Modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film as well as preparation method and application thereof

Abstract

The invention discloses a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film as well as a preparation method and application thereof, and relates to the technical field of wood adhesives. The specific preparation method comprises the following steps: preparing a deep-eutectic solvent from citric acid, choline chloride and water, and adding a crushed bleached pulp board into the deep-eutectic solvent to obtain a modified cellulose suspension; carrying out solid-liquid separation, washing and dispersing on the modified cellulose suspension to obtain a modified cellulose nanofiber dispersion liquid; adding polyvinyl alcohol into water, heating and stirring to obtain a polyvinyl alcohol aqueous solution; adding the modified cellulose nanofiber dispersion liquid into a polyvinyl alcohol aqueous solution, and then pouring and drying to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film; the adhesive film has excellent bonding performance, thermal stability and mechanical property, is environment-friendly and convenient to store and transport, and can greatly reduce energy consumption, save cost and reduce pollution when the adhesive film prepared by the invention is used for preparing plywood.

Description

A modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, its preparation method and application Technical Field

[0001] This invention relates to a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film, its preparation method and application, belonging to the field of wood adhesive technology. Background Technology

[0002] Adhesives have long held a core position in the wood processing industry, playing a crucial role, especially in the manufacture of engineered wood products. However, formaldehyde-based resin adhesives (such as urea-formaldehyde, phenol-formaldehyde, and melamine-formaldehyde resins) still dominate the market due to their cost and performance advantages, but they suffer from drawbacks such as non-renewable raw materials, poor biodegradability, and the potential release of carcinogens (such as formaldehyde) during production and use. Therefore, to address these shortcomings, a series of green and environmentally friendly formaldehyde-free adhesives have been widely developed and utilized, such as cellulose adhesives, lignin adhesives, starch adhesives, and polyvinyl alcohol adhesives. However, most of these adhesives are liquid adhesives (using water as a solvent), and their low solids content and high viscosity lead to drawbacks in the production process, hindering their industrialization and large-scale practical application. Furthermore, the drying and curing process of liquid adhesives requires a large amount of energy for water evaporation, which not only prolongs the production cycle but also significantly increases carbon emissions, failing to meet the current industrial requirements for green manufacturing. Based on this, green and environmentally friendly formaldehyde-free adhesive films, with their solvent-free physical form, are expected to solve the problems existing in the production and use of traditional liquid adhesives, and can also reduce transportation costs, prevent the pre-curing and deterioration of solvent-containing adhesives, and extend the shelf life.

[0003] Polyvinyl alcohol (PVA), as the world's most produced water-soluble polymer, can be mass-produced through non-petroleum routes at a low price. It possesses excellent film-forming properties, biodegradability, and chemical stability, making it a promising candidate for applications in films and adhesives. However, films made from pure PVA suffer from insufficient strength / toughness and poor water resistance. Traditional reinforcing materials, such as silica, graphite, and carbon nanotubes, while addressing these issues to some extent, often suffer from reduced ductility or elongation at break. Furthermore, many of these introduced materials are not biodegradable, leading to increasingly prominent environmental pollution problems.

[0004] Cellulose nanofibers, as a green emerging material integrating high strength, high specific surface area, thermal stability, biocompatibility, and functionalization potential, have shown great application prospects in the field of composite material reinforcement. In particular, the abundant hydroxyl groups on their surface facilitate chemical modification, and materials can be endowed with more ideal properties through oxidation, esterification, and grafting methods. Taking modified cellulose nanofibers prepared through modification treatment as an example, due to their slender morphology, fine diameter, and functional groups, the problem of poor dispersibility in aqueous solutions can be overcome by electrostatic repulsion, and resin films can be prepared by mixing them with a polyvinyl alcohol matrix. However, the preparation of modified cellulose nanofibers usually faces problems such as harsh reaction conditions, high reagent costs, and environmental pollution. Therefore, in order to develop commercially viable formaldehyde-free polyvinyl alcohol films reinforced with modified cellulose nanofibers, a scalable, economical, and environmentally friendly manufacturing method is urgently needed. Summary of the Invention

[0005] To address the shortcomings of related technologies, this invention provides a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, its preparation method, and its application. It has the advantages of high efficiency, environmental friendliness, and suitability for industrial scale-up, solving the problems of high energy consumption and low economic benefits in plywood production.

[0006] One of the objectives of this invention is to provide a method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film. The preparation schematic diagram is shown in Figure 1. The method includes the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and water are mixed and heated and stirred to obtain a eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent and heated and stirred to obtain a modified cellulose suspension; the modified cellulose suspension is subjected to solid-liquid separation (preferably by centrifugation), the solid material is collected, the solid material is washed to obtain the washed solid material, the washed solid material is dispersed in water and subjected to ultrasonic treatment to obtain a modified cellulose nanofiber dispersion.

[0007] (2) Preparation of polyvinyl alcohol aqueous solution: Add polyvinyl alcohol to water, heat and stir to obtain polyvinyl alcohol aqueous solution.

[0008] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: The modified cellulose nanofiber dispersion was added to the polyvinyl alcohol aqueous solution and stirred to obtain a film-forming solution. The film-forming solution was poured and dried to prepare the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0009] Preferably, in step (1), citric acid, choline chloride, and water are mixed in a mass ratio of (1~4):1:(1~3); the conditions for heating and stirring the citric acid, choline chloride, and water mixture are: heating and stirring at 50~80℃ for 10~30 min; the pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:(10~30) between the pulverized bleached pulp board and the mixed liquid; the conditions for continued heating and stirring are: heating and stirring at 100~130℃ at a speed of 300~600 rpm for 1~3 h.

[0010] Preferably, the conditions for ultrasonic treatment in step (1) are: ultrasonication at 20~30kHz for 5~10min; and the mass percentage concentration of the modified cellulose nanofiber dispersion is 1~5%.

[0011] Preferably, the heating and stirring conditions in step (2) are: heating and stirring at 90~100℃ and 300~600rpm for 30~300min; and the mass percentage concentration of the polyvinyl alcohol aqueous solution is 8~15%.

[0012] Preferably, the mass of modified cellulose nanofiber dispersion added to the film-forming solution in step (3) is 5-15% of the polyvinyl alcohol aqueous solution; the drying conditions are: drying at 40°C for 5-24 hours.

[0013] The second objective of this invention is to provide a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared by the method of this invention.

[0014] The third objective of this invention is to provide an application of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film in the preparation of plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of oxidant, and apply the aqueous solution of oxidant to the surface of the wood veneer for activation, thereby obtaining the activated wood veneer.

[0015] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0016] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent activated wood veneers (the structural schematic diagram is shown in Figure 2) and pre-compressed to obtain a pre-compressed sample.

[0017] (4) Hot pressing and curing: The pre-pressed sample is hot-pressed to obtain plywood.

[0018] Preferably, the aqueous solution of the oxidant in step (1) is any one of sodium periodate, potassium dichromate, potassium ferricyanide, nitric acid, hydrogen peroxide and peracetic acid aqueous solution with a mass percentage concentration of 8%; the wood veneer is poplar veneer.

[0019] Preferably, the pre-compression condition in step (3) is: pre-compressed at a pressure of 1 MPa for 30 minutes.

[0020] Preferably, the hot pressing conditions in step (4) are: hot pressing for 4 to 8 minutes at a hot pressing pressure of 1 MPa and a hot pressing temperature of 120 to 200°C.

[0021] Mechanism of the invention: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film prepared by the present invention achieves the construction of a uniform three-dimensional network reinforcement skeleton through the synergistic effect of modified cellulose nanofiber and polyvinyl alcohol. Water wetting can activate the surface of the adhesive film. After water activation, the hydrogen bond network formed inside the film opens, and the film produces an adhesive effect, acting on the poplar veneer that has been activated by a low concentration of oxidant. The plywood prepared by the present invention, through the synergistic effect of formaldehyde-free adhesive film activation, poplar veneer activation, high temperature and pressure heat treatment, enables the hydroxyl and carboxyl groups in the film molecules to interact with the hydroxyl and aldehyde / carboxyl groups generated after the wood surface is activated. Combined with hydrogen bonding, the water resistance and bonding strength of the plywood are improved, which meets the requirements of GB / T 9846-2015 Class II plywood.

[0022] The beneficial effects of the present invention are as follows: (1) In terms of composition, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film contains only polyvinyl alcohol and modified cellulose nanofiber. The raw materials are non-toxic, green and environmentally friendly, and there is no release of volatile organic compounds.

[0023] (2) In terms of preparation, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film preparation process is simple, does not rely on complex equipment conditions, is safe and environmentally friendly, and is easy to achieve large-scale production, and has good prospects for promotion and application.

[0024] (3) In terms of application, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film can be activated by rapid water activation treatment, and high-performance plywood can be prepared under different hot pressing temperatures and times. If combined with the use of low-concentration oxidant to activate the wood surface, the film can be used to prepare wood plywood with good hot water resistance by high-temperature hot pressing. Attached Figure Description

[0025] Figure 1 is a schematic diagram of the preparation of plywood using the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film of the present invention.

[0026] Figure 2 is a schematic diagram of the three-layer plywood structure prepared by the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film of the present invention. Detailed Implementation

[0027] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Unless otherwise specified, all chemical reagents used in the embodiments and comparative examples of this invention are of commercially available analytical grade. The bleached pulp boards used in the embodiments and comparative examples of this invention were purchased from Yunnan Yunjing Forestry & Paper Co., Ltd., and the size of the pulverized bleached pulp boards was 20-100 nm.

[0028] Example 1 A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board and eutectic solvent, and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min, the solid material is collected, the solid material is washed with deionized water until neutral, the washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0029] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0030] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 5% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 12h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.08mm.

[0031] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 1 of the present invention for plywood preparation, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 1 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of sodium periodate with a mass percentage concentration of 8%, and apply the aqueous solution of sodium periodate with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer activated with sodium periodate.

[0032] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0033] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by sodium periodate, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0034] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 6 minutes to obtain plywood.

[0035] The plywood prepared in Example 1 was subjected to physical and mechanical property tests on a universal testing machine according to the requirements of the national standard GB / T 17657-2013 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels". The main test was on the water resistance of the plywood. The specific steps are as follows: The plywood was cut into 24 specimens and divided into 3 equal parts for testing. The dry strength, 24-hour cold water strength (the specimen was immersed in cold water for 24 hours), and 3-hour 63°C wet strength (the specimen was immersed in 63°C water for 3 hours) were tested. The results showed that the dry strength of the plywood prepared in this example was 1.91 MPa, the 24-hour cold water strength was 1.31 MPa, and the 3-hour warm water (63°C) wet strength was 1.35 MPa.

[0036] This embodiment constructs a uniform three-dimensional network reinforcement framework by combining nanocellulose with polyvinyl alcohol, significantly improving the mechanical properties and thermal stability of the adhesive film. During hot pressing, this film can more effectively wet the wood surface and enhance adhesion through mechanical interlocking and chemical bonding. A synergistic effect is achieved through optimized hot pressing and wood surface activation pretreatment. This synergistic effect activates the wood surface, exposing more active reactive groups; it also promotes the full flow, uniform spreading, and complete curing of the adhesive film, forming a dense, defect-free, and tightly bonded interface with the wood surface. This not only significantly reduces interfacial voids but also promotes the formation of high-density hydrogen bonds and chemical bonds between the active groups of the adhesive film and the groups on the wood surface, thereby improving the bonding strength and water resistance of the plywood.

[0037] Example 2 A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material. The washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0038] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0039] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 5% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 5h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.06mm.

[0040] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film of Example 2 of the present invention in the preparation of plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film prepared in Example 2 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of potassium dichromate with a mass percentage concentration of 8%, and apply the aqueous solution of potassium dichromate with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer after potassium dichromate activation.

[0041] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0042] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by potassium dichromate, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0043] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 4 minutes to obtain plywood.

[0044] The plywood prepared in this embodiment was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this embodiment was 1.60 MPa, the 24-hour cold water strength was 1.28 MPa, and the 3-hour warm water (63°C) wet strength was 1.33 MPa.

[0045] This embodiment constructs a uniform three-dimensional network reinforcement framework by combining nanocellulose with polyvinyl alcohol, significantly improving the mechanical properties and thermal stability of the adhesive film. During hot pressing, this film can more effectively wet the wood surface and enhance adhesion through mechanical interlocking and chemical bonding. A synergistic effect is achieved through optimized hot pressing and wood surface activation pretreatment. This synergistic effect activates the wood surface, exposing more active reactive groups; it also promotes the full flow, uniform spreading, and complete curing of the adhesive film, forming a dense, defect-free, and tightly bonded interface with the wood surface. This not only significantly reduces interfacial voids but also promotes the formation of high-density hydrogen bonds and chemical bonds between the active groups of the adhesive film and the groups on the wood surface, thereby improving the bonding strength and water resistance of the plywood.

[0046] Example 3 A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material. The washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0047] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 30 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0048] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 5% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 24h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.07mm.

[0049] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 3 of the present invention for plywood preparation, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 3 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of potassium ferricyanide with a mass percentage concentration of 8%, and apply the aqueous solution of potassium ferricyanide with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer activated by potassium ferricyanide.

[0050] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0051] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by potassium ferricyanide, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0052] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 140℃, hot pressing is carried out for 8 minutes to obtain plywood.

[0053] The plywood prepared in this embodiment was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this embodiment was 1.59 MPa, the 24-hour cold water strength was 1.18 MPa, and the 3-hour warm water (63°C) wet strength was 1.14 MPa.

[0054] This embodiment constructs a uniform three-dimensional network reinforcement framework by combining nanocellulose with polyvinyl alcohol, significantly improving the mechanical properties and thermal stability of the adhesive film. During hot pressing, this film can more effectively wet the wood surface and enhance adhesion through mechanical interlocking and chemical bonding. A synergistic effect is achieved through optimized hot pressing and wood surface activation pretreatment. This synergistic effect activates the wood surface, exposing more active reactive groups; it also promotes the full flow, uniform spreading, and complete curing of the adhesive film, forming a dense, defect-free, and tightly bonded interface with the wood surface. This not only significantly reduces interfacial voids but also promotes the formation of high-density hydrogen bonds and chemical bonds between the active groups of the adhesive film and the groups on the wood surface, thereby improving the bonding strength and water resistance of the plywood.

[0055] Example 4 A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material. The washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0056] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 300 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0057] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 10% of that of polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 24h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.07mm.

[0058] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film of Example 4 of the present invention in the preparation of plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film prepared in Example 4 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare a nitric acid aqueous solution with a mass percentage concentration of 8%, and apply the nitric acid aqueous solution with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer after nitric acid activation.

[0059] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0060] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by nitric acid, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0061] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 160℃, hot pressing is carried out for 6 minutes to obtain plywood.

[0062] The plywood prepared in this embodiment was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this embodiment was 1.38 MPa, the 24-hour cold water strength was 1.27 MPa, and the 3-hour warm water (63°C) wet strength was 1.30 MPa.

[0063] This embodiment constructs a uniform three-dimensional network reinforcement framework by combining nanocellulose with polyvinyl alcohol, significantly improving the mechanical properties and thermal stability of the adhesive film. During hot pressing, this film can more effectively wet the wood surface and enhance adhesion through mechanical interlocking and chemical bonding. A synergistic effect is achieved through optimized hot pressing and wood surface activation pretreatment. This synergistic effect activates the wood surface, exposing more active reactive groups; it also promotes the full flow, uniform spreading, and complete curing of the adhesive film, forming a dense, defect-free, and tightly bonded interface with the wood surface. This not only significantly reduces interfacial voids but also promotes the formation of high-density hydrogen bonds and chemical bonds between the active groups of the adhesive film and the groups on the wood surface, thereby improving the bonding strength and water resistance of the plywood.

[0064] Example 5 A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and deionized water are mixed in a mass ratio of 4:1:1 and heated and stirred at 50°C for 25 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:10 to pulverized bleached pulp board and heated and stirred at 100°C at 300 rpm for 2 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material. The washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 20 kHz for 10 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 2%.

[0065] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 90°C at 500 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 8%.

[0066] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 2% by mass of modified cellulose nanofiber dispersion was added to 8% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 15% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 24h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.07mm.

[0067] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 5 of the present invention for plywood preparation, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 5 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare a hydrogen peroxide aqueous solution with a mass percentage concentration of 8%, and apply the hydrogen peroxide aqueous solution with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer after hydrogen peroxide activation.

[0068] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0069] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by hydrogen peroxide, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0070] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 4 minutes to obtain plywood.

[0071] The plywood prepared in this embodiment was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this embodiment was 1.74 MPa, the 24-hour cold water strength was 1.13 MPa, and the 3-hour warm water (63°C) wet strength was 1.16 MPa.

[0072] This embodiment constructs a uniform three-dimensional network reinforcement framework by combining nanocellulose with polyvinyl alcohol, significantly improving the mechanical properties and thermal stability of the adhesive film. During hot pressing, this film can more effectively wet the wood surface and enhance adhesion through mechanical interlocking and chemical bonding. A synergistic effect is achieved through optimized hot pressing and wood surface activation pretreatment. This synergistic effect activates the wood surface, exposing more active reactive groups; it also promotes the full flow, uniform spreading, and complete curing of the adhesive film, forming a dense, defect-free, and tightly bonded interface with the wood surface. This not only significantly reduces interfacial voids but also promotes the formation of high-density hydrogen bonds and chemical bonds between the active groups of the adhesive film and the groups on the wood surface, thereby improving the bonding strength and water resistance of the plywood.

[0073] Example 6 A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and deionized water are mixed in a mass ratio of 1:1:3 and heated and stirred at 60°C for 10 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:30 to pulverized bleached pulp board and heated and stirred at 120°C at 400 rpm for 1 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material. The washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 30 kHz for 5 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 5%.

[0074] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 100°C at 300 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 15%.

[0075] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 5% by mass of modified cellulose nanofiber dispersion was added to 15% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 10% of that of polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 24h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.07mm.

[0076] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 6 of the present invention for plywood preparation, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Example 6 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an 8% peracetic acid aqueous solution, and apply the 8% peracetic acid aqueous solution to the surface of a poplar veneer with a thickness of 1.38 mm for activation, to obtain poplar veneer activated by peracetic acid.

[0077] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0078] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by peracetic acid, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0079] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 4 minutes to obtain plywood.

[0080] The plywood prepared in this embodiment was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this embodiment was 1.70 MPa, the 24-hour cold water strength was 1.17 MPa, and the 3-hour warm water (63°C) wet strength was 1.20 MPa.

[0081] This embodiment constructs a uniform three-dimensional network reinforcement framework by combining nanocellulose with polyvinyl alcohol, significantly improving the mechanical properties and thermal stability of the adhesive film. During hot pressing, this film can more effectively wet the wood surface and enhance adhesion through mechanical interlocking and chemical bonding. A synergistic effect is achieved through optimized hot pressing and wood surface activation pretreatment. This synergistic effect activates the wood surface, exposing more active reactive groups; it also promotes the full flow, uniform spreading, and complete curing of the adhesive film, forming a dense, defect-free, and tightly bonded interface with the wood surface. This not only significantly reduces interfacial voids but also promotes the formation of high-density hydrogen bonds and chemical bonds between the active groups of the adhesive film and the groups on the wood surface, thereby improving the bonding strength and water resistance of the plywood.

[0082] Comparative Example 1: A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride, and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board, and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material. The washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0083] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0084] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 5% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 12h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.08mm.

[0085] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film of Comparative Example 1 in preparing plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Comparative Example 1 was used to prepare plywood, specifically including the following steps: (1) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was quickly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0086] (2) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent poplar veneers and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0087] (3) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 6 minutes to obtain plywood.

[0088] The plywood prepared in this comparative example was subjected to mechanical property testing in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this comparative example was 1.34 MPa. After immersing the plywood prepared in this comparative example in cold water for 24 hours, the adhesive layer of the plywood cracked. After immersing the plywood prepared in this comparative example in warm water at 60°C for 3 hours, the adhesive layer of the plywood also cracked. This indicates that the surface active groups of poplar veneer that have not been activated by oxidant are not exposed and cannot fully react and crosslink with the resin film. Therefore, the plywood is not water resistant.

[0089] Comparative Example 2: A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride, and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board, and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material, and the washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0090] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0091] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 5% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 12h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.08mm.

[0092] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film of Comparative Example 2 in the preparation of plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Comparative Example 2 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of sodium periodate with a mass percentage concentration of 8%, and apply the aqueous solution of sodium periodate with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer activated with sodium periodate.

[0093] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0094] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by sodium periodate, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0095] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 3 minutes to obtain plywood.

[0096] The plywood prepared in this comparative example was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this comparative example was 1.45 MPa, the 24-hour cold water strength was 0.65 MPa, and the 3-hour warm water (63°C) wet strength was 0.55 MPa.

[0097] This comparative example only used a 3-minute hot-pressing time to prepare plywood. The hot-pressing time was too short, the resin film was not fully cured, the cross-linking density with the activated wood surface was low, resulting in low wet strength that did not meet the national standard and poor water resistance.

[0098] Comparative Example 3: A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride, and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board, and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material, and the washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0099] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0100] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 1% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 12h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.08mm.

[0101] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film of Comparative Example 3 in the preparation of plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Comparative Example 3 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of sodium periodate with a mass percentage concentration of 8%, and apply the aqueous solution of sodium periodate with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer activated with sodium periodate.

[0102] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0103] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by sodium periodate, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0104] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 6 minutes to obtain plywood.

[0105] The plywood prepared in this comparative example was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this comparative example was 1.15 MPa, the 24-hour cold water strength was 0.45 MPa, and the 3-hour warm water (63°C) wet strength was 0.56 MPa.

[0106] In the comparative example, the mass of modified cellulose nanofiber dispersion added to the film was only 1% of the polyvinyl alcohol aqueous solution. This resulted in sparse dispersion of nanocellulose in the polyvinyl alcohol matrix, which could not form an effective network structure. Consequently, the resin film did not show significant reinforcement or toughening effects, and the wet bonding strength was low, failing to meet international standards.

[0107] Comparative Example 4: A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride, and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board, and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material, and the washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0108] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0109] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 20% of that of polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 12h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.08mm.

[0110] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film of Comparative Example 4 in the preparation of plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Comparative Example 4 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of sodium periodate with a mass percentage concentration of 8%, and apply the aqueous solution of sodium periodate with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer activated by sodium periodate.

[0111] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0112] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by sodium periodate, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0113] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 120℃, hot pressing is carried out for 6 minutes to obtain plywood.

[0114] The plywood prepared in this comparative example was subjected to mechanical property tests in the same manner as in Example 1. The results showed that the dry strength of the plywood prepared in this comparative example was 0.78 MPa, the 24-hour cold water strength was 0.56 MPa, and the 3-hour warm water (63°C) wet strength was 0.35 MPa.

[0115] The modified cellulose nanofiber dispersion in this comparative example was added at 20% of the polyvinyl alcohol aqueous solution. If the amount of nanocellulose added to the polyvinyl alcohol matrix is ​​too high, the nanocellulose in the prepared film is prone to agglomeration. Not only will it not be uniformly dispersed, but it will also produce defects in the film, resulting in poor film performance. Therefore, the film prepared in this comparative example has poor water resistance and does not meet international standards.

[0116] Comparative Example 5: A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, specifically including the following steps: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride, and deionized water are mixed in a mass ratio of 3:1:1 and heated and stirred at 80°C for 30 min to obtain a homogeneous and transparent eutectic solvent; then, pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:20 to pulverized bleached pulp board, and heated and stirred at 130°C at 600 rpm for 3 h to obtain a modified cellulose suspension; the modified cellulose suspension is centrifuged at 10000 rpm for 8 min to collect solid material, and the solid material is washed with deionized water until neutral to obtain washed solid material, and the washed solid material is dispersed in deionized water and ultrasonically treated at a frequency of 25 kHz for 8 min to obtain a modified cellulose nanofiber dispersion with a mass percentage concentration of 1%.

[0117] (2) Preparation of polyvinyl alcohol aqueous solution: Polyvinyl alcohol was added to deionized water and heated and stirred at 95°C at 600 rpm for 100 min to obtain a polyvinyl alcohol aqueous solution with a mass percentage concentration of 12%.

[0118] (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: 1% by mass of modified cellulose nanofiber dispersion was added to 12% by mass of polyvinyl alcohol aqueous solution and stirred at room temperature to obtain film-forming solution. The added mass of modified cellulose nanofiber dispersion was 5% of the polyvinyl alcohol aqueous solution. The film-forming solution was poured into a 15×15×3cm glass container and then dried at 40℃ for 12h to obtain a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film with a thickness of 0.08mm.

[0119] To verify the application performance of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film of Comparative Example 5 in the preparation of plywood, the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared in Comparative Example 5 was used to prepare plywood, specifically including the following steps: (1) Wood surface activation: Prepare an aqueous solution of sodium periodate with a mass percentage concentration of 8%, and apply the aqueous solution of sodium periodate with a mass percentage concentration of 8% to the surface of poplar veneer with a thickness of 1.38mm for activation, and obtain poplar veneer activated with sodium periodate.

[0120] (2) Water activation: The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film is rapidly immersed in water to obtain the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

[0121] (3) Pre-compression: The activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film was placed between three adjacent layers of poplar veneer activated by sodium periodate, and pre-compressed at 1 MPa for 30 min at room temperature to obtain the pre-compressed sample.

[0122] (4) Hot pressing curing: Under the conditions of hot pressing pressure of 1MPa and hot pressing temperature of 100℃, hot pressing is carried out for 6 minutes to obtain plywood.

[0123] The plywood prepared in this comparative example was subjected to mechanical property tests in the same manner as in Example 1. The test results showed that the dry strength of the plywood prepared in this comparative example was 1.56 MPa, the 24-hour cold water strength was 0.68 MPa, and the 3-hour warm water (63°C) wet strength was 0.45 MPa.

[0124] The temperature of the hot-pressed curing film in this comparative example was only 100℃, which did not reach the curing temperature of the resin film. The resin film was not fully cured, and the cross-linking density with the activated wood surface was low. Therefore, the water resistance was poor, resulting in the wet strength not meeting the international standard.

[0125] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing a modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film, characterized in that, Specifically, the following steps are included: (1) Preparation of modified cellulose nanofibers: Citric acid, choline chloride and water were mixed and heated and stirred to obtain a eutectic solvent; then pulverized bleached pulp board was added to the eutectic solvent and heated and stirred to obtain a modified cellulose suspension. The modified cellulose suspension was subjected to solid-liquid separation, the solid material was collected, the solid material was washed, the washed solid material was dispersed in water and subjected to ultrasonic treatment to obtain a modified cellulose nanofiber dispersion. (2) Preparation of polyvinyl alcohol aqueous solution: Add polyvinyl alcohol to water, heat and stir to obtain polyvinyl alcohol aqueous solution; (3) Preparation of modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film: Add modified cellulose nanofiber dispersion to polyvinyl alcohol aqueous solution, stir and mix to obtain film-forming liquid, pour and dry the film-forming liquid to prepare modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film.

2. The method for preparing the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film according to claim 1, characterized in that, In step (1), citric acid, choline chloride, and water are mixed in a mass ratio of (1~4):1:(1~3); the conditions for heating and stirring the citric acid, choline chloride, and water mixture are: heating and stirring at 50~80℃ for 10~30 min; the pulverized bleached pulp board is added to the eutectic solvent in a mass ratio of 1:(10~30); the conditions for continued heating and stirring are: heating and stirring at 100~130℃ at a speed of 300~600 rpm for 1~3 h.

3. The method for preparing the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film according to claim 1, characterized in that, The conditions for ultrasonic treatment in step (1) are: ultrasonication at 20-30 kHz for 5-10 min; and the mass percentage concentration of the modified cellulose nanofiber dispersion is 1-5%.

4. The method for preparing the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film according to claim 1, characterized in that, The heating and stirring conditions in step (2) are as follows: heating and stirring at 90~100℃ and 300~600rpm for 30~300min; the mass percentage concentration of the polyvinyl alcohol aqueous solution is 8~15%.

5. The method for preparing the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film according to claim 1, characterized in that, The mass of modified cellulose nanofiber dispersion added to the film-forming solution in step (3) is 5-15% of the polyvinyl alcohol aqueous solution; the drying conditions are: drying at 40℃ for 5-24 hours.

6. The modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film prepared by the method according to any one of claims 1 to 5.

7. The application of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film according to claim 6 in the preparation of plywood, characterized in that, Specifically, the following steps are included: (1) Activation of wood surface: Prepare an aqueous solution of oxidant and apply the aqueous solution of oxidant to the surface of wood veneer to activate it, and obtain activated wood veneer; (2) Water activation: Immerse the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film in water to obtain activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film; (3) Pre-pressing: Place the activated modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free film between three adjacent activated wood veneers and pre-press it to obtain pre-pressed sample; (4) Hot pressing curing: Hot press the pre-pressed sample to obtain plywood.

8. The application of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film according to claim 6 in the preparation of plywood, characterized in that, In step (1), the aqueous solution of the oxidant is any one of sodium periodate, potassium dichromate, potassium ferricyanide, nitric acid, hydrogen peroxide and peracetic acid aqueous solution with a mass percentage concentration of 8%; the wood veneer is poplar veneer.

9. The application of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film according to claim 6 in the preparation of plywood, characterized in that, The pre-compression condition in step (3) is: pre-compress at a pressure of 1 MPa for 30 minutes.

10. The application of the modified cellulose nanofiber reinforced polyvinyl alcohol formaldehyde-free adhesive film according to claim 6 in the preparation of plywood, characterized in that, The hot pressing conditions in step (4) are: hot pressing for 4 to 8 minutes at a hot pressing pressure of 1 MPa and a hot pressing temperature of 120 to 200°C.

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