Bodies containing superabsorber polymers, methods of producing such bodies, and the use of such bodies

Bodies containing superabsorber polymers, having superabsorber polymers bonded together with polymer fibers, wherein at least part of the superabsorber polymers are present as a microporous open-celled foam. The bodies are used for producing diapers, sanitary napkins, tampons and other articles of personal hygiene.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to bodies containing superabsorber polymers,
 methods of producing such bodies, and the use of such bodies, in
 particular for producing diapers, sanitary napkins, tampons and other
 articles of personal hygiene.
 2. Description of Related Art
 Absorbent structures, as are used for example in baby diapers, adult
 diapers and feminine hygiene, preferably contain so-called superabsorbers
 to increase the quantity of absorbable body fluid, make the diapers
 thinner and bind body fluids chemically within the developing gel body of
 the superabsorber. Leakage is thus prevented.
 The SAP content in absorbent structures has been increased more and more
 over the past few years, with the following objectives: to reduce the
 thickness of the absorbent products, and to bind the body fluids in such a
 way that they are not released again even under mechanical stress (while
 being worn on the body, during movement, under pressure). Here the
 introduction of SAP in such products has proved successful because the
 fluid is bound to the SAP chemically, and not by capillary means. In this
 context, the fibers or fiber materials essentially have the function of
 transporting the fluid as rapidly as possible to the SAP, and the SAP
 itself has the function of receiving the fluid (reservoir function). It
 has been possible largely to avoid the problem of leakage and keep the
 skin dry by using special design features in the diaper, sanitary napkin,
 etc., primarily by backing the article with a thin non-fluid-permeable
 medium (a film) and further providing on the side toward the body a
 covering with an open structure, such as a thin, hydrophilic or
 hydrophilized nonwoven fabric or film with vacuum-expanded,
 three-dimensional perforations, and by using barrier leg cuffs and
 incorporating elastic elements. However, the advantage of the SAP--very
 high, pressure-resistant fluid uptake--is offset by the drawback of gel
 blocking and the associated slowing of fluid transport. Known means of
 enhancing gel stability, or endowing the SAP particles with a core/sheath
 structure in which the core is more tightly cross-linked than the sheath,
 result in a reduction of fluid absorption.
 SAP (standing for superabsorber polymer) is known in a variety of
 embodiments, such as homophilic fibers composed 100% of SAP, bicomponent
 fibers having a core of nonabsorbent thermoplastic and a sheath of SAP,
 and finer and coarser powders.
 SAP fibers blended with other fibers may be processed using known dry
 laying techniques to produce a fluid-absorbent nonwoven fabric. Although
 ease of processing argues in favor of such fibers made of SAP or
 containing an SAP component, one drawback is their relatively high price,
 particularly in comparison to fine and coarse powders or other
 particulates. An SAP staple fiber based on polyacrylic acid (or its sodium
 salt) costs approximately three times the price of SAP in powder form. For
 that reason, such fibers have not been able to achieve widespread
 acceptance, particularly for hygiene applications. Bicomponent fibers, in
 which only the sheath is made of SAP, have proved to be especially
 disadvantageous from the price standpoint. The sheath component
 constitutes only one-third to at most one-half of the total fiber mass.
 The absorption capacity is thus reduced accordingly and the cost of the
 fiber in relationship to absorbent effect is even less advantageous.
 Using known dry compacting methods, such as mechanical interlacing or
 calendering in sheets or patterns, SAP fibers mixed with other fibers may
 be made into nonwoven fabrics, in which the risk of the SAP particles
 escaping is substantially reduced compared to SAP in other particulate
 forms. The SAP powder is added either by sprinkling onto a dense sheetlike
 body, such as tissue paper, or by dispersion within an open-pored,
 voluminous fiber structure. A loose deposition of fine SAP powder, or of
 nonfibrous SAP particles in other form, in an absorbent layer, is not
 advantageous. The position of the particles may change undesirably during
 use. In the worst case, the particles may escape from the absorbent
 product, if open-structured surfaces and/or open edges are present.
 Numerous proposed solutions exist for fixing SAP particles to the fibers of
 the absorbent structures, or in other words, either within the sheetlike
 body or on the surface of the sheetlike body.
 Diverse methods for moist pretreatment of the sheetlike body prior to the
 application of SAP powder, as well as dry (i.e., thermal) fixation
 methods, have been proposed in order to localize SAP particles on or
 within the absorbent article.
 From European Patent A-0 719 531 it is known how to pretreat the support
 medium for the SAP with moisture/water. After the application or
 incorporation of the SAP particles, the water-soluble components of the
 SAP (inherent components) are activated to form a bond. After drying, the
 SAP particles are firmly bonded to the fibers of the absorbent layer.
 In U.S. Pat. No. 3,070,095, a process is described in which SAP is
 sprinkled onto a tissue paper, then covered with a second tissue paper and
 compressed into a laminate by pressing between smooth rollers. This
 method, however, has the drawback that very little SAP can be applied, and
 the SAP is not penetrated with fibers and is thus only very weakly bonded.
 SAP powder escapes under the slightest mechanical stress. Moreover, such a
 liminate has an extreme tendency toward gel blocking, a characteristic of
 grave concern in hygiene applications.
 WO 90/11181 and WO 91/10413 propose that fibers should be premoistened with
 an aqueous polymer dispersion instead of water. After drying, both the
 bonding forces of the synthetic polymer in the dispersion and the inherent
 bonding forces of the SAP particles come into play.
 In all cases described thus far, the SAP particles are bonded to the fibers
 or incorporated into or onto the sheetlike body. But drying proves to be
 difficult and requires considerable outlay, because the water bound up in
 the SAP strongly resists re-release.
 A great drawback of these SAP fixation methods is the fact that bonding is
 guaranteed only in the dry state. As soon as a gel body or hydrogel is
 formed from the SAP after the adsorption of fluid, the bonding force drops
 to zero or nearly zero.
 European Patent A-0 720 488 describes a dry method for the fixation of SAP
 powder within a nonwoven fabric as the support medium for that powder. It
 proposes bonding fibers, either single-component (homophilic fibers) or
 bicomponent fibers having a higher-melting-point core and a
 lower-melting-point or lower-softening-point sheath. SAP is bonded to the
 fibers by fusion onto the bonding fibers, but in this case as well, the
 bonding force is almost entirely lost after the SAP is charged with fluid.
 Solutions have also been proposed for giving fibers a coating of
 superabsorber. According to a method described in U.S. Pat. No. 4,721,647,
 a monomer, such as acrylic acid, methacrylic acid or vinylsulfonic acid,
 dissolved in water, is applied by drops to the fibers of an absorbent
 article, and is cross-linked there in the presence of initiators and
 cross-linking agents, to form a fiber coating of SAP. The concept is based
 on the fact that the SAP firmly envelops the fiber and is firmly bonded to
 the fiber even after swelling into a hydrogel-like state.
 All methods described above for the application of SAP, via
 non-water-soluble precursors or olefin-unsaturated monomers, result in a
 substantial hardening in a nonwoven fabric, in comparison to the state of
 the fabric prior to the application of SAP, and at times cause a
 considerably lower application than when SAP powder is dispersed in the
 fabric.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide absorbent structures
 containing SAP that avoid the drawbacks of the known structures, avoid
 hardening in a nonwoven fabric and gel blocking during use, and retain
 absorbed fluid well.
 The object is accomplished according to the present invention by bodies
 containing superabsorber polymers, having superabsorber polymers bonded
 together with polymer fibers, at least part of the superabsorber polymers
 being present as a microporous open-celled foam.
 The body containing SAP according to the present invention eliminates or
 reduces the aforementioned drawbacks because the SAP particles have a
 special microporous, open-celled structure and are distributed in this
 structure within the absorbent product.

DETAILED DESCRIPTION OF THE INVENTION
 The structure may also be called a microporous sponge structure. By
 particles, here we mean any kind of form for deposition on and
 distribution within the absorbent product. The microporous SAP may
 surround the fibers uniformly, constitute a pearly coating over the fibers
 of the absorbent product, have an entirely irregular distribution around
 and between the fibers, and/or be positioned at the points of intersection
 between fibers and there perform the function of a bonding substance for
 the fibers. The polymer fibers may already have adequate integrity in a
 prior stage of production, particularly prior to the addition of SAP, so
 that a further enhancement of integrity is not necessary, although it may
 be desired. The stage of the absorbent product prior to the application of
 SAP may therefore be a nonwoven fabric, which has adequate integrity for
 final use thanks to known nonwoven laying methods and known compacting
 methods.
 But webs or sheets of loose unbonded fibers may also be used for SAP
 application. Such webs or sheets cannot be transported, or otherwise
 handled or processed further, without being supported. In this case the
 invention is configured in such a way that all, or part of the total, of
 the microporous foam or microporous SAP particles are solely or primarily
 responsible, as a bonding agent, for the integrity of the absorbent
 product.
 The foam structure of the SAP particles is open-celled, or predominantly
 open-celled. By this is meant the fact that up to 100% of the pores
 contained within the SAP connect with one another. Because of the
 penetrating microchannels within the SAP particles in the absorbent
 product, the fluid is transported more rapidly into the core of the
 product than with conventional compact SAP masses. Because of the
 extremely hydrophilic nature of the microporous SAP structure, aqueous
 media are very rapidly transported into the core of the particle, despite
 the small pore size. The pore size is in the range from 0.2 to 100 .mu.m,
 and preferably from 0.5 to 50 .mu.m.
 Within and on the SAP, wetting agents or other surfactants may be applied
 or integrated, further reducing the interfacial tension between the SAP
 polymer and water. This accelerates fluid transport both in the micro
 range--i.e., within the SAP--and in the macro range--i.e., within the
 volume of the absorbent product as a whole.
 The absorbent body according to the present invention may be used directly,
 or may be a component of an absorbent product having a multilayer
 structure. Such multilayer products are used, for example, in baby
 diapers, adult diapers and sanitary napkins. It is known how to place an
 absorbent core between a top sheet and a non-fluid-permeable backing film.
 Between the core and the top sheet may be interposed a further layer which
 spontaneously absorbs body fluid and transmits it to the core, after a
 time lag, for final storage, primarily within the SAP and to some extent
 in the intercapillary spaces between the fibers. This intermediate layer
 can largely prevent gel blocking.
 The present invention also relates to such a multilayer body containing, in
 the following order:
 1) a fluid-permeable top sheet
 2) a sheetlike layer of a body according to the present invention, and
 3) a non-fluid-permeable layer.
 The body containing superabsorber polymers may assume any desired shape.
 Preferably, for uses in diapers, sanitary napkins or panty liners, it is
 in a sheetlike shape. For use in the form of tampons, it may also, for
 example, be in a cylindrical or similar shape.
 In the body according to the present invention, at least part of the SAP is
 present as a microporous, open-celled foam. Together with this, other
 non-foam SAP particles may also be present. However, preferably all the
 SAP, or essentially all the SAP, is present as a microporous, open-celled
 foam.
 The distribution of the spongy, microporous SAP within the absorbent body
 may extend throughout the cross-section. But its distribution may also be
 limited to only part of the cross-section, so that one side is entirely
 free from SAP. It may be more concentrated on one side than on the other
 side, so that a distribution gradient exists perpendicular to the surface,
 from one surface to the other. The side with the higher open-celled SAP
 particle density will then preferably be turned toward the core or the
 non-fluid-permeable film backing.
 In a further embodiment of the present invention, entirely poreless SAP
 particles may be embedded in and/or deposited on, as indicated above, the
 SAP having a microporous foam structure, or be distributed in any other
 manner within or on the microporous structure. This may be an advantage
 when it is less important to cause the SAP to swell quickly, i.e., to have
 greatly accelerated fluid uptake, and instead for reasons of application
 technology it is opportune to increase the quantity of superabsorbent
 material per unit volume of the absorbent product as a whole.
 The product according to the present invention is characterized not only by
 a very greatly accelerated fluid uptake in the micro range of the SAP, but
 additionally by an extreme softness, conformability and deformability.
 These characteristics are especially prized in absorbent products such as
 diapers in particular, so as to prevent leakage after the emission of the
 body fluid. With such products, conformation to body shapes, in
 combination with elastic, is much better than in the case of stiffer, less
 pliant and more rigidly bonded products.
 The SAP may in principle be composed of polymers such as absorb at least 20
 times their own dry weight in fluid, while forming no hydrogel. The
 superabsorber polymer is preferably selected from among hydrophobically
 modified hydrocarbon polymers, poly(vinyl alcohol-co-vinyl acetate),
 poly(meth)acrylic acid, cyanoethylated or partially formylated poly(vinyl
 alcohol), poly-N-vinyl-2-oxazolidone, polypeptides, (meth)acrylate
 copolymers or N-alkyl(meth)acrylamide derivatives. Such polymers include
 salts of polyacrylic acid or copolymers contained by the copolymerization
 of acrylic acid and such comonomers as maleic acid, itaconic acid,
 acrylamide, 2-methylpropanesulfonic acid, 2-methacrylic ethanesulfonic
 acid, 2-hydroxyethyl methacrylate or styrenesulfonic acid, at a
 copolymerization ratio that does not adversely affect the characteristics
 of the SAP.
 The method described below for producing the absorbent product requires the
 use of water-soluble polymer or prepolymer substances having reactive,
 cross-linkable groups. Typical functional groups that may easily be
 brought into reaction with suitable cross-linking agents are hydroxyl,
 amino and carboxyl groups. The water-soluble polymers lose their water
 solubility through cross-linkage, and are converted to a non-water-soluble
 but swellable state.
 Examples of such water-soluble polymers are hydroxyalkylated starches,
 hydroxyalkylated guar and hydroxyalkylated dextran, copolymers of vinyl
 alcohol and vinyl acetate, polymethacrylic acid, acrylate or methacrylate
 copolymers such as poly(hydroxypropyl acrylate-co-acrylamide),
 poly(hydroxyethyl acrylate-co-diacetone acrylamide) and poly(hydroxypropyl
 acrylate-co-hydroxyethyl acrylate).
 The SAP is preferably selected from among hydroxypropyl dextran,
 hydroxypropyl guar, hydroxypropyl starch, hydroxypropyl cellulose,
 hydroxyethyl cellulose, methylcellulose, hydroxypropyl methylcellulose,
 and ethylhydroxyethyl cellulose.
 Examples of suitable polypeptides are poly(L-proline) and
 poly(valine-proline-lycine-X-glycine) where X=any desired amino acid.
 Hydroxypropyl cellulose or hydroxyethyl cellulose, especially hydroxypropyl
 cellulose, is used by particular preference. In the finished body
 containing SAP, the SAPs are cross-linked. Examples of suitable
 cross-linking agents are acetaldehyde, formaldehyde, glutaraldehyde,
 diglycidyl ether, divinyl sulfone, diisocyanate, dimethyl urea,
 epichlorohydrin, oxalic acid, phosphoryl chloride, trimetaphosphate,
 trimethylol melamine, polyacrolein-like compounds. Formaldehyde,
 glutaraldehyde, divinyl sulfone and epichlorohydrin are preferred.
 According to the present invention, all suitable polymer fibers may be used
 as polymer fibers. Examples of suitable fibers are polyethylene fibers,
 polypropylene fibers, polyethylene terephthalate fibers and other fibers
 that are usually used for producing diapers and similar products.
 The bodies containing SAP according to the present invention are produced
 by:
 a) Mixing a cross-linkable SAP with a solvent to form an SAP solution which
 can be separated into phases, and impregnating the polymer fibers with
 this solution,
 b) Inducing a phase separation of the SAP solution into a phase containing
 fibers and enriched with polymer, and a phase impoverished in polymer,
 c) During the phase separation, cross-linking the SAP in the
 polymer-enriched phase containing fibers, to form a microporous,
 open-celled foam on the fibers, and
 d) Drying the resulting body containing SAP while maintaining the
 microporous, open-celled structure in the SAP.
 Such a method for producing SAP foams that are not in contact with polymer
 fibers is described, for example, in WO 95/31500. The method described in
 that publication is preferably followed to produce the bodies according to
 the present invention.
 The induction of phase separation is preferably accomplished by heating, if
 applicable in a water-vapor atmosphere, to a temperature in the range up
 to 98.degree. C., or by the addition of volatile nonsolvents for the SAP.
 The method is based on causing a solution of a cross-linkable polymer to
 undergo phase separation by initiating cross-linking, and if applicable by
 adding agents to accelerate phase separation. One phase becomes
 impoverished in polymer, and the other becomes enriched with polymer. In
 the polymer-enriched phase, cross-linking is continued until a microporous
 foam has formed through coagulation. Finally drying is performed. The
 process up to the stage of pore formation preferably takes place in a
 previously produced, hermetically sealed mold. The wet sponge is removed
 from the mold for drying.
 The preferred process according to the present invention is based upon
 slowly heating to 98.degree. C., in a water-vapor atmosphere, a loose
 fiber web or already previously bonded nonwoven fabric, after the
 application of a cross-linkable, aqueous polymer solution mixed with a
 cross-linking agent, the cross-linking and the associated precipitation of
 the microporous structure having already been initiated during the heating
 period and being completed, likewise in a water-vapor atmosphere, after
 the final temperature is reached. The separation of the SAP here may be
 accelerated by the addition of electrolytes or other known nonsolvents for
 the polymer dissolved in water. After the cross-linked product has been
 entirely precipitated, drying is performed. To accelerate drying, a
 solvent exchange, known per se, may previously be performed, such that
 water in the micropores is initially displaced by a first water-soluble
 solvent like ethanol, which in turn is displaced by a second solvent, such
 as heptane, that has a very low boiling point compared to the first
 solvent.
 The further process parameters may be found in WO 95/31500. The heating of
 the polymer solution preferably proceeds at a temperature above the lower
 separation temperature. Precise information about the lower separation
 temperature, as well as the concentrations of polymer and cross-linking
 agent in the solution, may be found in the above publication. Information
 about the employed reaction times and temperature curves may also be
 obtained from WO 95/31500.
 The bodies or multilayer bodies containing SAP, according to the present
 invention, are preferably used for producing diapers, sanitary napkins,
 tampons, or other articles of personal hygiene.