WO2012119229A1

WO2012119229A1 - Highly charge group-modified cellulose fibers which can be made into cellulose nanostructures or super-absorbing cellulosic materials and method of making them

Info

Publication number: WO2012119229A1
Application number: PCT/CA2012/000200
Authority: WO
Inventors: Theodorus G. M. VAN DE VEN; Nur ALAM; Miro ANTAL; Alvaro TEJADO
Original assignee: The Royal Institution For The Advancement Of Learning/Mcgill University
Priority date: 2011-03-08
Filing date: 2012-03-08
Publication date: 2012-09-13

Abstract

The invention relates to highly charge group modified cellulose fibers which can be directly used as novel strengthening agent in papermaking, or as super-absorbent cellulosic material, or very easily made into cellulose nanostructures, such as nanofibrillar cellulose (NFC) and nanocrystalline cellulose (NCC), and the method of making them, which consists on providing a raw material cellulose, producing dialdehyde cellulose in a first chemical process with an improved protocol and converting the aldehyde groups into charge groups in a second chemical process. The surface charge concentration of the highly charge group modified cellulose products, including the cellulose nanostructures, can range from 0.1 to 11 mmol per gram of cellulose, and the cellulose nanostructures comprise an aspect ratio, defined as length over diameter, in a range from less than 10 to more than 1,000.

Description

HIGHLY CHARGE GROUP-MODIFIED CELLULOSE FIBERS WHICH CAN BE MADE INTO CELLULOSE NANOSTRUCTURES OR SUPER-ABSORBING CELLULOSIC

MATERIALS AND METHOD OF MAKING THEM

FIELD OF THE INVENTION

The invention relates to a highly charge group modified cellulose fiber, which can be directly used as a novel strengthening agent (e.g. in papermaking) or very easily made into super-absorbent cellulosic material or into cellulose nanostructures and/or dissolved cellulose derivatives, and the method of making it.

BACKGROUND OF THE INVENTION

Chemical Modification of Cellulose Fibers

Fibrillar cellulose has been functionalized in the past by different methods that introduce charge groups into cellulose molecular structure. Most of these methods use basic chemistry, like oxidizing some of the hydroxyl groups to carboxyl by using oxidizing agents such as chlorine, hydrogen peroxide, chlorite, and the like. Also the sulfation of cellulose with sulfuric acid in the presence of an aliphatic alcohol giving water-soluble cellulose sulfates has been disclosed in several patents: a) US2539451 by C.J. Malmand and C.L. Cram (1951 ), and b) US2559914 by G. Frank (1951 ). Other traditional methods deal with attaching molecules containing charge groups to cellulose fibers especially via esterification or etherification, like for example carboxymethylation with a-halo-acetic compounds as disclosed in patents a) JP72014128-B issued Dec. 5, 1968 to Sanyo Pulp Co. Ltd., and b) JP52069990-A; JP81016161 -B issued Dec 9, 1971 to Kao Soap Co. Ltd.

The preparation of dialdehyde polysaccharides by using periodic acid or periodates is also well known, as disclosed for example in U.S. Pat. No. 3,096,969, issued to J.E. Slager on Apr. 23, 1963. The preparation of cationic dialdehyde polysaccharides using certain metal salts is disclosed in U.S. Pat. No. 3,236,721 , filed on June 1 1 , 1964 by J.H. Curtis. Several addition reactions on aldehydes, such as reaction with hydroxyls or with primary amino groups, are also well-known and can be applied to dialdehyde cellulose. Another interesting addition reaction is the sulfonation of dialdehyde cellulose, which has been addressed before, as shown in Q.X. Hou et al., "Characteristics of wood cellulose fibers treated with periodate and bisulfite", Ind. Chem. Chem. Res. 46, 7830-7837 (2007). Recently, a common manner of introducing charge groups into cellulose fibers is through 2,2,6,6-tetramethylpiperidine-1 -oxyl (TEMPO) radical-mediated oxidation, where one of the three hydroxyl groups of accessible anhydroglucopyranose (AGU) repeating units of cellulose is converted to a carboxyl group. The use of such nitroxyl radicals and nitrosonium salts as an oxidative route to transform hydroxyl functions into carboxyl and/or aldehyde groups is disclosed in an article entitled "Organic nitrosonium salts as oxidants in organic chemistry", by J.M. Bobbitt and C.L. Flores, in Heterocycles 27(2), 509-533 (1988). Patent WO 95/07303 discloses the use of similar chemistry on carbohydrates having a primary hydroxyl group, which are oxidized under aqueous conditions to form products having a high content of carboxyl groups (greater than 90%). Similarly, this oxidation process has been used to prepare various polysaccharides with high carboxyl group content, as described in "Oxidation of primary alcohol groups of naturally occurring polysaccharides with 2,2, 6, 6-tetramethyl-1-piperidine oxoammonium ion" by P.S. Chang and J.F. Robyt, in J. Carbohydrate Chemistry 15(7), 819-830 (1996).

All these traditional procedures of introducing charge groups into cellulose are efficient when they are used in dissolved cellulosic compounds, like dissolved cellulose or low molecular weight polysaccharides, but are far less reactive in non-water-soluble cellulose products, these being either pulp fibers or cellulose micro- or nanostructures, and thus in producing highly modified species.

Production of Cellulose Nanostructures

There is not a clearly defined nomenclature for the non-water-soluble cellulose micro- and nanostructures, and terms like microcrystalline cellulose, cellulose nanocrystals, microfibrillated cellulose, cellulose nanofibrils and others, are often used indistinctly for what is generally the same product.

Cellulose fibers as they exist in plants generally comprise a 20-40 micron thick and 0.5-4 mm long fibrillar structure (referred hereafter as cellulose fibers) that can be longitudinally split into finer threads of around 3-20 nm in diameter and several microns in length. In the present description these finer threads will be referred to as nanofibrillar cellulose (NFC). Shorter nano-sized structures of around 100-300 nm in length, usually called nanocrystalline cellulose (NCC), may also be obtained by the present method. Hereinafter both NFC and NCC are understood to be "cellulose nanostructures". NCC and NFC of the prior art are difficult to derivatize (i.e. grafted with other molecules) due primarily to the small number of reactive functional groups present in their structure (because the reactivity of hydroxyl groups is relatively low). The lack of functional groups significantly reduces the possibility of modifying the physicochemical characteristics of the raw material cellulose and thus its possibility of being used in varied applications, like for example in polymer nanocomposites.

Known methods of producing nanofibrillar cellulose from cellulose fibers include purely mechanical processes, such as peeling off the nanofibrils from the fibers using high shear dispersion devices, or a combination of mechanical plus chemical and/or biological methods; in these, the fibers are first weakened, by either introducing charge groups into the structure or by partially digesting the amorphous regions of the fibers with enzymes, and then disintegrated by shear stress. The shear stress is applied by using energy intensive equipment such as blenders, high-shear homogenizers, ultrasounds or similar devices.

Several examples of these techniques as described in the following articles: a) F.W. Herrick et al., "Microfibrillated cellulose: morphology and accessibility", J. Appl. Polym. Sci.: Applied Polymer Symposium 37, 797-813 (1983); b) A.F. Turbak et al., "Microfibrillated cellulose, a new cellulose product: properties, uses, and commercial potential", J. Appl. Polym. Sci.: Applied Polymer Symposium 37, 815-827 (1983); c) M.A. Hubbe et al., "Cellulosic nanocomposites: a review", Bioresources 3(3), 929-980 (2008); d) S.W. Lee et al., "Preparation of cellulose nanofibrils by high-pressure homogenizer and cellulose-based composite films", J. Ind. Eng. Chem. 15, 50-55 (2009); e) M. Henriksson et al., "An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers", Eur. Polym. J. 43, 3434-3441 (2007); f) M. Henriksson et al., "Cellulose nanopaper structures of high toughness", Biomacromolecules 9, 1579-1585 (2008); g) T. Saito et al., "Individualization of nano- sized plant cellulose fibrils by direct surface carboxylation using TEMPO catalyst under neutral conditions", Biomacromolecules 10, 1992-1996 (2009); h) N. Tamura et al., "TEMPO-mediated oxidation of (1-3)-fi-D-glucans", Carbohydrate Polymers 77, 300-305 (2009); i) Y. Fan et al., "TEMPO-mediated oxidation of β-chitin to prepare individual nanofibrils", Carbohydrate Polymers 77, 832-838 (2009). The methods that include cellulose pretreatments require less mechanical energy to liberate the nanofibrils than purely mechanical processes, but even with the pretreatments of the prior art a substantial amount of mechanical energy is still needed (typically in the range of 1-10 kWh/kg). To date, the isolation of NFC by purely chemical methods i.e. without a high mechanical energy step added to the process, has not been achieved.

In recent years, the use of oxidizing techniques on cellulose has become popular with the aim of reducing the amount of mechanical energy required to liberate cellulose nanostructures; the introduction of carboxyl groups leads to the appearance of repulsive forces between them that weakens the structure. The preferred pathway is the TEMPO- mediated oxidation route, and the most common oxidizing agents are hypochlorite and chlorite. Some examples of the use of this oxidizing technique are found in: a) WO2009107795-A1 by M. Hirota, assigned to the University of Tokio (2009); b) JP2008001728-A by T. Saito et al., assigned to Ashahi Kasei KK (2008); c) WO2009084566-A1 by H. Abe et al., assigned to Nippon Paper Ind. Co. Ltd. (2009), and d) WO2009021688-A1 by J. Engelhardt et al., assigned to Dow Wolff Cellulosics GmbH (2009), which includes TE PO-mediated oxidation and phosphorylation up to a charge density of 1 .5 mmol/g (see line A in Figure 2). No data is available in the literature on decreasing the energy required to liberate NFC by the introduction of charge groups other than carboxyl or phosphoric.

However, the TEMPO-mediated oxidation technique, as it has been applied to date, leads to a maximum achievable carboxyl content of around 2.2 mmol per gram of cellulose as disclosed in JP2008001728-A (see line B in Figure 2); consequently, the use of this method alone cannot reduce further the amount of mechanical energy required to isolate the NFC. Depending on the extent of the reaction, the minimum amount of energy needed after this treatment will still be in the order of 2-10 kWh per kg of cellulose as disclosed in WO2009021688-A1.

The use of mechanical energy to liberate the NFC from cellulose fibers also has the inherent drawback of unavoidably damaging the nanofibrils, breaking them into lengths of a few micrometers at best. As a result of this breakage, and given that the typical nanofibril diameter is around 5-10 nm, the highest aspect ratio (fibre length/fibre diameter) ever reported for NFC in the prior art is less than 1000. Furthermore, the need for this high amount of mechanical energy, together with the relatively low yields obtained, impedes in the practice the industrial exploitation of NFC.

In terms of production, NCC is easily obtained by subjecting the cellulose fibers to a hydrolysis treatment, typically with a strong acid.

Various highly charge group modified water-soluble cellulose derivatives are produced as by-products upon the application of this method. Water-soluble 2,3,6- tricarboxycellulose (TCC) has been made before and is used as a haemostatic agent, called "Supercel". It is presently produced by N0₂ oxidation of 2,3-dialdehyde cellulose as shown in the article by T.J. Sinha and P. Vasudevan, "Blood-cellulosics interactions", Biomater. Med. Devices Artif. Organs 12(3-4), 273-287A (1984-1985). Patent WO2009107795-A1 also discloses the production of cellouronic acids obtained by substituting portions of the primary hydroxyls in cellulose crystals sufaces by COOH through TEMPO-mediated oxidation process.

According to recent literature (J.F. Beecher, Wood, trees and nanotechnology, Nature Nanotechnology 2, 466-467, 2007) two steps need to be further developed before cellulose nanofibrils are ready for real-world applications: a new production process free of strong acids or intensive energetic treatments, and the ability of the nanofibers to be dispersed in hydrophobic media, i.e. to be easily derivatized.

Dry Strength Agents in Papermaking

The two main strategies used to increase the dry strength of paper, widely reported in the literature, are mechanical beating of the pulp and addition of strengthening agents to the pulp suspension. The application of mechanical action (called beating or refining) on pulp suspensions causes fiber fibrillation, which results in a substantial increase of fiber-fiber contact surface and thus in bonding density. The increase in strength can reach more than 20 times that of non-treated paper, depending on the extent of the refining treatment. On the other hand, the addition of individual polyelectrolytes (from which carboxymethyl cellulose and cationic starch are the most widely used) to the papermaking furnish, provides upon drying an effective bridging mechanism between the fibers that leads to an enhanced adhesion. There is a need for a single product that, through simple addition to the papermaking furnish at the wet end, can combine both effects of an increased fiber-fiber contact and a covalent bridging mechanism, thus overcoming the reported incompatibilities of those treatments (fibrils created during beating often result being crosslinked back to the fibers by polyelectrolytes) and at the same time avoiding the highly intensive energetic refining process.

Super-Absorbing Materials

Super-absorbent paper already exists in the market; however, in most cases they are produced by incorporating super-absorbing polymeric particles to the fibers, which are either based on synthetic polymers, such as polyacrylates, sulfonated polystyrene, polyvynil alcohol, etc, or on natural polymers, such as carboxyalkyl cellulose, gum, carboxyalkyl starch, cellulose sulphate, etc. All these products suffer from several disadvantages, from which the most relevant one is physical dislodgement of the particles from the fibers during manufacturing and transportation, what leads to a diminished efficiency. Maximum absorbency of such materials can range from 10 to 100 g/g, and even more.

Synthetic super-absorbent fibers have been developed to overcome this problem, but they suffer from others, such as difficulty to be processed, limited absorbency under certain conditions and poor tensile strength. A few super-absorbing products have been developed based on chemical modification of cellulosic materials in the fiber form. U.S. Pat. No. 6,500,947 describes the production of sulfonated cellulose fibers, while U.S. Pat. No. 6,844,066 describes the production of crosslinked cellulose fibers previously modified with an ethylenically unsaturated monomer. There is a need for a super- absorbent material which is based on biodegradable and renewable starting materials and which is free of solvent-based processes, that combines the advantages of liquid absorbent capacity of conventional super-absorbent polymers and the advantageous liquid distribution and stability properties of cellulosic fibers.

The method and products described herein aim to increase the industrial use of cellulose fibers, consolidating the emerging fields and improving the established ones. SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is provided a cellulose product comprising a cellulosic structure with at least one charge group attached to the cellulosic structure, wherein the at least one charge group is selected from the group consisting of HS0₃, H₂P0₃, H₂P0₄, primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and carboxyl, wherein if only carboxyl is present in the cellulosic structure the cellulose product has a concentration of more than 2.2 to 11 mmol of charge groups per gram of cellulose product and if the carboxyl is absent or present with another charge group the concentration charge groups per gram of cellulose product is more than 1.5 to 1 1 mmol/gram of cellulose product.

In accordance with another aspect of the product described herein, there is nanofibrillar cellulose, NFC comprising a concentration of charge groups of 2 to 3 mmol/g (with a diameter of 3-20nm and a length of 0.5 to 5 microns).

In accordance with yet another aspect of the product described herein, there is nanocrystalline cellulose, NCC, comprising a concentration of charge groups of more than 2 mmol/g (with a diameter of 3-20nm and a length of 100 to 300 nm).

In accordance with still another aspect of the product described herein, there is an electrosterically stabilized nanocrystalline cellulose (ENCC) comprising a concentration of charge from 3.5 to 7.0 mmol/g. with a diameter around 10 nm and a length in the range 120 - 200 nm).

In accordance with yet still another aspect of the present invention, there is provided a 2,3-dicarboxycellulose, DCC, substituted in cellulose structure with at least one charge group selected from the group consisting of carboxyl, sulfonate (HS0₃), phosphate (H₂P0₄), phosphite (H₂P0₃), phosphonic acid (H₂P0₃), primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and combinations thereof, the charge groups linked to the cellulose structure either directly or through selected from the group consisting of a C1 -C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl. ln accordance with a further aspect of the present invention, there is provided a 2,3,6-tricarboxycellulose, TCC, substituted in cellulose structure with at least one charge group selected from the group consisting of carboxyl, sulfonate (HS0₃), phosphate (H₂P0₄), phosphite (H₂P0₃), phosphonic acid (H₂P0₃), primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and combinations thereof, the charge groups linked to the cellulose structure either directly or through selected from the group consisting of a C1 -C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl.

In accordance with yet a further aspect of the present invention, there is provided a method for producing cellulose products with a high concentration of charge groups attached covalently, the method comprising providing a raw material cellulose 2; oxidizing the raw material cellulose 2 in a medium comprising a salt and a periodate producing a dialdehyde cellulose 12; reacting the dialdehyde cellulose in a reaction selected from the group consisting of a halous oxidation, a sulfonation, a cationization reaction, an imino or acetal bond introducing reaction, a N-oxyl catalyzed oxidation and combinations thereof.

In accordance with still a further aspect of the method described herein, the high concentration of charge groups in the cellulose products is greater than 2.2 to 1 1 mmol of charge groups per gram of cellulose products if the charge group is carboxyl alone in the cellulose products.

In accordance with yet still a further aspect of the method described herein, the high concentration of charge groups in the cellulose products is greater than 1.5 to 1 1 mmol/gram of cellulose product if carboxyl is absent or present with another charge group in the cellulose products.

In accordance with still a further aspect of the method described herein, the high concentration of charge groups is from 3.5 to 7.0 mmol/g.

The invention relates to modified cellulose fibers comprising a high number of charge groups that can be directly used as novel strengthening agent in papermaking or can be made very easily into cellulose nanostructures, such as nanofibrillar cellulose (NFC) and nanocrystalline cellulose (NCC), or superabsorbent cellulosic material. The method of making the cellulose nanostructure, comprises providing a raw material cellulose, producing dialdehyde cellulose in a first chemical process with an improved protocol and converting the aldehyde groups into charge groups in a second chemical process. The number of introduced charge groups determines the uses that can be made from the modified fibers. Making fibers that can be used as strengthening agents in paper products requires charges (charged groups and/or dissociated weak acid groups) in the range 1.0-2.5 meq/g (the number of charge groups can be larger, as some of them may be protonated or dissociated and not contributing to the fiber's overall charge). Making superabsorbing fibers requires optimum charge group densities in the range 1.5-5 mmol/g. Making cellulose nanostructures requires charge densities above 2 meq/g. Charge densities slightly above 2 meq/g result in long nanofibers (NFC), whereas higher charge densities lead to short nanofibers (NCC). The higher the charge, the shorter the nanofibers. The charge concentration of the highly charge group modified cellulose products, including the cellulose nanostructures, can range from 0.1 to 1 1 mmol per gram of cellulose, and the cellulose nanostructures comprise an aspect ratio, defined as fiber length over diameter, in a range from less than 10 to more than 1 ,000.

In accordance with one aspect of the present invention, there is provided a non- water-soluble cellulose product (which includes, hereinafter, cellulose fibers and cellulose nanostructures) with at least one charge group attached to the cellulosic structure in a concentration of more than 1.5 to 1 1 mmol per gram of cellulose if the at least one charge group is selected from the group consisting of HS0₃, H₂P0₃, H₂P0₄, primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn) and zirconium (Zr), or is a combination of two or more of them, or is a combination of carboxyl (COOH) plus one or more of them, or in a concentration of more than 2.2 to 1 1 mmol per gram of cellulose if the charge group is COOH. ln accordance with another aspect of the present invention, there is provided a non-water-soluble cellulose product having general formula IV:

wherein Ri, R₂ and R₃ can be one of the following groups: CHO, COOH, R- COOH, R-HSO₃, R-NH₂, R-NH, R-N, R-N+ (+ representing a positive charge), Ri and R₂ can be also Ti, Zn, Cr, Sr, Sn or Zr, and R₃ can be also CH₂OH, R-H₂P0₄ or R-H₂P0₃, Ri being different or equal to R₂, and R₂ different or equal to R₃, wherein R is a C1 -C6 substituted or unsubstituted alkyl, alkyl-ether or aldimine, or a C2-C6 substituted or unsubstituted alkenyl;wherein one or more -OH groups or -H present in the cellulosic structure are unsubstituted or substituted with a halogen, an alkali metal, a C1-C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl; wherein the minimum charge density is 2.2 mmol/g when all charge groups present are COOH, or 1.5 mmol/g when at least one charge group is different from COOH; wherein the maximum charge group content is 1 1 mmol/g; wherein m and n are in any sequence and all permutations are equally contemplated, and (n+m) is an integer from 100 to 10,000.

In accordance with still another aspect of the present invention, there is provided a method of producing highly charge group modified non-water-soluble cellulose products comprising: providing a raw material cellulose, producing dialdehyde cellulose in a first chemical process and converting aldehyde groups into charge groups in a second chemical process, with the overall charge group content laying in any case above a defined threshold. BRIEF DESCRIPTION OF THE DRAWINGS AND TABLES

Further aspects and advantages will become better understood with reference to the description in association with the following drawings in which:

Figure 1A is a block diagram of the proposed method for producing highly charge group modified cellulose fibers, according to an embodiment of the present invention.

Figure 1 B illustrates three different products into which the main product can be converted (c represents the concentration of charge groups).

Figure 2 is a graph of disintegration energy (kWh/kg) versus charge concentration of a cellulose product (mmol/g) comparing untreated pulp (0), WO2009/021688 (A), JP2008001728 (B) and the chemically modified cellulose products of the present application requiring limited amounts of disintegration energy (C) and entirely converted into nanostructures without applying any disintegration energy (D);

Figure 3(a) is a photograph that illustrates a bleached softwood kraft pulp dispersed in water according to one embodiment of the present invention;

Figure 3(b) is a photograph that illustrates a dialdehyde product of the pulp dispersed in Figure 3(a) after periodate oxidation, where two hydroxyl groups have been converted into aldehyde groups;

Figure 3(c) is a photograph that illustrates a dicarboxyl cellulose product after chlorite oxidation of the dialdehyde pulp of Figure 3(b) (aldehyde groups converted into carboxyl groups) and purification;

Figure 3(d) is a photograph that illustrates a pure NFC suspension after TEMPO- mediated oxidation reaction of the dicarboxyl pulp of Figure 3(c) (the remaining hydroxyl group converted to a carboxyl group) allowed its spontaneous formation;

Figure 3(e) is a photograph that illustrates a pure NFC powder after being precipitated the product of Figure 3(d) in an ethanol/water mixture, washed with acetone and dried; Figure 4 illustrates a conductometric titration curve of a highly charged NFC (carboxyl content = 3.5 mmol/g cellulose) according to one embodiment of the present invention after completing one chemical treatment that leads to their spontaneous liberation.

Figure 5 shows a Scanning Electron Micrograph (SEM) image of NFC liberated by chemical treatment, after precipitation (sample corresponding to that of Figure 3(e)). The diameter of the nanofibers ranges from 5 to -20 nm.

Figure 6 shows an Atomic Force Micrograph (AFM) image of NCC obtained by the chemical treatment comprising a salt-assisted periodate oxidation followed by a TEMPO-mediated chlorite oxidation.

Figure 7 shows a ³C-NMR spectrum of TCC, with traces of NFC and NCC. The area under the peak labelled COOH is 2.8 times the area of the peak labelled C1 , implying cellulose was converted to TCC. The value is somewhat less than 3 because the presence of traces of NFC and NCC, as evidenced by the peak corresponding to C4 and C5 of TCC.

Figure 8 is an SEM of the carboxylated cellulose with a charge concentration of 2 mmol/g before exposure to high shear;

Figure 9 is an SEM of carboxylated cellulose (2 mmol/g) after exposure to high shear;

Figure 10 is an (AFM) height image of electrosterically stabilized nanocrystalline cellulose (ENCC) particles (at a charge density of 6.6 meq/g);

Figure 1 1 (a) is the equivalent hydrodynamic diameter of ENCC; in nm

Figure 1 1 (b) is a hydrodynamic diameter of ENCC in the presence of 1 NaCI;

Figure 1 1 (c) is the decrease of hydrodynamic diameter of ENCC with increasing NaCI concentration (at a charge density of 6.6 meq/g);

Figure 12(a) is the hydrodynamic diameter of ENCC after hydrolysis without salt; Figure 12(b) is the hydrodynamic diameter of ENCC after hydrolysis in the presence of 0.5 M NaCI (at a charge density of 1.4 meq/g); and

Figure 13 is an AFM height image of ENCC after hydrolysis (at a charge density of 1.4 meq/g).

DETAILED DESCRIPTION OF THE INVENTION

"Charge groups" refer to chemical species that hold a charge (either positive or negative), such as sulfonate groups, or that can be charged by varying the conditions of the medium (pH, concentration, ionic strength...), such as carboxyl groups.

"Electrosterically Stabilized Nanocrystalline Cellulose (ENCC)" is a highly charged NCC with charged groups, such as carboxyl groups protruding from the surface that stabilize the ENCC. ENCC also includes rod-like particles with a length in the range of 120-200 nm, and a diameter of approximately 10 nm.

The proposed method brings various known chemical reactions together, one of which is largely improved by a new protocol, in such a way that a cellulose fiber with charge-group concentration higher than any other previously reported is obtained. The resulting product has outstanding properties and versatility.

The invention relates to a cellulose fiber which has been chemically modified to have a high concentration of a charge group, this being a chemical group that either holds a charge (positive or negative) or is susceptible of being charged upon changing the conditions of the medium, e.g. the pH. The concentration of such charge group ranges from 1 .5 to 1 1 mmol/g (milimoles of chemical group per gram of cellulose), preferably from 2.2 to 7 mmol/g and most preferably from 2.5 to 3.5 mmo/g. The charge group is at least one of the following list: COOH, HS0₃, H₂P0₃, H₂P0₄, primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn) and zirconium (Zr). The highly charge group modified cellulose fiber can be directly used as novel strengthening agent (e.g. in papermaking) or can be easily made into novel cellulose nanostructures, such as highly charged nanofibrillar cellulose (NFC) and nanocrystalline cellulose (NCC), or can be easily made into super-absorbent cellulosic material. The invention also relates to the method of making such fibers. Highly charged water-soluble cellulose derivatives that will be described in greater detail later are produced as by-products upon the application of this method under certain conditions; they can include cellulose structures containing charge groups such as carboxyl (COOH), sulfonate (HS0₃), phosphate (H₂P0₄), phosphite (H₂P0₃), phosphonic acid (H₂P0₃), primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and others, linked to the cellulose structure either directly or through a larger molecule such as a C1 -C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl. When the charge groups in the water-soluble cellulose derivatives are COOH, such compounds are usually referred to as cellouronic acids. Two examples of this kind of products are 2,3-dicarboxycellulose (DCC) and 2,3,6-tricarboxycellulose (TCC) that are produced in a preferred embodiment of the present invention.

This new method is based on a surprising finding that a combination of different chemical reactions carried out in various orders and at specific conditions introduces into the cellulosic structure of the cellulose fiber product a charge group concentration higher than existing methods.

The chemical modification described herein can be carried out to different degrees, which in one aspect of the present invention allows reducing (in the case of a moderate modification) or virtually eliminating (in the case of a intensive modification) the mechanical energy required to liberate the cellulose nanostructures in the prior art. A moderate modification, thus, produces a modified cellulose fiber that retains its original shape and gets disintegrated into cellulose nanostructures (plus water-soluble cellulose derivatives) only when a certain mechanical energy is applied to it; such product is aimed to be used as strengthening agent, preferably in the papermaking industry. An intensive modification directly produces highly charged cellulose nanostructures (plus water- soluble cellulose derivatives) without the need of any mechanical action. Preferably to be used in polymeric nanocomposites.

Thus, this method allows overcoming the aforementioned difficulties and disadvantages of the prior art related to the production and derivatization of cellulose nanostructures, by providing a new method for its liberation through a chemical processes free of strong acids. Specifically, it eliminates the need for at least one energy intensive mechanical process step (in the case of NFC) and the need for treatment with strong acids (in the case of NCC), and produces novel highly functionalized cellulose nanostructures. The formation of either NFC or NCC depends on the reactions and the conditions that are used to produce them, as well as on the order or precise sequence of reactions that are applied.

In another aspect of the present invention, the method gives rise to a cellulose product with super-absorbent properties, which is achieved by minimizing the dissociation of the charge groups and thus the overall charge of the cellulose product.

Reference will now be made to the accompanying drawings, showing by way of illustration a particular embodiment of the present invention.

Figure 1A illustrates a block flow diagram of the process for producing highly charge group modified cellulose fibers. Figure 1 B shows schematically the three different products in which the charge group modified fiber can be easily made, by simply varying the charge group concentration and the degree of dissociation of such groups. The actual ranges of concentration (identified with the letter "c") and pH differ for each charge group, and also can slightly vary depending on the exact operation conditions. Data shown correspond to a particular modification where all the charge groups are carboxyl groups.

The method illustrated in Figure 1A comprises two chemical reactions: 10 a salt- assisted periodate oxidation of cellulose to produce a 2,3-dialdehyde cellulose intermediate, and 20 a chemical reaction by which the aldehyde groups are converted into charge groups and that is at least one of the reactions that will be described in detail later. Reaction 10 is necessarily a salt-assisted periodate oxidation but reaction 20 can vary (as described later), being the preferred one a salt-assisted chlorite oxidation of aldehydes into carboxyl groups. The method also allows the combination and/or repetition of the reactions.

A first step in the method illustrated in Figure 1A described herein is a salt- assisted periodate oxidation 10, that opens an anhydroglucopyranose (AGU) ring of the cellulosic structure of the raw material cellulose 2 to produce a dialdehyde cellulose fiber 12. This oxidation 10 begins with a raw material cellulose 2, from any one of a variety of sources, that is mixed in a solution that generally comprises a salt 5, with a periodate reagent 3. The raw material cellulose 2 may be derived from any plant source. In a preferred embodiment, the raw material cellulose 2 derives from hardwood, softwood or combinations thereof, where a particularly preferred embodiment, the raw material cellulose 2 is a bleached hardwood Kraft pulp, a bleached softwood Kraft pulp or a combination thereof.

The periodate reagent 3 used in the oxidation 10 is at least one of periodic acid, periodic acid salts, metaperiodic acid, metaperiodic acid salts and combinations thereof, and preferably sodium metaperiodate alone or in combination with any of the above listed oxidants. The salt 5 may be any salt, preferably an alkali metal halide, particularly LiCI, NaCI, KCI and combinations thereof and more preferably NaCI. The ionic nature of the salt 5 appears to allow easier access to the anhydroglucopyranose ring of the raw material cellulose 2 inside the fiber wall, thus allowing an improved ring opening and aldehyde production via the periodate oxidant reagent 3.

In reaction 10, 1 to 150 mole % (relative to moles of cellulose) of oxidant can be used. The dialdehyde cellulose can be obtained with any one of the following compounds: periodic acid, periodic acid salts, metaperiodic acid, metaperiodic acid salts and combinations thereof, and preferably sodium metaperiodate. The ionic strength of the medium can be increased from 0 to 2M by the addition of any salt, preferably an alkali metal halide, particularly LiCI, NaCI, KCI and combinations thereof and more preferably NaCI. The temperature in reaction 10 can range from about 5 to 45°C, preferably 20°C, and the pH from 3.5 to 8, and preferably from 4.5 to 6. The time required for reaction 10 is 1 to 24 h. The final aldehyde content that can be obtained averages 0.1 to 8 mmol per gram of cellulose.

The presence of salt in reaction 10 increases the ionic strength of the medium allowing the oxidizing agent to penetrate with a higher concentration into the cellulose fiber microstructure and resulting in higher conversion yields for a given amount of reactants. Alternatively when using salt, one can reach the same conversion as that obtained with traditional methods with far less periodate.

The second step in the process illustrated in Figure 1A is a reaction by which aldehyde groups are totally or partially converted into charge groups. Reaction 20 is at least one of the following reactions: i) a halous oxidation, which produces a dicarboxyl cellulose 22 by oxidizing the aldehyde moieties from the dialdehyde cellulose 12. The halous oxidation begins with a solution that comprises the dialdehyde cellulose 12, a halous oxidizing agent 13; ii) a sulfonation reaction, wherein a sulfonating agent 3, such as sodium bisulfite, converts the dialdehyde cellulose fibers 12 into sulfonated cellulose fibers by introducing a sulfite groups; iii) a cationization reaction, by which a cationizing agent 13, such as water-soluble acidic salts of titanium, zinc, strontium, tin, chromium and zirconium, and especially salts of zirconium such as zirconium chloride, zirconium oxy-chloride and zirconium sulphate, is attached to the dialdehydes cellulose 12; iv) a reaction of the dialdehyde cellulose 12 with a molecule having a charge group plus either a primary amino group, such as (2-aminoethyl)trimethylammonium chloride, or a hydroxyl group, such as 3-hydroxypropionic acid, that introduces charge groups via addition reactions on aldehydes through the formation of, respectively, imino or acetal bonds; v) a N-oxyl catalyzed oxidation in which a dialdehyde cellulose 12 is carboxylated by mixing it in a solution with a first oxidizing agent (A) 17 and a second oxidizing agent (B) 18. The first oxidizing agent (A) 17 being a halous acid or its salt, perhalogen acid or its salt, hydrogen peroxide or per-organic acid or its salt, laccase or peroxidase, and preferably sodium chlorite. The second oxidizing agent (B) 18 is a hypohalous acid or its salt, hydrogen peroxide or oxygen, preferably sodium hypochlorite. The N-oxyl catalyzed oxidation is undertaken in the presence of any N-oxyl compound 13, such as a 2,2,6,6- tetramethyl-1 -piperidine-N-oxyl (TEMPO) or 4-acetamide-2,2,6,6-tetramethyl-1 - piperidine-N-oxyl. The reaction is preferably undertaken at neutral pH; vi) a phosphorylation reaction resulting in the introduction of H₂P0₃ or H₂P0₄ groups. The phosphorylating agent 13 can be either H₃P0₃ or H₃P0₄ and the presence of a co- reactant 17 such as urea is also needed. All reactions can optionally incorporate a salt 15 and a pH adjustment reagent 19.

In reaction 20, 2 to 2.5 moles (per mole of aldehyde groups in 2,3-dialdehyde cellulose) of a compound X capable of introducing charge groups -anionic or cationic, and of any valence- to the cellulose structure either by direct reaction with the aldehyde functionalities or through a combination of reactions leading to the same result are used; the salt concentration of the medium can be varied from 0 to 2M, the salt being preferably NaCI; depending on the specific reaction, the reaction temperature can range from about 5 to 95°C and the reaction time from 1 minute to 72 h. The compound named X may be one of the following: i) an oxidizing agent that leads to the conversion of aldehyde to carboxyl groups, such as hypohalous acid or its salt, halous acid or its salt, perhalogen acid or its salt, hydrogen peroxide, nitrogen dioxide or per-organic acid or its salt, or combinations of them; ii) a sulfonating agent, such as sodium bisulfite, that leads to the sulfonation of dialdehyde cellulose introducing sulfite group; iii) a cationizing agent, such as water-soluble acidic salts of titanium, zinc, strontium, tin, chromium and zirconium, and especially salts of zirconium such as zirconium chloride, zirconium oxy- chloride and zirconium sulphate; iv) a molecule having a charge group plus either a primary amino group, such as (2-aminoethyl)trimethylammonium chloride, or a hydroxyl group, such as 3-hydroxypropionic acid, that introduces charge groups via addition reactions on aldehydes through the formation of, respectively, imino or acetal bonds. Alternatively reaction 20 can be v) a N-oxyl mediated oxidation. In that case it uses from 0.2 to 0.7, and preferably 0.55, mole % of catalyst referred to moles of cellulose, the catalyst being any N-oxyl compound, like 2,2,6,6-tetramethyl-1 -piperidine-N-oxyl (TEMPO) or 4-acetamide-2,2,6,6-tetramethyl-1 -piperidine-N-oxyl, from 1 to 160 mole % of oxidizing agent A and 15 to 20 mole % of oxidizing agent B both referred to the moles of cellulose; the oxidizing agent A being a halous acid or its salt, perhalogen acid or its salt, hydrogen peroxide or per-organic acid or its salt, laccase or peroxidase, and preferably sodium chlorite, and oxidizing agent B a hypohalous acid or its salt, hydrogen peroxide or oxygen, preferably sodium hypochlorite; the reaction temperature ranges from 5 to 60°C, preferably 55°C and the reaction time can be varied from 1 to 72 h. Reaction 20 can be also vi) a phosphorylation reaction using pentavalent phosphorous reagents (H₃P0₄, P₂0₅, POCI₃, some organic phosphates, etc.); the preferred route involves reacting the cellulose with an excess of 5 wt% of P₂0₅ in N,N- dimethylformamide at 25°C for about 0.5 to 48h, and hydrolyzing the pyrophosphate formed to sodium phosphate by putting the product in 10 wt% NaOH solution at 25°C for about 0.5h. Preferably, reaction 20 uses a combination of 2.35 moles of sodium chlorite and 2.35 moles of hydrogen peroxide.

Reaction 20 can be carried out under different conditions, depending on the desired characteristics of the final product. Allowing the pH to drop below 3 during the reaction will cause the hydrolysis of the cellulose molecules, thus obtaining shorter fibrils and/or NCC at the end of the process. Even if the pH is kept between 3 and 8, a similar effect might occur if the conversion of aldehydes is not complete by the end of the reaction and the resulting product is further subjected to a chemical process involving radical species. The remaining aldehyde groups undergo depolymerisation reactions when put in contact with radicals, like those present in the TEMPO-mediated oxidation, leading again to shorter fibrils and/or NCC. In order to maximize the amount of long NFC, the pH throughout reaction 20 should be kept between 3 and 8, and preferably between 6 and 7 for case i), between 4 and 5 for case ii), between 3 and 5 for cases iii) and iv), between 6and 8, and preferably 6.8 for case v) and between 4 and 8 for case vi).

After reaction 20 is completed, the presence of charge groups in or attached to C2 and C3 of AGU is believed to cause the appearance of repulsive forces acting between the fibrils that swell the fiber structure. In this open structure, a larger number of AGU are accessible, and thus the yield of further modifications, i.e. a different reaction 20, is higher than with non-treated pulp.

All the reactions carried out in this method occur under mild agitation conditions, and converting the subsequent products into cellulose nanostructures is possible without the energy intensive shearing or blending that is used in obtaining similar cellulose products by the methods known in the prior art.

The combination of reactions 10 and 20, allows a maximum content of charge groups in NFC higher than the highest ever reported (2.2 mmol/g), disclosed in patent JP2008001728-A by T. Saito et al., assigned to Ashahi Kasei KK. As we have discovered, only when the overall charge content surpasses a particular level of charge concentration (typically about 3.0 mmol per gram of cellulose or higher) does the NFC get spontaneously liberated (see Tables 1 and 2 below), i.e. there is no need to apply any extra mechanical energy. This particular level of charge concentration is strongly dependent on the type of process applied (the nature and order of the reactions) and on the precise operating conditions, with the concentration of the reactants being one important parameter. Cellulose fibers with charge contents between 2.2 mmol/g and such particular level are also of great interest, since the energy required to obtain nanostructures from them is very much reduced and, on top of this, their high functionalization makes them an extremely interesting new product with many potential applications.

The present description sets out a chemical method for producing highly charge group modified cellulose fibers, which permits liberating cellulose nanostructures (NFC and NCC) using much less mechanical energy than any other existing method (Figure 2).

When the method described herein is employed under certain conditions, the NFC and NCC produced can be obtained by purely chemical processes i.e. without the use of mechanical energy as a separate step (Figure 2, zone D) and therefore the method is free of a peeling step. Furthermore, the NFC and NCC obtained can reach a surface charge greater than any other previously reported. After the process the NFC and NCC are easily precipitated into a powder, allowing a fast recovery. The high functionalization of the resulting cellulose nanostructures that is achieved with this method facilitates their further derivatization following traditional and well known chemical reactions, i.e. esterification of carboxyl groups. In the same processes highly charged water-soluble cellulose products e.g. 2,3-dicarboxycellulose (DCC), are produced as the only reaction by-products. The amount of these compounds, as well as the physicochemical characteristics of the non-water-soluble cellulose products, can be controlled by modifying the operation conditions. The overall process is greener than presently used methods for producing either NFC or highly charged water-soluble cellulose products.

Figure 2 shows in a schematic diagram the relation between the charge density of cellulose fibers and the energy required to liberate cellulose nanostructures from them, for various methods. The diagram is divided into four zones along the horizontal axis, of charge concentration (mmol/g). The first zone, adjacent the vertical axis on the left, border the charge concentration from approximately 0 to 1 .5 mmol/g. The second zone adjacent the first zone, is from greater than 1.5 to 2.2 mmol/g. The third zone is from greater than 2.2 to about 3.5 mmol/g. The fourth zone is greater than about 3.5 mmol/g charge concentration.

The cellulose products of the first zone (illustrated with the uppermost sloped line A) may be made by methods of the prior art, particularly the processes described in WO2009/021688A1. Similarly, other existing methods, particularly the one described in JP2008/001728A, give rise to the cellulose products of the second zone (identified by the sloped line B).

The method described herein may also produce cellulose products in the two regions mentioned before, and it is the first method that produces non-water-soluble cellulose products in the third and forth zones (identified by lines C and D, respectively). In the third region, highly charge group modified cellulose fibers 22 which preserve their original macroscopic structure are created. These fibers can be directly used as a paper strength agent: the neutral pH of the wet end of the papermaking process will promote the dissociation of the charge groups, allowing the disintegration of the fibers into cellulose nanostructures upon being sheared in the headbox, just before the paper being formed. If instead these highly charge group modified cellulose fibers 22 are kept under slightly acidic conditions and then precipitated in a low surface tension solvent (such as ethanol) they show, when dried, super-absorbent properties, being able to absorb more than 50 times their weight of water. In the forth region, cellulose nanostructures (NFC or NCC depending on the reaction conditions) are spontaneously formed without any further mechanical treatment (free of a peeling step). It should be noted that B includes A, and that the present method (C+D) also includes both A and B. All values in the graph are approximations only meant to illustrate the varying energies required to peel the cellulose as the charge concentration increases.

Uses of highly charge group modified cellulose products obtained by means of this method include but are not limited to papermaking, industrial materials, biomedical applications, nanomaterials, polymeric nanocomposites, etc.

Thus the invention comprises a method that allows reducing the mechanical energy required to liberate NFC from cellulose to low levels, the final level of energy required depending on the precise conditions under which it is performed; the term "mechanical energy" is here defined as intensive mechanical energy applied as a separate step, i.e. using a blender to liberate the NFC, where peeling of the cellulose fibres occurs. It does not refer to the mechanical energy employed in stirring the mixtures during the chemical reactions.

The reactions are carried out under the specific conditions that were described before. The series of photographs found in Figures 3(a) to 3(e) illustrates the appearance of the product at different steps of the production and isolation of NFC, using a reaction approach composed of 10 periodate oxidation + 20i chlorite oxidation + 20v TEMPO-mediated oxidation, from bleached softwood kraft pulp. However, the method is applicable to all types of cellulose fibers extracted from all kinds of lignocellulosic compounds, including all types of gymnosperm, angiosperm, algae or bacterial cellulose, with or without previous treatments such as oxidation, sulfation or any other.

More specifically, the periodate oxidation 10 converts the cellulose fiber that has a general structure of formula I to a 2,3-dialdehyde cellulose having a general structure of formula II (all formulas disclosed herein are simplified representations of the different AGU that can exist within each particular cellulose structure and hence n and m can be in any permutation, that is, they are not confined to any precise sequence, but instead all combinations are equally contemplated):

wherein (n+m) is an integer from 100 to 10,000, in both formulas I and II. (The six carbons of cellulose AGU have been numbered in formula I according to the accepted nomenclature; such notation will be kept throughout the patent e.g. C6 representing carbon number 6, and so on).

Figure 3(b) is a photograph illustrating a suspension of the dialdehyde product of general formula II produced in reaction 10, after which two hydroxyl groups (C2 and C3) are converted into aldehyde groups.

In reaction 20 aldehydes represented in formula II are converted into charge groups either by directly modifying them or by grafting new species on them; such reaction can be oxidation, sulfonation, cationization, addition and others, leading to all the possibilities contemplated in formula IV; in a preferred embodiment, reaction 20 is a salt-assisted oxidation of the 2,3-dialdehyde cellulose by reaction with halous oxidant at neutral conditions, and leads to the formation of 2,3-dicarboxycellulose of general formula III:

wherein (n+m) is an integer from 100 to 10,000.

Figure 3(c) is a photograph illustrating a suspension of the 2,3-dicarboxycellulose product of general formula III after halite oxidation, by which the aldehyde groups of formula II were converted into carboxyl groups, and purification.

Figure 3(d) is a photograph illustrating a pure NFC suspension after: 10 salt- assisted periodate oxidation, 20i salt-assisted chlorite oxidation and 20v TEMPO- mediated chlorite/hypochlorite oxidation. By the combination of these reactions an important amount of all three types of hydroxyl groups of cellulose have been converted to carboxyl groups (final carboxyl content = 3.5 mmol/g -Figure 4), and this has led to the spontaneous liberation of the NFC.

Regarding the series of photographs in Figures 3(a) to 3(e) that illustrate the different steps of the isolation of NFC from bleached softwood kraft pulp using reactions 10, 20i and 20v, it is important to note that although the same cellulose concentration was employed in cases (a-d), that is 15 mL of 1 wt% aqueous pulp suspension, the mixture becomes transparent only after the last treatment (TEMPO) brings the charge content above 3.0 mmol/g allowing the spontaneous liberation of the NFC. The amount of NFC was increased in Figure 3(e) to make the final product more clearly visible in the picture. This sample also corresponds to that shown in Figure 5.

Up to a certain degree of modification the fibers remain intact (see third region in Figure 2) and beyond that limit no extra mechanical treatment is needed to liberate the NFC (see forth region in Figure 2 and photographs in Figure 3), but instead the presence of an increased amount of charge groups on the fibers (Figure 4) leads to their spontaneous break up into nanofibers (Figure 5). At the same time, the lack of intensive mechanical energy throughout the process allows obtaining long NFC although changing the procedure conditions also permits to obtain highly charged NCC (Figure 6). In the same processes highly modified water-soluble cellulose compounds are produced (Figure 7). Furthermore, the final cellulose products are highly functionalized and thus can be readily derivatized by a large number of well known reactions.

In one aspect described herein there is a cellulose nanostructure comprising lengths from 100 nm to several microns and aspect ratios in a range from less than 10 to more than ,000, and preferably from 200 to 1 ,000.

In another aspect described herein there is a non-water-soluble cellulose product, including cellulose fibers and cellulose nanostructures, comprising a concentration of charge groups of 0.1 -1 1 mmol of X per gram of cellulose, where X can be one of the following groups or the sum of several of them: COOH, HS0₃, R-NH₂, R-NH, R-N, R-N+ (+ representing here a positive charge), H₂P0₃, H₂P0₄, Ti, Zn, Cr, Sr, Sn or Zr.

In a further aspect described herein there is a cellulose product having the general formula IV:

wherein R^ R₂ and R₃ can be one of the following groups: CHO, COOH, R- COOH, R-HS0₃, R-NH₂, R-NH, R-N, R-N+ (+ representing a positive charge), R and R₂ can be also Ti, Zn, Cr, Sr, Sn or Zr, and R₃ can be also CH₂OH, R-H₂P0₄ or R-H₂P0₃, Ri being different or equal to R₂, and R₂ different or equal to R₃, wherein R is a C1-C6 substituted or unsubstituted alkyl, alkyl-ether or aldimine, or a C2-C6 substituted or unsubstituted alkenyl; wherein one or more -OH groups or -H present in the cellulosic structure are unsubstituted or substituted with a halogen, an alkali metal, a C1 -C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl; wherein the minimum charge density is 2.2 mmol/g when all charge groups present are COOH, or 1.5 mmol/g when at least one charge group is different from COOH; wherein the maximum charge group content is 1 1 mmol/g; wherein m and n are in any sequence and all permutations are equally contemplated, and (n+m) is an integer from 100 to 10,000.

The cellulose nanostructures that can be obtained through the different combinations of reactions mentioned in this patent have lengths ranging from 100 nm up to several microns and concentration of charge groups of 0.1-1 1 mmol per gram of cellulose. Figure 6 shows an example of NCC obtained by one of these combinations, in contrast to the long NFC shown in Figure 5. The relative proportions of hydroxyl, aldehyde and charge groups present on them can also be controlled by modifying the conditions and/or the order of the oxidizing reactions, as exemplified in Tables 1 and 2 below. Similar variations also allow controlling the length, the charge and the relative ratio to NFC of the water-soluble modified cellulose products (e.g. tricarboxycellulose (TCC)) that are obtained as the only by-products of such processes. In Figure 7, a ¹³C- NMR spectrum of a reaction mixture after applying one of the method approaches show the presence of TCC, and Table 1 and 2 also show how its amount varies with the process conditions.

Table 1

a⁾ Total NFC and TCC recovered, referred to the initial sample weight

b⁾ May contain traces of NFC

Table 1 shows the effect of TEMPO-oxidation reaction time (after periodate and chlorite oxidations) on carboxyl content, and on NFC and TCC yields obtained without applying any mechanical energy. Table 2

a⁾ Total NFC and TCC recovered, referred to the initial sample weight

b⁾ May contain traces of NFC

Table 2 shows the effect of TEMPO-oxidation reaction time (after periodate and chlorite oxidations) on carboxyl content, and on NFC and TCC yields obtained without applying any mechanical energy.

EXAMPLES

EXAMPLE 1 : Method to obtain highly charged high aspect ratio NFC at high yield from softwood Kraft pulp without the addition of any extra mechanical energy, in two reaction steps.

Reaction 10. Preparation of 2,3-dialdehyde cellulose by salt-assisted periodate oxidation.

The process was carried out in aqueous media using a glass beaker with overhead stirrer, by using the following reaction conditions: bleached softwood kraft pulp (3.5 g), sodium metaperiodate (4.6 g; 21.5 mmol; 100 mole % based on moles of cellulose) and sodium chloride (8.75 g; 1 N in the overall solution) were added in 150 mL deionised water. The reaction mixture adjusted at pH 4.5 by addition of NaOH and gently stirred at room temperature in the dark for 72 h. After this time, the modified pulp was filtered out and thoroughly washed with deionised water repeatedly.

The aldehyde content of the cellulose was then calculated using the hydroxylamine-hydrochloride (NH₂OH'HCI) standard titration method, by which the HCI released from the reaction of aldehydes and NH₂OH^«HCI is back-titrated with NaOH solution of known normality. Specifically, a water suspension of periodate-oxidized cellulose (20 mL; 0.65 g dry basis) was mixed with 40 mL of isopropanol (making a final proportion of isopropanol/water of 2/1 v/v)) and the mixture was sufficiently stirred to prepare a well-dispersed slurry. The pH of the mixture was then approximated to 2-3 by adding a few drops of concentrated HCI and then carefully adjusted to 3.5 with NaOH 0.1 . 10 mL of 10 wt% NH₂OH.HCI solution was added to this mixture, allowing it to react for 10 min. After that time the HCI released was titrated with 0.5N NaOH solution until pH 3.5 is reached again. The aldehyde content can then be calculated using the following equation:

Aldehyde content (mmol/g cellulose) = Volume of NaOH (mL) required for the titration X molarity of the NaOH (mol/L) / Weight of dry cellulose initially dissolved (g).

In the example, the aldehyde content was determined to be 6.6 mmol/g referred to the weight of cellulose.

Reaction 20i. Preparation of 2,3-dicarboxycellulose by chlorite oxidation reaction, and spontaneous liberation of the NFC during the process.

Periodate-oxidized pulp (3.5g), sodium chlorite (80% pure; 2.76 g; 24.5 mmol) and hydrogen peroxide (30 wt% solution, 2.76 g; 24.5 mmol) were mixed in 150 ml water. The reaction mixture was stirred at room temperature for 20 h and kept at pH 5 by drop wise addition of NaOH solution (necessary during first 3 h). After the reaction was completed, all the cellulose had been spontaneously converted into NFC. The nanofibers were coagulated by adding 2 volumes of ethanol and separated by filtration. The product was washed with acetone twice and subsequently dried.

The carboxyl content of the NFC was determined by the conductometric titration method (Figure 4). To a water suspension of periodate oxidized, chlorite-oxidized NFC (120 ml; 20.4 mg dry basis) 2.5 mL of a 0.02 M sodium chloride solution were added, and the mixture was sufficiently stirred to prepare a well-dispersed solution. Then 0.1 M HCI was slowly added to the mixture to set the pH value in the range of 2.5-3.0. A 0.005 M NaOH solution was then added at a rate of 0.05 mL/min until the mixture had reached pH 1 1 , as determined by a 836 Titrando (Metrohm, Switzerland) titrator. The carboxylate content of the sample was determined from the conductivity curves (an example of such curve is shown for a different sample in Figure 3) to be approximated 5.5 mmol/g cellulose. The following equation was used: Carboxylate content (mmol/g cellulose) = Volume of NaOH (ml.) required for deprotonation of carboxylic groups X molarity of NaOH (eq/L) / amount of dry cellulose sample initially used (g).

EXAMPLE 2: Method to obtain high aspect ratio NFC with different charge contents from softwood kraft pulp without the addition of any extra mechanical energy, in three reaction steps.

The process was carried out following the same methodology detailed in Example 1. The specific reaction conditions in this case were: bleached softwood kraft pulp (3.5 g), sodium metaperiodate (2.3 g; 10.75 mmol; 50 mole % based on moles of cellulose) and sodium chloride (1.7 g; 0.2 N in the overall solution) in 150 mL deionised water. The reaction time was 40 h.

The aldehyde content was determined in the same way as described in Example 1. In this case, the aldehyde content was found to be 2.6 mmol/g referred to the weight of cellulose.

Reaction 20i. Preparation of 2,3-dicarboxycellulose by chlorite oxidation reaction.

Periodate-oxidized pulp (3.5 g), sodium chlorite (80% pure; 2.45 g; 21.5 mmol) and hydrogen peroxide (30 wt% solution; 2.45 g; 21.5 mmol) were mixed in 150 ml water. The reaction mixture was treated as was described in Example 1 , with the only difference of the reaction time, this time being 20 h. In these conditions, after the reaction was completed the fibers still maintained their original macroscopic structure i.e. they were not spontaneously liberated to any extent. The modified fibers were isolated by adding 2 volumes of ethanol, which coagulates them and makes them easier to recover, and separated by filtration. The product was washed with acetone twice and subsequently dried.

The carboxyl content of the modified cellulose was determined by the conductometric titration method that was detailed before, using exactly the same conditions. The carboxylate content of this sample was determined to be 2.41 mmol/g cellulose, with a total consumption of 5 mM NaOH (9.83 mL). Reaction 20v. Preparation of 2,3,6-tricarboxycellulose by TEMPO oxidation, and spontaneous liberation of the NFC during the process.

Periodate oxidized, chlorite oxidized pulp (3.5g) was taken to a 3 neck flask and suspended in 0.05 M sodium phosphate buffer (315 mL; pH 6.8). In this mixture, TEMPO (0.056 g, 0.36 mmol) and sodium chlorite (80% pure; 4 g; 35 mmol) were dissolved. A 4.2 M sodium hypochlorite solution (0.83 mL; 3.5 mmol) was diluted to 0.35 M with the same 0.05 M phosphate buffer (35 mL), and added to the reaction mixture. The reaction mixture was stirred at 500 rpm and allowed to react at 60°C for alternatively 5, 10, 5, 20 and 45 h. After each time, nanofibrillar cellulose (NFC) which was spontaneously liberated (without the application of any extra mechanical energy) was separated from the rest of the cellulosic material by filtration through a 20 μιη pore nylon cloth and kept in water suspension. The NFC was then flocculated from the water suspension by adding 2 volumes of ethanol, and separated by filtration. This precipitate was washed with acetone twice and subsequently dried. Lengths over 5 Mm were produced.

The carboxyl content of the NFC was calculated using the conductometric titration described before. Table 1 shows the conditions and the results obtained for these experiments.

EXAMPLE 3. Method to obtain highly-charged non-water-soluble softwood kraft cellulose pulp, in two reaction steps.

Reaction 10. Preparation of 2,3-dialdehyde cellulose by periodate oxidation.

The recipe for preparation of 2,3-dialdehyde cellulose was the same as described in Example 1 except for the reaction time that in this case was 24 h.

The aldehyde content of the dialdehyde cellulose (0.56 g) was determined to be 3.4 mmol/g cellulose, corresponding to a total consumption of 0.5 NaOH (3.8 mL) in the hydroxylamine-hydrochloride method (described in Example 1 ).

The recipe for preparation of 2,3-dicarboxyl cellulose was the same as described in Examples 1 and 2, except for a different amount of sodium chlorite (80% pure; 3.70 g; 32.75 mmol) and hydrogen peroxide (30 wt% solution; 3.7 g; 32.75 mmol) used. The carboxyl content of the chlorite-oxidized cellulose (14.5 mg) was 3.2 mmol/g cellulose, corresponding to a total consumption of 5 mM NaOH solution (9.3 mL) in the conductrometric titration test (described in Example 1 ).

The highly charged softwood kraft pulp obtained herein requires a very small amount of mechanical energy to be disintegrated into NFC but is below the "spontaneous liberation" limit at these particular conditions (~3.5 mmol/g) and so it keeps the original fibrillar macrostructure of the unmodified cellulose pulp.

EXAMPLE 4. Method to obtain high aspect ratio NFC with different charge contents and TCC from softwood kraft pulp without the addition of any extra mechanical energy, in three reaction steps.

The sample used initially in this example is the product of Example 3, this is, a 2,3-dicarboxylated cellulose pulp with a content of carboxyl groups of 3.2 mmol per gram of cellulose obtained after 10) salt-assisted periodate oxidation and 20i) chlorite oxidation reactions.

Reaction 20v. Preparation of 2,3,6-tricarboxycellulose by TEMPO oxidation

The recipe for preparation of 2,3,6-tricarboxycellulose was the same as described in Example 2, and again it was repeated at different reaction times (0, 5, 15, 20 and 45h).

The carboxyl content of the NFC was again calculated by the total consumption of NaOH, using the conductrometric titration (as described in Example 1 ). The results are listed in Table 2.

EXAMPLE 5: Method to obtain highly charged NFC of intermediate aspect ratio and TCC from softwood kraft pulp, in two reaction steps.

The recipe for preparation of 2,3-dialdehyde cellulose was the same as described in Example 2. The aldehyde content, determined in the same way as described before, was 2.6 mmol/g referred to the weight of cellulose. Reaction 20i. Preparation of 2,3,6-tricarboxyl NFC and TCC by TEMPO-mediated oxidation.

The recipe for preparation of 2,3,6-tricarboxycellulose was the same as described in Example 2, except for the dose. Periodate-oxidized pulp (3.5g) was taken to a 3 neck flask and suspended in 0.05 M sodium phosphate buffer (315 mL, pH 6.8). In this mixture, TEMPO (0.1 12 g; 0.72 mmol) and sodium chlorite (7.9 g, 80% pure; 70 mmol) were dissolved. A 4.2 M sodium hypochlorite solution (1.75 mL, 7 mmol) was diluted to 0.35 M with the same 0.05 M phosphate buffer (35 mL), and added to the reaction mixture. The reaction mixture was stirred at 500 rpm and allowed to react at 60°C for 45 h. After this time, the nanofibrillar cellulose (NFC) which was spontaneously liberated (without the application of any extra mechanical energy) was collected by filtration through a 20 μιη pore nylon cloth. The NFC was then isolated by centrifugation at 5000 rpm for 15 min. The precipitate was washed with 1 N NaCI solution and its yield determined gravimetrically after drying (-60 wt%). AFM showed NFC average lengths of 1 -2 μηι.

The carboxyl content of the NFC was calculated using the conductometric titration described before. The carboxylate content of the sample was determined from the conductivity curves to be 1.78 mmol/g cellulose.

During the process, water-soluble TCC is also formed, as was concluded from the solid state NMR (Figure 7) of films resulting from drying the supernatant solution.

EXAMPLE 6: Method to obtain highly charged nanocrystalline cellulose (NCC) and TCC from softwood kraft pulp without the addition of any extra mechanical energy, in two reaction steps.

The recipe for preparation of 2,3-dialdehyde cellulose was the same as described in Example 1 , except for the reaction time that in this case was 40 h. The aldehyde content was determined in the same way as described in Example 1 , being in this case 5.08 mmol/g referred to the weight of cellulose. Reaction 20v. Preparation of 2,3,6-tricarboxyl NCC and TCC cellulose by TEMPO oxidation.

The recipe for preparation of 2,3,6-tricarboxycellulose was the same as described in Example 2, except for the dose. Periodate-oxidized pulp (3.5g) was taken to a 3 neck flask and suspended in 0.05 M sodium phosphate buffer (315 ml_, pH 6.8). In this mixture, TEMPO (0.168 g; 1.08 mmol) and sodium chlorite (80% pure, 1 1.85 g; 105 mmol) were dissolved. A 4.2 M sodium hypochlorite solution (2.625 mL; 10.5 mmol) was diluted to 0.35 M with the same 0.05 M phosphate buffer (35 mL), and added to the reaction mixture. The reaction mixture was stirred at 500 rpm and allowed to react at 60°C for 45 h. Subjecting the reaction mixture to centrifugation at 5000 rpm for 15 min after this time did not lead to the appearance of any precipitate, showing that all the initial cellulose sample had been converted into soluble and/or submicron-sized cellulose species. Both AFM (Figure 6) and SEM showed cellulose nanostructures in the range of 200 nm long and a few nanometers wide and thick, what is usually referred to as nanocrystalline cellulose (NCC). Also, NMR of the supernatant solution showed the presence of TCC.

It should be appreciated that the invention is not limited to the particular embodiments described and illustrated herein but includes all modifications and variations falling within the scope of the invention as defined in the appended claims.

EXAMPLE 7: Method to obtain superabsorbent cellulose fibre from softwood kraft pulp in two reaction steps.

Reaction 0. Preparation of 2,3-dialdehyde cellulose by periodate oxidation.

The recipe for preparation of 2,3-dialdehyde cellulose was the same as described in Example 2 except for the reaction time that in this case was 72 h. The aldehyde content, determined in the same way as described before, was 4 mmol/g referred to the weight of cellulose.

The recipe for preparation of 2,3-dicarboxyl cellulose was the same as described in Example 3. The carboxyl content of the chlorite oxidized cellulose was 3.4 mmol/g of cellulose, using the conductometric titration method described before. ln the following, extreme care was taken to keep the aqueous suspension of the modified fibers at pH lower than 6, in order to avoid dissociation of carboxyl groups and thus charging and disintegration of the fibers. Such modified fibers were isolated by adding to the aqueous suspension 2 volumes of ethanol, which coagulates them and makes them easier to recover, and separated by filtration. The product was washed with acetone twice and subsequently dried.

When put in contact with water (free swell), these modified fibers absorb about 50 g/g of distilled water per gram of fiber.

EXAMPLE 8: Self-fibrillating fibers.

Self-fibrillating fibers can be used as strength agents in papermaking. Samples were prepared with varying amounts (% w/w) of fibrillated fiber added to the paper. The samples were fibrillated during the papermaking process, thus providing increased bonding. The break-up in fibrils is shown in Figures 8 and 9.

The tensile strength of handsheets is shown in the Table 3.

Table 3

Adding self-fibrillating fibers to paper also results in smoother sheets. EXAMPLE 9: Superabsorbing fibers

Oxidized pulp is precipitated in an ethanol/water mixture, washed with acetone and dried at room temperature to keep the structure open. Without this procedure water take-up is slower. Typical carboxyl contents in the pulp vary from 1 - 3 mmol/g. Table 4 shows the water take-up of bleached kraft pulp for various carboxyl contents. Above 3 mmol/g the yield is less as some fibers disintegrate.

Table 4

Table 5 shows results for different pulps.

Table 5

Notes: 1 ) Na-form carboxylated fibre

2) Never dried Tate Kraft Softwood (Domtar Inc Espanola mill)

3) Eucalyptus (Cenibra Ceo Itochu international) As can be seen from Table 5, uptake of distilled (Dl) water can be up to 125 times the weight of the dry fiber. For tap water at 3 mmol/g water uptake varies from 54-78 times the fiber weight, depending on type of fiber.

EXAMPLE 10: Electrosterically stabilized nanocrystalline cellulose (ENCC)

The charge content of ENCC varies from 3.60 mmol/g to 6.60 mmol/g depending on the levels of oxidation. The charge content of NCC produced by our method is much larger than the theoretical maximum content. Therefore, these NCC are stabilized with highly charged DCC (Dissolved carboxylated cellulose) chains. We refer to this NCC as electrosterically stabilized nanocrystalline cellulose, or ENCC.

The morphology and size distribution of ENCC were examined by Atomic force microscopy (AFM) and dynamic light scattering (DLS). As shown in the AFM image (Figure 10), the ENCC contains rod-like particles with a length in the range 120 - 200 nm and a diameter around 0 nm.

As shown in (Figure 1 1 (a)), the hydrodynamic diameter of ENCC obtained by DLS is around 200 nm, which is decreased to 97 nm when the salt concentration increases to 1 M (Figure 1 1 (b)). The decrease of the hydrodynamic diameter of ENCC with increasing the salt concentration is given in Figure 1 1 (c).

However, the hydrodynamic diameter of ENCC after hydrolysis by 3N HCI at 45 °C for 2 hours shows around 76 nm (Figure 12(a)), which does not show much change even at high salt concentration; it is around 71 nm with 0.5 M NaCI (Figure 12(b)). The theoretical equivalent diameter for a particle with a length of 200 nm and an axis ratio of 20 is 67 nm, close to the observed value. These results suggest the ENCC consists of a hard core and a number of charged polymer chains attached to this core. At low salt concentration, the coils stretch and repel each other, leading to large particles with a smaller axis ratio, thus increasing the equivalent hydrodynamic diameter. For instance a length of 500 nm and an axis ratio of 12 nm would yield the observed hydrodynamic diameter of 200 nm. These estimates suggest that the carboxylated cellulose chains protruding from the NCC are quite large, in agreement with their large charge density. For ENCC with a charge of 4.8 meq/g, almost 75% of the charges reside in the protruding chains. After hydrolysis the polymer chains are cut off, leaving only the hard core, and one does not expect a large decrease in hydrodynamic diameter with high salt concentration. In addition to DLS, the AFM image shows the length of ENCC after hydrolysis is around 100 - 200 nm and the height is around 10 nm (Figure 13), which is consistent with the dimension of ENCC before hydrolysis. However, the charge content of ENCC before and after hydrolysis is dramatically different: 6.6 mmol/g and 1.4 mmol/g, respectively. This finding also supports our suggested morphology of ENCC. Since the stabilized polymer chains are highly charged dicarboxyl cellulose, the absence of these chains after hydrolysis would lead to a huge decrease in the charge content.

In conclusion, all the results strongly confirm that the nanocellulose is a new kind of nanocrystalline cellulose (NCC) which is electrosterically stabilized by highly charged DCC chains. We refer to this new structure as electrosterically stabilized nanocrystalline cellulose (ENCC), which is suitable for further modification or cross-linking due to numerous versatile carboxyl groups.

Claims

CLAIMS:

1 . A cellulose product comprising a cellulosic structure with at least one charge group attached to the cellulosic structure, wherein the at least one charge group is selected from the group consisting of HS0₃, H₂P0₃, H₂P0₄, primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and carboxyl, wherein if only carboxyl is present in the cellulosic structure the cellulose product has a concentration of more than 2.2 to 1 1 mmol of charge groups per gram of cellulose product and if the carboxyl is absent or present with another charge group the concentration charge groups per gram of cellulose product is more than 1.5 to 1 1 mmol/gram of cellulose product.

2. The product of claim 1 , is nanofibrillar cellulose, NFC, comprising a concentration of charge groups of 2 to 3.5 mmol/g (with a diameter of 3-20nm and a length of 0.5 to 5 microns).

3. The product of claim 1 , is an electrostericaily stabilized nanocrystalline (ENCC) comprising a concentration of charge groups from 3.5 to 7.0 mmol/g (with a diameter of 3-20nm and a length of 100 to 300 nm).

4. A method for producing cellulose products with a high concentration of charge groups attached covalently, the method comprising providing a raw material cellulose (2); oxidizing the raw material cellulose (2) in a medium comprising a salt and a periodate producing a dialdehyde cellulose (12); reacting the dialdehyde cellulose in a reaction selected from the group consisting of a halous oxidation, a sulfonation, a cationization reaction, an imino or acetal bond introducing reaction, a N-oxyl catalyzed oxidation and combinations thereof.

5. The method of claim 4, wherein the concentration of charge groups in the cellulose products is greater than 2.2 to 11 mmol of charge groups per gram of cellulose products if the charge group is carboxyl alone in the cellulose products.

6. The method of claim 4, wherein the concentration of charge groups in the cellulose products is greater than 1.5 to 11 mmol/g of cellulose product if carboxyl is absent or present with another charge group in the cellulose products.

7. The method of claim 4, wherein the concentration of charge groups in the cellulose product is from 3.5 to 7.0 mmol/g.

8. A 2,3-dicarboxycellulose, DCC, substituted in cellulose structure with at least one charge group selected from the group consisting of carboxyl, sulfonate (HS0₃), phosphate (H₂P0₄), phosphite (H₂P0₃), phosphonic acid (H₂P0₃), primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and combinations thereof, the charge groups linked to the cellulose structure either directly or through selected from the group consisting of a C1-C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl.

9. A 2,3,6-tricarboxycellulose, TCC, substituted in cellulose structure with at least one charge group selected from the group consisting of carboxyl, sulfonate (HS0₃), phosphate (H₂P0₄), phosphite (H₂P0₃), phosphonic acid (H₂P0₃), primary amine (NH₂), secondary amine (NH), tertiary amine (N), quaternary amine (N+), titanium (Ti), zinc (Zn), chromium (Cr), strontium (Sr), tin (Sn), zirconium (Zr) and combinations thereof, the charge groups linked to the cellulose structure either directly or through selected from the group consisting of a C1-C6 substituted or unsubstituted alkyl or a C2-C6 substituted or unsubstituted alkenyl.