Process for the production of pitch-type carbon fibers

A carbon fiber having a cross-sectional crystal structure of substantially uniform mesh form orientation as observed by a polarizing microscope.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a carbon fiber having a novel 
cross-sectional structure and improved strength. 
2. Discussion of Background 
Carbon fibers have high specific strength and high specific modulus, and 
they are expected to be most prospective as filler fibers for high 
performance composite materials. Among them, pitch-type carbon fibers have 
various advantages over polyacrylonitrile-type carbon fibers in that the 
raw material is abundantly available, the yield in the carbonization step 
is high, and the elastic modulus of fibers is high. 
Various studies have been made for the preparation of pitch material having 
good orientation properties for spinning, since it has been reported that 
it is possible to obtain pitch-type carbon fibers having high quality by 
using a pitch wherein carbonaceous raw material is heat-treated to develop 
anisotropy and readily orientable molecular seeds are formed, instead of 
an isotropic pitch which has been commonly used as the pitch material for 
spinning (Japanese Examined Patent Publication No. 8634/1974). 
It is well known that when a carbonaceous raw material such as heavy oil, 
tar or pitch is heated at a temperature of from 350.degree. to 500.degree. 
C., there form, in the material, small spherical particles which have a 
particle size of from a few microns to a few hundred microns and which 
exhibit an optical anisotropy under polarized light. When further heated, 
these small spherical particles grow and are integrated to form a 
structure having an optical anisotropy. This anisotropic structure is 
considered to be a precursor for a graphite crystal structure, wherein 
planar polymeric aromatic hydrocarbon layers formed by the thermal 
polycondensation of the carbonaceous raw material are laminated and 
oriented. 
A heat-treated product including such an anisotropic structure is generally 
called mesophase pitch. 
As a method for using such mesophase pitch as the pitch material for 
spinning, there has been proposed a method wherein e.g. petroleum pitch is 
subjected to heat treatment at a temperature of from about 350.degree. to 
about 450.degree. C. under a stand-still condition to obtain a pitch 
containing from 40 to 90% by weight of a mesophase, which is used as the 
pitch material for spinning (Japanese Unexamined Patent Publication No. 
19127/1974). 
However, it takes a long period of time to convert an isotropic 
carbonaceous raw material to the mesophase pitch by such a method. Under 
the circumstances, there has been proposed a method wherein the 
carbonaceous raw material is preliminarily treated with a sufficient 
amount of a solvent to obtain an insoluble component, which is then 
subjected to heat treatment at a temperature of from 230.degree. to 
400.degree. C. for a short period of time, i.e. for 10 minutes or less, to 
form a so-called neomesophase pitch which is highly oriented and contains 
at least 75% by weight of the optical anisotropic component and at most 
25% by weight of quinoline-insoluble components, and the neomesophase 
pitch is used as the pitch material for spinning (Japanese Unexamined 
Patent Publication No. 160427/1979). 
As other pitch materials having good orientation properties for the 
production of high performance carbon fibers, there have been proposed a 
so-called pre-mesophase pitch, i.e. a pitch which is obtainable by 
subjecting e.g. coal tar pitch to hydrogenation treatment in the presence 
of tetrahydroquinoline, followed by heat treatment at a temperature of 
about 450.degree. C. for a short period of time and which is optically 
isotropic and capable of being changed to have an optical anisotropy when 
heated at a temperature of at least 600.degree. C. (Japanese Unexamined 
Patent Publication No. 18421/1983), or a so-called dormant mesophase, i.e. 
a pitch which is obtainable by subjecting a mesophase pitch to 
hydrogenation treatment e.g. by the Birch reduction method and which is 
optically isotropic and, when an external force is applied, exhibits an 
orientation to the direction of the external force (Japanese Unexamined 
Patent Publication No. 100186/1982). 
It is possible to obtain pitch fibers by melt spinning such pitch material 
having good orientation properties through spinning nozzles. Then, the 
pitch fibers may be subjected to infusible treatment and carbonization, 
and optionally to graphitization, to obtain pitch-type high performance 
carbon fibers 
When the above-mentioned pitch material having good orientation properties 
is melt-spun, the laminar structure of planar polymeric hydrocarbon in the 
resulting pitch fibers is likely to have radial orientation in the 
cross-section of each fiber. Carbon fibers are commonly used for various 
fiber-rainforced composites, of which matrixes are made of e.g., an epoxy 
resin, a phonol resin or aluminum. In such cases, not only the strength of 
carbon fibers but also the bonding properties of carbon fibers with the 
matrix are important. As mentioned above, the carbon fibers having radial 
orientation in their cross-section generally have good bonding properties 
with the matrix, and they are preferable in such as aspect. In such pitch 
fibers, however, there have been drawbacks such that when tensile stress 
is exerted in the circumferential direction of the cross-section of each 
fiber due to the carbonization shrinkage during the subsequent infusible 
treatment and carbonization treatment, wedge-shaped cracks extending in 
the axial direction of each fiber are likely to form in the cross-section 
of the resulting carbon fiber, whereby the strength of the fiber tends to 
deteriorate. In an extreme case, the commercial value of the carbon fibers 
is impaired. 
Conventional commercially available pitch-type carbon fibers have radial 
orientation or random orientation in their cross-sectional structure. 
Thus, they are weak against compression in a radial direction, although 
they are strong in a longitudinal direction. Accordingly, when they are 
used for a composite, the mechanical strength of the composite tends to be 
poor. As a reason for this, it is believed that since stractural units in 
the cross-section of such commercially available carbon fibers are coarce, 
they are likely to cleave along such structures when a radial force is 
exerted to them. 
SUMMARY OF THE INVENTION 
The present inventors have conducted extensive researches to solve the 
above difficulties, and have found that carbon fibers having a 
cross-sectional structure of regular mesh form orientation as observed by 
a polarizing microscope are free from such drawbacks. 
The present invention provides a carbon fiber having a cross-sectional 
structure of regular mesh form orientation as observed by a polarizing 
microscope.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
There is no particular restriction as to the pitch material for the carbon 
fiber of the present invention, so long as it gives an optically 
anisotropic carbon fiber wherein readily orientable molecular seeds are 
formed. Various conventional pitch materials as mentioned above may be 
employed. 
As the carbonaceous raw material to obtain such pitch material, there may 
be mentioned, for instance, coal-originated coal tar, coal tar pitch or 
liquefied coal, or petroleum-originated heavy oil, tar or pitch. These 
carbonaceous raw materials usually contain impurities such as free carbon, 
non-dissolved coal or ash contents. It is desired that these impurities 
are preliminarily removed by a conventional method such as filtration, 
centrifugal separation or sedimentation separation by means of a solvent. 
Further, the above-mentioned carbonaceous raw material may be pre-treated 
by a method wherein it is subjected to heat treatment, and then soluble 
components are extracted with a certain solvent, or by a method wherein it 
is subjected to hydrogenation treatment in the presence of a 
hydrogen-donative solvent and hydrogen gas. 
In the present invention, the above-mentioned carbonaceous raw material or 
pre-treated carbonaceous raw material is heat treated usually at a 
temperature of from 350.degree. to 500.degree. C., preferably from 
380.degree. to 450.degree. C. for from 2 minutes to 50 hours, preferably 
from 5 minutes to 5 hours in an inert gas atmosphere such as nitrogen or 
argon or while introducing such an inert gas, to obtain a pitch containing 
at least 40% by weight, particularly more than 70% by weight of an 
optically anisotropic structure, which is suitable for use as the 
mesophase pitch. 
The proportion of the optically anisotropic structure of the mesophase 
pitch in the present invention is a value obtained as the proportion of 
the surface area of the portion exhibiting an optical anisotropy in the 
mesophase pitch, as observed by a polarizing microscope at normal 
temperature. 
Specifically, for instance, a pitch sample is crushed into particles having 
a size of a few millimeter, and the sample particles are embedded on the 
almost entire surface of a resin having a diameter of about 2 cm in 
accordance with a conventional method, and the surface was polished and 
then the entire surface was thoroughly observed by a polarizing microscope 
(100 magnifications), and the ratio of the surface area of the optically 
anisotropic portion to the entire surface area of the sample is obtained. 
For the determination of the compression strength, two carbon fiber 
monofilaments were arranged in parallel with each other with a distance of 
2 mm from each other, a small glass sheet was placed thereon, and a 
compressive load was put progressively thereon, whereby a load P at which 
a breaking sound due to acoustic emission was detected, was obtained. 
The compression strength .sigma. t is caluculated in accordance with the 
following formula: 
EQU .sigma. t=2P/.pi.DL 
where D is the diameter of the monofilament, and L is the length of the 
small glass sheet. 
The carbon fibers of the present invention are obtainable by a method 
wherein the above-mentioned pitch material for spinning is passed through 
a mesh filter layer, and then supplied to spinning nozzles for spinning 
(Japanese Patent Application No. 96975/1985). 
Here, the mesh filter layer is provided at an upstream portion of each 
spinning nozzle, in the flow passageway of the pitch material. When the 
molten pitch material passes through the mesh filter layer, the flow of 
the pitch material is finely divided, and the laminar state of the 
mesophase of the pitch material is regularly divided during the passage 
through the mesh filter layer, whereby pitch fibers having a 
cross-sectional structure of regular and fine mesh form orientation as 
observed by a polarizing microscope, as shown in FIGS. 1 and 2, will be 
formed. 
In the accompanying drawings, FIG. 1 is a photograph of the cross-section 
of a carbon fiber of the present invention as taken by a polarizing 
microscope (about 4,000 magnefications). FIG. 2 is a diagramatic 
illustration of the same cross-section. FIG. 3 is a diagramatic 
illustration of the cross-section of a conventional carbon fiber which has 
a cross-sectional crystal structure of random orientation. As in evident 
from the drawings, the carbon fiber of the present invention has a 
cross-sectional crystal structure, which is different from the 
conventional random structure. 
Namely, in the present invention, the cross-sectional crystal structure has 
substantially uniform mesh form orientation. Here, the mesh form means a 
structure as shown in FIGS. 1 or 2. It is desirable that the mesh form 
orientation is regular. The mesh size is at most 1 .mu.m, preferably from 
0.1 to 1 .mu.m. In other words, it is desirable that the cross-section of 
a monofilament is substantially unformely divided into at least 100 
sections, preferably from 100 to 7,000 sections. 
As a mesh screen constituting the mesh filter layer, there may be mentioned 
a net made of a metal material such as stainless steel, copper or 
aluminum, or a net made of an inorganic material such as ceramics, glass 
or graphite, which is sufficiently durable at a temperature of from 
350.degree. to 400.degree. C. 
It is preferred to employ a net obtained by weaving fine fibers of the 
above-mentioned metal or inorganic material by plain weave, twill weave or 
tatami weave. However, it is also possible to employ a net obtained by 
punching out a flat metal plate to form numerous perforations, or a net 
like an expanded metal obtained by expanding a metal plate provided with a 
number of slits. 
If the openings of the net are too large, the effects for finely dividing 
the cross-sectional structure of the fibers to avoid the radial 
orientation, tend to diminish. Therefore, the smaller the mesh openings, 
the better. Specifically, it is usual to employ a net having openings 
smaller than 50 mesh, preferably smaller than 100 mesh, more preferably 
smaller than 200 mesh. Such a net may be used in a single sheet. It is 
also possible to use a plurality of nets in a laminated state. However, it 
is preferred that the mesh filter layer has a thickness of at most 2 mm. 
FIGS. 4 to 8 show enlarged views of the portions in the vicinity of the 
spinning nozzles in various embodiments in which the mesh filter layers of 
the present invention are provided. Reference numeral 1 designates a 
spinneret, numeral 2 designates a spinning nozzle, numeral 3 designates a 
supply hole, numeral 4 designates a mesh filter layer, and numeral 5 
designates a space. 
As shown in these Figures, the mesh filter layer 4 is located above the 
spinning nozzle. If the pitch material passed through the mesh filter 
layer 4 is maintained in the molten state for a long period of time, the 
finely divided flow units of the pitch material are likely to be 
integrated again to return to the original state prior to the passage 
through the mesh filter layer 4. Accordingly, it is preferably to locate 
the mesh filter layer above the nozzle with interposition of the space 5 
therebetween so that the time required for the pitch material passed 
through the mesh filter layer 4 to reach the spinning nozzle is as short 
as possible i.e. not longer than one minute, preferably not longer than 30 
seconds, more preferably not longer than 10 seconds. 
The time required for the pitch material passed through the mesh filter 
layer 4 to reach the spinning nozzle, is represented by a volume obtained 
by dividing the volume from the lower side of the mesh filter layer 4 to 
the upper end of the inlet of the spinning nozzle i.e. the internal volume 
of the space 5, by the discharge amount of pitch material. 
Various shapes may be employed for the space 5, as shown in FIGS. 4 to 8. 
However, it is preferred to adjust the angle .theta. from the space 5 to 
the inlet of the spinning nozzle 2 to be at least 90.degree., preferably 
at least 120.degree., whereby the effects for finely dividing the 
cross-sectional structure of the resulting fibers to avoid the radial 
orientation, can be increased. The joint portion of the space 5 and the 
inlet of the spinning nozzle 2 may be curved so that the angle .theta. 
will not be less than 90.degree. C. 
There is no particular restriction as to the spinning nozzles to be used in 
the present invention. For example, spinning nozzles having a nozzle hole 
diameter of from 0.05 to 0.5 mm and a length of from 0.01 to 5 mm, may be 
used. 
The spinning nozzle means a fine hole through which the pitch material 
passes through immediately prior to being spun and which determines the 
fiber diameter, and the nozzle hole diameter means the diameter of the 
fine hole discharging the pitch material. 
Nozzles to be used in the presetn invention may be of a straight tubular 
type or of a type wherein the center portion of the nozzle is expanded, or 
of a type wherein the lower portion of the nozzle is expanded, which 
satisfies the above conditions. 
The pitch material passes through the mesh filter layer 4 and is discharged 
from the spinning nozzle 2 to be spun. By providing the mesh filter layer 
4, it is possible to conduct the spinning while exerting a pressure of at 
least 0.5 kg/cm.sup.2 G, preferably at least 2 kg/cm.sup.2 G to the pitch 
material, at the time of discharging the pitch material. 
In the present invention, when the pitch material in a molten state passes 
through the mesh filter layer 4, the flow of the pitch material is finely 
divided and the laminar state of the mesophase is regularly divided by the 
mesh filter layer 4, whereby pitch fibers having a cross-sectional fiber 
structure of regular and mesh form orientation as observed by a polarizing 
microscope can be obtained. 
Accordingly, the flowability of the pitch material can be improved by the 
mesh filter layer 4, and at the same time, the formation of gas or bubbles 
generated from the pitch material at the spinning temperature can be 
suppressed by the pressurizing operation within the above-mentioned range 
during the spinning, whereby the stability for spinning is improved, and 
pitch fibers having improved properties can be produced constantly for a 
long period of time as uniform fibers having no size deviation among the 
nozzle holes. 
The obtained pitch fibers are then subjected to infusible treatment and 
carbonization, and optionally graphitization, whereby high performance 
pitch-type carbon fibers having a regular and fine orientation structure 
with substantially uniform fine domains, free from wedge-shaped cracks 
extending in the axial direction of the fibers, are obtainable. These 
cross-sectional fiber structures are as measured by a polarizing 
microscope. 
The compression strength in a radial direction of these fibers is several 
times higher than that of commercially available pitch-type carbon fibers 
or pitch-type carbon fibers having a random structure. The reason for this 
is believed to be such that domain sizes are not uniform in a usual random 
structure, and axially extending voids are likely to form where large 
domains are present, whereby the compression strength of the fibers 
deteriorates. Now, the present invention will be described in further 
detail with reference to Examples. However, it should be understood that 
the present invention is by no means restricted by these specific 
Examples. 
EXAMPLES 1 to 3 
Into a 5 liter autoclave, 2 kg of coal tar pitch and 2 kg of a hydrogenated 
aromatic oil were introduced and heat-treated at 450.degree. C. for 1 
hour. The treated product was distilled under reduced pressure to obtain 
residual pitch. Then, 200 g of this residual pitch was subjected to heat 
treatment at 430.degree. C. for 125 minutes while bubbling nitrogen gas. 
The mesophase pitch thereby obtained had an optical anisotropy of 100%. 
Then, by using a spinneret as shown in FIG. 4, a stainless steel metal net 
(i.e. a network layer) 4 having the size as identified in Table 1 was 
provided in each supply hole 3 thereof. 
The position of the metal net was adjusted so that the time required for 
the pitch material passed through the mesh filter layer 4 to reach the 
spinning nozzle 2, i.e. the retention time in the space 5, was as shown in 
Table 1. 
Then, by using this spinneret, the above-mentioned mesophase pitch was 
melt-spun within a temperature range of from 325.degree. to 360.degree. C. 
In each case, pitch fibers having a diameter as small as 7 .mu.m were 
obtained constantly over a long period of time by adjusting the winding up 
speed at the optimum temperature. 
Pitch fibers obtained by melt spinning at a temperature of 336.degree. C., 
were subjected to infusible treatment in air at 310.degree. C., and then 
carbonization treatment in an argon atmosphere at 1400.degree. C., to 
obtain carbon fibers. The tensile strength and the crosssectional 
structure of the carbon fibers were measured. The results are shown in 
Table 1. 
With respect to the fibers in Examples 1 and 2, the compression strength of 
the respective monofilaments was measured. The results are shown in Table 
2. 
EXAMPLE 4 
The melt spinning and carbonization treatment were conducted in the same 
manner as in Example 1 except that by using a spinneret as shown in FIG. 5 
(i.e. a spinning nozzle 2 having a diameter of 0.2 mm and a length of 0.1 
mm), a 200 mesh stainless steel metal net was provided as a mesh filter 
layer 4 in each supply hole 3 thereof, at a position where the retention 
time of pitch material in the space 5 was 3.8 seconds. In the spinning, 
pitch fibers having a diameter as small as 7 .mu.m were obtained 
constantly over a long period of time. The results thereby obtained are 
shown in Table 1. 
EXAMPLE 5 
The melt spinning and carbonization treatment were conducted in the same 
manner as in Example 1 except that by using a spinneret as shown in FIG. 6 
(i.e. a spinning nozzle 2 having a diameter of 0.1 mm and a length of 0.1 
mm), a 635 mesh stainless steel metal net was provided as a network layer 
4 in each supply hole 3 thereof, at a position where the retention time of 
pitch material at the space 5 was 0.2 second. In the spinning, pitch 
fibers having a diameter as small as 7 .mu.m were obtained constantly over 
a long period of time. The results thereby obtained are shown in Table 1. 
COMATIVE EXAMPLE 1 
The melt spinning was conducted in the same manner as in Example 1 except 
that no mesh filter layer was employed, whereby pitch fibers having a 
diameter of 7 .mu.m or less could not be obtained constantly. The physical 
values of the carbon fibers obtained in the same manner as in Example 1 
are shown in Table 1. 
COMATIVE EXAMPLE 2 
Spinning was conducted in the same manner as in Example 1 except that a 
coralliform metal powder of stranless steel sieved to have a particle size 
within a range of from 60 to 65 mesh (from 0.208 to 0.246 mm) was filled 
in a thickness of about 10 mm in a supply hole without using a mesh filter 
layer. Pitch fibers having a diameter of up to 10 .mu.m could constantly 
be obtained. 
The physical property values of the carbon fibers are shown in Table 2. As 
shown in the Table, they were inferior in both the tensile strength and 
the compression strength. 
COMATIVE EXAMPLE 3 The physical property values of commercially availabe 
pitch-type carbon fibers are shown in Table 2. They had a radial 
structure, and showed poor physical properties as shown in the Table. 
TABLE 1 
__________________________________________________________________________ 
Spinning temper- 
ature range 
Tensile strength 
Spinneret within which 
of carbon fibers 
Spinning nozzle 
Space Network 
pitch fibers of 
having a 
Cross 
Nozzle hole 
Retention opening 
7 .mu.m can be 
diameter of 
sectional 
diameter 
Length 
time Angle .theta. 
size Obtained 9 .mu.m structure of 
(mm) (mm) 
(sec.) 
(degree) 
(mesh) 
constantly (.degree.C.) 
(kg/mm.sup.2) 
carbon 
__________________________________________________________________________ 
fibers 
Example 1 
0.1 0.05 
4.0 150 500 25.0 356 Mesh form 
structure 
(630) 
Example 2 
0.2 0.4 4.0 180 500 15.8 331 Mesh form 
structure 
(630) 
Example 3 
0.2 0.4 4.0 150 200 14.2 323 Mesh form 
structure 
(100) 
Example 4 
0.2 0.1 3.8 180 200 5.5 312 Mesh form 
structure 
(100) 
Example 5 
0.1 0.1 0.2 180 635 27.4 374 Mesh form 
structure 
(1100) 
Comparative 
0.1 0.05 
-- 150 -- -- -- Entirely 
Example 1 radial 
structure; 
cracks were 
observed 
__________________________________________________________________________ 
The numeral in parenthesis indicates the number of sections in the 
crosssection of the fiber. 
TABLE 2 
__________________________________________________________________________ 
Spinning temperature 
range within which 
Tensile strength of 
Compression strength 
pitch fibers of 7 .mu.m 
carbon fibers 
of carbon fibers 
Cross sectional 
can be obtained 
having a diameter 
having a diameter 
structure of 
constantly (.degree.C.) 
of 9 .mu.m (kg/mm.sup.2) 
of 9 .mu.m (kg/mm.sup.2) 
carbon fibers 
__________________________________________________________________________ 
Example 1 
25.0 356 16.1 Mesh form 
structure (630) 
Example 2 
15.8 331 16.7 Mesh form 
structure (630) 
Comparative 
15.0 298 3.4 Random structure 
Example 1 
Comparative 
-- 239* 3.4 Radial structure 
Example 2 
__________________________________________________________________________ 
The numeral in parenthesis indicates the number of sections in the 
crosssection of the fiber. 
*The diameter of carbons fibers was 10.5 .mu.m.