ABSORBENT ARTICLE CONTAINING SUPERABSORBENT MATERIAL

Abstract: Absorbent structure in an absorbent article such as a diaper, sanitary napkin, incontinence guard, wound dressing, bed protection, etc., said structure containing a superabsorbent material possibly in combination with other absorption- or carrier materials and where the superabsorbent material has a content of renewable and/or biodegradable raw material of at least 20 % by weight, calculated on the dry weight of superabsorbent material, and has a phase angle delta </=8 DEG defined as the phase shift between deformation and strain measured at rheological measurements.

Description: Absorbent article containing superabsorbent material Technical field The present invention refers to an absorbent structure in an absorbent article, such as a diaper, sanitary napkin, incontinence guard, wound dressing, bed protection etc., said structure containing a superabsorbent material possibly in combination with other absorbent and/or carrier materials and where the superabsorbent material has a content of renewable and/or biodegradeable raw material of at least 20 % by weight, calculated on the dry weight of superabsorbent material. The superabsorbent material can be in particulate form, e g grains, granules, flakes or fibers, or as a film, a foam or a coating on fibers and has a high capability to absorb liquids, such as waer and body liquids, e g urine and blood while swelling and forming a non water-soluble gel. Background of the invention So called superabsorbents are crosslinked polymers having the capability to absorb liquid several times, 10 times or more, their own weight. They further have capability to retain liquid even when they are exerted to an external pressure. They are extensively used in hygiene article such as diapers, incontinence guards, sanitary napkins and the like, at which they are usually present in particulate form such as e g grains, granules, flakes or fibers and are mixed or layered with other absorbent material, usually cellulosic fibers. Superabsorbents usually are crosslinked polymers of different chemical composition. Commonly occurring superabsorbents are crosslinked polyacrylates. Acrylic acid and polyacrylates are synthetic products based on oil raw material, i e a non renewable raw material. The increasing environmental conciousness however involves a desire to change over to materials based on renewable and/or biodegradeable raw materials. This applies especially to disposable products, which after use are thrown away. Poly- saccharides such as cellulose .and starch are examples of materials which are both biodegradeable and renewable and can be used as starting materials for superabsorbents. Other examples of useful polysaccharides .are xantane, alginate, chitosane, pectin, guar gum. Proteins .and peptides are also examples of polymers from a renewable source. The performance of a certain superabsorbent in an absorbent article depends on many factors, such as where and how the superabsorbent is mixed into the absorbent structure, particle shape, particle size and physical and chemical properties such as absorption capacity, gel strength and liquid retaining capacity. A phenomena called gelblocking can also negaitvely effect the absoφtion capacity in for example a fibrous structure containing superabsorbents. Gelblocking means that superabsorbent particles when wetted forms a gel which blocks the pores in the fibrous structure or between the particles and by that obstructs liquid transport from the wetting area to the rest of the absorbent article and liquid transport to all particles. This involves a poor utilization of the total absoφtion capacity of the absorbent body and involves a risk for leakage. A high gel strength - cross linking of the superabsorbent involves a decreased risk that gelblocking will occur. A too high gel strength however results in a too low absoφtion capacity. With gel strength is meant the tendency to gel deformation, which can be defined as the shear modulus of the gel. This is measured in a rheometer and is calculated as the relationship between (a) the shear stress/deformation that is applied to the sample and (b) the relative deformation/shear stress showed by the sample. The shear modulus, G', is measured in the unit Pa. This is described in e g EP-B-254 476. Another common method to characterize a superabsorbent and its capacity to perform in an absorbent article such as a diaper, incontinence guard, sanitary napkin and the like is to measure its capacity to absorb liquid under pressure, so called AUL (absorbency under load). This is disclosed i e g EP-B- 0,339,461. One measures here in a special apparatus the ability of the superabsorbent to absorb a saline solution according to the so called demand wettability principle under a certain load, usually 2kPa, which is an established standard load within this technical field. Other common measuring parameters is the free swell capacity (FSC) which is measured as g absorbed liquid per g superabsorbent when this is allowed to swell freely, and the capacity to retain liquid after centrifugation, centrifuge retention capacaity (CRC). According to the later method the superabsorbent is allowed to absorb liquid freely until saturation, after which the sample is centrifugated and it is measured how much of the absorbed liquid that is retained after centrifugation. None of these methods of characterizing the properties of a superabsorbent however tells the whole truth and does not always correlate to the performance of the super- absorbent in a product concept. This especially applies to superabsorbents wholly or partly based on renewable and/or biodegradeable raw materials, which usually have a low AUL value and a low gel strength and where neither AUL or any of the other above stated measuring methods is a good measure of the performance of the superabsorbent in an absorbent article. The object and most important features of the invention The object of the present invention is to provide .an absorbent structure in an absorbent article of the above mentioned kind where the absorbent structure contains a super- absorbent material wholly or partly based on renewable and/or biodegradeable raw materials and which shows a very good performance in the absorbent structure measured as absoφtion capacity and acquisition rate. This has according to the invention been achieved by the fact that the superabsorbent material has a phase angle δ <8°defined as the phase shift between deformation and strain measured at rheological measurements. The superabsorbent material preferably has a phase angle δ <6°, preferably <4°and most preferably < 3 ° . The superabsorbent material should preferably have a content of renewable and/or biodegradeable raw material of at least 40, preferably at least 60 and most preferably at least 80% by weight, calculated on the dry weight of the superabsorbent material. According to an alternative the superabsorbent material comprises a crosslinked polymer, where the polymer is based on renewable and/or biodegradeable raw material and the crosslinking agent either is renewable and/or biodegradeable or non-renewable/ non-biodegradeable. The absorbent structure can according to an embodiment contain a combination of hydrophilic fibers and superabsorbent material, at which the hydrophilic fibers preferably are cellulosic fluff pulp, tissue or hydrophilic synthetic fibers, and the absorbent structure contains between 5 and 80 % by weight superabsorbent material calculated on the total weight of the structure. According to another embodiment the absorbent structure contains a combination of an absorbent foam material and a superabsorbent material. According to a further embodiment the superabsorbent materialet is in the form of a foam and according to another embodiment in the form of a coating on a fibrous rmaterial The superabsorbent material can have a relatively low absoφtion capacity under load (AUL), more precisely no more than 25, preferably no more than 20 and most preferably no more than 18 ml of an acqueous solution containing 0,9 % by weigh of sodium chloride, per g superabsorbent material under a load of 2kPa. Description of the invention The absorbent structure according to the invention is intended to be used in an absorbent article of the kind comprising a liquid pervious cover sheet, a liquid impervious bottom sheet and an absorbent structure applied therebetween. The absorbent article can be a diaper, a sanitary napkin, an incontinence guard, a wound dressing, a bed protection etc. The shape and construction of the absorbent article is of course completely optional and the invention can be applied to all types of such articles. The increasing environmental conciousness has however involved that there is an aim towards replacing the materials in absorbent articles, e g diapers, that are based on non- renewable/non-biodegradeable raw materials with materials that are completely or partly based on renewable and/or biodegradeable raw materials. The materials concerned are the liquid impervious bottom sheet in the form of a plastic film, the liquid pervious cover sheet in the form of a nonwoven and the superabsorbent. Superabsorbents based on renewable and/or biodegradeable raw materials, such as starch, cellulose, alginate etc often have a low gel strength and a low AUL value. In tests that have been performed it has proved that those superabsorbents having essentially equal FSC (free swell capacity), CRC (centrifugation retention capacity) and AUL can have very different absoφtion capacity if they are combined with hydrophilic fibers, usually cellulosic fluff pulp, in an absorbent structure. The inventors of the present invention have therfore searched for another parameter showing a clear correlation with the absoφtion capacity of the superabsorbenten when incoφorated in an absorbent structure, as it is intended to be in an absorbent article, e g a diaper. Rheology is the science of the deformation and and flow properties of materials. The term rheology includes all deformation of a material that occurs when it is exerted to forces, both the usual gravitation or if higher forces are needed. Rheology deals with the relationship between FORCE, DEFORMATION and TIME. In the cases where all deformation energy is spread as heat in the material this is denoted as VISCOUS and one says that the material flows. In the cases where all deformation energy is stored in the material the material is denoted as ELASTIC and no flowing ocurs. A material in which a certain part of the deformation energy is stored and a certain part in spread as heat is denoted as VISCOELASTIC. At measurements of viscoelastic material the material is exerted to sinus-shaped strain (or deformation, depending on which type of instrument that is used) and the sinus- shaped deformationen (or strain) in the material is measured. The phase shift between deformation and strain is called the phase angle δ. This is 0° for an ideally elastic material and 90° for an ideally viscous material. By making a vector analysis of the strain/deformation realtionship there is obtained a component, G', in phase with the deformationen and a component, G", 90° offset in phase with the deformation. The higher values the modules G ' and G" have the more elastic and viscous respectively the material is. According to the above mentioned EP-B-254476 it is the module G' that is measured, which according to the invention has proved to be insufficient in order to foresee the function of the superabsorbent in an absorbent structure. It has now according to the invention shown that it is the phase angle δ defined as tan δ = G'VG' that is such a parameter that shows a clear correlation with the properties of the superabsorbent in a concept in an absorbent structure. Those superabsorbents having a value of δ < 8° show an especially good product function in the form of absoφtion capacity and acquisition rate. It is especially siuprising that this applies also to superabsorbents having such a low AUL value as lOg/g at 2 kPa, which is evident form table 1 below. The rheological measurements are made on a Carri Med CSL 2 100 from TA Instruments. The rheometer is a "controlled stress"- instrument. The measurment are performed according to established rheological methods, so that the person skilled in the art should easily be able to repeat the measurments. In all measurements the materials are prepared as samples according to the principle: lg absorbent material to 20 g synthetic urine. In order to test the materials at an equilibrium position in the swelling process the mixture sample + synthetic urine is allowed to stand for 2 hours before tested, however within 6 hours from the mixture was prepared The measurement is divided into two steps, where step one includes a so called "stress- sweep" with which the linear area of the material is defined. The linear viscoelastic area area occurs when the deformation is so small that the material only neglectably changes from its equilibrium position. From this linear position the shear stress is chosen, at which the material is to be tested in the coming frequency sweep. From the frequency sweep 0,l-10Hz the quantities G', G" and δ are obtained. All G', G" and δ-values shown in the table below are taken at 1,13 Hz. The superabsorbent based on renewable raw material that were tested were a number of test substance based on crosslinked CMC (carboxy methyl cellulose): Al, A2, A3, A4, A5, A6, A7, A8, A9, A10, Al 1, A12, A13 och A14. Preparation of superabsorbents based on crosslinked CMC is descrubed in e g EP-B-201 895. Some test substances based on crosslinked carboxy methyl starch: Bl, B2 och B3, were also tested, and which can be produced analogously with crosslinked carboxy methyl cellulose (CMC). Further some test substance based on crosslinked polyaspartate: Cl, C2 och C3, were tested and which e g may be produced according to WO96/08523, As reference material there was also tested a commercially available superabsorbent from Hoechst called Sanwet IM 7100 which is a crosslinked sodium polyacrylate and pure cellulosic fluff pulp without admixture of superabsorbent. The absoφtion concept tested consisted of 20 % by weight of a superabsorbent mixed with cellulosic fluff pulp of chemical type. The basis weight of the pulp structure was 500 g/m 2 . Besides the δ-value there was also measured FSC (g/g), CRC (g/g) and AUL (g/g) at 2 kPa of the superabsoerbents. The test liquid was in all cases synthetic urine according to the following recipe : 60 mmol KC1 (4.5 g), 130 mmol NaCl (7.6 g), 3.5 mmol MgSO 4 (0.42 g) , 2.0 mmol CaSO 4 *2H 2 0 (0.34 g), 300 mmol urea (18 g) and 1 g 0.1% triton were dissloved in 1 liter destilled water. The measuring methods used for testing the function of the superabsorbent in an absorbent structure according to above were the following: The absorption capacity in a sloping plane: The sample body was placed in a 30° inclination in order to imitate the position of the diaper during use and the lower portion was in contact with a liquid bath (synthetic urine) and was absorbing liquid by suction. The sample body was weighed before and after liquid absoφtion and the absoφtion was noted as g absorbed liquid per g absorbent body. Acquisition time: Three additions of each 60 ml liquid (synthetic urine) was made with a time interval of 10 minutes. The time it took before all liquid had been absorbed was measured (visual observation). The result of the performed measurements are shown in Table 1 below. Table 1 Correlation studies on these show that the δ-value correlates positively with the acquisition times 2 and 3, i e low acquisition times at low δ- values, and negatively with the absoφtion capacity, i e high absoφtion capacity at low δ-values. Multiple correlation studies show that CRC in combination with δ-values explain the result of absoφtion capacity and of acquisition time 3. In the absove stated tests the superabsorbent was in the form of particles which were mixed with cellulosic fluff pulp for forming an absorbent body. The invention is however not limited to such absorbent bodies, but it would also be possible to apply superabsorbent particles in the form of a layer between layers of cellulosic fluff pulp or other optional hydrophilic fibers, natural or synthetic. The superabsorbent can also be applied between layers of soft paper (tissue), nonwoven or other material. The superabsorbent can also be present in the form of fibers, as a film or as a coating on another material or as a foam. Instead of or besides cellulosic fluff pulp the absorbent body can contain other types of hydrophilic fibers, absorbent foam material or the like.

Claims: Claims 1. Absorbent structure in .an absorbent article such as a diaper, sanitary napkin, incontinence guard, wound dressing, bed protection etc., said structure containing a superabsorbent material possibly in combination with other absoφtion- or carrier materials and where the superabsorbent material has a content of renewable and/or biodegradeable raw material of at least 20 % by weight, calculated on the dry weight of superabsorbent material, characterized in that the superabsorbent material has a phase angle δ <8° defined as the phase shift between deformation .and strain measured at rheological measurements. 2. Absorbent structure according to claim 1, characterized in that the superabsorbent material has a phase angle δ ≤6°, preferably <4° and most preferably ≤3°. 3. Absorbent structure according to claim 1 or 2, characterized in that the superabsorbent material has a content of renewable and/or biodegradeable raw material of at least 40, preferably at least 60 and most preferably at least 80 % by weight, calculated on the dry weight of superabsorbent material. 4. Absorbent structure according to any of the preceding claims, characterized in that the superabsorbent material comprises a crosslinked polymer, where the polymer is based on a renewable and/or biodegradeable raw material and the crosslinking agent either is renewable and/or biodegradeable or non-renewable/non-biodegradeable. 5. Absorbent structure according to any of the preceding claims, characterized in that it contains a combination of hydrophilic fibers and superabsorbent material. 6. Absorbent structure according to claim 5, characterized in that the hydrophilic fibers comprises cellulosic fluff pulp, tissue or hydrophilic synthetic fibers. 7. Absorbent structure according to claim 5 or 6, characterized in that it contains between 5 and 80 % by weight of superabsorbent material calculated on the total weight of the structure. 8. Absorbent structure according to claims 1-4, characterized in that it contains a combination of an absorbent foam material and a superabsorbent material. 9. Absorbent structure as claimed in any of claims 1-4, characterized in that the superabsorbent material is in the form of a foam. 10. Absorbent structure as claimed in any of claims 1-4, characterized in that the superabsorbent material is in the form of a coating on a fibrous material. 11. Absorbent structure as claimed in any of the preceding claims, characterized in that the superabsorbent material has an absoφtion capacity under a load of 2kPa (AUL) of no more than 25, preferably no more than 20 and most preferably no more than 18 ml of an acqueous solution containing 0,9 % by weight of sodium chloride, per g superabsorbent material. 12. Absorbent article such as a diaper, a sanitary napkin, incontinence guard, wound dressing, bed protection etc comprising a liquid pervious cover layer, a liquid impervious bottom layer and an absorbent structure enclosed therebetween, characterized in that the absorbent structure is of the kind stated in any of claims 1-11.

Inventors: ANNERGREN JEANETTE, HIMMELMAN GUNILLA, HJERPE ANGELA, KALENTUN PIA, OESTERDAHL EJE, SAMUELSSON ANNIKA, STROEMBOM EVA, SVENSSON AASA, ANNERGREN, JEANETTE, HIMMELMAN, GUNILLA, HJERPE, ANGELA, KALENTUN, PIA, OESTERDAHL, EJE, SAMUELSSON, ANNIKA, STROEMBOM, EVA, SVENSSON, AASA
Applicants: ANNERGREN JEANETTE, HIMMELMAN GUNILLA, HJERPE ANGELA, KALENTUN PIA, OESTERDAHL EJE, SAMUELSSON ANNIKA, SCA HYGIENE PROD AB, STROEMBOM EVA, SVENSSON AASA, ANNERGREN, JEANETTE, HIMMELMAN, GUNILLA, HJERPE, ANGELA, KALENTUN, PIA, OESTERDAHL, EJE, SAMUELSSON, ANNIKA, SCA HYGIENE PRODUCTS AB, STROEMBOM, EVA, SVENSSON, AASA
Application year: 1998
Main classification: A61F13
IPC codes: A61F5, A61F13, A61L15, B01J20