A method for preparing a nanocellulose and a polyethylene plate containing the nanocellulose

Abstract

The present application belongs to the field of nanocellulose and polyethylene plate preparation, and particularly relates to a nanocellulose and a preparation method of a polyethylene plate containing the nanocellulose. The nanocellulose is obtained by mixing and reacting commercial microcrystalline cellulose, mixed acid and sodium dodecyl sulfate or sodium dodecyl benzene sulfonate or polyoxyethylene octanol, and then by freeze-drying, ultrasonic extraction and Soxhlet extraction. The nanocellulose is melted into polyethylene to obtain a polyethylene plate. The preparation time of the nanocellulose is shortened, and the yield of the nanocellulose is improved. The nanocellulose is uniformly distributed in the polyethylene matrix material, and the mechanical properties of the polyethylene matrix are enhanced. The polyethylene composite material is endowed with the ability of self-degradation at room temperature, and has a wide application prospect in industrial production.

Description

Technical Field

[0001] This invention belongs to the field of nanocellulose and polyethylene sheet preparation, specifically relating to a method for preparing nanocellulose and polyethylene sheets containing the nanocellulose. Background Technology

[0002] Nanocellulose is a novel, non-polluting, and biodegradable polymeric functional material extracted from natural cellulose. It has a unique structure and excellent properties. In addition, its lightweight, biodegradable, and renewable characteristics as a biomaterial have attracted much attention from researchers in the fields of nanocomposite materials, food, and biomedicine.

[0003] However, the current method for preparing nanocellulose uses highly concentrated inorganic acids, such as sulfuric acid, hydrochloric acid, and phosphoric acid, for acid hydrolysis. Acid hydrolysis has problems such as severe hydrolysis of cellulose, poor water solubility of cellulose, slow reaction between the two, and low yield.

[0004] Polyethylene is widely used in all aspects of daily life in the world today. It is mainly used to manufacture films, packaging materials, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can also be used as a high-frequency insulating material for televisions, radar, etc. With the development of petrochemicals, polyethylene production has developed rapidly, accounting for about 1 / 4 of the total plastic production. Due to the huge production of polyethylene, its recycling and degradation has become a problem. Although polyethylene can be degraded, its molecular structure determines that it is easy to cross-link but not easy to decompose. Its decomposition needs to be carried out at high temperatures, with a degradation temperature of about 450°C. Its degradation rate is also very slow, which inevitably leads to environmental pollution and high costs for centralized recycling and degradation.

[0005] Polyethylene sheets, due to their excellent chemical stability, are particularly difficult to degrade, and they are also highly sensitive to environmental stresses, often failing to meet the mechanical properties required for daily production. Therefore, it is essential to find a polyethylene sheet that is biodegradable at room temperature and possesses good mechanical properties. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention proposes a method for preparing nanocellulose and polyethylene sheets containing nanocellulose, which can accelerate the preparation of nanocellulose, increase the yield of nanocellulose, enhance the mechanical properties of polyethylene sheets containing nanocellulose, and further endow the polyethylene composite material with self-degradation ability in a room temperature environment.

[0007] The technical solution adopted by this invention to solve its technical problem is:

[0008] A method for preparing nanocellulose, the method comprising the following steps:

[0009] 1) Mix commercial microcrystalline cellulose, catalyst and mixed acid, react at 60-95℃ for 1-2 hours, then add weak alkaline solution to pH 7, centrifuge and wash with water to obtain suspension, wherein the weight-volume ratio of commercial microcrystalline cellulose, catalyst and mixed acid is 1g:0.001-0.05g:50-100ml;

[0010] 2) Freeze-dry the suspension obtained in step 1), add ethanol solvent, extract by ultrasonication at 200-300W for 20-40 min, and then extract by Soxhlet extraction for 3-6 h to obtain nanocellulose. The weight-volume ratio of the microcrystalline cellulose to ethanol is 1g:100-150ml.

[0011] In this invention, commercial microcrystalline cellulose, catalyst and mixed acid are mixed, and more preferably the reaction is carried out at 80-90°C for 1-2 hours, at which temperature is more conducive to the occurrence of cellulose acid hydrolysis reaction.

[0012] Furthermore, the mixed acid comprises hydrochloric acid, formic acid, and citric acid, each with an independent concentration of 3-6 M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =(0-1):(1.5-7.5):(7.5-1).

[0013] Furthermore, the concentrations of the hydrochloric acid, formic acid, and citric acid are all 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) = 1:2:7.

[0014] Furthermore, the weakly alkaline solution is selected from either 1-2% ammonia water or 0.5-1% sodium bicarbonate solution.

[0015] Furthermore, the catalyst is sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, or polyoxyethylene octanol.

[0016] The catalyst can increase the contact angle of acid on the surface of microcrystalline cellulose, allowing the acid to fully contact the microcrystalline cellulose and introduce negative charge to improve the surface stability of microcrystalline cellulose, promote the reaction, reduce the reaction time, and increase the reaction yield.

[0017] The ultrasonic extraction plus Soxhlet extraction method improves extraction efficiency while ensuring the extraction effect of this product.

[0018] Furthermore, the preparation method of the nanocellulose includes the following steps:

[0019] 1) Commercial microcrystalline cellulose, sodium dodecyl sulfate, and mixed acid were mixed and reacted at 80°C for 1 hour. A weakly alkaline solution was then added, and the mixture was centrifuged and washed with water to obtain a suspension with a pH of 7. The weight-to-volume ratio of the commercial microcrystalline cellulose, sodium dodecyl sulfate, and mixed acid was 1 g:0.001 g:100 ml. The mixed acid consisted of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3 M, and their volume ratio was V. (盐酸) :V (甲酸) :V (柠檬酸) =1:2:7;

[0020] 2) Freeze-dry the suspension obtained in step 1), add 1000 ml of ethanol solvent, extract by ultrasonication at 300 W for 20 min, and then extract by Soxhlet extraction for 3 h to obtain nanocellulose.

[0021] A method for preparing polyethylene sheets, comprising the following steps:

[0022] 1) Filling the mold: Mix the nanocellulose and polyethylene powder evenly at a mass ratio of 1:50-100, and spread it evenly in the mold;

[0023] 2) Hot pressing and melting: Set the upper mold temperature to 140-150℃ and reduce the molding pressure to 1-10MPa to improve the uniformity of the molding material curing. Set the lower mold temperature to 230-240℃. Place the mold on the lower mold that has reached the set temperature. Move the upper mold down to apply a pressure of 1-50MPa to the mold.

[0024] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0025] The beneficial effects of this invention are:

[0026] 1) It shortens the preparation time of nanocellulose and increases the extraction rate of nanocellulose.

[0027] 2) The nanocellulose molecules have a large number of functional groups, which are uniformly distributed in the polyethylene matrix material, thereby enhancing the mechanical properties of the polyethylene matrix;

[0028] 3) Nanocellulose is a fully biodegradable nanofiller that endows polyethylene composites with the ability to self-degrade at room temperature, and has broad application prospects in industrial production. Detailed Implementation

[0029] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Furthermore, it should be understood that after reading the disclosure of this invention, those skilled in the art can make various modifications or alterations to the present invention, and these equivalent forms also fall within the scope of protection defined by this invention.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0031] This invention provides a method for preparing nanocellulose, the specific steps of which include: 1) mixing commercial microcrystalline cellulose, catalyst and mixed acid, reacting at 60-95℃ for 1-2 hours, then adding a weak alkaline solution to pH 7, centrifuging and washing with water to obtain a suspension, wherein the weight-volume ratio of commercial microcrystalline cellulose, catalyst and mixed acid is 1g:0.001-0.05g:50-100ml;

[0032] 2) Freeze-dry the suspension obtained in step 1), add ethanol solvent, extract by ultrasonication at 200-300W for 20-40 min, and then extract by Soxhlet extraction for 3-6 h to obtain nanocellulose. The weight-volume ratio of the microcrystalline cellulose to ethanol is 1g:100-150ml.

[0033] Among them, commercial microcrystalline cellulose, catalyst and mixed acid are preferably reacted at 80-90℃ for 1-2 hours, which is conducive to the acid hydrolysis reaction of cellulose.

[0034] The catalyst is sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, or polyoxyethylene octanol, preferably sodium dodecyl sulfate. In the acid hydrolysis reaction of microcrystalline cellulose and mixed acid, the catalyst reduces the surface tension, allowing a thin film to form on the surface of the poorly hydrophilic microcrystalline cellulose, which then reacts fully with the acid solution, making it easier for the acid hydrolysate to penetrate the cellulose. The addition of the catalyst can change the pH value of the mixed acid reaction and buffer the ionic strength of the reaction solution, preventing the microcrystalline cellulose from gelling or precipitating due to ionic action.

[0035] The weakly alkaline solution includes ammonia water and sodium bicarbonate solution, preferably sodium bicarbonate solution, to adjust the pH value of the suspension and reduce the corrosive effect of the acidic solution.

[0036] This invention uses a combination of ultrasonic extraction and Soxhlet extraction, performing ultrasonic extraction first and then Soxhlet extraction, which accelerates the extraction efficiency while maintaining the separation rate.

[0037] The preparation of a polyethylene sheet containing the nanocellulose includes the following steps: 1) filling the mold: the nanocellulose and polyethylene powder are mixed evenly at a mass ratio of 1:50-100 and spread evenly in the mold;

[0038] 2) Hot pressing and melting: Set the upper mold temperature to 140-150℃ and reduce the molding pressure to 1-10MPa to improve the uniformity of the molding material curing. Set the lower mold temperature to 230-240℃. Place the mold on the lower mold that has reached the set temperature. Move the upper mold down to apply a pressure of 1-50MPa to the mold.

[0039] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0040] The present invention will now be further illustrated with specific examples. The following embodiments are only for explaining the present invention and do not constitute a limitation thereof. The test samples and test procedures used in the following embodiments include the following (if the specific experimental conditions are not specified in the embodiments, they are usually performed according to conventional conditions or the conditions recommended by the reagent company; the reagents, consumables, etc. used in the following embodiments can be obtained commercially unless otherwise specified).

[0041] Example 1:

[0042] A method for preparing nanocellulose, comprising the following steps:

[0043] 1) Mix 10g of commercial microcrystalline cellulose, 0.1g of sodium dodecyl sulfate, and 1000ml of mixed acid. React in a water bath at 85℃ for 1 hour. Then add 0.5% sodium bicarbonate solution until the pH reaches 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:2:7;

[0044] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and the mixture was extracted by ultrasonication at 300 W for 20 min and then by Soxhlet extraction for 3 h to obtain nanocellulose. The nanocellulose extraction rate was 83%.

[0045] A method for preparing a polyethylene sheet containing the aforementioned nanocellulose, comprising the following steps:

[0046] 1) Filling the mold: Mix 5g of nanocellulose and 250g of polyethylene powder evenly and spread them evenly in the mold;

[0047] 2) Hot pressing and melting: Set the upper mold temperature to 150℃, reduce the molding pressure to 10MPa to improve the uniformity of the molding material curing, set the lower mold temperature to 240℃, place the mold on the lower mold that has reached the set temperature, and move the upper mold down to apply a pressure of 40MPa to the mold;

[0048] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0049] Example 2:

[0050] A method for preparing nanocellulose, comprising the following steps:

[0051] 1) Mix 10g of commercial microcrystalline cellulose, 0.1g of sodium dodecylbenzenesulfonate, and 1000ml of mixed acid, react at 95℃ for 1h, then add 0.5% sodium bicarbonate solution until the pH equals 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:2:7;

[0052] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and the mixture was extracted by ultrasonication at 300 W for 20 min and then by Soxhlet extraction for 3 h to obtain nanocellulose. The nanocellulose extraction rate was 89%.

[0053] A method for preparing a polyethylene sheet containing the aforementioned nanocellulose, comprising the following steps:

[0054] 1) Filling the mold: Mix 10g of nanocellulose and 250g of polyethylene powder evenly and spread them evenly in the mold;

[0055] 2) Hot pressing and melting: Set the upper mold temperature to 140℃ and reduce the molding pressure to 10MPa to improve the uniformity of the molding material curing. Set the lower mold temperature to 230℃, place the mold on the lower mold that has reached the set temperature, and move the upper mold down to apply a pressure of 40MPa to the mold.

[0056] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0057] Example 3:

[0058] A method for preparing nanocellulose, comprising the following steps:

[0059] 1) Mix 10g of commercial microcrystalline cellulose, 0.01g of polyoxyethylene octanol, and 1000ml of mixed acid, react at 80℃ for 1h, then add 0.5% sodium bicarbonate solution until the pH equals 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:2:7;

[0060] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and the mixture was extracted by ultrasonication at 300 W for 20 min and then by Soxhlet extraction for 3 h to obtain nanocellulose. The nanocellulose extraction rate was 83%.

[0061] A method for preparing a polyethylene sheet containing the aforementioned nanocellulose, comprising the following steps:

[0062] 1) Filling the mold: Mix 15g of nanocellulose and 250g of polyethylene powder evenly and spread them evenly in the mold;

[0063] 2) Hot pressing and melting: Set the upper mold temperature to 140℃ and reduce the molding pressure to 5MPa to improve the uniformity of the molding material curing. Set the lower mold temperature to 230℃, place the mold on the lower mold that has reached the set temperature, and move the upper mold down to apply a pressure of 40MPa to the mold.

[0064] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0065] Example 4:

[0066] A method for preparing nanocellulose, comprising the following steps:

[0067] 1) Mix 10g of commercial microcrystalline cellulose, 0.5g of sodium dodecyl sulfate, and 1000ml of mixed acid. React in a water bath at 80℃ for 1 hour. Then add 0.5% sodium bicarbonate solution until the pH reaches 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 6M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:2:7;

[0068] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and the mixture was extracted by ultrasonic extraction at 150 W for 20 min and then by Soxhlet extraction for 3 h to obtain nanocellulose. The nanocellulose extraction rate was 86%.

[0069] A method for preparing a polyethylene sheet containing the aforementioned nanocellulose, comprising the following steps:

[0070] 1) Filling the mold: Mix 20g of nanocellulose and 250g of polyethylene powder evenly and spread them evenly in the mold;

[0071] 2) Hot pressing and melting: Set the upper mold temperature to 140℃ and reduce the molding pressure to 5MPa to improve the uniformity of the molding material curing. Set the lower mold temperature to 230℃, place the mold on the lower mold that has reached the set temperature, and move the upper mold down to apply a pressure of 40MPa to the mold.

[0072] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0073] Example 5:

[0074] The steps are similar to those in Example 1, except that V (盐酸) :V (甲酸) :V (柠檬酸) = 1:7.5:1.

[0075] The final nanocellulose extraction rate was 78%.

[0076] Example 6:

[0077] The steps are similar to those in Example 1, except that V (盐酸) :V (甲酸) :V (柠檬酸) = 1:6:3.

[0078] The final nanocellulose extraction rate was 80%.

[0079] Example 7:

[0080] The steps are similar to those in Example 1, except that V (盐酸) :V (甲酸) :V (柠檬酸) =1:4:5.

[0081] The final extraction rate of nanocellulose was 82%.

[0082] Example 8:

[0083] The steps are similar to those in Example 1, except that V (盐酸) :V (甲酸) :V (柠檬酸) =1:3:6.

[0084] The final nanocellulose extraction rate was 84%.

[0085] Example 9:

[0086] The steps are similar to those in Example 1, except that V(盐酸) :V (甲酸) :V (柠檬酸) = 1:1.5:7.5.

[0087] The final extraction rate of nanocellulose was 88%.

[0088] Comparative Example 1:

[0089] A method for preparing nanocellulose, comprising the following steps:

[0090] 1) Mix 10g of commercial microcrystalline cellulose with 1000ml of mixed acid, react in a water bath at 85℃ for 1h, then add 0.5% sodium bicarbonate solution until the pH equals 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:7:2;

[0091] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and the mixture was extracted by ultrasonication at 300 W for 20 min and then by Soxhlet extraction for 3 h to obtain nanocellulose. The nanocellulose extraction rate was 70%.

[0092] Comparative Example 2:

[0093] A method for preparing nanocellulose, comprising the following steps:

[0094] 1) Mix 10g of commercial microcrystalline cellulose, 0.1g of sodium dodecyl sulfate, and 1000ml of mixed acid. React in a water bath at 80℃ for 1 hour. Then add 0.5% sodium bicarbonate solution until the pH reaches 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:7:2;

[0095] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and the suspension was extracted by ultrasonic extraction at 300 W for 40 min to obtain nanocellulose. The nanocellulose extraction rate was 60%.

[0096] Comparative Example 3:

[0097] A method for preparing nanocellulose, comprising the following steps:

[0098] 1) Mix 10g of commercial microcrystalline cellulose, 0.1g of sodium dodecyl sulfate, and 1000ml of mixed acid. React in a water bath at 80℃ for 1 hour. Then add 0.5% sodium bicarbonate solution until the pH reaches 7. After centrifugation and washing with water, obtain a suspension. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:7:2;

[0099] 2) The suspension obtained in step 1) was freeze-dried, 1000 ml of ethanol solvent was added, and Soxhlet extraction was performed for 6 h to obtain nanocellulose with an extraction rate of 75%.

[0100] Comparative Example 4:

[0101] The procedure was similar to that of Comparative Example 3, except that nanocellulose was obtained by ultrasonic extraction at 300W for 120 minutes, with an extraction rate of 75%.

[0102] Comparative Example 5:

[0103] The procedure was similar to that of Comparative Example 3, except that nanocellulose was obtained by Soxhlet extraction for 12 hours, with an extraction rate of 85%.

[0104] Comparative Example 6:

[0105] A method for preparing a polyethylene sheet containing the aforementioned nanocellulose, comprising the following steps:

[0106] 1) Filling the mold: Spread 250g of polyethylene powder evenly into the mold;

[0107] 2) Hot pressing and melting: Set the upper mold temperature to 140℃ and reduce the molding pressure to 5MPa to improve the uniformity of the molding material curing. Set the lower mold temperature to 230℃, place the mold on the lower mold that has reached the set temperature, and move the upper mold down to apply a pressure of 40MPa to the mold.

[0108] 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain the polyethylene sheet containing the nanocellulose.

[0109] Table 1 shows the tensile yield strength and tensile modulus of the nanocellulose polyethylene sheets prepared in Examples 1-4 and Comparative Example 6.

[0110] Table 1. Mechanical strength of nanocellulose polyethylene sheets prepared in Examples 1-4 and Comparative Example 6

[0111] Sample number Tensile yield strength (MPa) Tensile modulus of elasticity (MPa) Example 1 17.8 465.3 Example 2 18.1 489.2 Example 3 17.6 457.4 Example 4 17.9 462.1 Comparative Example 6 16.1 442.5

[0112] It is evident that the nanocellulose polyethylene board prepared by this invention has higher tensile yield strength and tensile modulus of elasticity compared to ordinary non-nanocellulose polyethylene board. This is because a strong interfacial interaction is generated between nanocellulose and the polyethylene matrix, which greatly enhances the mechanical strength of the nanocellulose polyethylene board of this invention. Moreover, the more nanocellulose added, the greater the mechanical strength.

[0113] As can be seen from the comparison between Comparative Example 1 and Example 1, the catalyst of the present invention can effectively improve the extraction rate of the present invention.

[0114] As can be seen from Examples 1-9, for the same concentration of formic acid, hydrochloric acid, and citric acid, when the amount of hydrochloric acid added is the same, the smaller the ratio of formic acid to citric acid added, the higher the extraction rate of nanocellulose, and then tends to stabilize.

[0115] As can be seen from Comparative Examples 2-5, ultrasonic extraction of nanocellulose has a fast extraction speed but a low extraction rate, while Soxhlet extraction of nanocellulose has a slow extraction speed but a high extraction rate. Combining the two methods greatly shortens the extraction time and achieves a high extraction rate.

[0116] The preparation method of nanocellulose and polyethylene sheets containing nanocellulose provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing nanocellulose, characterized in that, The specific steps of this method are as follows: 1) Mix commercial microcrystalline cellulose, catalyst and mixed acid, react at 60-95℃ for 1-2 hours, then add weak alkaline solution to pH 7, centrifuge and wash with water to obtain suspension, wherein the weight-volume ratio of commercial microcrystalline cellulose, catalyst and mixed acid is 1g:0.001-0.05g:50-100ml; 2) Freeze-dry the suspension obtained in step 1), add ethanol solvent, extract by ultrasonication at 200-300W for 20-40 min, and then extract by Soxhlet extraction for 3-6 h to obtain nanocellulose. The weight-volume ratio of the commercial microcrystalline cellulose to ethanol is 1g:100-150ml. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with an independent concentration of 3-6 M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) =1:(1.5-7.5):(1-7.5) The catalyst is sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, or polyoxyethylene octanol.

2. The method for preparing nanocellulose according to claim 1, characterized in that: The concentrations of the hydrochloric acid, formic acid, and citric acid are 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) = 1:2:

7.

3. The method for preparing nanocellulose according to claim 1, characterized in that: The weakly alkaline solution is selected from either ammonia water with a concentration of 1-2% or sodium bicarbonate solution with a concentration of 0.5-1%.

4. The method for preparing nanocellulose according to claim 1, characterized in that, The steps of this method are as follows: 1) Mix commercial microcrystalline cellulose, sodium dodecyl sulfate, and mixed acid, react at 80°C for 1 hour, then add... Add 0.5% sodium bicarbonate solution, centrifuge, and wash with water to obtain a suspension with a pH of 7. The weight-to-volume ratio of the commercial microcrystalline cellulose, sodium dodecyl sulfate, and mixed acid is 1g:0.001g:100ml. The mixed acid is a mixture of hydrochloric acid, formic acid, and citric acid, each with a concentration of 3M, and their volume ratio is V. (盐酸) :V (甲酸) :V (柠檬酸) = 1:2:7; 2) Freeze-dry the suspension obtained in step 1), add ethanol solvent, extract by ultrasonication at 300W for 20 min and then by Soxhlet extraction for 3 h to obtain nanocellulose. The weight-volume ratio of the microcrystalline cellulose to ethanol is 1g:100ml.

5. A method for preparing a polyethylene sheet, characterized in that, The method includes the following steps: 1) Filling the mold: Mix the nanocellulose and polyethylene powder as described in claim 1 evenly at a mass ratio of 1:50-100, and spread them evenly in the mold; 2) Hot pressing and melting: Set the upper mold temperature to 140-150℃, reduce the molding pressure to 1-10MPa to improve the uniformity of the molding material curing, set the lower mold temperature to 230-240℃, place the mold on the lower mold that has reached the set temperature, and move the upper mold down to apply a pressure of 1-50MPa to the mold; 3) Cooling and curing: Take out the mold and place it on a flat vulcanizing machine for cold pressing and shaping. Cool to room temperature and take out the mold to obtain a polyethylene sheet containing the nanocellulose.