Production method of high-strength high-modulus small-tow polyacrylonitrile-based carbon fiber precursor

Abstract

The invention relates to the technical field of carbon fibers, and discloses a production method of a high-strength high-modulus small-tow polyacrylonitrile-based carbon fiber precursor. The PAN stock solution enters a single-stage wet spinning coagulating bath through a spinning assembly to be cured and formed, a PAN nascent fiber is obtained after drafting, and the PAN precursor for the high-strength and high-modulus carbon fiber is obtained after the nascent fiber is subjected to water washing drafting, oiling, drying densification, steam drafting and heat setting. According to the method, primary coagulating bath forming is adopted, spinning is carried out in cooperation with lower coagulating bath concentration and higher draft multiple, molecular chains can be arranged more regularly on the basis that the forming rate is increased, distortion or poor orientation of the molecular chains is avoided, the crystallinity and compactness of the fibers can be improved, and the spinning quality is improved. Further, the strength, modulus and other mechanical properties of the fiber are improved.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber technology, specifically to a method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor. Background Technology

[0002] Carbon fiber is widely used in aerospace, sporting goods, wind power generation, and other fields due to its excellent specific strength, specific modulus, and corrosion resistance. The performance of carbon fiber largely depends on the quality of its precursor—polyacrylonitrile (PAN) precursor fiber; high-quality carbon fiber precursor fiber is the foundation for the preparation of high-performance carbon fiber.

[0003] Carbon fibers can be classified into high-strength, high-strength medium-modulus, high-modulus, and high-strength high-modulus types according to their mechanical properties. The high-strength high-modulus carbon fiber involved in this invention typically has a tensile strength greater than 3.5 GPa and an elastic modulus greater than 350 GPa. Because it combines high strength and high rigidity with good dimensional stability, it has wide applications in aerospace, defense, and high-end civilian fields.

[0004] The groove structure on the surface of the precursor fiber produced by wet spinning can be retained in the carbon fiber, thereby strengthening the bonding strength between the carbon fiber and resin during the composite material preparation process and ultimately improving the mechanical properties of the composite material. Therefore, high-strength, high-modulus carbon fiber precursors are usually produced by wet spinning. However, in existing wet spinning technologies, the depth of the grooves on the precursor fiber and fiber surface varies, and there may even be broken "ribs" or defects, resulting in a reduction in the area of ​​the fiber that can bear external forces and a decrease in tensile strength.

[0005] The formation of nascent fibers from polyacrylonitrile dope in a coagulation bath directly affects the overall performance of the precursor and carbon fibers. Wet spinning coagulation bath formation relies primarily on double diffusion as the forming power, a direct and intense process. Traditional wet spinning methods often employ multi-stage coagulation (two or three stages) with high coagulation bath concentrations (60%-70%) to minimize the presence of a core-sheath structure in the nascent fibers produced under these conditions. However, this method cannot completely eliminate the core-sheath structure and easily leads to non-circular (e.g., kidney-shaped) cross-sections in the precursor fibers. These core-sheath structures become stress concentration points during subsequent high-temperature carbonization, causing cracks and defects in the carbon fibers. Furthermore, the kidney-shaped cross-section reduces the fiber's load-bearing efficiency, severely limiting the improvement of its mechanical properties. Therefore, avoiding core-sheath structures and improving the homogeneity and cross-sectional regularity of the nascent fibers are crucial for producing high-quality carbon fibers.

[0006] In existing technologies, wet spinning often uses dopes with low solid content (15%-18%), molecular weight (8w-10w), and rotational viscosity (4w-6w cP). Compared with dopes with high solid content, molecular weight, and rotational viscosity, the precursor fibers and carbon fibers produced under these conditions have poor density and the ability to withstand external forces, which limits the production of high-quality carbon fibers.

[0007] Furthermore, current industrial wet spinning production often uses larger tows of 24k or even 48k, but large tow carbon fibers cannot meet the demands of high-end applications for high-strength, high-modulus carbon fibers. Small tow carbon fibers (less than 12k), due to their stable and superior performance, processing flexibility, and reliability, are gradually becoming the preferred material for high-end applications.

[0008] Chinese patent CN116005279B discloses a method for preparing high-strength, high-modulus carbon fiber precursor, which employs a three-stage coagulation molding process. By controlling the hot water drawing method and tension of each segment during the precursor production process, it solves the problem of easy fuzzing of carbon fibers during graphitization. Chinese patent CN116024676A discloses a method for preparing high-strength, high-modulus carbon fiber precursor by wet spinning. It mainly ensures the consistency of the coagulation environment and coagulation state by adding a coagulation bath solution perpendicular to the fiber bundle direction at the spinneret outlet. Chinese patent CN111621878A discloses a large-diameter high-strength, medium-modulus, and high-strength, high-modulus carbon fiber with a surface groove structure and its preparation method. It mainly introduces the preparation of large-diameter precursor using wet spinning, which improves the uniformity of the radial optical density value of the pre-oxidized fiber by increasing the volume concentration of oxygen in the gas atmosphere during the pre-oxidation process. Although the aforementioned patent proposes a method for producing high-strength, high-modulus carbon fibers, the spinning solution, coagulation bath molding conditions, and other aspects all employ commonly used technologies in the field. Under these conditions, problems such as the core-sheath structure and irregular morphology of the raw fiber cannot be effectively solved. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a method for producing high-strength, high-modulus polyacrylonitrile-based carbon fiber precursor. By employing a polyacrylonitrile dopant solution with high solids content (18%-25%), high molecular weight (12w-20w), and high rotational viscosity (8w-15w cP), combined with primary coagulation bath molding conditions of low coagulation bath concentration (35-50wt%) and relatively high coagulation bath draw ratio (0.6-0.9 times), wet spinning is carried out. This method utilizes the rapid, homogeneous precipitation of the high-solids-content, high-viscosity dopant solution in a low-coagulation bath, solving the problems of core-sheath structure, irregular cross-sectional morphology, structural defects, and insufficient mechanical properties in carbon fiber precursor produced by existing wet spinning processes, effectively improving the overall performance of both the precursor and the carbon fiber.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: a method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor: PAN dopant is fed into a single-stage wet spinning coagulation bath via a spinneret and solidified. After drawing, PAN nascent fibers are obtained. The nascent fibers are then washed, drawn, oiled, dried and densified, steam-drawn, and heat-set to obtain PAN precursor for high-strength, high-modulus carbon fiber. The coagulation bath is a single-stage coagulation bath, and the solvent in the coagulation bath is a mixture of dimethyl sulfoxide (DMSO) and deionized water (DW). The concentration of DMSO is 35-50 wt%, the coagulation bath temperature is 45-70℃, and the draw ratio of the filament in the coagulation bath is 0.6-0.9 times.

[0011] Furthermore, the PAN stock solution has a solid content of 18wt%-25wt%, a molecular weight of 12w-20w, a rotational viscosity of 8w-15w cP, and a temperature of 60-80℃.

[0012] Furthermore, the spinneret assembly includes a metering pump and a spinneret plate. The metering pump has a range of 20cc / rpm and a rotation speed of 1-30rpm / min. The spinneret plate has 1k-12k holes with a hole diameter of 50-75μm.

[0013] Furthermore, the PAN stock solution enters the coagulation bath at a rate of 8-20 m / min.

[0014] Furthermore, the residence time of the nascent fibers in the coagulation bath is 8-20 seconds.

[0015] Furthermore, the water washing stretching includes a water washing section and a hot water stretching section.

[0016] Furthermore, the washing section adopts a 5-7 level washing process with a temperature of 40-65℃ and a total draw ratio of 1-1.05.

[0017] Furthermore, the hot water stretching section is divided into 3-4 stages, with a temperature of 75-95℃ and a total stretching ratio of 1.5-3.5.

[0018] Furthermore, the oiling temperature is 20-35℃, and the draw ratio is 0.9-1.1; the drying and densification are divided into 3-5 stages of drying, with a temperature of 120-200℃, the temperature increasing step by step, and the drying residence time is 45-120s.

[0019] Furthermore, the steam drawing uses saturated steam to draw the raw filament, with a steam pressure of 0.3-0.6 MPa and a draw ratio of 1.5-4.5, preferably 2.5-3.8; the heat setting uses steam with a pressure of 0.01-0.15 MPa and a draw ratio of 0.9-1.0.

[0020] The beneficial effects of this invention are: (1) By increasing the solid content, molecular weight and rotational viscosity of PAN raw solution, this invention can optimize fiber structure and performance, improve molecular chain orientation and increase carbonization efficiency, which together improve the strength, modulus and thermal stability of carbon fiber, and meet the demand for high-performance materials in aerospace, high-end manufacturing and other fields.

[0021] (2) The present invention uses a first-stage coagulation bath for molding, and uses a lower coagulation bath concentration and a higher draw ratio for spinning. This can improve the molding rate and make the molecular chains more regular, avoiding molecular chain distortion or poor orientation. This is beneficial to improve the crystallinity and density of the fiber, thereby improving the strength, modulus and other mechanical properties of the fiber.

[0022] (3) Lower coagulation bath concentration and higher draw ratio can make the phase separation process on the fiber surface slower and more uniform, avoiding the formation of a core-sheath structure on the surface, and the cross-section has a regular circular structure; at the same time, it helps to reduce surface void defects, and the distribution and depth of the groove structure are more uniform, resulting in higher fiber density. This has a positive impact on the subsequent composite performance with matrix materials such as resins, and helps to improve interfacial bonding and reduce stress concentration points.

[0023] (4) Under controllable rapid coagulation conditions, the spinning speed can be increased accordingly, thereby increasing the capacity of a single production line and improving production efficiency; low coagulation bath concentration can also reduce the total amount of solvent in the system, reduce the cost of raw materials, and improve environmental friendliness. Attached Figure Description

[0024] Figure 1 This is a cross-sectional optical microscope image of the PAN precursor obtained in Example 1 of this invention; Figure 2 This is a cross-sectional optical microscope image of the PAN precursor fiber prepared in Comparative Example 1. Detailed Implementation

[0025] The preparation method of the PAN stock solution of this invention refers to patent CN102517671A; the preparation method of the carbon fiber refers to patent CN111621878 A.

[0026] Example 1:

[0027] Preparation of high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor: A PAN dope solution with a solid content of 20 wt%, molecular weight of 13 w, rotational viscosity of 10 w cP, and temperature of 60 °C was spun out into a coagulation bath through a 3K spinneret with an aperture of 50 μm. The metering pump speed was 5.5 rpm, and the spun-out speed was 18.5 m / min. The DMSO concentration in the coagulation bath was 40 wt%, the temperature was 55 °C, the draw ratio of the fiber tow in the coagulation bath was 0.75, and the residence time was 9.3 s, yielding nascent PAN fibers.

[0028] The nascent PAN fibers undergo a washing and drawing process, divided into a washing stage and a hot water drawing stage. The washing process employs six stages, with washing temperatures for P1-P6 ranging from 45-60℃, increasing sequentially, and a total draw ratio of 1.02. The hot water drawing process consists of three stages, with hot water temperatures for P7-P9 at 75℃, 85℃, and 90℃ respectively, and a total draw ratio of 2.5. Following this, the fibers undergo a 25℃, 0.95 drawing and oiling treatment, and then proceed to a fourth stage of drying and densification, with temperatures of 130℃, [missing information]. The drying and densification processes were carried out at 140℃, 150℃, and 170℃, with the same residence time for each stage, for a total residence time of 50s. A 2.9-fold steam draw was performed using saturated steam at 0.3MPa, followed by heat setting using saturated steam at 0.01MPa, resulting in a draw ratio of 0.96. Finally, the yarn was wound up at 100m / min, yielding a precursor filament with a fineness of 0.80 dtex and a breaking strength of 7.6 cN / dtex. Optical microscope images of the precursor filament cross-section are shown below. Figure 1 As shown: PAN precursor fibers are evenly distributed and have good density. The cross-section of the precursor fibers is a regular circle and there is no core-sheath structure.

[0029] Using the above-mentioned precursor fibers, 3k high-strength, high-modulus carbon fibers with a tensile strength of 5.1 GPa and an elastic modulus of 386 GPa were obtained through pre-oxidation and carbonization treatment.

[0030] Example 2:

[0031] Preparation of high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor: A PAN dope solution with a solid content of 22 wt%, molecular weight of 15 w, rotational viscosity of 14 w cP, and temperature of 80 °C was spun out into a coagulation bath through a 3K spinneret with an aperture of 75 μm. The metering pump speed was 5.6 rpm, and the spun-out speed was 8.4 m / min. The DMSO concentration in the coagulation bath was 50 wt%, the temperature was 60 °C, the draw ratio of the fiber tow in the coagulation bath was 0.8 times, and the residence time was 19.8 s, yielding nascent PAN fibers.

[0032] The nascent PAN fibers undergo a washing and drawing process, divided into a washing stage and a hot water drawing stage. The washing process employs six stages, with washing temperatures for P1-P6 ranging from 45-60℃, increasing sequentially, and a total draw ratio of 1.05. The hot water drawing process consists of three stages, with hot water temperatures for P7-P9 at 80℃, 85℃, and 95℃ respectively, resulting in a total draw ratio of 3.5. Following this, the fibers undergo a 30℃, 0.98 drawing and oiling treatment, and then proceed to a fourth stage of drying and densification at 130℃. The drying and densification processes were carried out at 140℃, 150℃, and 170℃, with the same residence time for each stage, for a total residence time of 75s. Saturated steam at a pressure of 0.45MPa was used for a 3.8-fold steam stretching, followed by heat setting with saturated steam at a pressure of 0.01MPa, resulting in a stretch ratio of 0.98. Finally, the yarn was wound up at a speed of 90m / min, yielding a raw yarn with a single filament fineness of 1.0 dtex and a breaking strength of 6.8 cN / dtex.

[0033] Using the above-mentioned precursor fibers, 3k high-strength, high-modulus carbon fibers with a tensile strength of 4.3 GPa and an elastic modulus of 438 GPa were obtained through pre-oxidation and carbonization treatment.

[0034] Example 3:

[0035] Preparation of high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor: A PAN dope solution with a solid content of 18.5 wt%, molecular weight of 15 w, rotational viscosity of 10 w cP, and temperature of 70 °C was spun out into a coagulation bath through a 6K spinneret with an aperture of 50 μm. The metering pump speed was 10.1 rpm, and the spun-out speed was 17 m / min. The DMSO concentration in the coagulation bath was 45 wt%, the temperature was 55 °C, the draw ratio of the fiber tow in the coagulation bath was 0.65, and the residence time was 10.7 s, yielding nascent PAN fibers.

[0036] The nascent PAN fibers undergo a washing and drawing process, divided into a washing stage and a hot water drawing stage. The washing process employs six stages, with washing temperatures for P1-P6 ranging from 45-60℃, increasing sequentially, and a total draw ratio of 1.05. The hot water drawing process consists of three stages, with hot water temperatures for P7-P9 at 80℃, 90℃, and 95℃ respectively, resulting in a total draw ratio of 2.8. Following this, the fibers undergo a 30℃, 0.95 drawing and oiling treatment, and then proceed to a fourth stage of drying and densification, with temperatures of 140℃, [missing information]. The drying and densification processes were carried out at 150℃, 165℃, and 180℃, with the same residence time for each stage, for a total residence time of 58s. Saturated steam at a pressure of 0.35MPa was used for a 2.9-fold steam stretching, followed by heat setting with saturated steam at a pressure of 0.01MPa, resulting in a stretch ratio of 0.95. Finally, the yarn was wound up at a speed of 85m / min, yielding a raw yarn with a single filament fineness of 0.80dtex and a breaking strength of 7.2cN / dtex.

[0037] Using the above-mentioned precursor fibers, 6k high-strength, high-modulus carbon fibers with a tensile strength of 5.0 GPa and an elastic modulus of 392 GPa were obtained through pre-oxidation and carbonization treatment.

[0038] Comparative Example 1: A PAN stock solution with a solid content of 16 wt%, a molecular weight of 10 W, a rotational viscosity of 5 W cP, and a temperature of 60 °C was used. The solution was extruded through a 3K spinneret with a 50 μm aperture into the first-stage coagulation bath. The metering pump speed was 5.5 rpm, and the extrusion speed was 18.5 m / min. The DMSO concentration in the first-stage coagulation bath was 65 wt%, the temperature was 70 °C, the fiber tow draw ratio was 0.5, and the residence time was 9.8 s. The DMSO concentration in the second-stage coagulation bath was 50 wt%, the temperature was 60 °C, the fiber tow draw ratio was 1, and the residence time was 8.4 s. The DMSO concentration in the third-stage coagulation bath was 40 wt%, the temperature was 45 °C, the fiber tow draw ratio was 1.4, and the residence time was 5.4 s, yielding nascent PAN fibers.

[0039] The nascent PAN fibers undergo a washing and drawing process, divided into a washing stage and a hot water drawing stage. The washing process employs six stages, with washing temperatures for P1-P6 ranging from 45-60℃, increasing sequentially, and a total draw ratio of 1.02. The hot water drawing process consists of three stages, with hot water temperatures for P7-P9 at 75℃, 85℃, and 90℃ respectively, and a total draw ratio of 2.5. After oiling at 25℃ and 0.95, the fibers enter a fourth stage of drying and densification at 130℃. The yarn was dried at 140℃, 150℃, and 170℃, with the same residence time for each stage of densification, for a total residence time of 57s. A 2.6-fold steam draw was performed using saturated steam at 0.3MPa, followed by heat setting with saturated steam at 0.01MPa, resulting in a draw ratio of 0.98. Finally, the yarn was wound up at 80m / min, yielding a precursor filament with a fineness of 0.80 dtex and a breaking strength of 4.8 cN / dtex. Optical microscope images of the precursor filament cross-section are shown below. Figure 2 As shown: PAN precursor fibers are randomly distributed and have poor density. The cross-section of the precursor fibers is obviously kidney-shaped and has a core-skin structure.

[0040] Using the above-mentioned precursor fibers, 3k carbon fibers with a tensile strength of 3.6 GPa and an elastic modulus of 296 GPa were obtained through pre-oxidation and carbonization treatment.

[0041] As can be seen from the PAN precursor fibers and carbon fibers obtained through the above embodiments and comparative examples, the PAN precursor fibers prepared by the present invention have a regular circular cross-section, uniform groove structure distribution and depth, and high fiber density, thereby significantly improving the strength, modulus and other mechanical properties of the precursor fibers and carbon fibers, and achieving good technical results.

[0042] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications are possible without departing from the technical solutions described in the claims.

Claims

1. A method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor, characterized in that: Polyacrylonitrile (PAN) dope is fed into a single-stage wet spinning coagulation bath via a spinneret and solidified. After drawing, PAN nascent fibers are obtained. The nascent fibers are then washed, drawn, oiled, dried and densified, steam drawn, and heat-set to obtain PAN precursor fibers for high-strength, high-modulus carbon fibers. The coagulation bath is a single-stage coagulation bath. The solvent in the coagulation bath is a mixture of dimethyl sulfoxide and demineralized water. The concentration of dimethyl sulfoxide is 35-50 wt%, the coagulation bath temperature is 45-70℃, and the draw ratio of the fiber bundle in the coagulation bath is 0.6-0.9 times.

2. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The PAN stock solution has a solid content of 18wt%-25wt%, a molecular weight of 12w-20w, a rotational viscosity of 8w-15w cP, and a temperature of 60-80℃.

3. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The spinneret assembly includes a metering pump and a spinneret plate. The metering pump has a range of 20cc / rpm and a rotation speed of 1-30rpm / min. The spinneret plate has 1k-12k holes with a hole diameter of 50-75μm.

4. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The PAN stock solution enters the coagulation bath at a rate of 8-20 m / min.

5. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The residence time of the nascent fibers in the coagulation bath is 8-20 seconds.

6. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The water washing and stretching process includes a water washing section and a hot water stretching section.

7. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 6, characterized in that: The washing section uses 5-7 stages of washing at a temperature of 40-65℃ and a total draw ratio of 1-1.

05.

8. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 6, characterized in that: The hot water stretching section is divided into 3-4 stages, with a temperature of 75-95℃ and a total stretching ratio of 1.5-3.

5.

9. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The oiling temperature is 20-35℃, and the draw ratio is 0.9-1.1; the drying and densification are divided into 3-5 stages of drying, with a temperature of 120-200℃, the temperature increasing step by step, and the drying residence time is 45-120s.

10. The method for producing high-strength, high-modulus small-tow polyacrylonitrile-based carbon fiber precursor according to claim 1, characterized in that: The steam drawing process uses saturated steam to draw the raw filament at a steam pressure of 0.3-0.6 MPa and a draw ratio of 1.5-4.5; the heat setting process uses steam at a pressure of 0.01-0.15 MPa and a draw ratio of 0.9-1.0.

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