1. A preparation method of a high-transparency high-strength nanocellulose flexible membrane is characterized by comprising the following steps: the method comprises the following steps:

1) oxidizing hydroxyl of cellulose in the enzymolyzed softwood pulp into aldehyde group to obtain dialdehyde nano cellulose;

2) oxidizing aldehyde groups in the dialdehyde nanocellulose into carboxyl groups, and then homogenizing under high pressure to obtain the dicarboxyl nanocellulose;

3) uniformly mixing the dicarboxyl nano-cellulose, the polyaldehyde compounds and the polyalcohol compounds to obtain a mixed solution, and forming a film to obtain a high-transparency high-strength nano-cellulose flexible film;

the dicarboxyl nanocellulose in the step 2) is used in a suspension form, and the mass percentage concentration of the dicarboxyl nanocellulose in the suspension is 0.5-3%;

in the mixed solution in the step 3), the mass percent of the polyaldehyde compounds is 5-15%, the mass percent of the polyalcohol compounds is 0-20%, and the balance is a suspension of the dicarboxyl nanocellulose.

2. The method for preparing the high-transparency high-strength nanocellulose flexible film according to claim 1, characterized in that: the multi-aldehyde compound is more than one of glyoxal and 1, 5-glutaraldehyde; the polyalcohol compound is more than one of glycerol, ethylene glycol, triethylene glycol, 1, 3-butanediol and diethylene glycol;

the content of aldehyde groups in the dialdehyde nano cellulose in the step 1) is 0.5-3.0 mmol/g;

the content of carboxyl in the dicarboxyl nano-cellulose in the step 2) is 0.5-3.0 mmol/g;

oxidizing aldehyde groups in the dialdehyde nanocellulose into carboxyl groups in the step 2) refers to oxidizing the aldehyde groups in the dialdehyde nanocellulose into carboxyl groups by adopting sodium chlorite and glacial acetic acid; the molar ratio of aldehyde group to sodium chlorite to glacial acetic acid in the dialdehyde nano cellulose is 1 to (1-6) to (0.5-3); the conditions for oxidizing aldehyde groups into carboxyl groups are as follows: the reaction temperature is 20-60 ℃, and the reaction time is 8-48 h;

the dialdehyde nano-cellulose is used in the form of suspension, and the mass percentage concentration of the dialdehyde nano-cellulose in the suspension is 0.5-3.0%.

3. The method for preparing the high-transparency high-strength nanocellulose flexible film according to claim 1, characterized in that: and the film forming in the step 3) is to perform tape casting on the mixed solution to form a film, dry and press the film for 2-10 hours to obtain the nano cellulose flexible film.

4. The method for preparing the high-transparency high-strength nanocellulose flexible film according to claim 3, characterized in that: drying for 2-10 h at 40-60 ℃ in the drying condition in the step 3); the pressing is carried out for 2-10 hours under a certain pressure, and specifically is carried out for 2-10 hours under the pressure of a weight of 25-40 kg.

5. The method for preparing the high-transparency high-strength nanocellulose flexible film according to claim 1, characterized in that:

the high-speed homogenization is to adopt high-pressure microfluid nanometer dispersing equipment with a homogenizing head of a high-speed homogenizer of D8 or D10 and pressure of 15000-30000 psi to carry out high-pressure homogenization; homogenizing for 2-20 times;

the length of the dicarboxyl nano-cellulose is 200-600 nm, and the diameter of the dicarboxyl nano-cellulose is less than 10 nm.

6. The method for preparing the high-transparency high-strength nanocellulose flexible film according to claim 1, characterized in that: the step 1) of oxidizing the hydroxyl of the cellulose in the enzymolyzed softwood pulp into aldehyde group means that sodium periodate is adopted to oxidize the hydroxyl into aldehyde group;

wherein the enzymolyzed softwood pulp: the mass ratio of the sodium periodate is 1 to (0.5-4);

the condition of hydroxyl group oxidation to aldehyde group: the reaction temperature is 40-60 ℃, the reaction time is 2.5-5 h, water is used as a reaction medium in the reaction, and the mass concentration of the needle-leaved wood pulp subjected to enzymolysis in the water is 0.5-3%; the oxidation of hydroxyl group into aldehyde group is carried out under the condition of stirring, and the stirring speed is 250-450 r/min.

7. A high-transparency high-strength nanocellulose flexible film obtained by the preparation method of any one of claims 1 to 6.

8. The use of the high transparent high strength nanocellulose flexible film of claim 7, characterized by: the high-transparency high-strength nano cellulose flexible film is applied to food packaging.

9. The application of the dicarboxyl nano-cellulose is characterized in that: the dicarboxyl nanocellulose is used as a humectant or a solid particle stabilizer, and the dicarboxyl nanocellulose is used as the solid particle stabilizer and can play a role of a surfactant; the dicarboxy nanocellulose is as defined in claim 1.

10. Use according to claim 9, characterized in that: when the dicarboxyl nanocellulose is used in the form of a suspension, the dicarboxyl nanocellulose suspension is used for preparing a cosmetic emulsion, and is used as a solid particle stabilizer to stabilize the emulsion.

Background

With the development of economy and the increasing living standard of people, the pressure on resources and environment is greater and greater. Currently, products using petroleum as a raw material still dominate daily life, but many products using petroleum as a raw material cause huge pollution to the environment. Meanwhile, petroleum is used up as an irreproducible resource for one day. Therefore, the development of environmentally friendly products using sustainable resources is a future development trend. Cellulose is used as a main component of cell walls of higher plants, billions of tons of plants on the earth can be produced by photosynthesis every year, and the cellulose has degradability and biocompatibility, so that the pressure of resource shortage and environmental pollution is greatly reduced if the cellulose is used as a resource. In addition, the cellulose is further nanocrystallized, so that a series of advantages of high strength, high modulus, high specific surface area and the like can be fully exerted, and high-performance materials are continuously developed.

Up to now, the methods for preparing nanocellulose have been mainly mechanical, chemical and biological methods. The mechanical method is mainly prepared by grinding, ultrasonic, high-pressure homogenization or a combination of multiple methods. The chemical method is mainly to react chemicals with cellulose. However, most of the chemicals used in the chemical method are toxic and have great harm to the body and the environment. The biological method is mainly an enzymolysis method, and the method does not generate substances harmful to human or environment, and is a real environment-friendly method.

In addition, the film material prepared from cellulose or nano-cellulose has the defects of high brittleness and breakage after folding although the strength is high, and the application of the film material is also very challenging. How to prepare the flexible nano cellulose membrane material with high strength and high light transmittance becomes one of the problems to be solved by people.

Disclosure of Invention

In order to overcome the defects and shortcomings of the prior art, the invention aims to provide a high-transparency high-strength nanocellulose flexible membrane and a preparation method thereof. The method can prepare the high-performance film material with high light transmittance, high strength, small brittleness and high elongation at break.

The invention also aims to provide application of the high-transparency high-strength nanocellulose flexible film. The high-transparency high-strength nano cellulose flexible film is applied to food packaging.

The purpose of the invention is realized by the following technical scheme:

a preparation method of a high-transparency high-strength nanocellulose flexible membrane comprises the following steps:

1) oxidizing the enzymolyzed softwood pulp by sodium periodate to obtain dialdehyde nanocellulose, and oxidizing the dialdehyde nanocellulose by sodium chlorite and homogenizing at high pressure to finally obtain dicarboxyl nanocellulose;

2) uniformly mixing the dicarboxyl nano-cellulose, the polyaldehyde compounds and the polyalcohol compounds to obtain a film forming solution, and adopting a tape casting film forming technology to further obtain the high-transparency high-strength nano-cellulose flexible film.

The multi-aldehyde compound is more than one of glyoxal and 1, 5-glutaraldehyde; the polyalcohol compound is more than one of glycerol, ethylene glycol, triethylene glycol, 1, 3-butanediol and diethylene glycol.

The step 2) of uniformly mixing is ultrasonic mixing.

The dialdehyde nano cellulose in the step 1) is prepared by selectively oxidizing hydroxyl groups at C2 and C3 positions on cellulose molecules into aldehyde groups under the directional oxidation of sodium periodate.

The content of aldehyde groups in the dialdehyde nano cellulose in the step 1) is 0.5-3.0 mmol/g.

The carboxyl content of the dicarboxyl nanocellulose in the step 1) is 0.5-3.0 mmol/g.

Preparing the dicarboxyl nanocellulose in the step 1): oxidizing aldehyde groups in the dialdehyde nanocellulose into carboxyl groups, specifically oxidizing the aldehyde groups in the dialdehyde nanocellulose into carboxyl groups by adopting sodium chlorite and glacial acetic acid; the molar ratio of aldehyde group to sodium chlorite to glacial acetic acid in the dialdehyde nano cellulose is 1: 1-6: 0.5-3, preferably 1: 4: 2; conditions for oxidation of aldehyde groups to carboxyl groups: the reaction temperature is 20-60 ℃, and preferably 55-60 ℃; the reaction time is 8-48 h.

The dialdehyde nano-cellulose is used in the form of suspension, and the mass percentage concentration of the dialdehyde nano-cellulose in the suspension is 0.5-3.0%.

After the oxidation is finished, centrifuging and washing for 4-6 times.

And (3) carrying out high-pressure homogenization on the high-pressure homogenizing head in the step 1) by using a high-pressure microjet nano dispersing device with the model D8 or D10 and the pressure of 15000-30000 psi for 2-20 times of homogenization.

The length of the dicarboxyl nano-cellulose is 200-600 nm, and the diameter of the dicarboxyl nano-cellulose is less than 10 nm.

The dialdehyde nanocellulose in the step 1) is specifically prepared by mixing needle wood pulp subjected to enzymolysis with sodium periodate, stirring for reaction, centrifuging, and washing.

The mass ratio of the enzymolyzed softwood pulp to the sodium periodate is 1: 0.5-4, and preferably 1: 2-4. During the reaction, the solid content of the needle-leaved wood subjected to enzymolysis in water is 0.5-4%. The reaction takes water as a reaction medium.

The stirring reaction is carried out in a dark place at 40-60 ℃ for 2.5-5 h; the stirring speed is 250 to 450 r/min.

The dicarboxyl nanocellulose in the step 1) is used in a suspension form, and the mass percentage concentration of the dicarboxyl nanocellulose in the suspension is 0.5-3%.

The mass percent of the polybasic aldehyde compounds in the mixed solution obtained in the step 2) is 5-15%, the mass percent of the polyhydric alcohol compounds is 0-20%, and the balance is the suspension of the dicarboxyl nanocellulose.

The mass percentage of the polyaldehyde compounds is preferably 5-10%, and more preferably 5%; the mass percentage of the polyol compound is preferably 10-15%, and more preferably 10%.

And the step 2) of film forming refers to that the mixed solution is subjected to tape casting to form a film, the film is dried and pressed for 2-10 hours under certain pressure, and the nano cellulose flexible film is obtained.

The drying condition is drying for 2-10 h at 40-60 ℃; the pressing for 2-10 hours under a certain pressure refers to pressing for 2-10 hours under the pressure of a weight of 25-50 kg.

The thickness of the nano cellulose flexible film is 35-50 mu m.

The invention introduces carboxyl into cellulose, greatly promotes the nano fibrillation of the cellulose, and prepares a high-transparency film material. According to the invention, the dicarboxylated nano-cellulose is matched with polyaldehydes and polyols for use to prepare the high-transparency high-strength nano-cellulose flexible membrane material.

The dicarboxyl nanocellulose is used as a humectant or a solid particle stabilizer, and the solid particle stabilizer can play a role of a surfactant.

When the dicarboxyl nano-cellulose is used in a suspension form, the dicarboxyl nano-cellulose suspension (with the mass percentage concentration of 0.5-3%) is used for preparing cosmetic emulsion and is used as a solid particle stabilizer to stabilize the emulsion.

Compared with the prior art, the invention has the following characteristics and advantages:

(1) the method is characterized in that:

the fiber raw material is enzymolyzed softwood pulp (provided by Hangzhou chemical research institute Co., Ltd.), and the enzymolyzed softwood pulp enables beta-1, 4 glycosidic bonds among more celluloses to be broken and more hydroxyl groups to be exposed, so that in the next step, the reaction process of the fibers is deeper, the carboxyl content of the dicarboxyl nano-cellulose is increased, and the performance of the membrane material is finally improved. The aldehyde nanocellulose is obtained by oxidizing hydroxyl in the enzymolysis plant fiber; the enzymolysis plant fiber is obtained by carrying out enzymolysis on plant fiber.

The oxidation of the hydroxyl groups in the plant fibers subjected to enzymolysis refers to the oxidation of the hydroxyl groups in the plant fibers by sodium periodate.

The high transparency of the high-transparency high-strength nano cellulose flexible membrane is realized by reducing the size of cellulose to a nano level and simultaneously introducing carboxyl into the cellulose, so that the distribution of the cellulose is more uniform due to electrostatic repulsion among fibers. And meanwhile, through hot pressing and drying, the fibers are combined more tightly, the porosity is reduced, and the light scattering and absorption are reduced.

The high strength of the high-transparency high-strength nanocellulose flexible membrane is that a stronger hydrogen bond can be formed by utilizing carboxyl groups existing in the dicarboxy nanocellulose than hydroxyl groups, and the polyatomic aldehyde substance is used as a cross-linking agent, so that the carboxyl groups in the dicarboxy nanocellulose and the hydroxyl groups in the polyalcohol substances can be induced, and the carboxyl groups and the hydroxyl groups of different molecular chains of the dicarboxy nanocellulose can be induced to form multiple hydrogen bonds with each other, so that the membrane material can keep higher strength.

The flexibility of the high-transparency high-strength nano cellulose flexible membrane is realized by adding a polyalcohol substance as a plasticizer and by the interaction between the polyalcohol substance and the fiber, the brittleness of the fiber can be greatly reduced, and the flexibility of the membrane material is improved.

(2) The advantages are that:

when the film-forming solution is not added with polyaldehydes and polyalcohols, i.e. the film-forming solution only contains the biscarboxyl nanocellulose suspension, the tensile strength of the obtained film material is high and can reach about 160 MPa; however, the transmission of light at 550nm, the elongation at break and the recovery angle after 180 ° folding were all low, lower than the values obtained with the addition of polyaldehydes and polyols, and a relevant characterization was made in example 1.

Compared with the film material prepared from pure dicarboxyl nano-cellulose, after the polyatomic aldehyde and the polyatomic alcohol are added, the tensile strength of the film material is still kept at higher strength, the transmittance of 500nm light, the elongation at break and the angle recovered after being folded at 180 degrees are all greatly improved:

the preparation method can prepare the high-performance film material with the highest light transmittance of 90 percent (550nm), the tensile strength of about 130MPa and the highest breaking elongation of 13 percent under the condition of the optimal proportion.

② the nano cellulose membrane material prepared by the invention can be applied to food packaging.

The carboxylated nano cellulose suspension prepared by the invention can be applied to cosmetics, can be used in combination with other moisturizing components, even can replace certain moisturizing components, and has the functions of moisturizing and reducing the cost or can be used as a solid particle stabilizer, stabilize the emulsion and play a role of a surfactant.

Detailed Description

The present invention will be described in further detail with reference to specific examples, but the embodiments of the present invention are not limited thereto.

And (3) characterization: the film material is subjected to light transmittance test by using an ultraviolet-visible spectrophotometer, and the transmittance at the wavelength of 550nm is taken as a characteristic value of the film material transmittance. Secondly, a universal material electronic testing machine is used for testing the physical properties of the film material, such as tensile strength, breaking elongation and the like. Flexible representation: cutting the membrane material to be 8mm wide, folding the membrane material for 180 degrees, pressing the membrane material for 3-5 seconds, releasing the membrane material to enable the membrane material to recover naturally, finally measuring the recovered angle, and measuring for 3 times to obtain an average value.

Examples dialdehyde nanocellulose was prepared by the following method:

weighing 10g of enzymolysis needle wood pulp and 15-40 g of sodium periodate, enabling the mass concentration of the pulp to be 0.5-3%, stirring at the speed of 300-400 r/min, and reacting for 2.5-5 h in a dark place at the temperature of 40-60 ℃. After the reaction is finished, adding ethylene glycol to terminate the reaction, and finally, centrifugally washing the product in a centrifugal machine of 3500-4500 r/min for 20-30 min, washing, measuring the moisture and storing.

Determining aldehyde group content of dialdehyde nanocellulose: hydroxylamine hydrochloride method.

Taking 25mL of 0.25mol/L hydroxylamine hydrochloride solution into a beaker, measuring the initial pH value of the solution by using a pH meter, then weighing 100mg of dialdehyde nano-cellulose into the beaker, stirring the solution for 2 hours at room temperature by using a magnetic stirrer, filtering the solution by suction to obtain filtrate, titrating the filtrate by using 0.01mol/L sodium hydroxide solution, titrating the pH value to the initial pH value, recording the volume of the consumed sodium hydroxide, and calculating the content (mmol/g) of aldehyde groups.

Example 1

(1) Oxidation of dicarboxylic acid: 4.068g of sodium chlorite and 71.96mL of 0.25mol/L glacial acetic acid are added into 7.213g of dialdehyde nanocellulose with the aldehyde group content of 1.240mmol/g (the dialdehyde nanocellulose is used in a solid-liquid or suspension form, 346.8g of suspension is used, and the solid content is 2.08%), the aldehyde group, the chlorite ion and the glacial acetic acid are reacted for 10h at room temperature, and then the dicarboxyl nanocellulose is prepared; finally, homogenizing for 5 times by using high-pressure microjet nano-dispersion equipment under the conditions of 22000psi and D8 to obtain fibers with the lengths of 200-400 nm and the diameters of less than 10 nm;

(2) preparing a high-permeability high-strength film: method of film formation using tape casting: carrying out ultrasonic treatment on the homogenized dicarboxyl nano-cellulose suspension (with the solid content of 2%) for 2min to obtain a film-forming solution, wherein the mass concentration of a polybasic aldehyde substance in the film-forming solution is 0%, the mass concentration of a polyalcohol substance in the film-forming solution is 0%, and the mass concentration of the dicarboxyl nano-cellulose suspension is 100%; placing the film-forming solution in a plastic culture dish with a diameter of 40mm, and quantifying to 60g/m²And then standing overnight at room temperature, transferring to an oven, drying at 50 ℃ for 4h, transferring to a hot press, and hot pressing for 4h, wherein the weight is 30 kg. The resulting film product was tested and found to have a transmittance of 83.67% at a wavelength of 550nm, a tensile strength of 162.27Mpa, an elongation at break of 3.27%, and a recovery angle of 60.33 ° after folding 180 °.

Example 2

(1) Oxidation of dicarboxylic acid: adding 2.148g of sodium chlorite, 38.00ml of 0.25mol/L of glacial acetic acid and aldehyde group, chlorite ion and glacial acetic acid in a molar ratio of 1: 2: 1 into 7.8g of dialdehyde nanocellulose with the aldehyde group content of 1.219mmol/g (solid liquid or suspension, 352.87g of suspension and solid content of 2.21 percent), and reacting in a water bath at 60 ℃ for 8 hours to prepare the dicarboxyl nanocellulose; homogenizing for 5 times by using high-pressure microjet nano-dispersion equipment under the conditions of 22000psi and D8 to obtain fibers with the length of 350-550 nm and the diameter of less than 10 nm;

(2) preparing a high-permeability high-strength film: method of film formation using tape casting: mixing the homogenized dicarboxyl nano-cellulose suspension (solid content is 2%), glyoxal and glycerol, and carrying out ultrasonic treatment for 2min to obtain a film forming solution, wherein the mass concentration of the glyoxal in the film forming solution is 10%, the mass concentration of the glycerol is 20%, and the mass concentration of the dicarboxyl nano-cellulose suspension is 70%; placing the deposition solution in the diameterThe quantitative determination was 60g/m in a 40mm plastic culture dish²(ii) a And standing overnight at room temperature, transferring to an oven, drying at 50 ℃ for 4h, and transferring to a hot press for hot pressing for 4h (the pressure is equivalent to a weight of 30 kg), so that the membrane material is combined more tightly, the porosity is reduced, the strength and the light transmittance of the membrane material are improved, and the flexible membrane is obtained. The brittleness of the resulting film product was greatly improved and it was found that the transmittance at a wavelength of 550nm was 83.32%, the tensile strength was 84.73Mpa, the elongation at break was 12.02%, and the recovery angle after folding 180 ° was 128.67 °.

Example 3

(1) Oxidation of dicarboxylic acid: adding 3.134g of sodium chlorite and 55.44mL of 0.25mol/L glacial acetic acid into 7.516g of dialdehyde nanocellulose with the aldehyde group content of 0.922mmol/g (used in a solid-liquid or suspension form, 533.07g of suspension, and the solid content of 1.41%), reacting in a water bath at 60 ℃ for 48h to prepare the dicarboxyl nanocellulose; homogenizing for 5 times by using high-pressure microjet nano-dispersion equipment under the conditions of 22000psi and D8 to obtain fibers with the length of 200-400 nm and the diameter of less than 10 nm;

(2) preparing a high-permeability high-strength film: method of film formation using tape casting: mixing the homogenized dicarboxylic acid nanocellulose suspension (solid content is 2%), glyoxal and triethylene glycol, and carrying out ultrasonic treatment for 2min to obtain a film forming solution, wherein the mass concentration of the glyoxal, the mass concentration of the triethylene glycol and the mass concentration of the dicarboxy nanocellulose suspension in the film forming solution are respectively 10%, 15% and 75%; placing the film-forming solution in a plastic culture dish with a diameter of 40mm, and quantifying to 60g/m²(ii) a And standing overnight at room temperature, transferring to an oven, drying at 50 ℃ for 4h, and transferring to a hot press for hot pressing for 4h (the pressure is equivalent to a weight of 30 kg), so that the membrane material is bonded more tightly, the porosity is reduced, the strength and the light transmittance of the membrane material are improved, and the flexible membrane is obtained. The resulting film product was tested and found to have a transmittance of 86.448% at a wavelength of 550nm, a tensile strength of 115.36Mpa, an elongation at break of 10.24%, and a recovery angle of 120.33 ° after folding 180 °.

Example 4

(1) Oxidation of dicarboxylic acid: 4.860g of sodium chlorite and 86.00mL of 0.25mol/L glacial acetic acid are added into 9.93g of dialdehyde nanocellulose with the aldehyde group content of 1.444mmol/g (used in a solid-liquid or suspension form, 424.48g of suspension, and the solid content is 2.34%), the aldehyde group, the chlorite ion and the glacial acetic acid are reacted for 8 hours in a water bath at 60 ℃, and then the dicarboxyl nanocellulose is prepared; finally, homogenizing for 5 times by using high-pressure microjet nano-dispersion equipment under the conditions of 22000psi and D8 to obtain fibers with the lengths of 200-400 nm and the diameters of less than 10 nm;

(2) preparing a high-permeability high-strength film: method of film formation using tape casting: mixing the homogenized dicarboxy nano-cellulose suspension (solid content is 2%), glyoxal and 1, 3-butanediol, and carrying out ultrasonic treatment for 2min to obtain a film-forming solution, wherein the mass concentration of the glyoxal in the film-forming solution is 10%, the mass concentration of the 1, 3-butanediol in the film-forming solution is 10%, and the mass concentration of the dicarboxy nano-cellulose suspension is 80%; placing the film-forming solution in a plastic culture dish with a diameter of 40mm, and quantifying to 60g/m²And then standing overnight at room temperature, transferring to an oven, drying at 50 ℃ for 4h, transferring to a hot press, and hot pressing for 4h, wherein the weight is 30 kg. The resulting film product was tested and found to have a transmittance of 84.540% at a wavelength of 550nm, a tensile strength of 110.32Mpa, an elongation at break of 9.51%, and a recovery angle of 120.00 ° after folding 180 °.

Example 5

(1) Oxidation of dicarboxylic acid: 4.068g of sodium chlorite and 71.96mL of 0.25mol/L glacial acetic acid are added into 7.213g of dialdehyde nanocellulose with the aldehyde group content of 1.240mmol/g (the dialdehyde nanocellulose is used in a solid-liquid or suspension form, 346.8g of suspension is used, and the solid content is 2.08%), the aldehyde group, the chlorite ion and the glacial acetic acid are reacted for 10h at room temperature, and then the dicarboxyl nanocellulose is prepared; finally, homogenizing for 5 times by using high-pressure microjet nano-dispersion equipment under the conditions of 22000psi and D8 to obtain fibers with the lengths of 200-400 nm and the diameters of less than 10 nm;

(2) preparing a high-permeability high-strength film: method of film formation using tape casting: the homogenized dicarboxy nanocellulose suspension (solid content 2%) Mixing glyoxal and diglycol, and performing ultrasonic treatment for 2min to obtain a film-forming solution, wherein the mass concentration of the glyoxal in the film-forming solution is 10%, the mass concentration of the diglycol in the film-forming solution is 5%, and the mass concentration of the dicarboxyl nanocellulose suspension is 85%; placing the film-forming solution in a plastic culture dish with a diameter of 40mm, and quantifying to 60g/m²And then standing overnight at room temperature, transferring to an oven, drying at 50 ℃ for 4h, transferring to a hot press, and hot pressing for 4h, wherein the weight is 30 kg. The resulting film product was tested and found to have a transmittance of 85.265% at a wavelength of 550nm, a tensile strength of 100.91Mpa, an elongation at break of 8.55%, and a recovery angle of 106.33 ° after folding 180 °.

Example 6

(1) Oxidation of dicarboxylic acid: 305.78g of the above sample (2.26% solids content), i.e. 6.9g of dialdehyde nanocellulose with aldehyde group content of 1.186mmol/g, were weighed out, 3.7062g of sodium chlorite, 65.56mL of 0.25mol/L glacial acetic acid were added. The process followed here is: aldehyde group, chlorite ion and glacial acetic acid in the molar ratio of 1 to 4 to 2, and reacting at 60 deg.c for 8 hr to prepare the nanometer cellulose dicarboxyl. And finally, homogenizing for 5 times by using high-pressure microjet nano-dispersion equipment under the conditions of 22000psi and D8 to obtain the fiber with the length of 200-400 nm and the diameter of less than 10 nm.

(2) Preparing a high-permeability high-strength film: method of film formation using tape casting: taking the homogenized dicarboxyl nano-cellulose suspension (solid content is 2%) as a film-forming solution, wherein the mass concentration of 1, 5-glutaraldehyde in the film-forming solution is 10%, the mass concentration of ethylene glycol is 10%, and the mass concentration of the dicarboxyl nano-cellulose suspension is 80%; placing the film-forming solution in a plastic culture dish with a diameter of 40mm, and quantifying to 60g/m²And then standing overnight at room temperature, transferring to an oven, drying at 50 ℃ for 4h, transferring to a hot press, and hot pressing for 4h, wherein the weight is 30 kg. The resulting film product was tested and found to have a transmittance of 85.794% at a wavelength of 550nm, a tensile strength of 105.57Mpa, an elongation at break of 10.24%, and a recovery angle of 123.57 ° after folding 180 °.