UI{IQUE PROPERTIES OF FLEXIBLE CARBON FIBERS
Gnonen
National
E. Cn¡Ncn
Carbon Compang,
(Manuscript
Fostor,ict,, Ohio
received September 18, f 96f )
A comparison is made between the physical properties of flexible carbon fibers in batting, felt, yarn,
andclothandconventionalcarbonandgraphite,
Thenatureoftheproductimposesnoveltestingmethods
for the carbon industry. A description of these methods is also given. The unusual properties of these
materials such as their strength properties, strength to weight ratios, thennal mass behavior, thermal
and electrical conduct'ivity are discussed in relation to various laboratory and engineering applications.
I. INTRODUCTION
The title "{Jnique Properties" may seem
presumptuous but we hope you will agree that
there are some unique properties in the
flexible carbon products described in this
paper.
It is appropriate to take a short detour to
some non-carbon viewpoint to see the generally unique position of our specialty in the
landscape of physics and chemistry. What
other element in our earth is light, strong,
infusible, non-toxic, electrically conductive,
rigid, machinable, chemically inert in elemental form, chemically unsurpassed in organic
materials for variety of compounds? We are
dealing with the most exciting element in the
whole periodic table. The same char of cave
man campfires is the moderator in atomic
reactors. The energy cycle in the sun requires
carbon so that it literally lights our world but
it is still the economically available, widely
distributed, black raw material of human
industry.
But, specifically, we know about massive
carbon as crystalline natural graphite. We
use it in electrodes, blocks, pipe, and many
forms analogous to other massive ceramic
materials. The technology is actually that of
ceramics. In lamp filaments, carbon has been
seen in thin, relatively rigid strips. To weave
these slivers into cloth or form them in yarn
or felt would not be possible. Research
Iaboratories have produced fantastically strong
single crystal whiskers of graphite but where
can you get a foot long whisker let alone a $
mile of flexible carbon? Now yarn can be
made in these lengths.
Sinceearly 1959,l{ational CarbonCompany
has been producing strong, flexible cloth with
high purity graphite filaments. These webs
are far smaller than lamp filaments. Diameters
range from 4 to l5 ¡.r,in our textiles compared
with 25-160 ¡z in lamps. In 1960 a group of
carbon and graphite felts and battings were
made commercially. In 1961, development
quantities of yarn have been shipped.
II.
RAW MATDB,IAL
Graphite electrodes can be made from
almost any carbonaceous raw material that
leaves a high carbon residue on heating. Much
the same is true in the manufacture of carbon
and graphite fibers and fabrics. However,
with graphite fibers there is an additional
requirement; the raw material must char, not
melt, when processed. The typical starting
materials of today's graphite fahrics are
purified cellulosic and regenerated cellulosic
materials. A complex production process, for
which patents have been applied, converts
fiber or fabric to graphite in an electrically
heated furnace operated at 2700'C. The
fibers are already woven, felted or spun before
the furnacing operation. The product from
589
590
FIFTH CABBON CONFERENCE
TABLE I
Basi,cDófferencesBetweenFleni,bleCorbon Form,s
Textile fom
Volatile
content,
(to 2700"C)
Graphite
Carbon
Carbonaceous
Linear
shrinkage
(to 2700"C)
Strength
of cloth
1 in. strips
Temperature for
19¿,weight loss
inShr
0
6Yo
t80a
25 lbs
5 lbs
100 lbs
500"c
350"C
the furnace needs only to be cleaned of loose
pa,rticles and inspected before packaging for
shipment. The process has produced tonnage
quantities of cloth and felt.
III.
GENERAL
PROPER,TIES
n'lexible fibers are made in three degreesof
pyrolysis. All participate in non-fusible, nonflammable properties. They can be distinguished by volatile content and by threshold
oxidation temperature. For the purpose of'
this discussion these can be called carbonaceous, carbon and graphite forms. The
most extensive property determinations have
been made with the graphite form and the
bulk of the discussion will center on the
graphite. Table I shows the basic distinctions between these classesof textiles.
Commercial interest in the carbonaceous
form has centred principally upon composites
or laminates formed with resin binders for
relatively low temperature thermal service.
The main advantage of the carbonaceous
material over other more widely used resin
fillers is that there is no melting point so
that in high temperature service the fibers
retain their entity. It also has a high strength
to weight ratio.
The carbon forms are used principally
where low electrical conductivity, thermal
insulators are required. For example, a carbon
felt crucible liner performs very well in
induction heated vacuum fusion analytical
apparatus.
Graphite fibers have also been very successful when used in resign bonded laminates and
r00"c
in thermal insulation applications.
In
addition such graphite fibers have unique
application possibilities as high and low
temperature electrical heating elements, filters, teflon composite bearings, soft gaskets,
corrosion resistant electrical conductors, carbon bonded in structural parts, flexible conveyors in high temperature service and as
thin mold protectors in metallurgy.
Regardless of the form in which graphite
textiles are produced, all have a number of
properties in common with any elemental
carbon. Certain properties are distinctive of
fibers. Tahle II lists a number of these properties.
TABLE II
Properties Common to all Grad,es Gro,phóte Fi,bers
1.5-1.8
Density of fibers, g/cm3
0.04 max.
Ash, wto/o
3
Surface area, m2/gm
4to 15 ¡t
Fiber diameter
Specific heat B.t.u.i(lb) ("F) at 70'F 0 . 1 6
0.40
mean to 2700'F
approx. 0.9
Emissivity
Melting point: does not melt, sublimes at about
6600"F(3650'C)
Yapor pressure,¡r at (2000"C)
(2250"c)
(2500'c)
Thermal conductivity:
B.t.u./hr/sq ftl'F/ft
0.006
0.21
3.8
22 (of fiber)
Coefficient of thermal expansion (estimated)
Ito4
20'C x r0-6/'C
2lo 5
20 to 600"C x l0-6/"C
4to9 x 106
Modulus of elasticity, psi
4.5
Work function, electron V
26,000
Ileat of sublimation, B.t.u./lb
20'C 0.0033
Electrical resist'ivity, ohm.cm
Tensile strength, Ib/in.z
500'c 0.0025
r500'c 0.0015
50 x 103
UNIQUE PROPDR,TIES ON' N'LEXIBLE CARBON N'IBERS
591
Frc. l. Table üop view of cloth, felt, nei and yarn.
The high surface area of 3 ^rlg and the
small diameter of the filaments (4-15 ¡r) would
suggest that this material would be more
readily oxidized than massive graphite in hot
air. Ifowever, the high purity excludes most
of the oxidation promoting catalysts. This
results in measured oxidation rates in still air
given in Table III.
TABLE III
Oridation oJ Graphite Cloth in StilL A'r
Temperature
fc)
34[3
360
371
399
482
Days to lose
1% wt
co
showing the arrangement of fibers and their
separate structure in flexible cloth.
n'igure 4 showsa magnified endview of fibers
of a typical graphite cloth. The structure of
the individual flber is much finer grained than
usual forms of carbon such as coke, natural
graphite and other massive forms of carbon"
I{ew methods had to be devised to support,
polish and magnify a fiber specimen for
photomicrographic
study.
Conventional
microtome techniques can not be used on the
non-ductile graphite material.
The densenature ofthe fibers is again illustrated in X'ig.5. A few longitudinal holescan b e
identified but the basic integrity ofthe filament
r00
t0
1.0
0.1
Now is the time to get a closer look at the
kind of material we are discussing. A number
of forms have beenprod.uced.
In Fig. I we see cloth weighing about
8 ozfydz, net weighing I! oz, felt weighing
L4 oz, and yarn of about 4000 denier. This
non-carbon industry term is a ,.lengthdensity" figure. Denier is the gram weight of a
9000 meter length of yarn. Higher numbers
mean heavier yarn.
X'igure 2 shows the varietyof weaves which
can be used for cloth. X'elt and yarn are exact
pseudomorphs of their cellulosic raw materials.
Figure 3 shows a photomicrograph of the
edge view of plain we&ve graphite cloth
Fre. 2. Composite view of plain, basket, satin
and net weaves.
592
FIFTII
CARBON
CONFDRENCE
can be seen. This is an additional reason for
the unexpected resistance to oxidation mentioned earlier.
IY.
SPDCIFIC
PRODIICTS
The properties of a popular commercial
grade of'cloth (WCB) are given in Table IV.
Breaking strength in tension derived from
thesefiguresis about 8600psi. This is 10 times
TABLE I\¡
Clnracteri,zati.on o.f Stand,ard, Gra,phóte Cloth
Grctcle WCB
Fre. 3. Edge view- of cloth a,b150 magnification.
a Weight, oz/ydz
a Gage, in.
a Count, yarns/in. warp
fiIl
tr'ilaments/yarn bundle
Filament diameter, in.
a Tensile strength, Ib/in. width
¡ Permeability, Ctr'M air/ftz
b 0.5 in HzO AP
Electric resistanceohm/sqin.
70'F
r000'F
3000"¡'
/.t)
o.025
26.9
22.7
t440
0.0003
L20
0.49
0.38
0.20
A ASTM D39.59.
TJASTM D737.46
Frc. 4. End view of fibers in graphite cloth at
4000 magnification.
Fre. 5. C)biique section of flexible
libers at 250 masnification.
graphite
UNIQUE
PR,OPERTIES
OF FLEXIBLD
CARBON
FIBERS
593
the strength of a typical 12 in. diameter and adapted to these products. A complete
gra,phite electrode for melting steel. Tensile textile laboratory was added to the normal
strengths of individual filaments, not woven carbon testing facilities. In order to specify
in cloth, have been measured at 50,000- suitable raw materials, laboratory personnel
100,000psi. This is a long way from 3,000,000 had to become familiar with textile manupsi for laboratory single crystal graphite facturing methods and to adopt such textile
whiskers but is certainly unique in commer- property terms as gage, count, filaments/
cially available graphite.
yarn, etc. Textile tests for "crimp', (waviness
A graphite ya,rn now being developed for in woven yarn) have been useful in defining
use in filament windine is characterized in permanent set in graphite yarns. The rela_
Table V.
tion of number of yarns to permeability of final
graphite product can be defined with textile
TABLE V
tests. The tests often has to be adapted to
Graphite Y arn-Grad,e I7B-0030
graphite with special techniques. X'or example, the absenceof a yield point in graphite
Nominal filament diameter, in.
0.00018
made
in necessary to apply load more slowly
Filaments per ply
1600
Plies per yarn
l0
to get a breaking strength value rather than
Yarn gage (diameter), in.
0.040
an impact strength number. Furthermore,
Yarn denier
4320
it was necessaryin some instances,i.e. meaYarn weight, yd/lb
1040
surement of specific resistance, to combine
Room
textile
test methods with non-textile phvsical
Temperature 3000'Fa
units.
Averagé breaking strength, lb
20
40
Average rosistance, ohmiin.
3.1
1.6
a Estimated
With a strength of about 20 lb in untreated
form and much greater in resin impregnated
form, filament winding is a very practical
possibility. This then becomes the high
strength to weight ratio fiber needed in
missiles. The temperature limit of the fiber is
not 2500"n'but above 2500"C(4500'F).
The relation of graphite fibers to other
fibrous materials is shown in Table VI.
Thus at room temperature the strength
to weight ratio of fibrous graphite is in the
same order as the other fibrous materials
with which it is compared in Table VI. However, at temperatures above 1700'C, carbon
and graphite are the only materials with
useful mechanical strength.
V. CARBON TECHNOLOGY
Measurement of flexible graphite properties
made it necessary that the entire field of
textile testing and terminology be reviewed
38
VI.
CONCLUSIONS
A completely new family of previously
unavailable flexible carbon and graphite
fibrous materials, including woven and knit
fabrics, felt and yarn has been developed and
made in commercial quantities. These materials are characterized by unique flexibility,
high purity, good strength-increasing with
temperature, light weight and easy fabrication, low thermal expansion, good resistance
to thermal shock, extreme high temperature
stability and cbrrosion resistance and good
neutron moderating ability. Electrical resistivity varies from the practically non-conductive carbonaceous fibres to the typically
low resistivity of massive graphite. Thermal
conductivity, while theoretically covering a
similarly wide range, actually depends largely
on the bulk density of the specific material.
The unique physical, chemical and electrical
properties ofthese new materials suggesttheir
use in many valuable and novel applications.
Among these the following are typical: (l)
594
FIT'TH
CARBON
CONX'EIiENCE
Resistance heating-space heaters, water
heaters, etc. (2) Filtration of hot, corrosive
gases and liquids. (3) High strength, low
density laminates for nose cones and other
missile parts. (4) Reinforced plastics. (5)
Bearings, gaskets and packing. (6) High
temperature thermal insulation and radiation
shields. (7) Nuclear moderators. (8) Strongn
light weight structures made from yarns by
filament winding techniques. The evaluation
of these fibrous forms of carbon and graphite
has greatly expanded the potential for the
industrial, structural and military uses of
elemental carbon.
TABLE VI
Flenible GraphóteCompareilwith other Fibroz¿sMa'terials
Tenacity
g/denier
Graphite fiber
Carbon fiber
Carbonaceous fiber
Graphite beam
Piano wiro
Structural steel
Refrasil
Fiberglass
Nylon
Rayon
Wool
2.1
I.5
0.94
0.3
6.3-6.9
4.5-8.0
2.0-4.6
L.2-t.7
¡ Shrinks at 300"C. Does not melt.
Tensile
strength
(lb/in.'z x
103)
37
qn
t7
0.9
350
200
7.9
2r0
65-l 17
36-83
20-29
Speciflc
gmvity
t.4
1.4
t.4
1.8
7.8
7.0
qq
2.54
l.I4
1.5
1.32
Strength
to weight
ratio
(in.)
0.71
0.53
0.33
0.01
t.25
0.7I
0.r0
2.29
2.t8
r.1I
Max temp
(MP or
decomp.)
("c)
3000
3000
300e
3000
1500
1500
1700
600
260
250
300