WO2013076372A1

WO2013076372A1 - Nanocellulose composites

Info

Publication number: WO2013076372A1
Application number: PCT/FI2012/051159
Authority: WO
Inventors: Päivi LAAKSONEN; Markus Linder
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2011-11-24
Filing date: 2012-11-23
Publication date: 2013-05-30
Anticipated expiration: 2014-05-24
Also published as: FI20116175A7; FI20116175L

Abstract

Composite materials that utilize cellulose fibers are disclosed. The attachment between the cellulose fibers is strengthened by means of particles of graphite and/or graphene acting as binder. The composite material is prepared from a mixture containing water and cellulose, and graphite and/or graphene as an additive.

Description

NANOCELLULOSE COMPOSITES Technical Field

The present invention relates to cellulose based composite materials and their manufacture and use. Background Art

Papermaking Chemistry (ISBN 952-5216-04-7), Chapter 12, Dry-strength additives teaches that dry strength is a structural property of the paper web, which is mainly derived from the formation of bonds between fibers as the fiber network forms and dries. The strength of the actual paper depends on the strength of the individual fibers and of the bonds between fibers, on the number of bonds, and on the distribution of the bonds and fibers. Substances called dry- strength additives enhance one or more of these features but not, however, the strength of individual fibers.

According to the above-referred book, the strength of the fiber network can also be increased mechanically by more beating or refining, in which case the larger amount of micro-fibrils promotes the formation of bonds between fibers. The strength of paper can be enhanced by altering the fiber composition, for instance, by raising the proportion of long-fiber chemical pulp, by reducing the filler content, or by adding dry-strength additives. Process changes can also boost the strength, for example, by improving the formation, raising the pH, or adding wet pressing. Nevertheless, more refining and beating probably remains the most common way of increasing strength. However, when the negative effects of beating and refining are to be avoided, dry- strength additives remain the practical option. Dry-strength additives are generally water-soluble, hydrophilic polymers, either natural or synthetic. The most prominent commercial products are: starch, vegetable gums, carboxymethyl cellulose and synthetic polymers. WO 2011/059398 Al discloses a nanopaper that has a structure where micro fibrillated cellulose (MFC) and clay form a layered structure arranged substantially parallel to the surface of the paper. The clay particles, or platelets, are in the nanometer range and the length of the nanofibres of the MFC is in the micrometer range. Further the clay particles, or platelets, are preferably substantially isolated from each other. The length of the micro fibrillated cellulose nanofibres can be in the range of 5-20μιη and, according to an embodiment, the amount of microfibrillated cellulose is more than 10wt% and also the amount of clay is more than 10wt%. In yet another embodiment the nanopaper further comprises a water soluble cross-linker, the amount of which is more than 5wt%, based on the total weight of the nanopaper. The cross-linker is either chitosan or hyaluronic acid.

A method for preparing the clay-microfibrillated cellulose nano fibre nanopaper comprises:

-preparing a suspension of clay and microfibrillated cellulose nanofibres -mixing said suspension -filtrating said suspension

-obtaining or forming a film of said filtrated suspension -drying of said film.

Disclosure of Invention It is an object of the present invention to create, starting from e.g. the disclosure of WO 2011/059398 Al, which is incorporated herein by reference, new methods and composite materials that utilize cellulose fibers.

In particular, it is intended to improve or promote the formation of bonds between cellulose fibers, or strengthen the bonds that are formed, so that the resultant composite materials were stronger than prior composite materials with equivalent cellulose fibers.

The object of the invention is achieved by preparing the composite material from a mixture containing water and cellulose, and graphite and/or graphene as an additive.

The resultant composite material comprises cellulose fibers and/or fibrils and particles of graphite and/or graphene dispersed in the material such that at least a portion of the cellulose fibers and/or fibrils are attached to each other by means of said particles of graphite and/or graphene acting as binder. The use of graphite/graphene particles as binder or strength additive improves adhesion between the cellulose fibers and/or fibrils and therefore also the strength of the composite material.

The cellulose fibers themselves are, of course, one of the determining factors as to the strength of the composite, and therefore different fibers compositions give different "basic strength" to the composite.

It is also an object of at least some of the embodiments to improve the strength of the composite without too much processing or modifying the cellulose chemically in order not to deteriorate the properties of the cellulose fibers. According to these embodiments, the cellulose used, or at least most of the cellulose used, is native cellulose. Native cellulose refers to chemically unmodified cellulose and/or non-oxidized and non- functionalized cellulose.

Brief Description of Drawings

For a more complete understanding of the present invention and the advantages thereof, the invention is now described with the aid of the embodiments and examples and with reference to the following drawings, in which:

Figures 1A, IB and 1C present nanocellulose films with a graphene/graphite content of 1 wt-%, 5 wt-% and 29 wt-%, respectively;

Figure 2 presents a TEM image of a graphene flake embedded in a MFC suspension; Figures 3A and 3B present respectively an optical microscope image and a Raman spectrum of a composite surface;

Figures 4A, 4B, 4C, 4D and 4E present different mechanical properties of composites containing different amounts of graphene;

Fig. 5 depicts results of measurements in one comparative example; Fig. 6A, Fig. 6B and Fig. 6C describe results of another experiment; and

Fig. 7A and Fig. 7B are respective a TEM image and a hexagonal electron diffraction (ED) pattern in a further experiment. Modes for Carrying Out the Invention

All embodiments include a mixture of cellulose and graphite and/or graphene in some form, and therefore it is useful first to discuss these materials.

Basic materials

Graphene

Graphene is an allotrope of carbon shaped into one-atom-thick planar sheets of carbon atoms bound together into a "chicken wire" -type ring structure. In theory, when several graphene sheets are stacked together, they form the crystalline or "flake" form with an interplanar spacing between the graphene sheets of 0.335 nm. In practice, however, it is often difficult to manufacture uniform one-atom-thick sheets and therefore the naming of the material is often decided on the basis of its properties. Thus, even though graphene is basically one layer of atoms, in context of practical applications the term graphene is also used to refer to particles that have two or several layers of atoms but still exhibit properties clearly deviating from those of graphite in the application in question. However, when there are several layers of atoms and the relevant properties change, the material is called graphite.

Graphene can be produced, for example, by abrasion of graphite, by sonication of graphite, by cutting from nanotubes, or by epitaxial growth on various substrates or by growth from carbon-containing melts. A further alternative that can be utilized is exfoliation, which is easily carried out in large scale. High yields of graphene can also be obtained by burning magnesium metal in dry ice. However, the exfoliation is a simpler alternative for bringing graphene to solution environment. According to embodiments, the graphene can also be produced during the present methods from graphite by means of exfoliation, for instance.

U.S. Patent Application Publication No. US 2008/0279756 Al discloses a method of exfoliating a layered material (e.g., graphite and graphite oxide) to produce nano-scaled platelets. The method comprises dispersing graphite or graphite oxide particles in a liquid medium containing therein a surfactant or dispersing agent to obtain a suspension or slurry, and exposing the suspension or slurry to ultrasonic waves (ultrasonication) to produce the separated nano-scaled platelets. WO 2010/097517 A2 discloses another method of exfoliating graphene platelets from graphite. The method comprises facilitating exfoliation by treatment with proteins. In an embodiment, the proteins adhere to the surface of graphene and then the produced platelets may contain a graphene layer and a protein layer on the surface of the graphene layer.

Another commonly used process for producing graphene is the reduction of graphene oxide. However, this procedure will not provide an even quality with an evenly reduced graphene surface, but instead a surface still containing functional groups typical for the oxide. This is not advantageous for the use according to the present invention. One method using reduced graphene oxide, and trying also to benefit from said functional groups, is disclosed in Luong et al. "Graphene/cellulose nanocomposite paper with high electrical and mechanical performances", J. Mater. Chem., 2011, 21, 13991.

Graphite

Graphite is naturally found in three types, which are crystalline flake graphite, amorphous graphite, and lump graphite. In the embodiment, the preferred type is crystalline flake graphite, due to its particular capability of exfoliating into graphene layers.

Cellulose

The cellulose used in the embodiments is intended to include regular cellulose or nanocellulose, or a mixture thereof.

Thus, the term "cellulose" includes all cellulosic materials containing cellulose or nanocellulose, or mixtures thereof, obtained from any cellulose-containing source material, such as wood-based materials, including pulp from softwood or hardwood trees, or plant materials, including agricultural sources, grasses or other non-wood plant materials. The cellulose is generally isolated from these source materials by chemical, mechanical or thermo -mechanical pulping to provide cellulose fibers of a diameter of 15 - 30 um.

The cellulose can include other components, such as lignin and hemicelluloses, particularly when the cellulose has been prepared from wood pulp. However, these are present in a total content of less than 50 % by weight of the cellulose or cellulose mixture. Preferably, the used cellulose has undergone at least one treatment stage to remove such other components. Chemical reactions affecting the other components such as removal of hemicellulose or lignin may be carried out. However, any treatment stages are preferably selected from those leaving the surface of the cellulose unmodified, i.e. leaving the functional groups on said surface intact. The resulting cellulose is referred to as chemically unmodified cellulose. Further, according to a preferred embodiment, no oxidation of the cellulose is carried out, and no reactive functional groups are added to the surface of the cellulose. The resulting cellulose is referred to as non-oxidized and non-functionalized cellulose. This is due to the fact that no additional reactivity is required, since the method according the embodiments will function well based on pure hydrophobic interactions.

Native cellulose

The term native cellulose refers to cellulose that fulfils both of the above requirements, i.e. that the cellulose is chemically unmodified, non-oxidized and non-functionalized. However, native cellulose does not preclude other treatments such as fibrillation and pulping. Thus, native cellulose can also nanofibrillated cellulose (NFC) or micro fibrillated cellulose (MFC).

Nanocellulose

The term "nanocellulose" is intended to mean fine cellulose fibers, nanofibrillated cellulose (NFC) or microfibrillated cellulose (MFC), and refers to any cellulose fibers with an average diameter of < 10 μιη, preferably 5 nm - 1 μιη, which consist of, fibrils 5 - 200 nm in diameter. The use of the separated fibrils is preferred. However, partial flocculation generally occurs, providing also a portion of larger bundles. Common to such cellulose fibers is that they have a high specific surface area, resulting in a high contact area between the fibrils in the product, such as the mixture of cellulose and nanocellulose. The specific area of the nanocellulose used according to an embodiment is preferably at least 15 m²/g, particularly at least 30 m²/g.

Generally, nanocellulose is prepared by isolating the nano- or micro-sized fibrils from cellulose fibers using homogenizers, such as refiners or fibrillators. The raw-material can be bleached or unbleached cellulose, preferably unbleached cellulose obtained by chemical pulping. In addition, nanocellulose produced partly or entirely by bacterial processes can be used (bacterial cellulose).

As stated above, it is preferred not to use any chemical modification of the prepared (or isolated) cellulose. Such chemical treatment can also be avoided in the preparation of nanocellulose. It is generally known that said preparation can include also chemical pre- treatment steps, among others to reduce the high energy consumption normally associated with the preparation of nanocellulose. Thus, a preferred type of nanocellulose is cellulose that is native cellulose as defined above. Therefore, according to an embodiment, the cellulose, or at least a portion of the cellulose, is native nanocellulose. One method that can be used to prepare nanocellulose is described in WO 2011/059398 Al that has already been discussed above. Other processes for the manufacture of nanocellulose are disclosed in US 2007/0207692, WO 2007/91942 and US 7381294.

Embodiments

According to an embodiment, the method of preparing a composite material comprises preparing a mixture that contains water, cellulose and additives. The additives contain at least graphite and/or graphene as an additive that strengthens adhesion between the cellulose fibrils. Also other additives can be used but are not required according to the embodiments. After the preparation of the mixture, a sufficient amount of water is removed so that a solid composite material is formed. The formed mixture can contain, for example (wt% of the dry matter): cellulose 50-99.995 wt%, such as 75-99.5 wt%; graphene and graphite (in total) 0.005-50 wt%, for example 0.005-10 wt%, such as 0.1-5 wt%,; other dry-matter constituents 0-49,995 wt%, such as 0.1-20 wt%; and sufficient amount of water.

According to an embodiment, the mixture is homogenized, for example by a pressure homogenizer, fluidizer or rotor-stator mixer. Also sonication can be used. According to an embodiment, at least a portion of the cellulose comprises nanocellulose. The portion of the nanosellulose can be 0-100 wt%, such as 10-100 wt% (of the total weight of the cellulose).

According to an embodiment, the weight of the graphite and graphene is 0.005-10 %, such as 0.1-5 %, for example 0.2-4 %, of the weight of the cellulose. The weight of the graphite and graphene refers here to the total combined weight of these ingredients i.e. the sum of the weights of graphite and graphene.

In most practical embodiments, the graphene/graphite is originally added into the mixture in the form of graphite powder. Then, the relative portions of the graphite can be very high in the beginning but decrease during the manufacture as part of the graphite disintegrates as graphene. In the final product, the relative portion of the graphene can thus be higher. In other words, the relative portions of the graphite and graphene can vary during the manufacture, at least in embodiments that apply sonication or promote exfoliation in the mixture itself. The relative weight portion of graphene/graphite in the final product is for example 5/95 - 95/5, particularly 20/80 - 80/20.

It is assumed that it is beneficial to have the graphene/graphite mostly, or at least partly, in the form of small platelets, such as platelets having thicknesses in the order of nanometres, or at the most few micrometers. According to an embodiment, the portion of platelets having their thickness less than 100 nm can be, for example at least 5 %, such as at least 15 % or at least 50 % of the total weight of the graphene/graphite. According to another embodiment, the thickness of less than 100 nm referred to above is replaced by a thickness of less than 10 nm.

According to an embodiment, the method comprises first preparing the mixture containing water, cellulose and graphite powder and then sonicating the mixture to effect exfoliation of graphene from surfaces of particles of graphite powder.

According to an embodiment, the method comprises applying the mixture on a surface and allowing to dry, wherein the composite forms a coating on the surface. Drying may also be facilitated by filtration, hot-pressing or warming the film otherwise. According to an embodiment, the method comprises forming the mixture into a pulp and removing the water in a paper-making process, wherein the composite is a wood- based fiber product.

According to an embodiment, a composite material is formed that comprises cellulose fibers and particles of graphite and/or graphene dispersed in the material. A special characteristic of the material is that at least a portion of the cellulose fibers are attached to each other such that said particles of graphite and/or graphene are used as binder. The particles of graphite and/or graphene makes the adhesion between the cellulose fibers stronger and therefore the resultant composite material is stronger and stiffer than without the graphite/graphene binder.

According to an embodiment, at least a portion of the cellulose fibers in the composite material are nanocellulose fibrils.

According to an embodiment, the portion of the nanocellulose fibers from the cellulose fibers in the composite material is between 0.1-100 %, such as 1-100%, for example 5- 100 % in weight.

According to an embodiment, the weight of the graphite and graphene is 0.005-10 %, such as 0.1-5 %, for example 0.2-4 %, of the weight of the cellulose in the composite material.

According to an embodiment, the composite material is used in the form of a coating. Thus, a new kind of a coating can be provided that has the properties described above.

According to another embodiment, the composite material is used to make a package. Then, the package is made of, or comprises as a part thereof, the composite material that has the properties described above.

According to a further embodiment, the composite material is used in construction elements. Then, the construction element is made of, or comprises as a part thereof, the composite material that has the properties described above.

According to an embodiment, there is provided also a paper product that is made of, or comprises as a part thereof, the composite material that has the properties described above. As is apparent from the above, the products according to the embodiments are applicable not only as separate composite structures and products but also as coatings on surfaces and reinforcing layers or parts in different kinds of products that also comprise other materials. Therefore, also the mixture itself has individual utility. Therefore, according to an embodiment, there is provided a mixture for making a composite material or a coating. The mixture comprises water, cellulose and particles of graphite and/or graphene such that the weight of the graphite and graphene is 0.005-10 %, such as 0.1-5 %, for example 0.2-4 %, of the weight of the cellulose.

According to an embodiment, at least a portion of the cellulose in the mixture comprises nanocellulose.

According to a further embodiment of the mixture, the portion of the nanocellulose fibrils from the cellulose fibers is between 0.1-100 %, such as 1-100%, for example 5- 100 % in weight.

According to an embodiment, the graphene/graphite in the mixture is in the form of substantially planar flakes, sometimes called also as platelets.

According to a further embodiment, the planar graphene/graphite flakes or platelets have thicknesses less than 1 μιη, preferably less than 100 nm, such as less than 10 nm. According to further embodiments, the portion of such platelets having their thickness separately within each of the above ranges can be, for example at least 5 %, such as at least 15 % or at least 50 % of the total weight of the graphene/graphite.

According to an embodiment, the cellulose used in the above methods, products and mixtures is chemically unmodified cellulose.

According to another embodiment, the cellulose used in the above methods, products and mixtures is non-oxidized and non-functionalized cellulose. According to a further embodiment, the cellulose used in the above methods, products and mixtures is chemically unmodified nanocellulose.

According to an even further embodiment, the cellulose used in the above methods, products and mixtures is non-oxidized and non-functionalized nanocellulose. According to an embodiment, at least 50 wt-% of the cellulose used in the above methods, products and mixtures is chemically unmodified cellulose.

According to another embodiment, at least 50 wt-% of the cellulose the cellulose used in the above methods, products and mixtures is non-oxidized and non-functionalized cellulose.

According to a further embodiment, at least 50 wt-% of the cellulose used in the above methods, products and mixtures is chemically unmodified nanocellulose.

According to an even further embodiment, at least 50 wt-% of the cellulose used in the above methods, products and mixtures is non-oxidized and non-functionalized nanocellulose.

Examples

Preparation of nanocellulose

Nanocellulose, in this example microfibrillated cellulose (MFC), was obtained from UPM-Kymmene Corporation as a dilute hydrogel (solids content 1.9 %). The sample was prepared by mechanical disintegration of bleached birch pulp by ten passes through a M7115 Fluidizer (Micro fluidics M. Paakko, M. Ankerfors, H. Kosonen, A. Nykanen, S. Ahola, M. Osterberg, J. Ruokolainen, J. Laine, P. T. Larsson, O. Ikkala, T. Lindstrom, Biomacromolecules 2007 8, 1934-1941 Corp.) according to previous reports. Composite preparation

An appropriate amount of Kish graphite powder/flakes was weighted and mixed with an aqueous suspension of micro fibrillated cellulose (MFC). Dispersion of graphite and MFC was sonicated with a tip sonicator (Vibra-Cell VCX 750, Sonics & Materials Inc.) with a 450 W nominal power until a homogenous dispersion of MFC and graphite and/or graphene was created, for example until the energy directed to the sample was 33 kJ. Water was removed from the dispersion by vacuum filtration through a filter membrane. The resulting free-standing film of MFC and graphite and/or graphene was compressed with a gentle weight and dried in an oven (+ 65°C) at least for 16 hours. Examples of resulting films are shown in Fig. 1. Fig. 1A shows a MFC/graphene film containing 1 wt-% of graphene/graphite, Fig. IB shows a corresponding film with 5 wt- % graphene/graphite content, and in the film od Fig. 1C the graphene/graphite content is 29 wt-%.

Characterisation Transmission electron microscopy (TEM): Graphene/MFC dispersions were characterized with JEOLS JEM-3200FSC Cryo -Transmission Electron Microscope. Specimens were vitrified for cryo-imaging using vitrobot (FEI). An example of a thin, a few layers thick graphene flake exfoliated into the MFC dispersion is shown in Fig. 2.

Raman microscopy: Composite films were characterized by confocal Raman microscopy (WITec Alpha 300 RA) with a 532 nm laser. Graphene and thin graphite flakes were observed on the surfaces of the films. A typical Raman spectrum measured from a surface of a composite is shown in Fig. 3. Fig. 3 A is an optical microscope image from a composite surface. The size of the red rectangle is 10 μιη x 10 μιη. Fig. 3B is a Raman spectrum showing the characteristic bands of a few layered graphene flake, deviating from those of bulk graphite, measured at the indicated spot.

Tensile tests

Mechanical tests were carried out on a minitester (Deben) equipped with a 20 N load cell. All measurements were conducted at room temperature and an ambient humidity. The specimen sizes used were typically in the range of 2 cm x 3 mm x 4-6 μιη. The cross section widths and thicknesses were determined by optical microscopy and scanning electron microscopy, respectively.

Fig. 4A shows Young's moduli of the specimens with different graphene/graphite contents. The graphene/graphite contents of the specimens are expressed as the ratio of the weight of graphene/graphite to the weight of the cellulose. The measured Young's moduli are shown at contents of 0.00 %, 0.63 %, 1.25 %, 2.50 %, 5 %, 25 % and 50 %. Based on this example, it appears that best improvements in Young's modulus can be attained at graphene/graphite contents between approximately 0.7-3 %, more particularly between 0.8-2 % and especially at about or exactly 1.25 %, when using nanocellulose and graphene/graphite as prepared above. Fig. 4B shows ultimate tensile strengths of the specimens with different graphene/graphite contents corresponding to those of Fig. 4A. Based on this example, it appears that best improvements in the ultimate tensile strength can be attained at graphene/graphite contents between approximately 1-2 %, and especially at about or exactly 1.25 %, when using nanocellulose and graphene/graphite as prepared above.

Fig. 4C shows ultimate strains of the specimens with different graphene/graphite contents corresponding to those of Fig. 4A. This example indicates that, even though the strength increased, the ultimate strain did not substantially decrease when using nanocellulose and graphene/graphite as prepared above. Fig. 4D shows works of fracture of the specimens with different graphene/graphite contents corresponding to those of Fig. 4A. Based on this example, it appears that best improvements in the work of fracture can be attained at graphene/graphite contents between approximately 1-2 %, and especially at about or exactly 1.25 %, when using nanocellulose and graphene/graphite as prepared above. Fig. 4E shows yield strengths of the specimens with different graphene/graphite contents corresponding to those of Fig. 4A. Based on this example, it appears that best improvements in the yield strength can be attained at graphene/graphite contents between approximately 1-2 %, and especially at about or exactly 1.25 %, when using nanocellulose and graphene/graphite as prepared above. Fig. 5 depicts measurements of a nanocellulose film without graphene and a film containing 2.5 w-% of graphene. Fig. 5 shows reinforcement of nanocellulose film (grey) by addition of graphene (black). Stiffness and strength of the material increases significantly by the addition of graphene.

Based on the above described examples, it appears the particularly preferred graphene/graphite contents are those between 1-2 %, more specifically between 1.0-2.0

%, and preferably between 1.0-1.5 % and even more preferably between 1.1-1.3 %.

However, it is assumed that the optimal range is related to the several factors, such as size and shape of the graphene/graphite flakes and the quality of the cellulose.

Therefore, we expect that practically applicable ranges are also found that are remote to those presented above. Therefore, we present that generally the weight of the graphite and graphene can be 0.005-10 %, such as 0.1-5 %, for example 0.2-4 %, of the weight of the cellulose. However, when more narrow ranges are desired, the ranges 1.0-2.0 %, 1.0-1.5 % and 1.1-1.3 % can be used at least as starting points.

Exfoliation of graphene in nanocellulose matrix

Fig. 6A, Fig. 6B and Fig. 6C describe results of another experiment. In the experiment, exfoliation of graphene was observed as appearance of brown colour to the nanocellulose/graphene dispersions after exfoliation. This was followed by measuring transmission of light by the solutions. The results are shown in Fig. 6A. The energy used in the sonication varied from 1.65 to 33 kJ (from top to bottom curve). NFC concentration was 2mg/ml and initial amount of graphite 0.4 mg/ml. The solutions were diluted ten times before measurement. Fig. 6B shows the corresponding transmission values at 660 nm as a function of the exfoliation energy. Fig. 6C is a picture of the graphene/NFC solution after 33 kJ exfoliation. The solution was diluted ten times.

Fig. 7A is a TEM image of the solution containing graphene flakes in the nanocellulose matrix. Fig. 7B shows a hexagonal electron diffraction (ED) pattern measured from the graphene flake. The pattern indicates to a crystalline few-layer graphene flake.

The above description is only to exemplify the invention and is not intended to limit the scope of protection offered by the claims. The claims are also intended to cover the equivalents thereof and not to be construed literally.

Claims

Claims:

1. A method of preparing a composite material, comprising: preparing a mixture containing water and cellulose, and at least one of graphite and graphene as an additive; and removing water to form the composite.

2. The method of claim 1, comprising homogenizing the mixture.

3. The method of claim 1 or 2, wherein at least a portion of the cellulose comprises nanocellulose.

4. The method according to any one of claims 1-3, wherein the weight of the graphite and graphene is 0.005-10 %, such as 0.1-5 %, for example 0.2-4 %, of the weight of the cellulose.

5. The method of claim 4, wherein the mixture contains graphene in the form of substantially planar flakes.

6. The method of claims 4 or 5, wherein at least 5 %, such as at least 15 % or at least 50 % of the total weight of the graphene and graphite is in the form of flakes with their thicknesses less than 100 nm.

7. The method according to any one of claims 1-6, comprising: first preparing a mixture containing water, cellulose and graphite powder; and then sonicating the mixture to effect exfoliation of graphene from surfaces of particles of graphite powder.

8. The method according to any one of claims 1-7, wherein at least 50 wt-% of the cellulose is native cellulose.

9. The method according to any one of claims 1-8, wherein at least 50 wt-% of the cellulose is chemically unmodified cellulose.

10. A composite material, comprising cellulose and particles of graphite and/or graphene dispersed in the material, wherein at least a portion of the cellulose is attached together such that said particles of graphite and/or graphene are used as binder.

11. The composite material of claim 10, wherein at least a portion of the cellulose comprises nanocellulose fibrils.

12. The composite material of claim 11, wherein the portion of the nanocellulose fibrils from the cellulose is between 0.1-100 %, such as 1-100%, for example 5-100 % in weight.

13. The composite material according to any one of claims 10-12, wherein the weight of the graphite and graphene is 0.005-10 %, such as 0.1-5 %, for example 0.2-4 %, of the weight of the cellulose.

14. The composite material of claim 13, wherein the mixture contains graphene in the form of substantially planar flakes.

15. The composite material of claim 13 or 14, wherein at least 5 %, such as at least 15 % or at least 50 % of the total weight of the graphene and graphite is in the form of flakes with their thicknesses less than 100 nm.

16. The composite material according to any one of claims 10-15, wherein at least 50 wt-% of the cellulose is native cellulose.

17. The composite material according to any one of claims 10-16, wherein at least 50 wt-% of the cellulose is chemically unmodified cellulose.

18. A mixture for making a composite material or a coating, the mixture comprising water, cellulose and particles of graphite and/or graphene, wherein the weight of the graphite and graphene is 0.005-10 %>, such as 0.1-5 %>, for example 0.2-4 %>, of the weight of the cellulose.

19. The mixture of claim 18, wherein at least a portion of the cellulose comprises nanocellulose.

20. The mixture of claim 19, wherein the portion of the nanocellulose from the cellulose is between 0.1-100 %>, such as 1—100%, for example 5-100 %> in weight.

21. The mixture according to any one of claims 18-20, wherein the mixture contains graphene is in the form of substantially planar flakes.

22. The mixture according to any one of claims 18-21, wherein at least 5 %, such as at least 15 % or at least 50 % of the total weight of the graphene/graphite is in the form of flakes with their thicknesses less than lOOnm

23. The mixture according to any one of claims 18-22, wherein at least 50 wt-% of the cellulose is native cellulose.

24. The mixture according to any one of claims 18-23, wherein at least 50 wt-% of the cellulose is chemically unmodified cellulose.