Note: Descriptions are shown in the official language in which they were submitted.
TITLE
Sheath-Core Spinning of
Multilobal Conductive Core Filaments
BACKGROUND OF THE INVENTION
Synthetic filaments having antistatic
properties comprising a continuous nonconducting sheath
of synthetic polymer surrounding a conductive polymeric
core containing carbon black have been taught by Hull in
U.S. Patent No. 3,803,453. The cross-section of the
core shown in said patent is circular. Need has arisen
in certain end-use applications, such as career apparel __
worn in clean rooms, for even greater reduction of
static propensity, and contrary to the desires expressed-
by others to conceal the fiber blackness, is a desire
for greater visibility of the core.
Sheath-core filaments wherein the cross-section
of the core is trilobal are known. They can be prepared
with a spinneret of the type shown in U.S. Patent No.
2,936,483. While useful products of the invention can
be prepared with such spinnerets, improvements in
preserving definition of the trilobal core through the
spinning process is a worthwhile objective. The present
invention offers an improved spinning technique as well
as providing a novel filament which rapidly dissipates
electrical charges.
DESCRIPTION OF THE DRAWINGS
Figures 1 and 2 are schematic cross-sectional
views of sheath-core filament of the invention
illustrating trilobal and tetralobal cores as well as
showing how the required structural parameters are
determined.
Figure 3 is a fragmentary section of a
distribution and spinneret plate taken along line 3,3 of
Figure 4.
DP-4445
AUG-23-00 15:30 From:DIMOCK STRATTON CLARIZIO 4169716638 T-305 P.03/03 Job-376
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Figure 4 is a bottom view of the distribution
plate of Figure 3. ,
SUIII~IARY OF THE I NVENTI ON
In one aspect, the present invention provides a navel synthetic
sheath-core bicomponent filament having antistatic properties comprising a
continuous nonconductive sheath of a synthetic thermoplastic fiber Forming
polymer selected from the group consisting of polyester and polyarnide
surrounding an electrically conductive polymeric core, constituting from 0.3%
to 35% of the filament cross-section, said polymeric core comprised of 20% to
35% of electrically conductive carbon black dispersed in polyethylene, the
cross-section of said core having from three to six lobes and a modification
ratio of at least 2, with each lobe having an L/n ratio of from 1 to 20, where
L
is the length of a line drawn from the center point of the line between low
points of adjacent valleys on either side of the lobe to the farthest point on
said lobe, and D is the greatest width of the lobe as measured perpendicular
to
L. In a second $spect, the present invention provides an
improved process for better maintaining the core
definition during melt-.spinning of a sheath-core fiber
wherein one polymer composition constitutes the sheath
component and a different polymer composition
constitutes the core cpmponent and in which the core has
three or more Iobea. The process comprises
aimultenevusly extruding the molten sheeth and core
component compositions through a spinning orifice with
the sheath component completely surrounding the core
component, the improvement comprising, maintaining the
core cross-sectional configuration by
1) feeding the molten cere Component
composition in the desired multilobal crass-section
through a channel opening above a spinneret capillary,
2) feeding the molten sheath component from
a31 directions against the core along the periphery of
the entrance to the spinneret capillary to completely
surround the core component,
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3) controlling the flow of molten sheath
component composition at spaced sections along the
periphery of the spinneret capillary entrance to allow
more to flow to zones between the lobes than to zones at
the lobes, and
4) solidifying the molten components after
leaving the spinneret orifice.
DETAILED DESCRIPTION OF THE INVENTION
Static dissipating fibers are well-known in the
art and have been used for many years in textiles. A
particularly successful fiber has been the fiber __
described in U.S. Patent No. 3,803,453. This fiber is a
sheath-core bicomponent fiber prepared by melt co-
extrusion of two thermoplastic compositions as sheath
and core, respectively. The sheath is nonconductive.
The core polymer is made conductive by incorporation of
electrically conductive carbon black. The sheath
provides strength to the fiber, hides the black core,
and protects the core against chipping and flaking which
can occur if the core were exposed at the fiber surface.
Certain present day end-use applications require greater
anti-static effect with less concern for color. In
distinction, there is a greater desire to see more core
color as a means of distinguishing in use those garments
which are protected from those which are not.
Applicants have found that this can be accomplished by
modifying the sheath-core fiber of U.S. Patent No.
3,803,453. The modification consists primarily of
employing a core, of the same composition as in said
patent but having a cross-section with from three to
six lobes, a modification ratio of at least 2, and with
each lobe having an L/D ratio of from 1 to 20. Figure 1
shows such a cross-section.
Figure 1 is a schematic cross-sectional
representation of a sheath-core fiber wherein a trilobal
core is surrounded by a sheath as might be seen on an
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enlargement of a photomicrograph. The nature of the
core and sheath will be discussed in greater detail
below. The determination of modification ratio is known
in the art but, for convenience, it can be defined by
reference to Figure 1. The modification ratio is the
ratio of the radius of the smallest circle
circumscribing the trilobal core to the radius of the
largest circle which can be inscribed in the trilobal
core where the lobes meet. In Figure 1, this is A/B.
Determination of the L/D ratio for the lobes is
also illustrated by reference to Figure 1. A first line
is drawn connecting the low points of adjacent valleys
on either side of a lobe and another line L is drawn
from the center of the first line to the farthest point
of said lobe. The value D represents the greatest width
of the lobe as measured perpendicular to L. Figure 2 is
a schematic showing a cross-section of a round fiber
having a tetralobal core.
Spinning of the filaments of the invention can
be accomplished by conventional two-polymer sheath-core
spinning equipment with appropriate consideration for
the differing properties of the two components. The
filaments are readily prepared by known spinning
techniques and with polymers as taught, for example, in
U.S. Patent No. 2,936,482. Additional teaching of such
spinning with polyamides is found in U.S. Patent No.
2,989,798. A new improved process has been developed to
better preserve the definition of sheath-core
bicomponent fibers having tri-, tetra-, penta- or
hexalobal cores as they are extruded. This is described
below.
The improved process employed for spinning the
sheath-core bicomponent yarn of Examples 1 and 2 below,
is a modification of a conventional sheath-core bi-
component melt-spinning process. In the conventional
process, the core feed polymer stream and the sheath
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feed polymer stream are fed to a spinneret pack
including filters and screens, and to a plate which
distributes the molten polymer streams to orifices that
shape the core and surround it with sheath. Reference
5 to Figures 3 and 4 will assist in the understanding of
the modified process. Core polymer is fed to channel 2
and exits over the entrance to capillary 3 of spinneret
plate 5. Sheath polymer is fed through passageway 7 of
plate 8 into the space between plates 5 and 8,
maintained by shims not shown. This polymer is fed from
all directions against the core polymer stream in the
vicinity of the entrance to the spinneret capillary 3
and both streams pass through capillary 3 in sheath-core
relation, finally exiting from the spinneret orifice,
not shown, at the exit of capillary 3. The improved
process maintains better definition of the core lobes.
This is accomplished by controlling the flow of molten
sheath component composition against the core polymer
stream at spaced sections along the periphery of the
entrance to the capillary to allow more sheath polymer
to flow to zones between the lobes than to zones at the
lobes. This can be achieved by enlarging the passageway
for the sheath polymer to the capillary only in those
sections leading to zones between lobes. Thus, as shown
in Figures 3 and 4, depressions 10 were etched in plate
8 to permit increased sheath polymer flow to regions
between lobes.
The filament sheath may consist of any
extrudable, synthetic, thermoplastic, fiber-forming
polymer or copolymer. This includes polyolefins, such
as polyethylene and polypropylene, polyacrylics,
polyamides and polyesters of fiber-forming molecular
weight. Particularly suitable sheath polymers are
polyhexamethylene adipamide, polycaprolactam, and
polyethylene terephthalate.
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Tensile strength and other physical properties of the
filaments of the invention are primarily dependent on
the sheath polymer. For high strength filaments,
polymers of higher molecular weight and those permitting
higher draw ratios are used in the sheath. While
undrawn filaments of the invention may provide adequate
strength for some purposes, the drawn filaments are
preferred. In some applications, for example where the
filaments of the invention are to be subjected to high
temperature processing with other filaments such as in
hot fluid jet bulking or other texturing operations, ir-
is important that the sheath polymer have a sufficiently
high melting point to avoid undue softening or melting---
under such conditions.
The filament core of the antistatic fibers
consists of an electrically conductive carbon black
dispersed in a polymeric, thermoplastic matrix material.
The core material is selected with primary consideration
for conductivity and processability as described in
detail in U.S. Patent No. 3,803,453. Carbon black
concentra-tions in the core of 15 to 50 percent may be
employed. It is found that 20 to 35 percent provides
the preferred level of high conductivity while retaining
a reasonable level of processability.
The core polymer may also be selected from the
same group as that for the sheath, or it may be
non-fiber forming, since it is protected by the sheath.
In the case of non-antistatic fibers, the core of the
bicomponent fiber will, of course, be non-conductive.
The cross-sectional area of the core in the
composite filament need only be sufficient to impart the
desired antistatic properties thereto and may be as low
as 0.3 percent, preferably at least 0.5 percent and up
to 35 percent, by volume. The lower limit is governed
primarily by the capability of manufacturing sheath/core
filaments of sufficiently uniform quality while
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maintaining adequate core continuity at the low core
volume levels.
Conventional drawing processes for the
filaments can be used but care should be exercised to
avoid sharp corners which tend to break or damage the
core of the antistatic fibers. In general, hot drawing,
i.e., where some auxiliary filament heating is employed
during drawing, is preferred. This tends to soften the
core material further and aid in drawing of the
filaments. These antistatic filaments may be plied with
conventional synthetic, undrawn filaments and codrawn.
For general applications, the filaments of this
invention have a denier per filament (dpf) of less than-
50 and preferably less than 25 dpf.
The filaments of this invention are capable of
providing excellent static protection in all types of
textile end uses, including knitted, tufted, woven and
nonwoven textiles. They may contain conventional
additives and stabilizers such as dyes and antioxidants.
They may be subjected to all types of textile processing
including crimping, texturing, scouring, bleaching, etc.
They may be combined with staple or filament yarns and
used as staple fibers or as continuous filaments.
Said filaments may be combined with other
filaments or fibers during any appropriate step in yarn
production (e. g., spinning, drawing, texturing, plying,
rewinding, yarn spinning), or during fabric manufacture.
Care should be taken to minimize undesirable breaking of
the antistatic filaments in these operations.
Upon exiting the spinneret orifice, the
bicomponent stream cools and begins to solidify. It is
generally not desirable to apply too high a spin stretch
with the conductive fibers since quality as an
antistatic fiber diminishes. This is not a limitation
with other bicomponent fibers.
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TEST PROCEDURES
Tenacity and elongation of yarns were measured
using ASTM D-2256-80. The method for determining
relative viscosity (LRV) of polyester polymers is
described in U.S. 4,444,710 (Most). The method for
determining relative viscosity (RV) of polyamides is
disclosed in U.S. 4,145,473 (Samuelson). Surface
resistivity of fabrics is determined using AATCC Test
Method 76-1987. Electrostatic propensity of carpets is
measured using AATCC Test Method 139-1986. Static decay
data are measured using Method 4096 (March 13, 1980), __
Federal Test Method Std. No. lOlC. The modification
ratios and L/D ratios were measured from cross-sections_
on photomicrographs as well understood in the art.
The following examples, except for controls,
are intended to illustrate the invention and are not to
be construed as limiting. Multilobal core filaments of
the invention are described in each of Examples 1 to 3.
w a rw r ~ 1
Sheath-core filaments having a sheath of 23.5
LRV polyethylene terephthalate and a polyethylene core
that contained 28.9% carbon black were spun and wound up
without drawing at 1200 meters per minute. The conduc-
tive core constituted 6% by weight of these filaments,
and the yarns, which contained six filaments, were
subsequently heated to 140°C and drawn at the ratios
listed in Table I. Samples with a round conductive core
were spun using a spinneret assembly similar to that
shown in Figure 11 of U.S. 2,936,482, whereas those
having trilobal shaped cores were spun by the improved
process of this invention using the spinneret assembly
and plate shown in Figures 3 and 4. The modification
ratio of the trilobal conductive core was 5 and the L/D
ratio was 3. The trilobal core yarns were darker than
the round core yarns. After drawing, these yarns were
incorporated into a 100% polyester 28 cut jersey knit by
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feeding in the conductive core yarns at 5/16 inch
intervals. Yarn and fabric properties measured on these
samples are shown in Table I:
Table I
Core Shape Round Trilobal
Draw Ratio 2.35X 2.lOX
Total Denier 35.9 40.0
Tenacity, g/d 1.81 1.61
% Elongation 28.9 21.4
Fabric Properties
Surface Resistivity -
ohms/unit sq. 1.5 X 101' 1.9 X 101z
Federal Test Method 4046
Standard lOlC (90$ Decay)
Time in sec./2 sec. ch arge level
From: +5KV 33/900 0.23/275
-5KV 9.5/-950 0.20/-300
The fabric containing the yarn with the trilobal
shaped conductive core had significantly lower surface
resistivity and much faster static decay times than that
made with the yarns having round conductive cores.
ERAMPLE 2
Sheath-core filamentary yarns (40 denier 6
filaments) having a sheath of 46 RV 66-nylon and either
round or trilobal shaped conductive cores similar to
those described in EXAMPLE 1 were prepared, except they
were drawn at 110°C using a 3.2X draw ratio. The
modification ratio of the trilobal conductive core was
4 and the L/D ratio was 2. These conductive core fibers
were plied with 1225 denier nylon carpet yarn and direct
tufted into level loop carpets. Both carpets were
evaluated in the AATCC Test Method 134. The carpet
containing the yarns with trilobal shaped cores had a
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significantly lower measurement of 0.8 KV versus 1.2 KV
for the carpet made from yarns having round conductive
cores.
~vnMnr_~ Z
5 Utilizing spinneret assemblies as described in
Figure 11 of U.S. 2,936,483, sheath-core products were
produced having a 24% central conductive core surrounded
by a 76% sheath of polyethylene terephthalate. Filaments
having either round or trilobal (modification ratio of
10 2.0, L/D of 1.0) shaped conductive cores were prepared,
and the cores contained 32.0% carbon black ("Vulcan P", --
available from Cabot Corp.), compounded into a film grade
equivalent high melt index, low density polyethylene.
The resulting fibers were air quenched at 21°C,
drawn 1.84X and wound up at 1372 meters per minute as a
35 denier 6 filament product. After heat annealing
(130°C) to reduce shrinkage, the products were Woven into
fabric for static dissipation evaluation.
Woven fabrics were prepared as follows:
Non-Conductive Yarns - 150 denier, 34 filaments
- 3.3Z twist polyester fiber.
Static Dissipative Yarns - 100 denier, 39
filaments - 4S twist polyester fiber plus
one static dissipative yarn as
described above.
Weaving:
96 ends, 88 picks, 8 x 8 herringbone
Warp - 1 Static dissipative yarn and 23
non-conductive ends.
Filling - 2 Static dissipative yarns and
22 non-conductive picks.
Fabrics:
A. Contains Trilobal Core
B. Contains Round Core
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Electrostatic Properties
Yarn Resistivity, ohms/cm (length)-as
prepared.
A. 3.7 x 101'
B. 7.4 x 1011
Fabric Resistivity (AATCC 76-1987) ohms/
unit square after heat-setting and
scouring.
A. warp-2.9 x 1012, fill-2.7 x 1012
B. warp->1 x 101', fill->1 x 1013
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