Hydrophilic hydrogels with a high swelling capacity and...

Details Hydrophilic hydrogels with a high swelling capacity and... Hydrophilic hydrogels with a high swelling capacity and...

2001-04-06

2003-08-05

Lovering, Richard D. (Department: 1712)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C252S194000, C264S177170, C516S105000, C524S833000, C524S916000, C525S054230, C525S054260, C526S932000, C604S368000, C604S372000, C604S904000

Reexamination Certificate

active

06602950

ABSTRACT:

This is a 371 of PCT/EP99/07308 filed Oct. 1, 1999.
The present invention relates to hydrophilic, highly swellable hydrogels having high absorbency for water and aqeuous fluids, to a process for their preparation and to the use of these hydrogels.
Hydrophilic hydrogels obtainable by polymerization of unsaturated acids, for example acrylic acid, methacrylic acid, acrylamidopropanesulfonic acid, etc., in the presence of small amounts of multiply olefinically unsaturated compounds are already known as superabsorbent polymers. Also known are hydrophilic hydrogels obtainable by graft copolymerization of the olefinically unsaturated acids onto different matrices, for example polysaccharides, polyalkylene oxides and derivatives thereof.
The hydrogels mentioned are notable for high absorbency for water and aqueous solutions and are therefore widely used as absorbents in hygiene articles.
Such water-swellable hydrophilic polymers are generally prepared by free-radical polymerization in an aqueous solution which contains the monomers with or without a grafting base and crosslinker.
The water-swellable hydrophilic polymers produced for use in the hygiene and sanitary sector have a degree of neutralization in the range from 50 to 85 mol % based on the polymerized acid-functional monomer units, so that the hydrogels formed in use are pH neutral with regard to the skin.
The degree of neutralization is generally set prior to the polymerization, since this avoids the technically difficult neutralization of an acidic hydrogel of high viscosity. However, the polymerization of, for example, acrylic acid in the neutral pH range is slower, and leads to lower molecular weights, than the polymerization in the acidic range. This is explained by the electrostatic repellency between the most recently incorporated monomer unit and the next monomer unit to be incorporated, which repellency arises only minimally, if at all, in the case of a polymerization in the acidic pH range, since the monomer units are present in the uncharged, acidic form.
The trend toward ever thinner diaper constructions requires water-swellable hydrophilic polymers providing better and better performance characteristics with regard to absorption capacity, fluid acquisition and fluid transportation within the hygiene article, especially under a confining load.
EP-A-0 640 330, WO 95/22358, WO 95/26209 and WO 97/12575 describe a test for measuring the gel permeability of swollen hydrogel particles (Saline Flow Conductivity, SFC). This test determines the flow of a sodium chloride solution through a preswollen hydrogel particle layer under a pressure of 0.3 psi. Since, in this test method, the sodium chloride solution will flow through the swollen gel layer in the direction of the force of gravity, this method is less suitable for characterizing any capillary forces. In practice, however, it is relevant for the quality of hygiene articles that fluid can be transported under load even in a direction opposite to the force of gravity, i.e., capillary forces do play an important part.
It is an object of the present invention to provide hydrophilic, highly swellable hydrogels having improved absorption properties, especially improved capillarity under load, and a process for the production thereof.
We have found that this object is achieved by a hydrophilic, highly swellable hydrogel based on (co)polymerized monomers or based on graft (co)polymers obtainable by
a) free-radically (co)polymerizing one or more hydrophilic monomers or graft (co)polymerizing one or more hydrophilic monomers onto a grafting base, the average degree of neutralization of the acid-functional monomers being from 0 to 40 mol %,
b) comminuting the acidic hydrogel,
c) neutralizating the acidic hydrogel to an ultimate degree of neutralization of 50-85 mol % by adding a neutralizing agent,
d) drying, grinding and sieving the hydrogel particles characterized by
a centrifuge retention of at least 29 g/g for a 0.9% aqueous NaCl solution and
a vertical absorption (1 g) swell height of at least 3.5 cm coupled with an extractables content (16 h value) of less than 5% or
a centrifuge retention of at least 23 g/g for a 0.9% aqueous NaCl solution and
a vertical absorption (3 g) swell height of at least 5 cm
an extractables content (16 h value) of less than 4%.
The hydrophilic, highly swellable hydrogels of the invention and also the process for their production will now be more particularly described.
Hydrophilic monomers useful for preparing the water-swellable hydrophilic polymers of the invention include for example acids capable of addition polymerization, such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, styrenesulfonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid and their amides, hydroxyalkyl esters, amino- or ammonio-functional esters and amides. Water-soluble N-vinylamides or else diallyldimethylammonium chloride are also suitable.
Preferred hydrophilic monomers are compounds of the general formula (I)
where
R
1
is hydrogen, methyl or ethyl,
R
2
is —COOR
4
, hydroxysulfonyl, phosphonyl, a (C
1
-C
4
)-alkanol-esterified phosphonyl group or a group of the general formula (II)
R
3
is hydrogen, methyl, ethyl or carboxyl,
R
4
is hydrogen, amino-(C
1
-C
4
)-alkyl or hydroxy-(C
1
-C
4
)-alkyl, and
R
5
is hydroxysulfonyl, phosphonyl or carboxyl.
Examples of (C
1
-C
4
)-alkanols are methanol, ethanol, n-propanol and n-butanol.
Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid.
When the monomers used are acids, their alkali metal or ammonium salts may be used as comonomers in a fraction of up to 40% by weight.
Useful grafting bases may be of natural or synthetic origin. Examples are starch, cellulose or cellulose derivatives and also other polysaccharides and oligosaccharides, polyvinyl alcohol, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and also hydrophilic polyesters. Useful polyalkylene oxides have for example the formula (III)
where
R
6
and R
7
are independently hydrogen, alkyl, alkenyl or aryl,
X is hydrogen or methyl, and
n is an integer from 1 to 10000.
R
6
and R
7
are each for example linear or branched (C
1
-C
10
)-alkyl such as methyl, ethyl, propyl, isopropyl, n-butyl, (C
2
-C
6
)-alkenyl or aryl such as unsubstituted or (C
1
-C
4
)-alkyl-substituted phenyl.
R
6
and R
7
are each preferably hydrogen, (C
1
-C
4
)-alkyl, (C
2
-C
6
)-alkenyl or phenyl.
The hydrophilic, highly swellable hydrogels are preferably in a crosslinked state, i.e., they contain units polymerized into the polymer network that are derived from compounds having at least two double bonds.
Useful crosslinkers include in particular methylenebisacrylamide, methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, such as diacrylate or triacrylate, e.g., butanediol diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate and also trimethylolpropane triacrylate, allyl compounds such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, pentaerythritol triallyl esters or allyl esters of phosphoric acid and also vinyl compounds such as vinyl acrylate, divinyl adipate, divinylbenzene and vinylphosphonic acid derivatives, as described for example in EP-A-0 343 427.
The polymerization may be initiated using high energy electromagnetic radiation or the customary chemical polymerization initiators, for example organic peroxides such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide, cumene hydroperoxide, azo compounds such as azodiisobutyronitrile and also inorganic peroxy compounds such as ammonium persulfate, potassium persulfate or hydrogen peroxide, with or without reducing agents such as sodium bisulfite, and iron(II) sulfate or redox systems where the reducing component is an aliphatic or aromatic sulfi

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