Polyacrylonitrile pre-oxidized fiber as well as preparation method and application thereof

Abstract

The invention discloses a preparation method and application of polyacrylonitrile fiber pre-oxidized filaments, the pre-oxidized filaments comprise a core part located at the axis and a skin part wrapping the outer side of the core part, and the characteristic tensile strength of the polyacrylonitrile fiber pre-oxidized filaments is 475-580 cN / mu m. Wherein the polyacrylonitrile pre-oxidized fiber with the characteristic tensile strength of 475-580cN / mu m is applied to subsequent low-temperature carbonization, high-temperature carbonization and graphitization treatment, so that the structural defect on the carbon fiber, which is inherited by the radial chemical structure difference of the fiber in the carbonization process, can be avoided, and the tensile mechanical property of the graphite fiber can be further improved; and the method has a relatively good application range.

Description

Technical Field

[0001] The present invention belongs to the field of polyacrylonitrile pre-oxidation, and further relates to the preparation of carbon fiber from polyacrylonitrile. Specifically, it relates to a polyacrylonitrile pre-oxidized fiber and its preparation method and application. Background Art

[0002] High-strength and high-modulus polyacrylonitrile (PAN)-based carbon fibers can be widely used in high-end fields such as aerospace, mainly due to the following reasons: (1) High-strength and high-modulus PAN-based carbon fiber composites have high stiffness. Since spacecraft such as satellites and space stations have to withstand large accelerations and severe vibrations during the launch process of launch vehicles, when designing the structure of spacecraft, the main problem to be considered is to solve the stiffness problem under the condition of meeting the strength requirements. Only by using a structure with high stiffness can the integrity and functionality of the spacecraft structure be ensured. Usually, the specific modulus of the unidirectional material of PAN-based high-modulus carbon fiber composites is 5-7 times greater than that of aluminum alloy. Therefore, high-modulus carbon fiber composites are the best materials to meet the structural stiffness requirements of spacecraft such as satellites. (2) High-modulus carbon fiber composites can meet the requirements of high dimensional stability of spacecraft. The main problems faced by spacecraft in the space environment are high and low temperature alternation. For example, spacecraft operating in low Earth orbit will face a maximum temperature of 120 °C and a minimum temperature of up to -160 °C. Therefore, the structural components inside the spacecraft have extremely high requirements for the high precision and dimensional stability of the structure in the high and low temperature alternation environment. By using high-strength and high-modulus carbon fibers as reinforcing materials, composites with an almost zero coefficient of thermal expansion can be obtained through reasonable ply design, thus meeting the requirements of spacecraft for dimensional stability in the high and low temperature alternation environment. (3) High-strength and high-modulus carbon fiber composites can meet the lightweight requirements of spacecraft. The fuel used by spacecraft costs about $5 million per ton. Carrying a large amount of fuel will increase the takeoff and flight weight of the spacecraft, greatly increasing the manufacturing cost and flight cost of the spacecraft. It is estimated that for every 1 kg of mass saved by a satellite, the launch vehicle can reduce 500 kg of fuel and reduce the launch cost by $20,000. Compared with metal materials, PAN-based high-modulus carbon fiber composites have significant lightweight effects while meeting the high stiffness and dimensional stability of spacecraft, which helps to reduce the launch and operation costs.

[0003] The preparation process of high-strength and high-modulus polyacrylonitrile-based carbon fiber is extremely complex. First, a spinning dope is obtained through a polymerization reaction mainly based on acrylonitrile (AN) monomer; then, the PAN precursor fiber is spun; finally, the PAN precursor fiber is subjected to pre-oxidation treatment, low-temperature and high-temperature carbonization treatment, graphitization treatment, and surface treatment of high-strength and high-modulus carbon fiber. Since the PAN precursor fiber with straight and long molecular chains needs to obtain good heat resistance for carbonization and graphitization treatment, the pre-oxidation treatment of the precursor fiber plays an important role. During the pre-oxidation process, the long and straight polyacrylonitrile molecular chains in the polyacrylonitrile precursor fiber gradually form a "ladder"-type polymer structure with excellent heat resistance; in the subsequent carbonization process, non-carbon elements are gradually removed by releasing small-molecule gases, and the molecular chains of these network structures will gradually form the prototype of the graphene structure, so as to form graphite lamellae during the high-temperature carbonization treatment at a higher temperature, improve the internal structure of the fiber, and enhance the macroscopic mechanical properties of the final carbon fiber.

[0004] However, at present, the pre-oxidation process of PAN precursor fiber is a diffusion process of oxygen in the radial direction of the fiber, which is affected by environmental factors such as heat treatment temperature and heat treatment atmosphere. The most common and inevitable defect - "skin-core" structure will be formed at this stage. Such structural defects will be further inherited to the fibers after low-temperature carbonization, high-temperature carbonization, and even graphitization treatment, thereby affecting the fiber quality of the final product. Summary of the Invention

[0005] In order to overcome the problems existing in the prior art, the present invention provides a polyacrylonitrile pre-oxidized fiber and its preparation method and application. The polyacrylonitrile pre-oxidized fiber has an appropriate "characteristic tensile strength". By controlling the "characteristic tensile strength" of the pre-oxidized fiber as the structural quality parameter of the pre-oxidized fiber, the fibers obtained after low-temperature carbonization, high-temperature carbonization, and even graphitization treatment of the pre-oxidized fiber can avoid the structural defects in brittle materials caused by chemical defects remaining after carbonization, and can further improve the mechanical properties of the carbon fiber, and have a good scope of application.

[0006] One of the purposes of the present invention is to provide a polyacrylonitrile pre-oxidized fiber, which includes a core part located at the axis and a skin part wrapped outside the core part. Among them, the characteristic tensile strength of the polyacrylonitrile pre-oxidized fiber is 475 - 580 cN / μm (for example, 475 cN / μm, 480 cN / μm, 490 cN / μm, 500 cN / μm, 510 cN / μm, 520 cN / μm, 530 cN / μm, 540 cN / μm, 550 cN / μm, 560 cN / μm, 570 cN / μm or 580 cN / μm), and the acquisition of the characteristic tensile strength is as shown in formula (1):

[0007]

[0008] In formula (1), T.S. c represents the characteristic tensile strength (cN / μm) of the polyacrylonitrile pre-oxidized fiber, and the polyacrylonitrile pre-oxidized fiber is the final polyacrylonitrile pre-oxidized fiber; T represents the average breaking strength (cN) of the polyacrylonitrile pre-oxidized fiber, η represents the area ratio of the cortical radial cross-section to the overall radial cross-section of the polyacrylonitrile pre-oxidized fiber, and d represents the average diameter (μm) of the polyacrylonitrile pre-oxidized fiber.

[0009] In the present invention, the average breaking strength is obtained by testing according to GB / T 14337-2008.

[0010] In the present invention, η can be obtained as follows: the average area ratio of the cortical region in the cross-sectional image of the polyacrylonitrile pre-oxidized fiber to the entire fiber cross-section. Specifically, the cortex is the part with a darker contrast on the periphery of the cross-sectional slice of the polyacrylonitrile pre-oxidized fiber, and η is the ratio of the sum of all parts with a darker contrast on the periphery to the circular area of the entire fiber cross-section.

[0011] In a preferred embodiment, in formula (1), 1.5 cN ≤ T ≤ 8.0 cN, preferably, 2.5 cN ≤ T ≤ 5.2 cN. For example, T can be any value among 1.5 cN, 2 cN, 2.5 cN, 3 cN, 3.5 cN, 4 cN, 4.5 cN, 5 cN, 5.5 cN, 6 cN, 6.5 cN, 7 cN, 7.5 cN or 8 cN, or any range composed of any two of these values.

[0012] In a preferred embodiment, in formula (1), 55% ≤ η ≤ 95%, preferably, 74% ≤ η ≤ 91%. For example, η can be any value among 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90% or 95%, or any range composed of any two of these values.

[0013] In a preferred embodiment, in formula (1), 5.0 μm ≤ d ≤ 15.0 μm, preferably, 7.0 μm ≤ d ≤ 10.0 μm. For example, d can be any value among 5.0 μm, 6.0 μm, 8.0 μm, 10.0 μm, 12.0 μm, 14.0 μm or 15.0 μm, or any range composed of any two of these values.

[0014] In a preferred embodiment, the characteristic tensile strength of the polyacrylonitrile pre-oxidized fiber is 480 - 575 cN / μm, preferably 482.5 - 571.5 cN / μm. For example, it can be any value among 480 cN / μm, 490 cN / μm, 500 cN / μm, 510 cN / μm, 520 cN / μm, 530 cN / μm, 540 cN / μm, 550 cN / μm, 560 cN / μm, 570 cN / μm or 575 cN / μm, or any range composed of any two of these values.

[0015] After in-depth research, the present inventor found that by controlling the "characteristic tensile strength" of the pre-oxidized fiber as a structural quality parameter of the pre-oxidized fiber, the pre-oxidized fiber obtained after low-temperature carbonization, high-temperature carbonization, or even graphitization treatment can avoid structural defects in brittle materials caused by chemical defects remaining after carbonization, and can further improve the mechanical properties of carbon fiber, with a good scope of application. The structural quality parameters mainly include: (1) the area fraction of the "skin part" of the radial cross-section of the "skin part" of the pre-oxidized fiber in the overall radial cross-section of the pre-oxidized fiber, which is called the "skin part" ratio. When the fiber diameter is fixed, the higher the skin part ratio of the cross-section, the more fully the PAN fiber is affected by oxygen diffusion, and the relatively richer the internal thermal stabilization structure, which can withstand the temperature of subsequent carbonization treatment, thereby improving the strength of the final carbon fiber. If the skin part ratio of the fiber cross-section is low, it means that oxygen diffusion in the fiber is insufficient, and there are more non-thermal stabilization structures inside the fiber. Such structures will undergo cracking reactions as chemical structure defects during low-temperature carbonization, releasing small-molecule gases, coal tar, etc., and leaving defects inside the fiber; (2) the diameter of the PAN raw fiber. When the pre-oxidation treatment process is the same, it means that the process of oxygen propagation inside the fiber is considered the same. Under the same pre-oxidation treatment, fibers with different diameters will show different oxygen reaction processes, and thus pre-oxidized fiber cross-sections with different skin part ratios will be obtained. Finally, the carbon fiber prepared from the pre-oxidized fiber with a small skin part ratio should have poor mechanical properties; (3) the breaking strength of the pre-oxidized fiber. Generally, the breaking strength value of the pre-oxidized fiber can reflect the mechanical properties of the pre-oxidized fiber, including tensile strength, tensile modulus, etc.

[0016] Therefore, the pre-oxidized fiber with the above-mentioned structure / quality parameter - characteristic tensile strength proposed by the present invention can well and quickly reflect the quality of the PAN raw fiber after pre-oxidation treatment, solve the problems of slow feedback of pre-oxidized fiber structure parameters and inaccurate structure results, and well lay a solid foundation for subsequent carbon fiber preparation. Finally, the mechanical properties of the graphitized carbon fiber are further improved.

[0017] The second object of the present invention is to provide a method for preparing polyacrylonitrile pre-oxidized fiber, preferably used for preparing the polyacrylonitrile pre-oxidized fiber described in the first object of the present invention. The preparation method includes: successively performing low-temperature pre-oxidation, medium-temperature pre-oxidation, and high-temperature pre-oxidation on the polyacrylonitrile raw fiber; wherein, the conditions for the low-temperature pre-oxidation include: controlling the pressure of the furnace atmosphere to be 18-55 Pa, the temperature to be 180-220 °C, and the time to be 9-18 min; the conditions for the medium-temperature pre-oxidation include: controlling the pressure of the furnace atmosphere to be 25-65 Pa, the temperature to be 226-250 °C, and the time to be 18-32 min; the conditions for the high-temperature pre-oxidation include: controlling the pressure of the furnace atmosphere to be 35-85 Pa, the temperature to be 248-262 °C, and the time to be 12-28 min.

[0018] Among them, the temperature of the medium-temperature oxidation is higher than that of the low-temperature oxidation, and the temperature of the high-temperature oxidation is higher than that of the medium-temperature oxidation.

[0019] For example, the temperature of the low-temperature pre-oxidation is any point value or a range composed of any two point values among 180 °C, 185 °C, 190 °C, 195 °C, 200 °C, 205 °C, 210 °C, 215 °C, or 220 °C; the temperature of the medium-temperature pre-oxidation is any point value or a range composed of any two point values among 226 °C, 230 °C, 235 °C, 240 °C, 245 °C, or 250 °C; the temperature of the high-temperature pre-oxidation treatment is any point value or a range composed of any two point values among 248 °C, 250 °C, 255 °C, 260 °C, or 262 °C. The time of the low-temperature pre-oxidation is 9 min, 10 min, 12 min, 16 min, or 18 min; the time of the medium-temperature pre-oxidation is 18 min, 20 min, 22 min, 24 min, 26 min, 28 min, 30 min, or 32 min; the time of the high-temperature pre-oxidation is 12 min, 14 min, 16 min, 18 min, 20 min, 22 min, 24 min, 26 min, or 28 min.

[0020] In a preferred embodiment, the number of the polyacrylonitrile raw fibers is 1-20 K, and the fineness is 0.5-1.0 d / tex.

[0021] For example, the number of the polyacrylonitrile raw fibers is any value or a range composed of any two values among 1 K, 2 K, 4 K, 6 K, 8 K, 10 K, 12 K, 14 K, 16 K, 18 K, or 20 K, and the fineness is any value or a range composed of any two values among 0.5 d / tex, 0.6 d / tex, 0.7 d / tex, 0.8 d / tex, 0.9 d / tex, or 1 d / tex.

[0022] In a further preferred embodiment, the number of polyacrylonitrile filaments is 3 - 12K, and the fineness is 0.76 - 0.86 d / tex.

[0023] In an even more preferred embodiment, the polyacrylonitrile filaments are obtained by dry spinning or wet spinning, preferably by wet spinning.

[0024] In a preferred embodiment, the average diameter of the polyacrylonitrile filaments is 5.0 - 15.0 μm, preferably 7.0 - 10.0 μm, for example, any value or any range composed of any two values among 5.0 μm, 6.0 μm, 8.0 μm, 10.0 μm, 12.0 μm, 14.0 μm or 15.0 μm.

[0025] In a preferred embodiment, the conditions for pre - oxidation in the low - temperature stage include: the temperature is 180 - 210 °C, and the time is 10 - 15 min.

[0026] In a preferred embodiment, the conditions for pre - oxidation in the medium - temperature stage include: the temperature is 230 - 245 °C, and the time is 20 - 35 min.

[0027] In a preferred embodiment, the conditions for pre - oxidation in the high - temperature stage include: the temperature is 250 - 260 °C, and the time is 15 - 25 min.

[0028] In a preferred embodiment, the pre - oxidation is carried out in a pre - oxidation furnace: when pre - oxidizing in the low - temperature stage, the pressure of the furnace atmosphere is controlled to be 18 - 55 Pa; and / or, when pre - oxidizing in the medium - temperature stage, the pressure of the furnace atmosphere is controlled to be 25 - 65 Pa; and / or, when pre - oxidizing in the high - temperature stage, the pressure of the furnace atmosphere is controlled to be 35 - 85 Pa.

[0029] For example, the pre - oxidation is carried out in a pre - oxidation furnace: when pre - oxidizing in the low - temperature stage, the pressure of the furnace atmosphere is controlled to be any value or any range composed of any two values among 15 Pa, 20 Pa, 25 Pa, 30 Pa, 35 Pa, 40 Pa, 45 Pa, 50 Pa or 55 Pa; and / or, when pre - oxidizing in the medium - temperature stage, the pressure of the furnace atmosphere is controlled to be any value or any range composed of any two values among 25 Pa, 30 Pa, 35 Pa, 40 Pa, 45 Pa, 50 Pa, 55 Pa, 60 Pa or 65 Pa; and / or, when pre - oxidizing in the high - temperature stage, the pressure of the furnace atmosphere is controlled to be any value or any range composed of any two values among 35 Pa, 40 Pa, 45 Pa, 50 Pa, 55 Pa, 60 Pa, 65 Pa, 70 Pa, 75 Pa, 80 Pa or 85 Pa.

[0030] Among them, the pre - oxidation in the low - temperature stage, the medium - temperature stage and the high - temperature stage are carried out in different pre - oxidation furnaces.

[0031] In a further preferred embodiment, the pre-oxidation is carried out in a pre-oxidation furnace (preferably different pre-oxidation furnaces): when pre-oxidizing in the low-temperature stage, the atmosphere pressure in the furnace is controlled to be 20-50 Pa; and / or, when pre-oxidizing in the medium-temperature stage, the atmosphere pressure in the furnace is controlled to be 30-60 Pa; and / or, when pre-oxidizing in the high-temperature stage, the atmosphere pressure in the furnace is controlled to be 40-80 Pa.

[0032] In a preferred embodiment, when pre-oxidizing in the low-temperature stage, the medium-temperature stage, and the high-temperature stage, the draw ratio is independently controlled to be 0.8-1.2, preferably 0.9-1.1, more preferably 0.98-1.05, for example, any value or the range composed of any two values among 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, or 1.2.

[0033] Among them, the inventors found through a large number of studies that when pre-oxidizing, controlling the furnace pressure in each stage of pre-oxidation within the range defined in the present invention (especially the preferred range) and cooperating with the temperature, time, draw ratio, pressure, etc. of each stage of pre-oxidation, polyacrylonitrile pre-oxidized fibers with appropriate characteristic tensile strength can be obtained. Such pre-oxidized fibers can avoid the structural defects in brittle materials caused by chemical defects remaining after carbonization in fibers after low-temperature carbonization, high-temperature carbonization, and even graphitization treatment, and can further improve the mechanical properties of carbon fibers, having a good scope of application.

[0034] The preparation method of the present invention includes: dividing the pre-oxidation stage of PAN fibers into three basic stages: low-temperature stage pre-oxidation, medium-temperature stage pre-oxidation, and high-temperature stage pre-oxidation.

[0035] Samples are taken from the fibers that have completed the high-temperature stage pre-oxidation treatment, and after rapid resin embedding and ultramicrotomy, they are observed under an optical microscope. Drawing and counting operations are carried out on the skin-core structure of the cross-section of the pre-oxidized fibers, and data on the proportion of the skin part of the fiber cross-section is obtained; in addition, when obtaining pre-oxidized fiber samples, the single-filament tensile strength can also be tested according to GB / T 14337-2008 at the same time.

[0036] The polyacrylonitrile pre-oxidized filaments obtained by using the preparation method of the present invention have appropriate characteristic tensile strength, 5.0 μm ≤ d ≤ 15.0 μm, preferably, 7.0 μm ≤ d ≤ 10.0 μm.

[0037] In the present invention, by controlling the pressure inside the pre-oxidation furnace and regulating the "characteristic tensile strength" of the pre-oxidized fiber, it is made to reach an optimal value of 475 - 580 cN / μm. Applying the polyacrylonitrile pre-oxidized fiber with such structural parameters to subsequent low-temperature carbonization, high-temperature carbonization, and graphitization treatments can avoid structural defects "inherited" from the radial chemical structure differences of the fiber to the carbon fiber during the carbonization process, and can further improve the tensile mechanical properties of the graphite fiber, making it exceed the high-strength and high-modulus carbon fiber of Toray M55J grade in Japan, and having a good scope of application.

[0038] A third object of the present invention is to provide a polyacrylonitrile pre-oxidized fiber obtained by using the preparation method described in the second object of the present invention.

[0039] A fourth object of the present invention is to provide the application of the polyacrylonitrile pre-oxidized fiber described in the first object of the present invention or the preparation method described in the second object of the present invention in the preparation of carbon fibers.

[0040] A fifth object of the present invention is to provide a method for preparing carbon fibers, including: successively performing low-temperature carbonization treatment, high-temperature carbonization treatment, and graphitization treatment on the polyacrylonitrile pre-oxidized fiber, wherein the polyacrylonitrile pre-oxidized fiber is selected from the polyacrylonitrile pre-oxidized fiber described in the first object of the present invention, or the polyacrylonitrile pre-oxidized fiber is first obtained by using the preparation method described in the second object of the present invention.

[0041] In a preferred embodiment, the conditions of the low-temperature carbonization treatment include: the temperature is 200 - 1000°C, the time is 0.5 - 10 min, and the draw ratio is 0.8 - 1.2.

[0042] For example, the conditions of the low-temperature carbonization treatment include: the temperature is any value or a range composed of any two values among 200°C, 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 900°C, or 1000°C, the time is any value or a range composed of any two values among 0.5 min, 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, or 10 min, and the draw ratio is any value or a range composed of any two values among 0.8, 0.85, 0.9, 0.95, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.1, 1.15, or 1.2.

[0043] In a further preferred embodiment, the conditions of the low-temperature carbonization treatment include: the temperature is 300 - 800°C, the time is 1 - 4 min, and the draw ratio is 1.00 - 1.06.

[0044] In a further preferred embodiment, the low-temperature carbonization is carried out in multiple temperature zones (for example, 2 to 8 or 3 to 6), and the temperature of each temperature zone is independently any value within 200 to 1000 °C, preferably 300 to 800 °C, and the temperature of the subsequent temperature zone is higher than that of the previous temperature zone.

[0045] In a preferred embodiment, the conditions for the high-temperature carbonization treatment include: the temperature is 800 to 1800 °C, the treatment time is 0.5 to 10 min, and the draw ratio is 0.8 to 1.

[0046] For example, the conditions for the high-temperature carbonization treatment include: the temperature is any value or a range composed of any two values among 800 °C, 900 °C, 1000 °C, 1100 °C, 1200 °C, 1300 °C, 1400 °C, 1500 °C, 1600 °C, 1700 °C or 1800 °C, the treatment time is any value or a range composed of any two values among 0.5 min, 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min, and the draw ratio is any value or a range composed of any two values among 0.8, 0.85, 0.9, 0.95, 0.96, 0.97, 0.98, 0.99 or 1.0.

[0047] In a further preferred embodiment, the conditions for the high-temperature carbonization treatment include: the temperature is 1000 to 1500 °C, the treatment time is 1 to 4 min, and the draw ratio is 0.96 to 0.99.

[0048] Among them, the temperature of the high-temperature carbonization treatment is higher than that of the low-temperature carbonization treatment.

[0049] In a further preferred embodiment, the high-temperature carbonization is carried out in multiple temperature zones (for example, 2 to 8 or 3 to 6), and the temperature of each temperature zone is independently any value within 800 to 1800 °C, preferably 1000 to 1500 °C, and the temperature of the subsequent temperature zone is higher than that of the previous temperature zone.

[0050] In a preferred embodiment, the conditions for the graphitization treatment include: the temperature is 2200 to 3000 °C, the treatment time is 0.2 to 15 min, and the draw ratio is 0.9 to 1.5.

[0051] For example, the conditions for the graphitization treatment include: the temperature is any value or a range composed of any two values among 2200°C, 2300°C, 2400°C, 2500°C, 2600°C, 2700°C, 2800°C, 2900°C, or 3000°C; the treatment time is any value or a range composed of any two values among 0.2 min, 0.5 min, 0.8 min, 1 min, 2 min, 4 min, 6 min, 8 min, 10 min, 12 min, or 15 min; and the draw ratio is any value or a range composed of any two values among 0.9%, 1.0%, 1.02%, 1.04%, 1.06%, 1.08%, 1.10%, 1.12%, 1.14%, 1.15%, 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, or 1.5%.

[0052] In a further preferred embodiment, the conditions for the graphitization treatment include: the temperature is 2500 - 2700°C, the treatment time is 1 - 5 min, and the draw ratio is 1.04 - 1.15.

[0053] In a preferred embodiment, the preparation method includes:

[0054] (1) The pre-oxidized fiber described in one of the objects of the present invention or the pre-oxidized fiber obtained by using the preparation method described in the second object of the present invention is subjected to low-temperature carbonization treatment, with an average temperature of 300 - 800°C, a treatment time of about 1 - 4 min, and a draw ratio of 1.00 - 1.06.

[0055] (2) The fiber after the low-temperature carbonization treatment is subjected to high-temperature carbonization treatment, with an average temperature of 1000 - 1500°C, a treatment time of 1 - 4 min, and a draw ratio of 0.96 - 0.99.

[0056] (3) Finally, the fiber after the high-temperature carbonization treatment is subjected to graphitization treatment, with an average temperature of 2500 - 2700°C for the graphitization treatment, a treatment time of 1 - 5 min, and a draw ratio of 1.04 - 1.15.

[0057] The process for preparing polyacrylonitrile pre-oxidized fiber is optimized based on the prior art. The skin part of the final pre-oxidized fiber has a larger proportion, and the stabilization reaction is evenly distributed in the skin part area of the pre-oxidized fiber, without loss of the mechanical properties of the final carbon fiber due to over-oxidation.

[0058] By appropriately controlling the characteristic tensile strength of the pre-oxidized fiber through the present invention, the mechanical properties of the graphitized fiber can be made better (tensile strength above 4000 MPa, such as 4000 - 5000 MPa, and tensile modulus above 540 GPa, such as 540 - 600 GPa), which has important guiding significance for the polyacrylonitrile-based high-strength and high-modulus carbon fiber industry.

[0059] In the ranges and any values disclosed in the present invention, the endpoints and any values of the ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. In the following text, in principle, the various technical solutions can be combined with each other to obtain new technical solutions, which should also be regarded as specifically disclosed herein.

[0060] Compared with the prior art, the present invention has the following beneficial effects: By appropriately controlling the characteristic tensile strength of the pre-oxidized fiber through the present invention, the mechanical properties of the graphitized fiber can be made better (tensile strength above 4000 MPa, such as 4000 - 5000 MPa, and tensile modulus above 540 GPa, such as 540 - 600 GPa), which has important guiding significance for the polyacrylonitrile-based high-strength and high-modulus carbon fiber industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 Optical microscope cross-sectional view of the polyacrylonitrile pre-oxidized fiber obtained in Example 28.

[0062] Figure 2 Optical microscope cross-sectional view of the polyacrylonitrile pre-oxidized fiber obtained in Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0063] The present invention will be specifically described below in conjunction with specific embodiments. It is necessary to point out here that the following embodiments are only for further illustration of the present invention and cannot be understood as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention still fall within the protection scope of the present invention.

[0064] In addition, it should be noted that in the following detailed description, the various specific technical features described can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0065] In addition, any combination can be made among various different embodiments of the present invention as long as it does not violate the idea of the present invention. The technical solutions formed thereby belong to a part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0066] For the raw materials used in the examples and comparative examples, if not specifically defined, they are all disclosed in the prior art. For example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0067] In the examples and comparative examples, the characteristic tensile strength was obtained as follows: The polyacrylonitrile pre-oxidized fiber was ultrathin sectioned and the cross-section of the fiber section was observed under an optical microscope. The proportion η of all the dark parts with contrast in the concentric circles of the cross-section slice of the polyacrylonitrile pre-oxidized fiber cross-section to the area of the entire fiber cross-section circle, and the average diameter d of the pre-oxidized fiber were quantified. At the same time, the single-filament tensile test of the polyacrylonitrile pre-oxidized fiber was carried out according to the test standard GB / T 14337-2008, and finally the characteristic tensile strength of the polyacrylonitrile pre-oxidized fiber was calculated.

[0068]

Comparative Example 1

[0069] The raw silk of the binary copolymer polyacrylonitrile fiber spun by the domestic 6K wet method was used. The copolymer chemical composition of the raw silk was 95.5 wt% acrylonitrile, 3.5 wt% methyl acrylate, and 1 wt% itaconic acid, and its fineness was 0.8551 dtex. First, the PAN raw silk was pre-oxidized in the low-temperature section at a temperature of 180 °C and a furnace atmosphere pressure of 15 Pa, with a draw ratio of 1.03 and a treatment time of 15 min; the fiber after the low-temperature section pre-oxidation treatment was continuously fed into the pre-oxidation treatment in the intermediate temperature section, with a treatment temperature of 230 °C, a furnace pressure of 20 Pa, a draw ratio of 1.03, and a treatment time of 15 min; the fiber after the intermediate temperature section pre-oxidation treatment was continuously fed into the pre-oxidation treatment in the high-temperature section, with a treatment temperature of 250 °C, a furnace pressure of 25 Pa, a draw ratio of 1.00, and a treatment time of 30 min to obtain the polyacrylonitrile pre-oxidized fiber.

[0070] The polyacrylonitrile pre-oxidized fiber is subjected to low-temperature carbonization and high-temperature carbonization treatments in high-purity nitrogen in sequence to prepare the final polyacrylonitrile-based carbon fiber. The temperatures of the four temperature zones for low-temperature carbonization are 320°C - 400°C - 550°C - 700°C in sequence, the treatment time is 3 min, and the draw ratio is 1.06; the temperatures of the four temperature zones for high-temperature carbonization are 1100°C - 1200°C - 1300°C - 1450°C in sequence, the heat treatment time is 2 min, and the draw ratio is 0.96; the average temperature for graphitization is 2500°C, the draw ratio is 1.04, and the treatment time is 3 min. Finally, the polyacrylonitrile-based graphitized fiber is obtained. The mechanical properties of the polyacrylonitrile-based graphite fiber multifilament are tested according to the national standard GB / T 25749-2011, and the test results are shown in Table 1.

[0071]

Comparative Example 2

[0072] First, the PAN raw fiber is pre-oxidized in the low-temperature section at a temperature of 160°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 15 min. The remaining operations are the same as those in Comparative Example 1.

[0073]

Comparative Example 3

[0074] First, the PAN raw fiber is pre-oxidized in the low-temperature section at a temperature of 160°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as those in Comparative Example 1.

[0075]

Comparative Example 4

[0076] First, the PAN raw fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 8 min. The remaining operations are the same as those in Comparative Example 1.

[0077]

Comparative Example 5

[0078] First, the PAN raw fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 15 min; the fiber that has completed the low-temperature section pre-oxidation treatment is continuously fed into the pre-oxidation treatment in the intermediate temperature section. Its treatment temperature is 235°C, the furnace pressure is 20 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as those in Comparative Example 1.

[0079]

Comparative Example 6

[0080] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa, with a draw ratio of 1.03 and a treatment time of 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 225 °C, the in-furnace pressure is 25 Pa, the draw ratio is 1.03, and the treatment time is 15 min. The remaining operations are the same as in Comparative Example 1. However, the pre-oxidized fiber obtained after completing the high-temperature section pre-oxidation treatment in Comparative Example 1 cannot pass through the low-temperature carbonization furnace.

[0081]

Comparative Example 7

[0082] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa, with a draw ratio of 1.03 and a treatment time of 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 235 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 10 min. The remaining operations are the same as in Comparative Example 1. However, the pre-oxidized fiber obtained after completing the high-temperature section pre-oxidation treatment in Comparative Example 1 cannot pass through the low-temperature carbonization furnace.

[0083]

Comparative Example 8

[0084] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa, with a draw ratio of 1.03 and a treatment time of 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 255 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as in Comparative Example 1. However, the mechanical properties of the final graphitized fiber are not good.

[0085]

Comparative Example 9

[0086] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa, with a draw ratio of 1.03 and a treatment time of 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 235 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 10 min. The remaining operations are the same as in Comparative Example 1. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 250 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.00, and the treatment time is 30 min. The remaining operations are the same as in Comparative Example 1.

[0087]

Comparative Example 10

[0088] First, the PAN precursor fibers are pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 15 minutes. The fibers that have completed the pre-oxidation treatment in the low-temperature section are continuously fed into the pre-oxidation treatment in the intermediate temperature section. The treatment temperature is 235°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 10 minutes. The remaining operations are the same as in Comparative Example 1. The fibers that have completed the pre-oxidation treatment in the intermediate temperature section are continuously fed into the pre-oxidation treatment in the high-temperature section. The treatment temperature is 245°C, the furnace pressure is 50 Pa, the draw ratio is 1.00, and the treatment time is 20 minutes. The remaining operations are the same as in Comparative Example 1. The polyacrylonitrile fibers treated by this process cannot pass through the low-temperature carbonization furnace smoothly, and no graphitized fibers can be obtained.

[0089]

Comparative Example 11

[0090] First, the PAN precursor fibers are pre-oxidized in the low-temperature section at a temperature of 200°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 15 minutes. The fibers that have completed the pre-oxidation treatment in the low-temperature section are continuously fed into the pre-oxidation treatment in the intermediate temperature section. The treatment temperature is 235°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 10 minutes. The remaining operations are the same as in Comparative Example 1. The fibers that have completed the pre-oxidation treatment in the intermediate temperature section are continuously fed into the pre-oxidation treatment in the high-temperature section. The treatment temperature is 265°C, the furnace pressure is 50 Pa, the draw ratio is 1.00, and the treatment time is 30 minutes. The remaining operations are the same as in Comparative Example 1. The pre-oxidation treatment in the high-temperature section is relatively violent, and the polyacrylonitrile fibers are ablated and broken in the furnace, and the subsequent carbonization treatment cannot be carried out to obtain the final graphitized fiber sample.

[0091]

Example 1

[0092] First, the PAN precursor filaments (same as in Comparative Example 1) were pre-oxidized in the low-temperature section at a temperature of 180 °C and a furnace atmosphere pressure of 20 Pa. The draw ratio was 1.03 and the treatment time was 10 min. The fibers that completed the pre-oxidation treatment in the low-temperature section were continuously fed into the pre-oxidation treatment in the intermediate temperature section. The treatment temperature was 230 °C, the furnace pressure was 30 Pa, the draw ratio was 1.03, and the treatment time was 20 min. The fibers that completed the pre-oxidation treatment in the intermediate temperature section were continuously fed into the pre-oxidation treatment in the high-temperature section. The treatment temperature was 250 °C, the furnace pressure was 40 Pa, the draw ratio was 1.00, and the treatment time was 20 min, obtaining polyacrylonitrile pre-oxidized filaments. The PAN fibers that completed the pre-oxidation treatment were subjected to low-temperature carbonization. The temperatures of the four temperature zones were 320 °C - 400 °C - 550 °C - 700 °C in sequence, the treatment time was 3 min, and the draw ratio was 1.06; for high-temperature carbonization, the temperatures of the four temperature zones were 1100 °C - 1200 °C - 1300 °C - 1450 °C in sequence, the heat treatment time was 2 min, and the draw ratio was 0.96; the average temperature of graphitization was 2500 °C, the draw ratio was 1.04, and the treatment time was 3 min, finally obtaining polyacrylonitrile-based graphitized fibers.

[0093]

Example 2

[0094] First, the PAN precursor filaments were pre-oxidized in the low-temperature section at a temperature of 180 °C and a furnace atmosphere pressure of 35 Pa. The draw ratio was 1.03 and the treatment time was 10 min. The remaining operations were the same as in Example 1.

[0095]

Example 3

[0096] First, the PAN precursor filaments were pre-oxidized in the low-temperature section at a temperature of 180 °C and a furnace atmosphere pressure of 50 Pa. The draw ratio was 1.03 and the treatment time was 10 min. The remaining operations were the same as in Example 1.

[0097]

Example 4

[0098] First, the PAN precursor filaments were pre-oxidized in the low-temperature section at a temperature of 200 °C and a furnace atmosphere pressure of 20 Pa. The draw ratio was 1.03 and the treatment time was 12 min. The remaining operations were the same as in Example 1.

[0099]

Example 5

[0100] First, the PAN precursor filaments were pre-oxidized in the low-temperature section at a temperature of 210 °C and a furnace atmosphere pressure of 20 Pa. The draw ratio was 1.03 and the treatment time was 15 min. The remaining operations were the same as in Example 1.

[0101]

Example 6

[0102] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 210°C and a furnace atmosphere pressure of 50 Pa. The draw ratio is 1.03, and the treatment time is 15 min. The remaining operations are the same as those in Example 1.

[0103]

Example 7

[0104] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as those in Example 1.

[0105]

Example 8

[0106] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 35 min. The remaining operations are the same as those in Example 1.

[0107]

Example 9

[0108] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230°C, the furnace pressure is 50 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as those in Example 1.

[0109]

Example 10

[0110] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03, and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245°C, the furnace pressure is 60 Pa, the draw ratio is 1.03, and the treatment time is 35 min. The remaining operations are the same as those in Example 1.

[0111]

Example 11

[0112] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 190°C and a furnace atmosphere pressure of 50 Pa. The draw ratio is 1.03 and the treatment time is 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230°C, the furnace pressure is 60 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as those in Example 1.

[0113]

Example 12

[0114] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 210°C and a furnace atmosphere pressure of 50 Pa. The draw ratio is 1.03 and the treatment time is 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230°C, the furnace pressure is 60 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The remaining operations are the same as those in Example 1.

[0115]

Example 13

[0116] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 210°C and a furnace atmosphere pressure of 50 Pa. The draw ratio is 1.03 and the treatment time is 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245°C, the furnace pressure is 60 Pa, the draw ratio is 1.03, and the treatment time is 35 min. The remaining operations are the same as those in Example 1.

[0117]

Example 14

[0118] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 250°C, the furnace pressure is 40 Pa, the draw ratio is 1.00, and the treatment time is 15 min. The remaining operations are the same as those in Example 1.

[0119]

Example 15

[0120] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 255 °C, the in-furnace pressure is 40 Pa, the draw ratio is 1.00, and the treatment time is 20 min. The remaining operations are the same as those in Example 1.

[0121]

Example 16

[0122] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the in-furnace pressure is 40 Pa, the draw ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0123]

Example 17

[0124] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 250 °C, the in-furnace pressure is 60 Pa, the draw ratio is 1.00, and the treatment time is 15 min. The remaining operations are the same as those in Example 1.

[0125]

Example 18

[0126] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 250 °C, the in-furnace pressure is 80 Pa, the draw ratio is 1.00, and the treatment time is 15 min. The remaining operations are the same as those in Example 1.

[0127]

Example 19

[0128] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 255 °C, the in-furnace pressure is 60 Pa, the draw ratio is 1.00, and the treatment time is 20 min. The remaining operations are the same as those in Example 1.

[0129]

Example 20

[0130] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and an in-furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 230 °C, the in-furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the in-furnace pressure is 60 Pa, the draw ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0131]

Example 21

[0132] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate-temperature section, where the treatment temperature is 230°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate-temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 255°C, the furnace pressure is 80 Pa, the draw ratio is 1.00, and the treatment time is 20 min. The remaining operations are the same as in Example 1.

[0133]

Example 22

[0134] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 1 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate-temperature section, where the treatment temperature is 230°C, the furnace pressure is 30 Pa, the draw ratio is 1.03, and the treatment time is 20 min. The fiber that has completed the pre-oxidation treatment in the intermediate-temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260°C, the furnace pressure is 80 Pa, the draw ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as in Example 1.

[0135]

Example 23

[0136] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180°C and a furnace atmosphere pressure of 20 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate-temperature section, where the treatment temperature is 245°C, the furnace pressure is 45 Pa, the draw ratio is 1.03, and the treatment time is 25 min. The fiber that has completed the pre-oxidation treatment in the intermediate-temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260°C, the furnace pressure is 80 Pa, the draw ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as in Example 1.

[0137]

Example 24

[0138] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and a furnace atmosphere pressure of 20 Pa. The drawing ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245 °C, the furnace pressure is 60 Pa, the drawing ratio is 1.03, and the treatment time is 25 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the furnace pressure is 80 Pa, the drawing ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0139]

Example 25

[0140] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 180 °C and a furnace atmosphere pressure of 20 Pa. The drawing ratio is 1.03 and the treatment time is 1 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245 °C, the furnace pressure is 60 Pa, the drawing ratio is 1.03, and the treatment time is 35 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the furnace pressure is 80 Pa, the drawing ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0141]

Example 26

[0142] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 200 °C and a furnace atmosphere pressure of 20 Pa. The drawing ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245 °C, the furnace pressure is 60 Pa, the drawing ratio is 1.03, and the treatment time is 35 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the furnace pressure is 80 Pa, the drawing ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0143]

Example 27

[0144] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 200 °C and a furnace atmosphere pressure of 35 Pa. The draw ratio is 1.03 and the treatment time is 10 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245 °C, the furnace pressure is 60 Pa, the draw ratio is 1.03, and the treatment time is 35 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the furnace pressure is 80 Pa, the draw ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0145]

Example 28

[0146] First, the PAN precursor fiber is pre-oxidized in the low-temperature section at a temperature of 210 °C and a furnace atmosphere pressure of 50 Pa. The draw ratio is 1.03 and the treatment time is 15 min. The fiber that has completed the pre-oxidation treatment in the low-temperature section is continuously fed into the pre-oxidation treatment in the intermediate temperature section, where the treatment temperature is 245 °C, the furnace pressure is 60 Pa, the draw ratio is 1.03, and the treatment time is 35 min. The fiber that has completed the pre-oxidation treatment in the intermediate temperature section is continuously fed into the pre-oxidation treatment in the high-temperature section, where the treatment temperature is 260 °C, the furnace pressure is 80 Pa, the draw ratio is 1.00, and the treatment time is 25 min. The remaining operations are the same as those in Example 1.

[0147] Table 1. Mechanical properties of pre-oxidized fibers and corresponding polyacrylonitrile-based graphitized carbon fibers in comparative examples and examples

[0148]

[0149]

[0150] The present invention has been described in detail above in combination with specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications, or improvements can be made to the technical solutions and implementation manners of the present invention, and all of these fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A polyacrylonitrile pre-oxidized fiber, which comprises a core portion located at the axis and a skin portion wrapped outside the core portion. Wherein, The characteristic tensile strength of the polyacrylonitrile pre-oxidized fiber is 475 - 580 cN / μm, and the acquisition of the characteristic tensile strength is as shown in formula (1): In formula (1), T.S. c represents the characteristic tensile strength of the polyacrylonitrile pre-oxidized fiber, and the polyacrylonitrile pre-oxidized fiber is the final polyacrylonitrile pre-oxidized fiber; T represents the average breaking strength of the polyacrylonitrile pre-oxidized fiber, η represents the area ratio of the skin part radial cross-section to the overall radial cross-section of the polyacrylonitrile pre-oxidized fiber, and d represents the average diameter of the polyacrylonitrile pre-oxidized fiber.

2. The polyacrylonitrile pre-oxidized fiber according to claim 1, Characterized in that, In formula (1): 1.5 cN ≤ T ≤ 8.0 cN, preferably, 2.5 cN ≤ T ≤ 5.2 cN; and / or, 55% ≤ η ≤ 95%, preferably, 74% ≤ η ≤ 91%; and / or, 5.0 μm ≤ d ≤ 15.0 μm, preferably, 7.0 μm ≤ d ≤ 10.0 μm.

3. The polyacrylonitrile pre-oxidized fiber according to claim 1 or 2, Characterized in that, The characteristic tensile strength of the polyacrylonitrile pre-oxidized fiber is 480 - 575 cN / μm, preferably 482.5 - 571.5 cN / μm.

4. A method for preparing a polyacrylonitrile pre-oxidized fiber, preferably used for preparing the polyacrylonitrile pre-oxidized fiber according to any one of claims 1 - 3, the preparation method Comprises: Performing low-temperature pre-oxidation, medium-temperature pre-oxidation and high-temperature pre-oxidation on the polyacrylonitrile raw fiber in sequence; wherein, the conditions of the low-temperature pre-oxidation include: controlling the furnace atmosphere pressure to be 18 - 55 Pa and the time to be 9 - 18 min; the conditions of the medium-temperature pre-oxidation include: controlling the furnace atmosphere pressure to be 25 - 65 Pa and the time to be 18 - 32 min; the conditions of the high-temperature pre-oxidation include: controlling the furnace atmosphere pressure to be 35 - 85 Pa and the time to be 12 - 28 min.

5. The preparation method according to claim 4, Characterized in that, The conditions of the low-temperature pre-oxidation include: the temperature is 180 - 220 °C, preferably 180 - 210 °C, and the time is 10 - 15 min; and / or, the conditions of the medium-temperature pre-oxidation include: the temperature is 226 - 250 °C, preferably 230 - 245 °C, and the time is 20 - 35 min; and / or, the conditions of the high-temperature pre-oxidation include: the temperature is 248 - 262 °C, preferably 250 - 260 °C, and the time is 15 - 25 min.

6. The preparation method according to claim 4, Characterized in that, The pre-oxidation is carried out in a pre-oxidation furnace: during the low-temperature pre-oxidation, controlling the furnace atmosphere pressure to be 20 - 50 Pa; and / or, during the medium-temperature pre-oxidation, controlling the furnace atmosphere pressure to be 30 - 60 Pa; and / or, during the low-temperature pre-oxidation, controlling the furnace atmosphere pressure to be 40 - 80 Pa.

7. The preparation method according to any one of claims 4 - 6, Characterized in that, During the low-temperature pre-oxidation, the medium-temperature pre-oxidation and the high-temperature pre-oxidation, controlling the draw ratio to be independently 0.8 - 1.2, preferably 0.9 - 1.

1.

8. A polyacrylonitrile pre-oxidized fiber obtained by using the preparation method according to any one of claims 4 - 7.

9. Application of the polyacrylonitrile pre-oxidized fiber according to any one of claims 1 - 3 or the preparation method according to any one of claims 4 - 7 in the preparation of carbon fibers.

10. A method for preparing carbon fiber, comprising: performing low-temperature carbonization treatment, high-temperature carbonization treatment, and graphitization treatment on polyacrylonitrile pre-oxidized fiber in sequence, wherein the polyacrylonitrile pre-oxidized fiber is selected from the polyacrylonitrile pre-oxidized fiber described in any one of claims 1 to 3, or the polyacrylonitrile pre-oxidized fiber is obtained by using the preparation method described in any one of claims 4 to 7 first.

11. The preparation method according to claim 10, wherein, the conditions for the low-temperature carbonization treatment include: the temperature is 200 to 1000 °C, the time is 0.5 to 10 min, and the draw ratio is 0.8 to 1.2; and / or, the conditions for the high-temperature carbonization treatment include: the temperature is 800 to 1800 °C, the treatment time is 0.5 to 10 min, and the draw ratio is 0.8 to 1.

12. The preparation method according to claim 10 or 11, wherein, the conditions for the graphitization treatment include: the temperature is 2200 to 3000 °C, the treatment time is 0.2 to 15 min, and the draw ratio is 0.9 to 1.5.

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