Patent Application
20220228317
The invention relates to a fabric for a machine for producing a tissue web according to the preamble of claim 1, and a machine and a method for producing a tissue web with such a fabric.
The production of tissue or hygienic papers is still a rapidly growing market. A machine such as is typically used in tissue production is described in EP 1 167 115 B1. As is generally the case in paper production, here, too, a fibrous material suspension is applied to a fabric or between two fabrics and dewatered by evacuation. After that, the fibrous web is further dewatered in a press and then further dried thermally. The fibrous web is transported into the press on a water-absorbing fabric. The water pressed out of the web is absorbed by the fabric and, after the fibrous web has been removed, is removed from the fabric again. This is carried out by means of suction boxes, so-called Uhle boxes, as described by way of example in EP 2 602 387 B1.
Modern tissue machines are operated with very high production speeds, as a result of which the residence times of the tissue web in the press nip are very short. Insufficient dewatering of the web in the press is therefore generally the limiting factor which prevents an increase in the production speed of the machine.
It is therefore an object of the invention to propose a machine and a method for its operation which ensures improved dewatering of the tissue web.
It is a further object of the invention to propose a conversion solution for existing installations which can be implemented with little or no outlay.
It is a further object of the invention to propose a fabric which enables a significantly improved dewatering of the tissue web.
The objects are achieved completely by a fabric as claimed in the characterizing clause of claim 1, a machine as claimed in claim 8 and a method as claimed in claim 11. Further advantageous features of the embodiment according to the invention will be found in the sub-claims.
With regard to the fabric, the object is achieved by a fabric, in particular a felt, for use in a machine for producing a tissue web. The fabric comprises a base structure, which has or consists of a woven textile structure with MD threads and CD threads. Furthermore, the fabric comprises at least one layer of nonwoven fibers. According to the invention, provision is made for the MD threads wholly or predominantly to have a diameter between 0.25 mm and 0.45 mm, in particular between 0.3 mm and 0.35 mm, and for the thread density of the MD threads to be more than 37%, in particular between 37% and 45%.
The term “wholly or predominantly” is to be understood such that at least 90% of the MD threads, preferably 95%, in particular all the MD threads, have a diameter in the specified range.
Within the context of this application, the term “diameter of a thread” is used. In the case of round threads, this term is well-defined.
For monofilaments which deviate from the round shape, or else for threads twisted from multiple monofilaments, the diameter of the thread is to be understood to be the diameter of the smallest circle which encloses the cross section of the thread.
To determine the thread density of the MD threads, the number of threads per unit length is multiplied by their diameter and the value set in relation to the unit length.
Thus, with 10 threads/cm and a thread diameter of 0.40 mm, the result is a thread density of 10*0.4 mm/10 mm=40%.
By means of the combination of fine MD threads and a high MD thread density, the base structure can perform part of the functionality which is otherwise performed in the fabric by one or more nonwoven layers, such as, for example, evening out the pressure. The base structure permits a uniform pressure distribution through the felt, which was previously thus not possible. Experiments by the applicant have surprisingly shown that fabrics, in particular felts with such base structures, can manage with a lesser nonwoven overlay. This is not only economically advantageous as a result of the saved nonwoven layers, but permits improved dewatering even when used in a tissue machine. With a thinner felt, for example in the press of a tissue machine, nip dewatering can also further be achieved in addition to the Uhle box dewatering.
In preferred embodiments base structure is composed of two endless woven layers or two woven layers which have been made endless, wherein in both woven layers the MD threads wholly or predominantly have a diameter between 0.25 mm and 0.45 mm, in particular between 0.3 mm and 0.35 mm, and the thread density of the MD threads is more than 37%, in particular between 37% and 45%. This may be a two-layer woven material or two separate layers of woven material.
In preferred embodiments it may be provided for the textile structure to be composed of flatweave, wherein the warp threads of the loom are the MD threads of the fabric. The flatweave can, for example, have a plain weave.
Advantageously, the textile structure can comprise a woven material, a leno weave, a scrim or a knitted material or comprise such a material.
In the event that a woven material is used as the base structure, it should be pointed out that tubular woven materials are usually used for base structures of fabrics. These tubular woven materials are rotated by 90° for installation in the machine. As a result, the weft threads of the loom become the MD threads of the fabric, and the warp threads become the CD threads.
One alternative to this is the use of a flatweave.
In EP 0 425 523 B1, Paul Sudre describes the principle of producing a seamed fabric by means of a flatweave.
It is, however, also possible to use a flatweave which has the length of the fabric to be produced, and to connect the two front ends to one another. This also gives rise to an endless woven material structure which can be used as base structure or as part thereof.
Since, as a rule, the length of fabrics is considerably greater in the machine direction than their width in the CD direction, and the width of the loom is sufficiently large for the width of the machine, the necessary flatweave can most simply be implemented by weaving a sufficiently long piece, wherein the warp threads then become the MD threads in the fabric. A rotation by 90°, as in a tubular woven material, is not necessary. According to one aspect of the present invention, this is very advantageous for a fabric. While in a loom the warp threads can in principle be arranged as close beside one another as desired, two adjacent weft threads are necessarily spaced apart from each other as a result of the fact that the warp threads, for example in a plain weave, each change from top to bottom or vice versa between each weft thread pair. As a result of this forced spacing of the weft threads, together with the 90° rotation, such an MD thread density is impossible or possible only with very great difficulty when using very thin threads with conventional tubular woven materials. In the case of flatweaves, on the other hand, the warp threads serving as MD threads can be arranged as close to one another as desired, as a result of which fabrics having the properties described in the invention are comparatively simple to produce.
When flatweaves are used in the base structure, in advantageous embodiments the flatweave can be made endless by connecting the front ends. This connection can be made in particular by a welded connection. Here, ultrasonic welding and laser welding, in particular laser transmission welding, have proven to be suitable welding methods. If necessary, it may be advantageous to connect the two front ends with the aid of a connecting element. This can be, for example, one or more thread/threads, which is/are arranged in the CD direction and connected, in particular welded, to the MD threads of the two ends.
In advantageous embodiments, it may be provided for the base structure to be composed of two flatweaves which have been made endless. The two woven material loops can then be combined to form a two-layer structure. The two woven material loops can then be connected to one another for example by needling with one or more nonwoven layers. Alternatively or additionally, still further forms of connection may also be provided, such as for example sewing or welding the two layers.
The base structure can consist of a textile structure or comprise still further elements. In particular, the base structure can have still further textile structures, such as, for example, further woven materials.
Various materials can be used for the MD threads. Polyamides such as, for example, PA 6, PA 6.6 are suitable but also other polymers such as, for example, PET. If there are CD threads present, these can be built up from the same or another polymer.
Provision can be made for all the MD threads to be of the same type. However, different types of MD threads can also be used. As a result, for example, the fiber anchoring or the dimensional stability can be influenced. Advantageously, MD threads are monofilaments.
There are also various possibilities in the selection of the CD threads. In addition to monofilaments, multi-filaments or twisted threads can also be used.
Suitable twisted threads can consist of 4 or 6 filaments. The filaments used can have a diameter of 0.15—0.25 mm. For example, 0.2×2×2 twisted threads are very suitable.
In advantageous embodiments, all the CD threads can be designed as twisted threads. Here, both all the CD threads can consist of the same twisted thread. Alternatively, various twisted threads can also be used.
It may also be advantageous if both twisted threads and monofilaments are used as CD threads. In this case, preferably around 50% of the CD threads or more are designed as twisted thread.
A preferred embodiment, twisted threads and monofilaments can be provided alternately as CD threads. In this case, the proportion of twisted threads in the CD threads is 50% or less if, for technical reasons, for example to form seam loops, individual CD threads have to be removed.
In advantageous embodiments, the fabric can have one or more nonwoven layers. The layer or layers can be arranged on the paper side and/or the running side of the fabric.
Preferably, the weight of the fabric is between 750 g/m2 and 1250 g/m2, in particular between 900 g/m2 and 1100 g/m2. In special cases, weights of up to 1400 g/m2 are also possible. This is the total weight of base structure and nonwoven layers, if present.
It is very advantageous for the invention if the proportion by weight of the nonwoven layers corresponds at most to the proportion by weight of the base structure, in particular at most ⅔ of the proportion by weight of the base structure.
Usually, in felts for tissue production, the proportion by weight of the nonwoven fibers is greater than that of the base structure. Normally, 60% of the total weight is nonwoven fibers and 40% is base structure.
Since, in the fabrics proposed here, as a result of the combination of MD fiber density and MD fiber diameter, part of the functionality of the nonwoven layer is performed by the base structure, it is possible to reduce the quantity of nonwoven fibers. As a result, lighter and thinner felts are possible.
In preferred embodiments, the fabric can have a thickness of 3.5 mm and less, preferably between 2.5 mm and 3 mm! For this purpose, the thickness is determined under a pressure of 0.1 MPa.
The relative proportion of the nonwoven fibers on the overall fabric additionally decreases. Therefore, for example, it is possible to use felts in which the base structure and nonwoven layers each make up 50% of the proportion by weight. The base structure can even represent the greater proportion of the weight of the felt. If 60% of the total weight stems from the base structure and only 40% from the nonwoven fibers, then the proportion by weight of the nonwoven layers corresponds to only ⅔ of the proportion by weight of the base structure.
Over the service life of the fabric, the nonwoven layers are additionally highly compressed, as a result of which important fabric properties, such as the permeability, are reduced. As a result of a lower proportion of nonwoven fibers in the overall fabric, these losses resulting from compression of the nonwoven fibers are lower, and the properties of the fabric remain for longer in a tolerable range.
It may, however, also be advantageous if the paper side of the fabric has a certain quantity of nonwoven fibers. This quantity should not lie below 10% of the total weight of the fabric. It is also advantageous, at least for the upper side of the fabric touching the paper, to provide nonwoven fibers which have a fineness between 11 and 22 dtex.
The nonwoven fibers can be made of any suitable material, in particular from a polyamide but also from an elastomer such as, for example, thermoplastic polyurethane (TPU), melt fibers, bicomponent fibers or mixtures thereof.
With regard to the machine, the object is achieved by a machine for producing a tissue web which comprises a press device with at least one press nip. The machine has at least one fabric according to one aspect of the invention. This fabric runs through the at least one press nip together with the tissue web during the operation of the press device.
Tissue papers normally consist of pulp and are very lightweight papers. The mass per unit area is normally between 15 g/m2 and 30 g/m2. However, values of 10 g/m2 or 5 g/m2 are also likewise possible, as are papers with more than 30 g/m2.
In advantageous embodiments, the machine comprises what is known as a Crescent Former. Following the initial dewatering in the former, the fibrous web can then normally be transported on a fabric such as a felt into a press device, where further dewatering is carried out in the press nip.
Preferably, the press device has an extended nip, in particular a shoe nip. As compared with a likewise possible roll nip, here the residence time of the web in the nip is longer. As a result, in particular lower press pressures can be used. This is important during the production of tissue papers, since, as a result, the bulk of the web is maintained, which is an important quality parameter in tissue papers.
Furthermore, it may be advantageous if the press device has a wastewater trap which is set up to collect water which has been removed from the tissue web in the at least one press nip. As a result of using a felt according to one aspect of the invention—in particular in the extended nip or shoe presses—the dewatering performance of the press can also be increased by the fact that, in addition to the usual Uhle box dewatering, nip dewatering also takes place. Water (or a water-air mixture) from the tissue web is not only pressed into the fabric but also through the fabric. While the remaining part of the water pressed out is transported onward with the fabric to a Uhle box normally placed downstream, after the press nip this water enters the surroundings as splash water or spray mist. In addition, an escape of splash water forward counter to the running direction of the web is possible. This proportion of the dewatering is designated as nip dewatering. In order to prevent soiling of the press or uncontrolled wetting of the surroundings, it is advantageous to provide one or more wastewater traps in the press device. These are arranged such that the splash water can be collected and transported away.
By means of the fabric according to one aspect of the invention, and if necessary following the installation of a water trap, the dewatering performance of the press device can be increased considerably with very little outlay, even in existing tissue machines.
With regard to the method, the object is achieved by a method for producing a tissue web using a machine according to one aspect of the invention.
Advantageously, provision can be made for some of the dewatering to be carried out in the press device in the form of nip dewatering, and for this water thus removed to be wholly or partly collected by a wastewater trap.
In particular, provision can be made for the nip dewatering to make up more than 10%, in particular make up between 20% and 50%, of the total dewatering quantity of the press device.
In particular, it is advantageous if the tissue machine is operated at a high speed of more than 1200 m/min, in particular more than 1500 m/min or 1800 m/min. It is precisely at high production speeds that too low a dryness after the press is the limiting factor for an increase in the production speed. By means of the methods proposed here, the dryness after the press can be increased, which means that high production speeds are stably possible.
The invention will be explained in more detail below with reference to schematic figures, not to scale.
The fabric 2 of the machine 1 shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102019111441.1 | May 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/056701 | 3/12/2020 | WO | 00 |
