Process for manufacturing cellulose nanocrystals

Abstract

Method for manufacturing cellulose nanocrystals The invention relates to a method (1) for manufacturing cellulose nanocrystals (3) comprising a preparation (10) of a deep eutectic solvent (102) by mixing a quaternary ammonium salt (100) and a hydrogen bond donor compound (101, in a mechanochemical reactor (4), a formation (11) of a reaction medium (110) comprising cellulose fibers (2) and the deep eutectic solvent (102), and a mechanochemical treatment (12) of the reaction medium (110) so as to obtain cellulose nanocrystals (3) from the cellulose fibers (2). This mechanochemical treatment (12) allows the acid hydrolysis of the amorphous cellulose and the surface modification of the cellulose nanocrystals (3), while activating this reaction so as to limit the temperature and the treatment time compared to existing solutions. Figure for the abstract: Fig. 1

Description

Title of the invention: Method for manufacturing cellulose nanocrystals Technical field

[0001] The present invention relates to the field of manufacturing cellulose nanocrystals from cellulose fibers. It finds particularly advantageous and non-limiting application in the field of packaging, medical, paper, or even composite materials. STATE OF THE ART

[0002] Nanocelluloses are so-called "green" materials: bio-sourced, biodegradable and renewable. Nanocelluloses also have very good mechanical properties. They are divided into two categories: cellulose nanocrystals (abbreviated as CNC, from the English Cellulose NanoCrystals) and cellulose nanofibrils (abbreviated as CNF, from the English Cellulose NanoFibers). CNCs and CNFs have been the subject of increasing interest in recent decades. Indeed, between 1990 and 2019, publications concerning them increased annually by 29% and 26% per year. These data demonstrate the interest in these bio-sourced nanomaterials.

[0003] Conventionally, CNCs are manufactured by acid hydrolysis from cellulose fibers. For this, mineral acids are generally used, such as acids of formula H2SO4, HCl, H3PO4 or HBr. The CNCs can then be chemically treated to functionalize them. The main chemical treatments used are TEMPO oxidation, which allows the creation of carboxylic acid groups -COOH in the C6 position on the cellulose, or cationization which induces the creation of a positive charge on the surface of the CNCs.

[0004] Today, the use of cellulose nanocrystals is limited, in particular because of their cost. This is explained by their still difficult manufacturing on an industrial scale, in particular due to the corrosion of equipment by mineral acid, the hydrolytic degradation of cellulosic materials (reducing yields) and the difficulty in recovering the acid during effluent treatment. In addition, these pretreatments can involve toxic products, thus reducing the "green" aspect of these materials, and potentially limiting their use.

[0005] Several solutions are being considered to limit this. Organic acids can be used as a replacement for mineral acids. Organic acids are less corrosive and easier to regenerate. These organic acids can be used in aqueous solution or in their liquid form but remain expensive and difficult to recycle.

[0006] Another very recent solution involves the use of a new class of More environmentally friendly solvents: deep eutectic solvents. These solvents, which are slightly volatile or non-volatile, allow the isolation of CNCs while retaining the advantages of organic solvents.

[0007] However, their use requires longer processing times and higher reaction temperatures than those used in usual processes with mineral acids. These solutions therefore remain limited in allowing industrialization of the manufacture of CNCs.

[0008] An object of the present invention is therefore to propose an improved method for manufacturing cellulose nanocrystals, and in particular a method more suited to the constraints of industrial manufacturing, for example in terms of processing time, temperature, yield and / or cost.

[0009] Other objects, features and advantages of the present invention will become apparent from a consideration of the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0010] To achieve this objective, according to one embodiment, a method for manufacturing cellulose nanocrystals is provided comprising: - a supply of a deep eutectic solvent obtained by mixing a quaternary ammonium salt and a hydrogen bond donor compound, the hydrogen bond donor compound being capable of forming the deep eutectic solvent with the quaternary ammonium salt, - in a mechanochemical reactor, formation of a reaction medium comprising cellulose fibers and the deep eutectic solvent, - a mechanochemical treatment of the reaction medium in order to obtain cellulose nanocrystals from cellulose fibers.

[0011] The deep eutectic solvent thus allows a chemical treatment of the cellulose fibers which is here combined with a mechanical treatment in the mechanochemical reactor. This combined mechanochemical treatment allows the acid hydrolysis of the amorphous cellulose and the surface modification of the CNCs, while activating this reaction so as to limit the temperature and the treatment time compared to existing solutions. The CNCs can further be functionalized during their preparation. Indeed, the hydrogen bond donor can be condensed with the surface groups of the cellulose, and in particular the C6 groups of the cellulose. This has been observed in particular when the deep eutectic solvent is prepared from a hydrogen bond donor compound comprising at least one carboxylic acid, and preferably two carboxylic acids.A single step can thus allow the obtaining of CNCs as well as their functionalization to give them other properties.

[0012] During the development of the invention, it was further demonstrated that this mechanochemical treatment made it possible to achieve higher functionalization rates and yields than what would be obtained by carrying out a chemical treatment with a deep eutectic solvent and a mechanical treatment in a temporally dissociated manner. In particular, yields greater than 60% by mass could be obtained relative to the initial quantity of cellulose fibers introduced into the reactor.

[0013] The process is therefore improved compared to existing solutions implementing only deep eutectic solvent treatment. In particular, the process is better suited to industrial manufacturing constraints, for example in terms of treatment time, temperature and / or cost. BRIEF DESCRIPTION OF THE FIGURES

[0014] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0015] [Fig.l] [Fig.l] represents a diagram of the manufacturing process, according to an exemplary embodiment.

[0016] [Fig.2] [Fig.2] represents a flowchart of the manufacturing process, according to a example of realization.

[0017] [Fig.3A][Fig.3B] Figures 3A and 3B represent a schematic view of the CNCs obtained, according to two examples of implementation of the process.

[0018] [Fig.4] [Fig.4] represents a graph of the yield in mass percentage of the manufacturing process depending on the duration of the mechanochemical treatment, according to an exemplary embodiment.

[0019] [Fig.5] [Fig.5] represents a transmission electron microscopy image (MET) of the CNCs obtained, according to an example of implementation of the process.

[0020] [Fig.6] [Fig.6] represents an atomic force microscopy (AFM) image of the CNCs obtained, according to an example of implementation of the process.

[0021] [Fig.7A][Fig.7B] Figures 7A and 7B represent graphs of the distribution of the dimensions of the CNCs obtained, respectively of their length and their diameter, according to an exemplary embodiment of the method.

[0022] [Fig.8] [Fig.8] represents a nuclear magnetic resonance (NMR) spectrum of 13C cellulose fibers and CNCs obtained according to an example of implementation of the process.

[0023] [Fig.9A][Fig.9B] Figures 9A and 9B represent an X-ray diffractogram of the CNCs obtained according to two exemplary embodiments of the method, in comparison with that of residual fibers.

[0024] [Fig. 10] [Fig. 10] represents an FT-IR absorbance spectrum of cellulose fibers and CNCs obtained according to two examples of implementation of the process.

[0025] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular the relative dimensions between the CNCs, the cellulose fibers, the reactor, are not representative of reality. DETAILED DESCRIPTION

[0026] Before commencing a detailed review of embodiments of the invention, optional features are set out below which may optionally be used in combination or alternatively.

[0027] According to one example, the method comprises preparing the deep eutectic solvent by mixing the quaternary ammonium salt and the hydrogen bond donor compound, the hydrogen bond donor compound being capable of forming the deep eutectic solvent with the quaternary ammonium salt.

[0028] According to one example, the quaternary ammonium salt is choline chloride. During the development of the invention, it was indeed demonstrated that choline chloride was particularly suitable for the formation of the deep eutectic solvent to obtain the nanocrystals.

[0029] According to one example, the hydrogen bond donor compound comprises at least one carboxylic acid group. A carboxylic acid group makes it possible to obtain a deep eutectic solvent which can induce functionalization of the C6 carbon of the cellulose.

[0030] According to one example, the hydrogen bond donor compound comprises at least two carboxylic acid groups. Two carboxylic acid groups on the hydrogen bond donor make it possible to obtain a deep eutectic solvent introducing a carboxylic group -COOH on the C6 carbon of the cellulose by condensation to form an ester bond. At a neutral and / or basic pH, this makes it possible to introduce anionic groups on the CNCs and therefore to improve their stability. This solution makes it possible in a single step to obtain the CNCs while functionalizing them and thus providing new properties to the CNCs. The method therefore allows the manufacture of functionalized cellulose nanocrystals.

[0031] According to one example, at the end of the mechanochemical treatment, the cellulose nanocrystals have a quantity of carboxylate group greater than 100 peq / g of cellulose nanocrystals. The quantity of carboxylate group may be substantially less than 3000 peq / g of cellulose nanocrystals. Preferably, the quantity of carboxylate group may be substantially equal to 1500 peq / g of cellulose nanocrystals. Good stability of the CNCs is thus obtained, thanks to the mechanochemical treatment by NADES prepared from a hydrogen bond donor compound comprising at least two carboxylic acid groups.

[0032] According to one example, the hydrogen bond donor compound is selected from the group consisting of citric acid and oxalic acid, preferably the hydrogen bond donor compound is oxalic acid. The mechanochemical treatment resulting from the deep eutectic solvent derived from citric acid or oxalic acid, in particular with choline chloride, makes it possible to increase the yield and the functionalization rate of the CNCs. Oxalic acid, in comparison with citric acid, has a lower pKa and thus makes it possible to further improve the yield and the properties of the CNCs obtained.

[0033] According to one example, at least one carboxylic group of the hydrogen bond donor compound has a pKa less than or equal to 4.

[0034] According to one example, during the mechanochemical treatment, the reaction medium is free of additional solvent. According to one example, at least during the mechanochemical treatment, the reaction medium comprises only cellulose fibers and the deep eutectic solvent, as well as the reaction products resulting from this treatment.

[0035] According to one example, the mechanochemical treatment is configured to obtain a mass yield of CNC greater than or equal to 50%, preferably greater than or equal to 60% relative to the initial quantity of cellulose fibers introduced into the reactor 4.

[0036] According to one example, the mechanochemical treatment is carried out for a duration of less than or equal to 2 hours. The treatment time is thus limited compared to existing solutions, reducing the cost and further improving the compatibility of the process with industrial constraints.

[0037] According to one example, the mechanochemical treatment is carried out at a temperature between 15°C and 30°C, preferably substantially equal to 25°C. The treatment temperature is thus limited compared to existing solutions, here also reducing the cost and further improving the compatibility of the process with industrial constraints. According to one example, the process does not include a step of heating the reaction medium at least during the mechanochemical treatment. According to one example, during the mechanochemical treatment, the temperature of the reaction medium is maintained by a temperature regulation device, at a temperature less than or equal to 30°C, preferably 25°C.

[0038] According to one example, the mechanochemical reactor is a mechanochemical mill, for example a ball mill.

[0039] According to one example, the mechanochemical treatment is carried out at a vibration frequency of between 5 Hz and 100, preferably between 5 Hz and 50 Hz, preferably substantially equal to 30 Hz.

[0040] According to one example, after the mechanochemical treatment, the method comprises washing the reaction medium. The washing makes it possible to reduce the quantity, or even eliminate, the deep eutectic solvent from the reaction medium. The washing can therefore be carried out with a washing solvent distinct from the deep eutectic solvent. The washing solvent can be an aqueous solution, and preferably water.

[0041] According to one example, the washing is done by dialysis of the reaction medium by a dialysis membrane. Thus, the deep eutectic solvent can be replaced by the dialysis solvent to facilitate the subsequent use of the CNCs, functionalized or not. The dialysis of the reaction medium is preferably done with an aqueous solution, and preferably water.

[0042] According to one example, after the mechanochemical treatment, and where appropriate after washing the reaction medium, the method comprises separating the reaction medium into a first fraction comprising the cellulose nanocrystals and a second fraction comprising residual fibers. The first fraction comprising the cellulose nanocrystals is thus recovered in isolation from the second fraction. The CNCs obtained are therefore purer.

[0043] According to one example, at the end of the mechanochemical treatment, and preferably before washing the reaction medium, the reaction medium comprises a mass fraction of cellulose nanocrystals greater than or equal to 20%, preferably greater than or equal to 40%.

[0044] According to one example, the separation of the reaction medium comprises centrifugation of the reaction medium followed by sampling one of the first and second fractions, for example the first fraction.

[0045] According to one example, the second fraction is recovered for use in a process for manufacturing cellulose nanofibers. The process thus allows for the revalorization of residual fibers for the manufacture of CNF.

[0046] Several embodiments of the invention implementing successive steps of the manufacturing method are described below. Unless explicitly stated, the terms “successive”, “following”, “following” and their equivalents do not necessarily imply, even if this is generally preferred, that the steps follow one another immediately, intermediate steps being able to separate them.

[0047] Furthermore, the term “step” means the carrying out of a part of the method, and can designate a set of sub-steps.

[0048] Furthermore, the term "step" does not necessarily mean that the actions carried out during a step are simultaneous or immediately successive. Certain actions of a first step may in particular be followed by actions linked to a different step, and other actions of the first step may be repeated subsequently. Thus, the term step does not necessarily mean unitary and inseparable actions in the time and in the sequence of process phases.

[0049] The term “compound or material based on” a material A means a compound or material comprising this material A, and possibly other materials.

[0050] The word “bio-sourced” designates materials of natural origin, for example from renewable resources, and more particularly materials from biomass of animal, algal or plant origin.

[0051] The term "cellulose" or "cellulose fibers" means a polysaccharide that forms the main constituent of the cell wall of plant tissues and contributes to their support and rigidity. Cellulose comes from wood (which is the main source), cotton (whose fibers are almost pure cellulose), flax, hemp and other plants. It is also a constituent of several algae and some fungi.

[0052] It is known to form cellulose nanocrystals from natural cellulose fibers, and in particular from cellulose fibers derived from softwood or hardwood pulps. Cellulose nanocrystals are naturally present in cellulose fibers in the form of crystalline domains and typically comprise at least 50% by number of nano-objects, and typically CNCs are nano-objects (i.e., objects of which at least one of the dimensions is between 1 and 100 nanometers -nm). Cellulose nanocrystals (CNCs) are also commonly referred to as crystalline nanocellulose, cellulose nanocrystals, nanocrystalline cellulose, and cellulose nanowhiskers. CNCs typically have a diameter of between 1 and 50 nm and a length of between 100 nm and 2000 nm.

[0053] It is also known to form cellulose nanofibers from cellulose fibers, and in particular from cellulose fibers derived from softwood or hardwood pulp. Cellulose nanofibers are more particularly in the form of microfibers or microfibrils, MFC, or CMF (abbreviated from the English cellulose microfibrils), or nanofibers or nanofibrils, NFC or CNF (abbreviated from the English cellulose nanofibrils). Cellulose micro- or nanofibrils typically have a diameter of between 5 and 100 nm and a length of between 0.2 and 5 μm. It is noted that, in the context of the present invention, the terms “nanofibrillated cellulose” or “cellulose nanofibers” are used interchangeably to designate nanofibrillated cellulose, or cellulose nanofibers (NFC), and microfibrillated cellulose, or cellulose microfibers (MFC).

[0054] A parameter “substantially equal / greater / less than” a given value means that this parameter is equal / greater / less than the given value, to within plus or minus 10%, or even plus or minus 5%, of this value.

[0055] The method 1 is now described in more detail according to several exemplary embodiments with reference to the figures. [Fig.2] shows in dotted lines op- tional aspects of the process.

[0056] The method 1 comprises the provision of a deep eutectic solvent 102. Deep eutectic solvents (DES, also called Natural Deep Eutectic Solvents, NADES) are solvents which are increasingly used as a “green” alternative to ionic liquids. NADES are solvents formed by mixing at least two solid compounds 100, 101 at a proportion corresponding to the eutectic point. This mixture then behaves like a pure substance. NADES are generally liquid at room temperature, which facilitates their use as a reaction solvent at low temperature, for example at room temperature. It can be provided that this solvent is purchased as is from the mixture of the two compounds 100, 101. Alternatively, illustrated for example by FIGS. 1 and 2, the method can comprise the preparation 10 of the NADES by mixing.

[0057] In a mechanochemical reactor 4, a reaction medium 110 is then formed 11 with the NADES and cellulose fibers 2. The cellulose of the cellulose fibers 2 comprises crystalline domains 2a which will give the CNCs by the method 1, and amorphous fibrous domains 2b. The reaction medium 110 is then subjected to a mechanochemical treatment 12 configured so as to obtain cellulose nanocrystals 3 by hydrolysis from the cellulose fibers 2. It can be considered that the CNCs 3 are “extracted” from the cellulose fibers 2. The reaction medium is in particular maintained under the experimental conditions of the mechanochemical treatment 12 until the CNCs 3 are obtained.

[0058] Six phenomena occur during the mechanochemical treatment 12, causing morphological, structural and chemical changes in the cellulose fibers 2. During the first moments of grinding, the NADES 102 penetrates into the cellulose fibers 3 through the defects in the walls. During the same period, the mechanical treatment causes the fibers to fragment into micrometric particles. This fragmentation thus facilitates the penetration of the NADES 102. The NADES 102 hydrolyzes the amorphous part 2b of the cellulose, thus reducing its degree of polymerization and releasing CNC aggregates. In parallel, the NADES 102 functionalizes the CNCs 3, as illustrated by the passage between the aggregates 3a and 3b. Finally, mechanical actions allow the disintegration of the CNC aggregates 3a, 3b into individualized CNCs 3. Note that to simplify the illustration in [Fig.l], these steps are shown as successive and distinct.In reality, these phenomena can take place in parallel, or more or less successively depending on the progress of the mechanochemical treatment 12 and the level of grinding of the cellulose fibers 3. .

[0059] This combined chemical and mechanical action makes it possible to facilitate the action of NADES 102 on the cellulose fibers 2, by mechanical grinding of the fibers 2 on the one hand and in addition by providing energy to the reaction medium 110. Thus, the temperature and the processing times are limited compared to existing solutions. The CNC 3 are advantageously functionalized during their preparation. A single step thus allows the CNC 3 to be obtained as well as their functionalization to give them other properties. In the mechanochemical treatment, the mechanical treatment is at least partly simultaneous, and preferably totally simultaneous, with the chemical action of the NADES. Process 2 may be free of additional mechanical treatment of the reaction medium 110.

[0060] The mechanochemical treatment 12 can furthermore form residual fibers 4, in particular from the amorphous parts 2b of the cellulose.

[0061] The mechanochemical reactor 4 is a reactor configured to exert a mechanical stress on the reaction medium 110 that it contains. There are several types of mechanochemical reactors that can be used within the scope of the invention. According to one example, the mechanochemical reactor 4 is a ball mill 40. It can be provided that the mechanochemical reactor is another type of mechanochemical mill, for example a planetary mill. In the following, it is considered, without limitation, that the mechanochemical reactor 4 is a ball mill.

[0062] The preparation 11 of the NADES and the mechanochemical treatment 12 can be carried out in the mechanochemical reactor 4. Alternatively, the preparation 11 of the NADES can be carried out prior to its introduction into the mechanochemical reactor 4, for example in another mechanochemical reactor 4 or any other element allowing the mixing of the compounds 100, 101 to form the NADES. The cellulosic fibers 2 can be introduced into the reactor 4 after the introduction and / or the preparation of the NADES in the reactor 4. Alternatively, it can be provided that the compounds 100, 101 to form the NADES and the cellulosic fibers 4 are introduced into the reactor 4, in any order relative to each other, and that the mixing of the compounds 100, 101 is then carried out, in the presence of the cellulosic fibers.

[0063] Each step of method 1 is now described in more detail.

[0064] The preparation 11 of NADES 102 is first described. The preparation of NADES is easy compared to that of ionic liquids which require several stages of chemical synthesis and purification. It is a simple mixture of compounds 100, 101 composing NADES in good proportion, until a homogeneous liquid is obtained. These components are a pair of a hydrogen bond donor and an acceptor of this bond. The table below summarizes the main types of NADES known today, and the nature of the compounds mixed for their formation. [Tables 1] NADES Type Mixed Compounds Type I Quaternary Ammonium Salt + Metal Chloride Type II Quaternary Ammonium Salt + Hydrated Metal Chloride Type III Quaternary Ammonium Salt + Hydrogen Bond Donor Type IV Hydrated Metal Chloride + Hydrogen Bond Donor

[0065] In the context of the present invention, NADES 102 is a type III NADES, prepared from the mixture of a quaternary ammonium salt 100, and a hydrogen bond donor compound 101. The quaternary ammonium salt 100, and the hydrogen bond donor compound 101 are capable of forming together the NADES. A person skilled in the art knows which compound 100, 101 to choose to obtain a type III NADES. Examples are further given below.

[0066] The quaternary ammonium salt 100 comprises an ammonium cation carrying four groups attached to a nitrogen atom N, and a counterion. For example, the quaternary ammonium salt 100 may comprise the choline cation, associated with a counterion. For example, the quaternary ammonium salt 100 is choline chloride, having the following chemical formula. [Chem.l]

[0067] The hydrogen bond donor compound 101 may be any hydrogen bond donor compound capable of forming a NADES 102 with the quaternary ammonium salt 100. The hydrogen bond donor compound 101 may, for example, be an alcohol. Preferably, the hydrogen bond donor compound 101 comprises at least one carboxylic acid group. This facilitates good functionalization of the C6 carbon of the cellulose of the CNCs 3, by formation of an ester bond, as for example illustrated by the groups 30 in FIGS. 3A and 3B. In [Fig.3A] for example, a carboxylic acid of formula R-COOH was used to form the NADES. Functionalization of the CNC by the R group can be observed via an ester bond. This example is more generally applicable depending on the NADES used. We therefore understand that the properties of CNC 3 can be modified depending on their surface functionalization.

[0068] According to one example, the hydrogen bond donor compound 101 comprises at least two carboxylic acid groups. Thus, a first group can be used for the functionalization of the CNCs 3. The remaining group(s) can form carboxylate groups on the surface of the CNCs so as to improve their stability, as illustrated for example in [Fig.3B] following functionalization by a NADES formed from oxalic acid. At the end of the mechanochemical treatment 12, the CNCs 3 can have a quantity of carboxylic group (which can be deprotonated to carboxylate) of between 100 and 3000 peq / g of CNC, preferably substantially equal to 1500 peq / g. The microequivalents peq are given in molar quantity relative to the mass of CNC. This measurement can be carried out by conductometric assay. For this, according to an example, a known quantity of CNC is dispersed in a volume of water.The pH of the medium is then reduced by adding a known quantity of acid. A sodium hydroxide solution is then added to the reaction medium in small successive quantities. With each addition, the conductivity of the medium is measured. A conductivity curve of the medium is thus obtained as a function of the quantity of sodium hydroxide added. This curve makes it possible to deduce the molar quantity of carboxylic groups.

[0069] According to one example, the hydrogen bond donor compound 101 may be oxalic acid, having the following chemical formula. [Chem. 2]

[0070] According to one example, the hydrogen bond donor compound 101 may be citric acid, having the following chemical formula. [Chem. 3] OH

[0071] Oxalic acid, in comparison with citric acid, has a lower pKa and thus makes it possible to further improve the yield and properties of the CNCs obtained, as illustrated in more detail by way of example below. Oxalic acid is therefore used in preference to citric acid.

[0072] The molar ratio between the quaternary ammonium salt 100 and the hydrogen bond donor compound 101 is chosen so as to form the eutectic mixture of NADES 102. For the choline chloride-oxalic acid and choline chloride-citric acid couples, the molar ratio is preferably 1:1.

[0073] NADES 102 may be prepared by mixing compounds 100, 101. According to one example, NADES 102 is prepared by mixing in the mechanochemical reactor 4 under the action of the mechanical stress exerted in the reactor, so as to obtain NADES in liquid form. This mixing may advantageously be done at room temperature, without temperature control. The temperature obtained in the reactor at the end of the mechanochemical treatment may, for example, be between 40°C and 50°C. Alternatively, the mixture may be maintained at a given temperature by a temperature control device, for example included in the reactor 4.

[0074] The reaction medium 110 may be formed 11 by mixing the NADES 102 and the cellulosic fibers 2. The reaction medium 110 may be free of a solvent additional to the NADES, the NADES forming a liquid medium in which the mechanochemical treatment 12 may take place. The solvent of the reaction medium 110 may comprise only the NADES. The mass ratio between the cellulosic fibers 2 and the NADES 102 (mflbres / mNADEs) may be between 50% and 63% by mass during the formation of the reaction medium 110.

[0075] The cellulose fibers 2 may be of different natures. These fibers 2 may be bleached or not. The cellulose fibers may be ground so as to form fibers commonly referred to as fluffed, which corresponds to a dry mechanical treatment, typically grinding, limiting, and preferably avoiding, degradation of the fibers and making their surface available. The fibers then have the appearance of cellulose wadding or even cotton. The cellulose fibers 2 may be, for example, cotton fibers.

[0076] The reaction medium 110 formed is subjected to the mechanochemical treatment 12, in the mechanochemical reactor 4. The mechanochemical treatment 12 can be carried out for a duration t sufficient to obtain the CNCs 3 from the cellulose fibers 2. More particularly, the mechanochemical treatment 12 can be carried out for a duration t sufficient to achieve a mass yield greater than or equal to 50%, preferably greater than or equal to 60%, relative to the initial quantity of cellulose fibers introduced into the reactor 4. According to one example, this duration is less than or equal to 2 hours, and preferably greater than or equal to 0.5 hours. The treatment time necessary to obtain the CNCs 3 can in fact be limited thanks to the mechanochemical action. During the development of the invention, times of 0.5, 1 h and 2 h were tested, as described later with reference to the particular examples.

[0077] The mechanochemical treatment 12 can be carried out at a temperature T° between 15°C and 30°C, preferably substantially equal to 25°C. The mechanochemical treatment 12 can be carried out without temperature regulation, and therefore at room temperature (approximately 25°C). The temperature of the reaction medium 110 can be between 40°C and 50°C at the end of the mechanochemical treatment. The mechanochemical treatment 12 can alternatively be carried out by regulating the temperature by a temperature regulation device, for example as described previously. The temperature can be regulated to a temperature below 30°C. The mechanochemical action is in fact sufficient to obtain and functionalize the CNCs 3 without having to heat the reaction medium 110 to higher temperatures to provide energy to this medium.

[0078] Finally, the mechanochemical treatment can be carried out at a vibration frequency F sufficient to obtain the CNCs 3 from the cellulosic fibers 2, and preferably in the time ranges t indicated above. More particularly, the frequency F can be chosen so as to achieve a mass yield greater than or equal to 50%, preferably greater than or equal to 60%, relative to the initial quantity of cellulosic fibers introduced into the reactor 4. For this, the vibration frequency can in particular be between 5 Hz and 100 Hz, preferably between 5 Hz and 50 Hz, more preferably substantially equal to 30 Hz.

[0079] Washing 13 is now described. Washing 13 of the reaction medium 110 can be carried out at the end of the mechanochemical treatment. Washing 13 can be configured to reduce the quantity, or even eliminate, the NADES 102 from the reaction medium 110. Washing can therefore be done with a washing solvent distinct from the NADES 102. Since the NADES 102 is electrically conductive, washing can be done so as to bring the conductivity of the reaction medium 110 closer to, and preferably make it substantially equal to, that of the washing solvent. The washing solvent can for example be an aqueous solution, and for example water.

[0080] According to one example, the washing 13 of the reaction medium 110 is carried out by dialysis 130 of the reaction medium 110. The dialysis 130 can be carried out until the conductivity of the reaction medium 110 is brought closer to, and preferably made substantially equal to, that of the washing solvent. For this, the dialysis 130 can be carried out on a dialysis membrane. The dialysis membrane preferably has a cut-off threshold chosen so as to allow the NADES ions to pass through, while blocking the passage of the prepared CNCs. The membrane can in particular have a cut-off threshold of between 6 kDa and 8 kDa (with 1 Da « 1 g / mol).

[0081] After the mechanochemical treatment 12, and preferably after the washing 13, the method 1 may comprise a step 14 of separating the reaction medium 110 into two fractions. The first fraction 110a then comprises the cellulose nanocrystals 3. The second fraction 110b may comprise residual fibers 4. The first fraction 110a comprising the CNCs 3 is thus recovered in isolation from the second fraction 110b. An example of separation is described below without limitation. Note that any other separation method can be envisaged.

[0082] The separation 14 may comprise a dispersion 140 of the reaction medium 110. The dispersion 140 may be made by sonication. The dispersion makes it possible to resuspend the CNCs 3 in the medium 110, which then form a stable colloidal suspension in solution.

[0083] The separation 14 may comprise, preferably following the dispersion 140, a centrifugation 141 of the reaction medium so as to precipitate the elements present in the reaction medium other than the CNCs 3. In particular, this makes it possible to precipitate the residual fibers 4. Thus, the first fraction 110a may correspond to the supernatant at the end of the centrifugation 141, and the second fraction 110b may correspond to the pellet. Note that the separation 14 may comprise the centrifugation 141 without prior dispersion 140.

[0084] The first fraction 110a can be recovered 142. The CNCs isolated from the residual fibers 4 are thus obtained. For example, the supernatant can be collected at the end of the centrifugation 141. The second fraction 110b can be recovered 143. The second fraction 110b comprising the residual fibers can then be used in a process 15 for manufacturing CNF. For this, the second fraction 110b can for example be left to stand to sediment a portion of the residual fibers 4. Preferably, the sedimented fibers are then used in a process 15 for manufacturing CNF.

[0085] Specific examples

[0086] Two particular examples of carrying out method 1 are now described.

[0087] In these examples, the cotton fibers are commercially available bleached and mechanically treated cotton fibers from the paper industry. This pulp was fluffed under dry conditions at room temperature using a Forplex® apparatus.

[0088] The ball milling process is carried out using a vibrating ball reactor 4 (CryoMill®, Retsch GmbH). The grinding chamber (20 ml) and the balls 40 are made of zirconium dioxide (ZrO2). The balls are 50 in number, and their diameter is 5 mm.

[0089] In these examples, the hydrogen bond donor 101 is either citric acid (CAM) or oxalic acid (OAD), depending on the example. The quaternary ammonium salt 100 is choline chloride (ChCl). These two compounds 100, 101 were added to the grinding chamber of reactor 4 with 50 zirconium balls and ground at 30 Hz. After only 15 seconds of grinding, the two NADES ChCkOAD (ratio molar ratio 1:1) and ChCkCAM (molar ratio 1:1) can be obtained and cover the surface of the balls and the grinding chamber. The masses added to the grinding jar allow the production of 3 mmol of NADES and are given in the table below. [Tables 2] NADES Choline chloride (ChCl) (in g) Hydrogen bond donor CAM or OAD (in g) NADES (in g, equivalent to 3 mmol) ChCkOAD (1:1) 0.42 0.38 0.8 ChCkCAM (1:1) 0.42 0.58 1.0

[0090] For both examples, 0.5 grams of cotton fibers (dry matter percentage = 94%) are added to the grinding chamber containing the NADES and ground at 30 Hz. After 3 and 5 minutes of treatment, the grinding chamber is opened and the cellulosic material homogenized. Finally, the mechanochemical treatment 12 is carried out for a total treatment time of 0.5, 1 or 1.5 hours.

[0091] After the mechanochemical treatment 12, approximately 5 mL of water is added into the grinding chamber, and 15 seconds of grinding at 30 Hz is performed to disperse the treated fibers. Then, the suspension is recovered and dialyzed with deionized water until the sample conductivity is the same as that of deionized water (dialysis membrane molecular weight cutoff: 6-8 kDa). Then, the suspension was sonicated using a 250-watt sonication probe (Sonifer® 250, Branson) with a dispersive energy of approximately 4.22 kJ per g of materials. Then, the suspension was centrifuged for 10 minutes at 2600 g at 20°C. Finally, the supernatant containing the stable colloidal CNC 3 was removed and stored in the refrigerator. The mass concentration of the CNC suspension is then 0.2%. The precipitate containing the residual cellulose fibers (denoted RP in the following) was dispersed in deionized water and stored in the refrigerator.After one week of sedimentation, two distinct suspensions were obtained: one containing sedimented particles (denoted SP in the following) and the second non-sedimented particles (denoted nSP in the following).

[0092] The CNC samples are called “CNC-ChCl:OAD-MC” and “CNC-ChCl:CAM-MC” for CNCs obtained by the process using the NADES ChCkOAD and ChCkCAM, respectively. The CNC-ChCkOAD-MCs are obtained with three different treatment times (0.5, 1 and 1.5 hours). Characterizations other than yield were only made for 1.5 hours of mechanochemical treatment. The experimental conditions are summarized in the table below. For comparison, CNC samples obtained by NADES without mechanochemical treatment (CNC-ChCl:OAD and CNC-ChCl:CAM) are described in the table below [Tables3] Sample Time, (h) Mass concentration of CNC obtained in NADES after treatment (%wt) Temperature Mechanochemical treatment (MC) CNC-ChCl:O AD-MC 0.5 1 1.5 65% Ambient without temperature control Yes CNC-ChCl:C AM-MC 1.5 27% CNC-ChCl:O AD 6 36% 95°C No CNC-ChCl:C AM 6 8% 120°C No

[0093] The CNC 3, sedimented particles (SP) and non-sedimented particles (nSP) were separated after washing, which made it possible to determine the yield of each suspension obtained. The CNC yield (YCnc) is calculated as the ratio between the weight of CNC (mCNc) and the initial weight of cellulose fibers (m0), as expressed in the following equation. [Math.l] x 100

[0094] The evolution of the yield in CNC YCNc 50 as a function of the duration 51 of the mechanochemical treatment 12 is illustrated in [Fig.4] for: 52: the CNC-ChCl:OAD-MC, - 53: the sedimented SP particles obtained by the manufacturing process of CNC-ChCl:OAD-MC, - 54: non-sedimented particles nSP obtained by the manufacturing process CNC-ChCl:OAD-MC.

[0095] CNCs with a yield of 64.5 ± 5.3% were successfully obtained after only 1.5 hours of treatment using NADES ChCkOAD (initial fiber concentration in reactor 4 is about 62%).

[0096] For the CNC-ChCl-CAM-MC sample, the yields obtained after 1.5 hours are 27.1% for the CNCs, and 0.7% and 79.8% for the residual nSP and SP particles, respectively. Lower yields of CNCs are obtained compared to the ChCl-OAD-MC treatment. This result confirms that the acid hydrolysis of the amorphous part of the cellulose is more efficient with oxalic acid than with citric acid.

[0097] The dimensions of the CNCs produced after 1.5 hours of mechanochemical treatment were analyzed by SEM and by AFM, as shown respectively in Figures 5 and 6. The CNC-ChCl:OAD-MCs have typical dimensions for cotton cellulose nanocrystals with lengths and diameters of 143 ± 28 nm and 7 ± 2 nm, respectively. Figures 7A and 7B show respectively occurrence distribution diagrams as a function respectively of the length (in nm) and of the diameter (in nm).

[0098] The Zeta potential · of the CNC suspensions was measured with a Zetasizer® PRO device (Malvern Panalytical®). The folded capillary cell was kept at 20°C during the measurement. 1 mL of the sonicated CNC suspension was diluted by adding 8 mL of deionized water and 1 mL of NaOH solution (C=0.0125 mol / L) to adjust the pH and conductivity of the sample. Three sets of measurements of ten acquisitions were performed for each sample, and the average value was calculated.

[0099] The measurement of the Zeta potential Ç confirmed the hypothesis of the surface functionalization of the CNCs. A high value of the surface charges is observed for both CNCs with the ChCl:CAM-MC and ChCl:OAD-MC treatments. Thus, this method allows the obtaining of anionic particles with Zeta potential values ​​of -42.0 ± 3.3 mV and -41.2 ± 1.6 mV for the ChCl:CAM-MC and ChCl:OAD-MC treatments respectively.

[0100] 13C solid state NMR spectra were performed on cotton fibers 72 and on CNC-ChCl:OAD-MC 71 after 1.5 hours of grinding, as shown in [Fig.8]. The x-axis corresponds to the chemical shift 70 in ppm. The 13C NMR spectra were performed on an Avance® III 400 MHz spectrometer at a temperature of 298 K. The instrument was equipped with cross-polarization, high-power proton decoupling and magic angle spinning (CP-MAS), and the sample rotation speed is 12,000 Hz. Acquisitions were performed over a spectral width of 29,761 Hz with an acquisition time of 36 ms and 7,400 scans.

[0101] Both spectra are characteristic of a cellulosic sample. The observed peaks can be attributed to the different carbons of the anhydroglucose unit with the contribution of Cl (103.8 ppm), C2, C3 and C5 (merged in the large peak around of 74.4 ppm), C4 (81.9 ppm for cellulose fibers 72, 88.9 ppm for CNC 71), and C6 (61.5 for cellulose fibers and 64.7 ppm for CNC 71). An additional peak is present at 57.87 ppm which could be associated with residual choline chloride.

[0102] Figures 9A and 9B represent the X-ray diffraction diagrams for: - 80: the residual RP fibers obtained after ChCl:OAD-MC treatment, - 81: CNCs obtained after ChCl:OAD-MC treatment, - 82: residual RP fibers obtained after ChCl:CAM-MC treatment, - 83: CNCs obtained after ChCl:CAM-MC treatment.

[0103] The crystallinity index (CI) is calculated by the Segal method after 1.5 hours of mechanochemical treatment at 30 Hz, with the NADES ChCkOAD and ChCkCAM. This is an empirical method that allows a rapid comparison between cellulosic samples. The CI was calculated after background subtraction from the ratio of the peak height (1002) to the minimum height (Iam) located between peaks 002 and 101, as stated in the following equation. The measurements were carried out in the dry cellulosic samples (overnight, 105°C) using an X'Pert Pro MDP instrument (Malvern Panalytical®) in reflection mode with Bragg Brentano geometry. The anode was made of copper, and the wavelength was 1.5419 Angstrom. [Math.2] CI (%) = x 100 1002

[0104] All the studied samples show the characteristic diffractograms of the arrangement of cellulose I. Initial LTC of cotton cellulose fibers is 92%. The CNCs obtained after ChCl:OAD-MC and ChCl:CAM-MC treatment show a crystallinity of 93% and 87%, respectively. In comparison, the IC values ​​of the residual particles obtained after ChCl:OAD-MC and ChCl:CAM-MC are 94% and 93%, respectively. The CNCs of the ChCl:CAM-MC treatment show a lower crystallinity than for the ChCl:OAD-MC treatment. Note that the additional peak at 30.2 20 corresponds to contamination by zirconium dioxide during the mechanochemical treatment.

[0105] In [Fig. 10], FT-IR (Fourier Transform Infrared Spectroscopy) absorbance spectra as a function of wave number k were obtained for: - 90: cotton cellulose fibers, - 91: CNCs obtained after ChCl:CAM-MC treatment, - 92: CNCs obtained after ChCl:OAD-MC treatment.

[0106] FT-IR spectra are obtained using a Perkin-Elmer Spectrum 65 instrument (PerkinElmer®, USA). This technique is used to determine esterification between cellulose and organic acids. Given the proximity between the carbonyl peak and the ester peak (respectively, ~1720 cm1 and ~1740 cm1), each sample was basified using a NaOH solution to convert the carboxylic acid groups into carboxylate groups (approx. 1600 cm1) and dried at room temperature overnight before analysis. Spectra were recorded in Attenuated Total Reflectance (ATR) mode between 4000 and 600 cm1 with 16 scans and normalized for better comparison. At least two measurements were made per sample.

[0107] The FT-IR spectra of cotton 90 fibers show the classic absorption peaks for a cellulosic material with a peak at 1640 cm 1 due to water adsorption. The CNCs obtained by the mechanochemical treatment show an additional peak at 1744 cm1, attributed to the C=O stretching. This contribution, associated with a slight increase in the frequency of the C=O stretching, is due to the presence of a water layer. Associated with a slight increase in the CH stretching at 2853 cm1, this shows the functionalization of the CNCs by the oxalic acid and citric acid molecules according to the NADES used.

[0108] Furthermore, the thermal degradation temperature Td of cellulose can also be affected by the decrease in the degree of polymerization and the chemical modification of the surface. Thus, it is known that the introduction of carboxylate groups by TEMPO-mediated oxidation, the most used post-treatment to obtain anionic CNCs, decreases the thermal stability of cellulosic materials. For this reason, the use of CNCs produced from such solutions in bionanocomposites is limited.

[0109] However, it has been observed that the CNCs obtained by ChCl:OAD-MC and ChCl:CAM-MC treatment have a degradation temperature close to that of the original cotton cellulosic fibers: 338, 337 and 365°C, respectively. These degradation temperatures are similar to those obtained for the CNCs produced by molten oxalic acid and those reported in the literature for the ChCkOAD treatment without mechanochemical treatment. In comparison, the CNCs obtained by H2SO4 acid hydrolysis have a degradation temperature of 303°C under the same conditions.

[0110] The table below summarizes the results obtained for each sample. [Tables 4] Sample Mass yield (wt%) IC (%) Length (nm) Td (°C) UmV) Cellulosic cotton fibers NA 92 NA 364 NA CNC-ChCl:OAD-M Ca 65 + 5 93 143 + 28 337 -42.0 + 3.3 CNC-ChCl:CAM-M Ca 27 87 >400 335 -41.2 + 1.6

[0111] In view of the foregoing description, it is clear that the invention provides an improved method for manufacturing cellulose nanocrystals, and in particular a method better suited to the constraints of industrial manufacturing, for example in terms of processing time, temperature and / or cost.

[0112] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention. The present invention is not limited to the examples previously described. Many other variant embodiments are possible, for example by combining features previously described, without departing from the scope of the invention. In addition, the features described in relation to one aspect of the invention may be combined with another aspect of the invention.

Claims

Claims

1. A method (1) for manufacturing cellulose nanocrystals (3) comprising: • providing a deep eutectic solvent (102) obtained by mixing a quaternary ammonium salt (100) and a hydrogen bond donor compound (101), the hydrogen bond donor compound (101) being capable of forming the deep eutectic solvent (102) with the quaternary ammonium salt (100), • in a mechanochemical reactor (4), forming (11) a reaction medium (110) comprising cellulose fibers (2) and the deep eutectic solvent (102), • mechanochemical treatment (12) of the reaction medium (110) so as to obtain cellulose nanocrystals (3) from the cellulose fibers (2).

2. Method (1) according to the preceding claim in which the quaternary ammonium salt (100) is choline chloride.

3. A method (1) according to any preceding claim wherein the hydrogen bond donor compound (101) comprises at least one carboxylic acid group.

4. A method (1) according to any preceding claim, wherein the hydrogen bond donor compound (101) comprises at least two carboxylic acid groups.

5. Method (1) according to the preceding claim, wherein the hydrogen bond donor compound (101) is selected from the group consisting of citric acid and oxalic acid, preferably the hydrogen bond donor compound is oxalic acid.

6. Method (1) according to any one of the two preceding claims, in which, at the end of the mechanochemical treatment, the cellulose nanocrystals have a quantity of carboxylate group of between 100 and 3000 peq / g of cellulose nanocrystals, preferably substantially equal to 1500 peq / g.

7. Method (1) according to any one of the preceding claims in which the mechanochemical treatment (12) is carried out for a duration less than or equal to 2 hours.

8. Method (1) according to any one of the preceding claims in in which the mechanochemical treatment (12) is carried out at a temperature between 15°C and 30°C, preferably substantially equal to 25°C.

9. A method (1) according to any preceding claim wherein the mechanochemical reactor (4) is a mechanochemical mill, for example a ball mill (40).

10. Method (1) according to the preceding claim in which the mechanochemical treatment (12) is carried out at a vibration frequency of between 5 Hz and 50 Hz, preferably substantially equal to 30 Hz.

11. Method (1) according to any one of the preceding claims in which, after the mechanochemical treatment (12), the method comprises washing (13) the reaction medium (110).

12. Method (1) according to the preceding claim in which the washing is done by dialysis (130) of the reaction medium (110) by a dialysis membrane.

13. Method (1) according to any one of the preceding claims, in which, at the end of the mechanochemical treatment, the reaction medium (110) comprises a mass fraction of cellulose nanocrystals (3) greater than or equal to 20%, preferably greater than or equal to 40%.

14. Method (1) according to any one of the preceding claims wherein, after the mechanochemical treatment (12), the method (1) comprises a separation (14) of the reaction medium (110) into a first fraction (110a) comprising the cellulose nanocrystals (3) and a second fraction (110b) comprising residual fibers (4).

15. Method (1) according to the preceding claim, in which the separation (14) of the reaction medium (110) comprises a centrifugation (140) of the reaction medium (110) followed by a collection (142) of one of the first and second fractions (110a, 110b).

16. Method (1) according to any one of the two preceding claims, in which the second fraction (110b) is recovered for its use (15) in a process for manufacturing cellulose nanofibers.

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