WO2013009225A1

WO2013009225A1 - Absorbent composite material and an absorbent article comprising the absorbent composite material

Info

Publication number: WO2013009225A1
Application number: PCT/SE2011/050936
Authority: WO
Inventors: Shabira Abbas; Anette Larsson; Mikael Larsson; Johan HJÄRTSTAM; Ingrid Gustafson
Original assignee: SCA Hygiene Products AB
Current assignee: Essity Hygiene and Health AB
Priority date: 2011-07-08
Filing date: 2011-07-08
Publication date: 2013-01-17
Anticipated expiration: 2014-01-08

Abstract

An absorbent composite material comprising an intimate mixture of: a) network- forming cellulose fibrils having a diameter of from 1 nm to 100 μm, b) a hydrophilic absorbent polymer component bonded to the cellulose fibrils; and c) a water soluble polymer component. The hydrophilic absorbent polymer component remains bonded to the cellulose fibrils when the absorbent composite material is wet and in that water soluble polymer component is solubilized when the absorbent composite material is wet, the water soluble polymer component being a compound having the capability of increasing the viscosity of the aqueous portion of body fluids such as menses and runny feces.

Description

ABSORBENT COMPOSITE MATERIAL AND AN ABSORBENT ARTICLE COMPRISING THE ABSORBENT COMPOSITE MATERIAL

TECHNICAL FIELD

The present invention relates to an absorbent composite material comprising an intimate mixture of network-forming cellulose fibrils having a diameter of from 1 nm to 100 μητ, a hydrophilic absorbent polymer component bonded to the cellulose fibrils; and a water soluble polymer component. The invention also relates to an absorbent article comprising the absorbent composite material and to the use of the composite material for absorbing blood and feces and as an active agent release material.

BACKGROUND

Advances in absorbent article technology have stimulated the search for absorbent (often superabsorbent) materials with desirable properties such as high absorption, high storage capacity, high gel strength and high mechanical strength.

The absorbent materials may comprise two or more layers such as liquid acquisition layers, storage layers and distribution layers.

In order to obtain good liquid acquisition capability a liquid absorbent material should have high instant liquid intake capacity. It is common to use open bulky structures with large capillaries as such structures have rapid liquid acquisition. Some examples of such materials are cellulosic fluff pulp of the thermo-mechanical or chemical thermo- mechanical (CTMP) type, chemically stiffened cellulosic fibres, waddings of synthetic fibres and porous foam materials. It is usually preferred that the acquisition materials retain their open structure even under some pressure and/or that they are resilient so that they may regain a bulky structure after having been compressed.

A good liquid storage and retention capacity may be obtained by the use of

superabsorbent materials. Superabsorbent materials are commonly understood as being crosslinked polymers with the capacity to absorb liquid many times their own weight upon swelling.

The superabsorbent material in an absorbent structure, i.e. a diaper core, is often present in the form of small particles, which are arranged and contained in a fibrous matrix. The fibrous matrix usually consists of cellulosic fluff pulp of thermo-mechanical, chemical or chemical thermo-mechanical type, but a certain amount of synthetic fibres may also be used, e.g. to provide the structure with increased wet resiliency and/or the ability of being thermally stabilized.

A problem with absorbent structures containing superabsorbent material is that the commonly used superabsorbents are generally less well suited for absorption of bodily fluids such as menses and runny feces that comprise viscous components. Currently, most commercially available absorbent superabsorbents are adapted for urine absorption. When using such superabsorbents in an absorbent article for absorption of body fluids such as menses or runny feces, highly viscous components of the fluid will not be absorbed by the superabsorbent particles. When menstrual fluid or runny feces is absorbed by a superabsorbent, only water and water soluble components of the fluid may be absorbed by the superabsorbent while a part of the fluid containing the high-viscosity components remains unabsorbed, although somewhat dewatered. Accordingly, in a conventional absorbent structure the dewatered body fluid with the high-viscosity components may easily be squeezed out of the absorbent structure when it is exposed to pressure such as may occur when a wearer of an absorbent article containing the absorbent structure sits down or when the absorbent article is compressed between the legs of the wearer. The sticky dehydrated fluid may cause highly unpleasant rewetting and smearing of the wearer's skin and may be difficult to wash off from the skin.

A further problem with traditional absorbent structures containing superabsorbent particles is that it is difficult to achieve a desired distribution of the superabsorbent material in the structure and to retain the superabsorbent in a desired location in the absorbent structure during transport, storage and use of the article. It has been suggested in the art to combine superabsorbent polymers and fibrous material, such as cellulose fibres in order to overcome the problems with retention of the superabsorbent in an absorbent structure. An example of a material comprising cellulosic fibers and superabsorbent polymers is disclosed in US 2003/01 1 1 163, which describes a process for making an absorbent fibrous web composite including a stable and controllable dispersion of superabsorbent polymer. Two polymer precursors, for example, acrylic acid or methacrylic acid, are added in separate stages to form a superabsorbing polymer on or in a pre-formed fibrous web, which includes a plurality of hydrophilic fibres, e.g. microfibrillar cellulose or

microcrystalline cellulose.

Similarly, US 2003/01 1 1774 discloses a process for making an absorbent fibrous composite nonwoven web including e.g. superabsorbent polymers and a plurality of hydrophilic fibres. The polymerization of the superabsorbent polymer is integrated into the process of forming the absorbent composite nonwoven web.

Further, EP 1 207 914 discloses an absorbent structure comprising an open-cell foam structure wherein the pore walls of the structure comprise a liquid-storing material, e.g. polyacrylate. The absorbent structure is characterized in that the pores of the foam structure contain hydrophilic fibres, e.g. cellulose fibres, at which at least the main part of the hydrophilic fibres are firmly anchored in the pore walls of the foam structure, and that the fibre amount is at least 10 % by weight of the total weight of the open-cell foam in dry condition.

However, despite these prior efforts, there remains a need for an absorbent structure having improved capability of acquiring, absorbing and retaining viscous fluids such as menses and runny feces. Furthermore, there is a need for an absorbent structure having improved capability of preventing rewet from viscous fluids.

DEFINITIONS AND EXPLANATIONS

The term "cellulosic" in connection with fibers or fibrils refers to fibrils or fibers from natural sources such as wood, cotton, etc., regenerated cellulose and the derivatives from these fibers by means of chemical, mechanical, thermal treatment or any combination of these. Further, cellulosic fibers also includes cellulosic or cellulose-containing fibers produced by microorganisms.

The term "nanofibrils" means individual fibrils having a diameter equal to or less than 100 nm at all points along the nanofibril. The practical lower limit for the fiber diameter is approximately 1 nm. The diameter may vary along its length. The nanofibrils may exist as individual fibres and/or as clusters of nanofibrils. The term "nanofibrillated cellulose (NFC)" \s used interchangeably with the term "nanofibrils". "Whiskers" are a particular form of nanofibrils having an aspect ratio of at least 2. Nanofibrils having a smaller aspect ratio do not qualify as whiskers.

The term "microfibres" means individual fibres having a diameter equal or greater than 100 nm but less than or equal to 100 ym at all points along the microfibre. More specifically, the microfibres may have a diameter greater than 100 nm but less than or equal to 10 pm or a diameter greater than 100 nm but less than or equal to 1 m. The diameter may vary along the length of the microfibre. The microfibres may exist as individual microfibres and/or as clusters of microfibres in the composite. The term MFC (microfibrillated cellulose) is used interchangeably with the term "microfibers".

Microfibrillated cellulose may comprise a fraction of nanofibrils.

The term "porous" is used herein to describe a material that has pores and, which admits the passage of gas or liquid through these pores.

The term "composite" as used herein means a structure made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct, where said constituent materials are intimately integrated in each other such that they surround each other completely or in-part. Other components may also be present in the composite, as disclosed herein. The term "polymer" includes, but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term "polymer" shall include all possible configurational isomers of the material. These configurations include, but are not limited to isotactic, syndiotactic and atactic symmetries. "Superabsorbent polymer" is used herein to refer to water-swellable, water-insoluble organic or inorganic materials capable of absorbing at least about 20 times their own weight in an aqueous solution containing 0.9 weight percent (wt %) of sodium chloride. Organic materials suitable for use as a superabsorbent material can include natural materials such as polysaccharides (including modified polysaccharides such as CMC: carboxymethyl cellulose), polypeptides and the like, as well as synthetic materials such as synthetic hydrogel polymers. Such hydrogel polymers include, for example, alkali metal salts of polyacrylic acids (pAA), polyacrylamides, polyvinyl alcohol, polyacrylates, polyvinyl pyridines, and the like. Other suitable polymers include hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, and isobutylene maleic anhydride copolymers and mixtures thereof. The hydrogel polymers are preferably lightly crosslinked to render the material substantially water insoluble. Preferred superabsorbent materials may be further surface crosslinked so that the outer surface or shell of the superabsorbent particle, fibre, flake, sphere, etc. possesses a higher crosslink density than the inner portion of the superabsorbent.

The term "crosslinked" is used herein to describe a material composed of linear polymer chains which have been submitted to crosslinking by means of a cross-linking agent so that the linear polymer chain has been transformed into a three-dimensional network structure.

The term "bonded" as used herein includes both "intrinsic bonding" arising from direct bonding by means of physical and chemical interactions between the components of the composites according to the invention as well as bonds created by the use of a cross- linking agent.

The term "particle" includes powders, granules, flakes, spheres, short fibers, and the like. The term "foam" as used herein is a material that has been formed by trapping gas bubbles in a liquid or solid. Foams that are used for absorption purposes are of the open- cell type containing pores that are connected to each other and form an interconnected network.

"Menstrual fluid" is a body fluid containing mucus, tissue, microorganisms and blood. The composition of menstrual fluid varies between individuals. Blood is a fluid having high viscosity as compared to urine or water. The composition of blood is approximately 45 % blood cells and approximately 55 % blood plasma. The blood plasma is an electrolyte and contains salts, water and proteins (e.g. albumin, IgG and fibrinogen). The proteins add up to about 7-8 % of the blood plasma. The amount of protein in blood is in the order of 10¹⁷/ml blood, and the amount of blood cells is in the order of 10⁹/ml blood. As used herein, the high-viscous part or the high viscous phase of the menstrual fluid consists of the mucus, tissue, microorganisms and blood cells/blood clots while the low viscous or aqueous phase is made up of the blood plasma.

"Runny feces" is fecal matter that is a mixture of a low viscous, watery phase and more solid or high viscous parts.

The term "absorbent article" includes diapers, incontinence guards, sanitary napkins, panty liners, wound dressings, bed protectors, seat covers, and the like. SUMMARY OF THE INVENTION

In accordance with the invention, there is provided an absorbent composite material comprising an intimate mixture of:

a) network-forming cellulose fibrils having a diameter of from 1 nm to 100 pm, preferably from 3 nm to - 100 pm, such as from 100 nm - 100 pm;

b) a hydrophilic absorbent polymer component bonded to said cellulose fibrils; and c) a water soluble polymer component.

The hydrophilic absorbent polymer component remains bonded to the cellulose fibrils when the absorbent composite material is wet and the water soluble polymer component is solubilized when the absorbent composite material is wet. The water soluble polymer component is a compound having the capability of increasing the dynamic viscosity of the aqueous portion of body fluids such as menses and runny feces.

Preferably, the water soluble polymer component has the capability of increasing the dynamic viscosity of the aqueous portion of body fluids such as menses and runny feces at least 10 times, preferably at least 100 times and most preferably at least 1000 times.

The absorbent composite material of the invention has excellent ability of handling both the low viscosity phase and the high-viscosity components of body fluids such as menses and runny feces. When exposed to body fluid comprising highly viscous components contained in a low- viscosity phase, the composite material according to the invention will absorb and retain both low viscosity parts and high viscosity parts of the fluid by a combination of absorption and immobilization. Accordingly, in the absorbent composite according to the invention, in addition to low-viscosity parts of the body fluids being absorbed by the hydrophilic absorbent component in the composite, the water soluble polymer component in the composite is solubilized and released into the body fluid when the composite is wetted by body fluid. When the water soluble polymer component is dissolved in the aqueous phase of the body fluid that has not been absorbed by the hydrophilic absorbent component, the viscosity of the aqueous phase is raised and the high-viscosity components in the body fluid become trapped and immobilized. In this way, body fluids comprising high viscosity components are retained within the absorbent composite and are prevented from being squeezed out of the absorbent composite. The cellulose fibrils in the absorbent composite material of the invention may be selected from microfibrillated cellulose (MFC), nanofibril cellulose (NFC), nanocrystalline cellulose (NCC), whiskers or combinations and mixtures thereof.

The absorbent composite material according to the invention may comprise at most 25 wt. % cellulosic fibers having an average diameter greater than 100 μιη, preferably at most 10 wt.%, more preferably at most 5 wt. % and most preferably 0 wt % cellulosic fibers having an average diameter greater than 100 pm.

The water soluble polymer component is preferably a linear hydrophilic polymer having sufficient molecular weight to achieve a functional raise in the viscosity of the aqueous phase of body fluids such as menses and runny feces. The polymer component may be an electrically charged component or may be non-charged.

The water soluble polymer component in the composite material according to the invention may be selected from: polyacrylic acid, polyacrylates and polymetacrylates, sodium alginate, pectin, polysaccharides including chitosan, starch and modified polysaccharides such as modified starch, methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxyethylcellulose (HEC) hydroxypropylmethyl cellulose (HPMC),

hydroxypropyl cellulose (HPC), ethyl hydroxyethylcellulose (EHEC), and polyvinylalcohol (PVOH), polyvinylacetate (PVA), polyethylene oxide, polyacrylamides, hydrolyzed polyacrylamides, polyvinyl pyridines, hydrolyzed acrylonitrile, and isobutylene maleic anhydride copolymers and mixtures thereof.

Further suitable water soluble viscosity increasing polymers may be found in WO

5 2003/041752 A1.

Suitable polymers for use as the hydrophilic absorbent component that is bonded to the cellulose fibrils in the absorbent composite of the invention may include: natural materials such as polysaccharides including chitosan, starch and modified polysaccharides such as0 modified starch, methyl cellulose (MC), carboxymethyl cellulose (CMC),

hydroxyethylcellulose (HEC) hydroxypropylmethyl cellulose (HPMC), hydroxypropyl cellulose (HPC), and ethyl hydroxyethylcellulose (EHEC). Other suitable polymers may be derived from the group consisting of acrylic acid (AA) monomers and its salts, methacrylic acids and its salts and combinations thereof, acrylonitrile, acrylic esters (e.g. tert-butyl- or5 methylacrylic acid ester), unsaturated lactones, anhydrides, acrylamide monomers, secondary or tertiary acrylamides, or other alkenes, monomers having at least one alkene (olefin) group and at least one sulfonate or sulfonic acid group, ethylene sulfonate esters, ethylene sulfonic halides and heterocyclic monomers containing sulphonamide linkages are suitable monomers. Combination of these monomers with each other, and with other0 monomers, is possible when forming the polymers according to the invention. Other superabsorbent polymers may be polymeric sulfonic acids such as styrene sulfonic acid, sodium styrene sulfonate and/or similar compounds. If neutral monomers are used these may be hydrolysed in order to achieve a charged polymer, as known in the art. 5 The water soluble polymer component may be different from the hydrophilic absorbent polymer component or the hydrophilic absorbent polymer component and the water soluble polymer component may be parts of the same polymer composition. In the latter case, the polymer composition may either be a polymer composition having the intrinsic ability of interacting with and binding to the cellulose fibrils such that part of said polymer0 remains bonded to said cellulose fibrils when said composite material is in a wet state and part of said polymer is solubilized when said composite material is in a wet state.

Preferred polymer compositions are those which a chemical structure that is similar to that of the cellulose fibrils, such as polymer compositions based on modified cellulose.

Particularly preferred polymer compositions are those selected from: hydroxypropylmethyl5 cellulose (HPMC), hydroxylpropyl cellulose (HPC), hydroxyethyl cellulose (HEC), ethylhydroxyethyl cellulose (EHEC), and mixtures thereof. It is believed that such polymer compositions interact with the cellulose fibrils and may contribute to increasing the network strength of the composite material of the invention. The hydrophilic absorbent polymer component may be a crosslinkable component that has been bonded to the cellulose fibrils by crosslinking as defined herein. The

crosslinkable component may be selected from one or more of: polyacrylic acid, grafted starch, carboxy methyl cellulose and polyvinyl alcohol. The absorbent composite material of the invention may comprise more than one hydrophilic absorbent polymer component that is bonded to the cellulose fibrils and more than one water soluble polymer component. Polymer compositions providing both bonding/absorption and polymer release/viscosity increase may be combined with polymers providing only one of these functions.

In case of using polymer compositions providing both bonding/absorption and polymer release/viscosity the polymer composition, such as hydroxypropylmethyl cellulose (HPMC), hydroxylpropyl cellulose (HPC), hydroxyethyl cellulose (HEC), ethylhydroxyethyl cellulose (EHEC), and mixtures thereof, may constitute from 20-70 wt % of the absorbent composite based on the dry weight of the composite.

Based on the dry weight of the composite, the absorbent composite material of the invention may be composed of 1 -99 wt % of the fibrils, preferably 20-99 wt %, more preferably 30-70 wt %.

The cross-linked hydrophilic absorbent polymer component may be present in the absorbent composite material of the invention in an amount of 5-95 wt %, preferably 10- 70 wt %, more preferably 30-60 wt % based on the dry weight of the composite.

The water soluble polymer component may be present in the absorbent composite material of the invention in an amount of 5-95 wt %, preferably 10-70 wt %, more preferably 30-60 wt % based on the dry weight of the composite.

The absorbent composite material of the invention may further comprise at least one active agent selected from: odor inhibiting agents, anti-microbial agents, perfumes, skin care agents, odor reducing agents, nutrients for lactic acid producing bacteria, etc. The active agent will be released from the composite material when it becomes wetted by body fluid.

The absorbent composite material of the invention may be a component of an absorbent article and in particular of an absorbent article for personal care such as a diaper, a pant diaper, an incontinence guard, a sanitary napkin, a panty liner, or the like. The absorbent composite material of the invention is particularly suited for use in absorbent articles intended for blood absorption such as sanitary napkins, panty liner and hybrid

incontinence/catamenial articles.

The absorbent article of the invention is preferably of the kind that comprises a liquid pervious topsheet, a liquid impervious backsheet and an absorbent core arranged between the liquid pervious topsheet and the liquid impervious backsheet and wherein the absorbent core comprises the absorbent composite material.

The invention is also directed to the use of an absorbent composite material as set out herein for absorbing blood and to the use of the composite material as a combined liquid absorbing structure and active agent release material. The absorbent composite material of the invention may have the ability of practically one- dimensional swelling of the structure when it is wetted with body fluid. By one-dimensional swelling as used herein is implied that the material swells to a considerable extent in a first direction, such as up to 20 times or more of its original unloaded dimension in that direction and only to a negligible extent in the directions perpendicular to the first direction. By way of example, a sheet made from the absorbent composite of the invention may swell less than 1.2 in the x-y plane of the sheet and more than 20 times in the z-direction or thickness direction of the sheet.

The absorbent composite of the invention may be in the form of a foam. If in the form of a foam, the nanofibrils may be incorporated into the pore walls of the foam. The foam may have a pore size gradient. Additionally, the foam may comprise one or more substances selected from the group consisting of; plasticizers, surfactants and blowing agents. The composites of the invention comprising a polymeric compound that has the inherent ability of binding to cellulose fibrils may be made according to a method comprising the steps of:

a. providing cellulosic fibrils having a diameter of from 1 nm to 100 pm suspended in water;

b. adding a polymer selected from one or more of hydroxypropylmethyl cellulose (HPMC), hydroxylpropyl cellulose (HPC), hydroxyethyl cellulose (HEC), ethylhydroxyethyl cellulose (EHEC)

c. stirring the mixture until said polymer is solubilized in said solvent,

d. pouring the solution containing said fibrils onto a flat surface, and

e. drying the fibril/polymer mixture to form an absorbent composite.

The composites of the invention comprising a polymeric compound that is bonded to the cellulose fibrils by means of a cross-linking agent may be made according to a method comprising the steps of:

b. adding one or more neutralized monomer

c. adding a crosslinker,

d. adding an initiator, and

e. polymerizing the monomers and crosslinker forming a gel

f. adding a linear hydrophilic polymer;

g. letting said linear hydrophilic polymer dissolve in said gel;

h. drying the fiber/gel mixture to form an absorbent composite.

The initiator may be selected from the group consisting of oxidizing initiators, azo initiators, photoinitiators and/or thermal initators.

The drying step may be carried out with any suitable means as known in the art such as by convective heating, by IR or by freeze drying.

If particles are to be formed, the above methods may further comprise a step of forming the composite into particles. If a foam is to be formed, the above methods may further comprise a step of forming the composite into a foam. The method for making the foam may further comprise the steps of adding one or more substances selected from the group consisting of; plasticizers, surfactants and blowing agents. The absorbent article according to the invention may be a feminine product, such as sanitary napkins and pantyliners, as well as baby diapers and incontinence guards. In other words, the invention provides an absorbent article, wherein said absorbent article is a diaper, a pant diaper, an incontinence guard, a sanitary napkin or the like and of the kind comprising a liquid pervious topsheet, a liquid impervious backsheet having an absorbent core comprising the absorbent composite arranged between the topsheet and the backsheet. The absorbent core may further comprise additional materials and components such as foam, superabsorbents, binders, absorbent and non-absorbent fibrous material, e.g. cellulose fibers, synthetic fibers, tissue layers or non-woven materials in combination with the absorbent polymer composite.

The composites of the invention may be incorporated into the absorbent article in the form of foams, films, webs, or particles or mixtures of these forms of the composite. Depending on the ratio between the cellulose fibrils and the absorbing and releasing polymers in the composite, the dry composite may be more film-like if the fibre content is low or more like a fibrous web if the composite has a higher content of fibrils. After formation of the composite material, which will usually be in the form of a two-dimensional sheet-like foamed or non-foamed structure, the dried sheet-like structure may be ground or divided into smaller pieces by means of any suitable method such as cutting or shredding. DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates swelling behavior for MFC-HPMC composites, depending on

HPMC content. The water uptake per initial dry weight for initial HPMC contents of; 0 % ( A ), 20 % (♦), 35 % (o), 50 % (^χ), and 65 % (·) w/w is shown.

Figure 2 shows percentage of HPMC released at different times for MFC-HPMC composites with 20 % (^χ), 35 % (o) and 65 % ( A ) w/w HPMC. Figure 3 shows SEM images of swollen and freeze dried MFC HPMC

composites. Top row images are at a magnification of 10000X for samples with initial HPMC contents of (a) 0 %, (b) 20 % and (c) 50 % w/w. Bottom row is images are at a magnification of 100000X for samples with initial HPMC contents of (d) 0 %, (e) 20 % and (f) 50 % w/w.

Figure 4 shows a SEM image of the cross-section of a swollen and freeze dried

MFC HPMC composite having initial HPMC content of 50 % w/w.

EXAMPLES

The following examples illustrate a MFC/HPMC composite. HPMC is an associative polymer that forms complexes between and within polymer chains, and could potentially bind strongly to the MFC in the composites. It is theorized that the similarity of the materials may make it possible for non-substituted regions of the HPMC to form hydrogen bonds with the MFC surfaces. If HPMC were to bind strongly to the MFC, it would not be able to leave the composite, but would gel inside the composite.

Materials

Micro fibritlated cellulose (MFC) was bought from the Paper and Fibre Research Institute PFI, Trondheim, Norway. The MFC had been prepared from commercial bleached kraft pulp using a mechanical pre-treatment followed by homogenization and has previously been characterized as being highly heterogeneous. Hydroxypropyl methyl cellulose (HPMC) was of the grade 90 SH 100 SR, Shin-Etsu Chemical Co., Ltd., Tokyo, Japan. The swelling and release experiments were performed using deionized water. Phosphate buffer for SEC-RI analysis was prepared from analytical grade ingredients. Composite preparation

Stock solutions of HPMC and MFC were prepared having a concentration of 0.4 % w/v. These stock solutions were mixed to get HPMC concentrations of 0, 20, 35, 50, 65 and 80 % w/w in the composites. Composites were obtained by pouring 45 g solution into 100 ml weigh boats (VWR, Stockholm, Sweden) and evaporating the water at 30 °C in a desiccator with freshly dried silica gel orange (Sigma-Aldrich, Steinheim, Germany) until dry (about 3 weeks), the silica gel was replenished every three days. The composites were stored in desiccators with Silica gel orange in between analyses.

Swelling of the composites

The swelling of the composites was analysed using both gravimetrical analysis and microscopy observations. For analysis of swelling, rectangular composite pieces were cut out and the dry weights (4-10 mg) were recorded using a Shimadzu AUW220D

(Shimadzu Philippines Manufacturing Inc., Philiphines). The composites were swollen in 5 ml H20 in Non-Tissue Culture Treated Plates, 12 Well, Flat bottom with Low Evaporation Lid (Becton Dickinson and Company, Franklin Lakes, USA). The swelling studies were performed at 37 °C and shaking at 300 rpm using a Grant-bio PHMP-4 (Grant Instruments Ltd., Cambridg, UK). At specified times (10 min to 24 h) the weights of the swollen samples were recorded and the swelling degree per initial weight was calculated as:

( - m . )

~ ·^■» : (4) where m is the wet mass of the composite and mi is the initial dry weight of the composite. Swelling analyses were conducted on composites containing 0-65% w/w HPMC. All samples showed very quick initial swelling, and the swelling per initial dry weight increased dramatically for HMPC contents > 30% w/w. (Figure 1 ) The dimensional changes of composites swollen for 3 h showed that most of the swelling took place by increasing the thickness (h) of the composites

Release of HPMC

The release of HPMC from the composites was analyzed using size exclusion

chromatography, multiangle light scattering, coupled with refractive index detection (SEC- MALS-RI) and through gravimetrical analysis.

For analysis of weight loss during swelling, rectangular composite pieces were cut out and the dry weights (4-10 mg) were recorded using a Shimadzu AUW220D (Shimadzu

Philippines Manufacturing Inc. , Philiphines). The composite pieces were then subjected to USP release studies; 500 ml phosphate buffer, pH 6.8, 75 rpm, 37 °C. Samples were taken out at specified time point and were analyzed with regard to HPMC concentration using a SEC-MALS-RI system. The column used was a TSKgelGMPWxl 7.8mmx300mm 13um (TOSOHAAS, Germany). The MALS used was a DAWN EOS (Wyatt Technology, Santa Barbara, USA) and the Rl detector was an OPTILAB rEX (Wyatt Technology, Santa Barbara, USA). SEC-MALS-RI analysis of HPMC release from composites (20, 35 and 65 % w/w HPMC) revealed that some, but not all, of the HPMC was released from the composites; see Fig. 2. The HPMC released determined by SEC-MALS-RI was in good agreement with calculations based on weight loss after swelling for pure MFC composites and composites containing HPMC. Interestingly, a larger percentage of the HPMC was released from composites having higher initial HPMC content, indicating that interactions between MFC and HPMC are a cause for the retention of some HPMC. Given that MFC exists as particles such interactions should occur at the surfaces of the MFC. Thus, samples with a higher surface area of the MFC should release less HPMC per MFC. Indeed, the mass HPMC retained within the composites per mass of initial MFC after 24 h was 0.14, 0.26 and 0.41 for composites with 20, 35 and 65 % w/w HPMC, respectively. Based on the swelling results, it seems as if composites with higher HPMC content had formed different structures during the composite forming process, and as if those structures had an increased MFC surface area, which could be because HPMC prevents aggregation of MFC during composite formation, similar to the effect of hemicelluloses in preventing the coalescence of cellulose microfibrils during drying.

Characterization of composite structure after swelling

To get information on composites structure in the swollen state, the structure after the swelling experiments were preserved by freeze-drying. The freeze-dried composite samples with 0, 20, and 50 % w/w HPMC were investigated using SEM analysis. (SEM LEO Ultra 55 SEM equipped with a field emission gun (LEO Electron Microscopy Group, Germany) in secondary electron detection mode.)

The SEM analysis of freeze-dried composites revealed a highly porous structure for the pure MFC, having 100s of nanometers in pore sizes. HPMC containing composites had more dense structures with increasing MFC content (Fig. 3). This was not expected as the pore size generally increases with decreasing concentration of network within the sample.

There are however several logical explanations for the observed decrease in pore size.

First, the presence of HPMC hinders the aggregation of MFC into larger structures, resulting in a decrease in effective fiber radii which should promote a decreased pore size. Also, not all the HPMC was released from the composites. Any remaining HPMC should lead to a smaller observed pore size as it will also form a dry network structure upon drying. Finally, SEM analysis of a swollen and freeze-dried composite displayed a structure having stacked layers in the z-direction (see Fig. 4). Care should be taken when interpreting freeze-dried structures as describing the true structure of a wet sample.

However, the swelling results did show that almost all of the composite swelling occurred in the z-direction. This would be expected for a layered structure where the individual layers exhibited low swelling and deformability, but where the interlayer regions are swellable. Such behavior has previously been reported for a novel anisotropic hydrogel. In such systems swelling would mainly occur when water occupies the inter-layer spaces, pushing the layers apart. The swelling results in combination with the layered structure observed in SEM, strongly suggests that the network structure of the wet samples was of the layered form. The hypothesis is further supported by that a lamellar structure has been previously observed for dry MFC composites.

Claims

1 . An absorbent composite material comprising an intimate mixture of: a) network-forming cellulose fibrils having a diameter of from 1 nm to 100 μιτι, preferably from 3 nm to - 100 pm, such as from 100 nm - 100 pm;

b) a hydrophilic absorbent polymer component bonded to said cellulose fibrils; and

c) a water soluble polymer component

characterized in that said hydrophilic absorbent polymer component remains bonded to said cellulose fibrils when said absorbent composite material is wet and in that water soluble polymer component is solubilized when said absorbent composite material is wet, said water soluble polymer component being a compound having the capability of increasing the viscosity of the aqueous portion of body fluids such as menses and runny feces.

2. An absorbent composite material according to claim 1 , wherein said water soluble polymer component has the capability of increasing the dynamic viscosity of the aqueous portion of body fluids such as menses and runny feces at least 10 times, preferably at least 100 times and most preferably at least 1000 times.

3. An absorbent composite material according to claim 1 or 2, wherein said cellulose fibrils are selected from microfibrillated cellulose (MFC), nanofibril cellulose (NFC), nanocrystalline cellulose (NCC), whiskers or combinations and mixtures thereof.

4. An absorbent composite material according to claims 1 -3, wherein said composite material comprises at most 25 wt % cellulosic fibers having an average diameter greater than 100 pm, preferably at most 10 wt%, more preferably at most 5 wt. % and most preferably 0 wt % cellulosic fibers having an average diameter greater than 100pm.

5. An absorbent composite material according to any one of claims 1 -4, wherein said water soluble polymer component is different from said hydrophilic absorbent polymer component.

6. An absorbent composite material according to any one of claims 1 -4, wherein said hydrophilic absorbent polymer component and said water soluble polymer component are parts of the same polymer composition.

7. An absorbent composite material according to any one of the preceding claims, wherein said water soluble polymer component is selected from: polyacrylic acid, polyacrylates and polymetacrylates, sodium alginate, pectin, polysaccharides including chitosan, starch and modified polysaccharides such as modified starch, methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC) hydroxypropylmethyl cellulose (HPMC), hydroxypropyl cellulose (HPC), ethyl hydroxyethyl cellulose (EHEC), and polyvinylalcohol (PVOH), polyvinylacetate (PVA), polyethylene oxide, polyacrylamides, hydrolyzed polyacrylamides, polyvinyl pyridines, hydrolyzed acrylonitrile, and isobutylene maleic anhydride copolymers and mixtures thereof.

8. An absorbent composite material according to claim 7, wherein said polymer composition has the intrinsic ability of bonding to said cellulose fibrils such that part of said polymer remains bonded to said cellulose fibrils when said composite material is in a wet state and part of said polymer is solubilized when said composite material is in a wet state.

9. An absorbent composite material according to claim 8, wherein said polymer composition is selected from: hydroxypropylmethyl cellulose (HPMC), hydroxylpropyl cellulose (HPC), hydroxyethyl cellulose (HEC), ethylhydroxyethyl cellulose (EHEC), and mixtures thereof.

10. An absorbent composite material according to claim 8 or 9, wherein said polymer composition may constitute from 20-70 wt % of the absorbent composite based on the dry weight of the composite.

1 1. An absorbent composite material according to any one of claims 1 -7, wherein said hydrophilic absorbent polymer component is a crosslinkable component that has been bonded to said cellulose fibrils by crosslinking.

12. An absorbent composite material according to any of the preceding claims, wherein the absorbent material is composed of 1 -99 wt % of said fibrils, preferably 20- 99 wt %, more preferably 30-70 wt %, based on the dry weight of the composite.

13. An absorbent composite material according to any of the preceding claims, wherein said cross-linked hydrophilic absorbent polymer component is present in an amount of 5-95 wt %, preferably 10-70 wt %, more preferably 30-60 wt %, based on the dry weight of the composite.

14. An absorbent composite material according to any of the preceding claims, wherein said water soluble polymer component is present in an amount of 5-95 wt %, preferably 10-70 wt %, more preferably 30-60 wt %, based on the dry weight of the composite.

15. An absorbent composite material according to any of the preceding claims, wherein said composite further comprises at least one active agent selected from: odor inhibiting agents, anti-microbial agents, perfumes, skin care agents, odor reducing agents, nutrients for lactic acid producing bacteria, etc.

16. An absorbent article characterized in that it comprises an absorbent composite material as defined in any one of the preceding claims.

17. An absorbent article according to claim 16, wherein said absorbent article comprises a liquid pervious topsheet, a liquid impervious backsheet and an absorbent core arranged between said liquid pervious topsheet and said liquid impervious backsheet and wherein said absorbent core comprises said absorbent composite material.

18 An absorbent article according to claim 16 or 17, wherein said absorbent article is a sanitary napkin or a panty liner.

19. Use of an absorbent composite material according to any of claims 1 -15 for absorbing blood.

20. Use of an absorbent composite material according to any of claims 1 -15 as a combined liquid absorbing structure and active agent release material.