Precursor fibers of lignin-based carbon fibers, their production and use

Abstract

Described are precursor fibers of lignin-based carbon fibers having a content in water-soluble lignin salt (A). A special characteristic of these precursor fibers is the inclusion of water-soluble polyvinylpyrrolidone or a derivative (B) thereof. The invention further relates to an advantageous method for producing these precursor fibers and their advantageous use for producing carbon fibers by carbonization, optionally followed by graphitization.

Description

[0001] Precursor fibers of lignin-based carbon fibers, their production and use

[0002] The invention relates to a process for the production and use of precursor fibers made of lignin, which are dry-spun from water and can subsequently be converted into carbon fibers.

[0003] Carbon fibers, in the form of fiber composites, combine excellent mechanical properties with low density, leading to significant energy savings potential. A large proportion of manufactured carbon fibers are based on polyacrylonitrile (PAN) (> 95%), which is produced by polymerizing acrylonitrile with a small proportion of comonomers. The required acrylonitrile is synthesized from petroleum-derived propene with ammonia. The precursor fiber is produced using a special wet spinning process. Due to the high costs of the energy-intensive production of carbon fibers, the poor carbon dioxide balance of the overall process, and the use of problematic, toxic solvents in the wet spinning process, alternative materials and processes have long been sought.

[0004] Lignin has long been investigated as a potential alternative to PAN-based carbon fibers. The decisive factor was the 50 million tons of lignin produced annually as a byproduct of paper production and biorefineries. After cellulose, lignin is the second most abundant biopolymer in nature. While approximately 2% of lignin is used for commercial applications, the majority is used solely as fuel.

[0005] In addition to its economic importance, lignin exhibits a high carbon yield during carbonization (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). However, the use of lignin as a precursor material is associated with some difficulties due to its chemical makeup, structure, and purity. Depending on the various pulping processes, lignin is divided into organosolv, kraft, soda lignin, and lignin sulfonate. Lignin sulfonate is obtained as a by-product in a sulfite process during pulp production. In this process, the free hydroxyl groups of the lignin are replaced by sulfonate groups. This makes the lignin water-soluble and can therefore be used for the water-based dry spinning process.The sulfite process can produce lignin sulfonates with weight-average molecular weights of up to several 10,000 g / mol, since the sulfite digestion is usually carried out in an acidic environment and therefore the ß-O-4 bonds in the lignin are not broken.

[0006] As early as 1960, the first processes for producing carbonized lignin fibers were disclosed in US Pat. No. 3,461,082 A (Otani et al.). The lignin fibers mentioned therein could be produced using melt spinning, dry spinning, and wet spinning processes. In the dry spinning process, alkali lignin, thiolignin, or ligninsulfonate are dissolved in a solvent such as water, alkaline sodium and potassium hydroxide solution, acidic sulfuric acid, or hydrochloric acid solution. The ligninsulfonate could be spun by adding polyvinyl alcohol, PAN, or viscose as a plasticizer. Y. Fukuoka of Nippon Kayaku Co. first reported on commercial lignin-based carbon fibers, which were marketed under the name "Kayacarbon" (see US Pat. No. 3,461,082 A). DE 2118488 A and US 3723609 A describe further methods of co-spinning lignosulfonate with polyethylene oxide or acrylic acid-acrylamide copolymers as additives, in which fibers with a tensile strength of 0.8 GPa were achieved.Zhang et al., "Carbon," 2014, pp. 626-629, and "Journal of Applied Polymer Science," 2016, pp. 43663 (1-10), investigated the dry spinning of acetylated softwood kraft lignin with acetone and the influence of processing temperature and spinning concentration on fiber processability. Microscopic examinations revealed that the fiber surfaces exhibited indentations and did not possess a uniform, round cross-section. This could be explained by the rapid evaporation of the acetone. The resulting fibers exhibited a strength of 1.04 GPa and a Young's modulus of 52 GPa. Since fiber fusion was avoided only by 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 could be reduced from 40 h to 4 h (see Zhang et al., Carbon Fibers from UV-Assisted Stabilization of Lignin-Based Precursors, "Fibers", 2015, pp. 185-194). However, the mechanical properties deteriorated (tensile strength of 900 MPa and Young's modulus of 34 GPa).

[0007] Jin et al. (Carbon Fibers Derived from Fractionated-Solvated Lignin Precursors for Enhanced Mechanical Performance, ACS Sustainable "Chemistry & Engineering", 2018, pp. 14135-14142) reported on a fractionated lignin 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 was between 20 and 30 m / min. The fibers exhibited a strength of 1.39 GPa and a Young's modulus of 98 GPa.

[0008] In the 1990s, cost-effective lignin fibers were already obtained through simple melt spinning processes (Kubo et al., "Journal of Polymers and the Environment", 2005, pp. 97-105; Alexy et al., "Polymer", 2000, pp. 4901-4908). Currently, numerous patents and publications exist on the production of lignin-based carbon fibers. While the melt spinning process offers several advantages over the dry spinning process, such as the elimination of solvents and rapid take-off speed, lignin, due to its amorphous and branched structure, has no melting point and is therefore prone to decomposition (see Jin et al., "Chemistry & Engineering", 2018, pp. 14135-14142). Furthermore, the amorphous structure prevents crystalline orientation of the fiber, which can negatively impact its mechanical properties.For this reason, lignin is chemically modified or melt-spun with additional polymers to counteract its poor spinnability. To date, there is no commercially available lignin-based carbon fiber that meets current requirements.

[0009] CN 111321487 A describes a process for producing carbon nanofibers using an electrospinning process. These carbon nanofibers are said to have graphene structures on their surface and be suitable for applications in energy storage, catalysis, and adsorption. In the examples of CN 111321487 A, carbon nanofibers are produced from a mixture of lignosulfonate and polyvinyl alcohol.

[0010] Based on the above-mentioned prior art, the object of the invention is to propose advantageous precursor fibers of lignin-based carbon fibers that eliminate the disadvantages described in connection with the above prior art. This is to be achieved by an advantageous dry spinning process, thus obtaining water-based, dry-spun lignin fibers. These, in turn, are to be advantageously convertible into carbon fibers, combined with high carbon yield, low porosity, and improved mechanical properties, in particular better tensile strength and Young's modulus values.

[0011] The aim of the present invention is therefore also to obtain carbon fibers with excellent properties in terms of strength and modulus of elasticity from the dry-spun precursor fibers obtained by means of a cost-effective process.

[0012] The invention solves this problem by precursor fibers of lignin-based carbon fibers containing water-soluble lignin salt (A) and water-soluble polyvinylpyrrolidone or derivative (B).

[0013] Advantageous designs of these precursor fibers can be represented as follows:

[0014] It is advantageous if the water-soluble lignin salt (A) is represented by the formula LR z (I) in which the residue -R z represents a sulfonate, phosphate, phosphonate, phosphinate, phosphite, phosphonite, and / or phosphinite residue, wherein the water-soluble lignin salt (A) is used in particular as a lignin sulfonate. It has proven advantageous if the cation in the water-soluble lignin salt (A) is a sodium, ammonium, calcium, and / or magnesium ion, in particular an ammonium ion.

[0015] It is also important to consider the weight-average molecular weight M wof the water-soluble lignin salt (A). It is useful if the lignin salt (A) has a weight-average molecular weight M w of about 5,000 to 1,000,000, in particular of about 10,000 to 800,000 g / mol, and / or the water-soluble polyvinylpyrrolidone or derivative (B) has a weight-average molecular weight M w of approximately 10,000 to 2,000,000 g / mol, in particular of approximately 50,000 to 100,000 g / mol. The molecular weight is to be determined within the scope of the invention described here using GPC and suitable standards (e.g., polystyrene).

[0016] In the practical implementation of the present invention, it has been found that the water-soluble polyvinylpyrrolidone is advantageously present as a homopolymer. It may also be a derivative, in particular a copolymer, wherein the polyvinylpyrrolidone derivative should essentially correspond to the latter in terms of physical properties, not only with regard to the above-specified parameters for the weight-average molecular weight Mw, but in particular with regard to the further physical parameters specified in detail below. Suitable copolymers are: poly-C 1 -vinyl pyrrolidone (co-vinyl acetate) and poly-(N-vinylcaprolactam-co-N-vinylpyrrolidone). The comonomers are present in the polyvinylpyrrolidone copolymer in a molar amount, expressed as mol%, of 0.1 to 40 mol%, in particular of 10 to 30 mol%.

[0017] It is particularly preferred if the water-soluble polyvinylpyrrolidone or derivative (B) has a softening point of about 100°C to 175°C, in particular about 140°C to 160°C.

[0018] Particularly advantageous effects are achieved with the precursor fibers according to the invention when they contain a water-soluble, thermally activatable crosslinker (C) in the form of a formaldehyde-releasing compound. It is considered advantageous if the thermally activatable crosslinker (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).

[0019] For an advantageous embodiment of the precursor fibers according to the invention, approximately 0.1 to 1 part by weight, in particular approximately 0.3 to 0.7 parts by weight, of water-soluble polyvinylpyrrolidone or polyvinylpyrrolidone derivative (B) are used in the precursor fibers for 1 part by weight of water-soluble lignin salt (A). An advantageous quantitative relationship is that approximately 0.01 to 0.3 parts by weight, in particular approximately 0.05 to 0.15 parts by weight, of water-soluble crosslinker (C) are used in the precursor fibers for 1 part by weight of water-soluble lignin salt (A).

[0020] The following process for producing the precursor fibers according to the invention also falls within the scope of the invention:

[0021] The present invention therefore also provides a process for producing precursor fibers from lignin-based carbon fibers according to at least one of the advantageous embodiments of the precursor fibers described above. This process is characterized in that an aqueous solution (D) of the water-soluble lignin salt (A) and the water-soluble polyvinylpyrrolidone or derivative (B) is prepared, the resulting aqueous solution (D) is dry-spun into filaments to form precursor fibers from carbon fibers, and the filaments are drawn off.

[0022] It should be emphasized that the physical properties of the components in the form of the water-soluble lignin salt (A), the water-soluble polyvinylpyrrolidone (B) and the water-soluble crosslinker, in particular their chemical-structural properties, described in connection with the precursor fibers, also apply here.

[0023] Compared to other additional polymers, such as polyvinyl alcohol (see US 3461082 A), the inclusion of polyvinylpyrrolidone allows the precursor fiber of the invention to be processed at a higher heating rate in the stabilization process described in detail later, preferably between 5-20 K / min. This is due to the thermal behavior of polyvinylpyrrolidone, in particular its glass transition temperature of 150 °C. Compared to PVA, whose glass transition temperature is only approximately 85 °C, higher stabilization temperatures can be used. With polyvinylpyrrolidone, the fiber can be heated to 150 °C without increasing the chain mobility in the amorphous segments of the precursor fiber. This can prevent the precursor fibers from sticking together.In addition to the advantage that stable spinning solutions are obtained through the use of water-soluble polyvinylpyrrolidone, this additional polymer can be dissolved at room temperature.

[0024] The present invention is not subject to any significant restrictions in the selection of the respective water-soluble lignin salts (A). Commercially available water-soluble lignin sulfonates, in particular ammonium, calcium, magnesium, or sodium lignin sulfonates, are particularly suitable for this purpose, but other soluble or dispersible water-soluble lignin salts such as phosphates, phosphonates, phosphinates, phosphites, phosphonites, or phosphinites are also suitable. It is advantageous to use water-soluble lignin sulfonates with a high molecular weight of 30,000 g / mol or more and a low sugar content of <10%, in particular <3%. By processing higher molecular weight water-soluble lignin sulfonates, better mechanical properties, such as strength and Young's modulus, are achieved.

[0025] A particularly advantageous development of the invention consists in incorporating a water-soluble crosslinking agent (C) into the aqueous solution (D).

[0026] The dry spinning process according to the invention can preferably be carried out as follows: In the dry spinning process, the spinning solution is pressed through defined nozzle holes (100-200 μm diameter) with a spinning pump and dried in a spinning shaft at 40-60°C. The precursor fibers can be wound onto a spool using a driven winder. The fibers advantageously have a diameter in the range of approximately 6 μm to 60 μm, preferably 6 to 20 μm, in particular from 6 μm to 20 μm and more preferably 6 to 13 μm and particularly preferably 10 to 13 μm. The carbon fibers subsequently produced from the precursor fibers preferably have a diameter of 2 μm to 30 μm, particularly preferably a diameter of 4 μm to 8 μm.

[0027] In order to promote the spinning process, it is advantageous that the aqueous solution (D) is adjusted to a zero shear viscosity (measured according to DIN 53019-4) of about 50 to 800 Pa.s, preferably of about 100 to 600 Pa.s, in particular of about 150 to 350 Pa.s, measured at a temperature of 22°C.

[0028] To optimize this process, it is also advisable to concentrate the aqueous solution (D), in particular under vacuum and preferably using a rotary evaporator, until the zero-shear viscosity (measured according to DIN 53019-4 at a temperature of 22°C) of approximately 50 to 800 Pa.s, preferably of approximately 100 to 600 Pa.s, in particular of approximately 150 to 350 Pa.s, has been established, and the resulting concentrated aqueous solution (E) is dry-spun. Preferably, the concentration of the aqueous solution (D) is carried out under a vacuum of approximately 10 to 80 mbar, in particular of approximately 45 to 75 mbar. The spinning solution should preferably be free of particles and not have a gel-like character.

[0029] It has proven very advantageous to produce a mixture in which 1.) about 0.1 to 1 part by weight, in particular about 0.3 to 0.7 part by weight, of water-soluble polyvinylpyrrolidone or derivative (B) are added to 1 part by weight of water-soluble lignin salt (A), or 2.) about 0.1 to 1 part by weight, in particular about 0.3 to 0.7 part by weight, of water-soluble polyvinylpyrrolidone or derivative (B) and about 0.01 to 0.3 part by weight, in particular about 0.05 to 0.15 part by weight, of water-soluble crosslinking agent (C) are added to 1 part by weight of water-soluble lignin salt (A), and the respective mixture is dissolved in water and converted into the aqueous solution (D).

[0030] However, it is also possible to first incorporate the water-soluble crosslinker (C) into the aqueous solution (E). The quantitative relationships shown between the water-soluble lignin salt (A), the water-soluble polyvinylpyrrolidone or derivative (B), and the water-soluble crosslinker (C) should also be considered here. It is therefore particularly advantageous for the implementation of the present invention to incorporate the water-soluble crosslinker (C) into the aqueous solution (D) and / or the aqueous solution (E).

[0031] As already mentioned, it is particularly advantageous for the realization of the present invention to incorporate the water-soluble crosslinker (C) into the aqueous solution (D) and / or the aqueous solution (E). Optionally, the mechanical fiber properties can be improved by chemical crosslinking or by introducing additional aromatic groups. Therefore, the above-mentioned water-soluble crosslinkers (C), such as dimethyloldihydroxyethyleneurea (DMDHEU), trioxane, or urotropin, can be used in particular. The water-soluble crosslinkers (C) contain formaldehyde-releasing compounds, which, upon thermal activation, lead to crosslinking of the lignin. A fundamental advantage is that the water-soluble crosslinker (C) can be added to the polymer-containing spinning solution and spun along with it.

[0032] An advantageous further development of the process according to the invention should be noted, in which the concentrated aqueous solution (E) is dry-spun in the spinning shaft at a temperature of about 30°C to 100°C, in particular of about 40°C to 70°C.

[0033] The water content of the aqueous solution (D) also has advantageous effects on the success of the process according to the invention. It is preferred that the aqueous solution (D) contains at least about 40 wt.%, in particular about 45 to 65 wt.%, of water.

[0034] The water-soluble crosslinker (C) incorporated into the precursor fibers has already been discussed above. This crosslinker is present in an activatable form. Therefore, activation is particularly advantageously initiated when the water-soluble crosslinker (C) incorporated into the precursor fibers is activated by thermal stabilization of the precursor fibers, in particular by oxidative thermal stabilization, at a temperature of approximately 100 to 400°C, in particular approximately 200 to 300°C, preferably 250°C + / - 20°C. The residence times of the precursor fibers in the thermal stabilization are between 10 minutes and 4 hours, preferably between 10 minutes and 60 minutes, particularly preferably between 10 minutes and 40 minutes, depending on the heating rate and the temperature profile.

[0035] A particular objective of the present invention is to produce advantageous carbon fibers from the precursor fibers described above, optionally with prior thermal stabilization, in particular oxidative thermal stabilization, and / or with prior stabilization with high-energy radiation and optionally with subsequent graphitization. It is also advantageous to use ultraviolet radiation (UV), vacuum ultraviolet radiation (VUV), electron beams, X-rays, plasma stabilization, and / or gamma radiation as high-energy radiation. Accordingly, this application is attributable to the inventive concept.

[0036] In a further aspect, the present invention accordingly relates to a process for producing carbon fibers from precursor fibers as specified above, wherein the process includes thermal stabilization, in particular oxidative thermal stabilization, and / or stabilization with high-energy radiation and / or plasma stabilization, followed by carbonization, optionally with subsequent graphitization. Ultraviolet radiation (UV), vacuum ultraviolet radiation (VUV), electron beams, X-rays, and / or gamma radiation are preferably used as the high-energy radiation.

[0037] For the specified process, it is furthermore advantageous if the thermal stabilization of the precursor fibers is carried out in a temperature range of 100°C to 400°C, in particular from 200°C to 300°C, in particular at 250°C + / - 20°C.

[0038] The carbonization of the precursor fibers according to the invention is preferably carried out in the context of the process specified here at temperatures between 300°C and 1800°C, expediently under an inert gas atmosphere, and particularly preferably under a nitrogen atmosphere. For carbonization, it is further advantageous if the stabilized precursor fibers are subjected to low-temperature carbonization, in particular in a temperature range from 300°C to 1200°C, and preferably in a range from 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 (low-temperature LT furnace), whereby this carbonization is preferably carried out under an inert gas atmosphere, in particular a nitrogen atmosphere.

[0039] In a preferred embodiment, the precursor fibers are drawn in a low-temperature oven, in particular with a draw factor between 1% and 35%, preferably at least 2% and 30%, particularly preferably at least 5% and 25%. In this way, a further increase in the mechanical properties of the fibers can be achieved.

[0040] It is further advantageous if the fibers are subjected to high-temperature carbonization following low-temperature carbonization, wherein 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. The high-temperature carbonization is advantageously carried out in an inert gas atmosphere.

[0041] The carbon fibers resulting from the high-temperature furnace have diameters in the range of 2 to 30 pm, with a preferred diameter of 4 to 8 pm.

[0042] For the graphitization provided for in the process, it is preferred if it is carried out in an inert gas atmosphere, especially an argon atmosphere, at a maximum temperature of 3000°C with hot stretching. This treatment can further increase the orientation of the carbon fibers and their modulus of elasticity through simultaneous hot stretching. Graphitization can conveniently be carried out in a graphitization furnace (ultra-high-temperature furnace, UHT furnace).

[0043] In yet another aspect, the present invention relates to a use of the precursor fibers as specified above for producing carbon fibers by carbonization, optionally with prior thermal stabilization, in particular oxidative thermal stabilization, and / or with prior stabilization with high-energy radiation and / or plasma stabilization, optionally with subsequent graphitization.

[0044] The use of water-soluble polyvinylpyrrolidone as an auxiliary polymer not only improves spinnability but also explicitly introduces nitrogen into the carbon fibers, providing them with a nitrogen-containing surface, similar to carbon fibers made of polyacrylonitrile, for bonding resins in the production of composite components. In the current state of the art for precursor fibers made of cellulose or lignin with auxiliary polymers, such as PVA, the nitrogen atoms must be introduced in additional processing steps via plasma processes or chemical functionalization. Oxidative thermal stabilization generally takes place in an oxygen atmosphere up to a final temperature of 100 °C to 400 °C. The heating rate is between 20 and 0.05 K / min.

[0045] As already shown, the invention is characterized by numerous advantages: For example, the invention enables the production of multifilaments and can be scaled up to large-scale plants. The process according to the invention is economically and ecologically feasible, with the goal of producing environmentally friendly carbon fibers with low production costs. With regard to the production of cost-effective carbon fibers, the carbon yield after carbonization plays an important role. Therefore, there is great interest in keeping the lignin content in the precursor fiber as high as possible. The precursor fibers therefore preferably have a lignin content of at least 70 wt.%, although 80 wt.% has also been demonstrated. The invention thus relates to a continuous dry spinning process from preferably water-soluble lignin sulfonate with polyvinylpyrrolidone, using water as the solvent.

[0046] The polyvinylpyrrolidone enables, in particular, a high spinning speed in the range of at least 75 m / min to 100 m / min, as it stabilizes the rather brittle and amorphous lignin-based spinning dope. The polyvinylpyrrolidone facilitates the orientation of the lignin polymer in the fiber direction and reduces brittleness, thereby facilitating handling, especially winding, etc. The polyvinylpyrrolidone or derivative (B) added to the spinning solution also allows for a very high stabilization speed of 10-20 m / min, which is nowhere near achieved by the prior art. This saves considerable amounts of energy and costs in the production of carbon fibers according to the invention. The invention is explained in more detail below using examples:

[0047] Example 1:

[0048] (Preparation of a spinning solution with 1 part by weight of ammonium ligninsulfonate (softwood) and 0.43 parts by weight of polyvinylpyrrolidone and carrying out a dry spinning process)

[0049] To prepare the spinning solution, a mixture according to the invention was dissolved in water. For each 500 g of the mixture, one part by weight (350 g) of ammonium ligninsulfonate (softwood) and 0.43 part by weight (150 g) of polyvinylpyrrolidone with a weight-average molecular weight of 50,000 g / mol (PVP K30 from Sigma Aldrich) were dissolved in 1.4 l of water while stirring. After stirring for 30 minutes at room temperature, the solution was homogenized. Any residual particles were removed by filtration. The resulting spinning solution was concentrated under vacuum in a rotary evaporator until a zero-shear viscosity of 229 Pa.s at 22 °C was obtained. The rheological measurement was performed using a Physica MCR 301 rheometer from Anton Paar with a plate-on-plate geometry with a plate diameter of 25 mm and a gap distance of 0.5 mm.

[0050] The dry spinning process was conducted on a pilot scale using a multi-hole spinneret. The spinneret diameter was 100 μm with 90 filaments. The screw speed was 7.6 rpm. The spinning temperature was 22°C. The fibers were dried in the spinneret in air at temperatures between 40 and 50°C. The fibers were wound at a winding speed of 80 m / min and stored under constant climatic conditions (23°C, 40% relative humidity). The water content of the fibers after the dry spinning process was between 8 and 12 wt.%. The determination was carried out by Karl Fischer titration at 140°C. Example 2:

[0051] (Preparation of a spinning solution with 1 part by weight of ammonium ligninsulfonate (Hardwood) and 0.43 parts by weight of polyvinylpyrrolidone and carrying out a dry spinning process)

[0052] Example 2 was carried out with the exception that an ammonium lignin sulfonate (hardwood) was used instead of the ammonium lignin sulfonate (softwood). Accordingly, a 500 g mixture consisting of 1 part by weight (350 g) of ammonium lignin sulfonate (hardwood) and 0.43 part by weight (150 g) of polyvinylpyrrolidone with a weight-average molecular weight of 50,000 g / mol (PVP K30 from Sigma Aldrich) was mixed in 1.4 l of water, analogously to the procedure described in Example 1. After stirring for 30 minutes at room temperature, the solution was homogenized. Any residual particles were removed by filtration. The resulting spinning solution was concentrated under vacuum in a rotary evaporator until a zero-shear viscosity of 282 Pa.s at 22 °C was achieved. The rheological measurement was carried out using a Physica MCR 301 rheometer from Anton Paar with plate-plate geometry with a plate diameter of 25 mm and a gap distance of 0.5 mm.

[0053] The dry spinning process was conducted on a pilot scale using a multi-hole spinneret. The spinneret diameter was 100 μm with 90 filaments. The screw speed was 7.6 rpm, and the spinning temperature was 22°C. The fibers were dried in the spinneret in air at temperatures between 45 and 55°C. The fibers were wound at a winding speed of 90 m / min and stored under constant climatic conditions (23°C, 40% relative humidity).

[0054] The water content of the fibers after the dry spinning process was between 8 and 12 wt.%. This was determined by Karl Fischer titration at 140 °C.

[0055] Example 3:

[0056] (Preparation of spinning solutions with 60-80 wt.% ligninsulfonate and 40-20 wt.% polyvinylpyrrolidone and dry spinning) 60-80 wt.% ligninsulfonate and 40-20 wt.% polyvinylpyrrolidone were dissolved in water and concentrated using a rotary evaporator. The water content of the resulting spinning solution was 44-55 wt.% with a zero-shear viscosity of 170-500 Pa.s. The spinneret comprised 90 to 250 nozzle holes with a diameter of 100 μm, resulting in rovings of 90 to 250 filaments. The maximum spinning speed was between 75 and 100 m / min, with the spinning shaft heated between 40°C and 70°C. For continuous carbonization, a portion of the spun coils was plied 4 times under tension control to form an assembled roving (lk filaments).

[0057] Example 4:

[0058] (Preparation of a spinning solution with 1 part by weight of ammonium ligninsulfonate, 0.36 parts by weight of polyvinylpyrrolidone and 0.07 parts by weight of crosslinker (DMDHEU) and carrying out a dry spinning process

[0059] For a 500 g mixture, one part by weight (350 g) of ammonium ligninsulfonate (softwood), 0.36 part by weight (125 g) of polyvinylpyrrolidone with a weight-average molecular weight of 50,000 g / mol (PVP K30 from Sigma Aldrich), and 0.07 part by weight (25 g) of crosslinker (DMDHEU) were dissolved in 1.4 l of water while stirring. The procedure of Example 1 was repeated with the following modifications: the amount of polyvinylpyrrolidone was reduced from 0.43 part by weight to 0.36 part by weight, and an additional 0.07 part by weight of crosslinker (DMDHEU) was added to the aqueous solution.

[0060] After stirring for 30 minutes at room temperature, the solution was homogenized. Any residual particles were removed by filtration. After concentration by evaporating water under vacuum, the zero-shear viscosity of the spinning solution was adjusted to 200–300 Pa s (measured according to DIN 53019-4 at a temperature of 22°C). The solution was dry-spun through a multi-hole nozzle (100 μm) and wound up at 80–90 m / min. Table 1: Dry-spinning tests with different mixing ratios

[0061] *Note: c|o zero shear viscosity

[0062] Example 5: (Production of carbon fibers: discontinuous)

[0063] The precursor fibers obtained according to Examples 1 to 3 above were stabilized discontinuously in a muffle furnace at temperatures up to 250 °C. The heating rate was varied between 0.5 K / min and 20 K / min, with no fiber adhesion occurring even at 20 K / min.

[0064] The tensile strength and Young's modulus of the precursor fibers were determined using a Favimat from Textechno. Individual filaments with a clamping length of 12 mm were clamped between two clamps, and their fineness was determined by measuring the resonant frequency. The test speed was 1 mm / min. At least 20 valid measurements were used to calculate the mean values. The fiber diameter of the individual filaments was determined using a scanning electron microscope (SEM).

[0065] The stabilized precursor fibers had a tensile strength of 8-10 cN / tex and an elastic modulus of 400 cN / tex.

[0066] The subsequent carbonization was carried out discontinuously in a batch furnace between 1000 °C and 1400 °C under a nitrogen atmosphere. The heating rate was 10 K / min. The carbonized fibers had a diameter between 10-14 pm (SEM images). After cooling, carbon fibers with a carbon yield of 45% were obtained. Table 2 below shows some examples of the carbon fiber values ​​achieved. Continuous carbonization is expected to result in significantly higher mechanical properties, as this is the only way to achieve precursor stretching.

[0067] Table 2: Mechanical properties after carbonization at 1400 °C

[0068] Mixture Diameter Elongation Strength Elastic Modulus

[0069] Example ratio [pm] [%] [GPa] [GPa]

[0070] Lignosulfonate

[0071] 1 (Softwood) & PVP 17.0 ± 1.6 1.3 ± 0.1 0.98 ± 0.2 84 ± 18

[0072] (70:30)

[0073] Lignosulfonate

[0074] 2 (Hardwood) & PVP 10.l ± 1.0 2.4 ± 0.5 0.97 ± 0.2 41 ± 3

[0075] (70:30)

[0076] Lignosulfonate

[0077] 3 (Softwood) & PVP &12.7 ± 2.0 2.1 ± 0.3 1.5 ± 0.3 75 ± 16

[0078] DMDHEU (70:25:5)

[0079] Lignosulfonate

[0080] 4 (Hardwood) & PVP &14.1 ± 2.0 1.8 ± 0.3 1.39 ± 0.24 81 ± 19

[0081] DMDHEU (70:25:5)

[0082] Example 6:

[0083] The precursor fibers obtained according to Example 3 above were first plied into a 1k roving under controlled tension and then stabilized discontinuously in a muffle furnace at up to 250 °C. A heating rate of 1k / min and a residence time of 4 hours were used.

[0084] The subsequent continuous carbonization took place in the low-temperature furnace at a temperature range between 300 °C and 750 °C, and in the high-temperature furnace up to various final carbonization temperatures under a nitrogen atmosphere. The mechanical properties of individual carbon fiber monofilaments produced in a continuous carbonization process at various maximum temperatures in the high-temperature furnace, from 1000 °C to 1600 °C, are shown in the following table. It can be seen that the highest tensile strengths were achieved in a range between 1200 and 1600 °C, with the optimum at 1400 °C.

[0085] Table 3: Mechanical properties after continuous carbonization at different maximum temperatures in the HT furnace.

[0086] The intensity ratio (ID / IG) of the defect band (Dl band, 1340 cm 1 ) and the graphite band (G-band, 1590 cm 1), determined by Raman spectroscopy, depend on the carbonization temperature and provide information about defects in the graphite structure. The lower the ID / IG ratio, the fewer the number of defects in the carbon structure. With increasing maximum carbonization temperature (coking temperature), the ID / IG ratio decreases from 9.5 to 2.3 (-76%), as shown in Table 4. The spectra and data were determined and calculated from carbon fibers produced at different maximum temperatures in a high-temperature furnace from 1000°C to 1600°C.

[0087] Table 4: ID / IG values ​​of carbine fibers produced by continuous carbonization at different maximum carbonization temperatures. Sample ID Maximum l D / l G temperature [°C]

[0088] CF1000 1000 9.5

[0089] CF12OO 1200 4.0

[0090] CF1400 1400 3.3

[0091] CF1600 1600 2.3

[0092] Example 7

[0093] (Continuous production of carbon fibers with different stretch factors)

[0094] Analogous to the processes described above (Example 3), fibers (60 wt.% ligninsulfonate hardwood / 40 wt.% PVP) with 250 filaments were spun and plied into an IK roving and stabilized in a drying oven up to 250 °C at 1 K / min (4 h residence time). The fibers were then wound onto spools and carbonized in an LT oven (300 °C to 750 °C) at a draw factor of 1% and 5%. Table 6 summarizes the mechanical properties of the fibers. The fiber values ​​increase with increasing draw factor. At a draw factor of 5%, the mean tensile strength was 1.4 GPa, the El modulus 104 GPa, and the elongation 1.3%.

[0095] Table 5: Mechanical properties after continuous carbonization at different stretchings:

[0096]

[0097] The crystallite dimensions (L a , L c ), the layer plane distance (dooz) and the number of stacked layers (N c ), which were determined and calculated by wide-angle X-ray scattering (WAXS) of the corresponding carbon fibers at different draw factors at 1400 °C, are shown in the following Table 7. It can be seen that the crystallite dimensions L a and L c With a 5% stretch, the crystallites grow, while the interplanar spacing (d0o2) decreases. The increase in orientation that accompanies the increase in the stretch factor is particularly advantageous.

[0098] Table 7: WAXS measurement or the orientation of the lignin-based CF with stretch factor 0-5%:

[0099] Sample maximum DR L a [nm] £ c [nm] doo2 N c PO [%]

[0100] ID temperature [nm]

[0101] [°C]

[0102] CF1400 1400 1.00 3.49 0.79 0.375 3.7

[0103] CF1400. 1400 1.01 3.49 0.79 0.374 3.7 53

[0104] 2

[0105] CF1400. 1400 1.05 3.83 0.82 0.371 3.7 61

[0106] 3

[0107] Furthermore, two-dimensional wide-angle X-ray scattering images of the carbon fibers with different draw factors were taken. It can be seen that the crescent shape becomes more pronounced the higher the draw factor at 1400 °C. Example 8:

[0108] (Comparison example / spinning solution with pure lignosulfonate without polyvinylpyrrolidone)

[0109] 500 g of ammonium ligninsulfonate (softwood) was dissolved in 1.4 l of water. After stirring for 30 minutes at room temperature, the solution was homogenized. Any residual particles were removed by filtration. After concentration by evaporation of water under vacuum, the viscosity of the spinning solution was adjusted. The spinning solution could not be spun because it lacked sufficient thread pulling power.

Claims

Patent claims 1. Precursor fibers of lignin-based carbon fibers containing water-soluble lignin salt (A) and water-soluble polyvinylpyrrolidone or derivative (B).

2. Precursor fibers according to claim 1, characterized in that the water-soluble lignin salt (A) is represented by the formula LR z (I) in which the residue -R z a sulfonate, phosphate, phosphonate, phosphinate, phosphite, phosphonite and / or a phosphinite residue.

3. Precursor fibers according to claim 2, characterized in that the water-soluble lignin salt (A) is present as lignin sulfonate.

4. Precursor fibers according to at least one of the preceding claims, characterized in that the cation in the water-soluble lignin salt (A) is a sodium, ammonium, calcium and / or magnesium ion, in particular an ammonium ion.

5. Precursor fibers according to at least one of the preceding claims, characterized in that the water-soluble lignin salt (A) has a weight-average molecular weight M w of about 5,000 to 1,000,000, in particular of about 10,000 to 800,000 g / mol, and / or the water-soluble polyvinylpyrrolidone or derivative (B) has a weight-average molecular weight M w from about 10,000 to 2,000,000 g / mol, in particular from about 50,000 to 200,000 g / mol, and particularly preferably from about 50,000 to 100,000 g / mol.

6. Precursor fibers according to at least one of the preceding claims, characterized in that the water-soluble polyvinylpyrrolidone is present as a homopolymer.

7. Precursor fibers according to at least one of the preceding claims, characterized in that the water-soluble polyvinylpyrrolidone or derivative (B) has a softening point of about 100°C to 175°C, in particular about 140°C to 160°C.

8. Precursor fibers according to at least one of the preceding claims, characterized in that the precursor fibers contain a water-soluble, thermally activatable crosslinker (C) in the form of a formaldehyde-releasing compound.

9. Precursor fibers according to claim 8, characterized in that the thermally activatable crosslinker (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. Precursor fibers according to at least one of the preceding claims, characterized in that in the precursor fibers, about 0.1 to 1 part by weight, in particular about 0.3 to 0.7 parts by weight, of water-soluble polyvinylpyrrolidone or derivative (B) are present for 1 part by weight of water-soluble lignin salt (A).

11. Precursor fibers according to at least one of claims 8 to 10, characterized in that about 0.01 to 0.3 parts by weight, in particular about 0.05 to 0.15 parts by weight of water-soluble crosslinking agent (C) are used for 1 part by weight of water-soluble lignin salt (A).

12. A process for producing precursor fibers of lignin-based carbon fibers according to at least one of claims 1 to 11, characterized in that an aqueous solution (D) of the water-soluble lignin salt (A) and of the water-soluble polyvinylpyrrolidone or derivative (B) is prepared, the resulting aqueous solution (D) is dry-spun into filaments to form precursor fibers of carbon fibers, and the filaments are drawn off.

13. The method according to claim 12, characterized in that a water-soluble crosslinking agent (C) is incorporated into 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 according to DIN 53019-4 at a temperature of 22°C) of about 50 to 800 Pa.s, preferably of about 100 to 600 Pa.s, in particular of about 150 to 350 Pa.s.

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

16. The method according to any one of claims 12 to 15, characterized in that a mixture is prepared in which 1.) about 0.1 to 1 part by weight, in particular about 0.3 to 0.7 part by weight, of water-soluble polyvinylpyrrolidone or derivative (B) are added to 1 part by weight of water-soluble lignin salt (A), or 2.) about 0.1 to 1 part by weight, in particular about 0.3 to 0.7 part by weight, of water-soluble polyvinylpyrrolidone or derivative (B) and about 0.01 to 0.3 part by weight, in particular about 0.05 to 0.15 part by weight, of water-soluble crosslinking agent (C) are added to 1 part by weight of water-soluble lignin salt (A), and the respective mixture is dissolved in water and transferred into the aqueous solution (D).

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 to 80 mbar, in particular of about 45 to 75 mbar.

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

19. The process 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 about 45 to 65% by weight, of water.

20. Method according to at least one of claims 13 to 19, characterized in that the water-soluble Crosslinker (C) is activated by thermal stabilization of the precursor fibers, in particular by oxidative thermal stabilization, at a temperature of about 100 to 400°C, in particular about 200 to 300°C.

21. A process for producing carbon fibers from precursor fibers 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 with high-energy radiation and / or plasma stabilization is carried out and subsequent carbonization, optionally with subsequent graphitization, is carried out.

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

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

24. Method according to one of claims 21 to 23, characterized in that the stabilized precursor fibers are subjected to low-temperature carbonization in a temperature range from 300°C to 1200°C, in particular in a range from 300°C to 1000°C.

25. The method according to claim 24, characterized in that the precursor fibers are stretched in a low-temperature oven to increase the mechanical properties, preferably with a stretching factor between 1% and 35%, preferably at least 2% to 30%, particularly preferably at least 5% to 25%.

26. A method according to claim 24 or 25, characterized in that the fibers obtained from the low-temperature carbonization are subjected to a high-temperature Carbonization, preferably in a temperature range of 1200 °C to 1600 °C, more preferably up to 1500 °C and even more preferably at 1400 °C + / - 50 °C, in particular in an inert gas atmosphere.

27. A method according to claim 26, characterized in that the fibers obtained from the high-temperature carbonization are graphitized in an inert gas atmosphere, in particular an argon atmosphere, at a temperature of up to a maximum of 3000°C under hot drawing.

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

Images