CA1055662A

CA1055662A - Process for producing carbon fibers from mesophase pitch

Info

Publication number: CA1055662A
Application number: CA213,674A
Authority: CA
Inventors: Edgar R. Mchenry
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1973-12-11
Filing date: 1974-11-14
Publication date: 1979-06-05
Also published as: US4026788A

Abstract

IMPROVED PROCESS FOR PRODUCING
CARBON FIBERS FROM MESOPHASE PITCH

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, pitch of a given mesophase content, suit-able for producing carbon fibers, is produced in substan-tially shorter periods of time than heretofore possible, at a given temperature, by passing an inert gas through the pitch during formation of the mesophase.

Description

; 9330 . :
~L~S56~i2 ~ BACKGROUND OF THE INVENTION
.' , ' . 1. Field of the Invention This invention relates to an ilmproved process for producing carbon fibers from pitch which 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 ~; pitch of this type wherein the mesophase content is pro-duced in substantially shorter periods of time than hereto-fore possible, at a given temperature, by passing an inert gas through the plteh during formation of the mesophase.

2. Description of the Prior Art As a result of the rapidly expanding growth of the a~rcraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and ex-traordinary combination of physical propertles. Thus ma-terials characterized by high strength and stif~ness, and at the same time of light weight, were required for use in sueh applications as the fabrication of aircraft struc-- 20 tures, re-entry vehicles, and space ve~icles, as well as ,...................................................................... .
in the preparation of marine deep-submergence pressure ves-` sels and like structures, Existing technology was incapa-. ble o supplying such materials and the search to satisfy i this need centered about the fabrication of cornposite ;~ articles.

One of the most promising materials suggested for -2~
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: use in composite form was high strength, high modulus car . . .
bon textiles, which were introduced into the market place . ..
a~ 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 pro- -duce composites having extraordinary high-strength- and :~!i ` high-modulus-to-weight ratios and other exceptional proper- ~
..... . .
~ ties. However 3 the high cost of producing the high-,..... . .
, strength, h~gh-modulus carbon textiles employed in such ~;-. ......................................................................... .
composites has been a major deterren~ to their widespread ~j 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 de-. .
- scribed in United States patent 4,005, 183, entitled '~igh Modulus, High Strength Carbon Fibers Produced .,:.; . , .
From Mesophase Pitch". Such method comprises irst spin-,.,"... .. .
'j ning a carbonaceous fiber from a carbonaceous pitch which ,, . ~ . .
'~ has been transformed, in part, to a liquid crystal or so-`~ 20 called "mesophase" state, then thermosetting the fiber so ,.~.~ ; .
produced by heating the fiber in an oxygen-containing at- ~
:......................................................................... ...
; mosphere for a time sufficient to render it infusible 3 and ; . i finally carbonizing the thermoset fiber by heating in an ~ -I~ inert atmosphere to a temperature sufficiently elevated to ...
~ remove hydrogen and other volatiles and produce a substan-,., i .
-,,, tial~y all-carbon fiber. The carbon fibers produced in . .
:..
~:i this manner have a highly oriented structure characterized ''.'``~;'' ~ 3 -:,...
,: ~,, ~
h, ....
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~L055662 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 of poly- ;
; crystalline graphite and graphitic-like properties associ-ated therewith, such as high density and low electrical resistivity. At all stages of their development from the as-drawn condltion to the graphitized state, the fibers are characterized by the presence of large oriented elongated graphitizable domains preferentially aligned parallel to the fiber axi~.
When natural or synthetic pitches having an aro-matic base are heated under quiescent condi~ions at a tem-perature of about 350C.-500C., either at constant tem-perature or with gradually increasing 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 9pheres are shown to consist of layers o~
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 another and gradually coalesce wi~h 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 wherein the transition from one oriented domain to another sometimes ' :` 1055G62 ~ occurs smoothly and continuously through gradually curving ; .1 . . .
lamellae and sometimes through more sharply curving lamellae The 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 il discontinuity in molecular alignment. ~he ultimate size -: of the oriented domains produced is dependent upon the vis-cosity, and the rate of increase of the viscosi~y, of the mesophase from which they are formed, which, in turn are dependent upon the particular pitch and the heating rate.
In certain pitches, domains having sizes in excess of two hundred microns up to ln excess of one thousand mlcrons are . , .
~ produced. In other pitches, the viscosit~ of the mesophase ; is such that only limited coalescence and structural rear-rangement of layers occur, so that the ultimate domain size ,'r does not exceed one hundred microns.
. The highly oriented, optically anisotropic, in-soluble material produced by treating pitches in this man-ner has been given the term "mesophase", and pitchs contain-ing such ma~erial are known as "mesophase pltches". Such pitches, when heated above their softening points, are mix-i tures of two assentially immiscible liquids, one the op--; tically anisotropic, oriented mesophase portion, and the i other the isotropic non-mesophase portion. The term ::, ~ "mesophase" is derived from the ~reek "mesos" or "inter-.,.
rnediate" and indicates the pseudo-crystalline nature of this highly-oriented, optically anisotropic material.
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, l~SS66 ~ i Carbonaceous pitches having a mesophase content f "
o~ from about 40 per cent by weight to about 90 per cent by weight are suitable for spinning into fi.bers which can sub-~, ~; sequently be converted by heat treatment into carbon fl-; bers having a high Young's modulus of elasticity and high tensile strength. In order to obtain the desired fibers .
from such pitch, however, it is not only necessary that such amount of mesophase be present, but also that it form, under quiescent conditions, a homogeneous bulk mesophase having large coalesced domains, l.e., domains of aligned molecules in e~cess of two hundred microns up to in excess o one tho~sand microns in size. Pitches whlch form stringy bulk mesophase under quiescent conditions, having small oriented ;,.. .
domains, rather than large coalesced domains, are unsuit-able. Such pitches form mesophase havlng a high viscosity which undergoes only limited coalescense, insufficient to :, .
produce large coalesced domains having sizes in excess of , .. .
two hundred microns. Instead, small oriented domains of mesophase agglomerate to produce clumps or stringy masses l 20 wherein the ultimate domain size does not exceed one hundred .~. microns. Certain pitches which polymerize very rapidly are ;
1 of this type. Likewise, pitches which do not orm a homo .;
~,.
` geneous bulk mesophase are unsuitable. The latter phenome-non is caused by the presence of infusible solids (which . are either present in the original pitch or which dlevelop ~ on heating) which are enveloped by the coalescing mlesophase .`~ and serve to interrupt the homogeneity and uniformity o ., ~.
r --6--~!' `"'":`.,'.. , , ' " ' ' '. ",.,: ,.,: i , "; ," . . : .

, ...
~SS66Z ~' the coalesced domains, and the boundaries between them.
Another requirement is ~hat the pitch be non-thixotropic under the conditions employed in the spinning of the pitch into fibers, i.e., it must exhibit a non-thixotropic flow behavior so that the flow is uniform and well 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 therefrom. Pitches, on the other hand, ! 10 which do not exhibit nonthixotroplc flow behavior at , . . . .
the temperature of spinning, do not permit uni~orm iber~ to be spun therefrom which can be converted by further heat treatment into carbon fibe~s having a high Young's modulus of elasticity and high tensile strength.
` Carbonaceous pitches having a mesophase content ~;l of from about 40 per cent by we~ght to about 90 per cent by weight can be produced in accordance with known tech-niques, as disclosed in aforementioned Unlted States patent 4,005,183 by heating a carbonaceous pitch in an inert atmosphere at a temperature above about 350C.
for a time sufficient to produce the desired quantity of mesophase. By an inert atmosphere is meant an atmosphere ~x¦ which does not react with the pitch under the heating condi-... .
tions employed, such as nitrogen, argon, xenon, helium, and the like. The heating period required to produce the de-sired mesophase content varie~ with the particulclr pitch and temperature employed, with longer heating periods required at ' :.,~ ' .

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i 9330 ~5662 lower temperatures than at higher temperatures. At 350C., the minimum temperature generally required to produce meso-phase, at least one week of heating is usually necessary to produce a mesophase content of a~out 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 generally employed for this reason. Temperatures above about 500C. are un-desirable, and heating at this temperature should not be em-'.:
ployed for more than about 5 minutes to avoid conversion r o the pitch to coke.
,.: ;
Although the time required to produce a mesophase ~
,. . . .
~ pitch having a given mesophase content is reduced as the tem-,' : t , perature of preparation rises, it has been found that heat-i. .
ing at elevated temperatures adversely affects the rheologi- ;~
~; cal properties of the pitch by alte~ ng the molecular weight .. ;
distribution of both the mesophase and ~on-mesophase por-tions of the pitch. Thus, heating at elevated temperatures ~; 20 tends to increase the amount of high molecular weight mole-cules in the mesophase portion of the pitch. At the same time, heating at such temperatures also results in an in-,~;" .
; creased amount of low molecular weight molecules in the non-mesophase portion of the pitch. As a result, mesophase ~ `
~; pitches of a given mesophase content prepared at elevated . , temperatures in relatively short periods of time have been i found to have a higher average molecular weight in the . ..................................................................... .
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mesophase portion of the pitch and a lower average molecu-lar weight in the non-mesophase portion of the pitch, than mesophase pltches of like mesophase content: prepared at more moderate temperatures over more extended periods. This :~
wider molecular weight distribution has been found to have an adverse effect on the rheology and spinnability of the pitch, evidently because of a low degree of compatibility between the very high molecular weight fraction of the .
mesophase portion of the pitch and the very low molecular weight fraction of the non-mesophase portion of the pitch. ~

The very high molecular weight material in the mesophase ~
por~lon o~ ~he pitch can only be adequately plasticized at very high temperatures where the tendency of the very low molecular weight molecules in the non-mesophase portion of the pitch to volatilize is greatly increased. As a result, when such pitches are heated to a temperature where they have a viscosity suitable for spinning and attempts are made to produce ~ibers therefrom, excessive expulsion of volatile9 occurs which greatly ~n~er~eres with the proce~ss- -ability of the pitch into fibers of small and uniform diameter. For these reasons, means have been sought for shortening the time required to produce mesophase pitch at relatively moderate temperatures of preparation where more favorable rheological properties are imparted to the pitch.

SUMMARY OF THE INVENTION

In accordance with the present inventlon, lt has . .

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now been discovered that mesophase pitch of a given meso-phase content can be prepared in substantially shorter periods of time than heretofore possible9 at a given tem~ ~-perature 3 if an inert gas is passed through the pitch dur-ing formation of the mesophase. Treating the pitch with an inert gas in this manner aids in the removal of vola-tile low molecular weight components initially present, together with low molecular weight polymerization by-products of the pitch, and results in the more efficient conversion of the precursor pitch to mesophase pitch. Mesophase pitches having a mesophase content of from about 40 per cent by weight to about 90 per cent by weight can be pre-pared in this manner, at a given temperature, at a rate of up to more than twice as fast as that normally requLred in the absence of such treatment, i.e., in periods of time as little as less than one-half of that normally required when mesophase is produced without an inert gas being passed through the pitch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As a carbonaceous pitch is heated to a tempera-ture sufficiently elevated to produce mesophase, the more volatile low molecular weight molecules present therein are slowly volatilized rom the pitch. As heating is continued above a temperature at which mesophase is produced9 the more reactive higher molecular weight molecules polymerize to orm still higher molecular weight molecules, which then ; ' ~ ' .

- 10 - ~ .

~ . , ~(~55662 orient themselves to form mesophase. While the less reac-tive lower molecular weight molecules which have not been volatilized can also polymerize, they often form hydrogenated and/or substituted polymerization by-products having a molecular weight below about 600 which do not orient to form mesophase. Although these low molecular weight poly-merization by-products are gradually volatilized as heating o~ the pitch is continued, the presence of large amounts of these by-products during much of the time that the pitch is being converted to mesophase has been found to impede the formation of mesophase by the more reactlve molecules, and, as a result, to considerably lengthen the time neces-sary to produce a pitch o~ a given mesophase content. Fur-ther, because of their small size and low aromaticity, these polymerization by-products are not readily compatible with the larger, higher molecular weight, more aromatic molecules present ln the mesophase portion of the pitch, and the lack of compatibility between these high and low molecu-lar weight molecules adversely affects the rheology and spinnability of the pitch. As pointed out previously, the very high molecular weight fraction of the mesophase portion of the pitch can only be adequately plasticlzed at very high temperatures where the tendency of the very low molecular weight molecules in the non-mesophase portion of the pitch to volatilize is greatly increased, and when pitches hav-ing large amounts of such materlals are heated to a tempera-ture where they have a viscosity suitable for spinning and .
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~C~55662 attempts are made to produce fibers therefrom, excessive expulsion of volatiles occurs which greatly interferes with the processability of the pitch into fibers of small and uniform diameter.
This invention takes advantage of the differences in molecular weight and volatility between the mesophase-forming molecules present in the pitch and those low molecular weight components and polymerization by-products which do not form mesophase to effect removal of the un-desirable more volatile low molecular weight materials and more rapidly convert the pitch to mesophase. The molecules which do not convert to mesophase are of lower molecular weight than the higher molecular weight mesophase-forming molecules and, facilitated by the inert gas purge during conversion of the pitch to mesophase, are preferentially :
volatilized from the pitch during formation of the meso-phase, allowing the pitch to obtain a given mesophase con;
tent in substantially reduced periods o~ time. Thus, in addition to shortening the time required to produce a pitch of a given mesophase content, this procedure has the effect of lessening the amounk of low molecular weight molecules in the non-mesophase portion of the pitch and raising the average molecular weight thereof. Consequently, such pitches can more easily be spun into fibers of small and uniform diameter with little evolution of volatiles.
Removal of the more volatile components of the pitch which do not convert to mesophase is effected by , :
:.................................... , . . , ,:
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passing an inert gas through the pitch, during preparation of the mesophase, at a rate of at least O.S sc~h. per pound of pitch, preferably at a rate of 0.7 sc~h. to 5.0 scfh. per pound of pitch. Any inert gas which does not react with the pitch under the heating conditions employed can be used to facilitate removal of these components. Illustrative of such gase9 are nitrogen, argon, xenon, helium, steam and ~h~ like.
As aforementioned, removal of the undesir~le more volatlle low molecular weight materials hastenc~ con-.
ver8ion o~ the pitch to mesophase, and when mesophase i3 '' produced while pas~ing an inert ~as through the pitch in this manner, the time required to produce a pitch of a given mesophase content, at a given temperature, is reduced by as much as one-half o~ that normally required in the absence o such trea~ment or even more. Genera~ly~ the time required to produce a pitch of a given mesophase con-tent is reduced by at least 25 per cent, usually from 40 per cent to 70 per cent, when the mesophase is prepared while passing an inert ga~ through the pitch as described as oppo~ed to when it is prepared under identical conditions but in the absence of such treatment.
While any temperature above about 350C. up to about 500~C. can be employed to convert the precursor pitch to mesophase, it has been found that mesophase pitches posæss improved rheological and spinning char- -acteristics when they are prepared at a temperature of from 380C. to 440C., most preferab}y from 380C.

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~ 0 5 5 6 ~ Z

to 410C., so as to produce a mesophase content of from 50 per cent by weight to 65 per cent by weight. Usually from 2 hours to 60 hours of heating are required at such tem-per~tures to produce the desired amount o~ mesophase.
Mesophase pitches prepared under these conditions have been found to possess a smallar differential between the number average molecular weights of the mesophase and non-mesophase portions of the p~tch, than mesophase pitches having the same mesophase content which have been prepared at more elevated temperatures in qhorter periods of time. ' The attendant rheological and ~pinning properties acc~mpany-ing this narrower molecular weight dlstribu~ion have bee~
found ~o substantially facilitate the processability of the pitch into fibers of small and uniform diameter. ;
The mesophase pitches prepared under the preferred conditions, i.e., by heating at a temperature of from 380C.
to 440~C. so as to produce a mesophase content of from 50 per cent by weight to 65 per cent by weight possess a lesser amount o high molecular weight molecules i~ the mesophase portion of the pitch and a lesser amount of low molecular weight molecules in the non-mesophase portion of the pitch, and have a lower number average molecular weight in the mesophase portion of the pitch and a higher number average molecular welght in the non-mesophase portion of the pitch, than mesophase pitches having the same mesophase content which ha~e been prepared at more elevated temperatures in shorter periods of time. When mesophase pitches are pre-.' .... ; ~ . . . . ...................................... . .
- , . ., ~ . . . ..

1~5S66;~
pared under such conditions, less than 50 per cent of the molecules in the mesophase portion of the pitch have a molecular weight in excess of 4000, while the remaining molecules have a nu~ber average molecular weight of rom 1400 to 2800. The molecules in the non-mesophase portion of such pitches have a number average molecular weight of from 800 to 1200, with less than 20 per cent of such mole-cules having a molecular weight of less than 600. When such pitches are prepared by heating at the most preferred tem-perature range of from 380C. to 410C., from 20 per cent ::
to 40 per cent o the molecules in the mesophase portion of the pitch have a molecular welght in excess o 4000, while the remalning molecules have a number average molecu-lar weight o from 1400 to 2600. The molecules in the non-mesophase portion of pitches prepared by heating at the most preferred temperature range have a number average molecular weight of from 900 to 1200, with from 10 per cent to 16 per cent of such molecules having a molecular weight of less than 600. When mesophase pitches are prepared at temperature9 ln exce~s of 440C., on the other hand, more than 80 per cent of the molecules in the mesophase portion of the pitch have a molecular weight in excess of 4000, while in excess of 25 per cent o the molecules in the .
non-mesophase portion of the pitch have a molecular weight of less than 600. The molecules in the non-mesophase por-tion of the pitch have a number average molecular weight of less than 800, while the number average molecular weight o . ~ . . .

~1~55/66Z

the molecules in the mesophase portion of the pitch which do not have a molecular weight in excess of 4000 is from 1400 to 2800. `
Mesophase pitches prepared by heating at a tem-perature of from 380C. to 440C. so as to produce a meso-phase content of from 50 per cent by weight to 65 per cent by weight usually exhibit a viscosity of from 10 poises to 200 poises at a temperature of from 320C. to 440C., and can readily be spun into fibers of small and uniform diame-ter at such temperatures with little evolution of volatiles.Because of thelr excellent rheological properties, such pltches are eminently suitable for spinning carbonaceous ~ibers which may subsequently be conver~ed by heat treatment into ibers having a high Young's modulus of elasticity and high tensile strength.
In order to produce pitches having the preferred mesophase content and molecular weight characteristics, it i9 usually necessary to heat a carbonaceous pitch at a tem-perature o ~rom 380C. to 440C. for at least 2 hours, preferably or ~rom 2 hours to 60 hours. Excessive heating should be avoided so as not to produce a mesophase content in excess of 65 per cent by weight, or adversely affect the desired molecular weight distribution. To obtain the de-sired molecular weight characteristics it is also necessary - that the pltch be agitated during formation of the mesophase so as to produce a homogeneous emulsion of the immiscible mesophase and non-mesophase portions of the pitch. Such , ~L~)5566;~

agitation can be effected by any conventional means, e.g., by stirring or rotation of the pitch, so long as it is suffi-cient to efEectively intermix the mesophase and non-meso-phase portions of the pitch.
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-meso-phase portion of the pitch is readily soluble in organicsolvents such as quinoline and pyridlne, while the mesophase portion is essentially in~oluble.( ) In the case o~ pitches which do not 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 corresponds essentially to the mesophase con-(2) tent. In the case ofpitches 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 no~ solely due to the conversion oE the pitch to mesophase, but also ~-represents non-mesophase insolubles which are produced along with the mesophase during the heat treatment. Pitches (1) The per cent of quinoline insoluble (Q.I.) oE a given pitch is determined by quinoline extraction at 75~C. The per cent of pyridine insolubles (P.I.) is determined by Soxh-let extraction in boiling pyridine (115C.).
(2) The insoluble content oE the untreated pitch ls gen-erally less than 1 per cent (except Eor certain coal tar pitches) and consists largely of coke and carbon black :Eound in the original pitch.

1~55~;6Z
which contain infusible non-mesophase insolubles (either present in the original pitch or developed by heating) in amounts sufficient to prevent the developmlent of homogeneous bulk mesophase are unsuitable for use in tlhe present inven~
tion, as noted above. Generally, pitches which contain in excess of about 2 per cent by weight of such infusible ma-terials 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 visually observed by polarized light microscopy examina- ;
tion of the pitch (see, e.g., Brooks, J. D , and Taylor, G. H., "~he Formation of Some Graphitizing Carbons,i' Chemistry and Ph~slcs 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 o~ Graphitizable Organic Materials, t~ Metal-lography 3, pp. 337-369, 1970) The amounts of each of these materials may also be visually estimated in this manner.
Conventional molecular weight analysis techniques, can be employed to determlne the molecular weight char-acteristics of the mesophase pitches produced ln accordance with the present invention. In order to permit molecular weight determinations to be conducted independently on both the mesophase and non-mesophase portions of the pi~ch, the two phases may be conveniently separated through the use of a suitable organic solvent. As noted above, except for certain non-mesophase insolubles present in the original .. , ,. . ~, . . . , ., . . .: . . ..
. . . .. . . .

l~SS Ei6Z
pitch or which, in some instances, develop on heating, the non-mesophase portion of the pitch is reaclily soluble in organic solv~nts such as quinoline and pyridine, while the mesophase portion is essentially insoluble.( ) After separation of the two phases with a solvent in this manner, the non-mesophase portion of the pitch may be recovered from the solvent by vacuum distillation of the solvent.
One means which has been employed to determine the number average molecular weight of the mesophase pitches produced in accordance with the present invention involves the use of a vapor phase osmometer. The utilization of instruments of this type for molecular welght determinations ;~
has been desc.ribed by A, P, Brady et al. (Brady, A, P , Huff, H., and McGain, J. W., J. Phys. & Coll. Chem 55, 304, (195L)) The osmometer measures the difference in electri-cal resistance between a sensitive reference thermistor in contact with a pure solvent, and a second thermistor in con-tact with a solution of said solvent having dissolved therein a known concentration of a material whose molecular weight is to be determined. l~e difference in electrical resistance between the two thermistors is caused by a dif-ference in temperature between the thermistors which is produced by the different vapor pressures of the solvent and the solution. By comparing this value with the dif-

(3) The non-mesophase portion of the pitch m~y be readily separated from the mesophase portion by extraction with quinoline at 75C. or by Soxhlet extraction in boillng pyridine (115C.).

9330 ~ ~
, ~, 1~556~Z
ferences in resistance obtained with said solvent and standard solutions of said solvent containimg known con-centrations of compounds of known molecular weights, it is possible to calculate the molecular weight of the solute material. A drop of pure solvent and a drop of a solution of said solvent having dissolved therein a known concen- ~`
tration of the material whose molecular weight is being determined are suspended side by side on a reference thermis-tor and sample thermistor, respectively, contained in a closed thermostated chamber saturated with solvent vapor, and the resistance o the two thermistors is measured and the dif~erence between the two recorded. Since a solution o a given solvent will always have a lower vapor pressure than the pure solvent, a differential mass transfer occurs between the two drops and the solvent vapor phase, resulting in greater overall condensation on (and less evaporation from) the solution drop than on the solvent drop. This dif-ference in mass transfer causes a temporary temperature difference between the two thermistors (due to differences in loss o heat of vaporization between the two drops) which ~s proportional to the difference in vapor pressure between ~`
the two drops. Since the difference in vapor pressure be-tween the two drops, and hence the difference in tempera-ture and resistance, (~R~, between the two thermistors de-pends solely upon the number of molecules o the solute material dissolved in the solvent, and is independent of the chemical composition of the molecules, the mole rac-~ 9330 , .
lOSSG62 tion of solute in the solution, (N), can be de~ermined from a plot of~R vs. N for such solvent and solutions of such solvent containing known concentrations of compounds of known molecular weight.~ ) a R and N bear a direct linear relationship to each other, and from a determination of N
it is possible to calculate the calibration constant, (K), for the solvent employed from the formula:

K = R

Having determined the value of K, the molecular welght of the material may be determined from the formula:

(K - ~R) . ~ W~
aR ~ Wy wherein Mk is the molecular weight o~ the material upon which the determination is being made, K is the calibra-tion constant for the solvent emplo~ed, ~R is the differ-ence in resistance between the two thermistors, ~ ls the molecular weight of the solvent, Wy is the weight of the solvent, and Wx is the weight of the material whose molecu-lar weight is being determined. Of course, having once determined the value of the calibration constant of a given solvent, (K), the molecular weight of a given material may be determined directly from the formula.
While the molecular weight of ~he soluble portion .

(4) By the mole fraction of a given material in a solu-tion, (N), is meant the number of moles of such material in thesolution divided by the number of moles of such material in the solution plus the number of moles of the solvent.

9330 ~

",~ .
~L~55662 of the pitch can be determined directly on a solution thereof, in order to determine the molecular weight of the insoluble por~ion, it is necessary that it first be solubilized, e.g., by chemical reduction of the aromatic bonds of such material with hydrogen. A suitable means for solubilizing coals and carbons by reduction of the aromatic ~`
bonds of these materials has been described by J. D. Brooks et al. (Brooks, J. D., and Silberman, H., "The Chemical Reduction of Some Cokes and Chars", Fuel 41, pp. 67-69, 1962). This method involves the use of hydrogen generated by the reaction of lithium with ethylenediamine, and has been found to effectively reduce the aromatic bonds of carbona-ceous materials wit~out rupturing carbon-carbon bonds. Such method has been suitably employed to solubilize the in-soluble portion of the pitches prepared in accordance with the invention.
Another means which has been employed to de-termine the molecular weight characteristics of the meso-phase pitches produced in accordance with the present inven-tion i9 gel permeation chromatography (GPC). This tech-nique has been described by L. R. Snyder ~Snyder, L. R., "Determination of Asphalt Molecular Weight Distributions by Gel Permeation Chromatography", Anal. Chem 41, pp.
1223-1227, 1969). A gel permeation chromatograph is em-ployed to fractionate a solution of polymer or polymer re-lated molecules of various sizes, and the molecular weight distribution of the sample is determined with the aid of . , ~ . :, ~ , . .. .

` 9330 ~S5~;62 a detection system which is linearly responsive to solute concentration, such as a differential refractometer or a differential ultraviolet absorption spectrometer. As in ;
the case of the vapor phase osmometry technique, in order to permit molecular weight determinations to be conducted independently on both the mesophase and non-mesophase por-tions of the pitch, the two phases must first be separated through the use of a suitable organic solvent. Again, while the molecular weight of the soluble portion of the pitch can be determined directly on a solution thereof, in order to determine the molecular weight of the insoluble portion, it is necessary that it flrst be solubilized.
Fractionatlon of the sample whose molecular weight distribution ~s being determined is effected by dissolving the sample in a suitable solvent and passing the solution through the chromatograph and collecting measured fractions of the solution which elute through the separation column of the chromatograph. A given volume of solvent is required to pass molecules of a given molecular size through the chromato~raph, so that each fraction of solution which elutes from the chromatograph contains molecules of a given molecular size. The fractions which flow through the column first contain the higher molecular weight molecules, while the fractlons which take the longest time to elute through the column contain the lower molecular weight molecules.
After the sample has been fractionated, the con-:, centration of solute in each fraction is determined by means o~ a suitable detection system, such as a differential refractometer or a differential ultraviolet absorption spectrometer. When a differential refractometer is employed, the refractive index of each fraction is au~omatically com-pared to that of the pure solvent by means of two photo-electric cells which are sensitive to the intensity of light passing through such frac~ions and solvent, and the di~fer-ences in signal intensities between the two cells are auto-matically plotted against the cumulative elution volume of the solution. Since the magnitude o~ these differences in signal lntensity is linearly related to the concentration by weight of solute molecules present, the rela~ive concen-tration by weight of molecules in each fraction can be de-termined by dividing the differential signal intensity ~or that ~raction by the total integrated differential signal intensity of all the fractions. This relative concentration may be graphically depicted by a plot of the diferential signal intensi~y for each fraction against the cumulative elution volume of the sample.
The molecular weight of the molecules in each fraction can then be determined by standard techniques, e.g., by the osmometry techniques described above. Since most conventional pitches are composed of similar types of molecular species, once the molecular weights of the vari-ous fractions o~ a particular sample have been determined, that sample may be used as a standard and the molecular 9330 ;

~ 0 556 ~Z
weights of the fractions of subsequent samples can be de-termîned from the known molecular weights of like fractions of the standard. Thus, molecular weight determinations need not be repeatedly made on each fraction of each sam-ple, but may be obtained from the moleeular weights de-termined for like fractions of the standard. For con-venience, a molecular weight distribution curve depicting the relationship of the molecular weight to the elution volume of the standard may be prepared by plotting the molecular welghts determîned for the standard fractions against the cumulative elution volume of the standard. The molecular weights of the molecules of the various chromato-graphlc fractions o~ any glven sample can then be directly read from this curve. As aforementioned, the relative con-centration by weight of solute molecules in each fraction can be determined by differential refractive inde~ measure-ments.
To facilitate the molecular weight determinations J
the differential signal intensities and elu~ion vol~e values obtained on a given sample, together with previously determined molecular weight data relating to the various chromatographic fractions of a standard pitch, can be pro-cessed by a computer and transcribed into a complete molecu-lar weight distribution analysis. By this procedure, com-plete printouts are routinely provided of number average molecular weight (Mn)~ weight average molecular weight (Mw), ~ i molecular weight distributlon parameter (MW/Mn), as well ~ 9330 .
~LOS5662 as a compilation of molecular weight and percentage by weight of solute present in each chromatographic fraction of a sample.
Aromatic base car~onaceous pitches having a car-bon 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 employed to produce fibers capable of being heat treated to produce fibers having a high Young's modulus of elas ticity and a high tensile streng~h. Elements other than carbon and hydrogen, such as oxygen, sul~ur and nitrogen, are undesirable and should not be pre~ent in excess of about 4 per cent by weight. The presence of more than such amount of extraneous elements may disrupt the formation of carbon ;
crystallites during subsequent heat treatment and prevent the development of a graphltic-like structure within the fibers produced from these materials. In addition, the pre~ence of extraneous elements reduces the carbon content ~:
o the pitch and hence the ultlmate yield of carbon fiber.
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, which are well-graphitizing pitches, are preferred starting materials for producing the mesophase pitches ... . .

~5566Z

which are employed to produce the fibers of the instant in-vention. Petroleum pitch, of course, is the residuum carbonaceous material obtained from the distillation of crude oils or the catalytic cracking of petroleum distil-lates. Coal tar pitch is similarly obtained by the dis-tillation of coal. Both of these materials are commer-cially 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 preferred because of its ability to pro-duce excellent fibers. Acenaphthylene pitch can be pro-duced by the pyrolysis of polymers of acenaphthylene as described by Edstrom et al. in U. S Pa~ent 3,574,653.
Some pitches, such as fluoranthene pitch, poly-merize very rapidly when heated and fail to develop large coalesced domains of mesophase, and are, therefore, not suitable precursor materials. Likewise, pitches having a high infusible non-mesophase insoluble content in organic solvents such as quinoline or pyridine, or thosè whlch de-velop 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 hom~geneous bulk mesophase necessary to pro-duce highly oriented carbonaceous fibers capable of being converted by heat treatment into carbon fibers having a high Young's modulus of elasticity and high tensile strength. For this reason, pitches having an infusible . . ~ . .

: .'' ''.
~ ~ S S 6 6Z

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 be filtered to remove this mater~al before being heated to produce meso-phase. Preferably, such pitches are filtPred when they contain more than about 1 per cent by weight of such in- ~ ~
fusible, 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.
A~ the pitch ls 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 composltion and structure of the starting material. Generally, however, after heating a carbonaceous pitch for a ti~e suffieient ~o produce a meso-phase content oE from about 40 per cent by weight to about 9Q per cent by weight, the resulting pitch will contain a carbon content of from about 94-96 per cent by weight and a hydrogen content of from about 4-6 per cent by wPight.
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 ., .. , -............... , , : .. .. :- . . ~ .

--~ 9330 ~ ~ 556 ~2 cent by weight, the balance being hydrogen.
After the desired mesophase pitch has been pre-pared, it is spun into fibers by conventional techniques 3 e.g., by melt spinning, centrifugal spinning, blow spin-ning, or in any other known manner. As noted above, in order to obtain highly oriented carbonaceous fibers capable of being heat treated to produce carbon fibers having a high Young's modulus of elasticity and high tensile strength, the pitch must, under quiescent condi~ions, form a homogeneous bulk mesophase having large coalesced do-mains, and be nonthixotropic under the conditions employed ~n the spinning. Further, in order to obtain uni~orm Ei-bers from such pitch, the pitch should be agitated l~nedi-ately prior to spinning so as to effectively intermix the immiscible mesophase and non-mesophase portions of the pitch.
The temperature at which the p~tch is spun de-pends, of course, upon the temperature at which the pitch exhlbits a suitable viscosity. Since the sotening tem-perature of the pitch, and i~s viscoslty at a given tem-perature, increases as the mesophase content of the pitch increases, the mesophase content should not be permitted to rise to a point which raises the softening point of the pitch to excessive levels. For this reason, pitches having a mesophase content of more than about 90 per cent are generally not employed. Pitches containing a meso- ;
phase content of about 40 per cent by weight usually have ~ 9330 .
l~SS6~:2 a viscosity of about 200 poises at about 300C. and about 10 poises at about 375C., while pitches containing a meso-phase content of about 90 per cent by weight exhibit similar viscosities at temperatures above 430C. Within this viscosity range, fibers may be conveniently spun from such pitches at a rate of from about 50 feet per min~lte to about 1000 feet per minute and even up to about 3000 feet per minute. Preferably, the pitch employed has a mesophase content of from about 50 per cent by weight to about 65 per cent by weight and exhibits a viscosity of `
rom about 30 poises to about lS0 poises at temperatures of from about 340C. to about 380C. At such viscosity and temperature, unifonm flbers having diameters of from about 5 microns to about 25 microns c~n be easily spun.
As previously mentioned, however, in order to obtain ~he desired fibers, it is important that the pitch exhibit nonthixotropic flow behavior during the spinning of the fibers.
The carbonaceous fibers produced in thls man-ner are highly oriented graphitizable materials having a high degree of preferred orientation of their molecules parallel to the fiber axis. By "graphitizable" is meant that these fibers are capable of being converted thermally (usually by heating to a temperature in excess of about 2500C., e.g., from about 2500C. to about 3000C.) to a structure having the three-dimensiona~ order character-istic o~ polycrystalline graphite.

. . . . . . . . . . . . .
. . . . .

~ ~ S 56 6 ~

The fibers produced in this manner, of course, have the same chemical composition as the pitch from which they were drawn, and like such pitch contain from about 40 per cent by weight to about 90 per cent by weight meso~
phase. When examined under magnification by polarized light microscopy techniques, the fibers exhibit textural varia-tions which gi~e them the appearance of a "mini-composite".
Large elongated anisotropic domains, having a fibrillar-shaped appearance, can be seen distributed throughout the fiber. These anisotropic domains are highly oriented and pr~ferentially aligned parallel to the flber axis. It i8 belleved that ~hese anisotroplc domains, whlch are elongated by the shear forces exerted on the pitch during spinn~ng of the fibers, are not composed entirely of meso-phase, but are also made up of non-mesophase. Evidently, the non-mesophase Is or~ented, as well as drawn into elongated domains, during spinning by these shear forces and the orien~ing effects exerted by the mesophase doma-lns as ~hey are elongated. Isotropic regions may also be present, although they may not be visible and are diffi-cult to differentiate from those anisotropic regions which happen to show extinction. Characteristically, ~he ori-ented elongated domains have diameters in excess of 5000 A, generally from about 10,000 A ~o about 40,000 A, and be-cause of thelr large size are easily observed when examined by conventional polarized light microscopy techniques at a magnification of 1000. (The maxlmum resolving power :.

~ ~ 5566 2 of a standard polarized light microscope having a magnifica-tion factor of 1000 is only a few tenths of a micron [1 mi-cron - 10,000 A] and anisotropic domains having dimensions of 1000 A or less cannot be detected by this technique.) While fibers spun from a pitch containing in ex- -cess of about 85 per cent by weight mesophase often re-tain their shape when carbonized without any prior thermo-setting, fibers spun from a pitch containing less than about 85 per cent by weight mesophase require some thermo-setting before they can be carbonized Thermosetting o~ ~-the ibers iR re~dily effected by heating the ibers ln an oxygen-cont~ining atmosphere ~or a time sufficient to render them lnfusible. The ox~gen-containlng atmosphere employed may be pure oxygen or an oxygen-rich atmosphere.
Most conveniently, air is employed as the oxidizing at-mosphere.
The time required to efect thermosetting of the ibers will, of course, vary with such factors as the partlcular oxid~zlng atmosphere, ~he temperature employed, the diameter of the fibers, the particular pitch from which the fibers are prepared, and the mesophase content of such p~tch. Generally, however, thermosetting of the fibers can be e~fected in relatively short periods of time, usually in from about 5 minutes to about 60 minutes.
The temperature employed to effect~thermosetting of the fibers must, of course, not exceed the temperature at which the fibers will soften or distort. The maximum .. . . . . .

g330 , ~ .
~556~;~

temperature which can be employed will thus depend upon the particular pitch from which the fibers were spun, and the mesophase content of such pîtch. The higher the mesophase content of the pitch, the higher wlll be its softening temperature, and the higher the temperature which can be employed to effect thermosetting of the fibers. At higher temperatures, of course~ fibers of a given diameter can be ~ j thermoset in less time than is possible at lower tempera-tures. Fibers prepared from a pitch having a lower meso-phase content, on the other hand, require relatively longer heat treatment at somewhat lower temperatures to render them inf~slble.
A minimum temperature o at least 250C. is gen-erally necessary to effectively thermoset the carbonaceous fibers produced in accordance with the invention. Tempera-tures in excess of 400C. may cause melting and/or exces-sive burn-off of the ibers and should be avoided. Prefer-ably, temperatures of from about 275C. to about 350C. are employed. At 9uch temperatures, ~hermosetting can gen-erally be effected within from about S minutes to about 60 minutes. Since it is undesirable to oxidize the fibers more than necessary to render them totally infusible, thè
fibers are generally not heated for longer than about 60 minutes, or at temperatures in excess of 400C.
After the fibers have been thermoset, the in-fusible fibers are carbonized by heating in an inert at-mosphere, such as that described above, to a temperature 9330 ~

~05566Z -:

sufficiently elevated to remove hydrogen and other volatiles and produce a substantially all-carbon fiber. Fibers hav-ing a carbon content greater than about 98 per cent by weight can generally be produced by heating to a tempera-~ure in excess of about 1000C., and at temperatures in excess of about 1500C., the fibers are completely car-bonized.
Usually, carbonization is effected at a tempera-ture of from about 1000C. to about 2000C., preferably rom about 1500C. to about 1900C. Generally, residence times o~ from about 0.5 minute to about 25 minutes, prefer-ably from about 1 minute to about 5 minutes, are employed.
Whlle more extended heating times can be employed with good results, such residence times are uneconomical and, as a practical matter, there is no advantage in employing such long periods. ~ i In order to ensure that the rate of weight loss of the fibers does not become so excessive as to disrupt the fiber structure, it is preferred to heat the fibers for a brie~ period at a temperature of from about 700C. to about 900C. before they are heated to their final car-bonization temperature. Residence times at these tempera-tures of from about 30 seconds to about 5 minutes are usually sufficient. Preferably, the fibers are heated at a temperature of about 700C. for about one-half minute and then at a temperature of about 900C. for like time. In any event, the heating rate must be controlled so that the -34- ;

9 3 3 o volatization does not proceed at an excessive rate.
In a preferred method of heat treatment, con-tinuous fibers are passed through a series of heating zones which are held at successively higher temperatures.
If desired, the first o such æones may contain an oxidiz-ing atmosphere where thermosetting of the fibers is effected Several arrangements of apparatus can be utilized in pro-viding the series of heating zones. Thus, one furnace can be used with the fibers being passed through the fur-nace several times and with the temperature being increaset each time. Alternatively, the fibers may be given a single pa8s through several fu~naces, with each successive furnace bein~ main~ained at a hlgher ~emperature than that o the previous furnace. Also, a single furnace with several heating zones maintained at successively higher temper-atures in the direction of tra~el of the fibers, can be used.
The carbon fibers produced in this manner have a highly oriented structure characterized by the presence of carbon crystallites preferentially aligned parallel to the fiber ~is, and are graphitizable materials which when heated to graphitizing temperatures develop the three-di-mensional order characteristic of polycrystalline graphite and graphitic-like properties associated therewith, such as high density and low electrical resistivity.
If desired, the carbonized ibers may be further heated in an inert at~osphere, as described hereinbe~ore, ~-to a still higher temperature in a range of from about ~ -35-.~'' . ~ .
~05566Z

2500C. to about 3300C., preferably from about 2800C. to about 3000C., to produce fibers having not only a high degree of preferred orientation of ~heir carbon crystal-lites parallel to the fiber axis, but also a structure characteristic of polycrystalline graphite. A residence 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 times longer than 5 minutes are uneconomical and unnecessary, but may be employed if desired.
The fibers produced by heating at a tempera~ure above about 2500C., preferably above about 2800C., are characterized as having the three-dimensional order of poly-crystalline graphite. This three-dimensional order is es-tablished 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 (00~) bands of the pattern show ~he carbon crystallites of the ibers to be preferentially aligned parallel to the fiber axis. Microdensitometer scanning of the (002) band of the exposed X-ray film indicate this preferred orienta-tion to be no more than about 10, usually from about 5 to about 10~ (expressed as the full width at half maximum of the azimuthal intensity distribution). Apparent layer si~e (La) and apparent stack height (Lc) of ~he crys~al-lites arein excess of 1000 A and are thus too large to be ~C~55662 measured by X-ray techniques. The interlayer spacing (d) of the crystallites, calculated from the distance between the corresponding (oOR) diffraction arcs, is no more than O O O
3.37 A, usually from 3.36 A to 3.37 A.

EXAMPLE

T~e following example is set forth for purposes of illustration so that those skilled in the art may better understand the invention. It should be understood that it ls exemplary only, and should not be construed as limit ing the invention in any manner.

A commercial petroleum pitch was employed to produce a pitch having a mesophase content of about 53 per ; -~cent by weight. The precursor pitch had a number average molecular weight of 400, a density of 1.23 grams/cc., a softening temperature of 120C., and contained 0.83 per cent by weight quinoline insolubles (Q.I. was determined by quinoline extraction at 75C.). Chemical analysis showed a carbon content of 93.0%, a hydrogen content of S.6%, a sulfur content of 1.1% and 0.044% ash.
The mesophase pitch was produced by heating 60 grams o the precursor pitch in a 86 cc. reactor to a temperature of about 200C. over a one hour period, then increasing the temperature of the pitch from about 200C.
to about 400C. at a rate of about 30C. per hour, and main-~ 9330 :~S5662 taining the pitch at àbout 400C. for an additional 12 hours. The pitch was continuously stirred during this time and nitrogen gas was continuously bubbled through the pitch at a rate of 0.2 scfh.
The pitch produced in this mamner had a pyridine insoluble content of 53 per cent, indicati~g a mesophase content of close to 53 per cent. The pitch could be easily spun into fibers, and a considerable quantity of flber was ~
produced by spinning the pitch through a spinnerette -(0.015 inch diameter hole) at a temperature of 368C. The' fiber passed through a ni~rogen atmosphere as it left the ~pinnerette and beore it was taken up by a reel.
A portion o~ the ~iber produced in this manner was heated in oxygen for six minutes at 390C. The re-sulting oxidized fibers were totally infusible and could be heated at elevated temperatures without sagging. After heating the infusible fibers to 1900C. over a period of about 10 minutes in a nitrogen atmosphere, the fibers were found to have a tensile strength of 171 x 103 psi. and a Young's modulus of elasticity o~ 46 x 106 psi. (Ten-sile strength a~d Young's modulus are the average values o~ 10 samples.) For comparative purposes, a mesophase pitch was prepared fr~m the same precursor pitch and in the same manner described above except that while the pitch was pre- ;
pared under a nitrogen a~mosphere, the nitrogen was not allowed to bubble through the pitch. Thirty-two hours ~ S ~ 66 ~

of heating at 400C. were required to produce a mesophase pitch having a pyridine insoluble content of 50 per cent.

-39- .

'` ' ", " .' ' ' , , ' , ' ' ,~

Claims

WHAT IS CLAIMED IS:

1. In a process for producing a high-modulus, high-strength carbon fiber which comprises spinning a car-bonaceous fiber from a nonthixotropic carbonaceous meso-phase pitch which under quiescent conditions forms a homo-geneous bulk mesophase having large coalesced domains, thermosetting the fiber so produced by heating the fiber in an oxygen-containing atmosphere for a time sufficient to render it infusible, and carbonizing the thermoset fiber by heating it in an inert atmosphere, the improve-ment which comprises spinning the carbonaceous fiber from a mesophase pitch which has been prepared while passing an inert gas through the pitch during formation of the mesophase at a rate of at least 0.5 scfh. per pound of pitch while heating the pitch in an inert atmosphere for a time sufficient to produce a mesophase content of from 40 per cent by weight to 90 per cent by weight.

2. A process as in claim 1 wherein the inert gas is passed through the pitch at a rate of 0.7 scfh.
to 5.0 scfh. per pound of pitch.