A method for producing carbon fibers by melt spinning of lignin-modified polyacrylonitrile

Abstract

The present application belongs to the technical field of polymer blend melt spinning, and particularly relates to a method for preparing carbon fibers by melt spinning of lignin modified polyacrylonitrile. The modified lignin, polyacrylonitrile and plasticizer are melt mixed and then added to a melt spinning machine, and the yarn is extruded under pressure in a nitrogen atmosphere. The fiber yarn is then subjected to pre-oxidation treatment and high-temperature carbonization. Through ion liquid plasticizer-mediated molecular bridging, lignin-initiated controllable cyclization and synergistically constructed ordered condensed state structure, the three components are synergized to obtain carbon fibers. The carbon fibers prepared by the present application have a smooth surface without wrinkles, a circular cross-section and a dense structure without pores and cracks. The present application does not involve the use of organic solvents in melt spinning, has no environmental pollution, is green and environmentally friendly, and has low cost.

Description

Technical Field

[0001] This invention belongs to the field of polymer blending melt spinning technology, specifically relating to a method for preparing carbon fibers by melt spinning of lignin-modified polyacrylonitrile. Background Technology

[0002] In the new era of global dual-carbon strategy and rapid development of high-end manufacturing, carbon fiber, with its excellent comprehensive properties such as low density, ultra-high specific strength, corrosion resistance, and high temperature resistance, has become an indispensable core strategic new material in aerospace, new energy wind power, hydrogen energy storage, and automotive lightweighting, and the industry demand continues to grow rapidly.

[0003] Polyacrylonitrile (PAN)-based carbon fibers account for over 90% of global carbon fiber production, possessing advantages such as high strength, high modulus, and mature manufacturing processes. This is primarily attributed to the abundant nitrile groups in the PAN precursor molecular chain, which undergo cyclization, dehydrogenation, and oxidation reactions during pre-oxidation to form a thermally stable trapezoidal structure, thus ensuring the morphological stability and mechanical properties of the fiber during subsequent carbonization. However, PAN is derived from non-renewable fossil resources, resulting in high prices and significant susceptibility to fluctuations in crude oil prices, leading to high production costs and limited applications for carbon fibers. Furthermore, traditional PAN spinning heavily relies on solution spinning, requiring the use of toxic organic solvents (such as DMF and DMSO), causing environmental pollution and high solvent recovery costs. To achieve low-cost, green, and large-scale development of carbon fibers, utilizing modified agricultural and forestry biomass waste to replace part of the PAN raw material has become a cutting-edge research hotspot.

[0004] Lignin is the second most abundant natural aromatic polymer in the world. As a byproduct of pulp and paper making and biorefining industries, its global annual production exceeds 100 million tons. It is inexpensive, renewable, and possesses a high carbon content (60%~65%) and aromatic ring structure. During pyrolysis, it easily forms a graphite-like disordered layer structure, making it an ideal green precursor for carbon fiber preparation. However, currently, industrial lignin is mainly used for low-value combustion for energy or directly discarded, with a low rate of high-value utilization, and its application potential as a renewable aromatic carbon resource has not been fully realized. For carbon fiber preparation, lignin's three-dimensional amorphous network structure, wide molecular weight distribution, and strong intermolecular forces lead to poor melt flowability, a narrow thermal processing window, and insufficient continuous spinnability. Therefore, how to improve the processing rheological properties of lignin and achieve stable continuous spinning is the core bottleneck restricting the direct preparation of high-performance carbon fibers from pure lignin.

[0005] Chemical modification of lignin (acetylation, esterification) can reduce the number of active hydroxyl groups in lignin molecules to a certain extent, weaken intermolecular hydrogen bonding, improve the compatibility of lignin with the matrix, introduce functional monomers into lignin molecules, reduce intermolecular interactions, improve its thermoplastic processing behavior, and enhance lignin melt flowability and spinnability.

[0006] Melt spinning, as a green and efficient process, avoids the use of organic solvents. However, PAN's melting temperature is close to its decomposition temperature, making direct melt processing difficult. Plasticizers are needed to improve its melt flowability, enabling the melt spinning preparation of PAN-based carbon fiber precursors. Ionic liquids are room-temperature molten salts with low volatility, high thermal stability, strong polarity, and excellent solubility and plasticizing properties, and are considered green solvents and efficient polymer plasticizers. In lignin-modified polyacrylonitrile systems, ionic liquids can disrupt the hydrogen bonds between PAN molecular chains through ion-dipole interactions, lowering its glass transition temperature and melting temperature, improving melt flowability, and enabling melt spinning. Simultaneously, they can promote the molecular compatibility of lignin and PAN, inhibit lignin aggregation, and improve the stability of the blend spinning solution and the quality of the precursor fiber. Compared to traditional plasticizers, ionic liquids have high plasticizing efficiency and good thermal stability. They are not easily decomposed or volatilized during spinning and pre-oxidation processes, can stably exert their plasticizing effect, and are easily recovered through water washing, aligning with green chemistry principles. In addition, ionic liquids contain nitrogen, and after carbonization, they can form nitrogen-doped carbon structures, which helps to improve the mechanical and electrochemical properties of carbon fibers and provides a new path for the preparation of high-performance carbon fibers.

[0007] In summary, the following problems exist in the preparation of carbon fibers from lignin-modified polyacrylonitrile melt-spun fibers: (1) Polyacrylonitrile and lignin have poor compatibility. Lignin is an aromatic random branched polymer, while PAN is an aliphatic linear polymer. They are thermodynamically incompatible and prone to macro / micro phase separation. Therefore, plasticizers are needed to improve their compatibility. (2) Pure lignin directly melt-spun has problems with poor melt flowability and spinnability. Therefore, lignin needs to be modified to improve its poor flowability and spinnability.

[0008] However, how to select appropriate cellulose modification structures and plasticizer structures so that PAN, modified cellulose, and plasticizer can work synergistically to improve the fiber strength, elongation at break, and orientation of carbon fibers is the technical problem that this invention needs to solve. Summary of the Invention

[0009] To address the problems existing in the prior art, this invention provides a method for preparing carbon fibers by melt spinning of lignin-modified polyacrylonitrile. Modified lignin, polyacrylonitrile, and a plasticizer are melt-mixed and added to a melt spinning machine. The mixture is then extruded under pressure under nitrogen atmosphere. The fibers undergo pre-oxidation treatment and high-temperature carbonization. Through molecular bridging mediated by the ionic liquid plasticizer, controlled cyclization initiated by lignin, and the synergistic construction of an ordered condensed-state structure, the three components work together to obtain carbon fibers. The carbon fibers prepared by this invention have a smooth surface, no wrinkles, and a circular cross-section, possessing the advantages of a dense structure without pores or cracks. This invention uses melt spinning, which does not involve the use of organic solvents, resulting in no environmental pollution, making it green, environmentally friendly, and low-cost.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first aspect of this invention provides a method for preparing carbon fibers using lignin-modified polyacrylonitrile melt spinning, comprising the following steps: Step S1: Add acetylated modified lignin, polyacrylonitrile and ionic liquid plasticizer into a mixer and physically blend them under heating conditions to obtain a blended material; Step S2: Add the obtained blended material to the spinning assembly in the melt spinning machine, heat and melt the blended material, spray out the fiber under gas extrusion, shape it in the air, and wind and collect the yarn; Step S3: The fibers are immersed in water for water bath washing, then rinsed with ethanol and vacuum dried; Step S4: The fibers are placed in a muffle furnace and calcined to form pre-oxidized fibers; Step S5: The pre-oxidized fiber is placed in a tube furnace and calcined to obtain carbon fiber.

[0011] Further, in step S1, the mass of the acetylated modified lignin accounts for 2-40% of the total mass of the acetylated modified lignin, polyacrylonitrile, and ionic liquid plasticizer, and the mass ratio of the polyacrylonitrile to the ionic liquid plasticizer is 1:1.

[0012] Further, in step S1, the heating temperature is 130~180 ℃, and the physical blending time is 20~50 min.

[0013] Furthermore, in step S2, the heating and melting temperature is 150~200 ℃, and the winding speed is 10~50 r / s.

[0014] Furthermore, in step S3, the water bath washing time is 10~60 min, and the vacuum drying time is 2~24 h.

[0015] Furthermore, in step S4, the calcination temperature is 170~250 ℃ and the time is 1~4 h.

[0016] Furthermore, in step S5, the calcination temperature is 1000~1500 ℃ and the time is 1~4 h.

[0017] Furthermore, the lignin is at least one of enzymatically hydrolyzed lignin, alkali lignin, and lignin sulfonate.

[0018] Furthermore, the acetylation modification refers to at least one of phenolic hydroxyl acetylation modification, aliphatic hydroxyl acetylation modification, complete acetylation modification, and enzymatic acetylation modification.

[0019] Further, the ionic liquid plasticizer is at least one of 1-butyl-3-methylimidazolium chloride ([BMIM]CL), 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), 1-butyl-3-methylimidazolium bromide ([BMIM]Br), and 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl).

[0020] Polyacrylonitrile (PAN) exhibits extremely strong dipole forces between its intermolecular chains of cyano groups (-CN), resulting in a thermal decomposition temperature below its melting temperature, making it impossible to process by melting alone. At room temperature, [BMIM]CL is highly hygroscopic, and its anion (Cl...)... - The imidazole ring preferentially forms strong hydrogen bonds with water, while exhibiting no strong interaction with the cyano group (C≡N) of PAN (the C≡N peak only shifts slightly). However, at high temperatures (>120 °C), water is completely removed, and the C2-H on the [BMIM]CL imidazole ring acts as a hydrogen bond donor, forming new strong hydrogen bonds (C2-H···N≡C) with the polar C≡N groups (hydrogen bond acceptors) on the PAN molecular chain. The establishment of these new hydrogen bonds weakens the original C≡N dipole-dipole physical crosslinking points between PAN molecular chains, providing a molecular-level driving force for plasticization.

[0021] Building upon the research on binary systems, the introduction of acetylated lignin into the PAN / [BMIM]CL matrix marks a leap in the plasticizing system from a simple binary interaction of "plasticizer-polymer" to a ternary dynamic network of "small molecule ionic liquid-rigid aromatic oligomer-polar linear polymer". When the mixing temperature exceeds 100 °C and stabilizes in the 160~180 °C range, the adsorbed water in [BMIM]CL is completely removed, and the C2-H active sites on its imidazole ring are fully exposed. At this point, [BMIM]CL no longer acts solely as a plasticizer for PAN, but evolves into a bifunctional hydrogen-bonded bridging medium: one end anchors the cyano group on the PAN molecular chain through C2-H···N≡C hydrogen bonds, while the other end, through the strong polar interaction between C2-H and the small amount of residual hydroxyl groups (-OH) and abundant ester carbonyl groups (C=O) in acetylated lignin, "grafts" the originally thermodynamically incompatible rigid lignin macromolecules onto the molecular motion units of PAN. This bridging effect creates a homogeneous ternary composite melt.

[0022] From the perspective of aggregate thermodynamics and free volume regulation, this ternary system exhibits a synergistic plasticizing effect far exceeding that of the binary system. [BMIM]CL, with its high penetration efficiency, continues to disrupt the regular stacking of PAN quasicrystalline regions, promoting lamellar melting and amorphization of crystalline regions. Meanwhile, the introduced acetylated lignin, due to its large size and rigid aromatic skeleton, cannot enter the crystal lattice, but it effectively expands the already loose molecular chain spacing in the amorphous regions of PAN, significantly increasing the free volume within the system. The two complement each other in terms of spatial scale and mode of action: [BMIM]CL focuses on "crystallization," while acetylated lignin focuses on "chain extension." This dual perturbation leads to a significant reduction in the cooperative motion barrier of PAN molecular chain segments. As reflected in thermodynamic parameters, the glass transition temperature (Tg) and melting point (Tm) of the system are expected to decrease further. This allows for the production of a stable melt with good flowability within a processing window (approximately 100–180 °C) that is far below the traditional thermal degradation temperature of PAN. This provides a very promising processing path for low-temperature, low-energy-consumption green melt spinning.

[0023] Finally, a potential "structural template and interface guidance" synergy exists between [BMIM]CL and acetylated lignin. Under high-temperature mixing and subsequent stretching and orientation, the hydrogen bond network established by [BMIM]CL not only promotes the orientation of PAN molecular chains along the stretching direction, but also, through polar induction, causes acetylated lignin molecules with aromatic structures to oriented and attach along the PAN backbone. After cooling and solidification, this constructed "PAN ladder layer-lignin aromatic ring layer" hybrid framework, during subsequent pre-oxidation and carbonization processes, allows the high aromaticity of lignin to act as a seed crystal for graphitization, guiding the orderly growth of carbon layers, while its high residual carbon content synergistically increases the overall carbon yield. Simultaneously, the slow pyrolysis and dissipation of [BMIM]CL at this stage endows the final carbon fibers with additional functionalization sites.

[0024] In summary, this ternary system is not a simple superposition of the properties of each component, but rather opens up a new multi-component synergistic pathway for the preparation of high-performance, low-cost, and process-friendly carbon fiber precursors through [BMIM]CL-mediated molecular bridging, lignin-induced controllable cyclization, and synergistically constructed ordered condensed-state structures.

[0025] Compared with the prior art, the beneficial effects of the present invention are: This invention employs imidazole-based ionic liquid plasticizers with optimal compatibility and plasticizing effect with polyacrylonitrile (PAN), and adds acetylated modified lignin to improve interfacial compatibility, enhance fiber mechanical properties and stability, and simultaneously achieve high-value utilization of lignin. The pre-oxidation and carbonization conditions were controlled to prepare spherical, dense carbon fibers without pores or core-skin structures, exhibiting complete structure and excellent performance. Melt spinning was used to avoid the use of toxic organic solvents, and the addition of lignin also reduced the production cost of the carbon fibers. Attached Figure Description

[0026] Figure 1 This is a SEM image of the carbon fiber in Example 1.

[0027] Figure 2 This is a SEM image of the carbon fiber in Example 2.

[0028] Figure 3 This is a SEM image of the carbon fiber in Example 3.

[0029] Figure 4 The tensile strength test results of carbon fibers in Examples 1 and 2 are shown.

[0030] Figure 5 The images show the XRD patterns of the carbon fibers in Examples 1, 2, and 3.

[0031] Figure 6The images show the Raman spectra of the carbon fibers in Examples 1(a), 2(b), and 3(c). Detailed Implementation

[0032] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0033] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0034] Example 1 Polyacrylonitrile (MW150000) powder and plasticizer ([BMIM]CL) were mixed at a mass ratio of 50:50 in a high-speed rotary mixer at 130 °C for 40 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and melted at 160 °C to obtain a homogeneous blend melt. The melt stream flowing from the spinneret through the spinning assembly was set with hot air at 26 °C and wound at a winding speed of 25 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum dried for 12 h. The prepared fibers were pre-oxidized at 170 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h to obtain polyacrylonitrile carbon fibers. SEM images of the carbon fibers are shown below. Figure 1 As shown, the carbon fiber surface is smooth and free of wrinkles and cracks; the carbon fiber cross-section is circular, dense, and free of pores and defects; the cross-section diameter is approximately 107 μm. The obtained carbon fiber strength is 0.022 GPa, the elongation at break is 7.6%, and the orientation degree is 84.3%.

[0035] Example 2 Enzymatically hydrolyzed lignin (modified by phenolic hydroxyl acetylation) dried under vacuum at 60 °C for 24 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 2:49:49 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 170 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h. h yields lignin / polyacrylonitrile blended carbon fibers. SEM images of the carbon fibers are shown below. Figure 2 As shown, the carbon fiber surface is smooth and free of wrinkles and cracks; the carbon fiber cross-section is circular, dense, and without obvious pore defects; the cross-section diameter is approximately 86 μm, yielding a carbon fiber strength of 0.030 GPa, an elongation at break of 8.4%, and an orientation degree of 80.5%. The tensile strength test results of the carbon fibers in Example 1 and Example 2 are as follows... Figure 4 As shown, Example 1 has a greater elongation, but Example 2 has a higher strength, indicating that lignin may enhance the strength of carbon fibers but weaken their tensile properties.

[0036] Example 3 Enzymatically hydrolyzed lignin (modified by phenolic hydroxyl acetylation) dried under vacuum at 60 °C for 24 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 4:48:48 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 170 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h. h yields lignin / polyacrylonitrile blended carbon fibers. SEM images of the carbon fibers are shown below. Figure 3As shown, the carbon fiber surface is smooth and free of wrinkles and cracks; the carbon fiber cross-section is circular, dense, and without obvious pores or defects; the cross-section diameter is approximately 62 μm, yielding a carbon fiber strength of 0.064 GPa, an elongation at break of 9.24%, and an orientation degree of 85.2%. The XRD patterns of the carbon fibers in Example 1, Example 2, and Example 3 are shown below. Figure 5 As shown, all three curves exhibit a strong diffraction peak at 25° (corresponding to d002 in graphite crystals). The introduction of lignin reduces the d002 of the fibers, indicating a tighter interlayer arrangement and improved graphitization. The Raman spectra of the carbon fibers in Examples 1, 2, and 3 are shown below. Figure 6 As shown in (a), (b), and (c), it can be seen that all samples are within 1350 cm. -1 and 1580 cm -1 Typical D and G peaks appear, corresponding to the disorder / defect structure of carbon materials and graphite lattice vibrations, respectively. The ID / IG ratio of carbon fibers containing lignin is significantly lower than that of carbon fibers without lignin, indicating that the addition of lignin can effectively promote the ordering of carbon structures, reduce defect density, and increase the degree of graphitization.

[0037] Example 4 Enzymatically hydrolyzed lignin (modified by phenolic hydroxyl acetylation) dried under vacuum at 60 °C for 12 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 2:49:49 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 165 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 180 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 1 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber had a strength of 0.024 GPa, an elongation at break of 7.1%, and an orientation degree of 74.3%.

[0038] Example 5 Enzymatically hydrolyzed lignin (modified by aliphatic hydroxyacetylation) dried under vacuum at 60 °C for 18 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 4:48:48 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous blend melt. The blend melt stream flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 200 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h. The lignin / polyacrylonitrile blend carbon fiber was obtained. The resulting carbon fiber had a strength of 0.048 GPa, an elongation at break of 8.0%, and an orientation degree of 77.0%.

[0039] Example 6 Enzymatically hydrolyzed lignin (modified by phenolic hydroxyl acetylation) dried under vacuum at 60 °C for 6 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 6:47:47 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 190 °C (heating at 0.5 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h. The lignin / polyacrylonitrile blend carbon fiber was obtained. The resulting carbon fiber had a strength of 0.082 GPa, an elongation at break of 8.7%, and an orientation degree of 71.1%.

[0040] Example 7 Enzymatically hydrolyzed lignin (modified by aliphatic hydroxyacetylation) dried under vacuum at 60 °C for 2 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 8:46:46 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 170 °C (heating at 0.5 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 5 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber exhibited a strength of 0.080 GPa, an elongation at break of 9.3%, and a degree of orientation of 74.9%.

[0041] Example 8 Enzymatically hydrolyzed lignin (modified by aliphatic hydroxyacetylation) dried under vacuum at 60 °C for 24 h, polyacrylonitrile (MW150000) powder, and plasticizer ([EMIM]Cl) were mixed at a mass ratio of 4:48:48 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous blend melt. The blend melt stream flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 170 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1200 °C (heating at 3 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber had a strength of 0.070 GPa, an elongation at break of 10.7%, and an orientation degree of 70.4%.

[0042] Example 9 Enzymatically hydrolyzed lignin (modified by phenolic hydroxyl acetylation) dried under vacuum at 60 °C for 12 h, polyacrylonitrile (MW150000) powder, and plasticizer ([EMIM]Cl) were mixed at a mass ratio of 4:48:48 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 175 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 190 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1200 °C (heating at 5 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber exhibited a strength of 0.073 GPa, an elongation at break of 6.4%, and an orientation degree of 77.2%.

[0043] Example 10 Polyacrylonitrile (MW150000) powder and plasticizer ([EMIM] Cl) were mixed at a mass ratio of 50:50 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous blend melt. The blend melt stream flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The fibers were then vacuum dried for 12 h. The prepared fibers were pre-oxidized at 180 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1200 °C (heating at 3 °C / min) for 2 h to obtain lignin / polyacrylonitrile blended carbon fibers. The obtained carbon fiber has a strength of 0.020 GPa, an elongation at break of 12.3%, and an orientation degree of 74.9%.

[0044] Example 11 Alkali lignin (using a fully acetylated modification method) dried under vacuum at 60 °C for 12 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 6:47:47 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 185 °C to obtain a homogeneous blend melt. The blend melt stream flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 180 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 5 °C / min) for 2 h. The lignin / polyacrylonitrile blend carbon fiber was obtained. The resulting carbon fiber had a strength of 0.087 GPa, an elongation at break of 11.5%, and an orientation degree of 81.6%.

[0045] Example 12 Alkali lignin (using a fully acetylated modification method) dried under vacuum at 60 °C for 24 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 2:49:49 in a high-speed rotary mixer at 130 °C for 30 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous blend melt. The blend melt stream flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 200 °C (heating at 1 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber had a strength of 0.039 GPa, an elongation at break of 11.9%, and an orientation degree of 77.8%.

[0046] Example 13 Lignosulfonate (modified by phenolic hydroxyacetylation), polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were vacuum-dried at 60 °C for 24 h and mixed at 130 °C for 30 min in a high-speed rotary mixer at a mass ratio of 4:48:48 until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 170 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 220 °C (heating at 0.5 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 3 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber had a strength of 0.066 GPa, an elongation at break of 11.3%, and an orientation degree of 76.0%.

[0047] Example 14 Lignosulfonate (modified by phenolic hydroxyacetylation), polyacrylonitrile (MW150000) powder, and plasticizer ([EMIM]Cl) were vacuum-dried at 60 °C for 24 h and mixed at 130 °C for 30 min in a high-speed rotary mixer at a mass ratio of 6:47:47 until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous melt blend. The melt blend flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 min for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 220 °C (heating at 0.5 °C / min) for 2 h, and then carbonized at 1000 °C (heating at 5 °C / min) for 2 h. The lignin / polyacrylonitrile blend carbon fiber was obtained. The resulting carbon fiber had a strength of 0.068 GPa, an elongation at break of 9.7%, and an orientation degree of 75.0%.

[0048] Example 15 Lignosulfonate (using a fully acetylated modification method) dried under vacuum at 60 °C for 24 h, polyacrylonitrile (MW150000) powder, and plasticizer ([BMIM]CL) were mixed at a mass ratio of 2:49:49 in a high-speed rotary mixer at 130 °C for 40 min until homogeneous. The mixture was then added to a closed hopper of a spinning machine and blended at 160 °C to obtain a homogeneous blend melt. The blend melt stream flowing from the spinneret through the spinning assembly was air-set at 26 °C and wound at a winding speed of 20 r / s to obtain melt-spun fibers. The fibers were immersed in water for 30 h for water bath washing, followed by ethanol rinsing. The resulting fibers were then vacuum-dried for 12 h. The prepared fibers were pre-oxidized at 220 °C (heating at 0.5 °C / min) for 2 h, and then carbonized at 1500 °C (heating at 5 °C / min) for 2 h. The resulting lignin / polyacrylonitrile blend carbon fiber was obtained. The carbon fiber exhibited a strength of 0.043 GPa, an elongation at break of 8.2%, and an orientation degree of 78.6%.

[0049] The properties of the carbon fibers prepared in Examples 1-15 above are shown in Table 1 below.

[0050] Table 1 The results in Table 1 show that the fiber strength of Example 1 is much lower than that of the other examples, indicating that an appropriate amount of lignin addition may enhance fiber strength. Examples 2 (2% lignin), 3 (4% lignin), and 13 (4% lignin) exhibit balanced strength, elongation, and orientation. Examples 6, 11, 14 (6% lignin), and 7 (8%) show that the orientation generally drops to the 71%–75% range, and fiber regularity decreases. Excessive lignin content increases melt inhomogeneity, restricts the orientation of spinning molecular chains, and easily generates porosity defects during heat treatment, thus slowing the strength improvement. Under pre-oxidation conditions at 170℃, the fiber orientation is generally above 80%, with the highest molecular chain regularity and the fewest internal defects, balancing molding stability and final mechanical properties. Above 170℃, regardless of whether the heating rate is reduced, the orientation cannot be restored to the 170℃ level.

[0051] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing carbon fibers by melt spinning lignin-modified polyacrylonitrile, characterized in that, Includes the following steps: Step S1: Add acetylated modified lignin, polyacrylonitrile and ionic liquid plasticizer into a mixer and physically blend them under heating conditions to obtain a blended material; Step S2: Add the obtained blended material to the spinning assembly in the melt spinning machine, heat and melt the blended material, spray out the fiber under gas extrusion, shape it in the air, and wind and collect the yarn; Step S3: The fibers are immersed in water for water bath washing, then rinsed with ethanol and vacuum dried; Step S4: The fibers are placed in a muffle furnace and calcined to form pre-oxidized fibers; Step S5: The pre-oxidized fiber is placed in a tube furnace and calcined to obtain carbon fiber.

2. The method according to claim 1, characterized in that, In step S1, the mass of the acetylated modified lignin accounts for 2-40% of the total mass of the acetylated modified lignin, polyacrylonitrile, and ionic liquid plasticizer, and the mass ratio of the polyacrylonitrile to the ionic liquid plasticizer is 1:

1.

3. The method according to claim 1, characterized in that, In step S1, the heating temperature is 130~180 ℃, and the physical blending time is 20~50 min.

4. The method according to claim 1, characterized in that, In step S2, the heating and melting temperature is 150~200℃, and the winding speed is 10~50 r / s.

5. The method according to claim 1, characterized in that, In step S3, the water bath washing time is 10-60 min, and the vacuum drying time is 2-24 h.

6. The method according to claim 1, characterized in that, In step S4, the calcination temperature is 170~250 ℃ and the time is 1~4 h.

7. The method according to claim 1, characterized in that, In step S5, the calcination temperature is 1000~1500℃ and the time is 1~4 h.

8. The method according to claim 1, characterized in that, The lignin is at least one of enzymatically hydrolyzed lignin, alkali lignin, and lignin sulfonate.

9. The method according to claim 1, characterized in that, The acetylation modification refers to at least one of phenolic hydroxyl acetylation modification, aliphatic hydroxyl acetylation modification, complete acetylation modification, and enzymatic acetylation modification.

10. The method according to claim 1, characterized in that, The ionic liquid plasticizer is at least one of 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, and 1-hexyl-3-methylimidazolium chloride.

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