High-densification polyacrylonitrile pre-oxidized fiber as well as preparation method and application thereof

Abstract

The invention discloses a high-densification polyacrylonitrile pre-oxidized fiber, a preparation method and application, the preparation method of the high-densification polyacrylonitrile pre-oxidized fiber comprises the steps of catalyst loading, first-stage pre-oxidation, plasma-assisted pre-oxidation and second-stage pre-oxidation which are carried out in sequence, and the high-densification polyacrylonitrile pre-oxidized fiber is obtained. According to the preparation method of the high-densification polyacrylonitrile pre-oxidized fiber, plasma is introduced to assist pre-oxidation, high-activity particles generated by the plasma can synchronously act on the surface of the fiber and an internal matrix, a two-way synergistic oxidation mechanism from inside to outside and from outside to inside is formed, the skin-core structure difference is greatly reduced, and the high-densification polyacrylonitrile pre-oxidized fiber is prepared. The pre-oxidized fiber is endowed with excellent structure uniformity; the uniform oxidation reaction can disperse internal stress concentration and inhibit formation of microcracks and pores, and the surface smoothness of the fiber is improved by matching with a plasma micron-order etching effect, so that the product is low in defect density and excellent in comprehensive performance.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber manufacturing technology, and more specifically, to high-density polyacrylonitrile pre-oxidized fibers, their preparation methods, and applications. Background Technology

[0002] Polyacrylonitrile-based carbon fiber, with its superior specific strength, specific modulus, fatigue resistance, and corrosion resistance, has become a key material in high-end manufacturing fields such as aerospace, sporting goods, and wind power generation. As a precursor to carbon fiber, the quality of pre-oxidized fiber directly determines the performance indicators of the final product. Pre-oxidized fiber is prepared by heat-treating polyacrylonitrile (PAN) precursor fibers in air at 200-300℃, causing a series of reactions including cyclization, dehydrogenation, and oxidation, ultimately forming a fiber material with flame-retardant properties.

[0003] High-quality pre-oxidized fibers should meet the following key technical requirements: (1) It has a high degree of cyclization and oxidation: This ensures that it maintains structural stability during subsequent high-temperature carbonization and obtains a high carbonization yield; (2) Forming a homogeneous core-skin structure: It is necessary to minimize the structural differences between the skin and the core as much as possible, avoid significant core-skin delamination, and thus reduce the probability of internal defects in carbon fibers. (3) Good structural integrity: The fiber surface should be smooth and free of defects, such as grooves or cracks, and the internal structure should be dense and free of internal defects such as pores or bubbles.

[0004] While the multi-stage gradient heating pre-oxidation process commonly used in the industrial sector can meet the basic production requirements of pre-oxidized fibers, several technical challenges still need to be addressed: (1) Distinct core-sheath structure: Since the reaction begins on the fiber surface, heat and oxygen are transferred from the outside to the inside in a diffusion pattern, resulting in a high degree of cyclization and dense structure in the outer layer, while the reaction in the inner layer is insufficient, forming a significant core-sheath difference. This structural inhomogeneity is prone to internal cracks or pores due to shrinkage differences during carbonization, becoming a key factor restricting the improvement of carbon fiber performance; (2) Narrow process parameter window: The pre-oxidation process is accompanied by a violent exothermic reaction. If the temperature is not controlled carefully, it may lead to local overheating, causing fiber melting and breakage or the formation of a thermal core-sheath structure. In severe cases, it may even cause a combustion accident. (3) High energy and time costs: The pre-oxidation process is the most time-consuming and energy-intensive process in the entire carbon fiber production process, usually requiring 60-120 minutes of processing time.

[0005] In summary, developing a preparation process that can significantly improve the uniformity of the core-sheath structure, effectively shorten the reaction time, reduce energy consumption, and enhance the overall performance of pre-oxidized fibers is of great strategic significance for realizing the industrial production of high-performance carbon fibers. Summary of the Invention

[0006] The purpose of this invention is to provide highly dense polyacrylonitrile pre-oxidized fibers, their preparation method, and their applications, overcoming the problems of poor uniformity and low cyclization degree in the existing polyacrylonitrile carbon fiber sheath-core structure.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing highly dense polyacrylonitrile pre-oxidized fibers, comprising the following steps: Step S1: The polyacrylonitrile-based carbon fiber precursor is impregnated with a pretreatment solution and dried to obtain catalyst-supported precursor. The pretreatment solution includes a catalyst and a surfactant. Step S2: Perform a first-stage pre-oxidation on the supported catalyst precursor to obtain a first-stage pre-oxidized precursor. Step S3: Perform plasma-assisted pre-oxidation on the first-stage pre-oxidized precursor fiber to obtain plasma-assisted pre-oxidized precursor fiber; Step S4: Perform a second-stage pre-oxidation on the plasma-assisted pre-oxidized precursor fiber to obtain the high-density polyacrylonitrile pre-oxidized fiber.

[0008] In an optional implementation, step S1 satisfies at least one of the following features: a. The catalyst is selected from metal ion chelates; b. The concentration of the catalyst in the pretreatment solution is 0.1-1.0 wt%; c. The surfactant is selected from organic sulfonates; d. The concentration of the surfactant in the pretreatment solution is 0.05-0.3 wt%; e. The drying temperature is 50-80℃.

[0009] In an optional embodiment, the pre-oxidation temperature of the first stage is 180-220°C, and the atmosphere is an oxidizing atmosphere containing oxygen. And / or, the first stage of pre-oxidation time is 10-30 min; And / or, apply 5-15% stretching tension to the supported catalyst precursor fiber; And / or, the catalyst is selected from metal acetylacetone complexes.

[0010] In an optional embodiment, the plasma is a dielectric barrier discharge plasma; And / or, the power of the plasma processing device used is 100-500 W, and the processing frequency is 10-30 kHz; And / or, the plasma-assisted pre-oxidation time is 0.5-5 min; And / or, the catalyst is selected from at least one of iron acetylacetone and cobalt acetylacetone; And / or, during the first stage of pre-oxidation, an 8-12% stretching tension is applied to the supported catalyst precursor fiber; And / or, apply 0-3% relaxation tension to the plasma-assisted pre-oxidized precursor fiber.

[0011] In an optional embodiment, the pre-oxidation temperature of the second stage is 240-280°C, and the atmosphere is an oxidizing atmosphere containing oxygen. And / or, apply a relaxation tension of 0.5-2% to the plasma-assisted pre-oxidized precursor fiber.

[0012] Secondly, the present invention provides a highly dense polyacrylonitrile pre-oxidized fiber, which is prepared by the preparation method described in any one of the foregoing embodiments.

[0013] In an optional implementation, the degree of cyclization is ≥91%.

[0014] Thirdly, the present invention provides a method for preparing highly dense polyacrylonitrile carbon fiber, wherein the highly dense polyacrylonitrile pre-oxidized filament described in the foregoing embodiments is carbonized to obtain the highly dense polyacrylonitrile carbon fiber.

[0015] In an optional embodiment, the carbonization includes primary carbonization and secondary carbonization; The primary carbonization temperature is 300-900℃ and the time is 5-20 min; and / or the secondary carbonization temperature is 1000-1600℃ and the time is 1-5 min.

[0016] Fourthly, the present invention provides a highly dense polyacrylonitrile carbon fiber, which is prepared by the preparation method described in the foregoing embodiments, wherein the tensile strength of the highly dense polyacrylonitrile carbon fiber is ≥6.6 GPa.

[0017] The present invention has the following beneficial effects: The preparation method of high-density polyacrylonitrile pre-oxidized fiber in this application introduces plasma-assisted pre-oxidation. The highly active particles generated by plasma can act simultaneously on the fiber surface and internal matrix, forming a bidirectional synergistic oxidation mechanism "from the inside out" and "from the outside in", which significantly reduces the difference between the core and sheath structure and endows the pre-oxidized fiber with excellent structural uniformity. The uniform oxidation reaction can disperse the concentration of internal stress and inhibit the formation of microcracks and pores. Combined with the plasma micron-level etching effect, it can improve the surface smoothness of the fiber, resulting in low defect density and excellent overall performance. At the same time, plasma can significantly improve the kinetic rate of oxidation reaction, shortening the total pre-oxidation cycle by 30%-50% compared with traditional processes, achieving a win-win situation of efficiency improvement and energy consumption reduction. Moreover, through precise control of the three stages of "stress induction", "plasma activation" and "relaxation stabilization", the chemical composition and physical structure of the fiber can be accurately controlled, resulting in higher process controllability. Attached Figure Description

[0018] 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.

[0019] Figure 1 This is a process flow diagram of Embodiment 1 of this application; Figure 2 Micrograph of the polyacrylonitrile pre-oxidized fiber prepared in Example 1 of this application; Figure 3 Micrograph of the polyacrylonitrile pre-oxidized fiber prepared in Example 2 of this application; Figure 4 Micrograph of the polyacrylonitrile pre-oxidized fiber prepared in Example 3 of this application; Figure 5 A micrograph of the polyacrylonitrile pre-oxygenated fiber prepared in Comparative Example 1 of this application; Figure 6 Micrograph of the pre-oxygenated polyacrylonitrile fiber prepared in Comparative Example 2 of this application; Figure 7 A micrograph of the polyacrylonitrile pre-oxidized fiber prepared in Comparative Example 3 of this application. Detailed Implementation

[0020] 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.

[0021] It should be noted that the terms "and / or" in this application, such as "feature 1 and / or feature 2", all refer to the three cases of "feature 1" alone, "feature 2" alone, and "feature 1" plus "feature 2".

[0022] In addition, in the description of this application, unless otherwise stated, "at least one" includes "one or more", and "one or more" means two or more; the range of "numerical value a - numerical value b" includes the two endpoints "a" and "b", and the "unit of measurement" in "numerical value a - numerical value b + unit of measurement" represents the "unit of measurement" of both "numerical value a" and "numerical value b".

[0023] This invention provides a method for preparing highly dense polyacrylonitrile pre-oxidized fibers, comprising the following steps: Step S1: The polyacrylonitrile-based carbon fiber precursor is impregnated with a pretreatment solution and dried to obtain catalyst-supported precursor. The pretreatment solution includes a catalyst and a surfactant. Step S2: Perform a first-stage pre-oxidation on the supported catalyst precursor to obtain a first-stage pre-oxidized precursor. Step S3: Perform plasma-assisted pre-oxidation on the first-stage pre-oxidized precursor fiber to obtain plasma-assisted pre-oxidized precursor fiber; Step S4: Perform a second-stage pre-oxidation on the plasma-assisted pre-oxidized precursor fiber to obtain the high-density polyacrylonitrile pre-oxidized fiber.

[0024] The preparation method of high-density polyacrylonitrile pre-oxidized fiber in this application introduces plasma-assisted pre-oxidation. The highly active particles generated by plasma can act simultaneously on the fiber surface and internal matrix, forming a two-way synergistic oxidation mechanism "from the inside out" and "from the outside in", which significantly reduces the difference between the core and sheath structure and endows the pre-oxidized fiber with excellent structural uniformity. The uniform oxidation reaction can disperse the concentration of internal stress and inhibit the formation of microcracks and pores. Combined with the plasma micron-level etching effect, it can improve the surface smoothness of the fiber, resulting in low defect density and excellent overall performance. At the same time, plasma can significantly improve the kinetic rate of oxidation reaction, shortening the total pre-oxidation cycle by 30%-50% compared with traditional processes, achieving a win-win situation of efficiency improvement and energy consumption reduction. Moreover, through precise control of the three stages of "stress induction" - "plasma activation" - "relaxation stabilization", the chemical composition and physical structure of the fiber can be accurately controlled, and the process controllability is higher.

[0025] In an optional embodiment, the catalyst is selected from metal ion chelates, such as metal acetylacetone complexes, specifically at least one of iron acetylacetone and cobalt acetylacetone.

[0026] In an optional embodiment, the concentration of the catalyst in the pretreatment solution is 0.1-1.0 wt%, for example 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, or 1.0 wt%.

[0027] In an optional embodiment, the surfactant is selected from organic sulfonates, such as sodium dodecylbenzenesulfonate.

[0028] In an optional embodiment, the concentration of the surfactant in the pretreatment solution is 0.05-0.3 wt%, for example 0.05 wt%, 0.08 wt%, 0.11 wt%, 0.14 wt%, 0.17 wt%, 0.20 wt%, 0.23 wt%, 0.26 wt%, 0.29 wt%, or 0.3 wt%.

[0029] In an optional embodiment, the drying temperature is 50-80°C, for example 50°C, 53°C, 56°C, 59°C, 62°C, 65°C, 68°C, 71°C, 74°C, 77°C, or 80°C.

[0030] After the PAN precursor fibers are impregnated in an impregnation tank containing a pretreatment solution under a certain tension, they are dried at 50-80℃ to ensure that the catalyst components and surfactants are uniformly loaded on the surface and shallow layer of the precursor fibers.

[0031] The catalyst in the pretreatment solution is a metal ion chelate, which has high catalytic efficiency. The metal ions can coordinate with the cyano groups on the PAN macromolecular chain, effectively reducing the cyclization reaction initiation temperature and increasing the degree of cyclization. Secondly, the catalyst is uniformly dispersed on the fiber surface and shallow layer. When the active particles generated by the plasma penetrate into the fiber interior, they can react randomly, which helps to maintain the consistency of the reaction rate from the inside to the outside, thereby weakening the core-sheath structure and improving the uniformity of the core-sheath structure.

[0032] In an optional embodiment, the pre-oxidation temperature of the first stage is 180-220°C, such as 180°C, 184°C, 188°C, 192°C, 196°C, 200°C, 204°C, 208°C, 212°C, 216°C, or 220°C, preferably 190-210°C; the atmosphere is an oxidizing atmosphere containing oxygen, such as air or oxygen-enriched air.

[0033] In an optional implementation, the first-stage pre-oxidation time is 10-30 min, for example 10 min, 12 min, 14 min, 16 min, 18 min, 20 min, 22 min, 24 min, 26 min, 28 min, or 30 min.

[0034] In an optional embodiment, a stretching tension of 5-15% is applied to the supported catalyst precursor fiber, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%; preferably 8-12%.

[0035] The first stage of pre-oxidation aims to initiate a preliminary cyclization reaction and orient the macromolecular chains along the fiber axis under tension, laying the structural foundation for subsequent uniform reactions.

[0036] In an optional embodiment, the plasma is a dielectric barrier discharge plasma.

[0037] In an optional embodiment, the power of the plasma processing device used is 100-500 W, for example 100 W, 140 W, 180 W, 220 W, 260 W, 300 W, 340 W, 380 W, 420 W, 460 W, 500 W; and the processing frequency is 10-30 kHz, for example 10 kHz, 12 kHz, 14 kHz, 16 kHz, 18 kHz, 20 kHz, 22 kHz, 24 kHz, 26 kHz, 28 kHz, 30 kHz.

[0038] In an optional embodiment, the plasma-assisted pre-oxidation time is 0.5-5 min, for example 0.5 min, 1.0 min, 1.5 min, 2.0 min, 2.5 min, 3.0 min, 3.5 min, 4.0 min, 4.5 min, or 5.0 min.

[0039] Highly reactive particles generated by low-temperature plasma (such as O3, O·, OH·, NO) x (etc.) can efficiently and non-selectively penetrate into the fiber interior, initiating and accelerating cyclization / oxidation reactions from the "inside." This greatly weakens the core-sheath effect caused by traditional heat conduction from the outside in, achieving a uniform bulk reaction. At the same time, the etching effect of plasma can smooth the fiber surface and reduce existing grooves.

[0040] In an optional embodiment, the pre-oxidation temperature of the second stage is 240-280°C, such as 240°C, 244°C, 248°C, 252°C, 256°C, 260°C, 264°C, 268°C, 272°C, 276°C, or 280°C, preferably 250-270°C; the atmosphere is an oxidizing atmosphere containing oxygen, such as air or oxygen-enriched air.

[0041] In an optional embodiment, a relaxation tension of 0-3% is applied to the plasma-assisted pre-oxidized precursor fiber, for example, 0%, 0.3%, 0.7%, 1.0%, 1.3%, 1.7%, 2.0%, 2.3%, 2.7%, 3.0%; preferably 0.5-2%.

[0042] In the second stage of pre-oxidation, the tension is reduced to allow only a small amount of shrinkage, so that the fiber can complete the final cyclization and cross-linking and structural stabilization in a relaxed state, releasing internal stress.

[0043] The present invention also provides a highly dense polyacrylonitrile pre-oxidized fiber, which is prepared by the preparation method described in any one of the foregoing embodiments.

[0044] In an optional implementation, the degree of cyclization is ≥85%, for example 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, or 94%.

[0045] The present invention also provides a method for preparing highly dense polyacrylonitrile carbon fiber, wherein the highly dense polyacrylonitrile pre-oxidized fiber described in the foregoing embodiments is carbonized to obtain the highly dense polyacrylonitrile carbon fiber.

[0046] In an optional implementation, the carbonization includes primary carbonization and secondary carbonization.

[0047] In an optional embodiment, the primary carbonization temperature is 300-900℃, for example 300℃, 360℃, 420℃, 480℃, 540℃, 600℃, 660℃, 720℃, 780℃, 840℃, 900℃; and the time is 5-20 min, for example 5 min, 6.5 min, 8 min, 9.5 min, 11 min, 12.5 min, 14 min, 15.5 min, 17 min, 18.5 min, 20 min.

[0048] In an optional embodiment, the secondary carbonization temperature is 1000-1600℃, for example 1000℃, 1060℃, 1120℃, 1180℃, 1240℃, 1300℃, 1360℃, 1420℃, 1480℃, 1540℃, 1600℃; and the time is 1-5 min, for example 1 min, 1.4 min, 1.8 min, 2.2 min, 2.6 min, 3.0 min, 3.4 min, 3.8 min, 4.2 min, 4.6 min, 5 min.

[0049] The present invention also provides a highly dense polyacrylonitrile carbon fiber, which is prepared by the preparation method described in the foregoing embodiments. The tensile strength of the highly dense polyacrylonitrile carbon fiber is ≥6.6 GPa, for example 6.6 GPa, 6.8 GPa, 7.0 GPa, 7.2 GPa, or 7.4 GPa.

[0050] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0051] Example 1 This application provides a method for preparing polyacrylonitrile carbon fiber, the process of which is as follows: Figure 1 As shown, the specific steps include: S1: The PAN precursor fiber is pretreated with an aqueous solution containing 0.5 wt% ferric acetylacetone and 0.1 wt% sodium dodecylbenzenesulfonate, and then dried at 60°C. S2: Pass the treated raw yarn into the first pre-oxidation furnace at a temperature of 200°C in air atmosphere, apply 10% stretching tension, and treat for 20 minutes. S3: The fiber from the first oxidation furnace enters the DBD plasma device at a power of 300 W and a frequency of 15 kHz, and is treated in air for 2 minutes. S4: The fibers treated by S3 are passed into the second pre-oxidation furnace at a temperature of 260°C in air atmosphere, with 1% relaxation tension applied, and treated for 30 minutes. S5: The polyacrylonitrile pre-oxidized fiber prepared in S4 is subjected to carbonization treatment to obtain polyacrylonitrile carbon fiber. The carbonization includes primary carbonization and secondary carbonization. The primary carbonization temperature is 600℃ and the time is 10 min. The secondary carbonization temperature is 1300℃ and the time is 3 min.

[0052] Example 2 This application provides a method for preparing polyacrylonitrile carbon fiber, the process of which is as follows: Figure 1 As shown, the specific steps include: S1: The PAN precursor fiber is pretreated with an aqueous solution containing 0.1 wt% cobalt acetylacetonate and 0.05 wt% sodium dodecylbenzenesulfonate, and then dried at 50°C. S2: Pass the treated raw yarn into the first pre-oxidation furnace at a temperature of 220°C in air atmosphere, apply 5% stretching tension, and treat for 20 minutes. S3: The fiber passing through the first oxidation furnace enters the DBD plasma device at a power of 100 W and a frequency of 10 kHz, and is treated in air for 0.5 minutes; S4: The fibers treated by S3 are passed into the second pre-oxidation furnace at a temperature of 280°C in air atmosphere, with a relaxation tension of 2% applied, and treated for 30 minutes. S5: The polyacrylonitrile pre-oxidized fiber prepared in S4 is carbonized to obtain polyacrylonitrile carbon fiber. The carbonization includes primary carbonization and secondary carbonization. The primary carbonization temperature is 300℃ and the time is 20 min. The secondary carbonization temperature is 1600℃ and the time is 1 min.

[0053] Example 3 This application provides a method for preparing polyacrylonitrile carbon fiber, the process of which is as follows: Figure 1 As shown, the specific steps include: S1: The PAN precursor fiber is pretreated with an aqueous solution containing 1 wt% iron acetylacetone and 0.3 wt% sodium dodecylbenzenesulfonate, and then dried at 80°C. S2: Pass the treated raw yarn into the first pre-oxidation furnace at a temperature of 180°C in air atmosphere, apply 15% stretching tension, and treat for 20 minutes. S3: The fiber passes through the first oxidation furnace into the DBD plasma device, with a power of 500 W and a frequency of 30 kHz, and is treated in air for 5 minutes; S4: The fibers treated by S3 are passed into the second pre-oxidation furnace at a temperature of 240°C in air atmosphere, with a relaxation tension of 3% applied, and treated for 30 minutes. S5: The polyacrylonitrile pre-oxidized fiber prepared in S4 is carbonized to obtain polyacrylonitrile carbon fiber. The carbonization includes primary carbonization and secondary carbonization. The primary carbonization temperature is 900℃ and the time is 5 min. The secondary carbonization temperature is 1000℃ and the time is 5 min.

[0054] Comparative Example 1 This application provides a comparative example of a method for preparing polyacrylonitrile carbon fiber, which differs from Example 1 only in that the PAN precursor fiber does not undergo the S3 pretreatment step and directly enters the second pre-oxidation furnace.

[0055] Comparative Example 2 This application provides a comparative method for preparing polyacrylonitrile carbon fiber, which differs from Example 1 only in that step S1 is not performed.

[0056] Comparative Example 3 This application provides a comparative method for preparing polyacrylonitrile carbon fiber, which differs from Example 1 only in that steps S1 and S3 are not performed.

[0057] Experimental Example 1 (1) Cyclocyclization degree test of polyacrylonitrile pre-oxidized fiber Test method: The polyacrylonitrile pre-oxidized fibers prepared in Example 1 and Comparative Examples 1-3 were used as samples, and the degree of cyclization reaction of each sample was tested. The test results are summarized in Table 1. The specific test steps were as follows: the dried polyacrylonitrile pre-oxidized fibers were cut into small pieces, and KBr was mixed with the polyacrylonitrile pre-oxidized fibers at a mass ratio of 200:5 and ground to obtain KBr pellets for infrared analysis; the degree of cyclization was determined according to Formula I. C＝N / (I C＝N+ I C≡N ) calculate; where, I C≡N The characteristic absorption peak intensity of the cyano group corresponds to a wavenumber of 2240 cm⁻¹. -1 I C＝N The characteristic absorption peak intensity for C=N corresponds to a wavenumber of 1580 cm⁻¹. -1 .

[0058] Table 1

[0059] Referring to Table 1, the test results of Examples 1-3 and Comparative Examples 1-3 show that the polyacrylonitrile pre-oxidized fibers prepared according to the preparation process provided in the embodiments of this application have a higher degree of cyclization, indicating a higher degree of pre-oxidation. The degree of cyclization of Comparative Example 2, which was not impregnated, is lower than that of Comparative Example 3. This is because without the help of a catalyst, the cyclization reaction depends entirely on the high-energy excitation of plasma. The penetration depth of plasma active particles is limited, and the reaction rate is slow. The fiber surface reacts violently because it is directly exposed to plasma, while the fiber interior lacks catalyst guidance, resulting in a delayed reaction and a worse core-sheath structure.

[0060] (2) Core-sheath structure test of polyacrylonitrile pre-oxidized fiber Test method: The polyacrylonitrile pre-oxidized fibers prepared in Example 1 and Comparative Examples 1-3 were used as samples. Each PAN pre-oxidized fiber sample was then embedded in a mixture of epoxy resin and curing agent at a mass ratio of 15:2. After curing at 60°C for 2 h, the samples were sectioned using an ultramicrotome with a thickness of approximately 400 nm. The sections were then observed under a high-power optical microscope with a 20 X objective lens, and cross-sectional micrographs of the pre-oxidized fibers were taken.

[0061] See Figures 2-7 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.

[0062] (3) Mechanical property testing of polyacrylonitrile carbon fiber Test method: The polyacrylonitrile carbon fibers prepared in Example 1 and Comparative Examples 1-3 were used as samples, and the tensile strength of each sample was tested. The test results are summarized in Table 2. The test standard is based on GB / T3362.

[0063] Table 2

[0064] Referring to Table 3, the test results of Examples 1-3 and Comparative Examples 1-3 show that the polyacrylonitrile carbon fibers prepared according to the preparation process provided in the embodiments of this application have superior mechanical properties.

[0065] 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 highly dense polyacrylonitrile pre-oxidized fibers, characterized in that, Includes the following steps: Step S1: The polyacrylonitrile-based carbon fiber precursor is impregnated with a pretreatment solution and dried to obtain catalyst-supported precursor. The pretreatment solution includes a catalyst and a surfactant. Step S2: Perform a first-stage pre-oxidation on the supported catalyst precursor to obtain a first-stage pre-oxidized precursor. Step S3: Perform plasma-assisted pre-oxidation on the first-stage pre-oxidized precursor fiber to obtain plasma-assisted pre-oxidized precursor fiber; Step S4: Perform a second-stage pre-oxidation on the plasma-assisted pre-oxidized precursor fiber to obtain the high-density polyacrylonitrile pre-oxidized fiber.

2. The method for preparing high-density polyacrylonitrile pre-oxidized fibers according to claim 1, characterized in that, Step S1 satisfies at least one of the following characteristics: a. The catalyst is selected from metal ion chelates; b. The concentration of the catalyst in the pretreatment solution is 0.1-1.0 wt%; c. The surfactant is selected from organic sulfonates; d. The concentration of the surfactant in the pretreatment solution is 0.05-0.3 wt%; e. The drying temperature is 50-80℃.

3. The method for preparing high-density polyacrylonitrile pre-oxidized fibers according to claim 1, characterized in that, The first stage of pre-oxidation is carried out at a temperature of 180-220℃ in an oxidizing atmosphere containing oxygen. And / or, the first stage of pre-oxidation time is 10-30 min; And / or, apply 5-15% stretching tension to the supported catalyst precursor fiber; And / or, the catalyst is selected from metal acetylacetone complexes.

4. The method for preparing high-density polyacrylonitrile pre-oxidized fibers according to claim 1, characterized in that, The plasma is a dielectric barrier discharge plasma; And / or, the power of the plasma processing device used is 100-500 W, and the processing frequency is 10-30 kHz; And / or, the plasma-assisted pre-oxidation time is 0.5-5 min; And / or, the catalyst is selected from at least one of iron acetylacetone and cobalt acetylacetone; And / or, during the first stage of pre-oxidation, an 8-12% stretching tension is applied to the supported catalyst precursor fiber; And / or, apply 0-3% relaxation tension to the plasma-assisted pre-oxidized precursor fiber.

5. The method for preparing high-density polyacrylonitrile pre-oxidized fibers according to claim 1, characterized in that, The second stage of pre-oxidation is carried out at a temperature of 240-280℃ in an oxidizing atmosphere containing oxygen. And / or, apply a relaxation tension of 0.5-2% to the plasma-assisted pre-oxidized precursor fiber.

6. A high-density polyacrylonitrile pre-oxidized fiber, characterized in that, It is prepared by the preparation method according to any one of claims 1-5.

7. The high-density polyacrylonitrile pre-oxidized fiber according to claim 6, characterized in that, The degree of cyclization is ≥91%.

8. A method for preparing highly dense polyacrylonitrile carbon fiber, characterized in that, The high-density polyacrylonitrile pre-oxidized filament of claim 6 is carbonized to obtain the high-density polyacrylonitrile carbon fiber.

9. The method for preparing highly dense polyacrylonitrile carbon fiber according to claim 8, characterized in that, The carbonization includes primary carbonization and secondary carbonization; The primary carbonization temperature is 300-900℃ and the time is 5-20 min; and / or the secondary carbonization temperature is 1000-1600℃ and the time is 1-5 min.

10. A highly dense polyacrylonitrile carbon fiber, characterized in that, The highly dense polyacrylonitrile carbon fiber prepared by the preparation method described in claim 8 or 9 has a tensile strength ≥ 6.6 GPa.

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