# Patent

## Title
Absorbent core with high superabsorbent material content

## Abstract

An absorbent core (28) and an absorbent article (20) comprising the core. The absorbent core comprises a core wrap (16, 16′) enclosing an absorbent material (60), wherein the absorbent material comprises at least 80% of superabsorbent polymers (“SAP”) by weight of the absorbent material. The absorbent core has a Relative Wet Caliper Increase (RWCI) after compression of less than 10% as measured by the Wet Caliper And Compression Force (WCACF) Test as described herein, and wherein the core wrap (16, 16′) is at least partially sealed so that substantially no absorbent material leaks out of the core wrap while performing the WCACF Test.

## Background
FIELD OF THE INVENTION
The invention is for an absorbent core for personal hygiene absorbent products such as, but not limited to, baby diapers, training pants, feminine pads or adult incontinence products.
BACKGROUND OF THE INVENTION
Absorbent articles for personal hygiene, such as disposable diapers for infants, training pants for toddlers or adult incontinence undergarments, are designed to absorb and contain body exudates, in particular large quantity of urine. These absorbent articles comprise several layers providing different functions, for example a topsheet, a backsheet and in-between an absorbent core, among other layers.
The function of the absorbent core is to absorb and retain the exudates for a prolonged amount of time, for example overnight for a diaper, minimize re-wet to keep the wearer dry and avoid soiling of clothes or bed sheets. The majority of currently marketed absorbent articles comprise as absorbent material a blend of comminuted wood pulp with superabsorbent polymers (SAP) in particulate form, also called absorbent gelling materials (AGM), see for example U.S. Pat. No. 5,151,092 (Buell). Absorbent articles having a core consisting essentially of SAP as absorbent material (so called “airfelt-free” cores) have also been proposed but are less common than traditional mixed cores (see e.g. WO2008/155699 (Hundorf), WO95/11652 (Tanzer), WO2012/052172 (Van Malderen)).
Absorbent articles comprising an absorbent core with slits or grooves have also been proposed, typically to increase the fluid acquisition properties of the core or to act as a folding guide. WO95/11652 (Tanzer) discloses absorbent articles which include superabsorbent material located in discrete pockets having water-sensitive and water-insensitive containment structure. WO2009/047596 (Wright) discloses an absorbent article with a slit absorbent core.
Absorbent products which are flexible in the crotch region provide the benefits of improved freedom of movement for the wearer, especially when the user's legs compress the crotch region of the article laterally. However the inventors have found that highly flexible products may in generally have a poor resiliency when becoming wet, and thus tend to lose their shape when compressed by the movement of the wearer's legs. As the absorbent core is deformed, the product can fail performing properly and this increases the chance of failure such as fluid leakages. The inventors have now found a new absorbent core structure which can provide the benefit of good flexibility combined with good resiliency when loaded with fluid.
SUMMARY OF THE INVENTION
The invention is for an absorbent core as defined in the claims and an absorbent article comprising this absorbent core. The absorbent core comprises a core wrap enclosing an absorbent material, wherein the absorbent material comprises at least 80% of superabsorbent polymers (“SAP”) by weight of the absorbent material. The absorbent core comprises a front edge, a back edge and two longitudinal edges, and has a longitudinal axis oriented in a longitudinal direction. The absorbent material comprises at least one channel, in particular at least one pair of channels at least partially oriented in the longitudinal direction. The absorbent core has a Relative Wet Caliper Increase (RWCI) value of less than 10.0% as measured by the Wet Caliper And Compression Force (WCACF) Test as described herein, and the core wrap is at least partially sealed so that substantially no absorbent material leaks out of the core wrap while performing the WCACF Test. The absorbent core may further have a Wet Compression Force below 5.00 N, in particular from 1.00 to 3.00N, as measured by the WCACF Test.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of an absorbent article in the form of a diaper comprising an exemplary absorbent core according to the invention;
FIG. 2 is a transversal cross-section of the diaper of FIG. 1;
FIG. 3 is a top view of the exemplary absorbent core of the diaper of FIG. 1 taken in isolation;
FIG. 4 is a transversal cross-section of the core of FIG. 3;
FIG. 5 is a longitudinal cross-section of the core of FIG. 3;
FIG. 6 shows a top view of an alternative absorbent core of the invention.
FIG. 7 is a schematic description of an apparatus used to carry out the Wet Caliper And Compression Force Test, further detailed below.



DETAILED DESCRIPTION OF THE INVENTION
Introduction
As used herein, the term “absorbent article” refers to disposable products such as infant or adult diapers, training pants, and the like which are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Typically these articles comprise a topsheet, backsheet, an absorbent core and optionally an acquisition layer and/or distribution layer and typically other components, with the absorbent core normally placed between the backsheet and the acquisition system or topsheet.
As used herein, the term “absorbent core” refers to an individual component, which is placed or is intended to be placed within an absorbent article and which comprises an absorbent material enclosed in a core wrap. The term “absorbent core” does not cover an acquisition or distribution layer or any other component of an absorbent article which is not either integral part of the core wrap or placed within the core wrap. The absorbent core is typically the component of an absorbent article which comprises all, or at least the majority of, superabsorbent particles (SAP) and has the most absorbent capacity of all the components of the absorbent article.
A “nonwoven web” as used herein means a manufactured sheet, web or batt of directionally or randomly orientated fibers, bonded by friction, and/or cohesion and/or adhesion, excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling, whether or not additionally needled. The fibers may be of natural or man-made origin and may be staple or continuous filaments or be formed in situ. Commercially available fibers have diameters ranging from less than about 0.001 mm to more than about 0.2 mm and they come in several different forms such as short fibers (known as staple, or chopped), continuous single fibers (filaments or monofilaments), untwisted bundles of continuous filaments (tow), and twisted bundles of continuous filaments (yam). Nonwoven webs can be formed by many processes such as meltblowing, spunbonding, solvent spinning, electrospinning, carding and airlaying. The basis weight of nonwoven webs is usually expressed in grams per square meter (g/m2 or gsm).
“Comprise,” “comprising,” and “comprises” are open ended terms, each specifies the presence of the feature that follows, e.g. a component, but does not preclude the presence of other features, e.g. elements, steps, components known in the art or disclosed herein. These terms based on the verb “comprise” should be read as encompassing the narrower terms “consisting essential of” which excludes any element, step or ingredient not mentioned which materially affect the way the feature performs its function, and the term “consisting of” which excludes any element, step, or ingredient not specified. Any preferred or exemplary embodiments described below are not limiting the scope of the claims, unless specifically indicated to do so. The words “typically”, “normally”, “advantageously” and the likes also qualify features which are not intended to limit the scope of the claims unless specifically indicated to do so.
Unless indicated otherwise, the description refers to the absorbent core and absorbent article before use (i.e. dry, and not loaded with a fluid) and conditioned at least 24 hours at 21° C.+/−2° C. and 50+/−20% Relative Humidity (RH).
General Description of the Absorbent Article 20
An exemplary absorbent article 20 in which the absorbent core 28 of the invention can be used is an infant taped diaper 20 as represented in FIG. 1. FIG. 1 is a top plan view of the exemplary diaper 20, in a flat-out state, with portions of the structure being cut-away to more clearly show the construction of the diaper 20. This diaper 20 is shown for illustration purpose only as the invention may be used for making a wide variety of diapers or other absorbent articles.
The absorbent article 20 comprises a liquid permeable topsheet 24, a liquid impermeable backsheet 25, an absorbent core 28 between the topsheet 24 and the backsheet 25. The absorbent article may also comprise further typical components such as an acquisition layer 52 and/or a distribution layer 54 (collectively referred to as acquisition-distribution system “ADS”, designated as 50 in FIG. 2), and elasticized gasketing cuffs 32 present between topsheet and backsheet and upstanding barrier leg cuffs 34, which will be further detailed in the following. FIGS. 1-2 also show other typical taped diaper components such as a fastening system comprising fastening tabs 42 attached towards the back edge 12 of the article and cooperating with a landing zone 44 towards the front edge of the article. The absorbent article may also comprise other typical components, which are not represented in the Figures, such as a back elastic waist feature, a front elastic waist feature, transverse barrier cuff(s), a lotion application, etc. . . .
The absorbent article 20 comprises a front edge 10, a back edge 12, and two longitudinal edge edges. The front edge 10 is the edge of the article which is intended to be placed towards the front of the user when worn, and the back edge 12 is the opposite edge. The absorbent article may be notionally divided by a longitudinal axis 80 extending from the front edge to the back edge of the article and dividing the article in two substantially symmetrical halves relative to this axis, when viewing the article from the wearer facing side in a flat out configuration, as exemplarily shown in FIG. 1. If some part of the article is under tension due to elasticized components, the article may be typically flattened using clamps along the periphery of the article and/or a sticky surface, so that the topsheet and backsheet can be pulled taut so as to be substantially flat. The absorbent article 20 can also be notionally divided by a transversal axis 90 in a front region and a back region of equal length measured on the longitudinal axis, when the article is in such a flat state. This article's transversal axis 90 is perpendicular to the longitudinal axis 80 and placed at half the length of the article. The length of the article can be measured along the longitudinal axis 80 from front edge 10 to back edge 12.
The topsheet 24, the backsheet 25, the absorbent core 28 and the other article components may be assembled in a variety of well known configurations, in particular by gluing and/or heat embossing. Exemplary diaper assemblies are for example generally described in U.S. Pat. Nos. 3,860,003, 5,221,274, 5,554,145, 5,569,234, 5,580,411, and 6,004,306. The absorbent article is preferably thin. The article may be advantageously thin at the intersection of the longitudinal and transversal axis, for example with a caliper of from 1.0 mm to 8.0 mm, in particular from 1.5 mm to 6.0 mm, as measured using the Absorbent Article Caliper Test described below.
These and other components of the articles will now be discussed in more details.
Absorbent Core 28
The absorbent core of the invention comprises absorbent material with a high amount of superabsorbent polymers (herein abbreviated as “SAP”) enclosed within a core wrap. The SAP content represents at least 80% by weight of the absorbent material contained in the core wrap. The core wrap is not considered as absorbent material for the purpose of assessing the percentage of SAP in the absorbent core. The absorbent material defines an absorbent material deposition area 8 as seen when the core is placed substantially flat. As used herein, the term “absorbent core” does not include the topsheet, the backsheet and (if present) an acquisition-distribution system or layer which is not integral part of the absorbent core, in particular which is not placed within the core wrap. The core may consist essentially of, or consist of, the core wrap, the absorbent material and optionally glue. The term “absorbent core” and the term “core” are herein used interchangeably.
The exemplary absorbent core 28 of the absorbent article of FIG. 1 is shown in isolation in FIGS. 3-5. The absorbent core of the invention comprises a front edge 280, a back edge 282 and two longitudinal edges 284, 286 joining the front edge 280 and the back edge 282. The front edge 280 of the core is the edge of the core intended to be placed towards the front edge 10 of the absorbent article. Typically the absorbent material will be advantageously distributed in higher amount towards the front edge than towards the back edge as more absorbency is required at the front. Typically the front and back edges of the core 280, 282 are shorter than the longitudinal edges 284, 286 of the core. The absorbent core may also comprise a top side and a bottom side. The top side 288 of the core is the side intended to be placed towards the topsheet and the bottom side 290 the side intended to be placed towards the backsheet in the finished article 20. The top side 288 of the core is typically more hydrophilic than the bottom side 290. The width of the core at the crotch point as measured between the two longitudinal edges 284,286 should be sufficient for the WCACF Test to be conducted, i.e. should be at least 40 mm. The width of the core at the crotch point may in particular be of from 45 mm to 200 mm, or from 50 mm to 150 mm.
The absorbent core may be notionally divided by a longitudinal axis 80′ extending from the front edge to the back edge of the core and dividing the core in two substantially symmetrical halves relative to this axis, when viewing the core from the topside in a flat out configuration, as exemplarily shown in FIG. 3. Typically the longitudinal axis 80′ of the core and the longitudinal axis 80 of the article in which the core is intended to be placed will be contiguous, when viewed from the top as in FIG. 1. The transversal axis of the core (herein also referred to as “crotch line”), is perpendicular to the longitudinal axis and is passing through the crotch point C of the core. The crotch point C is the point of the absorbent core placed at a distance of 0.45 of L from the front edge of the absorbent core, L being the length of the core as measured from its front edge to its back edge on the longitudinal axis 80′, as illustrated in FIG. 3. The full length L of the core is measured from the front edge 280 to the back edge 282 of the core along its longitudinal axis 80′ and also includes the region of the core wrap which does not enclose the absorbent material, in particular at the front and back end seals when present. The length of the core L is of at least 320 mm, for example from 320 mm to 600 mm.
The crotch region 81 is defined herein as the region of the core extending from the crotch line, i.e. at the level of the crotch point C, towards the back edge and front edge of the core by a distance of a quarter of L (L/4) in both directions for a total length of L/2. The front region 82 and back region 83 of the core are the remaining regions of the deposition area towards the front and back edges of the core respectively.
The core wrap may be formed by two nonwoven material 16, 16′ which may be at least partially sealed along the edges of the absorbent core. The core wrap may be at least partially sealed along the core's front edge, back edge and two longitudinal edges so that substantially no absorbent material leaks out of the absorbent core wrap when performing the compression step of the WCACF test described below. It is not excluded that the core wrap can be sealed with a seal line further inboard than the core's edge, for example as in a gift wrapping if the core wrap comprises a single substrate. The absorbent core may also advantageously achieve an SAP loss of no more than about 70%, 60%, 50%, 40%, 30%, 20%, 10% according to the Wet Immobilization Test described in WO2010/0051166A1. Further aspects of the absorbent core will now be described in further details.
The absorbent core of the invention may be relatively thin and thinner than can conventional airfelt cores. In particular the caliper of the core (before use) as measured at the crotch point (C) according to the Core Caliper Test as described herein may be from 0.25 mm to 5.0 mm, in particular from 0.5 mm to 4.0 mm.
By “absorbent material” it is meant a material which has at least some absorbency and/or liquid retaining properties, such as SAP, cellulosic fibers as well as some hydrophilically treated synthetic fibers. Typically, glues used in making absorbent cores have no absorbency properties and are not considered as absorbent material. The SAP content may be higher than 80%, for example at least 85%, at least 90%, at least 95% and even up to and including 100% of the weight of the absorbent material contained within the core wrap. This high SAP content may provide a relatively thin core compared to conventional core typically comprising between 40-60% SAP and the rest of cellulose fibers. The absorbent material of the invention may in particular comprises less than 10% weight percent, or less than 5% weight percent, or even be substantially free of natural and/or synthetic fibers. The absorbent material may advantageously comprise little or no airfelt (cellulosic) fibers, in particular the absorbent core may comprise less than 15%, 10%, or 5% airfelt (cellulose) fibers by weight of the absorbent core, or even be substantially free of cellulose fibers.
The absorbent core of the invention may further comprise adhesive for example to help immobilizing the SAP within the core wrap and/or to ensure integrity of the core wrap, in particular when the core wrap is made of two or more substrates. The core wrap will typically extend to a larger area than strictly needed for containing the absorbent material within.
Cores comprising relatively high amount of SAP with various core designs have been proposed in the past, see for example in U.S. Pat. No. 5,599,335 (Goldman), EP1,447,066 (Busam), WO95/11652 (Tanzer), US2008/0312622A1 (Hundorf), WO2012/052172 (Van Malderen). In some embodiments, the absorbent material may be continuously present within the core wrap. In this case, the absorbent material may be for example obtained by the application of a single continuous layer of absorbent material. In other embodiments, the absorbent material may be comprised of individual pockets or stripes of absorbent material enclosed within the core wrap and separated by junction areas.
The continuous layer of absorbent material, in particular of SAP, may also be obtained by combining two “half” absorbent layers having discontinuous absorbent material application pattern wherein the resulting layer is substantially continuously distributed across the absorbent particulate polymer material area, as taught in US2008/0312622A1 (Hundorf) for example. The absorbent core 28 may for example, as illustrated in FIG. 5, comprise a first absorbent layer and a second absorbent layer, the first absorbent layer comprising a first substrate 16 and a first layer 61 of absorbent material, which may be 100% SAP, and the second absorbent layer comprising a second substrate and a second layer of absorbent material, which may also be 100% SAP, and a fibrous thermoplastic adhesive material 51 at least partially bonding each layer of absorbent material 61, 62 to its respective substrate. The first substrate 16 and the second substrate 16′ form the core wrap. The first and second absorbent layers may be deposited on their respective substrate in a deposition pattern comprising land areas comprising absorbent material and junction areas between the land areas which are free of absorbent material. The land areas as exemplified in FIG. 5 for example may be for example transversally orientated and span the width of the absorbent material deposition area 8. The fibrous thermoplastic adhesive material 51 may be at least partially in contact with the absorbent material 61, 62 in the land areas and at least partially in contact with the substrate layer in the junction areas. This imparts an essentially three-dimensional structure to the fibrous layer of thermoplastic adhesive material 51, which in itself is essentially a two-dimensional structure of relatively small thickness, as compared to the dimension in length and width directions. Thereby, the fibrous thermoplastic adhesive material may provide cavities to cover the absorbent material in the land area, and thereby immobilizes this absorbent material, which as already indicated may be 100% SAP.
The thermoplastic adhesive material may comprise, in its entirety, a single thermoplastic polymer or a blend of thermoplastic polymers, having a softening point, as determined by the ASTM Method D-36-95 “Ring and Ball”, in the range between 50° C. and 300° C., and/or the thermoplastic adhesive material may be a hotmelt adhesive comprising at least one thermoplastic polymer in combination with other thermoplastic diluents such as tackifying resins, plasticizers and additives such as antioxidants.
The thermoplastic polymer has typically a molecular weight (Mw) of more than 10,000 and a glass transition temperature (Tg) usually below room temperature or −6° C.<Tg<16° C. Typical concentrations of the polymer in a hotmelt are in the range of about 20 to about 40% by weight. The thermoplastic polymers may be water insensitive. Exemplary polymers are (styrenic) block copolymers including A-B-A triblock structures, A-B diblock structures and (A-B)n radial block copolymer structures wherein the A blocks are non-elastomeric polymer blocks, typically comprising polystyrene, and the B blocks are unsaturated conjugated diene or (partly) hydrogenated versions of such. The B block is typically isoprene, butadiene, ethylene/butylene (hydrogenated butadiene), ethylene/propylene (hydrogenated isoprene), and mixtures thereof. Other suitable thermoplastic polymers that may be employed are metallocene polyolefins, which are ethylene polymers prepared using single-site or metallocene catalysts. Therein, at least one comonomer can be polymerized with ethylene to make a copolymer, terpolymer or higher order polymer. Also applicable are amorphous polyolefins or amorphous polyalphaolefins (APAO) which are homopolymers, copolymers or terpolymers of C2 to C8 alpha olefins.
The tackifying resin may exemplarily have a Mw below 5,000 and a Tg usually above room temperature, typical concentrations of the resin in a hotmelt are in the range of about 30 to about 60%, and the plasticizer has a low Mw of typically less than 1,000 and a Tg below room temperature, with a typical concentration of about 0 to about 15%.
The thermoplastic adhesive 51 used for the fibrous layer preferably has elastomeric properties, such that the web formed by the fibers on the SAP layer is able to be stretched as the SAP swell. Exemplary elastomeric, hotmelt adhesives include thermoplastic elastomers such as ethylene vinyl acetates, polyurethanes, polyolefin blends of a hard component (generally a crystalline polyolefin such as polypropylene or polyethylene) and a Soft component (such as ethylene-propylene rubber); copolyesters such as poly (ethylene terephthalate-co-ethylene azelate); and thermoplastic elastomeric block copolymers having thermoplastic end blocks and rubbery mid blocks designated as A-B-A block copolymers: mixtures of structurally different homopolymers or copolymers, e.g., a mixture of polyethylene or polystyrene with an A-B-A block copolymer; mixtures of a thermoplastic elastomer and a low molecular weight resin modifier, e.g., a mixture of a styrene-isoprenestyrene block copolymer with polystyrene; and the elastomeric, hot-melt, pressure-sensitive adhesives described herein. Elastomeric, hot-melt adhesives of these types are described in more detail in U.S. Pat. No. 4,731,066 issued to Korpman on Mar. 15, 1988.
The thermoplastic adhesive material is advantageously applied as fibers. The fibers may exemplarily have an average thickness of about 1 to about 50 micrometers or about 1 to about 35 micrometers and an average length of about 5 mm to about 50 mm or about 5 mm to about 30 mm. To improve the adhesion of the thermoplastic adhesive material to the substrate or to any other layer, in particular any other nonwoven layer, such layers may be pre-treated with an auxiliary adhesive. The fibers adhere to each other to form a fibrous layer, which can also be described as a mesh.
In certain embodiments, the thermoplastic adhesive material will meet at least one, or several, or all of the following parameters. An exemplary thermoplastic adhesive material may have a storage modulus G′ measured at 20° C. of at least 30,000 Pa and less than 300,000 Pa, or less than 200,000 Pa, or between 140,000 Pa and 200,000 Pa, or less than 100,000 Pa. In a further aspect, the storage modulus G′ measured at 35° C. may be greater than 80,000 Pa. In a further aspect, the storage modulus G′ measured at 60° C. may be less than 300,000 Pa and more than 18,000 Pa, or more than 24,000 Pa, or more than 30,000 Pa, or more than 90,000 Pa. In a further aspect, the storage modulus G′ measured at 90° C. may be less than 200,000 Pa and more than 10,000 Pa, or more than 20,000 Pa, or more then 30,000 Pa. The storage modulus measured at 60° C. and 90° C. may be a measure for the form stability of the thermoplastic adhesive material at elevated ambient temperatures. This value is particularly important if the absorbent product is used in a hot climate where the thermoplastic adhesive material would lose its integrity if the storage modulus G′ at 60° C. and 90° C. is not sufficiently high.
G′ can be measured using a rheometer as indicated in WO2010/27719. The rheometer is capable of applying a shear stress to the adhesive and measuring the resulting strain (shear deformation) response at constant temperature. The adhesive is placed between a Peltier-element acting as lower, fixed plate and an upper plate with a radius R of e.g., 10 mm, which is connected to the drive shaft of a motor to generate the shear stress. The gap between both plates has a height H of e.g., 1500 micron. The Peltier-element enables temperature control of the material (+0.5° C.). The strain rate and frequency should be chosen such that all measurements are made in the linear viscoelastic region.
Absorbent Material Deposition Area 8
The absorbent core may comprise an absorbent material deposition area 8 defined by the periphery of the layer formed by the absorbent material 60 within the core wrap, as seen from the top when the absorbent core is laid flat, as illustrated in FIG. 3. The absorbent material 60 may be applied continuously or discontinuously in the absorbent material deposition area 8. If absorbent material free channels or junction areas between pockets or stripes of absorbent material are present, these are considered to be part of the absorbent material deposition area 8, for example for the purpose of measuring the width or the length L of the absorbent material deposition area.
The shape of the absorbent material deposition area 8 can vary, in particular it can be rectangular as shown in FIG. 3 or shaped with a so-called “dog bone” or “hour-glass” shape, which shows a tapering along its width at least in the crotch region 81 of the absorbent material deposition area, as shown in FIG. 6. When shaped (non-rectangular), the absorbent material deposition area 8 may have a relatively narrow width in the crotch region 81 of the core as this may provide for example better wearing comfort in the finished article incorporating the core. The absorbent material deposition area 8 may thus have a width (as measured in the transversal direction perpendicular to the longitudinal axis 80′) at its narrowest point which is less than about 100 mm, 90 mm, 80 mm, 70 mm, 60 mm or even less than about 50 mm. This narrowest width may typically be in the crotch region and may further be for example at least 5 mm, or at least 10 mm, or at least 20 mm smaller than the maximum width of the absorbent material deposition area 8 at its largest point in the front region 82 and/or back region 83 of the absorbent core.
The basis weight (amount deposited per unit of area) of the absorbent material may also be varied along the absorbent material deposition area 8 to create a profiled distribution of the absorbent material in the longitudinal direction, in the transversal direction, or both directions of the core. Hence the basis weight of the absorbent material may vary along the longitudinal axis of the core 80′, as well as along the transversal axis, or any axis parallel to any of these axes. The basis weight of absorbent material in area of relatively high basis weight such as the crotch point may thus be for example at least 10%, or 20%, or 30%, or 40%, or 50% higher than in an area of relatively low basis weight. In particular the absorbent material present in the absorbent material deposition area 8 at the level of the crotch point C may have more SAP per unit of area deposited as compared to any other area of the front region 82 or back region 83 of the deposition area 8. The basis weight of the SAP may be at least 10%, or 20%, or 30%, or 40%, or 50% higher at the crotch point (C) of the core than at an another point of the absorbent material deposition area on the longitudinal axis, in particular in the front or back region of the core.
The absorbent material 60 may be deposited using known techniques, which may allow relatively precise deposition of SAP at relatively high speed. In particular the SAP printing technology as disclosed for example in US2006/24433 (Blessing), US2008/0312617 and US2010/0051166A1 (both to Hundorf et al.) may be used. This technique uses a printing roll to deposit SAP onto a substrate disposed on a grid of a support which may include a plurality of cross bars extending substantially parallel to and spaced from one another so as to form channels extending between the plurality of cross-bars. This technology allows high-speed and precise deposition of SAP on a substrate. The channels of the absorbent core can be formed for example by modifying the pattern of the grid and receiving drums so that no SAP is applied in areas corresponding to the channels. EP application number 11169396.6 for example discloses this modification in more details.
Superabsorbent Polymer (SAP)
“Superabsorbent polymers” (“SAP”) as used herein refer to absorbent material which are cross-linked polymeric materials that can absorb at least 10 times their weight of an aqueous 0.9% saline solution as measured using the Centrifuge Retention Capacity (CRC) test (EDANA method WSP 241.2-05E). The SAP of the invention may in particular have a CRC value of more than 20 g/g, or more than 24 g/g, or of from 20 to 50 g/g, or from 20 to 40 g/g, or 24 to 30 g/g. The SAP useful in the present invention include a variety of water-insoluble, but water-swellable polymers capable of absorbing large quantities of fluids.
The superabsorbent polymer can be in particulate form so as to be flowable in the dry state. Typical particulate absorbent polymer materials are made of poly(meth)acrylic acid polymers. However, e.g. starch-based particulate absorbent polymer material may also be used, as well polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile. The superabsorbent polymer may be polyacrylates and polyacrylic acid polymers that are internally and/or surface cross-linked. Suitable materials are described in the PCT Patent Application WO07/047598 or for example WO07/046052 or for example WO2009/155265 and WO2009/155264. In some embodiments, suitable superabsorbent polymer particles may be obtained by current state of the art production processes as is more particularly as described in WO 2006/083584. The superabsorbent polymers are preferably internally cross-linked, i.e. the polymerization is carried out in the presence of compounds having two or more polymerizable groups which can be free-radically copolymerized into the polymer network. Useful crosslinkers include for example ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane as described in EP-A 530 438, di- and triacrylates as described in EP-A 547 847, EP-A 559 476, EP-A 632 068, WO 93/21237, WO 03/104299, WO 03/104300, WO 03/104301 and in DE-A 103 31 450, mixed acrylates which, as well as acrylate groups, include further ethylenically unsaturated groups, as described in DE-A 103 31 456 and DE-A 103 55 401, or crosslinker mixtures as described for example in DE-A 195 43 368, DE-A 196 46 484, WO 90/15830 and WO 02/32962 as well as cross-linkers described in WO2009/155265. The superabsorbent polymer particles may be externally surface cross-linked, or: post cross-linked). Useful post-crosslinkers include compounds including two or more groups capable of forming covalent bonds with the carboxylate groups of the polymers. Useful compounds include for example alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyglycidyl compounds as described in EP-A 083 022, EP-A 543 303 and EP-A 937 736, polyhydric alcohols as described in DE-C 33 14 019, cyclic carbonates as described in DE-A 40 20 780, 2-oxazolidone and its derivatives, such as N-(2-hydroxyethyl)-2-oxazolidone as described in DE-A 198 07 502, bis- and poly-2-oxazolidones as described in DE-A 198 07 992, 2-oxotetrahydro-1,3-oxazine and its derivatives as described in DE-A 198 54 573, N-acyl-2-oxazolidones as described in DE-A 198 54 574, cyclic ureas as described in DE-A 102 04 937, bicyclic amide acetals as described in DE-A 103 34 584, oxetane and cyclic ureas as described in EP1,199,327 and morpholine-2,3-dione and its derivatives as described in WO03/031482.
In some embodiments, the SAP are formed from polyacrylic acid polymers/polyacrylate polymers, for example having a neutralization degree of from 60% to 90%, or about 75%, having for example sodium counter ions.
The SAP useful for the present invention may be of numerous shapes. The term “particles” refers to granules, fibers, flakes, spheres, powders, platelets and other shapes and forms known to persons skilled in the art of superabsorbent polymer particles. In some embodiments, the SAP particles can be in the shape of fibers, i.e. elongated, acicular superabsorbent polymer particles. In those embodiments, the superabsorbent polymer particles fibers have a minor dimension (i.e. diameter of the fiber) of less than about 1 mm, usually less than about 500 μm, and preferably less than 250 μm down to 50 μm. The length of the fibers is preferably about 3 mm to about 100 mm. The fibers can also be in the form of a long filament that can be woven.
Typically, SAP are spherical-like particles. In contrast to fibers, “spherical-like particles” have a longest and a smallest dimension with a particulate ratio of longest to smallest particle dimension in the range of 1-5, where a value of 1 would equate a perfectly spherical particle and 5 would allow for some deviation from such a spherical particle. The superabsorbent polymer particles may have a particle size of less than 850 μm, or from 50 to 850 μm, preferably from 100 to 710 μm, more preferably from 150 to 650 μm, as measured according to EDANA method WSP 220.2-05. SAP having a relatively low particle size help to increase the surface area of the absorbent material which is in contact with liquid exudates and therefore support fast absorption of liquid exudates.
The SAP may have a particle sizes in the range from 45 μm to 4000 μm, more specifically a particle size distribution within the range of from 45 μm to about 2000 μm, or from about 100 μm to about 1000, 850 or 600 μm. The particle size distribution of a material in particulate form can be determined as it is known in the art, for example by means of dry sieve analysis (EDANA 420.02 “Particle Size distribution).
In some embodiments herein, the superabsorbent material is in the form of particles with a mass medium particle size up to 2 mm, or between 50 microns and 2 mm or to 1 mm, or preferably from 100 or 200 or 300 or 400 or 500 μm, or to 1000 or to 800 or to 700 μm; as can for example be measured by the method set out in for example EP-A-0,691,133. In some embodiments of the invention, the superabsorbent polymer material is in the form of particles whereof at least 80% by weight are particles of a size between 50 μm and 1200 μm and having a mass median particle size between any of the range combinations above. In addition, or in another embodiment of the invention, said particles are essentially spherical. In yet another or additional embodiment of the invention the superabsorbent polymer material has a relatively narrow range of particle sizes, e.g. with the majority (e.g. at least 80% or preferably at least 90% or even at least 95% by weight) of particles having a particle size between 50 μm and 1000 μm, preferably between 100 μm and 800 μm, and more preferably between 200 μm and 600 μm.
Suitable SAP may for example be obtained from inverse phase suspension polymerizations as described in U.S. Pat. Nos. 4,340,706 and 5,849,816 or from spray- or other gas-phase dispersion polymerizations as described in US Patent Applications No. 2009/0192035, 2009/0258994 and 2010/0068520. In some embodiments, suitable SAP may be obtained by current state of the art production processes as is more particularly described from page 12, line 23 to page 20, line 27 of WO 2006/083584.
The surface of the SAP may be coated, for example, with a cationic polymer. Preferred cationic polymers can include polyamine or polyimine materials. In some embodiments, the SAP may be coated with chitosan materials such as those disclosed in U.S. Pat. No. 7,537,832. In some other embodiments, the SAP may comprise mixed-bed Ion-Exchange absorbent polymers such as those disclosed in WO 99/34841 and WO 99/34842.
The absorbent core will typically comprise only one type of SAP, but it is not excluded that a blend of SAPs may be used. The fluid permeability of a superabsorbent polymer can be quantified using its Urine Permeability Measurement (UPM) value, as measured in the test disclosed European patent application number EP12174117.7. The UPM of the SAP may for example be of at least 10×10−7 cm3·sec/g, or at least 30×10−7 cm3·sec/g, or at least 50×10−7 cm3·sec/g, or more, e.g. at least 80 or 100×10−7 cm3·sec/g. The flow characteristics can also be adjusted by varying the quantity and distribution of the SAP used in the second absorbent layer.
For most absorbent articles, the liquid discharge occurs predominately in the front half of the article, in particular for diaper. The absorbent core may be thus placed in the absorbent article so that the front half of the absorbent article comprises most of the absorbent capacity of the core. Thus, at least: 60%, or 65%, or 70%, or 75%, or 80% of the SAP by weight may be present in the front half of the absorbent article, the remaining SAP being disposed in the back half of the absorbent article. The front half region of the absorbent article can be defined as the region between the front edge 10 of the absorbent article and the transversal axis 90 of the absorbent article. The transversal axis 90 is perpendicular to the longitudinal axis 80 and placed at a distance of half the length of the article as measured on longitudinal axis of the article from the front or back edge thereof.
The total amount of SAP present in the absorbent core may also vary according to expected user. Diapers for newborns may require less SAP than infant or adult incontinence diapers. The amount of SAP in the core may be for example comprised from about 5 to 60 g, in particular from 5 to 50 g. The average SAP basis weight within the (or “at least one”, if several are present) deposition area 8 of the SAP may be for example of at least 50, 100, 200, 300, 400, 500 or more g/m2. The areas of the channels present in the absorbent material deposition area 8 are deduced from the absorbent material deposition area to calculate this average basis weight.
Core wrap (16, 16′)
The core wrap may be made of a single substrate folded around the absorbent material, or may advantageously comprise two (or more) substrates which are attached to another. Typical attachments are the so-called C-wrap and/or sandwich wrap. In a C-wrap, as exemplarily shown in FIGS. 2 and 4, the longitudinal and/or transversal edges of one of the substrate are folded over the other substrate to form flaps. These flaps are then bonded to the external surface of the other substrate, typically by gluing.
The core wrap may be formed by any materials suitable for receiving and containing the absorbent material. Typical substrate materials used in the production of conventional cores may be used, in particular paper, tissues, films, wovens or nonwovens, or laminate of any of these. The core wrap may in particular be formed by a nonwoven web, such as a carded nonwoven, spunbond nonwoven (“S”) or meltblown nonwoven (“M”), and laminates of any of these. For example spunmelt polypropylene nonwovens are suitable, in particular those having a laminate web SMS, or SMMS, or SSMMS, structure, and having a basis weight range of about 5 gsm to 15 gsm. Suitable materials are for example disclosed in U.S. Pat. No. 7,744,576, US2011/0268932A1, US2011/0319848A1 or US2011/0250413A1. Nonwoven materials provided from synthetic fibers may be used, such as PE, PET and in particular PP.
If the core wrap comprises a first substrate 16 and a second substrate 16′ these may be made of the same type of material, or may be made of different materials or one of the substrate may be treated differently than the other to provide it with different properties. As the polymers used for nonwoven production are inherently hydrophobic, they are preferably coated with hydrophilic coatings if placed on the fluid receiving side of the absorbent core. It is advantageous that the top side of the core wrap, i.e. the side placed closer to the wearer in the absorbent article, be more hydrophilic than the bottom side of the core wrap. A possible way to produce nonwovens with durably hydrophilic coatings is via applying a hydrophilic monomer and a radical polymerization initiator onto the nonwoven, and conducting a polymerization activated via UV light resulting in monomer chemically bound to the surface of the nonwoven. An alternative possible way to produce nonwovens with durably hydrophilic coatings is to coat the nonwoven with hydrophilic nanoparticles, e.g. as described in WO 02/064877.
Permanently hydrophilic nonwovens are also useful in some embodiments. Surface tension, as described in U.S. Pat. No. 7,744,576 (Busam et al.), can be used to measure how permanently a certain hydrophilicity level is achieved. Liquid strike through, as described in U.S. Pat. No. 7,744,576, can be used to measure the hydrophilicity level. The first and/or second substrate may in particular have a surface tension of at least 55, preferably at least 60 and most preferably at least 65 mN/m or higher when being wetted with saline solution. The substrate may also have a liquid strike through time of less than 5 s for a fifth gush of liquid. These values can be measured using the test methods described in US7,744,576B2: “Determination Of Surface Tension” and “Determination of Strike Through” respectively.
Hydrophilicity and wettability are typically defined in terms of contact angle and the strike through time of the fluids, for example through a nonwoven fabric. This is discussed in detail in the American Chemical Society publication entitled “Contact angle, wettability and adhesion”, edited by Robert F. Gould (Copyright 1964). A substrate having a lower contact angle between the water and the surface of substrate may be said to be more hydrophilic than another.
The substrates may also be air-permeable. Films useful herein may therefore comprise micro-pores. The substrate may have for example an air-permeability of from 40 or from 50, to 300 or to 200 m3/(m2×min), as determined by EDANA method 140-1-99 (125 Pa, 38.3 cm2). The material of the core wrap may alternatively have a lower air-permeability, e.g. being non-air-permeable, for example to facilitate handling on a moving surface comprising vacuum.
In the present invention, the core wrap may be at least partially sealed along all the sides of the absorbent core or otherwise so that substantially no absorbent material leaks out of the core wrap while performing the WCACF Test indicated below. By “substantially no absorbent material” it is meant that less than 5%, advantageously less than 2%, or less than 1% or 0% by weight of absorbent material escapes the core wrap. In particular the core wrap should not in an appreciable way burst open while the test is conducted.
The term “seal” is to be understood in a broad sense. The seal does not need to be continuous along the whole periphery of the core wrap but may be discontinuous along part or the whole of it, such as formed by a series of closely spaced apart seal points on a line. While the seal may be at the periphery of the core, it is not excluded that a seal may also be at other locations of the core, for example close to the longitudinal centerline 80′. Typically a seal may be formed by gluing and/or thermal bonding.
If the core wrap is formed by two substrates 16, 16′, one seal per edge of the core may be typically be used to enclose the absorbent material 60 within the core wrap. This is exemplified in the FIGS. 4 and 5. As shown in FIG. 4, for example, the first substrate 16 may be placed on one side of the core (the top side as represented therein) and extends around the core's longitudinal edges to at least partially wrap the opposed (bottom) side of the core. The second substrate 16′ can be present between the wrapped flaps of the first substrate 16 and the absorbent material 60 of the core. The flaps of the first substrate 16 may be glued to the second substrate 16′ to provide a strong seal. This so called C-wrap construction can provide benefits such as improved resistance to bursting in a wet loaded state compared to a sandwich seal. The front edge and back edge of the core wrap may then also be sealed for example by gluing the first substrate and second substrate flat to another to provide more complete enclosure of the absorbent material across the whole of the periphery of the core. It can be advantageous to use the C-wrap at least on the longitudinal edges of the core which are longer than the front and end edges. In the so-called sandwich construction, the first and second substrates may also extend outwardly on all edges of the core and be sealed flat along the whole or parts of the periphery of the core typically by gluing and/or heat/pressure bonding. Typically neither first nor second substrates need to be shaped, so that they can be rectangularly cut for ease of production but of course other shapes are possible.
The core wrap may also be formed by a single substrate which may enclose as in a parcel wrap the absorbent material and be for example sealed along the front edge and back edge of the core and one longitudinal seal.
Channels 26, 26′
The absorbent core comprises at least one channel which is at least partially oriented in the longitudinal direction of the core. If the following the plural form “channels” will be used to mean “at least one channel”. The channels may be formed in various ways. For example the channels may be formed by zones within the absorbent material deposition area which may be substantially or completely free of absorbent material, in particular SAP. In addition or alternatively, the channel(s) may also be formed by continuously or discontinuously bonding the material forming the top side of the core wrap to the material forming the bottom side of the core wrap through the absorbent material deposition area. The channels may be advantageously continuous but it is not excluded that the channels are intermittent. The acquisition-distribution system or any sub-layer between the topsheet and the absorbent core, or another layer of the article, may also comprise channels, which may or not correspond to the channels of the absorbent core. The channels may be in particular fully encompassed within the absorbent material deposition area 8.
The channel or channels may in particular be present within the crotch region (81) of the core, in particular at least at the same longitudinal level as the crotch point C, as represented in FIG. 3 by the two longitudinally extending channels 26, 26′. Some channels may also extend from the crotch region 81 into the back region 82 and/or front region 83 of the core or may be solely present in the front region and/or in the back region of the core, as represented in FIG. 6 by the smaller channels 27, 27′.
The absorbent core 28 may also comprise more than two channels, for example at least 3, or at least 4 or at least 5 or at least 6. Shorter channels may also be present, for example in the back region or the front region of the core as represented by the pair of channels 27, 27′ in FIG. 6 towards the front of the core. The channels may comprise one or more pairs of channels symmetrically arranged relative to the longitudinal axis 80′.
The channels may be particularly useful in the absorbent core when the absorbent material deposition area is rectangular, as the channels can improve the flexibility of the core to an extent that there is less advantage in using a non-rectangular (shaped) core. Of course channels may also be present in a layer of SAP having a shaped deposition area.
The channels may extend substantially longitudinally, which means typically that each channel extends more in the longitudinal direction than in the transverse direction, and typically at least twice as much in the longitudinal direction than in the transverse direction (as measured after projection on the respective axis). The channels may have a length L′ projected on the longitudinal axis 80′ of the core that is at least 10% of the length L of the absorbent material deposition area 8. It may be advantageous that at least some or all of the channels are not completely or substantially completely transversely oriented channels in the core.
The channels may be completely oriented longitudinally and parallel to the longitudinal axis but also may be curved. In particular some or all the channels, in particular the channels present in the crotch region, may be concave towards the longitudinal axis 80′, as for example represented in FIGS. 3 and 7 for the pair of channels 26, 26′. The radius of curvature may typically be at least equal (and preferably at least 1.5 or at least 2.0 times this average transverse dimension) to the average transverse dimension of the absorbent material deposition area 8; and also straight but under an angle of (e.g. from) 5° up to 30°, or for example up to 20°, or up to 10° with a line parallel to the longitudinal axis. The radius of curvature may be constant for a channel, or may vary along its length. This may also includes channels with an angle therein, provided said angle between two parts of a channel is at least 120°, preferably at least 150°; and in any of these cases, provided the longitudinal extension of the channel is more than the transverse extension. The channels may also be branched, for example a central channel superposed with the longitudinal axis in the crotch region which branches towards the back and/or towards the front of the article.
In some embodiments, there is no channel that coincides with the longitudinal axis 80′ of the core. When present as symmetrical pairs relative to the longitudinal axis, the channels may be spaced apart from one another over their whole longitudinal dimension. The smallest spacing distance may be for example at least 5 mm, or at least 10 mm, or at least 16 mm. Furthermore, in order to reduce the risk of fluid leakages, the longitudinal main channels typically do not extend up to any of the edges of the absorbent material deposition area 8, and are therefore fully encompassed within the absorbent material deposition area of the core. Typically, the smallest distance between a channel and the closest edge of the absorbent material deposition area is at least 5 mm.
The channels may have a width We along at least part of its length which is at least 2 mm, or at least 3 mm or at least 4 mm, up to for example 20 mm, or 16 mm or 12 mm. The width of the channel may be constant through substantially the whole length of the channel or may vary along its length.
At least some or all the channels are advantageously permanent channels, meaning their integrity is at least partially maintained both in the dry state and in the wet state. Permanent channels may be obtained by provision of one or more adhesive material, for example the fibrous layer of adhesive material or a construction glue that helps adhering for example a substrate with an absorbent material within the walls of the channel. Permanent channels may be also in particular formed by bonding the upper side and lower side of the core wrap (e.g. first substrate 16 and the second substrate 16′) together through the channels. Typically, an adhesive can be used to bond both sides of the core wrap through the channels, but it is possible to bond via other known means, such as pressure bonding, ultrasonic bonding or heat bonding or combination thereof. The core wrap can be continuously bonded or intermittently bonded along the channels. The channels may advantageously remain or become visible at least through the topsheet and/or backsheet when the absorbent article is fully loaded with a fluid as disclosed in the Wet Channel Integrity Test below. This may be obtained by making the channels substantially free of SAP, so they will not swell, and sufficiently large so that they will not close when wet. Furthermore bonding the core wrap to itself through the channels may be advantageous. The Wet Channel Integrity Test described below can be used to test if channels are permanent and visible following wet saturation and to what extent. Advantageously, a permanent channel according to the invention has a percentage of integrity of at least: 20%, or 30%, or 40%, or 50%, or 60, or 70%, or 80%, or 90%, according to the Wet Channel Integrity Test described below.
Topsheet 24
The topsheet 24 is the part of the absorbent article that is directly in contact with the wearer's skin. The topsheet 24 can be joined to the backsheet 25, the core 28 and/or any other layers as is known in the art (as used herein, the term “joined” encompasses configurations whereby an element is directly secured to another element by affixing the element directly to the other element, and configurations whereby an element is indirectly secured to another element by affixing the element to intermediate member(s) which in turn are affixed to the other element). Usually, the topsheet 24 and the backsheet 25 are joined directly to each other in some locations (e.g. on or close to the periphery of the article) and are indirectly joined together in other locations by directly joining them to one or more other elements of the article 20.
The topsheet 24 is preferably compliant, soft-feeling, and non-irritating to the wearer's skin. Further, at least a portion of the topsheet 24 is liquid permeable, permitting liquids to readily penetrate through its thickness. A suitable topsheet may be manufactured from a wide range of materials, such as porous foams, reticulated foams, apertured plastic films, or woven or nonwoven materials of natural fibers (e.g., wood or cotton fibers), synthetic fibers or filaments (e.g., polyester or polypropylene or bicomponent PE/PP fibers or mixtures thereof), or a combination of natural and synthetic fibers. If the topsheet 24 includes fibers, the fibers may be spunbond, carded, wet-laid, meltblown, hydroentangled, or otherwise processed as is known in the art, in particular spunbond PP nonwoven. A suitable topsheet comprising a web of staple-length polypropylene fibers is manufactured by Veratec, Inc., a Division of International Paper Company, of Walpole, Mass. under the designation P-8.
Suitable formed film topsheets are also described in U.S. Pat. Nos. 3,929,135, 4,324,246, 4,342,314, 4,463,045, and 5,006,394. Other suitable topsheets may be made in accordance with U.S. Pat. Nos. 4,609,518 and 4,629,643 issued to Curro et al. Such formed films are available from The Procter & Gamble Company of Cincinnati, Ohio as “DRI-WEAVE” and from Tredegar Corporation, based in Richmond, Va., as “CLIFF-T”.
Any portion of the topsheet may be coated with a lotion as is known in the art. Examples of suitable lotions include those described in U.S. Pat. Nos. 5,607,760, 5,609,587, 5,643,588, 5,968,025 and 6,716,441. The topsheet 24 may also include or be treated with antibacterial agents, some examples of which are disclosed in PCT Publication WO95/24173. Further, the topsheet, the backsheet or any portion of the topsheet or backsheet may be embossed and/or matte finished to provide a more cloth like appearance.
The topsheet 24 may comprise one or more apertures to ease penetration of exudates therethrough, such as urine and/or feces (solid, semi-solid, or liquid). The size of at least the primary aperture is important in achieving the desired waste encapsulation performance. If the primary aperture is too small, the waste may not pass through the aperture, either due to poor alignment of the waste source and the aperture location or due to fecal masses having a diameter greater than the aperture. If the aperture is too large, the area of skin that may be contaminated by “rewet” from the article is increased. Typically, the total area of the apertures at the surface of a diaper may have an area of between about 10 cm2 and about 50 cm2, in particular between about 15 cm2 and 35 cm2. Examples of apertured topsheet are disclosed in U.S. Pat. No. 6,632,504, assigned to BBA NONWOVENS SIMPSONVILLE. WO2011/163582 also discloses suitable colored topsheet having a basis weight of from 12 to 18 gsm and comprising a plurality of bonded points. Each of the bonded points has a surface area of from 2 mm2 to 5 mm2 and the cumulated surface area of the plurality of bonded points is from 10 to 25% of the total surface area of the topsheet.
Typical diaper topsheets have a basis weight of from about 10 to about 28 gsm, in particular between from about 12 to about 18 gsm but other basis weights are possible.
Backsheet 25
The backsheet 25 is generally that portion of the absorbent article 20 which forms the majority of the external surface of the article when worn by the user. The backsheet is positioned towards the bottom side of the absorbent core and prevents the exudates absorbed and contained therein from soiling articles such as bedsheets and undergarments. The backsheet 25 is typically impermeable to liquids (e.g. urine). The backsheet may for example be or comprise a thin plastic film such as a thermoplastic film having a thickness of about 0.012 mm to about 0.051 mm. Exemplary backsheet films include those manufactured by Tredegar Corporation, based in Richmond, Va., and sold under the trade name CPC2 film. Other suitable backsheet materials may include breathable materials which permit vapors to escape from the diaper 20 while still preventing exudates from passing through the backsheet 25. Exemplary breathable materials may include materials such as woven webs, nonwoven webs, composite materials such as film-coated nonwoven webs, microporous films such as manufactured by Mitsui Toatsu Co., of Japan under the designation ESPOIR NO and by Tredegar Corporation of Richmond, Va., and sold under the designation EXAIRE, and monolithic films such as manufactured by Clopay Corporation, Cincinnati, Ohio under the name HYTREL blend P18-3097. Some breathable composite materials are described in greater detail in PCT Application No. WO 95/16746 published on Jun. 22, 1995 in the name of E. I. DuPont; U.S. Pat. No. 5,938,648 to LaVon et al., U.S. Pat. No. 4,681,793 to Linman et al., U.S. Pat. No. 5,865,823 to Curro; and U.S. Pat. No. 5,571,096 to Dobrin et al, U.S. Pat. No. 6,946,585B2 to London Brown.
The backsheet 25 may be joined to the topsheet 24, the absorbent core 28 or any other element of the diaper 20 by any attachment means known in the art. Suitable attachment means are described above with respect to means for joining the topsheet 24 to other elements of the article 20. For example, the attachment means may include a uniform continuous layer of adhesive, a patterned layer of adhesive, or an array of separate lines, spirals, or spots of adhesive. Suitable attachment means comprises an open pattern network of filaments of adhesive as disclosed in U.S. Pat. No. 4,573,986. Other suitable attachment means include several lines of adhesive filaments which are swirled into a spiral pattern, as is illustrated by the apparatus and methods shown in U.S. Pat. Nos. 3,911,173, 4,785,996; and 4,842,666. Adhesives which have been found to be satisfactory are manufactured by H. B. Fuller Company of St. Paul, Minn. and marketed as HL-1620 and HL 1358-XZP. Alternatively, the attachment means may comprise heat bonds, pressure bonds, ultrasonic bonds, dynamic mechanical bonds, or any other suitable attachment means or combinations of these attachment means as are known in the art.
Acquisition-distribution System
The absorbent articles of the invention may comprise an acquisition layer 52, a distribution layer 54, or combination of both (all herein collectively referred to as acquisition-distribution system “ADS”). The function of the ADS is typically to quickly acquire the fluid and distribute it to the absorbent core in an efficient manner. The ADS may comprise one, two or more layers, which may form a unitary layer or remain discrete layers which may be attached to each other. In the examples below, the ADS comprises two layers: a distribution layer 54 and an acquisition layer 52 disposed between the absorbent core and the topsheet, but the invention is not restricted to this example.
Typically, the ADS will not comprise SAP as this may slow the acquisition and distribution of the fluid. The prior art discloses many type of acquisition-distribution system, see for example WO2000/59430 (Daley), WO95/10996 (Richards), U.S. Pat. No. 5,700,254 (McDowall), WO02/067809 (Graef). The ADS may comprise, although not necessarily, two layers: a distribution layer 54 and an acquisition layer 52, which will now be exemplified in more details.
Distribution Layer 54
The function of a distribution layer 54 is to spread the insulting fluid liquid over a larger surface within the article so that the absorbent capacity of the core can be more efficiently used. Typically distribution layer are made of a nonwoven material based on synthetic or cellulosic fibers and having a relatively low density. The density of the distribution layer may vary depending on the compression of the article, but may typically range from 0.03 to 0.25 g/cm3, in particular from 0.05 to 0.15 g/cm3 measured at 0.30 psi (2.07 kPa). The distribution layer 54 may also be a material having a water retention value of from 25 to 60, preferably from 30 to 45, measured as indicated in the procedure disclosed in U.S. Pat. No. 5,137,537. The distribution layer may typically have an average basis weight of from 30 to 400 g/m2, in particular from 100 to 300 g/m2.
The distribution layer may for example comprise at least 50% by weight of cross-linked cellulose fibers. The cross-linked cellulosic fibers may be crimped, twisted, or curled, or a combination thereof including crimped, twisted, and curled. This type of material has been used in the past in disposable diapers as part of an acquisition system, for example US 2008/0312622 A1 (Hundorf). The cross-linked cellulosic fibers provide higher resilience and therefore higher resistance to the first absorbent layer against the compression in the product packaging or in use conditions, e.g. under baby weight. This provides the core with a higher void volume, permeability and liquid absorption, and hence reduced leakage and improved dryness.
Exemplary chemically cross-linked cellulosic fibers suitable for a distribution layer are disclosed in U.S. Pat. Nos. 5,549,791, 5,137,537, WO9534329 or US2007/118087. Exemplary cross-linking agents include polycarboxylic acids such as citric acid and/or polyacrylic acids such as acrylic acid and maleic acid copolymers. For example, the crosslinked cellulosic fibers may have between about 0.5 mole % and about 10.0 mole % of a C2-C9 polycarboxylic acid cross-linking agent, calculated on a cellulose anhydroglucose molar basis, reacted with said fibers in an intrafiber ester crosslink bond form. The C2-C9 polycarboxylic acid cross-linking agent may be selected from the group consisting of:

    
    
        aliphatic and alicyclic C2-C9 polycarboxylic acids having at least three carboxyl groups per molecule; and
        aliphatic and alicyclic C2-C9 polycarboxylic acids having two carboxyl groups per molecule and having a carbon-carbon double bond located alpha, beta to one or both of the carboxyl groups, wherein one carboxyl group in said C2-C9 polycarboxylic acid crosslinking agent is separated from a second carboxyl group by either two or three carbon atoms. The fibers may have in particular between about 1.5 mole % and about 6.0 mole % crosslinking agent, calculated on a cellulose anhydroglucose molar basis, reacted therewith in the form of intrafiber ester crosslink bonds. The cross-linking agent may be selected from the group consisting of citric acid, 1, 2, 3, 4 butane tetracarboxylic acid, and 1, 2, 3 propane tricarboxylic acid, in particular citric acid.
    
    


Polyacrylic acid cross-linking agents may also be selected from polyacrylic acid homopolymers, copolymers of acrylic acid, and mixtures thereof. The fibers may have between 1.0 weight % and 10.0 weight %, preferably between 3 weight % and 7 weight %, of these cross-linking agents, calculated on a dry fiber weight basis, reacted therewith in the form of intra-fiber crosslink bonds. The cross-linking agent may be a polyacrylic acid polymer having a molecular weight of from 500 to 40,000, preferably from 1,000 to 20,000. The polymeric polyacrylic acid cross-linking agent may be a copolymer of acrylic acid and maleic acid, in particular wherein the weight ratio of acrylic acid to maleic acid is from 10:1 to 1:1, preferably from 5:1 to 1.5:1. An effective amount of citric acid may be further mixed with said polymeric polyacrylic acid cross-linking agent.
The distribution layer comprising cross-linked cellulose fibers may comprise other fibers, but this layer may advantageously comprise at least 50%, or 60%, or 70%, or 80%, or 90% or even up to 100%, by weight of the layer, of cross-linked cellulose fibers (including the cross-linking agents). Examples of such mixed layer of cross-linked cellulose fibers may comprise about 70% by weight of chemically cross-linked cellulose fibers, about 10% by weight polyester (PET) fibers, and about 20% by weight untreated pulp fibers. In another example, the layer of cross-linked cellulose fibers may comprise about 70% by weight chemically cross-linked cellulose fibers, about 20% by weight lyocell fibers, and about 10% by weight PET fibers. In another example, the layer may comprise about 68% by weight chemically cross-linked cellulose fibers, about 16% by weight untreated pulp fibers, and about 16% by weight PET fibers. In another example, the layer of cross-linked cellulose fibers may comprise from about 90-100% by weight chemically cross-linked cellulose fibers.
Acquisition Layer 52
The absorbent article 20 may comprise an acquisition layer 52, whose function is to quickly acquire the fluid away from the topsheet so as to provide a good dryness for the wearer. The acquisition layer 52 is typically placed directly under the topsheet. If present, the distribution layer may be at least partially disposed under the acquisition layer. The acquisition layer may typically be or comprise a non-woven material, for example a SMS or SMMS material, comprising a spunbonded, a melt-blown and a further spunbonded layer or alternatively a carded chemical-bonded nonwoven. The non-woven material may in particular be latex bonded. Exemplary upper acquisition layers 52 are disclosed in U.S. Pat. No. 7,786,341. Carded, resin-bonded nonwovens may be used, in particular where the fibers used are solid round or round and hollow PET staple fibers (50/50 or 40/60 mix of 6 denier and 9 denier fibers). An exemplary binder is a butadiene/styrene latex. Non-wovens have the advantage that they can be manufactured outside the converting line and stored and used as a roll of material.
Further useful non-wovens are described in U.S. Pat. No. 6,645,569 to Cramer et al., U.S. Pat. No. 6,863,933 to Cramer et al., U.S. Pat. No. 7,112,621 to Rohrbaugh et al., and co patent applications US2003/148684 to Cramer et al. and US2005/008839 to Cramer et al.
The acquisition layer 52 may be stabilized by a latex binder, for example a styrene-butadiene latex binder (SB latex). Processes for obtaining such lattices are known, for example, from EP 149 880 (Kwok) and US 2003/0105190 (Diehl et al.). In certain embodiments, the binder may be present in the acquisition layer 52 in excess of about 12%, about 14% or about 16% by weight. SB latex is available under the trade name GENFLOTM 3160 (OMNOVA Solutions Inc.; Akron, Ohio).
A further acquisition layer may be used in addition to a first acquisition layer described above. For example a tissue layer may be placed between the first acquisition layer and the distribution layer. The tissue may have enhanced capillarity distribution properties compared to the acquisition layer described above. The tissue and the first acquisition layer may be of the same size or may be of different size, for example the tissue layer may extend further in the back of the absorbent article than the first acquisition layer. An example of hydrophilic tissue is a 13-15 gsm high wet strength made of cellulose fibers from supplier Havix.
Fastening System 42-44
The absorbent article may include a fastening system. The fastening system can be used to provide lateral tensions about the circumference of the absorbent article to hold the absorbent article on the wearer. This fastening system is not necessary for training pant article since the waist region of these articles is already bonded. The fastening system usually comprises a fastener such as tape tabs, hook and loop fastening components, interlocking fasteners such as tabs & slots, buckles, buttons, snaps, and/or hermaphroditic fastening components, although any other known fastening means are generally acceptable. A landing zone is normally provided on the front waist region of the article for the fastener to be releasably attached. Some exemplary surface fastening systems are disclosed in U.S. Pat. Nos. 3,848,594, 4,662,875, 4,846,815, 4,894,060, 4,946,527, 5,151,092 and 5,221,274 issued to Buell. An exemplary interlocking fastening system is disclosed in U.S. Pat. No. 6,432,098. The fastening system may also provide a means for holding the article in a disposal configuration as disclosed in U.S. Pat. No. 4,963,140 issued to Robertson et al.
The fastening system may also include primary and secondary fastening systems, as disclosed in U.S. Pat. No. 4,699,622 to reduce shifting of overlapped portions or to improve fit as disclosed in U.S. Pat. Nos. 5,242,436, 5,499,978, 5,507,736, and 5,591,152.
Front and Back Ears 46, 40
The absorbent article may comprise front ears 46 and back ears 40 as is known in the art. The ears can be integral part of the chassis, for example formed from the topsheet and/or backsheet as side panel. Alternatively, as represented on FIG. 1, they may be separate elements attached by gluing and/or heat embossing. The back ears 40 are advantageously stretchable to facilitate the attachment of the tabs 42 on the landing zone 40 and maintain the taped diapers in place around the wearer's waist. The back ears 40 may also be elastic or extensible to provide a more comfortable and contouring fit by initially conformably fitting the absorbent article to the wearer and sustaining this fit throughout the time of wear well past when absorbent article has been loaded with exudates since the elasticized ears allow the sides of the absorbent article to expand and contract.
Barrier Leg Cuffs 34 and Gasketing Cuffs 32
Absorbent articles such as diapers or training pants may typically further comprise components that increase the fit of the article around the legs of the wearer, in particular barrier leg cuffs 34 and gasketing cuffs 32. The barrier leg cuffs 32 may be formed by a piece of material, typically a nonwoven, which is partially bonded to the rest of the article and can be partially raised away and thus stand up from the plane defined by the topsheet, when the article is pulled flat as shown e.g. in FIGS. 1-2. The barrier leg cuffs can provide improved containment of liquids and other body exudates approximately at the junction of the torso and legs of the wearer. The barrier leg cuffs extend at least partially between the front edge and the back edge of the absorbent article on opposite sides of the longitudinal axis and are at least present at the level of the crotch point (C).
The barrier leg cuffs may be delimited by a proximal edge 64 joined to the rest of the article, typically the topsheet and/or the backsheet, and a free terminal edge 66, which is intended to contact and form a seal with the wearer's skin. The barrier leg cuffs 34 may be joined at the proximal edge 64 with the chassis of the article by a bond 65 which may be made for example by gluing, fusion bonding or combination of known bonding means. The bond 65 at the proximal edge 64 may be continuous or intermittent.
The barrier leg cuffs 32 can be integral with (i.e. formed from) the topsheet or the backsheet, or more typically be formed from a separate material joined to the rest of the article. Typically the material of the barrier leg cuffs may extend through the whole length of the article but is “tack bonded” to the topsheet towards the front edge and back edge of the article so that in these sections the barrier leg cuff material remains flush with the topsheet. Each barrier leg cuff 34 may comprise one, two or more elastic strings 35 close to this free terminal edge 66 to provide a better seal. In addition to the barrier leg cuffs 34, the article may comprise gasketing cuffs 32, which are formed in the same plane as the chassis of absorbent article, in particular may be at least partially enclosed between the topsheet and the backsheet, and may be placed transversely outwardly relative to the barrier leg cuffs. The gasketing cuffs can provide a better seal around the thighs of the wearer. Usually each gasketing leg cuff will comprise one or more elastic string or elastic element comprised in the chassis of the diaper for example between the topsheet and backsheet in the area of the leg openings.
U.S. Pat. No. 3,860,003 describes a disposable diaper which provides a contractible leg opening having a side flap and one or more elastic members to provide an elasticized leg cuff (a gasketing cuff). U.S. Pat. Nos. 4,808,178 and 4,909,803 issued to Aziz et al. describe disposable diapers having “stand-up” elasticized flaps (barrier leg cuffs) which improve the containment of the leg regions. U.S. Pat. Nos. 4,695,278 and 4,795,454 issued to Lawson and to Dragoo respectively, describe disposable diapers having dual cuffs, including gasketing cuffs and barrier leg cuffs. All or a portion of the barrier leg and/or gasketing cuffs may be treated with a lotion.
Elastic Waist Feature
The absorbent article may also comprise at least one elastic waist feature (not represented) that helps to provide improved fit and containment. The elastic waist feature is generally intended to elastically expand and contract to dynamically fit the wearer's waist. The elastic waist feature preferably extends at least longitudinally outwardly from at least one waist edge of the absorbent core 28 and generally forms at least a portion of the back side of the absorbent article. Disposable diapers can be constructed so as to have two elastic waist features, one positioned in the front waist region and one positioned in the back waist region. The elastic waist feature may be constructed in a number of different configurations including those described in U.S. Pat. Nos. 4,515,595, 4,710,189, 5,151,092 and 5,221,274.
Relations Between the Layers and Components
Typically, adjacent layers will be joined together using conventional bonding method such as adhesive coating via slot coating or spraying on the whole or part of the surface of the layer, or thermo-bonding, or pressure bonding or combinations thereof. This bonding is not represented in the Figures (except for the bonding 65 between the raised element of the leg cuffs 34 with the topsheet 24) for clarity and readability but bonding between the layers of the article should be considered to be present unless specifically excluded. Adhesives may be typically used to improve the adhesion of the different layers, for example between the backsheet and the core wrap. The glue may be any standard hotmelt glue as known in the art.
If an acquisition layer 52 is present, it may be advantageous that this acquisition layer is larger than or least as large as the distribution layer 54 in the longitudinal and/or transversal dimension. Thus the distribution layer 54 can be deposited on the acquisition layer 52. This simplifies handling, in particular if the acquisition layer is a nonwoven which can be unrolled from a roll of stock material. The distribution layer may also be deposited directly on the absorbent core's upper side of the core wrap or another layer of the article. Also, an acquisition layer 52 larger than the distribution layer allows to directly glue the acquisition layer to the storage core (at the larger areas). This can provide an increased article integrity and better liquid communication.
The absorbent core and in particular its absorbent material deposition area 8 may advantageously be at least as large and long and advantageously at least partially larger and/or longer than any of the layer in the ADS. This is because the absorbent material in the core can usually more effectively retain fluid and provide dryness benefits across a larger area than the ADS. The absorbent article may have a rectangular SAP layer and a non-rectangular (shaped) ADS. The absorbent article may also have a rectangular (non-shaped) ADS and a rectangular layer of SAP.
Method of Making
The absorbent cores and articles of the invention may be made by any conventional methods known in the art. In particular the articles may be hand-made or industrially produced at high speed on a modern converting line.
Experimental Settings
The values indicated herein are measured according to the methods indicated herein below, unless specified otherwise. All measurements are performed at 21° C.±2° C. and 50%±20% RH, unless specified otherwise. All samples should be kept at least 24 hours in these conditions to equilibrate before conducting the tests, unless indicated otherwise. All measurements should be reproduced on at least 4 samples and the average value obtained indicated, unless otherwise indicated.
Centrifuge Retention Capacity (CRC)
The CRC measures the liquid absorbed by the superabsorbent polymer particles for free swelling in excess liquid. The CRC is measured according to EDANA method WSP 241.2-05.
Dry Absorbent Core Caliper Test
This test may be used to measure the caliper of the absorbent core (before use i.e. without fluid loading) in a standardized manner.
Equipment: Mitutoyo manual caliper gauge with a resolution of 0.01 mm—or equivalent instrument.
Contact Foot: Flat circular foot with a diameter of 17.0 mm (±0.2 mm). A circular weight may be applied to the foot (e.g., a weight with a slot to facilitate application around the instrument shaft) to achieve the target weight. The total weight of foot and added weight (including shaft) is selected to provide 2.07 kPa (0.30 psi) of pressure to the sample.
The caliper gauge is mounted with the lower surface of the contact foot in an horizontal plane so that the lower surface of the contact foot contacts the center of the flat horizontal upper surface of a base plate approximately 20×25 cm. The gauge is set to read zero with the contact foot resting on the base plate.
Ruler: Calibrated metal ruler graduated in mm.
Stopwatch: Accuracy 1 second
Sample preparation: The core is conditioned at least 24 hours as indicated above.
Measurement procedure: The core is laid flat with the bottom side, i.e. the side intended to be placed towards the backsheet in the finished article facing down. The point of measurement (e.g. the crotch point C) is carefully drawn on the top side of the core taking care not to compress or deform the core.
The contact foot of the caliper gauge is raised and the core is placed flat on the base plate of the caliper gauge with the top side of the core up so that when lowered, the center of the foot is on the marked measuring point.
The foot is gently lowered onto the article and released (ensure calibration to “0” prior to the start of the measurement). The caliper value is read to the nearest 0.01 mm, 10 seconds after the foot is released.
The procedure is repeated for each measuring point. If there is a fold at the measuring point, the measurement is done in the closest area to this point but without any folds. Ten articles are measured in this manner for a given product and the average caliper is calculated and reported with an accuracy of one tenth mm.
Absorbent Article Caliper Test
The Absorbent Article Caliper Test can be performed as for the Dry Absorbent Core Caliper Test with the difference that the caliper of the finished absorbent article is measured instead of the caliper of the core. The point of measurement may be the intersection of the longitudinal axis (80) and transversal axis (90) of the absorbent article. If the absorbent articles were provided folded and/or in a package, the articles to be measured are unfolded and/or removed from the center area of the package. If the package contains more than 4 articles, the outer most two articles on each side of the package are not used in the testing. If the package contains more than 4 but fewer than 14 articles, then more than one package of articles is required to complete the testing. If the package contains 14 or more articles, then only one package of articles is required to perform the testing. If the package contains 4 or fewer articles then all articles in the package are measured and multiple packages are required to perform the measurement. Caliper readings should be taken 24±1 hours after the article is removed from the package, unfolded and conditioned. Physical manipulation of product should be minimal and restricted only to necessary sample preparation.
Any elastic components of the article that prevent the article from being laid flat under the caliper foot are cut or removed. These may include leg cuffs or waistbands. Pant-type articles are opened or cut along the side seams as necessary. Apply sufficient tension to flatten out any folds/wrinkles Care is taken to avoid touching and/or compressing the area of measurement.
Wet Channel Integrity Test
This test is designed to check the integrity of a channel in an absorbent core following wet saturation.
1. The full length (in millimeters) of the channel is measured in the dry state (if the channel is not straight, the curvilinear length through the middle of the channel is measured).
2. The absorbent core is then completely immersed in a large excess (e.g. 5 liters) of synthetic urine “Saline”, with a concentration of 9.00 g NaCl per 1000 ml solution prepared by dissolving the appropriate amount of sodium chloride in distilled water. The temperature of the solution must be 20+/−5° C.
3. After 1 minute in the saline, the core is removed and held vertically by one end for 5 seconds to drain, then extended flat on an horizontal surface with the top side (the side intended to be facing the wearer in the article) facing up. If the core comprises stretch elements, it is pulled taut so that no contraction is observed. The core can be fixed to an horizontal surface by clamps at its front edge and back edge, so that no contraction can happen.
4. The absorbent core is covered with a rectangular suitably weighted rigid plate, with dimensions as follows: length equal to the full length of the core, and width equal to the maximum core width at the widest point.
5. A pressure of 18.0 kPa is applied for 30 seconds over the area of the rigid plate above mentioned. Pressure is calculated on the basis of overall area encompassed by the rigid plate. Pressure is achieved by placing additional weights in the geometric center of the rigid plate, such that the combined weight of the rigid plate and the additional weights result in a pressure of 18.0 kPa over the total area of the rigid plate.
6. After 30 seconds, the additional weights and the rigid plate are removed.
7. Immediately afterwards, the cumulative length of the portions of the channel which remained intact is measured (in millimeters; if the channel is not straight, the curvilinear length through the middle of the channel is measured). If no portions of the channel remained intact then the channel is not permanent.
The percentage of integrity of the permanent channel is calculated by dividing the cumulative length of the portions of the channel which remained intact by the length of the channel in the dry state, and then multiplying the quotient by 100.
Wet Caliper And Compression Force (WCACF) Test
This test measures a) the percentage of increase in caliper of a saturated absorbent core following one lateral compression, and b) the force required to laterally compress the saturated absorbent core to a width of 40 mm. The WCACF Test is to be performed on an absorbent core according to the following instructions.

    
    
        1. Mark the longitudinal axis on the absorbent core on the top side of the core. The longitudinal axis generally divides the top side of the core into two roughly symmetric pieces along the length of the absorbent core when the core is viewed from the top as exemplarily shown on FIG. 3. The top side of the core is the side intended to be placed towards the wearer-facing side of the absorbent article. In doubt, the top side is normally more hydrophilic than the bottom side. If the top side still cannot be identified, the test is then conducted on an equal number of cores on alternative sides and the results averaged. Marking can be made with any pen taking care not to damage the core while marking
        2. Mark the crotch line on the same side of the absorbent core as the longitudinal centerline. The crotch line is perpendicular to the longitudinal axis and crosses the longitudinal axis at a distance equal to 45% of the length L of the absorbent core (0.45 L). This distance is measured from the front side of the absorbent core (see FIG. 3 for an exemplary illustration). The front side of the absorbent core is the side of the core intended to be placed towards the front of the absorbent article. If the intended orientation of the core is not known, the front edge is on the side of the core where the amount of SAP is higher. If the front edge can still not be identified, then half the samples can be tested with the distance starting from one side and the other half with the distance starting from the other side, and the results averaged. The intersection of the crotch line and the longitudinal axis is the crotch point C.
        3. The absorbent core is then immersed in a large excess, e.g. 5 l, of synthetic urine “Saline”, with a concentration of 9.00 g NaCl per 1000 ml solution prepared by dissolving the appropriate amount of sodium chloride in distilled water. The container must be large enough to accommodate the core in a flat configuration. The marked side of the core faces up during the immersion.
        4. After 1 minute in the saline, the absorbent core is removed and held vertically by the front side for 10 seconds to drain.
        5. The absorbent core is left to equilibrate for 10 minutes by pulling it flat on a horizontal surface, with the top side facing down. Clamps placed on the front and back sides of the core may be used to keep the loaded core flat.
        6. The caliper of the loaded absorbent core before compression is then measured at the crotch point and reported as Cinitial. For this purpose, a presser foot with a diameter of 17.0 mm is used, and a pressure of 2.07 kPa (0.30 psi) is applied. The absorbent core is laid flat on a plexiglas plate the marked side facing up, and the presser foot is gently lowered so that it is centered on the crotch point C. The thickness Cinitial is measured 30±2 seconds after initial contact between the foot and the core and reported to the nearest 0.1 mm.
        7. The loaded absorbent core with its top side facing up is then fixed on a rigid-plastic cylinder as schematically represented in FIG. 7. The cylinder 600 has a diameter d of 150 mm (+−1 mm). The last 20.0 mm (+−0.5 mm) of the front edge 280 of the core 28 is first attached to the external surface of the cylinder closest to the operator via a double sided tape previously applied on the cylinder or other fastening means so that the absorbent core can be securely and releasably attached to the cylinder. The last 20.0 mm (+−0.5 mm) of the back edge 282 of the core 28 is then attached at the diametrically opposed external surface of the cylinder at a high sufficient for the crotch point C to coincide with the central axis 610 of the cylinder 600.
        8. One understands that the cylinder needs to be sufficiently high so that the back edge of the core can be attached to it.
        9. The absorbent core is then laterally compressed as detailed below. Compressive forces are applied to the absorbent core by an assembly comprised of a pair of compression plates 630, 640, which simulate the portion of the legs compressing the absorbent core during use. Each compression plate should have dimensions 90 mm (+−1 mm)×90 mm (+−1 mm). The plates can be made from any suitable material that can be formed into the required flat, square shape (e.g. aluminum, Plexiglas). The plates should be placed lined up opposite one another. The compression plates are placed so that the Crotch Line on the top side of the core and the geometrical center of each compression plate are aligned and are in a horizontal plane.
        10. Each compression plate is driven toward the crotch point at constant rate of 100 mm/min. (total closing speed is 200 mm/min). The gap between both compression plates starts at a distance of 140.0 mm+−0.5 mm, or more if the width of the core so requires, and then narrows to a final gap of 40.0 mm+−0.5 mm when the absorbent core is compressed. The compression plates may for example use an apparatus such as a Zwick Z 1.0 or similar. The testing instrument includes a right clamp for securing one compression plate, and a left clamp for securing another compression plate. The equipment should include a force cell with an appropriate measurement range e.g. up to 100 N and a precision of at least +/−0.01 N.
        11. Once the absorbent core has been compressed to 40 mm, compression is maintained for 30 seconds. The force at the end of the 30 seconds immediately before the compression is released is recorded to the nearest 0.01 N and reported as the “Wet Compression Force”. The compression plates can then be returned to their initial positions at a speed of 100 mm/min for each plate.
        12. Immediately afterwards, the absorbent core is removed from the cylinder 600, taking care to not touch the area that has been compressed. If some absorbent material leaked out of the core wrap during the compression step this leaked out absorbent material is collected and weighted.
        13. The caliper at the crotch point C is measured again using the thickness measuring procedure as described above on step 6. This caliper value is reported as Cfinal.
    
    


This procedure is repeated for at least 4 core samples. The Relative Wet Caliper Increase (RWCI) of the absorbent core is then calculated as follows:

Relative Wet Caliper Increase (%)=(ΣCfinal−ΣCinitial)*100/ΣCinitial

where ΣCfinal is the sum of Cfinal values measured for all the samples and ΣCinitial is the sum of the Cinitial values measured for all the samples. The Relative Wet Caliper Increase value of the cores according to the invention is less than 10.0%, in particular it may range of from 1.0% to 9.5%, or 2.0% to 9.0%, or from to 2.5% to 8.0%.
If some absorbent material leaked at step 12, the rest of the absorbent material still contained in the core can be extracted and also weighted. If the amount that leaked represents less than 5% by weight of the total absorbent material of the core (leaked and extracted) then it is considered that “substantially no absorbent material” leaked during the test. Advantageously less than 2%, or less than 1% or even 0% by weight of absorbent material escapes the core wrap during step 12. In particular the core wrap should not in an appreciable way burst open while the test is conducted.
EXPERIMENTALS
The following absorbent core according to the invention was prepared:
Invention Example 1
The absorbent cores tested in this example were similar to the core illustrated in FIG. 3. The cores contained SAP as absorbent material, without cellulosic fibers. The core wrap comprised two substrates forming the top and bottom sides of the core, the upper substrate forming a C-wrap along the longitudinal edges of the core and the front and back edges of the core being attached flat. The core comprised two absorbent material free channels in the crotch region. The channels were symmetric in relation to the longitudinal axis 80 had a projected length thereon of about 227 mm, a width of about 8 mm and a shortest distance from each other of 20 mm. The core wrap was further attached to itself through the channels.
The absorbent core comprised in total 14.1 g fast absorbing SAP applied in an area of deposition having a length of 360 mm and a width of 110 mm (rectangular profile). The SAP was distributed so that the basis weight of SAP was higher in the crotch region than at the front region and still lower towards the back region. There was no profiling of the SAP in the transversal direction (“cross-machine direction” or “CD”, except of the channels which were free of absorbent material). The absorbent core was formed by SAP printing technology, as disclosed in US2010/0051166A1, which combines two nonwoven substrates each supporting a SAP layer and having a microfiber elastic glue applied on each SAP layer which immobilizes the SAP layer on the substrate. The channels were formed by using a suitable printing drum delimiting the channels shape, further information on how to form channels can be found in EP application number EP12174117.7 using printed SAP technology.
Auxiliary glue was applied between the SAP layer and the upper substrate 16, and was slot coated with 41 slots 1 mm wide with a distance of 1 mm between the slots along the whole length of the core wrap (390 mm). 0.211 g and 0.168 g of microfiber glue (from H. B. Fuller) were respectively applied on the upper and lower SAP layer, the area of application having a width of 110 mm and length of 390 mm on each SAP layer.
The core wrap had a length of 390 mm with two end flaps free of absorbent material having a length of 15 mm at the back and at the front edge of the absorbent core. The front and back end seals of the core were slot glued together, the glue slots having a length of 30 mm from the front end seal and 20 mm from the back end seal. The folded width of the core wrap was 120 mm.
The upper substrate 16 was a 10 gsm hydrophilically treated SMMS nonwoven and the lower substrate 16′ was a 10 gsm SMMS nonwoven. The upper substrate was cut at a length of 390 mm and a cut width of 165 mm. The lower substrate had a cut length of 390 mm and a cut width of 130 mm. The upper substrate was C-wrapped around the lower substrate on the lateral edges of the core and the lateral edges of the lower layer was slightly formed upwards on the edge of the absorbent material of the core so that the overall width of the folded core wrap was about 120 mm. The C-wrap was made permanent by application between the substrates of a core folding glue applied at 20 gsm with 2 slots having a slot width of 3 mm and 390 mm long on each side of the core.
The two substrates were additionally bonded together through the channels. The bond was formed by applying pressure and the auxiliary and microfiber glue. The bond was strong. The core wrap seals resisted compression and no absorbent material escaped the core wrap during the WCACF Test.
Invention Example 2
The cores tested in this example had two pair of channels and a shaped deposition area similar to the one shown in FIG. 6. The width of the absorbent material deposition area was 110 mm at the front and the back region and 90 mm at the crotch point of the absorbent material deposition area.
The projected lengths of the long and short channels on the longitudinal axis of the core were about 170 mm and 40 mm respectively. The smallest distance between the longer channels was about 16 mm. The smallest distance between the shorter channels was about 14 mm. The cores comprised 11.53 g of SAP. The core wrap comprised two nonwovens, the top substrate (16) was a 10 gsm SMMS nonwoven treated by a surfactant to be hydrophilic. The lower substrate (16′) was a 11 gsm SMMS nonwoven. Auxiliary glue was applied between the lower SAP layer and its respective lower substrate which was slot coated with 41 slots 1 mm wide with a distance of 1 mm between the slots along the whole length of the core wrap (390 mm). The microfiber glue (from H. B. Fuller) applied on each SAP layer was uniformly applied at width of 108 mm and length of 390 mm on each SAP layer, 0.211 g of microfiber glue was used on the core cover side and 0.211 g on the dusting layer side. The rest of the core construction was substantially similar as the cores in Invention Example 1.
Comparative Example
The comparative example 1 was substantially similar to Invention Example 2 with the difference that the absorbent core did not comprise material free channels.
Test Results
Four samples of each above mentioned products were tested according to the WCACF Test described above to measure the Relative Wet Caliper Increase and the Wet Compression Force of the core. The averaged results are compiled below:

















ΣCfinal/
Relative
Wet



ΣCinitial/4
4
wet caliper
Compression



[mm]
[mm]
increase
Force (N)




















Invention Example 1
13.0
13.8
6.2%
4.83


Invention Example 2
10.8
11.5
6.5%
2.81


Comparative
10.8
11.9
10.2%
3.05


example 1









Discussion

While not wishing to be bound by theory it is believed that the following features can provide alone or in combinations an increase in the relative wet caliper to an absorbent core missing one or more of the below features. None of these features should be considered as being limited the scope of the claims, unless specifically claimed.

    
    
        1) The top side of the wrap and the bottom side of the wrap may advantageously be at least partially bonded to each other through the channels. These bonds may be continuous or intermittent, and may be made via gluing and/or heat bonding, and may advantageously be sufficiently strong to at least partially resist delamination upon fluid loading (“permanent channels”), as discussed above. By constraining the core wrap in the channels, these bonds increase the strain of the core wrap and can diminish the wet caliper increase upon core loading.
        2) The core wrap may comprise a first substrate (16) and a second substrate (16′), both typically made of a nonwoven, wherein the first substrate forms a C-wrap around the second substrate. The first substrate may form the top side of the core wrap and the second substrate may form at least part of the bottom side of the core wrap. Typically the substrates may be bonded, for example by gluing, along the wrapped flaps of the first substrate together with the bottom side of the second substrate. The inventors believe that a C-wrap, especially along part or whole of the longitudinal sides of the absorbent core, can better restrain the absorbent material from breaking out of the core upon compression.
        3) The Wet Compression Force is influenced by the amount of absorbent material and the shape of the deposition area of the core in the crotch region. It is believed that a lower amount of absorbent material and/or a narrower deposition area at the crotch region of the core (as in a shaped area) can provide a decreased Wet Compression Force. The absorbent core of the invention may exemplarily have a Wet Compression Force below 5.00 N, in particular less than 3.00 N, or from 1.00 N to 5.00 N, as measured by the WCACF Test.
    
    


The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.”
Every document cited herein, including any cross referenced or related patent or application, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

## Claims

1. An absorbent core, wherein the absorbent core comprises a core wrap enclosing an absorbent material, wherein the absorbent material comprises at least 80% of superabsorbent polymers by weight of the absorbent material;
the absorbent core comprising a front edge, a back edge, two longitudinal edges, a front region, a back region and a crotch region, the absorbent core having a longitudinal axis oriented in a longitudinal direction, and a length (L) as measured between the front edge and back edge along the longitudinal axis which is at least 320 mm; the absorbent core further comprising:
a first absorbent layer and a second absorbent layer, wherein the first absorbent layer comprises a first substrate and a first layer of superabsorbent polymers, wherein the second absorbent layer comprises a second substrate and a second layer of superabsorbent polymers, wherein the absorbent core comprises a fibrous thermoplastic adhesive material positioned between the first absorbent layer and the second absorbent layer, wherein the fibrous thermoplastic adhesive material at least partially contributes to bonding each layer of the superabsorbent polymers to its respective substrate, and wherein the first substrate and the second substrate form the core wrap; and
at least a pair of channels symmetrically disposed relative to the longitudinal axis of the core, wherein neither channel coincides with the longitudinal axis and wherein the channels have a length projected on the longitudinal axis of the core which is at least 10% of the length L of the absorbent core, wherein the channels are substantially free of the absorbent material, wherein the width of a portion of the channels is at least about 2 mm, and wherein the channels are formed by bonding the first substrate directly to the second substrate;
wherein the channels are present at least at the same longitudinal level as a crotch point, C, wherein the crotch point is the point placed at a distance of 45% of the length L from the front edge of the absorbent core, and wherein the channels extend from the crotch region into the back region;
wherein the absorbent core has a Relative Wet Caliper Increase (RWCI) after compression of less than 10.0%; and wherein the core wrap is at least partially sealed along the edges of the core.

2. The absorbent core of claim 1, wherein the core has a Wet Compression Force of less than about 5.00 N.

3. The absorbent core of claim 2, wherein the Wet Compression Force is of from about 1.00 to about 3.00 N.

4. The absorbent core of claim 1, wherein the first substrate comprises a first nonwoven and the second substrate comprises a second nonwoven, and wherein the first nonwoven forms a C-wrap around the second nonwoven.

5. The absorbent core of claim 1, wherein the absorbent material defines an absorbent material deposition area within the core wrap, wherein the absorbent material deposition area is rectangular or shaped with a width having a minimum in the crotch region of the absorbent core, wherein the crotch region is the region of the core extending from the crotch point towards the back edge and the front edge of the absorbent core by a distance of a quarter of L in both directions towards the back edge and towards the front edge.

6. The absorbent core of claim 5, wherein the basis weight of the superabsorbent polymers is not homogenously distributed along the longitudinal axis of the core within the absorbent material deposition area, and wherein the basis weight of the superabsorbent polymers is at least 10% percent higher at the crotch point of the core than at another point of the absorbent material deposition area on the longitudinal axis.

7. The absorbent core of claim 1, wherein the absorbent material comprises at least about 90% of the superabsorbent polymers by total weight of the absorbent material.

8. The absorbent core of claim 1, wherein the absorbent material comprises less than about 10% of natural or synthetic fibers by total weight of the absorbent material.

9. The absorbent core of claim 8, wherein the absorbent material is substantially free of natural or synthetic fibers.

10. The absorbent core of claim 1, comprising from about 5 g to about 60 g of superabsorbent polymers.

11. The absorbent core of claim 10, comprising from about 10 g to about 50 g of superabsorbent polymers.

12. The absorbent core of claim 1, wherein the caliper of the core, as measured at the crotch point, is from about 0.25 mm to about 5.0 mm.

13. The absorbent core of claim 12, wherein the caliper of the core, as measured at the crotch point, is from about 0.5 mm to about 3.0 mm.

14. An absorbent article comprising: a liquid permeable topsheet, a liquid impermeable backsheet, and an absorbent core positioned between the topsheet and the backsheet;
wherein the absorbent core comprises the absorbent core of claim 1.