CA1055664A - Rapid thermosetting of carbonaceous fibers produced from mesophase pitch - Google Patents

Abstract

RAPID THERMOSETTING OF CARBONACEOUS
FIBERS PRODUCED FROM MESOPHASE PITCH
ABSTRACT OF THE DISCLOSURE

An improved process for producing carbon fibers from pitch which has been transformed, in part, to a liquid crystal or so-called "mesophase" state. According to the process, the time required to thermoset carbonaceous fibers spun from such pitches can be substantially reduced by treating the fibers with an aqueous chlorine solution before they are processed by heating in an oxygen atmo-sphere. After the fibers have been thermoset, the infusi-ble fibers are carbonized by heating in an inert atmosphere.

Description

~47 .

~0~5 BACKGROUND OF mE INVENTION
1. Field of the Invention This invention relates to an improved process for producing carbon fibers from pitch w~ich has been trans-formed, in part~ to a liquid crystal or so-called "meso-phase" state. More particularly, this :invention relates to an improved process for producing carbon fibers from pitches of this type wherein ~he carbonaceous fibers spun from such pitches are thermoset in substantially shorter periods of time than heretofore possible.

2. Description of the Prior Art As a result of the rapidly expanding growth of the aircraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and ex-traordinary combination of physical properties. Thus, ma-terials characterized by high strength and stiffness, and at the same time of light weight, were required for use in such applications as the fabrication of aircraft structures, -re-entry vehicles, and space vehicles, as well as in the preparation of marine deep-submergence pressure vessels and like structures. Existing technology was incapable of supplying such materials and the search to satisfy this need centered about the fabrication of composits articles.
One of the most promising materials suggested for use in composite form was high strength, high modulus car-bon textiles, which were introducPd into the market place -2~

~ 9474 ,~ .
55~

at the very time this rapid growth in the aircraft, space and missile industries was occurring. Such textiles have been incorporated in both plastic and metal matrices to produce composites having extraordinary high-strength- and high-modulus-to-weight ratios, and other exceptional proper-ties. However, the high cost of producing the high strength, high modulus carbon textiles employed in such composit~s has been a major deterrent to their widespread use, in spite of the remarkable properties exhibited by such composites.
One recently proposed method of providing high modulus, high strength carbon fibers at low cost is described in Canadian patent 1,019,919, entitled "Hlgh Modulus, High Strength Carbon Fibers Produced From Mesophase Pitch". Such method comprises first spinning a carbonaceous fiber from a carbonaceous pitch which has been transformed, in part to a liquid crystal or so-called "mesophase" state, then ther~osetting the fiber so-produced by heating the fiber in an oxygen-containing atmo-sphere for a time su~ficient to render it infusible, and finally carbonizing the thermoset fiber by heating in an inert atmosphere to a temperature sufficiently elevated to remove hydrogen and other volatiles. The carbon fibers produced in this manner have a highly oriented structure characterized by the presence of carbon crystallites pre~erentially aligned parallel to the fiber axis, and are graphitizable materials which when heated to graphitizing temperatures develop the three-dimensional order character-

-3-."~,,, ,;. .

5566~
.. ~ ..~..
istic of polycrystalline graphite and graphitic-like proper-ties associated therewith, such as high density and low electrical resistivity.
While carbonaceous fibers produced in accordance with aforementioned Canadian patent 1,019,919, i.e., by spinning from a carbonaceous pitch which has been trans-formed, in part, to a liquid crystal or so-called "meso-phase" state, can be thermoset in considerably shorter times than heretofore possible in other processes for producing ;
carbon fibers from pitch materials, the thermosetting time required is still of longer duration than is desired or co~mercial operations. For this reason, means have been sought for still urther reducing the heat treatment times necessary to thermoset the carbonaceous fibers produced in accordance with said process.
SUMMARY OF THE INVENTION
In accordance with the instant invention it has now been discovered that the time required to thermoset carbonaceous fibers which have been spun from carbonaceous pitches of the type described in aforementioned Canadian patent 1,019,919, i.e., carbonaceous pitches which have been transformedg in part, to a liquid crystal or so-called "mesophase" state, can be substantially re-duced by treating the ~ibers with an aqueous chlorine solutlon before they are processed by heating in an oxygen atmosphere. As a result of such pretreatment, the fibers can be thermally set, at any given temperature, in sub-:~' ' ' ~ S 5~ 6 stantially shorter periods of time than heretofore possible.
DESCRIPTION OF THE PREFERRED EMB()DIMENTS
When natural or synthetic carbonaceous pitches having an aromatic base are heated in an inert atmosphere at a temperature of above about 350C., either at constant temperatures or with gradually increas;ng temperature, small insoluble liquid spheres begin to appear in the pitch and gradually increase in size as heating is continued. When examined by electron diffraction and polarized light tech- -niques, these spheres are shown to consist of layers of oriented molecules aligned in the same direction, As these 9pheres continue to grow in size as heating is continued, they come in contact with one another and gradually coalesce with 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 form a bulk mesophase where-in the transition from one oriented domain to another some-times occurs smoothly and continuously through gradually curving lamellae and sometimes through more sharply curving lamellae. The differences in orientation between the do-mains 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 of the oriented domains prod~ced is ;
dependent upon the viscosity, and the rate of incrlease of the viscosity, of the mesophase from which thley are ~ ... .. , ~ .. . ... .

~L~5566~ , formed, which, in turn are dependent upon the particular pitch and the heating rate. In certain pltches, 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 rearrangement of layers occur, ; ~-so that the ultimate domain si7e does not exceed one hundred microns.
The highly oriented, optically anisotropic, in-soluble material produced by treating pitches in ~his manner has been given the tenm "mesophase", and pitches containing such material are known as "mesophase pitches". Such pitches, when heated above thelr sof~ening polnts, are mixtures of two immiscible liquids, one the optically ani-sotropic, oriented mesophase portion, and the other the isotropic non-mesophase portion. The tenm "mesophase"
is derived from the Greek "mesos" or "intermediate" and indicates the pseudo-crystalline nature of thls 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 highly oriented carbona-ceous fibers capable of being rapidly thermoset and heat treated to produce fibers having the three-dimensional order characteristic of polycrystalllne graphite accord-ing to the invention. In order to obtain the desired fibers from such pitch, however, the mesophase contairled . .. . . . . . :

~ 55~6~
therein, must, under quiescent conditions, form a homo-geneous 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 coalesoed domains, are unsuitaible. Such pitches form mesophase having a high viscosity which undergoes only limited coalescence, insufficient to produce large coalesced domains having sizes in excess of two hundred microns.
Instead, small oriented domains of mesophase agglomerate to produce cl~ips o~ stringy masses wherein the ultimate domain size does not exceed one hundred microns, Certain pltches which polymer~ze very rapidly are of this type.
Likewise, pitches which do not form a homogeneous bulk mesophase are unsuitable. The latter phenomenon is caused by the presence of infusible solids (which are either present in the original pitch or which develop on heating) w~ich are enveloped by the coalescing mesophase and serve to interrupt the homogeneity and uniformity of the coalesced domains, and the boundariè~ between them.
Another requirement is that the pitch be nonthixo-tropic under the conditions employed in the spinning of the pitch into fibers, i.e., it must exhibit a nonthixotropic ~.
flow behavior so that the flow is uniform and weill behaved.
When such pitches are heated to a temperature where they exhibit a viscosity of from about 10 poises to about 200 poises, uniform fibers may be readily spun there-from. Pitches, on the other hand, which do not exhibit nonthixotropic flow behavior at the temperature of spinning, do not permit uniform fibers to be spun there ~6 ~ ~
.
~rom which can be converted by further heat treatment into fibers having the three-dimensional order characteristic of polycrystalline graphite.
Carbonaceous pitches having a mesophase content o~
from about 40 per cent by weight to about 90 per cent by weight can be produced in accordance with known techniques, as disclosed in aforementioned Canadian patent 1,019,919, by heating a carbonaceous pitch in an inert atmosphere at a temperature above about 350C. for a time su~fi.cient 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 em-ployed, such as nltrogen, argon, xenon, helium, and the like. The heating period required to produce the desired mesophase content varies with the particular pitch and temperature employed, with;longer heating periods required at lower temperatures than at higher temperatures. At 350C., 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 450Co 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 preferred for this reason. Temperatures above about 500C. are undesirable, and heating at this temperature should not be employed for more than about 5 minutes to r.l ; .

'; . . ; : ~ . . ... .
.

9~74 ~ ~ 5 ~66 avoid conversion of the pitch to coke.
The degree to which the pitch has been converted to mesophase can readily be detenmined 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-meso-phase portion of the pitch is readily soluble in organic solvents such as ~uinoline and pyridine, while t~e mesophase portion is essentially insoluble.(l) In the case of pitches which do not develop non-mesophase insolubles when heated, the insoluble content of ~he heat treated pitch over and above the insoluble content o the pitch before Lt has been heat treated corresponds essentially to the mesophase con-tent.( ) In the case of pitches which do develop non-meso-phase 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 mesophase during the heat treatment.
Pitches which contain infusible non-mesophase insolubles (1) The per c~nt of quinoline insolubles (Q.I.) of a given pitch is determined by quinoline extraction at 75C. The per cent of pyridine insolubles (P.I.) is determined by Soxhlet extraction in boiling pyridine (115C.).

(2) The insoluble content of the untreated pitch is gen-erally less than 1 per cent (except for certain coal tar pitches) and consists largely of coke and carbon bLack found in the original pitch.

_g_ ~~ 9474 5566~ ~
(either prese~t in the original pitch or developPd by heating) in amounts sufficient to prevent the development of homogeneous bulk mesophase are unsuitable for producing highly orientad carbonaceous fibers capable of bei~g rapidly thermoset and heat treated to produce fibers having the three-dimensional order characteristic of polycrystalline graphite, as noted above. Generally, pitches which con-tain in excess of about 2 per cent by weig~t of such in-fusible materials are unsuitable. The presence or absence of such homogeneous bulk mesophase regions, as well as the presence or absence of infusible non-mesophase insolubles, can be vlsually observed by polarized light mlcroscopy examination of the pitch (see, e.g., Brooks, J~D., and Taylor, G. H., "The Formatlon 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 Ma~erials," Metal-lography ~ pp. 337 - 369, 1970). The amounts of each of these materials may also be vlsually estimated in thls 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 gen- ;
erally suitable for producing mesophase pitches which can be employad to produce fibers capable of being rapidly ther-moset and heat treated to produce fibers having the three- ;`
dimensional order characteristic of polycrystalline graphite - 10- "

' , ' ' . ' ' :, ' ' _~ 9474 ~ 5 ~ ~ 4 according to the invention. Elements other than carbon and hydrogen, such as oxygen, sulfur and nitrogen, are undesirable and should not be present in excess of about

4 per cent by weight. When such extranPous 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 whlch are employed to produce the fibers employed in the instant invention. Petroleum pitch can be derived from the thermal or catalytic cracking of petroleum fractions. Coal tar pitch is similarly obtained by the destructive distillation of coal. Both of these materials are commercially availa~le natural pitches in which meso-phase can easily be produced, and are preferred for this reason. Acenaphthylene pitch, on the other hand, is a ~ynthetic pitch which is preferred because of its ability to produce excellent fibers. Acenaphthylene pitch can be produced by the pyrolysis of polymers of acenaphthylene as described by Edstrom et al. in U.S. Patent 3,574,653.
Some pitches, such as fluoranthene pitch, polymerize very rapidly when heated and fail to develop large coalesced regions of mesophase, and are, therefore, not suitable pre-cursor materials. Likewise, pitches having a high infusi-ble non-mesophase insoluble content in organic solvents '''' ' ' , .

such as quinoline or pyridine, or those which develop a high infusible non-mesophase insoluble content when heated, should not be employed as starting materials, as explained above, because these pitches are incapable of developing the homogeneous bulk mesophase necessary to produce highly oriented carbonaceous fibers capable of being rapidly ther-moset and heat treated to produce fibers having the three-dimensional ~rder characteristic of polycrystalli~e graphite~
For this reason, pitches having an infusible quinoline-insoluble or pyridine-insoluble content of more than about 2 per cent by weight (determined as described above) should not be employed, or should ~e ~iltered to remove this ma-terial before being heated to produce mesophase. Preferably, such pitches are filtered when they contain more than about 1 per cent by weight of such infusible, insoluble material.
Most petroleum pitches and synthetic pitches have a low infusible, insoluble content and can be used directly without such filtration. Most 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 ~
350C. and 500C. to produce mesophase, the pitch will, -of course, pyrolyze to a certain extent and the composition of the pitch will be altered, depending 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 .

,. , ., , , . . . .~ , ,- ' , 1~55664 mesophase content of from about 40 per cent by weight to about 90 per cent by weight, the resulting pitch will con-tain a carbon content of from about 94-96 per cent by weight and a hydrogen content of from albout 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, ~he mesophase pitch will generally have a carbon content of from about ` `
92-95 per cent by weight, the balance being hydrogen.
Afte~ the desired mesophase pitch has been pre-pared, it iB spun into fibers by conventional techniques~
e,g., by melt splnning, centrifugal spinning, blow spinning, or in any other known manner. As noted above, in order to obtain highly oriented carbonaceous fibers capable o~ being ~-rapidly thenmoset and heat treated to produce fibers having the three-dimensional order characteristic of polycrystal-line graphite the pitch must, under quiescent conditions, form a homogeneous bulk mesophase having large coalesced domains, and be nonthixotropic under the conditlons employed ;`
in the spinning. Further, in order to obtain uniform ~ibers ;
from such pitch, the pitch should be agitated immediately prior to spinning so as to effectively intermix the immisci- `~
ble mesophase and non-mesophase portions of the pitch.
The temperature at which the pitch is spun de-pends, 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 de- ~ ;

~55~;64 formed and oriented. Since the softening 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 rlse to a point w~ich raises the softening pOill~ of the pitch to -~
excessive levels. For this reason, pitches having a meso-phase content of more than about 90 per cent are generally 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 310C, to above about 450C. 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 lS0 poises at temperatures of from about 340C to about 440C.
At such viscosity and temperature, uniform fibers having diameters of from about 5 micrometers to about 25 micro-meters can be easily spun. As previously mentioned, how ever, in order to obtain the desired fibers, it is important that the pitch exhibit nonthixotropic flow behaviDr during the spinning of the fibers, The carbonaceous fibers produced in this manner are highly oriented graphitizable materials having a high degree of preferred orientation of their molecules parallel ~955~i6~ :

to the fiber axis. By "graphitizable" is meant that these fibers are capable of being converted thermally (us~lly ~ -by heating to a temperature in excess of about 2500C., e.g., from about 2500C. to about 3000C,.) to a structure having the three-dimensional order charaLcteristic of poly-crystalline graphite.
Because of the thermoplastic nature of carbonaceous fibers produced in this manner, it is necessary that they - ;~
be thermoset before they can be carbonized. As disclosed in aforementioned Canadian patent 1,019,919, thermosetting can be readily effected by heatlng the fibers in an oxygen-contalning atmosphere for a time sufficient to render them infusible.
According to the present invention9 the time re-quired to thermoset carbonaceous fibers prepared in accord ance with aforementioned Canadian patent 1,019,919 and the present invention can be substantially reduced by treating the fibers with an aqueous chlorine solution before they are ~
proces~ed by heating in an oxygen atmosphere. As a result `;
2U of such pretreatment, the fibers can be thermally set, at any given temperature, in substantially shorter periods of time than heretofore possible.
The aqueous solutions of chlorine employed in the present invention can be prepared by simply bubbling gaseous chlorine into water. The chlorine should be added :Ln an amount sufficient to provide a chlorine concentration of at least 0.2 per cent by weight, preferably from about ~ ~ ~is : '~ .. li, ` .

9~74 , }55664 0.5 per cent by weight to about 1 per cent by weight (the upper solubility limit of chlorine in water~. The tempera- -ture of the solution is preferably maintained between about 10C. and 60C. Temperatures in excess of about 60C. are generally not employed because of the reduced solubility of chlorine in water at such temperatures, while at tempera-tures below about 10C., the chlorine precipitates out of the solution as chlorine hydrate (C12~8H20).
After the chlorine water solution has been pre-pared, it is maintained at a temperature between about 10C.
and 60~C., preferably between about 20C. and 40C., and the fibers are Lmmersed therein and allowed ~o soak for a time sufficient to allow them to partially thermoset, i.e., to form a thin skin on their surfaces. When continuous - ' filaments are being processed, the filaments may be fed through the chlorine water solution by means of a payof reel and a take-up reel. Alternatively, the flbers may be wrapped around a spool or simi.lar object before being immersed in the solutlon. The time the fibers are allowed to soak depends upon the temperature and concentration of chlorine in the bath, as well as upon such other factors as the diameter of the fibers, the particuk~r pitch from whlch the fibers are prepared, and the mesophase content of such pitch. Generally, fibers having diameters of from about 5 micrometers to about 25 micrometers need not be soaked for longer than about four minutes. In any event, in order to produce carbon fibers having adequate strength, 947~

~!~355664 : :~
.
the soaking time should not be allowed to exceed ten minutes, and preferably no more than five minutes. Longer soaking times result in carbon fibers w~lich are weak and :
brittle. On the other hand, a minimum soaking time of one-half minute is necessary to produce fibers having a tensile strength in excess of 1.38 GPa. Preferablyl the fibers are soaked in the bath for from one to three minutes.
In order to ensure that all the carbonaceous fibers are thoroughly wetted by the chlorine water solution during 10 - substantially the entire treating time, the solution may be circulated in the bath, e.g., by means of ultrasonic agitation. If desired, a suitable surfactant, e.g., an amphoteric or anionlc fluorocarbon, such as Fluorad FC-408 or Fluorad FC-423 (registered trademarks of the Minnesota Mining and Manufacturing Company), may be added to the solution to facilitate wetting o~ the fibers. The wetting agent is suitably employed Ln an amount of from about 0.001 parts by weight to 0.1 parts by welght per 100 parts by weight of the solution.
After the fibers have been partially thermoset in the chlorine water bath, they are removed from the bath and dried. While fibers treated in this manner are capable of being carbonized without any further thermosetting, the resulting carbonized fibers are characterized by a tensile strength below 1.38 GPa., usually below 0.69 GPa. In order to produce fibers having tensile strengths in excess of 1.38 GPa., therefore, it is necessary to further the~moset '' -17- ~

C~S5664 ~, the fibers by heating in oxygen before they are carbonized.
Likewise, in order to obtain such tensile strengths, the fibers should not be pretreated in halogen solutions other than chlorine water. Thus, e.g., pretreatment in bromine water results in the production of fibers having tensile strengths below 1.38 GPa., and usually below 0.69 GPa.
The temperature at which the fibers are heated in oxygento complete thermosetting must, of course, not exceed :
the temperature at which the fibers will soften or distort.
The maximum temperature which can be employed will thus depend upon the particular pitch rom which the fibers were spun, the mesophase content of such pitch, and the degree to which ~he fibers have been thermoset in the chlorine water bath. The higher the mesophase content of the fibers, and the greater the degree to which they have been thermoset, the higher will be their sotening temperature and the higher the temperature which can be employed to complete thermosetting At higher temperatures, o~ course, ~ibers of a given diameter can be thermoset in less time than is possible at lower temperatures. Fibers having a lower mesophase content, or which have been ther-moset to a lesser degree in the chlorine water bath~ on the other hand, require relatively longer heat ~reatment at somewhat lower temperatures to render them infusible.
A minimum temperature of at least 225C. is gener-ally necessary to complete thermosetting of the fibers Temperatures in excess of 400C. may cause melting and/or ,: :

~ . .
~055G64 ~ ~
..... : .

excessive burn-off of the fibers, as well as some reduction in the tensile strength of the carbonized product, and should be avoided~ Preferably, temperatures of at least 300C. are employed as thermosetting proceeds at a much faster rate at such temperatures. Fibers having diameters o from about 5 micrometers to about 25 micrometers can generally be thermoset at temperatures of 300C. or higher -within from about one minute to about four minutes. Since it is undesirable to oxidize the fibers more than necessary, the fibers are generally not heated for longer than about five minutes.
In order to ensure that all the carbonaceous fibers are effectively subjected to the oxygen atmosphere, the gas 1QW Of oxygen over the fibers should be adequate to permit full diffusion of the gas into the fibers and effect removal of all reaction products from the surace of the fibers. If the gas flow rate is too slow, poorly thermoset fibers and/or ignition of fiber volatiles and the fibers may result. Generally, gas flow rates of from about 0.14 standard cubic meters/hour to about 0.85 standard cubic metars/hour, preferably from about 0.54 standard cubic meters/hour to about 0.65 standard cubic meters/hour, per ,- .
570 cc. of furnace volume are suitable.
After the fibers have been thermoset, they are carbonized by heating in an inert atmosphere, such as that described above, to a temperature sufficiently elevated to remove hydrogen and other volatiles. Fibers having a 9~74 1~5S664 carbon content greater than about 98 per cent by weight can generally be produced by heating to a temperature in ex-cess of about 1000C., and at temperatures in excess of about 1500C., the fibers are completely carbonized~ -Usually, carb~nization is effected at a tempera- f ture of from about 1000C. to about 2500C.~ preferably from about 1400C. to about 1800C. Generally, residence times of from about 0.5 minute to about 60 minutes are employed. While ~ore extended heating times can be employed with good results, such residence times are un-economical and, as a practical matter, there is no ad-vantage in employing such long perlods. In order to ensure that the rate o~ weight loss of the fLbers cloes not become so excessive as to disrup~ the fiber structure, `~
it is preferred to gradually heat the fibers to their final carbonization temperature.
In a preferred method of heat treatment, con-tinuous fibers are passed through a series of heating zones which are held at successively higher tempera-tures. If desired, the first of such zones may contain an oxidizing atmosphere through which the fibers are passed after first passing through a chlorine water bath. Several `
arrangements of apparatus can be utilized in providing the series of heating zones. Thus, one furnace can be used with the fibers being passed through the furnace several times and with the temperature being increased each time.
~lternatively, the fibers may be given a single pass '''' ' , ' `

9~7~

~ 0556G4 -: :
through several furnaces, with each successive furnace being maintained at a higher temperature than that of the previous furnace. Also, a single furnace with several heating zones maintained at successively higher temperatures in the direc-tion of travel of the fibers, can be used.
The carbon fibers produced in this matmer have a highly oriented structure characterized by the presence of carbon crystallites preferentially aligned parallel to the fiber axis, and are graphitizable materials which when heated to graphitizing temperatures develop the three-dimensional order characteristic o polycrystalline graphite and graphite-like properties associated therewith, such as high density and low electrical resistivity. Fibers heated to about 1600C. have been found to be characterized by tensile strengths of greater than about 1.38 GPa. and by a Young's modulus of elasticity of at least about 207 GPa.
The fibers heated to a temperature of about 1~00C.
are quite dense7 exhibiting a density in excess of 2.0 grams/cc., usually from about 2.0 grams/cc. to about 2.2 grams/cc. Electrical resistivity of such fibers is gener-ally from about 800 x 10 6 ohm centimeters to about ;
1200 x 10-6 ohm centimeters. ;~
If desired, the carbonized fibers may be further heated in an inert atmosphere, as described hereinbefore, ;
to still higher temperature in a range of from about 2500C. to about 3300C., preferably from about 2800C.

'' "

~L055G64 to about 3000C., to produce fibers having not only a high degree of preferred orientation of their carbon crystallites parallel to the fiber axis, but also a structure charact~ristic of polycrystalline graphite.
A residence time of about 1 minute is satisfactory, al-though both shorter and longer times may be employed, e.g., from about 10 seconds to about 5 minutes~ or longerO
Residence times longer than 5 minutes are uneconomical and unnecessary, but may be employed if desired.
The fibers produced by heating at a temperature above about 2500C., preferably above about 2800C., are characterized as having the three-dimensional order of polycrystalline graphite. This three-dimensional order is clearly established by the X-ray diffraction pattern of the fibers, specifically by the presence of the (112) cross-lattice line and the resolution of the (10) band into two distinct lines, (100) and (101). The short arcs which constitute the (OOR) bands of the pattern show the carbon crystallites of the fibers to be preferentially aligned parallel to the fiber axis. Microdensitometer scanning of the (002) band of the exposed X-ray film indi-cates this preferred orientation to be no more than about 10, usually from about 5 to about 10 (expressed as the full width at half maxim~1m of the azimuthal intensity distribution). The interlayer spacing (d) of the crystal-lites, calculated from the distance between the correspond-ing (ooQ~ difraction arcs, is no more than 3.37A, usually .i ,,, ` ' 1~1155664 from 3.36 A to 3.37 A.
In addition to having a structure characteristic of that of polycrystalline graphite, the fibers are charac-terized by graphitic-like properties associated with such structure, such as high density and low electrical resis- ~ -tivity. Typically, these fibers have a density in excess of 2.1 grams/cc. up to 2.2 grams/cc., and higher. Electri-cal resistivity of the fibers has been found to be less than 250 x 10-6 ohm centimeter, usually from about 150 x 10-6 ohm centimeters to about 200 x 10-6 ohm centimeters.
The fibers are also character~zed by high mo~ulus and high tenslle strengths. Thus, these fibers have been found to be characterized by tensile strengths in excess of about 1.38 GPa. and by a Young's modulus of elasticity in excess of about 345 GPa. Usually such fibers have a tensile strength in excess of about 1.72 GPa., e.g., -from about 1.72 GPa. to about 2.41 GPa., and a Young's modulus in excess of about 517 GPa., e.g., from about 517 GPa. to about 828 GPa. `
The instant invention thus provides an improved method of preparing high strength, high modulus fibers in high yield from inexpensive, readily available, high ear-bon content precursors. The fibers can be used in the same applications where high strength, high modulus fibers have previously been employed, such as in the preparation of composites. The fibers are especially useful in applica-.. . , .. . -, .............. ~ . . . . ~
, : . , . :~ .. . . .

, ~
~1SS664 tions where high electrical conductivity and thermal con-ductivity along the axis of the fibers is important, e.g., they can be used to produce graphitic cloth heating ele-ments. Because of their extremely low electrical resis-tlvity, the fibers can be employed as filler material in the production of graphite electrodes.
The following examples are set forth for purposes of illustration so that those skilled in the art may better understand the invention. It should be ~nderst~od that they are exemplary only, and should not be construecl as limiting the invention ln any manner. Tensile strengths reerred to in the examples and throughout the speciEicatlon, unless otherwise indicated, are short gauge tensile strengths measured on 3 mm~ samples. Young's modulus was measured on 2.0 cm. sections unless otherwise indicatéd.

EXA~LE 1 A commercial petroleum pitch was employed to pro-duce a pitch having a mesophase content of about 56 per cent by weight. The precursor pitch had a density of 1.23 Mg./m.3, a softening t~mperature of 122C. and contained 5 0.5 per cent by weight quinoline insolubles (Q.I. was determined by quinoline extraction at 75C.). Chemical analysis showed a carbon content of 9401%, a hydrogen con-tent of 5.56%, a sulfur content of 1.82%,and 0.19% ash.
The mesophase pitch was produced by heating the precursor petroleum pitch at a temperature of about 380C.

.. , . . . ~ " , : . . .. .

~474 ~L~5S6~4 ~ . . .
. ... .. .
for about 45 hours under flowing nitrogen. The pitch was continuously stirred during this time and nitrogen gas was continuously bubbled through the pitch. After h~ating~
the pitch exhibited a softening point oiE 318C. and con-tained 56.7 per cent by weight pyridine insolubles, indi-cating that the pitch had a mesophase content of close to 56 per cent. -A portion of the pitch produced in this manner was then melt spun into fibers at a rate of 229 meters per minute through a 128 hole spinnerette (0.10 mm. diameter holes) at a temperature of 392C. The flbers passed through a nitrogen atmosphere as they left the spinnerette and were then taken up by a reel.
A portion of the spun fibers was cut into lengths 178-250 mm. long and submerged in a glass vessel ;
filled with a saturated solution of chlorine water con-taining 0.02 per cent by weight of a wetting agent (Fluorad FC-408, registered trademark of the Minnesota Mining and Manufacturing Company)~ The chlorine water solution was prepared by slowly bubbling chlorlne into water at 23C.
After soaking in the bath at 23C. for one minute, the fibers were re~Loved, dipped into distilled water for another minute, and dried at room temperature.
A portion of the fibers treated in this manner were then heated for two minutes in a furnace m~intained at a temperature of 300C. while oxygen was continuously passed through the furnace. The resulting fibers were .. , . . . ~ . . ..

5 ~6~ ~

sufficiently thermoset to be heated at elevated tempera-tures without sagging.
The infusible fibers were carbonized lmder nitrogen by first heating to a temperature of 90l)C. at a rate of 15C./minute, and then at 1650~C. for five minutes~ The resulting fibers had an average tensile strength of 1.7 ~Pa. and an average Young's modulus o elasticity of 207 GPa. (Tensile strength and Young's modulus are an average of 5 and 6 samples, respectively). The average fiber diameter was 13 micrometers.
When the spun fibers were immersed in the chlorine water bath or three minutes and heated in oxygen for ~our minutes at 300C., as described above, and then carbonized in the same manner, the resulting fibers had an average tensile strength of 2.23 GPa. and an average Young's modulus of elasticity of 283 GPa. ~Tensile strength and Young's modulus are an average of 7 and 5 samples, respec-tively).
When the spun fibers were heated at 300C. for two minutes in oxygen, as described above, without having first been treated in chlorine water, they softened and fused indicating that thermosetting was not complete.

A commercial petroleum pitch was employed to pro-duce a pitch having a mesophase content of about 54 per cent by weight. The precursor pitch had a density of '':

, , .. . . . . ..

~ ~ 9~7~

`- ~CI 5S66~ , 1~23 Mg./m.3 a softening temperature of 122C~ and con-tained 0.5 per cent by weight quinoline insolubles (Q.I.
was detenmined by quinoline extraction at 75C.). Chemical analysis showed a carbon content of 94.1%, a hydrogen con- ;
tent of 5.56%, a sulfur content of 1. 82~/o and 0.19% ash.
The mesophase pitch was produced by heating the precursor petroleum pitch under flowing nitrogen to a temperature of about 380C. at a rate of 5C./minute, maintaining the pitch at this temperature for 36 hours, and then further heating the pitch to about 430C. at a rate of 5C./minute where the temperature was maintained for 2 hours. The pitch was continuously stirred during this time and nitrogen gas was continuously bubbled through the pitch. Ater heating, the pitch exhibited a softening point of 338C. and contained 54.0 per cent by weight pyridine insolubles, indicating that the pitch had a meso-phase content of close to 54 per rent.
A portion of the pitch produced in this manner was then melt spun into fibers at a rate o 128 meters per minute through a one hole splnnerette (0.10 mm. diameter hole) at a temperature of 381C. The fibers were taken up by a reel as they left the spinnerette hole.
A portion of the spun fibers was cut into lengths 178-250 mm. long and submerged in a glass vessel filled with a saturated solution of chlorine water containing 0.02 per cent by weight of a wetting agent (Fluorad FC-423, registered trademark of the Minnesota Mining and . . , , .. , ., ... ,.. . ~, . . ,,. ~ ~ . .i ";~ .

Manufacturing Company). The chlorine water solution was prepared by slowly bubbling chlorine into water at 23C.
After soaking in the bath at 23C. for 0.5 minute, the fibers were removed, dipped into distilled water for one minute, and dried at room temperature.
A portion of the fibers treated in this manner ~;
were then heated for two minutes in a furnace maintained at a temperature of 350C. while oxygen was continuously passed through the furnace. The resulting fibers were sufficiently thermoset to be heated at elevated tempera-tures without sagging.
The in~uslble ~ibers were carbonized under nitrogen by first heating to a temperature o 925C. at a rate of 15C./minute, and then at 1750C. for five minutes. The resulting fibers had an average tensile strength of 2.92 GPa. and an average Young's modulus of elasticity of 207 GPa~ (Tensile strength and Young's modulus are an average of 15 and 5 samples, respectively). The average filament diameter was 6.8 micrometers.

A commercial petroleum pitch was employed ~o pro-duce a pitch having a mesophase content of about 62 per cent by weight. The precursor pitch had a density of 1.23 Mg./m.3, a softening temperature o~ 122C. and contained 0.5 per cent by weight quinoline insolubles (Q.I. was detenmined by quinoline extraction at 75C.) Chemical ` ' ~55664 `

analysis showed a carbon content of 94.:L%, a hydrogen content of 5056%, a sulfur content of 1.82% and 0.19% ash.
The mesophase pitch was produced by heating the precursor petroleum pitch at a temperature of about 410C.
for about 11.8 hours under flowing nitrogen. The pitch was continuously stirred during this time and steam was continuously bubbled through the pitch. After heating, the pitch exhibited a softenting point of 353C. and con- ~
tained 62.5 per cent by weight pyridine insolubles, i.ndi- -cating that the pitch had a mesophase content of close to

6~ per cent.
A portion of the pitch produced in this manner was then melt spun into flbers at a rate o 229 meters per minute through a 128 hole spinnerette (0.10 mm. diameter holes) at a temperature of 403C. The fibers passed through a nitrogen atmosphere as they left the spinnerette and were then taken up by a reel.
A portion of the spun fibers was continuously fed through a bath containing a saturated solution of chlorine water at room temperature at a rate of 0.30 meters/minute. The residence time of the fibers in the bath was 80 seconds. The chlorine water solution was prepared by slowly bubbling chlorine into water at 23C.
After passing through the chlorine water solution, the fibars were dried by heating at 100C., and the dried fibers were heated for two minutes in a furnace maintained at a temperature of 300C. while oxygen was continuously '~

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

~ - 9474 ~!~S566gl passed through the furnace. The r~sulting fibers were then carbonized under nitrogen by continuously passing them through a first heating zone maintained at 1000C.
and then through a second zone maintained at 1650C. at a rate of 0.30 meters/minute so as to allow a residence time of 1 minute in each zone. ~he resulting fibers had an average tensile strength o 1.57 GPa. and an average J`
Young's modulus of elasticity of 290 GPa. (Tensile strength was determined on epoxy impregnated strands 2.5 cm. long and is the average of 10 samples. Young's modulus was determined on 12.5 cm. strands and i9 the average o~ 2 samples.) '~he average fiber diameter was 10 micro-meters.
When the spun fibers were processed in the same manner through a bath containing a solution of 0.5 per cent by weight of bromine in water at a rate of 0.30 meters/
minute to allow a residence time of 30 seconds in the bath, and then dried, heated in oxygen, and carbonized, as described above, the resulting fibers had an average tensile strength of 0.34 GPa and an average Young's modulus of elasticity of 175 GPa. (Tensile strength was determined on epoxy impregnated strands 2.5 cm. long and is the average of 10 samples. Youngls modulus was detenmined -on 12.5 cm. strands and is the average o~ 2 samples.) Longer residence times than 30 seconds resulted in embrittlement and breakage of the fibers.
,,":,; "

~ 3~-.i . ~ ,

Claims (12)

°

WHAT IS CLAIMED IS:

1. In a process for producing a graphitizable carbon fiber which comprises spinning a carbonaceous fiber from a nonthixotropic carbonaceous pitch having a mesophase content of from 40 per cent by weight to 90 per cent by weight which under quiescent conditions forms a homogeneous bulk mesophase having large coalesced domains; thermosetting the spun fiber produced in this manner so as to render it infusible; and carbonizing the thermoset fiber by heating in an inert atmosphere; the improvement which comprises thermosetting the spun fiber by immersing the fiber in an aqueous chlorine solution for from 0.5 minute to 5 minutes, drying the fiber, and then heating it in an oxygen atmosphere at a temperature of at least 300°C. for from 1 minute to 5 minutes.

2. A process as in claim 1 wherein the aqueous chlorine solution has a temperature of from 10°C. to 60°.
and the fiber is immersed in the solution for from 0.5 minute to 4 minutes and heated in the oxygen atmosphere at a temperature of from 300°C. to 400°C. for from 1 minute to 4 minutes.

3. A process as in claim 2 wherein the aqueous chlorine solution has a temperature of from 20°C. to 40°C.
and the fiber is immersed in the solution for from 1 minute to 3 minutes.

4. A process as in claim 1 wherein the aqueous chlorine solution has a chlorine concentration of at least 0.2 per cent by weight.

5. A process as in claim 4 wherein the aqueous chlorine solution has a temperature of from 10°C. to 60°C.
and the fiber is immersed in the solution for from 0.5 minute to 4 minutes and heated in the oxygen atmosphere at a temperature of from 300°C. to 400°C. for from 1 minute to 4 minutes.

6. A process as in claim 5 wherein the aqueous chlorine solution has a temperature of from 20°C. to 40°C.
and the fiber is immersed in the solution for from 1 minute to 3 minutes.

7. A process as in claim 1 wherein the aqueous chlorine solution has a chlorine concentration of from 0.5 per cent by weight to 1 per cent by weight.

8. A process as in claim 7 wherein the aqueous chlorine solution has a temperature of from 10°C. to 60°C.
and the fiber is immersed in the solution for from 0.5 minute to 4 minutes and heated in the oxygen atmosphere at a temperature of from 300°C. to 400°C. for from 1 minute to 4 minutes.

9. A process as in claim 8 wherein the aqueous chlorine solution has a temperature of from 20°C. to 40°C.
and the fiber is immersed in the solution for from 1 minute to 3 minutes.

10. A process as in claim 1 wherein the aqueous chlorine solution is saturated with chlorine.

11. A process as in claim 10 wherein the aqueous chlorine solution has a temperature of from 10°C. to 60°C.
and the fiber is immersed in the solution for from 0.5 minute to 4 minutes and heated in the oxygen atmosphere at a temperature of from 300°C. to 400°C. for from 1 minute to 4 minutes.

12. A process as in claim 11 wherein the aqueous chlorine solution has a temperature of from 20°C. to 40°C.
and the fiber is immersed in the solution for from 1 minute to 3 minutes.