CA1324468C - Balanced ultra-high modulus and high tensile strength carbon fibers - Google Patents

Abstract

TITLE
BALANCED ULTRA-HIGH MODULUS
AND HIGH TENSILE STRENGTH CARBON FIBERS

ABSTRACT OF THE DISCLOSURE
Carbon fibers having balanced properties comprising an ultra-high modulus greater than 100 Mpsi and a high tensile strength of greater that 500 kpsi. The carbon fibers are derived from a solvent fractionated pitch having a mesophase content of greater than 90% by weight and a quinoline insolubles content of less than 1% by weight. A crude pitch feed is sequentially heat soaked, solvent fractionated, and extruded to form as-spun fibers. The latter are stabilized or infusibilized and then carbonized by multistage heat treatments involving separate precarbonization, carbonization, and graphitization steps with an intermediate cooling step.

Description

TI ~LE
~ALANCED ULTRA-~IIG~ ~ODUI.US
AND E~IGI~ TENSILE STRISNGT~ CARBON ~IBERS
BAC~GROVND OF THE INVENTION
The present invention relat~s to a carbon fiber product having b~lanc~d ultra-h~gh Young'~
~odulu~ and high ten~ile strengtb propertle~ More particularly, the invention p~rtains to carbon fibers having a modulu~ of great~r than 100 Mp~i ...
and a t~n~ tr~ngth of at l~a~t 500 ~p8i d~rived from a ~ol~nt fractlonated, m~opha~ piteh havlng a lou quinolln- ln~oluble~ content ~h- lnv-ntion i~ al~o conc~rned ~ith thc proce~ for prsiparlng rueh ultra-high modulut and hlgh t~nr$1~ rtr~ngth pitch carbon flb-rr In r~e~nt y-ar~ technlcal lit~ratur~ and pat~nte are repl~te ~ith dlrelorure~ of carbon flberr pr~pared from polyaerylonitrlle and earbonac ou- plteb ~roe~ r utll~lng polyaerylonltrlle and proe~rr~r utlll~lng a pltch fraetlon, 1 c ~ropha~ plteh, a~ preeurror~ ar~
ln eo~a-reial produetlon Th- u~ of ~ rophar~
plteh ar the fe~d ~at~rlal ~ould hav~ nu~rou~
advantage~, ~lnee polyaerylonltrll- ~PAN) flber~
r-qulr- expen~l~a and eo-plleat-d proeedur-r, lneludlng th- handllnq of toxle gar by-produet~
~lg~ Young'~ ~odulur i~ one of th-err-ntlal eharaeterlrtler of eo-~erelal earbon f~ber~ ne- blgh rtiffn-rr ~r an l~portant eonrld-rat~on ~h-n th- carbon flberr ar~ ~mploy~d ln th- re~nforeement of poly~er, ~tal, and other ~ ``
matrlee- to pro~ld- advane-d t-ehnology eompo~lt~
Con~-qu-ntly, th-r- ha- been eon~ld-rabl~ r~i~-areh on method~ for nhanelng modulur ~-onard S
Sing-r ~u~narl~-d th~ rtat- of th- prlor art ~L :.`.,, ~F , " ~
_ _ .:-.
` `' in his article entitled ~Carbon Fiber~ from Me~ophase Pitch" published in Fue~ Vol 60 (1981, September) pp 839-847 Whil~ PAN b~ed fiber~ provid~ h~gh 5 tensile strengths at low ~odulu~ (30 to 40 Mp~i ), attaining high ~odulu~ PAN carbon f~b~r~ ha~ proven difficult For exa~ple, a 1986 r~v~w of curr~nt carbon fib~r-, report~ th~ high-~t ~odulu~ PAN
fiber has a t-nacity of 355 kps~ at ~1 ~p~l ~odulu~ (J D ~ Hugh~, Carbon, Volu~ 2~, page 551 (1986) It ~ould b- highly advantagcou~ to produce carbon fib~r~ that havc h~gh t-n~
~tr-ngth~, i - , gr-at-r than 500 ~p~l, at as ultra-h~gh ~odulu~ Such balanccd propcrt~
~ an ~xc~ nt combination of ultra-hlgh ~odulu~
and h$gh t-nr~ trcngth, ~ould bc h~ghly d~lrablc for a var~ty of co~crc~al appl~cat~on~
Prev~ou~ ~nv~t~gat~on~ to l~prov~ th-Young'~ ~odulu~ of carbon ~ber~ der~vcd ro~ p~tch hav~ involv~d th- ur- of h-at treat~ nt t-~p-ratur~r ranglng fro- 1500' to 3000-C O
cour~c, graph~t~at~on ~ncr-a~e~ at the hlgh-r tc~pcratur~
Fl~eh-r ~nd Ruland ln ~Thc ~n1u-ncc of Graph~t~rat~on on the M~chan~cal ~ropert~-~ of Carbon Flber~, Collold and ~oly~er 8e~encc~, vol 250, No 0, pp 917 to 920 (1980) r~portcd that graph1tl~at~on ha~ an unfavorabl~ ffcct on th~
~eh~nleal prop-rt~ nclud~ng tcnrll- ~tr-ngth, of carbon fib-rc Ng ~t al ~n ~xt~nd-d Abrtr~ct~
of th- 16th Bi-nn~al Conf-r-nc- on Carbon~ Aa Ch-~ Soc , pp 515-516 (19~3) tat~d that hlgh ~odulur earbon flb-r# ~pun roa n-~opha~c pltch pcrfora-d 1~ ati~factorily than PAN-ba~cd fib~r A 1986 pap~r by Gulgon and Ob-rlln, Co~po-it-~ Scl~nc- and T~chnology, 25 ~1986) pp 231-241 r~v~al~ that th~ Younq'~ modulu~ of '''''.' '`

.. . _.. `

~3~ 1 324468 mesophase pitch based carbon fibers increa~s with --the average degree of graphitization However, page 240 reports that the ten~ile ~trength i~
always low The graphitization of polyacrylonitrile f~ber~ al60 affect~ adver~ely the een~ile ~trength Publi~h~d Brit$~h P~t~nt N~.
2,170,491 ~Pepper and Patton) di~clo~e~ on page 1, `~ lines 26-~0, that the ~tr~ngth and ~odulu~ of the~e carbonized fiber~ incr~a~c rapidly up to about 1400DC Beyond 1~00 C, ho~vcr, while the Young'~
~odulu~ lncrca~c~, tcncil- ~tr~ngth d~crca~-r, reportedly b~cau~e th~ rtructur- of th~ carboniz~d fib~rc b~com-~ ~or- r~pr~ntativc of truc ~raph~t~ A~ a r~sult of th-~ occurr~nc~, th~ -sr~t~h Pat~nt ~ay~ that co~-rclal PAN fib~r~ ar-u~ually off~r~d ln a carbon~d for~ ~lth lo~
~odulu~ and high ~tr-ngth or $n graph$t$~-d fora ~ith high ~odulu~ and lo~ ~tr-ngth ~`
On th~ othcr hand, Publ$~h-d ~urop-an Application 0159315 by Na~atani t al e~phaciz~ `
th~ n-~d to balancc thc ~odulur of clarticity and tcn~ilc rtrcngth prop~rti-~ of carbon fibcrr ba~d on acrylonltrllc-typc poly~cr~ Thc inv-ntor~
acco~pll~h tbi~ by ~ub~ctlng thc flbcrr to a co~plcx cycla o~ fla~--r~ tlng tr-at~cnt~, ~hlch includ~ tba pplication of ~longatlon and th-n a ~rl~r of carboni~ation tr~at~nt~ that do not ~xcaod 1600-C
R~c~nt inv-~tlgation~ (~ g U S
4,50-,~5~ - ~lgg~) to producc ~c~opha~c pitch-ba~-d carbon fibcr~ ~ith high-r t~n~ rtr~n~th~ involv~
~olv~nt fractionation trcat~cnt ~ith thc initial u~- of ~no~n organ$c ~olv-nt~ having a ~olubility para~tcr fro~ 9 ~ to 11 to ~parat- in~olubl~
and thcn tr~atlng th~ ~olution ~lth an organlc ~ 1 324468 ~olvent having a ~olubility parameter from 7 4 to 9 0 in order to recover insoluble~ that are convertible to the carbon fiber~ Although increased tensile ~trength~ were obtain~d, Young'~
moduli were substantially le~ than 100 Mp~i The 3 production of fiber~ with high ~odulur ~a~ not ; explored noth~r approach ha~ be~n to ~mploy pecial fe~d mat~rial~ On~ exampl~ ir the ~ynth~tic co~pound d~-crib~d in U S Pat-nt 4,C70,129 ~Tat~ et al ) Anoth~r iB to hydrog~nat~
~ixtur~ of coal tar or coal tar piteh and an aromatic oil, h~at the hydrog~nat~d product in th~ `
pr~nce of a crac~ing catalyrt, and th-re tr~at lS th~ ~olubl~ fraction of th~ r--ulting r~action produet to for~ ophase, a~ d~rcribed in ~ritirh Pat-nt 2,129,325 Th~e are exp~n~iv- proc~ s Con-id-rabl- ~ffort to i~prov- ~-chanical prop~rt~e~ of ~eropha~ pltch ba--d carbon flber~
ha- involv~d ~odifying the hap- of a eonvent1onal, c~rcular cro~ ection rpinn-r~t ~o that it har an enlarg-d no~ outl~t and th~r-by produe-r a ~avy ero~ -etional flb~r ~truetur~ (Japan~e publirh~d Pat-nt Applleatlon 62-~2320 Na~a~i~a t al - "
~a~hi~a Oil) ~arll-r ~ork ~ploy-d rpinn-r-tr nodifi-d in ueh a nann-r that th~y giv-lllproldal or ~ultilobal flbarr ~ith ~l~afy la~ella~ nlero~truetura Typieal pat-nt~ dir~et-d to ueh faatur-r ar~ a~ follo~r~
Japane~e Patent Applieat$on Publlcat$on ~ ``
61-27542C Ohyabu t al (Mlt~ui Cok~
U S Pat~nt 4,628,001, Sa~a~i et al ¦ (Tei~in Li~it~d) ~uropa~n ~atent Applieation Publieation 021996~ t al (Cl~mron Univ~r~ity) Spinnerets described in these references have narrow ~lots and are difficult to manufacture and maintain In the March/April 1987 i66ue of the S SAMPE Journal, pp 27-31, David A. Schulz ~n "Advance~ in UHM Carbon Fiber~ Production, Propertie~ and Applications~ conclude~ that the production of ultra-high ~odulu~ (UHM) carbon fibers fro~ pitch i6 a eompl~x proces~ ~nvolving ~any op~rationt and ~xtr~m~ eondition6 According to the author, it ~ ~noun that ultra-h~gh ~odulus earbon fiber~ d-riv-d from ~ophar~ piteh po~6r higher ery~tallinity and r-aeh bigh~r ~odulu~
l~v~l~ than fibcr~ ~ad~ fro~ other pr~eur~or~
Tabl- II of th~ art~el~ ~t~ fort~ th~ prop~rt~
of Amoco'~ UHM Thorncl P-100 f~bcr~ Thc qrand av~rag~ t~nr~le ~tr~ngth ~a~ 356 ~ ~p~l and th~
grand av~rag~ t-n~il- aodulu~ wa~ 111 Mp~l, ar ~a~ur~d by 6trand t~ting Thu~, d~pit~ ~any y-ar~ of ~xp-r~-ne-, Aaolco'r proc~r~ ha~ not l~d to l~provcd t~n~ tr-ngth via th~ ~arll~r patent dl~elo~ur~
It ~ould b- d~sirable, thereforc,` to hav~
earbon flbar~ ~hieh cxhlblt balanecd hlqh t-n-~l-~trength and ultra-high ~odulu- Mor-ov-r, lt ~ould be highly d-~ra~ to h~v- thc ability to produea ~ueh earbon fiber~ ~hilc avoldinq thc probl~ p-elal equlp~ont, and ~peeial or cxotie p~t~h~ r~qulr~d by th- prlor art ln th-~anufaetur- of ultra-hlgh ~odulu~ earbon fib~r~
SUMMI~RY OF THE tNVENT~ON
In aeeordancc ~lth tho pr-~nt inv~ntion th-r- are provid-d ~opha~- plteh-ba6~d earbon flber~ eharaet-rl~d by an ultra-high Young'~
modulu~ of gr~atcr than 100 Mp~i, prcf~rably greatcr than 110 and a balanecd t~n~ile 6trength of greater than 500 kp~i The preferred carbon fibers of the invention have an e~sentially round or circular cro~s-~ection The pitch precur~or i8 a high (> 9o%) me~ophase pitch fraction having a low quinol~n~
in601ublee cont~nt of l~s~ than about 1%, preferably 1~ than 0 3% by w~ight, ~olv~nt fractionat~d fro~ a crude pitcb fe~d that ha~ b~en pr~heated to a t~mperatur~ of from about 350 to ~xt, the ~olv~nt fractionat~d, ~gh me~ophas~ pitch fraetion is ~xtrud~d through a ~pinn~r~t h~ving convent~onal round cros~-s~ctional nozzl~ to provide a plurality of so-ealled gr-~n fib~r~ or as-spun fiber~
~hese gre-n f$berr ar~ ~tabili~-d, or ~nfuslbllired in an oxidati~ ga~ at~osph~r-, pr~earbonir-d at a t~p~ratur~ of 00-1000-C, and tben earbonired at a t-~p-ratur- of fro~ about 1000 to 2000'C Carbonir-d fibers ar~ sub~equently graphit$~d at a te~peratur- of fro~ inert 2500- to 3000-C. All but the stabiliration rtep are earri~d out in ~nert at~osphere~
D~TASL~D D~SCRSPTION OF TH~
SNV~NTSON AND T~E PREFERR~D ~M~ODSM~N~ .
St 1~ kno~n that a variety of pltehe~ ~ay be o~ployod to furnlsh hlgh ~esophas~ fraetion~
that ar- ur~ful for the preparaeion of earbon flb-r- Sueh piteh-s lnelud~ peeroleu~ pitehe~, eoal tar piteh-s, natural asphalts, pitehe~
obtained ~ a eoproduet of naphtha erae~ng, mlddl-dlstlllat- erae~lng, gas oil erae~lng, and fraetion~ having high aro~atie earbon eontent obtalned ~ro- exeraetion proees~e~ ~ueh a- furfural extraetlon Petroleu~ proeess~s which ean produee sultable petrol-um piteh~s includ~

"` 1 32~468 catalytic cracking, thermal cracking, and visbreaking.
since it is believed that all of the steps in preparation of carbon fibers including the heat treatment of the crude pitch feed to convert it into the mesophase fraction, the recovery of the mesophase fraction from the undesirable portion of the crude pitch feed, the spinning of the mesophase fraction into as-spun fibers, the stabilization or infusibilization of the as-spun fibers, and the heat treatments involved in converting the stabilized fibers into carbon or graphite fibers may have an influence on the ultimate properties of the carbon ~iber product, the present invention utilizes certain specific sequential treatments that lead to carbon fibers of the invention characterised by an outstanding balance of tensile properties. ~any of these treatments, although not necessarily in the sequence employed herein, are found in the patent or technical literature and, when possible, representative prior art disclosures will be noted.
H~AT TR~a~
In order to increase the mesophase content in the precursor feed for forming carbon ~ib~rs, the raw pitch is heated in accordanca with the procedure described and illustrated in U.S. Patent 4,184,942 (Angier et al.).
The heating can t~ke place in a reactor or autoclave at a temperature within thQ range of about 350 to 480C. For most purposes, the heating will be carried out at ambient pressures, although reduced pressures also can be utilized. Preferred pressures are from l psi to .... . .. . . . . . .

-8- l 324468 20 psi, while the time of heating may vary from 1 to 20 hours However, and a~ ~et forth in U S
Patent ~,184,9~2, it i~ particularly preferr~d to terminate the heating of the pitch ~u~t ~hort of the transformation of the pitch into ~ph~rule6 that are observable by polarized ~icroscopy An in~rt ~tripping gas ~uch a~ n~trogen and the li~ can be ~mploycd during th~ h~at ~oaking etep to as~i~t in thc r~moval of lo~r ~olecular ~eight and volat~ ub~tanc~ from the pitch SOLVENT FRACTIONAT~ON
Tb~ h~at tr-at~d pitch produce i~
pulv~ri2~d, g-n~rally in an inert at~oeph~r-, and flux~d ~ith an organic ~olvcnt ryrt~ to r~cov~r the ~e~opha~e fraetlon of th- pitch S~ U S
Pat-nt~ 4,208,267 (Dl-f~ndorf ~t al ) a~ ~all a~
~ ,9~2 ~Angi~r at al ) A~ di~elo~ed by Angi~r t al , tha a~ount of ~olv-nt ~ploy-d ~ill b~
sufflclcnt to g~ve a ~ol~ent ~nrolubla fract~on capablc of belng eonv~rtad to gr-at~r than 90%
optically an~otropie uat~rial, ~hich i- c~in~ntly ~ultable a~ a earbon fibcr prccurror The partlcularly pr~f~rred olv-nt fraetlonation proc~dur- ~ployad ln the practic~ of th~ pr-~ant lnvantlon l~ dl~elo~-d and illurtratcd ln V 8 t~tcnt ~,277,32~ (Gr~n~ood) Aecording to thl~ patcnt, thc hcat ~oa~d plteh produet i~
ad~ixad ~ith ~ non-reaeti~e, org~nle fluxlng llqùldt ~hich, ~han ~ix~d ~ith piteh in ~uffie~-nt a~ount~, ~ill rendar the pitch ~uffici-ntly fluid ~o that it can be e-~lly handl-d and ~hieh eaur-~
~ub~tantially all of th- guinolin- in~olubl-co~pon-nt~ to b- ~urp-nded in th- fluid pitch ~xa~ple- of suitabl- fluxing liguid~ arc tctrahydrofuran, light aromatic gar oil~, h~avy g aromatic gas oils, toluene and tetralin In general, the amount of organic fluxing liquid employed will be in the range of about 0 5 to 3 parts by weighe of the orqanic fluxing liquid p~r part by wei~ht of pitch; the preferred weiqht ratio being in the rang~ of 1 1 to 2 1 Solid material~, which con~i~t of all th~
quinoline in~olubl~ component~ ~uch a~ cok~, cataly~t, ~nd oth~r quinolin~ in~olubl~ for~d during th~ heat oa~ng t~p, ar~ ~parat~d fro~
the fluid pitch by ~di~ntation, c~ntrifugation or f~ltration Follo~ing ~paration of th~ ~u~p~nded ~olid ~at~rlal, th~ fluld pitch i~ tr~at~d with an anti--olv-nt to pr~cipitat~ and flocculat- that portion of th~ fluid pitch that i8 n~o~opha~ and ~p~cially ur~ful for conv-rrion into carbon fib~r~ Solv~nt or ~olv~nt ~ixtur~r havlng a ~olubility para~t~r b~t~-~n 8 0 and 9 5, pr-f-rably b-t~--n 8 7 to 9 2, at 25'C ar~
r-quir~d ~llu~tratlv~ xa~pl~- ar~ aro~atic hydrocarbon~ ruch a~ b~nr~n~, tolu~n~, and xyl~n~
a~ ~11 a- ~xtur-~ th-r-of ~lth allphatlc hydrocarbon~, ruch ar tolu~nc/h~ptan- lxtur~
Th~ pr~f-rr-d rolv-ntr ar- lxtur-~ of ~ol~-nt~ ar~
tolu-n- or tolu~n-/h~ptan- vixtur-r ~h-r- th~
a~ount of tolu-n~ i- at l~art C0 volu~c %
Ar d-rcrib-d by Gr~-nwood, th~
antl-rolv-nt ~111 b- cnploy~d ln a-ount~ rufficl~nt to provid- a rolv~nt in~olubl~ fraction, whlch i~
capabl- of b~ing thcrually convcrt-d to gr-at~r than 90% of an optlcally anl~otroplc ~at-rlal ln ~ than 10 vlnut-r Th- ratio of th-antl--olv-nt to pitch ~111 g~n-rally b~ fro~ about 5 ~1 to 150 ~1 of olv-nt p-r gra~ of pitch _g_ -` 1 324468 After precipitation of the neomesophase or mesophase fraction of the pitch, the precipitate can be recovered 5 by sedimentation, centrifugation or filtration. The quinoline insolubles content has been lowered to less than about 0.1~. The precipitate is then dried in, for example, a rotary-vacuum oven, and for ease of handling may be extruded at elevated temperatures to form pellets.
SPINNING
Spinning is carried out by feeding the precipitated mesophase pitch fraction, generally in the form of pellets, into a screw extruder and through a spinneret to form essentially round or circular cross-section fibers, quenching the filaments in air, and collecting the ~ilaments conventionally. The spinning apparatus may be of the conventional type, but for the present invention it can be advantageous to use the spinneret shown and described in U.S. Patent 4,576,811 (Riggs et al.) See espQcially Figs. 1 and 2 as well as Example 2 of Riggs et al. The form~r are described in column 2, line 50 to column 4, line 10; while the latter is found in column 4, lin~ 49, to column 5, line 7.
~': '' The rate of spinning is generally 100 to 1000 ~et~rs/minute. In general, the spun fiber diameter will rango from about 5 to 20 microns.

STABILIZa~ION
In the next processing step the as-spun or green fibers are sub~ected to stabilization or ` ~ ;
infusibilisation. The method and apparatus of U.S.
' `"~''`'' ' :" ' "'`"~,' Patent 4,576,810 (Redick) are employed. As known in the art, the as-spun fibers are collected in the usual manner on a spinning spool or bobbin. U.S. Patents 4,351,816 and 4,527,754 illustrate such spools, which would be useful for this operation.
According to the process of this invention, and using Redick's method and apparatus, the as-spun or green fibers are oxidized directly on the spinning spool with air or a mixture of oxygen and an inert gas. In general, the amount of oxygen in the gaseous mixture will vary from about 1 to 21~ by volume, the higher figure being reached when air is used.
The stabilization te~perature may vary from 200 to 340C. and the stabilization will generally take place over several hours. It will be understood that some minor experimentation may be necessary to determine optimum stabilisation tiDas and temperatures, and that shorter times are requirQd at higher temperatures while longer times ar~ required at lower temperatures. "
CARBQNIZ~IQ~ ``
The prQcarbonisation and carbonisation procedures are very important featurQs of the present process. In general, precarbonization is carried out at a temperature of from about 400-800C, while primary carbonization is carried out at about 1000-2000C, preferably 1500-1900C.
Precarbonigation i8 carried out for 0.1 to 1 minute and carbonization for about 0.3 to 3 minutes. Longer treatment times would not be detrimental.
The thus treated carbon fib~rs may also be coated with an epoxy resin solution from an :

~A~ `

.
applicator taught in u.s. Patent 4,624,102 (Bell, Jr.), utilizing as well the apparatus of this patent. This treatment reduces broken fibers on the surface of a carbon fiber yarn bundle. It will be understood, however, that this particular treatment may be omitted since it is not an essential feature of the present invention. Similarly, the apparatus and method of U.S.
Patent 4,689,947 (Winckler~ may also be employed for reducing broken fibers on the surface of a carbon fiber yarn bundle.
Between the carbonization and the subsequent graphitization at temperatures of at least 2~00C, i.e.
the so-called graphitization treatment, it may be advantageous for certain purposes to cool the fibers, wind the fibres on a spool or bobbin, and then unwind them. The exact significance of this procedure is not fully understood at this time, but it does appear to be of some benefit to ensure the obtention of high strength carbon fiber products.
The second carbonization or graphitization treatment is attained by sub~ecting the carbonized fiber to temperatures ranging fro~ about 2400 to 3300, preferably 2S from 2600 to 3000C. The time period for achieving grap~itisation may vary over a wide range, as illustrated in t~e exa~plQs.
TherQ i8 not~ing critical in the type of graphitizing equipaQnt employed in carrying out the sQcond carbonis~tion treatment at the higher temperatures. Various electric furnaces or ovens, such -as the Tammann electric furnace or a Centorr Associates -oven, under an inert atmosphQrQ such as argon, or the like, may be employed. Thus, bobbins ,:"

.: .
, .

.: :' or spools or carbonized fibers may be piddled into circular packages and graphitized in a Centorr Associates oven under an argon atmo~phere The carbonized fibers, under zero ten~ion, are next ; 5 subjected to the el~vated temperatur~
The graphite fiber product~ ar~ cool~d to ambient temp~rature and rewound onto bobbins or ~pool 8 .
The graphit~ fib~r~ have a numb~r of out~tanding charact~ri~tic~ that di~t~ngu~rh th~m fro~ fiber~ h~r~tofor~ di~clo~ed or ava~labl-co~reially Mor- ~p-eifieally, not only ~a- -Young'~ aodulu~ gr~at~r than 100 ~p~i but t-n~
~tr~ngth ~-r gr~at~r than about 500 kp~i Sueh a balaneed ultra-high aodulu~ and high t~nril~
~tr-ngth }ib~r ad~ froa ~opha~ piteh ~r uniqu~
ln~ofar a~ lt d~d not r~qulr~ th- u-~ of ~p~cial f~d ~at~r~al and rp~e~al ~qu~pa~nt and a rp~elal ~p~nner-t to obta~n ~avy ero~r-~et~onal, ~ pro~dal, or ~ultilobal f~b~rr~ Rath~r, th~
f~ber~ of th- lnv-nt~on have a ~ub~tantially eireular or round ero~ etional ~trueturo ~lth averag~ d~a~eterr of fro~ about 5 to 20 aieronr Thu~ tho prcr~nt f~bcrr havc laprov~d long-t~on eharaet~rl~tie~ ar a r--ult of thelr balaneed t-n~l- prop~rtl~ ~hlr ~-anr that yarn~
are oa~i-r to ~andle and ean bo par~-d ov~r guld~r ~ithout br~a~lng It ~ill be und~r~tood by thor~
~ d ~n tho art that ~aprov~d longat$on ~axin~re~ yl~ldr in th~ produetlon proe~ and in `;
th~ for~at~on of eoapo~t- aat-rial~
At tho pr-~nt tl~o, the r~aron~ ~hy th-earbon f~ber~ of thl~ lnv~ntlon have th-r- -xeallent fibor prop-rtl~r are not ntlr~ly under~tood5 it i~ b~ vad, how-v~r, that th~ f~b~r properti-r sten froa an unu~ual fiber fine . .

~tructure, which has a high crystal order and contains few di~rupting ~tructural defect~
Carbon fiber product~ of the invention were tested and found to have a pr~ferred ery~tal - 5 orientation angle of 1~8s than 6 d~gre~s at mea~ured by ~ide angle X-ray diffract~on (WAXD) Cry~tal orientation angle~ of le~r than 6 d~gree~
are cbaracteri~tic of th~ present f~bers and are h~ghly d~irabl~, ~ince they ar~ an ~nd~cium of ultra-h~gh modulu~ Thi~ ~aeur~ment i~ perfor~ed eonventionally, a~ d~crib~d, for ~xa~pl~, in U S
3,869,~29 (Blade~
Saall angl- X-ray ~cattarlng (SAXS) ~ a ~ea~ur~ of flb~r vo~dt and defeets SAXS int~n~ity vr eatt-rlng angl- data ~re obtain-d in th~
~guatorlal dir~etlon u~lng a ~rat~y ea~era X-ray ryrt~ a~ d~rerib~d in U S ~at-nt ~,639,3~7 ~ith the xeeption that a Phllip~-Noreleo high int-nsity line foeu~ X-ray tub~ ~lth a 2 5 by 10 uu foeal pot ~a- u~d in~tead of the Si-a-n~ tub~ ~ `
Th~ ~p~e~nr ~r~ pr~par-d by ~inding th~ fiber on a r-etangular fraae ~ith an opening ~uffleient to parr th~ X-ray b~a~ Th~ flb~r ~a~
~ound ~lth rufflel~nt t~n~ion to yl~ld a unlfor~
thle~ne-~ of ~r-ntlally parall~l flb-r~ In eertaln e~ tha fibars ~ara too brlttle to b~
~ound on th~ ~raae~ ln th~r~ ear~r the flber~ wer-eut to tha approprlate lenqth, ~rr~nq~d ~o that th~
fllaaent~ ~r~ p~rallel and ~ttaeh~d to th~ fram-ao ~lth tapa Speeln~n thie~n~rr after ~lndlng ~a~
rufflelent that tran~irrlon of Cu~ radiat~on approaehed l~e - 0 36a Thi~ enrur~r that dlffraeted lnt~n~lty ~111 be n~ar the ~ax~mua obtalnable ~ach ~a~pl~ wa~ zcann~d b~tw~en 0 1 and 5- 2~ ~catt~ring anglo Data w~re digitiz~d for .....

``'"'.~ .:

computer analy6i~, smoothed, ~nd corroct~d for sample thickne6s and background by the procedures described in u s 2atent q,639,3~7 The slope in the Porod region wa~
obtained fro~ a linear l~a~t tguar~s fit of the ln (inten~ity) ver~u~ ln(h) (where h i~ the ~att~ring vector - 4 6in~/~) plot in th~ 1 to ~ 20 r~gion, wher~ the Cu~, wave length Thi~ slop~
corr~latc~ ~ith t~n~ tr~ngtb SAXS slop~s of greatcr than about -2 1 ~re ~ea~ur~d ~ith fib~r~
of th~r invcntion By ~ay of contra~t, Amoco'8 P-120 had a SAXS ~lop~ of le~ than -2 . 2 .
La~cr Ra~an ~p~ctro~copy al~o rho~-d outatanding ~tructur- rcgularity ~or~
particularly, thc fib-r~ of thlr lnv-ntion hav~ a far ~orc unifora ~tructur- tban lo~cr ~odulu~
carbon fib~r~ ~ca 30 Mp~i) or A~oco'~ P-120 ~n at l~a~t thrcc rc~pcct~
(a) pr-r-nt fibcr~ hav~ th~ ra~-, highly graphitic tructur-, fro~ r~ln to corc7 (b) tha diffcr~nccr bct~-cn fibcr~ or along flbcr~ arc ~-ry ~allt and (c) th~ra are no rhift~ du- to r-~idual ~tr--~
Th~ la~ar Ra~an rp-ctro~co W ~a~urcmcnt~
~crc ~ado ln accordanc- ~ith th- follo~lng Flbar~ ~cr- ~bcddad in cpoxy rcrln, cut at an anglc to tho flbcr ax~r and poli~hcd to provida an llipt~cal ~cctlon ~lth an a~pcct ratio of about t-n Aft-r ultra-~onic clcanlng in rolvcnt to rc~ovc potcntial conta~inant~, la~cr Ra~an dyna~ic ~cattcring (1~20 to 1680 c--l) from ravcral arca~ of thc ~-ctlon ~a~ dat-ra~incd by a ~Ra~anor U-100~ ~icroprob~ with an ~rgon-Ion la~-r flltcrad to provldc Sl~ 532 n~ llght for illu~lnatlon Long axi~ of thc ~cction wa~ align~d parallel to the la~er polarization; a lens ~y~tem wa~ u~ed to focus the laser to a 2-3 micron diameter ~pot on the ~ection Great care was taken to assure that the spot ~i2e and po~ition were constant during data acqui~ition and that incid~nt light inten~ity ~as lnsuffici~nt to damag~ the ~p~cimen ~he inv~nt~on will b~ mor~ fully under~tood by r~fcrcnc~ to the follow~ng embodiment~, ~hic~, ho~cver, are not intend~d to be li~itativc EXAMP~g 2 A co~-rc~ally availabl~ p~trol~u~ pitch (A~hland 240) war ~acuu~ str~pp-d and then ~cat~d at a temp~rature of 177-C and plae-d ~n a r~aetor, a vacuu~ of about 29 lnehe~ ~g war drawn, thc pitch b-at-d to 363-C, and h-ld at th~t t~p-ratur~ until the toluen- in~olubl~ eontent war about 20% The total ti~- wa~ about 13 hour~
Thcr~ft-r, the vaeuu~ ~a~ bro~en with nitrog-n, the plteh h~ated to 391-C, held at that t~p~ratur- for about 1 hour, eool-d to 363-C, ~a~pl-d to eonflr~ a tolu-n- lneolubl~ eont-nt of 24-26%, and th-n eool-d to roo~ t-~p-ratur-T~- piteh o obtain-d war pulv-r$~ed, fluxed ~lth tolu-n- (lsl ~ ight ratio of ~olv-nt to plteh, by ~ lght~ by h-atlng to th- r-flux t-~p-ratura for about on- hour Th- ~olution wa~ ``
pa~r-d through a 5 uieron filt-r, and admix~d with 3~ ~uffiei-nt tolu-n-/h-ptan- (83sl7) (~anti-rolv-nt~
to provld- (a) an 85sl5 by ~olu~- tolu-nc/h-ptanc ~ixtur- and (b) an 8 1 ix-d rolvent/piteh ratio, by volu~e/~elght ~fter refluxing for 1 hour, th~ ~ixtur-~a~ eooled to a bient t~peratur- and th-pr-cipitat-d ~olldr wer~ irolat-d by -16- ;

centrifugation The cake was washed with additional anti-solvent and then dried in a rotary-vacuum oven Several ~uch batches were bl~nded, melt~d at about ~00C, pa~sed through a 2 micron filter, and extruded into pellet~ At thi~ point, the pitch pell~t~ have a quinolin~ in~olubl~s (ASTM
75C) of 1~ th~n 0 1 % by weight and ar~ 100%
me~opha~e, a~ determin~d by th~ polarized light micro~copy ~ethod Th- p~ t~ wer~ r~lt~d when f~d to a ~cr~w ~xtrud~r with an ~xit t~p~ratur~ of 350-C, rpun at about 360-C through a 4 inch dia~t~r/480 hol~ ~pinn~r~t Th~ hol~ ar~ round and array~d in 5 conc~ntrlc rlngc (96 hol~s p~r rlng) locat~d ln th- outer 1~2 inch of th- *pinn~r-t fac~ ~a~h holc ha~ a ~ount~rborc dia~t~r of 0 055 inch, a caplllary dia~t~r of 200 nicron~, a capillary l~ngth of 800 ~icron~ (~/D ~qual~ ~), and an ~ntranc~ angl- of 80/60 d~gr~, a~ d~f~n~d in Rlgg- t al~ U S Pat-nt ,576,811 ~S--partlcularly, ~xa-pl~ 2) Th~ ~pinn~r~t 1~ ~xt~rnally hcat-d to a~out 3CO'C, and th- ~p~nning c~ll co~prl~-~ an outcr qu-nc~ tub~ about 6 lnch-~ in dla~-t-r, 5 ~--d long, with top 6 inch-- ~cr~n~d to p-rnit ntry of qucnch air at roon tc~p~ratur~
A~pi~at$on 1~ provid~d by a tap~r~d (3 to 2-1~2 lnchc~) c~nt-r colu n that 1~ 4 lnch~ long ~at-r 1~ ~uppl$-d to th- air-cool~d a~-~pun ! 30 fila~-nt~ or gr-~n fib~r~, which arc wound at 550 yard~ p-r ninuto onto a ~pool di~clo~-d in U S
Pat-nt ,527,75~ (Flynn~
S~v-ral ~pool pac~ag-~, ach contain~ng about 1 pound of yarn, w-r- batch ~tabiliz~d by h~atlng ln alr All w-r~ h~ld for 30 minut~ at 225'C, hcat~d to 255 C ov~r 30 minut~, th~n h~ld at 255c for at least 2 hours Most ~pool~ were treated for 3 hour6 Carbonization wa~ carried out by combining the yarn from 6 stabilized packages 5 mounted in a creel to for~ a 2880 filament tow (nominally ~3~) forward~d at 4 feet/minut~ under the t~n~ion of it~ own weight ~about 150 gram~) through a 3 foot long precarbon~zatlon ov~n at 600-800'C, th~n through a 19 foot long, earbon-r~ tanc~ ov~n hav~ng a 1000^-1200 C cntranc~
~on~, a 1600'C carbon~atlon ~on~, and an ~x~t~ng 1000--1200 C ~on~. Th- fib~r~ ~r~ at carboni~atlon t~p~raeur~ for about ~ ~nut~. -The earboni~d yarn ~as n-xt pa~r~d through a 19 foot long eha~b-r eonta~ning dr~d, roo~ t~p~rature alr ad~ix~d ~lth 0 098~ (980 ppm) of o~on~ ruppl~d at a rat~ of 1 ef~ Th- yarnr ar~ ov~rlay~d ~lth a 1% ~olutlon of ~poxy r~ln (CMD-N55-5003, rold by the C-lan~- Corporation) in ~at~r, u~lng th- n~thod and apparatur ~ho~n ln U S
Pat-nt 4,62~,102 IB-ll~ Jr ) Th- thu~ tr-at~d yarn~ ~ere dr~ad at 350~C for ~ uinut-~ and th~n el~aned by parr~ng th- yarn through th- guid-d~#erlb~d and lllu~trat~d ln U ~ Pat~nt 4,689,947 (Nlnekler) At thlr point, a yarn fro~ a repr~entatl~ ~pool had a t-naeity of 370 ~prl and a nodulur of about 30 ~p~l.
A qroup of 8 bobblnr of th~r~ earbon~z~d yarn~ ~ r- plddl-d lnto e~reular pae~ager on graphite tray~ and graphitlred ln a Centorr A~oelat-~ o~-n und-r an argon at~o~ph-r- Th~
yarn~ ~-r- not r-rtraln-d (r-ro tenrion~
Te~p~ratur-~ ~er- iner-ar-d to lSOO-C over an 85 ~inut~ period, th-n to 2800-C ov-r 60 m~nut~r, and h-ld for 20 nlnut-r at 2800- to 2890 C

.
.

-19- ` 1 324468 sob~ins of graphitized yarn were wound from the piddled packages Single fib~r tenslle strength was determined for each bobbin at a 1 gauge length following ASTM 3379 The average ~ingle fiber t~n6il~ ~trength for fib~rs on all 8 bobbin6 wa~ 530 ~p~i The highe~t ringl~ bobbin , average ~a~ 600 ~p~i Modulu~ of repr~sentat~v~
; bobbin~ was deter~ln~d ultra-~onically following the ~ethod d~cribed by ~by ~J J Smlth, H Jiang and R. ~. Eby Poly~r Co~unicationr Vol 2a, p.
1987) Th~ av~rag~ fib~r ~odulur ua~ great-r than 125 Mp~ls av-rag- ~odulu~ of th~ highest r~ngl-bobbin ua~ 135 Mp~i Ba~d on reanning l~ctron micrograph (SE~) of fractur~ rurfac~, th-~ ~ib~rs app-ar~ to xhibit a unlqu- ~iero~truetur~, g-n-rally ~radial~ in charaet~r, ~ith hlgh fr~qu-ney, low a~plitud- ~lnklng vident in ~o~t la~lla, ~ith oeea~ional high a~plltude klnk~ that ar- ln r-gi~try ~ith adj~eent la~alla- No ~heath-eore eharacter i~ dl~eernlble) th~ la~alla- ~xtend fro~ the c-nt-r of th- flbar to lt~ periph-ry Th- abov- data re~-al th- produetlon of high t-n~ile ~tr~ngth a~ ~ell a~ ultra-high ~odulus earbon fib-r~ That th~ earbon flber~ of thl~
invention hava exe-llent ten~ prop-rti~ ln eo~p~ri~on ~ith a~ailable eo---reial flb-rr beeom-z raadlly appar-nt fro~ r~ lng A~oeo'~ T-chnleal ~ulletin F-7010 (~-v 2/1/87) Th- latt-r'~
eo~narelal fib-r P 120 har a typieal fib-r t-n~
~tr-ngth of 325 ~p~l at 120 Mp~i ~odulu~
Furth-r~ora, th- in~tant earbon fib-r~ alro hav- a I hlgher brea~ longation than P 120 fib~rr) improv~d j longation ~-an~ that yarnr ar- a~i-r to handl-and ean be pa~-d ov*r guid-r without br~akinq A~ pr-vlou~ly not-d, thi~ i~ an important eharaet-ri-tle both Eor maxi~izing yields in th--19- `

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

production proces~ and in forming reinforced compo~ite material~ Thus, a representative fiber sample was tested by hand pulling the yarn over cylinder~ of various diameter~t it 6howed no brok~n fibers at a 0 19 inch radius of curvature In contra~t, Amoco'~ P 120 ~howed ~om~ broak~ at 0 25 inch radiu~ and ~any brcak~ at 0 19 inch radius A r~pre~entativ~ inv~nt~v~ fib~r al~o ha6 a preferred cry~tal or~entation angl~ of 5 degr~e~
a~ ~arur~d by ~id~ angl~ X-ray d~ffraction ~NAXD) S~all angle X-ray ~eatt~ring ~SAXS~ ln (int~nrity)/ln (rcattar~ng v~ctor) rlop~
~arured for 6 of th- a ~a~pl~ Av~rag~ valu~ ~ar -1 98 (-1 8B to -2 05 rang-) P-120, by contra~t, ha~ a ~lop~ of lcr~ than -2 3 La~-r Ra~an ~poetro~eopy ~how-d high ~tructur~ r~gularity Thu~, for xa~pl-, carbon flb~r~ ~ad- ~i~ilarly to tho~- of ~xa~pl~ 1 ~ho~-d a ~harp graphit~ p-a~ at tho 158~ ~av~ nu~b-r Th~
p~a~ harpn-~ and loe~t~on ~how~ that th~r~ ar~
f-~ r~du~l ~train~ in th- fib~r~ r-~$dual trainr rcduc- ~tr-ngth No p-a~- eorr-~ponding to d~ord-r-d carbon ~-r- ob~-rv~d Analy~i~ of th-~c ~p-etra al~o por~it d~t~r~lnation of e-rta$n unifor~$ty para-~tor~, ~hor~ tho largor nu~b-r indieata~ tho groat-r un$for~$ty ~ `
CARBONI~F~ uDnrrr --~ AMOCO P-120 FIa~R _ FI~ER(GRAPHITIZED) - PtA~ ~IDT~ 1 7 5 0 1 2 .
Th-~o data claarly $nd$cat~ that th~ fib~r~ of this $nvont$on aro highly unifor~, $ c , ~tructurally ho~ogonoouc, and that ~uch un$formity of ~tructur~

appears to be achleved during th~ graphitlzatlon step EXAMPLE II
Thi~ example describes th~ re~ult~ of a - 5 production run and illustrate~ the consi~t~ntly ¦ good re~ult~ obtain~d Sev~ral hundred packag~ of yarn were prepared following the ~thod~ d~6crib~d in Exampl~
; I exc~pt that a propri~tary ~'licon~ oll fln'~h (DP-9503-1; Ta~i~oto Oil ~ Fat Co ) ~a~ u-~d in~t~ad of ~at~r Fib~r~ ~r- pr~par-d fro~ a bl~nd of ~v~ral bateh-- of h~at-~o~k~d pltch prepar-d fro~ a r~fln-ry d~cant o~l r~ldu- M'nor ~odi~icationr ln ~olv~nt ratio~ ~r- n-~d~d to obtain th~ d-rlr~d pltch ~-ltlng polnt Mlnor ~odlfication~ ~er~ nad- ln rtabillratlon to obtain optl~u~ earbon~d rtr~ngth Yarn ~a~ ~ound onto ~pool~ aft-r th- pri~ar~ carbonl~atlon d~rcrib~d ln ~xa~pl~ I ~xc~pt that hlgh~t t~p~ratur- ~a~
1530'C and th- flb~rr ~r- tr~at~d ~lth n-'th~r oron~ nor ~poxy fini~h Carbonl~d yarnr ~r~
plddl-d lnto pae~ag~r and bateh graphltlr~d ar d-~erlb~d ln ~x~upla S Slxty r~pr~r~ntatlv~
pac~ag-r ~te tenrll~ t~rted (~nglo f~la~nt) a~
1~ gauge l-ngth follo~lng ASTM D 3379 Av~rag~
tenaelty of all 60 pae~ag-r ~ar 5~9 ~prl, av~r~g~
~odulu~ ~xceoded 130 Mp#l 95~ of th- 60 pae~ag~r h~d t~naclticr abovc 500 ~p~l t~AMP~F ~I~
o Tbia exa~ple lllurtrat~r a r-cond produetlon run ln ~hleh flb~rr ~r- graphit~z~d ln a eontlnuou~ rather than a bateh op~ratlon Flb-r production ~a~ tbc ~- a~ ~xa~plc II up to wlnd-up aftcr priuary carbonlratlon. S-v-ral hundr~d ~-~poola ~cr~ pr-par~d by grapbitiring continuou~ly ln thc ovcn ry~t~ d-~crib-d ln Exa~pl- I such that " ' residence time at the highect temperature (2700C) was about 1 minute Thirty two repre~entative spools were tested as in Example II Single fiber tensile strength averaged 511 kpsi; averag~ modulu~
exceeded 120 Mp~i. Ten~ile strength of 69% of th~
item~ was abov~ 500 ~p~i Prop~rti~s though --exc~llent, wer~ ~omewhat lower than Exampl~
indicating that the higher ~raphiti2ation temp~rature and/or long~r tim~6 arc b~n~ficta ~XAMPLE tV
Co~posit~ unibarr wcr- pr~parcd follo~ing the general ~thod of Chang U S 4,681,911, ~xampl~
I, u~ing a~ ~atrix poly~r th~ co~potition nu~b~r 2 fro~ Tablc 1 (both colu~n ~ inforcing fib~rr w~r~ pr-par~d ar in ~xa~pl- I, rupra, or purcharcd (A~oco P-120) Thc tc~t rp-cl~cn~ wcrc 1/2~ ~d~, 6~ long and ca 100 ~ thlc~ and cach containcd ca~ 58 volun- pcr c~nt of rc~nforclng f~bcr T-~t~ng ~a~ conductcd ln according to th- AS~M
t--t- r-f-r-nc-d in U S ~,681,911 and thc follo~$ng r-~ult- w-rc obt-ln-ds Rcinforcing ~l~x ~odulu~ Flcx Strcngth sass ~ibcr (Mp~ prl) ~kp-i) Thi~ inv~ntion 55 77 ~ 3 A~oco P-120 ~0 53 3 8 Si~ilar ~hort bca~ rhcar ~trcngthr ~SBSS) arc indlcatlv- of rurfac- trcat~-ntr ~for adh-~ion) which ar- couparabl- Accordingly, about ~0 iuprov-d conpo~lt- fl-x ~odulur and rtr~ngth for thc inrtant flb-rr furthcr and dramatically lllu~trat- th-lr utillty in tbc applicationr (l c , vatrlx r-lnforc---nt~ for whlch th-y arc intcnd-d tnd--d, wh-n th- rauo pair of fibcrr wcrc compared in ~-tal ~atricc~, 1 c , in ~agn~lu~ alloy wir~
and rtrand~, ri~ilar i~provc~cntr alro wcr-ob~crv-d `
.~ .

Claims (9)

1. A substantially round or circular cross-sectional, pitch-derived carbon fiber with balanced tensile strength and modulus characteristics having a crystal orientation angle of less than 6 degrees, a small angle x-ray scattering slope (SAXS) between about 1.8 and -2.1, and a fiber tensile strength of at least about 500 kpsi.

2. A substantially round or circular cross-sectional carbon fiber product having balanced ultra-high tensile properties comprising an ultra-high modulus greater than 100 Mpsi and a high tensile strength greater than 500 kpsi, derived from a solvent fractionated mesophase pitch precursor, which is characterized by a mesophase content greater than 90% by weight and a quinoline insolubles content of less than 1% by weight; said solvent fractionated pitch precursor, after extrusion into fibers, being initially heated to an elevated temperature of at least 1000°C to effect carbonization, cooled to a lower temperature, and then heated to a higher temperature of at least 2000°C to effect graphitization

3. The carbon fiber product of claim 2 wherein the solvent fractionated mesophase pitch precursor has a quinoline insolubles content of less than 0.3 by weight.

4. The carbon fiber product of claim 2 wherein the solvent fractionated pitch precursor is obtained by treating a heated soaked pitch feed with an organic fluxing liquid, separating solids from the resulting fluid pitch, treating said separated fluid pitch with an organic solvent system having a solubility parameter at 25°C of between about 8.0 and 9.5.

5. A method for producing a substantially circular cross-sectional carbon fiber product with balanced tensile properties, comprising an ultra-high modulus greater than about 100 Mpsi and a high tensile strength greater than about 500 kpsi, which comprise the steps of:
(a) heat soaking a pitch feed material to increase the mesophase content;
(b) solvent fractionating the heat soaked pitch with a solvent system having a solubility parameter within the range of 8 to 9.5;
(c) recovering insoluble material from the solvent fractionated pitch, said insoluble material having a mesophase content of greater than 90% and a quinoline insolubles content of less than about 1% by weight;
(d) extruding the insoluble material through a spinneret with nozzles suitable for producing a multiplicity of green fibers with substantially round or circular cross-sectional structure;
(e) stabilizing said green fibers by heating them at an elevated temperature in an oxidative gas;
(f) carbonizing the stabilized green fibers by heat treatment at a temperature of at least 1000°C;
(g) cooling the carbonized fibers to a temperature below the carbonization temperature;
(h) graphitizing the cooled carbonized fibers by heat treatment at a temperature of at least 2400°C; and (i) recovering the resulting carbon fiber product with balanced ultra-high modulus and tensile strength properties.

6. The method of claim 5 in which the solvent fractionating step (b) is carried out by treating the heated soaked pitch feed material with an organic fluxing liquid, separating solids from the resulting fluid pitch, treating said separated fluid pitch with an organic solvent system having a solubility parameter at 25°C of between about 8.0 and 9.5.

7. The method of claim 5 in which the carbonized fibers in step (g) are cooled to ambient temperature.

8. The method of claim 5 in which following step (f) and prior to step (h) the carbonized fibers are wound onto and then unwound from a spool.

9. The method of claim 5 in which step (g) involves sizing the carbonized fibers.