Heat-treated flame-resistant fiber, heat-treated flame-resistant fiber sheet, and manufacturing method thereof, as well as graphite fiber and graphite fiber sheet

Abstract

To provide heat-treated flame-resistant fiber, that is a precursor of graphite fiber, that can produce graphite fiber with high molecular chain rigidity and high crystallinity even when graphitized at low temperatures.SOLUTION: Heat-treated flame-resistant fiber to be manufactured satisfies a following formula (1) (x-1.3)×(y-65.7)-1.5>0 by heat-treating flame-resistant fiber under predetermined conditions (however, in the formula (1), x is density of the heat-treated flame-resistant fiber (g / cm3), and y is a crystal orientation degree Π002 (%) of the heat-treated flame-resistant fiber).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a heat-treated flame-resistant fiber, a heat-treated flame-resistant fiber sheet, and methods for producing them, as well as to a graphite fiber and a graphite fiber sheet. [Background technology]

[0002] Carbon fibers have superior specific strength and specific modulus compared to other fibers, and are widely used industrially as reinforcing fibers to be combined with resins, taking advantage of their light weight and excellent mechanical properties.

[0003] Conventionally, carbon fibers are produced as follows. First, flame-resistant fibers are produced by flame-resistant treatment of precursor fibers such as acrylic fibers. The flame-resistant treatment is carried out, for example, by heating in an oxidizing atmosphere at 200 to 300°C for 30 to 100 minutes. This flame-resistant treatment causes a cyclization reaction of the nitrile groups in the acrylic fibers, increasing the amount of oxygen bonded. Next, carbon fibers are produced by carbonizing the resulting flame-resistant fibers. Carbonization is carried out, for example, by firing in an inert atmosphere using a firing furnace at 1000 to 2800°C while applying a temperature gradient. Graphite fibers are produced by further firing the carbon fibers at 2000 to 3200°C.

[0004] Graphite fibers have excellent electrical conductivity, and coupled with their excellent mechanical properties, they are expected to be used as various battery materials, such as sodium-sulfur batteries and fuel cells. When applied to various battery materials, graphite fibers are often processed into sheet-like materials such as felt and nonwoven fabrics. Generally, graphite fibers are brittle and not highly formable, so they are shaped in a state prior to carbonization or graphitization, i.e., in the form of flame-resistant fibers. Furthermore, since graphite fibers are required for battery materials to have low electrical resistance, they must be highly crystalline. To make graphite fibers highly crystalline, methods such as calcination at high temperatures for long periods of time are used, but such calcination at high temperatures for long periods of time tends to make the shaped sheet-like material brittle.

[0005] Patent Document 1 states that "it is made of polyacrylonitrile carbon fiber, and when the thickness is compressed to 50% of the thickness before compression, the repulsive force is 2 to 4 kg / cm 2 The document discloses a "carbon fiber felt for electrode materials, characterized in that it has a thickness recovery rate of 98% or more after pressure is released and a specific resistance value in the thickness direction of the carbon fiber felt of 0.11 Ω cm or less." It also states that this carbon fiber felt has appropriate formability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-279566 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a heat-treated flame-resistant fiber as a precursor of graphite fiber, which can produce graphite fiber with high crystallinity even when graphitized at a low temperature, and a heat-treated flame-resistant fiber sheet and a method for producing the same; and to provide graphite fiber and a graphite fiber sheet obtained by graphitizing the heat-treated flame-resistant fiber and the heat-treated flame-resistant fiber sheet. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that a heat-treated flame-resistant fiber having a predetermined relationship between density and degree of crystal orientation Π002 can solve the above problems, and have completed the present invention.

[0009] The present invention, which solves the above problems, is as follows.

[0010] [1] The following formula (1) (x-1.3)×(y-65.7)-1.5 > 0 Formula (1) (In formula (1), x is the density of the heat-treated flame-resistant fiber (g / cm3 ), y is the degree of crystal orientation of the heat-treated flame-resistant fiber Π002 (%)) A heat-treated flame-resistant fiber characterized by satisfying the above.

[0011] [2] The heat-treated flame-resistant fiber according to [1], wherein the x is 1.45 to 1.60.

[0012] [3] The heat-treated flame-resistant fiber according to [1] or [2], wherein y is 73.0 to 79.0.

[0013] [4] The heat-treated flame-resistant fiber according to any one of [1] to [3], wherein the oxygen content is 16.0 to 22.0 (mass %).

[0014] [5] The heat-treated flame-resistant fiber according to any one of [1] to [4], wherein the single fiber diameter is 4.0 to 30.0 (μm).

[0015] The heat-treated flame-resistant fiber described in [1] above is a heat-treated flame-resistant fiber having a high density, or a high crystal orientation degree Π002 even if the density is low. The density is 1.45 to 1.60 (g / cm 3 ) (above [2]), the degree of crystal orientation Π002 is preferably 73.0 to 79.0 (%) (above [3]), the oxygen content is preferably 16.0 to 22.0 (mass %) (above [4]), and the single yarn diameter is preferably 4.0 to 30.0 (μm) (above [5]).

[0016] [6] A heat-treated flame-resistant fiber sheet, characterized in that the heat-treated flame-resistant fiber according to any one of [1] to [5] is formed into a sheet.

[0017] The heat-treated flame-resistant fiber sheet described in [6] above is a heat-treated flame-resistant fiber sheet produced by subjecting the heat-treated flame-resistant fiber of the present invention to felting or papermaking.

[0018] [7] A method for producing a heat-treated flame-resistant fiber according to any one of [1] to [5], The carbon fiber precursor fiber is subjected to a flame retardant treatment in an oxidizing atmosphere at a draw ratio of less than 1.00 and a maximum temperature of 220 to 320°C to obtain a flame retardant fiber. A method for producing a heat-treated flame-resistant fiber, comprising heat-treating the heat-treated flame-resistant fiber in an oxidizing atmosphere at a draw ratio of 1.00 to 1.20 times and at a temperature of 220 to 350°C.

[0019] The invention described in [7] above is a method for producing a heat-treated flame-resistant fiber of the present invention. The heat-treated flame-resistant fiber of the present invention is produced by subjecting a flame-resistant fiber produced by a conventional method to a heat treatment at a predetermined temperature while applying a predetermined tension.

[0020] [8] A method for producing graphite fiber, which comprises graphitizing the heat-treated flame-resistant fiber according to any one of [1] to [5] in an inert atmosphere at a temperature of 2000 to 3200°C.

[0021] [9] A method for producing a graphite fiber sheet, comprising graphitizing the heat-treated flame-resistant fiber sheet according to [6] at a temperature of 2000 to 3200°C in an inert atmosphere.

[0022] The inventions described in [8] and [9] above are methods for producing the graphite fiber or graphite fiber sheet of the present invention. The graphite fiber or graphite fiber sheet of the present invention is produced by graphitizing the highly crystalline heat-treated flame-resistant fiber or heat-treated flame-resistant fiber sheet of the present invention. [Effects of the Invention]

[0023] The heat-treated flame-resistant fiber of the present invention has high crystallinity. By graphitizing this heat-treated flame-resistant fiber, it is possible to produce graphite fiber with high crystallinity, i.e., high electrical conductivity. Because the crystallinity is increased at the stage of the heat-treated flame-resistant fiber, which is a precursor of graphite fiber, it is possible to produce graphite fiber with sufficiently high crystallinity even if the graphitization conditions are relaxed. DETAILED DESCRIPTION OF THE INVENTION

[0024] The heat-treated flame-resistant fiber, heat-treated flame-resistant fiber sheet, and methods for producing them, as well as the graphite fiber and graphite fiber sheet, of the present invention will be described in detail below. In the present invention, density refers to the value at 25°C.

[0025] (1) Heat-treated flame-resistant fiber The heat-treated flame-resistant fiber of the present invention has a thermal conductivity represented by the following formula (1): (x-1.3)×(y-65.7)-1.5 > 0 Formula (1) (In formula (1), x is the density of the heat-treated flame-resistant fiber (g / cm 3 ), y is the degree of crystal orientation of the heat-treated flame-resistant fiber Π002 (%)) The present invention is characterized in that: The value of (x-1.3) x (y-65.7) - 1.5 is preferably greater than 0.10, more preferably greater than 0.30. There is no particular upper limit to the value of (x-1.3) x (y-65.7) - 1.5, but it may be less than 3.0 or less than 2.0. If the value of (x-1.3) × (y-65.7)-1.5 is 0 or less, the density or degree of crystal orientation Π002 is low, and it is difficult to produce highly crystalline graphite fibers even if such heat-treated flame-resistant fibers are used as precursors.

[0026] The density x of the heat-treated flame-resistant fiber is 1.45 to 1.60 (g / cm 3 ), and 1.48 to 1.60 (g / cm 3 ) is more preferably 1.45 (g / cm 3 If the tensile strength is less than 1.60 (g / cm), the degree of crystal orientation must be significantly increased to satisfy the above formula (1). However, in order to achieve such a degree of crystal orientation by heat treatment, the draw ratio during heat treatment must be extremely high, which makes the yarn more susceptible to breakage during the process. 3 ) may have poor drapeability and poor formability, making it difficult to use as a battery material.

[0027] The crystal orientation degree y of the heat-treated flame-resistant fiber is preferably 73.0 to 79.0%, more preferably 74.0 to 78.0%, and particularly preferably 74.5 to 77.5%. If the crystal orientation degree y is less than 73.0%, it is difficult to produce highly crystalline graphite fiber even when such a flame-resistant fiber is used as a precursor. To obtain highly crystalline graphite fiber using a flame-resistant fiber having a crystal orientation degree of less than 73.0% as a precursor, a graphitization treatment at high temperature for a long period of time is required. If the graphitization treatment is performed at high temperature for a long period of time, the resulting graphite fiber may become brittle or have poor handleability. Heat-treated flame-resistant fiber exceeding 79.0% may have poor drapeability and formability. This makes it difficult to use the fiber as a battery material.

[0028] The oxygen content of the heat-treated flame-resistant fiber is preferably 16.0 to 22.0 (mass%), more preferably 16.5 to 21.5 (mass%), and particularly preferably 17.0 to 21.0 (mass%). If the oxygen content is less than 16.0 (mass%), it is difficult to produce highly crystalline graphite fiber even if such flame-resistant fiber is used as a precursor. Heat-treated flame-resistant fiber with an oxygen content exceeding 22.0 (mass%) may have poor drapeability and poor shapability.

[0029] The single fiber diameter of the heat-treated flame-resistant fiber is preferably 4.0 to 30.0 (μm), more preferably 5.0 to 28.0 (μm), and particularly preferably 6.0 to 26.0 (μm). If it is 6.0 (μm) or more, it is less likely to break during processing. If it is 26.0 (μm) or less, it is easier to increase the degree of crystal orientation by heat treatment.

[0030] The heat-treated flame-resistant fiber of the present invention having the above formula (1) can be produced by subjecting raw material fibers to flame-resistant treatment in a conventional manner to obtain flame-resistant fiber, and then subjecting the flame-resistant fiber to heat treatment under predetermined conditions.

[0031] <Raw fiber> The raw fiber can be a PAN-based fiber obtained by spinning a spinning solution obtained by homopolymerizing acrylonitrile or copolymerizing a monomer composition containing 90% by mass or more, preferably 95% by mass or more, of acrylonitrile using a wet or dry-wet spinning method, followed by washing with water, drying, and stretching. Preferred copolymerizable monomers are polar monomers such as methyl acrylate, itaconic acid, methyl methacrylate, methacrylic acid, and acrylic acid.

[0032] The raw fiber may be a raw fiber strand or tow formed by bundling a plurality of continuous fibers. From the viewpoint of production efficiency, the number of single fibers in the raw fiber strand is preferably 1,000 to 1,000,000, and more preferably 3,000 to 600,000.

[0033] <Flame resistance> The PAN-based fiber is used as a precursor fiber, which is then flame-resistant treated in an oxidizing atmosphere at a draw ratio of less than 1.00 and a maximum temperature of 220 to 320°C for 10 to 100 minutes. This flame-resistant treatment causes a cyclization reaction of the nitrile groups within the PAN-based fiber molecules, further increasing the amount of oxygen bonded, resulting in a flame-resistant fiber. The oxidizing atmosphere is preferably oxygen, ozone, nitrogen dioxide, or air, and more preferably air from the perspective of cost.

[0034] The stretching ratio is preferably 0.80 or more and less than 1.00.

[0035] The maximum temperature to be reached is preferably 230 to 300°C, more preferably 240 to 280°C.

[0036] The density of the flame-resistant fiber obtained by this flame-resistant treatment is 1.25 (g / cm 3 ) super, 1.45 (g / cm 3 ) is preferred.

[0037] <Heat treatment> The flame-resistant fiber is heat-treated in an oxidizing atmosphere, specifically in air, at a draw ratio of 1.00 to 1.20 times and a maximum temperature of 220 to 350°C for 10 to 100 minutes to obtain the heat-treated flame-resistant fiber of the present invention.

[0038] The draw ratio is preferably 1.03 to 1.17. If the draw ratio is less than 1.00, the crystallinity may not be sufficiently high. If the draw ratio exceeds 1.20, the flame-resistant fiber is likely to break during heat treatment.

[0039] Although it depends on the heat treatment tension, the maximum temperature is preferably 230 to 320°C, more preferably 240 to 300°C. If the temperature is lower than 220°C, the crystallinity may not be sufficiently high. If the temperature exceeds 350°C, the flame-resistant fiber is likely to break during the heat treatment.

[0040] Although the reason why such a heat treatment can produce a heat-treated flame-resistant fiber with high crystallinity is not clear, it is believed that the cyclization reaction of the nitrile groups in the flame-resistant fiber molecules is further promoted, increasing the crosslink density, thereby reducing the flexible portions of the molecular chains and rigidifying the molecular chains of the flame-resistant fiber, thereby increasing the initial orientation. Furthermore, by using a heat-treated flame-resistant fiber with such an increased initial orientation as a raw material for graphite fiber, the structure of the resulting graphite fiber becomes more orderly and highly crystalline, which is believed to facilitate a further reduction in the volume resistivity of the graphite fiber.

[0041] (2) Graphite fiber The graphite fiber of the present invention is obtained by carbonizing and graphitizing the heat-treated flame-resistant fiber in an inert gas atmosphere. The inert gas atmosphere may be an inert gas such as nitrogen or argon. The oxygen concentration in the inert gas is preferably 1000 ppm by volume or less, more preferably 500 ppm by volume or less.

[0042] The temperature during graphitization is 1000 to 3200°C, preferably 1200 to 3000°C, and more preferably 1500 to 2800°C.

[0043] The graphitization time is preferably 10 seconds to 10 hours as the holding time at the maximum temperature.

[0044] The heat-treated flame-resistant fiber of the present invention preferably undergoes graphitization at a relatively low temperature for a short period of time because the initial molecular orientation is enhanced by the heat treatment. Specifically, graphite fiber with a sufficiently high crystallinity can be obtained at 1000 to 2500°C for 10 seconds to 4 hours. Because graphite fiber with a sufficiently high crystallinity can be obtained under such relatively mild conditions, the graphite fiber is less damaged, its conductivity is easily maintained, and it is easy to handle.

[0045] The graphite fiber of the present invention has a volume resistivity of 1.05×10 -3 [Ω·cm] or less, and -3 It is more preferable that the resistance is less than [Ω·cm].

[0046] The graphite fiber of the present invention preferably has a crystal size Lc of 2.20 nm or more, more preferably 2.30 nm or more, and even more preferably 2.40 nm or more. If Lc is less than 2.20 nm, electrical conductivity is likely to decrease.

[0047] The heat-treated flame-resistant fiber sheet of the present invention is not particularly limited as long as it is in sheet form, and examples thereof include woven fabrics, nonwoven fabrics, felts, mats, and papers. These may be produced by known methods. For example, the felting method may involve opening fibers by carding, multilayering, and then needle-punching the multilayered web. The graphite sheet of the present invention can be produced by graphitizing the heat-treated flame-resistant fiber sheet of the present invention. This graphite sheet is made from flame-resistant fibers with high initial orientation, and therefore can be made into a graphite sheet with high crystallinity at a relatively low temperature. As a result, the graphite fibers are less likely to break, and high electrical conductivity is more likely to be maintained. [Example]

[0048] The present invention will be described in more detail below with reference to examples. The physical properties of the fibers in each example and comparative example were evaluated by the following methods.

[0049] [1] Crystallite size Lc and crystal orientation Π002 Using a Rigaku Corporation RINT2000 X-ray diffractometer, the sample was set on a sample stage with the fiber axis perpendicular to the equatorial plane, and measurements were performed under the following conditions: The X-ray source used was CuKα radiation generated at an accelerating voltage of 40 kV and a current of 30 mA. The scanning range 2θ was from 10° to 40°. The straight line connecting the 10° and 40° points on the diffraction pattern was used as the baseline. The calculation was carried out using the following formula (2) from the half width of the crystal peak obtained at about 2θ=26° by the above-mentioned method. Crystallite size Lc (nm) = 0.9λ / βcosθ Equation (2) where λ is the wavelength of the X-ray, β is the apparent full width at half maximum, and θ is the diffraction angle. The full width at half maximum of the intensity distribution obtained by fixing 2θ at the position of the crystal peak obtained at about 2θ=26° by the above-mentioned method and scanning in the circumferential direction was used to calculate using the following formula (3). Crystal orientation degree Π002=(180-FWHM)×100 / 180...Formula (3) However, FWHM represents the apparent full width at half maximum (deg)

[0050] [2] Density Measurement was performed using the Archimedes method after degassing in acetone.

[0051] [3] Oxygen content After drying and dehydrating the sample, the amounts of carbon, hydrogen, and nitrogen were determined using an elemental analyzer (Elementer Vario EL cube). Anything other than carbon, hydrogen, nitrogen, and ash was counted as oxygen.

[0052] [4] Volume resistivity Measurements were taken in accordance with JIS R7609:2007.

[0053] Example 1 PAN-based fibers with a fineness of 1.1 dtex were heated in air in a flame-resistant furnace and subjected to oxidation treatment. The draw ratio was 0.8436 times (-15.64%), and the maximum temperature reached was 272°C. The single fiber diameter of the obtained flame-resistant fiber was 10.7 μm, the degree of crystal orientation Π002 was 72.2%, and the density was 1.41 g / cm 3 ), and the oxygen content was 14.8%. This flame-resistant fiber was heat-treated for 60 minutes at 260°C, a tension of 0.6 cN / dtex, and a draw ratio of 1.1255 (+12.55%). The single fiber diameter of the heat-treated flame-resistant fiber obtained was 9.7 μm, the crystalline orientation degree Π002 was 74.7%, and the density was 1.53 g / cm 3 ), and the oxygen content was 19.6%. The value of (x - 1.3) x (y - 65.7) - 1.5 calculated by formula (1) was 0.570. This heat-treated flame-resistant fiber was graphitized in a nitrogen atmosphere at a temperature of 2000°C for 4 hours. The crystallite size Lc of the obtained graphite fiber was 2.44 nm, and the volume resistivity was 0.905 (10 -3 The results are shown in Table 1.

[0054] (Examples 2 to 7, Comparative Examples 1 to 7) Heat-treated flame-resistant fibers and graphite fibers were produced in the same manner as in Example 1, except that the fineness of the raw PAN-based fibers, the flame-resistant treatment conditions, and the heat treatment conditions were changed as shown in Tables 1 and 2. The results are shown in Tables 1 and 2.

[0055] (Comparative Example 7) Flame-resistant fibers and graphite fibers were produced in the same manner as in Example 1, except that no heat treatment was performed and the graphitization temperature was set to 2600° C. The results are shown in Table 2.

[0056] [Table 1]

[0057] [Table 2]

[0058] In Comparative Example 1, the heat treatment temperature was too low, resulting in insufficient cyclization of the nitrile groups, and the density was not sufficiently high. In Comparative Example 2, the draw ratio during heat treatment was low, and the degree of crystal orientation was not sufficiently high. In Comparative Examples 3, 4, and 7, no heat treatment was performed. In Comparative Examples 5 and 6, the heat treatment temperature was too high, resulting in fiber breakage during the heat treatment. Therefore, none of the flame-resistant fibers satisfied the requirement of mathematical formula (1). Although the graphite fiber of Comparative Example 7 exhibited high electrical conductivity, it did not satisfy the requirement of mathematical formula (1), and therefore required a relatively high graphitization temperature of 2600°C. In contrast, in Examples 1 to 7, the heat treatment was performed under predetermined conditions, and therefore all of the heat-treated flame-resistant fibers satisfied the relationship of formula (1). The graphite fibers produced using these heat-treated flame-resistant fibers exhibited high electrical conductivity under the relatively mild conditions of 2000°C.

Claims

1. The following formula (1) (x-1.3) x (y-65.7) - 1.5 > 0 Formula (1) (In formula (1), x is the density of the heat-treated flame-resistant fiber (g / cm ３ ), y is the degree of crystal orientation (%) of the heat-treated flame-resistant fiber. Fulfilling The x is 1.45 to 1.60, A heat-treated flame-resistant fiber characterized in that the oxygen content is 16.0 to 22.0 (mass%).

2. 2. The heat-treated flame-resistant fiber according to claim 1, wherein y is 73.0 to 79.

0.

3. 3. The heat-treated flame-resistant fiber according to claim 1, wherein the single fiber diameter is 4.0 to 30.0 (μm).

4. A heat-treated flame-resistant fiber sheet, comprising the heat-treated flame-resistant fiber according to any one of claims 1 to 3 formed into a sheet.

5. The following formula (1) (x-1.3) x (y-65.7) - 1.5 > 0 Formula (1) (wherein, in formula (1), x is the density (g / cm 3 ) of the heat-treated flame-resistant fiber, and y is the degree of crystal orientation (%) of the heat-treated flame-resistant fiber.) Fulfilling A method for producing a heat-treated flame-resistant fiber, wherein x is 1.45 to 1.60, The carbon fiber precursor fiber is subjected to a flame retardant treatment in an oxidizing atmosphere at a draw ratio of less than 1.00 and a maximum temperature of 220 to 320°C to obtain a flame resistant fiber, The method for producing a heat-treated flame-resistant fiber comprises subjecting the flame-resistant fiber to a heat treatment in an oxidizing atmosphere at a draw ratio of 1.00 to 1.20 times and at a temperature of 220 to 350°C.

6. The following formula (1) (x-1.3) x (y-65.7) - 1.5 > 0 Formula (1) (wherein, in formula (1), x is the density (g / cm 3 ) of the heat-treated flame-resistant fiber, and y is the degree of crystal orientation (%) of the heat-treated flame-resistant fiber.) Fulfilling A method for producing graphite fibers, comprising graphitizing a heat-treated flame-resistant fiber in which x is 1.45 to 1.60 at a temperature of 2000 to 3200°C in an inert atmosphere.

7. The following formula (1) (x-1.3) x (y-65.7) - 1.5 > 0 Formula (1) (wherein, in formula (1), x is the density (g / cm 3 ) of the heat-treated flame-resistant fiber, and y is the degree of crystal orientation (%) of the heat-treated flame-resistant fiber.) Fulfilling A method for producing a graphite fiber sheet, comprising graphitizing a heat-treated flame-resistant fiber sheet obtained by forming heat-treated flame-resistant fibers, wherein x is 1.45 to 1.60, into a sheet at a temperature of 2000 to 3200°C in an inert atmosphere.

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