High-strength cellulose film and preparation method thereof

Abstract

The invention relates to a preparation method of a high-strength cellulose film, which comprises the following steps: putting cellulose into a DMSO / alkali mixed reagent for pre-swelling to obtain a cellulose dispersion liquid, and then preparing the cellulose film by adopting a vacuum filtration or film casting method, sequentially washing, hot-pressing and drying the cellulose film to obtain a high-strength cellulose film; the reagent subjected to suction filtration can be recycled and repeatedly used for pre-swelling of cellulose, and then the high-strength cellulose film can still be prepared through film suction, water washing and hot-pressing drying in sequence.

Description

Technical Field

[0003] The present invention relates to the field of natural polymer materials, and particularly to a high-strength cellulose film and a preparation method thereof. Background Art

[0004] Cellulose is the most abundant natural polymer material in nature, with an annual output of over 75 billion tons, and is widely present in wood, cotton, and agricultural waste. Its molecular chain is connected by β-1,4 glycosidic bonds, and it has excellent biodegradability, renewability, and chemical modifiability. In recent years, with the advancement of the global plastic ban policy, the demand for degradable materials represented by cellulose films has increased sharply in fields such as food packaging, medical dressings, and flexible electronic substrates. However, traditional petroleum-based plastic alternatives face two core challenges: one is to achieve the simultaneous improvement of high strength (>200 MPa) and high toughness (fracture strain >5%) through low-cost and green processes; the other is to solve the problems of high energy consumption and difficult recovery of the solvent system during the processing to meet the sustainability requirements of industrial production.

[0005] Currently, the preparation of cellulose films mainly relies on the dissolution-regeneration method and the TEMPO oxidation method. Among them, the process route of the dissolution-regeneration method can be divided into three categories:

[0006] Ionic liquid system: Ionic liquids such as [BMIM]Cl can dissolve cellulose by breaking hydrogen bonds, but the cost of ionic liquids is high (>100 US dollars / kg) (for example, in US Patent US20180327544A1, ionic liquids are used to prepare nanofibrillated cellulose films, but the solvent recovery requires multiple processes such as distillation and extraction, and the recovery energy consumption accounts for more than 40% of the total production cost), the recovery rate is less than 70%, and the residual ions easily cause the performance attenuation of the film (the tensile strength is usually <180 MPa);

[0007] Alkali / urea low-temperature system: Cellulose is dissolved in an aqueous solution of NaOH / urea at -12°C. Although the cost is low, it requires long-term freezing pretreatment (>8 hours), and during the regeneration process, microporous defects are easily formed due to the too fast phase separation rate, resulting in a decrease in the denseness of the film (porosity >15%). At the same time, the process is relatively complex and requires multiple steps of treatment (such as freezing pretreatment, gradient replacement drying). Taking Chinese Patent CN112125999A as an example, after dissolution with alkali / urea, it needs to go through 5 steps of water washing-ethanol replacement processes, significantly increasing the time and energy consumption costs;

[0008] N-methylmorpholine-N-oxide (NMMO) system: Although it can achieve direct melt processing of cellulose, the process temperature needs to exceed 130°C, with high energy consumption, and the thermal decomposition of NMMO will produce toxic by-products such as nitrosamines, limiting its environmentally friendly applications.

[0009] TEMPO oxidation method: Although it can selectively oxidize the C6 hydroxyl group of cellulose to generate carboxyl groups and form a stable cross-linked structure, compared with the previous enzymatic hydrolysis method, it can significantly improve the tensile strength and toughness of the film (breaking strength ~ 150 MPa, modulus ~ 6 GPa). However, under the condition of not being compounded with other materials, it still cannot meet the performance requirements for preparing high-strength cellulose flexible substrates and packaging materials that can replace petroleum-based plastics. Moreover, TEMPO catalysts and oxidation systems (such as NaClO / NaBr) need to be used, the reaction conditions are harsh, the reagent cost is high, and halogen-containing wastewater may be generated, causing serious environmental pollution.

[0010] In recent years, researchers have tried to reduce costs and increase efficiency by simplifying the process path. For example, a Chinese team once reported using the DMSO / NaOH system to prepare transparent films, but it was necessary to stir at 60 °C for 24 hours to achieve complete dissolution, and no solvent recycling device was designed. It should be noted that due to the low toxicity and high boiling point (189 °C) of DMSO, theoretically, it can be efficiently recovered by vacuum distillation. However, in the existing technology, the recovery rate of DMSO is generally lower than 80%, increasing the cost of its industrialization. Summary of the Invention

[0011] The purpose of the present invention is to solve the problems existing in the prior art and provide a method for preparing a high-strength cellulose film with solvent recoverability and a simple process.

[0012] To achieve the above purpose, the inventor provides a method for preparing a high-strength cellulose film, which includes the following steps:

[0013] Step 1: Add cellulose to the DMSO / alkali or DMAc / LiCl system, and continuously stir for 3 - 100 h for pre-swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰ - 1%; the molar ratio of the alkali reagent to the cellulose structural unit is 0.4 - 2.2:1

[0014] Step 2: Filter the cellulose micro / nanofiber dispersion with a mass concentration of 4‰ - 1% obtained in Step 1 into a film by vacuum filtration or casting, wash away the residual solvent in water, and then hot-press and dry to obtain the high-strength cellulose film;

[0015] Step 3: Recover the filtrate obtained in Step 2 and use it as a solvent to repeat Steps 1 and 2 to continue preparing the high-strength cellulose film.

[0016] In the swelling treatment in Step 1 of the present invention, under certain other conditions, the ratio of microfibers to nanofibers in the dispersion can be adjusted by adjusting the pre-swelling time.

[0017] In Step 1, under certain other conditions, the ratio of microfibers to nanofibers in the dispersion can be adjusted by controlling the molar ratio of LiCl / alkali reagent to cellulose structural units. LiCl / alkali reagent can selectively intercalate between cellulose chain sheets, increasing the layer spacing and promoting fiber cleavage to accelerate the preparation of cellulose micro / nanofibers.

[0018] In Step 3, the filtered recovery solution is still dimethyl sulfoxide / dimethylacetamide. Dimethyl sulfoxide / dimethylacetamide is prone to absorbing water, and attention should be paid to preventing water absorption, as water absorption may cause its denaturation, resulting in a decrease in the mechanical properties of the recovered swollen new cellulose film.

[0019] In Step 3, the filtered recovery solution can be used as dimethyl sulfoxide / dimethylacetamide to continue swelling the cellulose raw material to prepare high-strength cellulose films.

[0020] Preferably, in Step 1, the cellulose is one of the following: softwood cellulose, hardwood cellulose, cotton, and flax cellulose.

[0021] Preferably, in Step 1, the added alkali reagent is one of the following: KOH, LiOH, and NaOH.

[0022] Preferably, in Step 1, the added alkali reagent is KOH or LiOH, and the molar ratio of the alkali reagent to cellulose structural units is 1:1. When the molar ratio of KOH or LiOH to cellulose structural units is 1:1, the intercalation effect is the best, and the shortest stirring time required to reach the ideal effect is 60 - 90 h.

[0023] Preferably, in Step 1, the dispersion with a pre-swelling time between 60 - 90 h contains a large amount of cellulose nanofibers and a small amount of microfibers, forming a three-dimensional cellulose network with microfibers as the "trunk" and nanofibers as the "branches". The film prepared in this swollen state has a relatively high tensile strength (>250 MPa).

[0024] Preferably, in Step 3, the filtered recovery solution can be reused 10 - 30 times, and the cellulose films prepared by multiple recycling can still maintain good mechanical properties.

[0025] Preferably, in Step 2, the vacuum filtration conditions are: the pressure is -0.3 to -0.09 MPa; the filter membrane used is a nylon filter membrane with a pore size of 0.1 μm - 0.5 μm.

[0026] Preferably, the hot pressing conditions are: under negative pressure for 20 min - 40 min at 60 - 100 °C, and the pressure is -0.3 to -0.09 MPa, thereby enhancing the strength of the cellulose film.

[0027] In the present invention, by controlling the molar ratio of LiCl / alkali reagent to the cellulose structural unit and the stirring duration, taking the DMSO / LiOH system as an example, Li + The intercalation effect of cellulose molecular chains and the strong polar swelling effect of DMSO selectively destroy the hydrogen bond network in the amorphous region of cellulose, prompting the microfibrils to be gradually peeled off from the macroscopic fibers. During the pre-swelling process, LiOH forms a complex with cellulose hydroxyl groups, weakening the intermolecular force, while DMSO expands the fiber spacing through osmotic swelling, forming a dynamic equilibrium of "swelling - disentanglement". Under the action of the shear force field during stirring, the fibers are delaminated and peeled: the outer layer is completely dissociated into cellulose nanofibers (diameter 10 - 100 nm), while the inner layer retains some incompletely dissociated cellulose microfibers (diameter 1 - 10 μm). This multi-level peeling behavior can be controlled by regulating the swelling time and alkali concentration, and finally forms a three-dimensional network structure with cellulose microfibers as the framework and cellulose nanofibers as the filling. The improvement of the mechanical properties of the cellulose film made from this dispersion is due to the cross-scale synergistic effect between cellulose microfibers and cellulose nanofibers. Under tensile load, cellulose microfibers act as the main load-bearing body, and cellulose nanofibers form a three-dimensional network in the gaps between cellulose microfibers. Through the bridging action of nanofibers, the local stress is dispersed to a larger volume area, effectively inhibiting stress concentration, thus greatly improving the strength of the composite film.

[0028] Different from the prior art, the above technical solution has the following advantages:

[0029] 1. The method for preparing cellulose micro / nanofiber film in the present invention has simple process, recyclable solvent, and is economical and environmentally friendly.

[0030] 2. The elongation at break of the cellulose micro / nanofiber film prepared in the present invention is 4.77 - 12.63%, the breaking strength is 183.12 - 312.63 MPa, the Young's modulus is 5.07 - 15.66 GPa, and the loss energy is 7 - 18.93 MJ / m 3 ; after soaking in water for 2 min, the elongation at break is 8.62 - 11.31%, the breaking strength is 30.53 - 50.41 MPa, and the Young's modulus is 0.41 - 0.5 GPa; the high-strength cellulose film prepared by recycling the solvent has an elongation at break of 4.53 - 12.61%, a breaking strength of 200.79 - 290.17 MPa, a Young's modulus of 5.92 - 13.22 GPa, and a loss energy of 5.2 - 13.03 MJ / m 3 , all of which are higher than those of single nanofiber film (~150 MPa, 5.5 MJ / m 3 ) and microfiber composite materials (~180 MPa, 6.3 MJ / m 3)。Its performance improvement can be attributed to: ① The cellulose microfibers provide a rigid framework to resist plastic deformation, and the cellulose nanofibers enhance the interfacial bonding and crack resistance; ② The hierarchical structure realizes the load distribution at the micro- and nano-scales, avoiding the premature failure of fibers at a single scale. Description of the Drawings

[0031] Figure 1 Picture of the cellulose dispersion prepared in Example 1;

[0032] Figure 2 SEM picture of the coexistence of cellulose micro / nanofibers in the cellulose dispersion prepared in Example 1;

[0033] Figure 3 Comparison chart of the mechanical properties of cellulose films with different swelling durations prepared in Example 1;

[0034] Figure 4 Comparison chart of the mechanical properties of cellulose films with different recovery times of dimethyl sulfoxide prepared in Example 1. Detailed Implementation Modes

[0035] To describe in detail the technical content, structural features, achieved objectives and effects of the technical solution, the following is a detailed description in conjunction with specific embodiments and accompanied by drawings.

[0036] In each embodiment of the present invention, the cellulose raw material is pre-treated to obtain cellulose. The treatment operation can adopt the following method: Purify the cellulose raw material: Boil and soak it in an alkali solution to remove lignin, rinse it thoroughly with deionized water, and finally boil and soak it in a bleaching solution to remove pigments.

[0037] The specific steps of boiling and soaking in the alkali solution to remove lignin, rinsing thoroughly with deionized water, and finally boiling and soaking in the bleaching solution to remove pigments are as follows: Immerse the cellulose raw material in a mixed aqueous solution of sodium hydroxide (2.5 mol / L) and sodium sulfite (0.4 mol / L), and keep boiling for 12 h. Then rinse it three times with hot deionized water to remove most of the lignin and hemicellulose. Then put the treated cellulose raw material into a bleaching solution (aqueous solution of H 2 O 2 ), and keep it in a non-stirring boiling state. When the yellow color of the sample disappears, take out the sample and rinse it with cold water. Finally, store the sample with lignin removed in ethanol for use.

[0038] Example 1

[0039] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of DMSO, and stir at room temperature for 1 day for swelling treatment to obtain a cellulose dispersion with a mass concentration of 4‰.

[0040] Step 2: Take 7 g of the cellulose dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under a pressure of -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water through the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a cellulose film.

[0041] Example 2

[0042] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of DMSO, and stir at room temperature for 2 days for swelling treatment to obtain a cellulose dispersion with a mass concentration of 4‰.

[0043] Step 2: Take 7 g of the cellulose dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under a pressure of -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water through the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a cellulose film.

[0044] Example 3

[0045] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of DMSO, and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0046] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under a pressure of -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water through the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a high-strength cellulose film.

[0047] Step 3: Label the filtrate obtained in Step 2 as "DMSO-1" for recovery, seal it and store it for later use to prevent water absorption.

[0048] In this example, the cellulose micro / nanofiber dispersion is prepared in Step 1, and through further operations in Step 2, a high-strength cellulose film is obtained.

[0049] In this example, the recovery rate of the dimethyl sulfoxide recovered in Step 3 is higher than 98%.

[0050] After testing, the density of the high-strength cellulose film is 1.03 g / m 3, the tensile strength is 250 MPa and the toughness is 15 MJ / m 3 .

[0051] Figure 1 This is a photo after bottling the cellulose micro / nanofiber dispersion prepared in Step 1 of this example.

[0052] After bottling the cellulose micro / nanofiber dispersion prepared in Step 1, it was left for 7 days without obvious flocculation.

[0053] The cellulose micro / nanofiber dispersion prepared in Step 1 was subjected to gradient replacement with tert-butanol solution, then freeze-dried under vacuum at -60 °C, and photographed using a field emission scanning electron microscope. The specific photo is shown in Figure 2 . It can be seen from Figure 2 that the cellulose micro / nanofiber dispersion prepared in Step 1 has an interpenetrating network structure with the coexistence of cellulose microfibers and nanofibers under a field emission scanning electron microscope, indicating that the dispersion does indeed have both cellulose microfibers and nanofibers and their network structure.

[0054] Example 4

[0055] Step 1: 80 g of cellulose was mixed with 11.8 g of LiOH and 20 L (22 kg) of DMSO, and stirred at room temperature for 5 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0056] Step 2: 7 g of the cellulose micro / nanofiber dispersion was vacuum filtered using a nylon filter membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then 7 g of deionized water was vacuum filtered on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. The obtained wet cellulose membrane was put into a hot press and hot pressed at -0.01 MPa negative pressure and 60 °C for 20 min to obtain a cellulose film.

[0057] Example 5

[0058] Step 1: 80 g of cellulose was mixed with 11.8 g of LiOH and 20 L (22 kg) of DMSO, and stirred at room temperature for 30 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0059] Step 2: 7 g of the cellulose micro / nanofiber dispersion was vacuum filtered using a nylon filter membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then 7 g of deionized water was vacuum filtered on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. The obtained wet cellulose membrane was put into a hot press and hot pressed at -0.01 MPa negative pressure and 60 °C for 20 min to obtain a cellulose film.

[0060] The cellulose films prepared in Examples 1-5 were tested for their mechanical properties using a 68TM-10 high and low temperature material testing machine produced by Instron Corporation, USA. Before testing, the films were cut into rectangles of 5 cm × 1 cm, the stretching rate was set at 2 mm / min, and each sample was tested 3 times. Origin software was used to plot the results. The results are shown in Figure 3 . It can be seen from Figure 3 that the elongation at break, breaking strength, and loss energy of the high-strength cellulose film prepared in Example 3 are all better than those of the other four different swelling durations. This is because the swelling effect of the DMSO / LiOH system is time-dependent: Initial swelling (0-24 hours): Li + preferentially embeds into the amorphous region of cellulose, weakens the hydrogen bonds between molecular chains through electrostatic interaction, and DMSO molecules penetrate into the fiber gaps, triggering the initial disentanglement of microfibrils. At this time, only surface swelling of the fiber occurs, and the micron fiber skeleton remains intact, but the generation amount of nanofibers is insufficient, resulting in low stress transfer efficiency when the film is stressed; Medium swelling (24-72 hours): Li + gradually diffuses to the edge of the cellulose crystalline region, and the strong polar swelling effect of DMSO promotes the hierarchical peeling of the fiber. The microfibrils dissociate layer by layer from the outside to the inside, forming a gradient distribution structure of micron fibers (diameter 1-3 μm) and nanofibers (diameter 10-100 nm). At this time, the nanofibers can not only act as an "adhesive" to enhance the interfacial bonding but also disperse stress through nanofiber bridging; When over-swelling (>72 hours): Li + excessively erodes the cellulose crystalline region, resulting in excessive dissociation of the micron fiber skeleton and a decrease in the rigidity of the film.

[0061] The 3-day duration is used as the swelling balance point: The micron fibers retain sufficient mechanical strength as the main load-bearing body, and the nanofibers form a continuous network to achieve uniform stress distribution, and the ratio of the two reaches the optimal ratio of strength-toughness.

[0062] Example 6

[0063] Step 1: 80 g of cellulose was mixed with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-1", and swelling treatment was carried out by stirring at room temperature for 3 days to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0064] Step 2: 7 g of the cellulose micro / nanofiber dispersion was vacuum filtered using a nylon filter membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose film. Then 7 g of deionized water was vacuum filtered on the cellulose film to wash away the residual dimethyl sulfoxide in the cellulose film. The obtained wet cellulose film was put into a hot press and hot pressed at a negative pressure of -0.01 MPa and a temperature of 60 °C for 20 min to obtain a high-strength cellulose film.

[0065] Step 3: Label the filtrate obtained in Step 2 as "DMSO-2" for recycling, seal it and store it for future use to prevent water absorption.

[0066] Example 7

[0067] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-2", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0068] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration with a nylon filter membrane with a pore size of 0.22 μm at -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Put the obtained wet cellulose membrane into a hot press and hot press it at a negative pressure of -0.01 MPa and a temperature of 60 °C for 20 min to obtain a high-strength cellulose film.

[0069] Step 3: Label the filtrate obtained in Step 2 as "DMSO-3" for recycling, seal it and store it for future use to prevent water absorption.

[0070] Example 8

[0071] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-3", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0072] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration with a nylon filter membrane with a pore size of 0.22 μm at -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Put the obtained wet cellulose membrane into a hot press and hot press it at a negative pressure of -0.01 MPa and a temperature of 60 °C for 20 min to obtain a high-strength cellulose film.

[0073] Step 3: Label the filtrate obtained in Step 2 as "DMSO-4" for recycling, seal it and store it for future use to prevent water absorption.

[0074] Example 9

[0075] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-4", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0076] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then, filter 7 g of deionized water through the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the obtained wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a high-strength cellulose film.

[0077] Step 3: Label the filtrate obtained in Step 2 as "DMSO-5" for recycling, seal it and store it for later use to prevent water absorption.

[0078] Example 10

[0079] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-5", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0080] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then, filter 7 g of deionized water through the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the obtained wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a high-strength cellulose film.

[0081] Step 3: Label the filtrate obtained in Step 2 as "DMSO-6" for recycling, seal it and store it for later use to prevent water absorption.

[0082] Example 11

[0083] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-6", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0084] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then, filter 7 g of deionized water through the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the obtained wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a high-strength cellulose film.

[0085] Step 3: Label the filtrate obtained in Step 2 as "DMSO-7" for recycling, seal it and store it for later use to prevent water absorption.

[0086] Example 12

[0087] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-7", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0088] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration with a nylon filter membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Put the obtained wet cellulose membrane into a hot press and hot press it at a negative pressure of -0.01 MPa and a temperature of 60 °C for 20 min to obtain a high-strength cellulose film.

[0089] Step 3: Label the filtrate obtained in Step 2 as "DMSO-8" for recovery, seal it and store it for later use to prevent water absorption.

[0090] Example 13

[0091] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-8", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0092] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration with a nylon filter membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Put the obtained wet cellulose membrane into a hot press and hot press it at a negative pressure of -0.01 MPa and a temperature of 60 °C for 20 min to obtain a high-strength cellulose film.

[0093] Step 3: Label the filtrate obtained in Step 2 as "DMSO-9" for recovery, seal it and store it for later use to prevent water absorption.

[0094] Example 14

[0095] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-9", and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0096] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then, filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the obtained wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a high-strength cellulose film.

[0097] Step 3: Label the filtrate obtained in Step 2 as "DMSO-10" for recycling, seal it for storage and keep it for future use to prevent water absorption.

[0098] Example 15

[0099] Step 1: Mix 80 g of cellulose with 11.8 g of LiOH and 20 L (22 kg) of "DMSO-10" and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0100] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration using a nylon membrane with a pore size of 0.22 μm under -0.098 MPa to obtain a cellulose membrane. Then, filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethyl sulfoxide in the cellulose membrane. Place the obtained wet cellulose membrane in a hot press and hot press it for 20 min under a negative pressure of -0.01 MPa and at 60 °C to obtain a high-strength cellulose film.

[0101] The high-strength cellulose films prepared in Examples 6 - 16 were respectively subjected to tensile strength, elongation at break, modulus, and toughness tests. Among them, the test methods for tensile strength and elongation at break were as follows: Use a 68TM-10 type high and low temperature material testing machine produced by Instron Corporation of the United States to test the tensile strength and elongation at break of the composite film. Before testing, cut the film into rectangles of 5 cm × 1 cm, set the tensile rate to 2 mm / min, and measure each sample 3 times. The modulus and toughness were calculated using origin software. The test results are shown in Figure 4 . It can be seen from Figure 4 that the elongation rate of the solvent-recyclable high-strength cellulose film prepared by the present invention is 4.53 - 12.61%, the breaking strength is 200.79 - 290.17 MPa, the Young's modulus is 5.92 - 13.22 GPa, and the loss energy is 5.2 - 13.03 MJ / m 3 It can be seen that using recycled DMSO in this system does not affect the mechanical properties of the prepared cellulose film, and the cost can be greatly reduced by the method of recycling the solvent.

[0102] Example 16

[0103] Step 1: Mix 80 g of cellulose with 20.9 g of LiCl and 21.3 L (20 kg) of DMAc, and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0104] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration with a nylon membrane with a pore size of 0.22 μm at -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethylacetamide in the cellulose membrane. Put the obtained wet cellulose membrane into an oven and dry it at 60 °C for 20 min to obtain a high-strength cellulose film.

[0105] Step 3: Label the filtrate obtained in Step 2 as "DMAc-1" for recovery, seal it and store it for future use to prevent water absorption.

[0106] Example 17

[0107] Step 1: Mix 80 g of cellulose with 20.9 g of LiCl and 21.3 L (20 kg) of DMAc, and stir at room temperature for 3 days for swelling treatment to obtain a cellulose micro / nanofiber dispersion with a mass concentration of 4‰.

[0108] Step 2: Take 7 g of the cellulose micro / nanofiber dispersion and perform vacuum filtration with a nylon membrane with a pore size of 0.22 μm at -0.098 MPa to obtain a cellulose membrane. Then filter 7 g of deionized water on the cellulose membrane to wash away the residual dimethylacetamide in the cellulose membrane. Dry the obtained wet cellulose membrane at room temperature to obtain a high-strength cellulose film.

[0109] Step 3: Label the filtrate obtained in Step 2 as "DMAc-1" for recovery, seal it and store it for future use to prevent water absorption.

[0110] Examples 16 and 17 show that this swelling method is still applicable in the DMAc / LiCl system, and the solvent can be recycled without reducing the strength of the cellulose film. Even if a more environmentally friendly room temperature drying method is selected, the strength of the cellulose film can still be maintained at 200 MPa. It should be noted that although the above embodiments have been described in this article, the patent protection scope of the present invention is not limited thereby. Therefore, based on the innovative concept of the present invention, changes and modifications to the embodiments described in this article, or equivalent structural or equivalent process transformations made using the content of the specification and drawings of the present invention, directly or indirectly applying the above technical solutions to other related technical fields, are all included in the patent protection scope of the present invention.

Claims

1. A method for preparing a high-strength cellulose film, characterized in that: It includes the following steps: Step 1: adding cellulose to a DMSO / alkali system or a DMAc / LiCl system, stirring continuously for 3 to 100 hours for pre-swelling treatment, and obtaining a cellulose micro / nano fiber dispersion with a mass concentration of 4‰-1%; the molar ratio of the alkali agent to the cellulose structural unit is 0.4 to 2.2:1; Step 2: forming a film by vacuum filtration or casting the cellulose micro / nano fiber dispersion with a mass concentration of 4‰-1% obtained in step 1, washing the residual solvent in water, and then hot pressing and drying to obtain the high-strength cellulose film; Step 3: Recover the filtrate obtained in step 2 and use it as a solvent to repeat steps 1 and 2 to continue preparing the high-strength cellulose film.

2. The method for preparing a high-strength cellulose film according to claim 1, characterized in that: In the step 1, the cellulose is one of the following: coniferous wood cellulose, broadleaf wood cellulose, cotton and flax cellulose.

3. The method for preparing a high-strength cellulose film according to claim 1, characterized in that: In the step 1, the added alkaline reagent is one of the following: KOH, LiOH, NaOH.

4. The method for preparing a high-strength cellulose film according to claim 1, characterized in that: The water absorption of the cellulose, dimethyl sulfoxide, alkali, dimethylacetamide and lithium chloride in step 1 should be strictly controlled, and the cellulose should be placed in a vacuum oven at 40-120° C. overnight to remove water.

5. The method for preparing a high-strength cellulose film according to claim 1, characterized in that: The cellulose dispersion obtained in step 1 contains both micron fibers and nano fibers, and the two types of fibers coexist in a three-dimensional network structure.

6. The method for preparing a high-strength cellulose film according to claim 1, characterized in that: The hot pressing temperature is 60 to 120° C., the hot pressing pressure is -5 to -0.1 MPa, and the hot pressing time is 6 to 24 minutes.

7. A high-strength cellulose film, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 6.

Images