Carbon fiber sheet and process for production thereof

Details Carbon fiber sheet and process for production thereof Carbon fiber sheet and process for production thereof

2002-07-24

2004-11-02

Jones, Deborah (Department: 1775)

Fabric (woven, knitted, or nonwoven textile or cloth, etc.)

Nonwoven fabric

Including strand or fiber material which is of specific...

C442S368000, C442S365000, C442S179000, C442S102000, C442S103000, C442S021000, C428S408000, C428S098000, C428S219000, C428S220000, C423S447100, C423S447200, C264S029100, C264S640000, C264S642000, C264S319000

Reexamination Certificate

active

06812171

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a carbon fiber sheet obtained by carbonizing an oxidized polyacrylonitrile fiber sheet, as well as to a process for production of the carbon fiber sheet. More particularly, the present invention relates to a carbon fiber sheet which has a high carbon fiber content, is thin, has excellent shape ability, is superior in electrical conductivity of through-plane direction, and is suitable as a conductive material such as earth material, battery electrode material and the like, as well as to a process for production of the carbon fiber sheet.
This carbon fiber sheet is suitably used as an electrode material for cell or battery such as polymer electrolyte fuel cell, redox flow battery, zinc-bromine battery, zinc-chlorine battery or the like, or as an electrode material for electrolysis such as sodium chloride electrolysis or the like.
BACKGROUND ART
A study for using a sheet-like carbon material having electrical conductivity and excellent corrosion resistance, as an earth material or a battery electrode material, is being made. A carbon sheet used in such applications is required to have a small electric resistance in the through-plane direction.
When a carbon fiber sheet is used particularly as a battery electrode material, the carbon fiber sheet must per se have a small thickness and a high bulk density so as to meet the recent movement of cell or battery to smaller size and lighter weight. These properties allow the carbon material to have a reduced electric resistance in the through-plane direction.
As the carbon fiber sheet used in such applications, there have been known a molded carbon material, a carbon fiber fabric, a carbon fiber nonwoven fabric, etc.
As a molded carbon material of sheet shape and high bulk density, there is known a carbon fiber-reinforced carbon material (c/c paper) (JP No. 2584497 and JP-A-63-222078). This sheet is produced by making chopped carbon fibers into a paper, impregnating the resulting paper with a phenolic resin or the like to obtain a phenolic resin composite material, and carbonizing the phenolic resin or the like, in the phenolic resin composite material.
This sheet is produced by press molding using a mold and, therefore, is superior in thickness accuracy and surface smoothness. However, this sheet is inferior in flexibility and is impossible to make into a roll. Therefore, the sheet is unsuitable for applications where a long sheet is needed.
Further, the sheet is fragile and easily broken owing to, for example, the impact applied during the transportation or processing. Furthermore, the sheet has a high production cost and, when used in a large amount as a conductive material, is expensive. The reason why the carbon fiber-reinforced carbon sheet is fragile and inferior in flexibility, is that the sheet contains the carbonization product of the impregnated resin in a large amount.
In order to obtain a sheet of flexibility and yet high bulk density, it is necessary to make high the content of carbon fiber in sheet.
As a sheet-shaped carbon material with flexibility, a carbon fiber fabric is known. As such a fabric, there is a filament fabric (JP-A-4-281037 and JP-A-7-118988) and a spun yarn fabric (JP-A-10-280246).
One of the features of these fabrics is that they have such flexibility as they can be made into a roll and that they are easily handled when stored or used as a long product.
The filament fabric is obtained by weaving a carbon fiber strand into a fabric. The number of the carbon fibers constituting the carbon fiber strand can be various. In the filament fabric, the direction of the carbon fiber axis is basically parallel to the in-plane direction of the fabric. Therefore, the electric resistance of the fabric is low in the in-plane direction but high in the through-plane direction.
Meanwhile, as the spun yarn fabric, there is known a carbon fiber spun yarn fabric obtained by producing an oxidized polyacrylonitrile (PAN) fiber fabric using an oxidized PAN fiber spun yarn and carbonizing it. This carbon fiber spun yarn fabric is generally more flexible than the carbon fiber filament fabric. Further, since being obtained by twisting short fibers, the spun yarn fabric is expected to have a lower electric resistance in the through-plane direction than the carbon fiber filament fabric. Furthermore, the spun yarn fabric has a lower production cost than the above-mentioned C/C paper.
However, conventional carbon fiber spun yarn fabrics are generally low in bulk density. Therefore, they show a high electric resistance in the through-plane direction in applications requiring conductivity, such as electrode and the like, although the electric resistance is lower than that of the C/C paper.
As the spun yarn fabric, there was also proposed a carbon fiber fabric obtained by cutting a PAN-derived carbon fiber into a given length cut fiber and weaving the cut fiber into a fabric (JP-A-10-280246). This fabric, however, has a low bulk density. Compression of this fabric for higher bulk density results in a finely ground material.
As a carbon fiber sheet having flexibility and good handle ability equivalent to those of the carbon fiber fabric, there is a carbon fiber nonwoven fabric. This nonwoven fabric, when subjected to punching, shows a higher shape retain ability than the C/C paper and the carbon fiber fabric, and is produced more easily and at a lower cost than the C/C paper and the carbon fiber fabric. In general, the carbon fiber nonwoven fabric is obtained by subjecting an oxidized PAN fiber to a water jet treatment, a needle punching treatment, etc. to produce an oxidized fiber nonwoven fabric and carbonizing the oxidized fiber nonwoven fabric; therefore, in the carbon fiber nonwoven fabric, the proportion of the fiber whose axis is parallel to the through-plane direction, is larger than in the carbon fiber-reinforced carbon fiber. As a result, the carbon fiber nonwoven fabric is expected to have smaller electric resistance in the through-plane direction than that of the carbon fiber-reinforced carbon sheet.
However, since conventional oxidized fiber nonwoven fabrics are generally low in bulk density, the carbon fiber nonwoven fabric obtained by carbonizing such an oxidized fiber nonwoven fabric has a high electric resistance in the through-plane direction when used in applications such as electrode and the like.
In, for example, JP-A-9-119052 is described a process for producing an oxidized fiber nonwoven fabric, which comprises a making a web using an oxidized PAN fiber and subjecting the web to a water jet treatment. The nonwoven fabric obtained by this process has a low bulk density.
National Publication of International Patent Application No. 9-511802 discloses a technique of producing a fabric or a felt using a two-portion stable fiber having an inner core portion made of a thermoplastic polymer composition and an outer covering portion made of a carbonaceous material, surrounding the inner core portion. This two-portion stable fiber has a relatively low specific gravity of 1.20 to 1.32. A fabric or felt produced using this fiber has a low bulk density.
DISCLOSURE OF THE INVENTION
The present inventors made studies on the specifications of oxidized fiber spun yarn and oxidized fiber sheet and further on the application of a resin treatment or a pressurization treatment to oxidized fiber sheet. As a result, the present inventors found out that a carbon fiber sheet can be produced which has, as compared with conventional products, a high bulk density, appropriate flexibility and a low electric resistance in the though-plane direction. The above finding has led to the completion of the present invention.
The present invention aims at providing a carbon fiber sheet which is suitable as a conductive material such as earth material, battery electrode material or the like, has a high bulk density, appropriate flexibility and a low electric resistance in the through-plane direction, and is superior in shapeability; and a process for producing a such a carbon fiber sheet.
The present invent

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