Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVEN~ION
Process for producing carbon fibers
BACKGROUND OF THE INVENTION
1. Field o~ the Invention
This invention relates to a process for producing
carbon fibers. More particularly, it relaies to a process
for producing filament yarns of high strength, high
modules carbon fibers having flawless, superior properties
from a pitch containing a specified amount of mesophase,
as a raw material, by melt-spinning using spinning no~zles
(spinnerettes) having a specified structure in extrusion
holes for spinning dope.
A term "mesophase" herein referred to is one of
the components constituting the pitch and it means an
optically anisotropic part of the pitch ~hich shines
brilliantly when the section of a lump of pitch solidified
at a temperature close to room temperature is polished and
observed through the crossed nicols of reflection type
polarizing microscopy~ A pitch mostly composed of meso-
phase is callad mesophase pitch. The content of mesophase
in a mesophase pitch is calculated from the percentage o~
the area of optically anisotropic part obtained by obser-
vation under a reflection type polarizing microscope.
2. Description o the Prior Art
Recently, there has been a demand for high strength
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and high modulus light-weight materials in various fields,
e.g. in aircraftr motor vehicle and other ~ndustries, and
in this connection, a demand for carbon fibers provided
with the above-mentioned properties is rapidly increasing.
S It is well known that ~he starting material for high
strength, high modulus carbon fibers available now in the
marke-t are mostly polyacrylonitrile fibers. However these
polyacrylonitrile fihers are not only expensive but also
give only a low yield of carbon fibers, e.g. about 45~.
This fact also increases the production cost of the ultimate
products of carbon fibers.
As one method for producing high strength, high
modulus carbon fibers at a low cost, there are descrip-
tions in the official ga2ette of Japanese Patent Publica-
tion No. 1810 (197~) issued to Union Carbide Corporation
and it is a well known fact that mesophase-containing
pitches are excellent raw materials for filament yarn's
of high strength, high modulus carb~n ~ibers~ In the raw
materials of high strength, high modulus carbon fibers,
the content and the physical properties of mesophase gi~e
a great influence upon the physical properties of car~on
fibers. The higher the mesophase content, the better the
quality of mesophase, and the greater the improvement o
the physical properties of carbon fibers. Further, pitch
o low m~sophase content i~ not adequate as a raw matarial
7~
for high strengthr hi~h modulus-carbon fibers because
both the strength and modulus of the carbon fibers obtained
therefrom are lo~l.
As for the structure of the cross-section of
pitch-derived carbon fibers, it has been known that roughly
random shape ~orderless), radial shape kadiated), onion
shape (concentric circle shape), etc. of carbon arrangement
exist (Examples of literature The l~th biennial conference
on carbon, July 329 (1975); Pittburg and Ceramics 11 (1976)
No. 7, Nos 612-621) These structures depend greatly upon
the physical properties o~ raw material pitch. When mel~-
spinning is carried out by using a spinning no~zle in which
a narrow channel, as a passage or molten pitch, is a
straight tube having a circular cross-section as commonly
used case, filaments of carbon fibers thus obtained show
a structure in which carbonaceous material is radially
oriented because the higher the mesophase content o~ a raw
material pitch, the higher the orientation degree of
carbonaceous material of the filament produced by melt-
spinning, and after thermosetting and carboni7ation,obtained carbon fibers have noticeable radial structure.
Filaments of carbon fi~e~s having radial structure ~orm
very often bi~ cracks extending from the circumference of
cross-section toward the center of a filament and resultant
carbon iber~ utterly lose their value as articles of
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commerce.
It is an object of the present invention to
provide a process for producing carbon fibers in which
the above-mentioned problems of prior art processes for
producing pitch-derived carbon fibers have been overcome
and products having excellent properties can constantly
be made without forming a crack or cracks~
It is another object of the present invention
to provide spinning nozzles capable of providing high
strength, high modulus carbon fibers having nearly circular
cross-section and containing no crackes at all.
The above mentioned objects can be attained by
the process and the nozzles of the present invention.
SU~ARY OF T~E INVENTION
The present invention resides in a process for
producing high strength, high modu].us filament yarns of
carbon fibers which comprises su~jecting a pitch having a
mesophase content of 70% or higher to melt-spinning using
spinning nozzles having a cross-sectional area at their
nozzle outlet part greater than the cross-sectional area
of the narrowest part inside the nozzles and suhjecting
the resultant filament yarns to thermosetting and carbon-
ization to obtain high strength high modulus filament
yarns without cracks in the cross-section of the carbon
25- fibers.
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In the process of the present invention, melt
spinning is carried out at a temperature which is higher
than the softening temperature of mesophase pitch ~y from
40C to 140C preferably from 55C to 120C using spinning
nozzles having a cross-sectional area at their nozzle
outlet greater than the cross-sectional area of the narrow-
est part inside the nozzles, preferably by two times or
more.
DETAILED DESCRIPTION OF THE INVENTION
The inventor of the present application has
discovered after comprehensive studies that the properties
of car~on fibers can be notably improved by eliminating
the formation of crac~s in the cross-section of carbon
fibers made from raw material mesophase pitch containing
mesophase in an amount of 70~ or higher.
As a process for eliminating cracks in the
cross-section of carbon fibers, it has been found that the
use of spinning nozzles having a cross-sectional area of
the outlet of the noz21es greater than the cross-sectional
2~ area of the narrowes-t part o the thin passage for spinning
dope inside the nozzles, praferably having a ratio of the
cross-sectiona-l area at tne outlet of the nozzle to that
of the narrowest part of the thin passage for spinning
dope inside the nozzles o two or greater as shown herein-
after in the drawing o Figures 1, 2 and 3 and also the
use of a spinning temperature higher than the softeningtemperature (as measured by Koka type flow tester) by
40 to 140C, preferably by 55~ - 120C in the melt-
spinning, followed by usual treatments of thermosetting
and carbonization provides filament yarns of carbon fiber
which are excellent in quality and have no crack at all,
and thus the process of the present invention has been
completed.
Detailed description will be given as to the
above mentioned spinning temperature. Although optimum
spinning temperature somewhat varies depending on the
mesophase content in the mesophase pitch and physical
properties of mesophase, the results of ~ experimentS
show that, if spinning is carried out at a temperature
which is not higher than the softening point of mesophase
pitch by 40C, the viscosity of the mesophase pitch is
too high for spinning resu~ting in poor spinnability.
On the other and if spinning is carried out at a temper-
ature higher than the softening temperature of the
0 mesophase pitch by 140C or ~ore, the reduction of
.s
Viscoslty of mesophase ~ eh, increase of ~e~am~tLo~
of spinning nozzles, and change of properties of mesophase
pitch occur, resulting in increase of breakaqe of spun
filaments and sta~le spinning becomes difficult. Accord-
ingly, it is proper to select a ~pinning temperatu.re in
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the range of 40 to 140C, preferably 55 to 120C
higher than -the softening temperature of~ ~ mesophase.~; t'
The softening temperature of mesophase~is in the range
of 190C to 240C.
As for a raw material of mesophase pitch used
in the process of the present invention, petroleum-origin
heavy oil, such as topped crude (reduced C. or long residue),
vacuum residue (short residue), residue of thermal catalytic
cracking of vacuum gas oil, tar or pitch produced, as a
by-product of heat treatment of these residues and a coal-
origin heavy oil such as coal tar, coal tar pitch and a
coal liquified product can be mentioned. Mesophase pitch
can be produced by subjecting one or more of these raw
materials to heat treatment under non-oxidative atmosphere
l, to form mesophase, causing the resulting mesophase to grow
by aging, and separating the part mostly consisting of
mesophase.
The inventors of the present application have
found that filaments of carbon fibers having superior
qualities can be produced at an inexpensive price according
to the process of the present invention if the content of
mesophase in mesophase pitch is 70% or greater, preferably
90~ or higher. A mesophase pitch containing lower than
70~ mesophase, when suhjected to spinning according to an
usual manner and then to thermosetting and carhonization,
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provides carbon ~iber filaments which do not form radial
structure in cross-section due to its low degree of
carbon orientation. Therefor, although no crack is formed,
both tensile strength and modulus of resulting filaments
are low, and the carbon fibers have little value as
articles of commerce.
When mesophase pitch is used, as a ra~Y material
of filament yarn of carbon fibers, the higher the mesophase
content, the better the quality of the carbon fibers
When mesophase pitch containing 70% or more,
preferably 90% or more mesophase is melt-spun by causing
velocity change to the flow of mesophase pitch inside the
nozzles by using spinning nozzles having a cross-sectional
area at their nozzle outlet part greater than that of the
narrowest part of the passage way ~or spinning dope inside
the nozzles, preEerably in a ratio of the areas of 2 or
grater, filament yarns of car~on fibers free of crackes
in the cross-section can be obtained.
BRIEF DESCRIPTION OF THE ~RAWING5
Figure 1 is a view of vertical cross-section
through the center of one type of a nozzle oE this inven-
tion. Figure 2 is also a vertical cross-section through
the center of another type of nozzle. Figure 3 is also a
vertical cross-section through the center of a further
C~ fe.
ty~e of nozzle o~ thi~i invention. Fig~res~4 ~ 7 ~a-
~
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_ g _
photograph~of a cross-section of the filament yarns of
carbon fibers made by using the nozzle of the present
invention and observed under a SEM. Figure 8 is a photo-
graph o~ the cross-section of the filaments of carbon
fibers made by using the nozzle of the referential example.
Examples o~ nozzle shapes of the spinnerettes
used in the process of the present invention are illust-
rated in the drawings. However, it is to be noted that
the shape of the spinning nozzles used in the present
invention should not be limited to tho-se shown in these
drawings. Further, the cross-section of the nozzle should
not be limited only to circular shape. It is only limited
to the condition defined in the scope o~ claim.
In each drawings, 1 is an inlet part of spinning
dope. 2 is narrowest tube part. 3 is an outlet part of
extruding hole.
Following examples are offered by way of illust-
ration and not by ~ay o~ limitation.
Example 1
A distillate fraction higher than 404C, as
initial distilling point, o~ residue of thermal calalytic
cracking o~ vacuum gas oil was su~ected to heat treatment
at 420C for 2 hours while sending there methane gas and
further to heating at 320C for 16 hours to cause mesophase
to grow by aging and a part consisting mostly of mesophase
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was separated. The mesophase content o~ this mesophase
pitch was 91% according to the measurement under a
reflection type polarizing microscope and the softening
point (as measured by a Koka type flow tester) was 215C.
Using this mesophase pitch as a raw material,
and using spinning nozzles shown in Figure l (having 100
extrusion holes i.e. passage ~or spinning dope, a diameter
at the inlet part of spinning dope of 2.5 mm, a diameter
at the narrowest thin tube part of 0.15 mm, the length of
the narrowest thin tube part of 0.3 mm, an angle of cone
expanding toward the outlet part of 90, a diameter at
the outlet part of 0.3 mm), spinning was carried out at a
spinning temperature of 300C, and a spinning velocity o
210 m/min Res~ltant filament yarns of pitch fibers were
subjected to thermosetting at 300C and then to carboniza-
tion at 2500C to procluce products. When the cross-section
of these filaments of carbon fibers was observed under a
scanning type electron microscope(SEM), it was found that
~ost O~
the structure of the cross-section thereof was of radial
shape and there was no crac~ formed. Further, resultant
filaments of carbon fibers had a tensile strength of
278 kg/mm2, a modulus of elasticity of 49 $tmm2 and an
elongation of 0 57~.
Example 2
Using the mesophase pitch as u~ed in Example 1
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as a raw material and using spinning nozzles of Figure L,
(100 extrusion holes), in which the diameter of spinning
dope introducing part is 2.5 mm~, the diameter of the
thinnest tube part is 0.1 mm~, the length of the thinnest
tube part is 0.1 mm, the cone angle of a frustum expanding
toward outlet is 45, and the diameter of outlet part is
0.2 mm~, filament yarns of carbon fibe~s were produced by
spinning at a spinning temperature of 307C and at a
spinninq velocity of 500 m/min followed by other proces-
sings in same manner as in Example 1. When the cross-
s~ction of the resultant carbon fiber was observed with a
scanning type electron microscope, it has an onion like
structure in the cross-section as shown in Fig. 5, and no
crack was found
Exam~le 3
Using the mesophase pitch used as in Example 1
as a raw material, and using spinning nozzles of.Fi~ure 2
(100 extrusion holes) in which the diameter of spinning
dope introducing part is 2.5 mm0, the diameter of the
thinnest tu~e part is 0.1 mm~, the length of the thinnest
tube part-is 0.1 mm.~t and the diameter ~t the outlet part
~ ,n/S~
is O . 25 mm~. (Expanding by fo;ming a ho~ u~, filame~t.
yarns of carbon fibers were ~roduced by sp.inning at a
spinning tempe~ratur~ oE 280~C and a sDinning velocity of
180 mlmin_ followed by other processings in the same
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manner as in Example 1~ The representative cross-sectional
structure of resultant carbon fibers was a mixture of
radial, onion, and random patterns as shown in Figure 6.
There was found no crack at all.
Example 4
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100% Mesophase pitch having a so~tening point
of 235~C was obtained after the same processing as in
Example 1 except that longer time was necessary for sepa-
rating the mesophase pitch. Using this pitch and nozzles
as used in Example 2, filament y~rns of carbon fibers were
produced by spinning at a spinning temperature of 304C
and a spinning velocity o~ 150 m/min. followed by other
processings in the same manner as in Example 1. Represent-
ative cross-sectional structuLe o resultant fibers was
lS a mixture of radial and random patterns as shown in
Figure 7. There was found no crack at all.
Comparative Example 1
Using a mesophase pitch as used in Example 1 as
a raw material and using spinning nozzles having e~trusion
holes, in which thin tube parts of the extrusion holes are
o~ a straight tube having a diameter of 0.3 mm in cross-
section and 0 3 mm in len~th and also having a diameter of
0 3 mm at the outlet part~ filament yarns of car~on Eibers
were- produced under the same conditions for spinninq,
tharmosetting and carbonization as in Example 1. When the
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cross-section of the resultant filaments of carbon fibers
was observed under a scanning type electron microscope,
the structure of the cross-section of the filaments yarn
of carbon fibers was of radial shape as shown in Figure 8
S but there were formed crac~s at an angle of about 90~. t
Resultant filaments of carbon fibers had a tensile strength
of 157 kg/mm a modulus of elasticity of 38 $/mm and an
elongation of 0.41~.
Comparative ~xample 2
Using the mesophase pitch as used in Example 4
as a raw material, and using spinning nozzles having
extrusion holes in which thin tube parts of the extrusion
hole are of a straight tube having a cross-sectional
diameter of 0.1 mm, a length of 0.1 mm and a diameter at
lS the outlet being also 0.1 mm, filament yarns of carbon
~ibers were produced under the same condition for spinning,
thermosetting and carbonization as in Example 4. Resultant
filaments of carbon fibers had a structure of a radial
pattern in cross-section as in Comparativ~ Example 1, and
20 cracks were formed.
