Carbon fiber manufacturing method

Abstract

To provide a carbon fiber and a manufacturing method thereof.SOLUTION: By controlling a ratio of an oil agent, relations between a surface tension of the oil agent and a particle size are controlled, and moreover, impregnation of the oil agent to the inside of a carbon fiber is avoided, thus a carbon fiber having both of a low oil agent residual amount and high strength is produced.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing carbon fibers, and more particularly to high-strength carbon fibers and a method for producing the same. [Background technology]

[0002] Carbon fiber has low density, acid and alkali resistance, electrical conductivity, resistance to thermal expansion and contraction, and excellent mechanical properties. Therefore, it is widely used in aerospace, high-pressure gas cylinders, wind turbine blades, the automotive industry, cable cores, civil engineering reinforcement, sports and leisure equipment, military industry, biomedical equipment, and other fields. In recent years, with growing awareness of environmental protection, the demand for high-pressure gas cylinders used in fuel cell vehicles has rapidly increased, leading to a growing demand for high-strength carbon fiber. The immediate goal is to increase the burst strength of gas cylinders to increase hydrogen payloads and reduce vehicle weight, thereby extending the driving range of fuel cell vehicles.

[0003] Carbon fibers can be classified into polyacrylonitrile (PAN), rayon, pitch, etc., depending on the raw material of the raw yarn. In conventional carbon fiber processes, the raw material is drawn into a raw yarn in a spinning process, and then stabilization treatments such as oxidation and cyclization are performed at 200°C to 300°C. Next, a carbonization reaction such as high-temperature calcination is performed at a temperature of 300°C to 2000°C in an inert gas (e.g., nitrogen gas, argon gas, helium gas) atmosphere, and non-carbon elements such as nitrogen, hydrogen, and oxygen are removed to produce a carbon fiber product. Summary of the Invention [Problem to be solved by the invention]

[0004] However, during the stabilization and high-temperature carbonization processes, the polymers can melt due to heat, causing problems such as fusion between the individual fibers of the tow or direct combustion of the raw yarn. Furthermore, the resulting carbon fiber can suffer from defects such as fuzz and yarn breakage. These defects can lead to uneven resin impregnation during subsequent processing to produce carbon fiber composites, resulting in reduced physical properties and poor appearance. Therefore, to prevent these problems, a high-temperature-resistant oil can be applied to the raw yarn during the raw yarn spinning process. The oil must be able to withstand temperatures above 200°C, and typically polydimethylsiloxane (silicone oil) or modified silicone oils that have been ammonized, epoxidized, or esterified are used.

[0005] Before the stabilization treatment such as oxidation or cyclization is completed, silicone oil or modified silicone oil is applied to the surface of the raw yarn to give the raw yarn a heat-resistant protective effect and prevent the single yarns from fusing together or burning. However, if the oil agent particles penetrate into the fiber, they will react during the subsequent high-temperature baking to form silicon oxide (SiO x ), silicon carbide (SiC), silicon nitride (Si x N y ) are produced. If such silicides remain inside the carbon fiber, the carbon-carbon bond is inhibited, preventing the formation of a graphite structure, resulting in structural defects and a decrease in the strength of the carbon fiber. Furthermore, when force is applied to the carbon fiber, the silicides act as impurities inside the carbon fiber, causing stress concentration and resulting in a decrease in the physical properties of the carbon fiber. In addition, the high hardness of the silicides can cause wear within the carbon fiber, expanding the defect size and further decreasing the physical properties of the carbon fiber.

[0006] In view of this, it is desirable to provide a method for producing carbon fibers that can maintain the oil adhesion rate of raw yarns, prevent oil from remaining inside the carbon fibers, simultaneously avoid defects due to fusion and burning of single yarns, and produce carbon fibers with high strength. [Means for solving the problem]

[0007] One aspect of the present invention provides a method for producing carbon fibers in which the relationship between the surface tension and particle size of an oil agent is controlled to reduce penetration of the oil agent into the interior of the carbon fibers, thereby producing carbon fibers with high strength.

[0008] Another aspect of the present invention provides a carbon fiber produced by the above-described aspect, which has both a low residual oil content and high strength.

[0009] According to one aspect of the present invention, there is provided a method for producing carbon fibers, the method comprising: A polyacrylonitrile copolymer is dissolved in a solvent to obtain a spinning solution. The spinning dope is then subjected to a coagulation process to obtain a tow. Subsequently, the tow is subjected to an oiling process with an oil to obtain an oiled raw yarn. The oil solution consists of 80 wt% ammoniated modified silicone oil and 20 wt% polyethylene oxide and polypropylene oxide copolymer. The surface tension of the oil (σ [mN / m]) is 30mN / m to 70mN / m, and the particle size of the oil (R [nm]) is 10nm to 72nm. The relationship between the surface tension of the oil and the particle size is , σ +(R / 2) 0.5 =36 The following formula is satisfied. The oiled yarn is subjected to a dry densification process to obtain carbon fiber yarn. Subsequently, the carbon fiber raw yarn is subjected to a calcination process to obtain carbon fiber. The residual silicon content in the carbon fiber is 500 ppm to 2500 ppm. According to ASTM D 4018-99 The strength of carbon fiber is greater than 5000 MPa.

[0010] According to one embodiment of the present invention, the polyacrylonitrile copolymer has an intrinsic viscosity of 1.5 to 3.5.

[0011] According to one embodiment of the present invention, the pore size of the tow is 20 nm to 140 nm.

[0015] According to another aspect of the present invention, there is provided a carbon fiber produced by the above aspect.

[0017] According to one embodiment of the present invention, the ratio of the internal silicon content to the surface silicon content of the carbon fiber is less than or equal to 0.7. [Effects of the Invention]

[0019] The carbon fiber manufacturing method and the manufactured carbon fiber to which the present invention is applied reduce the penetration of the oil agent into the carbon fiber by controlling the relationship between the surface tension and particle size of the oil agent, thereby manufacturing carbon fiber that has both low oil agent residual amount and high strength. [Brief explanation of the drawings]

[0020] Aspects of the present disclosure may be better understood by reference to the following detailed description, taken in conjunction with the accompanying drawings, in which: It should be noted that, in accordance with standard industry practice, many features are not drawn to scale. In practice, the size of many features may be arbitrarily increased or decreased for clarity of discussion. [Figure 1] 1 is a flowchart illustrating a method for manufacturing carbon fibers according to some embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] As described above, the present invention provides a method for producing carbon fibers, which reduces penetration of an oil agent into the interior of carbon fibers by controlling the relationship between the surface tension and particle size of the oil agent, thereby producing carbon fibers that have both a low residual oil amount and high strength, and the produced carbon fibers.

[0022] See FIG. 1, a flowchart illustrating a method 100 for producing carbon fiber according to some embodiments of the present invention. First, in operation 110, a polyacrylonitrile copolymer is dissolved in a solvent to obtain a spinning solution. In some embodiments, the polyacrylonitrile copolymer is prepared by copolymerizing a monomer solution containing acrylonitrile and one to three comonomers. In some embodiments, to improve the physical properties of the carbon fiber, the acrylonitrile concentration is preferably 95 wt% or more, and the total comonomer concentration is preferably less than 5 wt%.

[0023] In some embodiments, the comonomer is a monomer containing an unsaturated bond, such as acrylic acid, methacrylic acid, acrylamide, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, itaconic acid, citric acid, maleic acid, mesaconic acid, crotonic acid, 2-hydroxyethyl methacrylate, styrene, vinyl methyl methacrylate, vinyl acetate, chloroethylene, vinylidene chloride, ethylene bromide, vinyl fluoride, vinylidene fluoride, allyl sulfonic acid, styrene sulfonic acid, and amine salts or ester derivatives of the above compounds. Specific examples include itaconic acid, which is preferred in terms of the solubility of the acrylonitrile copolymer in solvents, the compactness of the resulting fiber, and the ability to promote oxidation during the stabilization process.

[0024] In some embodiments, the monomer solution may be polymerized by solution polymerization, suspension polymerization, or emulsion polymerization. The polyacrylonitrile copolymer produced by the polymerization reaction must be purified to remove impurities such as unreacted monomers, initiator residues, and excess high polymers. In some embodiments, the polyacrylonitrile copolymer has an intrinsic viscosity of 1.5 to 3.5, based on the drawability of the carbon fiber yarn and the physical properties of the carbon fiber. It should be understood that the intrinsic viscosity of the polyacrylonitrile copolymer depends on its molecular weight. When the intrinsic viscosity is 1.5 to 3.5, the strength of the polymer is sufficient for high-magnification drawing, resulting in high-strength carbon fiber. Furthermore, polymers within this viscosity range have good solubility and are less likely to break.

[0025] In some embodiments, the solution used in operation 110 may be an organic solvent such as dimethylformamide, dimethylacetamide, or dimethyl sulfoxide, or an aqueous solution of inorganic salts such as zinc dichloride and sodium thiocyanate. In particular, to avoid residual metals that may affect the physical properties of the carbon fiber, and based on the solvent's dissolving ability, dimethyl sulfoxide is preferred. In some embodiments, the polymer concentration of the spinning dope is 18 wt% to 25 wt%. When the polymer concentration is within this range, the spinning dope can withstand high-magnification drawing in the subsequent process, and the produced carbon fiber has high strength. Furthermore, the spinning dope has relatively good uniformity, appropriate viscosity, and fluidity, which results in a stable spinning process and consistent carbon fiber production.

[0026] Next, in operation 120, the spinning dope is subjected to a coagulation process to obtain a tow. The coagulation process involves discharging the spinning dope through a spinneret with circular discharge holes into a coagulation bath and coagulating it into a tow. In some embodiments, the coagulation process may be dry-discharge wet spinning or wet-discharge wet spinning, selected according to the subsequent application of the carbon fiber. In some embodiments, the solvent contained in the coagulation bath for the coagulation process is the same as the solvent for the spinning dope. The concentration of the solution in the coagulation bath depends on the type of solvent and the manufacturing process. In some embodiments, for example, dimethyl sulfoxide is used as the solvent, and the concentration of the solution is 20 wt% to 50 wt%. When the concentration of the solution is within this range, the precipitation and coagulation rate of the polyacrylonitrile copolymer from the spinning dope is appropriate, the tow can be completely coagulated, the carbon fiber structure is not rough, the surface pore size is good, and there is no sticking of the single yarn during water washing and drawing. Generally, lowering the coagulation temperature is advantageous for improving fiber compaction, and in some embodiments the coagulation temperature should be less than 40°C.

[0027] Subsequently, the tow may be selectively stretched at a draw ratio of 5x or less, and then passed through a water washing tank to replace the solvent before further stretching. Generally, what is obtained after the coagulation process is a primary fiber, and after the primary fiber is stretched in the water washing tank, it is called a tow or raw yarn. In some embodiments, the draw ratio in the water washing tank should be less than 5x, and a multi-stage drawing method is preferred. In some embodiments, the bath liquid in the water washing tank may be the same as the solvent in the coagulation tank. Generally, the water washing temperature should be increased as much as possible without causing adhesion of the single yarn. In some embodiments, the temperature of the water washing tank exceeds 70°C, preferably exceeds 90°C. Pores are formed to avoid solvent residue, and boiling water is more preferably used as the bath liquid. The above-mentioned draw ratio, concentration, and temperature of the bath liquid in the water washing tank may all be used to adjust the pore size of the fiber. In some embodiments, the pore size of the tow after water washing is 20 nm to 140 nm. The tow having the pore size range described above does not have an excessively dense or loose tow surface, allowing oxygen to efficiently diffuse into the fiber during the subsequent stabilization treatment, and the carbon fiber has high strength.

[0028] Subsequently, in operation 130, the tow is subjected to an oiling process with an oil to obtain an oiled raw yarn. The relationship between the surface tension (σ [mN / m]) of the oil and the particle size (R [nm]) must be within a specific range, as shown in the following formula (1). 20<σ+(R / 2) 0.5 <60 (1) If the value of the above formula (1) is less than 20, the amount of oil remaining in the carbon fiber may be too high, further reducing the strength of the carbon fiber. Conversely, if the value based on the above formula (1) is greater than 60, thread breakage is likely to occur during production, making stable production impossible. In some embodiments, the oil contains silicone oil, water, and an emulsifier. In some embodiments, the silicone oil is an ammoniated modified silicone oil. The surface tension of the oil can be adjusted by adjusting the molecular weight and degree of ammoniation of the silicone oil, or by adjusting the concentration of the emulsifier in the oil or the temperature of the oil. In some embodiments, the surface tension of the oil is 20 mN / m to 70 mN / m, allowing the oil to penetrate into the interior of the fiber in an appropriate amount. In some embodiments, when an ammoniated modified silicone oil is used, a polyethylene oxide and polypropylene oxide copolymer can be used as the emulsifier. For example, a homogenizer can be used to uniformly disperse silicone oil and an emulsifier in water to form an oil solution with uniformly dispersed emulsified droplets. The particle size (R) of the oil solution droplets can be adjusted by controlling the mixing ratio of the ammoniated modified silicone oil and the emulsifier. Generally, the higher the emulsifier ratio, the smaller the oil solution particle size. In some embodiments, the oil solution particle size is 10 nm to 500 nm. Since the oil solution particle size does not need to be adjusted specifically to correspond to the pore size of the carbon fiber, it is easy to adjust the oil solution within this particle size range. For example, for 100 parts by weight of the oil solution, the ratios are 10 to 60 parts by weight of silicone oil, 10 to 40 parts by weight of emulsifier, and 30 to 80 parts by weight of water.

[0029] Subsequently, in operation 140, the oiled yarn is subjected to a dry densification process to obtain a carbon fiber yarn. Typically, the dry densification process is performed using heated rollers. The temperature of the dry densification process is adjusted depending on the moisture content of the fiber, and in some embodiments, the temperature is 100°C to 200°C.

[0030] Subsequently, after the dry densification process, a secondary stretching process may optionally be carried out. The secondary stretching process may utilize a high-temperature heated roller, a high-temperature heated plate, or may involve stretching in high-temperature, high-pressure steam. In some embodiments, the stretching ratio of the secondary stretching is 2 times or more.

[0031] Finally, in operation 150, the carbon fiber yarn is subjected to a sintering process to obtain carbon fibers. The sintering process includes four stages: stabilization, carbonization, surface treatment, and sizing. The stabilization is performed by controlling the carbon fiber yarn with an appropriate tension in an air atmosphere at 200 to 300°C. In some embodiments, the density of the stabilized carbon fiber is 1.30 g / cm. 3 ~1.40g / cm 3 The carbon fibers are then carbonized at high temperatures in an inert atmosphere. In some embodiments, the carbonization temperature is greater than 1000°C, preferably greater than 2000°C. The carbon fibers are then surface-treated to improve the bonding ability between the carbon fibers and the resin. In some embodiments, the surface treatment includes methods such as chemical grafting, plasma treatment, electrolysis, and ozone treatment. Finally, the surface-treated carbon fibers are washed with water, dried, and then sized by impregnation. The sizing process can provide protective effects such as abrasion resistance and bundleability to the carbon fibers.

[0032] In some embodiments, the carbon fiber produced by method 100 can have a strength of greater than 5000 MPa. In some embodiments, the residual silicon content of the carbon fiber produced by method 100 is 500 ppm to 2500 ppm, preferably 500 ppm to 2000 ppm. When the residual silicon content is within the above range, and if the raw yarn has an appropriate oil adhesion rate, the oil agent not only has good protective effects on the carbon fiber, such as abrasion resistance, heat resistance, and cohesiveness, but also the oil agent particles are less likely to penetrate into the fiber in large quantities, making it less likely for defects such as fluffing and yarn breakage to occur during production.

[0033] In some embodiments, the carbon fiber produced by method 100 has a ratio of internal silicon content to surface silicon content of 0.7 or less, preferably 0.5 or less, and more preferably 0.3 to 0.5. When the ratio of internal silicon content to surface silicon content of the carbon fiber is 0.7 or less, there is no excessive oil solution penetrating from the fiber surface into the fiber interior, which solves the problem of excessive oil solution penetration in conventional fiber. Note that the interior of the carbon fiber refers to a depth of about 0.5 μm from the surface.

[0034] The application of the present invention will be described below using several examples, but they are not intended to limit the present invention, and those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention.

[0035] Example 1 Using dimethyl sulfoxide as the solvent, a solution copolymerization reaction was carried out using a monomer concentration of 98 wt% acrylonitrile and 2 wt% itaconic acid. The resulting spinning solution had a molecular weight of 22 wt%. The spinning solution was discharged into the air from the spinneret and then introduced into a coagulation tank for a coagulation process to obtain a tow. The coagulation tank temperature was 3°C, and the bath liquid was a 35 wt% aqueous dimethyl sulfoxide solution. The tow was washed with water and then stretched in two stages in boiling water to a total magnification of 3.5 times. An oil solution was then added to the tow in an oil tank to obtain an oiled yarn. The oil solution concentration was 1.5 wt%, and the temperature was 30°C. The oil solution was 80 wt% ammoniated modified silicone oil and 20 wt% polyethylene oxide / polypropylene oxide copolymer (emulsifier) ​​emulsified in an aqueous solution using a homogenizer. The oiled raw yarn was dried and densified using a heated roller at 175°C, and then stretched 3.5 times in high-pressure steam to produce a carbon fiber raw yarn.

[0036] The carbon fiber raw yarn was gradually heated from 240°C to 280°C in an air atmosphere, and the speed ratio of the front and rear pulling rollers was controlled to 1.0 to maintain the fiber tension, and the stabilization treatment was performed. The fiber density after the stabilization treatment was 1.35 g / cm 3Next, the fiber was gradually heated in nitrogen gas from 300°C to 800°C, and low-temperature carbonization was performed by controlling the speed ratio of the front and rear pulling rolls to 0.9, and then the temperature was gradually heated from 900°C to 1800°C, and high-temperature carbonization was performed by controlling the speed ratio of the front and rear pulling rolls to 0.95. Thereafter, the fiber was introduced into an acidic solution to perform electrolytic surface treatment, and finally, the fiber was washed with water, dried, and sized to produce the carbon fiber of Example 1.

[0037] (Examples 2 to 3 and Comparative Examples 1 to 2) Carbon fibers of Example 2 were obtained under the same process conditions as in Example 1, except that the oil concentration in the oil bath was increased to 3.5 wt %.

[0038] The carbon fiber of Example 3 was obtained under the same process conditions as in Example 2, except that the concentration in the coagulation bath was reduced to 20 wt%, the temperature of the coagulation bath was increased to 15°C, and the total ratio of water washing and drawing was reduced to 2.5 times.

[0039] The carbon fiber of Comparative Example 1 was obtained under the same process conditions as in Example 1, except that the oil composition was changed to 90 wt% ammoniated modified silicone oil and 10 wt% polyethylene oxide and polypropylene oxide copolymer.

[0040] The carbon fiber of Comparative Example 2 was obtained under the same process conditions as in Example 1, except that the oil composition was changed to 90 wt% ammoniated modified silicone oil and 10 wt% polyethylene oxide and polypropylene oxide copolymer, and the temperature of the oil bath was increased to 40°C.

[0041] <Evaluation method> [Fiber pore size] The washed but unoiled fiber samples were dried at 90°C for 2 hours and then analyzed using a BET (3Flex Physisorption, Micromeritics) specific surface area and pore size analyzer. The results are shown in Table 1.

[0042] [Oil particle size] The particle size of the oil solution was detected using a laser particle size analyzer (dynamic light scattering; DLS) (Brookhaven NanoBrook Omni). The detection results are shown in Table 1 below.

[0043] [Surface tension of oil] The surface tension of the oil solution was measured using a surface tensiometer (K100C, KRUSS (U is an umlaut) GmbH). The measurement results are shown in Table 1 below.

[0044] [Residual silicon content in carbon fiber] After the carbon fiber was nitrated (dissolved in nitric acid), the amount of silicon remaining in the carbon fiber was assayed using an inductively coupled plasma optical emission spectrometry (ICP-OES) (Ultima2, Horiba). The assay results are shown in Table 1 below.

[0045] [Ratio of silicon impurities in the inner and outer layers of carbon fiber (I / S)] The surface silicon content (S) of the carbon fiber was examined using an X-ray photoelectron spectroscopy (XPS) analyzer (PHI VersaProbe III). The raw sample was then examined using direct ion gun etching to measure the inner layer silicon content (I) at a depth of 0.5 μm from the surface. The silicon impurity content ratio (I / S) of the inner and outer layers of the carbon fiber is the ratio of the inner layer silicon content (I) to the surface silicon content (S). The test results are shown in Table 1 below.

[0046] [Strength of carbon fiber] The test was carried out according to the standard of ASTM D 4018-99, and the test results are shown in Table 1.

[0047] [Table 1]

[0048] As shown in Table 1, the relationship between the droplet particle size and surface tension of the oil used in Examples 1 to 3 all satisfies Equation (1). The residual silicon content in Examples 1 to 3 was less than 1400 ppm, the ratio of internal silicon content to surface silicon content (I / S) was less than 0.7, even less than 0.5, and the carbon fiber strength was 5000 MPa or greater. The fiber pore size in Example 3 was much larger than the oil particle size, but as can be seen from the I / S value, no significant penetration of the oil into the interior was observed. In Comparative Examples 1 and 2, the oil composition ratio was adjusted. In Comparative Example 1, both the oil particle size and surface tension increased, and the value calculated based on Equation (1) was greater than 60. Therefore, although the residual silicon content and I / S were both small in Comparative Example 1, numerous thread breakages occurred during production, making stable production impossible. Conversely, in Comparative Example 2, both the particle size and surface tension of the oil agent decreased, and the value calculated based on formula (1) was less than 20. As a result, although Comparative Example 2 could be produced normally, both the residual silicon amount and I / S value increased significantly, and the strength of the obtained carbon fiber was far less than 5000 MPa.

[0049] According to the above examples, the carbon fibers produced by the carbon fiber manufacturing method 100 to which the present invention is applied are manufactured by adjusting the composition ratio of the oil agent to control the relationship between the surface tension of the oil agent and the particle size, thereby reducing the penetration of the oil agent into the carbon fiber, and producing carbon fibers that combine low oil agent residual amounts with high strength.

[0050] Although the present invention has been disclosed above in several embodiments, they are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and therefore the scope of protection of the present invention shall be as defined by the appended claims. [Explanation of symbols]

[0051] 100: Method 110, 120, 130, 140, 150: Operation

Claims

1. dissolving a polyacrylonitrile copolymer in a solvent to obtain a spinning solution; subjecting the spinning dope to a coagulation process to obtain a tow; The composition is composed of 80 wt % ammoniated modified silicone oil and 20 wt % polyethylene oxide and polypropylene oxide copolymer, has a surface tension (σ [mN / m]) of 30 mN / m to 70 mN / m, a particle size (R [nm]) of 10 nm to 72 nm, and the relationship between the surface tension and the particle size is σ + (R / 2). ０．５ an oiling process for the tow using an oil satisfying the condition of 0.1% to 36 to obtain an oiled raw yarn; performing a dry densification process on the oiled raw yarn to obtain a carbon fiber raw yarn; performing a sintering process on the carbon fiber raw yarn to obtain carbon fiber; Including, The carbon fiber has a residual silicon content of 500 ppm to 2500 ppm, and the strength of the carbon fiber according to ASTM D 4018-99 is greater than 5000 MPa.

2. 2. The method for producing carbon fibers according to claim 1, wherein the polyacrylonitrile copolymer has an intrinsic viscosity of 1.5 to 3.

5.

3. 2. The method for producing carbon fibers according to claim 1, wherein the pore diameter of the tow is 20 nm to 140 nm.

4. 2. The method for producing carbon fibers according to claim 1, wherein the ratio of the internal silicon content to the surface silicon content of the carbon fibers is 0.7 or less.

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