WO2003053298A2

WO2003053298A2 - Hydrophilic open-celled resilient latex foams

Info

Publication number: WO2003053298A2
Application number: PCT/EP2002/014450
Authority: WO
Inventors: Samantha Champ; Robert Michael Calhoun; Jimmy Duane Cross
Original assignee: Basf Aktiengesellschaft
Priority date: 2001-12-21
Filing date: 2002-12-18
Publication date: 2003-07-03
Also published as: AU2002358154A8; WO2003053298A3; AU2002358154A1

Abstract

Hydrophilic open-celled resilient latex foams characterized by a droplet acquisition rate of less than 5 seconds and a Free Swell Capacity of at least 5 g/g are prepared by treating open-celled resilient latex foams with at least one hydrophilicizing agent and are used in hygiene articles to acquire, distribute and inmmobilize body fluids.

Description

Hydrophilic open-celled resilient latex foams

Description

This invention relates to hydrophilic open-celled resilient latex foams.

Latex foams from natural and synthetic latices and also mixtures thereof are known, cf. for example D.C. Blackley in Polymer

Latices, Science and Technology, 2nd edition, Volume 3, Chapman & Hall, London 1997, pages 260 - 295. The foam is produced for example by the Dunlop process by foaming a heat-sensitive latex and subsequently gelling the foam by the application of heat. In one version of the process, the already foamed latex has added to it a gelling agent, for example sodium silicofluoride. A latex is foamed by admixing it with a foaming agent such as ammonium oleate. The foamed latex, which contains a foaming agent and a gelling agent, can be introduced into any desired mold and gelled therein at room temperature or elevated temperature. The foam is then vulcanized and thereafter washed and dried. Latex foams thus produced are hydrophobic.

Another process for producing foams from latices is the Crown Rubber process. This process is mainly used for producing foamed sheets useful as carpet backing or underlay. The Crown Rubber process is especially useful for processing carboxylated synthetic latices. The foaming agent used is a surfactant and the latices are foamed as in the Dunlop process, for example by the mechanical blowing method of beating finely divided gas bubbles into the latices. However, the solids content of the latices should be more than 55% by weight. The foamed latex is gelled solely by evaporating the water from the foamed latex. Again, the foams produced by this process are hydrophobic.

EP-A-0 427 219 discloses water-absorbent latex polymer foams obtainable by foaming a latex, blending the foam with finely divided water-absorbent polymers having a particle diameter of from 0.5 to 1 000 μm and drying the blend of foamed latex and water-absorbent polymer particles. The latices mentioned include acrylic acid latex, styrene-butadiene latex, polyethylene latex, vinyl acetate/acrylic acid latex, polyvinyl chloride latex, nitrile latex and styrene/acrylic acid latex. The foams containing water-absorbent polymers are used for example in the medical field to cover wounds and to package biological material. Prior DE applications 100 47 717.8 and 100 47 719.4 disclose hydrophilic open-celled resilient foams from melamine-formaldehyde resins which are used in hygiene articles to acquire, distribute and immobilize body fluids. Such foams are produced from open-celled resilient foams of melamine-formaldehyde resins by treatment with at least one hydrophilicizing agent such as surfactants or polymers containing amino and/or ammonium groups .

It is an object of the present invention to provide improved foams for use in hygiene articles. The foams shall provide in particular for a high acquisition of aqueous media.

We have found that this object is achieved according to the invention by hydrophilic open-celled resilient latex foams having a droplet acqusition rate of less than 5 seconds and a Free Swell Capacity of at least 5 g/g. Such latex foams have for example a minimum extensibility of 230%. Their BET specific surface area is for example at least 0.074 m²/g. They have a minimum tensile strength of > 144 kPa and preferably a density of from 80 to 480 kg/m³.

As reported above in the discussion of the^" background art, known latex foams are hydrophobic. The process of the invention provides for their treatment with a hydrophilicizing agent. All latices of the abovementioned synthetic latices and also natural latices and mixtures of synthetic and natural latices can be hydrophilicized by the process of the invention.

Particular preference is given to the use of latices based on styrene and butadiene. Such latices contain for example from 70 to 20 mol% of styrene units and from 30 to 80 mol% of butadiene units. They may optionally contain further comonomers in polymerized form. Useful comonomers include for example acrylic acid, methacrylic acid, maleic acid, methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, the corresponding esters of methacrylic acid and dimethyl maleate. The carboxylic acids may also be present in partially or completely neutralized form.

The comonomer content of the styrene-butadiene latices may be up to 40% by weight for example. These latices mostly contain up to 20% by weight of at least one comonomer, preferably acrylic ester, in polymerized form. The polymer content of the aqueous latices is for example at least 50% and preferably at least 55% by weight. Preferred latices contain comonomers in polymerized form that are capable of reacting with sulfur.

The latices are foamed in the presence of surfactants by prior art processes, for example by the mechanical blowing method of beating finely divided gas bubbles into the latices. The foam is then gelled and vulanized by heating, for example to temperatures above 120°C.

The latex foams thus producible are hydrophilicized using as hydrophilicizing agent for example at least one surfactant, polymers containing amino and/or ammonium groups, polyalkylene glycols and/or water-soluble polymers of monoethylenically unsaturated carboxylic acids.

When the latex foam is treated with a hydrophilicizing agent, this may take the form for example of an adsorption of a more hydrophilic component for example of surfactants or hydrophilic polymers which optionally exhibit a hydrophobic modification, or a chemical attachment of hydrophilic reagents for example of polyamides, polyepoxides or polycarboxylic acids to the surface of the foam. Hydrophilicization of the surface of the foam may also be effected by applying crosslinked addition polymers or a crosslinked hydrophilic sheath, for example by having

reagents capable of forming a network with themselves such as condensation products of epichlorohydrin and polyamidoamines or polyamines or

monomers or polymers capable of reacting with an added crosslinker, for example polycarboxylic acids in combination with multifunctional epoxides, polyhydric alcohols or polyamines, polyamines in combination with multifunctional epoxides, acrylates or esters

act on the latex foams. The hydrophilicizing agents, or hydrophilicizers, are normally employed in dissolved form by dissolving them in a solvent. They may also be applied in the form of aqueous dispersions or dispersions in an organic solvent to the foams to be hydrophilicized. The hydrophilicization may be effected for example by dipping the latex foam body into the liquid which contains the hydrophilicizer in dissolved or in dispersed form. Alternatively the liquid with the dissolved or dispersed hydrophilicizer may also be sprayed on the foam surface. The solvent is thereafter removed from the hydrophilicized foam body, for example by drying the foam.

The hydrophilicizer reacts with the latex foam to be hydrophilicized and is adsorbed on the polymer surfaces. The amount of hydrophilicizer added is dimensioned in such a way that, on the one hand, a hydrophilicization is brought about without, on the other hand, disrupting the mechanical properties of the foam (flexibility) . Preferably the hydrophilicizer is added in such an amount that the resulting amount of hydrophilicizer is in the range from 0.05 to 100% by weight, preferably in the range from 0.1 to 50% by weight, especially in the range from 0.2 to 30% by weight, based on the latex foam.

Hydrophilicization of the latex foam is possible for example through the action of at least one surfactant on the foam. Particular preference is given to adding skin-friendly surfactants. Examples of skin-friendly hydrophilicizers are oil-soluble surfactants such as sorbitan fatty acid esters, polyglycerol fatty acid esters and polyoxyethylene. Examples of surfactants of the above type are TRIODAN^® 20, a commercially available polyglycerol ester, and EMSORB^® 2502, a sorbitan sesquioleate. Preferred sorbitan fatty acid esters are sorbitan laurate (e.g. SPAN^® 20), sorbitan monooleate (SPAN^® 80) and combinations of sorbitan trioleate (SPAN^® 85) and sorbitan monooleate (SPAN^® 80) . Particular preference is given to the combination of sorbitan monooleate and sorbitan trioleate in a weight ratio of not less than 3:1, particularly preferably of 4:1. Combinations of sorbitan laurate with certain polyglycerol fatty acid esters are also used as hydrophilicizers. Polyglycerol fatty acid esters are obtained from ester-forming polyglycerols and fatty acids, cf. for example US-A-3, 637,774. Polyglycerols are characterized by a high fraction of linear (especially acyclic) diglycerols, a low fraction of tri- or higher polyglycerols and a low fraction of cyclic diglycerols. The weight ratio of sorbitan laurate to polyglycerol fatty acid ester is normally in the range from 10:1 to 1:10, preferably in the range from 4:1 to 1:1.

Preference is further given to organomodified polydimethylsiloxanes of the type Nuwet^® 500 or modified silicones of the type Nuwet^® 300 (from OSi) . Both the polydimethylsiloxanes and the polysilicones are modified to be hydrophilic, for example the polysilicones and the polydimethylsiloxanes contain at least one hydrophilic substituent selected from the group of amino, carboxyl, hydroxyl, and polyethylene glycol substituents.

Useful hydrophilicizers further include acylated polyamines, obtainable for example by reaction of polyamines with monobasic carboxylic acids. Useful polyamines include for example polyalkylenepolyamines having average molar masses of from 300 to 1 million, preferably of from 500 to 500 000. Preferred polyalkylenepolyamines are polyethyleneimines .

The monobasic carboxylic acids usually have from 1 to 18 carbon atoms, for example formic acid, acetic acid, propionic acid, lauric acid, palmitic acid or stearic acid. In some cases it is advantageous to react mixtures of a long-chain monocarboxylic acid in succession or together with a polyalkylenepolyamine.

Instead of carboxylic acids it is also possible to use the esters of carboxylic acids. When the polyalkylenepolyamines are reacted with the carboxylic acids or esters, the NH₂ or NH groups of the polyalkylenepolyamines are amidated. This is a way of acylating for example from 5 to 100%, preferably from 15 to 85%, of the nitrogen atoms in the polyalkylenepolyamine.

Useful hydrophilicizers further include polymers containing

i) at least one polyisocyanate and

ii) at least one compound having at least two isocyanate-reactive groups and additionally at least one tertiary amino group

in built-in form. At least some of the tertiary amino groups of component ii) in the polymer are in the form of ammonium groups. Charged cationic groups can be produced from the tertiary amine nitrogens of the compounds of component ii) and/or of the polymer either by protonation or by quaternization. Then at least a portion of the tertiary amino groups in the polymer will be present in the form of its reaction products with at least one neutralizing (protonating) and/or quaternizing agent.

The polyisocyanates i) are preferably selected from compounds having from 2 to 5 isocyanate groups, isocyanate prepolymers having an average number of from 2 to 5 isocyanate groups and mixtures thereof. It is also possible to use compounds which in addition to or instead of free isocyanate groups have functional groups which release isocyanate groups or react like isocyanate groups. These include for example compounds having blocked isocyanate groups, uretdione groups, isocyanurate groups and/or biuret groups. The compounds having isocyanurate groups are in particular simple triisocyanatoisocyanurates, i.e., cyclic trimers of diisocyanates, or mixtures with their higher homologs having more than one isocyanurate ring. Useful compounds of component ii) further include for example tertiary amines where the amine nitrogen has three substituents, which are preferably hydroxyalkyl and/or aminoalkyl groups. Preferred compounds used for component ii) are for example bis (aminopropyl)methylamine, bis (aminopropyl)piperazine, methyldiethanola ine and mixtures thereof .

Useful cationic polymers include all cationic synthetic polymers containing amino and/or ammonium groups . Examples of such cationic polymers are vinylamine polymers, vinylimidazole polymers, polymers containing quaternary vinylimidazol units, condensates of imidazole and epichlorohydrin, crosslinked polyamidoamines, ethyleneimine-grafted crosslinked polyamidoamines, polyethyleneimines, alkoxylated polyethyleneimines, crosslinked polyethyleneimines, amidated polyethyleneimines, alkylated polyethyleneimines, polyamines, amine-epichlorohydrin polycondensates, water-soluble polyaddition products of multifunctional amines with multifunctional epoxides, alkoxylated polyamines, polyallylamines, polydimethyldiallylammonium chlorides, polymers containing basic (meth) acrylamide or (meth) acrylic ester units, polymers containing basic quaternary (meth) acrylamide or (meth) acrylic ester units, and/or lysine condensates.

Cationic polymers also include amphoteric polymers having a net cationic charge, i.e., the polymers contain not only anionic but also cationic monomers in polymerized form, but the molar fraction of cationic units contained in the polymer is larger than that of the anionic units.

Vinylamine polymers (i.e., polymers containing vinylamine units) are preparable for example from open-chain N-vinylcarboxamides of the formula

Ri

CH₂= CH N (I)

C — R²

where R¹ and R² are identical or different and are each hydrogen or Ci" to C₆-alkyl. Useful monomers include for example N-vinylformamide (R¹=R²=H in formula I) , N-vinyl -N-methylformamide, N-vinylacetamide, N-vinyl -N-methylacetamide, N-vinyl -N-ethylacetamide, N-vinyl -N-methylpropionamide and N-vinylpropionamide. To prepare the polymers, the monomers mentioned may be polymerized alone, mixed with each other or together with other monoethylenically unsaturated monomers . Homo- and copolymers of N-vinylformamide are preferred as starting material. Vinylamine polymers are known for example from US-A-4 421 602, US-A-5 334 287, EP-A-0 216 387 and EP-A-0 251 182. They are obtained by acid, base or enzymatic hydrolysis of polymers containing monomers of the formula I.

Useful monoethylenically unsaturated monomers for copolymerization with N-vinylcarboxamides include all compounds copolymerizable therewith. Examples thereof are vinyl esters of saturated carboxylic acids of from 1 to 6 carbon atoms such as vinyl formate, vinyl acetate, vinyl propionate and vinyl butyrate and vinyl ethers such as Cι~ to Cε-alkyl vinyl ethers, e.g., methyl or ethyl vinyl ether. Useful comonomers further include ethylenically unsaturated C₃- to Cg-carboxylic acids, for example acrylic acid, methacrylic acid, maleic acid, crotonic acid, itaconic acid and vinylacetic acid and also their alkali metal and alkaline earth metal salts, esters, amides and nitriles of the carboxylic acids mentioned, for example methyl acrylate, methyl methacrylate, ethyl acrylate and ethyl methacrylate.

Further useful carboxylic esters are derived from (polyalkylene) glycols where in each case only one OH group is esterified, for example hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate and also monoacrylate esters of polyalkylene glycols having a molar mass of from 500 to 10 000. Useful comonomers further include esters of ethylenically unsaturated carboxylic acids with aminoalcohols, for example dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate, diethylaminoethyl methacrylate, dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate, diethylaminopropyl acrylate, dimethylaminobutyl acrylate and diethylaminobutyl acrylate. Basic acrylates can be used in the form of the free bases, the salts with mineral acids such as hydrochloric acid, sulfuric acid or nitric acid, the salts with organic acids such as formic acid, acetic acid, propionic acid or sulfonic acids or in quaternized form. Useful quaternizing agents include for example dimethyl sulfate, diethyl sulfate, methyl chloride, ethyl chloride or benzyl chloride. Useful comonomers further include amides of ethylenically unsaturated carboxylic acids such as acrylamide, methacrylamide and also N-alkylmonoamides and diamides of monoethylenically unsaturated carboxylic acids with alkyl radicals of from 1 to 6 carbon atoms, for example N-methylacrylamide,

N,N-dimethylacrylamide, N-methylmethacrylamide, N-ethylacrylamide and N-propylacrylamide and tert-butylacrylamide and also basic (meth)acrylamides, for example dimethylaminoethylacrylamide, dimethylaminoethylmethacrylamide, diethylaminoethylacrylamide, diethylaminoethylmethacrylamide, dimethylaminopropylacrylamide, diethylaminopropylacrylamide, dimethylaminopropylmethacrylamide and diethylaminopropylmethacrylamide.

Useful comonomers further include N-vinylpyrrolidone, N-vinyleaprolactarn, acrylonitrile, methacrylonitrile,

N-vinylimidazole and also substituted N-vinylimidazoles, for example N-vinyl-2-methylimidazole, N-vinyl-4-methylimidazole, N-vinyl-5-methylimidazole, N-vinyl-2-ethylimidazole and N-vinylimidazolines such as N-vinylimidazoline, N-vinyl-2-methylimidazoline and N-vinyl-2-ethylimidazoline. N-Vinylimidazoles and N-vinylimidazolines are used not only in the form of the free bases but also after neutralization with mineral acids or organic acids or after quaternization, a quaternization being preferably effected with dimethyl sulfate, diethyl sulfate, methyl chloride or benzyl chloride. Also useful are diallyldialkylammonium halides, for example diallyldimethylammonium chlorides.

Useful comonomers further include sulfo-containing monomers, for example vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, the alkali metal or ammonium salts of these acids or 3-sulfopropyl acrylate. Since the amphoteric copolymers contain more cationic units than anionic units, they have a cationic charge overall.

The copolymers contain for example

from 99.99 to 1 mol%, preferably from 99.9 to 5 mol%, of N-vinylcarboxamides of the formula I and

from 0.01 to 99 mol%, preferably from 0.1 to 95 mol%, of other monoethylenically unsaturated monomers copolymerizable therewith

in copolymerized form. To prepare vinylamine polymers it is preferable to start from homopolymers of N-vinylformamide or from copolymers obtainable by copolymerization of

- N-vinylformamide with

vinyl formate, vinyl acetate, vinyl propionate, acrylonitrile, N-vinylcaprolactam, N-vinylurea, acrylic acid, N-vinylpyrrolidone or Cι~ to C₆-alkyl vinyl ethers

and subsequent hydrolysis of the homo- or copolymers to form vinylamine units from the copolymerized N-vinylformamide units, the degree of hydrolysis being for example in the range from 0.1 to 100 mol%.

The hydrolysis of the above-described polymers is effected according to known processes by the action of acids, bases or enzymes. This converts the copolymerized monomers of the above-indicated formula I through detachment of the group

— C — R²

(ID,

where R² is as defined for formula I, into polymers which contain vinylamine units of the formula

CH₂ — CH —

N (III)

/ \

H Rl

where R¹ is as defined for formula I. When acids are used as hydrolyzing agents, the units III are present as ammonium salt.

The homopolymers of the N-vinylcarboxamides of the formula I and their copolymers may be hydrolyzed to an extent in the range from 0.1 to 100 mol%, preferably to an extent in the range from 70 to 100 mol%. In most cases, the degree of hydrolysis of the homo- and copolymers is in the range from 5 to 95 mol%. The degree of hydrolysis of the homopolymers is synonymous with the vinylamine units content of the polymers. In the case of copolymers containing units derived from vinyl esters, the hydrolysis of the N-vinylformamide units can be accompanied by a hydrolysis of the ester groups with the formation of vinyl alcohol units. This is the case especially when the hydrolysis of the copolymers is carried out in the presence of aqueous sodium hydroxide solution. Copolymerized acrylonitrile is likewise chemically modified in the hydrolysis, for example converted into amide groups or carboxyl groups . The homo- and copolymers containing vinylamine units may optionally contain up to 20 mol% of amidine units, formed for example by reaction of formic acid with two adjacent amino groups or by intramolecular reaction of an amino group with an adjacent amide group, for example of copolymerized N-vinylformamide. The molar masses of vinylamine polymers range for example from 1 000 to 10 million, preferably from 10 000 to 5 million (determined by light scattering) . This molar mass range corresponds for example to K values of from 5 to 300, preferably from 10 to 250 (determined by the method of H. Fikentscher in 5% aqueous sodium chloride solution at 25°C and a polymer concentration of 0.5% by weight).

The vinylamine polymers are preferably used in salt-free form. Salt-free aqueous solutions of vinylamine polymers are preparable for example from the above-described salt-containing polymer solutions by means of ultrafiltration using suitable membranes having molecular weight cutoffs at for example from 1 000 to 500 000 daltons, preferably from 10 000 to 300 000 daltons. The hereinbelow-described aqueous solutions of other polymers containing amino and/or ammonium groups are likewise obtainable in salt-free form by means of ultrafiltration.

Polyethyleneimines are prepared for example by polymerizing ethyleneimine in an aqueous solution in the presence of acid-detaching compounds, acids or Lewis acids.

Polyethyleneimines have for example molar masses of up to 2 million, preferably of from 200 to 500 000. Particular preference is given to using polyethyleneimines having molar masses of from 500 to 100 000. Useful polyethyleneimines further include water-soluble crosslinked polyethyleneimines which are obtainable by reaction of polyethyleneimines with crosslinkers such as epichlorohydrin or bischlorohydrin ethers of polyalkylene glycols containing from 2 to 100 ethylene oxide and/or propylene oxide units. Also useful are amidic polyethyleneimines which are obtainable for example by amidation of polyethyleneimines with Cι~ to C -monocarboxylic acids . Useful cationic polymers further include alkylated polyethyleneimines and alkoxylated polyethyleneimines. Alkoxylation is carried out using for example from 1 to 5 ethylene oxide or propylene oxide units per NH unit in the polyethyleneimine. Useful polymers containing amino and/or ammonium groups also include polyamidoamines, which are preparable for example by condensing dicarboxylic acids with polyamines. Useful polyamidoamines are obtained for example when dicarboxylic acids having from 4 to 10 carbon atoms are reacted with polyalkylenepolyamines containing from 3 to 10 basic nitrogen atoms in the molecule. Useful dicarboxylic acids include for example succinic acid, maleic acid, adipic acid, glutaric acid, suberic acid, sebacic acid or terephthalic acid. Polyamidoamines may also be prepared using mixtures of dicarboxylic acids as well as mixtures of plural polyalkylenepolyamines. Useful polyalkylenepolyamines include for example diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenetriamine, tripropylenetetramine, dihexamethylenetriamine, aminopropylethylenediamine and bisaminopropylethylenediamine. The dicarboxylic acids and polyalkylenepolyamines are heated at an elevated temperature, for example at from 120 to 220°C, preferably at from 130 to 180°C, to prepare polyamidoamines. The water of condensation formed is removed from the system. The condensation may also employ lactones or lactams of carboxylic acids having from 4 to 8 carbon atoms. The amount of a polyalkylenepolyamine used per mole of a dicarboxylic acid is for example in the range from 0.8 to 1.4 mol.

Amino-containing polymers further include ethyleneimine-grafted polyamidoamines. They are obtainable from the above-described polyamidoamines by a reaction with ethyleneimine in the presence of acids or Lewis acids such as sulfuric acid or boron trifluoride etherates at for example from 80 to 100°C. Compounds of this kind are described for example in DE-B-24 34 816.

Useful cationic polymers also include crosslinked or uncrosslinked polyamidoamines which may additionally have been grafted with ethyleneimine prior to crosslinking. Crosslinked ethyleneimine-grafted polyamidoamines are water-soluble and have for example an average molar weight of from 3 000 to 1 million daltons. Customary crosslinkers include for example epichlorohydrin or bischlorohydrin ethers of alkylene glycols and polyalkylene glycols.

Further examples of cationic polymers that contain amino and/or ammonium groups are polydiallyldimethylammonium chlorides. Addition polymers of this kind are likewise known. Useful cationic polymers further include copolymers of for example 1 - 99 mol%, preferably 30 - 70 mol%, of acrylamide and/or methacrylamide and 99 - 1 mol%, preferably 70 - 30 mol%, of cationic monomers such as dialkylaminoalkylacrylamide, dialkylaminoalkyl acrylate, dialkylaminoalkylmethacrylamide and/or dialkylaminoalkyl methacrylate. Basic acrylamides and methacrylamides are preferably likewise present in acid-neutralized form or in quaternized form. Examples are N-trimethylammoniumethylacrylamide chloride, N-trimethylammoniumethylmethacrylamide chloride, N-trimethylammoniumethyl methacrylate chloride, N-trimethylammoniumethyl acrylate chloride, trimethylammoniumethylacrylamide methosulfate, trimethylammoniumethylmethacrylamide methosulfate, N-ethyldimethylammoniumethylacrylamide ethosulfate,

N-ethyldimethylammoniumethylmethacrylamide ethosulfate, trimethylammoniumpropylacrylamide chloride, trimethylammoniumpropylmethacrylamide chloride, trimethylammoniumpropylacrylamide methosulfate, trimethylammoniumpropylmethacrylamide methosulfate and

N-ethyldimethylammoniumpropylacrylamide ethosulfate. Preference is given to trimethylammoniumpropylmethacrylamide chloride.

Further useful cationic monomers for preparing (meth) acrylamide polymers are diallyldimethylammonium halides and also basic

(meth) acrylates . Useful examples are copolymers of 1 - 99 mol%, preferably 30 - 70 mol%, of acrylamide and/or methacrylamide and 99 - 1 mol%, preferably 70 - 30 mol%, of dialkylaminoalkyl acrylates and/or methacrylates such as copolymers of acrylamide and N,N-dimethylaminoethyl acrylate or copolymers of acrylamide and dimethylaminopropyl acrylate. Basic acrylates or methacrylates are preferably present in acid-neutralized form or quaternized form. Quaternization may be effected for example with methyl chloride or with dimethyl sulfate.

Useful cationic polymers having amino and/or ammonium groups also include polyallylamines. Addition polymers of this kind are obtained by homopolymerization of allylamine, preferably in acid-neutralized form or in quaternized form, or by copolymerizing allylamine with other monoethylenically unsaturated monomers described above as comonomers for N-vinylcarboxamides .

The cationic polymers have for example K values of from 8 to 300, preferably from 15 to 180 (determined by the method of

H. Fikentscher in 5% aqueous sodium chloride solution at 25% and a polymer concentration of 0.5% by weight) . At pH 4.5, for example, they have a charge density of at least 1, preferably at least 4, meq/g of polyelectrolyte.

Preferred cationic polymers are polydimethyldiallylammonium chloride, polyethyleneimine, polymers containing vinylamine units, copolymers of acrylamide or methacrylamide that contain units derived from basic monomers, polymers containing lysine units or mixtures thereof. Examples of preferred cationic polymers are:

polylysines of M„ 250-250 000, preferably 500-100 000, and also lysine cocondensates having M_w molar masses of from 250 to 250 000, the cocondensable component being selected for example from amines, polyamines, ketene dimers, lactams, alcohols, alkoxylated amines, alkoxylated alcohols and/or nonproteinogenic amino acids,

vinylamine homopolymers, 1-99% hydrolyzed polyvinylformamides, copolymers of vinylformamide and vinyl acetate, vinyl alcohol, vinylpyrrolidone or acrylamide, each having molar masses of 3 000 - 500 000,

vinylimidazole homopolymers, vinylimidazole copolymers with vinylpyrrolidone, vinylformamide, acrylamide or vinyl acetate having molar masses of from 5 000 to 500 000 and also quaternary derivatives thereof,

polyethyleneimines, crosslinked polyethyleneimines or amidated polyethyleneimines having molar masses of from 500 to 3 000 000,

amine-epichlorohydrin polycondensates which contain imidazole, piperazine, Ci-Cs-alkylamines, Ci-Cβ-dialkylamines and/or dimethylaminopropylamine as amine component and have a molar mass of from 500 to 250 000, and

polymers containing basic (meth) acrylamide or (meth) acrylate ester units, polymers containing basic quaternary (meth) acrylamide or (meth) acrylate ester units having molar masses of from 10 000 to 2 000 000.

Amino-containing polymers which have been applied as hydrophilicizers to the latex foams may optionally be crosslinked thereon. Crosslinking of the foams treated with polymers containing amino groups is obtained for example by reaction with at least bifunctional crosslinkers such as epichlorohydrin, bischlorohydrin ethers of polyalkylene glycols, polyepoxides , multifunctional esters, multifunctional acids or multifunctional acrylates.

The latex foams hydrophilicized according to the invention are used in hygiene articles to acquire, distribute and immobilize body fluids, especially blood. Their hydrophilic character permits spontaneous acquisition of aqueous body fluids. The open-celled structure ensures rapid transportation into the foam interior. Hygiene articles which include the foams to be used according to the invention are essentially infant diapers, incontinence products, femcare articles, wound contact materials or secondary wound dressings.

The melamine-formaldehyde resin foams for inventive use in the hygiene sector are open-pored and hydrophilic. The droplet acquisition rate of the melamine-formaldehyde foams according to the invention is less than 5 seconds, preferably less than 2 seconds, particularly preferably less than 1 second.

The open-celled resilient latex foams are preferably incorporated as sheetlike structures in the form of foam fleeces from 0.1 to 10 mm, preferably from 1 to 5 mm, in thickness into hygiene products such as infant diapers, incontinence and femcare articles or as wound contact materials or in dressing materials. Foam density is for example in the range from 80 to 480 kg/m³ and preferably in the range from 100 to 250 kg/m³. The foams preferably have a webbed structure, a BET specific surface area of more than 0.074 m²/g, for example from 1 to 7 m²/g, a Free Swell Capacity of more than 5 g/g, for example from 6 to 10 g/g, and a tensile strength of > 144 kPa, for example from 150 to 250 kPa, in the dry state.

A hygiene article generally constitutes a combination of a liquid-impervious backsheet, a liquid-pervious topsheet, and an absorbent interlayer core. Hygiene articles of this type are known and described for example in EP-A-0 689 818. The absorbent composition is fixed between topsheet and backsheet. Elastic cuffs and self-adhesive tabs may optionally be integrated in the hygiene article. A preferred hygiene article construction is known for example from US-A-3, 860, 003.

When the hydrophilic open-celled resilient foams are used in a hygiene article, there are for example two ways of configuring the absorbent interlayer core: 1. The latex foam layer is used as the absorbent interlayer core without further layers. It then acts simultaneously as acquisition or acquisition/distribution layer and as storage layer.

2. The absorbent interlayer core consists of (a) a latex foam layer, which acts as acquisition or acquisition/distribution layer, and (b) a storage layer containing 10-100% by weight of highly swellable hydrogel.

The storage layer either is a hydrogel layer or constitutes compositions which include highly swellable hydrogels or have them fixed to them. Any composition is suitable that is capable of accommodating highly swellable hydrogels and being integrated into the absorbent core. A multiplicity of such compositions is already known and described in detail in the literature. A composition for installing the highly swellable hydrogels can be for example a fiber matrix consisting of a cellulose fiber mixture (airlaid web, wet laid web) or of synthetic polymer fibers (meltblown web, spunbonded web) , or else of a fiber blend of the cellulose fibers and synthetic fibers. Furthermore, open-pored foams or the like may be used to install highly swellable hydrogels.

Alternatively, such a composition can be the result of fusing two individual layers to form one or, better, a multiplicity of chambers which contain the highly swellable hydrogels. In this case, at least one of the two layers should be water-pervious. The second layer may be either water-pervious or water-impervious. The layer material used may be tissues or other fabrics, closed or open-celled foams, perforated films, elastomers or fabrics composed of fiber material. When the storage layer consists of a composition of layers, the layer material should have a pore structure whose pore dimensions are small enough to retain the highly swellable hydrogel particles. The above examples on the composition of the storage layer also include laminates composed of at least two layers between which the highly swellable hydrogels can be installed and fixed.

Furthermore, the storage layer can consist of a carrier material, for example a polymer film, on which the highly swellable hydrogel particles are fixed. The fixing can be effected not only on one side but also on both sides. The carrier material can be water-pervious or water-impervious. In the above compositions of the storage layer, the highly swellable hydrogels can have a weight fraction of from 10 to 100% by weight, preferably from 40 to 100% by weight and particularly preferably from 70 to 100% by weight. When the above-described storage layer composition constitutes a fiber matrix, then the absorbent composition results from a mixture of fiber materials and highly swellable hydrogels.

The storage layer may contain manifold fiber materials, which are used as fiber network or matrices. The present invention encompasses not only fibers of natural origin (modified or unmodified) but also synthetic fibers.

Examples of cellulose fibers include cellulose fibers which are customarily used in absorption products, such as fluff pulp and pulp of the cotton type. The materials (hard- or softwoods) , production processes, such as chemical pulp, semichemical pulp, chemothermomechanical pulp (CTMP) and bleaching processes, are not particularly restricted. For example, natural cellulose fibers such as cotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate are used.

Suitable synthetic fibers are produced for example from polyvinyl chloride, polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride, polyacrylic compounds such as ORLON , polyvinyl acetate, polyethyl vinyl acetate, soluble or insoluble polyvinyl alcohol. Examples of synthetic fibers include thermoplastic polyolefin fibers, such as polyethylene fibers (PULPEX^®) , polypropylene fibers and polyethylene-polypropylene bicomponent fibers, polyester fibers, such as polyethylene terephthalate fibers (DACRON^® or KODEL^®) , copolyesters, polyvinyl acetate, polyethyl vinyl acetate, polyvinyl chloride, polyvinylidene chloride, polyacrylics, polyamides, copolyamides, polystyrene and copolymers of the aforementioned polymers and also bicomponent fibers composed of polyethylene terephthalate-polyethylene isophthalate copolymer, polyethyl vinyl acetate/polypropylene, polyethylene/polyester, polypropylene/polyester, copolyester/polyester, polyamide fibers (nylon) , polyurethane fibers, polystyrene fibers and polyacrylonitrile fibers. Preference is given to polyolefin fibers, polyester fibers and their bicomponent fibers. Preference is further given to thermally adhesive bicomponent fibers composed of polyolefin of the core-sheath type and side-by-side type on account of their excellent dimensional stability following fluid absorption. The synthetic fibers mentioned are preferably used in combination with thermoplastic fibers. In the course of the heat treatment, the latter migrate to some extent into the matrix of the fiber material present and so constitute bond sites and renewed stiffening elements on cooling. Additionally the addition of thermoplastic fibers means that there is an increase in the present pore dimensions after the heat treatment has taken place. This makes it possible, by continuous addition of thermoplastic fibers during the formation of the absorbent core, to continuously increase the fraction of thermoplastic fibers in the direction of the topsheet, which results in a similarly continuous increase in the pore sizes. Thermoplastic fibers can be formed from a multiplicity of thermoplastic polymers which have a melting point of less than 190°C, preferably in the range from 75°C to 175°C. These temperatures are too low for damage to the cellulose fibers to be likely.

The above-described synthetic fibers may for example be from 1 to 200 mm in length and from 0.1 to 100 denier (gram per 9 000 meters) in diameter. Preferred thermoplastic fibers are from 3 to 50 mm in length, particularly preferred thermoplastic fibers are from 6 to 12 mm in length. The preferred diameter for the thermoplastic fiber is in the range from 1.4 to 10 decitex, and the range from 1.7 to 3.3 decitex (gram per 10 000 meters) is particularly preferred. The form of the fiber may vary; examples include woven types, narrow cylindrical types, cut/chopped yarn types, staple fiber types and continuous filament fiber types.

The fibers in the absorbent composition of the invention can be hydrophilic or hydrophobic. Combinations of both fiber foams are possible. According to the definition of Robert F. Gould in the 1964 American Chemical Society publication "Contact angle, wettability and adhesion", a fiber is referred to as hydrophilic when the contact angle between the liquid and the fiber (or the fiber surface) is less than 90° or when the liquid tends to spread spontaneously on the same surface. The two processes are generally coexistent. Conversely, a fiber is termed hydrophobic when a contact angle of greater than 90° is formed and no spreading is observed.

Preference is given to using hydrophilic fiber material. Particular preference is given to using fiber material which is weakly hydrophilic on the body side and most hydrophilic in the region surrounding the highly swellable hydrogels. In the manufacturing process, layers having different hydrophilicities are used to create a gradient which channels impinging fluid to the hydrogel, where it is ultimately absorbed.

Suitable hydrophilic fibers for use in the absorbent core of the invention include for example cellulose fibers, modified cellulose fibers, rayon, polyester fibers, for example polyethylene terephthalate (DACRON®) , and hydrophilic nylon (HYDROFIL^®) . Suitable hydrophilic fibers may also be obtained by hydrophilicizing hydrophobic fibers, for example the treatment of thermoplastic fibers obtained from polyolefins (e.g. polyethylene or polypropylene, polyamides, polystyrenes, polyurethanes, etc.) with surfactants or silica. However, for cost reasons and ease of availability, cellulosic fibers are preferred.

The highly swellable hydrogel particles are embedded into the fiber material described. This can be done in various ways, for example by using the hydrogel material and the fibers together to create an absorbent layer in the form of a matrix, or by incorporating highly swellable hydrogels into fiber mixture layers, where they are ultimately fixed, whether by means of adhesive or lamination of the layers .

The fluid-acquiring and -distributing fiber matrix may comprise synthetic fiber or cellulosic fiber or a mixture of synthetic fiber and cellulosic fiber, in which case the mixing ratio may vary from (100 to 0) synthetic fiber : (0 to 100) cellulosic fiber. The cellulosic fibers used may additionally have been chemically stiffened to increase the dimensional stability of the hygiene article.

The chemical stiffening of cellulosic fibers may be provided in different ways. A first way of providing fiber stiffening is by adding suitable coatings to the fiber material. Such additives include for example polyamide-epichlorohydrin coatings (Kymene^® 557 H) , polyacrylamide coatings (described in US-A-3, 556,932 or as the Parez 631 NC commercial product) , melamine-formaldehyde coatings and polyethyleneimine coatings .

Cellulosic fibers may also be chemically stiffened by chemical reaction. For instance, suitable crosslinker substances may be added to effect crosslinking taking place within the fiber. Suitable crosslinker substances are typical substances used for crosslinking monomers including but not limited to C₂-C₈-dialdehydes, C -Cs-monoaldehydes having acid functionality and in particular C -Cg-polycarboxylic acids. Specific substances from this series are for example glutaraldehyde, glyoxal, glyoxylic acid, formaldehyde and citric acid. These substances react with at least 2 hydroxyl groups within any one cellulose chain or between two adjacent cellulose chains within any one cellulose fiber. The crosslinking causes a stiffening of the fibers, to which greater dimensional stability is imparted as a result of this treatment. In addition to their hydrophilic character, these fibers exhibit uniform combinations of stiffening and elasticity. This physical property makes it possible to retain the capillary structure even under simultaneous contact with fluid and compressive forces and to prevent premature collapse.

Chemically crosslinked cellulose fibers are known, cf. for example WO-A-91/11162. The chemical crosslinking imparts stiffening to the fiber material, which is ultimately reflected in improved dimensional stability for the hygiene article as a whole. The individual layers are joined together by methods known to one skilled in the art, for example by intermelting by heat treatment, addition of hot-melt adhesives, latex binders, etc.

Generally, the invention utilizes a hydrophilicized fleece of an open-celled resilient latex foam as or in the absorbent interlayer core. The thickness of the absorbent interlayer when used as an absorbent core is generally in the range from 0.5 to 10 mm, preferably in the range from 1 to 5 mm. When used as an acquisition and distribution layer in combination with a storage layer, the thickness is in the range from 0.1 to 10 mm, preferably in the range from 0.5 to 3 mm.

The topsheet can be produced in various ways, for example as a woven, nonwoven, spun or combed fiber mixture. Preference is given to using a combed fiber mixture 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. It has a tensile strength of at least 400 g/cm in the dry state and 55 g/cm in the wet state.

The backsheet is usually a liquid-impervious material, for example polyolefin (a polyethylene backsheet for example) to protect the user's clothing from possible leakage.

The individual layers from which the hygiene articles are constructed are joined together by known methods, for example by intermelting the layers by heat treatment, addition of hot-melt adhesives, latex binders, etc. The absorbent interlayer core is positioned between topsheet and backsheet. Methods of measurement

Droplet acquisition rate

A single droplet of a 0.9% sodium chloride solution is pipetted onto a foam layer about 1-5 mm in thickness and the time taken for the droplet to disappear into the foam is recorded. The foam was rated hydrophilic when the absorption time was < 5 sec.

Density

Any suitable gravimetric method can be used for determining the density of the foam. What is determined is the mass of solid foam per unit volume of foam structure. A method for density determination of the foam is described in ASTM Method No.

D 3574-86, Test A. This method was originally developed for the density determination of urethane foams, but can also be used for this purpose. By this method, the dry mass and volume of a preconditioned sample is determined at 22 +/- 2°C. Volume determinations of larger sample dimensions are carried out under atmospheric pressure.

Free Swell Capacity (FSC)

This method is used to determine the free swellability of the open-celled resilient latex foam. To determine FSC, a testpiece of suitable size, for example with an area of approximately 1 cm x 1 cm, is cut out of a foam blank and weighed. The testpiece is placed in an excess of 0.9% by weight NaCl solution (at least 0.83 1 of sodium chloride solution / 1 g of foam) for 30 minutes. The testpiece is subsequently allowed to drip for 10 minutes before it is hung up by one corner, avoiding compression at all costs. The amount of liquid absorbed is determined by weighing the testpiece.

Acquisition time

The open-celled resilient hydrophilicized stryene-butadiene foam is cut into layers 1.5 mm, 2 mm or 4 mm in thickness. A commercially available sanitary napkin is carefully cut open, the highloft used as acquisition medium is removed and instead the open-celled resilient hydrophilicized styrene-butadiene foam layer inserted. The sanitary napkin is resealed. Difibrinated sheep blood is applied to it through a plastic plate having a ring in the middle (inner diameter of the ring 2.3 cm). The plate is loaded with additional weights, so that the total load on the napkin is 12.5 g/cm². The plastic plate is placed on the napkin in such a way that the center of the napkin is also the center of the application ring. 10 ml of difibrinated sheep blood are applied once. The blood is measured out in a graduated cylinder and applied to the napkin through the ring in the plate all at once. At the same time, the time taken for the blood to penetrate completely into the napkin is recorded. The time measured is noted as acquisition time.

Specific surface area

Specific surface area is determed by the BET method as set forth in German standard specification DIN 66132.

Examples

Latex foam 1

Preparation of a styrene-butadiene foam

To 100 parts of an aqueous 30% stryene and 70% butadiene latex having a polymer content of 70% were added with continuous mixing

1.5 parts of a 20% aqueous solution of potassium oleate,

2.25 parts of a 60% aqueous dispersion of sulfur, 1.5 parts of a 50% aqueous dispersion of zinc diethyl dithiocarbamate,

1.85 parts of a 50% aqueous dispersion of zinc

2-mercaptobenzothiazolate,

0.8 part of a 50% aqueous dispersion of 1, 3-diphenylguanidine, 0.3 part of a 50% aqueous dispersion of dimethylpolysiloxane,

2 parts of a 50% aqueous dispersion of zinc oxide,

1 part of an antioxidant based on phenol (Wingstay L) as a 50% aqueous dispersion, and

0.1 part of a preswelled polyacrylate thickener as a 13% aqueous solution.

The above-described mixture is hereinafter referred to as component A.

Component B was a 20% aqueous dispersion of sodium fluorosilicate. This is the gelling agent customarily used in latex foam production. Usually from 10 to 20 parts of component B are needed to gel 100 parts of component A.

Component A was foamed for 3 to 4 minutes in a mechanical foam blower until the foam had the desired density. The foaming operation was interrupted. After cooling to room temperature. component B was added, the mixture was subsequently homogenized for 1 minute at a higher speed, the machine speed was reduced and a uniform distribution of major bubbles in the mixture was ensured in the course of a further minute. The mixture was then transferred immediately to a stainless steel belt, on which it gelled in the course of 10 minutes. The gelling of the mixture can be speeded up by heating to higher temperatures, for example by heating to 135°C for 1 minute. After gelling, the latex foam was vulcanized by heating it to 135°C for 20 minutes under atmospheric air pressure. (The vulcanization time depends on the thickness of the latex foam. At 135°C it is about 10-30 minutes.) The vulcanized latex foam was cooled in the mold, demolded, washed and dried. The density of the foam was 0.143 g/cm³. The foam was cut into individual pieces which had the following dimensions: 210 mm x 280 mm.

Inventive example 1

Latex foam 1 was cut into samples having the hereinbelow reported dimensions:

(a) Sample 1: 200 cm x 270 cm x 1 mm

(b) Sample 2: 200 cm x 270 cm x 2 mm

A 1% solution of Nuwet 100 (organomodified polymethylsiloxane) in ethanol was prepared and the two foam samples were placed in the solution. After 30 minutes, the samples had absorbed liquid. They were taken from the solution and dried initially overnight in the air and then for 1 hour in a drying cabinet at 70°C for 1 hour.

Determination of hydrophilicity of latex foams

Each foam sample has 0.9% sodium chloride solution pipetted onto it from a 1 ml pipette and the time needed by the foam to absorb the droplet is determined. If the time was more than 300 seconds, the test was terminated. Such foam samples were classed as hydrophobic. The above-described latex foam samples (a) and (b) treated according to the invention and the untreated samples 1 and 2 gave the results reported in Table 1: Table 1

Inventive example 2

The open-celled resilient styrene-butadiene foam prepared according to inventive example 1 was cut into layers 2 mm in thickness. A commercially available sanitary napkin was carefully cut open, the material serving as aquisition medium removed and instead the 2 mm thick open-celled resilient styrene-butadiene foam layer inserted. The napkin was resealed. The time was then taken for 10 ml of defibrinated sheep blood to be absorbed.

Comparative example 2

A commercially available sanitary napkin was carefully cut open, the material serving as acquisition medium removed and then reinserted and the napkin resealed. This operation is intended to ensure perfect comparability.

Table 2

Claims

We claim: -

1. Hydrophilic open-celled resilient latex foams characterized 5 by a droplet acquisition rate of less than 5 seconds and a

Free Swell Capacity of least 5 g/g.

2. Hydrophilic open-celled resilient latex foams as claimed in claim 1, characterized by a minimum extensibility of 230%.

10

3. Hydrophilic open-celled resilient latex foams as claimed in claim 1 or 2, characterized by a BET specific surface area of at least 0.074 m²/g.

15 4. Hydrophilic open-celled resilient latex foams as claimed in any of claims 1 to 3 , characterized by a minimum tensile strength of > 144 kPa.

5. Hydrophilic open-celled resilient latex foams as claimed in 20 any of claims 1 to 4, characterized by a density of from 80 to 480 kg/m³.

6. A process for producing hydrophilic open-celled resilient foams as claimed in any of claims 1 to 5, which comprises

25 treating a latex foam with a hydrophilicizing agent.

7. A process as claimed in claim 6, wherein the latex foam used is based on styrene and butadiene.

30 8. A process as claimed in claim 6 or 7, wherein the hydrophilicizing agent used is at least one surfactant.

9. A process as claimed in any of claims 6 to 8, wherein the hydrophilicizing agent used is polymers containing amino

35 and/or ammonium groups.

10. A process as claimed in any of claims 6 to 9, wherein the hydrophilicizing agent used is polyalkylene glycols and/or water-soluble polymers of monoethylenically unsaturated

40 carboxylic acids.

11. A process as claimed in claim 9, wherein the hydrophilicizing agent used is addition polymers containing vinylamine units.

45 12. A process as claimed in claim 8, wherein the hydrophilizing agent used is at least one polydimethyldiloxane which contains at least one hydrophilic substituent selected from the group consisting of amino, carboxyl, hydroxyl and polyethylene glycol substituents.

13. The use of the hydrophilic open-celled resilient latex foams of any of claims 1 to 5 in hygiene articles to acquire, distribute and immobilize body fluids.

14. A use as claimed in claim 13, wherein the hygiene articles are infant diapers, incontinence products, femcare articles, wound contact materials or secondary wound dressings.