High-strength cellulose membrane and preparation method thereof

Abstract

The invention relates to the technical field of natural polymer material processing, in particular to a high-strength cellulose membrane and a preparation method thereof.The preparation method comprises the following steps that a solvent is prepared from an alkaline aqueous solution and a stabilizer to dissolve cellulose, and a cellulose solution is obtained after filtering and defoaming; the cellulose solution is regenerated in a salting-out bath after being extruded through a slit, and a carbonate solution or a mixed salt solution of carbonate and sulfate is selected as the salting-out bath; and washing, drafting and drying the regenerated product to obtain the high-strength cellulose membrane. Carbonate and sulfate ions in the salting-out bath can polarize hydrated water molecules around cellulose molecular chains, destroy hydrogen bond interaction between the cellulose molecular chains and water and water molecules and interfere hydrophobic interaction of the cellulose molecular chains, so that the water molecules are discharged from the cellulose molecular chains; the interaction among hydroxyl groups on a cellulose skeleton is enhanced, compact and uniform nanofibers are formed, and the mechanical property of the film is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of natural polymer material processing, and particularly relates to a high-strength cellulose film and a preparation method thereof. Background Art

[0002] Films are a type of material products widely used in industry and daily life, and among them, regenerated cellulose films are an important category of products. Similar to cellulose fibers, the viscose method and the cuprammonium method are mainly used in industry for production, and the products obtained thereby are respectively called Cellophane and Cuprophane. These two film products have the advantages of strong mechanical properties, good hydrophilic properties, good heat resistance, and excellent biological stability. Secondly, compared with common synthetic polymer film products such as PE, PVC, and PET, regenerated cellulose films also have the advantages of raw materials being free of chemical synthesis, non-toxic degradation, and recyclability. Therefore, cellulose films are a very promising macromolecular material. Unfortunately, the two methods commonly used in industry at present will use or generate toxic CS2 and heavy metal ions during the production process, posing a serious threat to human health and the ecological environment.

[0003] The method for regenerated cellulose films usually adopts the phase separation method of immersing the cellulose solution in a coagulation bath after casting. Currently, the types of coagulation baths mainly used are aqueous alcohol solutions, aqueous acid solutions, or mixed aqueous solutions of acids and their salts (sulfates, acetates, etc.) as coagulation baths. Compared with cellulose spinning, due to the limitation of the specific surface area of the membrane material, preparing membrane materials usually requires a longer coagulation time and more efficient coagulation performance of the coagulation bath itself. Due to the limitation of the coagulation bath performance, gels are not easily formed during the stretching and winding process of continuous production and are difficult to roll up, seriously affecting the preparation efficiency of cellulose films and thus limiting the application of cellulose films. Secondly, organic reagents such as alcohols used in traditional coagulation baths are volatile, bringing safety hazards to the production line, and the generation of volatile gases will also cause damage to human health and inevitable environmental pollution, and there is a large loss during the regeneration process and poor reuse rate; for acid coagulation baths, due to their strong corrosiveness, high requirements are imposed on the container equipment materials, and there is also a risk of exposure corrosion for operators. On the other hand, the neutralization of alkali and acid in the solution not only generates a large amount of heat causing temperature fluctuations but also significantly dilutes the concentration of the coagulation bath, greatly increasing the cost while the large amount of waste liquid generated is difficult to treat and utilize. Therefore, there is an urgent need to seek a more safe, environmentally friendly, recyclable, and highly efficient regenerated coagulation bath. Summary of the Invention

[0004] One of the objectives of the present invention is to provide a method for preparing a high-strength cellulose film, which uses an efficient salting-out method to rapidly and continuously produce high-strength cellulose films. In terms of the preparation mechanism, carbonate ions and sulfate ions can effectively increase the number and density of hydrogen bonds between cellulose molecular chains through strong hydration, promoting the formation of nanofibers. Since the solubility of carbonate at low temperatures is higher than that of sulfate, carbonate solutions or mixed salt solutions of carbonate and sulfate can be used to successfully prepare strong salting-out solutions with high concentrations at low temperatures, which is conducive to the efficient and rapid production of high-strength cellulose films. Secondly, carbonate and sulfate aqueous solutions have low costs, do not react with solvents, are non-toxic, have simple temperature control and can be recycled, and are particularly suitable for the preparation of high-strength cellulose films. The film forms rapidly and has a short production cycle, which is conducive to the rapid and efficient preparation of a large number of high-strength cellulose films.

[0005] Another objective of the present invention is to provide a high-strength cellulose film.

[0006] The solution adopted by the present invention to achieve the first objective is: a method for preparing a high-strength cellulose film, comprising the following steps:

[0007] (1) Using an alkaline aqueous solution and a stabilizer to prepare a solvent to dissolve cellulose, and obtaining a cellulose solution after filtration and degassing;

[0008] (2) The cellulose solution is extruded through a slit and regenerated in a salting-out bath, and the salting-out bath is selected from carbonate solutions or mixed salt solutions of carbonate and sulfate;

[0009] (3) The regenerated product is washed, stretched, and dried to obtain a high-strength cellulose film.

[0010] Preferably, in step (1), the solvent is pre-cooled to -20 to 0 °C and then cellulose is added, and it can be completely dissolved after high-speed stirring.

[0011] Preferably, in step (1), the alkali used is at least one of LiOH and NaOH, the concentration of the alkali in the solvent is 3 wt% to 12 wt%, the concentration of the stabilizer in the solvent is 0 to 20 wt%, and the stabilizer is at least one of urea and thiourea.

[0012] Preferably, in step (1), the concentration of cellulose in the cellulose solution is 2 wt% to 12 wt%.

[0013] Preferably, in step (1), it further includes adding a functional filler to the cellulose solution to obtain a cellulose / functional filler composite solution, and the functional filler is an organic or inorganic additive, including at least one of metal oxides, metal nitrides, graphene and its derivatives, carbon nanotubes and their derivatives, organic framework compounds, plasticizers, pore-forming agents, reinforcing agents, refractory additives, and dyes.

[0014] Preferably, in step (2), the salting-out bath is a single salting-out bath or a multi-stage salting-out bath.

[0015] Preferably, in step (2), the total concentration of salt in the salting-out bath is 5 wt% to 60 wt%, carbonate and sulfate are selected from potassium salt, sodium salt and lithium salt, and the salting-out bath is prepared in any proportion within the solubility range of the salt.

[0016] Preferably, in step (2), the temperature of the salting-out bath is -15 to 20°C.

[0017] Preferably, the stretching in step (3) is a single-stage stretching or a multi-stage stretching.

[0018] The technical solution adopted by the present invention to achieve the second purpose is: a high-strength cellulose film is prepared by adopting the preparation method.

[0019] The regeneration mechanism of salt solution is different from that of alcohol and acid. The carbonate ions and sulfate ions in the salt solution can polarize the hydrated water molecules around the cellulose molecular chain, destroy the hydrogen bond interaction between the cellulose molecular chain and its water and water molecules, interfere with the hydrophobic interaction of the cellulose molecular chain, make the water molecules discharged from the cellulose molecular chain, enhance the interaction between the hydroxyl groups on the cellulose skeleton, cause the lateral aggregation and recrystallization of the cellulose molecular chain, and form dense and uniform nanofibers; the higher the ion concentration, the more intense the salting-out effect will be. The concentration of the salt solution plays a key role in the forming speed and mechanical properties of the film; in the gelation process of the cellulose solution dissolved in alkali / urea at a higher temperature, the hydrophobic aggregation effect dominates, and the cellulose nanofibers gradually wrap and aggregate to form nanofiber aggregates, resulting in a decrease in the mechanical properties of the material. Low-temperature salt solution is more conducive to reducing the hydrophobic aggregation effect, promoting the formation of hydrogen bonds between chains and effectively inhibiting the generation of water vapor, ensuring the accuracy of the salting-out bath concentration during the preparation process.

[0020] Due to the reaction inertness of salt solution and alkali system, the residual solvent in the salting-out bath is easy to separate from the salt, and the recovered components can be further used to prepare the solvent and the salting-out bath. Experiments have shown that it can be recycled many times and the properties of the salting-out bath remain unchanged. While saving a lot of costs, it also has the potential to reduce carbon emissions. The excellent mechanical properties brought by using brine solution as the salting-out bath can enable the continuous production of regenerated cellulose film, greatly broadening the application prospects of regenerated cellulose.

[0021] The present invention has the following advantages and beneficial effects:

[0022] (1) In the salting-out bath used in the present invention, carbonate and sulfate salt ions can polarize the hydrated water molecules around the cellulose molecular chain, disrupt the hydrogen bond interaction between the cellulose molecular chain and its hydrated water molecules, interfere with the hydrophobic interaction of the cellulose molecular chain, cause the water molecules to be discharged from between the cellulose molecular chains, enhance the interaction between the hydroxyl groups on the cellulose backbone, lead to the lateral aggregation and recrystallization of the cellulose molecular chain, form dense and uniform nanofibers, and significantly improve the mechanical properties of the film.

[0023] (2) The salting-out bath used in the present invention has more excellent regeneration performance compared with the alcohol and acid components in the traditional coagulation bath. At the same time, it has better safety and stability, lower process difficulty and lower cost. The preparation method is green and environmentally friendly, with a high safety factor, which is conducive to industrialization.

[0024] (3) The film prepared in the present invention has a rapid film formation, a short production cycle, and excellent mechanical properties, which is conducive to the rapid and efficient preparation of a large number of high-strength cellulose films.

[0025] (4) The cellulose film prepared by the present invention through the dissolution and efficient salting-out regeneration preparation method has a simple preparation method. Secondly, it is convenient to incorporate various functional fillers. It is biodegradable while having excellent mechanical properties, avoiding environmental pollution. It has potential application value in the fields of packaging engineering, optical devices, and electrical engineering. Description of the Drawings

[0026] Figure 1 Shown is the SEM image of the cellulose membrane hydrogel obtained after water washing in Example 6;

[0027] Figure 2 Shown is the SEM image of the cellulose membrane hydrogel obtained after water washing in Comparative Example 1. Detailed Embodiments

[0028] To better understand the present invention, the following examples are further descriptions of the present invention, but the content of the present invention is not limited to the following examples only.

[0029] <Example 1>

[0030] Prepare an aqueous solution of LiOH / urea with a concentration of 3wt% / 15wt%, pre-cool it to 0 °C, add the cellulose raw material, and obtain a cellulose solution with a final concentration of 2wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film by a 35wt% K2CO3 solution at 0 °C. After passing through a stretching unit with a draw ratio of 1.1, it is immersed in a second 5wt% K2CO3 solution at 0 °C, and then after passing through a second stretching unit with a draw ratio of 1.2, it passes through a water washing unit. After being washed clean, it is drum-dried in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0031] Example 2

[0032] Prepare an aqueous solution of LiOH / NaOH / urea with concentrations of 2.5 wt% / 3.5 wt% / 10 wt% respectively. After pre-cooling to -10 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 6 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then coagulated into a film by a 0 °C 50 wt% K2CO3 solution. After passing through a stretching unit with a draw ratio of 1.1, it passes through a water washing unit. After being washed clean, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0033] Example 3

[0034] Prepare an aqueous solution of LiOH / NaOH with concentrations of 6 wt% / 3 wt% respectively. After pre-cooling to -15 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then coagulated into a film by a -15 °C 40 wt% K2CO3 solution. After passing through a stretching unit with a draw ratio of 1.1, it is then immersed in a second -15 °C 20 wt% K2CO3 solution, and then passes through a second stretching unit with a draw ratio of 1.1. After passing through a water washing unit, after being washed clean, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0035] Example 4

[0036] Prepare an aqueous solution of LiOH / urea with concentrations of 12 wt% / 20 wt% respectively. After pre-cooling to -20 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 12 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then coagulated into a film by a -15 °C 20 wt% K2CO3 solution. After passing through a stretching unit with a draw ratio of 1.3, it is then immersed in a second 5 °C 7 wt% Na2CO3 solution, and then passes through a second stretching unit with a draw ratio of 1.4. After passing through a water washing unit, after being washed clean, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0037] Example 5

[0038] Prepare aqueous solutions of LiOH / urea with concentrations of 7 wt% / 12 wt% respectively. After pre-cooling to -20 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 7 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film after passing through a 15 °C 45 wt% K2CO3 solution. After passing through a stretching unit with a draw ratio of 1.2, it is then immersed in a second 20 °C 60 wt% mixed salt (15 wt% Na2CO3 / 45 wt% K2CO3) solution. After passing through a second stretching unit with a draw ratio of 1.3, it passes through a water washing unit. After being washed clean, it passes through a third stretching unit with a draw ratio of 1.1 and then passes through a five-stage drying oven for air drying and is wound up to obtain a regenerated cellulose film.

[0039] <Example 6>

[0040] Prepare aqueous solutions of LiOH / urea with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film after passing through a first 5 °C 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution. After passing through a stretching unit with a draw ratio of 1.1, it is then immersed in a second 10 °C 43 wt% mixed salt (35 wt% K2CO3 / 8 wt% Na2SO4) solution. After passing through a second stretching unit with a draw ratio of 1.2, it passes through a third 15 °C 25 wt% K2CO3 solution and then passes through a third stretching unit with a draw ratio of 1.1 and enters a water washing unit. After being washed clean, it passes through a five-stage drying oven for air drying and is wound up to obtain a regenerated cellulose film. The surface of the cellulose film hydrogel obtained after water washing is observed using a scanning electron microscope (SEM, Sigma, Zeiss Germany), as Figure 1 shown. The hydrogel shows a dense network structure composed of cellulose nanofibers with uniform pores and small pore diameters, making the structure dense, thus providing good mechanical properties for the cellulose film, demonstrating that carbonates and sulfates have a strong salting-out hydration effect on cellulose.

[0041] <Example 7>

[0042] Prepare aqueous LiOH / urea solutions with concentrations of 3 wt% / 20 wt% respectively. After pre-cooling to 0 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 5 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and coagulated into a film in a 20 °C 10 wt% mixed salt solution (5 wt% Na2CO3 / 5 wt% Na2SO4). After passing through a stretching unit with a draw ratio of 1.1, it is immersed in a second 0 °C 5 wt% Na2CO3 solution, and then after passing through a second stretching unit with a draw ratio of 1.1, it passes through a water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0043] <Example 8>

[0044] Prepare aqueous LiOH / thiourea solutions with concentrations of 4 wt% / 20 wt% respectively. After pre-cooling to 0 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 6 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and coagulated into a film in a 5 °C 10 wt% mixed salt (5 wt% Li2CO3 / 5 wt% Li2SO4) solution. After passing through a stretching unit with a draw ratio of 1.1, it is immersed in a second 2.5 °C 7 wt% Li2CO3 solution, and then after passing through a second stretching unit with a draw ratio of 1.1, it passes through a water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0045] <Example 9>

[0046] Prepare aqueous LiOH / NaOH solutions with concentrations of 5 wt% / 4 wt% respectively. After pre-cooling to 0 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 5.5 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and coagulated into a film in a 5 °C 15 wt% mixed salt (10 wt% K2CO3 / 5 wt% K2SO4) solution. After passing through a stretching unit with a draw ratio of 1.1, it is immersed in a second 0 °C 25 wt% K2CO3 solution, and then after passing through a second stretching unit with a draw ratio of 1.3, it passes through a water washing unit. Then, after passing through a third 15 °C 32.5 wt% mixed salt (27.5 wt% K2CO3 / 5 wt% K2SO4) solution and a third stretching unit with a draw ratio of 1.4, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0047] <Example 10>

[0048] Prepare aqueous solutions of LiOH / NaOH / urea with concentrations of 6 wt% / 5 wt% / 25 wt% respectively. After pre-cooling to 0 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film in a -5 °C 30 wt% mixed salt (15 wt% K2CO3 / 15 wt% (NH4)2SO4) solution. After passing through a stretching unit with a draw ratio of 1.2, it is immersed in a second 0 °C 5 wt% Na2CO3 solution, and then after passing through a second stretching unit with a draw ratio of 1.4, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0049] <Example 11>

[0050] Prepare aqueous solutions of LiOH / NaOH / urea with concentrations of 4 wt% / 4 wt% / 15 wt% respectively. After pre-cooling to -10 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 4.5 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film in a -10 °C 25 wt% K2CO3 solution. After passing through a stretching unit with a draw ratio of 1.1, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0051] <Example 12>

[0052] Prepare aqueous solutions of LiOH / NaOH with concentrations of 6 wt% / 6 wt% respectively. After pre-cooling to -15 °C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 5 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film in a -10 °C 30 wt% K2CO3 solution. After passing through a stretching unit with a draw ratio of 1.1, it is immersed in a second 40 °C 25 wt% mixed salt (10 wt% K2CO3 / 15 wt% Na2SO4) solution, and then after passing through a second stretching unit with a draw ratio of 1.3, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0053] <Example 13>

[0054] Prepare NaOH / urea aqueous solutions with concentrations of 9 wt% / 15 wt% respectively. After pre-cooling to -20 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 7 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film by a 25 wt% mixed salt (5 wt% Na2CO3 / 20 wt% K2CO3) solution at -15 °C. After passing through a stretching unit with a draw ratio of 1.3, it is then immersed in a second 25 wt% mixed salt (5 wt% Na2CO3 / 20 wt% K2CO3) solution at -15 °C. After passing through a second stretching unit with a draw ratio of 1.3, it enters a water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0055] <Example 14>

[0056] Prepare LiOH / thiourea aqueous solutions with concentrations of 7 wt% / 12 wt% respectively. After pre-cooling to -20 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 7 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film by a 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% K2CO3) solution at 10 °C. After passing through a stretching unit with a draw ratio of 1.2, it is then immersed in a second 40 wt% mixed salt (10 wt% Na2CO3 / 30 wt% K2CO3) solution at 10 °C. After passing through a second stretching unit with a draw ratio of 1.4, it enters a water washing unit. After being washed clean, it passes through a third stretching unit with a draw ratio of 1.1 and then is dried by blowing in a five-stage drying oven and wound up to obtain a regenerated cellulose film.

[0057] <Example 15>

[0058] Prepare LiOH / NaOH / urea aqueous solutions with concentrations of 7.5 wt% / 5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 6 wt% after high-speed stirring. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and solidified into a film by a first 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution at 15 °C. After passing through a stretching unit with a draw ratio of 1.2, it is then immersed in a second 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution at 15 °C. After passing through a second stretching unit with a draw ratio of 1.2, it passes through a third 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution at 15 °C and then through a third stretching unit with a draw ratio of 1.2 and enters a water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and wound up to obtain a regenerated cellulose film.

[0059] <Example 16>

[0060] Prepare an aqueous solution of LiOH / urea with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15°C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Add 5 wt% of nano-hexagonal boron nitride powder to the solution, and after filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then passes through the first 5°C 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution to form a film. After passing through a stretching unit with a stretching ratio of 1.1, it is then immersed in the second 10°C 43 wt% mixed salt (35 wt% K2CO3 / 8 wt% Na2SO4) solution, and then passes through a second stretching unit with a stretching ratio of 1.2 and then through the third 15°C 25 wt% K2CO3 solution. Finally, after passing through a third stretching unit with a stretching ratio of 1.1, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated nano-hexagonal boron nitride / cellulose film.

[0061] <Example 17>

[0062] Prepare an aqueous solution of LiOH / urea with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15°C, add the cellulose raw material and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Add 5 wt% of nano-silica powder to the solution, and after filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then passes through the first 5°C 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution to form a film. After passing through a stretching unit with a stretching ratio of 1.1, it is then immersed in the second 10°C 43 wt% mixed salt (35 wt% K2CO3 / 8 wt% Na2SO4) solution, and then passes through a second stretching unit with a stretching ratio of 1.2 and then through the third 15°C 25 wt% K2CO3 solution. Finally, after passing through a third stretching unit with a stretching ratio of 1.1, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated nano-silica / cellulose film.

[0063] <Example 18>

[0064] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Add 5 wt% of graphene oxide powder to the solution, and after filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then passes through a first 5 °C 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution to form a film. After passing through a stretching unit with a draw ratio of 1.1, it is then immersed in a second 10 °C 43 wt% mixed salt (35 wt% K2CO3 / 8 wt% Na2SO4) solution, and then passes through a second stretching unit with a draw ratio of 1.2 and then through a third 15 °C 25 wt% K2CO3 solution. Finally, after passing through a third stretching unit with a draw ratio of 1.1, it enters a water washing unit. After being washed clean, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated graphene oxide / cellulose film.

[0065] <Example 19>

[0066] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Add 5 wt% of carbon nanotubes to the solution, and after filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then passes through a first 5 °C 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution to form a film. After passing through a stretching unit with a draw ratio of 1.1, it is then immersed in a second 10 °C 43 wt% mixed salt (35 wt% K2CO3 / 8 wt% Na2SO4) solution, and then passes through a second stretching unit with a draw ratio of 1.2 and then through a third 15 °C 25 wt% K2CO3 solution. Finally, after passing through a third stretching unit with a draw ratio of 1.1, it enters a water washing unit. After being washed clean, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated carbon nanotube / cellulose film.

[0067] <Example 20>

[0068] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Add 5 wt% of ZIF-8 type metal-organic framework material (MOFs) to the solution, and use it after filtration and centrifugal defoaming. The cellulose solution is extruded through a slit and then passes through the first 5 °C 25 wt% mixed salt (10 wt% Na2CO3 / 15 wt% Li2SO4) solution to form a film. After passing through a stretching unit with a draw ratio of 1.1, it is then immersed in the second 10 °C 43 wt% mixed salt (35 wt% K2CO3 / 8 wt% Na2SO4) solution, and then passes through a second stretching unit with a draw ratio of 1.2 and then through the third 15 °C 25 wt% K2CO3 solution. Finally, after passing through a third stretching unit with a draw ratio of 1.1, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated MOFs / cellulose film.

[0069] <Comparative Example 1>

[0070] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Use it after filtration and centrifugal defoaming. The cellulose solution is extruded through a slit and then passes through the first 0 °C 45 wt% NaBr solution to form a film. After passing through a stretching unit with a draw ratio of 1.1, it enters the water washing unit. After being washed clean, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film. Use a scanning electron microscope (SEM, Sigma, Zeiss Germany) to observe the surface of the cellulose film-like hydrogel obtained after water washing, as Figure 2 shown. Comparing with Example 6, due to the lack of efficient salting-out effect, the nanofiber arrangement of the cellulose hydrogel regenerated by NaBr is relatively loose, and the pore size inside the hydrogel is also larger, thus affecting the mechanical properties of the cellulose film.

[0071] <Comparative Example 2>

[0072] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15 °C, add cellulose raw materials and obtain a cellulose solution with a final concentration of 8 wt% after high-speed stirring. Use it after filtration and centrifugal defoaming. The cellulose solution is extruded through a slit and then passes through the first 0 °C absolute ethanol solution to form a film. After passing through a stretching unit with a draw ratio of 1.1, it passes through the second 0 °C 50 wt% ethanol solution to form a film, and then passes through the water washing unit. After being washed clean after a draw ratio of 1.1, it is dried by blowing in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0073] <Comparative Example 3>

[0074] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15°C, add the cellulose raw material. After high-speed stirring, a cellulose solution with a final concentration of 8 wt% is obtained. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then coagulated into a film after passing through a first 40°C 30 wt% KCl solution. After passing through a stretching unit with a draw ratio of 1.3, it enters and is coagulated into a film after passing through a second 40°C 20 wt% KCl solution. Then, after being washed clean by a washing unit, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0075] <Comparative Example 4>

[0076] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15°C, add the cellulose raw material. After high-speed stirring, a cellulose solution with a final concentration of 8 wt% is obtained. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then coagulated into a film after passing through a first 30°C 20 wt% Na2SO4 solution. After passing through a stretching unit with a draw ratio of 1.2, it enters and passes through a second 20°C 15 wt% Na2SO4 solution. Then, after being washed clean by a washing unit, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0077] <Comparative Example 5>

[0078] Prepare aqueous LiOH / urea solutions with concentrations of 7.5 wt% / 12 wt% respectively. After pre-cooling to -15°C, add the cellulose raw material. After high-speed stirring, a cellulose solution with a final concentration of 8 wt% is obtained. After filtration and centrifugal defoaming, it is ready for use. The cellulose solution is extruded through a slit and then coagulated into a film after passing through a first 10°C 35 wt% NaAc solution. After passing through a stretching unit with a draw ratio of 1.2, it enters and passes through a second 10°C 20 wt% NaAc solution. Then, after being washed clean by a washing unit, it is subjected to air drying in a five-stage drying oven and then wound up to obtain a regenerated cellulose film.

[0079] Characterize the mechanical properties of the cellulose films prepared in Examples 1 to 20 and Comparative Examples 1 to 3. The test results are shown in Table 1.

[0080] Table 1

[0081] Case Tensile strength / (MPa) Tensile strain / (%) Example 1 30 4 Example 2 131 23 Example 3 192 18 Example 4 238 13 Example 5 226 11 Example 6 298 10 Example 7 116 19 Example 8 118 13 Example 9 176 11 Example 10 179 15 Example 11 78 20 Example 12 143 22 Example 13 196 13 Example 14 172 11 Example 15 174 10 Example 16 316 9 Example 17 357 8 Example 18 391 10 Example 19 340 11 Example 20 327 10 Comparative Example 1 53 16 Comparative Example 2 140 10 Comparative Example 3 83 9 Comparative Example 4 104 8 Comparative Example 5 96 8

[0082] It can be seen from the data in Table 1 that higher cellulose concentrations and more appropriate salting-out bath concentrations can significantly improve the mechanical properties of cellulose films. By comparing with the comparative examples, it can be found that the mechanical properties of the films salted out with carbonate ions are significantly better than those regenerated with ethanol and other salts.

[0083] The above are the preferred embodiments of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications are also regarded as the protection scope of the present invention.

Claims

1. A method for preparing a high-strength cellulose membrane, characterized in that, It includes the following steps: (1) Prepare a solvent by using an alkaline aqueous solution and a stabilizer to dissolve cellulose. After filtration and defoaming, a cellulose solution is obtained; (2) The cellulose solution is extruded through a slit and regenerated in a salting-out bath. The salting-out bath is selected from a carbonate solution or a mixed salt solution of carbonate and sulfate; (3) The regenerated product is washed with water, stretched, and dried to obtain a high-strength cellulose membrane.

2. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (1), the solvent is precooled to -20 to 0 °C and then cellulose is added. It can be completely dissolved after high-speed stirring.

3. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (1), the alkali used is at least one of LiOH and NaOH. The concentration of the alkali in the solvent is 3wt% - 12wt%, and the concentration of the stabilizer in the solvent is 0 - 20wt%. The stabilizer is at least one of urea and thiourea.

4. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (1), the concentration of cellulose in the cellulose solution is 2wt% - 12wt%.

5. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (1), it also includes adding a functional filler to the cellulose solution to obtain a cellulose / functional filler composite solution. The functional filler is an organic or inorganic additive, including at least one of metal oxides, metal nitrides, graphene and its derivatives, carbon nanotubes and their derivatives, metal-organic framework compounds, plasticizers, pore-forming agents, reinforcing agents, refractory additives, and dyes.

6. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (2), the salting-out bath is a single salting-out bath or a multi-stage salting-out bath.

7. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (2), the total concentration of salts in the salting-out bath is 5wt% - 60wt%. The carbonate and sulfate are arbitrarily selected from potassium salts, sodium salts, and lithium salts. The salting-out bath is prepared in any proportion within the solubility range of the salts.

8. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (2), the temperature of the salting-out bath is -15 to 20 °C.

9. The method for preparing a high-strength cellulose membrane according to claim 1, characterized in that: In the step (3), the stretching is single-stage stretching or multi-stage stretching.

10. A high-strength cellulose membrane, characterized in that: It is prepared by using the preparation method described in any one of claims 1 to 9.

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