U.S. patent number 3,699,210 [Application Number 04/757,964] was granted by the patent office on 1972-10-17 for method of graphitizing fibers.
This patent grant is currently assigned to Monsanto Research Corporation. Invention is credited to Robert C. Binning, Leo P. Parts, Robert J. Peresie, Margaret L. Rodenburg.
United States Patent |
3,699,210 |
Binning , et al. |
October 17, 1972 |
METHOD OF GRAPHITIZING FIBERS
Abstract
A method for carbonizing and/or graphitizing precursor fibers
selected from the group consisting of polyacrylonitrile and
aromatic polyamide fibers, wherein the fibers are pretreated with
an oxygen-containing atmosphere at 180.degree.-550.degree.C. and
thereafter heated in a laser beam in a non-oxidizing atmosphere at
700.degree.-1,200.degree.C. for carbonizing and at
1,200.degree.-3,600.degree.C. for graphitizing.
Inventors: |
Binning; Robert C. (Kettering,
OH), Parts; Leo P. (Dayton, OH), Peresie; Robert J.
(Brecksville, OH), Rodenburg; Margaret L. (Kettering,
OH) |
Assignee: |
Monsanto Research Corporation
(St. Louis, MO)
|
Family
ID: |
25049902 |
Appl.
No.: |
04/757,964 |
Filed: |
September 6, 1968 |
Current U.S.
Class: |
423/447.8;
204/157.41; 423/448; 264/DIG.19; 423/447.4; 264/482 |
Current CPC
Class: |
D01F
9/32 (20130101); D01F 9/30 (20130101); D01F
9/22 (20130101); Y10S 264/19 (20130101) |
Current International
Class: |
D01F
9/22 (20060101); D01F 9/14 (20060101); D01F
9/32 (20060101); D01F 9/30 (20060101); C01b
031/07 () |
Field of
Search: |
;23/209.1,209.3,209.4
;204/157 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Sharkey et al. "Nature" Vol. 202, June 6, 1964, pages 988-989 .
Smith et al. "The Laser" Copyright 1966 by McGraw-Hill, Inc., page
460.
|
Primary Examiner: Meros; Edward J.
Claims
What we claim is:
1. In a method for preparing a graphitized fiber from a precursor
fiber selected from the group consisting of (1) acrylonitrile
polymer and (2) an aromatic polyamide consisting of repeating units
represented by the general formula:
wherein R and R' are selected from the group consisting of
hydrogen, lower alkyl of up to three carbon atoms, phenyl, lower
alkoxy containing up to three carbon atoms and nitro, and wherein
the R groups can be the same or different and the R' groups must be
the same, and wherein X and Y are selected from the group
consisting of hydrogen, lower alkyl containing up to three carbon
atoms and phenyl, the phenylene radicals of said general formula
being oriented other than ortho, having the steps of:
a. pretreating the fiber by heating at a temperature of from
180.degree. to 550.degree.C. in an oxygen-containing atmosphere for
a time sufficient to blacken the fiber;
b. heating the blackened fiber in a non-oxidizing atmosphere at a
temperature between 700.degree.C. and about 1,200.degree.C. to
carbonize the fiber; and thereafter
c. heating the carbonized fiber in a non-oxidizing atmosphere at a
temperature between about 1,200.degree. and 3,600.degree.C. for a
period of time greater than one-tenth of a second to graphitize the
carbonized fiber; wherein the improvement in the carbonizing and
graphitizing heating steps comprises irradiating substantially all
sides of the fiber at least once in a CO.sub.2 laser beam the time
of exposure being dependent upon the temperature.
2. In a method for preparing a graphitized fiber from a precursor
fiber as disclosed in claim 1 having the steps of
a. pretreating the fiber by heating at a temperature of from
180.degree. to 550.degree.C. in an oxygen containing atmosphere for
a time sufficient to blacken the fiber; and thereafter
b. heating the blackened fiber in a non-oxidizing atmosphere at a
temperature between 700.degree.C. and about 1,200.degree.C. to
carbonize the fiber;
the improvement further comprising the further step of irradiating
at least once substantially all sides of the carbonized fiber with
a CO.sub.2 laser beam in a non-oxidizing atmosphere at a
temperature between about 1,200.degree. and 3,600.degree.C. for a
period of time greater than one-tenth of a second, said time being
dependent upon the temperature.
3. A method of claim 1 in which the precursor fiber is
acrylonitrile polymer.
4. A method of claim 1 in which the precursor fiber is an aromatic
polyamide.
5. A method of claim 2 in which the precursor fiber is
acrylonitrile polymer.
6. A method of claim 2 in which the precursor fiber is an aromatic
polyamide.
Description
BACKGROUND OF THE INVENTION
The invention pertains to a process for preparing carbonized and/or
graphitized fibers by heating suitable precursors in a laser beam,
and particularly provides a continuous process for carbonizing or
graphitizing a precursor yarn.
Previously, carbonized or graphitic fibers have been prepared by
thermal degradation of various fibers, e.g., cellulose,
polyacrylonitrile, aromatic polyamide, etc. (see Ezekiel and Spain,
"Preparation of Graphite Fibers from Polymeric Fibers," Journal of
Polymer Science, Part C, No. 19, pp. 249-265 (1967)). The process
of pretreating an acrylonitrile precursor by heating in an
oxygen-containing atmosphere and thereafter carbonizing at
700.degree.-1200.degree.C. and graphitizing at
1,200.degree.-3,600.degree.C. was disclosed by Tsunoda in U.S. Pat.
No. 3,285,696, issued Nov. 15, 1966. A process for continuously
graphitizing a carbonaceous thread by passing an electric current
through it to heat it is disclosed by Cranch and Shinko in U.S.
Pat. No. 3,313,597, issued Apr. 11, 1967.
SUMMARY OF THE INVENTION
An object of this invention is to provide a process for preparing
flexible carbonized or graphitized fibers by a process utilizing a
laser beam, said fibers being useful in reinforcing plastic
composites.
These and other objects hereinafter defined are met by the
invention wherein there is provided a method for preparing a
carbonized fiber from a precursor fiber selected from the group
consisting of (1) acrylonitrile polymer and (2) an aromatic
polyamide consisting of repeating units represented by the general
formula:
wherein R and R' are selected from the group consisting of
hydrogen, lower alkyl of up to three carbon atoms, phenyl, lower
alkoxy containing up to three carbon atoms and nitro, and wherein
the R groups can be the same or different and the R' groups must be
the same, and wherein X and Y are selected from the group
consisting of hydrogen, lower alkyl containing up to three carbon
atoms and phenyl, the phenylene radicals of said general formula
being oriented other than ortho, which comprises (a) pretreating
the fiber by heating at a temperature of from 180.degree. to
550.degree.C. in an oxygen-containing atmosphere for a time
sufficient to blacken the fiber, and thereafter (b) heating in a
laser beam in a non-oxidizing atmosphere at a temperature between
700.degree. and about 1,200.degree.C. for a period of time greater
than one-tenth of a second, said time being dependent upon the
temperature. The present method for preparing graphitized fibers
comprises the additional step of (c) heating in a laser beam in a
non-oxidizing atmosphere at a temperature between about
1,200.degree. and 3,600.degree.C. for a period of time greater than
one-tenth of a second, said time being dependent upon the
temperature.
It is well known that carbon-base fibers are useful in reinforced
plastic composites (see Schmidt and Jones, "Carbon-Base Fiber
Reinforced Plastics," Chemical Engineering Progress, Vol. 58, No.
10, pp. 42-50 (1962)). For such purposes it is desirable that the
fibers be flexible and have a high tensile strength and elastic
modulus. Generally, graphitized carbon fibers are preferred over
carbonized fibers for their greater mechanical strength, higher
modulus, and higher thermal stability.
It is further known that the term "carbon-base fibers" includes a
wide range of materials varying in chemical composition within the
range 90-100 percent carbon, and varying considerably in crystal
structure, e.g., from a highly disordered or essentially amorphous
structure in the "carbonized fibers" to a more ordered but not
highly crystalline structure characterizing the "graphitized
fibers" (see Franklin, "The Structure of Graphitic Carbons," Acta
Crystallographica, Vol. 4, pp. 253-261 (1951)) and representing a
point in the continuum from amorphous carbon to highly crystalline
three-dimensionally ordered graphite.
The term "carbonized fibers" is used herein for fibers containing
at least 90 percent carbon, but showing essentially no (002)
reflection of graphite by X-ray diffraction analysis. "Graphitized
fibers" refers to fibers containing at least 95 percent carbon and
showing at least some degree of ordering by X-ray diffraction
analysis, e.g., the (002), (100), (004) and (110) reflections of
the graphitic carbons. Generally, such fibers do not show the
highly ordered structure of crystalline graphite.
The present process for carbonizing or graphitizing fibers
generally yields fibers having tensile strengths of over 100
.times. 10.sup.3 p.s.i. and elastic moduli of over 20 .times.
10.sup.6 p.s.i. Such fibers find ready application in plastic
composites for structural members, filament-wound tanks, ablative
nose cones, rocket exhaust nozzles, electric brushes, etc., where
they may be employed with epoxy, phenolic, silicone, polyimide, and
other resin systems.
Novel energy sources, lasers, have become available during the
recent years. Some types of lasers, e.g., the CO.sub.2 laser,
convert electrical energy with high efficiency to intense,
collimated beams of electromagnetic radiation. The laser output
beams can be readily focused with great efficiency onto objects for
generating high temperatures.
The improvement of the present invention over older methods of
carbonizing and graphitizing fibers lies in the use of laser
radiation for effecting the chemical and physical changes in the
fibers. The lasers offer advantages for this operation as compared
with conventional methods of heating, viz. (1) convenience in
start-up and shut-down without significant time lag, (2) rapid
response and sensitive control of power output by simple optical,
electronic, and electrical means, and (3) efficient energy
utilization and optical manipulation inherent in utilizing a
collimated beam of coherent radiation for heating. The beam is
readily manipulated by optical means so as to generate desired
energy flux densities and density gradients, using lenses or
reflectors. The temperature of fibers exposed to the radiation is
readily controlled either by changing the energy output of the
laser or by changing the energy flux density in the irradiated zone
by optical means. Laser heating permits carbonizing and
graphitizing fibers to be conducted as a continuous process. Fast
production rates can be attained by the use of lasers which provide
high power output.
According to the invention, precursor fibers which are preferably
either acrylonitrile homopolymer or copolymers, or an aromatic
polyamide (previously disclosed in U.S. Pat. No. 3,232,910, issued
Feb. 1, 1966), are first pretreated by heating in an
oxygen-containing atmosphere at between 180.degree. and
550.degree.C. for a time sufficient to partially oxidize and
blacken the fibers, and are then carbonized by heating in a laser
beam in a non-oxidizing atmosphere at between 700.degree. C. and
about 1,200.degree.C. The time of exposure of the fibers to the
laser beam during carbonizing varies with the temperature, being
about three minutes at 1,060.degree.C. and somewhat longer at
700.degree.C. The optimum time is readily determined by
experimentation so that carbonized fibers are thereby produced
having suitably high carbon content and satisfactory physical
properties.
As precursor fibers, various carbonaceous materials may be employed
in addition to the preferred acrylonitrile polymer or aromatic
polyamide aforementioned. Thus, cellulose in either its natural or
regenerated form, e.g., rayon, may be used; likewise copolymers of
acrylonitrile with up to 15 mol percent of .alpha.-monovinyl
compound such as methyl acrylate, methyl methacrylate, vinyl
acetate, vinyl chloride, vinylidine chloride, 2-methyl-2-vinyl
pyridine, etc. Catalysts may be incorporated in the fiber, to lower
the decomposition temperature; e.g., in cellulose fibers there may
be used ammonium phosphates, boric acid, zinc chloride, etc. The
fibers may be used as single fibers or as monofilaments, or loose
bundles or in the form of roving. For a continuous process the
preferred form is yarn. Other forms which are adaptable to the
process are tape, woven fabric, matted fibers, paper, etc.
For convenience, the precursor fibers may be heated in the
pretreating step in the oxygen-containing atmosphere by
conventional, relatively low-temperature, heating methods, viz.
resistance-heating, flame-heating, radiative-heating, etc.
well-known in the art. The pretreating may be done in bulk, in a
batch operation, or in a continuous operation.
As a further description of the invention, precursor fibers which
have been pretreated and carbonized, preferably but not necessarily
as described above using laser irradiation, are graphitized by
heating in a laser beam in a non-oxidizing atmosphere at between
about 1,200.degree. and 3,600.degree.C. The time of exposure of the
fibers to laser radiation during graphitizing varies with the
temperature, being about 3 minutes at 2,270.degree.C. The optimum
time is readily determined by experimentation. Excessive exposure
at high temperatures (above 3,000.degree.C.) is to be avoided, as
causing weakening of the fibers through sublimation of the
graphitized carbon.
In order that the carbonized fibers may be heated at temperatures
above 1,200.degree.C. they are preferably surrounded by a
non-oxidizing atmosphere such as nitrogen, hydrogen, helium,
methane, etc. or mixtures thereof. However, for purposes of
facilitating the carbonization or graphitization of the organic
fibers, small amounts of air, chlorine, hydrogen chloride, etc. may
be added as desired. Furthermore, it may be desirable to perform
one or more of the operations in a partial or full vacuum or even
under increased pressure, any of which are permitted through the
convenience of laser irradiation.
It is advantageous to heat fibers under tension, to stretch them,
and thereby orient the microcrystalline regions for attaining
superior strength. Means may be provided for rotating the fibers
around their longitudinal axis.
In addition to laser-irradiative heating during the carbonizing and
graphitizing steps, it may also be employed for the pretreating or
oxidation step. In order to attain desired temperature gradients
during the heating, a plurality of laser beams may be employed, or
a single laser beam may be split by the use of partly transmitting
beam-splitters.
Although the process is admirably adapted to the continuous
preparation of carbonized or graphitized yarn, other products may
be produced which are novel, utilizing the unique source of energy
provided by the laser beam. Thus, only one or more segments of a
precursor yarn may be carbonized or graphitized, leaving the ends
attached to the original unchanged yarn. In another application, a
surface of a bonded fibrous body may be graphitized without
affecting its interior structure; such a graphitized surface may
provide a useful bearing surface having lubricating properties. In
another application, a fibrous mat, e.g., a sheet of paper, or a
woven cloth may be exposed to a laser beam so as to carbonize
precise areas for decorative or aesthetic purposes in various
degrees of blackening, to simulate black or grey tones of a
charcoal drawing, by suitably controlling the energy of the beam
and the time of exposure at each area. In a similar manner, paper
or cloth may be carbonized in very specific and narrowly defined
areas, to produce symbols, letters, figures, bits of information,
etc. for the purpose of communication.
BRIEF DESCRIPTION OF THE DRAWING
Some of the novel features of the present invention will become
apparent from the following description which is to be considered
in connection with the accompanying drawing wherein:
FIGS. 1 and 2 are representations of two embodiments of a
continuous process for carbonizing or graphitizing organic fibers
by laser-heating.
In FIG. 1 there is shown a laser beam, focused by a condensing
lens, impinging upon a yarn of organic fibers. The laser beam 1
emanates from CO.sub.2 -laser tube 2 through infrared
radiation-transmitting lens 3. It passes through water-cooled
germanium condensing lens 4, thence through infrared
radiation-transmitting window 5 in housing 6, and impinges upon
yarn 7. The feedstock yarn is unwound from supply reel 8 which is
turned by an unreeling motor controlled by "dancer" 9. The dancer
consists of a pulley which floats on the yarn and operates a
microswitch. As downstream tension develops on the yarn, the dancer
moves upward and starts the unreeling motor to supply more yarn and
thereby maintain a predetermined tension, e.g., 1-3,000 grams. On
leaving housing 6, the carbonized or graphitized yarn passes over
pulley 10 and is wound on reel 11. Housing 6 is provided with an
entry port 12 for gases which may be non-oxidizing, e.g., nitrogen,
hydrogen, argon, helium, etc. to protect the hot fibers from
oxidation, or may contain reactive gases or mixtures thereof, e.g.,
air, chlorine, hydrogen chloride, etc. to facilitate the
carbonization or graphitization of the organic fibers. Housing 6 is
also provided with a second infrared radiation-transmitting window
13 so that laser radiation which is not intercepted and absorbed by
the yarn may be transmitted outside of the housing and subsequently
harmlessly absorbed. Windows 5 and 13 are sealed against housing 6
by O-rings 14 and suitable clamping means.
In FIG. 2 there is shown a modification in which a hollow
cylindrical reflector directs the fraction of laser radiation not
absorbed from the incident beam back onto the fibers. A condensing
lens may or may not be used to concentrate the laser beam; in this
embodiment it has been omitted. Thus, the collimated laser beam 21
emanates from laser tube 22 through radiation-transmitting lens 23.
It passes through infrared radiation-transmitting window 24 in
housing 25 and impinges upon yarn 26. Means are provided for moving
yarn 26 at a controlled rate under tension through laser beam 21,
as for example by the method illustrated in FIG. 1 or by simply
employing a motor-driven take-up reel at the upper end of the yarn
and by hanging a weight on the lower end. Cylindrical reflector 27
is a polished infrared radiation-reflecting hollow cylinder having
a hole cut in its front wall for the entering beam. It is supported
by rod 28 which passes through plate 29 and is manipulated for
focusing and alignment by handle 30. For maximum utilization of the
reflected energy the cylinder is positioned so that the yarn is in
the focal plane, which is parallel to the rear wall and spaced
therefrom by a distance equal to one-half of the radius of the
cylinder. Reflectors of other configurations, such as parabolic,
can also be used for focusing the radiation onto the yarn. The yarn
passes over yarn guides 31 which are rods, e.g., glass, graphite,
Teflon, etc., equipped with smooth notches. Bottom plate 32 is
provided with gas entry port 33. O-rings 34 are employed for seals
as in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is further illustrated by, but not limited to, the
following examples.
EXAMPLE 1
This example illustrates carbonization at a temperature below
1,200.degree.C.
The apparatus was essentially as represented in FIG. 2 in which a
reflector 27 having an internal diameter of 23 mm. was employed.
The upper end of the yarn went to a motor-driven take-up reel.
Tension was applied to the yarn by attaching a 10-gram weight to
the lower end of yarn 26.
The feedstock consisted of preoxidized acrylonitrile homopolymer
yarn having about 250 fibers in the bundle. For the preoxidation
treatment the yarn was wrapped on a thin-walled glass cylinder
taking care not to overlap the yarn. The yarn was then heated in an
air-circulating oven in which the temperature was raised from
25.degree.to 280.degree.C. during 2 hours and thereafter held at
280.degree. C. for 3 hours. The now-blackened yarn was cooled and
washed in distilled water at the boil for 1 hour. At this stage the
fibers had the following physical properties:
Tensile strength 31 .times. 10.sup.3 p.s.i.
Elastic modulus 1.4 .times. 10.sup.6 p.s.i.
Elongation (%) 5.8
Chemical analysis showed 55.1 percent carbon, 2.35 percent hydrogen
and 20.2 percent nitrogen, whereby the oxygen content was about 22
percent by difference.
The laser beam was produced by a CO.sub.2 laser made by Korad
Corp., Model K-G3, wavelength = 10.6.mu., operating at up to about
70 watts output.
The yarn was heated by two passes through the laser beam at an
average temperature of about 1,060.degree.C. Residence time in the
heated zone was about 1.6 minutes for each pass. Argon was blown
through the housing.
The physical properties of the partially graphitized carbon fibers
were determined on single fibers. For these as well as the products
obtained in the remaining examples, the tests and method used were
as follows: tensile strength and modulus were determined by the
method of S. Schulman reported in the Journal of Polymer Science,
Part C, Polymer Symposia, "High Temperature Resistant Fibers,"No.
19, pp. 211-225 (1967): "Methods of Single Fiber Evaluation." The
data reported here are the average of replicate determinations,
usually six or more.
Physical Properties of Product I
__________________________________________________________________________
Tensile strength 117 .times. 10.sup.3 p.s.i. Elastic modulus 20
.times. 10.sup.6 p.s.i.
__________________________________________________________________________
EXAMPLE 2
This example illustrates the use of a condensing lens.
The apparatus consisted of a modification of that represented by
FIG. 1: the yarn supply reel 8 and "dancer" 9 were removed and
tension was applied to the yarn simply by attaching a 5-gram weight
to the lower end of the yarn 7. A condensing lens 4 having a focal
length of 6 inches was used.
The feedstock consisted of preoxidized acrylonitrile homopolymer
yarn prepared as described in Example 1.
The yarn was passed repeatedly through the laser beam, increasing
the power of the laser generator after each second pass so that the
yarn attained the following observed temperatures (.degree. C):
820, 890, 1,010, 1,340, 1,890 and 1,930. The power output for the
highest temperature was 80 watts. During this operation, argon was
blown through the housing. The yarn moved at a rate of 0.37 inches
per minute. The residence time for the yarn in the heated zone was
about 13.8 seconds for each pass when the lens was at a distance of
6.87 inches from the yarn. The yarn was rotated 180.degree. after
the first pass at each temperature.
Physical Properties of Product II
__________________________________________________________________________
Tensile strength 134 .times. 10.sup.3 p.s.i. Elastic modulus 23.8
.times. 10.sup.6 p.s.i. Elongation (%) 0.57
__________________________________________________________________________
EXAMPLE 3
This example illustrates the use of a preoxidized and precarbonized
yarn.
The apparatus and method were essentially the same as in Example 2.
The tension was applied by a 10-gram weight.
The feedstock consisted of preoxidized and precarbonized
acrylonitrile homopolymer yarn. The preoxidation conditions were
the same as in Example 1. For the carbonization treatment, instead
of using laser irradiation, the yarn was heated by a conventional,
relatively low-temperature, method in a furnace at 950.degree.C.
for about 6 minutes in a nitrogen atmosphere. The fibers had the
following physical properties:
Tensile strength 89 .times. 10.sup.3 p.s.i.
Elastic modulus 12.5 .times. 10.sup.6 p.s.i.
Elongation (%) 0.77
Laser irradiation was then applied to heating the yarn by
successive passes through the laser beam, with two passes at each
of the following temperatures (.degree. C.): 1,160, 1,340, 1,950
and 1,990. At the highest temperature the 6-inch focal length
condensing lens was at a distance of 6.87 inches. During the
heating operation, argon was blown through the housing.
Physical Properties of Product III
__________________________________________________________________________
Tensile strength 118 .times. 10.sup.3 p.s.i. Elastic modulus 26
.times. 10.sup.6 p.s.i. Elongation (%) 0.43
__________________________________________________________________________
EXAMPLE 4
This example illustrates the use of a reflector, with stepwise
heating in seven steps to a maximum of 1,420.degree.C.
The apparatus was essentially as represented in FIG. 2 in which a
reflector 27 was employed. The upper end of the yarn went to a
motor-driven take-up reel. Tension was applied to the yarn by
attaching a 10-gram weight to the lower end of the yarn 26. The
condensing lens was not used.
The feedstock consisted of preoxidized acrylonitrile homopolymer
yarn prepared as in Example 1.
The yarn was heated by successive passes through the laser beam
with two passes at each of the following temperatures (.degree.
C.): ca. 400, ca. 500, 890, 1,000, 1,210, 1,320 and 1,420. Argon
was blown through the housing.
Physical Properties of Product IV
__________________________________________________________________________
Tensile strength 164 .times. 10.sup.3 p.s.i. Elastic modulus 26.2
.times. 10.sup.6 p.s.i.
__________________________________________________________________________
EXAMPLE 5
This example illustrates the use of a reflector in one-step heating
at a temperature of about 2,000.degree.-2,200.degree.C.
The apparatus was the same as in Example 4. The feedstock was
preoxidized acrylonitrile homopolymer prepared as in Example 1.
In Example 5-A, the yarn was heated by two passes through the laser
beam at an average temperature of about 2,010.degree.C. The yarn
was not rotated between passes. Argon was blown through the
housing.
Physical Properties of Product V-A
__________________________________________________________________________
Tensile strength 117 .times. 10.sup.3 p.s.i. Elastic modulus 23.9
.times. 10.sup.6 p.s.i.
__________________________________________________________________________
In Example 5-B, the yarn was heated by one pass at an average
temperature of about 2,170.degree.C.
physical Properties of Product V-B
__________________________________________________________________________
Tensile strength 108 .times. 10.sup.3 p.s.i. Elastic modulus 25.0
.times. 10.sup.6 p.s.i.
__________________________________________________________________________
EXAMPLE 6
This example illustrates heating to a maximum of about
2,270.degree.C.
The apparatus was the same as in Example 4. The feedstock was
preoxidized acrylonitrile homopolymer prepared as in Example 1.
The yarn was heated by two passes through the laser beam at each
temperature: first at about 1,030.degree.C., then at about
2,270.degree.C. Residence time in the heated zone was about 1.6
minutes for each pass. Argon was blown through the housing.
The product was found by chemical analysis to be 97.76 percent
carbon.
In another run, under similar conditions, a product was obtained
with the following properties:
Physical Properties of Product VI-B
__________________________________________________________________________
Tensile strength 112 .times. 10.sup.3 p.s.i. Elastic modulus 30
.times. 10.sup.6 p.s.i.
__________________________________________________________________________
EXAMPLE 7
This example illustrates the use of a condensing lens and reflector
in combination.
The apparatus consisted of the laser source and condensing lens as
represented in FIG. 1, and the reflector and housing as represented
in FIG. 2. The upper end of the yarn went to a motor-driven take-up
reel. Tension was applied to the yarn by a 10-gram weight. The yarn
moved at a rate of 0.37 inches per minute. The residence time for
the yarn in the heated zone was about 66 seconds for each pass when
the lens was at a distance of 10.5 inches.
The feedstock consisted of preoxidized acrylonitrile homopolymer
yarn prepared as in Example 1.
The yarn was heated by successive passes through the laser beam
with two passes at each of the following temperatures (.degree.
C.): ca. 500, 1,000, 1,330, 1,530, 1,690, 1,930 and 2,080. Argon
was blown through the housing.
Physical Properties of Product VII
__________________________________________________________________________
Tensile strength 139 .times. 10.sup.3 p.s.i. Elastic modulus 29.0
.times. 10.sup.6 p.s.i.
__________________________________________________________________________
EXAMPLE 8
This example illustrates the use of an aromatic polyamide as the
feedstock.
The apparatus was essentially as represented in FIG. 1 as modified
in Example 2: the yarn supply reel 8 and "dancer" 9 were removed
and tension was applied to the yarn in the form of a 10-gram
weight. A condensing lens was at a distance of between 6.5 to 9
inches from the yarn; at the highest temperature it was at a
distance of 6.5 inches. Under these conditions, the residence time
for the yarn in the heated zone was about 3 seconds for each
pass.
The feedstock consisted of preoxidized aromatic polyamide yarn
having about 300 fibers in a bundle. The fibers were derived from
poly N,N'-m-phenylenebis(m-benzamide)isophthalamide as disclosed in
U.S. Pat. No. 3,232,910, issued Feb. 1, 1966. For the preoxidation
treatment the yarn was heated in air at 420.degree.C. for 3 hours.
At this stage the now-blackened fibers had the following physical
properties:
Tensile strength 20 .times. 10.sup.3 p.s.i.
Elastic modulus 1.8 .times. 10.sup.6 p.s.i.
Elongation (%) 1.3
The yarn was heated by eight passes through the laser beam, as the
temperature was raised in stages from 1,520.degree. to
2,500.degree.C.
physical Properties of Product VIII
__________________________________________________________________________
Tensile strength 80 .times. 10.sup.3 p.s.i. Elastic modulus 8.2
.times. 10.sup.6 p.s.i. Elongation (%) 1.0
__________________________________________________________________________
EXAMPLE 9
This example illustrates graphitizing the surface of a bonded
fibrous body.
A layer of an epoxy resin-bonded carbon fiber composite is built up
on a steel shaft by winding a carbon yarn liberally coated with
curable epoxy resin. The composite is cured to a hard tough
condition. The outer surface of the composite, which is
substantially cylindrical in shape, is then exposed to laser
radiation in an inert atmosphere. Upon increasing the level of
irradiation gradually, to produce a maximum temperature of about
2,700.degree.C., the surface layers of carbon yarn are graphitized.
Upon fitting the structure to a bearing, the shaft is found to turn
smoothly because of the graphitized surface which results in a
decreased coefficient of friction.
It is to be understood that although the invention has been
described with specific reference to particular embodiments
thereof, it is not to be so limited since changes and alterations
therein may be made which are within the full intended scope of
this invention as defined by the appended claims.
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