Carbon fiber structure and process for producing the same

An infusiblized, or infusiblized and slightly carbonized fiber of optically anisotropic pitch type is combined with a phenolic resin fiber to produce a high bulk density carbon fiber structure in the form of a laminate of mutually entangled carbon fiber sheets with improved handleability of fiber sheets and improved stability of a laminate structure formed through entanglement. A high flexural strength carbon-carbon composite material with a high volume fiber content is produced by impregnation with a precursor of carbon and subsequent carbonization of the carbon fiber structure of the kind as described above or a fiber laminate of mutually entangled sheets of the infusiblized, or infusiblized and slightly carbonized fiber of optically anisotropic pitch type blended with the phenolic resin fiber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a high bulk density carbon fiber structure 
comprising a carbon fiber of optically anisotropic pitch as the main 
component, and to a process for producing the same. 
More particularly, the present invention relates to a high bulk density 
carbon fiber structure and a process for producing the same, in which 
utilization of the large elongation and shrinkage of a phenolic resin 
fiber in combination with an optically anisotropic pitch fiber produces a 
carbon fiber structure comprising a carbon fiber of optically anisotropic 
pitch as the main component and increased in bulk density, while improving 
the handleability of sheets of the phenolic resin fiber and the pitch 
fiber, which may be either blended with each other in sheets or constitute 
respective sheets, as we11 as stabilizing a laminate structure of the 
sheets formed through entanglement thereof. 
2. Prior Art 
Carbon fiber structures have heretofore been produced by mutual 
superposition and adhesion of necessary pieces of a carbon fiber sheet 
material in a thinly and flatly spread form of fiber, such as a mat, a 
paper-like material, a non-woven fabric, a woven fabric, or a knitted 
fabric. 
The kind of adhesive used in the production of such a carbon fiber 
structure greatly differs from one use application of the structure to 
another. For example, where the carbon fiber structure is used together 
with a plastic material to form a composite material, a choice is made of 
an adhesive having a good affinity for the plastic material and is 
occasionally having to be made of an adhesive which is substantially the 
same resin as the plastic material. Where the carbon fiber structure is 
used together with a metallic material to form a composite material, a 
choice is often desired to be made of an adhesive which is decomposed into 
vaporizable matter during the course of production of the composite 
material. 
Where the carbon fiber structure is used together with a carbon material to 
form a composite material, a choice is desired to be made of an adhesive 
high in yield of carbon in carbonization thereof and capable of forming a 
high strength carbon material. 
Carbon fibers have recently been increasingly evaluated as fibers for use 
in industrial materials. With an eye to such use of carbon fibers, there 
has been an increasing demand for a carbon fiber structure free of foreign 
matter and contaminants. 
Taking as one example of industrial materials heat-insulating materials to 
be used at high temperatures, fibrous materials have heretofore been 
frequently used as such heat-insulating materials, while foamed materials 
have not so often been used. In the case of porous ceramic materials to be 
used as heat-insulating materials at high temperatures, the presence of 
any pores confined inside the materials entails a danger of explosive 
destruction of the materials when the internal pressure of the pores 
varies in keeping with a temperature change. Accordingly, open-cell porous 
ceramic materials as well as fibrous ones are evaluated to be better. 
Under conditions necessitating a highly heat-resistant material, a high 
level of heat resistance inherent in carbon is highly evaluated. Since 
processed carbon materials which have heretofore been used as 
heat-insulating materials are mostly produced from natural materials as 
starting materials, however, they involve inherent problems, including 
contamination of themselves and surroundings with impurities derived from 
the natural materials. 
The use of a carbon fiber high in purity of carbon may naturally be 
considered an effective solution to the problem of contamination. When an 
adhesive is used in the course of production of a processed carbon 
material in the form of a carbon fiber structure, however, the adhesive 
becomes a source of contaminants in most cases. 
When an adhesive capable of complete decomposition upon heating is used to 
produce a processed carbon material in the form of a carbon fiber 
structure, the decomposition of the adhesive causes destruction of the 
processed carbon material. 
An effective means as a solution to the foregoing problems may involve 
elimination of adhesives. From such considerations, interfiber 
entanglement has heretofore been a solution to the same problems with 
organic fibers. 
Carbon fibers of optically anisotropic pitch type are very useful by virtue 
of their high strengths and high moduli of elasticity. However, they 
involve an essential difficulty in interfiber entangelment through 
needling or the like because of their small elongations and linear fiber 
morphology. 
An object of the present invention is to provide a solution to such 
problems with production of a carbon fiber structure of optically 
anisotropic pitch that entanglement of a carbon fiber of optically 
anisotropic pitch through needling or the like is liable to be ineffective 
because of its comparatively poor twisting, bending and crimping 
properties, unlike those of common organic fibers, as well as because of 
its substantially circular cross section; and that the entanglement cannot 
go on smoothly because the carbon fiber of optically anisotromic pitch is 
liable to break sooner than move during the course of entanglement because 
of its very small elongation. 
Another object of the present invention is to provide a carbon fiber 
structure of optically anisotropic pitch type which can be produced 
without the foregoing problems with the production thereof being involved 
and is increased in bulk density. 
SUMMARY OF THE INVENTION 
As a result of intensive investigations on the foregoing problems, the 
authors of the present invention have found out that mutual superposition 
and entanglement of sheets of an infusiblized, or infusiblized and 
slightly carbonized fiber of optically anisotropic pitch together with 
sheets constituted of the abovementioned fiber and a phenolic resin fiber 
enables a high bulk density carbon fiber structure to be easily produced 
with very good handleability of the sheets while stabilizing the laminate 
structure of the sheets. The present invention has been completed based on 
such findings. 
More specifically, in accordance with a processing-related aspect of the 
present invention, there is provided (1) a process for producing a high 
bulk density carbon fiber structure, comprising the step of piling up 
sheets of an infusiblized, or infusiblized and slightly carbonized fiber 
of optically anisotropic pitch blended with a phenolic resin fiber, the 
step of entangling the piled-up sheets with each other to form a fiber 
laminate, and the step of carbonizing the fiber laminate. 
In accordance with the same aspect of the present invention, there also is 
provided (2) a process for producing a high bulk density carbon fiber 
structure, comprising the step of piling up one or more sheets of an 
infusiblized, or infusiblized and slightly carbonized fiber of optically 
anisotropic pitch together with one or more sheets of a phenolic resin 
fiber either alone or blended with an infusiblized, or infusiblized and 
slightly carbonized fiber of optically anisotropic pitch, the step of 
entangling the piled-up sheets with each other to form a fiber laminate, 
and the step of carbonizing the fiber laminate. 
In accordance with the same aspect of the present invention, there is 
further provided (3) a process for producing a carbon-carbon composite 
material with a high volume fiber content, comprising the step of piling 
up sheets of an infusiblized, or infusiblized and slightly carbonized 
fiber of optically anisotropic pitch type blended with a phenolic resin 
fiber, the step of entangleing the piled-up sheets with each other to form 
a fiber laminate, the step of impregnating the fiber laminate with a 
precursor of carbon, and the step of carbonizing the impregnated fiber 
laminate. 
In accordance with a product-related aspect of the present invention, there 
is provided (4) a high bulk density carbon fiber structure comprising 
mutually tightly laminated and entangled sheets of a carbon fiber of 
optically anisotropic pitch blended with a carbon fiber of phenolic resin. 
In accordance with the same aspect of the present invention, there also is 
provided (5) a high bulk density carbon fiber structure comprising one or 
more sheets of a carbon fiber of optically anisotropic pitch and one or 
more sheets of a carbon fiber of phenolic resin either alone or blended 
with a carbon fiber of optically anisotropic pitch, wherein all the sheets 
are mutually tightly laminated and entangled. 
In accordance with the same aspect of the present invention, there is 
further provided (6) a high flexural strength carbon-carbon composite 
material with a high volume fiber content, comprising a fiber laminate of 
mutually laminated and entangled sheets wherein a carbon fiber of 
optically anisotropic pitch is combined with a carbon fiber of phenolic 
resin, and wherein the fiber laminate is in a state of impregnation with 
carbon in the form of a matrix. 
DETAILED DESCRIPTION 
The present invention will now be described in more detail. 
The infusiblized, or infusiblized and slightly carbonized fiber of 
optically anisotropic pitch type to be used in the present invention may 
be produced through a customary infusiblization treatment or customary 
infusiblization and slight carbonization treatments of a pitch fiber 
melt-spun from petroleum, coal or like pitch either comprising optically 
anisotropic components or being easily convertible into optically 
anisotropic matter under stress or heat, which pitch is usually used to 
produce a carbon fiber. 
Examples of the pitch easily convertible into optically anisotropic matter 
under stress or heat include a variety of optically isotropic pitchs 
comprising components easily convertible into optically anisotropic matter 
which are collected through extraction of heavy oil or pitch, or obtained 
through reduction of optically anisotropic pitch into optically isotropic 
pitch easily convertible into optically anisotropic matter. 
Methods of melt-spinning optically anisotropic pitch to produce a pitch 
fiber include a spun bonding method wherein pitch is spun from a common 
spinneret and drawn with the aid of a gaseous stream or a roller; a melt 
blowing method wherein pitch is spun from spinning orifices or slits 
having respective pitch outlets in a high-speed gaseous stream (air, 
steam, combustion waste gas, or the like) and drawn into fine fibers with 
the aid of the high-speed gaseous stream; and a centrifugal spinning 
method wherein pitch in a pot or a dish being revolved at a high speed is 
sprayed into fine liquid pitch streams by the centrifugal force thereof to 
form a fiberous material therefrom. For production of a mat-like material, 
the melt bowing method is preferable from the viewpoint of cost and 
quality. 
The infusiblized fiber of optically anisotropic pitch that may be used in 
the present invention is a fiber produced by heat-treating a fiber, 
melt-spun from optically anisotropic spinning pitch, in an oxidative 
atmosphere at relatively low temperatures including a maximum temperature 
of 200.degree. to 400.degree. C. according to a customary method to render 
the melt-spun fiber infusible for the purpose of preventing the fiber from 
undergoing interfiber fusion bonding. 
The infusiblized and slightly carbonized fiber of optically anisotropic 
pitch that may be used in the present invention is a fiber produced by 
heating the infusibilized fiber of optically anisotropic pitch up to at 
most 1,000.degree. C., preferably at most 800.degree. C., at a heat-up 
rate of 10.degree. to 100.degree. C./min in an atmosphere of an inert gas 
according to a customary method to slightly carbonize the infusiblized 
fiber for the purpose of improving the strength thereof so as to make the 
fiber well adapted for handling thereof in the piling-up, entanglement and 
carbonization steps involved in the process of the present invention. 
The sheets to be used in the present invention may be in the form of a mat, 
a woven fabric, a knitted fabric, a non-woven fabric, a paper-like 
material, an arrangement of fiber bundles such as slivers, or any other 
arbitrary fibrous sheet such as a fibrous flat material represented by a 
random web without any particular restrictions. Especially preferred are 
mats formed directly in association with the melt spinning step. For 
example, mats formed by the melt blowing method can be conveniently used. 
The infusiblization treatment, or infusiblization and slight carbonation 
treatments of an optically anisotropic pitch fiber may be done either 
immediately after a melt-spun pitch fiber is transferred to a 
fiber-collecting unit, or after the melt-spun pitch fiber is formed either 
into a cotton-like material through appropriate cutting or into a 
non-woven fabric through interfiber self-adhesion and/or entanglement. 
Since the optically anisotropic pitch fiber before the infusiblization 
treatment thereof is so very weak as to be often incapable of resisting 
the processing thereof to form it into a configuration corresponding to 
the fiber structure, it is necessary to use the infusiblized, or 
infusiblized and slightly carbonized fiber of optically anisotropic pitch 
for formation of fiber into a configuration corresponding to the fiber 
structure. 
The slight carbonization temperature may be arbitrarily set in accordance 
with the characteristics, such as strength and elongation, of the 
optically anisotropic fiber to be used. However, the slight carbonization 
temperature is preferably at most about 800.degree. C. to ensure as much 
as possible that it can avoid being substantially the same as the 
carbonization temperature to be used in the latter step involved in the 
process of the present invention. 
Any phenolic resin fiber of arbitrary choice such as a novolak fiber can be 
used in the present invention. However, it is preferable to use a phenolic 
resin fiber which is great in both elongation and shrinkage during heat 
treatment for carbonization thereof (when compared to the pitch-derived 
fiber) and is capable of having a high purity of carbon after 
carbonization thereof, examples of which fiber include Kynol (trade name 
of a product manufactured by Gun-ei Chemical Industry Co., Ltd.). 
A phenolic resin fiber having a small elongation is not preferable because 
it may be so ineffective in improving the processability of the 
infusiblized, or infusiblized and slightly carbonized fiber of optically 
anisotropic pitch when formed into the configuration corresponding to the 
carbon fiber structure that damage to the carbon fiber structure may be 
increased. 
A specific description will now be made of the processes for producing a 
carbon fiber structure or a carbon-carbon composite material according to 
the present invention. 
In the process (1) of the present invention, the infusiblized, or 
infusiblized and slightly carbonized fiber of optically anisotropic pitch 
may be blended with the phenolic resin fiber by a customary method such as 
blending of staple fibers formed from the two kinds of fibers or blending 
through carding of mat-like materials formed from the two kinds of fibers. 
The blending proportion of the infusiblized, or infusiblized and slightly 
carbonized fiber of optically anisotropic pitch relative to the blend 
thereof with the phenolic resin fiber may be arbitrarily set in accordance 
with the characteristics, such as bulk density, required of the carbon 
fiber structure to be produced according to the present invention. In 
order to make the most of the advantages, such as large elongation and 
large shrinkage, of the phenolic resin fiber as well as the advantages, 
such as high strength and high modulus of elasticity, of the infusiblized, 
or infusiblized and slightly carbonized fiber of optically anisotropic 
pitch, however, it is preferable that the blending proportion of the 
infusiblized, or infusiblized and slightly carbonized fiber of optically 
anisotropic pitch type relative to the blend thereof with the phenolic 
resin fiber be about 60 to 90 wt. % and the blending proportion of the 
phenolic resin fiber relative to the abovementioned blend be about 10 to 
40 wt. % accordingly. 
When the blending proportion of the phenolic resin fiber relative to the 
blend is less than about 10 wt. %, the advantageous physical properties, 
such as large elengation and large shrinkage, of the phenolic resin fiber 
may not be manifested satisfactorily. When it exceeds about 40 wt. %, the 
advantageous physical properties, such as high strength and high modulus 
of elasticity, of the infusibilized, or infusibilized and slightly 
carbonized fiber of optically anisotropic pitch tends not to be manifested 
satisfactorily. 
The foregoing description in connection with the process (1) of the present 
invention can substantially apply to the processes (2) and (3) of the 
present invention if some points thereof are read in such a changed 
context as to be adapted for the latter processes. 
The sheets of such fiber material are piled up and entangled with each 
other before the carbonization treatment thereof. 
The sheets may be piled up by a customary method such as hand lay-up of an 
appropriate number of sheets in a predetermined position. 
The entanglement of the piled-up sheets with each other may be done by any 
one of various methods, examples of which include needle punching, 
entanglement using a jet of water stream, collision of a fluid, such as 
water, containing a fiber suspended therein against piled-up sheets, and 
combinations thereof. 
Since the carbon fiber structure of the present invention is generally used 
in the form of a thick sheet in most cases, the needle punching method 
capable of entangling thick sheets of fiber with each other up to a 
considerable depth thereof provides good results, as compared with the 
methods of entanglement using a fluid which are capable of efficiently 
entangling thin sheets of fiber with each other. 
The resulting fiber laminate of entangled sheets is carbonized in an 
atmosphere of an inert gas at a temperature of at least about 800.degree. 
C. according to a customary method. The carbonization temperature may be 
arbitrarily set in accordance with the characteristices, such as strength 
and modulus of elasticity, required of the carbon fiber structure to be 
produced according to the present invention. 
The phenolic resin fiber is characterized by a shrinkage of about 25% when 
heat-treated at a temperature of at least 800.degree. C. for carbonization 
thereof. 
When the sheets of the infusiblized, or infusiblized and slightly 
carbonized fiber of optically anisotropic pitch blended with the phenolic 
resin fiber is piled up, entangled with each other and heat-treated to be 
carbonized according to the process (1) of the present invention, the 
shrinking effect of the phenolic resin fiber increases the bulk density of 
the resulting carbon fiber structure as against the bulk density of a 
carbon fiber structure produced from only the infusiblized, or 
infusiblized and slightly carbonized fiber of optically anisotropic pitch 
type, while enhancing the effect of entanglement to increase the 
fiber-holding power of the carbon fiber structure. This improves the 
abrasion resistance and vibration resistance of the carbon fiber 
structure. The same is true of the processes (2) and (3) of the present 
invention. 
In the process (3) of the present invention, examples of the precursor of 
carbon usable for impregnation therewith of the fiber laminate include 
easily carbonizable petroleum pitch, coal pitch and thermosetting resins, 
which are commonlly used to produce carbon-carbon composite materials. 
The fiber laminate is impregnated with a liquid carbonizable substance such 
as a resin or a pitch, which is the precursor of matrix carbon, under 
reduced pressure, under increased pressure, or under reduced and 
subsequently increased pressures or vice versa. The impregnation may be 
followed by press molding if necessary. 
The extent of the impregnation may be appropriately chosen in accordance 
with the desired physical properties, such as flexural strength and bulk 
density, of the carbon-carbon composite material to be produced according 
to the present invention. 
Subsequently, the impregnated fiber laminate is carbonized and, if 
necessary, graphitized in an atmosphere of an inert gas under atmospheric 
or increased pressure to form the carbon-carbon composite material endowed 
with necessary physical properties (strength, bulk density, etc.). 
As described hereinbefore, large shrinkage of the phenolic resin fiber 
during the course of carbonization treatment increases the bulk density of 
the carbon fiber structure according to the present invention. In the same 
context, the fiber laminate impregnated with the precursor of carbon, when 
carbonized, can be formed into the carbon-carbon composite material 
endowed with a high volume fiber content and a high flexural strength. The 
same is true of the case where the carbon fiber structure produced 
according to the process (1) or (2) of the present invention is 
impregnated with a precursor of carbon and subjected to further 
carbonization. 
Additionally stated, the fibers to be used in the present invention usually 
have an average filament diameter which is in the range of 1 .mu.m to 50 
.mu.m, while the total number of sheets thereof to be entangled with each 
other in the present invention is preferably 2 to 150, more preferably 8 
to 110. 
The carbon fiber structure of the present invention, which is constituted 
of the sheets of carbon fiber tightly entangled with each other, exhibits 
an excellent performance as a heat-insulating material that may be used 
particularly in an inert atmosphere at high temperatures at which radiant 
heat transfer predominates over convectional heat transfer and conductive 
heat transfer, because it is mainly constituted of the carbon fiber of 
optically anisotropic pitch type excellent in heat resistance and is mixed 
with very little amounts of components other than carbon. Accordingly, the 
carbon fiber structure of the present invention can be used as a 
heat-insulating material that may be provided just outside of the body of 
an electric furnace or in contact with the high temperature part of a 
nuclear reactor. 
The carbon fiber structure of the present invention can also be suitably 
used as a filter medium particularly in a non-oxidizing but 
high-temperature atmosphere or in a relatively low-temperature but 
severely corrosive atmosphere. 
The high bulk density carbon fiber structure of the present invention, when 
impregnated with an easily carbonizable substance and subjected to further 
carbonization, can be formed into a carbon-carbon composite material with 
a high volume fiber content. 
The advantages of the present invention will be summarized as follows. 
A laminate of carbon fiber sheets can be produced by piling up sheets of a 
carbon fiber and bonding the sheets to each other. When sheets of a carbon 
fiber of pitch type is bonded to each other through entanglement, however, 
damage to the carbon fiber is often caused and the efficiency of 
entanglement is poor because the carbon fiber of pitch type is small in 
elongation. By contrast, according to the present invention, the large 
elongation and shrinkage of the phenolic resin fiber is made the most of 
in the production of the carbon fiber structure constituted mainly of the 
carbon fiber of optically anisotropic pitch type to achieve improvements 
in handleability of fiber sheets and stability of a laminate structure 
formed through entanglement of fiber sheets without any substantial 
decrease in purity of carbon in the carbon fiber structure. 
The shrinking action of the phenolic resin fiber during heat treatment for 
carbonization makes the resulting carbon fiber structure high in bulk 
density. This is translated into an increase in volume fiber content in 
the case of a carbon-carbon composite material produced either by the 
process (3) of the present invention or by impregnating the carbon fiber 
structure of the present invention with a precursor of carbon and 
subjecting the impregnated structure to further carbonization.

BEST MODES FOR CARRYING OUT THE INVENTION 
The following Examples will now specifically illustrate the present 
invention in more detail, but should not be construed as limiting the 
scope of the invention. 
EXAMPLE 1 
Petroleum pitch having a softening point of 275.degree. C. and an optically 
anisotropic components content of 95 wt. %, as a starting material, was 
spun by the centrifugal spinning method, heat-treated by a customary 
method to be rendered infusible, and slightly carbonized by a customary 
method (maximum temperature: 700.degree. C.) to produce mat-like sheets of 
an infusiblized and slightly carbonized fiber of optically anisotropic 
pitch having a unit weight of 120 g/m.sup.2 and a thickness of 5 mm. 
Using six mat-like sheets of the pitch type fiber thus produced and four 
felt sheets (unit weight: 100 g/m.sup.2) of a phenolic resin fiber (Kynol 
manufactured by Gun-ei Chemical Industry Co., Ltd.) with the total number 
of sheets being 10, three sets each of two mat-like sheets of the pitch 
type fiber were piled up altenatively with the four felt sheets of the 
phenolic resin fiber in such a way as to position two felt sheets of the 
phenolic resin fiber on the respective two sides of the resulting assembly 
of the sheets, followed by entanglement of the sheets with each other 
through needle punching at a punching density of 35 times/cm..sup.2 
The resulting fiber laminate was carbonized in an inert gas at high 
temperatures including a maximum temperature of 1,600.degree. C. The 
resulting carbon fiber structure, which underwent no inter-sheet debonding 
and was in a tightly entangled state, had a bulk density of 0.13 g/cc and 
showed excellent heat-insulating properties at high temperatures. 
Substantially the same procedure as described above except that cloth 
sheets (unit weight: 190 g/m.sup.2) of the phenolic resin fiber (Kynol 
manufactured by Gun-ei Chemical Industry Co., Ltd.) were used instead of 
the felt sheets of the phenolic resin fiber was repeated to produce a 
carbon fiber structure having a performance comparable to that of the 
carbon fiber structure produced first. 
Additionally stated, when the same phenolic resin fiber as used in the 
above-mentioned felt sheets and cloth sheets was carbonized by heating in 
an inert gas up to 900.degree. C. at a heat-up rate of 5.degree. C./min, 
the shrinkage of the fiber was 24%. 
EXAMPLE 2 AND COMATIVE EXAMPLE 1 
Petroleum pitch having a softening point of 284.degree. C. and an optically 
anisotropic components content of 100 wt. %, as a starting material, was 
spun by the melt blowing method, heat-treated by a customary method to be 
rendered infusible, and slightly carbonized by a customary method (maximum 
temperature: 600.degree. C.) to produce a mat-like material of an 
infusiblized and slightly carbonized fiber of optically anisotropic pitch 
type having a unit weight of 120 g/m..sup.2 
In carding the mat-like material of the pitch type fiber thus produced and 
a mat-like material (unit weight: 200 g/m.sup.2) of a phenolic resin fiber 
(Kynol manufactured by Gun-ei Chemical Industry Co., Ltd.), 85 wt. % of 
the infusiblized and slightly carbonized fiber of optically anisotropic 
pitch type was blended with 15 wt. % of the phenolic resin fiber to form 
card webs, ten of which were then entangled with each other through needle 
punching at a punching density of 35 times/cm..sup.2 
The resulting fiber laminate was carbonized in an inert gas at high 
temperatures including a maximum temperature of 2,000.degree. C. The 
resulting carbon fiber structure, which underwent no inter-web debonding 
and was in a tightly entangled state, had a bulk density of 0.16 g/cc and 
showed excellent heat-insulating properties at high temperatures. 
Substantially the same procedure as described above except that the 
infusiblized and slightly carbonized fiber of optically anisotropic pitch 
type alone was used in carding was repeated to produce a carbon fiber 
structure having a bulk density of 0.048 g/cc. During the course of the 
carbonization treatment, a liability to inter-web debonding was observed 
by sight. 
EXAMPLE 3 AND COMATIVE EXAMPLE 2 
Petroleum pitch having a softening point of 284.degree. C. and an optically 
anisotropic components content of 100 wt. % was spun out of a common-type 
spinneret for melt spinning and sucked into an aspirator to be formed into 
a fiber, which was then collected on a net conveyor, heat-treated by a 
customary method to be rendered infusible, and slightly carbonized by a 
customary method (maximum temperature: 600.degree. C.) to produce sheets 
of an infusiblized and slightly carbonized fiber of optically anisotropic 
pitch having a unit weight of 120 g/m..sup.2 
The sheets of the pitch fiber thus produced were piled up and entangled 
with each other. The resulting fiber laminate was impregnated with 
petroleum pitch having a softening point of 147.degree. C. by a customary 
method, followed by carbonization thereof at high temperatures including a 
maximum temperature of 1,000.degree. C. 
The resulting carbon-carbon composite material had a volume fiber content 
of 2.5 wt. % and a flexural strength of 3.9 kg/mm.sup.2 as measured 
according to JIS K6911. 
The same sheets of the pitch fiber as used in the foregoing procedure was 
used together with the same phenolic resin fiber as used in Example 2 to 
form blended card webs in the same manner as in Example 2. 50 card webs 
thus formed were piled up and then entangled with each other through 
needle punching at punching density of 40 times/cm..sup.2 The resulting 
fiber laminate was impregnated with the same petroleum pitch as used in 
the foregoing procedure, followed by carbonization thereof at high 
temperatures including a maximum temperature of 1,000.degree. C. 
The resulting carbon-carbon composite material had a volume fiber content 
of 7.2 wt. % and a flexural strength of 5.6 kg/mm.sup.2 as measured 
according to JIS K6911. This flexural strength was a great improvement 
over that of the carbon-carbon composite material produced using the fiber 
laminate of the infusibilized and slightly carbonized fiber of optically 
anisotropic pitch alone.