Methods for reducing tar content during carbon fiber production, methods for preparing carbon fiber, and the resulting carbon fiber.

Abstract

This invention relates to a method for reducing tar during carbon fiber production, a method for preparing carbon fiber, and the resulting carbon fiber. The method for reducing tar during production includes drying and pre-oxidizing the precursor fiber to obtain pre-oxidized fiber, and then subjecting the pre-oxidized fiber to low-temperature carbonization. In these steps, the fiber tension is controlled based on the product of the tow number and fineness to reduce the amount of tar generated during carbon fiber preparation. After low-temperature carbonization, high-temperature carbonization is performed to obtain carbon fiber. This method for preparing carbon fiber with reduced tar during production not only effectively reduces the amount of tar generated during the production of large-tow carbon fiber, and effectively solves the problems of excessive tar and easy blockage of the low-temperature carbonization furnace exhaust outlet in existing processes for preparing large-tow carbon fiber, but also improves carbon yield and the mechanical properties of carbon fiber.

Description

Technical Field

[0001] This invention relates to the technical field of carbon fiber preparation, and more specifically, to a method for reducing tar during carbon fiber production, a method for preparing carbon fiber, and the resulting carbon fiber. Background Technology

[0002] During the low-temperature carbonization process, polyacrylonitrile-based carbon fibers generate various pyrolysis products. These products easily condense into liquid, relatively large-molecule, and easily agglomerated pyrolysis products at the waste outlet, known as tar. The tar generated during low-temperature carbonization leads to the following problems: Tar condenses below 300℃ and easily adheres to the inner wall of the waste outlet pipe. Accumulation to a certain extent can clog the pipe, affecting the pressure inside the low-temperature carbonization furnace and the safe operation of the waste outlet system. Simultaneously, the tar at the waste outlet of the low-temperature carbonization furnace causes the fiber bundles to adhere to each other, resulting in incomplete carbonization within the carbon fibers and affecting their performance.

[0003] Tar not only affects the structure and structure of carbon fibers, reducing carbon yield, but also has drawbacks such as corroding equipment, polluting the operating space, and clogging waste discharge pipes. Currently, the main research direction in China is to control tar formation by understanding its mechanism and to improve equipment, but the results have not been significant, often requiring shutdowns for tar removal. Summary of the Invention

[0004] To address the series of problems caused by tar production during carbon fiber production in existing technologies, this invention proposes a method for reducing tar during carbon fiber production, a method for preparing carbon fiber, and the resulting carbon fiber. The carbon fiber preparation method of this invention, which reduces tar during production, not only effectively reduces the amount of tar generated during the production of large-tow carbon fibers, and better solves the problems of excessive tar and easy blockage of the waste outlet of the low-temperature carbonization furnace during the preparation of large-tow carbon fibers in existing processes, but also improves carbon yield and the mechanical properties of carbon fibers.

[0005] One of the objectives of this invention is to provide a method for reducing tar during carbon fiber production, comprising the steps of drying and pre-oxidizing the precursor fiber to obtain pre-oxidized fiber, and then subjecting the pre-oxidized fiber to low-temperature carbonization treatment.

[0006] In the above steps, the fiber tension is controlled by the product of the number of filament bundles and the fineness, where the unit of fiber tension is cN and the unit of fineness is dtex.

[0007] The fiber tension before and after the raw filament drying step may be the same or different, and is independently selected from 0.01 to 0.04 times the product of the number of filament bundles and the fineness; wherein the unit of fiber tension is cN and the unit of fineness is dtex; for example, the fiber tension can be 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, or 0.04 times the product of the number of filament bundles and the fineness.

[0008] In this invention, the method for reducing tar during carbon fiber production is applicable not only to large-tow polyacrylonitrile precursor fibers but also to small-tow precursor fibers. Large-tow polyacrylonitrile precursor fibers, in particular, generate a large amount of tar during preparation, clogging the low-temperature carbonization waste outlet. This problem is quite prominent in actual production.

[0009] In the raw silk drying step of the present invention, preferably, the fiber tension before and after the raw silk drying step is independently selected from 0.02 to 0.03 times the product of the number of filaments and the fineness; wherein the unit of fiber tension is cN and the unit of fineness is dtex; for example, the fiber tension can be 0.02, 0.021, 0.022, 0.023, 0.024, 0.025, 0.026, 0.027, 0.028, 0.029, or 0.03 times the product of the number of filaments and the fineness.

[0010] Preferably,

[0011] The precursor fiber includes polyacrylonitrile precursor fiber; and / or,

[0012] The drying step employs hot air drying, more preferably...

[0013] The hot air is blown in the direction of the yarn feed for drying, and / or,

[0014] The temperature of the hot air is 100–120°C; and / or,

[0015] The effective heating time of the hot air is 10–60 seconds.

[0016] In this invention, the direction of hot air blowing also has a certain impact on tar removal; drying along the wire feeding direction is beneficial for tar removal.

[0017] In the pre-oxidation step described in this invention, preferably,

[0018] The pre-oxidation includes initial pre-oxidation, intermediate pre-oxidation and late pre-oxidation;

[0019] Preferably, the fiber tension decreases gradually from the initial stage to the later stage during the pre-oxidation stage; more preferably:

[0020] During the initial pre-oxidation stage, the fiber tension during the spinning process is 0.17 to 0.22 times the product of the number of filaments and the fineness, where the unit of fiber tension is cN and the unit of fineness is dtex; more preferably, it is 0.18 to 0.20 times; for example, the fiber tension can be 0.17, 0.18, 0.19, 0.2, 0.21, or 0.22 times the product of the number of filaments and the fineness.

[0021] During the intermediate pre-oxidation stage, the fiber tension during the spinning process is 0.14 to 0.20 times the product of the number of filaments and the fineness, where the unit of fiber tension is cN and the unit of fineness is dtex; more preferably, it is 0.16 to 0.18 times; for example, the fiber tension can be 0.14, 0.15, 0.16, 0.17, or 0.18 times the product of the number of filaments and the fineness.

[0022] During the later pre-oxidation stage, the fiber tension during the spinning process is 0.12 to 0.18 times the product of the number of filaments and the fineness, where the unit of fiber tension is cN and the unit of fineness is dtex; more preferably, it is 0.14 to 0.16 times; for example, the fiber tension can be 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, or 0.18 times the product of the number of filaments and the fineness.

[0023] Preferably,

[0024] The initial pre-oxidation temperature is 210-230℃; for example, it can be 210℃, 215℃, 220℃, 225℃, or 230℃.

[0025] The intermediate pre-oxidation temperature is 230-250℃; for example, it can be 230℃, 235℃, 240℃, 245℃, or 250℃.

[0026] The temperature for the subsequent pre-oxidation is 250-270℃; for example, it can be 250℃, 255℃, 260℃, 265℃, or 270℃.

[0027] Preferably,

[0028] The initial pre-oxidation time accounts for 40-60% of the total pre-oxidation time;

[0029] The intermediate pre-oxidation time accounts for 20-30% of the total pre-oxidation time;

[0030] The time for the later pre-oxidation accounts for 20-30% of the total pre-oxidation time.

[0031] Generally, in existing technologies, the total pre-oxidation time varies depending on the type of raw fiber or its parameters. In a preferred embodiment of the present invention, the total pre-oxidation time is 30-90 min, the initial pre-oxidation time is 15-45 min, the intermediate pre-oxidation time is 7.5-22.5 min, and the final pre-oxidation time is 7.5-22.5 min.

[0032] Preferably,

[0033] The oxygen content of the pre-oxidized fiber in the initial pre-oxidation is 3-5 wt%; for example, it can be 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, or 5 wt%.

[0034] The oxygen content of the pre-oxidized fiber in the intermediate pre-oxidation process is 5-7 wt%; for example, it can be 5 wt%, 5.5 wt%, 6 wt%, 6.5 wt%, or 7 wt%.

[0035] The oxygen content of the pre-oxidized fiber in the later pre-oxidation process is 7-9 wt%; for example, it can be 7 wt%, 7.5 wt%, 8 wt%, 8.5 wt%, or 9 wt%.

[0036] Preferably,

[0037] The density of the pre-oxidized fiber in the initial pre-oxidation stage is 1.18–1.22 g / cm³. 3 For example, it could be 1.18 g / cm³. 3 1.19 g / cm 3 1.20g / cm 3 1.21 g / cm 3 1.22g / cm 3 ;

[0038] The density of the pre-oxidized fiber in the intermediate pre-oxidation process is 1.28–1.32 g / cm³. 3 For example, it can be 1.28 g / cm³. 3 1.29g / cm 3 1.30g / cm 3 1.31 g / cm 3 1.32g / cm 3 ;

[0039] The density of the pre-oxidized fiber body after the subsequent pre-oxidation is 1.36–1.38 g / cm³. 3 For example, it could be 1.36 g / cm³. 3 1.37g / cm 3 1.38g / cm 3 .

[0040] This invention controls the temperature, time, and tension of the three stages of pre-oxidation, thereby enabling precise control of the oxygen content and bulk density of the pre-oxidation process, which in turn helps reduce the amount of tar in the subsequent low-temperature carbonization process.

[0041] In the low-temperature carbonization step described in this invention, preferably,

[0042] The low-temperature carbonization process employs a multi-stage gradient heating method, with the temperature difference between adjacent temperature zones not exceeding 100℃; and / or,

[0043] During the low-temperature carbonization treatment, the maximum temperature rise shall not exceed 800°C; and / or,

[0044] The initial temperature of the low-temperature carbonization is 20–30°C higher than the temperature of the subsequent pre-oxidation; and / or,

[0045] The coke discharge port of the low-temperature carbonization is located at a temperature of 450-550°C during the low-temperature carbonization process.

[0046] Preferably, the starting temperature for low-temperature carbonization is selected from 270-300℃, and the maximum temperature for heating is selected from 700-760℃; more preferably, a six-temperature-zone low-temperature carbonization is adopted; even more preferably, the starting temperature of the six-temperature-zone low-temperature carbonization is selected from 280-290℃, and the ending temperature is selected from 720-750℃.

[0047] Preferably, the fiber tension during the low-temperature carbonization process is 0.06 to 0.09 times the product of the number of filaments and the fineness; where the unit of fiber tension is cN and the unit of fineness is dtex, preferably 0.07 to 0.08 times; for example, the fiber tension can be 0.06, 0.062, 0.065, 0.068, 0.07, 0.71, 0.72, 0.073, 0.074, 0.075, 0.076, 0.077, 0.078, 0.079, 0.08, 0.082, 0.084, 0.086, 0.088, or 0.09 times the product of the number of filaments and the fineness.

[0048] A second objective of this invention is to provide a carbon fiber preparation method based on the method for reducing tar as described in one objective of this invention, wherein the carbon fiber preparation method further includes the following steps:

[0049] After low-temperature carbonization, high-temperature carbonization is carried out to obtain carbon fibers.

[0050] Preferably,

[0051] The high-temperature carbonization temperature is 900-1400℃.

[0052] A third objective of this invention is to provide carbon fibers produced using the tar reduction method described in one objective of this invention, or carbon fibers prepared using the preparation method described in another objective of this invention.

[0053] Preferably,

[0054] The carbon fiber has a tensile strength ≥ 4.0 GPa and a tensile modulus ≥ 245 GPa.

[0055] In this invention, the carbon fibers after high-temperature carbonization are also subjected to surface treatment and sizing and drying according to conventional practices to obtain the finished carbon fiber product. Surface treatment and sizing and drying can be performed using existing commonly used methods.

[0056] In this invention, fiber tension is strictly controlled according to the number of filament bundles and fineness; by coordinating the fiber tension in each stage of drying, pre-oxidation, and low-temperature carbonization, the chemical structure of the pre-oxidation and low-temperature carbonization stages is controlled, thereby reducing the amount of tar.

[0057] In this invention, the fiber tension is achieved by the drive devices before and after the equipment at each stage of the carbon fiber production process. The drive devices can be set with different rotation speeds. The drive devices are conventional devices, and the adjustment of fiber tension is very easy.

[0058] In this invention, the carbon fiber generation equipment includes the following units: a winding machine, a drying furnace, a pre-oxidation furnace, a low-temperature carbonization furnace, a high-temperature carbonization furnace, carbon fiber production equipment, surface treatment and sizing drying equipment, and a winding machine. Specifically, the precursor fiber is unwound on the winding machine, then passes through the drying furnace, pre-oxidation furnace, low-temperature carbonization furnace, and high-temperature carbonization furnace to obtain carbon fiber. The surface treatment and sizing drying equipment, along with the winding machine, ultimately produce the finished carbon fiber product. The tar content is significantly reduced throughout the entire preparation process.

[0059] Compared with the prior art, the present invention has at least the following advantages:

[0060] Low-temperature carbonization tar is a major problem in normal production. When it accumulates to a certain level, the production line must be shut down for treatment, resulting in a large amount of waste fiber. This invention strictly controls fiber tension based on the number of filament bundles and fineness. By coordinating the fiber tension in each stage of drying, pre-oxidation, and low-temperature carbonization, the chemical structure of the pre-oxidation and low-temperature carbonization stages is controlled, significantly reducing the amount of tar during carbon fiber production. This allows for longer production runs with virtually no need for tar cleaning.

[0061] The production method of the present invention does not require additional equipment, is easy to implement, and has low cost. Detailed Implementation

[0062] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection 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 are still within the scope of protection of the present invention.

[0063] Raw material source:

[0064] All raw materials used in this invention are conventional commercially available products.

[0065] Test method:

[0066] The bulk density of the fibers during the pre-oxidation stage was measured using a density gradient tube.

[0067] The oxygen content of the fiber during the pre-oxidation stage was measured using an elemental analyzer.

[0068] The tensile strength and tensile modulus of carbon fiber were determined according to GB / T3362-2005 Test Method for Tensile Properties of Carbon Fiber Multifilament.

[0069] Example 1

[0070] After unwinding, 48k large-tow polyacrylonitrile precursor fibers with a fineness of 1.2 dtex are dried in a drying device between the unwinding machine and the pre-oxidation furnace. Hot air heating is used, with the air blowing along the fiber feeding direction. The hot air temperature is 108℃, and the effective heating time is 50 seconds. The tension between the unwinding machine and the drying furnace is 1500 cN, and the tension between the drying furnace and the oxidation furnace is also 1500 cN. The fibers then enter a three-stage continuous box-type pre-oxidation furnace with a gradient temperature increase: initial pre-oxidation temperature 225℃, intermediate pre-oxidation temperature 245℃, and final pre-oxidation temperature 265℃. The total pre-oxidation heat treatment time is 60 minutes, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the intermediate and final pre-oxidation times each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 10900 cN; during the intermediate pre-oxidation, the fiber tension is 9800 cN; and during the final pre-oxidation, the fiber tension is 8650 cN. The fiber oxygen content was 4.52 wt% in the initial stage of pre-oxidation, 6.55 wt% in the middle stage, and 8.27 wt% in the later stage. The fiber density was 1.212 g / cm³ in the initial stage of pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.304 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.372 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage of the heat treatment lasts for 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4200 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage of the heat treatment lasting for 15 seconds.

[0071] The carbon yield of large-tow carbon fiber is 51.5%. The furnace pressure of the low-temperature carbonization furnace increases by 6% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three months without stopping to clean the tar during carbon fiber production.

[0072] The tensile strength prepared by the above method is 4200 MPa; the tensile modulus is 248 GPa.

[0073] Example 2

[0074] After unwinding, 48k large-tow polyacrylonitrile precursor fibers with a fineness of 1.2 dtex are dried in a drying device between the unwinding machine and the pre-oxidation furnace. Hot air heating is used, with the air blowing along the fiber feeding direction. The hot air temperature is 110℃, and the heating time is 45 seconds. The tension between the unwinding machine and the drying furnace is 1400 cN, and the tension between the drying furnace and the oxidation furnace is also 1400 cN. The fibers then enter a three-stage continuous box-type pre-oxidation furnace with a gradient temperature increase: 220℃ in the initial pre-oxidation stage, 240℃ in the middle stage, and 260℃ in the later stage. The total pre-oxidation heat treatment time is 60 minutes, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the middle and later stages each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 10900 cN; during the middle pre-oxidation stage, the fiber tension is 9800 cN; and during the later pre-oxidation stage, the fiber tension is 8650 cN. The fiber oxygen content was 4.25 wt% in the initial stage of pre-oxidation, 6.36 wt% in the middle stage, and 8.12 wt% in the later stage. The fiber density was 1.206 g / cm³ in the initial stage of pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.311 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.365 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage of the heat treatment lasts for 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4200 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage of the heat treatment lasting for 15 seconds.

[0075] The carbon yield of large-tow carbon fiber is 51.3%. The furnace pressure of the low-temperature carbonization furnace increases by 7% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three months without stopping to clean the tar during carbon fiber production.

[0076] The tensile strength prepared by the above method is 4060 MPa; the tensile modulus is 245 GPa.

[0077] Example 3

[0078] After unwinding, 48k large-tow polyacrylonitrile precursor fibers with a fineness of 1.2 dtex are dried in a drying device between the unwinding machine and the pre-oxidation furnace. Hot air heating is used, with the air blowing along the fiber feeding direction. The hot air temperature is 108℃, and the heating time is 50 seconds. The tension between the unwinding machine and the drying furnace is 1600 cN, and the tension between the drying furnace and the oxidation furnace is also 1600 cN. The fibers then enter a three-stage continuous box-type pre-oxidation furnace with a gradient temperature increase: initial pre-oxidation temperature 218℃, intermediate pre-oxidation temperature 236℃, and final pre-oxidation temperature 256℃. The total pre-oxidation heat treatment time is 60 minutes, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the intermediate and final pre-oxidation times each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 10589 cN; during the intermediate pre-oxidation process, the fiber tension is 9657 cN; and during the final pre-oxidation process, the fiber tension is 8268 cN. The fiber oxygen content was 4.05 wt% in the initial stage of pre-oxidation, 5.56 wt% in the middle stage, and 7.66 wt% in the later stage. The fiber density was 1.188 g / cm³ in the initial stage of pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.285 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.362 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage of the heat treatment lasts for 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4425 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage of the heat treatment lasting for 15 seconds.

[0079] The carbon yield of large-tow carbon fiber is 51.7%. The furnace pressure of the low-temperature carbonization furnace increases by 6% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three months without stopping to clean the tar during carbon fiber production.

[0080] The tensile strength prepared by the above method is 4165 MPa; the tensile modulus is 248 GPa.

[0081] Example 4

[0082] After unwinding, 48k large-tow polyacrylonitrile precursor fibers with a fineness of 1.2 dtex are dried in a drying device between the unwinding machine and the pre-oxidation furnace. Hot air heating is used, with the air blowing along the fiber feeding direction. The hot air temperature is 108℃, and the heating time is 50 seconds. The tension between the unwinding machine and the drying furnace is 1620 cN, and the tension between the drying furnace and the oxidation furnace is 1622 cN. The fibers then enter a three-stage continuous box-type pre-oxidation furnace with a gradient temperature increase: initial pre-oxidation temperature 225℃, intermediate pre-oxidation temperature 245℃, and final pre-oxidation temperature 265℃. The total pre-oxidation heat treatment time is 60 minutes, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the intermediate and final pre-oxidation times each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 10986 cN, during the intermediate pre-oxidation process it is 9965 cN, and during the final pre-oxidation process it is 9108 cN. The fiber oxygen content was 4.72 wt% in the initial stage of pre-oxidation, 6.67 wt% in the middle stage, and 8.18 wt% in the later stage. The fiber density was 1.192 g / cm³ in the initial stage of pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.296 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.376 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage of the heat treatment lasts for 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4267 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage of the heat treatment lasting for 15 seconds.

[0083] The carbon yield of large-tow carbon fiber is 51.8%. The furnace pressure of the low-temperature carbonization furnace increases by 5% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three months without stopping to clean the tar during carbon fiber production.

[0084] The tensile strength prepared by the above method is 4265 MPa; the tensile modulus is 247 GPa.

[0085] Example 5

[0086] After unwinding, 48k large-tow polyacrylonitrile precursor fibers with a fineness of 1.2 dtex are dried in a drying device between the unwinding machine and the pre-oxidation furnace. Hot air heating is used, with the air blowing along the fiber feeding direction. The hot air temperature is 108℃, and the heating time is 50 seconds. The tension between the unwinding machine and the drying furnace is 1280 cN, and the tension between the drying furnace and the oxidation furnace is 1285 cN. The fibers then enter a three-stage continuous box-type pre-oxidation furnace with gradient heating: initial pre-oxidation temperature 226℃, intermediate pre-oxidation temperature 245℃, and final pre-oxidation temperature 267℃. The total pre-oxidation heat treatment time is 60 minutes, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the intermediate and final pre-oxidation times each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 11209 cN; during the intermediate pre-oxidation process, the fiber tension is 9925 cN; and during the final pre-oxidation process, the fiber tension is 8759 cN. The fiber oxygen content was 4.62 wt% in the initial stage of pre-oxidation, 6.26 wt% in the middle stage, and 8.01 wt% in the later stage. The fiber density was 1.195 g / cm³ in the initial stage of pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.296 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.378 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage of the heat treatment lasts for 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4396 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage lasting for 30 seconds.

[0087] The carbon yield of large-tow carbon fiber is 51.2%. The furnace pressure of the low-temperature carbonization furnace increases by 8% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three months without stopping to clean the tar during carbon fiber production.

[0088] The tensile strength prepared by the above method is 4080 MPa; the tensile modulus is 246 GPa.

[0089] Example 6

[0090] After unwinding, 48k large-tow polyacrylonitrile precursor fibers with a fineness of 1.2 dtex are dried in a drying device between the unwinding machine and the pre-oxidation furnace. Hot air heating is used, with the air blowing along the fiber feeding direction. The hot air temperature is 106℃, and the heating time is 48 seconds. The tension between the unwinding machine and the drying furnace is 1300 cN, and the tension between the drying furnace and the oxidation furnace is also 1300 cN. The fibers then enter a three-stage continuous box-type pre-oxidation furnace with a gradient temperature increase: initial pre-oxidation temperature 228℃, intermediate pre-oxidation temperature 246℃, and final pre-oxidation temperature 262℃. The total pre-oxidation heat treatment time is 60 minutes, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the intermediate and final pre-oxidation times each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 11056 cN; during the intermediate pre-oxidation, the fiber tension is 9921 cN; and during the final pre-oxidation, the fiber tension is 8893 cN. The fiber oxygen content was 4.76 wt% in the initial stage of pre-oxidation, 6.91 wt% in the middle stage, and 8.57 wt% in the later stage. The fiber density was 1.216 g / cm³ in the initial stage of pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.317 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.373 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage of the heat treatment lasts for 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4452 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage lasting for 30 seconds.

[0091] The carbon yield of large-tow carbon fiber is 51.8%. The furnace pressure of the low-temperature carbonization furnace increases by 9% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three months without stopping to clean the tar during carbon fiber production.

[0092] The tensile strength prepared by the above method is 4100 MPa; the tensile modulus is 246 GPa.

[0093] Comparative Example 1

[0094] The preparation method is basically the same as that of Example 6, except that the tension between the feeding machine and the drying furnace in Comparative Example 1 is 2500 cN, and the tension between the drying furnace and the oxidation furnace is 2550 cN. The fiber enters a three-stage continuous box-type pre-oxidation furnace with gradient heating. The initial pre-oxidation temperature is 228°C, the middle pre-oxidation temperature is 246°C, and the later pre-oxidation temperature is 262°C. The total pre-oxidation heat treatment time is 60 min, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the middle and later pre-oxidation times each accounting for 25%. During the initial pre-oxidation feeding process, the fiber tension is 10900 cN, the middle pre-oxidation fiber tension is 9800 cN, and the later pre-oxidation fiber tension is 8650 cN. The fiber oxygen content is 4.53% in the initial pre-oxidation, 6.53 wt% in the middle pre-oxidation, and 8.28 wt% in the later pre-oxidation. The fiber bulk density is 1.228 g / cm³ in the initial pre-oxidation. 3 The fiber density during the pre-oxidation stage was 1.325 g / cm³. 3 The fiber density in the later stage of pre-oxidation was 1.388 g / cm³. 3 .

[0095] The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃, and each stage lasting 15 seconds. The coke discharge port is located at 500℃ during the low-temperature carbonization process, and the fiber tension is 4456 cN. Following the low-temperature carbonization, high-temperature carbonization is carried out at temperatures ranging from 900℃, 1000℃, 1100℃, 1200℃, 1300℃, to 1400℃, with each stage lasting 15 seconds.

[0096] The carbon yield of large-tow carbon fiber is 50.8%. The furnace pressure of the low-temperature carbonization furnace increases by 22% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. In this embodiment, the carbon fiber production process requires shutdown and tar cleaning after half a month of continuous operation.

[0097] The tensile strength prepared by the above method is 4021 MPa; the tensile modulus is 240 GPa.

[0098] As can be seen from the data of Comparative Example 1 of the present invention, due to the significant change in fiber tension during the drying stage of Comparative Example 1, the fiber density of the pre-oxidation process is significantly increased, which in turn leads to a significant increase in the amount of tar produced during the low-temperature carbonization process; and if the comparative example uses the same fiber tension as the pre-oxidation stage of Example 6, the risk of fiber breakage is greatly increased. Therefore, Comparative Example 1 uses a smaller fiber tension than Example 6.

[0099] Comparative Example 2

[0100] The preparation method is basically the same as that in Example 5, except that: the initial pre-oxidation temperature is 226℃, the intermediate pre-oxidation temperature is 245℃, and the final pre-oxidation temperature is 267℃; the total pre-oxidation heat treatment time is 60 min, with the initial pre-oxidation heat treatment time accounting for 50% of the total pre-oxidation time, and the intermediate and final pre-oxidation times each accounting for 25%. During the initial pre-oxidation spinning process, the fiber tension is 9700 cN, the intermediate pre-oxidation fiber tension is 9986 cN, and the final pre-oxidation fiber tension is 8787 cN. The fiber oxygen content is 3.24 wt% in the initial pre-oxidation stage, 6.09 wt% in the intermediate pre-oxidation stage, and 7.26 wt% in the final pre-oxidation stage. The fiber bulk density is 1.145 g / cm³ in the initial pre-oxidation stage. 3 The fiber density during the mid-stage of pre-oxidation was 1.275 g / cm³, and the fiber density during the late-stage of pre-oxidation was 1.356 g / cm³. 3The low-temperature carbonization process employs a multi-stage gradient heating method, with temperatures ranging from 280℃, 380℃, 480℃, 580℃, 680℃, to 750℃. Each stage lasts for 15 seconds. The coke outlet is located at 500℃ during low-temperature carbonization, and the fiber tension during this process is 4396 cN. Following low-temperature carbonization, high-temperature carbonization is performed at temperatures of 900℃, 1000℃, 1100℃, 1200℃, 1300℃, and 1400℃, with each stage lasting 15 seconds. The carbon yield of the large-tow carbon fiber is 49.4%. The furnace pressure in the low-temperature carbonization furnace increases by 100% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; lower furnace pressure indicates less tar produced. In this embodiment, the carbon fiber production process requires shutdown and tar cleaning after ten consecutive days of operation.

[0101] The tensile strength prepared by the above method is 3522 MPa; the tensile modulus is 226 GPa.

[0102] As can be seen from the data of Comparative Example 2 of the present invention, due to the significant change in fiber tension in the early stage of pre-oxidation in Comparative Example 2, the fiber density of the prepared pre-oxidation process is significantly reduced, which in turn leads to a significant increase in the amount of tar produced in the low-temperature carbonization process.

[0103] Comparative Example 3

[0104] The preparation method is basically the same as that in Example 4, except that: the low-temperature carbonization treatment adopts a multi-stage gradient heating with temperatures of 280℃, 400℃, 500℃, 700℃, 800℃, and 950℃, and the heat treatment time for each stage is 15 seconds. The coke discharge port is located at 700℃ during the low-temperature carbonization process, and the fiber tension is 5287cN. After the low-temperature carbonization treatment, high-temperature carbonization is carried out with temperatures of 900℃, 1000℃, 1100℃, 1200℃, 1300℃, and 1400℃, and the heat treatment time for each stage is 15 seconds.

[0105] The carbon yield of large-tow carbon fiber is 48.3%. The furnace pressure of the low-temperature carbonization furnace increases by 200% within 48 hours. In this invention, the furnace pressure of the low-temperature carbonization furnace characterizes the change in tar content; the lower the furnace pressure, the less tar is produced. The method of this embodiment can operate continuously for three days without stopping to clean the tar during carbon fiber production.

[0106] The tensile strength prepared by the above method is 3178 MPa; the tensile modulus is 227 GPa.

[0107] As can be seen from the data in Comparative Example 3 of this invention, different temperatures and fiber tensions during low-temperature carbonization treatment can lead to an increase in tar content and a decrease in fiber strength.

[0108] As can be seen from the comparative data above, this invention strictly controls fiber tension based on the number of filaments and fineness, and controls the chemical structure of the pre-oxidation stage and the low-temperature carbonization stage through fiber tension control at each stage, thereby reducing tar content and possessing significant application value. In summary, the method of this invention can achieve the goal of reducing tar in the production process of large-tow carbon fibers, especially reducing tar in the production process of large-tow carbon fibers, which has significant technical advantages and can be used in the industrial production of large-tow carbon fibers.

Claims

1. A method for reducing tar during carbon fiber production, comprising the steps of drying and pre-oxidizing the precursor fiber to obtain pre-oxidized fiber, and subjecting the pre-oxidized fiber to low-temperature carbonization treatment; The fiber tension before and after the raw fiber drying step is independently selected from 0.01 to 0.04 times the product of the fiber bundle number and fineness. The pre-oxidation includes initial pre-oxidation, intermediate pre-oxidation and late pre-oxidation; during the yarn feeding process in each pre-oxidation stage, the fiber tension gradually decreases from the initial stage to the late stage. In the initial pre-oxidation stage, the fiber tension during the spinning process is 0.17 to 0.22 times the product of the number of filaments and the fineness; in the intermediate pre-oxidation stage, the fiber tension during the spinning process is 0.14 to 0.20 times the product of the number of filaments and the fineness; in the late pre-oxidation stage, the fiber tension during the spinning process is 0.12 to 0.18 times the product of the number of filaments and the fineness; where fiber tension is measured in cN and fineness is measured in dtex. The low-temperature carbonization process employs a multi-stage gradient heating method, with the temperature difference between adjacent temperature zones not exceeding 100℃; during the low-temperature carbonization process, the fiber tension is 0.06 to 0.09 times the product of the number of filament bundles and the fineness. The precursor fiber is a large-tow polyacrylonitrile precursor fiber; During the low-temperature carbonization process, the maximum temperature rise shall not exceed 800°C; The coke discharge port of the low-temperature carbonization is located at a temperature of 450~550℃ during the low-temperature carbonization process.

2. The method for reducing tar during carbon fiber production according to claim 1, characterized in that, The fiber tension before and after the raw fiber drying step is independently selected from 0.02 to 0.03 times the product of the number of filament bundles and the fineness.

3. The method for reducing tar during carbon fiber production according to claim 1, characterized in that, The drying step uses hot air blowing for drying.

4. The method for reducing tar during carbon fiber production according to claim 3, characterized in that, The hot air is blown in the direction of the yarn feed for drying; and / or, The temperature of the hot air is 100~120℃; and / or, The effective heating time of the hot air is 10~60s.

5. The method for reducing tar during carbon fiber production according to claim 1, characterized in that, During the initial pre-oxidation stage, the fiber tension during the spinning process is 0.18 to 0.20 times the product of the number of filament bundles and the fineness. During the intermediate pre-oxidation stage, the fiber tension during the spinning process is 0.16 to 0.18 times the product of the number of filament bundles and the fineness. During the later pre-oxidation stage, the fiber tension during the spinning process is 0.14 to 0.16 times the product of the number of filament bundles and the fineness.

6. The method for reducing tar during carbon fiber production according to claim 5, characterized in that, The initial pre-oxidation temperature is 210-230℃; The temperature for the intermediate pre-oxidation is 230-250℃; The temperature for the subsequent pre-oxidation is 250-270℃.

7. The method for reducing tar during carbon fiber production according to claim 5, characterized in that, The initial pre-oxidation time accounts for 40-60% of the total pre-oxidation time; The intermediate pre-oxidation time accounts for 20-30% of the total pre-oxidation time; The time for the later pre-oxidation accounts for 20-30% of the total pre-oxidation time.

8. The method for reducing tar during carbon fiber production according to claim 5, characterized in that, The oxygen content of the pre-oxidized fiber in the initial pre-oxidation stage is 3-5 wt%; The oxygen content of the pre-oxidized fiber in the intermediate pre-oxidation process is 5-7 wt%; The oxygen content of the pre-oxidized fiber in the later pre-oxidation process is 7-9 wt%.

9. The method for reducing tar during carbon fiber production according to claim 5, characterized in that, The density of the pre-oxidized fiber in the initial pre-oxidation stage is 1.18~1.22 g / cm³. 3 ; The density of the pre-oxidized fiber in the intermediate pre-oxidation process is 1.28~1.32 g / cm³. 3 ; The density of the pre-oxidized fiber body after subsequent pre-oxidation is 1.36~1.38 g / cm³. 3 .

10. The method for reducing tar during carbon fiber production according to claim 1, characterized in that, The starting temperature of the low-temperature carbonization is 20-30°C higher than the later temperature of the pre-oxidation.

11. The method for reducing tar during carbon fiber production according to claim 10, characterized in that, The starting temperature for low-temperature carbonization is selected from 270-300℃, and the maximum temperature for heating is selected from 700-760℃.

12. The method for reducing tar during carbon fiber production according to claim 11, characterized in that, Low-temperature carbonization in six temperature zones is employed.

13. The method for reducing tar during carbon fiber production according to claim 12, characterized in that, The starting temperature of the six-temperature zone low-temperature carbonization is selected from 280-290℃, and the ending temperature is selected from 720-750℃.

14. The method for reducing tar during carbon fiber production according to claim 1, characterized in that, During low-temperature carbonization, the fiber tension is 0.07 to 0.08 times the product of the number of filament bundles and the fineness.

15. A method for preparing carbon fiber using the method according to any one of claims 1-14, characterized in that, The carbon fiber preparation method further includes the following steps: After the low-temperature carbonization treatment, high-temperature carbonization is carried out to obtain carbon fibers.

16. The carbon fiber preparation method according to claim 15, characterized in that, The high-temperature carbonization temperature is 900-1400℃.