Preparation method of nanocellulose film with high wet strength and high light transmittance and nanocellulose film prepared thereby

Abstract

The application discloses a preparation method of a nanocellulose film with high wet strength and high light transmittance, and comprises the following steps: dissolving the surface of the nanocellulose film by using a cellulose solvent; and then solidifying the nanocellulose film after the surface is dissolved, so as to obtain the nanocellulose film with a regenerated cellulose layer on the surface. In the preparation method, the crystalline structure of cellulose is destroyed by ions, so that the cellulose is dissolved; the method is simple in operation, short in processing time and low in reagent consumption. The application further discloses the nanocellulose film with high wet strength and high light transmittance prepared by the preparation method.

Description

Technical Field

[0001] This invention relates to nanocellulose membranes. More specifically, it relates to a method for preparing a nanocellulose membrane with high wet strength and high light transmittance, and the nanocellulose membrane obtained therefrom. Background Technology

[0002] Cellulose, as the most abundant polymer in nature, boasts advantages such as biodegradability, renewability, and wide availability, and holds promise for partially replacing non-degradable plastics and mitigating the environmental hazards caused by their large-scale use. In recent years, with the maturation of nanocellulose preparation methods, nanocellulose membranes, prepared via vacuum filtration, exhibit high strength, high light transmittance, and a low coefficient of thermal expansion, showing broad application prospects in food packaging, solar cells, lithium-ion battery separators, and flexible displays. However, due to the network structure formed by nanocellulose in the membrane, it readily absorbs large amounts of water through capillary action. In high humidity or aqueous environments, the interaction between fibers significantly decreases, leading to a reduction in material strength. Therefore, increasing the wet strength and improving the water resistance of nanocellulose membranes is a crucial area for expanding their application range.

[0003] Chinese patent application CN114230867A discloses a method for preparing a high-wet-strength cellulose composite material. This method involves modifying nanocellulose with a synthesized vegetable oil-based polymer. The amide and epoxy groups in the vegetable oil-based polymer effectively improve the poor interfacial compatibility between hydrophilic and hydrophobic interfaces in the composite, constructing a highly water-resistant cross-linked network system through hydrogen bonding and covalent bonding. This significantly improves the wet mechanical properties and surface hydrophobicity of the cellulose material with minimal impact on its rigidity and strength, expanding its application in wet environments. However, this method has drawbacks such as cumbersome steps and the need for organic solvents.

[0004] Chinese patent application CN110551300A discloses a water-resistant, transparent cellulose-based film and its preparation method. The method involves preparing a transparent cellulose-based film via sol-gel casting or vacuum filtration, immersing it in glacial acetic acid and subjecting it to a chemical reaction under ultrasonication, and then washing the reacted cellulose-based film with an organic solvent to obtain the water-resistant, transparent cellulose-based film. Its wet tensile strength is 15-30 MPa, and its transmittance in the visible light region is between 90-91%. However, this method has a long operation time (2-4 hours), and the cellulose requires modification treatment.

[0005] Chinese patent application CN109385928A discloses a nanocellulose / aramid nanofiber membrane composite nanopaper and its preparation method. The method involves: oxidizing cellulose with 2,2,6,6-tetramethylpiperidine oxide (TEMPO) to prepare nanocellulose, followed by dehydration to obtain cellulose nanopaper. The cellulose nanopaper is then impregnated with a polydiallylammonium chloride solution. Subsequently, aramid nanofibers and nanocellulose are self-assembled layer-by-layer to form a film in situ, effectively improving the wet strength, UV shielding, temperature resistance, and aging resistance of the nanocellulose-based material. However, this method is complex, requires the use of organic solvents, and the aramid fibers are non-degradable. Summary of the Invention

[0006] To address the above problems, one objective of this invention is to provide a method for preparing a nanocellulose membrane with high wet strength and high light transmittance, as well as the resulting nanocellulose membrane. In this preparation method, the crystalline structure of cellulose is disrupted by ions, causing it to dissolve, and a dense cellulose layer is regenerated on the surface. This method is simple to operate, has a short processing time, and uses few reagents.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] On one hand, the present invention provides a method for preparing a nanocellulose membrane with high wet strength and high light transmittance, comprising the following steps:

[0009] The surface of the cellulose nanofilm was dissolved using a cellulose solvent;

[0010] The dissolved nanocellulose membrane is then solidified to obtain a nanocellulose membrane with a layer of regenerated cellulose on its surface.

[0011] Further, the cellulose solvent is selected from one or more of ZnCl2·2-4H2O, CaCl2 / ZnCl2·3-4H2O, AlCl3 / ZnCl2·4H2O, DMAc / LiCl, NaOH / urea, NMMO, or ionic liquids. ZnCl2·2-4H2O refers to the 2-4 hydrate of ZnCl2, corresponding to a zinc chloride concentration of 65.4-79.1 wt%, within which cellulose can be dissolved. CaCl2 / ZnCl2·3-4H2O refers to the 2-4 hydrate of ZnCl2 containing CaCl2. AlCl3 / ZnCl2·4H2O refers to the tetrahydrate of ZnCl2 containing AlCl3.

[0012] Furthermore, the dissolution time is 5-60 seconds.

[0013] Furthermore, the dissolution time is 5-10 seconds.

[0014] Furthermore, the solidification is carried out in a regenerated solvent, which is selected from one or more of anhydrous ethanol, anhydrous propanol, or anhydrous isopropanol.

[0015] Furthermore, the solidification time is 10-60 minutes.

[0016] Furthermore, the solidification time is 5-15 minutes.

[0017] Furthermore, the basis weight of the nanocellulose membrane is 20-100 g / m³. 2 The dry strength is 100-200MPa, the light transmittance is 80-92%, and the haze is 77-92%.

[0018] Further, the preparation of the nanocellulose membrane includes: obtaining a wet membrane from the nanocellulose dispersion using a vacuum filtration method, followed by drying to obtain the nanocellulose membrane; or

[0019] The nanocellulose membrane was prepared by casting method using nanocellulose dispersion as raw material.

[0020] Furthermore, in the nanocellulose dispersion, the height of the nanocellulose is 5-200 nm and the length is greater than 5 μm.

[0021] Furthermore, in the nanocellulose dispersion, the height of the nanocellulose includes, but is not limited to, 5-20 nm, 50-200 nm, 20-50 nm, etc.

[0022] Furthermore, in the nanocellulose dispersion, the length of the nanocellulose is greater than 10 μm.

[0023] Furthermore, the concentration of the nanocellulose dispersion is 0.1-4 wt%.

[0024] Furthermore, the concentration of the nanocellulose dispersion is 0.1-1 wt%.

[0025] Furthermore, the concentration of the nanocellulose dispersion is 0.5-4 wt%.

[0026] Furthermore, the nanocellulose dispersion is prepared by dispersing cellulose raw materials in water and then mechanically grinding them.

[0027] Furthermore, the cellulose raw material is selected from natural cellulose and extracted from at least one of broadleaf wood, coniferous wood, bamboo, straw, seaweed, or sea squirt.

[0028] Furthermore, the mechanical grinding time is 20-240 min.

[0029] On the other hand, the present invention provides a nanocellulose membrane with high wet strength and high light transmittance prepared by the preparation method described above.

[0030] The beneficial effects of this invention are as follows:

[0031] Compared with existing technologies, the preparation method provided in this invention has the advantages of simple operation, short processing time, and fewer reagents. It utilizes only cellulose itself to enhance the wet strength of the membrane without introducing other components, achieving complete biodegradability. The wet strength is significantly improved without reducing the original dry strength. Simultaneously, it maintains the high light transmittance (≥85%) of the nanocellulose membrane and allows for haze adjustment within a certain range (49-88%). Attached Figure Description

[0032] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0033] Figure 1 The morphology of nanocellulose in Example 1 is shown.

[0034] Figure 2 The image shows a physical image of the high-transparency, high-haze nanocellulose membrane prepared in Example 1.

[0035] Figure 3 The image shows the nanofiber membrane prepared in Example 1 after being soaked in water for 24 hours.

[0036] Figure 4 The morphology of nanocellulose in Example 2 is shown.

[0037] Figure 5 The image shows a physical photograph of the high-transparency, medium-haze nanocellulose membrane prepared in Example 2.

[0038] Figure 6 The morphology of nanocellulose in Example 3 is shown.

[0039] Figure 7 The morphology of nanocellulose in Example 4 is shown.

[0040] Figure 8 A photograph of the high-transparency, high-haze nanocellulose membrane prepared in Comparative Example 1 is shown.

[0041] Figure 9 The image shows the nanofiber membrane prepared in Comparative Example 1 after being soaked in water for 24 hours. Detailed Implementation

[0042] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further explains the invention. Similar components in the drawings are indicated by the same reference numerals. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0043] Example 1

[0044] This embodiment provides a method for preparing a nanofiber cellulose membrane with high dry / wet strength, high light transmittance, and high haze, including the following steps:

[0045] 0.5g of hardwood pulp cellulose raw material and 20g of deionized water were weighed and added to a grinder for mechanical treatment. The mixture was ball-milled at 300 rpm for 40 min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose nanoparticles had a height between 50-200 nm and a length greater than 10 μm. The AFM morphology and height distribution of the nanocellulose nanoparticles are shown in the figure below. Figure 1 As shown.

[0046] The nanocellulose dispersion was diluted to 0.1 wt%, and a nanocellulose membrane was prepared by vacuum filtration. The dried nanocellulose membrane was immersed in ZnCl2·3H2O solution for 10 s, followed by regeneration in anhydrous ethanol for 10 min, to obtain a nanocellulose membrane with regenerated cellulose on its surface. A photograph of the membrane is shown below. Figure 2 As shown in the image. The actual product after soaking in water for 24 hours is shown in the image below. Figure 3 As shown.

[0047] The membrane has a dry tensile strength of 138.3 MPa, a wet tensile strength of 25.3 MPa, a light transmittance of 91%, and a haze of 81.7%.

[0048] The testing method is as follows:

[0049] Tensile strength: The test size of the specimen is 25×5mm. 2 The tensile rate was 2 mm / min. The dry test environment was 23℃ and 50RH. The wet test involved immersing the sample in water for 24 hours, then absorbing the surface moisture with filter paper, and performing the tensile test under the same conditions as the dry test.

[0050] Transmittance: The transmittance of the film at all angles and the linear transmittance were measured using a UV-Vis-NIR spectrometer. The haze was calculated using the following formula.

[0051] Haze = (1 - linear transmittance / full-angle transmittance) × 100%.

[0052] Example 2

[0053] This embodiment provides a method for preparing a nanofiber cellulose membrane with high dry / wet strength, high transmittance, and moderate haze, including the following steps:

[0054] 0.5 g of hardwood pulp cellulose raw material and 20 g of deionized water were weighed and added to a grinder for mechanical treatment. The mixture was ball-milled at 300 rpm for 120 min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose nanoparticles had a height between 5-20 nm and a length greater than 10 μm. The AFM morphology and height distribution of the nanocellulose nanoparticles are shown in the figure. Figure 4 As shown.

[0055] The nanocellulose dispersion was diluted to 0.1 wt%, and a nanocellulose membrane was prepared by vacuum filtration. The dried nanocellulose membrane was immersed in ZnCl2·3H2O solution for 10 s, followed by regeneration in anhydrous ethanol for 10 min, to obtain a nanocellulose membrane with regenerated cellulose on its surface. A photograph of the membrane is shown below. Figure 5 As shown.

[0056] The membrane has a dry tensile strength of 170 MPa, a wet tensile strength of 28.6 MPa, a light transmittance of 85%, and a haze of 49%.

[0057] Example 3

[0058] This embodiment provides a method for preparing a nanofiber cellulose membrane with high dry / wet strength, high transmittance, and high haze, including the following steps:

[0059] 0.5g of hardwood pulp cellulose raw material and 20g of deionized water were weighed and added to a grinder for mechanical treatment. The mixture was ball-milled at 300 rpm for 60 min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose nanoparticles had a height between 20-50 nm and a length greater than 10 μm. The AFM morphology and height distribution of the nanocellulose nanoparticles are shown in the figure below. Figure 6 As shown.

[0060] The nanocellulose dispersion was diluted to 1 wt% and dried at 60 °C using a casting method to prepare a nanocellulose membrane. The dried nanocellulose membrane was immersed in CaCl2 / ZnCl2·3H2O solution for 5 s, and then regenerated in anhydrous ethanol for 10 min to obtain a nanocellulose membrane with regenerated cellulose on its surface.

[0061] The membrane has a dry tensile strength of 151 MPa, a wet tensile strength of 31.7 MPa, a light transmittance of 89%, and a haze of 78.5%.

[0062] Example 4

[0063] This embodiment provides a method for preparing a nanofiber cellulose membrane with high dry / wet strength, high transmittance, and high haze, including the following steps:

[0064] 0.5g of bamboo pulp cellulose raw material and 20g of deionized water were weighed and added to a grinder for mechanical treatment. The mixture was ball-milled at 300 rpm for 60 min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose nanoparticles had a height between 5-20 nm and a length greater than 5 μm. The AFM morphology and height distribution of the nanocellulose nanoparticles are shown in the figure below. Figure 7 As shown.

[0065] The nanocellulose dispersion was diluted to 0.1 wt%, and a nanocellulose membrane was prepared by vacuum filtration. The dried nanocellulose membrane was immersed in ZnCl2·3H2O solution for 10 s, and then regenerated in anhydrous ethanol for 10 min to obtain a nanocellulose membrane with regenerated cellulose on its surface.

[0066] The membrane has a dry tensile strength of 123 MPa, a wet tensile strength of 27.5 MPa, a light transmittance of 89%, and a haze of 82%.

[0067] Example 5

[0068] This embodiment provides a method for preparing a nanofiber cellulose membrane with high dry / wet strength, high transmittance, and high haze, including the following steps:

[0069] Weigh 0.5g of bamboo pulp cellulose raw material and add 20g of deionized water to a grinder for mechanical treatment. Ball mill at 300rpm for 60min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose has a height between 5-20nm and a length greater than 5μm.

[0070] The nanocellulose dispersion was diluted to 0.1 wt%, and a nanocellulose membrane was prepared by vacuum filtration. The dried nanocellulose membrane was immersed in NaOH / urea solution for 10 s, and then regenerated in anhydrous isopropanol for 10 min to obtain a nanocellulose membrane with regenerated cellulose on its surface.

[0071] The membrane has a dry tensile strength of 110 MPa, a wet tensile strength of 24.3 MPa, a light transmittance of 85%, and a haze of 72%.

[0072] Comparative Example 1

[0073] This comparative example provides a method for preparing a nanofiber cellulose membrane with high dry strength, low wet strength, high light transmittance, and high haze, including the following steps:

[0074] Weigh 0.5g of hardwood pulp cellulose raw material and add 20g of deionized water to a grinder for mechanical treatment. Ball mill at 300rpm for 40min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose has a height between 50-200nm and a length greater than 10μm.

[0075] The nanocellulose dispersion was diluted to 0.1%, and a nanocellulose membrane was prepared by vacuum filtration. The actual image is shown below. Figure 8 As shown in the image, the actual product after soaking in water for 24 hours is as follows. Figure 9 As shown, the membrane has a dry tensile strength of 111 MPa, a light transmittance of 91%, and a haze of 88%. Its wet tensile strength is only 1.5 MPa.

[0076] Comparative Example 2

[0077] This comparative example provides a method for preparing a nanofiber cellulose membrane with high dry strength, low wet strength, high light transmittance, and high haze, including the following steps:

[0078] Weigh 0.5g of hardwood pulp cellulose raw material and add 20g of deionized water to a grinder for mechanical treatment. Ball mill at 300rpm for 60min to obtain a nanocellulose dispersion. In this dispersion, the nanocellulose has a height between 20-50nm and a length greater than 10μm.

[0079] A nanocellulose dispersion was diluted to 1 wt% and dried at 60 °C using a casting method to prepare a nanocellulose membrane. The membrane exhibited a dry tensile strength of 95 MPa, a light transmittance of 83%, and a haze of 82%. However, its wet tensile strength was only 1.2 MPa.

[0080] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for preparing a nanocellulose membrane with high wet strength and high light transmittance, characterized in that, It consists of the following steps: The surface of the cellulose nanofilm was dissolved using a cellulose solvent; The dissolved nanocellulose membrane is then solidified to obtain a nanocellulose membrane with a layer of regenerated cellulose on its surface. The cellulose solvent is selected from one or more of ZnCl2·2-4H2O, CaCl2 / ZnCl2·3-4H2O, AlCl3 / ZnCl2·4H2O, DMAc / LiCl, NaOH / urea, NMMO, or ionic liquids; The dissolution time is 5-60 seconds; The solidification is carried out in a regenerated solvent, which is selected from one or more of anhydrous ethanol, anhydrous propanol or anhydrous isopropanol. The solidification time is 10-60 minutes; The preparation of the nanocellulose membrane includes: obtaining a wet membrane from a nanocellulose dispersion using a vacuum filtration method, followed by drying to obtain the nanocellulose membrane; or The nanocellulose membrane was prepared by casting method using nanocellulose dispersion as raw material.

2. The preparation method according to claim 1, characterized in that, The basis weight of the nanocellulose membrane with a layer of regenerated cellulose on its surface is 20-100 g / m³. 2 The dry strength is 100-200 MPa, the light transmittance is 80-92%, and the haze is 77-92%.

3. The preparation method according to claim 1, characterized in that, In the nanocellulose dispersion, the height of the nanocellulose is 5-200 nm and the length is greater than 5 μm; The concentration of the nanocellulose dispersion is 0.5-4 wt%.

4. The preparation method according to claim 1 or 3, characterized in that, The nanocellulose dispersion is prepared by dispersing cellulose raw materials in water and then mechanically grinding them.

5. The preparation method according to claim 4, characterized in that, The cellulose raw material is selected from natural cellulose and is extracted from at least one of broadleaf wood, coniferous wood, bamboo, cotton fiber, hemp fiber, straw, seaweed, or sea squirt.

6. The preparation method according to claim 4, characterized in that, The mechanical grinding time is 20-240 min.

7. A nanocellulose membrane with high wet strength and high light transmittance prepared by the preparation method according to any one of claims 1-6.

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