A method for preparing carbon fibers based on a two-step process of bio-oil

Abstract

The application discloses a method for preparing carbon fibers based on a two-step method of biological oil, and does not add any modifier and co-carbonization agent. Aromatic carbon of pyrolysis lignin in the biological oil is used as a skeleton, and oxygen-containing functional groups such as hydroxyl and methoxyl are used as reactive sites. After thermal polycondensation and oxidation, the content of hydroxyl is reduced, the content of carbonyl is increased, the melt viscosity of the precursor is improved, and the melt spinning is more favorable. The introduction of oxygen increases the ether bond content of the fiber filament. Through pre-oxidation and carbonization, the carbon fibers with excellent performance are obtained.

Description

Technical Field

[0001] This invention belongs to the field of bio-based carbon fiber technology, and relates to a method for preparing carbon fiber based on a two-step bio-oil process. Background Technology

[0002] The performance of carbon fibers is highly dependent on the characteristics of the precursors. While pitch-based carbon fibers can be produced continuously at low cost through melt spinning in existing precursor systems, obtaining high-performance products depends on highly aromatized mesophase pitch. This pitch requires precise catalytic polycondensation and deep purification of coal tar or petroleum residue, resulting in high technical barriers, complex processes, and the often high-modulus, low-strength fibers, limiting their application in structural materials. Pyrolytic lignin in bio-oils, as a novel bio-based precursor, exhibits unique substitution and complementary value. Derived from the rapid pyrolysis of biomass for oil production, it is a phenolic oligomer formed by the thermal cracking and recondensation of natural lignin macromolecules. Compared to mesophase pitch, pyrolytic lignin has significant advantages: the raw material is renewable biomass waste, independent of fossil resources; its formation process naturally completes "pre-carbonization" and "pre-purification" during pyrolysis, resulting in a product with high carbon content and extremely low ash content, reducing microscopic defects in fibers from the source. Simultaneously, the extensive removal of aliphatic side chains endows it with excellent melt flowability, allowing direct compatibility with existing melt spinning equipment without the need for a solvent recovery system, making the process simpler and more environmentally friendly. Therefore, the development of pyrolytic lignin-based carbon fibers essentially draws upon the mature melt spinning process of pitch-based fibers while utilizing the structural characteristics of biomass itself to explore a low-cost, sustainable new path for bio-based carbon fibers. Summary of the Invention

[0003] The purpose of this invention is to provide a two-step method for preparing carbon fibers based on bio-oil. To improve the spinning properties of biomass precursors and enhance the mechanical properties of biomass carbon fibers, this invention employs a method of first thermal polycondensation followed by oxidation to improve the melt spinning properties, aromaticity, and molecular weight distribution of the precursors, thereby enhancing the mechanical properties of bio-based carbon fibers. This results in the development of a low-cost, sustainable new method for producing bio-based carbon fibers. To achieve the above objectives, the present invention provides the following technical solution: 1. A method for preparing carbon fiber based on a two-step bio-oil process, comprising the following steps: (1) Extraction of pyrolytic lignin: mix bio-oil and methanol in a 1:1 ratio, then add the mixture dropwise to cold water at 2-4℃ at a rate of 0.2-0.4 ml / min in an oil-water ratio of 1:10, collect the precipitate, and freeze dry. (2) Preparation of melt spinning precursor: Pyrolytic lignin was added to a four-necked flask, and argon gas of 0.6-0.8 L / min was introduced to polymerize and remove light components at 300-340℃. The mixture was kept at this temperature for 1-4 h, and then air was introduced at 300-340℃ at 0.2-0.4 ml / min for 1-2 h to obtain the bio-based melt spinning precursor. (3) Melt spinning: A certain mass of precursor is ground and then placed in a melt spinning machine. The precursor is spun at a rate of 800-1300 rpm under the conditions of 0.3-0.5 MPa and 160-230℃ to obtain bio-based carbon fiber precursor. (4) Pre-oxidation of raw fibers: The temperature is increased from room temperature to 60-100℃ at a heating rate of 1-3℃ / min, and then increased to 210-260℃ at a heating rate of 0.05-0.3℃ / min. The temperature is kept for 1-2 hours to obtain pre-oxidized fibers. (5) Pre-oxidized fiber carbonization: The pre-oxidized fiber is heated to 800-1000℃ at a heating rate of 5-10℃ / min under an inert atmosphere and held for 5-30min to obtain bio-based carbon fiber. This invention has the following technical effects: The two-step method for preparing carbon fiber based on bio-oil developed in this invention has superior precursor spinning performance and significantly improved fiber mechanical properties compared to using thermal polymerization or direct oxidative polymerization alone. It can replace traditional pitch-based general-purpose carbon fiber, reduce the use of fossil energy, select renewable carbon as carbon source, and reduce environmental pollution. Attached Figure Description Figure 1 Scanning electron microscope images of bio-based carbon fibers prepared under different conditions according to this invention: (a)PL-1 (b)PL-2 (c)PL-3 PL-1 is a carbon fiber sample prepared by thermal polymerization alone, PL-2 is a carbon fiber sample prepared by a two-step method of thermal polymerization and oxidation, and PL-3 is a carbon fiber sample prepared by oxidation alone. Figure 2 TG diagrams of bio-based carbon fibers prepared under different conditions according to the present invention. Figure 3 The mechanical properties of bio-based carbon fibers prepared under different conditions according to the present invention are shown in the figure. Detailed Implementation The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description. Example 1 The preparation of a comparative bio-based carbon fiber is carried out according to the following steps: (1) Extraction of pyrolytic lignin: mix bio-oil and methanol in a 1:1 ratio, then add the mixture dropwise to cold water at 2-4℃ at a rate of 0.2-0.4 ml / min in an oil-water ratio of 1:10, collect the precipitate, and freeze dry. (2) Preparation of melt spinning precursor: Pyrolytic lignin was added to a four-necked flask, and argon gas of 0.6-0.8 L / min was introduced to polymerize at 300-340℃ and remove light components. The mixture was kept at this temperature for 1-4 h to obtain a bio-based melt spinning precursor. (3) Melt spinning: A certain mass of precursor is ground and then placed in a melt spinning machine. The precursor is spun at a rate of 800-1300 rpm under the conditions of 0.3-0.5 MPa and 160-230℃ to obtain bio-based carbon fiber precursor. (4) Pre-oxidation of raw fibers: The temperature is increased from room temperature to 60-100℃ at a heating rate of 1-3℃ / min, and then increased to 210-260℃ at a heating rate of 0.05-0.3℃ / min. The temperature is kept for 1-2 hours to obtain pre-oxidized fibers. (5) Carbonization of pre-oxidized fiber: The pre-oxidized fiber is heated to 800-1000℃ at a heating rate of 5-10℃ / min under an inert atmosphere and held for 5-30min to obtain bio-based carbon fiber. Example 2 A two-step method for preparing carbon fiber based on bio-oil is carried out as follows: (1) Extraction of pyrolytic lignin: mix bio-oil and methanol in a 1:1 ratio, then add the mixture dropwise to cold water at 2-4℃ at a rate of 0.2-0.4 ml / min in an oil-water ratio of 1:10, collect the precipitate, and freeze dry. (2) Preparation of melt spinning precursor: Pyrolytic lignin was added to a four-necked flask, and argon gas of 0.6-0.8 L / min was introduced to polymerize and remove light components at 300-340℃. The mixture was kept at this temperature for 1-4 h, and then air was introduced at 300-340℃ at 0.2-0.4 ml / min for 1-2 h to obtain the bio-based melt spinning precursor. (3) Melt spinning: A certain mass of precursor is ground and then placed in a melt spinning machine. The precursor is spun at a rate of 800-1300 rpm under the conditions of 0.3-0.5 MPa and 160-230℃ to obtain bio-based carbon fiber precursor. (4) Pre-oxidation of raw fibers: The temperature is increased from room temperature to 60-100℃ at a heating rate of 1-3℃ / min, and then increased to 210-260℃ at a heating rate of 0.05-0.3℃ / min. The temperature is kept for 1-2 hours to obtain pre-oxidized fibers. (5) Carbonization of pre-oxidized fiber: The pre-oxidized fiber is heated to 800-1000℃ at a heating rate of 5-10℃ / min under an inert atmosphere and held for 5-30min to obtain bio-based carbon fiber. Example 3 The preparation of a comparative bio-based carbon fiber is carried out according to the following steps: (1) Extraction of pyrolytic lignin: mix bio-oil and methanol in a 1:1 ratio, then add the mixture dropwise to cold water at 2-4℃ at a rate of 0.2-0.4 ml / min in an oil-water ratio of 1:10, collect the precipitate, and freeze dry. (2) Preparation of melt spinning precursor: Pyrolytic lignin was added to a four-necked flask and kept at 300-340℃ with air introduced at 0.2-0.4 ml / min for 1-2 h to obtain bio-based melt spinning precursor. (3) Melt spinning: A certain mass of precursor is ground and then placed in a melt spinning machine. The precursor is spun at a rate of 800-1300 rpm under the conditions of 0.3-0.5 MPa and 160-230℃ to obtain bio-based carbon fiber precursor. (4) Pre-oxidation of raw fibers: The temperature is increased from room temperature to 60-100℃ at a heating rate of 1-3℃ / min, and then increased to 210-260℃ at a heating rate of 0.05-0.3℃ / min. The temperature is kept for 1-2 hours to obtain pre-oxidized fibers. (5) Carbonization of pre-oxidized fiber: The pre-oxidized fiber is heated to 800-1000℃ at a heating rate of 5-10℃ / min under an inert atmosphere and held for 5-30min to obtain bio-based carbon fiber. From the appendix Figure 1 SEM characterization of bio-based carbon fibers prepared under different conditions: When only thermal polymerization is performed, the precursor melt spinning performance is poor, with frequent fiber breakage and only a low take-up rate can be maintained. Although the fiber surface is smooth, the mechanical properties are poor in subsequent tests due to the large diameter. When the two-step synthesis is performed, the precursor melt spinning performance is excellent, and it can be stably spun at a rate of 1200 rpm for 30 minutes without fiber breakage. The fiber surface is smooth. When only oxidation reaction is performed, although the precursor melt spinning performance is excellent, the carbon fiber surface has defects because a lot of light components are not blown out. From the appendix Figure 2PL-1 exhibits rapid weight loss around 400–450℃, with a residual mass of approximately 31% at 800℃. This indicates that more small-molecule gases may be released during carbonization, easily leading to micropores, cracks, or localized structural collapse. Such defects significantly impact the fiber's mechanical properties. PL-2 shows the highest char yield at 800℃, approximately 38%. This indicates its superior thermal stability and char-forming ability, with relatively less volatile component release during carbonization, which is more conducive to forming dense, low-porosity carbon fibers. PL-3 shows significant mass loss at approximately 200–350℃, indicating a higher proportion of low-molecular-weight, side-chain, or oxygen-containing functional groups, resulting in relatively poor thermal stability. Although its final char yield is approximately 33%, slightly higher than PL-1, the earlier and wider weight loss process may cause continuous gas release during stabilization and low-temperature carbonization stages. Insufficient gas escape can easily lead to internal porosity, thus potentially affecting its strength. From the appendix Figure 3 As can be seen, compared with bio-based carbon fibers prepared by a single method, bio-based carbon fibers prepared by a two-step method have superior mechanical properties, with an average tensile strength of 1.1 GPa and an average tensile modulus of 71 GPa. Obviously, the above description is merely an embodiment of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent.

Claims

1. A method for preparing carbon fiber based on a two-step bio-oil process, characterized in that: Using a single renewable bio-oil as raw material, a precursor is obtained through cold water extraction followed by a two-step process of polycondensation and oxidation. This precursor is then processed into carbon fiber via spinning, pre-oxidation, and carbonization, comprising the following steps: The bio-oil was purified by filtration, and after purification, it was thoroughly mixed with methanol and extracted with cold water to obtain pyrolytic lignin. The pyrolytic lignin was dried at low temperature and then added to a four-necked flask for thermal condensation under an inert atmosphere. Then, air was introduced to carry out an oxidation reaction, and the mixture was cooled to obtain a bio-based carbon fiber precursor. Bio-based fibers were prepared using melt spinning. The bio-based fibers are pre-oxidized to obtain pre-oxidized fibers; The pre-oxidized fiber is subjected to high-temperature carbonization to obtain the bio-based carbon fiber. The method for preparing carbon fiber based on a two-step bio-oil process as described in claim 1 is characterized in that: The bio-oil was purified by filtration using 0.22 μm polytetrafluoroethylene. The method for preparing carbon fiber based on a two-step bio-oil process as described in claim 1 is characterized in that: The purified bio-oil was mixed with methanol and then added to cold water at 2°C at a rate of 0.2-0.8 ml / min. The method for preparing carbon fiber based on a two-step bio-oil process as described in claim 1 is characterized in that: The pyrolytic lignin obtained from the extraction is dried at low temperature to remove moisture. The method for preparing carbon fiber based on a two-step bio-oil process as described in claim 1 is characterized in that: The dried pyrolytic lignin was kept at 300-340℃ for 1-4 hours under an argon atmosphere of 0.6-0.8 L / min. The method for preparing carbon fiber based on a two-step bio-oil process as described in claim 1 is characterized in that: The oxidation air flow rate is 0.2-0.4 L / min, and the temperature is maintained for 1-2 hours. The method for preparing carbon fiber based on a two-step bio-oil process as described in claim 1 is characterized in that: The melt spinning parameters are 160-230℃, spinning pressure is 0.3-0.5MPa, and take-up rate is 800-1300rpm. The method for preparing carbon fiber based on a two-step bio-oil process according to claim 1 is characterized in that: The pre-oxidation process is as follows: under an oxygen atmosphere, the temperature is first increased from room temperature to 60-100℃ at a heating rate of 1-3℃ / min, and then increased to 210-260℃ at a heating rate of 0.05-0.3℃ / min, and held at that temperature for 1-2 hours. The method for preparing carbon fiber based on a two-step bio-oil process according to claim 1 is characterized in that: The pre-oxidized fiber is heated to 800-1000℃ under an inert atmosphere at a heating rate of 5-10℃ / min and held at that temperature for 5-30min. A two-step method for preparing carbon fiber based on bio-oil, characterized in that, The bio-based carbon fiber prepared according to any one of claims 1 to 9 has a tensile strength of 530 to 1400 MPa and a tensile modulus of 30 to 100 GPa.

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