WO2005084596A1

WO2005084596A1 - Absorbent tissue product and method for its manufacture

Info

Publication number: WO2005084596A1
Application number: PCT/SE2004/000307
Authority: WO
Inventors: Magnus Falk; Anna MÅNSSON; Sverker Albertsson
Original assignee: Sca Hygiene Products Ab
Priority date: 2004-03-04
Filing date: 2004-03-04
Publication date: 2005-09-15

Abstract

An absorbent tissue product of lignocellulose containing material provided with openings, and an absorbent tissue product made thereof. The tissue sheet comprises a plurality of wet stable cavities (2) with openings towards both sides if the sheet,. Each cavity (2) has a larger opening (7), and a smaller opening (9). Method for manufacturing of an absorbent structure from lignocellulose containing material with wet stable cavities and intermediate regions in which a wet paper web is formed on a forming fabric and is transferred to a patterning fabric (82). The wet paper web is shaped and provided with openings against the patterning fabric by suction or blowing of air through the wet paper web and the patterning fabric, and thereafter dried on the patterning fabric.

Description

Absorbent tissue product and method for its manufacture

Technical field

The present invention relates to an absorbent tissue product, a method for manufacturing of a tissue sheet and method for manufacturing of a tissue product .

Background

Absorbent tissue products should have high strength, high bulk, high absorbency, and be soft. High absorption is achieved through i.e. high bulk. Paper products having spaces between the fibres contain more room for absorption of liquids in spaces. The product should also present wet stability.

Perforated or embossed tissue sheets that are provided with openings or cavities are presently used for the manufacture of certain types of household tissue paper or industrial tissue paper. The openings or cavities in the tissue sheet capture dirt and enhances the absorbency of the tissue paper. The perforation may be conducted during the converting, by leading a dry paper web between a roll, provided with protruding spikes and a rubber roll or by forcing long needles through the paper web or by removing small areas of the paper web by punching. In all these cases the fibres break, hydrogen bonds are broken and the structures of both the fibres and the paper are destroyed. A paper web thus perforated, loses strength and does not show wet stability, i.e. it does not retain its shape while wet.

Summary of the invention

The present invention relates to an absorbent tissue product containing lignocellulose material, comprising at least one tissue sheet provided with openings, which is provided with a plurality of cavities, having the shape of a funnel or a crater. The cavities extend from one side of the sheet, through the sheet and out on the other side of the sheet as a protrusion, by means of the tissue sheet bending outwards from one side, towards the other side of the sheet, in the area of the cavity.^" The protrusions comprise an opening at their top and the product also shows wet stability.

The present invention further relates to the manufacturing of a wet stable tissue sheet of lignocellulose containing material being provided with openings, wherein a wet paper web is formed, shaped and provided with openings against a patterning paper machine fabric, which is provided with indentations that preferably are funnel shaped. The openings are accomplished by sucking and/or blowing air through the wet paper web and the patterned fabric, by means of vacuum and/or overpressure, whereby openings appear in the paper web in the areas that are formed against the indentations.

The present invention further relates to a method for the manufacture of a tissue product of lignocellulose containing material, wherein at least one tissue sheet is provided with openings according to the above method and is subsequently brought together with one or more additional tissue sheets.

The present invention provides a tissue product, which has excellent absorbency. Since the tissue product contains at least one tissue sheet provided with openings, which sheet comprises funnel shaped cavities, which extend outwards on the other side of the sheet as protrusions, an excellent absorbency is achieved. The funnel shaped cavity and its protrusion has an excellent ability to absorb liquids through its capillary forces, whereby the liquid is absorbed from the base of the funnel shaped cavity, which base is in the plane of the tissue sheet. Thanks to the tapering funnel shape of the cavity, liquid is more easily led in one direction, i.e. from the base opening towards the top opening.

The method for manufacturing of a tissue sheet according to the present invention provides for a strong tissue sheet, wherein no hydrogen bonds or fibres are broken during forming of the openings. Moreover, the tissue sheet is wet stable. The present invention will now be described in further detail, by means of the drawings.

Brief description of the drawings

Figure 1 shows a schematic representation of a tissue sheet provided with cavities with openings towards both sides of the sheet. This is a cross sectional view of Fig. 10, along line I-I,

Figure 2 shows a schematic cross sectional representation of one embodiment of the tissue product, comprising two tissue sheets, both being provided with cavities with openings towards both sides of the sheet,

Figures 3 and 4 show cross sectional representations of further embodiments of the tissue product, comprising two tissue sheets, both being provided with cavities with openings towards both sides of the sheet,

Figure 5 shows a cross sectional representation of an embodiments of the tissue product, which comprises two tissue sheets, one of which is provided with cavities with openings towards both sides of the sheet and the other being substantially flat,

Figure 6 shows a schematic cross sectional representation of another embodiment of the tissue product, comprising two tissue sheets, one of which is provided with cavities with openings towards both sides of the sheet and the other being substantially flat,

Figure 7 shows a schematic cross sectional representation of another embodiment of the tissue product, comprising two tissue sheets being provided with cavities with openings towards both sides of the sheet and one substantially flat sheet without cavities, Figure 8 shows a schematic cross sectional representation of one embodiment of a patterning forming fabric, which may be used when manufacturing a tissue sheet provided with cavities with openings, according to the invention,

Figure 9 shows a schematic cross sectional representation of another embodiment of a patterning forming fabric, which may be used when manufacturing a tissue sheet provided with cavities with openings according to the invention,

Figure 10 shows a schematic top view of a tissue sheet, according to the invention

Figure 11 shows a TAD-fabric as seen from above, according to the invention.

Detailed description of the present invention

The present invention relates to an absorbent tissue product, made of lignocellulose containing material, which product comprises at least one tissue sheet 1, which is provided with a plurality of cavities with openings towards both sides of the sheet and with intermediate regions. The tissue sheet has a local thickness x, which is the thickness of the base sheet in the intermediate regions. The cavities 2 have a depth y which is the distance between the two openings 7, 9. The depth y is indicated in Fig. 1.

A tissue sheet 1 as indicated above is shown in Fig. 1, which shows a cross sectional view along the line I-I in Fig. 10. The depth y of a cavity 2 preferably exceeds the local thickness x of the tissue sheet. The depth y is preferably 1.1, more preferably 1.5 and most preferably 2 times the local thickness x. The tissue sheet suitably has a local thickness of 20-400 μm, preferably 20-200 μm, and more preferably 30-150 μm. The cavities may be tapered. The larger openings 7 have an area ai and the smaller openings 9 have an area a₂, where ai is more than approximately 1, preferably more than 1,5 and more preferably more than 2 times a₂. The area ai of the larger opening 7 is 0,01-50 mm², 9 9 preferably 0,05-28 mm , and more preferably 0,2-20 mm , whereas the smaller opening 9 9 9 9 has an area a₂ of 0,01- 50 mm , preferably 0,05-20 mm and more preferably 0,2-13 mm . The size of the cavities, as well as the size of their openings, may vary depending on the intended use of the product. In a tissue product intended for wiping liquids of high viscosity, such as oil, the area of the larger opening 7 should be 0,8-50 mm², preferably 9 9 2-28 mm and more preferably 3-20 mm and the area of the smaller opening 9 should be 0,8-50 mm², preferably 2-20 mm² and more preferably 3-13 mm². Less viscous liquids, such as water, are more easily absorbed when the area of the larger opening 7 is 0,1-28 mm², preferably 0,05-13 mm² and more preferably 0,2-7 mm², and the area of the smaller opening 9 is 0,01- 13 mm², preferably 0,05-7 mm² and more preferably 0,2-3 mm². The size of the cavity and its openings can additionally be adapted such that the cavities 2 can accommodate impurity particles, as they enter the larger opening 7. In such applications the tissue sheet is oriented in the tissue product such that the side 3 of the tissue sheet with larger openings, will be turned towards the surface to be wiped. The wiping and liquid absorbing capacity for a multi-ply product is at its best when the sheets are oriented in the product such that the larger openings 7 of the cavities 2 are directed towards the surface to be wiped.

The shape of the cavities may be designed in different ways. The openings may be either symmetrical or unsymmetrical, and may also be circular, oval or cornered. When the opening area is small, typically less than 4 mm it more depends on the orientation of the fibres around the opening, and is typically substantially circular. The cavity may have the shape as a frustum of a cone or a crater, and may also be softly tapered in a hyperbolic shape or it may have a cornered shape. The cavity may also be designed so as to include a funnel part that tapers from the larger opening, followed by a cylindrical part towards the end of the smaller opening.

In order to achieve an excellent absorbency, there should be 0,5-40 cavities with 9 9 openings towards both sides 2 per cm in the tissue sheet, preferably 1-20 per cm , and more preferably 2-10 per cm . The cavities 2 are distributed over the surface of the tissue sheet, preferably in a regular pattern, although an irregular pattern would also do. Figure 10 shows a tissue sheet 1 with cavities 2, having circular openings, distributed evenly over the surface area in a regular pattern. Openings having other shapes and other regular or irregular patterns may of course also be used. There may also be cavities that not have openings towards both sides of the sheet. This can be a deliberate result of the manufacturing process, or may be an unintentional result of variations in the manufacturing process, which lead to the creation of some cavities or openings not being successful.

If desired, the tissue sheet may also comprise some areas without any pattern of cavities and openings.

The absorbency of the tissue product is influenced by the capillary action of the cavities, as well as the bulk of the material. By the provision of protruding cavities in the tissue sheet and a relatively large number of cavities, an excellent bulk is achieved in the product.

The distance from the centre of the opening of one cavity to that of an adjacent cavity has great influence on the bulk of the material, and thus also on the absorbency. This distance should preferably be 0,5-40 mm, more preferably 1-20, and most preferably 2-10 mm. Moreover, the cavities 2, which may have protrusions 5, that are provided in the tissue sheet result in the tissue sheet having a larger specific area for a given weight, which in turn results in an increased absorption in the materials.

The bulk of an absorbent tissue sheet provided with openings according to the present invention is preferably 3-30 cm /g, more preferably 5-25 cm³/g and most preferably 10- 20 c Vg. The basis weight is preferably 10-80 g/m², more preferably 10-50 g/m² and most preferably 15-40 g/m² and the local thickness of the tissue sheet is 50-800 μm.

The effective thickness of a tissue sheet or ply is determined according to SCAN-P 47:83, modified in that it is performed on one sheet or ply instead of eight sheets or plies. The effective thiclαiess of a tissue sheet according to the present invention is in many cases a measure of the depth y of the cavities. The density of a tissue sheet or ply is determined by means of the thickness and basis weight values, density = basis weight / thickness. The wet strength of the product is determined according to SCAN-P 58:56, as the tensile strength for a strip of 50 mm width, which was soaked in water during 15 seconds before the measurement. The geometrical mean value is calculated from the wet strength values measured in the machine direction and the transversal direction respectively. The dry strength of the product is determined according to SCAN-P 44:81. The relative wet strength is calculated as the quotient between the geometrical wet strength mean value and the geometrical dry strength mean value and is expressed in %. A product is considered being wet strong when the relative wet strength exceeds 15 %.

Wet bulk is measured by a thickness measuring device, which meet the requirements of SCAN-P 47:83. The measurement is performed on a tissue sheet, which is thoroughly moistened, whereupon the measurement is performed. The bulk is calculated as thickness/conditioned basis weight (conditioning performed according to SCAN-P 2:75).

As the cavities are wet stable, they will maintain their shape also when wet. The wet stability is achieved by means of the design of the cavities and that the cavities and their openings were provided in the tissue sheet when it still was wet, and that the sheet was then dried in this shape, as is described below. The wet stability may be enhanced by the addition of wet strength agents during the manufacturing of the tissue sheet. The paper then retains at least some of its strength when wet, typically 25 % is retained.

The present invention also relates to a method of manufacturing of a lignocellulosic tissue sheet provided with wet stable cavities with openings, which method includes the steps of: a) depositing an aqueous fibre suspension on a paper machine forming fabric or between two forming fabrics and dewatering it, so as to obtain a wet paper web 81, b) transferring of the wet paper web 81 to a patterning paper machine fabric 82, where the web is shaped and provided with openings against the patterning fabric 82, which comprises a plurality of indentations 83 intended to create cavities with openings. Each such indentation has an opening 85, , which is in the plane of the side 90 of the patterning fabric 82, which faces the wet paper web. The wet paper web is provided with openings by suction and/or blowing of air through the paper web 81 and the patterning fabric 82 in the direction from the paper facing side 90, by means of vacuum and/or over-pressure such that openings 87 are formed in those parts of the wet paper web that are formed against the indentations 83, and c) drying the paper web on the patterning fabric 82.

A preferred way of drying the paper web in step c) is to blow hot air through the paper web.

The forming and patterning fabrics are preferably woven fabric clothing, but may also be moulded polymer belts or perforated plastic foils. Figure 8 shows a patterning fabric 82 according to the invention, having a wet paper web on top of it, which paper web is being shaped against the patterning fabric according to the method described above. The paper facing side 90 of the patterning fabric is the side that is in contact with the paper web 81. The patterning fabric is provided with indentations 83 intended to create cavities with openings in a specific desired pattern.

There is a height difference between the upper plane of the patterning fabric and the bottom 86 of the indentations 83 intended to create cavities with openings, i.e. between the plane on the paper facing side 90 of the patterning fabric, on which the intermediate part of the web is formed in contact with the fabric, and the lowest point of the indentation where a cavity is created. The fibres in the paper web will deflect or bend downwards at the indentations 83 intended to create cavities with openings. The bending of the fibres in the downward direction is increased by vacuum and/or air streams and to some extent also by any possible water originating from the fibre suspension that might remain in the paper web, and which flows downwards in the indentations 83.

Vacuum can be achieved by placing suction boxes under the patterning fabric and air streams are obtained by blowing air from above, through the paper web and the patterning fabric, in the direction of the bottom 86 of the indentations, by means of blowing devices, e.g. one or more air gaps that guide the concentrated air flow. The fabric may also have other indentations, which are intended to give the intermediate area a three-dimensional structure. Such indentations are made in the fabric in such a way, so that they will not create any cavities with openings of the kind mentioned above.

The forming fabric, upon which the fibre suspension is deposited and the wet paper web is formed, may travel at a higher speed than the patterning fabric. This facilitates the forming of the three-dimensional structure. When the paper web reaches the patterning fabric, it is impeded by the lower travelling speed of the patterning fabric, which results in an excess of material in the paper web.

This excessive material must move in some direction and moves then typically in the z- direction, and is then advantageously sucked down into the indentations in the patterning fabric by means of the vacuum that is present in connection with the transfer between the forming fabric and the patterning fabric. The paper web including the excessive material is thus formed against the patterning fabric. The vacuum as such also contributes to the forming of the paper web against the patterning fabric.

A vacuum box, a so-called moulding box, placed after transferring the web to the patterning fabric, can enhance the forming of the three-dimensional structure. Such a vacuum box can work with vacuum up to or even exceeding 60 kPa. The high vacuum will help pulling the wet web into the structure of the patterning fabric. The vacuum needs to be adjusted considering e.g. choice of fibres and basis weight. As the fibres become bent along the walls 88 of the indentations 83 intended to create cavities with openings, and are partly drawn down into these indentations, the spaces between the fibres increase and the density in these parts of the paper web decreases. This leads to a lower air resistance in these parts and air that is sucked down by the under-pressure acting from below, or is blown downwards through the paper web and the patterning fabric, will choose to flow through these areas and accordingly blows the fibres away from each other, such that openings 87 are formed. The openings 87 are formed in the bottom area 86 of the indentations 83 intended to create cavities with openings and correspond to the openings of the tissue sheets, shown in other drawings.

Figures 8 and 9 show simplified schematic illustrations of different patterning fabrics. The patterning fabric 82 in Figure 8 is designed such that the bottom areas 89 of the indentations intended to create cavities with openings have a lower air resistance than the rest of the patterning fabric, thanks to the weaving technique, and the air streams will thus choose to travel through these areas. By reason of this, the fibres in the paper web will be blown apart and openings 88 will be formed in these areas.

In an alternative design, shown in Figure 9, the indentation 93 intended to create cavities with openings extend throughout the thickness of the patterning fabric 92, thereby constituting a hole 93 through the patterning fabric.

As compared to perforation of a dry paper web, the method of the present invention presents a lenient way of providing openings in a paper web. No hydrogen bonds will be broken and no fibres will break when openings are provided in a wet paper web according to this method. As the fibres bend or deflect downwards into the cavities and are "dragged" down into the indentations, the specific surface of the paper web, i.e. the tissue sheet, increases and the density may also decrease to some extent in this area, which contributes to the absorbency of the product. Apart from the advantages previously described, such as improved absorbency and wiping characteristics, the tissue sheet obtained by the method of this invention will thus have an improved strength, as compared to tissue sheets that have been perforated by conventional methods,. Due to deliberate design of the manufacturing process, or as an unintentional result of variations in the manufacturing process, the creation of some cavities and their openings may not be successful. That is, some of the cavities may lack an opening. This will, however, not affect the advantageous features of the product obtained.

Since the tissue sheet with its cavities and their openings is formed while the tissue sheet is still wet and is dried in the desired shape, the fibres are locked in that position. Thanks to this, the thus obtained shape of the cavities is wet stable. Conventional perforated tissue sheets have no such wet stable cavities, since the perforation is effected by tearing up fibres in a dry paper web, i.e. the perforation is not carried out while the sheet is wet and dried subsequently after shaping.

In the method for manufacturing of an absorbent structure with wet stable cavities and intermediate regions an aqueous fibre suspension is deposited on a paper machine forming fabric or between two forming fabrics, and is dewatered so as to obtain a wet paper web 81. The wet paper web is then transferred to a patterning fabric 82 against which it is shaped. The patterning fabric comprises a plurality of indentations 83, and the wet paper web 81 is provided with openings by suction and/or blowing of air through the paper web and the patterning fabric 82. The air is blown in the direction from the paper facing side 90, by means of vacuum and/or over-pressure, such that openings 87 are formed in those parts of the wet paper web, which are formed against the indentations 83. Finally the paper web is dried while still on the patterning fabric.

The indentations in the patterning fabric 83 intended to create cavities with openings of the patterning fabric 82 have openings 85, which are situated in the upper plane of the fabric and have an area of 0,01-50 mm², preferably 0,05-28 mm² and most preferably 0,2- 20 mm². The number of indentations 83 in the patterning fabric 82 is 0,5-40 per cm², 9 9 preferably 1-20 per cm and most preferably 2-10 per cm and the distance between the centre of each indentation 83 is 0,5-40 mm, preferably 1-20 mm and most preferably 2- 10 mm. The distance between the indentations and the number of indentations results in a tissue sheet with high bulk and excellent absorbency. The indentations intended to create cavities with openings are evenly distributed in the patterning fabric, preferably in a regular pattern. However, in some cases an irregular pattern may be advantageous.

The patterning fabric used in the present invention is preferably a TAD-fabric (Through Air Drying fabric). This fabric is preferably used also for drying the paper web after shaping the cavities. Drying may thus take place in the same production stage as the forming of openings in the paper web, but the paper web must necessarily be wet when starting the forming of openings. Being wet means that the dry content is 50% or lower. The paper web 81 is preferably dried by blowing hot air through it, when it is still in contact with the fabric. This contributes greatly to the wet stability of the tissue sheet, since the drying operation locks the tissue sheet and its cavities and protrusions in the desired shape.

One example of a TAD fabric, which is suitable for provision of openings in a tissue sheet according to the present invention, is disclosed in PCT application WO 00/63489. This fabric presents a three-dimensional surface pattern including indentations intended to create cavities with openings on the side of the fabric clothing intended to be in contact with the paper web. There are also other kinds of indentations, which are intended to give the intermediate area a three-dimensional structure. These later indentations will not give any holes. This patterning fabric thus has the characteristics required for provision of openings in a wet paper web, when used in the method of the present invention, provided that a sufficient under-pressure by means of vacuum is used or that an air stream of sufficiently high pressure is blown through the paper web and the patterning fabric.

In such a TAD fabric the weft and warp threads are arranged such that at certain points in the clothing, the warp threads are on top in three successive crossings between weft and warp threads, and at certain other points in the clothing, the weft threads are on top in three successive crossings between weft and warp threads. By this arrangement the threads are lifted at certain places and a surface pattern having relatively deep indentations is formed on the side of the fabric or clothing that is in contact with paper web. Figure 12 shows a surface profile of a TAD fabric such as the one described in WO 00/63489, as seen from above, obtained by a UBM measuring system (UBM Messtechnik GmbH (Ottostr. 2, D-76275 Ettlingen, Germany)). The surface profile indicates, by means of a grey-scale, the height level of the thread that has the uppermost position in the fabric at a specific measuring point and the surface profile is calculated from several measuring points, whereby a light colour indicates a higher level than a dark colour. In Figure 11, the threads positioned closest to the paper web appear as white lines, which form a square. Within some squares (121), there are supporting threads, and no openings are formed in the paper web in these areas. This kind of indentation is supposed to give the intermediate area a three-dimensional structure. Within other squares (122), there are no supporting threads, and in these areas openings are formed in the paper web. These indentations are intended to create openings.

The method for manufacturing of the tissue sheet provided with openings starts by forming a wet paper web from an aqueous fibre suspension. The fibre suspension may contain wet strength agents in order to achieve a further improved wet strength in the final tissue sheet. Conventional wet strength agents may be used, such as polyamide- amine-epichlorohydrin resins, cross-linked polymer formaldehyde resins or aldehyde derivatives of poly amide resins.

All kinds of paper pulp may be used in the invention, including different fibre types. Synthetic fibres, such as various thermoplastic fibres, e.g. polyester, polypropylene, polyamide, polyvinyl alcohol; vegetable fibres, e.g. flax, abbaca, cotton; or regenerated fibres, e.g. viscose, lyocell, acetate or other man-made fibres may also be used.

In the method described above, the wet paper web is formed on a separate forming fabric, and is subsequently transferred to a patterning fabric. As an alternative, the wet paper web may instead be formed directly on the patterning fabric.

The invention also relates to a method for manufacturing a lignocellulose containing tissue product, wherein at least a first tissue sheet is provided with openings according to the present invention and is brought together with a second tissue sheet provided with openings and/or a substantially flat tissue sheet without cavities. A single tissue sheet can also be used as a 1-ply product. Multi-ply products can contain one or more plies with openings.

It is possible that both sides of the sheet have openings with the same size, but it is prefered that the openings of one side are larger than on the other side.

The absorbent tissue product of the present invention may be built up in different ways. The best effect is achieved by a multi-ply product. Figures 2 to 7 show cross sectional schematic views of multi-ply products, which for example include two tissue sheets or plies 21, 31, 41, 22, 32, 42, provided with openings, one ply being superimposed on the other. In Figures 2-7 the tissue sheets are shown schematically and the cavities of each sheet or ply have openings that are larger on one side than on the other..

The sheets or plies may be arranged such that the sides with small openings 23, 24 and 33, 34, respectively, are directed towards each other, as in the embodiment shown in Figure 3, or away from each other, as in the embodiment shown in Figure 2, or a combination thereof, i.e. both sheets 41, 42 have the same direction, as in the embodiment shown in Figure 4.

When the small openings 35 of both plies are directed towards each other, as in Figure 3, a product having excellent absorbency and high bulk is achieved. The high absorbency is related to the suction of liquid into the large openings 36 of the cavities, and both sides of the product can be used against the surface to be wiped. Both plies 31, 32 have their large openings 36 directed outwards and the liquid is sucked into the large opening and is sucked further towards the centre of the multi-ply product. The liquid will be retained in the product, between the plies, since capillary forces prevent the liquid from entering the small openings 35. The liquid cannot leave the product through the opposite ply, since this ply is also directed with its small openings 35 facing inwards. In the case the small openings 35 of one ply reach the inner side of the other ply 32, the inner side being the side facing towards the interior of the product, the absorbency is further improved. This is even more pronounced in the case that the small opening is surrounded by protrusions. Liquid will then be sucked through the fibres in the cavity walls right in to the other ply, which absorbs the liquid.

In the alternative embodiment shown in Figure 2, the small openings 25 are instead directed away from each other in an absorbent multi-ply product. This absorbent product is primarily suitable for wiping of dirt or impurities that are difficult to remove from a surface, such as various kinds of dirt in the industry, but it has also a reasonable absorbency, thanks to its relatively high bulk. The small openings 25 will be directed towards the surface to be wiped, irrespective of which side is used against the surface. The protrusions 25 easily capture dirt or impurities and are able to absorb liquid to some extent. As an additional advantage, the spaces between the small openings 25 form pockets, in which dirt can be collected and transported away from the wiped surface.

In the further alternative embodiment, shown in Figure 4, the plies may be arranged in the absorbent product such that the small openings of both plies are directed in the same direction. The side 43 of the first ply 41, on which the small openings 45 are, is then arranged such that it faces the side 44 of the second ply 42, that has large openings. This embodiment provides a product that is highly absorptive from one side, i.e. from the side on which the ply 41 is arranged, having its large openings 46 directed outwards from the product, and has an excellent dirt capturing ability from the other side, which has the ply 42 arranged such that the small openings 45 are directed outwards. The product of this embodiment is preferably provided with some kind of indication as to what side is to be used for what purpose, in order to facilitate the determination of which side is primarily intended for absorption (e.g. provided with a marking symbolising water droplets), and which side is primarily intended for wiping of dirt (e.g. provided with a marking symbolising dirt). As an alternative the different plies may be made of paper webs of different colours.

The embodiments shown in the drawings have their tissue sheets or plies arranged such , that the openings are not positioned directly above each other. If desired, the plies may however alternatively be arranged such that the openings are positioned directly above each other, although not shown in the drawings.

A further embodiment of an absorbent product is shown in Figure 5, in which a ply 51 provided with openings is brought together with a substantially flat tissue ply 52. As the side 53 of the ply 51, with small openings is directed towards the interior of the product and towards the flat ply 52, the absorbent product will attain excellent absorbency. In use the ply provided with large openings should be directed against the surface to be wiped, and the large openings 56 are then directed towards the liquid to be wiped and absorbed. By means of capillary forces the liquid is sucked into the openings. The small openings should preferably be in direct contact to the flat tissue ply 52. This further contributes to the suction of liquid towards the flat tissue ply 52, since the fibres in the tops 57 of the protrusions 55 that are in contact with the flat ply draw the liquid further into the substantially flat ply 52. The substantially flat ply 52 has the further advantage of having higher strength than a ply provided with openings. When used for wiping the product is directed with the ply provided with openings turned towards the surface to be wiped, and the flat ply, having no openings is directed upwards. No openings, through which dirt or impurities might reach the user's hand, are thus directed towards the user, and the liquid is better retained in the product, in the ply nearest to the user. The product may advantageously be marked, in order to indicate to the user, which of the sides that is intended to be directed towards the surface to be wiped. This applies to all multi-ply products having different outer sides.

Figure 6 shows yet another embodiment of an absorbent product, comprising one ply 61, provided with cavities with openings, and one substantially flat ply 62. Ply 61 is arranged such that the side with small openings is arranged with small openings directed outwards. If the openings are surrounded by protrusions 65, they will point outwards. This absorbent product is highly suitable for removal of dirt from a surface, since the protrusions 65 of the funnel shaped cavities "scratch" dirt off the surface. The removed dirt particles can be transported in the pockets between the protrusions 65. The other side of the product has a relatively good absorbency, although there is a risk that liquid might leak out through the cavities. Accordingly this embodiment is thus primarily intended for wiping of dirt or impurities.

All the above embodiments may additionally comprise one or more substantially flat tissue plies. The embodiments comprising two plies that are provided with openings, as described above, may include at least one substantially flat tissue ply 73, preferably arranged between the plies 71, 72, provided with openings, as indicated in Figure 7. If the small openings are surrounded by protrusions, they should be directed inwards and towards the flat ply 73. Excellent absorbency is achieved due to the capillary forces discussed above, and the suction capacity through the fibres in the protrusions 75 that may be in contact with the flat ply 73 and which lead the liquid into the flat ply is also improved. Several combinations of plies, in addition to the above mentioned, lie within the concept of the present invention.

The tissue sheet or ply provided with cavities with openings used in the absorbent products is manufactured by the method of the present invention. This new method of manufacturing a tissue sheet with cavities with openings results in the provision of tissue sheets having excellent properties, such as strength, wet stability and absorption characteristics. This in turn, leads to highly improved properties of the products in which the tissue sheets are utilised.

When the tissue sheet is oriented in the product such that the small openings are directed inwards towards another tissue sheet, whereby the large openings of the cavities are directed against the surface to be wiped, a tissue product having excellent absorbency is obtained, since liquid is effectively absorbed into the cavities, via their large openings, by means of capillary forces.

When the tissue sheet is oriented in the product such that the small openings are directed outwards and away from another tissue sheet, , the tissue product obtained has a good ability remove dirt or impurities by means of the cavity protrusions and their top openings. The method according to the present invention produces a good and strong tissue sheet, provided with openings.

Example

A tissue sheet was manufactured on a pilot machine with twin-wire forming, a TAD section and a Yankee cylinder. In the examples two different TAD fabrics of the type described in WO 00/63489 were used. Both fabrics were similarly woven, such as having the same yarn count and yarn diameter. The fabric used in example 1-3 has two kinds of indentations: the ones intended to create cavities with openings and the ones intended to give the intermediate area a three-dimensional structure. The former indentations have high local air permeability in the indentations intended to create openings. The indentations therefore have normal local air permeability. The first fabric had 5,6 indentations per cm² indented to give openings and equally as many for three- dimensionality. The second fabric had altogether 11,2 indentations per cm². The following machine settings applied to all examples 1-5:

Machine speed: 600 m/min Speed difference between the inner fabric and the TAD fabric: + 6% Pick-up vacuum: 17 kPa

Pulp: wood-free deinked pulp (P2D from Unifibres Ltd) Wet strength agent: Kymene SLX (Polyamide-amine epichlorohydrin): 10 kg active/ton

Table 1

Table 2

The Examples show that tissue sheets manufactured on a TAD fabric like in Examples 1- 3 achieve a considerably larger number of openings than a corresponding tissue sheet manufactured on the fabric used in Examples 4-5. Although comprising a large number of openings, the tissue sheets in Examples 1-3 nevertheless have a strength, wet strength and wet bulk that are quite comparable to the tissue sheet of Examples 4-5.

Claims

1. An absorbent tissue product comprising at least one sheet of an absorbent structure consisting of lignocellulose fibres, said structure having cavities and intermediate regions characterised in that said cavities have openings towards both sides of said structure and show wet stability, i.e. that the distance (y) between the openings at each end of a cavity and the areas (ai) and (a₂) of said openings are essentially not decreased upon wetting of said structure, except for what is caused by swelling of the fibres.

2. The absorbent tissue product of claim 1, wherein the distance (y) between the openings of the cavities (2) exceeds the local thickness (x) of the intermediate regions.

3. The absorbent tissue product of claim 2, wherein the distance (y) between the openings is at least 1,1, preferably 1,5 and most preferably 2 times the local thickness (x) of the tissue sheet.

4. The absorbent tissue product of claim 1, wherein the wet bulk exceeds 5 g/cm³, preferably 8 g/cm³ and most preferably 10 g/cm³.

5. The absorbent tissue product of any one of the preceding claims, wherein the larger openings (7) have an area ai and the smaller openings (9) have an area a₂ and that ai is larger than a₂, preferably 1,5 times larger, and most preferably 2 times larger than a₂.

6. The absorbent tissue product of any one of the preceding claims, wherein the cavities are tapered, such that the cross sectional area is decreasing through the structure from the larger opening to the smaller opening.

7. The absorbent tissue product of any one of the preceding claims, wherein ai is 0,01-50 mm², preferably 0,05-28 mm² and most preferably 0,2-20 mm² and a₂ is 0,01- 50 mm², preferably 0,05-20 mm² and most preferably 0,2-13 mm².

8. The absorbent tissue product of any one of the preceding claims, wherein the number of cavities (2) in the tissue sheet is 0,6-40 per cm², preferably 1-20 per cm² and most preferably 2-10 per cm² and the distance between the centres of adjacent cavities (2) is 0,5-40 mm, preferably 1-20 mm and most preferably 2-10 mm.

9. The absorbent tissue product of any one of the preceding claims, wherein the intermediate regions mainly are located in the same plane as the openings with smaller area.

10. The absorbent tissue product of any one of the preceding claims, wherein the cavities (2) are distributed over the absorbent structure (1) in a regular pattern.

11. The absorbent tissue product of any one of claims 1 -9, wherein the cavities (2) are distributed over the absorbent structure in some areas, whereas other areas of the absorbent structure are free from cavities.

12. The absorbent tissue product of any one of the preceding claims, wherein the product comprises at least two absorbent structures of which at least one with cavities according to claims 1-11 and where the planar structure with cavities is arranged as an outer layer.

13. A method for manufacturing of an absorbent structure from lignocellulose containing material with wet stable cavities and intermediate regions which method comprises the steps of: a) depositing an aqueous fibre suspension on a paper machine forming fabric or between two forming fabrics, and dewatering it so as to obtain a wet paper web 81 , b) transferring of the wet paper web 81 to a patterning fabric 82, where the web is shaped against the patterning fabric(82), which comprises a plurality of indentations (83), whereupon the wet paper web is provided with openings by suction and/or blowing of air through the paper web (81) and the patterning fabric (82) in the direction from the paper facing side (90), by means of vacuum and/or over-pressure, such that openings (87) are formed in those parts of the wet paper web, which are formed against the indentations (83), and c) drying the paper web on the patterning fabric (82).

14. The method of claim 13 for manufacturing of a sheet with wet stable cavities, wherein the wet paper web is shaped and provided with openings against the patterning fabric (82), the indentations of which have one opening (85), having an area of 9 9 9

0,01-50 mm , preferably 0,05-28 mm and most preferably 0,2-20 mm , and the number 9 9 of indentations (83) in the patterning fabric is 0,5-40 per cm , preferably 1-20 per cm and most preferably 2-10 per cm , the distance between the centre of adjacent indentations (83) is 0,5-40 mm, preferably 1-20 mm and most preferably 2-10 mm.

15. The method of claim 13 or 14 for manufacturing of a wet stable tissue sheet, wherein the funnel shaped indentations are distributed over the patterning fabric.

16. The method of any one of claims 13 - 15 for manufacturing of a sheet with wet stable cavities, wherein the patterning fabric (82) has a lower air resistance in the area (89) of each indentation intended to give cavities (83) than in the rest of the patterning fabric (82).

17. The method of any one of claims 13 - 16 for manufacturing of a sheet with wet stable cavities, wherein each indentations (93) extends throughout the entire depth of the patterning fabric (92) such that through holes appear in the patterning fabric (92).

18. The method of any one of claims 13 - 17 for manufacturing of a sheet with wet stable cavities, wherein the product obtained is wet strong.

19. The method of any one of claims 13- 18 for manufacturing of a wet stable tissue sheet, wherein the wet paper web (81) is dried in step c) by being blown-through by hot air.

20. A method for manufacturing of a tissue product of lignocellulose containing material, characterized in that at least one tissue sheet is provided with openings by the method of claims 13-19 and is brought together with at least one other tissue sheet provided with cavities and openings and/or at least one substantially flat tissue sheet without cavities.