MATERIAL FOR ACQUISITION OF LIQUIDS AND DISPOSABLE ABSORBENT ARTICLE

COMPRISING THE MATERIAL

FIELD OF THE INVENTION The present invention relates to a material for acquisition of liquids, absorbent articles containing this material and methods for reducing the electrolyte concentration of aqueous mediums. The material comprises individualized, cellulosic fibers crosslinked with a polymeric acid and the material comprises at least one basic polymer.

BACKGROUND OF THE INVENTION

Disposable absorbent articles are broadly available and consumers are used to a high performance for the collection and retention of menses (in the case of sanitary napkins or panty liners) or for the collection and retention of urine and fecal material (in the case of e.g. disposable diapers). However, consumers do not only expect a superior absorbency behaviour, but place more and more emphasis on the wearing comfort of such articles, and namely on the dryness of those articles. Typically, such articles comprise multiple absorbent layers, at least one layer being primarily designed to store liquid (storage layer), and at least one other layer primarily designed to acquire and/or distribute liquid (acquisition layer). The storage layer typically comprises super- absorbent material, which is admixed with the traditionally used pulp fiber material. Such super- absorbent materials can absorb many times (e.g. 10, 20, or 30 times) their own weight and are therefore very helpful when designing an article of improved fluid handling properties. Many recent products employ higher and higher concentrations of super-absorbent materials, that are concentrations in excess of 50% of the total weight of the storage member. These products achieve a high absorbing capacity with a very thin storage member and are thereby typically overall thin products. While super-absorbent materials can store very large amounts of liquid, they are often not able to distribute the liquid from the point of impact to more remote areas of the absorbent article and to acquire the liquid as fast as it may be received by the article. For this reason acquisition layers are used, which provide for the interim acquisition of large amounts of liquid and which often also allow for a quick distribution of liquid. After initial acquisition by the acquisition layer, the liquid is subsequently absorbed by and finally stored in the storage layer.

Thereby the acquisition layer plays an important role in using the whole absorbent capacity provided by the storage layer.

WO 95/34710 describes individualized cellulosic fibers crosslinked with acrylic acid polymers. WO 97/00354 describes individualized cellulosic fibers crosslinked with monomeric polycarboxylic acids, e.g. citric acid. These crosslinked cellulosic fibers may be used in the acquisition layer for a rapid initial acquisition of fluids.

Besides initial acquisiton and distribution of liquids, another important factor for the performance of disposable absorbent articles is the absorbance capacity of the super-absorbent material in the storage layer. Super-absorbent materials are typically super-absorbent polymers (SAPs) which are lightly crosslinked hydrophilic polymers that can absorb up to about one hundred times their own weight, or more, of distilled water. An important requirement for an SAP in a hygienic article, such as a diaper, is the ability to retain the absorbed fluid under a confining pressure. The dramatic swelling and absorbent properties of SAPs are attributed to (a) electrostatic repulsion between the charges along the polymer chains, and (b) osmotic pressure of the counter ions. It is known, however, that these absorption properties are drastically reduced in solutions containing electrolytes, such as saline, urine or blood. The SAPs function much less effectively in the presence of such physiologic fluids. This dramatic decrease in absorption capacity is termed "salt poisoning". The most commonly used SAP for absorbing electrolyte-containing liquids such as urine, is partially crosslinked, neutralized polyacrylic acid, typically containing for example 50% to 75% or 70% to 100% neutralized carboxyl groups.

There have been attempts to couteract the salt poisoning effect by removing salts. US 2003/0144379 describes multicomponent superabsorbent gel particles comprising microdomains of acidic water-absorbent resins and microdomains of basic water-absorbent resin. The particles are described to have an improved ability to absorb and retain electrolyte-containing liquids. The particles are used as liquid storage material in absorbent articles without acquisition and distribution layers. WO 99/33843 and WO98/37149 describe mixed-bed ion-exchange polymer compositions useful in the absorption of body fluids such as urine, menses and the like. The compositions comprise a cation-exchange absorbent polymer that contains acid groups in their unneutralized form

and that contains an anion-exchange absorbent polymer having a multiplicity of unneutralized amine groups. WO 92/20735 describes salt tolerant super-absorbents. Example 3 describes a combination of a gel containing poly(acrylic acid) and a gel containing poly(methacrylamido propyltrimethylammonium hydroxide) having a higher swelling power in the presence of salt solutions than the singel gels. EP 0 210 756 Al describes absorbent structures comprising hydrogel, fibrous anion exchange material, fibrous cation exchange material and conventional absorbent material. JP 57-35938 A2 describes water absorbing materials by using a combination of a powdery or granular water-absorptive resin and an ion exchange resin. JP 11-89878 A2 describes absorption products such as diapers that are constituted by laminating a liquid permeable top sheet, a second sheet and a liquid impermeable back sheet and interposing an absorber between the second sheet and the back sheet. The second sheet is formed from a web which consists of natural fibers or synthetic fibers with ion exchange materials incorporated into the web. JP 01-164436 A2 describes a water absorbing material for electrolytic solutions comprising a builder with ion blocking capacity or materials with ion exchange capacity impregnated in or adhered on a base material.

Besides salt poisoning, the absorption capacity of SAPs can also be reduced by reducing the degree of neutralization, e.g by acidifying liquids. US 4,657,537 describes a disposable absorbent articles having an ion-exchanging topsheet. This topsheet exchanges only kations against protons. It does not remove anions and it acidifies the liquid by lowering the pH. Therefore, another object of the invention is to provide an improved acquisition material based on acid crosslinked cellulosic fibers which reduces the electrolyte concentration of a liquid without acidifying the liquid.

It is therefore one object of the invention to provide material for the acquisition of electrolyte containing liquids with good acquisition, distribution and/or absorption properties.

SUMMARY OF THE INVENTION

The present invention is directed to a material for acquisition of liquids comprising individualized, crosslinked cellulosic fibers. These fibers are crosslinked by having an effective amount of at least one acidic crosslinking agent reacted with said fibers in intra-fiber crosslink ester bond form, wherein said acidic crosslinking agent is a polymer comprising a plurality of

acidic functional groups. The material further comprises at least one basic polymer. The basic polymer can for example be selected from the group consisting of polymers containing a plurality of amine groups and polymers containing a plurality of quaternary ammonium hydroxide groups.

The present invention is further directed to a disposable absorbent article comprising the above mentioned material for acquisition of liquids. The present invention is further directed to a method of reducing the electrolyte concentration of aqueous mediums containing electrolytes, comprising contacting an aqueous medium which contains electrolytes with the above-mentioned material for acquisition of liquids.

In one embodiment, the invention is directed to a material for acquisition of liquids, said material comprising individualized, crosslinked cellulosic fibers, said fibers having an effective amount of at least one acidic crosslinking agent reacted with said fibers in intra-fiber crosslink ester bond form, wherein said acidic crosslinking agent is a polymer comprising a plurality of acidic functional groups; and wherein said material further comprises at least one conductivity reducing substance.

A conductivity reducing substance is a substance which, when added in an effective amount to said material, effects that said material reduces the electrical conductivity of a 0.9 wt. % NaCl solution when said NaCl solution is contacted with said material according to the test method described below. The conductivity is preferably reduced by at least 0.3 mS/cm, or by at least 1 mS/cm, or by at least 2 mS/cm. A reduction in conductivity is believed to correlate with a reduction of NaCl concentration, or generally with a reduction of electrolyte concentration.

These and other features, aspects, and advantages of the present invention will become evident to those skilled in the art from a reading of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims pointing out and distinctly claiming the present invention, it is believed the same will be better understood by the following drawings

taken in conjunction with the accompanying specification wherein like components are given the same reference number.

Figure 1 is a top plan view of a disposable diaper, with the upper layers partially cut away.

Figure 2 is a cross-sectional view of the disposable diaper shown in Figure 1

DETAILED DESCRIPTION OF THE INVENTION

Each of the components, as well as preferred or optional components and the method of making, is described in detail hereinafter. All percentages, parts and ratios are based upon the total weight of the compositions of the present invention, unless otherwise specified. All such weights as they pertain to listed ingredients are based on the active level and therefore do not include solvents or by-products that may be included in commercially available materials, unless otherwise specified. All molecular weights as used herein are weight average molecular weights expressed as grams/mole, unless otherwise specified.

Definitions

As used herein, the following terms have the following meanings: The terms "material for acquisition of liquids" and "liquid acquisition material" are used herein interchangeably.

"Absorbent article" refers to devices that absorb and contain liquid, and more specifically, refers to devices that are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Absorbent articles include but are not limited to diapers, adult incontinent briefs, training pants, diaper holders and liners, sanitary napkins and the like. Absorbent articles also include wipes, such as household cleaning wipes, baby wipes, and the like

"Disposable" is used herein to describe articles that are generally not intended to be laundered or otherwise restored or reused i.e., they are intended to be discarded after a single use

and, possibly, to be recycled, composted or otherwise disposed of in an environmentally compatible manner.

"Disposed" is used to mean that an element(s) is formed (joined and positioned) in a particular place or position as a unitary structure with other elements or as a separate element joined to another element.

"Diaper" refers to an absorbent article generally worn by infants and incontinent persons about the lower torso.

The terms "thickness" and "caliper" are used herein interchangeably.

"Attached" or "Joined" encompasses configurations whereby an element is directly secured to another element by affixing the element directly to the other element, and configurations whereby an element is indirectly secured to another element by affixing the element to intermediate member(s) which in turn are affixed to the other element.

"Comprise," "comprising," and "comprises" is an open ended term that specifies the presence of what follows e.g. a component but does not preclude the presence of other features, elements, steps or components known in the art, or disclosed herein.

The term "hydrophilic" describes fibers or surfaces of fibers, which are wettable by aqueous fluids (e.g. aqueous body fluids) deposited on these fibers. Hydrophilicity and wettability are typically defined in terms of contact angle and the strike through time for the fluids, for example through a nonwoven fabric. This is discussed in detail in the American Chemical

Society publication entitled "Contact angle, wettability and adhesion", edited by Robert F. Gould (Copyright 1964). A fiber or surface of a fiber is said to be wetted by a fluid (i.e. hydrophilic) when either the contact angle between the fluid and the fiber, or its surface, is less than 90°, or when the fluid tends to spread spontaneously across the surface of the fiber, both conditions are normally co-existing. Conversely, a fiber or surface of the fiber is considered to be hydrophobic if

the contact angle is greater than 90° and the fluid does not spread spontaneously across the surface of the fiber.

The terms "fiber" and "filament" are used interchangeably. The terms "nonwoven", "nonwoven fabric" and "nonwoven web" are used interchangeable.

The term "electrolyte" means an ionic substance which increases the electrical conductivity of water when dissolved in water.

Cellulosic fibers

The term "individualized, crosslinked fibers", refers to fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between polymer (e.g. cellulose) molecules of a single fiber, rather than between polymer molecules of separate fibers.

The term "cellulosic fiber" is a collective term for fibers made from natural cellulose, from regenerated cellulose or from cellulose esters. Cellulosic fibers from natural cellulose can be e.g. seed fibers or bast fibers. Regenerated cellulose can be made e.g. by dissolving and re- precipitating cellulose. Cellulosic fibers of diverse natural origin are applicable to the invention. Digested fibers from softwood, hardwood or cotton linters are preferably utilized. Fibers from Esparto grass, bagasse, kemp, flax, and other ligneous and cellulosic fiber sources may also be utilized as raw material in the invention. The fibers may be supplied in slurry, unsheeted or sheeted form. Fibers supplied as wet lap, dry lap or other sheeted form are preferably rendered into unsheeted form by mechanically disintegrating the sheet, preferably prior to contacting the fibers with the crosslinking agent. Also, preferably the fibers are provided in a wet or moistened condition. Most preferably, the fibers are never-dried fibers. In the case of dry lap, it is advantageous to moisten the fibers prior to mechanical disintegration in order to minimize damage to the fibers. The optimum fiber source utilized in conjunction with this invention will depend upon the particular end use contemplated. Generally, pulp fibers made by chemical pulping processes are preferred. Completely bleached, partially bleached and unbleached fibers are applicable. It may frequently be desired to utilize bleached pulp for its superior brightness and consumer appeal. Wood fibers that have been at least partially bleached are preferred for use in

the process of the present invention. For products such as paper towels and absorbent pads for diapers, sanitary napkins, catamenials, and other similar absorbent paper products, it is especially preferred to utilize fibers from southern North America softwood pulp due to their premium absorbency characteristic.

Crosslinking agent

Suitable acidic crosslinking agents according to the invention are agents having at least three acidic groups per molecule, wherein the acidic groups can react with hydroxyl groups of cellulosic fibers to form ester bonds. At least two of the acidic groups of one crosslinking agent molecule can react with hydroxyl groups of at least two cellulosic fiber molecules. Preferably, the reaction is with two cellulose molecules of the same fiber to form intra- fiber ester bonds. The acidic crosslinking agent is a polymer having a plurality (i.e. three or more) of acidic functional groups. Acidic functional groups may be for example carboxylic acid, sulfonic acid, or phosphoric acid groups. In one embodiment, the acidic functional groups are carboxylic acid groups. The polyacrylic acid polymers and copolymers described above can be used as acidic crosslinking agents alone or in combination with other polycarboxylic acids such as citric acid.

The acidic cross-linking agent can be a homopolymer, obtainable from a single type of monomer, wherein the monomer has at least one acidic group. The acidic cross-linking agent can also be a copolymer obtainable from at least two different types of monomers, wherein at least one type of monomer has at least one acidic group and further types of monomers may have no acidic groups. The acidic cross-linking agent may be derived from natural or from synthetic sources. Suitable monomers containing acid groups are, for example, acrylic acid, methacrylic acid, crotonic acid, maleic acid, maleic acid monoesters. Preferred are acrylic acid polymers, i.e. polymers obtainable by polymerizing acrylic acid or by co-polymerizing acrylic acid with at least one other monomer, different from acrylic acid. Co-monomers that are not substituted with acid groups are, for example, acrylamide, methacrylamide, alkyl acrylamide, dialkyl acrylamide, alkyl methacrylamide, dialkyl methacrylamide, alkyl acrylate, alkyl methacrylate, vinylcaprolactone, vinylpyrrolidone, vinyl ester, vinyl alcohol, wherein the alkyl groups of these monomers are preferably Cl to ClO alkyl groups, with Cl, C2, C3 or C4 alkyl groups being especially preferred. The alkyl groups can be linear or branched.

Preferred acidic crosslinking agents include polyacrylic acid polymers, copolymers of acrylic acid, and mixtures thereof. Particularly preferred polyacrylic acid crosslinking agents include copolymers of polyacrylic acid and maleic acid and the low molecular weight monoalkyl substituted phosphinate and phosphonate copolymers described in US 5,256,746. These polymers are preferred for their ability to crosslink individualized cellulose fibers as described in this invention and their non-negative effect on cellulose brightness when used in a crosslinking process. Also, these polymeric acidic crosslinking agents are preferred for their positive influence on absorption capacity of the resulting crosslinked cellulosic fibers due to a lower glass transition temperature when compared to monomeric acidic crosslinking agents such as e.g. citric acid. Polyacrylic acid polymers are made by polymerizing acrylic acid CH ₂ =CH-COOH to form the repeating chain

-CH2 - CH(COOM)- wherein M is an alkali metal, ammonium or hydrogen. In the final, crosslinked cellulosic fibers, M is preferably hydrogen or at least predominantly hydrogen without metal ions or metal ions present only in such minor amounts that they do not significantly reduce the cation reducing capacity of the material according to the invention. Polymers of this type useful in the present invention are available for example from the Rohm and Haas Company. Other polymers that are applicable to this invention are copolymers of polyacrylic acid and maleic acid. Preferably, the molecular weights of these copolymers range from 500 - 40,000, more preferably from about 1, 000 to about 20,000. The weight ratio of acrylic acid to maleic acid can range from about 10: 1 to about 1:1, more preferably from about 5:1 to 1. 5:1. A particularly preferred copolymer contains about 65% by weight acrylic acid and 35% by weight maleic acid. Another group of acrylic acid copolymers that are applicable to this invention are the low molecular weight monoalkyl substituted phosphinate and phosphonate copolymers described in U.S. Patent 5,256,746. These copolymers are especially preferred, since they provide fibers with high levels of absorbency, resiliency and brightness, and are safe and non-irritating to human skin. These copolymers are prepared with hypophosphorus acid and its salts (commonly sodium hypophosphite) and/or phosphorus acid as chain transfer agents. Molecular weights of these types of copolymers are preferably below 20,000, and more preferably, below 3,000, and most preferably between about 1,000 and 2,000.

The molecular weight of the polymeric, acidic crosslinking agents suitable for use in the present invention is preferably from 500 to 40,000, more preferably from 1,000 to 20,000. In case of copolymers of acrylic acid, the weight ratio of acrylic acid to further monomers (e.g. maleic acid) can range froml0:l to 1: 1, more preferably from 5:1 to 1.5:1.

Crosslinking of cellulosic fibers

In one embodiment, the crosslinked cellulosic fibers are those described in WO 95/34710. The crosslinked cellulosic fibers and the methods of making them as described in WO 95/34710 are incorporated herein by reference.

The individualized, crosslinked cellulosic fibers have an effective amount of the polymeric acid crosslinking agent reacted with the fibers in the form of intra- fiber crosslink bonds. As used herein, "effective amount of crosslinking agent" refers to an amount of crosslinking agent sufficient to provide an improvement in at least one significant absorbency property of the fibers themselves and/or absorbent structures containing the individualized, crosslinked fibers, relative to conventional, uncrosslinked fibers. One example of a significant absorbency property is drip capacity, which is a combined measurement of an absorbent structure's fluid absorbent capacity and fluid absorbency rate as described in WO 95/34710. The crosslinked cellulosic fibers can for example have from 1 wt. % to 50 wt. %, or from 5 wt. % to 30 wt. %, or from 10 wt. % to 20 wt. % crosslinking agent, calculated on a dry fiber basis, reacted with the fibers. Preferably, the crosslinking agent is contacted with the fibers in a liquid medium, under such conditions that the crosslinking agent penetrates into the interior of the individual fiber structures. However, other methods of crosslinking agent treatment, including spraying or spray and press, dip and press, etc., of the fibers while in individualized, fluffed form, or sheeted form are also within the scope of the invention.

Once the fibers are treated with crosslinking agent (and catalyst if one is used), the crosslinking agent is caused to react with the fibers in the substantial absence of inter- fiber bonds, i.e., while inter- fiber contact is maintained at a low degree of occurrence relative to unfluffed pulp fibers, or the fibers are submerged in a solution that does not facilitate the formation of inter- fiber bonding. This results in the formation of crosslink bonds which are intra-fiber in

nature. Under these conditions, the crosslinking agent reacts predominantly to form crosslink bonds between hydroxyl groups of a single cellulose chain or between hydroxyl groups of proximately located cellulose chains of a single cellulosic fiber. Although not presented or intended to limit the scope of the invention, it is believed that the acid groups on the acidic polymeric crosslinking agent react with the hydroxyl groups of the cellulose to form ester bonds. The formation of ester bonds, believed to be the desirable bond type providing stable crosslink bonds, is favored under acidic reaction conditions. Therefore, acidic crosslinking conditions, i. e., pH ranges of from about 1.5 to about 5, are preferred for the purposes of this invention. The fibers are preferably mechanically defibrated into a low density, individualized, fibrous form known as "fluff" prior to reaction of the crosslinking agent with the fibers. Mechanical defibration may be performed by a variety of methods which are presently known in the art.

The crosslinked cellulosic fibers may have unique combinations of stiffness and resiliency, which allow absorbent structures made from the fibers to maintain high levels of absorptivity, and exhibit high levels of resiliency and an expansionary responsiveness to wetting of a dry, compressed absorbent structure. In addition to having the levels of crosslinking within the stated ranges, the crosslinked fibers may be characterized by having water retention values (WRVs) of up to 100, e.g. less than about 60, or between about 25 to about 50, or between about 30 and about 45. The WRV of a particular fiber is indicative of the level of crosslinking for a particular crosslinking chemistry and method. A procedure for measuring WRV is given in WO 95/34710.

Neutralization

Not all acidic groups of the acidic crosslinking polymer undergo ester bond reactions with cellulosic hydroxyl groups, i.e. non-reacted, free acid groups remain in the individualized, crosslinked cellulosic fibers. According to the invention, these remaining acid groups can at least partially become neutralized by the at least one basic polymer, at least when the fibers are wetted. The polymers comprise at least four, preferably at least eight monomeric units. The basic polymers have a plurality (e.g. three or more) of base groups. Preferred base groups are primary, secondary, tertiary amine groups or quaternary ammonium hydroxide groups. Examples of polymers suitable for use herein include those which are prepared from polymerizable monomers

which contain base groups, or groups which can be converted to base groups after polymerization. Thus, such monomers include those which contain primary, secondary and/or tertiary amine groups; or the corresponding phosphines, or quaternary ammonium groups. The amount of the basic substance can be such, that the acid groups of the crosslinked cellulosic fiber can for example become neutralized from 50 to 100%, or from 70 to 100% or from 90 to 100% or to 100%. The degree of neutralization can be controlled by the appropriate selection of the amount of basic polymer. The absolute amount of basic polymer can for example be from 0,5 to 65 wt.% or from 1 to 50 wt.% or from 2 to 40 wt.%, based on the total amount of the material. The basic polymer can be used as neutralizing agent alone or in combination with other, water soluble, organic or inorganic non-polymeric bases. Preferred base groups of the basic polymer are amine groups which can be primary, secondary or tertiary amine groups.

The basic polymer can be a homopolymer, obtainable from a single type of monomer, wherein the monomer has at least one base group. The basic polymer can also be a copolymer obtainable from at least two different types of monomers, wherein at least one type of monomer has at least one base group and further types of monomers may have no base groups. The basic polymer may be derived from natural (e.g. comprising nucleobases) or from synthetic sources. The basic polymers may also be random, graft, or block copolymers, and may have linear or branched architectures. Suitable monomers containing base groups include, but are not limited to, are, for example, vinylamine, allylamine, diallylamine, ethyleneimine (aziridine), 4- aminobutene, alkyl oxazolines, 5-aminopentene, carbodiimides, formaldazine, melamine, dialkylaminoalkyl acrylate, dialkylaminoalkyl methacrylate, dialkylaminoalkyl acrylamide, dialkylaminoalkyl methacrylamide, vinylguanidine, allylguanidine and the like, as well as their secondary or tertiary amine derivatives, e.g. N-monoalkyl- or N,N-di-lkyl compounds with preferably from 1 to 4 carbon atoms.

Basic polymers derived from natural sources include, for example, diethyl amino ethyl ("DEAE") cellulose, polyethyleneimine ("PEI") cellulose, amino ethyl cellulose, triethyl amino ethyl cellulose, guanidoethyl cellulose, paraaminobenzyl cellulose, ECTEOLA cellulose (triethanolamine coupled to cellulose through glyceryl and poly glyceryl chains), benzoylated DEAE cellulose, and benzoylated-naphthoylated DEAE cellulose prepared by conventional

techniques. DEAE cellulose, for example, can be prepared by treating cellulose with a solution of 2-(diethylamino) ethyl chloride.

Synthetic basic polymers include, for example, poly(vinylamine), poly(allylamine), polyethylenimine, poly(dialkylaminoalkyl acrylamide), poly(dialkylaminoalkyl methacrylamide), poly(dialkylaminoalkyl acrylate), poly(dialkylaminoalkyl methacrylate) or polymeric resins containing quaternary ammonium hydroxide groups. Basic polymers can for example be prepared from at least one monomer having the general structure of formula (I)

R _I HC=CR ₂ -CC=O)-NH-Y-NR ₃ R ₄ (I) or of at least one monomer having the general structure of formula (II)

RiHC=CR ₂ -C(=O)-O- Y-NR ₃ R ₄ (II) wherein Ri and R ₂ , independently, are selected from the group consisting of hydrogen and methyl, Y is a divalent organic radical that can be linear or branched having 1 to 8 carbon atoms, and R ₃ and R ₄ , independently, are alkyl radicals having 1 to 4 carbon atoms. Preferably, Ri is hydrogen, R ₂ is hydrogen or methyl, Y has 2 or 3 carbon atoms, and R3 and R4 are equal and have 1 or 2 carbon atoms. Further examples of basic polymers are poly(vinylguanidine) and poly(allylguanidine) .

Preferred basic polymers include a variety of water-insoluble, but water-swellable polymers. These are typically lightly crosslinked polymers which contain a multiplicity of base functional groups, such as primary, secondary and/or tertiary amines; or the corresponding phosphines. The polymers can be rendered water- insoluble, but water-swellable, by a relatively low degree of crosslinking. This may be achieved by including the appropriate amount of a suitable crosslinking monomer during the polymerization reaction. Examples of crosslinking monomers include N,N'- methylenebisacrylamide, ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, triallylamine, diaziridine compounds, and the like. Alternatively, the polymers can be crosslinked after polymerization by reaction with a suitable crosslinking agent such as di- or poly-halogenated compounds and/or di- or poly-epoxy compounds. Examples include diiodopropane, dichloropropane, ethylene glycol diglycidyl ether, and the like.

Another preferred basic polymer is crosslinked divinylbenzene/styrene copolymer containing quaternary ammonium groups in its hydroxide form, e.g. AMBERLYST A26 OH of Rohm and Haas or any other kind of weak base or strong base anion exchange polymer.

While the basic polymer is preferably of one type (i.e. homogeneous), mixtures of base polymers can also be used in the present invention. For example, mixtures of polyethylenimine (which may be crosslinked) and polyallylamine (which may be crosslinked) can be used in the present invention.

Before neutralizing acidic groups of the crosslinked cellulosic fiber, the basic polymer may be from about 50% to about 100%, preferably about 80% to about 100%, more preferably from about 90% to about 100%, in the un-neutralized base form. In order to maximize the electrolyte concentration reducing capacity of the liquid acquisition material of the invention, it is desirable that the basic polymer has a high amount of amine groups per gram of dry polymer. Thus, it is preferred that the amine group density of the basic polymer component is at least 4 meq/g, or at least 6 meq/g, or at least 10 meq/g, more preferably at least about 15 meq/g, and most preferably at least about 20 meq/g.

In order to further maximize the electrolyte concentration reduction capacity of the liquid acquisition material of the invention, it is desirable that the material comprises approximately equal equivalents of acid groups and base groups. However, it may be desirable to have somewhat more equivalents of acid groups or of base groups, e.g., to compensate for differences in pK, to compensate for differences in neutralization, to alter the pH of (for example to acidify) the liquid to be acquired, etc. The approximate electrolyte concentration reducing capacity of the liquid acquisition material of the invention can be calculated from the acid and base strength of the constituent acidic crosslinked cellulosic fiber material and the basic polymer.

It is preferred that the electrolyte concentration reducing capacity of the liquid acquisition material of the invention is at least 0.05 meq/g, or at least 0.1 meq/g, more preferably at least about 0.3 meq/g.

Disposable absorbent article

One embodiment of the present invention is a disposable absorbent article comprising the above described material for acquisition of liquids. In one embodiment of the present invention the disposable absorbent article is a diaper comprising a liquid pervious topsheet, a liquid impervious backsheet, a liquid storing absorbent core layer comprising super-absorbent material positioned between said topsheet and said backsheet and a liquid acquiring and distributing layer comprising a material for acquisition of liquids as described above. At least part and preferably all of the material for acquisition of liquids according to the invention is positioned within the disposable absorbent article such, that the liquid to be absorbed (e.g. urine) is contacted with the material for acquisition of liquids before it is contacted with the liquid storing absorbent core layer. For example, the material for acquisition of liquids is positioned between said topsheet and said core layer.

One embodiment of the invention is shown in Figures 1 and 2. Figure 1 is a plan view of a diaper 20 as an embodiment of an absorbent article according to the present invention. The diaper is shown in its flat out, uncontracted state (i.e., without elastic induced contraction). Portions of the structure are cut away to more clearly show the underlying structure of the diaper 20. The portion of the diaper 20 that contacts a wearer is facing the viewer. The chassis 22 of the diaper 20 in Figure 1 comprises the main body of the diaper 20. The chassis 22 comprises an outer covering including a liquid pervious topsheet 24 and a liquid impervious backsheet 26. The chassis may also include most or all of the absorbent core 28 encased between the topsheet 24 and the backsheet 26. The chassis can further include side panels 30, leg cuffs 32 and a waist feature 34. The leg cuffs and the waist feature typically comprise elastic members 33. One end portion of the diaper 20 is configured as the front waist region 36 of the diaper 20. The opposite end portion is configured as the rear waist region 38 of the diaper 20. An intermediate portion of the diaper 20 is configured as the crotch region 37, which extends longitudinally between the front and rear waist regions 36 and 38. The crotch region 37 is that portion of the diaper 20 which, when the diaper 20 is worn, is generally positioned between the wearer's legs. The diaper 20 has a longitudinal axis 100 and a transverse axis 110. The periphery of the diaper 20 is defined by the outer edges of the diaper 20 in which the longitudinal edges 44 run generally parallel to the

longitudinal axis 100 of the diaper 20 and the end edges 46 run generally parallel to the transverse axis 110 of the diaper 20.

For unitary absorbent articles, the chassis 22 comprises the main structure of the diaper with other features added to form the composite diaper structure. The topsheet 24, the backsheet 26, and the absorbent core 28 may be assembled in a variety of well-known configurations. Specific diaper configurations are described generally in U.S. Pat. No. 5,569,234 entitled "Disposable PuIl-On Pant" issued to Buell et al. on October 29, 1996; and U.S. Patent No. 6,004,306 entitled "Absorbent Article With Multi-Directional Extensible Side Panels" issued to Robles et al. on December 21 , 1999.

The topsheet 24 in Figure 1 may be fully or partially elasticized or may be foreshortened to provide a void space between the topsheet 24 and the absorbent core 28. Exemplary structures including elasticized or foreshortened topsheets are described in more detail in U.S. Pat. No. 5,037,416 entitled "Disposable Absorbent Article Having Elastically Extensible Topsheet" issued to Allen et al. on August 6, 1991; and U.S. Pat. No. 5,269,775 entitled "Trisection Topsheets for Disposable Absorbent Articles and Disposable Absorbent Articles Having Such Trisection Topsheets" issued to Freeland et al. on December 14, 1993.

The backsheet 26 may be joined with the topsheet 24. The backsheet 26 prevents the exudates absorbed by the absorbent core 28 and contained within the article 20 from soiling other external articles that may contact the diaper 20, such as bed sheets and undergarments. Often, the backsheet 26 is substantially impervious to liquids (e.g., urine) and comprises a laminate of a nonwoven and a thin plastic film such as a thermoplastic film having a thickness of about 0.012 mm (0.5 mil) to about 0.051 mm (2.0 mils). Suitable backsheet films include those manufactured by Tredegar Industries Inc. of Terre Haute, IN and sold under the trade names X15306, X10962, and X 10964. Other suitable backsheet materials may include breathable materials that permit vapors to escape from the diaper 20 while still preventing exudates from passing through the backsheet 26. Exemplary breathable materials may include materials such as woven webs, nonwoven webs, composite materials such as film-coated nonwoven webs, and microporous

films such as manufactured by Mitsui Toatsu Co., of Japan under the designation ESPOIR NO and by EXXON Chemical Co., of Bay City, TX, under the designation EXXAIRE.

The absorbent core 28 in Figure 1 generally is disposed between the topsheet 24 and the backsheet 26. The absorbent core 28 may comprise any absorbent material that is generally compressible, conformable, non-irritating to the wearer's skin, and capable of absorbing and retaining liquids such as urine and other certain body exudates. The absorbent core 28 may be manufactured in a wide variety of sizes and shapes (e.g., rectangular, hourglass, "T"-shaped, asymmetric, etc.) and may comprise a wide variety of liquid-absorbent materials commonly used in disposable diapers and other absorbent articles such as comminuted wood pulp, which is generally referred to as air felt. Examples of other suitable absorbent materials include creped cellulose wadding; melt blown polymers, including co-form; chemically stiffened, modified or cross-linked cellulosic fibers; tissue, including tissue wraps and tissue laminates, absorbent foams, absorbent sponges, superabsorbent polymers, absorbent gelling materials, or any other known absorbent material or combinations of materials. The absorbent core may further comprise minor amounts (typically less than 10%) of non-liquid absorbent materials, such as adhesives, waxes, oils and the like. In one embodiment, the core comprises superabsorbent polymers and is air felt free.

Exemplary absorbent structures for use as the absorbent assemblies are described in U.S.

Patent 4,834,735, entitled "High Density Absorbent Members Having Lower Density and Lower Basis Weight Acquisition Zones", issued to Alemany et al. on May 30, 1989; and U.S. Patent No. 5,625,222 entitled "Absorbent Foam Materials For Aqueous Fluids Made From high Internal Phase Emulsions Having Very High Water-To-Oil Ratios" issued to DesMarais et al. on July 22, 1997.

The diaper 20 may also include such other features as are known in the art including front and rear ear panels, waist cap features, elastics and the like to provide better fit, containment and aesthetic characteristics. Such additional features are well known in the art and are described in U.S. Pat. No. 3,860,003 entitled "Contractable side portions for disposable diaper" issued to Buell et al. on January 14, 1975 and U.S. Patent No. 5,151,092 entitled "Absorbent article with

dynamic elastic waist feature having a predisposed resilient flexural hinge" issued to Buell et al. on September 29, 1992.

In order to keep the diaper 20 in place about the wearer, the waist regions 36 and 38 may include a fastening system comprising fastening members 40 attached to the rear waist region 38. In one embodiment the fastening system further comprises a landing zone 42 attached to the front waist region 36. The fastening member is attached to the front waist region 36, often to the landing zone 42, to form leg openings and an article waist. Diapers 20 according to the present invention may be provided with a re-closable fastening system or may alternatively be provided in the form of pant- type diapers. The fastening system and any component thereof may include any material suitable for such a use, including but not limited to plastics, films, foams, nonwoven webs, woven webs, paper, laminates, fiber reinforced plastics and the like, or combinations thereof. In some embodiments, the materials making up the fastening device are flexible. The flexibility is designed to allow the fastening system to conform to the shape of the body and thus, reduces the likelihood that the fastening system will irritate or injure the wearer's skin.

Figure 2 shows a cross-sectional view of Figure 1 taken in the transverse axis 110. Starting from the wearer facing side the diaper comprises the topsheet 24, the components of the absorbent core 28, and the backsheet 26. An acquisition system 50 is comprised between the topsheet 24 and the backsheet 26, preferably between the topsheet 24 and the absorbent core 28. The acquisition system 50 may comprise an upper acquisition layer 52 facing towards the wearer and a lower acquisition layer 54.

The material for acquisition of liquids according to the invention is preferably comprised in the acquisition system 50, either in the upper acquisition layer 52 or in the lower acquisition layer 54 or in both. In one embodiment the upper acquisition layer 52 comprises a nonwoven fabric whereas the lower acquisition layer 54 comprises the material according to the present invention. In another embodiment both acquisition layers are provided from the material according to the present invention. In case the acquisition system 50 comprises a non-woven fabric, this non-woven fabric is preferably hydrophilic. The acquisition layer can be in direct contact with the absorbent core 28.

The storage layer 60 can be wrapped by a core wrap material. In one embodiment the core wrap material comprises a top layer 56 and a bottom layer 58. The top layer 56 and the bottom layer 58 can be provided from a non-woven material. One useful material is a so-called SMS material, comprising a spunbonded, a melt-blown and a further spunbonded layers. The top layer 56 and the bottom layer 58 may be provided from two or more separate sheets of materials or they may be alternatively provided from a unitary sheet of material. Such a unitary sheet of material may be wrapped around the storage layer 60, e.g. in a C-fold. The top layer 56 and the bottom layer 58 may also be joined to each other, for example along their periphery. In another option both layers are joined along their longitudinal peripheries, in other embodiments they are joined along the transversal peripheries, or along the longitudinal and the transversal peripheries. The joining can be achieved my multiple means well known in the art, e.g. by adhesive means, using a continuous or a discontinuous pattern, for example a linear or curvilinear pattern. The storage layer 60 can comprise fibrous materials, mixed with superabsorbent, absorbent gelling materials. Other materials described above as suitable for the absorbent core 28 may also be comprised. In one embodiment, the storage layer 60 has reduced amounts of fibrous materials or is free of fibrous materials and the concentration of superabsorbent, absorbent gelling materials in the storage layer 60 is at least 40 wt.%, at least 60 wt.% or at least 90 wt.%, based on the total amount of absorbent material in the storage layer 60.

METHOD OF USE

One embodiment of the invention is a method of reducing the electrolyte concentration of an aqueous medium which contains electrolytes, comprising contacting the aqueous medium which contains electrolytes with a material for acquisition of liquids according to the invention as described above.

METHOD OF MAKING

The invention also relates to a process of making the above-described material. The process comprises the steps of a) providing a cellulosic based fiber, b) impregnating the fiber with the at least one acidic crosslinking agent and with the at least one basic substance, and

c) heating the resulting mixture to temperatures of at least the boiling point of water. A further process step can be baling of the resulting crosslinked fiber mixture.

EXAMPLES The materials illustrated in the following examples illustrate specific embodiments of the present invention, but are not intended to be limiting thereof. Other modifications can be undertaken by the skilled artisan without departing from the spirit and scope of this invention.

All exemplified amounts are listed as weight percents and exclude minor materials such as diluents, preservatives, colour solutions, imagery ingredients, botanicals, and so forth, unless otherwise specified. If a trade name is mentioned as ingredient and the respective product is itself a mixture (e.g. a solution, emulsion, dispersion etc.), then the exemplified amount relates to this mixture, unless otherwise specified.

In the following examples, 4 g patches, made of individualized, crosslinked cellulosic fibers are used. The fiber material is made according to Example II of WO 95/34710 with the difference, that the crosslinked fibers contain 8 wt.% polyacrylic acid (i.e. 0.32 g of polyacrylic acid are in a 4 g fiber patch), calculated on a dry fiber weight basis, reacted with the fibers in the form of intrafiber crosslink bonds.

Polyallylamine (PAAm) is commercially available as 20 wt.% aqueous solutions. Crosslinked quaternary ammonium hydroxide divinylbenzene/styrene copolymer is commercially available (Amberlyst A-26 OH) as water containing spherical resin beads. Both chemicals were used without further purification. The density of the 20 wt.% aqueous solution of polyallylamine was estimated to be ~ lg/ml.

Different saline (NaCl) solutions from 0.8 to 0.9 wt.% were prepared and their conductivity was measured. Within this concentration range, there is an almost linear relationship between conductivity and NaCl concentration. This relationship was used as reference to determine the NaCl concentration of the test samples by measuring the conductivity of the test samples.

The conductivity of 0.9 wt.% saline solutions before and after contact with the test material is measured, corrected for the conductivity effect of the basic material (polyallylamine or Amberlyst A-26 OH, respectively) and corrected for the dilution effect of water. The decrease in NaCl concentration (desalting effect) of the test samples is determined from the conductivity/concentration relationship.

Conductivity Measurements

Equipment:

Conductivity Meter: WTW LF 320 Stirrer and Hot Plate: IKA RH-KT/C

Eppendorf Pipette

The fiber patch is washed several times with distilled water and dried at 50 ⁰ C to remove all ex tractable components that might influence the conductivity measurements.

All conductivity measurements are carried out at room temperature or at 37 ⁰ C but the conductivity meter is used in "auto-correlation" mode so that the given conductivities are automatically correlated to 25°C. 4 g PAA-crosslinked cellulosic fiber pads are merged into 200 ml 0.9 wt.% saline (NaCl) solution and after 5 minutes stirring the conductivity was measured.

This conductivity was set as the base for the series of measurements and for the following calculations.

Example 1

4 g washed and dried polyacrylic acid crosslinked cellulosic fiber pad is merged into 200 ml 0.9 w% saline solution at room temperature and after 5 min manually stirring the conductivity is measured. Polyallylamine (PAAm) is added. The mixture is stirred and the conductivity is observed. From the conductivity change the concentration change of the NaCl solution is calculated and corrected for dilution effects.

Amount PAAm added 0.28 g

Difference in conductivity - 0.95 mS/cm Effective NaCl reduction - 0,051 wt.%

Example 2

4 g washed and dried polyacrylic acid crosslinked cellulosic fiber pad is merged into 200 ml 0.9 w% saline solution at room temperature and after 5 min manually stirring the conductivity is measured. Amberlyst A-26(OH) is added. The mixture is stirred and the conductivity is observed. From the conductivity change the concentration change of the NaCl solution is calculated and corrected for dilution effects.

Amount Amberlyst A-26(OH) added 1 ,8 g Difference in conductivity - 1.08 mS/cm Effective NaCl reduction - 0,065 wt.%

Example 3

4 g washed and dried polyacrylic acid crosslinked cellulosic fiber is strongly mixed for 10 minutes with 1.5 g Amberlyst A-26(OH) and then gently compressed with a pistil and 0.3 psi. A 0.9 wt.% saline solution is tempered to 37°C and 100 ml of the saline solution is poured in one gush on top the cellulosic fiber pad / A-26(OH) mixture to allow the liquid to flow through the pad and the solution was collected afterwards. This is repeated two more times and the conductivity before and afterwards as well as the amount of liquid is measured.

From the conductivity change the amount of absorbed NaCl is determined to be 1.249 mmol after 3 gushes of 100 ml (280 ml recovered).

The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as "40 mm" is intended to mean "about 40 mm".