Absorbent structures

Abstract: Absorbent structures comprising a continuous matrix of hydrophilic fibers, having dispersed therein particles of a wa­ ter-insoluble hydrogel, are disclosed. The distribution of hydrogel particles is showed toward the back face of the structure, resulting in the back face half containing at least 60% of the hydrogel material.

Description: Technical Field This invention relates to absorbent structures which are used to absorb fluids, especially body fluids and wastes. The absorbent structures herein are employed as the fluid absorbing member in products such as baby diapers, adult incontinence items, bandages, catamenials, bed pads, and the like. Background of the Invention Water-insoluble hydrogels have been proposed for use in absorbent structures. A major drawback of the hydrogel materials is their relatively high cost, as compared to conventional absorbent materials like cotton and wood pulp fibers. A second drawback is a phenomenon called gel-blocking: when exposed to a fluid, the outer layers of a hydrogel particle swell, thus preventing fluid from reaching the inner layers of the particle other than by a relatively slow diffusion process. Because of this gel blocking, hydrogel materials have relatively poor absorption kinetics. One general approach towards solving this problem is to provide a multilayer structure, comprising one or more hydrogel containing layers and one or more wicking layers. Examples of this approach are disclosed in: US-A-4.364.992, US-A-4.102.340, and EP-A-0.108.637. The last-named document discloses a composite structure formed of a wicking layer and an absorbent layer that are superposed on one another and compressed so as to bind the layers together by fiber entanglement. The absorbent containing the hydrogel is a low density, resilient fibrous mass while the wicking layer is a high density hydrophilic fibrous mass. The two layers can be of the same fibrous type but the absorbent layer preferably comprises bonded synthetic fibers. A drawback of this approach is that the wicking layer does not have the excellent rewet properties of hydrogel material. Another drawback is the relatively high manufacturing cost of multilayer structures. EP-A-0.122.042 discloses absorbent structures comprised of a hydrophilic fibrous matrix having dispersed therein discrete particles of a water-insoluble hydrogel material. The structures are compressed to a relatively high density, so as to ensure good wicking. Although this latter approach provides absorbent structures having a good performance in terms of rewet and absorbent capacity, there is still room for improvement. The high cost of hydrogel materials makes a further increase of the absorbent capacity per gram of material highly desirable. It is therefore an object of the present invention to provide a hydrogel-comprising absorbent structure having an improved absorbent capacity per gram of hydrogen material used. Summary of the Invention The present invention relates to an absorbent structure having a top face and a back face, and which is a continuous airlaid matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel, wherein the front and back face halves of the absorbent structure each contain the hydrogel and the back face half of the absorbent structure contains at least 60% of the water-insoluble hydrogel material and the matrix includes a thin area of low hydrogel concentration immediately adjacent the back face. Detailed Description of the Invention This invention relates to absorbent structures comprising hydrogel particles dispersed in a continuous matrix of hydrophilic fibers. The basis of this invention is the discovery that the absorbent capacity of such structures can be increased by preferentially placing the hydrogel particles near the face of the structure which is turned away from the point of fluid discharge. Absorbent structures intended for use in e.g., diapers or catamenials typically have a more or less planar configuration. For convenience, the face of the structure intended to be placed nearest to the body of the person using it will be referred to herein as the "top face". The opposing face will be referred to as the "back face". The absorbent structures herein comprise a continuous matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel. The distribution of hydrogel particles is skewed towards the back face of the structure. If the structures herein were to be cut into two halves along a plane substantially parallel to the top face and the back face, the resulting back face half would contain at least 60 % of the total amount of hydrogel material, whereas the top face half would contain not more than 40 % of the total amount. Preferably, the back face half contains at least 70 % of the total amount of hydrogel, most preferably at least 75 %. The hydrogel distribution in any given plane substantially parallel to the top face and back face can be homogeneous or nonhomogeneous. Any hydrogel distributions resulting in 60 % or more of the hydrogel to be dispersed in the back face half of the structure are suitable for the puposes of the present invention. Examples of such hydrogel distributions include structures which, in the Z-direction (from top face to bottom face) comprise an area of, e.g., 1-3 mm of low hydrogel concentration followed by a gradually increasing hydrogel concentration. Instead of a gradual increase of hydrogel concentration, there can be a rather abrupt increase. The structures have, immediately adjacent to the back face, a thin (eg. 0.5-2mm) area of low (eg., less than 10%, preferably less than 5%) hydrogel concentration. Structures of this type minimize the risk of hydrogel particles perforating the backsheet material of articles (diapers, catamenials, etc) in which the structures are used. Moreover, this area of low hydrogel concentration acts as a wicking layer. By "hydrogel" as used herein is meant a water-insoluble polymeric organic compound capable of absorbing aqueous fluids and retaining them under pressure. For good results, the hydrogel should be a cross-linked polymer. Cross-linking may be by covalent, ionic, vander Waals, or hydrogen bonding. Examples of polymers include polyacrylamides, polyvinyl alcohol, ethylene/maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, carboxymethyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid. polyacrylates, polyacrylamides, polyvinyl pyridine and the like. Other suitable hydrogels are those disclosed in US-A-3,901,236. Particularly preferred polymers for use herein are hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, polyacrylates, and isobutylene/maleic anhydride copolymers, or mixtures thereof. Processes for preparing hydrogels are disclosed in US Patents 4,076,663, 4,286,082, 3,734,876, 3,661,815, 3,670,731, 3,664,343, 3,783,871, and Belgian Patent 785,858. As used herein "Particles" include particles of any shape. e.g. spherical or semi-spherical, cubic, rod-like, polyhedral, etc.; but also shapes having a large greatest dimension/smallest dimension ratio, like needles, flakes and fibers, are contemplated for used herein. By "particle size" as used herein is meant the weight average of the smallest dimension of the individual particles. Conglomerates of hydrogel particles may also used, provided the weight average size of such conglomerates is within the limits set forth hereinbelow. Although the absorbent structures of the present invention are expected to perform well with hydrogel particles having a particles size varying over a wide range, other considerations may preclude the use of very small or very large particles. For reasons of industrial hygiene, (weight) average particle sizes smaller than 30 micrometers are less desirable. Particles having a smallest dimension larger than 4 mm may cause a feeling of grittiness in the absorbent structure, which is undesirable from a consumer standpoint. Preferred for use herein are particles having an (weight) average particle size of from 50 micrometers to 1 mm. The type of hydrophilic fibers is not critical for use in the present invention. Any type of hydrophilic fiber which is suitable for use in conventional absorbent products is also suitable for use in the absorbent structure of the present invention. Specific examples include cellulose fibers, rayon, polyester fibers. Other examples of suitable hydrophilic fibers are hydrophilized hydrophobic fibers, like surfactant-treated or silica-treated thermoplastic fibers. Also, fibers which do not provide webs of sufficient absorbent capacity to be useful in conventional absorbent structures, buy which do provide good wicking properties, are suitable for use in the absorbent structures of the present invention. This is so because, for the purposes of the present invention wicking properties of the fibers are far more important than their absorbent capacity. For reasons of availability and cost, cellulose fibers, in particular wood pulp fibers, are preferred. Preferred are absorbent structures which contain from 3 % to 40 %, by total dry weight of the structure, of the hydrogel material. More preferred are structures containing from 8 % to 20 % of hydrogel material. Since the fibrous matrix and the hydrogel particles are chemically different, the hydrogel distribution can be verified by chemical analysis, using well established chemical techniques. Typically, representative samples are taken from the top face half and and the back face half, respectively, of the absorbent structures. The samples are then analyzed for hydrogel content. The hydrogel content of the back face half sample should at least 1.5 times the hydrogel content of the sample taken from the top face half of the structure. As an example, a structure containing 15 % hydrogel material (by total dry weight of the structure), 80 % of which is disposed in the back face half of the structure, has a hydrogel content as follows: back face half : 0.8 x 0.15 0.5 x 100% = 24% top face half : 0.2 x 0.15 0.5 x 100% = 6% the back face half containing four times as much hydrogel as the top face half. Since many of the commercially available hydrogel materials contain acid (carboxylic, sulfonic, etc) groups, an acid-base indicater can be used for a semi-quantitative determination of the hydrogel distribution. For example, a crosslinked polyacrylate hydrogel has carboxy radicals as its functional groups. These carboxy groups are predominantly in the salt (e.g., sodium) form. The acid-base indicator bromocresol purple (5', 5'' - dibromo - O - cresolsulfone phthalein; yellow - pH 5.2 to 6.8 - purple) turns to its alkaline (purple) color when contacted with a polyacrylate hydrogel. The distribution of a polycarboxylate hydrogel in an absorbent structure can be made visible as follows: 50 mg bromocresol purple is dissolved in 1 liter distilled water. The pH of the solution is adjusted to 5, using HCl. 400 ml of the yellow solution are poured evenly onto the absorbent structure to be tested. The solution is allowed to penetrate for about two minutes. The intensity of the purple color is a measure of the amount of hydrogel present. The hydrogel distribution can be studied by comparing the color intensities at the top face and the back face of the absorbent structure. In an alternate procedure, the structure is cut in the Z-direction (= from top face to back face) when still in the dry condition. After application of the indicator solution, the hydrogel distribution in the Z-direction can be estimated. The assembly of baby diapers, catamenials and the like, using the herein-described absorbent structures, employs various liquid permeable topsheet materials, liquid impermeable backsheet materials, leg bands, tape fasteners and the like, all of which are described in the patent literature. See U.S. Patents 3,848,594 (tape fasteners); 3,860,003 (diaper construction); 4,081,301 (attachment of leg elastics); 3,929,135, 4,041,951 and 4,342,314 (improvements in topsheets) for thorough instructions regarding the assembly of diapers, catamenials, and the like. The manufacture of absorbent mats, especially "air-felt", is well-known and involves using an air stream to conduct fibrous material (e.g. cellulose) into a concave template of any desired depth and shape. The depth of the template determines the thickness of the resulting mat. Moisture or urine-impervious resins may be used to hold the mat in shape and the mat may be compressed to any desired Taber stiffness ; see U.S. Patent No. 3,860,003, cited above. INDUSTRIAL APPLICATION The following Example illustrates a preferred diaper made according to the present invention. The dimensions listed in the Example are for a diaper intended for use with a child in the 6 to 10 kilogram size range. These dimensions can be modified proportionately for different size children, or for adult incontinence briefs, according to standard commercial practice. As is well-known in the art, such diapers will comprise: a backsheet (the sheet used outermost from the skin, and which is generally made from urine-impenetrable plastic material to give the containment effect of plastic pants); an absorbent structure that will absorb and help contain urine and/or fecal matter; and, generally, a topsheet (the sheet closest to the skin) that is urine-permeable. While materials used in the assembly of disposable diapers and the like are well-known, the following may be mentioned, solely by way of example. Backsheet : The backsheet can comprise a urine-impervious polymer sheet, for example polyethylene or polypropylene, that is thin enough to be flexible. A polyethylene sheet 0.01-2 mm thick is typical. Topsheet : The diaper topsheet can comprise any loosely-woven or nonwoven cloth or scrim-type material that is urine-porous and comfortable to the skin. A nonwoven sheet comprising polypropylene fibres is typical. Elastic Members : The elastic used to provide leg gathers in the diapers can comprise elastic bands or threads, or elastic adhesive applied as a band or ribbon. One or more elastics can be applied longitudinally along both sides of the diaper, and laid-down either on the topsheet, on the backsheet, or sandwiched between said sheets. In a typical mode, the elastic is pre-stretched, then glued to the diaper using an elastic adhesive, all in well-known fashion. (see U.S. Patent 4 081 301). Fasteners : The diapers can be fastened by any convenient means, such as pins, snaps and the like. Typical fasteners comprise adhesive tapes, especially tapes in the "Y" configuration described in the patent literature. Assembly Means : The diapers herein can be assembled by any convenient bonding means, such as heat-sealing, ultrasonic sealing, and the like. Typically, urine-stable adhesives are used to assemble disposable diapers. Assembly Methods and Apparatus : The apparatus used to form the diapers herein constitutes no part of the present invention. Indeed, such articles may be assembled by hand. EXAMPLE I In a conventional air-laying method, absorbent structures are formed by air-conveying a mass of dry wood pulp fibers onto a rotating wire mesh drum. A pressure gradient is maintained over the wire mesh to ensure proper deposition of the fluff. The structures of the present invention can be made by metering hydrogel particles into the wood pulp fiber stream. Metering can be done by means of e.g., a weighing belt or a screw pump. Feeding of the hydrogel particles onto the laydown drum can be by gravity or forced air feed. In either case, a nozzle is used to spread the hydrogel particles over the crotch-width of the absorbent structure. Since the lay-down drum rotates while wood pulp fibers are being deposited onto it, the position of the hydrogel feed nozzle with respect to the drum and the wood pulp infeed largely determines the distribution of the hydrogel particles in the structure. The face of the structure which is in contact with the drum will become the back face. Seen from a vantage point from which the drum appears to have a clock-wise rotation,the left hand part of the wood pulp stream reaches the drum first. By placing the hydrogel in a way as to ensure predominant mixing with this left hand part of the wood pulp stream, the desired hydrogel distribution is obtained. Absorbent structures were made, comprised of wood pulp fibers ("airfelt") and particulate crosslinked polyacrylate hydrogel. The structures had a target weight of 43.58 g, 14 % of which was hydrogel, and 85.1 % of which was airfelt. The structures were calendered to a thickness of about 6.5 mm. The hydrogel distribution was determined, using the bromocresol purple indicator method described hereinabove. Structures made with gravity feed of the hydrogel had the following distribution (Z-direction, going from top face to back face). Top 1-2 mm : hydrogel/airfelt ratio 0/100 - 10/90 next 4-5 mm : hydrogel/airfelt ratio 20/80 - 50/50 back face 0.5 mm : hydrogel/airfelt ratio 0/100 - 10/90 Based on the above estimates, the back face half of The structure was calculated to contain 67 % of the total amount of hydrogel material. Having a thin area of low hydrogel concentration immediately adjacent to the back face, this structure is an example of the highly preferred structures herein. Structures made with forced air feed had the following hydrogel distribution (Z-direction, from top face to back face). Top 4-5 mm: hydrogel/airfelt ratio 0/100-5/95 Back 1.5 - 2.5 mm : hydrogel/airfelt ratio 50/50. Based on these estimates, the back face half of the structure was calculated to contain at least 85 % of the total amount of hydrogel material. Both structures, when incorporated in an otherwise conventional diaper, had improved leakage performance and wicking performance as compared to homogenous hydrogel/airfelt blend structures containing the same amounts of hydrogel and airfelt. EXAMPLE II A diaper is assembled from the following materials. 1. Backsheet : 0.025-0.070 mm polyethylene: width at top and bottom 33 cm; notched inwardly on both sides to a width-at-center of 28.5 cm; length 50.2 cm. 2. Topsheet : spun-bonded polypropylene, basis weight 21.5g/m 2 -24.5g/m 2 ; width at top and bottom 33 cm; notched inwardly on both sides to a width-at-center of 28.5 cm; length 50.2 cm. 3. Absorbent structure: prepared in the manner of Example I;, Taber range 7 - 9.5; 8.4 mm thick, calendered; width at top and bottom 28.6 cm; notched inwardly at both sides to a width-at-center of 10.2 cm; length 44.5 cm. 4. Elastics : four individual rubber strip (2 per side); width 4.77 mm; length 370 mm; thickness 0.178 mm (all the foregoing dimensions being in the relaxed state.) The diaper of Example II is assembled by overlaying the topsheet on the top face of said absorbent structure; overlaying the backsheet over the back face of said absorbent structure; and gluing the assembly together with urine-stable adhesive. The elastic bands (designated "inner" and "outer", corresponding to the bands closest to, and farthest from, the core, respectively) are stretched to ca. 50.2 cm and positioned between the topsheet:backsheet along each longitudinal side (2-bands per side) of the core. The inner bands along each side are positioned ca. 55 mm from the narrowest width of the core (measured from the inner edge of the elastic band). The bands are affixed with glue. The tape functions are affixed to the assembled diaper, which is then ready for use.

Claims: An absorbent structure having a top back face and which is a continuous airlaid matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel, wherein the front and back face halves of the absorbent structure each contain the hydrogel and the back face half of the absorbent structure contains at least 60% of the water-insoluble hydrogel material and the matrix includes a thin area of low hydrogel concentration immediately adjacent the back face. An absorbent structure according to claim 1 comprising from 3% to 40%, by dry weight of the structure, of water-insoluble hydrogel. An absorbent structure according to claim 1 or 2 wherein the water-insoluble hydrogel is selected from hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, polyacrylates, copolymers of isobutylene and maleic anhydride, and mixtures thereof. An absorbent structure according to any of the preceding claims wherein the hydrogel particles have an average particle size of from 30 micrometer to 4 mm. An absorbent structure according to any of the preceding claims wherein the hydrophilic fibers are cellulosic fibers, preferably wood pulp fibers. An absorbent structure according to any of the preceding claims wherein the back face half contains at least 70% of the water-insoluble hydrogel material. An absorbent structure according to any of the preceding claims comprising from 8% to 20%, by weight of the structure, of water-insoluble hydrogel. An absorbent structure according to any of the preceding claims wherein the back face half of the structure contains at least 75% of the total amount of hydrogel. A disposable diaper or catamenial product comprising: (a) a liquid impervious backing sheet; (b) a hydrophobic, liquid pervious top sheet; and (c) an absorbent structure according to any of the preceding claims, wherein the backing sheet overlays the back face of the absorbent structure and the top sheet overlays the top face of the absorbent structure.

Inventors: DUENK ELISABETH HENDRIKA, FISCHER WALTER JAKOB, DUENK, ELISABETH HENDRIKA, FISCHER, WALTER JAKOB
Applicants: PROCTER & GAMBLE, PROCTER & GAMBLE EUROP, PROCTER & GAMBLE EUROPEAN TECHNICAL CENTER, THE PROCTER & GAMBLE COMPANY
Application year: 1986
Main classification: A61F13
IPC codes: B01J20, A61F13, A61L15