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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention concerns water absorbent polymer articles and a process for
their production. More particularly, the invention discloses a starch or
cellulose-based water absorbent polymer combined with a foamable latex
polymer to form a latex foam polymer containing a water absorbent polymer
within its structure.
2. Prior Art
Recently there has been interest in producing absorbent materials
particularly for use in products such as sanitary napkins, diapers,
disposable dust cloths, etc. Some 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, highly water absorbent material.
Substitutes for these materials such as cross-linked polyethylene oxides,
cross-linked polyvinyl alcohols and hydrolyzed products of
starch-polyacrylonitrile-grafted polymers 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 their costs and difficulties of
production. In addition, some of these materials create disposal problems
because they are not biologically degradable.
Japanese Patent Application Kokai, No. 57-92,032 (1982) discloses a
polyurethane foam that contains a useful water absorbent polymer wherein
the size of the water absorbent resin is in the range from about 200 to
400 microns. This Kokai fails to disclose the combination of a water
absorbent polymer with a latex foam.
U.S. Pat. No. 4,725,428 discloses a process for making a crosslinked, super
absorbent polyurethane foam which contains a plurality of polycarbonyl
moieties convalently attached to the polyurethane through at least one
urethane, thiourethane or urea linkage. See also U.S. Pat. No. 4,725,629.
However, neither of these patents discloses the combination of a water
absorbent polymer with a latex foam.
A biodegradable, highly water absorbent polymer is 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
use of this water absorbent resin within a confined structure or for use
with other polymers within a latex foamed structure.
U.S. Pat. Nos. 4,454,268, 4,337,181 and 4,133,784 disclose various types of
films partially comprised of 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 water absorbent polymer disclosed herein or the mixture of a
water absorbent polymer with a conventional latex foamable polymer to form
a latex foam which exhibits high water absorbency.
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
discloses the use of a polyurethane foam as a support for the polymeric
absorbent. However, it fails to disclose the use of a water absorbent
polymer in general, the water absorbent polymer disclosed herein, or a
latex foam support.
Latex materials, particularly latex foams have been well known for many
years. The largest single use of latex today is in foam rubber. Latex
foams are frequently used in mattresses, pillows, seat cushions, carpet
backing and textile foam laminates. Latex foams also find use in providing
cushioning in many types of fabrics such as athletic clothing.
There are numerous processes for the production of latex foams, because of
their wide range of utility. See for example, U.S. Pat. Nos. 3,650,995,
4,205,103, and 4,174,415. While numerous latex foam patents exist, none
discloses the use of a latex foam containing within its structure a
cellulose based water absorbent polymer or a process for production of
such a material.
Accordingly, it is an object of this invention to prepare latex foams
containing within their structure water absorbent polymers.
It is a further object of this invention to disclose water absorbent latex
foams which contain water absorbent polymers within their structure which
are useful for the absorbance of fluids while retaining their shape.
It is a still further object of this invention to disclose a process for
preparing latex foams containing water absorbent polymers which are
secured to a backing to form a water absorbent latex foam laminate.
These and other objects, as well as the scope, nature, and utilization of
this invention, will be apparent from the following detail description.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a water
absorbent polymer latex foam which is produced by the following steps:
a. preparing a foamable latex polymer material;
b. foaming said latex polymer material;
c. blending a water absorbent polymer with the latex foam material; and
d. drying the latex foam material containing the water absorbent polymer
within its structure to form a water absorbent latex polymer foam.
The products produced by this process can be highly useful in those areas
where high water absorbance 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 products where high water absorbance of
a foamed material is important. In addition, this product can be combined
with a porous cover sheet to permit the water absorbing latex foam to draw
fluid through the porous cover sheet to form water absorbent materials for
use in food packaging etc. In addition, this water absorbent latex
material can be used with athletic garments such as foot wear, athletic
gloves, etc. This product not only absorbs moisture, but when backed with
a liquid impermeable product, keeps the moisture away from the skin
surface of the individual using the product.
DETAILED DESCRIPTION OF INVENTION
The water absorbent polymers used in the instant invention are solid, water
insoluble but water swellable polymers which are capable of absorbing many
times their own weight of water or aqueous solutions. These products are
polymers of water soluble acrylic or vinyl monomers which are slightly
crosslinked with a polyfunctional reactant. Such crosslinked 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
polyacrylonitrile 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 crosslinking agent. This process is
described in U.S. Pat. No. 4,076,663, which is incorporated by reference.
Water absorbing polymers can also be made as described in U.S. Pat. No.
4,340,706, incorporated by reference, by the inverse polymerization of
acrylic acid followed by crosslinking with a polyfunctional component,
e.g., epichlorohydrin. Other water absorbing polymers and processes for
their manufacture are disclosed, for example, 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 about 0.5 micron to about 450 microns and are
capable of absorbing at least about 15 times their weight of an aqueous
fluid. In a preferred embodiment superior absorption capabilities exist
where the water absorbing polymer particles are less than about 30 microns
in size. It is most preferred that the water absorbent polymer particles
are reduced to a fine powder preferably less than about 5 microns in size.
These particles show aqueous absorbance capability in excess of 35 times
their weight.
These water absorbing polymer particles swell when they absorb aqueous
fluids. The particles maintain their approximate shapes and geometry but
the dimensions thereof are greatly enlarged.
In preparing the articles of this invention, the water absorbing polymers
may also be mixed with other particulate material which is insoluble in
water and organic liquids which are capable of absorbing or adsorbing
liquids. One example of other particulate matter is naturally occurring
cellulose materials, such as saw dust, crushed corncobs, cotton linters,
wood pulp, and the like. Another type of particulate matter useful in this
invention is silica gel which can absorb fluids. Other useful absorbants
are molecular-sieve zeolites, activated alumina and calcium sulfate, also
known by the tradename Drierite.
Ion-exchange resins can also be used as another particulate material in
combination with the water absorbing polymers of this invention.
Particularly useful ion-exchange resins are the strong acid, cation
exchange resins.
Other particulates which can be mixed with the water absorbing polymers for
use in this invention are clay minerals, such as kaolin, montmorillonite,
illite, vermiculite, glauconite, attapulgite and the like. These clay
minerals are mixtures of metal oxides, e.g., aluminum oxide, magnesium
oxide, potassium oxide, and silicon oxide, and generally exist in the
amorphous state.
Once the water absorbent polymer is prepared, and combined with other
particulate material, if appropriate, it is blended with a foamed latex
material. Any conventional latex can be used in the preparation of the
water absorbent polymer latex foam. For example the latex can be selected
from the group consisting of acrylic, styrene-butadiene rubber,
polyethylene, vinyl acetate, vinyl acetate/acrylic copolymers, polyvinyl
chloride copolymers, nitriles, vinyl acetate homopolymers and styrene
acrylic polymers, with acrylic or styrene-butadiene rubber latex the
preferred latexes.
The preferred latex foam has a low water and high air content. In a
preferred embodiment the percentage of water in the latex foam is less
than about 60 percent.
Prior to foaming there can be added to the latex certain foaming agents to
enhance the foaming tendency of the latex or to add crosslinking or other
specific traits to the foam structure. Any conventional latex foaming
agent or crosslinking agent can be used such as ammonium stearate,
phosphate esters, ethozylated alcohols, azodicarbonamide, sodium laurel
sulfate, sulfosuccinamates and mono-ester sulfosuccinates. Preferable
latex foaming agents include sodium laural sulfate and sulfosuccinamates.
Prior to foaming of the foamable latex polymer, additional products may
also be added to the foamable latex such as surfactants, fillers or
non-woven fibers to further enhance the latex foam's properties. For
example, in a preferred embodiment to assist in surface absorption,
surfactants such as Pluronic-type surfactants may be added prior to the
foaming operation. These products will enhance the capability of the latex
foams in their absorbance by increasing the rate at which the water passes
through the surface of the foam.
The latex material is foamed by any conventional foaming procedures and
preferably is foamed by mechanical means using commercially available
equipment manufactured by companies such as Oakes, Latex Equipment Sales
and Services, XKG (Reddy, Pennsylvania) or Gaston County Sales and
Services Corporation (Stanley, North Carolina).
The thickness of the latex foam can be any conventional thickness used in
the casting of latex foams. However, in a preferred embodiment the
thickness of the foam should be less than about 100 mls. By limiting the
thickness of the foam to less than 100 mls, the latex foam will maintain
its high water absorbency. When the thickness of the latex foam exceeds
about 100 mls, the water absorbent capabilities of the foam may be reduced
or the time of absorption may be increased.
After the latex material is foamed, the water absorbing polymer materials
are blended with the latex foam. Any conventional method of blending known
in the industry can be used for the blending of the water absorbent
polymer with the latex foam. For example, the water absorbent polymer can
be mechanically blended into the foam. In a preferred embodiment, the
water absorbent polymer particulates are sprayed into the latex foam which
has been cast onto a non-woven substrate. To effect the spraying of the
water absorbent particles, the particles are suspended in the spraying
medium, such as air, nitrogen or other gaseous environment, and then under
pressure transported to a conventional spray nozzle. The water absorbent
polymer particulates are then sprayed into the latex foam. The water
absorbent particulates are anchored to the foam by the spraying process by
penetrating the surface of the foam and becoming embedded in the foam
structure, which distributes the water absorbent particulates evenly
throughout the structure of the latex foam.
When mixing the water absorbent polymer with the foamed latex polymer, the
percentage of the water absorbent polymer in relation to the foamed latex
polymer may vary depending upon the degree of fluid absorption that is
desired. Obviously the greater the percentage of water absorbent polymer
within the latex foam, the greater the absorbent capabilities of the latex
foam. However, when the percentage of the water absorbent polymer is too
high, i.e. greater than about 75 percent, the structure of the latex foam
begins to fall apart. Thus, in a preferred embodiment the percentage of
water absorbent polymer within the water absorbent polymer latex foam
should be from about 5 to about 45 percent and preferably from about 5 to
about 25 percent of the overall structure.
In prior art water absorbent composites, the water absorbent particulates
are generally distributed through a fibrous web with no chemical or
physical means for attachment. This technique generated areas in a
material which did not contain water absorbent materials, thus reducing
the absorbency of the overall product. The instant procedure for producing
water absorbent latex foams provides evenly distributed cellular residence
for the water absorbent polymer particulates within the foam. In addition,
the method of the instant invention allows a one-step continuous process
for combining a latex foam with a water absorbent polymer particulate.
Following blending of the water absorbent polymer in the latex foam, the
latex foam is dried in a conventional drying oven, preferably a hot air
oven, with temperatures less than 200.degree. C. until sufficient water
has been driven off, i.e. for a period of about 15 seconds to 2 minutes
depending on the temperature of the oven and the thickness of the foam
material.
The latex containing the water absorbent polymer may be used alone or it
may be secured to a non-woven substrate. Any conventional non-woven
substrate can be used which will adhere or stay in contact with the latex
foam. The preferred substrate onto which is cast the latex foam material
is a flexible fabric which is permeable to liquid and can be bound easily
to the latex foam material. The fabric can be made of any of the well
known textile materials such as cotton, wool, rayon, acetate, acrylic,
propylene, copolypropylene, polyester, nylon etc. with the preferred
materials including polyesters, polypropylenes and nylons. The fabric can
be woven or knitted, though non-woven materials such as those made by the
chemical and mechanical bonding of dry laid webs, by wet processing using
modified paper making techniques or spun bonding techniques are preferred.
Of the non-woven materials, spun bonded fabrics are more preferable. In
addition, the material of this layer can be produced from combinations of
porous materials such as the combination of polyesters and cotton. This
material exhibits good wicking qualities to transfer fluid through the
material to the latex foam layer.
The latex foam with water absorbent polymer blended in their structure
shows significant water absorbance depending on the thickness of the latex
foam and the percentage of water absorbent polymer contained within the
structure. With about 5 percent water absorbent polymer within the latex
foam structure, the distilled water absorbance of the material is at least
about 35 times the weight of the water absorbent polymer and at least
about 12 times the weight of the water absorbent polymer in a one percent
saline solution. When percentages of the water absorbent polymer in the
latex foam approach 25 percent the absorption of distilled water increases
to about 65 times the weight of the water absorbent polymer and up to
about 14 times the weight in a one percent saline solution.
The following examples are given as specific illustration of the invention.
All parts and percentages are by weight unless otherwise stated. It is
understood however that the invention is not limited to specific details
set forth in the examples.
EXAMPLE
A styrene-butadiene rubber (SBR) latex foam was cast on a wet-lay non-woven
substrate. The SBR latex chosen was Unocal #83026, a hydrophilic SBR
latex. Using a conventional stirring mechanism, 200 grams of SBR latex was
foamed until the ratio of air to SBR latex was about 7:1. The SBR latex
was agitated for about 3 to 4 minutes.
The substrate used to support the foam was a wet lay web comprised of 80
percent polyester and 20 percent wood pulp. Following the drying of this
wet-lay web, the foamed SBR latex was spread while still wet on the
substrate. Three different thicknesses of the SBR latex were prepared,
respectively 20 mls, 80 mls and 40 ml. After the SBR latex was spread and
the thickness was set, the water absorbent polymer was sprayed by a
Nordson Air Fluidized Powder Spray apparatus into the SBR latex. The water
absorbent material was a graft copolymer of about 91 percent acrylic acid
and 9 percent oxidized starch crosslinked with 0.1 percent
N,N'-methylene-bis-acrylamide made by the process described in U.S. Pat.
No. 4,076,663. The water absorbent material comprised respectively 5 to 10
percent, 15 to 20 percent and 20 to 25 percent of the SBR latex mixture.
The particulate size of the water absorbent material was generally less
than about 30 microns.
After the water absorbent polymer was sprayed on to the SBR latex foam, the
material was dried in an oven at 250.degree. F. for approximately 15 to 20
seconds to 5 minutes. After the drying of the SBR latex material, tests
were run on the foam to determine its absorbency in both distilled water
and a 1 percent saline solution. Results of these test are shown on Table
I.
TABLE I
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WATER ABSORBENT SBR LATEX FOAM
Water Absorbant
polymer percentage 5-10 15-20 20-25
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THICKNESS, mls. 20 80 40
STATIC LIQUID ABSORPTION
DISTILLED WATER
% by weight 3500 4500 6500
1% SALINE SOLUTION
% by weight 1200 1000 1400
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As is apparent from these result, latex foams containing water absorbent
polymers which absorb a large amount of water in relation to the weight of
the foam can be produced. Latex foams of this type can be prepared using
various types of substrates, various amounts of the water absorbent
polymer and having different thicknesses. These foams will have great
utility because of their high degree of water absorbance in comparison
with conventional latex foams.
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