Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
9339-1
~o~l36Z
BACKGROUND OF T~iE INVE~TION
1) Field of the Invention
~ .....
This inventton relates to selfrbonded webs o~ non woven
carbon fibers in the ~orm o~ blankets, felt, paper, fiber-
board, and the like.
. 2) Description of the Prior Art
Non-woven webs o~ carbon fiber, such as carbon ~iber
~el~ or batting, are known in the art and have been described
in the literature~ e.g., by Wessendorf et al. in U. S. patent
3,844,877~ However, the nature o~ such webs requlres that
- they be bonded together by some ~orm of binder in order to
form use~ul products. The requirement o~ a binder~ and the
processing di~ficulties attendant its use, however, renders
the use of such products commercially unattractive.
SUMMARY OF ~HE INYENTION
In accordance with the present invention~ it has now ;~
been discovered that webs composed of non-woven carbo~aceous
~ibers disposed in lntimately contacting relationship can
be prepared, and the fibers thereof bonded to each other
by infusible carbon bonds without the addition of a~y
external binder~ by spinnlng a carbonaceous pitch having
: a mesopha~e content o~ from about 40 per cent by weight to
about 90 per cent by wetght to form carbonaceous pitch :
~iber, disposing staple lengths o~ the spun fiber in
intlmately contacting relatlonship with each other in a
non-woven flbrous~we~, heating the web produced in this
manner in an oxidlzlng atmospAere~to thermoset the sur~aces ;-
of the fibers to an extent which will allow ~he fi~ers to
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9339~1
~ 362
maintain their shape upon heating to more elevated temper
atures but insuf~icient to thermoset the lnterior portions
of the ~ibers, heating the web containing the externally
thermoset fibers under corllpressive pressure in an oxygen-
~ree atmosphere to a temperature suf~iciently elevated to
cause the mesophase pitch in the unoxidized lnterior portions
of the ~ibers to undergo liquid ~low and exude through sur-
~ace pores or flaws in the ~ibers and contact the surfaces
o~ the ad~acent fibers, and further heating the web to a
carbonizing ~emperature in an oxygen-~ree atmosphere so as
to sxpel hydrogen and other volatiles and produce a carbon
body wherein the fibers are bonded to each other by in~usi-
ble carbon bonds.
. DESC IPTION OF THE PREFERRED EMBODIMENTS
While carbonaceous fibers can be spun from non-
` mesphase pltches, only mesophase pitches are employed
- in the present invention because o~ their a~ ty to produce
highly-oriented, high-modulus, high-strength ~ibers which
can be easily thermoset. Mesophase pitches are pitches
which have been trans~ormed, in whole or in part, to a
llquid ory~tal or so-called "mesophase" stabe. Such
pitches by nature contain highly orien~ed molecules, and
when these pitches are spun into fibers, the pitch molecules
are preferentially aligned by the spinning process along
the longitudlnal axis of the fiber to produce a highly
,
oriented ~iber.
; Mesophase pitches can be produced in accor~ance with
known techniques by heating a natural or synthetic carbona-
ceous pitch having an aroma~lc base ln an inert atmosphere
:
933~1
~ 7~L362
at a temperature of above about 350C. ~or a tlme su~fi-
cien~ to produce the deslred quantity of mesophase. When
such a pitch is heated in this manner under quiescent
conditions, either at cons~ant temperature or with grad
ually increasing temperature9 small insoluble liquid
spheres be~in to appear in the pikch which gradually
lncrease in size as heating is continued. When examined
by electron di~ract~on and polarized light techniques,
these spheres are shown to consis~ of layers of oriented
moleeules all~ned in the same direction. As these spheres
continue to grow in size as heating is continued, they
come in contac~ with one another and gradually coalesce
wlth each other to produce larger masses of aligned layers.
As coalescence continues, domains of aligned molecules much
larger than those of the original spheres are formed.
These domains come together to ~orm a bulk mesophase where~
in the transition from one oriented domain to another
sometimes occurs smoothly and continuously through grad~-
- ually curvin~ lamellae and sometimes through more sharply
curving lamellae. The di~ferences in orientation between
the domains create a complex array of polarlzed light
extinction contours in the bulk mesophase corresponding
- to varlous types of linear discontinuity in molecular
alignment. The ultimate size of the oriented domains
produced is dependent upon the viscosity, and the rate
of increase o~ the viscosity, of the mesophase from which
they are formed, which, in turn are dependent upon the
particular pitch and the heating rate. In certain pitches
domalns havin~ sizes in excess o~ two hundred microns and
~339-
~7 ~ 3~'~
as large as several thou~and microns are produced~ In
other pi~ches J the viscosity of the mesophase is such
that only limited coalescence and structural rearrange-
ment of layers occur, so that the ultimate domain size
does not exceed one hundred microns.
The highly oriented, optically anisotropic, insoluble
material produced by treating pitches in thls manner has
been given the term l'mesophas~", and pitches containing
such mat~rial are known as "mesophase pitches". Such
pitches, when heated above their so~tening points, are
m~xtures o~ two immlscible liquids, one the optically
anlsotropic, oriented mesophase portion, and the other
the isotropic non-mesophase portion. The term "mesophase"
is derived ~rom the Greek "mesos" or "intermedlate" and
indicates the pseudo-crystalline nature of this highly-
oriented, optically anisotropic materialO
Carbonaceous pitches having a mesophase content of
from about 40 per cent by weight to about 90 per cent by
weight are suitable for producing the highly oriented
carbonaceous fibers ~rom which the sel~-bonded webs of
the present invention can be producedO In order to obtain
the desired ~ibers from such pitch, however~ the mesophase
contalned therein must, under quiescent conditions~ ~orm
a homogeneous bulk mesophase having large coalesced domains,
i.e., domains o~ aligned molecules in excess of two hundred
microns. Pltches which ~orm stringy bulk mesophase under
quiescent conditions, hav~ng small orientPd~domains, rather
than large coalesced domains, are unsuitable. Such pitches
form mesophase having a high viscosity which undergoes
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9339-1
~07~36Z
only limited coalecence~ insufficient to produce large
coalesced domains having sizes in excess o~ two
hundred microns. Instead, small oriented domains of
mesophase agglomerate to produce clumps or stringy
masses wherein the ultimate domain size does not exceed
one hundred microns. Certain pltches which polymerize
very rapidly are of this type. Likewise, pltches
which do not form a homogeneous bulk mesophase are
unsuitable. The latter phenomenon is caused by the
presence o~ in~usible sollds (which are either precent
in the original pitch or which deYelop on heating)
which are enveloped by the coalescing mesophase and
serve to interrupt the homogeneity and unlformity of
the coale~ced domains, and the boundaries between them.
Another requirement is that the pitch be non-
thixotropic under the conditions employed in the
spinning of the pitch into fibers, i.e., it must
exhlbit a ~ewtonian or plastic flow behavior so that
the flow is uniform and well beha~ed. When such pitche6
are heated to a temperature where they exhibit a
viscosity of ~rom about 10 poises to about 200 poises,
uniform fibe~rs may be readily spun therefrom. Pitches,
on the other hand~ which do not exhibit Newtonian or
plastlc flow behavior at the temperature of spinning9
do not permit unlform fibers to be spun therefrom~
Carbonaceous pitches havlng a mesophase content
of f'rom about 40 per cent by weight to about 90 per
cent by weight can be produced in accordance wlth
known techniques, as aforesaid, b~ heating a natural
or syn~het1c car~onaceous pitch having an aromatic
9339-1
~ 6 2
base in an inert atmosphere at a temperature above
about 350C. for a tilr.e sufficient to produce the
desired quantity o~ mesophase. By an inert atmosphere
is meant an atmosphere which does not react with the
pitch under the heating conditions employed, such as
nitrogen, argon, xenon, helium~ and the like~ Th~
heating period required to produce the desired meso-
phase content varies wlth the partlcular pitch and
temperature employed, with longer heating periods
required at lower temperatures than at higher temper-
atures. At 350C. 9 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 per cent. At temperatures of
from about 400C. to 450C., conversion to mesophase
proceeds more rapidly, and a 50 per cent mesophase
content can usually be produced at such temperatures
within about 1-40 hours~ Such temperatures are pre-
ferred for this reason. Temperatures above about 500C.
are undesirable, and heating a~ this temperature should
not be employed for more than about 5 minutes to avoid
conversion of the pitch to coke.
The degree to whlch 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 wh~chg in some instances~ develop on
heating, ~he non-mesophase portion of the pitch ls
readily soluble in organic solvents such as quinoline
-7-
. . : . . . .... . .
9339-L
~ 3 6 Z
and pyridine, while the mesophase portion is essen-
tially insoluble~( ) In the case of pitches which d~
not de~elop non-mesophase insolubles when heated, the
~nsoluble conten~ of the heat trea~ed pitch over and
above the insoluble content o~ the pitch berore it has
been heat treated corresponds essentially to the mesophase
content.(2) In the case of pitches which do develop
non-mesophase insolubles when heated, the insoluble
content of the heat treated pitch over and abo~e the
insoluble content of the pitch be~ore it has been heat
treated is not solely due to the conversion of the
pitch to me~ophase, but also represents non-mesophase
insolubles wh~ch are produced along wi~h the mesophase during
the heat treatment. Pitches which contain inrusib}e non-
mesophase insolubles (either present in the original pitch
or developed by heating) in amounts suf~icient to prevent
the development o~ homogeneous bulk mesophase are unsuit-
able ~or producing highly oriented carbonaceous f~bers use-
ful in the present invention, as noted aboYe. Generally~
2a pitche~ which contain in excess o~ about 2 per cent by
weight of such infusible ma~erials are unsuitable. The pres-
ence or absence of such homogeneous bulk mesophase regions,
as well as the presence or absence of in~usible non-mesophase
lnsolubles, can be visually observserved by polarized
~1) The per oent of quinoline insolubles ~Q.`I.)--or a given
! . pitch is determined by quinoline extraction at 75C. The
per cent of pyridine insolubles (P.I.) is determined by
Soxhlek extraction in boll1ng pyridlne (115C.).
(2) The insoluble content of the untreated pitch is
generally less than 1 per cent (except ~or certain coal
tar pitches) and consists largely of coke and carbon
black ~ound in the original pitch.
. , "~.
.:. - ,,, :. .. ~ , . ,
933~-1
~ ~ 7 ~ 3 6 ~
l~gh~ microscopy examination of the pitCh (see, e,g,,
Brooks~ J. D" and Taylor~ G, H.t "The Formation of
Some Graphitizing Carbons,"~
Carbon~ Vol, 4? Marcel Dekker~ Inc~ New York~ 1968,
pp. 243-2~8; and Dubois, J~, Agache~ C.~ and White~
J, L~ "The Carbonaceous Mesophase Formed ln the
Pyrolysis o~ Graphitizable Organic Materlals~1 Metal-
lography 3, pp~ 337-369~ 1970)o The amounts o~ each of
these materials may also be ~isuall~ estimated in this
manner.
Aromatlc base carbonaceous pitches having a carbon
content of ~rom about 92 per cent by weight to about 96
: per cent by weight and a hydrogen conkent of from about
4 per cent by w~lght to about 8 per cent by weight are
generally suitable for producin~ mesophase pitches which
can be employed to produce the fibers u~e~ul in the
instant invention. Elements other than carbon and hydro-
gen, such as o~ygen~ sulfur and nitrogen, are undesirable
and should not be present in excess of about 4 per Cent
by weight. When ~uch extraneous elements are present in
amounts o~ from about 0.5 per cent by weight tQ about 4
per cent by weight, the pitches generally have a carbon
content of ~rom about 92-95 per cent by weight, the
: balanc~ being h~dro~en.
Petroleum pitch? coal tar pitch and acenaphthylene
pltch are pre~erred starting materials for producing the
mesophase pitches which-are employed to produce the ~ibers
useful in the instant inventionO Petroleum pitch can
be derived ~rom the thermaI or catalytic cracking of
petroleum ~ractionsO Coal tar pitch ~s similarly obtained
~ ~ 7 ~ 3 6 2 9339-C-l
by the des~ructive distîllatiDn of coal. Both of these
ma~erials are commercial?y available natural pltches in
which mesophase can easily be produced, and are preferred
for this reason Acenaph~thylene pitch, on the other hand7
is a synthetic pitch which is preferred becau~e ~f its
ability to produce excellent fibers. Acenaphthylene pitch
can be pr~duced by the p~rolysis of polymers of acenaphthy-
lené as described by Edstrom et al in U.S~ Patent
3,5749653.
Some pitches, such as fluoranthene pitch, pDlymerize
very rapidly when heated and fail to develDp large coalesced
domains of mesophase, and are, therefore, no~ suitable
precursor materials. Likewise3 pitches having a high
i~fusible non-mesophase con~ent insolub~e in organic
sDlvents such as quinDline or pyridine, or those which
develDp an insoluble high infusible non-mesophase content
when hea~ed, should no~ be employed as starting materials,
as explained above, because these pitches are incapable of
developing the homogeneous bulk mesophase ~ecessary to
produce highly ori2nted carbonaceous fibersc For ~his
reason, pitches having an lnfus~ble quinoline-insDluble
or pyrldine-insoluble content of more ~han about 2 per cent
by weight (determined as described above) should not be
employed, Dr should be filtered to remove this material
before being heated to produce mesophase. Preferably,
such pitches are filtered when ~hey contain more than a-
bout 1 percen~ ~y weight of such infusible, insoluble
material. Mose petroleum pitches a~d synthetic pitches
have a lnw infusible, insolubLe conten~ and can be used
.,' ~
, 10 ,,
.
9339-~
~7~ 3 ~
directly without such ~iltration. Most coal tar
pitches, on the other hand~ have a hlgh infusible~
insoluble content and require ~i}tration be~ore they
can be employed,
As the pitch is heated at a temperature between
350C, and 500~C, to produce mesophase~ the pitch
wlll, of course, pyrolyze to a certa~n extent and
the composition o~ the pitch will be ~ltered, depend-
ing upon the temperature~ the heating time, and the
composltlon and structure Q~ the starting materlal~
Generally, howe~er? a~ter heating a carbonaceous pitch
*or a tim~ su~icient to produce a mesopha~e content
o~ ~rom about 40 per cent by weight to about 90 per
cent by weight, the resulting pitch will contain a
carbon content o~ ~rom about 94-96 per cent by weighk
and a h~drogen content o~ from about 4_6 per cent ~y
~ weight. When such pitches contain elements other than
;~ carbon and hydrogen in amounts o~ ~rom about r5 per
cent by weight to about 4 per cent by weight, the
mesophase pltch wil} generally have a carbon content
o~ from about 92-95 per cent by weight, the balance
being hydrogen,
A~ter the desired mesophase pitch has been pre-
pared9 it is spun into fiber by conventional tech-
niques~ e.g., by melt spinning, centri~ugal spinning,
b~ow splnning, or in any other known manner, As
noted above, in order to obtain highly oriented car-
bonaceous ~ibers ~rom which the sel~-bonded webs o~
the present lnvention can be produced the pitch must?
; 30 under quiescent condit~ons, form a homogeneous buIk ` ~~
,~
11-
'
.. . . . . . . ..
'``~ 933g-1
L3~2
mesophase having large coalesced domains~ and be non-
thixotropic under the conditlons employed in the
spinning. Further, in order to obtain uni~orm fibers
from such pitch, the pitch should be agitated imme-
diately prior to spinning so as to ef~ectively inter-
mix the immiscible mesophase and non mesophase
portions of the pitch.
The temperature at which the pitch is spun
depends, o~ course~ upon the temperature at which the
pitch exhibits a suitable viscosity, and at which the
higher meltin~ mesophase portion o~ the pitch can be
easily deformed and oriented. Since the so~tening
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 soften
ing point of the pitch to excescive levels. For thls
reason, pitches having a mesophase content of more
than about 90 per cent are generally not employed~
Pitches containlng a mesophase content of ~rom about
40 per cent by weight to about 90 per cent by weight,
however, generally exhibit a viscoslty of ~rom about
10 poises to about 200 poises at temperatures of from
about 310C. to above about 450C. and can be readily spun
at such temperatures. Pre~erably, the pitch employed
has a mesophase content of ~rom about 45 per cent by
weight to about 75 per cent by weightD most preferably
from about 55 per cent by weight to about 75 per cent
by weigh~, and exhibits a yiscosity o~ from about 30
poises to abou~ 150 poises at temperatures of from
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9339 -~1
~ 3 6~
about 340C. to about 440C. At such viscosity and
te~perature, unirorm 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, i~ is important that the
pitch be nonthixotropic and exhibit Mewtonian or
plastic ~low behavior during the spinning o~ the
fibers.
The carbonaceous fibers produced in this manner
are highly oriented r,laterials having a high degree of
preferred orientation of their molecules parallel to
the ribers axis, as shown by their X-ray diffraction
patterns. This preferred orientation is apparent from
the short arcs which constitute the (002) bands of the
d~raction pattern. Microdensitometer scanning of the --
(002) bands of the exposed X-ray ~ilm indicate this
pre~erred orientation to be generally from about 20
to about 35, usually from about 25 to about 30
~expressed as the full width at half maximum of the
aæimuthal intensity distribution).
After the fiber has been spun~ staple lengths o~ the
fiber are formed into a non-woven web wherein the
staple fiber lengths are disposed in 1ntlmately
contacting relationship with each other~ Preferably
the staple fiber lengths are produced by blow spinning
of the pitch~ and the blow-spun ~ibers are disposed
into a web d~rectly from the spinnerette~ This can be
convenlently accomplished by positioning a screen in
the vicinity of the spinnerette and reducing the
9339 -~
1C~7~3~Z
pressure behind the screen so as to draw the blow-spun
~ibers onto the screen. The fibers are preferably
deposited on the screen so as to produce a web having
an areal density of about 0.05 - 0.5 kg./m2 of screen
surface. The screen employed is pre~erably in the
form o~ an endless wire mesh conveyor belt which can
be used to transport the web through an oxidizing
atmosphere O
Alternatively, continuous fiber can be spun and
then cut or chopped into a desired length before being
processed to form a web. Any method, either wet or
dry, which effects the disposition of such ~ibers in
intlmately contacting relation in a non-woven ~ibrous
web can be employed. Alr laying operations, such as
carding or garnetting, which e~fect a relatively
orien~ed disposition of fibers are suitable for this
purpose. When a more random dispos~tion of ~ibers is
desired, conventional textile devices which effect the
air laying of fibers in a random webbing can be employ-
ed.
The flbers ca~ also be formed into a web by water
layin~ the flbers using conventional paper making
techniques. When such techniques are employed, the
fibers are first cut to a length suitable for processing,
e.g., about 1/4 inch in length, homogeneously inter-
mixed with water and a suitable binder, such as starch
or other well known binder, to form an aqueous slurry,
and then deposited from the slurry on a sub~trate to
form a web. Generally, the web is formed elther by
running a dilute suspension o~ fibers onto the surface
.~
.
933~
~7~36;~
of a moving endless belt of wire cloth~ through which
excess water may be drawn, or by running an endless
belt of wire cloth through a suspension of the fibers.
In the first case, a part of the water is drawn of~
by gravity, a part is taken ~rom the web by suction~
and a part is removed by pressure. In the second
case, a vacuum is maintained below the stock level in
the cylinder in which the wlre cloth is rotating and
the web ~orms on the wire by suction. In either case,
the thickness of the web is controlled by the speed of
~he conveyor belk, by the consistency o~ the ~iber
suspension, and by the amount of suspension permitted
to ~low onto the belt.
After the non-woven fibrous web has been formed,
it is heated in an oxidizing atmosphere for a time suf-
~icient to thermoset the surfaces of the fibers of the
web to an extent which will allow the ~ibers to main-
t~ln their shape upon heating to more ele~ated temper-
a~ures but lnsufficient to thermoset the pitch in the
interior portions of the fibers to an extent which
will prevent the pitch from flowlng and exuding through
surface pores or flaws in the fibers upon such further
heatingO Generally, thermose~ting of the fibers to
an oxygen conkent of from about l per cent by weight
to about 6 per cent by weight is usually sufficient ~ ~
to allow the fibers to maintain their shape and at the -
same time nok prevent the pitch in the interior portions
of the fibers from ~lowlng and exuding through sur~ace
pores or ~laws in the fibers upon further heating at
more elevated tempera~ures. Upon such further heating,
, ':
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9339-1
~ ~ 7 ~ 3 62
small droplets of molten pitch exude from the fibers
at inter~als along the fiber lengths and contact the
surfaces of the adJacent fibers. By applying pressure
to the web during such heating to ef~ect greater ~iber-
to-fiber contact~ this bleeding effect can be conven-
iently utilized to bond the ~ibers together into a
cohesive, self-bonded mass. When the web is then
further heated to a carbonizing tempera~ure ln an
oxygen-~ree atmosphere so a~ to expel hydrogen and
other volatiles and produce a carbon body, infusible
carbon bonds are produced between the fibers.
As noted above, the non-woven fibrous web is
preferably produced by blow-spinning staple lengths of
~iber and collecting the blow-spun fibers on an endless
wire mesh conveyor belt which can be used to transport
the web through an oxidlzing atmosphere~ By varylng
the speed o~ this belt it is possible to expose the
web to th~ oxidizing atmosphere for any deslred length
o~ time and thereby thermoset the ~ibers contained
~hereln to any desired degreeO The extent to which
the fibers are oxidized, o~ course, will determine the
degree to which they wlll bleed when hea~ed to a tem-
pera~ure sufficiently elevated to cause the mesophase
pitch in the unoxidized lnterlor portions of the fibers
to undergo liquid flow, i.e., ~he degree to which ~he
pitch will exude through sur~ace pores or ~laws in the
fibers~ If desired, an oxidizing oven containing a
number o~ zones hav~ng progressively higher tempera
ture can be employed so as to allow the fibers to be
.
.. . .
9339 ~1
~7 ~ 3 6~
gradually heated to the desired ~inal oxidizing tem-
perature. Because the oxidation reaction is an
exothermic one, and hence difficult to control~ the
oven is suitably a convectlon oven in which the
oxidizing atmosphere may be passed through the web
and wire mesh conveyor belt so as to remove heat of
reaction from the immediate vicinity of the fibers
and maintain a more constant temperature. The
oxidizing gas, o~ course, may be recirculated through
the oven after passing through the web and conveyor
belt. To help maintain the web securely against the
belt and prevent the fibers from blowing around in the
oven, the oxidizing gas should be cîrculated downward
through the web and belt rather than upward. The
rate of flow of the gas, as well as the temperature,
should be independently controlled in each zone of the
oven to allow temperature and gas flow through the
web to be regulated as desired. Gas velocity through
the web is suitably maintained at a rate of from
about 1 to about 10 feet per minute. The temperature
of the zones is maintained, e.g.,at from about 175C.
in the first or entrance zone up to about 400C. in
the last or exit zone.
The oxldizing atmosphere employed to thermoset the
~ibers of the non-woven webs of the present invention
may be pure oxygen, nitric oxide, or any other appro- ;
pria~ oxidizing atmosphereO Most conveniently, air is
employed as the oxidi~ing atmosphere
The ~ime required to thermoset the surface of the
fibers will, or course, vary with such factors 2S the
- 17 -
9339 ~1
362
particular oxidizing atmosphere, the temperature
employed, the diameter of the fibers, the particular
pitch from which the fibers are prepared, and khe
mesophase content of such pitch. Generally, however,
thermosetting can be effected in relatively short
periods o~ time, usually in from about 5 minutes to
less than about 60 minutesO
The temperature employed to effect thermosetting
of the fibers must, o~ course, not exceed the tempera-
ture 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 such pitch.
The higher the mesophase content of the fiber, the higher
will be its s~ftening temperature, and the higher the
temperature which can be ernployed to effect thermoset-
ting. At higher temperatures, of course, thermosettlng
can be e~fected in less time than is possible at lower
temperatures. Fibers having a lower mesophase content3
on the other hand, require relatively longer heat
treatment at somewhat lower temperatures to render them
infusible~
A minimum temperature of at least 250Co is generally
necessary to ef~ectively ~hermoset the fibers. Tempera-
tures in excess of 500C~ may cause melting and/or
excessive burno~f o~ the fibers and should be avoided.
Preferably, temperatures of ~rom about 27~C. to about
390C. are employed. At such temperatures, the
requlred amount of thermosetting can usually be effected
- 18 -
9339 ol
~LID7~!L3~
within f~rom about 5 minutes to less than about 60
minutes.
After the fibers have been thermoset as requiredJ
they are heated under a compressive pressure to a
temperature sufficiently elevated to cause the
mesophase pitch in ~he unoxidized interior portlons o~
said ~ibers to undergo liquid flow and exude through
surface pores or flaws in the ~ibers, e.g~, a~ a
temperature of ~rom about 400C. to about 700C.
During such heating, small droplets of pitah appear at
intervals along the ~iber lengths and come into contact
with the surfaces of the ad~acent fibers. By applying
pressure to the web during such heating so as to
effect greater contact between the ~ibers, this bleeding
e~fect can be conveniently utilized to bond the fibers
together. When the web is then ~urther heated to a
carbonizing temperature in an o~ygen-free atmosphere
so as to expel hydrogen and other volatiles and produce
a carbon bodyy in~usible carbon bonds are formed between
the fibers and an integral, cohesive, self-bonded mass
is produced.
The extent to which the pitch will bleed or exude
through the sur~ace of the ~ibers depends~ of course,
upon the degree to which the fibers have been thermoset.
By controlling the areal denslty o~ the web and the
degree of thermosetting which the fibers are permitted
to undergo, lt is po~sible to produce a wide variety
o~ ~lnal products. Thus; when the web has a relatively
high areal density and the fibers are thermose to an
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9339~1
~07~362
extent which will allow only very limited flow of the
unoxidized, internal pitch during heat treatment, the
final product has the appearance o~ a loose 5 ~lu~fy,
low density blanket. Denser, better-bonded materials
resembling felt 9 ~iber-board and paper can be
produced from webs which have been thermoset to a
~omewhat lesser extent so as to permit more extensiYe
bleeding of internal pitcb~ with the exact product
produced also depending upon the areal density of the
web employed. By way of lllustration, by thermosetting
webs having an areal dPnsity o~ from about 0.05 kg./m.
; to about 0.5 kg./m.2 to an oxygen content of from about
1 per cent to about 3 per cent, a paper~like product
can be obtained. When webs havlng an areal density of
from about o.8 kg./m.2 to about 8.0 kg./m.2 are
thermoset to an oxygen content of from about 3 per
cent to about 5 per cenk, a product resemblin~ a stl~
~iberboard is obtained, while a ~elt-like materlal is
o~tained from webs ha~ing an areal de~sity of from about
0~05 kg./m.2 to about 8.0 kg./m.2 whlch have been
thermoset to an oxygen content o~ from about 4 per cent to
about 6 per oent. Products of greater thickness and stiff-
ness are obtained as the areal density of the webs increa~es.
I~ necessary, a number of webs may be superimposed
upon each other to increase the areal density. When
the oxygen content exceeds about 5 per cent, essentially
unbonded webs are formed. While these webs have some
strength due to mechanlcal entanglement of the fibers,
no bonding exists between thP ~ibers because no
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9339_1
~ ~ 7 ~ 36 Z
bleeding occurs durin~ the heating process~
In order to effect greater contact be~ween the
fibers so as to facilitate bonding of the fibers by
the pitch ~hich exudes from the fibers, a compressive
pressure is applied to the web during the heat treat-
ment. Generally pressures o~ from about 0.1 kPa to -
about 5 kPa are sufficient ~or ~his purpose~
Upon further heating, the fibers are evenkually
rendered totally infusible~ and upon heating to a
carbonizlng temperature, e~g. 9 a temperature of about
1000C.~ fibers having a carbon content greater than
about 98 per cent by weight are obtained. At tempera-
,tures in excess of about 1500C. 9 the fibers are sub-
stantially completely carbonized. Such heating should
- be conducted in an oxygen~ree atmosphere, such as the :
inert atmospheres described above, to prevent ~urther
. oxidation of the flbers.
Usually, carbonization is effected at a tempera-
ture o~ ~rom about 1000C. to about 2500C.~ pre~erably
from about 1500C. to abouk 1700C. Generally,
resldence times of from about 0.5 minute to about 60
minutes are emplo~ed. While more extended heating
times can be employed with good results, such resi~
dence 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 e:xcessive as to dlsrupt the
i riber structure, ~ is pre~erred ~o gradually heat the
. fibers to their final carbonization temperature
:- .
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21 - . .
9339-1
:~7~3/~;~
In a preferred embodiment of the invention, the
thermoset web is continuously transported through a
carbonizing oven on an endless carbon cloth conveyor
belt, i.e., on a belt consisting o~ either graphitic
or non-graphit~c carbon. Carbon cloth is particularly
sultable for use as a conveyor belt in a carbonizing
oven because o~ its strength~ ~lexibility, and high
tempeature resistance, as well as because it is soft,
nonabrasive and nonreacti~e with the fibers o~ the
web, and hence will not damage the web.
If desired, the carbonized web may be further
heated ln an inert atmosphere, as described herein-
before, to a graphitizing temperature in a range of
; from above about 2500C. to about 3300C, preferably
from about 2800C. to about 3000C. A resldence
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 ti~es longer than 5 minutes are uneconomical
and unnecessary, but may be employed if desired~
The products produced in accordance with the
invention can be used in a variety o~ applications,
e.g., for high temperature insulation purposes. The
blanket~like webs are particularly useful as rein-
forcing materials for producing composite structures.
; The paper-like webs are especially suitable for
producing speaker cones such as are described in
Canadian patent 1,009,156.
EXAMPLES
__
The following example is set forth for purposes
of illustration so that those skilled in the art may
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~ 9339 -1
107~L3G~
better understand the invention. It should be under-
stood that it is exemplary only, and should not be
construed as limiting the invention in any manner.
EXAMPLE 1
A commercial petroleum pitch was employed to
produce a pitch having a mesophase conte~t of about 64
per cent by weight. The precursor pitch had a
density of 1.25Mg. /m.3, a softening temperature Or
120C. and contained 0.7 per cent by weight quinoline
insolubles (Q.I. was determined by quinoline extraction
at 75C.). Chemical analysis showed a carbon content
of 93.8%~ a hydrogen content of 4.7%, a sulfur content
of 0.4%, and 0~1% ash.
The mesophase pitch was produced by heating the
precursor petroleum pitch at a temperature of about
400C. for about 15 hours under a nitrogen atmosphere.
Aft~r heating, the pitch contained 64 per cent by ;-~
weight quinoline insolubles, indicating that the pitch
had a mesophase content of close to 64 per cent. A
portion of this pitch was then blow-spun by means o~ a
spinnerette at a temperature of 380C, to produce staple
lengths o~ ~iber approximately 25 mm. in length and 10
mlcrons in diameter. The blow-spun fibers were
deposlted in intimately contacting relationship with ~ -each other on a wire mesh conveyor belt positioned
beside the spinnerette by reducing the pressure behind
, .
the conveyor belt so as to draw the blow spun ~ibers
onto the beltD The fibers were allowed to collect on
the belt until a fibrous web having an areal densi~y
,
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... ....
933g-~
7~36Z
of 0.1 - 0.3 kgO/m.2 of belt sur~ace accumulated.
The fibrous web produced in this manner was then
transported on the conveyor belt through a 12-meter
lon~ forced-air convection oven at a speed of 1 meter/
~inute. The oven contained eight zones~ each 1.5 meters
ln length, and the web was gradually heated f'rom 175C,
in the first or entrance zone to 350C. in the elghth
or exit zone while air was passed downward throug~ the
web and conveyor belt at a velocity of about 2 meters/
minute. The oxygen content of the fibers was increased
to 4.3 per cent as a result of this procedure.
The thermoset flbrous web was then cut into 250 mm.
b~ 280 mm. sections, and 8 of these sections were
stacked on top of one another in parallel ~ashion
.~ between two similarly siæed graphite platss. The
staoked webs were then subjected to a compress~ve
pressure of 2 kPa while they were heated under nltro-
gen to a temperature of 1600C. over a period of 60
minutes where the temperature was maintained for an
additional 60 m~nutes~
: The resulting carbonized webs were found to be
completely self-bonded and could be ~reely hand~ed
without loss of fibers. The webs werP 6 mm. thick,
and had a bulk density of 0.3 ~0/m.3~ appreciable
stif~ness characterlstic of fiberboard, and maintained
their shape well when handled.
When a sin~le web having an areal density of 0.1-
0.3 kg./mO2 was thermoset to an oxygen content of only 1.8
. ~ .
per cent and carbonized in the same manner, a dense, paper-
~ 30 like material was obtained.
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