ABSORBENT PRODUCTS WITH IMPROVED VERTICAL WICKING AND REWET CAPABILITY

Abstract: The present invention relates to an absorbent core for use in an absorbent article and methods to make the absorbent core and absorbent articles. The absorbent core has a discrete wicking layer made with compressible fibers such as eucalyptus or chemically treated fibers. The absorbent core and articles made with it have improved vertical wicking capacity and superior rewet characteristics.

Description: ABSORBENT PRODUCTS W ITH IMPROVED VERTICAL WICKING AND REWET CAPABILITY This application claims benefit of the filing date of U S Sei Nos 09/495,530 filed lanuan 31. 2000 and 60 21 1.091. filed June 12. 2000. each of which is hereby incorporated m its entπety FIELD OF THE INVENTION The piesent ention relates to disposable absorbent hygiene products like diapers, sanitary napkins, surgical drapes, wound dressings, and to absorbent cores foi use therein, and to a process for making these products with enhanced softness and fluid- handling capability \4ore particularly, this invention 1 elates to a multi-layer airlaid absorbent structure tng a discrete layei of compiessible fibrous wicking material placed between a moisture impermeable backsheet and a fluid storage layei, and to a continuous airlaid process foi the production thereof BACKGROUND OF THE INVENTION Disposable absorbent hygiene products such as premium baby diapers, training pants, adult incontinence de\ ices and feminine napkins are typically made with a cellulose fiber fluff-based absorbent core sandwiched between a liquid pervious top sheet and a low density acquisition or surge layer whose function is to allow the tempoiary storage and unobstructed passage of fluid to the absorbent core while acting as a barrier to the retransfer of liquid back to the skin of the user, and a liquid impervious backing sheet usually of plastic material, whose function is to contain the absorbed fluid and prevent it from passing through the absorbent core and soiling the undergarments of the wearer of the absorbent article The acquisition layer typically comprises chemically stiffened cellulose fluff or bonded synthetic fibers, wherein the bonding is through the medium of thermoplastic binder fibers or powder or via the application of a latex binder The absorbent core of these absorbent articles is usually constructed of defibeπzed wood pulp with or without superabsorbent polymer granules The absorbent core is typically formed on a pad-forming unit of a converting machine on a carrier tissue to facilitate processing. Some absorbent core forming units are equipped with layering capability in which a second discrete fluff layer may be laid ovei a pπmaiy fluff-based absoibent layer to fom a multi-layer absorbent coie In these absorbent coies, the pπmai y layer may include superabsorbent polymei granules With legard to conventionally produced absorbent cores, reference is made to U S Patent Nos 5,378 528, 5,128,082, 5,607,414, 5,147,343 5,149,335, 5.522,810, 5,041 ,104, 5 176,668 5,389.181 , and 4,596,567 In legard to the supei absorbent polymei component of absorbent cores, it is known from U S Pat Nos 3,669,103 and 3,670,731 to crosslink caiboxyhc polyelectrolytes to create hydrogel-formmg materials, now commonly leferred to as superabsorbents, and to use such materials to enhance the absorbency of disposable absoibent articles Fluid distribution, which is defined as wicking in both the horizontal and vertical planes, is dependent on effective capillary diametei, among othei factois It is well known that the effective size of capillaries formed between adjacent fibeis in a fibious stiuctuie is determined by fiber size and the density or extent of compaction of the fibious stiucture In U S Patent No 5,647,863, a combined storage and distribution member of an absorbent core is disclosed, that has higher capillary suction than a fluid acquisition member and a stain-mdicatmg member The storage/distribution layer not only draws the fluid insult out of the acquisition layer, but also preferably distributes the insult more in the longitudinal direction of the core than toward the sides This preferential movement of the fluid is a result of alignment of the fibrous materials in the machine direction during wet processing Wetlaid tn-component assemblies of chemically stiffened, cuiled, bulking fibers with high surface area fibers like eucalyptus and chemical binders oi thermal bonding fibeis, as disclosed m U S Patent Nos 5,549,589, 5,800,416, and 5,843,055, are especially preferred for the distribution/storage layer of U S Patent No 5,647,863 In U S Patent No 5,009,650, a multi-layered cellulose fluff absorbent structure is disclosed in which the layers of cellulosic fiber differ in density or average pore size In one aspect of the '650 disclosure, a separately-formed higher density or smaller pore size layer underlies at least a portion of a lower density layer to wick fluid from the fluid insult target area and transfer some of the fluid back to the lower density layer Naturally, only those pores in the lower density layer which are immediately adjacent to the higher density layer and of a smaller pore size than the largest pores in the higher density layei will accept fluid from the highei density layer Since the low ei density layer is mamly devoid of fluid, rewet problems aie lessened Rewet occuis when fluid is expressed under pressure back through the diapei top sheet and contacts the skin of the w eaiei While this wicking/stoiage layer can be achieλ ed using the same fibeis as in the lowei density layei by simply inci easing the density of the wicking layei it can also be obtained by using smaller diameter fibers so the pore size between fibers is smaller even without increasing the density \ anous hardwood fluff fibers, including eucalyptus, are suggested for the lower (wickmg/storage) layer In anothei aspect of U S Patent No 5,009,650, a higher density layer containing supei absorbent granules is placed undei the low-density fluff layer The wickmg/stotage layer comprises superabsoibent polymer gianules mixed w ith fibei and sandwiched between high-density fluff layers U S Patent No 5,009 650 illustrates seveial product designs intended to achieve fluid distnbution m absorbent products However, in each case, fluid distribution and ultimate fluid storage occui in the higher density layer, which may contain superabsorbent polymer As a different way of controlling capillary size, a gradient of fiber sizes fiom large at the top to small at the bottom is disclosed m U S Patent No 4,223,677 for fluid tiansfei into a disposable diapei In an entirely different approach, U S Patent No 5,188,624 teaches a liquid dispersion layer between an absorbent core and a backing sheet which the liquid dispersion layer has a lower density than the insert An enhanced vertical wicking rate is alleged for such a construction In like manner, U S Patent No 5, 401,267 interposes a lower density non-wickmg layer between two high wickmg layers and suggests that the inner layer transfers fluid from the first layer to the third layer The second layer, however, retains fluid only when the third layer is saturated U S Patent No 4,573,988 teaches an ultrathm and lightweight (less than 136 gsm or 4 oz vd 7 ), absorbent core in which a first, absorbent layer formed of synthetic staple fibers is held m a stabilized compressed state until released by wetting of the product Fluid distribution in the x, y plane occurs mainly in a higher density wickmg layer, which may be comprised of cellulose fibers The first, absorbent layer contains superabsorbent polymer rendered tacky during the process so as to act as a temporary binder for the compressed synthetic fibers. When the superabsorbent begins to swell after a fluid insult, the first layer pops open so the swelling of the superabsorbent is not impeded and the capillary suction of the first layer becomes significantly lower than that of the second layer. Effective wicking layers in absorbent products may be patterned or embossed to form discrete regions of higher density within an overall lower apparent density material. In U.S. Patent No. 3,938,522. hereby incorporated by reference in its entirety, a densified paper-like layer is formed on a cellulosic batt of wood pulp fibers by moistening the batt of fibers followed by heavy compaction. A densified layer created with moisture and pressure is commonly referred to as a "Burgeni" layer after the named inventor of U.S. Patent No. 3,017,304, hereby incorporated by reference in its entirety. The "Burgeni" layer disclosed in U.S. Patent No. 3, 938,522, hereby incorporated by reference in its entirety, is optionally formed with grooved compaction rolls to increase the available surface area of the paper-like layer and provide thickened ribs of densified material for transporting higher volumes of fluid than would be possible with a flat layer. The entire absorbent article is embossed on one or both sides to form spaced densified areas to enhance fluid spreading in U.S. Patent No. 4,443,512, hereby incoφorated by reference in its entirety,. Dry-laid or wet-laid fibrous webs of wood pulp fibers in U.S. Patent No. 4,612,231, hereby incorporated by reference in its entirety, are wetted with water and pressed between two heated cylinders, one of which is machined to impart a grid pattern into the fibrous web. A low density, soft, bulky, and absorbent paper sheet with a diamond pattern even after creping is described in U.S. Patent No.3. 905,863, hereby incorporated by reference in its entirety, in which the wet-laid sheet is pressed against a polymeric fabric of the desired texture. The densified areas in the pattern aid in fluid retention. Vertical wicking is not mentioned in U.S. Patent No.3,905.863. Similarly, in U.S. Patent No.3, 994,771 , hereby incoφorated by reference in its entirety, a wet-laid absorbent paper structure containing separate layers of long softwood fibers and short hardwood fibers is pressed against an open mesh drying/imprinting fabric to obtain softness and bulk. With these processes, as the density or degree of compaction of a material increases, so also does the bending modulus or stiffness of the material. SUMMARY OF THE INVENTION A need exists m the industry to provide vertical wickmg without loss of softness due to compaction of the matenal As the density or degiee of compaction of a material increases, so also does the bending modulus or stiffness of the material One consequence of the pi esent ention is dn orcmg ox eiall pioduct densitv and stiffness liom the ability of the pioduct to \ eitically ick fluid to high levels by including in a lelatively low-density product a fibrous layei of a density conducive to effective wickmg, pieferably located near the moistme impervious outer cover An essential feature of the wickmg layeis of this invention is the ability to effectiv ely tiansfer the vertically wicked fluid to the adjacent fluid storage layei This invention is dnected to an absorbent article made of a liquid permeable top sheet, a liquid impermeable back sheet, and an absoibent coie disposed bet een the topsheet and the backsheet The absoibent coie is made on a continuous an laid machine and includes an acquisition layer in effectiv e fluid communication with the topsheet and disposed beneath the topsheet, a storage layer having absorbent capacity, the storage layer contacting the acquisition layer and being disposed beneath the acquisition layei, and a wickmg layer having a density between 0 1 g/cc and 0 3 g/cc. the wickmg layer contacting the storage layer, and being disposed beneath the storage layer, the wickmg layei being made of compressible wood fibers, wherein the latio of the vertical wickmg height of the wickmg layer to the vertical wickmg height of the storage layei is greatei than 1 25, but is pieferably about 3 0 Additionally, this invention provides a method to make an absorbent article having a wickmg layer with the ability to effectively trans fei the vertically-wicked fluid to the adjacent fluid storage layer This invention provides an absorbent core comprising (1 ) an acquisition layer in effective fluid communication with (2) a storage layer having absorbent capacity disposed beneath and contacting the acquisition layer, and (3) a wickmg layer disposed beneath and contacting the storage layer, compπsing compressible hardwood pulp and having a density of from about 0 05 to about 0 4 g/cc, wheie the ratio of the vertical wickmg height of the wickmg layei to the vertical wickmg height of the storage layer is equal to or greater than 1 25 Within the scope of the aforementioned aspect of this invention is piovided an absoibent article comprising (A) a liquid pei meable top sheet, (B) a liquid impermeable back sheet, and (C) an absoibent core disposed between the topsheet and the backsheet, compiismg (1) an acquisition layer disposed beneath the topsheet and in effective fluid communication with the topsheet and in effective fluid communication with (2) a storage layer having absorbent capacity disposed beneath and contacting the acquisition layer, and (3) a wickmg layer disposed beneath and contacting the stoiage layei, compiismg compressible hardwood pulp and having a density of fiom 0 05 to 0 4 g/cc, where the ratio of the vertical wickmg height of the wickmg layei to the vertical wick g height of the storage layer is equal to or greater than 1 25 This invention provides a method of making an absorbent core compπsing (A) forming a wicking layer comprising compressible hardwood fibers, (B) compressing the wickmg layer to a density of between 0 05 to 0 4 g/cc, and, optionally, imprinting a compression pattern on the wickmg layer, (C) forming a storage layer having absorbent capacity in effectiv e fluid communication with the wicking layer, and (D) forming an acquisition layer in effective fluid communication with the storage layer, where the ratio of the vertical wicking height of the wickmg layer to vertical wickmg height of the storage layer is equal to or greater than 1 25 The present invention also provides a method of making an absorbent article comprising (A) providing a liquid impervious back sheet, (B) forming a wicking layer comprising compressible hardwood fibers, (C) compressing the wick g layer to a density of between 0 05 to 0 4 g cc, and, optionally, imprinting a compression pattern on the wickmg layei, (D) forming a storage layer having absorbent capacity in effective fluid communication with the wicking layer, (E) forming an acquisition layei in effective fluid communication w ith the stoiage layei, and (F) providing a liquid pervious top sheet in effective fluid communication w ith the acquisition layer, where the ratio of the vertical wickmg height of the wickmg layer to vertical wickmg height of the storage layer is equal to or greater than 1.25 Within the scope of this aspect of the invention is the core made by the afoiementioned method This mvention also provides a method of making an absorbent article compiismg (A) providing a liquid impervious back sheet, (B) forming a wickmg layer comprising compressible hardwood fibers. (C) compressing the wickmg layer to a density of between 0.05 to 0 4 g/cc. and, optionally, imprinting a compression pattern on the wicking layer, (D) forming a storage layer having absorbent capacity in effective fluid communication with the wickmg layer, (E) fonning an acquisition layer in effective fluid communication w ith the storage layer, and (F) providing a liquid pervious top sheet in effective fluid communication with the acquisition layer, where the ratio of the vertical wickmg height of the wicking layer to v ertical wicking height of the storage layer is equal to or greater than 1.25 Within the scope of this aspect of the invention is the absorbent article made by the aforementioned method In another embodiment, this invention provides an absorbent core comprising (1) an acquisition layer in effective fluid communication with (2) a storage layer having absorbent capacity disposed beneath and contacting the acquisition layer, and (3) a wickmg layei disposed beneath and contacting the stoiage layei compiismg compiessible haidw ood pulp The coie is made by a method of making an absoibent coie compiismg (A) forming a wickmg layei comprising compiessible hardwood fibers, (B) compiessing the w ick g lav ei to a densitv of betw een 0 05 to 0 4 g cc and, optionally, lmpimtmg a compiession pattern on the w ickmg la ei (C) forming a stoiage layei having absorbent capacity in eftectn e fluid communication with the wickmg layei, and (D) foπmng an acquisition layei m effective fluid communication w ith the storage layei In yet anothei embodiment, this m ention piovides an absorbent coie compiismg ( 1 ) an acquisition layei m effecti e fluid communication w ith (2) a stoiage layer hav mg absoibent capacity disposed beneath and contacting the acquisition layer, and (3) a web imprinted wickmg layei disposed beneath and contacting the storage layei, comprising compressible wood pulp in which there is a pattern of densified regions and less densified regions in the fibrous web The core is made by a method of making an absorbent core comprising (A) forming a wickmg layer comprising compressible wood fibeis, (B) compressing the wickmg lavei to a density of between 0 05 to 0 4 g/cc, wheie densification of the wicking layei is done between a forming oi transfei fabnc and a compaction roll or between a patterned compaction roll and a smooth roll or between two patterned compaction rolls to form a web imprinted wickmg layer ith a pattern of densified regions and less densified regions in the fibrous web, (C) forming a storage layer having absorbent capacity in effective fluid communication with the wickmg layer, and (D) forming an acquisition layer m effective fluid communication with the storage layer BRIEF DESCRIPTION OF THE FIGURES Fig 1 is a cioss-sectional view of the absorbent structure of an embodiment of the piesent mvention Fig 2 is a top v iew of a web imprinted wickmg member of an embodiment of the piesent invention Fig 3 is a side v tew of a web imprinted side membei of an embodiment of the piesent invention Fig 4 is a graph of wick g height versus density Fig 5 is a graph of rewet results Fig 6 is a graph of rewet results Fig 7 is a graph of rewet results Fig 8 is a graph of stain size results. Fig 9 (a) and (b) depicts perspective views of partially densified embodiments of the present invention DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS All patents, patent applications and publications cited in this specification are hereby incoφorated by reference in their entirety In case of conflict in temimology, the present disclosure controls It has now been discovered that by mcoφorating an independent wickmg layei into a disposable absorbent article, wicking height, fluid retention and rewet can be improved without compromising product softness This wickmg layei efficiently transports fluid vertically for a greater distance than is possible in a unitary or homogeneous structure of comparable overall density and also distributes the fluid laterally into an adjacent fluid storage layer and does so at an overall density and product stiffness such that the product remains soft, flexible, and comfortable to wear The wicking layer is physically independent of the other layers of the absorbent core, but is still in fluid communication with the adjacent storage layer. It is hypothesized that the osmotic pressure (fluid suction power) of the superabsorbent material in the storage layer is the driving force behind the effective transfer of the vertically-wicked fluid from the higher density or smaller pore size wicking layer back into the lower density storage layer. Superabsorbent polymers, being typically the sodium salt of poly (acrylic acid), are highly ionic and consequently exhibit high osmotic pressure. This high osmotic driving force overcomes the natural tendency for fluid to remain in regions of smaller pore size capillaries. In one embodiment of this invention, this improvement can be achieved by constructing the wicking layer from compressible wood pulp fibers such as hardwood pulp, typified by eucalyptus, or from softwood pulp treated to be readily compressible. It should be noted that mixtures of fibers, including hardwood and chemically treated softwood fibers, as well as minor amounts of synthetic fibers, may be used. The wicking layer typically contains from about 50 percent by weight to about 99.9 percent or 100 percent by weight of compressible wood pulp fibers, more desirably from about 80 percent to about 100 percent by weight, and preferably from about 90 percent to about 100 percent by weight of compressible wood pulp fibers, and, optionally, from about 0.1 percent to about 50 percent by weight of synthetic fibers, more desirably from about 0.1 percent to about 20 percent by weight, and preferably from about 0.1 percent to about 10 percent by weight, of synthetic fibers. The wicking layer may also contain binders, such as, for example, latex binders, or binding may be accomplished thermally with minor amounts of one or more synthetic fibers, such as, for example, a bicomponent fiber, or with a powder, such as, for example, polyethylene powder. In an alternative embodiment, various synthetic fibers can be substituted for the wood fibers of the wicking layer. Desirably, these are wettable fibers having a diameter of from about 5 microns to about 15 microns and a length of from about 0.5 mm to about 2.5 mm. Fibers of rayon, acrylic, polyester, polyamide and polyolefin, including polyethylene and polypropylene, may be suitable. A surfactant treatment may improve the wettability of the fibers. In a further embodiment of this invention, the wicking layer is densified to the desired degree for superior vertical wicking prior to forming the remainder of the product layers upon it, whereby the wicking heights achievable by the entire product are thereby enhanced. The measured density of the wicking layer will depend upon the pressure applied duimg the measurement After densification, the density of the wickmg lavei undei an applied pressure of 4 g/cm^ desirably is in the range from about 0 05 g cc to about 0 4 g/cc, moie desirably from 0 06 to 0 3 g/cc, even more desirably from 0 1 to 0 3 g/cc oi between 0 1 to 0 3 g/cc More particularly, with a hardwood pulp fiber such as eucalyptus, the density of the wickmg layei is piefeiably from about 0 05 to about 0 2 g cc and more piefeiably about 0 15 With compiessible softwood fibers the densitv desirably is from about 0 20 to about 0 40, and more preferably is about 0 25 At these density ranges the wickmg layer has limited absorbent capacity Further, in an alternative embodiment, the density of the wickmg layer under an applied pressure of 1 1 g cm- desnably is in the lange fiom about 0 08 g/cc to about 0 4 g/cc, more desirablv fiom 0 08 to 0 2 g/cc In yet another embodiment of the invention, the densification of the w ickmg layei is done between the fomung or transfer fabric and a compaction loll oi betw een a patterned compaction loll and a smooth roll or between two patterned compaction lolls to impose a pattern of densified regions and less densified regions in the fibrous vv eb The various compaction rolls may be heated In this embodiment, it has been suφπsmgly found that untreated softwood fibers can be used FIGS 2 and 3 show views of a web imprinted wicking layei The portion of the wickmg layer that has been compacted is shown as a plurality of stripes 22 and the noncompacted portion of the wickmg layer is shown as 24 The density of the web compacted stripes ranges fiom about 0 1 g/cc to 0 5 g/cc Although most of these procedures would be effective on wet-laid cellulose pulp sheets, a continuous airlaid web-forming process is ideally suited to the execution of all the embodiments of this invention In wet-laid sheet forming, a highly dilute slurry of fibers in water is deposited on a rapidly moving screen In airlaid sheet forming, the individualized fibers are dispersed in a stream of air and deposited dry on a moving screen Various aspects of the airlaid process are disclosed in U S Patent Nos 5,068,079, 5,269,049, 5,693,162, 5,922,163, 6,007,653, 5,927,051 , 5,956,926, 5,966,905, 5,921,064, 5,987,851 , 6,009,689, 6,067,775, 5,885,516, 5,028,224, 5,227,107. 5,316,601 , 4,908,175 4,927,582, 5,429 788, 5 445,777, 5,558 832, all ol w hich aie heieby incoipoiated by lefeience m then entnety In a prefeπed embodiment of this invention, the absoibent coie is a unιtaι> absoibent coie pioduced in a senes of unit opeiations a continuous piocess, pieferably an an laid pi ocess Wickmg can be impio ed along the machine-diiection ol the fibious w eb by embossing high-density lanes into the web This method imparts cioss-machme density giadients to the web This method does not negatively affect web stiffness as much as the method of densifymg the entire web However, this method also significantly reduces the v oid volume of the web, m that this method calls foi embossing through the entne web The present inv ention calls foi using an airlaid piocess with multiple heads and between-head embossing to pioduce a fibious web with improved w ickmg Wickmg can be impio ed with the piesent invention without saciificmg laige amounts of web void v olume Thus with the piesent inv ention wicking and fluid distnbution can be impioved without a laige negative effect on fluid acquisition rate The web according to the present invention can be manufactured, foi example, using an airlaid line with two forming heads The fust forming head can be used to unifonnly fomi a lower stratum of a two-strata unitaiy absorbent coie Next, an embossing roll can be used to emboss the lower stratum against the forming wne of the an laid line, thus placing high-density wickmg lanes into the lovvei stiatum Next, the second fonning head can be used to uniformly fonn the uppei stiatum of a two-stiata unitaiy absorbent core Finally, the entne web can be densified in a calendar stack to a unifomi thickness Densifymg the two-strata unitary absoibent core to a final, uniform thickness ensures that channels of relatively low density are present in the uppei stratum of the core, directly above the high-density wickmg lanes of the lower stratum Thus, high-density wicking lanes can be placed in unitary absorbent cores w hile at the same time providing for relatively low-density channels to preserve void v olume See Figures 9(a) and (b) Additionally, the wood pulp fibers can be treated by imprinting the wickmg layei with the pattern of the forming or transfer fabric using a compaction roll, which may be heated, 01 embossing the wickmg layer with a patterned and, optionally, heated compaction roll This treatment is referred to as web tieatment Heated compaction lolls are typically smooth-faced steel drums equipped with an internal heat souice and a hydraulic means of applying pressure to the compaction roll against another steel 01 haid l ubber 1 oil When the compaction 1 oil is engraved with a pattern w hich, undei pi essui c, transfers to the web being compacted, the engraved roll is typically known as an embossing roll and may be heated Thus, the selection of pulp type and/or w eb treatment allows the material of this invention to physically transport fluid vertically to an extent not practicable in homogeneous (nonlayered) structures due to stiffness limitations for product comfort An additional feature of this mvention is that the softness and hence comfort of the final absorbent products is not compromised by the densities needed ioi superior vertical wicking Now referring to FIG 1 , a cioss-sectional view of an an laid absorbent article is shown with moisture peπneable topsheet 10, acquisition layei 1 1 , stoiage layei 12. wickmg layer 13 and moisture impermeable backsheet 14 The topsheet 10 is liquid pervious and should be flexible and nonirπtating to the skm. The acquisition layer 1 1 , storage layer 12 and wickmg layer 13 form the absorbent core 15 The absorbent core 15 is used to collect bodily fluids such as menses or urine The topsheet 10, if employed, presents a body- facing surface which is compliant, soft-feelmg, and non-irπtatmg to the wearer's skm Further, the topsheet 10 is sufficiently porous to be liquid permeable, permitting liquid to readily peneti ate through its thickness A suitable topsheet 10 may be manufactured from a wide range of materials, such as porous foams, reticulated foams, apertured plastic films, natural fibers, such as, for example, wood or cotton fibers, synthetic fibers, such as, for example, polyester or polypropylene fibers, or a combination of natural and synthetic fibers The topsheet 10 is typically employed to help isolate the wearer's skm from liquids held the absorbent structure. The topsheet 10 can have a basis weight, ranging from about 10 0 gsm to about 100 gsm, and a density of between about 0 05 g/cc and about 0 5 g/cc The topsheet 10 can be treated with a selected amount of surfactant, or otherwise processed to impart the desired level of wettability and hydrophilicity. If a surfactant is used, it can be an internal additive or applied to the layer by any conventional means, such as spraying, brush coating and the like. Various woven and nonwoven fabrics can be used for the topsheet 10. For example, the topsheet may be composed of a meltblown or spunbonded web of polyolefin fibers. The topsheet may also be a bonded-carded-web composed of natural and/or synthetic fibers. The absorbent core may be manufactured in a variety of shapes. The core 15 is preferably confoπnable and nonirritating to the skin. The acquisition layer 1 1 is positioned between the topsheet 10 and the storage layer 12. The acquisition layer 1 1 functions to quickly collect and temporarily hold bodily fluids that have been deposited thereon or which have been traversed through the topsheet 10. Additionally, the acquisition layer 1 1 functions to transport those bodily fluids to the underlying storage layer 12. The acquisition 1 1 and storage 12 layers are thus in effective fluid communication. The acquisition layer 1 1 must have sufficient capillary suction to draw bodily fluids through the topsheet 10 but not have excessive fluid retention to make it difficult for the storage layer 12 positioned below the acquisition layer 1 1 to deabsorb the acquisition layer 1 1. Suitable material for the acquisition layer may include cross-linked cellulose fibers, synthetic fibers, or combinations thereof. The density of the acquisition layer is between 0.04 to 0.1 g/cc. The storage layer 12 functions to receive and ultimately contain bodily fluids passing through the acquisition layer 1 1. The acquisition 1 1 and storage 12 layers are thus in effective fluid communication. The density of the storage layer is between 0.05 to 0.25 g/cc. Examples of suitable materials for the storage layer 12 include synthetic or chemically treated cellulosic fibers and wood pulp and superabsorbent materials. The wicking layer 13 is positioned below the storage layer 12 and above the moisture impermeable backsheet 14. The wicking layer 13 functions to draw fluid out of the storage layer, to wick it to another less saturated area, and then to transfer a substantial portion of the fluid back to the storage layer 12. The storage layer 12 and the wicking layer 13 are thus in effective fluid communication. Vanous acquisition la eis and stoiage layers have been disclosed which aie suitable for use in this invention, such as, for example, the acquisition layei and the stoiage layer of U S Ser No 09/325,764, and corresponding PCT/US00/ 16001. both of which aie heieby incoφorated by reference m their entirety Various materials use in the consli uction of the acquisition layer and the stoiage layer and the lesultant piopeities and chaiactenstics of the layers are disclosed in U S Patent Nos 5,147,343, 5 378.528 5.795,439, 5,807,916, 5,849,21 1 , 2,929.154, 3,224,986, 3,332,909, and 4,076,673 all of which aie heieby incoφorated by reference in their entirety The backsheet 14 is substantially lmpeπi eable to liquids and is typically manufactuied fiom a thin plastic film, or other flexible liquid-impermeable material As used in the piesent specification, the term "flexible" lefeis to materials w hich are compliant and which will readily conform to the general shape and contouis of the w eaiei's body The backsheet 14 pi events the exudates contained in the absoibent stiuctuie fiom wetting articles such as bed sheets and ovei garments that contact the finished product The backsheet 14 may be, for example, a polyethylene film having a thickness of from about 0 012 mm (0 5 mil) to 0 051 mm (2 0 mils) Alternatively, the backsheet may be a woven or nonwoven fibrous web layer that has been constructed oi ti eated to impart the desired level of liquid impermeability Other alternative constiuctions for the backsheet 14 include laminates formed of a woven or nonw ov en fabnc and thermoplastic film The backsheet 14 may optionally be composed of a "breathable" mateπal w hich permits vapois to escape from the absorbent structure while still preventing liquid exudates from passing through the backsheet The backsheet can also be embossed and/or matte finished to provide a more aesthetically pleasing appearance In a pieferred embodiment of this invention, the wickmg layer comprises hardwood fiber, such as, for example, eucalyptus, birch, aspen, maple, cotton wood, willow, oak, beech, poplar and basswood, preferably, one or more of the several species of eucalyptus In another embodiment of this invention, the wickmg layer compπses a fluff pulp, either a hardwood pulp or a chemically treated softwood pulp , which is defined as a softwood pulp which has been softened or plasticized to be inherently more compiessible than unmodified fluff pulp fibei s The same pi essme applied to a plasticized fluff pulp web ill lesult in highei density than w hen applied to an unmodified fluff pulp web Additionally, the densified web of plasticized cellulose fibeis is inhei ently softei than a similar density web of unmodified fiber of the same wood type Softw ood pulps may be made mot e compi essible using cationic sui factants as debondei s to disiupt mterfiber associations Examples of debondei s ai e disclosed m U S Patent Nos 4,432,833, 4,425,186 and 5,776,308, all of which are herebv incoφorated by refeience in then entirety Plasticizers for cellulose, which can be added to a pulp slurry pπoi to foπmng wetlaid sheets, can also be used to soften pulp, although thev act by a different mechanism than debondmg agents Plasticizmg agents act w ithin the fibei , at the cellulose molecule, to make flexible oi soften amoφhous regions The lesultmg fibeis aie characterized as limp Since the plasticized fibei s lack stiffness the comminuted pulp is easiei to densify compared to fibers not heated with plasticizeis Plasticizers include polyhydπc alcohols such as glycerol, low moleculai weight polyglycol such as polyethylene glycols and polyhydroxy compounds These and othei plasticizers are described and exemplified m U S Patent Nos 4,098 996, 5547,541 and 4731 ,269, all of which are hereby incoφorated by reference m their entirety Ammonia, urea, and alkylammes are also known to plasticize wood pioducts, hich mainly contain cellulose (A J Stamm, Forest Products Journal 5(6) 413, 1955 herebv mcoipoi ated by l efeience in its entirety Desnably, the ratio of the wicking height of the wickmg layei to the wickmg height of the storage layer (homogeneous construction) is equal to or greatei than 1 25, more desirably, equal to or greater than 1 50, preferably, equal to or greater than 1 75, and more preferably, equal to or greater than 2 0, and even moie preferably, equal to or greater than 3 0 An unanticipated advantage of using a hardwood pulp wickmg layei under an absorbent structure is in the improvement of rewet performance Rewet or flowback is the interstitial fluid held in an absorbent structure, which may be released back through the topsheet under pressure Lower rewet relates to better dryness for the user of the pi oduct That a thin layei of fibeis undei a much hea iei fluid stoi age layei could impact the amount of fluid expressed back thiough the acquisition layei and topsheet was suφiismg It is theonzed that since the rewet test is pei foπned undei less than satuiated conditions, the stionger capillary suction powei of the low basis weight wickmg layei partially di ains the laigei capillaiies m the partially satui ated adjacent fluid stoi age lav ci theieby l educing the olume of fluid available foi expiession fiom the stiuctui e undei the modest piessui e of the rewet test The absoibent coies of this invention desnably hav e a lew et of about 2 5 g oi less, moie desirably of about 2 0 g or less, preferably of about 1 5 g oi less, moie pi efei bly of about 1 0 g oi less, and still more preferably, the absorbent core has a lewet of about 0 5 g Examples Test Methods foi Wicking Examples Except as may be noted m a specific test method, all tests should be conducted at 23 ° C (73 ° F) and 50% relative humidity and all samples should be conditioned at this tempeiature and humidity foi at least two hours pπoi to testing A Density Density (grams per cubic centimetei - g/cc) may be calculated from the basis weight (giams pei squaie metei - gsm) and the ca pei (centimetei s - cm) as measuied undei a given confining pi essure using the formula Density (g/cc) = Basis Weight (g/nx )/[ 10,000 cπr/irf x Cahpeι (cm)] Basis Weight (BW) is expressed m grams per square meter, density m grams pei cubic centimeter B Cahpei Cahper or thickness is measured as follows At least three measurements are taken on diffeient parts of a sample using a digital or analog thickness gauge made by AMES of Waltham, Mass The measurements are averaged The gauge has a 4 cm diametei foot and is equipped with a 50-gram weight so the pressuie applied to the sample is 4 giams/cnr Thickness is measured in inches and converted to centimetei s as needed for calculations C Vertical Wickmg Vertical wickmg is measuied by hanging the absorbent article or core vertically and dipping into a shallow pan of 0 9% saline The pan is equipped w ith a constant leveling device to maintain the level of saline The vertical use of the saline abov e the liquid level in the pan is measured after 30 minutes Samples are measured duplicate and the results averaged The units of vertical w lckmg are in millimeters The invention is illustrated by a series of experiments m which diapers and other like articles were made and tested. The experimental variables were the density of the ovetall product, the density of the wickmg layei , the type of cellulose fiber used in the wickmg layer, and the creation of a densified pattern in the wickmg layei Othei elements in the overall absorbent structure, such as the acquisition layei and the storage layer, were held constant for the puφose of illustrating the advantages of the particulai wicking layers of this invention. The wickmg layers, absorbent cores, and acquisition layers were prepared sequentially on a conventional laboratory air forming handsheet apparatus producing a pad of 35.6 cm (14 inches) square, sufficient, after trimming, to yield four test samples of 7 6 cm by 22.9 cm (3 by 9 inches) or three samples of 7 6 cm by 25.4 cm (3 by 10 inches). This laboratory preparation simulated the type of product that can be produced continuously in a single pass on a conventional airlaid machine with at least three forming heads. It is understood that the several layers constituting an entire absorbent product could be separately formed and assembled by a lamination process to achieve a comparable product However, such a product lacks certain advantages obtained m preparing a multilayer product in a continuous airlaid process In the first step, simulating the first forming head of an airlaid line, the wicking layer is deposited on a carrier tissue or directly on the forming wire If the wicking layer is densified at this time, the use of a between-the-heads compaction roll is simulated Otherwise, the fluid storage layer is laid down by the second forming head followed by the acquisition layer by the third forming head. The entire composite is sprayed with a latex binder, dried and pressed to the final density for the wicking measurement. The final density being between 0.1 to 0.3 g/cc. In the following experiments, an acquisition layer comprises 84 gsm of crimped polyester staple fiber (Type T-224 from Hoechst-Trevira of Salisbury. NC) to which is applied a latex binder (Airflex AF-181 from Air Products and Chemicals of Allentown PA). The absorbent cores, over which the acquisition layers are fomied. all contained 161 gsm of Southern softwood kraft cellulose fluff (Foley fluff from Buckeye Technologies Inc. of Memphis, TN) and 375 gsm of commercial acrylic acid-based superabsorbent (FAVOR SXM70) available from Stockhausen Inc. of Greensboro. NC. The wicking layers used in the following working examples were prepared with a target basis weight of at least 150 gsm. The compressible pulp ND416 is available from Weyerhaeuser Coφoration of Tacoma, WA. Sheeted bleached eucalyptus fiber is available from Sappi Saiccor of Johannesburg, South Africa and Aracruz Celulose (USA) Inc. of Raleigh, NC. In the homogeneous construction, the storage layer was the same as in the other products and there is an acquisition layer but no separate wicking layer. When the pre- densification of the wicking layer between forming heads one and two was simulated in the laboratory (Examples 1-3), the wicking layer was foraied and pre-densified to 0.30 g/cc and then the remainder of the structure was built upon it and compressed to the final overall density. The following samples were prepared and the vertical wicking results are shown in Table 1. Examples 1-3 in Table 1 show the vertical wicking performance of the wicking layer of this invention and the same combined with a fluid storage and fluid acquisition layers to represent an absorbent product. Performance of a product without the wicking layer of this invention is given by the homogeneous construction, which included only the acquisition layer and the fluid storage layer. The sequential formation is a simulation of the kind of product produced on an airlaid line with three forming heads with web compaction after the heads. The pre-densified wicking layer construction simulates an airlaid line with three foπriing heads and a compaction roll between heads one and two to bring the wicking layer to a density of 0.3 g/cc prior to forming the other layers on it. The density values in Table 1 are for the overall constructions, whether single layer, bi layer, or tπ layer Table 1 - Examples 1-3 Vertical Wickmg The examples in Table 1 show that at essentially the same densities for a given layer or construction, vertical wicking height increases based on fiber type, in the order Foley fluff < ND416 < eucalyptus. In Examples 1-3, the ratios of the wickmg height of the wicking layer to that of the storage layer (homogeneous construction) are 1 27, 1.49, and 1.64, respectively. The simulated pre-densification method to fabricate the article produces higher vertical wicking heights than the simple sequential formation approach in which the density of the wicking layer is not controlled but is determined by the compression applied to establish the overall density of the structures. The lowest wickmg performance was displayed by the homogeneous construction, lacking a discrete wicking layei . Table 2 (Examples 4-15) further illustrates the effect of the density of a wickmg layer on the vertical wicking measurement. A compressible fluff was made in situ, Examples 13-15, by spraying Foley pulp sheet with 4% by weight glycerin just before fiberization. The laboratory airlaid handsheet apparatus was again used for sample preparation followed by compaction in a laboratory press. Table 2 - Wicking Layers and Density Table 2 show s that even the unmodified Foley fluff can achieve high vertical wicking values at high density Unfortunately, the stiffness of the wickmg layei at gi eater than 0 3 g/cc density is such that it may not be acceptable m a disposable product intended to confoiTn to the human body The stiffness ranking of these matenals is Foley fluff > ND416 ~ Pulp/glycerm > eucalyptus The impact of patterning the densified wicking layei is demolish ated in Table 3 using the compiessible pulp ND416 available fiom Weyerhaeusei On an an laid pilot line, a 150-gsm web was foπned on an 18-gsm-carπer tissue and subjected to compaction to an initial density of 0 3 g/cc using a smooth compaction loll heated to 90°C The weave of the transfer fabric was impressed into the cellulose web This web was used as the wickmg layer m a laboratory-made 800-gsm-dιaper product containing 375 gsm of superabsorbent (Example 16) In Example 17 the same imprinted wickmg layei was cut into 0 25-inch stnps and placed 0 25 inch apart below the stoiage layei in a like diapei construction In Example 18, the same imprinted wicking layer was cut into 0 5-inch stnps and placed 0 5 inch apart below the storage layer In Example 19, the 150 gsm wicking layer was made in the laboratory and pressed against a piece of the transfer fabric 79 fiom the pilot plant to an appaient density of 0 3 in a laboiatoiy pi ess equipped with a gioo ed plate having 0 375 inch w ide gioo es 0 25 inch deep and 0 25 inch apart (See Figs 2 and 3) The sample and plate w eie pre-heated in an oven to 150°C This w ickmg lavei was included in an 800-gsm-dιaper product as befoie Table 3 - Patterned Wicking layers Examples 16-19 show that the inteiface aiea between layei s is important foi net fluid acquisition The planai wickmg layei in Example 16 wicked the entne length of the 10-ιnch sample Examples 17 and 18 w ith the 6 4 mm (0 25-inch) and 13 mm (0 5 inch) w ickmg stnps have half the contact aiea between the wickmg layer and the stoiage layei as m Example 16 The strips alone raised the saline level over 200 mm, but failed to adequately transfer this fluid to the storage layer as evidenced by the low vertical height in the storage layer and low grams of salme retained Example 19 with legions of intermediate density between the densified lanes efficiently moved fluid into the storage layei to a lespectable height of 148 mm In examples 17 19, the ratios of the wickmg height of the wickmg layer to that of the stoiage layer aie 1 58, 2 19, and 1 51 , lespectively In Example 20, the effect of imprinting a pattern in the wickmg layer is quite evident when a smooth wickmg layer prepared in the laboratory is pressed to different densities for vertical wicking measurement Extrapolating the straight line connecting the three points in FIG 4 shows that a density of about 0 5 g/cc is needed to reach 300 mm wickmg height m 30 minutes The pilot line produced wickmg layer used in the diaper construction of Example 16 wicked to 300 mm when tested alone at a density of 0 25 g/cc This vertical wickmg height suggests that in the densified regions of the fabric pattern, the density apparently equals or exceeds 0 5 g/cc The imprinted web was suφπsingly soft and conformable While it is important for product development for samples made at a certain density to be tested immediately, in the practical sense, materials must dehvei the desired pioperties after piolonged storage and particularly after an absorbent article has been worn on the body foi some time. During the use of absorbent products, motion of the weai ei may tend to disrupt the carefully engmeeied structuie so that the lequisite densities are no longer present In Example 21 , a 5 1 cm by 46 cm (2 inch by 18 inch) strip of the fabπc-impπnted 150 gsm wickmg layer of Example 16 is subjected to repeated crushing by hand and then checked for apparent density and lckmg performance along side a strip of the same web which had not been so handled The results of wickmg height versus time are shown in Table 4 Table 4 - Vertical Wicking Rate Table 4 clearly shows that even the "softened" imprinted compressible pulp wickmg layer managed to raise 0.9% NaCl to a height of 210 mm in 30 minutes It vv as able to do this despite an apparent density of only 0.12 g/cc and extreme softness from the mechanical working. These are representative of articles formed in accordance vv ith the v ention, however, it will be clear to those skilled in the art that the present invention may be incoφorated in other devices for the absoφtion of aqueous materials Test methods for Rewet Examples Except as may be noted in a specific test method, all tests should be conducted at 23 ° C (73 ° F) and 50% relative humidity. All samples should be conditioned at this temperature and humidity for at least two hours before testing. A. Basis weight Basis weight is the weight of the sample per unit area and is expressed in grams per square meter (g/m 2 or gsm). B Thickness Thickness is measured using an analog thickness gauge (B C Ames Co Waltham, MA) The gauge has a 4 1-cm diameter foot and is equipped with a 1 50-gιam weight so that the pressuie applied to the sample is 1 1 4 g/cm 7 Thickness is measuied in inches and is con erted to centimetei s as needed foi calculations C Densitv Density can be calculated from the basis weight and the thickness undei a giv en confining pressuie using the formula Density (g/cc) = Basis w eight (gsm) / [10,000 cm 7 / m 7 x Thickness (cm)] Density is expressed m grams per cubic centimetei (g/cc) D Synthetic menstrual fluid The synthetic menstrual fluid used in this woik contains the follow ing mgiedients in the designated amounts • Deionized water 903 3 g • Sodium chloride 9 0 g • Polyvmylpyrrohdone 122 0 g • Biebπch Scarlet 4 0 g Total solution volume 1 htei Biebπch Scat let (red dye) can be obtained from Sigma Chemical Co St Louis, MO Polyvinylpyπohdone (PVP, weight-average molecular weight approximately 55,000) can be obtained from Aldrich, Milwaukee, WI Sodium chlonde (ACS giade) can be obtained from J T Baker, Philhpsburg, NJ The dry ingredients are mixed in water for at least two hours to ensure complete dissolution The solution temperatme is adjusted to 22°C exactly Sixteen millihters of solution is pipetted into the UL adapter chamber of a Brookfield Model DV-II+ viscometer (Brookfield Engineering Laboratories, Inc , Stoughton, MA) The UL spmdle is placed into the chamber and the viscometer speed is set to 30 φm The target viscosity is between 9 and 10 centipoise Viscosity can be adjusted with additional water or PVP E Combination acquisition, le et, stain size test Equ ipment Electiomc balance (± 0 01 g precision) Fluid intake tester (FIT, Buckeye "BUI 44-97" design) Giade S22 blotter papei , 10 16 cm \ 24 13 cm (4 in \ 9 5 m ) Weight. 8408 5 g, 10 16 cm x 24 13 cm (4 in x 9 5 m ) Latex foam, 10 16 cm x 24 13 cm x 3 81 cm (4 in \ 9 5 in \ 1 5 m ) Rulei scaled in millimeters Synthetic menstiual fluid Topsheet, spunbond polypi opylene, 22 gsm, 25 4 cm x 10 16 cm ( 10 m x 4 in ) Latex foam can be obtained fiom Scott Fabncs, Memphis, TN Blottei papei can be obtained fiom Buckeye Technologies, Memphis, TN The topsheet matenal can be obtained fiom Av gol Nonwoven Industiies, Holon, Isiael The fluid intake testei (FIT), of Buckeye design, consists of a top plate and a bottom plate The top plate is a 29 7 cm x 19 0 cm x 1 3 cm plate of polycarbonate plastic The plate has a hole cut out of its center and a hollow intake cylinder is mounted the hole The innei diameter of the intake cylinder is 2 5 cm and the complete top plate weighs 872 grams The bottom plate of the FIT is essentially a 29 7 cm x 19 0 cm x 1 3 cm monolithic plate of polycarbonate plastic The sample is cut to 7 cm x 20 cm with the longei dimension m the machine dnection The sample weight and thickness are measured and lecoided An "X" is placed at the center of the top of the sample with a marking pen The sample is centered on the FIT bottom plate The topsheet is centered on the sample and the FIT top plate is loweied on top of the topsheet The top plate is centered on the sample so that the intake cylmdei is centered on the "X" marked on the sample A 10-ml insult of fluid is poured into the intake cylinder and the amount of time taken for the sample to acquire the fluid is measured and recorded This time, reported m seconds (s), is the acquisition time for the sample Simultaneous with the end of the acquisition time, a 20-mιnute waiting period begins At the end of the waiting period, the stain size is measured and recorded lengthwise (machine dn ection) on both the top and the bottom of the sample Stain size is leported in milhmeteis (mm) The polycaibonate plastic is clear so that the stain size can be viewed thiough the plastic The bottom stain can be measuied by temporarily flipping ov ei the FIT so that the bottom plate faces up Rewet is measured by lemovmg the top FIT plate, then placing a pie- eighed stack of eight S22 blottei papei s on the topsheet of the sample The foam is placed on the papei and the weight is placed on top of the foam (the weight, the foam and the paper constitute a 3 4 kPa (0 5) psi pi essuie on the sample) for two minutes The rew et, leported in giams (g), is calculated by subtracting the initial weight of the stack of papers from the final weight of the stack of papei s This combination test is usually performed in triplicate and the results ai e avei ged Examples A through H: Laboratory Samples for Rewet Measurement The invention is lllusti ated heie by pei forming a senes of experiments in which absoibent stiuctuies are constiucted and tested The experimental variables aie the type of cellulosic fibei used in the wicking layer of the structuie, the basis weight of the wickmg layei and the utilization of a web treatment on the wickmg layer Other elements m the oveiall absorbent structure, such as overall density, oveiall basis weight, composition of acquisition layer and composition of storage layer, are held constant foi the puφose of illustrating the advantages of the particular wickmg layers of this invention The temt "Unicoie" as used herein means a multi-layeied absorbent stiuctuie which could be manufactured on a continuous forming machine More specifically, the Unicore structures of this invention have discrete layei s for fluid acquisition, storage, and distribution (wicking) which layers are in contact thereby allowing fluid transfer between layers It is understood that such Unicore structures could also be fabricated from individually prepared layers of material In Examples A through H, the top acquisition layer comprises 35 gsm of polyester staple fiber ( 15 dpf x 6 mm, Grade 376X2, Wellman, Inc , Johnsonville, SC) to which is applied a latex bindei (Airflex 192, Air Products and Chemicals, Allentown, PA) The middle layer comprises 90 gsm of HPF fiber (a mercerized Southern softwood fibei available from Buckeye Technologies, Memphis, TN) and about 9 gsm bicomponent binder fiber (Grade AL-Adhesion-C, 1.7 dtex x 6 mm, FiberVisions. Covington, GA). These two layers were made separately on an airlaid pilot machine. Minimal compaction was used in the construction of these two layers. In these examples, the structures all contained the same top and middle layers. The bottom wicking layers for Examples A through H were made using a laboratory airlaid handsheet apparatus. For the wicking layers, we examined three effects. The first effect was the type of cellulosic fiber used in the wicking layer. Half of the wicking layers were made with Grade ND-416 fiber from Weyerhaeuser Co., Tacoma WA. The other half of the wicking layers were made with bleached kraft eucalyptus fiber from Aracruz Celulose (USA), Raleigh, NC. The wicking layers contained about 10% bicomponent binder fiber by weight. The second effect was the basis weight of the wicking layer, which was fixed at either 50 or 70 gsm of cellulosic. The third effect was the use of a web treatment; half of the samples received the web treatment. The web treatment involved pre-densification of the wicking layer, in which the wicking la er was foπned in the handsheet apparatus, then densified in a laboratory press. A piece of fom ing wire fabric was placed on the bottom platen of the press to impose a pattern of densified regions and less densified regions in the wicking layer. For the wicking layers subjected to the web treatment (Examples E through H), they were pre-densified. on average, to an overall, apparent density of about 0.06 to 0.07 g/cc, with the density in the pattern imposed by the forming wire fabric likely much higher than 0.10 g/cc. Unicore structures were assembled by hand in the laboratory. The structures in these examples were all densified to an overall, apparent density of 0.09 g/cc. These structures contained identical top and middle layers. Focusing on the wicking layer, we made eight structures using all possible combinations of the three effects. Table 5 shows test data for the eight structures, Examples A through H. Figure 5 is a graph of rewet data for these examples. Examples A through H can be divided into four head-to-head comparisons of eucalyptus and ND-416. These comparisons are indicated by the different textures used for the bars in Figure 5. In each comparison, eucalyptus fiber provides for lower rewet compared to ND-416. A statistical analysis of the data outlined in Table 5 shows that all of the effects of this laboratory study significantly influenced rewet. The ranking of the relative strength of the effects is web treatment (w ith is better than without) < basis eight (higher is better than lower) « choice of fiber (eucalyptus is much better than ND-416). Table 5. Results for laboratory evaluation of eucalyptus fiber in Lnicore. Examples I through K: Airlaid Pilot Samples for Rewet Measurements Examples I through K have the same top and middle layers. The top layer comprises 35 gsm of Wellman PET with about 6 gsm of Airflex 192 latex for bonding. The middle layer comprises 70 gsm HPF fiber. 56 gsm Favor 1 180 superabsorbent powder (Stockhausen, Inc., Greensboro, NC) and about 9.5 gsm FiberVisions bicomponent fiber, AL-Adhesion-C, 1.7 dtex x 4 mm. The bottom layer comprises 70 gsm of cellulosic fiber and about 5.3 gsm of the same FiberVisions bicomponent fiber. Examples I through K were made as unitary structures on a 0.6 meter-wide, three-head pilot line. Example I was made with ND-416 fiber and about 2.5 gsm of Airflex 192 latex was applied to the wire side of the web to control dusting. Example J was made identically to Example I, except that eucalyptus fiber was substituted for ND-416 fiber and a tissue earner ( 18 gsm, Cellu Tissue Co , East Hartford, CT) w as substituted foi the w ne-side latex Example K was made identically to I, except that the w eb tieatment w as used foi the eucalyptus (bottom) layei Table 6 shows test data foi thiee samples, Examples I thiough K Note the basis weight diffeience between Example 1 and Examples 1 and K This can be attnbuted to the cainei tissue, w hich imparts essentially no pei foπnance advantages to the samples Figure 6 shows lewet results foi Examples 1 through K Confiπnmg the laboiatoiy woik, the pilot samples show that eucalyptus and the web tieatment both help to impiove rewet Table 6. Table 6 Rewet Results foi Pilot Plant Samples Examples L through W 7 : survey of hardwood pulps In these examples, the experimental variables aie the type of haidwood fiber used in the wicking layer of the stmcture and the basis weight of the wickmg layei An example using the softwood fiber ND-416 in the wickmg layei is included for comparison purposes Other elements of the oveiall absorbent structure, such as oveiall density, overall basis weight, composition of acquisition layer and composition of storage layer, are held constant foi the purpose of illustrating the advantages of the particular wickmg layers of the present invention In Examples L through W, the top acquisition layer comprises 35 gsm of polyester staple fiber (15 dpf x 6 mm, Grade 376X2, Wellman, Inc , Tohnsonville, SC) and about 6 gsm of latex bindei (An flex 192, An Products and Chemicals, Allentown PA) The middle layei compπses 60 gsm of HPF fiber (a mercerized Southern softwood fibei a ailable fi om Buckeye Technologies, Memphis, TN) and about 7 gsm bicomponent bindei fibei (Giade AL-Adhesion-C, 1 7 dtex x 4 mm, FiberVisions, Covmgton. GA) These tw o layei s wei e made sepaiately on an an laid pilot machine Minimal compaction w as used in the consti uction of these tw o layei s In these examples, the stiuctui es all contained the same top and middle layers The bottom wickmg layers for Examples L through W weie made using a laboiatory airlaid handsheet apparatus For these examples, we examined two effects The fu st effect was the type of cellulosic fiber used m the wickmg layei Six cellulosic fibeis w ei e used, five haidwoods and one softwood Hardwoods • Bleached ki aft eucalyptus pulp fiom Aiacruz Celulose (USA), Raleigh, NC (Aracruz) • Dissolv mg eucalyptus pulp, Solucell-400, from Klabm Bacell, Camacan, Brazil (Solucell) • Bleached kraft eucalyptus pulp, Pπmacell, from Riocell, Guaiba, Brazil (Pπmacell) • Bleached kraft birch pulp from Kaukas Mill, UPM-Kymmene, Finland (Bnch) • Dissolving eucalyptus pulp from Sappi Saiccor, Johannesbuig, South Afiica (Saiccoi) Softwood • Bleached Southern softwood kraft pulp, Grade ND-416, from Weyerhaeusei Co , Tacoma, W A (ND-416) The wickmg layers contained about 10% bicomponent binder fiber by weight (Grade AL- Adhesion-C, 1 7 dtex x 4 mm, FiberVisions, Covmgton, GA) The second effect examined was the basis weight of the wickmg layer, which was fixed at either 50 or 80 gsm of cellulosic Unicore structures were assembled in the laboratory The structures in Examples L thiough W were densified to an overall, apparent density target of 0 085 g/cc Twelve structures were made (six pulps at two basis weights) Testing was perfoπned in tπphcate using the methods described in detail in the 4/4/00 disclosure Table 1 shows test data for Examples L through W Figure 1 is a graph of lewet data for these examples. Rew et results for both basis weights, 50 gsm and 80 gsm. are plotted in Figure 1 At each basis weight, the hardwood pulps generally provide foi lower rewet compared to the softvv ood pulp Figure 8 is a plot of bottom stam size data for these examples Although theie is some scatter in the data, at each basis weight, the bottom stam size for the softwood pulp is generally shorter than the bottom stain size for the hardwood pulps The hard ood pulps are more successful at vv lcking the fluid away from the insult site. Concerning the relation between rewet and bottom stam size, vv e hypothesize that improved wickmg (a larger bottom stam size) results in enhanced fluid retention (as measured by rewet) by moving fluid away from relatively saturated parts of the stiucture (the insult site) to less saturated parts of the structure (away from the msult site) Thus, a structure with exceptional wickmg would be capable of literally sucking fluid a ay from the insult site Such a structure could be described as a "high suction" core Table 7. Results foi laboratoiy evaluation of vaiious pulps as wickmg laveis in Unicoie

Claims: What is claimed is 1 An absoibent core comprising ( 1 ) an acquisition layei (2) a stoiage layei having absoibent capacity, disposed beneath and in fluid communication with the acquisition layei, and (3) a wickmg layer disposed beneath and in fluid communication with the storage layei, comprising compressible haidw ood pulp and having a density of between about 0 05 and about 0 4 g/cc, where the latio of the v ertical wickmg height of the wickmg lay ei to the vertical w ickmg height of the stoiage layei is equal to oi gieatei than 1 25 2 The absoibent coie of claim 1 , wheiein the latio of vertical wickmg height of the wickmg layei to the vertical wickmg height of the stoiage layei is equal to oi gieatei than 3 0 3 The absoibent core of one of claims 1 -2, wherein the compressible haidwood pulp is selected fiom the group consisting of eucalyptus, bnch, aspen, maple, cotton wood, willow, oak, beech, poplar, basswood and combinations theieof 4 The absorbent core of claim 3, wherein the compressible hardwood pulp is eucalyptus 5 The absoibent core of one of claims 1 -4 w herein the wickmg layei furthei comprises softwood fibers 6 The absorbent core of one of claims 1-5, wherein the wickmg layer is imprinted with a compression pattern 7 The absorbent core of one of claims 1-6, wherein the coie has a rewet value of about 3 0 g or less 8 The absorbent core of claim 7, wherein the core has a rewet value of about 2 0 g or less 9 The absorbent core of claim 8, wherein the core has a rewet value of about 1 0 g or less 10 The absoibent core of one of claims 1 -9, wheiein the wickmg laver has a density of between 0 1 and 0 3 g/cc 1 1 The absoibent core of one of claims 1-10 wheiein the absorbent coie is a unitaiy absoibent coie produced in a series of unit operations in a continuous piocess 12 An absoibent article comprising (A) a liquid permeable top sheet, (B) a liquid impermeable back sheet, and (C) an absoibent coie disposed between the topsheet and the backsheet, comprising ( 1 ) an acquisition layer disposed beneath and m fluid communication vv ith the topsheet, (2) a stoiage layei hav mg absorbent capacity disposed beneath and in fluid communication with the acquisition layei, and (3) a wicking layei disposed beneath and m fluid communication with the stoiage layei, compiismg compressible hardwood pulp and having a density of between 0 05 and 0 4 g/cc, where the ratio of the vertical wickmg height of the wickmg layer to the vertical wickmg height of the storage layer is equal to or greater than 1 25 13 The article of claim 12, wherein the article is selected from the group consisting of infant diapei s, training pants, adult incontinence briefs, feminine hygiene pads, surgical drapes and wound dressings 14 A method of making an absorbent core comprising (A) forming a wickmg layei comprising compressible hardwood fibers (B) compressing the wicking layer to a density of between 0 05 and 0 4 g/cc, (C) forming a storage layer having absorbent capacity and in effective fluid communication with the wickmg layer, and (D) forming an acquisition layer in fluid communication with the storage layer, where the ratio of the vertical wicking height of the wickmg layer to the vertical wicking height of the storage layer is equal to or greater than 1 25 15. The method of claim 14 wherein the ratio of vertical wicking height of the wicking layer to the vertical wicking height of the storage layer is equal to or greater than 3.0. 16. The method of one of claims 14-15, wherein the compressible hardwood pulp is selected from the group consisting of eucalyptus, birch, aspen, maple, cotton wood, willow, oak, beech, poplar, basswood and combinations thereof. 17. The method of claim 16, wherein the compressible hardwood pulp is eucalyptus. 18. The method of one of claims 14-17, wherein the wicking layer further comprises softwood fibers. 19. The method of claim 18 wherein the softwood fibers are chemically treated. 20. The method of one of claims 14-19, wherein the core has a rewet value of about 3.0 g or less. 21. The method of claim 20, wherein the core has a rewet value of about 2.0 g or less. 22. The method of claim 21 , wherein the core has a rewet value of about 1.0 g or less. 23. The method of one of claims 14-22, wherein the absorbent core is a unitary absorbent core produced in a series of unit operations in a continuous process. 24. The method of claim 23, wherein the process is an airlaid process. 25. The method of one of claims 14-24 wherein the wicking layer is imprinted with a compression pattern. 26. An absorbent core made by the method of one of claims 14-25. 27. A method of making an absorbent article comprising: (A) providing a liquid impervious back sheet; (B) forming a wicking layer comprising compressible hardwood fibers; (C) compressing the wicking layer to a density of between 0.05 and 0.4 g/cc; (D) forming a storage layer having absorbent capacity and in fluid communication with the wicking layer; and (E) forming an acquisition layer in fluid communication with the storage layer, and (F) providing a liquid pervious top sheet in fluid communication with the acquisition layer, where the ratio of the vertical wicking height of the wicking layer to the vertical wicking height of the storage layer is equal to or greater than 1 .25. 28. An absorbent article made by the method of claim 27. 29. An absorbent core comprising: ( 1 ) an acquisition layer; (2) a storage layer having absorbent capacity disposed beneath and in fluid communication with the acquisition layer; and (3) a wicking layer disposed beneath and in fluid communication with the storage layer, comprising compressible hardwood pulp. 30. The absorbent core of claim 29, wherein the wicking layer comprises from about 50 percent by weight to about 99.9 percent by weight of hardw ood fibers and from about 0.1 percent by weight to about 50 percent by weight synthetic fibers, the storage layer includes materials selected from the group consisting of synthetic fibers, chemically treated cellulosic fibers, wood pulp, superabsorbents and combinations thereof, and has a density of between 0.05 and 0.25 g/cc, and the acquisition layer includes materials selected from the group consisting of cross-linked cellulose fibers, synthetic fibers, and combinations thereof, and has a density of between 0.04 to 0.1 g/cc. 31. A method of making an absorbent core comprising: (A) forming a wicking layer comprising compressible hardwood fibers; (B) compressing the wicking layer to a density of between 0.05 and 0.4 g/cc; (C) forming a storage layer having absorbent capacity and in fluid communication with the wicking layer; and (D) forming an acquisition layer in fluid communication with the storage layer. 32. The method of claim 31 , wherein the absorbent core is a unitary absorbent core produced in a series of unit operations in a continuous process. 33. The method of one of claims 31 -32, wherein the process is an airlaid process. 34. The method of claim 31 , wherein the wicking layer is imprinted with a compression pattern. 35. An absorbent core comprising: ( 1 ) an acquisition layei , (2) a storage layei hav mg absoibent capacity disposed beneath and in fluid communication with the acquisition layei , and (3) a web imprinted wickmg layei disposed beneath and in fluid communication w ith the stoi age la ci compi ismg compi essible w ood pulp w hich theie is a pattern of densified l egions and less densified l egions 36 The absorbent core of claim 31 , w heiein the wicking layei comprises fiom about 50 pei cent by weight to about 99 9 pei cent by weight of wood fibeis and fiom about 0 1 peicent by eight to about 50 pet cent by weight synthetic fibeis, the storage layei includes materials selected fiom the gioup consisting of synthetic fibeis chemically heated cellulosic fibeis, w ood pulp, supei absoi bents, and combinations theieof, and has a density betw een 0 05 and 0 25 g/cc, and the acquisition layei includes matenal selected fiom the gioup consisting of ciosss nked cellulose fibei s, synthetic fibei s, and combinations theieof and has a density of bet een 0 04 and 0 1 g/cc 37 A method of making an absoibent core compiismg (A) forming a wickmg layei comprising compressible wood fibers, (B) compressing the wickmg layer to a density of between 0 05 and 0 4 g/cc, claim compiession taking place between a fonmng oi tiansfei fabnc and a compaction loll to foπn a web imprinted wickmg layei with a pattern of densified legions and less densified regions, (C) fonmng a stoiage layei having absorbent capacity m fluid communication with the wicking layer, and (D) forming an acquisition layer in fluid communication with the storage layer 38 The method of claim 37, wheiein the absorbent core is a unitary absoibent core produced in a series of unit operations in a continuous process 39 The method of claim 38, wherein the process is an airlaid process 40 The method of claim 37 wherein claim compression takes place between a patterned compaction roll and a smooth roll 41 . The method of claim 37 wherein claim compression takes place between two patterned compaction rolls. 42. An absorbent core made by the method of claim 31 or claim 37.

Inventors: BAKER JOHN PERRY, BOEHMER BRIAN E, ERSPAMER JOHN P, GROSS JAMES R, BAKER, JOHN, PERRY, BOEHMER, BRIAN, E., ERSPAMER, JOHN, P., GROSS, JAMES, R.
Applicants: BAKER JOHN PERRY, BKI HOLDING CORP, BOEHMER BRIAN E, ERSPAMER JOHN P, GROSS JAMES R, BAKER, JOHN, PERRY, BKI HOLDING CORPORATION, BOEHMER, BRIAN, E., ERSPAMER, JOHN, P., GROSS, JAMES, R.
Application year: 2001
Main classification: A61F13
IPC codes: A41B9, A61B19, A61F5, A61F13, D04H13