Bio-based precursor suitable for producing high-modulus carbon fibers
Patent Information
- Application Number
- PCT/CN2025/085252
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-03-27
- Publication Date
- 2026-10-01
Smart Images
Figure PCTCN2025085252-FTAPPB-I100001 
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Abstract
Description
A bio-based precursor suitable for producing high-modulus carbon fibers Technical Field
[0001] This invention relates to the field of carbon fiber preparation, specifically to a bio-based precursor suitable for producing high-modulus carbon fibers. Background Technology
[0002] Carbon fiber is used in the manufacture of high-performance composite materials and other technically demanding applications requiring excellent mechanical properties and lightweight characteristics. Compared to other materials such as glass fiber or steel, carbon fiber exhibits higher tensile strength and tensile modulus while being lighter in weight.
[0003] Carbon fibers are classified into "standard modulus" (SM), "medium modulus" (IM), "high modulus" (HM), or "ultra-high modulus" (UHM) carbon fibers according to their technical specifications. Standard modulus carbon fibers typically have a tensile strength between 3.5 and 4.5 GPa and a tensile modulus between 230 and 270 GPa. A typical application of standard modulus fibers is in pultruded laminates used to reinforce wind turbine rotor blades. Medium modulus carbon fibers typically have a tensile strength between 5.0 and 8.0 GPa and a tensile modulus between 280 and 300 GPa. They are used in applications requiring lightweight and high tensile strength, such as the main structure of aircraft and hydrogen pressure vessels. Standard modulus and medium modulus carbon fibers are typically made from polyacrylonitrile copolymer (PAN)-based precursor fibers and undergo thermal conversion at carbonization temperatures typically between 1500 and 1700 °C. High-modulus carbon fibers, also made from polyacrylonitrile precursors, typically exhibit tensile strengths between 2.7 and 5.0 GPa and tensile moduli between 400 and 600 GPa. They are used in applications requiring lightweight and high stiffness, such as golf clubs or robotic manufacturing equipment. Ultra-high modulus carbon fibers typically exhibit tensile strengths between 2.5 and 3.5 GPa and tensile moduli between 700 and 900 GPa. They are generally made from mesophase pitch precursors. Both high-modulus and ultra-high modulus carbon fibers require ultra-high temperature processing steps to achieve the desired tensile modulus levels, performed at temperatures of at least 2200°C. Ultra-high modulus carbon fibers are specifically designed for state-of-the-art engineering applications requiring extremely high stiffness per unit weight, such as satellites, drones, and sports equipment. The resistivity of both high-modulus and ultra-high modulus carbon fibers is 2–8 μΩ·m, making them suitable for applications requiring lightweight and high conductivity, such as electromagnetic interference (EMI) shielding to protect sensitive electronic components. Due to their high chemical resistance, they are suitable for producing so-called carbon paper in the gas diffusion layer of fuel cells. The high production cost of high-modulus and ultra-high-modulus carbon fibers makes them the most expensive carbon fibers on the market, which limits their market expansion potential despite their excellent material properties.
[0004] The high tensile modulus of carbon fiber originates from the linear orientation of sp along the fiber axis. 2 Hybridized carbon domains (or "graphite stacks"). During production, high-temperature heat treatment, and, where appropriate, the use of graphitization catalysts, promotes sp... 2 The formation of hybrid carbon. Carbon fibers are produced through the thermal conversion of precursor fibers. The precursor fibers undergo heat treatment at different temperatures and residence times as they pass through different furnaces. The first heat treatment step, known as stabilization or oxidation, involves the precursor being subjected to temperatures up to 350°C. After stabilization, the precursor is transferred to a low-temperature carbonization (LT) furnace, where it is treated at temperatures up to approximately 1000°C, followed by a high-temperature (HT) furnace, where it is treated at maximum temperatures typically between 1500 and 1700°C. In some cases, if high-modulus or ultra-high-modulus carbon fibers are required, the fibers are subsequently graphitized in an ultra-high-temperature (UHT) furnace, at temperatures up to 3000°C. This ultra-high-temperature process is extremely costly due to its high power consumption and the rapid thermal degradation of the heating elements in the UHT furnace (which typically require replacement after one month of production).
[0005] In terms of production volume, polyacrylonitrile (PAN)-based carbon fiber accounts for approximately 98% of the carbon fiber market, while pitch-based and cellulose-based carbon fibers account for the remaining 2%. Therefore, most carbon fibers today, especially those used in structural applications, are produced from petroleum-based feedstocks. The use of fossil-based feedstocks and the high energy consumption during thermal conversion result in a high carbon footprint for carbon fibers, especially for high-modulus (HM) and ultra-high-modulus (UHM) carbon fibers, as they are processed at extremely high temperatures. This makes it difficult for companies that prioritize environmental protection and sustainable development to find a reasonable justification for using carbon fibers, particularly high-modulus and ultra-high-modulus fibers.
[0006] Viscose-based carbon fiber, made from natural cellulose, is a typical bio-based carbon fiber and was the first commercially available carbon fiber used in structural applications. In the 1970s, Union Carbide launched it under the brand name Thornel. These carbon fibers have tensile moduli ranging from 170 to 690 GPa and tensile strengths up to 4 GPa (Morgan, P., *Carbon Fibers and Their Composites*; Taylor & Francis Publishers: Boca Raton, Florida, 2005; ISBN 0824709837). However, its production requires a stretching graphitization process, which involves stretching the fibers at ultra-high temperatures of approximately 3000°C. Due to the high processing temperature and a material yield of only 15-20%, production costs are extremely high. The advent of polyacrylonitrile (PAN)-based carbon fibers rendered Thornel carbon fibers obsolete, and its production ceased in 1978. Today, cellulose-based carbon fibers based on viscose precursors are used in non-structural applications such as insulation and filtration (activated carbon fibers).
[0007] In summary, existing precursor fibers require extremely high energy consumption in the process of preparing high-modulus carbon fibers, especially the use of fossil-based raw materials, which results in a high carbon footprint for carbon fiber production. Therefore, the market needs a bio-based precursor that can prepare bio-based high-modulus carbon fibers with lower energy consumption. Summary of the Invention
[0008] To address the above problems, this invention provides a bio-based precursor suitable for producing high-modulus carbon fibers. The specific solution is as follows:
[0009] A bio-based precursor suitable for producing high-modulus carbon fibers, characterized in that the bio-based precursor is a cellulose multifilament, wherein the filaments in the cellulose multifilament have a circular cross-section with a roundness error of 1 to 2, preferably 1.0 to 1.2, and particularly preferably 1 to 1.1;
[0010] The boron content of the cellulose multifilament is from 0.01% to 2.0% by weight, preferably from 0.1% to 1.0%, particularly preferably from 0.2% to 0.4% by weight, and most preferably from 0.2% to 0.3% by weight;
[0011] The crystallinity of the cellulose multifilament monofilament is 40% or more, preferably 45% or more, and more preferably 48% or more, and the degree of crystal orientation is 75% or more, preferably 80% or more.
[0012] For carbon fibers, a circular cross-sectional shape is superior to a non-circular shape because grooves and cracks along the fiber axis of carbon fibers with non-circular cross-sections can become defect points, leading to a decrease in tensile strength and tensile modulus. Theoretically, when tensile force is applied during thermal conversion, a circular cross-section can withstand a higher tensile force compared to a non-circular cross-section precursor because it allows for a more uniform load distribution. Grooves on the surface of a non-circular cross-section precursor can cause stress concentration, thus limiting the amount of tensile force that can be applied. The cross-sectional shape of the precursor fiber determines the cross-sectional shape of the resulting carbon fiber; carbon fibers with circular cross-sections are typically prepared from precursors with circular cross-sections. Therefore, the roundness tolerance of the precursor fiber cross-section is limited in this invention.
[0013] Cellulose is a material that is difficult to graphitize, and graphitization requires high temperatures. For such materials, impregnation with a catalyst that promotes graphitization can lower the graphitization temperature, thereby reducing the energy consumption in carbon fiber production. Boric acid is a commonly used catalyst for carbon fiber graphitization. However, existing research indicates that while the addition of boric acid lowers the graphitization temperature, it may also have an adverse effect on the properties of carbon fibers. For example, Ya et al. showed that for polyacrylonitrile-based carbon fibers, boric acid increases the tensile modulus when heat-treated in the temperature range of 1300-2900℃, but decreases the tensile strength at 2300℃, after which boric acid begins to increase the strength again (Ya Wen, Materials & Design, Vol. 36, April 2012, pp. 728-734); Jones and Thrower reported that boron reduces the tensile strength and modulus of pitch-based carbon fibers, which is attributed to stress concentration at the fiber skin-core interface caused by boron doping (Carbon, Volume 29, 1sue 2, 1991, Pages 251-269). Le et al. described the use of boric acid to improve material yield in the conversion of cellulose-lignin precursors into carbon fibers (Le et al., Cellulose, Volume 28, pages 729-739, 2021, DOI 10.1007 / s10570-020-03584-x). Hückstaedt et al. described the effect of boric acid on the graphitization of cellulose precursors prepared by viscose wet spinning (Polymers 2023, 15, 4310. https: / / doi.org / 10.3390 / polym15214310), in which the precursors were doped with boric acid solution, and the resulting impregnated precursor fibers contained 5.6% boric acid by weight (equivalent to 1% elemental boron). Subsequently, the precursor was heat-treated at a maximum temperature of 2000°C without stretching the fibers, resulting in carbon fibers with a tensile modulus of 137 GPa and a resistivity of 7.4 μΩ·m. The authors report that the stretching-carbonization treatment can increase the tensile modulus of carbon fibers obtained from boric acid-doped viscose precursors to 230 GPa. However, the authors also point out a drawback of this method: boric acid reduces the tensile strength of the carbon fibers from 1000 MPa to 750 MPa, due to the formation of boron carbide, which becomes a defect in the fiber. In this invention, the boron content of the cellulose multifilament is defined as 0.01% to 2.0% by weight, preferably 0.1% to 1.0%, particularly preferably in the range of 0.2% to 0.4% by weight, and most preferably 0.2% to 0.3%. Compared with the prior art, the boron content in the precursor can be reduced without affecting the mechanical properties of the resulting carbon fibers.Surprisingly, our research found that by lowering the graphitization temperature of the precursor, thermal conversion at a similar maximum carbonization temperature (2000℃) can increase the elastic modulus of the resulting carbon fibers without reducing their tensile strength. This may be attributed to the higher crystallinity and crystal orientation of the precursor fibers, which promotes the formation of oriented carbon domains in the final carbon fibers. Therefore, a lower graphitization temperature can be used during the thermal conversion process, and the amount of boron required for the catalytic formation of oriented carbon domains can be reduced. The latter avoids the formation of structural defects such as boron carbide in the carbon fibers, thereby avoiding a decrease in the strength of the resulting carbon fibers.
[0014] The addition of boron to the precursor can be achieved by adding boron to the spinning solution or by impregnating the precursor fibers in a boron-containing liquid. Preferably, the cellulose fiber precursor is impregnated in a boric acid solution to introduce boron into the precursor.
[0015] The cross-sectional shape and crystallinity of precursor fibers are determined by the spinning method and the diffusion process of solvent from the filament bundle into the non-solvent in the precipitation bath. In wet spinning (such as viscose spinning), the spinning nozzle is placed in the precipitation bath, which results in a non-circular cross-section of the precursor. In air-gap spinning (such as lyocell spinning or ionic liquid spinning), the spinning nozzle is located a few centimeters above the precipitation bath, which makes the cross-section closer to circular. Furthermore, compared to wet-spun viscose fibers, air-gap spun cellulose fibers have a higher crystalline structure, resulting in a higher tensile modulus and lower elongation at break.
[0016] The deviation from a perfectly circular cross-section can be quantified using roundness error. It is defined as follows:
[0017] Where L is the perimeter of the cross section and A is the cross-sectional area.
[0018] Furthermore, the coefficient of variation of the roundness error between filament monofilaments is less than 2%, preferably less than 1.5, and particularly preferably less than 1.2.
[0019] The coefficient of variation refers to the dispersion of the roundness error data, and is defined as the ratio of the standard deviation to the mean. In this invention, the coefficient of variation is calculated by measuring the roundness error of at least 50 fiber cross-sections. The smaller the coefficient of variation of the roundness error between filaments in the precursor, the better its uniformity, and the better the uniformity of stretching during heat treatment.
[0020] Furthermore, the diameter of the filaments in the cellulose multifilament is 5 to 30 μm, preferably 10 to 20 μm, and particularly preferably 12 to 17 μm. The fiber diameter is determined according to the measurement method described in GB / T 29762-2013 "Determination of Diameter and Cross-sectional Area of Carbon Fiber".
[0021] Because the precursor needs to be stretched in subsequent production processes, the cross-section of the fibers decreases during stretching. Commercial carbon fibers typically require a diameter of 5-7 μm, therefore the precursor fibers need to be sufficiently thick to allow for adequate stretching. Furthermore, thicker fibers can withstand greater stretching, and a higher degree of stretching is beneficial for carbon fiber crystallization and orientation, thus enabling the production of carbon fibers with better performance. However, the precursor fibers should not be too thick, as excessively thick fibers are difficult to oxidize sufficiently during the stabilization stage. Therefore, this invention limits the diameter of the precursor filaments.
[0022] The tensile strength of the precursor is 10 to 180 cN / tex, preferably 30 to 130 cN / tex, and particularly preferably 40 to 90 cN / tex;
[0023] The tensile modulus is in the range of 1000 cN / tex to 4000 cN / tex, preferably in the range of 1300 cN / tex to 3500 cN / tex, and particularly preferably in the range of 1700 cN / tex to 3000 cN / tex;
[0024] The elongation at break is 2% to 15%, preferably 4% to 12%, and particularly preferably 5% to 10%.
[0025] Inorganic metal content in the bio-based precursor:
[0026] Sodium (Na) content is less than 1% by weight, preferably less than 0.1% by weight, and particularly preferably less than 0.005% by weight;
[0027] Potassium (K) content is less than 1% by weight, preferably less than 0.1% by weight, and particularly preferably less than 0.005% by weight;
[0028] The calcium (Ca) content is less than 1% by weight, preferably less than 0.1% by weight, and particularly preferably less than 0.005% by weight.
[0029] The presence of certain inorganic metal atoms (such as sodium) in the precursor can negatively affect tensile strength because these atoms may form local hot spots and generate defects on the fiber during thermal conversion. Therefore, the inorganic metal content in the precursor is limited in this invention.
[0030] The bio-based precursor contains more than 70% bio-based carbon, preferably more than 80%, and particularly preferably more than 90%, as determined by EN 16640.
[0031] Cellulose is a natural polymer that is widely found in nature. The technology for producing fibers from cellulose is very mature. Compared with petroleum-based precursors, cellulose precursors have a smaller carbon footprint, better sustainability, and less environmental pollution.
[0032] There are three naturally occurring carbon isotopes on Earth: carbon-12 (… 12 C), accounting for 99% of all carbon on Earth; carbon-13 ( 13 C), accounting for 1%; and carbon-14 ( 14 C), its content is extremely low, only one in every 10 in the atmosphere. 12 There are approximately 1-1.5 carbon atoms per carbon atom. 14 C atom. 12 C and 13 C is stable; 14 Carbon (C) is radioactive and unstable, with a half-life of 5700 ± 30 years. When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons. 14 Carbon (C) is produced in the upper troposphere and stratosphere by nitrogen atoms absorbing neutrons. Plants absorb carbon dioxide during photosynthesis, therefore they contain C. 14 C. In addition, any product made from freshly harvested biomass also contains... 14 C. After plants and animals die, they decay due to radioactivity. 14 The content of C will decrease, and it will no longer exist after 45,000 years. 14 C residue. Fossil-based products are made from carbon raw materials that are millions of years old, and therefore contain no C residue. 14 C. Measurement standard EN 16640 utilizes this principle to determine the content of bio-based carbon in materials using accelerator mass spectrometry.
[0033] This invention also provides a process for producing high-modulus carbon fibers based on the above-mentioned bio-based precursor. Specifically, the process for producing the high-modulus carbon fibers includes the following steps:
[0034] (1) The cellulose multifilament precursor was heat-treated in a stabilization furnace;
[0035] (2) Subsequent heat treatment is carried out in a low-temperature carbonization furnace;
[0036] (3) Further heat treatment is carried out in a high-temperature carbonization furnace at a temperature of up to 2000°C, and the multifilament is subjected to a stretch of more than 30%.
[0037] After high-temperature carbonization furnace heat treatment, the fibers are no longer subjected to an ultra-high temperature treatment step. In this invention, the ultra-high temperature treatment refers to a treatment step with a temperature above 2200℃.
[0038] The tensile modulus of carbon fibers increases with increasing graphite stack size, decreasing stack spacing, and increasing orientation along the fiber axis. In this invention, the orientation is improved by stretching the fibers in a high-temperature carbonization furnace, thereby increasing the tensile modulus of the bio-based carbon fibers. To ensure the obtained carbon fibers achieve a high modulus, the stretch applied to the multifilaments during the high-temperature carbonization stage should be greater than 30%. The tensile strength of carbon fibers depends on the number density and size of defects (such as pores, lattice defects, or surface cracks, grooves, or pores). In this invention, the tensile strength of the bio-based carbon fibers is avoided by reducing the amount of boron used.
[0039] Furthermore, the heat treatment in the stabilization furnace is carried out in air at a temperature not exceeding 310°C, preferably not exceeding 290°C.
[0040] Furthermore, the heat treatment in the low-temperature carbonization furnace is carried out at a maximum temperature of 1000°C, and tension is applied to the fibers during the low-temperature carbonization process to avoid fiber shrinkage and performance loss.
[0041] Furthermore, the heat treatment steps in the low-temperature carbonization furnace and the heat treatment steps in the high-temperature carbonization furnace are carried out in a protective gas.
[0042] Using the bio-based precursors described above as raw materials, the high-modulus carbon fibers produced by the high-modulus carbon fiber production process described above possess the following characteristics:
[0043] The diameter of the carbon fiber is 5-10 μm, preferably 5 to 8 μm, and particularly preferably 5 to 7 μm;
[0044] The roundness error of the bio-based carbon fiber is 1.0 to 2.0, preferably 1.0-1.2, and particularly preferably 1.0-1.1, wherein the roundness error is calculated according to the following formula:
[0045] Where L is the perimeter of the cross section and A is the cross-sectional area.
[0046] The tensile strength is at least 2 GPa, preferably at least 2.5 GPa, and more preferably at least 2.9 GPa;
[0047] The tensile modulus is at least 220 GPa, preferably at least 270 GPa, and more preferably at least 300 GPa;
[0048] The elongation at break is 0.5% to 2.5%, preferably 0.7% to 2.0%, and particularly preferably 1% to 1.5%.
[0049] The bio-based carbon fiber has a bio-source carbon content of more than 80%, preferably more than 90%, and particularly preferably more than 98%, and the bio-source carbon content is determined according to EN16640.
[0050] The resistivity of the bio-based carbon fiber is less than 15 μΩ·m.
[0051] Existing commercially available carbon fibers typically have a diameter of 5 to 7 micrometers. To ensure the carbon fibers of this invention are compatible with the application scenarios of existing commercial carbon fibers, the diameter of the carbon fibers is limited. Compared to a circular cross-section, irregularly shaped carbon fibers have uneven surfaces, are prone to stress concentration, or experience localized heat concentration during thermal conversion, thus affecting the performance of the carbon fibers. Therefore, defining the roundness of the carbon fibers is crucial. The bio-based carbon content of the bio-based carbon fibers is limited to ensure that the carbon fibers are of biological origin, reducing the carbon footprint of the carbon fibers. The tensile strength, tensile modulus, and elongation at break of the carbon fibers are also limited to ensure that the performance of the carbon fibers meets the requirements of practical applications. Beneficial effects:
[0052] In the solution provided by this invention, the high crystallinity of the precursor reduces the graphitization temperature and the amount of boron used, avoiding the negative impact of excessive boron on the properties of the resulting carbon fibers. The addition of lower boron content in the precursor also helps to lower the graphitization temperature. Furthermore, the high roundness of the precursor allows for more uniform load distribution, enabling greater stretching of the yarn during thermal conversion without the need for ultra-high temperature (UHT) treatment. This greater stretching imparts higher modulus and higher conductivity to the resulting carbon fibers. Compared to precursors made from fossil-based raw materials, the bio-based precursor provided by this invention uses renewable raw materials and eliminates the need for ultra-high temperature (UHT) treatment during thermal conversion, resulting in lower energy consumption and enabling the production of high-modulus, conductive carbon fibers with a lower carbon footprint. Moreover, it enables the production of high-modulus carbon fibers with tensile modulus exceeding 400 GPa, tensile strength exceeding 2.9 GPa, and resistivity below 15 μΩ·m with lower energy consumption. Detailed Implementation
[0053] Different carbon fiber precursors with an oil content of 0.3% were prepared. The carbon fiber precursors were cellulose multifilament yarns containing multiple cellulose monofilaments. These precursors were converted into carbon fibers by first heat treatment in a stabilization furnace, then subsequent heat treatment in a low-temperature (LT) carbonization furnace, and finally further heat treatment in a high-temperature (HT) carbonization furnace.
[0054] The heat treatment during stabilization is carried out in air at a maximum temperature of 290°C for 60 minutes. The heat treatment during low-temperature carbonization is carried out at a maximum temperature of 1000°C, and a tension of 1.1 cN / tex is applied during the low-temperature carbonization process. The heat treatment during high-temperature carbonization is carried out at a maximum temperature of 2000°C, while the multifilament is stretched. In the examples below, the stretch applied to the multifilament is its maximum applicable stretch. Depending on the specific implementation process and process objective, the maximum applicable stretch is defined as 35%-99% of the elongation at break of the filament at 2000°C, preferably 50-99%, more preferably 80%-98%, and even more preferably 85%-95%. Specifically, in this invention, to obtain high-modulus carbon fibers, the maximum applicable stretch used in the examples below is 90% of the elongation at break of the filament. The maximum applicable stretch is determined by first measuring the elongation at break of the filament yarn at 2000°C. For different precursors, the maximum applicable stretch varies between 5% and 35%. For example, the carbon fiber in Comparative Example 4b has a breaking tensile strength of 13% during the high-temperature carbonization process; therefore, its maximum applicable tensile strength is 90% of this value, or 12%. The carbon fiber in Comparative Example 5b has a breaking tensile strength of 28% during the high-temperature carbonization process; therefore, its maximum applicable tensile strength is 90% of this value, or 25%.
[0055] Roundness error is determined according to the following formula:
[0056] Where L is the perimeter of the cross section and A is the cross-sectional area.
[0057] This error was obtained from optical microscope images using the imaging software ImageJ.
[0058] Boron content was determined using inductively coupled plasma atomic emission spectrometry (ICP-OES). This method involves introducing the sample as an aerosol into an argon plasma at a temperature up to 7000°C, where covalent bonds are completely dissociated, and atoms and ions are excited to emit light. The emission spectra are separated by a spectrophotometer and received by a detector. A calibration curve is established using a boric acid aqueous solution of known concentration to achieve a quantitative correlation between the intensity of characteristic spectral lines and the boron content. For the boron content detection of precursor fibers, the specific procedure is as follows: the fiber is cut into 5mm segments, digested in nitric acid under microwave irradiation, and the resulting solution is measured by introducing the aerosol into an argon plasma using an ICP-OES Optima 2100DV spectrometer (Perkin-Elmer).
[0059] The technical solution of the present invention will be illustrated more clearly below with a series of comparative examples and embodiments.
[0060] Comparative Example 1a
[0061] A carbon fiber precursor was prepared using a viscose process. A dissolving slurry with an intrinsic viscosity of 480 ml / g was activated with 17% caustic soda. The resulting activated cellulose had a cellulose content of 33% (mass fraction) and a sodium hydroxide content of 15% (mass fraction). After kneading and aging, the activated cellulose was reacted with carbon disulfide (CS2) at 32% (mass fraction) relative to the cellulose. The resulting cellulose xanthate had a γ value of 51 and was dissolved in a sodium hydroxide solution to adjust the final alkali content of the viscose solution to 7.1%. The resulting solution had a cellulose content of 9.1%.
[0062] The solution was spun into an acidic precipitation bath using a spinneret with 1000 orifices. The precipitation bath contained 100 g / L sodium sulfate, 200 g / L sulfuric acid, and 25 g / L zinc sulfate. The precipitated filament yarn was then fed into a decomposition bath containing a 15% sulfuric acid solution, washed with distilled hot water, and dried.
[0063] The filament has a strength of 40 cN / tex, a tensile modulus of 900 cN / tex, and an elongation at break of 12%. The filament has a leaf-shaped cross-section, with a roundness error of 2.0 and a coefficient of variation of 15%. Due to its leaf-shaped cross-section, its diameter cannot be determined. Wide-angle X-ray spectroscopy (WAXS) determined its crystallinity to be 30%, and its crystalline phase orientation along the fiber axis to be 52%.
[0064] Comparative Example 1b
[0065] The carbon fiber precursor from Comparative Example 1a was heat-treated and subjected to 10% of the maximum applicable tensile force during high-temperature carbonization to convert it into carbon fibers. The resulting carbon fibers had a tensile strength of 1050 MPa, a tensile modulus of 130 GPa, an elongation at break of 0.8%, and a resistivity of 22 μΩ·m. These carbon fibers had a leaf-shaped cross-section with a roundness error of 1.7 and a coefficient of variation of 13%. Due to their leaf-shaped cross-section, their diameter could not be determined.
[0066] Comparative Example 2a
[0067] Viscose-based multifilaments were prepared using the same method as described in Comparative Example 1a. To improve the tensile modulus and tensile strength of the corresponding carbon fibers, the viscose yarn was impregnated in a 4% (w / w) aqueous solution of boric acid. Boric acid is a well-known carbon fiber graphitization catalyst; after impregnation, the boric acid content in the viscose yarn reached 5.7%, equivalent to 1% (w / w) of boron. The filament strength was 42 cN / tex, the tensile modulus was 907 cN / tex, and the elongation at break was 12%. The roundness error was measured to be 2.0, and the coefficient of variation was 16%. Due to the petal-like cross-section, the diameter could not be determined. Microscopic images showed that fine solid crystals formed on the surface of the monofilament.
[0068] Comparative Example 2b
[0069] The viscose yarn impregnated in Comparative Example 2a was heat-treated and subjected to 15% of the maximum applicable tensile strength during high-temperature carbonization to convert it into carbon fiber.
[0070] The obtained carbon fibers have a tensile strength of 960 MPa, a tensile modulus of 248 GPa, an elongation at break of 0.4%, and a resistivity of 7 μΩ·m. These carbon fibers have a leaf-shaped cross-section with a roundness error of 1.7 and a coefficient of variation of 14%. Due to their leaf-shaped cross-section, their diameter cannot be determined.
[0071] Compared to Comparative Example 1b, this carbon fiber exhibits a significantly higher tensile modulus and a significantly lower resistivity, but its tensile strength is unexpectedly low. Similar to Hückstaedt's explanation, it is speculated that excessive boric acid content leads to deposits on the fiber surface, disrupting the formation of a continuous carbon structure and thus causing surface defects.
[0072] Comparative Example 3a
[0073] To reduce surface defects, the amount of boric acid used during impregnation was reduced. A viscose-based multifilament was prepared using the same method as in Comparative Example 2a. The viscose yarn was impregnated in a 2% (w / w) aqueous solution of boric acid, and the mass fraction of boric acid in the impregnated viscose yarn was determined by inductively coupled plasma mass spectrometry (ICP-OES) to be 2.7%, equivalent to a boron mass fraction of 0.5%.
[0074] The filament has a strength of 39 cN / tex, a tensile modulus of 895 cN / tex, and an elongation at break of 12%. The measured roundness error is 2.0, and the coefficient of variation is 15%. The images show that there are no visible crystals on the filament surface.
[0075] Comparative Example 3b
[0076] The viscose yarn impregnated in Comparative Example 3a was heat-treated and subjected to 13% of the maximum applicable tensile force during high-temperature carbonization to convert it into carbon fibers. The resulting carbon fibers had a tensile strength of 1150 MPa, a tensile modulus of 210 GPa, an elongation at break of 0.95%, and a resistivity of 8 μΩ·m. These carbon fibers had a leaf-shaped cross-section with a roundness error of 1.7 and a coefficient of variation of 13%. Due to the leaf-shaped cross-section, their diameter could not be determined.
[0077] Comparative Example 3 shows that reducing the amount of boric acid can slightly increase the tensile strength, but the tensile modulus will decrease. Furthermore, if the amount of boric acid is too low, it cannot catalyze graphitization during the thermal conversion process. Comparative Examples 1-3 show that viscose-based precursors have the potential to produce carbon fibers with high modulus and low resistivity, but they cannot meet the tensile strength requirements.
[0078] Comparative Example 4a
[0079] A carbon fiber precursor was prepared using the Lyocell process. Cellulose pulp was mixed with an N-methylmorpholine-N-oxide monohydrate (NMMO·H₂O) solution under vacuum and stirred at 90°C. The resulting spinning solution with a 10% cellulose concentration was pumped into a spinneret with 1000 spinneret holes, then passed through a 10 cm long air layer before entering a coagulation bath. The coagulation bath contained a mixture of NMMO and water at a ratio of 20:80. The precipitated filament yarn was washed with deionized water, dried at 150°C, oiled, and then wound.
[0080] Scanning electron microscopy (SEM) analysis revealed that the roundness error of the cellulose precursor filament was 1.05, the coefficient of variation was 1%, and the average diameter was 11 μm. The filament strength was measured to be 45 cN / tex, the tensile modulus was 2050 cN / tex, and the elongation at break was 6.3%. Wide-angle X-ray spectroscopy (WAXS) analysis showed that its crystallinity was 48%, and the crystalline phase orientation along the fiber axis was 82%.
[0081] Comparative Example 4b
[0082] The lyocell filament yarn from Comparative Example 4a was heat-treated and subjected to 12% of the maximum applicable tensile force during high-temperature carbonization to convert it into carbon fibers. The resulting carbon fibers had a tensile strength of 2250 MPa, a tensile modulus of 158 GPa, and an elongation at break of 1.3%. Compared to the carbon fibers prepared from the viscose precursor in Comparative Example 1b, these carbon fibers exhibited higher tensile strength and tensile modulus. This is attributed to the higher crystallinity of precursor filament 4a compared to precursor filament 1a, resulting in a higher degree of orientation in the obtained carbon fibers.
[0083] The resistivity of this carbon fiber is 19 μΩ·m. Its cross-section is circular with a roundness error of 1.03, a coefficient of variation of 1%, and a diameter of 4.4 μm, which is too small for commercially available carbon fibers.
[0084] Comparative Example 5a
[0085] A carbon fiber precursor was prepared using the process described in Comparative Example 4a. Lyocell filament yarn was impregnated in a 4% (w / w) aqueous solution of boric acid. Inductively coupled plasma mass spectrometry (ICP-OES) determined that the mass fraction of boric acid in the impregnated yarn was 1.4%, equivalent to a mass fraction of boron of 0.25%. Compared to the viscose-based precursor fiber in Comparative Example 2a, this yarn absorbed a lower amount of boric acid, which is attributed to its higher crystallinity.
[0086] The roundness error was 1.05, the coefficient of variation was 1%, and the average diameter was 11 μm, as determined by scanning electron microscopy.
[0087] Comparative Example 5b
[0088] The impregnated filament yarn from Comparative Example 5a was heat-treated to convert it into carbon fibers. The maximum applicable tensile strength applied during the high-temperature carbonization process was 25%. The resulting carbon fibers had a tensile strength of 2700 MPa, a tensile modulus of 250 GPa, an elongation at break of 1.0%, and a resistivity of 7 μΩ·m. The carbon fibers had a circular cross-section with a roundness error of 1.03, a coefficient of variation of 1%, and a diameter of 4.9 μm. For commercially available carbon fibers, this fiber diameter is slightly small and does not meet commercial requirements. Comparative Examples 5a and 5b show that, surprisingly, a much lower boron content in the precursor is sufficient to induce boric acid to catalyze graphitization compared to viscose fibers (Comparative Examples 3a and 3b). This may be attributed to the higher crystallinity of the precursor, which facilitates the formation of oriented graphite domains in the resulting carbon fibers. Furthermore, the inventors were also surprised to find that boric acid not only lowered the graphitization temperature but also increased the maximum applicable tensile strength of the fiber during the thermal conversion process, which helps to improve the modulus and conductivity of the carbon fibers.
[0089] Comparative Example 6a
[0090] To prepare carbon fibers with a diameter of at least 5 μm, a larger diameter lyocell carbon fiber precursor was prepared. Scanning electron microscopy (SEM) images showed a roundness error of 1.03, a coefficient of variation of 1%, and an average diameter of 14 μm. Measurements also revealed a filament strength of 43 cN / tex, a tensile modulus of 1900 cN / tex, an elongation at break of 7.0%, a crystallinity of 49%, and a crystal orientation degree of 84%.
[0091] Comparative Example 6b
[0092] The yarn in Comparative Example 6a was heat-treated to convert it into carbon fibers. Because the precursor in Comparative Example 6a had a larger diameter, a higher tensile strength (16%) could be applied during the high-temperature carbonization stage compared to the thermal conversion process in Comparative Example 4a, without reducing the diameter of the resulting carbon fibers to less than 5 μm. The resulting carbon fibers had a tensile strength of 2340 MPa, a tensile modulus of 184 GPa, an elongation at break of 1.2%, and a resistivity of 18 μΩ·m. These carbon fibers had a circular cross-section with a roundness error of 1.04, a coefficient of variation of 1%, and a diameter of 5.9 μm.
[0093] Example 1a
[0094] A larger diameter lyocell precursor was prepared according to the method of Comparative Example 6a. The yarn was impregnated in a 4% (w / w) aqueous solution of boric acid. Inductively coupled plasma mass spectrometry (ICP-OES) showed that the boric acid content in the impregnated yarn was 1.4%, equivalent to 0.27% boron. Scanning electron microscopy revealed a roundness error of 1.03, a coefficient of variation of 1%, and an average diameter of 14 μm. The filament strength was measured to be 43 cN / tex, the tensile modulus was 1950 cN / tex, and the elongation at break was 6.7%. According to EN 16640 standard, its bio-based carbon content was 100%.
[0095] Example 1b
[0096] The impregnated yarn from Example 1a was heat-treated to convert it into carbon fibers. Due to the larger diameter of the precursor in Example 1a, a higher maximum tensile strength (35%) could be applied during the high-temperature carbonization stage compared to the thermal conversion process of the precursor in Comparative Example 6a, without resulting in carbon fibers with a diameter less than 5 μm. The resulting carbon fibers had a tensile strength of 2920 MPa, a tensile modulus of 410 GPa, an elongation at break of 0.7%, and a resistivity of 4 μΩ·m. These carbon fibers had a circular cross-section with a roundness error of 1.02, a coefficient of variation of 1%, and a diameter of 5.4 μm. The bio-based carbon content was 100% as determined by EN 16640.
Claims
1. A bio-based precursor suitable for producing high-modulus carbon fibers, characterized in that, The bio-based precursor is cellulose multifilament; The filaments in the cellulose multifilament have a circular cross-section with a roundness error of 1 to 2, preferably 1.0 to 1.2, and particularly preferably 1 to 1.
1. The boron content of the cellulose multifilament is from 0.01% to 2.0% by weight, preferably from 0.1% to 1.0%, particularly preferably from 0.2% to 0.4%, and most preferably from 0.2% to 0.3%. The crystallinity of the cellulose multifilament monofilament is 40% or more, preferably 45% or more, and more preferably 48% or more, and the degree of crystal orientation is 75% or more, preferably 80% or more.
2. The bio-based precursor according to claim 1, characterized in that, The coefficient of variation of the roundness error of the filament monofilament is less than 2%, preferably less than 1.5, and particularly preferably less than 1.
2.
3. The bio-based precursor of claim 1, wherein, The diameter of the filament monofilament is 5 to 30 μm, preferably 10 to 20 μm, and particularly preferably 12 to 17 μm.
4. The bio-based precursor according to claim 1, characterized in that, The tensile strength of the bio-based precursor is 10 to 180 cN / tex, preferably 30 to 130 cN / tex, and particularly preferably 40 to 90 cN / tex; The tensile modulus is in the range of 1000 cN / tex to 4000 cN / tex, preferably in the range of 1300 cN / tex to 3500 cN / tex, and particularly preferably in the range of 1700 cN / tex to 3000 cN / tex; The elongation at break is 2% to 15%, preferably 4% to 12%, and particularly preferably 5% to 10%.
5. The bio-based precursor according to claim 1, characterized in that, Inorganic metal content in the bio-based precursor: Sodium content is less than 1% by weight, preferably less than 0.1% by weight, and particularly preferably less than 0.005% by weight; Potassium content is less than 1% by weight, preferably less than 0.1% by weight, and particularly preferably less than 0.005% by weight; The calcium content is less than 1% by weight, preferably less than 0.1% by weight, and particularly preferably less than 0.005% by weight.
6. The bio-based precursor according to claim 1, characterized in that, The bio-based precursor contains more than 70% bio-based carbon, preferably more than 80%, and particularly preferably more than 90%.
7. A process for producing high-modulus carbon fiber based on the bio-based precursor according to any one of claims 1-6, characterized in that, Includes the following steps: (1) The cellulose multifilament precursor was heat-treated in a stabilization furnace; (2) Subsequent heat treatment is carried out in a low-temperature carbonization furnace; (3) Further heat treatment is carried out in a high-temperature carbonization furnace at a temperature of up to 2000°C, and the multifilament is stretched by more than 30%. After high-temperature carbonization furnace heat treatment, the fibers are no longer subjected to ultra-high temperature treatment.
8. The high-modulus carbon fiber production process according to claim 7, characterized in that, The heat treatment in the stabilization furnace is carried out in air at a temperature not exceeding 310°C, preferably not exceeding 290°C.
9. The high-modulus carbon fiber production process according to claim 7, characterized in that, The heat treatment in the low-temperature carbonization furnace is carried out at a temperature of up to 1000°C, and tension is applied to the multifilaments during the low-temperature carbonization process.