U.S. patent application number 13/396885 was filed with the patent office on 2012-08-16 for absorbent, nonwoven material exhibiting z-direction density gradient.
This patent application is currently assigned to EAM CORPORATION. Invention is credited to Paul M. Ducker, Steven S. Harlen.
Application Number | 20120205061 13/396885 |
Document ID | / |
Family ID | 43307050 |
Filed Date | 2012-08-16 |
United States Patent
Application |
20120205061 |
Kind Code |
A1 |
Ducker; Paul M. ; et
al. |
August 16, 2012 |
ABSORBENT, NONWOVEN MATERIAL EXHIBITING Z-DIRECTION DENSITY
GRADIENT
Abstract
The present material contains density gradients which direct
fluid into the material and distribute it providing more effective
fluid transport and efficient utilization of storage capacity. The
material consists of two regions. In the first region, the material
has a low-density stratum adjacent to one surface, overlaying at
least one higher density stratum adjacent to the opposite surface
of the sheet. These strata create a density gradient in the
thickness direction (Z-direction) of the sheet. The second region
consists of a fluid distribution structure that has a higher
density than at least the lower density of the strata comprising
the first region. The fluid distribution structure is in direct
fluid communication with the adjacent strata in the first region,
along their boundaries.
Inventors: |
Ducker; Paul M.; (Brunswick,
GA) ; Harlen; Steven S.; (Jesup, GA) |
Assignee: |
EAM CORPORATION
|
Family ID: |
43307050 |
Appl. No.: |
13/396885 |
Filed: |
February 15, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12485293 |
Jun 16, 2009 |
|
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13396885 |
|
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Current U.S.
Class: |
162/109 |
Current CPC
Class: |
B32B 2307/50 20130101;
B32B 2262/067 20130101; A61F 13/533 20130101; B32B 2555/00
20130101; B32B 37/0076 20130101; B32B 2262/065 20130101; B32B 5/22
20130101; B32B 2307/726 20130101; B32B 5/26 20130101; B32B 2262/062
20130101; B32B 5/142 20130101; B32B 7/02 20130101; B32B 2307/72
20130101; B32B 3/263 20130101; B32B 2307/54 20130101; Y10T 156/10
20150115; B32B 5/022 20130101; B32B 2307/718 20130101; B32B 2250/20
20130101 |
Class at
Publication: |
162/109 |
International
Class: |
D21H 25/04 20060101
D21H025/04 |
Claims
1-12. (canceled)
13. A method making an absorbent, nonwoven material exhibiting a
Z-direction density gradient, comprising the steps of: providing a
cellulosic tissue layer; depositing cellulosic material on said
tissue layer; compacting said cellulosic material to form a fibrous
matrix of a first, relatively low-density stratum; providing a
second, relatively high-density stratum with hydrogen bonding
formed by applying heat and pressure providing a stable density,
comprising another fibrous matrix, on said first stratum; and
compacting said first and second strata by applying heat and
pressure in a defined pattern, to form an absorbent material with a
Z-direction density gradient, including forming a
liquid-distribution network extending through the entire thickness
of the material, and laterally adjacent to at least one portion of
said low-density and high-density strata, wherein said cellulosic
tissue layer is bonded to said liquid-distribution network for
integrating said absorbent material against delamination, and for
inhibiting release of the fibrous material of said first,
low-density stratum.
14. A method making an absorbent, nonwoven material in accordance
with claim 12, including forming said liquid-distribution network
with at least one longitudinally extending densified region.
15. A method making an absorbent, nonwoven material in accordance
with claim 12, including forming said liquid-distribution network
to comprise a land-and-sea densified region.
16. A method making an absorbent, nonwoven material in accordance
with claim 12, including forming at least one of said fibrous
matrices as a blend of cellulosic fibrous material and
superabsorbent polymeric material.
17. A method making an absorbent, nonwoven material in accordance
with claim 12, wherein said step of providing said high-density
stratum includes forming and compacting said high-density stratum
separately from said low-density stratum, and thereafter
positioning said high-density stratum on said low density
stratum.
18. A method of making an absorbent, nonwoven material in
accordance with claim 17, using an airlaid formation apparatus,
comprising a single formation section, and a single bonding
calender or multiple bonding calenders.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/485,293, filed Jun. 16, 2009.
TECHNICAL FIELD
[0002] The present invention relates generally to disposable
absorbent materials that have structures suitable for fluid
(liquid) intake, fluid storage and fluid distribution, and that
exhibit a high level of absorbency, and more particularly to an
improved, nonwoven material exhibiting a Z-direction density
gradient, comprising a first, relatively low-density stratum, and a
second, relatively high-density stratum, which are integrated and
stabilized by hydrogen bonding, without resort to binder
compositions or synthetic fibers, with the resultant material
exhibiting desirably high tensile strength, and resistance to
delamination. The absorbent material can be used in absorbent
articles such as feminine hygiene products, incontinence products,
and disposable diapers.
BACKGROUND OF THE INVENTION
[0003] Disposable absorbent articles, such as diapers, feminine
hygiene products, adult incontinence devices and the like have
found widespread acceptance. To function efficiently, such
absorbent articles must quickly absorb body liquids, distribute
those liquids within and throughout the absorbent article, and be
capable of retaining those body liquids when placed under
loads.
[0004] While the design of individual absorbent articles varies
depending on use, there are certain elements or components
typically common to such articles. The typical absorbent article
includes a liquid pervious top sheet or facing layer, which facing
layer is designed to be in contact with a body surface. The facing
layer is made of a material that allows for the unimpeded transfer
of fluid from the body into the core of the article. The facing
layer should not absorb fluid per se and, thus, should remain dry.
The typical article further includes a liquid impervious back sheet
or backing layer disposed on the outer surface of the article and
which layer is designed to prevent leakage of fluid out of the
article.
[0005] Disposed between the facing layer and backing layer is an
absorbent member referred to in the art as an absorbent core. The
function of the absorbent core is to absorb and retain body fluids
entering the absorbent article through the facing layer. Because
the origin of body fluids is localized, it is necessary to provide
a means for distributing liquid throughout the dimensions of the
absorbent core. This is typically accomplished either by providing
a distribution member in the core and/or by altering the
composition or structure of the absorbent core per se.
[0006] Since fluids can be presented more quickly to the absorbent
product than the absorbent core can absorb, a distinct acquisition
structure is frequently employed to capture liquid more quickly
than it is added and retain it long enough for remainder of the
core to absorb it out of the acquisition structure. This transfer
of fluid to the core restores the capacity of the acquisition
structure for subsequent fluid insults. It is advantageous for the
core to absorb fluids more quickly as the acquisition structure can
be smaller.
[0007] The absorbent core is frequently formulated of a cellulosic
wood fiber matrix or pulp, which pulp is capable of absorbing large
quantities of fluid. Fluid absorption and retention properties can
be enhanced by disposing superabsorbent materials in amongst the
fibers of the wood pulp. Superabsorbent polymeric materials (SAP)
are well known in the art as substantially water-insoluble,
absorbent polymeric compositions that are capable of absorbing
large amounts of fluid in relation to their weight and forming
hydrogels upon such absorption. Absorbent articles containing
blends or mixtures of pulp and superabsorbents are known in the
art. An example of this is taught in U.S. Pat. No. 3,670,731
(Harmon).
[0008] Fluid can be preferentially directed through an absorbent
core by use of wettability and or density gradients existing in the
core structure. Various embossed structures employ high-density
wicking lines or other continuities that due to their density
attract liquid from the adjacent core and then direct it away from
the point of insult.
[0009] U.S. Pat. No. 4,781,710 (Megison et al) teaches an absorbent
pad with low-density "tuft" regions, designed to quickly imbibe
liquid in fluid communication with very dense fluid transport
channels, which transport liquid away from the tuft regions, and
enclosed by the transport channels are storage structures of a
medium density. While this provides three differentiated structures
to accomplish the various functions, the structures are coplanar,
and do not direct fluid from the fluid receiving side of the core
to the opposite face in the thickness direction.
[0010] Fluid can be transported away from the fluid receiving
surface of an absorbent core by providing a density gradient in the
thickness direction. U.S. Pat. No. 5,525,407 (Yang) provides a core
with a density and wettability gradient in the thickness direction.
The low-density layer on the fluid receiving side acquires liquid
and directs it to higher density layers below. The fluid receiving
side becomes desirably drier, and the denser layers below also
spread the liquid in the in-plane directions utilizing portions of
the core that are not directly beneath the point of fluid
insult.
[0011] This technology produces the density gradients by forming
various strata using a device called a transverse webber and
differentiating the fiber types in each stratum using various
blends of synthetic fibers or binder fibers and cellulosic fibers
in different proportions. Ovens are used to activate the binding
fibers followed by heated calenders. The synthetic fibers are
expensive, they are not hydrophilic, and the ovens required to bond
the material at high speeds are energy and capital intensive.
[0012] Absorbent materials made using commercial multibonded
airlaid technology provides a method of manufacturing pre-formed
structured absorbent cores to the absorbent article converting
process. The use of pre-formed structured cores increases the
efficiency of the converting operation by taking the complexity of
forming or combining several core structures off of the converting
machine. U.S. Pat. No. 6,420,626 (Erspamer, Buckeye) teaches a
pre-formed unitary multibonded airlaid core with differentiated
acquisition, fluid storage, and fluid transport strata with
associated density gradients in the thickness direction. As in the
previous example, this technology requires the use of expensive
synthetic fibers and the requirement for large capital and
energy-intensive bonding ovens to bond the material.
[0013] U.S. Pat. No. 5,866,242 (Tan) teaches an airlaid material,
sometimes referred to as a hydrogen bonded airlaid, comprising
cellulosic fibers, and optionally superabsorbent polymer that is
bonded using heat and pressure to form hydrogen bonds. No synthetic
binder fibers, heat fusible thermoplastics or other chemical
binders are used. In commercial practice, a heated calender roll
applies the pressure and heat required to form hydrogen bonds
between the fibers. Compared to the multibonded airlaid process
previously cited, this bonding arrangement is much simpler to
operate, has significantly less energy consumption, and requires
much less capital expenditure than the bonding ovens used in the
multibonded airlaid process. Additionally no synthetic binder
fiber, fusible thermoplastic materials or chemical binders are
required which would add cost to the material and are non-absorbent
components. These binders can restrict the swelling of the SAP
particles reducing their absorbency.
[0014] While the hydrogen bonded airlaid process and material as
taught by Tan is very simple and cost-effective, the process does
not produce strong density gradients in the thickness direction.
Therefore, it would be desirable to have a hydrogen bonded airlaid
material comprising only cellulose and optionally SAP with no
chemical binders or synthetic bonding fibers, that has good sheet
integrity, minimal fiber dusting, and a strong density gradient in
the thickness direction.
SUMMARY OF THE INVENTION
[0015] The present invention relates to an absorbent material that
can be used as an absorbent core in absorbent articles such as
sanitary napkins, pantiliners, incontinence products or disposable
diapers. The material of the present invention is a nonwoven sheet,
consisting of cellulosic fibers and SAP, containing no binders,
latexes, etc, relying on hydrogen bonding to produce the necessary
structure.
[0016] The material contains density gradients which direct fluid
into the material and distribute it providing more effective fluid
transport and efficient utilization of storage capacity. The
material comprises two regions. In the first region, the material
has a low-density stratum adjacent to one surface overlaying at
least one higher density stratum. These strata create a density
gradient in the thickness, Z-direction of the sheet. The second
region includes a fluid distribution structure that has a higher
density than at least the lower density one of the strata
comprising the first region, and extends through the entire
thickness of the sheet. The fluid distribution structure is in
direct fluid communication with both the adjacent strata in the
first region, along their boundaries.
[0017] In another aspect of the invention, the density ratio in the
thickness direction is greater than 1.2:1. In another aspect of the
invention, the types of cellulose comprising the various strata in
the Z-direction can be differentiated. In another aspect of the
present invention, the droplet absorption time differs between the
two surfaces of the sheet with a droplet absorption time
ratio>1.5:1. In another aspect of the present invention, the
sheet has an effective containment mechanism for fibers in the
low-density stratum in the first region to prevent dusting.
[0018] In another aspect of the present invention, the material has
a tensile strength of at least 10 (N/50 mm) providing useful sheet
integrity for use in converting processes to make absorbent
articles. In another aspect of the present invention, liquid
spreads in the X-Y direction in the various core structures to an
extent according to their density. In a preferred embodiment, the
material is produced using a hydrogen bonded airlaid process using
heated calenders to provide the heat and pressure to effect the
bonding.
[0019] In accordance with the illustrated embodiments, the present
absorbent, nonwoven material comprises a first, relatively
low-density stratum comprising a fibrous matrix of cellulosic
fibrous material substantially free of synthetic fibers and binder
compositions. The present material further comprises a second,
relatively high-density stratum, juxtaposed to the first stratum in
liquid-transferring relationship therewith. The second stratum
comprises a fibrous matrix of cellulosic fibrous material
substantially free of synthetic fibers and binder compositions.
[0020] The absorbent material further includes a
liquid-distribution network extending substantially through the
Z-direction of the absorbent material, which is formed from the
first stratum and the second stratum. The liquid-distribution
network thus extends substantially through the entire thickness of
the material, and is provided so as to be laterally adjacent to at
least some portions of the first, low-density stratum and the
second, high-density stratum in liquid-transferring relationship
therewith.
[0021] The present material further includes a cellulosic fiber
tissue layer positioned on top of the low-density stratum, which
tissue layer is bonded to the liquid-distribution network for
integrating the absorbent material against delamination, and for
inhibiting release of the fibrous material of the low-density
stratum.
[0022] Notably, the present material is integrated and stabilized
by hydrogen boding, formed by the application of heat and pressure,
to thereby provide a nonwoven sheet with a machine direction (MD)
tensile strength of at least 10 (Newtons/50 millimeter wide sample)
and a vertical delamination strength of greater than 5N, with
hydrogen boding serving to stabilize the density of the strata and
the density gradient, and to stabilize the integrity of the
liquid-distribution network and bonded tissue layer. Without being
bound to any particular theory, it is believed that the hydrogen
bonding between a portion of the fibers in the cellulosic fiber
matrix are configured so as to hold it in a state of compression in
the thickness direction. The resiliency of the remainder of the
fibers pushes back against these compressive forces forming an
equilibrium density. It is believed that this is a distinctive
characteristic of hydrogen bonding formed by using a heated
calender to form bonds on an airlaid cellulosic web since the web
is compressed to form the bonds, but bonds are only formed between
some of the fibers, and the rest rebound against those bonds
according to their resiliency. It is believed that the beneficial
effect of this equilibrium is that when external mechanical forces
are applied to the structure such as compression, the tension and
resiliency in the structure makes it tend to spring back to its
equilibrium density. A distinctive feature of the material of the
present invention is that the integrity of the densities of each of
the various structures in the material of the present invention are
in this way believed to be maintained as the material is formed
into roll or festooned packages, converted into absorbent products,
and manipulated during end use. In this way, it is believed the
desirable functional aspects of these densities are likewise
maintained.
[0023] In the preferred embodiment of the present invention, the
absorbent material exhibits an apparent Z-direction density
gradient greater than 1.1:1. In one embodiment, the
liquid-distribution network comprises at least one longitudinally
extending densified region. In an alternative embodiment, the
liquid-distribution network comprises a land-and-sea densified
region.
[0024] The liquid-distribution network of the present absorbent
material comprises between about 5% and 50% of the surface area of
the absorbent material, more preferably, between about 10% and 35%
of the surface area of the absorbent material.
[0025] While it is within the purview of the present invention that
the present absorbent material be formed only from cellulosic
fibrous material, such as comminuted wood pulp, at least one of the
strata of the absorbent material can include superabsorbent
polymeric material. In such an embodiment, the absorbent material
can be provided with a basis weight of about 100 to 2000 gsm (grams
per square meter), and comprise between about 0% and 70%, by
weight, of superabsorbent polymeric material. The low-density
strata can be provided with a density in the range of 0.08 g/cc
(grams per cubic centimeter), to about 0.30 g/cc, and the
high-density strata provided with a density in the range of about
0.25 g/cc to 0.50 g/cc.
[0026] More preferably, when the present material includes
superabsorbent polymeric material, the absorbent material can be
provided with a basis weight of about 150 to 1000 gsm, and
comprises about 10% and 55%, by weight, of the superabsorbent
polymeric material. The low-density strata can be provided with a
density in the range of about 0.10 g/cc to 0.17 g/cc, and the
high-density stratum provided with a density in the range of about
0.25 g/cc to 0.40 g/cc.
[0027] In the preferred embodiment, the high-density stratum of the
present absorbent material includes another cellulosic tissue layer
at the lower surface thereof, with the cellulosic tissue layers
being bonded together along the liquid-distribution network of the
material.
[0028] It is within the purview of the present invention that the
fibrous material of the low-density stratum is different than the
fibrous material of the high-density stratum.
[0029] In accordance with the disclosed testing protocols, the
absorbent material in accordance with the present invention has a
droplet absorption time ratio of greater than or equal to
1.5:1.
[0030] A method of making the present absorbent, nonwoven material
comprises the steps of providing a cellulosic tissue layer, and
depositing cellulosic material on the tissue layer. The cellulosic
material is compacted to form a fibrous matrix of a first,
relatively low-density stratum.
[0031] The present method further includes providing a second,
relatively high-density stratum with hydrogen bonding formed by
applying heat and pressure providing a stable density, with the
second stratum comprising another fibrous matrix, and with the
second stratum provided on the first stratum.
[0032] The method further comprises compacting the first and second
strata by applying heat and pressure in a defined pattern, to form
an absorbent material with a Z-direction density gradient.
Formation includes forming a liquid-distribution network extending
through the entire thickness of the material, which network is
laterally adjacent to at least one portion of the low-density and
high-density strata. The cellulosic tissue layer of the material is
bonded to the liquid-distribution network for integrating the
absorbent material against delamination, and for inhibiting release
of the fibrous material of the first, low-density stratum. In
accordance with illustrated embodiments, the liquid-distribution of
the material is formed with at least one longitudinally extending
densified region, or alternatively, is formed to comprise a
land-and-sea densified region. As noted, at least one of the
fibrous matrices of the present absorbent material can be provided
in the form of a blend of cellulosic fibrous material and
superabsorbent polymeric material.
[0033] In the preferred method of making the present absorbent
material, the step of providing high-density stratum includes
forming and compacting the high-density stratum separately from the
low-density stratum, and thereafter positioning the high-density
stratum on the low-density stratum.
[0034] In the most preferred form, the absorbent material is formed
using an airlaid formation apparatus, comprising a single formation
section, and a single bonding calender positioned downstream of the
single formation section.
[0035] Other features and advantages of the present invention will
become readily apparent from the following detailed description,
the accompanying drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a diagrammatic, cross-sectional view of an
absorbent, nonwoven material exhibiting a Z-direction gradient
embodying the principals of the present invention; and
[0037] FIG. 2 is a diagrammatic, cross-sectional view of an
alternative embodiment of the present absorbent, nonwoven
material.
DETAILED DESCRIPTION
[0038] While the present invention is susceptible of embodiment in
various forms, there is shown in the drawings and will hereinafter
be described presently preferred embodiments of the invention, with
the understanding that the present disclosure is to be considered
as an exemplification of the invention, and is not intended to
limit the invention to the specific embodiments illustrated.
[0039] The present invention provides a novel absorbent nonwoven
material suitable for use in absorbent articles that comprises
cellulosic fibers and optionally superabsorbent polymer (SAP) with
no synthetic fibers or chemical bonding agents. The density
structure of the material is maintained through hydrogen bonding
and provides improved absorbency characteristics by directing the
flow of liquid through the structure along the density gradients.
Included in the structure is a strong density gradient in the
thickness direction over at least a portion of the surface.
[0040] Referring to FIG. 1, cellulosic fibers (51, 53, and 54) are
enclosed between two sheets of cellulosic tissue on top (52) and
bottom (not shown). The material is divided into two regions, which
in this particularly preferred embodiment alternate in a parallel
pattern, including at least one longitudinally extending densified
region, which preferably extends in the machine direction (MD) of
the material. Any number of patterns may be suitable, however,
depending on the desired physical and absorbent characteristics of
the sheet. The first region comprises a stratum of relatively
low-density fibers (54) adjacent to one face of the sheet
overlaying at least one higher density stratum of more heavily
bonded fibers (51), forming a density gradient in the thickness
direction. The low-density fibers (54), many of which are largely
unbonded, are contained by the top tissue (52) to prevent dusting
during handling. In the second region, cellulosic fibers bonded to
a relatively high density extend through the entire thickness of
the material, and are in fluid communication both strata (51, 54)
of the first region at the boundary between the two regions. The
top tissue (52) is strongly bonded to the fibers 51 in the second
region providing effective delamination strength for the tissue
layer. Any of the fiber structures 51, 53, or 54 can optionally
contain superabsorbent polymeric material SAP granules (not shown).
In this preferred embodiment, the high-density bonded second region
extends in the machine direction of the sheet, providing a strong
preferential longitudinal wicking of liquid. In alternative
embodiments, the second region can be in a series of discrete
disconnected shapes, with the second region providing little
wicking in the in-plane directions or in an alternative embodiment,
shaped to have continuities in both the length and width direction
providing in-plane wicking in both directions. The second region
can comprise between 1% to 90% of the surface, but is desirably
between 5% and 40% of the surface area, and more desirably between
15% and 35% of the surface.
[0041] Cellulosic fibers that can be used in the process of the
present invention are well known in the art and include wood pulp,
cotton, flax, and peat moss. Comminuted wood pulp is usually
preferred. Pulps can be obtained from mechanical or
chemi-mechanical, sulfite, kraft, pulping reject materials, organic
solvent pulps, etc. Both softwood and hardwood species are useful.
Softwood pulps are preferred. The pulp is most desirably provided
in a package that can be processed by the airlaid equipment used to
create the material of the present invention.
[0042] In another aspect of the present invention, the cellulosic
fibers used to create the upper and lower strata that form the
gradient can be differentiated in order to enhance the
effectiveness of the gradient. One example of this would be to use
cellulosic fibers, at least some of which have been made by a
process that includes the step of treating a liquid suspension of
pulp at a temperature of from about 15.degree. C. to about
60.degree. C. with an aqueous alkali metal salt solution having an
alkali metal salt concentration of from about 2 weight percent to
about 20 weight percent of the absorbent material.
[0043] Superabsorbent polymers (SAP) are well known in the art. As
used herein, the term "superabsorbent polymeric material" means a
substantially water-insoluble polymeric material capable of
absorbing large quantities of fluid in relation to their weight.
The superabsorbent material may be in the form of particulates,
fibers, flakes, granules, or aggregates. Exemplary and preferred
superabsorbent materials include salts of cross-linked polyacrylic
acid such as sodium polyacrylate. Superabsorbent materials are
commercially available (e.g., from Stockhausen GmbH, Krefeld,
Germany). A wide range of types of are used in various disposable
absorbent products; the appropriate grade depends very much on the
required absorbency characteristics of the end use article. Those
skilled in the art can select the optimal grade for the particular
end use design.
[0044] The absorbent material of the present invention can
incorporate an optional carrier tissue, and another optional tissue
layer on top of the web. Suitable tissue materials for use are well
known to those of ordinary skill in the art. Preferably, such
tissue is made of bleached wood pulp and has an air permeability of
about 273-300 CFM. Tissue for use in air-laying absorbent materials
is commercially available (e.g. From Cellu Tissue in East Hartford,
Conn.).
[0045] The absorbent material of the present invention can be
configured in a uniform manner or can be configured with many
strata of differing compositions of cellulose and/or
superabsorbent. Those skilled in the art of making airlaid
absorbent materials can design the optimal configuration for any
given end-use product application. A preferred material may be
configured with a top tissue and a carrier tissue, and have a
substantially uniform homogeneous mix of cellulosic fibers and
SAP.
[0046] A preferred method of producing the material of the present
invention is to use the hydrogen bonded airlaid process. For
purposes of this patent, a hydrogen bonded airlaid material is any
nonwoven comprising cellulosic fibers and optionally superabsorbent
polymer that is formed by suspending individualized fibers in an
air-stream and depositing them in an undensified web by sending
them through the forming heads of an airlaid web forming machine.
Then hydrogen bonds are formed in the material.
[0047] Examples of several airlaid web forming machines are
described in detail in U.S. Pat. No. 5,527,171 (Soerensen), hereby
incorporated by reference. The forming heads may include rotating
or agitated drums which serve to maintain fiber separation until
the fibers are pulled by a vacuum onto a foraminous condensing drum
or foraminous forming conveyor (or forming wire). Where multiple
defined strata are desired, such as those having different
compositions or densities, separate forming heads may be used to
sequentially form each stratum on top of the stratum previously
formed. As the fibers are airlaid the resulting structure is
densified and the fibers are bonded together. by applying heat and
pressure to the web to form hydrogen bonds, increasing the density
and strength of the material compared to its undensified state. No
chemical or thermoplastic binder materials are used. A preferred
method of applying heat and pressure is to use a heated calender
roll. Optionally, a cellulosic carrier tissue and a cellulosic top
tissue can be used, which will bond and integrate into the web
under this process.
[0048] In a preferred method to form the material of the present
invention, cellulosic fibers are defiberized in a hammermill and
deposited onto a moving forming wire covered with a carrier tissue.
Superabsorbent polymer is optionally metered and blended in the
forming head.
[0049] This web is then densified using a heated calender with a
flat surface at a preferred temperature of >100 C but more
preferably in the range of 150-200 C and a pressure necessary to
obtain the desired density of the bottom stratum.
[0050] Additional cellulosic fibers and optionally SAP are
deposited by a forming head onto a moving wire covered with a
carrier tissue. The web thus formed is then combined with the first
web at an embossed calender station. In a preferred embodiment, the
embossed calender station has a temperature >100.degree. C. and
preferably in the range of 150.degree.-200.degree. C. and a
pressure necessary to obtain the desired density and bonding in the
embossed regions of the material.
[0051] In accordance with the preferred method of practicing the
present invention, a cellulosic tissue layer is provided, on which
cellulosic material is deposited, and compacted to form a fibrous
matrix of a first, relatively low-density stratum. A second,
relatively high-density stratum is thereafter provided on the first
stratum, which second stratum comprises another fibrous matrix.
Hydrogen bonding formed by applying heat and pressure provide a
stable density.
[0052] The first and second strata are compacted by applying heat
and pressure in a defined pattern, to form an absorbent material
with a Z-direction density gradient, including forming a
liquid-distribution network extending through the entire thickness
of the material. The liquid-distribution network is provided
laterally adjacent to at least one portion of the low-density and
high-density strata, with the cellulosic tissue layer bonded to the
liquid-distribution network for integrating the absorbent material
against delamination, and for inhibiting release of the fibrous
material of the first, low-density stratum.
[0053] Preferably, the step of providing the high-density stratum
includes forming and compacting the high-density stratum separately
from the low-density stratum, and thereafter positioning the
high-density stratum on the low-density stratum. Since a typical
airlaid apparatus has a single forming section, and it is desirable
to avoid additional capital to reconfigure the machine, one method
of forming the two strata is to form them one alongside the other
on the same forming section, slit them apart into two webs, and
then combine them after calendering one web first. A calender with
an embossed pattern on one half of the roll, along with appropriate
web routing can be used to produce the desired configuration with a
single calender. Another approach using a single forming section is
to form part of the web and then remove it from the wire at it's
midpoint and then form the second stratum on the remainder of the
wire. While there are many approaches, it is preferable to use an
airlaid formation apparatus, comprising a single formation section,
and a single bonding calender positioned downstream of the single
formation section.
[0054] The embossed, densified regions can be provided in the form
of a "land and sea" pattern of highly bonded areas surrounded by
essentially non-bonded areas, or alternatively, more sophisticated
patterns comprising intermediate densities or gradients. A
particularly advantageous pattern comprises parallel lines of high
density, oriented in the longitudinal direction, (Illustrated in
FIG. 1) which preferentially distributes liquid along the
orientation of the lines, which can be useful to avoid side leakage
in an absorbent product. Useful patterns that maximize the intake
function have land-sea patterns less than 50% bonded, and more
preferably less than 30% bonded, but desirably more than 10%
bonded, and generally can yield materials with useful mechanical
integrity.
[0055] The material of the present invention has a basis weight of
100-2000 gsm, and more desirably in the range of 150-1000 gsm. The
SAP content can range from 0-70%, but is more desirably in the
range of 10-55% The density of the low-density stratum, as
indicated by the basis weight and density procedure described below
can range from 0.08 g/cc to 0.30 g/cc, with a more desirable range
being from 0.10 g/cc to 0.17 g/cc. The apparent density of the high
density stratum as indicated by the procedure below can range from
0.25 g/cc to 0.50 g/cc, with a more desirable range of 0.25-0.40
g/cc. The thickness direction density gradient in the first region,
defined by the ratio of the densities of these two strata, is
preferably in the range of 1.2-5.0, and even more preferably in the
range of 1.5 to 2.5. The density of the material in the second
region, as measured by the small area density test procedure
described below, is ideally in the range of 0.30 g/cc to 1.2 g/cc,
and more preferably 0.60 g/cc to 0.95 g/cc.
[0056] The embossed second region desirably comprises on average
5-50% of the material surface, more desirably comprising on average
10-35% of the surface area and more desirably comprises a repeating
pattern, although this is not a requirement for the material of the
present invention. The pattern of embossments can have no lateral
continuity (as in discreet dots, or other geometric shapes), can
have lateral continuity in one direction, (such as parallel lines,
non-intersecting zigzags, etc) or have lateral continuity in both
directions (outlines of diamonds, squares, hexagons, etc). FIG. 1
illustrates a second region comprising pattern of higher-density
parallel lines, while FIG. 2 illustrates a material with a first
region comprising lower-density circles surrounded by the higher
density second region. The pattern of embossment can also be
designed to correspond to the shape of the core in the finished
absorbent article, being used in a phased manner to provide
particular properties such as mechanical strength or fluid
transport in particular locations in the finished product where
they are most suitable.
[0057] In one attempt by the applicants to produce a hydrogen
bonded material comprising only pulp and SAP with a density
gradient in the thickness direction, an airlaid mat of fluff and
SAP was formed and then was run through a calender nip, with only
the bottom roll of the nip heated. A mild density gradient was thus
formed under process conditions that did not appreciably bond the
fibers of the lower density top stratum. The unbonded fibers on
this surface created unacceptable clouds of fiber dust when the web
was handled. A top tissue layer to contain the dust would not bond
to the web unless higher pressures were applied that caused the
density gradient to disappear. The material of the present
invention solves this problem in a very practical manner by having
strong bonds available in the dense second region attach a top
tissue layer in a robust manner to the web which then provides very
effective containment of the fiber in the low-density stratum of
the first region. The density of the second region required to form
these effective bonds is dependent on the percent of the surface
area that the second region comprises.
[0058] The prior art literature contains references to cores with
density gradients in the thickness direction exhibiting greater
wicking in the X and Y directions (in-plane) on the side of the
sheet that has the higher density than the side of the sheet with
the lower density. An example of this is in U.S. Pat. No. 5,525,407
(Yang). The material of the present invention clearly exhibits
similar behavior, with liquid added at one location being spread
furthest in the high density wicking lines of the second region,
followed by the higher density stratum in the first region, with
the least amount of spreading observed in the low-density stratum
of the first region. As indicated in the prior art literature, this
property is useful in hiding unsightly stains in various sanitary
napkin and pantiliner products.
[0059] This behavior was quantified for the material of Example 1,
by cutting a 12-inch.times.1-inch strip of material, suspending it
vertically with one end in a pan of 0.9% saline solution and
measuring the vertical wicking height after 30-minutes. The wicking
height was the average of the highest vertical extent of liquid and
the lowest vertical extent of liquid seen in the one-inch strip,
within each density region.
[0060] The average vertical wicking height for the lower density
stratum in the first region was 11.7 cm. The average vertical
wicking height for the higher density stratum in the first region
was 16.4 cm. (as viewed on the other side of the sample strip) The
average vertical wicking height for the parallel embossed lines in
the second region was 17.9 cm. The difference in vertical wicking
height between the two strata in the first region as viewed on
opposing sides of the sample strip is believed to be the effect of
the density gradient in the sheet thickness direction in this
region.
[0061] An unexpected additional benefit of the density gradient was
observed when small amounts of viscous liquid were applied to the
material of the present invention, according to the droplet
absorption time test procedure outlined below. Since the small
measured amounts of liquid added do not fully saturate the
material, there is a strong component of fluid flow in the
thickness direction, along the axis of the density gradient of the
present invention, quantifying its effects on the rate of fluid
flow through the thickness of the material. It was found that
measured droplets applied to one side of the sheet were absorbed
more slowly than similar amounts of liquid applied to the other
side of the sheet. Without being bound by any particular theory, it
is believed that the density gradient in the thickness direction is
drawing the liquid into the sheet in a preferred direction, with
the tendency to enhance flow rate from the low-density stratum to
the higher-density stratum. Additionally, it was found that
materials with similar basis weight and features as the material of
the present invention, but without the density gradient in the
thickness direction, exhibited none of the asymmetry of absorption
time, one side versus the other, and the speed of absorption was
slower than that of the material of the present invention in the
direction believed to be enhanced by the density gradient in the
thickness direction.
[0062] Since absorbency rate is critical to avoiding leakage, this
property of the material of the present invention can be very
useful in making improved absorbent cores for absorbent articles,
particularly those that handle more viscous liquids such as
sanitary napkins.
[0063] Test Procedures:
[0064] Basis weight and density: A 300 mm.times.200 mm hand sheet
of sample material is cut using an Atom model SE 20C die press from
Associated Pacific Company of Camarillo Calif. using an
appropriately sized cutting die. The sample is weighed on a lab
balance readable to 0.001 g. The sample is then placed in an Emveco
Microgauge, with a foot pressure of 0.7 psi. Caliper is measured in
6 locations about the sheet and the average is taken using the
average function.
[0065] The basis weight in grams per square meter is divided by the
caliper in millimeters and this is divided by 1000 to yield density
in units of grams per cubic centimeter.
[0066] Z-Direction Density Ratio:
[0067] A sample of web is taken immediately after the smooth
calender, but prior to any additional fiber being added, with a
stable process operating at constant speed, which is the intended
operation for a commercial airlaid process. This material becomes
the higher density stratum in the first region, when the process is
completed. Taking the sample at this point allows this stratum to
be measured without having to separate it from the remainder of the
material. 200 mm.times.300 mm samples (quantity: 10) are taken from
this material and the basis weight and density test listed above is
performed. The average value is representative of the
structure.
[0068] With the process still operating at the same constant speed
and stable state, a sample of the finished material is taken. 200
mm.times.300 mm samples (quantity: 10) are taken and again the
basis weight and density test is performed on these samples and
average values recorded.
[0069] Calculations:
[0070] Average measurements were taken for the basis weight and
caliper of the entire sheet, and for the basis weight and caliper
of the higher density stratum, taken directly after it is bonded by
the smooth calender in the process but before any additional fiber
is added.
[0071] The following calculations are then done to calculate the
basis weight and caliper of the lower density stratum, which cannot
be separated to be measured by itself:
average basis weight(entire sheet)-average basis weight(high
density stratum)=basis weight(low density stratum)
average caliper(entire sheet)-average caliper(high density stratum
caliper(low density stratum)
[0072] The density of the high density stratum and the low density
stratum are then calculated:
density(high density stratum g/cc)=basis weight(high density
stratum, gsm)/caliper(high density stratum, mm)/1000
density(low density stratum g/cc)=basis weight(low density stratum,
gsm)/caliper(low density stratum, mm)/1000
[0073] Finally, the density ratio between the high density stratum
and the low density stratum is calculated.
density(high density stratum, g/cc)/density(low density stratum,
g/cc)=Z-direction density ratio
[0074] Small Area Density:
[0075] The small dense areas of the second region are generally too
small to be measured using the Emveco Microgauge. Therefore, an MHC
brand mechanical dial indicator gauge on a magnetic base or the
equivalent with a dial indicator readable to 0.001 inch and a
0.09-inch ball end probe was used. When the dial indicator was
allowed to push downwards vertically on the weigh pan of a
Sartorius lab balance, 73 g was the reading. The dial indicator is
placed on a flat metal surface and the magnetic base is engaged.
The mounting bracketry is then adjusted for the dial indicator to
be oriented vertically with the ball in contact with the smooth
metal surface. The bezel is then rotated to yield a zero reading.
Caliper readings are taken by lifting the probe and placing a
sample beneath it so that the probe rests on the dense portion of
the second region. The caliper of is read directly in
millimeters.
[0076] Sampling for Small Area Density Testing:
[0077] A 300 mm.times.200 mm hand sheet of sample material is cut
using an Atom model SE 20C die press from Associated Pacific
Company of Camarillo Calif. using an appropriately sized cutting
die. The sample is weighed on an electronic lab balance (Sartorius,
or the equivalent) readable to 0.001 g. The weight is divided by
the area to yield a calculated average basis weight in grams per
square meter. Then 5 caliper readings are taken using the dial
indicator gauge according to the above procedure, taken from
equivalent locations in the second region. (in the case of the
example materials, the second region consists of parallel lines
embossed by a process intended to yield a constant caliper. The
caliper readings were taken along the centerline of these parallel
lines) The basis weight for the sheet in grams per square meter is
divided by the average of the 5 caliper measurements in millimeters
and divided by 1000 to yield the average density of the
readings.
[0078] Tensile:
[0079] A 240 mm.times.50 mm sample is cut using an Atom Model SE
20C die press from Associated Pacific Company of Camarillo, Calif.
and an appropriately sized cutting die. A tensile test is then done
on the strip using a Zwick Model Z005 tensile tester from
Zwick/Roell in Ulm, Germany, or the equivalent. The test starts at
a 200 mm jaw separation. The sample is placed in the jaws and the
force is zeroed. The tester program then applies 2N pre-load to the
strip at a rate of 100 mm/min and then proceeds to pull the sample
at a rate of 100 mm/minute until failure, recording the maximum
force in Newtons per the 50 mm wide test strip.
[0080] Droplet Absorption Time:
[0081] This test measures the time required for the capillary
action of the core to draw a measured dose of a viscous liquid into
it. This test uses a test fluid consisting of 0.9% saline solution
(available as a prepared solution from Lab Chem, of Pittsburgh,
Pa., Catalog No. 07933), with enough Sodium Carboxymethylcellulose
(Hercules Chemical, Type 7a) fully dissolved to yield a homogeneous
solution with a stable viscosity of 30+/-2 centipoise, measured
with a Brookfield Syncro-Lectric viscometer at 75 degrees C.
[0082] A 300 mm.times.200 mm sample of material is cut using an
Atom model SE 20C die press from Associated Pacific Company of
Camarillo Calif. using an appropriately sized cutting die. The
sample is weighed on a lab balance readable to 0.001 g. The weight
is divided by the area of the sample in square meters to yield the
basis weight in grams per square meter. The dose of test fluid in
cubic centimeters required is the basis weight of the sheet
multiplied by a factor of 0.00044. The calculated dose of test
fluid is drawn up into a 1 cc tuberculin syringe without needle
(available from BD Medical of Franklin Lakes, N.J. Catalog No
309602) with graduations readable to 0.01 cc. The end of the
syringe is held one droplet diameter above the material sample
oriented vertically and the dose is dispensed in about 1 second (to
avoid squirting and unduly spreading the droplet over a larger
surface). At the beginning of the dosing, a stopwatch is started
and the time required for the droplet to completely absorb into the
sheet is recorded. The end point is when the last specular liquid
surface disappears into the material.
[0083] Sampling for the Droplet Absorption Time Test:
[0084] Ten (10) droplets are placed in separate un-wetted locations
on one side of the material according to the procedure above and
the average and standard deviation of the absorption time is
recorded. In the case of materials in which the first and second
region dimensions are large relative to the droplets, the droplets
should be placed in the first region, which contains the density
gradient in the thickness direction. The sample is then turned over
and ten (10) droplets are placed on the other side, in un-wetted
locations, again according to the procedure above, and likewise the
average and standard deviation of the absorption time is recorded.
The average absorption time of the less dense side is then divided
by the average absorption time of the more dense side. This is the
droplet absorption time ratio.
Example 1
[0085] A nonwoven sheet material was made according to the present
invention. The material comprised cellulosic fiber (Rayfloc J-LDE)
from Rayonier, Jesup, Ga.), SAP (SA65s from Sumitomo Seika in
Singapore) and 17 gsm 3995 tissue, (Cellu tissue, East Hartford,
Conn.). The first stratum formed had a total basis weight of 150
gsm, comprising cellulosic fiber and 15% SAP and included a layer
of 17 gsm carrier tissue. Except for the carrier, the stratum was a
homogeneous mix of SAP and cellulose, and was densified using a
calender with a smooth surface on one roll and a linen pattern on
the other heated to 170.degree. C. at a sufficient pressure to
yield a density of 0.31 g/cc. To this was added an additional
stratum of material, with a total basis weight of 150 gsm, again
comprising cellulosic fiber and SAP and including a layer of 17 gsm
tissue, this time on the top. Except for the top tissue, the second
stratum was likewise a homogeneous mix of SAP and Cellulose. This
web was run through an embossed calender, heated to 170.degree. C.,
comprising a pattern of parallel circumferential raised ridges of
sinusoidal section, using the engraving pattern designated as
57RE80 from BF Perkins, of Rochester, N.Y. This creates a pattern
of parallel embossed lines in the material running in the machine
direction. The roll pressure was sufficient to produce a small area
density measurement along the centerline of the embossed lines of
0.75 g/cc. The material had an overall basis weight of 300 gsm, an
overall SAP content of around 15%, and an overall density of 0.22
g/cc.
Example 2
[0086] A material was made according to the present invention. The
material comprised the same raw materials as Example 1. The first
stratum had a total basis weight of 116 gsm, comprising cellulosic
fiber and 30% SAP by weight and included a layer of 17 gsm carrier
tissue. Except for the carrier, the stratum was a homogeneous mix
of SAP and Cellulose, and was densified using a calender with a
smooth surface on one roll and a linen pattern on the other heated
to 170.degree. C. at a sufficient pressure to yield a density of
0.28 g/cc. To this was added an additional stratum of material with
a total basis weight of 111 gsm, again comprising cellulosic fiber
and 25% SAP by weight and including a layer of 17 gsm tissue, this
time on the top. Except for the top tissue, the second stratum was
likewise a homogeneous mix of SAP and Cellulose and the web was run
through an embossed calender, with the same embossing pattern as
Example 1, heated to 17.degree. C., with a sufficient force to
produce a small area density measurement along the centerline of
the embossed lines of 0.86 g/cc. The material had an overall basis
weight of 227 gsm, a SAP content of around 30%, and an overall
density of 0.20 g/cc.
Example 3
[0087] A material was made according to the present invention. The
material comprised the same raw materials as Example 1. The first
stratum had a total basis weight of 106 gsm, comprising cellulosic
fiber and 25% SAP by weight and included a layer of 17 gsm carrier
tissue. Except for the carrier, the stratum was a homogeneous mix
of SAP and cellulose, and was densified using a calender with a
smooth surface on one roll and a linen pattern on the other heated
to 170.degree. C. at a sufficient pressure to yield a density of
0.28 g/cc. To this was added an additional stratum of material with
a total basis weight of 107 gsm, again comprising cellulosic fiber
and 25% SAP by weight and including a layer of 17 gsm tissue, this
time on the top. Except for the top tissue, the second stratum was
likewise a homogeneous mix of SAP and cellulose. The web was run
through an embossed calender, with the same embossing pattern as
Example 1, heated to 170 C, with a sufficient force to produce a
small area density measurement along the centerline of the embossed
lines of 0.81 g/cc. The material had an overall basis weight of 213
gsm, a SAP content of around 25%, and an overall density of 0.17
g/cc.
Example 4
[0088] A material was made according to the present invention. The
material comprised the same raw materials as Example 1. The first
stratum had a total basis weight of 91 gsm, comprising cellulosic
fiber and 10% SAP by weight and included a layer of 17 gsm carrier
tissue. Except for the carrier, the stratum was a homogeneous mix
of SAP and cellulose, and was densified using a calender with a
smooth surface on one roll and a linen pattern on the other heated
to 170.degree. C. at a sufficient pressure to yield a density of
0.29 g/cc. To this was added an additional stratum of material with
a total basis weight of 110 gsm, again comprising cellulosic fiber
and SAP and including a layer of 17 gsm tissue, this time on the
top. Except for the top tissue, the second stratum was likewise a
homogeneous mix of SAP and cellulose. This web was run through an
embossed calender, with the same embossing pattern as Example 1,
heated to 170.degree. C., with a sufficient force to produce a
small area density measurement along the centerline of the embossed
lines of 0.82 g/cc. The material had an overall basis weight of 201
gsm, a SAP content of around 20%, and an overall density of 0.19
g/cc.
Example 5
[0089] A material was made according to the present invention. The
material comprised the same raw materials as Example 1. The first
stratum had a total basis weight of 91 gsm, comprising cellulosic
fiber and 10% SAP by weight and included a layer of 17 gsm carrier
tissue. Except for the carrier, the stratum was a homogeneous mix
of SAP and cellulose, and was densified using a calender with a
smooth surface on one roll and a linen pattern on the other heated
to 170.degree. C. at a sufficient pressure to yield a density of
0.29 g/cc. To this was added an additional stratum of material with
a total basis weight of 87 gsm, again comprising cellulosic fiber
and SAP and including a layer of 17 gsm tissue, this time on the
top. Except for the top tissue, the second stratum was likewise a
homogeneous mix of SAP and cellulose. This web was run through an
embossed calender, with the same embossing pattern as Example 1,
heated to 170.degree. C., with a sufficient force to produce a
small area density measurement along the centerline of the embossed
lines of 0.91 g/cc. The material had an overall basis weight of 178
gsm, a SAP content of around 20%, and an overall density of 0.19
g/cc.
[0090] Control 1:
[0091] A material was made to serve as a control to Example 3. It
was intended to have a similar basis weight, SAP percentage, used
similar raw materials, and had tissue on the top and bottom. The
single stratum comprised cellulosic fiber and SAP and included a
layer of 17 gsm carrier tissue on both the top and bottom. Except
for the carrier layers, the stratum was a homogeneous mix of SAP
and cellulose, and was densified using a calender with a smooth
surface on one roll and a linen pattern on the other heated to
170.degree. C. at a sufficient pressure to yield a density of 0.32
g/cc, giving it density features similar to the higher density
stratum in the first region. The overall material had a basis
weight of 217 gsm and a SAP content of around 25%.
[0092] Control 2:
[0093] A material was made to serve as an alternative control to
Example 3. It was intended to have similar basis weight, SAP
percentage, used similar raw materials and had tissue on the top
and bottom. The entire material, however, was formed as one stratum
and was bonded using the embossed calender, creating the features
of the second region, but did not produce a density gradient in the
first region. The single stratum comprised cellulosic fiber and SAP
and included a layer of 17 gsm carrier tissue on both the top and
bottom, Except for the carrier layers, the stratum was a
homogeneous mix of SAP and cellulose, and was densified using a
calender with a smooth surface on one roll and a linen pattern on
the other heated to 170 C at a sufficient pressure to yield a
density of 0.15 g/cc. The overall material had a basis weight of
225 gsm and a SAP content of around 25%.
[0094] Basis Weight and Caliper measurements were taken on the 5
example materials, according to the sampling and procedures
explained above. Table 1 contains the measured and calculated
densities obtained for the various samples and the resulting
apparent density ratio in the thickness (Z-direction).
TABLE-US-00001 TABLE 1 Calculated Average Density of Calculated
Average Density of Low-Density Z-direction Density High Density
Stratum Density (g/cc) Stratum (g/cc) (g/cc) Ratio Example 1 0.22
0.31 0.17 1.8 Example 2 0.20 0.28 0.16 1.7 Example 3 0.17 0.28 0.12
2.4 Example 4 0.19 0.29 0.15 1.9 Example 5 0.19 0.29 0.14 2.1
[0095] It can be seen from the values above that the material of
the present invention contains a substantial density gradient in
the thickness direction, as quantified by the calculated
Z-direction density ratio values.
[0096] The small area density was measured for the 5 example
materials according to the method and sampling described above,
measured along the center of the dense lines in the second region.
Table 2 reports these average values obtained for Examples 1-5
TABLE-US-00002 TABLE 2 Small Area Average Density (Center of Second
Region) g/cc Example 1 0.75 Example 2 0.86 Example 3 0.81 Example 4
0.82 Example 5 0.91
[0097] Despite the differences in apparatus, the data suggests that
the density in the second region is higher than the density in the
first region.
[0098] Tensile measurements were taken for the various examples of
the material of the present invention. The average tensile values
are reported in Table 3 below:
TABLE-US-00003 TABLE 3 Average Tensile (N/50 mm) Example 1 38
Example 2 17 Example 3 18 Example 4 23 Example 5 20
[0099] These data suggest that the hydrogen bonding in the
materials of the present invention provides robust tensile
strength.
[0100] Examples of the present invention and control materials were
tested according to the droplet absorption time test procedure and
sampling described above. These values are reported in Table 4
below:
TABLE-US-00004 TABLE 4 Droplet Absorption Time (sec) Average
Average Examples of Value, Low- Value Low- the Present Density Side
Density Side Invention Up Down Ratio Example 1 3.8 9.1 2.4 Example
2 3.6 7.1 2.0 Example 3 3.5 7.9 2.3 Example 4 3.2 7.5 2.4 Example 5
3.3 8.9 2.7 Control Top Side Samples Top Side Up Down Ratio Control
1 6.7 6.5 .97 Control 2 5.2 5.8 1.1
[0101] Examples 1-5 all exhibit a substantial difference in average
droplet absorption times when comparing adding the liquid to the
low-density side as opposed to adding the liquid to the
high-density side.
[0102] Control 1 has a similar basis weight and SAP content as
Example 3, but it has a generally uniform density throughout its
thickness that is formed in a similar manner and looks like the
higher density stratum in the first region of Example 3. As would
be expected, the droplet absorption time is very similar for liquid
added on one side of the sheet compared to being added on the other
side. The absorption time for liquid added to Example 3 on the
low-density side is faster than for this control, which does not
have a gradient in the thickness direction.
[0103] Control 2 has a similar basis weight and SAP content as
Example 3, but it is entirely bonded using the embossed calender.
While the density and shape of the embossed second region is
similar to that of the second region in Example 3, the density of
the material in the first region looks more like the low-density
stratum of the first region in Example 3. There is no density
gradient in the thickness direction. As would be expected, the
droplet absorption time is very similar for liquid added on one
side of the sheet as compared to being added on the other, even
though the surface relief resulting from the embossing is higher
with the "top side up" than with the "top side down". The
absorption time for liquid added to Example 3 on the low-density
side is faster than for this control, which has all of the features
of Example 3 except for the gradient in the thickness
direction.
[0104] These and other data suggest that the density gradient of
the material of the present invention allow the material to exhibit
a faster droplet absorption rate than for similar materials that do
not have the density gradient in the thickness direction drawing
the liquid down into the material.
[0105] In order to effectively contain the unbonded cellulosic
fibers and SAP in the low-density surface of the material of the
present invention during normal web handling, a layer of cellulosic
tissue is hydrogen bonded to the low-density side of the sheet. By
bonding this tissue to the high-density second re, effective
bonding can be achieved while leaving the low-density stratum in
the first region relatively uncompressed. The vertical delamination
test, explained below, is a useful indicator of this bond strength.
Vertical Delamination values are advantageously greater than 5N,
and more desirably above 10N in order for the tissue to remain
bonded during web handling and splicing typical of converting
operations for disposable absorbent articles.
[0106] Vertical Delamination:
[0107] A strip of Spectape ST01 double sided adhesive tape is
attached to one surface of the material to be tested. A 50 mm
circular sample is cut from the taped portion using an Atom Model
SE 20C die press from Associated Pacific Company of Camarillo,
Calif. and an appropriately sized cutting die. A test is then
performed using a Zwick Model Z005 tensile tester from Zwick/Roell
in Ulm, Germany, or the equivalent. In the lower compression
portion of the machine, a 50 mm diameter circular platen is
attached to the load cell on the moveable crossbeam and a second
larger fixed circular platen is mounted to the frame below,
opposite the 50 mm moveable platen. The release paper is removed
from the taped sample and it is attached to the 50 mm moveable
platen using the adhesive surface. A second strip of double-sided
tape is applied to the lower platen surface and the release paper
is likewise removed. The platens are brought together, adhering the
sample faces to both of them, and then moved apart, delaminating
the sample. To do this without damaging the load cell, the moveable
platen is carefully brought down onto the fixed platen with the
sample between them at 30 mm/min until a force of 0.5N is read,
then at 2 mm/min until a force of 5N is read, and then at 0.5
mm/min until a force of 35N is read. Then the moveable platen is
moved upwards at 75 mm/min, while recording the maximum force
applied as the sample delaminates. This maximum force is the
vertical delamination force. Examination of the failed sample
reveals whether the failure was within the sample or if the sample
strength exceeded that of one of the taped bonds.
[0108] Vertical delamination testing was performed on the example
materials to show the integrity of the bonds between the strata and
particularly between the carrier tissue on the lower-density side
of the sheet. The average of 5 values for each was recorded. Please
find these in table 5 below:
TABLE-US-00005 TABLE 5 Average Vertical Delamination (N) Example 2
17.3 Example 3 17.0 Example 4 22.9 Example 5 22.8
[0109] From the forgoing, we observe the numerous modifications and
variations can be effected without departing from the true spirit
and scope of the novel concept of the present invention. It is to
be understood that no limitation with respect to the specific
embodiments illustrated here is intended or shown be inferred. The
disclosure is intended to cover by the appended claims all such
modifications should fall within the scope of the claims.
* * * * *