Note: Descriptions are shown in the official language in which they were submitted.
NON~OVBN W~B ~IT~ L0~ POI880N RATI0
B~kground of th~ I~vo~tion
This invention re:Lates generally to nonwoven
webs of thermoplastic fibers and more particularly
concerns a nonwoven web of thermoplastic fibers which
has been stabilized to a low Poisson ratio.
Absorbent personal care products such as
disposable diapers and incontinence garments are
generally constructed with an impermeable outer cover,
an absorbent system, and an inner liner. Such
products sometimes leak at the leg, top front, or top
back areas. Leakage can occur due to a variety of
shortcomings in the products, one being an
insufficient rate of fluid uptake by the absorbent
system especially on the second or thixd liquid surge.
Such failure may not be due to the absorbent capacity
of the product, but instead may be due to the
inability of the absorbent system to uptake liquid
rapidly enough to prevent puddling and leaking.
Therefore, there is a need for a nonwoven liner which
can improve the handling of liquid surges and
ef~ectively uptake and retain repeated loadings of
liquid during use until the underlying absorbent
system can accommodate the li~uid loading.
A survey of the prior art relating to such
liners is laid out in detail in Application Ser. No.
757,760, filed Septe~ber 11, 1991, entitled Thin
Absorbent Article Having Rapid Uptake Of Liquid,
which is assigned to Rimberly-Clark Corporation, the
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assignee of the present invention. In addition,
Application Ser. No. 757,760, discloses an improved
nonwoven surge management fabric formed of bicomponent
fibers, particularly for disposable diapers, that
provides the necessary surge capabilities required to
insure against leakage until the underlying absorbent
system of the disposable diaper can accommodate the
liquid loading. The disclosure of Application Ser.
No. 757,760 is therefore incorporated herein by
reference as if fully set forth herein.
While the surge management fabric described
in Application Ser. No. 757,7~0 is particularly
effective for accomplishing the liquid handling -~
capabilities required to protect against puddling and
leakage in a disposable diaper, the surge management
fabric tends to stretch or neck when subjected to
stress during the manufacturing process of the
disposable diaper. Necking is a phenomena which
results when a nonwoven web is subjected to stress in
the machine direction causing it to narrow or neck in
the cross machine direction thereby resulting in a web
that is smaller in width than the untensioned web.
The tendency of a nonwoven fabric to neck is
indicated by a dimensionless value called the Poisson
ratio. The Poisson ratio is the ratio of horizontal
strain to vertical strain for a material subjected to
a uniform vertical stress. The stress is the unit
loading on the material caused by an applied tension.
For nonwovens, the thickness of the web is generally
not taken into account when calculating stress, and
thus, the stress is typically reported a~ a force per
unit width of the nonwoven web. The strain is the
change in dimension caused by an applied stress,
divided by the original dimension. Thus, the equation
for Poisson ration (P) is: P = _ (-W/W?
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(-L/L)
where, W= material width
L= material length
W = change in width
L = change in length
In order to accommodate such undesirable
necking, it is often necessary to provide a web that
is wider than required so that when the web is
stretched, its necked width is still sufficient to
extend the full width of the converting machine used
to manufacture the disposable diaper. When the stress
is released after the disposable diaper has been
manufactured, the relaxed web returns to its original
width producing unsightly gathers in the finished
diaper. Consequently, there is a need in the art to
provide a surge management fabric in accordance with
the disclosure of Application Ser. No. 757,760 which
is dimensionally stabilized in the cross machine
direction so that when the surge management fabric is
subjected to stress in the machine direction necking
is minimized.
8u~ y o~ the Inv-~tion
It is therefore an object of the present
invention to provide a method fox dimensionally
- stabilizing a nonwoven web of thermoplastic fibers so
that necking is minimized.
It i9 also an object of the present
invention to provide a machine for producing a
di~ensionally stabilized nonwoven web of thermoplastic
fibers.
It is an object of the present invention to
provide a dimensionally stabilized nonwoven web of
thermoplastic fibers which web is characterized by low
necking and a low Poisson ratio.
It is particularly an object of the present
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invention to provide a dimensionally stabilized
nonwoven web of thermoplastic fibers in which some of
the fibers are bicomponent fibers.
It is further an object of the present
invention to provide a dimensionally stabilized
nonwoven web of thermoplastic fibers having a
predetermined thickness.
The foregoing objectives are achieved by a
process for dimensionally stabilizing a nonwoven web
of thermoplastic bicomponent fibers which process
includes laying a web of the fibers on a carrier, -
subjecting the web of fibers to a source of heat,
consolidating the web to a predetermined thickness by
passing the web through a predetermined fixed gap of
a caliper roll, and cooling the web ~o set the web at
the predetermined thickness. me foregoing objectives
are also achieved by a machine in which a fiber
forming head first deposits thermoplastic bicomponent
fibers onto a moving carrier to form a nonwoven web.
The carrier moves the web downstream to a through-air
bonder in which the web is subjected to heated air of
sufficient temperature to soften the outer surface of
the fibers. A caliper roll is located at the exit of
the through-air bonder. The caliper roll has a
predetermined fixed gap through which the heated web
passes. A cooling zone is located downstream of the
caliper roll in which cooling zone the nonwoven web is
then subjected to cooling air (generally ambient
temperature~ in order to set the web at a
predetermined thickness. The predeter~ined thickness
is established by the fixed gap of the caliper roll.
The foregoing objectives are also achieved
by a nonwoven liner composed of bicomponent
thermoplastic fibers and made in accordance with the
above-identified process. The resulting web has a
Poisson ratio of less than 2.0 and generally less than
1Ø Generally when calculating the Poisson ratio for
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a material, the material is strained along its length.
To achieve a ratio as mentioned above, the material is
placed under a strain of between about 1 and 10
percent with the target being about 5 percent strain.
As previously explained, the strain is the change in
dimension, in this case the length, divided by the
original dimension as a result of the applied stress.
Such a web therefore exhibits low necking when
subjected to tension during winding and converting
operations.
Other objects and advantages of the
invention will become apparent upon reading the
following detailed description and upon reference to
the drawings.
Brief Description of the Dr~wings
Figure l is a schematic drawing showing a
machine in accordance with the present invention for
dimensionally stabilizing a nonwoven web containing
thermoplastic bicomponent fibers.
Figure 2 is a graph of stress versus
neckdown as determined for a preferred embodiment of
the present invention and a comparative material.
Figure 3 is a graph of compression versus
caliper for the same materials used to obtain the data
shown in Figura 2.
Figure 4 is a graph of stress versus strain
for the same materials used to obtain the data shown
in Fi~ure 2.
D-tailed Desoription or th- Invention
While the invention will be described in
connection with a preferred embodiment and method, it
will be understood that we do not intend to limit the
invention to that embodiment or method. On the
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contrary, we ~ntend to cover all alternatives,
modifications, and equivalents as may be included
within the spirit and scope of the invention as
defined by the appended claims.
As disclosed in Application Ser. No.
757,760, a surge management fabric is configured for
placement either in contact with the skin of the
wearer as a liner/surge fabric or as an underlying
surge fabric between a top sheet and an underlying
absorbent system. When the surge management fabric is
used as an underlying surge fabric located between the
top sheet and the absorbent system of a disposable
diaper, the underlying surge fabric generally has a
basis weight within the range of about 17-102 grams
per square metèr (gsm) and includes at least about 25
percent by weight of bicomponent fibers to provide a
desired bicomponent fiber bond matrix. Up to 100
percent by weight of the underlying surge fabric can
be composed of bicomponent fibers, and accordingly,
0-75 percent by weight of the underlying surge fabric
may comprise non-bicomponent fibers. In addition, the
underlying surge fabric can comprise a blend of
smaller diameter fibers and relatively larger diameter
fibers. The smaller sized fibers have a denier of
less than about 3 denier, and preferably have a denier
within the range of about 0.9 - 3 denier. The larger
sized fibers have a denier of greater than about 3
denier, and preferably have a denier within the range
of about 3 - 18 denier. The lengths of the fibers
employed in the underlying surge fabric are within the
range of about 1-3 inches. The bond-matrix and the
blend of fiber deniers can advantageously provide for
and substantially maintain a desired pore size
structure.
For example, the underlying surge fabric may
comprise a nonwoven fibrous web which includes about
75 percent polyester fibers of at least 6 denier, such
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as PET (polyethylene terephthalate) fibers available
from Hoechst Celanese of Charlotte, North Carolina.
The polyester fibers have a length ranging from about
1.5 - 2.0 inches in length. The remaining 25 percent
of the fibrous web can be composed of bicomponent
binder fibers of not more than 3 denier, and
preferably about 1.5 denier. The bicomponent fiber
length ranges from about 1.5 - 2 inches. Suitable
bicomponent fibers are wettable or nonwettable,
polyethylene/polypropylene bicomponent fibers,
available from Chisso Corporation located in Osaka,
Japan. The bicomponent fiber can be a composite,
sheath-core type with the polypropylene forming the
core and polyethylene forming the sheath of the
composite fiber, or the bicomponent fiber can be a
side by side type fiber with the polypropylene forming
one side and the polyethylene forming the other side.
The polyester fibers and bicomponent fibers are
generally homogeneously blended together and are not
in a layered configuration. The fibers can be formed
into a carded web which is thermally bonded, such as
by through-air bonding or infrared bonding.
As another example, the underlying surge
fabric may be composed of a bonded carded web which
has a basis weight of about 50 gsD and includes a
mixture of polyester (PET) single-component fibers and
PET/polyethylene bicomponent fibers. The PET fibers
comprise about 60 percent by weight of the nonwoven
fabric, and are about 6 denier with an average fiber
length of about 2 inches. The PET/polyethylene
bicomponent fibers comprise about 40 percent by weight
of the fabric, and are about 1.8 denier with an
average fiber length of about 1.5 inches. The PET
forms the core and the polyethylene forms the sheath
of the bicomponent fiber. In optional constructions,
the larger-sized, PET single-component fibers may be
replaced by bicomponent fibers. In further optional
,
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arrangements, polypropylene/polyethylene bicomponent
fibers may be employed to form the bicomponent fiber
portion of any of the described underlying surge
fabrics. In addition, the bicomponent fibers may be
flat crimped or helically crimped.
The surge management fabric as previously
stated can also be positioned in the diaper so that it
is in contact with the skin of the wearer. Where the
surge management fabric is in contact with the skin of
the wearer, it may be configured as a two layer
liner/surge fabric. The composite liner/surge fabric
includes a bodyside liner layer adjacent the skin of
the wearer and an inner surge layer positioned toward
the underlying absorbent system. The layers can be
separately laid and can have different structures and
compositions. The fibers within each layer and the
intermingling fibers between the layer portions are
then suitably interconnected (such as by powder
bonding, point bonding, adhesive bonding, latex
bonding, or by through-air or infrared thermal
bonding) to form a composite liner/surge fabric. The
resultant composite liner/surge fabric has a total
basis weight of not more than about 102 gsm.
Preferably the total basis weight is within the range
of about 24 - 68 gsm, and more preferably is within
the range of about 45 - 55 gsm. In addition, the
- total average density of the composite liner/surge
fabric is not more than about 0.10 g/cc, and
preferably is not more than about 0.05 g/cc (as
determined at .05 psi).
The inner surge layer has a basis weight
within the range of about 17 - 85 gsm and includes at
least about 25 percent by weight of bicomponent fibers
to insure a bicomponent fiber bond-matrix. The inner
surge layer also comprises a blend of smaller diameter
fibers and relatively larger diameter fibers. The
smaller sized fibers have a denier within the range of
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about o.9 - 3 denier, and the larger sized fibers have
a denier within the range of about 3 - 18 denier. The
bond-matrix and the blend of fi~er deniers can
advantageously provide for and substantially maintain
a desired pore size structure within the inner surge
layer. Conversely, in certain applications the surge
layer may be used in conjunction with a conventional
liner material in which case the basis weight of the
surge layer will range between about 17 and about 102
lo gsm.
More particularly, the inner surge layer may
be composed of a carded web which has a basis weight
of about 34 gsm and includes a mixture of polyester
(PET) single-component fibers, available from
Hoechst-Celanese of Charlotte, North Carolina, and
polyethylene/PET (PE/PET) sheath-core bicomponent
fibers, available from BASF Corporation, Fibers
Division, of Williamsburg, Virginia. The PET fibers
can comprise about 60 percent by weight of the inner
surge layer and have a denier of about 6 with an
average fiber length of about 2 inches. The
polyethylene/PET bicomponent fibers comprise about 40
percent by weight of the outerside layer, and have a
denier of about 1.8 with an average fiber length of
about 1.5 inchec. Optionally, the larger-sized, PET
single-component fibers may be replaced by bicomponant
fib ers. As a fu rth er op tion,
polyethylene/polypropylene (PE/PP), sheath-core
bicomponent fibers may be employed to form the
bicomponent fiber portion of any of the described
fabrics. Suitable PE/PP bicomponent fibers are
available from Chisso Corporation located in Osaka,
Japan.
The bodyside liner layer of the liner/surge
fabric includes at least some bicomponent fibers to
provide desired levels of tactile softness and
abrasion resistance. The bodyside liner layer has a
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basis weight of at least about lo gsm, and the
bicomponent fiber size is within the range of about
0.9 - 3 denier with a fiber length with the range of
about 1 - 3 inches. Preferably, the fiber denier is
within the range of about 1.5 - 3.0, and more
preferably, is about 3.0 denier. A preferred fiber
length is about 1.5 inches. For example, bodyside
liner layer may comprise a carded web which has a
basis weight of about 17 gsm and is composed of 100%
PET/polyethylene, sheath-core bicomponent fibers,
obtained from BASF Corporation of Williamsburg,
Virginia, with a fiber denier of about 1.8 and fiber
lengths of about 1.5 inches.
In a particular embodiment of the composite
liner/surge fabric, the inner surge layer forms
approximately 65 weight percent of the composite
liner/surge web and is composed of a blend of
polyester fibers and bicomponent fibers. With respect
to this blended inner surge layer, about 60 percent by
weight of the blended inner surge layer is composed of
polyester fibers of at least about 6 denier and with
a fiber length within the range of about 1.5 - 2
inches. The remaining 40 percent of the blended inner
surge layer is composed of bicomponent fibers of not
more than about 3 denier, and preferably about 1.8
denier, with fiber lengths within the range of about
1.5 - 2 inches. The bodyside liner layer comprises
the remaining 35 weight percent of the composite
liner/surge fabric, and is composed of bicomponent
fibers have a denier within the range of about 0.9 -
3 to provide a soft liner type material appointed for
placement against the wearer s skin. In a particular
embodiment, the bodyside liner layer of the composite
liner/surge fabric has a basis weight of about 15 gsm
and is composed of bicomponent fibers of about 2
denier.
Another embodiment of the composite
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liner/surge fabric can comprise a bodyside liner layer
composed of about 100 percent polyethylene/polyester
sheath-core bicomponent fibers of not more then about
3 denier. The bodyside liner layer has a basis weight
of about 15 gsm. In addition, this embodiment of
composite liner/surge web includes an inner surge
layer composed of a 50/50 blend of polyester fibers of
about 6 denier and polyester/polyethylene, sheath-core
bicomponent fibers of not more than about 3 denier.
While the foregoing surge management fabrics
including the underlying surge fabric and the
liner/surge fabric provide excellent attributes for
controlling and managing the distribution of liquids
in a disposable diaper, the fabrics or webs described
above when stretched in the machine direction exhibit
significant necking during winding and converting
operations. Particularly, such fabrics exhibit
Poisson ratios above 2Ø We have found that surge
management fabrics such as those described above can
be produced with significant reduction in the necking
and resultant reduction of Poisson ratio to near 1Ø
Such dimensional stabilization is achieved by
subjecting the surge management fabrics to a source of
heat and then passing the fabrics through a fixed gap
of a caliper roll prior to cooling and setting the
fibers.
- It also should be appreciated that while the
foregoing description of materials has been in
conjunction with the use of fibrous webs employing
staple fibers, it al50 may be possible to utilize the
present invention with other types of fibrous webs.
Examples of such other fibrous webs include spunbonded
webs, melt blown webs, air laid webs, and solution
spun webs to mention but a few.
Turning to Fig. 1, there is shown a surge
management fabric forming machine 10. The fabric
forming machine 10 comprises a web forming station 11,
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a foraminous carrier belt 18, a through-air bonder 28,
and caliper roll 50.
The web forming station 11 includes a fiber
forming head 12 which produces a curtain 14 of web
forming fibers 16. The fibers 16 are the fibers
described previously in connection with the surge
managemen~- fabric and at least some of the fibers 16
are bicomponent fibers. The fibers 16 are deposited ~-~
on the moving foraminous carrier belt 18 above a
vacuum box 20 in which reduced atmospheric pressure is
attained by a vacuum pump 22. Under the influence of
the reduced pressure in vacuum box 20, the fibers 16
are deposited on the foraminous carrier belt 18 to
form a nonwoven web 24.
The foraminous belt 18 moves in the
direction indicated by arrow 26 and carries the
nonwoven web downstream from the web forming station
11 to the through-air bonder 28. The foraminous
carrier belt 18 passes around drum 30 of the -
through-air bonder 28 and around idlers 32, 34, 36,
and 40 before returning to the web forming station.
The through-air bonder 28 is conventional in
design such as the through-air bonders manufactured by
Honeycomb Systems of Biddeford, Maine. The
2S through-air bonder 28 consists of the rotating drum 30
which has a porous outer surface 42 and a pipe 44 at
the center of the drum through which a vacuum is
drawn. A hood 46 is disposed around the majority of
the circumference of the drum 30 and provides a source
of heated air shown schematically by arrows 4~. The
heated air 48 passes from the hood 46, through the web
24, the carrier belt 18, through the porous surface 42
of the drum 30, and out of pipe 44 to a source of
vacuum (not shown). The heated air 48 serves to
soften the external sheath of the bicomponent fibers
thereby rendering them tacky. A caliper roll 50 is
disposed at exit S2 of the through-air bonder 28. The
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caliper roll 50 is mounted on a pivoting lever arm 54
which is moved toward and away from the surface 42 of
the drum 30 by means of cylinder 56. Consequently,
the caliper roll 50 can be set at a predetermined
fixed gap 58 from the belt 18 which may or may not be
backed by the surface 42 of the drum 30. In
alternative arrangements, the gap 58 can be formed
between the caliper roll 50 and the through-air bonder
28 surface or another roll. The size of the
predetermined fixed gap 58 determines the Poisson
ratio of the resulting web 24 because the compression
of the web, while the fibers in the web are still
tacky from the through-air bonder 28, affects the
number and strength of the bonds between the fibers in
the web. When`the web 24 passing through the gap 58,
the caliper roll 50 compresses the web and the tacky
fibers together to create more and stronger bonds
between the fibers than if the web is uncompressed.
The enhanced bonding of the fibers in the compressed
state produces a web with a lesser propensity to neck
when strained.
The gap 58 is sized so that the desired web
thickness is obtained and the Poisson ratio of the
resulting web 24 is less than 2, and generally less
than l. The particular size of the gap 58 required to
obtain a particular web thickness and Poisson ratio
varies depending on factors including the composition
of the web 24 and the speed of the web through the
gap. Typically, the thickness of the resulting web 24
will be equal to or greater than the thickness of the
gap 58 and the faster the web moves through the gap,
the thicker is the resulting web. In other words, to
achieve a desired web thickness, a more narrow gap
typically must be used at higher line speeds than at
lower line speeds. The number and strength of the
bonds created resist forces which would tend to reduce
the wildth of the fabric and cause necking when the
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fabrlc is strained. In addition, the bonds created
resist compressive forces so that the fabric better
retains its loft.
On the downstream side of the caliper roll
50 there is located a vacuum seal 60 which allows
ambient air to be drawn through the web 24 along a
segment 62 of the surface 42 of the drum 30. The
vacuum seal 60 assures that ambient air drawn into the
drum 30 over the segment 62 passes through the
nonwoven web 24 to cool and thereby set the bonds that
have formed between the fibers.
Once the fabric has passed through cooling
zone 62 adjacent the exit 52 of the through-air bonder
28, the web 24 has achieved sufficient integrity that
it may be pulled from the carrier belt 18. The web 24
then passes around idle rolls 64 and 66 and then to a
winding roll (not shown).
When the web 24 is formed on the machine 10
shown in Fig. 1, the resulting web exhibits low
Poisson ratios of less than 2 and generally less than
1. The machine 10 shown in Fig. 1 was used to make
webs in accordance with the following example.
~xam~le 1
A dimensionally stable, liner/surge,
- nonwoven web having a basis weight of 1.5 osy was made
according to the process described above and
illustrated in Fig. 1. The liner layer had a basis
weight of 0.5 osy and was positioned ad;acent the
carrier belt. The liner layer comprised 100% M-1050,
3 denier, polyethylene/PET bicomponent fibers from
3ASF Corporation of Williamsburg, Virginia. The surge
layer had a basis weight of 1.0 osy and was positioned
to pass adjacent the caliper roll. The surge layer
comprised 60% by weight T-295, 6 denier, PET
homofibers from Hoechst-Celanese of Charlotte, North
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Carolina, 35% by weight M-1051, 1.8 denier,
polyethylene/PET bicomponent fibers from BASF
Corporation, and 5% by weight ES-HB, 2.0 denier,
polyethylene/polypropylene bicomponent fibers from
Chisso corporation of Osaka, Japan. Both the liner
and surge layers were formed by blending and carding.
The line speed was 450 feet per minute, the
temperature of the hot air in the through-alr bonder
was 262 F, the hood pressure in the through-air bonder
was 0.9" HzO, the size of the gap between the caliper
roll and the carrier belt was 0.030 inches, and the
cooling zone vacuum was less than 1" H2O. The
resulting liner/surge material had a caliper of 0.080
inches, a density of 0.022 g/cc, strip tensile
lS strengths (as measured on a 3 inch sample in
accordance with ASTM D 1117-6) of 5400 g/3'' (MD) and
450 g/3" (CD), and a Poisson ratio of 0.99.
The low Poisson ratio of 0.99 for the sample
fabric from Example 1 demonstrates the dimensional
stability of that fabric. The dimensional stability
of the sample fabric from Example 1 is also
illustrated by the graphs shown in Figs. 2-4 which
compare properties of that fabric to a comparative
sample fabric made without using the caliper roll.
Figure 2 shows that as the stress exerted on
the sa~ples is increased, the fabric sample from
Example 1 bonded with the use of the caliper roll has
a lower percentage of neckdown than the comparative
sample. Figure 3 shows that, under lower pressure,
the fabric sample from Example 1 is more resistant to
compression than the comparative sample. Figure 4
shows that the sample of fabric from Example
exhibits a much lower strain for a given stress. This
indicates that nonwoven fabric is much more
effectively bonded when a caliper roll is used in
accordance with the present invention.
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