U.S. patent number 3,919,376 [Application Number 05/318,482] was granted by the patent office on 1975-11-11 for process for producing high mesophase content pitch fibers.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to David A. Schulz.
United States Patent |
3,919,376 |
Schulz |
November 11, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Process for producing high mesophase content pitch fibers
Abstract
An improved process for producing carbon fibers from pitch which
has been transformed, in part, to a liquid crystal or so-called
"mesophase" state. According to the process, the mesophase content
of fibers spun from such pitch is increased before the fibers are
thermoset and carbonized by vacuum distillation of the
non-mesophase content of the fibers.
Inventors: |
Schulz; David A. (Fairview
Park, OH) |
Assignee: |
Union Carbide Corporation (New
York, NY)
|
Family
ID: |
23238360 |
Appl.
No.: |
05/318,482 |
Filed: |
December 26, 1972 |
Current U.S.
Class: |
264/102;
264/DIG.19; 423/447.4; 264/29.2 |
Current CPC
Class: |
D01F
9/00 (20130101); D01F 9/145 (20130101); Y10S
264/19 (20130101) |
Current International
Class: |
D01F
9/145 (20060101); D01F 9/00 (20060101); B24c
025/00 () |
Field of
Search: |
;423/447
;264/29,87,DIG.19,102 ;204/164,192 ;106/273R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Woo; Jay H.
Attorney, Agent or Firm: Piscitello; J. S.
Claims
What is claimed is:
1. A process for producing a pitch fiber having a high mesophase
content which comprises spinning a carbonaceous fiber from a
nonthixotropic carbonaceous pitch containing from 40 percent by
weight to 70 percent by weight mesophase, said mesophase being
present in the form of large, homogeneous, coalesced regions, and
heating the spun fiber under a pressure of less than 100 microns Hg
at a temperature and for a time sufficient to volatilize at least a
portion of the non-mesophase content of the fiber and produce a
fiber of higher mesophase content.
2. A process as in claim 1 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
3. A process as in claim 1 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
4. A process as in claim 1 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize at least 5
percent by weight of the non-mesophase content of the fiber.
5. A process as in claim 4 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
6. A process as in claim 4 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
7. A process as in claim 1 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize from 10 percent
by weight to 40 percent by weight of the non-mesophase content of
the fiber.
8. A process as in claim 7 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
9. A process as in claim 7 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
10. A process as in claim 1 wherein the fiber is heated under a
pressure of less than 30 microns Hg.
11. A process as in claim 10 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
12. A process as in claim 10 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
13. A process as in claim 10 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize at least 5
percent by weight of the non-mesophase content of the fiber.
14. A process as in claim 13 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
15. A process as in claim 13 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
16. A process as in claim 10 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize from 10 percent
by weight to 40 percent by weight of the non-mesophase content of
the fiber.
17. A process as in claim 16 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
18. A process as in claim 16 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
19. A process as in claim 1 wherein the pitch contains from 50
percent by weight to 65 percent by weight mesophase.
20. A process as in claim 19 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
21. A process as in claim 19 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
22. A process as in claim 19 wherein the fiber is heated at a
temperature and for a time sufficinet to volatilize at least 5
percent by weight of the non-mesophase content of the fiber.
23. A process as in claim 22 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
24. A process as in claim 22 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
25. A process as in claim 19 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize from 10 percent
by weight to 40 percent by weight of the non-mesophase content of
the fiber.
26. A process as in claim 25 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
27. A process as in claim 25 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
28. A process as in claim 19 wherein the fiber is heated under a
pressure of less than 30 microns Hg.
29. A process as in claim 28 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
30. A process as in claim 28 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
31. A process as in claim 28 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize at least 5
percent by weight of the non-mesophase content of the fiber.
32. A process as in claim 31 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
33. A process as in claim 31 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
34. A process as in claim 28 wherein the fiber is heated at a
temperature and for a time sufficient to volatilize 10 10 percent
by weight to 40 percent by weight of the non-mesophase content of
the fiber.
35. A process as in claim 34 wherein the fiber is heated at a
temperature of from 250.degree.C. to 400.degree.C.
36. A process as in claim 34 wherein the fiber is heated at a
temperature of from 300.degree.C. to 390.degree.C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved process for producing carbon
fibers from pitch which has been transformed, in part, to a liquid
crystal or so-called "mesophase" state. More particularly, this
invention relates to an improved process for producing carbon
fibers from pitch of this type wherein the mesophase content of
fibers spun from such pitch is increased before the fibers are
thermoset and carbonized by vacuum distillation of the
non-mesophase content of the fibers.
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 extraordinary combination of
physical properties. Thus, materials 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 composite articles.
One of the most promising materials suggested for use in composite
form was high strength, high modulus carbon 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 been incorporated in both plastic and metal matrices
to produce composites having extraordinary high-strength- and
high-modulus-to-weight ratios and other exceptional properties.
However, the high cost of producing the high strength, high modulus
carbon textiles employed in such composites 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 copending
application Ser. No. 239,490, entitled "High 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 thermosetting the fiber so-produced by
heating the fiber in an oxygen-containing atmosphere for a time
sufficient to render it totally infusible, and finally carbonizing
the thermoset fiber by heating in an inert atmosphere to a
temperature sufficiently elevated to remove hydrogen and other
volatiles and produce a substantially all-carbon fiber. The carbon
fibers produced in this manner 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 of polycrystalline graphite
and graphitic-like properties associated therewith, such as high
density and low electrical resistivity.
Since carbonaceous fibers drawn from pitches having a high
mesophase content can be thermoset in less time than carbonaceous
fibers drawn from pitches having a lower mesophase content, it is
desirable to employ pitches of high mesophase content in such
process. However, because the spinning of mesophase-containing
pitches becomes increasingly difficult as the mesophase content of
the pitch increases, and must be done at higher and higher
temperatures, the fibers are usually prepared from pitches having a
mesophase content of only from about 40 percent by weight to about
70 percent by weight.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has now been
discovered that pitch fibers having a high mesophaase content can
be prepared from pitch fibers of lower mesophase content which have
been spun from pitches of the type described in aforementioned
copending application Ser. No. 239, 490, i.e., carbonaceous pitches
which have been transformed, in part, to a liquid crystal or
so-called "mesophase" state, by subjecting the fibers to
low-pressure heat treatment so as to volatilize at least a portion
of the non-mesophase content of the fiber; and that the so-treated
fibers can be converted by further heat treatment into carbon
fibers having a high young's modulus of elasticity and high tensile
strength. The invention takes advantage of the differences in
molecular weight and volatility between the molecules of the
mesophase portion of the fiber and the molecules of the
non-mesophase portion to effect removal of the non-mesophase
portion and produce a fibrous residue of higher mesophase content.
The non-mesophase molecules are of lower molecular weight than the
higher molecular weight mesophase molecules and are preferentially
volatilized from the fiber during the initial heat treatment to
produce a fiber of higher mesophase content. This non-mesophase
material can be substantially completely removed by the
distillation or only partially, depending upon the relative amounts
of mesophase and non-mesophase material present in the fibers, the
diameter of the fibers, the heat treatment temperature, the
pressure employed during the heat treatment, and the duration of
the heat treatment. Surprisingly, it has been found from weight
loss data on the fibers, together with solubility and polarized
ligh microscopy studies, that the increase in mesophase content of
the heat-treated fibers is not due solely to volatilization of the
non-mesophase material, but results, in part, from conversion of
this non-mesophase to mesophase.
The fibers produced in this manner have a high degree of preferred
orientation of their molecules parallel to the fiber axis and can
be converted by heat treatment into carbon fibers having a high
Young's modulus of elasticity and high tensile strength. The carbon
fibers so-produced have a highly oriented structure characterized
by the presence of carbon crystallites preferentially aligned
parallel to the fiber axis, and when heated to graphitizing
temperatures develop the three-dimensional order characteristic of
polycrystalline graphite and graphitic-like properties associated
therewith, such as high density and low electrical resistance. At
all stages of their development from the as-drawn condition to the
graphitized state, the fibers are characterized by the presence of
large oriented graphitizable domains preferentially aligned
parallel to the fiber axis, with the fibers after vacuum
distillation, however, containing a lesser amount of non-mesophase
material than before such distillation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
When natural or synthetic pitches having an aromatic base are
heated at a temperature of about 350.degree.C.-450.degree.C.,
either at constant temperature or with gradually increasing
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 techniques,
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 heating is continued, they come in contact with one another
and gradually coalesce with each other to produce larger masses of
continuous aligned layers. Eventually, substantially the entire
pitch coalesces and takes on the superficial appearance of a mosaic
structure where, however, the transition from one oriented region
to another occurs smoothly and continuously through gradual curving
lamellar regions rather than through sharp boundaries between
uniform areas of oriented lamellae.
The highly oriented, optically anisotropic, insoluble material
produced by treating pitches in this manner has been given the term
"mesophase," and pitches containing such material are known as
"mesophase pitches". Such pitches, when heated above their
softening points, are mixtures of two immiscible liquids, one the
optically anisotropic, oriented mesophase portion in either
spherulite or coalesced form, and the other the isotropic
nonmesophase portion. The term "mesophase" is derived from the
Greek "mesos" or "intermediate" and indicates the
pseudo-crystalline nature of this highly-oriented, optically
anisotropic material.
Carbonaceous pitches having a mesophase content of from about 40
percent by weight to about 70 percent by weight can easily be spun
into fibers which can subsequently be converted by heat treatment
into carbon fibers having a high Young's modulus of elasticity and
high tensile strength. Although fibers can also be spun from
pitches having a mesophase content in excess of about 70 percent by
weight, e.g., up to about 90 percent by weight, these pitches are
exceedingly difficult to work with because of their high softening
temperatures, and fibers can only be spun from such pitches at
elevated temperatures where the pitches readily undergo
polymerization and/or coking.
In order to obtain highly oriented carbonaceous fibers capable of
being heat treated to produce fibers having the three-dimensional
order characteristic of polycrystalline graphite from carbonaceous
pitches having a mesophase content of from about 40 percent by
weight to about 90 percent by weight, however, it is not only
necessary that such amount of mesophase be present, but also that
it be present in the form of large, homogeneous, coalesced regions.
Pitches which polymerize very rapidly develop small or stringy
mesophase regions rather than large coalesced regions and are
unsuitable. Likewise, pitches which do not form homogeneous
coalesced regions of mesophase are unsuitable. The latter
phenomenon is caused by the presence of non-mesophase insolubles
(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 uniformity of the coalesced
domains.
Another requirement is that the pitch be nonthixotropic under the
conditions employed in the spinning of the pitch into fibers i.e.,
it must exhibit a Newtonian or plastic flow behavior so that the
viscosity coefficient is independent of the shear rate of the pitch
during the spinning process. 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 therefrom.
Thixotropic pitches, on the other hand, which do not exhibit
Newtonian or plastic flow behavior when attempts are made to spin
fibers therefrom, but rather undergo changes in apparent viscosity,
do not permit uniform fibers to be spun therefrom which can be
converted by further heat treatment into fibers capable of
developing the threedimensional order characteristic of
polycrystalline graphite.
Carbonaceous pitches having a mesophase content of from about 40
percent by weight to about 90 percent by weight can be produced in
accordance with known techniques, as disclosed in aforementioned
copending application Ser. No. 239, 490, by heating a carbonaceous
pitch in an inert atmosphere at a temperature above about
350.degree.C. for a time sufficient 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
employed, such as nitrogen, argon, zenon, 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 350.degree.C., 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 percent. At temperatures of from about 400.degree.C. to
450.degree.C., conversion to mesophase proceeds more rapidly, and a
50 percent mesophase content can usually be produced at such
temperatures within about 1-40 hours. Such temperatures are
preferred for this reason. Temperatures above about 500.degree.C.
are undesirable, and heating at this temperature should not be
employed for more than about 5 minutes to avoid conversion of the
pitch to coke.
The degree to which the pitch has been converted to mesophase can
readily be determined 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-mesophase portion of the pitch is readily soluble
in organic solvents such as quinoline and pyridine, while the
mesophase portion is insoluble. .sup.1 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 of the pitch before it has been heat treated is due to
conversion of the pitch to mesophaase. .sup.2 In the case of
pitches which do develop non-mesophase 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
such non-mesophase insolubles (either present in the original pitch
or developed by heating) in amounts sufficient to prevent the
development of homogeneous coalesced mesophase regions are
unsuitable for use in the present invention, as noted above.
Generally, pitches which contain in excess of about 2 percent by
weight of such materials are unsuitable. The presence or absence of
such homogeneous coalesced mesophase regions, as well as the
presence or absence of non-mesophase insolubles, can be visually
observed by polarized light microscopy examination 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 York, 1968, pp. 243-268; and Dubois,
J., Agache, C., and White, J. L., "The Carbonaceous Mesophase
Formed in the Pyrolysis of Graphitizable Organic Materials,"
Metallography 3, 337-369, 1970). The amounts of each of these
materials may also be visually estimated in this manner. .sup.1 The
per cent of quinoline insolubles (Q.I.) of a given pitch is
determined by quinoline extraction at 75.degree.C. The percent of
pyridine insolubles (P.I.) is determined by Soxhlet extraction in
boiling pyridine. (115.degree.C.,). .sup.2 The insoluble content of
the untreated pitch is generally less than 1 percent (except for
certain coal tar pitches) and consists largely of coke and carbon
black found in the original pitch.
Aromatic base carbonaceous pitches having a carbon content of from
about 92 percent by weight to about 96 percent by weight and a
hydrogen content of from about 4 percent by weight to about 8
percent by weight are generally suitable for producing mesophase
pitches which can be employed to produce fibers capable of being
heat treated to produce fibers having the three-dimensional order
characteristic of polycrystalline graphite. Elements other than
carbon and hydrogen, such as oxygen, sulfur and nitrogen, are
undesirable and should not be present in excess of about 4 percent
by weight. The presence of more than such amount of extraneous
elements may disrupt the formation of carbon crystallites during
subsequent heat treatment and prevent the development of a
graphitic-like structure within the fibers produced from these
materials. In addition, the presence of extraneous elements reduces
the carbon content of the pitch and hence the ultimate yield of
carbon fiber. When such extraneous elements are present in amounts
of from about 0.5 percent by weight to about 4 percent by weight,
the pitches generally have a carbon content of from about 92-95
percent 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 which are employed to produce the
fibers of the instant invention. Petroleum pitch, of course, is the
residuum carbonaceous material obtained from the distillation of
crude oils or the catalytic cracking of petroleum distillates. Coal
tar pitch is similarly obtained by the distillation of coal. Both
of these materials are commercially 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 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. Pat. 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 precursor materials.
Likewise, pitches having a high non-mesophase insoluble content in
organic solvents such as quinoline or pyridine, or those which
develop a high 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 regions
of coalesced mesophase which are necesary to produce highly
oriented carbonaceous fibers capable of developing the
three-dimensional order characteristic of polycrystalline graphite.
For this reason, pitches having a quinoline-insoluble or
pyridine-insoluble content of more than about 2 percent by weight
(determined as described above) should not be employed, or should
be filtered to remove this material before being heated to produce
mesophase. Preferably, such pitches are filtered when they contain
more than about 1 percent by weight of such insoluble material.
Most petroleum pitches and synthetic pitches have a low insoluble
content and can be used directly without such filtration. Most coal
tar pitches, on the other hand, have a high insoluble content and
require filtration before they can be employed.
As the pitch is heated at a temperature between 350.degree.C. and
500.degree.C. 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 mesophase content of from about 40 percent by weight
to about 90 percent by weight, the resulting pitch will contain a
carbon content of from about 94-96 percent by weight and a hydrogen
content of from about 4-6 percent by weight. When such pitches
contain elements other than carbon and hydrogen in amounts of from
about 0.5 percent by weight to about 4 percent by weight, the
mesophase pitch will generally have a carbon content of from about
92-95 percent by weight, the balance being hydrogen.
After the desired mesophase pitch has been prepared, it is spun
into fibers by conventional techniques, e.g., by melt spinning,
centrifugal spinning, blow spinning, or in any other known manner.
As noted above, in order to obtain highly oriented carbonaceous
fibers capable of developing the three-dimensional order
characteristic of polycrystalline graphite the pitch must contain
large homogeneous regions of coalesced mesophase and be
nonthixotropic under the conditions employed in the spinning.
The temperature at which the pitch is spun depends, of course, upon
the temperature at which the pitch exhibits a suitable viscosity.
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
rise to a point which raises the softening point of the pitch to
excessive levels. For this reason, pitches having a mesophase
content of more than about 70 percent are usually not employed.
Pitches containing a mesophase content of about 40 percent by
weight usually have a viscosity of about 200 poises at about
250.degree.C. and about 10 poises at about 300.degree.C., while
pitches containing a mesophase content of about 70 percent by
weight exhibit similar viscosities at about 390.degree.C. and
440.degree.C., respectively. Within this viscosity range, fibers
may be conveniently spun from such pitches at a rate of from about
20 feet per minute to about 100 feet per minute and even up to
about 3000 feet per minute. Preferably, the pitch employed has a
mesophase content of from about 50 percent by weight to about 65
percent by weight and exhibits a viscosity of from about 30 poises
to about 60 poises at temperatures of from about 340.degree.C. to
about 380.degree.C. At such viscosity and temperature, uniform
fibers having diameters of from about 10 microns to about 20
microns can be easily spun. As previously mentioned, however, in
order to obtain the desired fibers, it is important that the pitch
be nonthixotropic and exhibit Newtonian or plastic flow behavior so
that the viscosity coefficient is independent of the shear rate of
the pitch 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 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.degree.C., e.g., from about 2500.degree.C. to about
3000.degree.C.) to a structure having the three-dimensional order
characteristic of polycrystalline graphite.
The fibers produced in this manner, of course, have the same
chemical composition as the pitch from which they were drawn, and
like such pitch contain from about 40 percent by weight to about 90
percent by weight mesophase. When examined under magnification by
polarized light and scanning electron microscopy techniques, large
fibrillar-shaped domains of mesophase interspersed with large
elongated non-mesophase regions can be seen distributed throughout
the fiber, giving the fibers the appearance of a "mini-composite".
These fibrillar mesophase domains are highly oriented and
preferentially aligned parallel to the fiber axis.
Characteristically, these domains have diameters in excess of 5,000
A, generally from about 10,000 A to about 40,000 A, and because of
their large size are easily observed when examined by conventional
polarized light microscopy techniques at a magnification of 1000.
(The maximum resolving power of a standard polarized light
microscope having a magnification factor of 1000 is only a few
tenths of a micron [1 micron = 10,000 A] and anisotropic domains
having dimensions of 1000 A or less cannot be detected by this
technique.)
After the fibers have been spun, as hereinbefore described, they
are heated under reduced pressure so as to volatilize at least a
portion of the non-mesophase portion of the fiber. As previously
stated, the invention takes advantage of the differences in
molecular weight and volatility between the molecules of the
mesophase portion of the fiber and the molecules of the
non-mesophase portion to effect removal of the non-mesophase
portion and produce a fibrous residue of high mesophase content. As
has been noted, the non-mesophase molecules are of lower molecular
weight than the higher molecular weight mesophase molecules and are
preferentially volatilized from the fiber during the heat treatment
to produce a fiber of higher mesophase content. This non-mesophase
material can be substantially completely removed by the
distillation or only partially, depending upon the relative amounts
of mesophase and non-mesophase present in the fibers, the diameter
of the fibers, the heat treatment temperature, the pressure
employed during the heat treatment, and the duration of the heat
treatment. The extent of which non-mesophase has been volatilized
can readily be determined by the loss in weight which the fibers
undergo during the heat treatment.
Removal of the non-mesophase portion of the fibers is effected by
heating the fibers under a pressure of less than about 100 microns
Hg, preferably less than 30 microns Hg, at a temperature and for a
time sufficient to volatilize as much of the non-mesophase material
from the fibers as desired. The temperature employed must be
sufficiently high to effect the desired degree of volatilization
but must not, of course, exceed the temperature at which the fibers
will soften or distort, or the temperature at which sintering of
fibers in contact with each other occurs. Higher temperatures
permit more complete volatilization of the non-mesophase material
in a given time than do lower temperatures. By employing
sufficiently high temperatures for an appropriate time, it is
possible to substantially completely remove the entire
non-mesophase content of the fibers.
A minimum temperature of at least 250.degree.C. is generally
necessary to volatilize non-mesophase material from the fibers.
Temperatures in excess of 400.degree.C. may cause melting of the
fibers and should be avoided. Preferably, temperatures of from
about 300.degree.C. to about 390.degree.C. are employed.
The time required to effect removal of the non-mesophase portion of
the fibers will, of course, vary with such factors as the relative
amounts of mesophase and non-mesophase material present in the
fibers, the diameter of the fibers, the heat treatment temperature,
and the pressure employed during the heat treatment. Relatively
thick fibers and/or fibers having a relatively high non-mesophase
content require longer heating times to effect the removal than do
thinner fibers or fibers having a lower non-mesophase content.
Likewise, the use of higher temperatures and/or lower pressures
permit a given amount of non-mesophase material to be removed in
shorter periods of time than is possible at lower temperatures
and/or higher pressures. Removal of at least 5 percent by weight of
the non-mesophase content of the fibers can usually be effected by
heating at an appropriate temperature within from about 5 minutes
to about 30 minutes. Removal of from about 10 percent by weight to
about 40 percent by weight of the non-mesophase content usually
requires more protracted heating times, e.g., from about 0.5 hour
to about 100 hours or more.
The fibers produced in this manner, like their precursors, are
characterized by the presence of large oriented graphitizable
domains preferentially aligned parallel to the fiber axis, with the
fibers after vacuum distillation, however, containing a lesser
amount of non-mesophase material than before such distillation. By
further heat treatment, these fibers can be converted into carbon
fibers having a high Young's modulus of elasticity and high tensile
strength.
While heat-treated fibers containing in excess of about 85 percent
by weight mesophase are, at times, sufficiently infusible to permit
them to be carbonized without any prior thermosetting treatment,
fibers containing less than about 85 percent by weight mesophase
require some thermosetting before they can be carbonized.
(Evidently, the fibers containing more than 85 percent by weight
mesophase are sufficiently reinforced by their fibrillar structure
to allow them to be carbonized directly without any prior
thermosetting treatment.) In any event, because of the higher ratio
of mesophase to non-mesophase of the heat treated fibers compared
to their precursors, they can be thermoset, at any given
temperature, in shorter periods of time than said precursors.
Thermosetting of the fibers is readily effected by heating the
fibers in an oxygen-containing atmosphere for a time sufficient to
render them totally infusible. The oxygen-containing atmosphere
employed may be pure oxygen or an oxygen-rich atmosphere. Most
conveniently, air is employed as the oxidizing atmosphere.
The time required to effect thermosetting of the fibers will, of
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 content of the fibers. Generally, however, thermosetting
of the fibers can be effected in relatively short periods of time,
usually in from about 4 minutes to about 50 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 temperature which can be
employed will thus depend upon the particular pitch from which the
fibers were spun, and the mesophase content of the fibers. The
higher the mesophase content of the fiber, the higher will be its
softening temperature, and the higher the temperature which can be
employed to effect thermosetting. At higher temperatures, of
course, fibers of a given diameter can be thermoset in less time
than is possible at lower temperatures. Fibers having a lower
mesophase content, on the other hand, require relatively longer
heat treatment at somewhat lower temperatures to render them
infusible.
A minimum temperature of at least 250.degree.C. is generally
necessary to effectively thermoset the heat-treated fibers produced
in accordance with the invention. Temperatures in excess of
400.degree.C. may cause melting and/or excessive burn-off of the
fibers and should be avoided. Preferably, temperatures of from
about 325.degree.C. to about 390.degree.C. are employed. At such
temperatures, thermosetting can generally be effected within from
about 4 minutes to about 50 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 50 minutes, or at temperatures in excess of 400.degree.C.
After the fibers have been thermoset, the infusible fibers are
carbonized by heating in an inert atmosphere, such as that
described above, to a temperature sufficiently elevated to remove
hydrogen and other volatiles and produce a substantially all-carbon
fiber. Fibers having a carbon content greater than about 98 percent
by weight can generally be produced by heating to a temperature in
excess of about 1000.degree.C., and at temperatures in excess of
about 1500.degree.C., the fibers are completely carbonized.
Usually, carbonization is effected at a temperature of from about
1000.degree.C. to about 2000.degree.C., preferably from about
1500.degree.C. to about 1900.degree.C. Generally, residence times
of from about 0.5 minute to about 25 minutes, preferably 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
advantage 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
from about 700.degree.C. to about 900.degree.C. before they are
heated to their final carbonization temperature. Residence times at
these temperatures of from about 30 seconds to about 5 minutes are
usually sufficient. Preferably, the fibers are heated at a
temperature of about 700.degree.C. for about one-half minute and
then at a temperature of about 900.degree.C. for like time. In any
event, the heating rate must be controlled so that the
volatilization does not proceed at an excessive rate.
In a preferred method of heat treatment, continuous filaments of
the fibers are passed through a series of heating zones which are
held at successively higher temperatures. 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. 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 temperatures in the direction of
travel of the fibers, can be used.
The carbon fibers produced in this manner 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 of
polycrystalline graphite and graphite-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
temperature in a range of from about 2500.degree.C. to about
3300.degree.C., preferably from about 2800.degree.C. to about
3000.degree.C., to produce fibers having not only a high degree of
preferred orientation of their carbon crystallites parallel to the
fiber axis, but also by a structure characteristic of
polycrystalline graphite. A residence time of about 1 minute is
satisfactory, although both shorter and longer times may be
employed, e.g., from about 10 seconds to about 5 minutes, or
longer. 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
2500.degree.C., preferably above about 2800.degree.C., are
characterized as having the three-dimensional order of
polycrystalline graphite. This three-dimensional order is
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 (00l) 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 indicate this preferred orientation to be no more than about
10.degree., usually from about 5.degree. to to about 10.degree.
(expressed as the full width as half maximum of the azimuthal
intensity distribution). Apparent layer size (L.sub.a) and apparent
stack height (L.sub.c) of the crystallites are in excess of 1000 A
and are thus too large to be measured by X-ray techniques. The
interlayer spacing (d) of the crystallites, calculated from the
distance between the corresponding (00l) diffraction arcs, is no
more than 3.37 A, usually from 3.36 A to 3.37 A.
EXAMPLE 1
A commercial petroleum pitch was employed to produce a pitch having
a mesophase content of about 59 percent by weight. The precursor
pitch had a density of 1.24 grams/cc., a softening temperature of
120.degree.C. and contained about 1 percent by weight pyridine
insolubles (P. I. was determined by Soxhlet extraction in boiling
pyridine.). Chemical analysis showed a carbon content of about 93%,
a hydrogen content of about 6%, a sulfur content of about 1% and
0.15% ash.
The mesophase pitch was produced by heating the precursor petroleum
pitch at a temperature of about 400.degree.C. for about 20 hours
under a nitrogen atmosphere. After heating, the pitch contained
59.8 percent by weight pyridine insolubles, indicating that the
pitch had a mesophase content of close to 59 percent.
A portion of this pitch was transferred to an extrusion cylinder
and spun into fiber by applying pressure to the pitch with an argur
while the molten pitch was extruded through a pin-hole orifice
(diameter 0.015 inch) at the bottom of the extruder at a rate of
between 200 to 400 feet/minute. The filament passed through a
nitrogen atmosphere as it left the extruder orifice and was then
taken up by a reel. A considerable quantity of fiber 35 microns in
diameter was produced in this manner at a temperature of
380.degree.C.
A portion of the as-drawn fiber was heated at a temperature of
315.degree.C. for 64 hours under a pressure of 20 microns Hg. The
fiber showed a loss in weight of 11.5 percent as a result of the
heat treatment.
Surprisingly, the fiber contained 90 percent by weight pyridine
insolubles after heat treatment, indicating that the fiber had a
mesophase content of about 90 percent. Based on the mesophase
content of the as-drawn fiber and the loss in weight during the
heat treatment, the mesophase content of the fiber should have been
only 68 percent. This indicated that a portion of the non-mesophase
present in the as-drawn fiber had been converted to mesophase
during the heat treatment. Polarized light microscopy studies of
the fiber also indicated a substantial increase in mesophase
content as a result of the heat treatment.
The heat treated fiber essentially fully retained the integrity of
the as-drawn fiber and showed no serious disruptions in the fiber
surface.
Another portion of the as-drawn fiber was heated at a temperature
of 360.degree.C. for a total of 2.5 hours under a pressure of 20
microns Hg. After each half hour period, the fiber was removed from
the oven, cooled, and weighed. The weight loss of the fiber during
each half hour period is indicated below: Time, minutes Weight
Loss, % ______________________________________ 30 minutes 11.2 60
minutes 13.4 90 minutes 14.3 120 minutes 14.7 150 minutes 14.8
______________________________________
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