Patent Application
20090242158
1. Field of the Invention
The invention relates to a process and an apparatus for producing a fibrous web, in particular a tissue web.
2. Description of the Related Art
Tissue paper ideally has high absorbency or a high water absorption capacity in conjunction with a high tear resistance. The absorbency and the water absorption capacity are determined substantially by the volume and porosity of the tissue paper.
In order to increase the volume, it has already been proposed to press the tissue paper web only zonally during its production, in order, in addition to the pressed regions of higher tear resistance, and to obtain more lightly pressed or unpressed more voluminous regions.
During the production of tissue paper, in a last drying step, the tissue web is led over the circumferential surface of a heated Yankee drying cylinder, before the finished product is creped off the latter. While the tissue web is being led over the Yankee drying cylinder, it is held by a fabric.
In particular during the production of tissue paper with voluminous regions which have been compressed only slightly during the dewatering, there is, however, the problem that the tissue paper comes into contact with the hot circumferential surface of the Yankee drying cylinder with an excessively low dryness. This problem occurs to a greater extent at high machine running speeds, since here the dewatering times are reduced further and the voluminous regions accordingly carry still more moisture with them.
On account of the excessively low dryness, during contact of the tissue web with the heated circumferential surface of the Yankee drying cylinder, water vapor is produced between the hot circumferential surface and the tissue web, which can lead to the web lifting off the roll circumferential surface.
As a result of the tissue web lifting off the roll circumferential surface, it is possible for problems to occur with account to the runnability, up to breaking of the tissue web.
Furthermore, on account of the formation of water vapor between the tissue web and the heated circumferential surface of the Yankee drying cylinder, it is possible for the formation of bubbles and holes in the tissue web to occur.
It is already known to coat the drying or Yankee cylinder in order to counter the problems occurring during a transfer of the fibrous web from a TAD (through air drying) fabric to the surface of the Yankees cylinder. In addition, a doctor arrangement has already been proposed in which a doctor crepes the fibrous web and doctors it off the Yankee cylinder, and at least one further doctor is provided for the purpose of removing a layer of the roll coating containing dirt.
At present, there exist two different processes for producing tissue paper. Firstly, there is the conventional tissue production process, in which the fibrous web is formed, pressed and dried on the Yankee cylinder. Secondly, there is what is known as the TAD production process (TAD=through air drying), in which the fibrous web is dried between the sheet forming zone and the press section by way of an air stream. This method is associated with a high paper quality.
To address the aforementioned problems, various parameters, in particular those which relate to the region of the Yankee cylinder, must be chosen in a suitable way.
Typical values for some known parameters are listed in the following table:
The high quantity of coating material previously required to coat the Yankee cylinder is obviously associated with economic disadvantages. The same applies to the relatively high blade or doctor loading previously required.
What is needed in the art is an improved process and an improved apparatus of the type mentioned at the beginning. What is needed in the art is an improved process and an improved apparatus which ensures the highest possible quality of the tissue paper and, at the same time, ensures that this high product quality can also be achieved with a lower quantity of coating material for the Yankee cylinder and a lower blade or doctor loading.
The present invention provides a process for producing a fibrous web, in particular a tissue web, having the following steps:
a) in a pressing zone, the fibrous web is pressed lying between the structured belt and a circulating, unstructured permeable supporting belt,
b) the fibrous web and the structured belt are fed to a press nip provided on a Yankee cylinder,
c) the fibrous web is transferred from the structured belt to the surface of the Yankee cylinder in the region of the press nip,
d) the surface of the Yankee cylinder is doctored off continuously and then recoated again, so that a renewed coating is always present in the press nip.
With this process according to the invention, a high quality of the tissue paper is achieved with, at the same time, a reduced required quantity of coating material for the Yankee drying cylinder and reduced blade or doctor loading. The fact that optimal results can be achieved in particular even with a reduced quantity of coating material and reduced doctor loading can be attributed to various factors.
After the fibrous web has been pressed in the pressing zone, lying between the structured belt and a circulating unstructured, that is to say relatively smooth, permeable supporting belt, the tissue web can be brought into contact with the Yankee drying cylinder with a relatively smooth side, while on the other hand, on account of the structured belt lying on the other side of the web, only part of the fibrous or tissue web is pressed. In a departure from the conventional TAD machines, the tissue web can therefore come into contact with the Yankee cylinder with approximately 100% of the surface of the relevant side, while only part thereof is pressed.
During the pressing of the fibrous web in the pressing zone between the structured belt and the circulating unstructured, that is to say relatively smooth, permeable supporting belt, the fibrous web is dewatered in the direction of the permeable supporting belt. In this case, the pressing pressure is preferably 1.5 bar or less.
Because the fibrous web is dewatered further toward the side facing away from the structured belt through the supporting and/or dewatering belt, the fibers are forced in the direction of the relatively flat or smooth surface of the supporting belt, formed in particular by a dewatering belt. The dewatering can be carried out for example by way of an appropriately high vacuum and/or mechanically, for example by way of a tensioning belt, by way of which the structured belt, the fibrous web and the supporting belt are pressed against a preferably smooth surface. According to the invention, the relevant gas stream therefore flows firstly through the permeable structured belt, then the fibrous web and finally the permeable supporting belt. By contrast, in the case of a conventional TAD process, the relevant gas stream flows firstly through the fibrous web and then the structured belt. In such a conventional TAD process the fibrous web is therefore not given a smooth surface. Apart from this, the differential pressure produced in the conventional TAD process is relatively low.
According to the present invention, the coating of the Yankee cylinder additionally ensures improved transfer of the fibrous web from the structured belt to the surface of the Yankee cylinder. By way of the coating, the heat transfer from the Yankee cylinder to the fibrous web is improved considerably. In addition, bubble formation, lifting of the web and so on are counteracted. Since at least part of the coating has been removed continuously again by way of the relevant doctor, no dirt can accumulate.
For optimal support of the aforementioned transfer of the fibrous web from the supporting belt to the surface of the tissue cylinder, an adhesive coating material is applied to the surface of the Yankee cylinder.
According to another embodiment of the present invention, the quantity of material applied continuously to the surface of the Yankee cylinder preferably lies in a range from about 3 to about 9 mg/m2.
In order to doctor off the surface of the Yankee cylinder continuously, a doctor or the like is loaded in such a way that the result is a line force in a range from about 3 to about 7 kN/m, preferably in a range from about 5 to about 6 kN/m. The relevant doctor loading can thus be reduced considerably without any kind of costs in terms of quality.
The fibrous web is formed on a circulating permeable structure belt, as a result of which the depressions of the structured belt are filled up with fibers, instead of the fibers of a web already formed being sucked into the depressions of the structured belt in a conventional TAD process. In this way, a voluminous fibrous web is produced as compared with the process in which a smoothly formed fibrous web is pressed into the depressions of a structured belt. Furthermore, as a result of forming the fibrous web between the permeable structured belt and a smooth unstructured forming fabric, the effect brought about by the dewatering of the fibrous web in the pressing zone in the direction of the supporting belt is further intensified such that approximately 100% of the surface of the relevant side of the fibrous web can come into contact with the Yankee cylinder.
As already mentioned, the pressing zone is expediently delimited on the side adjacent to the supporting belt by an at least substantially smooth surface. In this case, this smooth surface can in particular be formed by a rotating roll.
In the region of the pressing zone, a gas stream can be produced which flows through the structured belt, the fibrous web and the supporting belt, the gas stream flowing first through the structured belt, then the fibrous web and finally the supporting belt.
The gas stream that is produced can in particular be an air stream and/or steam stream.
According to another embodiment of the present invention, the gas stream is at least partly produced by way of a suction zone of a rotating suction roll, which delimits the pressing zone on the side adjacent to the supporting belt.
However, the gas stream can also at least partly be produced by way of a flat or curved suction box or the like, which delimits the pressing zone on the side adjacent to the supporting belt.
Furthermore, the gas stream can be produced at least partly by way of a positive pressure hood arranged on the side of the permeable structured belt. The positive pressure hood can be, for example, a steam blower box.
Alternatively or additionally, the structured belt, the fibrous web and the supporting belt can also be pressed against an at least substantially smooth surface by way of a press belt under tension. In this case, the smooth surface can in particular again be formed by a rotating roll.
The structured belt used is expediently a structured fabric.
The structured belt used can also be a (TAD) fabric, for example.
The fibrous web is formed in a way as has been described previously. However, in specific cases it may also be expedient to form the fibrous web by the fibrous stock suspension being sucked into the structure of the permeable structured belt by way of a vacuum device.
The supporting belt, as already mentioned, can in particular be a dewatering belt.
The relatively depressed and the relatively elevated regions of the structured belt are formed and arranged relative to one another in such a way that at most 35% and preferably at most 25% of the structured belt is pressed in the press nip. The gentlest possible pressing is achieved by the press nip provided on the Yankee cylinder being a shoe press nip. If the fibrous web to be produced does not require any bulk but does have a high dryness with a high production output, as an alternative to this the press nip formed on the Yankee cylinder can be formed with a suction press roll or a press roll.
According to another embodiment of the present invention, the Yankee cylinder is assigned only one doctor, by way of which, firstly, at least part of the coating is removed and, secondly, the fibrous web is creped and lifted off the cylinder surface.
According to another embodiment of the present invention, the Yankee cylinder is assigned a first doctor for removing at least part of the coating and a second doctor, by means of which the fibrous web is creped and lifted off the cylinder surface.
The outlet pocket between fibrous web and structured belt, occurring at the outlet from the press nip, is acted on by way of an air knife.
According to another embodiment of the present invention, in the region in which the structured belt runs off a mating roll forming the press nip with the Yankee cylinder, the outlet pocket between the structured belt and the mating roll is acted on by way of an air knife. As a result, fibers situated in the depressions of the structured belt are released, which assists the transfer of the fibrous web on the Yankee cylinder. In addition, the formation of bubbles on the fibrous web is counteracted.
The fibrous web removed from the Yankee cylinder again is subsequently wound up.
According to the present invention, an apparatus for producing a fibrous web, in particular a tissue web, includes a sheet forming zone, in which the fibrous web is formed on a circulating permeable structured belt, a pressing zone, through which the fibrous web is led lying between the structured belt and a circulating unstructured permeable supporting belt, and a press nip provided on a Yankee cylinder, through which the fibrous web is led together with the structured belt, the fibrous web in the region of the press nip being transferred from the structured belt to the surface of the Yankee cylinder, at least one doctor, which doctors off the surface of the Yankee cylinder continuously, and a coating device, by way of which the Yankee cylinder is subsequently recoated again, so that a renewed roll coating is always present in the press nip.
The coating device is driven or regulated in such a way that the quantity of coating material applied continuously to the surface of the Yankee cylinder lies in the range from about 3 to about 9 mg/m2.
A doctor is provided to scrape off the surface of the Yankee cylinder continuously and to crepe the fibrous web, this doctor being loaded in such a way that the result is a line force in a range from about 3 to about 7 kN/m, preferably in a range from about 5 to about 6 kN/m.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring now to the drawings, and more particularly to
A headbox 12 delivers a stock suspension jet into an inlet gap 14, which is formed in the region of a forming roll 16 between an inner circulating permeable structured belt 18 and an outer circulating forming fabric 20, which run together in the region of the forming roll 16 and are subsequently led jointly around this forming roll 16.
The structured belt 18 can be in particular a three-dimensionally structured fabric.
Facing the tissue web 22, the forming fabric 20 has a side that is relatively smooth as compared with the relevant side of the structured belt 18.
In contrast, the side of the structured belt 18 facing the tissue web 22 has depressed regions and regions elevated with respect thereto, the tissue web 22 being formed in these depressed and elevated regions of the structured belt 18.
The structured belt 18 can be formed by a TAD fabric, for example.
In the region of the forming roll 16, the tissue web 22 is dewatered substantially through the outer forming fabric 20. Then, in the region of a deflection roll 24, the forming fabric 20 is separated from the tissue web 22 again which, together with the structured belt 18, is led further to a pressing zone 26, in which the tissue web 22 is pressed lying between the structured belt 18 and a circulating unstructured permeable supporting belt 28.
The permeable supporting belt 28 can in particular be a felt. In the region of the pressing zone 26, pressure is exerted on the structured belt 18, the tissue web 22 and the supporting belt 28, the tissue web 22 being dewatered in the direction of the supporting belt 28 formed, for example, by a felt.
Since the tissue web 22 is dewatered in the direction of the permeable supporting belt 28 in the pressing zone 26, and the structured belt 18 led through this pressing zone 26 is identical with the structured belt on which the tissue web 22 was formed, the more voluminous sections of the tissue web 22 are compressed less highly than the less voluminous sections, so that, as a result, the voluminous structure of the relevant more voluminous sections is maintained.
The dewatering pressure for the tissue web 22 in the pressing zone 26 is produced simultaneously, at least in some sections, by a gas stream and by a mechanical pressing force.
The gas stream flows firstly through the structured belt 18, then the tissue web 22 and finally the permeable supporting belt 28.
As can be seen from
The mechanical force applied alternatively or additionally is produced by the structured belt 18, the tissue web 22 and the supporting belt 28 in the pressing zone 26 being led between a press belt 34 under tension and a smooth surface, which is formed here by the roll 32, for example.
The pressing zone 26 is at least substantially defined by the wrap region of the press belt 34 around the circumferential surface of the suction roll 32, this wrap region being defined by the distance between the two deflection rolls 36, 38.
In the region 40 through which the tissue web 22 is led together with the structured belt 18, the tissue web 22 can be subjected to at least one further drying step.
Following that, the tissue web 22 is led together with the structured belt 18 through a press nip 44 formed on a drying cylinder, specifically a Yankee cylinder 42. In this press nip 44 the tissue web 22 lies between the structured belt 18 and the smooth surface of the Yankee cylinder 42. The press nip 44 is formed by a shoe press nip. The Yankee cylinder 42 is therefore assigned a shoe press unit, here a shoe press roll 46, in order to form the press nip 44. As can be seen from
The fact that the tissue web has been formed between the structured belt 18 and a forming fabric 20 which is relatively smooth in relation thereto, means only the side of the tissue web 22 formed on the structured belt 18 has an undulating surface. By contrast, the surface of the tissue web 22 formed on the smooth forming fabric 20 is relatively smooth. The tissue web 22 now comes with this smooth side into contact with the surface of the Yankee cylinder 42 in the press nip 44. The tissue web 22 therefore touches the Yankee cylinder with a relatively large area. Since the structured belt 18 in the press nip 44 is identical with the structured belt on which the tissue web 22 was formed, it is moreover ensured that the more voluminous regions of the tissue web 22 are also virtually not pressed in this press nip 44. On the other hand, the less voluminous regions of the tissue web 22 are pressed, which means that the strength of the tissue web 22 is increased further.
Following the press nip 44, the structured belt 18 is separated from the tissue web 22, which is led on the Yankee cylinder 42 as far as a doctor 48, by which the tissue web 42 is creped and lifted off the Yankee cylinder 42. The tissue web 22 is subsequently fed to a reeler 50, in which it is wound up with the aid of a pressure roll 52 to form a roll 54.
Therefore, in the region of the press nip 44, the tissue web 22 is transferred from the structured belt 18 to the surface of the Yankee cylinder 42. The surface of the Yankee cylinder 42 is doctored off continuously, for example by way of the doctor 48, and is subsequently recoated again by way of a coating device 56, so that a renewed coating is always present in the press nip 44. An adhesive coating material can be applied to the surface of the Yankee cylinder 42.
The quantity of coating material applied continuously to the surface of the Yankee cylinder 42 expediently lies in a range from about 3 to about 9 mg/m2. In order to doctor off the surface of the Yankee cylinder 42 continuously, a doctor or the like, here for example the doctor 48 simultaneously creping the tissue web, can be loaded in such a way that the result is a line force in a range from about 3 to about 7 kN/m, preferably in a range from about 5 to about 6 kN/m.
The gas stream that is produced which, in the pressing zone 26, flows firstly through the structured belt 18, then the tissue web 22 and finally the supporting belt 28, can in particular be an air stream and/or steam stream.
Alternatively or additionally to the suction roll 32, the gas stream can in particular be produced at least partly by way of a positive pressure hood 58 arranged on the side of the permeable structured belt 18.
As already mentioned, the structured belt 18 used can in particular be a structured fabric, for example a TAD fabric.
The supporting belt 28 provided can in particular be a dewatering belt, as already mentioned.
The relatively depressed and the relatively elevated regions of the structured belt 18 are preferably formed and arranged relative to one another in such a way that at most 35% and preferably at most 25% of the structured belt 18 is pressed in the press nip 44 formed with the Yankee cylinder 42.
In
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claim.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 046 903.5 | Sep 2005 | DE | national |
This is a division of U.S. patent application Ser. No. 11/527,248, entitled “PROCESS AND APPARATUS FOR PRODUCING A TISSUE WEB”, filed Sep. 26, 2006, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11527248 | Sep 2006 | US |
| Child | 12481236 | US |
