Redispersible nanocellulose composite material, preparation method and application

Abstract

The present invention discloses a redispersible nanocellulose composite material, a preparation method and an application. The nanocellulose composite material is prepared by mixing nanocellulose and a water-soluble thioctic acid salt compound in an aqueous phase and then drying. The nanocellulose composite material is recyclable, the thioctic acid salt component can be redissolved and depolymerized into small molecules in the aqueous phase, and the nanocellulose component can be redispersed in the aqueous phase. Each component in the nanocellulose composite material of the present invention is a bio-based compound, which is green, environmentally friendly, safe and non-toxic. During the drying process, thioctic acid salt can be effectively coated on the surface of nanocellulose, which has the effect of hindering the formation of dense hydrogen bonds between nanocelluloses. The prepared dried nanocellulose composite material has controllable performance and can achieve effective separation and recovery of components in the aqueous phase. It has the advantages of high production efficiency, low cost, simple process, controllable performance, etc., and is of great significance in promoting the industrial application of nanocellulose.

Description

Technical Field

[0001] The invention relates to a nano cellulose material, in particular to a redispersible nano cellulose composite material. Background Art

[0002] Cellulose is the most abundant biomacromolecule in nature, possessing excellent mechanical properties, thermal stability, biocompatibility, and biodegradability. With technological advancements, nanocellulose with diameters below 100 nanometers has been industrialized and exhibits broad application potential in a wide range of fields, including papermaking, food, electronics, pharmaceuticals, and environmental management. Nanocellulose is primarily categorized into cellulose nanocrystals and cellulose nanofibers based on preparation processes and morphology. When nanocellulose is dispersed in water, the numerous hydroxyl groups on its surface hydrogen bond with water molecules, forming a cellulose-H2O-cellulose structure. As water evaporates, the distances between the nanocellulose nanoparticles decrease, increasing van der Waals forces and forming dense hydrogen bonds between the nanocellulose nanoparticles. When dried nanocellulose is rewetted, the hydrogen bonds are not completely broken, resulting in significant changes in its morphology and properties. To avoid irreversible property changes during the drying process, nanocellulose products are currently primarily delivered as aqueous dispersions or gels with solids content below 35%. However, products with excessive water content not only significantly increase the cost of transportation and storage, but also cause degradation of cellulose molecules by microorganisms or fungi in the aqueous phase, which greatly limits the application of nanocellulose in industry.

[0003] The reason why dried nanocellulose is difficult to redisperse is the hydrogen bonding between fibers. Therefore, minimizing the number of hydrogen bonding connections is crucial to improving the redispersibility of nanocellulose. Based on the necessary conditions for the formation of hydrogen bonds between nanocelluloses (i.e., free hydroxyl groups with similar distances (0.25nm-0.35nm)), researchers have gradually developed redispersibility strategies for modifying the surface hydroxyl groups of nanocellulose and increasing the steric hindrance between nanocelluloses, targeting the two important factors of free hydroxyl groups and spatial distance. With the increase of the surface charge of nanocellulose after hydroxyl modification, the electrostatic repulsion between nanocelluloses is enhanced, and the hydrogen bonding between fibers is effectively reduced. At the same time, grafting or embedding macromolecules on the surface of nanocellulose to increase the spatial distance between nanocelluloses can also effectively reduce the formation of hydrogen bonds. In addition, some research teams have developed process methods based on solvent exchange and controlled drying process to improve the redispersibility of nanocellulose.

[0004] Although there have been many reports on the research on the recovery and dispersion of dried nanocellulose, the redispersion effect of nanocellulose has not been effectively controlled. The following difficulties still exist: ① It is difficult to remove additives, ② the performance of modified nanocellulose is quite different from that of original nanocellulose, ③ the energy consumption is high, ④ the process is cumbersome, and ⑤ the process method has poor universality.

[0005] 1. Patent CN103275336B, "A Method for Preparing Easily Redispersible Cellulose Crystals After Drying," discloses a method for preparing aqueous redispersible cellulose crystals by mixing a nonionic surfactant with a cellulose crystal suspension. However, this method suffers from the difficulty of removing the surfactant, hindering the application of nanocellulose.

[0006] 2. Patent CN106905437B, "Highly Heat-Resistant Redispersible Powder Cellulose Nanocrystals and Preparation Method Thereof," discloses adding an ionic liquid to a cellulose nanocrystal suspension to form ionic bonds between the ionic liquid and the cellulose nanocrystals to produce dry nanocellulose with heat resistance and water redispersibility. However, this method has the disadvantages of being complex and costly, and difficult to separate the ionic liquid components.

[0007] 3. Patent CN110272503B, "A Method for Preparing Aqueous Solution-Redispersible Cellulose Nanofibrils," discloses a method for adding potassium chloride to a CNF suspension and vacuum drying to produce aqueous-redispersible dry nanocellulose. However, this method suffers from high energy consumption and difficulty in large-scale industrialization.

[0008] 4. Patent CN110818806B, "A Method for Preparing Water-Redispersible Cellulose Nanocrystals," discloses a method for preparing aqueous redispersible nanocellulose by performing an acid hydrolysis reaction at 150-180°C and a pressure of 5-20 MPa, followed by separation of the product in an organic solvent. However, this method suffers from complex processes and environmental pollution.

[0009] 5. Patent CN111187433A, "A Thioctic Acid-Modified Ethyl Cellulose Film and Its Preparation Method," discloses a method for improving the flexibility of film materials by modifying ethyl cellulose with lipoic acid. The difference lies in the raw materials used: ethyl cellulose, a cellulose derivative, is not water-soluble, and lipoic acid is also not water-soluble. The reaction system for the two is glacial acetic acid. However, the disadvantage is that the mechanical strength of the ethyl cellulose film is lower than that of nanocellulose film materials, and the reaction conditions for modifying ethyl cellulose with lipoic acid are harsh and the process is complex.

[0010] Removing water from nanocellulose dispersions while preserving the properties of nanocellulose is a major challenge in the commercialization of nanocellulose materials. It is very important to develop lossless, cost-effective, scalable and environmentally friendly redispersible dry nanocellulose products. Summary of the Invention

[0011] The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and provide a redispersible dry nanocellulose composite material without changing the original physical and chemical properties of nanocellulose. While maintaining usability, it solves the problems of poor redispersibility of dry nanocellulose, complex recycling process and high energy consumption, thereby providing conditions for the widespread application of nanocellulose products.

[0012] The present invention is achieved through the following technical solutions.

[0013] The invention provides a redispersible nano-cellulose composite material, the main components of which are nano-cellulose and a water-soluble lipoic acid salt compound.

[0014] The nanocellulose is a nanoscale microfibrillated cellulose with a diameter of less than 100 nanometers, which has a large specific surface area and excellent mechanical properties. Nanocellulose is not limited to a single type and includes cellulose nanocrystals, cellulose nanofibers, bacterial cellulose, etc., preferably short rod-shaped cellulose nanocrystals or fibrous cellulose nanofibers.

[0015] The water-soluble lipoate compound has excellent recyclability and can achieve dynamic covalent ring-opening polymerization and depolymerization by regulating the moisture content. The water-soluble lipoate compound includes any one or a combination of at least two of sodium lipoate, potassium lipoate, ammonium lipoate, and triethanolamine lipoate.

[0016] Based on the dry weight of the nanocellulose as 100%, the water-soluble lipoate compound is added in an amount of 2.5% to 300%. The amount of the water-soluble lipoate compound added significantly affects the mechanical properties and redispersibility of the dried nanocellulose material. If the amount is too low, the dense hydrogen bonding between the nanocelluloses cannot be effectively reduced, which is not conducive to subsequent recovery and redispersibility. If the amount is too high, the mechanical strength of the nanocellulose composite is too low and the cost is increased, which is not conducive to practical use. Different types of water-soluble lipoate compounds have different effects on the nanocellulose composite material. By adjusting the type and content of the salt ions, the mechanical properties and redispersibility of the dried nanocellulose material can be customized. Depending on the size of the salt ions and the strength of the ionic bond, the addition amount of sodium lipoate is preferably 100% to 300%, the addition amount of potassium lipoate is preferably 50% to 150%, the addition amount of ammonium lipoate is preferably 25% to 150%, and the addition amount of triethanolamine lipoate is preferably 2.5% to 75%.

[0017] The nanocellulose composite material is one of particles, films or gel materials.

[0018] At the same time, the present invention provides a preparation method of a nanocellulose and water-soluble lipoic acid salt composite material, the main steps of which are: blending the nanocellulose and the water-soluble lipoic acid salt compound in an aqueous phase, and then drying to prepare the nanocellulose composite material.

[0019] The mixing methods of the aqueous phase system include but are not limited to magnetic stirring, mechanical stirring, vortexing and homogenization. Based on convenience and mixing effect, the mixing methods of the aqueous phase system are preferably magnetic stirring and vortexing.

[0020] The drying treatment method includes but is not limited to oven drying, spray drying, freeze drying and supercritical drying.

[0021] The drying method significantly influences hydrogen bond formation in dried nanocellulose. Oven drying and spray drying can lead to severe aggregation of nanocellulose due to capillary forces. Freeze drying and supercritical drying, by eliminating capillary forces, effectively preserve the dispersed nanostructure of nanocellulose in the aqueous phase. Different drying methods can be selected to prepare different forms of dried nanocellulose composites, depending on the intended use and application environment.

[0022] The recovery process of the nanocellulose composite material in the present invention is to soak the nanocellulose composite material in water, replace and collect the water phase multiple times to obtain the lipoic acid salt component, and then mechanically oscillate and shear the swollen nanocellulose sample to obtain the redispersed nanocellulose component.

[0023] The redispersion medium includes, but is not limited to, deionized water, an aqueous solution with a pH value of 5 or above, a mixed solvent of water and other solvents, and the like.

[0024] The redispersion process of dried nanocellulose occurs when solvent molecules enter the nanocellulose's three-dimensional network and disrupt weak hydrogen bonds. The stronger the affinity between the redispersion medium and the nanocellulose, the more pronounced the nanocellulose swelling effect, which in turn causes more hydrogen bonds between the nanocellulose to break, improving the redispersion effect. Based on the basic physicochemical properties of nanocellulose, a neutral aqueous solution is the preferred redispersion medium.

[0025] The mechanical oscillation shearing treatment includes but is not limited to one or a combination of methods such as magnetic stirring, mechanical stirring, vortexing, homogenization, and ultrasonic dispersion.

[0026] The main mechanism of action of the present invention is explained as follows: Water-soluble lipoate compounds can be thoroughly and evenly mixed with nanocellulose dispersed in the aqueous phase. In the presence of salt ions, water-soluble lipoate molecules do not form stable hydrogen bonds with the nanocellulose. Due to the strong hydrophilicity of nanocellulose, during the drying process, as water evaporates, water-soluble lipoate molecules gradually aggregate on the nanocellulose surface. The carboxyl groups of lipoate experience electrostatic repulsion with the partially positively charged nanocellulose, while lipoate adhering to the nanocellulose surface provides steric hindrance. As the lipoate content increases, the accessibility of free hydroxyl groups on the nanocellulose surface decreases, significantly reducing hydrogen bonding and significantly improving the redispersibility of the nanocellulose in the aqueous phase. When the dried nanocellulose material is immersed in the aqueous phase, a large amount of water molecules enter, inducing the depolymerization of the lipoate polymers, and the lipoate components are completely converted into water-soluble small lipoate molecules. Simultaneously, hydrogen bonding between the nanocellulose molecules weakens, causing the composite material to swell in the aqueous phase. The rich porous network structure of the nanocellulose facilitates the efficient diffusion of the small lipoate molecules. Therefore, simple aqueous elution can be used to separate the lipoate component from the nanocellulose component. Because the hydration of the hydrophilic groups causes some weaker hydrogen bonds to break, and the mechanical shearing effect further promotes the breakage of the remaining relatively strong hydrogen bonds, the nanocellulose can be redispersed in an aqueous environment. Lipoic acid significantly inhibits the hydrogen bonding of nanocellulose, significantly reducing the energy consumption of the nanocellulose recovery process in this invention.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) In the present invention, the nanocellulose and water-soluble lipoic acid salt are both bio-based compounds, and the nanocellulose composite material prepared by blending the two is green, safe and environmentally friendly.

[0029] (2) In the present invention, the water-soluble lipoic acid salt component is coated on the surface of the nanocellulose, which can effectively reduce the hydrogen bonding between the nanocelluloses, significantly improve the redispersion efficiency of the nanocellulose, and greatly reduce the energy consumption of the redispersion process.

[0030] (3) In the present invention, the water-soluble lipoate compound has the unique property of water-induced ring-opening self-polymerization and depolymerization, which allows for effective separation from the nanocellulose during the aqueous swelling of the composite material. The hydrogen bonds between the nanocelluloses are largely destroyed during the swelling and mechanical dispersion process. The separation and recovery process of the nanocellulose and lipoate composite material is characterized by high efficiency and convenience.

[0031] (4) In the present invention, the process for preparing a recyclable nanocellulose composite material by blending a water-soluble lipoic acid salt compound with nanocellulose and then drying the mixture is universally applicable to different types of nanocellulose. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of chemical structure changes during the water molecule-driven ring-opening covalent bond self-polymerization and depolymerization process of the lipoic acid salt compound of the present invention;

[0033] Figure 2 Schematic diagram of the present invention relating to the hindering effect of the lipoic acid salt component in the dry nanocellulose material on the hydrogen bonding between nanocelluloses;

[0034] Figure 3 It is the infrared spectrum of sodium thioctic acid recovered in the present invention;

[0035] Figure 4 is an atomic force microscopy image of the redispersed cellulose nanocrystals of the present invention;

[0036] Figure 5 This is an atomic force microscope image of the redispersed cellulose nanofibers in the present invention. DETAILED DESCRIPTION

[0037] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0038] The main components of the redispersible nanocellulose composite material of the present invention are nanocellulose and a water-soluble lipoic acid salt compound. The preparation method is to blend the nanocellulose and the lipoic acid salt compound in a dispersion medium and then dry them to prepare the composite material.

[0039] Among the main components, nanocellulose is water-dispersible nanocellulose with a diameter of less than 100 nanometers. It includes short, rod-shaped cellulose nanocrystals with diameters of 10-100 nanometers and fibrous cellulose nanofibers with diameters of 2-100 nanometers. Cellulose nanocrystals are prepared by using acid or enzymatic hydrolysis to degrade the relatively loose, amorphous regions of cellulose pulp to obtain hydrolyzed cellulose with high crystallinity. Mechanical treatment is then used to further isolate the cellulose microcrystals into nanoscale crystals. Strategies for preparing cellulose nanofibers include "top-down" exfoliation and "bottom-up" assembly. The "top-down" preparation strategy includes both mechanical treatment alone and a combination of chemical pretreatment and mechanical treatment. Chemical pretreatment methods primarily involve oxidation and grafting of hydroxyl groups. Oxidation methods include TEMPO oxidation, sodium periodate oxidation, and hypochlorite oxidation. Grafting modifications include amination, esterification, and silanization. Mechanical treatment methods include high-pressure homogenization, ball milling, microfluidization, and ultrasonication. The "bottom-up" preparation strategy includes microbial fermentation, biological tissue culture, and dissolution regeneration. Among them, the cellulose nanofibers produced by microbial fermentation are bacterial cellulose.

[0040] Among the main components, water-soluble lipoic acid salt compounds are formed by the neutralization reaction of lipoic acid and alkaline compounds. Specifically, sodium lipoate, potassium lipoate, lipoic acid ammonium, lipoic acid triethanolamine, cesium lipoate, etc. can be listed.

[0041] In the preparation method, water is typically used as a mixing medium for the nanocellulose and water-soluble lipoic acid salt. The nanocellulose aqueous dispersion is directly blended with the lipoic acid salt aqueous solution, and the system is uniformly mixed using mechanical oscillation and shearing methods, such as magnetic stirring, mechanical stirring, vortexing, and homogenization. There is no particular limitation on the concentration of the nanocellulose and lipoic acid salt in the mixed system, as long as they can be fully mixed.

[0042] Water-soluble lipoic acid salts have unique dynamic covalent ring-opening polymerization properties, which can achieve self-polymerization and self-depolymerization driven by water molecules ( Figure 1 The carboxyl groups in lipoic acid salts dissolve in the form of carboxylates (there is an electrostatic repulsion between them and some nanocelluloses with negatively charged groups). Lipoic acid salts can form a stable and uniform mixed system with nanocellulose in the aqueous phase. During the drying process, as the water evaporates, the lipoic acid salt components will migrate to the surface of the nanocellulose, and some lipoic acid salts with appropriate spatial distances will self-polymerize to form polymers after drying. The size of lipoic acid molecules is greater than 0.35 nanometers, which can effectively hinder the formation of hydrogen bonds between nanocelluloses ( Figure 2 ).

[0043] Different drying methods can be used to prepare composite materials with significantly different distributions of nanocellulose and lipoic acid salts. Oven drying is suitable for preparing nanocellulose composite film materials, as the drying process is relatively slow and the distribution of lipoic acid salt and nanocellulose is more uniform. Spray drying is suitable for preparing nanocellulose composite granular materials, as it has high drying efficiency and good lipoic acid salt coating on the nanocellulose surface. Freeze drying and supercritical drying are suitable for preparing nanocellulose composite gel materials. Because the composite system is converted into a solid-phase frozen sample or alcohol gel sample before drying, the three-dimensional pore structure of the nanocellulose network is rich.

[0044] According to the differences in usage and performance requirements, the types, ratios and drying methods of nanocellulose and lipoic acid salts can be specifically adjusted.

[0045] The recovery and redispersion steps for dried composite materials are relatively simple, consisting of aqueous phase replacement and mechanical treatment. The appropriate treatment method can be selected based on the material's form. The aqueous swelling of nanocellulose disrupts some hydrogen bonds, making mechanical redispersion of loosely structured gels relatively gentle. However, denser particles or films experience more intense shearing.

[0046] The present invention will be further described below with reference to the embodiments.

[0047] Example 1

[0048] In this example, the research results on the effect of sodium lipoate content on the mechanical properties of cellulose nanocrystal composite film materials are described.

[0049] An appropriate amount of cellulose pulp was added to a 64% sulfuric acid solution at a mass ratio of 1:10. Acid hydrolysis was performed in a 45°C water bath with stirring. After 40 minutes, 10 volumes of cold deionized water were added to terminate the reaction. The mixed dispersion was cleaned by multiple centrifugation steps. The supernatant was decanted and the precipitate was collected. The precipitate was dialyzed in a dialysis bag until the pH was neutral. The cleaned suspension was ultrasonically dispersed, centrifuged, and the supernatant, representing the cellulose nanocrystal dispersion, was collected.

[0050] Add equal molar amounts of lipoic acid and sodium hydroxide into a beaker filled with an appropriate amount of deionized water, stir the mixture with a magnetic stirrer, and react for 1 hour to obtain a transparent yellow solution with a pH value of about 7-8, i.e., a sodium lipoate aqueous solution.

[0051] Different amounts of cellulose nanocrystal dispersion (0.2g dry weight) at a 2% concentration were added to the cellulose nanocrystal dispersion. Aqueous sodium lipoate solutions containing 2.5%, 7.5%, 15%, 25%, 50%, 75%, 100%, 150%, and 300% cellulose by dry weight were weighed and added to the cellulose nanocrystal dispersion. The different mixtures were stirred on a magnetic stirrer for one hour to obtain composite systems with varying sodium lipoate contents after uniform mixing.

[0052] The different mixed systems were added into polyethylene plastic culture dishes with a diameter of 9 cm, respectively, and placed in a forced air oven at 40° C. to dry for 24 hours to obtain a dry nanocellulose composite film material.

[0053] The mechanical properties of different composite film samples were tested using a universal testing machine, and their main performance data are shown in Table 1.

[0054] Table 1 Mechanical properties of nanocellulose composite films with different sodium lipoate contents

[0055]

[0056]

[0057] The results show that the sodium lipoate content significantly affects the tensile strength and elongation of the nanocellulose composite film. Low addition levels promote uniform distribution of the nanocellulose, improving both strength and toughness. High addition levels increase the brittleness of the nanocellulose composite film and reduce its tensile strength.

[0058] Example 2

[0059] In this embodiment, the research results on the effect of thioctic acid triethanolamine content on the mechanical properties of cellulose nanocrystal composite film materials are described.

[0060] Disperse 5 g of bleached hardwood pulp in 500 ml of water and stir evenly with a magnetic stirrer. Then, weigh 0.5 g of sodium bromide and 0.08 g of TEMPO and dissolve them in the cellulose suspension. After the mixture is stirred evenly, add 30 mmol of sodium hypochlorite to the system for oxidation. As the oxidation proceeds, the pH of the reaction system decreases. During this time, add 0.5 M sodium hydroxide solution to maintain the pH at 10. After 6 hours of oxidation, add 2 ml of ethanol to terminate the reaction. The oxidized cellulose is then filtered, and the precipitate is washed in 1 liter of deionized water and filtered. This washing process is repeated five times. After thorough cleaning, add an appropriate amount of oxidized cellulose to 100 ml of deionized water, stir and disperse evenly, and ultrasonically disperse the system using an ultrasonic disruptor. The ultrasonically dispersed oxidized cellulose suspension is centrifuged, and the supernatant is collected to form the cellulose nanofiber dispersion.

[0061] Add equal molar amounts of lipoic acid and triethanolamine into a beaker filled with an appropriate amount of deionized water, stir the mixture with a magnetic stirrer, and react for 1 hour to obtain a transparent yellow solution with a pH value of about 7-8, which is a lipoic acid triethanolamine aqueous solution.

[0062] Take different portions of cellulose nanofiber dispersion with a dry weight of 0.16 g and a concentration of 2%, and weigh sodium lipoate aqueous solutions with a dry weight of 2.5%, 7.5%, 15%, 25%, 50%, 75%, 100%, 150%, and 300% of cellulose and add them to the cellulose nanofiber dispersion respectively. Place the different mixed systems on a mechanical stirrer and stir for 1 hour. After mixing evenly, composite systems with different sodium lipoate contents can be obtained.

[0063] The different mixed systems were added into polyethylene plastic culture dishes with a diameter of 6 cm, respectively, and placed in a forced air oven at 50° C. for 16 hours to obtain dry nanocellulose composite film materials.

[0064] The mechanical properties of different composite film samples were tested using a universal testing machine, and their main performance data are shown in Table 1.

[0065] Table 1 Mechanical properties of nanocellulose composite films with different sodium lipoate contents

[0066] Lipoic acid triethanolamine / nanocellulose (%) Tensile strength (MPa) Elongation (%) 0 140.5 3.21 2.5 138.3 3.54 7.5 132.7 3.97 15 126.2 4.46 25 120.6 5.04 50 82.7 8.78 75 41.6 17.6 100 29.1 21.4 150 19.6 27.8 300 20.4 36.1

[0067] The results show that the content of thioctic acid triethanolamine has a significant effect on the tensile strength and elongation of the nanocellulose composite film. As the content of thioctic acid triethanolamine increases, the tensile strength of the composite film material shows a downward trend, while the plasticity continues to improve and the elongation at break gradually increases.

[0068] Example 3

[0069] In this example, the research results on the effects of lipoic acid salt types on the mechanical properties of cellulose nanofiber composite film materials are described.

[0070] An appropriate amount of bleached softwood kraft pulp was added to deionized water and stirred to prepare a 1% cellulose suspension. The cellulose suspension was homogenized five times using a nano-microfluidizer at a pressure of 15,000 bar to obtain a cellulose nanofiber dispersion.

[0071] Multiple portions of lipoic acid in equal molar amounts are dispersed in deionized water, and equal molar amounts of sodium hydroxide, potassium hydroxide, ammonia water, and triethanolamine are respectively added to the lipoic acid dispersion. The mixed system is stirred with a magnetic stirrer. After reacting for 1 hour, a transparent yellow solution with a pH value of about 7-8 is obtained, which is an aqueous solution of sodium lipoate, potassium lipoate, lipoic acid ammonia, and lipoic acid triethanolamine.

[0072] Take multiple portions of cellulose nanocrystal dispersion with a dry weight of 0.2 grams and a concentration of 0.6%, weigh a thioctic acid salt aqueous solution with a dry weight of 50% of cellulose and add them to the cellulose nanofiber dispersion respectively. Place the different mixed systems on a vortex instrument and shake for 2 minutes. After mixing evenly, a nanocellulose composite system containing different thioctic acid salts can be obtained.

[0073] The different mixed systems were added into polyethylene plastic culture dishes with a diameter of 9 cm, respectively, and placed in a forced air oven at 45° C. to dry for 24 hours to obtain a dry nanocellulose composite film material.

[0074] The mechanical properties of different composite film samples were tested using a universal testing machine, and their main performance data are shown in Table 1.

[0075] Table 1 Mechanical properties of nanocellulose composite films with different sodium lipoate contents

[0076] Types of lipoic acid salts Tensile strength (MPa) Elongation (%) Sodium lipoate 24.4 3.04 Potassium lipoate 30.8 2.96 Lipoic acid 28.7 2.81 Lipoic acid triethanolamine 15.2 9.65

[0077] The results show that the type of lipoic acid salt has a significant effect on the mechanical properties of the nanocellulose composite film, and the difference in the properties of the salt ions directly affects the tensile strength and elongation at break of the composite film.

[0078] Example 4

[0079] In this example, the research results on the influence of the type of nanocellulose on the mechanical properties of the composite film material are described.

[0080] The cellulose nanocrystals are the same as the sample in Example 1 and are labeled as nanocellulose①.

[0081] The carboxylated cellulose nanofibers are the same as the sample in Example 2 and are labeled as nanocellulose②.

[0082] The cellulose nanofibers prepared by simple mechanical treatment are the same as the sample in Example 3 and are marked as nanocellulose ③.

[0083] Bacterial cellulose is labeled as nanocellulose ④: First, the seed culture medium and fermentation medium are prepared with a ratio of 20 grams of glucose, 5 grams of yeast powder, 5 grams of peptone, 2.7 grams of disodium hydrogen phosphate, and 1.15 grams of citric acid per liter. Place the Acetobacter xylinum in a constant temperature incubator at 30°C for activation for 30 minutes, use an inoculation loop to pick up a trace of colonies from the slant and transfer them to the seed culture medium, place the inoculated seed culture medium in a shaking incubator, and culture it at 30°C and 150rpm for 24 hours. Subsequently, the cultured seed liquid is further inoculated into the fermentation medium at an inoculation rate of 8%, and the inoculated fermentation medium is placed in a constant temperature shaking incubator for shaking culture for 12 hours. Thereafter, the fermentation liquid containing the flocculent precipitate is poured into a stainless steel shallow dish and statically cultured at 30°C in a constant temperature incubator. The bacterial cellulose membrane prepared by static fermentation was placed in a sink and rinsed continuously with running tap water for 48 hours. The membrane was then immersed in a 100-ml mixed solution containing 3.0 g of sodium hydroxide and 3 ml of hydrogen peroxide in an 80°C waterbath for 5 hours. The membrane was then repeatedly rinsed with deionized water until the pH was near neutral. The white gel-like component was dispersed in deionized water and subjected to high-pressure homogenization to obtain a bacterial cellulose dispersion.

[0084] Take 0.18 grams of nanocellulose dispersion of different types, weigh 50% cellulose dry weight of lipoic acid triethanolamine aqueous solution and add different nanocellulose dispersions respectively, place different mixed systems on a magnetic stirrer and stir for 2 hours, and after mixing evenly, a composite system containing different types of nanocellulose can be obtained.

[0085] The different mixed systems were added into polyethylene plastic culture dishes with a diameter of 9 cm, respectively, and placed in a forced air oven at 50° C. for 18 hours to obtain dry nanocellulose composite film materials.

[0086] The mechanical properties of different composite film samples were tested using a universal testing machine, and their main performance data are shown in Table 1.

[0087] Table 1 Mechanical properties of nanocellulose composite films with different sodium lipoate contents

[0088] Types of nanocellulose Tensile strength (MPa) Elongation (%) Nanocellulose① 14.6 5.86 Nanocellulose② 82.7 8.78 Nanocellulose③ 17.3 9.95 Nanocellulose④ 22.8 9.24

[0089] The results show that due to the differences in the physicochemical properties of different types of nanocellulose, the mechanical properties of the nanocellulose composite film materials prepared from them are significantly different.

[0090] Example 5

[0091] In this embodiment, different forms of nanocellulose composite materials are described.

[0092] The nanocellulose mixed system is the mixed system of cellulose nanofibers and potassium lipoate in Example 3.

[0093] The preparation process of the nanocellulose composite film material is the same as that in Example 3, and the drying method is oven drying. During the drying process, the nanocellulose slowly stacks under the action of gravity to form a dense film connected by hydrogen bonds.

[0094] An appropriate amount of the mixed system was added to a centrifuge tube, frozen in a refrigerator, and then placed in a freeze dryer for 72 hours to obtain a dry nanocellulose composite aerogel material. Because the aqueous mixture system had already transformed into a solid phase before drying, the spatial structure of the nanocellulose and lipoic acid salt was fixed, and the dried aerogel material had a relatively rich pore structure.

[0095] An appropriate amount of the mixed system was added to a centrifuge tube, and ethanol was added for solvent replacement to prepare a nanocellulose composite ethanol gel. The alcohol gel was placed in the chamber of a supercritical dryer and treated under a stream of liquid carbon dioxide at 15°C for 8 hours. The chamber temperature was then raised to 40°C to obtain a supercritical phase and treated for another hour. The pressure was then gradually reduced at 40°C for another hour. The resulting aerogel was a supercritically dried nanocellulose composite aerogel material. Compared to freeze-drying, due to solvent replacement, the aerogel material prepared by supercritical drying has a richer pore structure and a relatively more uniform structure, and the hydrogen bonds formed between the nanocelluloses are also relatively more uniform.

[0096] An appropriate amount of the mixed system is added to a spray dryer, where the atomizer disperses the raw liquid into a mist of droplets. These droplets, dried by hot gases, transform into nanocellulose composite particles. Due to the rapid evaporation of the solvent during the spray drying process, the nanocellulose droplets shrink significantly as they transform into particles, forming dense hydrogen bonds between the nanocellulose particles.

[0097] Example 6

[0098] In this example, the research results on the recycling of some cellulose nanocrystal composite film materials in Examples 1 and 2 are described.

[0099] Take 0.15 grams of nanocellulose composite film material and add it to a beaker filled with deionized water. With the self-depolymerization of the thioctic acid salt polymer and the wetting of the nanocellulose, the film swells. Pour the collected aqueous phase at intervals of 20 minutes and replace with new deionized water to wash the swollen film, and repeat the process 5 times. The collected aqueous phase system is a thioctic acid salt aqueous solution. The thioctic acid salt can be converted into thioctic acid for recycling or directly stored in the form of thioctic acid salt according to needs. Add the cleaned swollen nanocellulose material to new deionized water and homogenize the system with a high-speed homogenizer for 1 minute. After shearing, the nanocellulose will be redispersed in the aqueous phase and converted into a nanocellulose aqueous phase dispersion. The recovered thioctic acid salt was characterized by an infrared spectrometer, as shown in FIG. Figure 3As shown in Figure 2, its spectrum is almost the same as that of the original lipoic acid salt. Atomic force microscopy was used to characterize the redispersed nanocellulose, and its morphology is shown in Figure 2. Figure 4 As shown, it is in a completely isolated and dispersed state, which is consistent with the original undried dispersion state.

[0100] Example 7

[0101] In this example, the results of the research on the recycling of part of the cellulose nanofiber composite film material in Example 3 are described.

[0102] Take 0.1 gram of nanocellulose composite film material and add it to a beaker filled with deionized water. Pour the collected aqueous phase every 20 minutes and replace it with new deionized water to wash the swollen film. Repeat the process 5 times. The collected aqueous phase system is a thioctic acid aqueous solution. The thioctic acid can be converted into thioctic acid for recycling or directly stored in the form of thioctic acid according to needs. Add the cleaned swollen nanocellulose material to new deionized water and stir the system with a magnetic stirrer for 1 hour. The nanocellulose film can be converted into a nanocellulose aqueous phase dispersion. Atomic force microscopy was used to characterize the redispersed nanocellulose, and its morphology is as follows Figure 5 As shown, it is consistent with the original undried dispersion state.

[0103] Example 8

[0104] In this example, the results of the research on the recycling of nanocellulose composite film materials prepared from some different nanocellulose types in Example 4 are described.

[0105] 0.15 grams of nanocellulose composite film was added to a beaker filled with deionized water. The aqueous phase was decanted and replaced with fresh deionized water every 20 minutes to rinse the swollen film. This process was repeated eight times. The decanted aqueous phase, a lipoic acid salt solution, was dried and recovered. The cleaned, swollen nanocellulose material was added to fresh deionized water and mechanically treated with an ultrasonicator. The nanocellulose film was converted into an aqueous nanocellulose dispersion. Comparison of the redispersed nanocellulose with the original nanocellulose revealed nearly identical morphology and chemical properties.

[0106] Example 9

[0107] In this example, the results of the research on the recycling of some nano-cellulose composite particulate materials in Example 5 are described.

[0108] 0.15 grams of nanocellulose composite particles were added to a beaker filled with deionized water. The aqueous phase was centrifuged every 20 minutes and replaced with fresh deionized water for washing. This process was repeated five times. The aqueous phase collected by centrifugation was a lipoic acid salt solution, which was then dried and recovered. The cleaned, swollen nanocellulose material was added to fresh deionized water and mechanically treated using a magnetic stirrer, a vortexer, a high-speed homogenizer, and an ultrasonic disruptor. The nanocellulose particles were converted into an aqueous nanocellulose dispersion. Comparative testing revealed that the redispersed nanocellulose was virtually identical to the original nanocellulose in terms of morphology and chemical properties.

[0109] Example 10

[0110] In this example, the results of the research on the recycling of part of the nanocellulose composite gel material in Example 5 are described.

[0111] 0.15 grams of the nanocellulose composite gel material was added to a beaker filled with deionized water. The aqueous phase was collected by filtration at 5-minute intervals and replaced with fresh deionized water for rinsing. This process was repeated three times. The filtered and collected aqueous phase was a lipoic acid salt aqueous solution, which was then dried and recovered. The cleaned, swollen nanocellulose material was added to fresh deionized water and mechanically treated using a magnetic stirrer, a vortexer, a high-speed homogenizer, and an ultrasonic disruptor. The nanocellulose gel was converted into a nanocellulose aqueous dispersion. Comparative tests revealed that the redispersed nanocellulose was virtually identical to the original nanocellulose in terms of morphology and chemical properties.

Claims

1. A method for preparing a redispersible nanocellulose composite material, characterized in that: Nanocellulose and a water-soluble lipoate compound are blended in an aqueous phase and then dried to form a nanocellulose composite material; based on 100% mass of the nanocellulose, the mass of the water-soluble lipoate compound is 2.5%-300%; the water-soluble lipoate compound can achieve self-polymerization and self-depolymerization under the drive of water molecules.

2. The method for preparing a redispersible nanocellulose composite material according to claim 1, wherein: Nanocellulose is short rod-shaped cellulose nanocrystals or fibrous cellulose nanofibers with a diameter of less than 100 nanometers.

3. The method for preparing a redispersible nanocellulose composite material according to claim 1, wherein: The water-soluble lipoic acid salt compound is formed by the neutralization reaction of lipoic acid and alkaline substances, including any one of sodium lipoate, potassium lipoate, lipoic acid ammonium, and lipoic acid triethanolamine, or a combination of at least two thereof.

4. The method for preparing a redispersible nanocellulose composite material according to claim 1, wherein: The nanocellulose composite material is a kind of particle, film or gel material.

5. The method for preparing a redispersible nanocellulose composite material according to claim 1, wherein: The nanocellulose composite material can be transformed into a mixed system of water-soluble lipoic acid salt compound and nanocellulose dispersion in an aqueous medium and is recyclable.

6. The method for preparing a redispersible nanocellulose composite material according to claim 5, characterized in that: The recovery process involves soaking the redispersible nanocellulose composite material in an aqueous phase, replacing and collecting the aqueous phase multiple times to obtain a water-soluble lipoic acid salt compound component, and then subjecting the swollen nanocellulose sample to mechanical oscillation shear treatment to obtain a redispersible nanocellulose component.

7. The method for preparing a redispersible nanocellulose composite material according to claim 1, wherein: The blending method of the nanocellulose and the water-soluble lipoic acid salt compound is a mixing treatment method based on mechanical shearing; the drying method is one of spray drying, oven drying, freeze drying or supercritical drying.

8. A redispersible nanocellulose composite material prepared by the preparation method according to any one of claims 1 to 7.

9. Use of a redispersible nanocellulose composite material prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The redispersible nanocellulose composite material is used in the fields of coating materials, packaging materials, preparation of medical dressings, preparation of drug carriers, and electronic substrates.

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