Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR ISOLATING MESOPHASE PITCH Case No. 8018
Back~rounct and Summary of the Invention
Tt is well known that carbon fibers having excellent
properties suitable for commercial use can be produced
from mesophase
pitch, hiesophase pitch derived carbon fibers are light
weight,
strong, stiff, thermally and electrically conductive,
and both
chemically and thermally inert. The mesophase-derived
carbon fibers
perform well as reinforcements in composites, and have
found use in
aerospace applications and quality sporting equipment.
Low cost carbon fibers produced from isotropic pitch
exhibit
little molecular orientation and relatively poor mechanical
properties. In contrast, carbon fibers produced from
mesophase pitch
exhibit highly preferred molecular orientation and relatively
excellent mechanical properties.
The term "pitch" as used herein means petroleum pitches,
natural asphalt and heavy oil obtained as a by-produot
in tho naphtha
cracking industry, pitches of high carbon content obtainod
from
petroleum asphalt and other substances having properties
of pitches
produced as by-products in various industrial production
processes.
The term "petroleum pitch" refers to the residuum
carbonaceous material obtained from the thermal and catalytic
cracking
of petraleum distillates or residues.
The term "anisotropic pitch" or "mesophase pitch" means
pitch comprising molecules having aromatic structure
which through
interaction have associated together to form optically
ordered liquid
crystals.
The term "isotropic pitch" means pitch comprising molecules
which are not aligned in optically ordered liquid crystals.
The term "mesogens" means mesophase-forming materials or
mesophase precursors.
Mesophase pitch is not ordinarily available in existing
hydrocarbon fractions, such as refining fractions, or in coal
fractions, such as coal tars. Mesophase pitch, however, may be
derived from isotropic pitch containing mesogens. Isotropic pitch
containing mesogens is usually prepared by the treatment of aromatic
feedstocks. Such treatment, which is well known in tha art, may
involve one or more heat soaking steps, with or without. agitation, and
CA 02026488 1999-08-11
2
with or without gas sparging or purging. Gas sparging may be carried out with
an
inert gas or with an oxidative gas, or with both types of operations. Numerous
patents describe various aspects of the treatment of aromatic containing
feedstocks
to obtain isotropic pitch. Included are: U.S. Patent Nos. 4,209,500, heat
soaking;
3,976,729 and 4,017,327, agitation during heat treatment; 3,974,264 and
4,026,788, inert sparge gas during heat treatment; 4,283,269, heat soaking of
fluxed pitch; Japanese Patent 65090/85, heating in the presence of an
oxidizing
gas; 4,464,248, catalytic heat soaking; 3,595,946 and 4,066,737, use of
oxidative
reactive material; and 4,474,617, use of oxidizing gas; and many others.
Mesophase pitch may be obtained from isotropic pitch containing
mesogens by solvent fractionation, which is carried out by the following
steps:
( 1 ) Fluxing the isotropic pitch in a hot solvent.
(2) Separating flux insolubles by filtration, centrifugation, or
other suitable means.
(3) Diluting the flux filtrate with an anti-solvent (comix solvent) to
precipitate a mesophase-forming (mesogen containing) pitch.
(4) Washing and drying the precipitated pitch.
(5) Fusing the precipitated pitch to form mesophase.
The solvent fractionation procedure described is well known in the art and
is set forth in some detail in numerous patents, including U.S. Patent No.
4,277,324. This patent sets forth numerous solvent and anti-solvents which can
be
employed in solvent fractionation and the operating conditions and procedures
which may be used.
Separation of mesogens from isotropic pitch may also be effected by the
solvent extraction process described in U.S. Patent 4,208,267. In this patent
fractionation is accomplished without fluxing or flux filtration. The mesogen-
containing isotropic pitch is extracted with a comix type solvent and the
mesogens
are collected as an insoluble residue. Solvents used in this process are
similar to
those employed in the process of U.S. Patent 4,277,324.
It is desirable to provide an alternative process for obtaining mesophase
pitch from isotropic pitch which does not involve the use of a comix solvent,
and
CA 02026488 1999-08-11
thus eliminates the need for storage and pumping facilities for two solvents
and
separation facilities for separating the solvents.
According to the present invention, isotropic pitch containing mesogens is
combined with a solvent and subjected to dense phase or supercritical
conditions
to effect phase separation of the mesogens from the pitch. In one aspect of
the
invention, isotropic pitch containing mesogens is fluxed with a solvent to
solubilize the mesogens, the flux mixture is then filtered to remove
insolubles, and
the solubilized mesogens are phase separated from the flux mixture under dense
phase or supercritical conditions of temperature and pressure. The dense phase
or
supercritical conditions employed are such that the mesogens are recovered as
mesophase.
Further aspects of the present invention are as follows:
A process for the preparation of mesophase pitch which comprises:
(a) combining an isotropic pitch containing mesogens with a solvent,
(b) effecting phase separation of the mesogens from the isotropic pitch under
solvent supercritical conditions of temperature and pressure, wherein said
mesogens associate together under solvent super-critical conditions of
temperature and pressure to form mesophase pitch; and
(c) recovering mesophase pitch.
A process for the preparation of mesophase pitch which comprises:
(a) subjecting an isotropic pitch containing mesogens to fluxing with a
solvent
to solubilize the mesogens,
(b) filtering the flux mixture to remove insolubles,
(c) separating the solubilized mesogens from the flux solvent under solvent
supercritical conditions of temperature and pressure, wherein said
mesogens associate together under solvent supercritical conditions of
temperature and pressure to form mesophase pitch; and
(d) recovering mesophase pitch.
A process for producing mesophase pitch which comprises:
(a) subjecting a pitch to heat soaking to form an isotropic pitch containing
mesogens but substantially free of mesophase,
CA 02026488 1999-08-11
3a
(b) fluxing the isotropic pitch with a solvent to form a mixture and to
solubilize
the mesogens,
(c) filtering the flux mixture in step (b) to remove insolubles and any
mesophase,
(d) phase separating the solubilized mesogens from the solvent in step (b)
under solvent supercritical conditions of temperature and pressure, wherein
said mesogens associate together under solvent supercritical conditions of
temperature and pressure to form mesophase pitch; and
(e) recovering mesophase pitch.
The Prior Art
U.S. Patent No. 4,581,124 discloses treatment of a pitch (containing a
substantial amount of mesophase, i.e. 5 to 25 weight percent) with solvent
extraction under supercritical conditions to recover a mesophase rich pitch
containing at least 30 percent mesophase and preferably at least 50 percent
mesophase by weight.
Japanese Patent No. 60-170694 discloses the preparation of precursor pitch
for carbon fibers by extracting coal tar pitch with an aromatic solvent in a
critical
state. The extracted pitch is then subjected to heat treatment with sparging
of inert
gas to give the desired product.
U.S. Patent No. 4,277,324 discloses converting an isotropic pitch to
anisotropic (mesophase) pitch by solvent fractionation. Isotropic pitch is
first
mixed with an organic fluxing solvent. Suspended insoluble solids in the flux
mixture are then removed by physical means, such as filtration. The solids-
free
flux liquid is then treated with an antisolvent to precipitate a mesophase-
forming
pitch which is fused to form mesophase. The patent further discloses heat
soaking
the pitch prior to solvent fractionation.
U.S. Patent No. 4,208,267 discloses extracting isotropic pitches with a
comix (antisolvent) solvent to provide a solvent insoluble fraction. This
fraction
when heated to 230°C to 400°C is converted to greater than 75%
mesophase.
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Brief Description of the Drawing
Figure 1 is a schematic diagram of a process unit suitable
for producing mesophase pitch which illustrates the invention.
Detailed Description of the Invention
Suitable isotropic pitches for use in carrying out the
process o.f the invention are obtained by various treatments of heavy
aromatic fractions, including heat soaking. While heavy fractions
generally may be used, the preferred materials are petroleum pitches
as previously defined. On a weight basis, particularly useful pitches
will contain from about 88 percent to about 93 percent carbon, and
from about 9 percent to about LF percent hydrogen. While elements
other than carbon and hydrogen such as sulfur and nitrogen are
normally present in such pitches, it is important that these other
elements do not exceed about 5 percent by weight of the p:Ltch. Also,
these particularly useful pitches typically w:Lll have an average
molecular rveight on the order of about 200 to about 1000,
Useful starting materials in addition to the preferred
petroleum pitches include ethylene cracker tars, coal derivatives,
petroleum thermal tars, and aromatic distillates having a boiling
range of from 650 to 950°F.
When heat soaking is employed to obtain suitable isotropic
pitch, this procedure is usually accomplished at a temperature in the
range of about 370 to about 500°C for about 0.10 to about 240 hours.
Lower soak temperatures require longer soak times and vice versa. The
preferred soaking conditions are from about 2 to about 24 hours at a
temperature range of about 390 to about 430°C. As mentioned
previously, the heat soaking step may be carried out with or without
agitation and with or without the presence of a sparge or purge gas.
In the preferred aspect of the invention, isotropic pitch
containing mesogens is mixed with a fluxing solvent and is fluxed to
solubilize the mesogens.
A variety of solvents are suitable for use as the fluxing
material. They include such compounds as aromatics such as benzene
and naphthalene, naptheno-aromatics such as tetralin and
9,10-dihydroanthracene, alkyl aromatics such as toluene, xylenes and
methyl naphthalenes, hetero-aromatics such as pyridine, quinoline and
tetrahydrofuran; and combinations thereof. Also suitable are simple
~~c~=~~'~~~3
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halo carbons, including chloro and fluoro derivatives of paraffin
hydrocarbons containing 1 to 4 carbon atoms such as chloroform and
trichloroethane and halogenated aromatics such as trichlorobenzene.
In general, any organic solvent having a critical temperature below
about 500°C, which is non-reactive with the pitch and which, when
mixed with the pitch in sufficient amounts, is capable of solubilizing
the mesogens may be used in carrying out the process of the invention.
At temperatures above about 500°C undesirable reactions can take
place
with or 'between aromatic compounds in the pitch.
The amount of fluxing solvent used will vary depending upon
the temperature at which mixing is conducted and the composition of
the pitch. In general, the amount of solvent used will be in the
range of between about 0.05 parts by weight of solvent per part by
weight of pitch to about 2.5 parts by weight of solvent per part by
weight of pitch, Preferably, the weight ratio of flux solvent to
pitch will be in the range of from about 0.7 to 1 to about 1.5 to 1.
The fluxing operation is usually carried out at an elovated
temperature and at sufficient pressure to maintain the system in the
liquid St<lte, 1'liXing or agitation era provided during the fluxing
operation to aid in the solubilization of the mesogens. Usually the
fluxing operation is performed at a temperature in the range of
between about 30 and about 150°C and for a time period of between
about 0.1 and about 2.0 hours. However, fluxing may be carried out up
to the boiling point of the solvent at system pressure. If desired,
the flux mixture may be stored in tankage indefinitely.
Upon completion of the fluxing step, the solubilized
mesogens are separated from the insoluble portion of the pitch by the
usually techniques of sedimentation, centrifugation or filtration. If
filtration is the selected separation technique used, a filter aid may
be employed, if desired, to facilitate the separation of the fluid
material from the solids.
The solid materials which are removed from the fluid pitch
consist of materials such as coke and catalyst fines which were
present in the pitch prior to heat soaking, as well as those
insolubles generated during heat soaking. If heat soaking conditions
are not carefully controlled, mesophase may be generated in the pitch
during heat soaking. This mesophase is partially lost in the process
~~~~~~~c~
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since it is predominantly insoluble in the flux mixture and is removed
with the other insolubles during the separation process. In the
process of the invention, isotropic pitch, which is substantially free
of mesophase, is preferred since this means that the prior treatment
of the pitch has been accomplished in a manner to provide for a
maximum amount of mesogens in the pitch prior to solvent
fractionation.
After removal of the solids from the system, the remaining
pitch solvent mixture containing dissolved mesogens is subjected to
supercritical temperature and pressure, i.e. temperature and pressure
at or above the critical temperature and critical pressure of the flux
solvent to effect phase separation of the mesogens from the pitch. In
the case of toluene, for example, the critical conditions are 319°C
and s11 psia. The time required to separate mesogens fxom the system
will vary, depending on the particular 'pitch and tho solvent employed
and the geometry of the separation vessel. If desired, additional
fluxing solvent may be added to the system. The amount of such added
solvent may be up to about 12 parts of solvent by weight per 'part by
weight of pitch and preferably from about O.S to about 6 parts of
solvent per part of pitch. If additional fluxing solvent is added,
agitation or mixing is desirable to promote intimate interphase
contact.
In the prior art method of solvent fractionation of
isotropic pitch, which included the use of a comix or anti-solvent, a
fusing operation served to convert the mesogens to mesophase pitch.
In the process of this invention, fusing is not necessary to
accomplish this conversion since the product obtained from the
supercritical phase separation step is mesophase rather than mesogens.
The supercritical conditions applied in carrying out the
process of the invention will vary depending on the solvent used, the
composition of the pitch and the temperature employed. The level of
supercritical pressure may be used to control the solubility of the
pitch in the solvent and thus established the yield and the melting
point of the mesophase product. For example, at a given temperature
and solvent-to-pitch ratio, if the pressure on the system is
increased, the solubility of the pitch in the solvent also increases.
This results in a lower yield of 'higher melting point mesophase
_,_
product. Lowering the pressure gives the opposite result. Generally
the supercritical temperature employed will be at or somewhat above
the critical temperature of the solvent, e.g. from 0 to about 100°C
above the solvent critical temperature. If desired, higher
temperatures may be used; however, they are riot required. The
pressure maintained on the system will vary over a wider rang: since
it is most conveniently used for controlling product properties and
yield. Thus the pressure applied an the system may be up to twice as
high as the critical pressure or higher if des:Lred.
The temperature and pressure required for the process herein
are the same as or higher than the critical temperature and pressure
of the solvent used in the process. Suitable solvents are those
solvents which have critical temperatures in the range of from about
100°C to about 500°C. The upper temperature limit is controlled
by
the thermal stability of the pitch and/or solvent mixture. The lowor
temperature limit is set by the critical temperature of tho particular
solvent used. Preferred solvents have critical temperatures above
200°C; however, other solvents such as the halocarbons have lower
critical temperatures. for example chlorotrifluoromethane has a
critical temperature of 29°C. The process temperature is typically up
to about 100°C above the critical temperature of the solvent or
higher.
The process pressure is generally from about 300 psig to
about 5.000 psig, preferably from about 500 psig to about 3,000 psig.
It should be noted however, that some pitch/solvent process systems
may utilize higher or lower pressures. The system pressure varies
over a wide range since it is most conveniently used for controlling
product properties and yield. Thus, the pressure applied to the
system may be up to twice as high as the critical pressure of the
solvent or higher.
The amount of solvent used in the process and the
temperature employed also affect the solubility of the pitch in the
solvent which in turn affects the melting point of the mesophase
product. For example, increasing the amount of solvent increases the
amount of pitch solubilized and a similar effect is obtained with
increasing temperature. Both of these variations result in a reduced
yield of mesophase product of increased melting point.
_8_
Upon completion of phase separation of the mesogens (now
mesophase) from the pitch, flux solvent dissolved in the mesophase may
be removed by reducing the system pressure while maintaining the
temperature at a sufficient level to maintain the mesophase in the
liquid state. Solvent removal is usually carried out at a temperature
of between about 300 and about 400°C for between about 0.01 and about
2 hours, depending on the type of solvent removal procedure used. For
example, with thin film evaporation only very short residence times
are required.
The mesophase pitch product obtained in the process of the
invention can be spun into continuous and isotropic carbon fibers by
conventional procedures, such as melt spinning, followed by the
separate steps of stabilization and carbonization. These are known
techniques and consequently they do not constitute a critical feature
of the present invention.
In addition to the conventional solvent Fluxing, the process
of this invention also includes enhanced fluxing. Enhanced fluxing
employs elevated temperatures and pressures up to the critical
conditions for the flux mixture. Enhanced fluxing offers higher
solubility leading to improved yields. It also offers process
advantages such as greater compatibility with the supercr:ttical
conditions employed in the process and easier flux filtering of less
viscous mixtures. The solvent ratio employed with enhanced fluxing
will vary from between about 0.5 and about 2.5 parts by weight of
solvent per part of weight by pitch.
After removal of the solvent the liquid mesophase recovered
under the supercritical conditions of the invention may be spun
directly, or alternatively this material may be cooled to a solid
phase material for transport in storage. If desired, the mesophase
product may be solvent washed and dried as in the conventional two
solvent process.
In the preferred aspect of the invention, as
afore-described, solvent fluxing of the heat soaked isotropic pitch
and filtration of the flux mixture removes inorganic contaminants and
flux insoluble components from the desired product. This results in a
high quality mesophase having a very low quinoline insolubles content.
Dense phase or supercritical separation of the mesogens from the pitch
'~~'~~t~~~
_ 9 _
may also be effected without the fluxing or filtration steps to
provide a desirable mesophase product. While the mesophase obtained
by this simplified process is not of as high quality as that resulting
from fluxing and filtration, it is suitable for use in many
applications and is of higher quality than mesophase obtained from
isotropic pitch by other processes such as gas sparging, gravity
separation. Tn this aspect o.f the invention the heat soaked isotropic
pitch containing mesogens is combined with the solvent in a suitable
manner. For example, the pitch may be melted and cornbined with heated
solvent and the combination than subjected to supercritical
conditions. Alternatively the pitch may be subjected to supercritical
conditions of the particular solvent used and then combined with
solvent, also provided under supercritical conditions. After they are
combined the pitch and solvent are subjected to mixing or agitation to
provide an intimate admixture of the materials prior 'to effecting
phase separation. Thereafter the procedure followed is tho same as
that previously described for tho preferred embodiment of the
invention subsequent to the filtration step. The solvents employed in
this aspect of the invention are the same as those previously listed
for the preferred embodiment. The amount of solvent used is up to
about 12 parts per part 'by weight of pitch and preferably from about
0.5 to about 8.0 parts of solvent per part of pitch.
The process of the invention may be further exemplified by
reference to the flow scheme shown in the drawing. Referring to the
drawing, filtered flux liquid, which is a mixture of isotropic pitch,
solvent, and solubilized mesagens, is introduced through line 2 to
mixer 5 and 3s joined by solvent provided via line 28. Both of these
streams are increased in pressure and temperature to supercritical
conditions prior to their introduction to the mixer. After thorough
mixing the materials are introduced to phase separator 4, wherein
phase separation takes place to provide a mixture of isotropic pitch
and solvent in the upper portion of the separator and mesophase
containing dissolved solvent in the lower portion of the separator.
The bottom phase in the separator is removed through line 6 and
introduced to stripper 8 where separation and recovery of the solvent
is effected. For this purpose, stripping gas is introduced to the
stripper through line 10. Mesophase pitch product is withdrawn from
~ ~~ .'~ ~a
_
the bottom of the stripper through line 12 and stripping gas and
solvent are removed overhead through line 14 and passed to flash drum
16. The solvent and stripping gas in the flash drum are ,joined by
isotropic pitch and solvent removed overhead from phase separator 4
5 through line 18. In the flash drum conditions of temperature and
pressure are maintained to provide separation of solvent and stripping
gas from the isotropic pitch, which is withdrawn from the bottom of
the flash drum through line 20. The solvent and stripping gas are
taken overhead -through line 22 and introduced to separator 24 where
10 the solvent and stripping gas era separated. Tha gas is withdrawn
overhead through line 30 and solvent is removed from the bottom of the
separator and is recycled to the fluxing operation through line 26. A
part of the solvent is also transferred through Line 28 for
combination with the filtered flux entering mixer 5 as previously
described.
The drawing has boen described by reference to the preferred
embodiment of the invention; however, the same process procedure :Ls
followed when fluxing and filtration are not employed. In this cuss
the feed to mixer 5 through line 1 is isotropic patch containing
mesogens rather than filtered flux.
The following examples illustrate the results obtained in
carrying out the invention.
Example 1
An isotropic feedstock was prepared by heat soaking an
850+°F cut of decant oil from an FCC unit for six hours at
741°F. The
heat soaked pitch was then fluxed by conventional means by combining
the pitch and flux solvent (toluene) in about equal amounts at the
reflux temperature of toluene. Flux filtration of the mixture removed
particles down to submicron size. The filtered flux liquid was then
vacuum distilled to remove the toluene. A clean, solid heat soaked
pitch with a hot stage melting point of 123°C resulted from this
procedure. 285 gm of this pitch were mixed with an initial 950 gm of
toluene in a 2-liter high pressure stirred autoclave. The system was
heated to a processing temperature of 340°C under autogenous pressure.
Upon reaching the operating temperature, 834 gm of additional toluene
were added to raise the operating pressure to 1215 psia. The
resulting mixture of about 22.8 percent pitch in toluene was then
2~~~~~~
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agitated at 500 rpm for a period of one hour. Processing conditions
during agitation were 340°C and 1215 psia pressure. After one hour,
the agitator was turned off and the mixture was permitted to
equilibrate and settle for 30 minutes. Following the settling period,
samples were obtained at operating pressure from the top and bottom of
the autoclave using heated sample containers. These samples were the
basis of all subsequent analyses.
The top equilibrated phase was 81.9 weight percent toluene,
with the remainder being extracted pitch oils, The bottom phase was
24.9 weight percent toluene, with the remainder being non-volatile
mesophase pitch. Product yield in the bottom phase as a percentage of
feed weight was 27 percent on a toluene-free basis. The non-volatile
material from the bottom phase was remo~=ed from the sample container
and heated to 360°C and held for 30 minutes under vacuum to remove the
volatiles.
The mesophase content of the product from the bottom phase
by hot stage examination was determined from a polished section, using
optical image analysis. The product was 100 percent mesophase. The
hot stage melting point of the material was 337°C. Tha material was
successfully press spun into a continuous fiber at a spinning
temperature of 360°C. The fiber was stabilized and carbonized by
conventional means. Properties from samples of the fiber were as
follows:
Tensile Strength (Kpsi) 320
Modulus (Mpsi) 33
Elongation (~) 0.81
These properties are indicative of a fiber of acceptable quality.
Example 2
A 1000 gm sample of the heat-soaked aromatic pitch 'prepared
in Example 1 was fluxed 1:1 in toluene at 110°C. Flux filtering
netted 4.6~ insolubles. The flux filtrate was diluted with comix
solvent (toluene/heptane) at a ratio of 8 ml per gram of pitch feed.
This rejection mixture was cooled to 30°C and the precipitate was
isolated by filtration, washed and dried. The yield, melting
temperature and mesophase content of the precipitate and the
toluene:heptane comix ratio are shown below:
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Comix-toluene:heptane (ml:ml) 88:12
Precipitate Properties
Yield, wt$ 20.1
Melting temp., °C 322
Mesophase content, ~ 100
The properties of the masophase pitch obtained in this
example using the prior art solvent fractionation process are
comparable to the 27 wt$ yield, 337°C melting temperature and 100
percent mesophase content obtained in Example 1 using the process of
the invention.
The comix toluene:heptane ratio may be used to control the
melting point of the precipitate. Increasing the amount of heptane
during rejection will precipitate a softer (lower melting) product and
result in a slightly higher yield.
Example 3
Two tests wero carriod out with the foadstock of Examplo 1.
Heat soaking, flux filtration and recovery of mesophase warp carried
out in the same manner and under the same conditions as in Example 1,
except that the operating pressure and solvent-to-pitch ratio wero
zp varied as shown in the following table.
TABLE 1
PROCESS CONDITIONS
Percent Mesophase
Heat Hot Stage
Temp, Pressure Soaked Melt Temp.
Test C Asia Pitch* C
Control (Ex. 1) 340 1215 22.8 337
1 340 2710 24.6 428
2 340 1420 43.7 310
*Percent heat soaked pitch in mixture of solvent and pitch
subjected to supercritical conditions of temperature and
pressure.
Test 1 illustrates the effect of pressure on solubility and
thus the pitch melting point. Increasing the pressure increases the
solubility of the pitch in the solvent which provides a separated
mesophase product having a higher melting point.
~~12~~~~ ,
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Test 2 illustrates the effect of solvent-to-pitch ratio on
solubility and the mesophase melting point. :.Reducing the amount of
solvent decreases the solubility of the pitch in the solvent which
results in a separated mesophase product of lower melting point.
While certain embodiments and details have been shown for
the purpose of illustrating the present invention, :Lt will be apparent
to those skilled in the art that various changes and modifications may
be made herein without departing from the spirit or scope of the
invention.
In the process of the invention all of the above variables
interact and are controlled to provide the desired mesophase product
and ultimately the properties of 'the fiber made from such product.
Obviously, many modifications and variations of the
invention, as hereinbefore set forth, may be made without departing
from the spirit and scope thereof, and therefore only such limitations
should be imposed as are indicated in the appended claims.
