CN108822315B

CN108822315B - High-strength transparent hydrophobic cellulose nano-film and preparation method thereof

Info

Publication number: CN108822315B
Application number: CN201711455421.3A
Authority: CN
Inventors: 刘超; 李滨; 张跃冬; 于光; 冯晓燕
Original assignee: Qingdao Institute of Bioenergy and Bioprocess Technology of CAS
Current assignee: Qingdao Institute of Bioenergy and Bioprocess Technology of CAS
Priority date: 2017-12-28
Filing date: 2017-12-28
Publication date: 2021-02-09
Anticipated expiration: 2037-12-28
Also published as: CN108822315A

Abstract

The invention discloses a high-strength transparent hydrophobic cellulose nano-film and a preparation method thereof, belonging to the fields of green cellulose materials and environment-friendly nano-materials. The method takes natural cellulose as a raw material, firstly hydrolyzes the cellulose raw material through formic acid, and the separated cellulose solid is subjected to solvent replacement, mechanical treatment and drying forming in sequence to obtain the high-strength transparent hydrophobic cellulose nano-film. The nano cellulose membrane prepared by the method has excellent property, compact structure, high strength and high transparency, the tensile strength of the membrane can be more than 200MPa, and the transparency can reach 95 percent; the hydrophobic modification process of the invention does not need complex chemical reaction and expensive chemical reagent, and molecular rearrangement and recrystallization can occur on the surface of cellulose in the processes of mechanical treatment and film formation; the water contact angle of the obtained nano cellulose membrane can reach 99 degrees, the original property can be still maintained after the nano cellulose membrane is soaked in water for a long time, and the nano cellulose membrane has better water resistance and can be used in a high-humidity environment.

Description

High-strength transparent hydrophobic cellulose nano-film and preparation method thereof

Technical Field

The invention relates to the field of green cellulose materials and environment-friendly nano materials, in particular to a high-strength transparent hydrophobic cellulose nano-film and a preparation method thereof.

Background

In recent years, resource and environmental issues have been receiving more and more attention, and there is a trend of necessity to develop and utilize renewable resources instead of conventional fossil resources. Cellulose is the most abundant biopolymer on earth, and it is mainly present in the cell walls of higher plants. The nano-scale cellulose not only has the advantages of wide source, no toxicity, good biocompatibility, high tensile strength, biodegradability and the like, but also has the characteristics of high specific surface area, high reaction activity, small size effect, low thermal ductility and the like of nano materials. The nano-cellulose can replace expensive and non-biodegradable artificial synthetic materials (such as carbon nano-tubes), and is considered as a new generation raw material with great potential. The cellulose nano-film is a thin-film material obtained by performing processes such as suction filtration, drying, spin coating and the like on the nano-cellulose dispersion liquid, has the advantages of high barrier property, flexibility, transparency, high strength and the like, and has good application prospect in the fields of flexible solar cells, lithium ion battery diaphragms, conductive transparent electrodes, flexible screen displays, wearable electronic devices, green packaging materials and the like.

Nanocellulose can be divided into two categories: (I) mainly Cellulose Nanofibers (CNF) prepared by a combination of pretreatment and mechanical treatment, and (II) Cellulose Nanocrystals (CNC) prepared by chemical hydrolysis. CNC is mainly prepared by hydrolyzing amorphous and partially crystalline regions in cellulose, and strong inorganic acid hydrolysis (e.g., sulfuric acid, hydrochloric acid, etc.) is the most commonly used method. The inorganic strong acid hydrolysis method easily causes equipment corrosion, and a large amount of waste acid is difficult to treat, thereby causing various environmental problems. In addition, the small aspect ratio of the CNC causes the strength of the prepared nano film to be low, so the CNC is rarely applied to the preparation of the nano film.

The cellulose nano-film prepared by using the CNF with large major diameter has high strength and good stability, and related reports are more. Hsieh et al (Scientific reports.2017,7) use commercial wood pulp as a raw material, homogenize the raw material under the condition of 245MPa for 100 times, and then obtain the high-transparency cellulose nano-film by a filtering and drying method; the invention patent 201410000430.3 discloses that CNF is obtained by high-intensity ultrasonic (800W ultrasonic pulverization for 1-2 h) and high-intensity high-pressure homogenization treatment (100-300 MPa for 15-40 times), and finally, a transparent cellulose nano-film is obtained by suction filtration and drying. The invention patent 201610199891.7 discloses that a high-strength cellulose nano-film can be prepared by taking bleached wood pulp, bleached non-wood pulp, cotton pulp and the like as raw materials and adopting TEMPO oxidation pretreatment, cellulase method pretreatment or inorganic acid pretreatment combined with mechanical post-treatment; petroudy et al (Carbohydrate polymers.2017,157,1883-1891) use bagasse as raw material, and use the method of combination of enzymolysis and mechanical treatment to make CNF, then use direct drying and dynamic forming method to make transparent cellulose nano-film. The energy consumption for preparing the nano-cellulose by direct mechanical treatment is high, and the uniformity and the strength of a formed film are poor; the enzyme method has long treatment period and higher cost; although the TEMPO oxidation method has good product performance, the used chemical reagent is toxic and expensive, and the industrialization is difficult.

The surface of the cellulose nano-film prepared by the common method has a large number of hydrophilic groups, the cellulose film material is sensitive to humidity and poor in water tolerance, and the cellulose film has the problems of swelling deformation, strength reduction, poor barrier property and the like after moisture absorption. In addition, hydrophilic cellulose also has a problem of poor compatibility with other hydrophobic matrices when preparing composite materials. There are a number of reports in the literature on hydrophobic modification of nanocellulose, with physical adsorption and chemical modification being common methods. Physical surface adsorption is mainly achieved by adsorbing a surfactant, an electrically-opposite substance, or a polymer electrolyte on the surface of the CNF through the action of aqueous affinity, electrostatic attraction, hydrogen bonds, or van der waals force. The chemical modification is to introduce various small molecules or macromolecules on the surface of the cellulose by means of covalent bond linkage, and common methods are as follows: esterification, silylation, carbonylation, etherification, etc. These treatments generally require heating or long treatment times, sometimes require the addition of some toxic chemicals, and are inefficient and difficult to scale up.

In summary, there are still many problems in the cellulose nanocrystallization process, which prevent the large-scale production and application, such as: long pretreatment process period, no environmental pollution, expensive chemical, difficult recovery, high preparation cost and the like. In addition, the cellulose nano-film prepared by the traditional method has poor water resistance and is not easy to use in a humid environment. To perform the hydrophobic modification, a complicated, costly or environmentally unfriendly subsequent treatment is required. Therefore, there is an urgent need to develop an efficient, sustainable, low-cost and environmentally friendly method for preparing high-performance cellulose nanomembranes.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a preparation method of a high-strength transparent cellulose nano-film.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a preparation method of a high-strength transparent hydrophobic cellulose nano-film is characterized in that natural cellulose is used as a raw material, the cellulose raw material is hydrolyzed by formic acid, and separated cellulose solids are subjected to solvent replacement, mechanical treatment and drying forming in sequence to obtain the high-strength transparent hydrophobic cellulose nano-film. The method specifically comprises the following steps:

(a) mechanically scattering a natural cellulose raw material, adding the natural cellulose raw material into a formic acid solution, and carrying out hydrolysis reaction under the heating and stirring conditions to obtain a mixed material of a cellulose solid product and a hydrolysate;

(b) cooling the mixed material obtained in the step (1) to room temperature, then carrying out solid-liquid separation through centrifugation or filtration treatment, and washing the separated cellulose solid to be neutral by using water; collecting the separated hydrolysate, recovering formic acid by reduced pressure distillation, and collecting byproducts such as glucose, glucose oligosaccharide and hemicellulose sugar;

(c) adding the cellulose solid washed in the step (b) into an aprotic polar organic solvent for displacement treatment, adjusting the content of the cellulose solid, and performing multiple displacement treatments to obtain a cellulose suspension;

(d) carrying out high-pressure homogenization, ultrasonic crushing, disc grinding or ball milling treatment on the cellulose suspension obtained in the step (c) to enable the cellulose to be completely nanocrystallized and realize partial dissolution of cellulose surface molecules to obtain a CNF suspension;

(e) directly drying and molding the CNF suspension obtained in the step (d) or performing suction filtration and molding to obtain a wet film, wherein the filtrate or the evaporated solvent can be recycled; and drying the obtained wet film in vacuum to obtain the high-strength transparent hydrophobic cellulose nano film.

In the step (a), the natural cellulose raw material is one or a mixture of several of commercial paper pulp (including bleached wood pulp, bleached straw pulp and the like), microcrystalline cellulose, cotton and cotton linters; the concentration of the formic acid solution is 50-100 wt%, and the concentration of the formic acid solution is preferably 80-90 wt%; the weight ratio of the added natural cellulose raw material to the formic acid solution is 1 (5-100), preferably 1 (20-40);

in the step (a), in the hydrolysis esterification reaction process, the reaction temperature is 60-160 ℃, and the preferable reaction temperature is 80-100 ℃; the reaction time is 0.5 to 12 hours, and the preferable reaction time is 2 to 8 hours; the stirring speed is from 100rpm to 1000rpm, preferably from 300rpm to 500 rpm.

In the step (c), the aprotic polar organic solvent is dimethyl sulfoxide (DMSO), Dimethylformamide (DMF), Dimethylacetamide (DMAC), 1, 3-dimethyl-2-imidazolidinone (DMI), or N-methylpyrrolidone (NMP).

In the step (d), when the cellulose suspension is subjected to high-pressure homogenization treatment, the cellulose content in the cellulose suspension is 0.1-3 wt%, preferably 0.2-1.0 wt%; in the high-pressure homogenizing treatment process, the selected homogenizing pressure is 30-150MPa, preferably 80-100MPa, and the homogenizing times are 4-30 times, preferably 6-10 times; when the cellulose suspension is treated by ultrasonic crushing, the cellulose content in the cellulose suspension is 0.1-5 wt%, preferably 0.2-1 wt%; the selected ultrasonic power is 300-; the ultrasonic treatment time is 20-90min, preferably 30-60 min.

In the step (d), when the cellulose suspension is treated by ball milling, the cellulose content in the cellulose suspension is 5-60 wt%, preferably 20-40 wt%; the grinding time is 2-36h, preferably 4-12 h; when the cellulose suspension is subjected to disc grinding treatment, the cellulose content in the cellulose suspension is 5 to 30 wt%, preferably 10 to 20 wt%, and the grinding treatment is performed 5 to 60 times, preferably 20 to 35 times.

In the step (e), when the CNF suspension is directly dried and formed, the drying time is 6-96h, preferably 36-72h, and the drying temperature is 30-90 ℃, preferably 50-70 ℃; when the CNF suspension is formed by suction filtration, the wet film formed by suction filtration is naturally dried for 2-10h at room temperature, preferably for 4-8h, and then is dried in vacuum; the vacuum drying temperature is 40-70 ℃, the drying time is 0.5-8h, and the preferable vacuum drying time is 1-3 h.

The high-strength transparent hydrophobic cellulose nano-film prepared by the method has the tensile strength of more than 198MPa, the transparency of more than 90 percent and the water contact angle of more than or equal to 87 degrees.

The invention has the following advantages and beneficial effects:

1. the method adopts specific natural cellulose as a raw material, and simultaneously achieves the purposes of destroying the compact structure in the cellulose and esterifying the cellulose by using a formic acid hydrolysis one-pot method; the reaction temperature of formic acid hydrolysis is preferably lower than the boiling point of formic acid (100.8 ℃), the hydrolysis can be carried out under normal pressure, and the whole reaction process has low requirements on equipment; under the induction of a certain solvent environment, mechanically treating the cellulose treated by formic acid, and partially dissolving and completely nanocrystallizing molecules on the surface of the cellulose; during the subsequent drying and forming process of the nano-cellulose, the dissolved cellulose is partially recrystallized and molecules on the surface of the nano-cellulose are recombined, and finally the self-reinforced cellulose nano-film with hydrophobic property is prepared. In comparative example 1, the strength of the film formed after hydrolysis of cellulose with formic acid and subsequent homogenization in water was only 34.96Mpa, and the water contact angle was only 35 °. The results show that in the mechanical treatment and subsequent drying film-forming processes in the solvent environment of the invention, the cellulose interaction and the surface molecular properties are obviously changed, and the film-forming performance is obviously improved; in comparative example 3, the crystallinity of the CNF suspension was 62% by direct lyophilization and 71% by drying the same into a film under the conditions described in the present invention, which indicates that some of the cellulose molecules recrystallized during the film formation process and the film product formed a more dense structure after molding.

2. The invention directly adopts natural cellulose as raw material, and has wide source, regeneration and low price. The chemical medicine used in the preparation process can be recovered, the formic acid used in the hydrolysis process of the formic acid can be efficiently recovered through reduced pressure distillation, the aprotic polar organic solvent used in the subsequent treatment process can be recycled, and the whole process is sustainable and environment-friendly.

3. The invention can realize the high-efficiency nanocrystallization of cellulose, and the formic acid hydrolysis process can destroy the compact structure in the cellulose; the aprotic polar organic solvent in the mechanical treatment process can effectively swell the cellulose treated by formic acid, so that the energy consumption in the nanocrystallization process of the mechanically treated cellulose is reduced, for example: the preferable high-pressure homogenizing pressure is 80-100MPa, and the homogenizing times are 6-10 times, so that uniform suspension can be obtained, and the complete nanocrystallization of the cellulose is realized.

4. The nano cellulose membrane prepared by the method has excellent property, compact structure, high strength and high transparency, the tensile strength of the membrane can be more than 200MPa, and the transparency can reach 95 percent; in addition, the hydrophobic modification process of the invention does not need complex chemical reaction and expensive chemical reagent, and molecular rearrangement and recrystallization can occur on the surface of the cellulose in the processes of mechanical treatment and film formation; the water contact angle of the obtained nano cellulose membrane can reach 99 degrees, and the nano cellulose membrane can still keep the original properties after being soaked in water for a long time (figure 3), has better water resistance and can be used in a high-humidity environment.

Drawings

FIG. 1 is a flow chart of the preparation process of the cellulose nano-film of the invention.

FIG. 2 is a graph comparing the stress-strain curve of the cellulose nanomembrane of example 1 and the contact angle of different cellulose nanomembranes; wherein: (a) a stress-strain curve; (b) contact angle of cellulose nano-film.

Fig. 3 is a graph comparing stability in water of a cellulose nanomembrane prepared by a conventional method and a cellulose nanomembrane prepared in an embodiment of the present invention.

FIG. 4 is a comparative photograph of CNF suspensions after mechanical treatment of cellulose with formic acid in different solvent environments in each example.

FIG. 5 is a comparative picture of CNF film formation after mechanical treatment of cellulose in different solvent environments with formic acid treatment in each example.

FIG. 6 is a transmission electron micrograph of a CNF prepared in example 4.

Fig. 7 is a water contact angle picture of the cellulose nano-film prepared in example 4.

FIG. 8 is a transmission electron micrograph of a CNF prepared in example 5.

FIG. 9 is a transmission electron micrograph of a CNF prepared in example 6.

The specific implementation mode is as follows:

the following examples are given by way of illustration of embodiments of the invention and are not to be construed as limiting the invention, and it will be understood by those skilled in the art that modifications and extensions may be made without departing from the spirit and scope of the invention.

The invention relates to a high-strength transparent hydrophobic cellulose nano-film and a preparation method thereof, and the preparation process is shown in figure 1. The method adopts natural cellulose with wide sources and low price as raw materials, and utilizes formic acid to hydrolyze and destroy a compact structure in the cellulose and partially esterify the cellulose; under the induction of a certain solvent environment, mechanically treating the cellulose treated by formic acid, and partially dissolving and completely nanocrystallizing molecules on the surface of the cellulose; during the subsequent drying and forming process of the nano-cellulose, the dissolved cellulose is partially recrystallized and molecules on the surface of the nano-cellulose are recombined, and finally the self-reinforced cellulose nano-film with hydrophobic property is prepared. Formic acid and solvent in the whole process can be recycled, no waste liquid is discharged, and cellulose treated by formic acid can be effectively swelled in the solvent environment, so that mechanical energy consumption in the cellulose nanocrystallization process is reduced. The film prepared by the method has excellent performance, and has the advantages of high strength, water resistance, high barrier, high transparency, flexibility and the like.

Example 1

The method for preparing the high-strength transparent cellulose nano-film by using the commercial bleached softwood pulp as a raw material mainly comprises the following steps:

1) the bleached softwood pulp was broken up with a pulverizer, then 50g of the broken up material was placed in a reaction kettle, and 1000mL of 88 wt% formic acid solution was added. And then carrying out hydrolysis reaction for 6h at 100 ℃, adjusting the stirring speed to 600rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. The separated solid is continuously filtered and washed by water to be neutral to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar in the liquid.

2) Displacing the washed cellulose solid solvent into DMAC (dimethylacetamide), and adjusting the mass concentration of the cellulose DMAC suspension to 0.2 wt%; the DMAC replacement liquid and the cellulose formate contained therein can be recovered and utilized. And then, carrying out high-pressure homogenization treatment on the suspension under the following treatment conditions: homogenization at 100MPa for 10 times yielded a CNF suspension (shown in FIG. 4).

3) And (3) taking a certain amount of the CNF suspension in the step 2) to be placed in a vacuum suction filter for forming, and then naturally drying the wet membrane at room temperature for 6 hours, and then performing vacuum drying at 60 ℃ for 2 hours to obtain the cellulose nano membrane (shown in figure 5). The cellulose nano-film has the tensile strength of 227.97MPa, the transparency of 95 percent and the water contact angle of 87 degrees. The specific stress strain is shown in the D-FCN curve in FIG. 2a (D-FCN is a film-forming sample homogenized in DMAC after formic acid hydrolysis).

Example 2

The method for preparing the high-strength cellulose nano-film by taking the commercial bleached wheat straw pulp as the raw material comprises the following steps:

1) the bleached straw pulp sheet was mechanically dispersed by a pulverizer, and then 50g of the above raw material was placed in a reaction kettle, into which 1200mL of 95 wt% formic acid solution was added. And then carrying out hydrolysis reaction for 5h at 85 ℃, adjusting the stirring speed to 200rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. Washing the separated solid with water to neutrality to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar dissolved out in the reaction process.

2) The washed cellulose solid solvent was replaced into DMF and the mass concentration thereof was adjusted to 0.4 wt%. And then, carrying out high-pressure homogenization treatment on the suspension under the following treatment conditions: homogenization at 90MPa for 8 times yielded a CNF suspension (as shown in FIG. 4).

3) Taking 100g of the CNF suspension in the step 2), placing the CNF suspension in a suction filter for forming, and then naturally drying the wet membrane at room temperature for 5h, and then drying the wet membrane at 60 ℃ for 4h in vacuum to obtain the cellulose nano-membrane (shown in figure 5). The tensile strength of the nano cellulose membrane is 213.04MPa, the transparency is 93 percent, and the water contact angle is 94 degrees. .

Example 3

A method for preparing transparent high-strength cellulose nano-film by taking bleached hardwood pulp as a raw material comprises the following steps:

1) after the paddle was broken up by a pulverizer, 50g of the above raw material was put into a reaction vessel, and 1500mL of 80 wt% formic acid solution was added. And then carrying out hydrolysis reaction for 8h at 100 ℃, adjusting the stirring speed to 400rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. Filtering and washing the separated solid with deionized water to neutrality to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar products.

2) The washed cellulose solid solvent was replaced with DMSO, and the mass concentration thereof was adjusted to 0.3 wt%. Subsequently, the suspension is subjected to ultrasonic crushing treatment under the following conditions: 1000W ultrasonication for 50min gave a CNF suspension (as shown in FIG. 4).

3) 100g of the CNF suspension in step 2) was placed in a suction filter for shaping, and then the wet membrane was naturally air-dried at room temperature for 6h and then vacuum-dried at 50 ℃ for 8h to obtain a nano-cellulose membrane (as shown in FIG. 5). The tensile strength of the nano cellulose membrane is 198.43MPa, the transparency is 95%, and the water contact angle is 92 degrees.

Example 4

A method for preparing a high-strength nano cellulose membrane from microcrystalline cellulose comprises the following steps:

1) 50g of microcrystalline cellulose are taken directly into the reaction vessel, and 1200mL of 88 wt% formic acid solution and 1mL of hydrochloric acid are added. And then carrying out hydrolysis reaction for 2h at the temperature of 98 ℃, adjusting the stirring speed to 400rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. Filtering and washing the separated solid with deionized water to neutrality to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar products.

2) The washed cellulose solid was replaced with DMAC and its mass concentration was adjusted to 0.4 wt%. And then, carrying out high-pressure homogenization treatment on the suspension under the following treatment conditions: homogenizing at 90MPa for 10 times to obtain CNF suspension (transmission electron microscopy image of CNF is shown in FIG. 6).

3) And (3) taking 100g of the CNF suspension in the step 2), directly drying the CNF suspension in a glass dish for 8h for forming, and then drying the wet film in vacuum at 60 ℃ for 6h to obtain the nano cellulose film. The tensile strength of the nano cellulose film was 198.28MPa, the transparency was 94%, and the water contact angle was 99 ° (the contact angle is shown in FIG. 7).

Example 5

A method for preparing a high-strength transparent cellulose nano-film by taking cotton as a raw material comprises the following steps:

1) pulverizing cotton to 2-10mm length with a pulverizer, placing 50g of the above raw materials in a reaction kettle, and adding 1000mL of 90 wt% formic acid solution. And then carrying out hydrolysis reaction for 10h at 90 ℃, adjusting the stirring speed to be 500rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. Filtering and washing the separated solid with deionized water to neutrality to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar products.

2) The washed crude cellulose solvent was replaced with DMAC, and its mass concentration was adjusted to 15%. The slurry was then subjected to a grinding treatment 30 times in a disk mill to obtain a CNF suspension (the CNF transmission electron microscope is shown in fig. 8).

3) Diluting 100g of the CNF suspension in the step 2) with DMAC (dimethylacetamide) to a mass concentration of 0.3 wt%, then placing 100g of the CNF suspension in a suction filter for forming, and then naturally drying the wet membrane at room temperature for 6h, and then drying the wet membrane at 70 ℃ for 3h in vacuum to obtain the nano cellulose membrane. The tensile strength of the nano cellulose membrane is 228.55MPa, the transparency is 92%, and the water contact angle is 96 degrees.

Example 6

A method for preparing a high-strength nano cellulose membrane from a commercial cotton pulp plate comprises the following steps:

1) a commercial cotton pulp board was broken up by a pulverizer, and then 50g of the above raw materials were put into a reaction kettle, to which 1000mL of 88 wt% formic acid solution was added. And then carrying out hydrolysis reaction for 3h at 110 ℃, adjusting the stirring speed to be 500rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. Filtering and washing the separated solid with deionized water to neutrality to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar products.

2) The washed cellulose solids were directly solvent-displaced into DMAC without pulping. Then ball milling the suspension under the following conditions: the solid content was 30 wt%, and the treatment time was 12 hours, to obtain a CNF suspension (the transmission electron microscope of the CNF is shown in FIG. 9)

3) Diluting the CNF suspension in the step 2) with DMAC (dimethylacetamide) to a mass concentration of 0.3 wt%, then placing 100g of the suspension in a suction filter for molding, and then naturally drying the wet membrane at room temperature for 6h, and then drying the wet membrane at 60 ℃ for 5h in vacuum to obtain the nano cellulose membrane. The tensile strength of the nano cellulose membrane is 208.05MPa, the transparency is 94%, and the water contact angle is 92 degrees.

Comparative example 1

The method for preparing the high-strength transparent cellulose nano-film by using the bleached softwood pulp comprises the following steps:

2) The washed cellulose solid is not subjected to solvent replacement, the mass concentration of the cellulose suspension is directly adjusted to 0.2 wt% by water, and then the suspension is subjected to high-pressure homogenization treatment under the following conditions: 100MPa, 10 times, resulting in a CNF suspension (sample Water in FIG. 4).

3) And (3) putting a certain amount of the CNF suspension in the step 2) into a vacuum suction filter for forming, and then naturally drying the wet membrane at room temperature for 6 hours, and then drying the wet membrane at 60 ℃ for 2 hours in vacuum to obtain the cellulose nano membrane (shown as a water sample in figure 5). The tensile strength of the cellulose nano-film is 34.96MPa (W-FCN in figure 2a is the stress-strain curve of a film-forming sample after homogenization in water after formic acid hydrolysis), the water contact angle is 35 degrees (W-FCN in figure 2 b), and the transparency of the film is reduced to 85%. The mechanical properties of the cellulose nano-film formed by the same drawing in the comparative example 1 (D-FCN sample in figure 2) can be seen to be greatly influenced by the mechanical treatment under the condition of selecting a proper solvent.

In addition, from fig. 2b, which is the water contact angle of the cellulose nanomembranes prepared by different processes, it can be seen that D-FCN prepared by the method of the present invention has a significantly larger water contact angle relative to other samples (SCN is a sulfuric acid hydrolysis CNC prepared cellulose nanomembrane, and TCN is a TEMPO oxidation CNF prepared cellulose nanomembrane). As shown in FIG. 3, the water resistance of D-FCN prepared by the method of the invention in water is also higher than that of other samples, the D-FCN can still keep the original shape after being soaked for a long time, and the stable structures of other cellulose nano-films are damaged.

Comparative example 2

The bleached softwood pulp is directly mechanically scattered and crushed without formic acid hydrolysis treatment, and is dispersed into a DMAC solvent, and the mass concentration of the DMAC solvent is adjusted to be 0.4 wt%. Then, carrying out high-pressure homogenization treatment on the suspension, wherein the slurry directly blocks a homogenizer and cannot be subjected to subsequent treatment; in order to continue mechanical crushing, the cellulose DMAC suspension needs to be pretreated for 6 times under 30MPa to ensure that the fiber structure is initially crushed, and then is homogenized for 10 times under 100MPa to obtain the cellulose suspension.

Taking 100g of the cellulose suspension, placing the cellulose suspension in a suction filter for forming, and then naturally drying the wet membrane for 5 hours at room temperature and then drying the wet membrane for 6 hours at 60 ℃ in vacuum to obtain the cellulose membrane. The tensile strength of the obtained cellulose film was 25.66MPa, the transparency was 78%, and the contact angle was 28 °. The performance of the cellulose film was significantly lower than the strength of the nanocellulose film in example 1. This example illustrates that formic acid hydrolysis treatment is very necessary to facilitate efficient subsequent mechanical nanocrystallization, and that formic acid treatment has a great influence on the properties of cellulose nanomembranes. The reason is mainly because the cellulose treated by formic acid has better compatibility with solvents such as DMAC and the like and is easier to swell. The formic acid treatment makes the nanocrystallization of the cellulose easier in the mechanical treatment process, and partial dissolution and recrystallization of cellulose surface molecules occur in the mechanical treatment process and the subsequent drying film forming process, so that the formed cellulose nano film has more excellent performance.

Comparative example 3

The hydrolysis and high pressure homogenization processes used conditions similar to those in example 1, and the cellulose samples obtained by different drying processes had significant differences in properties, as follows:

1) a commercial bleached softwood pulp board is taken as a raw material, and after being scattered by a crusher, 50g of scattered raw material is placed in a reaction kettle, and 1000mL of 88 wt% formic acid solution is added. And then carrying out hydrolysis reaction for 6h at 100 ℃, adjusting the stirring speed to 600rpm, cooling to room temperature after the reaction is finished, and separating solid and liquid in a suction filtration mode. The separated solid is continuously filtered and washed by water to be neutral to obtain cellulose solid; the separated liquid is decompressed and distilled to recover formic acid and simultaneously recover sugar in the liquid.

2) Displacing the washed cellulose solid solvent into DMAC (dimethylacetamide), and adjusting the mass concentration of the cellulose DMAC suspension to 0.2 wt%; the DMAC replacement liquid and the cellulose formate contained therein can be recovered and utilized. And then, carrying out high-pressure homogenization treatment on the suspension under the following treatment conditions: homogenizing 10 times at 100MPa to obtain CNF suspension.

3) Carrying out vacuum filtration, drying and molding on the suspension obtained in the step 2) according to the steps in the example 1 to obtain a cellulose nano film with the tensile strength of 227.97MPa, the transparency of 95% and the water contact angle of 87 degrees, wherein the crystallinity of the film is 71%;

4) if the solvent of the CNF suspension obtained in step 2) is replaced by water and then dried to form a film, only a cellulose nano-film with tensile strength of 80MPa, transparency of 76% and water contact angle of 62 ° can be obtained, and the crystallinity of the film is 62%. Therefore, the solvent environment of the cellulose nano-film drying process is very important, and the process has important influence on the molecular arrangement and the film forming performance of the nano-cellulose surface.

5) If the CNF suspension obtained in step 2) is directly freeze-dried, solid cellulose powder with a crystallinity of 63% can be obtained, which is lower than the cellulose nanomembrane obtained in step 3). This shows that, during the drying and forming process of the cellulose nano-film of the invention, the partially disordered cellulose molecules are rearranged and recrystallized. The more stable and compact structure enables the cellulose nano-film obtained by the invention to have excellent performance.

Claims

1. A preparation method of a high-strength transparent hydrophobic cellulose nano-film is characterized by comprising the following steps: the method takes natural cellulose as a raw material, firstly, the cellulose raw material is hydrolyzed by formic acid, and the separated cellulose solid is subjected to solvent replacement, mechanical treatment and drying forming in sequence to obtain the high-strength transparent hydrophobic cellulose nano-film;

the method comprises the following steps:

(b) cooling the mixed material obtained in the step (1) to room temperature, then carrying out solid-liquid separation through centrifugation or filtration treatment, and washing the separated cellulose solid to be neutral by using water; collecting the separated hydrolysate, recovering formic acid by reduced pressure distillation, and collecting byproducts;

2. The method for preparing a high-strength transparent hydrophobic cellulose nanomembrane according to claim 1, wherein: in the step (a), the natural cellulose raw material is one or a mixture of several of commercial paper pulp, microcrystalline cellulose, cotton and cotton linter, and the commercial paper pulp is bleached wood pulp or bleached straw pulp; the concentration of the formic acid solution is 50-100 wt%; the weight ratio of the added natural cellulose raw material to the formic acid solution is 1 (5-100).

3. The method for preparing a high-strength transparent hydrophobic cellulose nanomembrane according to claim 1, wherein: in the step (a), in the hydrolysis esterification reaction process, the reaction temperature is 60-160 ℃, the reaction time is 0.5-12h, and the stirring speed is 100-1000 rpm.

4. The method for preparing a high-strength transparent hydrophobic cellulose nanomembrane according to claim 1, wherein: in the step (c), the aprotic polar organic solvent is dimethyl sulfoxide, dimethylformamide, dimethylacetamide, 1, 3-dimethyl-2-imidazolidinone or N-methylpyrrolidone.

5. The method for preparing a high-strength transparent hydrophobic cellulose nanomembrane according to claim 1, wherein: in the step (d), when the cellulose suspension is subjected to high-pressure homogenization treatment, the cellulose content in the cellulose suspension is 0.1-3 wt%; in the high-pressure homogenizing treatment process, the selected homogenizing pressure is 30-150MPa, and the homogenizing times are 4-30 times; when the cellulose suspension is treated by ultrasonic crushing, the cellulose content in the cellulose suspension is 0.1-5 wt%, the selected ultrasonic power is 300-.

6. The method for preparing a high-strength transparent hydrophobic cellulose nanomembrane according to claim 1, wherein: in the step (d), when the cellulose suspension is treated by ball milling, the cellulose content in the cellulose suspension is 5-60 wt%, and the grinding time is 2-36 h; when the cellulose suspension is ground by a disc grinder, the cellulose content in the cellulose suspension is 5-30 wt%, and the grinding treatment is carried out for 5-60 times.

7. The method for preparing a high-strength transparent hydrophobic cellulose nanomembrane according to claim 1, wherein: in the step (e), when the CNF suspension is directly dried and formed, the drying time is 6-96h, and the drying temperature is 30-90 ℃; when the CNF suspension is formed by suction filtration, the wet film formed by suction filtration is naturally dried for 2-10h at room temperature, and then is dried in vacuum; the temperature of the vacuum drying is 40-70 ℃, and the drying time is 0.5-8 h.

8. A high strength transparent hydrophobic cellulose nanofilm prepared by the method of any one of claims 1-7, characterized by: the cellulose nano-film has tensile strength of more than 198MPa, transparency of more than 90 percent and water contact angle of more than or equal to 87 degrees.