Microfibrillated cellulose foams
11680370 · 2023-06-20
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Inventors
Cpc classification
International classification
Abstract
The present invention relates to porous foam materials comprising or essentially consisting of microfibrillated cellulose (“MFC”). These porous foam materials are light weight and can be tailored to specific uses. The present invention also relates to a process for making porous foam materials according to the present invention.
Claims
1. Method for making porous, microfibrillated cellulose based materials, said method comprising at least the following steps: (i) mixing a predetermined amount of microfibrillated cellulose (MFC) in a solvent together with a predetermined amount of at least one water soluble salt, so that a homogenous mixture results; (ii) bringing the mixture of (i) into a desired shape and drying this mixture in an oven until dry (first drying step); (iii) after completion of step (ii), immersing the dried material of step (ii) in a solvent thus leaching out at least 95% of the at least one water soluble salt added in step (i); (iv) after completion of step (iii), drying this mixture from step (iii) in an oven until dry (second drying step), wherein a resulting material after step (iv) is porous.
2. Method according to claim 1, wherein each of the at least one water soluble salt is characterized in that the solubility of said salt in water changes by less than 25% when changing the temperature from 20° C. to 100° C.
3. Method according to claim 1, wherein the method does not comprise a step of freeze-drying.
4. Method according to claim 1, wherein an amount of microfibrillated cellulose is from 1% to 30% by weight relative to a weight of solvent in the mixture of (i).
5. Method according to claim 1, wherein a weight ratio of salt present in the mixture in (i) and a solids content of MFC in the same mixture is in a range from 500:1 to 1:1.
6. Method according to claim 1, wherein the at least one water soluble salt as present in the mixture of step (i) is present in the form of particles that have an average particle size from 5 μm to 5 mm.
7. Method according to claim 6, wherein at least a portion of said at least one water soluble salt remains in the form of these particles during steps (i) and (ii).
8. Method according to claim 1, wherein the solvent is water.
9. Method according to claim 1, wherein each of the at least one water soluble salt has a solubility in water, at 20° C., of from 15 g/100 ml to 100 g/100 ml.
10. Method according to claim 1, wherein the method does not comprise use of any solvent other than water.
Description
DETAILED DESCRIPTION OF THE INVENTION
(1) The invention is described in more detail in the following, with reference to the enclosed figures, which are only meant to be illustrative, wherein:
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(6) In principle, any type of microfibrillated cellulose (MFC) can be used to make the porous materials in accordance with the present invention, as long as the fiber bundles as present in the original cellulose pulp are sufficiently disintegrated in the process of making MFC so that the average diameter of the resulting fibers/fibrils is in the nanometer-range and therefore more surface of the overall cellulose-based material has been created, vis-à-vis the surface available in the original cellulose material. MFC may be prepared according to any of the processes described in the art, including the prior art specifically cited in the “Background”-Section above.
(7) Origin of the Cellulose Used to Prepare the MFC
(8) In accordance with the present invention, there is no specific restriction in regard to the origin of the cellulose, and hence of the microfibrillated cellulose. In principle, the raw material for the cellulose microfibrils may be any cellulosic material, in particular wood, annual plants, cotton, flax, straw, ramie, bagasse (from sugar cane), suitable algae, jute, sugar beet, citrus fruits, waste from the food processing industry or energy crops or cellulose of bacterial origin or from animal origin, e.g. from tunicates.
(9) In a preferred embodiment, wood-based materials are used as raw materials, either hardwood or softwood or both (in mixtures). Further preferably softwood is used as a raw material, either one kind or mixtures of different soft wood types. Bacterial microfibrillated cellulose is also preferred, due to its comparatively high purity.
(10) Modified (Derivatized) and Non-Modified (Un-Derivatized) Cellulose/MFC
(11) In principle, the microfibrillated cellulose in accordance with the present invention may be unmodified in respect to its functional groups or may be physically modified or chemically modified, or both.
(12) However, in preferred embodiments of the present invention, the microfibrillated cellulose is not modified, in particular not TEMPO-oxidized, as the pore-forming effect of the salt particles may be reduced if the microfibrillated cellulose is modified, in particular oxidized in accordance with the TEMPO process.
(13) Chemical modification of the surface of the cellulose microfibrils may be achieved by various possible reactions of the surface functional groups of the cellulose microfibrils and more particularly of the hydroxyl functional groups, preferably by: oxidation, silylation reactions, etherification reactions, condensations with isocyanates, alkoxylation reactions with alkylene oxides, or condensation or substitution reactions with glycidyl derivatives. Chemical modification may take place before or after the defibrillation step.
(14) The cellulose microfibrils may, in principle, also be modified by a physical route, either by adsorption at the surface, or by spraying, or by coating, or by encapsulation of the microfibril. Preferred modified microfibrils can be obtained by physical adsorption of at least one compound. The MFC may also be modified by association with an amphiphilic compound (surfactant).
(15) However, in preferred embodiments, the microfibrillated cellulose is not physically modified.
(16) In a preferred embodiment of the present invention, the microfibrillated cellulose as used in step (i) is prepared by a process, which comprises at least the following steps: (a) subjecting a cellulose pulp to at least one mechanical pretreatment step; (b) subjecting the mechanically pretreated cellulose pulp of step (a) to a homogenizing step, which results in fibrils and fibril bundles of reduced length and diameter vis-à-vis the cellulose fibers present in the mechanically pretreated cellulose pulp of step (a), said step (b) resulting in microfibrillated cellulose; wherein the homogenizing step (b) involves compressing the cellulose pulp from step (a) and subjecting the cellulose pulp to a pressure drop.
(17) The mechanical pretreatment step preferably is or comprises a refining step. The purpose of the mechanical pretreatment is to “beat” the cellulose pulp in order to increase the accessibility of the cell walls, i.e. to increase the surface area.
(18) A refiner that is preferably used in the mechanical pretreatment step comprises at least one rotating disk. Therein, the cellulose pulp slurry is subjected to shear forces between the at least one rotating disk and at least one stationary disk.
(19) Prior to the mechanical pretreatment step, or in addition to the mechanical pretreatment step, enzymatic (pre)treatment of the cellulose pulp is an optional additional step that may be preferred for some applications. In regard to enzymatic pretreatment in conjunction with microfibrillating cellulose, the respective content of WO 2007/091942 is incorporated herein by reference. Any other type of pretreatment, including chemical pretreatment is also within the scope of the present invention.
(20) In the homogenizing step (b), which is to be conducted after the (mechanical) pretreatment step, the cellulose pulp slurry from step (a) is passed through a homogenizer at least once, preferably at least two times, as described, for example, in PCT/EP2015/001103, the respective content of which is hereby incorporated by reference.
EXAMPLES
Example 1
(21) Preparation of Microfibrillated Cellulose
(22) MFC as used to make the porous materials in accordance with the present invention is commercially available and commercialized by Borregaard as “Exilva F01-V”, based on cellulose pulp from Norwegian spruce (softwood).
(23) The MFC in step (i) was present as a paste, having a solids content of 10%. The solvent was water.
Example 2
(24) Preparation of Cellulose Foam
(25) 50 g of the MFC from Example 1 (solids content: 10%) was carefully mixed with 50 g of NaCl (Aldrich 31434N; D50 particle size: 400 μm). The resulting paste was formed into a disk shape form and dried at 105° C. overnight. The dried disk was then immersed in distilled water (200 ml) and kept for 4 h. The water was changed 3 times, after which steps the disk was dried at 105° C. overnight, resulting in porous disk (4.8 g).
Example 3
(26) Re-Dispersion Test of Cellulose Foam
(27) 2 g of the material obtained from Example 2 was mixed with 198 g of distilled water. The mixture was mixed with Ultra Turrax 4 min/10000 rpm, resulting in a suspension with visible phase separation meaning that the product is not re-dispersible.
Example 4
(28) Preparation of Cellulose Foam
(29) 60 g of MFC from Example 1 (solids content: 10%) was carefully mixed with 60 g of NaCl. The paste was spread onto a glass petri dish with a diameter of 9 cm and height of 1.2 cm and dried at 45° C. overnight and then at 105° C. overnight. The dry disk was immersed in distilled water (250 ml) for 1 hour. The water was changed 3 times, after which the disk was dried at 105° C. for 8 hours, resulting in a porous disk (5.84 g), 8.7 cm diameter and 0.8 cm thickness.(see
Example 5
(30) Preparation of Cellulose Foam
(31) 50 g of Exilva F 01-V from Example 1 (10% solids content) was carefully mixed with 100 g of NaCl. The paste was spread to a glass petri dish with a diameter of 9 cm and height of 1.2 cm and dried at 45° C. overnight and then at 105° C. overnight. The dry disk was immersed in distilled water (400 ml) for 1 hour. The water was changed 3 times, after which the disk was dried at 105° C. for 8 hours, resulting in a porous disk (4.76 g), 8.8 cm diameter and 0.9 cm thickness (see
Example 6
(32) Water Absorption of Cellulose Foam
(33) 321 mg (approximately 2 cm×2 cm) of the material obtained from Example 4 was immersed in distilled water. After 5 min the piece was removed from water, carefully tapped dry from excess water. The weight of the piece was 1.44 g.
Example 7
(34) Water Absorption of Cellulose Foam
(35) 259 mg (approximately 2 cm×2 cm) of the material obtained from Example 5 was immersed in distilled water. After 5 min the piece was removed from water, carefully tapped dry from excess water. The weight of the piece was 2.22 g.
Example 8
Comparative Example
(36) Water Absorption of Cellulose Film
(37) 10 g of MFC from Example 1 (10% solids content, no salt added) was carefully mixed with 40 g of water. The suspension was dried on a glass petri dish with a diameter of 9 cm and height of 1.2 cm and dried at 105° C. overnight, resulting in a thin film. A piece of film (486 mg) was immersed in distilled water. After 5 min the film piece was removed from water, carefully tapped dry from excess water. The weight of the film was 816 mg. (see
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