JP7765876B2 - Heat-treated flame-resistant fiber, heat-treated flame-resistant fiber sheet, and manufacturing method thereof, as well as graphite fiber and graphite fiber sheet - Google Patents

Description

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.

Carbon fiber has superior specific strength and specific modulus compared to other fibers, and its light weight and excellent mechanical properties mean that it is widely used industrially as a reinforcing fiber to be combined with resins.

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

Graphite fiber has excellent electrical conductivity, and combined with its excellent mechanical properties, it is expected to be used as a material for various batteries, such as sodium-sulfur batteries and fuel cells. When applied to various battery materials, graphite fiber is often processed into sheet-like materials such as felt or nonwoven fabric. Generally, graphite fiber is brittle and not highly formable, so it is shaped in its pre-carbonization or pre-graphitization state, i.e., flame-resistant fiber state. Furthermore, battery materials require that the graphite fiber itself have low electrical resistance, so the graphite fiber itself must be highly crystalline. To achieve high crystallinity in graphite fiber, methods such as calcination at high temperatures for long periods of time are used, but such high-temperature, long-term calcination can easily make the shaped sheet brittle.

Patent Document 1 discloses "a carbon fiber felt for electrode materials, which is made of polyacrylonitrile carbon fiber, has a repulsive force of 2 to 4 kg/ ^cm2 when compressed to 50% of the thickness before compression, has a thickness recovery rate of 98% or more after decompression, and has a specific resistance value of 0.11 Ω cm or less in the thickness direction of the carbon fiber felt." It is also stated that this carbon fiber felt has appropriate formability.

Japanese Patent Application Laid-Open No. 2001-279566

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.

After extensive research, the inventors discovered that heat-treated flame-resistant fibers that satisfy a specific relationship between density and crystalline orientation Π002 can solve the above-mentioned problems, leading to the completion of the present invention.

The present invention, which solves the above problems, is as follows:

[1] The following formula (1)
(x-1.3) x (y-65.7) - 1.5 > 0 Formula (1)
(In the formula (1), x is the density (g/cm ³ ) of the heat-treated flame-resistant fiber, and y is the degree of crystal orientation Π002 (%) of the heat-treated flame-resistant fiber.)
A heat-treated flame-resistant fiber characterized by satisfying the above.

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

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

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

[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).

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

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

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

[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 resistant fiber,
The method for producing a heat-treated flame-resistant fiber comprises 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.

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

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

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

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.

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.

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 are described in detail below. Note that, in the present invention, density refers to the value at 25°C.

(1) Heat-Treated Flame-Resistant Fiber The heat-treated flame-resistant fiber of the present invention has a thermal resistance represented by the following formula (1):
(x-1.3) x (y-65.7) - 1.5 > 0 Formula (1)
(In the formula (1), x is the density (g/cm ³ ) of the heat-treated flame-resistant fiber, and y is the degree of crystal orientation Π002 (%) of the heat-treated flame-resistant fiber.)
The present invention is characterized in that:
The value of (x-1.3)×(y-65.7)-1.5 is preferably greater than 0.10, and more preferably greater than 0.30. There is no particular upper limit to the value of (x-1.3)×(y-65.7)-1.5, but it may be less than 3.0 or less than 2.0.
When the value of (x-1.3) × (y-65.7) - 1.5 is 0 or less, the density or the 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.

The density x of the heat-treated flame-resistant fiber is preferably 1.45 to 1.60 (g/cm ³ ), and more preferably 1.48 to 1.60 (g/cm ³ ). If the density x is less than 1.45 (g/cm ³ ), the degree of crystal orientation must be quite high to satisfy the above mathematical formula (1). However, to achieve such a degree of crystal orientation by heat treatment, the draw ratio during heat treatment must be extremely high, making the fiber more susceptible to breakage during the process. Heat-treated flame-resistant fibers with a density exceeding 1.60 (g/cm ³ ) may have poor drapeability and poor formability. This makes them difficult to use as battery materials.

The crystalline 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 crystalline orientation degree y is less than 73.0%, it is difficult to produce highly crystalline graphite fiber even when using such flame-resistant fiber as a precursor. To obtain highly crystalline graphite fiber using a flame-resistant fiber with a crystalline orientation degree of less than 73.0% as a precursor, graphitization treatment at high temperature for a long period of time is required. Graphitization treatment at high temperature for a long period of time may result in the resulting graphite fiber becoming brittle or having reduced handleability. Heat-treated flame-resistant fiber with a crystalline orientation degree exceeding 79.0% may have poor drapeability and formability, making it difficult to use as a battery material.

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 when using such flame-resistant fiber as a precursor. Heat-treated flame-resistant fiber with an oxygen content exceeding 22.0 (mass%) may have poor drapeability and poor formability.

The single yarn 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 especially 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 crystalline orientation by heat treatment.

The heat-treated flame-resistant fiber of the present invention, which satisfies the above formula (1), can be produced by subjecting raw fiber to flame-resistant treatment using conventional methods to obtain flame-resistant fiber, and then subjecting the resulting fiber to heat treatment under specified conditions.

<Raw fiber>
The raw fiber may 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 drawing. Preferred copolymerizable monomers are polar monomers such as methyl acrylate, itaconic acid, methyl methacrylate, methacrylic acid, and acrylic acid.

The raw fiber may be a raw fiber strand or tow made by bundling multiple continuous fibers. From the standpoint 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.

<Flame resistance>
The PAN-based fiber is used as a precursor fiber, and this precursor fiber is 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.

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

The maximum temperature reached is preferably 230 to 300°C, and more preferably 240 to 280°C.

The density of the flame-resistant fiber obtained by this flame-resistant treatment is preferably more than 1.25 (g/cm ³ ) and less than 1.45 (g/cm ³ ).

<Heat treatment>
The flame-resistant fiber of the present invention is obtained by heat-treating the above-mentioned flame-resistant fiber 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.

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

Although it depends on the heat treatment tension, the maximum temperature reached is preferably 230 to 320°C, and more preferably 240 to 300°C. If the temperature is below 220°C, the degree of crystallinity may not be sufficiently high. If the temperature exceeds 350°C, the flame-resistant fiber is more likely to break during heat treatment.

The reason why such heat treatment results in heat-treated flame-resistant fiber with high crystallinity is unclear, but it is thought that this is because the cyclization reaction of nitrile groups in the flame-resistant fiber molecules is further accelerated, increasing the crosslink density, reducing the flexible portions of the molecular chains and rigidifying the molecular chains of the flame-resistant fiber, resulting in a higher initial orientation. Furthermore, by using heat-treated flame-resistant fiber with such an increased initial orientation as the raw material for graphite fiber, the resulting graphite fiber's structure becomes more orderly and highly crystalline, making it easier to reduce the volume resistivity of the graphite fiber.

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

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

The graphitization time is preferably 10 seconds to 10 hours, with the maximum temperature being maintained.

Because the heat-treated flame-resistant fiber of the present invention has an enhanced initial molecular orientation due to the heat treatment, it is preferable to graphitize it at a relatively low temperature for a short period of time. Specifically, graphite fiber with a sufficiently high crystallinity can be obtained by heating it at 1000 to 2500°C for 10 seconds to 4 hours. Because graphite fiber with a sufficiently high crystallinity can be obtained under these relatively mild conditions, the graphite fiber is less damaged, its conductivity is easily maintained, and it is easy to handle.

The graphite fiber of the present invention preferably has a volume resistivity of less than 1.05×10 ⁻³ [Ω·cm], and more preferably less than 1.03×10 ⁻³ [Ω·cm].

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.

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.

The present invention will now be described in more detail using examples. The physical properties of the fibers in each example and comparative example were evaluated using the following methods.

[1] Crystallite size Lc and crystal orientation degree Π002
Using a Rigaku Corporation RINT2000 X-ray diffractometer, a sample stage was attached on which the sample was set with the fiber axis direction perpendicular to the equatorial plane, and measurements were performed under the following conditions: CuKα rays were used as the X-ray source, generated at an acceleration 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 in the diffraction pattern was used as the baseline.
The half width of the crystal peak obtained in the above-mentioned method at about 2θ=26° was calculated using the following formula (2).
Crystallite size Lc (nm) = 0.9λ/β cos θ Formula (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)

[2] Density: Measured by Archimedes' method. The sample was degassed in acetone and then measured.

[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). The amount of carbon, hydrogen, nitrogen, and ash other than the amount of oxygen was determined.

[4] Volume resistivity: Measured in accordance with JIS R7609:2007.

Example 1
A PAN-based fiber having a fineness of 1.1 dtex was heated in air in a flame-resistant furnace for oxidation treatment. The draw ratio was 0.8436 (-15.64%), and the maximum temperature reached was 272°C. The resulting flame-resistant fiber had a single yarn diameter of 10.7 (μm), a crystalline orientation degree Π002 of 72.2 (%), a density of 1.41 (g/cm ³ ), and an oxygen content of 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 resulting heat-treated flame-resistant fiber had a single yarn diameter of 9.7 (μm), a crystalline orientation degree Π002 of 74.7 (%), a density of 1.53 (g/cm ³ ), and an oxygen content of 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 Ω·cm). The results are shown in Table 1.

(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 PAN-based fibers used as raw materials, the flame-resistant treatment conditions, and the heat treatment conditions were changed as shown in Table 1 or 2. The results are shown in Table 1 or 2.

(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.

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 requirements of formula (1). Although the graphite fiber of Comparative Example 7 exhibited high electrical conductivity, it did not satisfy the requirements of 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 (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
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. The heat-treated flame-resistant fiber according to claim 1 , wherein y is 73.0 to 79.0. 3. The heat-treated flame-resistant fiber according to claim 1, wherein the single fiber diameter is 4.0 to 30.0 (μm). 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. 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. 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. 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.