WO2004020008A1 - Superabsorbent polymer particles - Google Patents

Abstract

Superabsorbent polymer particles having a particles size distribution of about 300 to about 5000 μm and a mass median particle size of about 700 μm or greater are disclosed. Improved diaper cores and absorbent articles containing the large particle size superabsorbent polymer particles also are disclosed.

Description

SUPERABSORBENT POLYMER PARTICLES

FIELD OF THE INVENTION

The present invention relates to superabsorbent polymer (SAP) particles having (a) a particle size distribution of about 300 to about 5000 μm and (b) a mass median particle size greater than about 700 μm. The particles can be (a) surface crosslinked conventional SAP particles (e.g., surface crosslinked poty acrylic acid) neutralized 25 to 100%), (b) multicomponent superabsorbent polymer particles having at least one microdomain of an acidic resin in contact with, or in close proximity to, at least one microdomain of a basic resin, (c) a mixture of (i) particles of an unneutralized acidic water-absorbing resin and (ii) particles of an unneutralized basic water-absorbing resin, or (d) a mixture containing at least two of (a), (b), and (c).

BACKGROUND OF THE INVENTION

Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.

These polymers, and others, are known in the art by various names, such as superabsorbent polymers, hydrogels, hydrocolloids, and water-absorbent hy- drophilic polymers, for example. As used herein, the term "SAP" refers to a superabsorbent polymer, and collectively refers to such water-absorbing materials. As used herein, the term "SAP particles" refers to superabsorbent polymer particles in the dry state, more specifically, particles containing from no water up to an amount of water less than the weight of the particles, and typically less than about 5%, by weight, water. The terms "SAP gel," "SAP hydrogel," or "hydrogel" refer to a superabsorbent polymer in the hydrated state, more specifically, particles that have absorbed at least their weight in water, and typically several times their weight in water.

The term "surface-crosslinked SAP particle" refers to an SAP particle having its molecular chains present in the vicinity of the particle surface crosslinked by a polyfunctional compound applied to the surface of the particle. The term "surface crosslinking" means that the level of functional crosslinks in the SAP particle in the vicinity of the surface of the particle generally is higher than the level of functional crosslinks in the SAP particle in the interior of the particle.

SAPs are lightly crosslinked hydrophilic polymers, and are discussed generally in Goldman et al. U.S. Patent Nos. 5,669,894 and 5,559,335, each incorporated herein by reference. Multicomponent SAP particles are disclosed in U.S. Patent Nos. 6,072,101; 6,159,591 ; 6,222,091 ; 6,235,965; and 6,342,298, each incorporated herein by reference.

SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, such as a diaper.

The development of highly absorbent, SAP-containing articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products, such as sanitary napkins, is the subject of substantial commercial interest. A highly desired characteristic of such absorbent articles is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. Article packaging also is more compact, which makes the diapers easier for the consumer to carry and store. Packaging compactness also results in reduced distribution costs for the manu- facturer and distributor, including less required shelf space per diaper unit.

A variety of parameters effect the ability of an SAP particle to rapidly absorb a large amount of a fluid, and then to retain the absorbed fluid under various stresses. Optimization of these parameters allows a reduction in amounts of cellulosic fiber present in a diaper core, which in turn reduces the overall bulk of the diaper. SAP particles, therefore, are designed in an attempt to optimize absorption capacity, absorption rate, acquisition time, gel strength, and permeability.

The present invention is directed to the surprising and unexpected find- ing that SAP particles having a relatively large particle size distribution and a relatively large mass median particle size improve absorption and retention properties, and reduce or eliminate the amount of cellulosic fibers, or fluff, in a diaper core. The SAP particles of the present invention have a high absorption capacity and a convective flow that allows rapid uptake of a fluid. Absorbent articles contain a relatively low amount (e.g., less than about 50% by weight) of SAP particles for several reasons. First, SAPs employed in present absorbent articles lack an absorption rate that allows the SAP particles to quickly absorb body fluids, especially in "gush" situations. This necessitates the inclusion of cel- lulosic fibers, typically wood pulp fibers, in the absorptive core of the article as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.

In order to manufacture diaper cores substantially, or completely, free of cellulosic fiber, a continuous zone of SAP particles is required. However, because of the nature of SAP particles, it is impossible to combine features like high absorption capacity and high gel strength within one SAP product because improving one feature adversely affects the other. For example, in order to provide a high absorption capacity, the degree of SAP crosslinking has to be sufficiently low to enable long flexible polymer chains to absorb large quantities of fluids. But, the degree of crosslinking also determines the gel strength of the superabsorbent polymer. In personal care products, SAP hydrogels of a relatively high gel strength are needed because of the mechanical forces applied by individuals wearing personal care products. High gel strength is obtained by higher degrees of crosslinking, and for that reason, a well-defined lower crosslinking limit exists to produce useful superabsorbents.

More importantly, many SAPs exhibit gel blocking. "Gel blocking" occurs when the SAP particles are wetted, and the SAP particles swell to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other re- gions of the absorbent member takes place via a slow diffusion process. Gel blocking can be a particularly acute problem if the SAP particles do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. In practical terms, the acquisition of fluids by the absorbent article is much slower than the rate at which a fluid is discharged, especially in a gush situation. Leakage from the absorbent article can take place well before the SAP particles in the absorbent article are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the remainder of the absorbent core.

The gel blocking phenomena necessitates the use of a fibrous matrix, i.e., fluff, in which the SAP particles are dispersed. The fibrous matrix separates the SAP particles from one another. The fibrous matrix also provides a capillary structure that allows fluid to reach SAP located in regions of the core remote from the initial fluid discharge point. However, dispersing a relatively low amount of the SAP in a fibrous matrix to minimize or avoid gel blocking reduces the overall fluid storage capacity of absorbent cores. Overall, using lower amounts of an SAP limits the advantage of the SAP, i.e., an ability to absorb and retain large quantities of body fluids per given volume.

WO 98/37149 discloses SAP particles having a mass median particle size equal to or greater than about 400 microns mixed with hydrophilic fibrous materials. This admixture minimizes gel blocking and helps maintain an open capillary structure within the absorbent structure to enhance planar transport of fluids away from the area of initial discharge to the remainder of the absorbent core. In addition, the particle size distribution of the SAP has been adjusted in an attempt to improve absorbent ca- pacity and efficiency of the particles employed in the absorbent structure (see U.S.

Patent Nos. 5,047,023 and 5,061 ,259). However, U.S. Patent No. 5,047,023 discloses that increasing the mass median particle size does not, by itself, provide absorbent articles having a high absorption capacity and rapid an acquisition rate.

For absorbent cores containing a relatively high amount of SAP particles, other SAP properties also are important. It has been found that the openness, or porosity, of the hydrogel layer formed when the SAP swells in the presence of body fluids helps determine the ability of an SAP to acquire and transport a fluid, especially when the SAP is present in high amounts in the absorbent core. Porosity refers to the fractional volume of a particle that is not occupied by solid material. For a hydrogel layer formed entirely from an SAP, porosity is the fractional volume of the layer that is not occupied by hydrogel. For an absorbent structure containing the hydrogel, as well as other components, porosity is the fractional volume (also referred to as void volume) that is not occupied by the hydrogel or other solid components (e.g., cellulosic fibers).

It would be desirable to provide SAP particles that exhibit exceptional water absorption and retention properties, especially with respect to electrolyte- containing liquids. In addition, it would be desirable to provide SAP particles that have an ability to absorb large amounts of fluids quickly, demonstrate good fluid permeability and conductivity into and through an SAP particle and an absorbent core containing SAP particles, and have a high gel strength, such that a hydrogel formed from the SAP particles does not deform or flow under an applied stress or pressure.

SUMMARY OF THE INVENTION

The present invention is directed to SAP particles having (a) a particle size distribution of about 300 to about 5000 μm and (b) a mass median particle size of greater than about 700 μm. The SAP particles can be (a) surface crosslinked conventional SAP particles; (b) mufticomponent superabsorbent particles disclosed in U.S. Patent Nos. 6,072,101 ; 6,159,591 ; 6,222,091 ; 6,235,965; and 6,342,298, each incorpo- rated herein by reference; (c) a mixture of (i) particles of an unneutralized acid water- absorbing resin and (ii) particles of an unneutralized basic water-absorbing resin, or (d) a mixture containing at least two of (a), (b), and (c).

Accordingly, one aspect of the present invention is to provide SAP particles having a large, defined particle size, and that have a high absorption capacity, a high absorption rate, and good permeability and gel strength, and that demonstrate an improved ability to absorb and retain electrolyte-containing liquids, such as saline, blood, urine, and menses.

More particularly, in one embodiment, the present invention is directed to surface crosslinked conventional SAP particles having (a) a particle size distribution of about 300 to about 5000 μm and (b) a mass median particle size of greater than 700 μm. In another embodiment, the present invention is directed to multicomponent SAP particles containing at least one discrete microdomain of at least one acidic water- absorbing resin in contact with, or in close proximity to, at least one microdomain of at least one basic water-absorbing resin, and (a) having a particle size distribution of about 300 to about 5000 μm and (b) a mass median particle size of greater than about 700 μm. The multicomponent SAP particles can contain a plurality of microdomains of the acidic water-absorbing resin and/or the basic water-absorbing resin dispersed throughout the particle. The acidic resin can be a strong or a weak acidic resin. Similarly, the basic resin can be a strong or a weak basic resin. A preferred SAP contains one or more microdomains of at least one weak acidic resin and one or more microdomains of at least one weak basic resin.

Another aspect of the present invention is to provide an SAP material comprising a mixture containing (i) particles of an unneutralized acidic water-absorbing resin and (ii) particles of an unneutralized basic water-absorbing resin, and (a) having a particle size distribution of about 300 to about 5000 μm and (b) a mass median particle size of greater than about 700 μm. The mixture contains about 10% to about 90%, by weight, acidic resin particles and about 10% to about 90%, by weight, basic resin particles.

Yet another aspect of the present invention is to provide an SAP mate- rial comprising a mixture containing at least two of (i) surface crosslinked conventional SAP particles, (ii) multicomponent SAP particles, and (iii) a mixture comprising particles of an unneutralized acidic water-absorbing resin and particles of an unneutralized basic water-absorbing resin, each type of particle having a particle size distribution of about 300 to about 5000 μm and a mass median particle size of greater than about 700 μm. Typically (i), (ii), or (iii), if present in the mixture at all, is present in an amount of about 10% to about 90%, by weight.

In a preferred embodiment of the present invention, the SAP comprises particles of a partially neutralized, surface crosslinked SAP, e.g., poly(acrylic acid), containing at least 25%, and up to 100%, neutralized carboxyl groups. In another preferred embodiment, the SAP comprises multicomponent SAP particles comprising unneutralized poly(acrylic acid) and unneutralized poly(vinylamine). Alternatively, the SAP comprises a mixed bed of unneutralized poly(acrylic acid) particles and unneutral- ized pofy(vinylamine) particles. In each embodiment, the particles have a particle size distribution of about 300 to about 5000 μm and a mass median particle size of greater than 700 μm.

Yet another aspect of the present invention is to provide an absorbent sheet material comprising SAP particles having (a) a mass median particle size of greater than 700 μm and (b) a particle size distribution of about 300 to about 5000 μm. The sheet optionally can contain up to 40% by weight of a cellulosic fluff and/or a non- woven fiber.

Still another aspect of the present invention is to provide absorbent articles, such as diapers, incontinence pads, and catamenial devices, having an absorbent core comprising SAP particles having the recited particle size range and median. The absorbent article comprises a core, wherein the core contains greater than 50%, and up to 100%, by weight, of the SAP particles.

These and other aspects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an absorbent article having a core containing 100% by weight SAP particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to surface crosslinked conventional SAP particles, to SAP particles containing an unneutralized acidic water-absorbing resin and an unneutralized basic water-absorbing resin, either in the same particle or as an admixture of particles, and to mixtures of such SAP particles. As used herein, the term "unneutralized" is defined as a water-absorbing resin neutralized 0% to 25%. The term "conventional SAP" is defined as a lightly crosslinked acidic or basic resin, neutralized from 25% to 100%, and capable of absorbing several times its weight of an aqueous fluid. All SAP particles of the present invention have a particle size distribution of about 300 to about 5000 μm, and a relatively large mass median particle size of greater than about 700 μm.

The conventional SAP is surface crosslinked. The multicomponent superabsorbent polymer particles and the unneutralized acidic water-absorbing resin and unneutralized basic water-absorbing resin are optionally surface crosslinked.

The surface treatment of SAP particles is well known. As understood in the art, surface crosslinked SAP particles have a higher level of crosslinking in the vicinity of the surface than in the interior. As used herein, "surface" describes the outer- facing boundaries of the particle. For porous SAP particles, exposed internal surfaces also are included in the definition of surface. Surface crosslinking of SAPs is generally discussed in F.L. Buchholz et al., ed., "Modern Superabsorbent Polymer Technology," Wiley-VCH, New York, NY, pages 97-108 (1998).

An SAP of the present invention is limited only in that the SAP is capa- ble of absorbing several times its weight of an aqueous fluid and swells to form a hydrogel. The SAP can be an acidic water-absorbing resin (anionic), a basic water- absorbing resin (cationic), or a mixture thereof. If the SAP component of the present composition comprises an acidic or a basic water-absorbing resin, the SAP is neutralized 25%o to 100%, i.e., has a degree of neutralization (DN) of 25 to 100. If the SAP component contains an acidic and a basic resin, the SAP can be neutralized or unneutralized, i.e., DN=0 up to DN=100. The SAP component also can contain a mixture of neutralized and unneutralized SAPs.

In one embodiment, the SAP particles having the disclosed mass me- dian particle size and particle size distribution is a surface crosslinked conventional SAP. A conventional SAP can be an anionic (an acidic water-absorbing resin) or a cationic (a basic water-absorbing resin). Conventional SAPs are 25% to 100% neutralized.

A preferred surface crosslinked conventional SAP is an acidic water- absorbing resin neutralized 25% to 100%. The acidic water-absorbing resin can be a single resin, or a mixture of resins. The acidic resin can be a homopolymer or a copolymer. The identity of the acidic water-absorbing resin is not limited as long as the resin is capable of swelling and absorbing at least ten times its weight in water, when in a neutralized form. Monomers useful in the preparation of an SAP are disclosed in U.S. Patent No. 5,149,750 and WO 01/68156, each incorporated herein by reference.

The anionic SAPs are based on an acidic water-absorbing resin. The anionic SAPs, either strongly acidic or weakly acidic, can be any resin that acts as an SAP in its neutralized form. The acidic resins typically contain a plurality of carboxylic acid, sulfonic acid, phosphonic acid, phosphoric acid, and/or sulfuric acid moieties.

The acidic water-absorbing resin typically is a lightly crosslinked acrylic resin, such as lightly crosslinked poly(acrylic acid). The lightly crosslinked acidic resin typically is prepared by polymerizing an acidic monomer containing an acyl moiety, e.g., acrylic acid, or a moiety capable of providing an acid group, i.e., acrylonitrile, in the presence of an internal crosslinking monomer, i.e., a polyfunctional organic compound. The acidic resin can contain other copolymerizable units, i.e., other mono- ethylenically unsaturated comonomers, well known in the art, as long as the polymer is substantially, i.e., at least 10%, and preferably at least 25%, acidic monomer units. To achieve the full advantage of the present invention, the acidic resin contains at least 50%, and more preferably, at least 75%, and up to 100%, acidic monomer units.

Ethylenically unsaturated carboxylic acid and carboxylic acid anhydride monomers useful in the acidic water-absorbing resin include acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, α-cyanoacrylic acid, /3-methylacrylic acid (cro- tonic acid), α-phenylacrylic acid, -acryloxy-propionic acid, sorbic acid, α-chlorosorbic acid, angelic acid, cinnamic acid, p-chlorocinnamic acid, 3-stearylacrylic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fur- maric acid, tricarboxyethylene, and maleic anhydride. Acrylic acid is the most preferred ethylenically unsaturated carboxylic acid for preparing the SAP.

Ethylenically unsaturated sulfonic acid monomers include aliphatic and aromatic vinyl sulfonic acids, such as vinyl sulfonic acid, allyl suifonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, acrylic and methacrylic sulfonic acids, such as sul- foethyl acrylate, sulfoethyl meth-acrylate, sulfopropyl acrylate, sulfopropyl meth- acrylate, 2-hydroxy-3-methacryloxypropyl sulfonic acid, and 2-acryIamide-2- methylpropane sulfonic acid. Phosphate-containing acidic resins are prepared by ho- mopolymerizing or copolymerizing ethylenically unsaturated monomers containing a phosphoric acid moiety, such as methacryloxy ethyl phosphate. An extensive list of suitable SAP-forming monomers can be found in U.S. Patent No.4,076,663, incorporated herein by reference. The polymerization of acidic monomers, and copolymerizable monomers, if present, most commonly is performed by free radical processes in the presence of a polyfunctional organic compound. The acidic resins are crosslinked to a sufficient extent such that the polymer is water insoluble. Cross-linking renders the acidic resins substantially water insoluble, and, in part, serves to determine the absorption capacity of the resins. For use in absorption applications, an acidic resin is lightly crosslinked, i.e., has a crosslinking density of less than about 20%, preferably less than about 10%, and most preferably about 0.01% to about 7%.

Conventional SAPs include neutralized, water-swellable polymers or copolymers containing monomeric units of (meth)acrylic acid, maleic acid, itaconic acid, fumaric acid (meth)acrylamide, (meth)acrylonitrile, vinyl acetate and hydrolysis products thereof, vinylpyrrolidone, vinylpyridine, vinylsulfonic acid and esters and amides thereof, and N-alkyl and N,N-dialkyl-substituted esters and/or amides of (meth)acrylic acid, or anhydrides and quaternary ammonium compounds of these monomers. In addition, natural water-swellable polymers useful as conventional SAP particles include, but are not limited to, carboxymethylcellulose, hydroxyethylcellulose, hy- droxypropylcellulose, methylcellulose, guar seed meal, a xanthan, an alginate, starch and derivatives thereof, as well as neutralized graft polymers of these natural polymers and the above-listed monomers.

The anionic SAPs can be, for example, a poly(acrylic acid), a hydrolyzed starch-acrylonitrile graft copolymer, a starch-acrylic acid graft copolymer, a saponified vinyl acetate-acrylic ester copolymer, a hydrolyzed acrylonitrile copolymer, a hydro- lyzed acrylamide copolymer, an ethylene-maleic anhydride copolymer, an isobutylene- maleic anhydride copolymer, a poly(vinylsulfonic acid), a poly(vinylphosphonic acid), a poly(vinylphosphoric acid), a poly(vinyIsulfuric acid), a sulfonated polystyrene, and mixtures thereof. Other lightly crosslinked hydrophilic polymers are disclosed in Goldman et al. U.S. Patent Nos. 5,669,894 and 5,559,335, each incorporated herein by refer- ence. The preferred anionic SAP is a poly(acryIic acid).

Analogous to the acidic resin, the conventional basic water-absorbing resin, i.e., cationic SAP, in the present absorbent particles can be a strong or weak basic water-absorbing resins. The basic water-absorbing resin can be a single resin or a mixture of resins. The basic resin can be a homopolymer or a copolymer. The identity of the basic resin is not limited as long as the basic resin is capable of swelling and absorbing at least ten times its weight in water, when in a charged form. The weak conventional basic resin preferably is present in its cationic form, i.e., 25% to 100% of the basic moieties, e.g., amino groups, are present in a charged form. The strong ba- sic resins typically are present in the hydroxide (OH) or bicarbonate (HCO₃) form. The basic water-absorbing resin typically is a lightly crosslinked resin. The lightly cross-linked basic water-absorbing resin can contain other copolymerizable units and is crosslinked using an internal crosslinking monomer, as set forth above with respect to the acidic water-absorbing resin.

A basic water-absorbing resin used in the present SAP particles typically contains an amino or a guanidino group. Accordingly, a water-soluble basic resin also can be crosslinked in solution by suspending or dissolving an uncrosslinked basic resin in an aqueous or alcoholic medium, then adding a di- or polyf unctional compound capable of cross-linking the basic resin by reaction with the amino groups of the basic resin. Such crosslinking agents are disclosed in U.S. Patent Nos. 5,085,787 and 6,235,965, incorporated herein by reference, and in EP 450 923. Preferred crosslinking agents are ethylene glycol diglycidyl ether (EGDGE), a water-soluble diglycidyl ether, and a dibromoalkane, an alcohol-soluble compound.

The basic resin, either strongly or weakly basic, therefore, can be any resin that acts as an SAP in its charged form. Examples of basic resins include a poly(vinylamine), a polyethylenimine, a poly(vinylguanidine), a poly(allylamine), a poly- (allylguanidine), or a poly(dialkylaminoalkyl (meth)acrylamide). Preferred basic resins include a poly(vinylamine), polyethylenimine, poly(vinylguanidine), poly(dimethylaminoethyl acrylamide) (poly(DAEA)), and poly(dimethylaminopropyl methacrylamide) (poly(DMAPMA)), a poly(dimethyldiallylammonium hydroxide), a qua- temized polystyrene derivative, a guanidine-modified polystyrene, a quatemized poly((meth)acrylamide) or ester analog. See U.S. Patent No. 6,235,965, incorporated herein by reference.

Copolymerizable monomers for introduction into the acidic resin, or into the basic resin, include, but are not limited to, ethylene, propylene, isobutylene, d. ₄alkyl acrylates and methacrylates, vinyl acetate, methyl vinyl ether, and styrenic compounds having a formula:

wherein R represents hydrogen or a C_halky! group, and wherein the phenyl ring optionally is substituted with one to four C_1-4alkyl or hydroxy groups. Suitable C_halky! acrylates include, but are not limited to, methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate, and the like, and mixtures thereof. Suitable C^alkyl methacrylates include, but are not limited to, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n- propylmethylmethacrylate, n-butyl methacrylate, and the like, and mixtures thereof or with C_1-4alkyl acrylates. Suitable styrenic compounds include, but are not limited to, styrene, α-methylstyrene, p-methylstyrene, t-butyl styrene, and the like, and mixtures thereof or with

acrylates and/or methacrylates.

A conventional SAP utilized in the present invention is surface crosslinked. Surface crosslinking or annealing of an SAP is known in the art, as set forth in U.S. Patent No. 6,222,091 , incorporated herein by reference, which discloses compounds and conditions for surface crosslinking and/or annealing an acidic or a ba- sic SAP.

In general, surface crosslinking is achieved by contacting SAP particles with a solution of a surface crosslinking agent to wet predominantly only the outer surfaces of the SAP particles. Surface crosslinking and drying of the SAP particles then is performed, preferably by heating at least the wetted surfaces of the SAP particles.

Typically, SAP particles are surface treated with a solution of a surface crosslinking agent. The solution contains about 0.01 % to about 4%, and preferably about 0.4% to about 2%, by weight, surface crosslinking agent in a suitable solvent, for example, water or an alcohol. The solution can be applied as a fine spray onto the surface of freely tumbling SAP particles at a ratio of about 1 :0.01 to about 1 :0.5 parts by weight SAP particles to solution of surface crosslinking agent. To achieve the desired absorption properties, the surface crosslinker is distributed evenly on the surfaces of the SAP particles. For this purpose, mixing is performed in suitable mixers, e.g., fluidized bed mixers, paddle mixers, a rotating disc mixer, a ribbon mixer, a screw mixer, milling rolls, or twin-worm mixers.

The surface crosslinking agent is present in an amount of 0.001 % to about 5%, by weight of the SAP particles, and preferably 0.01 % to about 2% by weight. To achieve the full advantage of the present invention, the surface crosslinking agent is present in an amount of about 0.05% to about 1 % by weight.

The crosslinking reaction and drying of the surface-treated SAP particles are achieved by heating the surface-treated polymer at a suitable temperature, e.g., about 25°C to about 250°C, and preferably about 105°C to about 200°C, for about 60 to about 180, and preferably for about 60 to about 150 minutes. However, any other method of reacting the crosslinking agent to achieve surface crosslinking of the SAP particles, and any other method of drying SAP particles, such as microwave energy, can be used.

Surface treating with a surface crosslinking agent, and subsequent or simultaneous heating, provides additional polymer crosslinks in the vicinity of the surface of the SAP particles. The gradation ion crosslinking from the surface of the SAP particles to interior, i.e., the anisotropy of crosslink density, can vary, both in depth and profile. Thus, for example, the depth of surface crosslinking can be shallow, with a relatively sharp transition from a high level to a low level of crosslinking. Alternatively, for example, the depth of surface crosslinking can be a significant fraction of the dimensions of the SAP particle, with a broader transition.

Surface crosslinking generally is performed after the final boundaries of the SAP particles are essentially established (e.g., by grinding, extruding, or foaming). However, it is also possible to effect surface crosslinking concurrently with the creation of final boundaries. Furthermore, some additional changes in SAP particle boundaries can occur even after surface crosslinks are introduced.

Suitable surface crosslinkers include, but are not limited to, di- or poly- glycidyl compounds, such as diglycidyl phosphonates, ethylene glycol diglycidyl ether, and bischlorohydrin ethers of polyalkylene glycols; alkoxysilyl compounds; polyaziri- dines based on polyethers or substituted hydrocarbons, for example, bis-N- aziridinomethane; polyamines or polyamidoamines and their reaction products with epichlorohydrin; polyols, such as ethylene glycol, 1 ,2-propanediol, 1 ,4-butanediol, glyc- erol, methyltriglycol, polyethylene glycols having an average molecular weight M_w of 200-10,000, di- and polyglycerol, pentaerythritol, sorbitol, the ethoxylates of these polyols and their esters with carboxylic acids or carbonic acid such as ethylene car- bonate or propylene carbonate; carbonic acid derivatives, such as urea, thiourea, gua- nidine, dicyandiamide, 2-oxazolidinone and its derivatives, bisoxazoline, polyoxazoli- nes, di- and polyisocyanates; di- and poly-N-methylol compounds such as, for example, methylenebis(N-methylolmethacrylamide) or melamine-formaldehyde resins; compounds having two or more blocked isocyanate groups such as, for example, trimethyl- hexamethylene diisocyanate blocked with 2,2,6,6-tetramethylpiperidin-4-one.

Particularly suitable postcrosslinkers are di- or polyglycidyl compounds, such as ethylene glycol diglycidyl ether. See U.S. Patent No. 6,159,591 , incorporated herein by reference, for additional surface crosslinking agents for anionic and cationic SAPs, and method of surface crosslinking and annealing SAP particles. Suitable surface crosslinking agents are capable of reacting with the acid moieties or amino groups of an acidic or a basic water-absorbing resin, respectively, and crosslinking the resin. Preferably, the surface crosslinking agent is alcohol soluble or water soluble, and possesses sufficient reactivity with the resin such that crosslinking occurs in a controlled fashion, preferably at a temperature of about 25°C to about 180°C.

Nonlimiting examples of suitable surface crosslinking agents for acidic resins include, but are not limited to:

(a) polyhydroxy compounds, such as glycols and glycerol;

(b) metal salts;

(c) quaternary ammonium compounds; (d) a multifunctional epoxy compound;

(e) an alkylene carbonate, such as ethylene carbonate or propylene carbonate;

(f) a plyaziridine, such as 2,2-bishydroxymethyl butanol tris[3-(1- aziridine propionate]); (g) a haloepoxy, such as epichlorohydrin;

(h) a polyamine, such as ethylenediamine; (i) a polyisocyanate, such as 2,4-toluene dissocyanate;

(j) hydroxyalkylamides, hydroxyalkylamines, and oxazolinium ion, disclosed in U.S. Patent No. 6,376,618, U.S. Patent No. 6,391 ,451 , and WO 01/8959, for example, bis[N,N-di(β- hydroxyethyl)]adipamide, available commercially as PRIMID™ XL-552, EMS-CHEMIE, Domet, Switzerland, bis[N,N-di(β- hydroxypropyl)]succinamide, bis[N,N-di(β- hydroxyethyl)]azelamide, bis[N,N-di(β-hydroxy- propyl)]adipamide, bis[N-methyl-N-(β-hydroxyethyl)]oxamide, and PRIMID™ QM-1260; (k) other crosslinking agents for acidic water-absorbing resins known to persons skilled in the art.

Nonlimiting examples of suitable surface crosslinking agents for basic resins include, but are not limited to:

(a) dihalides and disulfonate esters, for example, compounds of the formula Y-(CH₂)_P-Y,

wherein p is a number from 2 to 12, and Y, independently, is halo (preferably bromo), tosylate, mesylate, or other alkyl, or aryl sulfonate esters;

(b) multifunctional aziridines;

(c) multifunctional aldehydes, for example, glutaraldehyde, triox- ane, paraformaldehyde, terephthaldehyde, malonaldehyde, and glyoxal, and acetals and bisulfites thereof; (d) halohydrins, such as epichlorohydrin;

(e) multifunctional epoxy compounds, for example, ethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, and bisphenol F diglycidyl ether,

(f) multifunctional carboxylic acids and esters, acid chlorides, and anhydrides derived therefrom, for example, di- and polycarbox- ylic acids containing 2 to 12 carbon atoms, and the methyl and ethyl esters, acid chlorides, and anhydrides derived therefrom, such as oxalic acid, adipic acid, succinic acid, dodecanoic acid, malonic acid, and glutaric acid, and esters, anhydrides, and acid chlorides derived therefrom;

(g) organic titanates, such as TYZOR® AA, available from E.I. Du- Pont de Nemours, Wilmington, DE;

(h) melamine resins, such as the CYMEL® resins available from

Cytec Industries, Wayne, NJ; (i) hydroxymethyl ureas, such as N, N'-dihydroxymethyl-4,5- dihydroxyethylene urea; (j) multifunctional isocyanates, such as toluene diisocyanate, iso- phorone diisocyanate, methylene diisocyanate; and (k) other crosslinking agents for basic water-absorbing resins known to persons skilled in the art.

A preferred surface crosslinked conventional SAP is neutralized surface crosslinked poly(acrylic acid), i.e., surface crosslinked PAA. It should be noted that although this disclosure is directed primarily to PAA, other acidic and basic water- absorbing resins can be utilized as the SAP. The surface crosslinked conventional SAP particles of the present invention have a mass median particle size greater than about 700 μm and a particle size distribution of about 300 to about 5000 μm.

In another embodiment, the present invention is directed to multicompo- nent SAP particles containing at least one microdomain of an acidic water-absorbing resin (i.e., anionic SAP) in close proximity to, and preferably in contact with, at least one microdomain of a basic water-absorbing resin (i.e., cationic SAP). Each particle contains one or more microdomains of an acidic resin and one or more microdomains of a basic resin. The microdomains can be distributed nonhomogeneously or homoge- neously throughout each particle. The multicomponent SAP particles of the present invention have a particle size distribution of about 300 to about 5000 μm and have a mass median particle size greater than about 700 μm.

Each multicomponent SAP particle contains at least one acidic water- absorbing resin and at least one basic water-absorbing resin. In one embodiment, the SAP particles consist essentially of acidic resins and basic resins, and contain micro- domains of the acidic and/or basic resins. In another embodiment, microdomains of the acidic and basic resins are dispersed in an absorbent matrix resin.

The multicomponent SAP particles of the present invention are not limited to a particular structure or shape. However, it is important that substantially each multicomponent SAP particle contains at least one microdomain of an acidic water- absorbing resin and at least one microdomain of a basic water-absorbing resin in close proximity to one another. Improved water absorption and retention, and improved fluid permeability through and between multicomponent SAP particles, are observed as long as the acidic resin microdomain and the basic resin microdomain are in close proximity within the particle. In a preferred embodiment, the microdomains of acidic and basic resin are in contact. Multicomponent SAP particles, and their method of manufacture, are discussed in U.S. Patent Nos. 6,072,101 ; 6,159,591 ; 6,222,091 ; 6,235,965; and 6,342,298, each incorporated herein by reference.

The multicomponent SAP particles of the present invention comprise an acidic resin and a basic resin in a weight ratio of about 90:10 to about 10:90, and preferably about 20:80 to about 80:20. To achieve the full advantage of the present inven- tion, the weight ratio of acidic resin to basic resin in a multicomponent SAP particle is about 30:70 to about 70:30. The acidic and basic resins can be distributed homogeneously or nonhomogeneously throughout the SAP particle.

The present multicomponent SAP particles contain at least about 50%, and preferably at least about 70%, by weight of acidic resin plus basic resin. To achieve the full advantage of the present invention, a multicomponent SAP particle contains about 80% to 100% by weight of the acidic resin plus basic resin. Components of the present SAP particles, other than the acidic and basic resin, typically, include a matrix resin or other minor optional ingredients. The surface crosslinked conventional and the multicomponent SAP particles, and the SAP particles used in other embodiments of the present invention, can be in any form, either regular or irregular, such as granules, fibers, beads, powders, or flakes, or any other desired shape. In embodiments wherein the multicomponent SAP is prepared using an extrusion step, the shape of the SAP is determined by the shape of the extrusion die. The shape of the SAP particles also can be determined by other physical operations, such as milling or by the method of preparing the particles, such as agglomeration.

In accordance with an important feature of the present invention, the

SAP particles utilized in the present invention have a particle size distribution of about 300 to about 5000 microns (μm), and preferably about 300 to about 2000 μm. To achieve the full advantage of the present invention, the SAP particles have a particle size distribution of about 400 to about 1500 μm. The SAP particles also have a mass median particle size of greater than about 700 μm, and preferably greater than about 750 μm. To achieve the full advantage of the present invention, the SAP particles have a mass median particle size of greater than about 800 μm. In preferred embodiments, the SAP particles are in the form of a granule or a bead.

For the SAP particles described above, particle size distribution is defined as the dimension determined by sieve size analysis. Thus, for example, a particle that is retained on a U.S.A. Standard Testing Sieve with 250 micron openings (e.g., No. 60 U.S. Series Alternate Sieve Designation) is considered to have a size greater than 250 microns; a particle that passes through a sieve with 250 micron openings and is retained on a sieve with 125 micron openings (e.g., No. 120 U.S. Series Alternate

Sieve Designation) is considered to have a particle size between 125 and 250 microns; and a particle that passes through a sieve with 125 micron openings is considered to have a size less than 125 microns.

The mass median particle size of a given sample of SAP is defined as the particle size that divides the sample in half on a mass basis, i.e., one-half of the sample has a particle size greater than the mass median size. A standard particle-size plotting method (wherein the cumulative weight percent of the particle sample retained on or passed through a given sieve size opening is plotted versus sieve size opening on probability paper) typically is used to determine median particle size when the 50% mass value does not correspond to the size opening of a U.S.A. Standard Testing Sieve. Methods for determining the particle size of the SAP particles are further described in U.S. Patent No. 5,061,259, incorporated by reference. In another embodiment, the multicomponent SAP particles are in the shape of a fiber, i.e., an elongated, acicular SAP particle. The fiber is in the shape of a cylinder, for example, having a minor dimension (i.e., diameter) and a major dimension (i.e., length). Cylindrical multicomponent SAP fibers have a minor dimension (i.e., di- ameter of the fiber) greater than about 300 μm, and up to about 1000 μm. The cylindrical SAP fibers have a relatively long major dimension, for example, about 300 to about 5000 μm.

Like the surface crosslinked conventional SAPs, an acidic water- absorbing resin present in a multicomponent SAP particle can be either a strong or a weak acidic water-absorbing resin. The acidic water-absorbing resin can be a single resin, or a mixture of resins. The acidic resin can be a homopolymer or a copolymer as described above with respect to the conventional SAPs. The identity of the acidic water-absorbing resin is not limited as long as the resin is capable of swelling and absorb- ing at least ten times its weight in water, when in a neutralized form. The acidic resin is present in its acidic, or unneutralized, form, i.e., about 75% to 100% of the acidic moieties are present in the free acid form. Although the free acid form of an acidic water- absorbing resin is generally a poor water absorbent, the combination of an acidic resin and a basic resin either in a multicomponent SAP particle or a mixed bed system pro- vides excellent water absorption and retention properties.

Analogous to the acidic resin, the basic water-absorbing resin in the present SAP particles can be a strong or weak basic water-absorbing resins. The basic water-absorbing resin can be a single resin or a mixture of resins. The basic resin can be a homopolymer or a copolymer, as previously discussed with respect to conventional SAPs. The identity of the basic resin is not limited as long as the basic resin is capable of swelling and absorbing at least ten times its weight in water, when in a charged form. The weak basic resin typically is present in its free base, or unneutralized, form, i.e., about 75% to about 100% of the basic moieties, e.g., amino groups, are present in a neutral, uncharged form.

The multicomponent SAPs can be prepared by various methods, and the exact method of preparing a multicomponent SAP is not limited. Any method that provides a particle having at least one microdomain of an acidic resin in contact with or in close proximity to at least one microdomain of a basic resin is suitable. Persons skilled in the art are aware of methods of preparation wherein the multicomponent SAP contains at least one microdomain of an acidic resin and at least one microdomain of a basic resin in contact or in close proximity with each other. In embodiments wherein an acidic resin and a basic resin are present as microdomains within a matrix of a matrix resin, particles of an acidic resin and a basic resin are admixed with a rubbery gel of a matrix resin, and the resulting mixture is extruded, then dried, to form multicomponent SAP particles having microdomains of an acidic resin and a basic resin dispersed in a continuous phase of a matrix resin. Alternatively, rubbery gels of an acidic resin, basic resin, and matrix resin can be coex- truded to provide a multicomponent SAP containing microdomains of an acidic resin, a basic resin, and a matrix resin dispersed throughout the particle. In this embodiment, the acidic resin, basic resin, and resulting multicomponent SAP, each can be optionally surface crosslinked and/or annealed.

In some embodiments, one or more of the acidic resin, the basic resin, and the multicomponent SAP particles optionally are surface crosslinked and/or annealed, as disclosed in U.S. Patent No. 6,235,965.

A strong acidic resin can be used with either a strong basic resin or a weak basic resin, or a mixture thereof. A weak acidic resin can be used with a strong basic resin or a weak basic resin, or a mixture thereof. Preferably, the acidic resin is a weak acidic resin and the basic resin is a weak basic resin. In most preferred embodi- ments, the acidic resin comprises unneutralized poly(acrylic acid), and the basic resin comprises unneutralized poly(vinylamine), polyethylenimine, or a mixture thereof. Examples illustrating the preparation of multicomponent SAP particles can be found in U.S. Patent No. 6,222,091.

In another embodiment, the multicomponent SAP particles can be admixed with particles of a surface crosslinked conventional SAP, and/or a mixture of an unneutralized acidic water-absorbing resin and an unneutralized basic water-absorbing resin to provide an SAP material having improved absorption properties. In this embodiment, all the SAP particles have a particle size distribution of about 300 to about 5000 μm, and a median particle size of greater than about 700 μm. The unneutralized acidic and basic resins are neutralized 0% to 25%. The surface crosslinked conventional SAP is neutralized 25% to 100%.

The SAP material of this embodiment comprises about 10% to about 90%, and preferably about 25% to about 85%, by weight, multicomponent SAP particles and about 10% to about 90%, and preferably, about 25% to about 85%, by weight, surface crosslinked conventional SAP particles and/or a mixture of unneutralized acidic resin particles and unneutralized basic resin particles. More preferably, the SAP material contains about 30% to about 75%, by weight, multicomponent SAP particles. To achieve the full advantage of the present invention, the SAP material contains about 35% to about 75%, by weight, the multicomponent SAP particles. The SAP particles of this embodiment can be of any shape, e.g., granular, fiber, powder, or platelets.

In yet another embodiment, a superabsorbent material comprises an admixture of particles of an unneutralized basic water-absorbing resin, like a PVAm, and particles of an unneutralized acidic water-absorbing resin, like PAA, wherein both the acidic and basic water-absorbing resins have a particle size distribution of about 300 to about 5000 μm and a mass median particle size of greater than about 700 μm. Both the acidic and basic water-absorbing resin are neutralized 0% to about 25%. The acidic and basic water-absorbing resins can be any of the previously discussed acidic and basic resins used in the preparation of a multicomponent SAP, and either or both are optionally surface crosslinked or annealed. The superabsorbent material of this embodiment also can contain surface crosslinked conventional SAP particles having the mass median particle size and particle size distribution disclosed above.

The SAP material of this embodiment comprises about 10% to about 90%, and preferably about 25% to about 85%, by weight, acidic water-absorbing resin particles and about 10% to about 90%, and preferably, about 25% to about 85%, by weight, basic water-absorbing resin particles. More preferably, the SAP material con- tains about 30% to about 75%, by weight, acidic resin particles. To achieve the full advantage of the present invention, the SAP material contains about 35% to about 75%, by weight, the acidic resin particles. If present, the surface crosslinked conventional SAP is present in an amount of one to 100 weight parts surface crosslinked conventional SAP per 100 weight parts of the total acidic and basic resins.

A preferred acidic water-absorbing resin is unneutralized poly(acrylic acid) (PAA), e.g., DN up to about 25. Preferred basic water-absorbing resins are unneutralized poly(vinylamine) (PVAm) or unneutralized polyethylenimine. An example of an SAP material of this embodiment comprising particles of an acidic and a basic wa- ter-absorbing resin is a mixture of unneutralized (DN=0) PAA particles and unneutralized PVAm particles. The PVAm/PAA weight ratio of this SAP material is 30/70. Blends containing at a mixture of an acidic resin and a basic resin, and surface crosslinked conventional SAP also can be used. Preferred surface crosslinked conventional SAPs are neutralized PAAs, i.e., PAA of DN 65 to 100.

Superabsorbent materials containing large size acidic resin and basic resin particles demonstrate unexpected water absorption and retention properties. Such SAP materials comprise a mixture of discrete particles of two uncharged, slightly crosslinked polymers. When contacted with water or an aqueous electrolyte-containing medium, the two uncharged resins neutralize each other to form a superabsorbent ma- terial. This also reduces the electrolyte content of the medium absorbed by polymer, further enhancing the polyelectrolyte effect. Neither polymer in its uncharged form behaves as an SAP by itself when contacted with a fluid. However, a superabsorbent material, which contains a simple mixture of two resins, one acidic and one basic, is capable of acting as an absorbent material because the two resins are converted to their polyelectrolyte form. Prior superabsorbent mixed bed systems have demonstrated good water absorption and retention properties. However, the present SAP materials, which contain large particle size resins, exhibit an improved fluid absorption capacity, improved permeability and acquisition rate, and avoid gel blocking.

In the test results set forth below, the SAP particles of the present invention were tested for absorption under load at 0.30 psi, 0.70 psi, and 0.90 psi. Absorption under load (AUL) is a measure of the ability of an SAP to absorb fluid under an applied pressure. The AUL was determined by the following method, as disclosed in U.S. Patent No. 5,149,335, incorporated herein by reference.

An SAP (0.160 g +/-0.001 g) is carefully scattered onto a 140-micron, water-permeable mesh attached to the base of a hollow Plexiglas cylinder with an internal diameter of 25 mm. The SAP sample is covered with a 100 g cover plate and the cylinder assembly weighed. This gives an applied pressure of 20 g/cm² (0.30 psi). Alternatively, the sample can be covered with a 250 g or a 300 g cover plate to give an applied pressure of 43 g/cm² (0.70 psi or 66 g/cm² (0.90 psi)). The screened base of the cylinder is placed in a 100 mm petri dish containing 25 milliliters of a test solution (usually 0.9% saline), and the SAP sample is allowed to absorb for 1 hour. By re- weighing the cylinder assembly, the AUL (at a given pressure) is calculated by dividing the weight of liquid absorbed by the dry weight of the SAP sample before liquid contact.

In addition to an ability to absorb and retain relatively large amounts of a liquid, it also is important for an SAP to exhibit good permeability, and, therefore, rap- idly absorb the liquid. Therefore, in addition to absorbent capacity, or gel volume, useful SAP particles also have a high gel strength, i.e., the particles do not deform after absorbing a liquid. In addition, the permeability or flow conductivity of a hydrogel formed when SAP particles swell, or have already swelled, in the presence of a liquid is extremely important property for practical use of the SAP particles. Differences in per- meability or flow conductivity of the absorbent polymer can directly impact the ability of an absorbent article to acquire and distribute body fluids.

Many types of SAP particles exhibit gel blocking. "Gel blocking" occurs when the SAP particles are wetted and swell, which inhibits fluid transmission to the interior of the SAP particles and between absorbent SAP particles. Gel blocking can be a particularly acute problem if the SAP particles lack adequate gel strength, and deform or spread under stress after the SAP particles swell with absorbed fluid.

Accordingly, an SAP particle can have a satisfactory AUL value, but will have inadequate permeability or flow conductivity to be useful at high concentrations in absorbent structures. In order to have a high AUL value, it is only necessary that the hydrogel formed from the SAP particles has a minimal permeability such that, under a confining pressure of 0.28 psi, gel blocking does not occur to any significant degree. The degree of permeability needed to simply avoid gel blocking is much less than the permeability needed to provide good fluid transport properties. Accordingly, SAPs that avoid gel blocking and have a satisfactory AUL value can still be greatly deficient in these other fluid handling properties.

An important characteristic of the large-size SAP particles of the present invention is permeability when swollen with a liquid to form a hydrogel zone or layer, as defined by the Saline Flow Conductivity (SFC) value of the SAP particles. SFC measures the ability of an SAP to transport saline fluids, such as the ability of the hydrogel layer formed from the swollen SAP to transport body fluids. A material having relatively high SFC value is an air-laid web of wood pulp fibers. Typically, an air-laid web of pulp fibers (e.g., having a density of 0.15 g/cc) exhibits an SFC value of about 200 x 10^"7 cm³sec/g. In contrast, typical hydrogel-forming SAPs exhibit SFC values of 1 x 10^"7 cm³sec/g or less. When an SAP is present at high concentrations in an absorbent structure, and then swells to form a hydrogel under usage pressures, the boundaries of the hydrogel come into contact, and interstitial voids in this high SAP concentration region become generally bounded by hydrogel. When this occurs, the permeability or saline flow conductivity properties in this region is generally indicative of the permeability or saline flow conductivity properties of a hydrogel zone formed from the SAP alone. Increasing the permeability of these swollen high concentration regions to levels that approach or even exceed conventional acquisition/distribution materials, such as wood pulp fluff, can provide superior fluid handling properties for the absorbent structure, thus decreasing incidents of leakage, especially at high fluid loadings.

Accordingly, it would be highly desirable to provide SAP particles having an SFC value that approaches or exceeds the SFC value of an air-laid web of wood pulp fibers. This is particularly true if high, localized concentrations of SAP particles are to be effectively used in an absorbent article. High SFC values also indicate an ability of the resultant hydrogel to absorb and retain body fluids under normal usage conditions. A method for determining the SFC value of SAP particles is set forth in Goldman et al. U.S. Patent No. 5,599,335, incorporated herein by reference. The present SAP particles also were subjected to other tests, and the results compared to standard size SAP particles, i.e., SAP particles having a particle size distribution of about 104 to about 850 μm and a mass median particle size of about 100 to about 300 μm. All tested SAP particles were PAA (DN=70).

The CRC (centrifuge retention capacity) test is designed to measure the amount of saline solution retained inside an absorbent material subjected to a specific centrifuge force. The measurement of CRC is disclosed in U.S. Patent No. 6,187,828 and U.S. Patent No. 5,633,316, each incorporated herein by reference.

The rewet/acquisition time tests were performed on diaper cores made as follows and using the following test procedure:

The cores were prepared using a conventional laboratory procedure wherein a laboratory core-forming unit comprising a two-chamber vacuum system forms an airlaid fluff pulp-absorbent composite matrix to produce a 12 cm x 21 cm diaper core. The core-forming unit comprises a roller brush on a variable-speed laboratory motor, a fiber distribution screen in close proximity to the brush, a forming screen on an adjustable damper, and a vacuum system capable of supplying a consistent and continuous negative pressure between 8 and 15 inches of water.

The core-forming unit is contained such that the vacuum pulls the fibers and granular material from an adjustable introduction slide, through the rotating brush and distribution screen, directly onto the forming screen. The vacuum exhaust is recir- culated through the inlet of the formation slide, thereby controlling the temperature and humidity of the operation.

When forming a core, the desired amount of defiberized fluff pulp is evenly disbursed in small pieces onto the brush roller in the upper chamber. In the lower chamber, a rectangular tissue, or topsheet (21 cm x 12 cm), is placed onto the forming screen. For most cores, the sliding upper chamber lid is partially closed to leave about a one-half inch gap. In the case of a homogeneous pulp/SAP core, the SAP is sprinkled through the gap into the upper chamber immediately after the brush begins rotating. In order to achieve a homogeneous distribution, a small amount of SAP is added to the fluff prior to beginning the motor. The amount of time used to introduce the remainder of the SAP varies with the amount of fluff pulp utilized. After the fiber and absorbent polymer material are deposited, the motor is turned off, and the damper unit containing the laboratory core is removed from the lower chamber. The uncompressed core then is placed on a backsheet made from a polymeric film, and put into a compression unit. At this time, another rectangular tissue and a nonwoven cov- erstock is placed on top of the core. Absorbent cores are compressed for a given amount of time, typically 5 minutes, with a hydraulic press at pressures of between about 5,000 pounds and about 10,000 pounds, and typically about 7,000 pounds, to achieve the desired density. After 5 minutes, the laboratory-prepared absorbent cores are removed from the press, weighed, and measured for thickness.

In particular, the fluff containing diaper cores were prepared as follows. The fluff and large particle size SAP particles are admixed and introduced into in the pad core forming machine in the relative amounts desired. Top and bottom tissues are placed on opposing surfaces of the core, then the core is compressed at 10,000 pounds pressure (260 psi) for five minutes.

The cores were tested for rewet under a 0.7 psi load and liquid acquisition time. The following describes the procedures to determine the acquisition time and rewet under load of a hygienic article, like a diaper. These tests exhibit the rate of absorption and fluid retention of a 0.9%, by weight, saline solution, by a hygienic article over 3 to 5 separate fluid insults while under a load of 0.7 psi.

Apparatus:

100 ml separatory funnel, configured to deliver a flow rate of 7 ml/sec, or equivalent;

3.642 kg circular weight (0.7 psi) 10 cm diameter, with 2.38 cm ID perspex dose tube through the center of weight;

VWR Scientific, 9 cm filter paper or equivalent;

2.5 kg circular weight (0.7 psi)-8 cm diameter; Digital timer;

Electronic balance (accuracy of a 0.01 gram);

Stopwatch.

Procedure:

1. Preparation

(a) Record the weight (g) of the hygienic article, e.g., diaper, to be tested;

(b) Place hygienic article flat on the bench top, for example, by removing any elastics and/or taping the ends of the article to the bench top;

(c) Place the 3.64 kg circular weight onto the hygienic article with the opening of the perspex dose tube positioned at the insult point (i.e., 5 cm to- ward the front from the center). 2. Primary Insult and Rewet Test

(a) Measure 100 ml of 0.9% NaCl solution (i.e., 0.9% by weight sodium chloride in deionized or distilled water) into separatory funnel. Dispense the NaCl solution into the perspex tube of the weight at a flow rate of 7 ml/sec and start the timer immediately. Stop the timer when all of the

NaCl solution has completely disappeared from the surface of the hygienic article at the base of the perspex tube. Record this time as the primary acquisition time (sec).

(b) After 10 minutes have elapsed, remove the weight and conduct the re- wet test procedure:

(i) Weigh a stack of 10 filter papers, record this value (dry weight).

(ii) Place the filter papers over the insult point on the hygienic article. Set the timer for 2 minutes. Place the 2.5 kg weight onto the filter papers and start timer immediately.

(iii) After 2 minutes have elapsed, remove the weight and re- weigh the filter papers (wet weight). Subtract the dry weight of the filter papers from the wet weight, this is the rewet value. Record this value as the primary rewet value (g).

3. Secondary Insult and Rewet Test

(a) Place the 3.64 kg weight back onto the hygienic article in the same position as before. Repeat step 2a using 50 ml NaCl solution (recoding the absorption time as the secondary acquisition time) and steps 2b (i)-(iii) using 20 filter paper (recording the rewet values as the secondary re- wet).

4. Tertiary, and additional, Insult and Rewet Tests

(a) Place the load back onto the diaper in the same position as before.

Repeat step 2a using 50 ml of NaCl solution (recording the absorption time as the tertiary acquisition time) and steps 2b (i)-(iii) using 30 filter paper (recording the rewet value as the tertiary or subsequent rewet).

Example 1

An aqueous monomer mixture containing 27 wt% acrylic acid, 0.15 wt% methylenebisacrylamide based on acrylic acid (boaa), 0.28 wt% sodium persulfate boaa, 0.075 wt% 2-hydroxy-2-methyl-1 -phenyl-1 -propanone (DAROCURE® 1173, available from Ciba Additives) boaa, and 0.025 wt% IRGACURE® 651 was prepared, then cooled to 15°C. The resulting monomer mixture then was polymerized for 12.5 minutes under a UV light (UV intensity=20 mW/cm²). The resulting PAA hydrogel was extruded through a KitchenAid Model K5SS mixer fitted with a meat grinder attachment. Next, sodium carbonate was added to the PAA hydrogel to neutralize the acrylic acid groups to 75 mol%, followed by two additional extrusions. The PAA hydrogel was dried at 150°C for one hour, then milled and sized to 400-1500 μm. The dry SAP particles then were surface crosslinked by spraying a solution containing 0.1 wt% ethylene glycol diglycidyl ether, 3.35 wt% water, and 1.65 wt% propylene glycol, all based on the weight of SAP particles, onto the SAP particles, followed by heating at 150°C for one hour. The surface crosslinked conventional SAP particles were sifted again from 300- 1500 μm to remove fines and agglomerates, and the mass median particle size of the sifted SAP particles was 810 μm. The absorbing properties of the resulting SAP parti- cles are summarized in Table 1 , and results of the rewet/acquisition time under pressure test are summarized in Table 2.

Comparative Example 1

The procedure of Example 1 was duplicated, except the SAP hydrogel was milled and sized to 106-850 μm. After surface crosslinking as in Example 1 , the surface crosslinked conventional SAP particles were sifted again from 150-850 μm to remove fines and agglomerates, and the mass median particle size of the sifted SAP particles was 460 μm. The absorbing properties of the resulting SAP particles are summarized in Table 1 , and results of the rewet/acquisition time under pressure test are summarized in Table 2.

Comparative Example 2

The procedure of Example 1 was duplicated, except the SAP hydrogel was milled and sized to 100-300 μm. After surface crosslinking as in Example 1 , the surface crosslinked SAP particles were sifted again from 100-300 μm to remove fines and agglomerates and the mass median particle size of the sifted SAP particles was 180 μm. The absorbing properties of the resulting SAP particles are summarized in Table 1 , and results of the rewet/acquisition time under pressure test are summarized in Table 2. Comparative Example 3

The procedure of Example 1 was duplicated, except the conventional SAP particles were not surface crosslinked. The mass median particle size of the sifted conventional SAP particles (not surface crosslinked) was 830 μm. The absorbing properties of the resulting SAP particles are summarized in Table 1 , and results of the rewet/acquisition time under pressure test are summarized in Table 2.

PSD-particle size distribution

AH acquisition times reported in seconds.

Example 2

A 30 L capacity polyethylene vessel, well insulated by foamed polymer material, was charged with 14,340 g demineralized water and 36 g pentaerythritol trial- lyl ether as an internal crosslinking monomer. Sodium bicarbonate (5172 g) was suspended in the initial charge, and 5990 g of acrylic acid was gradually metered in at a rate such that foaming of the reaction solution was controlled. The reaction solution then was cooled to a temperature of about 3°-5°C. At a temperature of 4°C, the initia- tors, 2,2'-azobisamidinopropane dihydrochloride (6.0 g) dissolved in 60 g demineralized water, peroxodisulfate (12.0 g) dissolved in 450 g demineralized water, and ascorbic acid (1.2 g) dissolved in 50 g demineralized water, were added in succession, and thoroughly stirred into, the reaction solution. The reaction solution then was allowed to stand without stirring. The polymerization proceeded adiabatically, with the temperature rising to about 85°C and a hydrogel formed. The hydrogel was transferred into a kneader, admixed with ethylene glycol diglycidyl ether (15 g), comminuted, dried in an airstream in 170°C, ground and screened to a particle size distribution ranging from 500 to 1600 μm. The resulting SAP particles (1 kg) were sprayed in a plowshare mixer with a solution of 2 g polyglyceryl polyglycidyl ether (DENACOL® EX-512 from Nagase Chemicals Ltd.), 0.3 g citric acid, 60 g demineralized water, and 40 g 1 ,2-propanediol, then heated at 150°C for 40 minutes. The SAP particle size fraction 500-1600 μm was screened out, and the mass median particle size was 890 μm. The absorbing properties of the resulting surface crosslinked conventional SAP particles are summarized in Table 3, and results of the rewet/acquisition time under pressure test are summarized in Table 4.

Comparative Example 4

Example 2 was duplicated, except the SAP particles were screened to a particle size distribution ranging from 200 to 1000 μm. The SAP particles (1 kg) were sprayed in a plowshare mixer with a solution of 2 g polyglyceryl polyglycidyl ether (DENACOL® EX-512), 0.3 g citric acid, 60 g demineralized water, and 40 g 1 ,2- propanediol, then heated at 150°C for 40 minutes. The SAP particle size fraction 200- 1000 μm was screened out, and the mass median particle size was 610 μm. The absorbing properties of the resulting SAP particles are summarized in Table 3, and re- suits of the rewet/acquisition time under pressure test are summarized in Table 4.

Comparative Example 5

Example 2 was duplicated, except the SAP particles were screened to a particle size distribution ranging from 200 to 500 μm. The SAP particles (1 kg) were sprayed in a plowshare mixer with a solution of 2 g polyglyceryl polyglycidyl ether (DE- NACOL® EX-512 from Nagase Chemicals Ltd.), 0.3 g citric acid, 60 g demineralized water, and 40 g 1 ,2-propanediol, then heated at 150°C for 40 minutes. The SAP particle size fraction 200-500 μm was screened out, and the mass median particle size was 360 μm. The absorbing properties of the resulting SAP particles are summarized in Table 3, and results of the rewet/acquisition time under pressure test are summarized in Table 4. Comparative Example 6

Example 2 was duplicated, except the SAP particles were not surface crosslinked. The conventional SAP particles (not surface crosslinked) were ground and screened to a particle size distribution ranging from 500 to 1600 μm. The mass median particle size of the SAP particles was 900 μm. The absorbing properties of the resulting SAP particles are summarized in Table 3, and results of the rewet/acquisition time under pressure test are summarized in Table 4.

Example 3

Preparation of Poly(acrylic acid) (DN=0)

A monomer mixture containing acrylic acid (270 grams), deionized water (810 grams), methylenebisacrylamide (0.4 grams), sodium persulfate (0.547 grams), and 2-hydroxy-2-methyl-1-phenyl-propan-1-one (0.157 grams) was prepared, then sparged with nitrogen for 15 minutes. The monomer mixture was placed into a shallow glass dish, then the monomer mixture was polymerized at an initiation temperature of 10°C under 20 mW/cm² of UV light for about 12 to about 15 minutes. The resulting PAA was a rubbery gel. The rubbery PAA gel was cut into small pieces, then extruded two times through a KitchenAid Model K5SS mixer with meat grinder attachment.

Preparation of a crosslinked Poly(vinylamine) Resin

To 100 g of an 8% by weight aqueous poly(vinylamine) solution was added about 2 mol% (0.66 g) of ethylene glycol diglycidyl ether. The resulting mixture was stirred for about 5 minutes to dissolve the ethylene glycol diglycidyl ether, then the homogeneous mixture was placed in an oven, heated to about 60°C, and held for two hours. The resulting PVam hydrogel was cut into small pieces, then extruded two times through a KitchenAid Model K5SS mixer with meat grinder attachment.

Preparation of a Multicomponent Superabsorbent Polymer Particles

One part of the chopped PAA hydrogel was mixed with 3.1 parts of the chopped PVam hydrogel and coextruded three times through a KitchenAid Model K5SS mixer with meat grinder attachment. The multicomponent superabsorbent polymer hydrogel was dried in a laboratory drying oven at 90°C, ground, and screened to a particle size distribution ranging from 400 to 1300 μm. The mass median particle size was 800 μm. The absorbing properties of the resulting multicomponent superabsorbent polymer particles are summarized in Table 5, and results of the rewet/acquisition time under pressure test are summarized in Table 6.

Comparative Example 7

Poly(acrylic acid) (DN=0) and crosslinked poly(vinylamine) hydrogels were prepared as in Example 3.

A multicomponent superabsorbent polymer was prepared as in Example

3, except the multicomponent superabsorbent polymer hydrogel was dried in a laboratory drying oven at 90°C, ground, and screened to a particle size distribution ranging from 200 to 850 μm. The mass median particle size was 490 μm. The absorbing properties of the resulting multicomponent superabsorbent polymer particles are sum- marized in Table 5, and results of the rewet/acquisition time under pressure test are summarized in Table 6.

* Measured in Jayco synthetic urine

The above data demonstrates that the large particle size SAPs of the present invention have an excellent ability to absorb and retain large amounts of a fluid. Large particle size SAPs exhibit better acquisition times and only slightly worse rewet because are better able to they open the core structure.

The large particle size SAP particles of the present invention are useful in hygienic products, such as diapers, adult incontinence articles, feminine napkins, general purpose wipes and cloths, and in aqueous waste solidification. In accordance with an important feature of the present invention, a hygienic product, or other absorbent article, has a core containing about 15% to 100%, better about 50% to 100%, preferably about 60% to 100%, more preferably about 75% to 100%, and most preferably about 75% to 95% of SAP particles having a large particle size, as defined above.

An absorbent core of the present invention can range from heavily loaded cores (e.g., 60-95 wt % superabsorbent polymer particles/5-40 wt % fluff) to f luffless cores (i.e., 100% SAP particles). The fluff less cores typically are constructed of alternate layers of (a) tissue and (b) SAP particles, having a PSD of about 300 to about 5000 μm, and a mass median particle size of greater than 700 μm. The SAP particles often are present as a pressed sheet containing the SAP particles, and optionally fluff and/or nonwoven fibers. Additionally, a top, or acquisition, layer of standard particle size superabsorbent polymer (i.e., particle size range of about 104 to about 850 μm) optionally can be used to provide faster acquisition rates.

Present-day diapers generally consist of a topsheet made from a non- woven material that is in contact with the skin of the wearer, an acquisition layer below (i.e., opposite the skin of wearer) the topsheet, a core that is below the acquisition layer, and a backsheet below the core. This construction is well known in the industry. In a preferred embodiment, the present diaper consists essentially of a topsheet, a core, and a backsheet, i.e., an acquisition layer is not present. As illustrated below, the improvements provided by the present large particle size SAP particles permit an acquisition layer to be omitted from a disposable diaper. Such a result is important in the art because an expensive acquisition layer can be omitted, the diaper is lighter and thinner, and absorptive properties are not adversely affected.

A single absorbent layer or sheet containing large particle size SAP particles can be used as the absorbent component of an absorbent core. Preferably, a plurality of absorbent layers or sheets are used in the absorbent core, more preferably together with a wicking layer (e.g., a tissue layer) between absorbent layers or sheets to provide improved wicking of a fluid between and through the absorbent sheets. In more preferred embodiments, at least one of the absorbent layers or sheets in an absorbent core contains nonwoven fibers to improve wet strength of the absorbent core and assist in wicking.

A preferred core contains two to five absorbent layers or sheets. By utilizing a laminate of thin absorbent layers or sheets, as opposed to a single, thicker absorbent layer or sheet, horizontal expansion of the core is decreased, and vertical expansion is promoted. This feature provides a good fluid transport through the core, provides a better fitting diaper after an initial insult, and avoids leaking when the diaper is subsequently rewet by a second and additional insult. In more preferred embodi- ments, the core contains a laminate of two or more absorbent layers or sheets wherein a wicking layer is positioned between each absorbent sheet layer or sheet, and on top and at the bottom of the laminate.

An absorbent layer or sheet containing SAP particles of the present invention, or a laminate comprising such layers or sheets, is present in an absorbent core to provide a desired basis weight (i.e., weight of SAP in the core) of about 50 to about 800 gsm (grams/square meter), and preferably about 150 to about 600 gsm. To achieve the full advantage of the present invention, the basis weight is about 300 to about 550 gsm. The desired basis weight of the core is related to the end use of the core. For example, diapers for newborns have a low basis weight, as opposed to a medium basis weight for toddlers, and a high basis weight for overnight diapers.

A fluffless core of the present invention is illustrated in FIG. 1. FIG. 1 shows a cross section of an absorbent article 30 having a topsheet 32, a backsheet 36, and an absorbent core indicated by 40 positioned between topsheet 32 and backsheet 36. As shown in FIG. 1 , core 40 comprises a plurality of layers 42. Layers 42 comprise large particle size SAP particles, and are separated from one another by tissue layers 44. The fluffless core in FIG. 1 can include additional layer and tissue layer

(not shown) disposed between topsheet 32 and layer 42. This optional additional layer serves as an acquisition/distribution layer and contains a conventional SAP, e.g., PAA (DN=70) having a particle size range of about 104 μm to 850 μm. A fluffless core illustrated in FIG. 1 can contain one to five, and preferably two to four layers 42, i.e., one to five layers of large particle size SAP particles.

An example of a topsheet 32 is staple length polypropylene fibers having a denier of about 1.5, such as Hercules-type 151 polypropylene marketed by Hercules, Inc., Wilmington, DE. As used herein, the term "staple length fibers" refers to fibers having a length of at least about 15.9 mm (0.62 inches). The backsheet 36 is impervious to liquids, and typically is manufactured from a thin plastic film, although other flexible liquid impervious materials also can be used. The backsheet prevents exudates absorbed and contained in the absorbent core 40 from wetting articles, such as bed sheets and undergarments, that contact the diaper 30.

In a preferred embodiment, a present diaper core consists essentially of a topsheet, a core, and a backsheet, i.e., an acquisition layer is not present. Improvements provided by present absorbent sheet materials permit an acquisition layer to be omitted from a disposable diaper. Such a result is both new and unexpected in the art in that an expensive acquisition layer can be omitted, the diaper is lighter and thinner, and absorptive properties are not adversely affected.

Cores referred to as "fluffless" cores contain 100% of an SAP and are free of cellulosic fibers, e.g., are free of "fluff" materials. Typically, high loading commercial diapers contain 45% to 60% by weight of a cellulosic fibers to achieve rapid absorption of a liquid.

For an absorbent article having a core containing a "fluff" component, the "fluff" comprises a fibrous material in the form of a web or matrix. Fibers include naturally occurring fibers (modified or unmodified). Examples of suitable unmodified/modified naturally occurring fibers include cotton, Esparto grass, bagasse, kemp, flax, silk, wool, wood pulp, chemically modified wood pulp, and jute. See WO 98/37149 and U.S. Patent No. 5,859,074, each incorporated herein by reference, for a complete discussion of "fluff" components for use in an absorbent sheet article.

The cores also can include an optional nonwoven fiber, for example, polypropylene, polyethylene, polyethylene terephthalate, viscose, and mixtures thereof. Also, an open fiber mesh of nonwoven fibers can be used, for example, cellulose ace- tate fiber. Nonwoven fibers can be made by drylaid thermobonded, carded air-through bonded, spunbond, or spun-meltblown-spun processes. Nonwoven fibers impart additional wet strength to an absorbent layer or sheet when used in an amount of about 10 to about 20 grams per square meter (gsm) of sheet material.

Suitable fibers, and fiber meshes, can be made from polyvinyl chloride, polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride, polyacrylics such as ORLON®, polyvinyl acetate, polyethylvinyl acetate, nonsoluble or soluble polyvinyl alcohol, polyolefins such as polyethylene (e.g., PULPEX®) and polypropylene, polyam- ides (e.g., nylon), polyesters (e.g., DACRON® or KODEL®), polyurethanes, polysty- renes, and the like.

Hydrophilic fibers are preferred, and include rayon, polyester fibers, such as polyethylene terephthalate (e.g., DACRON®), hydrophilic nylon (e.g. HYDRO- FIL®), and the like. Suitable hydrophilic fibers can also be obtained by hydrophilizing hydrophobic fibers, such as surfactant-treated or silica-treated thermoplastic fibers derived from, for example, polyolefins, such as polyethylene or polypropylene, polyacrylics, polyamides, polystyrenes, polyurethanes, and the like.

Overall, the data presented herein demonstrates that a diaper core con- taining large particle size SAP particles of the present invention demonstrate accept- able to excellent, rewet values and acquisition times. The practical result of these properties is a core having an improved ability to prevent leakage in gush situations and in rewet situations, even in the absence of an acquisition layer.

The improved results demonstrated by a core of the present invention permit the thickness of the core to be reduced. Typically, cores contain 50% or more fluff or pulp to achieve rapid liquid absorption while avoiding problems like gel blocking. The present cores, which contain large particle size SAP particles acquire liquids sufficiently fast to avoid problems, like gel blocking, and, therefore, the amount of fluff or pulp in the core can be reduced, or eliminated. A reduction in the amount of the low- density fluff results in a thinner core, and, accordingly, a thinner diaper. Therefore, a core of the present invention can contain at least 50% particles, preferably at least 75%, and up to 100% of SAP particles. In various embodiments, the presence of a fluff is no longer necessary, or desired.

In addition to a thinner diaper, the present cores also allow an acquisition layer to be omitted from the diaper. The acquisition layer in a diaper typically is a nonwoven or fibrous material, usually having a high degree of void space of "loft," that assists in the initial absorption of a liquid. The present cores acquire liquid at a suffi- cient rate such that diapers free of an acquisition layers are practicable.

Many modifications and variations of the invention as hereinbefore set forth can be made without departing from the spirit and scope thereof and, therefore, only such limitations should be imposed as are indicated by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A particulate superabsorbent polymer having particle size of about 300 to about 5000 μm and a mass median particle size of greater than about 700 μm.

2. The particulate superabsorbent polymer of claim 1 having a particle size distribution of about 300 to about 2000 μm.

3. The particulate superabsorbent polymer of claim 1 having a particle size distribu- tion of about 400 to about 1500 μm.

4. The particulate superabsorbent polymer of one of the claims 1 to 3 having a mass median particle size of greater than about 750 μm.

5. The particulate superabsorbent polymer of one of the claims 1 to 3 having a mass median particle size of greater than about 800 μm.

6. The particulate superabsorbent polymer of one of the claims 1 to 5 selected from the group consisting of

(a) surface crosslinked conventional superabsorbent polymer particles;

(b) multicomponent superabsorbent polymer particles having at least one microdomain of an unneutralized acidic resin in contact with, or in close proximity to, at least one microdomain of an unneutralized basic resin; (c) a mixture of (i) particles of unneutralized acidic water-absorbing resin and

(ii) particles of unneutralized basic water-absorbing resin; and (d) a mixture comprising at least two of (a), (b), and (c).

7. The particulate superabsorbent polymer of one of the claims 1 to 6 comprising surface crosslinked poly(acrylic acid) neutralized about 25% to about 100%.

8. The particular superabsorbent polymer of one of the claims 1 to 6 comprising multicomponent superabsorbent particles comprising (a) unneutralized poly(acrylic acid) and (b) poly(vinylamine), polyethylenimine, or a mixture thereof.

9. The particulate superabsorbent polymer of one of the claims 1 to 6 comprising discrete particles of unneutralized poly(acrylic acid) and discrete particles of poly(vinylamine), polyethylenimine, or a mixture thereof.

10. The particulate superabsorbent polymer of one of the claims 1 to 6 comprising one or more of polyacrylic acid, a hydrolyzed starch-acrylonitrile graft copolymer, a starch-acrylic acid graft copolymer, a saponified vinyl acetate-acrylic ester copolymer, a hydrolyzed acrylonitrile polymer, a hydrolyzed acrylamide copolymer, an ethylene-maleic anhydride copolymer, an isobutylene-maleic anhydride copolymer, a poly(vinylphosphonic acid), a poly(vinylsulfonic acid), a po- ly(vinylphosphoric acid), a polyfvinylsulfuric acid), a sulfonated polystyrene, a poly(aspartic acid), a poly(lactic acid), a poly(vinylamine), a poly(dialkylaminoalkyl(meth)acrylamide), a polymer prepared from the ester analog of an N-(dialkylamino(meth)acrylamide), a polyethylenimine, a poly(vinylguanidine), a poly(allylguanidine), a poly(allylamine), a poly(dimethyl- dialkylammonium hydroxide), a guanidine-modified polystyrene, a quaternized polystyrene, a quaternized poly(meth)acrylamide or ester analog thereof, po- ly(vinylalcohol-co-vinylamine), or salts thereof.

11. A method of absorbing an aqueous medium comprising contacting the medium with the particulate superabsorbent polymer of one of the claims 1 to 10.

12. The method of claim 11 wherein the aqueous medium contains electrolytes.

13. The method of claim 12 wherein the electrolyte-containing aqueous medium is selected from the group consisting of urine, saline, menses, and blood.

14. An absorbent article comprising the particulate superabsorbent polymer of one of the claims 1 to 10.

15. The article of claim 14 wherein the article is a diaper or a catamenial device.

16. A diaper having a core, said core comprising at least 15% by weight of the particulate superabsorbent polymer of one of the claims 1 to 10.

17. The diaper of claim 16 wherein the core comprises at least 50% by weight of the composition of the particulate superabsorbent polymer.

18. The diaper of claim 17 wherein the core comprises at least 75% by weight of the composition of the particulate superabsorbent polymer.

19. The diaper of claim 18 wherein the core comprises 75 - 95% by weight of the particulate superabsorbent polymer.

20. The diaper of one of the claims 15 to 19 further comprising a topsheet in contact with a first surface of the core, and a backsheet in contact with a second surface of the core, said second core surface opposite from said first core surface.

21. The diaper of claim 20 further comprising an acquisition layer disposed between the topsheet and the core.

22. The diaper of claim 20 wherein the diaper is free of an acquisition layer.