RU2790986C2 - Modified lignin, method for its preparation and application - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention relates to modified lignin based on substituted and/or unsubstituted lignin, and the lignin molecules in substituted and unsubstituted lignin (A) are connected by binding blocks (B) to increase the molecular weight. Lignin differs in that the binding blocks (B) are aromatic and are connected to lignin molecules (A) by means of a methylene group. The binding block (B) comes from the interaction of lignin (A) with an aromatic binding compound (C), which contains at least two hydroxymethyl groups. The specified modified lignin has an average calculated molecular weight from 2,000 to 50,000 g/mol. The invention also reveals a method for producing modified lignin. In addition, the invention relates to the use of modified lignin to produce fiber precursors for wet, dry-wet or dry spinning. In addition, the use of modified lignin for the production of fiber precursors by spinning from a melt is disclosed.

EFFECT: obtaining carbon fibers with valuable properties, in particular, with respect to tensile strength, as well as elastic modulus, by means of preferred fiber precursors.

24 cl, 3 dwg, 1 tbl, 8 ex

Description

The invention relates to a modified lignin based on substituted and unsubstituted lignin (A), wherein the molecules of substituted and unsubstituted lignin (A) are connected by binding blocks (B) to increase the molecular weight, to a method for producing modified lignin and its use for obtaining fiber precursors, which are then can be converted into carbon fibers.

There is a growing demand for carbon fibers to produce lightweight structural parts from carbon fiber reinforced synthetic materials (CFRP). In particular, high growth rates are expected in such a mass market as the automotive industry. The high prices so far, in particular in the manufacture of these lightweight structural CFK parts, as well as the relatively high prices of carbon fibers, are due to the fact that such products are not yet generally available to the mass market. In the field of manufacturing lightweight structural CFK parts, promising methods have been developed in recent years which enable fast, precise, automated and thus favorable manufacturing. In the case of carbon fibers, some efforts have been made so far to keep prices down while maintaining quality. This concerns both the optimization of the production method and the use of precursor materials. Currently, carbon fibers are mainly (>95%) formed from polyacrylonitrile. It is obtained by the method of synthesis in solution by polymerization of acrylonitrile with a small part of the comonomers. Acrylonitrile itself is obtained under the conditions of the Sohio process and is synthesized from ammonia and petroleum-derived propene. The transformation into polymeric fibers is carried out by means of a special wet spinning process. It requires large amounts of additional solvents and precipitants and is also very slow.

Therefore, many research efforts are focusing on the development of alternative precursors from renewable, oil-independent feedstocks for carbon fibers. These include biopolymers cellulose and lignin. In particular, lignin is an attractive potential precursor since it naturally has a high content of aromatic structures and thus also provides a high yield of carbon. Lignin does not compete with food and is formed during the manufacture of paper, and is a waste from biorefining. A significant problem when using lignin as a raw material is the possibility of significant variations in its properties and purity, as well as the chemical composition and structure of these biopolymers. For the use of lignin as a polymer, it turned out to be significant that lignin, as a by-product of the pulp and paper industry, although it has a polymeric part, however, on average, we are talking about low molecular weight oligomers with a very low degree of polymerization and a highly branched, partially crosslinked structure. This is due to the fragmentation transformations that occur when the wood is cooked. In fact, the goal here is to obtain as intact as possible cellulose in the form of a polymer, for which the lignin surrounding it is chemically cleaved. Therefore, it has a low molecular weight and high polydispersity. To use lignin as the polymeric material, membrane filtration or fractional settling can be used to obtain fractions with desired molecular weights and properties. Unfortunately, this results in a high proportion of waste. The product, as before, has a highly branched structure. The low molecular nature of lignin is manifested primarily in mechanically weak load capacity and brittleness.

For the use of lignin as a starting material for carbon fibers, a structure that is as linear and polymeric as possible is desirable. This structure allows the molecules to align along the fiber axis during the spinning process and increase the mechanical load capacity of both the precursor fiber and the carbon fiber formed from it. In particular, the still too low mechanical load capacity of the precursor fiber is largely responsible for why there is no lignin-based carbon fiber as a market product to date. Precursor fibers are brittle and inflexible, which, among other things, is due to the low molecular weight. The addition of flexible auxiliary polymers is advisable only to a small extent, since as a result, as a rule, the yield of carbon from the entire material decreases, and the quality of the carbon fiber deteriorates sharply due to the defects caused by this. Therefore, based on the prototype, it is necessary to develop a method that allows you to controllably build a linear structure with an increase in the molecular weight of lignin as a carbon fiber precursor. Previous methods for polymerizing lignin, and thus producing polymers with covalently bonded lignin, do not generally focus on their use as a carbon fiber precursor and are therefore unsuitable. Some of these methods should be briefly presented.

US Pat. No. 4,769,434 A describes in detail the basic catalyzed condensation of lignin with divalent electrophiles. The electrophiles mentioned in it are aldehydes, hydroxymethylated amines, phenolic compounds, and isocyanates. US Pat. No. 3,984,363 A discusses the epoxidation of lignin under alkaline conditions and subsequent curing using (polymeric) amines. Also discussed as crosslinking compounds are carboxylic acid anhydrides, as well as the esterification of lignin with the formation of double bonds suitable for crosslinking. In contrast, US Pat. No. 5,102,992 A discloses the preparation and use of polymerizable prepolymers from lignin, wherein alkylene oxide-modified lignin is reacted with an acrylate-functional compound to form polymerizable prepolymers. An additional field of polymers from lignin are polyurethanes based on lignin. According to US Pat. No. 9,598,529 B2, a polyurethane precursor is mixed with lignin and heated, causing the mixture to polymerize. Lignin is also reacted with isocyanates in the presence of polyols to form a polymer (see US 6,025,452 A). As another area of lignin polymers, copolymers of lignin and polylactic acid (PLA) can be mentioned. Thus, for example, US 20130281582 Al describes how an ester of lignin with butyric acid is heated in the presence of polylactic acid to obtain a PLA-grafted lignin copolymer.

A similar chemical approach is described in US Pat. No. 9,567,432 B2, according to which graft copolymers of lignin and polylactide are synthesized. They can react with crosslinking agents such as hexamethylene diisocyanate, for example. The radical polymerization of lignin to form a polymer is presented in US 5,608,040 A, which forms polymers from lignin and organic compounds with at least 3 carbon atoms in the presence of radical oxidizable enzymes. Two less specific methods to obtain polymeric lignin as a precursor for carbon fibers are disclosed in US 20130255216 Al. Oxypropylation of lignin followed by propargylation in multiple steps results in a lignin-based product formed by chain extension (H. Sadeghifar, S. Sen, SV Patil, DS Argyropoulos, ACS Sus. Chem. Eng ., 2016, vol. 4, pp. 5230-5237 ). In addition, thermally stable copolymers of poly(aryleneethersulfones) with lignin have been reported, which are prepared from lignin and aromatic difluorodiphenylsulfone (DFDPS). The product has improved thermal stability compared to untreated lignin (DS Argyropoulos, H. Sadeghifar, C. Cui, S. Sen, ACS Sus. Chem. Eng ., 2014, vol. 2, pp. 264-271). All of these processes result in a largely highly branched product or graft polymer which cannot be linearized based on lignin. Moreover, aliphatic compounds such as polylactic acid are used as copolymers and comonomers. During the thermal conversion of lignin to carbon fibers, these aliphatic compounds lead to small residual masses of carbon and, accordingly, low yields. The consequence of this is a large loss of materials during the thermal transformation into carbon fibers, but also a high porosity of the carbon fibers and thus poor mechanical properties thereof.

The invention is based on the task of eliminating the disadvantages described above in connection with the prototype, in particular, to propose a modified lignin of the type indicated at the beginning, which has a linear orientation and can be advantageously converted into carbon fibers with a high carbon yield, low porosity and improved mechanical characteristics of carbon fibers. .

According to the invention, this problem is solved by means of a modified lignin of the type indicated at the outset, which is characterized in that the linking blocks (B) are aromatic and are linked to the lignin molecules (A) via a methylene group.

Suitable unsubstituted lignin (A) are, in particular, hardwood lignin, softwood lignin or cereal straw lignin, whether underivatized or derivatized. Suitable pulp lignins are, in particular, all conventional forms, organo-solvent delignification lignin and kraft lignin being preferred.

In various situations, it is preferred that the modified lignin according to the invention is derived from a substituted lignin, the substituted lignin being represented in particular by the formulas

and/or

in which they represent: the residue L-O in formulas (I) and (II) is an esterified or esterified lignin residue, the residue -CO-R _x in formula (I) is an acyl residue, the residues R _x and R _y are an aliphatic and/or aromatic residue. For this, specific preferred embodiments are available. Thus, one such embodiment is characterized in that the aromatic radicals R _x and R _y represent an aryl radical, in particular a phenolic, benzyl, methoxybenzyl and/or 3-phenylpropenyl radical. Furthermore, it is preferred that the aliphatic radicals R _x and R _y represent a straight chain or branched C _1-6 alkyl radical, in particular a methyl, ethyl, n-propyl, isopropyl, tert-butyl or neopentyl group.

With respect to the derivatives of formulas (I) and (II) described above, short chain methyl (C1)- and ethyl (C2) ethers and lignin acetyl esters have proved to be favorable, as already shown, with the methyl ethers according to the formula ( II). In this embodiment, lignins from the organo-solvent delignification process are preferred in particular, which are already partly esterified and/or esterified during their production process, for example by treatment with acetic acid. Also considered as a lignin derivative is, in particular, phenolized lignin, in which phenol or phenol derivatives are covalently bonded to lignin at reactive aromatic positions, as a result of which the number of reactive positions on the lignin increases. In this case, phenol, as a rule, replaces the aliphatic hydroxyl groups of lignin and corresponds to the formula L-O-phenol (III). Considerations regarding the substitution of the hydroxyl groups of the lignin derivative in the above formulas (I) and (II) are also important. It turned out to be preferable that 1% to 100%, preferably 10% to 90%, in particular 30% to 80% of the hydroxyl residues of the lignin according to formulas (I) and (II) are substituted. With regard to phenolized lignin, it is recommended to replace phenol and, accordingly, phenol derivatives between 1% and 80%, preferably from 10% to 60%, in particular between 20% and 50%, of aliphatic hydroxyl groups according to the formula L-O-phenol (III ).

An essential part of the invention is the connecting block (B). It is aromatic in the broadest sense and is linked to the lignin (A) molecules, substituted or unsubstituted, via a methylene group. In the process according to the invention, it is introduced into the modified lignin according to the invention by means of a binder compound (C). As for the details regarding the binder compound (C), reference should be made to the following explanations accordingly.

With this in mind, it turned out to be preferable that the binder block (B) comes from the interaction of lignin (A) with an aromatic binder compound (C), which has at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, and is a monomer, oligomer and/or a polymer, in particular 2,6-bis(hydroxymethyl)-para-cresol, para-cresol-based resol, phenol-based resol and/or a hydroxymethyl-terminated Novolac type compound.

It turns out to be favorable when attention is paid to the molar ratio of the binder block (B) to the lignin molecules (A). Thus, preferably, when the molar ratio of the binder block (B) to the lignin molecules (A), substituted or unsubstituted, is from about 1:1 to 1:7, in particular from 1:2 to 1:4, and particularly preferably about 1:3.

The modified lignin (A) according to the invention exhibits numerous important beneficial properties:

Thus, the modified lignin is characterized in particular in that its ash content, determined according to DIN EN ISO 3451-4, is less than 0.1% by weight. This circumstance is important because it is known that a high content of foreign ions can have a detrimental effect on the structure of carbon and the stability of carbon fibers. Therefore, it is advantageous when the ash content is maintained in the above preferred rule, whereby less than 0.1 wt.% and/or less than 1000 ^ppm foreign ions are present, in particular less than 500 ^ppm foreign ions, in particular in the form alkali metal ions. An accurate quantification of the salts and metals contained in the modified lignin according to the invention can be performed using ICP-MS (Inductively Coupled Plasma Mass Spectrometry), after suitable calibration. There is a very low detection limit here.

Of particular importance is the complex viscosity of the modified lignin according to the invention, in each case according to the specific purpose. Thus, it is preferred that the modified lignin for the purpose of wet, dry-wet and dry spinning has a complex viscosity in solution from 10 to 500 Pa.s, in particular from 50 to 300 Pa.s, determined according to DIN 53019-4, while for the purpose of melt spinning, the complex viscosity is preferably from 50 to 2000 Pa.s, in particular from 100 to 1000 Pa.s, determined according to DIN 53019-4,

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

In principle, the number average molecular weight also plays a significant role according to the invention: for example, the lignin according to the invention is characterized in that it preferably has a number average molecular weight of 2,000 to 50,000 g/mol, in particular 4,000 to 40,000 g/mol. In this case, the weighted average molecular weight varies between 10,000 and 250,000 g/mol, in particular from 20,000 to 100,000 g/mol. The above guidelines help the person skilled in the art to adjust the structural characteristics of the product according to the invention, such as desired linearity, degree of branching and crosslinking. They can be expertly controlled by adjusting the optimum stoichiometric ratios between lignin (A) and binder (B).

Thus, in the context of the practice of the invention, it is also important when attention _is paid to the number average molecular weight (M _n ) of the starting lignin and, accordingly, of the lignin A derivative. /mol, in particular from 300 to 10000 g/mol. The weight average molecular weight (M _w ) preferably varies between 1000 and 40000 g/mol, in particular between 2000 and 20000 g/mol. The lignin and the lignin derivative used also comprise fractions of the lignin and the lignin derivative (A) respectively, which are obtained by fractionating the lignin and the lignin derivative, respectively, in particular according to particle size. Thus, fractionated precipitation of lignin, which can be carried out already in the process of obtaining and, accordingly, isolation of lignin, may be appropriate, as well as sequential extraction of lignin, as well as ultrafiltration of lignin solutions. The fractionated lignin thus has a lower polydispersity than the original lignin, which facilitates the calculation of the conversion stoichiometry in the context of the process according to the invention described below, as well as a starting molecular weight that is as high as possible. It is therefore preferred that the lignin (A), substituted or unsubstituted, used has as little polydispersity as possible, preferably <8, in particular <6. In order to enable the process according to the invention to be carried out in a favorable manner and, accordingly, the interaction of the lignin (A) with the binder compound (C), reactive aromatic positions in the lignin (A) are preferred. With regard to these positions, we can talk about: aromatic positions occupied by hydrogen atoms (H) in the α-position to the phenolic hydroxy group of lignin (respectively, the guaiacyl fragment), as well as aromatic positions occupied by hydrogen atoms (H) in the ortho-position to ether groups lignin, and in relation to these ether groups, as a rule, we are talking about naturally occurring methoxy groups, as well as lignin-binding ether bridges. Free aromatic positions of phenol and phenol derivatives, which were covalently bound by lignin during its phenolization, are also considered here as reactive positions. Therefore, it is preferable to detect the minimum number of these free positions.

Intended use also plays a role here, ie whether the modified lignin is to be used for wet, dry-wet or dry spinning, or for melt spinning of fiber precursors. Thus, preferably, for the purpose of wet, dry-wet and dry spinning to form fiber precursors, the number average molecular weight is from 2000 to 50000 g/mol, in particular from 4000 to 40000 g/mol. If, however, it is used for melt spinning of fiber precursors, the number average molecular weight is advantageously between 2000 and 15000 g/mol, in particular between 4000 and 10000 g/mol.

In addition, the subject of the invention is a method for producing modified lignin, in particular a modified lignin of the type described in detail above according to the invention, which is characterized in that the substituted and/or unsubstituted lignin (A) is dissolved in a solvent and reacted in the solution with an aromatic binder a compound (C) which has at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, in the presence of an organic acid (D) at an elevated temperature, in particular from 60°C to 160°C, to form a modified lignin.

The essential reactants in the process according to the invention are the following: lignin (A), substituted and/or unsubstituted in the sense of the invention, is reacted with an aromatic binder compound (C) which has at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups . With respect to lignin (A), reference should be made to the above embodiments with the formation of the modified lignin according to the invention. The aromatic binder compound (C), when reacted according to the process according to the invention, leads to the formation of the binder block (B) of the modified lignin according to the invention. The binder compound (C) is characterized in that, as emphasized, it has at least two terminal hydroxymethyl groups. It is aromatic in nature and may be monomeric, oligomeric or polymeric in nature. Preferred examples have been given above, which should be referred to. Thus, these compounds are distinguished by the hydroxymethyl groups introduced at the ends. The images shown below represent the chemical structures of possible binders based on 2,6-bis-hydroxymethyl-p-cresol (n=1) and p-cresol resol.

The second formula preferably holds the ratio 1<m+n<40, in particular 1<m+n<10, and most preferably 1<m+n<5.

In principle, we can talk about an aromatic ring system, which is based on the benzene ring. Other (condensed) aromatic systems may also be involved, in particular naphthalene, anthracene and the like. As shown in the above images, at least two terminal hydroxymethyl groups are preferably placed in the meta position relative to each other. In principle, a para-position of these two groups would also be possible, with the ortho-position not being favorable due to spatial difficulties. Finally, suitable substituents in the aromatic system are questionable. Such a statement of the question does not constitute the essence of the present invention. The person skilled in the art can easily select suitable substituents here, such as hydroxyl groups or lower alkyl groups, in particular an alkyl group with from 1 to 6 carbon atoms.

In addition, with regard to the above chemical structures of the binder (C): the core of the repeating structural unit in these binders form aromatic monomers such as phenol, cresol, hydroxyphenol and also aromatic systems that do not belong to benzene, such as furan , pyrrole, thiophene, and other aromatic heterocycles.

Here para-cresol proved to be the most preferred. In principle, the number of repeating structural units is possible between 1 and many hundreds, however, the ranges from 1 to 60, preferably from 1 to 40, in particular from 1 to 20, are favorable. cresol with the number of repeating structural units equal to 1, according to the invention, occupies a special place, since the reaction described later proceeds with an increase in molecular weight in an extremely satisfactory way, and constitutes only a small mass fraction in the total mass of the product. The polydispersity of the oligomeric and/or polymeric binder should preferably be as low as possible, since an optimized calculation of the stoichiometric ratios between lignin (A) and binder (C) can thus be carried out.

In general, it can be seen that the process according to the invention is advantageously controlled by reacting the lignin (A), whether unsubstituted or substituted, with an aromatic binder compound C under experimentally controlled stoichiometric conditions of the compound in the presence of an organic acid (D) . In this case, the main contribution is made by the chemical functional groups essential for the interaction in the binder compound (C), and not to the same extent in lignin (A). Therefore, the interaction of lignin A with itself essentially does not occur. The transformation is essentially controlled by the controlled addition of a binder (C). Thus, as a result of this transformation, products based on lignin can be obtained, in particular, with an increase in more than 20 times the number average molecular weight. With optimally adjusted stoichiometric ratios, the product obtained in this way has an uncrosslinked and highly linear structure. In particular, the reaction product by partial evaporation of the solvent used can be directly transferred to the spinning mass. However, it is also possible to carry out evaporation until a solid is formed, in order to then dissolve it in another suitable solvent, in order to obtain the most technologically suitable spinning mass in this way. But here, too, it is possible to carry out a chemical functionalization in a special way in order to make this product meltable and thus recyclable by melt spinning.

It is particularly advantageous when the reaction is carried out at a temperature of from 80°C to 120°C, in particular from 90°C to 110°C. The reaction, which is based on the method according to the invention, can be carried out both at atmospheric pressure and at elevated pressure in a steel autoclave, a non-oxidizing atmosphere, for example under nitrogen, being preferred. The duration of the interaction is not significant. However, it has proven to be preferable when the reaction is carried out for a period of 5 to 24 hours, in particular 7 to 15 hours. In this case, for a particularly favorable conversion, the duration of the reaction depends on various factors, such as, for example, the grade and type of lignin (A) and binder (C) used.

Since water is released during the reaction, it is preferable to add a water absorbent to the reaction medium in order to remove the water formed during the reaction. The addition of water absorbents can serve to speed up the reaction, since water is formed as a condensation product. In cases where 1,4-dioxane is used as a solvent and a polar aprotic solvent as a reaction medium, they perform this function to some extent. If the solvent is not sufficiently hygroscopic, for example, molecular sieves (eg 3A) or a water binding system can be used to remove the water formed during the condensation and thus during the reaction and thereby accelerate the conversion.

With regard to the specific object of the invention and, accordingly, the requirements related to this in the method according to the invention, substituted lignin (A), but also unsubstituted lignin (A) can be used as starting material. For this, reference should be made to the above explanations regarding unsubstituted and substituted lignin. The aromatic binder compound (C) used in the process according to the invention is characterized, as already discussed, in particular in that it has at least two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, which is particularly true for oligomeric and monomeric aromatic binders (C). The rationale for this is that the use of two terminal hydroxymethyl groups can achieve linear chain extension, while the multiple hydroxymethyl groups in compound C lead to branching.

As may already be understood, the binder compound (C) may be monomeric, oligomeric and/or polymeric. These are, in particular, 2,6-bis-(hydroxymethyl)-para-cresol, para-cresol-based resole, phenol-based resole and/or a hydroxymethyl-terminated novolac type compound. In the practice of the process according to the invention, it has been found that the oligomeric and/or polymeric binder compound (C) advantageously has a polydispersity below 2, in particular below 1.5. The rationale for this is that, in particular, an exact calculation of the stoichiometric ratio in the reaction of lignin and binder can be achieved, and thus a precise control of the properties of the product formed according to the invention by the method according to the invention.

When carrying out the process according to the invention, attention must also be paid to the molar ratio of binder compound (C) to lignin (A), whether substituted or unsubstituted. Thus, it is preferred that the binder compound (C) and the lignin (A) are reacted to form the modified lignin in a molar ratio of from about 1:1 to 1:7, in particular from 1:2 to 1:4, particularly preferably about 1:3.

When choosing an organic acid (D) for a specialist there is no critical limitation. He can easily determine by experience and accordingly use a particularly suitable organic acid (D). It is particularly advantageous when the organic acid (D), in particular in the form of p-toluenesulfonic acid (p-TsOH) and/or methanesulfonic acid (MSA), has a pK _s value of <3, in particular <2.

Next, a specific presentation of the method according to the invention will be given to explain it in detail. In doing so, reference is made to particularly useful experimental data which have been obtained in connection with the invention.

In general, it has been shown that, in particular, the reaction of about 8.5 weight parts of non-derivatized Alcell lignin as lignin (A) with about 1 weight part of 2,6-bis-hydroxymethyl-p-cresol as component (C ) in the presence of about 1.0 parts by weight of methanesulfonic acid as organic acid (D) results in a very favorable product for the purpose of use as a polymeric linearized precursor of increased molecular weight carbon fibers, the solvent being 1,4-dioxane, and the duration reaction is about 7 hours at a temperature of about 100°C. This reaction product does not have branched structures and completely and easily dissolves in the solvents used for the interaction. Partially soluble, up to cross-linked reaction products are obtained due to weight ratios of 2,6-bis-hydroxymethyl-para-cresol to lignin (A) above 1:8.5, such as 1:6, that is, with a greater number of weight parts of the binder compound (C) per part of the lignin (A).

Based on the fact that not only monomeric compounds (C) but also polymeric compounds (C) are suitable, it is recommended to set not only the weight ratios of the reaction components to each other, but also the molar ratios of lignin (A) to the hydroxymethyl groups of the binder compound (C). The following ratio is therefore considered to be preferred: about 10 g of lignin (A) at number average molecular weight correspond to 20 mmol of lignin (A) used. From this, according to the above embodiment, follow 7 mmol of the binder compound (C) containing two terminal hydroxymethyl groups, which corresponds to approximately 3.5 mmol of hydroxymethyl groups. Therefore, it has proved useful to represent favorable molar proportions between binder (C) and lignin (A) (lignin and/or lignin derivative) as follows: preferably, 1 weight part of binder (C) with two reactive terminal hydroxymethyl groups is from 1 to 6, in particular 2 to 4 parts by weight of lignin (A), particularly preferably 3 parts by weight of lignin (A). The amount of the compound (D), that is, the organic acid (D), can likewise be expressed as follows: for 3 parts by weight of lignin, there are about 2 parts by weight of the compound (D), in particular when it is monofunctional.

The molar ratio between lignin (A) and organic acid (D) must also be optimized. Therefore, it is preferable that about 0.2 to 1 mole of organic acid (D) is used per 1 mol of lignin (A), in particular 0.5 to 1 mol.

The method according to the invention is carried out using a solvent which can be easily determined experimentally, in particular for dissolving lignin (A), substituted and/or unsubstituted. For this, a large number of suitable solvents are available to the skilled person. Especially 2,4-dioxane, mixtures of 1,4-dioxane and water, tetrahydrofuran (THF), but in particular also polar aprotic solvents such as dimethylformamide (DMF), dimethylsulfoxide (DMSO) and dimethylacetamide (DMAc ). The cyclic ether 1,4-dioxane, which is capable of extensive hydrogen bridging with lignin and has a boiling point of only 101° C. under normal conditions, has proven to be particularly suitable, and is also highly miscible with water.

In principle, it is still possible to further optimize the process according to the invention by using additional polymers, in particular with regard to improving the spinning characteristics and improving the mechanical properties of the fiber precursors obtained from the reaction product in the process according to the invention. It can be advantageous here, when the modified lignin according to the invention is transferred to the spinning mass, that it contains an auxiliary polymer E. ), polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP). Cellulose esters such as, for example, 2,5-acetate and cellulose acetate butyrate have proven to be particularly suitable. In addition, it has proven advantageous when the auxiliary polymer (E), based on the weight of the modified lignin according to the invention, is present in an amount of at least 5% by weight, in particular from 10 to 30% by weight.

For a further explanation of the method according to the invention, reference is made to the following FIG. 1. It shows the reaction scheme in the reaction carried out according to the invention and a typical possible chemical structure of lignin (A) and the structure of the binder compound (C). Low molecular weight lignin (A) and/or lignin derivative (A) is reacted with a monomeric, oligomeric or polymeric binder compound in dissolved form in the presence of an organic acid to form high molecular weight lignin.

The particular significance of the lignin according to the invention and, accordingly, the modified lignin obtained in the manner described, is that it can be flexibly used for the preparation of fiber precursors. Thus, it is generally true that modified lignin can be advantageously used for the production of fiber precursors for melt spinning, in particular using a plasticizer, in particular in the form of monomeric compounds, in particular DMF, DMSO, phenol, resorcinol, vanillin. and phthalic acid esters, or for wet, dry-wet or dry spinning.

Fiber precursors are particularly suitable for the production of carbon fibers by charring, optionally with preliminary oxidative thermal stabilization and, if necessary, with subsequent graphitization. These preferred possibilities for using modified lignin include in particular: for the purpose of fiber spinning, a non-crosslinked product with as much as possible a linear structure is particularly advantageous, since the preferred molecular orientations in the fiber precursor can thus be better achieved. It is particularly advantageous to orient the linear chain molecules along the fiber axis in order to obtain a mechanically stable fiber precursor. The orientation is also maintained in the final carbon fibers by heat treatment, which are thus also given improved properties. The product must therefore be completely uncrosslinked and readily soluble in the solvents described above, in particular 1,4-dioxane, mixtures of 1,4-dioxane and water, tetrahydrofuran (THF), and in polar aprotic solvents, in particular such as DMF, DMSO and DMAc. The spinning mass, which contains the modified lignin according to the invention, depending on the embodiment, can be processed either by the melt spinning process, as shown, by melt spinning, or by solution spinning. The spinning mass for solution spinning contains the modified lignin according to the invention and a suitable solvent, which may be either used directly in the reaction mixture during synthesis or introduced specially. Other polymeric compounds may additionally be added to the spinning mass.

As solvents, the solvents already discussed above are particularly suitable, and of course also ionic liquids. These solvents are preferably 1,4-dioxane, mixtures of 1,4-dioxane and water, tetrahydrofuran (THF), and polar aprotic solvents such as DMF, DMSO and DMAc. The solvent in the form of dimethylformamide (DMF) has proven to be particularly useful in the practice of the invention. The complex viscosity of the spinning mass at the treatment temperature is preferably between 10 and 500 Pa.s, in particular between 50 and 300 Pa.s. In this case, the loss modulus exceeds the accumulation modulus, determined by rheological measurements in the frequency range from 1 to 100 1/sec and a strain constant of 10%.

The spinning mass preferably does not contain particles and, in particular, should not have a gelatinous character. The preferred spinning mass in terms of the concentration of the lignin modified according to the invention in the solvent is determined as follows: for example, it has proven to be favorable for fiber spinning when about 1 weight part of lignin is between about 0.5 and 3, preferably between 0.5 and 2, particularly preferably between 0.8 and 1.0 weight parts of the solvent. This is true for both the solution spinning possibilities mentioned above, in particular in the form of wet spinning, air-gap spinning or dry spinning. In order to use the lignin modified according to the invention in the form of a spinning mass for processing under melt spinning conditions, it is preferable to carry out a simple chemical derivatization of it. This involves masking transformations of the hydroxyl groups of the product, preferably a simple esterification using carboxylic acids and carboxylic acid derivatives. In this case, the introduction of hexanoyl (C ₆ ) residues proved to be especially effective. As a rule, we are talking about the formula LO-CO-R _x (IV), where L represents the (polycondensed) lignin (A) according to the invention, and x preferably represents an aliphatic C ₃ -C ₁₁ residue. In this case, aliphatic residues that contain unsaturated fragments are of particular importance. The content of non-esterified hydroxyl groups still present in the product according to the invention can vary between 1% and 100%, preferably from 1% to 90%, in particular from 30% to 60%.

The aim of the present invention is to obtain, by means of the preferred fiber precursors, carbon fibers with valuable properties, in particular with regard to tensile strength as well as modulus of elasticity, in a favorable technological way. In this case, it is expedient to first subject the fiber precursors according to the invention to conventional oxidative thermal stabilization. The thermally stabilized material is then subjected to charring in the usual manner to produce carbon fibers. This may be followed by graphitization if necessary.

Oxidative thermal stabilization is preferably carried out to a final temperature of from 100°C to 400°C, in particular from 200°C to 300°C. Preferred heating rates for oxidative heat setting are not critical. The heating rate is preferably kept below 10 K/min, in particular below 5 K/min, the minimum heating rate being in particular at least 0.05 K/min. It may be advantageous to expose the heat stabilized fiber precursors to crosslinking with high energy radiation. Crosslinking can be performed with high energy radiation, preferably by means of ultraviolet (UV), vacuum ultraviolet (VUV), electron beam, x-ray or gamma radiation. The properties of the fiber precursors obtained according to the invention have a significant influence on the properties of the carbon fibers formed from them. The resulting stabilized fiber precursors are then charred to form carbon fibers. If necessary, they are then graphitized.

During charring, which is carried out at gradually increasing temperatures between 300° C. and 1700° C., in particular from 1000° C. to 1500° C., in particular in a nitrogen atmosphere, the carbon content constantly increases and reaches between about 85-95%. As a result of subsequent graphitization, the carbon content of the carbon fibers can be increased to >95%. The optionally carried out graphitization is carried out by heat treatment at temperatures from 1700° C. to 3000° C., in particular from 2000° C. to 2500° C., in a protective gas atmosphere.

Hereinafter the invention will be further explained by means of examples:

Example 1 (Polycondensation of Alcell Hardwood Lignin with 2,6-bis-(hydroxymethyl)-p-cresol (laboratory scale))

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

For an explanation of the modified lignin according to the invention obtained by the above method, reference should be made to the following FIG. 2. It shows thermogravimetric analysis (TGA) measurements (helium, 10 K/min) of polycondensed hardwood lignin with 2,6-bis-hydroxymethyl-p-cresol as binder Compound C from this Example 1, and raw lignin synthesized using 1 weight part binder to 3 weight parts lignin and 1 weight part methanesulfonic acid (MSA). An increased weight of residual carbon of 35 to 46% by weight in the polycondensed reaction product according to the invention is clearly visible.

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;IR spectrum attenuated total reflection (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-bis-(hydroxymethyl)-p-cresol (pilot plant))

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)-para-cresol (120 g) and methanesulfonic acid (93 ml), the reaction mixture was heated with stirring with a mechanical stirrer for 7 hours at 95°C. After completion of the interaction, the reaction mixture was precipitated in ice water, and the precipitated solid was separated. To clean it, it was repeatedly washed abundantly with water, and then freeze-dried.

Example 3 (synthesis of para-cresol resol)

para-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 with sodium hydroxide solution to pH 9, the reactor was purged with nitrogen and plugged. The mixture was heated at 95° C. for about 5 hours until the stirrer torque was increased, after which 1,4-dioxane (3 L) and additional formaldehyde (37%, 200 ml) were added. The interaction was continued for an additional 2 hours. Upon completion of the reaction, the solution was precipitated in cold water, the solid was washed copiously with water and methanol, and then freeze-dried. ¹ H-NMR (dmso-d ₆ , 400 MHz): δ ( ^ppm )=2.0-2.3 (CH ₃ ), 3.48 (phenolic 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 spectrum: v (cm ^-1 )=3195, 2915, 2877, 2326, 2326, 1606, 1482, 1371, 1206, 1154, 1062, 996, 959, 913, 858.

Example 4 (polycondensation of Alcell-lignin from hardwood with para-cresol resol)

Lignin (1.5 g) was dissolved with stirring in a previously prepared separately solution of para-cresol resole (5.2 g) in 1,4-dioxane (30 ml). After addition of methanesulfonic acid (70 μl), the reaction vessel was filled with nitrogen, and stirring was carried out for 22 hours at 100°C. After completion of the interaction, the reaction mixture was precipitated in ice water, and the precipitated solid was separated. To clean it, it was repeatedly washed abundantly with water, and then freeze-dried.

(Methanesulfonic acid (MSA), 10 mol%):

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 spectrum: v (cm ^-1 )=3238, 3006, 2611, 2860, 2345, 2116, 1703, 1603, 1779, 1452, 1286, 1222, 1117, 1030, 858, 783, 558.

Fig. 3 shows the molecular weight distributions of the inventive products from Examples 1 and 4 compared to standard untreated lignin. Table 1 shows a listing of the molecular weights of the starting materials and products of the transformation according to the invention in terms of increasing molecular weight. In particular, the weight average molecular weight of the interaction of hardwood lignin with 2,6-bis-hydroxymethyl-para-cresol and 20 mol.% methanesulfonic acid (MSA) increases more than 100 times to 57,000 g/mol.

Example 4a (polycondensation of fractionated Alcell hardwood lignin with 2,6-bis-(hydroxymethyl)-p-cresol)

Hardwood lignin was suspended in ethanol for 15 minutes and filtered. In this case, low molecular weight lignin passed into solution. Ethanol extraction could produce fractionated lignin with a number average molecular weight of 2200 g⋅mol^-1 (from the original 500 g⋅mol^-1) with 66% by weight yield. Polycondensation of this fractionated lignin with 2,6-bis-hydroxymethyl-p-cresol and under the same conditions as in Example 4 resulted in a product withM _n 33000 g mol^-1, as well as withM _w 133000 g mol^-1.

In this regard, reference should be made to FIG. 3: it shows the molecular weight distributions of 2,6-bis-hydroxymethyl-p-cresol polycondensed hardwood lignin, untreated lignin, fractionated lignin, and 2,6-bis-hydroxymethyl-p-cresol polycondensed fractionated lignin from hardwood. The measurements were carried out using gel permeation chromatography (GPC) in a mixture of N, N-dimethylacetamide (DMAc)/lithium bromide (LiBr) with a concentration of 5 g l ^-1 .

Example 5 (Polycondensation of UPM Biopiva softwood lignin with 2,6-bis-hydroxymethyl-p-cresol)

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 for 22 hours at 95°C. After cooling to room temperature, the mixture was precipitated in water. For purification, the precipitate was extracted three times with hot water for 2 hours each time to remove p-toluenesulfonic acid (TsOH) and 1,4-dioxane.

Example 6: (obtaining a spinning mass and spinning the material according to Example 2)

The product (110 g) from Example 2 was dissolved in DMF (100 ml) while stirring with a mechanical stirrer by slowly adding to the solvent over 24 hours. The solution was kept for 1 week in a refrigerator at 5°C, and then at room temperature for 4 hours at a pressure of 100 mbar (10 kPa). The spinning mass in the rheological measurements proved to be shear thinning with viscosities between 100 and 1000 Pa.s in the frequency range of 100-0.1 1/s (10% strain). Then, the bubble-free material using a ram spinner and a multifilament spinneret (150 µm×64) could be spun at 20° C. into fine fibers under dry spinning conditions. The withdrawal speed on the spinneret was 5 m/min, the winding speed was 75 m/min.

Example 7 (stabilization and charring of fibers according to Example 6)

Lignin fibers were subjected to oxidative thermal stabilization. While the fibers were heated from room temperature with a heating rate of 0.25 K/min in air in a muffle furnace (Nabertherm GmbH) to 250°C. In this case, the fibers were used in the form of a monofilament. The charring was carried out in pure nitrogen by heating from room temperature to 1000° C. at a heating rate of 10 K/min in a high-temperature furnace (manufactured by Gero).

Example 8 (derivatization of the reaction product according to Example 2)

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 slowly added dropwise through an addition funnel. After complete addition, the reaction mixture was heated for 3 hours at 80° C., then stirred overnight at room temperature. For working up, the reaction mixture was concentrated and precipitated in dilute hydrochloric acid (0.1N). The resulting solid was washed with demineralized water and then stirred for 24 hours with a saturated aqueous sodium hydrogen carbonate solution, then again with demineralized water and 0.1 N NaOH. hydrochloric acid to remove excess caproic acid in the form of sodium hexanoate, and also to extract the residual pyridine in the form of hydrochloride. The product was freeze-dried under reduced pressure of 1×10 ^-2 mbar (1 Pa) for 24 hours. A dark brown to black solid is obtained.

The following Table 1 lists the molecular weights of the starting materials and products from Examples 1-5.

Table 1 M _n [g⋅mol^-1] M _w [g⋅mol ^-1 ] Growth PDI Lignin, raw 500 2800 5.1 para-cresol resol 5000 6600 1.3 Polycondensed lignin (para-cresol resol as binder, 1 part MSA to 3 parts lignin) 9000 33800 3.9 Polycondensed fractionated lignin (2,6-bis(hydroxymethyl)-p-cresol as binder, 1 part MSA to 3 parts lignin) 33000 133000 4.0 Polycondensed lignin (2,6-bis-hydroxymethyl-p-cresol as binder, 1 part MSA to 3 parts lignin) 3200 24000 7.5 Polycondensed lignin (2,6-bis-hydroxymethyl-p-cresol as binder, 2 parts MSA to 3 parts lignin) 4600 57000 12.0 Polycondensed softwood lignin (UPM Biopiva) (2,6-bis-hydroxymethyl-p-cresol as binder, 2 parts p-TsOH to 3 parts lignin) 4800 40800 8.4 Polycondensed Alcell-methyl lignin from hardwood (2,6-bis-hydroxymethyl-p-cresol as binder, 2 parts p-TsOH to 3 parts lignin) 5000 37600 7.5

Note: Molecular weights were determined by GPC measurements in DMAc/LiBr, 5 g⋅l ^-1 , and standard calibration against PMMA standards. The products according to the invention have always been synthesized using a binder:lignin molar ratio of 1:2.85, and 1-2 moles of acid per 3 moles of lignin.

Claims (32)

1. Modified lignin based on substituted and/or unsubstituted lignin, wherein the lignin molecules in the substituted and unsubstituted lignin (A) are connected by linking blocks (B), characterized in that the linking blocks (B) are aromatic and are connected to the lignin molecules (A) via the methylene group, wherein the linking block (B) is derived from the reaction of lignin (A) with an aromatic linking compound (C) which contains at least two hydroxymethyl groups, where the specified modified lignin has a number average molecular weight from 2000 to 50000 g/mol. 2. Modified lignin according to claim 1, characterized in that the two hydroxymethyl groups are terminal hydroxymethyl groups. 3. Modified lignin according to claim 1 or 2, characterized in that the substituted lignin is represented by the formulas LO-COR _x (I) and/or L-OR _y (II), in which mean: residue L-O in formulas (I) and (II) esterified or esterified lignin residue, residue -CO-R _x in formula (I) acyl residue, residues R _x and R _y aliphatic and/or aromatic residue. 4. Modified lignin according to claim 3, characterized in that the aromatic residues R _x and R _y represent an aryl residue, in particular a phenolic, benzyl, methoxybenzyl and/or 3-phenylpropenyl residue. 5. Modified lignin according to claim 3, characterized in that the aliphatic residues R _x and R _y represent a linear chain or branched C _1-6 alkyl residue, in particular methyl, ethyl, n-propyl, isopropyl, tert-butyl or neopentyl group. 6. Modified lignin according to any one of the preceding claims, characterized in that the binding block (B) is derived from the interaction of lignin (A) with an aromatic binding compound (C) which has at least two terminal hydroxymethyl groups and is a monomer, an oligomer and /or a polymer, in particular 2,6-bis(hydroxymethyl)-para-cresol, para-cresol-based resol, phenol-based resol and/or a hydroxymethyl-terminated Novolac type compound. 7. Modified lignin according to any one of the preceding claims, characterized in that the molar ratio of the binder block (B) to the lignin molecules (A), substituted or unsubstituted, is from about 1:1 to 1:7, in particular from 1:2 up to 1:4, and particularly preferably about 1:3. 8. Modified lignin according to any one of the preceding claims, characterized in that its ash content, determined according to DIN EN ISO 3451-4, is less than 0.1% by weight. 9. Modified lignin according to any one of the preceding claims, characterized in that it has a number average molecular weight of from 4000 to 40000 g/mol. 10. The method for producing modified lignin, in particular modified lignin according to any of the preceding claims, characterized in that the substituted or unsubstituted lignin (A) is dissolved in a solvent, and in the solution is introduced into interaction with an aromatic binder compound (C), which has two hydroxymethyl groups, in particular two terminal hydroxymethyl groups, in the presence of an organic acid (D) at an elevated temperature, in particular from 60° C. to 160° C., to form a modified lignin. 11. The method according to p. 10, characterized in that the interaction is carried out at a temperature of from 80°C to 120°C, in particular, from 90°C to 110°C. 12. Method according to claim 10 or 11, characterized in that the reaction is carried out for 5 to 24 hours, in particular 7 to 15 hours. 13. The method according to any one of paragraphs. 10-12, characterized in that a water absorber is added to the reaction solution, in particular, to remove the water formed during the interaction. 14. The method according to any one of paragraphs. 10-13, characterized in that the substituted lignin (A) is converted according to the definitions of the preceding paragraphs. 15. The method according to any one of paragraphs. 10-14, characterized in that the binder compound (C) is a monomer, oligomer and/or polymer, in particular 2,6-bis(hydroxymethyl)-para-cresol, para-cresol-based resol, phenol-based resol and/or a hydroxymethyl terminated novolac type compound. 16. The method according to any one of paragraphs. 10-15, characterized in that the oligomeric and/or polymeric binder compound (C) has a polydispersity below 2. 17. The method according to any one of paragraphs. 10-16, characterized in that the binder compound (C) and lignin (A) interact with the formation of modified lignin in a molar ratio of about 1:1 to 1:7. 18. The method according to at least one of paragraphs. 10-17, characterized in that the organic acid (D), in particular in the form of p-toluenesulfonic acid (p-TsOH) and/or methanesulfonic acid (MSA), has a pK _s value <3. 19. The method according to any one of paragraphs. 10-18, characterized in that for 1 mol of lignin (A), from about 0.2 to 1 mol of organic acid (D) is used. 20. The use of modified lignin according to any one of paragraphs. 1-9 to obtain fiber precursors for wet, dry-wet or dry spinning. 21. The use according to claim 20, characterized in that said modified lignin for the purpose of wet, dry-wet and dry spinning with the formation of fiber precursors has a complex viscosity in solution from 10 to 500 Pa s, in particular from 50 to 300 Pa ⋅sec, determined according to DIN 53019-4. 22. The use of modified lignin according to any one of paragraphs. 1-9 for the production of fiber precursors by melt spinning. 23. Use according to claim 22, characterized in that said lignin for the purpose of melt spinning to form fiber precursors has a complex viscosity of from 50 to 2000 Pa.s, in particular from 100 to 1000 Pa.s. 24. Application according to any one of paragraphs. 20-23, according to which fiber precursors are used to obtain carbon fibers by charring, if necessary with preliminary oxidative thermal stabilization and, if necessary, with subsequent graphitization.

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