U.S. patent application number 16/806326 was filed with the patent office on 2020-09-10 for method of making polyacrylonitrile based carbon fibers and polyacrylonitrile based carbon fiber fabric.
The applicant listed for this patent is University of Kentucky Research Foundation. Invention is credited to John Craddock, Matthew Weisenberger.
Application Number | 20200283932 16/806326 |
Document ID | / |
Family ID | 1000004722511 |
Filed Date | 2020-09-10 |
United States Patent
Application |
20200283932 |
Kind Code |
A1 |
Weisenberger; Matthew ; et
al. |
September 10, 2020 |
METHOD OF MAKING POLYACRYLONITRILE BASED CARBON FIBERS AND
POLYACRYLONITRILE BASED CARBON FIBER FABRIC
Abstract
Methods to produce a polyacrylonitrile-based carbon fiber and
polyacrylonitrile-based carbon fiber fabric with physical
characteristic closely resembling rayon-based carbon fibers are
disclosed. A polyacrylonitrile-based carbon fiber and
polyacrylonitrile-based carbon fiber fabric with a unique
combination of physical properties are also disclosed.
Inventors: |
Weisenberger; Matthew;
(Lexington, KY) ; Craddock; John; (Lexington,
KY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Kentucky Research Foundation |
Lexington |
KY |
US |
|
|
Family ID: |
1000004722511 |
Appl. No.: |
16/806326 |
Filed: |
March 2, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62813359 |
Mar 4, 2019 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D03D 15/0083 20130101;
D01D 5/253 20130101; D10B 2101/12 20130101; D10B 2321/10 20130101;
D01F 6/18 20130101 |
International
Class: |
D01F 6/18 20060101
D01F006/18; D03D 15/00 20060101 D03D015/00; D01D 5/253 20060101
D01D005/253 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with government support under Grant
No. W31P4Q-17-C-0009 awarded by the US Army AMRDEC and Materials
Sciences Corporation. The government has certain rights in the
invention.
Claims
1. A method of producing polyacrylonitrile (PAN)-based carbon
fibers, comprising: extruding a PAN dope through a spinneret to
produce a tow of PAN fibers having a lobed cross section; washing,
drying and winding the tow of PAN fibers without subjecting the PAN
fibers to hot drawing; and stabilizing and carbonizing the tow of
PAN fibers at a temperature of less than 1,000.degree. C. to
produce PAN-based carbon fibers.
2. The method of claim 1, including using a spinneret with a
+-shaped opening to produce PAN fiber filaments having a cross
section with a lobed shape.
3. The method of claim 2, wherein the stabilizing includes heating
the tow of PAN fibers to 250.degree.-280.degree. C.
4. The method of claim 2, wherein a highest fiber temperature
reached during carbonization is between 850.degree. C. and
950.degree. C.
5. The method of claim 2, wherein a highest fiber temperature
reached during carbonization is between 885.degree. C. and
915.degree. C.
6. A method of producing a polyacrylonitrile (PAN)-based carbon
fiber fabric, comprising: extruding a PAN dope through a spinneret
to produce a tow of PAN fibers having a lobed cross section;
washing, drying and winding the tow of PAN fibers without
subjecting the PAN fibers to hot drawing; weaving the tow of PAN
fibers into a woven PAN fiber fabric; and stabilizing and
carbonizing the woven PAN fiber fabric at a temperature of less
than 1,000.degree. C. to produce PAN-based woven carbon fiber
fabric.
7. The method of claim 6, including using a spinneret with a
+-shaped opening to produce PAN fiber filaments having a cross
section with a lobed shape.
8. The method of claim 7, wherein the stabilizing includes heating
the woven PAN fiber fabric to 265.degree. C.
9. The method of claim 6, wherein a highest fiber temperature
reached during carbonization is between 850.degree. C. and
950.degree. C.
10. The method of claim 6, wherein a highest fiber temperature
reached during carbonization is between 885.degree. C. and
915.degree. C.
11. The method of claim 6, including using a spinneret with a
+-shaped opening to produce PAN fibers having a cross section with
a lobed shape.
12. The PAN-based carbon fibers made by the method of claim 1.
13. The PAN-based carbon fibers of claim 12, wherein lobes of PAN
fiber filaments have a characteristic width greater than 10% of a
width of said PAN fiber filaments at a largest dimension of said
PAN fiber filaments.
14. The PAN-based carbon fiber fabric made by the method of claim
7.
15. The PAN-based carbon fiber fabric of claim 14, wherein lobes of
said PAN fiber filaments have a characteristic width greater than
10% of a width of said PAN fiber filaments at a largest dimension
of said fiber.
16. A PAN-based carbon fiber, comprising a thermal diffusivity of
less than 2 mm.sup.2/sec along a longitudinal axis of the PAN-based
carbon fiber, a modulus of less than 100 GPa along the longitudinal
axis in tension, a break stress of less than 1 GPa along the
longitudinal axis in tension and a lobe width W.sub.1/max fiber
diameter W.sub.f greater than 0.1.
17. The PAN-based carbon fiber of claim 16, wherein the thermal
diffusivity is less than 1.6 mm.sup.2/sec along the longitudinal
axis of the PAN-based carbon fiber.
18. The PAN-based carbon fiber of claim 17, wherein the modulus is
less than 65 GPa along the longitudinal axis in tension.
Description
RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 62/813,359 filed on Mar. 4, 2019 which is
hereby incorporated by reference in its entirety.
TECHNICAL FIELD
[0003] This document relates generally to the field of carbon
fibers and, more particularly to a new and improved method for
making polyacrylonitrile-based carbon fibers with low thermal
conductivity and a lobed profile for ablative composite and other
applications.
BACKGROUND
[0004] Rayon-based carbon fibers have long been used for ablative
composite applications such as for heat shields and the like
required for space vehicles and other technologically advanced
vehicles and applications. Environmental concerns related to the
production of rayon have led to the cessation of the domestic
production of rayon. As a result, the United States has had to rely
on old stock and foreign production for the rayon used to produce
carbon fibers having the desired characteristics for ablative
composite applications. This is a strategic concern for which, to
date, there has been no clear solution.
[0005] This document relates to the production of polyacrylonitrile
or PAN-based carbon fibers having a unique combination of
properties for PAN-based carbon fibers that approach those
characteristic of rayon-based carbon fibers. More particularly, the
PAN-based carbon fibers of the present method are characterized by
(a) a lobed cross section that provides for enhanced mechanical
interlocking between fibers, (b) a relatively low thermal
conductivity characteristic of rayon-based carbon fibers, and (c)
tensile properties similar to rayon-based carbon fibers.
SUMMARY
[0006] In accordance with the purposes and benefits described
herein, a new and improved method is provided for the production of
PAN-based carbon fibers. That method comprises the steps of: (a)
extruding a PAN dope through a spinneret to produce a tow of PAN
fibers having a lobed cross section, (b) washing, drying and
winding the tow of PAN fibers without subjecting the PAN fibers to
hot drawing and (c) carbonizing the tow of PAN fibers at a
temperature of less than 1,000.degree. C. to produce PAN-based
carbon fibers.
[0007] The method may include using a spinneret with a +-shaped
opening to produce PAN fiber filaments having a cross section with
a lobed shape.
[0008] The method may further include the step of stabilizing the
tow of PAN fibers prior to carbonization. That stabilizing may
include heating the PAN fibers to 250.degree.-280.degree. C.
[0009] In some embodiments, during carbonizing, the highest fiber
temperature reached may be between 850.degree. C. and 950.degree.
C. In some embodiments, during carbonizing, the highest fiber
temperature reached may be between 885.degree. C. and 915.degree.
C.
[0010] PAN-based carbon fibers made by this method are also
claimed. Those PAN-based carbon fibers may be characterized by the
lobes of the PAN fiber filaments having a characteristic width
greater than 10% of a width of the PAN fiber filaments at the
largest dimension of the PAN fiber filaments.
[0011] In accordance with an additional aspect, a method is
provided for producing a polyacrylonotrile-based/(PAN)-based carbon
fiber fabric, comprising: (a) extruding a PAN dope through a
spinneret to produce a tow of PAN fibers having a lobed cross
section, (b) washing, drying and winding the tow of PAN fibers
without subjecting the PAN fibers to hot drawing, (c) weaving the
tow of carbon fibers into a woven PAN fiber fabric and (d)
carbonizing the woven PAN fiber fabric at a temperature of less
than 1,000.degree. C. to produce a woven PAN-based carbon fiber
fabric.
[0012] The method may include using a spinneret with a +-shaped
opening to produce PAN fiber filaments having a cross section with
a lobed shape.
[0013] The method may further include the step of stabilizing the
tow of PAN fibers prior to carbonizing. That stabilizing may
include heating the PAN fibers to 250.degree.-280.degree. C.
[0014] In some embodiments, during carbonizing, the highest fiber
temperature reached may be between 850.degree. C. and 950.degree.
C. In some embodiments, during carbonizing, the highest fiber
temperature reached may be between 885.degree. C. and 915.degree.
C.
[0015] PAN-based carbon fiber fabrics made by this method are also
claimed. Those PAN-based carbon fiber fabrics may be characterized
by the lobes of the PAN fiber filaments having a characteristic
width greater than 10% of a width of the PAN fiber filaments at the
largest dimension of the PAN fiber filaments.
[0016] In the following description, there are shown and described
several embodiments of the methods as well as the PAN-based carbon
fiber and PAN-based carbon fiber fabric products made from the
methods. As it should be realized, the methods and the products of
those methods are capable of other, different embodiments and their
several details are capable of modification in various, obvious
aspects all without departing from the methods and products as set
forth and described in the following claims. Accordingly, the
drawings and descriptions should be regarded as illustrative in
nature and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0017] The accompanying drawing figures incorporated herein and
forming a part of the specification illustrates several aspects of
the methods and products of those methods and together with the
description serves to explain certain principles thereof.
[0018] FIG. 1 is a schematic representation of the carbon fiber
production process.
[0019] FIG. 2 illustrates the geometry of a +-shaped spinneret
opening of the type that may be utilized to produce the lobed PAN
fibers used in the production of the PAN-based carbon fibers.
[0020] FIG. 3 is a schematic representation of the carbon fiber
fabric production method.
[0021] FIG. 4 is a cross section illustrating multiple fibers
produced as a result of the lobed shaped cross section of those
fibers.
[0022] FIG. 5 illustrates the lobe characteristics of the PAN-based
carbon fibers.
DETAILED DESCRIPTION
[0023] Reference is now made to FIG. 1 which schematically
illustrates the new and improved method 10 for producing
polyacrylonitrile-based carbon fibers/PAN-based carbon fibers. For
purposes of this document, the terminology "PAN-based carbon
fibers" includes carbon fibers made from polymer precursor fibers
with a chemical composition consisting greater than 80% of the
acrylonitrile monomer repeat units.
[0024] That method includes the step 12 of extruding a PAN dope of
a type known in the art through a spinneret to produce a tow of PAN
fibers wherein the filaments thereof have a lobed cross section.
The spinneret 14 may include a plurality of openings 16 through
which the PAN dope is extruded into a water coagulation bath of a
type known in the art to be useful in the spinning of PAN fibers.
More particularly, as illustrated in FIG. 2, each spinneret opening
or hole 16 may have a +-shape adapted to produce fibers having the
appearance of crenulated fibers. For example, each leg 18 may have
a width W of 0.03 mm with the opposed legs having a total length L
of 0.09 mm.
[0025] Following extruding, the method includes the washing, drying
and winding 20 of the tow of PAN fibers without subjecting the PAN
fibers to hot drawing. The absence of hot drawing is an important
factor in producing PAN-based carbon fibers having the desired
physical characteristics for use in ablative composite
applications.
[0026] Following winding, the method includes stabilizing 22 the
tow of PAN fibers by heating to a stabilizing temperature of, for
example, 250.degree.-280.degree. C. in air and then cooling back to
room temperature. In one particularly useful embodiment, the tow of
PAN fibers is heated to 265.degree. C.
[0027] Following stabilizing, the tow of PAN fibers is subjected to
carbonizing 24 in an inert atmosphere at a temperature of less than
1,000.degree. C. to produce PAN-based carbon fibers from the PAN
fiber tow. In one possible embodiment of the method, the highest
fiber temperature reached during carbonization is between
850.degree. C. and 950.degree. C. In another possible embodiment of
the method, the carbonization temperature is between 885.degree. C.
and 915.degree. C. In still another, the carbonization temperature
is 900.degree. C. In one possible embodiment, the fibers are
maintained at peak carbonization temperature for between one and
ten minutes.
[0028] Reference is now made to FIG. 3 which illustrates the new
and improved method 30 of producing a polyacrylonitrile-based
carbon fiber fabric/PAN-based carbon fiber fabric. That method
includes the step 32 of extruding a PAN dope of a type known in the
art through a spinneret to produce a tow of PAN fibers having a
lobed cross section.
[0029] As noted above, the spinneret 14 may include a plurality of
openings 16 through which the PAN dope is extruded into a water
coagulation bath of a type known in the art to be useful in the
spinning of PAN fibers. More particularly, as illustrated in FIG.
2, each spinneret opening or hole 16 may have a +-shape adapted to
produce fibers having the appearance of crenulated fibers. For
example, each leg 18 may have a width W of 0.03 mm with the opposed
legs having a total length L of 0.09 mm.
[0030] Following extruding, the method includes the washing, drying
and winding 34 of the tow of PAN fibers without subjecting the PAN
fibers to hot drawing. The absence of hot drawing is an important
factor in producing PAN-based carbon fibers having the desired
physical characteristics for use in ablative composite
applications.
[0031] Following winding, the method includes the step 36 of
weaving the tow of PAN fibers into a woven PAN fiber fabric.
Durable weave patterns are those with more drape, such as 8 harness
satin.
[0032] Following weaving, the method includes stabilizing 38 the
tow of PAN fibers by heating in air to a desired stabilizing
temperature of, for example, 250-280.degree. C. In one particularly
useful embodiment, the tow of PAN fibers is heated to 265.degree.
C.
[0033] Next, the woven PAN fiber fabric is subjected to carbonizing
40 in an inert atmosphere at a temperature of less than
1,000.degree. C. to produce a PAN-based carbon fiber fabric from
the PAN woven PAN fiber fabric. In one possible embodiment of the
method, the highest fiber temperature reached during carbonization
is between 850.degree. C. and 950.degree. C. In another possible
embodiment of the method, the carbonization temperature is between
885.degree. C. and 915.degree. C. In still another, the
carbonization temperature is 900.degree. C.
[0034] As should be appreciated, the crenulated or lobed cross
section of the PAN fiber filaments allows the fibers F to better
mechanically interlock into the matrix material of the final
composite.
[0035] The PAN-based carbon fiber filaments produced by the method
disclosed herein include lobes or legs having a characteristic
width W.sub.1 greater than 10% of a width W.sub.f of the fiber
filaments at the largest dimension of the fiber filaments: that is
W.sub.1/W.sub.f>=0.1 (see FIG. 5).
[0036] The PAN-based carbon fibers are characterized by a unique
combination of physical characteristics that make them particularly
useful for ablative composite applications. The PAN-based carbon
fibers have a thermal diffusivity of less than 2 mm.sup.2/sec along
the fiber axis. More specifically, with the low-fire carbonization
temperature of 900.degree. C. an axial thermal diffusivity of as
low as 1.503 mm.sup.2/sec has been achieved. The incumbent rayon
based carbon fiber has an axial thermal diffusivity of 1.944
mm.sup.2/sec.
[0037] The PAN-based carbon fibers have a modulus of less than 100
GPa along the fiber axis in tension. More specifically, under the
no hot drawing condition, a modulus of as low as 60.06 GPa has been
achieved. The rayon based carbon fiber is approximately 39.7
GPa.
[0038] The PAN-based carbon fibers have a break stress of less than
1 GPa along the fiber axis in tension. More specifically, under the
no hot drawing condition, a break stress of 0.506 GPa has been
achieved. The rayon based carbon fiber is approximately 0.454
GPa.
[0039] Finally, the PAN-based carbon fibers have a lobe width
W.sub.1/max fiber diameter W.sub.f of greater than 0.1. More
specifically, under the indicated processing conditions, a W.sub.1
of 6 microns and a W.sub.f of 12 microns for a W.sub.1/W.sub.f of
0.5 has been achieved.
[0040] Experimental Section:
[0041] Dope Preparation:
[0042] 14 wt % of polyacrylonitrile (PAN) polymer powder (relative
to the solvent, dimethylsulfoxide, DMSO plus the PAN masses) was
mixed and prepared into a spinning solution or dope. For example,
196 g of PAN was mixed with 1204 g of DMSO. It is recommended that
the PAN powder be dried under vacuum for 1 hour at 50.degree. C. to
remove moisture. After hand mixing to break up any large polymer
powder agglomerates, the dope was poured into a purpose-built dope
mixer and heated to 55.degree. C. for overnight mixing. The dope
was then extracted from the mixer with a 15 psi overpressure of
nitrogen, directly into the spinning pump system. Next, the dope in
the pump system was placed under vacuum to remove any entrained air
bubbles in the dope. This degassing step is important for good
fiber spinning.
[0043] Spin Preparation:
[0044] First the spinneret was inspected for cleanliness by optical
microscopy, to ensure that no capillaries were clogged. Inspection
of individual capillaries is best done under the microscope. If any
are clogged, the spinneret is placed into a container of clean
dimethylacetamide (DMAc) and sonicated using a wand sonicator at an
intensity of 20% for 15 minutes. Similarly, after a spin run, this
cleaning process was repeated and the spinneret was allowed to
sonicate for one hour.
[0045] A new 3 micron filter element was placed in the filter
housing. Then the spinning pump system, the filter housing, and the
spinneret die head were heated to 50.degree. C. On back to back
spin runs, the same filter element is typically used. Down the
line, the heated godet dryer rollers were heated to 90.degree. C.
The pump and heated godet were given an hour to fully heat up. The
adapter hardware for the wet spinning spinneret was also placed in
an oven at 50.degree. C.
[0046] A 0.65 wt % silicone oil based emulsion spin finish was
prepared by mixing 3.25 g of a proprietary spin finish concentrate,
with 196.75 g of deionized water (1.625 wt %). As well a 0.5 wt %
polyvinyl alcohol (PVA) spin/warp finish was prepared by mixing 11
g of a warp sizing, with 189 g of deionized water (5.5 wt %).
[0047] Spinning Procedure:
[0048] The spinning pump was connected to the filter manifold and
die head by flexible pressure hoses. 40-50 ml of dope was purged
through all components to eliminate contaminates and air bubbles.
On back to back spin runs only 20 ml of dope was purged through the
system. The spinneret was attached to the preheated adapter
hardware and then attached to the die head. At an initial flow rate
of 3.0 ml/min the spinneret (500 holes) was submerged into a 78 wt
% DMSO/DI water coagulation bath and the nascent 500 filament tow
is formed. The spinneret used was a special shaped hole spinneret
as to generate fibers with cross sectional shapes that emulated
crenulated fiber--(as observed in lobed fibers). Out of coagulation
the tow was pulled on to the Y0 (initial) godet set to 0.9 m/min.
Upon stable spinning, the flow rate was incrementally stepped down
from approximately 4 ml/min (for start-up) to 1.38 ml/min. After
coagulation, the fiber tow was then stretched 2.1.times. through a
50 wt % DMSO/DI water bath and pulled onto the Y1 (second) godet at
1.9 m/min. The fiber tow then travelled down the line through six
deionized water wash baths at 1.9 m/min speeding up slightly to
2-2.2 m/min to keep the tow taught. No hot drawing was applied to
the tow (typically precursor PAN fiber is significantly drawn). The
fiber tow was then passed through the silicone emulsion spin finish
and onto the heated godet (90.degree. C.). After drying on the
first four rollers, the fiber tow passed through the PVA spin
finish bath at the fifth roller. The fiber was then dried on the
remaining three rollers of the heated godet and taken onto
cardboard spools at 2 m/min. The tow was traverse wound on
cardboard cores and accumulated for subsequent weaving
processing.
[0049] Weaving
[0050] The spun tow, in continuous form, was woven into a
satin-harness fabric. An approximately 6 inch wide by 10s of feet
long fabric was woven from 180 warp ends of the low k PAN precursor
fiber.
[0051] Thermal Conversion:
[0052] Thermal conversion can be done on the spun PAN fiber, or on
the woven fabric. For the purposes of ablative carbon composites,
the woven is preferred.
[0053] Stabilization
[0054] The woven fabric was placed in an air convection oven,
unconstrained, and heated at a rate of 10 C/min to 175.degree. C.,
then 0.5.degree. C./min to 265.degree. C. Then it was allowed to
cool to room temperature.
[0055] Carbonization
[0056] The stabilized fabric was carbonized under an inert
atmosphere by ramping to 900.degree. C. at 5.degree. C./min. We
consider this a `low-fired` carbonization temperature. Higher
carbonization temperatures increased the resulting carbon fiber
thermal diffusivity (and thermal conductivity) beyond values of
interest, see Table 1. The HTT needs to be <1000 C.
[0057] The primary goal was to demonstrate a PAN-based carbon fiber
with as similar as possible thermal, and mechanical properties to
the incumbent rayon-derived carbon fiber--shown in Table 1 as Rayon
derived carbon fiber. First, the data suggested that a highest heat
treatment temperature (HTT) during carbonization of 900 C resulted
in the PAN-based carbon fiber with thermal diffusivity most similar
to the Rayon derived carbon fiber. Second, the 900 C HTT resulted
in the modulus of the PAN-based carbon fiber most similar to the
Rayon derived carbon fiber. Third, the no-draw condition of the PAN
precursor, coupled with the low fire HTT of 900 C resulted in the
PAN-based carbon fiber most similar to the Rayon derived carbon
fiber. Lastly, the lobed (or crenulated) shape of the PAN precursor
fiber, rendered a PAN-based carbon fiber similar to the Rayon
carbon fiber.
[0058] Together these three issues; low fire carbonization
temperature, no precursor draw, and lobed shape, result in a unique
low k PAN-based carbon fiber, similar thermally, mechanically, and
in cross sectional shape, to the Rayon based carbon fiber.
TABLE-US-00001 TABLE 1 Thermal and mechanical properties of low-k
PAN derived carbon fibers as a function of highest heat treatment
temperature, compared to Rayon derived carbon fiber. Standard
derivations are shown in parenthesis. Corrected Heat Diffusivity
Stress at Break Modulus Strain at Capacity at HTT (mm.sup.2/s)
(MPa) (GPa) Break (%) H/C Ratio 25C (J/gC) Rayon N/A 1.944 (0.052)
454 (144) 39.7 (2) 1.14 (0.35) 0.01645 0.8740 derived carbon fiber
Baseline 900 1.549 (0.079) 981 (381) 135.3 (15) 0.71 (0.24) 0.02337
1.0589 Low-k PAN 1000 3.805 (0.242) 1786 (404) 204 (13) 0.87 (0.19)
0.00862 0.6802 (367) 1200 5.83 (0.127) 1889 (411) 206 (14) 0.92
(0.19) 0.00643 0.6302 1400 9.713 (0.573) 1774 (693) 231 (13) 0.76
(0.28) 0.00435 0.6647 1600 13.879 (1.744) 2010 (570) 248 (23) 0.81
(0.23) 0.01383 0.6343 Unstretched 900 1.503 (0.108) 1167 (469) 92
(14) 1.27 (0.50) 0.02432 0.8310 Low k PAN 1000 3.662 (0.285) 1678
(443) 175 (9) 0.95 (0.23) 0.00605 0.6222 (368) 1200 5.868 (0.199)
1863 (441) 191 (12) 0.98 (0.22) 0.00865 0.7299 1400* 7.964 (0.226)
2025 (511) 210 (17) 0.96 (0.20) 0.00402 0.8092 1600 13.706 (0.465)
1134 (387) 228 (29) 0.49 (0.15) 0.00581 0.6167 No Draw 900 506.31
(168.8) 60.06 (3.43) 0.833 (0.237) (374)
[0059] Typically the fibers were hot stretched in the spinning
process in 2 stages, which involved a hot water stretch (stage 1)
followed by a hot glycerol stretch (stage 2). Here
"Unstretched"=stretched in hot water, but not stretch in hot
glycerol, or no 2nd stage stretch. "No Draw"=not stretched in hot
water or hot glycerol (no stage 1 or stage 2). However, in all
cases, the fibers were drawn approximately 2.times. just after
their initial coagulation in the first step of the washing
process.
[0060] The foregoing has been presented for purposes of
illustration and description. It is not intended to be exhaustive
or to limit the embodiments to the precise form disclosed. Obvious
modifications and variations are possible in light of the above
teachings. All such modifications and variations are within the
scope of the appended claims when interpreted in accordance with
the breadth to which they are fairly, legally and equitably
entitled.
* * * * *