U.S. patent application number 11/027556 was filed with the patent office on 2006-07-06 for absorbent composites containing biodegradable reinforcing fibers.
Invention is credited to Richard W. Tanzer, Katie Veith, Raj Wallajapet.
Application Number | 20060147689 11/027556 |
Document ID | / |
Family ID | 36190467 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060147689 |
Kind Code |
A1 |
Wallajapet; Raj ; et
al. |
July 6, 2006 |
Absorbent composites containing biodegradable reinforcing
fibers
Abstract
An absorbent composite contains a superabsorbent material and
thermoplastic biodegradable reinforcing fibers, and may also
contain pulp fibers. The superabsorbent material may be
biodegradable, providing a biodegradable absorbent composite. The
superabsorbent material may have a gel strength from about 500
dynes/cm.sup.2 to about 80,000 dynes/cm.sup.2. The biodegradable
reinforcing fibers may contain fibers of a polyester such as
poly(lactic acid). The biodegradable reinforcing fibers may be
wettable, reducing or eliminating the need for treating the surface
of the fibers with a wetting agent such as a surfactant, and may
provide desirable performance without being bonded to the other
components.
Inventors: |
Wallajapet; Raj; (Neenah,
WI) ; Veith; Katie; (Fremont, WI) ; Tanzer;
Richard W.; (Neenah, WI) |
Correspondence
Address: |
K. Shannon Mrksich, Ph. D.;Brinks Hofer Gilson & Lione
P.O. Box 10395
Chicago
IL
60610
US
|
Family ID: |
36190467 |
Appl. No.: |
11/027556 |
Filed: |
December 30, 2004 |
Current U.S.
Class: |
428/292.1 ;
428/364; 428/369 |
Current CPC
Class: |
Y10T 428/2913 20150115;
Y10T 428/2922 20150115; A61L 15/26 20130101; Y10T 428/249924
20150401; C08L 67/04 20130101; A61L 15/60 20130101; A61L 15/26
20130101 |
Class at
Publication: |
428/292.1 ;
428/369; 428/364 |
International
Class: |
D04H 13/00 20060101
D04H013/00; D02G 3/00 20060101 D02G003/00 |
Claims
1. An absorbent composite, comprising: a biodegradable
superabsorbent material; and a plurality of thermoplastic
biodegradable reinforcing fibers.
2. The absorbent composite of claim 1, wherein the thermoplastic
biodegradable reinforcing fibers comprise poly(hydroxyalkanoate)
fibers.
3. The absorbent composite of claim 2, wherein the thermoplastic
biodegradable reinforcing fibers comprise poly(lactic acid)
fibers.
4. The absorbent composite of claim 1, wherein the thermoplastic
biodegradable reinforcing fibers are un-bonded.
5. The absorbent composite of claim 1, wherein the thermoplastic
biodegradable reinforcing fibers are wettable.
6. The absorbent composite of claim 5, wherein the composite is
free of wetting agents.
7. The absorbent composite of claim 1, further comprising pulp
fibers.
8. The absorbent composite of claim 1, wherein the biodegradable
superabsorbent material comprises carboxymethyl cellulose.
9. The absorbent composite of claim 1, wherein the biodegradable
superabsorbent is present in a loading from about 10 wt % to about
70 wt %.
10. The absorbent composite of claim 7, wherein the pulp fibers are
present in a loading from about 25 wt % to about 85 wt %.
11. The absorbent composite of claim 1, wherein the thermoplastic
biodegradable reinforcing fibers are present in a loading from
about 5 wt % to about 30 wt %.
12. The absorbent composite of claim 1, wherein the gel strength of
the superabsorbent is from about 500 dynes/cm.sup.2 to about 80,000
dynes/cm.sup.2.
13. An absorbent composite, comprising: a superabsorbent material
having a gel strength from about 500 dynes/cm.sup.2 to about 80,000
dynes/cm.sup.2; and a plurality of thermoplastic biodegradable
reinforcing fibers.
14. The absorbent composite of claim 13, wherein the thermoplastic
biodegradable reinforcing fibers comprise poly(hydroxyalkanoate)
fibers.
15. The absorbent composite of claim 14, wherein the thermoplastic
biodegradable reinforcing fibers comprise poly(lactic acid)
fibers.
16. The absorbent composite of claim 13, wherein the thermoplastic
biodegradable reinforcing fibers are un-bonded.
17. The absorbent composite of claim 13, wherein the thermoplastic
biodegradable reinforcing fibers are wettable.
18. The absorbent composite of claim 17, wherein the composite is
free of wetting agents.
19. The absorbent composite of claim 13, wherein the superabsorbent
material comprises carboxymethyl cellulose.
20. The absorbent composite of claim 13, wherein the superabsorbent
is present in a loading from about 10 wt % to about 70 wt %.
21. The absorbent composite of claim 13, further comprising pulp
fibers.
22. The absorbent composite of claim 21, wherein the pulp fibers
are present in a loading from about 25 wt % to about 85 wt %.
23. The absorbent composite of claim 13, wherein the thermoplastic
biodegradable reinforcing fibers are present in a loading from
about 5 wt % to about 30 wt %.
24. An absorbent composite, comprising: from about 10 wt % to about
70 wt % of a biodegradable superabsorbent material; from about 25
wt % to about 85 wt % of pulp fibers; and from about 5 wt % to
about 30 wt % of poly(lactic acid) reinforcing fibers.
25. The absorbent composite of claim 24, wherein the poly(lactic
acid) reinforcing fibers are un-bonded.
26. The absorbent composite of claim 24, wherein the poly(lactic
acid) reinforcing fibers have a length from about 2 mm to about 60
mm.
27. The absorbent composite of claim 24, wherein the poly(lactic
acid) reinforcing fibers have a diameter from about 1.5 denier to
about 6 denier.
28. The absorbent composite of claim 24, wherein the poly(lactic
acid) reinforcing fibers have from about 0 crimps per inch to about
12 crimps per inch.
29. The absorbent composite of claim 24, wherein the composite has
a permeability of at least 10 darcies.
30. The absorbent composite of claim 24, wherein the composite has
a density from about 0.09 grams per cubic centimeter to about 0.3
grams per centimeter.
31. The absorbent composite of claim 24, wherein the composite is
free of wetting agents.
32. The absorbent composite of claim 24, wherein the weight ratio
of pulp fibers to poly(lactic acid) reinforcing fibers is from
about 1:1 to about 5:1; and the weight ratio of biodegradable
superabsorbent material to poly(lactic acid) reinforcing fibers is
from about 1:1 to about 4:1.
33. A method of forming an absorbent composite, comprising:
combining a superabsorbent material and a plurality of
biodegradable reinforcing fibers into a mixture; and compressing
the mixture in a dry state into a composite having a density from
about 0.09 grams per cubic centimeter to about 0.3 grams per
centimeter; wherein the biodegradable reinforcing fibers remain
un-bonded.
34. The method of claim 33, wherein the biodegradable reinforcing
fibers comprise poly(hydroxyalkanoate) fibers.
35. The method of claim 34, wherein the biodegradable reinforcing
fibers comprise poly(lactic acid) fibers.
36. The method of claim 33, wherein the composite is free of
wetting agents.
37. The method of claim 33, wherein the combining comprises
air-forming the superabsorbent material with the biodegradable
reinforcing fibers.
38. The method of claim 33, wherein the combining further comprises
combining pulp fibers with the superabsorbent material and the
biodegradable reinforcing fibers.
39. The method of claim 38, wherein the combining comprises
air-forming the superabsorbent material with the biodegradable
reinforcing fibers and the pulp fibers.
Description
BACKGROUND
[0001] Disposable absorbent products are used extensively for body
waste management. These disposable absorbent products include one
or more absorbent structures to manage body waste effectively. The
absorbent structure within the disposable absorbent product takes
up and retains the body waste within the absorbent product. A
variety of other components may also be present in a typical
absorbent product, including liquid impermeable backing sheets,
liquid permeable liners, wicking layers, and components for
securing the product to the user. The particular combination and
configuration of components in an absorbent product will depend on
the intended use of the product.
[0002] Absorbent structures typically include a superabsorbent
material, which can absorb large amounts of water or other aqueous
liquids. One ongoing effort in the development of superabsorbent
materials has been to increase the stiffness of the gel formed when
the superabsorbent material has absorbed an aqueous liquid.
Increased gel stiffness, also referred to as gel strength, can
increase the porosity of the absorbent structure, thereby
increasing the liquid intake rate and distribution within the
structure. These and other factors can provide for better
utilization of the entire absorbent capacity of the structure.
[0003] When superabsorbent materials are to be employed in personal
care products such as diapers, it is generally desirable to employ
a superabsorbent possessing both a high absorbent capacity and a
high gel stiffness. High capacity is desired because it allows for
the use of a smaller mass of superabsorbent material. High gel
stiffness is desirable to prevent the formation of flowable
gelatinous masses of superabsorbent, which may leak from the
product or form a barrier to the transport of liquids through the
matrix in which such superabsorbent materials are generally
located. Superabsorbent properties such as gel stiffness and
absorbent capacity are described, for example, in U.S. Pat. No.
5,082,723.
[0004] One approach for compensating for low gel stiffness of a
superabsorbent material is to reinforce the absorbent structure
with thermoplastic binder fibers. A common thermoplastic binder
fiber for this application is a fiber having a sheath/core
structure of polyethylene and poly(ethylene terephthalate) (PET).
Polyethylene is used as the sheath polymer due to its low melting
point, good rheology and low cost. PET is used as the core due to
its higher melting point and overall stability. These stabilized
absorbent composites containing reinforcing fibers can be used as
absorbent structures in disposable absorbent products.
[0005] One potential drawback to stabilized absorbent composites is
that conventional thermoplastic binder fibers tend to have low
wettability due to their hydrophilic surfaces. This low wettability
can reduce the overall liquid intake and distribution of the
absorbent structure, despite increasing the porosity. The
wettability of thermoplastic binder fibers can be increased by
treating their surfaces with a wetting agent such as a surfactant
or a fiber spin finish agent. However, these wetting agents can be
dissolved in water, reducing the surface tension of the absorbed
liquid and actually decreasing both the wicking of liquid into the
composite and the subsequent distribution of the absorbed
liquid.
[0006] Another ongoing effort in the development of superabsorbent
materials has been to make the superabsorbent biodegradable, such
that the material either can be disposed of by composting or can be
recycled into useful raw materials. Examples of biodegradable
superabsorbent materials are disclosed in U.S. Pat. Nos. 4,952,550;
5,847,089 and 6,540,853; and in U.S. Patent Application Publication
No. 2004/0157734 A1.
[0007] Biodegradable superabsorbent materials typically have less
desirable absorbent properties relative to conventional
non-biodegradable superabsorbent materials such as polyacrylates.
This difference in absorbent properties is believed to be related
to the lower gel stiffness of the hydrogels formed from
biodegradable superabsorbent materials. Typically, absorbent
composites made with biodegradable superabsorbents have tended to
exhibit inferior intake, distribution and retention of liquids
relative to absorbent composites based on polyacrylate
superabsorbents. As with conventional superabsorbents, the gel
stiffness of biodegradable superabsorbents can be increased by
reinforcement with thermoplastic binder fibers. However,
reinforcing a biodegradable superabsorbent material with
non-biodegradable fibers would undermine the goal of constructing a
compeletely biodegradable absorbent product.
[0008] There is thus a need for improved absorbent structures for
disposable absorbent products. Improved absorbent structures may
have improved wicking and distribution of aqueous liquids, while
exhibiting other absorbent properties that are at least as good as
those of conventional structures. Ideally, the structures could be
made completely of biodegradable materials to help provide
biodegradable absorbent products.
BRIEF SUMMARY
[0009] In an embodiment of the invention, there is provided an
absorbent composite, comprising a biodegradable superabsorbent
material, and a plurality of thermoplastic biodegradable
reinforcing fibers.
[0010] In another embodiment of the invention, there is provided an
absorbent composite, comprising a superabsorbent material having a
gel strength from about 500 dynes/cm.sup.2 to about 80,000
dynes/cm.sup.2, and a plurality of thermoplastic biodegradable
reinforcing fibers.
[0011] These embodiments may further include absorbent composites
wherein the thermoplastic biodegradable reinforcing fibers comprise
poly(hydroxyalkanoate) fibers, wherein the thermoplastic
biodegradable reinforcing fibers comprise poly(lactic acid) fibers,
wherein the thermoplastic biodegradable reinforcing fibers are
un-bonded, wherein the thermoplastic biodegradable reinforcing
fibers are wettable, and wherein the composite is free of wetting
agents.
[0012] These embodiments may further include absorbent composites
wherein the biodegradable superabsorbent material comprises
carboxymethyl cellulose, wherein the biodegradable superabsorbent
is present in a loading from about 10 wt % to about 70 wt %,
wherein the absorbent composite further comprises pulp fibers,
wherein the pulp fibers are present in a loading from about 25 wt %
to about 85 wt %, wherein the thermoplastic biodegradable
reinforcing fibers are present in a loading from about 5 wt % to
about 30 wt %, and wherein the gel strength of the superabsorbent
is from about 500 dynes/cm.sup.2 to about 80,000
dynes/cm.sup.2.
[0013] In another embodiment of the invention, there is provided an
absorbent composite, comprising from about 10 wt % to about 70 wt %
of a biodegradable superabsorbent material, from about 25 wt % to
about 85 wt % of pulp fibers, and from about 5 wt % to about 30 wt
% of poly(lactic acid) reinforcing fibers.
[0014] These embodiments may further include absorbent composites
wherein the poly(lactic acid) reinforcing fibers are un-bonded,
wherein the poly(lactic acid) reinforcing fibers have a length from
about 2 mm to about 60 mm, wherein the poly(lactic acid)
reinforcing fibers have a diameter from about 1.5 denier to about 6
denier, wherein the poly(lactic acid) reinforcing fibers have from
about 0 crimps per inch to about 12 crimps per inch, wherein the
composite has a permeability of at least 10 darcies, wherein the
composite has a density from about 0.09 grams per cubic centimeter
to about 0.3 grams per centimeter, wherein the composite is free of
wetting agents, wherein the weight ratio of pulp fibers to
poly(lactic acid) reinforcing fibers is from about 1:1 to about
5:1, and wherein the weight ratio of biodegradable superabsorbent
material to poly(lactic acid) reinforcing fibers is from about 1:1
to about 4:1.
[0015] In another embodiment of the invention, there is provided a
method of forming an absorbent composite, comprising combining a
superabsorbent material and a plurality of biodegradable
reinforcing fibers into a mixture, and compressing the mixture in a
dry state into a composite having a density from about 0.09 grams
per cubic centimeter to about 0.3 grams per centimeter, wherein the
biodegradable reinforcing fibers remain un-bonded.
[0016] These embodiments may further include a method wherein the
biodegradable reinforcing fibers comprise poly(hydroxyalkanoate)
fibers, wherein the biodegradable reinforcing fibers comprise
poly(lactic acid) fibers, wherein the composite is free of wetting
agents, wherein the combining comprises air-forming the
superabsorbent material with the biodegradable reinforcing fibers,
wherein the combining further comprises combining pulp fibers with
the superabsorbent material and the biodegradable reinforcing
fibers, and wherein the combining comprises air-forming the
superabsorbent material with the biodegradable reinforcing fibers
and the pulp fibers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graph of permeability of absorbent composites
containing a variety of superabsorbent materials, both with and
without biodegradable reinforcing fibers;
[0018] FIG. 2 is a graph of the mass of liquid absorbed over time
by absorbent composites containing a biodegradable superabsorbent
material;
[0019] FIG. 3 is a graph of the mass of liquid absorbed over time
by absorbent composites containing a high gel stiffness
superabsorbent material;
[0020] FIG. 4 is a graph of liquid distribution within an absorbent
composite containing a high gel stiffness superabsorbent material
with and without biodegradable reinforcing fibers; and
[0021] FIG. 5 is a graph of liquid distribution within an absorbent
composite containing a biodegradable superabsorbent material with
and without biodegradable reinforcing fibers.
DETAILED DESCRIPTION
[0022] An absorbent structure includes a superabsorbent material
and a plurality of biodegradable reinforcing fibers together as an
absorbent composite. The presence of reinforcing fibers can
increase the stiffness and improve the resiliency of the composite
structure. Biodegradable reinforcing fibers may be used in
conjunction with biodegradable superabsorbent materials to produce
a composite that is biodegradable. Reinforcing fibers that are
inherently wettable may allow for a reduction or elimination in the
amount of surfactant used in the composite. In one example, fibers
made of aliphatic polyesters can be incorporated readily into an
absorbent structure, can provide desirable absorbent properties,
and also can be biodegradable. Aliphatic polyesters that can be
used as reinforcing fibers include biodegradable
poly(hydroxyalkanoates) such as poly(lactic acid).
[0023] Absorbent composites include a superabsorbent material and a
plurality of reinforcing fibers. In addition, absorbent composites
can include pulp fibers. An absorbent composite can be a simple
mixture of these components, or the reinforcing fibers can be
bonded to the other components of the composite, for example by
heating the mixture at an elevated temperature or by treating the
mixture with a bonding agent.
[0024] As used herein, the term "superabsorbent material" refers to
a water-swellable, water-insoluble organic or inorganic material
having an absorbent capacity for a 0.9 percent by weight (0.9 wt %)
aqueous sodium chloride solution of at least 10 grams of solution
per gram of polymer. That is, a superabsorbent material is capable
of absorbing at least about 10 times its own weight in a 0.9 wt %
aqueous sodium chloride solution. Preferably, a superabsorbent
material is capable of absorbing at least about 20 times its
weight, more preferably at least about 30 times its weight, even
more preferably at least about 40 times its weight, even more
preferably at least about 50 times its weight, even more preferably
at least about 60 times its weight in a 0.9 wt % aqueous sodium
chloride solution. The term "hydrogel" refers specifically to
superabsorbent material in the water-swollen state.
[0025] Examples of organic superabsorbent materials include natural
materials such as agar, pectin, guar gum and the like, as well as
synthetic materials such as synthetic superabsorbent polymers.
Superabsorbent polymers include, for example, polyacrylamides,
polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl
ethers, hydroxypropyl cellulose, polyvinylmorpholinone, alkali
metal salts of polyacrylic acids, and polymers and copolymers of
vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl
pyridines, and the like. Other exemplary superabsorbant polymers
include hydrolyzed acrylonitrile grafted starch, acrylic acid
grafted starch, and isobutylene maleic anhydride copolymers and
mixtures thereof.
[0026] Specific examples of polyacrylate superabsorbent materials
include SANWET ASAP 2300 polymer (Chemdal, Portsmouth, Va.), DOW
DRYTECH 2035LD polymer (Dow Chemical Co., Midland, Miss.), FAVOR
SAB 870M and FAVOR SAB 880 polymers (Stockhausen, Inc., Greensboro,
N.C.), and the high gel stiffness polymer SXM 9543
(Stockhausen).
[0027] Specific examples of biodegradable superabsorbent materials
include carboxymethyl cellulose materials, such as biodegradable
superabsorbents available from Stochkausen. Carboxymethyl cellulose
based biodegradable superabsorbent materials are described, for
example, in U.S. Patent Application Publication No. 2004/0157734
A1, which is incorporated by reference herein. As noted in this
patent document, a partially neutralized, uncrosslinked,
carboxyl-containing polysaccharide can be preswelled and
subsequently dried, and the dried polycarboxypolysaccharide can be
surface-post crosslinked by means of a surface crosslinker.
Polycarboxypolysaccharides may inherently contain carboxyl groups,
or they may be derived from polysaccharides without carboxyl groups
but that are provided with carboxyl groups by subsequent
modification. Polycarboxypolysaccharides may be modified to contain
other groups, particularly groups that improve the solubility in
water, such as hydroxyalkyl and especially hydroxyethyl groups and
also phosphate groups. Specific examples of
polycarboxypolysaccharides include carboxymethylguar, carboxylated
hydroxyethyl or hydroxypropylcellulose, carboxymethylcellulose and
carboxymethylstarch, oxidized starch, carboxylated phosphatestarch,
xanthan and mixtures thereof. Polycarboxy-polysaccharide
superabsorbent polymers may be modified by addition of
carboxyl-free polysaccharides, such as polygalactomannans or
hydroxyalkylcelluloses, and/or by addition of other additives. The
polycarboxypolysaccharide may be preswollen in an aqueous phase to
form a hydrogel, and the aqueous phase may also include additive
substances. Following thermal drying, comminution and
classification of the hydrogel, the surface of the
polycarboxypolysaccharide powder can be crosslinked with covalent
and/or ionic crosslinkers which react with surface moieties,
preferably carboxyl, carboxylate or hydroxyl groups, preferably by
heating. The resulting particulate superabsorbent polymers can
exhibit very good retention and absorption ability, significantly
improved absorbency for water and aqueous fluids against an
external pressure, and excellent ageing stability.
[0028] The superabsorbent materials may be in any form suitable for
use in absorbent composites including particles, fibers, flakes,
films, foams, or spheres. Preferably, the superabsorbent material
includes particles of hydrocolloids, preferably an ionic
hydrocolloid. The superabsorbent polymers preferably are lightly
crosslinked to render the material substantially water insoluble.
Crosslinking may be accomplished, for example, by irradiation
and/or by covalent, ionic, van der Waals, or hydrogen bonding.
Superabsorbent materials may be shell crosslinked so that the outer
surface or shell of the superabsorbent particle, fiber, flake,
film, foam, or sphere possesses a higher crosslink density than the
inner portion of the superabsorbent.
[0029] As used herein, the term "fiber" or "fibrous" refers to a
particulate material wherein the ratio of the length of the
particulate material to the diameter of the particulate material is
greater than about 10. Conversely, a nonfiber or nonfibrous
material refers to a particulate material wherein the length to
diameter ratio is about 10 or less. Both the reinforcing fibers and
the pulp fibers are fibrous materials.
[0030] A wide variety of fibers can be used as, or in the
preparation of, the fibrous pulp. Examples of pulp fibers include,
but are not limited to, cellulosic fibers such as wood and wood
products, e.g., wood pulp fibers. Examples of pulp fibers include
non-woody paper-making fibers from cotton; from straws and grasses,
such as rice and esparto; from canes and reeds, such as bagasse;
from bamboos; from stalks with bast fibers, such as jute, flax,
kenaf, cannabis, linen and ramie; and from leaf fibers, such as
abaca and sisal. Examples of pulp fibers include man-made fibers
obtained from regenerated cellulose or cellulose derivatives, such
as cellulose acetate. The fibrous pulp also can use mixtures of
such materials, e.g., mixtures of one or more cellulosic fibers.
Other materials from which the fibrous pulp may be made include
non-cellulosic fibers such as wool and glass, and synthetic fibers,
such as polyethylene, polypropylene and polyester. Pulp fibers
generally may have lengths from about 0.5 mm to about 20 mm. For
example, pulp fibers may have lengths from about 1 mm to about 10
mm, and may have lengths from about 2 mm to about 5 mm.
[0031] Biodegradable reinforcing fibers can be any thermoplastic
fiber made of a biodegradable material. As used herein, the term
"thermoplastic" refers to a polymeric material that can be
processed by melting, forming, and shaping. This is in contrast to
a thermoset polymeric material, which cannot be melted and shaped
again after its original shaping is complete. As used herein, the
term "biodegradable" refers to a polymeric material that, when
composted under standard conditions for 180 days, at least 60% of
the organic carbon in the material is converted to carbon dioxide,
relative to a positive reference material (cellulose=100%). The
American Society for Testing and Materials (ASTM) Standard Test
Method for Determining Aerobic Biodegradation of Plastic Materials
Under Controlled Composting Conditions, designation D 5338, is used
for this determination. Consistent with this test procedure,
samples are initially incubated for 45 days; and, if significant
biodegradation of the test substance is still being observed, the
incubation time may be extended to 90 days or 180 days.
[0032] Biodegradable reinforcing fibers may be continuous in
length, that is, individual fibers may extend the length or width
of the absorbent composite. Spunbond fibers are an example of
continuous fibers. Biodegradable reinforcing fibers may also be
non-continuous. Non-continuous fibers include staple fibers,
linters, and melt blown fibers, and may range from 2 mm to 60 mm in
length. In certain embodiments of the invention, the non-continuous
reinforcing fibers may range from 5 mm to 40 mm in length. Other
embodiments include biodegradable reinforcing fibers which are
between 7 mm and 25 mm long. Biodegradable reinforcing fibers may
have diameters from about 1.5 denier to about 6 denier and may have
from zero to 12 crimps per inch.
[0033] It is desirable that the biodegradable reinforcing fibers
are wettable. As used herein, the term "wettable" refers to a
material such as a fiber that, without any separate substance on
its surface, exhibits a water-in-air contact angle of less than
90.degree. (i.e., 0.degree. to 90.degree.). Preferably, the
biodegradable reinforcing fibers exhibit a water-in-air contact
angle from about 0.degree. to about 85.degree., and more preferably
from about 0.degree. to about 80.degree.. Preferably, wettable
biodegradable fibers exhibit a water-in-air contact angle of less
than 90.degree., at a temperature from about 0.degree. C. to about
100.degree. C., and preferably at typical use conditions, such as
from about 20.degree. C. to about 40.degree. C. In some
conventional absorbent composite systems, the addition of wetting
agents to the absorbent composite reduces the absorbent properties
of the composites. Preferably, the biodegradable reinforcing fibers
do not contain any separate substance on the fiber surface other
than the fiber material.
[0034] Examples of biodegradable reinforcing fibers include
biodegradable aliphatic polyesters. The term "polyester" means a
polymer having ester (--C(.dbd.O)--O--) linkages periodically in
the backbone. Aliphatic polyesters are polyesters that contain
alkyl groups, including alkanes, alkenes and alkynes, but do not
contain aromatic groups. The alkyl groups may be linear, branched
and/or cyclic.
[0035] A preferred class of aliphatic polyesters is the
poly(hydroxyalkanoate) family (PHAs). PHAs have the general
structural formula (I): ##STR1## where R is hydrogen or an alkyl
group, x is an integer from 0 to 10, and n is the number of
repeating units. Typically, R is hydrogen or an alkyl group
containing from 1 to 15 carbon atoms. The physical properties of
PHAs can be controlled by altering the R group and/or by altering
the number (x) of --CH.sub.2-- groups between the ester groups.
Different repeating units may also be combined into a single
polymer, and the nature and distribution of the repeating units can
also affect the final properties. Typical examples of PHAs include
poly(3-hydroxybutyrate), poly(4-hydroxybutyrate),
poly[(3-hydroxybutyrate)-co-(3-hydroxyvalerate)], poly(glycolic
acid), poly(lactic acid), and poly(caprolactone). A general
description of PHAs, and of the relationships of chemical structure
with mechanical properties and biodegradation behavior, is given in
U.S. Pat. No. 6,548,569.
[0036] One specific example of biodegradable reinforcing fibers
that are wettable are fibers made of poly(lactic acid) (PLA). PLA
is a biodegradable PHA having the general formula (I) where R is
methyl (--CH.sub.3) and x is zero. Fibers of PLA can be made to
have a tensile modulus of at least 2 gigaPascals (GPa), providing
for good resiliency of the fibers and of absorbent composites
containing the fibers. Since PLA is thermoplastic, exposure of a
composite containing PLA to elevated temperatures can provide for
softening or melting of the outer surface of the fibers and
subsequent bonding of the fibers to the other components of the
composite. It has been found surprisingly that PLA fibers can be
used as reinforcing fibers in absorbent composites to yield
composites having increased absorbent capacity, improved liquid
wicking, better liquid intake, higher resiliency of the absorbent
structure, and improved permeability of the structure. Examples of
biodegradable PLA fibers that may be used as reinforcing fibers
include the PLA fibers disclosed in U.S. Pat. Nos. 6,506,873 and
6,177,193, and the PLA/PLA bicomponent fibers disclosed in U.S.
Pat. No. 5,698,322.
[0037] The high modulus of PLA fibers can impart increased
resiliency to stabilized composites. There have been efforts to
increase the resiliency of absorbent composites by increasing the
gel stiffness, also referred to as the gel strength, of the
superabsorbent material. However, an increase in gel stiffness in a
superabsorbent material can result in a decrease in the absorbent
capacity of the superabsorbent in conventional systems. Absorbent
capacity is the amount of liquid absorbed per unit mass of
superabsorbant material.
[0038] Gel stiffness may be defined as the ratio of the absorbency
under a load of 0.9 pounds per square inch (psi) to the centrifuge
retention capacity. The measurement of gel stiffness is described
in U.S. Pat. No. 5,415,643, which is incorporated herein by
reference. Alternatively, direct measurements of the response of a
swollen gel to shear forces may be used to determine gel strength;
Onwumere et al. (U.S. Pat. No. 5,047,456, granted 10 Sep. 1991,
assigned to Kimberly-Clark Corporation), incorporated herein by
reference, provides such a test method. In particular, gel strength
may be represented by the shear or storage modulus G' of a swollen
gel at a strain of 1% and frequency of 1 rad/sec measured using a
RDS-II Rheometer (Rheometrics, Inc. Buffalo Grove, Ill.) and
expressed in units of dynes/cm.sup.2.
[0039] It has been observed surprisingly that PLA fibers can be
used in a composite with high gel stiffness superabsorbent
materials to improve the absorbent capacity of the composite. PLA
fibers can also be used in a composite with superabsorbent
materials having lower gel stiffness to increase the resiliency of
the composite. Absorbent composites comprising superabsorbents with
a gel strength in the range of 500-80,000 dynes/cm.sup.2 can
advantageously benefit from the inclusion of PLA fibers.
Furthermore, absorbent composites comprising superabsorbents with a
gel strength in the range of 1000-40,000 dynes/cm.sup.2 or in the
range of 2000-20,000 dynes/cm.sup.2 can benefit from the inclusion
of PLA fibers.
[0040] Resiliency in absorbent composites is useful in keeping the
absorbent structure open and in preventing collapse of the
capillary structure in the composite upon absorption of liquid.
This open structure is believed to allow better penetration of
liquid into the composite, thereby improving permeability.
Improving the permeability of a composite is beneficial in
improving the overall performance of an absorbent product and can
result in better liquid intake, more efficient utilization of the
absorbent capacity, and increased skin dryness for the user.
[0041] The resilient nature of PLA helps to reinforce the composite
structure and provides for greater void volume under an applied
external load. This can lead to a higher absorbent capacity of the
composite compared to composites containing only superabsorbent
material and pulp fibers. The ability of PLA fibers to reinforce
the absorbent composite enables superabsorbents with low gel
stiffness to function effectively in structures stabilized with
PLA. This is particularly beneficial for obtaining satisfactory
performance using biodegradable superabsorbents, which typically
have lower gel stiffness compared to conventional polyacrylate
superabsorbents.
[0042] The wettability of PLA can reduce or eliminate the need for
surfactant treatments. To stabilize absorbent composites containing
superabsorbent material and pulp fibers, the contact angle of the
reinforcing fibers ideally is low enough to be wetted by the liquid
insult, so that the liquid can be taken up efficiently into the
composite. The advancing contact angle of PLA is about 82 degrees,
and the receding contact angle is about 68 degrees. Thus, PLA
fibers are sufficiently wettable in the absence of wetting agents,
surfactants, spin finish agents, or other surface treatments. The
wettability of PLA also enables larger quantities of PLA
reinforcing fibers to be incorporated into an absorbent composite
without impairing the wicking behavior of the composite. For
example, conventional polyethylene/PET bi-component binder fibers
typically cannot be incorporated into an absorbent composite at
levels greater than 5 wt %, as higher loadings reduce the rate of
liquid absorption of the composite. In contrast, it has been found
surprisingly that PLA reinforcing fibers can be added at levels as
high as 25 wt % of the absorbent composite without reducing the
wicking of liquids.
[0043] The biodegradability of PLA fibers can allow for the
development of biodegradable absorbent composites. The absorbent
properties of biodegradable superabsorbent materials typically are
not as good as those of non-biodegradable superabsorbents, such as
the conventional polyacrylate systems. Biodegradable
superabsorbents tend to have lower gel stiffness than the
non-biodegradable materials, which can limit the rate of absorption
as well as the overall absorbent capacity. PLA fibers can be used
successfully as reinforcing fibers in biodegradable superabsorbent
materials, and the entire absorbent composite can thus be disposed
of as biodegradable waste. Moreover, the use of PLA fibers as
reinforcing fibers in biodegradable absorbent composites
surprisingly can improve the absorbent properties of biodegradable
systems to levels comparable to those of polyacrylate
superabsorbents.
[0044] Biodegradable reinforcing fibers also include fibers
containing PLA together with other polymers, which may or may not
be biodegradable. It is noted that a fiber may contain a
significant amount of non-biodegradable material, yet still fall
within the definition of "biodegradable" as set forth above when
the entire fiber is considered. Examples of biodegradable
reinforcing fiber materials containing PLA together with other
polymers include blends of PLA with non-biodegradable polymers such
as polyethylene, polypropylene, polystyrene, and poly(ethylene
terephthalate). Examples also include composite fibers of PLA and
other polymers. Preferably the biodegradable reinforcing fibers
contain less than 10 wt % of non-biodegradable material. More
preferably the biodegradable reinforcing fibers contain less than 7
wt % of non-biodegradable material, still more preferably contain
less than 5 wt % of non-biodegradable material, and still more
preferably contain less than 1 wt % of non-biodegradable
material.
[0045] The individual components of the absorbent composite can be
present in varying amounts. However, it has been found that the
following percentages work well in forming an absorbent composite
for use in an absorbent structure. The pulp fibers can range from
about 25 wt % to about 85 wt % of the absorbent composite. The
reinforcing fibers can range from about 5 wt % to about 30 wt % of
the absorbent composite. The superabsorbent can range from about 10
wt % to about 70 wt % of the absorbent composite. In one example,
an absorbent composite for absorbing and retaining urine contains
about 58 wt % pulp fibers, about 10 wt % of reinforcing fibers, and
about 32 wt % of superabsorbent. In another example, an absorbent
composite for absorbing and retaining a variety of aqueous body
wastes, including urine, contains from about 35 wt % to about 60 wt
% of pulp fibers, from 5 wt % to about 25 wt % of reinforcing
fibers, and from about 15 wt % to about 40 wt % of
superabsorbent.
[0046] The pulp fibers preferably are present in the absorbent
composite in a greater weight percentage than the reinforcing
fibers. Desirably the weight ratio of pulp fibers to reinforcing
fibers is in the range of 1:1 to 5:1. Alternatively the weight
ratio of pulp fibers to reinforcing fibers is in the range of 1.5:1
to 3:1. This can help to reduce the overall cost of the absorbent
composite, as cellulosic pulp fibers are generally much less
expensive than reinforcing fibers, and to ensure that an absorbent
article has sufficient liquid absorbing capacity. Preferably, the
reinforcing fibers are present in a loading of at least 5 wt % of
the absorbent composite to ensure that the absorbent composite has
sufficient mechanical properties.
[0047] It is desirable that the weight ratio of superabsorbent to
reinforcing fibers be in the range of 1:1 to 1:4. Alternatively,
absorbent composites with the weight ratio of reinforcing fibers to
superabsorbent be in the range of 0.33:1 to 0.75:1 may be used. In
these ranges there are sufficient reinforcing fibers to enhance
absorbent performance without adding excessive cost.
[0048] The absorbent composite can be formed by mixing the
superabsorbent, biodegradable reinforcing fibers, and optional
fibrous pulp. This mixture, in a substantially dry condition, can
then be compressed to a density ranging from about 0.09 grams per
cubic centimeter (g/cm.sup.3) to about 0.3 g/cm.sup.3. Preferably,
the absorbent composite is compressed to a density ranging from
about 0.15 g/cm.sup.3 to about 0.22 g/cm.sup.3. More preferably,
the absorbent composite is compressed to a density of about 0.2
g/cm.sup.3. This compression of the absorbent composite will assist
in forming the absorbent article.
[0049] Absorbent composites can be made by a variety of methods,
including airlaid, carding, wetlaid and coform processes. Exemplary
embodiments of airlaid processes are described in U.S. Pat. Nos.
4,666,647; 5,028,224; 6,207,099; 6,479,061. The carding process
uses a "card" which is a machine consisting of a series of rolls,
the surfaces of which are covered with many projecting wires or
metal teeth. See, for example, the "Dictionary of Fiber &
Textile Technology", Hoechst Celanese Corp., Charlotte, N.C., 1990.
Carding separates, aligns, and delivers fibers as a nonwoven web.
The wetlaid process consists of dispersing fibers in an aqueous
suspension and then filtering the fibers onto a screen belt or
perforated drum. Wetlaid nonwovens generally utilize shorter fibers
than carding. The wetlaid process results in a more random
orientation of the fibers and more isotropic properties than
carding. Exemplary embodiments of coform processes are described in
U.S. Pat. Nos. 4,100,324 and 5,952,251.
[0050] If thermal bonding is desired, the mixture can be heat cured
prior to compression, during the compression, or after the
composite has been compressed. For example, heat curing can be
carried out by heating the mixture to a temperature of about
165.degree. C. for a time from about 8 seconds to about 10 seconds.
Alternatively, microwave radiation may be used to heat the
absorbent composite, using methods such as those disclosed in U.S.
Pat. No. 5,916,203. The thermoplastic nature of PLA allows PLA
reinforcing fibers to be thermally bonded to one or more other
components of the absorbent composite. PLA fibers can be fabricated
to have a wide range of melting points, allowing for optimization
of the time and temperature of the bonding process.
[0051] In some absorbent composite systems, PLA reinforcing fibers
provide better absorbent properties when they are not thermally
bonded. For example, it has been found surprisingly that absorbent
composites containing un-bonded PLA fibers may provide more rapid
vertical wicking than composites containing no reinforcing fibers
or containing PLA fibers that have been thermally bonded into the
composite. This is in contrast to conventional binder fibers, which
provide their optimum performance only when the binder fibers have
been thermally bonded. The elimination of thermal bonding from the
manufacturing process may reduce production time and cost and may
reduce the variability of properties of absorbent composites within
a particular system.
[0052] In one embodiment, stabilized absorbent composites
containing biodegradable reinforcing fibers have minimal tensile
strength. Composites of this type can be incorporated into a
disposable absorbent product by, for example, depositing a portion
of the composite onto a substrate and depositing a layer over the
composite to secure the composite within the product. In another
embodiment, stabilized absorbent composites containing
biodegradable reinforcing fibers can have sufficient tensile
strength in the machine direction to allow the composite to be
wound into rolls. Rolls of this type can be unwound later, and the
unwound composite can be processed on conventional converting
equipment.
[0053] The tensile strength of the composite can be adjusted by
changing parameters including the concentration of the reinforcing
fibers, the conditions used for the optional thermal bonding, the
specific density to which the composite is compacted, and other
parameters known to those skilled in the art. Tensile strengths of
absorbent composites can be tested using a model MTS/Sintech 1/S
which is commercially sold by MTS Systems Corporation (Research
Triangle Park, N.C.). The tensile strength at peak load is measured
by securing a 50 mm strip of stabilized material between two
movable jaws of a tensile tester. A distance of about 10 cm
initially separates the two jaws, and the two jaws are then moved
outward away from one another at a rate of 25 cm/minute until the
strip of material breaks. The tensile strength is recorded as peak
load. Absorbent composites that have been thermally bonded may have
a tensile strength of at least 12 Newtons per 50 mm (N/50 mm).
Absorbent composites that have not been thermally bonded typically
will have a tensile strength of less than 12 N/50 mm.
[0054] The following specific examples are given by way of further
illustration, and are not meant to limit the above disclosure or
the claims that follow.
EXAMPLES
Examples 1-12
Formation of Absorbent Composites
[0055] Individual mixtures were prepared containing 40 wt %
superabsorbent and 60 wt % fibrous component, which included
fibrous pulp and biodegradable reinforcing fibers. The
superabsorbent was one of three superabsorbent
materials--biodegradable superabsorbent, high gel stiffness SXM
9543, or FAVOR SAB 880--all obtained from Stockhausen, Inc.
(Greensboro, N.C.). The fibrous pulp was CR 1654, a southern
softwood kraft pulp made by Alliance Corporation (a unit of The
Aaron Group of Companies, Plymouth Meeting, Pa.).
[0056] The PLA fibers were mono-component staple PLA fibers
produced by Fiber Innovations Technology (FIT, Johnson City,
Tenn.). These fibers had a melting point of about 162.degree. C., a
fiber length of 1.5 inch (3.8 cm), 3 denier fiber diameter, and
about 9 crimps/inch. Two types of PLA fibers were used. Referring
to Table 1, the fibers indicated as "neat" were fibers of pure PLA
without any separate substance on the fiber surface. The fibers
indicated as having a spin finish contained residual surfactant on
the fiber surface, since these fibers were treated with a spin
finish containing 0.03% surfactant.
[0057] Absorbent composites were then air-formed using a hand sheet
former to produce air-formed absorbent composites, each composite
having a basis weight of 400 grams per square meter (gsm). Each
air-formed composite was then transferred to a Carver Press and
densified to 0.2 grams per cubic centimeter (g/cc) density. As
indicated in Table 1, for some of the composites the PLA fiber was
thermally bonded by heating the air-formed composite in a forced
air oven (also referred to as a through-air bonder). TABLE-US-00001
TABLE 1 Absorbent Composites Example No. Superabsorbent PLA (wt %)
Treatment 1 Biodegradable 15 neat 2 Biodegradable 15 spin finish 3
Biodegradable 25 spin finish 4 Biodegradable 25 bonding 5
Biodegradable -- -- 6 SXM 9543 7.5 neat 7 SXM 9543 15 neat 8 SXM
9543 15 spin finish 9 SXM 9543 15 bonding 10 SXM 9543 -- -- 11
FAVOR SAB 880 15 neat 12 FAVOR SAB 880 -- --
[0058] Permeability Measurements
[0059] Permeability is a measure of the ease with which liquid can
pass through a material. Abosorbent composites through which liquid
can pass more easily should have a higher value of permeability,
and are said to be more "open."
[0060] The permeability test uses Darcy's Law to calculate the
permeability by measuring the flow of liquid through a fully
swollen composite. This test was carried out as described in U.S.
Pat. No. 6,437,214, which is substantially equivalent to Federal
Test Method Standard FTMS 191 Method 5514, dated Dec. 31, 1968. In
measuring permeability, absorbent samples were cut into 23/8 inch
(6.0 cm) diameter circles and placed in a cup with a mesh screen at
the bottom. The cup with the sample was placed in 0.9 wt % aqueous
sodium chloride solution (saline) for 60 minutes. The cup was then
removed from the saline bath and filled to 7.8 cm with saline. A
fluid delivery pump was adjusted to maintain this liquid height for
60 seconds while liquid flowed through the sample into a container
placed on a balance. The flow rate was measured and was then used
to calculate permeability (K) using Darcy's law: K = H Q .mu. A p
.DELTA. .times. .times. P ##EQU1## where H is the height of the
composite after swelling in cm; Q is the mass flow rate in g/s,
.mu. is the liquid viscosity in poise; A is the cross-sectional
area in cm.sup.2, p is the liquid density in g/cm.sup.3; and
.DELTA.P is the hydrostatic head in dyne/cm.sup.2. Through
conversion factors, the composite permeability is reported in units
of darcies (1 darcy=10.sup.-8 cm.sup.2).
[0061] The effect of biodegradable reinforcing fibers on the
permeability of the absorbent composite is shown in FIG. 1. For
each of the three superabsorbents, the permeability was measured
for absorbent composites with no reinforcing fibers and with 15 wt
% PLA reinforcing fibers. Each of the systems showed an increase in
permeability with the inclusion of PLA reinforcing fibers. These
results indicate that adding PLA reinforcing fibers should enhance
the permeability of any absorbent composite containing a
superabsorbent material.
[0062] Vertical Wicking Measurements
[0063] Vertical wicking is a measure of the ease with which a
liquid is absorbed into a material. The ability of a composite to
wick liquid vertically and to distribute the absorbed liquid is
related to its capillary tension, which in turn is a function of
the surface tension between the composite and the liquid and of the
pore size within the composite. In measuring vertical wicking,
12.5.times.3 inch (31.8.times.7.6 cm) absorbent composite samples
were placed vertically into a pool of saline for 30 minutes. The
mass of liquid absorbed by the sample in a given amount of time was
recorded.
[0064] The effect of PLA staple fibers on vertical wicking is
illustrated in the graphs of FIGS. 2 and 3, each of which plot the
mass of liquid absorbed as a function of time. FIG. 2 shows the
results for Example Nos. 1-5, containing biodegradable
superabsorbent. The composite containing 15 wt % PLA without
surfactant treatment ("neat PLA") had more rapid liquid absorption
compared to the composite containing 15 wt % PLA with surfactant
treatment. Also, liquid absorption was higher at 15 wt % PLA fiber
content than at 25 wt % PLA fiber content. FIG. 3 shows the results
for Example Nos. 6-10, containing the high gel stiffness
superabsorbent. Here also PLA without surfactant treatment provided
for more rapid liquid absorption. In both the biodegradable
superabsorbent system and the high gel stiffness system, thermal
bonding of the PLA reinforcing fibers resulted in the lowest
measurements of vertical wicking.
[0065] X-Ray Densitometry Measurements
[0066] After a sample has been analyzed in the vertical wicking
test, liquid distribution within the sample can be determined using
x-ray densitometry. The sample containing the absorbed liquid is
placed flat in an x-ray unit and exposed. The x-ray image is
captured and analyzed for liquid distribution using software from
OPTIMUS. The amount of liquid absorbed in the composite for a range
of heights above the saline pool is measured and plotted to obtain
the liquid profile of the product.
[0067] Liquid distribution profiles for the absorbent composites of
Example Nos. 7 and 10 (high gel stiffness superabsorbent) and of
Example Nos. 1 and 5 (biodegradable superabsorbent) are shown in
FIGS. 4 and 5, respectively. FIG. 4 illustrates that the liquid
distribution profile for composites containing a high gel stiffness
superabsorbent with 15 wt % PLA fibers is a more even distribution
than for the same composite without PLA reinforcing fibers. This is
observed in that more liquid is present at greater heights above
the saline pool for the PLA-containing composite than for the
control. A similar improvement in the liquid distribution with
incorporation of 15 wt % PLA fibers is also illustrated in FIG. 5
for the lower gel stiffness biodegradable superabsorbent
system.
[0068] The liquid wicking and distribution results illustrated in
FIGS. 2-5 all indicate a surprising and unexpected benefit in using
PLA fibers to improve liquid distribution. This is in addition to
the surprising and unexpected benefit in using PLA fibers to
increase the overall permeability of absorbent composites.
[0069] It is therefore intended that the foregoing detailed
description be regarded as illustrative rather than limiting, and
that it be understood that it is the following claims, including
all equivalents, that are intended to define the spirit and scope
of this invention.
* * * * *