Method for producing highly rigid ex-cellulose carbon fibres, and carbon fibres

Abstract

Carbon fibres (CF) combine high strength, rigidity and low density in a unique manner. On account of this combination of properties, they are considered indispensable for use in lightweight construction, a technology which is key to achieving the desired energy transition with greater climate neutrality and resource efficiency. CFs are used in particular in the mobility sector, for example in aerospace or in the automotive sector, where significant weight savings can be achieved by using CFs, thus enabling fuel consumption to be reduced. In addition, CFs are also used in other end applications, including in the rotors of wind turbines, hydrogen pressure tanks, sports equipment and medical products. Almost all CFs used in these applications are produced from polyacrylonitrile copolymer (PAN)-based precursor fibres (precursors), which together make up a market share of 96 to 98%. However, CFs based on PAN precursors (ex-PAN CF) are petroleum-based, cost-intensive and CO2-intensive to produce and, during the conversion process, cause large amounts of toxic substances such as hydrocyanic acid (~20 wt.%). Bio-based alternatives based on natural resources such as cellulose or CF produced therefrom (ex-cellulose CF) are therefore experiencing a renaissance since they do not have the disadvantages listed for PAN and, in comparison to same, potentially enable a reduction in CO2 emissions and production costs.

Description

[0001] Processes for the production of high-stiffness ex-cellulose carbon fibers and carbon fibers

[0002] Carbon fibers (CF) uniquely combine high strength, stiffness, and low density. This combination of properties makes them indispensable for lightweight construction, a key technology on the path to the desired energy transition with greater climate neutrality and resource efficiency. CFs are used particularly in the mobility sector, for example in aerospace and automotive, where their use enables significant weight savings and thus reduces fuel consumption. Furthermore, CFs are also used in other end applications, including wind turbine rotors, hydrogen pressure tanks, sporting goods, and medical devices. Almost all CFs used in these applications are produced from polyacrylonitrile copolymer (PAN)-based precursor fibers, which together account for a market share of 96 to 98%.However, carbon sulfates based on PAN precursors (ex-PAN CF) are petroleum-based, costly, and carbon-intensive to produce, and generate large quantities of toxic substances such as hydrogen cyanide (~20 wt%) during the conversion process. Therefore, bio-based alternatives based on natural resources such as cellulose, or carbon sulfates derived from it (ex-cellulose CF), are experiencing a renaissance, as they do not have the disadvantages listed for PAN and, compared to PAN, potentially enable a reduction in carbon emissions and production costs.

[0003] A significant disadvantage of ex-cellulose CFs, however, is their low material yield of approximately 15 wt% and their poor mechanical properties, particularly their stiffness (tensile modulus) of a maximum of 130 GPa. In comparison, commercially available CFs based on PAN (ex-PAN CF) exhibit a tensile modulus of 230-270 GPa (HT type) or 280-300 GPa (IM type) and a material yield of approximately 45 wt%.

[0004] Approaches to increasing the material yield of ex-cellulose CFs are described in the scientific literature and represent the state of the art. These approaches include the addition of Lewis acids or flame-retardant substances to the cellulosic starting fiber material, such as ammonium tosylate, ammonium dihydrogen phosphate, or phosphoric acid, etc. (Parry, A.; Windle, A. Carbon fibers from cellulosic precursors: A review. J. Mater. Sci. 2012, 47, 4236-4250; Spörl, JM; Beyer, R.; Abels, F.; Cwik, T.; Müller, A.; Hermanutz, F.; Buchmeiser, MR Cellulose-Derived Carbon Fibers with Improved Carbon Yield and Mechanical Properties. Macromol. Mater. Eng. 2017, 302, 1700195.), or of additives such as lignin or carbon black (Bengtsson, A.; Hecht, P.; Sommertune, J.; Ek, M.; Sjöholm, E. Carbon Fibers from Lignin-Cellulose Precursors: Effect of Carbonization Conditions. ACS Sustain. Chem. Eng.2020, 8, 6826-6833; Zhang, Xin; Lu, Yonggen; Xiao, Hao; Peterlik, Herwig (2014): Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers. In: Journal of Materials Science 49 (2), pp. 673-684. DOI: 10.1007 / sl0853-013-7748-0), the material yield, starting from 15 wt.%, can be increased to 40 wt.%. However, all these approaches have in common that, with regard to the mechanical properties, only a tensile modulus of elasticity < 130 GPa is achieved, and therefore an application as a reinforcing fiber is not practical, since the requirements regarding the tensile modulus of elasticity are at least 170 GPa.

[0005] The only known method for producing ex-cellulose CF with a tensile modulus of elasticity greater than 170 GPa is shown in Fig. 1 (Process A). Fig. 1 shows a schematic representation of the thermo-mechanical process for producing ex-cellulose CF with a tensile modulus of elasticity of 130 to 600 GPa and p less than 25 pQm. The process direction is from left to right. While Process A describes the prior art procedure with four thermal process stages, Process B shows an example of a process according to the invention with only three thermal process stages, which is discussed further below.

[0006] According to the prior art method described in Fig. 1 (process path A), a fourth process stage, the so-called graphitization or UHT stage (6), is carried out in the CF manufacturing process as shown in Fig. 1 (process path A) at temperatures of 2000 to 3000 °C (see also: Zhang, Xin; Lu, Yonggen; Xiao, Hao; Peterlik, Herwig (2014): Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers. In: Journal of Materials Science 49 (2), pp. 673-684. DOI: 10.1007 / sl0853-013-7748-0 (Morgan, P. Carbon Fibers and Their Composites; Taylor & Francis: Boca Raton, FL, USA, 2005; ISBN 0824709837). In this temperature range, the previously carbonized fiber material becomes plastic and can be stretched. and be oriented along the fiber axis.That this process works technically was demonstrated as early as the 1970s by Union Carbide, using commercially available ex-cellulose CF fiber types such as Thornel 25 and Thornel 50, which exhibited Young's modulus values ​​of 170–375 GPa. However, the fourth, or additional, UHT process stage (6) resulted in extremely high production and investment costs (CAPEX and OPEX), so that the production of ex-cellulose CF manufactured in this way was completely discontinued for economic reasons in 1978 (Morgan, P. Carbon Fibers and Their Composites; Taylor & Francis: Boca Raton, FL, USA, 2005; ISBN 0824709837). However, such a process is extremely complex, as high temperatures are essential. Furthermore, at these high temperatures, additional material loss in the carbon fibers and enormous wear and tear on the technical processing equipment are often observed.

[0007] Based on the known state of the art, the object of the present invention is therefore to provide an efficient and less expensive method with which higher tensile E-modulus values ​​(E > 130 GPa) and lower specific electrical resistances can be achieved in carbon fibers.

[0008] This problem is solved by the features of claim 1. Claim 13 specifies carbon fibers according to the invention. The respective dependent claims represent advantageous further developments.

[0009] The present invention thus relates in a first aspect to a process for the production of carbon fibers, in which cellulosic multifilament yarn is thermally stabilized, after stabilization is carried out in a first carbonization stage and after completion of the first carbonization stage is fed to a second carbonization stage, wherein the second carbonization stage is carried out at higher temperatures than in the first carbonization stage, wherein at least one boron-containing substance is applied to and / or incorporated into the cellulosic multifilament yarn before the second carbonization stage is carried out, and in the second carbonization stage the cellulosic multifilament yarn is stretched.

[0010] Based on the invention and the underlying process, ex-cellulose CF with tensile modulus values ​​of 130-600 GPa and a specific electrical resistance < 25 pQm can be produced at process temperatures between 1000 and 2000 °C (Figure 1, process path B). Surprisingly, the uneconomical process step of UHT treatment (6) at 2000 to 3000 °C can be avoided, and ex-cellulose CF can be produced on conventional carbonization plants designed for, e.g., ex-PAN CFs.

[0011] Carbon fibers (CFs) represent an intermediate product in the value chain, which is further processed into technically sophisticated and high-strength composite materials or components, particularly in lightweight construction. The claimed process leads to ex-cellulose CFs with the special technical characteristic of a high tensile modulus of elasticity (E-modulus) of 130-600 GPa in combination with a very low specific electrical resistance < 25 pQm. Furthermore, the process is characterized in that, compared to the prior art, the resulting CFs can be produced more cost-effectively and with a better carbon footprint (lower CO2 emissions). The CFs produced according to the claimed process and the resulting components would be characterized by lower weight (lightweight construction) and better electrical and thermal conductivity properties, as well as a better carbon footprint.

[0012] Essential to the process according to the invention is the timing of the introduction or application of the substance containing at least one boron (hereinafter also referred to synonymously as: treatment with the substance containing at least one boron). This treatment takes place before the second carbonization stage and can be carried out once or several times. For example, treatment is possible even before thermal stabilization. This includes, for example, treating a multifilament yarn with a boron-containing substance and then stabilizing it. The introduction or application of the substance containing at least one boron can also take place during the upstream production of the fiber, for example, by adding at least one boron-containing substance to a spinning solution for fiber production. It is also possible to add the substance containing at least one boron to a precipitation bath used in a spinning process.

[0013] Alternatively or additionally, the treatment—that is, the incorporation or application of the boron-containing substance onto or into the multifilament yarn—can take place during or after thermal stabilization and / or during or after the first thermal treatment stage. The process can be carried out continuously by passing quasi-continuous multifilament yarn through the described stages. However, it is also conceivable to first produce a precursor (i.e., a cellulosic multifilament yarn treated with a boron-containing substance) in a separate process, which is then thermally stabilized and / or carbonized at a later stage. The incorporation or application of the boron-containing substance and the thermal stabilization or carbonization can thus also be carried out locally at different locations.For example, it is possible to first impregnate the cellulosic multifilament yarn. Thermal stabilization and carbonization can then take place at a different time and location.

[0014] An advantageous embodiment provides that the total boron content of the cellulosic multifilament yarn after application or incorporation is at least 0.01 wt.%, preferably 0.01 wt.% to 5.0 wt.%, preferably 0.1 wt.% to 3.0 wt.%, and particularly preferably 0.3 wt.% to 1.5 wt.%.

[0015] The total boron content refers to the total weight content of elemental boron, based on the cellulosic multifilament yarn. The boron content can be determined, for example, using common analytical methods known to those skilled in the art, such as inductively coupled plasma emission spectroscopy (ICP-OES), etc.

[0016] In particular, the substance containing at least one boron is selected from the group consisting of inorganic and organic boron compounds, preferably selected from the group consisting of borates, boranes, boric esters, borneols, boron oxides, in particular orthoboric acid (H3BO3), metaboric acid (HBO2, 3 modifications), diboron trioxide (B2O3), trimethyl borate, and mixtures and combinations thereof.

[0017] The incorporation or application of the boron-containing substance is preferably carried out by impregnating the cellulosic multifilament yarn with an aqueous solution containing the boron-containing substance and / or by incorporating it into the spinning solution. In particular, the fiber made of cellulosic material is immersed or passed through an aqueous solution containing the boron-containing substance, for example, and particularly advantageously, before thermal stabilization.

[0018] In particular, the cellulosic fiber is pretreated with an aqueous solution of boric acid. The cellulosic yarn can be continuously passed through a suitable bath and then dried.

[0019] The application and / or incorporation of at least one boron-containing substance onto or into the cellulosic multifilament yarn can also take place directly after a solution spinning process for the production of the multifilament yarn. In this case, it is possible for the boron-containing substance to be applied and / or incorporated into the still-wet multifilament yarn (the so-called "never-dried" multifilament yarn). Alternatively, it is also possible for the spun multifilament yarn to be dried first, and the boron-containing substance to be applied and / or incorporated into the multifilament yarn after it has been re-moistened, if necessary.

[0020] Similarly, the multifilament yarn can be mixed with the substance containing at least one boron atom during the manufacturing process, i.e., during the solution spinning process. In this case, the substance containing at least one boron atom is added to the spinning solution, and the solution spinning process for the cellulosic multifilament yarn is carried out with this solution.

[0021] Alternatively or additionally, the boron-containing substance can also be incorporated after the stabilization stage or after the first carbonization stage. The only important thing is that a certain proportion of a boron-containing substance is present in or on the fiber before the second carbonization stage.

[0022] Thermal stabilization is carried out, for example, at temperatures of 150 to 400 °C, especially 200 to 300 °C.

[0023] Preferred temperatures for the first carbonation stage are in the range of 300 to 1000 °C, while higher temperatures, for example 1000 to 2000 °C, are maintained in the second carbonation stage.

[0024] During the second carbonization stage, the cellulosic multifilament yarn is stretched, preferably by a factor of 1.01 to 1.5, more preferably by 1.05 to 1.30.

[0025] In the second carbonization stage, the cellulosic multifilament yarn is subjected to a fiber tension of 1.0 cN / tex, preferably at least 2.5 cN / tex, particularly preferably at least 5.0 cN / tex in the fiber direction.

[0026] Stretching can also occur during the first carbonization stage.

[0027] In particular, the cellulosic multifilament yarn is not exposed to temperatures exceeding 2000 °C during the process.

[0028] The process according to the invention thus makes it possible to produce carbon fibers with a high tensile modulus and low specific electrical resistance in a significantly more economical way; exposure of the fibers to temperatures above 2000 °C (and thus the performance of a UHT stage) is therefore not necessary.

[0029] In particular, the process is carried out in such a way that, apart from the process stages stabilization, first carbonization stage and second carbonization stage, no further thermal process stages are carried out.

[0030] The cellulosic multifilament yarn comprises at least 50 wt.% cellulose, preferably at least 90 wt.% cellulose, and more preferably at least 95 wt.% cellulose. The cellulosic multifilament yarn can, for example, be made entirely of cellulose.

[0031] In a particularly preferred embodiment, only the cellulosic multifilament yarn is treated with an aqueous solution of the boron-containing substance, for example, boric acid. Preferably, the cellulosic multifilament yarn was produced prior to thermal stabilization by solution spinning, in particular by the viscose process, lyocell process, carbamate process, cold alkali process, cuproxide-ammonia (cuoxam) process, or by spinning the cellulosic multifilament yarn from ionic liquids. These processes are described, for example, in the textbook "Fibers - History, Production, Properties, Market", ed. Dieter Veit, Springer Verlag, 2023, ISBN 978-3-031-15308-2 (https: / / doi.org / 10.1007 / 978-3-031-15309-9).

[0032] According to a further aspect, the present invention relates to a cellulose-based carbon fiber produced as described above. The carbon fiber is characterized in particular by a high tensile modulus and low specific electrical resistance, a combination previously unknown in the prior art. Only the process according to the invention enables the corresponding properties of the carbon fiber.

[0033] Preferably, the carbon fiber has a tensile modulus of elasticity of at least 130 GPa, preferably 130 to 600 GPa, and more preferably 150 to 500 GPa. The tensile modulus of elasticity can be measured, for example, according to ISO 11566:1996.

[0034] The specific electrical resistance of the carbon fiber according to the invention can, for example, have values ​​of < 25 pQm, preferably of 1 to 20 pQm, and more preferably of 4 to 12 pQm. The specific electrical resistance can be measured, for example, according to ISO 13931:2013.

[0035] Likewise, the carbon fiber obtained is characterized by a total boron content (based on elemental boron) of 0.01 wt.% to 5.0 wt.%, preferably 0.1 wt.% to 3.0 wt.%, particularly preferably 0.3 wt.% to 1.5 wt.%.

[0036] The present invention is explained in more detail with reference to the following embodiments, without limiting the invention to the specific parameters shown. The present invention relates to a thermo-mechanical process at temperatures of max. 1000 to 2000 °C for the continuous production of ex-cellulose CF with a tensile modulus (E) between 130 GPa and 600 GPa and a specific electrical resistance (p) of less than 25 pQm based on cellulosic continuous multifilament yarns having a boron concentration of at least 0.01 wt.% (Figure 1, process path B). The thermo-mechanical process is characterized in that, in the HT stage (5) in the temperature range of 1000-2000 °C, the fiber is stretched in the fiber direction, e.g., by fiber transport devices (2), thereby inducing a mechanical force or fiber tension.This results in the surprising formation of a graphitically ordered carbon structure in the presence of boron at comparatively low temperatures. This structure is oriented parallel to the fiber axis and thus produces highly advantageous properties in the fiber direction. The graphitically ordered carbon structures can be unambiguously detected using X-ray diffraction methods. The higher the distortion ratio (fiber stretching) and the resulting fiber stress, the better the properties of the resulting ex-cellulose CF with regard to tensile modulus and specific electrical conductivity. Such an effect is not disclosed in the scientific literature or in the prior art. For a positive effect to occur, a boron content of at least 0.01 wt% is required, ideally in or on the processed cellulosic feedstock fiber.precursor (1) must be present, or a boron content of at least 0.01 wt% must be present in the intermediate fibers, e.g. between stage (3) and (4), between stage (4) and (5) or in the final C fiber (7).

[0037] As examples 1 to 12 demonstrate, the surprising effect of the disproportionately strong increase in the ex-cellulose CF properties (tensile modulus and specific electrical resistance p) occurs when a) boron is contained in the fiber material, e.g., in a concentration of 0.3 wt% or 1 wt%, and additionally b) in the temperature range between 1000 and 2000 °C, the fiber material is stretched, thereby inducing a high fiber tension.

[0038] Examples: In the following examples, an endless cellulosic multifilament yarn consisting of 1000 individual filaments (lk) was spun using the viscose process (solution spinning process) and impregnated with a boron-containing substance (boric acid) by immersing the filaments in an aqueous boric acid solution, dried, and wound onto the spool (1). The boron content relative to the yarn material was 0.3 wt% (Table 2) or 1.0 wt% (Table 3). This fiber material was then used for CF production according to process (B). The precursor was unwound from the spool (1) and continuously transported first through the stabilization oven (3) at 200 to 300 °C using a fiber transport device (2), then through the LT carbonization oven (4) at temperatures of 300 to 1000 °C and finally through the HT carbonization oven (5) at temperatures of 1000 to 2000 °C.In the HT carbonization furnace, different distortions (v4 / v3) were set by adjusting the speed ratio of the fiber transport unit positioned at the furnace inlet (v3) and outlet (v4), resulting in varying fiber tensions (Table 1-3). As a result, CF (7) were obtained whose tensile modulus (E-modulus) increased significantly with increasing distortion ratio / fiber tension o, and whose specific electrical resistance p decreased significantly.

[0039] Table 1: Process parameters: Deformation ratio E and yarn tension o in HT carbonization (1000 to 2000 °C) and resulting CF properties: Tensile E-modulus E and specific electrical resistance p based on a cellulosic lk multifilament yarn (precursor) without boron content produced according to the viscose solution spinning process (Comparative examples: ■

[0040] Table 2: Process parameters: Deformation ratio E and yarn tension o in HT carbonization (1000-2000 °C) and resulting CF properties: tensile E-modulus E and specific electrical resistance p based on a boron-containing (0.3 wt.% boron) cellulosic lk multifilament yarn (precursor) produced according to the viscose solution spinning process.

[0041] Table 3: Process parameters distortion ratio E and yarn tension o in HT carbonization (1000 to 2000 °C) and resulting CF properties tensile E-modulus E and specific electrical resistance p based on a boron-containing (1.0 wt.% boron) cellulosic lk multifilament yarn (precursor) produced according to the viscose solution spinning process.

[0042] Legend:

[0043] (A) Procedure A - State of the art

[0044] (B) Method B - claimed advantageous method

[0045] (1) Starting fiber material, cellulosic multifilament yarn

[0046] (2) Fiber transport unit Process stage 1 Stabilization (200-300 °C)

[0047] (3) Process stage 2 LT Carbonization (300-1000 °C)

[0048] (4) Process stage 3 HT carbonization (1000-2000 °C)

[0049] (5) Process stage 4 UHT carbonization (2000-3000 °C)

[0050] (6) Carbonized multifilament yarn (carbon fiber CF)

Claims

Patent claims 1. A process for producing carbon fibers in which cellulosic multifilament yarn is thermally stabilized, undergoes a first carbonization stage after stabilization, and is subjected to a second carbonization stage after completion of the first carbonization stage, wherein the second carbonization stage is carried out at higher temperatures than in the first carbonization stage, characterized in that, prior to carrying out the second carbonization stage, at least one boron-containing substance, selected from the group consisting of inorganic and organic boron compounds, is applied to and / or incorporated into the cellulosic multifilament yarn, and in the second carbonization stage, the cellulosic multifilament yarn is stretched by a factor of 1.01 to 1.

5.

2. Method according to claim 1, characterized in that the total boron content of the cellulosic multifilament yarn after application or incorporation is at least 0.01 wt.%, preferably 0.01 wt.% to 5.0 wt.%, preferably 0.1 wt.% to 3.0 wt.%, particularly preferably 0.3 wt.% to 1.5 wt.%.

3. A method according to one of the preceding claims, characterized in that the inorganic and organic boron compounds are selected from the group consisting of borates, boranes, boric esters, borneols, boron oxides, in particular orthoboric acid (H3BO3), metaboric acid (HBO2, 3 modifications), diboron trioxide (B2O3), trimethyl borates and mixtures and combinations thereof.

4. Method according to one of the preceding claims, characterized in that the application and / or incorporation of the substance containing at least one boron is carried out by impregnating the cellulosic multifilament yarn with an aqueous solution containing the substance containing at least one boron and / or by incorporating it into the spinning solution.

5. Method according to one of the preceding claims, characterized in that the application and / or incorporation of at least one boron-containing substance onto or into the cellulosic multifilament yarn takes place directly after a solution spinning process for the production of the multifilament yarn and after washing, if necessary.

6. Method according to one of the preceding claims, characterized in that the application and / or incorporation of the at least one boron-containing substance is carried out by adding the at least one boron-containing substance to a spinning solution which is spun into cellulosic multifilament yarn in a solution spinning process.

7. Method according to one of the preceding claims, characterized in that the stabilization is carried out at temperatures of 150 to 400 °C.

8. Method according to one of the preceding claims, characterized in that the first carbonization stage is carried out at temperatures of 300 to 1000 °C and / or the second carbonization stage is carried out at temperatures of 1000 to 2000 °C.

9. Method according to one of the preceding claims, characterized in that in the second carbonization stage the stretching of the cellulosic multifilament yarn is carried out by a factor of 1.05 to 1.

30.

10. Method according to one of the preceding claims, characterized in that in the second carbonization stage the cellulosic multifilament yarn is subjected to a fiber tension of 1.0 cN / tex, preferably at least 2.5 cN / tex, particularly preferably at least 5.0 cN / tex in the fiber direction.

11. Method according to one of the preceding claims, characterized in that during the method the cellulosic multifilament yarn is not exposed to temperatures of more than 2000 °C.

12. Method according to one of the preceding claims, characterized in that no further thermal process steps are carried out in addition to the process stages stabilization, first carbonization stage and second carbonization stage.

13. Method according to one of the preceding claims, characterized in that the cellulosic multifilament yarn contains at least 50 wt.% cellulose, preferably at least 90 wt.% cellulose, more preferably at least 95 wt.% cellulose.

14. Carbon fiber produced by a process according to any of the preceding claims.

15. Carbon fiber according to the preceding claim, characterized by a tensile modulus of elasticity, measured according to ISO 11566:1996, of at least 130 GPa, preferably 130 to 600 GPa, more preferably 150 to 450 GPa.

16. Carbon fiber according to one of the two preceding claims, characterized by a specific electrical resistance of < 25 pQm, preferably of 1 to 20 pQm, more preferably of 4 to 12 pQm, measured according to ISO 13931:2013.

17. Carbon fiber according to one of claims 14 to 16, characterized by a total boron content of 0.01 wt.% to 5.0 wt.%, preferably 0.2 wt.% to 2.0 wt.%, particularly preferably 0.3 wt.% to 1.5 wt.%.

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