A nanocellulose-based highly transparent composite barrier film and its preparation method

Abstract

The present invention discloses a nanocellulose-based highly transparent composite barrier film and a preparation method thereof. The method comprises the following steps: uniformly dispersing a TEMPO-oxidized nanocellulose gel in deionized water, uniformly mixing the dispersion with a PVA solution, and adding a sodium montmorillonite suspension and a boric acid solution to mix evenly. The mixed solution is vacuum degassed and then dried at room temperature, 42-48°C, and 56-63°C, respectively. The composite film is immersed in a polymethyl methacrylate solution, removed to remove excess solution and solidified, sprayed with a modified nanosilica solution, and then dried at room temperature and 80-90°C, respectively, to obtain the nanocellulose-based highly transparent composite barrier film. The nanocellulose-based highly transparent composite barrier film achieves uniform compounding of organic and inorganic components while forming a high barrier and well-compatible imitation brick-mud structure, which has high transparency and good mechanical properties.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of biomass-based nanocomposite materials, and specifically relates to a nanocellulose-based highly transparent composite barrier film and a preparation method thereof. Background Art

[0002] In recent years, the demand for encapsulation materials has been increasing, and a growing number of them are being used in a wide range of applications, such as food preservation, pharmaceutical encapsulation, electronic component packaging for biochemical protection, OLED packaging, and in aerospace and military applications. Encapsulation of materials maintains the factory quality of the contents, preventing performance damage from environmental influences, and their exceptional barrier properties can extend the material's service life. Barrier properties represent a material's ability to block penetrating objects, such as oxygen and water vapor, from diffusing from high to low concentrations due to concentration differences. The most commonly used encapsulation materials rely on excellent gas, moisture, and UV resistance. Furthermore, high light transmittance and strong mechanical properties are also key criteria for barrier materials in electronics and photovoltaic applications.

[0003] Oxygen and moisture barrier properties are among the most essential of many barrier properties. The permeation process of oxygen and moisture barrier materials is primarily divided into three stages: adsorption from one surface, diffusion through the middle, and desorption from the other surface. This process primarily increases the diffusion path for foreign molecules by reducing permeability, moisture sensitivity, and the free volume created by free groups within the material. Barrier properties are related to the material's inherent molecular polarity, molecular orientation, molecular hygroscopicity, and crystallinity. Particularly in the emerging field of flexible electronics, high-oxygen and moisture barrier films can effectively block water vapor and oxygen, ensuring the stability of flexible devices, extending their service life, and enhancing their impact resistance. However, the oxygen and water vapor barrier requirements for solar cell and OLED encapsulation are far higher than those for food and pharmaceuticals, a key obstacle to the successful application of solar cells and OLEDs in next-generation flexible electronics and displays. Therefore, research on high-oxygen and moisture barrier and highly transparent films is crucial. The performance of barrier films is closely related to the material itself. High-barrier matrices often possess high crystallinity, a regular molecular chain structure, low molecular polarity, and low moisture sensitivity. The greater the molecular polarity, the less likely non-polar oxygen molecules are to pass through, resulting in a higher gas barrier property. The greater the material's moisture resistance, the less likely it is to interact with water molecules in the environment, and the higher its water vapor barrier properties. However, the active groups in highly moisture-sensitive materials improve oxygen resistance but reduce water vapor resistance, which is a major challenge in the current widespread replacement of petroleum-based barrier films with bio-based materials.

[0004] While the use of petroleum-based polymers can impart high barrier properties, flexibility, and hydrophobicity to materials, environmental constraints on fossil fuel resources, poor recyclability, and waste generation are increasingly problematic. Packaging waste made from petroleum-based polymers accounts for over 40% of plastic waste, contributing to significant pollution and exacerbating the marine crisis. Furthermore, these microplastics can enter the terrestrial food chain and ultimately the human body, posing a threat to human health. The demand for sustainability and existing regulations prohibiting the use of single-use plastics are forcing the search for alternative solutions. In recent years, cellulose, with its abundant resources, renewable nature, and excellent barrier properties, has been widely used in the preparation of flexible functional composite membranes and is often employed as an advanced biomass material in various cutting-edge applications. Cellulose is a polymer rich in hydroxyl sites. The cellulose backbone's β-(1,4) glycosidic bonds, through their long chains or polycyclic structures, provide sufficient structural rigidity and build a stable network. The hydroxyl groups at positions 2, 3, and 6 can not only form intramolecular and intermolecular hydrogen bonds themselves, but can also form hydrogen, ionic, and covalent bonds with other polymers, such as amino-rich proteins, chitosan, and hydroxyl-containing polyvinyl alcohol. This increases the density of the hybrid network, reduces internal free volume and gas diffusion rate, thereby extending the diffusion path of gases within the barrier material and reducing the permeation of water vapor and oxygen. Nanosilver particles, silica nanospheres, and montmorillonite nanosheets can all be incorporated into the cellulose network. By virtue of their dispersibility and hydrogen bonding with cellulose molecular chains, they enhance the barrier properties of the network structure and reduce internal defects. Inorganic particles, due to their inherent lattice structure, are naturally impermeable to water vapor and oxygen. From zero-dimensional to two-dimensional inorganic materials, most exhibit barrier properties to gases; the accumulation of crystals contributes to the formation of a stable barrier. A well-compatible barrier membrane structure has low porosity and high density. Furthermore, inorganic materials must be evenly dispersed, minimizing stacking and uniformly loading the cellulose polymer chains, thus reducing defects in the composite membrane's internal brick-and-mortar structure. When cellulose forms strong interactions with other polymer molecules, it can bind inorganic materials and reduce the free volume within the membrane material.

[0005] Cellulose-based barrier films primarily consist of organic polymers and functional fillers, forming a hybrid network structure that controls the adsorption and desorption of water vapor and oxygen on the exterior of the packaging film, as well as the diffusion rate within it. The first stage of surface adsorption is likely related primarily to the hydrophilicity of the barrier film surface. Although this theory has not been fully confirmed, some research suggests that hydrophobic modification of the membrane surface is effective. The more hydrophobic the membrane, the less likely liquid water is to adhere, and the less likely gaseous water vapor molecules are to form hydrogen bonds with the membrane surface. This prevents the membrane's hydrogen bonds from breaking, creating pores or defects that provide permeation pathways for water vapor and oxygen. Therefore, the nanoparticles or polymers on the membrane surface form a micro-nanohydrophobic structure that mimics the surface of a lotus leaf, endowing the barrier film with a lower surface energy and thus reducing surface adsorption. The second stage, molecular diffusion, is primarily determined by the membrane's internal structure, namely, its free volume and internal binding energy. When polymers or fillers are combined with micro- and nano-cellulose, the micro- and nano-structures themselves possess a high specific surface area. Nanocellulose also possesses a high aspect ratio. Through cross-linking, electrostatic bonding, or hydrogen bonding, they form a dense network or layered nano-brick-and-mud structure. This extends the gas diffusion path and reduces its diffusion rate within the barrier membrane. The brick-and-mud structure, composed of cellulose polymers and inorganic nanosheets or spherical particles within the barrier membrane material, forms a critical barrier to water vapor and oxygen.

[0006] However, there is currently no report on a novel method for preparing nanocellulose-based barrier films to improve the compatibility between multi-component materials and ensure a balance between the mechanical properties and barrier properties of the composite films while achieving high barrier properties. Summary of the Invention

[0007] In response to the problems existing in the prior art, the present invention provides a nanocellulose-based highly transparent composite barrier film and a preparation method thereof, which achieves uniform compounding of organic and inorganic components while forming a high barrier and well-compatible imitation brick-mud structure with high transparency and good mechanical properties.

[0008] In order to achieve the above purpose, the present invention adopts the following technical solutions to achieve

[0009] A method for preparing a nanocellulose-based highly transparent composite barrier film comprises the following steps:

[0010] S1, uniformly dispersing a TEMPO-oxidized nanocellulose gel in deionized water to obtain a TEMPO-oxidized nanocellulose dispersion, then uniformly mixing the dispersion with a PVA solution, wherein the ratio of PVA to TEMPO-oxidized nanocellulose gel is (0.24-0.36) g: (24-36) mL, and finally adding a sodium montmorillonite suspension and a boric acid solution and uniformly mixing, wherein the boric acid accounts for 8% by weight of the PVA and the sodium montmorillonite accounts for 10% by weight of the total weight of the PVA and TEMPO-oxidized nanocellulose gel, to obtain a mixed solution;

[0011] S2, vacuum degassing the mixed solution and drying it at room temperature, 42-48° C., and 56-63° C. in sequence to obtain a composite film, immersing the composite film in a polymethyl methacrylate solution, removing excess polymethyl methacrylate solution, and then curing the composite film to form a PMMA coating on the composite film;

[0012] S3, spraying a modified nano-silica solution on the PMMA coating, wherein the modified nano-silica solution is obtained by replacing the silicon-oxygen-hydrogen bonds on the silica with the alkyl chains on the hexamethyldisilazane, and then drying at room temperature and 80-90°C in turn to obtain a nanocellulose-based highly transparent composite barrier film.

[0013] Preferably, S1 is to ultrasonicate the TEMPO-oxidized nanocellulose gel in deionized water for 25 to 35 minutes and stir at 500 to 800 rpm / min for 0.5 to 1.5 hours to obtain a TEMPO-oxidized nanocellulose dispersion with a mass percentage of 0.5%.

[0014] Preferably, S1 heats a mixture of PVA and deionized water in an oil bath at 80-90° C. to form a transparent solution with a mass percentage of 3.5%, then cools to room temperature, and vacuum degasses for 3-5 times to obtain a PVA solution.

[0015] Preferably, S1 disperses sodium montmorillonite in deionized water, stirs at a rate of 900-1100 rpm / min for 0.5-1.5 h, then ultrasonicates for 25-35 min, and centrifuges at a rate of 2500-3500 rpm / min for 8-12 min to remove the lower layer of particles to obtain a supernatant, and stirs, ultrasonicates, and centrifuges the supernatant to obtain a sodium montmorillonite suspension with a concentration of 6.7 mg / mL.

[0016] Preferably, S1 stirs the TEMPO-oxidized nanocellulose dispersion and PVA solution at a rate of 700-900 rpm / min for 2.5-3.5 hours at 40-60°C, and adds a boric acid solution with a concentration of 0.1 mol / L. After adding the boric acid solution and Na-MMT suspension, stir for 4.5-5.5 hours, and then stir at a rate of 400-600 rpm / min for 60-120 minutes to obtain a mixed solution.

[0017] Preferably, S2 vacuum degasses the mixed solution and then dries it at room temperature for 22 to 26 hours, then dries it at 42 to 48° C. for 22 to 26 hours, and finally dries it at 56 to 63° C. for 45 to 50 hours to obtain a composite membrane.

[0018] Preferably, S2 dissolves 25 g of polymethyl methacrylate particles in 100 mL of N,N-dimethylformamide by ultrasonication for 2.5 to 3.5 hours to form a polymethyl methacrylate solution, and the composite film is immersed in the solution for 8 to 12 seconds. After removing excess polymethyl methacrylate solution, the composite film is cured at 80 to 90° C. for 20 to 25 minutes to form a PMMA coating on the composite film.

[0019] Preferably, the modified nano-silica solution described in S3 is obtained by the following process:

[0020] Dissolve 4 mL of ethyl orthosilicate in 60 mL of anhydrous ethanol, drop 3.2 mL of hexamethyldisilazane and react at room temperature for 8 to 10 hours. After terminating the reaction, let it stand for 64 to 80 hours to obtain a modified nano-silica solution.

[0021] Furthermore, in S3, the modified nano-silica solution is diluted with 2 to 4 times the volume of anhydrous ethanol and then sprayed on the PMMA coating, and then dried at room temperature and 80 to 90° C. in sequence.

[0022] A nanocellulose-based highly transparent composite barrier film obtained by any one of the above methods for preparing a nanocellulose-based highly transparent composite barrier film.

[0023] Compared with the prior art, the present invention has the following beneficial technical effects:

[0024] The present invention discloses a method for preparing a nanocellulose-based highly transparent composite barrier film. The cross-linking synergistic surface modification method is used to improve the barrier properties and comprehensive performance of the composite film. The surface of the composite film is impregnated with polymethyl methacrylate (PMMA) and sprayed with a modified nanosilica solution. After curing and spraying, a multilayer structure is assembled. The PMMA moisture-resistant coating can fill pores, reduce the permeation of water vapor and oxygen, and the polarity of surface hydroxyl groups, providing a protective barrier. The multilayer structure is constructed to improve the surface moisture sensitivity of the composite film, which is beneficial to improving the barrier properties. Ultimately, the free energy of the composite barrier film surface is reduced, thereby improving the comprehensive performance of the nanocellulose-based composite barrier film. The modified nanosilica has a high hydrophobic property and can increase the contact angle and water resistance after spraying. Multiple hydrogen bonds and BOC covalent bonds are constructed by cross-linking TEMPO-oxidized nanocellulose and boric acid, and the density of the composite barrier film is improved by multiple interfacial interactions. The present invention utilizes water molecules to induce hydrogen bonds between cellulose nanofibers, and prepares a nanocellulose-based highly transparent composite barrier film by solvent evaporation self-assembly film formation. TEMPO-oxidized nanocellulose can provide excellent mechanical properties, and polyvinyl alcohol (PVA) serves as a film-forming agent and a binder as a basis for high compatibility, and both have certain oxygen barrier properties. The well-dispersed two-dimensional lamellar structure of sodium montmorillonite serves as a two-dimensional filler that is impermeable to natural gas. It is oriented in the composite barrier film and plays a role as a barrier, forming a gas barrier path. The presence of hydrogen bonds promotes the formation of a brick-mud-like structure, giving the composite film good gas barrier properties and mechanical properties. Boric acid (BA) is used as a cross-linking agent, and borate groups are introduced to form covalent bonds and hydrogen bonds with polyvinyl alcohol, TEMPO-oxidized nanocellulose, and sodium montmorillonite, consuming the free hydroxyl groups present in the system to form a brick-mud layered structure that imitates brick shells, thereby improving the density of the membrane interior. The present invention consumes free hydroxyl sites through covalent bonds to reduce the internal polarity of the composite membrane, extend the gas diffusion path, and cooperate with multiple low-surface-energy coatings to construct a barrier protection for the exposed hydroxyl groups on the surface of the composite barrier membrane, thereby reducing gas adsorption. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1a This is a cross-sectional SEM image of the composite membrane obtained without using boric acid according to the present invention.

[0026] Figure 1b for Figure 1a Enlarged view of the circled area.

[0027] Figure 1c This is a cross-sectional SEM image of the composite membrane obtained in Example 1 of the present invention.

[0028] Figure 1d for Figure 1c Enlarged view of the circled area.

[0029] Figure 1eThis is a cross-sectional element distribution diagram of the composite membrane obtained in Example 1 of the present invention.

[0030] Figure 1f This is a diagram of element content in the cross section of the composite membrane obtained in Example 1 of the present invention.

[0031] Figure 2a This is the surface morphology of the composite membrane obtained in Example 1 of the present invention.

[0032] Figure 2b This is a surface morphology diagram after the PMMA coating is formed in Example 1 of the present invention.

[0033] Figure 2c This is a cross-sectional morphology diagram of the composite film before coating of the present invention.

[0034] Figure 2d This is a cross-sectional morphology diagram after the PMMA coating is formed in Example 1 of the present invention.

[0035] Figure 2e for Figure 2d Magnified image of .

[0036] Figure 3a This is a surface morphology of the composite barrier film obtained in Example 1 of the present invention.

[0037] Figure 3b This is the surface Si element distribution diagram of the composite barrier film obtained in Example 1 of the present invention.

[0038] Figure 3c This is a graph showing the surface element content of the composite barrier film obtained in Example 1 of the present invention.

[0039] Figure 4a This is the XPS C 1s fine spectrum of the composite film obtained without using boric acid according to the present invention.

[0040] Figure 4b This is the XPS C 1s fine spectrum of the composite film obtained in Example 1 of the present invention.

[0041] Figure 4c This is a C1s fine spectrum after the PMMA coating is formed in Example 1 of the present invention.

[0042] Figure 4d This is the XPS B 1s fine spectrum of the composite film obtained in Example 1 of the present invention.

[0043] Figure 5a This is the full XPS spectrum of the composite barrier film obtained in Example 1 of the present invention.

[0044] Figure 5b This is the XPS Si 2p fine spectrum of the composite barrier film obtained in Example 1 of the present invention.

[0045] Figure 6 This is a histogram of water vapor transmission rates of different composite films after cross-linking and coating treatments of the present invention.

[0046] Figure 7 The figure is a histogram of oxygen transmission rates of different composite films after cross-linking and coating treatment according to the present invention.

[0047] Figure 8 JV curve of composite film used for solar cell encapsulation test. DETAILED DESCRIPTION

[0048] The present invention will be further described in detail below with reference to specific embodiments, which are intended to explain the present invention rather than to limit it.

[0049] The present invention provides a method for preparing a nanocellulose-based highly transparent composite barrier film, comprising the following steps:

[0050] Step 1: ultrasonically and stirring a 1.0% by mass TEMPO-oxidized nanocellulose gel in deionized water to achieve uniform dispersion. Specifically, the mixture is treated with an ultrasonic cell disruptor for 25 to 35 minutes, and then stirred at 500 to 800 rpm / min for 0.5 to 1.5 hours to obtain a 0.5% by mass TOCN-CNF dispersion. The TOCN-CNF dispersion includes TEMPO-oxidized nanocellulose and deionized water, and the ratio of the nanocellulose gel to the deionized water is 1:1.

[0051] Step 2: Stirring the mixture of PVA and deionized water in an oil bath at 80-90° C. to dissolve the PVA to form a transparent solution with a mass percentage of 3.5%, cooling to room temperature, and vacuum degassing 3-5 times to obtain a PVA solution;

[0052] Step 3, sodium montmorillonite (Na-MMT) is dispersed in deionized water, stirred at a rate of 900-1100 rpm / min for 0.5-1.5 h, ultrasonicated for 25-35 min, and centrifuged at 2500-3500 rpm / min for 8-12 min, the lower layer of particles is removed, and then stirred, ultrasonicated, and centrifuged for a total of 4-6 times to obtain a Na-MMT suspension with a concentration of 6.7 mg / mL;

[0053] Step 4, dissolving boric acid in deionized water to obtain a transparent boric acid solution with a concentration of 0.1 mol / L;

[0054] Step 5: PVA solution and TOCN-CNF dispersion were blended at a ratio of (0.24-0.36) g PVA to TEMPO-oxidized nanocellulose gel: (24-36) mL. The mixture was stirred at a rate of 700-900 rpm / min in a water bath at 40-60°C for 2.5-3.5 h. Boric acid solution and Na-MMT suspension were added and stirred for 4.5-5.5 h, so that Na-MMT accounted for 10% of the total mass of PVA and TEMPO-oxidized nanocellulose gel and boric acid accounted for 8% of the mass of PVA. An overhead stirrer was used to stir at a rate of 400-600 rpm / min for 60-120 min.

[0055] Step 6: The obtained mixed solution is vacuum degassed and cast into a 90 mm PS culture dish. A three-stage drying method is used: first drying at room temperature for 22 to 26 hours, then drying at 42 to 48° C. for 22 to 26 hours, and finally drying at 56 to 63° C. for 45 to 50 hours. The gradient temperature is used to achieve thorough drying from the inside out, which is beneficial to reducing internal pores caused by rapid drying, to obtain a composite membrane, which is recorded as BA-CMP.

[0056] Step 7: 25 g of polymethyl methacrylate (PMMA) particles were dissolved in 100 mL of N,N-dimethylformamide (DMF) by ultrasonic treatment for 2.5 to 3.5 h to form a transparent viscous solution. The composite membrane prepared in step 6 was immersed in the transparent viscous solution. After 8 to 12 seconds, the composite membrane was taken out and placed on a clean glass plate. The transparent viscous solution was evenly distributed on the composite membrane. One end of the glass rod was pulled out along the end away from the composite membrane. At the same time, one end of the composite membrane was pulled out along the end away from the glass rod at the other end of the glass rod to remove excess transparent viscous solution on the surface. The resulting composite membrane was then placed in a vacuum drying oven at 80 to 90° C. and cured for 20 to 25 minutes to volatilize the DMF. A PMMA coating was formed on the composite membrane, which was recorded as PMMA-CMP.

[0057] Step 8: 4 mL of tetraethyl orthosilicate (TEOS) was dissolved in 60 mL of anhydrous ethanol and stirred for 30 to 40 minutes. 3.2 mL of hexamethyldisilazane (HMDS) was added dropwise with a pipette and reacted at room temperature for 8 to 10 hours. 6 mL of deionized water was then added and stirred for 30 to 40 minutes to terminate the reaction. The resulting solution was allowed to stand at room temperature for 64 to 80 hours to obtain a modified nano-silica solution. The alkyl chains on the HMDS replaced the silicon-oxygen-hydrogen bonds on the silica through an alkylation reaction to form highly hydrophobic nano-silica, which was recorded as SiO2NPs.

[0058] Step 9: dilute the obtained modified nano-silica solution with 2 to 4 times the volume of anhydrous ethanol, that is, the anhydrous ethanol is 2 to 4 times the volume of the modified nano-silica solution, and spray the diluted solution on a PMMA coating of 90×90 mm in size with a spray gun. The spraying distance is 25 to 35 cm, and the spraying amount is 10 to 20 mL. That is, the ratio of the spraying amount to the area of the PMMA coating is (10 to 20) mL: 81 cm 2 The resulting three-layer film was then dried at room temperature and then at 80-90°C. The solvent ethanol was allowed to evaporate naturally during drying at room temperature, allowing the hydrophobic nanosilica coating to dry completely, forming a highly hydrophobic barrier and obtaining a nanocellulose-based highly transparent composite barrier film, designated PMMA-SiO2NPs.

[0059] Example 1

[0060] The present invention provides a method for preparing a nanocellulose-based highly transparent composite barrier film, comprising the following steps:

[0061] Step 1: 36 mL of 1.0% by mass TEMPO-oxidized nanocellulose gel was ultrasonicated and stirred in 36 mL of deionized water, specifically using an ultrasonic cell disruptor for 30 minutes and stirring at 700 rpm / min for 1 hour to obtain a 0.5% by mass TEMPO-CNF dispersion;

[0062] Step 2: A mixture of 0.24 g of PVA and 6.85 mL of deionized water was stirred in an oil bath at 85° C. to dissolve the PVA into a transparent solution with a mass percentage of 3.5%. The solution was cooled to room temperature and vacuum degassed three times to obtain a PVA solution.

[0063] Step 3: Disperse 1 g of sodium montmorillonite (Na-MMT) in 100 mL of deionized water, stir at 1000 rpm / min for 1 hour, ultrasonicate for 30 minutes, and centrifuge at 3000 rpm / min for 10 minutes to remove the lower layer of particles. Stir, ultrasonicate, and centrifuge for a total of 5 times to obtain a highly dispersed Na-MMT suspension with a concentration of 6.7 mg / mL.

[0064] Step 4: dissolving 0.3 g of boric acid in 48 mL of deionized water to obtain a transparent boric acid solution with a concentration of 0.1 mol / L;

[0065] Step 5: Add the PVA solution obtained in step 2 to the TOCN-CNF dispersion obtained in step 1, stir at 800 rpm / min in a 50°C water bath for 3 h, add boric acid solution and 8.95 mL of Na-MMT suspension, and continue stirring for 5 h until the boric acid in the boric acid solution accounts for 8% of the mass of PVA and the Na-MMT accounts for 10% of the total mass of PVA and TEMPO-oxidized nanocellulose gel. Stir at 500 rpm / min for 90 min using an overhead stirrer;

[0066] Step 6: vacuum degassing the obtained mixed solution, pouring it into a 90 mm PS culture dish, drying it at room temperature for 24 h, then drying it at 45° C. for 24 h, and finally drying it at 60° C. for 48 h to obtain a composite membrane.

[0067] Step 7: 25 g of PMMA particles were dissolved in 100 mL of N,N-dimethylformamide (DMF) by ultrasonication for 2 h to form a transparent viscous solution. The composite film prepared in step 6 was immersed in the transparent viscous solution. After 10 seconds, the composite film was taken out and placed on a clean glass plate. The transparent viscous solution was evenly distributed on the composite film. One end of the glass rod was pulled out along the end away from the composite film. At the same time, one end of the composite film was pulled out along the end away from the glass rod at the other end of the glass rod. The resulting composite film was then placed in a vacuum drying oven at 85° C. and cured for 25 minutes to form a PMMA coating on the composite film.

[0068] Step 8: Dissolve 4 mL of tetraethyl orthosilicate (TEOS) in 60 mL of anhydrous ethanol and stir for 30 minutes. Add 3.2 mL of hexamethyldisilazane (HMDS) dropwise with a pipette and react at room temperature for 9 hours. Then, add 6 mL of deionized water and continue stirring for 30 minutes to terminate the reaction. Let the resulting solution stand at room temperature for 3 days to obtain a modified nano-silica solution.

[0069] Step 9: Dilute the resulting modified nanosilica solution with 3 volumes of anhydrous ethanol. Spray the diluted solution onto a 90 x 90 mm PMMA coating using a spray gun at a distance of 30 cm and a spray volume of 15 mL. The resulting three-layer film was then dried at room temperature and 85°C, respectively.

[0070] Example 2

[0071] The present invention provides a method for preparing a nanocellulose-based highly transparent composite barrier film, comprising the following steps:

[0072] Step 1: 30 mL of 1.0% by mass TEMPO-oxidized nanocellulose gel was ultrasonicated and stirred in 30 mL of deionized water, specifically using an ultrasonic cell disruptor for 30 minutes and stirring at 700 rpm / min for 1 hour to obtain a 0.5% by mass TOCN-CNF dispersion;

[0073] Step 2: A mixture of 0.3 g of PVA and 8.57 mL of deionized water was stirred in an oil bath at 85° C. to dissolve the PVA into a transparent solution with a mass percentage of 3.5%, cooled to room temperature, and vacuum degassed three times to obtain a PVA solution;

[0074] Step 3: Disperse 1 g of sodium montmorillonite (Na-MMT) in 100 mL of deionized water, stir at 1000 rpm / min for 1 hour, ultrasonicate for 30 minutes, and centrifuge at 3000 rpm / min for 10 minutes to remove the lower layer of particles. Stir, ultrasonicate, and centrifuge for a total of 5 times to obtain a highly dispersed Na-MMT suspension with a concentration of 6.7 mg / mL.

[0075] Step 4: dissolving 0.3 g of boric acid in 48 mL of deionized water to obtain a transparent boric acid solution with a concentration of 0.1 mol / L;

[0076] Step 5: Add the PVA solution obtained in step 2 to the TOCN-CNF dispersion obtained in step 1, stir at 800 rpm / min in a 50°C water bath for 3 h, add boric acid solution and 8.95 mL of Na-MMT suspension, and continue stirring for 5 h until the boric acid in the boric acid solution accounts for 8% of the mass of PVA and the Na-MMT accounts for 10% of the total mass of PVA and TEMPO-oxidized nanocellulose gel. Stir at 500 rpm / min for 90 min using an overhead stirrer;

[0077] Step 6: vacuum degassing the obtained mixed solution, pouring it into a 90 mm PS culture dish, drying it at room temperature for 24 h, then drying it at 45° C. for 24 h, and finally drying it at 60° C. for 48 h to obtain a composite membrane.

[0078] Step 7: 25 g of PMMA particles were dissolved in 100 mL of N,N-dimethylformamide (DMF) by ultrasonication for 2 h to form a transparent viscous solution. The composite film prepared in step 6 was immersed in the transparent viscous solution. After 10 seconds, the composite film was taken out and placed on a clean glass plate. The transparent viscous solution was evenly distributed on the composite film. One end of the glass rod was pulled out along the end away from the composite film. At the same time, one end of the composite film was pulled out along the end away from the glass rod at the other end of the glass rod. The resulting composite film was then placed in a vacuum drying oven at 85° C. and cured for 25 minutes to form a PMMA coating on the composite film.

[0079] Step 8: Dissolve 4 mL of tetraethyl orthosilicate (TEOS) in 60 mL of anhydrous ethanol and stir for 30 minutes. Add 3.2 mL of hexamethyldisilazane (HMDS) dropwise with a pipette and react at room temperature for 9 hours. Then, add 6 mL of deionized water and continue stirring for 30 minutes to terminate the reaction. Let the resulting solution stand at room temperature for 3 days to obtain a modified nano-silica solution.

[0080] Step 9: Dilute the resulting modified nanosilica solution with 3 volumes of anhydrous ethanol. Spray the diluted solution onto a 90 x 90 mm PMMA coating using a spray gun at a distance of 30 cm and a spray volume of 15 mL. The resulting three-layer film was then dried at room temperature and 85°C, respectively.

[0081] Example 3

[0082] The present invention provides a method for preparing a nanocellulose-based highly transparent composite barrier film, comprising the following steps:

[0083] Step 1: 24 mL of 1.0% by mass TEMPO-oxidized nanocellulose gel was ultrasonicated and stirred in 24 mL of deionized water, specifically using an ultrasonic cell disruptor for 30 minutes and stirring at 700 rpm / min for 1 hour to obtain a 0.5% by mass TOCN-CNF dispersion;

[0084] Step 2: A mixture of 0.36 g of PVA and 10.28 mL of deionized water was stirred in an oil bath at 85° C. to dissolve the PVA into a transparent solution with a mass percentage of 3.5%. The solution was cooled to room temperature and vacuum degassed three times to obtain a PVA solution.

[0085] Step 3: Disperse 1 g of sodium montmorillonite (Na-MMT) in 100 mL of deionized water, stir at 1000 rpm / min for 1 hour, ultrasonicate for 30 minutes, and centrifuge at 3000 rpm / min for 10 minutes to remove the lower layer of particles. Stir, ultrasonicate, and centrifuge for a total of 5 times to obtain a highly dispersed Na-MMT suspension with a concentration of 6.7 mg / mL.

[0086] Step 4: dissolving 0.3 g of boric acid in 48 mL of deionized water to obtain a transparent boric acid solution with a concentration of 0.1 mol / L;

[0087] Step 5: Add the PVA solution obtained in step 2 to the TOCN-CNF dispersion obtained in step 1, stir at 800 rpm / min in a 50°C water bath for 3 h, add boric acid solution and 8.95 mL of Na-MMT suspension, and continue stirring for 5 h until the boric acid in the boric acid solution accounts for 8% of the mass of PVA and the Na-MMT accounts for 10% of the total mass of PVA and TEMPO-oxidized nanocellulose gel. Stir at 500 rpm / min for 90 min using an overhead stirrer;

[0088] Step 6: vacuum degassing the obtained mixed solution, pouring it into a 90 mm PS culture dish, drying it at room temperature for 24 h, then drying it at 45° C. for 24 h, and finally drying it at 60° C. for 48 h to obtain a composite membrane.

[0089] Step 7: 25 g of PMMA particles were dissolved in 100 mL of N,N-dimethylformamide (DMF) by ultrasonication for 2 h to form a transparent viscous solution. The composite film prepared in step 6 was immersed in the transparent viscous solution. After 10 seconds, the composite film was taken out and placed on a clean glass plate. The transparent viscous solution was evenly distributed on the composite film. One end of the glass rod was pulled out along the end away from the composite film. At the same time, one end of the composite film was pulled out along the end away from the glass rod at the other end of the glass rod. The resulting composite film was then placed in a vacuum drying oven at 85° C. and cured for 25 minutes to form a PMMA coating on the composite film.

[0090] Step 8: Dissolve 4 mL of tetraethyl orthosilicate (TEOS) in 60 mL of anhydrous ethanol and stir for 30 minutes. Add 3.2 mL of hexamethyldisilazane (HMDS) dropwise with a pipette and react at room temperature for 9 hours. Then, add 6 mL of deionized water and continue stirring for 30 minutes to terminate the reaction. Let the resulting solution stand at room temperature for 3 days to obtain a modified nano-silica solution.

[0091] Step 9: Dilute the resulting modified nanosilica solution with 3 volumes of anhydrous ethanol. Spray the diluted solution onto a 90 x 90 mm PMMA coating using a spray gun at a distance of 30 cm and a spray volume of 15 mL. The resulting three-layer film was then dried at room temperature and 85°C, respectively.

[0092] Remove the description of boric acid and re-implement the remaining step 6 of Example 1 to obtain a composite barrier film before cross-linking. Figure 1a and Figure 1b It can be seen that there are obvious interlayer defects in the cross-sectional structure of the composite membrane, and the interlayer spacing is large.

[0093] Combine Figure 1c and Figure 1dIt can be seen that the cross-section of the composite membrane after boric acid cross-linking is tighter because the introduction of borate covalent bonds and more hydrogen bonds reduces the generation of voids and narrows the interlayer spacing.

[0094] In order to verify the introduction of boric acid, the cross-sectional structure of the composite membrane after boric acid cross-linking was scanned. Figure 1e The first row is the element distribution diagram of C, O, and B, and the second row is the element distribution diagram of Si, Al, and Na. It can be clearly seen that the B element is introduced and evenly dispersed. Figure 1f The element contents of C, O, B, Si, Al and Na are shown in FIG.

[0095] from Figure 2a It can be seen that before the PMMA coating is formed, the surface of the composite membrane presents a fiber-interwoven morphology and still has obvious pores.

[0096] from Figure 2b It can be seen that after the PMMA coating is formed, the resin coating adheres to the surface of the composite membrane, filling and covering the pores between the fibers, which is beneficial to reducing the permeation of water vapor and oxygen.

[0097] contrast Figure 2c and Figure 2d 、 Figure 2e It can be seen that the composite film after the PMMA coating is formed has a two-layer structure with an obvious heterogeneous layer separation structure, which proves the successful introduction of the PMMA coating.

[0098] from Figure 3a It can be seen that the surface of the composite film after nano-silicon dioxide spraying is covered with a dense and evenly distributed layer of material, especially Figure 2a and Figure 2b In comparison, the fibers on the membrane surface are completely covered, the porosity is reduced, and the surface of the composite membrane is relatively smooth, which is conducive to improving the barrier properties.

[0099] In order to verify the presence of the hydrophobic nano-silica coating, a surface scan of the barrier composite film was performed. Figure 3b It can be observed that Si element is evenly distributed on the surface of the composite film. Figure 3c It can be seen that the Si element content is 44.01%.

[0100] from Figure 4a From the XPS spectrum, we can see that the C-OH binding energy in the composite film before boric acid crosslinking is 286.3 eV, while Figure 4b From the XPS spectrum, we can see that the C-OH binding energy in the composite film after boric acid cross-linking is 286.4eV, and compared with Figure 4a The decrease in peak intensity indicates that the formation of covalent bonds and hydrogen bonds after boric acid cross-linking reduces the polarity of free hydroxyl groups.

[0101] from Figure 4cIt can be seen that after the PMMA coating is formed, the C-OH peak intensity is Figure 4b It shows that the PMMA coating reduces the polarity of the hydroxyl groups on the surface of the composite barrier film.

[0102] from Figure 4d The fine spectrum of the B element shows that BO bonds are formed in the composite film after boric acid cross-linking, indicating that BOC covalent bonds are indeed formed by cross-linking.

[0103] In order to verify the introduction of nano-silica coating, full spectrum and fine spectrum analysis were performed. Figure 5a The presence of Si element on the composite barrier film can be observed. Figure 5b This indicates that Si-O bonds are formed in the composite barrier film.

[0104] from Figure 6 and Figure 7 Figure 3 is a bar graph of the water vapor permeability and oxygen permeability of different composite films after cross-linking and coating treatment. With the internal cross-linking and external synergistic modification treatment, the internal structure of the composite film becomes tighter and a hydrophobic moisture-proof barrier is formed on the outside. Therefore, the water vapor and oxygen permeabilities become lower and lower, that is, the vertical axis gradually decreases. Among them, CNF / PVA / MMT is the blank control group in step 6, which is the material obtained without adding boric acid; BA-CNF / PVA / MMT is the material obtained in step 6; PMMA-BA-CNF / PVA / MMT is the material obtained in step 7; SiO2NPs-PMMA-BA-CNF / PVA / MMT is the material obtained in step 9.

[0105] The composite barrier film obtained in this embodiment was simply packaged with the solar cell, and its performance parameters were tested. 5g of polyvinyl alcohol was dissolved in 100mL of deionized water, stirred at 90°C for 1h to obtain a polyvinyl alcohol solution, which was used as an adhesive for the composite film and the solar cell. Cut 84×61cm of the composite film for use, use a dropper to take 1-1.5mL of polyvinyl alcohol solution and apply it on the battery cell in a vacuum glove box, then wait for 10s and gently cover it with the composite barrier film to avoid the appearance of bubbles. After drying at room temperature, place it in a constant temperature and humidity box at 23°C and 50% RH for one week, and place an unpackaged battery cell for comparison. The test conditions are light irradiation intensity of 100mW / cm2, atmospheric mass coefficient AM1.5, and temperature of 25°C. As Figure 8 The working parameters of two groups of solar cells were tested under the same conditions. It can be seen that after a week of humidity test, the open circuit voltage of the cells encapsulated with the prepared high-transparency and high-hydrophobic barrier film increased from 5.8622V to 6.0386V, and the short-circuit current density increased from 30.2694mA / cm 2 Increased to 30.9217mA / cm 2, its conversion efficiency is 13.6889%, while the photoelectric conversion efficiency of the bare cell without barrier film protection is 13.6059%, that is, the composite barrier film increases the conversion efficiency of the solar cell by 0.083%, which is a relatively high increase.

Claims

1. A method for preparing a nanocellulose-based highly transparent composite barrier film, characterized in that: The steps include: S1, uniformly dispersing a TEMPO-oxidized nanocellulose gel in deionized water to obtain a TEMPO-oxidized nanocellulose dispersion, then uniformly mixing the dispersion with a PVA solution, wherein the ratio of PVA to TEMPO-oxidized nanocellulose gel is (0.24-0.36) g: (24-36) mL, and finally adding a sodium montmorillonite suspension and a boric acid solution and uniformly mixing, wherein the boric acid accounts for 8% by weight of the PVA and the sodium montmorillonite accounts for 10% by weight of the total weight of the PVA and TEMPO-oxidized nanocellulose gel, to obtain a mixed solution; S2, vacuum degassing the mixed solution and drying it at room temperature, 42-48° C., and 56-63° C. in sequence to obtain a composite film, immersing the composite film in a polymethyl methacrylate solution, removing excess polymethyl methacrylate solution, and then curing the composite film to form a PMMA coating on the composite film; S3, spraying a modified nano-silica solution on the PMMA coating, wherein the modified nano-silica solution is obtained by replacing the silicon-oxygen-hydrogen bonds on the silica with the alkyl chains on the hexamethyldisilazane, and then drying at room temperature and 80-90°C in turn to obtain a nanocellulose-based highly transparent composite barrier film.

2. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: S1: ultrasonically treating the TEMPO-oxidized nanocellulose gel in deionized water for 25 to 35 minutes and stirring at 500 to 800 rpm / min for 0.5 to 1.5 hours to obtain a TEMPO-oxidized nanocellulose dispersion with a mass percentage of 0.5%.

3. The method for preparing the nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: S1: Heat a mixture of PVA and deionized water in an oil bath at 80-90° C. to form a transparent solution with a mass percentage of 3.5%, then cool to room temperature, and vacuum degas 3-5 times to obtain a PVA solution.

4. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: S1 disperses sodium montmorillonite in deionized water, stirs at a rate of 900-1100 rpm / min for 0.5-1.5 h, then ultrasonicates for 25-35 min, and centrifuges at a rate of 2500-3500 rpm / min for 8-12 min to remove the lower layer of particles to obtain a supernatant. The supernatant is stirred, ultrasonicated, and centrifuged to obtain a sodium montmorillonite suspension with a concentration of 6.7 mg / mL.

5. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: S1: The TEMPO-oxidized nanocellulose dispersion and PVA solution were stirred at 40-60°C at a rate of 700-900 rpm / min for 2.5-3.5 h. The concentration of the added boric acid solution was 0.1 mol / L. After adding the boric acid solution and Na-MMT suspension, the mixture was stirred for 4.5-5.5 h, and then stirred at a rate of 400-600 rpm / min for 60-120 min to obtain a mixed solution.

6. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: S2 vacuum degasses the mixed solution and first dries it at room temperature for 22 to 26 hours, then dries it at 42 to 48° C. for 22 to 26 hours, and finally dries it at 56 to 63° C. for 45 to 50 hours to obtain a composite membrane.

7. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: S2: 25 g of polymethyl methacrylate particles are dissolved in 100 mL of N,N-dimethylformamide by ultrasonication for 2.5 to 3.5 hours to form a polymethyl methacrylate solution. The composite film is immersed in the solution for 8 to 12 seconds. After removing excess polymethyl methacrylate solution, the composite film is cured at 80 to 90° C. for 20 to 25 minutes to form a PMMA coating on the composite film.

8. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 1, characterized in that: The modified nano-silica solution described in S3 is obtained by the following process: Dissolve 4 mL of ethyl orthosilicate in 60 mL of anhydrous ethanol, drop 3.2 mL of hexamethyldisilazane and react at room temperature for 8 to 10 hours. After terminating the reaction, let it stand for 64 to 80 hours to obtain a modified nano-silica solution.

9. The method for preparing a nanocellulose-based highly transparent composite barrier film according to claim 8, characterized in that: S3: diluting the modified nano-silica solution with 2 to 4 times the volume of anhydrous ethanol and spraying the solution on the PMMA coating, and then drying the solution at room temperature and 80 to 90° C. in turn.

10. A nanocellulose-based highly transparent composite barrier film obtained by the preparation method of the nanocellulose-based highly transparent composite barrier film according to any one of claims 1 to 9.

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