MODIFIED LIGNIN, METHOD FOR ITS PREPARATION AND USE

Description

[0001] The invention relates to a modified lignin based on substituted and unsubstituted lignin (A), wherein the lignin molecules of the substituted and unsubstituted lignin (A) are linked with linking units (B) to increase the molecular weight, a process for producing the modified lignin and its use for producing precursor fibers which can subsequently be converted into carbon fibers.

[0002] The demand for carbon fibers for the production of CFRP lightweight components is steadily increasing. Particularly in mass markets such as the automotive industry, significant growth rates are expected. To date, the high costs, especially in the manufacturing of these CFRP lightweight components, as well as the relatively high cost of carbon fibers themselves, have prevented such products from being affordable for the mass market. In recent years, promising processes have been developed in the field of CFRP lightweight component manufacturing, enabling fast, precise, automated, and therefore cost-effective production. With regard to carbon fiber, considerable efforts have been made to maintain quality while reducing costs. This includes both optimizing the manufacturing process and the use of precursor materials. Currently, carbon fibers are predominantly (> 95%) made from polyacrylonitrile.This is produced using a wet chemical process by polymerizing acrylonitrile with a small proportion of comonomers. Acrylonitrile itself originates from the Sohio process and is synthesized from ammonia and petroleum-based propene. The conversion of the polymer powder into polymer fiber is achieved through a special wet spinning process. This requires large quantities of additional solvent and precipitant and is also very slow.

[0003] Therefore, much research focuses on the development of alternative, renewable, petroleum-independent precursors for carbon fibers. These include the biopolymers cellulose and lignin. Lignin, in particular, represents an attractive potential precursor because it naturally contains a high content of aromatic structures and thus exhibits a high carbon yield. Lignin does not compete with food crops and is a byproduct of paper production and biorefineries. A major challenge in the material use of lignin is its potentially highly variable properties and purity, as well as the chemical composition and structure of this biopolymer.For the polymeric use of lignin, one faces the challenge that while lignin, as a waste product of the paper industry, does contain polymeric components, these are on average low-molecular-weight oligomers with a very low degree of polymerization and a highly branched, partially cross-linked structure. This is a consequence of the fragmentation reactions that occur during pulping from wood. Indeed, the goal there is to obtain cellulose as a polymer with as little damage as possible, for which the surrounding lignin is chemically fragmented. It therefore exhibits low molecular weights and high polydispersity. To utilize lignin as a polymer, membrane filtration or fractional precipitation can be used to obtain fractions with the desired molecular weights and properties. Unfortunately, this results in a high waste rate. The product still has a highly branched structure.The low molecular weight of lignin is primarily manifested in the material's weak mechanical strength and brittleness.

[0004] For the use of lignin as a starting material for carbon fibers, a highly linear and polymeric structure is desirable. This structure allows the molecules to align along the fiber axis during the spinning process and increases the mechanical strength of both the precursor fiber and the resulting carbon fiber. The significantly insufficient mechanical strength of the precursor fiber is a key reason why no lignin-based carbon fiber exists as a marketable product to date. The precursor fibers are brittle and inflexible, due in part to their low molecular weight. Adding flexible auxiliary polymers is only beneficial to a limited extent, as this typically reduces the carbon yield of the overall material and drastically degrades the quality of the carbon fiber due to resulting defects.Therefore, based on the current state of the art, it is necessary to develop a method that allows for a controlled, linear molecular weight build-up of lignin as a carbon fiber precursor. Existing methods for the polymerization of lignin or for the production of polymers with covalently bonded lignin are mostly not designed for its use as a carbon fiber precursor and are therefore unsuitable. Some of these methods will be briefly presented below.

[0005] US patent 4769434 A describes in detail the base-catalyzed condensation of lignin with bivalent electrophilic compounds. The electrophiles mentioned therein are aldehydes, hydroxymethylated amines, phenolic compounds, and isocyanates. US patent 3984363 A addresses the epoxidation of lignin under alkaline conditions followed by hardening with (polymeric) amines. Carboxylic anhydrides and the esterification of lignin with crosslinkable double bonds are also considered as crosslinking compounds. The preparation and use of polymerizable prepolymers from lignin are disclosed in US patent 5102992 A, in which lignin modified with alkylene oxide is reacted with a compound containing an acrylate group to obtain polymerizable prepolymers. Lignin-based polyurethanes represent another area of ​​application for polymers derived from lignin.According to US 9598529 B2, polyurethane precursors are mixed with lignin and heated, causing the mixture to polymerize. Lignin also reacts with isocyanates in the presence of polyols to form a polymer (see US 6025452 A). Another area of ​​lignin polymerization involves copolymers of lignin and polylactic acid (PLA). For example, US 20130281582 A1 describes how a butanoic acid ester of lignin is heated in the presence of polylactic acid to produce a lignin-PLA graft copolymer.

[0006] A similar chemistry is described in US 9567432 B2, according to which lignin-polylactide graft copolymers are synthesized. These are then reacted with crosslinking agents, such as hexamethylene diisocyanate. A radical polymerization of lignin to a polymer is described in US 5608040 A, in which polymers are formed from lignin and organic compounds with at least three carbon atoms in the presence of radically oxidizing enzymes. Two of the few concrete methods for producing polymeric lignin as a precursor for carbon fibers are disclosed in US 20130255216 A1. An oxypropylation of lignin, followed by propargylation, leads in several steps to a chain-extended lignin-based product (H. Sadeghifar, S. Sen, SV Patil, DS Argyropoulos, ACS Sus. Chem. Eng. 2016, 4, 5230-5237). Furthermore, thermally stable polyarylene ethersulfone lignin copolymers are reported, which are produced from lignin and aromatic difluorodiphenyl sulfone (DFDPS).The product exhibits increased thermal stability compared to untreated lignin (DS Argyropoulos, H. Sadeghifar, C. Cui, S. Sen, ACS Sus. Chem. Eng. 2014, 2, 264-271). However, all these methods typically result in a highly branched product or a graft polymer, which cannot be linearized based on lignin. Furthermore, aliphatic compounds, such as polylactic acid, are used as copolymers and comonomers. During the thermal conversion of lignin to carbon fiber, these aliphatic compounds lead to low residual carbon masses and yields. This results in high material loss during the thermal conversion to carbon fiber, as well as high carbon fiber porosity and consequently poor mechanical properties.

[0007] The invention is based on the objective of overcoming the disadvantages described above in connection with the prior art, in particular to propose a modified lignin of the type mentioned at the outset, which is linearly oriented and can be advantageously converted into carbon fibers, with high carbon yield, low porosity and improved mechanical properties of the carbon fiber.

[0008] According to the invention, this problem is solved by a modified lignin of the type described above, characterized in that the linking units (B) are aromatic and are connected to the lignin molecules (A) via a methylene group, wherein the linking unit (B) is derived from a reaction of the lignin (A) with an aromatic linking compound (C) containing two hydroxymethyl groups.

[0009] Suitable non-substituted lignin A include hardwood, softwood, and grass lignins, both underivatized and derivatized. Suitable lignin types include all common forms, with organosolv and kraft lignins being preferred.

[0010] In various cases, it is preferred that the modified lignin according to the invention is based on a substituted lignin, wherein the substituted lignin is represented in particular by the formulas LO-COR x (I) and / or L-OR y (II), where: the residue LO in formulas (I) and (II) represents an esterified or etherified lignin residue, the residue -CO-R x in formula (I) represents an acyl residue, and the residues R x and R y represent an aliphatic and / or aromatic residue. There are several advantageous embodiments of this. One such embodiment is characterized in that the aromatic residues R x and R y represent an aryl residue, in particular a phenol, benzyl, methoxybenzyl, and / or a 3-phenylpropenyl residue. Furthermore, it is considered preferred that the aliphatic residues R x and R y comprise a straight-chain or branched C 1-6 alkyl residue, in particular a methyl, ethyl, n-propyl, isopropyl, tert.-Butyl- or a neo-pentyl residue.

[0011] As already described, short-chain methyl (C1) and ethyl (C2) ethers and acetyl esters of lignin have proven advantageous in derivatives of the formulas (I) and (II) described above, with the methyl ether according to formula (II) being the most prominent. In this embodiment of the invention, organosolv lignins are particularly advantageous, as they are partially etherified and / or esterified as a consequence of their manufacturing process, e.g., by acetic acid digestion. Phenolized lignin, in which phenol or phenol derivatives with reactive aromatic sites are covalently bound to lignin, thereby increasing the number of reactive sites on the lignin, is also a suitable lignin derivative. In this case, the phenol has generally substituted the aliphatic hydroxyl groups of the lignin and corresponds to the formula LO-phenol (III).Considerations regarding the occupation of the hydroxyl groups of the lignin derivative in the aforementioned formulas (I) and (II) are also important. It is preferred that 1% to 100%, preferably 10% to 90%, and particularly 30% to 80%, of the hydroxyl residues of the lignin are occupied according to formulas (I) and (II). With regard to the phenolized lignin, it is recommended that between 1% and 80%, preferably 10% to 60%, and particularly between 20% and 50%, of the aliphatic hydroxyl groups be occupied with phenol or phenol derivatives according to formula LO-phenol (III).

[0012] An essential component of the invention is the linking unit (B). This unit is aromatic in the broadest sense and is linked to the lignin molecules (A), substituted or unsubstituted, via a methylene group. In the process according to the invention, it is introduced into the modified lignin by means of a linking compound (C). For further details, please refer to the following explanations regarding the linking compound (C).

[0013] Taking this into account, the linking unit (B) is based on a reaction of the lignin (A) with an aromatic linking compound (C) which has at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, and is monomeric, oligomeric and / or polymeric, in particular a 2,6-bis(hydroxymethyl)-p-cresol, a p-cresol-based resol, a phenol-based resol and / or a novolac-like compound with terminally arranged hydroxymethyl groups.

[0014] It is advantageous to pay attention to the molar ratio of the linking unit (B) to the lignin molecules (A). It is advantageous if the molar ratio of the linking unit (B) to the lignin molecules (A), whether substituted or unsubstituted, is approximately 1:1 to 1:7, particularly 1:2 to 1:4, and most preferably approximately 1:3.

[0015] The modified lignin (A) according to the invention exhibits a multitude of relevant advantageous properties: In particular, the modified lignin is characterized by an ash content, determined according to DIN EN ISO 3451-4, of less than 0.1 wt.%. This aspect is important because it is known that an excessively high proportion of foreign ions can have a detrimental effect on the carbon structure and the stability of the carbon fiber. Therefore, it is considered advantageous if the ash content complies with the above preferred rule, according to which less than 0.1 wt.% and / or less than 1000 ppm of foreign ions, in particular less than 500 ppm of foreign ions, especially in the form of alkali metal ions, are present. The precise quantification of the salts and metals contained in the modified lignin according to the invention can be carried out by ICP-MS (inductively coupled plasma mass spectrometry) after suitable calibration. A very low detection limit is achieved using this method.

[0016] Of particular importance is the complex viscosity of the modified lignin according to the invention, in accordance with the respective purpose. It is preferred that the modified lignin for wet, dry-wet, and dry spinning has a complex viscosity in solution of 10 to 500 Pa·s, particularly 50 to 300 Pa·s, determined according to DIN 53019-4, while for melt spinning a complex viscosity of 50 to 2000 Pa·s, particularly 100 to 1000 Pa·s, determined according to DIN 53019-4, is advantageous.

[0017] Furthermore, the lignin modified according to the invention for the purpose of melt spinning is characterized by the fact that it preferably has a glass transition temperature (Tg) of 80 °C to 220 °C, in particular of 100 °C to 180 °C, determined by differential calorimetry according to DIN EN ISO 11357-2.

[0018] In principle, the number-average molecular weight also plays a significant role according to the invention: The lignin according to the invention is characterized by preferably having a number-average molecular weight of 2000 to 50000 g / mol, particularly of 4000 to 40000 g / mol. The weight-average molecular weight ranges between 10000 and 250000 g / mol, particularly between 20000 and 100000 g / mol. The above information helps those skilled in the art to adjust specific structural properties of the product according to the invention, such as desirable linearity, the degree of branching, and the degree of cross-linking. These can be controlled by those skilled in the art by adjusting the optimal stoichiometric ratio between lignin (A) and cross-linking compound (B).

[0019] Therefore, in the practical implementation of the invention, it is also important to pay attention to the number-average molecular weight (Mn) of the starting lignin or the lignin derivative A. It has proven advantageous to start with a number-average molecular weight (Mn) of 200 to 30,000 g / mol, particularly 300 to 10,000 g / mol. The weight-average molecular weight (Mw) preferably ranges between 1,000 and 40,000 g / mol, particularly between 2,000 and 20,000 g / mol. The lignin or lignin derivative used also includes fractions of lignin or lignin derivative (A) that result from the fractionation of lignin or lignin derivative, particularly according to particle size. It is therefore useful to use the fractional precipitation of lignin, which already occurs during the manufacturing or...The isolation process of lignin is feasible, as are the sequential extraction of lignin and the ultrafiltration of lignin solutions. Fractionated lignin thus has a lower polydispersity than the starting lignin, which is advantageous for calculating the stoichiometry of the reaction within the framework of the inventive process described below, and possibly a higher starting molecular weight. Therefore, it is preferred if the lignin (A) used, whether substituted or unsubstituted, has the lowest possible polydispersity, preferably < 8, in particular < 6. To advantageously carry out the inventive process described below, or to advantageously enable the reaction of lignin (A) with linking compound (C), reactive aromatic sites on the lignin (A) are beneficial.These sites can include: hydrogen-occupied aromatic positions in the α-position to the phenolic hydroxyl group of lignin (corresponding to a guaiacyl unit), as well as hydrogen-occupied aromatic positions ortho to ether groups of lignin, which are generally the naturally occurring methoxy groups, and lignin-linking ether bridges. Also considered reactive sites are the free aromatic positions of phenol and phenol derivatives, which have been covalently bound to lignin through a phenolization reaction. Therefore, a minimum number of these free sites is preferably present in the lignin.

[0020] Here, too, the intended purpose plays a role, i.e., whether the modified lignin is used for wet, dry-wet, or dry spinning, or for melt spinning of precursor fibers. For wet, dry-wet, and dry spinning of precursor fibers, it is preferred that it has a number-average molecular weight of 2,000 to 50,000 g / mol, particularly 4,000 to 40,000 g / mol. If it is used for melt spinning of precursor fibers, then the number-average molecular weight is advantageously between 2,000 and 15,000 g / mol, particularly 4,000 to 10,000 g / mol.

[0021] The invention further relates to a process for producing a modified lignin, in particular a modified lignin of the type described in detail above according to the invention, characterized in that the substituted and / or unsubstituted lignin (A) is dissolved in a solvent and reacted in the solution with an aromatic linking compound (C) having at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, in the presence of an organic acid (D) at elevated temperature, in particular from 60°C to 160°C, to form modified lignin.

[0022] The essential reaction components of the process according to the invention are as follows: A lignin (A), substituted and / or unsubstituted according to the invention, is reacted with an aromatic linking compound (C) having at least two hydroxyl-methyl groups, in particular two terminal hydroxyl-methyl groups. For information on the lignin (A), reference is made to the preceding descriptions of the modified lignin according to the invention. The aromatic linking compound (C), through reaction according to the process of the invention, leads to the linking unit (B) of the modified lignin according to the invention. The linking compound (C) is characterized by having, as emphasized, at least two terminal hydroxymethyl groups. It has aromatic character and can be monomeric, oligomeric, and / or polymeric in nature. Preferred examples have been described above, to which reference is made.These compounds are therefore characterized by terminally attached hydroxymethyl groups. The following figures depict chemical structures of possible linkage compounds based on 2,6-bis-hydroxymethyl-p-cresol (n = 1) and a p-cresol-resol.

[0023] For the second formula, preferably 1 ≤ m+n ≤ 40, in particular 1 ≤ m+n ≤ 10, and most preferably 1 ≤ m+n ≤ 5.

[0024] In principle, the system can be an aromatic ring system based on a benzene ring. Other (condensed) aromatic systems can also be used, in particular naphthalene, anthracene, and the like. As shown in the figures above, the at least two terminal hydroxymethyl groups are preferably arranged in the meta position relative to each other. A para position of these two groups would also be possible, although an ortho arrangement does not appear preferred for steric reasons. Finally, the question arises as to suitable substituents on the aromatic system. This question does not affect the core of the present invention. It is readily possible for a person skilled in the art to find suitable substituents, such as a hydroxyl group or a lower alkyl group, in particular an alkyl group with 1 to 6 carbon atoms.

[0025] Further information on the chemical structures of the linking compound (C) shown above: The core of the repeating unit of these linking compounds consists of aromatic monomers such as phenol, cresol, hydroxyphenol and also aromatic systems that are not based on benzene, such as furans, pyrroles, thiophenes and other aromatic heterocycles.

[0026] p-Cresol has proven particularly advantageous in this context. In principle, repeating units between 1 and several hundred are possible, but a range of 1 to 60, preferably 1 to 40, and especially 1 to 20, is advantageous. The monomeric 2,6-bis-hydroxymethyl-p-cresol with a number of repeating units of 1 occupies a special position according to the invention, since the conversion to molecular weight build-up described later proceeds in a highly satisfactory manner and constitutes only a small mass fraction of the total mass of the product. The polydispersity of the oligomeric or polymeric linking compound should preferably be as low as possible, as this allows for an optimized calculation of the stoichiometry between lignin (A) and the linking compound (C).

[0027] In general, it can be emphasized that the process according to the invention is advantageously controlled by reacting lignin (A), whether unsubstituted or substituted, with an aromatic linking compound C under precisely controlled stoichiometric conditions in the presence of an organic acid (D). The linking compound (C), and not the lignin (A), contributes significantly to the chemical functionalities essential for the reaction. Therefore, lignin A does not essentially react with itself. The reaction is primarily controlled by the controlled addition of linking compound (C). As a result of this reaction, lignin-based products, particularly those with an increased number-average molecular weight of more than a factor of 20, can be obtained.The resulting product, with its optimally adjusted stoichiometry, has a non-crosslinked and largely linear structure. The reaction product can be directly converted into a spinning compound, particularly by partially evacuating the solvent used. However, it is also possible to continue evaporation until a solid is reached, which can then be dissolved in other suitable solvents to obtain a suitable spinning compound. Alternatively, a chemical functionalization can be carried out to make the product meltable, allowing it to be processed by melt spinning.

[0028] It is particularly advantageous if the reaction is carried out at a temperature of 80°C to 120°C, especially 90°C to 110°C. The reaction underlying the process according to the invention can be carried out under atmospheric pressure as well as under elevated pressure in a steel autoclave, with a non-oxidative atmosphere being advantageous, for example, under nitrogen. The duration of the reaction is not critical. However, it is preferred if the reaction is carried out for 5 to 24 hours, especially 7 to 15 hours. The particularly advantageous reaction duration depends on various factors, such as the type and nature of the lignin (A) and the linking compound (C) used.

[0029] Since water is produced during the reaction, it is advantageous to add a water-removing agent to the reaction medium to remove the reaction water. The addition of water-removing agents can accelerate the reaction because water is formed as a condensation product. In cases where 1,4-dioxane is used as the solvent and polar aprotic solvents are used as the reaction medium, these solvents fulfill this function to a certain extent. If the solvent is not sufficiently hygroscopic, molecular sieves (e.g., 3A) or a water-carrying system can be used to remove the condensation water or reaction water and thus accelerate the reaction.

[0030] With regard to the specific objective of the invention and the related requirements, both substituted and unsubstituted lignin (A) can be used as starting materials in the process according to the invention. Reference is made to the preceding explanations concerning unsubstituted and substituted lignin. As already mentioned, the aromatic linking compound (C) used in the process according to the invention is characterized in particular by having at least two hydroxymethyl groups, especially two terminal hydroxymethyl groups, which applies especially to oligomeric and monomeric aromatic linking compounds (C). The reason for this is that the use of two terminal hydroxymethyl groups allows for linear chain elongation, whereas multiple hydroxymethyl groups per compound C lead to branching.

[0031] As already indicated, the linking compound (C) can be monomeric, oligomeric, and / or polymeric. It is, in particular, a 2,6-bis(hydroxymethyl)-p-cresol, a p-cresol-based resol, a phenol-based resol, and / or a novolac-like compound with terminally arranged hydroxymethyl groups. Practical application of the process according to the invention has shown that the oligomeric and / or polymeric linking compound (C) advantageously exhibits a polydispersity of less than 2, and in particular less than 1.5. This is because, in particular, this allows for an accurate stoichiometric calculation of the reaction between lignin and the linking bond, and thus precise control of the properties of the product according to the invention.

[0032] When carrying out the process according to the invention, attention should also be paid to the molar ratio of the linking compound (C) and the lignin (A), whether substituted or unsubstituted. It is advantageous if the linking compound (C) and the lignin (A) are reacted to form modified lignin in a molar ratio of approximately 1:1 to 1:7, in particular 1:2 to 1:4, and most preferably approximately 1:3.

[0033] In selecting the organic acid (D), the person skilled in the art is not subject to any critical restrictions. It is readily possible for them to determine the most suitable organic acid (D) and use it accordingly. It is advantageous if the organic acid (D), particularly in the form of p-toluenesulfonic acid (p-TsOH) and / or methanesulfonic acid (MSA), has a pKa value of < 3, and especially < 2.

[0034] A more detailed description of the method according to the invention will follow for further explanation. Reference will be made to particularly useful experiences gained in connection with the invention.

[0035] In general, it has been shown that the reaction of approximately 8.5 parts by weight of underivatized Alcell lignin as lignin (A) with approximately 1 part by weight of 2,6-bishydroxymethyl-p-cresol as component (C) in the presence of approximately 1.0 part by weight of methanesulfonic acid as organic acid (D) yields a highly advantageous product for the intended application as a polymeric linearized carbon fiber precursor with increased molecular weight, where the solvent is 1,4-dioxane and the reaction time is approximately 7 hours at approximately 100°C. This reaction product exhibits no cross-linked structures and is completely and readily soluble in the reaction solvents used. Partially soluble to cross-linked reaction products are therefore obtained at weight ratios of 2,6-bis-hydroxymethyl-p-cresol to lignin (A) greater than 1:8.5, such as 1:6, i.e., more parts by weight of cross-linking compound (C) per part lignin (A).

[0036] Since not only monomeric compounds (C) but also polymeric compounds (C) are suitable, it is recommended to specify not only the weight ratios of the reactants but also the molar ratios of lignin (A) to the hydroxymethyl groups of the linking compound (C). Therefore, the following relationship is considered preferred: approximately 10 g of lignin (A) corresponds to 20 mmol, assuming a number-average molecular weight of the lignin (A) used. According to the above configuration, this is followed by 7 mmol of linking compound (C), containing two terminal hydroxymethyl groups, which corresponds to approximately 3.5 mmol of hydroxymethyl groups.Therefore, it has proven advantageous to express the favorable molar ratios between the linking compound (C) and lignin (A) (lignin and / or lignin derivative) as follows: Preferably, approximately 1 part by weight of linking compound (C) with two reactive terminal hydroxymethyl groups 1 to 6, in particular 2 to 4, parts by weight of lignin (A), and particularly preferably 3 parts by weight of lignin (A), are present for every 1 part by weight of linking compound (C) with two reactive terminal hydroxymethyl groups 1 to 6, in particular 2 to 4, parts by weight. The amount of compound (D), i.e., the organic acid (D), can be expressed similarly: Approximately 2 parts by weight of compound D are present for every 3 parts by weight of lignin, particularly if the latter is monofunctional.

[0037] The molar ratio between lignin (A) and organic acid (D) should also be optimized. It is therefore advantageous to use approximately 0.2 to 1 mol of organic acid (D), particularly 0.5 to 1 mol, for every 1 mol of lignin (A).

[0038] The process according to the invention is carried out using a solvent that can be readily determined by a person skilled in the art, in particular one for dissolving the lignin (A), substituted and / or unsubstituted. A large number of suitable solvents are available to the person skilled in the art. Particularly suitable are 2,4-dioxane, mixtures of 1,4-dioxane and water, tetrahydrofuran (THF), and especially polar aprotic solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and dimethylacetamide (DMAc). The cyclic ether 1,4-dioxane is particularly suitable, as it is capable of forming strong hydrogen bonds with lignin, has a boiling point of only 101°C under normal conditions, and is also readily miscible with water.

[0039] In principle, there is also the possibility of further optimizing the process according to the invention by incorporating additive polymers, particularly with regard to improved spinning behavior and improved mechanical properties of the precursor fibers obtained from the reaction product of the process according to the invention. Here, it can be advantageous if the lignin modified according to the invention is converted into a spinning mass that contains an auxiliary polymer E. It is expedient for this to be a polyester (PES), a polyamide (PA), a polymethyl methacrylate (PMMA), polyvinyl acetate (PVAc), polyvinyl alcohol (PVA), or polyvinylpyrrolidone (PVP). Esters of cellulose, such as 2,5-acetate and cellulose acetate butyrate, are particularly suitable. Furthermore, it is considered advantageous if the auxiliary polymer (E), based on the mass of the lignin modified according to the invention, contains an amount of at least 5 wt.%, and in particular 10 to 30 wt.%, of a specific polymer.-%, is present.

[0040] For a further explanation of the method according to the invention, please refer to the following Figure 1 Reference is made to this figure. This figure shows the reaction scheme of the reaction proceeding according to the invention and a typical possible chemical structure of lignin (A) and the structure of the linking compound (C). Low molecular weight lignin (A) and / or lignin derivative (A) are reacted with a monomeric, oligomeric, or polymeric linking compound in dissolved form in the presence of an organic acid to form a higher molecular weight lignin.

[0041] The particular value of the lignin according to the invention, or of the modified lignin produced according to the described process, lies in its flexibility in the production of precursor fibers. In general, the modified lignin can be advantageously used for the production of precursor fibers by melt spinning, particularly with the inclusion of a plasticizer, especially in the form of monomeric compounds, in particular DMF, DMSO, phenol, resorcinol, vanillin and phthalic acid esters, or by wet, dry-wet or dry spinning.

[0042] The precursor fibers are particularly well-suited for the production of carbon fibers by carbonization, optionally with prior oxidative thermal stabilization and optionally with subsequent graphitization. Regarding these advantageous applications of the modified lignin in detail: For fiber spinning, an uncrosslinked product with a structure that is as linear as possible is particularly beneficial, as this allows for better generation of preferred molecular orientations in the precursor fiber. Orienting the chain-like molecules along the fiber axis is especially advantageous for obtaining a mechanically stable precursor fiber. This orientation is also transferred throughout the thermal process to the final carbon fiber, which consequently exhibits improved properties.The non-crosslinked product should therefore be completely and readily soluble in the solvents described above, in particular 1,4-dioxane, mixtures of 1,4-dioxane and water, tetrahydrofuran (THF), and polar aprotic solvents, such as DMF, DMSO, and DMAc. Depending on the embodiment, the spinning mass containing the modified lignin according to the invention can be processed either by melt spinning, as shown, or by solution spinning. The spinning mass for solution spinning contains the modified lignin according to the invention and a suitable solvent, which can either be derived directly from the reaction mixture of the synthesis or added separately. Other polymeric compounds can also be added to the spinning mass.

[0043] Suitable solvents include those already mentioned above, but also ionic liquids. Preferably, these solvents are 1,4-dioxane, mixtures of 1,4-dioxane and water, tetrahydrofuran (THF), and polar aprotic solvents such as DMF, DMSO, and DMAc. In practical implementation of the invention, a solvent in the form of dimethylformamide (DMF) proved particularly suitable. The complex viscosity of the spinning compound at the processing temperature is preferably between 10 and 500 Pa·s, and particularly between 50 and 300 Pa·s. In this case, the loss modulus predominates over the storage modulus, as determined by rheological measurements in the frequency range of 1 to 100 Hz and a constant deformation of 10%.

[0044] The spinning compound is preferably free of particles and should, in particular, not have a gel-like character. A preferred spinning compound, in light of the concentration of the lignin modified according to the invention in the solvent, can be defined as follows: For fiber spinning, it has proven advantageous if, for approximately 1 part by weight of lignin, there are between approximately 0.5 and 3, preferably between 0.5 and 2, and particularly preferably between 0.8 and 1.0 parts by weight of solvent. This applies to the aforementioned methods of solution spinning, in particular in the form of wet spinning, air-gap spinning, or dry spinning. For the use of the lignin modified according to the invention in the form of a melt-processable spinning compound, a simple chemical derivatization thereof is preferably carried out.This involves masking reactions of the hydroxyl groups of the product, preferably simple esterifications with aliphatic carboxylic acids and carboxylic acid derivatives. The inserted hexanoyl (C6) residues have proven particularly effective. The general formula is LO-CO-Rx (IV), where L represents the (polycondensed) lignin (A) according to the invention and x preferably represents an aliphatic residue C3-C11. Aliphatic residues containing unsaturated moieties are of particular value. The proportion of unesterified, remaining hydroxyl groups in the product according to the invention can vary between 1% and 100%, preferably from 1% to 90%, and particularly from 30% to 60%.

[0045] The aim of the present invention is to obtain carbon fibers with valuable properties, particularly with regard to tensile strength and modulus of elasticity, using advantageous precursor fibers and a cost-effective process. It is advantageous to first subject the precursor fibers according to the invention to oxidative thermal stabilization in the usual manner. The thermally stabilized material is then subjected to carbonization in the usual manner to produce carbon fibers. Graphitization can optionally follow.

[0046] Oxidative thermal stabilization is preferably carried out up to a final temperature of 100°C to 400°C, particularly from 200°C to 300°C. The preferred heating rates for oxidative thermal stabilization are not critical. A heating rate of less than 10 K / min, particularly less than 5 K / min, is preferably maintained, with the minimum heating rate being at least 0.05 K / min. It can be advantageous to subject the thermally stabilized precursor fibers to crosslinking with high-energy radiation. Crosslinking can be carried out with high-energy radiation, preferably using ultraviolet radiation (UV), vacuum ultraviolet radiation (VUV), electron beam radiation, X-rays, or gamma radiation. The properties of the precursor fibers produced according to the invention have a significant influence on the properties of the carbon fibers produced therewith.The stabilized precursor fibers obtained are then carbonized to form carbon fibers. These are then optionally graphitized.

[0047] During carbonization, which is carried out at gradually increasing temperatures between 300°C and 1,700°C, particularly between 1,000°C and 1,500°C, especially in a nitrogen atmosphere, the carbon content steadily increases, reaching approximately 85–95%. Subsequent graphitization allows the carbon content of the carbon fibers to be increased to > 95%. Graphitization, if carried out, is achieved through a thermal treatment at 1,700°C to 3,000°C, particularly between 2,000°C and 2,500°C, under a protective gas atmosphere.

[0048] The invention will be explained in more detail below using examples: Example 1 (Polycondensation of Alcell hardwood lignin with 2,6-bis(hydroxymethyl)-p-cresol (laboratory scale)):

[0049] Lignin (3 g) was dissolved in 1,4-dioxane (20 ml), and 2,6-bis(hydroxymethyl)-p-cresol and methanesulfonic acid (0.270 ml) were added. The reaction mixture was stirred at 100°C for 22 h. After cooling to room temperature, the mixture was precipitated in water. For purification, the precipitate was extracted three times for 2 h each with hot water to remove the methanesulfonic acid (MSA) and 1,4-dioxane.

[0050] To explain the modified lignin obtained in the above manner according to the invention, reference is made to the following Figure 2Reference is made to this. This shows TGA measurements (helium, 10 K / min) of polycondensed hardwood lignin with 2,6-bis-hydroxymethyl-p-cresol as linking compound C from this Example 1 and of untreated lignin, synthesized using 1 part by weight of linking compound to 3 parts by weight of lignin and 2 parts by weight of methanesulfonic acid (MSA). The increase in residual carbon mass from 35 to 46 wt% of the inventive polycondensed reaction product is clearly visible.

[0051] Polycondensate: ( 1< H-NMR (dmso-d 6 , 400 MHz): δ (ppm) = 0.61, 0.99, 1.71, 3.54, 6.69; 13< C-NMR (dmso-d 6 , 101 MHz): δ (ppm) = 14.4, 20.8, 29.6, 56.4, 60.2, 106.9, 127.6, 148.4; ATR-IR: v (cm -1 < ) = 3345, 2933, 2842, 2114, 1702, 1605, 1512, 1455, 1309, 1211, 1113, 1029, 910, 870. Example 2 (Polycondensation of Alcell hardwood lignin with 2,6-(hydroxymethyl)-p-cresol (pilot plant)):

[0052] Lignin (1 kg) was dissolved in 1,4-dioxane (3.5 l) and placed in a 5 l steel reactor. After adding 2,6-bis(hydroxymethyl)-p-cresol (120 g) and methanesulfonic acid (93 ml), the reaction mixture was heated to 95 °C for 7 h with mechanical stirring. After completion of the reaction, the mixture was precipitated in ice water and the precipitated solid was separated. For purification, it was washed several times with plenty of water and then freeze-dried. Example 3 (Synthesis of a p-cresol-resol):

[0053] p-cresol (1 kg) was melted at 50 °C and transferred to a 5 L steel reactor. After adding an aqueous solution of formaldehyde (37%, 1 L) and adjusting the pH to 9 with sodium hydroxide solution, the reactor was purged with nitrogen and sealed. The mixture was heated to 95 °C for approximately 5 h until the stirring torque increased, after which 1,4-dioxane (3 L) and a further 200 mL of formaldehyde (37%, 200 mL) were added. The reaction was continued for another 2 h. After completion of the reaction, the solution was precipitated in cold water, the solid was washed with ample water and methanol, and finally freeze-dried. 1< H-NMR (dmso-d 6 , 400 MHz): δ (ppm) = 2.0 - 2.3 (CH 3 ), 3.48 (Phenol-OH), 3.83 (-CH 2 -), 4.4 - 4.9 (-CH 2 -O-CH 2 -), 5.22 (-CH 2 -OH), 6.6 - 7.2 (aromatic.H); 13<C-NMR (dmso-d 6 , 101 MHz): δ (ppm) = 21, 26, 31, 58, 60, 69, 129, 150; ATR-IR: v (cm -1< ) ​​= 3195, 2915, 2877, 2326, 2326, 1606, 1482, 1371, 1206, 1154, 1062, 996, 959, 913, 858. Example 4 (Polycondensation of Alcell hardwood lignin with p-cresol-resol):

[0054] Lignin (1.5 g) was dissolved in p-cresol resol (5.2 g) previously dissolved separately in 1,4-dioxane (30 ml) under stirring. After adding methanesulfonic acid (70 µl), the reaction vessel was sealed under nitrogen and stirred for 22 h at 100°C. After completion of the reaction, the mixture was precipitated in ice water and the precipitated solid was separated. For purification, it was washed several times with plenty of water and then freeze-dried. (10 mol% methanesulfonic acid (MSA)):

[0055] 1< H-NMR (dmso-d 6 , 400 MHz): δ (ppm) = 1.23, 2.09, 3.40, 3.58, 3.87, 4.86, 5.26, 6.66; 13<C-NMR (dmso-d 6 , 101 MHz): δ (ppm) = 21, 31, 67, 128, 150; ATR-IR: v (cm -1< ) ​​= 3238, 3006, 2611, 2860, 2345, 2116, 1703, 1603, 1779, 1452, 1286, 1222, 1117, 1030, 858, 783, 558.

[0056] The Figure 3 Figure 1 shows the molecular weight distributions of the reaction products according to the invention from Examples 1 and 4 in comparison with conventional untreated lignin. Table 1 lists the molecular weights of the reactants and products of the reaction according to the invention. In particular, the weight-average molecular weight of the reaction of hardwood lignin with 2,6-bishydroxymethyl p-cresol and 20 mol% methanesulfonic acid (MSA) increased by more than a factor of 100 to 57,000 g / mol. Example 4a (Polycondensation of fractionated Alcell hardwood lignin with 2,6-bis(hydroxymethyl)-p-cresol)

[0057] Hardwood lignin was suspended in ethanol for 15 minutes and filtered off. The low molecular weight lignin dissolved in the solution. Extraction with ethanol yielded a fractionated lignin with a number-mean molecular weight of 2200 gmol⁻¹ (original 500 gmol⁻¹) in a 66 wt% yield. Polycondensation of this fractionated lignin with 2,6-bishydroxymethyl- p -cresol under the same conditions as in Example 4 led to a product with a M n of 33,000 gmol -1< as well as a M w of 133,000 gmol -1< .

[0058] In this context, the following should be noted Figure 3Referenced: This shows the molecular weight distributions of hardwood lignin polycondensed with 2,6-bis-hydroxymethyl-p-cresol, untreated lignin, fractionated lignin, and fractionated hardwood lignin polycondensed with 2,6-bis-hydroxymethyl-p-cresol. The determinations were carried out by gel permeation chromatography (GPC) in N,N-dimethylacetamide (DMAc) / lithium bromide (LiBr) 5 g / l. Example 5 (Polycondensation of UPM Biopiva softwood lignin with 2,6-(hydroxymethyl)-p-cresol):

[0059] Softwood lignin (3 g) was dissolved in 1,4-dioxane (20 ml). Then, 2,6-bis(hydroxymethyl)-p-cresol (0.35 g) and p-toluenesulfonic acid (0.36 g) were added. The reaction mixture was stirred at 95°C for 22 h. After cooling to room temperature, the mixture was precipitated in water. For purification, the precipitate was extracted three times for 2 h each time with hot water to remove p-toluenesulfonic acid (TsOH) and 1,4-dioxane. Example 6(Preparing the spinning mass and spinning the material according to example 2):

[0060] The product (110 g) from Example 2 was dissolved in DMF (100 ml) by slow addition to the solvent over 24 hours with mechanical stirring. The solution was stored in a refrigerator at 5°C for one week and then degassed at room temperature for 4 hours at 100 mbar. Rheological measurements showed that the spinning mass exhibited shear thinning with viscosities between 100 and 1000 Pa·s in the frequency range of 100–0.1 1 / s (10% deformation). The bubble-free material was then continuously dry-spun into fine fibers at 20°C using a piston spinning machine and a multifilament spinneret (150 µm x 64). The exit velocity at the spinneret was 5 m / min, and the winding speed was 75 m / min. Example 7 (Stabilization and carbonization of the fibers according to example 6):

[0061] The lignin fibers were oxidatively thermostabilized. In this process, the fibers were heated from room temperature to 250°C in a muffle furnace (Nabertherm GmbH) at a heating rate of 0.25 K / min. The fibers were used as monofilaments. Carbonization was carried out in a high-temperature furnace (Gero) under pure nitrogen from room temperature to 1000°C at a heating rate of 10 K / min. Example 8 (Derivatization of the conversion product according to example 2):

[0062] Lignin (1 mol hydroxyl groups, 100 g) was placed in a 1 L single-necked round-bottom flask and dissolved in pyridine (500 mL) with stirring. Hexanoyl chloride (1 mol, 134.6 g) was added slowly dropwise via a dropping funnel. After complete addition, the reaction mixture was heated to 80°C for 3 hours and then stirred overnight at room temperature. For work-up, the reaction mixture was concentrated and precipitated in dilute hydrochloric acid (0.1 N). The resulting solid was washed with demineralized water and then stirred for 24 h with saturated aqueous sodium bicarbonate solution, followed by demineralized water and 0.1 N hydrochloric acid to remove excess hexanoic acid as sodium hexanoate and to extract any remaining pyridine as hydrochloride. The product was freeze-dried under reduced pressure at < 1 × 10⁻² mbar for 24 h. The result was a dark brown to black solid.

[0063] Table 1 below lists the molecular weights of the reactants and products from Examples 1 to 5. Table 1 M n [gmol -1< ] MW [gmol -1< ] PDI Lignin, untreated 500 2.800 5,1 p-cresol-resol 5.000 6.600 1,3 Polycondensed lignin ( p -Cresol-resol linkage compound, 1 part MSA to 3 parts lignin) 9.000 33.800 3,9 Polycondensed fractionated lignin (2,6-bis(hydroxymethyl)-p-cresol linkage compound, 1 part MSA to 3 parts lignin) 33.000 133.000 4,0 Polycondensed lignin (2,6-bishydroxymethyl- p -cresol linkage compound, 1 part MSA to 3 parts lignin)) 3.200 24.000 7,5 Polycondensed lignin (2,6-bishydroxymethyl- p -cresol linkage compound, (2 parts MSA to 3 parts lignin) 4.600 57.000 12,0 Polycondensed lignin (UPM Biopiva) softwood (2,6-bishydroxymethyl- p- cresol coupling compound, 2 parts 4.800 40.800 8,4 p -TsOH to 3 parts lignin) Polycondensed Alcell hardwood methyl lignin (2,6-bishydroxymethyl- p -cresol coupling compound, 2 parts p -TsOH to 3 parts lignin) 5.000 37.600 7,5

[0064] Note: The molecular weights were determined by GPC measurements in DMAc / LiBr 5 gl -1< and conventional calibration against PMMA standards. The products according to the invention were always synthesized using a linker:lignin molar ratio of 1:2.85 and 1–2 mol of acid per 3 mol of lignin.

Claims

1. A modified lignin based on substituted and / or unsubstituted lignin, wherein the lignin molecules of the substituted and unsubstituted lignin (A) are linked to linking units (B) to increase the molecular weight, characterized in that the linking units (B) are aromatic and are linked to the lignin molecules (A) via a methylene group, wherein the linking unit (B) is derived from a reaction of the lignin (A) with an aromatic linking compound (C) which contains at least two hydroxymethyl groups, and wherein the modified lignin has a number-average molecular weight of 2000 to 50,000 g / mol, the number-average molecular weight being determined by means of the methods in the description.

2. The modified lignin according to claim 1, characterised in that the substituted lignin is represented by the formulae         L-O-CORx     (I) and / or         L-ORy     (II), in which: the residue L-O in formulae (I) and (II) represents an esterified or etherified lignin moiety, the moiety -CO-Rx in formula (I) represents an acyl moiety, the moieties Rx and Ry represent an aliphatic and / or aromatic moiety.

3. The modified lignin according to claim 2, characterised in that the aromatic moieties Rx and Ry represent an aryl moiety, in particular a phenol, benzyl, methoxybenzyl and / or a 3-phenylpropenyl moiety.

4. The modified lignin according to claim 2, characterised in that the aliphatic moieties Rx and Ry represent a straight-chain or branched C1-6 alkyl moiety, in particular a methyl, ethyl, n-propyl, isopropyl, tert-butyl or neo-pentyl moiety.

5. The modified lignin according to at least one of the preceding claims, characterized in that the linking unit (B) is based on a reaction of the lignin (A) with an aromatic linking compound (C) which has at least two terminal hydroxymethyl groups and is monomeric, oligomeric and / or polymeric, in particular a 2,6-bis(hydroxymethyl)-p-cresol, a p-cresol-based resol, phenol-based resol and / or a novolak-like compound with terminally arranged hydroxymethyl groups.

6. The modified lignin according to at least one of the preceding claims, characterised in that the molar ratio of the linking unit (B) to the lignin molecules (A), substituted or unsubstituted, is about 1:1 to 1:7, in particular 1:2 to 1:4, and particularly preferably about 1:3.

7. The modified lignin according to at least one of the preceding claims, characterised in that its ash content, determined according to DIN EN ISO 3451-4, is less than 0.1% by weight.

8. The modified lignin according to at least one of the preceding claims, characterised in that the modified lignin has a number-average molecular weight of 4000 to 40,000 g / mol.

9. A process for the preparation of a modified lignin according to claim 1-8, characterized in that, the substituted and / or unsubstituted lignin (A) is dissolved in a solvent and reacted in the solution with an aromatic linking compound (C) which has at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, in the presence of an organic acid (D) at elevated temperature, in particular from 60°C to 160°C, to give modified lignin.

10. The process according to claim 9, characterised in that the reaction is carried out for 5 to 24 h, in particular 7 to 15 h.

11. The process according to at least one of claims 9 or 10, characterised in that a substituted lignin (A) is reacted according to the definitions of the preceding claims.

12. The process according to at least one of claims 9 to 11, characterised in that the linking compound (C) is monomeric, oligomeric, and / or polymeric, and represents in particular a 2,6-bis(hydroxymethyl)-p-cresol, a p-cresol-based resol, phenol-based resol, and / or a novolak-like compound with terminally arranged hydroxymethyl groups.

13. The process according to at least one of claims 9 to 12, characterised in that the oligomeric and / or the polymeric linking compound (C) has a polydispersity below 2, in particular below 1.5.

14. The process according to at least one of claims 9 to 13, characterised in that the linking compound (C) and the lignin (A) are reacted to modified lignin in a molar ratio of about 1:1 to 1:7, in particular 1:2 to 1:4, particularly preferably of about 1:3.

15. The process according to at least one of claims 9 to 14, characterised in that the organic acid (D), in particular in the form of p-toluenesulfonic acid (p-TsOH) and / or methanesulfonic acid (MSA), has a pKs value of < 3, in particular of < 2.

16. The process according to at least one of claims 9 to 15, characterised in that about 0.2 to 1 mol, in particular about 0.5 to 1 mol, of organic acid (D) are used per 1 mol of lignin (A).

17. A use of the modified lignin obtainable by a process according to at least one of claims 9 to 16 for the production of precursor fibres by melt spinning, in particular with the inclusion of a softener, or by wet, dry-wet, or dry spinning.

18. The use of the modified lignin according to claim 17, characterized in that, for the purpose of wet, dry-wet, and dry spinning into precursors, it has 1. a complex viscosity in solution in the range from 10 to 500 Pa.s, in particular from 50 to 300 Pa.s, as determined according to DIN 53019, and / or 2. a number-average molecular weight in the range from 2000 to 50,000 g / mol, in particular from 4000 to 40,000 g / mol.

19. The use of the modified lignin according to claim 17, characterized in that, for the purpose of melt spinning into precursor fibres, it has 1. a complex viscosity in the range from 50 to 2000 Pa.s, in particular from 100 to 1000 Pa.s, as determined according to DIN 53019-4, and / or 2. a glass transition temperature (Tg) in the range of 80°C to 220°C, in particular of 100°C to 180°C, as determined by differential scanning calorimetry according to DIN EN ISO 11357-2, and / or 3. a number-average molecular weight in the range of 2000 to 15,000 g / mol, in particular of 4000 to 10,000 g / mol.

20. The use of the modified lignin according to any one of the preceding claims 17 to 19, according to which the precursor fibres are used for the production of carbon fibres by carbonisation, optionally preceded by oxidative thermostabilization, and optionally followed by graphitisation.