Preparation method of high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film

Abstract

The invention relates to a preparation method of a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane, which comprises the following steps: reacting a mixture of polyvinyl alcohol, KH550, carboxylated nanocellulose and water at 50-75 DEG C for 30-120 minutes, carrying out vacuum defoaming, casting into a mold, and drying to obtain the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane. Wherein the carboxyl content of the carboxylated nanocellulose is not less than 1.0 mmol / g, the average length-diameter ratio is 10-50, the addition amount of the carboxylated nanocellulose is 0.5 wt%-10 wt% of the mass of the polyvinyl alcohol, and the addition amount of the KH550 is 1 wt%-6 wt% of the mass of the polyvinyl alcohol. The method is simple in preparation, and the prepared composite film is high in strength and toughness.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of composite films, and relates to a preparation method of a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film. BACKGROUND

[0002] Polyvinyl alcohol, as a biodegradable water-soluble polymer material, has been widely used in many fields such as food packaging, medical protection, and flexible electronic substrates due to its excellent film-forming property, biocompatibility, and good mechanical basic performance. However, pure polyvinyl alcohol film has some obvious shortcomings in practical application, such as insufficient tensile strength, poor water resistance, and limited toughness. Especially in scenarios with high requirements for material mechanical properties, such as high-strength packaging film and medical suture film, pure polyvinyl alcohol film is difficult to meet the actual demand.

[0003] In the field of material science, the reinforcement and toughening mechanisms are often mutually exclusive. Materials with high strength and stiffness usually have low deformation ability and poor toughness. Therefore, how to simultaneously realize the reinforcement and toughening of polymer materials has been one of the main challenges in this field, and polyvinyl alcohol materials are no exception.

[0004] Currently, there have been some attempts to reinforce and toughen polyvinyl alcohol materials. Publication No. CN202510529389.7 discloses a method for preparing a high-strength and high-toughness polyvinyl alcohol composite film using waste polyimide recycling. The method recycles and upgrades waste polyimide (PI) materials into a hydrogen bond crosslinking agent. This hydrogen bond crosslinking agent is composed of a multimer containing a large number of amide bonds and amino groups, which can easily form strong hydrogen bonds with polyvinyl alcohol, thereby changing the orientation and arrangement of polyvinyl alcohol molecular chains, making them more compact and ordered, and thus improving the tensile strength and modulus of the film. Meanwhile, the reversible breaking and rebuilding characteristics of hydrogen bonds enable the film to better disperse stress when subjected to external force, avoiding stress concentration and thus improving the toughness of the film. However, this method has the disadvantage of complex preparation process, as it requires the additional preparation of a hydrogen bond crosslinking agent.

[0005] In addition, Document 1 (Research Status of Nanocellulose Crystal Reinforced Polyvinyl Alcohol Film [J]. Packaging Engineering, 2023.2, 23.) mentions that nanocellulose crystals can be crosslinked with polyvinyl alcohol through chemical crosslinking agents to prepare reinforced and toughened composite films. Commonly used chemical crosslinking agents include aldehydes, acid anhydrides, and carboxylic acids. However, during the reaction process, these crosslinking agents may preferentially react with polyvinyl alcohol, resulting in insufficient crosslinking of nanocellulose, which weakens the interfacial bonding between nanocellulose and polyvinyl alcohol. Insufficient interfacial bonding weakens stress transfer efficiency, limits the reinforcing effect of nanocellulose, and ultimately limits the mechanical properties of the film.

[0006] In view of the deficiencies of the prior art, it is of great significance to study a preparation method of high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film. SUMMARY

[0007] The purpose of the present application is to solve the problems in the prior art and provide a preparation method of high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0008] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0009] A preparation method of high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film, wherein a mixture of polyvinyl alcohol, KH550, carboxylated nanocellulose and water is reacted at 50-75℃ for 30-120min, then vacuum degassed, cast into a mold, and dried to obtain the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0010] In the present application, the carboxyl content of the carboxylated nanocellulose is not less than 1.0mmol / g, the average aspect ratio is 10-50, and the addition amount of the carboxylated nanocellulose is 0.5wt%-10wt% of the mass of the polyvinyl alcohol, and the addition amount of the KH550 is 1wt%-6wt% of the mass of the polyvinyl alcohol.

[0011] In the prior art, polyvinyl alcohol, nanocellulose and crosslinking agent are mixed uniformly to obtain a mixed solution, and then the mixed solution is cast into a film. The obtained film is difficult to have both high strength and high toughness, and the fundamental reasons are as follows:

[0012] ① The dispersibility of nanocellulose in polyvinyl alcohol is poor, and aggregation is easy to occur. The surface of nanocellulose has a large number of hydroxyl groups, which can easily form aggregates through hydrogen bonding. The aggregates can cause stress concentration in the film and become the starting point of crack initiation, and at the same time, reduce the reinforcing efficiency of nanocellulose, so that the strength and toughness of the film cannot be improved simultaneously.

[0013] ② The crosslinking reaction is not synchronized. In the mixed system, the crosslinking agent (such as aldehyde, acid anhydride, carboxylic acid, etc.) may preferentially react with polyvinyl alcohol, and nanocellulose is not fully crosslinked, which further leads to weak interfacial bonding between nanocellulose and polyvinyl alcohol. Insufficient interfacial bonding can weaken the stress transfer efficiency, so that the reinforcing effect of nanocellulose cannot be effectively exerted, and the mechanical properties of the film are limited.

[0014] ③ The crosslinking degree is not properly controlled. When the crosslinking degree is insufficient, the strength of the film is low; when the crosslinking degree is too high, the toughness of the film decreases.

[0015] To address the aforementioned issue ①, this invention utilizes carboxylated nanocellulose with a high carboxyl content and controls its aspect ratio. Specifically, using carboxylated nanocellulose with a high carboxyl content and controlling its aspect ratio ensures sufficient negative charge on the nanocellulose surface, resulting in electrostatic repulsion in water. Therefore, uniform dispersion of nanocellulose in polyvinyl alcohol can be achieved without the addition of compatibilizers or other additives. If the carboxyl content of the carboxylated nanocellulose is too low, the electrostatic repulsion is insufficient, causing the fibers to easily aggregate and become stress concentration points, thus reducing the mechanical properties of the membrane. If the aspect ratio of the carboxylated nanocellulose is too low, the crystal structure is severely damaged, weakening the reinforcing effect; conversely, it easily entangles in bundles within the membrane, increasing the difficulty of dispersion and also causing stress concentration.

[0016] This invention also strictly controls the addition amount of carboxylated nanocellulose to 0.5wt%-10wt% of the mass of polyvinyl alcohol, because this range allows for the formation of sufficient cross-linking nodes and avoids agglomeration. If the addition amount of carboxylated nanocellulose is too low, the reinforcing effect is not obvious; conversely, the carboxylated nanocellulose is prone to agglomeration, forming aggregates, which also causes stress concentration.

[0017] To address reasons ② and ③ above, this invention solves the problems by controlling the type of crosslinking agent, the amount of crosslinking agent used, and the conditions of the crosslinking reaction.

[0018] Unlike existing technologies, this invention uses KH550 as a crosslinking agent because the terminal amino groups of KH550 can form amide bonds with the surface carboxyl groups of carboxylated cellulose nanofibers, and the ethoxy groups of KH550 can form a covalent siloxane network with the hydroxyl groups of polyvinyl alcohol, achieving simultaneous crosslinking at both interfaces. If the same crosslinking agent as in existing technologies is used, it would be difficult to guarantee simultaneous crosslinking at both interfaces.

[0019] The present invention controls the addition amount of KH550 to be 1wt%-6wt% of the mass of polyvinyl alcohol, because this range can form a moderate cross-linking network; if the addition amount of KH550 is too low, the cross-linking points are insufficient and the tensile strength is not significantly improved; conversely, the cross-linking density is too high, the film brittleness increases and the film toughness decreases significantly.

[0020] This invention controls the crosslinking reaction temperature to 50-75℃ and the reaction time to 30-120 min. These conditions ensure that the ethoxy groups of KH550 are fully hydrolyzed into silanol, forming a stable siloxane network with polyvinyl alcohol and carboxylated nanocellulose. If the crosslinking reaction temperature is too low or the reaction time is too short, the crosslinking will be incomplete, and the membrane strength will decrease; conversely, the degradation of polyvinyl alcohol segments will be accelerated, and the membrane's elongation at break will decrease.

[0021] As a preferred technical solution:

[0022] The method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane as described above has a thickness of 20 μm, a tensile strength of 82-130 MPa, and an elongation at break of 201%-307%.

[0023] The method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane as described above, wherein the degree of polymerization of polyvinyl alcohol is 1700-2000 and the degree of alcoholysis is 88%-99%.

[0024] The method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane as described above, wherein the carboxyl content of the carboxylated cellulose nanofiber is not higher than 1.8 mmol / g, the average length is 210-301 nm, and the crystallinity is about 80%.

[0025] The preparation method of the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane described above involves the following steps: the preparation process of the mixture of polyvinyl alcohol, KH550, carboxylated nanocellulose and water is as follows: the aqueous dispersion of carboxylated nanocellulose and KH550 are added to the aqueous solution of polyvinyl alcohol (obtained by dissolving polyvinyl alcohol in water at 50-75°C) and stirred until homogeneous (for approximately 30-120 minutes).

[0026] The method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane as described above involves a concentration of 1 wt% for the aqueous dispersion of carboxylated nanocellulose and a concentration of 5 wt%-12 wt% for the aqueous solution of polyvinyl alcohol.

[0027] The preparation method of the high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanoparticle composite membrane described above includes the following steps for preparing the aqueous dispersion of carboxylated cellulose nanoparticles:

[0028] (a) Mix cellulose raw materials (microcrystalline cellulose, wood pulp, paper pulp, cotton linters, cotton fabric, corn cob, corn stalk, bamboo fiber, wood, etc.) with a eutectic solvent (DES), stir and react at 80-120℃ for 1-6 hours, cool, add water, centrifuge to obtain pretreated cellulose, wash repeatedly until neutral, and dry at 40-60℃ for later use.

[0029] The eutectic solvent consists of a hydrogen bond donor and a hydrogen bond acceptor; the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is one or more of oxalic acid, citric acid, and lactic acid.

[0030] (b) Disperse the pretreated cellulose in water to prepare a suspension, then add NaOH solution to adjust the pH to 10-12, stir and activate for 30-60 min, then add chloroacetic acid, react at 50-70℃ for 2-6 h, then add hydrochloric acid to adjust the pH to 7, centrifuge and wash until no chloride ions are present to obtain carboxylated cellulose;

[0031] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 30-60min to obtain an aqueous dispersion of carboxylated nanocellulose.

[0032] The carboxylated cellulose nanoparticles prepared by this invention have a higher carboxyl content in their aqueous dispersion, while retaining a complete crystal structure and high strength. This overcomes the bottleneck of existing technologies where "carboxyl content and crystal integrity cannot be simultaneously achieved," as detailed below:

[0033] Existing strong acid hydrolysis methods (such as sulfuric acid and hydrochloric acid) break glycosidic bonds in cellulose and destroy the crystal structure by strong acid. Although this can reduce the degree of polymerization, it will cause the crystal skeleton to collapse and introduce impurities such as sulfate ester groups. In the subsequent carboxylation reaction, the activity of hydroxyl groups is inhibited and the degree of carboxyl substitution is low. As a result, the carboxylated nanocellulose produced has a low carboxyl content and the integrity of the crystal structure is damaged.

[0034] The existing TEMPO oxidation method relies on the sodium hypochlorite-sodium bromide system to oxidize the hydroxyl groups on the surface of cellulose. It requires strict control of pH (9-11) and the ratio of oxidant. The oxidation process is prone to irregular breakage of cellulose chains, damage to the integrity of crystal structure, and carboxyl groups are only distributed on the fiber surface.

[0035] Pretreatment of neutral / alkaline DES in existing technologies: Existing DES mostly use neutral hydrogen bond donors such as urea and glycerol, which can only loosen the cellulose aggregate state and cannot effectively destroy the hydrogen bonds inside the crystallization region. After pretreatment, the cellulose still retains high crystallinity, and chloroacetic acid is difficult to penetrate into the crystal during subsequent carboxylation, resulting in a low degree of carboxylation.

[0036] Existing ionic liquid methods can dissolve cellulose, but ionic liquids (such as 1-butyl-3-methylimidazolium chloride) are expensive, and recovery requires multiple extractions. After pretreatment, the cellulose chains are prone to excessive degradation, and the integrity of the crystals is destroyed.

[0037] If etherification is performed directly without pretreatment, chloroacetic acid will have difficulty penetrating into the crystal interior, resulting in a low degree of carboxylation.

[0038] Without pretreatment, direct ultrasonic recarboxylation still cannot achieve both high carboxyl content and crystal integrity. This is because although ultrasonic crushing can refine the fibers, the high crystallinity of untreated cellulose raw materials makes subsequent carboxylation insufficient, and ultrasound can easily cause crystal chain breakage.

[0039] This invention employs an acidic DES system of choline chloride + oxalic acid / citric acid / lactic acid, which has the following advantages: The acidic hydrogen bond donor (such as oxalic acid) can gently disrupt the intermolecular hydrogen bonds of cellulose through protonation, exposing a large number of active hydroxyl groups (the utilization rate of C2, C3, and C6 hydroxyl groups is increased to over 60%) without damaging the six-membered ring skeleton of the glucose unit, thus preserving the "high-strength skeleton" of the crystal. Subsequently, chloroacetic acid can rapidly undergo an etherification reaction with the exposed large number of active hydroxyl groups under alkaline conditions (pH=10-12), thereby increasing the degree of carboxylation. The alkaline reaction environment only promotes the etherification reaction without damaging the preserved crystal skeleton. Therefore, the carboxylated nanocellulose of this invention achieves both high carboxyl content and crystal integrity.

[0040] Existing ultrasonic fragmentation techniques are mostly "violent dispersions," which can refine the size but destroy the crystal integrity and lose the reinforcing ability. This invention utilizes the electrostatic repulsion of carboxyl groups, combined with the gentle mechanical force of pulsed ultrasound, to disperse carboxylated cellulose into nanoscale while retaining the crystal structure, thus facilitating the subsequent preparation of high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membranes.

[0041] In the preparation method of the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane as described above, in step (a), the mass ratio of cellulose raw material to eutectic solvent is 1:10-50, and the molar ratio of hydrogen bond donor to hydrogen bond acceptor is 1-3:1.

[0042] In the preparation method of the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane as described above, in steps (a) and (b), the concentration of the suspension is 1 wt%, and the molar ratio of cellulose raw material to chloroacetic acid is 1:1.5-2.5.

[0043] In the preparation method of the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane described above, drying refers to vacuum drying at 60°C for 2 hours.

[0044] Beneficial effects:

[0045] (1) In the preparation method of the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane of the present invention, an acidic DES system of choline chloride + oxalic acid / citric acid / lactic acid is used to pretreat the cellulose raw material, replacing the traditional strong acid and toxic solvent, and overcoming the problems of high pollution and difficult recycling of existing methods. The DES can be efficiently recovered and reused by distillation, which significantly reduces environmental pollution and meets the requirements of green chemical industry.

[0046] (2) In the preparation method of a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane of the present invention, an acidic DES system of choline chloride + oxalic acid / citric acid / lactic acid is used to pretreat the cellulose raw material, which preserves the integrity of the cellulose crystals and allows the hydroxyl groups to be fully exposed, thereby achieving high carboxylation in the subsequent carboxylation process, and thus obtaining highly carboxylated and crystal-complete hydroxy cellulose.

[0047] (3) In the preparation method of a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane of the present invention, carboxylated nanocellulose with high carboxyl content is used, and the aspect ratio of carboxylated nanocellulose is controlled to ensure that the surface of nanocellulose has sufficient negative charge and forms electrostatic repulsion in water. Therefore, the uniform dispersion of nanocellulose in polyvinyl alcohol can be achieved without adding compatibilizers or other additives.

[0048] (4) The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite membrane prepared by the present invention has a significant improvement over pure polyvinyl alcohol membrane and has both high strength and high toughness characteristics. Attached Figure Description

[0049] Figure 1 Transmission electron microscopy image of the carboxylated cellulose nanoparticles prepared in Example 1. Detailed Implementation

[0050] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0051] In the following embodiments:

[0052] The carboxyl content of carboxylated nanocellulose was determined by titration.

[0053] The morphology and size of carboxylated cellulose nanoparticles were observed using TEM. The length and diameter of the cellulose nanoparticles were measured and statistically analyzed, and the aspect ratio was calculated.

[0054] The tensile strength and elongation at break of the film were evaluated using a tensile test (ASTM D882-18).

[0055] Example 1

[0056] A method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, the specific steps of which are as follows:

[0057] (1) Preparation of raw materials:

[0058] Chloroacetic acid;

[0059] water;

[0060] KH550;

[0061] Cellulose raw material: wood pulp fiber;

[0062] Eutectic solvent: composed of hydrogen bond donor and hydrogen bond acceptor in a molar ratio of 1:1; the hydrogen bond acceptor is choline chloride and the hydrogen bond donor is oxalic acid;

[0063] An aqueous solution of polyvinyl alcohol with a concentration of 5 wt%, wherein the degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 88%;

[0064] (2) Preparation of an aqueous dispersion of carboxylated cellulose nanoparticles;

[0065] (a) The cellulose raw material is mixed with a eutectic solvent, stirred and reacted at 100°C for 2 hours, cooled, water is added, and centrifuged to obtain pretreated cellulose; wherein the mass ratio of cellulose raw material to eutectic solvent is 1:10.

[0066] (b) The pretreated cellulose was dispersed in water to prepare a suspension with a concentration of 1 wt%, the pH was adjusted to 10, the mixture was stirred and activated for 45 min, chloroacetic acid was added, the mixture was reacted at 50 °C for 2 h, the pH was adjusted to 7, and the mixture was centrifuged and washed to obtain carboxylated cellulose; wherein the molar ratio of cellulose raw material to chloroacetic acid was 1:1.5.

[0067] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 30min to obtain an aqueous dispersion of carboxylated nanocellulose with a concentration of 1wt%.

[0068] The prepared aqueous dispersion of carboxylated cellulose nanoparticles contained 1 mmol / g of carboxyl groups, with an average length of 219 nm and an average diameter of 17 nm. Figure 1 The image shown is a transmission electron microscope (TEM) image of the prepared carboxylated cellulose nanoparticles.

[0069] (3) Add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol, stir evenly, react at 60°C for 120 min, degas, cast into a mold, and vacuum dry at 60°C for 2 h to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0070] The amount of carboxylated nanocellulose added is 0.5 wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 1 wt% of the mass of polyvinyl alcohol.

[0071] The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film prepared has a thickness of 20 μm, a tensile strength of 82.5 MPa, and an elongation at break of 201.4%.

[0072] Example 2

[0073] A method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, the specific steps of which are as follows:

[0074] (1) Preparation of raw materials:

[0075] Chloroacetic acid;

[0076] water;

[0077] KH550;

[0078] Cellulose raw material: wood pulp fiber;

[0079] Eutectic solvent: composed of hydrogen bond donor and hydrogen bond acceptor in a molar ratio of 1:1; the hydrogen bond acceptor is choline chloride and the hydrogen bond donor is citric acid;

[0080] An aqueous solution of polyvinyl alcohol with a concentration of 5 wt%, wherein the degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 88%;

[0081] (2) Preparation of an aqueous dispersion of carboxylated cellulose nanoparticles;

[0082] (a) The cellulose raw material is mixed with a eutectic solvent, stirred and reacted at 110°C for 3 hours, cooled, water is added, and centrifuged to obtain pretreated cellulose; wherein the mass ratio of cellulose raw material to eutectic solvent is 1:10.

[0083] (b) The pretreated cellulose was dispersed in water to prepare a suspension with a concentration of 1 wt%, the pH was adjusted to 12, the mixture was stirred and activated for 60 min, chloroacetic acid was added, the mixture was reacted at 65 °C for 3 h, the pH was adjusted to 7, and the mixture was centrifuged and washed to obtain carboxylated cellulose; wherein the molar ratio of cellulose raw material to chloroacetic acid was 1:2.

[0084] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 60min to obtain an aqueous dispersion of carboxylated nanocellulose with a concentration of 1wt%.

[0085] The carboxyl content of the prepared carboxylated cellulose nanoparticles in the aqueous dispersion was 1.8 mmol / g, the average length was 265 nm, and the average diameter was 28 nm.

[0086] (3) Add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol, stir evenly, react at 70°C for 60 min, degas, cast into a mold, and vacuum dry at 60°C for 2 h to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0087] The amount of carboxylated nanocellulose added is 8 wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 4 wt% of the mass of polyvinyl alcohol.

[0088] The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film prepared has a thickness of 20 μm, a tensile strength of 130.1 MPa, and an elongation at break of 307.1%.

[0089] Example 3

[0090] A method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, the specific steps of which are as follows:

[0091] (1) Preparation of raw materials:

[0092] Chloroacetic acid;

[0093] water;

[0094] KH550;

[0095] Cellulose raw material: wood pulp fiber;

[0096] Eutectic solvent: composed of hydrogen bond donors and hydrogen bond acceptors in a molar ratio of 1:1; the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is lactic acid;

[0097] An aqueous solution of polyvinyl alcohol with a concentration of 8 wt%, wherein the degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 88%;

[0098] (2) Preparation of an aqueous dispersion of carboxylated cellulose nanoparticles;

[0099] (a) The cellulose raw material is mixed with a eutectic solvent, stirred and reacted at 80°C for 1 h, cooled, water is added, and centrifuged to obtain pretreated cellulose; wherein the mass ratio of cellulose raw material to eutectic solvent is 1:20.

[0100] (b) The pretreated cellulose was dispersed in water to prepare a suspension with a concentration of 1 wt%, the pH was adjusted to 11, the mixture was stirred and activated for 30 min, chloroacetic acid was added, the mixture was reacted at 55 °C for 4 h, the pH was adjusted to 7, and the mixture was centrifuged and washed to obtain carboxylated cellulose; wherein the molar ratio of cellulose raw material to chloroacetic acid was 1:2.

[0101] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 35min to obtain an aqueous dispersion of carboxylated nanocellulose with a concentration of 1wt%.

[0102] The carboxyl group content of the prepared carboxylated nanocellulose in the aqueous dispersion was 1.2 mmol / g, the average length was 227 nm, and the average diameter was 18 nm.

[0103] (3) Add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol, stir evenly, react at 60°C for 45 min, degas, cast into a mold, and vacuum dry at 60°C for 2 h to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0104] The amount of carboxylated nanocellulose added is 2 wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 3 wt% of the mass of polyvinyl alcohol.

[0105] The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film prepared has a thickness of 20 μm, a tensile strength of 94.6 MPa, and an elongation at break of 235.6%.

[0106] Example 4

[0107] A method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, the specific steps of which are as follows:

[0108] (1) Preparation of raw materials:

[0109] Chloroacetic acid;

[0110] water;

[0111] KH550;

[0112] Cellulose raw material: wood pulp fiber;

[0113] Eutectic solvent: composed of hydrogen bond donor and hydrogen bond acceptor in a molar ratio of 3:1; the hydrogen bond acceptor is choline chloride and the hydrogen bond donor is oxalic acid;

[0114] An aqueous solution of polyvinyl alcohol with a concentration of 10 wt%, wherein the degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 88%;

[0115] (2) Preparation of an aqueous dispersion of carboxylated cellulose nanoparticles;

[0116] (a) The cellulose raw material is mixed with a eutectic solvent, stirred and reacted at 80°C for 6 hours, cooled, water is added, and centrifuged to obtain pretreated cellulose; wherein the mass ratio of cellulose raw material to eutectic solvent is 1:20.

[0117] (b) The pretreated cellulose was dispersed in water to prepare a suspension with a concentration of 1 wt%, the pH was adjusted to 12, the mixture was stirred and activated for 60 min, chloroacetic acid was added, the mixture was reacted at 60 °C for 4 h, the pH was adjusted to 7, and the mixture was centrifuged and washed to obtain carboxylated cellulose; wherein the molar ratio of cellulose raw material to chloroacetic acid was 1:2.

[0118] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 45min to obtain an aqueous dispersion of carboxylated nanocellulose with a concentration of 1wt%.

[0119] The carboxyl content of the prepared carboxylated cellulose nanoparticles in the aqueous dispersion was 1.6 mmol / g, the average length was 210 nm, and the average diameter was 16 nm.

[0120] (3) Add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol, stir evenly, react at 50°C for 90 min, degas, cast into a mold, and vacuum dry at 60°C for 2 h to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0121] The amount of carboxylated nanocellulose added is 6 wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 5 wt% of the mass of polyvinyl alcohol.

[0122] The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film prepared has a thickness of 20 μm, a tensile strength of 110.8 MPa, and an elongation at break of 250.9%.

[0123] Example 5

[0124] A method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, the specific steps of which are as follows:

[0125] (1) Preparation of raw materials:

[0126] Chloroacetic acid;

[0127] water;

[0128] KH550;

[0129] Cellulose raw material: wood pulp fiber;

[0130] Eutectic solvent: composed of hydrogen bond donor and hydrogen bond acceptor in a molar ratio of 2:1; the hydrogen bond acceptor is choline chloride and the hydrogen bond donor is citric acid;

[0131] An aqueous solution of polyvinyl alcohol with a concentration of 12 wt%, wherein the degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 88%;

[0132] (2) Preparation of an aqueous dispersion of carboxylated cellulose nanoparticles;

[0133] (a) The cellulose raw material is mixed with a eutectic solvent, stirred and reacted at 120°C for 4 hours, cooled, water is added, and centrifuged to obtain pretreated cellulose; wherein the mass ratio of cellulose raw material to eutectic solvent is 1:50.

[0134] (b) The pretreated cellulose was dispersed in water to prepare a suspension with a concentration of 1 wt%, the pH was adjusted to 12, the mixture was stirred and activated for 50 min, chloroacetic acid was added, the mixture was reacted at 70 °C for 5 h, the pH was adjusted to 7, and the mixture was centrifuged and washed to obtain carboxylated cellulose; wherein the molar ratio of cellulose raw material to chloroacetic acid was 1:2.5.

[0135] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 55min to obtain an aqueous dispersion of carboxylated nanocellulose with a concentration of 1wt%.

[0136] The carboxyl content of the prepared carboxylated cellulose nanoparticles in the aqueous dispersion was 1.3 mmol / g, the average length was 248 nm, and the average diameter was 25 nm.

[0137] (3) Add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol, stir evenly, react at 75°C for 30 min, degas, cast into a mold, and vacuum dry at 60°C for 2 h to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0138] The amount of carboxylated nanocellulose added is 10 wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 6 wt% of the mass of polyvinyl alcohol.

[0139] The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film prepared has a thickness of 20 μm, a tensile strength of 100.4 MPa, and an elongation at break of 227.5%.

[0140] Example 6

[0141] A method for preparing a high-strength and high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, the specific steps of which are as follows:

[0142] (1) Preparation of raw materials:

[0143] Chloroacetic acid;

[0144] water;

[0145] KH550;

[0146] Cellulose raw material: wood pulp fiber;

[0147] Eutectic solvent: composed of hydrogen bond donors and hydrogen bond acceptors in a molar ratio of 1:1; the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is a mixture of oxalic acid and citric acid in a molar ratio of 1:1.

[0148] An aqueous solution of polyvinyl alcohol with a concentration of 12 wt%, wherein the degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 88%;

[0149] (2) Preparation of an aqueous dispersion of carboxylated cellulose nanoparticles;

[0150] (a) The cellulose raw material is mixed with a eutectic solvent, stirred and reacted at 90°C for 3 hours, cooled, water is added, and centrifuged to obtain pretreated cellulose; wherein the mass ratio of cellulose raw material to eutectic solvent is 1:30.

[0151] (b) The pretreated cellulose was dispersed in water to prepare a suspension with a concentration of 1 wt%, the pH was adjusted to 10, the mixture was stirred and activated for 60 min, chloroacetic acid was added, the mixture was reacted at 60 °C for 6 h, the pH was adjusted to 7, and the mixture was centrifuged and washed to obtain carboxylated cellulose; wherein the molar ratio of cellulose raw material to chloroacetic acid was 1:1.5.

[0152] (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic disruptor (power 1000W, working mode is pulse, working for 2s, interval 2s) for 40min to obtain an aqueous dispersion of carboxylated nanocellulose with a concentration of 1wt%.

[0153] The carboxyl content of the prepared carboxylated cellulose nanoparticles in the aqueous dispersion was 1.4 mmol / g, the average length was 301 nm, and the average diameter was 27 nm.

[0154] (3) Add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol, stir evenly, react at 55℃ for 120 min, degas, cast into a mold, and vacuum dry at 60℃ for 2 h to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film.

[0155] The amount of carboxylated nanocellulose added is 1 wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 2 wt% of the mass of polyvinyl alcohol.

[0156] The high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film prepared has a thickness of 20 μm, a tensile strength of 85.7 MPa, and an elongation at break of 210.8%.

Claims

1. A method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanofiber composite membrane, characterized in that, A mixture of polyvinyl alcohol, KH550, carboxylated nanocellulose and water is reacted at 50-75℃ for 30-120 min, degassed, cast into a mold, and dried to obtain a high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film. The carboxyl content of the carboxylated nanocellulose is not less than 1.0 mmol / g, the average aspect ratio is 10-50, the amount of carboxylated nanocellulose added is 0.5wt%-10wt% of the mass of polyvinyl alcohol, and the amount of KH550 added is 1wt%-6wt% of the mass of polyvinyl alcohol.

2. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 1, characterized in that, The tensile strength of the high-strength and high-toughness polyvinyl alcohol / carboxylated nanocellulose composite film is 82-130 MPa, and the elongation at break is 201%-307%.

3. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 1, characterized in that, Polyvinyl alcohol has a degree of polymerization of 1700-2000 and a degree of alcoholysis of 88%-99%.

4. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 1, characterized in that, The carboxyl content of carboxylated nanocellulose is no higher than 1.8 mmol / g, and the average length is 210-301 nm.

5. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 4, characterized in that, The preparation process of the mixture of polyvinyl alcohol, KH550, carboxylated nanocellulose and water is as follows: add the aqueous dispersion of carboxylated nanocellulose and KH550 to the aqueous solution of polyvinyl alcohol and stir evenly.

6. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 5, characterized in that, The concentration of the aqueous solution of polyvinyl alcohol is 5wt%-12wt%.

7. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 5, characterized in that, The preparation steps of the aqueous dispersion of carboxylated cellulose nanoparticles are as follows: (a) The cellulose raw material is mixed with a eutectic solvent and stirred at 80-120°C for 1-6 hours. After cooling, water is added and the mixture is centrifuged to obtain pretreated cellulose. The eutectic solvent consists of a hydrogen bond donor and a hydrogen bond acceptor; the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is one or more of oxalic acid, citric acid, and lactic acid. (b) Disperse the pretreated cellulose in water to prepare a suspension, adjust the pH to 10-12, stir and activate for 30-60 min, add chloroacetic acid, react at 50-70℃ for 2-6 h, adjust the pH to 7, centrifuge and wash to obtain carboxylated cellulose; (c) Disperse carboxylated cellulose in water and treat it with an ultrasonic homogenizer for 30-60 min to obtain an aqueous dispersion of carboxylated nanocellulose.

8. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 7, characterized in that, In step (a), the mass ratio of cellulose raw material to eutectic solvent is 1:10-50, and the molar ratio of hydrogen bond donor to hydrogen bond acceptor is 1-3:

1.

9. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 7, characterized in that, In steps (a) and (b), the molar ratio of cellulose raw material to chloroacetic acid is 1:1.5-2.

5.

10. The method for preparing a high-strength, high-toughness polyvinyl alcohol / carboxylated cellulose nanocomposite membrane according to claim 1, characterized in that, Drying refers to vacuum drying at 60℃ for 2 hours.

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