Method for efficiently preparing modified polyacrylonitrile-based carbon fiber

Abstract

The invention belongs to the technical field of polyacrylonitrile-based fibers, and relates to a method for efficiently preparing modified polyacrylonitrile-based carbon fibers, which comprises the following steps: carrying out solution polymerization on a carboxyl-containing comonomer and acrylonitrile, and mixing with a guanidine carbonate solution to obtain a modified spinning solution; and carrying out spinning, heat treatment, oxidation, low-temperature carbonization and high-temperature carbonization to obtain the modified polyacrylonitrile-based carbon fiber. Compared with the prior art, the method has the advantages that the guanidine salt modifier is added into the carboxyl-containing acrylonitrile copolymer solution, so that the defect that the heat resistance of a core layer is insufficient when the fibers are subjected to an inert atmosphere pretreatment process can be overcome, and the carbon yield is increased to 52.5% or above from unmodified 48.4%. Meanwhile, the modifier can be uniformly distributed in the fiber, and compared with impregnation modification, the sheath-core structure can be reduced, and the mechanical property of the fiber can be improved. Especially for large tows of 24k or above, the cv value of a single fiber in the tows can be obviously reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyacrylonitrile-based fibers, and relates to a method for efficiently preparing modified polyacrylonitrile-based carbon fibers. Background Art

[0002] The production of polyacrylonitrile (PAN)-based carbon fibers involves processes such as polymerization, spinning, oxidation, and carbonization. Among them, the oxidation process usually takes 60 minutes. Limited by the oxidation efficiency, the oxidation equipment is bulky, and the energy consumption accounts for about 40% of the entire carbonization process. In recent years, with the increasingly fierce price competition in carbon fibers, the research on processes and equipment for improving the oxidation efficiency of carbon fibers has become a hot topic in the industry.

[0003] Chinese Patent CN201780096765.1 proposes a rapid oxidation fiber preparation process. The raw silk is first heat-treated in a nitrogen atmosphere for a certain period of time to form fibers with a certain degree of cyclization, and then enters the air atmosphere for oxidation reaction, which can significantly shorten the oxidation time. However, the inventor's research found that if only pre-cyclization treatment is carried out in an inert atmosphere, for large tows of 24K and above, the mass transfer and heat transfer of the tow will be restricted, and the cv value of the cyclization reaction between single filaments is relatively high, and obvious color differences can be observed visually.

[0004] Chinese Patent CN201310100469.8 adopts a pre-treatment process before low-carbonization to improve the carbon yield. By increasing the degree of low-temperature carbonization reaction and improving the heat resistance of the fibers, the purpose of improving the carbon yield is achieved. However, this method is essentially an extension of low-temperature carbonization, which reduces the fiber production efficiency and increases the fiber production cost.

[0005] Chinese Patent CN202011540059.1 proposes a method of using phosphoric acid as a modification additive to improve the carbon yield of PAN-based carbon fibers. However, the inventor's research found that due to the good water solubility of phosphoric acid, it is very easy to be lost during the double-diffusion process, resulting in a decline in the modification effect.

[0006] Chinese Patent CN202010762400.1 proposes an impregnation modification method, using a nitrogen-phosphorus compound as a modifier to impregnate and modify the raw silk, which can significantly improve the carbon yield. However, the disadvantage of this process is that due to the relatively dense structure of the raw silk, small molecule modifiers mainly adhere to the outer surface of the fiber and are difficult to penetrate into the fiber interior. Especially during the rapid oxidation process, the high-concentration small molecule modifiers on the surface will accelerate the cortical reaction, aggravating the skin-core structure.

[0007] However, under this process, due to the rapid formation of the skin-core structure of the fiber, the diffusion rate of oxygen in the fiber radial direction is inhibited. Even if the fiber bulk density is sufficient to pass through low-temperature carbonization, due to the lack of oxygen structure in the core layer, insufficient intermolecular cross-linking cannot be formed, resulting in insufficient heat resistance of the fiber during the low-carbonization process and affecting the carbon yield. Summary of the Invention

[0008] The object of the present invention is to provide a method for efficiently preparing modified polyacrylonitrile-based carbon fibers. By adding guanidine salts to an acrylonitrile copolymer solution and performing spinning, modified precursor fibers are obtained. The introduction of guanidine salts improves the fiber cyclization rate and the degree of intermolecular crosslinking, solves the shortcoming of insufficient core layer heat resistance in the inert gas pretreatment oxidation process of the existing AN-IA copolymerization system, and improves the carbon yield. Modification is carried out by adding guanidine salts to the spinning solution. The amount of the modifier used is small and it is evenly distributed in the fiber, which is beneficial to improving the carbonization performance of the fiber.

[0009] The object of the present invention can be achieved by the following technical solutions:

[0010] A method for efficiently preparing modified polyacrylonitrile-based carbon fibers, comprising: performing solution polymerization of a carboxyl-containing comonomer and acrylonitrile, and then mixing with a guanidine carbonate solution to obtain a modified spinning solution; spinning, heat treatment, oxidation, low-temperature carbonization, and high-temperature carbonization to obtain modified polyacrylonitrile-based carbon fibers.

[0011] Specifically, the method includes: performing solution polymerization of at least one carboxyl-containing comonomer and acrylonitrile to obtain an acrylonitrile copolymer solution; adding a guanidine carbonate solution to the polymer solution to obtain a modified spinning solution; spinning the modified spinning solution to obtain modified precursor fibers; pretreating the modified precursor fibers in an inert atmosphere, and then sequentially performing air oxidation, low-temperature carbonization, and high-temperature carbonization to obtain modified polyacrylonitrile-based carbon fibers.

[0012] Further, the carboxyl-containing comonomer is selected from one or more of acrylic acid, methacrylic acid, itaconic acid, monomethyl itaconate, monoethyl itaconate, monon-propyl itaconate, monoisopropyl itaconate, monon-butyl itaconate, monoisobutyl itaconate, monoter-butyl itaconate, maleic acid, methyl maleic acid, maleic anhydride, fumaric acid, or methyl fumaric acid;

[0013] The mass ratio of the carboxyl-containing comonomer to acrylonitrile is 20:0.3 - 0.5, the polymerization reaction temperature is 60 - 70 °C, and the reaction time is 20 - 28 h.

[0014] Further, the polymerization solvent used is selected from one or more of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, aqueous sodium thiocyanate solution, or aqueous zinc chloride solution.

[0015] Further, after the solution polymerization, a polymerization product mixture is obtained. The polymerization product mixture is subjected to reduced-pressure heating to remove acrylonitrile monomers, and then defoamed by heating and standing.

[0016] The guanidine carbonate solution is added between the degassing outlet and the degassing inlet, or the guanidine carbonate solution is added between the degassing outlet and the pre-spinning storage tank.

[0017] Furthermore, the total acrylonitrile monomer content in the polymer solution after degassing is 3000 ppm.

[0018] Preferably, the total acrylonitrile monomer content in the polymer solution after degassing is 2000 ppm.

[0019] Furthermore, the molar ratio of the guanidine carbonate to the carboxyl group in the copolymer is 0.2 to 2.

[0020] Preferably, the molar ratio of the guanidine carbonate to the carboxyl group in the copolymer is 0.5 to 1.5.

[0021] Furthermore, the guanidine carbonate is added in an intermittent or continuous manner.

[0022] Furthermore, the modified polymer solution is spun by a wet spinning process or a dry-jet wet spinning process.

[0023] When the modified raw silk is heat-treated in an inert atmosphere, the gas used can be nitrogen or argon at a temperature of 200 to 350 °C, preferably nitrogen; the treatment time is 0.5 to 10 min, preferably 1 to 5 min; the draw ratio is 0.9 to 1.5, preferably 0.95 to 1.2.

[0024] During the heat treatment in an inert atmosphere, the tow tension range is controlled within 60 to 240 cN / k (k is the tow specification). Preferably, the tension range is controlled within 80 to 200 cN / k.

[0025] At a certain temperature, the modified raw silk first undergoes cyclization and dehydrogenation reactions in an inert or vacuum and other oxygen-free atmospheres, accompanied by a small amount of cracking reactions. Usually, FT-IR is used to calculate the cyclization index and dehydrogenation index of the fiber. Preferably, the cyclization index (Ic) of the fiber after heat treatment in an inert atmosphere is 0.5 to 0.7, and the dehydrogenation index (Id) is 0.3 - 0.6.

[0026] Furthermore, the heat treatment is carried out in a protective atmosphere, entering an air oxidation device with 2 to 8 temperature zones, preferably 2 to 6 temperature zones; the temperature range is 180 to 290 °C, preferably 200 to 280 °C, the total effective residence time is 8 to 45 min, preferably 8 to 30 min; the total draw ratio is 0.9 to 1.1 to obtain pre-oxidized fibers. More preferably, a 2 - 4 temperature zone device, a temperature range of 210 to 275 °C, and the total effective residence time is controlled within 8 to 20 min are used to achieve higher production efficiency. The density of the pre-oxidized fibers obtained through the above process is controlled at 1.30 to 1.42 g / m. Preferably, the density of the pre-oxidized fibers is controlled within 1.31 to 1.38 g / m 3 。

[0027] Further, the oxidation is carried out in air. At 180 - 250 °C, the tension of the precursor fiber is controlled at 80 - 120 cN / k; at 250 - 280 °C, the tension of the precursor fiber is controlled at 100 - 160 cN / k.

[0028] Further, in the low-temperature carbonization, the low-temperature carbonization is set with 2 - 8 temperature zones, the temperature is 300 - 1000 °C, the effective residence time is 0.5 - 6 min, and the draw ratio is 0.9 - 1.2. Preferably, it is set with 3 - 6 temperature zones, the temperature is 350 - 850 °C, the effective residence time is 0.5 - 3 min, and the draw ratio is 0.95 - 1.15.

[0029] Further, in the high-temperature carbonization, it is set with 2 - 10 temperature zones, the temperature is 900 - 1500 °C, the effective residence time is 0.5 - 6 min, and the draw ratio is 0.93 - 0.98. Preferably, the high-temperature carbonization is set with 3 - 8 temperature zones, the effective residence time is 0.5 - 3 min, and the draw ratio is 0.94 - 0.97.

[0030] Compared with the prior art, the present invention has the following characteristics:

[0031] By adding a guanidine salt modifier to the acrylonitrile copolymer solution containing carboxyl groups, the present invention can improve the disadvantage of insufficient heat resistance of the core layer when the fiber is pretreated with an inert atmosphere, and increase the carbon yield from 48.4% of the unmodified one to more than 52.5%. At the same time, the present invention can achieve uniform distribution of the modifier in the fiber. Compared with impregnation modification, it can reduce the skin-core structure and improve the mechanical properties of the fiber. Especially for large tows above 24k, the present invention can significantly reduce the cv value of single fibers in the tow. Description of the Drawings

[0032] Figure 1 It is a process flow chart of a method for efficiently preparing modified polyacrylonitrile-based carbon fiber in an embodiment;

[0033] Figure 2 It is a schematic diagram of an implementation device of a method for efficiently preparing modified polyacrylonitrile-based carbon fiber in an embodiment;

[0034] Description of the marks in the figure:

[0035] 1 - polymerization kettle, 2 - monomer removal kettle, 3 - degassing kettle, 4 - metering pump, 5 - first plunger pump, 6 - static mixer, 7 - precision filter, 8 - second plunger pump. Detailed Embodiments

[0036] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are implemented on the premise of the above technical solutions of the present invention, and detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0037] In the following embodiments, the parts are all by mass. The tensile test standard is GB / T 3362-2017.

[0038] Embodiment

[0039] As Figure 1 shown, a method for efficiently preparing modified polyacrylonitrile-based carbon fiber, and the device used is as Figure 2 shown, including a polymerization kettle 1, a degassing kettle 2, and a defoaming kettle 3 connected in series in sequence. The two kettles are connected in series through a metering pump 4, and it includes:

[0040] 1. Preparation and modification of spinning solution

[0041] Put 20 parts of acrylonitrile, 0.4 part of itaconic acid, and 80 parts of dimethyl sulfoxide (DMSO) into the polymerization kettle 1, stir and heat up to 65 °C, add 0.12 part of azobisisobutyronitrile, and react at a constant temperature for 24 h to obtain a copolymer solution with a solid content of 19.5% and an intrinsic viscosity of 1.8 dl / g.

[0042] Transport the above copolymer solution into the degassing kettle 2 at a rate of 50 kg / h, and remove the unreacted acrylonitrile monomer at 65 °C and a pressure of 1 kPa. The solid content of the polymer solution is 20.5%, the intrinsic viscosity is 1.73 dl / g, and the acrylonitrile monomer content is 1500 ppm.

[0043] Preparation of guanidine carbonate solution: Mix 0.6 part of guanidine carbonate with 0.9 part of water evenly, and then add 3 parts of DMSO and mix evenly.

[0044] Set the flow rate of the polymer solution to 50 kg / h, the flow rate of the guanidine carbonate solution to 1.5 kg / h, and the molar ratio of guanidine carbonate to carboxyl group to be R g / c = 0.8. As Figure 2 shown, use the first plunger pump 5 to continuously pump the guanidine carbonate solution into it, and enter the static mixer 6 together with the polymer solution at the outlet of the degassing kettle 2. The mixed material enters the defoaming kettle 3, and keep the temperature in the kettle at 65 °C and the pressure at 1 kPa to complete defoaming.

[0045] The defoamed polymer solution is sent to the spinning pre-spinning unit through a precision filter 7.

[0046] 2. Preparation of precursor fiber

[0047] Using a 24k wet spinning plate with an aperture of 0.055μm, passing through three coagulation baths with concentrations of 75%, 45%, and 25% in sequence, drawing 1.6 times, then successively washing with water at 70°C, drawing 2 times in a water bath at 90°C, drying at 150°C, drawing 3 times with steam at 140°C, and finally drying and shaping at 120°C to obtain the raw fiber.

[0048] 3. Oxidation and carbonization

[0049] The obtained 24k raw fiber is input into a pretreatment furnace filled with nitrogen at a constant tension of 700 cN through a yarn rack. The pretreatment temperature is 255°C, the time is 3 min, the drawing ratio is 1.05 times, and the tow tension is 2800 cN. The cyclization index of the pre-cyclized fiber is 0.59, and the dehydrogenation index is 0.43. After pretreatment, the fiber enters a four-temperature zone oxidation furnace, and the oxidation process is shown in Table 1.

[0050] Table 1

[0051] Oxidation temperature zone Draft ratio Temperature (°C) Tension (cN) Residence time Temperature zone 1 1 247 2500～2700 3 min Temperature zone 2 0.99 255 2500～2700 3 min Temperature zone 3 0.985 265 2500～2700 3 min Temperature zone 4 0.985 272 2500～2700 3 min

[0052] The parameters of the obtained oxidized fiber are as follows: bulk density 1.36 g / cm 3 , oxygen content 9.4%, modulus 8.5 GPa, elongation 19%.

[0053] The pre-oxidized fiber enters a six-temperature zone low-temperature carbonization furnace with a temperature profile of 350, 400, 450, 500, 600, and 650°C, drawing ratio 1.05, tension 1200 cN, and residence time 75 s to complete low-temperature carbonization.

[0054] The low-temperature carbonized fiber enters a four-temperature zone high-temperature carbonization furnace with a temperature profile of 1050, 1150, 1250, and 1350°C, drawing ratio 0.965, tension 3000 cN, and residence time 50 s to complete high-temperature carbonization.

[0055] The density of the obtained carbon fiber is 1.79 g / cm 3 , and the mechanical properties of the single carbon fiber filament are as follows: tensile strength is 4.2 GPa, tensile modulus is 240 GPa, and elongation at break is 1.76%. The carbonization yield is 53%.

[0056] The preparation steps of other embodiments of the present invention are basically the same as those of Example 1, except as shown in Table 2. Among them, point A is added through the first plunger pump 5, and B is added through the second plunger pump 8.

[0057] Table 2 Examples 2 - 7 and Comparative Examples

[0058]

[0059]

[0060] Comparative Example 1:

[0061] The addition amount of guanidine carbonate is 0, and the rest is the same as in Example 1. The density of the pre-oxidized fiber body is 1.33 g / cm 3 , Ic = 0.41, Id = 0.31, the carbon yield is 48.4%, and the amount of low-carbon furnace tar is large. After carbonization of the pre-oxidized fiber, the tensile strength is 3.88 Gpa, the modulus is 202 Gpa, and the elongation at break is 2.1%.

[0062] Comparative Example 2:

[0063] Guanidine carbonate is added through the second plunger pump 8, and the rest is the same as in Example 3. The obtained pre-oxidized fiber has Ic = 0.58, Id = 0.45, the carbonization yield of the fiber is 52.5%, the tensile strength is 4.3 Gpa, the modulus is 235 Gpa, and the elongation at break is 1.77%.

[0064] Comparative Example 3:

[0065] In the raw silk preparation stage, the spinning solution preparation and modification stage are the same as in Example 1; in the oxidation and carbonization stage: the temperature of the pretreatment furnace is room temperature, the atmosphere is air, and the rest of the conditions are the same as in Example 1. The density of the oxidized fiber body is 1.25 g / cm 3 , and the heat resistance of the oxidized fiber is insufficient and it cannot pass through the low-temperature carbonization furnace.

[0066] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A method for efficiently preparing modified polyacrylonitrile-based carbon fiber, characterized in that, comprising: performing solution polymerization on a carboxyl-containing comonomer and acrylonitrile, and then mixing with a guanidine carbonate solution to obtain a modified spinning solution; spinning, heat treatment, oxidation, low-temperature carbonization, and high-temperature carbonization to obtain modified polyacrylonitrile-based carbon fiber.

2. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, the carboxyl-containing comonomer is selected from one or more of acrylic acid, methacrylic acid, itaconic acid, monomethyl itaconate, monoethyl itaconate, monon-propyl itaconate, monoisopropyl itaconate, monon-butyl itaconate, monoisobutyl itaconate, monoter-butyl itaconate, maleic acid, methyl maleic acid, maleic anhydride, fumaric acid, or methyl fumaric acid; the mass ratio of the carboxyl-containing comonomer to acrylonitrile is 20:0.3 - 0.5, the polymerization reaction temperature is 60 - 70 °C, and the reaction time is 20 - 28 h.

3. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, the polymerization solvent used is selected from one or more of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, aqueous sodium thiocyanate solution, or aqueous zinc chloride solution.

4. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, after the solution polymerization, a polymerization product mixture is obtained, and the acrylonitrile monomer in the polymerization product mixture is removed by reduced pressure heating, and then defoamed by heating and standing; the guanidine carbonate solution is added between the monomer removal outlet and the defoaming inlet, or the guanidine carbonate solution is added between the defoaming outlet and the spinning pre-storage tank.

5. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, the molar ratio of guanidine carbonate to carboxyl in the copolymer is 0.2 - 2.

6. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 5, characterized in that, the molar ratio of guanidine carbonate to carboxyl in the copolymer is 0.5 - 1.

5.

7. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, the heat treatment is carried out in a protective atmosphere, and enters a 2 - 8 temperature zone air oxidation device; the temperature range is 180 - 290 °C, the total effective residence time is 8 - 45 min; the total draw ratio is 0.9 - 1.1, to obtain pre-oxidized fiber.

8. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, the oxidation is carried out in air, at 180 - 250 °C, the raw fiber tension is controlled at 80 - 120 cN / k; at 250 - 280 °C, the raw fiber tension is controlled at 100 - 160 cN / k.

9. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, in the low-temperature carbonization, the low-temperature carbonization is set with 2 - 8 temperature zones, the temperature is 300 - 1000 °C, the effective residence time is 0.5 - 6 min, and the draw ratio is 0.9 - 1.

2.

10. The method for efficiently preparing modified polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, in the high-temperature carbonization, a temperature zone setting of 2 to 10 is adopted, the temperature is 900 to 1500 °C, the effective residence time is 0.5 to 6 min, and the draw ratio is 0.93 to 0.98.

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