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CROSS-REFERENCE TO RELATED APPLICATIONS
A superabsorbent polyurethane foam based on an interpenetrating polymer
network of a crosslinked hydrophilic polyurethane and a crosslinked
addition polymer containing a plurality of carbamoyl, substituted
carbamoyl, or carboxy groups or the alkali metal or ammonium salts
thereof, is described in application Ser. No. 888,545, entitled
SUPERABSORBENT POLYURETHANE FOAMS, filed of even date in the names of Chad
E. Garvey and Jose F. Pazos and assigned to the assignee of the present
application.
A superabsorbent polyurethane foam containing a plurality of polycarbonyl
moieties covalently attached to the polyurethane through at least one
urethane, thiourethane, or urea linkage, said polycarbonyl moieties
comprising from about 5 to about 100 carbonyl-containing repeating units,
the carbonyl portions being independently selected from the group
consisting of carbamoyl, substituted carbamoyl, and carboxy and alkali
metal and ammonium salts thereof, is described in application Ser. No.
888,301, entitled SUPERABSORBENT POLYURETHANE FOAMS, filed of even date in
the names of Chad E. Garvey, Jose F. Pazos, and Gerard J. F. Ring and
assigned to the assignee of the present application.
BACKGROUND OF THE INVENTION
The present invention relates to polyurethane foams. More particularly, the
present invention relates to polyurethane foams having greatly enhanced
absorbent capabilities, i.e., superabsorbent polyurethane foams, which are
based on quasi-interpenetrating polymer networks.
Polyurethane foams are, of course, well known to those having ordinary
skill in the art. Indeed, a voluminous body of literature has accumulated
over the years as researchers explored combinations of starting materials
and correlated starting materials with foam properties. Moreover, numerous
efforts have been made to either modify the physical properties of
polyurethane foams or to prepare specialized polyurethane foams having
unique properties. A particularly sought-after property is increased water
absorbency. Polymers having this property often are referred to as
hydrogels or superabsorbents.
The nature and utility of superabsorbent are illustrated by U.S. Pat. No.
4,449,977, although it will be appreciated by those having ordinary skill
in the art that numerous other references also could be cited. According
to this reference, an apparently desirable feature of a superabsorbent is
the presence of acrylate or methacrylate groups which can be salts,
amides, esters, or the free acids.
As a practical matter, many hydrogels are based on acrylate and
methacrylate polymers and copolymers. See, by way of example only, U.S.
Pat. Nos. 2,976,576, 3,220,960, 3,993,616, 4,154,898, 4,167,464,
4,192,727, 4,192,827, and 4,529,739. The last-cited patent is of
particular interest since the disclosed water-absorbent polymers are
foamed. Other hydrogels are based on starch or a modified starch, as shown
by U.S. Pat. Nos. 4,069,177, 4,076,663, 4,115,332, and 4,117,222. Still
other hydrogels are based on poly(oxyalkylene) glycols; see, e.g., U.S.
Pat. No. 3,783,872. Hydrogels prepared from hydrolyzed crosslinked
polyacrylamides and crosslinked sulfonated polystyrenes are described in
U.S. Pat. No. 4,235,237. Finally, polymers based on maleic anhydride (or
similar compounds) have been described in U.S. Pat. Nos. 2,988,539,
3,393,168, 3,514,419, 3,557,067, and 4,401,793. U.S. Pat. No. 3,900,378 is
of interest since it describes hydrogels from radiation crosslinked blends
of hydrophilic polymers and fillers, many of the polymers being those
described in the foregoing patents. However, such materials are not
necessarily well suited for the uses described in U.S. Pat. No. 4,449,977,
supra.
Polyurethane hydrogels are, of course, known and frequently are based on
the reaction of a poly(oxyalkylene) polyol with a diisocyanate. However,
relatively few of such hydrogels contain acrylate or methacrylate
moieties, or even carboxylate groups. Examples of carboxylate-containing
polyurethanes and polyurethane hydrogels are noted below.
U.S. Pat. No. 3,928,299 describes the reaction of a hydroxy
group-containing polymer with an unsaturated isocyanate. Suitable hydroxy
group-containing polymers can be obtained by the polymerization of
hydroxyalkyl esters of acrylic acid or alpha-alkyl-substituted acrylic
acids or by the copolymerization of these compounds with other vinyl or
vinylidene compounds. Suitable hydroxyalkyl esters are the monoesters of
the foregoing acids with ethylene glycol, propylene glycol,
propane-1,3-diol, butanediol, diethylene glycol, and higher polyethylene
glycols. Such polymers also can be polyesters of polybasic aliphatic or
aromatic carboxylic acids with polyhydric alcohols, polyurethanes which
contain hydroxy groups, or epoxy resins which contain hydroxy groups. The
resulting polymers are crosslinkable by vinyl polymerization and are
useful for the preparation of coating and molded products. Foams,
especially polyurethane foams, are not mentioned. See also U.S. Pat. No.
4,210,713.
Disclosures similar to that of the above patent are found in U.S. Pat. Nos.
3,871,908, 3,856,830, 3,954,714, and 4,082,710.
U.S. Pat. No. 4,131,602 describes radiation-curable acrylated polyurethane
coating compositions. The compositions are prepared by reacting an
isocyanate-terminated urethane intermediate with an amount of a
hydroxyalkyl, hydroxycycloalkyl, or hydroxyaryl ester of acrylic acid or
methacrylic acid which is sufficient to react with at least 80 percent of
the isocyanate groups of the urethane intermediate. The urethane
intermediate is the reaction product of an organic diisocyanate, an
organic triol or tetraol which is either a polyester or a polyether, and
an organic diol which is either a polyester or a polyether. The
diisocyanate is used in an equivalent excess to the other two components.
In addition, when the triol or tetraol is a polyester, the diol must be a
polyether, and when the triol or tetraol is a polyether, the diol must be
a polyester.
U.S. Pat. No. 4,153,778 describes acrylyl-capped urethane oligomers which
readily cure by thermal or radiation means and are useful as coatings,
binders, and adhesives. The oligomers are the reaction products of a
poly(oxytetramethylene) diol or a polycaprolactone polyol, an organic
diisocyanate, a di- or trimethylol carboxylic acid, and an acrylyl
compound.
Polyester urethane-containing molding compositions are described in U.S.
Pat. No. 4,287,116. Briefly, an ethylenically unsaturated monomer solution
having dissolved therein a polyester urethane resin and an organic polyol
polyurethane precursor is gelled by the addition of a polyisocyanate
polyurethane precursor. The gelled mixture then is molded and cured by
copolymerization of the ethylenically unsaturated monomer and the resin.
The resin typically is a condensation product of a dihydroxy-terminated
poly(oxyalkylene) bisphenol A maleate or fumarate and a polyisocyanate
which is further reacted with a hydroxy-terminated ester of acrylic or
methacrylic acid. The polyol precursor is a saturated aliphatic polyol or
alkoxylated derivative thereof. The ethylenically unsaturated monomer can
be, by way of illustration, styrene, vinyltoluene, divinylbenzene, esters
of acrylic or methacrylic acid, vinyl acetate, divinyl ether, and the
like. Finally, the polyisocyanate precursor is an aliphatic,
cycloaliphatic, or aromatic compound having at least two isocyanate
groups.
U.S. Pat. No. 4,480,079 discloses molded plastic products which are
produced by the in-mold copolymerization of methyl methacrylate with a
polyurethane acrylate or methacrylate. The latter material is derived from
a hydroxylakyl acrylate or methacrylate by reaction of the hydroxy groups
thereof with the isocyanate groups of either a polyisocyanate free of
urethane groups and having an isocyanate functionality greater than two or
a urethane polyisocyanate having an isocyanate functionality greater than
two and derived from a polyisocyanate by reaction thereof with the hydroxy
groups of an aliphatic alcohol having up to three hydroxy groups.
A series of ten patents relates to polyurethane polyene or diacrylate
polymers. These ten patents, discussed briefly below, appear to relate to
interpenetrating polymer networks, although such terminology does not
appear to have been applied to the polymer compositions described by these
references. For general discussions of interpenetrating polymer networks,
see, by way of example only, D. Klempner et al., J. Elastoplastics, 5, 196
(1973); A. A. Donatelli et al., Macromolecules, 9, 671 and 676 (1976); L.
H. Sperling et al., Macromolecules, 9, 743 (1976); L. H. Sperling, J.
Polymer Science, 12, 141 (1977); and D. L. Siegfried et al., J. Polymer
Science, 16, 583 (1978).
U.S. Pat. No. 4,359,558 discloses hydrophilic polyurethane diacrylate
compositions. The compositions, which form hydrogels upon immersion in
water, are prepared by reacting a diacrylate in the presence of a
hydrophilic polyurethane. A free radical initiator may be present. The
polyurethane typically is the reaction product of one or more diols having
a number average molecular weight in the range of from about 200 to about
20,000, selected from the group consisting of ethylene glycol and long
chain poly(oxyalkylene) diols, with a urethane precursor selected from the
group consisting of organic polyisocyanates and nitrile carbonates in the
presence of an organic tin catalyst. Optionally, polyfunctional lactone
also may be present in amounts of from 0.1 to 30 percent by weight, based
on the weight of the total reaction mixture. The long chain diols
typically are the condensation products of either ethylene oxide or
propylene oxide. The diacrylate may be prepared by reacting acrylic acid
chloride (propenoyl chloride) or methacrylic acid chloride
(2-methylpropenoyl chloride) with a glycol such as ethylene glycol or a
condensation product of either ethylene oxide or propylene oxide. The two
components are dissolved in a suitable solvent, cast as a film, and cured
by heat or ultraviolet radiation. See also U.S. Pat. Nos. 4,408,023,
4,424,305, 4,439,583, 4,439,584, and 4,439,585.
Hydrophilic polyurethane acrylate compositions are disclosed in U.K. Patent
Application No. GB 2,150,938A. The disclosure is similar to that of U.S.
Pat. Nos. 4,359,558 et al., except that the diacrylate is replaced with an
acrylate which is the monoacrylic or monomethacrylic ester of an alcohol
having less than 13 carbon atoms. The preferred acrylates are stated to be
hydroxyethyl acrylate, methyl methacrylate, and methyl acrylate. The
polyurethane and acrylate components are dissolved in a solvent,
optionally in the presence of a free radical initiator, cast as a film,
and cured thermally or by ultraviolet radiation. Shaped articles can be
made by removing the solvent under reduced pressure, molding the residual
mixture, and curing the molded article thermally.
Hydrophilic polyurethane polyene compositions are disclosed in U.S. Pat.
No. 4,454,309. The compositions are prepared by reacting a polyene in the
presence of a hydrophilic polyurethane. The polyene is either a polyallyl
ester of a polybasic acid or a polyacrylic or polymerthacrylic ester of a
polyhydric alcohol. The polyurethane is that described in U.S. Pat. No.
4,359,558. As with the compositions of such earlier patent, the components
are dissolved in a suitable solvent, cast as a film, and cured with heat
or ultraviolet radiation. Alternatively, the solvent may be removed under
reduced pressure and the residual mixture molded and cured thermally. See
also U.S. Pat. Nos. 4,490,423 and 4,496,535.
In each of the foregoing reference, the unsaturated monomers are
polymerized in the presence of an existing polymer, i.e., a polyurethane.
A similar approach is disclosed in U.S. Pat. No. 4,551,486. According to
the patent, hardenable dental compositions are prepared by polymerizing
crosslinking oligomers in the presence of a crosslinked polymer and one or
more of a filler, an initiator, and a monofunctional monomer. The
crosslinked polymer can be a polyurethane, although the preferred polymers
are derived from aliphatic, cycloaliphatic, phenyl, and substituted phenyl
esters of acrylic acid and homologs thereof. The crosslinking agents which
are useful in the preparation of the crosslinked polymer can be selected
from a wide variety of polyfunctional materials. The preferred
functionality apparently is an ethylenic function, presumably because the
preferred polymers are prepared by the addition polymerization of
unsaturated monomers. The crosslinking aligomers also tend to be
polyunsaturated compounds, such as acrylic and lower alkyl acrylic acid
diesters, acrylic and lower acrylic acid esters of alcohols having a
second reactive function, urethane diacrylates and dimethacrylates,
polyvinyl compounds, divinyl aromatic compounds, and the like. Preferred
compounds include allyl acrylate, allyl methacrylate, vinyl acrylate,
vinyl methacrylate, dimethallyl fumarate, N-allylacrylamide, crotyl
acrylate, allyl crotonate, allyl cinnamate, diallyl maleate, acrylate and
methacrylate esters of polyols, and the like.
The reverse approach is described in European Patent Application No.
85105252.2, published as No. 0,163,150. In general, a polyurethane foam is
prepared in the presence of a polyelectrolyte polymer. Preferably, the
polyurethane is prepared from an isocyanate-terminated poly (oxyalkylene)
polyol, such as the HYPOL(.RTM.) precursors sold by W. R. Grace & Co. The
crosslinking agents are selected to react with the carboxylic acid groups
of the polyelectroyte polymer and include polyhaloalkanols,
haloepoxyalkenes, polyglycidyl ethers, defined di- and triaziridines, and
the like. The polyelectrolyte polymers tend to be polymers or copolymers
of acrylic and methacrylic acid with such monomers as acrylates,
methacrylates, acrylamide, olefins, vinyl aromatic compounds,
styrenesulfonic acid, vinyl ethers, vinyl acetate, vinyl alcohol, maleic
acid, fumaric acid, and the like. The examples used a polyacrylic acid
which had been treated with sodium hydroxide.
It perhaps should be noted at this point that interpenetrating polymer
networks ideally do not include any grafting of the first polymer to the
second, although, as noted by Donatelli et al., some grafting may take
place accidentally. Because of the selection of polymer types described in
the foregoing interpenetrating polymer network reference, little, if any,
grafting should have taken place.
Acrylic monomers containing carbamate (urethane) functionality are
described in U.S. Pat. Nos. 3,297,745, 3,425,988, 4,129,667, and
4,279,833. An acrylic monomer containing isocyanate functionality,
isocyanatoethyl methacrylate, is described in Adhesives Age, Oct. 1984.
The article summarizes there areas in which isocyanatoethyl methacrylate
has been used: (1) applications where the methacrylate group is
polymerized first, leaving the isocyanate group for a later reaction, (2)
applications where the isocyanate group is reacted with a polyfunctional
material first, leaving the vinyl group for a later reaction, and (3)
applications where the isocyanate group is reacted with a monofunctional
reagent first to make a new monomer which can be polymerized later.
Because a significant amount of the voluminous polyurethane foam literature
relates to the use as a starting material of what may be termed polyether
polyols, polyglycolethers, or poly(oxyalkylene) polyols and such starting
material has acquired a singularly important status in the polyurethane
art, a discussion of representative reference relating thereto is deemed
necessary for the sake of completeness.
One of the earliest reference describing such materials is U.S. Pat. No.
2,948,691. According to this patent, polyglycolethers having a molecular
weight of at least 500 and at least two terminal hydroxy groups can be
reacted with mono- or polyfunctional isocyanates to give products which
may be used for producing plasticizers, lubricants, plastics, spongy
materials, gel formers, thickening agents, and the like. The patent
describes the preparation of hydrogels and foams.
Subsequent studies with these poly (oxyalkylene) polyols demonstrated a
high suitability for the preparation of hydrogels and foams having
particular properties, as illustrated by the reference described below.
U.S. Pat. No. 3,861,993 describes a composite foam scouring pad, one
component of which is a hydrophilic foam composition prepared by reacting
an isocyanate-capped poly(oxyethylene) polyol having an isocyanate
functionality of at least two with an aqueous solution containing a
blowing agent such as a polyisocyanate, a nonionic surfactant, and, when
the isocyanate-capped poly(oxyethylene) polyol isocyanate functionality is
about two, a crosslinking agent. The ratio of moles of water to moles of
isocyanate functionality in the polyol can range from about 6.5 to about
390. The same hydrophilic foam is employed to prepare a laminated fabric
as described in U.S. Pat. No. 3,874,964 and a horticultural foam structure
as described in U.S. Pat. No. 3,889,417. The reticulated crosslinked
polyurethane foam described in U.S. Pat. No. 3,890,254 appears to differ
from that described above in that particular types of surfactants are
required and the isocyanate-capped poly(oxyethylene) polyol is derived
from a poly(oxyethylene) polyol having a weight average molecular weight
of from about 200 to about 20,000 and a hydroxy group functionality of
from about 2 to about 8. See also U.S. Pat. No. 4,160,076.
Compressed foams which are restored to their original volume in the
presence of water or heat are disclosed in U.S. Pat. No. 3,903,232; see
also U.S. Pat. No. 3,854,535. The foams are similar to those described in
U.S. Pat. No. 3,861,993. Briefly, a mixture of from 0 to about 97 percent
by weight of an isocyanate-capped hydrophilic poly(oxyethylene) polyol
having an isocyanate functionality of two and an isocyanate-capped
poly(oxyethylene) polyol having an isocyanate functionality of from about
3 to about 8 and a weight average molecular weight of from about 200 to
about 1,500 (20,000 according to claim 1) is reacted with water,
optionally in the presence of a crosslinking agent. The ratio of moles of
water to moles of isocyanate groups can range from about 6.5 to about 390.
See also U.S. Pat. Nos. 4,156,592 and 4,292,412 which disclose the use of
such foams in the preparation of expandable fabric softener-containing
articles and hydrophilic fabric softener foam compositions, respectively.
Similar foams are disclosed in U.S. Pat. Nos. 4,110,508 and 4,137,200 in
which the poly(oxyethylene) polyol moiety of the isocyanate-capped polyol
has a weight average molecular weight of from about 200 to about 20,000.
See also U.S. Pat. Nos. 4,201,846, 4,258,137, and 4,309,509 which describe
the incorporation into the foam of U.S. Pat. No. 4,137,200 hydrophilic
fibers prepared from vinyl alcohol homopolymers and copolymers, an epoxy
resin, and an odorant, respectively. In addition, U.S. Pat. No. 4,127,516
describes the inclusion of a polyurea in the reaction mixture which yields
the foams of U.S. Pat. No. 4,110,508. The polyurea is prepared by, for
example, the reaction between a linear poly(oxyethylene) polyol which has
been capped with a polyisocyanate and a polyamine in an organic solvent.
U.S. Pat. No. 3,904,557 describes a method for producing a multicolored
polyurethane sponge. A poly(oxyethylene) polyol having a weight average
molecular weight of from about 200 to about 20,000 and a hydroxy
functionality of from about 2 to about 8 is capped with a polyisocyanate.
At least two distinctly different coloring agents are added to at least
two different portions of isocyanate-capped polyol or water. The colored
portions then are reacted with separate portions of water or
isocyanate-capped polyol, respectively, to form separate colored foaming
masses which then are mixed together under laminar flow conditions to
yield a multicolored variegated polyurethane foam.
A polyurethane hydrogel is described in U.S. Pat. No. 4,118,354. The
hydrogel is produced by dispersing into an aqueous liquid phase a product
obtained by the reaction of a polyisocyanate having at least two
isocyanate groups with a polyether. The polyether results from the
polycondensation of at least two alkylene oxides with a polyalcohol having
at least two hydroxy groups and has an average molecular weight per
hydroxy group of from 1,000 to 4,000. Preferably, 75 to 85 percent of the
alkylene oxides is ethylene oxide. The resulting hydrogel is stated to
have a greater water content and to be highly elastic and highly stable,
even in the presence of a corrosive electrolyte solution.
Urethane foams having low resiliency are described in U.S. Pat. No.
4,158,087. The foams are obtained by reacting a poly(oxyalklene) urethane
prepolymer containing at least 40 mole percent of oxyethylene units in the
oxyalkylene portion of the prepolymer, water, and from about 40 to about
150 parts by weight on a solids basis per 100 parts by weight of the
prepolymer of a synthetic polymer latex. The prepolymer is an
isocyanate-capped poly(oxyethylene) polyol of the type described in U.S.
Pat. Nos. 3,903,232 et seq.
U.S. Pat. No. 4,181,770 describes the preparation of a hydrophilic foam
from an isocyanate-terminated branched polyethylene polyol, an
isocyanate-terminated polyester prepolymer, a minor amount of a
4,4-diphenylmethanediisocyanate/polycarbodiimide liquid condensation
product which has 30 percent free isocyanate groups, and water. The foam
is stated to have improved firmness and scuff resistance properties.
Isocyanate-capped urethane-containing prepolymers prepared from polyols
obtained from an epihalohydrin are described in U.S. Pat. Nos. 4,273,913
and 4,297,482. The polyol can be, for example, a polyalkylene glycol
composed of the same or different oxyalkylene units or a mixture of
different polyalkylene glycols.
U.S. Pat. Nos. 4,314,034, 4,365,025, 4,377,645, 4,384,050, and 4,384,051
describe variations of a general concept which involves mixing a resin
phase and an aqueous phase. The resin phase comprises an isocyanate-capped
poly(oxyalkylene) glycol of the type described in U.S. Pat. Nos. 3,903,232
et seq. and diphenylmethane diisocyanate and/or polymeric forms or
isocyanate-containing derivatives thereof.
Finally, mention should be made of U.S. Pat. Nos. 3,412,054 and 4,156,066.
The first patent describes water-dilutable polyurethanes which are useful
as surface coatings and printing inks. Such polyurethanes contain
carboxylic acid groups which can be neutralized with ammonia or an amine.
The carboxylic acid groups are provided by incorporating into the
polyurethane a 2,2-bis(hydroxymethyl)-substituted carboxylic acid.
Examples of suitable acids include 2,2-bis(hydroxymethyl)acetic acid,
2,2,2-tris(hydroxymethyl) acetic acid, 2,2-bis(hydroxymethyl)propionic
acid, 2,2-bis(hydroxymethyl)butyric acid, 2,2-bis(hydroxymethyl)pentanoic
acid, and the like. The polyurethanes are prepared by known techniques,
such as by adding organic diisocyanate to a mixture of a suitable
carboxylic acid and a polyol polyurethane precursor.
The second patent, U.S. Pat. No. 4,156,066, discloses polyurethanes
characterized by lactone groups and hydroxy groups in the polymer
backbone. The polyurethanes are prepared by reacting an organic
polyisocyanate with a poly(oxyalkylene) polyol and a polyfunctional
lactone having excess hydroxy groups. The free hydroxy groups which are
present in the formed polyurethane are available for crosslinking the
polymer. The lactone groups can be hydrolyzed to form free carboxylic acid
groups or carboxylate groups.
In addition to efforts directed at altering the nature of the polyurethane
per se, as noted at length above, other efforts have been directed at
either incorporating into the polyurethane foam a material which will give
the desired property or preparing a foam of an entirely different type.
Both of these approaches are illustrated by the references which are
summarized in the paragraphs which follow.
U.S. Pat. No. 3,900,030 describes a polyurethane foam of approximately the
same type as those disclosed in U.S. Pat. No. 3,903,232 et. seq. which has
dispersed throughout the foam a particulate, water-swellable polymer
containing a plurality of hydrophilic groups such as carboxy, carboxamide,
sulfonate salt, or hydroxy groups. The particulate polymer is included to
increase the water absorbency of the foam.
According to U.S. Pat. No. 4,377,160, a sheet or strip of a polyurethane
foam is dipped first into a polyvinyl alcohol solution and then into a
reactive gelling agent solution in order to gel the polyvinyl alcohol in
the foam. The resulting gel-impregnated foam is useful as a cooling
compression bandage.
Finally, U.S. Pat. No. 4,098,728 discloses foams prepared by the
copolymerization of polyvinyl alcohol and formaldehyde. The foams are
stated to be useful as surgical sponges. The patent notes deficiencies
with polyurethane sponges which are related to the generally hydrophobic
nature of polyurethanes. The patent also states that fast wicking and high
liquid holding capacity are desirable qualities of surgical sponges,
qualities which apparently are lacking in polyurethane sponges.
It is evident that the polyurethane foams of the prior art, while certainly
admirable for many applications, suffer from various disadvantages for a
number of uses. Many of these disadvantages are associated with the
generally hydrophobic nature of polyurethanes. Consequently, in spite of
the prior art efforts to prepare superabsorbent foams, there still is a
need for improvements with regard to such materials.
SUMMARY OF THE INVENTION
Consequently, it is an object of the present invention to provide a
superabsorbent polyurethane foam.
It is also is an object of the present invention to provide a
superabsorbent polyurethane foam which is based on a
quasi-interpenetrating polymer network.
It also is an object of the present invention to provide a superabsorbent
polyurethane foam which is prepared from an isocyanate-terminated
poly(oxyalkylene) polyol.
It is a further object of the present invention to provide a superabsorbent
polyurethane foam prepared from an isocyanate-terminated poly(oxyalkylene)
polyol, which foam is based on a quasi-interpenetrating polymer network.
Still another object of the present invention is to provide a
superabsorbent polyurethane foam prepared from an isocyanate-terminated
poly(oxyalkylene) polyol, which foam is based on a quasi-interpenetrating
polymer network of crosslinked polyurethane and substantially linear
addition polymer having a plurality of carbamoyl, substituted carbamoyl,
or carboxy groups or alkali metal or ammonium salts thereof.
These and other objects will be readily apparent to those having ordinary
skill in the art from a reading of the specification and claims which
follow.
Accordingly, the present invention provides a superabsorbent polyurethane
foam which comprises a quasi-interpenetrating polymer network of a
crosslinked polyurethane and a substantially linear addition polymer, the
molecules of which addition polymer are comprised of functional
group-containing repeating units which may be the same or different, said
functional groups being selected from the group consisting of carbamoyl,
substituted carbamoyl, and carboxy and alkali metal and ammonium salts
thereof, the number of repeating units per molecule being in the range of
from about 10 to about 300, the superabsorbent polyurethane foam being
essentially free of grafting of addition polymer to polyurethane.
In preferred embodiments, the polyurethane is derived from an
isocyanate-terminated poly(oxyalkylene) polyol having an isocyanate
functionality greater than two and the addition polymer is the
polymerization product of at least one compound selected from the group
consisting of acrylamide, substituted acrylamides, and acrylate and
methacrylate salts, i.e., alkali metal and ammonium salts of acrylic and
methacrylic acid.
The present invention also provides a method of preparing a superabsorbent
polyurethane foam which comprises mixing an isocyanate-terminated
polyurethane prepolymer having an isocyanate functionality greater than
two with (1) at least one compound having (a) at least one carbon-carbon
double bond capable of undergoing addition polymerization with itself and
other compounds which may be present and (b) at least one functional group
selected from the group consisting of carbamoyl, substituted carbamoyl,
and carboxy and the alkali metal and ammonium salts thereof; and (2)
water, in the presence of at least one thermally activated free radical
initiator, with the resulting polyurethane foam being essentially free of
grafting of addition polymer to polyurethane.
In preferred embodiments, said compound is selected from a group consisting
of acrylamides, methacrylamides, and alkali metal and ammonium salts of
acrylic and methacrylic acid. In additional preferred embodiments, said
compound is selected from the group consisting of acrylamide,
methacrylamide, potassium acrylate, and potassium methacrylate. In yet
other preferred embodiments, the prepolymer is an isocyanate-terminated
poly(oxyalkylene) polyol.
DETAILED DESCRIPTION OF THE INVENTION
The term "ammonium" is used herein to mean any quaternary ammonium ion
derived from ammonia or primary, secondary, or tertiary amines. The choice
of such amines is limited only to the extent that any given amine
significantly adversely affects foam properties.
For convenience, the term "precursor" will be used herein to mean a
polyfunctional polyurethane precursor which is reacted with a
polyisocyanate to give an isocyanate-terminated material, whereas the term
"prepolymer" will refer to the isocyanate-terminated material. As will be
made clear later, each term is intended to include both a single compound
or material and a mixture of two or more compounds or materials.
The phrases "at least one compound," "at least one thermally activated free
radical initiator," and the like are intended to include the use of a
single compound or a mixture of two or more compounds. For convenience,
however, the use hereinafter of such terms as "compound," "free radical
initiator," and the like shall be read as encompassing both single
compounds and mixtures of two or more compounds.
The nature of the polyurethane is not known to be critical. Thus, the
polyurethane can contain other moieties, such as polyesters, polyethers,
and the like. However, the polyurethane must be crosslinked, as will be
made more clear hereinafter, in order to assure foam integrity. Because of
the wide variety of polyurethanes which can be prepared within the spirit
and scope of the present invention, it is not practical to define precise
ranges for the degree of crosslinking which will result in suitable foams.
However, one having ordinary skill in the art can readily prepare foams
coming within the scope of the present invention, without the need for
undue experimentation, by following the guidelines contained herein.
Stated differently, the precursors can be any of the precursors known to
those having ordinary skill in the art for the preparation of polyurethane
foams. Because the foam must be crosslinked, the precursor should have at
least two isocyanate-reactive groups per molecule (referred to hereinafter
as precursor isocyanate-reactive functionality, or PICRF). The
isocyanate-reactive groups can be the same or different and can be any of
the groups known to be reactive with an isocyanate. Such groups include
primary aliphatic amines, secondary aliphatic amines, primary aromatic
amines, secondary aromatic amines, hydrazines, amides, ureas, urethanes,
imides, amidines, nitramines, diazoamino compounds, phenylhydrazones,
aminooximes, sulfimides, acylureas, thioureas, isothioureas, primary
alcohols, secondary alcohols, phenols, carboxylic acids, malonic esters,
nitroalkanes, acetoacetic esters, primary mercaptans, secondary
mercaptans, thiophenols, and the like. For a review of groups which are
reactive with isocyanates, see J. H. Saunders and K. C. Frisch,
"Polyurethanes: Chemistry and Technology, Part I. Chemistry," Vol. XVI,
Part I, Interscience Publishers, New York, 1962, pp. 63-128.
The preferred isocyanate-reactive groups are primary and secondary
alcohols, primary and secondary amines, and primary and secondary
mercaptans. The more preferred groups are primary alcohols and amines,
with primary alcohols being most preferred.
As a practical matter, polyrethane foams prepared from a poly(oxyalkylene)
polyol are preferred, largely because of the water miscibility and ready
commercial availability of such polyols and the generally acceptable
properties of the resulting foams. Especially useful are the
isocyanate-terminated polyurethane prepolymers which are based on such
polyols, such as the HYPOL(.RTM.) prepolymers available from W. R. Grace &
Co., Organic Chemicals Division, Lexington, Mass.
As used herein, the terms "poly(oxyalkylene) glycol" and "poly(oxyalkylene)
polyol" are intended to be synonymous and to include any poly(oxyalkylene)
condensation product containing at least two hydroxy groups. Moreover,
such materials may be considered to be derived from one or more alkylene
oxides. Thus, the oxyalkylene moiety can be all of one type or a mixture
of two or more types. A mixture may be either random or block. In
addition, such materials can contain other moieties, such as polyesters,
polyamides, and the like, as already indicated.
Because the preparation of prepolymers is well known in the art and
prepolymers are commercially available, the reaction by which the
prepolymers are formed is not deemed to form a part of the method of the
present invention. Nevertheless, some discussion regarding the
relationships between the functionality of the precursor and the
functionality of the prepolymer perhaps is appropriate.
As already noted, the PICRF should be at least two. When the PICRF is two,
crosslinking of the polyurethane during the foam-forming reaction can be
achieved by using either a polyisocyanate having at least three isocyanate
groups per molecule or a mixture of a diisocyanate and one or more
polyisocyanates having at least three isocyanate groups per molecule.
Alternatively, or in addition, a separate crosslinking agent can be added,
as is well known to those having ordinary skill in the art. When the PICRF
is three or more, crosslinking can be controlled by adding one or more
compounds having but one isocyanate-reactive group.
Usually, and preferably, the PICRF will be between two and three. This is
achieved, as is well known in the art, by employing a mixture of two or
more precursors, one of which has a PICRF of two and one of which has a
PICRF of three or more. The ratio of the amounts of precursor present and
the PICRF values of each determine the average PICRF value for the
mixture.
It should be noted, however, that crosslinking still can be achieved when
the PICRF is less than two by preparing the prepolymer with an isocyanate
having three or more isocyanate groups per molecule. Such a procedure will
yield a prepolymer having a prepolymer isocyanate functionality (or PPICF)
greater than two. However, a PICRF of less than two requires the presence
of precursor molecules having but one isocyanate-reactive group. Such
molecules are chain terminating and, as a consequence, can have
deleterious effects on polyurethane properties. Such effects may be
sufficiently minor when the precursor molecular weight is relatively high.
Accordingly, the present invention is deemed to include the use of
precursors having a PICRF of less than two, provided that acceptable
superabsorbent polyurethane foams can be prepared therewith.
The prepolymer isocyanate functionality (or PPICF) clearly is affected by
the PICRF. If a diisocyanate is used to prepare the prepolymer, the PPICF
and the PICRF will be the same. However, the PPICF can be either lower or
higher than the PICRF by using an appropriate mixture of isocyanates. For
example, the use of a mixture of a monoisocyanate and a diisocyanate will
reduce the PPICF relative to the PICRF, the extent of such reduction being
a function of the amount of monoisocyanate present. Alternatively, a
mixture of a diisocyanate and a triisocyanate (or any polyisocyanate
having at least three isocyanate groups per molecule) will increase the
PPICF relative to the PICRF, the extent of such increase being a function
of the amount of triisocyanate (or polyisocyanate) present. As already
indicated, however, the PPICF must be greater than the two. Preferably,
the PPICF will be equal to or greater than about 2.3.
As already stated, the superabsorbent polyurethane foam of the present
invention is prepared by mixing an isocyanate-terminated polyurethane
prepolymer having an isocyanate functionality greater than two with water
and at least one compound in the presence of at least one thermally
activated free radical initiator.
Said compound must have (1) at least one carbon-carbon double bond capable
of undergoing addition homopolymerization and copolymerization with other
compounds which may be present and (2) at least one functional group
selected from the group consisting of carbamoyl, substituted carbamoyl,
and carboxy and the alkali metal and ammonium salts thereof.
In general, said compound is selected from the group consisting of
carboxylic acid amides in which the amido moiety can be substituted or
unsubstituted, i.e., carbamoyl and substituted carbamoyl, and carboxylic
acids and alkali metal and ammonium salts thereof, which compounds have at
least one carbon-carbon double bond capable of undergoing addition
polymerization as already described.
Said compound can contain more than one functional group as long as there
is present in the compound at least one carbamoyl, substituted carbamoyl,
or carboxy group or alkali metal or ammonium salt thereof and the
additional functional groups do not react with isocyanate groups in
aqueous media and do not significantly adversely affect either the
polymerization reactions or the properties of the resulting foam. Thus,
when two or more functional groups are present in any first compound, such
groups can be the same or different. Moreover, all functional groups
present in the compound can be selected from the foregoing group of
required functionality, if desired. When all of the functional groups are
carboxy groups, such groups can be present partly as the free acid and
partly as a salt.
By way of illustration only, suitable compounds include, among others,
acrylic acid, methacrylic acid, 2-butenoic acid, 4-chloro-2-butenoic acid,
3-butenoic acid, 5-allyl-3-methoxybenzoic acid, cinnamic acid,
2-carboxycinnamic acid, 4-carboxycinnamic acid, 1-cyclohexenylcyanoacetic
acid, cyclohexene-1-carboxylic acid, fumaric acid, chlorofumaric acid,
mesaconic acid, 2,4-hexadienedioc acid, 3-hexenoic acid, itaconic acid,
linoleic acid, maleic acid, allylmalonic acid, 4-methyl-2-pentenoic acid,
allylacetic acid, propenoic acid, the amides thereof, the substituted
amides thereof, the alkali metal salts thereof, the ammonium salts
thereof, and the like. Among the ammonia salts, those derived from ammonia
and the primary lower alkyl amines are preferred, with those derived from
ammonia being most preferred. The preferred compounds are acrylic acid,
methacrylic acid, acrylamide, methacrylamide, and the sodium and potassium
salts of acrylic and methacrylic acid. Acrylamide and the potassium salts
of acrylic and methacrylic acid are the most preferred compounds.
The amount of the compound employed is primarily dependent upon the degree
of superabsorbency desired and the solubility of each compound in water or
prepolymer. As a practical matter, the approximate lower limit is about
1.times.10.sup.-3 mole of the compound(s) per g of prepolymer. Preferably,
such amount will be at least about 2.times.10.sup.-3, and most preferably
at least about 3.times.10.sup.-3, mole per g of prepolymer. It must be
emphasized, though, that these values are given by way of illustration
only, in part because the superabsorbency obtained with any given compound
is affected by the nature of the precursor.
In view of the foregoing discussion, it should be apparent that the amount
of the compound on weight basis can vary widely. By way of illustration
only, such amount often will be in the range of from about 20 to about 50
percent by weight, based on the weight of prepolymer.
Said compound can be dissolved in either the prepolymer or the water. The
choice of phase primarily is a function of compound solubility.
The mixing of prepolymer, compound, and water must take place in t | | |
