METHOD AND FORMULATION FOR PREPARING LIGNIN FIBRES

Abstract

A formulation for melt-preparation of lignin-based fibres, which are precursors of carbon fibres. The formulation includes lignin, a plasticiser and a cross-linking agent capable of cross-linking with the lignin at a temperature at least 10 C. higher than the glass-transition temperature of the intimate blend of the lignin and the plasticizer. A method for preparing lignin-based fibres using this formulation includes the hot extrusion spinning of an intimate blend of the components of the formulation, under adequate conditions for cross-linking the cross-linking agent and the lignin in the terminal area of the extrusion device used.

Claims

1. A method for melt-preparation of lignin-based fibers, comprising: a/ the intimate blending of the components of a formulation comprising: lignin, a plasticizer miscible with said lignin, and a cross-linking agent capable of cross-linking with said lignin, in conditions in which no cross-linking of the cross-linking agent and of the lignin occurs, b/ the spinning of the intimate blend thus obtained in a hot extrusion device comprising an extrusion head in a terminal part, in order to form continuous threads, by application in said extrusion device of conditions such that: the temperatures applied in said extrusion device are higher than the glass-transition temperature of said intimate blend, and the temperatures, the residence time of said intimate blend in said extrusion device and the residence time of said intimate blend in said extrusion head are such that the cross-linking reaction of the cross-linking agent and lignin is initiated only in the extrusion head, c/ when appropriate, the drawing of the threads obtained in order to form lignin-based fibers of great length, with the cross-linking agent being chosen in such a way that the cross-linking reaction thereof with said lignin is initiated in a time less than said residence time of said intimate blend in said extrusion head at temperatures above a temperature at least 10 C. greater than the glass-transition temperature of said intimate blend of said lignin and of said plasticizer.

2. The method according to claim 1, wherein the step a/ of intimate blending of the components of said formulation comprises the following sub-steps: a1/ preparation of said formulation, by blending of said lignin, said plasticizer and said cross-linking agent, a2/ the blending by extrusion of the components of said formulation, at a temperature greater than the glass-transition temperature of the lignin and than a softening temperature of the plasticizer, in order to form an extrudate in which the lignin, the plasticizer and the cross-linking agent are in an intimate blend.

3. The method according to claim 1, wherein the step b/ of spinning of the intimate blend of the components of the formulation, it is applied in the extrusion head a temperature that is greater than the glass-transition temperature of said intimate blend by a value of between 10 and 150 C.

4. The method according to claim 1, wherein the lignin and the plasticizer are such, and in proportions such, that the glass-transition temperature of said intimate blend of said lignin and of said plasticizer is between 0 and 180 C.

5. Method according to claim 1, wherein the lignin has a weight average molecular weight between 1,000 and 100,000 g/mol.

6. Method according to claim 1, wherein the plasticizer is a polymer, or blend of polymers.

7. Method according to claim 1, wherein the plasticizer is a fusible acrylonitrile polymer.

8. Method according to claim 1, wherein the plasticizer has a glass-transition temperature, where applicable glass-transition temperatures, of less than 150 C.

9. Method according to claim 1, wherein the cross-linking agent is chosen in the group consisting of: benzoxazines, epoxies, oxazolines, polyoxymethylenes, aldehydes, hexamethylenetetramine and hexamethylenemethoxymelamine; compounds that have a functionality greater than or equal to two, of which the chemical functions are chosen in the group consisting of the functions: benzoxazine, epoxy (glycidyl ether in particular), isocyanate, anhydride, carboxylic acid, methylol and ester, with these chemical functions being identical or different, or any blend of such compounds.

10. Method according to claim 1, wherein the cross-linking agent is capable of initiating a cross-linking reaction with the lignin in less than 10 minutes at a temperature between 30 and 190 C.

11. Method according to claim 1, wherein the cross-linking agent is capable of cross-linking with the lignin at ambient temperature.

12. Method according to claim 1, wherein said formulation comprises the following percentages by weight, with respect to the total weight of the formulation: 50 to 98% of lignin, and/or 1 to 49% of plasticizer, and/or 1 to 25%, preferably 2 to 10%, preferentially 2.5 to 5%, of cross-linking agent.

13. Method according to claim 1, wherein said formulation comprises one or several additives chosen in the group consisting of carbon fillers of nanometric size, alone or in a mixture.

14. Method according to claim 1, wherein a final step of storage of said lignin-based fiber at ambient temperature.

15. Formulation for the implementing of a method of melt-preparation of lignin-based fibers according to claim 1, comprising: lignin that is not chemically modified, a plasticizer miscible with said lignin, and a cross-linking agent capable of initiating a cross-linking reaction with said lignin in less than 10 minutes at temperatures above a temperature at least 10 C. greater than the glass-transition temperature of said intimate blend of said lignin and of said plasticizer.

16. Formulation according to claim 15, wherein the plasticizer is a fusible acrylonitrile polymer.

17. Lignin-based extrudate obtained at the end of the step a/ of intimate blending of the components of the formulation of a method according to claim 1.

18. Lignin-based fiber obtainable by a method according to claim 1, having a length greater than or equal to 1 m.

19. A method for the manufacture of carbon fibers, comprising a step of using a lignin-based fiber according to claim 18.

20. Method for the manufacture of a carbon fiber, comprising: 1/ the melt-preparation of lignin-based fiber, comprising: a/ the intimate blending of the components of a formulation comprising: lignin, a plasticizer miscible with said lignin, and a cross-linking agent capable of cross-linking with said lignin, in conditions in which no cross-linking of the cross-linking agent and of the lignin occurs, b/ the spinning of the intimate blend thus obtained in a hot extrusion device comprising an extrusion head in a terminal part, in order to form continuous threads, by application in said extrusion device of conditions such that: the temperatures applied in said extrusion device are higher than the glass-transition temperature of said intimate blend, and the temperatures, the residence time of said intimate blend in said extrusion device and the residence time of said intimate blend in said extrusion head are such that the cross-linking reaction of the cross-linking agent and lignin is initiated only in the extrusion head, c/ when appropriate, the drawing of the threads obtained in order to form lignin-based fibers of great length, with the cross-linking agent being chosen in such a way that the cross-linking reaction thereof with said lignin is initiated in a time less than said residence time of said intimate blend in said extrusion head at temperatures above a temperature at least 10 C. greater than the glass-transition temperature of said intimate blend of said lignin and of said plasticizer; and 2/subjecting the lignin-based fiber thus obtained to a carbonization treatment.

21. Method for the manufacture of a carbon fiber according to claim 20, wherein said lignin-based fiber is not subjected to any heat treatment step prior to the implementing of the carbonization treatment.

22. Method for the manufacture of a carbon fiber according to claim 20, wherein prior to the treatment of said lignin-based fiber by carbonization, a step of heat treating said lignin-based fiber during a period between 10 and 60 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0167] The characteristics and advantages of the present disclosures shall appear more clearly in light of the examples of implementation hereinafter, provided simply for the purposes of illustration and in no way limiting of the present disclosure, with the support of FIGS. 1 to 9, wherein:

[0168] FIG. 1 diagrammatically shows, in the form of a block diagram, the various steps of a method for manufacturing a carbon fiber according to the present disclosure;

[0169] FIG. 2 shows a bargraph that illustrates the melt flow index (MFI) measured at different temperatures, for granules obtained in the first step of a method according to the present disclosure (with cross-linking agent) and for granules obtained by a similar method, but which does not implement a cross-linking agent (without cross-linking agent);

[0170] FIG. 3 shows a graph that represents, according to the temperature, the loss factor (tan delta) and the storage modulus (E) measured by dynamic mechanical analysis for a sample of granules obtained in the first step of a method for preparing lignin-based fibers according to the present disclosure: without additional heat treatment (E0), with a heat treatment of 130 C. for 5 min (E1) or a heat treatment of 130 C. for 5 min, then from 20 C. up to 220 C. for 100 min (E2);

[0171] FIG. 4 shows graphs that illustrate the change in the melt flow index (MFI) measured respectively at a/ 160 C., b/ 130 C., according to the sampling time, for granules obtained in the first step of a method according to the present disclosure; on each one of these graphs the reference area for the calculation of the MFI is indicated by a box;

[0172] FIG. 5 shows a graph that represents, according to the temperature, the loss factor (tan delta) and the storage modulus (E) measured by dynamic mechanical analysis for a sample of granules obtained in the first step of a method according to the present disclosure: without additional heat treatment (E0), with a heat treatment of 130 C. for 5 min (E1) or a heat treatment of 130 C. for 5 min then of 20 C. up to 220 C. for 100 min (E2);

[0173] FIG. 6 shows a scanning electron microscope image of a lignin-based fiber obtained by a method in accordance with the present disclosure;

[0174] FIG. 7 shows an optical microscope image of a carbon fiber obtained by direct carbonization of the lignin-based fiber of FIG. 5, by a method of manufacturing carbon fibers in accordance with the present disclosure;

[0175] FIG. 8 shows graphs that illustrate the change in the melt flow index (MFI) measured respectively at a/ 130 C., b/ 170 C., according to the sampling time, for granules obtained in the first step of a method similar to the method according to the present disclosure that does not implement any cross-linking agent; on each one of these graphs the reference area for the calculation of the MFI is indicated by a box;

[0176] and FIG. 9 shows a graph that represents, according to the temperature, the loss factor (tan delta) and the storage modulus (E) measured by dynamic mechanical analysis for a sample of granules obtained in the first step of a method similar to the method according to the present disclosure that does not implement any cross-linking agent, without additional heat treatment.

DETAILED DESCRIPTION

[0177] The various steps of a method for manufacturing a carbon fiber according to an aspect of the present disclosure are shown diagrammatically in FIG. 1, in the form of a block diagram.

[0178] This method first comprises the manufacturing of a lignin-based fiber, in accordance with the present disclosure.

[0179] In the first step a1/, shown as 11 in the figure, it comprises the blending of lignin, of a plasticizer that is hot miscible with the lignin and of a cross-linking agent capable of cross-linking with the lignin, more precisely to initiate a cross-linking reaction with the lignin, in a few minutes at a temperature which is at least 10 C. greater than the glass-transition temperature of the intimate blend of the lignin and of the plasticizer.

[0180] The following step a2/ of the method, shown as 12 in the figure, consists in carrying out the intimate blending of the components hereinabove, via the so-called compounding technique, at a temperature greater than the glass-transition temperature of the lignin and than the softening temperature, in particular the melting temperature when it is a crystalline or semi-crystalline material, of the plasticizer. At the end of this step, an extrudate is obtained, in particular in the form of granules, wherein the lignin, the plasticizer and the cross-linking agent are in an intimate blend. No cross-linking reaction has begun yet at this stage of the method.

[0181] The step a1/ and the step a2/ can be carried out successively or simultaneously, at least for some of the operations that they require.

[0182] The following step of the method for preparation of a lignin-based fiber according to a particular aspect of the present disclosure, shown as 13 in the figure, consists in a step of spinning b/ of the extrudate obtained, in an extrusion device comprising an extrusion head in the terminal part, in order to form continuous threads. To this effect, operating conditions of temperature and of residence time are applied in the device, which are such that the temperature applied in the extrusion device is higher than the glass-transition temperature of the intimate blend formed in the step a2/, so that this blend is in fluid form; and the cross-linking of the cross-linking agent and of the lignin is caused in the terminal part of the extrusion device, in the extrusion head, and only in this terminal part.

[0183] At the output of the extrusion device, a lignin-based fiber is obtained, which is subjected, if necessary, to a step c/ of drawing, shown as 14 in the figure, which is conventional in itself.

[0184] The lignin-based fiber obtained is then subjected to a step of winding, shown as 15 in the figure.

[0185] The method for manufacturing a carbon fiber according to the present disclosure then comprises a step of carbonization of the lignin-based fiber, shown as 16 in the figure, and conventional in itself.

[0186] Optionally, it can comprise an intermediate step, shown as 17 in FIG. 1, of heat treatment of the wound lignin-based fiber, aiming to increase its stability by progression of the cross-linking reaction of the cross-linking agent and of the lignin, at a temperature less than 350 C. and for a duration that does not exceed a few tens of minutes.

Example 1

[0187] 1/ Formulation

[0188] The formulation in accordance with the present disclosure implemented in this example contains the following contents of constituents, expressed as a percentage by weight, with respect to the total weight of the formulation:

[0189] 76% Protobind 2400 lignin marketed by the company GreenValue,

[0190] 20% polyethylene oxide polymer marketed under the name Alkox E30 by the company Meisei Chemical Works, of a number average molecular weight between 300,000 and 400,000 g/mol and of a weight average molecular weight between 400,000 and 550,000 g/mol (plasticizer),

[0191] and 4% bisphenol A diglycidylether, an epoxy compound marketed under the name Epolam 8056 R by the company Axson (cross-linking agent).

[0192] The properties of the plasticizer are as follows: glass-transition temperature Tg=50/60 C., melting temperature Tf=55-60 C., crystallization temperature Tc=30-40 C.

[0193] The properties of the cross-linking agent are as follows: glass-transition temperature Tg=20 C., molecular weight less than 700 g/mol.

[0194] The glass-transition temperature of this formulation, measured by DMA according to the protocol mentioned hereinabove, is 62 C. The operating cross-linking temperature thereof is greater than 120 C.

[0195] 2/ Method for Preparing Lignin Fibers

[0196] Using these ingredients, the following method for manufacturing lignin fibers in accordance with the present disclosure is implemented.

[0197] Compounding

[0198] The lignin and the plasticizer are blended in a prior step via mechanical action. An auger dozer is used to pour this blend into the feeding hopper of an extruder that is not heated.

[0199] The extruder used is a Eurolab twin-screw co-rotating screw extruder from Thermo Scientific. The operating parameters are as follows:

[0200] Screw diameter=16 mm

[0201] L/D ratio: 40

[0202] Screw speed=250 rpm

[0203] No degassing is implemented during the extrusion.

[0204] The temperatures of the various areas of the extruder are as follows:

[0205] Temperature of the supply area: 60 C.

[0206] Temperature of the blending areas: 100 C. and 120 C.

[0207] Temperature of the conveying areas: 120 C. and 135 C.

[0208] Temperature of the die: 130 C.

[0209] The temperature of the extrusion die is 30 C. less than the operating cross-linking temperature of the cross-linking agent with the lignin.

[0210] The cross-linking agent, which is in liquid form, is injected using a syringe pump into the penultimate heating area (i.e. at the level of the area at 135 C.). It therefore spends little time in the extruder, which makes it possible to prevent activating the cross-linking reaction with the lignin.

[0211] A flexible and drawable ring is obtained at the die exit. The total residence time of the lignin in the extruder was less than 5 min.

[0212] This ring is air-cooled before being granulated.

[0213] Analysis of the Granules Obtained

[0214] The granules thus obtained are analyzed in order to determine their melt flow index (MFI) at different temperatures. This analysis consists in measuring the mass of material passing through a given die under the action of a fixed pressure, during a given time and at a fixed temperature. The more fluid the material is, the more substantial the quantity of material exiting from the die during this given time is.

[0215] The conditions of analysis are as follows. The temperatures tested are 130, 140, 150, 160 and 170 C. Once the oven is at the desired temperature, 5 g of material are inserted into the hopper of the MFI measuring device. So that this quantity is really at the desired temperature, it is subjected to a preheating time of 5 min under a weight of 2.16 Kg, then a sample is taken every 10 seconds.

[0216] For the purposes of comparison, the same measuring protocol is implemented, for granules obtained as indicated hereinabove, but without the addition of a cross-linking agent.

[0217] The results obtained are shown in FIG. 2.

[0218] For the comparative granules without cross-linking agent (hatched bars on the graph), it is observed that the higher the temperature is, the higher the MFI index is.

[0219] For the granules obtained in accordance with the present disclosure (white bars on the graph): at 130 C., the MFI is low, but higher than for the granules obtained without cross-linking agent. At 130 C., the non-reacted cross-linking agent acts as a plasticizer and fluidizes the blend. After 6 minutes of flow under heating, a caking due to the cross-linking is observed (the cross-linking agent has played its role well and entirely cross-linked the formulation: no more flow is produced). A 140 C., the MFI is a little higher. The viscosity of the non-reacted blend is lower, because the temperature is higher. A caking however takes place faster, after 3 minutes of flow under heating. The kinetics of the reaction is accelerated as the temperature is higher. A 150 C., it is observed that the MFI has fallen, and the caking takes place after only 2 minutes of flow under heating. This indicates that the cross-linking started during the preheating time. Finally, at 160 C., there is no flow after 5 minutes, the caking therefore took place during the preheating time.

[0220] Thus, this demonstrates that the cross-linking agent reacts after a certain period of time and cross-links the blend, causing the viscosity to increase and the MFI to fall; furthermore, the time required for the reaction decreases when the temperature increases.

[0221] The granules are furthermore analyzed by dynamic mechanical analysis (DMA) in order to measure their viscoelastic and mechanical properties according to the temperature. To this effect, at the end of the compounding step, they are subjected to additional heat treatments that make it possible to simulate the behavior of the material during the subsequent steps of the method (spinning, then optional additional heat treatment): heat treatment at 130 C. for 5 min; heat treatment at 130 C. for 5 min then a second passage from 20 to 220 C. for 100 min. For the purposes of comparison, a sample obtained directly at the end of compounding is also analyzed.

[0222] A DMA device 1 from Mettler Toledo is implemented. The operating parameters are as follows:

[0223] Type of solicitation: tension

[0224] Sample solicited: parallelepiped; L=5 mm; W=2 mm; thickness=0.8 mm

[0225] Temperature gradient: 2 C./min

[0226] Frequency: 1 Hz

[0227] Displacement: 2 m

[0228] The displacement is chosen in such a way as to obtain a signal while still remaining in the linear viscoelastic domain.

[0229] The results obtained are shown in FIG. 3.

[0230] It is observed here that the sample such as obtained after the compounding (E0) creeps after the glass transition, so that it is suitable for a later spinning. The sample heated to 130 C. for 5 min (E1) has a slightly offset glass transition, and the material does not creep after the latter; after the glass transition, the storage modulus E is from 5 to 10 times greater than that of the sample E0. The material has started to cross-link. The cross-linking is not completed, since a rise in E is observed between 130 C. and 220 C. This result confirms that it will be possible to carry out the following step of the method according to the present disclosure at a temperature greater than or equal to 130 C. to start the step of cross-linking. For the sample that underwent the first temperature sweeping and a subsequent temperature sweeping between 20 and 220 C. (E2), the glass transition is substantially offset towards the high temperatures and after this transition, E remains 10 times greater than that of E0. The material is completely cross-linked and does not creep. E reached by E1 at 220 C. is very close to E reached by E2. This result shows the importance of the temperature of the die in the following step: an excessively hot die results in a complete cross-linking and in an obstruction of the extrusion head by the material.

[0231] Spinning

[0232] The granules obtained at the end of the step of compounding are subjected to a step of hot extrusion spinning then drawing of the fibers, in the following way.

[0233] A Scamex single-screw table horizontal extruder of the Rheoscam type is implemented with a L/D ratio=11, with 3 heating areas. The third heating area makes it possible to control the temperature of the extrusion head, and is adjusted to initiate the cross-linking.

[0234] The temperatures used are more precisely as follows:

[0235] Temperature of the supply area: 120 C.

[0236] Temperature of the conveying area: 140 C.

[0237] Temperature of the extrusion head: 180 C.

[0238] The granules are introduced into the extruder, and the melted blend is directly extruded through the extrusion head at the screw output, to form a thread, which is drawn while being cooled in the ambient air.

[0239] The thread then joins a set of feeding rollers, at a speed of 20 m/min, passes through an oven at the temperature of 160 C., the n over a set of drawing rollers, at a speed of 40 m/min. It undergoes a drawing between the two sets of rollers, according to the ratio of the rotation speed of the feeding rollers and of the drawing rollers. In the last step, the lignin fiber obtained is wound on a cardboard tube.

[0240] This fiber has the form of a monofilament. It has a diameter of about 20 m. Its length is about 1,000 meters.

[0241] This lignin fiber can be directly subjected to a step of carbonization, in order to form a carbon fiber, without undergoing any intermediate step of oxidative thermal treatment.

[0242] By way of example, it can to this effect be subjected to a static carbonization under nitrogen and under tension with a temperature gradient of 20 to 1200 C. at 3 C./min, conventionally in itself. The carbon fiber then obtained has the form of a monofilament, and has a diameter of about 10 m.

Example 2

[0243] 1/ Formulation

[0244] The formulation in accordance with the present disclosure implemented in this example contains the following contents of constituents, expressed as a percentage by weight, with respect to the total weight of the formulation: [0245] 75% Protobind 2400 lignin, [0246] 20% polyethylene oxide polymer marketed under the name Alkox E30 (plasticizer), [0247] and 5% bisphenol F bisbenzoxazine marketed under the name Araldite MT 35700 by the company Huntsman (cross-linking agent).

[0248] The properties of the plasticizer are as follows: glass-transition temperature Tg=50/60 C., melting temperature Tf=55-60 C., crystallization temperature Tc=3040 C.

[0249] The properties of the cross-linking agent are as follows: glass-transition temperature Tg=73.6 C., melting temperature Tf=240 C.

[0250] The glass-transition temperature of this formulation, measured by DMA according to the protocol mentioned hereinabove, is 65 C. The operating cross-linking temperature thereof is greater than 180 C.

[0251] 2/ Method for Preparing Lignin Fibers

[0252] Using these ingredients, the method for manufacturing lignin fibers in accordance with the present disclosure is implemented such as described hereinabove in the example 1, except for the following operating parameters.

[0253] Compounding

[0254] The temperatures of the various areas of the extruder are as follows:

[0255] Temperature of the supply area: 60 C.

[0256] Temperature of the blending areas: 100 C. and 130 C.

[0257] Temperature of the conveying areas: 130 C. and 130 C.

[0258] Temperature of the die: 130 C.

[0259] The screw speed is 150 rpm.

[0260] The cross-linking agent used beings to cross-link with the lignin around 200 C. The extrusion temperatures being lower than its activation temperature, it plays the role of a filler in the formulation. The ring at the die output is flexible and can be drawn. It is air cooled before it is granulated.

[0261] Analysis of the Granules Obtained

[0262] The granules thus obtained are analyzed in order to determine their melt flow index MFI at different temperatures. The protocol used is the same as the one disclosed in the Example 1 hereinabove.

[0263] The temperatures tested are 160 and 190 C.

[0264] The results obtained are shown in FIG. 4.

[0265] For the sample treated at 160 C. (graph a/), for the reference area indicated by a box in the figure, a melt flow index MFI=0.940.41 g/10 min is calculated. For the sample treated at 130 C. (graph b/), for the reference area indicated by a box in the figure, a melt flow index MFI=0.820.29 g/10 min.

[0266] These results obtained at 160 and 190 C. are explicit: the MFI decreases over time for each temperature. This means that the cross-linking agent is starting to act. In the following step of spinning, the cross-linking in the last area of the extruder can be carried out, and this so as to obtain a lignin-based fiber that does not require a later step of thermo-oxidation.

[0267] The granules are furthermore analyzed by dynamic mechanical analysis (DMA), according to the protocol indicated in the example 1 hereinabove.

[0268] The results obtained are shown in FIG. 5.

[0269] It is observed here that the sample such as obtained after the compounding (E0) creeps after the glass transition, and is therefore suitable for the next step of the method according to the present disclosure. The sample treated at 130 C. for 5 min (E1) has exactly the same behavior: the cross-linking did not take place. In both cases, a rise in E towards 190 C. is noted, showing a starting of cross-linking. The result obtained for the sample that was subjected to an additional heat treatment from 20 C. to 220 C. (E2) confirms that the cross-linking has taken place: the glass transition takes place at a higher temperature, and E after this transition is 10 times higher than in the preceding cases, there is still the signal: the material has not crept.

[0270] This confirms that the cross-linking begins around 190 C.

[0271] Spinning

[0272] The granules obtained at the end of the step of compounding are subjected to a step of hot extrusion spinning then drawing of the fibers, according to the protocol described in the example 1 hereinabove.

[0273] The temperatures of the various areas of the extruder are as follows:

[0274] Temperature of the supply area: 130 C.

[0275] Temperature of the conveying area: 180 C.

[0276] Temperature of the extrusion head: 200 C.

[0277] A lignin fiber with a length of about 500 m and a diameter of about 100 m is obtained. A scanning electron microscope image of this lignin-based fiber is shown in FIG. 6.

[0278] 3/ Method for Preparing Carbon Fibers

[0279] This lignin fiber is directly subjected to a step of carbonization, in order to form a carbon fiber, without being subjected beforehand to any step of oxidative heat treatment.

[0280] This step is carried out via static carbonization under nitrogen and under tension, with a temperature gradient from 20 to 1200 C. at 3 C./min, conventionally in itself. The carbon fiber obtained has the form of a continuous monofilament with a length of about 20 cm, and has a diameter of about 50 m.

[0281] An optical microscope image of this carbon fiber is shown in FIG. 7.

Example 3

[0282] 1/ Formulation

[0283] The formulation in accordance with the present disclosure implemented in this example contains the following contents of constituents, expressed as a percentage by weight, with respect to the total weight of the formulation: [0284] 72% Protobind 2400 lignin marketed by the company GreenValue, [0285] 18% ether polycarboxylate polymer marketed under the name Ethacryl HF by the company Coatex, with a weight average molecular weight between 100,000 and 150,000 g/mol (plasticizer), [0286] and 10% bisphenol A diglycidylether, an epoxy compound marketed under the name Epolam 8056 R by the company Axson (cross-linking agent).

[0287] The properties of the plasticizer are as follows: glass-transition temperature Tg=45/60 C., melting temperature Tf=35-45 C., crystallization temperature Tc=0-15 C.

[0288] The glass-transition temperature of this formulation, measured by DMA according to the protocol mentioned hereinabove, is 55 C. The operating cross-linking temperature of the cross-linking agent with the lignin is greater than 130 C.

[0289] 2/ Method for Preparing Lignin Fibers

[0290] Using these ingredients, the following method for manufacturing lignin fibers in accordance with the present disclosure is implemented.

[0291] Compounding

[0292] An auger dozer is used to pour the lignin into the feeding hopper of an extruder that is not heated. The Ethacryl HF is injected into this same feeding hopper thanks to a peristaltic pump, while the cross-linking agent is injected only into the penultimate heating area before the die, using a syringe pump. It therefore spends little time in the extruder, which makes it possible to prevent activating the cross-linking reaction with the lignin.

[0293] The extruder used is an LTE 26-40 twin-screw co-rotating screw extruder from Labtech Engineering Company LTD. The operating parameters are as follows:

[0294] Screw diameter=26 mm

[0295] L/D ratio: 40

[0296] Screw speed=190 rpm

[0297] Three degassing openings are implemented during the extrusion.

[0298] The temperatures of the various areas of the extruder are as follows:

[0299] Temperature of the supply area: 80 C.

[0300] Temperature of the blending areas: 120 C.

[0301] Temperature of the conveying areas: 120 C.

[0302] Temperature of the die: 120 C.

[0303] The temperature of the extrusion die is 10 C. less than the operating cross-linking temperature of the cross-linking agent with the lignin.

[0304] A flexible and drawable ring is obtained at the die exit. The total residence time of the lignin in the extruder was less than 1 min.

[0305] This ring is air-cooled before being granulated.

[0306] Spinning

[0307] The granules obtained at the end of the step of compounding are subjected to a step of hot extrusion spinning then drawing of the fibers, in the following way. The operating protocol is such as described in the example 1, except for temperatures of the extruder, which are as follows:

[0308] Temperature of the supply area: 130 C.

[0309] Temperature of the conveying area: 130 C.

[0310] Temperature of the extrusion head: 140 C.

[0311] The thread obtained at the end of the extrusion step then join a set of feeding rollers, at a speed of 27 m/min, passes through an oven at a temperature of 100 C., then on a set of drawing rollers, at a speed of 81 m/min.

[0312] In the last step, the lignin fiber obtained is wound on a cardboard tube.

[0313] This fiber has the form of a monofilament. It has a diameter of about 30 m. Its length is about 500 meters.

[0314] This lignin fiber can be subjected directly to a step of carbonization, in order to form a carbon fiber, without undergoing any intermediate step of oxidative thermal treatment.

[0315] By way of example, it can to this effect be subjected to a static carbonization under nitrogen and under tension with a temperature gradient of 20 to 1200 C. to 3 C./min, conventionally in itself. The carbon fiber then obtained has the form of a monofilament, and has a diameter of about 15 m.

Example 4

[0316] 1/ Formulation

[0317] The formulation in accordance with the present disclosure implemented in this example contains the following contents of constituents, expressed as a percentage by weight, with respect to the total weight of the formulation: [0318] 63% Protobind 2400 lignin marketed by the company GreenValue, [0319] 27% poly(acrylonitrile-co-methyl acrylate)-block-poly(acrylonitrile-co-butadiene) copolymer marketed under the name Barex 210E. [0320] and 10% bisphenol F bisbenzoxazine marketed under the name Araldite MT 35700 by the company Huntsman (cross-linking agent).

[0321] The properties of the plasticizer are as follows: glass-transition temperature 1 Tg1=66 C., glass-transition temperature 2 Tg2=131 C.

[0322] The glass-transition temperature of this formulation is 85 C. The operating cross-linking temperature of the cross-linking agent with the lignin is greater than 180 C.

[0323] 2/ Method for Preparing Lignin Fibers

[0324] Using these ingredients, the following method for manufacturing lignin fibers in accordance with the present disclosure is implemented.

[0325] Compounding

[0326] The lignin and the plasticizer are blended in a prior step via mechanical action. An auger dozer is used to pour this blend into the feeding hopper of an extruder that is not heated.

[0327] The extruder used is a Eurolab twin-screw co-rotating screw extruder from Thermo Scientific. The operating parameters are as follows:

[0328] Screw diameter=16 mm

[0329] L/D ratio: 40

[0330] Screw speed=150 rpm

[0331] No degassing is implemented during the extrusion.

[0332] The temperatures of the various areas of the extruder are as follows:

[0333] Temperature of the supply area: 60 C.

[0334] Temperature of the blending areas: 100 C. and 150 C.

[0335] Temperature of the conveying areas: 150 C. and 150 C.

[0336] Temperature of the die: 150 C.

[0337] The temperature of the extrusion die is 30 C. less than the operating cross-linking temperature of the cross-linking agent with the lignin.

[0338] A flexible and drawable ring is obtained at the die exit. The total residence time of the lignin in the extruder was less than 5 min.

[0339] This ring is air-cooled before being granulated.

[0340] Spinning

[0341] The granules obtained at the end of the step of compounding are subjected to a step of hot extrusion spinning then drawing of the fibers, in the following way. The operating protocol is such as described in the example 1, except for temperatures of the extruder, which are as follows:

[0342] Temperature of the supply area: 160 C.

[0343] Temperature of the conveying area: 165 C.

[0344] Temperature of the extrusion head: 170 C.

[0345] The thread obtained is wound on a cardboard tube. This fiber has the form of a monofilament. It has a diameter of about 30 m. Its length is greater than 1,200 m. The fiber obtained is mechanically resistant and has increased drawability in relation to the fibers obtained in the preceding examples 1, 2, and 3. Because of that, it can advantageously be wound at a speed of 630 m/min.

[0346] This lignin fiber can be directly subjected to a step of carbonization, in order to form a carbon fiber, without undergoing any intermediate step of oxidative thermal treatment.

[0347] By way of example, it can to this effect be subjected to a static carbonization under nitrogen and under tension with a temperature gradient from 20 to 1200 C. at 3 C./min, conventionally in itself. The carbon fiber then obtained has the form of a monofilament, and has a diameter of about 20 m.

Example 5Comparative Example

[0348] For the purposes of comparison, a method for preparing lignin-based fibers similar to the method according to the present disclosure is implemented, which however does not implement any cross-linking agent.

[0349] 1/ Formulation

[0350] The formulation used in this example contains the following content in constituents, expressed as a percentage by weight, with respect to the total weight of the formulation: [0351] 80% Protobind 2400 lignin, [0352] 20% polyethylene oxide polymer marketed under the name Alkox E30 (plasticizer),

[0353] The glass-transition temperature of this formulation, measured by DMA according to the protocol disclosed hereinabove, is 74 C.

[0354] 2/ Method for Preparing Lignin Fibers

[0355] Using these ingredients, a method for manufacturing lignin fibers is implemented according to the operating conditions described hereinabove in the example 1, except for the following operating parameters.

[0356] Compounding

[0357] The temperatures of the various areas of the extruder are as follows:

[0358] Temperature of the supply area: 60 C.

[0359] Temperature of the blending areas: 100 C. and 120 C.

[0360] Temperature of the conveying areas: 120 C. and 130 C.

[0361] Temperature of the die: 130 C.

[0362] The screw speed is 250 rpm.

[0363] Analysis of the Granules Obtained

[0364] The granules thus obtained are analyzed in order to determine their melt flow index MFI at different temperatures. The protocol used is the same as the one disclosed in the Example 1 hereinabove.

[0365] The temperatures tested are 130 and 170 C. The results obtained are shown in FIG. 8, respectively in a/ and in b/.

[0366] The following is calculated: for the sample treated at 130 C. (graph a/), for the reference area indicated by a box in the figure, a melt flow index MFI=0.420.22 g/10 min; for the sample treated at 170 C. (graph b/), for the reference area indicated by a box in the figure, a melt flow index MFI=4.401.07 g/10 min.

[0367] These results obtained at 130 and 170 C. are explicit: the MFI is constant over time for a given temperature and the MFI is increasingly higher when the temperature increases. This means that the higher the temperature is, the more fluid the lignin/plasticizer blend is.

[0368] The granules are furthermore analyzed by dynamic mechanical analysis (DMA) according to the protocol indicated in the example 1 hereinabove, pour

[0369] The results obtained for the analysis of the sample coming from the compounding, without any additional heat treatment, are shown in FIG. 9.

[0370] It is observed here that the sample such as obtained after the compounding creeps after the glass transition (loss of signal), and is therefore suitable for the next step of the method.

[0371] The sample treated at 120 C. for 5 min has exactly the same behavior: the cross-linking did not take place. A second passage at 220 C. cannot be carried out, the sample having crept in the apparatus.

[0372] Spinning

[0373] The granules obtained at the end of the step of compounding are subjected to a step of hot extrusion spinning then drawing of the fibers, according to the protocol described in the example 1 hereinabove, with the operating parameters:

[0374] Temperature of the supply area: 160 C.

[0375] Temperature of the conveying area: 170 C.

[0376] Temperature of the extrusion head: 180 C.

[0377] A lignin-based fiber is obtained with a length of about 500 m and diameter about 100 m.

[0378] 3/ Method for Preparing Carbon Fibers

[0379] This lignin-based fiber is directly subjected to a step of carbonization, without being subjected beforehand to any step of oxidative heat treatment.

[0380] This step is carried out via static carbonization under nitrogen and under tension, with a temperature gradient from 20 to 1200 C. at 3 C./min, conventionally in itself. During this step, it is observed that the fiber melts and creeps at a temperature less than 250 C., and that it breaks in the oven.

[0381] This demonstrates that it is not possible to directly carbonize the lignin-based fiber obtained in the absence of a cross-linking agent.