Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF mE INVENTION
1. Field of the Invention
This invention relates to an improved process for
producing carbon fibers from pitch w~ich has been trans-
formed, in part~ to a liquid crystal or so-called "meso-
phase" state. More particularly, this :invention relates
to an improved process for producing carbon fibers from
pitches of this type wherein ~he carbonaceous fibers spun
from such pitches are thermoset in substantially shorter
periods of time than heretofore possible.
2. Description of the Prior Art
As a result of the rapidly expanding growth of the
aircraft, space and missile industries in recent years, a
need was created for materials exhibiting a unique and ex-
traordinary combination of physical properties. Thus, ma-
terials characterized by high strength and stiffness, and
at the same time of light weight, were required for use in
such applications as the fabrication of aircraft structures, -
re-entry vehicles, and space vehicles, as well as in the
preparation of marine deep-submergence pressure vessels
and like structures. Existing technology was incapable of
supplying such materials and the search to satisfy this
need centered about the fabrication of composits articles.
One of the most promising materials suggested for
use in composite form was high strength, high modulus car-
bon textiles, which were introducPd into the market place
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at the very time this rapid growth in the aircraft, space
and missile industries was occurring. Such textiles have
been incorporated in both plastic and metal matrices to
produce composites having extraordinary high-strength- and
high-modulus-to-weight ratios, and other exceptional proper-
ties. However, the high cost of producing the high strength,
high modulus carbon textiles employed in such composit~s
has been a major deterrent to their widespread use, in spite
of the remarkable properties exhibited by such composites.
One recently proposed method of providing high
modulus, high strength carbon fibers at low cost is
described in Canadian patent 1,019,919, entitled "Hlgh
Modulus, High Strength Carbon Fibers Produced From
Mesophase Pitch". Such method comprises first spinning
a carbonaceous fiber from a carbonaceous pitch which has
been transformed, in part to a liquid crystal or so-
called "mesophase" state, then ther~osetting the fiber so-
produced by heating the fiber in an oxygen-containing atmo-
sphere for a time su~ficient to render it infusible, and
finally carbonizing the thermoset fiber by heating in an
inert atmosphere to a temperature sufficiently elevated to
remove hydrogen and other volatiles. The carbon fibers
produced in this manner have a highly oriented structure
characterized by the presence of carbon crystallites
pre~erentially aligned parallel to the fiber axis, and are
graphitizable materials which when heated to graphitizing
temperatures develop the three-dimensional order character-
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.. ~ ..~..
istic of polycrystalline graphite and graphitic-like proper-
ties associated therewith, such as high density and low
electrical resistivity.
While carbonaceous fibers produced in accordance
with aforementioned Canadian patent 1,019,919, i.e., by
spinning from a carbonaceous pitch which has been trans-
formed, in part, to a liquid crystal or so-called "meso-
phase" state, can be thermoset in considerably shorter times
than heretofore possible in other processes for producing ;
carbon fibers from pitch materials, the thermosetting time
required is still of longer duration than is desired or
co~mercial operations. For this reason, means have been
sought for still urther reducing the heat treatment
times necessary to thermoset the carbonaceous fibers
produced in accordance with said process.
SUMMARY OF THE INVENTION
In accordance with the instant invention it has
now been discovered that the time required to thermoset
carbonaceous fibers which have been spun from carbonaceous
pitches of the type described in aforementioned
Canadian patent 1,019,919, i.e., carbonaceous pitches
which have been transformedg in part, to a liquid crystal
or so-called "mesophase" state, can be substantially re-
duced by treating the ~ibers with an aqueous chlorine
solutlon before they are processed by heating in an oxygen
atmosphere. As a result of such pretreatment, the fibers
can be thermally set, at any given temperature, in sub-
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stantially shorter periods of time than heretofore possible.
DESCRIPTION OF THE PREFERRED EMB()DIMENTS
When natural or synthetic carbonaceous pitches
having an aromatic base are heated in an inert atmosphere
at a temperature of above about 350C., either at constant
temperatures or with gradually increas;ng temperature, small
insoluble liquid spheres begin to appear in the pitch and
gradually increase in size as heating is continued. When
examined by electron diffraction and polarized light tech- -
niques, these spheres are shown to consist of layers of
oriented molecules aligned in the same direction, As these
9pheres continue to grow in size as heating is continued,
they come in contact with one another and gradually coalesce
with each other to produce larger masses of aligned layers.
As coalescence continues, domains of aligned molecules much
larger than those of the original spheres are formed.
These domains come together to form a bulk mesophase where-
in the transition from one oriented domain to another some-
times occurs smoothly and continuously through gradually
curving lamellae and sometimes through more sharply curving
lamellae. The differences in orientation between the do-
mains create a complex array of polarized light ,extinction
contours in the bulk mesophase corresponding to various
types of linear discontinuity in molecular alignment.
The ultimate size of the oriented domains prod~ced is ;
dependent upon the viscosity, and the rate of incrlease
of the viscosity, of the mesophase from which thley are
~ ... .. , ~ .. . ... .
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formed, which, in turn are dependent upon the particular
pitch and the heating rate. In certain pltches, domains
having sizes in excess of two hundred microns and as large
as several thousand microns are produced. In other pitches,
the viscosity of the mesophase is such that only limited
coalescence and structural rearrangement of layers occur, ; ~-
so that the ultimate domain si7e does not exceed one hundred
microns.
The highly oriented, optically anisotropic, in-
soluble material produced by treating pitches in ~his manner
has been given the tenm "mesophase", and pitches containing
such material are known as "mesophase pitches". Such
pitches, when heated above thelr sof~ening polnts, are
mixtures of two immiscible liquids, one the optically ani-
sotropic, oriented mesophase portion, and the other the
isotropic non-mesophase portion. The tenm "mesophase"
is derived from the Greek "mesos" or "intermediate" and
indicates the pseudo-crystalline nature of thls highly
oriented, optically anisotropic material.
Carbonaceous pitches having a mesophase content of
from about 40 per cent by weight to about 90 per cent by
weight are suitable for producing highly oriented carbona-
ceous fibers capable of being rapidly thermoset and heat
treated to produce fibers having the three-dimensional
order characteristic of polycrystalllne graphite accord-
ing to the invention. In order to obtain the desired
fibers from such pitch, however, the mesophase contairled
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therein, must, under quiescent conditions, form a homo-
geneous bulk mesophase having large coalesced domains,
i.e., domains of aligned molecules in excess of two hundred
microns. Pitches which form stringy bulk mesophase under
quiescent conditions, having small oriented domains, rather
than large coalesoed domains, are unsuitaible. Such pitches
form mesophase having a high viscosity which undergoes only
limited coalescence, insufficient to produce large coalesced
domains having sizes in excess of two hundred microns.
Instead, small oriented domains of mesophase agglomerate
to produce cl~ips o~ stringy masses wherein the ultimate
domain size does not exceed one hundred microns, Certain
pltches which polymer~ze very rapidly are of this type.
Likewise, pitches which do not form a homogeneous bulk
mesophase are unsuitable. The latter phenomenon is caused
by the presence of infusible solids (which are either
present in the original pitch or which develop on heating)
w~ich are enveloped by the coalescing mesophase and serve
to interrupt the homogeneity and uniformity of the coalesced
domains, and the boundariè~ between them.
Another requirement is that the pitch be nonthixo-
tropic under the conditions employed in the spinning of the
pitch into fibers, i.e., it must exhibit a nonthixotropic
~.
flow behavior so that the flow is uniform and weill behaved.
When such pitches are heated to a temperature where
they exhibit a viscosity of from about 10 poises to
about 200 poises, uniform fibers may be readily spun there-
from. Pitches, on the other hand, which do not exhibit
nonthixotropic flow behavior at the temperature of
spinning, do not permit uniform fibers to be spun there
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~rom which can be converted by further heat treatment into
fibers having the three-dimensional order characteristic
of polycrystalline graphite.
Carbonaceous pitches having a mesophase content o~
from about 40 per cent by weight to about 90 per cent by
weight can be produced in accordance with known techniques,
as disclosed in aforementioned Canadian patent 1,019,919,
by heating a carbonaceous pitch in an inert atmosphere
at a temperature above about 350C. for a time su~fi.cient
to produce the desired quantity of mesophase. By an
inert atmosphere is meant an atmosphere which does not
react with the pitch under the heating conditions em-
ployed, such as nltrogen, argon, xenon, helium, and the
like. The heating period required to produce the desired
mesophase content varies with the particular pitch and
temperature employed, with;longer heating periods required
at lower temperatures than at higher temperatures. At
350C., the minimum temperature generally required to
produce mesophase, at least one week of heating is usually
necessary to produce a mesophase content of about 40 per
cent. At temperatures of from about 400C. to 450Co
conversion to mesophase proceeds more rapidly, and a 50
per cent mesophase content can usually be produced at such
temperatures within about 1-40 hours. Such temperatures
are preferred for this reason. Temperatures above about
500C. are undesirable, and heating at this temperature
should not be employed for more than about 5 minutes to
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avoid conversion of the pitch to coke.
The degree to which the pitch has been converted
to mesophase can readily be detenmined by polarized light
microscopy and solubility examinations. Except for certain
non-mesophase insolubles present in the original pitch or
which, in some instances, develop on heating, the non-meso-
phase portion of the pitch is readily soluble in organic
solvents such as ~uinoline and pyridine, while t~e mesophase
portion is essentially insoluble.(l) In the case of pitches
which do not develop non-mesophase insolubles when heated,
the insoluble content of ~he heat treated pitch over and
above the insoluble content o the pitch before Lt has been
heat treated corresponds essentially to the mesophase con-
tent.( ) In the case of pitches which do develop non-meso-
phase insolubles when heated~ the insoluble content of the
heat treated pitch over and above the insoluble content of
the pitch before it has been heat treated is not solely
due to the conversion of the pitch to mesophase, but also
represents non-mesophase insolubles which are produced
along with the mesophase during the heat treatment.
Pitches which contain infusible non-mesophase insolubles
(1) The per c~nt of quinoline insolubles (Q.I.) of a given
pitch is determined by quinoline extraction at 75C. The
per cent of pyridine insolubles (P.I.) is determined by
Soxhlet extraction in boiling pyridine (115C.).
(2) The insoluble content of the untreated pitch is gen-
erally less than 1 per cent (except for certain coal tar
pitches) and consists largely of coke and carbon bLack
found in the original pitch.
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(either prese~t in the original pitch or developPd by
heating) in amounts sufficient to prevent the development
of homogeneous bulk mesophase are unsuitable for producing
highly orientad carbonaceous fibers capable of bei~g rapidly
thermoset and heat treated to produce fibers having the
three-dimensional order characteristic of polycrystalline
graphite, as noted above. Generally, pitches which con-
tain in excess of about 2 per cent by weig~t of such in-
fusible materials are unsuitable. The presence or absence
of such homogeneous bulk mesophase regions, as well as the
presence or absence of infusible non-mesophase insolubles,
can be vlsually observed by polarized light mlcroscopy
examination of the pitch (see, e.g., Brooks, J~D., and
Taylor, G. H., "The Formatlon of Some Graphitizing Carbons,"
ChemistrY and PhYsics of Carbon, Vol. 4, Marcel Dekker,
Inc., New York, 1968, pp. 243 - 268; and Dubois, J., Agache,
C , and White, J.L., "The Carbonaceous Mesophase Formed in
the Pyrolysis of Graphitizable Organic Ma~erials," Metal-
lography ~ pp. 337 - 369, 1970). The amounts of each of
these materials may also be vlsually estimated in thls manner.
Aromatic base carbonaceous pitches having a carbon
content of from about 92 per cent by weight to about 96
per cent by weight and a hydrogen content of from about 4
per cent by weight to about 8 per cent by weight are gen- ;
erally suitable for producing mesophase pitches which can
be employad to produce fibers capable of being rapidly ther-
moset and heat treated to produce fibers having the three- ;`
dimensional order characteristic of polycrystalline graphite
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according to the invention. Elements other than carbon
and hydrogen, such as oxygen, sulfur and nitrogen, are
undesirable and should not be present in excess of about
4 per cent by weight. When such extranPous elements are
present in amounts of from about 0.5 per cent by weight
to about 4 per cent by weight, the pitches generally have
a carbon content of from about 92-95 per cent by weight,
the balance being hydrogen.
Petroleum pitch, coal tar pitch and acenaphthylene
pitch are preferred starting materials for producing the
mesophase pitches whlch are employed to produce the fibers
employed in the instant invention. Petroleum pitch can be
derived from the thermal or catalytic cracking of petroleum
fractions. Coal tar pitch is similarly obtained by the
destructive distillation of coal. Both of these materials
are commercially availa~le natural pitches in which meso-
phase can easily be produced, and are preferred for this
reason. Acenaphthylene pitch, on the other hand, is a
~ynthetic pitch which is preferred because of its ability
to produce excellent fibers. Acenaphthylene pitch can be
produced by the pyrolysis of polymers of acenaphthylene
as described by Edstrom et al. in U.S. Patent 3,574,653.
Some pitches, such as fluoranthene pitch, polymerize
very rapidly when heated and fail to develop large coalesced
regions of mesophase, and are, therefore, not suitable pre-
cursor materials. Likewise, pitches having a high infusi-
ble non-mesophase insoluble content in organic solvents
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5664
such as quinoline or pyridine, or those which develop a
high infusible non-mesophase insoluble content when heated,
should not be employed as starting materials, as explained
above, because these pitches are incapable of developing
the homogeneous bulk mesophase necessary to produce highly
oriented carbonaceous fibers capable of being rapidly ther-
moset and heat treated to produce fibers having the three-
dimensional ~rder characteristic of polycrystalli~e graphite~
For this reason, pitches having an infusible quinoline-
insoluble or pyridine-insoluble content of more than about
2 per cent by weight (determined as described above) should
not be employed, or should ~e ~iltered to remove this ma-
terial before being heated to produce mesophase. Preferably,
such pitches are filtered when they contain more than about
1 per cent by weight of such infusible, insoluble material.
Most petroleum pitches and synthetic pitches have a low
infusible, insoluble content and can be used directly
without such filtration. Most coal tar pitches, on the
other hand, have a high infusible, insoluble content and
require filtration before they can be employed.
As the pitch is heated at a temperature between ~
350C. and 500C. to produce mesophase, the pitch will, -
of course, pyrolyze to a certain extent and the composition
of the pitch will be altered, depending upon the temperature,
the heating time, and the composition and structure of the
starting material. Generally, however, after heating a
carbonaceous pitch for a time sufficient to produce a
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mesophase content of from about 40 per cent by weight to
about 90 per cent by weight, the resulting pitch will con-
tain a carbon content of from about 94-96 per cent by
weight and a hydrogen content of from albout 4-6 per cent
by weight. When such pitches contain elements other than ~ -
carbon and hydrogen in amounts of from about 0.5 per cent
by weight to about 4 per cent by weight, ~he mesophase
pitch will generally have a carbon content of from about ` `
92-95 per cent by weight, the balance being hydrogen.
Afte~ the desired mesophase pitch has been pre-
pared, it iB spun into fibers by conventional techniques~
e,g., by melt splnning, centrifugal spinning, blow spinning,
or in any other known manner. As noted above, in order to
obtain highly oriented carbonaceous fibers capable o~ being ~-
rapidly thenmoset and heat treated to produce fibers having
the three-dimensional order characteristic of polycrystal-
line graphite the pitch must, under quiescent conditions,
form a homogeneous bulk mesophase having large coalesced
domains, and be nonthixotropic under the conditlons employed ;`
in the spinning. Further, in order to obtain uniform ~ibers ;
from such pitch, the pitch should be agitated immediately
prior to spinning so as to effectively intermix the immisci- `~
ble mesophase and non-mesophase portions of the pitch.
The temperature at which the pitch is spun de-
pends, of course) upon the temperature at which the pitch
exhibits a suitable viscosity, and at which the higher-
melting mesophase portion of the pitch can be easily de- ~ ;
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formed and oriented. Since the softening temperature of
the pitch, and its viscosity at a given temperature,
increases as the mesophase content of the pitch increases,
the mesophase content should not be permitted to rlse to
a point w~ich raises the softening pOill~ of the pitch to -~
excessive levels. For this reason, pitches having a meso-
phase content of more than about 90 per cent are generally
not employed. Pitches containing a mesophase content of
from about 40 per cent by weight to about 90 per cent by
weight, however, generally exhibit a viscosity of from
about 10 poises to about 200 poises at temperatures of from
about 310C, to above about 450C. and can be readily spun
at such temperatures. Preferably, the pitch employed has
a mesophase content of from about 45 per cent by weight to
about 75 per cent by weight, most preferably from about 55
per cent by weight to about 75 per cent by weight, and
exhibits a viscosity of from about 30 poises to about lS0
poises at temperatures of from about 340C to about 440C.
At such viscosity and temperature, uniform fibers having
diameters of from about 5 micrometers to about 25 micro-
meters can be easily spun. As previously mentioned, how
ever, in order to obtain the desired fibers, it is important
that the pitch exhibit nonthixotropic flow behaviDr during
the spinning of the fibers,
The carbonaceous fibers produced in this manner
are highly oriented graphitizable materials having a high
degree of preferred orientation of their molecules parallel
9474
~955~i6~ :
to the fiber axis. By "graphitizable" is meant that these
fibers are capable of being converted thermally (us~lly ~ -
by heating to a temperature in excess of about 2500C.,
e.g., from about 2500C. to about 3000C,.) to a structure
having the three-dimensional order charaLcteristic of poly-
crystalline graphite.
Because of the thermoplastic nature of carbonaceous
fibers produced in this manner, it is necessary that they - ;~
be thermoset before they can be carbonized. As disclosed
in aforementioned Canadian patent 1,019,919, thermosetting
can be readily effected by heatlng the fibers in an oxygen-
contalning atmosphere for a time sufficient to render them
infusible.
According to the present invention9 the time re-
quired to thermoset carbonaceous fibers prepared in accord
ance with aforementioned Canadian patent 1,019,919 and the
present invention can be substantially reduced by treating
the fibers with an aqueous chlorine solution before they are ~
proces~ed by heating in an oxygen atmosphere. As a result `;
2U of such pretreatment, the fibers can be thermally set, at
any given temperature, in substantially shorter periods of
time than heretofore possible.
The aqueous solutions of chlorine employed in the
present invention can be prepared by simply bubbling gaseous
chlorine into water. The chlorine should be added :Ln an
amount sufficient to provide a chlorine concentration of
at least 0.2 per cent by weight, preferably from about
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}55664
0.5 per cent by weight to about 1 per cent by weight (the
upper solubility limit of chlorine in water~. The tempera- -
ture of the solution is preferably maintained between about
10C. and 60C. Temperatures in excess of about 60C. are
generally not employed because of the reduced solubility
of chlorine in water at such temperatures, while at tempera-
tures below about 10C., the chlorine precipitates out of
the solution as chlorine hydrate (C12~8H20).
After the chlorine water solution has been pre-
pared, it is maintained at a temperature between about 10C.
and 60~C., preferably between about 20C. and 40C., and
the fibers are Lmmersed therein and allowed ~o soak for
a time sufficient to allow them to partially thermoset,
i.e., to form a thin skin on their surfaces. When continuous - '
filaments are being processed, the filaments may be fed
through the chlorine water solution by means of a payof
reel and a take-up reel. Alternatively, the flbers may
be wrapped around a spool or simi.lar object before being
immersed in the solutlon. The time the fibers are allowed
to soak depends upon the temperature and concentration of
chlorine in the bath, as well as upon such other factors
as the diameter of the fibers, the particuk~r pitch from
whlch the fibers are prepared, and the mesophase content
of such pitch. Generally, fibers having diameters of from
about 5 micrometers to about 25 micrometers need not be
soaked for longer than about four minutes. In any event,
in order to produce carbon fibers having adequate strength,
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~!~355664 : :~
.
the soaking time should not be allowed to exceed ten
minutes, and preferably no more than five minutes. Longer
soaking times result in carbon fibers w~lich are weak and
:
brittle. On the other hand, a minimum soaking time of one-
half minute is necessary to produce fibers having a tensile
strength in excess of 1.38 GPa. Preferablyl the fibers are
soaked in the bath for from one to three minutes.
In order to ensure that all the carbonaceous fibers
are thoroughly wetted by the chlorine water solution during
10 - substantially the entire treating time, the solution may
be circulated in the bath, e.g., by means of ultrasonic
agitation. If desired, a suitable surfactant, e.g., an
amphoteric or anionlc fluorocarbon, such as Fluorad FC-408
or Fluorad FC-423 (registered trademarks of the Minnesota
Mining and Manufacturing Company), may be added to the
solution to facilitate wetting o~ the fibers. The wetting
agent is suitably employed Ln an amount of from about 0.001
parts by weight to 0.1 parts by welght per 100 parts by
weight of the solution.
After the fibers have been partially thermoset in
the chlorine water bath, they are removed from the bath
and dried. While fibers treated in this manner are capable
of being carbonized without any further thermosetting, the
resulting carbonized fibers are characterized by a tensile
strength below 1.38 GPa., usually below 0.69 GPa. In order
to produce fibers having tensile strengths in excess of
1.38 GPa., therefore, it is necessary to further the~moset
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C~S5664 ~,
the fibers by heating in oxygen before they are carbonized.
Likewise, in order to obtain such tensile strengths, the
fibers should not be pretreated in halogen solutions other
than chlorine water. Thus, e.g., pretreatment in bromine
water results in the production of fibers having tensile
strengths below 1.38 GPa., and usually below 0.69 GPa.
The temperature at which the fibers are heated in
oxygento complete thermosetting must, of course, not exceed :
the temperature at which the fibers will soften or distort.
The maximum temperature which can be employed will thus
depend upon the particular pitch rom which the fibers
were spun, the mesophase content of such pitch, and the
degree to which ~he fibers have been thermoset in the
chlorine water bath. The higher the mesophase content
of the fibers, and the greater the degree to which they
have been thermoset, the higher will be their sotening
temperature and the higher the temperature which can be
employed to complete thermosetting At higher temperatures,
o~ course, ~ibers of a given diameter can be thermoset in
less time than is possible at lower temperatures. Fibers
having a lower mesophase content, or which have been ther-
moset to a lesser degree in the chlorine water bath~ on
the other hand, require relatively longer heat ~reatment
at somewhat lower temperatures to render them infusible.
A minimum temperature of at least 225C. is gener-
ally necessary to complete thermosetting of the fibers
Temperatures in excess of 400C. may cause melting and/or
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~055G64 ~ ~
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excessive burn-off of the fibers, as well as some reduction
in the tensile strength of the carbonized product, and
should be avoided~ Preferably, temperatures of at least
300C. are employed as thermosetting proceeds at a much
faster rate at such temperatures. Fibers having diameters
o from about 5 micrometers to about 25 micrometers can
generally be thermoset at temperatures of 300C. or higher -
within from about one minute to about four minutes. Since
it is undesirable to oxidize the fibers more than necessary,
the fibers are generally not heated for longer than about
five minutes.
In order to ensure that all the carbonaceous fibers
are effectively subjected to the oxygen atmosphere, the
gas 1QW Of oxygen over the fibers should be adequate to
permit full diffusion of the gas into the fibers and effect
removal of all reaction products from the surace of the
fibers. If the gas flow rate is too slow, poorly thermoset
fibers and/or ignition of fiber volatiles and the fibers
may result. Generally, gas flow rates of from about 0.14
standard cubic meters/hour to about 0.85 standard cubic
metars/hour, preferably from about 0.54 standard cubic
meters/hour to about 0.65 standard cubic meters/hour, per
,- .
570 cc. of furnace volume are suitable.
After the fibers have been thermoset, they are
carbonized by heating in an inert atmosphere, such as that
described above, to a temperature sufficiently elevated to
remove hydrogen and other volatiles. Fibers having a
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1~5S664
carbon content greater than about 98 per cent by weight can
generally be produced by heating to a temperature in ex-
cess of about 1000C., and at temperatures in excess of
about 1500C., the fibers are completely carbonized~ -
Usually, carb~nization is effected at a tempera- f
ture of from about 1000C. to about 2500C.~ preferably
from about 1400C. to about 1800C. Generally, residence
times of from about 0.5 minute to about 60 minutes are
employed. While ~ore extended heating times can be
employed with good results, such residence times are un-
economical and, as a practical matter, there is no ad-
vantage in employing such long perlods. In order to
ensure that the rate o~ weight loss of the fLbers cloes
not become so excessive as to disrup~ the fiber structure, `~
it is preferred to gradually heat the fibers to their
final carbonization temperature.
In a preferred method of heat treatment, con-
tinuous fibers are passed through a series of heating
zones which are held at successively higher tempera-
tures. If desired, the first of such zones may contain an
oxidizing atmosphere through which the fibers are passed
after first passing through a chlorine water bath. Several `
arrangements of apparatus can be utilized in providing the
series of heating zones. Thus, one furnace can be used
with the fibers being passed through the furnace several
times and with the temperature being increased each time.
~lternatively, the fibers may be given a single pass
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~ 0556G4
-: :
through several furnaces, with each successive furnace being
maintained at a higher temperature than that of the previous
furnace. Also, a single furnace with several heating zones
maintained at successively higher temperatures in the direc-
tion of travel of the fibers, can be used.
The carbon fibers produced in this matmer have a
highly oriented structure characterized by the presence of
carbon crystallites preferentially aligned parallel to the
fiber axis, and are graphitizable materials which when
heated to graphitizing temperatures develop the three-
dimensional order characteristic o polycrystalline graphite
and graphite-like properties associated therewith, such as
high density and low electrical resistivity. Fibers heated
to about 1600C. have been found to be characterized by
tensile strengths of greater than about 1.38 GPa. and by
a Young's modulus of elasticity of at least about 207 GPa.
The fibers heated to a temperature of about 1~00C.
are quite dense7 exhibiting a density in excess of 2.0
grams/cc., usually from about 2.0 grams/cc. to about 2.2
grams/cc. Electrical resistivity of such fibers is gener-
ally from about 800 x 10 6 ohm centimeters to about ;
1200 x 10-6 ohm centimeters. ;~
If desired, the carbonized fibers may be further
heated in an inert atmosphere, as described hereinbefore, ;
to still higher temperature in a range of from about
2500C. to about 3300C., preferably from about 2800C.
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~L055G64
to about 3000C., to produce fibers having not only a
high degree of preferred orientation of their carbon
crystallites parallel to the fiber axis, but also a
structure charact~ristic of polycrystalline graphite.
A residence time of about 1 minute is satisfactory, al-
though both shorter and longer times may be employed,
e.g., from about 10 seconds to about 5 minutes~ or longerO
Residence times longer than 5 minutes are uneconomical
and unnecessary, but may be employed if desired.
The fibers produced by heating at a temperature
above about 2500C., preferably above about 2800C., are
characterized as having the three-dimensional order of
polycrystalline graphite. This three-dimensional order
is clearly established by the X-ray diffraction pattern of
the fibers, specifically by the presence of the (112)
cross-lattice line and the resolution of the (10) band
into two distinct lines, (100) and (101). The short arcs
which constitute the (OOR) bands of the pattern show the
carbon crystallites of the fibers to be preferentially
aligned parallel to the fiber axis. Microdensitometer
scanning of the (002) band of the exposed X-ray film indi-
cates this preferred orientation to be no more than about
10, usually from about 5 to about 10 (expressed as the
full width at half maxim~1m of the azimuthal intensity
distribution). The interlayer spacing (d) of the crystal-
lites, calculated from the distance between the correspond-
ing (ooQ~ difraction arcs, is no more than 3.37A, usually
22
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9474
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1~1155664
from 3.36 A to 3.37 A.
In addition to having a structure characteristic
of that of polycrystalline graphite, the fibers are charac-
terized by graphitic-like properties associated with such
structure, such as high density and low electrical resis- ~ -
tivity. Typically, these fibers have a density in excess
of 2.1 grams/cc. up to 2.2 grams/cc., and higher. Electri-
cal resistivity of the fibers has been found to be less
than 250 x 10-6 ohm centimeter, usually from about
150 x 10-6 ohm centimeters to about 200 x 10-6 ohm
centimeters.
The fibers are also character~zed by high mo~ulus
and high tenslle strengths. Thus, these fibers have been
found to be characterized by tensile strengths in excess
of about 1.38 GPa. and by a Young's modulus of elasticity
in excess of about 345 GPa. Usually such fibers have a
tensile strength in excess of about 1.72 GPa., e.g., -from
about 1.72 GPa. to about 2.41 GPa., and a Young's modulus
in excess of about 517 GPa., e.g., from about 517 GPa. to
about 828 GPa. `
The instant invention thus provides an improved
method of preparing high strength, high modulus fibers in
high yield from inexpensive, readily available, high ear-
bon content precursors. The fibers can be used in the same
applications where high strength, high modulus fibers have
previously been employed, such as in the preparation of
composites. The fibers are especially useful in applica-
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, : . , . :~ .. . . .
9474
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~1SS664
tions where high electrical conductivity and thermal con-
ductivity along the axis of the fibers is important, e.g.,
they can be used to produce graphitic cloth heating ele-
ments. Because of their extremely low electrical resis-
tlvity, the fibers can be employed as filler material in
the production of graphite electrodes.
The following examples are set forth for purposes
of illustration so that those skilled in the art may better
understand the invention. It should be ~nderst~od that
they are exemplary only, and should not be construecl as
limiting the invention ln any manner. Tensile strengths
reerred to in the examples and throughout the speciEicatlon,
unless otherwise indicated, are short gauge tensile strengths
measured on 3 mm~ samples. Young's modulus was measured
on 2.0 cm. sections unless otherwise indicatéd.
EXA~LE 1
A commercial petroleum pitch was employed to pro-
duce a pitch having a mesophase content of about 56 per
cent by weight. The precursor pitch had a density of 1.23
Mg./m.3, a softening t~mperature of 122C. and contained 5
0.5 per cent by weight quinoline insolubles (Q.I. was
determined by quinoline extraction at 75C.). Chemical
analysis showed a carbon content of 9401%, a hydrogen con-
tent of 5.56%, a sulfur content of 1.82%,and 0.19% ash.
The mesophase pitch was produced by heating the
precursor petroleum pitch at a temperature of about 380C.
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~474
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~ . . .
. ... .. .
for about 45 hours under flowing nitrogen. The pitch was
continuously stirred during this time and nitrogen gas was
continuously bubbled through the pitch. After h~ating~
the pitch exhibited a softening point oiE 318C. and con-
tained 56.7 per cent by weight pyridine insolubles, indi-
cating that the pitch had a mesophase content of close to
56 per cent. -
A portion of the pitch produced in this manner
was then melt spun into fibers at a rate of 229 meters per
minute through a 128 hole spinnerette (0.10 mm. diameter
holes) at a temperature of 392C. The flbers passed
through a nitrogen atmosphere as they left the spinnerette
and were then taken up by a reel.
A portion of the spun fibers was cut into
lengths 178-250 mm. long and submerged in a glass vessel ;
filled with a saturated solution of chlorine water con-
taining 0.02 per cent by weight of a wetting agent (Fluorad
FC-408, registered trademark of the Minnesota Mining and
Manufacturing Company)~ The chlorine water solution was
prepared by slowly bubbling chlorlne into water at 23C.
After soaking in the bath at 23C. for one minute, the
fibers were re~Loved, dipped into distilled water for
another minute, and dried at room temperature.
A portion of the fibers treated in this manner
were then heated for two minutes in a furnace m~intained
at a temperature of 300C. while oxygen was continuously
passed through the furnace. The resulting fibers were
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.. , . . . ~ . . ..
9474
5 ~6~ ~
sufficiently thermoset to be heated at elevated tempera-
tures without sagging.
The infusible fibers were carbonized lmder nitrogen
by first heating to a temperature of 90l)C. at a rate of
15C./minute, and then at 1650~C. for five minutes~ The
resulting fibers had an average tensile strength of 1.7
~Pa. and an average Young's modulus o elasticity of 207
GPa. (Tensile strength and Young's modulus are an average
of 5 and 6 samples, respectively). The average fiber
diameter was 13 micrometers.
When the spun fibers were immersed in the chlorine
water bath or three minutes and heated in oxygen for ~our
minutes at 300C., as described above, and then carbonized
in the same manner, the resulting fibers had an average
tensile strength of 2.23 GPa. and an average Young's
modulus of elasticity of 283 GPa. ~Tensile strength and
Young's modulus are an average of 7 and 5 samples, respec-
tively).
When the spun fibers were heated at 300C. for
two minutes in oxygen, as described above, without having
first been treated in chlorine water, they softened and
fused indicating that thermosetting was not complete.
EXAMPLE 2
A commercial petroleum pitch was employed to pro-
duce a pitch having a mesophase content of about 54 per
cent by weight. The precursor pitch had a density of
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1~23 Mg./m.3 a softening temperature of 122C~ and con-
tained 0.5 per cent by weight quinoline insolubles (Q.I.
was detenmined by quinoline extraction at 75C.). Chemical
analysis showed a carbon content of 94.1%, a hydrogen con- ;
tent of 5.56%, a sulfur content of 1. 82~/o and 0.19% ash.
The mesophase pitch was produced by heating the
precursor petroleum pitch under flowing nitrogen to a
temperature of about 380C. at a rate of 5C./minute,
maintaining the pitch at this temperature for 36 hours, and
then further heating the pitch to about 430C. at a rate
of 5C./minute where the temperature was maintained for
2 hours. The pitch was continuously stirred during this
time and nitrogen gas was continuously bubbled through
the pitch. Ater heating, the pitch exhibited a softening
point of 338C. and contained 54.0 per cent by weight
pyridine insolubles, indicating that the pitch had a meso-
phase content of close to 54 per rent.
A portion of the pitch produced in this manner
was then melt spun into fibers at a rate o 128 meters
per minute through a one hole splnnerette (0.10 mm. diameter
hole) at a temperature of 381C. The fibers were taken
up by a reel as they left the spinnerette hole.
A portion of the spun fibers was cut into lengths
178-250 mm. long and submerged in a glass vessel filled with
a saturated solution of chlorine water containing 0.02
per cent by weight of a wetting agent (Fluorad FC-423,
registered trademark of the Minnesota Mining and
. . , ,
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9474
";~ .
5664
Manufacturing Company). The chlorine water solution was
prepared by slowly bubbling chlorine into water at 23C.
After soaking in the bath at 23C. for 0.5 minute, the
fibers were removed, dipped into distilled water for one
minute, and dried at room temperature.
A portion of the fibers treated in this manner ~;
were then heated for two minutes in a furnace maintained
at a temperature of 350C. while oxygen was continuously
passed through the furnace. The resulting fibers were
sufficiently thermoset to be heated at elevated tempera-
tures without sagging.
The in~uslble ~ibers were carbonized under nitrogen
by first heating to a temperature o 925C. at a rate of
15C./minute, and then at 1750C. for five minutes. The
resulting fibers had an average tensile strength of 2.92
GPa. and an average Young's modulus of elasticity of 207
GPa~ (Tensile strength and Young's modulus are an average
of 15 and 5 samples, respectively). The average filament
diameter was 6.8 micrometers.
EXAMPLE 3
A commercial petroleum pitch was employed ~o pro-
duce a pitch having a mesophase content of about 62 per
cent by weight. The precursor pitch had a density of 1.23
Mg./m.3, a softening temperature o~ 122C. and contained
0.5 per cent by weight quinoline insolubles (Q.I. was
detenmined by quinoline extraction at 75C.) Chemical
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9474
` '
~55664 `
analysis showed a carbon content of 94.:L%, a hydrogen
content of 5056%, a sulfur content of 1.82% and 0.19% ash.
The mesophase pitch was produced by heating the
precursor petroleum pitch at a temperature of about 410C.
for about 11.8 hours under flowing nitrogen. The pitch
was continuously stirred during this time and steam was
continuously bubbled through the pitch. After heating,
the pitch exhibited a softenting point of 353C. and con- ~
tained 62.5 per cent by weight pyridine insolubles, i.ndi- -
cating that the pitch had a mesophase content of close to
6~ per cent.
A portion of the pitch produced in this manner
was then melt spun into flbers at a rate o 229 meters per
minute through a 128 hole spinnerette (0.10 mm. diameter
holes) at a temperature of 403C. The fibers passed
through a nitrogen atmosphere as they left the spinnerette
and were then taken up by a reel.
A portion of the spun fibers was continuously
fed through a bath containing a saturated solution of
chlorine water at room temperature at a rate of 0.30
meters/minute. The residence time of the fibers in the
bath was 80 seconds. The chlorine water solution was
prepared by slowly bubbling chlorine into water at 23C.
After passing through the chlorine water solution,
the fibars were dried by heating at 100C., and the dried
fibers were heated for two minutes in a furnace maintained
at a temperature of 300C. while oxygen was continuously
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, . . ~ . ~ .: . . . . . .
~ - 9474
~!~S566gl
passed through the furnace. The r~sulting fibers were
then carbonized under nitrogen by continuously passing
them through a first heating zone maintained at 1000C.
and then through a second zone maintained at 1650C. at
a rate of 0.30 meters/minute so as to allow a residence
time of 1 minute in each zone. ~he resulting fibers had
an average tensile strength o 1.57 GPa. and an average J`
Young's modulus of elasticity of 290 GPa. (Tensile strength
was determined on epoxy impregnated strands 2.5 cm. long
and is the average of 10 samples. Young's modulus was
determined on 12.5 cm. strands and i9 the average o~ 2
samples.) '~he average fiber diameter was 10 micro-
meters.
When the spun fibers were processed in the same
manner through a bath containing a solution of 0.5 per
cent by weight of bromine in water at a rate of 0.30 meters/
minute to allow a residence time of 30 seconds in the bath,
and then dried, heated in oxygen, and carbonized, as
described above, the resulting fibers had an average tensile
strength of 0.34 GPa and an average Young's modulus of
elasticity of 175 GPa. (Tensile strength was determined
on epoxy impregnated strands 2.5 cm. long and is the
average of 10 samples. Youngls modulus was detenmined -
on 12.5 cm. strands and is the average o~ 2 samples.)
Longer residence times than 30 seconds resulted in
embrittlement and breakage of the fibers.
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