Field of the invention

The present invention relates to absorbent members in absorbent cores for absorbent arti¬ cles, which are intended to receive and retain bodily discharges such as urine. Such arti¬ cles are disposable hygiene articles like baby diapers, training pants, adult incontinence articles, feminine care articles and the like. The improvement essentially is based on the recognition that replacing most or all of the cushioning fibrous absorbent material in an absorbent core by a liquid storage material capable of retaining liquid while maintaining or improving acquisition behavior is desirable as the reduction in cushioning is more than compensated by the gain in comfort. The comfort however can only be achieved if the more fundamental requirements of a diaper in respect to liquid handling are satisfied or improved. Especially if this liquid handling performance is improved beyond the per¬ formance of conventional absorbent structures in order to allow creation of thinner and drier absorbent articles, the users of such articles would experience them as providing a more than expected comfort improvement relative to the thinness gain. To provide such absorbent cores and articles made therewith only became possible with the development of new highly permeable absorbent gel materials especially capable to acquire and con¬ duct liquids at super absorbent polymer concentrations, which are not used today. The second aspect allowing this breakthrough development is the ability to maintain the com¬ fort and performance of such high super absorbent polymer concentration articles during the full usage cycle of the article, from dry to fully loaded, especially by improving the ability of the absorbent members to maintain permeability of such articles during use.

Background of the invention

Absorbent Articles for receiving and retaining bodily discharges such as urine or faeces such as disposable diapers, training pants, and adult incontinence articles are well known in the art, and significant effort has been spent against improving their performance. Such improvements generally aim at addressing the primary function of such articles, namely retaining body fluids, but also at minimizing the negatives associated with wearing such articles by increasing the comfort of the wearer. One aspect contributing to such comfort is how to pick up and retain the body waste (primarily liquid) in an "absorbent structure", whereby the waste material is acquired by the article, then conducted away from the loca¬ tion of acquisition (distribution, permeation), and then stored (retained).

It is also well established that reducing the thickness of the article by reducing the pri¬ mary thickness cause, i.e. the absorbent structure helps to improve comfort. This however was always a question of balance between liquid handling performance and thickness. Also a substantial amount of cushioning was considered necessary for comfortable dia¬ pers. Finally the skilled person considered it impossible to reduce or even remove the fibrous material to a point where the modern particulate super absorbent materials would take over part or all of the liquid acquisition and distribution functionalities previously provided by fibrous matrixes. Finally even if there were structures which could possibly provide all such beneficial aspects when dry it was completely in-conceivable that this could be build into an absorbent structure such that the liquid handling and comfort per¬ formance would be maintained even after the first gushes of liquid had been absorbed.

The development of absorbent structures of particular thinness has also other beneficial aspects making such a development the subject of substantial commercial interest. For example, thinner diapers are not just less bulky to wear and fit better under clothing they are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.

As indicated the ability to provide thinner absorbent articles such as diapers has been con¬ tingent on the ability to develop relatively thin absorbent structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of absorbent polymers often referred to as "hydrogels, "hydrogel-forming polymer "super absorbents" "hydrocolloid", "super absorbent polymer or SAP" has been particularly im¬ portant. See, for example, the discussion contained in EP 1 393 757 A chapters 8 to 22.

An important property at higher concentrations of these SAPs is their permeability/flow conductivity. Permeability/flow conductivity can be defined in terms of Saline Flow Con¬ ductivity (SFC) values. SFC is well established in the art and defined in great detail e.g. in PCT WO-95-26209, page 69 following or EP-B-752892 paragraph 224 following. SFC measures the ability of a material to transport saline fluids, such as the ability of the hy- drogel layer formed from the swollen SAP to transport body fluids. Typically, an air-laid web of pulp fibers (e.g., having a density of 0.15 g/cc) will exhibit an SFC value of about 200 xlO-7 cm3sec/g. By contrast, typical SAPs such as Aqualic L-74 (made by Nippon Shokubai Co., LTD) and Nalco-1180 (made by Nalco Chemical Co.) exhibit SFC values of at most 1 xlO-7 cm3sec/g. Accordingly, it would be highly desirable to be able to use SAPs that more closely approach an air-laid web of wood pulp fibers in terms of SFC.

Accordingly, it would be desirable to be able to provide an absorbent member compris¬ ing: (1) a region or regions having a relatively high concentration of SAP particles; (2) with relatively high permeability/flow conductivity properties more like an air-laid fi¬ brous web; (3) that can readily acquire fluids from or even provide the function of high capillary suction acquisition layers under typical usage conditions / pressures even when subjected to normal use forces. It would also be highly desirable to be able to use SAPs in these absorbent members that, when swollen by body fluids, continue to have a good permeability, improved wicking and/or improved swelling properties.

SUMMARY OF THE INVENTION

The present invention and its characteristics are defined in the independent claims and preferred embodiments are specified in the dependant claims. In particular the present invention relates to absorbent cores useful in the provision of absorbent incontinence arti¬ cles such as baby diapers or adult incontinence articles, which articles preferably com¬ prise a topsheet with the absorbent core positioned (immediately) adjacent to it and fur¬ ther optionally a backsheet which together with the topsheet sandwiches the absorbent core.

The absorbent cores of the present invention are especially useful for collection of bodily liquids such as urine. The absorbent core is a component of an absorbent article that is primarily responsible for the fluid handling properties of the article, including acquiring, transporting, distributing and storing body fluids. As such, the absorbent core typically does not include the topsheet or backsheet of the absorbent article. The absorbent core can be provided by a number of absorbent members. An absorbent member refers to a component of the absorbent core that typically provides one or more fluid handling prop- erties, e.g., fluid acquisition, fluid distribution, fluid transportation, fluid storage, etc. An absorbent member can provide the entire absorbent core or only a portion of the absorbent core, i.e., the absorbent core can comprise one or more absorbent members.

According to the present invention an absorbent member, comprised in an absorbent core, is provided with a swellable hydrogel-forming polymer. This swellable hydrogel-forming polymer comprises in combination at least one hydrophilic polymer with dendritic struc¬ ture and at least one water-insoluble phosphate.

In a preferred embodiment the hydrophilic polymer with dendritic structure is a polyester made of a polyol and 2,2-dimethylolpropione acid, alternatively the hydrophilic polymer can be selected from the group of polypropylenimine, polyamidoamine, polyesteramide or combinations thereof.

In preferred embodiments the water-insoluble phosphate is a calcium phosphate. In other preferred alternatives or in combination with the previous embodiment the hydrogel- forming polymer can comprise a powdered and/or dusty additive, preferably the additive is selected from the group of a metallic salt, a pyrogenic silicic acid, a polysaccharide, a non-ionic tenside, a wax, diatomaceous earth or combination thereof.

The hydrogel forming polymer according to the present invention preferably has a high saline flow conductivity, preferably a saline flow conductivity of at least 80 x 10"7 cm3 s g "1J preferably at least 12O x 10"7 cm3 s g ~\

In particularly beneficial embodiments of the absorbent core according to the present in¬ vention the absorbent member comprises at least 50%, preferably at least 60%, more pref¬ erably at least 75%, and most preferably at least 85%, by weight of the hydrogel-forming polymer. When the absorbent core is imployed in an absorbent article comprising a liquid pervious topsheet for receiving body liquids such as urine, the core is preferably posi¬ tioned adjacent to the topsheet such that the absorbent member is located as close as pos¬ sible to the topsheet.

DETAILED DESCRIPTION OF THE INVENTION.

A. Definitions As used herein, the term "absorbent core" refers to a component of an absorbent article that is primarily responsible for fluid handling properties of the article, including acquir¬ ing, transporting, distributing and storing body fluids. As such, the absorbent core typi¬ cally does not include the topsheet or backsheet of the absorbent article.

As used herein, the term "absorbent member" refers to the components of the absorbent core that typically provide one or more fluid handling properties, e.g., fluid acquisition, fluid distribution, fluid transportation, fluid storage, etc. The absorbent member can pro¬ vide the entire absorbent core or only a portion of the absorbent core, i.e., the absorbent core can comprise one or more absorbent members.

As used herein, the term "comprising" means that e.g. various components, members, steps and the like can be conjointly employed according to the present invention. Ac¬ cordingly, the term "comprising" encompasses the more restrictive terms "made of and "consisting of," these latter, more restrictive terms having their standard meaning as un¬ derstood in the art.

All percentages, ratios and proportions used herein are by weight unless otherwise speci¬ fied.

B. Material and Components of the Absorbent Core

a. Chemical Composition of hydrogel forming absorbent polymers

The SAP, super absorbent polymers (SAPs) or hydrogel-forming polymers, useful in the present invention include a variety of water-insoluble, but water-swellable polymers ca¬ pable of absorbing large quantities of fluids. Such polymer materials are generally known in the art and include all those well-known polymers used or deemed useful in the context of disposable absorbent article technology. Particularly the SAPs disclosed in EP-A-752 892 or those disclosed in a textbook entitled "Modern Super Absorbent Technology" by F.L. Buchholz and A.T. Graham, published by Wiley VCH, New York, 1998 are useful in the context of the present invention. Preferred polymer materials for use in making SAPs are slightly network cross linked polymers of partially neutralized polyacrylic acids and starch derivatives thereof. Pref¬ erably, the SAPs comprise from about 50 to about 95%, preferably about 75%, neutral¬ ized, slightly network cross linked, polyacrylic acid (i.e., poly (sodium acrylate/acrylic acid)). Network cross-linking renders the polymer substantially water-insoluble and, in part, determines the absorptive capacity and extractable polymer content characteristics of the hydrogel-forming absorbent polymers. Processes for network cross linking these polymers and typical network cross-linking agents are described in greater detail in U.S. Patent 4,076,663 or references cited supra.

While the SAP is preferably of one type (i.e., homogeneous), mixtures of polymers can also be used in the present invention. The SAPs useful in the present invention can have a size, shape, and/or morphology varying over a wide range. According to a preferred em¬ bodiment of the present invention these polymers are in the form of particles that do not have a large ratio of greatest dimension to smallest dimension (e.g., granules, pulveru- lents, inter-particle aggregates, inter-particle cross linked aggregates, and the like). The SAPs can also comprise mixtures with low levels of one or more additives, such as for example powdered silica, surfactants, glue, binders, and the like.

For particles as described above, particle size is defined as the dimension determined by sieve size analysis. The methods for determining particle sizes of the SAP particles are described in U.S. Patent 5,061,259 (Goldman et. al), issued October 29, 1991.

Surprisingly it was now found that including hydrophilic polymers with aborescent, espe¬ cially dendritic structure in the production of SAPs is useful to provide superabsorbers with a small abrasion dust content, in particular after mechanical stressing, with an im¬ proved ability of binding powdered and/or dust-like additives, with a high swelling speed, with a high rate of internal liquid transfer and with optimal flow characteristics

According to Rompp's Encyclopedia of Chemistry, published by Georg Thieme Verlag in Stuttgart Germany, 10th edition, page 898 (Rδmpp, Lexikon-Chemie, Georg Thieme Ver¬ lag, Stuttgart, 10. Auflage, Seite 898), dendritic polymers are synthetic macromolecules constructed by step-for-step linkage of two or more monomers in each case with every already bound monomer, with the result that the number of monomer end-groups in¬ creases exponentially with every step and ultimately a spherical tree structure is created. The hydrophilic polymers with dendritic structure, which can be used according to the present invention are polyols with at least 8, preferably at least 16 and most preferably 32 hydroxyl groups as well as a non linear, preferably at least 14-fold and most preferably 30-fold branched fundamental structure.

Hydrophilic polymers with dendritic structures are for example polyesters obtained from a starting polyol by esterification with a C3-C20 hydroxycarboxylic acid, preferably with a C4-C12 hydroxycarboxylic acid and most preferably with a C4-C8 hydroxycarboxylic acid wherein the hydroxycarboxylic acid exhibits at least two hydroxyl groups, preferably two hydroxyl groups and/or at least two carboxylic acid groups. Of particular preference are hydroxycarboxylic acids with two hydroxyl groups and one carboxylic acid group, in particular 2,2-dimethylol propionic acid. Polyols are compounds with at least two hy¬ droxyl groups, such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, butylene glycol, 1,3-propanediol, 1,4-butanediol, bisphenol A, glycerine, trimethylol pro¬ pane, pentaerythritol and/or sorbitol. Preferred are dendritic polyesters, of particular pref¬ erence are Boltorn® 20, Boltorn® 30, Boltorn® 40 und Boltorn® 310 (Perstorp Specialty Chemicals AB, SE).

Hydrophilic polymers with dendritic structure also include polymers obtainable by con¬ densation of polyols with at least two hydroxyl groups and subsequent alkoxylation. Ex¬ amples in this case are branched polyethylene glycols obtainable via condensation of glycerin molecules and subsequent ethoxylation.

In addition to the above, the hydrophilic polymers with dendritic structure also include all polymers obtainable through polymerization of a monomer with at least one hydroxyl group and subsequent alkoxylation. Polymerization is preferably carried out in the pres¬ ence of a cross-linking agent. Polymer particles which are hydrophilic on their surface and exhibit numerous hydroxyl groups can be obtained. So-called star-shaped polyethyl¬ ene glycols, obtained by radical polymerization of p-hydroxyethylstyrene and subsequent alkoxylation, are as examples of the above in Makromol. Chem. 189, 2885 (1988).

Further examples of polymers used according to the invention are highly branched poly¬ mers of trade mark HYBRANE® as well as Astramol-Dendrimere® (DSM N. V., NL). Included in this category are particularly highly branched poly(propylene imines), start¬ ing, for example, with butylene diamine and obtained via repeated multiple Michael con- densation with acrylonitrile and hydrogenation, star-polycaprolactons, star-nylon-6, highly branched polyester amides, based, for example, on the addition product of succinic anhydride and diethanolamine in 1:1 molar ratio. In the process according to the inven¬ tion, so-called PAMAM Dendrimere based on poly(amidoamine), starting for example, with ammonia and obtained via repeated multiple conversion with methyl acrylate and ethylene diamine can also be used.

Preferred in accordance with the present invention are hydrophilic polymers with den¬ dritic structure and a glass transition temperature Tg between 20 and 200°C, particularly preferable between 25 and 5O0C and/or an average molecular weight between 1000 and 10000 g/mol, most preferably between 2000 and 6000 g/mol.

According to the invention, 0.005 to 10 weight percent of the hydrophilic polymers are preferred. Better yet between 0.01 and 5 weight percent and more preferred between 0,05 and 1 weight percent, most preferred between 0.10 to 0.80 weight percent of hydrophilic polymer with dendritic structure are employed, with the percentages refering the swella- ble hydrogel-forming polymer.

Hydrophilic polymers with dendritic structure are preferably mixed with the dried water- absorbing hydrogel. Dry preferably means less than 20% by weight of water content, par¬ ticularly preferred are less than 10% by weight of water content. The hydrophilic polymer with dendritic structure may, however, be added to the swellable hydrogel-forming poly¬ mer prior to, during or after the surface cross-linking process. Preferably, however, addi¬ tion or/and coating takes place during a surface cross-linking process. The method of in¬ termixing is subject to no limitations.

Where the coating is accompanied by surface cross-linking of the base hydrogel-forming polymer, the surface cross-linking agent may be brought into solution together with the dendritic polymer, or alternatively by separate addition into the surface cross-linking mixer. Where dust-like or powdered additive are also coated onto the base polymer, the possibility exists that the dendritic polymer is dissolved in a solvent wherein the dust-like or powdered additive is also dispersed. As an option, this mixture may also contain the surface cross-linking agent. Suitable solvents are all those known to any person skilled in the art which may be used for surface cross-linking, preferably water. In a preferred embodiment according to the invention the swellabel hydrogel-forming polymers comprise a fraction of particles of diameter less than 10 μm and less than 100 ppm by weight, preferably less than 50 ppm by weight and more preferably less than 10 ppm by weight wherein the swellable hydrogel-forming polymer comprises at least one powdered or dust-like additive such as metal salts, aluminum sulfate and/or magne¬ sium sulfate, pyrogenic silicic acids such as Aerosil® 200, polysaccharides and their de¬ rivatives, non-ionic surfactants, waxes, diatom earths and/or microscopic hollow inclu¬ sions.

In accordance with the present invention the hydrogel-forming polymers also comprise water insoluble phosphates such as calcium phosphate. The term "Water insoluble" signi¬ fies a solubility of less than 1 g, preferably less than 0.1 g and more preferably less than 0.01 g in 100 ml of water at 25°C.

Microscopic hollow inclusions are described in Chem. Ing. Techn. 75, 669 (2003). Mi¬ croscopic hollow inclusions are gas-filled or evacuated globular solid-state particles with diameters between 1 and 1000 μm. Typical wall-thicknesses of the microscopic hollow inclusions are between 1 and 10% of their diameter. The wall materials are subject to no limitations. Possible wall materials are glass, ceramic-forming oxides or mixed oxides, silicates, alumosilicates, polymers, polycondensates and metals.

Swellable hydrogel-forming polymers according to the invention typically have a Saline flow conductivity (SFC) of at least 40 x 10"7 cm3s/g, preferably of at least 60 x 10"7 cmVg, of particular preference 80 x 10"7 cmVg, even more preferably of at least 12O x 10"7 cm3s/g and most preferably of at least 130 x 10"7 cmVg.

Swellable hydrogel-forming polymers according to the invention typically have a cen¬ trifugal retention capacity (CRC) of at least 24 g/g, preferably at least 25 g/g, more pref¬ erably at least 26 g/g, even more preferably at least 28 g/g and most preferably at least 30 g/g.

Powdered additives preferably have an average particle size of less than 200 μm, of par¬ ticular preference less than 400 μm. Dust-like additives preferably have an average parti¬ cle size of less than 200 μm, preferably of less than 50 μm and of particular preference less than 10 μm. b. Physical Properties

(1). Saline Flow Conductivity (SFC)

An important characteristic and mandatory for the SAPs useful in the present invention is their permeability or flow conductivity when swollen with body fluids so as to form a hydrogel zone or layer. This permeability or flow conductivity is defined herein in terms of the Saline Flow Conductivity (SFC) value of the SAP. SFC measures the ability of a formed hydrogel layer to transport or distribute body fluids under usage pressures. The method for determining a SFC value is provided e.g. in EP-B-752 892 paragraph 224 fol¬ lowing. The SFC value of the SAPs useful in the present invention has already been men¬ tioned in terms of units. As expressed herein the numerical value of a unit is 10-7 cm3sec/g (or cm3sec/g/10,000,000). In other words 40 SFC units means an SFU value of 40 times 10-7 cm3sec/g.

(2). Centrifuge Retention Capacity (CRC)

For most hydrogel-forming absorbent polymers, gel volume as a measurement of absor¬ bent capacity is determined by the method described in U.S. Reissue Patent 32,649 (Brandt et al), reissued April 19, 1988 but using 0.9% saline solution instead of synthetic urine. The gel volume as well as the CRC capacity is calculated on a dry-weight basis. An alternative method for measuring gel volume can be used for SAPs that adsorb Blue Dex- tran (see gel volume method in Re 32,649) to the surfaces of the formed hydrogel (e.g., polymers prepared from cationic monomers). For these hydrogel-forming polymers, the Absorptive Capacity test is used, but the dry weight of the hydrogel-forming polymer is used in the calculation instead of the as-is weight. See e.g. U.S. Patent 5,124,188 (Roe et al), issued June 23, 1992 at Columns 27-28 for description of the Absorptive Capacity test.

For the evaluation of the centrifuge retention capacity it has been found that the so-called tea-bag-evaluation or measurement (hereinafter CRC measurement) is most appropriate to reflect the maintenance of capillary pressure at situations approaching saturation of the absorbent capability of a SAP material. For the test standard laboratory conditions (21-23 0C, 50% relative humidity) are used. Sample SAP material is kept dry in a tightly closing flask or other container, which is only opened upon start of the evaluation. Other material used in the evaluation (tissues, equipment etc.) is conditioned for 24 hours prior to meas¬ urements at the above laboratory conditions.

For the CRC measurement 0.2 +/- 0.0050 g of SAP particles are put into a tea bag (the bag needs to be freely liquid pervious and must retain the particles, i.e. the tea bag pores need to be not larger than the smallest particles. The tea bag should have a size of 60mm x 85mm and is sealed by welding after filling. The tea bag is then immersed for 30 min¬ utes in a 0.9% saline solution such that there is at least 0.83 1 of solution per gram of SAP; preferably there is a substantial excess of this ratio. After the 30 minute immersion the tea bag is centrifuged at 250 g for 3 minutes to remove excess saline solution. The bag is weight to the nearest 0.01 g and the absorbed liquid is calculated. The result is reported by using the amount of dry SAP, which was put into the tea bag, as grams absorbed per gram of SAP particles.

c. Absorbent Cores containing SAPs

According to the present invention absorbent members as part of absorbent cores for dis¬ posable absorbent articles comprise the previously described SAPs, with or without other optional components such as fibers, distribution / acquisition or thermoplastic material, foams, scrims etc. These absorbent members function as fluid storage members, even when subjected to pressures and tensions and torsions normally encountered as a result of the wearer's movements of absorbent articles made therewith. It should be understood, however, that such polymer-containing absorbent members can also serve several func¬ tions integrated in a single member.

An important aspect of these absorbent members according to the present invention is that they contain one or more regions having a high concentration of these SAPs in order to provide relatively thin absorbent articles capable of acquiring, distributing, absorbing and retaining large quantities of body fluids. A high concentration of SAPs, in accordance with the present invention, is desirable to reduce the level of other components, in par¬ ticular fibrous components.

In measuring the concentration of SAP an absorbent member or in a given region of an absorbent member, the percent by weight of the SAP relative to the combined weight of SAP and any other components (e.g., fibers, thermoplastic material, etc.) that are present in the member, respectively in the same region containing the polymer is used. With this in mind, the concentration of the SAPs in a given region of an absorbent member accord¬ ing to the present invention can be in the range of from about 50 to 100%, preferably from about 60 to 100%, more preferably from about 75 to 100%, and most preferably from about 85% to 100%.

Absorbent members according to the present invention comprising high concentrations of SAPs are useful alone or in combination with other absorbent members as part of the ab¬ sorbent cores and articles according to the present invention. Depending on the intended use, the preferred cores according the present invention comprise the SAPs according to the present invention in a basis weight of at least 20 g/m2, preferably at least 100 g/m2 and even more preferably of at least 150 g/m2.

The preferred absorbent members in the cores according to the present invention can in¬ clude those useful for initially acquiring the discharged body fluids and distributing these fluids to the fluid storage member. These preferred absorbent members can provide mul¬ tiple fluid handling properties (e.g., fluid acquisition and distribution) or single fluid han¬ dling properties (e.g., fluid distribution). Other absorbent members can also comprise lower concentrations of the SAPs that have the physical properties previously specified or can comprise SAPs having different physical properties. The preferred absorbent mem¬ bers are preferably located close, more preferably immediately adjacent , to the topsheet, which is exposed to the source of the liquid to be absorbed.

d. Absorbent Articles

Because of the unique absorbent properties of the absorbent cores of the present inven¬ tion, they are especially suitable for use in disposable absorbent articles for absorption of urine (also referred to as disposable absorbent incontinence articles). These absorbent articles typically comprise a liquid impervious (but preferably gas pervious) backsheet, a fluid pervious topsheet joined to, or otherwise associated with the backsheet, and the ab¬ sorbent core according to the present invention positioned between the backsheet and the topsheet. Such articles are well known in the art and fully disclosed in various documents mentioned throughout the description e.g. in EP 752 892. While particular embodiments have been shown and explained in the present specifica¬ tion the limit of the present invention is defined by the appended claims and all those em¬ bodiments falling within them. Further all information disclosed in references cited in the specification is herewith incorporated by reference to the desirable extend and/or useful for understanding the present invention.

Parallel with the present specification a detailed description of the making and evaluating processes used for making SAP particles according to the present invention is filed in a patent application entitled "Absorbent member for absorbent articles comprising swella- ble polymers of high permeability which are capable of forming hydrogels" by appli¬ cant/assignee BASF, Ludwigshafen of Germany. A copy of this application is available in the file of the European patent office of the present application.