Crosslinked, hydrophilic, highly swellable hydrogels,...

Details Crosslinked, hydrophilic, highly swellable hydrogels,... Crosslinked, hydrophilic, highly swellable hydrogels,...

1999-02-05

2001-10-02

Zitomer, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S278000, C526S287000, C526S240000, C526S303100, C526S318200, C526S320000

Reexamination Certificate

active

06297335

ABSTRACT:

SPECIFICATION
The present invention relates to crosslinked, hydrophilic, highly swellable hydrogels having a Pressure Absorbency Index <100 and a vertical Absorption of not less than 12 g/g under a pressure of 1922.8 Pa, a process for production thereof and use thereof.
Hydrophilic hydrogels obtainable by polymerization of unsaturated acids, for example acrylic acid or methacrylic acid or acrylamidopropanesulfonic acid, in the presence of small amounts of polyolefinically unsaturated compounds are already known as superabsorbent polymers. They are described for example in U.S. Pat. Nos. 4,057,521, 4,062,817, 4,525,527, 4,286,082, 4,340,706 and 4,295,987.
Also known are hydrophilic hydrogels obtainable by graft copolymerization of olefinically unsaturated acids on different matrices, for example polysaccharides, polyalkylene oxides and derivatives thereof; cf. for example U.S. Pat. Nos. 5,011,892, 4,076,663 and 4,931,497.
The hydrogels mentioned have high absorption capacity for water and aqueous solutions and are therefore the preferred superabsorbent polymers for use as absorbents in hygiene articles.
The increasing tendency to make hygiene articles such as baby and adult incontinence diapers ever smaller and thinner while preserving the same total absorption capacity is contingent on the ability to reduce the proportion of bulky wood pulp fluff and to raise the proportion of highly swellable hydrogel. As a consequence, the superabsorbent polymers have to perform additional functions with regard to liquid acquisition, transportation and distribution which were previously performed by the wood pulp fluff in order that any leakage of the hygiene article due to the phenomenon known as gel blocking may be prevented.
U.S. Pat. No. 5,147,343 discloses absorbent compositions comprising a porous fiber matrix and an amount of superabsorbent polymer dispersed in the pores of the fiber matrix. The superabsorbent polymer can absorb at least 27 ml of a 0.9% strength by weight sodium chloride solution under a restraining pressure of 21,000 dyn/cm
2
. The amount of superabsorbent polymer in the absorbent composition is preferably 10-60% by weight.
U.S. Pat. No. 5,149,335 describes absorbent structures comprising 60-100% by weight of superabsorbent polymer. The superabsorbent polymer is characterized by a Free-Swell Rate of less than 60 s and a 5-minute AUL (absorption under load) value of at least 15 g/gs
EP-A-0 532 002 discloses absorbent compositions comprising a porous fiber matrix and at least 30% by weight of superabsorbent polymer, the superabsorbent polymer having a Deformation Under Load-of 0.60 mm or less and a Wicking Index of 10 cm or more.
EP-A-0 615 736 relates to absorbent compositions comprising 30-100% by weight of superabsorbent polymer having a Pressure Absorbency Index of at least 100 and an extractables content (16 h extraction in 0.9% strength by weight aqueous sodium chloride solution) of less than 13% by weight.
EP-A-0 761 191 describes absorbent compositions comprising a fiber matrix and at least 30% by weight of superabsorbent polymer having a Wicking Parameter of 700 or more.
U.S. Pat. No. 5,562,646 discloses an absorbent composition comprising at least one region having a superabsorbent polymer in a concentration of 60-100% by weight, the superabsorbent polymer having a porosity of at least 0.15 and a Performance Under Pressure value of at least 23 g/g under a confining pressure of 0.7 psi (4826.5 Pa).
U.S. Pat. Nos. 5,599,335 and 5,669,894 describe absorbent compositions comprising at least one region having a superabsorbent polymer in a concentration of 60-100% by weight, the superabsorbent polymer having a Saline Plow conductivity value of at least 30×10
−7
cm
3
sec/g and a Performance Under Pressure value of at least 23 g/g under a confining pressure of 0.7 psi.
Although the use of prior art superabsorbent polymers has improved the quality of highly hydrogel-loaded hygiene articles compared with the use of minimally crosslinked, non-surface-postcrosslinked superabsorbent polymer products of the 1st generation, prior art superabsorbent polymers in these hygiene articles all exhibit disadvantages with regard to liquid acquisition, transportation and distribution, so that there are limits to increasing the proportion of highly swellable hydrogel in the diaper due to the attendant loss of quality.
It is an object of the present invention to provide a highly swellable hydrogel which does not have the above-described disadvantages and which exhibits excellent acquisition and retaining characteristics for body fluids even when used in diaper constructions having a very high proportion of superabsorbent polymer.
We have found that this object is achieved by a crosslinked, hydrophilic, highly swellable hydrogel based on polymerized monomers or based on graft polymers, characterized by a Pressure Absorbency Index <100 and a vertical Absorption of not less than 12 g/g under a pressure of 1922.8 Pa. Particular preference is here given to hydrogels having a Performance under Pressure value of less than 23 g/g under a confining pressure of 0.7 psi (4826.5 Pa), and/or an Absorbency Under Load (AUL) value of less than 27 g/g under a confining pressure of 21,000 dyn/cM
2
(2100 Pa). The extremely high crosslinking of the hydrogels is obtained using a process for producing the crosslinked, hydrophilic, highly swellable hydrogels, which comprises free-radically polymerizing hydrophilic monomers which contain acid groups, or their alkali metal or ammonium salts, with
(a) a copolymerization crosslinker which contains at least two ethylenically unsaturated double bonds in the molecule, and
(b) a reactive crosslinker A which contains an ethylenically unsaturated double bond and at least one functional group capable of forming covalent bonds with the acid groups of the hydrophilic polymers, a reactive crosslinker B which contains at least two functional groups capable of forming covalent bonds with the acid groups of the hydrophilic polymers, and/or with ions of polyvalent metals
optionally in the presence of at least one grafting base to form a crosslinked, hydrophilic base polymer, comminuting said base polymer and postcrosslinking the surface of the particles of said base polymer. A surface postcrosslinking of the base polymers initially obtained increases the crosslinking density of the surface even further.
In what follows, the highly crosslinked, hydrophilic, highly swellable hydrogels of the present invention and the process for their production will be more particularly described.
Hydrophilic monomers useful for producing the water-swellable hydrophilic polymers of the present invention include for example polymerization-capable acids, such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, styrenesulfonic acid, maleic acid including its anhydride, fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido -2-methylpropanephosphonic acid and also their amides, hydroxyalkyl esters and amino group- or ammonium group-containing esters and amides. Also, water-soluble N-vinylamides or else diallyldimethylammonium chloride.
Preferred hydrophilic monomers are compounds of the general formula (I)
where
R
1
is hydrogen, methyl or ethyl,
R
2
is a —COOR
4
group, a sulfonyl group, a phosphonyl group, a phosphonyl group esterified with (C
1
-C
4
) alkanol or a group of the formula
R
3
is hydrogen, methyl, ethyl or carboxyl,
R
4
is hydrogen, an alkali metal ion or an ammonium ion, and
R5 is sulfonyl, phosphonyl or carboxyl.
Examples of (C
1
-C
4
) alkanols are methanol, ethanol, n-propanol, isopropanol and n-butanol.
Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid.
Suitable grafting bases may be of natural or synthetic origin. Examples are starch, cellulose and cellulose derivatives, and also other polysaccharides and oligosaccharides, polyvinyl alcohol, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides or block copolymers of ethylene oxide and propyle

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