Method for producing a carbon fiber precursor from lignin and cellulose xanthate as well as a carbon fiber precursor

Abstract

A method for producing a precursor fiber (1) for the production of a carbon fiber and such a precursor fiber (1) are disclosed. The method comprising extruding a spinning dope comprising lignin having an average molecular weight selected within an interval of from 1,500 up to 25,000 g / mol and cellulose xanthate through a spinning nozzle into a spinning bath comprising from 20 g / L up to 400 g / L of sulfuric acid and from 10 g / L up to 120 g / L of divalent cation sulfate salt to produce a precursor fiber (1) having a lignin and cellulose containing core (2) and a cellulose-rich surface layer (5).

Description

[0001] METHOD FOR PRODUCING A CARBON FIBER PRECURSOR FROM LIGNIN AND CELLULOSE XANTHATE AS WELL AS A CARBON FIBER PRECURSOR

[0002] TECHNICAL FIELD

[0003] The present invention generally relates to carbon fiber precursors and in particular to a method of producing lignin and cellulose containing precursor fibers suitable for the production of carbon fibers and to such lignin and cellulose containing precursor fibers.

[0004] BACKGROUND

[0005] Lignin is a polyaromatic polyol and is, after cellulose, the second most common material component in wood and other lignocellulosic plants. During chemical pulping, cellulosic fibers are separated from softwoods, hardwoods, and annual plant biomass, for further processing to paper, board and tissue products. With more than 80 % occurrence, Kraft pulping is the dominant chemical pulping process, whereas other pulping processes include soda pulping, sulfite pulping and the organosolv process. In alkaline pulping, i.e., Kraft and soda pulping, large quantities of lignin become dissolved in the alkaline pulping liquor, known as black liquor. This black liquor is a highly alkaline complex mixture containing used cooking chemicals, solubilized wood lignin, carbohydrates, and organic acids. The lignin can be further processed from the black liquor to energy by combustion of the partly evaporated black liquor or, alternatively, be isolated in solid form by addition of acid. The amount of carbon in lignin is approximately 60-65%.

[0006] Cellulose fibers produced by the viscose process and such cellulose fibers containing lignin have been suggested as raw material for production of carbon fibers, see for instance EP 2 889 401 . Compared to the traditional raw materials for making carbon fibers, lignin is more cost-competitive. However, lignin is not a fiber forming material in its unmodified form. Accordingly, cross-linked forms of lignin, such as formaldehyde cross-linked sodium lignosulfonate or sodium lignate have been suggested in the art, such as in US 4,215,212. However, such cross-linking of lignin increases the complexity and cost of the production process and additionally leads to the inclusion of undesired cross-linking chemicals in the fibers.

[0007] Another shortcoming of the prior art processes for producing cellulose and lignin containing fibers as precursors for carbon fibers is large lignin losses during formation of the cellulose and lignin containing fibers as lignin is leaching from the fibers during the production. SUMMARY

[0008] It is a general objective to reduce lignin losses during manufacture of cellulose and lignin containing precursor fibers.

[0009] This and other objectives are met by the present invention.

[0010] The present invention is defined by the independent claims. Further embodiments of the invention are defined by the dependent claims.

[0011] An aspect of the invention relates to a method for producing a precursor fiber for the production of a carbon fiber. The method comprises extruding a spinning dope comprising lignin having an average molecular weight selected within an interval of from 1 ,500 up to 25,000 g / mol and cellulose xanthate through a spinning nozzle into a spinning bath comprising from 20 g / L up to 400 g / L of sulfuric acid and from 10 g / L up to 120 g / L of divalent cation sulfate salt to produce a precursor fiber having a lignin and cellulose containing core and a cellulose-rich surface layer.

[0012] Another aspect of the invention relates to a precursor fiber for the production of a carbon fiber. The precursor fiber comprises a lignin and cellulose containing core and a cellulose-rich surface layer and is obtainable by a method according to above.

[0013] The invention discloses the manufacture of precursor fibers that are suitable for production of carbon fibers. The precursor fibers are manufactured from cellulose and lignin in a process wherein lignin leakage and losses are reduced without using lignin in a cross-linked form. This is possible by the formation of a cellulose-rich surface layer on the lignin and cellulose containing core of the precursor fiber and where this cellulose-rich surface layer effectively restricts lignin leakage from the core. The invention therefore achieves a more cost effective and less complex production of precursor fibers that can be used in the production of carbon fibers.

[0014] BRIEF DECRIPTION OF THE DRAWINGS

[0015] The embodiments, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

[0016] Fig. 1 is an electron micrograph of a cross-sectional view of a precursor fiber produced according to Example 3; Fig. 2 is an electron micrograph of a cross-sectional view of a precursor fiber produced according to Example 4;

[0017] Fig. 3 is an electron micrograph of a cross-sectional view of a precursor fiber produced according to Example 6;

[0018] Fig. 4 is an electron micrograph of a cross-sectional view of a precursor fiber produced according to Example 7;

[0019] Fig. 5 is a flow chart illustrating a method for producing a precursor fiber for the production of a carbon fiber according to various embodiments;

[0020] Fig. 6 is a schematic illustration of a precursor fiber for the production of a carbon fiber; and

[0021] Fig. 7 is a transmission electron microscopy (TEM) image of a cross-section view of a portion of a precursor fiber according to the invention.

[0022] DETAILED DESCRIPTION

[0023] The present invention generally relates to carbon fiber precursors and in particular to a method of producing lignin and cellulose containing precursor fibers suitable for the production of carbon fibers and such lignin and cellulose containing precursor fibers.

[0024] The method of the invention enables a cost-efficient production of precursor fibers that are suitable for production of carbon fibers. The method produces precursor fibers with very low lignin losses during formation of the precursor fibers but without the need for using cross-linked forms of lignin. This is possible by forming a cellulose-rich surface layer on the precursor fibers that prevent or at least significantly inhibit lignin losses from the cellulose and lignin containing core of the precursor fibers. In fact, embodiments of the invention can reduce the lignin losses during production of the precursor fibers to less than 1 % by weight due to the presence of the cellulose-rich surface layer.

[0025] This cellulose-rich surface layer is formed by extruding a spinning dope comprising cellulose and lignin into a spinning bath comprising sulfuric acid and divalent cation sulfate salt. The presence of the divalent cation sulfate salt in the spinning bath induces formation of the "protecting” cellulose-rich surface layer, which forms a barrier around the cellulose and lignin containing fiber core and thereby reduces lignin leakage and losses. As shown herein, replacing the divalent cation sulfate salt with a monovalent cation sulfate salt significantly increased the lignin losses since no corresponding cellulose-rich surface layer was formed on the fiber surface.

[0026] An aspect of the invention therefore relates to a method for producing a precursor fiber for the production of a carbon fiber, see Figs. 5 and 6. The method comprises extruding, in step S5 of Fig. 5, a spinning dope comprising lignin having an average molecular weight selected within an interval of from 1 ,500 up to 25,000 g / mol and cellulose xanthate through a spinning nozzle into a spinning bath. The spinning bath comprises from 20 g / L up to 400 g / L sulfuric acid (H2SO4) and from 10 g / L up to 120 g / L of a divalent cation sulfate salt. The extrusion of the spinning dope into the spinning bath produces a precursor fiber 1 , see Fig. 6, having a lignin and cellulose containing core 2 and a cellulose-rich surface layer 5. Fig. 7 is a transmission electron microscopy (TEM) image of a cross-section view of a portion of a precursor fiber showing the lignin and cellulose containing core 2 and a cellulose-rich surface layer 5. In Fig. 7, the light gray parts are lignin particles whereas the dark gray parts represent cellulose.

[0027] The extrusion of the spinning dope into the spinning bath of the invention thereby creates a comparatively, when compared to the average diameter of the precursor fiber 1 and the average diameter of the lignin and cellulose containing core 2, thin cellulose-rich surface layer or sheath 5 on the lignin and cellulose containing core 2. The cellulose-rich surface layer 5 typically has an average thickness of at least 2.5 nm, preferably at least 5 nm and more preferably at least 10 nm. This should be compared to the average diameter of the precursor fiber 1 , which is typically in the pm range as shown in Figs. 1 and 2. In fact, the cellulose-rich surface layer or sheath 5 is so thin that it cannot be distinguished in Figs. 1 and 2. The TEM image of Fig. 7 indicates the thin cellulose-rich surface layer 5 on the lignin and cellulose containing core 2 containing a cellulose matrix with lignin particles.

[0028] The formed cellulose-rich surface layer 5 has higher weight percentage of cellulose as compared to the lignin and cellulose containing core 2. In an embodiment, the cellulose-rich surface layer 5 has a cellulose content of at least 75 % by weight (w / w), preferably at least 80 % by weight, and more preferably at least 90 % by weight.

[0029] This means also that the cellulose-rich surface layer 5 has a higher weight ratio of cellulose to lignin as compared to the lignin and cellulose containing core 2. In an embodiment, the lignin and cellulose containing core 2 comprises a weight ratio of cellulose to lignin selected within an interval of from 1 :0.8 up to 1 :2, preferably within an interval of from 1 :0.85 up to 1 :2, and more preferably within an interval of from 1 :0.9 up to 1 :2, such as within an interval of from 1 :1 up to 1 :2. Hence, the lignin and cellulose containing core 2 preferably contains substantially the same amount (weight percentage) of lignin as the amount (weight percentage) of cellulose, or a higher amount (weight percentage) of lignin. For instance, a spinning dope comprising substantially the same amount (weight percentage) of lignin and cellulose or a cellulose to lignin weight ratio of 1 :1.5 could be extruded into the spinning bath in step S5 to form a precursor fiber 1 with substantially the same weight percentage of lignin and cellulose or with a cellulose to lignin weight ratio of 1 : 1.5 in the lignin and cellulose containing core 2 but significantly higher weight percentage of cellulose than lignin in the cellulose-rich surface layer 5.

[0030] The method of the invention is, due to the formation of the cellulose-rich surface layer 5 on the precursor fiber 1 , characterized by a very low lignin leakage and loss during the formation of the precursor fiber 1 . In an embodiment, the method is characterized by a lignin loss equal to or less than 5 % by weight, preferably equal to or less than 3 % by weight, and more preferably equal to or less than 2 % by weight, such as equal to or less than 1 % by weight. As is shown in the Example section, the method of the invention can achieve a lignin loss even equal to or less than 1 % by weight. This should be compared to prior art methods, such as disclosed in US 4,215,212, which had almost 100 % by weight of lignin loss if the lignin was present in a non-crosslinked form as sodium lignosulfate or sodium lignate.

[0031] In an embodiment, the spinning dope as extruded in step S5 is formed by mixing a lignin solution comprising lignin in an alkaline aqueous solution with a viscose solution comprising cellulose xanthate in an alkaline aqueous solution to form the spinning dope in step S3 of Fig. 5.

[0032] Hence, in an invention the lignin solution and the viscose solution are mixed in step S3 prior to extruding the spinning dope S5. It is believed that such a pre-mixing of the lignin and viscose solutions prior to extrusion promotes the formation of the cellulose-rich surface layer 5 on the lignin and cellulose containing core 2 during extrusion in step S5. In an embodiment, the mixing of the lignin solution and the viscose solution in step S3 is preferably performed at least 30 s prior to extrusion of the spinning dope in step S5, preferably at least 1 min, more preferably at least 5 min, such as at least 10 min, at least 15 min, at least 30 min, at least 45 min or even longer, such as at least 1 hour prior to extrusion in step S5.

[0033] The mixture of the lignin solution and the viscose solution is optionally processed in step S4 prior to extrusion in step S5. Illustrative, but non-limiting, examples of such a processing include filtration of the mixture of the lignin solution and the viscose solution and / or degassing the mixture of the lignin solution and the viscose solution. The filtration may then remove larger cellulose and / or lignin particles or aggregates that may otherwise impede or even obstruct the extrusion of the spinning dope through a spinning nozzle in step S5. Correspondingly, degassing of the mixture of the lignin solution and the viscose solution is preferred to remove any gas bubbles in the mixture, which may otherwise block the spinning nozzle during extrusion.

[0034] In an embodiment, the method also comprises preparing the lignin solution in step S1 of Fig. 5. This step S1 preferably comprises dissolving or dispersing, preferably dissolving, lignin having an average molecular weight selected within an interval of from 1 ,500 up to 25,000 g / mol in the alkaline aqueous solution at an amount of lignin selected within an interval of from 15 up to 45 % by weight. In a particular embodiment, the alkaline aqueous solution prepared in step S1 comprises from 25 up to 45 % by weight of lignin, preferably from 25 up to 35 % by weight of lignin, such as about 30 % by weight of lignin.

[0035] In an embodiment, the alkaline aqueous solution is a lye, i.e., an alkali metal hydroxide, and preferably an aqueous sodium hydroxide (NaOH) solution. In an embodiment, the alkali metal hydroxide, such as NaOH, content in the alkaline aqueous solution is preferably selected within an interval of from 3 up to 15 % by weight, more preferably within an interval of from 3 up to 10 % by weight.

[0036] In an embodiment, a basicity of the lignin solution prepared in step S1 is substantially the same as a basicity of the viscose solution. Hence, the pH of the lignin solution prepared in step S1 is preferably substantially the same as the pH of the viscose solution. Substantially the same pH as referred to herein encompass a difference in pH of no more than 10%, preferably of no more than 5 %, and more preferably of no more than 2.5%, such as of no more than 1 %.

[0037] In an embodiment, the method also comprises preparing the viscose solution in step S2 of Fig. 5. This preparation in step S2 can be performed according to any known process to produce cellulose xanthate and a viscose solution from cellulose.

[0038] In an embodiment, the preparation in step S2 comprises an activation of cellulose by alkalization, followed by treatment with carbon disulfide (CS2) to produce cellulose xanthate. For instance, cellulose in the form of pulp is treated with aqueous sodium hydroxide to form "alkali cellulose", which has the approximate formula [C6HgO4-ONa]n. The alkali cellulose is then typically allowed to depolymerize, also referred to as ripe or mature in the art, to an extent. The alkali cellulose is then treated with carbon disulfide to form sodium cellulose xanthate [C6HgO4-ONa]n+ nCSg — > [C6H5(OH)4-OCS2Na]n. The cellulose xanthate is then dissolved or dispersed, preferably dissolved, in an alkaline aqueous solution, i.e., a lye, and preferably an aqueous NaOH solution to form the viscose solution in step S2.

[0039] As an illustrative example, cellulose in the form of pulp, such as Kraft pulp, and preferably prehydrolyse Kraft (PHK) pulp, is activated by NaOH to produce alkali cellulose, which following depolymerization preferably has a degree of polymerization (DPcuox) within an interval of from 200 up to 500, preferably within an interval of from 300 up to 400. Carbon disulfide is then added to the alkali cellulose, preferably at an amount selected within an interval of from 18 up to 42 % based on the weight of cellulose, preferably within an interval of from 26 up 35 % based on the weight of cellulose, to produce the cellulose xanthate. The cellulose xanthate is then dissolved or dispersed in aqueous sodium hydroxide at a temperature lower than room temperature (20-25°C), preferably at a temperature lower than 10°C to prepare the viscose solution in step S2.

[0040] The viscose solution preferably comprises cellulose xanthate at an amount of from 6 up to 14 % by weight, preferably from 8 up to 10 % by weight.

[0041] In an embodiment, the alkali metal hydroxide, such as NaOH, content in the viscose solution is preferably selected within an interval of from 3 up to 15 % by weight, more preferably within an interval of from 5 up to 10 % by weight.

[0042] The lignin solution, preferably as prepared in step S1 , can be mixed with the viscose solution, preferably as prepared in step S2, in step S3 at different stages of depolymerization, i.e., ripeness or maturity, of the viscose solution. In a preferred embodiment, the mixing in step S3 is performed when the viscose solution has a stage of depolymerization selected within an interval of from 3 up to 20 degree Hottenroth (°H), also referred to as Hottenroth index in the art (Hottenroth (1915) Chem. Ztg. 39: 119 in: Gbtze, Chemiefasern, Nach dem Viskoseverfahren, Springer Verlag, Berlin, pp 118-120, 209, 214 (1967)).

[0043] The so-produced spinning dope preferably has a total polymer content, i.e., total amount of lignin and cellulose, of at least 8 % by weight, preferably at least 10 % by weight.

[0044] The lignin used in the method of Fig. 5 is preferably in the form of lignin from a chemical pulping process, preferably from an alkaline pulping process, and more preferably from a Kraft pulping process, i.e., Kraft lignin (KL). Such a Kraft lignin generally has an average molecular weight within the preferred interval of from 1,500 up to 25,000 g / mol. This range of average molecular weight encompasses lignin molecules that achieve a desired lignin solubility in the lignin solution and the spinning dope and further promote the formation of the lignin and cellulose containing core 2 with a cellulose-rich surface layer 5 during extrusion in step S5 into the spinning bath.

[0045] In a preferred embodiment, the lignin has an average molecular weight selected within an interval of from 2,000 up to 20,000 g / mol, more preferably selected within an interval of from 5,000 up to 10,000 g / mol and most preferably selected within an interval of from 5,000 up to 8,000 g / mol.

[0046] In an embodiment, the lignin has an ash content of no more than 0.8 % by weight.

[0047] In an embodiment, the lignin has a sulfur (S) content of at least 1 % by weight.

[0048] In an embodiment, the spinning dope comprises non-cross linked lignin and cellulose xanthate. Hence, the lignin in the lignin solution and in the spinning dope and thereby in the produced precursor fiber 1 is preferably in a non-cross linked form. This is in contrast to the prior art as represented by US 4,215,212, which requires the lignin to be cross-linked with formaldehyde in the form of sodium lignosulfonate or sodium lignate cross-linked with formaldehyde.

[0049] The spinning bath, into which the spinning dope is extruded in step S5 of Fig. 5, comprises from 10 g / L up to 120 g / L divalent cation sulfate salt. In an embodiment, the divalent cation sulfate salt is selected from the group consisting of zinc sulfate (ZnSO ), magnesium sulfate (MgSO ), calcium sulfate (CaSO ), copper sulfate (CuSO ), and any mixture or combination thereof. In a preferred embodiment, the divalent cation sulfate salt is zinc sulfate or a mixture or combination of zinc sulfate and at least one divalent cation sulfate salt selected from the group consisting of magnesium sulfate, calcium sulfate and copper sulfate. In a currently preferred embodiment, the divalent cation sulfate salt is zinc sulfate.

[0050] In an embodiment, the spinning bath also comprises at least one monovalent cation sulfate salt. In such an embodiment, the spinning bath comprises at least one divalent cation sulfate salt, preferably zinc sulfate, and at least one monovalent cation sulfate salt.

[0051] In an embodiment, the monovalent cation sulfate salt is selected from the group consisting of sodium sulfate (Na2SC>4), potassium sulfate (K2SO4), and a mixture or a combination thereof. In a preferred embodiment, the monovalent cation sulfate salt is sodium sulfate. Hence, in a particular embodiment, the spinning bath preferably comprises zinc sulfate and sodium sulfate.

[0052] In an embodiment, the spinning bath comprises an amount of monovalent cation sulfate salt selected within an interval of from 80 up to 240 g / L, preferably selected within an interval of from 100 up to 240 g / L.

[0053] In an embodiment, the spinning bath has a total concentration of inorganic salts of no more than 400 g / L. In a particular embodiment, the spinning bath has a total concentration of inorganic salts selected within an interval of from 10 g / L up to 400 g / L, preferably selected within an interval of from 10 g / L up to 300 g / L, and more preferably selected within an interval of from 20 g / L up to 300 g / L.

[0054] The total concentration of inorganic salts in the spinning bath seem to affect the cross-sectional shape of the produced precursor fiber 1. In more detail, reducing the total concentration of inorganic salts from 270 g / L down to 135 g / L resulted in a precursor fiber 1 with a more circular cross section as is evident by comparing Figs. 1 and 2.

[0055] In an embodiment, the inorganic salts present in the spinning bath are the divalent cation sulfate salt(s) and the optional monovalent cation sulfate salt(s).

[0056] In an embodiment, the spinning dope comprises a lignin content selected within an interval of from 3 up to 9 % by weight.

[0057] In an embodiment, the spinning dope comprises a weight ratio of cellulose to lignin selected within an interval of from 1 :0.8 up to 1 :2, preferably within an interval of from 1 :0.85 up to 1 :2, and more preferably within an interval of from 1 : 1 up to 1 :2. Thus, the spinning dope preferably comprises substantially the same amount of cellulose as lignin or up to twice the amount of lignin as compared to cellulose.

[0058] In an embodiment, step S5 of Fig. 5 comprises extruding the spinning dope through the spinning nozzle into the spinning bath having an average temperature selected within an interval of from 1 up to 60°C, preferably within an interval of from 1 up to 50°C, such as within an interval of from 5 up to 50°C, and more preferably within an interval of from 1 up to 45°C, such as within an interval of from 5 up to 45°C. Experimental data as presented herein indicates that the temperature of the spinning bath may have an effect on the cross-sectional shape of the precursor fiber 1 . In particular, reducing the temperature of the spinning bath from 42°C down to 7°C resulted in a more circular cross section of the precursor fiber 1 as is evident by comparing Figs. 1 and 2. This reduction in the temperature of the spinning bath is preferably accompanied by a reduction in the total concentration of inorganic salts as mentioned in the foregoing.

[0059] The spinning nozzle used in step S5 of Fig. 5 comprises at least one hole but may optionally comprise multiple, i.e., at least two, holes, through which the spinning dope is extruded as a liquid filament jet or ray. The at least one hole of the spinning nozzle is preferably mounted submerged into the spinning bath so that the liquid filament jet or ray is formed by extruding the spinning dope through the spinning nozzle in step S5 into the spinning bath. The extruded spinning dope immediately solidifies by coagulation and is typically at the same time stretched, so called jet stretching. This jet stretch represents the ratio between the extrusion velocity and the take-up velocity of the formed precursor fiber 1. In a preferred embodiment, the jet stretch is selected within an interval of from 0.4 up to 2.9.

[0060] In an embodiment, the method comprises an additional step S6, which comprises introducing the precursor fiber 1 into a decomposition bath comprising sulfuric acid.

[0061] The decomposition bath preferably comprises an amount of sulfuric acid selected within an interval of from 5 up to 200 g / L. Further, the decomposition bath preferably has an average temperature selected within an interval of from 70 up to 100°C.

[0062] The precursor fiber 1 is preferably drawn in the decomposition bath in step S7 with a drawing factor selected within an interval of from 1.1 up to 2.5. A drawing factor of 1 .1-2.5 means that the length of the precursor fiber 1 drawn in step S7 is 1.1-2.5 of the length of the precursor fiber 1 introduced into the decomposition bath in step S6.

[0063] Optionally, hot water steam can be applied to the decomposition bath during the drawing in step S7.

[0064] This drawing of the precursor fiber 1 in the decomposition bath cleaves off the xanthate groups from the precursor fiber 1. The precursor fiber 1 from step S5 or the optional steps S6 and S7 is optionally, but preferably, washed to remove any by-products. Hence, in an embodiment, the method comprises step S8 of Fig. 5, which comprises washing the precursor fiber 1 with water or an aqueous solution at a temperature selected within an interval of from 40 up to 80°C, preferably selected within an interval of from 50 up to 70°C, such as about 60°C.

[0065] The washing in step S8 is preferably conducted for at least 1 min, preferably at least 2 min, and more preferably at least 5 min, such as from 5 up to 30 min, more preferably from 5 up to 10 min.

[0066] A preferred washing solution is water or a mixture of water and an organic solvent. The washing solution used in step S8 preferably has a pH selected within an interval of from 1 up to 10, preferably selected within an interval of from 5 up to 8.

[0067] The precursor fiber 1 is preferably dried in step S9 at a temperature equal to or above 40°C, preferably equal to or above 50°C, and more preferably equal to or above 60°C.

[0068] The drying in step S9 is preferably conducted for at least 30 s, preferably at least 1 min, and more preferably at least 2 min, such as from 2 up to 15 min, more preferably from 2 up to 10 min.

[0069] The stretched and washed precursor fiber 1 is optionally treated, before drying in step S9, after drying in step S9 or both before and after drying in step S9, with a spinning oil with an antistatic effect.

[0070] Another aspect of the invention relates to a precursor fiber 1 for the production of a carbon fiber. The precursor fiber 1 comprises a lignin and cellulose containing core 2 and a cellulose-rich surface layer 5. In an embodiment, the precursor fiber 1 is obtainable by a method according to the invention, such as described in the foregoing in connection with Fig. 5.

[0071] In an embodiment, the lignin and cellulose containing core 2 comprises a weight ratio of cellulose to lignin selected within an interval of from 1 :0.8 up to 1 :2, preferably within an interval of from 1 :0.85 up to 1 :2, and more preferably within an interval of from 1 :1 up to 1 :2.

[0072] In an embodiment, the cellulose-rich surface layer 5 has an average thickness of at least 2.5 nm, preferably at least 5 nm, and more preferably at least 10 nm. In an embodiment, the cellulose-rich surface layer 5 has a higher weight ratio of cellulose to lignin as compared to the lignin and cellulose containing core 2.

[0073] In an embodiment, the cellulose-rich surface layer 5 has a cellulose content of at least 75 % by weight.

[0074] In an embodiment, the precursor fiber 1 has a water retention value (WRV) of less than 100 %.

[0075] WRV was determined by mixing 0.5 g of the precursor fiber with 50 mL deionized water and allowed to stand for 15 min in room temperature. The sample was then centrifuged in a filter dish for 15 min at 3000 g. The mass of this sample was measured (Mi). The sample was then dried overnight at 105°C in a convection oven and the mass of the dried sample was measured (M2). The WRV is then calculated in [%] as 100x(Mi - M2) / M2.

[0076] In an embodiment, the precursor fiber 1 has a sulfur content of no more than 2 % by weight.

[0077] In an embodiment, the precursor fiber 1 has an ash content of no more than 0.02 % by weight.

[0078] The precursor fibers 1 of the invention are suitable as precursors or starting material for the production of carbon fibers and activated carbon fibers. Carbon fibers are high-performance reinforcing fibers, which are used in composite materials in various fields, such as aircraft construction, high-performance vehicle construction, for sports equipment, wind energy plants, etc.

[0079] Carbon fibers are produced by heat treatment above 1 ,000°C of organic precursor fibers. Today, polyacrylonitrile (PAN) or copolymers of polyacrylonitrile are the dominating polymers for the production of precursors for carbon fibers. PAN are products of the petrochemical industry and thereby not an environmental or sustainable precursor for carbon fibers. The precursor fibers 1 of the invention are thereby suitable as more environmentally friendly and sustainable precursors for carbon fibers.

[0080] EXAMPLES

[0081] EXAMPLE 1 - Viscose solution preparation

[0082] A viscose solution was produced by activating a prehydrolyse Kraft (PHK) pulp with a degree of polymerization (DPcuox) of 610 by sodium hydroxide (NaOH). The activated cellulose had a DPcuox of 370, a cellulose content of 32.7 % (w / w) and a sodium hydroxide content of 15.1 % (w / w). The activated cellulose was derivatized with 32 % (w / w) of carbon disulfide (CS2) in relation to cellulose to form cellulose xanthate. The viscose solution was prepared by adding cellulose xanthate to an aqueous solution of sodium hydroxide to obtain a final alkali content of 7.1 % (w / w) in the viscose solution. The viscose solution had a cellulose content of 9.1 % (w / w).

[0083] EXAMPLE 2 - Spinning dope preparation

[0084] For the preparation of the spinning dope, the viscose solution from Example 1 containing 9.1 % (w / w) of cellulose and 7.1 % (w / w) of sodium hydroxide was mixed with a lignin solution comprising 30 % (w / w) of Kraft lignin and 4.9 % sodium hydroxide (w / w). Spinning dopes with lignin contents between 3 % (w / w) and 9 % (w / w) turned out to be most promising for the desired precursor fiber quality.

[0085] EXAMPLE 3 - Precursor fiber production

[0086] For precursor fiber production, the lignin containing spinning dope from Example 2 having 7.3 % (w / w) of cellulose, 7.3 % (w / w) of Kraft lignin and 6.5 % (w / w) sodium hydroxide was spun into an acidic spinning bath. The filaments were spun through spinneret holes having a round shaped cross-section with a capillary diameter of 60 pm and a total hole number of 300.

[0087] The filaments were spun into a spinning bath comprising 210 g / L sodium sulfate, 60 g / L zinc sulfate and 60 g / L sulfuric acid and having a temperature of 42°C.

[0088] The tow was stretched by 30 % in a 2 % aqueous sulfuric acid solution at 95°C, washed with distilled 60°C hot water for 8 minutes and dried for 4 minutes at 60°C.

[0089] The obtained 3000 filaments had a single filament titer of 4.7 dtex, a filament tenacity of 12.1 cN / tex and an elongation at break of 16.4 %.

[0090] Fig. 1 is a cross-sectional view of a precursor fiber produced in accordance with Example 3 having a lobulated cross section. Table 1 - Textile physical properties of precursor fibers as a function of the jet-stretch using a sodium sulfate and zinc sulfate containing spinning bath according to Example 3

[0091] Titer Jet-stretch Drawing Tenacity Modulus Elongation at

[0092] [dtex] [cN / tex] cN / tex break [%]

[0093] 6.1 0.8 1.1 9.5 385 17.7

[0094] 5.9 0.7 1.4 13.7 610 8.4

[0095] 5.3 0.9 1.2 11.2 490 13.1

[0096] 4.6 0.7 1.7 11.1 802 5.0

[0097] 2.2 2.2 1.1 9.4 501 14.1

[0098] 1.9 2.3 1.3 14.1 681 14.2

[0099] Titer in Table 1 was determined by the vibroscope method according to ISO 1973:2021 , Textile fibres, Determination of linear density, Gravimetric method and vibroscope method.

[0100] Tenacity, modulus and elongation at break in Table 1 was determined via the single filament tensile test according to ASTM D3822 / D3822M-14 (2020), Standard test method for tensile properties of single textile fibers.

[0101] EXAMPLE 4 - Precursor fiber production

[0102] For precursor fiber production, the lignin containing spinning dope from Example 2 having 7.3 % (w / w) of cellulose, 7.3 % (w / w) of Kraft lignin and 6.5 % (w / w) sodium hydroxide was spun into an acidic spinning bath. The filaments were spun through spinneret holes having a round shaped cross-section with a capillary diameter of 60 pm and a total hole number of 300.

[0103] The filaments were spun into a spinning bath comprising 140 g / L sodium sulfate, 35 g / L zinc sulfate and 60 g / L sulfuric acid and having a temperature of 7°C.

[0104] The tow was stretched by 30 % in a 2 % aqueous sulfuric acid solution at 95°C, washed with distilled 60°C hot water for 8 minutes and dried for 4 minutes at 60°C.

[0105] The obtained 1000 filaments had a single filament titer of 2.1 dtex, a filament tenacity of 13.9 cN / tex and an elongation at break of 13.2 % measured as described in Example 3. Fig. 2 is a cross-sectional view of a precursor fiber produced in accordance with Example 4 having a circular cross section.

[0106] EXAMPLE 5 - Precursor fiber production

[0107] Precursor fiber production was performed in accordance with Example 3 but with the cellulose to lignin content ratio changed from 1 : 1 into 1 : 1.5.

[0108] EXAMPLE 6 - Precursor fiber production - comparative example

[0109] For precursor fiber production, the lignin containing spinning dope from Example 2 having 7.3 % (w / w) of cellulose, 7.3 % (w / w) of Kraft lignin and 6.5 % (w / w) sodium hydroxide was spun into an acidic spinning bath. The filaments were spun through spinneret holes having a round shaped cross-section with a capillary diameter of 60 pm and a total hole number of 300.

[0110] The filaments were spun into a spinning bath comprising 264 g / L ammonium sulfate and 80 g / L sulfuric acid and having a temperature of 42°C.

[0111] The tow was stretched by 30 % in a 2 % aqueous sulfuric acid solution at 95°C, washed with distilled 60°C hot water for 8 minutes and dried for 4 minutes at 60°C.

[0112] The obtained 300 filaments had a single filament titer of 4.6 dtex, a filament tenacity of 13.3 cN / tex and an elongation at break of 7.8 % measured as described in Example 3.

[0113] Fig. 3 is a cross-sectional view of a precursor fiber produced in accordance with Example 6 having a circular cross section.

[0114] EXAMPLE 7 - Precursor fiber production - comparative example

[0115] For precursor fiber production, the lignin containing spinning dope from Example 2 having 7.3 % (w / w) of cellulose, 7.3 % (w / w) of Kraft lignin and 6.5 % (w / w) sodium hydroxide was spun into an acidic spinning bath. The filaments were spun through spinneret holes having a round shaped cross-section with a capillary diameter of 60 pm and a total hole number of 300.

[0116] The filaments were spun into a spinning bath comprising 140 g / L sodium sulfate and 80 g / L sulfuric acid and having a temperature of 35°C. The tow was stretched by 30 % in a 2 % aqueous sulfuric acid solution at 95°C, washed with distilled 60°C hot water for 8 minutes and dried for 4 minutes at 60°C.

[0117] The obtained 300 filaments had a single filament titer of 4.9 dtex, a filament tenacity of 11 .6 cN / tex and an elongation at break of 15.3 % measured as described in Example 3.

[0118] Fig. 4 is a cross-sectional view of a precursor fiber produced in accordance with Example 7 having a lobular cross section.

[0119] Elemental analysis was performed by determining the CHNS (carbon, hydrogen, nitrogen, sulfur) content using a FlashEA 1112 CHNS / O Automatic Elemental Analyser with 2 Autosampler MAS200R (Thermo Scientific). Samples weighed into thin-walled tin capsules were transferred into a quartz combustion tube with a constant helium flow. The samples were burnt over tungsten oxide with the addition of high-purity oxygen at temperature of approximately 1020°C. The resulting nitrogen oxides and sulfur trioxide were reduced to nitrogen and sulfur dioxide on copper filings. The four components N2, CO2, H2O and SO2 obtained were separated by gas chromatography in a packed Porapack PQS column and detection was carried out by means of a thermal conductivity detector.

[0120] The lignin content of the precursor fibers from Examples 3 to 7 were determined by elemental analysis as described above and summarized in Table 2.

[0121] Table 2 - Lignin content of precursor fibers determined via elemental analysis

[0122] The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

CLAIMS1. A method for producing a precursor fiber (1 ) for the production of a carbon fiber, the method comprising extruding (S5) a spinning dope comprising lignin having an average molecular weight selected within an interval of from 1 ,500 up to 25,000 g / mol and cellulose xanthate through a spinning nozzle into a spinning bath comprising from 20 g / L up to 400 g / L of sulfuric acid and from 10 g / L up to 120 g / L of divalent cation sulfate salt to produce the precursor fiber (1) having a lignin and cellulose containing core (2) and a cellulose-rich surface layer (5).

2. The method according to claim 1 , wherein the spinning bath has a total concentration of inorganic salts of no more than 400 g / L.

3. The method according to claim 1 or 2, wherein the method is characterized by a lignin loss of equal to or less than 5 % by weight, preferably equal to or less than 3 % by weight, more preferably equal to or less than 2 % by weight, and most preferably equal to or less than 1 % by weight.

4. The method according to any one of claims 1 to 3, further comprising mixing (S3) a lignin solution comprising lignin in an alkaline aqueous solution with a viscose solution comprising cellulose xanthate in an alkaline aqueous solution to form the spinning dope.

5. The method according to claim 4, further comprising dissolving or dispersing (S1) lignin having an average molecular weight selected within an interval of from 1 ,500 up to 25,000 g / mol in the alkaline aqueous solution, preferably aqueous sodium hydroxide solution, at an amount of lignin within an interval of from 15 up to 45 % by weight, preferably within an interval of from 25 up to 35 % by weight.

6. The method according to claim 4 or 5, further comprising processing (S4) the mixture of the lignin solution and the viscose solution by filtering and / or degassing the mixture of the lignin solution and the viscose solution.

7. The method according to any one of claims 1 to 6, wherein the spinning dope comprises at least 10 % by weight of lignin and cellulose xanthate.

8. The method according to any one of claims 1 to 7, wherein the lignin has an average molecular weight selected within an interval of from 2,000 up to 20,000 g / ml, preferably selected within an interval of from 5,000 up to 8,000 g / ml.

9. The method according to any one of claims 1 to 8, wherein the spinning dope comprises non-cross linked lignin and cellulose xanthate.

10. The method according to any one of claims 1 to 9, wherein the divalent cation sulfate salt is selected from the group consisting of zinc sulfate, magnesium sulfate, calcium sulfate, copper sulfate, and any mixture thereof.

11. The method according to claim 10, wherein the divalent cation sulfate salt is zinc sulfate or a mixture of zinc sulfate and at least one divalent cation sulfate salt selected from the group consisting of magnesium sulfate, calcium sulfate and copper sulfate, preferably zinc sulfate.

12. The method according to any one of claims 1 to 11 , wherein the spinning bath further comprises a monovalent cation sulfate salt.

13. The method according to claim 12, wherein the spinning bath comprises an amount of monovalent cation sulfate salt selected within an interval of from 80 up to 240 g / L.

14. The method according to claim 12 or 13, wherein the monovalent cation sulfate salt is selected from the group consisting of sodium sulfate, potassium sulfate, and a mixture thereof, preferably sodium sulfate.

15. The method according to any one of claims 1 to 14, wherein the spinning dope comprises a lignin content selected within an interval of from 3 up to 9 % by weight.

16. The method according to any one of claims 1 to 15, wherein the spinning dope comprises a weight ratio of cellulose to lignin selected within an interval of from 1 :0.8 up to 1 :2, preferably within an interval of from 1 :0.85 up to 1 :2, and more preferably within an interval of from 1 :1 up to 1 :2.

17. The method according to any one of claims 1 to 16, wherein extruding (S5) the spinning dope comprises extruding (S5) the spinning dope through the spinning nozzle into the spinning bath having an average temperature selected within an interval of from 1 up to 45°C.

18. The method according to any one of claims 1 to 17, further comprising introducing (S6) the precursor fiber (1 ) into a decomposition bath comprising sulfuric acid.

19. The method according to any one of claims 1 to 18, further comprising washing (S8) the precursor fiber (1) with water or an aqueous solution at a temperature selected within an interval of from 40 up to 80°C.

20. The method according to any one of claims 1 to 19, further comprising drying (S9) the precursor fiber (1) at a temperature equal to or above 40°C, preferably equal to or above 60°C.21 . A precursor fiber (1) for the production of a carbon fiber, wherein the precursor fiber (1) comprises a lignin and cellulose containing core (2) and a cellulose-rich surface layer (5) and is obtainable by a method according to any one of claims 1 to 20.

22. The precursor fiber according to claim 21 , wherein the lignin and cellulose containing core (2) comprises a weight ratio of cellulose to lignin selected within an interval of from 1 :0.8 up to 1 :2, preferably within an interval of from 1 :0.85 up to 1 :2, and more preferably within an interval of from 1 : 1 up to 1 :2.

23. The precursor fiber according to claim 21 or 22, wherein the cellulose-rich surface layer (5) has an average thickness of at least 2.5 nm, preferably at least 5 nm, and more preferably at least 10 nm.

24. The precursor fiber according to any one of claims 21 to 22, wherein the cellulose-rich surface layer (5) has a higher weight ratio of cellulose to lignin as compared to the lignin and cellulose containing core (2).

25. The precursor fiber according to any one of claims 21 to 24, wherein the cellulose-rich surface layer (5) has a cellulose content of at least 75 % by weight.

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