Mixed-bed ion-exchange hydrogel-forming polymer compositions...

Details Mixed-bed ion-exchange hydrogel-forming polymer compositions... Mixed-bed ion-exchange hydrogel-forming polymer compositions...

1999-08-19

2001-07-10

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, C604S358000

Reexamination Certificate

active

06258996

ABSTRACT:

TECHNICAL FIELD
This application relates to absorbent members for absorbing body fluids such as urine and menses. This application particularly relates to mixed-bed ion-exchange hydrogel-forming polymer compositions and absorbent members having at least one region comprising a relatively high concentration of these compositions.
BACKGROUND OF THE INVENTION
The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as “hydrogels,” “superabsorbents” or “hydrocolloid” material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter “hydrogel-forming absorbent polymers”) in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example. U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These hydrogel-forming absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. These polymers are rendered water-insoluble yet water-swellable, by slightly cross-linking the carboxyl group-containing polymer chains with conventional di- or poly-functional monomer materials, such as N,N′-methylenebisacrylamide, trimethylol propane triacrylate or triallyl amine. These slightly crosslinked absorbent polymers still comprise a multiplicity of anionic (charged) carboxyl groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
These hydrogel-forming absorbent polymers are also often made by initially polymerizing unsaturated amines or derivatives thereof such as diallyldimethylammonium chloride, N,N-dimethylaminoethylmethacrylate.HCl, N,N-dimethylaminoethylacrylate.HCl, methacrylamido-propyltrimethyl-ammonium hydroxide and the like. These polymers are rendered water-insoluble, yet water-swellable, by slightly cross-linking the polymer chains with conventional di- or poly-functional monomer materials, such as N,N′-methylenebisacrylamide, trimethylol propane triacrylate or triallyl amine. These slightly crosslinked absorbent polymers still comprise a multiplicity of cationic (charged) amine groups attached to the polymer backbone. It is these charged amine groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
The degree of cross-linking determines not only the water-insolubility of these hydrogel-forming absorbent polymers, but is also an important factor in establishing two other characteristics of these polymers: their absorbent capacity and gel strength. Absorbent capacity or “gel volume” is a measure of the amount of water or body fluid that a given amount of hydrogel-forming polymer will absorb. Gel strength relates to the tendency of the hydrogel formed from these polymers to deform or “flow” under an applied stress. Hydrogel-forming polymers useful as absorbents in absorbent structures and articles such as disposable diapers need to have adequately high gel volume, as well as adequately high gel strength. Gel volume needs to be sufficiently high to enable the hydrogel-forming polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength needs to be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution throughout the structure/article. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al). issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. No. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
These hydrogel-forming polymers are typically lightly-crosslinked polyelectrolytes that swell in aqueous electrolyte solutions primarily as a result of an osmotic driving force. The osmotic driving force for hydrogel-forming polymer swelling results primarily from polyelectrolyte counterions that are dissociated from the polyelectrolyte but are kept inside the swollen polymer due to electroneutrality considerations. Hydrogel-forming polymers that comprise weak-acid or weak-base polyelectrolytes (e.g., carboxylic acid or mono/di/tri- amine functional groups) in their un-neutralized forms are only slightly dissociated in urine solutions. These weak-acid or weak-base hydrogel-forming polymers must be at least partially neutralized with base or acid, respectively, in order to generate substantial concentrations of dissociated counterions. Without neutralization to e.g., ~70%, these weak-acid or weak-base hydrogel-forming polymers do not swell to their maximum potential absorbent capacity or gel volume. In contrast, the absorbent capacity of hydrogel-forming polymers comprising strong-acid or strong-base functional groups (e.g., sulfonic acid or quaternary ammonium hydroxide) are much less sensitive to their degree of neutralization. However, the use of these strong-acid or strong-base hydrogel-forming polymers in their un-neutralized forms have the potential to shift the pH of the urine solution to unacceptably low or high values, respectively.
Even after neutralization, the osmotic driving force for swelling and thus the absorbent capacity or gel volume of polyelectrolyte hydrogel-forming polymers is greatly depressed by the high concentration of dissolved electrolyte normally present in urine. The concentration of this dissolved electrolyte, expressed as wt % NaCl, can be as high as 0.9% (physiological saline) or higher. It is known that reducing the concentration of dissolved electrolyte in urine (e.g., by dilution with distilled water) can greatly increase the absorbent capacity of a polyelectrolyte hydrogel-forming polymer. Thus, for example, when Jayco synthetic urine is used to measure the gel volume of a partially-neutralized sodium polyacrylate hydrogel-forming polymer, a ten-fold dilution of Jayco with distilled water can results in approximately a three-fold increase in gel volume.
It is known that the concentration of dissolved electrolyte in an aqueous solutions can be lowered by “reaction” of the solution with a mixed-bed ion-exchange resin. (Ion-exchange columns are often used commercially to deionize water.) Electrolyte concentration is reduced by the combined effect of (i) exchange of dissolved cations (e.g., Na
+
) in the aqueous solution with H
+

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