Preparation method of polyacrylonitrile pre-oxidized filament, polyacrylonitrile pre-oxidized filament and carbon fiber

Abstract

This invention discloses a method for preparing polyacrylonitrile pre-oxidized fibers, the pre-oxidized fibers themselves, and carbon fibers, relating to the field of carbon fiber manufacturing technology. The invention involves gradient-temperature polymerization of vinyl phenolic compounds and acrylate compounds containing active hydrogen with acrylonitrile. This gradient copolymerization introduces vinyl phenolic compounds and oxygen-containing polar monomers into the polyacrylonitrile molecular chain, lowering the cyclization initiation temperature at the molecular level and constructing oxygen-loving channels. During the drying and densification stage, a gas-phase dynamic crosslinking treatment is introduced, causing the crosslinking agent to form a gradient distribution in the fiber radial direction, constructing a structure with moderate surface crosslinking and a retained active core. In the multi-segment gradient temperature field pre-oxidation process, dynamic tension and ultrasonic physical field-assisted treatment are combined to force a secondary uniform diffusion of heat and oxygen at high temperatures. The degree of cyclization of the pre-oxidized polyacrylonitrile fibers reaches over 90%, the core-sheath structure is essentially eliminated, and the tensile strength of the carbon fibers after subsequent carbonization is increased by more than 20% compared to traditional methods.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber manufacturing technology, and more specifically, to a method for preparing polyacrylonitrile pre-oxidized fibers, polyacrylonitrile pre-oxidized fibers, and carbon fibers. Background Technology

[0002] Polyacrylonitrile (PAN)-based carbon fibers are widely used in aerospace and high-end civilian applications due to their superior properties such as high specific strength and high specific modulus. Pre-oxidation is a key intermediate step in the preparation of PAN-based carbon fibers, aiming to transform the linear molecular chains of PAN into a heat-resistant, infusible trapezoidal structure. However, the pre-oxidation process faces two well-known technical challenges: First, the heat release is concentrated. The cyclization reaction of PAN is a strong exothermic reaction, and the heat is released rapidly in a short period of time, which can easily lead to local overheating, melting, or even fiber breakage, seriously restricting production efficiency and product stability.

[0003] Secondly, there is the "skin-core" structure. Since the pre-oxidation reaction begins in the skin layer, and oxygen diffuses from the outside in, the skin layer first forms a dense oxide layer, hindering further diffusion of oxygen to the fiber core, resulting in insufficient pre-oxidation in the core. This radial structural inhomogeneity is inherited by the final carbon fiber, forming structural defects and becoming a bottleneck restricting the improvement of carbon fiber tensile strength.

[0004] To alleviate these problems, various improvement schemes have emerged in existing technologies. For example, monomers such as acrylic acid and itaconic acid are introduced through copolymerization to lower the cyclization initiation temperature, but single copolymerization modification has limited improvement on radial oxygen diffusion capacity. Other technologies promote mass transfer by adding nanoparticles to the spinning solution or impregnating the fiber, but impregnating agents often preferentially adhere to the fiber surface, potentially exacerbating the formation of the core-sheath structure. Furthermore, traditional pre-oxidation processes often employ static temperature control, making precise regulation of the reaction process difficult, especially at high temperatures. Once the sheath structure is fixed, core defects cannot be remedied.

[0005] Therefore, there is an urgent need to optimize the preparation process of polyacrylonitrile pre-oxidized fibers in order to obtain pre-oxidized fibers with high cyclization degree and uniform radial structure.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing polyacrylonitrile pre-oxidized fibers, polyacrylonitrile pre-oxidized fibers and carbon fibers, with the aim of obtaining pre-oxidized fibers with high cyclization degree and uniform radial structure.

[0008] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing polyacrylonitrile pre-oxidized fibers, comprising: Gradient copolymerization: Acrylonitrile, a first comonomer, and a second comonomer are subjected to a gradient temperature polymerization reaction in a solvent to obtain a gradient copolymerized modified polyacrylonitrile spinning solution; wherein, the first comonomer is a vinyl phenol compound, and the second comonomer is an acrylate compound containing active hydrogen. Spinning and dynamic crosslinking drying densification: The gradient copolymerized polyacrylonitrile spinning solution is spun into polyacrylonitrile precursor fibers, and then the polyacrylonitrile precursor fibers are dried and densified in a crosslinking agent atmosphere to obtain densified precursor fibers. Multi-field coupling pre-oxidation treatment: The densified precursor fibers are pre-oxidized in a multi-segment gradient temperature field, and ultrasonic physical field treatment is applied in the final homogenization stage of pre-oxidation.

[0009] In an optional embodiment, the first comonomer is selected from at least one of 4-vinylphenol and 2-methoxy-4-vinylphenol; And / or, the amount of the first comonomer is 0.5%-3.0% of the total mass of the monomers.

[0010] In an optional embodiment, the second comonomer is selected from at least one of acrylic acid and hydroxyethyl methacrylate; And / or, the amount of the second comonomer is 1.0%-5.0% of the total mass of the monomers.

[0011] In an optional embodiment, the gradient heating polymerization process includes: first reacting at 60℃-65℃ for 2h-4h, and then heating to 75℃-80℃ to continue the reaction for 1h-2h. And / or, before the gradient temperature polymerization reaction, an initiator is added, and then the polymerization reaction is carried out under an inert atmosphere; the initiator is selected from at least one of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; the amount of initiator added accounts for 0.3%-0.8% of the total mass of the monomer; And / or, the solvent is selected from at least one of dimethyl sulfoxide and dimethylformamide; And / or, by adjusting the amount of solvent, the solid content of the initial reaction system is made to be 15%-25%.

[0012] In an optional embodiment, the crosslinking agent atmosphere is selected from at least one of boric acid saturated vapor and ammonium phosphate saturated vapor; And / or, after oiling the polyacrylonitrile precursor fiber, immediately perform a drying and densification treatment at a temperature of 100℃-150℃ for a time of 0.5min-2.0min. And / or, polyacrylonitrile precursor fibers are prepared using wet spinning or dry-jet wet spinning processes.

[0013] In an optional implementation, the multi-segment gradient temperature field includes at least four temperature zones, and the temperature gradually increases from the first temperature zone to the fourth temperature zone. The temperature of the first temperature zone is 180℃-200℃, the residence time is 20min-30min, and hot air circulation is used. The temperature in the second temperature zone is 210℃-230℃, the residence time is 20min-30min, and a mixed atmosphere of hot air and superheated steam is used, with a volume ratio of hot air to superheated steam of 1:(0.5-1.5). The temperature in the third temperature zone is 235℃-250℃, the residence time is 15min-25min, and hot air circulation is used. The temperature in the fourth temperature zone is 245℃-255℃, the dwell time is 10min-20min, and ultrasonic vibration treatment with a frequency of 20kHz-40kHz is applied in the fourth temperature zone.

[0014] In an optional implementation, a dynamic tension field and a multi-segment gradient temperature field are used to perform pre-oxidation treatment in combination, so that the fiber runs continuously in the pre-oxidation furnace and a gradient stretching from weak to strong is applied to control the total stretching ratio to be 0.95-1.05. Preferably, in the first temperature zone, the tension is controlled at 0.3cN-0.8cN, and the stretching is -1%-0%; in the second temperature zone, the tension is controlled at 0.6cN-1.0cN, and the stretching is 0%-1%; in the third temperature zone, the tension is controlled at 0.8cN-1.2cN, and the stretching is 0.5%-1.5%; and in the fourth temperature zone, the tension is controlled at 1.0cN-1.5cN, and the stretching is 0%-1%.

[0015] In an optional embodiment, the pre-oxidized fibers are cooled by cooling rollers and then wound up by a winding machine.

[0016] Secondly, the present invention provides a polyacrylonitrile pre-oxidized fiber, which is prepared by any of the preparation methods described in the foregoing embodiments; Preferably, the degree of cyclization of the polyacrylonitrile pre-oxidized fiber is greater than or equal to 90%.

[0017] Thirdly, the present invention provides a carbon fiber obtained by carbonization treatment of polyacrylonitrile pre-oxidized filaments as described in the foregoing embodiments.

[0018] This invention offers the following advantages: It utilizes a gradient-temperature polymerization process involving vinyl phenolic compounds and acrylate compounds containing active hydrogen with acrylonitrile. This gradient copolymerization introduces vinyl phenols and oxygen-containing polar monomers into the polyacrylonitrile molecular chain, lowering the cyclization initiation temperature at the molecular level and constructing oxygen-loving channels. During the drying and densification stage, a gas-phase dynamic crosslinking treatment is introduced, causing the crosslinking agent to form a gradient distribution in the fiber's radial direction, creating a structure with moderate surface crosslinking and maintained core activity. In the multi-stage gradient temperature field pre-oxidation process, dynamic tension and ultrasonic physical field-assisted treatment are combined to force a secondary uniform diffusion of heat and oxygen at high temperatures. The synergistic effect of these three stages results in a pre-oxidized fiber with a high degree of cyclization and highly uniform radial chemical structure.

[0019] Infrared testing revealed that the polyacrylonitrile pre-oxidized fibers provided by this invention exhibit a cyclization degree (Ar) exceeding 90%, essentially eliminating the core-sheath structure. After subsequent carbonization, the tensile strength of the carbon fibers is increased by more than 20% compared to traditional methods. This invention offers strong process controllability, produces products with excellent homogeneity, and is suitable for the industrial production of high-performance carbon fibers. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 A process flow diagram of the preparation method of polyacrylonitrile pre-oxidized fiber provided in the embodiments of this application; Figure 2 Micrograph of the polyacrylonitrile pre-oxidized fiber prepared in Example 1 of this application; Figure 3 A micrograph of the polyacrylonitrile pre-oxidized fiber prepared in Comparative Example 1 of this application. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0023] It should be noted that in this application, "and / or," such as "feature 1 and / or feature 2," refers to three possibilities: feature 1 alone, feature 2 alone, or feature 1 plus feature 2. Furthermore, in the description of this application, unless otherwise stated, "multiple" in "one or more" means two or more.

[0024] This invention addresses the technical challenges of existing pre-oxidized fibers, such as the tendency to develop a core-shell structure and concentrated exothermic cyclization reactions. It proposes a synergistic preparation method integrating chemical modification, gradient crosslinking, and physical homogenization.

[0025] This invention provides a method for preparing polyacrylonitrile pre-oxidized fibers, such as... Figure 1 As shown, by constructing a three-pronged technical architecture of "chemical modification to regulate reactivity, physical crosslinking to build a gradient structure, and external field-assisted forced mass transfer," the formation of the core-shell structure is suppressed at its source, resulting in pre-oxidized fibers with high cyclization and uniform radial structure. The steps are as follows: S1. Preparation of gradient copolymerized modified polyacrylonitrile spinning solution Acrylonitrile (AN), a first comonomer, and a second comonomer were subjected to a gradient-temperature polymerization reaction in a solvent to obtain a gradient copolymerized modified polyacrylonitrile spinning solution. The first comonomer is a vinylphenol compound used to lower the cyclization initiation temperature; the second comonomer is an acrylate compound containing active hydrogen to improve the oxygen affinity of the fiber. By adding the first and second comonomers to acrylonitrile, a "binary gradient copolymerization" method is used to provide a "molecular channel" for oxygen diffusion at the molecular level.

[0026] In some embodiments, the first comonomer is selected from at least one of 4-vinylphenol and 2-methoxy-4-vinylphenol, and the first comonomer can be any one or more of the above. The phenolic hydroxyl group of the first comonomer has an active hydrogen, which can initiate a cyclization reaction of the molecular chain at a lower temperature, dispersing the concentrated exothermic peak into a gentle, broad peak. The amount of the first comonomer accounts for 0.5%-3.0% of the total mass of the monomers, such as 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, etc. Specifically, the total mass of the monomers refers to the total mass of acrylonitrile, the first comonomer, and the second comonomer.

[0027] In some embodiments, the second comonomer is selected from at least one of acrylic acid and hydroxyethyl methacrylate, and the second comonomer can be any one or more of the above. The carboxyl or hydroxyl groups of the second comonomer can improve the hydrophilicity of the fiber. More importantly, its polar side groups construct "oxygen-friendly channels" inside the fiber, providing a molecular-level pathway for oxygen to diffuse into the core. The amount of the second comonomer accounts for 1.0%-5.0% of the total mass of the monomers, such as 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, etc.

[0028] The solvent can be a commonly used polar solvent, and the specific type is not limited. In some embodiments, the solvent is selected from at least one of dimethyl sulfoxide and dimethylformamide, and the solvent can be any one or more of the above. By adjusting the amount of solvent, the solid content of the initial reaction system is made to be 15%-25%, such as 15%, 18%, 20%, 23%, 25%, etc. Specifically, the solid content refers to the mass fraction of the total monomer in the system.

[0029] To facilitate the polymerization reaction, an initiator is added before the gradient heating process, followed by polymerization under an inert atmosphere (such as nitrogen or argon). The initiator is selected from at least one of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; any one or more of these initiators can be used. The amount of initiator added is 0.3%-0.8% of the total monomer mass, such as 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0030] In some embodiments, the gradient temperature polymerization process includes: first, reacting at 60℃-65℃ for 2h-4h for low-temperature initiation to allow the comonomers to be uniformly embedded in the molecular chains; then, heating to 75℃-80℃ for another 1h-2h to improve the polymerization conversion rate. After the reaction, the comonomers are removed and defoamed to obtain the modified PAN spinning solution. Specifically, the reaction temperature in the first stage can be 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, etc.; the reaction time can be 2h, 3h, 4h, etc.; the reaction temperature in the second stage can be 75℃, 76℃, 77℃, 78℃, 79℃, 80℃, etc.; the reaction time can be 1.0h, 1.3h, 1.5h, 1.8h, 2.0h, etc.

[0031] S2, spinning and dynamic crosslinking drying densification Polyacrylonitrile (PA) precursor fibers were obtained by spinning gradient copolymerized polyacrylonitrile (PAC) spinning solution. These PAC precursor fibers were then dried and densified in a crosslinking agent atmosphere, causing the crosslinking agent to form a gradient distribution in the fiber radial direction, resulting in a densified precursor fiber. A gradient crosslinking structure was constructed in the fiber radial direction through "dynamic gas-phase crosslinking," avoiding excessively dense skin layers.

[0032] The specific spinning process is not limited; for example, PAN precursor yarn can be produced by wet spinning or dry-jet wet spinning processes.

[0033] In some embodiments, the crosslinking agent atmosphere is selected from at least one of boric acid saturated vapor and ammonium phosphate saturated vapor, and the crosslinking agent atmosphere can be any one or a combination of the above. The boric acid saturated vapor is prepared using conventional methods, such as the following: boric acid is added to deionized water, heated to 70-90°C and stirred to dissolve, preparing a boric acid saturated solution, filtered, and then pumped into a closed steam generator; the steam generator is heated to 110-130°C, causing the boric acid to volatilize along with the water vapor, forming boric acid saturated vapor.

[0034] In some embodiments, after oiling, the polyacrylonitrile precursor fibers are immediately subjected to a drying and densification treatment. This treatment is carried out in a dynamic crosslinking atmosphere: the polyacrylonitrile precursor fibers are passed through a saturated vapor atmosphere containing boric acid or ammonium phosphate at a temperature range of 100°C-150°C for 0.5 min-2.0 min. During this process, the crosslinking agent vapor molecules form a natural concentration gradient from the surface to the core of the fiber in the radial direction, inducing slight crosslinking of the fiber surface molecular chains, forming a gradient structure of "moderate surface crosslinking and core remaining active." Specifically, the drying and densification treatment temperature can be 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, etc.; the drying and densification treatment time can be 0.5 min, 1.0 min, 1.5 min, 2.0 min, etc.

[0035] S3, Multi-field Coupling Pre-oxidation Treatment The densified precursor fibers undergo pre-oxidation treatment under the synergistic effect of a dynamic tension field and a multi-segment gradient temperature field. Ultrasonic physical fields are introduced in the final homogenization stage to physically break down the cortical barrier at high temperatures through "ultrasonic-assisted homogenization".

[0036] In some embodiments, the multi-segment gradient temperature field comprises at least four temperature zones, with the temperature gradually increasing from the first temperature zone to the fourth temperature zone. The operating parameters for the first temperature zone to the fourth temperature zone are as follows: The temperature of the first temperature zone (low-temperature initiation zone) is 180℃-200℃, such as 180℃, 185℃, 190℃, 195℃, 200℃, etc.; the residence time is 20min-30min, such as 20min, 23min, 25min, 28min, 30min, etc.; the first temperature zone uses hot air circulation, which is mainly for the initiation of cyclization reaction.

[0037] The temperature in the second temperature zone (intermediate-temperature circulatory zone) is 210℃-230℃, such as 210℃, 215℃, 220℃, 225℃, 230℃, etc.; the residence time is 20min-30min, such as 20min, 23min, 25min, 28min, 30min, etc.; the second temperature zone uses a mixed atmosphere of hot air and superheated steam, and the volume ratio of hot air to superheated steam is 1:(0.5-1.5), such as 1:0.5, 1:0.8, 1:1.0, 1:1.3, 1:1.5, etc. The introduction of superheated steam can significantly improve heat transfer efficiency and participate in the regulation of the surface chemical microenvironment, inhibiting the excessive densification of the skin layer.

[0038] The temperature of the third temperature zone (high-temperature oxidation zone) is 235℃-250℃, such as 235℃, 240℃, 245℃, 250℃, etc.; the residence time is 15min-25min, such as 15min, 18min, 20min, 23min, 25min, etc.; the third temperature zone uses hot air circulation to further oxidize and stabilize the fiber in the third temperature zone.

[0039] The fourth temperature zone (homogenization treatment zone) has a temperature of 245℃-255℃, which can be 245℃, 250℃, 255℃, etc.; the residence time is 10min-20min, which can be 10min, 13min, 15min, 18min, 20min, etc.; the fourth temperature zone is supplemented with ultrasonic vibration treatment at a frequency of 20kHz-40kHz (such as 20kHz, 25kHz, 30kHz, 35kHz, 40kHz, etc.). The high-frequency vibration energy of the ultrasound can act on the inside of the fiber, causing the already formed rigid skin to undergo microscopic relaxation, temporarily opening or widening the channels for oxygen and heat to be transferred to the core, thereby achieving secondary oxidation and homogenization of the core.

[0040] In some embodiments, the dynamic tension field is a gradient dynamic stretching applied to the fiber. The pre-oxidation treatment is carried out in conjunction with the dynamic tension field and a multi-segment gradient temperature field, so that the fiber runs continuously in the pre-oxidation furnace and a gradient stretching from weak to strong is applied to control the total stretching ratio to be 0.95-1.05, such as 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, etc., to control the orientation and shrinkage of fiber molecules.

[0041] Furthermore, in the first temperature zone, the tension is controlled at 0.3cN-0.8cN, and the stretch is -1%-0%; in the second temperature zone, the tension is controlled at 0.6cN-1.0cN, and the stretch is 0%-1%; in the third temperature zone, the tension is controlled at 0.8cN-1.2cN, and the stretch is 0.5%-1.5%; and in the fourth temperature zone, the tension is controlled at 1.0cN-1.5cN, and the stretch is 0%-1%. By adjusting the tension and stretch in each temperature zone, the orientation and shrinkage of fiber molecules can be better controlled.

[0042] S4, Cooling and winding The pre-oxidized fibers are cooled by cooling rollers and then wound up by a winding machine to obtain high-homogeneity polyacrylonitrile pre-oxidized fibers.

[0043] In summary, the method for preparing polyacrylonitrile pre-oxidized fibers provided by this invention has the following advantages: (1) Fundamentally suppressing the core-sheath structure: "Binary gradient copolymerization" provides "molecular channels" for oxygen diffusion at the molecular level; "Dynamic gas-phase crosslinking" constructs a gradient crosslinking structure in the fiber radial direction, avoiding excessively dense sheath; "Ultrasonic-assisted homogenization" physically breaks down the sheath barrier at high temperature. The synergistic effect of these three factors results in a highly uniform radial chemical structure of the pre-oxidized fiber. Infrared testing shows that the degree of cyclization (Ar) is as high as 90% or more, far superior to traditional processes.

[0044] (2) Significantly improves process safety and controllability: Vinylphenol comonomers reduce the cyclization initiation temperature to below 180℃ and broaden the exothermic peak, avoiding the "explosive polymerization" and fiber breakage phenomena caused by concentrated exothermic reactions in traditional processes. Combined with the gradient crosslinking structure, the thermal stability of the fiber in the high-temperature region is greatly improved, and the fuzz and fiber breakage rate are reduced by more than 80% during the production process.

[0045] (3) Achieving a leap in the performance of pre-oxidized fibers and subsequent carbon fibers: The obtained pre-oxidized fibers have a breaking strength of over 4.8 cN / dtex and a uniform radial structure. After carbonization under the same conditions, the tensile strength of the resulting carbon fibers is more than 20% higher than that of carbon fibers obtained by carbonization of conventional pre-oxidized fibers, providing a high-quality intermediate for the preparation of high-performance carbon fibers.

[0046] This invention also provides a polyacrylonitrile pre-oxidized fiber, which is prepared by the preparation method provided in this invention. The degree of cyclization of the prepared polyacrylonitrile pre-oxidized fiber is greater than or equal to 90%, and the difference between the core and sheath structure is indistinguishable to the naked eye.

[0047] This invention also provides a carbon fiber, which is obtained by carbonization treatment of the above-mentioned polyacrylonitrile pre-oxidized fiber. The specific steps of the carbonization treatment are not limited, and conventional carbonization processes for polyacrylonitrile pre-oxidized fiber are all within the protection scope of this invention.

[0048] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0049] Example 1 This application provides a method for preparing high-homogeneity polyacrylonitrile pre-oxidized fibers, comprising the following steps: (1) Preparation of modified spinning solution: Acrylonitrile (AN), 1.5% of 4-vinylphenol (by mass of total monomers), and 2.5% of hydroxyethyl methacrylate (by mass of total monomers) were dissolved in dimethyl sulfoxide, with a solid content of 20%. Azobisisobutyronitrile (AIB) was added at 0.5% of total monomers, and the mixture was reacted at 65°C for 3 hours under nitrogen protection, followed by a further increase to 78°C for 1.5 hours. After the reaction was completed, the modified PAN spinning solution was obtained by removing monomers and bubbles.

[0050] (2) Spinning and dynamic crosslinking drying: The dry-jet wet spinning method is adopted. The spinning solution is extruded through the spinneret and enters the coagulation bath through the air layer to form nascent fibers. After washing and oiling, the fibers are sent to the dynamic crosslinking treatment zone. Boric acid saturated steam is introduced into the treatment zone, the temperature is controlled at 120℃, and the treatment time is 1 minute to dry and densify.

[0051] (3) Multi-field coupled pre-oxidation: The dried fibers are fed into a five-temperature zone pre-oxidation furnace. The process parameters for each temperature zone are as follows: Zone 1: 190℃ / 25min (hot air circulation); Zone 2: 220℃ / 25min (hot air and superheated steam with a volume ratio of 1:1 are introduced); Zone 3: 240℃ / 20min (hot air circulation); Zone 4: 250℃ / 15min (hot air circulation), and the ultrasonic generator is turned on at a frequency of 28kHz; Zone 5 is the cooling zone. The total draw ratio of the fibers throughout the entire process is 1.02. In Zone 1, the tension is controlled at 0.5cN and the draw is -0.5%; in Zone 2, the tension is controlled at 0.8cN and the draw is 0.5%; in Zone 3, the tension is controlled at 1.0cN and the draw is 1.0%; in Zone 4, the tension is controlled at 1.3cN and the draw is 0.5%.

[0052] (4) Winding: The pre-oxidized fiber is cooled by a cooling roller and then wound up by a winding machine to obtain pre-oxidized fiber.

[0053] Example 2 This application provides a method for preparing high-homogeneity polyacrylonitrile pre-oxidized fibers, comprising the following steps: (1) Preparation of modified spinning solution: Acrylonitrile (AN), 0.5% of 4-vinylphenol (by mass of total monomers), and 1.0% of hydroxyethyl methacrylate were dissolved in dimethyl sulfoxide, with a solid content of 15%. Azobisisobutyronitrile (AIB) was added at 0.3% of total monomers, and the mixture was reacted at 60°C for 4 hours under nitrogen protection, followed by a reaction at 75°C for 2 hours. After the reaction was completed, the modified PAN spinning solution was obtained by removing monomers and bubbles.

[0054] (2) Spinning and dynamic crosslinking drying: The dry-jet wet spinning method is adopted. The spinning solution is extruded through the spinneret and enters the coagulation bath through the air layer to form nascent fibers. After washing and oiling, the fibers are sent to the dynamic crosslinking treatment zone. Boric acid saturated steam is introduced into the treatment zone, the temperature is controlled at 100℃, and the treatment time is 2 minutes to dry and densify.

[0055] (3) Multi-field coupled pre-oxidation: The dried fibers are fed into a five-temperature zone pre-oxidation furnace. The process parameters for each temperature zone are as follows: Zone 1: 180℃ / 30min (hot air circulation); Zone 2: 210℃ / 30min (hot air and superheated steam with a volume ratio of 1:1 are introduced); Zone 3: 235℃ / 25min (hot air circulation); Zone 4: 245℃ / 20min (hot air circulation), and the ultrasonic generator is turned on at a frequency of 20kHz; Zone 5 is the cooling zone. The total draw ratio of the fibers throughout the entire process is 0.95. In Zone 1, the tension is controlled at 0.3cN and the draw is -1.0%; in Zone 2, the tension is controlled at 0.6cN and the draw is 0%; in Zone 3, the tension is controlled at 0.8cN and the draw is 0.5%; in Zone 4, the tension is controlled at 1.0cN and the draw is 0%.

[0056] (4) Winding: The pre-oxidized fiber is cooled by a cooling roller and then wound up by a winding machine to obtain pre-oxidized fiber.

[0057] Example 3 This application provides a method for preparing high-homogeneity polyacrylonitrile pre-oxidized fibers, comprising the following steps: (1) Preparation of modified spinning solution: Acrylonitrile (AN), 4-vinylphenol (3.0% by mass of total monomers), and hydroxyethyl methacrylate (5.0% by mass) were dissolved in dimethyl sulfoxide, with a solid content of 25%. Azobisisobutyronitrile (0.8% by mass of total monomers) was added, and the mixture was reacted at 65°C for 2 hours under nitrogen protection, followed by a reaction at 80°C for 1 hour. After the reaction was completed, the modified PAN spinning solution was obtained by removing monomers and bubbles.

[0058] (2) Spinning and dynamic crosslinking drying: The dry-jet wet spinning method is adopted. The spinning solution is extruded through the spinneret and enters the coagulation bath through the air layer to form nascent fibers. After washing and oiling, the fibers are sent to the dynamic crosslinking treatment zone. Boric acid saturated steam is introduced into the treatment zone, the temperature is controlled at 150℃, and the treatment time is 0.5 minutes to dry and densify.

[0059] (3) Multi-field coupled pre-oxidation: The dried fibers are fed into a five-temperature zone pre-oxidation furnace. The process parameters for each temperature zone are as follows: Zone 1: 200℃ / 20min (hot air circulation); Zone 2: 230℃ / 20min (hot air and superheated steam with a volume ratio of 1:1 are introduced); Zone 3: 250℃ / 15min (hot air circulation); Zone 4: 255℃ / 10min (hot air circulation), and the ultrasonic generator is turned on at a frequency of 40kHz; Zone 5 is the cooling zone. The total draw ratio of the fibers throughout the entire process is 1.05. In Zone 1, the tension is controlled at 0.8cN and the draw is 0%; in Zone 2, the tension is controlled at 1.0cN and the draw is 1.0%; in Zone 3, the tension is controlled at 1.2cN and the draw is 1.5%; in Zone 4, the tension is controlled at 1.5cN and the draw is 1.0%.

[0060] (4) Winding: The pre-oxidized fiber is cooled by a cooling roller and then wound up by a winding machine to obtain pre-oxidized fiber.

[0061] Example 4 The only difference from Example 1 is that 4-vinylphenol is replaced with an equal amount of 2-methoxy-4-vinylphenol, and hydroxyethyl methacrylate is replaced with an equal amount of acrylic acid.

[0062] Example 5 The only difference from Example 1 is that the amount of 4-vinylphenol added is 0.5% of the total mass of the monomer.

[0063] Example 6 The only difference from Example 1 is that the amount of 4-vinylphenol added accounts for 3.0% of the total mass of the monomer.

[0064] Example 7 The only difference from Example 1 is that the amount of 4-vinylphenol added accounts for 0.3% of the total mass of the monomer.

[0065] Example 8 The only difference from Example 1 is that the amount of 4-vinylphenol added accounts for 5.0% of the total mass of the monomer.

[0066] Example 9 The only difference from Example 1 is that the amount of hydroxyethyl methacrylate added accounts for 1.0% of the total mass of the monomers.

[0067] Example 10 The only difference from Example 1 is that the amount of hydroxyethyl methacrylate added accounts for 5.0% of the total monomer mass.

[0068] Example 11 The only difference from Example 1 is that the amount of hydroxyethyl methacrylate added accounts for 0.5% of the total mass of the monomers.

[0069] Example 12 The only difference from Example 1 is that the amount of hydroxyethyl methacrylate added accounts for 8.0% of the total monomer mass.

[0070] Comparative Example 1 Conventional PAN precursor fibers (comonomer is only acrylic acid, content 1.5%) were used. No dynamic crosslinking pretreatment was performed. The process was carried out in a conventional air atmosphere pre-oxidation furnace with the following temperature zones: 200℃ / 30min, 220℃ / 30min, 240℃ / 20min. The total draw ratio was 1.02.

[0071] Comparative Example 2 The only difference from Example 1 is that 4-vinylphenol is not added, but is replaced with an equal mass of hydroxyethyl methacrylate.

[0072] Comparative Example 3 The only difference from Example 1 is that hydroxyethyl methacrylate is not added, and it is replaced with an equal mass of 4-vinylphenol.

[0073] Comparative Example 4 The only difference from Example 1 is that step (1) does not involve gradient heating, but reacts at 65°C for 4.5 hours.

[0074] Comparative Example 5 The only difference from Example 1 is that step (1) does not involve gradient heating, but reacts at 78°C for 4.5 hours.

[0075] Comparative Example 6 The only difference from Example 1 is that in step (2), the drying and densification process is carried out in an air atmosphere.

[0076] Comparative Example 7 The only difference from Example 1 is that ultrasonic physical field processing is not introduced in step (3).

[0077] Test case (1) Cyclocyclization degree test of polyacrylonitrile pre-oxidized fiber Test Method: The polyacrylonitrile pre-oxidized fibers prepared in the examples and comparative examples were used as samples. The degree of cyclization reaction of each sample was then tested, and the test results are summarized in Table 1. The specific test steps were as follows: After cutting the dried polyacrylonitrile pre-oxidized fibers into small pieces, KBr was mixed with the polyacrylonitrile pre-oxidized fibers at a mass ratio of 200:5 and ground to prepare KBr pellets for infrared analysis; where IC≡N represents the characteristic absorption peak intensity of the cyano group, corresponding to a wavenumber of 2240 cm⁻¹. -1 IC=N represents the characteristic absorption peak intensity of C=N, corresponding to a wavenumber of 1580 cm⁻¹. -1 .

[0078] Table 1. Performance test results of polyacrylonitrile pre-oxidized fibers prepared in the examples and comparative examples.

[0079] Referring to Table 1, the test results of the examples and comparative examples show that the pre-oxidized polyacrylonitrile fibers prepared according to the preparation process provided in the examples of this application have a higher degree of cyclization and a higher breaking strength, indicating a higher degree of pre-oxidation.

[0080] (2) Core-sheath structure test of polyacrylonitrile pre-oxidized fiber Test method: The polyacrylonitrile pre-oxidized fibers prepared in the examples and comparative examples were used as samples. Each PAN pre-oxidized fiber sample was then embedded in a mixture of epoxy resin and curing agent at a ratio of 15:2 (mass ratio). After curing at 60°C for 2 h, the samples were sliced ​​using an ultramicrotome with a thickness of approximately 400 nm. The slices were then observed under a high-power optical microscope with a 20X objective lens, and cross-sectional micrographs of the pre-oxidized fibers were taken.

[0081] See Figure 2 , Figure 3 It is evident that the polyacrylonitrile pre-oxidized fiber prepared according to the process provided in the embodiments of this application has significantly fewer core-sheath delamination defects.

[0082] Carbon fiber properties: The pre-oxidized fibers of the examples and comparative examples were carbonized under the same conditions (nitrogen atmosphere, gradient temperature increase from 400 to 1350℃). The tensile strength test results of the obtained carbon fibers were as follows: the tensile strength of the carbon fiber of Example 1 was 7.0 GPa, and the tensile strength of the carbon fiber of Comparative Example 1 was 5.5 GPa. The strength of the carbon fiber prepared by the method of the present invention was improved by 27.3%.

[0083] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing polyacrylonitrile pre-oxidized fibers, characterized in that, include: Gradient copolymerization: Acrylonitrile, a first comonomer, and a second comonomer are subjected to a gradient temperature polymerization reaction in a solvent to obtain a gradient copolymerized modified polyacrylonitrile spinning solution; wherein, the first comonomer is a vinyl phenol compound, and the second comonomer is an acrylate compound containing active hydrogen. Spinning and dynamic crosslinking drying densification: The gradient copolymerized polyacrylonitrile spinning solution is spun to obtain polyacrylonitrile precursor fibers, and then the polyacrylonitrile precursor fibers are dried and densified in a crosslinking agent atmosphere to obtain densified precursor fibers. Multi-field coupling pre-oxidation treatment: The densified precursor fiber is pre-oxidized in a multi-segment gradient temperature field, and ultrasonic physical field treatment is applied in the final homogenization stage of pre-oxidation.

2. The preparation method according to claim 1, characterized in that, The first comonomer is selected from at least one of 4-vinylphenol and 2-methoxy-4-vinylphenol; And / or, the amount of the first comonomer accounts for 0.5%-3.0% of the total mass of the monomers.

3. The preparation method according to claim 1, characterized in that, The second comonomer is selected from at least one of acrylic acid and hydroxyethyl methacrylate; And / or, the amount of the second comonomer is 1.0%-5.0% of the total mass of the monomers.

4. The preparation method according to any one of claims 1-3, characterized in that, The gradient heating polymerization process includes: first reacting at 60℃-65℃ for 2h-4h, and then heating to 75℃-80℃ to continue the reaction for 1h-2h; And / or, before the gradient temperature polymerization reaction, an initiator is added, followed by polymerization under an inert atmosphere; the initiator is selected from at least one of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; the amount of initiator added is 0.3%-0.8% of the total mass of the monomers; And / or, the solvent is selected from at least one of dimethyl sulfoxide and dimethylformamide; And / or, by adjusting the amount of the solvent, the solid content of the initial reaction system is made to be 15%-25%.

5. The preparation method according to claim 1, characterized in that, The crosslinking agent atmosphere is selected from at least one of boric acid saturated vapor and ammonium phosphate saturated vapor; And / or, after the polyacrylonitrile precursor fiber is oiled, it is immediately subjected to drying and densification treatment at a temperature of 100℃-150℃ for a time of 0.5min-2.0min. And / or, polyacrylonitrile precursor fibers are prepared using wet spinning or dry-jet wet spinning processes.

6. The preparation method according to claim 1, characterized in that, The multi-segment gradient temperature field contains at least four temperature zones, and the temperature gradually increases from the first temperature zone to the fourth temperature zone. The temperature of the first temperature zone is 180℃-200℃, the residence time is 20min-30min, and hot air circulation is used. The temperature in the second temperature zone is 210℃-230℃, the residence time is 20min-30min, and a mixed atmosphere of hot air and superheated steam is used, with a volume ratio of hot air to superheated steam of 1:(0.5-1.5). The temperature in the third temperature zone is 235℃-250℃, the residence time is 15min-25min, and hot air circulation is used. The temperature of the fourth temperature zone is 245℃-255℃, the residence time is 10min-20min, and the fourth temperature zone is supplemented with ultrasonic vibration treatment with a frequency of 20kHz-40kHz.

7. The preparation method according to claim 6, characterized in that, Pre-oxidation treatment is carried out in conjunction with dynamic tension field and multi-segment gradient temperature field, so that the fiber runs continuously in the pre-oxidation furnace and a gradient stretching from weak to strong is applied to control the total stretching ratio to be 0.95-1.

05. Preferably, in the first temperature zone, the tension is controlled at 0.3cN-0.8cN, and the stretching is -1%-0%; in the second temperature zone, the tension is controlled at 0.6cN-1.0cN, and the stretching is 0%-1%; in the third temperature zone, the tension is controlled at 0.8cN-1.2cN, and the stretching is 0.5%-1.5%; and in the fourth temperature zone, the tension is controlled at 1.0cN-1.5cN, and the stretching is 0%-1%.

8. The preparation method according to claim 1, characterized in that, The pre-oxidized fibers are cooled by cooling rollers and then wound up by a winding machine.

9. A polyacrylonitrile pre-oxidized fiber, characterized in that, Prepared by the preparation method according to any one of claims 1-8; Preferably, the degree of cyclization of the polyacrylonitrile pre-oxidized fiber is greater than or equal to 90%.

10. A carbon fiber, characterized in that, It is prepared by carbonization treatment of the polyacrylonitrile pre-oxidized fiber as described in claim 9.

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