Triaxial fabric composed of carbon fiber strands and method for production thereof

A triaxial fabric composed of carbon fiber strands is provided, which has a crimp releasing ratio (L'-L)/L satisfying (L.sub.0 -L)/L.sub.0 !.times.0.7.gtoreq.(L'-L)/L.gtoreq.0. Since the crimps of the fiber strands composing the fabric are preliminarily fixed, fiber breakage due to abrasion in the intersecting portions of the fabric is prevented, whereby a high quality triaxial fabric is obtained. A method for the production of such a fabric is also provided.

This application claims benefit of international application 
PCT/JP95/02248, filed Nov. 2, 1995, published as WO96/14455 May 15, 1996. 
1. Technical Field 
The present invention relates to a triaxial fabric composed of carbon fiber 
strands suitable for various members used in the space industry, such as 
those of an artificial satellite, which requires reduced weight and a high 
specific modulus. 
2. Background Art 
Carbon fiber-reinforced plastics have been widely used in various fields 
including the sport/leisure industry, the general industry and the 
aerospace industry, because they are light in weight and excellent in 
specific strength and specific modulus. Recently, the application of such 
materials to the aerospace industry, particularly to various members of 
artificial satellites, has been highlighted due to their characteristics. 
Carbon fibers used as reinforcement are classified as PAN-based or 
pitch-based carbon fibers in accordance with the starting materials 
thereof. The pitch-based carbon fibers are more suitable for the space 
industry than the PAN-based carbon fibers because a graphite crystal 
structure easily grows in the fiber, to result in a high elastic modulus, 
high thermal conductivity and low thermal expansion coefficient, which are 
favorable in the space industry. A reinforcement substrate used for a 
composite may have various configurations such as unidirectional fibers, 
two dimensional fabrics or three dimensional fabrics in accordance with 
the use of the resultant composites. An ordinary fabric wherein two sets 
of fiber strands are interwoven with each other at a right angle 
(generally referred to as a "biaxial fabric") is usually used for a carbon 
fiber reinforced plastic material. Because this fabric exhibits a strong 
anisotropic property, it is necessary, when used in a portion wherein an 
isotropic property is required, to ply a plurality of such fabrics while 
varying orientation angles so that a so-called cross ply structure is 
obtained. Accordingly, there is a tendency for increased weight and for 
generating delamination between plies. 
On the other hand, a triaxial fabric shown in FIGS. 1 and 2 has a structure 
wherein two sets of warp yarns (bias yarns) 2 are interwoven with one set 
of weft yarns (0.degree. directional yarns) 1 at .+-.60.degree. relative 
to a widthwise direction of the fabric. This fabric is inherently 
quasi-isotropic even as a single ply and thus needs no laying up, which 
results in the weight reduction and the prevention of delamination. It is 
also easy to further reduce the thickness of the single fabric. In 
addition, this fabric has advantages in that the shape-retaining ability 
is excellent due to its high shearing rigidity, and the strength per unit 
weight and the compression strength after damage by impact are high, and 
further it has a good aesthetic appearance. 
The triaxial fabric, however, has a larger crimp (wave configuration of 
fiber strand) then the ordinary fabric, which means that fiber strands in 
the triaxial fabric are forcibly bent. Accordingly, in the triaxial fabric 
of carbon fiber strands, the fiber strands are abrasive in the 
intersection thereof and liable to be damaged to cause fiber breakages as 
shown in FIG. 3. The fabric having fiber breakages is not only unfavorable 
from the viewpoint of surface quality, but also has a risk of fabric 
breakage when the fabric is stretched even under a relatively low load. It 
is surmised that such fiber breakages are caused by the following 
mechanisms; one is that a large recovery force generates in the carbon 
fiber strand when the same is bent because of its high elastic modulus, 
which causes a large abrasive force between the intersecting portion of 
the fiber strands; and other is that, since the elongation at break of 
carbon fiber is small, the fiber is easily broken under the abrasion. 
Therefore, when carbon fibers having a high elastic modulus and a smaller 
elongation at break are used, the generation of fiber breakage will be 
further accelerated. 
Accordingly, a triaxial fabric having less fiber breakages and excellent in 
surface quality has been required. Although such a demand is particularly 
aimed at a fabric composed of high elastic modulus carbon fibers, to 
manufacture the triaxial fabric made from the high elastic modulus carbon 
fibers is difficult because fiber breakage is liable to occur during the 
manufacturing process. At present, it is said that carbon fibers capable 
of forming a triaxial fabric in an industrial sense are solely those 
having an elastic modulus of 40 tf/mm.sup.2 or less (see Science & 
Industry, vol. 66, page 273, published in 1992). The weaving operation 
becomes more difficult if fiber strands having a smaller linear density 
are used for the purpose of weight reduction, because the tensile strength 
(durable force) of the fiber strand itself is lowered. 
Notwithstanding such circumstances, triaxial fabrics of reduced weight 
woven from high elastic modulus carbon fiber strands are highly desired. 
Particularly in the aerospace industry, not only reduced weight but also 
other properties are sometimes required, such as a high rigidity, 
heat-dispersibility or thermal dimensional stability, for the purpose of 
obtaining composites of a mono-layered type or a multi-layered type used 
for a skin member (surface layer) of a honeycomb sandwich structure of an 
artificial satellite. To realize such properties, high elastic modulus 
carbon fibers having a high elastic-modulus, high thermal conductivity and 
low coefficient of thermal expansion are suitably used. Thornel p-758, 
P-100 and P-120 available from Amoco Performance Products are examples of 
such carbon fibers having high elastic modulus and high thermal 
conductivity. 
It has long been desired to weave a triaxial fabric which is easily reduced 
in weight from the carbon fibers of the above properties and to use the 
same as a reinforcement substrate for a composite. 
In this case, it is desired that the areal weight of the fabric (fabric 
weight per unit area) be 300 g/m.sup.2 or less and the elastic modulus 
thereof be 3 tf/mm.sup.2 or more in view of the balance between weight 
reduction and improvement of the elastic modulus. However, there are no 
triaxial fabrics having such properties in the prior art, because the 
weaving thereof is impossible, as stated above. 
Thus, the production of triaxial fabrics improved in surface quality and 
those composed of carbon fibers having a high elastic modulus which have 
not been obtained in the prior art is a serious problem in this field. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to solve the above drawbacks in the 
prior art and to provide high quality triaxial fabrics composed of carbon 
fibers and a method for producing the same, which are suitable for a skin 
member of a honeycomb sandwich structure used in an artificial satellite 
or a solar cell panel. 
That is, according to the present invention, a triaxial fabric is provided, 
wherein two sets of warp yarns (bias yarns) of carbon fiber strands are 
interwoven with one set of weft yarns (0.degree. directional yarns) of 
carbon fiber strands having the same cross-sectional area as the bias yarn 
and arranged in the widthwise direction of the fabric, characterized in 
that the fabric has a crimp releasing ratio (L'-L)/L, after being 
subjected to a heat treatment at 1800.degree. C. in an inert gas 
atmosphere, satisfying the following equation: 
EQU (L.sub.0 -L)/L.sub.0 .times.0.7.gtoreq.(L'-L)/L.gtoreq.0 
wherein L is an apparent length of the weft yarn composing the triaxial 
fabric; L.sub.0 is a length of the waft yarn along the crimp; and L' is an 
apparent length of the weft yarn when removed from the fabric. 
As shown in FIGS. 4(a) and (4b), the crimp releasing ratio (L'-L)/L defined 
herein represents a proportion of the length change of the weft yarn when 
the same is optionally pulled out from the fabric. In other words, it 
shows the extent to which the crimping degree (L.sub.0 -L)/L, of the 
fabric is released. If the crimp were completely fixed, the crimping 
degree would not change even after the yarn is been removed from the 
fabric and the crimp releasing ratio becomes zero. While, if the crimp is 
completely released, the crimp releasing ratio would be (L.sub.0 -L)/L. 
The present invention further provides a method for producing a triaxial 
fabric characterized in that mesopitch-based carbon fiber strands having a 
tensile strength of 300 kgf/mm.sup.2 or more, a tensile strain at break of 
0.6% or more and a tensile elastic modulus of 15 tf/mm.sup.2 or more are 
woven into a triaxial fabric, which then is subjected to a heat treatment 
in an inert gas atmosphere at a temperature of 1800.degree. C. or more. 
In addition, the present invention provides, as the use of the triaxial 
fabric described above, composites suitable for a single-layered or a 
multi-layered skin member of a honeycomb sandwich structure for an 
artificial satellite. 
The present invention will be described in more detail below with reference 
to the attached drawings. 
Fibers used in the present invention may be of any kind used in 
conventional triaxial fabrics, such as carbon fibers, aramid fibers or 
glass fibers. Of them, carbon fibers are particularly suitable for the 
present invention because of their high elastic modulus and small 
elongation at break. 
Generally speaking, as shown in FIGS. 4(a) and 4(b), fiber strands in a 
fabric show bending, i.e., crimps. The degree of crimping is represented 
by the following equation (1) as the crimping degree. 
EQU (L.sub.0 -L)/L.sub.0 ( 1) 
A triaxial fabric has a larger crimping degree than the conventional plain 
weave fabric due to its structure. 
The fiber strand is forcibly bent in the fabric. Therefore, when fibers, 
especially carbon fibers having a high elastic modulus and small 
elongation at break, are used, the fiber portion in the crimp generates a 
force straightening the fiber, which results in a compression between the 
adjacent fiber strands intersecting each other. The higher the elastic 
modulus, the larger this force. Under the circumstances, if relative 
movement occurs in the fabric between the fiber strands intersecting each 
other by the handling thereof, etc., the fiber strands may be abraded in 
the intersecting portion, resulting in the damage or breakage of fibers. 
This is particularly significant if carbon fibers having a high elastic 
modulus are used. 
To suppress the generation of fiber breakages, it is conceived to use 
fibers having a low elastic modulus or those having a large elongation at 
break. However, the use of fibers having a high elastic modulus is 
indispensable for obtaining articles having a reduced weight and high 
rigidity. In addition, generally speaking, since the elongation at break 
of carbon fibers decreases as the elastic modulus thereof increases, there 
are few fibers having large values of both elastic modulus and elongation 
at break. 
As another means for suppressing the generation of fiber breakages, it is 
conceived to prepare the fabric with fiber strands inherently having 
original crimps therein to mitigate the compression between the fiber 
strands in the intersecting portion and to prevent fiber damage during 
abrasion. 
However, there have been no methods in the prior art to produce a fiber 
strand inherently having crimps therein. Also it has been thought that, 
even though there were such methods, fiber strands inherently having 
crimps therein would be unsuitable for a reinforcement material because 
the fibers in the fiber strand would be oriented at a maximum angle 
.theta. to the reinforcement direction, which significantly reduces the 
tensile strength thereof. 
According to the study made by the present inventors, it was found that, 
although the fiber strand pulled off from the fabric so that the crimps 
are maintained has a tensile strength and tensile modulus somewhat smaller 
than those of the original straight strand, there is no problem in the 
handling of the fabric itself and also there is almost no reduction in 
either tensile strength or tensile modulus of a resin matrix composite 
using the fabric as a reinforcement substrate. 
It is surmised that this is because, since the fiber is completely coated 
with resin in the resin matrix fabric composite, stresses in the 
respective portions of the fiber are transmitted via the resin, and not 
locally concentrated at a certain point. 
In this case, it is unnecessary to completely fix the crimps so that the 
crimp releasing ratio is zero, but at least 30% of the fabric crimping 
degree must be maintained. If the crimp releasing ratio is smaller than 
30% of the fabric crimping degree (in other words, if the crimp releasing 
ratio is larger than (L.sub.0 -L)/L.sub.0 .times.0.7), the compression 
between the fiber strands in the intersecting portion becomes larger, 
unfavorably increasing the abrasive force. 
In this regard, the value of (L.sub.0 -L)/L.sub.0 can be obtained by 
measuring the linear distance L between two optional points in the fiber 
strand by an optical-microscopic photograph or the like and a length 
L.sub.0 thereof along the crimps by an image processor, a Curvimeter or 
others. 
Values of the areal weight and the elastic modulus of the fabric may be 
optionally selected. However, the areal weight of the triaxial fabric is 
preferably in a range between 45 g/m.sup.2 and 300 g/m.sup.2, more 
preferably between 45 g/m.sup.2 and 180 g/m.sup.2, and the tensile modulus 
of a resin matrix composite wherein the triaxial fabric is impregnated 
with a resin is preferably 3 tf/mm.sup.2 or more, more preferably 4.5 
tf/mm.sup.2 or more. That is, when the composite is used for an artificial 
satellite, the areal weight of the fabric-(fabric weight per unit area) is 
preferably less than 180 g/m.sup.2 to obtain both weight reduction and a 
favorable elastic modulus, but it is difficult to produce a fabric having 
a basis weight of less than 45 g/m.sup.2. From the viewpoint of obtaining 
both weight reduction and a favorable elastic modulus, the tensile elastic 
modulus of the resin matrix composite wherein the triaxial fabric is 
impregnated with a resin is 3 tf/mm.sup.2 or more, preferably 4.5 
tf/mm.sup.2 or more. The triaxial fabric having such characteristics has 
not been known in the prior art. 
This triaxial fabric is produced in the following manner. Carbon fiber 
strands are originated from a mesophase pitch to have a tensile strength 
of 300 kgf/mm.sup.2 or more, a tensile strain at break of 0.6% or more and 
a tensile modulus of 15 tf/mm.sup.2 or more, and woven into a greige 
triaxial fabric. Thereafter, the greige fabric is subjected to a heat 
treatment in an inert gas atmosphere at a temperature of 1800.degree. C. 
or more. This heat treatment temperature of 1800.degree. C. or more is 
selected based on the relationship between the temperature and the crimp 
releasing ratio shown in the graph of FIG. 5. 
The carbon fiber composing the triaxial fabric is obtained from a pitch, 
preferably a mesophase. 
By using the mesophase pitch, crimps in the fiber strand can be well fixed 
by heat treatment at a temperature of 1800.degree. C. or more. Further, 
the carbon fiber prepared from the mesophase pitch is suitable for an 
artificial satellite material, because the properties thereof, such as a 
thermal conductivity, coefficient of thermal expansion, tensile modulus or 
others are superior to those of other carbon fibers. Pitches used as a 
starting material for the mesophase pitch include various kinds, such as 
coal pitch, liquidized coal pitch, ethylene tar pitch, petroleum pitch 
such as decant oil pitch obtained from a decomposition residue of fluid 
catalytic cracking, or synthetic pitch made of naphthalene or the like by 
using a catalyst. These pitches are treated in the conventional manner to 
be in a mesophase so that the optically anisotropic pitch appears therein. 
The carbon fiber has a tensile strength of 300 kgf/mm.sup.2, a tensile 
strain at break of 0.6% or more and a tensile modulus of 15 tf/mm.sup.2 or 
more. 
Even though there may be cases wherein the triaxial fabric can be woven 
from carbon fiber strands having a tensile modulus of less than 15 
tf/mm.sup.2, such a fabric is unfavorable because the weave structure may 
be slackened to generate gaps in the intersecting portions between the 
fiber strands due to the significant volume loss in the fiber strand after 
the graphitization. 
Generally speaking, it is unnecessary to define an upper limit of the 
tensile modulus. However, since the tensile elongation at break becomes 
smaller as tensile modulus becomes higher, weaving of the triaxial fabric 
while using the fiber strands having an excessively high elastic modulus 
becomes difficult. Accordingly, the fiber strands having a tensile 
modulus, for example, of 40 tf/mm.sup.2 or less are suitably used for this 
purpose. Nevertheless, carbon fiber strands having a tensile modulus 
higher than the above limit may be used if the weaving operation is 
possible. 
When carbon fiber strands having a tensile modulus of 15 tf/mm.sup.2 are 
used for weaving the triaxial fabric, the tensile strength thereof must be 
300 kgf/mm.sup.2 or more and the tensile strain at break must be 0.6% or 
more. If the tensile strength is less than 300 kgf/mm.sup.2, the fiber 
strand may not be resistant to weaving tension, and if the tensile strain 
at break is less than 0.6%, fibers in the fiber strand are liable to be 
bent and broken during the weaving process, thereby the production of the 
triaxial fabric becomes difficult. 
Such carbon fibers suitable for the present invention can be produced by a 
conventional method, including a spinning process, an infusion treatment, 
carbonization and/or graphitization. 
The carbon fiber strands are woven into a triaxial fabric by a triaxial 
fabric loom. The weave structure, weaving density, linear density of the 
carbon fiber strand or others may be optionally selected. However, the 
triaxial fabric is usually woven into a basic structure at a weaving 
density of 9.25 end/inch. 
To achieve a basic weight in a final product of 300 g/m.sup.2 or less, 
preferably 180 g/m.sup.2 or less for the purpose of weight reduction, it 
is necessary that the linear density of the fiber strand (defined by a 
weight per unit length or the fiber strand) be 300 g/km or less, 
preferably 180 g/km or less, when the carbon fiber strands (having a 
density of about 2.1) made of a mesophase pitch are woven into the 
triaxial fabric of the basic structure at the weaving density of 9.25 
end/inch. However, if the linear density of the carbon fiber strand is 
less than 40 g/km, the weaving operation becomes difficult because the 
fiber strand is not resistant to a force applied thereon during the 
weaving operation. Accordingly, the linear density of the fiber stand is 
preferably in a range between 40 g/km and 300 g/km, more preferably 
between 40 g/km and 160 g/km. 
The triaxial fabric thus woven is subjected to a heat treatment in an inert 
gas atmosphere at a temperature of 1800.degree. C. or more. This heat 
treatment may be of a batch type wherein the triaxial fabric is 
preliminarily cut into pieces of a size suitable for being loaded into a 
furnace and processed therein in an inert gas atmosphere, or of a 
continuous type wherein the triaxial fabric is continuously supplied into 
a heat zone filled with an inert gas. The inert gas is, for example, 
N.sub.2, Ar or others which are chemically inactive. 
The inert gas atmosphere is used for preventing depletion by oxidation of 
the carbon fibers during the heat treatment. Therefore, the inclusion of a 
small amount of oxidative gas may be allowed as far as no depletion 
occurs. For example, there are cases wherein a carbon dioxide gas 
atmosphere exhibiting a weak acidic property can be used, provided the 
heat treatment is carried out for a short time. 
The furnace may be of any type, provided it is possible to heat the fabric 
of a predetermined size at a predetermined temperature. 
It is possible to fix the crimp in the fabric in the inert gas atmosphere 
at 1800.degree. C. or more as stated above. When carbon fibers having a 
tensile modulus of 15 tf/mm.sup.2 are used, the heat treatment must be 
carried out at 1800.degree. C. or more for crimp fixation. A certain 
relationship is observed between the ease of crimp fixation and the growth 
of graphite crystal (see FIG. 5). That is, carbon fibers wherein graphite 
crystal growth is high (i.e., having a higher elastic modulus) need to be 
heat treated at a higher temperature. However, the duration of the heat 
treatment also influences the crimp fixation, and therefore, there are 
cases wherein a long heat treatment at a low temperature is equivalent in 
effect to a short heat treatment at a high temperature. 
Accordingly, in such cases a heating profile can be experimentally 
determined in view of the crystal conditions of graphite in the carbon 
fiber strand used for weaving the fabric and the desired crystal 
conditions in the final fabric. For example, it is possible to heat the 
fabric at a temperature lower than that for the graphitization during the 
production of the carbon fiber strand used for weaving the fabric. In this 
regard, since graphite crystals are liable to grow as a result of the heat 
treatment, the elastic modulus of the fabric may become higher, but never 
lower. 
As stated above, according to the present invention, it is possible to 
produce triaxial fabrics having a high tensile modulus that could not be 
attained by conventional methods, by adopting a heating profile capable of 
promoting graphite crystal growth in the fabric. For example, triaxial 
fabrics having both reduced weight and high elastic modulus can be 
produced, which was not shown in the prior art, and having a basis weight 
in the range between 45 g/m.sup.2 and 300 g/m.sup.2, preferably between 45 
g/m.sup.2 and 180 g/m.sup.2, and a tensile modulus of 3 tf/mm.sup.2 or 
more, preferably 4.5 tf/mm.sup.2 or more. This is achievable by weaving a 
triaxial fabric of basic structure at a weaving density of 9.25 end/inch 
with carbon fiber strands having a tensile modulus of 15 tf/mm.sup.2 or 
more and a linear density in a range between 40 g/km and 300 g/km, 
preferably between 40 g/km and 160 g/km, which fabric is then subjected to 
a heat treatment in accordance with a predetermined heating profile in an 
inert gas atmosphere. The heating profile used therefor is experimentally 
determined as stated before. 
The resultant triaxial fabric has no slackness in the fabric structure 
caused by the volume reduction of carbon fiber strands during the heat 
treatment, and crimps in the fiber strands constituting the fabric are 
fixed. Therefore, the fabric has a good shape-retaining property and a 
favorable handling property, causing less fiber breakages during the 
handling thereof. 
The tensile modulus of the triaxial fabric used in this text is obtained by 
a tensile test carried out while using, as a test piece, a resin matrix 
composite prepared by impregnating the triaxial fabric with resin. In the 
preparation of the triaxial fabric composite, 100 parts of epoxy resin 
(Epikote 828; produced by Yuka-Shell Epoxy K.K.) mixed with 3 parts of a 
hardening agent (Monethylamine boron trifluoride complex) is dissolved in 
150 parts of a solvent (methyl ethyl ketone) (all parts are by weight), 
which is then impregnated in the triaxial fabric. The fabric thus 
impregnated with resin is dried by air for 8 hours or more to have a fiber 
content of about 60% by volume, and thereafter cured in an autoclave at 
120.degree. C. for 2 hours under a pressure of 5 atmospheres to result in 
the target composite. Test places are prepared by cutting the composite 
into pieces having a size of 400 mm long.times.35 mm wide so that the 
stretching direction coincides with the longitudinal direction of one of 
the fiber strands. 
A configuration of the test piece is shown in FIG. 6. The dimensions 
thereof are as follows: 
Length of test piece (L): 400 mm 
Width of test piece (B): 35 mm.+-.2 mm 
Thickness of test piece (C): the same as the thickness of triaxial fabric 
resin matrix composite after being cured. 
Length of gauge (D): 300 mm 
Thickness of tab (F): 1 mm 
Length of tab (G): 50 mm 
An adhesive is coated on the back surface of a strain gauge described 
below, and the test piece of composite is directly adhered thereon. 
Strain gauge: PC-20-11-1L (biaxial type), manufactured by Kyowa Dengyo K.K. 
Size of substrate: 30 mm.times.30 mm 
Length of gauge: 20 mm 
Electric resistance: 120.+-.4.OMEGA. 
The test piece is conditioned in accordance with Procedure A defined in 
ASTM D618 before the tensile test. 
The tensile test is carried out by a universal tester at a strain speed in 
a range between 0.01 mm/mm min and 0.02 mm/mm min. 
The thickness and width are measured at five points of the gauge section of 
the test piece including a center thereof, and data thus obtained are 
simply averaged to determine mean values, respectively. 
The elastic modulus (E) is calculated by the following equation. 
EQU E=(.increment.P/A)/.increment.e 
wherein .increment.P is an increment of load between 10% and 30% of the 
maximum load at break, A is a cross-sectional area of the test pieces, and 
.increment.e is an increment of strain between 10% and 30% of the maximum 
load at break.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention will be described with reference to the preferred 
embodiments. 
Preferred Embodiments 
EXAMPLE 1 
As one example of the present invention, a high elastic modulus triaxial 
fabric was prepared in the following manner. The triaxial fabric was woven 
from carbon fiber strands with a linear density of 95 g/km by a triaxial 
fabric loom (manufactured by Barber Colman Inc.), which strands originated 
from a mesophase pitch and had a tensile strength of 300 kg/mm.sup.2, a 
tensile modulus of 35 tf/mm.sup.2 and a tensile elongation at break of 
0.92%. According to the observation of the appearance thereof, the 
resultant fabric had no fiber breakages and was very flexible. The greige 
fabric thus obtained was then cut into pieces, each having a size of 500 
mm.times.500 mm, which pieces were then subjected to a graphitization 
treatment in a graphitization furnace in a N.sub.2 and Ar gas atmosphere 
under the conditions of 2000.degree. C..times.5 hours, 2500.degree. 
C..times.10 hours and 2900.degree. C..times.4 hours, respectively. The 
graphitized pieces of the triaxial fabric were free from fiber loss due to 
the heat treatment and had a good appearance. In addition, no significant 
fiber breakages occurred during the handling thereof. 
As a comparative example, the greige triaxial fabric was subjected to a 
graphitization treatment at 1500.degree. C. for 5 hours. The resultant 
triaxial fabric had a good appearance but fiber breakages sometimes 
occurred during handling. 
Then, to obtain the elastic modulus of the triaxial fabric thus produced, 
the fabric was processed to be a triaxial fabric resin composite, on which 
was carried out a tensile test. 
Various physical properties of the triaxial fabric and the reinforced 
plastic composite thereof are shown below in Table 1. 
TABLE 1 
______________________________________ 
Fabric Composite 
Basis Tensile 
Heat Treatment 
Crimp rel. wt. Fiber Vf modulus 
Conditions 
ratio (g/m) breakage 
(%) (tf/mm.sup.2) 
______________________________________ 
2000.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
107 very 61 4.6 
5 hours 0.2 slight 
2500.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
99 slight 58 7.6 
10 hours 00.5 
2900.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
98 slight 60 8.5 
4 hours 00.3 
1500.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
107 present 
60 2.8 
5 hours 0.9 
(Comparative 
Ex.) 
______________________________________ 
EXAMPLE 2 
As another example of the present invention, a high elastic modulus 
triaxial fabric was prepared in the following manner. The triaxial fabric 
was woven from carbon fiber strands by a triaxial fabric loom, which 
strands originated from mesophase pitch and have a tensile strength of 300 
kg/mm.sup.2, a tensile modulus of 30 tf/mm.sup.2 and a tensile elongation 
at break of about 1%. In this connection, the carbon fiber strand was 
composed of 3000 carbon fibers, each having a 7 .mu.m diameter, for a 
total linear density of 270 g/km, and the triaxial fabric was of a basic 
structure having a weaving density of 9.25 end/inch. 
The resulting greige fabric had a good appearance having no fiber breakages 
and was flexible. Thereafter, the greige fabric was cut into pieces, each 
having a size of 500 mm.times.500 mm, which pieces were then subjected to 
graphitization treatment in a graphitization furnace in an N.sub.2 and Ar 
gas atmosphere under the conditions of 2000.degree. C..times.5 hours, 
2500.degree..times.10 hours and 2900.degree..times.4 hours, respectively. 
The graphitized places of the triaxial fabric were free from fiber loss 
due to heat treatment, and had a good appearance. In addition, no 
significant fiber breakages occurred during the handling thereof. 
As a comparative example, the greige triaxial fabric was subjected to a 
graphitization treatment at 1500.degree. C. for 5 hours. The resultant 
triaxial fabric had a good appearance but fiber breakages sometimes 
occurred during handling. 
Also as another comparative example, a trial was made for weaving a 
triaxial fabric from carbon fiber strands composed of 3000 carbon fibers, 
each having a 7 .mu.m diameter, originated from a mesophase pitch, having 
a tensile strength of 350 kg/mm.sup.2, a tensile modulus of 80 tf/mm.sup.2 
and a tensile elongation at break of 0.4%. However, the weaving operation 
was impossible because many yarn breakages occurred in the weaving 
process. Then, the weaving was manually carried out while using a simple 
jig to obtain a small fabric sample of a size of 30 mm.times.30 mm, which 
had many fiber breakages therein, and which had a deteriorated appearance. 
Then, to obtain the elastic modulus of the triaxial fabric thus produced, 
the fabric was processed into a triaxial fabric resin composite, on which 
a tensile test was carried out. That is, 100 parts epoxy resin (Epikote 
828; produced by Yuka-Shell Epoxy K.K.) mixed with a 3 parts of hardening 
agent trifluoro monoethylamine boron trifluoride complex) was dissolved in 
150 parts of a solvent (methyl ethyl ketone) (all parts are by weight), 
which is then impregnated in the triaxial fabric. The fabric thus 
impregnated with resin was dried by air for 8 hours or more to have a 
fiber content of about 60% by volume, and thereafter cured in an autoclave 
at 120.degree. C. for 2 hours under 5 atmospheres to result in the target 
composite. Test pieces were prepared by cutting the composite into a size 
of 400 mm long.times.35 mm wide so that the stretching direction coincides 
with the orientation of weft yarns, and adhering a strain gauge 
(PC-20-11-1L (biaxial type), produced by Kyowa Dengyo K.K.) on one surface 
of the test piece. The tensile test was carried out after tabs were 
attached to the test piece as shown in FIG. 6. The tensile modulus was 
obtained from a stress-strain curve depicted through the strain gauge as 
an inclination between two points on the curve corresponding to 10% and 
30% of the strain at break. The cross-sectional area of the test piece was 
calculated from the width and thickness of the test piece. 
Various physical properties of the triaxial fabric and the reinforced 
plastic composite thereof are shown below in Table 2. 
TABLE 2 
______________________________________ 
Fabric Composite 
Basis Tensile 
Heat Treatment 
Crimp rel. wt. Fiber Vf modulus 
Conditions 
ratio (g/m) breakage 
(%) (tf/mm.sup.2) 
______________________________________ 
2000.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
207 slight 61 4.0 
5 hours 0.25 
2500.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
265 slight 62 6.8 
10 hours 0.1 
2900.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
260 slight 60 7.5 
4 hours 0.1 
1500.degree. C. .times. 
(L.sub.0 - L)L.sub.0 .times. 
207 present 
58 2.8 
5 hours 0.9 
(Comparative 
Ex.) 
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Next, a honeycomb sandwich panel was prepared, as a mock solar cell panel. 
A triaxial fabric was woven from carbon fiber strands by a triaxial fabric 
loom, which strands are originated from a mesophase pitch and have a 
tensile strength of 300 kg/mm.sup.2, a tensile modulus of 30 tf/mm.sup.2 
and a tensile elongation at break of about 1%. In this connection, the 
carbon fiber strand was composed of 1000 carbon fibers, each having a 7 
.mu.m diameter for a total linear density of 95 g/km, and the triaxial 
fabric was of a basic structure having a weaving density of 9.25 end/inch. 
The greige fabric was cut into pieces, each having a size of 500 
mm.times.300 mm, which pieces were then subjected to graphitization 
treatment in a graphitization furnace in an N.sub.2 and Ar gas atmosphere 
at 2500.degree. C. for 10 hours. The resultant fabric had a basis areal 
weight of 95 g/m.sup.2. The triaxial fabric was impregnated with a 
hot-melt type epoxy resin (R521 produced by Shin-Nittetsu Kagaku K.K.) and 
cured in an autoclave at 120.degree. C. for 2 hours under a pressure of 5 
atm to produce a reinforced plastic composite. A sandwich panel was 
prepared using the resultant composite as a skin member and an aluminum 
honeycomb (produced by Al CORE, 1/2 inch thickness, 1/8 inch cell size and 
0.0007 inch foil thickness) as a core member, both being adhered to each 
other via a film adhesive Redux 312 UL (produced by CIBA GEIGY) by hot 
pressing. 
As a comparative example, a composite was prepared in the following manner. 
A unidirectional prepreg (T300 produced by Toray Industries, having a 
tensile modulus of 23 tf/mm.sup.2 and a areal weight 50 g/m.sup.2) was cut 
into pieces sized 500 mm.times.300 mm, so that three pairs were obtained, 
oriented in the 0.degree., +60.degree. and -60.degree. directions, 
respectively. The six pieces were layered with each other in such a manner 
that they were symmetrically arranged as seen in the normal direction; 
0/+60/-60/-60/+60/0, and cured in an autoclave at 120.degree. C. for 2 
hours under a pressure of 5 atms to prepare the target composite. There 
were problems in this case that the cutting of the pregreg was troublesome 
and there was much loss in the cutting operation. A sandwich panel was 
prepared from the resultant composite in the same manner as in the above 
example. The weights of the panel and rates of thermal conduction of the 
skin member are shown in Table 3. It is apparent that the panel according 
to the present invention has a reduced weight and a high thermal 
conductivity, and is suitable for a solar cell panel. The rate of thermal 
conduction used in this text is the amount of heat flowing through a 
material 1 m long per unit time with a temperature gradient of 1.degree. 
k/m, which is represented by multiplying the thermal conductivity by the 
thickness of the material. 
TABLE 3 
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Rate of thermal 
Panel weight 
conduction 
______________________________________ 
Example 860 g/m.sup.2 
40 .times. 10.sup.4 
Comparative 1500 g/m.sup.2 
3 .times. 10.sup.4 
Example 
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INDUSTRIAL APPLICABILITY 
A triaxial fabric having a reduced weight and a high tensile modulus is 
provided according to the present invention. The fabric of the present 
invention is excellent in the shape retaining ability, whereby the fiber 
breakage is greatly minimized in the intersecting portion between fiber 
strands during the handling thereof. Further, the fabric according to the 
present invention is useful as a reinforcement substrate for a composite 
requiring a reduced weight. Particularly the fabric is most suitable for 
satellite antennas and skin members of solar cell panels.