Cellulose nanofiber production process

Abstract

------ 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 carry out the functionalization reaction; 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.

Description

Title of the invention: Process for producing cellulose nanofibers

[0001] The present invention relates to a process 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 their 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 over 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 the fibers until nanofibers with a diameter of 5–15 nm and a length of a few microns are obtained. This step requires very high energy consumption to fractionate the fibers to the nanometer level and constitutes a technological barrier that limits the industrial applications of cellulose nanofibers. In order to limit this energy consumption and circumvent this industrial barrier, two biomass pretreatment strategies are currently used: 1. The first method 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 fiber's condition, particularly its prior thermo / chemical history. Depending on the process, the quality of the nanocellulose, especially 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. 2. The second strategy consists of introducing charged groups onto the surface of the fiber. These charges create electrostatic repulsions, Delamination is facilitated and extremely efficient. The most commonly used chemical pretreatment is the oxidation of cellulosic fibers catalyzed by the radical 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). TEMPO-catalyzed oxidation aims to oxidize the primary C6 alcohol of the glucose unit to a carboxylic acid, allowing the introduction of charge onto the surface. However, the removal of reaction products leads to large quantities of highly polluted effluents and residual reactants in the final product, which continue to react and ultimately degrade the properties of the nanocellulose.

[0005] Another alternative pretreatment is carboxymethylation. The carboxymethylation reaction takes place 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 objective is not to completely solubilize the fiber but to defibrillate it to obtain the nanofibers. To achieve this, the fibers are first wetted in monochloroacetic acid and then added to a NaOH-isopropanol solution. This limits the solubilization of the fiber by the attack of sodium hydroxide and allows the nanofibers to be obtained without a significant decrease in crystallinity or significant degradation.

[0007] According to application WO 2023 / 156348, a process for preparing multifunctionalized cellulose fibers is known, 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 function. 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, which makes the process polluting and increases 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 present inventors sought to circumvent this key industrial barrier in order to propose a new manufacturing process for cellulose nanofibers that facilitates the final fibrillation or delamination step – without going as far as solubilization. Complete fiber processing – by lowering the energy cost without involving toxic chemical reagents like monochloroacetic acid. One of the limitations of carboxymethylation lies in the solvent exchange from water to isopropanol via ethanol, and it has become apparent that this ethanol step is 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: A. disperse the starting cellulose fibers in water; B. filter to obtain water-activated cellulose fibers; C. impregnate 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 their esters; D. introduce the impregnated cellulose fibers into at least one C2-C4 alcohol at a temperature of 60-70 °C and stir the mixture to allow the functionalization reaction to take place; E. purify by filtration the functionalized fibres obtained in step (D) and wash them; F. redisperse them in the water and agitate them; and G. subject the functionalized cellulosic fibers resulting from step (F) to a mechanical nanofibrillation treatment.

[0011] In step (B), it may be envisaged to wash the activated cellulose fibers with alcohol, such as isopropanol or ethanol.

[0012] In the present invention, the following may be used:

[0013] as phenyl acrylic acid(s) at least one compound of formula (I): O (I) QH

[0014] in which R represents H or a phenyl nucleus substituent selected from -OH, -OCH3 or -CH=CH-COOH or at least two phenyl nucleus substituents, identical or different, selected from -OH, -OCH3 or -CH=CH-COOH;

[0015] 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,

[0016] such as benzoic acid(s) or their esters at least one compound of formula (II): O (II) g" R

[0017] in which R represents H or a phenyl nucleus substituent selected from -OH, -OCH3, -NH2 or -CH=CHCH3 and R' represents H or CH3;

[0018] 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,

[0019] such as imidazole acrylic acid, trans-urocanic acid

[0020] as imidazole carboxylic acid(s), at least one compound of formula (III): g (ni)

[0021] in which R represents H or a substituent of the aromatic ring chosen from -OH, -OCH3 or -NH2;

[0022] such as 5-benzylimidazolecarboxylic acid,

[0023] as furylacrylic acid(s), at least one compound of formula (IV): ? (IV) R

[0024] in which R represents H or a substituent of the furyl ring chosen from -OH, -OCH3 or -NH2;

[0025] such as 3-(2-furyl)acrylic acid.

[0026] 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.

[0027] At each of the steps (C) and (D), at least one C2-C4 alcohol can be used, chosen from propanol and isopropanol.

[0028] At step (C), the mooring can be conducted for 10 min - 1 h.

[0029] Step (D) can be brought to a temperature that can reach reflux, being particularly of 60-70°C.

[0030] Step (D) can be carried out in the presence of sodium hydroxide, the ratio of sodium hydroxide in millimoles / mass of fiber in grams being between 4 and 20, in particular 4.

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

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

[0033] In 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. It can, for example, be carried out by homogenization or high-pressure microfluidization. For example, the functionalized cellulose fibers can be made to pass through a piston pump that applies high pressure, 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 about 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] On this drawing:

[0037] Fig. 1 represents FTIR spectra of control cellulosic fibers (BK) and of different fibers functionalized according to the present invention.

[0038] Fig. 2 represents the visual appearance (top) and the polarized light optical microscopy morphology (bottom) of the control cellulosic fibers (BK) and 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] Fig. 3 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] Fig. 4 represents polarized light optical microscope observations of starting and cinnamic acid functionalized cellulosic fibers: 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] Fig. 5 represents SEM observations of starting and cinnamic acid functionalized cellulosic fibers: 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] Fig. 6 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] Fig. 7 represents the distribution of fibers according to their width (pm) of the fibers according to Fig. 6,

[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 control cellulosic 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 a to a: Production of cellulose nanofibers without NaOH 1. Acid JlPA, TA, 30 min ■> <7%: s. ipa, / □“c, 1 u .¾ y . <7, Cinnamic acid, p-coumaric acid, ferulic acid General operating procedure

[0059] A. 100 mg of cellulose fibers were dispersed in water using a mixer to obtain water-activated starting fibers. B. Water-activated fibers were filtered to 0.22 µm using a vacuum filtration system and washed with ethanol. C. The fibers were then impregnated with an acid according to the invention (0.10 mmol) dissolved in 0.5 mL of IPA for 30 min. at room temperature. The ratio of the acid according to the invention (mmol) / fibers (g) was 1. D. 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. E. The fibers obtained were purified by filtration followed by washing with IPA (x3) and water (x3) until neutral pH. F. The fibers were redispersed in the water under agitation. G. The resulting suspensions were then homogenized using a Panda Plus 2000 homogenizer (GEA Niro Soavi, Italy) to a concentration of 1% (w / w, dry matter content). The mixture was then passed through a piston pump applying high pressure. This homogenizer is equipped with two interaction chambers operating between 150 and 200 bar maximum pressure 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 a to Ih: Preparation of cellulose nanofibers

[0061] As an acid according to the invention, cinnamic acid (Ex. 1a), p-coumaric acid (Ex. 1b), ferulic acid (Ex. 1e), 1,4-diphenylenediacrylic acid (Ex. 1d), 4-aminobenzoic acid (Ex. 1e), trans-urocanic acid (Ex. 1i), 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 the Examples respectively la to le, but after impregnation, the fibers were added to a solution heated to 60°C of sodium hydroxide (16 mg, 0.4 mmol) in isopropanol (1 mL) in the round-bottomed trineck.

[0064] Example 3: Analysis of BK fibers, fibers according to Examples 1a to 1e obtained with and without NaOH, and fibers according to Examples Id to Ih obtained without NaOH.

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

[0066] The band at 1738 cm⁻¹ corresponds to the stretching vibration of the C=O bond of the COOH groups of the acids according to the invention. [Fig. 1] shows the presence of carboxyl groups, even though the band intensity is low, which demonstrates the presence of these carboxylic acids.

[0067] The next step in characterizing the functionalized fibers is to determine the rate of grafting of the acids according to the invention, by conductimetric titration. Conductimetry allows the 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 g1) With NaOH Without NaOH BK-CA 0.239 ± 0.029 0.118 + 0.020 BK-CoA 0.216 ± 0.076 0.264 + 0.060 BK-FA 0.293 ± 0.056 0.210 + 0.035

[0069] The charge of the starting cellulosic fibers is 0.212 mmol g⁻¹. After 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. Cinnamic acid (CA) ' ' ' oh r' c' 0^............+ n:' ' A ; p-C-pumaric acid (CoA) Ferulic acid (FA) O ÇH O GG - - J C1 "• .+-. ., . O -. -s . A '• •A'-' ■ lj-H Vyÿii-'OH •' ' .............;-xj HO HO ' aOO-kï HO HO' 'CH '

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

[0071] Fig. 2 of 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 the 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. Figure 3 shows the appearance of BK-CA, UBK-CA, and CTM-CA pulps prepared without NaOH, and demonstrates how the reaction with cinnamic acid facilitates the breakdown of fibers in the three types of pulp studied.

[0075] To demonstrate pulp fibrillation, the inventors performed high-pressure homogenization tests on 0.1 g L⁻¹ dispersions for 5 min, as few differences between functionalized and non-functionalized fibers had been observed under optical microscopy. Dispersion concentrations of 0.1 g L⁻¹ were homogenized for 5 min. These fibers were observed by MOLP and SEM before and after homogenization ([Fig. 4]).

[0076] In [Fig. 4], fiber fibrillation is observed after passing through the homogenizer. We then observed the same samples using SEM to examine the fiber surface in more detail and better detect fiber fibrillation. In [Fig. 5], 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 ([Fig.6]) and their distribution according to their width and compare their evolution before and after functionalization ([Fig.7]).

[0078] Thus, these results confirm that high-pressure homogenization leads to significant fibrillation of the fibers with or without pretreatment, as can be seen in [Fig.6] and [Fig.7].

[0079] Moreover, on [Fig.7], it can be seen 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 appears less significant than on BK fibers, since the fibers have already decreased in size.

Claims

Demands

1. - Process for producing cellulose nanofibers by fibrillation of cellulose fibers, characterized by the fact that it comprises the following successive stages: A. disperse the starting cellulose fibers in water; B. filter to obtain water-activated cellulose fibers; C. impregnate 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 their esters; D. introduce the impregnated cellulose fibers into at least one C2-C4 alcohol at a temperature of 60-70 °C and stir the mixture to allow the functionalization reaction to take place; E. purify by filtration the functionalized fibres obtained in step (D) and wash them; F. redisperse them in the water and agitate them; and G. subject to mechanical nanofibrillation treatment functionalized cellulosic fibers resulting from step (F).

2. - A method according to claim 1, characterized in that one used : as phenyl acrylic acid(s) at least one compound of formula (I): O ® OH 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; 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, such as benzoic acid(s) or their esters at least one compound of formula (II): O (II) GOLD R in which R represents H or a phenyl nucleus substituent chosen 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): Q (III) NU ri 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) ' ' OH R in which R represents H or a substituent of the furyl ring chosen from -OH, -OCH3 or -NH2; such as 3-(2-furyl)acrylic acid.

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10. - 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, selected 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 process according to any one of claims 1 to 6, characterized in that step (D) is carried out in the presence of sodium hydroxide, the ratio of sodium hydroxide in millimoles / mass of fibers in grams being between 4 and 20, in particular 4. - A method according to any one of claims 1 to 7, characterized in that at step (D), the mixture is stirred for 1-2 hours. - A process according to any one of claims 1 to 8, 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 9, 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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