Process for producing high-strength, high-modulus carbon fibers

A process of producing carbon fibers, which comprises oxidizing polyacrylonitrile filaments having a tensile strength at 240.degree. C. of 0.3 g/d or higher and a tensile modulus at 240.degree. C. of 2.0 g/d or higher in an oxidizing atmosphere under a high tension of 0.2 g/d or higher, and carbonizing the oxidized filaments under a high tension.

BACKGROUND 
Carbon fibers have been widely used as a structural material in the form of 
a composite thereof with a matrix material such as a resin or a metal. 
Since carbon fibers have excellent mechanical, thermal, electrical and 
antimicrobial properties, they are used as reinforcing fibers for 
structural members of aerospace vehicles such as crafts, rockets, etc., as 
well as structural members of sporting goods such as golf club shafts, 
tennis rackets, fishing rods, etc. A generally adopted process for 
producing such carbon fibers comprises heating acrylic fibers as the raw 
material (precursor) in an oxidizing atmosphere of about 200.degree. to 
300.degree. C. to convert the precursors into oxidized fibers, and 
subsequently heating the oxidized fibers in an atmosphere of at least 
about 1,000.degree. C. to carbonize the same. 
Investigations have recently been made on the use of carbon fibers in the 
fields where a higher performance is required, for example, in primary 
structural members of aircrafts, which use may be attained by further 
improving the performance, particularly the tensile strength, of carbon 
fibers while keeping the modulus of elasticity thereof on a high level. 
Thus, there has been a growing demand for higher quality and performance 
of carbon fibers. Many proposals have heretofore been made with a view to 
coping with such demand. However, the proposed processes have achieved an 
increase or improvement in tensile strength but no increase or improvement 
in modulus. Most of the proposed processes involve a problem that the 
tensile strength is lowered when an enhancement of or an improvement in 
the tensile modulus is intended. For example, as one of those proposals, 
Japanese patent application Kokai publication No. 55-163217 discloses a 
process of producing carbon fibers of a high performance which uses an 
acrylic precursor obtained by a dry-jet wet spinning and a multi-stage 
drawing. However, this Japanese publication does not disclose oxidation 
and carbonization steps operated under a very high tension. On the other 
hand, European application publication No. 0159365 Al discloses oxidation 
and carbonization steps operated under a very high tension, but does not 
disclose a dry-jet wet spinning and a multi-stage drawing. 
SUMMARY 
An object of the present invention is to provide a process for producing 
high-strength, high-modulus carbon fibers which are improved in both of 
tensile strength and tensile modulus and have highly balanced values of 
properties. 
Another object of the present invention is to provide a process for 
producing high-strength, high-modulus carbon fibers having a high quality 
of being free from filament breakage and fluffing. 
According to the present invention, there are obtained high-strength, 
high-modulus carbon fibers having a strand tensile strength of at least 
580 kg/mm.sup.2, a strand tensile modulus of 29 tons/mm.sup.2 or higher, 
and a degree of X-ray crystallographic orientation of 82%, and satisfying 
the following formula concerning the degree of orientation and the X-ray 
crystallographic perfectness: 
EQU .pi.(002)-93.0+B(002).times.2.0&gt;0. 
A remarkable feature of the process for producing carbon fibers according 
to the present invention consists in the use of an acrylic fiber precursor 
containing 99 wt. % or more of acrylonitrile units and having a tensile 
strength at 240.degree. C. of at least 0.3 g/d and a tensile modulus at 
240.degree. C. of 2.0 g/d or higher. Such acrylic precursor is oxidized 
under a tension of 0.2 g/d or higher, preferably 0.2 to 0.8 g/d at a 
temperature within the range of 200.degree. to 300.degree. C. The 
resulting oxidized fibers are then heated under a high tension of 0.03 to 
0.1 g/d in an inert atmosphere at a temperature within the range of 
300.degree. to 900.degree. C. to effect a preliminary carbonization. The 
fibers are further heated under a high tension of 0.2 to 0.8 g/d in an 
inert atmosphere maintained at a temperature of 1,000.degree. to 
1,500.degree. C. to complete carbonization. 
The tension mentioned here is calculated on the basis of the size of fibers 
before the oxidation and carbonization reactions.

DETAILED DESCRIPTION OF THE INVENTION 
When the tensile strength at 240.degree. C. of an acrylic precursor to be 
used in the present invention is lower than 0.3 g/d, a difficulty is 
encountered in oxidizing the precursor under a high tension. When the 
tensile modulus at 240.degree. C. is lower than 2.0 g/d, heating of the 
precursor under a high tension within the abovementioned range in the 
steps of oxidation and carbonization becomes impossible. As a result, the 
high-strength, high-modulus carbon fibers according to the present 
invention cannot be obtained. 
A tensile strength of 0.3 g/d or higher and a tensile modulus of 2.0 g/d or 
higher at 240.degree. C. are indispensable requisites for the precursor to 
reflect the influence of a high tension of the fiber during the oxidation 
and carbonization stages on an improvement in the quality of carbon 
fibers. When the precursor satisfies these requisites, it will become 
possible for the first time to produce high-strength, high-modulus carbon 
fibers having a high degree of X-ray crystallographic orientation and 
X-ray crystallographic perfectness as aimed at by the present invention. 
In a process for preparing an acrylic fiber precursor satisfying the 
above-described requisites of tensile strength and tensile modulus at 
240.degree. C., acrylonitrile and at least one comonomer preferably 
selected from the group consisting of acrylic acid, methacrylic acid, 
itaconic acid, and alkaline metal or ammonium salts and amide compounds 
thereof are used to form an acrylonitrile copolymer composed of 99 wt. % 
or more of acrylonitrile units and 1 wt. % or less of comonomer units. 
The acrylonitrile polymer should have an intrinsic viscosity of 1.3 to 3.0, 
preferably 1.5 to 2.0. Useful solvents for preparing a dope of the 
acrylonitrile copolymer include organic solvents such as dimethyl 
sulfoxide (DMSO), dimethylacetamide (DMAc), and dimethylformamide (DMF); 
and inorganic solvents such as aqueous solutions of nitric acid, zinc 
chloride, or sodium rhodanide, though the kind of the solvent is not 
particularly limited thereto. 
As for the spinning process, a dry-jet wet spinning has to be employed. The 
process comprising first extruding a dope or spinning solution of an 
acrylonitrile polymer solution through a spinneret into an inert 
atmosphere and then introducing the extrudate into a coagulating bath. The 
resulting swollen fibers contain voids, of which the diameter is smaller 
than that of conventional fibers, are drawn in multiple steps at a 
temperature of 100.degree. C. or higher to finally provide an overall draw 
ratio of 7 or higher, preferably 9 or higher, whereby the void size of the 
swollen fiber is decreased to 100 .ANG. or smaller. The degree of 
orientation of the resulting drawn filaments, as expressed by .pi.(400), 
is preferably 92% or higher. 
Where comonomers other than acrylic acid, methacrylic acid, itaconic acid, 
and alkaline metal or ammonium salts and amide compounds thereof are used, 
and where comonomers selected from the group consisting of acrylic acid, 
methacrylic acid, itaconic acid, and alkaline metal or ammonium salts and 
amide compounds thereof are used in an amount exceeding 1 wt. %, the 
hydrophilicity or plasticity or both of the resulting acrylic fibers are 
increased, with the result that no acrylic fiber satisfying the 
above-mentioned requisites of the tensile strength and tensile modulus at 
240.degree. C. cannot be obtained. In other words, the kind and the amount 
of comonomer as in the above-mentioned cases weaken the intermolecular 
force between the polymer chains constituting the fiber and reduce the 
structure perfectness of the fiber from the viewpoint of the resulting 
fiber structure, thus causing deterioration in the properties of the 
acrylic precursor at a high temperature of 240.degree. C. 
When the mean size of voids in the swollen acrylic fibers directly before 
collapsing obtained by dry-jet wet spinning exceeds 100 .ANG., not only 
are voids constituting a structural defect of the resulting carbon fiber 
formed but also the fibril structure of the precursor remains in the 
crosssection of the carbon fiber. In other words, the fiber structure of 
the swollen fiber before collapsing is reflected as such in the structure 
of the carbon fiber. Thus, a decrease in the void size is very important 
in attaining the objects of the present invention. 
The conditions for obtaining swollen fibers having the mean size of voids 
less than 100 .ANG. are multistep drawing in at least two steps, 
preferably 4 to 6 steps, and an overall draw ratio of at least 7, 
preferably 9 or more. 
Instances of multistep drawing include a process wherein drawing is 
effected using drawing baths consisting of water or an aqueous solution of 
a solvent common with a spinning solution while keeping the drawing baths 
at successively elevated temperatures. More specifically, there can be 
mentioned a process wherein drawing is effected using first to fourth 
drawing baths of a dimethyl sulfoxide (DMSO)-water system having a DMSO 
concentration of lower than 5% at draw ratios in the first to fourth 
drawing baths of 1.33, 1.33, 1.20, and 1.20, respectively, to provide an 
overall draw ratio of about 2.55 and maintained at temperatures of 
30.degree. C., 35.degree. C., 40.degree. C. and 50.degree. C., 
respectively. 
The fineness of filaments of the precursor to be used in the present 
invention may be about 0.1 to 3 d, preferably 0.1 to 0.8 d. The total 
number of filaments can be arbitrarily chosen within a range of 500 to 
30,000. 
In order to have the structure perfectness of the acrylic precursor in the 
raw yarn state reflected on that of carbon fiber bundles as much as 
possible, it is important to apply a tension of 0.2 g/d or higher 
preferably 0.2 to 0.8 g/d, in conversion of the precursor into the 
oxidized fiber. Where the tension applied to the precursor in this 
conversion is below the above-mentioned value, relaxation of the fiber 
structure occurs to merely form oxidized fibers having a poor degree of 
orientation no matter how high the structure perfectness of the precursor 
may be. As a result, only carbon fibers having poor strength 
characteristics are obtained. 
In carbonization of the oxidized fibers having a high degree of 
orientation, it is necessary that the oxidized fibers be heated under a 
high tension of about 0.05 to 0.1 g/d in an inert atmosphere within a 
range of 300.degree. to 900.degree. C., and subsequently heated under a 
tension of about 0.2 to 0.8 g/d in an inert atmosphere maintained at a 
temperature as low as possible, namely at a temperature usually of 
1,000.degree. to 1,500.degree. C., preferably 1,450.degree. C. or lower, 
to complete carbonization. 
The resulting carbon fibers according to the present invention 
characteristically have a strand tensile strength of 580 kg/mm.sup.2 or 
higher and a strand tensile modulus of 29 tons/mm.sup.2 or higher. The 
degree of X-ray crystallographic orientation as expressed by .pi.(002) is 
characteristically at least 82% or more. The following formula (I) is 
characteristically positive: 
EQU .pi.(002)-93.0+B(002).times.2.0 (I), 
wherein the degree of X-ray crystallographic orientation, .pi.(002), is a 
yardstick showing the degree of orientation in the fiber axis of graphite 
crystals constituting the carbon fibers, and the X-ray crystallographic 
perfectness, B(002), is a yardstick showing the degree of growth of 
graphite crystals. 
Since the carbon fibers according to the present invention are obtained by 
carbonization under a high tension of acrylic fibers having a high 
structure perfectness as the raw material precursor, it is characterized 
in that it has undergone no relaxation of the fiber structure during the 
carbonization. Therefore, the carbon fibers according to the present 
invention have a high degree of orientation, a positive value of the 
formula (I), as compared with conventional carbon fibers obtained at the 
same carbonization temperature. It has an extremely excellent mechanical 
properties including a strand tensile strength of 580 kg/mm.sup.2 or 
higher and a strand tensile modulus of 29 tons/mm.sup.2 or higher. 
Further, the carbon fibers according to the present invention have a high 
grade and a high quality since it is considerably free from fluff, 
scratches, and cracks. 
The following Examples will now specifically illustrate the present 
invention. The degree of X-ray crystallographic orientation, the X-ray 
crystallographic perfectness, the mean void size, the tensile strength and 
tensile modulus of a precursor at a high temperature, the strand tensile 
strength, and the strand tensile modulus as mentioned in the present 
invention are respectively measured by the following methods. 
(1) Degree of X-ray crystallographic orientation 
20 mg/4 cm of a sample is bound with collodion in a mold having a width of 
1 mm in preparation for a measurement. The measurement is made using as 
the X-ray source a K.sub.60 line (wavelength: 1.5418 A) of Cu made 
monochromatic with a Ni filter at an output of 35 kV and 15 mA. In the 
case of a precursor, a half-value width H (.degree.) of a peak is obtained 
by scanning a peak of Miller indices (400) observed around 2 
.theta.=17.0.degree. in the circumferential direction. The degree of 
orientation, .pi.%, is calculated from the half-value width according to 
the following equation: 
EQU .pi.=(180-H)/180 (%). 
A goniometer having a slit of 2 mm.phi. and a scintillation counter are 
used. The scanning speed is 4.degree./min and the time constant is 1 sec, 
while the chart speed is 1 cm/min. In the case of a carbon fiber, the 
degree of orientation, .pi.%, is calculated from a halfvalue width H 
(.degree.) of a peak obtained by scanning a peak of Miller indices (002) 
observed around 2 .theta.=25.5.degree. in the circumferential direction 
according to the above-mentioned equation. The scanning speed is 
8.degree./min. 
(2) X-ray crystallographic perfectness 
The half-value width H (.degree.) of a peak obtained by scanning a peak of 
Miller indices (002) measured in the same manner as in the measurement of 
the degree of X-ray crystallographic orientation, .pi., in the equatorial 
direction is defined as B(002). 
(3) Mean void size 
Filaments are sufficiently washed and stripped of water containing on the 
surfaces thereof by a centrifugal separator (3000 rpm.times.15 min). 
Thereafter, about 5 mg of the filaments are placed in a closed sample 
vessel. The melting point of water present in the voids of the sample was 
measured by a differential scanning calorimeter (DSC), which is operated 
from -60.degree. C. to ambient temperature. The mean void size is 
calculated from the value of a peak appearing at a temperature of 
0.degree. C. or lower according to the following equation. The temperature 
rise speed is 2.5.degree. C./min. Pure water is used for temperature 
correction, while indium is used for calory correction. 
EQU void size=164/[melting point] (.degree. C.) 
(4) Measurement of tensile strength and tensile modulus at high temperature 
of precursor 
A filament is introduced into an air heating furnace (effective furnace 
length: 2.6 m) set at 240.degree. C. at a speed of 1 m/min. The tension 
and elongation in the introduction are measured to find the tensile 
strength and the tensile modulus. The tensile modulus is calculated from 
the gradient of the most highly inclined line of the stress-elongation 
curve. 
(5) Strand tensile strength and strand tensile modulus 
The tensile strength and tensile modulus of strands impregnated with an 
epoxy resin are measured in accordance with the measurement method 
stipulated in JIS-R-7601. The average value of 10 measurement runs is 
shown. 
EXAMPLE 1 
A 20% DMSO solution of an acrylonitrile copolymer composed of 99 wt. % of 
acrylonitrile units and 1 wt. % of methacrylic acid units (solution 
viscosity at 45.degree. C.: 600 poises) was subjected to dry-jet wet 
spinning extruding into air through a spinneret having a hole diameter of 
0.1 mm and the number of holes of 1,500 under 5 levels of conditions Nos. 
1 to 5 as listed in Table 1. Coagulation was made by introducing spun 
filaments into a 30% aqueous DMSO solution, followed by withdrawal of the 
resulting coagulated filaments from the bath. The coagulated filaments 
were washed with water by the customary method, and drawn in three-step 
water baths of 30.degree. C., 40.degree. C., and 50.degree. C., followed 
by furnishing thereto with a heat-resistant silicone oil. The resulting 
filaments were dried to collapse the same, and further drawn in steam to 
provide an overall draw ratio of 12. Thus, precursors having a filament 
fineness of 0.7 d were prepared. Filaments prepared under the conditions 
No. 1 were broken in steam drawing, resulting in a failure of drawing at 
an overall draw ratio of 12. 
The obtained precursors Nos. 2 to 5 were respectively heated under a 
tension of 0.24 g/d in air having a temperature gradient in a range of 
245.degree. to 275.degree. C. to be converted into oxidized filaments, 
which were finally heated in an inert atmosphere heated up to 
1,350.degree. C. to obtain carbon fibers having properties as listed in 
Table 1. 
In the cases of the precursors Nos. 2, 3, and 5 in Table 1, the mean void 
size of filaments before drying was smaller than 100 .ANG., and the 
tensile strength and tensile modulus at a high temperature were enough to 
satisfy the requirements specified in the present invention. The carbon 
fibers obtained from these precursors had excellent tensile strength and 
tensile modulus. 
In contrast, the precursor No. 4 had a void size of larger than 100 .ANG., 
and did not satisfy the draw ratio, the tensile strength and tensile 
elasticity at a high temperature, etc. as specified in the present 
invention. The carbon fiber obtained from this precursor was found to have 
poor mechanical properties. 
For the purpose of comparison, substantially the same procedure of spinning 
as in the case of the precursor No. 3 except that in-bath drawing was done 
only in one step using a bath of 50.degree. C. was repeated to find that 
the mean void size of filaments before drying for collapsing was about 20 
.ANG. but the spinability of the filaments was so poor that only a very 
fluffy precursor can be obtained at a draw ratio of about 12. 
TABLE 1 
______________________________________ 
No. 1 2 3 4 5 
______________________________________ 
Dope 35 35 35 50 35 
Temperature 
(.degree.C.) 
Coagulation 
5 5 5 25 30 
Bath 
Temperature 
(.degree.C.) 
Draw Ratio 2 3 4 4 3 
Mean Void 100 30 25 150 90 
Size 
(.ANG.) 
Degree of -- 92.4 92.5 89.8 92.1 
Orientation 
(%) 
Tensile -- 0.35 0.35 0.27 0.33 
Strength 
at 240.degree. C. 
(g/d) 
Tensile -- 3.0 2.8 1.5 2.1 
Modulus 
at 240.degree. C. 
(g/d) 
Tensile -- 580 580 520 560 
Strength 
of Carbon 
Fibers 
(Kg/mm.sup.2) 
Tensile -- 29.5 29.7 28.3 29.0 
Modulus of 
Carbon Fibers 
(ton/mm.sup.2) 
______________________________________ 
EXAMPLE 2 
A 20% DMSO solution of an acrylonitrile copolymer composed of 99.3 wt. % of 
acrylonitrile units and 0.7 wt. % of itaconic acid units (solution 
viscosity at 45.degree. C.: 700 poises) was first extruded into an air 
atmosphere through a spinneret having a hole diameter of 0.1 mm and the 
number of holes of 3,000 at a temperature of 35.degree. C., and then 
introduced into a 30% aqueous DMSO solution of 5.degree. C. to effect 
coagulation, followed by withdrawal of the resulting coagulated filaments 
from the bath. The coagulated filaments were washed with water by the 
customary method, and drawn in five-step drawing baths providing a 
temperature gradient ranging from 30.degree. C. to 50.degree. C., followed 
by furnishing with oil. The resulting filaments were dried to collapse the 
same, and further drawn in steam to provide varied overall draw ratios as 
listed in Table 2. Thus, precursors Nos. 6, 7, and 8 having a filament 
fineness of 0.7 d were prepared. 
The precursors Nos. 6, 7, and 8 were oxidized and carbonized under the same 
conditions as in Example 1 to prepare carbon fibers having mechanical 
properties as listed in Table 2. 
COMATIVE EXAMPLE 1 
A spinning solution was directly introduced through a spinneret having a 
hole diameter of 0.05 mm into a 30% aqueous DMSO solution without 
extruding into an air atmosphere, while following substantially the same 
procedure as in Example 2. In this procedure, when the temperature of the 
coagulation bath was set at 5.degree. C., filaments were broken. 
Accordingly, the temperature of the coagulation bath was changed to 
45.degree. C. The spinning was done with the other conditions being the 
same as in Example 2. The overall draw ratio was varied in steam drawing 
to those as listed in Table 2. Thus, precursors Nos. 9, 10, and 11 having 
a filament fineness of 0.7 d were prepared. 
The precursors Nos. 9, 10, and 11 were oxidized and carbonized under the 
same conditions as in Example 1 to prepare carbon fibers having mechanical 
properties as listed in Table 2. 
TABLE 2 
______________________________________ 
No. 6 7 8 9 10 11 
______________________________________ 
Spinning dry-jet dry-jet dry-jet 
wet wet wet 
Method wet wet wet process 
process 
process 
process process process 
Overall 6.0 9.0 12.0 6.0 9.0 12.0 
Draw 
Ratio 
Mean Void 
30 30 30 140 140 140 
Size 
(.ANG.) 
Degree of 
84.5 92.1 92.5 84.0 88.6 90.8 
Orienta- 
tion 
(%) 
Tensile 0.20 0.30 0.38 0.15 0.20 0.27 
Strength 
at 240.degree. C. 
(g/d) 
Tensile 1.3 2.0 3.0 0.8 1.0 1.4 
Modulus 
at 240.degree. C. 
(g/d) 
Tensile 510 580 590 510 530 560 
Strength 
of Carbon 
Fibers 
(Kg/mm.sup.2) 
Tensile 27.0 29.0 29.7 23.5 25.0 26.0 
Modulus 
of Carbon 
Fibers 
(ton/mm.sup.2) 
______________________________________ 
EXAMPLE 3 
Spinning was done using acrylonitrile copolymers having varied itaconic 
acid unit contents under substantially the same conditions as in the 
preparation of the precursor No. 7 in Example 2. The tensile strength and 
tensile modulus at a high temperature (240.degree. C.) of the resulting 
precursors are listed in Table 3. 
TABLE 3 
______________________________________ 
Properties at 240.degree. C. 
Tensile Tensile 
Comonomer Strength Modulus 
wt. % (g/d) (g/d) 
______________________________________ 
0.3 0.40 2.9 
0.7 0.38 3.0 
1.0 0.32 2.0 
1.5 0.27 1.8 
______________________________________ 
EXAMPLE 4 
A 20% DMSO solution of an acrylonitrile copolymer composed of 99.3 wt. % of 
acrylonitrile units and 0.7 wt. % itaconic acid units (solution viscosity 
at 45.degree. C.: 700 poises) was first extruded into an air atmosphere 
through a spinneret having a hole diameter of 0.1 mm and the number of 
holes of 3,000 at a temperature of 35.degree. C., and then introduced into 
a 30% aqueous DMSO solution to effect coagulation, followed by withdrawal 
of the resulting coagulated filaments from the bath. The coagulated 
filaments were washed with water by the customary method, and drawn in a 
water bath having a temperature gradient ranging from 30.degree. C. to 
60.degree. C., followed by furnishing thereto with a silicone oil. The 
resulting filaments were dried to collapse the same, and further drawn in 
steam to provide an overall draw ratio of 12. Thus, five kinds of 
precursors Nos. 12 to 16 as shown in Table 4 which have a filament 
fineness of 0.7 d were prepared. 
The precursors Nos. 12 to 16 were oxidized and carbonized under conditions 
as listed in Table 4 to prepare carbon fibers having mechanical properties 
as listed in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Properties Properties of 
at 240.degree. C. 
Tension in 
Carboni- Carbonized Filaments 
Value of the 
Tensile 
Tensile 
Oxidation 
zation B .pi. 
Tensile 
Tensile 
left-side 
Spinning 
Strength 
Modulus 
Step Temperature 
(002) 
(002) 
Strength 
Modulus 
term of 
No 
Method 
(g/d) 
(g/d) 
(g/d) (.degree.C.) 
(.degree.) 
(%) 
(kg/mm.sup.2) 
(ton/mm.sup.2) 
formula 
__________________________________________________________________________ 
(I) 
12 
dry-jet 
wet 0.38 2.5 0.35 1350 5.40 82.8 
595 30.2 0.6 
process 
13 
dry-jet 
" " 0.24 1250 5.65 82.3 
580 29.2 0.7 
wet 
process 
14 
dry-jet 
" " " 1350 5.36 82.4 
585 29.5 0.1 
wet 
process 
15 
dry-jet 
" " " 1450 4.90 84.4 
580 30.1 1.2 
wet 
process 
16 
dry-jet 
" " 0.16 1350 5.30 81.5 
560 28.2 -0.9 
wet 
process 
17 
wet 0.27 1.4 0.35 yarn -- -- 
process breakage 
18 
wet " " 0.24 1250 5.60 81.3 
550 27.9 -0.5 
process 
19 
wet " " " 1350 5.35 81.1 
560 28.3 -0.6 
process 
20 
wet " " " 1450 4.90 82.0 
555 29.4 -1.2 
process 
21 
wet " " 0.16 1350 5.30 80.8 
560 28.8 -1.6 
process 
__________________________________________________________________________ 
COMATIVE EXAMPLE 2 
As in Example 3, substantially the same procedure as in Example 1 except 
that a dope of 35.degree. C. was directly extruded through a spinneret 
having a hole diameter of 0.05 mm into a 30% aqueous DMSO solution, was 
repeated to obtain precursors Nos. 17 to 21 showing mechanical properties 
at a high temperature (240.degree. C.) as listed in Table 4. The 
precursors Nos. 17 to 21 were respectively oxidized and carbonized under 
conditions of oxidation tension and carbonization temperature as listed in 
Table 4 to prepare carbon fibers having mechanical properties as listed in 
Table 4.