WATER-ABSORBENT CORES FOR WATER ABSORBENT ARTICLES

Abstract: Water-absorbent cores for water-absorbent articles and which contain superabsorbent polymeric material exhibit improved absorbency and reduced rewet if the material is provided in part in powder, particulate or powder and particulate form and in part in fibrous form. Water-absorbent articles containing said cores are also provided.

Description: WATER-ABSORBENT CORES FOR WATER ABSORBENT ARTICLES The present invention relates to absorbent articles. In particular, it relates to absorbent articles such as disposable diapers, sanitary napkins, pads and incontinence garments. In addition, the present invention relates to a water-absorbent core for an absorbent article. As used herein, the term "water" when used alone or in the phrases "water- absorbing", "water-absorbent", "water-swellable" and "water-management" is understood to mean not only water but also aqueous media such as, in particular, electrolyte solutions such as body fluids. Absorbent articles such as sanitary napkins, pads, incontinence garments and disposable diapers have been known for many years and much effort has been made to improve the functional efficiency of such articles to make them more absorbent, more comfortable to wear and less obtrusive to the wearer. In general, such products have a core which includes a water- absorbent layer. This water-absorbent layer may be formed from any suitable water-absorbent material including wood pulp, rayon and tissue. These water-absorbent materials commonly have an absorbency of about lOg per gram of water. It will be understood that the absorbency values are different for saline and other such solutions. It is known to improve the absorbency of the core by including in the core one of the polymer compositions commonly known as superabsorbent polymers. These polymers generally have a much greater level of absorbency. The absorbency for water may be in the region of lOOg per gram or even of lOOOg per gram. A number of superabsorbent polymers have been developed which exhibit the capacity to be water-absorbing. Known compositions may be in any suitable form including powders, particles and fibers. US 3,954,721 and US 3,983,095, which are incorporated herein by reference, disclose preparations for derivatives of copolymers of maleic anhydride with at least one vinyl monomer in fibrous form. The fibrous copolymers are rendered hydrophilic and water-swellable by reaction with ammonia or an alkali metal hydroxide. US 3,810,468, which is incorporated herein by reference, discloses lightly cross-linked olefin-maleic anhydride copolymers prepared as substantially linear copolymers and then reacted with a diol or a diamine to introduce cross-linking. The resultant lightly cross-linked copolymers are treated with ammonia or an aqueous or alcohol solution of an alkali metal hydroxide. US 3,980,663, which is incorporated herein by reference, describes water-swellable absorbent articles made from carboxylic polyelectrolytes via cross-linking with glycerine diglycidyl ether. European Published Application No. 0 268 498 (incorporated herein by reference) describes a water-absorbent composition formed by causing a substantially linear polymer of water-soluble ethylenically unsaturated monomer blends comprising carboxylic and hydroxylic monomers to cross-link internally. Further examples of superabsorbent polymer compositions are those produced from a copolymer of an α,β unsaturated monomer having at least one pendant unit selected from a carboxylic acid group and derivatives thereof and a copolymerisable monomer. A proportion of the pendant units are present in the final copolymer as the free acid and a proportion as the salt of the acid. These copolymers are capable of being cross-linked, either internally or with a variety of cross-linking agents, to form the water-swellable composition. Examples of water-swellable compositions of this type can be found in US 4,616,063, 4,705,773, 4,731,067, 4,743,244, 4,788,237, 4,813,945, 4,880,868 and 4,892,533 and EP 0 272 074, 0 264 208 and 0436 514 which are incorporated herein by reference. Derivatives of carboxylic acid groups include carboxylic acid salt groups, carboxylic acid amide groups, carboxylic acid imide groups, carboxylic acid anhydride groups and carboxylic acid ester groups. Other examples of water-absorbent polymers can be found in US 4,798,861, WO 93/17066, WO 93/255735, WO 93/24684, WO 93/12275, European Published Application Nos 0 401 044, 0 269 393, 0 326 382, 0 227 305, 0 101 253, 0 213 799, 0 232 121, 0 342 919, 0 233 014, 0 268 498 and 0 397 410, British Patent Application Nos 2 082 614, 2 022 505, 2 270 030, 2 269 602 and 2 126 591, U.S. Patent Nos 4,418,163, 4,418,163, 3,989,586, 4,332,917, 4,338,417, 4,420,588 and 4,155,957 and French Patent Application No. 2 525 121 which are all incorporated herein by reference. Superabsorbent materials of the kinds referred to in the above-mentioned patents and applications may be in any suitable form including powder, particulate, granular and fibers; the fibers may be straight or may be curled and/or crimped. Details of such curly/crimped fibers may be found in US 4,822,453, 4,888,453 5,462,793 and 4,898,462 which are incorporated herein by reference. Other kinds of water-absorbent materials may be included in the water-absorbent core. Suitable materials include naturally occurring water-absorbent materials. One such water-absorbent material is starchy material such as that proposed by the US Department of Agriculture in 1969-1970. Peat moss may also be used a water- absorbent material. In this connection, reference may be made to US 5,477,627 5,429,242 5,374,260 4,992,324 4,676,871 4,573,988 4,560,372 4,540,454 4,537,590 and 4,226,237 which are incorporated herein by reference. Alginates have also been proposed as suitable water-absorbent material. One example of the use of such alginates is that suggested by Beghin/Kayserberg. Although known water-absorbent cores are capable of absorbing substantial volumes of water, they do not satisfactorily overcome the problems of rewet once the water has been absorbed into the water-absorbent material. Rewet is a measure of how dry the article comprising the water-absorbent core feels to the touch after water- absorption. This is generally measured under a load of approximately 3-5kPa. Rewet is of particular importance in the applications to which the absorbent articles of the present invention are directed as if the article feels damp, the user will feel damp and uncomfortable. The problem of rewet is particularly acute where the water-absorbent layer is formed from or includes the superabsorbent polymers described above, in particular where it is formed from particularly fast absorbing superabsorbent polymers. Although the superabsorbent polymers described above can absorb a large volume of water and absorption can take place in a short time period, in some circumstances the superabsorbent polymer may become locally saturated because of the fast absorbency of the superabsorbent material and may thereby feel damp to the touch. We have now discovered that an improved core which exhibits high absorbency and low rewet may be obtained if the core comprises superabsorbent polymeric material in powder or particulate form and superabsorbent polymeric material in fibrous form. The term "fibrous form" includes material in the form of fibres and material in the form of filaments. Thus, according to one aspect of the present invention there is provided a core for use in an absorbent article wherein the core comprises (a) superabsorbent polymeric material in powder, particulate or powder and particulate form and (b) superabsorbent polymeric material in fibrous form. The superabsorbent polymeric material in powder, particulate or powder and particulate form and the superabsorbent polymer material in fibrous form may be formed from the same superabsorbent polymeric material or they may be formed from different superabsorbent polymeric material. Generally, the superabsorbent material in fibrous form will have a higher rate of absorbency, commonly in the range of 50g to lOOg per gram or even higher, than the superabsorbent polymeric material in powder, particulate or powder and particulate form, whose rate of absorbency may be, for example, about lOg per gram. Without wishing to be bound by any theory, it is believed that when the core is insulted with water, the superabsorbent material in fibrous form, which has the higher rate of absorbency, will absorb the greatest volume of the water and some of the superabsorbent material in powder, particulate or powder and particulate form will not absorb an appreciable amount of water due to its lower rate of absorbency. When the core is then put under pressure, for example by the movement of the user of the absorbent article, water may be expelled from the fibers. However, instead of the water being expelled from the core towards its upper surface it will be absorbed by the superabsorbent material in the core which did not become saturated during the initial insult of water, ie. the superabsorbent material in powder, particulate or powder and particulate form. It will therefore be understood that the core of the present invention is particularly suitable for use in an absorbent article. Thus, according to a second aspect of the present invention, there is provided an absorbent article including the core of the above first aspect. The water-absorbent core may be of non-uniform configuration. For example, where the absorbent article is a feminine hygiene product or a diaper, the water-absorbent core may be shaped such that it is thicker in the crotch region. Whatever material is used for the absorbent layer, it is generally backed by a fluid- impervious backing sheet to protect clothing and the surrounding area from soiling and to prevent leakage of the body fluid which would cause embarrassment to the user. Any suitable water-impervious backing material may be used. The article generally has a water-permeable non-woven type cover-sheet which defines the surface of the article which will, in use, be in contact with the user. The cover sheet is intended to insulate the wearer from contact with water that has been absorbed into the core, thus the cover sheet should allow water to pass through it into the core but remain dry and soft to the touch. Any suitable material may be used as the cover-sheet. One example of a suitable cover-sheet is described in US 5,257,982 which is incorporated herein by reference. As there is often a discharge of a substantial volume of water in a short time, the core may additionally include a distribution layer which is generally located above the water-absorbent layer. This distribution layer serves to wick the water away from the point of initial contact with the article and transport it to other parts of the water- absorbent layer. The use of a distribution layer of this type is advantageous in addressing the problem of pooling if a substantial volume of water is discharged in a short time and thereby gives the user a feeling of dryness. Examples of distribution layers include those described in EP 0 565 606, GB 2 266 465 and GB 2 278 371 which are incorporated herein by reference. As the distribution layer serves to wick the water away from the point of initial contact, it is accepted that the water should be transported in the plane of the layer as quickly as possible and thus, distribution layers are generally made of hydrophilic material. The concept of speeding up the rate at which water is transported in the plane of the distribution layer is discussed in detail in EP 0 565 606 assigned to The Procter & Gamble Company in which the distribution layer forms part of the multiple layer absorbent core. The article may additionally or alternatively include a water-management layer of the kind described in International Patent Application No. PCT/EP96/03530 which is incorporated herein by reference. Thus according to a third aspect of the present invention, there is provided an absorbent article comprising a liquid pervious cover sheet, a liquid impervious backsheet and a core described in the above second aspect wherein the cover sheet is joined to the backsheet to enclose the core. A particularly preferred embodiment of the present invention will now be described. A water-absorbing article comprises a water-absorbent core. The water-absorbent core may be of any suitable arrangement and will generally comprise, for example, bulking material such as wood fluff. Low level water-absorbent material such as wood pulp, rayon, tissue and the like may additionally be present. The core will additionally include superabsorbent material. From 1 to 99 % (of total superabsorbent material) by weight of this superabsorbent material is in particulate and/or powder form and from 99 to 1 % by weight of the superabsorbent material is in fibrous form. Preferably, from 10 to 90 % by weight of the superabsorbent material is in particulate and/or powder form and from 90 to 10 % by weight of the superabsorbent material is in fibrous form. Most preferably the total of superabsorbent material will comprise about one third material in powder, particulate or powder and particulate form and two thirds in fibrous form. The particles of superabsorbent material are preferably in the size range 100-300μm. The fibrous superabsorbent is preferably sized to have a denier of approximately 10 to 30, a cross-sectional diameter of approximately 10 to lOOμm and a length of approximately 6mm to 150mm, more preferably about 50mm. Any suitable superabsorbent material may be used to form the superabsorbent material in powder, particulate or fibrous form. Examples of some suitable polymers are detailed above. The superabsorbent material, particularly the superabsorbent material in fibrous form, is preferably based on a copolymer having (a) repeating units derived from an ,β- unsaturated monomer and containing pendant carboxylic groups and (b) repeating units of a monomer copolymerisable with the α,j8-unsaturated monomer. Preferably, a proportion of the pendant carboxylic groups are free carboxylic acid (-COOH) groups and a proportion are carboxylic acid salt groups. The α,j3-unsaturated monomer from which the copolymer is formed may be a monomer containing pendant groups selected from carboxylic acid groups and derivatives thereof. Examples of suitable derivatives of carboxylic acid groups include carboxylic acid salt groups, carboxylic acid amide groups, carboxylic acid imide groups, carboxylic acid anhydride groups and carboxylic acid ester groups. Suitable α,β unsaturated monomers having at least one pendant unit selected from a carboxylic acid group and derivatives thereof include maleic anhydride; crotonic acid; fumaric acid; mesaconic acid; the sodium salt of maleic acid; the sodium salt of 2-methyl, 2-butene dicarboxylic acid; the sodium salt of itaconic acid; maleamic acid; maleamide; N-phenyl maleimide; maleimide; maleic anhydride; fumaric anhydride; itaconic anhydride; citraconic anhydride; mesaconic anhydride; methyl itaconic anhydride; ethyl maleic anhydride; diethylmaleate; methylmaleate and their mixtures. Any suitable copolymer izable comonomer can be employed. Suitable copolymerizable comonomers include alkenes, eg. ethylene, propylene and isobutylene, C, to C 4 alkyl acrylates and methacrylates, vinyl esters such as vinyl acetate, vinyl ethers such as methyl vinyl ether and isobutyl vinyl ether, and styrenic compounds having the formula: wherein R represents hydrogen or an alkyl group having from 1 to 6 carbon atoms and wherein the benzene ring may be substituted. Suitable substituents include low molecular weight alkyl groups or hydroxy groups. Suitable C, to C 4 alkyl acrylates include methyl acrylate, ethyl acrylate, isopropyl aery late, n-propyl acrylate, n-butyl acrylate and the like and their mixtures. Suitable to C 4 alkyl methacrylates include methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propylmethacrylate, n-butyl methacrylate, and the like, and their mixtures. Suitable styrenic compounds include styrene, α-methylstyrene, p-methylstyrene, t- butyl styrene and their mixtures. Any suitable free radical polymerization catalyst can be used to catalyze the copolymerization reaction. Suitable catalysts include benzoyl peroxide, such as that sold under the trade name PERKADOX and the VAZO azobisisobutyronitrile catalysts which are commercially available from E. I. DuPont de Nemours. NAZO- 64 and NAZO-67 catalysts are suitable for use in this invention at amounts of from about 0.2 to about 2 mole percent or greater based on the amount of α,β unsaturated monomer such as maleic anhydride. The pendant unit or units on the α,/3-unsaturated monomer employed in the copolymerisation will determine what, if any, additional reactions must be carried out to obtain the desired copolymer in which a proportion of the pendant units are present as the free acid and a proportion are present as the salt of the acid. In general, if the α,/3-unsaturated monomer bears only carboxylic acid amide, carboxylic acid imide, carboxylic acid anhydride, carboxylic acid ester groups, or mixtures thereof, it is necessary to convert at least a portion of these carboxylic acid derivative groups to carboxylic acid groups by, for example, an hydrolysis reaction. If the α,j(3-unsaturated monomer bears only carboxylic acid salt groups, partial acidification to form carboxylic acid groups will be necessary. The copolymer after such treatment, if any, preferably comprises from about 20 to about 80 wt% pendant carboxylic acid groups and from about 80 to about 20 wt% pendant carboxylate salts. One copolymer that is particularly suitable for use in the production of the superabsorbent composition is a copolymer of maleic anhydride and isobutylene. Another suitable copolymer is that of maleic anhydride and styrene copolymer. Suitable copolymers will preferably have peak molecular weights of from about 5,000 to about 500,000 or more. Suitable copolymers of maleic anhydride and isobutylene can be prepared using any suitable conventional method. Such copolymers are also commercially available from Kuraray Isoprene Chemical Company, Ltd. , Tokyo, Japan, under the trade mark ISOBAM. ISOBAM copolymers are available in several grades which are differentiated by viscosity molecular weight: ISOBAM-10, 160,000 to 170,000; ISOBAM-06, 80,000 to 90,000; ISOBAM-04, 55,000 to 65,000; and ISOBAM-600, 6,000 to 10,000. To form the desired copolymer containing pendant carboxylic acid groups and pendant carboxylic acid salt groups, the copolymer of maleic anhydride and isobutylene may suitably be reacted with a controlled amount of alkali to convert the anhydride groups to a mixture of free carboxylic acid groups and carboxylic acid salt groups. The copolymer containing pendant carboxylic acid groups and pendant carboxylic acid salt groups may be combined with a cross-linking agent having two or more groups, for example hydroxyl or heterocyclic carbonate groups, reactive with functional groups on the polymer chain. In one alternative, however, the copolymer may crosslink intramolecularly in the absence of an external cross-linking agent by means of covalent or hydrogen bonds. Especially where the superabsorbent material is to be provided in fibrous form, it is preferred that the copolymer (and crosslinking agent, if present) is provided in the form of a syrup. After the copolymer has been prepared, but typically before it is cured, but in some instances as it is curing, it is processed into any desired form using conventional methods of fabrication. For example, it can be subjected to casting; spray drying; air-assisted spray drying; air attenuation; or wet, dry or flash spinning. In one preferred embodiment, the copolymer is provided in the form of a syrup from which fibre is obtained by spinning. The syrup may include crosslinking agent where desired and crosslinking may occur during or after spirming to form superabsorbent fibre or filament. Whilst the present invention has been described with reference to disposable diapers, sanitary napkins, pads and incontinence garments it will be understood that the invention is equally applicable to other products which require high water-absorption capability such as pant liners, training pads, tampons, adult incontinence pads, bandages, patient underpads (for example pads of the type described in US 3,814,101 US 4,342,314 and EP 0 052 403 which are incorporated herein by reference), mortuary pads, casket liners, forensic examination pads, meat trays, soaker pads for food use, medical tray pads, fenestration drapes, other medical related articles, seed germination pads, capillary mats, baby bibs, desiccant strips for anti-rust use, bath mats, packaging, sorbents, clothing, breast pads, underarm pads, surgical and dental sponges, bandages, industrial wipes, domestic wipes, wipes, filters, cable wrap, food preservation articles, roofing materials, automotive trim, furniture, gasket, sealants, pond liners, bedding, clothing, cement, household pet litter, soil modifiers, wound covers and the like.

Claims: CLAIMS; 1. A water-absorbent core for a water-absorbent article, said core including superabsorbent polymeric material in powder, particulate or powder and particulate form, and superabsorbent polymeric material in fibrous form. 2. A water-absorbent core as claimed in Claim 1 characterised in that the superabsorbent polymeric material in fibrous form has a higher rate of absorbency than the superabsorbent polymeric material in powder, particulate or powder and particulate form. 3. A water-absorbent core as claimed in Claim 1 or Claim 2, characterised in that 10 to 90% by weight of the superabsorbent polymeric material is in fibrous form and 90 to 10% by weight is in powder, particulate or powder and particulate form. 4. A water-absorbent core as claimed in any one of Claims 1 to 3 characterised in that the superabsorbent polymeric material comprises a copolymer having repeating units derived from α.β-unsaturated monomer and containing pendant carboxylic groups and repeating units of a monomer copolymerisable with said α,jS-unsaturated monomer. 5. A water-absorbent core as claimed in Claim 4 characterised in that the pendant carboxylic groups comprise free carboxylic acid groups and carboxylic acid salt groups. 6. A water-absorbent core as claimed in Claim 5 characterised in that from 20 to 80% of the pendant carboxylic groups are free carboxylic acid groups and from 80 to 20% are carboxylic acid salt groups. 7. A water-absorbent core as claimed in any one of Claims 4 to 6 characterised in that the copolymer is crosslinked. 8. A water-absorbent core as claimed in any one of Claims 4 to 7 characterised in that the copolymer is derived from a copolymer of isobutylene and maleic anhydride or a copolymer of styrene and maleic anhydride. 9. A water-absorbent article including a core as claimed in any one of Claims 1 to 8. 10. A water-absorbent article as claimed in Claim 9 selected from disposable diapers, sanitary napkins, pads, incontinence garments, pant liners, training pads, tampons, adult incontinence pads, bandages, patient underpads, mortuary pads, casket liners, forensic examination pads, meat trays, soaker pads for food use, medical tray pads, fenestration drapes, other medical related articles, seed germination pads, capillary mats, baby bibs, desiccant strips for anti-rust use, bath mats, packaging, sorbents, clothing, breast pads, underarm pads, surgical and dental sponges, bandages, industrial wipes, domestic wipes, wipes, filters, cable wrap, food preservation articles, roofing materials, automotive trim, furniture, gasket, sealants, pond liners, bedding, clothing, cement, household pet litter, soil modifiers, and wound covers.

Inventors: CHEYNE IAN, TETZ MARK, CHEYNE, IAN, TETZ, MARK
Applicants: CAMELOT SUPERABSORBENTS BV, CAMELOT TECHNOLOGIES LIMITED, CHEYNE IAN, TETZ MARK, CHEYNE, IAN, TETZ, MARK
Application year: 1999
Main classification: A61L15
IPC codes: A61F13, A61L15, C08F222, C08L101