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Description  |
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BACKGROUND OF THE INVENTION
The present invention is related to an absorbent material for absorbent
articles. More particularly, the present invention is related to an
absorbent material fabricated from starch.
Absorbent articles containing an absorbent core, e.g., diapers and sanitary
napkins, are widely utilized. In general, such products have an absorbent
core which contains one or more layers of fluid absorbent materials, such
as fluffed cellulose batt, e.g., wood pulp, cotton fluff, tissue and the
like. The absorbent core may additionally contain particles of a
superabsorbent. Superabsorbents are hydrocolloids that absorb at least
about 10 times its own weight of aqueous fluid. Since superabsorbents are
relatively expensive, provide only weak structural integrity and require
high surface area to be efficient, they are in general employed as small
particulates that are dispersed or imbedded in a matrix of fluffed
cellulosic fiber batt, e.g., wood or cotton fluff, or in a thermoplastic
foam, e.g., polyurethane foam.
However, these prior art approaches in incorporating a superabsorbent into
an absorbent core have disadvantages. For example, superabsorbent
particles dispersed in a fluffed cellulosic fiber batt may settle over
time, altering the absorption performance and deviating from the designed
criteria of the absorbent article. In addition, the fiber batt tends to
collapse and close interfiber-capillaries when the batt is expose to
liquid, thereby hindering subsequent insults of liquid from having an
access to unused portions of the batt and the superabsorbent particles
dispersed therein. Furthermore, since superabsorbent particles must be
allowed to swell for the particles to efficiently absorb liquid, the
collapsed batt which hinders the expansion of the superabsorbent particles
prevents efficient use of the particles. An example of such
absorbent-hinderance problems is addressed in U.S. Pat. No. 5,147,343 to
Kellengerger.
As for dispersing superabsorbent particles in a polyurethane foam, the
inherent hydrophobic nature and rigidity of the polyurethane foam cause
inefficient use of the absorbent materials. The intercellular structure of
the foam may not provide sufficiently large enough space to allow the
superabsorbent particles to fully swell, causing an inefficient use of the
absorbent capacity of the superabsorbent particles. Moreover, as disclosed
for example in U.S. Pat. No. 4,985,467 to Kelly et al., the intercellular
structure of the inherently hydrophobic foam must be carefully engineered
to allow liquid to have proper access to the imbedded superabsorbent
particles.
Consequently, it would be desirable to provide an economical and highly
efficient superabsorbent-containing absorbent material that does not have
the above-illustrated disadvantages of the prior art absorbent materials.
SUMMARY OF THE INVENTION
There is provided in accordance with the present invention a resilient
absorbent foam containing starch, a superabsorbent material and a
plasticizer. The absorbent foam is a highly absorbent article that
additionally provides high dimensional stability, resiliency and
flexibility. In addition, the foam absorbent article is biodegradable and
flushable. Additionally provided is an absorbent foam containing starch
and a plasticizer, which is highly resilient and flexible.
The present invention also provides an absorbent article containing the
resilient absorbent foam. The absorbent article is suitable for
application where the article is subjected to multiple insults of liquid
inflows. The article does not contain the conventional carrier cellulose
fluff for superabsorbents, which tends to collapse upon a liquid insult
and hinders the full utilization of the absorbent capacity of the
superabsorbent, so that the superabsorbent dispersed in the foam absorbent
is efficiently utilized.
The foam absorbent is highly suitable for use in absorbent articles, such
as diapers, sanitary napkins, adult care products, wound dressings and the
like.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a 20 times magnified view of a starch foam of the present
invention, showing the closed cell structure.
FIG. 2 is a 100 times magnified view of the starch foam of FIG. 1, showing
the domains of a superabsorbent within the cell walls of the foam.
FIG. 3 is a 20 times magnified view of a starch foam that contains starch
and a plasticizing agent.
FIG. 4 is a 100 times magnified view of the starch foam of FIG. 3.
FIG. 5 is an absorbent article containing a starch foam absorbent core.
FIG. 6 is a 20 times magnified view of a starch foam of the present
invention that was compacted to impart improved flexibility and resiliency
.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an economical and efficient absorbent
material suitable for use in absorbent articles, such as diapers, sanitary
napkins, adult care products, wound dressings and the like. The absorbent
material is a starch foam produced from a starch blend composition. The
starch blend composition contains, based on the total weight of the
composition, from about 20 wt % to about 80 wt %, desirably from about 25
wt % to about 70 wt %, more desirably from about 30 wt % to 50 wt %, of
starch; up to about 70 wt %, desirably from about 5 wt % to about 60 wt %,
more desirably from about 10 wt % to about 50 wt %, most desirably from
about 20 wt % to about 40 wt %, of a superabsorbent; and from about 5 wt %
to about 40 wt %, desirably between about 10 wt % and 30 wt %, more
desirably between 15 wt % and about 25 wt %, of a plasticizing agent.
The term starch as used herein indicates reserve polysaccharides found in
plants, e.g., corn, wheat, potato and the like, and is a mixture of linear
(amylose) and branched (amylopectin) polymers of .alpha.-D-glucopyranosyl
units. As is known in the art, natural starches from different plants
contain different levels of amylose and amylopectin. Although different
starches containing different levels of the two glucopyranosyl units can
be employed for the present absorbent material, desirable starches contain
at least about 20 wt %, more desirably at least about 25 wt %, based on
the total starch weight, of amylose. In general, high amylose content
starch provides a more resilient starch foam.
Superabsorbents suitable for the present invention include natural or
synthetic ionic hydrocolloids and nonionic hydrocollids that are water
insoluble but water swellable polymer solids, e.g., polyacrylamides,
polyarylic acids, metal salts of polyacrylic acid, polyacrylates, polymers
and copolymer of vinyl sulfonic acid, polyacrylate grafted starches,
polyvinyl ethers, polyvinylpyrrolidone, sulfonated polystyrene
polysulfoethyl acrylate, and the like. Additionally suitable
superabsorbents are starch and cellulose based superabsorbents, including
carboxymethyl starch, carboxymethyl cellulose and saponified
starch-polyacrylonitrile graft copolymers. Of these, more desirable
superabsorbent materials are metal salts, e.g., sodium salt, of
polyacrylic acids.
The starch foam of the present invention further contains a polymeric
plasticizing agent or plasticizer to provide improved flexibility and
resiliency. Suitable plasticizing agents must have a sufficient thermal
stability to withstand the temperature of the foam production process and
do not significantly interfere with the absorbent property of the foamed
starch. Plasticizing agents suitable for the present starch composition
include polyvinyl alcohol, ethylene vinyl alcohol copolymer, polyvinyl
acetate, ethylene vinyl acetate and blends thereof. Of the suitable
plasticizing agents, polyvinyl alcohol is particularly desirable.
Specifically, polyvinyl alcohol suitable for the present invention has a
molecular weight of from about 25,000 to about 190,000, desirably from
about 30,000 to about 150,000, and a degree of hydrolysis of at least
about 70%, desirably from about 85% to about 99%.
Suitable starch foams of the present invention are produced when the starch
composition with a defined amount of water is treated at an elevated
temperature in a closed container to form a pressurized melt and then the
melt is exposed to an environment that allows the melt to expand and cool,
e.g., ambient environment. This foaming process can be conveniently
carried out in an extruder, particularly in an unvented extruder, which is
equipped with a heating jacket or heating element. Extruders useful for
the present invention include conventional single-screw and twin-screw
extruders, although twin-screw extruders are more desirable in that they
promote more thorough mixing of the starch foam composition. The
temperature and pressure of the extruder are controlled to process the
starch composition at an elevated temperature, which may be between about
100.degree. C. and about 200.degree. C., desirably between about
110.degree. C. and about 150.degree. C., and at an elevated pressure,
which may be between about 500 psi and about 1,300 psi. The starch
composition is fed to an extruder and gradually becomes molten while the
water content of the composition becomes superheated as the composition is
passed through the conveying zone and the compression zone of the
extruder. The molten starch composition is then conveyed to and extruded
through a die to form shaped foam articles. The die opening may have any
desired configuration to produce shaped foam articles that are suitable
for absorbent articles, e.g., sheets and rods. As the molten starch
containing compressed, superheated water is extruded and exposed to a
lower pressure and temperature environment, e.g., ambient environment, the
molten starch expands while approximately retaining the circumferential
contour of the opening of the die and solidifies to produce a foam
material having a closed cell structure. The extruded foam articles from
the present starch composition have a relatively uniform closed cell
structure with low density and good resiliency, flexibility and
dimensional stability. The flexibility and resiliency of the articles can
be further improved by calendering or flexing the extruded foam articles.
Alternatively, the starch composition can be extruded into pellets. The
pellets can be subsequently molded into an absorbent foam article in a
mold by packing the mold with the pellets and exposing the pellets to
humidity, e.g., a stream of steam or mist, under moderate pressure to form
a fused, shaped absorbent structure. It has been found that the present
starch foam pellets are highly shapable and fusible without losing the
closed cell structure and absorbent capacity when the foam pellets, more
specifically exterior surfaces of the pellets, are moistened. During the
molding process, heat may be applied to dry or remove moisture from the
fused, shaped absorbent article.
In order to obtain proper starch foam structures, the total moisture
content of the starch composition should be from about 5 to about 30
weight %, more preferably from about 9 to about 20 weight %, most
preferably from about 14 to about 17 weight %, based on the dry weight of
starch in the composition. Typically, commercially produced starch
contains about 7 to 12% by weight residual moisture. Therefore, little or
no addition of water is required to produce the present starch foam
absorbent material.
Surprisingly, it has been found that subjecting the superabsorbent through
the extrusion process does not measurably diminish its absorbent capacity
and that the starch foam structure integrally and securely incorporates
the superabsorbent. The starch foam which has the superabsorbent uniformly
and securely dispersed therein is highly suitable for the absorbent layer
of absorbent articles. Unlike the prior art foam matrices holding
superabsorbent particles, e.g., polyurethane foam, the starch foam matrix
does not significantly impede the swelling movement of the dispersed
superabsorbent since the foamed starch itself is highly water absorbent
and becomes highly malleable and deformable upon absorbing liquid. In
addition, the dispersed superabsorbent, which is securely incorporated
into the cell wall of the starch foam, does not dislodge from the foam.
Turning to the figures, FIGS. 1-2 are magnified views of an exemplary
absorbent material of the present invention, and FIGS. 3-4 are magnified
views of a starch foam, Example 14, infra, that is produced from a starch
composition which contains starch and a plasticizing agent. As will be
further described in Example 2, the foam composition of FIGS. 1 and 2
contains about 57 wt % of starch, 23 wt % of a superabsorbent and 20 wt %
of a plasticizing agent. FIGS. 1 and 3, which are 20 times magnified
views, show the closed cell structure of the present starch foam, and
FIGS. 2 and 4, which are 100 times magnified views, show the starch foam
structure in greater detail. FIG. 2, when compared to FIG. 4, shows thick
and rough sections in the cell structure of the foam, and these sections
are believed to be the superabsorbent dispersed in the starch matrix. As
can be seen from FIG. 2, the superabsorbent is securely incorporated into
the starch foam matrix as an integral part of the foam structure, forming
well disbursed and permanently integrated superabsorbent domains
throughout the wall of the foam structure.
Additional advantages of the starch foam absorbent material, which is
derived from natural starch, include that the foam matrix is highly
biodegradable and provides limited integrity and resiliency when wetted
with saline solutions. Consequently, the absorbent material, especially
when a starch-based superabsorbent is utilized therein, is highly
biodegradable, alleviating the disposal problem. In addition, an
application of limited agitation or force disintegrates the liquid
saturated starch foam into small pieces or segments that can easily be
composted or flushed.
The starch foam absorbent material in its unwetted form is a highly
flexible and resilient absorbent material and, thus, is useful for various
absorbent articles, including diapers, sanitary napkins, training pants,
incontinence products, wound dressings, absorbent layers for meat-trays
and the like. For example, the starch foam absorbent material is extruded
into a sheet and placed between a liquid-impermeable backing material and
a liquid-permeable facing material to form an absorbent article. Turning
to FIG. 5, there is provided an absorbent article 10, such as a diaper,
containing a liquid-impermeable backing material 12, a liquid-permeable
facing material 14 and a starch foam absorbent layer 16. The absorbent
layer 16 is formed from the present absorbent starch foam and can be in
the form of, for example, a shaped sheet. The starch foam sheet may, in
addition, be perforated or apertured to increase the surface area of the
absorbent layer, thereby further improving the speed of liquid intake. The
liquid-impermeable backing material 12 can be a film produced from a
thermoplastic, such as polyethylene, polypropylene, polyester, polyamide,
polyvinyl chloride and the like. Alternatively, the backing material can
be a microporouos film or a highly liquid-resistant nonwoven fabric that
is impervious to liquid but pervious to vapor, thereby increasing the
comfort of the user of the absorbent article. The liquid-permeable facing
material 14 can be a nonwoven or woven fabric, perforated film or
nonwoven, or any other relatively porous material that is known to rapidly
pass fluid to the absorbent material.
In accordance with the present invention, the starch composition desirably
is thoroughly preblended, for example, in a tumble blender, before the
composition is fed to the extruder. The starch foam composition may
further contain various additive such as processing aids, lubricants,
crosslinking agents, blowing agents, pigments, fillers, reinforcement
fibers and the like. Suitable lubricants and processing aids include metal
stearates, paraffinic waxes, vegetable and mineral oils, glycerine,
glyceryl esters, e.g., glyceryl monostearate, and the like. Suitable
filler fibers that improve the strength of the starch foam include
cellulosic fibers, e.g., beet fibers, soy fibers, fine pulp fibers and the
like.
It is to be noted that although the composition for the present absorbent
material is described to contain a superabsorbent, a highly useful
absorbent material, although it is not as absorbent as the above-described
absorbent material, can be produced in accordance with the present
invention without the addition of a superabsorbent since natural starch
foam itself readily absorbs aqueous fluids. Such absorbent foam can be
produced from a starch composition containing, based on the total weight
of the composition, from about 60 wt % to about 90 wt % of starch and from
about 40 wt % to about 10 wt % of the above-described plasticizing agent
as well as the above-illustrated processing aids and other additives.
The following examples are provided for illustration purposes and the
invention is not limited thereto.
EXAMPLES
Examples 1-14
Absorbent starch foam sheets were produced from starch compositions as
disclosed in Table 1. In addition, each starch composition contained about
0.5 wt % of an edible vegetable oil, about 0.5 wt % of glycerine and about
0.3 wt % of glyceryl monostearate, Myvaplex.TM., which is available from
Eastman Chemical Products, Inc., as processing aids.
The components of each starch composition were thoroughly blended and then
extruded through a Wenger TX-52 twin-screw extruder, having four zones
and a die. The four zones were a feeding zone, which was set at about
24.degree. C., a mixing zone, which was set at about 24.degree. C., a
conveying zone, which was set at about 24.degree. C. and a compression
zone, which was set at about 125.degree. C. The die, which had two
circular openings of a 0.08 inch diameter, was set at about 110.degree. C.
The feed rate was about 100 lbs/hour.
The resulting starch foam was weighed and then immersed in water for 3
minutes. The water saturated foam was placed on a porous wire screen of a
vacuum apparatus for 1 minute to drain excess water. The vacuum apparatus
was equipped with a porous wire screen on the top and a neoprene lid that
contiguously covers the wire screen and closes the vacuum apparatus. Then
the lid was closed and 0.5 psi vacuum pressure was applied for 5 minutes.
The weight of the foam was then measured and the absorbent capacity of the
foam was calculated as follows: (weight of wet foam--weight of dry
foam)/weight of dry foam. The result is shown in Table 1.
TABLE 1
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Composition
(weight %)
Superabsorbent
Plasticizer
Capacity
Example
Starch Type Content
Type Content
(g/g)
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1 67 IM 23 PV3 10 11.5
2 57 IM 23 PV3 20 9.7
3 60 IM 20 PV3 20 13.6
4 50 IM 30 PV3 20 13.9
5 40 IM 20 PV3 30 13.6
6 40 IM 20 PV3 40 14.5
7 40 IM 40 PV3 20 15.8
8 40 FA 40 PV3 20 17.1
9 60 IM 20 PV5 20 13.3
10 50 IM 30 PV5 20 11.3
11 72 G4 8 PV3 20 7.6
12 60 IM 20 EPV 20 6.1
13 80 IM 20 -- 0 10
14 80 -- 0 PV3 20 6.3
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IM = superabsorbent Sandwet .TM. IM 5000, which is a polyacrylate
superabsorbent and is available from Hoechst Celanese.
FA = superabsorbent Favor .TM. 870, which is a polyacrylate superabsorben
and is available from Stockhousen.
G4 = polyacrylate grafted starch superabsorbent Waterlock .TM. G400, whic
is available from Grain Processing.
PV3 = polyvinyl alcohol Airvol .RTM. 325, which is available from Air
Products and Chemicals, Inc. and its hydrolysis level is about 98% and
molecular weight is between about 85,000 and 146,000.
PV5 = polyvinyl alcohol Airvol .RTM. 540, which is available from Air
Products and Chemicals, Inc. and its hydrolysis level is about 88% and
molecular weight is between about 124,000 and 186,000.
EPV = ethylene polyvinyl alcohol EVAL .TM. 400, which contains about 44 w
% of ethylene. EVAL is available from EVAL Corp of America.
All of the starch foams of the above examples, except Example 13, were
resilient as well as aqueous-fluid absorbent, making them highly suitable
for absorbent articles. The starch foam of Example 13, which did not
contain a plasticizing agent, was a rigid material that was irreversibly
compacted and disintegrated into an aggregate of powder or small clumps
when pressure was applied.
As stated above and illustrated in FIGS. 1-4, the present starch foam is a
resilient absorbent material, and in addition, the starch foam
incorporates superabsorbents into its wall matrix, improving the
absorbency of the foam and securely holding and evenly distributing the
superabsorbents throughout the matrix.
Example 15
A foam sheet was produced from the starch composition of Examp | | |
