Stock material or miscellaneous articles – Structurally defined web or sheet – Including aperture
Reexamination Certificate
1998-08-25
2001-06-12
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including aperture
C428S134000, C428S158000, C428S316600, C428S913000, C442S370000
Reexamination Certificate
active
06245410
ABSTRACT:
The present invention relates to absorber elements having anisotropic swelling behavior made of at least one composite material having absorbent elements on a support, and to their production and use.
Water-absorbent foamed crosslinked polymers, which are frequently termed superabsorbent polymers or foams or superabsorbents, are able to absorb many times their own weight of aqueous liquids, forming hydrogels. These polymers are therefore used, for example, in hygiene products, such as diapers, for absorbing urine. They have the property of retaining the absorbed liquid even under mechanical load.
The polymers mentioned are frequently employed in hygiene or sanitary products in the form of composite materials. Thus, WO-A-88/09801 describes a water-swellable polymeric absorption material, which can be used in the form of a laminate with a woven fabric. WO-A-94/07935 describes water-absorbent hydrophilic polyurethane gel foams which can be applied to sheet supports, such as woven fabrics, knitted fabrics, webs or films.
EP-A-427 219 and U.S. Pat. No. 4,990,541 disclose water-absorbent latex foams into which superabsorbent finely divided materials can be incorporated after the foaming process.
WO-A-94/22502 describes a superabsorbent polymer foam which can be employed for producing hygiene articles, such as diapers. For this purpose, the polymer foam is applied to a support, with the polymer foam being able to cover the surface of the support in part or completely and being able to be applied in any desired pattern, with the pattern being formed from relatively large joined surfaces of the polymer foam and having the purpose of distributing the absorbent areas of the hygiene article in the desired manner.
The superabsorbent polymers have the property of expanding essentially isotropically in all three directions in space when aqueous liquids are absorbed. Generally, the polymers expand in each direction by a factor of from 2 to 5, so that a total increase in volume of a factor from 8 to 125 may be observed. In practice, it has been found that this property is disadvantageous in the construction of hygiene articles, since an area expansion of the polymers over a relatively large range is generally impossible. In addition, consideration must be given to the fact that, with an area expansion by a factor of from 4 to 25, when a single foam layer is placed into a diaper, only between ¼ or {fraction (1/25)} of the diaper surface may be covered by the foam, in order to have enough room for the swelling. This means that effective absorption of the body fluids which are released is not ensured in every case in this manner, so that complete absorption, for example in the case of diapers, must be ensured in other ways. If, in contrast, all of the available area in a diaper is covered with a superabsorbent polymer layer, as mentioned, there is insufficient space available to enable effective and rapid swelling of the polymer layer and thus effective absorption of the body fluids.
Attempts to solve these problems by producing simple composite materials, as described in the prior art, have failed. Regardless of whether a foam layer is applied to a support, a foam layer is introduced between two supports or a composite is provided in which a support material is embedded in the foam, disintegration of the composite materials during the swelling process is always observed. The composite materials sometimes delaminate at the support/polymer layer interface, but more frequently it is observed that cracks form within the partially swollen polymer layers and thus extensive polymer pieces detach from the composite material. After breakdown of the composite, the detached polymer particles swell unhindered isotropically in all three spatial directions. It is an object of the present invention, therefore, to provide an absorber element which is based on a composite material of a support and a water-absorbent polymer foam and is able to absorb liquids effectively and rapidly without the composite breaking down during the swelling process.
We have found that this object is achieved, surprisingly, by an absorber element which expands essentially in only one dimension, that is perpendicularly to the support material.
The present invention therefore relates to an absorber element of at least one composite material having absorbent elements on a support in which a plurality of elements of a superabsorbent foam (foam elements) are arranged on at least one support in a grid pattern at distances in such a manner that the elements in the swollen state touch at their peripheries.
The grid pattern can be chosen without restriction. However, care must be taken to ensure that the individual foam elements are spaced from one another in a manner such that sufficient space is available to enable complete swelling of the foam elements. The distance between the foam elements is preferably selected in a manner such that they are in contact with one another after the swelling process is complete. If the isotropic swelling factor is 3.0, for example, the distance between the foam elements must—in the case of cylindrical foam elements—correspond to twice the diameter of the cylinders. Therefore, an open area 8 times that of the original foam element must be available in a ring shape around the original foam element.
However, it can also be advantageous to choose the distances between the foam elements to be up to 50%, in particular up to 20%, less than the distance required for the foam elements just to touch in the swollen state. An additional stability of the swollen composite material can be achieved in this manner.
However, on the other hand, it can also be advantageous to choose the distances to be up to 100%, preferably up to 50%, greater than the distance required for the foam elements to touch one another in the swollen state. Composite materials are obtained in this manner which, even in the swollen state, have a permeability to incident liquids.
By applying the polymer foams in the form of a grid in the manner described, the composite material essentially expands anisotropically, that is it expands significantly only in a direction perpendicular to the support material. Expansion of the composite material in the direction parallel to the surface of the support material, in contrast, essentially does not occur.
The shape of the foam elements can generally be chosen without restriction. They generally represent elevations on the support, which may be cylindrical, hemispherical, pyramidal, conical, truncated pyramidal, truncated conical, etc. If an increased permeability of the composite material is desired, the shape of the foam elements is chosen in a manner such that gaps (gusset volumes) remain between the swollen foam elements which ensure a high vertical permeability. This is preferably achieved by employing foam elements in the form of cylinders or cylindrical elevations or elevations having pentagonal cross-section.
If, in contrast, a composite material is desired in which the swollen foam elements form an essentially closed surface, i.e. the foam elements cover the surface, as foam elements, use is preferably made of elevations having triangular, square, rectangular or hexagonal cross-section.
Preference is given to foam elements having hexagonal or circular cross-section.
The flank angle of the foam elements, i.e. the angle between the support surface and the lateral surface of the foam element, can be chosen without restriction. Expediently, it is 90°. However, in some cases it is advantageous if the angle is smaller, preferably in the range 20-90°, and in particular in the range 50-90°. Foam elements are then present in the shape of a cone or a pyramid or in the shape of a truncated cone or a truncated pyramid.
The height of the foam elements depends on the application of the absorber element. For hygiene and sanitary articles it is generally in the range from about 0.1 mm to about 20 mm. The cross-sectional area of the foam elements is generally in the range from 1 to 400 mm
2
, preferably from 2 to
Anstock Thomas
Hahnle Hans-Joachim
Kremeskötter Jens
Schornick Gunnar
Tropsch Jurgen
BASF - Aktiengesellschaft
Copenheaver Blaine
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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