Method for producing cellulose nanofibers

Abstract

A method for producing cellulose nanofibers by fibrillation of cellulose fibers, characterized in that it comprises the following successive steps: A. dispersing the starting cellulose fibers in water; B. filtering to obtain water-activated cellulose fibers; C. impregnating said water-activated fibers with a solution, in at least one C2-C4 alcohol, of at least one functionalizing agent selected from phenyl acrylic acids, benzoic acids, imidazole acrylic acids, imidazole carboxylic acids, furylacrylic acids and esters thereof; D. introducing, into at least one C2-C4 alcohol at a temperature of 60-70°C, the impregnated cellulose fibers and stirring the mixture so that the functionalization reaction takes place; E. purifying by filtration the functionalized fibers obtained in step (D) and washing them; F. redispersing said functionalized fibers in water and stirring them; and G. subjecting the functionalized cellulose fibers resulting from step (F) to a mechanical nanofibrillation treatment.

Description

Cellulose nanofiber production process

[0001] The present invention relates to a method for producing cellulose nanofibers.

[0002] Cellulose nanofibers are widely used, for example, as reinforcement in the manufacture of composites, in the packaging industry, as coatings due to their barrier properties, in separation technologies such as filtration and purification, as well as in hydrogels and aerogels. On an industrial scale, cellulose nanofibers are produced by mechanical delamination preceded by enzymatic or oxidative pretreatment, as described below.

[0003] Nanocelluloses have been the subject of intense literature for about ten years due to their self-organizing, mechanical, and optical properties, as well as the versatility of their interaction capabilities and surfactant properties. They are usually classified into two categories: nanocrystals and nanofibers. Cellulose nanofibers represent a remarkable change of scale, namely 5-15 nm in diameter and a few microns in length, in the production of functionalized macromolecules and exhibit exceptional innovative properties enabling the creation of new materials or objects.

[0004] Cellulose nanofibers are obtained by mechanical delamination—or fibrillation—of fibers until they reach a diameter of 5–15 nm and a length of a few microns. This step requires significant energy consumption to fractionate the fibers to the nanoscale and represents a technological barrier that limits the industrial applications of cellulose nanofibers. To reduce this energy consumption and overcome this industrial barrier, two biomass pretreatment strategies are currently used: The first involves pretreating the fibers with enzymatic cocktails of cellulases, such as endoglucanases, which break down the fiber structure, thus facilitating subsequent mechanical delamination. This process is extremely versatile depending on the state of the fiber and, in particular, its prior thermo / chemical history.Depending on the process, the quality of the nanocellulose, particularly its dispersion state and therefore agglomeration, varies considerably, as do the energy yields, since the treated fiber requires a significant energy input to facilitate its defibrillation. The second strategy involves introducing charged groups onto the fiber surface. These charges create electrostatic repulsions, facilitating and extremely efficient delamination. The most commonly used chemical pretreatment is the oxidation of cellulosic fibers catalyzed by the radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). TEMPO-catalyzed oxidation aims to oxidize the primary C6 alcohol of the glucose unit to a carboxylic acid, thus introducing a charge onto the surface.However, the removal of reaction products leads to large quantities of highly polluted effluents and reagent residues in the final product which continue to react and ultimately degrade the properties of nanocelluloses.

[0005] Another alternative pretreatment is carboxymethylation. The carboxymethylation reaction occurs in two steps: swelling and partial dissolution of the fibers in a basic medium (NaOH), followed by etherification of the fibers. Swelling in NaOH and the introduction of fillers during the reaction facilitate the complete solubilization of the final carboxymethylcellulose.

[0006] In the case of cellulose nanofibers, the goal is not to completely solubilize the fiber but to defibrillate it to obtain the nanofibers. To achieve this, the fibers are first soaked in monochloroacetic acid and then added to a NaOH-isopropanol solution. This limits the solubilization of the fiber by the sodium hydroxide reaction and allows the nanofibers to be obtained without a significant decrease in crystallinity or substantial degradation.

[0007] Application WO 2023 / 156348 discloses a process for preparing multifunctionalized cellulose fibers consisting of grafting cellulose fibers with two functional groups in a single container and in an alkaline reaction medium. For grafting, two reagents are used, each containing a halogen atom and / or a vinyl group and / or a phenyl group and lacking a hydroxyl group. The first reagent also contains the first functional group, and the second reagent also contains the second functional group. The first reagent is monochloroacetic acid for grafting with a carboxymethyl group: –CH2CO2H. Trifunctionalization can be considered, but always with monochloroacetic acid as one of the grafting reagents.

[0008] Thus, in general, the preparation of nanofibers requires toxic chemical reagents and high mechanical energy to facilitate fibrillation, making the process polluting and increasing production costs. This limitation currently hinders the production of cellulose nanofibers, so the search for new green methods for nanofiber production remains an open question.

[0009] The inventors sought to circumvent this key industrial obstacle in order to propose a new manufacturing process for cellulose nanofibers that facilitates the final fibrillation or delamination step—without requiring complete solubilization of the fibers—while reducing energy costs and eliminating the need for toxic chemical reagents such as monochloroacetic acid. One of the limitations of carboxymethylation lies in the solvent exchange from water to isopropanol via ethanol, and it became apparent that this ethanol step was no longer necessary.

[0010] The present invention relates to a process for producing cellulose nanofibers by fibrillation of cellulose fibers, characterized in that it comprises the following successive steps: dispersing the starting cellulose fibers in water; filtering to obtain water-activated cellulose fibers; impregnating said water-activated fibers with a solution in at least one C2-C4 alcohol of at least one functionalizing agent selected from phenylacrylic acids, benzoic acids, imidazole acrylic acids, imidazole carboxylic acids, furylacrylic acids and their esters; introducing the impregnated cellulose fibers into at least one C2-C4 alcohol at a temperature of 60-70 °C and stirring the mixture to allow the functionalization reaction to occur; purifying the functionalized fibers obtained in step (D) by filtration and washing them; redispersing them in water and stirring them;and subject the functionalized cellulosic fibers resulting from step (F) to a mechanical nanofibrillation treatment.

[0011] Step (D) can be carried out without the addition of sodium hydroxide.

[0012] In step (B), it is possible to plan to wash the activated cellulose fibers with alcohol, such as isopropanol or ethanol.

[0013] In the present invention, the following may be used:

[0014] as phenyl acrylic acid(s) at least one compound of formula (I): (I)

[0015] in which R represents H or a phenyl nucleus substituent chosen from –OH, -OCH3 or –CH=CH-COOH or at least two phenyl nucleus substituents, identical or different, chosen from –OH, -OCH3 or –CH=CH-COOH;

[0016] such as cinnamic acid, p-coumaric acid, m-coumaric acid, o-coumaric acid, caffeic acid, ferulic acid, sinapic acid, 3,4-dimethoxycinnamic acid and 1,4-diphenylenediacrylic acid,

[0017] such as benzoic acid(s) or their esters at least one compound of formula (II): (II)

[0018] in which R represents H or a phenyl nucleus substituent chosen from –OH, -OCH3, -NH2 or –CH=CHCH3 and R' represents H or CH3;

[0019] such as benzoic acid, 2-aminobenzoic acid, 3-aminobenzoic acid, 4-aminobenzoic acid, 2-hydroxybenzoic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 2-methoxybenzoic acid, 3-methoxybenzoic acid, 4-methoxybenzoic acid and E-anethole,

[0020] such as imidazole acrylic acid, trans-urocanic acid

[0021] as imidazole carboxylic acid(s), at least one compound of formula (III): (III)

[0022] in which R represents H or a substituent of the aromatic ring chosen from –OH, -OCH3 or –NH2;

[0023] such as 5-benzylimidazolecarboxylic acid,

[0024] as furylacrylic acid(s), at least one compound of formula (IV): (IV)

[0025] in which R represents H or a substituent of the furyl ring chosen from –OH, -OCH3 or –NH2;

[0026] such as 3-(2-furyl)acrylic acid.

[0027] In step (C), a ratio of functionalizing agent(s) in millimoles / fiber mass in grams between 0.5 and 50, in particular between 1 and 20, can be used.

[0028] At each of the steps (C) and (D), at least one C2-C4 alcohol can be used, chosen from propanol and isopropanol.

[0029] At stage (C), the mooring can be conducted for 10 min - 1 h.

[0030] Step (D) can be conducted at a temperature that can reach reflux, in particular 60-70°C.

[0031] At step (D), the mixture can be stirred for 1-2 hours.

[0032] At step (E), the functionalized fibers can be washed with a C2-C4 alcohol, such as propanol or isopropanol.

[0033] At step (F), the functionalized fibers can be gently agitated at a temperature of 4-30°C for 1-24 h.

[0034] The nanofibrillation step of the process according to the present invention can be carried out by any conventional technique for mechanically breaking cellulose fibers to achieve the nanoscale. For example, it can be performed by homogenization or high-pressure microfluidization. For instance, the functionalized cellulose fibers can be passed through a piston pump that applies high pressure, and then several times through Z-shaped interaction chambers, for example, with internal diameters of 400, 200, and 100 µm, preferably at a constant flow rate, for example, of approximately 350 mL / min, and, for example, at pressures of 100, 1500, and 2000 bar, respectively.

[0035] The following examples illustrate the present invention without however limiting its scope, with reference to the attached drawing.

[0036] In this drawing:

[0037] Larepresents FTIR spectra of control cellulosic fibers (BK) and of different fibers functionalized according to the present invention.

[0038] Figure 1 represents the visual appearance (top) and the morphology by polarized light optical microscopy (bottom) of the control cellulosic fibers (BK) and of the fibers functionalized with cinnamic acid (BK-CA), p-coumaric acid (BK-CoA) and ferulic acid (BK-FA), with and without NaOH in the reaction medium.

[0039] Figure 1 represents the visual appearance (top) and the morphology by polarized light optical microscopy (middle) and by scanning electron microscopy (bottom) of cellulosic fibers functionalized with cinnamic acid: bleached Kraft paper pulp (BK-CA), unbleached Kraft paper pulp (UBK-CA) and chemically and mechanically treated pulp (CTM-CA), without NaOH in the reaction medium.

[0040] Larepresents observations under polarized light optical microscopy of starting and functionalized cellulosic fibers with cinnamic acid: bleached Kraft paper pulp (BK-CA), unbleached Kraft paper pulp (UBK-CA) and chemically and mechanically treated pulp (CTM-CA), before and after high pressure homogenization.

[0041] Larepresents SEM observations of starting and functionalized cellulosic fibers with cinnamic acid: bleached Kraft paper pulp (BK-CA), unbleached Kraft paper pulp (UBK-CA) and chemically and mechanically treated pulp (CTM-CA), before and after high-pressure homogenization.

[0042] Lare represents the evolution of the average width of the starting and functionalized fibers with cinnamic acid: bleached Kraft paper pulp (BK-CA), unbleached Kraft paper pulp (UBK-CA) and chemically and mechanically treated pulp (CTM-CA), before and after high pressure homogenization.

[0043] La represents the distribution of fibers according to their width (μm) of the fibers according to the.

[0044] In these examples, the following abbreviations have been used:

[0045] IPA: isopropanol

[0046] TA: ambient temperature

[0047] BK: bleached Kraft pulp or paper pulp from softwoods, constituting the reference cellulose fibers

[0048] UBK: unbleached Kraft pulp or paper pulp

[0049] CTM: chemically and mechanically treated pulp or paste

[0050] BK-CA: BK fibers according to the invention functionalized with cinnamic acid

[0051] BK-CoA: BK fibers according to the invention functionalized with p-coumaric acid

[0052] BK-FA: BK fibers according to the invention functionalized with ferulic acid

[0053] UBK-CA: UBK pulp or paste according to the invention functionalized with cinnamic acid

[0054] CTM-CA: CTM pulp or paste according to the invention functionalized with cinnamic acid

[0055] MOLP: Polarized Light Optical Microscopy

[0056] FITR: Infrared Spectroscopy

[0057] SEM: scanning electron microscope

[0058] Examples 1a to 1c: Production of cellulose nanofibers without NaOH General operating procedure

[0059] One hundred mg of cellulose fibers were dispersed in water using a mixer to obtain water-activated starting fibers. The water-activated fibers were filtered through a 0.22 µm vacuum filtration system and washed with ethanol. The fibers were then impregnated with an acid according to the invention (0.10 mmol) dissolved in 0.5 mL of IPA for 30 minutes at room temperature. The ratio of the acid according to the invention (mmol) to fibers (g) was 1. After impregnation, the fibers were added to 1 mL of IPA heated to 70°C in a round-bottom flask. The reaction was allowed to proceed for 60 min at 70°C under reflux. The fibers obtained were purified by filtration followed by washing with IPA (x3) and water (x3) until neutral pH. The fibers were redispersed in water under agitation. The suspensions obtained were then homogenized using a Panda Plus 2000 homogenizer (GEA Niro Soavi, Italy) to a concentration of 1% (w / w, dry matter content).The slurry was then passed through a piston pump that applied high pressure. This homogenizer is equipped with two interaction chambers operating at a maximum pressure of 150 to 200 bar and allows for a constant flow rate of approximately 150 mL / min. The paste suspension was forced through the two chambers for 5 minutes at operating pressures between 60 MPa and 100 MPa (600 and 1000 bar).

[0060] Examples 1a to 1h: Preparation of cellulose nanofibers

[0061] As an acid according to the invention, cinnamic acid (Ex. 1a), p-coumaric acid (Ex. 1b), ferulic acid (Ex. 1c), 1,4-diphenylenediacrylic acid (Ex. 1d), 4-aminobenzoic acid (Ex. 1e), trans-urocanic acid (Ex. 1f), 5-benzylimidazolecarboxylic acid (Ex. 1g) and 3-(2-furylacrylic acid) (Ex. 1h) were used respectively.

[0062] Examples 2a to 2c: Production of cellulose nanofibers using NaOH

[0063] We proceeded as in each of Examples 1a to 1c respectively, but after impregnation, we added the fibers to a solution heated to 60°C of sodium hydroxide (16 mg, 0.4 mmol) in isopropanol (1 mL) in the round-bottomed three-neck.

[0064] Example 3: Analysis of BK fibers, fibers according to Examples 1a to 1c obtained with and without NaOH, and fibers according to Examples 1d to 1h obtained without NaOH

[0065] The presence of the acid functionalities according to the invention of the above fibers according to the invention was detected by FITR. Lamontre shows the IR spectra of the fibers according to Examples 1a to 1c – respectively noted BK-CA, BK-CoA and BK-FA, obtained with and without NaOH, of the fibers according to Examples 1d to 1h without NaOH, as well as that of the BK fibers (control).

[0066] The band at 1738 cm -1This corresponds to the stretching vibration of the C=O bond of the COOH groups of the acids according to the invention. It demonstrates the presence of carboxyl groups, even if the band intensity is weak, thus proving the presence of these carboxylic acids.

[0067] The next step in characterizing the functionalized fibers is to determine the acid grafting rate according to the invention by conductimetric titration. Conductimetry allows measurement of the charge of the nanofibers, which originates from the acid groups according to the invention. The following table shows the charge of the fibers after functionalization.

[0068] Table. Charge of BK-CA, BK-CoA, and BK-FA fibers, with and without NaOH in the reaction medium. Charge (mmol g -1 )With NaOH Without NaOHBK-CABK-CoABK-FA0.239 ± 0.0290.216 ± 0.0760.293 ± 0.0560.118 ± 0.0200.264 ± 0.0600.210 ± 0.035

[0069] The charge of the starting cellulosic fibers is 0.212 mmol g -1After functionalization, the charge does not increase significantly, unlike with oxidation by TEMPO or carboxymethylation. This result can be explained by the presence of free electrons in the benzene ring, which can give rise to different resonance structures, where the carbonyl groups are in equilibrium with diols.

[0070] Example 4: Visual appearance and morphology of nanofibers

[0071] The attached drawing illustrates in the upper part the visual appearance of the BK reference fibers and the BK-CA, BK-CoA and BK-FA fibers with and without NaOH in the reaction medium, and in the lower part their morphology by MOLP.

[0072] It is thus shown that, compared to control nanofibers, fibrillation is indeed facilitated.

[0073] Example 5: Visual appearance and morphology

[0074] To demonstrate the applicability of the process, the inventors tested the reaction on cellulosic fibers selected from BK, UBK, and CTM pulps. The results show the appearance of BK-CA, UBK-CA, and CTM-CA pulps prepared without NaOH, and demonstrate how the reaction with cinnamic acid facilitates the breakdown of fibers in all three types of pulp studied.

[0075] To demonstrate pulp fibrillation, the inventors performed high-pressure homogenization tests on 0.1 g L dispersions -1 for 5 min, as few differences between functionalized and non-functionalized fibers had been observed under optical microscopy. Dispersion at 0.1 g L -1 were homogenized for 5 min. These fibers were observed by MOLP and SEM before and after homogenization ().

[0076] In the image, fiber fibrillation is observed after passing through the homogenizer. We then observed the same samples using a scanning electron microscope (SEM) to examine the fiber surface in more detail and better detect fiber fibrillation. In the image, fiber fibrillation is clearly visible after passing through the homogenizer for all pulp types.

[0077] To evaluate the impact of pretreatment on the fibers, fiber width measurements were carried out using ImageJ software and we were thus able to determine the average width of the fibers () and their distribution according to their width and compare their evolution before and after functionalization ().

[0078] Thus, these results confirm that high-pressure homogenization leads to significant fiber fibrillation with or without pretreatment, as can be seen on laet la.

[0079] Furthermore, on the, we observe that pretreatment positively influences fibrillation with an increase in the proportion of narrower fibers.

[0080] Furthermore, for UBK-CA and CTM-CA pulps, pretreatment appears to induce fibrillation even before mechanical treatment, as an increase in the proportion of fibers with a width less than 30 µm is observed. Thus, the effect of pretreatment on homogenized fibers seems less significant than on BK fibers, since the fibers have already decreased in size.

[0081] Example 6: Impact of the presence of traces of water

[0082] The conditions used were the same as those in Example 1 except that in one case the fibers were not washed with ethanol in step (B). Cinnamic acid was used in step (D).

[0083] The fibrillation of cellulosic fibers was visualized by the adsorption of dyes of different molar masses, a process known as Simons staining. The fibers were thus contacted with a 1% mixture of Direct Blue 1 ((6E)-4-amino-6-[[4-[4-[N′-(8-amino-1-oxo-5,7-disulfonato-2-naphthylidene)hydrazino]-3-methoxy-phenyl]-2-methoxy-phenyl]hydrazono]-5-oxo-naphthalene-1,3-tetrasodium disulfonate) and Direct Orange 15 (Acid (2Z,8Z,14Z,20Z)-2,3,14,15-tetraazapentacyclo[20.2.2.2~4,7~.2~10,13~.2~16,19~]dotriaconta-1(24),2,4,6,8,10,12,14,16,18,20,22,25,27,29,31-hexadecaene-6,11,18,23-tetrasulfonic), in a 1:1 ratio. Direct Blue 1 dye has a molar mass of 992.82 kDa and Direct Orange 15 has a molar mass of 25,000 kDa.

[0084] When the dye mixture comes into contact with the fibers, Direct Blue 1, which has a lower molar mass, penetrates the fibers, turning them blue, while Direct Orange 15, due to its higher molar mass, does not penetrate the fibers. When the fiber is disrupted and fibrillated by the functionalization reaction, the structure becomes more accessible, and the Direct Orange 15 dye can penetrate, turning the fibers orange.

[0085] Lamontre shows optical microscopy images of the fibers in contact with the dye mixture, before and after the functionalization reaction.

[0086] The images show how the reaction, under the tested conditions, gives an orange coloration to the fibers, in comparison with the starting fibers, which are colored blue.

[0087] Example 7: Impact of the amount of cinnamic acid

[0088] The impact of the cinnamic acid:cellulose fiber ratio was studied by applying the conditions from Example 1 to the conditions in the table below. Cellulose Fiber Cinnamic Acid (CA) Isopropanol CA:fiber Ratio Time 50 mg 0 0.5 mL 0 70°C 1 h 50 mg 3.75 mg 0.5 mL 0.5 mmol / g 70°C 1 h 50 mg 7.5 mg 0.5 mL 1 mmol / g 70°C 1 h

[0089] Lamontre shows the optical microscopy images after contact with the dyes as in Example 6.

[0090] In both conditions tested, the results obtained are satisfactory.

Claims

– A process for producing cellulose nanofibers by fibrillation of cellulose fibers, characterized in that it comprises the following successive steps: dispersing the starting cellulose fibers in water; filtering to obtain water-activated cellulose fibers; impregnating said water-activated fibers with a solution in at least one C2-C4 alcohol of at least one functionalizing agent selected from phenylacrylic acids, benzoic acids, imidazole acrylic acids, imidazole carboxylic acids, furylacrylic acids and their esters; introducing the impregnated cellulose fibers into at least one C2-C4 alcohol at a temperature of 60-70 °C and stirring the mixture to allow the functionalization reaction to occur; purifying the functionalized fibers obtained in step (D) by filtration and washing them; redispersing them in water and stirring them;and subject the functionalized cellulosic fibers resulting from step (F) to a mechanical nanofibrillation treatment. – A process according to claim 1, characterized in that at least one compound of formula (I) is used as phenylacrylic acid(s): (I) in which R represents H or a phenyl ring substituent selected from –OH, -OCH3 or –CH=CH-COOH or at least two phenyl ring substituents, identical or different, selected from –OH, -OCH3 or –CH=CH-COOH; such as cinnamic acid, p-coumaric acid, m-coumaric acid, o-coumaric acid, caffeic acid, ferulic acid, sinapic acid, 3,4-dimethoxycinnamic acid and 1,4-diphenylenediacrylic acid, as benzoic acid(s) or their esters at least one compound of formula (II): (II) wherein R represents H or a phenyl substituent selected from –OH, -OCH3, -NH2 or –CH=CHCH3 and R' represents H or CH3; such as benzoic acid, 2-aminobenzoic acid, 3-aminobenzoic acid, 4-aminobenzoic acid, 2-hydroxybenzoic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 2-methoxybenzoic acid, 3-methoxybenzoic acid, 4-methoxybenzoic acid and E-anethole, as acrylic imidazole acid, trans-urocanic acid as carboxylic imidazole acid(s), at least one compound of formula (III): (III) in which R represents H or a substituent of the aromatic ring chosen from –OH, -OCH3 or –NH2; such as 5-benzylimidazolecarboxylic acid, as furylacrylic acid(s), at least one compound of formula (IV): (IV) in which R represents H or a furyl ring substituent chosen from –OH, -OCH3 or –NH2; such as 3-(2-furyl)acrylic acid. – A process according to any one of claims 1 and 2, characterized in that in step (C), a ratio of functionalizing agent(s) in millimoles / fibre mass in grams is used between 0.5 and 50, in particular between 1 and 20. - A process according to any one of claims 1 to 3, characterized in that at each of the steps (C) and (D), at least one C2-C4 alcohol is used, chosen from propanol and isopropanol. - Method according to any one of claims 1 to 4, characterized in that in step (C), the wetting is carried out for 10 min - 1 h. - A process according to any one of claims 1 to 5, characterized in that step (D) is carried out at a temperature that can reach reflux, in particular 60-70°C. - A method according to any one of claims 1 to 6, characterized in that at step (D), the mixture is stirred for 1-2 hours. - A process according to any one of claims 1 to 7, characterized in that in step (E), the functionalized fibers are washed with a C2-C4 alcohol, such as propanol or isopropanol. - A process according to any one of claims 1 to 8, characterized in that in step (F), the functionalized fibers are gently agitated at a temperature of 4 - 30 °C for 1 - 24 h.

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