Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Stratified or layered articles
Reexamination Certificate
2002-04-19
2004-11-16
Staicovici, Stefan (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Stratified or layered articles
C264S119000, C264S120000, C264S121000, C264S145000, C264S153000, C264S257000, C264S258000, C264S293000, C264S319000, C156S196000, C156S276000
Reexamination Certificate
active
06818166
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of forming a web of fibres intended for use in absorbent products, such as sanitary towels, incontinence products, nappies or the like, by air-laying fibres. The invention also relates to a fibre web produced using this method.
BACKGROUND ART
Absorption bodies for use in disposable absorbent products of the type mentioned above are produced from what is known as fluff pulp made from cellulose. Fluff pulp comes in various forms, such as mechanical, thermomechanical, chemithermomechanical or chemical pulp. Fluff pulp is delivered in bales or rolls and is defibrated in mills before forming. The mat formation for forming absorption bodies is carried out by defibrated pulp being transported in an air flow towards a forming wire, usually in the form of what is known as a mat-forming wheel. This is air-permeable and the fibres remain on the periphery of the mat-forming wheel and form a fibre mat of low density, which fibre mat is compressed to the desired density in further processing.
During mat formation, a continuous web of fibre material can be formed, which web is compressed and cut to form individual absorption bodies. In further handling, these can be separated in the web direction for separate positioning between web materials running continuously in the web direction, which are intended to form a covering around separate absorption bodies to form individual products, such as nappies, sanitary towels, incontinence products or the like.
It is also common to combine the individual absorption bodies with a further absorption body of different density and extent. One purpose of having two or more absorption bodies of different density in an absorbent product can be to create a density gradient in the thickness direction of the product in order to control liquid transfer into the product. Another purpose can be that it is desirable to have a highly compressed spreading layer which has the capacity to spread received liquid over the length of the entire product for better utilization of available absorption material. This is because it is a common problem with absorbent products, such as nappies or the like, that the product becomes locally saturated with liquid and leakage occurs long before available absorption material is utilized. When two absorption bodies which are to work together are combined, however, it is necessary for the separate layers to cohere intimately so that an insulating interspace is not formed between the layers, that is to say areas in which the layers are separated and liquid transfer consequently does not take place effectively between the layers. If an insulating interspace is present between the two absorbent layers, the risk of leakage in the lateral direction of the product is of course great. In practice therefore, after two different layers of different density have been combined, it is necessary to compress the layers arranged one on the other. After the two layers of different density have been co-compressed in such a manner, the desired effects mentioned above are lost to a greater or lesser extent.
Mat formation can also be carried out by separate absorption bodies being airlaid in moulds arranged with a uniform mutual spacing over the periphery of the mat-forming wheel. Mat formation can then be effected either by continuous forming taking place on the mat-forming wheel and the fibre material outside the moulds being brushed away and fed back into the system or by the fibre material being guided only into the separate moulds. In the former case, the brushing can interfere with the absorption bodies formed and tear these. This problem is particularly marked if the absorption body formed is not homogeneous but there are irregularities caused by flock formation during forming. In such cases, large chunks or clumps can be torn out of the absorption bodies during brushing, so that the quality becomes variable, even to the extent that large hollows appear in the absorption bodies. When the fibre material is guided only to the moulds, there is a risk of great disruption in the air flow at the edge portions of the moulds, which can cause defects in the absorption bodies.
The development of mat-forming in moulds on mat-forming wheels has been going on for a very long time. As early as in 1970, U.S. Pat. No. 3,518,726 described mat-forming in moulds, where the air flow and the forming in the moulds is controlled by the bottom in the moulds having different hole density and/or different hole size in different areas, as a result of which the absorption bodies formed can have different thickness in different areas and/or different density in different areas. Already in this publication therefore, forming of anatomically adapted absorption bodies for use in, for example, sanitary towels is described.
It has been known for a long time, from inter alia the abovementioned publication, to have moulds with different depth in different areas for forming anatomically adapted absorption bodies. U.S. Pat. No. 4,598,441 deals with the problem that these absorption bodies with different depth in different areas formed in moulds on a mat-forming wheel have the limitation that one side of the absorption body formed is plane and thus not anatomically adapted. U.S. Pat. No. 4,598,441 proposes using two mat-forming wheels, where absorption elements with a profiled side are produced on each mat-forming wheel, after which the plane sides of absorption elements formed on one mat-forming wheel are applied to the plane sides of absorption elements formed on the other mat-forming wheel to create absorption bodies in which the entire outer contour is shaped. In this way therefore, optimally anatomically adapted absorption bodies can be produced.
Over the years, development has moved towards very complicated and large mat-forming wheels. This is due on the one hand to the fact that manufacturing speeds have increased and larger air flows are required for forming the absorption bodies and on the other hand to the fact that it became desirable to have absorption bodies with a number of layers. Mention was made above of the disruptive air flows which can occur in the edge areas of covers which separate the flow of airborne fibres from surrounding air. When a fibre layer is formed on top of a layer already formed in the moulds on a large mat-forming wheel, there is a great risk of disruptive air flows in the edge areas of the covers destroying the layer already formed. EP-B1-0292624, for example, describes a proposal for solving this problem. This document discloses a mat-forming wheel with two mat-forming covers arranged immediately after one another and with two negative-pressure chambers which interact with the covers. For the purpose of avoiding pressure differences at the transition between the two covers, the negative-pressure chamber for one cover extends in under the other cover. A disadvantage of this is that the fibre-forming in one mould is disrupted by the negative pressure from the negative-pressure chamber of an adjacent mould and that as a result the forming becomes uneven. Sealing problems are also described in U.S. Pat. No. 3,717,905.
U.S. Pat. No. 3,501,813 expresses the prejudice that using a number of mat-forming wheels for forming an absorption body with a thicker central part involves considerable disadvantages. This publication states that this procedure with a number of mat-forming wheels it too expensive on the one hand because separate forming units are required for forming each web and on the other hand because equipment is required for combining the webs to form the intended absorption body.
Forming absorption products with a varying basis weight and different thickness in different areas on a single mat-forming wheel is described in a great many patent publications. Examples of these are U.S. Pat. Nos. 4,016,628, 4,388,056, 4,761,258 and 4,859,388. In U.S. Pat. No. 4,761,258, the flow of fibres is controlled by arranging air holes in the forming wire in a selected pattern, in which way the weight per
Edwardson Gunnar
Göransson Claes
Burns Doane Swecker & Mathis L.L.P.
SCA Hygiene Products AB
Staicovici Stefan
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