U.S. patent application number 10/837956 was filed with the patent office on 2005-11-03 for process for making fine spunbond filaments.
Invention is credited to Bansal, Vishal.
Application Number | 20050241745 10/837956 |
Document ID | / |
Family ID | 34968881 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050241745 |
Kind Code |
A1 |
Bansal, Vishal |
November 3, 2005 |
Process for making fine spunbond filaments
Abstract
A method is provided for preparing webs of spunbond fibers
having reduced diameter. The spunbond fibers can be single
component fibers or multiple component fibers having a symmetric
cross-section, or combinations thereof. The fibers are re-heated
and drawn in a secondary drawing step after quenching to provide
fibers having at least a 5% reduction in average fiber diameter
compared to fibers that have not been re-heated and drawn in a
secondary drawing step.
Inventors: |
Bansal, Vishal; (Richmond,
VA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
34968881 |
Appl. No.: |
10/837956 |
Filed: |
May 3, 2004 |
Current U.S.
Class: |
156/167 ;
156/433 |
Current CPC
Class: |
D01D 5/0985
20130101 |
Class at
Publication: |
156/167 ;
156/433 |
International
Class: |
D04H 003/16 |
Claims
What is claimed is:
1. A method for preparing a spunbond nonwoven fabric, comprising
the steps of: a. melt spinning a plurality of continuous polymeric
filaments from a spinneret, wherein the continuous filaments are
selected from the group consisting of single component filaments
and multiple component filaments having a symmetric cross-section
and comprising at least a first polymeric component and at least a
second polymeric component; b. drawing the filaments in a first
drawing step; c. quenching the drawn filaments; d. passing the
quenched filaments through a pneumatic draw jet, e. supplying the
draw jet with a gaseous stream, the gaseous stream applying a
tension to the filaments as the filaments and the gaseous stream
pass through and exit the draw jet; f. heating the filaments while
under the tension applied by the gaseous stream to a temperature
sufficient to draw the filaments in a second drawing step, thereby
reducing the average filament diameter by at least 5 percent
compared to the average filament diameter that is achieved when the
filaments are not heated and drawn in a second drawing step in an
otherwise identical process; and g. collecting the filaments on a
collecting surface to form a nonwoven web.
2. The method of claim 1, wherein the heating step comprises
blowing hot air on the filaments as they exit the draw jet.
3. The method of claim 1, wherein the heating step comprises
exposing the filaments to radiant heat as they exit the draw
jet.
4. The method of claim 1, wherein the multiple component filaments
comprise bicomponent filaments that have a concentric sheath-core
cross-section.
5. The method of claim 4, wherein the sheath comprises a polymer
selected from the group consisting of polyethylenes and polyester
copolymers and the core comprises a polymer selected from the group
consisting of polypropylenes, polyesters, and polyamides.
6. The method of claim 5, wherein the sheath comprises a polymer
selected from the group consisting of polyethylenes and polyester
copolymers and the core comprises a polyester.
7. The method of claim 6, wherein the core comprises poly(ethylene
terephthalate).
8. The method of claim 7, wherein the sheath comprises linear low
density polyethylene.
9. The method of claim 5, wherein the sheath comprises a polyester
copolymer selected from the group consisting of poly(ethylene
terephthalate) modified with di-methyl isophthalic acid, and
poly(ethylene terephthalate) modified with
1,4-cyclohexanedimethanol.
10. The method of either of claims 1 or 6, wherein the filament
diameter is reduced by at least 10 percent compared to the average
filament diameter that is achieved when the filaments are not
heated and drawn in a second drawing step.
11. The method of either of claims 1 or 6, wherein an average
filament diameter of less than 14.5 micrometers is achieved after
the second drawing step compared to the average filament diameter
that is achieved when the filaments are not heated and not drawn in
a second drawing step in an otherwise identical process.
12. The method of claim 11, wherein an average filament diameter of
less than 10 micrometers is achieved after the second drawing
step.
13. The method of claim 1, wherein the continuous filaments
comprise single component filaments.
14. The method of claim 13, wherein the single component filaments
comprise polyester filaments.
15. The method of claim 14, wherein the polyester comprises
poly(ethylene terephthalate)
16. The method of claim 1, wherein the multiple component filaments
comprise bicomponent filaments that have a segmented pie
cross-section having an even number of alternating segments,
wherein adjacent segments comprise different polymers.
17. The method of claim 16, wherein adjacent polymer segment
combinations are selected from the group consisting of
polypropylene/polystyrene, polypropylene/poly(ethylene
terephthalate), polypropylene/polyamide, polyethylene/polystyrene,
polyethylene/poly(ethylene terephthalate), polyethylene/polyamide
polystyrene/polyamide and poly(ethylene
terephthalate)/polyamide.
18. The method of claim 17, wherein the adjacent polymer segment
combination is polypropylene/poly(ethylene terephthalate)
19. The method of claim 17, wherein the adjacent polymer segment
combination is polyethylene/poly(ethylene terephthalate).
20. An apparatus for forming a nonwoven web of polymeric continuous
filaments, comprising: a. a spinneret for spinning continuous
polymeric filaments; b. a quenching means positioned below the
spinneret; c. a pneumatic draw jet having a filament inlet and a
filament exit positioned below the quenching means, through which
the polymeric filaments are passed together with a gaseous stream
that is fed to the pneumatic draw jet; d. a means for heating the
polymeric filaments to a temperature sufficient to draw the
filaments after the filaments exit the draw jet, the heating means
located no more than 20 cm below the exit of the draw jet; and e. a
collecting surface for collecting the filaments to form a nonwoven
web.
21. The apparatus of claim 20, wherein the heating means comprises
a hot air jet.
22. The apparatus of claim 20, wherein the heating means comprises
a radiant heat source.
23. The apparatus of claim 22, wherein the radiant heat source
comprises an infrared heat source.
24. The apparatus of either of claims 21 or 22, wherein the heating
means is located no more than 0.5 cm from the exit of the draw jet.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for forming spunbond
filaments having reduced diameters.
[0003] 2. Description of the Related Art
[0004] In conventional spunbond methods, one or more extruders
supply molten polymer to a spin pack in which the polymer is spun
through openings to form a curtain of filaments. The filaments are
partially cooled in an air quenching zone and generally pass
through a pneumatic jet prior to being laid down on a moving belt,
scrim, or other fibrous layer. The tension applied to the fibers by
the pneumatic jet causes them to be drawn near the spinneret face
to reduce the fiber size and to increase fiber strength. Spunbond
fibers generally have a diameter greater than about 5 micrometers.
The fiber diameter can be reduced by lowering the polymer
throughput per hole, increasing the pneumatic draw jet pressure,
reducing the spin-line distance (distance between the exit of the
polymer capillaries in the spinneret and the entrance of the
pneumatic jet), lowering the polymer viscosity, and general
optimization of the spinning process. However, such methods are
limited by the degree of reduction in fiber diameter that can be
achieved due to process instabilities that can occur as a result of
these approaches. For example, the frequency of spinning defects
such as polymer drips and broken filaments can increase, which is
unacceptable in a commercial spunbond process. In addition,
reducing the throughput per hole has a negative impact on process
economics. Reducing the polymer viscosity can have a negative
effect on other fiber properties such as fiber strength.
[0005] U.S. Pat. No. 6,379,136 to Najour et al. describes an
apparatus for preparing sub-denier spunbond nonwovens comprising a
two-sided multilevel quench system and a vertically moveable draw
jet assembly with adjustable primary and secondary jet-nozzles and
a variable width draw jet slot. U.S. Patent Application Publication
No. 2003/0178741 describes a method for reducing spunbond filament
diameters in a spunbond process in which quench air fed to a
quenching chamber is divided into at least 2 streams in the
vertical direction, wherein the air velocity of the quench air in
the lowermost stream is set higher than that of the quench air in
the uppermost stream. Both of these methods are not easily
adaptable to pre-existing spunbond lines. In addition, some of the
process instabilities described above, such as increased drips and
fiber breaks, may occur using these processes.
[0006] U.S. Pat. No. 5,418,045 to Pike et al. describes a method
for making a nonwoven fabric comprising crimped bicomponent
continuous filaments having an asymmetric cross-section that
includes a heating step to activate the latent crimp of the
filaments prior to laydown. In one embodiment, heated air is used
in the aspirating jet to activate the latent crimp.
[0007] There remains a need for a low cost method for reducing the
diameter of spunbond fibers using currently existing spunbond
equipment.
BRIEF SUMMARY OF THE INVENTION
[0008] A method for preparing a spunbond nonwoven fabric comprising
the steps of:
[0009] melt spinning a plurality of continuous polymeric filaments
from a spinneret, wherein the continuous filaments are selected
from the group consisting of single component filaments and
multiple component filaments having a symmetric cross-section and
comprising at least a first polymeric component and at least a
second polymeric component;
[0010] drawing the filaments in a first drawing step;
[0011] quenching the drawn filaments;
[0012] passing the quenched filaments through a pneumatic draw
jet,
[0013] supplying the draw jet with a gaseous stream, the gaseous
stream applying a tension to the filaments as the filaments and the
gaseous stream pass through and exit the draw jet;
[0014] heating the filaments while under the tension applied by the
gaseous stream to a temperature sufficient to draw the filaments in
a second drawing step thereby reducing the average filament
diameter by at least 5 percent compared to the average filament
diameter that is achieved when the filaments are not heated and
drawn in a second drawing step in an otherwise identical process;
and
[0015] collecting the filaments on a collecting surface to form a
nonwoven web.
[0016] This invention is also an apparatus for practicing the
method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a side-elevation view of a conventional
spunbond apparatus for preparing a bicomponent spunbond web.
[0018] FIG. 2 shows a cross-sectional view of one embodiment of an
apparatus for practicing the method of the present invention that
includes a heating means below the exit of the pneumatic draw
jet.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention is directed toward a method for
preparing a spunbond nonwoven fabric comprising filaments having a
reduced diameter. The method of the present invention can be
performed using conventional spunbond equipment with only minor
modification being required.
[0020] The term "copolymer" as used herein includes random, block,
alternating, and graft copolymers prepared by polymerizing two or
more comonomers and thus includes dipolymers, terpolymers, etc.
[0021] The term "polyolefin" as used herein, is intended to mean
any of a series of largely saturated open chain polymeric
hydrocarbons composed only of carbon and hydrogen. Typical
polyolefins include, but are not limited to, polyethylene,
polypropylene, polymethylpentene and various combinations of the
ethylene, propylene, and methylpentene monomers.
[0022] The term "polyethylene" (PE) as used herein is intended to
encompass not only homopolymers of ethylene, but also copolymers
wherein at least 85% of the recurring units are ethylene units and
includes "linear low density polyethylenes" (LLDPE), which are
linear ethylene/.alpha.-olefin copolymers having a density of less
than about 0.955 g/cm.sup.3 and "high density polyethylenes"
(HDPE), which are polyethylene homopolymers having a density of at
least about 0.94 g/cm.sup.3.
[0023] The term "polypropylene" as used herein is intended to
encompass not only homopolymers of propylene, but also copolymers
wherein at least 85% of the recurring units are propylene
units.
[0024] The term "polyester" as used herein is intended to embrace
polymers wherein at least 85% of the recurring units are
condensation products of dicarboxylic acids and dihydroxy alcohols
with linkages created by formation of ester units.
[0025] The term "polyamide" as used herein is intended to embrace
polymers containing recurring amide (--CONH--) groups. One class of
polyamides is prepared by copolymerizing one or more dicarboxylic
acids with one or more diamines.
[0026] The term "nonwoven fabric, sheet, layer or web" as used
herein means a structure of individual fibers, filaments, or
threads that are positioned in a random manner to form a planar
material without an identifiable pattern, as opposed to a knitted
or woven fabric. The terms "fiber" and "filament" will be used
interchangeably throughout this application. Examples of nonwoven
fabrics include meltblown webs, spunbond webs, carded webs,
air-laid webs, wet-laid webs, and spunlaced webs and composite webs
comprising more than one nonwoven layer.
[0027] The term "spunbond fibers" as used herein means fibers that
are melt-spun by extruding molten thermoplastic polymer material as
fibers from a plurality of fine, usually circular, capillaries of a
spinneret with the diameter of the extruded fibers then being
rapidly reduced by drawing and then quenching the fibers. Spunbond
fibers are generally continuous fibers.
[0028] The term "meltblown fibers" as used herein, means fibers
that are melt-spun by meltblowing, which comprises extruding a
melt-processable polymer through a plurality of capillaries as
molten streams into a high velocity gas (e.g. air) stream.
Meltblown fibers generally have a diameter between about 0.5 and 10
micrometers and are generally discontinuous fibers but can also be
continuous.
[0029] The term "spunbond-meltblown-spunbond nonwoven fabric" (SMS)
as used herein refers to a multi-layer composite sheet comprising a
web of meltblown fibers sandwiched between and bonded to two
spunbond layers. Additional spunbond and/or meltblown layers can be
incorporated in the SMS fabric, for example
spunbond-meltblown-meltblown-spunbond (SMMS), etc.
[0030] The term "multiple component fiber" as used herein refers to
a fiber that is composed of at least two distinct polymeric
components that have been spun together to form a single fiber. The
at least two polymeric components are arranged in distinct
substantially constantly positioned zones across the cross-section
of the multiple component fibers, the zones extending substantially
continuously along the length of the fibers. An example of a
multiple component fiber is a bicomponent fiber that is made from
two distinct polymer components, such as sheath-core fibers that
comprise a first polymeric component forming the sheath and a
second polymeric component forming the core that is completely
surrounded by the sheath. Multiple component fibers are
distinguished from fibers that are extruded from a single
homogeneous or heterogeneous blend of polymeric materials. The term
"multiple component spunbond web" as used herein refers to a
spunbond web comprising multiple component spunbond fibers. The
term "bicomponent spunbond web" as used herein refers to a spunbond
web comprising bicomponent spunbond fibers. A multiple component
web can comprise both multiple component and single component
fibers.
[0031] The method of the present invention is suitable for
preparing spunbond nonwoven fibers having reduced diameter wherein
the spunbond fibers are selected from the group consisting of
single component fibers and multiple component fibers having a
substantially symmetric cross-section. The fibers may have a round
cross-section. The fibers can also have a multi-lobal or other
cross-section, however if the fibers are multiple component fibers
they are preferably substantially radially symmetric. By "radially
symmetric" cross-section is meant a cross-section for which
rotation of the fiber about its longitudinal axis by 360.degree./n,
in which "n" is an integer greater than 1 representing the "n-fold"
symmetry of the fibers, results in a fiber that is
indistinguishable from the fiber before rotation, including the
position of the distinct polymeric components. In determining the
symmetry of a fiber, a cross-section is taken perpendicular to the
fiber axis. For fibers having a non-round cross-section, the
"effective diameter" is equal to the diameter of a hypothetical
round fiber having the same cross sectional area. It is understood
that when the term average fiber diameter is used for fibers having
a non-round cross-section, that the average fiber diameter is the
average effective diameter.
[0032] In one embodiment, the fibers are bicomponent fibers with
the two polymers arranged in a concentric sheath-core
configuration.
[0033] Polymers suitable for forming the spunbond fibers include
polyolefins, polyesters, and polyamides, and copolymers thereof.
Examples of suitable polyolefins include polyethylenes (such as
LLDPE and HDPE) and polypropylene. Examples of polyesters include
poly(ethylene terephthalate) (PET), which is a condensation product
of ethylene glycol and terephthalic acid and poly(1,3-propylene
terephthalate), which is a condensation product of 1,3-propanediol
and terephthalic acid. Examples of polyamides suitable for use in
the present invention include poly(hexamethylene adipamide) (nylon
6,6) and polycaprolactam (nylon 6). Examples of polymer
combinations suitable for use in bicomponent fibers having a
symmetric cross-section include polyester/polyethylene,
polyester/polyester copolymer and polypropylene/polyethylene.
Preferred polymer combinations include poly(ethylene
terephthalate)/polyethylene, poly(ethylene terephthalate)/linear
low density polyethylene, poly(ethylene
terephthalate)/poly(ethylene terephthalate) copolymer and
polypropylene/linear low density polyethylene. When the fibers have
a symmetric sheath-core cross-section (e.g. concentric
sheath-core), the lower melting polymer (generally the second
polymer named in each polymer combination above) preferably forms
the sheath component. Poly(ethylene terephthalate) copolymers
suitable for use in making fibers in the process of the present
invention include amorphous and semi-crystalline poly(ethylene
terephthalate) copolymers. For example, poly(ethylene
terephthalate) copolymers in which between about 5 and 30 mole
percent based on the diacid component is formed from di-methyl
isophthalic acid, as well as poly(ethylene terephthalate)
copolymers in which between about 5 and 60 mole percent based on
the glycol component is formed from 1,4-cyclohexanedimethanol can
be used. Poly(ethylene terephthalate) copolymers that have been
modified with 1,4-cyclohexanedimethanol are available from Eastman
Chemicals (Kingsport, Tenn.) as PETG copolymers. Poly(ethylene
terephthalate) copolymers that have been modified with di-methyl
isophthalic acid are available from E. I. du Pont de Nemours and
Company (Wilmington, Del.) as Crystar.RTM. polyester
copolymers.
[0034] In another embodiment, the fibers have a segmented pie
cross-section having an even number of alternating segments of
distinct polymers arranged in a configuration that is substantially
radially symmetric. The segmented filaments are chosen so that they
do not readily split in the second drawing step. However, to
provide an even finer filament size, the filaments may be subjected
to further processing, such as hydroentangling, or the like, to
cause the filaments to split. It is preferable the adjacent polymer
segments have a solubility difference of two or more
(cal/cm.sup.3).sup.1/2. The polymer combination suitable for use is
polypropylene or polyethylene alternating with polystyrene,
poly(ethylene terephthalate) or polyamide. Alternatively,
polystyrene can alternate with polyamide and
poly(ethyleneterephthalate) can alternate with polyamide.
[0035] FIG. 1 shows a side-elevation view of a conventional
spunbond apparatus for preparing a bicomponent spunbond web. It is
understood that the method of the present invention can also be
used for preparing spunbond nonwovens comprising single component
fibers by using a spin block designed to spin single component
fibers or by feeding the same polymer into hoppers 10 and 12 shown
in FIG. 1. Alternately, multiple component spunbond webs comprising
more than two polymeric components can be prepared by introducing
one or more additional extruders and spinning multiple component
fibers from three or more polymers using an appropriately designed
spin block. To form bicomponent fibers in this apparatus, two
different thermoplastic polymers are fed into the hoppers 10 and
12, respectively. The polymer in hoppers 10 and 12 are fed to
extruders 14 and 16, respectively, which each melt and pressurize
the polymer contained therein and force it through filters 18 and
20 and metering pumps 22 and 24, respectively. The two polymer
streams are combined in spin block 26 by known methods to produce
the desired bicomponent filament cross-section. In one embodiment,
the multiple component fibers comprise bicomponent sheath-core
fibers wherein the fibers comprise between about 5 and 60 weight
percent of the sheath component and between about 40 and 95 weight
percent the core component. More preferably, the bicomponent fibers
comprise between about 15 and 40 weight percent of the sheath
component and between about 60 and 85 weight percent of the core
component. In one embodiment, the polymeric sheath component has a
lower melting point than the polymeric core component to facilitate
thermal bonding of the spunbond fabric. For example, the sheath
component can have a melting point that is at least 10.degree. C.
lower than the melting point of the highest-melting component and
more preferably has a melting point that is at least 20.degree. C.
lower than the melting point of the highest-melting component. For
example, the sheath component can be a polyethylene such as linear
low density polyethylene, high density polyethylene, or a blend
thereof and the core component can be a polyester such as
poly(ethylene terephthalate).
[0036] The melted polymers exit spin block 26 through a plurality
of capillary openings on the face of the spinneret 28 to form a
curtain of filaments 30. The capillary openings may be arranged on
the spinneret face in a conventional pattern, for example
rectangular, staggered, or other configuration. The filaments are
cooled with quenching air 32 and then passed through a pneumatic
draw jet 34 before being laid down to form a bicomponent spunbond
web. The quenching air is provided by one or more conventional
quench boxes that direct air against the filaments, generally at a
rate of about 0.3 to 2.5 m/sec and at a temperature in the range of
5.degree. C. to 25.degree. C. In one embodiment of the process of
the present invention, a two-sided quench system is used, wherein
quench air is directed onto the curtain of filaments from both
sides, to achieve a more uniform quench and reduce or eliminate
development of latent crimp which can occur when an asymmetric
(e.g. one-sided) quench is used. During the quenching step, the
temperature of the filaments is sufficiently reduced so that the
filaments do not stick to each other or to the inner walls of the
jet while passing through the jet. For example, when spinning
poly(ethylene terephthalate), the filaments can be quenched to a
filament temperature less than or equal to about 150.degree. C.
[0037] Air 36 is fed into draw jet 34 and provides the draw tension
on the filaments that causes them to be drawn (i.e., the primary
draw) near the spinneret face 28. The filaments 37 exiting the draw
jet are deposited onto a laydown belt or forming screen 38 to form
a web 40 of continuous filaments. In the present invention, the
filaments are re-heated while under the tension applied to them by
air 36 that is fed into pneumatic draw jet 34. The filaments are
re-heated to a temperature that causes the filaments to be drawn
(i.e., the secondary draw) and the average filament diameter to be
reduced by at least five percent, more preferably at least 10
percent compared to the average filament diameter that is achieved
when the fibers are not re-heated. In the re-heating step, however,
the filaments should not be heated to a temperature that is so high
as to cause the filaments to stick together when they contact each
other or to stick to the inner walls of the draw jet. For example,
when spinning poly(ethylene terephthalate) spunbond filaments,
heating the filaments to temperatures greater than about 70.degree.
C. and less than 225.degree. C. has been found to provide the
desired reduction in fiber diameter. The air velocity and air
pressure in the draw jet should also be sufficient to provide an
attenuation force sufficient to achieve the desired secondary fiber
draw. This secondary drawing of the fibers is believed to occur in
the pneumatic draw jet or essentially just after exiting the draw
jet (depending on whether the filaments are re-heated in the
pneumatic draw jet or after exiting the pneumatic draw jet).
[0038] For the re-heating step, air 36 that is introduced into the
pneumatic draw jet can be heated to a temperature that is
sufficient to achieve the desired reduction in fiber diameter.
However, because it can be costly to heat large quantities of air,
the filaments are preferably heated after they exit the pneumatic
draw jet. In a preferred embodiment, the filaments are heated by a
heating means after they exit the pneumatic draw jet while still
under tension imposed by the air 36. In the embodiment shown in
FIG. 2, the re-heating step is conducted by blowing heated air 42
through nozzles 44 onto both sides of the curtain of filaments as
they exit draw jet 34. Instead of using hot air, a radiant heating
means could be placed near the exit of the pneumatic jet.
Conventional infrared panel heaters would be a suitable heating
means. The heating means is preferably located no more than 20 cm
from the exit of the draw jet and more preferably no more than 0.5
cm from the exit of the draw jet. The undissipated velocity of the
air exiting the pneumatic draw jet contributes to the secondary
draw on the filaments at this stage and further reduces the
diameter.
[0039] The filaments are preferably single component filaments or
multiple component filaments having a substantially symmetric
cross-section. If the filaments are multiple component filaments
having an asymmetric cross-section, they will develop crimp during
the re-heating step due to differential shrinkage of the different
polymer components, which is believed to reduce or even eliminate
any secondary draw that is achieved in the re-heating step. The
method of the present invention is especially suitable for making
spunbond filaments having small diameters. In contrast, adjusting
other process parameters in an effort to reduce fiber diameter
generally results in the problems described above. The filaments 37
preferably have an average filament diameter of less than 14.5
micrometers after the re-heating step, more preferably less than
about 10 micrometers. After drawing the filaments in the secondary
drawing step, they are laid down to form a spunbond web, and
optionally bonded, as is known in the art. The method of the
present invention can be conducted on conventional spunbonding
equipment, with minimal modification to insert a heating means at
the exit of the pneumatic draw jet or to provide a source to heat
the air introduced into the pneumatic draw jet.
[0040] It has been found that spunbond webs prepared using the
method of the present invention have a softer, more pliable hand
than spunbond webs prepared in the absence of a secondary draw
step. The change in hand was surprisingly higher than expected
based on the reduction in fiber size alone. Without wishing to be
bound by theory, it is possible that the reheating and secondary
drawing step modifies the crystal morphology of the polymers
forming the spunbond fibers to provide a much more pliable hand
than spunbond fibers that are spun in a conventional spunbond
process.
[0041] In another embodiment of the present invention, a
multi-layer nonwoven sheet can be prepared by using multiple
spinblocks in series or alternating with meltblowing dies to form
spunbond-meltblown-spunbond nonwovens. Any number of spunbond and
meltblown layers can be laid down to form multi-layer nonwoven
sheets.
TEST METHODS
[0042] In the description above and in the examples that follow,
the following test methods were employed to determine various
reported characteristics and properties.
[0043] Average Fiber Diameter was measured by optical microscopy
and is reported as an average value in micrometers. The fibers were
mounted on an optical slide for measuring fiber size. For each
spunbond fabric, the diameters of about 100 fibers were measured
and averaged. The fibers used for fiber diameter measurements were
collected manually by removing fibers at four different locations
across the width of the curtain of filaments, prior to the
filaments contacting the collection belt. About twenty-five fibers
were collected from each of the four collection locations.
EXAMPLES
Examples 1A and 1B
[0044] A spunbond bicomponent sheet was made with a PET component
and a polyester copolymer component. The PET component had an
intrinsic viscosity of 0.53 dl/g (as measured in U.S. Pat. No.
4,743,504), and is available from DuPont as Crystar.RTM. polyester
(Merge 3949). The PET resin was dried in a through-air drier at an
air temperature of 120.degree. C., to a polymer moisture content of
less than 50 parts per million. The polyester copolymer was a
poly(ethylene terephthalate) copolymer modified with
1,4-cyclohexanedimethanol, available from Eastman Chemicals as
Merge PETG 20372. The PET polymer was heated to 290.degree. C. and
the polyester copolymer was heated to 275.degree. C. in separate
extruders. The two polymers were separately extruded and metered to
a spin-pack assembly, where the two melt streams were separately
filtered and then combined through a stack of distribution plates
to provide multiple rows of filaments having a concentric
core-sheath cross-section. The PET component formed the core and
the polyester copolymer component formed the sheath. The spin-pack
assembly consisted a total of 3360 round capillary openings and was
heated to 290.degree. C. Each capillary had a diameter of 0.23 mm
and length of 0.92 mm. The polymer throughput was of 0.5
g/hole/min. The fibers were 30 weight percent polyester
copolymer.
[0045] The filaments were cooled in a cross-flow quench (2-sided)
and an attenuating force was provided to the fibers by passing them
through a rectangular slot jet. The air pressure in the jet was 70
psig.
[0046] A secondary hot air drawing unit was attached about 0.5 cm
below the exit of the pneumatic jet as depicted in FIG. 2. The hot
air slots were 0.5 inch (1.27 cm) long in the vertical direction
and extended the entire width of the pneumatic draw jet. The exits
of the air jets were located about 2 inches (5.08 cm) from each
side of the curtain of filaments.
[0047] The fibers exiting the jet were collected on a forming belt.
Vacuum was applied underneath the belt to help pin the fibers to
the belt after laydown. The belt speed was adjusted to yield a
nonwoven sheet with basis weight of 70 g/m.sup.2. The fibers were
then thermally bonded between a set of embosser roll and anvil
roll. The bonding conditions were 150.degree. C. roll temperature
and 250 pounds per lineal inch (4475 kg per meter) nip pressure.
The sheet was then collected into rolls on the winder.
[0048] Two different combinations of hot air speed and temperature
in the secondary hot air drawing unit were evaluated as represented
by 1A and 1B. The results are shown in Table 1.
Comparative Example A
[0049] Comparative Example A was run under identical conditions as
Examples 1A and 1B, except that no air was passed through the
secondary drawing unit. The results are shown in Table 1.
Example 2A and 2B
[0050] A spunbond web was made using the process described in
Example 1 except that a PET polymer (Crystar.RTM. Merge 1988),
intrinsic viscosity of 0.58 dl/g) was fed through both the
extruders to form 100% PET filaments. Two different combinations of
hot air speed and temperature in the secondary hot air draw unit
were evaluated as represented by 2A and 2B. The results are shown
in Table 1.
Comparative Example B
[0051] Comparative Example B was run under identical conditions as
Examples 2A and 2B, except that no air was passed through the
secondary drawing unit. The results are shown in Table 1.
Example 3
[0052] A spunbond web was made using the process described in
Example 1, except that a PET polymer (Crystar.RTM. Merge 1988,
intrinsic viscosity of 0.58 dl/g) was fed through both the
extruders, and infrared panels were used instead of hot air jets to
heat the filaments in the secondary drawing step. Two infrared
heater panels (10.16 cm long in the vertical direction and
extending the entire width of the pneumatic draw jet) were attached
below the exit of the pneumatic draw jet, one panel on either side
of the exit of the jet. The infrared panels were ceramic infrared
heater panels made by Chromalux and had a surface temperature of
about 1400.degree. F. (760.degree. C.). The panels were located
about 3 inches (7.62 cm) from each side of the curtain of
filaments. The results are shown in Table 1.
Comparative Example C
[0053] Comparative Example C was run under identical conditions as
Example 3, except that no infrared heating step was used. The
results are shown in Table 1.
1TABLE 1 Average Fiber Size Average Standard Hot Air Hot Air Fiber
Deviation Temp Velocity Diameter of Fiber Example Number (.degree.
C.) (ft/sec)* (.mu.) Diameter Example 1A 200 21 6.4 0.9 Example 1B
132 30 8.2 1.1 Comparative Ex. A -- -- 9.2 2.1 Example 2A 200 21
6.6 1.3 Example 2B 132 30 7.8 0.8 Comparative Ex. B -- -- 9.5 2.2
Example 3 N/A N/A 7.6 1.4 Comparative Ex. C N/A N/A 9.5 2.2 *1
foot/second = 30.5 centimeter/second
* * * * *