Note: Descriptions are shown in the official language in which they were submitted.
N36-25 CANADA
~04672~
ACTIVATED PITCH BASED CARBON FIBERS
Background of the Invention
Because of the widespread availability, cheapness
and attractive chemical properties of various pyrogenous
residues such as pitch, they have found wide usage as
starting materials for the fabrication of binders and
coating materials. Pitches have been used as part of the
component mixtures in the fabrication of carbonaceous
articles of various kinds and in the formulation of carbon-
aceous fibers. One of the uses for carbonaceous fibersstems from the treatment of the fiber by appropriate means
to increase their surface area and absorptive properties.
Activated fibers of this type are useful in applications
involving the purification of fluids. At the present time,
fibers of this nature are derived by the carbonization and
activation of relatively expensive organic resins. While
pitch is inexpensive, it can be formed into stable fibers
only by oxidative processes which are difficult and time
consuming. No process is presently known for activating
such fibers to make them absorbent. It is an object of
the present invention therefore, to describe pitch fibers,
modified by small amounts of added resins, which can be
successfully made into highly absorbent activated carbon
fibers and to further describe a process for making the
fibers.
The invention pertains to a flexible absorbent carbon
fiber in which the fiber is made from a mixture comprising a
pitch and a phenol-formaldehyde novolac, the fiber having a
surface area in the range of about 300 to about 2500 square
meters per gram. The pitch-novolac mixture may comprise
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from about 5 to about 40 percent novolac, with the carbon
fiber having a diameter ranging from about 0.1 to about
300 microns. The fiber is made by fiberizing a molten
mixture of pitch and novolac, curing the fiber to make it
infusible, heating the fiber in air to a temperature ranging
from about 250C to about 450C, and subsequently heating
the fiber to a temperature ranging from about 700C to
about 900C under an inert atmosphere to form the adsorbent
carbon fiber. The surface area of the fiber may be further
increased by heating in steam at a temperature ranging from
about 800C to about 900C.
The starting pyrogenous residues that can be used in
the process of the present invention include a variety of
pitches such as coal tar pitches, pitches obtained by
distillation of oils, petroleum pitches, pyrogenous asphalts,
and a variety of pitch-like substances produced as by-products
of various industrial processes, such as distillation residues.
Preferably, the starting pyrogenous residue has a softening
point of from about 80C to about 200C, more preferably
from about 100C to about 150C. Preferably, the pyrogenous
residue has a carbon to hydrogen ratio based on weight per-
cent from about 18 to about 25. The content of aromatic and
unsaturated components varies, depending upon the source of
the raw material pyrogenous residue.
Preferably, pyrogenous residues or pitches used as
starting materials have a beta-resin content greater than
about 5 percent and preferably greater than lO percent.
The beta-resin is the benzene insoluble content of the pyro-
genous residue minus the quinoline insoluble content. In
making the determination, there are other solvents, such as
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toluene, which can be substituted for benzene, and pyridine
which can be substituted for quinoline. The beta-resin
; p~rtion of the pyrogenous residue appears to enhance the
binding and adhesive qualitites thereof. It is believed
" that a suitable amount of beta~resin contributes to rendering
the fusible fiber infusible by a short curing process. The
,~ upper limit of the percent of beta-resin in the starting
pyrogenous residue is not critical but is generally limited
, by the type of pitch used and process conditions. Most
,~ 10 commercially available pitches have a beta-resin content of
less than about 30 percent, but pitches with,a beta-resin
content higher than 45 percent can be used in the present
invention.
Generally, commercially available coal tar pitch has
a benzene insoluble content of about 20 to about 50 percent
and a quinoline insoluble content of about 10 to about 20
percent, with a resulting beta-resin content in the range of
about 10 to 30 percent. These pitches are suited for use
as a starting material in the process of the present invention
without further modification.
Petroleum pitches and pyrogenous asphalts often have
beta-resin contents less than about 5 percent. This is
generally due to a low percentage of benzene insolubles,
generally less than about 10 percent. In such a case, while
the fusible fiber of pyrogenous residue and novolac can be
rendered infusible by reacting with formaldehyde, the curing
process is comparatively slow. Alternatively, it is possible
to upgrade the pitch by increasing the beta-resin content.
Such upgrading can be done by reacting the pitch or asphalt
with an aldehyde and phenolic compound in the presence of
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an acid catalyst at a temperature sufficiently high to effect
condensation between the pitch or asphalt, aldehyde and
phenolic compound. Such a method is described in British
specification No. 1,080,866 and u. S. Patent No. 3,301,803.
The amount of aldehyde and phenolic compound that is
employed can vary widely depending on the degree of upgrading
necessary. The reaction is carried out at a temperature
from about 150F to about 600F for a suitable period of
time.
The amount of quinoline insoluble in the starting
pyrogenous residue should be less than about 20 percent and
preferably less than about 10 percent. As the percentage
of quinoline insoluble in the starting pyrogenous residue
is decreased, the ease of fiberization of the melt is
increased and the uniformity of the fibers is enhanced. The
most preferred starting pyrogenous residue contains zero or
a very low, percentage of quinoline insoluble. The quino-
line insolubles represent material which is not soluble
in the pyrogenous residue at the spinning temperature and
which form an undesirable second phase. Removal of the
quinoline insolubles can be accomplished by diluting the
pitch in an appropriate solvent and filtering or centrifuging
to remove the insolubles. Such a method is described in
U. S. Patent No. 3,595,946.
A wide variety of novolac resins may be used as
starting materials in the process of the present invention.
The term "novolac" refers to a condensation product of a -
phenolic compound with formaldehyde, the condensation being
carried out in the presence of a catalyst to form a novolac
resin, wherein there are virtually no methylol groups, such
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as present in resoles, and wherein the molecules of the
phenolic compounds are linked together by a methylene
group. The phenolic compound may be phenol, or phenol
wherein one or more of the non-hydroxylic hydrogens are
replaced by any of various substituents attached to the
benzene ring, a few examples of which are the cresols,
phenyl phenols, 3, 5-dialkylphenols, chlorophenols,
resorcinol, hydroquinone, chloroglucinol and the like.
The phenolic compound may instead be naphthyl or hydroxy-
phenanthrene or another hydroxyl derivative of a compoundhaving a condensed ring system.
For purposes of the present invention, any fusible
novolac which is capable of further polymerization with a
suitable aldehyde may be employed for the production of
fibers. Stated another way, the novolac molecules must have
two or more available sites for further polymerization.
Apart from this limitation, any novolac might be employed,
including modified novolacs, i.e., those in which a non-
phenolic compound is also included in the molecule, such as
the diphenyl o~ide or bis phenol-A modified phenol-formalde-
hyde novolac. Mixtures of novolacs may be employed or
novolacs containing more than one species of phenolic com-
pounds may be employed.
Novolacs generally have a number-average molecular
weight in the range from about 500 to about 1200, although
an exceptional case in which the molecular weight may be as
low as 300 or as high as 2000 or more may occur. Unmodified
phenol-formaldehyde novolacs usually have a number-average
weight in the range from about 500 to about 900, most of
the commercially available materials falling within this
range.
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Preferably, novolacs with a molecular weight from about
500 to about 1200 are employed in the method of the present
invention. The temperature at which low molecular weight
novolacs soften and become tacky is usually comparatively
low. Therefore, it is necessary to cure the fiberized
novolac at a very low temperature to avoid adherence and/or
deformation of the fiber. It is usually undesirable to
employ such curing temperatures since the curing rate
increases dramatically with the increase in temperature, and
a low curing temperature entails the practical disadvantages
of a prolonged curing cycle. It is generally preferred to
employ a novolac having a moderately high molecular weight
to permit curing in a reasonable time without adherence and/
or deformation, but to avoid the extreme upper end of the
molecular weight range to minimize problems in fiberizing due
to gelling.
A mixture of pyrogenous residue, or pitch, and novolac
may be formed by any convenient technique such as dry blending
or melting the pyrogenous residue and novolac by heating
together to form a homogenous mixture. Mixtures containing
from about 5 to about 40 percent novolac can be used for
preparing the fibers of the present invention. Since the
pyrogenous residue is the most economically available
component of the mixture it is preferred to employ less than
about 35 percent novolac. It is preferable that the novolac
content be at least about 10 percent and more preferably
that it be at least about 25 percent in the mixture so that
the spinability of the fiber is enhanced and the curing time
can be sufficiently reduced. Preferably, the mixture consists
essentially of the pyrogenous residue and novolac.
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The fiberization can be performed by any convenient
method, such as drawing a continuous filament downwardly
from an orifice in the bottom of a vessel containing a
molten mixture of pitch and novolac. The filament is
wound and collected on a revolving take-up spool mounted
below the orifice. The take-up spool also serves to
attenuate the filament as it is drawn from the orifice before
it cools and solidifies upon contacting the atmosphere bet-
ween the orifice and the spool. The melt can also be formed
into short staple fibers by methods known in the prior art,
such as by blowing the melt through a fiberizing nozzle
and collecting the cooled fibers, or by blowing a thin stream
of melt into the path of a hot blast of gas. These methods
produce a staple consisting of a multiplicity of fusible
uncured pitch-novolac fibers of variable length and diameter.
The diameter of such fibers can vary from 0.1 micron to about
300 microns.
When producing a continuous filament having a uniform
diameter by melt spinning, the fibers preferably have
diameters from about 10 to about 30 microns. The filament
diameter depends primarily upon two factors; the drawing
rate and the flow rate of the melt through the orifice.
The fiber diameter decreases as the drawing rate is increased,
and increases as the flow rate of the melt is increased.
The flow rate of the melt depends primarily upon the diameter
and length of the orifice, and the viscosity of the melt,
increasing as the orifice diameter is increased, decreasing
as the length of the orifice is increased, and increasing as
the viscosity of the melt is decreased. An increase of flow
rate may also be effected, if desired, by applying pressure
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to the melt to force it through the orifice. Alternatively,
fibers of the modified pitch material can be produced by
pouring a mixture of pitch and novolac resin in a vessel
which is connected to a nozzle. The nozzle in turn is
connected to a source of air pressure for forcing the
mixture through the nozzle. In this manner short staple or
blown fibers of appropriate diameter can be collected. The
fibers produced either by drawing or by blowing are fusible
and may be treated further to cause the fibers to cure or
crosslink at least to the point of infusibility.
The curing is generally effected by heating the fibers
in the presence of a source of methylene groups, such as
formaldehyde, and preferably also in the presence of a
suitable catalyst, such as an acid. Blowing produces a
staple comprising fibers of varying lengths and diameters,
diameters as small as about 0.1 micron or less being attain-
able, as well as considerably thicker fibers. Melt spinning
may be employed to produce fibers in the form continuous
filaments if desired, having diameters as small as about 4
microns, up to as large as about 300 microns or more. After
curing, the fibers may be processed by various techniques
to produce rovings and yarns, paper felt, woven or knitted
fabrics, and various other textile forms. A particularly
desirable method for the preparation of infusible cured
phenol-formaldehyde novolac fibers is set forth in detail
in U. S. Patent 3,650,102, issued to James Economy et al
which is assigned to The Carborundum Company. In accordance
with the present invention, the infusible modified pitch
fiber is carbonized by heating it in air from room temperature
(about 25C) up to an intermediate temperature in the range
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fr~m about 250c to about 450c, the temperature being
continually increased at a rate which may vary from about
50C per hour to about 200C per hour. Heating is then
continued in a non-oxidizing atmosphere such as nltrogen or
a similar inert gas, from said intermediate temperature to
a final ~emperature in the range from about 700~C to about
900C, the temperature being continually increased at a
rate of from about 50C per hour to about 200C per hour.
While the precise nature of the conversion effected thereby
has not been ascertained, it appears that the heating in air
results in enhanced crosslinking, partial pyrolysis and
carbonization of the starting fiber to produce a partially
crosslinked and carbonized fiber which is further carbonized
during the heating in the non-oxidizing atmosphere.
The method results in the production of a carbon fiber
wherein the carbon generally has a surface area of at least
300 m2/gm to about 800 m2/gm. It has further been found
that, if the starting cured modified pitch fiber is swelled
by immersing it in a highly polar sovent before carbonizing
it by the method just described, the resulting carbon fiber
generally has a somewhat greater surface area of at least
about 400 m2/gm and usually within the range from about
400 m2/gm to about 1000 m2/gm. A particularly desirable
feature of the method described is that carbon fibers may
be produced thereby which, in addition to having a carbon
surface area in the range from about 300 m2/gm to about
1000 m2/gm, are relatively strong and very flexible.
Another interesting feature of this invention is that if the
modified pitch fiber, after curing in the presence of
formaldehyde, is heated in air at a heating rate of 10C
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to 50C/hr. up to 250C, before heating rapidly to higher
temperatures, at rates of 50-200C/hr., the resulting
fibers, in addition to being absorbent and active are also
found to possess improved flexibility. Another particularly
important feature of the method described is that the carbon
surface area of the carbon fiber produced thereby may
subsequently be increased, if desired, from an initial sur-
face area of about 300-1000 m2/gm to as much as about
2500 m /gm by heating the fiber in steam at a temperature
in the range from about 800C to about 900C. This steam
treatment is in addition to, and subsequent to, the second
heating stage in nitrogen or similar inert gas. This is
quite important, since the adsorptive capacity of the carbon,
and the flexibility of the fiber, generally increase with
increasing surface area.
It should be noted that infusible fibers with pitch
contents approaching 95 percent may be successfully made by
the process of the invention and heat treated to give fibers
of greatly increased surface area. This behavior is entirely
unexpected in view of the behavior shown by fibers consisting
wholly of a pyrogenous residue or pitch. These latter fibers
are thermoplastic and pass through a fluid state when heated
to carbonization temperatures of 800C. When such fibers are
subjected to an activation treatment such as that described
for the activation of modified pitch fibers, products with
surface areas of less than 1-5 m2/gm are obtained. Con-
sidering this behavior, the marked change brought about by
the addition of as lit~le as 5 percent of resin to the
modified fiber is completely unexpected. While the exact
reasons are not completely ascertained, it is believed that
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crosslinking of the modified pitch material during the
treatment of curing and activation may be one of the factors
in producing the superior fibers of the invention.
The invention will be further described partly with
reference to the following examples, which are intended to
illustrate and not to limit the scope of the invention.
Example 1
A starting coal tar pitch (Allied Chemicals Company)
had a softening point of 125C, a beta-resin content of 22
percent, and a quinoline insoluble content of 13.6 percent.
The pitch was mixed with a novolac resin (Varcum) having a
molecular weight of about 800 to 1000, in the proportion of
70 percent of pitch to 30 percent of resin in the final
mixture. The novolac and pitch were heated together to
190C to form a homogeneous mixture and the resulting mixture
was poured into a fiberization vessel. The vessel was a
cylinder having an orifice at the bottom and a plunger at
the top for forcing liquid through the orifice. The vessel
was mounted onto the fiberization equipment, which included
a spool attached to the shaft of a variable speed electric
motor mounted beneath the vessel for gathering the fibers.
The vessel was surrounded by an electrical heating coil
connected to an adjustable source of electricity, whereby a
controlled amount of heat was imparted to the vessel and its
contents. The fibers were spun through an orifice of about
1.5 mm in length, having an internal diameter of about
0.3 mm The vessel containing the mixture of resin was
maintained at a temperature of about 120C while the bottom
portion with the orifice was maintained at about 150C.
The mixture of pitch and novolac was driven through the
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orifice by a ram at a pressure of about 110 p.s.i. The
resulting mixed pitch-novolac filament had an average dia-
meter of about 15 to 25 microns, and was taken up on a
graphite cylindrical cone at the rate of 500 r.p.m. The
~iber bundle thus obtained was cured by hanging the graphite
cone containing the fiber on a graphite support and immersing
in a curing solution. The solution was prepared by mixing
equal portions of an aqueous solution containing about an 18
percent concentration of hydrochloric acid and the same
concentration of paraformaldehyde. The curing solution with
the graphite cone containing the fiber immersed therein was
heated from room temperature to 100C by increasing the
temperature from 25C to 50C over a period of 1 hour,
increasing the temperature from 50C to 100C over a period
of 1/2 hour and maintaining the temperature at 100C for 4
hours for a total residence time of about 5-1/2 to 6 hours.
The cured fibers were removed, washed with water, and dried
in air at about 60C. The cured fibers were infusible and
did not soften when heated in an oven or in a flame.
Example 2
A mixture of pitch and novolac resin prepared according
to the procedure of Example 1 was poured into a fiberization
vessel equipped with a nozzle. The nozzle was connected to
a source of air pressure for forcing the mixture of pitch-
novolac in air through the nozzle. In this manner, short
staple fibers or blown fibers were collected on a plate
after being cooled by falling through the air. The nozzle
was heated at about 250C and the air pressure used was
about 20 p.s.i. The fibers collected in this manner were
cured by placing in a graphite container to form a mat and
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curing in a liquid state in a manner similar to Example 1.
The average diameter of the cured fiber was about 2 microns.
These fibers, as in Example 1, were infusible.
Example 3
A tow of infusible modified pitch filaments comprising
coal tar pitch and phenol-formaldehyde novolac was prepared
in substantial accordance with the procedure of Example 1,
the filaments having diameters of about 12-18 mi~rons. The
tow was cut into six-inch (a~out 15 cm) segments, and 5 gm.
of the cut fibers were placed in a tube furnace. The fibers
were heated from room temperature to an intermediate tempera-
ture of 400C at a rate of temperature rise of 200C per
hour while passing a slow current of air through the tube
to remove the volatiles dissipated by the fibers and to
maintain an air atmosphere. Upon reaching 400C, the air
current was replaced by a slow current of nitrogen and heating
was continued in the nitrogen atmosphere at a rate of tempera-
ture rise of 200C per hour up to a final temperature of 900C,
after which the resulting carbon fibers were allowed to cool
to room temperature, nitrogen being employed to provide a
non-oxidizing atmosphere during the cooling cycle to preclude
oxidation of the carbon. A yield of 3 gm of flexible carbon
fibers was obtained, the carbon having an average surface
area of 720 m2/gm.
Example 4
Example 3 was repeated, but the fibers were slowly
heated in air to an intermediate temperature of 200-250C,
at a rate of 10-50C/hr., and then more rapidly at 100C/hr.
to 450C with the heating continued in nitrogen to a final
temperature of 700C at a rate of 100C/hr. The resulting
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fibers were found to be very flexible. While the exact
mechanism has not been ascertained, it is believed that the
slow heating of the formaldehyde cured fibers to 250C
renders the fibers more completely cross-linked, which is
believed to improve mechanical properties. These fibers
had an average surface area of 705 m2/gm with individual
fibers having carbon surface areas up to about 1000 m2/gm.
Example 5
The Gurface area of the carbon fibers produced according
to the invention as illustrated in Examples 3-4 may be
increased, if desired, by heating the carbon fibers in
steam at a temperature in the ranges from about 800C to
about 900C. It is thereby possible to increase the surface
area from about 300-1000 m2/gm to as high as about 2500
m /gm. During the heating, the carbon fibers gradually
increase in porosity and surface area and lose weight as
carbon is burned off, and will be completely dissipated if
heated too long. Therefore, while the heating should be
carried out for a time sufficient to effect an increase in
surface area, it should not be unduly prolonged, and the
preferred time is generally that which gives the desired
surface area with minimum weight loss. The higher the
temperature within the range specified, the shorter the time
required for a given increase in surface area. Thus, for
example, at 800C approximately 90 minutes was required to
increase the surface area to 2000 m2/gm whereas only
about 20 minutes was required at 900C, the latter tempera-
ture being preferred. Slightly longer periods are required
to attain surface areas of about 2500 m2/gm.
Example 6
The carbon fibers produced in Example 4 were placed in
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a tube furnace at 800C under a slow current of steam and
held under these conditions for two hours, after which the
fibers were cooled to room temperature in a non-oxidizing
(nitrogen) atmosphere. The resulting carbon fibers had an
average carbon surface area of 2400 m2/gm and exhibited
good flexibility and mechanical properties.
The method of the invention may be carried out with
the fibers in virtually any desired form including, for
example tows, rovings and yarns, staple and batting, felt,
paper, woven or knitted fabrics and the like, the form of
choice depending primarily upon the intended use for the
carbon fibers. Similarily, the fiber may be of any desired
diameter, which is selected primarily with refexence to
the intended use for the resulting carbon fibers. While
the surface area does not appear to be dependent upon
diameter, the tensile strength of the carbon fibers tends
to increase with increasing diameter and their flexibility
tends to increase with decreasing diameter. It will be
apparent that while the examples illustrate the invention
as carried out in a batchwise fashion, suitable apparatus
for carrying out the steps in a continuous manner may be
readily devised.
The carbon in fibers produced according to the invention
is amorphous (glassy) and is of the type known as hard
carbon, i.e., highly cross-linked carbon which is very
difficult to graphitize. The high surface area carbon
fibers, in various forms, have numerous application. For
example, they are particularly useful as an adsorbent in gas
masks and in adsorbent protective clothing and filter media.
Percentages referred to herein are by weight unless
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otherwise expressly stated or clearly indicated by the con-
text. Surface areas as set forth herein are surface areas
as determined with a Model 2200 Automatic Surface Area
Analyzer (Micromeritics Instrumental Corp. Norcross, Georgia)
in accordance with the BET method and eq1~ation of srunauer,
Emmett and Teller (see J. Amer, Chem. Soc. 60; 309-316
(1938)), which involves a determination of the quantity of
a gas such as nitrogen which is required to form a
monomolecular layer adsorbed on the surface of the sample.
While the invention has been described herein with
reference to certain examples and preferred embodiments, it
is to be understood that various changes and modifications
may be made by those skilled in the art without departing
from the concept of the invention, the scope of which is
to be determined by reference to the following claims.
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