AN ABSORBENT SHEET OR WEB MATERIAL AND A METHOD OF PRODUCING THE MATERIAL BY DRY FORMING

Abstract: An inexpensive sheet or web shaped fibre material for use e.g. in sanitary napkins typically consists of a mixture of short cellulose fibres and some 15 % of relatively long, thermoplastic binder fibres. It is a noticeable problem that the short fibres "dust" out of the material, and according to the invention this is remedied in that the material, when made by dry forming, is provided with a surface coating of very thin layers of pure binder fibres. In addition to a marked holding back of the dust, these layers condition that the amount of binder fibres in the base material can be halved and that the breaking stress of the material is noticeably improved. Thereby the products are usable not only as inserts, but also as individual, self-contained units e.g. for wiping in domestic or industrial cleaning.

Description: An absorbent sheet or web material and a method of pro¬ ducing the material by dry forming. The present invention relates to a method of producing an absorbent sheet or web material by dry forming of a layer of short cellulose fibres in admixture with relatively long, thermoplastic binding fibres which are actuated by heating. Products of this type are typically used as absorbent inserts in sanitary articles, e.g. in sanitary napkins for women. With respect to both production price and absorption ca¬ pacity it is desirable to use a relatively low degree of ad¬ mixture of the binding fibres, normally some 15%, but in re¬ turn the problem arises that the products "dust" with the short fibres, which may amount to a serious problem, in par- ticular in relevant further processing operations. In prin¬ ciple, it is possible to avoid the dusting by applying a bon¬ ding substance onto the product surfaces, but this will com¬ plicate the manufacturing and make the products more expensi¬ ve. With the invention it has surprisingly been found that it is possible to achieve an inexpensably obtained surface se¬ aling which will effectively prevent any noticeable dusting from the products. To this end the production is arranged such that a laminated product is built up, comprising a first, very thin layer of pure binding fibres, e.g. with an amount of only some 3 g/m and thus with a rather open struc¬ ture conditioning a good penetration of liquid, a following layer of a fibre mixture of short cellulose fibres and longer binding fibres, though preferably with a reduced content of binding fibres, e.g. with a ratio of 93:7 or 95:5 of cellulo¬ se and binding fibres, respectively, and a final top layer corresponding to the thin layer of binding fibres as first laid out. The product thus shaped is passed through a heating zone for actuation of the binding fibres generally in the en- tire laminated product. The low content of binding fibres in the middle layer may result in a delamination in small areas of the product in re¬ sponse to folding (forming "elephant skin"). This can be avo- ided by calandering the product in a heated calander such that top and bottom layers of the product are welded toget¬ her, though only over 5-20% of the surface, preferably about 10%. With an amount of binding fibres of only 1-5 g/m 2 in the outer layers, preferably 2-4 g/m , it is easily undrstood that these layers will be readily liquid permeable, as the open surface area will amount to 80-90%. On the other hand, it is partly inexplainable how the same limited fibre coating can effect a marked reduction or almost total elimination of the dusting of short fibres from the products. However, both laboratory tests and test productions have verified that this effect with respect to dust binding is indeed achieved. In connection with the invention it has been found, sur- prisingly, that the very thin surface layers " provide for a marked increase of the tensile strength of the products, viz. as much as 3-4 times, and that the surface wear strength of the material is also considerably increased. From a quality point of view the material is hereby upgraded from an absor- bent insert material, which requires a surface protecting layer, to a self contained product that is usable e.g. for cleaning purposes domestically and in the industry, while still having a low content of binding fibres of only some 15% or less. From US-A-4, 054, 141 it is known to produce relatively thick pad members with a core layer of absorption fibres and with surface layers consisting of a mixture of these fibres and binding fibres. The surface layers are in no way "thin" layers with binding fibres oriented mainly in the plane of the surface, as to the contrary it is emphasized that a large part of the fibres project inwardly in or from the surface so as to enable an easier penetration of liquid and a certain cohesion in the outer layer. Thus, the the relatively thick core layer is not internally bonded, whereby it will easily delaminate, and for the formation of the surface layers it will be necessary to use a considerable amount of binding fi¬ bres, which will only partially be active as a coherent, li¬ quid permeable surface. It is well thinkable that in some way, not further specified, it is possible to achieve a cer¬ tain surface barrier effect against extrusion of the non- bonded, short fibres in the core layer, but then only with a rather large concentration of binding fibres in a surface layer of noticeable thickness. Moreover, the relatively large amount of binding fibres with this known method will not result in the said surprising increase of product strength, inasfar as this increase has to be connected with the fact that the binding fibres are pre- sent in a thin layer with the fibres oriented in the surface plane itself; fibres projecting inwardly from the surface cannot contribute to a strength increase and not either to a barring against extruding short fibre dust, which is an established problem in connection with aftertreatment of the products. The method known from the said US-A-4,054,141 is based on a cell filling of special moulds for forming shaped, limited pad mambers, while the present invention is based on a pro¬ duction of a relatively thin web material which can be manu- factured with much higher capacity and can be folded or plea¬ ded into a plural layer shape and then be cut to form pad members consisting of more layers. The web or sheet material, of course, also finds many other possibilities of applicati¬ on. In the following the invention is described in more de¬ tail with reference to the drawing, in which: Fig. 1 is a schematic view of a system for producing pro¬ ducts according to the invention, while Fig. 2 is a sectional view of a web produced thereby. Fig. 1 shows a dry forming system with a perforated for¬ ming wire 2, above which there is provided three consecutive forming heads 4, 6 and 8 for distribution of supplied fibres across the wire 2. Beneath the wire, fully conventionally, there is mounted a suction box 10 to which air is sucked down through the wire, partly for a rapid deposition of the fibres on the wire and partly for stabilizing the formed fiber layer or layers thereon. It is well known that with such an arrangement with more forming heads it is possible to produce laminated products, and with the invention this principle is used to the effect that there is supplied to the first and the last forming head a weak flow of air fluidized, relatively long binding fibres for the formation of outer product layers with very small thickness and density, e.g. only 2-3 g/m , while the interme¬ diate forming head is used for the formation of the basic fi¬ ber web. This web may be made with desired properties, thick- ness and density out of a mixture of cellulose fibres and binding fibres, preferably with a binding fibre content of only 2-4%. For stabilizing the very thin outer layers on the produ¬ ced web, the web together with the wire is passed through a pair of rollers 12,14, which are preferably heated for achie¬ ving a sligt compaction of the product, whereby it is conso¬ lidated sufficiently for a following conveying to a flow- through oven 16, in which the binding fibres are activated. From the oven 16 the web, now stabilized, is moved through a calander unit 18, the rollers of which are kept heated to a temperature a few degrees below the actuation temperature of the binding fibres. Thereafter the web is fi¬ nally reeled up at 19. In connection with the calandering the web may be subjec- ted to a point or line embossing for additional stabilization of the thin outer layers and for counteracting a delamination of the products. As shown in Fig. 2 the web product will consist of an ab¬ sorbent middle layer 20 of a desired thickness with surface layers 22 of binding fibers and of very small thickness. As mentioned, it has been found as a surprising fact that these surface layers even for an opening degree of 80-90% act re¬ taining on the short cellulose fibers as present in unbonded condition in the middle layer. It is undoubtedly contributory to this effect that the surface layers are provided as sepa- ratly laid out, thin layers, in which the binding fibres will predominantly be oriented in the layer plane itself, while the same fibres in the middle layer occur with random orien- tations so as to have no special barrier effect towards the loose short fibres. It is also the pronounced layer orientation of the cover fibres that will condition the said marked increase of strength, because of the strong mutual binding of the fibres. In this connection it is important that the binding fibre layers are as "clean" as possible, because the presence of even a small amount of cellulose fibres would weaken the bin¬ ding in the layers noticeably. It is not required to use precisely the same type of bin¬ ding fibres in the middle layer and the surface layers, re¬ spectively, and it can even be considered to optimize the surface fibres without heavy economical consequences, because they are used in very small amounts only. For certain products, e.g. for further mbnolateral lami¬ nation, it may be sufficient to use a surface layer 22 at one side only. It should be mentioned that it has been found by experi- ments that a layer thickness of 7-10 g/m in the surface lay- ers of the heat actuated binder fibres results in an unaccep¬ table reduction of the absorption capacity and the opacity. In a product of 75 g/m the total content of binding fibres will then be some 30%, rendering the product perceivably "synthetic" and unrealistically expensive. It has been found that it is possible to obtain a further and quite noticeable increase of the tensile strength by mo¬ ving the web material, after the calandering at the rollers 18, through a heating zone as shown in dotted lines at 16', such that a renewed actuation of the binding fibres can be effected. This also results in an improved barrier effect against dusting from the material as well as an improvement of the retention capacity, i.e. the ability to retain rest liquid after squeezing of wet material. The discussed properties will now be illustrated by two examples; Example 1 : With the use of 14,4% Al-Special-C Phil 65/35 1,7 x 6 (heat actuated binding fibres, Danaklon A/S, Denmark and 85,6% NF 405 (Softwood pulp, wood cellulose, Wyerhaeuser, USA) two different products are made by dry forming: A: With homogenous fibre mixture and conventional heat actuation of binding fibres. Bl: With bottom and top surface layer 3g/m 100% binding fibres and a middle layer of a homogenous mixture of cellulo¬ se and binding fibres in the ratio 93:7. B2: Bl after calandering. B3 : B2 after passage of heat tunnel 142°C. Relevant measuring results: Breaking - Waterab- Weight Thickness Dens ity strength Dust sorption Retention g/m 2 mm kq/rrr^ MD. α/2' ' mq Q/CΓ q/q A: 76 1,20 63 315 90 15 4,9 Bl: 74 1,10 67 750 18 14 5,0 B2: 73 0,60 122 1770 1,0 6,4 4,5 B3: 72 0,63 114 2010 0,6 6,7 5,3 The listed values should serve primarily for mutual com¬ parison, so it is deemed superfluous to describe the measu¬ ring methods in more detail. Example 2 : In the same manner, products A, Bl and B2 are produced based on 15,6% Al-Special-C Phil 65/35 1,7 x 6 binding fibres and 84,4% Rayfloc-X-J (Softwood pulp, wood cellulose; ITT Ra- yonier Inc. , USA) Breaking- Waterab- Weight Thickness Density strength Dust sorption Retention q/ /m2 mm kq/m MD. g/2' ' mg g/g g/g A: 101 1,22 83 785 61 14 6,1 Bl: 97 1,08 90 1020 19 13 6,0 B2: 102 0,76 134 2100 1,6 5,4 4,8

Claims: C L A I M S : 1. A sheet or web material of absorbent type consisting of a dry formed product of short cellulose fibres and relati¬ vely long, thermoplastic binding fibres, typically for use as inserts in sanitary articles, the material having at least at one side a surface layer with an upconcentrated content of binding fibres, characterized in that the web material com¬ prises a core or main layer of cellulose fibres in admixture with a small amount of binding fibres and, at one or both si- des, a very thin surface layer solely consisting of binding fibres, preferably in an amount of only l-5g/m and with the¬ se fibres predominantly oriented in the surface plane itself. 2. A material according to claim 1, characterized in that the content of binding fibres in the mixed main layer is as low as 2-10%, preferably 5-8%. 3. A material according to claim 1, characterized in that it is provided with the thin binding fibre layers at both si- des and appears as a trade product for direct use, e.g. as a wiper material. 4. A method of dry forming a material web according to claim 1, whereby a mixture of short cellulose fibres and re- latively long thermoplastic binding fibres is laid out on a forming wire from a forming head, whereafter the binding fi¬ bres are actuated by the web passing through a heat zone, characterized in that a very thin layer of binding fibres, preferably with an amount of only 1-5 g/m , is laid out from a separate forming head before or after the laying out of the said fibre mixture so as to form a very thin surface layer on the top or bottom side of the web. 5. A method according to claim 4, characterized in that from an additional forming head a corresponding thin binding fibre layer is laid out for coating also the opposite side of the web. 6. A method acording to claim 4, characterized in that the mono or preferably double sided coated web is subjected to a calandering after its passage through the heating zone. 7. A method according to claim 6, characterized in that the web is additionally moved through a heating zone after the calandering. 8. A method according to claim 5, characterized in that for the calandering there is used a heated calander providing a welding together of the two surface layers over 5-20% of the web area, preferably some 10% thereof. 9. A method according to claim 4, characterized in that the mixed fibre layer is used with a content of only 2-10% of binding fibres, preferably 5-8%.

Inventors: SCHILKOWSKI HELMUT ERWIN, SCHILKOWSKI, HELMUT, ERWIN
Applicants: SCAN WEB, SCHILKOWSKI HELMUT ERWIN, SCAN-WEB I/S, SCHILKOWSKI, HELMUT, ERWIN
Application year: 1997
Main classification: D21H27
IPC codes: D04H1, D04H5