Patent Grant
8366878
This invention relates to a structuring layer of a structuring belt used for structuring a wet fibrous web by means of pressing in a press section of a tissue papermaking machine for manufacturing high bulk tissue paper, wherein the structuring layer has a web-carrying side with a surface for cooperating with the fibrous web, said surface having depressions forming a three-dimensional structure of the surface. The invention also relates to a structuring belt for structuring a wet fibrous web in a press section by means of pressing in a tissue papermaking machine for manufacturing high bulk tissue paper. The invention further relates to a press section employing such a structuring belt, and to a tissue papermaking machine having such a press section.
The invention also relates to a method of manufacturing a structured high bulk tissue paper web and to such a high bulk tissue web.
The invention relates furthermore to a method of converting or upgrading an existing tissue papermaking machine.
The term “tissue paper” as used herein refers to soft paper with a basis weight usually of less than 25 g/m2. Tissue paper web having a basis weight of 10-50 g/m2 (more preferably 15-25 g/m2) forms a base paper from which certain single-ply and/or multi-ply products (e.g., napkins, towels, and toilet paper) can be manufactured. The term “high bulk” tissue paper means that the bulk is about 8-20 cm3/g and the single-ply tissue paper web thickness is about 160-400 μm.
Tissue paper is manufactured from a mixture of hardwood and softwood cellulose fibers, usually from a so-called “virgin” pulp constituting fresh fibers, as opposed to recycled fibers. Alternatively, recycled fibers can be mixed in with virgin fibers to make the pulp. Depending on the particular products to be made, various mixtures of fibers can be used. For example for toilet paper and/or facial tissue, the pulp may comprise 50-90% by weight of hardwood and 50-10% by weight of softwood fibers, a preferred mixture being about 70% hardwood and 30% softwood. For paper towels, the pulp may comprise 0-50% by weight hardwood and 100-50% by weight softwood fibers.
In manufacturing creped tissue paper, typically the formed wet paper web of cellulose fibers is dewatered before final drying on a Yankee cylinder, the dewatering usually being performed by either a pressing technique or a through-air-drying (TAD) technique. In some conventional tissue machines a suction pressure roll or a blind-drilled roll is used as a press roll that presses the web against the Yankee cylinder, but this compression of the web results in a final tissue product with relatively low thickness and low bulk such as 5-9 cm3/g. In other conventional tissue machines the web is pressed and dewatered in a double-felted pre-press before the reaching the Yankee cylinder, the pre-press being formed by two press rolls that define a press nip therebetween, but again, the rolls compress the paper web uniformly and it results in the web having relatively low thickness and bulk. It has been proposed to use an extended-nip press such as a shoe press as an alternative to the above-mentioned conventional pressing techniques, which extended- or long-nip press can apply lower pressure but provides a longer dwell time in the nip. As another alternative, it has been proposed to use a shoe press against the Yankee cylinder in order to decrease the compression of the web in the press nip, so as to increase the bulk or thickness of the web. The objective has been to achieve the same level of bulk or thickness as achieved by the TAD technique, but up to now this has not been possible. The thickness or bulk of the paper is important for the absorption ability of the paper and the feel of softness. The TAD technique therefore is still superior to the pressing technique in terms of achieving high bulk or thickness of the paper web, but it has the disadvantage that it necessarily requires higher energy consumption than is the case with a pressing technique, especially when TAD is used as a pre-drying process on a web containing a substantial amount of water to be removed. When TAD is used in place of the Yankee cylinder for final drying of the tissue web, the TAD technique requires less drying capacity than when it is used for pre-drying, and therefore has a lower energy consumption. Therefore, using TAD for final drying could be a viable alternative to the use of a Yankee cylinder.
In tissue papermaking machines that employ the pressing technique for dewatering the paper, a press felt runs together with the tissue web through the press nip, and the press felt receives water squeezed from the web and carries the water away. In order to achieve a high bulk, it is preferred to use only one press nip, but in some cases a single nip cannot achieve sufficient dewatering and hence one has to compromise and use a second press nip.
In tissue machines employing the pressing technique, it is also possible to use a “structuring clothing”, which is a clothing whose web-contacting surface has a lot of voids and top portions distributed between the voids. As the structuring clothing passes along with the tissue web through the press nip, the voids receive the fiber network of the tissue web, and therefore only those areas of the web contacted by the top portions of the clothing are compressed. Furthermore, in order to reduce compression in the nip and thereby increase the bulk or thickness of the fiber web in comparison with that obtainable using smooth roll presses, it is possible to use an extended-nip press as noted above. Such a structuring clothing can be a woven wire, belt, or fabric, including but not limited to a TAD fabric. The woven structure of such a fabric forms the voids and top portions of the web-contacting surface as described above. Such a fabric can further have a special coating (e.g., of a photo-sensitive resin or other type of material) to emphasize or form the pattern to be embossed into the wet tissue web during pressing in the press nip.
The term “structuring” of the paper as used herein refers to a process in which a three-dimensional pattern of the structuring layer of a structuring belt is embossed into the wet fibrous web during a pressing process when the fibrous network structure fills the three-dimensional (3D) pattern of the structuring belt layer, and in which fibers in the wet fibrous web are still movable relative to each other so that they are advantageously brought to new positions and directions relative to each other by the action of the elastically compressible press felt, which presses the wet fibrous web into the three-dimensional pattern or voids of the structuring belt layer, thereby promoting fiber binding between the fibers of the network and achieving partial dewatering of the wet fibrous web while achieving an increased bulk at the same basis weight, and MD and CD tensile strengths of the finally dried tissue paper web comparable to those of conventional tissue paper, and achieving an improved structure of this basic tissue paper.
U.S. Pat. No. 6,547,924 and U.S. Pat. No. 6,340,413 describe a tissue papermaking machine in which a structuring belt carries the fibrous web from the last press in the press section to the drying cylinder. However, the papermaking machine described in these references cannot produce a tissue paper of sufficiently high quality and high bulk while also achieving an acceptable dryness to make this machine concept commercially attractive/interesting. The described machines, because of the plurality of press nips required in order to meet the requirements for the dryness of the web for its runnability, do not meet the wishes of customers today to have a high bulk paper. Furthermore, there were problems with runnability of the machine, as either the web was too wet because the press felt was saturated with water and could not absorb a sufficient quantity in the nip, which led to paper breaks, or the dryness was sufficient and the runnability was good but the bulk or the quality of the final tissue paper web was too low. Additionally, the impermeable structuring belt described in these patents has a polymer web-contacting layer that includes grooves, but the dimensions of the grooves are such that the desired hydraulic pressure into the grooves cannot be created, which results in runnability problems, particularly web breaks as noted.
The present invention relates to the structuring of a wet web by a 3D structuring layer of a structuring belt, and to a structuring belt comprising such a structuring layer for use in pressing technology for manufacturing a high bulk tissue paper. The structuring belt itself is a non-woven or woven clothing. This means that 3D patterns of the structuring layer are created not by a woven structure, but by other means. The bulk of the paper is maintained in that depressions defined solely within the thickness of the structuring layer (i.e., not defined by the structure of any underlying layer beneath the structuring layer) of the belt receive the fibrous network of the wet tissue web thereon and those depressions prevent compaction of the entire web structure during the pressing.
Following comprehensive research, the present inventors have realized that the structure of the structuring layer that is in contact with the web during the pressing process has great and probably crucial importance from the point of view of being able to achieve a tissue paper with a higher bulk than that previously possible in a conventional papermaking machine using the conventional pressing technique with the multiple presses or with a press roll against a Yankee cylinder, and that the structure of this layer of the structuring belt can also be used as a parameter for controlling properties of the web after the nip and for achieving a high dryness of the web in the press section in which the actual structuring of the wet fibrous web occurs.
An object of at least some embodiments of the invention is to make it possible to manufacture, at a low energy cost, a tissue paper web of high bulk comparable to that of TAD-paper, and in particular having a bulk of about 12-20 cm3/g depending on the chosen basis weight of the web, as compared to a conventional tissue paper typically having a bulk of only 6-9 cm3/g for the same basis weight. Reduction of the energy cost and energy saving are achieved by attaining a relatively high dryness (about 40-52%) directly after the first, and preferably the only, press nip, which makes it possible to avoid having to use the TAD technique as a pre-drying process in order to increase the dryness of the fiber web prior to entering the press nip for ensuring the machine runnability and avoiding the web breaks in the press nip and prior to the final drying of the tissue web. Thus the required drying capacity (including but not limited to a size of the drying cylinder and its temperature, a hood with integrated fans, air supply and so on) of the machine is reduced by 20-35%. Energy savings in the final drying section of the machine could be up to about 35%.
High bulk of a tissue paper web is an important property for the absorption capacity of the web. The single-layered paper web can then be rewound into single-layered or multi-layered finished consumer products such as sanitary paper, napkins, towels, facial tissue and toilet paper. The quality of these products is determined at least in part by their absorption capacity and their soft feel to the consumer.
In accordance with some embodiments of the invention, a structuring belt is employed having a structuring layer that defines a web-contacting surface for cooperating with and structuring of the wet fibrous web in the pressing process. The web-contacting surface of the structuring layer has voids or depressions, or alternatively elevations, forming a three-dimensional structure of the web-contacting surface, the depressions or elevations being distributed over the web-contacting surface and together constituting 20-80% of the web-contacting surface. The web-contacting surface also includes a continuous flat top surface area between the depressions or voids, or alternatively includes a flat valley surface area between the elevations, the top surface area or valley surface area delimiting the depressions or elevations, respectively. Each depression or elevation has a dimension l in a first direction (x) in the plane of the top surface area, a dimension b in a second direction (y) in the plane of the top surface area, the first and second directions (x, y) being at right angles to each other, a mean depth or mean height d (in the z-direction, which is perpendicular to the x, y-directions and extends in the thickness direction of the structured layer), and an area a as measured in the plane of the top surface area, these dimensions being defined when the belt is in a compressed state in a press nip.
The invention is described further with reference to the drawings, in which:
The press section 3 comprises a main press 11 including a first press element 12 and a second press element 13 that cooperate with each other to form a press nip between them. The main press 11 may be a roll press, a long-nip or extended-nip press such as a shoe press (not shown in the figures), or any other type of extended-nip press known in the art. The press section 3 further comprises a structuring belt 14 with a structuring layer 60 according to the present invention running in an endless loop about a plurality of supporting rolls 15, about a smooth transfer roll 16 located in connection to the drying section 4, and through the press nip N1 of the main press 11 together and in contact with the formed fibrous web 1′ in order to provide pressing, dewatering, and structuring of the formed embryonic fibrous web 1′ when it passes through the press nip N1 so that a structured or embossed, partially dewatered fibrous web 1″ exits the press nip N1. The structured fibrous web 1″ is then carried by the structuring belt 14 up to a transfer nip N2 formed between the transfer roll 16 and a drying surface 20 of a drying cylinder 19. No pressing or dewatering takes place in the nip N2, but only the transfer of the partially dewatered, structured fibrous web 1″ to the surface 20 of the drying cylinder 19. In this case as illustrated, the drying cylinder 19 is a Yankee cylinder, but other types of drying sections known in the art are possible, such as a TAD-cylinder for final drying. The press section 3 further comprises a water-receiving press felt 17 that is elastically formable and compressible in the z-direction (perpendicular to the plane of the felt), running in an endless loop about a plurality of support rolls 18 and through the press nip N1 of the main press 11 together with the structuring belt 14 such that the formed fibrous web 1′ is sandwiched between the structuring belt 14 and the press felt 17. The first press element 12 is located in the loop of the structuring belt 14 and the second press element 13 is located in the loop of the press felt 17. In the embodiments shown in
Immediately before the first roll 18 after the main press 11, a spray device 53 for cleaning the press felt 17 is arranged on the inside of the press felt 17 for supplying fresh or clarified white water to the wedge-shaped narrowing space between the press felt 17 and the roll 18, the water being pressed into the press felt 17 and displacing the contaminated water in the press felt 17 after pressing in the main press 11 through and out of the press felt 17 when this runs about the roll 18. Upstream of the following roll 18, suction boxes 54 are arranged on the outside of the press felt in order to withdraw and remove water from the press felt 17. Alternatively, other suction devices known in the art could be used in this position.
Once the structuring belt 14 has left the transfer roll 16 and before it reaches the main press 11, the structuring belt 14 passes through a cleaning station 30 at which the web-contacting surface of the structuring layer 60 is cleaned from contaminations.
The drying section 4 comprises a first drying cylinder 19 having a drying surface 20, which, in the illustrated embodiments, is the only drying cylinder, and advantageously is a Yankee cylinder. Alternatively, the drying section may consist of a plurality of drying cylinders, a metal belt dryer, or a TAD-cylinder with a TAD-fabric wrapped thereabout. The particular type of final drying device used is not critical to the present invention. As noted, in the illustrated embodiments the drying surface 20 of the drying cylinder 19 cooperates with the transfer roll 16 to form the transfer nip N2, and also serves to perform final drying of the partially dewatered, structured fibrous web 1″. A creping doctor 21 is arranged at a downstream point along the drying surface 20 to crepe or remove the dried fibrous web 1″ from the drying surface 20 in order to obtain a tissue paper web 1′″ that is both structured and creped. The drying cylinder 19 is covered by a hood 21. The structuring belt 14 and the structured fibrous web 1″ run into the transfer nip N2 together but leave the transfer nip N2 separately because the structured fibrous web 1″ adheres to and is transferred to the drying surface 20 of the drying cylinder 19, while the structuring belt 14 diverges from the drying surface 20 after the transfer nip N2. The pressure in the transfer nip N2 is less than 1 MPa, and could be much lower (e.g., 10 kN/m) in order to ensure that no additional compression and dewatering of the web occurs as the web passes through the transfer nip N2. In order to ensure that the fibrous web 1″ is transferred and affixed to the drying surface 20, an adhesive is advantageously applied to the drying surface 20 by means of a spray device 23 at a point between the creping doctor 21 and the transfer nip N2 where the drying surface 20 is free, or by other means known in the art.
The main press 11 may be a roll press in which the two press elements 12, 13 are rolls with smooth solid mantle surfaces, or may be an extended-nip press (e.g., a shoe press) in which the first press element 12 is a smooth counter roll and the second press element 13 comprises a press shoe and an endless belt or a jacket running through the press nip of the shoe press in sliding contact with the press shoe, which exerts a predetermined pressure on the inside of the belt and on the counter roll 12. The press shoe thus constitutes a device forming an extended press nip. In a further preferred embodiment of the main press 11, the first press element 12 is a smooth counter roll and the second press element comprises a device for forming an extended press nip, said device including an elastically deformable and pressurized supporting element arranged to press in the direction towards the opposing counter roll and to generally conform its shape to a portion of the counter roll as disclosed in U.S. Pat. No. 7,527,708, which is hereby incorporated herein by reference. As an alternative to these types of extended-nip press devices, another other known type of extended-nip press could be employed.
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All of the depressions can be identical if desired. Alternatively, the depressions can comprise two or more groups of depressions, wherein the design of the depressions in the different groups differs, but the depressions within each group are identical.
Tests have shown that the form, extent, and volume of the depressions 63 are very important with respect to the runnability of the tissuemaking machine (and particularly with respect to the ability of the fibrous web to separate from the structuring layer 60 under the influence of hydraulic pressure communicated into the depressions) as well as its ability to produce a tissue paper web of good quality, i.e., having high bulk of 8-20 cm3/g and high softness, at a lower energy consumption compared to corresponding conventional or TAD-machines having comparable dimensions. Alternatively the process/structuring layer of the invention can allow decreasing the required drying capacity/energy supply (e.g., decreased drying section/machine dimensions or fewer fans for the hood/air supply) while keeping the same dimensions and speed as those of conventional or TAD-machines, or can allow increasing the machine speed (higher production) at the same energy consumption and the same dimensions as those of conventional or TAD-machines.
In order to achieve an optimum structure and dryness of the web, it is important that the structuring belt 14 allows the wet fibrous web 1′ to be formed into the depressions 63 when the fibrous web 1′ passes through the press nip N1 sandwiched between the press felt 17 and the structuring belt 14. It is also important that the press felt 17 can reach down into the depressions 63 during the pressing process in order to build up a sufficiently high hydraulic pressure so that water in the wet fibrous web 1′ can move into the press felt 17 and not remain in the fibrous web at the end of the pressing operation. The depressions 63 must be sufficiently large to allow the press felt 17 to penetrate into the depressions 63 together with the fibrous web. Each depression 63 must have an optimum depth that allows water in the bottom of the depression 63 to be transported away. In other words, the depth of the depression 63 must not be too great, as an excessive depth will prevent the desired hydraulic pressure from building up and thus will not facilitate the release of the web after leaving the press nip.
The structuring layer 60 with this specific well-defined, structured, web-contacting surface 61 is an important parameter for controlling the structure, thickness/bulk, and dryness of the structured and partially dewatered fibrous web 1″ after the press nip N1 before final drying.
The pressure in the press nip N1 should not be excessively high but rather should be within the normal ranges conventionally used for pressing, and the press felt 17 can be of the conventional elastically compressible type, which, in addition to its required water-receiving capacity during compression, is able to be elastically deformed into the web-contacting surface of the structuring layer 60 with the wet fibrous web 1′ located therebetween in the manner as described above for co-acting with the depressions 63 so as to create the hydraulic pressure therein.
Each depression 63 has a predetermined dimension l in the machine direction (MD) of the structuring layer 60 and a predetermined dimension b in the cross direction (CD) of the belt 14. The depressions 63 may be oriented in the machine direction, in which case l>b, or in the cross direction, in which case l<b. However, the depressions 63 are preferably oriented substantially in the machine direction, as this gives better and more-uniform creping and results in a softer tissue paper.
Each depression 63 also has a predetermined depth d, a predetermined area a, and a predetermined volume v. The depth d of each depression may be constant over substantially all of the depression 63, in which case the depression 63 has a bottom surface 71 that is flat and parallel to the top surface area 70. The depth d alternatively may vary over the surface of the depression 63, in which case the depth d represents an average or mean depth over the surface of the depression.
The depressions 63 are arranged at a predetermined distance from each other so that they are distributed in a uniform manner over the web-contacting surface 61 and cover a predetermined part thereof. Thus, the abovementioned continuous top surface area 70 constitutes the remaining part of the web-contacting surface 61 and delimits the depressions 63, and constitutes the part of the web-contacting surface 61 cooperating with the drying surface 20 when the fibrous web 1″ is transferred to the drying cylinder 19.
The above-mentioned parameters must therefore cooperate in order to obtain good runnability (such as no web breaks) and good quality of the tissue paper web 1′″. Tests have shown that the following parameters should exist in order to achieve this:
In the case where the depth of the depression 63 varies between being in the non-compressed state and in the compressed state in the nip depending on the material used, the value d refers to the mean depth of the depression. However, under no circumstances should the greatest depth value d of the depression exceed 0.6 mm, as measured when the structuring layer 60 of the structuring belt 14 is under compression in a press nip.
In addition to the abovementioned parameter values, the depressions preferably should collectively cover between about 20% and about 80% of the total web-contacting surface 61.
A tissue paper web (preferably creped) having the following properties on the reel can be manufactured in a tissue papermaking machine provided with a structuring belt with a structuring layer as above:
while the conventional creped tissue paper web has a bulk of 6-9 cm3/g and softness of 50-70 measured according to EMTEC TSA (Tissue Softness Analyzer with a scale from 0 to 100) for conditioned paper at 20° C. and 50% air humidity.
More specifically, tissue paper of a quality suitable for facial tissue, toilet paper, and household paper can be manufactured by a tissue papermaking machine according to the invention, the tissue paper having the following properties:
The structuring layer 60 is preferably made of a polymer material, e.g., polyurethane, in which layer 60 the depressions 63 preferably are formed by laser burning. The structuring layer 60 may alternatively be made of a different material, e.g., metal or carbon fiber, and other techniques may be used to form the depressions. The structuring layer 60 is preferably approximately 3-6 mm thick, but its thickness may be between 0.2 and 10 mm. The structuring layer 60 may be provided with a reinforcing member 57 and a wear layer 58.
The structuring belt 14 with the layer 60 is preferably substantially water-impermeable as mentioned for the tissue papermaking machines shown in the drawings. Alternatively, the structuring belt 14 may be made permeable. This can help control of the adhesion of the web to the belt. For example, the structuring layer 60 may be needled so that it has through holes. The depressions or the surface area surrounding the depressions, or both, may be needled. Like the structuring layer 60, the wear layer 58 also can be needled if desired.
In order to increase the service life of the structuring belt 14, the structuring belt 14 may comprise a wear layer 58, e.g., in the form of a felt layer which is needled into the structuring layer 60 and arranged on the side of the structuring belt 14 directed away from the fibrous web.
In order to increase the strength of the structuring belt 14, the structuring belt 14 may comprise reinforcing means 57, e.g., in the form of reinforcement wires arranged within the structuring layer 60, a metal strip or a fabric.
With the aid of a structuring belt 14 according to the invention, it is thus possible to manufacture a tissue paper web which, after creping from the drying surface 20 and conditioning at 20° C. and an air humidity of 50%, has a basis weight in the range of 10-50 g/m2, a thickness in the range of 160-400 μm, preferably 200-300 μm, a bulk in the range of 8-20 cm3/g, an MD tensile strength in the range of 50-300 N/m, a CD tensile strength in the range of 30-250 N/m, and a softness in the range of 70-90 as measured according to EMTEC TSA (Tissue Softness Analyzer) with a measuring scale of 0 to 100.
The fibrous web 1′, 1″ preferably comprises a short-fiber layer and a long-fiber layer, wherein the fibrous web 1′, 1″ is transferred to the drying surface 20 in the transfer nip N2 with the short-fiber layer directed towards the drying surface 20. The finished tissue paper web 1 thus preferably also has a short-fiber layer on one side 79, i.e., the side which has been in contact with the drying surface 20, and a long-fiber layer on its other side 80, i.e., on the side which has been in contact with the structuring belt 14.
The invention has been described above by way of a number of embodiments. However, it will be clear that other embodiments or variants are within the scope of the invention. For instance, it will be clear that alternative embodiments of the depressions are possible without going beyond the scope of the invention as defined in the claims. Alternative embodiments of this kind comprise, e.g. circular, rhombic or elliptical depressions, the longitudinal axes of which do not necessarily have to be situated in the machine or cross direction of the structuring belt, but may form an angle therewith.
The present application is a division of U.S. patent application Ser. No. 12/561,840 filed Sep. 17, 2009 which is a continuation of International Patent Application PCT/SE2008/051332 filed on Nov. 20, 2008 and published under PCT Article 21(2) in the English language, and also claims the benefit of the priority date of U.S. Provisional Patent Application Ser. No. 61/097,837 filed on Sep. 17, 2008, the entire disclosures of such applications being hereby incorporated herein by reference.
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| 6340413 | Nilsson et al. | Jan 2002 | B1 |
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| Number | Date | Country | |
|---|---|---|---|
| 20120267063 A1 | Oct 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61097837 | Sep 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12561840 | Sep 2009 | US |
| Child | 13544603 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/SE2008/051332 | Nov 2008 | US |
| Child | 12561840 | US |
