CA1309814C - Process for converting comminuted material into a solid body - Google Patents

Abstract

ABSTRACT

A process for hot isostatic pressing comminuted material in a pressure vessel, the process comprising the steps of: loading the material into an envelope; placing the envelope with the material into a receptacle; surrounding the envelope in the receptacle with a pressure transfer medium;
positioning the receptacle with the envelope and tranfer medium in the cavity of the pressure vessel; and applying heat and pressure to the tranfer medium in the receptacle at levels and for such time as is sufficient to compact the comminuted material into a substantially solid body.

Description

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This is a divisional application of copending application serial no. 528,519, filed January 29, 1987.

The present invention relates to carbon materials and more particularly graphite bulk articles formed by the pyrolysis of a plurality of consolidated preoxidized S fibers, particularly polymeric polyacrylonitrile fibers.
Carbon-carbon composites are generally carbon mat-rices reinforced with carbon fibers aligned or distri-buted therein. Such composites have been formed by a variety of methods, usually involving the impregnation of a porous carbon fiber structure with a resin, pyroly~
tic carbon or the like. For example, a mat, felt, tow or the like of carbon fibers may be impregnated by a pressure or evacuation technique with a binder of pitch or a synthetic carbon-yielding resin that is subsequent-ly polymerized. The impregnated body is then pyrolyzedby heating to temperatures sufficiently high to convert the impregnant binder to a carbon matrix.
ALternatively, a carbon matrix can be formed by impregnating a porous, carbon fiber body with a hydro-carbon gas that is then thermally decomposed to carbon.In either case, the carbonized body can be reimpregnated and repyrolyzed to increase density and improve other properties. The resulting carbon matrix, however, is generally not well bonded to the fibers because of shrinking of the matrix during pyrolysis. Further, the composite often tends to have a coarse structure with significant residual porosity and low Young's modulus.
In prior art manufacturing of carbon fibers, it is often preferred to use precursor fibers of acrylic poly-mers such as polyacrylonitrile (PAN). As used herein,the term PAN is intended to include acrylic fibers con~
taining at least 85% polyacrylonitrile, the balance ' i ~ 3 ~

including other polymers. Such PAN fibers do not melt prior to pyrolytic decomposition~ and pyrolyzed fibers produced from PAN have substantially greater strength than fibers produced from other inexpensive precursors such as pitch or regenerated cellulose-based materials.
It has long been known that yarns prepared from acrylonitrile will, during heating in an oxygen-rontaining a~mosphere at about 2000C, undergo a change resultin~ in a black color and fire~resistant properties for the yarn. It is believed that during such heating extensive dehydrogenation of the polymer backbone occurs and some of the penden~ nitrile groups are hydrolyzed to the amino or carboxylic structure, thereby catalyzing a thermal, block-type polymerization of properly oriented nitrile groups. Additionally, such heating also produ-ces molecular cross-linking, induced at least in part by oxidizing agents. Thus, apparently the oxidation pro-cess causes the polymer chains in the fiber to link intramolecularly to form a ladder structure, markedly altering the physical characteristcs of the fibers. For example, such oxidized fibers no longer are soluble in polyacrylonitrile Yolvents such as dimethyl ~ormamide or tetramethylene cyclic sulfone.
PAN and other fibers for use in carbon composites are usually heat-stabilized by a thermal oxidation pro-cess wherein the fibers are heated in an oxygen-containing atmosphere at between about 200 C and 400 C until a desired oxygen content, usually between about 5 to 15 weight percent, preferably around 10 weiyht percent, is achieved. Such heat-stabilized, oxygen-containing fibers are known as preoxidized fibers4 Strictly speaking, when preoxidized PAN is subjected .
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to temperatures above about 1000 C, it loses its non-carbon content, and because it does not melt, it chars.
According to Jenkins and Kawamura, Polymeric Carbons -_ Carbon Fiber~_Glass and Char r Cambridge University Press, London, 1976, the charred material is termed "polymeric carbon", a material that should be sharply differPn-tiated from graphitic carbon produced by pyrolysis of cokes formed from a liquid or tarry state. The foregoing appears to explain the comment of J. Hermann in his article "Electrical Conductivity of Vapor-Grown Carbon Fibers", Carbon~ Vol. 21, No. 4, pp.431, 435, that it is ",~. common knowledge that PAN fibers do not graphitize. n Polymeric carbon is characterized by having a turbo-stratic network of carbon atoms as opposed to the exten-sive graphite sheets that must necessarily exist in true graphitic carbon. Cf. Jenkins and Kawamura, supra, at page 2. These two forms of carbon can also be distinguished readily from one another by a number of tests based on the diff~rent crystalline structures of the materials.
For example, polymeric carbon made from PAN will have a relatively disordered structure and will typi-cally exhibit carbon basal planes that are concentric at the outer portions of the fiber, but are radial inker-nally. The density of the fiber will be around 1.7 to 1.8 g/cc.
On the other hand, graphitic carbon fibers made from pitch have a well ordered structure- and will typically provide graphite "planes" that are substantialLy all radially disposed out to the fiber surface. The density of such graphitic pitch fibers will typically be about .

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2.1 to 2.2 y/cc. Also, in true ~raphite, X-ray studies through scanning electron microscopy will show C-direction spacing to be below about 3D5 A, the theoreti-cal spacing being 3.354 Al . 5According to an aspect of the invention there is provide~ a process for hot isostatic pressing comminuted material in a pressure vessel, said process comprising the steps of:
loading said material into an enYelope;
10placing said envelope with said material into a receptacle;
surrounding said envelope in said receptacle with a pressure transfer medium;
positioning said receptacle with said envelope and tr~nsfer medium in the cavity of said pressure vessel;
applying heat and pressure to said transfer medium in said receptacle at levels and for such time as is sufficient to compact said comminuted material into a substantially solid body.

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The present invention constitutes an improved approach to the problem of using the expensive multiple cycle ~atrix impregnation~graphitization processing heretofore required to provide carbon bodies, and also S results in high values of Young's modulus, no~ hereto-fore achieved in carbon-carbon bodies- To these ends, the binder material employed in the present inventîon is derived in situ directly from preoxidized fibers-them-selves, The binder ma~erial is formed by infiltrating a plurality of preoxidized fibers with a liquid polar plasticizer such as water or an alcohol having from 2 to lO carbon atoms per molecule, the plasticizer and fibers reacting with one another to extract or leach a tarry leachate from the infiltrated fibers and coating the latter. The coated fibers are then consolidated or dif-fusion bonded to one another at high pressure, ~ypically at a temperature below 400~C as by pressing, hot isosta-tic pressing, autoclaving, extrusion or the like. After diffusion bonding, the bulk material formed is no longer fibrous in nature, but the bulk structure substantially retains the axial moLecular orientation of the original fibers. This bulk material can be carbonized at atmospheric ~ressure to obtain higher values of Young's modulus for the carbonized material than have been pre-viously achieved.
In an important aspect of the present invention, the coated fibers are both consolidated and pyrolyzed, for example at 600 C under pressure, all preferably by hot i 3 ~

isostatic pressing (HIP), while avoiding cooling between consolidation and pyrolysis. After the HIP process is complete, again the bulk mate.rial formed is no longer fibrous in nature, but the bulk structure substantially retains the molecular orientation of the original fibers.
This bulk material can be carbonized at lower tempera-tures than those heretofore required to obtain a given value of Young's modulus for the carbonized material.
When preoxidized PAN fibers have thus been consolidated and pyrolyzed under pressure, the carbonaceous product can be truly graphitized by subsequent heat treatment to obtain material with a modulus of at leas~ 40 x 106 psi, and a tensile strength of at least 20 x lQ3 psi.
A principal object of the present invention is therefore to provide a method of forming a bulk carbon structure from preoxidized fibers, which structure has a high modulus of elasticity. Yet another object of the present invention is to provide a bulk carbon structure from precursor preoxidized fibers, which structure is not grossly fibrous but retains the molecular orien-tation characteristic of the preoxidized fibers, and therefore can be carbonized or graphitized to produce high strength, high modulus bulk carbon or graphite bodies with minimal cracking~ .
Other objects of the present invention are to pro-vide such a method wherein preoxidized fibers are infused with a plasticizer to form a tarry exudate that serves as a binder in a subsequent consolidation step, and to provide such a method wherein the plasticizer 3Q employed is capable of extracting a tarry leachate from the infused preoxidized fiber~ and thus avoids the need to add any matrix material to the resultlng carbon body.
Another important obje~t of the present invention ig to 1 3 ~

provide such a method where;n although the structure is formed from preoxidized polyacylonitrile fibers, it can nevertheless be truly graphitized by subsequent heat treatment to produce high strength, high modulus graphite bodies of low porosity and minimal cracking.
Yet other objects of the present invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the processes comprising the several steps and relation of one or more of such steps with respect to the others, and the pro-ducts and compositions possessing the features, proper-ties and relatlon o elements, all of which are exemplified in the following detailed disclosure and the scope o~ the application of which will be indicated in the claims.
For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description.
Generally, in the process of the present invention, a plurality of preoxidized fibers are infused, preferably to saturation, with any polar liquid plasticizer capable of extracting a tarry leachate from the fibers. The ~ibers may be any carbonaceous precursor capable of being so infused, such as those formed of rayon and the like, but are preferably polyacrylonitriles. For example, typical precursor fibers are "Grafil S.A.F.~ from Hysol Grafil Co., a polyacryloni~rile believed to contain 5%
methyl acrylate and 1%~itaconic acid, "Dralon T", from Bayer Aktiengesselschaft, believed to be pure acrylic homopolymer, and many othersO The precur~or ~ibers should be stabilized by known processing to have about 7 to 14 weight percent oxygen.

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The plasticizer can be any of a large number of polar solvents such as water, ethylene carbonate, dimethyl sulfoxide, and alcohols e.y~ normal saturated alcohols such as ethyl alcohol, n-pentyl alcohol r n-hexyl alcohol, n-heptyl alcohol, n-octyl alcohol, n-nonyl alcohol and n-decyl alcohol, tertiary-pentyl alcohol, cyclo-pentanol and cyclohexanol; unsaturated alcohols such as ethylene glycol, propylene glycol, 1~3 propanediol and glycerol; and aromatic alcohols such as benzyl alcohol, a-phenylethyl alcohol and B-phenylethyl alcohol. While ethylene glycol is a preferred alcohol, at least rom a cost viewpoint the preferred plasticizer is simply water.
The infusion of preoxidized fibers with the plast~-cizer is continued at a temperature above, at or below the boiling point of the plasticizer for a sufficient time for a substantial amount of leachate to form on the surface of the fibers, iOe. until the pre-oxidized fibers have imbibed at leas~ 5 and up to as much as 80 percent by weight o~ the plasticizer in terms of the fiber weight. The minimum infusion time is, inter alia, a function of the fiber diameters and the infusion tem-perature and pressure. It i5 believed that during this period, the infused pLasticizer extracts short fragments of the polymer chain from the interior of the fiber, which fragments were ormed during the oxidation pro-cess. The exact composition of the tarry exudate is not known, but it is in the form of a dark, viscous, stic~y fluid. The iniltrated plas~icizer also causes some swelling and softening of the preoxidized ~lbers, rendering them much more flexible.
A~ter the preoxidized fibers have been appro-... . :

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priately infused with plasticizer to form the desired exudate on the fiber surfaces~ a plurality of the treated fibers can then be readily consolidated or diffusion-bonded to one another or other fibers by a variety of techniques at comparatively low temperatures and pressures, e.g. ~s low as ~00~C and 2000 psio Bonding can be achieved statically by orienting a plura-lity of the treated fibers in a mold and subjecting them to isos~atic pressing at relatively low temperatures and pressure. On re~oval of the pressed product from the press enclosure, some residual exudate may remain behind. Unlike the prior art, however, because of the high plasticity given to the fibers by their swollen and softened state when treated according to the present lS invention, and the presence of the binding exudate, a plurality of the treated ~ibers may be consolidated by the dynamic process of hot extrusion. In either case, the resulting bulk structure or shaped product retains the internal moLecular orientation present in the origi-nal preoxidized PAN fibers. The resulting bulk struc-ture also shows litt}e or no gross fiber/matrix differentiation or clear boundaries characteristic of prior art composites.
Fur~her processing of the consolidated fibers is desireable to ~ully utilize the in~usion treat~ent o the present invention. The shaped product produced by consolidating the leached and coated preoxidized fibers possesses the strength and modulus of the original pre-oxidized i~ers, eOg. a relatively low modulus of less than 1 x 106 psi and relatively low ~trength, typically around 2 x 104 psi or less. However, this material i~
conver~ible to a high modulus (e.g., up to 5 x lb7 p~i) F.~-30 CIP

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_ 9 _ and high strength (e.g. up to 8 x 105 psi) carbon body with appropriate heat treatment in an inert atmosphere.
Such heat treatments are generally determined by the end properties and shape configurations desired~ and in general call for gradual heating up to between 1400-C
and 3200 C for maximum strength and stiffness. Slow heating tha~ avoids sudden release of volatiles within the ~tructure, and maintenance of the shaped product under pressure during the carbonization cycle, both serve to reduce or minimize crack formation in the resulting carbonized bulk product.
U.S. Patent No. 3,817,700 teaches treatment of PA~
fibers with a catalytic amount of alkaline or alkaline earth metal substituted polyol in a polyol solvent prior to oxidation of the fiber, thus permitting thermal oxi-dation to occur at fairly high temperatures.
The problem of differential fiber/matrix dimensional changes in carbon composites has been addressed in U.S.
Patent 3,927,186 which suggests treating flexible urethane resin strands with a liquid polymerizable furan resin or resin precursor such as furfuryl alcohol, to swell the strand. After removal of all liquid resin from the surfaces of the strands, the swollen urethane is thermally carbonized. An alternative solution Z5 offered by U.S. Patent 4,350,672 to this problem, is to completely eliminate any binder or matrix by relying on the plasticity of precursor fibers to effect bonding by compression molding prior to pyrolysis. To that end, the latter patent teaches assembling a plurality of synthetic polymer fibers, preferably polyacrylonitrile (PAN) polymers or aromatic polycyclic polymers such as certain polyamides, polyimides, polybenzimidazoles, or .,. ,~; .

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polythiadiazoles, and subjecting the assembled fibers simultaneously to a temperature and pressure sufficient to cause heat distortion flow and bonding between con-tiguous fibers. The bonded fibers are then pyrolyæed in a non-oxidizing atmosphere at relatively high tem peratures, for example up to 3500 C.
Consideration of the conditions ~et forth in the Examples in U.S. Patent 4t350.672, (using preoxidized acrylic copolymer fibers of 8% oxygen content) reveals that the carbonization processing temperatures required to obtain a given Young's modulus are substantially higher than those needed to obtain similar ~esults in the present invention. For example, in U.S. Patent 4,350,672, heat treatment to 1700-C is required to pro-duce a carbon structure with a Young's modulus of 25X106 psi. In the present invention, heat treatment to lOOO C provides a carbon article with a Young's modulus of 24X106 psi; con~inued processing to 1400C raises the modulus to 28X106 psi- These values should be com-pared ~ith the Young's moduLus of typical fine-grained bulk graphite of from lX106 to 2X106 psi, and is con-sistent with the typical Youny's modulus of other prior art unidir~ctional, organic resin and metal matrix com-posite articles (15x106 to about 30xlO6 psi).
The infiltrated preoxidized fibers coated with the leachate of the present invention can also be utilized 2S a matrix precursor with fully carbonized or graphi-tized fibers as a conventional reinforcement. For example, one can prepare a composite layup of alternate layers of carbonized or graphitized fiber~ with pre~
oxidized PAN fibers. The entire layup may be infu~ed with plasticlzer according to the teachlngs of the pre-Ft~-30 CIP

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sent invention to produce a leachate in situ, or the preoxidized fibers can be pretreated in like manner prior to forming the layup. In either instance, the resulting layup is then consolidated at low temperatures and pressures using standard platen pressing, hot isostatic pressing, autoclave or extrusion techniques.
Final firing of the composites is then carried out to the required carbonization or graphitization temperature in an inert atmosphere. The layups can comprise aligned or random carbon fibers in a matrix precursor of aligned or randomly oriented preoxidized fibers. The matrix formed from the treated preoxidized fibers, being highly molecularly oriented, provides additional strength and stiffness, and also permits greater controL of the rela-tive thermal expansion values of the matrix and rein-forced material.
As noted above, an important variation of the present lnvention is the concurrent consoli2ation and pyrolysis of the infused fibers. For this variation, 1mportantly the preoxidized precursor fibers (with oxygen content between about 9 to 14 weight percent) are stabilized to have oxidized densities of between 1.35 and 1.45 g/cc for reasons elucidated later herein. In the preferred pro-cess, these preoxidized fibers in the form of tops, yarns, tows and the like are laid up unidirec~ionally and pulled into a plastic envelope or tube, typically of polytetra~luorethylene, polyolefin heat shrinkable material or the like. The fiberg can thus be packed into the envelope to a 55 to 60~ fiber volume maximally.
In order to improve the packing density, the packed envelope may be in~erted into a metal ~ube, (typlcally stainless steel with a 0.050" wall, 1~" out~ide Fl~-30 CIP

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~ 12 -diameter) and the latter drawn through a series of metal-drawing dies (e.g. 5 dies are required to provide a reduced outside diameter of about 1.1"). This serves to increase the fiber volume inside the envelope to as high as 75 to 80%.
The metal jacket is then removed, as by machining, and the compressed plastic tube is cut into short lengths, typically 9". One or more of these length~ is placed in a plastic bag (e.g. prepared from 1 mil polytetrafluorethylene film). Water, or example 70 weight percent with reference to the fiber weight, i5 added to the bag and the fiber is allowed to ~oak, typi-cally overnight. It has been found that lf the density o~ the preoxidized fibers is less than about 1.35 g/cc, the fibers tend to dissolve in the plasticizer on heating, leaving no fibrous structure. On the other hand, if the density of the fibers is greater than about 1.45, the reaction between the fibers and the plasti-cizer tends to be too slow or insufficient.
~0 Followin~ infusion of the fibers by the plasticizer, the bag i5 closed and pLaced in a receptacle such as an open metal can of 2~ gauge stainless steel, and held in spaced relation to the bottom of the can by an approp-riate steel barrier or tool. The can is then filled wi~h a pressure transfer medium such as comminuted re-fractory material (e.g. carbon black, sand or the like) or a metal alloy such as PbBi that preferably melts at a low temperature. In using such alloy, one simply pours the liquid metal in the can containing the specimen and allows it to chill cast. The can with the spaced speci~
men trapped in the ~rozen metal is then placed in the pressure vessel. It will be appreclated that in loading F.~-30 CIP

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the can with the transfer medium, the latter surrounds the bag in whole or in part. Thus when the can and con-tents are subjected to heat and pressure in the pressure vessel, isostatic compaction of the specimen occurs.
S Gaseous reaction products bubble through or diffuse to the surface of the pressure transfer medium. The use of metal alloy is preferred because it is easy to use, chill casts, and can berendered liquid at reasonably low temperature, accomodates well for shrinkage of the sampLe incurred in the subsequent processing.
In order to effect consolidation of the infused fibers and subsequent pyrolysis, the can with its con-tents is then preferably sub~ected to hot isostatic pressing at pressures that may be as high as 15000 psi lS and at tempera~ures brought up to above 400 C at a rela-tively slow rate, e.g~ 20-C/hour. Where the transfer medium is a metal alloy, the latter is selected to be molten at the temperature at which initial consolidation occurs, e.g. from about 150-C to 300 C. Above those tem peratures, the consolidated specimen will pyrolyze to basically form a carbon body. It is important to avoid both depressurization and cooling of the sample between consolidation and pyrolysis, because pyrolysis under pressure yields samples with fewer cracks. During pyrolysis, the specimen decomposes in par~ to yield a number of gases, such as ammonia, which collect within the can, ultimately providing a shrunken carbon skele-ton.
The can is allowed to cool under pressure to below about 200 C beore removal from the pres~ure ves3el. To remove conten~s of the can, one need only remelt the alloy surrounding the specimen thereby permittlng the spesimen and any holder to rise to the 9urface of the F~-30 CIP

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molten metal.
It is hypothesized that in this hot isostatic pro-cessing, as evidenced by the low carbon yields and microstructure of the resulting product, the less stable center regions of the preoxidized PAN fibers are "squeezed out" during consolidation and pyrolysis. The result following graphitization, is that there is a pre-dominantly relatively coarse lamellar microstructure (as compared to that of graphite fibers) consistiny of distorted ribbons extending several fiber diameters in the off-axis direction that have a general alignment in the longitudinal axis of the product. High axial modu-lus, high transverse modulus and high shear strength result from this graphitic, r;bbon-like structure.
For a better understanding of the present invention, representative examples are given as follows, all per-centages being by weight unless otherwise indicated.
Densities of samples were measured by the Archimedes ~ ; !
technique, typically using propanol to infiltrate the sample pores, to provide apparent densities.
EXAMPLE I
A two meter length of PAN Eiber~, preoxidized to approximately 8 weight percent oxygen~ was wound on a cylindrical glass mandrel and bathed in boiling ethylene glycol for 15 minutes. The mandrel and ibers were removed from the alcohol bath and permitted to cool to room temperature. Upon removal of the treated wound fibers from the mandrel, the resulting product main~
tained its cylindrical shape and appeared to have sintered into a substantially unified structure~
EXAMPLE II
A specimen, formed of 34 ends o a 60Q0 filament ~ow ~ 3 ~

of an oxidized PAN-based fiber (Hysol Grafil SA~, 10 wt.% Oxygen), was laid into a ~" x 5" area o~ a steel mold in a unidirectional fashion. Approximately 100 cc of ethylene glycol was poured over the fiber in order to completely saturate it. After an imbibition period of thirty ~inutes, the specimen was pressed to form a unified structure having a thickness of .056".
EXAMPLE III
A preform, about 50" in length, formed of 588 ends of 10 ply, Z-twist preoxidized PAN-based fiber (Courtelle), was wrapped on a vertical frame in a uni-directional ~ashion and pulled into a tube (1.23" inter-nal diameter) made of FEP fluoropolymer. A 9" specimen, cut from the filled tube, was plasticized by absorption of deioni2ed water in an amount of about 90~ of the dry weight o the fiber, and sealed in a bag formed of poly~
tetrafluorethylene film.
The bag was inserted into a stainless steel can and surrounded with molten PbBi alloy that was allowed to set. The can was then hot isostatically pressed at 15x103 psi, while the temperature was increased from room temperature to 600 C at a rate of about 5-C/15 minutes. The pressure and temperature were then reduoed to permit removal of the specimen from the can and alloy. Following removal of the specimen from the press, the specimen was subjected to high temperature pyrolysis up to 2500 C under argon in a closed-atmosphere, quartz and graphite apparatus utiliziny a Westinghouse R/F Generator as an inductive heat source~
After an ini~ial thorough atmo~phere purge with argon, heating was initiated, bringing the specimen fsom room temperature ~1 C) to 2500 C at a rate of 100-C/hour.

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The specimen was allowed to slowly cool in the furnace under ~he argon.
Following heat treatment, the density of the speci-men was measured in an isopropyl alcohol solution and found to be 2.14 g/cc. Young's modulus, measured ultra-sonically axially was 26.3x106 psi, and 1.3x106 psi, transversly.
X-ray measured crystal spacings taken on the sample confirmed the graphite nature of the fibrous carbon structure. The interlayer C-spacing was measured at 3.383 A.
EXAMPLE IV
A specimen was prepared as in Example III except that it was subjected to heat treatment to 3200-C.
X-ray measurement provided as crystal spacing of 3r359 A, extremely close to the theoretical crystal spacing o~
3.354 A for graphi~e. The Youngls modulus, measured by flexure, was 47.9x106 psi.
EXAMPLE V
A preform, about 50" in length, formed of 586 ends of 10 ply, Z-twist preoxidized PAN-based fiber (Courtelle), was wrapped on a vertical frame in a uni-directional fashion and pulled înto a tube ~1.23'l inter-nal diameter) made of FEP fluoropolymer. A 10"
specimen, cut from the filled tube, was plasticiæed by absorption o deionized water in an amount of about 80 of the dry weight of the fiber, and sealed in a bag ~ormed of polytetrafluorethylene film~
The bag was then hot isostatically pressed in a PbBi alloy at 15x103 p~i while increa~lng the temperature from room temperature to 700-C at a rate of about 5-C/15 minutes. The pressure and temperature were then reduced . .

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to permit removal of the specimen from the alloy and can. Following removal o~ the specimen from the pressure vessel, the density was measured in isopropyl alcohol as 1.67 g/cc. The specimen was then heat treated as in Example III, but only to 1600 C at a rate of lQO C/hour, and allowed to slowly cool in the furnace under the argon.
Following heat treatment, the density of the speci-men was measured in an isopropyl alcohol solution and found to be 2.03 g/oc. Flexure strength and modulus were measured as respectively 17.8xlU3 psi, and 8.24 X106 psi. Sonic modulus was measured in the axial direction at 10.6x106 psiO The flexural modulus was measured at 9.49xlO~ psi in the axial direction and 4.26x106 psi ln the transverse direction. Compression strengths were 18.1x103 psi axially and 2.4x103 psi transversely. Thermal conductivity at 1600-C was 0.360 w cm~l ~C-l. Diffusivi~y at 1600-C measured O.L06 cm2. Interlaminar ~hear o~ 1. gaxlo3 psi was ~ound.
Thermal expansion at 1600-C was measured as 0.34%. X-ray measurement of the interlayer spacing at 3~43 A
again confirmed the graphite nature of the fibrous car;~
bon structure.
EXAMPLE VI
A preform, about S0" in length, formed of 38 ends o~ 10 plyt Z-twist preoxidized PAN-based fiber (Courtelle), was hand-wrapped in a unidirectional hori-zontal fashion and pulled into a tube ~1.23" interna~
diameter) made of FEP fluQropolymer. The filled tube was ~hen lnserted into a ~tainless steel tube and the latter was drawn down to an internal diameter o~ 1.118".
Following drawing, a 9" specimen wa~ cut rom ~he tube * trade m~

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and the external stainless jacket was removed using a Bridgeport Millng Machine, restoring the FEP as the outer casing. The specimen was thPn plasticized by absorption of deionized water in an amount of about 70%
of the dry weight of the fiber, and sealed in a poly-tetrafluorethylene bag.
The bag was then hot isostatically pressed as described in Example III, the specimen was removed from the press and heat treated to 2500 C as in Example III.
Following heat treatment, the bulk density was measured as 1.77 g/cc. The specimen was then placed inside a metal can and the remaining space in the can was filled with petroleum-based Ashland 240 pitch. The specimen was impregnated with the pitch by hot isostatically pressing the can at 15x103 psi, while increasing the temperature from room temperature to 600 C at a rate of about 5-C/15 minutes. The pressure and temperature were then reduced to permit removal of the specimen from the canO Following removal o the specimen from the can, the specimen was sub~ected to high temperature pyrolysis as described in Example III.
Following ~he second heat treatment, a number of test~ were conducted on the resulting product. The bulk density was measured at 1.93 g/cc, a substantial incre~se over the density measured following the first heat treatment. Interlaminar shear strength measured greater than 3.47x103 p5i . Flexure strength of 33x103 psi, a modulus of 40X106 psi, and elongation of 0.085%
were also found in measuring the specimen following the second heat treatment.
EXAMPLE VII
To obtain comparative data, unplasticiZed ibers F~-30 CIP

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were treated by a process similar to that set forth in ,i Example III, To this end r as shown in the following Table, preoxidized fibers having a density of 1~47 (Grafil SAF ~rom ~ysol Grafil, a polyacronitrile fiber believed to include 5 wt. percent methyl acrylate and 1 wt. percent itaconic acid) were treated under various conditions by the hot isostatic process without any infusion of plasticizer. In the selected runs shown, the temperatures are in degrees C, the pressures in pounds/in2 and the resulting densities in grams/cc~
TABLE
Sample ~ Temp.Pressure Density 1 150 69 1.46 2 175 130 1.48 3 200 256 1~49 4 225 367 1.5 70015,000 1.72 Exemplary fibers from the process shown as sample #4 were treated at graphitization temperatures of 1600-C, 1750~C and 2300 C to yield respective produ~ts with den-sities of 1.85, 186 and and 1.89, considerably below the densities achieved in Examples III throuqh V above wherein a plasticizer was usedO
Since certain changes may be made in the above described processes and products without departing from the scope of the inventions herein ~nvolved, i~ .is intended that all ma~ter contained in ~he above descrip~
tion or shown in the accompanying drawing shall be interpre~ed in an illustrative and not in a limiting sense.

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Claims

THE EMBODIEMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for hot isostatic pressing comminuted material in a pressure vessel, said process comprising the steps of:
loading said material into an envelope;
placing said envelope with said material into a receptacle;
surrounding said envelope in said receptacle with a pressure transfer medium;
positioning said receptacle with said envelope and transfer medium in the cavity of said pressure vessel;
applying heat and pressure to said transfer medium in said receptacle at levels and for such time as is sufficient to compact said comminuted material into a substantially solid body.
2. A process as defined in claim 1 wherein said transfer medium is a metal that melts at a temperature below the temperature required to consolidate said material.
3. A process as defined in claim 2 wherein said metal is an eutectic alloy.
4. A process as defined in claim 3 wherein said alloy is a bismuth-lead alloy.
5, A process as defined in claim 1 wherein said comminuted material is carbonaceous.