U.S. patent number 3,841,079 [Application Number 05/244,544] was granted by the patent office on 1974-10-15 for carbon filaments capable of substantial crack diversion during fracture.
This patent grant is currently assigned to Celanese Corporation. Invention is credited to Michael J. Ram, John P. Riggs.
United States Patent |
3,841,079 |
Ram , et al. |
October 15, 1974 |
CARBON FILAMENTS CAPABLE OF SUBSTANTIAL CRACK DIVERSION DURING
FRACTURE
Abstract
An improved carbon filament is provided having an internal
structure capable of increasing the amount of work required to
break the filament. The internal structure of the carbon filaments
of the present invention as evidenced by the means apparent
fracture surface energy of the same facilitates the exhibition of
satisfactory strength properties even if accompanied by the
presence of gross inhomogenities and structural flows such as
commonly encountered in carbon filaments of the prior art.
Inventors: |
Ram; Michael J. (West Orange,
NJ), Riggs; John P. (Berkley Heights, NJ) |
Assignee: |
Celanese Corporation (New York,
NY)
|
Family
ID: |
26703817 |
Appl.
No.: |
05/244,544 |
Filed: |
April 17, 1972 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
28545 |
Apr 14, 1970 |
3657409 |
|
|
|
Current U.S.
Class: |
57/243;
423/447.2 |
Current CPC
Class: |
D01F
6/18 (20130101); D01F 6/38 (20130101); D01F
9/328 (20130101); D01F 9/225 (20130101) |
Current International
Class: |
D01F
9/22 (20060101); D01F 9/32 (20060101); D01F
6/18 (20060101); D01F 9/14 (20060101); D02g
003/02 () |
Field of
Search: |
;57/14R,14BY,139
;423/447 ;8/115.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Petrakes; John
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of our U.S. Ser. No. 28,545, filed
Apr. 14, 1970 entitled "Improved Process for the Production of
Acrylic Filaments" (now U.S. Pat. No. 3,657,409).
Claims
We claim:
1. An improved carbon filament comprising at least 90 percent
carbon by weight having an unusually highly developed microporous
and fibrillar internal structure substantially coextensive with the
length of said filament capable of diverting a propagating crack
during fracture thereby increasing the amount of work required to
break the filament as evidenced by a mean apparent fracture surface
energy of at least 50 joules per square meter.
2. An improved carbon filament according to claim 1 wherein said
filament is present in a multifilament yarn.
3. An improved carbon filament according to claim 1 wherein said
filament is present in a multifilament tow.
4. An improved carbon filament according to claim 1 wherein said
filament comprises at least 95 percent carbon by weight.
5. An improved carbon filament according to claim 1 wherein said
filament comprises at least 98 percent carbon by weight.
6. An improved carbon filament according to claim 1 which exhibits
a mean single filament Young's modulus of about 20 to 50 million
psi.
7. An improved carbon filament according to claim 1 wherein the
cross-sectional configuration thereof is substantially round.
8. An improved carbon filament according to claim 1 which exhibits
a mean apparent fracture surface energy of at least 60 joules per
square meter.
9. An improved carbon filament according to claim 1 which exhibits
a mean apparent fracture surface energy of at least 70 joules per
square meter.
10. An improved carbon filament comprising at least 95 percent
carbon by weight which exhibits a mean single filament Young's
modulus of about 20 to 50 million psi and has an unusually highly
developed microporous and fibrillar internal structure
substantially coextensive with the length of said filament capable
of diverting a propagating crack during fracture thereby increasing
the amount of work required to break the filament as evidenced by a
mean apparent fracture surface energy of at least 70 joules per
square meter.
Description
BACKGROUND OF THE INVENTION
Carbon filaments have long been known and are discussed in the
technical literature. It has generally been recognized that the
structure of the carbon filaments is influenced to some degree by
the nature of the fibrous material which is thermally converted
into the carbon filaments and by the processing conditions utilized
during the thermal conversion. It has also been recognized that
carbon filaments are known to possess an internal structure which
is somewhat fibrillar in nature and that some micropores (i.e.,
microvoids) in addition to the usual structural flaws may be
detected within the same. See, for instance, the article by R.
Perret and W. Ruland appearing in J. Appl. Cryst., Vol. 3, Pages
525-532 (1970), entitled "The Microstructure of PAN-Base Carbon
Fibres."
In the search for high performance materials considerable interest
has been focused upon carbon fibers. Industrial high performance
materials of the future are projected to make substantial
utilization of fiber reinforced composites, and carbon fibers
theoretically have among the best properties of any fiber for use
as a high strength reinforcement. Among these desirable properties
are corrosion and high temperature resistance, low density, high
modulus and high tensile strength. Carbon fiber reinforced
composites are commonly formed by incorporating carbon filaments in
a resinous or metallic matrix. Representative uses for carbon fiber
reinforced composites include aerospace structural components,
rocket motor casings, deep-submergence vessels and ablative
materials for heat shields on re-entry vehicles, etc.
Heretofore, those material scientists interested in attempting to
improve the internal structure of carbon filaments have directed
their attention largely to elimination of strength reducing flaws
within the same. See, for instance, the article by J.W. Johnson and
D.J. Thorne appearing in Carbon, Vol. 7, Pages 659-661 (1969),
entitled "Effect of Internal Polymer Flaws on Strength of Carbon
Fibres Prepared From an Acrylic Precursor," and the article by John
W. Johnson appearing in Applied Polymer Symposia, Vol 9, Pages
229-243 (1969), entitled "Factors Affecting the Tensile Strength of
Carbon-Graphite Fibres."
The present invention provides a novel approach to the improvement
of carbon filaments. The carbon filaments of the present invention
possess an internal structure unlike that exhibited by the carbon
filaments of the prior art as discussed in detail hereafter.
It is an object of the invention to provide improved carbon
filaments.
It is an object of the invention to provide carbon filaments
possessing an improved internal structure.
It is an object of the invention to provide carbon filaments
capable of substantial crack diversion upon fracture.
It is an object of the invention to provide an improved carbon
filament possessing an unusually highly developed microporous and
fibrillar internal structure.
It is another object of the invention to provide an improved carbon
filament possessing an internal structure which facilitates
exhibition of highly satisfactory strength properties even if
accompanied by the presence of structural flaws such as commonly
encountered in carbon filaments of the prior art.
These and other objects as well as the scope, nature, and
utilization of the invention will be apparent from the following
description and appended claims.
SUMMARY OF THE INVENTION
An improved carbon filament is provided comprising at least 90
percent carbon by weight having an unusually highly developed
microporous and fibrillar internal structure substantially
coextensive with the length of the filament capable of diverting a
propagating crack during fracture thereby increasing the amount of
work required to break the filament as evidenced by a mean apparent
fracture surface energy of at least 50 joules per square meter.
In a preferred embodiment of the invention the carbon filaments
contain at least 95 percent carbon by weight and additionally
exhibit a mean single filament Young's modulus of about 20 to 50
million psi.
DESCRIPTION OF DRAWINGS
FIG. 1 and FIG. 2 are photographs made with the aid of a scanning
electron microscope at a magnification of 5,740X of matching sides
of the primary fracture surface of a carbon filament of the present
invention formed in accordance with the procedure described in the
Example which shows a flaw having a maximum dimension of 1.4
micron.
FIG. 3 is a graph which illustrates for carbon filaments of 35
million psi Young's modulus having various average flaw sizes the
relationship between the mean apparent fracture surface energy and
the mean single filament tensile strength.
FIG. 4 is a schematic view of a representative apparatus
arrangement suitable for forming acrylic filaments which are
subsequently thermally transformed into the carbon filaments of the
present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
The improved carbon filaments of the present invention are derived
from polymeric fibrous materials. For instance, an acrylic polymer
may be formed into an acrylic filament possessing a requisite
internal structure, and the resulting acrylic filament converted to
a carbon filament possessing an improved internal structure through
appropriate thermal processing.
Acrylic filament precursors suitable for thermal conversion into
the improved carbon filaments of the present invention may be
formed in accordance with embodiments of the process described in
our commonly assigned U.S. Ser. No. 28,545, filed Apr. 14, 1970,
and entitled "Improved Process for the Production of Acrylic
Filaments" (now U.S. Pat. No. 3,657,409) which is herein
incorporated by reference. Such improved acrylic filaments are
claimed in our commonly assigned U.S. Ser. No. 244,541, filed
concurrently herewith, entitled "Improved Acrylic Filaments Which
Are Particularly Suited for Thermal Conversion to Carbon
Filaments."
More specifically, the fiber-forming acrylic polymer selected for
use in the formation of the acrylic precursor may be either an
acrylonitrile homopolymer or an acrylonitrile copolymer which
contains at least about 85 mol percent of acrylonitrile units and
up to about 15 mol percent of one or more monovinyl units
copolymerized therewith. An acrylonitrile homopolymer is
particularly preferred. Suitable copolymers commonly contain at
least about 95 mol percent of recurring acrylonitrile units and up
to about 5 mol percent of one or more monovinyl units copolymerized
therewith. Representative monovinyl units which may be incorporated
in the acrylonitrile copolymers include styrene, methyl acrylate,
methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene
chloride, vinyl pyridine, and the like. The acrylic polymers may be
formed by standard polymerization processes which are well known in
the art. Minor quantities of preoxidation or graphitization
catalysts may optionally be incorporated in the bulk acrylic
polymer prior to spinning.
The solvent utilized to form the spinning solution may be
dimethylacetamide. The solvent is sometimes identified as
N,N-dimethylacetamide or DMAC, and the chemical formula CH.sub.3
CON(CH.sub.3).sub.2. The standard technical or commercial grade of
dimethylacetamide may be employed as the solvent in the formation
of the spinning solution.
The spinning solution may be prepared by dissolving sufficient
acrylic polymer in the dimethylacetamide solvent to yield a
solution suitable for extrusion containing from about 15 to 30
percent arcylic polymer by weight based upon the total weight of
the solution, and preferably from about 18 to 25 percent by weight.
In a particularly preferred embodiment of the invention the
spinning solution contains the acrylic polymer in a concentration
of about 20 to 22 percent by weight based upon the total weight of
the solution. The low shear viscosity of the spinning solution
should be within the range of about 80 to 3,000 poise measured at
25.degree.C. and preferably within the range of about 125 to 1,500
poise measured at 25.degree.C. If the spinning solution low shear
viscosity is much below about 80 poise measured at 25.degree.C.,
spinning breakdowns commonly occur. If the spinning solution low
shear viscosity is much above about 3,000 poise measured at
25.degree.C., extremely high spinning pressures are required and
plugging of the extrusion orifice may occur.
In a preferred precursor formation technique the spinning solution
additionally contains about 0.1 to 5.0 percent by weight based upon
the total weight of the solution, and preferably about 0.5 to 2
percent by weight based upon the total weight of the solution of
lithium chloride dissolved therein. The incorporation of lithium
chloride serves the function of lowering and preserving upon
standing the viscosity of the spinning solution. The desired
solution fluidity and mobility are accordingly efficiently
maintained even upon the passage of time. For instance, it has been
found that a solution comprising 22 parts by weight acrylonitrile
homopolymer, 2 parts by weight lithium chloride, and 76 parts by
weight dimethylacetamide solvent commonly exhibits a relatively
constant low shear viscosity of about 150 poise measured at
25.degree.C. after standing for 250 hours. A solution containing an
even lesser concentration of acrylonitrile homopolymer and no
lithium chloride (i.e., 20 parts by weight polymer, and 80 parts by
weight dimethylacetamide) tends to increase in viscosity upon
standing and exhibits a low shear viscosity of about 1,000 poise
measured at 25.degree.C. after about 2 1/2 hours. The lithium
chloride may be dissolved in the dimethylacetamide solvent either
simultaneously with the acrylic polymer or before or after the
acrylic polymer is dissolved therein. Minor quantities of
preoxidation or graphitization catalysts may optionally be
incorporated in the spinning solution.
The spinning solution is preferably filtered, such as by passage
through a plate and frame press provided with an appropriate
filtration medium, prior to wet spinning in order to assure the
removal of any extraneous solid matter which could possibly
obstruct the extrusion orifice during the spinning operation.
The spinning solution containing the fiber forming acrylic polymer
dissolved therein is extruded into a coagulation bath under
conditions capable of forming an acrylic filament having an
internal structure which is capable upon subsequent thermal
treatment of yielding the improved carbon filaments of the present
invention.
It has been found that acrylic filaments possessing the requisite
internal structure are produced when an essentially non-aqueous
coagulation bath is utilized having a temperature of about
0.degree. to 45.degree.C. (preferably about 10.degree. to
35.degree.C.) which consists essentially of about 55 to 85 percent
by weight of ethylene glycol and about 15 to 45 percent by weight
of dimethylacetamide. When employing dimethylacetamide in the
coagulation bath in concentrations much greater than about 45
percent by weight, then filament breakage tends to occur at the
spinneret. When employing dimethylacetamide in the coagulation bath
in concentrations much less than about 15 percent by weight, then
the resulting filaments tend to lose their substantially round
cross-section and have a tendency to exhibit a more pronounced
bean-shaped configuration. In a preferred embodiment of the
invention the coagulation bath consists essentially of about 60 to
75 percent by weight of ethylene glycol and about 25 to 40 percent
by weight of dimethylacetamide. In a particularly preferred
embodiment of the invention the coagulation bath consists
essentially of about 60 percent by weight of ethylene glycol and
about 40 percent by weight of dimethylacetamide. At the relatively
low coagulation bath temperatures employed the coagulation rate
tends to be relatively slow and to enhance the formation of the
desired fiber internal structure.
The temperature of the spinning solution at the time of its
extrusion should be within the range of about 10.degree.C. to about
90.degree.C., and preferably at about 20.degree. to 30.degree.C. In
a particularly preferred embodiment of the invention the spinning
solution is provided at room temperature, e.g., about 25.degree.C.,
which thereby facilitates expeditious handling and storage of the
same.
The spinneret utilized during the extrusion may contain a single
hole through which a single filament is extruded, and preferably
contains a plurality of holes whereby a plurality of filaments may
be simultaneously extruded in yarn or tow form. For instance, tows
of up to 20,000, or more, continuous filaments may be formed. The
spinneret preferably contains holes having a diameter between about
50 to 150 microns when producing relatively low denier filaments
having an as-spun denier of about 8 to 24 denier per filament, and
holes of about 300 to 500 microns when producing relatively high
denier filaments having as as-spun denier of about 100 to 1,500
denier per filament. Extrusion pressures between about 100 and 700
psig may be conveniently selected, and preferably between about 100
and 400 psig. Spinning or extrusion speeds of about 0.5 to 10
meters per minute (e.g., 3 to 6 meters per minute) may be
employed.
Throughout the extrusion process the coagulation bath is preferably
circulated. A relatively constant composition within the
coagulation bath may be maintained through the continuous
withdrawal and purification of the same. Alternatively, additional
ethylene glycol may be continuously added to the coagulation bath
to preserve the desired proportion of dimethylacetamide to ethylene
glycol within the same. The length of the coagulation bath is
adjusted so that the resulting as-spun filaments are present within
the coagulation bath for a residence time of at least about 6
seconds. For instance, residence times of about 6 to 300 seconds
may be conveniently selected. Residence times less than about 6
seconds tend to result in an insufficiently developed fibrillar
structure within the as-spun filaments. Residence times for the
as-spun filaments in the coagulation bath in excess of 300 seconds
tend to yield no commensurate advantage. Particularly preferred
residence times for the as-spun filament in the coagulation bath
range from about 6 to 50 seconds.
The resulting as-spun filament is next washed with water to remove
dimethylacetamide solvent from the same. The as-spun filament is
preferably washed with water until substantially all residual
amounts of solvent, coagulation bath, and inorganic compound (e.g.,
lithium chloride), if any, are removed from the same. It is
essential that the filament first be exposed to a relatively cool
water wash medium at a temperature of about 10.degree. to
50.degree.C. and preferably at about 10.degree. to 30.degree.C.,
and most preferably at room temperature (e.g., about 25.degree.C.),
for at least about 25 seconds. The entire wash treatment may be
conducted at a temperature within the range of about 10.degree. to
50.degree.C. Alternatively the washing of the filament may be
subsequently continued at a more highly elevated temperature, e.g.,
in excess of about 50.degree.C. to remove additional solvent. In a
preferred embodiment of the invention the initial cold water wash
is conducted for at least about 50 seconds.
When the entire wash is conducted at a relatively cool wash
temperature of about 10.degree. to 50.degree.C., wash times of
about 25 to 240 seconds and preferably about 50 to 120 seconds are
commonly utilized depending upon the filament denier. Longer wash
times tend to yield no commensurate advantage.
It has been found that the initial cool water wash described above
is essential in order to preserve the requisite fiber homogeneity
in the acrylic filament precursor. During the cool water wash a
one-way transfer of residual quantities of the dimethylacetamide
spinning solvent out of the filament is believed to be promoted to
the substantial exclusion of the passage of the molecules of the
water wash medium into the filament. It has been found that if the
as-spun filament is initially washed at a temperature substantially
higher than about 50.degree.C., then the resulting washed filament
tends to contain a significant number of macrovoids and tends to
flatten. At temperatures below about 10.degree.C. the washing
procedure tends to be unduly slow. The residual dimethylacetamide
content of the washed acrylic filaments preferably is no more than
about 5 percent dimethylacetamide by weight, and most preferably no
more than about 0.1 percent by weight, prior to subsequent
processing.
The water wash treatment is conveniently conducted in an in-line
operation with the filament after it leaves the coagulation bath
being continuously passed through a water wash medium which is
continuously regenerated. Conventional filament wash rolls may be
utilized. The filament alternatively may be washed with water while
wound upon a perforated bobbin, or by the use of other washing
means as will be apparent to those skilled in the art.
The as-spun and washed acrylic filament is drawn or stretched from
about 1.5 times its original length up to the point at which the
filament breaks to orient the same and to thereby enhance its
tensile properties. Total draw ratios above about 1.5:1 to 15:1 may
commonly be selected. The drawing is commonly conducted at an
elevated temperature and preferably at a total draw ratio of
between about 3:1 and 12:1. The dense internal filament structure
makes possible the use of the relatively high total draw ratios
indicated. As will be apparent to those skilled in the art, the
drawing of the as-spun and washed acrylic filament may be conducted
by a variety of techniques. For instance, it is possible for the
drawing to be conducted while the filament is (a) immersed in a
heated liquid draw medium, (b) suspended in a heated gaseous
atmosphere, (e.g., at a temperature of about 120.degree. to
200.degree.C.) or (c) in contact with a heated solid surface (e.g.,
at a temperature of about 130.degree. to 170.degree.C.). If
desired, the total draw imparted to the filament may be conducted
by a combination of the foregoing techniques. When draw techniques
(b) and (c) are utilized, it is essential that the acrylic filament
be provided to the draw zone in an essentially dry form in order to
avoid formation. When draw technique (a) is employed, the acrylic
filament is subsequently washed to remove the draw medium and is
dried. Additionally, the liquid draw medium may also serve a
washing and/or coagulating function wherein residual quantities of
dimethylacetamide are removed from the water washed fiber.
In a preferred embodiment of the invention the washed acrylic
filament is at least partially drawn while immersed in a hot
glycerin bath. In a particularly preferred embodiment of the
invention the filament is drawn while immersed in a hot glycerin
bath at a temperature of about 80.degree. to 110.degree.C. and at a
draw ratio of about 1.5:1 to 3:1 (preferably at a temperature of
about 90.degree.C. and a draw ratio of about 2:1), washed in cool
water (e.g., at a temperature of about 10.degree. to 50.degree.C.),
and subsequently drawn at a draw ratio of about 3:1 to 6:1 while in
contact with a hot shoe at a temperature of about 100.degree. to
220.degree.C., and preferably at a temperature of about 150.degree.
to 160.degree.C.
The drawn acrylic filaments optionally may be plied to form yarns
or tows of increased total denier as will be apparent to those
skilled in the art prior to thermal conversion into the improved
carbon filaments of the present invention as described
hereafter.
The resulting acrylic filaments are converted to a stabilized or
heat-resistant form which is capable of undergoing carbonization.
The stabilization treatment renders the acrylic filaments
non-burning when subjected to an ordinary match flame while
retaining their original fibrous configuration essentially intact.
The stabilization reaction may be conducted by heating the acrylic
filaments at moderate temperatures in accordance with techniques
known in the art. Such a stabilization procedure is commonly
conducted in the presence of oxygen and results in the formation of
a cyclized and preoxidized product which exhibits a thermal
stability not exhibited by the unmodified acrylic filaments. While
it is possible that the stabilization reaction be conducted on a
batch basis, it is preferable that the stabilization reaction be
conducted on a continuous basis. Catalyzed stabilization reactions
optionally may be selected. The exact stabilization temperatures
employed will vary with the chemical composition of the acrylic
filaments. Preferred stabilization procedures are described in
commonly assigned U.S. Ser. No. 749,957, filed Aug. 5, 1968 (now
abandoned), of Dagobert E. Stuetz, and in U.S. Pat. No. 3,539,295,
of Michael J. Ram, which are herein incorporated by reference.
Other stabilization procedures capable of imparting thermal
stability to the acrylic filaments may be selected. The highly
fibrillar internal structure required to make possible the
formation of the claimed carbon filaments is retained throughout
the stabilization reaction.
The stabilized acrylic filaments are converted to the carbon
filaments of the present invention by thermal treatment at a more
highly elevated temperature of at least 1,000.degree.C., e.g.
1,000.degree. to 2,000.degree.C. in a non-oxidizing atmosphere.
Preferably inert atmospheres such as nitrogen, argon and helium are
employed. The stabilized acrylic filaments are subjected to such
highly elevated thermal treatment until carbon filaments containing
at least 90 percent carbon by weight are formed, and preferably
until carbon filaments containing at least about 95 percent carbon
by weight are formed. In a more particularly preferred embodiment
carbon filaments containing at least 98 percent carbon are formed.
Carbon filaments of optimum tensile strength are formed when the
maximum temperature provided in the heating zone is about
1,500.degree. to 1,900.degree.C. (e.g., 1,800.degree.C.). The
carbon fibers are preferably formed on a continuous basis by
continuous passage through a heating zone containing a
non-oxidizing atmosphere and a temperature gradient in which the
stabilized acrylic filaments are gradually raised to the maximum
carbonization temperature. During the thermal treatment at
1,000.degree. to 2,000.degree.C. a highly developed microporous
structure is inherently imparted to the resulting carbon filaments
wherein a large number of elongated micropores of up to about 25
Angstroms (e.g., about 5 to 15 Angstroms) in thickness are disposed
between the highly fibrillar internal structure which are largely
preserved during the thermal treatment. The increased presence of
the micropores within the highly fibrillar internal structure is
confirmed by small angle X-ray analysis.
The carbon filaments of the present invention commonly exhibit a
mean single filament Young's modulus of about 20 to 50 million psi.
As discussed hereafter, the internal structure of the carbon
filaments of the present invention provides carbon filaments of
improved tenacity in spite of structural flaws such as those
commonly encountered in the prior art. For instance, carbon
filaments commonly possess flaws of 0.5 micron and larger as
discussed in the Johnson, and Thorne and Johnson articles cited
earlier.
The improved carbon fibers of the present invention have an
unusually highly developed microporous and fibrillar internal
structure which is capable of diverting a propagating crack during
fracture thereby increasing the amount of work required to break
the filament as evidenced by a mean apparent fracture surface
energy (i.e., .gamma.a) of at least 50 joules per square meter
(e.g., 50 to 150, or more, joules per square meter). In a preferred
embodiment the carbon filaments exhibit a mean apparent fracture
surface energy of at least 60 joules per square meter. In a
particularly preferred embodiment the carbon filaments exhibit a
mean apparent fracture surface energy of at least 70 joules per
square meter.
It has been found that carbon/graphite filaments fail in a brittle
fashion and that the Griffith failure criterion can be utilized to
elucidate the fracture phenomenology. One form of the simple
Griffith equation which has been applied to the fracture of many
brittle materials is as follows:
Wtb = 1/2 .iota..sub.b .epsilon..sub.b = .iota..sub.b.sup.2 /2E =
.gamma..sub.a /.pi. C
where
WTB = work-to-break (energy to fracture).
.iota..sub.b = breaking stress
.epsilon..sub.b = breaking strain
E = Young's modulus
.gamma..sub.a = apparent fracture surface energy
C = size of critical flaw at which fracture initiated
.pi. = 3.14 . . .
For an ideally brittle material, .gamma..sub.a should correspond to
the energy required to break primary chemical bonds. It is
recognized, however, that in all but extremely rare cases of
apparently brittle fracture, large amounts of plastic work are done
at the crack tip, leading to observed values of .gamma..sub.a much
higher than would be expected from bond-breaking alone. See, for
example, "Fracture" edited by H. Liebowitz, Academic Press, New
York (1968). The apparent fracture surface energy, .gamma..sub.a,
is an intrinsic property of the carbon fiber and accordingly is
dependent upon the internal physical structure of the carbon
fiber.
A technique for the determination of the mean apparent fracture
surface energy, .gamma..sub.a, for a given carbon filament is
described in detail below. Generally stated the WTB and C for a
given carbon filament are determined and the mean apparent fracture
surface energy is calculated therefrom.
1. Single carbon filaments individually are broken while immersed
in glycerin and the stress, strain, and Young's modulus determined
by conventional fiber testing techniques. The use of a glycerin
bath minimizes the formation of secondary fracture surfaces which
tend to be formed in an open atmosphere.
2. The resulting pair of ends for each broken filament is examined
in the field of a scanning electron microscope and compared to
assure a match thus insuring that a primary fracture surface is
being examined. See, for instance, FIG. 1 and FIG. 2 which are
photographs made with the aid of a scanning electron microscope of
the primary fracture surface of a carbon filament of the present
invention formed in accordance with the procedure described in the
Example.
3. The critical flaw, C, which initiated the fracture is located
and the longest dimension on the primary fracture surface is
measured. In FIG. 1 and FIG. 2 the largest dimension of the flaw on
the primary fracture surface of each broken end was found to be 1.4
micron.
The above procedure is repeated a number of times for a given
sample of carbon filaments and the values obtained for WTB and C
are plotted as log WTB vs. log C. A least squares line is drawn
through the points assuming a slope of -1 in accordance with the
theoretical prediction of the Griffith equation. The intercept of
this line is accordingly log (.gamma..sub.a /.pi.) from the which
the mean .gamma..sub.a, or mean apparent fracture surface energy,
is determined. It is recommended that at least four carbon
filaments be broken as described above when determining the mean
apparent fracture surface energy for a given sample of carbon
filaments. The results obtained are, of course, more statistically
accurate as the number of breaks increases.
The mean apparent fracture surface energies of a wide variety of
carbon filaments have been determined. In all instances, the mean
apparent fracture surface energies were substantially below that of
the carbon filaments of the present invention. The following Table
I sets forth the average flaw size and mean apparent fracture
surface energy obtained for representative carbon filaments.
TABLE I
__________________________________________________________________________
Carbon Carbon Average Single Average Mean Apparent Fiber Source
Fiber Designation Filament Young's Flaw Fracture Surface Modulus
Size Energy Mpsi micron joules/sq.m.
__________________________________________________________________________
Great Lakes Carbon 3T.sup.1 31 0.8 33 Corp. 4T.sup.1 39 0.8 31
5T.sup.1 48 0.9 37 6T.sup.1 60 0.9 26 Hercules, Inc. HT-S.sup.2 39
0.7 41 A-S.sup.2 36 1.1 42 HM-S.sup.2 64 1.4 25 Johnson &
Thorne Acrilan 20 0.7 39 Carbon Fibers.sup.3 Celanese A.sup.4 35
1.1 33 B.sup.4 35 0.5 29 C.sup.4 35 1.4 29 D.sup.4 37 1.4 14
__________________________________________________________________________
1 Derived from a dry spun Orlon acrylic precursor. 2 Derived from a
wet spun Courtelle acrylic precursor (sodium thiocyanate spinning
solution). 3 Shown in FIG. 5, following Page 661, of Vol. 7, of
Carbon, article by J W. Johnson and D. J. Thorne cited earlier (wet
spun from DMAC spinning solution employing an aqueous coagulation
bath). 4 Derived from various dry spun acrylic precursors.
An unusually high mean apparent fracture surface energy enables a
larger average flaw size to be tolerated while still obtaining a
high strength carbon filament. As fracture is initiated and a
propagating crack meets a microvoid between fibrils, the crack is
diverted and additional energy is consumed. For example, see the
following Table II.
TABLE II
__________________________________________________________________________
Mean Apparent Fracture Surface Energy Average Flaw Size Average WTB
Average 6b
__________________________________________________________________________
14 joules/sq.m. 0.5 micron 1260 psi 304 Kpsi 27 joules/sq.m. 0.5
micron 2540 psi 434 Kpsi 65 joules/sq.m. 0.5 micron 5850 psi 650
Kpsi 65 joules/sq.m. 1.2 micron 2540 psi 434 Kpsi
__________________________________________________________________________
FIG. 3 illustrates for carbon filaments of 35 million psi Young's
modulus having various average flaw sizes the relationship between
the mean apparent fracture surface energy and the mean single
filament tensile strength.
The following example is given as a specific illustration of the
invention with reference being made to FIG. 4 of the drawings. It
should be understood, however, that it is not essential that the
improved carbon filaments of the present invention be formed
through the utilization of the exact processing parameters set
forth in the Example.
EXAMPLE
Twenty-two parts by weight of polyacrylonitrile homopolymer, 2
parts by weight of lithium chloride, and 76 parts by weight of
industrial grade dimethylacetamide are slurried at room temperature
for 120 minutes by use of a stirred vessel. The slurry is heated to
a temperature of 100.degree.C. over a period of about 90 minutes
where it is mixed with agitation for 2 hours. The resulting
solution is passed four times through a conventional filter press
while at 100.degree.C. over a period of 19 hours in order to remove
any solid contamination. The low shear viscosity (Brookfield) of
the resulting spinning solution after degassing is found to be
about 120 poise measured at 27.degree.C.
The spinning solution is provided in dope bomb 1 under an
atmosphere of nitrogen at 20 psig. The spinning solution is
conveyed to spinneret 2 via the line 4 where it is extruded into
coagulation bath 6. The spinneret 2 is of the standard cup type and
comprises a single circle of 400 holes each having a diameter of
100 microns.
The coagulation bath consists of 60 parts by weight ethylene glycol
and 40 parts by weight dimethylacetamide and is provided at a
temperature of 36.degree.C. The coagulation bath is caused to flow
concurrently with coagulated filament 8 and is maintained at a
relatively constant composition by the continuous addition of
ethylene glycol to the same and the continuous withdrawal of a
portion of the bath. The coagulation bath has a length of 37 inches
and the coagulated filaments are maintained in the same for a
residence time of about 9 seconds.
The coagulated filaments pass under guide 10 which is immersed in
coagulation bath 6 and are conveyed to a skewed roll 12 and wash
roll 14 which is partially immersed in water bath 16 which is
maintained at 23.degree.C. The coagulated filaments are taken up on
roll 12 at a rate of 6 meters per minute. The filaments are wrapped
about skewed roll 12 and wash roll 14 for a residence time of about
125 seconds during which time the filaments are immersed in water
for approximately 25 seconds and withdrawn with water adhering to
the same during which time substantially all residual amounts of
dimethylacetamide are removed from the same.
The washed filaments are next continuously passed through stretch
bath 17 having a length of 15 inches which is provided with
glycerin at 80.degree.C. Rollers 18 and 20 situated outside the
stretch bath and rollers 22 and 24 immersed within the stretch bath
guide the filaments during the stretching operation. The filaments
are next taken up on skewed roll 26 and wash roll 28 which is
partially immersed in a water wash bath 30 provided at 22.degree.C.
which is substantially identical to that of wash bath 16. The
filaments are taken up on skewed roll 26 at a rate of 12 meters per
minute and are accordingly drawn at a draw ratio of 2:1 while
immersed in the glycerin stretch bath. The filaments are immersed
in the water wash bath 30 for approximately 12.5 seconds during
which time residual quantities of glycerin are substantially
removed from the same. The water present in wash bath 30 is
circulated and is constantly regenerated. The washed filaments are
next passed to skewed roll 32 and drying roll 34 where residual
quantities of moisture are expelled from the same. Drying roll 34
is steam heated and maintained at a constant temperature of
approximately 95.degree.C.
The washed and dried filaments are next passed over a 2 -foot
heated draw shoe which is provided at a constant temperature of
145.degree.C. The residence time of the filaments while in contact
with the hot shoe 36 is 1.25 second. The drawn filaments are
collected on takeup roll 38 at a rate of 60 meters per minute.
Two of the drawn filament bundles are next plied to form a
continuous length of acrylic fibrous material consisting of 800
continuous filaments. This resulting continuous length is next
subjected to a brief thermal pretreatment in accordance with the
teachings of commonly assigned U.S. Ser. No. 17,962, filed Mar. 9,
1970 (now abandoned). More specifically, the continuous length is
passed continuously through an oven provided with an air atmosphere
at 195.degree.C. for a residence time of about 240 seconds while
maintaining the longitudinal tension thereon so that 10.5 percent
shrinkage in length takes place.
The continuous length of thermally pretreated acrylic fibrous
material is next passed for about 180 minutes through a multiple
roll oven provided with an air atmosphere at 266.degree.C. While
passing through this oven the acrylic fibrous material is thermally
stabilized and is rendered black and non-burning when subjected to
an ordinary match flame. The resulting stabilized fibrous material
retains its original fibrous configuration essentially intact, and
contains a bound oxygen of about 10.1 percent by weight when
subjected to the Unterzaucher analysis.
The continuous length of stabilized filaments is next converted to
the improved carbon filaments of the present invention by passage
through an Inductotherm induction furnace utilizing a 20 KW power
source. The induction furnace comprises a water cooled copper coil
and a hollow graphite tube suspended within the coil having a
length of 38 inches and an inner diameter of 0.75 inch through
which the continuous length of stabilized filaments is continuously
passed. The copper coil which encompasses a portion of the hollow
graphite tube is positioned at a location essentially equidistant
from the respective ends of the graphite tube. An inert atmosphere
of nitrogen is maintained within the induction furnace. Air is
substantially excluded from the induction furnace by purging with
nitrogen. The continuous length of stabilized filaments is passed
through the induction furnace at a rate of about 3 inches per
minute. A longitudinal tension of 0.2 grams per denier is exerted
upon the continuous length of fibrous material as it passes through
the induction furnace. The fibrous material is at a temperature of
about 150.degree.C. as it enters the induction furnace and is
raised to a temperature of 800.degree.C. in about 150 seconds, and
from 800.degree.C. to 1,500.degree.C. in about 200 seconds where it
was maintained at 1,500.degree.C. .+-. 25.degree.C. for about 48
seconds.
The resulting carbon filaments contain in excess of 98 percent
carbon by weight, and are found to possess a mean apparent fracture
surface energy of 82 joules per square meter. The mean apparent
fracture surface energy was calculated as heretofore described.
FIG. 1 and FIG. 2 are photographs made with the aid of a scanning
electron microscope at a magnification of 5,740X of matching sides
of the primary fracture surface of a representative carbon filament
which shows a flaw having a maximum dimension of 1.4 micron. The
average flaw size for the carbon filaments produced is found to be
2.7 microns.
The resulting carbon filaments additionally exhibit a specific
gravity per filament of about 1.75 , a denier per filament of 0.78,
a mean single filament tensile strength of 350,000 psi, a mean
single filament Young's modulus of 44 million psi, and an
elongation of 0.78 percent.
Although the formation of the improved carbon filaments have been
described with preferred embodiments, it is to be understood that
variations and modifications may be employed in the carbon filament
formation technique without departing from the concept of the
present invention.
* * * * *