Mechanically stable hydrogel-forming polymers

Details Mechanically stable hydrogel-forming polymers Mechanically stable hydrogel-forming polymers

1999-10-04

2002-07-02

Weiss, John G. (Department: 3761)

Surgery

Means and methods for collecting body fluids or waste material

Absorbent pad for external or internal application and...

C604S367000, C604S372000

Reexamination Certificate

active

06414214

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to hydrogel-forming polymers of improved mechanical stability which have an AUL 0.7 psi of at least 20 g/g, a Saline Flow Conductivity of at least 40×10
−7
cm
3
s/g and a Frangibility Index of at least 60%, their production and the use of these polymers for absorbing aqueous fluids, especially body fluids in hygiene articles for adults and infants.
DISCUSSION OF THE BACKGROUND
Hydrogel-forming polymers have high absorption capacity for water and aqueous solutions and are therefore the preferred superabsorbent polymers for use as absorbents in hygiene articles.
SUMMARY OF THE INVENTION
The increasing tendency to make hygiene articles such as 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 from the hygiene article due to the phenomenon known as gel blocking may be prevented.
Examples of hygiene articles containing a major fraction of highly swellable hydrogel are taught in U.S. Pat. No. 5,149,335, EP-A-532 002, EP-A-615 736, EP-A-761 191 and U.S. Pat. No. 5,562,646. However, the products have unsatisfactory properties with regard to fluid transportation in the swollen state.
U.S. Pat. No. 5,599,335 and U.S. Pat. No. 5,669,894 describe an absorbent composition comprising at least one region comprising a superabsorbent polymer in a concentration of 60-100% by weight, the superabsorbent polymer having a Saline Flow 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 (4826.5 Pa).
Hygiene articles having a high hydrogel content require hydrogels exhibiting good absorption capacity under load and adequate permeability in the swollen gel state. These properties are generally exhibited by hydrogel-forming polymers that are densely crosslinked.
However, dense crosslinking causes hydrogel-forming polymers to become very brittle, so that they are easily attrited or even fractured by the action of mechanical forces occurring for example in the course of pneumatic conveying, for example in the course of the manufacture of the hygiene article, in the unswollen state. Attrition due to mechanical stress gives rise, firstly, to dust fractions and, secondly, to a deterioration in the physico-chemical product properties. Fine dust less than 10 &mgr;m in particle size in undesirable for inhalation-toxic reasons. Fine dusts less than 100 &mgr;m in size are the cause of visually detectable dusting with all its consequences and lead to handling problems in the production and processing plant and are therefore likewise undesirable. An increased fines fraction also causes a deterioration in the absorption and fluid transmission properties, since the swollen fines particles plug the pores in the absorbent structure. In the case of surface-postcrosslinked hydrogel-forming polymers, furthermore, mechanical attrition destroys the more highly crosslinked surface layer of the polymer particle, causing the lightly crosslinked core to come to the surface, so that these particles exhibit gel blocking in the absorbent structure.
There have been various attempts to stabilize the densely crosslinked polymers to mechanical stresses through an additional coating.
For instance, EP-A-703 265 describes highly swellable hydrophilic hydrogels coated with nonreactive water-insoluble film-forming polymers and thus exhibiting improved attrition resistance. Preferred film-forming polymers are homo- or copolymers of vinyl esters and also homo- or copolymers of acrylic or methacrylic esters.
EP 755 964 teaches coating the hydrogels with nonreactive water-insoluble waxes for improved attrition resistance. Preferred waxes are montan waxes and polyethylene waxes or oxidates of polyethylene waxes.
The disadvantage of both the coatings is a hydrophobicization of the particle surface, which leads to a deterioration in fluid transmission in the hygiene article.
WO 94/22940 describes coating hydrogel-forming polymers with polyether polyols for dustproofing and improved attrition resistance. However, this coating may become dissolved off on contact with aqueous fluids and will then increase the viscosity of the fluid to be absorbed, reducing the amount of fluid absorbed by the absorbent composition.
EP-A-690 077 describes the polymerization of comonomers containing ether and hydroxyl groups, for example polyethylene glycol (meth)acrylates or polypropylene glycol (meth)acrylates, to improve the attrition resistance. To obtain the desired effect, however, relatively large fractions of these copolymers have to be used, even though they do not contribute to the osmotic swelling power of the hydrogels and are therefore responsible for a deterioration in the swelling capacity on a weight basis.
It is an object of the present invention to provide hydrogel-forming polymers which possess high mechanical stability, high absorption capacity under load and high permeability in the swollen state without the abovementioned disadvantages.
We have found that this object is achieved by hydrogel-forming polymers having an AUL 0.7 psi (4826.5 Pa) of at least 20 g/g, a Saline Flow Conductivity of at least 40×10
−7
cm
3
s/g and a Frangibility Index of at least 60%. The present invention further provides a process for producing such hydrogel-forming polymers and for their use for absorbing aqueous fluids, especially body fluids in hygiene articles for adults and infants.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polymers of the invention are obtainable by adjusting a hydrogel-forming polymer having a Saline Flow Conductivity (SFC) of at least 40×10
−7
cm
3
s/g and an AUL 0.7 psi (4826.5 Pa) of at least 20 g/g to a residual moisture content of at least 3% by weight, preferably 4% by weight, especially 5% by weight, based on the hydrogel-forming polymer.
As used herein, “residual moisture content” is the amount of moisture which takes 3 h to evaporate at 105° C. The residual moisture content is preferably water. The addition of small amounts, up to 20% by weight, of a water-miscible organic solvent in a mixture with water is possible, but generally does not provide a superior effect compared with pure water.
Polymers having an SFC ≧40×10
−7
cm
3
s/g and an AUL 0.7 psi ≧20 g/g are common knowledge and are described for example in U.S. Pat. No. 5,599,335 and U.S. Pat. No. 5,669,894. These polymers are surface-postcrosslinked hydrogels. The surface postcrosslinking is responsible for the high degree of crosslinking of the polymer and hence for the problems described at the beginning.
The base polymers which are subsequently surface-postcrosslinked are crosslinked polymers having acid groups which are predominantly present in the form of their salts, generally alkali metal or ammonium salts. Such polymers swell on contact with aqueous fluids to form gels.
Examples of such base polymers are graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked cellulose or starch ethers and esters bearing acid groups, crosslinked carboxymethylcellulose or natural products capable of swelling in aqueous fluids and having acid groups, for example alginates and carrageenans.
Suitable 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, polyamines, polyamides and also hydrophilic polyesters. Suitable polyalkylene oxides conform for example to the formula
where
R
1
and R
2
are independen

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