WO2018002446A1

WO2018002446A1 - Porous cellulose structure

Info

Publication number: WO2018002446A1
Application number: PCT/FI2017/050493
Authority: WO
Inventors: Henrikki Mertaniemi; Antti Laukkanen
Original assignee: Xylocel Oy
Priority date: 2016-06-30
Filing date: 2017-06-30
Publication date: 2018-01-04

Abstract

The present invention provides methods for manufacturing porous nanocellulose structures, and nanocellulose structures having connected porosity and high surface area.

Description

POROUS CELLULOSE STRUCTURE

BACKGROUND

Cellulose is a polysaccharide found in the plant cell wall, forming nanofibrils organized in a matrix of other biopolymers, such as hemicelluloses, pectin, and lignin. The individual nanofibrils, referred to as elementary fibrils, have a diameter of a few nanometers, and their length can exceed 1 μιτι. The elementary fibrils are highly crystalline and have remarkable mechanical properties, having a stiffness of ca. 140 GPa and a tensile strength of 2-3 GPa. In primary cell walls, especially in parenchyma cells, the cellulose nanofibrils are distributed randomly forming a flexible membrane layer together with pectin and hemicelluloses. In certain plant species, an additional secondary wall structure is formed after the cell is fully-grown, especially in various wood species. In the secondary cell walls, the nanofibrils are highly aligned mostly in the same direction and tightly bound to each other through hydrogen-bonding and covalent lignin bridges, forming a very rigid structure.

The cellulose nanofibrils can be separated from biomass using a chemical or enzymatical pretreatment combined with mechanical fibrillation. The product is commonly and interchangeably called nanocellulose, nanofibrillated cellulose (NFC), microfibrillated cellulose (MFC), or cellulose nanofibrils (CNF). Depending on the manufacturing process, nanofibrillated cellulose consists of individual elementary cellulose fibrils or bundles of a few such fibrils. These bundles typically have a diameter in the range from 10 to 50 nm and a length up to few micrometers. Although the secondary cell walls, for example in wood, are rich of cellulose nanofibrils, isolation of the structures without damaging the fibrils itself is very difficult. Also, the needed fibrillation process is complicated, expensive, and often a chemical pre-treatment is needed prior to fibrillation. Plant tissues made of primary cell walls, however, form an alternative source for the separation of the nanofibrils. Cells with primary walls are common for example in all fruit and vegetable species. These plants are mainly composed of parenchyma cells, i.e. ground tissue that generally constitutes the "filler" tissue in soft parts of plants. They have thin but flexible primary cell walls and the secondary cell wall is usually absent. The parenchyma tissue has a variety of functions, for example, to store starch in tubers, such as potato and cassava or storage of sucrose in sugar beet and sugar cane pith.

Nanofibrillated cellulose forms a gel in water already at a low concentration, ca. 0.1 wt%. The stiffness of the hydrogel increases with increasing nanocellulose concentration, but the gel remains strongly shear-thinning. The nanocellulose is typically produced at a low solids content lower than 2.5 wt%. Nanoporous materials are desirable in applications where a high specific surface area is required, including solid catalyst supports, supercapacitors, and gas or liquid adsorption. Porous nanocellulose in the form of aerogels, foams, and membranes have been typically produced using freeze-drying or supercritical drying of low concentration nanocellulose gel, often in combination with sequential solvent exchanges. (WO2010102802A1 , WO2012134378A1 , US20130330417)

The production of porous nanocelluloses is restricted and complex for several reasons. In order to achieve a high specific surface area, the starting material has typically been a hydrogel with a low solids content below 5%, most often smaller than 1 %. If such gel is frozen slowly for freeze-drying, large ice crystals will form, and the resulting freeze-dried structure will have large pores with a size ranging from 50 μιτι to above 0.1 mm. If smaller pores are desired in order to further increase the specific surface area, the sample has to freeze rapidly so that ice crystal size is limited due to the kinetics of freezing. US20130330417 describes freezing of nanocellulose hydrogel in ethanol/dry ice bath prior to freeze-drying, resulting to a foam with pores from 1 μιτι to about 100 μιτι. However, only very thin samples can be frozen quickly enough to produce pores smaller than a few micrometers, even if liquid nitrogen would be used. This limits the thickness of products that can be feasibly manufactured using this method to ca. 1 mm. Other problems with typical freezing methods, including the high cost of liquid nitrogen freezing, are discussed in WO2010102802. Another approach to produce micro- or nanoporous nanocellulose involves multiple solvent exchanges, optionally in combination with supercritical drying or freeze-drying. WO2012134378A1 describes the concentration of dilute nanocellulose dispersion by filtration in order to obtain a gel cake for producing a porous nanocellulose membrane using supercritical CO₂ drying. This approach is difficult to scale up and the product price will be high due to the complexity and time scale of the process.

SUMMARY

It is an object to provide porous nanocellulose structures of varying forms and dimensions.

According to a first aspect is provided a method of producing porous nanocellulose structure comprising i. providing nanofibrillated cellulose in an aqueous medium as a hydrogel; ii. concentrating the hydrogel to provide a concentrate; and iii. freeze drying the concentrate.

According to another aspect is provided a method of producing porous nanocellulose structure comprising i-ii. providing a nanofibrillated cellulose concentrate, having a solids content of at least 10%, in an aqueous medium as a hydrogel; iii. freeze drying the concentrate.

An advantage of the method is that objects much thicker than films or membranes can be manufactured. Another advantage is that as the hydrogel is concentrated, much less water has to be removed from the concentrate in drying. Another advantage of the method is that as freezing is carried out to the concentrate at a high solids content, formation of large ice crystals is prevented. Typically when low concentration nanocellulose gels are frozen, large ice crystals form, leading into formation of large cavities in the cellulosic structure. Typically such ice crystal cavities have a size ranging from 10 to Ι ΟΌμιτι that remain in the structure as ice sublimates during drying. Thus, the present method provides a different type of material as compared to prior nanocellulose materials such as foams and aerogels obtained by freeze-drying low concentration nanocellulose gels. In the structure obtained using the present method, a characteristic is absence of said ice crystal cavities after drying, even when freezing is carried out slowly. Because the large cavities typically found in freeze-dried cellulose structures are missing, the structures and materials obtained using the present method have increased specific surface area.

The method is also suitable for manufacturing structures of desired shape or texture that have good mechanic properties. Further, the method is economical, easy to scale up and avoids use of environmentally harmful chemicals, such as organic solvents that are required in methods involving solvent exchanges.

The method provides microporous material which has large surface area and is light-weight. The method makes it possible to manufacture porous nanocellulose material in various forms and it is not limited to manufacturing membranes only. The freeze drying step can be carried out for larger objects, resulting in microporous materials of desired shape and/or texture without the limitations described above.

According to a second aspect is provided a method of manufacturing a slow release system comprising: i. providing nanofibrillated cellulose in an aqueous medium as a hydrogel; ii. concentrating the hydrogel to provide a concentrate; iii. freeze drying the concentrate to obtain a slow release system; wherein the aqueous medium, the hydrogel, or the concentrate comprises an additional agent which is capable of being releasing from the slow release system.

According to another aspect is provided a method of manufacturing a slow release system comprising: i-ii. providing nanofibrillated cellulose concentrate having a solids content of at least 10% in an aqueous medium as a hydrogel; iii. freeze drying the concentrate to obtain a slow release system; wherein the aqueous medium, the hydrogel, or the concentrate comprises an additional agent which is capable of being releasing from the slow release system. According to a third aspect is provided a nanocellulose structure having a connected porosity; and a density of at least 50 mg/cm³.

According to a fourth aspect is provided a slow release system comprising i. the nanocellulose structure of the third aspect; ii. an additional agent.

According to a fifth aspect is provided a lateral flow device comprising the nanocellulose structure of the third aspect.

Brief description of figures:

Figure 1 . SEM picture of freeze-dried nanocellulose, low magnification. Bar 1 μιτι. Figure 2. SEM picture of freeze-dried nanocellulose. Bar 200 nm. Detailed description

In an embodiment the cellulose is parenchymal cellulose.

Parenchymal cellulose is particularly advantageous to obtain high concentrations of hydrogel before freeze drying. Parenchymal cellulose can be nanofibrillated in surprisingly high concentrations that allow freeze drying with little further concentrating, or even without a further concentrating step.

In an embodiment step ii. is carried out using pressure filtration. Alternatively, concentrating can be carried out using vacuum filtration, hot air drying, vacuum evaporation, dry steam drying, centrifugation, centrifugal filtration, centrifugal separation, or any suitable industrial drying method.

In an embodiment step ii. is carried out to a solids content of at least 10%. In another embodiment step ii. is carried out to a solids content of at least 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%. The upper limit of the solids content is preferably below 30%.

An advantage of the concentrating step to a value of at least 5% is that it allows reaching a higher solids content before freeze-drying. It may be advantageous to provide the nanofibrillated cellulose in step i. in a lower solids contents for example if the cellulose is derivatized or otherwise processed before adjusting the concentration of the hydrogel to a higher solids content for freeze drying. Water can be removed from the present nanofibrillated cellulose for example by pressure filtration, which makes it possible to reach high solids contents. In an embodiment the density of the product obtained after step iii. is at least 50 mg/cm³. In another embodiment the density is between 50 and 280 mg/cm³, such as 50, 55, 60, 70, 80, 90, 100, 1 10, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, or 280 mg/cm³.

The density of the product can be adjusted simply by adjusting the solids content during step ii. to a value which provides the freeze-dried product in a desired density. A skilled person is readily able to select an appropriate solids content. Thus, the mechanical properties of the freeze-dried product can be controlled by controlling the solids content in step ii. of the method.

In an embodiment the concentrate is provided in a mould before step iii. is started.

In another embodiment the concentrated is provided cut, extruded or shaped into a desired form before step iii. is started.

Advantageously objects of desired shape, forms or textures can be manufactured. Advantageously the structure substantially maintains the its shape during freeze drying. Because freeze-drying is carried out to a concentrated hydrogel, freeze-dried product is much stronger than thin membranes or foams obtained by prior methods. The product obtained by the present method is rigid and has plastic-like properties after freeze-drying. The present method can provide a "locked" structure of the nanocellulose structure which maintains after freeze-drying and during use. After the freeze-drying, the dried product can be coated, painted, or dyed according to the intended use of the product. For example a surface of the product can be coated with a hydrophobic coating such as wax.

The freeze-dried product can be impregnated with a substance, such as a monomer or a resin. Optionally, the impregnated substance can be subsequently chemically crosslinked or polymerized.

In an embodiment a cryoprotectant is added before step iii.

In an embodiment of the third aspect the nanocellulose structure comprises micropores connected by nanometer-sized open-cell porosity. In another embodiment the structure is free of micrometerscale pores.

In an embodiment of the third aspect the nanocellulose is derivatised.

In an embodiment of the third aspect the nanocellulose is fibrillated parenchymal cellulose.

In an embodiment the nanofibrillated cellulose is obtained by fibrillating parenchymal cellulose.

In another embodiment the fibrillating step involves using a fibrillation aid, such as a hard particle, for example ΤΊΟ₂.

In an embodiment the source of parenchymal cellulose is selected from sugar beet pulp, potato pulp, cassava pulp, citrus peel, bagasse pith, sweet potato, corn, fruits, vegetables or combination thereof.

In an embodiment the freeze drying is carried out to an object of hydrogel having a minimum dimension of at least 5 mm.

In an embodiment the hydrogel or the concentrate comprises additional agents, such as a drug, enzyme, fertilizer, solid catalyst, microcapsulated oil, fragrance, antibiotic, pesticide, herbicide, fungicide, nutrient, vitamin, flavouring, biologically active agent, diagnostic agent, antibody, antibody fragment, antigen, antigen fragment, amino acid, oligopeptide, polypeptide, protein, nucleoside, nucleotide, oligonucleotide, nucleic acid, monosaccharide, oligosaccharide, polysaccharide, or lipid. In an embodiment the porous structure is continuous open cell type porosity. In an embodiment the freeze-d ed product has a dense fibrous microstructure. The structure may be devoid of large cavities created by ice crystals.

In an embodiment the additional agent is selected from a drug, enzyme, fertilizer, solid catalyst, microcapsulated oil, fragrance, antibiotic, pesticide, herbicide, fungicide, nutrient, vitamin, flavouring, biologically active agent, diagnostic agent, antibody, antibody fragment, antigen, antigen fragment, amino acid, oligopeptide, polypeptide, protein, nucleoside, nucleotide, oligonucleotide, nucleic acid, monosaccharide, oligosaccharide, polysaccharide, or lipid.

In an embodiment the nanocellulose structure is provided in a form of a patch, granule, pellet, pill, extruded profile, film, sheet, or membrane.

In an embodiment of the fifth aspect the nanocellulose comprises nitrocellulose.

Examples

The following examples are provided to illustrate various aspects of the present invention. They are not intended to limit the invention, which is defined by the accompanying claims.

Example 1 i. Production of nanocellulose from birch pulp

Bleached birch pulp was nanofibrillated using a microfluidizer (Microfluidics Inc.) (6 pass, 1800 bar). ii. Batch production of nanofibrillar parenchymal cellulose based on potato pulp

Concentrated potato pulp from a starch factory was purified in a lye wash. Here, the potato pulp was taken to a 25 g/L suspension and heated to 60- 90°C. With gentle stirring, NaOH was added, e.g. 0.05 M. During this time, the hydrated potato clippings lost their solid-like morphology and broke down into a dark brown viscous mass within a minute. After 120 minutes of stirring, the reaction was cooled down and filtrated through a steel screen (0.25 mm pore size). The lye-washed pale grey cellulosic potato mass was further washed with copious amounts of water. The obtained material was suspended into water. The form and dryness of the raw material, extraction method, and fibrillation method varied. For example, potato pulp extracted according to this example was fibrillated mechanically using a rotor-rotor mixer (4 pass, 1800 rpm) or a homogenizer (4 pass, 600 bar). iii. Batch production of nanofibrillar parenchymal cellulose based on sugar beet pulp

Compressed sugar beet pulp (26wt% dry content) from a sugar factory was first washed in lye. Here, pulp was taken to a 25 dry g/L suspension and heated to 70-80°C. With gentle stirring, NaOH was added, e.g 0.05 M. During this time, the hydrated beet clippings lost their solid-like morphology and broke down into a dark brown viscous mass. After 120 minutes of stirring, the reaction was cooled down and filtrated through a steel screen (0.25 mm pore size). The lye-washed pale grey cellulosic sugar beet mass was further washed with copious amounts of water. The obtained material was suspended into water and fibrillated using a high-speed grinder at pH 8-10. The form and dryness of the raw material, extraction method, and fibrillation method varied. For example, pelleted sugar beet pulp extracted according to this example was fibrillated mechanically using a rotor-rotor mixer (4 pass, 1800 rpm).

Example 2: Nanofibrillated cellulose from birch pulp was diluted to 1 .5 wt% (300 ml) using high-shear mixing by a stick blender for 1 minute. Example 3: Titanium dioxide powder (22.5 g) was mixed with nanofibrillated cellulose from sugar beet (300 ml, 1 .5 wt%) using high-shear mixing by a stick blender for 1 minute.

Example 4: Mixtures from Examples 2 or 3 (300 ml), or nanofibrillated cellulose from potato (200 ml, 3.3 wt.%) were inserted into a pressure filtration device and filtered through a polymer fabric (PET, 1 μιτι hole size) applying a pressure of 5 bar for 1-2 hours. The solid contents of the filtered cakes were 26% (birch), 72% (beet + TiO₂), and 32% (potato).

Example 5: Concentrated nanofibrillated cellulose from Example 4 was diluted to target solid contents of 5, 10, 15, 20 and 25 wt% using UltraTurrax high shear laboratory mixer at 10,000 rpm for 1 minute. A sample of each concentration was molded to a disc-shaped mold with a diameter of 10 mm and a thickness of 1 mm.

Example 6: Zinc oxide powder (1 .6 g) was mixed with concentrated

nanofibrillated cellulose from Example 4 (potato, 10 g) and water (7.4 g) using UltraTurrax high shear laboratory blender at 10000 rpm for 1 minute. A sample was extruded to a tube-shaped mold with a diameter of 2 mm and a length of 30 mm.

Example 7: A 15-wt% sample (10 g) from Example 5 was extruded through a slit of 0.5 x 25 mm to form a ribbon. Example 8: A 20-wt% sample (100 g) from Example 5 was compression molded to a shape of a hollow spherical cap with a diameter of 140 mm, a height of 40 mm, and an average wall thickness of 5.3 mm.

Example 9: The samples from Examples 4-7 were frozen in a freezer at -22°C for 12 h. The frozen samples were dried by sublimation in vacuum for 12 h. The obtained products were strong and stiff in compression and had a matt white surface.

Example 10: The densities of freeze-dried samples from Example 9 were measured by weighing the samples before and after drying, and using a density value of 1 .5 g/cm³ for cellulose. The volume of the samples was not observed to change during freeze-drying. The results are shown in Table 1 .

Table 1 . Characteristics of the freeze-dried samples.

Example 11 : Microstucture of a freeze-dried sample with a density of 270 mg/cm³ was studied using a scanning electron microscope (SEM). Samples cut half-way through using a razor blade and the rest was torn apart using tweezers. The samples were glued on aluminum stubs and sputter coated with a 10-nm layer of Au-Pd. The torn surfaces were imaged using a Zeiss Sigma VP SEM. Scanning electron microscopy picture at 5000X magnification in Figure 1 shows that the sample has a dense fibrous microstructure. Large pores (10 - 100 μιτι) typically created by ice crystals were absent. Higher magnification in Figure 2 reveals that the at the sub-micron length scale, the structure consists of a network of nanofibrils and nanofibril bundles. At this level, the micropores are connected by nano-sized open-cell porosity.

Different non-binding example aspects and embodiments of the present invention have been illustrated in the foregoing. The above embodiments are used merely to explain selected aspects or steps that may be utilized in implementations of the present invention. Some embodiments may be presented only with reference to certain example aspects of the invention. It should be appreciated that corresponding embodiments may apply to other example aspects as well. Any appropriate combinations of the embodiments may be formed.

Without limiting the scope and interpretation of the patent claims, certain technical effects of one or more of the example embodiments disclosed herein are listed in the following: objects of desired shape, forms or textures can be manufactured, the average pore size is decreased and the surface area to volume ratio is increased; solvent exchange, rapid freezing, or a cryoprotectant is not required to achieve these effects.

Claims

1 . A method of producing porous nanocellulose structure comprising: i. providing nanofibrillated cellulose in an aqueous medium as a hydrogel; ii. concentrating the hydrogel to provide a concentrate; and iii. freeze drying the concentrate.

2. A method of producing porous nanocellulose structure comprising i-ii. providing a nanofibrillated cellulose concentrate, having a solids content of at least 10%, in an aqueous medium as a hydrogel; iii. freeze drying the concentrate.

3. The method of claim 1 or 2 comprising providing the concentrate in a mould before step iii. is started.

4. The method of preceding claims comprising providing the concentrate cut, extruded or shaped into a desired form before step iii. is started.

5. The method of preceding claims wherein the nanofibrillated cellulose is obtained by fibrillating parenchymal cellulose.

6. The method of preceding claims wherein the parenchymal cellulose is selected from citrus peel, potato, sugar beet, bagasse or cassava or combination thereof.

7. A method of manufacturing a slow release system comprising: i. providing nanofibrillated cellulose in an aqueous medium as a hydrogel; ii. concentrating the hydrogel to provide a concentrate; iii. freeze drying the concentrate to obtain a slow release system; wherein the aqueous medium, the hydrogel, or the concentrate comprises an additional agent which is capable of being releasing from the slow release system.

8. A method of manufacturing a slow release system comprising: i-ii. providing nanofibrillated cellulose concentrate having a solids content of at least 10% in an aqueous medium as a hydrogel; iii. freeze drying the concentrate to obtain a slow release system; wherein the aqueous medium, the hydrogel, or the concentrate comprises an additional agent which is capable of being releasing from the slow release system.

5 9. The method of claims 7-8 wherein the additional agent is selected from a drug, enzyme, fertilizer, solid catalyst, microcapsulated oil, fragrance, antibiotic, pesticide, herbicide, fungicide, nutrient, vitamin, flavouring, biologically active agent, diagnostic agent, antibody, antibody fragment, antigen, antigen fragment, amino acid, oligopeptide, polypeptide, protein, nucleoside, nucleotide, 10 oligonucleotide, nucleic acid, monosaccharide, oligosaccharide, polysaccharide, or lipid.

10. The method of preceding claims wherein the nanocellulose structure is provided in a form of a patch, granule, pellet, pill, extruded profile, film, sheet, or membrane.

15 1 1 . A nanocellulose structure having a connected porosity; and a density of at least 50 mg/cm³.

12. The nanocellulose structure of claim 1 1 having a surface area per volume between 0.5 and 50 m²/cm³, more preferably between 1 and 40 m²/cm³, even

20 more preferably between 2 and 20 m²/cm³.

13. The nanocellulose structure of claim 1 1 or 12 the nanocellulose comprises micropores connected by nanometer-sized open-cell porosity.

14. The nanocellulose structure of preceding claims wherein the nanocellulose is derivatised.

25 15. The nanocellulose structure of preceding claims wherein the nanocellulose is fibrillated parenchymal cellulose.

16. A slow release system comprising i. the nanocellulose structure of preceding claims; ii. an additional agent.

17. The slow release system of claim 16 wherein the additional agent is selected from a drug, enzyme, fertilizer, solid catalyst, microcapsulated oil, fragrance, antibiotic, pesticide, herbicide, fungicide, nutrient, vitamin, flavouring, biologically active agent, diagnostic agent, antibody, antibody fragment, antigen, antigen

5 fragment, amino acid, oligopeptide, polypeptide, protein, nucleoside, nucleotide, oligonucleotide, nucleic acid, monosaccharide, oligosaccharide, polysaccharide, or lipid.

18. A lateral flow device comprising the nanocellulose structure of the preceding claims.

10 19. The lateral flow device of claim 18 wherein the nanocellulose comprises

nitrocellulose.