CN110818806B - Preparation method of water-redispersible cellulose nanocrystals - Google Patents
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Abstract
A method for preparing water-redispersible cellulose nanocrystals, comprising the steps of: 1) mixing the raw materials in a ratio of 1: dispersing the fiber raw material into an acid solution with the acid concentration less than 0.1% at the ratio of 8-11 g/ml, wherein the acid solution is hydrochloric acid or sulfuric acid; carrying out reaction under the conditions of heating, pressurizing and stirring; the reaction temperature is 150 ℃ and 180 ℃, the reaction time is 50-80min, and the reaction pressure is 5-20 MPa; filtering the unhydrolyzed cellulose to obtain filtrate for later use; 2) adding the obtained filtrate into an organic solvent under stirring, performing centrifugal separation, and collecting centrifugal precipitate; 3) the resulting precipitate is dried to obtain a cellulose nanocrystalline solid with an initial moisture content of 10% to 20% by weight. The preparation method provided by the invention has the advantages that the water redispersibility of the cellulose nanocrystal is improved by depositing the byproduct soluble sugar on the surface of the cellulose nanocrystal in the preparation process, and the preparation method is environment-friendly and is simple to operate.
Description
Technical Field
The invention relates to the field of dispersion of cellulose nanocrystals, in particular to a method for dispersing cellulose nanocrystals in an aqueous medium.
Background
The Cellulose Nanocrystalline (NCC) is a renewable and recyclable carbon neutral material, and due to the factors and potential unique properties, the Cellulose Nanocrystalline material has wide application potential. The solid content concentration of the existing well-dispersed unmodified cellulose nanocrystal suspension is 1 percent at most, and the rest 99 percent is water completely, so that the storage and transportation cost of the cellulose nanocrystal is high, and the growth of bacteria and fungi in the suspension is difficult to prevent; in addition, high solids contents are required for the modification of cellulose nanocrystals by dispersion in solvents and for the preparation of composite materials. Therefore, the suspension of cellulose nanocrystals must be dried to reduce storage and transportation costs, avoid the growth of bacteria and fungi in the suspension, and increase the range of application of cellulose nanocrystals. However, the cellulose nanocrystal has small particle size, large specific surface area and rich surface hydroxyl content, during the drying and dehydration process, particles are flocculated under the action of hydrogen bond and van der waals force, and the flocculated cellulose nanocrystal is difficult to be redispersed to the original nanometer size by a physical or chemical method, so that some characteristics of the cellulose nanocrystal material are lost, and the research and application of the cellulose nanocrystal are greatly limited.
There are two main methods for improving the redispersibility of cellulose nanocrystals on the eye: the first is a physical method, for example, protective colloids such as surfactants and polyelectrolytes are added to generate chemical bonding effect between the protective colloids and active groups, hydrophilic ends of the surfactants are adsorbed on the surface of the nano-cellulose, hydrophobic ends are adsorbed in a matrix with proper dissolving conditions, and the sedimentation of nano-cellulose particles and the collision and aggregation among the particles are prevented through space stabilization effect and electrostatic repulsion effect. However, because the specific surface area of the NCC is large, the amount of the required surfactant is too large, and the addition of too much surfactant not only influences the further compounding of the NCC and the polymer, but also causes difficulty in the subsequent removal of the surfactant. And secondly, reducing the hydrophilicity of NCC and preventing the generation of hydrogen bonds among nanocellulose by introducing steric hindrance and electrostatic groups by adopting a chemical modification method, wherein the chemical modification of nanocellulose comprises graft copolymerization, silanization, TEMPO oxidation, acetylation, esterification, cationization and the like. However, the reaction conditions are relatively harsh, so the application range of the two methods in actual production is limited to a certain extent, and therefore, the development of a simple and environment-friendly preparation method of the water-redispersible cellulose nanocrystals is very important.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a simple and environment-friendly preparation method of water-redispersible cellulose nano-crystals. Firstly, preparing a mixed suspension containing cellulose nanocrystals and soluble sugars by adopting an extremely low acid concentration (less than or equal to 0.1%), then dropwise adding the mixed suspension into an organic solvent, carrying out solvent replacement to deposit the soluble sugars on the surface of the cellulose nanocrystals, and then evaporating and drying to obtain a water-redispersible cellulose nanocrystal solid form containing 10-20% of initial water content.
In order to achieve the purpose, the invention adopts the technical scheme that:
a preparation method of water-redispersible cellulose nano-crystals comprises the following steps:
1) dispersing 3-5g of fiber raw material in 35-40ml of hydrochloric acid or sulfuric acid (the ratio of the fiber raw material to the acid solution is 1: 8-11 g/ml, the unit g of the fiber raw material and the unit ml of the acid solution), and reacting in a high-pressure reaction kettle under the conditions of heating, pressurizing and stirring; the reaction temperature is 150 ℃ and 180 ℃, the reaction time is 50-80min, and the reaction pressure is 5-20 MPa; after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to obtain filtrate (mixed suspension containing cellulose nanocrystalline and soluble sugar) so as to separate unhydrolyzed cellulose;
2) adding the obtained filtrate (mixed suspension containing the cellulose nanocrystal and the soluble sugar) into an organic solvent, and performing solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystal; then carrying out centrifugal precipitation, and taking the lower-layer precipitate for later use;
3) drying the obtained precipitate at a constant temperature of 60 ℃ to obtain a water-redispersible cellulose nanocrystalline solid form containing an initial water content of 10% to 20%.
The soluble sugar is a byproduct in the process of preparing the cellulose nanocrystalline by acid hydrolysis of the fiber in the step 1).
The organic solvent is one or more of ethanol, propanol, tert-butanol, acetone, diethyl ether and n-butanol.
The cellulose raw material is bamboo pulp, wheat straw pulp, microcrystalline cellulose or cotton pulp. The requirements of each fiber are as follows:
the length of the bamboo pulp fiber is not less than 1.8mm and not more than 2.5mm, and the width of the bamboo pulp fiber is not less than 15 mu m and not more than 20 mu m.
The wheat straw pulp fiber is required to be not less than 0.8mm and not more than 1.5mm in length and not more than 10 mu m and not more than 15 mu m in width.
The length of the microcrystalline cellulose fiber is required to be less than or equal to 20 mu m and less than or equal to 100 mu m, and the width of the microcrystalline cellulose fiber is required to be less than or equal to 10 mu m and less than or equal to 16 mu m.
The cotton pulp fiber is required to have a length of 2mm or more and 6mm or less and a width of 20 mu m or more and 40 mu m or less.
The invention has the beneficial effects that:
the byproduct soluble sugar generated in the process of preparing the cellulose nanocrystals by acid hydrolysis of the fibers is deposited on the surface of the cellulose nanocrystals by solvent replacement, so that the redispersibility of the water of the cellulose nanocrystals is improved, and the method has the advantages of environmental friendliness and simple operation, and solves the obstacles in the industrial production of the existing nanocellulose crystals; has wide industrial application prospect.
Drawings
FIG. 1 is a TEM image of the by-product (soluble sugar) of the process of acid hydrolysis of fiber to prepare cellulose nanocrystals according to the present invention (the insert is SAED diffractogram for soluble sugar).
FIG. 2 is a TEM image of cellulose nanocrystals prepared according to the present invention (the insert is SAED diffraction pattern of cellulose nanocrystals).
FIG. 3 is a TEM image of water-redispersible cellulose nanocrystals prepared according to the present invention (the inset represented by area 1 is the SAED diffraction pattern of the cellulose nanocrystals; the inset represented by area 2 is the SAED diffraction pattern of the soluble sugars).
Detailed Description
A method for preparing water-redispersible cellulose nanocrystals comprising the steps of:
1) dispersing 3-5g of fiber raw material in 35-40ml of acid solution (the ratio of the fiber raw material to the acid solution is 1: 8-11 g/ml, in the ratio, the unit of the fiber raw material is g, and the unit of the acid solution is ml), wherein the acid solution is hydrochloric acid or sulfuric acid with the acid concentration of less than 0.1%, and reacting under the conditions of heating, pressurizing and stirring; the reaction temperature is 150 ℃ and 180 ℃, the reaction time is 50-80min, and the reaction pressure is 5-20 MPa; filtering the unhydrolyzed cellulose to obtain filtrate for later use;
2) adding the obtained filtrate into an organic solvent under stirring, performing centrifugal separation, and collecting centrifugal precipitate;
3) the resulting precipitate is dried to obtain a cellulose nanocrystalline solid with an initial moisture content of 10% to 20% by weight.
The organic solvent is one or more of ethanol, propanol, tert-butanol, acetone, diethyl ether and n-butanol.
The present invention will be described in further detail with reference to specific examples, in which the soluble sugars are byproducts of the hydrolysis of the fiber raw material in acid to prepare cellulose nanocrystals, i.e., the filtrate in step 1 contains cellulose nanocrystals and soluble sugars.
Example 1
1) Taking 4g of bamboo pulp fiber (the length is more than or equal to 1.8mm and less than or equal to 2.5mm, the width is more than or equal to 15 mu m and less than or equal to 20 mu m), and adding 38ml of hydrochloric acid with the mass fraction of 0.08%; the reaction temperature is 150 ℃, the reaction time is 50min, and the reaction pressure is 20 MPa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugar) into a tert-butyl alcohol solvent under stirring, and carrying out solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 20%.
Example 2
1) Taking 3g wheat straw pulp fiber (the length is more than or equal to 0.8mm and less than or equal to 1.5mm, the width is more than or equal to 10 mu m and less than or equal to 15 mu m), and adding 38ml hydrochloric acid with the mass fraction of 0.1% and 0.9ml distilled water; the reaction temperature is 160 ℃, the reaction time is 70min, and the reaction pressure is 10 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugar) into ethanol under stirring, and performing solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 20%.
Example 3
1) Taking 5g of microcrystalline cellulose fiber (the length is more than or equal to 20 mu m and less than or equal to 100 mu m, the width is more than or equal to 10 mu m and less than or equal to 16 mu m), and adding 39ml of sulfuric acid with the mass fraction of 0.08 percent and 0.6ml of distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 15 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugar) into propanol under stirring, and performing solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the layer was removed and dried at 60 ℃ to a constant temperature to a water-redispersible cellulose nanocrystalline solid form containing an initial water content of 18%.
Example 4
1) Taking 5g of bamboo pulp fiber (the length is more than or equal to 2mm and less than or equal to 4mm, the width is more than or equal to 20 mu m and less than or equal to 30 mu m), adding 39ml of sulfuric acid with the mass fraction of 0.1 percent and 0.7ml of distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugars) to an ethanol/tert-butyl alcohol mixed solvent (in a volume ratio of 5:3) under stirring, and carrying out solvent replacement to deposit the soluble sugars on the surfaces of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 10%.
Example 5
1) Taking 3g of cotton pulp fiber (the length is more than or equal to 2mm and less than or equal to 6mm, the width is more than or equal to 20 mu m and less than or equal to 40 mu m), and adding 39ml of sulfuric acid with the mass fraction of 0.1 percent and 0.7ml of distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 20 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugar) into an acetone solvent under stirring, and carrying out solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 15%.
Example 6
1) Taking 5g of bamboo pulp fiber (the length is more than or equal to 2mm and less than or equal to 4mm, the width is more than or equal to 20 mu m and less than or equal to 30 mu m), adding 39ml of sulfuric acid with the mass fraction of 0.1 percent and 0.7ml of distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugar) into an ether solvent under stirring, and carrying out solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water-redispersible cellulose nanocrystals containing an initial water content of 13%.
Example 7
1) Taking 5g of bamboo pulp fiber (the length is more than or equal to 2mm and less than or equal to 4mm, the width is more than or equal to 20 mu m and less than or equal to 30 mu m), adding 39ml of sulfuric acid with the mass fraction of 0.1 percent and 0.7ml of distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing cellulose nanocrystals and soluble sugar) to n-butanol under stirring, and performing solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 10%.
Example 8
1) Taking 5g wheat straw pulp fiber (the length is more than or equal to 0.8mm and less than or equal to 1.5mm, the width is more than or equal to 10 mu m and less than or equal to 15 mu m), adding 39ml sulfuric acid with the mass fraction of 0.1 percent and 0.7ml distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing cellulose nanocrystals and soluble sugar) into an ethanol/n-butanol mixed solvent (volume ratio of 5:3) under stirring, and performing solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water-redispersible cellulose nanocrystals containing an initial water content of 13%.
Example 9
1) Taking 5g wheat straw pulp fiber (the length is more than or equal to 0.8mm and less than or equal to 1.5mm, the width is more than or equal to 10 mu m and less than or equal to 15 mu m), adding 39ml sulfuric acid with the mass fraction of 0.1 percent and 0.7ml distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing cellulose nanocrystals and soluble sugar) into an ethanol/n-butanol/tert-butanol mixed solvent (volume ratio of 4:3:3) under stirring, and performing solvent replacement to deposit the soluble sugar on the surface of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water-redispersible cellulose nanocrystals containing an initial water content of 13%.
Example 10
1) Taking 5g wheat straw pulp fiber (the length is more than or equal to 0.8mm and less than or equal to 1.5mm, the width is more than or equal to 10 mu m and less than or equal to 15 mu m), adding 39ml sulfuric acid with the mass fraction of 0.1 percent and 0.7ml distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugars) to an acetone/diethyl ether mixed solvent (in a volume ratio of 2:3) while stirring, and performing solvent replacement to deposit the soluble sugars on the surfaces of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a water redispersible cellulose nanocrystalline solid form containing an initial water content of 18%.
Example 11
1) Taking 5g wheat straw pulp fiber (the length is more than or equal to 0.8mm and less than or equal to 1.5mm, the width is more than or equal to 10 mu m and less than or equal to 15 mu m), adding 39ml sulfuric acid with the mass fraction of 0.1 percent and 0.7ml distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugars) to an ethanol/diethyl ether/acetone mixed solvent (in a volume ratio of 2:1:2) under stirring, and carrying out solvent replacement to deposit the soluble sugars on the surfaces of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 12%.
Example 12
1) Taking 5g wheat straw pulp fiber (the length is more than or equal to 0.8mm and less than or equal to 1.5mm, the width is more than or equal to 10 mu m and less than or equal to 15 mu m), adding 39ml sulfuric acid with the mass fraction of 0.1 percent and 0.7ml distilled water; the reaction temperature is 180 ℃, the reaction time is 60min, and the reaction pressure is 5 Mpa;
2) after the reaction is finished, filtering the reaction mixture by a nylon filter screen (the aperture is less than 1 mu m) to separate unhydrolyzed cellulose; dropwise adding the obtained filtrate (containing the cellulose nanocrystals and the soluble sugars) into an ethanol/diethyl ether/acetone/tert-butyl alcohol mixed solvent (in a volume ratio of 2:1:1:1) under stirring, and carrying out solvent replacement to deposit the soluble sugars on the surfaces of the cellulose nanocrystals; the precipitate was then centrifuged and the lower layer was dried at 60 ℃ to a constant temperature to a solid form of water redispersible cellulose nanocrystals containing an initial water content of 10%.
And (3) respectively carrying out Transmission Electron Microscope (TEM) and electron diffraction (SAED) analysis on the cellulose nanocrystals in the obtained filtrate, the byproduct soluble sugar and the finally obtained product water redispersible cellulose nanocrystals:
FIG. 1 is a TEM image of a by-product soluble sugar, which is seen in FIG. 1 as an oval shape after drying, and in FIG. 1 as a SAED diffraction pattern of the soluble sugar, which is seen in the inset as an amorphous state without a clear diffraction ring; FIG. 2 is a TEM image of cellulose nanocrystals, wherein the cellulose nanocrystals are rod-shaped as can be seen from FIG. 2, and the insert image in FIG. 2 is a SAED diffraction pattern of the cellulose nanocrystals, wherein the cellulose nanocrystals have clear diffraction rings and are in a crystalline state as can be seen from the insert image; the obtained water-redispersible cellulose nanocrystals are shown in fig. 3, as can be seen from fig. 3, the periphery of the rod is wrapped with other substances, and in fig. 3, two insets are provided, wherein the insets represented by the area 1 are used for SAED data acquisition on the rod-shaped structure, and as can be seen from the SEAD diffractogram of the area 1, the SAED diffractogram of the rod-shaped structure has clear diffraction rings and is in a crystalline structure, and thus, is cellulose nanocrystals; the inset represented by region 2 was SAED data acquisition on non-rod-like structures, which had no clear diffraction rings and were amorphous structures, and thus were soluble sugars, as seen on the SAED diffractogram of region 2. In summary, by comparing fig. 1, fig. 2 and fig. 3, we can conclude that the soluble sugar is dissolved in the organic solvent and precipitated from the solvent after the suspension of cellulose nanocrystals and soluble sugar is added into the tert-butyl alcohol solvent, and the cellulose nanocrystals are used as seed crystals during the precipitation process and are deposited on the surface of the cellulose nanocrystals.
Claims (9)
1. A method for preparing water-redispersible cellulose nanocrystals, comprising the steps of:
1) mixing the raw materials in a ratio of 1: dispersing fiber raw materials into an acid solution with the acid concentration of less than 0.1% according to the proportion of 8-11 g/ml, wherein the unit g of the fiber raw materials and the unit ml of the acid solution are obtained; the acid solution is hydrochloric acid or sulfuric acid; carrying out reaction under the conditions of heating, pressurizing and stirring; the reaction temperature is 150 ℃ and 180 ℃, the reaction time is 50-80min, and the reaction pressure is 5-20 MPa; filtering the unhydrolyzed cellulose to obtain filtrate for later use;
2) adding the obtained filtrate into an organic solvent under stirring, performing centrifugal separation, and collecting centrifugal precipitate;
3) the resulting precipitate is dried to obtain a cellulose nanocrystalline solid with an initial moisture content of 10% to 20% by weight.
2. The method of claim 1, wherein the organic solvent is one or a mixture of two or more of ethanol, propanol, tert-butanol, acetone, diethyl ether and n-butanol.
3. The method for preparing water-redispersible cellulose nanocrystals according to claim 1, characterized in that the cellulose raw material is bamboo pulp, wheat straw pulp, microcrystalline cellulose or cotton pulp.
4. The method for preparing water-redispersible cellulose nanocrystals according to claim 1, characterized in that the filtration in step 1) is carried out using a nylon filter screen having a pore size of < 1 μm.
5. The method for producing water-redispersible cellulose nanocrystals according to claim 1, characterized in that the drying temperature in the step 3) is 60 ℃.
6. The method of claim 3, wherein the bamboo pulp fiber is required to have a length of 1.8mm or more and a length of 2.5mm or less and a width of 15 μm or more and a width of 20 μm or less.
7. The method of claim 3, wherein the wheat straw pulp fiber is required to have a length of 0.8mm or less and a length of 1.5mm or less, and a width of 10 μm or less and a width of 15 μm or less.
8. The method for producing water-redispersible cellulose nanocrystals as claimed in claim 3, wherein the microcrystalline cellulose fibers are required to have a length of 20 μm or less and a length of 100 μm or less and a width of 10 μm or less and a width of 16 μm or less.
9. The method of claim 3, wherein the cotton pulp fiber is required to have a length of 2mm or less and a length of 6mm or less and a width of 20 μm or less and a width of 40 μm or less.
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