Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
9330 -
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved process for
producing car~on fibers from pitch which has been trans-
formed3 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 ~rom
pitch of this type wherein the mesophase conteQt is pro-
~ duced in substantially shorter periods of time than hereto-
; 10 ore possible, at a glven temperature, by passing an inert
gas through the pitch during formation of the mesophase.
2. Descri~tion 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 struc-
tures, re-entry vehicles, and space vehicles, as well as
in the preparation of marine deep-submergence pressure ves-
sels and like structures, ~isting techno~ogy was incapa-
ble of supplying such materials and the search to satisfy
this need centered about the abrication of composite
articles.
One of the most promising materials suggested for
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.
9330-1
~L~585~
use in composite form was high strength, high modulus car-
bon textiles, which were introduced into the market place
at the very time this rapid growth in the aircraft, space
and missile industries was occurring. Such textiles have
~een incorporated in both plastic and m~tal matrices to pro-
duce 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
composites ha~ been a major deterrent to their widespread
use, in spite of the remarkable properties exhiblted by
such composites
One recently proposed method of providing high-
modulus, high-strength carbon fibers at low cost is de-
scribed in United States patent 4,005, 183, entitled
"High Modulus, High Strength Carbon Fibers Produced
From Mesophase Pitch". Such method comprises first spin-
ning a carbonaceous fiber rom a carbonaceous pitch which
has been tran~formed, in part, to a liquid crystal or so-
called "mesophase" state, then thermosetting the fiber so
produced by heating the fiber in an oxygen-containing at-
mosphere or a time sufficient`to render it inusible, and
finally carbonizing the thermoset iber by heating in an
inert atmosphere to a temperature suficiently elevated to
remove hydrogen and other volati~ s and produce a substan-
tially all-carbon fiber The carbon fibers produced in
this manner have a highly oriented structure characterized
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9330-1
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by the presence of carbon crystallites preferentially
aligned parallel to the fiber axls, and are graphitizable
materials which when heated to graphitizing temperatures
develop the three-dimensional order characterist~c of poly-
crystalline graphite and graphitic-like properties associ-
ated therewith, such as high density and low electrical
resistivity. At all stages o~ their development from the
as-drawn condition to the graphitized state, the fibers are
characterized by th~ presenee of large oriented elongated
graphitizable domains preferentially aligned parallel to the
fiber axis.
When natural or synthetic pitches having an aro-
matic base are heated under quiescent conditions at a tem-
perature of about 350C.-500C., either at constant tem-
pera~ure or with gradually increasing temperature, small
insoluble liquid spheres begin to appear in the pitch and
gradually increase in size as heating is continued. ~nen
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
spheres continue to grow in size as hea~ing is con~inued,
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 wherein the
transition from one oriented domain to another sometimes
9330 - 1
~S~3544
occurs smoothly and continuously through gradually curving
lamellae and sometimes through more sharply curving lamellae.
The differences in orientation between the domains create
a compleg array o~ polarized light extinction contours in
the bulk mesophase corresponding to various types of l~near
discontinuity in molecular alignment. The ultlmate size
of the oriented domains produced is dependent upon the vis-
cosity, and the rate of increase of the viscosity, of the
mesophase ~rom which they are formed, which, in turn are
depeQdent upon ~he particular pitch and the heating rate.
In certain pitches, domains having sizes in excess o~ two
hundred microns up to in excess of one thousand microns are
produced. In other pitches, the viscosity o the mesophase
is such that only limited coalescence and structural rear-
rangement o~ layers occur, so tha~ the ultimate domain size
does not exceed one hundred microns.
The highly oriented, optically anisotropic, in-
soluble material produced by treating pitches in this man-
ner has been given the term "mesophase", and pi~chs contain-
ing such material are known as "mesophase pitches". Such
pitches, when heated above their softening points, are mix-
tures of two essentially immiscible liquids, one the op-
tically anisotropic, oriented mesophase portion, and the
other the isotropic non-mesophase portion. The term
"mesophase'~ is derived from the Greek "mesos" or "inter-
mediate" and indicates the pseudo-crystalline nature of
this highly-oriented, optically anisotropic material.
9330-1
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Carbonaceous pitches having a mesophase content
of from about 40 per cent by waight to about 90 per cent by
weight are suitable for spinning into fibers which can sub-
sequently be converted by heat treatment into carbon fi-
bers having a high Young's modulus of elasticity and high
tensile strength. In order to obtain the desired fibers
from such pitch, however, it is not only necessary that such
amount of mesophase be present, but also that it ~orm, under
quiescent conditions, a homogeneous bulk mesophase having
large coalesced domains, i.e., domains of aligned molecules
in excess of two hundred microns up to in excess o one
thousand microns in size. Pitches which form stringy bulk
mesophase under quiescent conditions, having small oriented
domains, rather than large coalesced domains, are unsu~t-
able. Such pitches form mesophase ha~ ng a high viscosity
which undergoes only limited coalescense, insufficient to
produce large coalesced domains having sizes in excess of
two hundred microns. Instead, small oriented domalns of
mesophase agglomerate to produce clumps or stringy masses
wherein the ultimate domain size does not exceed one hundred
microns. Certa~n pitches which polymerize ~ery rapidly are
of this type. Likewise, pitches which do not fonm a homo-
geneous bulk mesophase are unsuitable. The latter phenome-
non is caused by the presence of infusible solids (which
are either present in the original pitch or which develop
on heating) which are enveloped by the coalescing mesophase
and serve to interrupt the homogeneity and unifonmity of
9330 -1
~058~i44
the coa~esced domains, and the boundaries between them.
Another requirement is that the pitch be non-
thixotropic under the conditions employed in the spinning
of the pitch into ~ibers, i e., it must exhibit a New-
tonian or plastic flow behavior so that the flow is uni-
form and well behaved. When such pi~ches 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 therefrom. Pitches, on the other hand, which
do not exhibit Newtonian or plastic flow behavior at the
temperature of spinning, do not permit uniform fibers to
be spun thererom which can be converted by further heat
treatment into carbon fibers having a high Young's modulus
of elasticity and high tensile strength.
Carbonaceous pitches having a mesophase content
o ~rom about 40 per cent by weight to about 90 per cent
by weight can be produced in accordance with known tPch-
niques, as disclosed in aforementioned United States
- patent 4,005, 183 by heating a carbonaceous pitch
20 in an inert atmosphere at a temperature above about 350C.
for a time suficient to produce the desired quantity o
mesophase. By an inert atmosphere is meant an atmosphere
which does not react with the pitch under the heating condi-
tions employed, such as nitrogen, argon, xenon, helium, and
~the like. The heating period required to produce the de-
sired mesophase content varies with the particular pitch and
tempera~ure employed, with longer heating periods required at
9333-1
~L051~544
lower temperatures than at higher temperatures. At 350C.,
the minimum temperature generally required to produce meso-
phase, 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 450C., 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 generally employed
for this reason. Temperatures above about 500C. are un-
desirable, and heating at this temperature should not be em-
ployed for more than about 5 minutes to avoid conversion
of the pitch to coke.
Although the time required to produce a mesophase
pitch having a given mesophase content is reduced as the tem-
perature of preparation rises, it has been found that heat-
ing at elevated temperatures adversely affects the rheologi-
cal properties of the pitch by alte~ ng the molecular weight
distribution of both the mesophase and non-mesophase por-
tions of the pitch. Thus, heating at elevated temperatures
tends to increase the amount of high molecular weight mole-
cules in the mesophase portion of the pitch. At the same
t~me, heating at such temperatures also results in an in-
creased amount of low molecular weight molecules in the non-
mesophase portion of the pitch. As a result, mesophase
pitches of a given mesophase content prepared at elevated
temperatures in relatively short periods of time have been
found to have a higher average molecular weight in the
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mesophase portion of the pitch and a lower average molecu-
lar weight in the non-mesophase portion of the pitch, than
mesophase pitches of like mesophase content prepared at more
moderate temperatures over more extended periods. This
wider molecular weight distribution has been ~ound to have
an adverse effect on the rheology and spinnabili~y of the
pitch, evidently because of a low degree of compatibility
between the very high molecular weight fractlon of the
mesophase portion of the pitch and the very low molecular
weight fraction o~ the non-mesophase portion of the pitch.
The very high molecular weight material in the mesophase
portion of the pitch can only be adequately plasticized at
very high temperatures where the tendency o the very low
molecular weight molecules in the non-mesophase portion of
the pitch to volatilize is greatly increased. As a result,
when such pitches are heated to a temperature where they
have a viscosity suitable for spinning and attempts are
made to produce fibers therefrom, excessive expulsion o
volatiles occurs which greatly interferes with the process-
ability of the pitch into ibers of small and uniform
diameter. For ~hese reasons, means have been sought or
shortening the time required to produce mesophase pitch at
relatively moderate temperatures of preparation where more
favorable rheological properties are imparted to the pitch.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has
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now been discovered that mesophase pitch o~ a given meso-
phase content can be prepared in substantially shorter
periods of time than heretofore possible, at a given tem-
perature, if an inert gas is passed through the pitcn dur-
ing formation of the mesophase. Treating the pitch with
an inert gas in this manner aids in the removal o~ vola-
tile low molecular weight components initi~lly present,
together with low molecular weight polymerization by-products
of the pitch, and results in the more efficient conversion
of the precursor pitch to mesophase pitch. Mesophase
pi~ches having a mesophase content of ~rom about 40 per
cent by weight to about 90 per cent by weight can be pre-
pared in this manner, at a glven temperature, at a rate of
up to more than twice as fast as that normally required in
the absence o such treatment, i.e., in periods of time as
little as less than one-half of that normally required when
mesophase is produced without an inert gas being passed
through the pitch.
; DESCRIPTION OF THE PREFERRED EMBODIMENTS
As a carbonaceous pitch is heated to a tempera-
ture suf~icientl~ elevated to produce mesophase, the more
volatile Low molecular weight molecules present therein are
slowly volatilized from the pitch. As heating is continued
above a temperature at which mesophase is produced, the
more reactive higher molecular weight molecules polymerize
to form still higher molecular weight molecules, which then
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g330 -1
~S~S~
orient themselves to form mesophase. While the less reac-
tive lower molecular weight molecules which have not been
volatilized can also polymerize, they often form hydrogenated
and/or substituted polymerization by-products having a
molecular weight below about 600 which do not orient to
form mesophase. Althollgh these low molecular weight poly-
merization by-products are gradually volatilized as heating
of the pitch is continued, the presence o~ large amounts of
these by-products during much of the time that the pîtch
is being converted to mesophase has been found to impede
the formation o~ mesophase by the more reactive molecules,
and, as a result, to considerably lengthen the time neces-
sary to produce a pitch of a given mesophase content. Fur-
ther, because of their small size and low aro~aticity,
these polymerization by-products are not readily compatible
with the larger, higher molecular weight, more aromatic
molecules present i~ the mesophase portion of the pitch, and
the lack o~ compatibility between these high and low molecu-
lar weight molecules adversely affects the rheology and
spinnability of the pitch. As painted out previously, the
very high molecular weight ~raction o the mesophase portion
of the pitch can only be adequately plasticized at very high
temperatures where the tendency of the very Iow molecular
weight molecules in the non-mesophase portion of the pitch
to volatilize is greatly increased, and when pitches hav-
ing large amounts of such materials are heated to a tempera-
ture where they have a viscosity suitable for spinning and
933() -
~CI 58544
attempts are made to produce fibers therefrom, excessive
expulsion of volatîles occurs which greatly interferes
with the processability of the pitch into fibers of small
and uniform diameter.
This invention takes advantage o the diferences
in molecular weight and volatility between the mesophase-
fonming molecules present in the pitch and those low
molecular weight components and polymerization by-products
which do not form mesophase to effect removal of the un-
desirable more volatile low molecular weight materîals and
more rapidly convert the pitch to mesophase. ~he molecules
which do not convert to mesophase are of lower molecular
weight than the higher molecular weight mesophase-forming
molecules and, facilitated by the inert gas purge during
conversion of the pitch to mesophase, are preferentially
volatilized from the pitch during formation of the meso-
phase, allowing the pitch to obtain a given mesophase con-
tent in substantially reduced periods of time. Thus, in
addition to shortening the t~me required to produce a
pitch of a given mesophase content, this procedure has
the e~fect of lessening the amount of low molecular weight
molecules in the non-mesophase portion of the pitch and
raising the average molecular weight thereof. Consequently,
such pitches can more easily be spun into fibers of small and
uniform diameter with little evolution of volatiles.
Removal of the more volatile components of the
pitch which do not convert to mesophase is efected by
` 9330-1
1a~5~354~
passing an inert gas through the pitch, during preparation
of the mesophase, at a rate of at least 0.5 sch. per pound
of pitch, preferably at a rate of 0.7 sc~h. to 5.0 scfh. per
pound of pitch. Any inert gas which does not react with the
pitch under the heating conditions employed can be used to
facilitate removal of these components Illustrative of such
gases are nitrogen, argon, xenon, helium, steam and th~ likeO
As aforementioned, removal of the undesirable
more volatile low molecular weight materials hastens con-
version of the pitch to mesophase, and when mesophase is
produced while passing an inert gas through the pitch in
this manner t the time required to produce a pitch of a
given mesophase content, at a given temperature, is reduced
by as much as one-half of that normally required in the
absence of such treatment or even more. Generally, the
time required to produce a pitch of a given mesophase con-
tent is reduced by at least 25 per cent, usually from 40
; per cent to 70 per cent, when the mesophase is prepared
while passing an inert gas through the pitch as described
as opposed to when it is prepared under identical conditions
but in the absence of such treabment.
While any temperature above about 350~C. up to
about 500C. can be employed to convert the precursor
pitch to mesoph se, it has been found that mesophase
pitches posæss improved rheological and spinning char-
acteristics when they are prepared at a temperature
of from 380C. to 440C., most preferably from 380C.
9330-1
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to 410C., so as to produce a mesophase content of from 50
per cent by weight to 65 per cent by weight. Usually from
2 hours to 60 hours of heating are required at such tem-
peratures to produce the desired ~mount of mesophase.
Mesophase pitches prepared under these condi~ions have
been found to possess a smaller differential between the
number average mole~ular weights of the mesophase and non-
mesophase portions of the pitch, than mesophase pitches
having the same mesophase content which have been prepared
at more elevated temperatures in shorter periods of time.
The attendant rheological and spinning properties accompany-
ing this narrower molecular weight distribution have been
found to substantially facilitate the processability of
the pitch into fibers of small and uniform diameter.
The mesophase pitches prepared under the preferred
conditions, i.e., by heating at a temperature of from 380C.
to 440C. so as to produce a mesophase content of from 50
per cent by weight to 65 per cent by weight passess a lesser
amount of high molecular weight molecules in the mesophase
portion o the pitch and a lesser amount of low molecular
weight molecules in the non-mesophase portion of the pitch,
and have a lower number average molecular weight in the
mesophase portion o~ the pitch and a higher number average
molecular weight in the non-mesophase portion of the pitch,
than mesophase pitches having the same mesophase content
which have been prepared at more elevated temperatures in
shorter periods of time. When mesophase pitches are pre-
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~L058S~
pared ~nder such conditions, less than 50 per cent of the
molecules in the mesophase portion of the pitch have a
molecular weight in excess of 4000, while the remaining
molecules have a number average molecular weight of from
1400 to 2800. The molecules in the non-mesophase portion
of such pitches have a number average molecular weight o
from 800 to 1200, with less than 20 per cent of such mole-
cules having a molecular weight of less than 600. When such
pitches are prepared by heating at the most preferred tem-
perature range of from 380C. to 410C., from 20 per cent
to 40 per cent of the molecules in the mesophase portion
of the pitch have a molecular weight in excess of 4000,
while the remaining molecules have a number average molecu-
lar weight of from 1400 to 2600. The molecules in the non-
mesophase portion of pitches prepared by heating at the most
preferred temperature range have a number average molecular
weight of from 900 to 1200~ with from 10 per cent to 16
per cent of such molecules having a molecular weight of
less than 600. When mesophase pitches are prepared at
temperatures in excess of 440C., on the other hand, more
than 80 per cent of the molecules in the mesophase portion
of the pitch have a molecular weight in excess of 4000,
while in excess o 25 per cent of the molecules in the
non-mesophase portion of the pitch have a molecular weight
of less than 600. The molecules in the non-mesophase por-
tion of the pitch have a number average molecular weight of
less than 800, while the n~mber average molecular weight of
-15-
9330 1
~ 5 ~ 5 4 ~
the molecules in the mesophase portion of the pitch which
do not have a molecular weight in e~cess of 4000 is from
1400 to 2800.
Mesophase pitches prepared by heating at a tem-
perature of from 380~C. to 440C. so as to produce a meso-
phase content of from 50 per cent by weight to 65 per cent
by weight usually exhibit a viscosity of from 10 poises to
200 poises at a temperature of from 320C. to 440C., and
can readily be spun into fibers of small and uni~orm diame-
~er at such temperatures with little evolution of volatiles.Because of their excellent rheological properties, suc~
pitches are eminently suitable for spinning carbonaceous
fibers which may subse~uently be converted by heat treatment
into fibers having a high Young's modulus of elasticity and
high tensile strength.
In order to produce pitches having the preferred
mesophase content and molecular weight characteristics, it
is usually necessary to heat a carbonaceous pitch at a tem-
perature of from 380C. to 44QC. for at least 2 hours,
preferably for from 2 hours to 6~ hours. Excessive heating
should be avoided 90 as not to produce a mesophase content
in excess of 65 per cent by weight, or adversely affect the
desired molecular weight distribution. To obtain the de-
sired molecular weight characteristics it is also necessary
that the pitch be agitated during formation of the mesophase
so as to produce a homogeneous emulsion of the immiscible
mesophase and non-mesophase portions of the pitch. Such
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.
~L05~54~
agitation can be e~fected by any conventional means, e.g.,
by stirring or rotation of the pitch, so long as it is suffi-
cient to effectively intermix the mesophase and non-meso-
phase portions of the pitch.
The degree to w~ich the pitch has been converted
to mesophase can readily be de~ermined by polariæed 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 organicsolvents such as quinoline and pyridine, while the mesophase
portion is essentially insoluble.( ) In the case of pitches
which do not develop non-mesophase insolubles when heated,
the insoluble content of the heat treated pitch over and
above the insoluble content o the pitch before it has been
heat treated corresponds essentially to the mesophase con-
tent. In the case ofpitches which do develop non-
mesophase insolubles when heated, the insoluble content of
the heat treated pitch over and above the insoluble content
o~ the pitch before it has been heat treated is not solely
due to the conversion o~ the pitch to mesophase, but also
represents non-mesophase insolubles which are produced along
with the mesophase during the heat treatment. Pitches
(1) The per cent o quinoline insoluble (Q.I.) of a given
pitch is determined by quinoline extraction at 75C. The
per cent of pyridine insolubles (P.I.) is determined by Soxh-
let extraction in boiling pyridine (115~C.).
(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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9330-l
~ ~ S ~ ~ 4 4
which contain infusible non-mesophase insolubles (either
present in the original pitch or developed by heating) in
amounts sufficien~ to prevent the development of homogeneous
bulk mesophase are unsuitable for use in the present inven-
tion, as noted above. Generally, pitches which contain in
excess of about 2 per cent by weight of such infusible ma-
terials are unsuitable. The presence or absence of such
homogeneous bulk mesophase regions, as well as the presence
or absence of inusible non-mesophase insolubles, can be
visually observed by polarized light microscopy examina-
tion of the pitch (see, e.g., Brooks, J. D., and Taylor,
G. H., "The Formation of Some Graphitizing Carbons,"
Chemistry and_PhYsics of Carbon, Vol. 4, Marcel Dekker, Inc.,
New Yor~, 1968, pp. 243-268, and Dubois, J., Agache, C.,
and White, J. L., "The Carbonaceous Mesophase Formed in the
Pyrolysis of Graphitizabl~ Organic Materials," Metal-
~lography 3, pp. 337-369, 1970). The amounts of each of these
materials may also be visually estimated in this manner.
Conventional molecular weight analysis techniques,
can be em~loyed to determine the molecular weight chàr-
acteristics of the mesophase pitches produced in accordance
with the present invention. In order to permit molecular
weight determinations to be conducted independently on both
the mesophase and non-mesophase portions of the pitch, the
two phases may be conveniently separated through the use of
a suitable organic solvent. As noted above, e~cept for
certain non-mesophase insolubles present in the original
-18-
9330-1
~585~
pitch or which, in some instances, develop on heating, the
non-mesophase portion of the pitch is readily soluble in
organic solvents such as quinoline and pyridine, while the
mesophase portion is essen~ially insoluble.( ) Af~er
separation of the two phases with a solvent in this manner,
the non-mesophase portion of the pitch may be recovered from
the solvent by vacuum distillation of the solvent.
One means which has been employed to determine
the number average molecular weight of the mesophase pitches
produced ln accordance with the present invention involves
the use of a vapor phase osmometer. The utilization of
instruments of this type for molecular weight determinations
has been described by A. P. Brady et al. (Brady, A. P.,
Huff, H., and McGain, J. W., J. Phys. & Coll. Che~ 55, 304,
(1951)). The osmometer measures the difference in electri-
cal resistance between a sensitive reference thermistor in
contact with a pure solvent, and a second thermistor in con-
tact with a solution of said solvent having dissolved
therein a known concentration of a material whose molecular
20 , weight is to be determined. The diference in electrical
resistance between the two thermistors is caused by a dif
~erence in temperature between the thermistors which is
produced by the different vapor pressures of the solvent
and the solution. By comparing this value with the dif
(3) The non-mesophase portion of the pitch may be readily
separated from the mesophase portion by extraction with
quinoline at 75~C. or by Soxhlet extraction in boiling
pyridine ~115C.).
-19-
9330 -1
1058544
ferences in resistance obtained with said solvent and
standard solutions of said solvent containing known con-
centrations of compounds of known molecular weights, it is
possible to calcu~ate the molecular weight of the solute
material. A drop of pure solvent and a drop of a solution
of said solvent having dissolved therein a known concen-
tration of the material whose molecular weight is being
determined are suspended side by side on a reference the~mis-
tor and sample thermistor, respectively, contained in a
closed thermostated chamber saturated with solvent vapor,
and the resistance of the two thermistors is measured and
the difference between the two recorded. Since a solution
of a given solvent will always have a lower vapor pressure
than the pure solvent, a diferential mass transfer occurs
between the two drops and the solvent vapor phase, resulting
in greater overall condensation on (and less evaporation
from) the solution drop than on the solvent drop. This di~
ference in mass transfer causes a temporary temperature
difference betwean the two thermistors (due to differences
in 109s of heat of vaporization between the two drops) which
is proportional to the difference in vapor pressure between
the two drops. Since the difference in vapor pressure be-
tween the two drops, and hence the difference in tempera-
ture and resistance, (~R), between the two thermistors de-
pends solely upon the number of molecules of the solute
material dissolved in the solvent, and is independent of
the chemical composition of the molecules, the mole frac-
-20-
9330-1
~S~S~
tion of solute in the solution, (N), can be determined from
a plot of ~R vs. N for such solvent and solutions of such
solvent containing known concentrations of compounds o
known molecular weight. ~ ) ~R and N bear a direct linear
relationship to each other, and from a determination of N
it is possible to calculate the calibration constant, (K),
for the solvent employed from the formula:
K N - ~
Having determined the value of K, the molecular weight of
the material may be determined from the formula:
= (K - ~R) My W~
aR Wy
wherein Mk is the molecular weight of the material upon
which the determination is being made, K i9 the calibra-
tion constant or the solvent employed, ~R is the differ-
ence in resistance between the two thermistors, My is the
molecular weight of the solvent, Wy is the weight of the
solvent, and Wx is the weight of the material whose molecu-
lar weight is being determined. Of course, having once
determined the value of the calibration constant of a given
solvent, (K), the molecular weight of a given material may
be determined directly from the ~ormula.
While the molecular weight of the soluble portion
(4) By the mole fraction of a given material in a solu-
tion, (N), is meant the number of moles of such material in
thesolution divided by the number of moles of such material
in the solution plus the number of moles of the solvent.
-21~
9330 -
10~854g~
of the pitch can be determined directly on a solution
thereof, in order to determine the molecular weight of the
insoluble portion, it is necessary that it first be
solubilized, e.g., by chemical reduction of the ~romatic
bonds of such material with hydrogen. A suitable means for
solubilizing coals and carbons by reduction of the aroma~ic
bonds o~ these materials has been described by J. D. Brooks
et al. (Brooks, J. D., and Silberman, H., t'The Chemical
Reduction o~ Some Cokes and Charsr', Fuel 41, pp. 67-69,
1962). Thls method involves the use of hydrogen generated
by the reaction of lithium with ethylenedlamine, and has been
found to effectively reduce the aromatic bonds of carbona-
ceous materials without rupturing carbon-carbon bonds. Such
method has been suitably employed to solubilize the in-
soluble portion of the pitches prepared in accordance with
the inver.tion.
Another means which has been employed to de~
termine the molecular weight characteristics o~ the meso-
phase p~stches produced in accordance with the present inven-
tion is gel permeation chromatography (GPC). This tech-
nique has been described by L. R. Snyder (Snyder, L. R.,
"Determination of Asphalt ~olecular Weight Distributîons
by Gel Permeation Chromatography", Anal. Chem. 41, pp.
1223-1227, 1969). A gel permeation chromatograph is em-
ployed to fractionate a solution of polymer or polymer re-
lated molecules of various sizes, and the molecular weight
distribution of the sample is determined with the aid of
9330-1
~ ~ 58 S ~ 4
a detection system which is linearly responsive to solute
concentration, such as a differential refractometer or a
differential ultraviolet absorption spectrometer. As in
the case of the vapor phase osmometry tecknique, in order
to permit molecular weight de~enminations to be conducted
independently on both the mesophase and non-mesophase por-
tions of the pitch, the two phases must first be separa~ed
through the use of a suitable organic solvent. Again,
while the molecular weight of the soluble portion of the
pitch can be determined directly on a solution thereof, in
order to determine the molecular weight of the insoluble
portion, it is necessary that it first be solubilized.
Fractionation of the sample whose molecular weight
distribution is being determined is ef~ected by dissolving
the sample ln a suitable solvent and passing the solution
through the chromatograph and collecting measured fractions
of the solution which elute through the separation column
of the chromatograph. A given volume of solvent is required
to pass molecules of a given molecular size through the
chromatograph, so that each fraction o~ solution which
elutes from the chromatograph contains molecules of a given
molecular size. The fractions which flow through the
column first contain the higher molecular weight molecules,
while the fractions which take the longest time to elute
through the column contain the lower molecular weight
molecules.
After the sam~le has been fractionated, the con-
-23-
9330 -1
~5~44
centration of solute in each fraction is determined by
means o a suitable detection system, such as a differential
refractometer or a diferential ultraviolet absorption
sp~ctrometer. When a differen~ial refractometer is employed,
the re~ractive index of each fraction is automatically com-
pared to that of the pure solvent by means of two photo-
electric cells which are sensitive to the intensity of light
passing through such fractions and solvent, and the differ-
ences ln signal intensities between the two cells are auto-
matically plotted against the cumulative elution volume of
the solution. Since the magnitude of these differences in
signal intenslty is linearly related to the concentration
by weight of solute molecules present, the relative concen-
tration by weight of molecules in each ~raction can be de-
termined by dividing the differential signal int~nsity for
that fraction by the total integrated different~ 1 signal
intensity of all the fractions. This relative concentration
may be graphically depicted by a plot of the differential
signal intensity for each fraction against ~he cumulative
elution volume of the sample.
The molecular weight of the molecules in each
fraction can then be determined by standard techniques,
e.g., by the osmometry techniques described above. Since
most conventional pitches are composed o~ similar types of
molecular species, once the molecular weights o the vari-
ous fractions of a particulax sample have been determined,
that sample may be used as a standard and the molecular
-24-
9330-1
~ ~ ~8 S 4 4
weights o~ the fractions of subsequent samples can be de-
termined from the known molecular weights of like fractions
of the standard. Thus, molecular weight determinations
need not be repeatedly made on each fraction of each sam-
ple, but may be obtained from the molecular weights de-
termined for like fractions of the standard. For con-
venience, a molecular weight diStribution curve depicting
the relationship of the molecular welght to the elution
volume of the standard may be prepared by plotting the
molecular weights determined for the standard fractions
against the cumulative elution volume of the standard. The
molecular weights of the molecules of the various chromato-
graphic fractions of any given sample can then be directly
read from this curve. As aforementioned, the relative con-
centration by weight of solute molecules in each fraction
can be determined by di~ferential re~ractive index measure-
ments.
To facilitate the molecular weight determinations,
the differential signal intensities and elution volume
values obtained on a given sample, together with previously
determined lecular weight data rèlating to the various
chromatographic ~ractions o~ a standard pitch, can be pro-
cessed by a computer and transcribed into a complete molecu-
lar weight distribution analysis. By this procedure, com-
plete printouts are routinely provided of number average
molecular weight (Mn)9 weight average molecular weight (Mw),
molecular weight distribution parameter (MW/Mn), as well
-25-
93~0-1
~ 5 4 4
as a compilation of molecular weight and percentage by
weight of solute present in each chromatographic fraction
of a sample.
Aromatic base carbonaceous pitches having a car-
bon content of from about 92 per cent by weight to about 96
per cent by weight and a hydrogen conten~ of from about 4
per cent by weight to about 8 per cent by weight are gen-
erally sui~able for producing mesophase pitches which can
be employed to produce fibers capable of being heat treated
to produce fibers having a high Young's modulus of elas-
ticity and a high tensile strength. 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 cen~ by weight. The presence of more than such amount
of extraneous elements may disrupt the ~ormation of carbon
crystallites during subsaquent heat treatment and prevent
the development of a graphitic-like structure within the
fibers produced from these materials. ln addition, the
presence of extraneous elements reduces the carbon content
o~ the pitch and hence the ultimate yield o~ carbon ~iber.
When such e~traneous elements are present in amounts of
from about 0.5 per cent by weight to about 4 per cen~ by
weight, the pitches generally have a carbon conten~ of from
about 92-95 per cent by weight, the balance being hydrogen.
Petroleum pitch, coal tar pitch and acenaphthylene
pitch, which are well-graphitizing pitches, are preferred
starting materials for producing the mesophase pitches
9330 -1
~ 5~ 5 ~ ~
which are employed to produce the ~ibers of the instant in-
vention. Petroleum pitch, of course, is ~he residuum
carbonaceous material obtained from the dis~illation of
crude oils or the catalytic cracking of petroleum distil-
lates. Coal tar pitch is similarly obtained by the dis-
tillation of coal. Bo~h of these materials are commer-
cially available natural pitches in which mesophase can
easily be produced, and are preferred for this reason.
Acenaphthylene pitch, on the other hand, is a synthetic
pitch which is pre~erred because of its ability to pro-
duce excellent fibers. Acenaphthylene pitch can be pro-
duced by the pyrolysis of polymers of acenaphthylene as
described by Edstrom et al. in U. S. Patent 3,574,6S3.
Some pitches, such as fluoranthene pitch, poly-
merize very rapidly when heated and fail to develop large
coalesced domains of mesophase, and are, therefore, not
suitable precursor materials. Likewise, pitches having a
high infusible non-mesophase insoluble co~tent in organic
solvents such as quinoline or pyridine, or those which de-
velop a high inusible non-mesophase insoluble conten~ 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 pro-
duce highly oriented carbonaceous fibers capable of being
converted by heat treatment into carbon fibers having a
high Young's modulus of elasticity and high tensile
strength. For this reason, pitches having an infusible
9330-1
~5854~
quinoline-insoluble or pyridine-insoluble co~tent of more
than about 2 per cent by weight (determined as described
above) should not be employed, or should be filtered to
remove this material be~ore being heated to produce meso-
phase. Preferably, such pitches are filtered when they
contain more than about 1 per cent by weight of such in-
fusible, insoluble material. Most petroleum pitches and
synthetic pitches have a low infusible, insoluble content
and can be used directly without su~h filtration. Most
coal tar pitches, on the o~her hand, have a high infusible,
insoluble content and require filtration before they can
be employad.
As the pitch is heated at a temperature between
350~C. and 500C. to produce mesophase, the pltch 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 heatin~ a
carbonaceous pitch for a ti~e sufficient to produce a meso-
phase content of from about 40 per cent by weight to about
90 per cent by weight, the resulting pitch will contain a
carbon content of from about 94-96 per cent by weight and
a hydrogen content of from about 4-6 per cent by weigh~.
When such pitches co~ntain elements other than carbon and
hydrogen in amounts of from about 0.5 per cent by weight
to about 4 per cent by weight, the mesophase pitch will
generally have a carbon content of from about 92-95 per
-2~-
9330-1
~L~58~4~
cent by weight, the balance being hydrogen.
After the desired mesophase pitch has been pre-
pared, it is spun into fibers by conventional techniques,
e.g., by melt spinning, centrifugal spinning, blow spin-
ning, or in any other known manner. As noted above, in
order to obta~n highly ori~nted car~onaceous fibers capable
of being heat treated to produce carbon fibers having a
high Young's modulus of elasticity and high tensile
strength, the pitch must, under quiescent conditions, form
a homogeneous bulk mesophase having large coalesced do-
mains, and be nonthixotropic under the condi~ions employed
in the spinning. Further, in order to obtain unifonm fi-
bers from such pitch, the pitch should be agitated immedi-
ately prior to spinning so as to efectively intermix the
immiscible mesophase and non-mesophase portions o the
pitch.
The temperature at which the pitch is spun de-
pends, o~ course, upon the temperature at which the pitch
exhibits a suitable viscosity. Since the softening tem-
perature of the pitch, and its viscosity at a given tem-
perature, increases as the mesophase content of the pitch
increases, the mesophase content should not be permitted
to rise to a point which raises the softening point of
the pitch to excessive levels. For this reason, pitches
having a mesophase content o~ more than about 90 per cent
are generally not employed. Pitches containing a meso-
phase content of about 40 per cent by weight usually have
-29-
~330-1
~ S~ 5 4 ~
a viscosity of about 200 poises at about 300~C. and about
10 poises at about 375GC., while pitches containing a meso-
phase content of about 90 per cent by weight exhibit
similar viscosities at temperatures above 430C. Within
this viscosity range, fibers may be conveniently spun from
such pitches at a rate of from about 50 feet per minute
to about lOOQ feet per minute and even up to about 3000
eet per minute. Preferably, the pîtch employed has a meso-
phase content of from abGut 50 per cent by weight to about
65 per cent by weight and exhibits a viscosity of from about
30 poises to about 150 poises at temperatures of from about
340C. to about 380C. At such viscosity and temperature,
uniform fibers having diameters of from about 5 microns to
about 25 microns can be easily spun. As previously men-
tioned, however, in order to obtain the desired ~ibers, it
is important that the pitch be nonthixotropic and exhibit
Newtonian or plastic ~low behavior during the spinning of
the fibers.
The carbonaceous ~ibers produced in this man~
ner are highly oriented graphitizable materials having a
high degree of preferred orientation of their ~olecules
parallel to the fiber axis. By "graphitizable" is meant
that these fibers are capable of being converted thermally
(usually by heating to a temperature in excess of about
2500~C., e.g., from about 2500C. to about 3000C.) to a
structure having the three-dimensional order character-
istic of polycrystalline graphite.
-30-
9330-1
~ ~ 5 ~ 5 ~ ~
The fibers produced in this manner, of course,
have the same chem cal composition as the pitch ~rom which
they were drawn, and like such pitch contain from about 40
per cent by weight to about 90 per cen~ by weight meso-
phase. When examined under magnification by polarized light
microscopy techniques, the fibers exhibit textural varia-
tions which give them the appearance of a "mini-composite".
Large elongated anisotropic domains, having a fibrillar-
shaped appearance, can be seen distributed throughou~ the
fiber. These anisotropic domains are highly oriented and
pre~erentially aligned parallel to the fiber a~is. lt
is believed that these anisotropic domains, which are
elongated by the shear forces exerted on the pitch during
spinning of the ~ibers, are not composed entirely of meso-
phase, but are also made up of non-mesophase. Evidently,
the non-mesophase is oriented, as well as drawn into
elongated domains, during spinning b7 these shear orces
aQd the orienting effects exerted by the mesophase domains
as they are elongated. Isotropic regions may also be
present, although they mày not be visible and are difi-
cult to dlfferentiate from those anisotropic regions which
happen to show ex~inctlon. Charac~eristically, the ori-
ented elongated domains have diameters in excess of 5000 A,
generally ~ro~ about 10,000 A to about 40,000 A, and be-
cause of ~heir large size are easily observed when examined
by conventio-nal polarized light microscopy techniques at
a magnification of 1000. (The maximum resolving power
7JJU ~
3S4~
of a standard polarized light microscope having a magnifica-
tion factor of 1000 is only a few tenths of a micron ~1 mi-
cron = 10,000 A] and anisotropic domains having dimensions
of 1000 A or less cannot be detected by ~his technique.)
While fibers spun rom a pitch containing in ex-
cess o~ about 85 per cent by weight mesophase often re-
tain their shape when carbonized without any prior thermo-
setting, fibers spun from a pitch containing less than
about 85 per cent by weight mesophase require soma thermo-
setting before they can be carbonized. Thermosetting of
the fibers is readily effected by heating the fibers in
an oxygen-containing atmosphere for a time suficient to
render them infusible. The oxygen-containing atmosphere
employed may be pure ox~gen or an oxygen-rich atmosphere
Most conveniently, air is employed as the oxidizing at-
mosphere.
~he time required to effect thermosetting of
the fibers will, o course, vary with such factors as the
particular oxidizing atmosphere, the temperature employed,
the diameter of the fibers, the particular pitch from
- which the fibers are prepared, and the mesophase con~ent
of such pitch. Generally, ho~ever, thermosetting of the
fibers can be effected in relatively short periods of time,
usuall~ in from about 5 minutes to about 60 minutes.
The temperature employed to effect thermosetting
of the fibers must, of course, not exceed the temperature
at which the fibers will soften or distort. The maximum
-32-
9330-1
~5~S4~
temperature which can be employed will thus depend upon the
particular pitch from which ~he fibers were spun, and the
mesophase content of such pitch. The higher the mesophase
content of the pitch, the higher will be its softening
temperature, and the higher the temperature which can be
employed to effect thermosetting of the fibers. At higher
temperatures, of course, fibers of a given diameter can be
thermoset in less time than is possible at lower tempera-
tures. Fibers prepared from a pitch having a lower meso-
phase content, on the other hand, require relatively longer
heat treatment at somewhat lower temperatures to render them
infusible.
A minimum temperature of at least 250C. is gen-
erally necessary to effectively thermoset the carbonaceous
fibers produced in accordance with the invention. Tempera-
tures in excess of 400C. may cause melting and/or exces-
sive burn-off of the fibers and should be avoided. Prefer-
ably, temperatures of from about 275C. to about 350C. are
employed. At such temperatures~ thermosetting can gen-
erally be effected within ~rom about 5 minutes to about 60
minutes. Since it is undesirable to oxidize the fibers
more than necessary to render them totally infusible, the
fibers are generally not heated for longer than about 60
minutes, or at temperatures in excess of 400C.
After the fibers have been thermoset, the in-
fusible fi~ers are carbonized by heating in an inert at-
mosphere, such as that described above, to a temperature
9330-1
~o5~44
sufficiently elevated to remove hydrogen and other volatiles
and produce a substantially all-carbon fiber. Fibers hav-
ing a carbon content greater than about 98 per cent by
weight can generally be produced by heating to a tempera-
ture in excess of about 1000C., and at temperatures in
excess of about 1500C , ~he fibers are completely car-
bonized.
UsualLy, carbonization i5 effected at a tempera-
ture of from about 1000C. to about 2000C., preferably
from about 1500C. to about 1900C. Generally, residence
times of from about 0.5 minute to about 25 minutes, prefer-
ably from about 1 minute to about 5 minutes, are employed.
While more extended heating times can be employed with good
results, such residence times are uneconomical and, as a
practical matter, there is no ad~antage in employing such
long periods.
In order to ensure that the rate of weight loss
of the;fibers does not become so excessive as to disrupt
the fiber structure, it is preferred to heat the fibers for
a brief period at a temperature of ~rom about 700C. to
about 900C. before they are heated to their final car-
bonization temperature. Residence times at these tempera-
tures of from about 30 seconds to about 5 minutes are
usually sufficient. Preferably, the fibers are heated at
a temperature of about 700C. for about one-half minute and
then at a temperature of about 900C. for like time. In
any event, the heating rate must be controlled so that the
9330-1
1~5~3544
vola~ization does not proceed at an excessive rate.
In a preferred method of heat treatment, con~
tinuous fibers are passed through a series of heating
zones which are held at successively higher temperatures.
If desired, the first of such zones may contain an oxidiz-
ing atmQsphere where thermosetting of the fibers is e~fected.
Several arrangements of apparatus can be utilized in pro-
viding the series of heating zones. Thus, one furnace
can be used with the fibers being passed through the fur-
nace several times and with the temperature being increased
each time. Alternatively, the fibers may be given a single
pass 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 temper-
atures in the direction of travel of the fibers, can be usedO
The carbon fibers produced in this manner have a
highly oriented structure characterized by the presence of
carbon crystallites preeren~ially aligned parallel to the
fiber axi9, and are graphitizable materials which when
heated to graphitizing temperatures develop the three-di-
mensional order characteristic of polycrystalline graphi~e
and graphitic-like properties associated therewith, such
as high density and low electrical resistivity.
If desired, the carbonized fibers may be further
heated in an inert atmosphere, as described hereinbefore,
to a still higher tempera~ure in a range of from about
-35-
9330-1
~ 5~ 5 ~ ~
2500C. to about 3300C., preferably from about 2800C. to
about 3000C., to produce fibers having no~ only a high
degree of preferred orientation of their carbon crystal-
lites parallel to the fiber axis, bu~ also a s~ructure
characteristic o~ polycrystalline graphite. A residence
time of about 1 minute is satisfactory, although both
shorter and longer times m~y be employed, e.g., from about
10 seconds to about 5 minutes, or longer. Residence times
longer than 5 minutes are uneconomical and unnec0ssary,
but ~ay 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-dlmensional order of poly-
crystalline graphite. This three-dimensional order is es-
tablished by the X-ray diffraction pattern of the fibers,
speci~ically 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 (00~) bands of the pattern show the carbon crystallites
of the fibers to be preferentially aligned parallel to the
fiber axis. MicrodensLtometer scanning of the (002) band
of the exposed X-ray ~ilm indicate this preferred orienta-
tion to be no more than about 10, usually ~rom about 5
to about 10 (expressed as the full width at half maximum
of the azimuthal intensity distribution). Apparent layer
size (La) and apparent stack height (Lc) of the crystal-
lites arein excess of 1000 A and are thus too large to be
-36-
9330-1
~ ~ S~ S ~ 4
measured by X-ray techniques. The lnterlayer spacing (d)
of the crystallites, calculated from the distance between
the corresponding (OOR) diffraction arcs, is no more than
O O O
3.37 A, usually from 3.36 A to 3.37 A.
EXAMPLE
The following example is set forth for purposes
o~ illustration so that those skilled in the art may be~ter
understand the invention. It should be understood that it
is exemplary only, and should not be construed as limit-
ing the invention in any manner.
EXAMPLE 1
A commercial petroleum pitch was employed to
produce a pitch having a mesophase content o~ about 53 per
cent by weight. The precursor pitch had a number average
molecular weight of 400, a de~sity of 1.23 grams/cc., a
softening temperature of 120C., and contained 0.83 per
cent by weight quinoline insolubles tQ,I. was determined
by quinoline extraction at 75C.). Chemical analysis
showed a carbon content of 93.0%, a hydrogen content of
5.6%, a sulfur content of 1.1% and 0~044~/O ash.
The mesophase pitch was produced by heating 60
grams o the precursor pitch in a 86 cc. reactor to a
temperature of about 200C. over a one hour period, then
increasing the temperature of the pitch from about 200C.
to about 400C. at a rate of about 30C. per hour, and main-
-37-
9330~1
5 8 ~ 4 4
~aining the pitch at àbout 400C. ~or an additional 12
hours. The pitch was continuously stirred during this
time and nitrogen gas was continuously bubbled through
the pitch at a rate of 0.2 scfh.
The pitch produced in this manner had a pyridine
insoluble content of 53 per cent, indicating a mesophase
content of close to 53 per cent. The pitch could be easily
spun into fibers, and a considerable qua~tity of ~iber was
produced by spinning the pitch through a spinnerette
(O.015 inch diamet~r hole) at a tem~erature of 368C. The
fiber passed through a nitrogen a~mosphere as it left
the spinnerette and before it was taken up by a reel.
A portion of the fiber produced in this manner
was heated in oxygen for six minutes at 390C, The xe-
sulting oxidized fibers were totally infusible and could
be heated at elevated temperatures without sagging. After
heating the infusible fibers to 1900C. over a period of
about 10 minutes in a nitrogen atmosphere, the fibers were
found to have a tensile strength of 171 x 103 psi. and
a Young's modulus of elasticity of 46 x 106 psi. (Ten-
sile strength and Young's modulus are the average values
of 10 samples.)
For comparative puxposes, a meso~hase pitch was
prepared ~rom the same precursor pitch and in the same
manner described above except that while the pitch was pre-
pared under a nitrogen atmosphere, the nitrogen was not
allowed to bubble through the pitch. Thirt~-two hours
-38-
9330-1
~ O 5~ 5 ~ ~
o~ heating a~ 400C. were required to produce a mesophase
pitch having a pyridine insoluble content o~ 50 per cent.
-39-
