Precursor Fibers of Lignin-Based Carbon Fibers, Their Production and Use

Abstract

The lignin-based carbon fiber precursor fibers containing water-soluble lignin salt (A) will be described. A special feature of these precursor fibers is that they contain water-soluble polyvinylpyrrolidone or its derivative (B). The present invention further relates to an advantageous method for producing these precursor fibers and an advantageous use of the precursor fibers for producing carbon fibers by carbonization and then optionally graphitization.

Description

Technical Field

[0001] The present invention relates to a method for producing and using a precursor fiber of lignin that is dry-spun using water, which can then be converted into carbon fiber.

Background Art

[0002] Carbon fibers in the form of fiber composites have excellent mechanical properties and low density, and have a high potential for energy savings. Most of the carbon fibers produced are based on polyacrylonitrile (PAN) (more than 95%) produced by polymerizing acrylonitrile with a small amount of comonomer. The required acrylonitrile is synthesized from petroleum-derived propene and ammonia. This precursor fiber is produced using a special wet-spinning process. Due to the high cost of energy-intensive production of carbon fibers in the wet-spinning process, the poor CO2 balance of the entire process, and the use of problematic toxic solvents, alternative materials and processes have been sought for a long time.

[0003] Lignin has been studied for a long time as a potential alternative to PAN-based carbon fibers. The decisive factor in this research is that 50 million tons of lignin are produced annually as a by-product of papermaking and in biorefineries. After cellulose, lignin is the second most abundant biopolymer in nature. Only about 2% of lignin is used for commercial purposes, and most of it is simply used as fuel.

[0004] Apart from its economic importance, lignin has a high carbon yield during carbonization (Non-Patent Document 1). Nevertheless, using lignin as a precursor material involves several difficulties due to its chemical composition, structure, and purity. Lignin is fractionated into organosolv lignin, kraft lignin, soda lignin, and lignosulfonate by various digestion processes. Lignosulfonate is obtained as a by-product in the sulfite process during the production of chemical pulp. In this process, the free hydroxyl groups of lignin are replaced by sulfonic acid groups. As a result, lignin becomes water-soluble and can be used in an aqueous dry spinning process. Since the sulfite digestion is usually carried out in an acidic environment, the β-O-4 bonds of lignin are not cleaved, and thus lignosulfonate with a weight-average molecular weight of up to several tens of thousands g / mol can be produced using the sulfite process.

[0005] The first method for producing carbonized lignin fibers was disclosed as early as 1960 in Patent Document 1 (Otani et al.). The lignin fibers described therein could be produced using a melt spinning process, a dry spinning process, and a wet spinning process. In the dry spinning process, alkali lignin, thio lignin, or lignin sulfonate was dissolved in a solvent such as water, an alkaline sodium hydroxide and potassium solution, acidic sulfuric acid, or a hydrochloric acid solution. It was possible to spin lignosulfonate by adding polyvinyl alcohol, PAN, or viscose as a plasticizer. Y. Fukuoka of Nippon Kayaku Co., Ltd. reported on commercial lignin-based carbon fibers for the first time, which are sold under the name "Kayanol." Patent Documents 2 and 3 describe another method of co-spinning lignosulfonate with polyethylene oxide or an acrylic acid-acrylamide copolymer as an additive, and in this method, fibers with a tensile strength of 0.8 GPa have been realized. Non-Patent Documents 2 and 3 investigated the dry spinning of acetylated softwood kraft lignin with acetone and the influence of processing temperature and spinning concentration on the processability of the fibers. Microscopic examination showed that there were notches on the fiber surface and that the cross-section was not uniform and round. This could be explained by the rapid evaporation of acetone. The strength of the produced fibers was 1.04 GPa and the elastic modulus was 52 GPa. Since fiber fusion could only be prevented at a low heating rate of 0.01 °C / min, this process was not suitable for large-scale production. By additional UV irradiation of the lignin fibers, the stabilization time was shortened from 40 hours to 4 hours (see Non-Patent Document 4). However, the mechanical properties decreased (tensile strength 900 MPa and elastic modulus 34 GPa).

[0006] Jin et al. (Non-Patent Document 5) reported on lignin fractionated from softwood kraft lignin with a weight average molecular weight of 28,600 g / mol, which was dry-spun into fibers using a mixture of 85% acetic acid and 15% water. The winding speed in this process was 20 m / min to 30 m / min. The strength of the fiber was 1.39 GPa, and the elastic modulus was 98 GPa.

[0007] In the 1990s, low-cost lignin fibers were already obtained using a simple melt-spinning process (Non-Patent Documents 6 and 7). Currently, there are multiple patents and publications regarding the production of lignin-based carbon fibers. The melt-spinning process actually has several advantages over the dry-spinning process, such as not requiring a solvent and having a high take-off speed. However, since lignin is amorphous and has a branched structure, it has no melting point and thus tends to decompose (see Non-Patent Document 5). Furthermore, the amorphous structure may hinder the crystal orientation of the fiber and potentially have an adverse effect on the mechanical properties. As a result, lignin is either chemically modified or spun using additional polymers in the melt-spinning process to compensate for its poor spinnability. To date, there are no commercially available lignin-based carbon fibers that meet the current requirements.

[0008] Patent Document 4 describes a method for producing carbon nanofibers using the electrospinning method. These carbon nanofibers have a graphene structure on the surface and are considered suitable for applications in the fields of energy storage, catalysis, and adsorption. In the example of Patent Document 4, the carbon nanofibers are produced from a mixture of lignosulfonate and polyvinyl alcohol.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

[0010] [Non-Patent Document 1] Fang et al., Manufacture and application of lignin-based carbon fibers (LCFs) and lignin-based carbon nanofibers (LCNFs), Journal of Green Chemistry, 2017, pp. 1794-1827 [Non-Patent Document 2] Zhang et al, "Carbon", 2014, pp. 626-629 [Non-Patent Document 3] "Journal of Applied Polymer Science", 2016, pp. 43663 (1-10) [Non-Patent Document 4] Zhang et al, Carbon Fibers from UV-Assisted Stabilization of Lignin-Based Precursors, "Fibers", 2015, pp. 185-194 [Non-Patent Document 5] Carbon Fibers Derived from Fractionated-Solvated Lignin Precursors for Enhanced Mechanical Performance, ACS Sustainable "Chemistry & Engineering", 2018, pp. 14135-14142 [Non-Patent Document 6] Kubo et al., "Journal of Polymers and the Environment", 2005. pp. 97-105

Non-Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0011] Based on the above prior art, an object of the present invention is to propose an advantageous precursor fiber for lignin-based carbon fibers that eliminates the drawbacks described in relation to the above prior art, which is achieved by an advantageous dry spinning process, whereby it is considered that aqueous dry-spun lignin fibers can be obtained. These should be advantageously convertible into carbon fibers, together with a high carbon yield, low porosity, and improved mechanical properties, particularly better values of tensile strength and modulus of elasticity.

[0012] Therefore, an object of the present invention is also to obtain carbon fibers having excellent properties in terms of strength and modulus of elasticity from the dry-spun precursor fibers obtained by a beneficial process.

Means for Solving the Problems

[0013] The present invention solves this problem by means of precursor fibers for lignin-based carbon fibers containing a water-soluble lignin salt (A) and a water-soluble polyvinylpyrrolidone or a derivative thereof (B).

[0014] Advantageous embodiments of these precursor fibers can be described as follows:

[0015] The water-soluble lignin salt (A) has the formula L-R z (I) (wherein -R zThe moiety is represented by a sulfonate, phosphate, phosphonate, phosphinate, phosphite, phosphonite, and / or phosphinate moiety), and the water-soluble lignin salt (A) is particularly advantageous when it is a lignin sulfonate. It has been found to be advantageous when the cation in the water-soluble lignin salt (A) is a sodium ion, ammonium ion, calcium ion, and / or magnesium ion, particularly an ammonium ion.

[0016] The weight average molecular weight M of the water-soluble lignin salt (A) w It is also important to note this. The weight average molecular weight M of the water-soluble lignin salt (A) w is from about 5000 g / mol to 1000000 g / mol, particularly from about 10000 g / mol to 800000 g / mol, and / or the weight average molecular weight M of the water-soluble polyvinylpyrrolidone or its derivative (B) w is from about 10000 g / mol to 2000000 g / mol, particularly from about 50000 g / mol to 100000 g / mol, which is useful. In the context of the invention described herein, the molecular weight is determined using GPC and appropriate standards (such as polystyrene).

[0017] In the actual practice of the present invention, it has been found advantageous for the water-soluble polyvinylpyrrolidone to be present as a homopolymer. Also, the water-soluble polyvinylpyrrolidone may be a derivative, particularly a copolymer. In this case, the derivative of polyvinylpyrrolidone needs to be essentially consistent with polyvinylpyrrolidone not only with respect to the above general conditions regarding the weight average molecular weight M w but also with respect to other physical quantities particularly defined in detail below. Suitable copolymers are poly-(1-vinylpyrrolidone-co-vinyl acetate) and poly-(N-vinylcaprolactam-co-N-vinylpyrrolidone). The comonomer is present in the copolymer of polyvinylpyrrolidone in a molar amount of 0.1 mol% to 40 mol%, particularly 10 mol% to 30 mol%, expressed in mol%.

[0018] It is particularly preferred that the softening point of the water-soluble polyvinylpyrrolidone or its derivative (B) is about 100°C to 175°C, particularly about 140°C to 160°C.

[0019] Particularly advantageous effects are achieved when the precursor fibers according to the invention contain a water-soluble thermally activatable crosslinking agent (C) in the form of a formaldehyde-releasing compound. It is considered advantageous for the thermally activatable crosslinking agent (C) to be present as 1,3,5-trioxane, paraformaldehyde, hexamethylenetetramine, dimethylol dihydroxyethyleneurea (DMDHEU), 1,3-bis(hydroxymethyl)imidazolidin-2-one (DMEU), and / or 1,3-bis(hydroxymethyl)urea (DMU).

[0020] In an advantageous embodiment of the precursor fibers according to the invention, about 0.1 part by weight to 1 part by weight, particularly about 0.3 part by weight to 0.7 part by weight, of the water-soluble polyvinylpyrrolidone or its derivative (B) is present in the precursor fibers per 1 part by weight of the water-soluble lignin salt (A). The advantageous quantitative relationship is that about 0.01 part by weight to 0.3 part by weight, particularly about 0.05 part by weight to 0.15 part by weight, of the water-soluble crosslinking agent (C) is present in the precursor fibers per 1 part by weight of the water-soluble lignin salt (A).

[0021] The following method described for producing the precursor fibers according to the invention is also within the scope of the invention:

[0022] Accordingly, an object of the present invention is also to provide a method for producing a precursor fiber of a lignin-based carbon fiber according to at least one of the advantageous embodiments of the precursor fiber described above. This method is characterized by preparing aqueous solutions (D) of the water-soluble lignin salt (A) and the water-soluble polyvinylpyrrolidone or its derivative (B) respectively, dry-spinning the obtained aqueous solutions (D) to form filaments for forming the precursor fibers of the carbon fibers, and stretching the filaments.

[0023] It should be emphasized that the physical properties, particularly their chemical structure properties, of the components in the form of the water-soluble lignin salt (A), water-soluble polyvinylpyrrolidone (B) and water-soluble crosslinking agent, as described in relation to the precursor fibers, also apply here.

[0024] Compared with other additive polymers such as polyvinyl alcohol (see Patent Document 1), the precursor fibers according to the present invention contain polyvinylpyrrolidone, and therefore, in the stabilization process described in detail later, they can preferably be processed at a high heating rate of 5 K / min to 20 K / min. The reason for this is the thermal behavior of polyvinylpyrrolidone, particularly its glass transition temperature being 150°C. Compared with PVA whose glass transition temperature is only about 85°C, a higher stabilization temperature can be used. When using polyvinylpyrrolidone, the fibers can be heated to 150°C without increasing the chain mobility of the amorphous segments of the precursor fibers. This prevents the precursor fibers from sticking to each other. In addition to the advantage that a stable spinning solution can be obtained by using water-soluble polyvinylpyrrolidone, this additional polymer can be dissolved at room temperature.

[0025] The present invention is not subject to any significant limitations in the selection of each water-soluble lignin salt (A). For this purpose, commercial water-soluble lignosulfonates, particularly ammonium, calcium, magnesium or sodium lignosulfonates, are mainly considered, but other soluble or dispersible water-soluble lignin salts such as phosphates, phosphonates, phosphinates, phosphites, phosphonites or phosphinates are also considered. It is advantageous to use water-soluble lignosulfonates with a high molecular weight starting from 30,000 g / mol and a low sugar content of less than 10%, particularly less than 3%. By processing higher molecular weight water-soluble lignosulfonates, mechanical properties such as strength and modulus of elasticity are improved.

[0026] A particularly advantageous further development of the present invention is that the water-soluble crosslinking agent (C) is contained in the aqueous solution (D).

[0027] The dry spinning process according to the present invention can preferably be carried out as follows: In the dry spinning process, the spinning solution is forced through a predetermined die hole (diameter 100 μm to 200 μm) using a spinning pump and dried at 40 °C to 60 °C in a spinning shaft. The precursor fibers can be wound onto a bobbin using a driven winder. The fibers have an advantageous diameter in the range of about 6 μm to 60 μm, preferably 6 μm to 20 μm, particularly 6 μm to 20 μm, more preferably 6 μm to 13 μm, and particularly preferably 10 μm to 13 μm. The carbon fibers subsequently produced from the precursor fibers preferably have a diameter of 2 μm to 30 μm, particularly preferably 4 μm to 8 μm.

[0028] To promote the spinning process, it is advantageous to adjust the aqueous solution (D) to a zero shear viscosity (measured according to DIN 53019-4) of about 50 Pa·s to 800 Pa·s, preferably about 100 Pa·s to 600 Pa·s, particularly about 150 Pa·s to 350 Pa·s, measured at a temperature of 22 °C.

[0029] To optimize this process, it is also useful to concentrate the aqueous solution (D), particularly under vacuum, preferably using a rotary evaporator, until it reaches a zero shear viscosity (measured at a temperature of 22 °C according to DIN 53019-4) of about 50 Pa·s to 800 Pa·s, preferably about 100 Pa·s to 600 Pa·s, particularly about 150 Pa·s to 350 Pa·s, and to dry-spin the resulting concentrated aqueous solution (E). Preferably, the concentration of the aqueous solution (D) is carried out under a vacuum of about 10 mbar to 80 mbar, particularly about 45 mbar to 75 mbar. The spinning solution should preferably be particle-free and should not have a gel-like nature.

[0030] 1.) Add from about 0.1 to 1 part by weight, particularly from about 0.3 to 0.7 part by weight, of a water-soluble polyvinylpyrrolidone or its derivative (B) to 1 part by weight of a water-soluble lignin salt (A), or 2.) Add from about 0.1 to 1 part by weight, particularly from about 0.3 to 0.7 part by weight, of a water-soluble polyvinylpyrrolidone or its derivative (B) and from about 0.01 to 0.3 part by weight, particularly from about 0.05 to 0.15 part by weight, of a water-soluble crosslinking agent (C) to 1 part by weight of a water-soluble lignin salt (A) to prepare a mixture, and it has been found to be highly advantageous to dissolve each mixture in water to form an aqueous solution (D).

[0031] However, it is also possible to first incorporate the water-soluble crosslinking agent (C) into an aqueous solution (E). The indicated quantitative ratios of the water-soluble lignin salt (A), the water-soluble polyvinylpyrrolidone or its derivative (B), and the water-soluble crosslinking agent (C) should also be considered here. For this reason, for the practice of the present invention, it is particularly advantageous to incorporate the water-soluble crosslinking agent (C) into the aqueous solution (D) and / or the aqueous solution (E).

[0032] As already mentioned, for the embodiments of the present invention, it is particularly advantageous to incorporate the water-soluble crosslinking agent (C) into the aqueous solution (D) and / or the aqueous solution (E). Optionally, the mechanical fiber properties can be improved by chemical crosslinking or the introduction of additional aromatic groups. Therefore, the above-mentioned water-soluble crosslinking agents (C), such as dimethylol dihydroxyethylene urea (DMDHEU), trioxane, or urotropin, can be used in particular. The water-soluble crosslinking agent (C) contains a formaldehyde-releasing compound that leads to the crosslinking of lignin when thermally activated. A highly important advantage is that the water-soluble crosslinking agent (C) can be added to the polymer-containing spinning solution in advance and spun together with it.

[0033] An advantageous further development of the method according to the invention should be noted in that the concentrated aqueous solution (E) is dry-spun at a temperature of about 30°C to 100°C, particularly about 40°C to 70°C, in a spinning shaft.

[0034] The water content of the aqueous solution (D) also has an advantageous effect on the success of the process according to the invention. Here, it is preferred that the aqueous solution (D) contains at least about 40% by weight, in particular about 45% to 65% by weight, of water.

[0035] The water-soluble crosslinking agent (C) incorporated into the precursor fibers has already been discussed above. The crosslinking agent is present in an activatable form. Thus, when the water-soluble crosslinking agent (C) incorporated into the precursor fibers is activated by thermal stabilization of the precursor fibers, in particular oxidative thermal stabilization, at a temperature of about 100 °C to 400 °C, in particular about 200 °C to 300 °C, preferably 250 °C ± 20 °C, the activation starts particularly advantageously. The residence time of the precursor fibers in the thermal stabilization is 10 minutes to 4 hours, preferably 10 minutes to 60 minutes, particularly preferably 10 minutes to 40 minutes, depending on the heating rate and the temperature profile.

[0036] A particular object of the present invention is to produce advantageous carbon fibers from the above-mentioned precursor fibers, optionally preceded by thermal stabilization, in particular oxidative thermal stabilization, and / or stabilization by high-energy radiation, and optionally followed by graphitization. It is also advantageous to use ultraviolet (UV), vacuum ultraviolet (VUV), electron radiation, X-ray radiation, plasma stabilization, and / or gamma radiation as high-energy radiation. Thus, the teaching of this use results from the concept according to the invention.

[0037] Thus, in a further aspect, the present invention relates to a process for producing carbon fibers from the above-mentioned precursor fibers, the process comprising thermal stabilization, in particular oxidative thermal stabilization, and / or stabilization by high-energy radiation, and / or plasma stabilization, and subsequent carbonization, followed optionally by graphitization. As high-energy radiation, ultraviolet (UV), vacuum ultraviolet (VUV), electron radiation, X-ray radiation, and / or gamma radiation are preferred.

[0038] Furthermore, it is advantageous for the above-mentioned process to carry out the thermal stabilization of the precursor fibers in a temperature range of 100 °C to 400 °C, in particular 200 °C to 300 °C, particularly 250 °C ± 20 °C.

[0039] The carbonization of the precursor fibers according to the present invention is preferably carried out at a temperature of 300°C to 1800°C, suitably in an inert gas atmosphere, particularly preferably in a nitrogen atmosphere, for the purposes of the methods disclosed herein. For carbonization, it is also advantageous to subject the stabilized precursor fibers to low-temperature carbonization, particularly in the temperature range of 300°C to 1200°C, preferably in the range of 300°C to 1000°C, and more preferably at 800°C ± 50°C. Such carbonization can be carried out, for example, in a low-temperature furnace (LT furnace), and this carbonization is preferably carried out in an inert gas atmosphere, particularly in a nitrogen atmosphere.

[0040] In a preferred embodiment, the precursor fibers are drawn in a low-temperature furnace with a draw ratio of particularly 1% to 35%, preferably at least 2% to 30%, particularly preferably at least 5% to 25%. In this way, further improvement of the mechanical properties of the fibers can be achieved.

[0041] Furthermore, it is advantageous to subject the fibers to high-temperature carbonization after low-temperature carbonization, and the temperature is preferably set in the range of 1200°C to 1600°C, more preferably up to 1500°C, and even more preferably up to 1400°C ± 50°C. High-temperature carbonization is suitably carried out in an inert gas atmosphere.

[0042] The diameter of the carbon fibers obtained from the high-temperature furnace is in the range of 2μm to 30μm, and the preferred diameter is 4μm to 8μm.

[0043] For graphitization provided in the context of this method, it is preferably carried out by thermal drawing in an inert gas atmosphere, particularly an argon atmosphere, at a temperature up to 3000°C. This treatment enables the orientation of the carbon fibers, and at the same time, the elastic modulus is further improved by carrying out thermal drawing. Graphitization can be carried out in a graphitization furnace (ultra-high-temperature furnace, UHT furnace).

[0044] In yet another aspect, the present invention relates to the use of the above-described precursor fibers for producing carbon fibers by optionally performing thermal stabilization, particularly oxidative thermal stabilization, and / or stabilization by high-energy radiation, and / or plasma stabilization before carbonization, followed optionally by carbonization with subsequent graphitization.

[0045] When water-soluble polyvinylpyrrolidone is used as an auxiliary polymer, not only is the spinnability improved, but nitrogen is clearly introduced into the carbon fibers, so that a nitrogen-containing surface for binding resins during the production of composite parts can be obtained, similar to carbon fibers made of polyacrylonitrile. In the current state of the art of precursor fibers made of cellulose or lignin and an auxiliary polymer such as PVA, it is necessary to introduce nitrogen atoms in a further process step via a plasma process or chemical functionalization. Oxidative thermal stabilization is generally carried out in an oxygen atmosphere up to a final temperature of 100 °C to 400 °C. The heating rate is 20 K / min to 0.05 K / min.

[0046] As already shown, the present invention is characterized by a number of advantages: For this reason, the present invention enables the production of multifilaments and can be transferred to an industrial-scale system. The method according to the present invention is economically and ecologically feasible with the aim of realizing environmentally friendly carbon fibers at low production costs. Regarding the production of cost-effective carbon fibers, the carbon yield after carbonization plays an important role. Therefore, it is very important to keep the lignin content in the precursor fibers as high as possible. Therefore, the precursor fibers preferably have a lignin content of at least 70% by weight, although 80% by weight has also been demonstrated. For this reason, the present invention relates to a continuous dry spinning process using water as a solvent, preferably a water-soluble lignosulfonate and polyvinylpyrrolidone.

[0047] In particular, polyvinylpyrrolidone enables a high spinning speed in the range of at least 75 m / min to 100 m / min in order to stabilize a somewhat brittle and amorphous spinning dope based on lignin. In particular, polyvinylpyrrolidone promotes the orientation of the lignin polymer in the fiber direction and reduces brittleness, making handling, particularly winding, etc. easier.

[0048] The polyvinylpyrrolidone or its derivative (B) added to the spinning solution enables a very high stabilization rate of 10 m / min to 20 m / min, which is completely different from the current state of the art. Thereby, in the production of carbon fibers according to the present invention, a significant amount of energy and cost are saved. The present invention will be described in more detail below with reference to examples:

Mode for Carrying Out the Invention

Examples

[0049] Example 1: (Preparation of a spinning solution containing 1 part by weight of ammonium lignosulfonate (softwood) and 0.43 part by weight of polyvinylpyrrolidone, and implementation of the dry spinning process) To prepare the spinning solution, the mixture according to the present invention was dissolved in water. The 500 g of the mixture contained 1 part by weight (350 g) of ammonium lignosulfonate (softwood) and 0.43 part by weight (150 g) of polyvinylpyrrolidone (PVP K30 manufactured by Sigma Aldrich) having a weight average molecular weight of 50,000 g / mol, and this was dissolved in 1.4 l of water while stirring. After stirring at room temperature for 30 minutes, the solution was homogenized. Any resulting residual particles were removed by filtration. The obtained spinning solution was concentrated in vacuo using a rotary evaporator until a zero shear viscosity of 229 Pa·s at 22 °C was obtained. Rheological measurements were performed using a Physica MCR 301 rheometer manufactured by Anton Paar in a plate-plate configuration with a plate diameter of 25 mm and a gap distance of 0.5 mm.

[0050] The dry spinning process was carried out on a pilot scale with a porous nozzle. The spinneret diameter was 100 μm, and the number of filaments was 90. The screw speed was 7.6 rpm. The spinning temperature was 22 °C. The fibers were air-dried at a temperature of 40 °C to 50 °C in the spinning shaft. The fibers were wound up at a winding speed of 80 m / min and stored under certain climatic conditions (23 °C, relative humidity 40%). The water content of the fibers after the dry spinning process was 8 wt% to 12 wt%. The measurement was carried out by Karl Fischer titration at 140 °C.

[0051] Example 2: (Preparation of a spinning solution containing 1 part by weight of ammonium lignosulfonate (broad-leaved tree) and 0.43 part by weight of polyvinylpyrrolidone, and implementation of the dry spinning process) Example 2 was carried out with the exception that ammonium lignosulfonate (broad-leaved tree) was used instead of ammonium lignosulfonate (coniferous tree). Therefore, in the same procedure as described in Example 1, 500 g of a mixture containing 1 part by weight (350 g) of ammonium lignosulfonate (broad-leaved tree) and 0.43 part by weight (150 g) of polyvinylpyrrolidone (PVP K30 manufactured by Sigma Aldrich) having a weight average molecular weight of 50,000 g / mol was mixed with 1.4 l of water. After stirring at room temperature for 30 minutes, the solution was homogenized. Any resulting residual particles were removed by filtration. The obtained spinning solution was concentrated in vacuo in a rotary evaporator until a zero-shear viscosity of 282 Pa·s was obtained at 22 °C. Rheological measurements were carried out using a Physica MCR 301 rheometer manufactured by Anton Paar in a plate-plate configuration with a plate diameter of 25 mm and a gap distance of 0.5 mm.

[0052] The dry spinning process was carried out on a pilot scale with a porous nozzle. The spinneret diameter was 100 μm, and the number of filaments was 90. The screw speed was 7.6 rpm, and the spinning temperature was 22 °C. The fibers were air-dried at a temperature of 45 °C to 55 °C in the spinning shaft. The fibers were wound up at a winding speed of 90 m / min and stored under certain climatic conditions (23 °C, relative humidity 40%).

[0053] The water content of the fiber after the dry spinning process was 8 wt% to 12 wt%. The measurement was carried out by Karl Fischer titration at 140 °C.

[0054] Example 3: (Preparation of a spinning solution containing 60 wt% to 80 wt% of lignosulfonate and 40 wt% to 20 wt% of polyvinylpyrrolidone, and implementation of the dry spinning process) 60 wt% to 80 wt% of lignosulfonate and 40 wt% to 20 wt% of polyvinylpyrrolidone were dissolved in water and concentrated using a rotary evaporator. The water content in the resulting spinning solution was 44 wt% to 55 wt%, and the zero shear viscosity was 170 Pa·s to 500 Pa·s. The spinneret had 90 to 250 nozzle holes with a diameter of 100 μm, and as a result, rovings of 90 to 250 filaments were obtained. The maximum spinning speed was 75 m / min to 100 m / min, and the spinning shaft was heated to 40 °C to 70 °C. For continuous carbonization, a part of the spinning coil was fanned out 4 times under tension control to form an assembled roving (1k filament).

[0055] Example 4: (Preparation of a spinning solution containing 1 part by weight of ammonium lignosulfonate, 0.36 part by weight of polyvinylpyrrolidone, and 0.07 part by weight of a crosslinking agent (DMDHEU), and implementation of the dry spinning process) The 500 g of the mixture contained 1 part by weight (350 g) of ammonium lignosulfonate (softwood), 0.36 part by weight (125 g) of polyvinylpyrrolidone (PVP K30 manufactured by Sigma Aldrich) with a weight average molecular weight of 50000 g / mol, and 0.07 part by weight (25 g) of a crosslinking agent (DMDHEU), which was dissolved in 1.4 l of water while stirring. Therefore, the process means of Example 1 was reproduced with the following changes. The amount of polyvinylpyrrolidone was reduced from 0.43 part by weight to 0.36 part by weight, and the aqueous solution further contained 0.07 part by weight of a crosslinking agent (DMDHEU).

[0056] After stirring at room temperature for 30 minutes, the solution was homogenized. Any resulting residual particles were removed by filtration. After concentration by evaporating water in vacuo, the zero shear viscosity of the spinning solution was adjusted to 200 Pa·s to 300 Pa·s (measured at a temperature of 22 °C according to DIN 53019-4). The solution was spun in a dry spinning process through a porous nozzle (100 μm) and wound up at 80 m / min to 90 m / min.

[0057] TIFF2025522096000001.tif58170

[0058] Example 5: (Manufacture of carbon fibers: discontinuous) The precursor fibers obtained according to Examples 1 to 3 above were discontinuously stabilized up to 250 °C in a muffle furnace. The heating rate was varied from 0.5 K / min to 20 K / min, but the fibers did not stick even at 20 K / min.

[0059] The tensile strength and modulus of elasticity of the precursor fibers were measured using a Favimat from Textechno. For this purpose, individual filaments were clamped between two clamps with a clamp length of 12 mm and their fineness was determined by measuring the resonance frequency. The test speed for the measurement was 1 mm / min. The average value was calculated using at least 20 valid measured values. The fiber diameter of the individual filaments was determined using a scanning electron microscope (SEM).

[0060] The tensile strength of the stabilized precursor fibers was 8 cN / tex to 10 cN / tex and the modulus of elasticity was 400 cN / tex.

[0061] Subsequent carbonization was carried out discontinuously in a batch furnace at 1000 °C to 1400 °C under a nitrogen atmosphere. The heating rate was 10 K / min. The diameter of the carbonized fibers was 10 μm to 14 μm (SEM image). After cooling, carbon fibers with a carbon yield of 45% were obtained. Table 2 below exemplarily shows some of the values of the obtained carbon fibers. In the case of continuous carbonization, mechanical parameters can be expected to be significantly higher since stretching of the precursor is only possible with continuous carbonization.

[0062] TIFF2025522096000002.tif83170

[0063] Example 6: First, the precursor fibers obtained according to Example 3 above were fanned out in a stress-controlled manner to form 1k roving, and then discontinuously stabilized up to 250 °C in a muffle furnace. A heating rate of 1 K / min was used, and the dwell time was 4 hours.

[0064] Thereafter, continuous carbonization was carried out in a nitrogen atmosphere in the LT furnace in the temperature range of 300 °C to 750 °C and at various final carbonization temperatures in the HT furnace.

[0065] The mechanical parameters of individual carbon fibers of monofilaments produced in a continuous carbonization process at various maximum temperatures in a high-temperature furnace of 1000 °C to 1600 °C are shown in the following table. It can be seen that the maximum tensile strength can be achieved in the range of 1200 °C to 1600 °C, and the optimum value is 1400 °C.

[0066] TIFF2025522096000003.tif47170

[0067] The intensity ratio (ID / IG) of the defect band (D1 band, 1340 cm -1 ) and the graphite band (G band, 1590 cm -1 ), determined by Raman spectroscopy, depends on the carbonization temperature and provides information on the defects of the graphite structure. The lower the ID / IG ratio, the fewer the number of defects in the carbon structure. As shown in Table 4, as the maximum carbonization temperature (coking temperature) increases, the ID / IG ratio decreases from 9.5 to 2.3 (-76%). The spectra and data were measured and calculated from carbon fibers produced at various maximum temperatures of 1000 °C to 1600 °C in a high-temperature furnace.

[0068] TIFF2025522096000004.tif31170

[0069] Example 7 (Continuous production of carbon fibers with various draw factors) Similar to the process of Example 3 above, fibers (lignosulfonate (hardwood) 60 wt% / PVP 40 wt%) were spun with 250 filaments, spread out fan-shaped to form 1K rovings, and stabilized by raising the temperature to 250 °C at 1K / min in a drying chamber (rest time 4 hours). Then, the fibers were wound onto bobbins and carbonized in an LT furnace (300 °C to 750 °C) at draw factors of 1% and 5%. Table 6 summarizes the mechanical properties of the fibers. Here, as the draw factor increases, the value of the fibers also increases. When the elongation rate was 5%, the average value of the tensile strength was 1.4 GPa, the elastic modulus E1 was 104 GPa, and the elongation rate was 1.3%.

[0070] TIFF2025522096000005.tif70170

[0071] Dimensions (L a , L c ) of crystallites, interlayer spacing (d 002 ), and stacking number (N c ) of the corresponding carbon fibers with various draw factors measured and calculated by wide-angle X-ray scattering (WAXS) at 1400 °C are shown in Table 7 below. It can be seen that with 5% draw, the dimensions L a and L c of the crystallites increase, the crystallites grow, and the interlayer spacing d 002 decreases. It is particularly advantageous that the orientation increases in conjunction with the increase in the draw factor.

[0072] TIFF2025522096000006.tif63170

[0073] Furthermore, two-dimensional wide-angle X-ray scattering images of carbon fibers with various draw factors were taken. It can be seen that the crescent shape becomes more prominent as the draw at 1400 °C increases.

[0074] Example 8: (Comparative example / Spinning solution containing pure lignosulfonate without polyvinylpyrrolidone) 500 g of ammonium lignosulfonate (softwood) was dissolved in 1.4 l of water. After stirring at room temperature for 30 minutes, the solution was homogenized. Any resulting residual particles were removed by filtration. After concentration by evaporating water in vacuo, the viscosity of the spinning solution was adjusted. The spinning solution could not be spun because it did not have sufficient thread drawability.

Claims

1. A precursor fiber for lignin-based carbon fiber, comprising a water-soluble lignin salt (A) and a water-soluble polyvinylpyrrolidone or a derivative thereof (B).

2. The water-soluble lignin salt (A) has the formula L-R ｚ (I) (wherein -R ｚ represents a sulfonate, phosphate, phosphonate, phosphinate, phosphite, phosphonite, and / or phosphinite moiety), the precursor fiber according to claim 1, characterized in that it is represented by

3. The precursor fiber according to claim 2, wherein the water-soluble lignin salt (A) is present as a lignosulfonate.

4. The precursor fiber according to at least one of claims 1 to 3, wherein the cation in the water-soluble lignin salt (A) is a sodium ion, an ammonium ion, a calcium ion, and / or a magnesium ion, particularly an ammonium ion.

5. The weight average molecular weight M of the water-soluble lignin salt (A) ｗ is from about 5,000 g / mol to 1,000,000 g / mol, particularly from about 10,000 g / mol to 800,000 g / mol, and / or the weight average molecular weight M of the water-soluble polyvinylpyrrolidone or its derivative (B) ｗ is from about 10,000 g / mol to 2,000,000 g / mol, particularly from about 50,000 g / mol to 200,000 g / mol, particularly preferably from about 50,000 g / mol to 100,000 g / mol, and the precursor fiber according to at least one of claims 1 to 4 is characterized by this.

6. The precursor fiber according to at least one of claims 1 to 5, wherein the water-soluble polyvinylpyrrolidone is present as a homopolymer.

7. The precursor fiber according to at least one of claims 1 to 6, wherein the softening point of the water-soluble polyvinylpyrrolidone or a derivative thereof (B) is about 100°C to 175°C, particularly about 140°C to 160°C.

8. The precursor fiber according to at least one of claims 1 to 7, wherein the precursor fiber contains a water-soluble thermally activatable crosslinking agent (C) in the form of a formaldehyde-releasing compound.

9. The precursor fiber according to claim 8, wherein the thermally activatable crosslinking agent (C) is present as 1,3,5-trioxane, paraformaldehyde, hexamethylenetetramine, dimethyloldihydroxyethyleneurea (DMDHEU), 1,3-bis(hydroxymethyl)imidazolidin-2-one (DMEU), and / or 1,3-bis(hydroxymethyl)urea (DMU).

10. The precursor fiber according to at least one of claims 1 to 9, wherein about 0.1 part by weight to 1 part by weight, particularly about 0.3 part by weight to 0.7 part by weight, of the water-soluble polyvinylpyrrolidone or a derivative thereof (B) is present in the precursor fiber per 1 part by weight of the water-soluble lignin salt (A).

11. The precursor fiber according to at least one of claims 8 to 10, wherein about 0.01 part by weight to 0.3 part by weight, particularly about 0.05 part by weight to 0.15 part by weight, of a water-soluble crosslinking agent (C) is present per 1 part by weight of the water-soluble lignin salt (A).

12. Manufacturing a precursor fiber of lignin-based carbon fiber according to at least one of claims 1 to 11, comprising preparing aqueous solutions (D) of the water-soluble lignin salt (A) and the water-soluble polyvinylpyrrolidone or its derivative (B) respectively, dry-spinning the obtained aqueous solution (D) to form filaments of a precursor fiber of carbon fiber, and stretching the filaments.

13. The method according to claim 12, characterized in that a water-soluble crosslinking agent (C) is contained in the aqueous solution (D).

14. The method according to claim 12 or 13, characterized in that the aqueous solution (D) is adjusted to a zero shear viscosity (measured at a temperature of 22 °C according to DIN 53019-4) of about 50 Pa·s to 800 Pa·s, preferably about 100 Pa·s to 600 Pa·s, particularly about 150 Pa·s to 350 Pa·s.

15. The method according to claim 14, characterized in that the aqueous solution (D) is particularly concentrated in vacuo to increase the zero shear viscosity until it reaches a zero shear viscosity (measured at a temperature of 22 °C according to DIN 53019-4) of about 50 Pa·s to 800 Pa·s, preferably about 100 Pa·s to 600 Pa·s, particularly about 150 Pa·s to 350 Pa·s, and the concentrated aqueous solution (E) is dry-spun.

16. 1.) Add about 0.1 part by weight to 1 part by weight, particularly about 0.3 part by weight to 0.7 part by weight of water-soluble polyvinylpyrrolidone or its derivative (B) to 1 part by weight of water-soluble lignin salt (A), or 2.) Add about 0.1 part by weight to 1 part by weight, particularly about 0.3 part by weight to 0.7 part by weight of the water-soluble polyvinylpyrrolidone or its derivative (B) and about 0.01 part by weight to 0.3 part by weight, particularly about 0.05 part by weight to 0.15 part by weight of water-soluble crosslinking agent (C) to 1 part by weight of water-soluble lignin salt (A) to prepare a mixture, and dissolve each mixture in water and transfer it to the aqueous solution (D). The method according to any one of claims 12 to 15.

17. The method according to claim 15 or 16, characterized in that the concentration of the aqueous solution (D) is carried out under a vacuum of about 10 mbar to 80 mbar, particularly about 45 mbar to 75 mbar.

18. The method according to claim 16 or 17, characterized in that the concentrated aqueous solution (E) is dry-spun at a temperature of about 30 °C to 100 °C, particularly about 40 °C to 70 °C, in a spinning shaft.

19. The method according to at least one of claims 12 to 18, characterized in that the aqueous solution (D) contains at least about 40% by weight, in particular from about 45% to 65% by weight of water.

20. The method according to at least one of claims 13 to 19, characterized in that the water-soluble crosslinking agent (C) incorporated into the precursor fiber is activated by thermal stabilization, in particular oxidative thermal stabilization, of the precursor fiber at a temperature of from about 100°C to 400°C, in particular from about 200°C to 300°C.

21. A method for producing carbon fibers from a precursor fiber according to at least one of claims 1 to 11, characterized in that for producing the carbon fibers, thermal stabilization, in particular oxidative thermal stabilization, and / or stabilization by high-energy radiation, and / or plasma stabilization is carried out, and then carbonization is carried out, optionally followed by subsequent graphitization.

22. The method according to claim 21, characterized in that the high-energy radiation used is ultraviolet (UV), vacuum ultraviolet (VUV), electron radiation, X-ray radiation, and / or gamma radiation.

23. The method according to claim 21 or 22, characterized in that the thermal stabilization of the precursor fiber is carried out in a temperature range of from 100°C to 400°C, in particular from 200°C to 300°C, in particular at 250°C ± 20°C.

24. The method according to any one of claims 21 to 23, characterized in that the stabilized precursor fiber is subjected to low-temperature carbonization in a temperature range of from 300°C to 1200°C, in particular from 300°C to 1000°C.

25. The method according to claim 24, characterized in that the precursor fiber is preferably drawn in a low-temperature furnace at a draw ratio of from 1% to 35%, preferably at least from 2% to 30%, particularly preferably at least from 5% to 25% to enhance the mechanical properties.

26. The method according to claim 24 or 25, characterized in that the fiber obtained from the low-temperature carbonization is further subjected to high-temperature carbonization, preferably in a temperature range of from 1200°C to 1600°C, more preferably up to 1500°C, particularly preferably at 1400°C ± 50°C, especially in an inert gas atmosphere.

27. The method according to claim 26, characterized in that the fiber obtained from the high-temperature carbonization is graphitized by thermal drawing at a temperature of up to 3000°C, in particular in an inert gas atmosphere, especially an argon atmosphere.

28. Use of a precursor fiber according to at least one of claims 1 to 11 for producing carbon fibers by optionally performing thermal stabilization, in particular oxidative thermal stabilization, and / or stabilization by high-energy radiation, and / or plasma stabilization before carbonization and optionally subsequent carbonization with graphitization.

Images