Preparation method for low-defect polyacrylonitrile-based carbon fiber

Abstract

The present disclosure discloses a preparation method of a low-defect polyacrylonitrile-based carbon fiber, and relates to the technical field of carbon fibers. The preparation method comprises: preparing a carbon fiber precursor; drawing the carbon fiber precursor to pass through a plurality of independent temperature-controlled pre-oxidation furnaces sequentially, the carbon fiber precursor in the pre-oxidation furnaces is given different degrees of drafting through the independent drive of each of the plurality of pre-oxidation furnaces, and the carbon fiber precursor is pre-oxidized to form a pre-oxidized tow; drawing the pre-oxidized tow to pass through a low-temperature carbonization furnace with a two-stage gas seal chamber structure to allow a low-temperature carbonization treatment to produce a low-carbon tow; drawing the low-carbon tow to pass through a high-temperature carbonization furnace with a tow dehumidification device to allow a high-temperature carbonization treatment to produce a high-carbon tow; drawing the high-carbon tow to pass through a one to two-stage surface treatment tank to allow a surface treatment; and the high-carbon tow is dried after sizing.

Description

[0001] The present disclosure is provided based on and claims priority to Chinese Patent Application 202111415584.5 titled "PREPARATION METHOD OF LOW-DEFECT POLYACRYLONITRILE-BASED CARBON FIBER" and filed on November 25, 2021, which is incorporated herein by reference in its entirety.Technical Field

[0002] The present disclosure relates to, but is not limited to, a preparation method of a low-defect polyacrylonitrile (PAN)-based carbon fiber.Background

[0003] Carbon fiber-reinforced epoxy resin composites have the highest specific strength and specific modulus among existing engineering materials. With the increasing maturity of the application technology of carbon fiber composites in fields such as pressure vessels and aerospace, a demand for high-performance carbon fibers on the market has increased hugely, especially a demand for high-performance PAN-based carbon fibers has increased most significantly. Therefore, how to further improve the performance of PAN-based carbon fibers has always been the focus and difficulty of technological development for carbon fibers.

[0004] During a low-temperature carbonization process, the weight loss of carbon fibers is usually 40% or more, non-carbon elements O, H and N in fibers escape in large quantities, and fibers undergoing a weight loss change from a solid state to a gas state, which inevitably leads to generation of a large amount of pyrolysis gas. If the waste gas and tar are not discharged in time, it will not only pollute the fibers running in the furnace chamber, produce local defects, and even lead to greater defects in the high temperature carbonization process, but also shorten the operation cycle of the production line and indirectly increase the operation cost of the production line. Therefore, the instantaneous waste discharge of a low-temperature carbonization furnace is usually the focus of the structural design and process optimization of the low-temperature carbonization furnace. From the perspective of improving the quality and efficiency, the single-line production capacity of carbon fibers is steadily increasing, which also puts forward advanced requirements for a waste discharge capacity of a low-temperature furnace. In addition, there is a contradiction in the control of a low-temperature carbonization process. That is, in order to reduce the adverse effects of waste gas and tar on tows, it is necessary to increase the exhaust gas discharge capacity as much as possible. However, with the improvement of waste discharge capacity, the sealing effect of the furnace head and tail will be greatly reduced, and will even cause the oxygen intake in the furnace to cause a sharp decline in performance of carbon fibers. Therefore, it often takes a long time to find the balance point of low-temperature carbonization state during daily process optimization, which is time-consuming and laborious and not conducive to the stability of product performance.

[0005] A low-temperature carbonization process (usually at 300°C to 1000°C) is dominated by a thermal decomposition reaction, and a high-temperature carbonization process (usually at 1000°C to 1500°C) is dominated by a thermal polycondensation reaction; and the two processes involve different drawing ratios. Therefore, a low-temperature carbonization furnace and a high-temperature carbonization furnace are usually designed separately, and an operation zone of 3 m to 5 m is usually left between the low-temperature carbonization furnace and the high-temperature carbonization furnace during actual production, such that high-heat fibers (usually with a temperature of higher than 150°C) at an outlet of the low-temperature carbonization furnace absorb the water vapor in the air when moving in the operation zone (usually about 1 min to 5 min) and the water vapor is brought into the high-temperature carbonization furnace. Although most of the water vapor brought by the tow will be blown out through the gas seal of the furnace mouth or discharged through a waste discharge pipe at the head of the high-temperature furnace, a part of the water vapor will still be brought into a furnace cavity, and then is combined with carbon atoms on fibers and escapes in the form of CO + H 2 or CO during high-temperature carbonization, resulting in defects such as voids and even pores of fibers.Summary

[0006] An overview of the subject described in detail in the present disclosure is provided below, which is not intended to limit the protection scope of the claims.

[0007] The present disclosure provides a preparation method of a low-defect PAN-based carbon fiber, comprising: preparing a carbon fiber precursor, wherein the carbon fiber precursor has a monofilament strength of higher than or equal to 7.0 cN / dtex and a fineness of 0.50 dtex to 0.70 dtex; drawing the carbon fiber precursor to pass through a plurality of independent temperature-controlled pre-oxidation furnaces sequentially to pre-oxidize the carbon fiber precursor to produce a pre-oxidized tow with a density of 1.340 g / cm 3< to 1.360 g / cm 3< , wherein a total drawing ratio of the plurality of pre-oxidation furnaces for the carbon fiber precursor is -12% to +5%; drawing the pre-oxidized tow to pass through a low-temperature carbonization furnace with a two-stage gas seal chamber structure to allow a low-temperature carbonization treatment to produce a low-carbon tow, wherein a head and a tail of the low-temperature carbonization furnace each are provided with a two-stage gas seal chamber, and a pressure difference between a first-stage gas seal chamber and a second-stage gas seal chamber of the two-stage gas seal chamber is 1 Pa to 10 Pa; and a drawing ratio of the low-temperature carbonization furnace for the pre-oxidized tow is +1% to +5%; drawing the low-carbon tow to pass through a high-temperature carbonization furnace with a tow dehumidification device to allow a high-temperature carbonization treatment to produce a high-carbon tow, wherein a drawing ratio of the high-temperature carbonization furnace for the low-carbon tow is -8% to -2%; drawing the high-carbon tow to pass through a one to two-stage surface treatment tank to allow a surface treatment, cleaning a high-carbon tow obtained after the surface treatment, and drying the high-carbon tow; and drawing the high-carbon tow into a sizing tank for sizing, and drying a sized high-carbon tow at 150°C.

[0008] Wherein, for each of the plurality of pre-oxidation furnaces, a pre-oxidation temperature is 200°C to 300°C, and a temperature control accuracy of a heating zone of each pre-oxidation furnace is within ±2°C.

[0009] Wherein, the first-stage gas seal chamber of the two-stage gas seal chamber of the low-temperature carbonization furnace is connected to a low-temperature furnace waste gas treatment system through a fan to maintain the pressure difference between the first-stage gas seal chamber and the second-stage gas seal chamber at 1 Pa to 10 Pa.

[0010] Wherein, a second-stage gas seal chamber located at the head of the low-temperature carbonization furnace comprises a heating device, and a heating temperature of the heating device is 150°C to 400°C.

[0011] Wherein, the low-temperature carbonization furnace comprises 6 to 8 first heating zones; and for each first heating zone, a carbonization temperature is 300°C to 1000°C and a heating rate is about 40°C / min to 100°C / min.

[0012] Wherein, the tail of the low-temperature carbonization furnace is provided with a first water-cooling system to control a temperature of the low-carbon tow drawn out from the low-temperature carbonization furnace at 150°C or lower.

[0013] Wherein, the tow dehumidification device is arranged at a furnace mouth of the high-temperature carbonization furnace, and there is a gap of 5 mm to 50 mm between the tow dehumidification device and the furnace mouth of the high-temperature carbonization furnace.

[0014] Wherein, the tow dehumidification device purges hot air from the middle of the high-temperature carbonization furnace to two ends of the high-temperature carbonization furnace, and the hot air has a temperature of 110°C to 150°C and a flow rate of 5 m / s or less.

[0015] Wherein, the high-temperature carbonization furnace comprises 4 to 8 second heating zones; and for each second heating zone, a carbonization temperature is 1000°C to 1600°C and a heating rate is 100°C / min to 150°C / min.

[0016] Wherein, the high-carbon tow is subjected to a surface treatment with an electrolyte, and the electrolyte is an ammonium salt electrolyte.

[0017] Wherein, the electrolyte comprises at least one selected from the group consisting of ammonium bicarbonate, monoammonium phosphate (MAP), and ammonium sulfate.

[0018] Wherein, the sizing tank is filled with a sizing agent, and the high-carbon tow is soaked in the sizing agent to size the high-carbon tow.

[0019] Wherein, the sizing agent comprises an epoxy resin, and a concentration of the sizing agent is 0.5% to 1.5%.

[0020] Other aspects of the present disclosure are understandable upon reading and understanding of the accompanying drawings and detailed description.Brief Description of the Drawings

[0021] The accompanying drawings incorporated into the specification and constituting a part of the specification illustrate the embodiments of the present disclosure, and are used together with the description to explain the principles of the embodiments of the present disclosure. In these accompanying drawings, similar reference numerals represent similar elements. The accompanying drawings in the following description illustrate some rather than all of the embodiments of the present disclosure. Those skilled in the art may obtain other accompanying drawings based on these accompanying drawings without creative efforts. FIG. 1 is a schematic diagram of a low-temperature carbonization furnace according to an exemplary embodiment; and FIG. 2 is a schematic diagram of a high-temperature carbonization furnace according to an exemplary embodiment. Reference numerals:

[0022] 1: first gas seal chamber, 2: second gas seal chamber, 3: fan; 4: nitrogen gas seal, 5: electric heating, 6: low-temperature furnace body, 7: third gas seal chamber, 8: first water-cooling system, 9: fourth gas seal chamber, 10: waste discharge pipe, 11: tow dehumidification device, 11-1: hot air pipe, 12: high-temperature furnace head gas seal, 13: high-temperature furnace head waste discharge pipe, 14: high-temperature furnace body, 15: second water-cooling system, and 16: furnace tail gas seal. Detailed Description of the Embodiments

[0023] The technical solutions in the embodiments of the present disclosure are described below clearly and completely with reference to the accompanying drawings in the embodiments of the present disclosure. Apparently, the described embodiments are merely some rather than all of the embodiments of the present disclosure. All other embodiments obtained by those skilled in the art based on the embodiments of the present disclosure without creative efforts should fall within the protection scope of the present disclosure. It should be noted that the embodiments in the present disclosure and features in the embodiments may be combined with each other in a non-conflicting manner.

[0024] An exemplary embodiment of the present disclosure provides a preparation method of a low-defect PAN-based carbon fiber, comprising the following steps: S110: Preparing a carbon fiber precursor, wherein the carbon fiber precursor has a monofilament strength of higher than or equal to 7.0 cN / dtex and a fineness of 0.50 dtex to 0.70 dtex.

[0025] The carbon fiber precursor can be supplied in a form of a coil, and the coil of the carbon fiber precursor is placed on an unwinding mechanism, such that the carbon fiber precursor can be continuously supplied through the unwinding mechanism.

[0026] S120: Drawing the carbon fiber precursor to pass through a plurality of independent temperature-controlled pre-oxidation furnaces sequentially to pre-oxidize the carbon fiber precursor to produce a pre-oxidized tow with a density of 1.340 g / cm 3< to 1.360 g / cm 3< , wherein a total drawing ratio of the plurality of pre-oxidation furnaces for the carbon fiber precursor is -12% to +5%.

[0027] Drawing the carbon fiber precursor to pass through two or more independent temperature-controlled pre-oxidation furnaces sequentially to undergo a pre-oxidization treatment, and a number of the pre-oxidation furnaces is set according to production needs. For example, the carbon fiber precursor can be drawn to pass through sequentially two pre-oxidation furnaces, three pre-oxidation furnaces, four pre-oxidation furnaces, or the like.

[0028] Each pre-oxidation furnace heats the carbon fiber precursor independently to pre-oxidize the carbon fiber precursor. In this embodiment, for each of the plurality of pre-oxidation furnaces, a pre-oxidation temperature is 200°C to 300°C, and a temperature control accuracy of a heating zone of each pre-oxidation furnace is within ±2°C. In this way, the carbon fiber precursor can be uniformly pre-oxidized by the plurality of pre-oxidation furnaces to produce the pre-oxidized tow.

[0029] Each pre-oxidation furnace is driven independently, and thus the plurality of pre-oxidation furnaces may exhibit different drawing ratios for the carbon fiber precursor. In this embodiment, a rate at which each pre-oxidation furnace is driven to draw the carbon fiber precursor can be adjusted to adjust a drawing ratio for the carbon fiber precursor, such that a total drawing ratio of the plurality of pre-oxidation furnaces for the carbon fiber precursor is -12% to +5%. For example, the total drawing ratio of the plurality of pre-oxidation furnaces for the carbon fiber precursor can be -12%, -10%, -8%, -6%, -4%, -2%, 0%, 1%, 3%, or 5%.

[0030] The pre-oxidized tow obtained by pre-oxidizing the carbon fiber precursor has a density of 1.340 g / cm 3< to 1.360 g / cm 3< . For example, the pre-oxidized tow may have a density of 1.340 g / cm 3< , 1.345 g / cm 3< , 1.350 g / cm 3< , 1.355 g / cm 3< , or 1.360 g / cm 3< .

[0031] S130: Drawing the pre-oxidized tow to pass through a low-temperature carbonization furnace with a two-stage gas seal chamber structure to undergo a low-temperature carbonization treatment to produce a low-carbon tow, wherein a head and a tail of the low-temperature carbonization furnace each are provided with a two-stage gas seal chamber, and a pressure difference between a first-stage gas seal chamber and a second-stage gas seal chamber of the two-stage gas seal chamber is 1 Pa to 10 Pa; and a drawing ratio of the low-temperature carbonization furnace for the pre-oxidized tow is +1% to +5%.

[0032] In a direction from the head of the low-temperature carbonization furnace to the tail of the low-temperature carbonization furnace, the pre-oxidized tow is drawn into the low-temperature carbonization furnace to allow the low-temperature carbonization treatment; and a second-stage gas seal chamber located at the head comprises a heating device, a heating temperature of the heating device is 150°C to 400°C, and a maximum temperature of the second-stage gas seal chamber located at the head is 400°C. The heating device is provided to heat the pre-oxidized tow, which is conducive to improving the low-temperature carbonization efficiency of the pre-oxidized tow.

[0033] In the direction from the head of the low-temperature carbonization furnace to the tail of the low-temperature carbonization furnace, the low-temperature carbonization furnace comprises 6 to 8 first heating zones arranged sequentially; and for each first heating zone, a carbonization temperature is 300°C to 1000°C and a heating rate is 40°C / min to 100°C / min. Carbonization temperatures of the first heating zones of the low-temperature carbonization furnace are roughly the same, such that the pre-oxidized tow is uniformly carbonized at a low temperature in the low-temperature carbonization furnace to produce the low-carbon tow. For example, the carbonization temperature of each first heating zone can be 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 900°C, or 1000°C.

[0034] The tail of the low-temperature carbonization furnace is provided with a first water-cooling system to control a temperature of the low-carbon tow drawn out from the low-temperature carbonization furnace at 150°C or lower.

[0035] A carbonization temperature of the low-temperature carbonization furnace is relatively high, and thus a temperature of the low-carbon tow produced after low-temperature carbonization of the pre-oxidized tow is also relatively high; and the low-carbon tow needs to enter the subsequent high-temperature carbonization furnace through the tail of the low-temperature carbonization furnace, and is cooled by the first water-cooling system to 150°C or lower.

[0036] In this embodiment, a rate at which the low-temperature carbonization furnace is driven to draw the pre-oxidized tow is controlled to control a drawing ratio; and a drawing ratio of the low-temperature carbonization furnace for the pre-oxidized tow is +1% to +5%, for example, the drawing ratio of the low-temperature carbonization furnace for the pre-oxidized tow can be +1%, +2%, +3%, +4%, or +5%.

[0037] S140: The low-carbon tow is drawn to pass through a high-temperature carbonization furnace with a tow dehumidification device to allow a high-temperature carbonization treatment to produce a high-carbon tow, wherein a drawing ratio of the high-temperature carbonization furnace for the low-carbon tow is -2% to -8%.

[0038] In this embodiment, the high-temperature carbonization furnace comprises a tow dehumidification device, and the tow dehumidification device is arranged at a side wherein a furnace mouth of the high-temperature carbonization furnace is located; and in a direction of travel of a tow, the tow dehumidification device is arranged at a front end of the furnace mouth of the high-temperature carbonization furnace, and there is a gap of 5 mm to 50 mm between the tow dehumidification device and the furnace mouth of the high-temperature carbonization furnace.

[0039] The tow dehumidification device has a height of 5 mm to 30 mm. For example, the tow dehumidification device can be a cylindrical structure with a height of 5 mm to 30 mm, wherein one opening of the cylindrical structure faces towards the low-temperature carbonization furnace, and the other opening of the cylindrical structure faces towards the furnace mouth of the high-temperature carbonization furnace; and the low-carbon tow enters from one opening of the cylindrical structure and enters the high-temperature carbonization furnace through the other opening of the cylindrical structure.

[0040] In a radial direction of the cylindrical structure, the tow dehumidification device purges hot air from a middle to two ends, and the hot air has a temperature of 110°C to 150°C and a flow rate of 5 m / s or less. The hot air is purged to remove moisture on a surface of the low-carbon tow, and the moisture on the surface of the low-carbon tow escapes between the tow dehumidification device and the furnace mouth of the high-temperature carbonization furnace, thereby ensuring that the low-carbon tow entering the high-temperature carbonization furnace is dry and does not have residual moisture. In this way, water vapor can be maximally prevented from entering the high-temperature carbonization furnace to damage a tow, such as to reduce or avoid surface defects such as microvoids and even micropores caused by a reaction between PAN-based carbon fibers and surface trace water at a high temperature during high-temperature carbonization of the PAN-based carbon fibers.

[0041] It can be understood that the cylindrical structure does not constitute a limitation to the tow dehumidification device, the tow dehumidification device can be a structure with a function of removing the moisture on the surface of the low-carbon tow, and the tow dehumidification device can be set to have any shape and any structure as needed.

[0042] In this embodiment, in a direction of travel of the tow, the high-temperature carbonization furnace comprises 4 to 8 second heating zones arranged sequentially, and 4, 5, 6, or 8 second heating zones can be arranged according to production needs. A carbonization temperature of each second heating zone can be 1000°C to 1600°C, such as 1000°C, 1200°C, 1500°C, or 1600°C; and a heating rate of each second heating zone can be 100°C / min to 150°C / min.

[0043] After entering the high-temperature carbonization furnace, the low-carbon tow passes through the second heating zones sequentially, and is uniformly carbonized at a high temperature in the high-temperature carbonization furnace to produce the high-carbon tow.

[0044] S150: The high-carbon tow is drawn to pass through a one to two-stage surface treatment tank to undergo a surface treatment, then cleaned, and dried.

[0045] The high-carbon tow out from the high-temperature carbonization furnace is subjected to the surface treatment with an electrolyte to reduce defects on a surface of the high-carbon tow, which facilitates the subsequent sizing for the high-carbon tow. The electrolyte is an ammonium salt electrolyte. For example, the electrolyte comprises at least one selected from the group consisting of ammonium bicarbonate, MAP, and ammonium sulfate.

[0046] An electric quantity for the surface treatment can be set according to application requirements, for example, the electric quantity can be set to 1 c / g to 50 c / g.

[0047] Then, the high-carbon tow is washed with deionized water to remove the residual electrolyte. Then the high-carbon tow is dried. In this embodiment, the high-carbon tow may be oven-dried, which does not constitute a limitation to the present disclosure.

[0048] S160: The high-carbon tow is drawn into a sizing tank for sizing, and then dried at 150°C.

[0049] The sizing tank is filled with a sizing tank, and after entering the sizing tank, the high-carbon tow is soaked in the sizing tank, and after entering the sizing tank, the high-carbon tow is soaked in the sizing tank, wherein the sizing agent is attached to a surface of the dry high-carbon tow to size the high-carbon tow.

[0050] The sizing agent can be an epoxy resin sizing agent, and the sizing agent comprises an epoxy resin. The sizing agent may have a concentration of 0.5% to 1.5%, for example, the concentration of the epoxy resin in the sizing agent is 0.5%, 0.8%, 1.2%, or 1.5%.

[0051] The sized high-carbon tow is dried at 150°C to obtain the low-defect PAN-based carbon fiber. The low-defect PAN-based carbon fiber prepared in this embodiment has few defects such as voids or pores on a surface and improved fiber strength.

[0052] In some embodiments, this embodiment is a description for the above embodiments, and this embodiment is different from the above embodiments in that a first-stage gas seal chamber of each two-stage gas seal chamber of the low-temperature carbonization furnace is connected to a low-temperature furnace waste gas treatment system through a fan to maintain a pressure difference between the first-stage gas seal chamber and a second-stage gas seal chamber at 1 Pa to 10 Pa. Through the pressure difference between the first-stage gas seal chamber and the second-stage gas seal chamber, pyrolysis gas generated due to low-temperature carbonization of the pre-oxidized tow in the low-temperature carbonization furnace is discharged through the fan to the waste gas treatment system of the low-temperature carbonization furnace, which improves the waste discharge capacity of the low-temperature carbonization furnace and avoids the adverse effect of pyrolysis gas on the pre-oxidized tow, thereby improving the quality of the produced low-defect PAN-based carbon fiber and further reducing defects of the carbon fiber.

[0053] The present disclosure is further described below in conjunction with embodiments.

[0054] In the preparation method of the low-defect PAN-based carbon fiber in this embodiment, after a carbon fiber precursor is unwound, the low defect polyacrylonitrile-based carbon fiber is obtained by pre-oxidation furnace, low temperature carbonization furnace, high temperature carbonization furnace, surface treatment, washing, sizing, drying and winding carbonization.

[0055] FIG. 1 shows a structure of a low-temperature carbonization furnace used in the preparation method of this embodiment. As shown in FIG. 1, the low-temperature carbonization furnace comprises a first gas seal chamber 1, a second gas seal chamber 2, a low-temperature furnace body 6, a third gas seal chamber 7, and a fourth gas seal chamber 9 sequentially. The first gas seal chamber 1 and the second gas seal chamber 2 constitute a two-stage gas seal chamber at the head of the low-temperature carbonization furnace, and the third gas seal chamber 7 and the fourth gas seal chamber 9 constitute a two-stage gas seal chamber at a tail of the low-temperature carbonization furnace. The second gas seal chamber 2 is provided with a nitrogen gas seal 4 and a heating device 5; the low-temperature carbonization furnace 6 is further provided with a waste discharge pipe 10, and a fan 3 is connected to the waste discharge pipe 10; and the third gas seal chamber 7 is provided with a first water-cooling system 8. The first gas seal chamber 1 and the fourth gas seal chamber 9 (tail) each are provided with a channel connected to the fan 3.

[0056] FIG. 2 shows a structure of a high-temperature carbonization furnace with a tow dehumidification device used in the preparation method of this embodiment. As shown in FIG. 2, the tow dehumidification device 11, a high-temperature furnace head gas seal 12, a high-temperature furnace body 14, and a high-temperature furnace tail gas seal 16 are arranged sequentially. The tow dehumidification device 11 is provided with a hot air pipe 11-1, a high-temperature furnace head waste discharge pipe 13 is provided at the high-temperature furnace head gas seal 12, and a second water-cooling system 15 is provided at the tail of the high-temperature carbonization furnace.

[0057] The preparation method of the low-defect PAN-based carbon fiber in this embodiment adopts the following implementation: A carbon fiber precursor is first prepared by a dry-jet wet spinning process, wherein the carbon fiber precursor has a monofilament strength of higher than or equal to 7.0 cN / dtex and a fineness of 0.50 dtex to 0.70 dtex.

[0058] The carbon fiber precursor is unwound and allowed to pass through three independent temperature-controlled pre-oxidation furnaces sequentially, wherein a pre-oxidation temperature of each pre-oxidation furnace is 200°C to 300°C, and a temperature control accuracy of an effective heating zone of a single pre-oxidation furnace is within ±2°C. The independent drive of each oxidation furnace allows different degrees of drawing for a tow in the oxidation furnaces, and a total drawing ratio of the three pre-oxidation furnaces is -12% to +5%. The pre-oxidized tow produced in this embodiment has a density of 1.340 g / cm 3< to 1.360 g / cm 3< .

[0059] Then, the pre-oxidized tow is drawn into a low-temperature carbonization furnace with an efficient tar-discharge system, and the pre-oxidized tow passes through the low-temperature carbonization furnace in a direction from a head of the low-temperature carbonization furnace to a tail of the low-temperature carbonization furnace. The head of the low-temperature carbonization furnace is provided with a first gas seal chamber 1 and a second gas seal chamber 2, wherein a pressure difference between the first gas seal chamber 1 and the second gas seal chamber 2 is 1 Pa to 10 Pa and a maximum heating temperature in the second gas seal chamber 2 at the head is 400°C; and the tail of the low-temperature carbonization furnace is provided with a third gas seal chamber 7 and a fourth gas seal chamber 9, wherein the third gas seal chamber 7 is provided with a first water-cooling device 8 to control a temperature of a tow out from the low-temperature carbonization furnace at 150°C or lower.

[0060] The low-temperature carbonization furnace is divided into 6 to 8 heating zones, and for each heating zone, a carbonization temperature is 300°C to 1000°C and a heating rate is about 40°C / min to 100°C / min; a drawing ratio of low-temperature carbonization is controlled by controlling a driving speed ratio of the low-temperature carbonization furnace before and after the low-temperature carbonization, and the drawing ratio is +1% to +5%; and the pre-oxidized tow is carbonized at a low temperature in the low-temperature carbonization furnace to produce a low-carbon tow. A temperature of the low-carbon tow out from the low-temperature carbonization furnace is lower than 150°C, for example, the temperature of the low-carbon tow out from the low-temperature carbonization furnace can be 145°C, 140°C, or lower.

[0061] The low-carbon tow is drawn to pass through a tow dehumidification device and then allowed to enter a high-temperature carbonization furnace.

[0062] There is a gap of 5 mm to 50 mm between the tow dehumidification device 11 and a furnace mouth of the high-temperature carbonization furnace, such that the moisture on the low-carbon tow can escape through the gap without being brought into the high-temperature carbonization furnace; an inner cavity of the tow dehumidification device 11 has a height of about 5 mm to 30 mm; the heating of the tow dehumidification device 11 is conducted by purging hot air from a middle to two ends, and the hot air has a temperature of 110°C to 150°C and a flow rate of 5 m / s or less; the tow dehumidification device 11 can heat air with heat resulting from a heat exchange between the first water-cooling device 8 of the low-temperature carbonization furnace and the low-carbon tow; and a head of the high-temperature carbonization furnace is provided with a nitrogen gas seal and a waste discharge pipe, and a tail of the high-temperature carbonization furnace is provided with a second water-cooling system to reduce a temperature of a tow out from the high-temperature carbonization furnace. In this embodiment, a temperature of a tow out from the high-temperature carbonization furnace is 150°C or lower. The high-temperature carbonization furnace is divided into 4 to 8 heating zones, and for each heating zone, a carbonization temperature is 1000°C to 1600°C and a heating rate is about 100°C / min to 150°C / min. The low-carbon tow is carbonized at a high temperature in the high-temperature carbonization furnace to produce a high-carbon tow. In this embodiment, a drawing ratio of high-temperature carbonization is controlled by controlling a driving speed ratio of the high-temperature carbonization furnace before and after the high-temperature carbonization, and the drawing ratio is -8% to -2%.

[0063] The high-carbon tow is then drawn to pass through a one to two-stage surface treatment tank, wherein the surface treatment tank is filled with an electrolyte to allow a surface treatment for the high-carbon tow. The electrolyte can comprise an ammonium salt electrolyte such as ammonium bicarbonate, MAP, and ammonium sulfate; an electric quantity for the surface treatment is set according to application requirements, and is preferably 1 c / g to 50 c / g; and a high-carbon tow obtained after the surface treatment is washed with water and then dried.

[0064] Then, the high-carbon tow is drawn to further pass through a sizing tank for sizing, wherein the sizing tank is filled with a sizing agent. The sizing agent can be an epoxy resin sizing agent; and the sizing agent comprises an epoxy resin, and the sizing agent may have a concentration of 0.5% to 1.5%, such as 0.5%, 0.8%, 1.2%, and 1.5%.

[0065] A sized high-carbon tow is dried in a drying oven at about 150°C to obtain the low-defect PAN-based carbon fiber.

[0066] Table 1 shows specific parameters of preparation in the preparation method of the low-defect PAN-based carbon fiber in Examples 1 to 5. It should be noted that the specific parameters in the preparation method of the low-defect PAN-based carbon fiber in the present disclosure are not limited to the data in Table 1. Table 1 Specific examples of the preparation method of the low-defect PAN-based carbon fiberExample 1Example 2Example 3Example 4Example 5Example 6Drawing of pre-oxidation0.950.950.950.920.920.94Temperature of pre-oxidation / °C274274274276276276Drawing of low-temperature carbonization1.021.021.021.021.021.01Temperature of low-temperature carbonization / °C700700700720720720Pressure of a first-stage gas seal chamber, Pa-2-4-5-5-4-3Pressure of a second-stage gas seal chamber, Pa-1-1-2-2-1-1Drawing of high-temperature carbonization0.960.960.960.970.970.95Temperature of high-temperature carbonization / °C145014501450140014001400Temperature of dehumidification in a high-temperature carbonization furnace / °C110110120120150150Dew point at a head of a high-temperature carbonization furnace, ppm-5.6-5.6-6.8-6.8-8.3-8.1

[0067] As a reference, Table 2 shows specific parameters of preparation of carbon fibers in Comparative Examples 1, 2, and 3. Table 2 Comparative examples of a preparation method of a carbon fiberComparative Example 1Comparative Example 2Comparative Example 3Drawing of pre-oxidation0.950.880.95Temperature of pre-oxidation / °C274274274Drawing of low-temperature carbonization1.021.021.02Temperature of low-temperature carbonization / °C700700700Pressure of a first-stage gas seal chamber, PaNoneNone-2Pressure of a second-stage gas seal chamber, Pa+1-1-1Drawing of high-temperature carbonization0.950.970.95Temperature of high-temperature carbonization / °C145014501450Temperature of dehumidification in a high-temperature carbonization furnace / °C / / / Dew point at a head of a high-temperature carbonization furnace, ppm8.38.38.3

[0068] A small-angle X-ray diffraction method was used to detect the size and distribution of voids or pores on each of the low-defect PAN-based carbon fibers prepared in Examples 1 to 5 and the carbon fibers prepared in Comparative Examples 1 to 3, and test results are shown in Table 3. Table 3 Test results of the carbon fibers prepared in Examples 1 to 5 and the carbon fibers prepared in Comparative Examples 1 to 3Exam- ple 1Exam- ple 2Exam- ple 3Example 4ExampleExam- ple 6Comparative Example 1Comparative Example 2Comparative Example 3Micropore length, Å605.9605.9582.7562.5571.0593.8786.2810.2651.2Micropore volume1.631.551.511.481.521.581.751.801.77Strength, MPa602161346215630462576183584557425888

[0069] It can be seen from Tables 1, 2 and 3 that a carbon fiber prepared by the preparation method of the low-defect PAN-based carbon fiber of the present disclosure has smaller micropore volume, few structural defects, and improved carbon fiber strength.

[0070] In the preparation method of the low-defect PAN-based carbon fiber of this embodiment, the discharge of pyrolysis gas in the low-temperature carbonization furnace is promoted through a pressure difference between the first gas seal chamber and the second gas seal chamber, which can avoid the adverse effect of pyrolysis gas on the pre-oxidized tow, improve a quality of the prepared low-defect PAN-based carbon fiber, and reduce defects of the carbon fiber; a tow entering the high-temperature carbonization furnace is dehumidified at a high temperature by the tow dehumidification device to prevent water vapor from damaging the tow during high-temperature carbonization and reduce defects generated during high-temperature carbonization; and the expansion of fiber defects caused by carbonization is further alleviated by adjusting drawing ratios of the pre-oxidation, low-temperature carbonization, and high-temperature carbonization.

[0071] The embodiments or implementations of this specification are described in a progressive manner, and each embodiment focuses on differences from other embodiments. The same or similar parts between the embodiments may refer to each other.

[0072] In the description of this specification, the description with reference to terms such as "an embodiment", "an exemplary embodiment", "some implementations", "a schematic implementation", and "an example" means that the specific feature, structure, material, or characteristic described in combination with the implementation(s) or example(s) is comprised in at least one implementation or example of the present disclosure.

[0073] In this specification, the schematic expression of the above term does not necessarily refer to the same implementation or example. Moreover, the described specific features, structures, materials, or characteristics may be combined with each other in an appropriate manner in any one or more implementations or examples.

[0074] It should be noted that in the description of the present disclosure, the terms such as "center", "top", "bottom", "left", "right", "vertical", "horizontal", "inner", and "outer" indicate the orientation or position relationships based on the accompanying drawings. These terms are merely intended to facilitate description of the present disclosure and simplify the description, rather than to indicate or imply that the mentioned apparatus or element must have a specific orientation and must be constructed and operated in a specific orientation. Therefore, these terms should not be construed as a limitation to the present disclosure.

[0075] It can be understood that the terms such as "first" and "second" used in the present disclosure can be used to describe various structures, but these structures are not limited by these terms. These terms are merely intended to distinguish one structure from another.

[0076] The same elements in one or more accompanying drawings are denoted by similar reference numerals. For the sake of clarity, various parts in the accompanying drawings are not drawn to scale. In addition, some well-known parts may not be shown. For the sake of brevity, a structure obtained by implementing a plurality of steps may be shown in one accompanying drawing. In order to understand the present disclosure clearly, many specific details of the present disclosure, such as the structure, material, size, processing process, and technology of the device, are described below. However, as those skilled in the art can understand, the present disclosure may not be implemented according to these specific details.

[0077] Finally, it should be noted that the above embodiments are merely intended to explain the technical solutions of the present disclosure, rather than to limit the present disclosure. Although the present disclosure is described in detail with reference to the above embodiments, those skilled in the art should understand that they may still make modifications to the technical solutions described in the above embodiments or make equivalent replacements to some or all technical features thereof; and these modifications or replacements do not make the essence of the corresponding technical solution depart from the scope of the technical solutions of the embodiments of the present application.Industrial applicability

[0078] In the preparation method of the low-defect PAN-based carbon fiber provided in this embodiment of the present disclosure, the discharge of pyrolysis gas in the low-temperature carbonization furnace is promoted through a pressure difference between the first gas seal chamber and the second gas seal chamber, which can avoid the adverse effect of pyrolysis gas on the pre-oxidized tow, improve the quality of the prepared low-defect PAN-based carbon fiber, and reduce defects of the carbon fiber; a tow entering the high-temperature carbonization furnace is dehumidified at a high temperature by the tow dehumidification device to prevent the water vapor from damaging the tow during high-temperature carbonization and reduce defects generated during high-temperature carbonization; and the expansion of fiber defects caused by carbonization is further alleviated by adjusting drawing ratios of the pre-oxidation, low-temperature carbonization, and high-temperature carbonization.

Claims

1. A preparation method of a low-defect polyacrylonitrile-based carbon fiber, comprising: preparing a carbon fiber precursor, wherein the carbon fiber precursor has a monofilament strength of higher than or equal to 7.0 cN / dtex and a fineness of 0.50 dtex to 0.70 dtex; drawing the carbon fiber precursor to pass through a plurality of independent temperature-controlled pre-oxidation furnaces sequentially, pre-oxidizing the carbon fiber precursor to produce a pre-oxidized tow with a density of 1.340 g / cm3 to 1.360 g / cm3, wherein a total drawing ratio of the plurality of pre-oxidation furnaces for the carbon fiber precursor is -12% to +5%; drawing the pre-oxidized tow to pass through a low-temperature carbonization furnace with a two-stage gas seal chamber structure to allow a low-temperature carbonization treatment to produce a low-carbon tow, wherein a head and a tail of the low-temperature carbonization furnace each are provided with a two-stage gas seal chamber, and a pressure difference between a first-stage gas seal chamber and a second-stage gas seal chamber of the two-stage gas seal chamber is 1 Pa to 10 Pa; and a drawing ratio of the low-temperature carbonization furnace for the pre-oxidized tow is +1% to +5%; drawing the low-carbon tow to pass through a high-temperature carbonization furnace with a tow dehumidification device to allow a high-temperature carbonization treatment to produce a high-carbon tow, wherein a drawing ratio of the high-temperature carbonization furnace for the low-carbon tow is -8% to -2%; drawing the high-carbon tow to pass through a one to two-stage surface treatment tank to allow a surface treatment, cleaning a high-carbon tow obtained after the surface treatment, and drying the high-carbon tow; and drawing the high-carbon tow into a sizing tank for sizing, and drying a sized high-carbon tow at 150°C.

2. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein for each of the plurality of pre-oxidation furnaces, a pre-oxidation temperature is 200°C to 300°C, and a temperature control accuracy of a heating zone of each pre-oxidation furnace is within ±2°C.

3. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the first-stage gas seal chamber of the two-stage gas seal chamber of the low-temperature carbonization furnace is connected to a low-temperature furnace waste gas treatment system through a fan to maintain the pressure difference between the first-stage gas seal chamber and the second-stage gas seal chamber at 1 Pa to 10 Pa.

4. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 3, wherein a second-stage gas seal chamber located at the head of the low-temperature carbonization furnace comprises a heating device, and a heating temperature of the heating device is 150°C to 400°C.

5. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the low-temperature carbonization furnace comprises 6 to 8 first heating zones; and for each first heating zone, a carbonization temperature is 300°C to 1000°C and a heating rate is about 40°C / min to 100°C / min.

6. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the tail of the low-temperature carbonization furnace is provided with a first water-cooling system to control a temperature of the low-carbon tow drawn out from the low-temperature carbonization furnace at 150°C or lower.

7. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the tow dehumidification device is arranged at a furnace mouth of the high-temperature carbonization furnace, and there is a gap of 5 mm to 50 mm between the tow dehumidification device and the furnace mouth of the high-temperature carbonization furnace.

8. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the tow dehumidification device purges hot air from the middle of the high-temperature carbonization furnace to two ends of the high-temperature carbonization furnace, and the hot air has a temperature of 110°C to 150°C and a flow rate of 5 m / s or less.

9. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1 or 5, wherein the high-temperature carbonization furnace comprises 4 to 8 second heating zones; and for each second heating zone, a carbonization temperature is 1000°C to 1600°C and a heating rate is 100°C / min to 150°C / min.

10. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the high-carbon tow is subjected to a surface treatment with an electrolyte, and the electrolyte is an ammonium salt electrolyte.

11. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 10, wherein the electrolyte comprises at least one selected from the group consisting of ammonium bicarbonate, monoammonium phosphate and ammonium sulfate.

12. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 1, wherein the sizing tank is filled with a sizing agent, and the high-carbon tow is soaked in the sizing agent to size the high-carbon tow.

13. The preparation method of the low-defect polyacrylonitrile-based carbon fiber according to claim 12, wherein the sizing agent comprises an epoxy resin, and a concentration of the sizing agent is 0.5% to 1.5%.

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