Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
HIGH SPEED MELT SPINNING OF FLUOROPOLYMER FIBERS
BACKGROUND OF THE INVENTION
The processes and apparatus of the present invention concern melt
spinning fluoropolymers into single filaments or mufti-filament yarns at high
spinning speeds.
Melt spinning of thermoplastic copolymers based on tetrafluoroethylene is
known. However, there is considerable economic incentive to drive fiber
spinning
rates ever higher for these high value polymers. One problem facing processes
of
melt spinning is that at high shear rates, melt fracture occurs which becomes
evident as surface roughness in the extruded fibers. Since the critical shear
rate for
the onset of melt fracture decreases with increasing melt viscosity, ways to
decrease melt viscosity have centered on raising the temperature of the melt.
However, in many polymers including thermoplastic copolymers based on
tetrafluoroethylene, the polymer exhibits thermal degradation before any
significant decrease in melt viscosity can be achieved.
Fibers of polytetrafluoroethylene (PTFE) homopolymer are also highly
valued, particularly for their chemical and mechanical properties, such as low
coefficient of friction, thermal stability and chemical inertness. However,
processing by melt spinning has proved elusive. Since polytetrafluoroethylene
homopolymer fibers are conventionally formed by a disperson spinning process
involving many steps and complicated equipment, there is great economic
incentive to find a method for melt spinning such fibers.
The problem of spinning fibers from high viscosity polymer melts has
been previously addressed for polyesters. In U.S. Patent 3,437,725 a spinneret
assembly is described having a top plate, a heating plate and a lower plate
with a
spacer providing air space between the top plate and the heating plate. Hollow
inserts, one for each filament to be spun, are placed in the top plate and
extend to
the bottom face of the lower plate. Molten polymer is fed into the inserts for
spinning through capillaries. An electrical heater supplies heat to maintain
the
lower plate, heating plate and lower portions of the inserts at a temperature
at least
60°C higher than the temperature of the supplied molten polymer. Heated
capillary temperatures ranging between 290 and 430°C were listed in
examples for
spinning polyesters. No mention is made of any fluoropolymer or temperatures
needed to melt spin fluoropolymers at high spinning speeds.
SUMMARY OF THE INVENTION
The present invention provides a process for melt spinning a composition
comprising a highly fluorinated thermoplastic polymer or a blend of such
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polymers, comprising the steps of melting a composition comprising a highly
fluorinated thermoplastic polymer or a blend of such polymers to form a molten
fluoropolymer composition; conveying said molten fluoropolymer composition
under pressure to an extrusion die of an apparatus for melt spinning; and
extruding
the molten fluoropolymer composition through the extrusion die to form molten
filaments, said die being at a temperature of at least 450°C, at a
shear rate of at
least 100 sec-1, and at a spinning speed of at least 500 m/min.
The present invention also provides a process for melt spinning a
composition comprising polytetrafluoroethylene homopolymer, comprising the
steps of melting a composition comprising a polytetrafluoroethylene
homopolymer to form a molten poiytetrafluoroethylene composition; conveying
said molten polytetrafluoroethylene composition under pressure to an extrusion
die of an apparatus for melt spinning; and extruding the molten
polytetrafluoroethylene composition through the extrusion die to form molten
I S filaments.
The present invention further provides an apparatus for melt-spinning
fibers comprising a spinneret assembly comprising means for filtering; a
spinneret; an elongated transfer line, said transfer line being disposed
between
said filtration means and said spinneret; means for heating said elongated
transfer
line; means for heating said spinneret; and an elongated annealer disposed
beneath
said spinneret assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure I is a cross-sectional view of a portion of a conventional
apparatus for melt spinning.
Figure 2 is a cross-sectional view of one embodiment of a portion of a
melt spinning apparatus of the present invention having an elongated
spinneret.
Figure 3 is a cross-sectional view of one embodiment of a portion of a
melt spinning apparatus having a shortened elongated spinneret.
Figure 4 is a cross-sectional view of one embodiment of a portion of a
melt spinning apparatus of the present invention having a shortened elongated
spinneret with heating means disposed within a center cavit)~ thereof and
heating
means disposed on an outer surface thereof.
Figure 5 is an exploded cross-sectional view of one embodiment of a
melt spinning apparatus of the present invention featuring an elongated
transfer
line disposed between a pack filter and a spinneret disc.
Figure 6 is an assembled cross-sectional view of the melt spinning
apparatus of Fig. S.
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Figure 7 is an exploded cross-sectional view one embodiment of a
melt spinning apparatus of the present invention featuring another embodiment
of
an elongated transfer line and spinneret disc.
Figure 8 is an assembled cross-sectional view of the melt spinning
apparatus of Fig. 7.
Figure 9 is a schematic of one embodiment of a melt spinning
apparatus of the present invention.
Figures l0A and l OB are cross-sectional views of one embodiment of
an annealer useful in the present invention. Fig. 10B is an enlarged view of a
portion of Fig. 10A.
Figure 11 is a graph plotting shear rate (1/sec) vs. SSF at 500°C
for a
composition of Example 1, wherein the darkened triangle represents the spin
stretch factor (SSF) at first filament break and the open triangle represents
the SSF
at the last filament break. Included is some data for
denier/tenacity/speed/gpm.
Figure 12 is a graph demonstrating that temperature exerts a positive
effect on SSF at first filament break at constant shear rate. The circle
represents
SSF at 420°C; the square represents SSF at 460°C; and the
triangle represents SSF
at 500°C (see also Example 1 ).
Figure 13 is a graphical representation of throughput vs. solidification
distance from a spinneret with and without an annealer using FEP-5100, a 30-
mil/30-filament spinneret, a 3-in diameter, 41-in long annealer, and spinneret
temperatures of 380°C (triangle), 430°C (square) and
480°C (circle), wherein the
open symbols represent no annealer and the darkened symbols represent use of
an
annealer.
Figure 14 is a graphical representation of distance from a spinneret
(inch) vs. yarn temperature with an annealer (darkened symbols) and without an
annealer (open symbols) using FEP-5100, a 39.4-mil/30-filament spinneret, a
spinneret temperature of 480°C, at 45.4 gpm/6.0 pph, wherein the square
represents the yarn temperature at a spinning speed of 400 mpm, the circle
represents the yarn temperature at S00 mpm, and the triangle represents the
yarn
temperature at 700 mpm.
Figure 1 S is a graphical representation of length of annealer (inch) vs.
first-filament-break speed in meters/minute (mpm). The following were used:
FEP-5100 fluoropolymer, a 30-mil/30-filament spinneret, a spinneret
temperature
of 480°C, and 44.8 grams/minute (gpm).
Figure 1 ~ is a graphical representation of temperature vs. first
filament break speed (mpm) for Example 23, wherein the darkened circle
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represents the sample of the present invention and the square represents the
comparative sample.
DETAILED DESCRIPTION
The process of the present invention affords the benefits of high
temperature spinning while avoiding the pitfalls thereof. In the process of
the
present invention, the composition comprising highly fluorinated thermoplastic
polymer or blend of such polymers can be exposed to temperatures above the
degradation temperature of the polymers for times Buff cient to cause a
decrease in
melt viscosity but insufficient for significant polymer degradation to occur.
In
melt spinning, the molten composition experiences the highest shear rate
during
its transit through the extrusion die, e.g. capillaries, of the spinneret of
the melt
spinning apparatus. In the process of the present invention, it is at that
point that
the molten composition can be heated to a temperature above the degradation
temperature of the highly fluorinated polymer. Because of the high throughput
speed achievable in the present invention due to the elevated temperature, the
residence time of the composition in the extrusion die is kept to a minimum.
Accordingly, the present invention provides a first process for melt
spinning a composition comprising a highly fluorinated thermoplastic polymer
or
a blend of such polymers, comprising the steps of melting a composition
comprising a highly fluorinated thermoplastic polymer or a blend of such
polymers to form a molten fluoropolymer composition; conveying said molten
fluoropolymer composition under pressure to an extrusion die of an apparatus
for
melt spinning; and extruding the molten fluoropolymer composition through the
extrusion die to form molten filaments, said die being at a temperature of at
least
450°C, at a shear rate of at least 100 sec-1, and at a spinning speed
of at least
500 m/min.
In the melting step, a composition including a highly fluorinated
thermoplastic polymer or a blend of such polymers is melted. Highly
fluorinated
thermoplastic polymers for the purpose of this first process include
homopolymers
other than polytetrafluoroethylene (PTFE), such as polyvinylidene fluoride
(PVDF), and copolymers, such as copolymers of tetrafluoroethylene (TFE)
prepared with comonomers including perfluoroolefins, such as a perfluorovinyl
alkyl compound, a perfluoroalkyl vinyl ether, or blends of such polymers. The
term "copolymer", for purposes of this invention, is intended to encompass
polymers comprising two or more comonomers in a single polymer. A
representative perfluorovinyl alkyl compound is hexafluoropropylene.
Representative perfluoroalkyl vinyl ethers are perfluoromethyl vinyl ether
(PMVE)5 perfluoroethyl vinyl ether (PEVE), and perfluoropropyl vinyl ether
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(PPVE). Preferred highly fluorinated polymers are the copolymers prepared from
tetrafluoroethylene and perfluoroalkyl vinyl ether and the copolymers prepared
from tetrafluoroethylene and hexafluoropropylene. Most preferred copolymers
are
TFE with 1-20 mol% of a perfluorovinyl alkyl comonomer, preferably 3-10 mol%
hexafluoropropylene or 3-10 mol% hexafluororpopylene and 0.2-2 mol% PEVE
or PPVE, and copolymers of TFE with 0.5-10 mol% perfluoroalkyl vinyl ether,
including 0.5-3 mol% PPVE or PEVE. Also suitable for the practice of this
invention are blends of the highly fluorinated thermoplastic polymers
including
blends of TFE copolymers.
The fluoropolymers suitable for the practice of the present invention
preferably exhibit a melt flow rate (MFR) of 1 to about 50 g/10 minutes as
determined at 372°C according to ASTM D2116, D3307, D1238, or
corresponding tests available for other highly fluorinated thermoplastic
polymers.
The composition comprising the highly fluorinated thermoplastic polymer
or a blend of such polymers can further comprise additives. Such additives can
include, for example, pigments and fillers.
In the present process the composition comprising the highly fluorinated
polymer or blend of such polymers, discussed above, is melted to form a molten
fluoropolymer composition. Any means known in the art for providing a melt can
be used. A representative method can include introducing the fluoropolymer
composition to an extruder which is heated to a temperature sufficient to melt
the
composition but below the degradation temperature of the highly fluorinated
thermoplastic polymer or blend of such polymers. This temperature is dependent
upon the particular polymers used.
Once the composition is in a molten state, it is conveyed under pressure to
an extrusion die, such as a spinneret, of an apparatus for melt spinning.
Means of
conveying compositions to the extrusion die are well known in the art and
include
apparatus with a ram or piston, a single screw or a twin-screw. In a preferred
embodiment of the process of the present invention, an extruder is employed to
melt and convey the molten composition suitable for the practice of this
invention
to a single or multi-aperture strand extrusion die to form, respectively a
monofilament or multifilament fiber product. The extruder barrel and screw,
and
the die are preferably made from corrosion resistant materials including high
nickel content corrosion resistant steel alloy, such as Hastelloy 0276 (Cabot
Corp., Kokomo, III. Many suitable extruders, including screw-type and piston
type, are know in the art and are available commercially. A metering device,
such
as a gear pump, may also be included to facilitate the metering of the melt
between the screw and the spinneret.
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In the process of the present invention, after the molten fluoropolymer
composition is conveyed to the extrusion die, it is extruded through the
apertures
of the extrusion die, said die being at a temperature of at least
450°C, at a shear
rate of at least 100 sec-/, and at a spinning speed of at least S00 m/min.
The apertures of the extrusion die can be of any desired cross-sectional
shape, with a circular cross-sectional shape preferred. The diameter of a
circular
cross-sectional aperture found suitable for use in the process of the present
invention can be in the range of about 0.5 to 4.0 mm, but the practice of this
invention is not limited to that range. The length to diameter ratio of the
extrusion
die aperture useful in the present invention is preferably in the range of
about 1:1
to about 8:1. Although the hole pattern is not critical, it is preferred if
the holes
are arranged in one or two concentric circles, with a single circle
arrangement
being more preferred.
Fig. 1 depicts a portion of a conventional melt spinning apparatus for
thermoplastic polymers, spinneret assembly 10. Shown are adaptor 1 which mav_
be heated with a cartridge heater inserted within space 9 located between the
dotted lines along adaptor 1, which is attached to means for conveying and
melting the fluoropolymer composition (not shown), filter pack 2 containing
melt
filtration means 3, typically screens, and conventional spinneret 4 having
face
plate 5, face plate 5 being disposed at one end of spinneret 4 at a distance,
h, from
the opposite end of spinneret 4. Spinneret 4 is disposed adjacent bottom face
8 of
filter pack 2, and together with filter pack 2 is affixed to adaptor 1 by
retaining nut
6. Spinneret assembly 10 is heated by band heater 7 circumferentially disposed
around retaining nut 6. In Fig. 1, spinneret 4 is generally heated by its
conductive
contact with retaining nut 6.
In the conventional spinneret assembly design of Fig. 1, there is no
convenient way to heat only face plate 5 of spinneret 4 because spinneret 4
resides
entirely within retaining ring 6. Any attempt to super-heat face plate 5 would
result in heating a considerable portion of other areas of spinneret assembly
10 to
a similar if somewhat lower temperature. This undesirable heating of areas
besides face plate 5 of spinneret assembly 10 to temperatures at or above the
degradation temperature of the fluoropolymer composition would result in an
undesirably long duration of exposure of the fluoropolymer composition to high
temperature and could lead to excessive polymer degradation under some
circumstances.
During extrusion in the present invention, the extrusion die is heated to a
temperature of at least 450°C. For certain fluoropolymer compositions
herein, the
extrusion die can be heated to temperatures greater than about 500°C.
Heating to
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these temperatures without degradation of the fluoropolymer composition can be
done by thermally isolating the extrusion die from other areas of the melt
spinning
apparatus that may contain the fluoropolymer composition. When the molten
fluoropolymer composition begins to pass through the extrusion die, the
elevated
S temperature of the die thereof induces a rapid decrease in polymer melt
viscosity,
permitting a high rate of transmission through the extrusion die. To avoid
thermal
degradation, it is necessary to reduce the residence time of the melt at the
high
temperatures. Since degradation is a function not only of temperature but also
of
time, if the temperature is high, it is preferred that the residence time be
minimized. Thus, the present invention provides the highest temperature in the
area where it would be most beneficial, namely the extrusion die, e.g. the
walls of
the spinneret capillary holes, which are in the face plate of the spinneret.
Therefore, the extrusion die can be kept thermally isolated from other areas
of the
melt spinning apparatus that may be in contact with the fluoropolymer
composition.
The spinneret or a portion thereof that includes the face plate can be heated
independently of other areas of the spinneret assembly. Any means for
providing
highly localized heating to a temperature of at least 450°C can be
employed for
the practice of the invention. Such means includes a coil heater, a cartridge
heater, a band heater, and apparatus for radio frequency, conduction,
induction or
convective heating, such as an induction heater. Insulation may be used, such
as
ceramic insulation, to provide off sets and thereby thermal isolation between
the
face plate and other areas of the melt spinning apparatus that may be in
contact
with the fluoropolymer composition. Use of one or more cooling jackets can
also
be used on areas of the spinneret or spinneret assembly other than the
extrusion
die to provide thermal isolation of the extrusion die.
In order to facilitate the thermal isolation of the extrusion die, it has been
found satisfactory in one embodiment of the present invention to offset the
spinneret face plate from the spinneret body by simply increasing the
distance, h,
between the ends of the conventional spinneret shown in Fig. 1. Increasing the
distance in this manner, shown in Fig. 2 as h', enables separate heating of
the
spinneret face plate from the bulk of the remainder of the spinneret assembly.
Thus, the spinneret face plate of the present invention in one embodiment is
separated from the bottom face of the filter pack by distance h' which
distance is
sufficient to allow separate heating of the spinneret face plate.
In Fig. 2 is shown spinneret assembly 20 having adapter 21 which is
attached to means for melting and/or conveying the fluoropolymer composition
(not shown), filter pack 22 containing screen 23 and bottom face 28, elongated
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spinneret 24 having face plate 25 being disposed at one end of spinneret 24 at
a
distance, h', from the opposite end of spinneret 24 at bottom face 28 of
filter pack
22, wherein h'>h other measurements of Fig. 1 and 2 held equal, to enable face
plate 25 to extend outside of retaining nut 26. With face plate 25 thus
protruding
from retaining nut 26, heating means 29 can be used to separately heat face
plate
25, and thus face plate 25 is thermally isolated from the remainder of the
spinneret
assembly. Heating means 27, such as a band or coil heater, is disposed
circumferentially around retaining nut 26.
An alternative embodiment of a spinneret assembly useful in the present
invention is shown in Fig. 3 as spinneret assembly 30. 1n this embodiment, the
bottom part of retaining nut 26 of Fig. 2 is reduced in size, e.g. the
retaining nut is
thinner, see retaining nut 36 in Fig. 3. Here, the body of elongated spinneret
34 is
shortened relative to the length of spinneret 24 of Fig. 2, and yet spinneret
34 is
elongated (relative to spinneret 4 of Fig. 1 ) so as to extend beyond
retaining nut 46
enabling face plate 35 to be heated separately, by means 39, from means 37
shown
for heating another area of the spinneret assembly. Also shown is adapter 31
which is attached to means for melting and/or conveying the fluoropolymer
composition (not shown), filter pack 32 and filtration means 33, and channel
38.
In the above embodiments of the present invention, molten composition
conveyed into the spinneret can be heated by means disposed around the outside
wall of the spinneret, and thus the temperature of the melt adjacent the walls
of the
apertures is higher than the temperature in the center of the melt. The effect
of
this temperature non-uniformity, highest at the outside and cooling toward the
center of the melt, can cause extruding filaments to bend toward the center of
the
spinneret. The bent angle has been observed higher than 45 degrees at high jet
velocity for certain fluoropolymer compositions. The impact of this phenomenon
can be reduction in attainable high speed filament continuity. In order to
reduce
any temperature gradient between the outermost and innermost parts of the
polymer melt, a heating means is provided within aperture 48, such as a
cartridge
heater, can be introduced into the center of elongated spinneret 44, as shown
in the
spinneret assembly 40 of Fig. 4. Also shown in Fig. 4 are adapter 41 which is
attached to means for melting and/or conveying the fluoropolymer composition
(not shown), filter pack 42, filtration means 43, retaining nut 46, heating
means 47
and 49, and face plate 45.
A further embodiment provided by the present invention, shown in Figs. 5
and 6 as spinneret assembly 50, is to heat the melt faster and through nanrow
channel 62 (relative to channel 38 of Fig. 3) provided within transfer line
583 and
reduce the volume directly upstream to spinneret face plate 55. By reducing
the
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volume, the residence time is reduced. This embodiment also provides the
opportunity to provide an intermediate temperature zone for the composition
while in channel 62 of transfer line 58 through use of heating means 60. Thus,
the
present process can further include exposing the fluoropolymer composition to
an
intermediate temperature ranging from the melt temperature of the
fluoropolymer
composition to a temperature less than the temperature of the extrusion die,
e.g. at
the face plate of the spinneret. As shown, the portion of transfer line 58
adjacent
filter pack 52 can be heated via heating means 57 disposed circumferentially
around retaining nut 56. The fluoropolymer composition within channel 62 of
transfer line 58 can be pre-heated to at least one intermediate temperature
which
can range from above the melting temperature of the fluoropolymer composition
to a temperature lower than the temperature at face plate 55 via heating means
57
andlor heating means 60. Face plate 55 is shown in this embodiment as being
separately heated via heating means 61 held in spinneret sleeve 59. Transfer
line
58 is disposed downstream of filter pack 52 and filtration means 53 and
followed
by spinneret 54, shown having a disc shape. Spinneret 54 can be removable for
cleaning and replacement without removal of pack filter 52. Also shown is
adapter 51 which is attached to means for melting and/or conveying the
fluoropolymer composition (not shown).
Figs. 7 and 8 show spinneret assembly 70 of the present invention which
embodiment permits removal of transfer line 78 and can accomodate larger
diameter disc spinnerets relative to the embodiment shown in Figs. 5 and 6,
such
as spinneret 74. Spinneret nut 79 holds disc spinneret 74 having face plate 75
to
the bottom of face 82 of transfer line 78. Narrow internal flow channel 83 in
transfer line 78 reduces the volume and residence time of the fluoropolymer
composition at high temperature to further reduce the chance of degradation.
Transfer line 78 also provides a means of stepping up to an intermediate
temperature between filtration means 73 and spinneret 74 via its separate
heating
means 80. At the same time, the transfer line embodiment shown provides more
uniform and faster heat transfer. An additional advantage of this embodiment
is
that disc spinneret 74 can be replaced without having to remove the filter
pack,
and the disc can be easier to fabricate. Also shown are adapter 71, which is
attached to means for melting and/or conveying the fluoropolymer composition
(not shown), plate 72 which has multiple distribution channels providing
support
for filtration means 73, retaining nut 76 surrounded by heating means 77,
chamber
84 disposed between filtration means 73 and transfer line 78, and face plate
75.
It is believed that the present process provides self melt lubricated
extrusion. By "self melt lubricated extrusion" is meant that only the skin of
the
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extrudate, the portion of the melt directly adjacent the walls of the
apertures,
becomes heated to extremely high temperature by the very hot die aperture
surface
resulting in very low viscosity of this portion of the melt while keeping the
bulk of
the extrudate to a lower temperature due to the short contact or residence
time.
S The considerably reduced viscosity of the outer layer skin behaves like a
thin
lubricating f lm thus permitting the extrusion to become plug flow, wherein
the
bulk of the extrudate experiences uniform velocity.
As used herein "shear rate" refers to the apparent wall shear rate,
calculated as 4Q/~R3 (Q = volumetric flow rate, R= radius of capillary). In
the
process of the present invention, the shear rate is at least 100/sec. The
shear rate
range over which satisfactory fiber melt-spinning can be achieved in a given
configuration and at a given temperature grows progressively narrower with
increasing polymer melt viscosity. The operating window can be expanded by
increasing the temperature which displaces the critical shear rate for the
onset of
melt fracture to higher rates, but care must be taken to avoid polymer
degradation.
The critical temperature/shear rate for melt fracture is determined herein by
increasing the throughput rate for a given temperature and die dimension until
surface roughness is visible as shown by the change in molten extrudate from a
transparent to a slightly opaqueness indicating the onset of melt fracture.
Further
increase in throughput rate would give an undesirable coarser surface
roughness
and poorer spinning performance and properties.
The spinning speed of the process of the present invention is at least 500
m/min and is determined herein as the spinning speed at the last roll, which
depending on the configuration of the melt-spinning apparatus may be a take-up
roll or may be a wind-up roll.
It is found in the practice of the present invention that both shear rate and
SSF have a large effect on the strength of the spun filament. The same
strength
can be maintained as the shear rate increases while the SSF decreases and vice
versa as demonstrated in Example l and shown graphically in Fig. 11.
The process of the present invention can further comprise shielding the
filaments. By shielding the filaments, the air surrounding the filaments
remains
warmer than if the filaments were exposed to unrestricted ambient air and thus
prevents rapid cooling of the filaments. Unrestricted ambient air, and in
particular, turbulent air can result in rapid cooling of the filaments which
is
undesirable because it can be detrimental to the amount of draw the filament
may
have. Thus shielding the filaments can permit higher attenuation of spin
stretch.
It has been observed herein that the achievement of high SSF for high spinning
can be obtained if the solidification of the molten threadline occurs at a
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greater than 50 times the diameter of the extrusion die (capillary diameter)
(see
also Fig. 13). Preferably, the solidification distance is greater than 500
times the
diameter of the capillary diameter. Shielding can be accomplished by running
the
molten filaments through an annealer. An annealer permits the high speed
extruded molten filaments to be spin stretched to a high degree and thus
increases
the spinning speed. Although a gentle suction of air can be generated by the
fast
moving yarn through the bottom of the annealer, the annealer still provides a
relatively quiescent environment against surrounding air turbulence which
partially cools but prevents rapid cooling of the extremely hot molten
filaments,
maintaining the filaments above their melting point for a much further
distance
from the spinneret than without an annealer. This is shown graphically in Fig.
13.
The use of an annealer also maintains the solidified yarn at a higher
temperature
than without the use of an annealer as shown in Fig. 14. In addition, the use
of an
annealer can permit higher spinning speeds as shown in Fig. 15 (note: 0-inch
represents no annealer).
One embodiment of an annealer useful in the present invention is shown in
Figs. 1 OA and l OB. As shown, annealer 200 includes inner tube 202 which is a
long tube concentrically disposed inside outer tuber 204, a slightly larger
diameter
tube which can be of substantially the same length. Inner tube 202 can be
positioned within outer tube 204 to extend below outer tube 204 and thus
provides
an exit for the molten filaments and further creates a cylindrical opening 205
at
the top of outer tube 204. Opening 205 permits air to be sucked into inner
chamber 206 of inner tube 202 which may have been pre-heated in annular space
208 between inner tube 202 and outer tube 204. Although external heat is not
provided, annular space 208 can be heated during spinning by the heat
radiating
from the extruded hot molten filaments. Top flange 210, which can have a
circular peripheral lip, sits on top of outer tube 204. Mesh tubing 212,
preferably
composed of a fine mesh screen, such as 20-mesh, can be attached to top flange
210 and is disposed adjacent the inner walls of inner tube 202. Mesh tubing
2I2
extends axially through inner chamber 206 beyond opening 205, but it is not
necessary to provide the mesh tubing for the entire length of the inner tube.
Mesh
tubing 212, which can further include a second finer mesh, such as 100-mesh,
attached to or in close proximity to the first mesh, serves to reduce incoming
air
turbulence and also facilitates a substantially uniform distribution of the
air so that
the air travels radially into inner chamber 206 through opening 205. There is
also
shown perforated annular plate spacers 214, disposed between inner tube 202
and
outer tube 204, and connected either to the outer surface of inner tube 202 or
to
the inner surface of outer tube 204, and can serve to prevent inner tube 202
from
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falling out of outer tube 204. Screens 216 of fine mesh can be placed on top
of
plate 214 to diffuse and distribute the air traveling up and into opening 205.
Such
spacers 214 and 216 are optional. An optional glass ring 220 permits visual
observation of the molten threadlines and spinneret face.
The inner and outer tubes of the annealer can be fabricated from materials
including metal, such as aluminum, or plastic, such as Lucite. The annealer
can
be self standing or held stable with a suitable mounting mechanism which can
be
attached to other elements of a melt spinning apparatus or affixed to other
materials to keep it held steady.
The process of the present invention can further comprise passing the
extrudate in the form of one or more strands through a quench zone to means
for
accumulating the spun fiber. The quench zone may be at ambient temperature, or
heated or cooled with respect thereto, depending upon the requirement of the
particular process configuration employed.
I S Any means for accumulating the fiber is suitable for the practice of the
present invention. Such means include a rotating drum, a piddler, or a wind-
up,
preferably with a traverse, all of which are known in the art. Other means
include
a process of chopping or cutting the continuous spun-drawn fiber for the
purpose
of producing a staple fiber tow or a fibrid. Still other means include a
direct on-
line incorporation of the spun-drawn fiber into a fabric structure or a
composite
structure. One means found suitable in the embodiments here in below described
is a high-speed textile type wind-up, of the sort commercially available from
Leesona Co., Burlington, NC.
Such other means as are known in the art of fiber spinning to assist in
conveying the fiber may be employed as warranted. These means include the use
of guide pulleys, take-up rolls, air bars, separators and the like.
An anti-static finish can be applied to the fiber. Such finish application is
well known in the trade.
The process of the present invention can further comprise drawing the
fiber, a relaxing stage, or both. The fiber can be drawn between take-up rolls
and
a set of draw-rolls. Such drawing is well known in the trade to increase the
fiber
tenacity and decrease the linear density. The take-up rolls may be heated to
impart
a higher degree of draw to the fiber, the temperature and the degree of draw
depending on the desired final fiber properties. Likewise additional steps,
known
to those of ordinary skill in the art, may be added to the present process to
relax
the fiber.
The present invention also provides a second process for melt spinning a
composition comprising polytetrafluoroethylene homopolymer, comprising the
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steps of melting a composition comprising a polytetrafluoroethylene
homopolymer to form a molten polytetrafluoroethylene composition; conveying
said molten tetrafluoroethylene composition under pressure to an extrusion die
of
an apparatus for melt spinning; and extruding the molten
polytetrafluoroethylene
composition through the extrusion die to form molten filaments.
In the method of melt spinning the homopolymer, polytetrafluoroethylene
(PTFE), preferred PTFE homopolymers are those that give a melt flow at
temperatures below 480°C. Preferred homopolymers include Zonyl~ fluoro-
additives, PTFE granular molding powder grades, such as Teflon~ PTFE TE-
6472, and PTFE lubricated paste extrusion resins, such as Teflon~ PTFE 62, all
available from E. I. du Pont de Nemours and Co., Wilmington, DE. Because of
the extreme temperatures required to exhibit melt flow characteristics which
border on the verge of thermal degradation, the present process is of
particular
importance in the successful melt processing and fiber spinning of PTFE
homopolymers.
The description above pertaining to the steps in the first process of melt
spinning the highly fluorinated thermoplastic composition and the apparatus
useful therefor are applicable to the process of melt spinning the
polytetrafluoroethylene composition. However, the same limitations on
extrusion
die temperature or shear rate or spinning speed found in the first process may
not
be applicable in the present PTFE process. Preferably, the temperature of the
extrusion die is at least 450°C. The spinning speed is preferably at
least 50 mpm;
more preferably at least 200 mpm; and most preferably at least 500 mpm.
The present invention further provides an apparatus for melt-spinning
fibers comprising a spinneret assembly comprising means for filtering; a
spinneret; an elongated transfer line, said transfer line being disposed
between
said filtration means and said spinneret; means for heating said elongated
transfer
line; means for heating said spinneret; and an elongated annealer disposed
beneath
said spinneret assembly.
Any means for filtering melt-spun fiber conventionally used in the art for
melt-spinning can be used in the present apparatus. The spinneret is
constructed
to allow separate heating of the face of the spinneret, e.g. the portion of
the
spinneret which includes the walls of the capillaries, which face may comprise
a
separate plate or be integral part of the body of the spinneret, from other
areas of
the melt-spinning apparatus. The length to diameter ratio of the capillaries
within
the spinneret are preferably about 1:1 to about 8:1. The capillary holes of
the
spinneret are preferably arranged to achieve uniform heating among all of the
holes. Preferably, the capillary holes are arranged in two concentric circles
or in
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WO 00/44967 PCT/US00/02108
one circle. Preferably the spinneret is separately removable from the transfer
line
to allow easy cleaning or replacement. Likewise, the transfer line is
preferably
removable from the filter pack and the spinneret. Means for heating the
transfer
line and means for heating the spinneret can include a band heater, a coil
heater. or
other conduction, convection or induction heaters known to those of skill in
the
art.
The elongated annealer, described in more detail above and in the
examples, preferably comprises an inner tube and an outer tube separated by an
annular space. Preferably the inside diameter of the inner tubes ranges from
about
3-inches to about 8-inches. The elongated annealer can further comprise a mesh
tube disposed adjacent the inner wall of the inner tube extending at least
partially
down the length of the inner tube. The elongated annealer can further comprise
at
least one perforated plate disposed within the annular space, extending
radially
with respect to the circumference of said outer tube, and attached to the
outer wall
1 S of said inner tube, the inner wall of said outer tube, or to both tubes.
Screens may be positioned on or in close proximity to these perforated
plates. Air can enter the annular space of the annealer through an opening or
port.
The anneaier can further comprise means for measuring or controlling the air
flow
rate, such as via a needle valve or a flow meter.
The present apparatus can fiwther comprise means for accumulating the
spun filaments. Any means conventionally known in the art can be used,
including but not limited to, a take-up roll, a draw-roll, and a wind-up roll.
One embodiment of an apparatus of the present invention for melt-
spinning is shown, as melt spinning apparatus 100 in Fig. 9. Shown are feed
hopper 102 into which the polymer composition is fed, preferably in the form
of
pellets. These pellets are heated and conveyed through screw extruder 103.
After
the polymer or blend composition is melted, it is conveyed under pressure to
pump block 104, through filter pack 105, transfer line i06 to spinneret 107
having
face 108. Glass sleeve 109 permits viewing of the molten filaments. Molten
fluoropolymer composition is extruded through one or more apertures of face
plate 108 in spinneret 107 to form a continuous strand which is directed
through
elongated annealer 110 wherein the strand is shielded to prevent rapid
cooling.
Upon leaving the annealer, the spun fiber travels through pigtail guides 111,
change of direction guides 116 to kiss roll 112 for an optional finish
application,
to a pair of take-up rolls 113, a pair of draw rolls 114, and a windup 115.
Additional draws may be added as well as relaxation rolls.
Fibers made by the process and apparatus of the present invention can be
useful in textiles. Such textiles can be used in high performance sporting
apparel,
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WO 00/44967 PCT/US00/02108
such as socks. Such fibers can be combined with other fibers in fabrics.
Fibers of
PTFE can be used for industrial quality yarn for wet filtration. PTFE fiber
can
also be chopped for dry lubricant bearings.
EXAMPLES
In the examples the following polymers (all available from E. I. du Pont de
Nemours and Company, Wilmington, DE) were used:
Teflon~ PFA 340, a copolymer of TFE and perfluoropropyl vinyl ether
Teflon~ FEP 5100, a copolymer of TFE, hexafluoropropylene, and perfluoroethyl
vinyl ether
Zonyl~ MP-1300 PTFE
Teflon~ TE-6462 PTFE
Teflon~ PTFE TE-6472, a granular molding powder
Teflon~ PTFE 62, a lubricated paste extrusion resin
Zonyl~ MP-1600N, PTFE
Unless otherwise indicated, the polymer used was Teflon~ PFA 340.
EXAMPLE 1
The effects of spinneret temperature, shear rate and spin stretch factor
(SSF) on spinning speed and fiber properties were tested.
Spinning was conducted using a 1.0-inch diameter steel single screw
extruder, to which was connected a spin pump block, which was in tum connected
to a spinneret pack adapter with the following features: a by-pass plate was
used
in place of a spin pump. An elongated spinneret was used, such as is depicted
in
Fig. 2, wherein "h"' was 2.0 in. A 30-mil 39-hole spinneret, wherein all of
the
holes were in only one circle, was used to cover the shear rate from low to
medium shear rates, e.g. about 60/sec to about 180/sec, while a 15-mil 25-hole
spinneret was used to cover the medium to high shear rates, e.g. about 350/sec
to
about 1,150/sec. A 1-inch high, 1.25-inch inside diameter coil heater
(Industrial
Heater Corp.) was wrapped around the lower 1-inch part of the elongated
spinneret and was used to separately heat a portion of the spinneret that
included
the face plate. Conventional take-up rolls were used along with a Leesona wind-
up.
The temperature profile prior to the spinneret was 350°C in the
screw
extruder, 380°C in the pump block to the pack filter located between
the extruder
and the spinneret. Three spinning operations were performed using Teflon~ PFA
340. The spinneret temperature was set at 420°C, 460°C, or
500°C.
At 420°C melt fracture (M.F.) occurred at about 180/sec shear
rate. The
highest possible spinning speed with all filaments intact without melt
fracture was
slightly less than 219 mpm at a shear rate of about 90/sec. The fiber tenacity
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this speed and shear was 1.02 gpd. The highest spinning speed at last filament
break was 490 mpm at a shear rate of about 60/sec, and the fiber tenacity was
1.68
gpd with a filament denier of 4Ø
At 460°C the spinnable shear rate increased to slightly less than
720/sec
before the onset of melt fracture. The highest measured spinning speed at
first
filament break was 435 mpm at a shear rate of 160/sec, and the fiber possessed
a
tenacity of 1.13 gpd. The highest spinning speed at last filament break was
850
mpm also at a shear rate of about 160/sec. The highest fiber tenacity for
fiber
spun to last filament break was 1.61 gpd spun at 580 mpm with a filament
denier
of2Ø
A graph of shear rate vs. spin stretch factor for the 500°C spinneret
sample
is shown in Fig. 11. The darkened triangle represents data at first filament
break
and the open triangle is data at last filament break. At 500°C, the
spinnable shear
rate was pushed to slightly less than 1,150/sec before the onset of melt
fracture.
The highest spinning speed at first filament break was 933 mpm at a shear rate
of
about 180/sec, and the fiber possessed a tenacity of 1.04 gpd. The highest
spinning speed at last filament break was 930 mpm also about 180/sec, and the
tenacity at this speed was of 1.15 gpd.
Thus, it is seen that as the temperature of spinneret increased from
420°C
to 500°C, the attainable spinning speed increased by a factor of 4.3X.
Temperature also exerted a positive effect on the SSF at first filament
break at constant shear rate, as shown in Fig. 12. The darkened circles show
SSF
at 420°C; the darkened squares show SSF at 460°C; and the
darkened triangles
show SSF at 500°C. A higher SSF meant that at the same throughput rate
and
given spinneret hole size, the take-up roll speed was higher in spinning
speed.
Unless otherwise stated in the remaining examples, spinning was
conducted using the equipment described above except that a 1.5-inch diameter
corrosion resistant single screw extruder, made by Killion Extruders, Inc.,
Cedar
Grove, N.J, was used. This extruder had three separate heating zones
designated
"Screw Zone l, 2 and 3" in the temperature profiles below. A clamp ring was
used to attach the extruder to a screw adapter holding them together, and the
screw
adapter was, in turn, attached to a spinneret adapter. The clamp ring was
heated
using a cylindrical rod camidge heater, and the screw adapter and spinneret
adapters were heated using cartridge heaters. A band heater was used to heat
the
filter pack. Unless otherwise indicated, a band or coil heater was used for
heating
any transfer line present, and the spinneret face. Conventional take-up and
wind-
up equipment was used, including a Ixesona wind-up.
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EXAMPLE 2
Spinning was conducted at a throughput rate of 1.3 grams per minute per
hole using a 30-mil 30-hole elongated spinneret at a jet velocity of 1.9 mpm.
The
equipment spinning temperature (°C) profile was:
Screw Zone Clamp Screw Spinneret Pack
1 2 3 Rind Adapter Adapter Filter Spinneret
350 350 350 380 353 480 480 500
The shear rate was 328/sec, and the maximum spinning speed achieved
was 1,100 mpm for a spin-stretch factor at first filament break (FFB) of 580.
The
denier, tenacity, elongation, and modulus of the resultant fibers were,
respectively:
11 d/0.76 gpd/61 %/5.6 gpd.
EXAMPLE 3
This spin was done similar to Example 2 except that a 5-foot tall tapered
aluminum annealer was added to the equipment downstream of the spinneret to
shield the molten filaments after their exit from the spinneret. The annealer
had a
square cross section, 12-inch square at the top and tapering down to a 1.0-
inch
square at the bottom. The same temperature profile was used as in Example 2
except for the following changes: 380°C screw adapter, 470°C
spinneret adapter,
470°C pack filter. The shear rate was 328/sec. At the same throughput
rate of 1.3
grams per minute per hole and using the same 30-mil, 30-hole elongated
spinneret
as was used in Example 2, the maximum spinning speed increased by 35%, or 385
mpm to 1,485 mpm, for a SSF at FFB of 782. The denier, tenacity, elongation
and
modulus of the resultant fibers were, respectively: 9.4 d/0.72 gpd/76%/5.1
gpd.
EXAMPLE 4
This spin was done similar to Examples 2 and 3 except that a different
annealer was used. For this spin, a 6-ft 3-in high self standing Lucite~
annealer
was used which had a 12-in x 12-in square cross section. The same temperature
profile was used as in Example 3. The shear rate was 328/sec. The maximum
spinning speed was increased to 1,756 mpm for a SSF at FFB of 924. This was a
60% increase in spinning speed compared to Example 2, or an 18% increase in
spinning speed compared to Example 3. The denier, tenacity, elongation and
modulus of the resultant fibers were respectively: 6.0 d/1.16 gpd/28%/10 gpd.
EXAMPLE 5
A spinneret assembly, such as shown in Fig. 3, having a shortened
elongated spinneret was used in this example. The distance between the bottom
face of the filter pack and the face plate of the spinneret was 1.25-inch. The
same
temperature profile and the same 6-ft 3-in Lucite~ annealer was used as in
Example 4. The shear rate was 328/sec. The maximum spinning speed achieved
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PCT/US00/02108
was 1,860 mpm for a SSF at FFB of 979. This high speed sample was not tested
for fiber properties, but another sample spun under the same conditions at a
shear
rate of 342/sec with a spinning speed of 1,701 mpm had fiber properties
(denier,
tenacity, elongation and modulus, respectively) of: 7.6 d/I.O1 gpd/68%/6.2
gpd.
EXAMPLE 6
Spinning was conducted as in Example S, except that the shortened
elongated spinneret was heated using an induction heating coil, and the
following
changes in the temperature profile were used: 440°C pack filter, 522-
531 °C
spinneret. The shear rate was 342/sec. The maximum spinning speed at FFB was
1,860 mpm. The denier, tenacity, elongation and modulus of the resultant
fibers
were, respectively: 9.6 d/1.06 gpd/49%/8.7 gpd.
EXAMPLE 7
Spinning was conducted as in Example 6, except that the annealer used
was the same tapered aluminum annealer used in Example 3. A 12-in cube clear
1 S Lucite~ box was added on top on the annealer for the purpose of viewing
the
threadlines. The shear rate was 342/sec. The maximum spinning speed at FFB
was 1,860 mpm. The denier, tenacity, elongation and modulus of the resultant
fibers were, respectively: 9.0 d/1.02 gpd/54%/7.7 gpd.
EXAMPLE 8
Spinning was conducted using a spinneret, such as is shown in Fig. 4,
having a cartridge heater (available from Industrial Heater Corp. Stratford,
CT) in
the center of the spinneret and a standard band heater on the outside of the
spinneret. The length of the spinneret from the bottom face of the filter pack
to
the face plate of the spinneret was 1.25-inch. The temperature profile used
was:
Screw Zone Clamp Screw Spinneret Pack Spinneret
I 2 3 Ring Adapter Adapter Filter Center Spinneret
350 350 350 380 380 411 410 496 S00
The spinneret used had 26 holes; however, the throughput per hole was kept
constant as in Examples 2 to 7. Thus, the shear rate was about the same, i.e.
342/sec. The maximum spinning speed was 1,976 mpm for a SSF of 1,040. The
6% increase in speed compared to Example 5 was attributed to the more uniform
heating of the melt across the spinneret. The fiber properties of denier,
tenacity,
elongation and modulus were, respectively: 5.6 d/1.09 gpd/55%/7.0 gpd.
Another sample spun with a 400°C temperature in the spinneret
adapter
and pack filter and the same 500°C in the spinneret gave a maximum
speed of
1,920 mpm for a SSF of 1,010. Fiber tenacity was higher with the fiber
properties
of denier, tenacity, elongation and modulus measured as follows: 5.6 d/I.25
gpd/54%/8.7 gpd.
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EXAMPLE 9
A spinneret assembly, such as is shown in Fig. 6, was used to test the
effectiveness of this embodiment in achieving high spinning speed. A 15-hole
1.0
in diameter disc spinneret with 30-mil diameter holes was used. The annealer
used was the 6-ft 3-in Lucite~ annealer used in Example 4. A band heater was
used for the pack filter. The transfer line measured from the bottom face of
the
filter pack to the spinneret disc was 3.125-inch.
At a screw rpm of 4.0, the total throughput rate was 20.3 grams per minute
(2.7 Ibs/hr) or 1.35 gpm per hole. This is substantially the same throughput
rate
per hole for the previous examples. A spinning speed of 1,816 mpm was achieved
with all filaments intact under the following conditions: the screw extruder
temperature was set at 350°C in all three zones; the clamp ring and the
screw
adapter were set at 380°C for a measured melt temperature of
389°C; the spinneret
adapter and pack filter were set at 430°C; the transfer line was set at
470°C; and
the spinneret was set at 500°C.
Decreasing the temperature of the spinneret adapter and pack filter and
increasing the transfer line temperature further improved the spinning speed:
Spinneret Adapter Transfer Maximum Properties
and Pack Filter Line Spinneret Speed _ Den/Ten/E/Mod
430°C 474°C 500°C 1,816 mpm 6.5/1.20/45%/10
420°C 471°C 500°C 1,969 mpm 5.5/1.24/24%/12
410°C 471°C 500°C 1,965 mpm 5.6/1.38/35%/I3
400°C 470°C 500°C 1,950 mpm 5.8/1.27/32%/12
400°C 480°C 500°C 1,994 mpm 5.3/1.48/48%/I2
A spinning speed of 1,994 mpm was achieved which was a 14%
improvement from the spinning speed of 1,756 mpm in Example 4. The shear
rate was 347/sec. Fiber tenacity improved by 28% from 1.16 gpd to 1.48 gpd.
This improvement in strength was attributed, besides the higher speed, to a
lesser
or no polymer degradation.
Several samples of yarn were collected at 1,000 mpm to test the long term
stability of the spinning process. Filament spinning continuity was excellent
allowing for a wind up of 60 minutes and 105 minutes with both voluntarily
doffed. The fiber properties of denier/tenacity/elongation and modulus were:
l ld/0.94-l.Olgpd/68-80%/7.Sgpd, respectively.
A sample, spun at 1,500 mpm and lasting 4 minutes, had filament
properties of denier/tenacity/elongation/modulus of 7.2d/1.20gpd/39%/l lgpd,
respectively. Another sample, spun at 1,000 mpm and drawn in-line at 1.4x at
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PCT/US00/02108
280°C, had the fiber properties of denier/tenacity/elongation/modulus
of
7.6d/1.41gpd/25%/l4gpd, respectively.
Measurements made on air samples collected at the annealer exit, along
the yarn path above the heated take-up rolls, and above the wind-up did not
detect
any evolved gases. Thermal polymer degradation would have produced gases.
Since evolved gases could also have been trapped or dissolved inside the
fibers,
the fibers were collected in vials and their head spaces, checked at various
time
intervals using infra-red spectroscopy, gas chromatograph/mass spectrometry,
and
ion chromatography, also did not contain any evolved gases. Additionally, the
fiber samples were heated to 200°C to release any dissolved gases, but
none were
detected. These results confirmed that in the present process, despite using
temperatures as high as 500°C to facilitate high shear rate, high
spinning speed
and high SSF, there was no polymer degradation. PFA polymer would have
degraded easily if subjected to a temperature as low as 425°C for more
than 1.0
I S minute.
EXAMPLE 10
This spin was similar to Example 9 except that an induction heater coil of
about 1/8-in was wrapped twice around the face of the spinneret. The
temperature
profile in the screw extruder up to the screw adapter were kept the same as in
Example 9. The shear rate was 347/sec. There was a 3.6% improvement in
maximum spinning speed (from 1,994 mpm in Example 9) to 2,065 mpm for a
SSF at FFB of 1,087. Maximum speed and properties obtained are shown below:
Spinneret Adapter Transfer Maximum Properties
and Pack Filter Line Spinneret S eed Den/Ten/E/Mod
430°C 470°C 520°C 1,9/0 mpm 6.9/1.04/59%/6.5
400°C 480°C 525°C 2,065 mpm 5.6/1.21/32%/I l
Spinning continuity proved excellent when a sample was spun for 90
minutes at 997 mpm and voluntarily doffed. Fiber properties of
denier/tenacity/elongation/ modulus were: 10.3d/0.97gpd/68%/3.6gpd,
respectively.
EXAMPLE 11
A spinneret assembly, as shown in Fig. 8, was used. The spinneret face
had a diameter of 1.75" and 60 holes of 30-mil diameter. Throughput rate per
hole was 1.35 gpm for a total throughput of 81 gpm or 10.7 pounds per hour
(pph). The tapered aluminum annealer with the 12-in cube Lucite~ box on top of
Example 7 was used. The temperature (°C) profile used was:
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WO 00/44967 PCT/US00/02108
Screw Zone Clamp Screw Spinneret Pack Transfer
I 2 3 Rine Adapter Adapter Filter Line Spinneret
350 350 350 380 380 400 400 477 500
The maximum spinning speed was 1,359 mpm. The shear rate was
347/sec. The fiber properties of denier/tenacity/elongation/modulus were 8.0
d/1.04 gpd/67%/7.1 gpd, respectively.
The cause of the decrease in spinning speed, compared to the spinneret
with 30 holes, such as in Example 7, was thought to be due to too much heat
retention in the annealer due to the 2x higher total throughput. The annealer
was
replaced with the larger capacity 6-ft 3-in Lucite~ box annealer, and the
maximum spinning speed increased to 1,500 mpm. The temperature (°C)
profile
used was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
350 350 350 380 380 420 420 500 520
The fiber properties of denier/tenacity/elongation/modulus were: 7.2 d/1.20
gpd/48%/9.4 gpd.
In order to reduce excessive heat retention within the annealer, the
annealer door, which ran lengthwise and nearly encompassed one side of the
annealer, was opened full and covered with a perforated screen to provide
quiescent air movement without turbulence. Using a perforated metal sheet with
3/32-inch diameter holes separated by 3/16-inch center-to center improved the
maximum spinning speed by 8% to 1,623 rnpm, compared to using the annealer
with the door closed, using the slightly different temperature (°C)
profile:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adat~ter Filter Line Spinneret
350 350 350 380 380 400 400 500 520
The fiber properties of denier/tenacity/elongation/modulus were 7.5 d/1.18
gpd/50%/8.9 gpd, respectively.
Some non-uniform air movement was observed in the perforated metal
sheet covered front annealer, described above, because there was diffused air
movement going in and out at the front while none at the other three sides. A
thermocouple placed near the spinneret face showed the temperature fluctuating
from 368°C to 390°C or a change of 22°C.
A larger Lucite~ annealer was used which measured 20-in x 24-in cross-
section and 71.5-inch in height with an opening at the top for the spinneret
and at
the bottom for access to threadline. During spinning, there was too much up
and
down air motion and the spinning speed was reduced.
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Inserts were placed at the bottom of the annealer to reduce the 20-in x 24-
in opening to a 20-in square. These inserts were tapered down so that the yarn
would fall out. The measured temperature fluctuation was still high at
25°C, but
the actual temperatures were significantly cooler, 240°C to
265°C (Note: while
the measured temperature was lower than in the smaller annealer, comparison
between the absolute temperature between the two annealers should not be taken
too exactly as the location of the thermocouple may not be exactly situated.)
The
air stability was visibly more quiescent. With the same temperature profile,
the
maximum spinning speed was improved and was slightly higher than that
recorded for the smaller annealer: 1,680 mpm. The fiber properties of
denier/tenacity/elongationlmodulus were 8.2 d/0.84 gpd/59%/5.9 gpd,
respectively.
EXAMPLE 12
With the preceeding designs for an annealer there was some difficulty in
reaching the yarn at the bottom of the annealer in order to bring it to a
sucker gun
for stringing up the yarn through all the yarn processing path to the wind-up.
In
addition, annealing of the molten threadline depended entirely on natural air
convection with no means of control. These two problems were solved with an
annealer design, such as is shown in Figs. i OA and l OB. This annealer easily
permitted picking up of the yam at its bottom conical exit. Incoming air from
a
compressed air source flowed through the annular space between the inner and
outer tubes and up through several fine mesh screens to eliminate eddy's
current
and into the top and radially toward the molten filaments. Air was allowed to
enter through a lower port in the annealer, and the air flow rate was
controlled
with a needle valve and measured by a flow meter. Temperatures within the
inner
tube along the top six inches could be monitored by thermocouples placed an
inch
apart. The height of the air inlet port between the inside and outside tube
was
adjustable between 1.0 in to 4.0 in. A 1.0 in high glass ring permitted visual
observation of the molten threadlines and the spinneret face.
Spinning was conducted using a spinneret assembly configured as in Fig. 8
and a 30-hole 39.4-mil diameter with a length/diameter of 3.0 spinneret.
Spinning
occurred at a throughput of 1.3 gpm with the following temperature profile:
350°C
from the screw extruder to the pack filter, 450°C in the transfer line
and 500°C in
the spinneret. The temperatures inside the annealer were: 268°C at 1.0-
in from
the spinneret face, 252°C at 2-in from the spinneret face, and
222°C at 6-in from
the spinneret face. The temperature fluctuation was negligible with a change
of
only 2°C versus up to 25°C observed in the annealers of the
previous examples
herein. The shear rate was 151/sec. Maximum spinning speed achieved was 1,737
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WO 00/44967 PCT/US00/02108
mpm. The fiber properties of denier/tenacity/elongation/modulus were: 4.2
d/1.17
gpd/57%/7.8 g;~d, respectively.
The robustness of this spinning system was confirmed when excellent
spinning continuity was demonstrated with a 3.5-hour package of yarn at 1,005
mpm with a 1.4x in-line draw from a 702 mpm at 240°C take-up roll
speed. The
yarn package had a net weight of over 20 pounds and a 2.0-in thick cake on a
6.0-
in diameter bobbin. The temperature (°C) profile was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
350 350 350 350 350 350 350 448 S00
The fiber properties of denier/tenacity/elongation/modulus were 12.6 d/0.80
gpd/92%/3.8 gpd, respectively.
EXAMPLE 13
Spinning was conducted as in Example 12 but instead of PFA 340,
Teflon~ FEP 5100 fluoropolymer was used. The temperature (°C)
profile was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ripe Adapter Adapter Filter Line Spinneret
315 319 325 325 325 325 325 401 480
The temperatures used were lower in this example than for the PFA polymer
because FEP is less stable than PFA. The shear rate was 161/sec. The maximum
spinning speed achieved was 1,290 mpm. The fiber properties of
denier/tenacity/elongation/modulus were 7.3 d/1.04 gpd/36%/10 gpd,
respectively.
EXAMPLE 14
This spin was made to test the process robustness developed in Example
13 for the Teflon~ FEP 5100 polymer. Excellent spinning continuity, using the
same equipment design as in Examples 12 and 13, was demonstrated with a 3.5-
hour bobbin obtained at the same take-up speed of 700 mpm as in Example 12 for
the PFA polymer. The yarn was drawn off line at the same draw ratio of 1.4x
but
at a lower temperature of 200°C because the melting point of FEP
(260°C ) is
lower than the melting point of PFA (305°C). The yam package was
similar to
that of the PFA 340 polymer spin in Example 12. The temperature (°C)
profile
used was lower than the one used in Example 13, namely:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
305 310 315 315 315 315 315 393 480
The shear rate was 163/sec. The drawn fiber properties of
denier/tenacity/elongation/modulus were 12.2 d/0.97 gpd/45%/5.8 gpd,
respectively.
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EXAMPLE 15
A spin of PTFE homopolymer was made using pelletized Zonyl~ MP-
1300 PTFE. The pelletized form of the homopolymer was compacted from fine
PTFE powder using a pelletizer comprising a male mold with 1,013 of 0.257-inch
diameter imbedded rods and a female mold, 2.0-inch thick. The powder which
had a density of about 0.36 g/ml was compacted under over 30 tons of pressure
in
a press to produce pellets having a 0.28-inch diameter, 0.50 inch length and a
density of I .58 g/ml. The same equipment and 30-hole spinneret as in Example
14
was used. The temperature (°C) profile used was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ripe Adapter Adapter Filter Line Spinneret
400 400 400 400 400 410 410 450 520
The molten filaments exiting from the spinneret face appeared translucent and
glittering, an indication of some degradation. The filaments, however, did not
1 S come out of the annealer in continuous form but rather in bits and pieces.
Varying
the throughput rate from 0.17 g/minlhole to I .33 g/minlhole did not result in
continuous filaments.
After the MP-1300 pellets ran out in the feed hopper, about 200 grams of
PTFE homopolymer TE-6462 in powder form was fed into the hopper and
extruded resulting in long, continuous filaments. The free-fall continuous
f laments were ductile and could be handled or gently pulled between fingers
without breaking. The measured denier of a filament was 349.
EXAMPLE 16
In order to spin Teflon~ PTFE TE-6472, the extruder and spinning
apparatus used in Example I S was brought to the following high temperature
(°C)
profile, and PFA 340 was used first to avoid degradation of the PTFE
homopolymer to follow due to stagnation during the heating-up process which
lasted 2.5 hrs:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
470 470 470 470 470 470 470 450 510
Compressed powder pellets of Teflon~ PTFE TE-6472, classified as a granular
molding powder, were added after the PFA pellets feed were gone and the screw
was turning at 14.0 rpm. Six minutes after the Teflon~ PTFE TE-6472 pellets
were added, the pack pressure was found rapidly rising from 204 psi to over
1,000
psi indicating that the Teflon~ PTFE TE-6472 had reached the pack. Screw
speed was constantly adjusted and spinneret temperature raised to 550°C
to
maintain pack pressure at 1,000 psi. Continuous transparent molten filaments
24
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WO 00/44967 PCT/US00/02108
were extruding but contained gas bubbles, an indication of thermal
degradation,
and solidifying into white filaments. At 2.0 rpm, the measured throughput was
7.6 gpm versus an expected 10.5 gpm. Even though the screw rpm was
maintained at 2.0 rpm, the throughput was found to continuously decrease to as
low as 0.4 gpm, and the continuous filaments began to break up into drips
connected between long (as long as 48-in) and very fine filaments. These very
fine filaments were visually similar to a light spider web, so light that they
floated
in the air. Measured filaments denier was between less than 0.6 and 18. This
clearly demonstrated that PTFE could be melt spun even to very fine filament
IO denier.
The cause of the reduction in throughput was ring pluggage at the entrance
to the barrel of the extruder, which effectively prevented the feeding of the
fluoropolymer pellets. In order to clear the pluggage, all of the polymer was
vacuummed out until the screw was visible. Then PFA pellets were added and
pushed down using a specially made rectangular plate, attached to a 0.5-inch
rod,
which had the dimension of the barrel opening. Turning the screw caused the
small PFA pellets to scrape off the stuck PTFE compressed powder from the
screw surface.
After the ring pluggage was cleared and feeding resumed, the PTFE
compressed powder pellets were added again. At a screw speed of 5.0 rpm, with
a
measured throughput of 9.3 gpm, continuous filaments from all 30 holes were
spun and taken up on take-up rolls at 30 mpm. Excellent spinning continuity
lasted about 15 minutes before ring pluggage occurred again as evidenced by a
drop in pack pressure. This experiment clearly demonstrated that homopolymer
PTFE can be melt-spun. The temperature (°C) profile was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Rina Adapter Adapter Filter Line Spinneret
420 440 480 485 485 485 485 495 500
The PTFE fiber samples were ductile permitting handling without brittle
failure
and permitted tensile testing.
Sample Filament Strength Tenacity
Identification Denier (gramsl (~d~
Free fall 686 36.0 0.05
Free fall 1,042 71.8 0.07
30 mpm 332 14.0 0.04
EXAMPLE 17
Spinning was conducted on Teflon~ PTFE 62, classified as a lubricated
paste extrusion resin. The powder was similarly compressed under 50 tons of
CA 02353074 2001-05-31
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pressure into cylindrical pellets 0.28-inch in diameter and 0.52-inch in
length and
with a density of about I .6 g/cc.
The same equipment and start-up procedure was used as in Example 16.
The Teflon~ PTFE 62 pellets were added at 3.8 rpm screw speed. Good feeding
was obtained at beginning and measured throughput was 9.9 gpm versus 20 gpm
expected. Screw speed was increased to 7.7 rpm. Pack pressure was found to
rise
continuously and was held at 1,200 psi by reducing the screw speed indicating
good feeding. Ring pluggage occured and pack pressure dropped. Rewing up the
screw to 30 rpm loosened the pluggage and the pack pressure rose. At 10 rpm,
the
pack pressure climbed to as high as 2,150 psi when continuous filaments were
spun at 55 mpm. Spinning continuity lasted about S minutes before ring
pluggage
occurred.
EXAMPLE 18
The fibers spun in Examples 16 and I 7 were hot drawn in a heated salt
bath. Filaments were cut to about one inch in length and were held between
pointed tweezers and drawn while briefly immersed in a salt bath. Draw
temperature ranged from 330°C to 400°C. The fiber could not be
drawn at 320°C.
The melting point of PTFE homopolymer ranged from 325°C to
342°C, thus the
fibers were drawn in the molten state. The filaments were easily drawn between
S.Ox to 8.Ox draw ratio. The filaments changed from a bright with no preferred
orientation, under cross-polars, to a intense blue color in one direction and
pinkish
red in a direction 90° to it, indicating preferred molecular
orientation along fiber
axis. A 340°C draw temperature gave the highest degree of orientation.
A drawn
filament with a measured denier of 7.7 gave 0.2 gpd in tenacity.
EXAMPLE 19
The spinneret assembly described in Example 9 and shown in Fig. 6 was
used to spin Teflon~ PFA 340 and to compare the spinning conditions found with
a conventional spinneret assembly design (see Fig. 1 ), where the spinneret
cannot
be heated separately, with spinning conditions in which the spinneret is
thermally
isolated from the pack filter. Thermal isolation was obtained in part in this
embodiment by adding a transfer line between the bottom face of the pack
filter
and the spinneret face.
Two control runs were made using the same spinneret system but keeping
the spinneret at the same constant temperature. A 10--hole 30-mil spinneret
was
used.
The first control spin was made by keeping the temperature (°C)
profile at
350°C as shown below:
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WO 00/44967 PCT/US00/02108
Screw Zone Clamp Screw Spinneret Pack Transfer
I 2 3 Ring Adapter Adapter Filter Line Spinneret
350 350 350 350 350 350 350 350 350
The throughput was increased until a slight melt fracture was observed at
0.178 gpm per hole. The shear rate at this maximum throughput was 45.7/sec,
and
the maximum spinning speed achieved was 58 mpm having a jet velocity of 0.26
mpm and a SSF of 223.
The second control spin was made at a higher temperature profile of
400°C
as shown below:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
350 350 350 350 350 350 400 400 400
The higher temperature of 400°C permitted higher throughput of 0.370
gpm per
hole before melt fracture. At a lower throughput, before melt fracture, of
0.238
gpm per hole, a maximum spinning speed of 206 mpm was obtained. At the
highest throughput and at the edge of melt fracture, the achieved maximum
spinning speed was 381 mpm at a shear rate of 95/sec, jet velocity of 0.54 mpm
and a SSF of 704.
The following temperature (°C) profile was used:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
325 330 335 335 335 335 335 450 500
With this temperature profile, the throughput could be pushed to as high as
1.125
gpm per hole, 3 times higher than the uniform 400°C control, and still
without
melt fracture. Achieved maximum spinning speed was 1,956 mpm, 5 times higher
than the uniform 400°C control, at a shear rate of 289/sec, jet
velocity of 1.645
mpm and a SSF of 1,189.
A control run was not simulated at 500°C because in a conventional
spinneret system, the pack filter has to be heated to the same 500°C
temperature.
With the pack filter at 500°C, the polymer would seriously degrade due
to the long
residence time, 10.1 minutes, in the pack filter. At 425°C, the polymer
would
begin degrading in less than 1.3 minutes.
EXAMPLE 20
The following experiment was conducted to detenmine the distance from
the spinneret face when the molten filaments would solidify. Solidification
was
determined to have occurred when it was visually observed that the transparent
molten filaments turned opaque. This observation was more clearly observed
with
a high intensity lamp shining directly at the bundle of filaments. The
transition
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from transparent to opaque was observable from free-fall (by gravity) to
speeds up
to 200 mpm. Extrusion of the molten filaments were conducted with and without
an annealing tube. In the case where an annealing tube was used, a special
clear
glass annealing tube was used in order to permit visual observation and which
measured 3.0-inch in diameter and 41-inch long. The spinneret used had 30
holes
of 30-mil diameter. Teflon~ FEP-5100 polymer was used.
The results plotted in Fig. 13 show the data without an annealer in opened
symbols while those using an annealer in filled symbols. The plot shows the
free-
fall distance as an increasing function of total throughput at three constant
spinneret temperatures: 380°C (triangle symbol), 430°C (square
symbol) and
480°C (circle symbol). It shows that the solidification distance
increases with
total throughput at constant spinneret temperature. It also shows that the
solidification distance increases with increasing spinneret temperature at the
same
throughput. Furthermore, it shows that with an annealing tube, the
solidification
distance is about twice as far as that without an annealing tube.
The effects of stringing up the filaments were shown in another
experiment to increase the solidification distance from about 6 inches to
about 15
inches without an annealing tube at a take-up speed of 200 mpm. Therefore, the
solidification distance shown in the Fig. 13 represents the shortest
solidification
distance.
The following temperature (°C) profile was used:
Screw Zones Clamp Screw Spinneret Pack Tranfer
1 2 3 Ring Adauter Adapter Filter Line Spinneret
275 285 295 315 315 315 315 380 380, 430, 480
EXAMPLE 21
PTFE homopolymer grade, Zonyl~ MP-1600N, was melt-processed and
spun into fibers, using a spinneret assembly as depicted in Fig. 8. The
polymer
powder was compressed in a 0.5-in high female mold with 0.25-in diameter
holes,
which were filled with the polymer powder, using less than 0.25-in diameter
rods
into thin discs of about 0.1-in thick. About two pounds of these thin disc
pellets
were made. The pellets were hand fed into the screw extruder just enough to
fill
the threads section of the screw as a precaution against being crushed and
causing
sticking and ring pluggage in the screw. The following temperature profile was
used.
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Ring Adapter Adapter Filter Line Spinneret
380 385 390 390 390 390 390 450 500°C
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At a screw speed of 1.94 rpm, the throughput was at 9.4 grams per minute with
a
pack pressure of 238-246 psi using a 10'~oles 30-mil diameter spinneret. The
shear rate was 242/sec. The annealer used in Example 12 and shown in Figs. l0A
and l OB, was used. No ring pluggage problems were experienced. The spin was
S cut short after running out of pellets.
The 10 filaments was initially picked up by hand and went over to the
take-up roll and after one wrap, a sucker gun was used to string up the yarn
all the
way to the Leesona windup. The initial spinning speed was 30 mpm and speed
was gradually increased to a maximum of 202 mpm. Filament denier
measurement on three filaments were: 33, 36 and 41. The measured as-spun
filament fiber properties for the 41 denier filament (denier/tenacity/break
elongation/modulus) were: 41 denier/0.05 gpdll.3%/3.7 gpd.
Teflon~ PTFE 62 was spun using cut-up pieces and thin disc pellets to
avoid the ring pluggage. The temperature (°C) profile used was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Rina Adapter Adapter Filter Line Spinneret
440 445 450 450 450 450 450 450 500°C
The cut-up pellets fed well with no pluggage. However, the pellet discs
eventually developed a ring pluggage problem. Spinning at up to 60 mpm was
achieved before the pluggage occurred at shear rate ranging from 183/sec to
614/sec.
EXAMPLE 22
Pellets of Zonyl~ MP-1600N PTFE homopolymer powder were similarly
prepared as in Example 21, using the same spinneret assembly. At the following
temperature profile, the effects of an annealer were studied by spinning
without
and with the annealer. Throughput rate was at 8.4 grams per minute through a
30-
mil diameter, 30-hole spinneret for a shear rate of 72/sec.
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Rin~~ Adapter Adapter Filter Line Spinneret
315 330 340 340 340 340 340 400 400°C
Without annealer. About 15% of these extruding filaments could not sustain
their
own weight at a vertical free fall distance of 5-ft 8-in. For those surviving
filaments, they were able to be spun at a maximum speed of only 1 S mpm before
they broke.
With a 48-in long annealer: All filaments were free falling continuously to
the
floor. The first filament-break (FFB) spinning speed was 50 mpm and the
maximum spinning speed (MSS) attained was 480 mpm. By raising the
temperature of the transfer line and spinneret to 450°C and
500°C, the FFB was
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WO 00/44967 PCT/US00/02108
improved to 85 mpm and the MSS was at 250 mpm. The yarn was visibly thick
and thin. The yarn uniformity was found to improve with the introduction of
room temperature air through the annealer jacket into the top of the annealer.
At
250 cfh (cubic feet per hour), the yarn became uniform. Under this condition
of
spinning, the MSS was improved to 404 mpm. Filament fiber properties
(denier/tenacity/break elongation/ modulus) were 5.8/0.16gpd/12%/8 gpd.
EXAMPLE 23
This experiment used Teflon~ FEP-5100 as the fluoropoIymer
composition and demonstrated the advantage of thermally isolating the
spinneret.
A spinneret assembly as depicted Fig. 8 was used. The control was run in the
same
assembly but keeping the temperature the same for all parts. The
temperature(°C)
profiles for the controls were:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Rin Ada ter Ada ter Filter Line S inneret
275 300 350 350 350 350 350 350 350
275 350 400 400 400 400 400 400 400
275 350 400 400 450 450 450 450 450
The temperature profile in the Screw Zones l and 2 was kept low and not at
test
temperature until Screw Zone 3 or Clamp Ring. The degradation would hve been
worse had Screw Zones 1 and 2 been at test temperature.
The temperature profile for the sample of the present invention was:
Screw Zone Clamp Screw Spinneret Pack Transfer
1 2 3 Rin Adapter Adapter Filter Line Spinneret
275 295 300 300 300 300 300 380 480
The shear rates were: 86/sec at 10 gpm, 232/sec at 27.2 gpm, 359/sec at 42
gpm,
and 385/sec at 45 gpm. As seen in Fig. 16, a spinning speed of 1,900 mpm,
without any noticeable degradation, was achieved at a spinneret temperature of
about 480°C. However, the control experienced slight thermal
degradation at a
spinneret temperature of 400°C attaining a spinning speed of about 600
mpm at
that temperature and severe thermal degradation at about 450°C with a
spinning
speed of 900 mpm.
