JP6864588B2 - Carbon fiber sheet laminate and its manufacturing method - Google Patents

Description

The present invention relates to a carbon fiber sheet laminate, and more particularly to a carbon fiber sheet laminate capable of suppressing fracture due to stress.

Since carbon fibers have excellent thermal stability and chemical stability, carbon fiber felts made by entwining carbon fibers and carbon fiber sheets obtained by impregnating carbon fiber felts with a resin material and carbonizing them are used as heat insulating materials. Widely used as a sound absorbing material. The carbon fiber felt has an advantage of being excellent in flexibility, and the carbon fiber sheet has an advantage of being excellent in shape stability and capable of fine processing. Further, when the carbon fiber sheet is used in an environment where oxygen gas or SiO gas is generated, the carbonized product of the resin material reacts with these gases prior to the carbon fiber, so that there is an advantage that the carbon fiber is not easily deteriorated. ..

Which one to use is appropriately selected according to the purpose of use and the intended use. Here, since the molded heat insulating material used by laminating carbon fiber sheets is excellent in thermal stability, heat insulating performance and shape stability, a single crystal silicon pulling device, a polycrystalline silicon cast furnace, a metal or a ceramics It is used as a heat insulating material for high temperature furnaces such as sintering furnaces and vacuum vapor deposition furnaces.

By the way, depending on the usage conditions, stress may be applied to the molded heat insulating material, but if the stress is excessively applied, cracks occur in the carbon fiber sheet constituting the molded heat insulating material. If the cracks progress, the carbon fiber sheet may be destroyed, and in such a case, the heat insulating function cannot be exhibited. When carbon fiber felt having excellent flexibility does not occur, such a problem does not occur, but there are cases where a molded heat insulating material must be used from the viewpoint of shape stability and the like.

Here, when external stress is applied to the molded heat insulating material, when the molded heat insulating material comes into contact with surrounding members, when internal stress is applied, when the molded heat insulating material is locally and rapidly heated. Etc. are assumed.

By the way, as a technique relating to a heat insulating material using carbon fiber, the following Patent Document 1 can be mentioned.

Japanese Unexamined Patent Publication No. 2008-196552

The technique of Patent Document 1 is a technique relating to a carbon fiber heat insulating material obtained by compression-molding and firing a laminate of a resin-impregnated carbon fiber felt impregnated or coated with a resin binder and a carbon fiber felt.

According to this technique, it is said that the deterioration of the heat insulating property is suppressed while increasing the rigidity, and the workability on a wall body such as a heating furnace can be easily performed.

However, in this technology, it is essential to include a part of carbon fiber felt that does not contain resin, but this part is mechanical because there is no resin component that contributes to adhesion, which is difficult to process finely. There are problems such as low strength and adhesive strength, and because it does not contain a component that oxidizes prior to carbon fibers, the skeleton of the carbon fibers collapses due to oxidative consumption and the heat insulating property deteriorates.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a carbon fiber sheet laminate capable of suppressing fracture due to stress.

The present invention relating to a carbon fiber sheet laminate for solving the above problems is configured as follows.
Carbon having a fiber felt in which carbon fibers are entangled three-dimensionally at random and a protective carbon layer covering the carbon fiber surface of the fiber felt, and a plurality of carbon fiber sheets made of carbonaceous material are laminated. The fiber sheet laminate, wherein the carbon fibers include isotropic pitch-based carbon fibers and polyacrylonitrile-based carbon fibers, and the mass of the isotropic pitch-based carbon fibers in the total mass of the carbon fibers. The proportion of the polyacrylonitrile-based carbon fibers to the total mass of the carbon fibers is 20% or more, and the isotropic pitch-based carbon fibers and the said isotropic pitch-based carbon fibers to the total mass of the carbon fibers. It is characterized in that the ratio of the total mass of the polyacrylonitrile-based carbon fibers is 90% or more.

In the above configuration, the carbon fibers constituting the carbon fiber sheet include isotropic pitch-based carbon fibers and polyacrylonitrile (PAN) -based carbon fibers, and are isotropic pitch-based with respect to the total mass of the carbon fibers. Carbon fibers are regulated to 20% by mass or more, PAN-based carbon fibers are regulated to 20% by mass or more, and the total of both is regulated to 90% by mass or more.

The strength of the carbon fiber sheet tends to increase as the number of protective carbon layers connecting the contacts between the carbon fibers increases. Here, the PAN-based carbon fiber has a property that the strength of the PAN-based carbon fiber alone is high and the fibers are not easily entangled with each other. For this reason, the carbon fiber sheet made of only PAN-based carbon fibers has low strength because the fibers are less likely to be entangled with each other and there are few contacts between the carbon fibers. However, after the protective carbon layer that binds the contacts of the carbon fibers is destroyed, the PAN-based carbon fibers maintain the strength of the carbon fiber sheets to a certain extent, so that if a crack occurs in one carbon fiber sheet, This crack does not easily progress to other (adjacent) carbon fiber sheets continuously, and the carbon fiber sheet laminate does not break at once.

On the other hand, isotropic pitch-based carbon fibers have the properties of high flexibility, easy entanglement of fibers, and lower strength of a single body than PAN-based carbon fibers. Therefore, the carbon fiber sheet using only isotropic pitch carbon fibers has many contacts between the carbon fibers and has high strength as a carbon fiber sheet. However, the strength of the carbon fiber sheet after the protective carbon layer that binds the carbon fiber contacts is broken is insufficient, and the cracks generated in one carbon fiber sheet continue to the other carbon fiber sheets. It is easy to proceed, and the carbon fiber sheet laminate is destroyed at once.

On the other hand, the isotropic pitch carbon fiber and the PAN carbon fiber are entwined with the isotropic pitch carbon fiber and the PAN carbon fiber by regulating the mass mixing ratio as described above and entwining the isotropic pitch carbon fiber and the PAN carbon fiber three-dimensionally at random. It is possible to realize a carbon fiber sheet laminate having the advantages of both of the carbon fibers. That is, while maintaining the strength of the carbon fiber sheet by the isotropic pitch carbon fiber, the strength of the carbon fiber sheet is maintained to a certain extent even after the crack is generated by the PAN carbon fiber, and the crack is propagated. It is possible to realize a carbon fiber sheet laminate in which

Here, if the amount of the isotropic pitch-based carbon fiber in the entire carbon fiber is too small, the effect of the isotropic pitch-based carbon fiber cannot be sufficiently obtained. Further, if the amount of the PAN-based carbon fiber in the entire carbon fiber is too small, the effect of the PAN-based carbon fiber cannot be sufficiently obtained. Therefore, the mass of the isotropic pitch carbon fiber in the entire carbon fiber is regulated to 20% or more, more preferably 25% or more, and most preferably 30%. Further, the mass of the PAN-based carbon fiber in the entire carbon fiber is regulated to 20% or more, more preferably 25% or more, and most preferably 30%.

Further, the carbon fibers may contain other carbon fibers such as anisotropic pitch carbon fibers and rayon carbon fibers, but in order to sufficiently obtain the effects of the isotropic pitch carbon fibers and the PAN carbon fibers. The total mass of isotropic pitch carbon fibers and PAN carbon fibers in the total carbon fibers is regulated to 90% or more, more preferably 95% or more, and most preferably 100% (other carbons). Does not contain fiber).

Here, the carbon fiber sheet laminate includes a plate-shaped carbon fiber sheet in which a plurality of plate-shaped carbon fiber sheets are laminated, a carbon fiber sheet in which one or a plurality of carbon fiber sheets are spirally wound and laminated, and the like. Is done.

Further, it is preferable that the carbon fiber sheets constituting the carbon fiber sheet laminate have the same bulk density, thickness, mass mixing ratio of carbon fibers, and the like.

Further, the carbon fiber sheet arranged on the surface (one or both) of the carbon fiber sheet laminate may be impregnated with pyrolytic carbon or may contain carbonaceous particles such as graphite particles and amorphous carbon particles. You may use it. Further, a surface layer having a high bulk density and a volume fraction of carbon fibers may be attached to the surface of the carbon fiber sheet laminate. With such a configuration, wear and dust generation of the carbon fiber sheet laminate can be further suppressed. Further, when these are not included, the manufacturing process can be simplified and the cost can be reduced. It is preferable that the carbon fiber sheet other than the surface does not contain components other than the carbon fiber and the protective carbon layer.

The carbon fiber sheet laminate according to the present invention can be used as a molding heat insulating material for a high temperature furnace such as a single crystal silicon pulling device, a polycrystalline silicon cast furnace, a metal or ceramics sintering furnace, and a vacuum vapor deposition furnace, and is also used for railway vehicles. It can also be used for sound absorbing heat insulating materials, heat insulating boards for ships, and the like.

Further, when the carbon fiber sheet laminate is used as a molded heat insulating material, if active gas (oxygen gas, SiO gas, etc.) mixed as an impurity or generated in the furnace is present around the carbon fiber sheet laminate, the protective carbon layer is used. Reacts with the active gas prior to carbon fiber. As a result, the reaction between the carbon fiber and the active gas is suppressed from deterioration.

Here, when the carbon substance reacts with oxygen gas, it is removed as carbon dioxide gas, and when it reacts with SiO gas, it becomes SiC and remains without being removed. In either case, it is composed of carbon fibers. Since the skeletal structure is maintained, the heat insulating effect obtained by the skeletal structure forming a large number of spaces is maintained.

The carbon fiber sheet laminate is formed by laminating a plurality of carbon fiber sheets composed of carbon, and therefore the carbon fiber sheet laminate does not contain any component other than carbon.

In the above configuration, the isotropic pitch carbon fiber may be a curved carbon fiber. When the carbon fibers are curved, the entanglement between the carbon fibers can be further enhanced.

Here, the curved carbon fiber means that the length when the fiber is pulled linearly (that is, the fiber length) is L1, the maximum length of the curved fiber in the natural state (or the maximum point in the natural state). When the dimension, that is, the length at which this distance becomes the largest when the distance between any two points on the curved fiber is measured is L2, L1 / L2 (the ratio of L1 and L2) is 1. Defined (or defined) as a carbon fiber having a curved shape of 3 or more. In addition, when the fiber is pulled, the curved shape of the fiber may not be temporarily maintained. Therefore, the length L2 is the maximum in the natural state of the curved fiber after the fiber of the length L1 is freely dropped from a predetermined height (for example, about 30 to 100 cm) in order to make the measurement conditions more accurate. It may be measured as length. Further, the maximum length L2 often has a variation in each curved carbon fiber, and usually has a plurality of measured values [for example, 5 or more (for example, 5 to 200), preferably 10 or more (for example, 5 or more). For example, it can be obtained as an average value (average maximum length) of measured values of 20 or more (for example, about 20 to 50), more preferably about 10 to 100).

Since it is difficult to make the PAN-based carbon fiber curved due to its manufacturing method, it is preferable to use a PAN-based carbon fiber that is not curved (straight).

The method for producing a carbon fiber sheet laminate according to the present invention for solving the above problems is configured as follows.
A fiber felt production step of entwining carbon fibers three-dimensionally and randomly to form a fiber felt, a prepreg production step of impregnating the fiber felt with a thermosetting resin to produce a prepreg of a carbon fiber sheet, and the prepreg. A lamination step of laminating a plurality of the prepreg laminates to form a prepreg laminate, and a binding step of heating the prepreg laminate while applying pressure to thermally cure the thermosetting resin and binding the prepregs. The prepreg laminate is heat-treated in an inert gas atmosphere to carbonize the thermosetting resin, and the carbon fibers include isotropic pitch-based carbon fibers and polyacrylonitrile-based carbon. The mass ratio of the isotropic pitch-based carbon fibers to the total mass of the carbon fibers including the fibers is 20% or more, and the mass ratio of the polyacrylonitrile-based carbon fibers to the total mass of the carbon fibers is 20% or more. A method for producing a carbon fiber sheet laminate, which is 20% or more and the ratio of the total mass of the isotropic pitch carbon fiber and the polyacrylonitrile carbon fiber to the total mass of the carbon fiber is 90% or more.

The carbon fiber sheet laminate according to the present invention can be produced by the above production method.

As described above, according to the present invention, it is possible to realize a carbon fiber sheet laminate capable of suppressing fracture due to stress.

FIG. 1 is a perspective view schematically showing the structure of the carbon fiber sheet laminate according to the present invention. FIG. 2 is a diagram showing an outline of a three-point bending test. FIG. 3 is a graph showing the results of the three-point bending test. FIG. 4 is a diagram showing an outline of the peeling test. 5A and 5B are microscopic cross-sectional photographs of the carbon fiber sheet laminate according to Example 1, in which FIG. 5A shows a plan view and FIG. 5B shows a side view.

(Embodiment)
FIG. 1 is a perspective view schematically showing the structure of the carbon fiber sheet laminate according to the present embodiment. The carbon fiber sheet laminate 100 according to the present embodiment has a fiber felt in which carbon fibers are entangled and a protective carbon layer made of carbon material that covers the carbon fiber surface of the fiber felt, and is composed of carbon material. The carbon fiber sheets 1 to be formed are laminated (8 carbon fiber sheets 1 are laminated in FIG. 1). Further, in the carbon fiber sheet 1, the carbon fibers are three-dimensionally and randomly oriented.

The carbon fibers constituting the carbon fiber sheet 1 include isotropic pitch-based carbon fibers and polyacrylonitrile (PAN) -based carbon fibers, and the ratio of the isotropic pitch-based carbon fibers to the total mass of the carbon fibers is 20% or more, the ratio of PAN-based carbon fiber to the total mass of carbon fiber is 20% or more, and the ratio of the total mass of isotropic pitch-based carbon fiber and PAN-based carbon fiber to the total mass of carbon fiber is 90% or more. Is regulated by.

Here, the PAN-based carbon fiber has high strength as a single substance, and the fibers are less likely to be entangled with each other. On the other hand, the isotropic pitch-based carbon fiber has high flexibility, the fibers are easily entangled with each other, and the strength of a single carbon fiber is lower than that of the PAN-based carbon fiber. By using the isotropic pitch-based carbon fiber and the PAN-based carbon fiber with the mass mixing ratio regulated as described above, the isotropic pitch-based carbon fiber has both the ease of entanglement and the strength of the PAN-based carbon fiber. A carbon fiber sheet laminate can be realized.

Here, if the amount of isotropic pitch-based carbon fibers in the total mass of carbon fibers is too small, the amount of PAN-based carbon fibers is too small, or the total mass of both is too small, these effects Is not sufficiently obtained.

Here, the isotropic pitch-based carbon fiber is a carbon fiber made from an isotropic pitch that has been infusibilized, and a commercially available one can be used. Pitch is a mixture of innumerable condensed polycyclic aromatic compounds chemically, such as liquid tar obtained by carbonization of wood and coal, bitumen obtained from oil sands, and oil obtained by carbonization of oil shale. Residual oil obtained by distillation of crude oil, tar produced by carbonization of petroleum distillates, etc. are heat-treated and polymerized to be solid at room temperature. Specific examples thereof include coal-derived pitches, petroleum-derived pitches, synthetic pitches obtained by polymerizing aromatic compounds such as naphthalene, and the like.

As the isotropic pitch-based carbon fiber, those produced by a known method can be used. For example, spinning a pitch derived from petroleum or coal and depositing it on a table gives a mat of pitch fibers. The resulting mat is an aggregate of pitch fibers having different lengths in the range of approximately 5 to 400 mm. The spinning method is not particularly limited, but is generally performed by melt spinning. As a method of melt spinning, there are a vortex method and a centrifugal method. The eddy current method gives curved fibers, and the centrifugal method gives non-curved (straight) fibers. Pitch fibers are infusible and carbonized to form a carbon fiber mat. The infusibilizing step is a step of introducing oxygen into the surface of the pitch fiber to oxidize it. The atmosphere of the infusibilization process can be air or NOx. The temperature of the carbonization treatment is not particularly limited, but may be 700 to 1200 ° C. in consideration of economic efficiency and the like. When curved fibers are used, the fibers are more likely to be entangled with each other in the fiber felt, and the strength is likely to be increased.

The isotropic pitch-based carbon fibers preferably have an average fiber diameter (diameter) of 7 to 20 μm, more preferably 9 to 18 μm, and even more preferably 11 to 15 μm. The length thereof is preferably 5 to 400 mm, more preferably 8 to 350 mm, and even more preferably 10 to 300 mm.

The polyacrylonitrile (PAN) -based carbon fiber is obtained by carbonizing the polyacrylonitrile fiber, and a commercially available one can be used. The PAN-based carbon fiber preferably has a fiber length of 20 to 200 mm, more preferably 30 to 80 mm. The average fiber diameter (diameter) is preferably 5 to 13 μm, more preferably 5 to 9 μm, and even more preferably 5 to 7 μm.

Further, any of the carbon fibers is not particularly limited as the microscopic structure of the carbon fibers, and only those having the same shape (curved shape, linear shape, cross-sectional shape, etc.) may be used, or different structures may be used. Although those may be mixed, it is preferable that the isotropic pitch-based carbon fibers are curved and the PAN-based carbon fibers are linear (not curved).

Further, the shape of the fiber felt constituting the carbon fiber sheet is not particularly limited, and the length and width are also not particularly limited. As the fiber felt, for example, one having a thickness of about 3 to 15 mm can be used. Further, as the microscopic structure of the carbon fiber felt, a structure in which carbon fibers oriented in three-dimensionally random directions are intricately intersected is used.

The protective carbon layer covers the entire surface of the carbon fibers constituting the fiber felt or a part of the surface of the carbon fibers. Further, the protected carbon layer may be carbonaceous (amorphous carbon or graphitic carbon), and the amorphous carbon may be either non-graphitizable or easy-graphitizable. The compound from which the protective carbon layer is derived is not particularly limited, but it is preferable to use a resin material that can be impregnated into the fiber felt. Of these, thermosetting resins such as phenol resins, furan resins, polyimide resins, and epoxy resins are preferable. When the thermosetting resin is used, the carbon fibers and the laminated carbon fiber sheets can be easily and firmly bonded to each other by thermosetting and carbonization.

Here, only one type of thermosetting resin may be used, or two or more types may be mixed and used. Further, the thermosetting resin may be contained in the fiber felt as it is, or may be diluted with a solvent and contained in the fiber felt. As the solvent, alcohols such as methyl alcohol and ethyl alcohol can be used.

Further, the fiber felt may be cut after producing the carbon fiber sheet laminate using a long or long fiber felt, or may be cut in advance to the size of the carbon fiber sheet laminate.

Here, the bulk density of the carbon fiber sheet laminate is preferably 0.05~0.30g / cm ^3, more preferably 0.07~0.20g / cm ^3, 0.10~ It is more preferably 0.18 g / cm ^3.

The mass ratio of the carbon fiber to the protective carbon layer in the carbon fiber sheet is preferably 100: 5 to 100: 100, more preferably 100: 10 to 100: 80, and 100: 15 to 100. : 60 is more preferable.

The thickness of each carbon fiber sheet is preferably 3 to 15 mm, more preferably 5 to 12 mm, and even more preferably 6 to 10 mm.

Next, a method for manufacturing the carbon fiber sheet laminate will be described.

(Preparation of fiber felt)
As the fiber felt, one produced by a known method can be used, and a method in which carbon fibers are easily oriented three-dimensionally randomly is adopted. As a method for forming the fiber felt, for example, a method in which a fiber in which an isotropic pitch carbon fiber and a PAN carbon fiber are mixed is opened by a fiber opener, raised by air pressure and deposited, and then a needle punch is used. Examples thereof include a method of stirring and mixing in a solution and depositing it on a papermaking net, a method of spinning fiber felt by a carding means such as a carding machine, and then using a needle punch. The fiber felt preferably has a thickness of 3 to 20 mm, more preferably 5 to 15 mm. Basis weight of the fiber felt, for example, is preferably from 100 to 2000 g / m ^2, and more preferably 300 to 1500 g / m ^2.

(Making prepreg)
After that, the thermosetting resin solution is sprayed on the fiber felt and immersed in the thermosetting resin solution, or the thermosetting resin solution is applied to prepare a prepreg. At this time, the amount of the synthetic resin is adjusted so that the mass ratio of the carbon fiber and the protective carbon layer becomes 100: 5 to 100: 100 after firing.

(Laminating step)
A plurality of prepregs prepared as described above are sequentially laminated so as to have a desired thickness. Further, one or a plurality of prepregs may be spirally wound around a cylindrical or cylindrical mandrel and laminated.

(Bounding / carbonization)
The laminate produced as described above is heated while being pressurized to heat-cure the thermosetting resin to bind the prepreg. Then, the thermosetting resin is carbonized by heating at 1500 to 2500 ° C. for a predetermined time (for example, 1 to 20 hours) in an inert gas atmosphere to obtain a carbon fiber sheet laminate.

Here, carbonization as used herein means a broad definition including graphitization. For example, it is conceivable that the graphite structure develops especially when the heat treatment is performed at a temperature of 2000 ° C. or higher. However, in the present invention, the carbon material constituting the carbon fiber sheet laminate is either amorphous carbon or graphite carbon. It may be.

The present invention will be described in more detail based on examples.

(Example 1)
(Making carbon fiber)
An isotropic pitch derived from coal was melt-spun by a vortex method to obtain a mat made of curved pitch fibers. The pitch fibers have a length of about 10 to 300 mm, and these fibers are aggregated to form a mat. This mat was heat-treated in an air atmosphere from room temperature to about 250 to 300 ° C. for a total of 30 minutes to insolubilize the pitch fibers to obtain a fiber mat. This fiber mat was carbonized at about 1000 ° C. in an inert gas atmosphere to obtain a mat of isotropic pitch carbon fibers (average diameter 13 μm). The length when the carbon fiber is pulled linearly (that is, the fiber length) is L1, the maximum length of the curved fiber in the natural state (or the maximum point dimension in the natural state, that is, on the curved fiber. When the distance between any two points was measured, L1 / L2 (the ratio of L1 to L2) was 2.1, where L2 was the length at which this distance was the largest.

(Making fiber felt)
The above isotropic pitch carbon fiber and polyacrylonitrile (PAN) carbon fiber (manufactured by Toray Industries, Inc., average fiber diameter 7 μm, length 40 mm) are mixed and opened at a mass ratio of 50:50. , The fiber felt (300 × 300 × 10 (thickness) mm, grain 560 g / m ² ) was produced by entanglement by the needle punch method.

(Making prepreg)
A prepreg was prepared by immersing the fiber felt in a resole-type phenol-resin-based thermosetting resin solution. At this time, a phenol resin-based thermosetting resin solution was added so that the carbon mass formed by carbonizing the thermosetting resin when heat-treated at 2000 ° C. was 40 parts by mass with respect to 100 parts by mass of carbon fibers. ..

(Laminating step)
Eight layers of the above prepregs were laminated to prepare a prepreg laminated body.

(Bounding / carbonization step)
The prepreg laminate was bound by thermosetting the phenol resin by pressurizing the prepreg laminate at 200 ° C. for 90 minutes while compressing the prepreg laminate with a spacer placed so as to have a thickness of about 40 mm. The obtained prepreg laminate was heat-treated at 2000 ° C. under an inert atmosphere to obtain a flat plate-shaped carbon fiber sheet laminate. The bulk density of the obtained carbon fiber sheet laminate was 0.15 g / cm ³ .

(Example 2)
^{A fiber felt (300 × 300 × 8 (thickness) mm, grain size 420 g / m 2} ) was prepared by setting the mass ratio of the isotropic pitch carbon fiber and the PAN carbon fiber to 30:70 in the preparation of the fiber felt. Except for the above, the carbon fiber sheet laminate according to Example 2 was obtained in the same manner as in Example 1 above. The bulk density of the obtained carbon fiber sheet laminate was 0.14 g / cm ³ .

(Comparative Example 1)
In the production of the fiber felt, the carbon fiber according to Comparative Example 1 was produced in the same manner as in Example 1 above, except that the fiber felt (300 × 300 × 5 (thickness) mm) was produced using only PAN-based carbon fibers. A sheet laminate was produced. The bulk density of the obtained carbon fiber sheet laminate was 0.15 g / cm ³ .

(Comparative Example 2)
In the production of the fiber felt, the same as in Example 1 above, except that the fiber felt (300 × 300 × 10 (thickness) mm, grain size 500 g / m ^{2) was produced using only isotropic pitch carbon fibers.} A carbon fiber sheet laminate according to Comparative Example 2 was produced. The bulk density of the obtained carbon fiber sheet laminate was 0.15 g / cm ³ .

(Three-point bending test)
The carbon fiber sheet laminates according to Examples 1 and 2 and Comparative Examples 1 and 2 were cut into a length of 250 mm, a width of 40 mm, and a height of 40 mm, respectively, to obtain a test piece 200. The test piece 200 was placed on a table 10 in which the distance between the fulcrums was set to 200 mm. Pressure was applied to the test piece 200 by the indenter 20, and the relationship between the pressure and the amount of displacement was measured. The results are shown in FIG. 3 and Table 1.

(Peeling test)
The carbon fiber sheet laminates according to Examples 1 and 2 and Comparative Examples 1 and 2 were cut into a length of 40 mm, a width of 40 mm, and a height of 40 mm, respectively, to obtain a test piece 300. As shown in FIG. 4, the carbon fiber sheet is laminated in the height direction of the test piece, and the carbon fiber sheet to be both end faces of the test piece 300 and the jig 30 are attached with an epoxy adhesive. Fixed using. The test piece 300 was pulled in the height direction using a jig 30, and the peeling strength (stress) when the laminated boundary surface of the test piece was peeled was measured. The results are shown in Table 1.

In Comparative Example 1, the maximum stress was as small as 0.38 MPa in the three-point bending test. Further, in Comparative Example 2, although the maximum stress was as large as 0.81 MPa, the carbon fiber sheet laminate was immediately destroyed when the maximum load was reached. On the other hand, in Examples 1 and 2, it can be seen that the maximum stress is as large as 0.74 MPa and 0.84 MPa, and the stress is not destroyed at once even after the maximum load is reached.

Further, in Comparative Example 1, the peeling strength was as small as 10 kPa in the peeling test. Further, in Comparative Example 2, the peel strength was as large as 55 kPa. On the other hand, in Examples 1 and 2, it can be seen that the peel strength is 26 kPa and 14 kPa, which are higher than those in Comparative Example 1. These things can be considered as follows.

In Comparative Example 1, only PAN-based carbon fibers, which have high strength as a single substance but are less likely to be entangled with each other, are used. Therefore, there are few contacts between the carbon fibers, and the strength of the carbon fiber sheet is low. However, the cracks generated in one carbon fiber sheet do not easily progress to other carbon fiber sheets continuously and do not break at once. This can be confirmed from the fact that in the test result of FIG. 3, the displacement progresses without a large change in stress in the region where the mutation amount is 13% or more.

Further, in Comparative Example 2, only isotropic pitch carbon fibers in which the fibers are easily entangled with each other are used. Therefore, there are many points of contact between the carbon fibers, and the strength of the carbon fiber sheet is high. However, the cracks generated in one carbon fiber sheet easily proceed to the other carbon fiber sheets continuously, and the carbon fiber sheet laminate is destroyed at once. This can be confirmed from the fact that in the test result of FIG. 3, the stress changes at once from 0.81 MPa to 0 MPa in the vicinity of the displacement amount of 25%.

On the other hand, in Examples 1 and 2, isotropic pitch-based carbon fibers and PAN-based carbon fibers are mixed and used. Therefore, the isotropic pitch-based carbon fibers and the PAN-based carbon fibers are entangled in the carbon fiber sheet, and the fibers are well entangled with each other by the action of the isotropic pitch-based carbon fibers. Therefore, while the three-point bending strength and peeling strength are maintained high, when cracks are generated due to the action of PAN-based carbon fibers, it is difficult for them to continuously proceed to other carbon fiber sheets, and in the bending test. The amount of displacement of is large. This can be confirmed from the fact that in the test result of FIG. 3, the displacement progresses without a large change in stress in the region where the displacement amount is 20% or more.

In Example 1 in which the ratio of the isotropic pitch-based carbon fibers is higher, the strength in the bending test is lower and the peel strength is higher than in Example 2. Therefore, the ratio of isotropic pitch-based carbon fibers and PAN-based carbon fibers to the total carbon fibers may be determined from the bending strength and peel strength required for the intended use. Here, the ratio of the isotropic pitch carbon fiber to the total carbon fiber is 20% by mass or more, the ratio of the PAN carbon fiber is 20% by mass, and the ratio of the total mass of the isotropic pitch carbon fiber and the PAN carbon fiber is. 90% by mass or more.

FIG. 5 shows a cross-sectional micrograph of the carbon fiber sheet laminate according to Example 1 in the vicinity of the surface layer. 5A and 5B are microscopic cross-sectional photographs of the carbon fiber sheet laminate according to Example 1, in which FIG. 5A shows a plan view and FIG. 5B shows a side view. As shown in FIGS. 5A and 5B, the carbon fiber sheet laminate has a relatively large diameter (average diameter of 13 μm) curved isotropic pitch carbon fiber 3 and a relatively large diameter. It can be seen that the thin PAN-based carbon fibers 4 (with an average diameter of 7 μm) are entangled three-dimensionally and randomly.

The carbon fiber sheet laminate according to the present invention is excellent in strength, self-supporting property and workability, and has a high stress relaxation effect. The carbon fiber sheet laminate having such properties is particularly suitable for use in an environment where stress failure is likely to occur, and its industrial significance is great.

1 Carbon fiber sheet 3 Isotropic pitch carbon fiber 4 Polyacrylonitrile carbon fiber 10 units 20 Indenter 30 Jigger 100 Carbon fiber sheet laminate 200 Test piece 300 Test piece

Claims (3)

Carbon having a fiber felt in which carbon fibers are entangled three-dimensionally at random and a protective carbon layer covering the carbon fiber surface of the fiber felt, and a plurality of carbon fiber sheets made of carbonaceous material are laminated. It is a fiber sheet laminate,
The carbon fibers include isotropic pitch-based carbon fibers and polyacrylonitrile-based carbon fibers.
The mass ratio of the isotropic pitch carbon fiber to the total mass of the carbon fiber is 20% or more.
The mass ratio of the polyacrylonitrile-based carbon fiber to the total mass of the carbon fiber is 20% or more.
The ratio of the total mass of the isotropic pitch-based carbon fiber and the polyacrylonitrile-based carbon fiber to the total mass of the carbon fiber is 90% or more.
A carbon fiber sheet laminate characterized by this. The isotropic pitch-based carbon fiber is a curved carbon fiber.
The carbon fiber sheet laminate according to claim 1. A fiber felt manufacturing process in which carbon fibers are randomly entangled three-dimensionally to form a fiber felt.
A prepreg preparation step of impregnating the fiber felt with a thermosetting resin to prepare a prepreg of a carbon fiber sheet,
A laminating step of laminating a plurality of the prepregs to form a prepreg laminated body,
A binding step of heating the prepreg laminate while applying pressure to heat-curing the thermosetting resin and binding the prepreg.
It has a carbonization step of heat-treating the bound prepreg laminate in an inert gas atmosphere to carbonize the thermosetting resin.
The carbon fibers include isotropic pitch-based carbon fibers and polyacrylonitrile-based carbon fibers.
The mass ratio of the isotropic pitch carbon fiber to the total mass of the carbon fiber is 20% or more.
The mass ratio of the polyacrylonitrile-based carbon fiber to the total mass of the carbon fiber is 20% or more.
A method for producing a carbon fiber sheet laminate, wherein the ratio of the total mass of the isotropic pitch-based carbon fiber and the polyacrylonitrile-based carbon fiber to the total mass of the carbon fiber is 90% or more.

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