Preparation method of fresh-keeping packaging film based on radiation refrigeration

Abstract

The invention designs a fresh-keeping packaging film based on radiation refrigeration. The fresh-keeping packaging film comprises an ultraviolet shielding layer, a radiation refrigeration layer, a modified atmosphere layer and an antibacterial and antioxidant layer. And the ultraviolet shielding layer is prepared by selecting a PVA material and adding a lignin-loaded nanofiber tube, so that the photooxidation effect of ultraviolet rays can be effectively reduced. The radiation refrigeration layer utilizes a specific material (such as silicon dioxide and titanium dioxide) to realize an efficient radiation refrigeration effect, and purple cabbage anthocyanin (PKA) is added as a pH response dye. The modified atmosphere layer is composed of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), silica gel nanoparticles (JSNC) and the like, the transmittance of oxygen and carbon dioxide can be effectively controlled, and meanwhile the film is coated with an ethylene scavenger to slow down maturation and decay of food. The antibacterial and antioxidant layer is prepared by mixing melanin, polyphenol, essential oil (such as tea polyphenol and cinnamon essential oil), polycaprolactone (PCL) and gelatin (Gel) through an electrostatic spinning technology, so that the antibacterial and antioxidant layer not only has antioxidant performance, but also can effectively inhibit the growth of microorganisms.

Description

Technical Field

[0001] The present invention relates to the technical field of food preservation, and specifically to a preparation method of a fresh-keeping packaging film based on radiative cooling. Background Art

[0002] The storage temperature significantly affects the spoilage rate and post-harvest life of fresh agricultural products. The respiration of agricultural products themselves will also reduce their nutritional value. At the same time, microbial contamination, oxidation, spoilage, etc. lead to the degradation of the original sensory properties of fresh agricultural products and increase the risk of foodborne diseases.

[0003] Traditional food preservation methods mostly rely on refrigeration equipment, but such equipment usually has high energy consumption and problems such as noise and uneven refrigeration. At the same time, the means of inhibiting food microbial contamination, oxidation, spoilage, etc. in traditional food preservation methods are relatively single and have poor effects. Radiative cooling technology uses the principle of photo-thermal conversion to convert solar radiant energy into a refrigeration effect, and has advantages such as low energy consumption, low noise, environmental protection and reliability. Therefore, applying radiative cooling technology to food fresh-keeping packaging films and attaching some antibacterial agents, antioxidants and adding certain gas regulation means on the fresh-keeping packaging films are expected to provide a more efficient and environmentally friendly solution for the field of food preservation. Summary of the Invention

[0004] Technical Objective: To overcome the deficiencies in the prior art, the present invention designs a fresh-keeping packaging film based on radiative cooling. This packaging film can reduce the temperature of food through radiative cooling. At the same time, the present invention effectively extends the storage time of things through methods such as gas regulation, adding antibacterial agents, antioxidants, etc. Moreover, the present invention uses radiative cooling to cool down, without energy consumption, no noise, and relatively uniform refrigeration, which is a green and environmentally friendly preservation method.

[0005] Technical Solution: To achieve the above objective, the present invention provides a preparation method of a fresh-keeping packaging film based on radiative cooling, which includes the following parts:

[0006] Ultraviolet shielding layer: Selecting PVA material has excellent transparency, wear resistance, thermal stability and high gas barrier performance. At the same time, adding nanofiber tubes loaded with lignin makes it have excellent ultraviolet shielding ability, which can effectively reduce the photo-oxidation effect of ultraviolet rays during storage and transportation, resulting in food spoilage and nutritional loss.

[0007] Radiative cooling layer: Using specific materials, such as silicon dioxide, titanium dioxide, etc., which have high emissivity in the mid-infrared light wavelength range and low absorbance and high transmittance in the visible light wavelength range, so as to achieve an efficient radiative cooling effect. At the same time, adding purple cabbage anthocyanin (PKA) as a pH-responsive dye to better observe the fresh-keeping situation of food.

[0008] Modified atmosphere layer: Composed of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and silica nanoparticles (JSNC), etc., the film not only has excellent mechanical properties but also can effectively control the oxygen and carbon dioxide permeation rates to inhibit the respiration of fruits and the growth of microorganisms. At the same time, an ethylene scavenger (Pt / CeO2 or Pt / DMS) is coated on the film to slow down the ripening and spoilage of food, thereby extending the shelf life of fruits.

[0009] Antibacterial and antioxidant layer: A nanofiber membrane with a high specific surface area and porous structure is prepared by electrospinning a mixed solution of polycaprolactone (PCL) and gelatin (Gel) added with melanin, polyphenols, and essential oils (such as tea polyphenols and cinnamon essential oil), which not only has antioxidant properties but also can effectively inhibit the growth of microorganisms.

[0010] The ultraviolet shielding layer selects a polyvinyl alcohol (PVA) film added with nanofiber tubes loaded with lignin, and the preparation method is as follows:

[0011] 1. Preparation of acetylated lignin

[0012] Lignin is mixed and stirred with acetic anhydride / pyridine solution (pyridine and acetic anhydride are mixed at a volume ratio of 3:10) for 24 h, and then a large amount of distilled water is added. The solid residue is obtained by centrifugation. After washing and drying, acetylated lignin (ALG) is obtained.

[0013] 2. Acidification of halloysite nanotubes

[0014] HNTs are dispersed in an acetic acid aqueous solution, and the dispersion is continuously stirred on a magnetic stirrer at a temperature of 50 °C for 3 d. Subsequently, the mixture is centrifuged and washed with deionized water until the pH value of the supernatant reaches neutral. The etched HNTs obtained after centrifugation are dried at 50 °C - 60 °C for 20 - 30 h, and the obtained product is labeled as e-HNTs.

[0015] 3. Extraction of chitin nanofibers (CNF)

[0016] The chopped Yimu mushroom mycelium is dispersed in an acetic acid aqueous solution. After homogenization treatment with a high-speed disperser, the suspension is transferred to a high-pressure dynamic nano-disperser and treated at 700 bar for 10 - 30 minutes in 5 cycles to obtain a stable CNF suspension, which is stored in the refrigerator for later use.

[0017] 4. Preparation of ALG@e-HNTs

[0018] e-HNTs are slowly dispersed in ALG and subjected to ultrasonic and magnetic stirring to achieve the encapsulation of ALG in e-HNTs. The precipitate is collected from the bottom after centrifugation. After repeated centrifugation and ethanol washing, the obtained ALG@e-HNTs are dried at a certain temperature for standby.

[0019] 5. Preparation of PVA film

[0020] After the CNF suspension is ultrasonically treated, TA is added, and after being fully mixed, the modified ALG@e-HNTs are added. The solution is mixed evenly under high-speed shearing, and a modified PVA film is obtained by spraying.

[0021] The radiation cooling layer is a titanium dioxide film prepared by electrostatic spinning technology, and the preparation method is as follows:

[0022] 1. Preparation of spinning solution

[0023] Tetrabutyl titanate, anhydrous ethanol, hydrochloric acid and water are mixed evenly in a certain molar ratio (e.g., the molar ratio of tetrabutyl titanate to anhydrous ethanol: 1:1.13-5; the concentration of hydrochloric acid: 37%; the molar ratio of water to tetrabutyl titanate: 3-5:1). Stir at room temperature until the solution is completely clarified, then add purple cabbage anthocyanin (PKA), keep warm at an appropriate temperature (35-60°C) and continue stirring for a period of time (12-24 hours) to ensure the uniformity and stability of the spinning solution.

[0024] 2. Electrospinning

[0025] Introduce the prepared spinning solution into the electrospinning device. Adjust the distance between the spinneret and the metal target (10-20 cm) and the voltage (15-30 kV) to obtain a stable and uniform jet split flow. After spinning for a period of time, when the fiber film on the metal target reaches a certain thickness, stop spinning.

[0026] 3. Post-treatment of fiber membrane

[0027] The obtained fiber membrane is kept at a certain temperature (90-110°C) for a period of time (more than 3 hours) to remove residual solvent and moisture. Then the fiber membrane is placed in a muffle furnace or other equipment that can reach the required temperature, and kept at an appropriate temperature (350-550°C) for a period of time (more than 3 hours) for heat treatment. Controlling the heating rate is also a key step, usually controlled at 4-6°C / min.

[0028] The gas conditioning layer is a composite film composed of polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and silica gel nanoparticles (JSNC), etc., and coated with an ethylene scavenger (Pt / CeO2 or Pt / DMS), and the preparation method is as follows:

[0029] 1. Synthesis of Silica Nanoparticles (SNPs)

[0030] Mix a certain proportion of absolute ethanol, ammonia water (about 30% aqueous solution), and deionized water, and stir for 10 - 20 minutes to ensure complete mixing of the solution. Add tetraethyl orthosilicate (TEOS) to the above solution and react at 30°C - 50°C. After reacting for 3 hours, add TEOS again and continue the reaction to synthesize SNPs. Wash the synthesized SNPs with absolute ethanol by high-speed centrifugation, and then dry them in an oven at 50°C - 70°C to prevent further growth of the particles.

[0031] 2. Synthesis of silica-coated jute fiber nanocomposite (JSNC)

[0032] (1) Pretreatment of jute fiber:

[0033] Clean and shear the jute fiber (JF), wash it with deionized water multiple times, then treat it with a 10% alkaline solution at 50°C - 70°C for 2 - 3 hours to remove impurities and partially delignify, wash it with deionized water and dry it at room temperature.

[0034] (2) Silica coating:

[0035] Mix deionized water, jute fiber, and NaOH, stir vigorously at 80°C - 90°C for more than 30 minutes. Add TEOS (the mass ratio of jute fiber to TEOS is 20 - 25:1), and incubate at 80°C - 90°C for 2 - 3 hours. Centrifuge the final composite material, wash it with deionized water multiple times, and then dry it in an oven at 60°C.

[0036] (3) Ball milling treatment:

[0037] Put the dried composite material into a ball mill and grind it in the ball mill tank at a speed of 200 rpm - 500 rpm for 6 hours. Take out the ground powder from the ball mill tank and store it for subsequent use.

[0038] 3. Preparation of hybrid nanocomposite film

[0039] Dissolve PVA and PVP powders in deionized water respectively, and make a PVA / PVP solution by stirring in a water bath. Mix the PVA solution and PVP solution according to a certain weight ratio to prepare a PVA / PVP double polymer solution. Add the SNPs and JSNC solutions to the PVA / PVP solution to obtain a mixed solution with a final concentration of 0.5%. Stir the mixed solution at 20°C - 30°C for 30 - 60 minutes, and then use ultrasonic treatment to remove bubbles. Pour the solution onto a plastic plate to form a film and dry it overnight in a vacuum oven.

[0040] 4. Spraying of ethylene remover

[0041] (1) Dissolve zein in 80% ethanol to prepare a zein solution. Use a coater to evenly coat the zein solution on the surface of the film. Zein, as an adhesive, can firmly fix the ethylene scavenger on the film surface.

[0042] (2) Spray the ethylene scavenger (Pt / CeO2 or Pt / DMS) on the film surface coated with zein through a powder sprayer. Hot press at 30 °C to 50 °C for 5 to 10 minutes to ensure that the ethylene scavenger is tightly bound to zein and prevent it from falling off during storage and use.

[0043] (3) Peel the obtained film from the plastic plate and condition it in an environment of 27 °C and 50% relative humidity for at least 48 hours.

[0044] The antibacterial and antioxidant layer is a composite nanofiber membrane of polycaprolactone (PCL) and gelatin (Gel) added with melanin, polyphenols, and essential oils (such as tea polyphenols and cinnamon essential oil). The preparation method is as follows:

[0045] 1. Extraction, hydrolysis, and purification of melanin

[0046] (1) Extraction of melanin

[0047] Wash the pecan shells with water and add ammonia water (NH3·H2O) to adjust the pH to 10.5 (final concentration 2 g / 100 g). Stir at 200 rpm to 300 rpm for 24 hours. Then filter the extract through a nylon membrane and use HCl to adjust the pH of the filtrate to 2.5. After filtration, crude melanin is obtained.

[0048] (2) Hydrolysis and purification of melanin

[0049] Hydrolyze the crude melanin in HCl at 100 °C and then centrifuge at 10000 rpm to obtain a precipitate. Resuspend the precipitate in NH3 and wash it successively with chloroform (>99.5%) and ethyl acetate (≥99.5%). Acidify the supernatant with HCl to pH 2.5 and filter to obtain purified melanin.

[0050] 2. Preparation of PCL / gelatin / melanin fibers

[0051] (1) Solution preparation

[0052] Dissolve PCL and gelatin in a mixed solution of glacial acetic acid (AA) and formic acid (FA) respectively and stir at room temperature for 24 hours. Then mix the PCL solution and the gelatin solution in a ratio of 1:1 (w / w) and stir at room temperature for 1 to 3 hours.

[0053] (2) Electrospinning process

[0054] A certain amount of tea polyphenols, cinnamon essential oil, and melanin were added to the PCL / gelatin mixed solution respectively. The polymer solution was injected into the electrospinning equipment at a flow rate of 0.3 - 0.5 mL / h. The distance between the nozzle and the collector was 15 cm, and the applied voltage was 10 kV - 20 kV. The collector was a rotating drum covered with an aluminum film, and the rotation speed was 300 rpm.

[0055] The obtained fiber membrane was kept at a certain temperature (such as 90°C - 110°C) for a certain period of time (such as more than 3 hours) to remove the residual solvent and moisture.

[0056] Laminating and curing:

[0057] 1. Use a coater or a doctor blade to evenly coat the surface of each film with an adhesive (such as maleic acid modified vinyl chloride resin, polyurethane aluminum plastic adhesive, etc.).

[0058] 2. Preheat the film coated with the adhesive to improve the bonding effect and composite quality. Under the conditions of heating and pressurization, the films were thermally laminated layer by layer to form a composite film. The composite air pressure was generally adjusted to 0.4 MPa - 0.7 MPa, and the temperature of the composite roller was adjusted to 60°C - 80°C.

[0059] 3. The film was cooled by a cooling roller to fix the bonding effect and composite quality.

[0060] 4. The composite film was cured in a curing chamber to improve the curing degree of the adhesive and the physical properties of the composite film. The curing temperature and time should be set according to the type of adhesive and the requirements of the composite film, generally curing at 20°C - 30°C for 24 h - 48 h. Description of the drawings

[0061] Figure 1 Schematic diagram of the radiation cooling film structure of an embodiment provided by the present invention

[0062] Reference numerals: 1, ultraviolet shielding layer; 2, radiation cooling layer; 3, gas modulation layer; 4, antibacterial and antioxidant layer. Detailed implementation manners

[0063] The following provides an embodiment in combination with the content of the present invention, but the present invention is not limited to this embodiment.

[0064] The ultraviolet shielding layer selects a polyvinyl alcohol (PVA) film added with nanofiber tubes loaded with lignin, and the preparation method is as follows:

[0065] 1. Preparation of acetylated lignin

[0066] Mix 1 g of lignin with 13 mL of acetic anhydride / pyridine solution (volume ratio 10:3), stir at a speed of 600 rpm at room temperature for 24 hours, then add a large amount of distilled water, and centrifuge to obtain a solid residue. Wash and dry to obtain acetylated lignin (ALG).

[0067] 2. Acidified halloysite nanotubes

[0068] Disperse 5 g of HNTs in 100 mL of acetic acid aqueous solution. The dispersion is continuously stirred on a magnetic stirrer at a temperature of 50 °C for 3 d. Subsequently, centrifuge the mixture, and thoroughly wash the HNTs with deionized water until the pH value of the supernatant reaches neutral. The etched HNTs obtained after centrifugation are dried at 60 °C for 1 day, and the resulting product is labeled as e-HNTs.

[0069] 3. Extraction of chitin nanofibers (CNF)

[0070] Weigh 5 g of chopped Yimu mushroom mycelium and disperse it in 100 mL of acetic acid aqueous solution. Use a high-speed disperser to homogenize at a speed of 20000 rpm for 30 minutes. Subsequently, transfer the suspension to a high-pressure dynamic nano-disperser and process it at 700 bar for 20 minutes in 5 cycles to obtain a stable CNF suspension, which is then stored in a refrigerator at 5 °C for later use.

[0071] 4. Preparation of ALG@e-HNTs

[0072] Slowly disperse 1.5 g of e-HNTs in 6 g of ALG, and then perform ultrasonic and magnetic stirring to achieve the encapsulation of ALG in e-HNTs. Centrifuge the resulting mixture at 7000 rpm and collect the precipitate from the bottom. After repeated centrifugation and ethanol washing, the obtained ALG@e-HNTs are dried at 70 °C for 3 h for later use.

[0073] 5. Preparation of PVA film

[0074] Ultrasonically treat 20 mL of 0.05 g / mL CNF suspension, add 40 mg of TA, mix well, and then add 3 g of modified ALG@e-HNTs. Uniformly mix the solution under high-speed shearing at 10000 rpm for 5 min, and obtain a modified PVA film by spraying method (equipment parameters: pressure is 0.1 bar, nozzle size is 1.0 mm, flow rate is 350 mL / min, spray distance is 35 cm).

[0075] The radiation cooling layer is selected as a titanium dioxide film prepared by electrospinning technology, and the preparation method is as follows:

[0076] 1. Mix 2 mol of tetrabutyl titanate, absolute ethanol, 37% hydrochloric acid, and water in a ratio of 3:6:5:1 evenly. Stir at room temperature until the solution becomes completely clear, then add 3% purple cabbage anthocyanin (PKA), keep it warm at 45 °C and continuously stir for 15 hours to ensure the uniformity and stability of the spinning solution.

[0077] 2. Introduce the prepared spinning solution into an electrospinning device. Adjust the distance between the spinneret and the metal target to 15 cm and the voltage to 20 kV to obtain a stable and uniform jet breakup flow. After spinning for a period of time, when the fiber membrane on the metal target reaches 80 μm, stop spinning.

[0078] 3. Keep the obtained fiber membrane warm at 100 °C for 4 hours to remove residual solvents and moisture. Then place the fiber membrane into a muffle furnace or other equipment capable of reaching the required temperature, keep it warm at 350 °C for 3.5 hours (heating rate is 4 °C / min) for heat treatment.

[0079] The gas barrier layer is a composite film composed of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and silica nanoparticles (JSNC), etc., coated with an ethylene scavenger (Pt / CeO2 or Pt / DMS). The preparation method is as follows:

[0080] 1. Synthesis of silica nanoparticles (SNPs)

[0081] (1) Mix 50 mL of absolute ethanol, 2 mL of ammonia water (about 30% aqueous solution), and 1 mL of deionized water, and stir for 10 minutes to ensure complete mixing of the solution.

[0082] (2) TEOS addition: Add 1.5 mL of tetraethyl orthosilicate (TEOS) to the above solution and react at 40 °C.

[0083] (3) Reaction progress: After reacting for 3 hours, add another 1 mL of TEOS and continue the reaction to synthesize SNPs.

[0084] (4) Purification: Wash the synthesized SNPs with absolute ethanol by high-speed centrifugation (13000 rpm, 15 minutes), and then dry them in an oven at 60 °C for 3 hours to prevent further growth of the particles.

[0085] 2. Synthesis of silica-coated jute fiber nanocomposites (JSNC)

[0086] (1) Jute fiber pretreatment:

[0087] The jute fibers (JF) were manually cleaned from agricultural waste and cut into lengths of approximately 6 mm. The jute fibers were washed several times with deionized water. The jute fibers were treated with a 10% alkaline solution at 60 °C for 2 hours to remove impurities and partially delignify. They were washed three times with deionized water (30 °C) and dried at room temperature.

[0088] (2) Silica coating:

[0089] 96 mL of deionized water, 400 mg of treated jute fibers, and 1.4 mL of NaOH were mixed and stirred vigorously at 80 °C for 40 minutes. 0.02 mL of TEOS was added and incubated at 80 °C for 150 minutes. The final composite was centrifuged, washed three times with deionized water (10000 rpm, 10 minutes), and then dried in an oven at 60 °C.

[0090] (3) Ball milling treatment:

[0091] The dried composite was placed in a ball mill and milled in a 250 mL ball mill jar at a speed of 300 rpm for 6 hours, with a 15-minute interval every 30 minutes. The milled powder was taken out of the ball mill jar and stored for subsequent use.

[0092] 3. Preparation of hybrid nanocomposite film

[0093] (1) Preparation of PVA solution:

[0094] 10 g of PVA powder was dissolved in 100 mL of deionized water and stirred in a water bath at 70 °C for 1 hour to prepare a 10% PVA solution.

[0095] (2) Preparation of PVP solution:

[0096] 1 g of PVP powder was dissolved in 100 mL of deionized water and stirred in a water bath at 70 °C for 1 hour to prepare a 1% PVP solution.

[0097] (3) Preparation of PVA / PVP mixed solution:

[0098] The PVA solution and the PVP solution were mixed in a weight ratio of 6:4 to prepare a PVA / PVP dual-polymer solution.

[0099] (4) Addition of nanoparticles:

[0100] SNPs and JSNC solution were added to the PVA / PVP solution to obtain a mixed solution with a final concentration of 0.5%.

[0101] (5) Solution mixing and degassing:

[0102] Stir the mixed solution at 27 °C for 40 minutes, and then use ultrasonic treatment for 40 minutes to remove air bubbles.

[0103] (6) Casting and drying of the thin film:

[0104] Pour 30 mL of the thin film forming solution onto a 20×20 cm plastic plate and dry it overnight at 38 °C in a vacuum oven.

[0105] 4. Spraying of ethylene remover

[0106] (1) Dissolve zein in 80% ethanol to prepare a 10% zein solution. Use a coater to uniformly coat the zein solution on the surface of the thin film, and the coating amount is 6 g / m 2 . Zein, as an adhesive, can firmly fix the ethylene scavenger on the surface of the thin film.

[0107] (2) Spray the ethylene scavenger (Pt / CeO2 or Pt / DMS) onto the surface of the thin film coated with zein through a powder sprayer. Hot press at 45 °C for 5 minutes to ensure that the ethylene scavenger is tightly combined with zein and prevent it from falling off during storage and use.

[0108] (3) Post-treatment of the thin film:

[0109] Peel the obtained thin film from the plastic plate and condition it in an environment of 27 °C and 50% relative humidity for at least 48 hours.

[0110] The antibacterial and antioxidant layer is a composite nanofiber membrane of polycaprolactone (PCL) and gelatin (Gel) added with melanin, polyphenols, and essential oils (such as tea polyphenols and cinnamon essential oil). The preparation method is as follows:

[0111] 1. Extraction, hydrolysis, and purification of melanin

[0112] (1) Extraction of melanin

[0113] Wash the pecan shells in water at a ratio of 1:10. Add 10 g / 100 g of ammonia water (NH3·H2O) to the washed pecan shells and adjust the pH to 10.5 (final concentration of 2 g / 100 g). Stir at 200 rpm for 24 hours. Then filter the extract through a 0.45 μm nylon filter membrane and adjust the pH of the filtrate to 2.5 with 2M HCl. Coarse melanin is obtained after filtration.

[0114] (2) Hydrolysis and purification of melanin

[0115] Hydrolysis: Hydrolyze the coarse melanin in 7M HCl at 100 °C for 6 hours.

[0116] Centrifugation: Centrifuge the hydrolyzed solution at 10,000 rpm to obtain a precipitate.

[0117] Resuspension: Resuspend the precipitate in 5.28 M NH₃.

[0118] Washing: Wash successively with chloroform (>99.5%) and ethyl acetate (≥99.5%).

[0119] Acidification and filtration: Acidify the supernatant with 1 M HCl to pH 2.5 and filter to obtain purified melanin.

[0120] 2. Preparation of PCL / gelatin / melanin fibers

[0121] (1) Solution preparation

[0122] Dissolve 15 wt% of PCL and gelatin separately in a mixed solution of glacial acetic acid (AA) and formic acid (FA) (9:1, w / w) and stir at room temperature for 24 hours. Then mix the PCL solution and the gelatin solution in a ratio of 1:1 (w / w) and stir at room temperature for 2 hours.

[0123] (2) Electrospinning process

[0124] Add 0.3% (w / v) of tea polyphenols, 0.1% (v / v) of cinnamon essential oil and 3.0 wt% of melanin to the PCL / gelatin mixed solution. Inject 6 mL of the polymer solution into the electrospinning equipment at a flow rate of 0.3 mL / h. The distance between the nozzle and the collector is 15 cm, and the applied voltage is 15 kV. The collector is a rotating drum covered with an aluminum film, and the rotation speed is 300 rpm.

[0125] Keep the obtained fiber membrane at 100 °C for 3 hours at a certain temperature to remove residual solvents and moisture.

[0126] Lamination and curing:

[0127] 1. Use a coater or a doctor blade to uniformly coat the surface of each film with an adhesive (such as maleic acid modified vinyl chloride resin, polyurethane aluminum plastic adhesive, etc.).

[0128] 2. Preheat the film coated with the adhesive to improve the adhesion effect and the composite quality. Under heating and pressurizing conditions, thermally laminate the films layer by layer to form a composite film. The composite air pressure is generally adjusted to 0.5 MPa, and the temperature of the composite roller is adjusted to 70 °C.

[0129] 3. Cool the film through a cooling roller to fix the adhesion effect and the composite quality.

[0130] 4. Cure the composite film in a curing chamber at 30 °C for 24 hours to improve the curing degree of the adhesive and the physical properties of the composite film.

[0131] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. The fresh-keeping packaging film based on radiative refrigeration is characterized in that: Its main constituent modules include an ultraviolet shielding layer (1), a radiative cooling layer (2), a gas modulation layer (3), and an antibacterial and antioxidant layer (4).

2. The fresh-keeping packaging film based on radiative cooling according to claim 1, wherein: The ultraviolet shielding layer (1) selects lignin to absorb ultraviolet rays and uses acidified halloysite nanotubes (e-HNTs) to carry acetylated lignin (ALG), and the mixing ratio is e-HNTs:ALG = 1:

4.

3. The fresh-keeping packaging film based on radiative cooling according to claim 1, wherein: For the radiative cooling functional layer (2), the titanium dioxide cooling film material is uniformly mixed according to the ratio of tetrabutyl titanate: absolute ethanol: 37% hydrochloric acid: water = 3:6:5:1, and the addition amount of purple cabbage anthocyanin (PKA) is 3% - 5%.

4. The fresh-keeping packaging film based on radiative cooling according to claim 1, wherein: The gas modulation layer (3) is composed of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and silica gel nanoparticles (JSNC), etc., where PVA:PVP = 10 - 20:

1.

5. The modified atmosphere layer (3) according to claim 4, characterized in that: The silica gel nanoparticles (JSNC) are prepared by treating jute fibers with an alkaline solution to remove impurities and partially delignify them, and then reacting with tetraethyl orthosilicate (TEOS) by centrifugation, drying, and grinding (where the mass ratio of jute fibers to TEOS is 20 - 25:1).

6. The fresh-keeping packaging film based on radiative cooling according to claim 1, wherein: The antibacterial and antioxidant layer (4) is made of a mixed solution of polycaprolactone (PCL) and gelatin (Gel) added with melanin, polyphenols, and essential oils (such as tea polyphenols and cinnamon essential oil). Among them, the 15wt% PCL solution and the gelatin solution are mixed in a ratio of 1:1 (w / w), and the ratio of tea polyphenols, cinnamon essential oil, and melanin is 3:1:30.

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