Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a PROCESS FOR THE
MANUFACTURE OF CARBON FIBERS AND FEEDSTOCK THEREFOR.
15 Background of the Invention
Carbon and graphite fibers and composites ma,de therefrom
are finding increasing uses in such diverse applications as
lightweight aircraft and aerospace structures, automobi]e parts, and
20 sporting equipment. Due to their high strength per weight ratio
further added uses of these composites can be expected in the
future.
Typically in the manufacture of carbon or graphite fibers
25 a carbonaceous material is melted, spun into a thread or filament by
conventional spinning techniques and therea~ter the filament is
converted to a carbon or graphite fiber. Conventionally the spun
filament is stabilized , i . e ., rendered infusible , through a heat
treatment in an oxidizing atmosphere and thereafter heated to a
30 higher temperature in an inert atmosphere to convert it into a
carbon or graphite fiber.
The prior art discloses many different carbonaceous
materials (sometimes called fiber precursors) that may be utilized to
35 manufacture a carbon or graphite fiber. However, the two most
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signiffcant commercial processes employ mesophase pitch or
polyacrylonitrile. Through the use of such materials high strength
graphite fibers can be produced.
In order for carbon or graphite fibers to be more widely
~- accepted in commercial applications, improved, more economical
fibers must be developed.- Three significant manufacturing costs
are the preparation of the feedstocks from which the fibers are
produced, spinning of the fibers, and the cost of stabilizing the
fibers and subsequently converting them to the end product.
d,
In the manufacture of relatively expensive, structured
high performance graphite fibers from mesophase pitch one of the
most significant costs is the cost of producing the mesophase pitch.
Most procesjses ordinarily require heating ~f a conventional pitch
material at elevated temperatures over a period of several hours.
For example, in Lewis et al U.S. Patent 3,967,729, Singer U.S.
Patent No. 4,005,183, and Schulz U.S. Patent No. i,O14,725, the
preparation of the mesophase pitch requires that the initial
`~ 20 feedstock be heated to an elevated temperature for a number of
hours. Obviously such a process is time consuming and costly.
Also care must be taken in heating for a specific time, as
mesophase pitch can increase in viscosity rapidly, making it
unsuitable for spinning.
The manufacture of graphite or carbon fibers from
polyacrylonitrile also employes a relatively expensive feedstock in
the process. It is generally thought that the overall cost of
producing fibers from polyacrylonitrile is about equal to the cost of
, 30 producing carbon or graphite fibers from mesophase pitch. With
either process the final cost of the graphite ffbers is currently $15
to S50 per pound.
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Most of the commerical fibers produced from
polyacrylonitrile or mesophase pitch have been fibers which have
subsequently been converted to graphite fibers. Because the
temperature of graphitization if higher than the temperature
5 required to prepare a carbon fiber, ~raphite fibers are much more
costly to produce than carbon fibers. However, certain mechanical
properties of graphite fibers are generally superjor ta those of
carbon fibers.
In the past attempts have been made to manufacture
carbon fibers from ptich materials without first converting the pitch
to the mesophase state. For various reasons these attempts have
not been al~ogether successful and today there exists a need for a
commerically economical process for manufacturing lower cost carbon
15 ffbers havinlJ intermediate mechanical properties from nonmesophase
pitch materials, e.g. for asbestos replacement markets.
Various desirable and undesirable characteristics of the
fiber precursor have been disclosed in the prior art. For example,
20 Fuller et al U.S. Patent No. 3,959,448 discloses that shorter
stabilization times can be obtained if the softening point of coal tar
pitch is increased. However, an at~endant disadvantage has been
recognized, namely that spinning fibers from coal tar pitch having a
softening point of above 200C is very difficult. See for example,
25 Turner et al U.S. Patent 3,767,741. Likewise, it has been
recognized that handling carbon fibers made from pitch is relatively
dimcult. See for example, Kimura et al U.S. Patent No. 3,639,953.
Otani U.S. Patent 3,629,379 teaches the use of heat
30 treatment at elevated temperature combined with high vacuum
distillation, and heat treatment at elevated temperature combined
with admixture of reactive species (peroxides, metal halides, etc. )
to produce pitches suitable for melt or centrifugal spinning. The
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heat treatment step is about one hour, the distillation step is about
three hours, and all operations are batch as opposed to continuous
operation. Otani also teaches the desirability of reducing the
aliphatic chain components to limit outgassing during carboni2ation,
5 and. .the use of the above cited reactive species to reduce the
stabilization time required to prepare the pitch fibers for
carbonization.
Besides the softening point, other properties of the pitch I.
material are also important. ~or example, the presence of
10impurities and particulatës, molecular weight and molecular weight- range, and aromaticity. Also, the chemical composi~ion of the pitch
material is important, expecially insofar as the stabilization of the
fiber prior to carbonization is concerned. In fact, various
additives and other techniques are taught- in the prior art for
15 addition to the pitch material in order to provide a pitch fiber that~
can be . quickly and easily stabilized. See for example Carbon
Vol. 16 pp. 439-444 (Pergamon Press 1979), and Otani, . ¦ii
,. I
U.S. 3,629,379. -- ---
Ob~ects oi the Invention - ~-
In contrast to the preoccupation of much of the prior art
toward the production of mesophase pitch for use in producing ' I25graphite fibers, the present invention is directed primarily to the . 1 j
production of nonmesophasic aromatic enriched pitches that can be
quickly processed into carbon fibers at a much lower cost and
which have excellent intermediate properties permitting them to be -,
used in many applications where asbestos is currently being used.. i
An important object of this invention has been to provide ¦.
an economically feasible process for manufacturing carbon ffbers
from conventional petroleum derived aromatic enriched pitch
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materials without first having to produce expensive mesophase
pitch. Another important objective of this inventlon has been to
provide an improved high softenlng polnt, i.e., 249C (480F) or
above and preferably 266C (510P) or above petroleum derlved
5 aromatic enriched pitch material having a hlgh reactivity that can
be easily stabilized and that can be carbonized to form carbon
fibers suitable for use in high strength composites. Another
objective has been to provide an asbestos replacement type carbon
fiber. Another important objective has been to provide a process
10 wherein the pitch is converted to a higher~ softening point material
in a very short period of distillation time, preferably from about 1
second to 30 seconds, more preferably from about 5 seconds to 25
seconds and most preferably from about 5 seconds to 15 seconds so
that the formation of mesophase pitch is avoided.
Other important objectives of this invention have been to
provide a carbon fiber manufactured from aromatic enriched pitch
material wherein the fibers are of small diameter thus enabling them
to be quic~ly stabilized and where they have the durability to be
20 handled in the process. These and other objectives of the
invention will be apparent to those skilled in the art from the
following description and examples.
Summary of the Invention
One feature of the present invention is to prepare and
utilize in a carbon fiber process a high softening point,
nonmesophase, quickly stabilizable aromatic enriched pitch material
having a normal heptane insolubles content (ASTM D 3279-78) of
30 about 80-90% by weight and the properites set forth in Table I.
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TABLE I
~STM Te~t
Pro~erty _ Number V lue
Softening Point, C D-3104 ~t le~t 249
Xylene Insoluble~, X D-3671 15-40
10 Co~ing Value, X D-2416 65-90
Helium Den~ity, g/om3 * At about
1.25-1.32
15 Sulfur, X D-1552 ~0.1-4.0
*Detenmined by BecbIan Pycnometer g/cc at 25C.
Percentage ~umber6 are weight percents.
Another feature of the present invention is to prepare the
above aromatic enriched pitch material from a pitch material which
may be an aromatic base unoxidized carbonaceous pitch material
obtained from distillation of crude oils or most preferably the
25 pyrolysis of heavy aromatic slurry oil from. catalytic cracking of
petroleum distillates. It can be further characterized as an
aromatic enriched thermal petroleum pitch. The manufacture of
various pitches not necessarily equivalent to the pitches of this
invention, is known and is taught in Nash U.S. Patent 2,768,119
30 and Bell U.S. Patent 3,140,249. The properties of these more
conventional pitches are more fully defined in Table II.
Another important aspect of the present invention is the
method by which the above described petroleum pitch is converted
35 to the higher softening point aromatic enriched pitch of the present
invention by the removal or elimination of lower molecular weight
species. A number of conventional techniques as previously
described in Otani, can be employed such as conventional batch
vacuum distillation, as pointed out previously, we prefer to use
continuous equilibrium flash distillation. A better way of
converting the pitch to the higher softening point material is to use
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a very short residence time wiped film evaporator of the type shown
in Monty U.S. Patent 3,348,600 and Month U.S. Patent 3,349,828.
It i8 especially preferred that over 25 percent by weight,
5 preferably 25% to 50% by weight and most preferably 45 to 55% of
the material having a molecular weight of below about 550 is
removed or eliminated.
Another important aspect of the present invention has
10 been to use the melt blowing process disd~osed in Keller et al U.S.
Patent 3,755,527, Harting et al U. S . Patent 3,825,380 and Buntin
U.S. Patent 3,849,241 to process the high softening point pitch into
the form of a continuous mat of fibers. Continuous filament fibers
can also be produced using the die technology cited above.
.15 ~ ^
While this technique has been sucessfully applied to
polymeric material, such as polypropylene, we have been successful
in modifying the melt blowing process to permit the production of
high quality pitch fiber mats. A survey of the literature fails to
20 reveal the use of melt blowing die technology to produce pitch
fibers.
The present invention enables one to manufacture fibers
having a very small diameter, e.g. from about 6 to 30, more likely
from about 8 to 20 and most selectively from about 10 to 14
25 microns. Fibers with such diameters admit of certain special
applications that larger diameter fibers are not adapted for.
Although not wishing to be bound by any theory, it is
believed that the improved results of the invention are due to the
30 fact that the treatment time for increasing the softening point and
aromatic enrichment, is purposely kept very short. In keeping the
time short and not overly treating the pitch material, the alkyl
groups present in the pitch material are not destroyed or removed
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by thermal dealkylation during the preparation of the high softening
point pitch. The percent alpha hydrogens of total hydrogen ls
about 20 to about 40, more preferably about 25 to about 35 and
most preferably from about 28 to about 32. The percentage of beta
5 hydrogen atoms of the total hydrogen atoms is thus preferably from
about 2% to 15%, more preferably from about 4% to 12% and most
preferably from about 6% to 10%. The percentage of gamma
hydrogen atoms of the total hydrogen atoms is thus preferably from
about 1% to 10%, more preferably from about 3% to 9% and most
10 preferably from about 5% to 8%.
In Barr et al, "Chemical Changes During the Mild Air
Oxidation of Pitch", Volume 16, Carbon, pp. 439-444 (1978) the
authors note that the greater reactivity of petroleum pitches as
15 compared to~ coal tar pitches is attributed tô a higher concentration
of alkyl (methyl, ethyl) side chains in the petroleum pitches. By
utilizing a method whereby the softening point of the pitches of the
present invention are substantially increased by only brief exposure
to high temperatures, these desirable alkyl side chains are
20 preserved. Moreover, as noted below, the chemical composition of
the pitch, from the standpoint of speed of stabilization, is
enhanced. This preserves the reactivity of the pitch and greatly
reduces the time required to stabilized the fiber.
The basic process steps involved in the process include
the following:
1. Producing a petroleum pitch from a highly aromatic
slurry oil, and subjecting ~aid pitch to vacuum flash distillation or
wiped film evaporation, to prepare an enriched unique pitch having
a softening point of preferably at least 249C (480F), more
preferably about 265C (510F) or above, and most preferably
254C to 266C (490F to 511F) by treating an unmodified thermal
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petroleum pitch having a softening point, as measured by Mettler
softening point apparatus by ASTM Method D-3104, of about 77C to
: 122C, and preferably about 122C.
2. Converting the high softening point aromatic
enriched pitch of Step 1 into a roving or mat of pitch ffbers,
preferably throught the use of a melt blowing process as described
in the just identified patents.
3. Stabilizing in less than 200 minutes without addition
of reactive species to the pitch, more preferably in less than 100
minntes and most preferably in about 50-90 minutes, the pitch fiber
roving or mat product resulting from Step 2 in an oxidizing
atmosphere at a temperature of between about 180C (356F) to
310C (590.F), preferably in a continuous, multi-stage heat
treatment apparatus under an oxidizing conditions.
4. Further heating the resulting infusible roving or mat
product of Step 3 to a temperature of about 1000C (1832F) to
3000C (5500F), more preferably from about 900C to 1500C and
most preferably from about 1000C to 1200C in an inert atmosphere
in order to carbonize or graphitize roving, mat or continuous
filament product.
Description of the Preferred Embodiments
Starting Pitch Material
The starting petroleum pitch utilized in the process of the
30 invention is an aromatic base unoxidized carbonaceous pitch
produced from heavy slurry oil produced in catalytic cracking of
petroleum distillates. It can be further characterized as unoxidized
thermal petroleum pitch of highly aromatic content. These pitches
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remain rigid at temperatures closely approaching thelr melting
points. The preferred procedure for preparlnç~ the unoxldized
starting petroleum pitch uses, as a startlng material, a clarifled
slurry oil or cyde oil from which substantially all parafflns have
5 been removed in fluid catalytic cracking. Where the fluid catalytic
- cracking is not sufffciently severe to remove substantially all
paraffins from the slurry oil or cycle oil, they must be extracted
with furfural. In either case, the resultant starting material is a
highly aromatic oil boiling at about 315 to 540C. This oil is
10 thermally cracked at elevated temperatures and pressures for a time
sufficient to produce a thermally cracked petroleum pitch with a
softening point of about 38.7 to about 126.7C. The manufacture of
some other unoxidized petroleum pitche products, although not
necessarily considered suitable for use as is Ashland Petroleum
15 Pitch 240, ig described in Nash U.S. Patent No. 2,768,119 and Bell
et al U.S. Patent No. 3,140,249, Table II presents comparative
properties of four unoxidized commercially available petroleum
pitches (A, B, C, and D) suitable for use as a starting material for
use in this invention.
Pitch AlPha and Beta Hvdrogens:
As mentioned elsewhere in the present specification, the
preservation of alpha and beta hydrogens (i.e. alkyl side chains) is
25 a special feature of the present invention. The percentage of alpha
and beta hydrogen mentioned above will be preserved in the pitch
after all processing is complete to form the pitch fibers.
Alpha and beta hydrogen content can be determined
30 analytically by nuclear magnetic resonance (NMR) techniques. This
technique also determines the concentration of other hydrogen types
(aromatic, etc.).
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Pitch Softening Point:
The softening point for the present invention will be
determined by methods well known to the Industry, preferably
5 ASTM No. D-3104, modified to use stainless steel balls and cup and
high temperature furnance in view of the high softening points of
the present pitches. Softening point will preferably be in the
range of at least 249C, more preferably from about 265~C to about
274C, and most preferably from about 254C to about 266C.
Pitch Xylene Insolubles:
The xylene insolubles content of the materials of the
present invention should preferably be in the range of from about 0
15 to about 40j percent by weight, more preferably from about 0 to
about 35 percent by weight, and most preferably from about 0 to
about 32 percent by weight. Xylene insolubles will be determined
by techniques well known to the industry, including ASTM No.
D-3671.
Pitch Quinoline Insolubles
Quinoline insolubles of the pitches of the present
invention will preferably be from about 0 to about 5 percent by
25 weight, more preferably from about 0 to about 1 percent by weight,
and most preferably from about 0 to about 0.25 percent by weight.
As quinoline insolubles generally represents either catalyst or free
carbon or mesophase carbon, the lowest possible quinoline insolubles
content is preferred.
Pitch Sulfur Content:
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- The sulfur content of the pitches of the present invention
will be determined by the content of the feed materlals, but wlll
preferably be as low as possible. Sulfur contents of from about 0.1
to about 4 percent by weight, more preferably from about 0.1 to
5 about 3 percent by weight, and most preferably from about 0.1 to
about 1.5 percent by weight can be used with the invention. Both
environmental considerations and the disruption of fiber qualit~v
;~' caused by the gasification of the sulfur from the pitch dictate this
preference for low sulfur content. Sulfur content is readily
10 determined by ASTM No. D-1551 or other techniques well known to
the industry.
Pitch Coking Value:
15The coking value of the pitches of the present invention
will generally be determined by ASTM No. D-2416 and will
preferably be in the range of about 65 to about 90 weight percent,
more preferably from about 70 to about 85 weight percent, and most
preferably from about 75 to about 85 weight percent coke based on
20 the total weight of the pitch. Even higher coking values are, of
course, as the coking value represents to a large degree the
percent carbon which will remain in the final carbon fiber after
stabilization and all other processing has been completed.
., .
25 Pitch MesoDhase Content:
The mesophase content of the pitch of the present
invention will preferably be as low as possible, though amounts of
as much as 5% or even more may be tolerated in special instances.
30 Generally, for economic considerations, amounts of from about 0 to
about 5 percent by weight mesophase, more preferably from 0 to
; about 1 percent by weight mesophase, and most preferably from
;~about 0 to about 0.25 percent by weight mesophase will be useful
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with the invention. The percent mesophase content of the pitches
can be determined by quinoline insolubles, or by optical
microscropic techr~iques, utilizing crossed polarization filters and
measuring the area (then calculating as vo~ume and as weight) of
5 the mesophase present under microscopic examination under
polarized light.
TABLE II
Test Pitch Pitch Pitch Pitch
Test Method A B C D
Softening ASTM
Point, C D-2319 78.3 115.6 115.6 126.7
Density, Beckman
G/cc Hc Pyc 1.192 1.228 1.210 1.239
Mod. Con. i ASTM
20Carbon Wt.X D-2416 37.8 51.0 50.4 53.1
Flash, ASTM
C0C, C D-92 316 307.2 312.8 312.8
25 Sulfur, AST~
Wt.X D-1551 2.73 2.0 1.03 2.5
Xylene ASTM
Ins. Wt.X D-2317 0.7 5.0 2.2 5.8
Quinoline ASTM
Ins. Wt.% D-2318 0.11 Nil Nil Nil
BROOKFIELD VISCOSITY USING
NO. 2 SPINDLE
. .
Temperature, F Viscosity, cps
350 40 ~ 395 515 2000
325 60 - -
300 140
.
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The preferred unoxidized enriched petroleum pitch used in this
invention has a carbon content of from about 93% by weight to
about 95% by weight and'a hydrogen content of from about 5% by
weight to about 7% by weight, excluslve of other elements.
Elements other than carbon and hydrogen such as oxygen, su]fur,
and nitrogen are undesirable and should not be present in excess
of about 4% by weight preferably less than 4%. The pitch, due to
processing, may likely contain a low concentration of hard particles.
The presence or absence of particulate matter can be' determined
analytically and is also quite undesirable~ Preferably particulate
matter shou2d be less than 0.1%, more preferably 0.01%, and most
preferably less than 0.001%. For example, a sample of the pitch
under consideration can be dissolved in an aromatic solvent such as
benzene, xylene or quinoline and filtered. The presence of any
residue on~the filter medium which does not soften at elevated
temperatures up to 400C (as measured by a standard capillary
melting point apparatus) indicates the presence of a hard particle
material. In another test fDr suitability, the pitch under
consideration is forced through a specially sized orifice. Plugging
of the orifice indicates the presence of unacceptably large particles.
Ash 'content can also be used to establish hard particle
contamination . '
:
A pitch supplied under the designation A-240 by Ashland
' 25 Oil , inc ., is a commercially available unoxidized petroleum pitch
meeting the above requirements. It is described in more detail in
Smith et al, "Characterization and Reproducibility of Petroleum
Pitches" (U.S. Dep. Com. N.T.I.S. 1974; Y-1921).
It has the following characteristics: .
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TABLE III
m ICAL ANALYSIS FOR A COMMERCIAL PITCH tA-240
Typical
Test Method _ _ ly~ 8
Softening Point ASTM D-2319 120C
Density, g/cc,
25C Becbman Pycno~eter 1.230
Co~ing Value ASTM D-2416 52
Fla6h, COC, C ASTM D-92 312
Ash, wtX ASTM D-2415 ~ 0.16
BI, wtX ASTM D-2317 5
QI, wtX ASTM D-2318 Nil
Sulfur, Wt.7, ASTM D-1552 2.5X
Distillation, wtX AST~ D-2569
0-270C O
270-300C O
300-360C 2.45
Specific Heat Calculated
Calories/gD at
-5C 0.271
38C 0.299
93C 0.331
- 140C 0.365
35 Viscosity, CPS Broo~field
RPM Thermosel, Model
325F 1.5 LVT, Spi~dle #18 2734
350F 1.5 866
375F 1.5 362
400F 3.0 162
Preparation of Improved Pitch Material Having
Inc eased Softening Point and High Reactivity
In order to produce the high softening point aromatic
enriched preferred pitch material of the present invention, the
pitche of Table III hereof is treated so as to increase the softening
point of the pitch material to about 249C (480F) or above and to
provide the characteristics as set forth in Table I hereof.
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The pitch 60 produced ls nonmesophase pitch. By
nonmesophase ls meant less than about 5% by weight of mesophase
pitch. Such a pitch would generally be referred to in the art as
an isotropic pitch, e.g., a pitch exhiblting physlcal properties such
5 as light transmission with the same values when measured along
axes in all directions.
In a~n effort to produce such a pitch material various
methods have been tried. As a result it was discovered that a
10 preferred technique involved the use of a wiped film evaporator.
This technique reduces the time- of thermal exposure of the
product, thus providing a better fiber precursor. A suitable wiped
film evaporator is manufactured by Artisan Industires, Inc. of
Waltham, Massachusetts and sold under the trademark Rototherm.
15 . It is a s~raight sided, mechanically aidêd, thin-film processor
operating on the turbulent film principle. Feed, as for example,
pitch material, entering the unit is thrown by centrifugal- force
against the heated evaporator walls to form a turbulent film between
the wall and rotor blade tips. The turbulent flowing film covers
20 the entire wall regardless of the evaporation rate. The material is
exposed to high temperatures for only a few seconds. The
Rototherm wiped-film evaporator is generally shown in Monty U. S .
Patent 3,348,600 and Monty U. S. Patent 3,349,828,
As noted in the '600 patent, the various inlet
25 and outlet positions may be changed. In fact, in actual operation
of the Rototherm wiped-film evaporator it has been determined that
the feed inlet (No. 18 in the patent) can be the product outlet.
The following will serve as examples as to how produce the high
softening point pitch of the present invention.
A number of runs are made using an Artisan Rototherm
wiped film evaporator having one square foot of evaporating surface
with the blades of the rotor being spaced 1/16" away from the wall.
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The evaporator employed is a horizontal model with a countercurrent
flow pattern, i.e., the liquid and vapors traveled in opposing
directions. The condensers employed are external to the unit and
for the runs two units are employed along with a cold trap before
5 the mechanical vacuum pump. The unit employed is heavlly
insulated with fibergalss insulation in order to obtain and maintain
the temperatures that are required. A schematic of the system
employed is shown in Figure 1 hereof.
Briefly described, A-240 pitch ma4terial is melted in a melt
tank 1. Prior thereto it is filtered to remove contaminants
including catalyst fines. It is pumped by 2enith pump 3 through
line 2 and through back pressure valve 4 into the wiped-fi1m
evaporator 5. The wiped-film evaporator 5 is heated by hot oil
15 contained ip reservoi~ 6 which is pumped into the thin-film
evaporator through line 7. As the pitch material is treated in the
thin-fflm evaporator 5 vapors escape the evaporator through line 8
and are condensed in a first condenser 9 and a second condenser
11 connected by line 10. The vapors then pass through conduit 12
20 into cold trap 13 and out through line 14. Vacuum is applied to
the system from vacuum pump 15. An auxiliary vacuum pump 16 is
provided in case of failure of the main vacuum pump.
Feed rates of between 15 to 20 pounds of pitch per hour
25 are utilized which produce about 10 pounds per hour of the higher
softening point pitch. The time it takes to increase the softening
point is only five to fifteen seconds. The absolute pressure
employed was between about 0 .1 torr and 0. 5 torr. The
temperature of the unit is stabilized at about 377C (710F). Table
III below shows the result of three runs designated Run 1008, Run
1009 and Run 1010;
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TABLE IV
Xylene
Run Insolu- Co~ing Heliu
Desig- S.P. bles Value Density Sulfur
ID08 245 15 . 2 78.1 1.260
1009 244 17.6 78.4 1.2B7 2.79
1010 261 29.1 81.3 1.260 2.61
ASTM No. D-3104 D-3671 D-2416 * D-1551
- 15 *Determined by Beckman Pyconometer g/cc at 25C.
For comparative purposes, pitch material is prepared in
the following fashion and the run is designated pitch A-410 VR.
, All products had softening points of about 210C (410F).
20 Conventional production A-240 pitch as described earlier is filtered
through a one micron fiberglass wound filter. About 250 pounds of
this pitch is loaded into a conventional vacuum still, subsequently
heated to 343-371C (650-700F) and evacuated to between one to
two torr. Tables IV (A) and (B) provide added information as to
25 the method of pitch preparation and the resultant properties.
TABLE V (A)
Run Number 5521 5522 5693 5855
Charge, ~g. to still 114 114 114 114
Overhead, X 30 29.6 28.2 32.0
Bottoms, % 68.8 70.4 72.0 69.4
Vacuum, mm H8 Abs 1 1 1 2
Final Pot Tem., C 364 364 335 342
Distillation Time, hr. 17.0 13.6 27.7 19.0
~' ~
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TABLE V ~B)
Test Method 5521 5522 5693 5855
S.P., C D-3104 208 212 212 212
XI, X D-3671 19.6 19.1 21.6 16.3
~ CV, X D-2416 ~ -- 73.5
.: 10
He Dens.,
gmlCc * 1.260 1.289 1.275 1.268
S, X D-1552 1.1-
1.25 1.14 1.19 1.33
Ash, X D-2415 0.04 0.04 0.03 0.05
*Detenmined by Be~kman Pyconmeter g/cc at 25C.
Fiber Processin~
;,
Without further processing, the increased softening point
pitch (AR-510-TF; Run 1009 of Table III) is fed to a melt blowing
extruder of the type disclosed in Buntin et al U. S . Patent
3,615,995 and Buntin et al 3,684,415. These patents describe a
technique for melt blowing thermoplastic materials wherein a molten
fiberforming thermoplastic polymer resin is extruded through a
plurality of orifices of suitable diameter into a moving stream of hot
inert gas which is issued from outlets surrounding or adjacent to
the orifices so as to attenuate the molten material into fibers which
form a ffber stream. The hot inert gas stream flows at a linear
' velocity parallel to and higher than the fflaments issuing from the
orifices so that the fflaments are drawn by the gas stream. The
fibers are collected on a receiver in the path of the fiber stream to
form a non-woven mat.
Fibers are prepared in a like manner using the A-410-VR
(Run 5521) pitch material.
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Stabilization and Carbonization
. . _ .
The fibers are then stabilized as follows. The fibers
made from the AR-510-TF pitch are successfully stabil~zed in air by
5 a special heat cycle found to be especially suitable. More
particularly, it was empirically determined that the stabilization
cycle as shown in Fig. 2 can be effectively employed to stabilize the
fibers in less than 100 minutes, a time consistent with commercial
criteria. More particularly, the 100 minute cycle consists of holding
10 the pitch fibers at approximately 11C .(20F) below the glass
transition temperature (Tg) of th`e precursor pitch (i . e. about
180C [356F] ) for about 50 minutes . This is followed by an
increase to about 200C (392F) and holding 30 minutes at that
temperature. The temperature is then increased to about 265C
(509F) and the fibers hold 10 minutes. ~inally, the fibers are
heated to about 305C (581F) and- held 10 minutes at this
temperature. The physical properties of these fibers after heating
them to about 1100C (2000F)- in a nitrogen atmosphere for two
hours in order to convert them to carbon fibers is presented in
20 Table Vlo
By "oxidizing" environment it is meant either an oxidizing
atmosphere or an oxidizing material impregnated within or on the
surface of the fiber. The oxidizing atmosphere can consist of gases
25 such as air, enriched air, oxygen, ozone, nitrogen oxides, sulfur
oxides, and etc. The impregnated oxidizing material can be any of
a number of oxidizing agents such as sulfur, nitrogen oxides,
sulfur oxides, peroxides, persulfates, and etc.
TAsI E VI
Property ~ AR-510-TF A-410~7
Tensile Strength, (10 psi) s3 41.2
ASTM D-3379
Young's Modulus, (106 psi) 4.3 4.1
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~STM D-3379
Di-~eter, Microns 13.4 22
Nu~ber of Fibers Tested 11 10
In order to stabilize fibers made from a A-410-VR pttch a
heating cyde extending over a perlod of 36 hours is required.
More particularly, they are air stabilized by holding them at a
10 temperature of about 152C (306F) for 24 hours and then
increasing the temperature to 301C (574F) where they are held
for a period of twelve (12) hours. If either temperature is
exceeded or time shortened, the fibers begin to melt and fuse
during subsequent processing. The fibers when treated properly
` 15 are carbonized by heating them to 1200C (2192F) in a nitrogen
atmosphere. The physical properties of ~arbon ffbers prepared
from the A ~410-VR pitch material are set forth in Table VI and are
approximately equal to, or slightly inferior to, the properties of the
fibers prepared from the AR-510-TF pitch material as set forth in
Table VI above.
As noted above, in the air stabilization of fibers made
from the AR-510-TF material or from other high softening point
pitch materials, it has been found that the air stabilization is much
more effective where the fibers are first heated to a temperature of
` about 6 to 11C (10 to 20F) below the glass transition temperature
of the pitch precursor and thereafter after a period of time of
appro~imately 50 minutes are then heated to 299-316C (570-600F)
until they are stabilized. As used herein, the "glass transition
30 point" represents the temperature of Young's modulus change. It
also is the temperature at which a glassy material undergoes a
change in coefficient of expansion and it is often associated with a
stress release. Thermal mechanical analysis is a suitable analytical
` technique for measuring tg. The procedure employed comprises
35 grinding a small portion of pitch fiber and compacting it into a
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0.25" diameter by 0.125" aluminum cup. A conical probe i6 placed
in contact with the surface and a 10 gram load i8 applied. The
penetration of the probe ls then measured as a f~unction of
- temperature as the sample ls heated at 10C/mlnute in a nitrogen
5 atmosphere. At 6-11C (10-20F) below the glass transition the
- fibers maintain their stiffness while at the same time the
temperature represents the highest temperatures allowable for
satisfactory stabilization to occur. Thls temperature is below the
point at which fiber-fiber fusion can occur. After the fiber has
10 been heated at this temperature for a suffic~ient time to form a skin,
the temperature can then be raised at a rate such that the
increased temperature is below the glass transition temperature of
the oxidized fibers. It has been discovered that during the
oxidation of the carbon fibers the glass transition temperature
` 15 increases a~id by maintaining the temperature during heat-up at a
point 6 to 11C (10 to 20F) below the glass transition temperature,
undesired slumping of the fibers does not occur. As the
temperature is increased the oxidation rate increases and conversely
the stabilization time decreases.
As noted in the Tables above, the AR-510-TF pitch fiber
can be stabilized in a much shorter period of time than can the
A-410-VR fiber. In fact, the time required to stabilize is
approximately twenty-five times longer for the fiber made from an
25 A-410-VR pitch. This decrease in stabilization time is in part due
to the increased softening point of the pitch fiber which enables it
to be heated to a much higher initial stabilization temperature. It
is also due in substantial part to the increased reactivity of the
precursor pitch material as contrasted to the lower softening point
30 pitch material from which it was prepared.
As noted, the use of a wiped-film evaporator is presently
the preferred method since the high thermal efficiency leads to a
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decreased exposure of the product to high temperatures, and thus
minimizes the formation of hlgher viscosltv dlspersed phases, e.g.,
mesophase, which can result In difflcultles In the flber forming
operation, and can result in discontinuous compositlonal areas In the
5 flnal product fiber.
In order to demonstrate that the shortened stabilization
; cycle is due in large part to the different chemical composition of
the pitch materials, the following tests are conducted. Two
10 pitches, samples AR-510-TF (Run 1009) and A-438-VR (Run 5053),
are crushed and screened to a -100 mesh ~200 mesh sizing (i.e.
; -150 ~75 microns) and then heated at 160C (320F), 182C (360F),
and 190C (375F) in circulating hot air. Samples are removed at
different times between 16 and 165 hours. The samples are
analyzed f~r both weight change and xylene insolubles content.
The rate constants are found by plotting xylene insolubles versus
time as a first order relationship. From this evaluation it is
determined that AR-510-TF (Run 1009) oxidizes substantially faster
than the A-430-VR (Run 5053). The calcu~ated rate constants are
about 25 times faster, a ffgure which correlates reasonably well with
the actual test results. The high softening point pitches of the
present invention prepared in 15 seconds or less have a
substantially higher reactivity than pitches of the prior art.
, .
Various methods besides wiped-fflm evaporation may be
employed to increase the softening point of the pitch without
adversely affecting its reactivity. Solvent extraction, oxidation,
nitrogen stripping and flash distillation may be employed. A brief
description of each will now be provided.
A method which can be used to produce a high softening
point pitch material is solvent extraction. Three extraction methods
can be used. They are: (1) supercritical extraction, (2)
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conventional extraction, and (3) anti-solvent extraction. These
methods greatly reduce the temperature to whlch the pitch is
subjected, thus providing a better fiber precursor. Extraction is a
method that removes lower molecular welght materlals thus leaving a
S high softening point high molecular weight fiber precursor.
,j In supercritical extraction the pitch is pumped into a
~! pressure vessel where it is continuously extracted with a solvent at
a pressure which is above the supercritical pressure of the solvent.
,' lO The usual solvents for this process ar,e normal hydrocarbons
although the process is not so limited. The solvent along with the
par~ of the pitch that is solubilized is removed to a series of
pressure step-down vessels where the solvent is flashed off. The
- insoluble part of the pitch is removed from the bottom of the
15 reactor. This insolub]e portion is used as the fiber precursor.
The softening point of the insoluble fraction is adjusted by varying
the temperature at which the extraction is conducted.
One advantage of supercritical extraction is that it can be
20 used to purify the fiber precursor pitch. It has been mentioned
; previously that the pitch contains inorganic impurities and
~, particulates. By using a solvent that will extract at least 95% of
-~ the pitch the inorganic impurities and particulates can be left in the
insoluble fraction which constitutes less than 5g6 of the pitch. The,
, 25 at least, 95% of the pitch obtained from the first extraction is then
- supercritically extracted as described above to yield a high
softening point fiber precursor pitch that is free of inorganic
impurities and particulates.
Another method of extraction that can be used is
anti-solvent extraction. This method of extraction can also be used
to produce a fiber precursor pitch which is free of inorganic
impurities and particulates. The starting pitch is dissolved in a
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I t77605
solvent such as chloroform which will dissolve at least 9596 of the
pitch. The pitch/chloroform solutlon ls then filtered through a
6mall pore filter. Thls flltration step removes the inorganic
impurlties and particulates. The pitch~chloroform solution ls then
S diluted wlth a solvent, such as a normal hydrocarbon which has a
limited solubility for pitch. Upon the addition of the normal
hydrocarbon solvent an insoluble pitch begins to precipitate. When
the addition of the normal hydrocarbon is complete, the solution is
filtered. The insoluble portion which is removed by filtration is a
10 high softening point fiber precursor pitch which is free of inorganic
impurities and particulates. The softening point of the insoluble
portion is adjusted by the amount of normal hydrocarbon added to
the pitch/chloroform solution.
Al~other extraction method that can be used to produce a
high softening point fiber precursor pitch is conventional solvent
extraction such as that used in refinery solvent deasphalting.
Pitch is extracted in an extraction vessel using an extraction
solvent at a given temperature and pressure. The usual solvents
20 for this process are normal hydrocarbons although the process is
not limited to these solvents. The solvent along with the part of
the pitch that is solubilized is removed to a flash chamber where
the solvent is removed. The insoluble part of the pitch is removed
out the bottom of the extractor. This insoluble fraction is used as
25 fiber precursor. The softening point of the insoluble fraction is
adjusted by varying the severity of the extraction conditions.
Another method which can be used to produce a high
softening point pitch fiber precursor is oxidation. Oxidation can be
30 catalytic or non-catalytic. The time the pitch is subjected to high
temperatures is quite long so care is necessary to prevent the
temperature of the oxidizer from becoming too high. lf care is
exercised it is possible to produce a mesophase free pitch.
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Oxidation is a method which ~oth removes lower molecular weight
molecules by distilling them and/or eliminates them by causing them
to react to form larger molecules. Oxidat~on can be either a batch
or a continuous reaction.
Pitch is oxidized in either a batch or continuous oxidizer
at a temperature of 250-3~0C. The oxidizing gas can be any
number of gases such as air, enriched air, NO2 and SO2. Care
must be taken not to allow the temperature of the oxidizer to go
10 above 300C to avoid the formation of unwanted mesophase. This
technique is one of the least desirable techniques since the amount
of ~ime which the pitch is subjected to fairly high temperatures is
great and there is a risk of mesophase formation. The oxidation
can be carried out catalytically by the addition of any number of
15 oxidation catalysts. These catalysts include FeC13, P2O5,
peroxides, Na2cO3, etc. The catalysts could also perform another
function in that they could act as catalysts for fiber stabilization.
Stabilization is simply an oxidation process.
Another method which can be used to produce a high
softening point fiber precursor is the reaction of the pitch with
sulfur. Sulfur performs much the same function as oxygen in that
it dehydrogenates and crosslinks the pitch molecules. It mostly
eliminates the small molecules by causing them to react. The sulfur
is added to the pitch slowly after the pitch has been heated to
2~0-300C. When the sulfur is added there is evolution of H2S so
care must be taken. Also, the temperature must be controlled
below 300C to avoid mesophase formation. This technique is one of
the least desirable also because the pitch is subjected to high
temperatures for an extended period of time, and sulfur is also
incorporated into the final product.
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1 ~77605
Another method conslsts of stripping wlth nitrogen while
the pitch is maintained at a temperature of about 300C. ~or
example~ the softening point of the pitch can be Increased by
stripping with nitrogen according to the foDowing procedure. A
S reactor, equipped wlth a 300 rpm stirrer, Is half-filled with
commercial A-240 pitch. The temperature of the reactor and its
stirred contents Is raised to 300C~ using an electrical heating
mantle. Nitrogen is sparged through the stirred pitch at a rate of
5 cubic feet/hour/ pound of pitch. The overhead material is vented
10 through a pipe in the top of the reactor a~nd is flared. After six
hours the pitch is removed from thè reactor and its softening point
is determined to be about 250C using the Mettler softening point
appara,us (ASTM D-3104) and the modified Conradson carbon
(ASTM 2416) is detérmined to be 81Ø The same procedure can be
15 repeated wit~ superheated steam as the stripping gas.
High softening point pitch can be produced by use of an
equilibrium flash distillation still. In such a unit, liquid A-240
pitch is pumped into a pre-heater zone where the feed is heated to
20 the flash tèmperature. Directly after heating, the feed enters the
flash zone. This zone is a large, well-heated vessei under vacuum
where the volatiles ar- e allowed to escape from the liquid phase.
The vapors are condensed and collected through an overhead line,
while the liquid bottoms are allowed to flow out a bottom opening to
25 be collected and used as a carbon fiber precursor.
.
Modiffcations: It will be understood that the examples are
merely illustrative and that the invention is suspectible to a variety I;
of modifications and variations which will become apparent to those
30 skilled in the art upon a reading of the application.
,
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