WO2004007601A1

WO2004007601A1 - Sheetlike structures of water-absorbent open-celled foams based on crosslinked acid-functional polymers

Info

Publication number: WO2004007601A1
Application number: PCT/EP2003/007226
Authority: WO
Inventors: Samantha Champ; Hans-Joachim HÄHNLE; Mariola Wanior
Original assignee: Basf Aktiengesellschaft
Priority date: 2002-07-12
Filing date: 2003-07-07
Publication date: 2004-01-22
Also published as: AU2003281029A1

Abstract

Sheetlike structures formed of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers having a Free Absorption Rate (FAR) of at least 11 g/g.sec for 0.9% by weight aqueous sodium chloride solution, the surface of said sheetlike structures having edge lengths x and y having been postcrosslinked so that the crosslink density decreases in the z direction between this surface and the opposite surface and the use of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers in the form of sheetlike structures having edge lengths x and y and wherein the crosslink density has a gradient in the z direction, as an acquisition, distribution and/or storage layer in hygiene articles.

Description

Sheetlike structures of water-absorbent open-celled foams based on crosslinked acid-functional polymers

Description

This invention relates to sheetlike structures of water- absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers and their use in hygiene articles .

Water-absorbent, predominantly open-celled foams based on crosslinked acid-functional monomers are known, cf . EP-B-0 858 478, WO-A-99/44648 and WO-A-00/52087. They are prepared for example by foaming a polymerizable aqueous mixture containing at least 50 mol% neutralized acid-functional monoethylenically unsaturated monomers, crosslinkers and at least one surfactant and subsequently polymerizing the foamed mixture. The foaming of the polymerizable mixture can be effected for example by dispersing fine bubbles of a gas which is inert toward free radicals or by dissolving such a gas under elevated pressure in the polymerizable mixture and decompressing the mixture. The water content of the foams is adjusted to 1-60% by weight for example. The foams can optionally be subjected to surface postcrosslinking by spraying a crosslinker onto the foamed material or immersing the foam therein and heating the crosslinker-laden foam to a higher temperature. The foams are used for example in hygiene articles to acquire, distribute and store body fluids .

WO-A-97/31600 discloses an absorber element for use in hygiene or sanitary articles wherein a plurality of elements of a superabsorbent foam are arranged on a support in a grid pattern at such distances that the elements in the swollen state touch at their peripheries. For example, a monomer foam can be applied to the support in the desired grid pattern and then polymerized or separately prepared foam elements can be fixed on the support in the desired grid pattern by chemical or physical means. However, the permeability of the superabsorbent foams is still in need of improvement .

It is an object of the present invention to provide water-absorbent articles having a high absorption capacity and improved permeability on the basis of crosslinked acid-functional open-celled polymer foams . We have found that this object is achieved by sheetlike structures formed of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers having a Free Absorption Rate (FAR) of at least 11 g/g.sec for 0.9% by weight aqueous sodium chloride solution, the surface of said sheetlike structures having edge lengths x and y having been postcrosslinked so that the crosslink density decreases in the z direction between this surface and the opposite surface.

Such sheetlike structures are obtainable for example by applying at least one crosslinker to one side of a sheetlike structure formed of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers and heating said sheetlike structure to temperatures where said crosslinker reacts with the polymer. The sheetlike structures are for example sheets, webs or films having a thickness (z direction) of from 0.5 to 50 mm and preferably of from 1 to 10 mm.

The present invention also provides for the use of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers in the form of sheetlike structures having edge lengths x and y and wherein the crosslink density has a gradient in the z direction, as an acquisition, distribution and/or storage layer in hygiene articles. The surface of the foam structure with the higher crosslink density is disposed in the hygiene article so that it is on the bodyfacing side.

Water-absorbent, predominantly open-celled crosslinked acid-functional polymer foams are known from the prior art cited at the beginning, cf . EP-B-0 858 478 page 2 line 55 to page 10 line 54, WO-A-99/44648 page 4 line 41 to page 27 line 42 and WO-A-00/52087 page 5 line 32 to page 28 line 45. They are also known as hydrogel foams and are obtainable for example by first preparing a polymerizable aqueous mixture containing

from 10 to 80% by weight of acid-functional monoethylenically unsaturated monomers which are partially neutralized, for example at least 20 mol% neutralized, - optionally up to 50% by weight of other monoethylenically unsaturated monomers, from 0.001 to 5% by weight of crosslinker, at least one initiator, - from 0.1 to 20% by weight of at least one surfactant, - optionally a solubilizer and optionally thickeners, foam stabilizers, polymerization regulators, fillers and/or nucleators.

The polymerizable aqueous mixture is foamed either by dispersing fine bubbles of a gas which is inert toward free radicals or by dissolving an inert gas under a pressure of from 2 to 400 bar and then decompressing the mixture to atmospheric . The foamed mixture is then in either case polymerized to form a hydrogel foam. This method makes it possible to obtain foam articles in any shape, although preference is given to blocks from which foam webs or sheets of for example from 0.5 to 10 mm in thickness can be cut, and also to sheets, webs or films. The thus obtainable water-absorbable, predominantly open-celled crosslinked addition polymer foams are postcrosslinked according to the invention so that there is a crosslink density gradient in the z-direction of the sheetlike structure, i.e., between the upper and lower surfaces of the sheetlike structure. To postcrosslink the foam articles, they are initially treated with a solution of a crosslinker, for example of a polyhydric alcohol such as propylene glycol or butylene glycol, bisepoxides or polyglycidyl compounds and the preferably only onesidedly crosslinker-solution-treated sheetlike structures formed of predominantly open-celled crosslinked acid-functional addition polymer foams are heated to temperatures of for example 120-200°C to postcrosslink the surface.

Useful acid-functional monoethylenically unsaturated monomers include for example acrylic acid, methacrylic acid, acrylamido- propanesulfonic acid or mixtures thereof. Particular preference is given to the use of acrylic acid as a monomer to prepare water-absorbent addition polymers. The acid-functional compounds are usually neutralized with the aid of aqueous sodium hydroxide solution or potassium hydroxide solution. Water-absorbent polymers can also be prepared by polymerizing the acid-functional monomers in the presence of natural products such as starch, cellulose, cellulose derivatives or degradation products of starch such as oxidized starch, enzymatically degraded starch or in the presence of acids or bases of destruc ured starch. Graft polymers are formed. The polymerization of the acid-functional monomers is always effected in the presence of at least one crosslinker, one initiator and one surfactant in an aqueous medium. These materials are present in the polymerizable aqueous mixture which is foamed for example by the mechanical foaming method (dispersing of fine bubbles of an inert gas into the polymerizable mixture) or by dissolving for example carbon dioxide in the polymerizable aqueous mixture under a pressure of for example 12 bar and decompressing this mixture to atmospheric. The flowable foam thus prepared can then be transferred for example onto a belt having side walls or into molds and polymerized into webs, sheets or blocks and subsequently dried. The polymerization is carried out by prior art processes. Depending on the initiator used, it can be effected by raising the temperature, by the action of light (UV rays) , by irradiation with electron beams or else by a combination thereof, for example by raising the temperature and UV irradiation.

Foam layers up to 1 mm thick are prepared for example by one sidedly heating or irradiating a polymerizable mixture. To produce sheetlike foam structures more than one centimeter in thickness, the polymerizable mixture is heated by the action of microwaves for preference. Sheetlike structures of foams for example from 1 mm to 5 cm and preferably up to 2 cm in thickness are preferably prepared by initiating the polymerization of the polymerizable foam mixture on both sides, for example by heating the foam on a belt having side walls while at the same time irradiating the foam from above with UV light. The density of the foam changes only little if at all during the polymerization. The water content of the foams has a major influence on their flexibility. The water content is generally in the range from 1 to 80% by weight and preferably in the range from 5 to 60% by weight .

Foams having particularly high flexibility are obtained when at least 20 mol% of the acid groups of water-absorbent crosslinked acid-functional polymer foams have been neutralized with alkanolamines, cf . WO-A-00/52087, page 25 line 1 to page 26 line 10. The degree of neutralization of the carboxyl groups of the hydrogel foams is for example in the range from 40 to 95 mol% and preferably in the range from 55 to 85 mol%. By predominantly open celled is meant that at least 80% of the hydrogel foam is open celled. The hydrogel foams are preferably 100% open celled.

The water-absorbent, predominantly open-celled crosslinked acid-functional polymer foams have for example a density of from 0.001 to 0.9 g/cm³ and preferably of from 0.05 to 0.5 g/cm3, a water absorption capacity of at least 5 g/g, a Free Absorption Rate (FAR) of from 4.0 to 100 g/g sec for a 0.9% by weight aqueous sodium chloride solution and a Vertical Wicking Time (VWT = time for 0.9% by weight aqueous sodium chloride solution to advance vertically in a foam) of from 0.2 to 120 seconds for a height of 4 cm. The above-described, prior art sheetlike constructs formed of hydrogel foams are subjected to a surface postcrosslinking operation on one side. This provides sheetlike constructs of open-celled hydrogel foams which have a postcrosslinking gradient between their upper and lower surfaces. The postcrosslinking operation can be carried out not only on the dried but also on the moist hydrogel foam after polymerization. To prepare a sheetlike structure in a hydrogel foam having a postcrosslinking gradient, the foam is fed as a sheetlike structure. This can be effected for example in the form of individual sheets, films, tapes or other sheetlike geometric forms of varying size. For instance, a polyacrylate foam in the form of an endless roll can be subjected to a surface postcrosslinking operation on one side only, on a moving belt, so that an inhomogeneous postcrosslinking takes place in the z direction, where the x and y directions define the area of the surface.

In the inhomogeneous postcrosslinking operation, the crosslinking reagents are applied to a surface of hydrogel foam, i.e., compounds having at least two reactive groups capable under suitable conditions, for example on heating to not less than 70°C, of reacting with the acid groups of the hydrogel foam. It is also possible in this case to achieve a modification of the inhomogeneous crosslink density by controlling the depthwise penetration of the crosslinker. Suitable crosslinkers combine with the carboxyl groups of the polymer matrix to form covalent or ionic bonds. Such compounds are preferably applied in the form of an aqueous solution to the surface of the sheetlike structure of a hydrogel foam. The aqueous solution can contain for example water-miscible organic solvents, such as alcohols such as methanol, ethanol or isopropanol, acetone, dimethylforma ide or dimethyl sulfoxide. Useful crosslinkers include in principle all compounds useful as crosslinkers for preparing hydrogels. Examples of suitable postcrosslinking agents are:

di- or polyglycidyl compounds such as phosphonic acid diglycidyl ether or ethylene glycol diglycidyl ether, bischlorohydrin ethers of polyalkylene glycols, alkoxysilyl compounds, - polyaziridines, compounds which contain aziridine units and are based on polyethers or substituted hydrocarbons, for example bis-N-aziridinomethane, polyamines or polyamidoamines or their reaction products with epichlorohydrin, - polyols such as ethylene glycol, 1, 2-propanediol ,

1, 4-butanediol, glycerol, methyltriglycol, polyethylene glycols having an average molecular weight M_w of 200 - 10 000, di- and polyglycerol , pentaerythritol, trimethylolpropane, sorbitol, the ethoxylates of these polyols, for example glycerol, pentaerythritol and/or trimethylolpropane ethoxylation products containing from 1 to 20 and preferably from 2 to 8 ethylene oxide units per OH group, and also esters thereof with carboxylic acids or carbonic esters such as ethylene carbonate or propylene carbonate, carbonic acid derivatives such as urea, thiourea, guanidine, dicyandia ide, 2-oxazolidinone and its derivatives, bisoxazoline, polyoxazolines, di- and polyisocyanates, di- and poly-N-methylol compounds such as for example methylenebis (N-methylolmethacrylamide) or melamine-formaldehyde resins, - compounds having two or more blocked isocyanate groups such as for example trimethylhexamethylene diisocyanate blocked with 2,2,3, 6-tetramethyl-4-piperidinone, - solutions of divalent or more highly valent metal salts of which the metal cations can react with the acid groups of the polymer to form ionic or covalent bonds or complexes.

Examples of divalent or more highly valent metal cations are Mg⁺, Ca²⁺, Al³⁺, Sc3⁺, Ti⁴⁺, Mn²⁺, Fe²⁺/3⁺, Co⁺, Ni²⁺, Zn²⁺, Zr⁴⁺, La³⁺ and Ce⁴⁺. Preferred metal cations used are Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺ and Zr⁴⁺. The metal cations may be used not only alone but also mixed with each other and also together with at least one other customary crosslinker (cf . above) . Of the metal cations mentioned, all metal salts are suitable that possess adequate solubility in the solvent to be used. Of particular suitability are metal salts with weakly complexing anions such as chloride, nitrate and sulfate.

Useful solvents for the metal salts include water, alcohols, acetone, dimethylformamide, dimethyl sulfoxide and also mixtures thereof. Particularly preferred solvents are water and water-alcohol mixtures such as water/methanol or water/l, 2-propanediol .

If necessary, the postcrosslinking operation can be carried out in the presence of acidic catalysts such as for example p-toluenesulfonic acid, phosphoric acid, boric acid or ammonium dihydrogenphosphate .

Particularly suitable postcrosslinking agents are di- or polyglycidyl compounds such as ethylene glycol diglycidyl ether, the reaction products of polyamidoamines with epichlorohydrin, polyvalent metal cations and 2-oxazolidinone. In a continuous production process, the crosslinker solution is preferably applied by spraying a solution of the crosslinker for example through parallel connected nozzles which spray onto one surface only of the sheetlike hydrogel foam. The solution of the crosslinker can be applied via any apparatus known to one skilled in the art. It can be augmented for example with compressed air or effected without compressed air. The compressed air is preferably produced using inert carrier gas, for example nitrogen, argon or helium. Furthermore, the area to be impregnated can be determined and set via spray angles . The spray angle can be chosen via an electronically adjustable nozzle opening. The setting of the droplet size of the solution to be sprayed can alternatively be effected via the setting of the viscosity of the crosslinker solution and/or via the compressed air. The surface of the sheetlike^" structure of hydrogel foam can be provided with the crosslinker homogeneously or - as already indicated above - inhomogeneously. The crosslinker or a solution of the crosslinker can also for example be printed in the form of a pattern onto the surface of the hydrogel foam or be applied in the form of a pattern in any other way. Similarly, one-sided application of the crosslinker is possible using a knife coater.

The postcrosslinker solution is applied for example in an amount per unit area which should not exceed 0.02 ml/cm². More preferably, the surface has a postcrosslinker solution rate in the range from 0.001 to 0.015 ml/cm² and most preferably in the range from 0.001 to 0.012 ml/cm². This application rate ensures that the depthwise penetration of the postcrosslinker solution does not exceed the thickness of the sheetlike construct of hydrogel foam, so that a postcrosslinking gradient can develop.

Generally, the postcrosslinker solution is applied in such a concentration that the solvent does not account for more than 50% by weight and the crosslinker quantity for not more than 40% by weight, each based on polymer. Preferably the surface receives a solvent quantity in the concentration range from 0.1 to 30% by weight, more preferably in the concentration range from 0.5 to 20% by weight and most preferably in the concentration range from 1 to 10% by weight, each based on polymer. The crosslinker quantity based on polymer foam is for example in the range from 0.1 to 25% by weight, preferably in the range from 0.5 to 10% by weight and mostly in the range from 0.5 to 8% by weight.

The postcrosslinking gradient can be controlled for example by controlling the depthwise penetration of the crosslinker solution via the application rate and crosslinker quantity depending on the layer thickness of the sheetlike hydrogel foam feed. Both the top surface and the bottom surface of the gel foam can be postcrosslinked, but in that case different amounts of crosslinker must be applied respectively to the top surface and the bottom surface in order that a postcrosslinking gradient is developed between these surfaces. To prepare a sheetlike structure of a hydrogel foam having a postcrosslinking gradient between top surface and bottom surface in accordance with the invention, it is also possible to apply at least one crosslinker or a solution containing at least one crosslinker to the top surface and to the bottom surface of the sheetlike structure in equal amounts, to carry out the postcrosslinking operation and subsequently to split the thus both sidedly surface-postcrosslinked sheetlike structure a single time by for example making a horizontal cut in the z direction of the sheetlike structure. If, for example, the cut is made in the middle of the z direction of the both sidedly postcrosslinked sheetlike structure, it is halved.

After the crosslinker solution has been applied, the crosslinker is reacted with the hydrogel foam, for example in a downstream drying zone, at from 80 to 190°C and preferably at from 100 to 160°C. The reaction time is for example in the range from 2 minutes to 6 hours, preferably in the range from 10 minutes to 2 hours and mostly in the range from 10 minutes to 1 hour, during which not only cleavage products but also solvent fractions can be removed. The drying and postcrosslinking operation can also be effected by blowing with a preheated carrier gas.

The subject sheetlike structure formed of hydrogel foam having a crosslink gradient between top and bottom surfaces is preferably used in hygiene articles so that the surface having the higher crosslink density faces the body. Such a structure has distinctly improved properties over homogeneously crosslinked sheetlike samples of the same size with regard to absorption rate and permeability. It is superior to sheetlike structures formed of hydrogel foams which have not been postcrosslinked at the surface by possessing very good mechanical properties.

Hygiene articles are for example infant diapers, incontinence products, femcare articles, wound contact materials or secondary wound dressings. A hygiene article generally comprises a combination of a liquid-impervious backsheet, a liquid pervious topsheet, and an absorbent core. Hygiene articles of this kind are known, for example from EP-A-0 689 818. The absorbent core is fixed between the topsheet and the backsheet. Optionally, leg cuffs and self-adhesive tabs can be integrated in the hygiene article. A preferred design for such hygiene articles is known for example from US patent 3,860,003.

The topsheet is a soft interlayer which does not irritate the skin. The topsheet is water pervious and permits rapid passage into the subsequent absorbent core of the body fluid to be absorbed. The topsheet can be prepared from a multiplicity of different materials, for example porous foams, perforated synthetic films, natural fibers (cellulose, cotton fibers), synthetic fibers (polyester, polypropylene fibers) or a combination of natural and synthetic fibers . Preferably the topsheet is made of hydrophobic material in order that the skin of the user may be protected against prolonged contact with aqueous fluids .

The topsheet can be prepared from different materials, for example as a woven, non-woven, spun or combed fiber blend. Preference is given to using combed fiber blend which is thermally bonded to form the topsheet. The basis weight of the topsheet is preferably in the range from 18 to 25 g/m², and it has a tensile strength of at least 400 g/cm in the dry state and 55 g/cm in the wet state.

Topsheet and backsheet are joined together in a suitable manner in a production operation known to one skilled in the art. The absorbent core is positioned between topsheet and backsheet.

The backsheet used is usually a liquid-impervious material, for example a polyolefin (e.g., polyethylene backsheets) , in order that the clothing of the wearer may be protected against possible leakage.

The open-celled hydrophilic foam formed from crosslinked acid-functional monomers is used within the absorbent core, according to the invention. Owing to its remarkable properties, such as liquid acquisition and transmission and also storage, the sheetlike hydrogel foam gradient-postcrosslinked constructs to be used according to the invention are predestined for use as an acquisition and distribution layer or generally completely as an absorbent core.

When used as an absorbent core, the subject foams described above perform various functions in hygiene articles, namely acquisition, distribution and storage. The absorbent core can also contain two or more, for example 3, 4 or 5, sheetlike gradient-postcrosslinked hydrogel foams to be used according to the invention. The individual functions can either be completely performed or be augmented by further constituents, for instance storage can be increased by the addition of superabsorbent granules or acquisition and distribution can be optimized by further constituents such as high loft nonwovens, polypropylene nonwovens, polyester nonwovens or chemically modified products.

Determination of monomer foam density

Precisely 100 ml of the monomer foam are introduced into a graduated cylinder and the weight of the foam volume is determined. Dividing the weight found in g by 100 provides the density of the foam in g/cm3.

Determination of the polymer foam density

The density of superabsorbent foams is determined gravimetrically. A uniform foam layer having a defined thickness in the range from 3 to 5 mm is cut for example with a sharp knife to obtain square shapes having an edge length of 5 cm. The samples are weighed and the weight obtained is divided by the volume calculated from the dimensions.

Determination of absorption capacity

The absorption capacity of the superabsorbent foam in terms of water per gram of superabsorbent is determined on pieces of foam having a thickness of 3 mm and each weighing 1 g. The absorption capacity is here tested by the teabag test. The liquid used is 0.9% by weight sodium chloride solution. 1 g of the foam material is introduced into a teabag, which is then sealed. Care must be taken to ensure that the teabag offers sufficient room for complete swelling. The teabag is then immersed for a certain period, for example 30 min, into the liquid and weighed back after a drip-off time of for example 10 minutes. The blank is determined by immersing the teabag without superabsorbent foam in the solution and determining the weight of the teabag under the conditions described above. The absorption capacity then follows from the following equation (1) :

c — Absorption capacity = (i), where

G_TS is ^{t e} weight of the teabag with superabsorbent foam G is the weight of the teabag in the blank test and G_s is the starting weight of the superabsorbent foam.

Determination of absorption rate

The Free Absorption Rate (FAR) is found by cutting out, using a sharp knife, rectangular samples weighing 0.5 g (Wi) from foam layers having a uniform thickness of 3 mm. These samples are placed in a Petri dish and 10 g (W ) of 0.9% sodium chloride solution are poured over. A stopwatch is used to determine the time required for the foam sample to completely absorb the 0.9% sodium chloride solution. The absorption rate (FAR) in g/g -sec is calculated from the following equation (2):

W,

FAR =- ^ (w_x X measured time in sec)

Vertical Wicking Time VWT

A Petri dish (10 cm in diameter and 1 cm in height) is filled with 0.9% sodium chloride solution up to a depth of 0.5 cm. A glass tube (1 cm in diameter and 15 cm in length) is then sited a short distance above the base of the dish. A foam strip 6 cm in length having a square base area of 5 x 5 mm is marked at 2.4 and 6 cm and placed inside the glass tube in the liquid. The time measurement is started at the same time. The time in seconds taken to reach the respective mark is determined.

Acquisition time

The open-celled polyacrylate foam is cut into layers 1.5 mm, 2 mm or 4 mm in thickness. A commercially available diaper is carefully cut open, the high loft used as an acquisition medium removed and instead the open-celled polyacrylate foam layer inserted. The diaper is resealed. Synthetic urine solution is applied to it through a plastic plate having a ring in the middle (inner diameter of the ring 6.0 cm, height 4.0 cm) . The plate is loaded with additional weights so that the total pressure on the diaper is 13.6 g/cm². The plastic plate is placed on the diaper in such a way that the center of the diaper is also the center of the application ring. 60 ml of 0.9% by weight sodium chloride solution are applied three times. The sodium chloride solution is measured out in a graduated cylinder and applied to the diaper in a continuous stream through the ring in the plate. At the same time, the time is taken for the solution to penetrate completely into the diaper. The time measured is noted as acquisition time 1. Thereafter, the diaper is loaded with a plate for 20 min, the pressure being maintained at 13.6 g/cm². This is followed by the second application of the liquid. The time measured is noted as acquisition time 2. The same method is employed to determine acquisition time 3.

5 The percentages in the examples are by weight, unless the context suggests otherwise.

Inventive example 1

10 (a) Preparation of a foam film

The following components were mixed in a beaker using a magnetic stirrer :

15 348.55 g of acrylic acid (4.84 mol)

135.51 g of 37.3% sodium acrylate solution in water (0.54 mol) 28.00 g of polyethylene glycol diacrylate of polyethylene glycol of molar mass 400 21.33 g of a 15% aqueous solution of an addition product of 20 80 mol of ethylene oxide with 1 mol of a linear saturated CiβCis fatty alcohol 65.70 g of water

With ice-cooling, 400.90 g (2.69 mol) of triethanolamine were 25 added to this solution in such a way that the internal temperature did not rise above 16°C. The resulting solution was transferred into a pressure vessel and saturated therein with carbon dioxide under a pressure of 12 bar for 25 min. Under pressure, 26.67 g of a 3% aqueous solution of 30 2 , 2 '-azobis (2-amidinopropane) dihydrochloride were added and mixed in using a fast stream of carbon dioxide until the mixture was homogeneous. Carbon dioxide was then passed through the reaction mixture for a further 5 min. The saturated reaction mixture was forced under a pressure of 12 bar through a 1 mm 35 diameter nozzle to form a finely celled free-flowing foam.

The monomer foam obtained was placed on an A3 size glass plate having edges 3 mm in height and covered with a second glass plate. The foam sample was irradiated synchronously from both 40 sides with two UV/VIS lamps (Hδhnle UV 1000) for 4 minutes.

The foam layer obtained was completely dried in a vacuum oven at 70°C. To determine the properties, a portion of the foam was then adjusted to a moisture content of 5% by spraying with water.

45

Solids content of reaction mixture: 81.04% Degree of neutralization: 60 mol% Monomer foam density: 0.18 g/cm³ Polymer foam density: 0.19 g/cm³ Foam structure: homogeneous , completely open celled, no skin Thickness of foam film: 3 mm

(b) Onesided surface postcrosslinking of foam film

The 3 mm polyacrylate foam film prepared according to (a) was admixed with a surface postcrosslinking solution which, each percentage being based on the polymer foam, contained 3.25% of water, 1.65% of 1, 2-propanediol, 0.03% of ethylene glycol diglycidyl ether and 0.075% of aluminum sulfate. This solution was applied uniformly to the top surface of the foam film using a 2 ml syringe. The foam film thus treated was then heat treated at 100°C in a through air drying cabinet for 2 hours. The performance data are given in table 1.

Inventive example 2

(a) Preparation of a foam film

The following components were mixed in a beaker using a magnetic stirrer :

303.24 g of acrylic acid (4.21 mol) 117.90 g of 37.3% sodium acrylate solution in water (0.47 mol) 24.36 g of polyethylene glycol diacrylate of polyethylene glycol of molar mass 500 5.57 g of addition product of 80 mol of ethylene oxide with

1 mol of a linear saturated C₁₅C₁₈ fatty alcohol

0.15 g of water

With ice-cooling, 348.79 g (2.69 mol) of triethanolamine were added to this solution in such a way that the internal temperature did not rise above 16°C. The resulting solution was transferred into a pressure vessel and saturated therein with carbon dioxide under a pressure of 12 bar for 25 min. Under pressure, 13.92 g of a 3% aqueous solution of 2, 2 ' -azobis (2-amidinopropane) dihydrochloride were added and mixed in using a fast stream of carbon dioxide until the mixture was homogeneous. Carbon dioxide was then passed through the reaction mixture for a further 5 min. The saturated reaction mixture was forced under a pressure of 12 bar through a 1 mm diameter nozzle to form a finely celled free-flowing foam. The monomer foam obtained was placed on an A3 size glass plate having edges 3 mm in height and covered with a second glass plate. The foam sample was irradiated synchronously from both sides with two UV/VIS lamps (Hδhnle UV 1000) for 4 minutes.

Solids content of reaction mixture: 88.67%

Degree of neutralization: 60 mol%

Monomer foam density: 0.19 g/cm³

Polymer foam density: 0.17 g/cm³

Foam structure : homogeneous , completely open celled, no skin

Thickness of foam film: 3 mm

(b) Onesided surface postcrosslinking of foam film

The foam film prepared according to inventive example 2 (a) was admixed with a surface postcrosslinking solution which, each percentage being based on the polymer foam, contained 2.9% of water, 1.95% of 1, 2-propanediol, 0.075% of ethylene glycol diglycidyl ether and 0.075% of aluminum sulfate. This solution was applied uniformly to the top surface of the film using a 2 ml syringe. The film thus treated was then heat treated at 100°C in a through air drying cabinet for 2 hours . The performance data are given in table 1.

Inventive example 3

(a) Preparation of a foam film

The following components were mixed in a beaker using a magnetic stirrer:

278.84 g of acrylic acid (3.87 mol)

108.41 g of 37.3% sodium acrylate solution in water (0.43 mol) 22.40 g of polyethylene glycol diacrylate of polyethylene glycol of molar mass 500

17.07 g of a 15% aqueous solution of an addition product of 80 mol of ethylene dioxide with 1 mol of a linear saturated Cι₆Ci₈ fatty alcohol 5.12 g of a 25% aqueous solution of the salt of diethanolamine and heptafluorooctanesulfonic acid 52 . 56 g of water

With ice-cooling, 320.72 g (2.15 mol) of triethanolamine were added to this solution in such a way that the internal temperature did not rise above 16°C. The resulting homogenous mixture was transferred into a pressure vessel and saturated therein with carbon dioxide under a pressure of 10 bar for 25 min. Under pressure, 21.33 g of a 3% aqueous solution of 2, 2 '-azobis (2-amidinopropane) dihydrochloride were added and mixed in using a fast stream of carbon dioxide until the mixture was homogeneous. Carbon dioxide was then passed through the reaction mixture for a further 5 min. The saturated reaction mixture was forced under a pressure of 12 bar through a 1 mm diameter nozzle to form a finely celled free-flowing foam.

The monomer foam obtained was placed on an A3 size glass plate having edges 3 mm in height and covered with a second glass plate. The foam sample was irradiated synchronously from both sides with two UV/VIS lamps (Hόhnle UV 1000) for 4 minutes.

Solids content of reaction mixture: 81.0%

Degree of neutralization: 60 mol%

Monomer foam density: 0.18 g/cm³

Polymer foam density: 0.18 g/cm³

Foam structure: homogeneous, completely open celled, no skin

Thickness of foam film: 3 mm

(b) Onesided surface postcrosslinking of foam film

The foam film prepared according to inventive example 3 (a) was admixed with a surface postcrosslinking solution which, each percentage being based on the polymer foam, contained 3.6% of water, 1.2% of 1, 2-propanediol and 0.2% of ethylene glycol diglycidyl ether. The crosslinker solution was applied uniformly to the top surface of the foam film using a 2 ml syringe. The foam film thus treated was then heat treated at 100°C in a through air drying cabinet for 1 hour. The performance data are given in table 1.

Comparative example 1 Foam film 3 mm in thickness, prepared according to inventive example 1(a) . (This film has no surface postcrosslinking. The same applies to the foam films of comparative examples 2 and 3) .

Comparative example 2

Foam film 3 mm in thickness, prepared according to inventive example 2 (a) .

Comparative example 3

Foam film 3 mm in thickness, prepared according to inventive example 3(a).

The absorption profiles of the polymers described in the preceding inventive and comparative examples are collated in the tables which follow. They show that the inventive products have a significantly improved absorption performance and better distribution performance than the products of the prior art.

Table 1

Table 2

Inventive example 4 The open-celled hydrophilic gradient-postcrosslinked foam film prepared according to inventive example 1 (b) was cut into layers 2 mm in thickness . A commercially available diaper was carefully cut open, the high loft used as an acquisition medium removed and instead the open-celled hydrophilic polyacrylate foam layer 2 mm in thickness inserted. The diaper was resealed. The times were then determined to absorb 3 successive applications of 60 ml of synthetic urine each time. The results are given in table 3.

Inventive example 5

The gradient-postcrosslinked foam film prepared according to inventive example 2(b) was cut into layers 2 mm in thickness. A test diaper was prepared and tested in accordance with inventive example 4. The results are given in table 3.

Comparative example 4

The open-celled foam film without postcrosslinking gradient prepared according to inventive example 1 (a) was cut into layers 2 mm in thickness, which were inserted into a test diaper and tested in accordance with inventive example 4. The results are given in table 3.

Comparative example 5

A commercially available diaper was carefully cut open, the high loft removed and then reinserted and the diaper resealed. This procedure should ensure optimum comparability. The testing was done as described in inventive example 4. The results are given in table 3.

Table 3

As table 3 reveals, the open-celled hydrophilic polyacrylate foam having a postcrosslinking gradient exhibits clearly improved acquisition times over the foams of comparative examples 4 and 5 alike.

Comparative example 6

To illustrate the formation of the crosslink gradient within the polyacrylate foam layer in the present process, a polyacrylate foam was first prepared according to inventive example 1(a) and then immersed for 5 seconds in an excess of the postcrosslinker solution described in inventive example 1 (b) . The foam sample was then heat treated at 100°C in a through air drying cabinet for 2 hours . The performance data are given in table 4.

Comparative example 7

The foam film prepared according to inventive example 2(a) was immersed for 5 seconds in the postcrosslinker solution described in inventive example 2(b). The foam sample was then heat treated at 100°C in a through air drying cabinet for 2 hours. The performance data are given in table 4.

Comparative example 8

The foam film prepared according to inventive example 3 (a) was admixed with the postcrosslinker solution described in inventive example 3 (b) on both sides . The impregnated foam sample was then heat treated at 100°C in a through air drying cabinet for 1 hour. The performance data are given in table 4. Table 4

Claims

We claim: -

1. Sheetlike structures formed of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers having a Free Absorption Rate (FAR) of at least 11 g/g. sec for 0.9% by weight aqueous sodium chloride solution, the surface of said sheetlike structures having edge lengths x and y having been postcrosslinked so that the crosslink density decreases in the z direction between this surface and the opposite surface.

2. Sheetlike structures as claimed in claim 1, obtainable by applying at least one crosslinker to one side of a sheetlike structure formed of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers and heating said sheetlike structure to temperatures where said crosslinker reacts with the polymer.

3. Sheetlike structures as claimed in claim 1 or 2 , being sheets, webs or films having a thickness (z direction) of from 0.5 to 50 mm and preferably of from 1 to 10 mm.

4. The use of water-absorbent, predominantly open-celled foams based on crosslinked acid-functional polymers in the form of sheetlike structures having edge lengths x and y and wherein the crosslink density has a gradient in the z direction, as an acquisition, distribution and/or storage layer in hygiene articles.

5. A use as claimed in claim 4, wherein the surface with the higher crosslink density is disposed in the hygiene article so that it is on the bodyfacing side.

6. A use as claimed in claim 5, wherein only the bodyfacing side of the sheetlike foam structure is postcrosslinked at the surface.

7. A use as claimed in any of claims 4 to 6, wherein the predominantly open-celled foams comprise crosslinked polymers of acrylic acid.

8. A use as claimed in any of claims 4 to 7, wherein the hygiene articles are infant diapers, incontinence products, femcare articles, wound contact materials or secondary wound dressings .