Note: Descriptions are shown in the official language in which they were submitted.
' 2009528
TITLE
IMPROVED PITCH CARBON FIBER SPINNING PROCESS
BACRGROUND OF THE INVENTION
This invention relates to a process for
producing pitch carbon fibers which avoids formation of
cracks which run in the axial direction of the fibers.
It is well recognized in the prior art that
carbon fibers prepared from pitch can be subject to axial
cracking which decreases the fibers' strengt~, and thus
their utility and value. The source of the cracking has
been identified as fiber microstructure which is radial
in nature rather than either random or "onion skin". See
U.S. 4,504,454 for a description and drawings and photos.'
of the cracking phenomenon and various fiber micro-
structures. There have been several approaches to the
resolution of this problem reported in the art. U.S.
4,504,454 concentrates on spinning conditions. Other
references such as U.S. 4,331,620, U.S. 4,376,747, and
U.S. 4, 717,331 focus on the placing of inserts in the
spinneret which yield modification of pitch flow in the
spinneret to produce the desired nonradial microstruc-
ture in the fiber. Operation of spinnerets with moving
parts on a commercial scale is very difficult. Similarly,
maintaining continuity and uniformity of fibers spun from
spinnerets having particulate or other very fine porous
structures inside the spinneret is a very difficult task
on a commercial scale.
Another approach to the problem has been to
alter the geometry of the spinneret itself. See for
example U.S. 4,576,811 and U.S. 4,628,001, as well as
Japanese patent applications Kokai 61(1986)-75820 and
75821, as well as Japanese patent application
168127-1984. The '811 patent maintains a typical
spinneret geometry, but examines the effects of various
modifications of internal angles in the zone which joins
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the counterbore and capillary. The '001 patent describes
the use of non-round spinnerets and produces mostly
non-round fibers, which may be less desirable for some
applications. While strong fibers are produced, including
some round small diameter fibers, the use of non-round
spinnerets might present manufacturing or operating
difficulties. The Japanese applications describe
spinnerets which provide variation in cross-sectional
area through which the pitch passes. These spinnerets can
produce round fibers, but the non-conventional spinneret
profile can lead to difficulties in manufacturing the
spinnerets, and in cleaning them.
This invention is capable of producing generally
round cross-section fibers with spinnerets which are
relatively simple to manufacture and maintain. The fibers
have high strength due to random microstructure which
prevents axial cracking. This is true, even for fibers of
large diameters. Strong large diameter continuous carbon
fibers have not been available heretofore due to the
difficulties in producing such fibers. Accordingly, this
invention includes both continuous fibers which are
strung and large in diameter, and the process of fiber
preparation, which is useful for fibers of both large and
small diameters.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial schematic section of a
melt spinning pack useful in the practice of this
invention. Figure 2 is a view through the spinneret
showing a rectangular opening to the spinneret.
SUMMARY OF THE INVENTION
Axial cracking in substantially round carbon
fibers can be avoided by use of the configuration of the
conduit of this invention through which the pitch is
spun. The process of this invention involves spinning
mesophase pitch through a spinneret having a round
cross-section discharge capillary, but having at its
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inlet an opening which has a high aspect ratio. The opening may be
trapeziodal,
elliptical, a parallelogram, or the like, provided it is long and narrow.
Rectangular
openings are preferred. Aspect ratios (length divided by width of the opening)
of at
least 3:1 are preferred, with ratios of at least 5:1 being more preferred. The
opening
must be larger than the cross-sectional area of the capillary. Ratios of these
areas of
at least 2:1 are preferred, with ratios of at least 8:1 more preferred. A
preferred
process will employ a spinneret with a counterbore upstream of and larger in
diameter
than the capillary at its outlet, and at its inlet the high aspect ratio
opening having an
area smaller than the cross-sectional area of the counterbore. The area of the
opening
1 o is preferably from 10% to 70% of the area of the countebore, and more
preferably in
the range of 25 to 45% of the area of the counterbore. For rectangular
openings it is
preferred that the length of the smaller side of the rectangle is
approximately equal to
the length of the diameter of the capillary of the spinneret.
The process of this invention is sufficiently effective in preventing the
formation of axial cracks in fibers that it can be used to prepare strong,
continuous,
substantially round cross-section, large diameter carbon fibers. These fibers
have a
diameter of from 30 to 100 micrometers and a strength after stabilization and
carbonization of at least 375 Kpsi minus the diameter of the fiber in
micrometers.
Fibers having a diameter of 40 to 80 micrometers are preferred. Such large
diameter
2o fibers are useful in the reinforcement of metal, ceramic or plastic
matrices.
Further aspects of the invention are as follows:
In a process for spinning round carbon fibers from pitch comprising
extruding molten mesophase pitch through a spinneret having a round cross-
section
discharge capillary wherein the flow of molten pitch is first directed through
an
opening with a high aspect ratio and an area substantially larger than the
cross-
sectional area of the capillary, characterized by a counterbore in the
spinneret
upstream of the capillary and downstream of the opening said counterbore being
larger in diameter than the capillary.
A carbon fiber having a cross section, wherein said cross section has a
3o cross-sectional aspect ratio of 1.1 or less, and wherein the diameters of
said cross
section range from 30 to 100 micrometers, a random microstructure and a
tensile
strength of at least 335 Kpsi.
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A carbon fiber having a cross section wherein said cross section has a
cross-sectional aspect ratio of 1.1 or less, a random microstructure and
wherein the
diameters of said cross section range from 30 to 100 micrometers, said fiber
being
prepared by the process of:
extruding molten mesophase pitch through an opening having a cross-
sectional aspect ratio of at least 3:1;
followed by passing said molten mesophase pitch into a counterbore;
followed by passing said molten mesophase pitch into a capillary, and
allowing said mesophase pitch to exit said capillary as a carbon fiber.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be further explained by referring to the drawings.
Figure 1 shows in schematic cross-section a spinning pack useful in the
practice of
this invention. The pack consists of spinneret 10, shim
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15, distribution plate 17 and screen pack 19 supporting
filtration medium 20, which is described fn U.S.
3,896,028 (Phillips). The screen and filtration medium
are optional elements. Associated support, gasketing,
heating and enclosing means are not shown in Figure 1.
Molten pitch supplied externally (means not shown) flows
through the pack elements in the reverse order and is
successively filtered through 20, is directed to one of a
plurality of spinneret counterbores 24 via ope of a
plurality of coaxial holes 18 in distribution plate 17,
passes through the opening 16 in shim 15 which forms the
flow of pitch into a ribbon configuration. The pitch is
then extruded through the spinneret capillary 22.
Refinements in the spinneret 10 consist of wide entrance
26 which has tapering neck 28 leading to counterbore 24.
Counterbore 24 communicates With capillary 22 via
entrance 30 with tapering neck 32. Figure 3 of US
4,576,811 describes in detail the capillary entrance 30
and features within the tapering neck 32. Reference to
Figure 2 further details the alignment of high aspect
ratio opening 16 (which in this preferred embodiment is
rectangular) of shim 15 to the axis of capillary 22 in
the spinneret 10. This arrangement is repeated for each
of the many capillaries in the spinneret, and provides
the beneficial formation of molten pitch flow into a
ribbon configuration in its path from the distribution
plate 17 to the spinneret 10. The pitch flow stream
generally remains within a plane that includes the axis
of the spinneret capillary 22. The drawings show a shim
plate separate from the body of the spinneret used to
provide the beneficial flow configuring opening. However
other arrangements in which the high aspect ratio opening
is incorporated in the spinneret body are within the
scope of this invention.
It is preferred that the opening provide a
reduction in cross-sectional area of pitch flow, as
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compared to the spinneret counterbore area, of about
10-70~, with from about 25 to 45% preferred. If the flow
configuring opening is too wide (i.e., the shim opening
has too low an aspect ratio) the benefits of the
5 invention may not be obtained. If the flow restriction
is too great (i.e., the shim opening is too narrow)
process continuity may be impacted. The aspect ratio may
be 25:1 or more, provided the continuous flow of pitch
through the opening is not impeded. The rectpngular
geometry is the preferred flow configuration, but other
configurations providing substantially ribbon-like flow
may be used. Equipment used to prepare pitch carbon
fibers has in general evolved empirically from the larger
body of melt-spinning art. Basic understanding has often
lagged such development. What is understood, however, is
that molten pitch, a discotic liquid crystalline
material, has quite long relaxation times ("memory")
relative to conventional organic polymers and that this
property is very likely responsible for the beneficial
results achieved by the practice of this invention.
The long relaxation time of pitch probably also
accounts for a slight variation from circular cross-
sections observed in fibers produced by the process of
this invention. While the fibers are substantially round,
the fibers, particularly the larger diameter fibers, spun
through a rectangular opening upstream of the round
spinneret exhibit a slight oval shape. They have an
aspect ratio of 1.1 or less. That is, the longer
dimension of the cross-section is 1.1 or less larger than
the shorter dimension of the cross-section.
Subsequent to spinning in the manner described,
fiber stabilization, carbonization and optional
graphitization is carried out conventionally. Subsequent
to preparing the as-spun or "green" filaments or yarns as
described above, a finish (either fugitive or durable)
may be applied to ease handling and/or provide
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protection. Stabilization in air is generally conducted
between 250 and 380 °C. and on bobbins (see, e.g., US
4,527,754) preferably following the procedure disclosed
in US 4,576,810. Larger diameter fibers will require
longer stabilization times; a useful "rule of thumb" is
that one hour of stabilization time is required for each
micron of larger fiber diameter. Accordingly, a 30 micron
fiber would be stabilized for ca. 30 hours, at least to
establish a point of reference in developing~the optimum
stabilization protocol for a fiber of that diameter.
After stabilization, the yarns or fibers can be
devolatilized or "precarbonized" in an inert atmosphere ~.
at temperatures between 800 and 1000 °C. so that
subsequent carbonization may proceed more smoothly and
that formation of strength-limiting voids is reduced or
eliminated entirely. Precarbonization is usually
accomplished with 0.1 to 1 minute. Carbonization in
inert atmosphere is carried out at 1000 to 2000°C. and
preferably between 1500-1950 °C. for about 0.3 to 3
minutes. At this point a surface treatment and/or finish
application may be beneficial to improve fiber
performance, e.g., adhesion, in its eventual application,
e.g.,in a composite. Graphitization, if desired, is
usually accomplished in an inert atmosphere by heating
between 2400 and 3300 °C., preferably between 2600-
3000 °C. for at least about a minute. During any of the
above-mentioned heating steps, longer times of treatment
do not appear to be detrimental.
A plot of tensile strength versus diameter for
carbon fibers of the prior art exhibits a curved line
with high tensile strengths for small fibers, declining
as fiber size is increased. For fiber diameters larger
than 30 micrometers the curve flattens, but continues to
. trend downward as fiber diameter is increased. A plot of
data for the large fibers of this invention provides a
similar curve, roughly parallel to that for the prior art
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fibers, but with higher tensile strengths. Treating the
graph for diameters of 30 micrometers and up as a
straight line, the strength versus diameter relationship
for the large diameter fibers of this invention is
approximated by the equation, S > or ~ 375 - D. In this
equation, S is strength in Kpsi, and D is fiber diameter
in micrometers.
The invention will be more fully understood by
reference to the following non-limiting examples.
BxAriPLE 1
Midcontinent refinery decant oil was topped to
produce an 850°F plus residue. The residue analyzed 91.8%
carbon, 6.5%, hydrogen, 35.1% Conradson carbon residue and
81.6% aromatic carbon by C1' NMR. The decant oil residue
was heat soaked 6.3 hours at 740°F, and then vacuum
deoiled to produce a heat soaked pitch. This pitch tested
16.4% tetrahydrofuran insolubles (1 gram pitch in 20 ml
THF at 75°F).
The pitch so obtained was pulverized, fluxed
with toluene (1:1 weight ratio of solvent to pitch) by
heating to the reflux temperature for about one hour.
The solution was passed through a 1 micron filter, and
admixed with sufficient toluene/heptane (98:2)
("anti-solvent") to provide (a) an 99:1 by volume
toluene/heptane mixture and (b) an 8:1 mixed solvent/
pitch ratio, by volume/weight.
After refluxing for 1 hour, the mixture was
cooled to ambient temperature and the precipitated solids
were isolated by centrifugation. The cake was washed
with additional anti-solvent and then dried in a
rotary-vacuum oven. Several such batches were blended,
melted at about 400°C, passed through a 2 micron filter,
and extruded into pellets. At this point, the pitch
pellets have a quinoline insolubles (ASTM 75°C) of less
than 0.1% by weight and are 100% mesophase, as determined
by the polarized light microscopy method.
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The pellets were remelted when fed to a screw
extruder with an exit temperature of 350°C, spun at about
360°C through a 4 inch diameter/480 hole spinneret. The
holes are round and arrayed in 5 concentric rings (96
holes per ring) located in the outer 1/2 inch of the
spinneret face. Each hole has a counterbore diameter of
0.055 inch, a capillary diameter of 200 microns, a
capillary length of 800 microns (L/D equals 4), and an
entrance angle of 80/60 degrees, as defined ,in Riggs et
al. U.S. Patent 4,576,811 (See particularly, Example 2).
Between the spinneret and the distribution plate a 0.005
inch thick shim is interposed. The shim has a plurality .
of 0.008 X 0.10 inch slots that align with each spinneret
hole as shown in figure 2. These slots form the pitch
into a ribbon-shaped flow configuration to the
spinnerets.
The spinneret is externally heated to about
360°C, and the spinning cell comprises an outer quench
tube about 6 inches in diameter, 5 feet long, with top 6
inches screened to permit entry of quench air at room
temperature. Aspiration is provided by a tapered (3 to
2-1/2 inches) center column that is 4 inches long. A
silicone oil finish supplied by Takemoto Oil and Fat Co.
is applied to the air-cooled as-spun filaments or green
fibers, which are wound at 550 yards per minute onto a
spool disclosed fn U.S. Patent 4,527,754 (Flynn).
Several spool packages, each containing about 1
pound of yarn, were batch stabilized by heating in air.
All were heated to 170°C for 80 minutes. The temperature
was then increased in stages to 245°C over several hours,
then held at 245°C for an additional period of almost 2
hours.
Carbonization was carried out by combining the
yarn from 6 stabilized packages mounted in a creel to
form a 2880 filament tow (nominally "3R") forwarded at 12
feet/minute under the tension of its own weight (about
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150 grams) through a 3 foot long precarbonization oven at
600-800°C, then through a 19 foot long, carbon-resistance
oven having a 1000° - 1200°C entrance zone, a 1950°C
carbonization zone, and an exiting 1000° - 1200°C zone.
The fibers were at carbonization temperatures for about 1
minute. The carbonized yarn was next passed through a 19
foot long chamber containing dried, room temperature air
admixed with 0.098% (980 ppm) of ozone supplied at a rate
of 1 cfm. The yarns are overlayed with a 1%,solution of
epoxy resin (CMD-W55-5003, sold by the Celanese
Corporation) in water, using the method and apparatus
shown in U.S. Patent 4,624,102 (Bell, Jr.). The thus ,
treated yarns were cured at 350°C and then cleaned by
passing the yarn through the guide described and
illustrated in U.S. Patent 4,589,947 (Winckler). Ten
representative bobbins of the carbonized yarn so produced
were selected and single fiber tensile properties were
determined at 1" gauge length following ASTM 3379 on 10
samples from each bobbin (average diameter was 9.4
microns). The average resulting properties were 478 Kpsi
strength, 52 Mpsi modulus and 0.9% elongation. Less than
1% of the filaments observed in photomicrographic cross-
section of the yarn bundles showed signs of longitudinal
cracking. The microstructure of the individual filaments
was in all cases random, an unusual level of microstruc-
tural control and homogeneity.
Ten representative bobbins of carbonized yarn
produced and characterized as above but were made from a
different batch of the same type of pitch without the
slotted shim produced the following average properties:
diameter 9.3 micron, 418 Kpsi strength, 53 Mpsi modulus
and 0.7% elongation. These are appreciable differences.
In addition, 33% of the filaments observed in photo-
micrographic cross-sections of the ye.rn bundles showed
signs of longitudinal cracking. The observed
microstructure Was generally radial in character.
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The above example Was repeated, with the
following changes: a different batch of the same type of
pitch was used and a different amount of "antisolvent"
was employed, such that the resulting mixture was 90:10
by volume of toluene/heptane.
The result of this change was that the pitch had
a "predicted spin temperature" of 355 °C. vs. 346 °C. for
the pitch useo in Example 1. The "predicted, spin
temperature" is the temperature at which the pitch
exhibits a melt viscosity of 630 poises, measured using
an Instron capillary viscometer. In addition: the
spinneret had 500 holes (vs. 480); and the entrance angle
Was 135 degrees (vs. 80/60).
The fibers were carbonized as in Example 1 then
graphitized using the same equipment run such that the
residence time at the highest temperature (2550 °C.) was
about 30 seconds. Resulting graphite fibers averaged (25
breaks/2 bobbins) 609 Kpsi strength, modulus 135 Mpsi and
elongation was 0.55% No longitudinal cracking was
observed; the microstructure was "random".
ERAMPLE 3
Example 1 can be repeated as follows: Pitch
similar to that used in Example 1 is employed. The
spinneret bores have the same configuration as in example
1 but are twice as large (i.e., the capillary is 0.016
in. in diameter, etc.). The shim opening is rectangular
and 0.010 in. wide. Fibers spun will have a diameter of
48 micrometers and a strength greater than 327 Kpsi.
Microscopic examination of the cross-section of the
fibers will reveal random microstructure, and the fibers
will have little or no axial cracking.
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