Method for producing conductive graphene-based inks and products thereof

Abstract

The present invention relates to a method for preparing a graphene-based conductive ink for additive manufacturing, said method being based on graphite exfoliation. The invention also relates to a graphene-based conductive ink for additive manufacturing, to a method for preparing a substrate coated with said conductive ink and to the resulting coated substrate.

Description

Technical Field

[0001] The present invention relates to a method for preparing a graphene-based conductive ink for additive manufacturing, the method being based on graphite exfoliation. The present invention also relates to a graphene-based conductive ink for additive manufacturing, a method for preparing a substrate coated with the conductive ink, and the resulting coated substrate. Background Art

[0002] Graphene is a material composed of pure carbon in which the carbon atoms are arranged in a flat monolayer with a thickness of one atom and in which the carbon atoms are in the sp 2 hybrid state. This atomic arrangement provides a series of interconnected orbitals suitable for conducting electrons in a very efficient manner. Graphene is also a lightweight material with good mechanical properties in terms of mechanical resistance. Graphene is therefore of great interest as a material component for applications in conductive materials, lightweight materials and / or mechanically resistant materials. In particular, graphene is considered a promising material for the formulation of conductive inks used in advanced manufacturing methods. This would allow a reduction in the use of metals in conductive structures such as printed circuits and microelectronic circuits. For economic, practical and environmental reasons, it is attractive to replace the metallic conductive parts in such products with graphene-based materials. Graphene, in turn, has a crystal structure that can be viewed as stacked layers of graphene, where the layers are connected via sp in each layer. 2 The carbon atoms form orbitals that interact with each other through electronic interactions. While graphite is abundant and readily available, it does not benefit from the same mechanical and electronic properties as graphene.

[0003] Methods for preparing graphene are known in the art. For example, graphite can be exfoliated by applying tape to the graphite and pulling it apart to remove a single layer of graphene. Chemical vapor deposition is also known in the art for the production of graphene. Although providing high-quality graphene in terms of the thickness of the graphene layer produced, these methods are poorly scalable on an industrial scale. Other methods have been developed, such as chemical exfoliation of graphite. This method has the advantage of being scalable, but at the expense of a reduced quality of the graphene obtained compared to the previously mentioned processes. In particular, when the exfoliation process allows the production of a material with reduced thickness, it is believed to provide high-quality graphene, thereby benefiting from high electrical conductivity. An alternative method is to reduce graphene oxide. However, this method does not achieve sufficient electrical conductivity when graphene oxide is chemically reduced.

[0004] Chemical graphite exfoliation methods have been reported in the art. Typically, these methods are performed in a liquid phase consisting of an organic solvent or water and an interfacially active material, also called an exfoliant, adapted to be intercalated between graphene layers in the graphite and to interact with the layers via non-covalent interactions such as hydrogen bonding, anionic-interactions, cationic-interactions or hydrophobic interactions. These interactions act synergistically to extract a layer of graphene from the graphite material. Such methods typically allow the formation of graphene nanosheets or nanoflakes with reduced graphite to graphene conversion.

[0005] For example, patent application US2015 / 0072162 discloses a method for exfoliating graphite using ethyl cellulose as an exfoliating agent, with a weight ratio of 1 to 2 grams of ethyl cellulose to 5 grams of graphite and using an ethyl cellulose concentration of 1 weight / volume % to 2 weight / volume %. The disclosed method allows the production of inks containing low to medium viscosity graphene after several iterative solvent exchange steps involving aqueous solvents. After the graphene powder is separated from the exfoliation process, the ink for inkjet printing is prepared by dissolving the powder in an organic solvent. After 10 inkjet printing cycles and high temperature annealing, a graphite with a thickness of about 140 nm and a conductivity of about 2.5×10 4 According to the authors, this performance level is achieved due to the decomposition of ethylcellulose into aromatic substances during the annealing process.

[0006] Carrasco et al. reported in Carbon 70, 2014, pp. 157-163, a graphite exfoliation process using cellulose nanocrystals as an exfoliating agent. The developed process used a weight ratio of cellulose nanocrystals to graphite ranging from 1:1 to 1:20, with optimal exfoliation yields obtained in the range of 1:1 to 1:4. The disclosed method allowed the preparation of aqueous dispersions of graphene sheets with concentrations above 1 mg / mL of graphene sheets. However, the authors did not indicate the use of cellulose nanocrystals in the preparation of inks for advanced manufacturing technologies.

[0007] Ferreira and co-workers also disclosed a graphite exfoliation method using cellulose as an exfoliating agent in Nanoscale, September 2017, pp. 10219-10226. An alkaline aqueous solution of cellulose was used as a graphite exfoliation medium, with a cellulose to graphite weight ratio of 2:5 to 5:2. In this case, cellulose was used as an adhesive for the cellulose substrate. Thus, the authors reported the preparation of paper-based electrodes coated with graphene obtained by graphite exfoliation.

[0008] It is also known in the art that nanocellulose is able to stabilize solutions or dispersions of carbonaceous materials. In this regard, patent application PCT / EP2021 / 054774 discloses a cellulose composite composition, which in particular comprises cellulose nanofibers obtained by enzymatic digestion of filter paper and reduced graphene oxide, thereby providing a hybrid material suitable for biocompatible devices. The authors disclose in particular a paper composite material with a conductivity of up to 25 S / m. The material is prepared by reducing graphene oxide embedded in a nanocellulose-based paper.

[0009] Liu and colleagues reported a method for exfoliation of graphite oxide in Composites Part B 225 (2021) 109250, which includes the following steps: (i) preparing low-oxidation expanded graphite (LOEG) from graphite; (ii) mixing LOEG with microfibrillated cellulose slurry in an alkaline medium; (iii) grinding the mixture of step (ii) to provide a low-oxidation graphene-nanocellulose hybrid material in a suspension. In this material, the mass ratio of graphite to microfibrillated cellulose is 2:1. A membrane is prepared from the resulting suspension by evaporating the supernatant. The best performing membrane reportedly has an in-plane conductivity of up to 5800 S / m. However, the disclosed method suffers from poor scalability due to the requirement for a ball milling step.

[0010] Xu and colleagues in Nanoscale, November 2019, pp. 11719-11729 disclose a method for exfoliating graphite using cellulose nanofibers, the method comprising the step of mixing a mixture of graphite and cellulose nanofibers at a weight ratio of 1 gram of cellulose nanofibers to 32.8 grams of graphite, the cellulose nanofibers being at a concentration of 1.5 grams per liter of water. In particular, the mixture does not contain any additional binder. However, the material prepared from this mixture is described as a poor conductor of electricity, and the disclosed method is described as unsuitable for preparing conductive inks. The graphene sheets obtained are mainly formed of multiple graphene layers (about 50% are 3 layers and 47% are multiple layers). These materials are disclosed as being suitable for forming nanopaper that can be used as moisture-responsive actuators.

[0011] According to the disclosure in the art, there is still a need to provide improved graphene-based conductive inks suitable for advanced manufacturing and methods for preparing the same, as well as uses in preparing conductive coated substrates. Summary of the invention

[0012] After exhaustive research, the inventors have developed a method for preparing a substrate coated with a graphene-based conductor, the method comprising a step of reducing the thickness of a coating on the substrate, the coating resulting from a coating step using an ink comprising graphite stripped with a stripper and a binder, thereby forming a film of the ink. The inventors have found that the combination of a stripper and a binder as described herein in the graphite-based ink according to the present invention achieves a two-step graphite stripping method. The first graphite stripping step is promoted by the stripper in the solution, resulting in the formation of graphene nanosheets. As is known in the art, the graphene nanosheets are materials with poor conductivity. It is believed that the above-mentioned thickness reduction step of the coating comprising the ink promotes the second stripping step of the graphene nanosheets formed in the solution, thereby improving the conductivity of the film. Once the ink of the present invention is transferred to the surface of the substrate, in particular when the thickness of the resulting coating is reduced in the compression step, the second stripping step occurs. During this second stripping step, the binder contained in the ink composition is necessary to maintain good adhesion of the coating composition to the substrate. In addition, it is believed that the solvent, stripping agent and adhesive interact with each other and with the graphene nanosheets at the supramolecular level to provide a viscous ink composition with sufficient viscosity for application in advanced manufacturing technologies. Therefore, the method of the present invention requires the use of an ink comprising exfoliated graphite and an adhesive, wherein the graphite is exfoliated by the intervention of an exfoliating agent, the exfoliating agent being selected from the group consisting of: lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins and mixtures thereof. The amount of the exfoliating agent, more particularly, the ratio of the exfoliating agent to graphite is an essential feature of the present invention, because on the one hand, too much exfoliating agent can lead to a reduction in the conductivity of the final material, and on the other hand, too little exfoliating agent can lead to inefficient exfoliation of graphite, resulting in poor conductivity. The exfoliating agent used in the present invention advantageously further allows the graphene nanosheets and nanosheets to be stabilized in the solution formed by exfoliation of graphite. In addition, the exfoliating agent further enhances the adhesion of the ink to the substrate. The binder is also an essential aspect of the present invention because when the ink is deposited on the substrate, the binder ensures the adhesion of the ink to the substrate during the second exfoliation step associated with the shear between the graphene flakes formed by the first exfoliation induced by the exfoliator. A further reduction in the thickness of the coating formed by the deposited ink provides a highly conductive coated substrate.

[0013] Thus, in a first aspect, the present invention relates to a method for producing an ink for forming a conductive coating, comprising the following steps:

[0014] (i) providing a mixture comprising graphite, an exfoliating agent and a first polar solvent, wherein:

[0015] - a stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins and mixtures thereof; and

[0016] - the weight ratio of the stripping agent to the graphite is in the range of 1:100 to 1:20;

[0017] (ii) homogenizing the mixture provided in step (i);

[0018] (iii) adding a binder different from the release agent to the mixture obtained from step (ii), wherein the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H; and wherein the amount of the binder is such that the weight ratio of the binder to the release agent is in the range of 4:1 to 1:2;

[0019] (iv) adding a second polar protic solvent to the mixture obtained from step (iii), wherein the boiling point of the second polar protic solvent is equal to or greater than the first polar solvent provided in step (i); and

[0020] (v) removing substantially all of the first polar solvent provided in step (i) while retaining substantially all of the second polar protic solvent added in step (iv).

[0021] Therefore, the second aspect of the present invention relates to an ink composition for forming a conductive coating, comprising:

[0022] - solvents,

[0023] - Carbonaceous materials consisting of one or more of graphene, exfoliated graphite and mixtures thereof with graphite;

[0024] - a stripping agent selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins, and mixtures thereof; and

[0025] a binder selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H,

[0026] wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2.

[0027] The third aspect of the present invention relates to an ink composition obtained by the method according to the first aspect of the present invention.

[0028] As mentioned above, the inks of the second and third aspects of the present invention can be used to prepare conductive coatings. Therefore, a fourth aspect of the present invention relates to a method for preparing a conductive coated substrate, comprising the following steps:

[0029] (i) providing a substrate for receiving a conductive ink for coating;

[0030] (ii) providing an ink composition as defined in the second or third aspect of the present invention;

[0031] (iii) depositing the ink provided in step (ii) on the substrate provided in step (i);

[0032] (iv) drying the ink deposited in step (iii) to form a film of the dried ink;

[0033] (v) reducing the thickness of the film obtained from step (iv) until the film thickness is less than 50% of the thickness of the film obtained from step (iv), and optionally, repeating steps (iii) to (v).

[0034] The fifth aspect of the present invention relates to a coated substrate composition obtained by the method according to the fourth aspect of the present invention.

[0035] A sixth aspect of the present invention relates to a coated substrate, wherein the coating comprises a carbonaceous material, a stripping agent and a binder, wherein the carbonaceous material is composed of one or more of graphene, exfoliated graphite and a mixture thereof with graphite; the stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannin and a mixture thereof; the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide. The water-soluble polysaccharide has a degree of polymerization higher than 5 and is optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6) alkyl, (C1-C6) alkylcarbonyl and (C1-C6) alkyl-CO2H, wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 A method according to the present invention is described, comprising the following steps: (A) providing a mixture comprising graphite (2), an exfoliating agent (1) and a first polar solvent; step (a) homogenizing the mixture to produce exfoliated graphite (3); step (b) adding a second polar protic solvent and a binder, followed by evaporating the first solvent in large quantities to produce ink (4); (B) transferring the ink (4) comprising the exfoliated graphite (3) to a substrate; step (c) producing a film (6) on the substrate with the deposited ink by shearing with a spatula or a scraper (5) followed by drying; and (C) a step of compressing the produced film (6) to form a conductive coating (7).

[0037] Figure 2 Depicted is the concentration of graphene and / or exfoliated graphite as measured by UV-Vis spectroscopy at 660 nm depending on the exfoliator concentration in a mixture comprising graphite and (upper panel) lignin sulfonate as exfoliator or (lower panel) cellulose nanocrystals (square symbols) as exfoliator or carboxymethyl cellulose as exfoliator (circular symbols, comparative example).

[0038] Figure 3 Described are (on the left) an ink according to the second or third aspect of the present invention (ink E) and (on the right) a substrate made of paper, which is coated with ink E according to the second or third aspect of the present invention by screen printing according to method B described below.

[0039] Figure 4Described are (top) images of a paper substrate coated with ink C according to the present invention before and after a step of reducing the thickness of the film formed in the method of the fourth aspect of the present invention, and (bottom) images made by microscopic examination of a paper substrate coated with ink C according to the present invention before and after a step of reducing the thickness of the film formed in the method of the fourth aspect of the present invention.

[0040] Figure 5 The surface resistivity (expressed in ohm per square) of a film formed according to the method of the fourth aspect of the present invention before and after a thickness reduction step (right y-axis, undashed bars), and the thickness (expressed in μm) of a film formed according to the method of the fourth aspect of the present invention before and after a thickness reduction step (left y-axis, dashed bars) are depicted using ink A as described below.

[0041] Figure 6 Depicted is the evolution of the surface resistivity (expressed in ohm per square) of a piece of paper coated with an ink according to the fourth aspect of the invention, wherein the ink was prepared using cellulose nanocrystals obtained from enzymatic digestion of filter paper as a stripping agent (Ink E, square symbols) or the ink was prepared using commercial cellulose nanocrystals as a stripping agent (Ink C, circle symbols).

[0042] Figure 7 Depicted is A) the evolution of the surface resistivity (expressed in ohms per square) of a piece of paper coated with an ink according to the fourth aspect of the invention, wherein the ink was prepared after a printing pass using cellulose nanocrystals obtained by enzymatic digestion of filter paper as a stripping agent (Ink E), said resistivity measured in contact with a surface (white squares) or after an indentation process (black squares); the grey circles provide similar data for aluminum for comparison purposes; B) the evolution of the surface resistivity (expressed in ohms per square) of a piece of paper coated with an ink according to the fourth aspect of the invention, wherein the ink was prepared using cellulose nanocrystals obtained by enzymatic digestion of filter paper as a stripping agent (Ink E), said resistivity measured in contact with a surface at different current values ​​(a: 10 mA, b: 20 mA, c: 70 mA), said resistivity measured in contact with a surface. The data for aluminum are also provided for comparison purposes.

[0043] Figure 8 Depicted are graphs of A) an ink according to the second or third aspect of the invention but not comprising any binder, which ink is the blank ink in Table 1 deposited on a piece of paper (Comparative Example), and B) a graph of Ink E (Comparative Example) deposited on a piece of paper.

[0044] Fig. 9A diagram depicting a coated substrate according to the fifth aspect of the invention, wherein the substrate is paper (left) or plastic (polyethylene terephthalate, right), and the ink is Ink C. DETAILED DESCRIPTION

[0045] Unless otherwise specified, all terms used in this application should be understood as having ordinary meanings known in the art. Other more specific definitions of certain terms used in this application are described below and are intended to be uniformly applied throughout the specification and claims, unless otherwise expressly provided.

[0046] For the purpose of the present invention, any range provided includes the lower and upper endpoints of the range. Unless otherwise specified, the ranges provided, such as temperature, time, molar ratio, volume ratio, etc., should be regarded as approximate values ​​(i.e., there is a 5% variation range near the indicated point).

[0047] The term "polysaccharide" refers to a molecule comprising multiple repeating units, which are derived from monosaccharide building blocks and are linked by glycosidic bonds. The number of monosaccharide repeating units contained in the polysaccharide compound is also referred to as "degree of polymerization" in this article. Examples of polysaccharides include arabinoxylan, callose, cellulose, hemicellulose, chitin, chitosan, alginic acid, alginates, pectin and agarose. In the context of the present invention, it is contemplated that the polysaccharide compound may be functionalized with one or more functional groups, such as (C1-C6) alkyl, (C1-C6) alkylcarbonyl and (C1-C6) alkyl-CO2H.

[0048] The term cellulose refers to a homopolymer of β-glucose.

[0049] The three main types of nanocellulose are cellulose nanofibers, cellulose nanocrystals, and bacterial cellulose. Bacterial cellulose can be the result of enzymatic digestion of paper, for example using ancestral endoglucanases, e.g. et al., Comm.Chem.2:76, 2019 (pp. 10-11) and Alonso-Lerma et al., Commun.Mater.1:57, 2020, which are incorporated herein by reference. Bacterial cellulose may consist of a mixture of cellulose nanofibers and cellulose nanocrystals. In the context of the present invention, unless otherwise specified, nanocellulose is a general term including cellulose nanofibers, cellulose nanocrystals, or a mixture of the two.

[0050] However, in the literature, the terms nanofibrillated cellulose, nanofibrillated cellulose, nanofibrillated cellulose, nanofibrils, nanofibrils, cellulose nanofibrils and cellulose nanofibers seem to be used interchangeably, including as synonyms for microfibrillated cellulose, microfibrils and microfibrils (MFC). Cellulose nanofibers are thin fibers longer than cellulose nanocrystals, and can reach several microns in length. In the context of the present invention, the term "cellulose nanofiber" refers to a fiber of cellulose with a thickness or diameter in the nanometer range (1 to 1000nm). Typically, the length of cellulose nanofibers is 1 μm to 10 μm, and the diameter (also referred to as width) is 5nm to 60nm. Cellulose nanofibers differ from cellulose nanocrystals at least in the aspect ratio (length / diameter).

[0051] Cellulose nanocrystals (cellulose nanocrystals, also called nanowhiskers) are needle-shaped crystalline particles with a length of hundreds of nanometers, depending on their source and separation treatment. Due to their crystalline structure, cellulose nanocrystals have higher mechanical stability than cellulose nanofibers. Cellulose nanocrystals have high surface area and tensile strength, comparable to other materials such as Kevlar or carbon nanotubes. In the context of the present invention, the term "cellulose nanocrystals" refers to particles of cellulose with a thickness or diameter and length (for non-spherical particles) in the nanometer range (i.e., 1 to 1000 nm). More particularly, the average diameter (also called width) of cellulose nanocrystals is between 2-60 nm and a length of 10 nm to 2 μm, depending on the cellulose source and separation treatment. The diameter and length of cellulose nanocrystals and / or cellulose nanofibers can be calculated using any suitable technique known to those skilled in the art, such as by atomic force microscopy (AFM).

[0052] As used herein, the term "chitin" refers to a β(1-4) polymer of N-acetyl-D-glucosamine. Chitin is a linear, highly crystalline homopolymer of β-1,4 N-acetylglucosamine (GlcNAc) composed of β-1,4 linked N-acetylglucosamine residues arranged in antiparallel (α), parallel (β) or mixed (γ, two parallel chains alternating with one antiparallel chain) chains, with the (α) configuration being the most abundant. The repeating unit of chitin is represented by the following formula:

[0053]

[0054] In the context of the present invention, the term "chitin nanofiber" refers to a fiber of chitin with a thickness or diameter in the nanometer range (1 to 1000nm). Typically, the length of chitin nanofibers is 1 μm to 10 μm, and the diameter (also referred to as width) is 5nm to 60nm. Chitin nanofibers differ from chitin nanocrystals at least in the aspect ratio (length / diameter). In the context of the present invention, the term "chitin nanocrystals" refers to particles of chitin with a thickness or diameter and length (for non-spherical particles) in the nanometer range (i.e. 1 to 1000nm). More particularly, the average diameter (also referred to as width) of chitin nanocrystals is between 2-60nm, and the length is 10nm to 2 μm, depending on the chitin source and separation process. The diameter and length of chitin nanocrystals and / or chitin nanofibers can be calculated using any suitable technique known to those skilled in the art, such as by atomic force microscopy (AFM). As reviewed by Lin et al., “Preparation, properties and applications of polysaccharide nanocrystals in advanced functional nanomaterials: a review” Nanoscale 4, pp. 3274-3294 (2012), nanocrystals and nanofibers of chitin can be obtained by enzymatic digestion of chitin.

[0055] In the context of the present invention, the term "lignin sulfonate" refers to sulfonated lignin compounds obtained from the production of wood pulp using sulfite pulping. Lignin sulfonates are water-soluble anionic polyelectrolyte polymers whose molecular formula contains repeating units, one fragment of which is

[0056]

[0057] Lignosulfonate compounds are well known in the art and are generally present in the form of salts with cations derived from alkali or alkaline earth metals.

[0058] In the context of the present invention, the term "hemicellulose" refers to a polysaccharide formed by at least two different sugar repeating units connected by bonds 1-4, such as xylose, arabinose, galactose, glucose, glucuronic acid and mannose. Typically, the length of hemicellulose nanofibers is 1 μm to 10 μm, and the diameter (also referred to as width) is 5nm to 60nm. Hemicellulose nanofibers differ from hemicellulose nanocrystals at least in terms of the length-width aspect ratio (length / diameter). In the context of the present invention, the term "hemicellulose nanocrystals" refers to particles of hemicellulose having a thickness or diameter and length (for non-spherical particles) in the nanometer range (i.e., 1 to 1000nm). More particularly, the average diameter (also referred to as width) of hemicellulose nanocrystals is between 2-60nm, and the length is 10nm to 2 μm, depending on the hemicellulose source and separation process. The diameter and length of hemicellulose nanocrystals and / or hemicellulose nanofibers can be calculated using any suitable technique known to those skilled in the art, such as by atomic force microscopy (AFM).

[0059] Arabinoxylan is an example of a hemicellulose compound and is a polysaccharide comprising a copolymer of arabinose and xylose.

[0060] As used herein, the term "chitosan" refers to a derivative of chitin obtained by deacetylation of solid chitin under alkaline conditions (such as concentrated NaOH) or enzymatic hydrolysis in the presence of chitin deacetylase. Chitosan is a linear polysaccharide consisting of randomly distributed β-(1-4) linked D-glucosamine (deacetylated units) and N-acetyl-D-glucosamine (acetylated units), characterized by its average molecular weight and degree of acetylation (the proportion of acetylated glucosamine units along the polymer backbone). Chitosan is represented by the following general formula:

[0061]

[0062] Chitosan derivatives according to the present invention include but are not limited to PEG-chitosan (copolymer), azidated chitosan, N-phthalyl chitosan, chitosan-C (6) -MPEG (copolymer), adipic acid chitosan, fumaric acid chitosan, lactic acid chitosan, acetic chitosan, hydrochloric acid chitosan, carboxymethyl chitosan, N-sulfonic acid-N, O-carboxymethyl chitosan, ascorbic acid chitosan, malic acid chitosan, glutamic acid chitosan, trimethyl chitosan (TMC), aryl chitosan, thiolated chitosan, N-succinyl-chitosan (Suc-Chi), thiosemicarbazide chitosan, N, O-carboxymethyl chitosan (NOCC) and hydroxypropoxylated chitosan (HPC), N-trimethylene chloride chitosan, phthalic acid chitosan, and trimethylammonium chitosan. Chitosan derivatives can be obtained from chitosan according to the present invention by methods known to those skilled in the art.

[0063] As used herein, the term "lignin" refers to a phenylpropane polymer of monolignol monomers (p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol) present as components of the secondary cell walls of plants and certain types of algae. Lignin may also refer to any lignin derivative, such as esters, amides, ethers, carboxylic acids and salts thereof, which are derived from natural products or modifications of naturally obtained lignin materials. The term "lignin" also refers to alkaline salts of the compound.

[0064] The term "tannin" is known in the art and refers to large polyphenolic compounds containing enough hydroxyl groups and other suitable groups (such as carboxyl groups) to form strong complexes with various macromolecules. Tannin can also refer to any tannin derivatives, such as esters, amides, ethers, carboxylic acids and salts thereof, which are derived from natural products or modifications of naturally obtained tannin materials. Exemplary genera of shrubs and / or trees from which suitable tannins can be derived can include, but are not limited to, Acacia, Chestnut, Vachellia, Catechu, Terminalia, Phyllanthus, Caesalpinia, Oak, Brocade, Hemlock, Rhus, Walnut, Hickory and Pine, any combination thereof or any mixture thereof. Suitable tannins include, for example, in a non-limiting manner, tannic acid and salts thereof, such as alkaline salts, with CAS number 1401-55-4.

[0065] Callose is a plant polysaccharide composed of glucose residues linked together by β-1,3 bonds, with the general formula

[0066]

[0067] Alginic acid is a linear copolymer in which homoblocks of (1→4) linked β-D-mannuronic acid (M) and α-L-guluronic acid (G) residues are covalently linked together in different sequences or blocks. The monomers can appear in homoblocks of continuous G-residues (G-blocks), homoblocks of continuous M-residues (M-blocks), or homoblocks of alternating M- and G-residues (MG-blocks). Thus, the general formula of alginic acid is:

[0068]

[0069] As known in the art, the term "pectin" refers to a heteropolysaccharide compound known in the art, particularly present in a main chain consisting of optionally esterified D-galacturonic acid in an α1-4 chain configuration, and optionally with other sugar compounds as side chains. The acid groups along the chain of D-galacturonic acid repeating units are esterified mainly with methoxy groups in natural products. Acetyl groups may also be present on the free hydroxyl groups.

[0070] As is known in the art, starch is a polysaccharide comprising glucose monomers joined in α-1,4 bonds. The simplest form of starch is a linear polymer amylose; amylopectin is a branched form. Typically, starch nanofibers have a length of 1 μm to 10 μm and a diameter (also called width) of 5 nm to 60 nm. Starch nanofibers differ from starch nanocrystals at least in terms of the aspect ratio (length / diameter). In the context of the present invention, the term "starch nanocrystals" refers to particles of starch having a thickness or diameter and a length (for non-spherical particles) in the nanometer range (i.e., 1 to 1000 nm). More particularly, the average diameter (also called width) of starch nanocrystals is between 2-60 nm and a length of 10 nm to 2 μm, depending on the starch source and separation process. The diameter and length of starch nanocrystals and / or starch nanofibers can be calculated using any suitable technique known to those skilled in the art, such as by atomic force microscopy (AFM). As reviewed by Lin et al., “Preparation, properties and applications of polysaccharide nanocrystals in advanced functional nanomaterials: a review,” Nanoscale 4, pp. 3274-3294 (2012), starch nanocrystals and nanofibers can be obtained by enzymatic digestion of starch.

[0071] As known in the art, the term "agarose" refers to a polysaccharide composed of repeating units of agarobiose, a disaccharide composed of D-galactose and 3,6-anhydride-L-galactopyranose, with the formula:

[0072]

[0073] In the context of the present invention, a mixture of components is considered to be "homogeneous" when the components forming the mixture are distributed in a substantially uniform manner throughout the mixture.

[0074] As mentioned above, the first aspect of the present invention relates to a method for producing an ink for forming a conductive coating, comprising the following steps:

[0075] (i) providing a mixture comprising graphite, an exfoliating agent and a first polar solvent, wherein:

[0076] - a stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins and mixtures thereof; and

[0077] - the weight ratio of the stripping agent to the graphite is in the range of 1:100 to 1:20;

[0078] (ii) homogenizing the mixture provided in step (i);

[0079] (iii) adding a binder different from the release agent to the mixture obtained from step (ii), wherein the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H; and wherein the amount of the binder is such that the weight ratio of the binder to the release agent is in the range of 4:1 to 1:2;

[0080] (iv) adding a second polar protic solvent to the mixture obtained from step (iii), wherein the boiling point of the second polar protic solvent is equal to or greater than the first polar solvent provided in step (i); and

[0081] (v) removing substantially all of the first polar solvent provided in step (i) while retaining substantially all of the second polar protic solvent added in step (iv).

[0082] In a preferred embodiment of the first aspect of the present invention, the first polar solvent of the mixture of step (i) is selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, acetone, methyl ethyl ketone, 3-pentanone, 2-pentanone and mixtures thereof.

[0083] In another preferred embodiment of the first aspect of the present invention, the first polar solvent of the mixture of step (i) is selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol and mixtures thereof.

[0084] In another more preferred embodiment of the first aspect of the present invention, the first polar solvent is a mixture of water and isopropanol; preferably in a volume ratio of 1:1.

[0085] In another preferred embodiment of the first aspect of the present invention, the graphite in the mixture of step (i) is selected from the group consisting of synthetic graphite powder, natural graphite flakes, natural graphite powder and mixtures thereof; preferably synthetic graphite powder.

[0086] In another preferred embodiment of the first aspect of the present invention, the particle size of the graphite in the mixture of step (i) is less than 30 μm.

[0087] In another preferred embodiment of the first aspect of the present invention, the exfoliating agent in the mixture of step (i) is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers and mixtures thereof.

[0088] In another preferred embodiment of the first aspect of the present invention, the exfoliating agent in the mixture of step (i) is selected from the group consisting of cellulose nanocrystals, cellulose nanofibers and mixtures thereof.

[0089] In another preferred embodiment of the first aspect of the present invention, the exfoliating agent in the mixture of step (i) is cellulose nanofibers or cellulose nanocrystals having an average length of 100 nm to 1500 nm and a diameter of 10 nm to 20 nm.

[0090] In another preferred embodiment of the first aspect of the present invention, the stripping agent in the mixture of step (i) is cellulose nanofibers or cellulose nanocrystals obtained by enzymatic digestion of filter paper; preferably obtained by digestion of filter paper mediated by one of endoglucanase, cellobiohydrolase and β-glucosidase optionally linked to a carbohydrate binding module; more preferably obtained by digestion of filter paper mediated by endoglucanase associated with the last common ancestor of Firmicutes, the common ancestor optionally linked to a carbohydrate binding module. Such enzymatic digestion methods are known in the art and have been disclosed in Alonso-Lerma, B., Barandiaran, L., Ugarte, L. et al., High performance crystalline nanocellulose using an ancestral endoglucanase, Commun Mater 1, 57 (2020), page 8 (the method section is incorporated herein by reference).

[0091] In another preferred embodiment of the first aspect of the present invention, the exfoliating agent in the mixture of step (i) is cellulose nanocrystals obtained by acid treatment of cellulose.

[0092] In another preferred embodiment of the first aspect of the present invention, the weight ratio of the exfoliant to the graphite is in the range of 1:90 to 1:30. In certain specific embodiments, the weight ratio of the exfoliant to the graphite is in the range of 1:40 to 1:30, preferably 1:32 to 1:30.

[0093] In another preferred embodiment of the first aspect of the present invention, in the mixture of step (i), the concentration of the stripping agent is 100 mg per liter of solvent to 10 grams per liter of solvent; preferably 1 gram per liter of solvent to 5 grams per liter of solvent. In certain embodiments, the concentration of the stripping agent is 2 grams per liter of solvent to 2.5 grams per liter of solvent.

[0094] In another preferred embodiment of the first aspect of the invention, step (ii) is performed by sonication using a gentle bath or tip, or by shear mixing the mixture of step (i).

[0095] In another preferred embodiment of the first aspect of the present invention, step (ii) is performed by sonication using a gentle bath.

[0096] In another preferred embodiment of the first aspect of the present invention, the binder of step (iii) is selected from the group consisting of polyaniline, arabinoxylan, callose, cellulose, chitin, chitosan, alginic acid, alkaline salts of alginate, pectin, agarose and mixtures thereof, wherein the available hydroxyl and amino groups in each of arabinoxylan, callose, cellulose, chitin, chitosan, pectin and agarose are optionally substituted by free radicals selected from the group consisting of: (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H.

[0097] In another preferred embodiment of the first aspect of the present invention, the binder of step (iii) is selected from the group consisting of polyaniline, cellulose, chitin, chitosan, alginic acid, alkaline salts of alginate, agarose and mixtures thereof, wherein the available hydroxyl and amino groups in each of cellulose, chitin, chitosan and agarose are optionally substituted by free radicals selected from the group consisting of: (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H.

[0098] In another preferred embodiment of the first aspect of the invention, the binder of step (iii) is selected from the group consisting of polyaniline and cellulose, wherein the available hydroxyl groups in the cellulose are optionally substituted by free radicals selected from the group consisting of: (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H.

[0099] In another preferred embodiment of the first aspect of the present invention, the binder of step (iii) is selected from the group consisting of carboxymethyl cellulose, polyaniline and mixtures thereof; preferably carboxymethyl cellulose. Carboxymethyl cellulose is cellulose in which at least one hydroxyl group of the repeating unit of cellulose is substituted by a -CH2CO2H group.

[0100] In another preferred embodiment of the first aspect of the present invention, the amount of adhesive in step (iii) is such that the weight ratio of adhesive to release agent is in the range of 2:1 to 1:2; preferably 3:2 to 2:3.

[0101] In another preferred embodiment of the first aspect of the present invention, the second polar solvent of step (iv) is selected from the group consisting of water, 1,2-propylene glycol, glycerol, diethylene glycol, 1,2-ethylene glycol, terpineol and mixtures thereof.

[0102] In another preferred embodiment of the first aspect of the present invention, the second polar solvent of step (iv) is 1,2-propylene glycol or water.

[0103] In another preferred embodiment of the first aspect of the present invention, the second polar solvent of step (iv) is 1,2-propylene glycol.

[0104] In another preferred embodiment of the first aspect of the invention, the second polar solvent of step (iv) is 1,2-propylene glycol in an amount such that the volume ratio of 1,2-propylene glycol to the first polar solvent provided in step (i) is from 1:5 to 1:10; preferably 1:8. This advantageously allows the production of inks having a high viscosity.

[0105] In another preferred embodiment of the first aspect of the present invention, the amount of the second polar solvent in step (iv) is such that the volume ratio of the second polar protic solvent to the first polar solvent provided in step (i) is 1:5 to 1:10.

[0106] In another preferred embodiment of the first aspect of the present invention, the second polar solvent of step (iv) and the first polar solvent of step (i) are the same solvent. In this embodiment, step (v) is a concentration step in such a manner that the volume of solvent remaining after the concentration step is 0.1 to 0.2 times the volume of solvent before step (v); preferably 0.125 times the volume of solvent before step (v).

[0107] In another preferred embodiment of the first aspect of the invention, step (v) is performed by heating and / or by applying a vacuum.

[0108] In another preferred embodiment of the first aspect of the present invention, step (v) is performed by heating at a temperature of 100°C.

[0109] As mentioned above, the second aspect of the present invention relates to an ink composition for forming a conductive coating, comprising:

[0110] - solvents,

[0111] - Carbonaceous materials consisting of one or more of graphene, exfoliated graphite and mixtures thereof with graphite;

[0112] - a stripping agent selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins, and mixtures thereof; and

[0113] a binder selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H,

[0114] wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2.

[0115] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises 85 wt% to 98 wt% of the carbonaceous material, wherein the weight percentages are expressed relative to the sum of the weights of the carbonaceous material, the release agent and the binder.

[0116] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises 90 wt% to 98 wt% of the carbonaceous material, wherein the weight percentage is expressed relative to the sum of the weights of the carbonaceous material, the release agent and the binder.

[0117] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises 92 wt% to 98 wt% of the carbonaceous material, wherein the weight percentage is expressed relative to the sum of the weights of the carbonaceous material, the release agent and the binder.

[0118] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises a solvent, which is a polar protic solvent; preferably, the solvent is a solvent S or an azeotropic mixture of the solvent S and a solvent selected from the group consisting of: water, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, acetone, methyl ethyl ketone, 3-pentanone, 2-pentanone and mixtures thereof, wherein S is selected from the group consisting of water, 1,2-propylene glycol, glycerol, diethylene glycol, 1,2-ethylene glycol, terpineol and mixtures thereof.

[0119] In a more preferred embodiment of the second aspect of the present invention, the ink composition comprises a solvent, which is a polar protic solvent; preferably, the solvent is a solvent S or an azeotropic mixture of the solvent S and a solvent selected from the group consisting of: water, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol and mixtures thereof, wherein S is selected from the group consisting of water, 1,2-propylene glycol, glycerol, diethylene glycol, 1,2-ethylene glycol, terpineol and mixtures thereof.

[0120] In a more preferred embodiment of the second aspect of the invention, the ink composition comprises a solvent, which is a polar protic solvent; preferably, the solvent is a solvent S or an azeotropic mixture of the solvent S and a solvent, wherein the solvent is a mixture of water and isopropanol, preferably in a volume ratio of 1:1; wherein S is selected from the group consisting of water, 1,2-propylene glycol, glycerol, diethylene glycol, 1,2-ethylene glycol, terpineol and mixtures thereof.

[0121] In a more preferred embodiment of the second aspect of the invention, the ink composition comprises a solvent, which is a polar protic solvent; preferably, the solvent is solvent S or an azeotropic mixture of the solvent S and a solvent, wherein the solvent is a mixture of water and isopropanol, preferably in a volume ratio of 1:1; wherein S is 1,2-propylene glycol.

[0122] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises a solvent which is 1,2-propylene glycol or an azeotropic mixture thereof with water and / or isopropyl alcohol.

[0123] In a preferred embodiment of the second aspect of the invention, the ink composition comprises a carbonaceous material consisting of a mixture of exfoliated graphite and graphite, the graphite being preferably synthetic graphite powder. The carbonaceous material of the ink composition of the second aspect of the invention is obtained by the effect of the exfoliating agent on the graphite suspension as defined in the first aspect of the invention. When completely exfoliated, the carbonaceous material of the second aspect of the invention may consist of graphene sheets, exfoliated graphite and mixtures thereof. When partially exfoliated, i.e. when all of the graphite is not exfoliated, the carbonaceous material of the second aspect of the invention may consist of a mixture of graphite and graphene sheets and / or exfoliated graphite.

[0124] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises a release agent as defined in any one of the preferred embodiments of the first aspect of the present invention.

[0125] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises a binder as defined in any one of the preferred embodiments of the first aspect of the present invention.

[0126] In a preferred embodiment of the second aspect of the present invention, the ink composition is an ink composition in which the weight ratio of the stripper to the graphite is in the range of 1:90 to 1:30. In certain specific embodiments, the weight ratio of the stripper to the graphite is in the range of 1:40 to 1:30, preferably 1:32 to 1:30.

[0127] In a preferred embodiment of the second aspect of the present invention, the ink composition is one in which the amount of binder is such that the weight ratio of binder to release agent is in the range of 2:1 to 1:2 (preferably 3:2 to 2:3).

[0128] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises 1 wt% to 4 wt% of the release agent, wherein the weight percentage is expressed relative to the sum of the weights of the carbonaceous material, the release agent and the binder.

[0129] In a preferred embodiment of the second aspect of the present invention, the ink composition comprises 1 wt% to 6 wt% of the binder, wherein the weight percentage is expressed relative to the sum of the weights of the carbonaceous material, the release agent and the binder.

[0130] In a preferred embodiment of the second aspect of the invention, the ink composition is an ink composition wherein the graphite concentration is between 40 and 1000 grams per liter of solvent (preferably 60 or 600 grams per liter of solvent). In a more preferred embodiment, the concentration is 600 grams per liter of solvent.

[0131] Therefore, in a preferred embodiment of the second aspect of the present invention, the ink composition is an ink composition having a kinematic viscosity between 300 Pa·s and 3000 Pa·s (preferably between 1000 Pa·s and 1500 Pa·s). This is advantageous because it allows the ink composition to be used in advanced manufacturing techniques such as screen printing, syringe extrusion and 3D printing.

[0132] In a more preferred embodiment of the second aspect of the invention, the ink composition comprises and preferably consists of 1,2-propylene glycol as a solvent and 93 wt% graphite, 1.7 wt% cellulose nanocrystals, 1.6 wt% polyaniline, 3.7 wt% carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent. This ink is further referred to herein as Ink A.

[0133] In a more preferred embodiment of the second aspect of the invention, the ink composition comprises and preferably consists of 1,2-propylene glycol as solvent and 94 wt% of graphite, 3 wt% of cellulose nanocrystals obtainable by acid treatment of filter paper, 3 wt% of carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent. This ink is further referred to herein as Ink C.

[0134] In a more preferred embodiment of the second aspect of the present invention, the ink composition comprises and preferably consists of 1,2-propylene glycol or water as a solvent and 94% by weight of graphite, 3% by weight of cellulose nanocrystals or nanofibers obtainable by enzymatic hydrolysis of filter paper, and 3% by weight of carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent. Methods for enzymatic hydrolysis of filter paper are known in the art and as defined in the first aspect of the present invention above. When the solvent is 1,2-propylene glycol, the ink is further referred to as ink E in this article. When the solvent is water, the ink is further referred to as ink H in this article.

[0135] In a more preferred embodiment of the second aspect of the invention, the ink composition comprises and preferably consists of 1,2-propylene glycol as solvent and 96.3 wt% of graphite, 1.8 wt% of cellulose nanocrystals or nanofibers obtainable by enzymatic hydrolysis of filter paper, 1.9 wt% of carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent. This ink is further referred to herein as Ink J.

[0136] In a more preferred embodiment of the second aspect of the invention, the ink composition comprises and preferably consists of 1,2-propylene glycol as solvent and 97.2 wt% of graphite, 1.2 wt% of cellulose nanocrystals or nanofibers obtainable by enzymatic hydrolysis of filter paper, 1.6 wt% of carboxymethyl cellulose, wherein the concentration of graphite is 600 g per liter of solvent. This ink is further referred to herein as ink K.

[0137] As further shown in the Examples below, Inks A, C, E, H, J, and K are particularly useful for preparing conductive coatings.

[0138] As mentioned above, the third aspect of the present invention relates to a composition obtainable by the method of the first aspect of the present invention.

[0139] A fourth aspect of the present invention relates to a method for preparing a conductive coated substrate, comprising the following steps:

[0140] (i) providing a substrate for receiving a conductive ink for coating;

[0141] (ii) providing an ink composition as defined in any one of the second or third aspects of the present invention;

[0142] (iii) depositing the ink provided in step (ii) on the substrate provided in step (i);

[0143] (iv) drying the ink deposited in step (iii) to form a film of the dried ink;

[0144] (v) reducing the thickness of the film obtained from step (iv) until the film thickness is less than 50% of the thickness of the film obtained from step (iv), and optionally, repeating steps (iii) to (v).

[0145] In a preferred embodiment of the fourth aspect of the invention, the substrate is made of cardboard, paper, cellulose acetate or a plastic material such as polypropylene, polyvinyl chloride, polyacrylonitrile, polyurethane or polyethylene terephthalate.

[0146] In a preferred embodiment of the fourth aspect of the invention, the thickness of the film produced in step (v) is at least 5 μm; preferably between 10 and 80 μm. As described in further detail in the Examples below, the thicker the film, the more conductive the coating.

[0147] In a preferred embodiment of the fourth aspect of the invention, the deposition step (iii) can be performed by screen printing, ink extrusion, 3D printing or scraper application. More preferably, the deposition step is performed by screen printing technology, such as serigraphy. As known in the art, different patterns can be advantageously printed using this technology.

[0148] In a further preferred embodiment of the fourth aspect of the invention, the depositing step (iii) may be performed by screen printing using a silk fabric having 43 to 120 threads per cm.

[0149] In a preferred embodiment of the fourth aspect of the invention, the drying step (iv) is carried out by heating, preferably at a temperature above the boiling point of the ink solvent provided in step (i). Thus, when the solvent is 1,2-propylene glycol, the drying step (iv) may be carried out at a temperature of 150°C.

[0150] In a preferred embodiment of the fourth aspect of the invention, the thickness of the film produced by drying step (iv) is higher than 40 μm.

[0151] In a preferred embodiment of the fourth aspect of the present invention, in step (v), the thickness of the film obtained in step (iv) is reduced until a film thickness of less than 40% of the thickness of the film obtained in step (iv) is reached; preferably, the thickness of the film obtained in step (iv) is reduced until a film thickness of less than 30% of the thickness of the film obtained in step (iv) is reached. As described above, it is believed that this step promotes shearing between the exfoliated graphite flakes contained in the ink, thereby inducing a secondary exfoliation of graphite within the deposited coating, which results in an increase in the conductivity of the coating.

[0152] In a preferred embodiment of the fourth aspect of the invention, the thickness of the film produced in step (v) is at least 5 μm; preferably between 10 and 80 μm.

[0153] In a preferred embodiment of the fourth aspect of the present invention, step (v) is performed by subjecting the film of step (iv) to compression rolling. The compression rolling is preferably performed at a rate of 1 to 5 cm per second.

[0154] In a preferred embodiment of the fourth aspect of the invention, the method comprises a further step (vi) of heating the coated substrate obtained from step (v); preferably at a temperature of 200 to 300° C. The heating step is preferably performed for a period of 5 to 10 minutes.

[0155] A fifth aspect of the present invention relates to a coated substrate obtainable by the method of the fourth aspect of the present invention as defined above in any one of the preferred embodiments of the fourth aspect.

[0156] In a preferred embodiment of the fifth aspect of the invention, the surface resistivity of the coating is less than 40 ohms per square; preferably less than 10 ohms per square; more preferably less than 5 ohms per square.

[0157] In a preferred embodiment of the fifth aspect of the invention, the electrical conductivity of the coating is at least 8000 S / m, preferably at least 15000 S / m.

[0158] In a sixth aspect, the present invention relates to a coated substrate, wherein the coating comprises a carbonaceous material, a stripping agent and a binder, wherein the carbonaceous material is composed of one or more of graphene, exfoliated graphite and a mixture thereof with graphite; the stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannin and a mixture thereof; the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide. The water-soluble polysaccharide has a degree of polymerization higher than 5 and is optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6) alkyl, (C1-C6) alkylcarbonyl and (C1-C6) alkyl-CO2H, wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2.

[0159] In a preferred embodiment of the sixth aspect, the present invention relates to a coated substrate, wherein the substrate is made of paper or a plastic material. In certain embodiments, the plastic material is polyethylene terephthalate. The substrate is preferably a paper sheet.

[0160] In a preferred embodiment of the sixth aspect of the invention, the coated substrate is a substrate in which the carbonaceous material is as defined in any one of the embodiments of the second aspect of the invention and / or the carbonaceous material is present in an amount as defined in any one of the embodiments of the second aspect of the invention.

[0161] In a preferred embodiment of the sixth aspect of the invention, the coated substrate is a substrate in which the stripping agent is as defined in any one of the embodiments of the second aspect of the invention and / or the stripping agent is present in an amount as defined in any one of the embodiments of the second aspect of the invention.

[0162] In a preferred embodiment of the sixth aspect of the invention, the coated substrate is a substrate wherein the binder is as defined in any one of the embodiments of the second aspect of the invention and / or the binder is present in an amount as defined in any one of the embodiments of the second aspect of the invention.

[0163] In a preferred embodiment of the sixth aspect of the invention, the coated substrate is a substrate wherein the thickness of the coating is at least 5 μm; preferably between 10 and 80 μm.

[0164] In a preferred embodiment of the sixth aspect of the invention, the surface resistivity of the coated substrate is less than 40 ohms per square; preferably less than 10 ohms per square; more preferably less than 5 ohms per square.

[0165] In another preferred embodiment of the sixth aspect of the present invention, the electrical conductivity of the coating is at least 8000 S / m, preferably at least 15000 S / m.

[0166] Throughout the specification and claims, the word "comprising" and variations of the word are not intended to exclude other technical features, additives, components or steps. In addition, the word "comprising" includes the cases of "consisting of" and "consisting essentially of". Other objects, advantages and features of the present invention will become apparent to those skilled in the art after reading the specification, or can be learned by practicing the present invention. The following examples are provided by way of illustration and are not intended to limit the present invention.

[0167] Example

[0168] General

[0169] UV-Vis spectra were measured on a NanoDrop apparatus (Thermo Scientific) in spectrophotometer mode, analyzing in the range of 700 to 200 nm and measuring absorbance at 660 nm.

[0170] Cellulose nanocrystals or nanofibers obtained by enzymatic hydrolysis were prepared by hydrolyzing filter paper using an ancestral endoglucanase and using the process disclosed in: et al., Comm. Chem. 2:76, 2019 (pp. 10-11) and Alonso-Lerma et al., Commun. Mater. 1:57, 2020, the above documents are incorporated herein by reference.

[0171] Cellulose nanocrystals or nanofibers obtained by acidic hydrolysis are prepared by hydrolyzing filter paper using sulfuric acid as described in Composites Science and Technology 92 (2014) 27-33, page 28, section 2.2, which is incorporated herein by reference.

[0172] The graphite is composed of synthetic graphite powder (CAS: 7782-42-5, particle size <30 μm) with a main diffraction peak at 26.6° (at 2) and a specific density of 2.2 g / cm 3 .

[0173] Screen printing apparatus and mesh (Brildor, #43 to #120). The mesh was coated with azo sensitized emulsion ink, dried for about 30 minutes, and then irradiated with the desired patterned photolithography (photolite). After irradiation under a UV lamp (about 1 meter below) for 15 minutes, the mesh was vigorously rinsed with water to remove the uncured ink and expose the desired pattern. A rubber scraper (75 Shore) was used to apply and spread the different graphene inks, and the pattern was retained after application due to the best rheological properties of the existing inks.

[0174] A commercial semiconductor characterization device (Jandel, RM3-AR model) was used to contact or press into the sample block in four-probe mode. Surface resistivity values ​​(in Ω / □) were collected by applying different current intensities between 10 nA and 50 mA. Samples with different inks, different numbers of printing passes, and different post-processing conditions (temperature and rolling) were measured as described in this article.

[0175] The viscosity was measured kinematically by measuring the time required for a representative ink (Ink E) to flow through a 10 mL plastic vial (equipped with mL graduations) with a hole in the bottom. These allowed the flow rate to be calculated. This time was compared to the time required for glycerol to flow through the same vial and the viscosity was estimated by the following formula:

[0176]

[0177] Compression rolling was performed using a 130mm long double stainless steel rolling mill, roller diameter 65mm, mirrored sheet, maximum opening: 0-5mm, flat area (+ / - 0.01mm): 130mm.

[0178] Method for evaluating stripping agents by sedimentation

[0179] The efficiency of graphite exfoliation triggered by exfoliants such as lignin sulfonates and cellulose nanocrystals was quantitatively determined in sedimentation experiments. In the first step, a suspension of natural graphite flakes with an average width of 500 μm and a concentration of 100 mg / mL water was prepared. Different amounts of exfoliants were added to the segmented samples of the above suspension to prepare several graphite suspensions with different exfoliant concentrations in the range of 0.1-10 mg / mL. The resulting mixture was ultrasonically treated in a water bath for two hours and then centrifuged at 100 rcf for 5 minutes. The supernatant was collected and its absorbance at 660 nm was measured. The concentration of exfoliated graphite or graphene in the supernatant was calculated according to the Beer-Lambert law based on the following: The molar extinction coefficient ε of graphene is 680 mL·g -1 ·m -1 , the values ​​are obtained by weight calibration.

[0180] Figure 2 The results of the measurements are shown using lignin sulfonate (upper panel), cellulose nanocrystals (lower panel, squares) or carboxymethyl cellulose (lower panel, circles - comparative example) as exfoliating agent. Figure 2 The results show that both lignin sulfonate and cellulose nanocrystals are suitable exfoliating agents, as they can both bring the concentration of exfoliated graphite and / or graphene to the range of 0.4 to 0.5 g / L. In contrast, carboxymethyl cellulose is not a suitable exfoliating agent, as the amount of exfoliated graphite and / or graphene in the supernatant decreases dramatically with the increase in the amount of exfoliating agent, especially in the range of 5 to 10 mg / mL.

[0181] General method for preparing ink

[0182] Method A: A method according to the first aspect of the present invention comprises the following steps:

[0183] (i) preparing a mixture of an exfoliating agent (cellulose nanocrystals CNC or enzymatic nanocellulose ENC) and synthetic graphite in a 1 / 1 (v / v) mixture of water and isopropanol such that the concentration of graphite is 75 grams per liter of the mixture and the weight ratio of the exfoliating agent to the graphite is as shown in Table 1;

[0184] (ii) homogenizing the mixture provided in step (i) by sonication in a water bath for one hour;

[0185] (iii) adding a binder as shown in Table 1, namely carboxymethyl cellulose (CMC) or a mixture thereof with polyaniline, to the mixture obtained in step (ii) in an amount such that the weight ratio of the binder to the graphite is as shown in Table 1;

[0186] (iv) adding 1,2-propylene glycol to the mixture obtained in step (iii) in an amount such that the volume ratio of 1,2-propylene glycol to the mixture of water and isopropyl alcohol is 1:8; and

[0187] (v) evaporating water and isopropyl alcohol from the mixture obtained in step (iv) at about 100°C using mild magnetic stirring to produce a viscous grey fluid, i.e., the ink according to the present invention.

[0188] General method for preparing coated substrates

[0189] Method B: A method according to the fourth aspect of the present invention comprises the following steps:

[0190] (i) providing a substrate for receiving the conductive ink for coating, the substrate being a sheet of paper or a sheet of polyethylene terephthalate (PET);

[0191] (ii) providing an ink composition prepared according to method A;

[0192] (iii) depositing the ink provided in step (ii) on the substrate provided in step (i) by screen printing using a #43 mesh; the mesh having been previously treated with a photosensitive ink (Zoicocoat WR, KOPIMASK) and irradiated with a photolite having a different printed pattern;

[0193] (iv) drying the ink deposited in step (iii) by heating at 150° C. to form a film of the dried ink;

[0194] (v) reducing the thickness of the film obtained in step (iv) by compression rolling using a stainless steel flat roll mill;

[0195] (vi) optionally, repeating steps (iii) to (v)—the sequence of steps (iii) to (v) is also referred to herein as “number of printing passes”; the number of printing passes for each prepared coated substrate is shown in Table 1;

[0196] (vii) heating the coated substrate obtained in step (vi) at a temperature of 220°C to 250°C for 10 to 15 minutes.

[0197] Methods for measuring coating thickness

[0198] Use a micrometer to calculate the thickness of the coated substrate by subtracting the measured thickness of the substrate before coating from the measured thickness of the substrate. (HDP356) Measure the thickness of the prepared coating.

[0199] Method for measuring the resistivity of coated substrates

[0200] For surface resistivity (and conductivity) measurements, such as Figure 3 (right) The well-established four-point method is used on a substrate coated with a circular pattern of 30 mm diameter (selected during the printing step of method B). Surface resistivity measurements were taken in areas near the center of these circles to minimize edge effects or distortions in the measurements.

[0201] The surface resistivity has been measured using a commercial semiconductor characterization equipment with four-point terminals specifically designed for measuring surface resistivity. Measurements have been made using a wide range of currents from 1 nA to 70 mA. The value of 10 mA has been chosen as a standard value for resistivity comparisons between different materials. Therefore, all values ​​in Table 1 are given for a current value of 10 mA. The surface resistivity values ​​have been measured by indenting the machining tip (the results of which are very close to the mass conductivity values ​​of the material) and by making a small contact of the tip with the coating surface. As can be seen from the values ​​reported in Table 1, the surface resistivity values ​​(R s,c ) is much lower than the value obtained by indentation processing (R s,i ).

[0202] According to the surface resistivity value R s and thickness t, the conductivity of the coating is estimated using the following relationship: σ(S / m) = 1 / (R s ·t).

[0203] Table 1 below summarizes different ink compositions prepared according to method A with a graphite concentration of 600 g per liter of solvent and different paper substrates coated with the inks according to method B and their physical properties.

[0204] Table 1

[0205]

[0206]

[0207] 1,2PD = 1,2-propylene glycol; * Blanks were made with inks made only from graphite

[0208] As will be apparent to the skilled person, Ink I does not contain any stripping agent and is provided as a comparative example. Data relating to blanks and aluminum coatings are also provided as reference examples.

[0209] Results

[0210] The results in Table 1 above show that the combination of a stripping agent such as ENC and a binder such as CMC surprisingly allows the formation of a conductive coating having a lower resistivity than a conductive coating prepared from an ink containing only a binder such as CMC according to Method B, when comparing Ink I (a comparative example without a stripping agent) to the results for Ink H or Ink J. The coating obtained with Ink I was slightly conductive (e.g., Figure 2 As shown, CMC can act as a release agent). In addition, Figure 8 The results show that the comparative ink formulations containing graphite and cellulose nanocrystals and / or nanofibers as release agents and without a binder are not suitable for forming a conductive coating because the deposited film formed does not adhere to the surface substrate. These data indicate that the combination of a release agent and a binder is essential in the ink formulation in order to form a conductive coating according to the method of the present invention.

[0211] Figure 5 The results show that the step of reducing the thickness of the conductive coating in the method of the invention (e.g. by compression) allows the production of a coating with reduced resistivity. This suggests that this step triggers a second stripping process of the ink once it has been deposited on the substrate.

[0212] The results of Table 1 also show that the number of printing passes employed in the conductive coating preparation process allows for the production of films of increasing thickness. Figure 7 The data show that the resistivity of the conductive coating decreases with increasing film thickness. This effect can also be seen from Figure 7 It can be concluded.

[0213] By comparing Ink E and Ink H, it can be concluded that 1,2-propylene glycol is more preferred as solvent than water, as it allows the formation of a conductive coating with similar resistivity, but using fewer printing passes and therefore advantageously using a smaller amount of ink material.

[0214] Figure 6 The results show that by comparing the resistivity of conductive coatings of different thicknesses prepared by Ink C or Ink E, when cellulose nanofibers or cellulose nanocrystals obtained by enzymatic digestion of filter paper are used as stripping agents, especially when cellulose nanofibers or cellulose nanocrystals obtained by endoglucanase-mediated digestion of filter paper associated with the last Firmicutes common ancestor (optionally linked to a carbohydrate binding module) are used, the resistivity of the resulting conductive coating is lower than when cellulose nanofibers and / or nanocrystals prepared by acid treatment of filter paper are used as stripping agents. Without being bound by theory, it is believed that this is because cellulose nanocrystals and / or nanofibers prepared by enzymatic digestion are more hydrophobic than cellulose nanocrystals and / or nanofibers prepared by acid treatment of filter paper because they carry a smaller amount of charged groups, such as sulfate groups.

[0215] Terms

[0216] The invention may also be defined in accordance with the following terms:

[0217] 1. A method for producing an ink for forming a conductive coating, comprising the following steps:

[0218] (i) providing a mixture comprising graphite, an exfoliating agent and a first polar solvent, wherein:

[0219] - a stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins and mixtures thereof; and

[0220] - the weight ratio of the stripping agent to the graphite is in the range of 1:100 to 1:20;

[0221] (ii) homogenizing the mixture provided in step (i);

[0222] (iii) adding a binder different from the release agent to the mixture obtained from step (ii), wherein the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H; and wherein the amount of the binder is such that the weight ratio of the binder to the release agent is in the range of 4:1 to 1:2;

[0223] (iv) adding a second polar protic solvent to the mixture obtained in step (iii), wherein the boiling point of the second polar protic solvent is equal to or greater than the first polar solvent provided in step (i), and preferably, the amount of the second polar protic solvent is such that the volume ratio of the second polar protic solvent to the first polar solvent provided in step (i) is 1:5 to 1:10; and

[0224] (v) removing substantially all of the first polar solvent provided in step (i) while retaining substantially all of the second polar protic solvent added in step (iv).

[0225] 2. The method according to clause 1, wherein the first solvent of the mixture of step (i) is selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, acetone, methyl ethyl ketone, 3-pentanone, 2-pentanone and mixtures thereof.

[0226] 3. The method according to any one of clauses 1 or 2, wherein the first solvent of the mixture of step (i) is selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol and mixtures thereof.

[0227] 4. The method according to any one of clauses 1 to 3, wherein the first solvent is a mixture of water and isopropanol; preferably in a volume ratio of 1:1.

[0228] 5. The method according to any one of clauses 1 to 4, wherein the graphite in the mixture of step (i) is selected from the group consisting of synthetic graphite powder, natural graphite flakes and natural graphite powder; preferably synthetic graphite powder.

[0229] 6. The method according to any one of clauses 1 to 5, wherein the exfoliating agent in the mixture of step (i) is selected from the group consisting of lignin sulfonates, cellulose nanocrystals, cellulose nanofibers and mixtures thereof.

[0230] 7. The method according to any one of clauses 1 to 6, wherein the exfoliating agent in the mixture of step (i) is cellulose nanocrystals and / or cellulose nanofibers.

[0231] 8. The method according to any one of clauses 1 to 7, wherein the exfoliating agent in the mixture of step (i) is cellulose nanofibers or cellulose nanocrystals having an average length of between 100 nm and 1500 nm and a diameter of between 10 nm and 20 nm.

[0232] 9. A method according to clause 7 or 8, wherein the cellulose nanofibers or cellulose nanocrystals are obtained by enzymatic digestion of filter paper; preferably by digestion of filter paper mediated by one of endoglucanase, cellobiohydrolase and β-glucosidase optionally linked to a carbohydrate binding module; more preferably by digestion of filter paper mediated by endoglucanase associated with the last common ancestor of Firmicutes, the common ancestor being optionally linked to a carbohydrate binding module.

[0233] 10. The method according to any one of clauses 1 to 7, wherein the exfoliating agent in the mixture of step (i) is cellulose nanocrystals obtained by acid treatment of cellulose.

[0234] 11. The method according to any one of clauses 1 to 10, wherein the weight ratio of exfoliant to graphite is in the range of 1:90 to 1:30.

[0235] 12. The method according to any one of clauses 1 to 11, wherein the concentration of the stripping agent in the mixture of step (i) is 100 mg per liter of solvent to 10 grams per liter of solvent; preferably 1 gram per liter of solvent to 5 grams per liter of solvent.

[0236] 13. A method according to any one of clauses 1 to 12, wherein step (ii) is performed by sonication with a gentle bath or tip, or by shear mixing the mixture of step (i), or by a combination of both.

[0237] 14. A method according to any one of clauses 1 to 13, wherein the binder of step (iii) is selected from the group consisting of polyaniline, arabinoxylan, callose, cellulose, chitin, chitosan, alginic acid, alginic salts of alginate, pectin, agarose and mixtures thereof, wherein the available hydroxyl and amino groups in each of arabinoxylan, callose, cellulose, chitin, chitosan, pectin and agarose are optionally substituted with free radicals selected from the group consisting of: (C1-C6) alkyl , (C1-C6) alkylcarbonyl and (C1-C6) alkyl-CO2H; preferably, the binder is selected from the group consisting of polyaniline, cellulose, chitin, chitosan, alginic acid, alkaline salts of alginate, agarose and mixtures thereof, wherein the available hydroxyl and amino groups in each of cellulose, chitin, chitosan and agarose are optionally substituted by free radicals, and the free radicals are selected from the group consisting of: (C1-C6) alkyl, (C1-C6) alkylcarbonyl and (C1-C6) alkyl-CO2H.

[0238] 15. A method according to any one of clauses 1 to 14, wherein the binder of step (iii) is selected from the group consisting of polyaniline and cellulose, wherein the available hydroxyl groups in the cellulose are optionally substituted by free radicals selected from the group consisting of: (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H.

[0239] 16. The method according to any one of clauses 1 to 15, wherein the binder of step (iii) is selected from the group consisting of carboxymethyl cellulose, polyaniline and mixtures thereof; preferably carboxymethyl cellulose.

[0240] 17. A method according to any one of clauses 1 to 16, wherein the amount of adhesive of step (iii) is such that the weight ratio of adhesive to release agent is in the range of 2:1 to 1:2; preferably 3:2 to 2:3.

[0241] 18. The method according to any one of clauses 1 to 17, wherein the second solvent of step (iv) is selected from the group consisting of water, 1,2-propylene glycol, glycerol, diethylene glycol, 1,2-ethylene glycol, terpineol and mixtures thereof.

[0242] 19. The process according to any one of clauses 1 to 18, wherein the second solvent of step (iv) is 1,2-propylene glycol.

[0243] 20. A process according to any one of clauses 1 to 19, wherein the amount of the second solvent in step (iv) is such that the volume ratio of 1,2-propylene glycol to the first polar solvent provided in step (i) is from 1:5 to 1:10; preferably 1:8.

[0244] 21. The method according to any one of clauses 1 to 19, wherein step (v) is performed by heating and / or by applying a vacuum.

[0245] 22. An ink composition for forming a conductive coating, comprising:

[0246] - solvents,

[0247] - Carbonaceous materials consisting of one or more of graphene, exfoliated graphite and mixtures thereof with graphite;

[0248] - a stripping agent selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins, and mixtures thereof; and

[0249] a binder selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H,

[0250] wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2.

[0251] 23. The composition according to clause 22, comprising 85% to 98% by weight of the carbonaceous material, wherein the weight percentages are expressed relative to the sum of the weights of the carbonaceous material, the exfoliant and the binder.

[0252] 24. The composition according to clause 22, comprising 90% to 98% by weight of the carbonaceous material, wherein the weight percentages are expressed relative to the sum of the weights of the carbonaceous material, the exfoliant and the binder.

[0253] 25. The composition according to clause 22, comprising 92% to 98% by weight of the carbonaceous material, wherein the weight percentages are expressed relative to the sum of the weights of the carbonaceous material, the exfoliant, and the binder.

[0254] 26. The composition according to any one of clauses 22 to 25, wherein the solvent is a polar protic solvent.

[0255] 27. The composition according to any one of clauses 22 to 26, wherein the solvent is solvent S or an azeotropic mixture of said solvent S and a solvent as defined in any one of clauses 2 to 4, wherein S is selected from the group consisting of water, 1,2-propylene glycol, glycerol, diethylene glycol, 1,2-ethylene glycol, terpineol and mixtures thereof.

[0256] 28. The composition according to any one of clauses 22 to 27, wherein the solvent is a polar protic solvent which is 1,2-propylene glycol or an azeotropic mixture thereof with water and / or isopropanol.

[0257] 29. The composition according to any one of clauses 22 to 28, wherein the carbonaceous material consists of a mixture of exfoliated graphite and graphite; the graphite is preferably synthetic graphite powder.

[0258] 30. The composition according to any one of clauses 22 to 29, wherein the release agent is as defined in any one of clauses 6 to 10.

[0259] 31. The composition according to any one of clauses 22 to 30, wherein the binder is as defined in clause 14 or 15.

[0260] 32. The composition according to any one of clauses 22 to 31, wherein the weight ratio of exfoliant to graphite is in the range of 1:90 to 1:30.

[0261] 33. A composition according to any of clauses 22 to 32, wherein the amount of binder is such that the weight ratio of binder to release agent is in the range of 2:1 to 1:2; preferably 3:2 to 2:3.

[0262] 34. A composition according to any one of clauses 22 to 33, having a kinematic viscosity between 300 Pa·s and 3000 Pa·s; preferably between 1000 Pa·s and 1500 Pa·s.

[0263] 35. The composition according to any one of clauses 22 to 34, comprising 1 wt% to 4 wt% of the exfoliant, wherein the weight percentages are expressed relative to the sum of the weights of the carbonaceous material, the exfoliant and the binder.

[0264] 36. The composition according to any one of clauses 22 to 35, comprising 1 wt% to 6 wt% binder, wherein the weight percentages are expressed relative to the sum of the weights of carbonaceous material, exfoliant and binder.

[0265] 37. A composition according to any of clauses 22 to 36, wherein the concentration of graphite is between 40 and 1000 grams per litre of solvent; preferably 60 or 600 grams per litre of solvent.

[0266] 38. A composition according to any one of clauses 22 to 37 comprising 1,2-propylene glycol as solvent and 93 wt% graphite, 1.6 wt% cellulose nanocrystals, 1.6 wt% polyaniline, 3.8 wt% carboxymethylcellulose, wherein the concentration of graphite is 60 grams per liter of solvent.

[0267] 39. A composition according to any one of clauses 22 to 37 comprising 1,2-propylene glycol as solvent and 94 wt% graphite, 3 wt% cellulose nanocrystals as defined in clause 10, 3 wt% carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent.

[0268] 40. A composition according to any one of clauses 22 to 37 comprising 1,2-propylene glycol as solvent and 94 wt% graphite, 3 wt% cellulose nanofibers as defined in clause 8 or 9, 3 wt% carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent.

[0269] 41. A composition according to any one of clauses 22 to 37, comprising water as solvent and 94 wt% of graphite, 3 wt% of cellulose nanofibers as defined in clause 8 or 9, 3 wt% of carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent.

[0270] 42. A composition according to any one of clauses 22 to 37 comprising 1,2-propylene glycol as solvent and 96.3 wt% of graphite, 1.8 wt% of cellulose nanofibers as defined in clause 8 or 9, 1.9 wt% of carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent.

[0271] 43. A composition according to any one of clauses 22 to 37 comprising 1,2-propylene glycol as solvent and 97.2 wt% of graphite, 1.2 wt% of cellulose nanofibers as defined in clause 8 or 9, 1.6 wt% of carboxymethyl cellulose, wherein the concentration of graphite is 600 grams per liter of solvent.

[0272] 44. A composition obtainable by a process according to any one of clauses 1 to 21.

[0273] 45. A method for preparing a conductive coated substrate, comprising the steps of:

[0274] (i) providing a substrate for receiving a conductive ink for coating;

[0275] (ii) providing an ink composition as defined in any one of clauses 22 to 42 or 43;

[0276] (iii) depositing the ink provided in step (ii) on the substrate provided in step (i); preferably by inkjet printing, screen printing or syringe extrusion and 3D printing;

[0277] (iv) drying the ink deposited in step (iii) to form a film of the dried ink; preferably, the deposited ink is dried by heating;

[0278] (v) reducing the thickness of the film obtained from step (iv) until the film thickness is less than 50% of the thickness of the film obtained from step (iv), and optionally, repeating steps (iii) to (v); preferably by compression rolling.

[0279] 46. ​​The method according to clause 45, wherein the substrate is made of paper or plastic material.

[0280] 47. A method according to clause 45 or 46, wherein the thickness of the film produced in step (v) is at least 5 μm; preferably between 10 and 80 μm.

[0281] 48. A coated substrate obtainable by a method according to any one of clauses 45 to 47.

[0282] 49. A coated substrate according to clause 48, wherein the surface resistivity of the coating is less than 40 ohms per square; preferably less than 10 ohms per square; more preferably less than 5 ohms per square.

[0283] 50. A coated substrate according to clause 48 or 49, wherein the electrical conductivity of the coating is at least 8000 S / m, preferably at least 15000 S / m.

[0284] 51. A coated substrate, wherein the coating comprises a carbonaceous material, an exfoliating agent and a binder, the carbonaceous material consisting of one or more of graphene, exfoliated graphite and mixtures thereof with graphite; the exfoliating agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannins and mixtures thereof; the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide, the water-soluble polysaccharide having a degree of polymerization greater than 5 and optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H,

[0285] wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2.

[0286] 52. A coated substrate according to clause 51, wherein the substrate is made of paper or a plastic material.

[0287] 53. A coated substrate according to clause 51 or 52, wherein the carbonaceous material is present in an amount as defined in any one of clauses 23 to 25.

[0288] 54. A coated substrate according to any one of clauses 51 to 53, wherein the release agent is as defined in any one of clauses 6 to 10.

[0289] 55. A coated substrate according to any one of clauses 51 to 54, wherein the adhesive is as defined in clause 14 or 15.

[0290] 56. A coated substrate according to any of clauses 51 to 55, wherein the amount of binder is as defined in clause 33 or 36.

[0291] 57. The coated substrate according to any one of clauses 51 to 56, wherein the amount of release agent is as defined in clause 35.

[0292] 58. A coated substrate according to any one of clauses 51 to 57, wherein the carbonaceous material, the binder and the release agent are present in amounts as defined in any one of clauses 38 to 43.

[0293] 59. A coated substrate according to any of clauses 51 to 58, wherein the thickness of the coating is at least 5 μm; preferably between 10 and 80 μm.

[0294] 60. A coated substrate according to any of clauses 51 to 59, wherein the surface resistivity of the coating is less than 40 ohms per square; preferably less than 10 ohms per square; more preferably less than 5 ohms per square.

[0295] 61. A coated substrate according to any of clauses 51 to 60, wherein the conductivity of the coating is at least 8000 S / m, preferably at least 15000 S / m.

Claims

1. A method for producing an ink for forming a conductive coating, comprising the following steps: (i) providing a mixture comprising graphite, an exfoliating agent and a first polar solvent, wherein: - the exfoliating agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannin and mixtures thereof; and - the weight ratio of the stripping agent to the graphite is in the range of 1:100 to 1:20; (ii) homogenizing the mixture provided in step (i); (iii) adding a binder different from the release agent to the mixture obtained from step (ii), wherein the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide, the water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H; and wherein the amount of the binder is such that the weight ratio of the binder to the release agent is in the range of 4:1 to 1:2; (iv) adding a second polar protic solvent to the mixture obtained from step (iii), wherein the boiling point of the second polar protic solvent is equal to or greater than the first polar solvent provided in step (i); and (v) removing substantially all of the first polar solvent provided in step (i) while retaining substantially all of the second polar protic solvent added in step (iv).

2. The method according to claim 1, wherein the first polar solvent of the mixture of step (i) is a mixture of water and isopropanol in a volume ratio of 1:

1.

3. The method according to any one of claims 1 or 2, wherein the stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals and cellulose nanofibers, wherein the cellulose nanofibers or cellulose nanocrystals are obtained by endoglucanase-mediated enzymatic digestion of filter paper.

4. The method according to any one of claims 1 to 3, wherein the weight ratio of the exfoliant to the graphite is in the range of 1:90 to 1:

30.

5. The method according to any one of claims 1 to 4, wherein step (ii) is performed by sonication with a gentle bath or tip, or by shear mixing the mixture of step (i).

6. The method according to any one of claims 1 to 5, wherein the binder of step (iii) is selected from the group consisting of carboxymethyl cellulose, polyaniline and mixtures thereof.

7. The process according to any one of claims 1 to 6, wherein the second solvent of step (iv) is 1,2-propylene glycol.

8. An ink composition for forming a conductive coating, comprising: - solvents, - Carbonaceous materials consisting of one or more of graphene, exfoliated graphite and mixtures thereof with graphite; - a stripping agent selected from the group consisting of lignin sulfonates, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, optionally sulfonated tannins, and mixtures thereof; and a binder selected from the group consisting of polyaniline and a water-soluble polysaccharide having a degree of polymerization higher than 5 and optionally substituted at any available position by one or more free radicals independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H, wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:

2.

9. The composition according to claim 8, comprising 85 to 98 wt% of the carbonaceous material, wherein: The weight percentage is expressed relative to the sum of the weights of the carbonaceous material, the stripping agent, and the binder.

10. The composition according to any one of claims 8 to 9, wherein the solvent is 1,2-propylene glycol or an azeotropic mixture thereof with water and / or isopropanol.

11. The composition according to any one of claims 8 to 10, having a kinematic viscosity between 1000 Pa·s and 1500 Pa·s.

12. The composition according to any one of claims 8 to 11, comprising 90 to 98 wt% of the carbonaceous material, 1 to 4 wt% of the exfoliant and 1 to 6 wt% of the binder, wherein: The weight percentage is expressed relative to the sum of the weights of the carbonaceous material, the stripping agent, and the binder.

13. Composition obtained by the method according to any one of claims 1 to 7.

14. A method for preparing a conductive coated substrate, comprising the steps of: (vi) providing a substrate for receiving the conductive ink for coating; (vii) providing an ink composition as defined in any one of claims 8 to 12 or 13; (viii) depositing the ink provided in step (ii) on the substrate provided in step (i); (ix) drying the ink deposited in step (iii) to form a film of the dried ink; (x) reducing the thickness of the film obtained from step (iv) until the film thickness is less than 50% of the thickness of the film obtained from step (iv), and optionally, repeating steps (iii) to (v).

15. Coated substrate obtainable by the method according to claim 14.

16. A coated substrate, wherein the coating comprises a carbonaceous material, a stripping agent and a binder, wherein the carbonaceous material is composed of one or more of graphene, exfoliated graphite and a mixture thereof with graphite; the stripping agent is selected from the group consisting of lignin sulfonate, cellulose nanocrystals, cellulose nanofibers, chitin nanocrystals, chitin nanofibers, hemicellulose nanocrystals, hemicellulose nanofibers, starch nanocrystals, starch nanofibers, lignin, tannin and a mixture thereof; the binder is selected from the group consisting of polyaniline and a water-soluble polysaccharide, and the water-soluble The polysaccharide has a degree of polymerization higher than 5 and is optionally substituted at any available position by one or more free radicals, the one or more free radicals being independently selected from the group consisting of (C1-C6)alkyl, (C1-C6)alkylcarbonyl and (C1-C6)alkyl-CO2H, wherein the binder and the stripping agent are different, and wherein the amount of the stripping agent is such that the weight ratio of the stripping agent to the carbonaceous material is in the range of 1:100 to 1:20, and wherein the amount of the binder is such that the weight ratio of the binder to the stripping agent is in the range of 4:1 to 1:2.

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