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The present invention is directed to an improvement in polyurethane foam
whereby the absorption and holding of water and aqueous solutions is
greatly increased over previously known polyurethane foam materials.
BACKGROUND OF THE INVENTION
Solid materials with a quantity of water and aqueous fluids have long found
utility in numerous arts. Both natural and synthetic materials have been
developed which will absorb various quantities of aqueous materials.
One of the earliest of such materials was obtained from the marine animal,
the sponge. Natural sponges were used in ancient Greece and Rome to apply
paint, as mops and by soldiers as substitutes for drinking vessels.
Natural sponges absorb considerable amounts of aqueous fluids and, as
such, are useful in medicine for absorbing blood and other body fluids,
and in industry wherever absorbent material is needed.
However, natural sponges have today been largely replaced by synthetic
"sponges". Advantages of synthetic sponges are their more constant supply
and their being available in any size or shape desired. Synthetic sponges
can be made from a variety of polymers including vinyls, viscose,
cellulose, rubber, polyurethane and so forth. Many of the materials
themselves are not inherently water absorbent per se and their water
holding capacity is a function of the sponge-like physical structure or by
use of some absorbing adjunct.
So-called sponge rubber may be made either from dry rubber or from latex.
Blowing produces one type of sponge rubber from dry rubber. During
vulcanization, the chemicals that have been added turn to gas and "blow"
tiny bubbles of air in the rubber compound. When the rubber gels, or sets,
in the mold, the bubbles are trapped in it. Blown sponge rubber may be
either hard or soft.
Foam rubber is a type of sponge rubber made by whipping air into latex,
much as a cook whips air into egg whites. Vulcanization takes place after
the foam gels in a mold. Foam rubber has millions of tiny cells filled
with air. Some types may be nine-tenths air and only one-tenth rubber.
Foam rubber is used for upholstery and foam strips for surgical use. Thus
both "sponge rubber" and "foam rubber" are closed cell foams wherein
closed voids or cells provide reservoir spaces for liquids.
Polyurethane foams are prepared by reacting a polyisocyanate with a
poly-hydroxy compound in the presence of a small amount of water as a
"blowing agent". The water reacts with isocyanate groups producing carbon
dioxide gas which forms the cells or trapped bubbles when the polyurethane
sets.
There has long been interest in producing more highly absorbent materials
particularly for use in such products as sanitary napkins, diapers,
disposable dust cloths, etc. Many of the prior art materials used to form
these products have been non-woven fabrics, papers, pulps, spongy urethane
resins, natural sponges and the like. However, these materials exhibit
relatively low water absorbency, thus failing to satisfy the need for a
low volume, high water absorbent material. Substitutes for these materials
such as cross-linked polyethylene oxides, cross-linked polyvinyl alcohols
and hydrolyzed products of starch-polyacrylonitrile-grafted polymer have
recently appeared on the market. While these products do show increased
water absorbency, they also suffer from significant disadvantages in that
their water absorbency is still not sufficiently high to justify the costs
and the difficulties of production. In addition, some of these materials
create disposal problems because they are not biologically degradable.
Thus, in the prior art, various proposals to increase the water absorbency
and holding capacity for synthetic foams have been advanced.
U.S. Pat. No. 4,104,435 relates to a synthetic sponge comprising a foamed
resilient material with a network of fibrous material containing therein.
U.S. Pat. No. 4,717,738 discloses a polyurethane resin based on a hydroxyl
containing polymer polyol and a process for making the same.
U.S. Pat. No. 4,725,629 discloses a super absorbent polyurethane foam based
on an interconnecting polymer network of a cross-linked polyurethane and a
cross-linked addition polymer containing a plurality of chain segments
made up of functional groups containing repeating units which may be the
same or different.
A biodegradable, high water absorbent polymer has been disclosed in U.S.
Pat. No. 4,076,663. While the resins of this patent do show increased
water absorbency, their use has been limited to mixing them with sanitary
napkins, diapers and other such products wherein the resins are used in
their particulate or powder form. Thus, this process fails to disclose the
combination of this water absorbent resin with a means for using these
polymers within a confined structure or for use with other polymers within
a foamed structure, thus retaining the absorbency of the water absorbent
material within the confines of a conventional foamed structure.
U.S. Pat. Nos. 4,454,268; 4,337,181 and 4,133,784 disclose various types of
films which contain water absorbent polymers. While these patents disclose
starch-based, water absorbent polymers prepared from a combination of
starch and ethylene acrylic acid copolymers, they fail to disclose the
particular type of water absorbent polymer disclosed herein or the mixture
of a water absorbent polymer with a conventional polymer which is
subsequently foamed to form a foam which exhibits high water absorbency
while retaining the characteristics of the polymer which form the polymer
foam.
U.S. Pat. No. 3,669,103 discloses water swellable, water insoluble
polymeric sorbents for the absorption of aqueous fluids wherein said
polymeric sorbents are lightly cross-linked polymers. This patent also
discloses the use of a water insoluble polyurethane foam as a support for
the polymeric absorbent.
U.S. Pat. No. 4,464,428, describes a closed cell foam of a cross-linked
plastic material (such as polyolefin) having a plurality of internal
canals which are filled with particulate material, fiber bundles and so
forth.
U.S. Pat. No. 4,394,930, relates to an absorbent foam product by reacting
under foaming conditions a solids, water insoluble, water-swellable
polymer (such as a polyacrylate), a solid blowing agent and a liquid
polyhydroxy compound.
U.S. Pat. No. 3,900,030 and U.S. Pat. No. 4,239,043 each describe a means
for increasing the absorbency of polyurethane foam catamenial devices. The
former suggests using a flexible open-celled foam in which a finely
divided, water-swellable polymer is uniformly dispersed and the latter
suggest improving foam absorbency by placing on the foam surface 5 to 35%,
by weight of cellulose fibers.
Japanese published application Ser. No. 55-168,104 (1982) discloses that
loss of water absorbing and water stopping properties of a polyurethane
foam can be minimized if the closed-cell polyurethane foam contains both
independent air bubbles as well as water-absorbing resin particles of a
specific size range. Water absorbing resins are disclosed to include
polymeric electrolyte prepared by grafting an acrylic acid type monomer
onto starch, acrylate polymer or copolymer hydrolysates, cross-linked
sodium polyacrylate and so forth.
U.S. Pat. No. 4,731,391, discloses a method of preparing a superabsorbent
polyurethane foam wherein polyurethane foam precursors are mixed and
reacted under foaming and free radical conditions with an ethylenically
unsaturated compound such as an acrylate, methacrylate or acryamide.
Japanese published application Ser. No. 57-92,032 (1982) disclosed a
polyurethane foam that contains a water absorbent polymer wherein the
percentage of the air bubble formation is in the range of 1 to 60 percent,
wherein the diameter of the cells is in the range of 200 to 400 microns
and wherein the size of the water absorbent resin is in the range from
about 200 to 400 microns.
Thus, the art has long sought a synthetic material which will absorb and
hold large quantities of water. It is therefore one object of the present
invention to provide a synthetic structure which will absorb and hold
large quantities of aqueous liquids. It is another object of this
invention to prepare polymer foams which are highly water absorbent and
which have the ability to retain and hold a high water content.
It is a further object of this invention to prepare foamed polyurethane
polymers containing highly water absorbent resin.
It is still a further object of this invention to provide water absorbent
foams which can be useful for the absorbence of fluids while retaining
their basic shape.
These and other objects, as well as the scope, nature, and utilization of
this invention, will be apparent from the following description.
BRIEF DESCRIPTION OF THE INVENTION
It has now been surprisingly discovered that a synthetic polymer skeletal
matrix can absorb and hold unexpectably large quantities of aqueous
liquids notwithstanding its completely open physical structure. The
skeletal matrix according to this invention is prepared by mixing a
hydrocolloid with polymer precursors, producing a polymer foam from that
mixture and then thermally reticulating the foam to produce the skeletal
matrix.
The foam produced by this process can be highly useful in those areas where
high water absorbence is critical, such as for use with diapers, sanitary
napkins and the like, packaging material for products which must avoid
exposure to water and other areas where high water absorbence of a
material in a foam structure is important.
DETAILED DESCRIPTION
Any type of polymer which can be foamed and subsequently reticulated can be
used as the polymer for producing the water absorbent polymer foam of this
invention. For example, cellular polystyrene, polyvinyl chloride,
copolymers of styrene N-acrylonitrile, and polyethylene foams can be
manufactured by a conventional physical stabilization process. Cellular
polystyrene, cellulose acetate, polyolefins and polyvinyl chloride foams
can be manufactured by any conventional decompression expansion process.
However, the most versatile method is a chemical stabilization process for
the production of foams such as polyurethane, polyisocyanates,
polyphenols, epoxy resins and silicon resins. Polyurethane is the
preferred polymer to be used in preparation of the water absorbent polymer
foam of this invention.
Thus, the present invention involves first producing a polymer foam which
contains in the polymer a superabsorbent material and then reticulating
that foam. The following disclosure will be expressed in terms of a
polyurethane foam but it should be understood that any suitable polymer
can be used in place of polyurethane.
The polyurethane foam (or other polymeric foam) which is subjected to
thermal reticulation in this invention can be prepared by known methods of
polymer foam production. To produce a foam suitable as a reticulation
precursor for the present invention, a mixture comprising a polyurethane
foam reaction mixture and a superabsorbent material dispersed therein is
prepared. The polyurethane foam reaction mixture is a conventional mixture
used to produce known polyurethane foams and comprises a polyisocyanate, a
polyhydroxy compound (polyol), a blowing agent and other conventional
additives such as catalysts, surfactants, and so forth, all of which are
well known to those of ordinary skill in the polymer foam art.
When the polyisocyanate and polyol react to form polyurethane, a gas or
vapor is generated in situ by the blowing agent while the reaction mixture
remains in the fluid or plastic state. The generation of this gas results
in the formation of bubbles, approximately spherical in shape, in the
plastic material. As the bubbles expand, cells are formed and the
resulting structure when cured comprises a foamed polyurethane with a
cellular structure with interconnecting cell membranes. A common blowing
agent is water which reacts with excess isocyanate groups to form, in
situ, carbon dioxide gas.
Additional materials may also be added to the foamable reaction mixture,
such as surfactants, fillers or non-woven fibers to further enhance foam
properties. For example, in one embodiment, surface absorption can be
assisted by adding surfactants such as Pluronic-type surfactants prior to
the foaming operation. These products will enhance the capability of the
foams in their absorbence by increasing the rate at which the water is
absorbed.
Suitable isocyanate reactants include, but are not limited to aromatic
polyisocyanates having a ratio of isocyanate groups to aromatic rings of
about 2:1 to about 1:1. Examples are toluene diisocyanate (TDI), phenylene
diisocyanate, xylene diisocyanate napthylene diisocyanate and diphenyl-4,
4-diisocyanate.
Polyols are generally categorized as polyether polyols or polyester
polyols. Polyether polyols are conventionally oxides, such as ethylene
oxide or propylene oxide, polymerized onto an active hydrogen compound
such as ethylene glycol, propylene glycol, glycerol and so forth.
Polyester polyols are conventionally polycondensation products of
polyhydric acids, such as adipic acid, maleric acid or phathalic acid,
with polyhydroxy compounds, such as ethylene glycol, propylene glycol,
glycerol and so forth.
In addition to the conventional foam-forming formulation of polyisocyanate,
polyol and blowing agent, the polyurethane foam used to prepare the
skeletal structure of the present invention must have incorporated therein
a solid superabsorbent material. A suitable superabsorbent material is one
which remains insoluble in the liquid it absorbs and will absorb at least
15 times its weight of the liquid. Both natural and synthetic
superabsorbent materials are known. Natural materials include guar gum,
other natural gums, starches, and so forth. Synthetic superabsorbents
include chemically modified cellulose such as carboxymethyl cellulose, and
acrylic-type polymers. Examples of acrylic-type superabsorbent materials
are starch grafted sodium polyacrylate and sodium polyacrylate.
The water absorbent polymers are solid, water insoluble, water swellable
polymers which are capable of absorbing many times their own weight of
water or aqueous solutions. Such materials include polymers of water
soluble acrylic or vinyl monomers which are slightly cross-linked with a
polyfunctional reactant. Such cross-linked polymers include
polyvinylpyrrolidone, sulfonated polystyrene, polysulfoethyl acrylate,
poly(2-hydroxyethylacrylate) polyacrylamide, polyacrylic acid, partial and
complete alkali metal salts of polyacrylic acid, and the like. Also
included are starch modified polyacrylic acids and hydrolyzed
plyacrylonitrile and their alkali metal salts.
Useful water absorbing polymers can be made by polymerizing acrylic acid
and starch in an aqueous medium using a polyfunctional monomer, e.g.,
N,N-alkylene-bis-acrylamide, as the cross-linking agent. This process is
described in U.S. Pat. No. 4,076,663. Water absorbing polymers can also be
made as described in U.S. 4,340,706 by the inverse polymerization of
acrylic acid followed by the cross-linking with a polyfunctional
component, e.g., epichlorohydrin. Other water absorbing polymers and
processes for their manufacture are disclosed in U.S. Pat. Nos. 4,654,039;
3,669,103 and 3,670,731. All of the aforesaid patents are hereby
incorporated by reference.
The water absorbing polymers particularly useful in this invention are
those described in U.S. Pat. No. 4,076,663. These water absorbing polymers
have a particle size of from 0.5 micron to about 450 microns and are
capable of absorbing at least about 15 times their weight of aqueous
fluid. In a preferred embodiment superior absorption capabilities exist
where the water absorbing polymer particles are less than about 30 microns
in size. These particles show absorbence capability in excess of 65 times
their weight.
These water absorbing polymer particles swell when they absorb aqueous
fluid. The particles maintain the approximate shapes and geometry they had
before contact with the fluid but the dimensions thereof are greatly
enlarged.
From about 1 to 100 parts of superabsorbent per hundred parts of polyol
foam forming material can be used. However, a preferred amount of
superabsorbent material is 5 to 75 parts per hundred parts polyol; 10 to
50 parts per hundred is more preferred and about 25 to 50 parts per
hundred parts polyurethane is most preferred.
Polyurethane foam containing the superabsorbent material is prepared by
conventional methods for urethane foam production. The superabsorbent
material in finely divided form is uniformly dispersed in a conventional
foam-forming mixture of polyisocyanate, polyol and blowing agent. Then the
mixture is reacted and the foam is formed. U.S. Pat. No. 4,731,391
describes a method for preparing a polyurethane foam having superabsorbent
material incorporated in the polyurethane.
To obtain the skeletal structure of the present invention, the polyurethane
foam containing superabsorbent material is subjected to thermal
reticulation whereby the windows or membranes are removed from individual
cells or bubbles which make up the foam structure thereby leaving an open
cell or skeletal structure. Reticulation results in a foam having in
excess of 80% open cells, preferably at least 95% open cells and most
preferably at least 99% open cells. Thermal reticulation of polyurethane
foam is a known procedure to those skilled in the art. See, for example,
U.S. Pat. No. 3,171,820 and U.S. Pat. No. 3,175,025. Fundamentally,
reticulation is achieved by providing a combustible mixture of an oxidizer
material and an oxidizable material within the cells of a polyurethane
foam and then igniting the combustible mixture whereupon the cell windows
or membranes are destroyed.
Thus, the skeletal structure of the present invention is completely open.
With such an open structure it is quite surprising that the structure of
this invention will more rapidly absorb large amounts of liquid (such as
water, blood, urine and so forth) than a corresponding non-reticulated
cellular polyurethane foam.
The following examples merely illustrate the present invention without
imposing limitations thereon.
EXAMPLES
A conventional polyurethane foam-forming formulation of toluene
diisocyanate and a polyether polyol was modified by adding various
indicated amounts of particulate superabsorbent material starch grafted
sodium polyacrylate (SGP). Specifically the SGP is a graft copolymer of
about 91 percent acrylic acid and 9 percent oxidized starch cross-linked
with 0.1 percent N,N'-methylene-bis-acrylamide made by the process
described in U.S. Pat. No. 4,076,663 with a particle size ranging from
about 30 to 140 microns.
Foams were made from each formulation and water holding capacity, on both
non-reticulated and reticulated foams, are listed below. To measure water
holding capacity, a foam test sample 5.times.3.times.1/4inches was
weighed, immersed in water for 1 hour, removed from water and let drain on
a screen for 3 minutes. The sample is then again weighed and the percent
water pick-up is calculated.
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Water Holding Capacity.sup.2
Example SGP.sup.1 Non-Reticulated
Reticulated
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CONTROL 0 19.1 17.3
1 5 13.7 18.5
2 10 16.1 25.4
3 15 16.1 29.1
4 20 18.1 29.9
5 25 20.8 36.7
6 50 19.9 30.3
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.sup.1 Parts of starch grafted sodium polyacrylate (SGP) per 100 parts
polyol.
.sup.2 Water Holding capacity in terms of grams water absorbed per gram o
foam.
Another test was run by modifying a conventional toluene
diisocyanate/polyether polyol formulation with four different
superabsorbent materials each in an amount of 50 parts per hundred parts
polyurethane.
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Water Holding Capacity
Non-
Example Superabsorbent
Reticulated
Reticulated
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CONTROL NONE 19.1 17.3
8 SGP 19.1 30.3
9 Na polyacrylate
20.6 38.6
10 Na carboxymethyl-
24.4 28.5
cellulose
11 guar gum 15.6 19.2
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To determine if water holding capacity increases with higher levels of
superabsorbent material over 25 parts per hundred parts polyurethane,
foams were prepared with six levels of SGP superabsorbent material from 25
to 50 parts per hundred and the resulting data is shown below:
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Water Holding Capacity
Example SGP Non-Reticulated
Reticulated
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12 25 20.8 36.9
13 30 19.8 33.0
14 35 19.3 33.0
15 40 20.5 34.5
16 45 19.6 34.8
17 50 19.9 31.7
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Thus, Examples 12-17 show that the water holding capacity does not increase
with an increase in SGP content from 25 to 50 parts per hundred and
remains substantially constant.
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