METHOD FOR PRODUCING A CARBON FIBER FROM A PAPER PRODUCT

Description

[0001] The present invention falls within the general field of the manufacture of carbon fibers, intended in particular for the preparation of composite materials, from bio-sourced materials.

[0002] More particularly, the present invention relates to a method for manufacturing a carbon fiber from a paper product. The invention also relates to a more general method for manufacturing an article made of a composite material based on carbon fibers distributed in a polymer organic resin matrix, comprising implementing a method for manufacturing a carbon fiber according to the invention.

[0003] Carbon fibers are used in many fields, taking advantage of their particularly advantageous mechanical, electrical and thermal properties and their low weight.

[0004] The manufacture of carbon fibers from renewable bio-sourced materials, in order to overcome the problem of the planned depletion of fossil resources, has been the subject of much research in recent decades. In particular, it has been proposed by the prior art to manufacture such fibers from cellulose, a macromolecular carbohydrate that forms the wall of plant cells and is the main component of wood. Cellulose is the most abundant organic matter on Earth. Carbon fibers obtained from cellulose have in particular the advantage of very good structuring.

[0005] The present invention aims to provide a method for manufacturing, at low cost and in the most environmentally friendly manner possible, from cellulose, continuous carbon fibers having sufficiently good mechanical properties to be usable for numerous applications, and in particular for the manufacture of articles made from composite materials.

[0006] Seeking to achieve this goal, the present inventors were interested in the paper recycling sector.

[0007] Paper is actually made up of about 70 to 85% cellulose fibers, extracted from wood or paper recycling. The remaining 15 to 30% are mineral filler-type additives, mainly composed of carbonates, as well as various additives such as glues, starches, pigments, etc., which have been added during the manufacture of the paper to give it specific properties.

[0008] A very large quantity of paper is currently consumed in the world, in particular for the manufacture of paper products such as printing paper, packaging, sanitary and household paper, but also hygiene or medical products. The recycling of paper products, particularly used paper products, is therefore of great interest, both from an ecological and economic point of view, and this is particularly true in the context of the manufacture of carbon fibers. The cost price of used paper is in fact much lower than that of conventional precursor materials used for the manufacture of carbon fibers, such as purified cellulose, particularly from the paper industry, or polyacrylonitrile. Document WO 2018 / 142025 describes a process for preparing cellulose fibers, usable as precursors of carbon fibers, from cellulosic waste from everyday life.This process comprises an initial step of treating the waste by cooking in an alkaline glycerol liquor, then dissolving the resulting cellulose pulp in an ionic liquid, and spinning the resulting solution to form a cellulose fiber. WO 2014 / 162062 describes a process for producing cellulose fibers from a lignocellulosic material. This process comprises dissolving the material in a particular solvent, a diazabicyclononene-based ionic liquid, and spinning the resulting solution to form a cellulose fiber.

[0009] US 5,601,767 describes a process for producing cellulose fibers from paper, comprising dissolving paper shreds in an aqueous amine oxide solution, removing the water by drying at high temperature, and forming a cellulose fiber by solvent spinning. However, this document makes no mention that this cellulose fiber can be used for the formation of carbon fibers.

[0010] Further relevant information can be found in GB 860 342 A, EP 2 947 200 A1, FR 2 997 097 A1 and WO 2010 / 104458 A1.

[0011] The present inventors have discovered that the cellulose contained in manufactured paper products can be recovered in a form which is sufficiently little degraded to allow its use in a conventional spinning process, usually applied from papermaking cellulose pulp, to form individual and continuous cellulose-based fibers, these cellulose-based fibers subsequently making it possible, by carbonization, to obtain carbon fibers whose mechanical properties are entirely satisfactory, this with a good carbon yield during the carbonization step. More particularly, it has been discovered by the present inventors that such a result can be obtained, under specific operating conditions, with a minimal number of steps, and in particular without implementing pretreatment steps such as cooking, washing, pulping, etc., of the manufactured product or of the cellulose it contains, before the spinning step, and without requiring a significant temperature input, other than for the carbonization step itself, and this contrary to what is recommended by the prior art concerning the manufacture of cellulose fibers precursors of carbon fibers. These carbon fibers can in particular be used for the manufacture of composite materials.

[0012] Thus, according to the present invention, there is provided a method of manufacturing a carbon fiber by recycling a paper product.

[0013] In this description, the term "paper product" means any manufactured product made from paper pulp, for example, but not limited to, office or printing paper products, including sheets and envelopes, paper towels, facial tissue, toilet paper, paper napkins, etc.

[0014] The term "paper product" does not include pulp obtained directly from wood, by chemical or mechanical processes of the paper industry.

[0015] The manufactured product from which the method according to the invention is applied may be either new or used, in particular at the end of its life.

[0016] The method for manufacturing a carbon fiber according to the invention comprises, after a step of collecting the paper product: a / the preparation of a cellulose-based fiber, comprising successive steps of: a1 / grinding / shredding this paper product, so as to obtain a ground paper; a3 / dissolving the ground material obtained at the end of step a1 / directly in an aqueous phosphoric acid solution as a solvent solution, in which the cellulose is soluble, to form a so-called spinning solution; a4 / from this spinning solution, manufacturing a continuous cellulose-based fiber by a solvent spinning process, also commonly called a wet spinning process; a5 / where appropriate, drawing the continuous cellulose-based fiber thus obtained, so as to form a fiber of greater length, the implementation of such a drawing step being particularly preferred in the context of the invention; b / and carbonizing the continuous cellulose-based fiber thus obtained to form a carbon fiber.

[0017] By "dissolving the ground material directly in an aqueous solution of phosphoric acid" is meant in the present description that the ground material obtained at the end of step a1 / is directly dissolved, as is, in the aqueous solution, without being subjected to one or more prior treatment steps.

[0018] The solvent-based spinning process, or wet spinning, implemented in step a4 / of the process according to the invention consists, in a conventional manner in itself, of carrying out an extrusion of the spinning solution through a spinning die, in a so-called coagulation bath containing a non-solvent for the cellulose, which is miscible with the solvent solution, i.e. the aqueous phosphoric acid solution, used to prepare the spinning solution.

[0019] The solvent spinning process implemented according to the invention may be a so-called "wet" process, according to which the spinning die is immersed in the coagulation bath, or a so-called "air gap" process, according to which the spinning die is placed at a distance, generally between 1 mm and 20 cm, above the coagulation bath.

[0020] The method for manufacturing a carbon fiber according to the invention is simple to implement, each of its steps being able to be carried out by techniques known to those skilled in the art. By an extremely reduced number of steps, it makes it possible to form carbon fibers having good qualities, in particular mechanical qualities, and this at a particularly advantageous cost, and much lower than that required to manufacture carbon fibers based on conventional precursor materials, such as for example purified cellulose, from the paper industry, or polyacrylonitrile. In particular, the method according to the invention, comprising a step of preparing a continuous fiber based on cellulose, which induces a rearrangement of the molecular structures within the fiber, resulting in a crystalline structure of the cellulose within the fiber, makes it possible to obtain a final carbon fiber having very good mechanical properties.The method according to the invention particularly advantageously makes it possible to obtain carbon fibers having a stress resistance greater than 1,200 MPa and a Young's modulus greater than 75 GPa, and even a stress resistance greater than or equal to 2,500 MPa and a Young's modulus greater than or equal to 200 GPa after graphitization. These mechanical properties make them entirely suitable for use in fields of application requiring high strength of the materials used. Nothing in the prior art suggested that such satisfactory mechanical properties could have been obtained with such a reduced number of steps, i.e. grinding of the manufactured product into paper, dissolution of the ground material, without any other pretreatment, in the aqueous phosphoric acid solution, and solvent spinning of the solution obtained.More generally, the carbon fibers obtained by the manufacturing process according to the invention find application in numerous fields, in particular for the production of materials or parts intended to be implemented in fields as varied as the fields of construction and infrastructure, industrial equipment, automobile, rail or naval transport, electricity and electronics, sports and leisure, renewable energies and in particular wind turbines, etc. For this purpose, they can be used as is, assembled in the form of non-wovens or in woven or knitted form, where appropriate in a mixture with other types of fibers.The carbon fibers obtained by the manufacturing process according to the invention can advantageously be used, due to their low cost, as a replacement for glass fibers in applications requiring the use of reinforcing fibers with moderate stress resistance and low production cost. For example, the carbon fibers obtained by the process according to the invention can be used for the manufacture of structures in the field of construction or the production of renewable energies, such as wind turbine or hydro turbine blades, as a replacement for all or part of the glass fibers used therein until now. The carbon fibers according to the invention can, for example, be used to replace 5 to 40%, in particular 10 to 30%, of these glass fibers.

[0021] The present invention thus relates in particular to a wind turbine blade, or a tidal turbine blade, of large dimensions, that is to say at least 30 meters in length, and typically 40 to 100 meters in length, formed from a composite material based on reinforcing fibers dispersed in a polymer resin matrix, and comprising as reinforcing fibers of the blade, the latter typically being formed of two half-blades, between 5 and 40%, and preferably between 10 and 30%, of carbon fibers derived from cellulose obtained by a manufacturing method according to the invention, the remainder of the reinforcing fibers being constituted by glass fibers. The density of these carbon fibers derived from cellulose is preferably advantageously between 1.3 and 1.8 g / m 3< , while the density of the glass fibers is approximately equal to 2.2 g / m 3< .The above percentage of carbon fibers derived from cellulose obtained according to the invention, relative to the total quantity of reinforcing fibers, is defined for the reinforcing fibers of the blade airfoil, and is understood in addition to the percentage of carbon fibers included in the resin-carbon composite spar (commonly referred to as the "spar cap"), which is incorporated for large blades inside the blade, in the volume defined by the half-blades.

[0022] The percentage and distribution of carbon fibers from cellulose in the blade are calculated in particular according to several criteria: dimensions and mechanical properties, weight, size, engine energy, target power of the wind turbine (advantageously 3 to 8 MW).

[0023] The blades of wind turbines or similar (for example, hydro turbines) thus obtained make it possible, for a target length and mechanical properties, to reduce the power required by the structure's motors.

[0024] The method according to the invention may also meet one or more of the characteristics described below, implemented in isolation or in each of their technically effective combinations.

[0025] The grinding / shredding of the paper product carried out in step a1 / of the method according to the invention can be carried out according to any method known to those skilled in the art, for example by means of a conventional grinder / shredding machine.

[0026] Preferably, in step a1 / the grinding is carried out so as to obtain a ground material formed of particles whose largest dimension is between 100 and 1000 µm, preferably between 200 and 500 µm. These particles are preferably non-agglomerated.

[0027] This step may be preceded by various cleaning and / or dusting operations on the paper product, as well as cutting.

[0028] The ground material thus obtained is directly dissolved in the spinning solution.

[0029] The process according to the invention thus proves to be particularly advantageous compared to the processes proposed by the prior art for preparing cellulose intended for the manufacture of carbon fibers from lignocellulosic biomass, which include advanced purification steps. The time and cost required for the manufacture of carbon fibers by the process according to the invention are thus significantly lower than those of conventional processes for preparing carbon fibers from lignocellulosic biomass. The cost of the cellulosic raw material subjected to the spinning step is in particular significantly lower than that of conventional processes for preparing carbon fibers.

[0030] In the method according to the invention, not comprising such a step a2 / of pretreatment of the paper shred, in comparison with the methods comprising such a pretreatment step, the implementation of the solvent spinning step is advantageously just as easy and the production yield of a carbon fiber is just as high. The method according to the invention thus advantageously combines speed, efficiency and low cost.

[0031] Preferably, the ground material dissolved in the solvent solution in step a3 / of the process according to the invention contains more than 90% by weight, preferably more than 95% by weight, of cellulose relative to the total weight of the ground material. Its water content is preferably less than or equal to 15% by weight, relative to the total weight of the ground material. Its mineral filler content is preferably less than or equal to 0.3%, and preferably less than or equal to 0.1%, by weight relative to the total weight of the ground material. Such a characteristic advantageously improves the mechanical properties of the carbon fiber obtained by the process according to the invention.

[0032] The paper product from which the method according to the invention is implemented can be formed from paper from recycling.

[0033] Alternatively, it can be made into paper from chemical pulp, particularly kraft or bisulfite pulp. Examples include writing or printing paper. Chemical pulps are made by cooking wood using chemicals. There are two main processes: the acid process, which produces bisulfite pulp, and the kraft process, which involves cooking the wood in a liquor containing soda and sodium sulfide.

[0034] Step a3 / of dissolving the ground material obtained at the end of step a1 / of the process uses, as a solvent solution for the cellulose, an aqueous solution of phosphoric acid.

[0035] Typical solvent solutions are zinc chloride solutions, formic acid, N-methylmorpholine-N-oxide (NMMO), ionic liquids, or any technically possible mixtures thereof.

[0036] As a solvent solution, an aqueous solution of phosphoric acid, preferably concentrated to 75 to 99% by volume, relative to the total volume of the aqueous solution, in phosphoric acid, alone surprisingly makes it possible to obtain, from a manufactured paper product, which is subjected to a simple grinding step before its dissolution in the solvent solution, carbon fibers with particularly advantageous mechanical properties.

[0037] The dissolution of the ground material in the solvent solution may be assisted by heat treatment, for example by heating under reduced pressure, then cooling to a very low temperature, in a conventional manner. This heat treatment is however preferably limited to a temperature less than or equal to 45°C.

[0038] More generally, during step a / of preparing a cellulose-based fibre, the temperature applied to the cellulose does not exceed 45°C. This results in particularly good mechanical properties of the final carbon fibre, with the advantage of low energy expenditure.

[0039] In particular embodiments of the invention, the spinning solution is subjected to the solvent spinning process without a prior drying step, aimed in particular at removing water. The number of steps of the process according to the invention is then advantageously reduced to a minimum.

[0040] The spinning solution is preferably not subjected to any other step prior to the spinning itself, with the exception of the incorporation into this solution of additional ingredients as described below. It may furthermore, if necessary, be subjected to a filtration step. In preferred embodiments of the invention, the spinning solution is not filtered prior to spinning.

[0041] In preferred embodiments of the invention, in step a3 / of dissolving the ground material in the solvent solution, consisting of the aqueous phosphoric acid solution, the ground material is mixed with purified cellulose.

[0042] In the present description, the term “purified cellulose” means cellulose obtained from lignocellulosic biomass, in particular so-called paper cellulose, i.e. resulting from a paper-making process and having a higher degree of purity than the cellulose contained in the ground material obtained according to the invention. Purified cellulose is commonly used, in the processes of the prior art, for the manufacture of carbon fibers.

[0043] The purified cellulose used in the context of the present invention may come from hardwood or softwood, from annual plants, such as straw or cotton, etc. It may have been obtained by any conventional method in itself, for example by a kraft or soda cooking process.

[0044] In the mixture of ground material and purified cellulose incorporated into the spinning solution, the purified cellulose is preferably present in an amount by weight of between 20 and 90%, preferably between 40 and 60%, for example approximately 50%, relative to the total weight of the mixture of ground material and purified cellulose.

[0045] The total quantity of ground material, or where appropriate of the mixture of ground material and the purified cellulose added thereto, which is dissolved in the solvent solution in step a3 / of the process, is between 1 and 50% by weight, preferably between 5 and 30% by weight and preferentially between 5 and 20% by weight, for example between 5 and 15% by weight, relative to the total weight of the spinning solution obtained.

[0046] Thus, in step a3 / of the process according to the invention, the concentration of ground material, where appropriate in a mixture of ground material and purified cellulose, dissolved in the solvent solution, is between 1 and 50% by weight, preferably between 5 and 30% by weight and preferentially between 5 and 20% by weight, for example between 5 and 15% by weight, relative to the total weight of the spinning solution obtained.

[0047] The paper or paper pulp shredder can also be mixed with substances other than purified cellulose, for example with polyacrylonitrile, advantageously making it possible to form carbon fibers with further improved mechanical properties, in particular having a stress resistance greater than 4,000 MPa after graphitization. These mechanical properties make them particularly suitable for use in fields of application requiring very high strength of the materials used, for example for the manufacture of hydrogen tanks.

[0048] The method according to the invention may comprise the addition to the spinning solution of one or more additives, aimed at better structuring the material, improving the mechanical properties of the fibers formed, etc.

[0049] Each of these additives may in particular be present in the spinning solution at a content of between 1 ppm and 10% by weight, relative to the total weight of the spinning solution, preferably between 1 ppm and 5% by weight, for example between 100 ppm and 1% by weight, relative to the total weight of the spinning solution. Examples of additives that may be added to the spinning solution according to the invention are compatibilizing agents, such as maleic anhydride graft polymers or copolymers. By way of example, mention may in particular be made of Lotader ®< 3300 marketed by the company Arkema, or Beiwa ®< 901 from DzBh.

[0050] In particular embodiments of the invention, the spinning solution contains a non-ionic emulsifying agent, preferably in a concentration of between 0.1 and 1% by weight, for example approximately 0.2% by weight, relative to the weight of ground material, where appropriate relative to the weight of the mixture of ground material and purified cellulose, dissolved in said solvent solution during said step a3 / . The spinning solution may, for example, contain one or more emulsifying agents sold under the name Emulan ®< by the company BASF.

[0051] The spinning solution may be filtered to remove solid particles before proceeding to the next step of the process according to the invention.

[0052] In particular embodiments of the invention, a nanometric-sized carbonaceous filler, or several nanometric-sized carbonaceous fillers, is (are) added to the spinning solution, during the implementation of step a3 / of the method according to the invention, or just before or after. The nanometric-sized carbonaceous fillers are preferably added to the spinning solution in an amount of between 1 ppm and 30% by weight relative to the weight of ground material, where appropriate relative to the weight of the mixture of ground material and purified cellulose, dissolved in the solvent solution during step a3 / . This concentration is preferably between 0.001 and 5%, and in particular between 0.01 and 5%.

[0053] Here, the term "nanometric-sized carbon filler" means a filler comprising an element from the group consisting of single- or multi-walled carbon nanotubes, carbon nanofibers, graphene, graphene oxide, reduced graphene oxide, fullerenes, cellulose nanofibrils, cellulose nanocrystals and carbon black, or any mixture of such elements. Preferably, the nanometric-sized carbon fillers integrated into the spinning solution according to the invention are carbon nanotubes, alone or in a mixture with graphene. Carbon nanotubes are, for example, marketed by the company Arkema under the name Graphistrength ®<.

[0054] The nanometric-sized carbon fillers according to the present invention may have a smallest dimension of between 0.1 and 200 nm, preferably between 0.1 and 160 nm, and preferentially between 0.1 and 50 nm. This dimension can for example be measured by light scattering.

[0055] According to the present invention, the term "graphene" means a flat, isolated and individualized graphite sheet, but also, by extension, an assembly comprising between one and a few dozen sheets and having a flat or more or less undulating structure. This definition thus encompasses FLGs (for Few Layer Graphene, i.e., weakly stacked graphene), NGPs (for Nanosized Graphene Plates, i.e., graphene plates of nanometric dimension), CNSs (for Carbon NanoSheets, i.e., graphene nanosheets), GNRs (for Graphene NanoRibbons, i.e., graphene nanoribbons). However, it excludes carbon nanotubes and nanofibers, which are respectively made up of the winding of one or more graphene sheets in a coaxial manner and the turbostratic stacking of these sheets.The nanometric-sized carbon fillers are preferably incorporated into the spinning solution according to the invention in the form of a liquid dispersion, which may be aqueous or solvent-based.

[0056] The dispersion of nanometric-sized carbon fillers can be carried out by an ultrasonic probe, a ball mill, a high-shear mixer, or any other device conventionally used, and where appropriate in the presence of a surfactant.

[0057] The spinning process implemented in step a4 / of the process according to the invention, to form a continuous cellulose fiber, may be of any type known to those skilled in the art.

[0058] As explained above, in this step, the spinning solution is injected through a spinning die consisting of one or more holes, into a coagulation bath, statically or in flow, directly into the bath (according to the so-called wet-spinning method) or through an air space (according to the so-called dry-jet wet spinning method). Upon contact with the coagulation bath, the fiber solidifies. This advantageously produces a continuous cellulose-based fiber.

[0059] The coagulation bath used can have any classic composition in itself, compatible with the phosphoric acid of the spinning solution and the type of particular solvent spinning process used.

[0060] For example, the coagulation bath can be formed from isopropanol, water, acetone, or any other solvent allowing coagulation of the cellulose on contact, or any of their mixtures.

[0061] In a particular embodiment of the invention, the spinning solution is formed from phosphoric acid, where appropriate in the presence of a non-ionic emulsifier, and the coagulation bath is formed from isopropanol, alone or mixed with water.

[0062] The continuous cellulose-based fiber obtained by the spinning process according to the invention, which may be in the form of monofilament or multifilament, can then be washed and dried.

[0063] If necessary, it is subjected to a step a5 / of drawing, to form a fiber of greater length.

[0064] The step of drawing the fibers can be carried out according to any method and by means of any apparatus known to those skilled in the art for carrying out such an operation. It can in particular be carried out at a temperature causing softening of the material constituting the fibers. For this purpose, the fibers pass successively over a bank of so-called feed rollers, through a furnace brought to said temperature, then over a bank of so-called draw rollers. They undergo drawing between the two banks of rollers, according to the ratio of the rotation speeds of the feed rollers and the draw rollers. They can otherwise be drawn on heated rollers rotating at different speeds.

[0065] Such stretching advantageously allows the polymer chains to be aligned along the axis of the fibers.

[0066] The fibers can optionally be treated at the end of spinning, by radiation treatments, such as gamma rays, beta rays, electron beams, UV rays.

[0067] Then, the resulting cellulose-based fiber, which can be of considerable length, can be put into a reel, for example on a cardboard tube.

[0068] The cellulose-based fiber obtained according to the present invention can then be sized, in a conventional manner in itself, before being subjected to the carbonization step in order to obtain a continuous carbon fiber.

[0069] In a variant of the invention, the nanometric-sized carbon fillers are introduced into the sizing bath, and not into the spinning solution as explained above.

[0070] Step b / of carbonization of the continuous cellulose-based fiber of the process according to the invention is carried out in a conventional manner in itself, using any combination of operating parameters described in the prior art for the carbonization of cellulose fibers.

[0071] It is preferably carried out under an inert atmosphere.

[0072] Carbonization can be carried out continuously, with the fiber passing through the carbonization furnace, or discontinuously, i.e. in static mode, with the fiber preferably being kept under tension in the furnace.

[0073] Prior to carbonization, the continuous cellulose-based fiber may be impregnated with one or more additives, in particular so-called carbonization agents, promoting an increase in the mechanical properties of the carbon fiber finally obtained and an increase in the carbon yield of the carbonization step. These additives are conventional in themselves. For example, the cellulose-based fiber may be impregnated with one or more of the following additives: Brönsted acid compounds or compounds releasing Brönsted acids upon temperature increase, in particular hydrogen halides, halides having any counterion such as sodium, potassium, ammonium, etc., sulfuric acid, sulfates having any counterion such as sodium, potassium, ammonium, etc., organic sulfonic acids carrying an alkyl group, such as methylsulfonic acid, or an aryl group, such as paratoluenesulfonic acid, or any other organic group, organic sulfonates corresponding to the above sulfonic acids and having any counterion such as sodium, potassium, ammonium, etc., phosphoric acid and polyphosphoric acids, and / or phosphates and polyphosphates having any counterion such as sodium, potassium, ammonium, etc.; Lewis acid compounds, in particular boric acid and / or metal halides such as aluminum chloride AlCl 3 or iron chloride FeCl 2; urea; carbon fillers of nanometric size; and / or formulations based on silicone polymers, for example based on polydimethylsiloxane, in particular combined with a crosslinking agent. Such a list is in no way limiting of the invention.

[0074] Impregnation additives based on silicone polymers are particularly preferred in the context of the present invention, since they make it possible to further improve the mechanical properties of the carbon fiber obtained according to the invention. The silicone-type impregnation additives are preferably used at a rate, defined as the mass percentage of additive deposited relative to the total weight of the cellulosic fiber and the additive, of between 0.01 and 20%, preferably between 0.5 and 10%, and more preferably between 1 and 5%.

[0075] Carbonization can be carried out at a temperature between 1000 and 1500°C, and preceded or not by a stabilization step in air at a temperature of around 250°C.

[0076] In particular embodiments of the invention, the carbonization furnace is hermetically sealed and evacuated to a value below 1.10 4< Pa (0.1 bar). It is then filled with an inert gas, such as nitrogen, argon, etc., and a gas leak is created so that the gas flow rate is between 50 and 500 volume renewals per hour. The pressure in the furnace is preferably between 1.10 3< Pa and 5.10 4< Pa above atmospheric pressure. The temperature applied in the carbonization furnace is preferably between 800°C and 1500°C.

[0077] At the end of this carbonization treatment, a carbon fiber is obtained. This carbon fiber can then be subjected, if necessary, to a graphitization treatment, so as to obtain a better structuring of the carbon, and therefore more advantageous mechanical properties. This treatment can for example be carried out by heating the fiber to a temperature between 2000 and 3000°C, under inert gas, for example for a period of between 30 seconds and 10 minutes.

[0078] The cellulose-based fiber according to the invention can otherwise be carbonized continuously, by passing it successively through different furnaces, including a carbonization furnace under an inert atmosphere at a temperature of between 800°C and 1500°C, then a graphitization furnace at a temperature of up to 2500°C. The speed of movement of the fiber in these furnaces is then preferably between 0.1 and 10 m / h.

[0079] The carbon fiber obtained from the process according to the invention can have a diameter of between 5 and 30 µm, and a length of several meters.

[0080] In particular embodiments of the invention, the method comprises a step of forming a sheet of cellulose-based fibers from a plurality of continuous cellulose-based fibers obtained in step a4 / or a5 / . Step b / of carbonizing this continuous cellulose-based fiber is then carried out by carbonizing the sheet of continuous cellulose-based fibers, to form a sheet based on carbon fibers.

[0081] The web formed from the cellulose-based fibers according to the invention can have any shape and size. The fibers can be arranged in fabrics of different weights and weaves, for example taffeta, twill, satin, etc., used alone or in combinations, or in nonwovens, for example in which the fibers are all oriented in the same direction, or in which the fibers are randomly oriented, such as sails, felts or nonwoven films. These are referred to as unidirectional webs.

[0082] The continuous cellulose-based fibers according to the invention can be used in the web, alone or in association with other types of fibers.

[0083] The carbonization of the web of continuous cellulose-based fibers can be carried out according to any carbonization method known to those skilled in the art, in static mode or in continuous movement in a carbonization furnace. The characteristics set out above with reference to the treatment of individual fibers apply in a similar manner to the carbonization of the web of continuous cellulose-based fibers according to the invention.

[0084] The continuous cellulose-based fiber sheets according to the invention can be subjected to carbonization individually, or in the form of a stack of sheets, flat or, if necessary, after shaping into a desired shape.

[0085] A carbon fiber obtained by a process according to the invention meets one or more of the above characteristics.

[0086] This carbon fiber is continuous, and it can have a diameter between 1 and 1000 µm, preferably between 15 and 30 µm, and a length of a few meters.

[0087] It can advantageously have a stress resistance greater than 1200 MPa, preferably greater than or equal to 2000 MPa after graphitization, and / or a Young's modulus greater than 75 GPa, preferably greater than or equal to 200 GPa after graphitization, these parameters being measured according to standard ISO 11566, method B.

[0088] A continuous cellulose-based fiber obtained as an intermediate product during the implementation of a process for manufacturing a carbon fiber according to the invention, at the end of step a / of this process, is made from recycled paper products, alone or in a mixture with other constituents, as listed above.

[0089] This continuous cellulose-based fibre can have a diameter between 10 and 30 µm, and / or a strength between 10 and 40 cN / tex, and / or a Young's modulus between 10 and 30 GPa, these parameters being measured according to the ISO 2062 standard.

[0090] It is advantageously storable and transportable.

[0091] A carbon fiber sheet can be obtained from carbon fibers manufactured according to the invention, said carbon fibers being woven or knitted together or being juxtaposed therein in the form of a non-woven fabric.

[0092] A sheet of carbon fibers can be obtained by a method for manufacturing a carbon fiber according to the invention, this method comprising a step of forming a sheet of cellulose-based fibers from a plurality of continuous cellulose-based fibers obtained in step a4 / or a5 / of the method, and a step of carbonizing said sheet of continuous cellulose-based fibers, to form a sheet based on carbon fibers.

[0093] A three-dimensional fibrous structure can be obtained by stacking a plurality of sheets of continuous cellulose-based fibers manufactured according to the invention, or by stacking a plurality of sheets of carbon fibers manufactured according to the invention, where appropriate shaped to the desired shape, for example to constitute a preform for the manufacture of an article made of composite material based on reinforcing fibers distributed in a binder.

[0094] A carbon fiber or a sheet of carbon fibers manufactured according to the invention can be used for the manufacture of an article made of composite material based on carbon fibers distributed in a matrix of organic polymer resin.

[0095] Another aspect of the invention relates to a method for manufacturing an article made of a composite material based on carbon fibers distributed in a polymer organic resin matrix, this method comprising: implementing a method for manufacturing a carbon fiber according to the invention, comprising manufacturing a carbon fiber by carbonizing an individual cellulose-based continuous fiber, and forming a carbon fiber sheet from a plurality of the carbon fibers thus obtained; or implementing a method for manufacturing a carbon fiber according to the invention, comprising forming a sheet of cellulose-based continuous fibers and carbonizing this sheet of fibers to form a carbon fiber-based sheet; and manufacturing an article made of a composite material from a plurality of carbon fiber sheets thus obtained.

[0096] The manufacture of an article made of composite material from a plurality of carbon fiber sheets obtained in accordance with the present invention can be carried out according to any method conventional in itself for those skilled in the art.

[0097] A composite material is defined throughout this description in a conventional manner, that is to say as consisting of the assembly of several different elementary materials or components linked together, more particularly long mechanically strong fibers, in this case carbon fibers, distributed in a matrix of organic polymer resin. The term resin here defines a polymer compound, which may be of the thermoplastic or thermosetting type, which acts as a structural glue in which the fibers are dispersed in a more or less organized manner. The composite material thus formed has mechanical properties of its own, which are entirely advantageous in terms of mechanical strength and lightness.

[0098] Schematically, the manufacture of such a composite material consists of molding into the desired shape a stack of a plurality of sheets of carbon fibers impregnated with the unpolymerized organic resin, under conditions, in particular temperature, causing the polymerization of this resin.

[0099] The article made of composite material according to the invention can, for example, be manufactured using the technique of draping plies pre-impregnated with resin, at least some of these plies being constituted by the carbon fiber sheets according to the invention, then polymerization of the assembly in an autoclave so as to form, in a conventional manner, the composite material; or even by resin injection or infusion techniques, in particular by the resin transfer technique, known as RTM, for "Resin Transfer Molding", on plies of dry fibers, some of these plies being constituted by the carbon fiber sheets according to the invention.

[0100] The composite material manufactured according to the present invention may be of the monolithic type and / or of the sandwich type, for example with a honeycomb structure.

[0101] The fibres can be arranged in fabrics of different weights and weaves, for example taffeta, twill, satin, etc., used alone or in combinations, or in non-woven fabrics, for example in which the fibres are all oriented in the same direction.

[0102] The carbon fibers obtained in accordance with the present invention can be used alone, or in association with one or more other types of fibers, any configuration of such an association falling within the scope of the present invention.

[0103] Any conventional resin in itself may be used within the scope of the invention, in particular thermosetting type resins, for example epoxy resins, phenolic resins or a mixture of the two, or even thermoplastic resins.

[0104] An article made of composite material based on carbon fibers distributed in an organic polymer resin matrix obtained by a manufacturing method according to the invention, meeting one or more of the above characteristics, finds advantageous application in numerous fields.

[0105] The characteristics and advantages of the invention will appear more clearly in light of the implementation examples below, provided for purely illustrative purposes and in no way limiting the invention. Comparative example 1 - office paper

[0106] For example, a carbon fiber manufacturing process is implemented using a paper product, such as office paper for printing and writing (A4 Clairefontaine Clairalpha 80g / m 2 paper).

[0107] First, this paper is stripped of the fillers it contains. To do this, it undergoes the following procedure.

[0108] The sheets of paper are first shredded automatically.

[0109] The resulting chips are then pre-treated to reduce their mineral content and other impurities. To do this, they are placed in a pulper with water to form a paper pulp suspended in water, at 20 g / L of dry matter. This pulp is cleaned with a pressurized water jet three times on a 50 µm sieve (hyperwashing), until clear water is obtained. The hyperwashed pulp is then steamed at 95°C overnight, then ground to form larger particles of approximately 250 µm.

[0110] Thermogravimetric analyses at the various stages of this process enable its effectiveness to be checked by determining the rate of mineral fillers contained in the material. To this end, a sample of material is first subjected to a temperature of 500°C in air, in order to determine its mineral filler content in general; then to a temperature of 900°C in air, in order to determine the proportion of carbonate in these mineral fillers.

[0111] It was thus determined that the starting paper contained 14% mineral fillers (composed of 94% carbonates) and 5% water. After pretreatment, the mineral filler content in the ground paper pulp was less than 1%.

[0112] The ground paper pulp is then dissolved in phosphoric acid, in an amount by weight of 10% by weight relative to the total weight of the mixture.

[0113] To this solution is added an aqueous dispersion of carbon nanotubes, formed by means of a reactor connected to a ball mill and an ultrasonic probe, by dispersing carbon nanotubes in an amount of 0.9% by weight, in the presence of the surfactant marketed under the name Brij ®< S20 at a concentration of 1.2% by weight, in water.

[0114] This aqueous dispersion is added to the spinning solution in an amount of 0.1% by weight relative to the weight of ground pulp contained in the spinning solution. The quality of dissolution is checked by optical microscopy and by viscosity measurements. This dispersion does not contain any aggregates larger than or equal to 1 µm.

[0115] A non-ionic emulsifier, such as the product marketed under the name Emulan ®< , is also added to this solution in an amount of 0.2% by weight relative to the weight of ground pulp contained in the spinning solution. Such a non-ionic emulsifier advantageously facilitates the impregnation of the cellulose of the recycled paper with phosphoric acid.

[0116] The mixture is heated to 45 °C for 15 min under reduced pressure of 100 mbar and stirring at 40 rpm, then cooled for 3 h at -10 °C under the same reduced pressure and stirring. It is then placed at 0 °C overnight, still under the same conditions of reduced pressure and stirring, and finally cooled to -10 °C.

[0117] The resulting spinning solution is extruded through a spinning die with 500 holes each of 80 µm in diameter, and injected directly into a coagulation bath composed of an isopropanol / water mixture (60 / 40 by volume).

[0118] Spinning parameters are for example: spinning solution temperature 0°C, transfer pump speed 800 rpm, coagulation bath temperature 20°C.

[0119] It forms in the coagulation bath cellulose fibers, in which carbon nanotubes are trapped and well dispersed.

[0120] The fibers thus formed are drawn into a neutralization bath, based on potassium hydroxide KOH, at 20°C, in order to eliminate the phosphoric acid remaining on the fibers, then into a water washing bath at 15°C, before being dried by hot air in an oven at 250°C.

[0121] They are then stretched on heated rollers at a temperature of 160°C, in a conventional manner.

[0122] The cellulose-based fibers are then wound at a winding speed of 12 m / min.

[0123] The result is a cellulose-based multifilament fiber with a diameter of approximately 25 µm and several meters in length.

[0124] This continuous cellulose-based fiber has a strength of between 10 and 40 cN / tex and a Young's modulus of between 10 and 30 GPa (these parameters are measured according to the protocols described in ISO 2062). This continuous cellulose-based fiber can then be subjected to a stabilization step in air at a temperature of around 250°C, prior to a carbonization step in nitrogen up to 1200°C.

[0125] This process produces a carbon fiber with, in the non-graphitized state, a stress resistance greater than 1,200 MPa and a Young's modulus greater than 75 GPa (these parameters being measured according to the protocols described in standard ISO 11566, method B). Example 1 - office paper

[0126] A method for manufacturing a carbon fiber in accordance with the invention is carried out using the same paper product as that used in Comparative Example 1 above, office paper for printing and writing (A4 Clairefontaine Clairalpha 80g / m 2 paper).

[0127] The paper sheets are first shredded automatically. The resulting shreds are ground to form particles approximately 250 µm in size.

[0128] In accordance with the invention, the ground paper is then directly dissolved in an 85% aqueous phosphoric acid solution (containing a phosphoric acid concentration of 85% by volume relative to the total volume of said aqueous solution), in an amount of 7% by weight relative to the total weight of the mixture, to form a spinning solution.

[0129] A non-ionic emulsifier, such as the product marketed under the name Emulan ®, is added to this solution in an amount of 0.2% by weight relative to the weight of ground paper contained in the spinning solution. Such a non-ionic emulsifier advantageously facilitates the impregnation of the cellulose of the recycled paper with phosphoric acid.

[0130] The mixture is heated to 45 °C for 15 min under reduced pressure of 100 mbar and stirring at 40 rpm, then cooled for 3 h at -10 °C under the same reduced pressure and stirring. It is then placed at 0 °C overnight, still under the same conditions of reduced pressure and stirring, and finally cooled to -10 °C.

[0131] In a variant of the process, the shredded paper may be mixed, in the phosphoric acid solution, with cellulose from wood obtained by a papermaking process, of a higher degree of purity, for example in a paper shred / high purity cellulose weight ratio of 20 / 80 or 50 / 50.

[0132] The spinning solution is extruded through a spinning die with 500 holes each of 80 µm in diameter, and injected directly into a coagulation bath composed of an isopropanol / water mixture (60 / 40 by volume).

[0133] Spinning parameters are for example: spinning solution temperature 0°C, transfer pump speed 600 rpm, coagulation bath temperature 20°C.

[0134] It is formed in the coagulation bath of cellulose fibers.

[0135] The fibers thus formed are drawn into a neutralization bath, based on potassium hydroxide KOH at 3% by weight in water, at 20°C, in order to eliminate the phosphoric acid remaining on the fibers, then into a water washing bath at 15°C, before being dried by hot air in an oven at 250°C. They are then stretched on heated rollers at a temperature of 160°C, in a conventional manner.

[0136] The cellulose-based fibers are then wound at a winding speed of 10 m / min.

[0137] The result is a cellulose-based multifilament fiber with a diameter of approximately 28 µm and several meters in length.

[0138] This continuous cellulose-based fiber has a strength of between 10 and 40 cN / tex and a Young's modulus of between 10 and 40 GPa (these parameters are measured according to the protocols described in ISO 2062). This continuous cellulose-based fiber can then be subjected to a stabilization step in air at a temperature of around 250°C, prior to a carbonization step in nitrogen up to 1200°C.

[0139] Prior to the carbonization step, the continuous cellulose-based fiber can be impregnated with so-called carbonization agents, promoting an increase in the mechanical properties of the carbon fiber finally obtained, and in the carbon yield of the carbonization step.

[0140] The carbonization step can be followed by a graphitization step, by heating the fiber to a temperature between 2000 and 3000°C, under inert gas, for a duration between 1 and 10 min.

[0141] At the end of this process, a carbon fiber is obtained having particularly satisfactory mechanical properties, in particular, in the non-graphitized state, a stress resistance greater than 1,200 MPa and a Young's modulus greater than 75 GPa (these parameters being measured according to the protocols described in standard ISO 11566, method B). These mechanical properties are as good as those of the carbon fibers obtained in Comparative Example 1, in which the paper shred was subjected to a pretreatment before its dissolution in the aqueous phosphoric acid solution.

[0142] Furthermore, without additives, the carbon fiber obtained according to the invention has a carbon yield of 15.4% at 1000°C under nitrogen and an inorganic content of less than 0.007% (these parameters being measured by thermogravimetric analyses at 10°C / min under nitrogen up to 1000°C then passage under air at 1000°C).

[0143] This carbon fiber was obtained at very low cost, compared to carbon fibers formed by conventional processes proposed by the prior art.

[0144] This carbon fiber can be used for many applications, for example for the manufacture of composite material articles, in which said fibers are dispersed in an organic polymer resin. Example 2 - paper towels

[0145] A method of manufacturing a carbon fiber according to the invention is carried out using paper towels.

[0146] This paper is ground before being dissolved in phosphoric acid (85% aqueous solution), in an amount by weight of 7%, relative to the total weight of the mixture.

[0147] A non-ionic emulsifier, such as the product marketed under the name Emulan ®< , is added to this solution in an amount of 0.2% by weight relative to the weight of ground pulp contained in the spinning solution.

[0148] The mixture is heated to 45°C for 30 min under reduced pressure of 100 mbar and stirring at 40 rpm, then cooled to -10°C under the same reduced pressure and stirring overnight.

[0149] The resulting spinning solution is extruded through a spinning die with 500 holes each of 80 µm in diameter, and injected directly into a coagulation bath composed of an isopropanol / water mixture (60 / 40 by volume).

[0150] Spinning parameters are for example: spinning solution temperature 0°C, transfer pump speed 600 rpm, coagulation bath temperature 20°C.

[0151] Cellulose fibers are formed in the coagulation bath.

[0152] The fibers thus formed are drawn into a neutralization bath, based on potassium hydroxide KOH, at 20°C, in order to eliminate the phosphoric acid remaining on the fibers, then into a water washing bath at 15°C, before being dried by hot air in an oven at 260°C.

[0153] They are then stretched on heated rollers at a temperature of 120°C, in a conventional manner.

[0154] These cellulose fibers are then subjected to a carbonization step, according to the protocol indicated in example 1 above.

Claims

1. Method for manufacturing a carbon fibre from a manufactured paper product, characterised in that it comprises: a / preparing a cellulose-based fibre, comprising successive steps of: a1 / grinding said paper product to obtain a ground paper material, a3 / directly dissolving the ground material obtained at the end of step a1 / in an aqueous phosphoric acid solution, to form a so-called spinning solution, a4 / from said spinning solution, manufacturing a cellulose-based continuous fibre by implementing a solvent spinning process, a5 / optionally, drawing the resulting cellulose-based continuous fibre, b / and carbonising said cellulose-based continuous fibre to form a carbon fibre.

2. Method for manufacturing a carbon fibre according to claim 1, wherein the spinning solution is subjected to said solvent spinning process without being subjected to a prior drying step.

3. Method for manufacturing a carbon fibre according to one of claims 1 or 2, wherein said aqueous phosphoric acid solution contains a concentration of phosphoric acid comprised between 75 and 99 vol%, based on the total volume of said aqueous solution.

4. Method for manufacturing a carbon fibre according to any one of claims 1 to 3, wherein, in step a / of preparing a cellulose-based fibre, the temperature does not exceed 45°C.

5. Method for manufacturing a carbon fibre according to any one of claims 1 to 4, wherein, in step a1 / the grinding is carried out in such a way as to obtain a ground material formed of particles, the largest dimension whereof is comprised between 200 and 500 µm.

6. Method for manufacturing a carbon fibre according to any one of claims 1 to 5, wherein said paper product is formed of paper derived from a chemical papermaking pulp.

7. Method for manufacturing a carbon fibre according to any one of claims 1 to 6, wherein, in step a3 / the ground material is mixed with purified cellulose.

8. Method for manufacturing a carbon fibre according to any one of claims 1 to 7, wherein, in step a3 / the concentration of ground material, or where appropriate of the mixture of ground material and purified cellulose, dissolved in the aqueous phosphoric acid solution, is comprised between 1 and 50 wt%, preferably between 5 and 30 wt%, based on the total weight of the spinning solution.

9. Method for manufacturing a carbon fibre according to any one of claims 1 to 8, wherein the spinning solution contains a non-ionic emulsifying agent, preferably at a concentration comprised between 0.1 and 1 wt%, based on the weight of ground material, where appropriate based on the weight of the mixture of ground material and purified cellulose, dissolved in said aqueous phosphoric acid solution in said step a3 / .

10. Method for manufacturing a carbon fibre according to any one of claims 1 to 9, wherein a nanosized carbonaceous filler, in particular chosen from among carbon nanotubes and graphene, alone or mixed together, is added to the spinning solution, preferably at a concentration comprised between 1 ppm and 5 wt%, based on the weight of the ground material, where appropriate based on the weight of the mixture of ground material and purified cellulose, dissolved in said aqueous phosphoric acid solution in said step a3 / .

11. Method for manufacturing a carbon fibre according to any one of claims 1 to 10, comprising a step of forming a web of cellulose-based fibres from a plurality of cellulose-based continuous fibres obtained in step a4 / or a5 / , and wherein step b / of carbonising said cellulose-based continuous fibre is carried out by carbonising said web of cellulose-based continuous fibres to form a carbon fibre-based web.

12. Method for manufacturing an article made of composite material containing carbon fibres distributed in an organic polymer resin matrix, characterised in that it comprises: - implementing a method for manufacturing a carbon fibre according to any one of claims 1 to 10 and forming a carbon fibre web from a plurality of the carbon fibres obtained, or implementing a method for manufacturing a carbon fibre according to claim 11 to form a carbon fibre-based web, - and manufacturing said article made of composite material from a plurality of carbon fibre webs thus obtained.