Note: Descriptions are shown in the official language in which they were submitted.
Z004370
CONTINUOUS, ULTRAHIGH MODULUS CARBON FIBER
R~CKGROUND OF T~F INVFNTION
This invention relates to carbon fibers and more particularly
to continuous pitch-based carbon fibers having a high modulus
and low electrical resistivity and methods for the production
of such fibers, and to composites comprising such fibers.
Carbon fibers have long been known, and methods for their
production from a variety of precursors are well described in
the art. Cellulosic precursors have been used for producing
carbon fiber since the early 1960's, with rayon being the
dominant carbon fiber precursor for nearly two decades. More
recently, as the art has developed methods for producing
carbon fiber derived from such materials as polyacrylonitrile
(PAN) and pitch, the importance of rayon-based carbon fiber
has declined. This shift has been due in part to the superior
toughness, tensile strength and stiffness exhibited by both
PAN-based and pitch-based carbon fiber. In addition, the
conversion yield of rayon to carbon fiber is low, and the
resulting carbon fiber is ordinarily lower in density than
carbon fiber based on PAN or pitch, which further limits its
potential uses.
It is known that the tensile modulus of carbon fiber generally
increases with increasing density, as does the thermal
conductivity, while the electrical resistivity of carbon fiber
decreases as fiber density is increased. Carbon fiber with
high thermal conductivity has found use in applications where
heat dissipation is a requirement such as, for example, in the
manufacture of heat sinks and in brake pad applications, while
fiber with a high degree of stiffness lends greater
dimensional stability to composites. Considerable effort has
therefore been expended to achieve carbon fibers with these
high densities reproducibly and with good control.
"~
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Z004370
Polyacrylonitrile fiber, when oxidized and carbonized under
appropriate conditions, provides tough, high strength, high
modulus carbon fiber. The overall conversion yield in
producing fiber from PAN is good, and the finished fiber is
capable of achieving the outstanding tensile strength needed
for producing the high performance composite materials used in
a variety of sports, automotive and aircraft applications.
However, the tensile modulus of commercially available PAN-
based fiber does not generally exceed about 50 X 106 psi,
which is somewhat deficient for use in applications that
require a high degree of stiffness. Moreover, PAN-based
carbon fibers generally exhibit densities of less than 1.9,
~ together with low thermal conductivity, ordinarily less than
200 w/m-K, and high electrical resistivity.
Pitch-based carbon fiber has generally been recognized as
capable of providing greater stiffness and higher thermal
conductivity than carbon fiber from other sources, and
considerable effort has been directed toward the development
of pitch-based ultra-high modulus carbon fibers with good
thermal conductivity. Such carbon fibers could find immediate
application in forming composites for use where good
dissipation of electrical charges or heat is desired. In
addition, the combin-ation of high stiffness and good thermal
conductivity with the negative coefficient of thermal
expansion characteristically exhibited by pitch-based fibers
would make such composites extraordinarily dimensionally
stable.
The continuous carbon fibers heretofore disclosed and
described in the art, including those carbon fibers having
tensile modulus values as great as about 120 to 125 X 106 psi
which have been designated as "ultra-high modulus", have
generally exhibited densities of less than about 2.2 gtcc,
thermal conductivities of less than about 1000 w/m-K and
electrical resistivities generally above about 1.8 micro-ohm-
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meter. For most high modulus, pitch-based carbon fibers
produced in commercial facilities, the thermal conductivity
ordinarily falls below about 700 w/m-K, and the electrical
resistivity is generally above 2.0 micro-ohm-meter. Although
there has recently been reported in the art pitch-based carbon
fiber having a tensile modulus substantially above about 125 X
106 psi, with single carbon fiber filament values as great 140
X 106 psi, these fibers also generally do not exhibit low
electrical resistivity characteristics, and the thermal
conductivity of these fibers is also reported to be low,
generally below 1000 w/m-K.
Crystalline graphite has a density of about 2.26 g/cc, and
generally exhibits excellent thermal conductivity, near 1800
w/m-K, and low electrical resistivity, well below 1.5 micro-
ohm-meter. However, even though methods for producing
graphite whiskers having extremely high modulus together with
conductivity and density properties near those of single
graphite crystals are known, the art has not suggested the
preparation of continuous carbon fibers from pitch or any
other source with a density of 2.2 g/cc or greater, a thermal
conductivity well above 1100 w/m-K and an electrical
resistivity significantly below 1.5 micro-ohm-meter, to as low
as 1.2 micro-ohm-meter and lower.
A carbon fiber having a density of about 2.2 or greater and an
electrical resistivity below 1.5 micro-ohm-meter, together
with a tensile modulus well above 125 X 106 psi and even as
great as 130 X 106 psi or greater would be a substantial
advance in the carbon fiber art. Such carbon fiber, and
particularly fiber exhibiting a thermal conductivity greater
than 1100 w/m-K, would find immediate wide acceptance for use
in a variety of composite applications, and would be
particularly useful for composites in which good dimensional
stability and low electrical resistivity are needed.
SU~M~RY OF T~F INVFNTION
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The carbon fibers of this invention are high modulus, pitch-
based continuous carbon fibers having a very high thermal
conductivity, and a low electrical resistivity. The carbon
fibers and woven fabric reinforcement made from such fibers
are particularly useful for the production of composites.
DFTAITF~ D~SCRIPTION
The carbon fibers of this invention are pitch-based continuous
carbon fibers having a density of not less than 2.18 g/cc, a
tensile modulus substantially above 120 X 106 psi, and an
electrical resistivity below about 1.6 micro-ohm-meter. More
particularly, the continuous carbon fibers of this invention
have a density in the range of from about 2.18 g/cc to the
limiting density of crystalline graphite, about 2.26 g/cc, a
tensile modulus greater than 125 X 106 psi and an electrical
resistivity below about 1.5 micro-ohm-meter. Preferably, the
continuous carbon fibers of this invention will have a density
in the range of from about 2.2 to about 2.26 g/cc, a tensile
modulus in the range of from about 125 X 106 psi to about 150
X 106 psi, and an electrical resistivity in the range of from
1.5 to about 0.95, more preferably from about 1.2 to about
0.95 micro-ohm-meter. The continuous carbon fibers of this
invention exhibit a thermal conductivity generally in the
range of about 950 to about 1800 w/m-K, preferably above
about 1000 w/m-K, and still more preferably above about 1100
w/m-K.
The high density, low electrical resistivity carbon fibers of
this invention may be further described as being highly
oriented and graphitic. The fibers have a three-dimensional
order and crystalline structure characteristic of
polycrystalline graphite, as will be apparent from an
examination of the X-ray diffraction pattern of the fibers.
Although the precise relationships are not understood, the
crystallite size and the degree of crystallite orientation in
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the fiber, as well as the degree of crystallinity, appear to
affect the level of electrical resistivity and thermal
conductivity that may be achieved.
The high density carbon fibers of this invention may be
produced from high purity, high softening temperature
mesophase pitch using improved spinning techniques and a
sequence of controlled heating steps whereby the pitch is spun
to form a fiber, infusibilized and then carbonized.
High purity, high softening temperature mesophase pitch
suitable for use in producing the carbon fibers according to
the practice of this invention can be obtained from petroleum
hydrocarbon or coal tar sources. A variety of methods for
preparing a suitable pitch are well known, including those
disclosed in U.S. patents 3,974,264, 4,026,788, and 4,209,500,
and any of these methods as well as the variety of solvent-
based methods known in the art may be employed for these
purposes. Several methods have been used in the art to
characterize the mesophase component of pitch, including
solubility in particular solvents and degree of optical
anisotropy. The mesophase pitch useful in the practice of
this invention preferably comprises greater than 90 wt%
mesophase, and preferably will be a substantially 100 wt %
mesophase pitch, as defined and described by the terminology
and methods disclosed by S. Chwastiak et al in Carbon 19, 357
- 363 (1981). The pitch can also be described as having a
high softening temperature, preferably greater than about 340
C, and more preferably above about 345 C, although when
derived from coal tar sources a pitch having a somewhat lower
softening temperature may also be useful. For the purposes of
this invention, the pitch will be thoroughly filtered to
remove infusible particulate matter and other contaminants
that may contribute to the formation of defects and flaws in
the fiber.
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-
The pitch is spun from the melt using conventional methods, in
general by forcing the molten pitch through a spinnerette
while maintaining the pitch at a temperature well above the
softening temperature. However, the temperatures useful for
spinning generally lie in a narrow range and will vary,
depending in part upon the viscosity and other physical
properties of the particular pitch being spun. Those skilled
in the art of melt-spinning will recognize that even though
the pitch may be in a molten state, it may be too viscous or
may have insufficient strength in the melt to form a filament
and may even decompose or de-volatilize to form voids and
other flaws when the pitch temperature is outside the
temperature range useful for spinning that pitch. Thus it has
long been a necessary and standard practice in the art to
conduct initial tests to establish the temperature range that
will be effective for melt spinning the particular pitch being
employed. For the purposes of this invention, the pitch will
preferably be spun at or near the highest temperature within
in the effective range of spinning temperatures at which the
pitch may be spun. The degree of orientation of the mesophase
domains in the spun pitch fiber appears to increase in
proportion to spinning temperature, and a high spinning
temperature is therefore desirable to obtain the very high
degree of orientation of the mesophase domains within the
fiber structure for the purposes of this invention.
While not wishing to be bound by any particular theory of
operation, it appears that the degree of crystallization that
may take place within the pitch fiber during the subsequent
thermal carbonization steps to form microcrystalline graphite,
as well as the size of the crystallites that may form, is
related to size of the mesophase domains in the pitch fiber
and the degree of orientation of the mesophase domains. Thus,
pitch fibers having large, well-oriented mesophase domains
tend form fibers comprising larger, more compact graphitic
29,702
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microcrystals upon being carbonized. The size, and
particularly the length of the mesophase domains as determined
by Lc, and the degree of domain orientation in the pitch fiber
appear in turn to be determined at least in part by the
conditions employed for spinning the pitch fiber, with the
domain size and degree of domain orientation in the pitch
fiber as well as the density of the resulting carbon fiber
appearing to increase as the temperature of fiber spinning is
increased. The spinning temperature range for a particular
pitch will generally rise as the softening temperature of the
pitch is increased, and the use of mesophase pitch materials
having a high softening temperature will thus be preferred.
It is well known that pitch tends to polymerize when heated,
and to coke, particularly when exposed to an oxidizing
environment while hot. Polymerization may in turn increase
the melt viscosity of the pitch, making spinning difficult or
impossible, while coking of the pitch forms infusible
particles that contribute to flaws in the fiber and may block
the spinnerette. The spinning process will therefore
preferably be conducted using melting and heating operations
designed and optimized to protect the molten pitch from
exposure to air or other oxidizing conditions during the
spinning operations, and to minimize the time the pitch is
exposed to elevated temperatures.
A variety of methods are known for converting pitch fiber to
carbon fiber, including those described for example in U.S.
patents 4,005,183, 4,209,500, 4,138,525 and 4,351,816.
In
the practice of conventional carbon fiber processes, it is
generally necessary to first infusibilize the thermoplastic
pitch fiber filaments in an oxidation step, such as by heating
in an oxidizing gas atmosphere at a temperature in the range
of from 200 to 400 C. The infusibilized pitch fiber is then
carbonized by further heating in the absence of any oxidizing
29,702
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--- gas. The carbonizing steps may be carried out by heating the
fiber in bulk, for example by winding the infusibilized yarn
on a bobbin prior to the heating step, by a threadline
operation, or by a combination of bulk and threadline
operations.
Conventionally, the carbonizing step has been conducted in the
art by heating in the substantial absence of air or other
oxidizing gases, and preferably in a substantially inert gas
atmosphere, to a temperature in the range of from 1000 to
1900 C, and graphitizing by heating at further elevated
temperatures. The heating steps are generally conducted to
specified temperatures at a carefully controlled rate,
particularly before and during the carbonizing step in order
to avoid melting or otherwise causing damage to the fiber.
Alternative processes for infusibilizing the pitch fiber have
been described more recently, for example in published
European patent application 85 200867.3. According to the
disclosure therein, the pitch fiber is infusibilized by
treatment with a liquid oxidizing composition, preferably
comprising aqueous nitric acid, and then carbonized. The
subsequent carbonizing and graphitizing operations using pitch
fiber infusibilized with liquid oxidizing composition may be
be conducted according to the processes disclosed and
described in the U.S. patents set forth herein above. In the
alternative, the infusibilized fiber may be carbonized and
graphitized in a single operation whereby the fiber is wound
on a suitable spool and heated under controlled conditions to
a temperature above 2000 C, preferably above 3000 C to
accomplish the graphitization step.
In a preferred embodiment of the aforesaid alternative process
for infusibilizing the pitch fiber, the liquid oxidizing
composition comprises an aqueous solution of nitric acid.
Nitric acid is relatively inexpensive and may be readily
obtained in concentrated form from commercial sources. The
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- concentrated acid will be diluted with water, preferably with
deionized or distilled water to avoid introducing undesirable
contaminants, to achieve the desired concentration.
The concentration of nitric acid employed will depend in part
upon the length of time the pitch will be exposed to the
nitric acid, as well as on the amount of aqueous nitric acid
that will be added per unit weight of fiber and the degree of
drying that will take place before the heat treatment is
carried out. Although a concentration of as low as 10 wt% may
be used, concentrations of at least 15 wt% will ordinarily be
needed to achieve adequate oxidation and reduced fiber
sticking. Still more preferred to accomplish adequate
treatment in a reasonable length of time will be
concentrations above about 20 wt% and preferably in the range
of from about 20 to about 30 wt%. For most commercial
operations, where the time between the application of the acid
to the pitch yarn and the heat treatment will be in the range
of from one to about five days, a concentration of
approximately 25 wt% will be suitable. Under circumstances
where the duration of the exposure of the fiber to acid before
the heat treatment is expected to be brief, thus requiring
that the oxidation be accomplished quickly, or when the amount
of aqueous nitric acid composition that will be added per unit
weight of fiber will be low in order to achieve a high rate of
fiber production, the concentration of the nitric acid may be
further increased above 30 wt% to as much as 40 wt%. However,
the treatment of carbonaceous materials such as pitch with
high nitric acid concentrations may increase the likelihood of
a rapid, exothermic and possibly sudden or even explosive
decomposition of the oxidized materials and hence excessive
concentrations of nitric acid are to be avoided.
Some form of surface treatment for the pitch fibers may be
desirable to minimize the occurrence of "sticking" or fusion
during the subsequent heat treatment. For example, the liquid
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oxidizing composition may include carbon black or colloidal
graphite particles and a surfactant for these purposes. The
particles serve to separate the pitch filaments and thereby
reduce sticking, and the surfactant may be useful for
maintaining the particles as a uniform dispersion in the
composition, as well as aiding the flow of the oxidizing
composition over the fibers. A variety of suitable anionic
and nonionic surfactants are well known and widely available,
typically including various water soluble sodium and ammonium
salts of compounds such as tetramethyl oleic acid, lauric acid
and the like. Other alternative surface treatments that may
be useful include the application of a sizing composition to
the pitch fibers, either with the liquid oxidizing composition
or in a subsequent step.
A variety of methods for applying the liquid oxidizing
composition to the pitch fibers including dipping, spraying,
misting and the like will be readily apparent to those skilled
in the art. A rotating kiss wheel, commonly employed for the
application of sizing to fibers, may also be conveniently used
for this purpose. The composition may also be applied to the
pitch yarn in bulk after the yarn has been accumulated, such
as for example by dipping or spraying the bobbin wound with
fiber. A relatively loose winding of the fiber on the bobbin
will be desired to allow the composition to flow more freely
through the fiber.
The package or spool comprising the fiber wet with nitric acid
may be heat treated directly. However, the wet fiber may
contain as much as 50 wt% aqueous acid, requiring the
evaporation of large quantities of water during the subsequent
heating steps. It may therefore be desirable to allow the
excess aqueous composition to fully drain from the spool, and
to carry out an initial low temperature heating step to
further dry the fiber. The drying step may be conducted in a
separate operation carried out in a low temperature oven, or
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- by placing the spool in the furnace and conducting an initial
low temperature heating cycle with a sweep of inert gas to
remove moisture before finally sealing the furnace, in order
to reduce the potential for furnace blow-out or other furnace
damage due to the presence of large quantities of steam.
Since the addition of heat cycles increases energy
consumption, it may be desirable as an alternative to permit
the spool to undergo drying at ambient temperatures during the
storage period. It will be desirable to exercise some care
during the drying and storage to ensure that the wound fiber
does not sag on the spool.
The heat treatment of the fiber infusibilized with aqueous
nitric acid or similar liquid oxidizing composition may be
conducted in a single heating step to a temperature in the
range of 3000 - 3500 C to produce the high modulus fiber of
this invention. The heat treatment will be conducted in a
substantially non-reactive atmosphere to ensure that the fiber
is not consumed. The non-reactive atmosphere may be nitrogen,
argon or helium, however for temperatures above about 2000 C,
argon and helium are preferred. Although the non-reactive
atmosphere may include a small amount of oxygen without
causing serious harm, particularly if the temperature is not
raised too rapidly, the presence of oxygen should be avoided.
In addition, yarn wet from being treated with liquid oxidizing
composition will produce an atmosphere of steam when heated,
which should be purged from the furnace before carbonizing
temperatures are reached, inasmuch as steam is highly reactive
at such temperatures. It may be desirable to include boron or
similar graphitizing components in the furnace atmosphere and
these will be regarded as non-reactive as the term is used
herein.
The heat treatment used in the carbonizing and graphitizing of
pitch fibers infusibilized with aqueous nitric acid or similar
oxidizing composition has three broad ranges which are
29,702
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2004370
- important in deciding a heating schedule. The rate of
temperature increase up to about 400 C should take into
account that the pitch fibers may not become completely
infusibilized until heated above that temperature, and too
rapid heating may result in fiber deformation due to
softening, fusion and disorientation of the mesophase. While
the temperature increase above about 400 C may take place at
a higher rate, it must be recognized that much of the gas loss
that occurs during the pyrolysis or carbonizing process takes
place as the fibers are heated in the range of 400 C to about
800 C, and too rapid an increase can result in damage due to
evolving gases. Above about 800 C, to the final temperature
in the range of 1100 - 2000 C for carbonized fibers, and up
to 3000 and above for graphitizing, the rate of heating may
be much greater, and conducted generally at as rapid a rate as
may be desired.
A convenient heating schedule includes heating at an initial
rate of 25 C/hr from room temperature to about 400 C, then
at 50 C/hr from 400 to 800 C, and finally at a rate of
100 C/hr, or even greater if desired, over the range of from
about 800 C to the final temperature. The heating schedule
also is determined in part upon the type of fiber, the size of
the spools, the effective loading of the furnace and similar
factors. Various further adjustments may be necessary for use
of specific equipment and materials, as will also be readily
apparent to those skilled in the art.
It will be recognized that although the heat treatment of the
infusibilized fiber has been described as a single step
process, the heating of the fiber may in the alternative be
conducted in a series of steps or stages, with cooling and
storage of intermediate materials such as carbonized fiber for
further processing at a later time. The infusibilized fiber
may also be carbonized using conventional carbonizing
processes such as those described herein above.
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The preparation of the ultra high modulus, high thermal
conductivity fibers of this invention will be better
understood by consideration of the following illustrative
examples. The following examples serve only to illustrate
methods for the preparation of fibers which are specific
embodiments of the practice of this invention, and are not
intended in any way to limit the scope of this invention.
F. XZ~MP T .F~ S
The test methods employed in the following examples for
determining strand tensile properties for continuous carbon
fiber are described in ASTM D4018 and D3800.
Electrical resistivity for carbon fibers was determined by
measuring the resistance per unit length of 50 and 100 cm
lengths of the yarn using an ohm-meter, then calculating the
yarn resistivity as the resistance multiplied by the cross-
sectional area. Cross-sectional area was in turn determined
from the weight per unit length, measured according to ASTM
D4018 and density, measured according to ASTM D3800 using o-
dichlorobenzene as the immersion liquid.
Methods for measurement of thermal conductivity of carbon
fiber have been described for single filaments by L. Peraux et
al in "The Temperature Variation of the Thermal Conductivity
of Benzene-derived Carbon Fibers", Soli~ St~te Communlc~tlons
50, 697 - 700 (1984), and for composites by B. Bozone and M.C,
Flanagan in Conference on Therm~l Con~uctlvity Metho~s,
Batelle Memorial Institute, pp 29 - 57, 1961.
Methods for determining the crystalline characteristics of
materials are well known, and such methods have long been used
for characterizing a variety of substances. The application
of such methods to the examination of graphite and of carbon
fibers has also been summarized, for example in U.S. patents
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Z004370
3,919,376 and 4,005,183, the teachings of which are
incorporated herein by reference.
~x~mple 1. Pitch fiber yarn having 2000 filaments was spun
from a 351 C softening point mesophase pitch, using an
average temperature of 401 C. The fiber was spun at an
extrusion rate of 8.9 lb/hr and a takeup speed of 590 ft/min
for 18 min, then at 12 lb/hr and 800 ft/min, to provide a
total fiber weight of 4.1 lb. A mixture containing aqueous
nitric acid (25 wt%) and 35 g/l of carbon black was applied to
the fiber during the spinning operation using a kiss wheel,
adding 2.6 lbs to the final weight of the pitch fiber. The
fiber was wound at a low crossing angle onto a graphite bobbin
covered with a 1/4" thick carbon felt pad to give a diameter
of 3.5". The final spool or package of fiber was tapered, 10
at the base and 4" at the top, and had an outside diameter of
6.5".
The package was placed in the top position of an induction
furnace and heated in an argon atmosphere at a rate of 25/hr
to 400 C, then at 50/hr to 800 C, and finally at 100/hr to
3200C. The spool was held at 3200 C for one hour, then
cooled.
The fiber had the following strand properties:
tensile strength 327,000 psi
tensile modulus 125,000,000 psi
yield 0.324 g/m
density 2.20 g/cc
resistivity 1. 51 micro-ohm-meter
29,702
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~x~le 2. Pitch fiber yarn having 2000 filaments was spun
from a 355 C softening point mesophase pitch, using an
average temperature of 412 C. The fiber was spun at an
extrusion rate of 12 lb/hr and 850 ft/min, to provide a total
fiber weight of 3.8 lb. A mixture containing aqueous nitric
acid (25 wt%) and 35 g/l of carbon black was applied to the
fiber during the spinning operation using a kiss wheel. The
fiber was wound at a low crossing angle onto a graphite bobbin
covered with a 1/4" thick carbon felt pad to give a diameter
of 3.5". The final spool or package of fiber was tapered, 10"
at the base and 4" at the top, and had an outside diameter of
6.5". The final weight of the pitch fiber package included 38
wt~ aqueous acid mixture.
The package was mechanically rotated and allowed to dry at
room temperature to a moisture content of about 15 wt%, and
then further to a final moisture content of less than 9 wt%.
The package was placed in the induction furnace and heated in
an nitrogen atmosphere at a rate of 25/hr to 400 C, then at
50/hr to 800 C, then to 1300 C and held for 24 hr before
being cooled, removed from the furnace and placed in a second
induction furnace. The package was again heated in an argon
atmosphere at 100/hr to 3230C, held at 3230 C for 2 hr,
then cooled.
The fiber had the following strand properties:
tensile strength 453,000 psi
tensile modulus 136,000,000 psi
yield 0.355 g/m
density 2.21 g/cc
resistivity 1.14 micro-ohm-meter
29,702
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Z004370
The resistivity of the carbon fiber is remarkably low,
indicating substantial improvement in thermal conductivity.
The combination of good conductivity, characterized by a
resistivity value less than 1.5 micro-ohm-meter and a high
tensile modulus, greater than 125,000,000 psi, found for these
fibers is considerably greater has been achievable in the art
to this time, and is quite surprising.
~x~m~le 3. Pitch fiber yarn having 2000 filaments was spun
from a 351 C softening point mesophase pitch, using an
average temperature of 400 C. The fiber was spun at an
extrusion rate of 15.4 lb/hr. The fiber was treated with
aqueous nitric acid, wound on a bobbin and subjected to a
first heat treatment substantially by the procedures of
Example 2. The fiber was then threadline processed in a 2400
furnace for about 5 sec. using 600 g of yarn tension, wound in
a parallel manner on a flanged graphite spool and heated in an
argon atmosphere at 100/hr to about 3310 C. The spool was
held at about 3310 C for one hour, then cooled.
The fiber had the following strand properties:
tensile strength 376,000 psi
tensile modulus 138,000,000 psi
yield 0.311 g/m
density 2.21 g/cc
resistivity 1.47 micro-ohm-meter
~x~les 4 - 6. Additional ultrahigh modulus, high density
continuous carbon fibers were prepared from high softening
temperature pitches, substantially following the processes of
29,702
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2004370
- Example 3. The fiber properties and the precursor pitch data
are summarized in Table I.
The use of a high softening pitch together with a high
spinning temperature will be seen to contribute to the
improvement of the conductivity and modulus of the fiber, as
is further confirmed by the following comparative examples.
29,702
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Z004370
Co~r~t;ve F.x~m~l es
Co~r~tive ~x~m~1e A. Pitch fiber yarn having 2000 filaments
was spun from a 331 C softening point mesophase pitch, using
an average temperature of 372 C. The fiber was spun at an
extrusion rate of 12 lb/hr, and thermoset by heating in air at
an average rate of 280 C/hr to 380 C and held for 5 min
before being cooled to room temperature and wound onto a
graphite bobbin.
The package was placed in the induction furnace and heated in
a nitrogen atmosphere at a rate of 50/hr to 800 C, and
finally at 100/hr to 1300 C and held at that temperature for
two hours before cooling. The fiber was threadline processed
in a 2400 furnace for about 5 sec. using 600 g of yarn
tension, then wound in a parallel manner on a flanged graphite
spool and heated in an argon atmosphere at 100/hr to 3080 C.
The spool was held at 3080 C for two hours, then cooled.
The carbon fiber had the following strand properties:
tensile strength 293,000 psi
tensile modulus 102,000,000 psi
yield 0.322 g/cc
density 2.16 g/cc
resistivity 2.73 micro-ohm-meter
Co ~r~tive Fx~mDles R ~n~ C. Additional prior art carbon
fibers were prepared following the procedure of Comparative
Example A. The processing temperatures and spin temperatures
used in preparing the pitch fibers and the physical properties
of the resulting carbon fibers are summarized in Table I,
together with the properties of ultrahigh modulus, low
resistivity fibers of this invention.
29,702
-19- 20~4370
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-20-
Z0043~0
- It will be seen from a consideration of the physical
properties of the carbon fiber of Comparative _xamples A - C
that the prior art high modulus carbon fibers exhibit a high
resistivity, well above the resistivity values for the carbon
fiber of this invention, and a density below about 2.2 g/cc.
In the following examples, commercial pitch-based carbon
fibers were heated at graphitizing temperatures to determine
the effect of repeated thermal treatment on prior art carbon
fiber modulus and electrical resistivity.
Co~rAt;ve Fx~m~le D. A commercial high modulus, continuous
pitch-based carbon fiber was obtained from Amoco Performance
Products Inc. as Thornel P-120 carbon fiber having a tensile
strength of 364 kpsi, a tensile modulus of 122 X 106 psi, a
resistivity of 1.801 micro-ohm-meter, a density of 2.173 g/cc,
and a d spacing Co (004) of 3.375 A. The fiber was heat
treated for about 2 hrs in a furnace held at 3330 C, after
which the resistivity was 1.776 micro-ohm-meter, the density
was 2.186 g/cc and the d spacing Co (004) was 3.371.
Com~r~tive FxA~DIe F. A commercial high modulus, continuous
pitch-based carbon fiber was obtained from Amoco Performance
Products Inc. as Thornel P-100 carbon fiber having a tensile
strength of 350 kpsi, a tensile modulus of 110 X 106 psi, a
resistivity of 2.31 micro-ohm-meter, a density of 2.168 g/cc,
and a d spacing Co (004) of 3.379 A. After the fiber was heat
treated about 1 hr in an oven maintained at 3000 C, the
tensile modulus was 110 X 106 psi, the resistivity was
measured as 2.05 micro-ohm-meter, the density was 2.167 g/cc
and the d spacing Co (004) was 3.377 A. A heat treatment of
the fiber for about 1 hr in an oven maintained at 3300 C gave
a tensile modulus of 126 X 106 psi, a resistivity of 1.73
micro-ohm-meter and a density of 2.180 g/cc.
Co~pArAt;ve FxAm~le F. A commercial high modulus, continuous
pitch-based carbon fiber was obtained from Amoco Performance
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Z004370
_ Products Inc. as Thornel P-75 carbon fiber having a tensile
strength of 279 kpsi, a tensile modulus of 73 X 106 psi, a
resistivity of 7.12 micro-ohm-meter, a density of 2.085 g/cc,
and a d spacing Co (004) of 3.418 A. After the fiber was heat
treated for about 2 hrs in an oven maintained at 3010 C, the
tensile modulus was 113 X 106 psi, the resistivity was
measured as 2.52 micro-ohm-meter, the density was 2.175 and
the d spacing Co (004) was 3.382 A.
Co~r~tive Fx~ple G. A commercial high modulus continuous
pitch-based carbon fiber was obtained from Amoco Performance
Products Inc. as Thornel P-55 carbon fiber having a tensile
strength of 315 kpsi, a tensile modulus of 56 X 106 psi, a
~ resistivity of 8.73 micro-ohm-meter, a density of 2.035 g/cc,
and a d spacing Co (004) of 3.429 A. After the fiber was heat
treated about 1 hr in an oven maintained at 3000 C, the
tensile modulus was 101 X 106 psi, the resistivity was
measured as 2.27 micro-ohm-meter, the density was 2.163 and
the d spacing Co (004) was 3.377 A. A heat treatment of the
fiber for about 1 hr in an oven maintained at 3300 C gave a
tensile modulus of 123 X 106 psi a resistivity of 1.81 micro-
ohm-meter and a density of 2.183 g/cc.
From a consideration of Comparative Examples D - G, it will be
apparent that extended heating of prior art carbon fiber may
serve to reduce the high electrical resistivity and improve
the modulus of such fibers, apparently by reducing the
amorphous carbon character of the fiber as shown by the
decreased d spacing. However, it will be seen that the
properties of prior art fibers appear to approach limiting
values during the thermal treatment, and that an increase in
thermal treatment alone is not sufficient to provide carbon
fibers having the modulus and thermal properties exhibited by
the fibers of this invention.
It will thus be seen that the present invention is a pitch-
based continuous carbon fiber having a density of not less
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- than 2.18 g/cc, a tensile modulus substantially above 120 X
106 psi, and an electrical resistivity below about 1.6 micro-
ohm-meter. More particularly, the invention is a continuous
carbon fiber having a density in the range of from about 2.18
g/cc to the limiting density of crystalliine graphite, about
2.26 g/cc, a tensile modulus in the range of from about 125 X
106 psi to about 150 X 106 psi, and an electrical resistivity
in the range of from 1.5 to about 0.95. The thermal
conductivity of the continuous carbon fibers of this invention
lies in the range of about 950 to about 1800 w/m-K, generally
above about 1000 w/m-K and more preferably above 1100 w/m-K,
and the fibers thus are particularly attractive for use in
fiber reinforced composites where good dimensional stability
and dissipation of heat is desired. The present invention is
further directed to methods for making such carbon fiber and
to composites comprising such carbon fiber. It will be
recognized by those skilled in the art that further
modifications, particularly in the processes described for
making the continuous pitch-based carbon fibers of this
invention, may be made without departing from the spirit and
scope of the invention, which is solely defined by the
appended claims.
29,702
