Porous, Absorbent Macrostructures of Bonded Absorbent Particles Surface Crosslinked With Cationic Amino-Epichlorohydrin Adducts

This application relates to porous, absorbent macrostructures that, upon contacting liquids such as water or body exudates (e.g., urine), swell and imbibe such liquids, and are useful in absorbent articles such as diapers, adult incontinence pads, sanitary napkins, and the like. This application particularly relates to porous macrostructures of bonded absorbent particles that are surface crosslinked with cationic, preferably polymeric, amino-epichlorohydrin adducts.

Particulate, absorbent, polymeric compositions are capable of absorbing large quantities of liquids such as water and body exudates (e.g., urine) and are further capable of retaining such absorbed liquids under moderate pressures. The absorption characteristics of such polymeric compositions make them especially useful for incorporation into absorbent articles such as diapers. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of particulate, absorbent, polymeric compositions (often referred to as "hydrogels", "superabsorbents", or "hydrocolloid materials") in absorbent articles.

Conventional particulate, absorbent, polymeric compositions, however, have the limitation that the particles are not immobilized and are free to migrate during processing and/or use. Migration of the particles can lead to material handling losses during manufacturing as well as nonhomogeneous incorporation of the particles into structures in which the particles are being used. A more significant problem, though, occurs when these particulate materials migrate during or after swelling in use. Such mobility leads to high resistance to liquid flow through the material due to the lack of stable interparticle capillary or liquid transport channels. This phenomenon is one form of what is commonly referred to as "gel blocking."

One attempt to overcome the performance limitations associated with absorbent particle mobility during use in absorbent articles is incorporation of the particulate, absorbent, polymeric compositions into tissue laminates, i.e. layered absorbent structures. By encapsulating the particles between tissue layers, the overall particle mobility within an absorbent structure is diminished. However, upon liquid contact, the particles within the laminate are often free to move relative to each other resulting in the breakdown of any preexistent interparticle capillary channels.

Another attempted solution is to immobilize the particulate, absorbent, polymeric compositions by the addition of large quantities of liquid polyhydroxy compounds that act as an adhesive to hold the particles together or to a substrate. See, for example, U.S. Pat. No. 4,410,571 (Korpman), issued Oct. 18, 1983. While this approach does limit migration before and, to some extent, during swelling, the particles eventually become detached from each other in the presence of excess liquid, resulting again in the breakdown of any preexisting capillary channels between the particles.

Another attempted solution to overcome the problem of absorbent particle mobility is to produce a superabsorbent film by extrusion of a solution of a linear absorbent polymer and subsequently crosslinking it. See, for example, U.S. Pat. No. 4,861,539 (Allen et al), issued Aug. 29, 1989 (crosslinked with a polyhydroxy compound such as a glycol or glycerol); and U.S. Pat. No. 4,076,673 (Burkholder), issued Feb. 28, 1978 (crosslinked with polyamine-polyamide epichlorohydrin adducts such as Kymene.RTM.). While these superabsorbent films may absorb significant quantities of liquids, they have limited liquid transport properties because they are essentially nonporous, i.e. lack internal capillary channels. Indeed, due to the lack of internal capillary channels, these superabsorbent films are especially prone to gel blocking.

A more recent solution proposed to overcome the problem of absorbent particle mobility is to form these particles into aggregate macrostructures, typically as sheets of bonded absorbent particles. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992. These aggregate macrostructures are prepared by initially mixing the absorbent particles with a solution of a nonionic crosslinking agent, water and a hydrophilic organic solvent such as isopropanol. These nonionic crosslinking agents include polyhydric alcohols (e.g., glycerol), polyaziridine compounds (e.g., 2,2-bishydroxymethyl butanoltris[3-(1-aziridine) propionate]), haloepoxy compounds (e.g., epicholorhydrin), polyaldehyde compounds (e.g., glutaraldehyde), polyamine compounds (e.g., ethylene amine), and polyisocyanate compounds (e.g., 2,4-toluene diisocyanate), preferably glycerol. See Column 11, lines 22-54, of Roe et al.

Particulate absorbent polymer compositions of the type used in making these aggregate macrostructures usually contain multiple carboxy groups and are typically derived from polycarboxy compounds such as the polyacrylates. When using glycerol as the crosslinking agent, the hydroxy groups of the glycerol typically react with the carboxy groups of the polymers present in the absorbent particles by an esterification reaction. The crosslinked, ester bond formed by glycerol occurs not only at the surface of the absorbent particles, but also inside particles. This is due to the fact that glycerol is a nonionic, relatively small molecule that can penetrate inside the absorbent particles. The resulting internal crosslinking leads to a lower absorbent capacity for the bonded particles of the aggregate macrostructures.

Moreover, the crosslinking reaction between the hydroxy groups of the glycerol and the carboxy groups of the polymers present in the absorbent particles is relatively slow. Indeed, the glycerol treated absorbent particles are typically cured at 200.degree. C. for 50 minutes. This provides relatively brittle sheets of bonded absorbent particles that are more difficult to handle, especially in making the ultimately desired absorbent structures. Accordingly, these brittle sheets need to be treated with a plasticizer, such as a mixture of water and glycerol, to make them relatively flexible and thus easier to handle in manufacturing absorbent structures.

Accordingly, it would be desirable to be able to make such aggregate macrostructures of bonded absorbent particles using a crosslinking agent that: (1) reacts rapidly with the carboxy groups of the polymer present in the absorbent particles and primarily at the surface thereof to minimize absorbency effects; (2) provides improved absorbency and mechanical (tensile) properties for the aggregate macrostructures; (3) provides flexible sheets of such aggregate macrostructures that can be easily made into absorbent structures used in diapers, adult incontinence pads, sanitary napkins and the like; and (4) does not necessarily require organic solvents such as isopropanol.

The present invention relates to improved porous, absorbent, macrostructures that comprise interparticle bonded aggregates. These aggregates comprise a multiplicity of precursor absorbent particles bonded to each other at the surface thereof, the particles comprising a substantially water-insoluble, absorbent hydrogel-forming, polymer material having anionic functional groups. These aggregates further comprise a cationic, preferably polymeric, amino-epichlorohydrin adduct reacted with the absorbent polymer material at the surface of the precursor particles and in an amount sufficient to effective surface crosslinking. These aggregates also have pores between adjacent precursor particles, the pores being interconnected by intercommunicating channels so as to form a liquid permeable macrostructure, the circumscribed dry volume of the macrostructure being greater than about 0,008 mm.sup.3.

The present invention further relates to a method for making such porous absorbent macrostructures by providing a multiplicity of the precursor absorbent particles that are then treated with a sufficient amount of the cationic, preferably polymeric, amino-epichlorohydrin adduct. The treated precursor particles are then physically associated to form aggregates and the adduct reacted with the absorbent polymer material of the precursor particles so as to cause effective surface crosslinking. The porous, absorbent macrostructures obtained are useful, alone, or in combination with other absorbent materials, in absorbent structures for various absorbent articles, including diapers, adult incontinence pads, sanitary napkins, and the like.

The porous absorbent macrostructures of the present invention, and method for making same, provide a number of significant advantages over prior porous absorbent macrostructures made with nonionic crosslinking agents, in particular glycerol. The use of cationic, preferably polymeric, amino-epichlorohydrin adducts as the crosslinking agent according to the present invention improves the cure rate and enhances the absorbent capacity of the particles by reducing or eliminating innerparticle crosslinking. This is due to the fact that these adducts, especially the polymeric resin versions, being relatively large, cationic molecules, are unable to penetrate inside the absorbent particles. In addition, the cationic functional (e.g., azetedinium) groups of these adducts are believed to react very rapidly with the artionic, typically carboxy, functional groups of the polymer material comprising the absorbent particles, even at ambient room temperature, e.g., at 18.degree.-25.degree. C.). As a result, lower levels of crosslinking agent are required, e.g., as low as 1% by weight of the absorbent particles, versus typically 4% by weight for glycerol as the crosslinking agent.

Use of these cationic, preferably polymeric, amino-epichlorohydrin adducts provides other significant advantages over porous absorbent macrostructures made with glycerol as the crosslinking agent. The porous absorbent macrostructures of the present invention have improved absorbency and mechanical (tensile) properties. Unlike glycerol-crosslinked absorbent macrostructures, flexible absorbent macrostructures (e.g., sheets) according to the present invention can be made by an essentially one-step process without the need of subsequent treatment with a plasticizer (e.g., adding a mixture of water and glycerol). In addition, organic solvents such as isopropanol are not required in making absorbent macrostructures according to the present invention.

FIG. 1 is a photomicrograph (magnification 34.9.times.) of a section of a porous, absorbent macrostructure according to the present invention;

FIG. 2 is an enlarged portion (magnification 75.times.) of the macrostructure shown in FIG. 1;

FIG. 3 is a further enlarged portion (magnification 200.times.) of the macrostructure shown in FIG. 2;

FIG. 4 is a further enlarged portion (magnification 400.times.) of the macrostructure shown in FIG. 3;

FIG. 5 is a perspective view of a disposable diaper embodiment according to the present invention wherein portions of the topsheet have been cut-away to more clearly show the underlying absorbent core (an embodiment of an absorbent member according to the present invention) of the diaper wherein the absorbent member comprises a porous, absorbent macrostructure according to the present invention;

FIG. 6 is a cross-sectional view of the absorbent core of the diaper shown in FIG. 5 taken along sectional line 6--6 of FIG. 8;

FIG. 7 is a perspective view of a disposable diaper embodiment according to the present invention wherein portions of the topsheet have been cut away to more clearly show an alternative dual-layer absorbent core embodiment;

FIG. 8 is a blown-apart view of the components of a diaper structure, one of the components being an alternative dual-layer absorbent core where the absorbent macrostructure is in the form of a plurality of strips; and

FIG. 9 is a simplified perspective view of an apparatus for making absorbent macrostructures of the present invention in the form of sheets.

Porous, absorbent macrostructures according to the present invention are structures capable of absorbing large quantities of liquids such as water and/or body exudates (e.g., urine or menses) and then retaining such liquids under moderate pressures. Because of the particulate nature of the precursor particles, the macrostructure has pores between adjacent precursor particles. These pores are interconnected by intercommunicating channels such that the macrostructure is liquid permeable (i.e., has capillary transport channels).

Due to the bonds formed between the precursor particles, the resultant aggregate macrostructure has improved structural integrity, increased liquid acquisition and distribution rates, and minimal gel-blocking characteristics. It has been found that when the macrostructure is contacted with liquids, the macrostructure swells generally isotropically even under moderate confining pressures, absorbs such liquids into the pores between the precursor particles, and then imbibes such liquids into the particles. The isotropic swelling of the macrostructure allows the precursor particles and the pores to maintain their relative geometry and spatial relationships even when swollen. Thus, the macrostructures are relatively "fluid stable" in that the precursor particles do not dissociate from each other, thereby minimizing the incidence of gel blocking and allowing the capillary channels to be maintained and enlarged when swollen so that the macrostructure can acquire and transport subsequent loadings of liquid, even excess liquid.

As used herein, the term "macrostructure" means a structure having a circumscribed volume when substantially dry (i.e., circumscribed dry volume) of at least about 0.008 mm.sup.3, preferably at least about 10.0 mm.sup.3, more preferably at least about 100 mm.sup.3, most preferably at least about 500 mm.sup.3. Typically, the macrostructures of the present invention will have a circumscribed dry volume much greater than about 500 mm.sup.3. In preferred embodiments of the present invention, the macrostructures have a circumscribed dry volume of between about 1000 mm.sup.3 and about 100,000 mm.sup.3.

While the macrostructures of the present invention can have a number of shapes and sizes, they are typically in the form of sheets, films, cylinders, blocks, spheres, fibers, filaments, or other shaped elements. The macrostructures will generally have a thickness or diameter between about 0.2 mm and about 10.0 mm. Preferably for use in absorbent products, the macrostructures are in the form of a sheet. The term "sheet" as used herein describes macrostructures having a thickness at least about 0.2 mm. The sheets will preferably have a thickness between about 0.5 mm and about 10 mm, typically from about 1 mm to about 3 mm.

As shown in FIGS. 1 through 4, the porous, absorbent macrostructures of the present invention comprise interparticle bonded aggregates. These interparticle bonded aggregates usually comprise about 8 or more previously independent precursor particles. For preferred circumscribed dry volumes and sizes of the individual precursor particles used herein, these interparticle bonded aggregates typically are formed from about 100,000 or more individual precursor particles. These individual precursor particles can comprise granules, pulverulents, spheres, flakes, fibers, aggregates or agglomerates.

As can be especially seen in FIGS. 1 and 2, the individual precursor particles can have a variety of shapes, such as cubic, rod-like, polyhedral, spherical, rounded, angular, irregular, randomly-sized irregular shapes, e.g., pulverulent products of grinding or pulverizing steps, or shapes having a large greatest dimension/smallest dimension ratio so as to be needle-like, flake-like, or fiber-like.

As particularly shown in FIGS. 3 and 4, the interparticle bonded aggregate comprising the macrostructures of the present invention are formed, in essence, by the joining or adhering together of adjacent particles. The adhesive agent is essentially the polymeric material that is present in the surface of these particles. When these precursor particles are treated and physically associated as described hereafter, the polymer material present in the surface of these particles is sufficiently plastic and cohesive (e.g., sticky) such that adjacent particles are adhered together, typically as discrete linking portions between the particles. The crosslinking reaction between the amino-epichlorohydrin adduct and the polymer material of the particles then sets this adhered structure such that the particles in the aggregate remain cohesively bonded together.

The macrostructures of the present invention are formed from polymer materials capable of absorbing large quantities of liquids. (Such .polymer materials are commonly referred to as "hydrogel", "hydrocolloid", or "superabsorbent" materials.) The macrostructures preferably comprise substantially water-insoluble, absorbent hydrogel-forming, polymer material. The specific polymer materials will be discussed herein with respect to those forming the precursor particles.

Although the precursor particles can have a size varying over a wide range, specific particle size distributions and sizes are preferred. For purposes of the present invention, particle size is defined for precursor particles that do not have a large greatest dimension/smallest dimension ratio such as fibers (e.g., granules, flakes, or pulverulents) as the dimension of a precursor particle which is determined by sieve size analysis. Thus, for example, a precursor particle that is retained on a standard #30 sieve with 600 micron openings is considered to have a particle size greater than 600 microns, a precursor particle that passes through the #30 sieve with 600 micron openings and is retained on a standard #35 sieve with 500 micron openings is considered to have a particle size between 500 and 600 microns, and a precursor particle that passes through a #35 sieve with 500 micron openings is considered to have a particle size less than 500 microns. In preferred embodiments of the present invention, the precursor particles will generally range in size from about 1 micron to about 2000 microns, more preferably from about 20 microns to about 1000 microns.

Further, for purposes of this invention, the mass average particle size of the precursor particles is important in determining the characteristics and properties of the resultant macrostructures. The mass average particle size of a given sample of precursor particles is defined as the particle size which is the average particle size of the sample on a mass basis. A method for determining the mass average particle size of a sample is described hereinafter in the Test Methods section. The mass average particle size of the precursor particles will generally be from about 20 microns to about 1500 microns, more preferably from about 50 microns to about 1000 microns. In preferred embodiments of the present invention, the precursor particles have a mass average particle size less than about 1000 microns, more preferably less than about 600 microns, most preferably less than about 500 microns. In especially preferred embodiments of the present invention, the mass average particle size of the precursor particles is relatively small (i.e. the precursor particles are fines). In these embodiments, the mass average particle size of the precursor particles is less than about 300 microns, more preferably less than about 180 microns. In an exemplary embodiment, at least about 95% by weight of the precursor particles have a particle size between about 150 microns and about 300 microns. In an alternative embodiment,. at least about 95% by weight of the precursor particles have a particle size between about 90 microns and about 180 microns. Narrow precursor particle size distributions are preferred because they result in a higher porosity macrostructure due to the higher void fraction when densified versus broader precursor particle size distributions with equivalent mass average particle sizes.

The particle size of materials having a large greatest dimension/smallest dimension such as fibers is typically defined by their largest dimension. For example, if absorbent, polymeric fibers (i.e. superabsorbent fibers) are used in the macrostructures of the present invention, the length of the fibers is used to define the "particle size." (The denier and/or the diameter of the fibers can also be specified.) In exemplary embodiments of the present invention, the fibers have a length greater than about 5 mm, preferably between about 10 mm and about 100 mm, more preferably between about 10 mm and about 50 mm.

The precursor particles comprise substantially water-insoluble, absorbent hydrogel-forming, polymer material having a multiplicity of anionic, functional groups, such as sulfonic acid, and more typically carboxy, groups. Examples of polymer materials suitable for use as the precursor particles herein include those which are prepared from polymerizable, unsaturated, acid-containing monomers. Thus, such monomers include the olefinically unsaturated acids and anhydrides which contain at least one carbon to carbon olefinic double bond. More specifically, these monomers can be selected from olefinically unsaturated carboxylic acids and acid anhydrides, olefinically unsaturated sulfonic acids, and mixtures thereof.

Some non-acid monomers can also be included, usually minor amounts, in preparing the precursor particles herein. Such non-acid monomers can include, for example, the water-soluble or water-dispersible esters of the acid-containing monomers, as well as monomers which contain no carboxylic or sulfonic acid groups at all. Optional non-acid monomers can thus include monomers containing the following types of functional groups: carboxylic acid or sulfonic acid esters, hydroxyl groups, amide-groups, amino groups, nitrile groups and quaternary ammomium salt groups. These non-acid monomers are well-known materials and are described in greater detail, for example, in U.S. Pat. No. 4,076,663 (Masuda et al), issued Feb. 28, 1978, and in U.S. Pat. No. 4,062,817 (Westerman), issued Dec. 13, 1977, both of which are incorporated by reference.

Olefinically unsaturated carboxylic acid and carboxylic acid anhydride monomers include the acrylic acids typified by acrylic acid itself, methacrylic acid, ethacrylic acid, .alpha.-chloroacrylic acid, .alpha.-cyanoacrylic acid, .beta.-methylacrylic acid (crotonic acid), .alpha.-phenylacrylic acid, .beta.-acryloxypropionic acid, sorbic acid, .alpha.-chlorosorbic acid, angelic acid, cinnamic acid, p-chlorocinnamic acid, .beta.-sterylacrylic acid, itaconic acid, citroconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric acid, tricarboxyethylene and maleic acid anhydride.

Olefinically unsaturated sulfonic acid monomers include aliphatic or aromatic vinyl sulfonic acids such as vinylsulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid and styrene sulfonic acid; acrylic and methacrylic sulfonic acid such as sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl acrylate, sulfopropyl methacrylate, 2- hydroxy-3-methacryloxypropyl sulfonic acid and 2-acrylamide-2-methylpropane sulfonic acid.

Preferred polymer materials for use in the present invention contain carboxy groups. These polymers include hydrolyzed starch-acrylonitrile graft copolymers, partially neutralized starch-acrylonitrile graft copolymers, starch-acrylic acid graft copolymers, partially neutralized starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylic ester copolymers, hydrolyzed acrylonitrile or acrylamide copolymers, slightly network crosslinked polymers of any of the foregoing copolymers, partially neutralized polyacrylic acid, and slightly network crosslinked polymers of partially neutralized polyacrylic acid. These polymers can be used either solely or in the form of a mixture of two or more different polymers. Examples of these polymer materials are disclosed in U.S. Pat. Nos. 3,661,875, 4,076,663, 4,093,776, 4,666,983, and 4,734,478.

Most preferred polymer materials for use in making the precursor particles are slightly network crosslinked polymers of partially neutralized polyacrylic acids and starch derivatives thereof. Most preferably, the precursor particles comprise from about 50 to about 95%, preferably about 75%, neutralized, slightly network crosslinked, polyacrylic acid (i.e. poly (sodium acrylate/acrylic acid)).

As described above, the precursor particles are preferably made from polymer materials that are slightly network crosslinked. Network crosslinking serves to render the polymer materials from which the precursor particles are made substantially water-insoluble and, in part, determines the absorptive capacity and extractable polymer content characteristics of the precursor particles and the resultant macrostructures. Processes for network crosslinking the, polymers and typical network crosslinking agents are described in greater detail in the hereinbefore-referenced U.S. Pat. No. 4,076,663.

The individual precursor particles can be formed in any conventional manner. Typical and preferred processes for producing, the individual precursor particles are described in U.S. Pat. No. Re. 32,649 (Brandt et al), issued Apr. 19, 1988, U.S. Pat. No. 4,666,983 (Tsubakimoto et al), issued May 19, 1987, and U.S. Pat. No. 4,625,001 (Tsubakimoto et al), issued Nov. 25, 1986, all of which are incorporated by reference.

Preferred methods for forming the precursor particles are those that involve aqueous solution or other solution polymerization methods. As described in the above-referenced U.S. Pat. No. 32,649, aqueous solution polymerization involves the use of an aqueous reaction mixture to carry out polymerization to form the precursor particles. The aqueous reaction mixture is then subjected to polymerization conditions which are sufficient to produce in the mixture, substantially water-insoluble, slightly network crosslinked polymer material. The mass of polymer material thereby formed is then pulverized or chopped to form the individual precursor particles.

More specifically, the aqueous solution polymerization method for producing the individual precursor particles comprises the preparation of an aqueous reaction mixture in which to carry out polymerization to form the desired precursor particles. One element of such a reaction mixture is the acid group-containing monomer material which will form the "backbone" of the precursor particles to be produced. The reaction mixture will generally comprise about 100 parts by weight of the monomer material. Another component of the aqueous reaction mixture comprises a network crosslinking agent. Network crosslinking agents useful in forming the precursor particles are described in more detail in the above-referenced U.S. Pat. No. Re. 32,649, U.S. Pat. No. 4,666,983, and U.S. Pat. No. 4,625,001. The network crosslinking agent will generally be present in the aqueous reaction mixture in an amount of from about 0,001 mole percent to about 5 mole percent based on the total moles of monomer present in the aqueous mixture (about 0.01 to about 20 parts by weight, based on 100 parts by weight of the monomer material). An optional component of the aqueous reaction mixture comprises a free radical initiator including, for example, peroxygen compounds such as sodium, potassium, and ammonium persulfates, caprylyl peroxide, benzoyl peroxide, hydrogen peroxide, cumene hydroperoxides, tertiary butyl diperphthalate, tertiary butyl perbenzoate, sodium peracetate, sodium percarbonate, and the like. Other optional components of the aqueous reaction mixture comprise the various non-acidic co-monomer materials including esters of the essential unsaturated acidic functional group-containing monomers or other co-monomers containing no carboxylic or sulfonic acid functionalities at all.

The aqueous reaction mixture is subjected to polymerization conditions which are sufficient to produce in the mixture substantially water-insoluble, absorbent, hydrogel-forming, slightly network crosslinked polymer materials. The polymerization conditions are also discussed in more detail in the three above-referenced patents. Such polymerization conditions generally involve heating (thermal activation techniques) to a polymerization temperature from about 0.degree. C. to about 100.degree. C., more preferably from about 5.degree. C. to about 40.degree. C. Polymerization conditions under which the aqueous reaction mixture is maintained can also include, for example, subjecting the reaction mixture, or portions thereof, to any conventional form of polymerization activating irradiation. Radioactive, electronic, ultraviolet, or electromagnetic radiation are alternative conventional polymerization techniques.

The acid functional groups of the polymer materials formed in the aqueous reaction mixture are also preferably neutralized. Neutralization can be carried out in any conventional manner which results in at least about 25 mole percent, and more preferably at least about 50 mole percent, of the total monomer utilized to form the polymer material being acid group-containing monomers that are neutralized with a salt-forming cation. Such salt-forming cations include, for example, alkali metals, ammonium, substituted ammonium and amines as discussed in further detail in the above-references U.S. Pat. No. Re. 32,649.

While it is preferred that the precursor particles be manufactured using an aqueous solution polymerization process, it is also possible to carry out the polymerization process using multi-phase polymerization processing techniques such as inverse emulsion polymerization or inverse suspension polymerization procedures. In the inverse emulsion polymerization or inverse suspension polymerization procedures, the aqueous reaction mixture as hereinbefore described is suspended in the form of tiny droplets in a matrix of a water-immiscible, inert organic solvent such as cyclohexane. The resultant precursor particles are generally spherical in shape. Inverse suspension polymerization procedures are described in greater detail in U.S. Pat. No. 4,340,706 (Obaysashi et al), issued Jul. 20, 1982, U.S. Pat. No. 4,506,052 (Flesher et al), issued Mar. 19, 1985, and U.S. Pat. No. 4,735,987 (Morita et al), issued Apr. 5, 1988, all of which are incorporated by reference.

In preferred embodiments of the present invention, the precursor particles used to form the bonded particle aggregates are substantially dry. The term "substantially dry" is used herein to mean that the precursor particles have a liquid content, typically water or other solution content, less than about 50%, preferably less than about 20%, more preferably less than about 10%, by weight of the precursor particles. In general, the liquid content of the precursor particles is in the range of from about 0.01% to about 5% by weight of the precursor particles. The individual precursor particles can be dried by any conventional method such as by heating. Alternatively, when the precursor particles are formed using an aqueous reaction mixture, water can be removed from the reaction mixture by azeotropic distillation. The polymer-containing aqueous reaction mixture can also be treated with a dewatering solvent such as methanol. Combinations of these drying procedures can also be used. The dewatered mass of polymer material can then be chopped or pulverized to form substantially dry precursor particles of substantially water-insoluble, absorbent, hydrogel-forming, polymer material.

Preferred precursor particles of the present invention are those which exhibit a high absorptive capacity so that the resultant macrostructure formed from such precursor particles also has a high absorptive capacity. Absorptive capacity refers to the capacity of a given polymer material to absorb liquids with which it comes into contact. Absorptive capacity can vary significantly with the nature of the liquid being absorbed and with the manner in which the liquid contacts the polymer material. For purposes of this invention, Absorptive Capacity is defined in terms of the amount of Synthetic Urine (as hereinafter defined) absorbed by any given polymer material in terms of grams of Synthetic Urine per gram of polymer material in a procedure hereinafter defined in the Test Methods section. Preferred precursor particles of the present invention are those which have an Absorptive Capacity of at least about 20 grams, more preferably at least about 25 grams, of Synthetic Urine per gram of polymer material. Typically, the polymer materials of the precursor particles herein have an Absorptive Capacity of from about 20 grams to about 70 grams of Synthetic Urine per gram of polymer material. Precursor particles having this relatively high absorptive capacity characteristic produce macrostructures that are especially useful in absorbent products, absorbent members, and absorbent articles since the resultant macrostructures formed from such precursor particles can, by definition, hold desirably high amounts of discharged body exudates such as urine.

While all of the precursor particles are preferably formed from the same polymer material with the same properties, this need not be the case. For example, some precursor particles can comprise a starch-acrylic acid graft copolymer while other precursor particles can comprise a slightly network crosslinked polymer of partially neutralized polyacrylic acid. Further, the precursor particles can vary in size, shape, absorptive capacity, or any other property or characteristic. In a preferred embodiment of the present invention, the precursor particles consist essentially of slightly network crosslinked polymers of partially neutralized polyacrylic acid, each precursor particle having similar properties.

A key component of the interparticle bonded aggregates that comprise the porous macrostructures of the present invention is an adduct of epichlorohydrin with certain types of monomeric or polymeric amines. These amino-epichlorohydrin adducts react with the polymer material of the absorbent precursor particles, and in particular the anionic, typically carboxy, functional groups of these polymer materials to form a covalent, ester-type bond. In other words, the amino-epichlorohydrin adduct serves to crosslink the polymer material present in the absorbent precursor particles. (The portions of the absorbent particle containing polymer material that has been effectively crosslinked with the amino-epichlorohydrin adduct swell less in the presence of aqueous body fluids relative to the other uncrosslinked portions of the particle.)

It is believed that these reacted amino-epichlorohydrin adducts primarily provide crosslinking at the surface of the absorbent precursor particles. This is due to the fact that these adducts, and especially the polymeric resin versions of these adducts, are relatively large, cationic molecules. As a result, they are unable to penetrate inside the absorbent particles, and therefore can only react with polymer material at the surface thereof. In addition, the cationic functional groups (e.g., azetedinium groups) of these adducts, particularly polymeric resin versions, are believed to react very rapidly with the artionic, typically carboxy, functional groups of the polymer material of the absorbent particles, even at room temperature (e.g., at from about 18.degree. to about 25.degree. C.). As a result, fairly modest levels (e.g., as low as about 1% by weight of the particles) of these amino-epichlorohydrin adducts are required to provide effective surface crosslinking of the polymer material present in the absorbent precursor particles.

As used herein, "cationic amino-epichlorohydrin adduct" refers to the reaction product between epichlorohydrin and a monomeric or polymeric amine such that the resulting reaction product has at least two cationic functional groups. These adducts can be in the form of monomeric compounds (e.g., the reaction product of epichlorohydrin and ethylene diamine), or can be in polymeric form (e.g., the reaction product between epichlorohydrin, and polyamide-polyamines or polyethyleneimines). The polymeric versions of these cationic amino-epichlorohydrin adducts are typically referred to as "resins."

One type of amino compound which can be reacted with epichlorohydrin to form adducts useful in the present invention comprises monomeric di-, tri- and higher amines having primary or secondary amino groups in their structures. Examples of useful diamines of this type include bis-2-aminoethyl ether, N,N-dimethylethylenediamine, piperazine, and ethylenediamine. Examples of useful triamines of this type include N-aminoethyl piperazine, and dialkylene triamines such as diethylenetriamine, and dipropylenetriamine.

Such amine materials are reacted with epichlorohydrin to form the cationic amino-epichlorohydrin adducts useful as crosslinking agents herein. Preparation of these adducts, as well as a more complete description of the adducts themselves, can be found in U.S. Pat. No. 4,310,593 (Gross), issued Jan. 12, 1982, and in Ross et al, J. Organic Chemistry, Vol. 29, pp. 824-826 (1964). Both of these documents are incorporated by reference.

In addition to monomeric amines, polymeric amines such as polyethyleneimines can also be used as the amino compound. A particularly desirable amino compound which can be reacted with epichlorohydrin to form preferred cationic polymeric adduct resins useful herein comprise certain polyamide-polyamines derived from polyalkylene polyamines and saturated C.sub.3 -C.sub.10 dibasic carboxylic acids. Epichlorohydrin/polyamide-polyamine adducts of this kind are water-soluble, thermosetting cationic polymers which are well known in the art as wet strength resins for paper products.

In the preparation of polyamide-polyamines used to form this preferred class of cationic polymeric resins, a dicarboxylic acid is first reacted with a polyalkylene-polyamine, preferably in aqueous solution, under conditions such as to produce a water-soluble, long chain polyamide containing the recurring groups --NH(C.sub.n H.sub.2n HN).sub.x --CORCO-- where n and x are each 2 or more and R is the C.sub.1 to C.sub.8 alkylene group of the dicarboxylic acid.

A variety of polyalkylene polyamines including polyethylene polyamines, polypropylene polyamines, polybutylene polyamines and so on can be employed to prepare the polyamide-polyamine, of which the polyethylene polyamines represent an economically preferred class. More specifically, preferred polyalkylene polyamines used to prepare the cationic polymeric resins herein are polyamines containing two primary amine groups and at least one secondary amine group in which the nitrogen atoms are linked together by groups of the formula --C.sub.n H.sub.2n -- where n is a small integer greater than unity and the number of such groups in the molecule ranges from two up to about eight and preferably up to about four. The nitrogen atoms can be attached to adjacent carbon atoms in the group --C.sub.n H.sub.2n -- or to carbon atoms further apart, but not to the same carbon atom. Also contemplated is the use of such polyamines as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenetriamine, and the like, which can be obtained in reasonably pure form. Of all the foregoing, the most preferred are the polyethylene polyamines containing from two to four ethylene groups, two primary amine groups, and from one to three secondary amine groups.

Also contemplated for use herein are polyamine precursor materials containing at least three amino groups with at least one of these groups being a tertiary amino group. Suitable polyamines of this type include methyl bis(3-aminopropyl)amine, methyl bis(2-aminoethyl)amine, N-(2-aminoethyl)piperazine, 4,7-dimethyltriethylenetetramine and the like.

The dicarboxylic acids which can be reacted with the foregoing polyamines to form the polyamide-polyamine precursors of the preferred cationic polymeric resins useful herein comprise the saturated aliphatic C.sub.3 -C.sub.10 dicarboxylic acids. More preferred are those containing from 3 to 8 carbon atoms, such as malonic, succinic, glutaric, adipic, and so on, together with diglycolic acid. Of these, diglycolic acid and the saturated aliphatic dicarboxylic acids having from 4 to 6 carbon atoms in the molecule, namely, succinic, glutaric and adipic are most preferred. Blends of two or more of these dicarboxylic acids can also be used, as well as blends of one or more of these with higher saturated aliphatic dicarboxylic acids such as azelaic and sebacic, as long as the resulting long chain polyamide-polyamine is water-soluble or at least water-dispersible.

The polyamide-polyamine materials prepared from the foregoing polyamines and dicarboxylic acids are reacted with epichlorohydrin to form the cationic polymeric amino-epichlorohydrin resins preferred for use herein as the crosslinking agent. Preparation of such materials is describe in greater detail in U.S. Pat. No. 2,926,116 (Keim), issued Feb. 23, 1960, U.S. Pat. No. 2,926,154 (Keim), issued Feb. 23, 1960, and U.S. Pat. No. 3,332,901 (Keim), issued Jul. 25, 1967, all of which are incorporated by reference.

The cationic polyamide-polyamine-epichlorohydrin resins preferred for use herein as crosslinking agents are commercially marketed by Hercules Inc. under the trade name Kymene.RTM.. Especially useful are Kymene.RTM. 557H, Kymene.RTM. 557LX and Kymene.RTM. 557 Plus, which are the epichlorohydrin adducts of polyamide-polyamines which are the reaction products of diethylenetriamine and adipic acid. They are typically marketed in the form of aqueous solutions of the cationic resin material containing from about 10% to about 33% by weight of the resin active.

In preparing the interparticle bonded aggregates that comprise the porous, absorbent macrostructures of the present invention, the absorbent precursor particles are treated with an sufficient amount of the cationic amino-epichlorohydrin adduct to react with the polymer material at the surface of the particles so as to cause effective crosslinking, i.e., the crosslinked surface of the particle swells less in the presence of aqueous body fluids relative to the uncrosslinked portions. What constitutes "a sufficient amount" of the adduct depends upon a number of factors, including the particular absorbent precursor particles treated, the particular amino-epichlorohydrin adduct used, the particular effects desired in forming the interparticle bonded aggregate, and like factors. In the case of monomeric amino-epichlorohydrin adducts, such as a piperazine-epichlorohydrin adducts, the amount of adduct used can be in the range of from about 0.1 to about 3 parts by weight, preferably from about 0.5 to about 1.5 parts by weight, most preferably from about 0.8 to about 1.2 parts by weight, per 100 parts by weight of the absorbent precursor particles. In the case of preferred polymeric amino-epichlorohydrin resins, such as Kymene.RTM. 557H, 557LX or Plus, the amount of resin used can be from about 0.1 to about 5 parts by weight, preferably from about 0.5 to about 2.5 parts by weight, most preferably from about 1 to about 2 parts by weight, per 100 parts by weight of the absorbent precursor particles.

Besides the absorbent precursor particles and the cationic amino-epichlorohydrin adduct, other components or agents can be used as aids in preparing the interparticle bonded aggregates. For example, water is typically used with the adduct to form an aqueous treatment solution thereof. Water promotes the uniform dispersion of the adduct on the surface of the precursor particles and causes permeation of the adduct into the surface regions of these particles. Water also promotes a stronger physical association between the treated precursor particles, providing greater integrity of the resultant interparticle bonded crosslinked aggregates. In the present invention, water is used in an amount of less than about 25 parts by weight (i.e. from 0 to about 25 parts by weight), preferably in the range of from about 3 to about 15 parts by weight, more preferably in the range of from about 5 to about 10 parts by weight, per 100 parts by weight of the precursor particles. The actual amount of water used can vary depending upon the type of adduct used, the type of polymer material used in forming the precursor particles, the particle size of these precursor particles, the inclusion of other optional components (e.g., glycerol) and like factors.

Although not absolutely necessary, organic solvents can be used, usually to promote uniform dispersion of the cationic amino-epichlorohydrin adduct onto the surface of the precursor particles. These organic solvents are typically hydrophilic, and can include lower alcohols such as methanol and ethanol; amides such as N,N-dimethylformamide and N,N-diethylformamide; and sulfoxides such as dimethylsulfoxide. If a hydrophilic solvent is used, it is in an amount of less than about 20 parts by weight (i.e. from 0 to about 20 parts by weight), preferably in the range of from about 5 to about 15 parts by weight, more preferably in the range of from about 8 to about 12 parts by weight, per 100 parts by weight of the precursor particles. The actual amount of hydrophilic solvent used can vary depending upon the adduct used, the polymer material used forming the precursor particles, the particle size of these precursor particles and like factors.

As previously noted, the use of hydrophilic organic solvents is not necessarily required in preparing bonded particle aggregates of the present invention. Indeed, it can be desirable to avoid the use of such organic solvents. Such solvents typically need to be removed from the aggregate before it is suitable for its intended use. The removal of organic solvents is frequently an energy and process intensive, and adds additional processing costs. Some hydrophilic solvents, such as isoproponal or t-butanol, can cause the amino-epichlorohydrin adduct to precipitate out of solution and are therefore undesirable for this reason. Indeed, the only solvents typically used in preparing the bonded particle aggregates of the present invention are the lower alcohols such as methanol and ethanol that are not too energy or process intensive to remove, and do not cause the amino-epichlorohydrin adduct to precipitate out of aqueous solution.

Other optional components can also be used with the cationic amino-epichlorohydrin adduct, and especially aqueous treatment solutions thereof. It is particularly preferred that the treatment solution comprising the cationic amino-epichlorohydrin adduct include a plasticizer, especially when the treated precursor particles are ambient temperature cured as described hereafter. In the absence of a plasticizer, the treated precursor particles, when formed into the interparticle bonded aggregates, can be relatively brittle, and thus more difficult to handle, especially in making the ultimately desired absorbent structures. Inclusion of a plasticizer in the treatment solution insures that the resulting interparticle bonded aggregates (when ambient temperature cured) form relatively flexible porous, absorbent macrostructures, particularly flexible, porous, absorbent sheets of the interparticle bonded aggregates. These flexible sheets are relatively easy to handle in making the ultimately desired absorbent structures.

Suitable plasticizers include water, alone or in combination with other components such as glycerol, propylene glycol (i.e. 1,2-propanediol), 1,3-propanediol, ethylene glycol, sorbitol, sucrose, polymeric solutions such as those involving polyvinyl alcohol, ester precursors of polyvinyl alcohol, or polyethylene glycol, or mixtures thereof. These other components in the plasticizer, such as glycerol, are believed to act as humectants, coplasticizers or both, with water being the primary plasticizer. The preferred plasticizer for use in the present invention is a mixture of glycerol and water, particularly when included as part of an aqueous treatment solution of the cationic amino-epichlorohydrin adduct, in a weight ratio of glycerol to water of from about 0.5:1 to about 2:1, preferably from about 0.8:1 to about 1.7:1.

The actual amount of plasticizer used can vary depending upon the particular plasticizer used, the type of polymer material used in forming the precursor particles, and the particular flexibility effects desired from the plasticizer. Typically, the plasticizer is used in an amount of from about 5 to about 100 parts by weight, preferably from about 5 to about 60 parts by weight, more preferably from about 10 to about 30 parts by weight, most preferably from about 15 to about 20 parts by weight, per 100 parts by weight of the precursor particles.

In the method of the present invention, the absorbent precursor particles can be treated with the cationic amino-epichlorohydrin adduct, typically an aqueous solution thereof, by any of a variety of techniques. These include any method for applying solutions to materials, including coating, dumping, pouring, dropping, spraying, atomizing, condensing, or immersing the absorbent precursor particles with the cationic amino-epichlorohydrin adduct, or solution thereof. As used herein, the term "applied" means that at least a portion of the surface area of at least some of the precursor particles to be bonded together has an effective amount of the adduct on it to cause surface crosslinking. In other words, the cationic adduct can be applied onto some of the precursor particles, all of the precursor particles, a portion of the surface of some or all of the precursor particles, or the entire surface of some or all of the precursor particles. Preferably, the adduct is coated onto the entire surface of most, preferably all, of the absorbent precursor particles so as to enhance the efficiency, strength, and density of the interparticle bonds between the precursor particles, as well as the desired surface crosslinking of the polymer material in the surface of these precursor particles.

In an embodiment of the method of the present invention, after the treatment solution has been applied onto the precursor particles, the treated precursor particles are mixed or layered together by any of a number of mixing or layering techniques to insure that the precursor particles are thoroughly coated with the treatment solution. Because the precursor particles are thoroughly coated with the treatment solution, the efficiency, strength, and density of the bonds between the precursor particles is enhanced, as well as surface crosslinking resulting from the reaction of the cationic adduct with the polymer material forming the precursor particles. This mixing can be accomplished using various techniques and apparatus, including various mixers or kneaders, as are known in the art.

Before, during, or after applying the treatment solution, the precursor particles are physically associated together to form an aggregate macrostructure. The term "physically associated" is used herein to mean that the precursor particles are brought together and remain in contact with each other as component parts in any of a number of various ways and spatial relationships so as to form a single unit (an aggregate macrostructure).

The precursor particles are preferably physically associated together by applying an associating agent onto the precursor particles and physically contacting the precursor particles at at least the portion of the surface of the precursor particles having the associating agent applied thereto. Preferred associating agents cause the polymer material of the precursor particles, when brought together, to adhere together by the action of fluid surface tension forces and/or the entanglement of polymer chains due to external swelling. Associating agents useful in the present invention include hydrophilic organic solvents, typically low molecular Weight alcohols such as methanol or ethanol; water; a mixture of hydrophilic organic solvents and water; the cationic amino-epichlorohydrin adducts previously described, or mixtures thereof. Preferred associating agents are water, methanol, ethanol, cationic polymeric amino-epichlorohydrin resins such as Kymene.RTM. 557H, or 557LX or Plus, or mixtures thereof. Typically the associating agent comprises a mixture including the cationic amino-epichlorohydrin adduct such that the step of applying the adduct is carried out simultaneously with the step of applying the associating agent.

The associating agents can be applied to the precursor particles by any of various techniques and apparatus used for applying solutions to materials including coating, dumping, pouring, spraying, atomizing, condensing, or immersing the associating agent on the precursor particles. The associating agent is applied to at )east a portion of the surface of at least some of the precursor particles to be bonded together. Preferably, the associating agent is coated onto the entire surface of most, preferably all, of the precursor particles. The associating agent is generally mixed with, or sprayed onto, the precursor particles by any of a number of mixing/spraying techniques and mixing/spraying apparatus to insure that the precursor particles are thoroughly coated with the associating agent.

When an associating agent has been applied to the precursor particles, the precursor particles can be physically contacted together in a number of different ways. For example, the associating agent alone can hold the particles together in contact. Alternatively, gravitational forces can be used to insure contact between the precursor particles, e.g., by layering precursor particles. Further, the particles can be placed in a container having a fixed volume so as to insure contact between the precursor particles.

The precursor particles can alternatively be physically associated together by physically constraining the precursor particles such that they are in contact with each other. For example, the precursor particles can be packed tightly into a container having a fixed volume such that the precursor particles physically contact each other. Alternatively or in combination with the above procedure, gravitational forces (e.g., layering) can be used to physically associate the precursor particles. The precursor particles can also be physically associated together by electrostatic attraction or by the introduction of an adhering agent (e.g., an adhesive material such as a water-soluble adhesive) to adhere them together. The precursor particles can also be attached to a third member (a substrate) such that the precursor particles are brought into contact with each other by the substrate.

In an alternative method of forming the macrostructures of the present invention, the aggregate of the precursor particles is shaped into various geometries, spatial relationships, and densities to form an aggregate having a defined shape, size, and/or density. The aggregate can be shaped by any conventional shaping techniques as are known in the art. Preferred methods for shaping the aggregate include casting, molding, or forming operations. Casting and molding techniques generally involve introducing the precursor particles into a prepared mold cavity and applying pressure to (compressing) the aggregate to cause the aggregate to conform to the shape of the mold cavity. Examples of specific molding techniques for use herein include compression molding, injection molding, extrusion or laminating. For example, a multiplicity of precursor particles can be added to a container having a fixed volume mold cavity and the aggregate compressed to conform to the shape of the mold cavity so that the resultant macrostructure has the same shape. Forming techniques involve performing various operations on the aggregate to modify its shape, and/or size, and/or density. Examples of specific forming techniques for use herein include rolling, forging, extruding, spinning, coating or drawing operations. For example, an aggregate mixture of the precursor particles and at least the cationic amino-epichlorohydrin adduct can be passed between a pair of compaction rolls to form an aggregate sheet. Alternatively, the aggregate mixture can be extruded through an orifice to form an aggregate having a shape corresponding to that of the the orifice. Further, the aggregate mixture can be cast on a surface to form an aggregate having a desired shape or surface morphology. Any or all of these techniques can also be used in combination to form the shaped aggregate. Any suitable apparatus as are known in the art can be used to carry out such operations, which can be performed with the material or portions of the apparatus either hot and/or cold.

In an embodiment of the method of the present invention, an aggregate mixture of precursor particles, the cationic amino-epichlorophydrin adduct, water, humectant/co-plasticizer (optional), and a hydrophilic organic solvent are added to the hopper of a conventional extruder apparatus. An example of an extruder apparatus is shown in FIGS. 12-14 of Principles of Polymer Materials, Second Edition, (McGraw Hill Book Company, 1982) at page 331, which publication is incorporated reference. The aggregate mixture is extruded through the orifice of the extruder apparatus to feed a pair of driven compaction rolls having a fixed (but variable) gap between the rolls so as to compress the aggregate into the form of a sheet. The sheet is then processed to specific lengths to provide macrostructures that have a specifically designed size, shape and/or density.

In forming the aggregate macrostructures of the present invention into particular shapes, and especially sheets, the density should be carefully controlled. If the density of the shape aggregate macrostructure is too high, it can be more prone to gel blocking. Conversely, if the density is too low, the shaped aggregate macrostructure can have insufficient tensile strength and integrity. Shaped aggregate macrostructures of the present invention usually have a density of from about 0.7 to about 1.3 g/cc, preferably from about 0.8 to about 1.1 g/cc, and most preferably from about 0.9 to about 1.0 g/cc.

A preferred method and apparatus for continuously forming the aggregate macrostructures of the present invention into sheets is described in U.S. application Ser. No. 935,693, to Michael S. Kolodesh et al, entitled "Method and Apparatus for Making Cohesive Sheets from Particulate Absorbent Polymeric Composition," filed Aug. 21, 1992, the disclosure of which is incorporated by reference. This

continuous method for making aggregate sheets can best be understood by reference to FIG. 9 which shows apparatus 301 for carrying it out. Apparatus 301 has frame 302 for supporting its various components. Apparatus 301 comprises a support means, shown in FIG. 9 as moving conveyor 303 which moves in the direction of arrow 310. Conveyor 303 first passes under an initial sprayer 304a. After passing under initial sprayer 304a, conveyor 303 passes under at least one means for continuously layering a predetermined amount of precursor particles onto the conveyor. This is shown in FIG. 9 as feeders 305a through 305e. Conveyor 303 also passes under at least one means for spraying a predetermined amount of treatment solution onto the layer of precursor particles on the conveyor. This is shown in FIG. 9 as sprayers 304b through 304f. Apparatus 301 further comprises a pair of non-planar opposing pressure applicators down stream from feeders 305 and sprayers 304. The pressure applicators are shown in FIG. 9 as a pair of compaction rolls 306. Also shown in FIG. 9 as being part of apparatus 301 is a slitting and transfer conveyor 307, knife and anvil rolls 308, and a sheet accumulator 309.

Conveyor 303 can be a flat belt conveyor that has good release properties, such as polyurethane, which is commonly used in the food industry. The width of the conveyor is determined by the desired sheet size. The conveyor generally moves in the direction of arrow 310 from point 311, where the initial sprayer 304a is located, to a point 312, where the knife and anvil rolls 308 are located. Conveyor 303 would typically be an endless conveyor as shown in FIG. 9.

Conveyor 303 first passes under an initial sprayer 304a, where the conveyor is sprayed with a predetermined amount of treatment solution so as to cover a predetermined area of the conveyor. This initial spraying insures that the bottom part of the first precursor particle layer is exposed to the treatment solution. Also, the wet conveyor surface will prevent the subsequently fed particles from bouncing away from their desired placement. However, the initial spraying step is not absolutely necessary, especially when the first layer of particles to be placed on the conveyor is relatively thin, or when the conveyor travels at slower speeds.

Sprayer 304a (as well as sprayers 304b through 304f) must deliver a substantially uniform mist, atomized spray and should have a low impact force to avoid possible blow off of precursor particles. One sprayer that has been found to work well is a model 6218-1/4 JAU atomized air actuated nozzle assembly, available from Spraying Systems Co., Wheaton, Ill. 60188.

Conveyor 303 then passes under feeder 305a where a predetermined amount of dry precursor particles is layered onto the predetermined area of the conveyor. The amount of precursor particles to be layered onto conveyor 303 depends on a number of factors including, but not limited to, the desired density of the resultant sheet, the number of layering steps to be performed, the size of the particles being used and the desired width of the resultant sheet. At a minimum the predetermined amount should be enough to substantially cover a predetermined area of the conveyor with a layer one particle in thickness.

Feeder 305a (as well as feeders 305b through 305e) must be capable of distributing the precursor particles in a thin and preferably wide layer. Thinner layers on the conveyor insure that all of the particles are treated during subsequent spraying steps and wider layers will increase production output. Vibrating feeders have been shown to be adequate for layering the dry precursor particles onto the conveyor. An example of a suitable vibrating feeder is a Super Feeder model #2106E-003S4, commercially available from Solids Flow Control, P.O. Box 410767, 14201-A South Lakes Drive, Charlotte, N.C. 28241-0767. This feeder has a weight feed-back control system for accuracy.

Conveyor 303 then passes under a second sprayer 304b. A predetermined area of conveyor 303 having the first layer of precursor particles is sprayed with a predetermined amount of the same treatment solution used in initial sprayer 304a. In general, the predetermined amount of treatment solution is related to the amount of particles in the layer. The greater the amount of particles in the layer, the more treatment solution is needed to treat substantially all of the particles.

The metering and spraying steps can then be repeated a number of times (e.g., using feeders 305b through 305e and sprayers 304c through 304f) depending on the desired density of the ultimate sheet. When the metering and spraying steps are repeated a number of times and the initial spraying step is performed, as described above, the first layer of particles is exposed to two spray applications. Therefore, the initial spraying step and the first post-layering spraying step each need only spray half the amount of treatment solution needed to treat that amount of particles in the first layer on conveyor 303. The other sprayers 304c through 304f will spray the normal amount of treatment solution, i.e. twice the amount of either the initial or first post-laying spray.

After all of the layering and spraying steps have been performed, the treated precursor particles typically loosely adhere together to form a web. Conveyor 303 then moves this web and delivers it to a pair of opposing pressure applicators. The pressure applicators shown in FIG. 9 take the form of compaction rolls 306. However, as will be appreciated to those skilled in the art, an intermittent conveyor method could be used, with opposing plates or platens used to compress the web.

Compaction rolls 306 can have a non-planar, rough surface. As the web passes through compaction rolls 306, the pressure on the web causes it to expand. The rough surface of rolls 306 reduces the sliding effect between the rolls and the web in contact with the rolls. This in turn reduces expansion of the web in both the machine direction 310 and cross-machine direction. Machine direction expansion is undesirable because it requires compaction rolls 306 to speed up in order to match the machine direction expansion. Compaction by rolls 306 densifies the web of freely deposited layers of precursor particles and sprayed treatment solution into a sheet.

Compaction rolls 306 can be in the form of cylindrical stainless steel rolls that are coated with a plasma coating, thereby giving the rolls a rough surface and causing them to release the web more easily after compaction. Examples of suitable coatings include coating #'s 934 and 936, available from Plasma Coatings, Inc., Waterbury, Conn. 06702. The gap between the compaction rolls determines the amount of compaction applied to the web.

Apparatus 301 can include a slitter to trim the web edges prior to compaction. The edges of the web can have a less uniform density than the rest of the web, and are typically subjected to inconsistent application of treatment solution and particles due to the conveyor belt movement in the cross-machine direction, thus making removal desirable. The slitter can be regular circular knife working against a hard surface such as a transfer conveyor belt, as indicated by 307.

After the web passes through compaction rolls 306, a sheet is formed and collected in accumulator 309. Accumulator 309 can take the form of a wind-up roll that rolls up the sheet into a single roll of a desired size. When the desired size roll is obtained apparatus 301 can have a second slitter to cut the sheet. This second slitter can take the form of knife and anvil roll 308.

Simultaneously or after the cationic amino-epichlorohydrin adduct has been applied, the precursor particles have been physically associated together to form an aggregate, and the aggregate has been shaped, the adduct is reacted With the polymer material of the precursor particles, while maintaining the physical association of the precursor particles, to provide effective surface crosslinking in the precursor particles in the aggregate macrostructure. Because of the relatively reactive cationic functional groups of the amino-epichlorohydrin adducts used in the present invention, this crosslinking reaction between the adduct and the polymer material of the precursor particles can occur at relatively low temperatures. Indeed, this crosslinking reaction (curing) can occur at ambient room temperatures. Such ambient temperature curing is particularly desirable when the treatment solution comprising the adduct additionally contains a plasticizer, such as a mixture of water and glycerol. Curing at significantly above ambient temperatures can cause the plasticizer to be driven off due to its volatility, thus necessitating an additional step to plasticize the resulting interparticle bonded aggregate. Such ambient curing is typically carried out at a temperature of from about 18.degree. to about 35.degree. C. for from about 12 to about 48 hours. Preferably, such ambient curing is carried out at a temperature of from about 18.degree. to about 25.degree. C. for from about 24 to about 48 hours.

Although the crosslinking reaction between the cationic amino-epichlorohydrin adduct and the polymer material of the precursor particles can occur at ambient temperatures, such curing can also be carried out at higher temperatures to speed up the reaction. Higher temperature curing typically involves heating the treated and associated precursor particles to cause the crosslinking reaction between the adduct and the polymer material of the precursor particles to occur in a shorter period of time, typically minutes. This heating step can be carried out using a number of conventional heating devices, including various ovens or dryers well known in the art.

Generally, heat curing can be carried out at a temperature above about 50.degree. C. for a period of time sufficient to complete the crosslinking reaction between the adduct and the polymer material of the precursor particles. The particular temperatures and times used in heat curing will depend upon the particular cationic amino-epichlorohydrin adduct used and the polymer material present in the precursor particles. If the cure temperature is too low, or the cure time too short, the reaction will not be sufficiently driven, resulting in macrostructures that have insufficient integrity and poor absorbency. If the cure temperature is too high, the absorbency of the precursor particles can be degraded or the network crosslinks of these precursor particles, depending upon the specific polymer materials used, can be degraded to such point that the resulting macrostructure is less useful for absorbing large quantities of liquids. In addition, if the cure time and temperatures are not appropriate, extractable levels of the resulting aggregates can be greater, thus increasing the incidence of that particular form of gel-blocking. Therefore, heat curing is generally carried out at a temperature in the range of front about 50.degree. to about 205.degree. C. for from about 1 to about 20 minutes. Preferably, heat curing is carried out at a temperature of from about 180.degree. to about 200.degree. C. for from about 5 to about 15 minutes. The actual time and temperatures used can vary depending upon the specific polymer materials used in making the precursor particles, the specific adducts used, the thickness or diameter of the macrostructure involved, and like factors.

The crosslinking reaction between the cationic amino-epichlorohydrin adduct and the polymer material of the precursor particles is sufficiently fast, even at ambient temperatures, such that it can be carried out in the absence of initiators and/or catalysts. However, an important factor relative to the reactivity of the amino-epichlorohydrin adduct is the pH of the treatment solution containing the adduct. Typically, the pH of the treatment solution is from about 4 to about 9, preferably from about 4 to about 6. Maintenance of the treatment solution at a pH within these ranges insures that the amino-epichlorohydrin adduct will be sufficiently reactive, even at ambient temperatures.

The physical association of the treated precursor particles needs to be maintained during the curing step so that, as crosslinking occurs, adjacent precursor particles become cohesively bonded together. If forces or stresses are sufficient to disassociate the precursor particles that are present during the crosslinking reaction, insufficient bonding of the precursor particles can occur. This can result in aggregates having poor structural integrity. The physical association of the precursor particles is typically maintained by insuring minimal dissociation forces or stresses are introduced during the curing step.

As previously noted, the steps in the method of the present invention for producing the macrostructures need not be carried out in any specific order, and can be carried out simultaneously. For example, the cationic amino-epichlorohydrin adduct can be applied simultaneously with the physical association of the precursor particles, shaped into a preferred shape and typically a desired density, and then the adduct reacted with the polymer material of the precursor particles, either immediately after the above steps are completed or after the aggregate has been left standing for a period of time, to simultaneously surface crosslink the precursor particles and form the aggregate macrostructure. Typically, the precursor particles are mixed or sprayed with a solution of the adduct, water, a humectant and/or coplasticizer (e.g., glycerol), and a hydrophilic organic solvent (e.g., methanol) to form an adhered together aggregate. The adduct, water, humectant/coplasticizer and hydrophilic organic solvent serve as the associating agent for the precursor particles, the adduct also serving as the crosslinking agent. The adhered aggregate (i.e. the associated precursor particles and the aqueous mixture) is subsequently shaped into a densified sheet by a combination of extruding and rolling techniques as described above. The adduct is subsequently reacted with the polymer material by ambient or heat curing to simultaneously cause crosslinking at the surface of the precursor particles and to form a cohesive interparticle bonded aggregate macrostructure.

Under certain conditions, especially if the treated precursor particles have been heat cured, the resultant macrostructures can be somewhat inflexible and potentially brittle. In such cases, the macrostructures can be made more flexible by treating it with a plasticizer. Suitable plasticizers include water, alone or in combination with the humectants/coplasticizers previously described, preferably glycerol. The plasticizer can be applied to the macrostructures in a number of different ways, including spraying, coating, atomizing, immersing, or dumping the plasticizer onto the macrostructure. Alternatively, in the case of water alone, the macrostructure can be placed in a high humidity environment (e.g., greater than 70% relative humidity). The amount of plasticizer applied to the macrostructure can be selected depending upon the specific plasticizer used, and the effects desired. Typically, the amount of plasticizer applied is from about 5 to about 100 parts by weight, preferably from about 5 to about 60 parts by weight, per 100 parts by weight of the macrostructure. A particularly preferred plasticizer comprises a mixture of glycerol and water in a weight ratio of from about 0.5:1 to about 2:1, preferably from about 0.8:1 to about 1.7:1.

As shown in FIGS. 1 through 4 and especially FIGS. 3 and 4, the macrostructures resulting from the method of the present invention have pores (the dark areas of the photomicrograph) between adjacent precursor particles. The pores are small interstices between adjacent precursor particles that allow the passage of liquid into the interior of the macrostructure. The pores are formed into the macrostructure because the precursor particles do not "fit" or pack tightly enough, even when compressed, to eliminate the pores. (The packing efficiency of the precursor particles is less than 1.) The pores are generally smaller than the constituent precursor particles and provide capillaries between the precursor particles to transport liquid into the interior of the macrostructure.

The pores are interconnected with each other by intercommunicating channels between the pores. The channels allow liquids contacting the macrostructure to be transported via capillary forces (i.e., capillary channels are formed) to other portions of the macrostructure so that the total volume of the macrostructure is used in absorbing such liquids. Further, when swollen, the pores and the intercommunicating channels allow liquids to pass through the macrostructure either to layers of precursor particles remote from the initial point of liquid contact or to other structures in contact with the macrostructure. Thus, the macrostructure is considered to be liquid permeable due to the pores and the intercommunicating channels.

The void fraction (i.e., the total volume of the macrostructure that comprises the pores and the channels) has a minimum value for a given precursor particle size distribution. In general, the narrower the precursor particle size distribution, the higher the void fraction will be. Thus, it is preferred, so as to provide higher void fractions in a densified state, that the precursor particles have a relatively narrow particle size distribution.

Another feature of the macrostructures of the present invention is that they swell generally isotropically, even under moderate confining pressures, when liquids are deposited onto or come into contact with them. Isotropic swelling is used herein to mean that the macrostructure swells generally equally in all directions when wetted. Isotropic swelling is an important property of the macrostructure because the precursor particles and the pores are able to maintain their relative geometry and spatial relationships even when swollen such that the existing capillary channels are maintained, if not enlarged, during use. (The pores and the precursor particles get larger during swelling.) Thus, the macrostructure can imbibe and/or transport through itself additional loadings of liquid while not gel blocking.

An indication that crosslink bonds are being formed at the surface of the precursor particles is that the resultant macrostructures are fluid (i.e., liquid) stable. "Fluid stable" is used herein to mean a macrostructure comprising an interparticle bonded aggregate that remains substantially intact (i.e., most of the previously independent component precursor particles remain bonded together) upon contact with or swelling (with and/or without stress) in an aqueous fluid. While this definition of fluid stability recognizes that most, preferably all, of the precursor particles remain bonded together, it, however, should be recognized that some of the precursor particles can dissociate themselves from the macrostructure if, for example, other particles have been subsequently water agglomerated onto it.

Fluid stability is an important feature of the macrostructures of the present invention because it allows the aggregate to maintain its relative structure in both the dry and swollen states, and because it immobilizes component precursor particles. In an end product such as an absorbent member or an absorbent article, fluid stability is beneficial in reducing gel blocking since precursor particles remain aggregated even when contacted with liquid, and allows one to use previously independent fine particles in an aggregate form to increase the rate of fluid uptake of the resultant macrostructure without introducing the element of gel blocking.

Fluid stability can be measured in an aggregate macrostructure by a two step process. The initial dynamic response of the aggregate macrostructure upon contact with the aqueous fluid is observed and then the fully swollen equilibrium condition of the aggregate macrostructure is observed. A test method for determining fluid stability based on these criteria is hereafter described in the Test Methods section.

In use, liquids that are deposited onto or come in contact with the macrostructures are imbibed by the precursor particles or are passed into the pores and transmitted to other portions of the macrostructure where they are imbibed by other precursor particles or transported through the macrostructure to other absorbent members adjacent thereto.

Various types of fiber material can be used as the reinforcing members in the macrostructures of the present invention. Any type of fiber material which is suitable for use in conventional absorbent products is also suitable for use in the macrostructures herein. Specific examples of such fiber material include cellulose fibers, modified cellulose fibers, rayon, polypropylene, and polyester fibers such as polyethylene terephthalate (DACRON), hydrophilic nylon (HYDROFIL), and the like. Examples of other fiber materials for use in the present invention in addition to some already discussed are hydrophilized hydrophobic fibers, such as surfactant-treated or silica-treated thermoplastic fibers derived, for example, from polyolefins such as polyethylene or polypropylene, polyacryl ics, polyamides, polystyrenes, polyurethanes and the like. In fact, hydrophilized hydrophobic fibers which are in and of themselves not very absorbent and which, therefore, do not provide webs of sufficient absorbent capacity to be useful in conventional absorbent structures, are suitable for use in the macrostructures of the present invention by virtue of their good wicking properties. This is because, in the macrostructures herein, the wicking propensity of the fibers is as important, if not more important, than the absorbent capacity of the fiber material itself due to the high rate of fluid uptake and lack of gel blocking properties of the macrostructures of the present invention. Synthetic fibers are generally preferred for use herein as the fiber component of the macrostructure. Most preferred are polyolefin fibers, preferably polyethylene fibers.

Other cellulosic fiber materials which can be useful in certain macrostructures herein are chemically stiffened cellulosic fibers. Preferred chemically stiffened cellulosic fibers are the stiffened, twisted, curled cellulosic fibers which can be produced by internally crosslinking cellulose fibers with a crosslinking agent. Suitable stiffened, twisted, curled cellulose fibers useful as the hydrophilic fiber material herein are described in greater detail in U.S. Pat. No. 4,888,093 (Dean et al), issued Dec. 19, 1989; U.S. Pat. No. 4,889,595 (Herron et al), issued Dec. 26, 1989; U.S. Pat. No. 4,889,596 (Schoggen et al), issued Dec. 26, 1989; U.S. Pat. No. 4,889,597 (Bourbon et al), issued Dec. 26, 1989; and U.S. Pat. No. 4,898,647 (Moore et al), issued Feb. 6, 1990, all of which are incorporated by reference.

As used herein, the term "hydrophilic" describes fibers or the surfaces of fibers which are wetted by the liquids deposited onto the fibers (i.e., if water or aqueous body fluid readily spreads on or over the surface of the fiber without regard to whether or not the fiber actually imbibes fluid or forms a gel). The state of the art respecting wetting of materials allows definition of hydrophobicity (and wetting) in terms of contact angles and the surface tension of the liquids and solids involved. This is discussed in detail in the American Chemical Society Publication entitled "Contact Angle, Wettability, and Adhesion" edited by Robert F. Gould and copyrighted in 1964. A fiber or surface of a fiber is said to be wetted by a liquid either when the contact angle between the liquid and the fiber or surface is less than 90.degree. or when the liquid will tend to spread spontaneously across the surface of the fiber; both conditions normally coexisting.

The fiber material can be added to the macrostructures by introducing the fibers into solution with the cationic amino-epichlorohydrin adduct, by mixing with the precursor particles prior to applying the adduct, or by adding the fiber material to the adduct/precursor particle mixture. For example, the fiber material can be kneaded into the adduct/precursor particle mixture. The fiber material is preferably thoroughly mixed with the solution so that the fiber material is uniformly dispersed throughout the macrostructure. The fibers are also preferably added before reacting the adduct with the polymer material of the precursor particles.

The relative amount of fiber material mixed with the precursor particles can vary widely. The fiber material is preferably added in a range from about 0.01 parts to about 50 parts, more preferably in the range of from about 0.5 parts to about 5 parts, by weight per 100 parts by weight of the precursor particles.

The porous, absorbent macrostructures can be used for many purposes in many fields of use. For example, the macrostructures can be used for packing containers; drug delivery devices; wound cleaning devices; burn treatment devices; ion exchange column materials; construction materials; agricultural or horticultural materials such as seed sheets or water-retentive materials; and industrial uses such as sludge or oil dewatering agents, materials for the prevention of dew formation, dessicants, and humidity control materials.

The porous, absorbent macrostructures of the present invention are useful when joined to a carrier. Carriers useful in the present invention include absorbent materials such as cellulose fibers. The carriers also can be any other carriers as are known in the art such as nonwoven webs, tissue webs, foams, polyacrylate fibers, apertured polymeric webs, synthetic fibers, metallic foils, elastomers, and the like. The macrostructures can be joined directly or indirectly to the carriers and can be joined thereto by chemical or physical bonding such as are known in the art, including adhesives or chemicals that react so as to adhere the macrostructures to the carriers.

Because of the unique absorbent properties of the porous, absorbent macrostructures of the present invention, they are especially suitable for use as absorbent cores in absorbent articles, especially disposable absorbent articles. As used herein, the term "absorbent article" refers to articles which absorb and contain body exudates and more specifically refers to articles which are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Additionally, "disposable" absorbent articles are those which are intended to be discarded after a single use (i.e., the original absorbent article in its whole is not intended to be laundered or otherwise restored or reused as an absorbent article, although certain materials or all of the absorbent article may be recycled, reused, or composted). A preferred embodiment of a disposable absorbent article, diaper 20, is shown in FIG. 5. As used herein, the term "diaper" refers to a garment generally worn by infants and incontinent persons that is worn about the lower torso of the wearer. It should be understood, however, that the present invention is also applicable to other absorbent articles such as incontinent briefs, incontinent pads, training pants, diaper inserts, sanitary napkins, facial tissues, paper towels, and the like.

FIG. 5 is a perspective view of the diaper 20 of the present invention in its uncontracted state (i.e., with all the elastic induced contraction removed) with portions of the structure being cut-away to more clearly show the construction of the diaper 20 and with the portion of the diaper 20 which contacts the wearer facing the viewer. The diaper 20 is shown in FIG. 5 to preferably comprise a liquid pervious topsheet 38; a liquid impervious backsheet 40 joined with the topsheet 38; an absorbent core 42 positioned between the topsheet 38 and the backsheet 40; elastic members 44; and tape tab fasteners 46. While the topsheet 38, the backsheet 40, the absorbent core 42, and the elastic members 44 can be assembled in a variety of well known configurations, a preferred diaper configuration is described generally in U.S. Pat. No. 3,860,003 (Buell), issued Jan. 14, 1975, which is incorporated by reference. Alternatively preferred configurations for disposable diapers herein are also disclosed in U.S. Pat. No. 4,808,178 (Aziz et al), issued Feb. 28, 1989; U.S. Pat. No. 4,695,278 (Lawson), issued Sep. 22, 1987; and U.S. Pat. No. 816,025 (Foreman), issued Mar. 28, 1989, all of which are incorporated by reference.

FIG. 5 shows a preferred embodiment of the diaper 20 in which the topsheet 38 and the backsheet 40 are co-extensive and have length and width dimensions generally larger than those of the absorbent core 42. The topsheet 38 is joined with and superimposed on the backsheet 40 thereby forming the periphery of the diaper 20. The periphery defines the outer perimeter or the edges of the diaper 20. The periphery comprises the end edges and the longitudinal edges 30.

The topsheet 38 is compliant, soft feeling, and non-irritating to the wearer's skin. Further, the topsheet 38 is liquid pervious permitting liquids to readily penetrate through its thickness. A suitable topsheet 38 can be manufactured from a wide range of materials such as porous foams, reticulated foams, apertured plastic films, natural fibers (e.g., wood or cotton fibers), synthetic fibers (e.g., polyester or polypropylene fibers) or from a combination of natural and synthetic fibers. Preferably, the topsheet 38 is made of a hydrophobic material to isolate the wearer's skin from liquids in the absorbent core 42.

A particularly preferred topsheet 38 comprises staple length polypropylene fibers having a denier of about 1.5, such as Hercules type 151 polypropylene marketed by Hercules, Inc. of Wilmington, Del. As used herein, the term "staple length fibers" refers to those fibers having a length of at least about 15.9 mm (0.62 inches).

There are a number of manufacturing techniques which can be used to manufacture the topsheet 38. For example, the topsheet 38 can be woven, nonwoven, spunbonded, carded or the like A preferred topsheet is carded, and thermally bonded by means well known to those skilled in the fabrics art. Preferably, the topsheet 38 has a weight from about 18 to about 25 grams per square meter, a minimum dry tensile strength of at least about 400 grams per centimeter in the machine direction, and a wet tensile strength of at least about 55 grams per centimeter in the cross-machine direction.

The backsheet 40 is impervious to liquids and is preferably manufactured from a thin plastic film, although other flexible liquid impervious materials may also be used. The backsheet 40 prevents the exudates absorbed and contained in the absorbent core 42 from wetting articles which contact the diaper 20 such as bedsheets and undergarments. Preferably, the backsheet 40 is polyethylene film having a thickness from about 0.012 mm (0.5 mil) to about 0.051 centimeters (2.0 mils), although other flexible, liquid impervious materials can be used. As used herein, the term "flexible" refers to materials which are compliant and which will readily conform to the general shape and contours of the wearer's body.

A suitable polyethylene film is manufactured by Monsanto Chemical Corporation and marketed in the trade as Film No. 8020. The backsheet 40 is preferably embossed and/or matte finished to provide a more clothlike appearance. Further, the backsheet 40 may permit vapors to escape from the absorbent core 42 while still preventing exudates from passing through the backsheet 40.

The size of the backsheet 40 is dictated by the size of the absorbent core 42 and the exact diaper design selected. In a preferred embodiment, the backsheet 40 has a modified hourglass-shape extending beyond the absorbent core 42 a minimum distance of at least about 1.3 centimeters to about 2.5 centimeters (about 0.5 to about 1.0 inch) around the entire diaper periphery.

The topsheet 38 and the backsheet 40 are joined together in any suitable manner. As used herein, the term "joined" encompasses configurations whereby the topsheet 38 is directly joined to the backsheet 40 by affixing the topsheet 38 directly to the backsheet 40, and configurations whereby the topsheet 38 is indirectly joined to the backsheet 40 by affixing the topsheet 38 to intermediate members which in turn are affixed to the backsheet 40. In a preferred embodiment, the topsheet 38 and the backsheet 40 are affixed directly to each other in the diaper periphery by attachment means (not shown) such as an adhesive or any other attachment means as known in the art. For example, a uniform continuous layer of adhesive, a patterned layer of adhesive, or an array of separate lines or spots of adhesive can be used to affix the topsheet 38 to the backsheet 40.

Tape tab fasteners 46 are typically applied to the back waistband region of the diaper 20 to provide a fastening means for holding the diaper on the wearer. The tape tab fasteners 46 can be any of those well known in the art, such as the fastening tape disclosed in U.S. Pat. No. 3,848,594 (Buell), issued Nov. 19, 1974, which is incorporated by reference. These tape tab fasteners 46 or other diaper fastening means are typically applied near the corners of the diaper 20.

The elastic members 44 are disposed adjacent the periphery of the diaper 20, preferably along each longitudinal edge 30, so that the elastic members 44 tend to draw and hold the diaper 20 against the legs of the wearer. Alternatively, the elastic members 44 can be disposed adjacent either or both of the end edges 32 of the diaper 20 to provide a waistband as well as or rather than leg cuffs. For example, a suitable waistband is disclosed in U.S. Pat. No. 4,515,595 (Kievit et al), issued May 7, 1985, which is incorporated by reference. In addition, a method and apparatus suitable for manufacturing a disposable diaper having elastically contractible elastic members is described in U.S. Pat. No. 4,081,301 (Buell), issued Mar. 28, 1978, which is incorporated by reference.

The elastic members 44 are secured to the diaper 20 in an elastically contractible condition so that in a normally unrestrained configuration, the elastic members 44 effectively contract or gather the diaper 20. The elastic members 44 can be secured in an elastically contractible condition in at least two ways. For example, the elastic members 44 can be stretched and secured while the diaper 20 is in an uncontracted condition. Alternatively, the diaper 20 can be contracted, for example, by pleating, and the elastic members 44 secured and connected to the diaper 20 while the elastic members 44 are in their unrelaxed or unstretched condition.

In the embodiment illustrated in FIG. 5, the elastic members 44 extend along a portion of the length of the diaper 20. Alternatively, the elastic members 44 can extend the entire length of the diaper 20, or any other length suitable to provide an elastically contractible line. The length of the elastic members 44 is dictated by the diaper design.

The elastic members 44 can be in a multitude of configurations. For example, the width of the elastic members 44 can be varied from about 0.25 millimeters (0.01 inches) to about 25 millimeters (1.0 inch) or more; the elastic members 44 can comprise a single strand of elastic material or can comprise several parallel or non-parallel strands of elastic material; or the elastic members 44 can be rectangular or curvilinear. Still further, the elastic members 44 can be affixed to the diaper in any of several ways which are known in the art. For example, the elastic members 44 can be ultrasonically bonded, heat and pressure sealed into the diaper 20 using a variety of bonding patterns or the elastic members 44 can simply be glued to the diaper 20.

The absorbent core 42 of the diaper 20 is positioned between the topsheet 38 and the backsheet 40. The absorbent core 42 can be manufactured in a wide variety of sizes and shapes (e.g., rectangular, hourglass, asymmetrical, etc.) and from a wide variety of materials. The total absorbent capacity of the absorbent core 42 should, however, be compatible with the design liquid loading for the intended use of the absorbent article or diaper. Further, the size and absorbent capacity of the absorbent core 42 can vary to accommodate wearers ranging from infants through adults. The absorbent core 42 comprises the porous, absorbent macrostructures of the present invention.

A preferred embodiment of the diaper 20 has a rectangular-shaped absorbent core 42. As shown in FIG. 6, the absorbent core 42 preferably comprises an absorbent member 48 comprising an envelope web 50 and a porous, absorbent macrostructure 52 disposed in the envelope web 50. The macrostructure 52 is encased in the envelope web 50 to minimize the potential for the precursor particles to migrate through the topsheet and to provide an additional liquid transport layer between the topsheet 38 and the macrostructure 52 to enhance liquid acquisition and minimize rewet. As shown in FIG. 6, a single envelope web 50 is wrapped about the macrostructure 52 by folding to form a first layer 54 and a second layer 56. The edges 58 of the envelope web 50 are sealed about its periphery by any conventional means such as an adhesive 59 (as shown), ultrasonic bonds, or heat/pressure bonds, to form a pouch. The envelope web 50 can comprise a number of materials including nonwoven webs, paper webs, or webs of absorbent materials such as tissue paper. The envelope web 50 preferably comprises a nonwoven web similar to the webs used to form the topsheet 38. The Donwoven web is preferably hydrophilic to allow liquids to rapidly pass through the envelope web 50. Similar layered absorbent members (laminates) are more fully described in U.S. Pat. No. 4,578,068 (Kramer et al), issued Mar. 25, 1986, which is incorporated by reference.

Alternatively, the absorbent cores 42 of the present invention can consist solely of one or more (a plurality of the) porous, absorbent macrostructures of the present invention; can comprise a combination of layers including the macrostructures of the present invention; or any other absorbent core configurations including one or more of the macrostructures of the present invention.

FIG. 7 shows an alternative embodiment of the diaper 120 comprising a dual-layer absorbent core 142 comprising a modified hourglass-shaped absorbent member 60 and a sheet 62 of the porous, absorbent macrostructure positioned subjacent the absorbent member 60 (i.e., between the absorbent member 60 and the backsheet 40).

The absorbent member 60 serves to quickly collect and temporarily hold discharged liquids and to transport such liquids by wicking from the point of initial contact to other parts of the absorbent member 60 and to the macrostructure sheet 62. The absorbent member 60 preferably comprises a web or batt of fiber materials. Various types of fiber material can be used i n the absorbent member 60 such as the fiber materials previously discussed herein. Cellulosic fibers are generally preferred for use herein, wood pulp fibers being especially preferred. The absorbent member 60 can also contain specific amounts of a particulate, absorbent, polymeric composition. The absorbent member 60, for example, can contain up to about 50% by its weight of the polymeric composition. In the most preferred embodiments, the absorbent member 60 contains from 0% to about 8% by its weight of a particulate, absorbent, polymeric composition. In alternatively preferred embodiments, the absorbent member 60 comprises chemically stiffened cellulosic fibers as previously discussed herein. Exemplary embodiments of the absorbent member 60 useful in the present invention are described in U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987; and U.S. Pat. No. 4,834,735 (Alemany et al) issued May 30 1989, both of which are incorporated by reference. Absorbent members having a storage zone and an acquisition zone having a lower average density and a lower average basis weight per unit area than the storage zone so that the acquisition zone can effectively and efficiently rapidly acquire discharged liquid are especially preferred for use herein.

The absorbent member 60 can be of any desired shape, for example, rectangular, oval, oblong, asymmetric or hourglass-shaped. The shape of the absorbent member 60 can define the general shape of the resulting diaper 120. In the preferred embodiments as shown in FIG. 7, the absorbent member 60 is hourglass-shaped.

The macrostructure sheet 62 of the present invention need not be the same size as the absorbent member 60 and can, in fact, have a top surface which is substantially smaller or larger than the top surface area of the absorbent member 60. As shown in FIG. 7, the macrostructure sheet 62 is smaller than the absorbent member 60 and has a top surface area from about 0.10 to about 1.0 times that of the absorbent member 60. Most preferably, the top surface area of the macrostructure sheet 62 will be only from about 0.10 to about 0.75, and most preferably from about 0.10 to about 0.5 times that of the absorbent member 60. In an alternative embodiment, the absorbent member 60 is smaller than the macrostructure sheet 62 and has a top surface area from about 0.25 to about 1.0 times, more preferably from about 0.3 to about 0.95 times that of the macrostructure sheet 62. In this alternative embodiment, the absorbent member 60 preferably comprises chemically stiffened cellulosic fibers, as previously described.

The macrostructure sheet 62 is preferably placed in a specific positional relationship with respect to the backsheet 40 and/or the absorbent member 60 in the diaper. More particularly, the macrostructure sheet 62 is positioned generally toward the front of the diaper so that the macrostructure sheet 62 is most effectively located to acquire and hold discharged liquids.

In alternatively preferred embodiments, a plurality of macrostructures, preferably from two to six macrostructure strips or sheets, can be substituted for the single macrostructure sheet 62 shown in FIG. 7. Further, additional absorbent layers, members, or structures can be placed into the absorbent core 142. For example, an additional absorbent member can be positioned between the macrostructure sheet 62 and the backsheet 40 to provide reserve capacity for the absorbent core 142 and/or a layer to distribute liquids passing through the macrostructure sheet 62 to other portions of the absorbent core 142 or to the macrostructure sheet 62. The macrostructure sheet 62 can also alternatively be positioned over the absorbent member 60 so as to be positioned between the topsheet 38 and the absorbent member 60.

FIG. 8 shows an alternative embodiment of a diaper 220 comprising an alternative dual-layer absorbent core 242 comprising a rectangular shaped absorbent member 260 and three elongated parallel spaced macrostructure strips 262 positioned between absorbent member 260 and backsheet 40.

The absorbent member 260 serves to quickly collect and temporarily hold discharged liquids and to transport such liquids by wicking from the point of initial contact to other parts of the absorbent member 260 and to macrostructure strips 262. This absorbent member 260 preferably comprises a web or bat of fiber materials, most preferably chemically stiffened cellulosic fibers as previously discussed herein. Macrostructure strips 262 together act to acquire and hold the discharged liquids. By spacing macrostructure strips 262 from one another, a more effective surface area is presented for acquiring and holding the discharge liquids. This is particularly true since the spaced macrostructure strips 262 can swell and expand in the direction of their widths without interfering with the ability of adjacent strips to acquire discharged liquids.

In use, the diaper 20 is applied to a wearer by positioning the back waistband region under the wearer's back, and drawing the reminder of the diaper 20 between the wearer's legs so that the front waistband region is positioned across the front of the wearer. The tape-tab fasteners 46 are then secured preferably to outwardly facing areas of the diaper 20. In use, disposable diapers or other absorbent articles incorporating the porous, absorbent macrostructures of the present invention tend to more quickly and efficiently distribute and store liquids and to remain dry due to the high absorbent capacity of the macrostructures. Disposable diapers incorporating the macrostructures of the present invention can also be thinner and more flexible.

The specific synthetic urine used in the test methods of the present invention is referred to herein as "Synthetic Urine". The Synthetic Urine is commonly known as Jayco SynUrine and is available from Jayco Pharmaceuticals Company of Camp Hill, Pa. The formula for the Synthetic Urine is: 2.0 g/l of KCl; 2.0 g/l of Na.sub.2 SO.sub.4 ; 0.85 g/l of (NH.sub.4)H.sub.2 PO.sub.4 ; 0.15 g/l (NH.sub.4).sub.2 HPO.sub.4 ; 0.19 g/l of CaCl.sub.2 and 0.23 g/l of MgCl.sub.2. All of the chemicals are of reagent grade. The pH of the Synthetic Urine is in the range of 6.0 to 6.4.

The polymeric composition is placed within a "tea bag", immersed in an excess of Synthetic Urine for a specified period of time, and then centrifuged for a specific period of time. The ratio of polymeric composition final weight after centrifuging minus initial weight (net fluid gain) to initial weight determines the Absorptive Capacity.

The following procedure is conducted under standard laboratory conditions at 23.degree. C. (73.degree. F.) and 50% relative humidity. Using a 6 cm.times.12 cm cutting die, the tea bag material is cut, folded in half lengthwise and sealed along two sides with a T-bar sealer to produce a 6 cm.times.6 cm tea bag square. The tea bag material utilized is a grade 1234 heat sealable material, obtainable from C. H. Dexter, Division of the Dexter Corp., Windsor Locks, Conn., U.S.A., or equivalent. Lower porosity tea bag material should be used if required to retain fine particles. 0.200 grams plus or minus 0.005 grams of the polymeric composition is weighed onto a weighing paper and transferred into the tea bag, and the top (open end) of the tea bag is sealed. An empty tea bag is sealed at the top and is used as a blank. Approximately 300 milliliters of Synthetic Urine are poured into a 1,000 millimeter beaker. The blank tea bag i s submerged in the Synthetic Urine. The tea bag containing the polymeric composition (the sample tea bag) is held horizontally to distribute the material evenly throughout the tea bag. The tea bag is laid on the surface of the Synthetic Urine. The tea bag is allowed to wet, for a period of no more than one minute, and then is fully submerged and soaked for 60 minutes. Approximately 2 minutes after the first sample is submerged, a second set of tea bags, prepared identically to the first set of blank and sample tea bags, is submerged and soaked for 60 minutes in the same manner as the first set. After the prescribed soak time is elapsed, for each set of tea bag samples, the tea bags are promptly removed (using tongs) from the Synthetic Urine. The samples are then centrifuged as described below. The centrifuge used is a Delux Dynac II Centrifuge, Fisher Model No. 05-100-26, obtainable from Fisher Scientific Co. of Pittsburgh, Pa., or equivalent. The centrifuge should be equipped with a direct read tachometer and an electric brake. The centrifuge is further equipped with a cylindrical insert basket having an approximately 2.5 inch (6.35 cm) high outer wall with an 8.435 inch (21.425 cm) outer diameter, a 7.935 inch (20.155 cm) inside diameter, and 9 rows each of approximately 106 3/32 inch (0.238 cm) diameter circular holes equally spaced around the circumference of the outer wall, and having a basket floor with six 1/4 inch (0.635) cm) diameter circular drainage holes equally spaced around the circumference of the basket floor at a distance of 1/2 inch (1.27 cm) from the interior surface of the outer wall to the center of the drainage holes, or an equivalent. The basket is mounted in the centrifuge so as to rotate, as well as brake, in unison with the centrifuge. The sample tea bags are positioned in the centrifuge basket with a folded end of the tea bag in the direction of the centrifuge spin to absorb the initial force. The blank tea bags are placed to either side of the corresponding sample tea bags. The sample tea bag of the second set must be placed opposite the sample tea bag of the first set; and the blank tea bag of the second set opposite the blank tea bag of the first set, to balance the centrifuge. The centrifuge is started and allowed to ramp up quickly to a stable speed of 1,500 rpm. Once the centrifuge has been stabilized at 1,500 rpm, a timer is set for 3 minutes. After 3 minutes, the centrifuge is turned off and the brake is applied. The first sample tea bag and the first blank tea bag are removed and weighed separately. The procedure is repeated for the second sample tea bag and the second blank tea bag. The Absorptive Capacity (ac) for each of the samples is calculated as follows: ac=(sample tea bag weight after centrifuge minus blank tea bag weight after centrifuge minus dry polymeric composition weight) divided by (dry polymeric composition weight). The Absorptive Capacity value for use herein is the average Absorptive Capacity of the two samples.

The objective of this method is to determine the stability of an aggregate upon exposure to Synthetic Urine.

The sample macrostructure is placed in a shallow dish. An excess amount of Synthetic Urine is added to the macrostructure. The swelling of the macrostructure is observed until equilibrium is reached. During the observation of the swelling macrostructure, the macrostructure is observed for small particles breaking off from the main aggregate, platelet-like particles floating away from the main aggregate, or particle expansion only in the two dimensional x-y plane with particles breaking and floating away from the main aggregate. If the aggregate has a large number of broken away component particles, the macrostructure is considered unstable. The macrostructure should also be observed for isotropic swelling. If the aggregate remains relatively stable and the relative geometry and spatial relationships of the precursor particles and the pores are maintained after the test procedure, the macrostructure is considered stable. Preferably, fluid stable macrostructures are capable of being picked up in their swollen state without breaking apart.

The particle size distribution on a weight percent basis of a 10 gram bulk sample of the precursor particles is determined by sieving the sample through a set of 19 sieves ranging in size from a standard #20 sieve (850 microns) through a standard #400 sieve (38 microns). The sieves are standard sieves as obtainable from the Gilson Company, Inc. of Worthington, Ohio. The procedure is carried out on three stacks of sieves at a time since the equipment used cannot hold all 19 sieves at one time. A first stack contains sieves #20, 25, 30, 35, 40, 45, and 50 plus the sieve pan; the second stack contains sieves #60, 70, 80, 100, 120, and 140 plus the sieve pan; the third stack contains sieves #170, 200, 230, 270, 325, and 400 plus the sieve pan. The precursor particles remaining on each of these sieves are then weighed to determine the particle size distribution on a weight percent basis.

The first stack of sieves is mounted on a shaker and 10.0 grams plus or minus 0.00 grams of the sample is placed on the #20 sieve. The shaker used is a Vibratory 3-inch Sieve Shaker Model SS-5 as obtainable from the Gilson Company, Inc. of Worthington, Ohio. The stack is shaken for 3 minutes at approximately 2100 vibrations per minute ("6" on the instrument dial). The sieve pan is then removed and the stack set aside for later weighing. Using a soft brush, the sample remaining on the sieve pan is transferred onto a weighing paper. The second stack of sieves is mounted on the shaker and the sample on the weighing paper is transferred onto the #60 sieve. The second stack is shaken for 3 minutes at approximately 2100 vibrations per minute, the sample remaining on the sieve pan being transferred to a weighing paper and the stack set aside. The third stack of sieves is mounted on the shaker and the sample on the weighing paper is transferred onto the #170 sieve. The third stack is shaken for 3 minutes at approximately 2100 vibrations per minute. A soft brush is used to transfer the contents of each given sieve onto a tared weighing paper. The sample is weighed on a standard three place scale and the weight of the sample on the specific sieve is recorded. This step is repeated, using a fresh weighing paper for each sample, for each sieve, and for the sample remaining on the sieve pan after the third stack of sieves has been shaken. The method is repeated for two additional 10 gram samples. The average of the weights of the three samples for each sieve determine the average particle size distribution on a weight percent basis for each sieve size.

The Mass Average Particle Size of the 10 gram bulk sample is calculated as follows: ##EQU1## wherein maps is the mass average particle size; Mi is the weight of the particles on the specific sieve; and D.sub.i is the "size parameter" for the specific sieve. The size parameter, D.sub.i of a sieve is defined to mean the size (in microns) of the next highest sieve. For example, a standard #50 sieve has a size parameter of 355 microns, which corresponds to the size of the openings in a standard #45 sieve (the next highest sieve). The Mass Average Particle Size for use herein is the average of the mass average particle size of the three samples.

A jacketed 10 liter twin arm stainless steel kneader measuring 220 mm.times.240 mm in the opening and 240 mm in depth, and having two Sigma type blades possessing a rotational diameter of 120 mm is sealed with a lid. An aqueous monomer solution is prepared consisting of 37 weight % monomer. The monomer consists of 75 mole % sodium acrylate and 25 mole % acrylic acid. 5500 grams of the aqueous monomer solution is charged to the kneader vessel, which is subsequently purged with nitrogen gas to remove the remaining entrapped air. Then, the two Sigma type blades are set rotating at rates of 46 rpm and the jacket is heated by the passage of 35.degree. C. water. 2.8 g of sodium persulfate and 0.14 g of L-ascorbic acid are added as polymerization initiators. Polymerization begins about four minutes after the addition of the initiators. A peak temperature of 82.degree. C. is reached inside the reaction system 15 minutes after the addition of the initiators. The hydrated gel polymer is divided into particles about 5 mm in size as the stirring is continued. The lid is removed from the kneader 60 minutes after the start of the polymerization and the material is removed from the kneader.

The resultant hydrated aqueous gel polymer thus obtained is spread on a standard #50 size metal gauze and dried with hot air at 150.degree. C. for 90 minutes. The dried particles are pulverized with a hammer type crusher and sifted with a standard #20 sieve 850 microns) to obtain particles that pass through the standard #20 sieve. The mass average particle size of these particles is 405 microns.

One hundred parts of precursor particles made in accordance with the Precursor Particle Example are placed into a 5 quart standing kitchen-type mixer. The precursor particles have a particle size such that the precursor particles pass through a standard No. 50 sieve (300 microns) and are retained on a standard No. 100 sieve (150 microns). An aqueous treatment solution is prepared from a mixture of 4.3 parts Kymene Plus (30% resin active), 2.6 parts water and 10.0 parts methanol. This treatment solution is sprayed onto the precursor particles with a Preval sprayer (available from The Precision Valve Corporation of Yonkers, N.Y.). The treatment solution is sprayed onto the precursor particles, while the mixer is operating at slow speed, for a period of about 4 minutes, i.e. until all of the solution is sprayed onto the particles. After spraying, the mixture of wet precursor particles is mixed at the highest speed setting for 2 to 5 minutes. During this high speed mixing, the methanol is evaporated, thus increasing the stickiness of the treated mixture of precursor particles so that they will remain adhered together. This sticky mixture of treated precursor particles is then fed to an extrusion/compaction unit. The extruder screw has a length of 8 inches (20.3 cm) and contains 5 flights, each flight being 1.5 inches (3.8 cm) in length. The outside diameter of the extruder screw is 1.75 inches (4.45 cm) and the screw-to-housing clearance is 0.20 inches (0.51 cm). The unit is activated such that the extruder screw turns at a rate of 47 rpm. The mixture is extruded between two smooth finish steel compaction rolls (nip rolls) with a fixed (but variable) gap. The compaction rolls have a diameter of 8.975 inches (22.8 cm) and are driven at a rate of 5.4 rpm. The gap between the compaction rolls is 0.015 inches (0.38 mm). The formed aggregate sheets are then separated into approximately 12 to 15 inch (30 to 40 cm) lengths. The resultant aggregate sheets are heated in a forced air convection oven at 200.degree. C. for about 10 minutes so that the Kymene Plus reacts with the polymer material in the surface of the precursor particles, thus causing effective crosslinking. The oven-cured sheets have a thickness (caliper) of about 0.031 inches (0.8 mm) and a width of about 1.95 inches (4.95 cm). A plasticizer solution containing 65 parts glycerol and 35 parts distilled water is sprayed onto the oven-cured sheets at the rate of 0.9 g. of plasticizer solution, per 1.0 g. of the oven-cured sheet. About 1/2 hour after treatment with the plasticizer solution, the sheets have sufficient flexibility and tensile strength to be picked up.

In this example, 100 parts of precursor particles made in accordance with the precursor particle example and having the particle size characteristics described in Example 1 are used. An aqueous treatment solution prepared from a mixture of 6.0 parts Kymene Plus (30% resin active), 3.5 parts water and 8.5 parts glycerol is also used.

A reciprocating table or shuttle is used in conjunction with a pair of sprayers that apply the treatment solution and a vibratory feeder that deposits the precursor particles. The sprayers and feeder are positioned above the reciprocating surface of the table. As the surface of this table moves underneath the sprayers, the treatment solution is sprayed onto the table surface (or layer of particles) in a predetermined pattern. As the surface of the table moves further in the same direction and underneath the feeder, a predetermined amount of precursor particles are deposited onto the table surface or previous layer of treated particles. After the particles have been deposited from the feeder to form a layer thereof, the surface of the table moves back in the opposite direction so that the sequence of applying treatment solution/depositing a layer of particles can be repeated.

Initially, a predetermined amount of the treatment solution is sprayed onto the moving surface of the table. After the surface of the table has been initially sprayed with treatment solution, five layers of precursor particles (0.2 g./in.sup.2 of particles per layer) are deposited from the feeder. After each layer of precursor particles has been deposited, a predetermined amount of the treatment solution is sprayed on top of each layer. The amount of treatment solution sprayed initially onto the surface of the table, as well as the first layer of precursor particles, is about 0.018 g/in.sup.2. The amount of treatment solution sprayed onto the other four layers of precursor particles is about 0.036 g/in.sup.2. In effect, each layer of precursor particles is treated with the same amount of solution.

After the layering of precursor particles and spraying with treatment solution is complete, a relatively cohesive sheet of particles is formed. This cohesive sheet is then fed by a belt to a compaction unit. The compaction unit consists of two coated steel compaction rolls (nip rolls) with a fixed (but variable) gap. The compaction rolls have a diameter of about 8 inches (20 cm) and are driven at a rate of about 20 rpm. The gap between the compaction rolls is 0.035-0.040 inches (0.9-1.0 mm). The resultant aggregate sheets (density of 0.9-1.0 g/cc) are stored in plastic bags at ambient room temperature (about 65.degree.-72.degree. F., 18.3.degree.-22.2.degree. C.) for about 24 hours. During this ambient temperature curing, the Kymene Plus reacts with the polymer material in the surface of the precursor particles, thus causing effective crosslinking. The ambient temperature cured sheets have a thickness (caliper) of about 0.050-0.060 inches (1.3-1.5 mm) and a width of about 4 inches (10 cm). These ambient temperature cured sheets have sufficient flexibility and tensile strength to be handled without breaking or tearing.

In this example, apparatus 301 shown in FIG. 9 is used. The precursor particles used are made in accordance with the precursor particle example and have a size between 150-250 microns. An aqueous treatment solution is prepared from a mixture of 5.0 parts Kymene Plus (30% resin active), 7.1 parts of water and 12.7 parts glycerol. Feeders 305 are Super Feeder model #210 SE-00354 vibrating feeders, available from Solids Flow Control, of Charlotte, N.C. Sprayers 304 are model 6218-1/4 JAU atomized air actuated nozzle assemblies, available from Spraying Systems, Co., of Wheaton, Ill. For the first two applications, sprayers 304a and 304b deliver the treatment solution to conveyor 303 at a rate of 39.8 grams/min. For subsequent applications, sprayers 304c through 304f deliver the treatment solution to conveyor 303 at a rate of 79.6 grams/min. Conveyor 303 is a moving conveyor made from polyurethane, and travels at a speed of 27 ft./min. The pressure applicators are a pair of compaction rolls 306 having 8 inch (20 cm) diameters and being 12 inches (30.5 cm) wide. The top and bottom rolls 306 are coated with a #934 Plasma Coating, available from Plasma Coatings, Inc., of Waterbury, Conn.

This example is carried out according to the following steps:

STEP 1: Initially spray a predetermined area of the conveyor with treatment solution in an amount substantially equal to 0.025 grams of solution per square inch of the conveyor.

STEP 2: Layer substantially continuously 0.2 grams of precursor particles per square inch of the conveyor onto the same predetermined area.

STEP 3: The first layer of precursor particles on the predetermined area of the conveyor is sprayed with treatment solution in an amount substantially equal to 0.025 grams of solution per square inch of conveyor.

STEP 4: Layer substantially continuously 0.2 grams of precursor particles per square inch of the conveyor onto the same predetermined area.

STEP 5: The second layer of precursor particles on the predetermined area of the conveyor is sprayed with treatment solution in an amount substantially equal to 0.050 grams of solution per square inch of the conveyor.

STEP 6: Steps 4 and 5 are repeated, in order, 3 more times, giving: (a) a total of one initial spraying step and five post-layering spraying steps for a total of 0.25 grams of treatment solution per square inch of the conveyor; and (b) a total of five layering steps for a total of 1 gram of precursor particles per square inch of the conveyor. A web is now formed.

STEP 7: The web is passed through the compaction rolls. The gap between the compaction rolls is 0.035 inches (0.9 mm). This produces a sheet having a density of 0.995 g/cc.

STEP 8: The sheet is cured by placing it in a plastic bag and allowing it to sit at ambient temperature (72.degree. F., 22.2.degree. C.) for 48 hours.

The resultant sheet has good flexibility, gel blocking and wet integrity properties.