U.S. patent number 3,754,957 [Application Number 05/065,454] was granted by the patent office on 1973-08-28 for enhancement of the surface characteristics of carbon fibers.
This patent grant is currently assigned to Celanese Corporation. Invention is credited to Melvin L. Druin, George R. Ferment, Velliyur N. P. Rao.
United States Patent |
3,754,957 |
Druin , et al. |
August 28, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
ENHANCEMENT OF THE SURFACE CHARACTERISTICS OF CARBON FIBERS
Abstract
An improved process is provided for modifying the suface
characteristics of a carbonaceous fibrous material (either
amorphous carbon or graphitic carbon) and to thereby facilitate
enhanced adhesion between the fibrous material and a matrix
material. The fibrous material is continuously passed at a
relatively rapid rate through a heating zone containing a minor
quantity of gaseous molecular oxygen under conditions found
suitable for bringing about the desired surface modification.
Composite articles of enhanced interlaminar shear strength may be
formed by incorporating the fibers modified in accordance with the
present process in a resinous matrix material.
Inventors: |
Druin; Melvin L. (West Orange,
NJ), Ferment; George R. (Dover, NJ), Rao; Velliyur N.
P. (North Plainfield, NJ) |
Assignee: |
Celanese Corporation (New York,
NY)
|
Family
ID: |
22062817 |
Appl.
No.: |
05/065,454 |
Filed: |
August 20, 1970 |
Current U.S.
Class: |
8/115.54;
264/345; 427/444; 428/400; 423/447.6; 428/367 |
Current CPC
Class: |
C08J
5/06 (20130101); D01F 11/122 (20130101); Y10T
428/2918 (20150115); Y10T 428/2978 (20150115) |
Current International
Class: |
D01F
11/00 (20060101); C08J 5/06 (20060101); C08J
5/04 (20060101); D01F 11/12 (20060101); C08h
017/08 (); C08h 017/10 () |
Field of
Search: |
;106/307
;23/209.1,209.2,29.1F |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3476703 |
November 1969 |
Wadsworth et al. |
|
Other References
Sach et al., Process and Apparatus for Treatment of Carbon or
Graphitic Fibers, Chem. Abstracts, Vol. 71, 1969, Col.
103026(h)..
|
Primary Examiner: Poer; James E.
Claims
We claim:
1. An improved process for the modification of the surface
characteristics of a carbonaceous fibrous material containing at
least about 90 per cent carbon by weight so as to improve its
ability to bond to a resinous matrix material comprising
continuously passing in the direction of its length a continuous
length of said carbonaceous fibrous material through a heating zone
provided at a temperature of about 1,000.degree. to 1,800.degree.
C. to which is introduced a gaseous atmosphere consisting of about
0.2 by volume of an inert gas for a residence time of about 5 to 60
seconds, with the mole ratio of said molecular oxygen provided in
said gaseous atmosphere to that of carbon present in said
carbonaceous fibrous material being at least about 0.02:1.
2. An improved process according to claim 1 wherein said
carbonaceous fibrous material contains at least about 95 per cent
carbon by weight.
3. An improved process according to claim 1 wherein said
carbonaceous fibrous material includes a substantial quantity of
graphitic carbon.
4. An improved process according to claim 1 wherein said
carbonaceous fibrous material is derived from an acrylic fibrous
material selected from the group consisting of an acrylonitrile
homopolymer and acrylonitrile copolymers which contain at least
about 85 mole per cent of acrylonitrile units and up to about 15
mole per cent of one or more monovinyl units copolymerized
therewith.
5. An improved process according to claim 1 wherein said continuous
langth of carbonaceous fibrous material is one or more continuous
multifilament yarn.
6. An improved process according to claim 1 wherein said inert gas
is selected from the group consisting of nitrogen, argon, and
helium.
7. An improved process according to claim 1 wherein said heating
zone is provided at a temperature of about 1,000.degree. to
1300.degree. C. and a gaseous atmosphere consisting of about 0.4 to
2.0 per cent by volume molecular oxygen and about 98 to 99.6 per
cent by volume of an inert gas is introduced therein.
8. An improved process according to claim 1 wherein said mole ratio
of molecular oxygen provided in said heating zone to that of carbon
present in said carbonaceous fibrous material ranges from about
0.02:1 to 0.25:1.
9. An improved process for the modification of the surface
characteristics of a carbonaceous fibrous material containing at
least about 95 per cent carbon by weight and exhibiting a
predominantly graphitic X-ray diffraction pattern so as to improve
its ability to bond to a resinous matrix material comprising:
a. continuously introducing a continuous length of said
carbonaceous fibrous material into a heating zone provided at a
temperature of about 1,000.degree. to 1800.degree. C.
b. continuously introducing a gaseous atmosphere consisting of
about 0.2 to 4 per cent by volume molecular oxygen and about 96 to
99.8 per cent by volume of an inert gas into said heating zone with
the mole ratio of said molecular oxygen provided in said heating
zone to that of carbon present in said carbonaceous fibrous
material ranging from about 0.02:1 to 0.25:1,
c. continuously withdrawing a portion of the gaseous atmosphere
from said heating zone,
d. continuously passing in the direction of its length a continuous
length of said carbonaceous fibrous material through said heating
zone at said temperature for a residence time of about 5 to 60
seconds, and
e. continuously withdrawing the resulting continuous length of
carbonaceous fibrous material from said heating zone.
10. An improved process according to claim 9 wherein said
carbonaceous fibrous material contains at least about 99 per cent
carbon by weight.
11. An improved process according to claim 9 wherein said
carbonaceous fibrous material is derived from an acrylic fibrous
material selected from the group consisting of an acrylonitrile
homopolymer and acrylonitrile copolymers which contain at least
about 85 mole per cent of acrylonitrile units and up to about 15
mole per cent of one or more monovinyl units copolymerized
therewith.
12. An improved process according to claim 9 wherein said
continuous length of carbonaceous fibrous material is one or more
continuous multifilament yarn.
13. An improved process according to claim 9 wherein said heating
zone is provided at a temperature of about 1,000.degree. to
1300.degree. C. and a gaseous atmosphere consisting of about 0.4 to
2.0 per cent by volume molecular oxygen and about 98 to 99.6 per
cent by volume of an inert gas is continuously introduced
therein.
14. An improved process according to claim 9 wherein said inert gas
is selected from the group consisting of nitrogen, argon, and
helium.
15. An improved process according to claim 13 wherein said inert
gas is selected from the group consisting of nitrogen, argon, and
helium.
Description
BACKGROUND OF THE INVENTION
In the search for high performance materials, considerable interest
has been focused upon carbon fibers. The term "carbon fibers" is
used herein in its generic sense and includes graphite fibers as
well as amorphous carbon fibers. Graphite fibers are defined herein
as fibers which consist essentially of carbon and have a
predominant X-ray diffraction pattern characteristic of graphite.
Amorphous carbon fibers, on the other hand, are defined as fibers
in which the bulk of the fiber weight can be attributed to carbon
and which exhibit an essentially amorphous x-ray diffraction
pattern. Graphite fibers generally have a higher Young's modulus
than do amorphous carbon fibers and in addition are more highly
electrically and thermally conductive.
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 high strength reinforcement. Among these desirable
properties are corrosion and high temperature resistance, low
density. high tensile strength, and high modulus. Graphite is one
of the very few known materials whose tensile strength increases
with temperature. 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.
In the prior art numerous materials have been proposed for use as
possible matrices in which carbon fibers may be incorporated to
provide reinforcement and produce a composite article. The matrix
material which is selected is commonly a thermosetting resinous
material and is commonly selected because of its ability to also
withstand highly elevated temperatures.
While it has been possible in the past to provide carbon fibers of
highly desirable strength and modulus characteristics, difficulties
have arisen when one attempts to gain the full advantages of such
properties in the resulting carbon fiber reinforced composite
article. Such inability to capitalize upon the superior single
filament properties of the reinforcing fiber has been traced to
inadequate adhesion between the fiber and the matrix in the
resulting composite article.
Various techniques have been proposed in the past for modifying the
fiber properties of a previously formed carbon fiber in order to
make possible improved adhesion when present in a composite
article. See, for instance, British Pat. No. 1,180,441 to Nicholas
J. Wadsworth and Willian Watt wherein it is taught to heat a carbon
fiber normally within the range of 350.degree. C. to 850.degree. C.
(e.g. 500.degree. to 600.degree. C.) in an oxidizing atmosphere
such as air for an appreciable period of time. Other atmospheres
contemplated for use in the process include an oxygen rich
atmosphere, pure oxygen, or an atmosphere containing an oxide of
nitrogen from which free oxygen becomes available such as nitrous
oxide and nitrogen dioxide.
It is an object of the invention to provide a continuous process
for rapidly and efficiently modifying the surface characteristics
of carbon fibers.
It is an object of the invention to provide an improved process for
improving the ability of carbon fibers to bond to a resinous matrix
material.
It is an object of the invention to provide a process for modifying
the surface characteristics of carbon fibers which eliminates the
need for extended treatment periods.
It is another object of the invention to provide composite articles
reinforced with carbon fibers exhibiting improved interlaminar
shear strength.
These and other objects, as well as the scope, nature, and
utilization of the invention will be apparent from the following
detailed description and appended claims.
SUMMARY OF THE INVENTION
It has been found that a process for the modification of the
surface characteristics of a carbonaceous fibrous material
containing at least about 90 per cent carbon by weight comprises
continuously passing a continuous length of the fibrous material
through a heating zone provided at a temperature of about
1,000.degree. to 1,800.degree. C. containing a gaseous atmosphere
consisting essentially of about 0.2 to 4 per cent by volume
molecular oxygen and about 96 to 99.8 per cent by volume of an
inert gas for a residence time of about 5 to 60 seconds, with the
mole ratio of said molecular oxygen provided in said gaseous
atmosphere to that of carbon present in said carbonaceous fibrous
material being at least about 0.02:1. The resulting carbon fibers
may be incorporated in a resinous matrix material to form a
composite article exhibiting enhanced interlaminar shear
strength.
DESCRIPTION OF THE DRAWINGS
FIG 1 is a photograph made with the aid of a scanning electron
microscope of a portion of graphite filament which has not
undergone surface modification.
FIG. 2 is a photograph made with the aid of a scanning electron
microscope of a portion of a graphite filament which has been
surface modified in accordance with the present process.
FIG. 3 is a photograph made with the aid of a scanning electron
microscope of a portion of a graphite filament which has undergone
excessive surface modification.
DESCRIPTION OF PREFERRED EMBODIMENTS
The Starting Material
The fibers which are modified in accordance with the present
process are carbonaceous and contain at least about 90 per cent
carbon by weight. Such carbon fibers may exhibit either an
armophous carbon or a predominantly graphitic carbon X-ray
diffraction pattern. In a preferred embodiment of the process the
carbonaceous fibers which undergo surface treatment contain at
least about 95 per cent carbon by weight, and at least about 99 per
cent carbon by weight in a particularly preferred embodiment of the
process.
The carbonaceous fibrous materials may be present as a continuous
length in a variety of physical configurations provided substantial
access to the fiber surface is possible during the surface
modification treatment described hereafter. For instance, the
carbonaceous fibrous materials may assume the configuration of a
continuous length of a multifilament yarn, tape, tow, strand,
cable, or similar fibrous assemblage. In a preferred embodiment of
the process the carbonaceous fibrous material is one or more
continuous multifilament yarn. When a plurality of multifilament
yarns are surface treated simultaneously, they may be continuously
passed through the heating zone while in parallel and in the form
of a flat ribbon.
The carbonaceous fibrous material which is treated in the present
process optionally may be provided with a twist which tends to
improve the handling characteristics. For instance, a twist of
about 0.1 to 5 tpi, and preferably about 0.3 to 1.0 tpi, may be
imparted to a multifilament yarn. Also, a false twist may be used
instead of or in addition to a real twist. Alternatively, one may
select continuous bundles of fibrous material which possess
essentially no twist.
The carbonaceous fibers which serve as the starting material in the
present process may be formed in accordance with a variety of
techniques as will be apparent to those skilled in the art. For
instance, organic polymeric fibrous materials which are capable of
undergoing thermal stabilization may be initially stabilized by
treatment in an appropriate atmosphere at a moderate temperature
(e.g. 200.degree. to 400.degree. C.), and subsequently heated in an
inert atmosphere at a more highly elevated temperature, e.g.
900.degree. to 1000.degree. C., or more, until a carbonaceous
fibrous material is formed. If the thermally stabilized material is
heated to a maximum temperature of 2,000.degree. to 3,100.degree.
C. (preferably 2,400.degree. to 3,100.degree. C.) in an inert
atmosphere, substantial amounts of graphitic carbon are commonly
detected in the resulting carbon fiber, otherwise the carbon fiber
will commonly exhibit an essentially amorphous X-ray diffraction
pattern.
The exact temperature and atmosphere utilized during the initial
stabilization of an organic polymeric fibrous material commonly
vary with the composition of the precursor as will be apparent to
those skilled in the art. During the carbonization reaction
elements present in the fibrous material other than carbon (e.g.
oxygen and hydrogen) are substantially expelled. Suitable organic
polymeric fibrous materials from which the fibrous material capable
of undergoing carbonization may be derived include an acrylic
polymer, a cellulosic polymer, a polyamide, a polybenzimidazole,
polyvinyl alcohol, etc. As discussed hereafter, acrylic polymeric
materials are particularly suited for use as precursors in the
formation of carbonaceous fibrous materials. Illustrative examples
of suitable cellulosic materials include the natural and
regenerated forms of cellulose, e.g. rayon. Illustrative examples
of suitable polyamide materials include the aromatic polyamides,
such as nylon 6T, which is formed by the condensation of
hexamethylenediamine and terephthalic acid. An illustrative example
of a suitable polybenzimidazole is poly-
2,2'-m-phenylene-5,5'-bibenzimidazole.
A fibrous acrylic polymeric material prior to stabilization may be
formed primarily of recurring acrylonitrile units. For instance,
the acrylic polymer should contain not less than about 85 mole per
cent of recurring acrylonitrile units with not more than about 15
mole per cent of a monovinyl compound which is copolymerizable with
acrylonitrile such as styrene, methyl acrylate, methyl
methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride,
vinyl pyridine, and the like, or a plurality of such monovinyl
compounds.
During the formation of a preferred carbonaceous fibrous material
for use in the present process multifilament bundles of an acrylic
fibrous material may be initially stabilized in an
oxygen-containing atmosphere (i.e., preoxidized) on a continuous
basis in accordance with the teachings of U.S. Ser. No. 749,957,
filed Aug. 5, 1968, of Dagobert E. Stuetz, which is assigned to the
same assignee as the present invention and is herein incorporated
by reference. More specifically, the acrylic fibrous material
should be either an acrylonitrile homopolymer or an acrylonitrile
copolymer which contains no more than about 5 mole per cent of one
or more monovinyl comonomers copolymerized with acrylonitrile. In a
particularly preferred embodiment of the process the fibrous
material is derived from an acrylonitrile homopolymer. The
stabilized acrylic fibrous material which is preoxidized in an
oxygen-containing atmosphere is black in appearance, contains a
bound oxygen content of at least about 7 per cent by weight as
determined by the Unterzaucher analysis, retains its original
fibrous configuration essentially intact, and is non-burning when
subjected to an ordinary match flame.
In preferred techniques for forming the starting material for the
present process a stabilized acrylic fibrous material is carbonized
and graphitized while passing through a temperature gradient
present in a heating zone in accordance with the procedures
described in commonly assigned U.S. Ser. Nos. 777,275, filed Nov.
20, 1968 of Charles M. Clarke; 17,780, filed Mar. 9, 1970 of
Charles M. Clarke, Michael J. Ram, and John P. Riggs; and 17,832,
filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and Arnold
J. Rosenthal. Each of these disclosures is herein incorporated by
reference.
In accordance with a particularly preferred carbonization and
graphitization technique a continuous length of stabilized acrylic
fibrous material which is non-burning when subjected to an ordinary
match flame and derived from an acrylic fibrous material selected
from the group consisting of an acrylonitrile homopolymer and
acrylonitrile copolymers which contain at least about 85 per cent
of acrylonitrile units and up to about 15 mole per cent of one or
more monovinyl units copolymerized therewith is converted to a
graphitic fibrous material while preserving the original fibrous
configuration essentially intact while passing through a
carbonization/graphitization heating zone containing an inert
gaseous atmosphere and a temperature gradient in which the fibrous
material is raised within a period of about 20 to about 300 seconds
from about 800.degree. C. to a temperature of about 1,600.degree.
C. to form a continuous length of carbonized fibrous material, and
in which the carbonized fibrous material is subsequently raised
from about 1,600.degree. C. to a maximum temperature of at least
about 2,400.degree. C. within a period of about 3 to 300 seconds
where it is maintained for about 10 seconds to about 200 seconds to
form a continuous length of graphitic fibrous material.
The equipment utilized to produce the heating zone used to produce
the carbonaceous starting material may be varied as will be
apparent to those skilled in the art. It is essential that the
apparatus selected be capable of producing the required temperature
while excluding the presence of an oxidizing atmosphere.
In a preferred technique the continuous length of fibrous material
undergoing carbonization is heated by use of an induction furnace.
In such a procedure the fibrous material may be passed in the
direction of its length through a hollow graphite tube or other
susceptor which is situated within the windings of an induction
coil. By varying the length of the graphite tube, the length of the
induction coil, and the rate at which the fibrous material is
passed through the graphite tube, many apparatus arrangements
capable of producing carbonization or carbonization and
graphitization may be selected. For large scale production, it is
of course preferred tht relatively long tubes or susceptors be used
so that the fibrous material may be passed through the same at a
more rapid rate while being carbonized or carbonized and
graphitized. The temperature gradient of a given apparatus may be
determined by conventional optical pyrometer measurements as will
be apparent to those skilled in the art. The fibrous material
because of its small mass and relatively large surface area
instantaneously assumes essentially the same temperature as that of
the zone through which it is continuously passed.
THE SURFACE TREATMENT
The continuous length of carbonaceous fibrous material is
continuously passed (e.g. in the direction of its length) through a
heating zone containing a gaseous atmosphere consisting essentially
of about 0.2 to 4 per cent by volume molecular oxygen (preferably
0.4 to 2.0 per cent by volume molecular oxygen) and about 96 to
99.8 per cent by volume of an inert carrier gas (preferably 98 to
99.6 per cent by volume inert carrier gas) under the conditions
described in detail hereafter. Suitable inert carrier gases include
nitrogen, argon, and helium, etc. The heating zone is effictively
isolated from the atmosphere thereby facilitating the presence of
the desired quantity of molecular oxygen within the heating zone
and the elimination of appreciable extraneous addition of molecular
oxygen to the heating zone.
The gaseous atmosphere (heretofore described) is provided in the
heating zone at a temperature of about 1,000.degree. to
1,800.degree. C. At temperatures much below about 1,000.degree. C.
the surface treatment reaction tends to be inordinately slow, and
inappropriate for continuous operation on an efficient basis. At
temperatures much above about 1,800.degree. C. the surface
treatment reaction becomes so rapid that it is difficult to
control. If desired, a temperature gradient may be provided within
the heating zone which rises to the desired surface treatment
temperature. The gaseous atmosphere preferably is preheated prior
to introduction into the heating zone and preferably is
continuously supplied to the heating zone with a portion of the
gaseous atmosphere being continuously withdrawn from the heating
zone whereby off gases are effectively expelled. In a preferred
embodiment of the process the gaseous atmosphere is provided at a
temperature of about 1,000.degree. to 1,300.degree. C.
The concentration of molecular oxygen and the quantity of
carbonaceous fibrous material provided in the heating zone are such
that the mole ratio of molecular oxygen to carbon in the
carbonaceous fibrous material undergoing treatment is at least
about 0.02:1 (e.g. 0.02:1 to 0.25:1). When the mole ratio is much
below about 0.02:1, then the desired surface treatment tends to be
inordinately slow. When the mole ratio is much above about 0.25:1,
then surface overtreatment accompanied by a significant loss in
single filament tenacity occurs.
The contact time during which the carbonaceous fibrous material is
passed through the heating zone commonly ranges from about 5 to 60
seconds. The minimum contact time varies with the concentration of
molecular oxygen in the gaseous atmosphere, the temperature of the
gaseous atmosphere, and the relative molar concentrations of
molecular oxygen and carbon present in the carbonaceous fibrous
material within the heating zone. Generally, the higher the
temperature of the gaseous atmosphere, the more rapid the surface
modification. Generally the higher the concentration of molecular
oxygen in the gaseous atmosphere, the more rapid the surface
modification. Also it has been observed that graphitic fibrous
materials of high single filament Young's modulus (e.g. in excess
of 50,000,000 psi) tend to require a slightly longer contact time
for optimum results than do carbonaceous fibrous materials of a
predominantly amorphous X-ray diffraction pattern which generally
exhibit a lower single filament Young's modulus. Also when the
carbonaceous fibrous material is provided as a relatively compact
assemblage of a plurality of fibers, then longer residence times
may be advantageously employed as will be apparent to those skilled
in the art.
The surface modification treatment of the present process generally
is terminated prior to achieving a fiber weight loss is excess of
10 per cent by weight. Greater fiber weight losses are to be
avoided since such weight losses are generally indicative of an
excessive surface treatment and yield no commensurate advantage. In
fact, the effectiveness of the surface treatment previously
achieved may actually be diminished under such circumstances. Fiber
weight losses of about 0.5 to 7 per cent by weight (e.g. 1 or 2 per
cent by weight) are commonly attained in preferred embodiments of
the present process.
A particularly preferred embodiment of the present process for the
modification of the surface characteristics of a carbonaceous
fibrous material containing at least about 95 per cent carbon by
weight and exhibiting a predominantly graphitic X-ray diffraction
pattern comprises: (a) continuously introducing a continuous length
of the fibrous material into a heating zone provided at a
temperature of about 1,000.degree. to 1,800.degree. C. containing a
gaseous atmosphere consisting essentially of about 0.2 to 4 per
cent by volume molecular oxygen and about 96 to 99.8 parts by
volume of an inert gas, (b) continuously introducing said gaseous
atmosphere into said heating zone with the mole ratio of said
molecular oxygen provided in the heating zone to that of carbon
present in the carbonaceous fibrous material ranging from about
0.02:1 to 0.25:1, (c) continuously withdrawing a portion of the
gaseous atmosphere from the heating zone, (d) continuously passing
a continuous length of the carbonaceous fibrous material through
the heating zone at said temperature for a residence time of about
5 to 60 seconds, and (e) continuously withdrawing the resulting
continuous fibrous material from the heating zone.
The theory whereby the surface of a carbonaceous fibrous material
is modified in the present process is considered complex and
incapable of simple explanation. It is believed, however, that the
resulting modification is attributable to a combination of physical
and chemical interactions between the gaseous molecular oxygen
atmosphere and the carbonaceous fibrous material. Such interaction
likely includes the chemical reaction of molecular oxygen with
carbon adjacent the surface of the fiber.
The surface modification imparted to the carbonaceous fibrous
material through the use of the present process has been found to
exhibit an appreciable life which is not diminished to any
substantial degree even after the passage of 30, or more days.
The surface treatment of the present process makes possible
improved adhesive bonding between the carbonaceous fibers, and a
resinous matrix material. Accordingly, carbon fiber reinforced
composite materials which incorporate fibers treated as heretofore
described exhibit enhanced shear strength, flexural strength,
compressive strength, etc. The resinous matrix material employed in
the formation of such composite materials is commonly a polar
thermosetting resin such as an epoxy, a polyimide, a polyester, a
phenolic, etc. The carbonaceous fibrous material is commonly
provided in such resulting composite materials in either an aligned
or random fashion in a concentration of about 20 to 70 per cent by
volume.
The following examples are given as specific illustrations of the
invention. It should be understood, however, that the invention is
not limited to the specific details set forth in the examples.
EXAMPLE I
A high strength-high modulus carbonaceous yarn derived from an
acrylonitrile homopolymer yarn in accordance with procedures
described in U.S. Ser. Nos. 749,957, filed Aug. 5, 1968, and
777,275, filed Nov. 20, 1968, was selected as the starting
material. The yarn consisted of a 1600 fil bundle having a total
denier of about 1,000, had a carbon content in excess of 99 per
cent by weight, exhibited a predominantly graphitic X-ray
diffraction pattern, a single filament tenacity of about 307,000
psi (12 gpd), and a single filament Young's modulus of about
77,000,000 psi (3000 gpd). A photograph of a filament of a
substantially similar untreated yarn made with the aid of a
scanning electron microscope at a magnification of 6400X is
provided as FIG. 1.
Portions of the yarn were continuously unwound from bobbins and 15
ends of the yarn were continuously passed while in parallel and in
the form of a flat ribbon through a heat treatment zone provided
with a temperature gradient containing an atmosphere of gaseous
oxygen and nitrogen in the concentrations indicated.
The heat treatment zone consisted of an 18 inch Inconel tube having
an inner diameter of about 1 inch which was positioned within a
resistance wound muffle furnace having a length of 12 inches. Three
inches of the Inconel tube protruded from each end of the muffle
furnace. A hot zone (maximum temperature portion of gradient)
having a length of about 3 inches was centrally located in the
Inconel tube through which the yarn continuously passed and was
adjusted to a constant temperature of about 1,050.degree. C.
The premixed gaseous atmosphere was continuously introduced into
the Inconel tube at the yarn feed end at a rate of 25.0 S.C.F.H.
(std. cu. ft. per hour). Air was excluded from the heat treatment
zone by means of a nitrogen padded chamber which enclosed the
surface treatment furnace. Off gases were continuously displaced
and withdrawn from the heat treatment zone by the continuously
introduced supply of the premixed gases. Off gases were withdrawn
from the surface treatment zone primarily at the yarn exit end of
the tube. The fiber weight losses which occurred during the surface
treatment were less than 10 per cent and commonly ranged from 1 to
3 per cent.
Composite articles were next formed employing the surface modified
yarn samples as a reinforcing medium in a resinous matrix. The
composite articles were rectangular bars consisting of about 50 per
cent by volume of the yarn and having dimensions of 1/8 inch
.times. 1/4 inch .times. 5 inches. The composite articles were
formed by impregnation of the yarn in a liquid epoxy resin-hardener
mixture at 50.degree. C. followed by unidirectional layup of the
required quantity of the impregnated yarn in a steel mold and
compression molding of the layup for 2 hours at 93.degree. C., and
2.5 hours at 200.degree. C. in a heated platen press at about 100
psi pressure. The mold was cooled slowly to room temperature, and
the composite article was removed from the mold cavity and cut to
size for testing. The resinous matrix material used in the
formation of the composite article was provided as a solventless
system which contained 100 parts by weight epoxy resin and 88 parts
by weight of anhydride curing agent.
The following data summarizes the surface treatment conditions
employed and the properties achieved. ##SPC1##
The horizontal interlaminar shear strengths reported were
determined by short beam testing of the carbon fiber reinforced
composite according to the procedure of ASTM D2344-65T as modified
for straight bar testing at a 4:1 span to depth ratio.
For comparative purposes a composite article was formed as
heretofore described employing an identical carbonaceous yarn
without subjecting the same to any form of surface modification.
The average horizontal interlaminar shear strength of the composite
article was only 3000 psi.
A photograph of a filament of the surface treated yarn of Sample B
(above) made with the aid of a scanning electron microscope at a
magnification of 6400X is provided as FIG. 2.
For comparative purposes a sample of the untreated yarn was passed
through the heating zone containing 2.0 per cent by volume oxygen
and 98.0 per cent by volume nitrogen at a rate of 10 in./min., and
maintained at 1,050.degree. C. for 15 seconds. A photograph of a
filament of the yarn made with the aid of a scanning electron
microscope at a magnification of 6,400X is provided at FIG. 3. The
large voids present upon the surface of the fiber indicate
overtreatment.
EXAMPLE II
Exampte I was repeated with the exception that the three inch hot
zone of the Inconel tube was provided at a temperature of
1,100.degree. C.
The following data summarizes the surface treatment conditions
employed and the properties achieved. ##SPC2##
EXAMPLE III
Example I was repeated with the exception that the three inch hot
zone of the Inconel tube was provided at a temperature of
1,260.degree. C.
The following data summarizes the surface treatment conditions
employed and the properties achieved. ##SPC3##
EXAMPLE IV
Example I was repeated with the exception that a different
apparatus was employed to produce a temperature gradient having a
10 inch hot zone at a temperature of about 1,700.degree. C.
The heat treatment zone consisted of a 48 inch long ceramic tube
having an inner diameter of about 0.5 inch which was positioned
within a graphite susceptor of an induction furnace having a length
of 42 inches.
The premixed gaseous atmosphere was continuously introduced into
the yarn feed end of the ceramic tube at a rate of 25.0 S.C.F.H.
Air was excluded from the heat treatment zone by means of a
nitrogen padded chamber which enclosed the heat treatment furnace.
Off gases were continuously displaced and withdrawn from the heat
treatment zone by the continuously introduced supply of the
premixed gases. Off gases were withdrawn from the surface treatment
zone primarily at the yarn exit end of the tube.
Portions of the yarn were surface treated and formed into
composites as described in Example I.
The following data summarizes the surface treatment conditions
employed and the properties achieved when the hot zone of the
ceramic tube was provided at 1,700.degree. C. ##SPC4##
Although the invention has been described with preferred
embodiments, it is to be understood that variations and
modifications may be resorted to as will be apparent to those
skilled in the art. Such variations are to be considered within the
preview and scope of the claims appended hereto.
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