CA1060612A

CA1060612A - Self-bonded webs of non-woven carbon fibers

Info

Publication number: CA1060612A
Application number: CA234,615A
Authority: CA
Inventors: David A. Schulz
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1974-09-27
Filing date: 1975-08-29
Publication date: 1979-08-21
Also published as: JPS5526219B2; DE2542966B2; DE2559533A1; JPS5160774A; JPS552571A; US4032607A; GB1479218A; IT1047598B; FR2286226B1; FR2286226A1; DE2542966A1

Abstract

SELF-BONDED WEBS OF NON-WOVEN CARBON FIBERS

ABSTRACT OF THE INVENTION

Self-bonded webs of non-woven carbon fibers in the form of blankets, felt, paper, fiberboard, and the like, are produced by spinning a carbonaceous pitch having a mesophase content of from about 40 per cent by weight to about 90 per cent by weight to form carbonaceous pitch fiber; dis-posing staple lengths of the spun fiber in intimately contacting relationship with each other in a non-woven fibrous web; heating the web produced in this manner in an oxidizing atmosphere to thermoset the surfaces of the fibers to an extent which will allow the fibers to main-tain their shape upon heating to more elevated temperatures but insufficient to thermoset the interior portions of the fibers; heating the web containing the externally thermoset fibers under compressive pressure in an oxygen-free atmosphere to a temperature sufficiently elevated to cause the mesophase pitch in the unoxidized interior portions of the fibers to undergo liquid flow and exude through surface pores or flaws in the fibers and contact the surfaces of the adjacent fibers; and further heating the web to a carbonizing temperature in an oxygen-free atmo-sphere so as to expel hydrogen and other volatiles and produce a carbon body wherein the fibers are bonded to each other by infusible carbon bonds.

Description

--` 9339 1060~lZ

BACKGROUND OF THE INVENTION
1) Field of the Invention . . _ . .
Thie invention relates to self-bonded webs of non-woven carbon fibers in the form of blankets, felt, paper, fiber- --board, and the like.

2) Description of the Prior Art Non-woven webs of carbon fiber, such as carbon fiber felt 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 of such webs requires that -they be bonded together by some form of binder in order to form useful products. The requirement of a binder, and the processing difficulties attendant its use, however, renders the use of such products commercially unattractive.

SUMMARY OF THE INVENTION
In accordance with the present invention, it has now ~ --been discovered that webs composed of non-woven carbonaceous fibers disposed in intimately contaeting relationship can be prepared, and the fibers thereof bonded to each other by infusible carbon bonds without the addition of any external binder, by spinning a carbonaceous pitch having a mesophase content of from about 40 per cent by weight to about 90 per cent by weight to form carbonaceous pitch fiber, disposing staple lengths of the spun fiber in -~
intimately contacting relationship with each other in a non-woven fibrous web, heating the web produced in this : .
manner in an oxidizing atmosphere to thermoset the surfaces of the fibers to an extent which will allow the fibers to :~ .. ' , t, - 2-933g ~0~
maintain their shape upon heating to more elevated temper-atures but insufficient to thermoset the interior portions of the fibers, heating the web containing the externally thermoset fibers under compressive pressure in an oxygen-free atmosphere to a temperature sufficiently elevated to cause the mesophase pitch in the unoxidized interior portions of the fibers to undergo liquid flow and exude through sur-face pores or flaws in the fibers and contact the surfaces of the adjacent fibers, and further heating the web to a carbonizing temperature in an oxygen-free atmosphere so as to expel hydrogen and other volatiles and produce a carbon body wherein the fibers are bonded to each other by infusi-ble carbon bonds.

' DESCRIPTION OF THE PREFE~RED EMBODIMENTS
While carbonaceous fibers can be spun from non- `
mesophase pitches, only me~ophase pitches are employed in the present invention because of their ability to produce highly-oriented, high-modulus, high-strength fibers which can be easily thermoset. Mesophase pitches are pitches which have been transformed, in whole or in part, to a liquid crystal or so-called "mesophase" state. Such pitches by nature contain highly oriented molecules, and when these pitches are spun into fibers, the pitch molecules are preferentially aligned by the spinning process along the longitudinal axis of the fiber to produce a highly oriented fiber. ~-Mesophase pitches can be produced in accordance with known techniques by he~ting a natural or synthetic carbona-ceous pitch having an aromatic base in an inert atmosphere , -106()61~
at a temperature of above about 350 C. for a time suffi-cient to produce the desired quantity of mesophase. When such a pitch is heated in this manner under quiescent conditions, either at constant temperature or with grad-ually increasing temperature, small insoluble liquid spheres begin to appear in the pitch which gradually increase in size as heating is continued. When examined by electron diffraction and polarized light techniques, these spheres are shown to consist of layers of oriented molecules aligned in the same direction. As these spheres continue to grow in size as heating is continued, they come in contact with one ànother and gradually coalesce with each other to produce larger m~sses 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 form a bulk mesophase where-in the transition from one oriented domain to another sometimes occurs smoothly and continuously through grad-ually curving lamellae and sometimes through more sharply curving lamellae. Th~ differences in orientation between the domains create a complex array of polarized light extinction contours in the bulk mesophase corresponding to various types of linear discontinuity in molecular - -~
alignment. The ultimate size o-f the oriented domains produced is dependent upon the viscosity, and the rate of increase of the viscosity, of the mesoph~se from which they are formed, which, in turn are dependent upon the particular pitch and ~he heating rate. In certain pitches domains having sizes in excess of two hundred microns and as large as several thousand microns are produced. In other pitches, 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 this manner has been given the tenm "mesophase", and pitches containing such material are known as "mesophase pitches". Such pitches, when heated above their softening points, are mixtures of two immiscible liquids, one the optically anisotropic, oriented mesophase portion, and the other the isotropic non-mesophase portion. The tenm "mesophasel' -is derived from the Greek "mesos" or "intermediate" and indicates the pseudo-crystalline nature of this highly-oriented, optically anisotropic material.
Carbonaceous pitches having a mesophase content of from about 40 per cent by weight to about 90 per cent by weight are suitable for producing the highly oriented carbonaceous fibers from which the self-bonded webs of the present invention can be produced. In order to obtain the desired fibers from such pitch, however, the mesophase contained therein must, under quiescent conditions, fonm a homogeneous bulk mesophase having large coalesced domains, i.e., domains of aligned molecules in excess of two hundred microns. Pitches which form stringy bulk mesophase under quiescent conditions, having small oriented domains, rather ~-than large coalesced domains, are unsuitable. Such pitches a form mesophase having a high vis/osi~y which undergoes ~ 93~9 1~6~

only limited coalescence, lnsuff~clent to produce large coalesced domains having sizes in excess o~ two hundred microns. Instead, small oriented domalns o~
mesophase agglomerate to produce clumps or stringy masses wherein the ultimate domain size does not exceed one hundred mlcrons. Certain pitches whlch polymerlze very rapidly are of this type. Llkewise, pitches whlch do not form a ho~ogeneous bulk mesophase are ~-unsuitable. The latter phenomenon ls caused by the presence of infusible solids (whlch are either present ln the orlginal pltch or whlch develop on heatlng) which are enveloped by the coalescing mesophase and serve to lnterrupt the homogeneity and uni~ormlty o~
the coalesced domains, and the boundaries between them.
Another requlrement is that the pitch be non-thixotroplc under the condltions employed ln the ~-~
splnnlng of the pitch into flbersg l.e., lt must ~-exhlbit a nonthlxotropic flow behavlor so that the flow ls uniform and well behaved. When such pitches are heated to a temperature where they exhlbit a viscoslty of ~rom about 10 poises to about 200 poises, unlrorm fibers may be readily spun therefrom. Pitches, on the other hand, whlch do not exhibit nonthixotroplc ~low behavior at the temperature of splnning, do not permit uni~orm ~ibers to be spun therefrom.
Carbonaceous pltches havlng a mesophase content o~ from about 40 per cent by welght to about 9~ per cent by welght can be produced in accordance with known techniques, as aforesaid, by heatlng a natural or synthetic carbonaceous pitch havlng an aromatlc '" ~ :.' - 6 - ~ ~
:' ~ ,. .

106(~61~
base in an inert atmosphere at a temperature above about 350 C. for a time sufficient to produce the desired quantity of mesophase. By an inert atmosphere is meant an atmosphere which does not react with the pitch under the heating conditions employed, such as nitrogen, argon, xenon, helium, and the like. The -~
heating period required ~o produce the desired meso-phase content varies with the particular pitch and temperature employed, with longer heating pexiods required at lower temperatures than at higher ~emper-atures. At 350 C., the minimum temperature generally required to produce mesophase, at least one week of heating is usually necessary to produce a mesophase content of about 40 per cent. At temperatures of -from about 400 C. to 450 C., 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 500 C.
are undesirable, and heating at this temperature should not be employed for more than about 5 minutes to avoid -conversion of the pitch to coke.
The degree to which the pitch has been converted to mesophase can readily be determined by polarized light microscopy and solubility examinations. Except for certain non-mesophase insolubles present in the original pitch or which, in some instances, develop on heating, the non-mesophase portion of the pitch is readily s~luble in organic solvents such as quinoline " . .. .

1~0~
and pyridine~ while the mesophase portion is essen- -tially insoluble. ( ) In the case of pitches which do not develop non-mesophase insolubles when heated, the ins~luble content of the heat treated pitch ~ver and above the insoluble content of the pitch before it has been heat treated corresponds essentially to the mesophase content. ( ) In the case of pitches which do develop non-mesophase insolubles when heated, the insoluble content of the heat treated pitch over and above the insoluble content of the pitch before it has been heat treated is not solely due to the conversion of the ~-~
pitch to mesophase, but also represents non-mesophase insolubles which are produced along with the mes~phase during the heat treatment. Pitches which contain infusible non-mesophase insolubles (either present in the original pitch or developed by heating) in amounts sufficient to prevent the development of homogeneous bulk mesophase are unsuit-able for producing highly oriented carbonaceous fibers use-ful in the present invention, as noted above. Generally, pitches which contain in excess of about 2 per cent by weight of such infusible materials are unsuitable. The pres-ence or absence of such homogeneous bulk mesophase regions, as well as the presence or absence of infusible non-mesophase insolubles, can be visually observed by polarized (1) The per cent of quinoline insolùbles rQ-~ of a given pitch is determined by quinoline extraction at 75 S. The per cent of pyridine insolubles (P.I.) iæ determined by Soxhlet extraction ~n boiling pyridine (115 C.).
(2) The insoluble content of the untreated pitch is :
generally less than 1 per cent (except for certain coal tar pitches) and consists largely of coke and carbon black found in the original pitch. ~
' ~ ' ":

': :

~ 6 1 ~ 9339 light microscopy examination of the pitch (see, e.g., Brooks, J. D., and Taylor, G. H., "The Formation of Some Graphitizing Carbons," Chemistry and Physics of Carbon, Vol. 4, Marcel Dekker, Inc., New York, 1968, pp. 243-268; and Dubois, J., Agache, C., and White, J. L., "The Carbonaceous Mesophase Formed in the Pyrolysis of Graphitizable Organic Materials ,~r Metal- ~ -lography 3, pp. 337-369, 1970~ The amounts of each of these materials may also be visually estimated in this manner.
Aromatic base carbonaceous pitches having a carbon content of from about 92 per cent by weight to about 96 per cent by weight and a hydrogen content of from about

4 per cent by weight to about 8 per cent by weight are generally suitable for producing mesophase pitches which can be employed to produce the fibers useful in the instant invention. Elements other than carbon and hydro-gen, such as oxygen, sulfur and nitrogen, are undesirable and should not be present in excess of about 4 per cent by weight. When such extraneous elements are present in amounts of from about 0.5 per cent by weight to about 4 per cent by weight, the pitches generally have a carbon content of from about 92-95 per cent by weight, the balance being hydrogen.
Petroleum pitch, coal tar pitch and acenaphthylene pitch are preferred starting materials for producing the ~ - -mesophase pitches which are employed to produce the fibers useful in the instant invention. Petroleum pitch can be derived from the thermal or catalytic cracking of ~-petroleum fractions. Coal tar pitch is similarly obtained _9- ~ -:.' :~:' ,:, ~ , by the destructive distlllatlon of coal. Both of these materials are commerclally available natural pitches in which mesophase can easily be produced, and are preferred for this reason. Acenaphthylene pitch, on the other hand, is a synthetic pitch which is pre-ferred because of its abillty to produce excellent fibers. Acenaphthylene pitch can be produced by the pyrolysis o~ polymers o~ acenaphthylene as descr~bed ~ -by Edstrom et al. in U.S. Patent 3,574,653.
~ome pitches, such as fluoranthene pitch, poly-merize very rapldly when heated and ~ail to develop large coalesced domalns of mesophase, and are, there-~ore, not sultable precursor materials. Likewise, pltches havlng a high in~uslble non-mesophase lnsoluble content in organic solvents such as quinoline or pyridine, or those which develop a high lnfusible non-mesophase insoluble content when heated, should not be employed as starting materials, as explained above, because these pltches are incapable o~ de~eloplng the homogeneous bulk mesophase necessary to produce highly oriented carbonaceous fibers. For thls reason, pitches havlng an lnfusible ~
qulnoline-insoluble or pyridine-~nsoluble content o~ -;
more than about 2 per cent by weight (determined as described above) should not be employed, or should be flltered to remove this material before being heated to produce mesophase. Preferably, such pitches are ~ltered when they contaln more than about 1 per cent by we~ght of such lnfuslble, lnsoluble materlal.
Most petroleum p~tches and synthetlc pltches have a low lnfusible, insoluble content and can be used - 10 - ~.

1~()61~
directly without such filtration. M~st coal tar pitches, on the other hand, have a high infusible, insoluble content and require filtration before they can be employed As the pitch is heated at a temperature between 350 C. and 500 C. to produce mesophase, the pi~ch will, of course, pyrolyze to a certain extent and the composition of the pitch will be altered, depend-ing upon the temperature, the heating time, and the composition and structure of the starting material.
Generally, however, after heating a carbonaceous pitch for a time sufficient to produce a mesophase content of from about 40 per cent by weight to about 90 per cent by weight, the resulting pitch will contain à
carbon content of from about 94-96 per cent by weight and a hydrogen content of from about 4-6 per cent by weight. When such pitches contain elements other than carbon and hydrogen in amounts of from about 0.5 per cent by weight to about 4 per cent by weight, the mesophase pitch will generally have a carbon content of from about 92-95 per cent by weight, the balance -being hydrogen.
After the desired mesophase pitch has been pre-pared, it is spun into fiber by conventional tech-niques, e.g., by melt spinning, centrifugal spinning, blow spinning, or in any other known manner. As noted above, in order to obtain highly oriented car-bonaceous fibers from which the self-bonded webs of .
the present invention can be produced the pitch must, under quiescent conditions, form a homogeneous bulk -' '.'' .. .

~06 ~ 9339 mesophase having large coalesced domains, and be non-thixotropic under the conditions employed in the spinning. Further, in order to obtain uniform fibers from such pitch, the pitch should be agitated imme-diately prior to spinning so as to effectively inter-mix the immiscible mesophase and non-mesophase portions of the pitch.
The temperature at which the pitch is spun depends, of course, upon the temperature at which the pitch exhibits a suitable viscosity, and at which the higher-melting mesophase portion of the pitch can be easily deformed and oriented. Since the softening temperature of the pitch, and its ~iscosity 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 excessive levels. For this reason, pitches having a mesophase content of more than about 90 per cent are gener~lly not employed.
Pitches containing a mesophase content of from about 40 per cent by weight to about 90 per cent by weight, however, generally exhibit a viscosity of from about 10 poises to about 200 poises at temperatures of from about 310 C. to above about 450 C. and can be readily spun at such temperatures. Preferably, the pitch employed has a mesophase content of from about 45 per cent by weight to about 75 per cent by weight, most preferably from about 55 per cent by weight to about 75 per cent by weight, and exhibits a viscosity of from about 30 poises to about 150 poises at temperatures of from .

ab~ut 340C. to about 440C. At such viscoslty and temperature, uni~orm ~lbers having diameters of from about lO mlcrons to about 20 microns can be easlly spun. As prev~ously mentioned, however, ln order to obtain the desired flbers, it is important that the pltch exhiblt nonthixotropic ~low behavior durlng the spinn~ng of the f~bers.
The carbonaceous flbers produced in thls manner are highly oriented materlals havlng a hlgh degree of preferred orientation o~ their molecules parallel to the flbers axlsJ as shown by their X-ray di~fraction ~
patterns. This preferred orientatlon is apparent from ~ ~-the short arcs whlch constltute the (002) bands o~ the dif~ractlon pattern. Microdensitometer scann~ng Or the (002) bands of the exposed X-ray ~ilm indicate this preferred orientation to be generally rrom about 20n ~
to about 3~, usually ~rom about 25 to about 30 ;; -(expressed as the ~ull wldth at half maximum of the azimuthal lntenslty distribution).
After the flber has been spun, staple lengths of the ~lber are formed lnto a non-woven web wherein the staple ~iber lengthR are disposed in intimately contacting relationship with each other. Prererably the staple fiber lengths are produced by blow-spinning of the pitchJ and the blow-spun ~lbers are dlsposed into a web dlrectly ~rom the spinnerette. ~hls can be convenlently accompllshed by posltioning a screen ln the vicinity o~ the splnnerette and reduclng the - .-. ., ..:
' . - , .:
.:.... . ...
.

. ~ .. .. . . . ~

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pressure behind the screen so as to draw the blow-spun fibers 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 preferably in the form of an endless wire mesh conveyor belt which can be used to transport the web through an oxidizing atmosphere.
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 fibers in intimately contacting relation in a non-woven fibrous web can be employed. Air laying operations, such as carding or garnetting, which effect a relatively oriented disposition of fibers are suitable ~or this purpose. When a more random disposition of fibers is desired, conventional textile devices which effect the air laying of fibers in a random webbing can be employ-ed. ~ -The fibers can also be formed into a web by water laying the fibers 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 fr~m the slurry on a substrate to form a web. Generally, the web is formed either by running a dilute suspension of fibers onto the surface 0 6~ 6 ~ Z 9339 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 off by gravity, a part is taken from 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 wire cloth is rotating and the web forms on the wire by suction. In either case, the thickness of the web is controlled by the speed of the conveyor belt, by the consistency of the fiber suspension, and by the amount of suspension permitted to flow onto the belt. ~ -After the non-woven fibrous web has been formed, it is heated in an oxidizing atmosphere for a time suf-ficient to thermoset the surfaces of the fibers of the web to an extent which will allow the fibers to main~
tain their shape upon heating to more elevated temper-atures but insufficient to thermoset the pitch in the in~erior portions of the fibers to an extent which will prevent the pitch from flowing and exuding through surface pores or flaws in the fibers upon such further heating. Generally, thermosetting of the fibers to -an oxygen content of from about 1 per cent by weight to about 6 per cent by weight is usually sufficient to allow the fibers t~ maintain their shape and at the same time not prevent the pitch in the interior portions of the fibers from flowing and exuding through surface pores or flaws in the fibers upon further heating at ;
more elevated temperatures. Upon such further heating, .

,,, ... , .... .. ~ . . . . .

-~0 6 ~ 9339 small droplets of molten pitch exude from the fibers at intervals along the fiber lengths and contact the surfaces of the adjacent fibers. By applying pressure to the web during such he~ting to effect greater fiber-to-fiber contact, this bleeding effect can be conven-iently utilized to bond the fibers together into a cohesive, self-bonded mass. When the web is then further heated to a carbonizing temperature in an oxygen-free atmosphere so as -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 fiber and collecting the blow-spun fibers on an endless wire mesh conveyor belt which can be used to transport the web through an oxidizing atmosphere. By varying the speed of this belt it is possible to expose the web to the oxidizing atmosphere for any desired length of time and thereby thermoset the fibers contained therein to any desired degree. The extent to which the fibers are oxidized, of course, will determine the degree to which they will bleed when heated to a tem-perature sufficiently elevated to cause the mesophase pitch in the unoxidized interior portions of the fibers to undergo li~uid flow, i.e., the degree to which the pitch will exude through surface pores or flaws in the fiber~. If desired, an oxidizing oven containing a number of zones having progressively higher tempera-ture can be employed so as to allow the fibers to be 06 ~ 9339 ~radually heated to the desired final oxidizing tem-perature. Because the oxidation reaction is an exothermic one, and hence difficult to control, the oven is suitably a convection 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, of 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 circulated 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 l to about 10 feet per minute. The temperature of the zones is maintained, e.g., at from about 175 C.
in the first or entrance zone up to about 400 C. in the last or exit zone.
The oxidizing atmosphere employed to thermoset the fibers of the non-woven webs of the present invention may be pure oxygen, nitric oxide, or any other appro- ;~
priate oxidizing atmosphere. Most conveniently, air is employed as the oxidizing atmosphere.
The time required to thermoset the surface of the fibers will, of course, vary with such factors as the -.,, ~''- ', .

~0~
particular oxidizing atmosphere, the temperature employed, the diameter of the fibers, the particular pitch from which the fibers are prepared, and the mesophase content of such pitch. Generally, however, thermosetting can be effected in relatively shor~
periods of time, usually in from about 5 minutes to less than about 60 minutes.
The temperature employed to effect thermosetting of the fibers must, of 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 softening temperature, and the higher the temperature which can be employed to effect thermoset-ting. At higher temperatures, of course, thermosetting can be effected in less time ~han is possible at lower temperatures. Fibers having a lower mesophase content, on the other hand, require relatively longer heat treatment at somewhat lower temperatures to render them infusible.
A minimum temperature of at least 250 C. is generally necessary to effectively thermoset the fibers. Tempera-tures in excess of 500 C. may cause melting and/or excessive burnoff of the fibers and should be avoided.
Preferably, temperatures of from about 275 C. to about 390 C. are employed. At such temperatures, the required amount of thermosetting can usually be effected '' ~06~6~'~ 9339 within from about 5 minutes to less than about 60 minutes.
After the fibers have been thermoset as required, they are heated under a compressive pressure to a temperature sufficiently elevated to cause the mesophase pitch in the unoxidized interior portions of -said fibers to undergo liquid flow and exude through surface pores or flaws in the fibers, e.g., at a temperature of from about 40~ C. to about 700 C.
During such heating, small droplets of pitch appear at intervals along the fiber lengths and come into contact -with the surfaces of the adjacent fibers. By applying pressure to the web during such heating so as to effect greater contact between the fibers, this bleeding effect ean be conveniently utilized to bond the fibers together. When the web is then further heated to a carbonizing temperature in an oxygen-free atmosphere so as to expel hydrogen and other volatiles and produce a carbon body, infusible 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 surface of the fibers depends, of course, upon the degree to which the fibers have been thermoset.
By controlling the areal density of the web and the degree of thermosettin~ which the fibers are permitted to undergo, it is possible to produce a wide variety -of final products. Thus, when the web has a relatively high areal density and the fibers are thermoset to an -,"'';

-19- ~ ~
~' ..

6V~l~
extent which will allow only very limited flow of the unoxidized, internal pitch during heat treatment, the final product has the appearance of a loose, fluffy, low density blanket. Denser, better-bonded materials resembling felt, fiber-board and paper can be produced from webs which have been thermoset to a somewhat lesser extent so as to permit more extensive bleeding of internal pitch, with the exact product produced also depending upon the areal density of the web employed. By way of illustration, by thermosetting webs having an areal density of from about 0.05 kg./m.2 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 having an areal density of from about 0.8 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 cent, a product resembling a stiff fiberboard is obtained, while a felt-like material is obtained from webs having an areal density of from about 0.05 kg./m.2 to about 8.0 kg./m.2 which have been thermoset to an oxygen content of from àbout 4 per cènt ~o about 6 per cent. Products of greater thickness and stiff- ;
ness are obtained as the areal density of the webs increases -If necessary, a number of webs may be superimposed .
upon each other to increase the areal density. When - ~
the oxygen content exceeds about 6 per cent, essentially -unbonded webs are formed. While these webs have some - -strength due to mechanical entanglement o~ the fibers, no bonding exists between the fibers because no -20~
,: .

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bleeding occurs during the heating process.
In order to effect greater contact between the fibers so as to facilitate bonding of the fibers by the pitch which exudes from the fibers, a compressive pressure is applied to the web during the heat treat- -ment. Generally pressures of from about 0.1 kPa to about 5 kPa are sufficient for this purpose.
Upon further heating, the fibers are eventually rendered totally infusible, and upon heating to a carbonizing temperature, e.g., a temperature of about 1000 C., fibers having a carbon content greater than about 98 per cent by weight are obtained. At tempera-tures in excess of about 1500 C., the fibers are sub-stantially completely carbonized. Sùch heating should be conducted in an oxygen-free atmosphere, such as the inert atmosphere described above, to prevent further oxidation of the fibers.
Usually, carbonization is effected at a tempera-ture of from about 1000 C. to about 2500 C., preferably from about lS00 C. to about 1700 C. Generally, residence times of from about 0.5 minute to about 60 minutes are employed. 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 excessi~e as to disrupt the fiber structure, it is preferred to gradually heat the fibers to their final carbonization temperature.
'' ~ ' ' ' J6~Z
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 ef either graphitic or non-graphitlc carbon. Carbon cloth is particularly suitable for use as a conveyor belt in a carbonizln~
oven because of its strength, ~lexibility, and high tempeature resistance, as well as because lt is soft, nonabrasive and nonreactive with the ~ibers of the web, and hence will not damage the web.
If desired, the carbonized web may be further heated in an inert atmosphere, as descrlbed ~ereln-be~ore, to a graphltizing temperature in a range of from above ~b~ut 2500C. to about 3300C, preferably ~rom about 2800C. to about 3000C. A resldence tlme of about 1 minute is satisfactory, althou~h both shorter and~longer tlmes may be employed, e.g., from about 10 seconds to about 5 minute3, or longer.
Residence tlmes longer than 5 minutes are uneconomical and unnecessary, but may be employed if desired.
The products produced in accordance wlth the lnvention can be used ln a varlety of appllcations, ~; - e~ for high temperature lnsulatlon purposes. The blanket-like webs are particularly useful as rein-~orcing~materials for produclng comp~site structures.
The paper-llke webs~are especla~l~ suitable ~or produclng speaker cones such as are descrlbed ln Canadlan patent 1, oag, I56.
EXAMPLES
30~ ~ The following example ls set forth ~or purposes of illustration so that those skllled ln the art may . ' 1 ~ 6~

better understand the invention. It should be under-stood that it is exemplary only, and should nat be construed as limiting the invention in any manner.

EXAMPLE l A commercial petroleum pltch was employed to produce a pitch having a mesophase content of about 64 per cent by weight. The precur~or pltch had a density o~ 1.25Mg, /m.3, a softening temperature of 120C. and contained 0.7 per cent by weight quinoline 19 insolubles (Q.I. was determlned by quinoline extraction at 75C.). C~emlcal analysis showed a carbon content of 93.8S, a hydrogen content of 4.7%, a sul~ur content o~ ~.4%, and 0.1% ash.
~he mesophase pltch wa~ produced by heatlng the precursor petroleum pitch at a temperature o~ about 400C. for about 15 hours under a nitrogen atmosphere.
A~ter heating, the pitch contained 64 per cent by w-ight ~uinoline insolubles~ indicatlng that the pitch - ~ .
had a mesophase content o~ close to 64 per cent. A
20~ portlon o~ this pitch was then b~ow-spun by means of a p~nnere~te at a temperaturè of' 380C . to produce staple lengths o~ ~lber approxlmately 25 m~. ln length and lO
microns in dlæmeter. The blow-spun ~ibers were ~ ~-depo~ited in lntimately eontactir~g re~lationship with ::
eaah~ other on a~ wire ~mesh c onveyor :belt positioned ~ ~.
beslde~ the spinnerette by reduclng the pressure behind t he~ conveyor: belt; so as to draw the blow-spun flbers onto the belt . ~ The ~lbers were allowed to collect on the belt ur~tll a fibrous web ha~lng an areal density , ~, .
~ 23-1()60~1~
of 0.1 - 0.3 kg./m.2 o~ belt surface accumulated.
The fibrous web produced in thi~ manner was then transported on the conveyor belt through a 12-meter long forced-air convection oven at a speed o~ 1 meter/
minute. The oven contained ei~ht zones, each 1.5 meters in length, and the web was gradually heated from 175C.
in the first or entrance zone to 350C. in the elghth or exit zone while air was pa~sed downward through the web and conveyor belt at a velocity of about 2 meters/
minute. The oxygen content of the fibers waæ lncreased to 4. 3 per cent as a result of this procedure.
The thermoset flbrous web was then cut lnto 250 mm.
by 280 mm. sections, and 8 o~ the~e sections were stacked on top of one another in paraIlel fashlon between two slmilarly slzed graphite plates. The stacked webs were then sub~ected to a compres~lve pres~ure of 2 kPa while they wer~ heated under nltr gen to a temperature of 1600C. over a perlod o~ 60 ~ mlnutes wbere the temperature was maintalned for an 2~ ~ additional 60 minutes.
The resulting carbonized webs were found to be completely sel~-bonded and could be rreely handled -without loss of flbers. The ~ebs were 6 mm, thick, and had a bulk den~ity of 0.3 ~/m.3, appreciable ~; stif~ness characterlstic of fiberboard, and maintained the~r shape well when handled.
When a slngle ~eb having an areal density of 0.1- -; 0.3 kg./m.2 was thermoset to an oxy~en content of only 1.8 per cent and carbonlzed in the same manner, a dense, paper-Iike material was obtalned.

Claims

WHAT IS CLAIMED IS:

1. A process for producing self-bonded webs of non-woven carbon fibers which comprises spinning carbona-ceous pitch fiber from a nonthixotropic carbonaceous pitch having a mesophase content of from 40 per cent by weight to 90 per cent by weight, which mesophase content, under quiescent conditions, forms homogenous bulk mesophase having large coalesced domains; dis-posing staple lengths of the spun fiber in intimately contacting relationship with each other in a non-woven fibrous web; heating the web produced in this manner in an oxidizing atmosphere for a time sufficient to thermoset the surfaces of the fibers of the web to an extent which will allow the fibers to maintain their shape upon heating to more elevated temperatures but insufficient to thermoset the interior portions of the fibers; heating the web containing the externally thermoset fibers under compressive pressure in an oxygen-free atmosphere to a temperature sufficiently elevated to cause the mesophase pitch in the unoxidized interior portions of the fibers to undergo liquid flow and exude through the surfaces of the fibers and con-tact the surfaces of the adjacent fibers; and further heating the web to a carbonizing temperature in an oxygen-free atmosphere to produce a carbon body wherein the fibers are bonded to each other by infusible carbon bonds.

2. A process as in claim 1 wherein the staple fiber lengths are produced by blow-spinning of the pitch, and the blow-spun fibers are disposed into a web directly from the spinnerette.

3. A process as in claim 2 wherein the blow-spun fibers are disposed in a web on an endless wire mesh conveyor belt by reducing the pressure behind the belt so as to draw the blow-spun fibers onto the belt.

4. A process as in claim 3 wherein the web is transported on the wire mesh conveyor belt through an oxidizing atmosphere wherein thermosetting of the surfaces of the web fibers is effected.

5. A process as in claim 4 wherein thermosetting is effected in a convection oven in which the oxidizing atmosphere is circulated downward through the web and wire mesh conveyor belt, and in which the web is gradually heated to the desired oxidizing temperature in a plurality of heating zones having progressively higher temperatures.

6. A process as in claim 5 wherein the thermoset web is transported on an endless carbon cloth conveyor belt through an oxygen-free atmosphere wherein the web is further heated and carbonized.

7. A process as in claim 5 wherein the oxidizing atmosphere is air.

8. A process as in claim 7 wherein the thermoset web is transported on an endless carbon cloth conveyor belt through an oxygen-free atmosphere wherein the web is further heated and carbonized.

9. A process as in claim 3 wherein the blow-spun fibers are deposited on the wire mesh conveyor belt to produce a web having an areal density of about 0.05 kg./m.2 to about 0.5 kg./m.2.

10. A process as in claim 9 wherein the web is transported on the wire mesh conveyor belt through an oxidizing atmosphere wherein the fibers of the web are oxidized to an oxygen content of from 1 per cent by weight to 6 per cent by weight.

11. A process as in claim 10 wherein thermosetting is effected in a convection oven in which the oxidizing atmosphere is circulated downward through the web and wire mesh conveyor belt, and in which the web is grad-ually heated to the desired oxidizing temperature in a plurity of heating zones having progressively higher temperatures.

12. A process as in claim 11 wherein the thermo-set web is transported on an endless carbon cloth conveyor belt through an oxygen-free atmosphere wherein the web is further heated and carbonized.

13. A process as in claim 11 wherein the oxidizing atmosphere is air.

14. A process as in claim 13 wherein the thermo-set web is transported on an endless carbon cloth conveyor belt through an oxygen-free atmosphere wherein the web is further heated and carbonized.

15. A process as in claim 9 wherein the web is transported on the wire mesh conveyor belt through an oxidizing atmosphere wherein the fibers of the web are oxidized to an oxygen content of from 1 per cent by weight to 3 per cent by weight.

16. A process as in claim 15 wherein thermo-setting is effected in a convection oven in which the oxidizing atmosphere is circulated downward through the web and wire mesh conveyor belt, and in which the web is gradually heated to the desired oxidizing temperature in a plurality of heating zones having progressively higher temperatures.

17. A process as in claim 16 wherein the thermoset web is transported on an endless carbon cloth conveyor belt through an oxygen-free atmosphere wherein the web is further heated and carbonized.

18. A process as in claim 16 wherein the oxidizing oven is air.

19. A process as in claim 18 wherein the thermoset web is transported on an endless carbon cloth conveyor belt through an oxygen-free atmosphere wherein the web is further heated and carbonized.