Note: Descriptions are shown in the official language in which they were submitted.
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THREE DIMENSIONAL NEEDLED NON-WOVEN ABSORBENT COMPOSITES
Field of the invention
The present invention relates to a three dimensional needled non-woven
absorbent
composite which is especially designed for use as disposable sanitary or
medical
article and which, thanks to its structure, has a plurality of advantages:
fast fluid
acquisition, effective x-y distribution, minimum leakage or runoff, low rewet,
low
stain view ability, high absorbency capacity, dimensional stability, wet
strength,
no chemical binder, no shake off of absorbents powders and extreme softness.
Background of the Invention
Absorbent structures used in sanitary napkins, diapers, incontinence pad and
training pants have common requirements including sufficient absorbency
capacity,
fast liquid acquisition, rapid liquid distribution in inner structure, good
liquid
retention under load, low liquid run-off, smoothness and softness.
To meet the above mentioned requirements, the absorbent product usually
consists
2 0 of different components such as a cover stock to provide clean and dry
surface for
the body skin, an .acquisition layer to quickly trap the insult liquid, a
distribution
layer to distribute the liquid to the full dimension of the absorption core,
an
absorbent core to absorb and retain the liquid, and an impermeable back sheet
to
prevent the leakage of the fluid out of the absorbent article. Substantial
efforts
have been made to improve the performance of each component. Improvements
are being made continuously.
It has been realized that to control the liquid flow direction it is necessary
to adopt
3-D (three dimensional) structure design. Perforated film (US Patent
3,929,135)
3 0 is a delicately designed cover stock with 3-D apertures having one way
valve effect
to reduce wet back. Flow control coversheet (US Patent 5,531,727) utilized the
concept that in the porous structure, liquid tends to be sucked towards the
finer
pores. By arranging layered fibers from coarse to fine denier, a pore gradient
in z
direction is created. It is said that the flow control coversheet can improve
the
liquid struck through and reduce the wet back.
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The current available absorbent cores are mainly special formulated cellulose
fiber
matrix or fluffy pulp. To large extent air-laid processing methods are
utilized to
produce this sheet type absorbent core. To increase the capacity of absorption
and
retention and to reduce the thickness of the products, super-absorbent
polymer,
normally in powder type, are added to the mixture. To get coherency and
sufficient
strength, the pulp sheet has to be compressed to certain density. With more
super-
absorbent polymer added, it is necessary to add thermal binding components,
which has adverse effect on absorbency and material cost. In forming practical
absorbent articles, the pulp sheet is either c-folded or multi-layered to
achieve
required absorption capacity. When used as multi-layer system, the pulp sheets
would have different density and different super-absorbent content (US Patent
5,866,242). To overcome the conflicts of softness and reducing thickness, the
pulp sheets some time need to be softened prior to its use by mechanically
defibering (US Patent 5,814,034). The pulp type absorbents do have good
wicking
and fluid retention properties. However this type of absorbent always have a
paper
like stiff feeling. The major problems of the pulp absorbents may be low
acquisition
speed and high run-off, resulting in leaking napkins, which are due to the
limitation
of density of the pulp sheet. The pulp type absorbency sheet may achieve truly
thin. But it -still needs the aids of acquisition or distribution components
when
fabricating the absorbent articles.
Peat moss composite board is also used as absorbent core. US patent 5,296,290
released an invention of combining a hydrophobic non-woven fibrous fleece
materials with absorbent hydrophilic layer, preferably peat moss composite
board.
The two layers are preferably bonded by needling. It was said that the
penetrated
fibers serve as wicks. The fluid can be carried away from the first layer to
hydrophilic layer. The fibrous fleece may improve the liquid distribution
speed for
the whole system. However the peat moss board is quite stiff and has very low
3 0 wet strength and hydrophobic fibers having high surface tension have
adverse
effect on fluid acquisition speed of the absorbent composite
Super absorbent polymer (SAP) is an efficient way to construct compact and
efficient absorbent core. However the processing of such structure usually
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accompanies the problems of poor integration of SAP powder and fluffy pulp.
The
SAP powders can have uneven amount over the surface of the core, can be shaken
off the core, or finally move in the absorbent structure resulting in variable
absorbency performance. SAP fibers have been developed for more than a decade.
But they have not really penetrated into the absorbent product market. Apart
from
the reason of price comparison with SAP powder, the other reasons are lack of
technical know how for its processing methodology and performance design when
combining with other fibers. Staple SAP fibers may be carded to web by
traditional
machinery mostly in conjunction with other staple fibers. However, not every
bonding method is suitable for the web containing SAP fibers. The possible
bonding
techniques are thermal, chemical or needle bonding. But thermal and chemical
bonding may adversely affect the product softness, cost, fluid behavior and
absorption performance.
Summary of the Invention
The object of the present invention is to solve in a very simple manner, the
above
mentioned, sophisticated absorption problems.
2 0 More specifically, the object of the present invention is to provide a
novel non-
woven absorbent composition with integrated performance of fluid acquisition,
distribution and absorption, and also with improved flexibility. The main
focus of
the invention is of using staple fibers and super absorbent polymer fibers to
built
up three-dimensional structural composites which are suitable for feminine
hygiene
2 5 products and other absorbent products.
In accordance with the invention, the above object is achieved with a liquid
absorbent composite comprising at least two layers bonded by needle punching.
The fibers and web combination of the composite can be selected to meet any
3 0 predetermined needs.
The absorbent composite according to the invention comprises at least two
layers
including an upper layer consisting of a loose needled non-woven material
forming
a coarse fiber web having a liquid acquisition and transportation ability. The
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composition also comprises a lower layer consisting of a relative dense non-
woven
material having a liquid absorption and retention ability. The lower layer
acts as an
absorbent core and stores the liquid. Both layers are intimately combined by
mechanical needling, also called needle punching, which provides transverse
fiber
tufts extending from the upper layers) to
the lower layer(s).
The upper or "acquisition/distribution" layer is made of relative coarse
fibers of 3
to 9 denier having a large pore size and volume and a low density ranging
between
0.04 and 0.10 g/cm3. Thus, the upper layer provides fast liquid acquisition
and
transportation. At the same time, its porous structure acts as a reservoir and
collects large amounts of incoming fluid, thereby reducing the liquid runoff.
The
fibers of the acquisition layer are wet-able synthetic fibers to provide fast
liquid
transport in the fibrous structure and reducing the wet back.
The lower or "absorbent" layer (also called "core") consists of a needled non-
woven material preferably made of SAP fiber, hydrophilic surface treated
synthetic
fiber and viscose. The density of the absorbent core is higher than that of
acquisition layer, between 0.10 g/cm3 and 0.25 g/cm3.
Needling bonds the acquisition/distribution layer and the absorbent layer
together.
The needling density is between 20 p/cmz (p/cmz stands for perforations per
square
centimer) and 100 p/cmz to give sufficient bonding and good z direction fiber
tufts,
which build up wicks to transport the liquid in contact with denser material
where
higher capillary force is available and the liquid absorption and retention
values are
high.
The fiber proportion in the absorption layer is selected to maximize the SAP
fiber
absorption capacity, best utilize the fibrous pore storage capacity, and
minimize the
3 0 cost. Rayon fiber provides the hydrophilicity. The synthetic fiber
increases the
compressive resistance of the absorbent core thus the retention capacity, and
improves the gel-blocking problem.
The acquisition/distribution layer fiber web can be produced with a
conventional
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textile carding equipment. A single denier fiber or multi-denier synthetic
fiber
system can be used to produce acquisition/distribution layers with different
performance, e.g. low rewet, fast fluid transportation, good coverage etc. The
web
is needled to keep it lofty and soft. The fiber may range from 3 denier to 9
denier,
5 and its weight ranges from 20 gsm to 70 gsm (gsm stands for grams per square
meter). The absorbent core is produced by traditional carding and needling
procedure. To achieve thinner structure, heating and thickness calibration
calendering can be used in line to produce the absorbent cores in one step.
The
needling parameters for the absorption core and combination is arranged such
that
a density gradient from low to high in z direction is built up and the SAP
fiber is
distributed more far away from the acquisition layer. The gradient density
gives
directional wicking effect. The far away arrangement of SAP fiber gives
enhanced
liquid retention to reduce rewet.
As another option, the composite includes the combination of needled non-woven
material with pulp/SAP airlaid non-woven material. There are two types of
needled
non-woven material that can be used. One is the acquisition layer described
above.
The other one is a blend of synthetic fibers and the SAP fiber. When combining
the
acquisition layer with the air-laid non-woven, the performance that is
obtained is
2 0 similar to the one obtained with a needled non-woven composite, but the
flexibility
is not as good as with a needled non-woven composite. When combining
synthetic/SAP fiber non-woven with the air-laid non-woven, the SAP selected
has
relative lower absorbent rate than that in the absorbent core. Thus the
absorbent
capacity increased without loosing other performance such as fast acquisition
speed etc.
The composite has a desired flow pattern, i.e. insult liquid rapidly flows and
spreads in the plane and fast penetrates to the web simultaneously. The
reservoir
effect dramatically improves the run-off performance under large flow rate
insult.
3 0 The acquisition seed is also significantly increased. The needled non-
woven
absorbent composite is very soft comparing with available absorbent products.
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The invention and its advantages will be better understood upon reading the
following non-restrictive description of preferred embodiments of the
invention,
made with reference to the accompanying drawings.
Brief description of the drawings
Figure 1 is a side elevational cross-section view of a liquid absorbent
composite material according to the invention; and
Figure 2 is a schematic representation of the equipment that was used for
carrying out absorption tests.
Description of preferred embodiments of the invention
As shown in the Fig. 1, the composite according to the invention comprises an
upper acquisition layer 1 and a lower absorbent layer or core 2. Needling
bonds the
two layers. The needling provides vertical fiber tufts 4 that extend from the
acquisition layer to the absorbent layer to form wicks. The density of the
acquisition layer 1 is lower than the one of the absorbent core 2. The
absorbent
core 2 also has a density gradient, with a higher density in its lower part 3.
The
liquid is easily trapped by acquisition layer 1 and quickly spreads in the x-y
plane
of the acquisition layer 1. The fiber tufts in the z direction provide a flow
path and
help the fluid to be absorbed by the absorbent core 2. The density gradient
within
the core 2 helps to retain the liquid away from the top surface.
The acquisition layer is made of coarse synthetic fibers and as of polyester,
polypropylene polyamide etc., whose surface is hydrophilic treated. The fibers
are
in the range of 3 to 9 denier. More preferably, the fibers are 4 to 6 denier.
The
acquisition layer has a basis weight of 30 gsm to 70 gsm. The fibrous web is
needled to give coherence. The needling ranges from 30 p/cm2 to 100 p/cm2
3 0 according to the fiber blend to give a density ranging between 0.04 g/cm3
and 0.10
g/cm3. The coarse fibers avoid most of liquid rewet. Because the acquisition
layer
consisting of coarse fibers has larger pore size as compared to the substrate
absorbent, it has a capillary break effect and it prevents the liquid absorbed
being
wet back. The fibers in the layer can be of identical denier or multi-denier.
In case
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of a blend of fibers with different sizes, the fibers should be hydrophilic
surface
treated to fasten the liquid through.
The absorbent core is made from a fiber blend of rayon, synthetic fiber and
super
absorbent fiber. The rayon fiber denier ranges from 1 to 4. The proportion of
the
rayon is between 20% to 50%. More preferred is 30%. The rayon fiber functions
as reducing the liquid contact angle to enhance the capillary drawing force.
It was
proved that more rayon may reduce the liquid retention because it is easily
collapsed when wet.
The synthetic fiber has a denier range of 1.5 to 4 and is used at a proportion
of
30% to 60%. The fiber could be polyester or polypropylene or the mixture of
the
two. The synthetic fiber should be treated with a hydrophilic spin finish.
Although
there is difference between hydrophilic viscose fiber and the synthetic fiber
in the
liquid intimacy, the amount of the liquid hold in the viscose fiber is very
small. Thus
the absorbent and retention capacities depend largely on the pore volume and
the
capillary force created by the fibrous system. Therefore the synthetic fiber
can be
used. There are several benefits for the usage of synthetic fiber. First the
synthetic
fiber can increase the compressive resistance, thereof the liquid retention
capacity.
Second, it may isolate the super absorbent fiber to avoid the gel locking.
Third, it
can reduce the cost of the absorbent core dramatically.
The super absorbent fibers can be supplied by Camelot Technologies, High River
Alberta, Canada, or Technical Absorbents, Grimsby, UK. In the absorbent core
the
9 denier and 51 mm super absorbent fiber was used in proportion of 10% to 40%.
The above three fibers are mixed thoroughly before going to the card. After
card
and cross lapper, the web is needled from one direction. With the needles
penetrating to the, web, fibers are carried through the web by the barbs on
the
needle. There are more inter-reaction between the bars and the fibers at the
3 0 needling side. Fibers are more densely packed at the needle entrance side.
Thus a
fiber density gradient is formed; i.e. the fiber density is high at the
needling side
and low at the backside. It makes the one direction needled non-woven have
distinct acquisition speed between the two surfaces. From the loose side the
acquisition speed is about 2 to 3 times faster than that from the dense side.
Owing
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to the difference of fiber denier and the flex rigidity between the super
absorbent
fiber and the other fibers, the super absorbent fiber tends to stay at the
needling
side. At certain needling depth condition, this trend becomes more
significant, i.e.
more super absorbent fibers are arranged at the dense side. This is another
benefit
when arranging the liquid flow and retention in one direction. The needling
density
ranges from 50 p/cm2 to 200 p/cm2. Needling penetration is between 9 and 1 1
mm. The needled non-woven's density is 0.1 g/cm3 to 0.25 g/cm3. To achieve
thinner absorbent cores, post calender process can be applied. The traditional
hot
calender is an available way. The temperature is between 80 °C to 110
°C to avoid
altering the softness of the absorbents. For instance one can use infrared
heating
plus cold calender on the needling line to increase the efficiency and
productivity.
When using calendering compaction, the needling density and penetration
conditions should be in the low side of the given range. The absorbent core
can be
from 100 gsm to 400 gsm depending on the absorbency capacity requirements.
The direction density gradient of the absorbent cone can also be achieved by
utilizing multi-layer design. Each layer has different density where the
lowest
density layer is first in contact with the result liquid.
2 0 The acquisition layer and the absorbent layer are bonded together by
needling. The
loose side of the absorbent layer contacts the acquisition layer. In this way
the
fiber density is from low to high in z direction. Therefore the capillary
force
increases from the top to the bottom. With more super-absorbent fiber at the
bottom, the liquid is hold away from the topside. The needling is conducted
from
the acquisition layer towards the absorbent core. The needling density is
between
20 p/cmz and 50 p/cm2. This gives reasonable bonding and introduces fiber
wicks
to assist the fluid flowing towards the dense material. Thus a three
dimensional
absorbents structure is built up. The test shown that the integrated
absorbents
dramatically increase the liquid acquisition speed, and has superior low
rewet, zero
3 0 runoff and extreme softness. The structure is very stable in both dry and
wet state.
The combination also includes the acquisition layer described above and an
airlaid
non-woven made 'of pulp and super-absorbent fiber. The airlaid non-woven are
selected with a soft handle. The two layers are bonded by the needling. The
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needles penetrate from the acquisition layer to the airlaid non-woven
absorbent
core with density of 20 p/cm2 to 50 p/cmz. The needling penetration varies
from
6 mm to 9mm depending on the density of the airlaid non-woven and the needle
used. By this arrangement, the acquisition layer is mechanically bonded with
the
absorbent core. The integration is therefore guarantied. At the same time the
airlaid
non-woven is softened to certain extent. The significant feature is that the
acquisition speed increased dramatically.
When combining needled non-woven with airlaid non-woven, a proportion of SAP
fiber can be added to the synthetic fiber acquisition layer to balance the
absorption
capacity and using thinner airlaid non-woven. In this case, the needled non-
woven
has a basis weight between 7° and 150 gsm, SAP fiber proportion is from
5% to
25%. Preferably for low absorption rate, coarse SAP fiber is used.
The present invention is further illustrated by the following examples.
Example 1
The composite absorbent material was made with a needled non-woven acquisition
layer and a needled non-woven absorbent core. The acquisition layer was made
from 3 den polypropylene, with basis weight of 70 gsm, under 40 needling
strokes/cm2 and 9 mm needling depth. A 150 gsm absorbent core was made of
fiber blend of 9 denier super-absorbent fiber, 1.5 denier rayon and 2.5 denier
polypropylene by proportion of 40%, 30% and 30% respectively. The absorbent
core was needled at 80 p/cm2 and 11 mm depth before combining with the
acquisition layer. The needling condition of 50 p/cmz and 7 mm depth was used
in
the combination.
Example 2
Another absorbent material was made of the needled non-woven composite with
3 0 similar structure to example 1. The acquisition layer was made of 6 denier
polyester with the basis weight of 70 gsm. It was pre-needled at 50 p/cm2 and
10
mm depth. The absorbent core was the same as the one in example 1 but with
basis weight of 180 gsm.
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Example 3
Another absorbent material was produced by needled non-woven composite. A 60
gsm acquisition layer was made of 75% 6 denier polyester and 25% 3 denier
polypropylene. The absorbent core was of 170 gsm and the mixture of super-
s absorbent fiber, rayon and polypropylene at the proportion of 30%, 30% and
40%
respectively. The absorbent core was pre-needled before compacted by
traditional
calendering. The two materials were combined by needling at 35 p/cmZ and 8 mm
penetration.
10 Example 4
Another absorbent material was made of needled non-woven acquisition layer
combining with an airlaid non-woven. The acquisition layer used was the same
as
the one in example 3. The airlaid non-woven had the basis weight 165 gsm. The
fiber content included 30% super-absorbent. The rest fibers were viscose pulp
and
thermal bonding fiber. The needling condition used to combine the acquisition
layer
and the airlaid non-woven were 35 p/cmz and 9 mm depth.
Example 5
Another absorbent material was a combination of a needled non-woven as top
layer
and an airlaid non-woven as bottom layer. The top layer non-woven consisted of
25% super-absorbent fiber, 45% 6 den polyester and 30% of 3 den polyester.
The basis weight of the top layer was 120 gsm. In top layer. The airlaid non-
woven
contained 30% super-absorbent fiber, 55% pulp and 15% thermal bonding fiber.
The basis weight of the airlaid non-woven was 95 gsm. The two components were
bonded by needling at 40 p/cm2 and 7.5 mm depth.
Comparison Example 6
This comparison example was one of top sale feminine napkins in the market. It
was made of a perforated cover stock, acquisition layer, absorbent layer and
an
3 0 impermeable back film. The acquisition layer is a pulp sheet (roughly 62
gsm). The
absorbent layer was a c-folded pulp sheet. About 90 gsm super-absorbent powder
was distributed in the interstice of the c-folded absorbent pulp sheet. For
comparison with the above absorbent composite, the cover stock was removed
before conducting test.
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Comparison Example 7
This comparison example was the airlaid non-woven used in the example 5.
Comparison Example 8
The comparison example was a needled non-woven. It consisted of 65% of 1.5
den rayon and 35% of super-absorbent fiber. The basis weight and the density
were 90 gsm and 0.08 g/cm3 respectively.
The examples were tested as follows:
Test liquid
A specific synthetic menstrual liquid was used in the tests. The test liquid
had the
viscosity of 10 cps, surface tension of 70.3 dyn/cm and pH value of 10.5.
Liquid acquisition test
The acquisition test method is a modified EDANA test method 150.3-96 for cover
stock strike through property. The difference is that the current test method
used
Perspex plate with a central oval hole of size 13mm by 30 mm in each
direction.
2 0 10 ml of test liquid was applied to the hole from 5 mm above the sample.
Weights
were exerted to the Perplex plate to give 0.25 psi pressure over the entire
surface
of the sample during the test. The time required for the 10 ml test liquid
being fully
absorbed is called acquisition speed.
2 5 Liquid absorption
The absorption test was conducted on a demand absorption tester. The test
equipment is shown in the Fig.2. It comprises a constant pressure liquid
supply
vessel 1 1, incorporating a pressure compensation device 12 to maintain the
liquid
outlet pressure constant. A sample support 13 incorporating a support plate 14
is
3 0 connected to the liquid supplier 11. The support plate 4 is provided with
a central
hole having a diameter of 13 mm. The pressure compensation device is
adjustable
so that it can provide different outlet pressure. The absorption test used a 0
hydraulic pressure to test absolute demand absorption characteristics of the
material. During the test, samples 15 were placed on the support plate 14.
After
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having been fully saturated, the samples were weighed. The absorbency capacity
is the value of absorbed liquid amount divided by samples' dry weight.
Liquid retention
The retention tests the sample liquid retention ability under certain
pressure. The
sample was saturated on the demand absorbency test rig before being
transferred
to a pile of standard filter papers. A plate with pressure of 1 psi was
applied over
the entire surface of the sample and maintained for 2 minutes. The sample was
weighed to calculate the remaining liquid. The amount of the remaining liquid
divided by the sample dry weight gives the retention capacity.
Wicking speed
The wicking speed was obtained from the demand absorbency test. The time used
by the liquid to fully spread to 100 cmz of the sample was recorded as the
wicking
speed.
Runoff
The runoff test is from internal test method as well. A 3 cm x 3 cm sample was
affixed on to a 45° plane. 2 ml of test liquid was applied on the near
top edge of
the sample in speed of 2 ml per second. The percentage of liquid runoff from
the
surface and edge of the sample is called runoff.
The results that were obtained are the following:
30
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TABLE 1
Examples Acqui- AbsorptionRetentionWicking Runoff
sition capacity capacity speed
speed
Seconds g/g g/g Minutes
Example 9 18.9 11.6 3.3 0
1
Example 4.5 16.0 10.0 7 0
2
Example 4.5 20.8 10.8 5.3 0
3
Example 7.6 19.6 10.1 4.5 0.1
4
Example 6.5 18.0 10.0 6 0
Example 45 12.2 10.8 4 25
6
Example 508 16.2 10.7 17 36.5
7
Example 328 26.0 12.1 20 0
8
5
As it can be seen in Table 1, the invented non-woven absorbent composites had
dramatically increased the acquisition speed. The pulp materials in the
comparison
examples 6 and 7 should have very good absorption ability. The comparison
needled homogeneous non-woven example 8 had relative large porosity. But none
of them could absorb large amount of instant liquid insult in short period.
While
with the invented absorbent composites, three-dimensional structure help to
absorb
large amount of instant liquid insult and distribute them into the absorbent
core in
very short time. This performance is very important for reducing the liquid
runoff
problem.
The absorption capacity results illustrate that the absorbent needled non-
woven
composites had superior absorbency capacity to the commercial product.
The retention capacity test results show that the presented three-dimensional
absorbent composites had similar liquid retention capacity to the commercial
product, single airlaid non-woven and single layer needled non-woven. It
illustrates
that the needled non-woven acquisition layer, although being loose compared
with
the absorbent core, did not diminish the liquid retention capacity of the
whole
system.
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Liquid wicking speed results show that there are apparent difference between
the
multi-layer and single-layer structure. The composite absorbent had fast
liquid
spread speed in x-y plane. This was the contribution of the needled non-woven
acquisition layer, which had large pore size to facilitate high linear and
volume flow
speed. Principally liquid is moved through the porous structure by gravity,
the
hydrostatic head of the liquid, and the drawing of the capillaries. The
acquisition
layer in example 1 relatively best balanced the pore size in capillary drawing
and
flow rate resulting in the fast liquid wicking speed.
Runoff is one of the major problems to be solved for the current sanitary
napkin
products. It can be seen from the Table 1 that none of the comparison examples
could get zero runoff. With the presented absorbent non-woven composites, zero
run-off had been achieved. The success was from the true three-dimensional
absorbents structure. The liquid insult was effectively trapped by the needled
non-
woven acquisition layer and rapidly distributed in three-dimensional way to
the
absorbent core. The z direction wicks and the high capillary drawing of the
absorbent core enhanced the absorption speed. Therefore the runoff was
effectively controlled to zero.
All the presented absorbent non-woven composites had lower rewet than the
comparison samples' rewet. One could easily feel difference between them. The
surface of the insulted absorbent composites were very dry.
