Fracture resistant superabsorbent polymers

Details Fracture resistant superabsorbent polymers Fracture resistant superabsorbent polymers

1998-12-18

2002-03-05

Weiss, John G. (Department: 3761)

Surgery

Means and methods for collecting body fluids or waste material

Absorbent pad for external or internal application and...

C604S365000, C604S385010, C604S385220, C604S385310, C524S379000, C524S530000, C524S831000, C526S240000

Reexamination Certificate

active

06353148

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to improved superabsorbent polymers demonstrating high fracture resistance. More particularly, the present invention is directed to improved superabsorbent polymers containing a dispersed phase comprising an elastomeric material. The resultant improved superabsorbent polymer is particularly useful in disposable personal care articles such as diapers, adult incontinence devices, and feminine napkins.
BACKGROUND OF THE INVENTION
Conventional absorbent articles such as baby diapers, adult incontinence devices, and feminine napkins are typically made with a cellulose fiber fluff-based absorbent core sandwiched between a liquid pervious top sheet whose function is to allow the unobstructed passage of fluid to the absorbent core, and a liquid impervious backing sheet, usually of plastic material, whose function is to contain the absorbed fluid and prevent it from passing through the absorbent core and soiling the undergarments of the wearer of the absorbent article.
The absorbent core of these absorbent articles is typically constructed of defiberized wood pulp combined with superabsorbent polymer granules. The absorbent core is typically formed on a carrier tissue in a pad forming unit of a converting machine. With regard to conventionally produced absorbent structures, reference is made to U.S. Pat. Nos. 5,009,650, 5,378,528, 5,128,082, 5,607,414, 5,147,343, 5,149,335, 5,522,810, 5,041,104, 5,176,668, 5,389,181, and 4,596,567, the disclosures of which are hereby incorporated herein by reference, as are the disclosures of all other patents, patent applications or references cited herein.
It is known from U.S. Pat. Nos. 3,669,103 and 3,670,731 that carboxylic polyelectrolytes may be crosslinked to create hydrogel-forming materials, now commonly referred to as superabsorbents, supersorbers or superabsorbent polymers (generally referred to herein as “SAPs”), and to employ such materials to enhance the absorbency of disposable absorbent articles. It is also known from U.S. Patent Nos. 3,980,663 and 4,076,673 that SAP may be formed by adding crosslinkers to solutions of carboxylated polyelectrolytes and then drying and curing the polymer. Unfortunately, the prior art approaches to making SAPs yield brittle, glassy, abrasive particulates. As a result, absorbent products generally incorporate SAPs in the form of discrete particles which may take the form of granules, flakes, powder, chunks, nuggets, pellets, needles, fibers, rods and the like. During further handling or processing, these brittle SAP materials tend to break into smaller particles, even dust particles that are small enough to become airborne. The fracture of SAP particles into smaller particles or dust creates an industrial hygiene problem. The airborne dust contaminates the air in the manufacturing and converting plants. In addition, small particles can foul the manufacturing and converting equipment. Sometimes fouling problems necessitate the incorporation of design features (such as carrier tissue) in the absorbent products simply to minimize the fouling of the equipment by small SAP particles. The brittleness of SAPs has traditionally poses a dusting problem not only while it is being processed into an absorbent article, but also in that the particle size distribution shifts toward smaller particles after the product leaves the manufacturer while it is being processed into an absorbent article.
Work place dust is handled by engineering controls such as air filtration and dust collection systems, but it would be better to eliminate or at least substantially reduce the formation of dust so that the engineering controls in the work place would be a secondary line of defense and not the primary dust-control means.
Accordingly, it would be desirable to provide a plastic SAP particle, which would be more resistant to fracture and disintegration during processing. Humectants, such as glycerol, have been suggested as plasticizers for SAPs. However, because they function well only in the presence of water, their use is impractical as the SAP becomes tacky in the presence of small amounts of water, and is designed to swell and gel in the presence of large amounts of water. Furthermore, water is not a practical plasticizer for SAPs in a commercial setting because the moisture level in the polymer and, consequently, its ductility or brittleness would fluctuate with changes in the relative humidity.
Another proposed dust control technique entails coating the brittle SAP particles with a non-penetrating hydrophilic liquid which would trap the microscopic dust particles as they form and prevent them from becoming airborne during subsequent handling. This approach is described in WO 94/22940, in which various polyethylene oxide adducts used in surface treatments of SAP particles are taught as de-dusting agents. A different approach is disclosed in EP 0 690 077 Al, wherein the fracture mechanics of the SAP particles are modified by using polyethylene oxide functional co-monomers in the conventional acrylic acid polymerization or through a process of solvent exchange, to distribute polyglycols throughout superabsorbent granules. The polyglycol materials are effective in minimizing the generation of polymer fines and aerosol dusts when the dry particles are subjected to high impact and shearing conditions. However, such polyglycol co-monomers are costly specialty chemicals and using them at levels high enough to effectively modify SAP properties would make the resultant SAP economically unattractive. Even the high molecular weight polyethylene oxide polymers which could be solvent-exchanged into the granules cost several times that of the SAP itself.
Accordingly, there is a need in the art for novel approaches for solving the brittleness problems of SAPs. Applicant has now surprisingly discovered that an aqueous dispersion of a rubbery material may be used to successfully overcome the brittleness problem.
SUMMARY OF THE INVENTION
The present invention is directed to a hydrogel-forming polymeric material including a first compound which is a superabsorbent polymer and a second compound including an elastomeric material (i.e. , soft, rubbery latex) effective to increase the fracture resistance of the superabsorbent. The second compound is present in the first as a dispersed phase. The first compound includes a water insoluble but water swellable polymer or a carboxylic-functional polyelectrolyte in combination with a crosslinker reactive with carboxyl or carboxylate groups. The second compound includes an effective amount of an aqueous dispersion of a rubbery material.
DETAILED DESCRIPTION OF THE INVENTION
All patents, patent applications and references cited herein are incorporated hereby by reference. In case of inconsistencies, the present disclosure governs.
The present invention provides a cost-effective, improved superabsorbent material having modified fracture mechanics to greatly minimize the formation of dust during the handling of the material. This is accomplished by thoroughly blending the superabsorbent polymer (SAP), at the time of its manufacture, with an elastomeric material which forms a dispersed elastomeric phase thereby lowering the ductile-brittle transition temperature of the SAP to below room temperature so that frequency of spontaneous particle fracture during shipping and handling is greatly diminished. The elastomeric material used in the present invention is a soft latex that has a rubbery appearance upon drying. In the context of this invention, a soft latex is one which is film-forming at room temperature unlike a hard latex which dries to a powder and must be heated above its softening point to become film-forming.
The hydrogel-forming material of the present invention is a blend of at least two immiscible polymers—(1) the superabsorbent polymer (SAP), and (2) the polymer present in an aqueous dispersion of an elastomer, which is combined with the SAP during manufacture. As used herein, the “hydrogel-forming polymeric material” refers to a highly absorbe

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