CA1071362A - Carbon cloth conveyor belt - Google Patents

Abstract

An endless carbon cloth conveyor belt for transporting thermoset webs of non-woven carbonaceous pitch fibers through a carbonizing oven.

Description

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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 '~..

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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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 ~ 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 . . : . . . .... . .

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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.
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.:. - ,,, :. .. ~ , . , 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-~

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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 ` ~~
,~

' .. . . . . . . ..

'``~ 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 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~

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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, , ':

~ ~ 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 9339 ~1 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 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 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 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 - . .

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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 ~ 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.

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 , ... ....

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.
:-:; .

Claims

1. An endless carbon cloth conveyor belt for transporting thermoset webs of non-woven carbonaceous pitch fibers through a carbonizing oven.