Polyacrylonitrile precursor fiber, method for producing the same, and use thereof

Abstract

This invention relates to the field of carbon fiber material preparation technology, specifically to a polyacrylonitrile precursor fiber, its preparation method, and its application. The preparation method includes: providing nascent polyacrylonitrile fibers, and sequentially subjecting them to coagulation molding, nine-stage water washing, two-stage hot water drawing, oiling treatment, multi-stage drying and densification, and steam drawing along the fiber transport direction to obtain the polyacrylonitrile precursor fiber. The nine-stage water washing temperature is 30-70℃, the two-stage hot water drawing temperature is 75-95℃ with a total draw ratio of 2-4 times, and the steam drawing temperature is 140-160℃ with a pressure of 3-5 Bar and a draw ratio of 3-5 times. Through the process matching between the nine-stage water washing, two-stage hot water drawing, and steam drawing, continuous control is achieved in the solvent removal, structural adjustment, and molecular chain orientation processes of the fiber, resulting in a polyacrylonitrile precursor fiber with a uniform fiber structure and stable mechanical properties. The obtained polyacrylonitrile precursor fiber can be used to prepare polyacrylonitrile-based carbon fibers.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber material preparation technology, specifically to a polyacrylonitrile precursor fiber, its preparation method, and its application. Background Technology

[0002] Polyacrylonitrile (PAN) precursor fibers are important precursor materials for the preparation of PAN-based carbon fibers. Their fiber structure and properties affect the structural transformation behavior during subsequent pre-oxidation and carbonization processes, and further influence the mechanical properties of the resulting carbon fibers. Therefore, PAN precursor fibers typically need to possess high structural stability, good fiber uniformity, and suitable mechanical properties.

[0003] In existing technologies, polyacrylonitrile precursor fibers are typically prepared through wet spinning or dry-spinning-wet spinning processes. During preparation, the polyacrylonitrile spinning solution is fed into a spinneret to form nascent fibers, which then undergo coagulation, washing, drawing, oiling, drying, and steam drawing processes to obtain polyacrylonitrile precursor fibers that meet the requirements for subsequent carbon fiber production.

[0004] However, during the preparation of polyacrylonitrile precursor fibers, the internal structure of the nascent fibers is still in a gradual formation and adjustment stage after solidification. The degree of solvent removal, molecular chain arrangement, and fiber densification all affect the final precursor fiber performance. In existing preparation processes, due to the varying effects of different parameters during spinning on the fiber structure, problems such as uneven internal fiber structure, insufficient molecular chain orientation, and poor fiber performance stability are prone to occur.

[0005] Therefore, how to improve the fiber structure stability during the preparation of polyacrylonitrile precursor fibers and enhance the orientation degree and overall performance of the precursor fibers has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] In order to address the aforementioned defects and shortcomings in the existing technology, this invention aims to provide a polyacrylonitrile precursor fiber, its preparation method, and its application, so as to solve the problems of uneven internal structure, insufficient molecular chain orientation, and poor fiber performance stability of polyacrylonitrile precursor fiber.

[0007] In a first aspect, the present invention provides a method for preparing polyacrylonitrile precursor fibers, comprising: A polyacrylonitrile virgin fiber is provided, and along the conveying direction of the virgin fiber, it is sequentially subjected to coagulation molding, nine-stage water washing, two-stage hot water drawing, oiling treatment, multi-stage drying densification and steam drawing to obtain polyacrylonitrile precursor fiber. in, The nine-stage water washing temperature range is 30~70℃. The nine-stage water washing includes a first temperature gradient water washing stage, a second temperature gradient water washing stage, and a third temperature gradient water washing stage, and the temperature of the first temperature gradient water washing stage, the second temperature gradient water washing stage, and the third temperature gradient water washing stage increases sequentially. The water temperature for the secondary hot water stretching is 75~95℃, the duration of each stage of hot water stretching is 14~16s, and the total stretching ratio of the secondary hot water stretching is 2~4 times. The steam temperature for steam stretching is 140~160℃, the steam pressure is 3~5 Bar, the steam stretching time is 2~3s, and the steam stretching ratio is 3~5 times.

[0008] According to the technical solution provided in the embodiments of this application, the solidification molding satisfies at least one of the following process parameters: (a) The DMSO solution in the coagulation bath has a mass concentration of 30% to 35%; (b) The temperature of the coagulation bath is 2~8℃; and, (c) The circulation rate of the coagulation bath is 0.7~1.2 m³. 3 / h.

[0009] According to the technical solution provided in the embodiments of this application, the temperature of the first temperature gradient water washing stage is 30~50℃; The temperature of the second temperature gradient water washing stage is 50~60℃; The temperature of the third temperature gradient water washing stage is 60~70℃.

[0010] According to the technical solution provided in the embodiments of this application, the oiling treatment satisfies at least one of the following process parameters: (a) The composite oil agent in the upper oil tank includes amino-modified silicone oil, polyether-modified silicone oil and emulsifier; (b) The mass concentration of the compound oil is 2%~5%; and, (c) The oil temperature in the upper oil tank is 20~25℃.

[0011] According to the technical solution provided in the embodiments of this application, the multi-stage drying and densification includes primary drying, secondary drying, tertiary drying and quaternary drying. Each drying zone is arranged sequentially along the fiber conveying direction. Each drying zone is equipped with a pressure roller mechanism and an air processor. The pressure roller mechanism is used to compact and densify the oiled fiber. The air processor is used to apply compressed air to the fiber to remove impurities.

[0012] According to the technical solution provided in the embodiments of this application, the multi-stage drying and densification satisfies at least one of the following process parameters: (a) The roller pressure of each drying roller is 6~12 Bar; (b) The drying time for each stage is 20~60s; and, (c) The compressed air pressure of each level of the air scraper mechanism is 0.2~1 Bar.

[0013] According to the technical solution provided in the embodiments of this application, the secondary hot water stretching includes a first-stage hot water stretching and a second-stage hot water stretching; The first-stage hot water stretching ratio is 1.5 to 2 times, and the second-stage hot water stretching ratio is 1.5 to 2 times.

[0014] According to the technical solution provided in the embodiments of this application, the total draw ratio in the preparation process of the polyacrylonitrile precursor is 9 to 12 times.

[0015] Secondly, the present invention provides a polyacrylonitrile precursor fiber, which is prepared by the preparation method described above.

[0016] Thirdly, the present invention provides an application of the polyacrylonitrile precursor described above, wherein the polyacrylonitrile precursor is used to prepare polyacrylonitrile-based carbon fibers.

[0017] The beneficial effects of this invention are: By setting up a nine-stage water washing process that includes three temperature gradients with progressively increasing temperatures, the residual solvent inside the nascent fiber gradually migrates outward and is removed. This avoids uneven internal structure of the fiber due to excessively rapid solvent removal and maintains the appropriate plasticization state required for the fiber in the subsequent stretching stage, which is beneficial to improving the stability of the fiber structure.

[0018] By controlling the water temperature, stretching time, and total stretching multiple of each stage of the two-stage hot water stretching process, the fibers after water washing gradually undergo molecular chain rearrangement and orientation under heat softening conditions. This increases the degree of alignment of polyacrylonitrile molecular chains along the fiber axis and reduces structural defects caused by local overstretching.

[0019] By setting up a steam drawing process and controlling the steam temperature, pressure, action time and drawing ratio, the fibers after hot water drawing undergo further chain segment movement and orientation improvement in a high temperature and high humidity environment, promoting the densification of the internal structure of the fiber and improving the orientation degree and mechanical properties of the precursor yarn.

[0020] This invention achieves continuous control over the process of solvent removal, structural adjustment, and molecular chain orientation of fibers through the matching of nine-stage water washing, two-stage hot water stretching, and steam stretching. This improves the problems of uneven internal fiber structure, insufficient molecular chain orientation, and poor performance stability of polyacrylonitrile precursor fibers in the prior art. The resulting polyacrylonitrile precursor fibers have a uniform fiber structure and stable mechanical properties, and are beneficial to improving the performance of carbon fibers formed in the subsequent pre-oxidation and carbonization processes. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to embodiments. The specific embodiments described herein are only for explaining the related invention and are not intended to limit the invention. Unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to embodiments.

[0022] Unless otherwise specified, the materials, reagents, and apparatus used in the following examples can be obtained commercially or prepared according to methods published in the literature.

[0023] In this document, when values ​​are described as ranges, it should be understood that such disclosure includes disclosure of all possible subranges within that range, as well as the specific numerical values ​​falling within that range, regardless of whether the specific numerical value or specific subrange is explicitly specified.

[0024] One objective of this invention is to provide a method for preparing polyacrylonitrile precursor fibers, comprising: A polyacrylonitrile virgin fiber is provided, and along the conveying direction of the virgin fiber, it is sequentially subjected to coagulation molding, nine-stage water washing, two-stage hot water drawing, oiling treatment, multi-stage drying densification and steam drawing to obtain polyacrylonitrile precursor fiber. in, The nine-stage water washing temperature is 30~70℃. The nine-stage water washing includes a first temperature gradient water washing stage, a second temperature gradient water washing stage, and a third temperature gradient water washing stage, and the temperature of the first temperature gradient water washing stage, the second temperature gradient water washing stage, and the third temperature gradient water washing stage increases sequentially. The water temperature for the secondary hot water stretching is 75~95℃, the duration of each stage of hot water stretching is 14~16s, and the total stretching ratio of the secondary hot water stretching is 2~4 times. The steam temperature for steam stretching is 140~160℃, the steam pressure is 3~5 Bar, the steam stretching time is 2~3s, and the steam stretching ratio is 3~5 times.

[0025] In this invention, the nine-stage water washing temperature is 30~70℃, for example: 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃ or 70℃.

[0026] Specifically, the nine-stage water washing is used to remove residual solvents and coagulation media from the interior of the nascent polyacrylonitrile fibers after coagulation and molding. By controlling the water washing temperature to 30~70℃, suitable mass transfer conditions are formed between the inside of the fiber and the external water washing medium, promoting the gradual diffusion and removal of residual solvents. When the nine-stage water washing temperature is below 30℃, the diffusion effect between the water washing medium and the residual solvent inside the fiber weakens, the solvent removal efficiency decreases, and the content of residual solvent inside the fiber is likely to increase, affecting the molecular chain arrangement during subsequent drawing. When the nine-stage water washing temperature is above 70℃, the fiber coagulation structure may be adjusted too quickly, increasing the range of changes in the fiber plasticization state, which is not conducive to stable orientation during subsequent drawing. At the same time, the temperatures of the first temperature gradient water washing stage, the second temperature gradient water washing stage, and the third temperature gradient water washing stage increase sequentially, which is beneficial to adjusting the solvent migration rate according to the changes in fiber structure.

[0027] In this invention, the water temperature of the secondary hot water stretching is 75~95℃, for example: 75℃, 80℃, 85℃, 90℃ or 95℃.

[0028] Specifically, the secondary hot water drawing is used to thermoplasticize the nascent polyacrylonitrile fibers after water washing, and promotes the migration and alignment of polyacrylonitrile molecular chains inside the fiber under heat, so that the molecular chains gradually align along the fiber axis. By controlling the water temperature of the secondary hot water drawing at 75~95℃, the fiber maintains a suitable flexible state, improves the molecular chain mobility during the drawing process, and promotes fiber structure adjustment. When the water temperature of the secondary hot water drawing is lower than 75℃, the mobility of the chain segments inside the fiber decreases, the migration and alignment of molecular chains are restricted, and it is difficult to fully achieve structural orientation under the drawing action. When the water temperature of the secondary hot water drawing is higher than 95℃, the degree of fiber softening due to heat increases, the fiber structural stability decreases, and the fiber morphology retention ability decreases during the drawing process.

[0029] In this invention, the duration of each hot water stretching stage is independently 14 to 16 seconds, for example: 14 seconds, 15 seconds or 16 seconds.

[0030] Specifically, the duration of each stage of hot water stretching is used to control the heating time of the fiber in the hot water environment, allowing the internal chain segments of the fiber to migrate and adjust their arrangement under the action of heat and stretching stress. By controlling the duration of each stage of hot water stretching to 14-16 seconds, the fiber is kept in a thermoplastic state for a sufficient period of time, promoting the orientation of polyacrylonitrile molecular chains along the fiber axis and maintaining fiber structural stability. When the duration of each stage of hot water stretching is less than 14 seconds, the fiber heating time is shortened, the chain segment movement and molecular chain rearrangement process are insufficient, and the degree of molecular chain orientation during stretching is reduced. When the duration of each stage of hot water stretching is greater than 16 seconds, the fiber is in the hot water environment for a long time, the degree of internal structural adjustment of the fiber increases, and the dimensional stability during stretching is affected.

[0031] In this invention, the total stretching ratio of the secondary hot water stretching is 2 to 4 times, for example: 2 times, 2.5 times, 3 times, 3.5 times or 4 times.

[0032] Specifically, the total draw ratio of the secondary hot water drawing is used to control the total stretching degree of the nascent polyacrylonitrile fibers in a hot water environment, causing the polyacrylonitrile molecular chains inside the fiber to adjust their arrangement under axial stretching. By controlling the total draw ratio of the secondary hot water drawing to 2 to 4 times, the fibers can achieve gradual molecular chain orientation while maintaining continuity, improving the internal structural arrangement of the fibers. When the total draw ratio of the secondary hot water drawing is less than 2 times, the axial stretching of the fibers is insufficient, the degree of molecular chain alignment decreases, and the fiber orientation structure is not fully formed. When the total draw ratio of the secondary hot water drawing is greater than 4 times, the degree of tensile deformation of the fibers increases, the degree of stress concentration inside the fibers increases, and the stability of the fiber structure is affected.

[0033] Furthermore, by controlling the total stretching ratio of the secondary hot water stretching to 2 to 4 times, and matching it with the hot water temperature and hot water stretching time, the fiber can complete the gradual orientation adjustment in the thermoplastic state, reducing the impact of a single stretching on the fiber structure, and providing a basis for further improving the degree of molecular chain orientation in subsequent steam stretching.

[0034] In this invention, the steam temperature for steam stretching is 140~160℃, for example: 140℃, 145℃, 150℃, 155℃ or 160℃.

[0035] Specifically, the steam drawing is used to further stretch the polyacrylonitrile fibers after the secondary hot water drawing. The high-temperature steam environment enhances the mobility of the fiber's internal chain segments, allowing the initially arranged polyacrylonitrile molecular chains to migrate and orient further. By controlling the steam drawing temperature to 140-160°C, the fiber undergoes further structural adjustment while in a softened state, increasing the degree of molecular chain alignment along the fiber axis. When the steam drawing temperature is below 140°C, the mobility of the fiber's internal chain segments decreases, molecular chain migration is restricted, and the degree of molecular chain orientation adjustment during steam drawing is reduced. When the steam drawing temperature is above 160°C, the degree of fiber heating increases, the stability of the fiber's internal structure decreases, and the morphological retention during steam drawing is affected.

[0036] In this invention, the steam pressure is 3 to 5 Bar, for example: 3 Bar, 3.5 Bar, 4 Bar, 4.5 Bar or 5 Bar.

[0037] Specifically, the steam pressure is used to regulate the thermal and humid environment of the fiber during steam drawing, allowing high-temperature steam to fully act on the interior of the polyacrylonitrile fiber, improving the mobility of the internal chain segments, and promoting the alignment of molecular chains along the fiber axis in conjunction with the drawing process. By controlling the steam pressure to 3-5 Bar, the steam temperature and humid heat effects meet the fiber structure adjustment requirements, further improving the initial orientation structure formed by hot water drawing. When the steam pressure is below 3 Bar, the heat carried by the steam and the humid heat effect weaken, reducing the mobility of the internal chain segments and decreasing the degree of molecular chain migration and orientation adjustment. When the steam pressure is above 5 Bar, the thermal and humid effects on the fiber are enhanced, increasing the degree of internal fiber structure adjustment and affecting the structural stability during steam drawing.

[0038] In this invention, the steam stretching time is 2 to 3 seconds, for example: 2 seconds, 2.3 seconds, 2.5 seconds, 2.8 seconds or 3 seconds.

[0039] Specifically, the steam drawing time is used to control the duration of contact between the polyacrylonitrile fiber and the high-temperature steam environment, allowing the internal chain segments of the fiber to undergo further migration and alignment under the influence of heat, humidity, and drawing stress. By controlling the steam drawing time to 2-3 seconds, the fiber completes molecular chain orientation adjustment while maintaining continuous transport, promoting the further improvement of the initial orientation structure formed by hot water drawing. When the steam drawing time is less than 2 seconds, the contact time between the fiber and the high-temperature steam environment is shortened, resulting in insufficient chain segment movement and molecular chain rearrangement, thus reducing the orientation adjustment effect of the steam drawing stage. When the steam drawing time is greater than 3 seconds, the fiber remains in a high-temperature and humid environment, increasing the degree of internal structural adjustment and affecting the dimensional stability during the steam drawing process.

[0040] Furthermore, by controlling the steam stretching time to 2-3 seconds and matching it with steam temperature, steam pressure and steam stretching ratio, polyacrylonitrile fibers can be efficiently stretched in a short time, achieving continuous adjustment of fiber structure.

[0041] In this invention, the steam stretching ratio is 3 to 5 times, for example: 3 times, 3.5 times, 4 times, 4.5 times or 5 times.

[0042] Specifically, the steam draw ratio is used to control the axial stretching degree of polyacrylonitrile fibers in a high-temperature steam environment, allowing the initial orientation structure formed by the two-stage hot water draw to undergo further stretching and orientation. By controlling the steam draw ratio to be 3 to 5 times, the polyacrylonitrile molecular chains inside the fiber are further aligned under the hot and humid environment and axial stretching, improving the orientation degree of the molecular chains along the fiber axis and perfecting the internal structure of the fiber. When the steam draw ratio is less than 3 times, the axial stretching effect on the fiber is insufficient, the further orientation degree of the molecular chains is reduced, and the effect of steam draw on fiber structure adjustment cannot be fully utilized. When the steam draw ratio is greater than 5 times, the degree of tensile deformation on the fiber increases, the stress concentration inside the fiber increases, and the stability of the fiber structure is affected.

[0043] Furthermore, by controlling the steam drawing ratio to 3 to 5 times and matching it with steam temperature, steam pressure, and steam drawing time, polyacrylonitrile fibers are drawn efficiently, achieving continuous adjustment of fiber structure. At the same time, it is matched with the total drawing ratio of 2 to 4 times of the secondary hot water drawing, so that the polyacrylonitrile fibers undergo a staged orientation process. Among them, the secondary hot water drawing achieves the initial arrangement of fiber molecular chains, and the steam drawing further improves the degree of molecular chain orientation, thereby obtaining structurally stable polyacrylonitrile precursor fibers.

[0044] In a preferred embodiment, the solidification molding satisfies at least one of the following process parameters: (a) The DMSO solution in the coagulation bath has a mass concentration of 30% to 35%; (b) The temperature of the coagulation bath is 2~8℃; and, (c) The circulation rate of the coagulation bath is 0.7~1.2 m³. 3 / h.

[0045] In this invention, the mass concentration of the DMSO solution in the coagulation bath is 30%~35%, for example: 30%, 31%, 32%, 33%, 34% or 35%; Specifically, the coagulation bath is used to coagulate and shape the nascent polyacrylonitrile fibers. By controlling the DMSO solution concentration in the coagulation bath to 30%~35%, the component exchange process between the nascent fibers and the coagulation bath is adjusted, allowing DMSO inside the nascent fibers to gradually diffuse into the coagulation bath, while simultaneously promoting the formation of a continuous fiber structure from the polyacrylonitrile molecular chains. When the DMSO solution concentration in the coagulation bath is below 30%, the concentration difference between the nascent fibers and the coagulation bath increases, the DMSO diffusion rate accelerates, leading to an increased coagulation speed on the surface of the nascent fibers and potentially causing differences in the degree of coagulation inside and outside the fibers. When the DMSO solution concentration in the coagulation bath is above 35%, the component exchange rate between the nascent fibers and the coagulation bath decreases, affecting the coagulation and shaping of the nascent fibers.

[0046] In this invention, the temperature of the coagulation bath is 2~8℃, for example: 2℃, 3℃, 4℃, 5℃, 6℃, 7℃ or 8℃; Specifically, the coagulation bath temperature is used to regulate the structure formation rate during the coagulation process of the nascent fiber. By controlling the coagulation bath temperature to 2~8℃, the nascent fiber maintains a continuous and stable structural transformation process during coagulation. When the coagulation bath temperature is below 2℃, the migration rate of the internal components of the nascent fiber decreases, the coagulation process proceeds slowly, and the fiber structure formation is affected. When the coagulation bath temperature is above 8℃, the migration rate of the internal components of the nascent fiber increases, the coagulation process accelerates, and the fiber structure adjustment is easily uneven.

[0047] In this invention, the circulation rate of the coagulation bath is 0.7~1.2m³. 3 / h, for example: 0.7m 3 / h, 0.8m 3 / h, 0.9m 3 / h or 1.2m 3 / h.

[0048] Specifically, the coagulation bath circulation rate is used to maintain the stability of the coagulation environment around the nascent fibers, and is controlled to be 0.7~1.2 m. 3 / h, to maintain a stable flow state of the coagulation bath around the nascent fibers, reducing the impact of local concentration and temperature differences on the coagulation process; when the circulation rate of the coagulation bath is less than 0.7m 3 When the circulation rate of the coagulation bath decreases, the component distribution around the nascent fibers is prone to change, affecting the stability of the coagulation process; when the circulation rate of the coagulation bath exceeds 1.2 m³ / h, the coagulation bath turnover rate decreases. 3 When the flow rate of the coagulation bath increases at a rate of / h, it can easily disturb the nascent fibers during transport, affecting the stability of fiber formation.

[0049] Furthermore, by matching and controlling the mass concentration, temperature, and circulation rate of the DMSO solution in the coagulation bath, the nascent polyacrylonitrile fibers are stably coagulated and formed, providing a structural basis for the removal of residual components during the subsequent nine-stage washing process and the adjustment of molecular chain orientation during the secondary hot water stretching process.

[0050] In a preferred embodiment, The temperature of the first temperature gradient water washing stage is 30~50℃; The temperature of the second temperature gradient water washing stage is 50~60℃; The temperature of the third temperature gradient water washing stage is 60~70℃.

[0051] More preferably, the nine-stage water washing includes a first-stage water washing to a ninth-stage water washing arranged sequentially, wherein the first-stage water washing to the third-stage water washing adopts a first temperature gradient, the fourth-stage water washing to the sixth-stage water washing adopts a second temperature gradient, and the seventh-stage water washing to the ninth-stage water washing adopts a third temperature gradient, and the first temperature gradient, the second temperature gradient and the third temperature gradient increase sequentially.

[0052] In this invention, the temperature of the first temperature gradient water washing stage is 30~50℃, for example: 30℃, 35℃, 40℃, 45℃ or 50℃.

[0053] Preferably, the temperature of the first temperature gradient water washing stage is 40°C.

[0054] Specifically, the first-stage washing to the third-stage washing uses a first temperature gradient to perform preliminary washing treatment on the polyacrylonitrile nascent fibers after coagulation and molding, so that the solvent and coagulation medium remaining inside the nascent fibers gradually diffuse outward, reducing the solvent content inside the fibers and adjusting the fiber structure from a coagulated state to a stable state.

[0055] In this invention, the temperature of the second temperature gradient water washing stage is 50~60℃, for example: 50℃, 55℃ or 60℃; Preferably, the temperature of the second temperature gradient water washing stage is 55°C.

[0056] Specifically, the fourth to sixth stage water washing adopts a second temperature gradient to further promote the migration and removal of residual DMSO inside the fiber, and enhances the mass transfer between the fiber and the washing medium by increasing the washing temperature, so as to gradually homogenize the internal structure of the fiber and provide a stable basis for the adjustment of molecular chain arrangement during the subsequent hot water drawing process.

[0057] In this invention, the temperature of the three-stage water washing is 60~70℃, for example: 60℃, 65℃, or 70℃.

[0058] Preferably, the temperature of the three-stage water washing is 65°C.

[0059] Specifically, the seventh to ninth stages of washing employ a third temperature gradient to further reduce the residual DMSO content inside the fiber and promote fiber structure stabilization, so that the washed polyacrylonitrile fiber has a suitable plasticized state when it enters the hot water stretching stage.

[0060] It is worth noting that since the nine-stage washing process includes a first temperature gradient washing stage, a second temperature gradient washing stage, and a third temperature gradient washing stage with successively increasing temperatures, there is a temperature increase relationship between these three stages. Therefore, when the temperature of the first temperature gradient washing stage is 50°C, the temperature of the second temperature gradient washing stage should be greater than 50°C; when the temperature of the second temperature gradient washing stage is 60°C, the temperature of the third temperature gradient washing stage should be greater than 60°C. This ensures that the washing temperature gradually increases along the fiber conveying direction during the nine-stage washing process, achieving gradient removal of residual solvents inside the fiber.

[0061] In a preferred embodiment, the oiling treatment satisfies at least one of the following process parameters: (a) The composite oil agent in the upper oil tank includes amino-modified silicone oil, polyether-modified silicone oil and emulsifier; (b) The mass concentration of the compound oil is 2%~5%; and, (c) The oil temperature in the upper oil tank is 20~25℃.

[0062] In this invention, the mass concentration of the compound oil is 2% to 6%, for example: 2%, 3%, 4%, or 5%.

[0063] Specifically, the oiling treatment is used to form an oil film layer on the surface of polyacrylonitrile fibers, reducing friction during subsequent conveying, drying, densification, and drawing processes, and improving the lubricity and bundle properties of the fiber surface. By controlling the mass concentration of the composite oil agent to 2%~6%, the composite oil agent adheres to the fiber surface, forming a continuous oil film structure on the fiber surface, reducing direct contact between fibers. When the mass concentration of the composite oil agent is below 2%, the oil film coverage on the fiber surface decreases, and friction during fiber conveying increases, affecting the integrity of the fiber surface. When the mass concentration of the composite oil agent is above 5%, the amount of oil agent adhering to the fiber surface increases, affecting oil removal during subsequent drying and densification processes and fiber heat treatment processes.

[0064] In this invention, the oil temperature in the upper oil tank is 20~25℃, for example: 20℃, 21℃, 22℃, 23℃, 24℃ or 25℃.

[0065] Specifically, the oil temperature in the upper oil tank is used to regulate the flow state of the composite oil and its wetting effect on the fiber surface. By controlling the oil temperature to 20~25℃, the composite oil maintains suitable fluidity and improves the uniformity of contact between the composite oil and the polyacrylonitrile fiber surface. When the oil temperature is below 20℃, the fluidity of the composite oil decreases, affecting the uniformity of wetting and adhesion of the oil on the fiber surface. When the oil temperature is above 25℃, the stability of the composite oil decreases, affecting the uniform distribution of the oil components on the fiber surface.

[0066] Furthermore, by controlling the matching relationship between the composition of the composite oil, the mass concentration of the composite oil, and the oiling temperature, a stable oil film layer is formed on the surface of the polyacrylonitrile fiber after the drawing treatment, which reduces the risk of fiber damage during subsequent multi-stage drying densification and steam drawing processes, and maintains the stability during the continuous fiber conveying process.

[0067] In a preferred embodiment, the multi-stage drying and densification includes primary drying, secondary drying, tertiary drying and quaternary drying. Each drying zone is arranged sequentially along the fiber conveying direction. Each drying zone is equipped with a drying roller and an air scraper mechanism. The air scraper mechanism is used to automatically clean the dirt on the surface of the drying roller to ensure the densification of the fiber bundle.

[0068] Specifically, the multi-stage drying and densification process is used to dry and densify the oiled polyacrylonitrile fibers. The primary, secondary, tertiary, and quaternary drying stages are sequentially arranged along the fiber conveying direction to gradually remove residual moisture and volatile components from the fibers, causing the fiber structure to gradually shrink and increase its density. Drying rollers in each drying zone apply pressure to the fibers, promoting densification. The air scraper mechanism cleans dirt adhering to the surface of the drying rollers, preventing dirt accumulation from affecting the drying and densification process.

[0069] In a preferred embodiment, the multi-stage drying densification satisfies at least one of the following process parameters: (a) The roller pressure of each drying roller is 6~12 Bar; (b) The drying time for each stage is 20~60s; and, (c) The compressed air pressure of each level of the air scraper mechanism is 0.2~1 Bar.

[0070] In this invention, the roller pressure of each drying roller is 6~12 Bar, for example: 6 Bar, 7 Bar, 8 Bar, 9 Bar, 10 Bar, 11 Bar or 12 Bar.

[0071] Specifically, the drying roller is used to apply a compaction effect to the polyacrylonitrile fiber during the drying process. By controlling the roller pressure to be 6~12 Bar, the fiber undergoes moderate compression during the gradual removal of moisture, promoting the shrinkage of internal pores and improving the density of the fiber structure. When the roller pressure is lower than 6 Bar, the compaction effect on the fiber is insufficient, and the degree of internal structure densification decreases. When the roller pressure is higher than 12 Bar, the compression effect on the fiber is too strong, which can easily lead to fiber cross-section deformation or internal stress concentration, affecting the stability of the original fiber structure.

[0072] In this invention, the drying time for each stage is 20 to 60 seconds, for example: 20 seconds, 30 seconds, 40 seconds, 50 seconds or 60 seconds.

[0073] Specifically, the drying time for each stage is used to control the removal of internal moisture and volatile components from the polyacrylonitrile fiber, allowing the fiber to complete structural shrinkage and densification adjustments during the gradual drying process. By controlling the drying time for each stage to be 20-60 seconds, the internal moisture of the fiber can be fully migrated to the outside of the fiber and removed, while avoiding excessively rapid water loss during the drying process, which could lead to large structural stresses inside the fiber. When the drying time for each stage is less than 20 seconds, the internal moisture of the fiber is not sufficiently removed, which can easily lead to an increase in residual moisture inside the fiber, affecting the structural adjustment during the subsequent steam drawing process. When the drying time for each stage is greater than 60 seconds, the fiber is in a dry environment for a long time, which can easily cause excessive shrinkage of the fiber surface, reducing the uniformity of the internal structure of the fiber.

[0074] In this invention, the compressed air pressure of each level of the air scraper mechanism is 0.2~1 Bar, for example: 0.2 Bar, 0.4 Bar, 0.6 Bar, 0.8 Bar or 1 Bar.

[0075] Specifically, the air scraper mechanism is used to clean the surface of the drying roller by blowing compressed air. By controlling the compressed air pressure to 0.2~1 Bar, dirt and residues adhering to the surface of the drying roller during the drying process are effectively removed, preventing dirt from accumulating on the surface of the drying roller and affecting the contact state between the drying roller and the fiber bundle, thus ensuring the densification of the fiber bundle. When the compressed air pressure is lower than 0.2 Bar, the airflow intensity is insufficient, making it difficult to effectively remove the adhering substances on the surface of the drying roller and affecting the cleaning effect of the drying roller. When the compressed air pressure is higher than 1 Bar, the airflow is too strong, which can easily disturb the fiber conveying state, affecting the stability of the fiber bundle operation and the continuity of the raw fiber preparation process.

[0076] In a preferred embodiment, the secondary hot water stretching includes a first-stage hot water stretching and a second-stage hot water stretching, wherein the first-stage hot water stretching ratio is 1.5 to 2 times, and the second-stage hot water stretching ratio is 1.5 to 2 times.

[0077] In this invention, the first-stage hot water stretching ratio is 1.5 to 2 times, for example: 1.5 times, 1.6 times, 1.7 times, 1.8 times, 1.9 times or 2 times.

[0078] Specifically, the first-stage hot water drawing is used to initially draw the polyacrylonitrile fibers after nine-stage washing. Under the action of hot water, the polyacrylonitrile molecular chains inside the fiber gain mobility and undergo initial alignment along the fiber axis, thereby improving the internal structural state of the fiber and providing a stable structural foundation for the subsequent second-stage hot water drawing. When the first-stage hot water drawing ratio is less than 1.5 times, the fiber is subjected to insufficient tensile force, and the improvement in the degree of molecular chain orientation is limited; when the first-stage hot water drawing ratio is greater than 2 times, the fiber is subjected to excessive tensile force in the initial drawing stage, which can easily cause local structural inhomogeneity and affect the stability of the subsequent drawing process.

[0079] In this invention, the second-stage hot water stretching ratio is 1.5 to 2 times, for example: 1.5 times, 1.6 times, 1.7 times, 1.8 times, 1.9 times or 2 times.

[0080] Specifically, the second-stage hot water drawing is used to further stretch the polyacrylonitrile fibers after the first-stage hot water drawing, allowing the initially aligned molecular chains to continue oriented along the fiber axis, increasing the degree of axial orientation and promoting fiber structure homogenization. When the second-stage hot water drawing ratio is less than 1.5 times, the further orientation effect on the fiber is insufficient, making it difficult to achieve sufficient alignment of the molecular chains; when the second-stage hot water drawing ratio is greater than 2 times, the tensile force on the fiber is enhanced, which can easily lead to increased internal stress in the fiber and reduce the structural stability of the precursor fiber.

[0081] In a preferred embodiment, the total draw ratio during the preparation of the polyacrylonitrile precursor fiber is 9 to 12 times.

[0082] In this invention, the total draw ratio during the preparation of the polyacrylonitrile precursor fiber is 9 to 12 times, for example, 9 times, 10 times, 11 times or 12 times.

[0083] Specifically, the total draw ratio is the product of the draw ratios of each stage in the preparation of polyacrylonitrile precursor fibers. Specifically, the total draw ratio of the second-stage hot water drawing is the product of the first-stage hot water drawing ratio and the second-stage hot water drawing ratio. The total draw ratio in the preparation of polyacrylonitrile precursor fibers is the product of the total draw ratio of the second-stage hot water drawing and the steam drawing ratio. By controlling the total draw ratio to 9-12 times, the polyacrylonitrile molecular chains are gradually oriented in the fiber axial direction, improving the degree of fiber orientation while avoiding excessive draw ratio that could lead to a decrease in fiber structural stability. When the total draw ratio is below 9 times, the orientation effect on the molecular chains during each stage of drawing is insufficient, reducing the degree of alignment of the polyacrylonitrile molecular chains along the fiber axial direction, affecting the mechanical properties of the precursor fiber and the structural transformation during subsequent carbon fiber preparation. When the total draw ratio is above 12 times, the overall tensile force on the fiber is enhanced, easily leading to increased internal stress and decreased structural stability, affecting the stability of the continuous precursor fiber production process.

[0084] A second objective of this invention is to provide a polyacrylonitrile precursor fiber, which is prepared by the method described above.

[0085] Specifically, the polyacrylonitrile precursor fiber is prepared through coagulation molding, nine-stage washing, two-stage hot water stretching, oiling treatment, multi-stage drying densification, and steam stretching. Coagulation molding forms a continuous fiber structure in the nascent polyacrylonitrile fibers. The nine-stage washing process, with progressively increasing temperatures in the first, second, and third temperature gradient washing stages, promotes the gradual removal of residual solvents from the fiber interior, stabilizing the fiber structure. Two-stage hot water stretching with a total stretching ratio of 2-4 times is performed in a 75-95℃ hot water environment, causing the polyacrylonitrile molecular chains to initially align along the fiber axis. After oiling treatment and multi-stage drying densification, a protective layer forms on the fiber surface, while internal moisture is gradually removed and the density of the structure is improved through compaction. Steam stretching with a ratio of 3-5 times is performed in a 140-160℃, 3-5 Bar steam environment, further orienting the initially aligned molecular chains, thereby obtaining a polyacrylonitrile precursor fiber with improved molecular chain orientation and a denser internal structure.

[0086] A third objective of this invention is to provide an application of polyacrylonitrile precursor fibers, which are used to prepare polyacrylonitrile-based carbon fibers.

[0087] Specifically, the polyacrylonitrile precursor fiber is pre-oxidized and carbonized to form polyacrylonitrile-based carbon fiber. The polyacrylonitrile molecular chains inside the polyacrylonitrile precursor fiber are oriented along the fiber axis, and the fiber structure is dried and densified to form a dense structure. This allows the precursor fiber to maintain a stable structural transformation process during subsequent pre-oxidation and carbonization, promoting the formation of an ordered carbon fiber structure along the fiber axis.

[0088] Example 1 This embodiment provides a method for preparing polyacrylonitrile precursor fibers, including: A primary polyacrylonitrile fiber is provided, and along the conveying direction of the primary fiber, it is sequentially subjected to coagulation molding, nine-stage water washing, two-stage hot water drawing, oiling treatment, multi-stage drying densification and steam drawing to obtain polyacrylonitrile precursor fiber.

[0089] During the solidification process, a 32% (w / w) DMSO aqueous solution was used as the solidification bath, the solidification bath temperature was 5℃, and the solidification bath circulation rate was 0.8 m³ / s. 3 / h.

[0090] The nine-stage water washing includes a first temperature gradient water washing stage, a second temperature gradient water washing stage, and a third temperature gradient water washing stage. The temperature of the first temperature gradient water washing stage is 40℃, the temperature of the second temperature gradient water washing stage is 55℃, and the temperature of the third temperature gradient water washing stage is 65℃.

[0091] The two-stage hot water stretching includes the first-stage hot water stretching and the second-stage hot water stretching. The hot water temperature is 85℃, and the hot water stretching time for each stage is 15s. The first-stage hot water stretching ratio is 1.5 times, and the second-stage hot water stretching ratio is 2 times.

[0092] During the oiling process, a 4% (w / w) composite oil agent was used at an oil temperature of 22°C.

[0093] Multi-stage drying and densification includes primary drying, secondary drying, tertiary drying and quaternary drying. The roller pressure of each drying stage is 9 Bar, the drying time of each stage is 40 seconds, and the compressed air pressure of each air scraper mechanism is 0.6 Bar.

[0094] During the steam stretching process, the steam temperature was 150℃, the steam pressure was 4 Bar, the steam stretching time was 2.5s, and the steam stretching ratio was 4 times.

[0095] Example 2 The difference from Example 1 is that: The coagulation bath used a 30% (w / w) DMSO aqueous solution, the coagulation bath temperature was 3℃, and the coagulation bath circulation rate was 0.6 m³ / s. 3 / h.

[0096] The temperature of the first temperature gradient water washing stage is 30℃, the temperature of the second temperature gradient water washing stage is 50℃, and the temperature of the third temperature gradient water washing stage is 60℃.

[0097] The hot water stretching temperature is 85℃, the hot water stretching time for each stage is 14s, the first stage hot water stretching ratio is 1.5 times, and the second stage hot water stretching ratio is 1.5 times.

[0098] The steam stretching temperature was 140℃, the steam pressure was 3 Bar, the steam stretching time was 2s, and the steam stretching ratio was 3 times.

[0099] The remaining conditions are the same as in Example 1.

[0100] Example 3 The difference from Example 1 is that: The coagulation bath uses a 35% (w / w) DMSO aqueous solution, the coagulation bath temperature is 7℃, and the coagulation bath circulation rate is 1m³. 3 / h.

[0101] The temperature of the first temperature gradient water washing stage is 50℃, the temperature of the second temperature gradient water washing stage is 60℃, and the temperature of the third temperature gradient water washing stage is 70℃.

[0102] The hot water stretching temperature is 95℃, the hot water stretching time for each stage is 16s, the first stage hot water stretching ratio is 2 times, and the second stage hot water stretching ratio is 2 times.

[0103] The steam stretching temperature was 160℃, the steam pressure was 5 Bar, the steam stretching time was 3s, and the steam stretching ratio was 5 times.

[0104] The remaining conditions are the same as in Example 1.

[0105] Example 4 The difference from Example 1 is that: During the two-stage hot water stretching process, the hot water temperature was 88℃, the stretching time for each stage was 15s, the stretching ratio for the first stage was 1.75 times, and the stretching ratio for the second stage was 1.75 times.

[0106] During the steam stretching process, the steam temperature was 145℃, the steam pressure was 3.5 Bar, the steam stretching time was 2.5 s, and the steam stretching ratio was 3.5 times.

[0107] The remaining conditions are the same as in Example 1.

[0108] Example 5 The difference from Example 1 is that: During the two-stage hot water stretching process, the hot water temperature was 92℃, the stretching time for each stage was 14s, the stretching ratio for the first stage was 1.75 times, and the stretching ratio for the second stage was 2 times.

[0109] During the steam stretching process, the steam temperature was 155℃, the steam pressure was 4.5 Bar, the steam stretching time was 2s, and the steam stretching ratio was 4.5 times.

[0110] The remaining conditions are the same as in Example 1.

[0111] Example 6 The difference from Example 1 is that: The coagulation bath uses a 31% DMSO aqueous solution, the coagulation bath temperature is 4℃, and the coagulation bath circulation rate is 0.7m³ / h.

[0112] The temperature for the first temperature gradient water washing stage is 38℃, the temperature for the second temperature gradient water washing stage is 56℃, and the temperature for the third temperature gradient water washing stage is 61℃.

[0113] The secondary hot water stretching temperature is 87℃, the stretching time for each stage of hot water is 14s, the stretching ratio for the primary hot water stage is 1.5 times, and the stretching ratio for the secondary hot water stage is 1.75 times.

[0114] The steam stretching temperature was 145℃, the steam pressure was 3.5 Bar, the steam stretching time was 2.5 s, and the steam stretching ratio was 3.5 times.

[0115] The remaining conditions are the same as in Example 1.

[0116] Example 7 The difference from Example 1 is that: The coagulation bath used a 33% (w / w) DMSO aqueous solution, the coagulation bath temperature was 6℃, and the coagulation bath circulation rate was 0.9 m³ / s. 3 / h.

[0117] The temperature of the first temperature gradient water washing stage is 42℃, the temperature of the second temperature gradient water washing stage is 58℃, and the temperature of the third temperature gradient water washing stage is 63℃.

[0118] The secondary hot water stretching temperature is 91℃, the stretching time for each stage of hot water is 15s, the stretching ratio for the primary hot water stage is 1.6 times, and the stretching ratio for the secondary hot water stage is 1.6 times.

[0119] The steam stretching temperature was 152℃, the steam pressure was 4.2 Bar, the steam stretching time was 2.5 s, and the steam stretching ratio was 4.2 times.

[0120] The remaining conditions are the same as in Example 1.

[0121] Example 8 The difference from Example 1 is that: The coagulation bath used a 34% (w / w) DMSO aqueous solution, the coagulation bath temperature was 6℃, and the coagulation bath circulation rate was 0.85 m³ / s. 3 / h.

[0122] The temperature for the first temperature gradient water washing stage is 43℃, the temperature for the second temperature gradient water washing stage is 59℃, and the temperature for the third temperature gradient water washing stage is 64℃.

[0123] The secondary hot water stretching temperature is 93℃, the stretching time for each stage of hot water is 16s, the stretching ratio for the first stage of hot water is 1.9 times, and the stretching ratio for the second stage of hot water is 1.9 times.

[0124] The steam stretching temperature was 158℃, the steam pressure was 4.8 Bar, the steam stretching time was 3s, and the steam stretching ratio was 4.8 times.

[0125] The remaining conditions are the same as in Example 1.

[0126] Example 9 The difference from Example 1 is that: During the oiling process, a 2% (by mass) composite oil agent is used at an oil temperature of 20°C.

[0127] During the multi-stage drying and densification process, the roller pressure of each drying roller is 6 Bar, the drying time of each stage is 20 seconds, and the compressed air pressure of each air scraper mechanism is 0.2 Bar.

[0128] The remaining conditions are the same as in Example 1.

[0129] Example 10 The difference from Example 1 is that: During the oiling process, a 5% (w / w) composite oil agent is used at an oil temperature of 25°C.

[0130] During the multi-stage drying and densification process, the roller pressure of each drying roller is 12 Bar, the drying time of each stage is 60 seconds, and the compressed air pressure of each air scraper mechanism is 1 Bar.

[0131] The remaining conditions are the same as in Example 1.

[0132] Example 11 The difference from Example 1 is that: The coagulation bath used a 31% (w / w) DMSO aqueous solution, the coagulation bath temperature was 4℃, and the coagulation bath circulation rate was 0.7 m³ / s. 3 / h.

[0133] The temperature for the first temperature gradient water washing stage is 37℃, the temperature for the second temperature gradient water washing stage is 56℃, and the temperature for the third temperature gradient water washing stage is 61℃.

[0134] The secondary hot water stretching temperature is 89℃, the stretching time for each stage of hot water is 15s, the stretching ratio for the first stage of hot water is 1.6 times, and the stretching ratio for the second stage of hot water is 1.6 times.

[0135] During the multi-stage drying and densification process, the roller pressure of each drying roller is 8 Bar, the drying time of each stage is 35 seconds, and the compressed air pressure of each air scraper mechanism is 0.5 Bar.

[0136] The steam stretching temperature was 148℃, the steam pressure was 3.8 Bar, the steam stretching time was 2.5 s, and the steam stretching ratio was 3.8 times.

[0137] The remaining conditions are the same as in Example 1.

[0138] Example 12 The difference from Example 1 is that: The coagulation bath used a 34% (w / w) DMSO aqueous solution, the coagulation bath temperature was 6℃, and the coagulation bath circulation rate was 0.9 m³ / s. 3 / h.

[0139] The temperature for the first temperature gradient water washing stage is 44℃, the temperature for the second temperature gradient water washing stage is 59℃, and the temperature for the third temperature gradient water washing stage is 64℃.

[0140] The secondary hot water stretching temperature is 94℃, the stretching time for each stage of hot water is 16s, the stretching ratio for the first stage of hot water is 1.8 times, and the stretching ratio for the second stage of hot water is 1.8 times.

[0141] During the oiling process, a 5% (w / w) composite oil agent is used at an oil temperature of 24°C.

[0142] During the multi-stage drying and densification process, the roller pressure of each drying roller is 10 Bar, the drying time of each stage is 50 seconds, and the compressed air pressure of each air scraper mechanism is 0.8 Bar.

[0143] The steam stretching temperature was 157℃, the steam pressure was 4.6 Bar, the steam stretching time was 2.8 s, and the steam stretching ratio was 4.6 times.

[0144] The remaining conditions are the same as in Example 1.

[0145] Comparative Example 1 The difference from Example 1 is that: During the nine-stage water washing process, the temperature of the first temperature gradient water washing stage is 65℃, the temperature of the second temperature gradient water washing stage is 55℃, and the temperature of the third temperature gradient water washing stage is 40℃.

[0146] The remaining conditions are the same as in Example 1.

[0147] Comparative Example 2 The difference from Example 1 is that: In the two-stage hot water stretching process, the first-stage hot water stretching ratio is 5 times, and the second-stage hot water stretching ratio is 1 time.

[0148] The remaining conditions are the same as in Example 1.

[0149] Comparative Example 3 The difference from Example 1 is that: In the two-stage hot water stretching process, the first-stage hot water stretching ratio is 3 times, and the second-stage hot water stretching ratio is 4 times.

[0150] The steam stretching step is cancelled.

[0151] The remaining conditions are the same as in Example 1.

[0152] Comparative Example 4 The difference from Example 1 is that: During the two-stage hot water stretching process, the hot water temperature is 70℃, the stretching time for each stage of hot water is 15s, the stretching ratio for the first stage of hot water is 1.5 times, and the stretching ratio for the second stage of hot water is 1.5 times.

[0153] The remaining conditions are the same as in Example 1.

[0154] Comparative Example 5 The difference from Example 1 is that: The secondary hot water stretching step is eliminated, and steam stretching is performed directly. The steam temperature is 150℃, the steam pressure is 4 Bar, the steam stretching time is 2.5s, and the steam stretching ratio is 9 times.

[0155] The remaining conditions are the same as in Example 1.

[0156] Comparative Example 6 The difference from Example 1 is that: In the nine-stage water washing process, the temperature of the first temperature gradient water washing stage is 65℃, the temperature of the second temperature gradient water washing stage is 65℃, and the temperature of the third temperature gradient water washing stage is 65℃.

[0157] The remaining conditions are the same as in Example 1.

[0158] Comparative Example 7 The difference from Example 1 is that: In the nine-stage water washing process, the temperature of the first temperature gradient water washing stage is 45℃, the temperature of the second temperature gradient water washing stage is 65℃, and the temperature of the third temperature gradient water washing stage is 65℃.

[0159] The remaining conditions are the same as in Example 1.

[0160] Comparative Example 8 The difference from Example 1 is that: During the steam stretching process, the steam temperature was 120℃, the steam pressure was 2 Bar, the steam stretching time was 2.5s, and the steam stretching ratio was 4 times.

[0161] The remaining conditions are the same as in Example 1.

[0162] Performance testing The polyacrylonitrile precursor fibers prepared in Examples 1-12 and Comparative Examples 1-8 were subjected to performance tests. The test items included linear density, fineness, fineness CV%, strength, strength CV%, modulus, modulus CV%, elongation, elongation CV%, diameter, diameter CV%, cross-sectional morphology, oil content, water content, boiling water shrinkage rate, and solvent residue.

[0163] The linear density was tested according to GB / T 14335; the tensile strength, tensile modulus and elongation at break were tested according to GB / T 14337; the fiber diameter was tested using a microscopic measurement method; the cross-sectional morphology was observed using a scanning electron microscope; and the oil content, water content and residual solvent content were detected using corresponding chemical analysis methods.

[0164] The performance test results of Examples 1-12 are shown in Table 1 below, and the performance test results of Comparative Examples 1-8 are shown in Table 2 below; Table 1

[0165] Table 2

[0166] The test results show that the polyacrylonitrile precursor fibers prepared in Examples 1-12 have high strength and modulus, while maintaining a suitable elongation. Furthermore, the fineness, diameter, oil content, water content, boiling water shrinkage rate, and residual solvent content are all within a stable range. This indicates that the present invention, through the process matching between coagulation molding, nine-stage water washing, two-stage hot water stretching, oiling treatment, multi-stage drying densification, and steam stretching, enables the polyacrylonitrile nascent fibers to achieve gradual solvent removal, gradual molecular chain orientation, and gradual fiber structure densification during processing, thereby obtaining polyacrylonitrile precursor fibers with uniform structure and stable performance.

[0167] The nine-stage washing process employs progressively increasing temperatures in the first, second, and third temperature gradient washing stages, allowing residual solvent inside the fiber to gradually migrate outward and reducing solvent residue. The second-stage hot water stretching, through two continuous stretching stages, causes the fiber to gradually undergo molecular chain movement and axial orientation under heating, avoiding structural inhomogeneity caused by excessive stretching in a single step. Steam stretching further promotes fiber structure adjustment, improving fiber strength and modulus. Simultaneously, during the multi-stage drying and densification process, the roller pressure of the drying rollers promotes fiber structure shrinkage and densification, and the air scraper mechanism cleans the surface of the drying rollers, ensuring the stable progress of the drying and densification process.

[0168] In contrast, in Comparative Example 1, the water washing temperature decreased progressively along the fiber conveying direction, affecting the removal of residual solvent inside the fiber and reducing the fiber structure adjustment effect during subsequent drawing. In Comparative Example 2, the excessively high first-stage hot water drawing ratio and the reduced second-stage hot water drawing ratio concentrated the tensile force on the fiber in the early stage, resulting in an uneven molecular chain orientation process and affecting the structural stability of the precursor fiber. In Comparative Example 3, the elimination of steam drawing deprived the fiber after hot water drawing of further structural adjustment, leading to a decrease in the degree of molecular chain orientation perfection. In Comparative Example 4, the reduced hot water drawing temperature restricted the movement of polyacrylonitrile molecular chains, affecting... The effects of steam stretching were as follows: Comparative Example 5 eliminated the secondary hot water stretching and directly performed steam stretching, resulting in a lack of gradual stretching and adjustment process for the fibers, which easily led to a decrease in fiber structure uniformity; Comparative Example 6 used constant temperature water washing, which could not form a gradually increasing temperature gradient, reducing the effect of gradual solvent migration and removal during the water washing process; In Comparative Example 7, the temperature changes between the first, second, and third temperature gradient water washing stages were discontinuous, which affected the fiber internal structure adjustment process; Comparative Example 8 reduced the steam stretching temperature and pressure, which reduced the promoting effect of high temperature and high humidity environment on molecular chain movement and orientation perfection.

[0169] This demonstrates that by controlling the process matching relationship between the nine-stage water washing, the two-stage hot water stretching, and the steam stretching, as well as the synergistic relationship between the multi-stage drying and densification processes, the present invention enables the gradual regulation of solvent removal, molecular chain orientation, and structural densification of polyacrylonitrile fibers during continuous processing, thereby improving the mechanical properties and structural stability of the precursor fiber. The resulting polyacrylonitrile precursor fiber can meet the requirements for subsequent preparation of polyacrylonitrile-based carbon fibers.

[0170] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A method for preparing polyacrylonitrile precursor fibers, characterized in that, include: A polyacrylonitrile virgin fiber is provided, and along the conveying direction of the virgin fiber, it is sequentially subjected to coagulation molding, nine-stage water washing, two-stage hot water drawing, oiling treatment, multi-stage drying densification and steam drawing to obtain polyacrylonitrile precursor fiber. in, The nine-stage water washing temperature is 30~70℃. The nine-stage water washing includes a first temperature gradient water washing stage, a second temperature gradient water washing stage, and a third temperature gradient water washing stage, and the temperature of the first temperature gradient water washing stage, the second temperature gradient water washing stage, and the third temperature gradient water washing stage increases sequentially. The water temperature for the secondary hot water stretching is 75~95℃, the duration of each stage of hot water stretching is 14~16s, and the total stretching ratio of the secondary hot water stretching is 2~4 times. The steam temperature for steam stretching is 140~160℃, the steam pressure is 3~5 Bar, the steam stretching time is 2~3s, and the steam stretching ratio is 3~5 times.

2. The method for preparing polyacrylonitrile precursor fiber according to claim 1, characterized in that, The solidification molding process satisfies at least one of the following process parameters: (a) The DMSO solution in the coagulation bath has a mass concentration of 30% to 35%; (b) The temperature of the coagulation bath is 2~8℃; and, (c) The circulation rate of the coagulation bath is 0.7~1.2 m³. 3 / h.

3. The method for preparing polyacrylonitrile precursor fiber according to claim 1, characterized in that, The temperature of the first temperature gradient water washing stage is 30~50℃; The temperature of the second temperature gradient water washing stage is 50~60℃; The temperature of the third temperature gradient water washing stage is 60~70℃.

4. The method for preparing polyacrylonitrile precursor fiber according to claim 1, characterized in that, The oiling treatment satisfies at least one of the following process parameters: (a) The composite oil agent in the upper oil tank includes amino-modified silicone oil, polyether-modified silicone oil and emulsifier; (b) The mass concentration of the compound oil is 2%~5%; and, (c) The oil temperature in the upper oil tank is 20~25℃.

5. The method for preparing polyacrylonitrile precursor fiber according to claim 1, characterized in that, The multi-stage drying and densification process includes primary drying, secondary drying, tertiary drying, and quaternary drying. Each drying zone is arranged sequentially along the fiber conveying direction. Each drying zone is equipped with a drying roller and an air scraper mechanism. The air scraper mechanism is used to automatically clean the dirt on the surface of the drying roller to ensure the densification of the fiber bundle.

6. The method for preparing polyacrylonitrile precursor fiber according to claim 5, characterized in that, The multi-stage drying and densification process satisfies at least one of the following process parameters: (a) The roller pressure of each drying roller is 6~12 Bar; (b) The drying time for each stage is 20~60s; (c) The compressed air pressure of each level of the air scraper mechanism is 0.2~1 Bar.

7. The method for preparing polyacrylonitrile precursor fiber according to claim 1, characterized in that, The secondary hot water drawing includes a first-stage hot water drawing and a second-stage hot water drawing; The first-stage hot water stretching ratio is 1.5 to 2 times, and the second-stage hot water stretching ratio is 1.5 to 2 times.

8. The method for preparing polyacrylonitrile precursor fiber according to claim 7, characterized in that, The total draw ratio during the preparation of the polyacrylonitrile precursor fiber is 9 to 12 times.

9. A polyacrylonitrile precursor fiber, characterized in that, The polyacrylonitrile precursor fiber is prepared by the preparation method according to any one of claims 1 to 8.

10. An application of the polyacrylonitrile precursor fiber as described in claim 9, characterized in that, The polyacrylonitrile precursor fiber is used to prepare polyacrylonitrile-based carbon fiber.

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