, and continue stirring for 15min to obtain polysaccharide sol.

[0056] Preparation of S2 core-shell type adsorption functional packing: Weigh 4g of ZIF-8 and montmorillonite compound packing (montmorillonite content 20wt%, ZIF-8 particle size 150~200nm), disperse it in 200mL gelatin-sodium alginate solution (gelatin to sodium alginate mass ratio 1:1, solid content 8%), use 300W ultrasonic emulsification for 30min, filter and dry at 60℃ for 2h to obtain core-shell type adsorption functional packing (coating layer thickness 18nm).

[0057] S3 Multi-filler Co-dispersion: Weigh 4g of core-shell type adsorption functional filler, 2g of nano silver, 4g of nano ZnO, and 5g of nano cellulose, add 300mL of deionized water, and sonicate for 30min to form a functional filler suspension.

[0058] S4 Blending and Homogenization: The functional filler suspension is slowly added dropwise to the polysaccharide sol, and magnetic stirring (300 r / min) and ultrasonic homogenization are alternated for 15 min (3 min stirring + 2 min ultrasonication) to form a composite sol.

[0059] S5 Casting Film Forming: The composite sol is placed in a vacuum degassing chamber and degassed for 20 minutes at -0.08 MPa. It is then cast onto a polytetrafluoroethylene plate with a thickness of 0.2 mm and dried at 45°C for 24 hours. After peeling, a polysaccharide-based nanocomposite preservation film with mechanics-triggered adsorption function is obtained.

[0060] Comparative Example 1

[0061] A method for preparing a common polysaccharide-based nanocomposite food preservation film includes the following steps:

[0062] S1 polysaccharide dissolution: Weigh 100g of chitosan and starch at a mass ratio of 4:1, add 1000mL of 1% dilute acetic acid solution, stir in a 45℃ water bath for 40min until completely dissolved, add 25g of glycerol, continue stirring for 15min to obtain polysaccharide sol.

[0063] S2 filler dispersion: Weigh 2g ZIF-8, 1g nano silver, 3g nano ZnO, and 3g nano cellulose, add 200mL deionized water, and sonicate for 25min to form a functional filler suspension.

[0064] S3 Blending and Homogenization: The functional filler suspension is slowly added dropwise to the polysaccharide sol, and magnetic stirring (400 r / min) and ultrasonic homogenization are alternated for 15 min to form a composite sol;

[0065] S4 Casting Film Forming: The composite sol was placed in a vacuum degassing chamber and degassed for 15 minutes at -0.09 MPa. It was then cast onto a polytetrafluoroethylene plate with a thickness of 0.15 mm and dried at 50°C for 18 hours. After peeling, a common polysaccharide-based nanocomposite food preservation film (without nano-coating layer) was obtained.

[0066] Performance testing

[0067] Adsorption function storage stability: The membrane materials of Examples 1-3 and Comparative Example 1 were stored under normal temperature and humidity conditions, and the ethylene adsorption rate was tested at 1 month, 3 months and 6 months, respectively.

[0068] Shelf life test: Fresh-cut roses (slow decay) and fresh-cut lisianthus (fast decay) were packaged with the film materials of Examples 1-3 and Comparative Example 1, respectively, stored at room temperature, and the shelf life (time without obvious yellowing or rotting) was recorded.

[0069] Mechanical property testing: Tensile strength was tested according to standard GB / T1040.3-2006;

[0070] Antibacterial performance test: The diameter of the inhibition zone against Escherichia coli and Staphylococcus aureus was tested using the inhibition zone method;

[0071] Safety testing: Heavy metal ion migration was tested according to standard GB4806.1-2016.

[0072] The test results are shown in the table below:

[0073] Test Item Example 1 Example 2 Example 3 Comparative Example 1 1-month Ethylene Adsorption Rate (%) 92 93 95 88 3-month Ethylene Adsorption Rate (%) 89 90 92 65 6-month Ethylene Adsorption Rate (%) 85 87 89 32 Fresh-cut Rose Shelf Life (days) 10 11 12 6 Fresh-cut Lisianthus Shelf Life (passively triggered, days) 89 10 4 Fresh-cut Lisianthus Shelf Life (rubbing-strengthened, days) 13 14 15 - Tensile Strength (MPa) 16.2 17.5 18.3 15.8 Escherichia coli Inhibition Zone Diameter (mm) 12.5 13.8 14.6 12.3 Staphylococcus aureus Inhibition Zone Diameter (mm) 11.8 13.1 14.0 11.5 Ag⁺ Migration (mg / kg) 0.03 0.04 0.03 0.03 Zn²⁺ Migration (mg / kg) 3.2 3.5 3.8 3.1 surface

[0074] The test results show that the preservation films of Examples 1-3 of the present invention still maintain a high ethylene adsorption rate after 6 months of storage, while the ordinary preservation film of Comparative Example 1 has an ethylene adsorption rate of only 32% after 6 months of storage. This indicates that the core-shell type adsorption functional filler of the present invention can effectively prevent the adsorption function from failing prematurely. In terms of shelf life, Examples 1-3 extend the shelf life of fresh-cut roses by more than 67% compared with Comparative Example 1, and extend the shelf life of fresh-cut lisianthus (passively triggered) by more than 100%, and extend it by more than 225% after rubbing and strengthening. At the same time, the film materials of Examples 1-3 have excellent mechanical properties, antibacterial properties and safety. All indicators are better than or equal to Comparative Example 1, which fully demonstrates the technical advantages of the present invention.

[0075] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A fresh-cut leaf preservation packaging material based on nanotechnology, characterized in that, The raw materials include 100 parts by weight of compound polysaccharide, 2-4 parts by weight of core-shell adsorption functional filler, 1-2 parts by weight of nano silver, 3-4 parts by weight of nano ZnO, 3-5 parts by weight of nano cellulose, and 20-30 parts by weight of plasticizer; the compound polysaccharide is a compound system of chitosan with starch, kudzu root powder, or sodium alginate, with a compounding mass ratio of 3:1 to 5:1; the core-shell adsorption functional filler consists of an adsorption functional core and a nano-coating layer, the adsorption functional core having a particle size of 5 mm. ZIF-8 with a thickness of 0–200 nm, or a composite filler of ZIF-8 and montmorillonite (montmorillonite content 10–20 wt%); the nano-coating layer is a force-induced fracturing biodegradable material with a thickness of 10–20 nm, selected from gelatinized starch nanospheres, low molecular weight polylactic acid nanoshells, or gelatin-sodium alginate nanocomposite layers (gelatin to sodium alginate mass ratio 1:1), and the fracturing threshold of the nano-coating layer is 3–10 N; the plasticizer is glycerol or sorbitol.

2. The nanotechnology-based fresh-cut leaf preservation packaging material according to claim 1, characterized in that, The low molecular weight polylactic acid has a molecular weight of 1000 to 3000.

3. The nanotechnology-based fresh-cut leaf preservation packaging material according to claim 1, characterized in that, The core-shell type adsorption functional filler has an overall particle size of 60-220 nm and is uniformly dispersed in the compound polysaccharide matrix without agglomeration.

4. A method for preparing a fresh-cut leaf preservation packaging material based on nanotechnology, characterized in that, Includes the following steps: S1. Dissolution of compound polysaccharides: Add the compound polysaccharides to a 1%–2% dilute acetic acid solution according to the proportion, stir in a water bath at 40–50℃ for 30–60 min until completely dissolved, add plasticizer, and continue stirring for 15 min to obtain polysaccharide sol; S2. Preparation of core-shell adsorption functional filler: Disperse the adsorption functional cores in a solution of nano-coating material, and use ultrasonic emulsification at 300–500W for 20–30 min to uniformly coat the surface of the adsorption functional cores with nano-coating layer, filter and dry to obtain core-shell adsorption functional filler; S3. Co-dispersion of multiple fillers: Mix the core-shell adsorption functional filler, nano silver, nano ZnO, and nano cellulose according to the proportion... Example: Mix, add deionized water, and sonicate for 20-30 min to form a functional filler suspension; S4. Blend and homogenize: Slowly add the functional filler suspension dropwise to the polysaccharide sol, and alternate between magnetic stirring (300-500 r / min) and sonic homogenization for 15 min to form a composite sol; S5. Casting and film formation: Place the composite sol in a vacuum degassing chamber and degas for 10-20 min under -0.08 to -0.1 MPa conditions, then cast it on a polytetrafluoroethylene plate with a thickness of 0.1-0.2 mm, dry at 40-60℃ for 12-24 h, and peel off to obtain a fresh-cut leaf preservation packaging material based on nanotechnology.

5. The preparation method according to claim 4, characterized in that, The solid content of the nano-coating material solution in S2 is 8% to 12%.

6. The preparation method according to claim 4, characterized in that, The alternation cycle of magnetic stirring and ultrasonic homogenization in S4 is 3 minutes of stirring + 2 minutes of ultrasonication.

7. The preparation method according to claim 4, characterized in that, The moisture content of the dried plastic wrap in S5 is controlled at 8% to 12%.