PRESS FELT

Description

[0001] In the production of paper and board, press felts are used to transport and dewater the fiber web in the press section. The main components of such felts are a load-bearing base structure and nonwoven layers, which are usually needled to the base structure. In most cases, fabrics are used for the base structures.

[0002] To increase the volume for liquid absorption and also to increase strength, the load-bearing base structure can comprise multiple fabric layers arranged one above the other. Felts with such base structures are described, for example, in EP 0 425 523 or EP 0 672 784 81. Likewise, as described in EP 2160495 B1, multiple layers of nonwoven fibers with different fiber counts can also be provided.

[0003] During operation, a press felt is exposed to repeated stresses in one or more press nips. In the press nip, the felt is compressed and after passing through the press nip, the felt expands again essentially to its original thickness. Because this process is repeated extremely frequently, the felt compacts after a short time. The state of the art explains this by the fleece layers being pressed together. Compaction of the fabric layers with the formation of a layer of reduced permeability also occurs. This changes important properties of the felt, such as its permeability. To compensate for this, it is known from EP2 678 472, for example, to provide a particularly fine fleece layer which is abraded during felt operation. The abrasion of the fine fleece fibers increases the permeability of the felt, while at the same time the compaction reduces the permeability.Although this allows a largely constant permeability of the felt to be achieved, the provision of the 'sacrificial fleece layer' involves additional costs and effort.

[0004] It is therefore an object of the invention to propose a press felt in which the tendency to compaction is reduced. It is also an object of the present invention to propose a felt that provides a large volume for liquid absorption.

[0005] The object is achieved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention can be found in the subclaims.

[0006] This proposes a press felt for a machine for producing a fibrous web, comprising a woven base structure and a nonwoven overlay attached thereto, wherein the base structure has a first fabric layer and a second fabric layer. According to the invention, at least the first fabric layer has longitudinal threads and transverse threads that intersect at intersection points, wherein the longitudinal threads and the transverse threads of the first fabric layer are welded together at at least 5% of the intersection points, in particular at at least 10% of the intersection points.

[0007] The terms "longitudinal thread" and "MD thread" as well as "transverse thread" and "CD thread" are synonyms.

[0008] As mentioned, basic structures with multiple fabric layers are advantageous in providing a large volume for liquid absorption ("void volume"). However, it has been shown that such felts also have a relatively strong tendency to compact. The inventor has recognized that part of this compaction effect is caused by the various fabric layers being partially pressed into each other by the loads in the nip. This is facilitated or reinforced by the fact that the threads of the fabrics are displaceable to a certain extent. Due to the continued loading in the nip, the individual MD or CD threads can be displaced in such a way that threads from one layer are pressed into the spaces between the other layer, thereby increasing the compaction of the felt and also reducing its permeability.To prevent this effect, or at least significantly reduce it, the inventor proposes that, at least in the first fabric layer, parts of the longitudinal threads and transverse threads crossing at intersection points be welded together. It is envisaged that at least 5% of the intersection points, in particular at least 10% of the intersection points, the longitudinal threads and the transverse threads of the first fabric layer are firmly bonded to one another, in particular welded to one another. In this way, the displacement of the threads is prevented or impeded, and the compaction effect described above is suppressed.

[0009] With an increase in the proportion of welded crossing points, e.g. to 15%, 20%, 25%, 30%, 35%, 40% or more, the fixation of the threads and the suppression of displaceability increases. However, this also increases the stiffness of the basic structure and thus of the entire felt. This is usually only possible or desired to a certain extent. Thus, it is provided that the longitudinal threads and the transverse threads of the first fabric layer are welded together at less than 60%, in particular at less than 50% of the crossing points. The advantageous effect of the invention can be increased if the second fabric layer also has longitudinal threads and transverse threads crossing at crossing points, wherein at at least 5% of the crossing points, in particular at at least 10% of the crossing points, the longitudinal threads and the transverse threads of the second fabric layer are materially connected to one another, in particular welded to one another.This further reduces the mobility of the threads in the two layers relative to each other. In the second layer, too, increasing the proportion of welded intersections, e.g., to 15%, 20%, 25%, 30%, 35%, 40%, or more, increases the fixation of the threads and suppresses their mobility. It is often advantageous if the longitudinal threads and the transverse threads of the second fabric layer are welded together at less than 60%, and especially at less than 50%, of the intersection points.

[0010] Even if in most of the examples described the basic structure has exactly two fabric layers, designs can also be provided in which the basic structure comprises one or more further layers, in particular one or more further fabric layers.

[0011] The material connection at the intersection points can be created in various ways.

[0012] For example, the use of bi-component fibers ("BiCo fibers") is possible. Bi-component fibers consist of two components, e.g., a core and a sheath. The two polymers have different softening or melting temperatures. The melting temperature of the core is higher than that of the sheath, so that the sheath can be melted at a certain temperature increase, thus creating connection points between the respective core fibers in the mixture of the existing matrix.

[0013] An alternative is to join the MD and CD fibers together using welded joints. Various processes, such as ultrasonic welding or transmission welding, are possible for creating the welded joint. NIR transmission welding is considered particularly advantageous. The polyamide threads commonly used are largely transparent to light in the NIR range between approximately 780 nm and 1100 nm.

[0014] It can now advantageously be provided that at least some of the longitudinal threads and / or the transverse threads of the first layer and / or the second layer absorb laser light of a wavelength in the range between 780 nm and 1100 nm completely or to a significant extent. (An absorption of more than 30%, in particular more than 40% of the corresponding light is considered significant absorption. Such threads are referred to below as absorbing threads.) When the fabric is suitably irradiated with light from this wavelength range, the light penetrates the non-absorbing threads relatively unhindered and is absorbed by the absorbing threads. This heats the contact point between the two threads to such an extent that welding occurs.

[0015] Advantageously, the absorbent threads can be made of the same polymer as the other threads, with an additional absorber additive added. This allows for particularly durable welded joints. Alternatively, compatible polymers can be used instead of the same polymer, e.g., polyamide 6 and polyamide 6.6.

[0016] Furthermore, quasi-simultaneous welding processes can also be used to create spot welds. This can sometimes eliminate the need for absorbers.

[0017] By selectively irradiating selected contact areas / crossing points, only these can be welded.

[0018] It is possible, for example, for this selective irradiation to take place in the form of regular patterns, e.g. in the form of straight lines, wavy lines, dot patterns, etc. The width of these lines or the diameter of the dots can in particular be chosen to be so large that several crossing points, in particular 2, 3, 4, 5 or more, are covered.

[0019] To achieve the desired effect, the intersection points with the material-fit connections are not located exclusively in a part of the covering - for example, a seam area - but are distributed over the entire surface of the covering and, in particular, are evenly distributed.

[0020] Such a uniform distribution can be achieved, for example, by weaving absorbent threads as CD threads or MD threads according to a fixed, predetermined pattern. For example, every 10th CD thread can be an absorbent thread. This results in a relatively small number of connection points. If every 4th CD thread, every 2nd CD thread, or even every CD thread is woven as an absorbent thread, the number of possible connection points increases.

[0021] Analogously, BiCo threads can also be woven according to the pattern described above.

[0022] There's considerable flexibility in the fabric layers described here. Some examples are listed below: The fabrics can be flat-woven. The fabrics can be processed in a roll-to-roll process. In particular, a fabric with the bonded joints can be produced as a roll and then cut to size during the production of the actual felt. The fabrics can be flat-woven and welded to form a continuous band. The fabrics can be endlessly woven. The fabrics can have a seam connection, preferably with a pin-thread seam. The fabrics can be single-layer or multi-layer. The fabrics can be woven from monofilaments and / or multifilaments and / or twisted yarns. The fabrics can also be leno fabrics.

[0023] In very advantageous embodiments, it can be provided that in the first fabric layer and / or the second fabric layer, at each crossing point at which the longitudinal threads and the transverse threads are connected to one another in a material-locking manner, there is no material-locking connection at the adjacent crossing points.

[0024] Adjacent crossing points are understood to be the four crossing points that are directly adjacent in the longitudinal and transverse directions.

[0025] Such an arrangement of the cohesive connections, in particular the welded connections, is advantageous because it allows a good fixation of the threads, but even with a comparatively high proportion of cohesively connected crossing points (e.g. 30%, 40% or 50%) the increase in the stiffness of the structure still remains tolerable.

[0026] Such a fabric layer is also easy to manufacture. For example, a plain weave fabric can be used as the first fabric layer. The fabric can be made of longitudinal threads that are transparent to light of a specific wavelength, while the transverse threads absorb this wavelength completely or partially. The bonded joints can then be realized as welded joints using transmission welding with light of this wavelength.

[0027] If the fabric is irradiated from one side with light of this wavelength, for example, using a laser, there are intersection points where the transparent thread lies above the absorbing thread. At these intersection points, the light penetrates the transparent thread and is absorbed by the absorbing thread, causing heating at the contact point and a cohesive bond.

[0028] However, due to the plain weave, the absorbent thread now lies above the transparent thread at the four adjacent intersection points. Therefore, the absorbent thread only heats up on its surface, not at the contact point. Thus, there is no material bond at these intersection points.

[0029] If the entire tissue is irradiated with the laser using this method, essentially 50% of the intersection points are welded together.

[0030] However, it is also possible to irradiate only parts of the tissue with the laser. This results in a smaller proportion of connected intersections.

[0031] Further advantageous embodiments of the invention are explained using exemplary embodiments with reference to the drawings. The features mentioned can be advantageously implemented not only in the illustrated combination, but also individually combined with one another. The figures show in detail: Figure 1 Fabric layer for a press felt according to one aspect of the invention Figures 2a, 2b, 2c show the compaction process suppressed by an embodiment according to the present invention. Figure 3 shows a press felt according to one aspect of the invention

[0032] The figures are described in more detail below.

[0033] Figure 1shows a fabric layer that can be used as a first fabric layer 1 or a second fabric layer 2 in a press felt according to one aspect of the invention. Shown is a plain weave fabric made of intersecting longitudinal threads 3 and transverse threads 4. Some of the transverse threads 6 are designed as absorbent threads 6. In the case of the Figure 1In the example shown, every second transverse thread 4 is designed as an absorbent thread 6. The remaining threads 3, 4 are made of a material such as a polyamide, which is completely or largely transparent to light in the NIR range. The absorbent threads 6 can, for example, consist of the same polymer to which an absorber additive is added. If the crossing points 5 are irradiated with light from a wavelength range that the absorbent threads 6 absorb - for example using a corresponding NIR laser - this light penetrates through the non-absorbing longitudinal threads 3 to the absorbing threads 6. These heat up primarily at the contact point between the two threads, resulting in a material-to-material connection between the two threads in the form of a welded joint.

[0034] Either all of these intersection points 5 can be welded, or only a part of them.

[0035] It should be noted that at some of the crossing points in Figure 1 the absorbing thread 6 runs above the longitudinal thread 3. If one also wants to create a material-tight connection at these intersection points, it is advantageous to irradiate the tissue from the opposite side.

[0036] If you irradiate only from one side, this also results in Figure 1 The fabric shown is a design in which at each crossing point 5, at which the longitudinal threads 3 and the transverse threads 4 are materially connected to one another, there is no materially connected connection at the adjacent crossing points 5.

[0037] In such a fabric layer 1, 2, both the displacement of the longitudinal threads 3 in the transverse direction and the displacement of the transverse threads 4 in the longitudinal direction are hindered or prevented.

[0038] The Figures 2a, 2b and 2cshow schematically the behavior of a press felt under load in which no crossing points 5 are materially connected to one another in any of the fabric layers 1, 2. The press felt has a first fabric layer 1 and a second fabric layer 2, which together provide the basic structure of the felt. A fleece layer 7 is provided on the first fabric layer 1. For the sake of simplicity of illustration, only the longitudinal threads 3.1, 3.2 of the fabric layers 1, 2 are shown. In the felt shown here, the first fabric layer 1 and the second fabric layer 2 are different, which is illustrated here by way of example by different diameters of the longitudinal threads 3.1 of the first layer 1 and the longitudinal threads 3.2 of the second layer 2. However, the two layers 1, 2 can also be of the same fabric type; in particular, they can be formed by folding and laying a single piece of fabric on top of one another, as described, for example, in EP 0 425 523.

[0039] Figure 2ashows the felt without external load. As in Figure 2b As indicated, for example, a load acts on the felt when passing through a press nip. The second fabric layer 2 is thereby pressed upward against the first fabric layer. This creates shear forces in the transverse direction on the longitudinal threads 3.1, 3.2, favored by the round shape of the threads.

[0040] Figure 2c shows the felt in compressed form. The external load compresses the fleece layer 7. On the other hand, the longitudinal threads 3.1, 3.2 also shift in the transverse direction, so that the two fabric layers 1, 2 are partially pressed into each other. The void volume of one fabric layer 1, 2 is thus partially filled by the threads of the other fabric layer 2, 1 and is no longer available for fluid absorption.

[0041] Figure 3 differs from Figure 2cmerely in that the felt is constructed according to one aspect of the present invention. This means that, in particular, at least 5% of the intersection points 5, in particular at least 10% of the intersection points 5, the longitudinal threads 3.1 and the transverse threads 4 of the first fabric layer 1 are integrally connected to one another, in particular welded to one another. Furthermore, it can also be provided that, at least 5% of the intersection points 5, in particular at least 10% of the intersection points 5, the longitudinal threads 3.2 and the transverse threads 4 of the second fabric layer 2 are integrally connected to one another, in particular welded to one another.

[0042] Here, too, the nonwoven overlay 7 is compressed. However, the longitudinal threads 3.1, 3.2 cannot deflect in the transverse direction. The transverse forces are absorbed by the cohesive connections at the intersection points 5. This prevents or at least reduces penetration of the first fabric layer 1 and the second fabric layer 2. The void volume of the fabric layers 1, 2 is barely reduced by penetrating threads 3, 4 of the other fabric layer 2, 1 and thus remains available for fluid absorption. List of reference symbols

[0043] 1First fabric layer 2Second fabric layer 3Longitudinal thread 3.1Longitudinal thread 3.2Longitudinal thread 4Cross thread 5Crossing point 6Absorbent thread 7Fleece overlay

Claims

1. Press felt for a machine for producing a fibrous web, comprising a woven base structure and a nonwoven support (7) attached thereto, the base structure having a first fabric layer (1) and a second fabric layer (2), at least the first fabric layer (1) having longitudinal threads (3, 3.1) and transverse threads (4) crossing at intersections (5), characterised in that at least 5% of the intersections (5), in particular at least 10% of the intersections (5), the longitudinal threads (3, 3.1) and the transverse threads (4) of the first fabric layer (1) are connected to one another in a material-locking manner, wherein the crossing points (5) with the material-locking connections are not located exclusively in a partial region of the press felt - in particular a seam region - but are distributed, in particular evenly distributed, over the entire surface of the press felt, and wherein the longitudinal threads (3, 3.1) and the transverse threads (4) of the first fabric layer (1) are welded together at less than 60%, in particular at less than 50%, of the crossing points (5).

2. Press felt according to one of the preceding claims, characterised in that the second fabric layer (2) also has longitudinal threads (3, 3.2) and transverse threads (4) crossing at intersections (5), the longitudinal threads (3, 3.2) and the transverse threads (4) of the second fabric layer (2) being connected to one another, in particular welded to one another, in a material-locking manner at at least 5% of the intersections (5), in particular at at least 10% of the intersections (5).

3. Press felt according to one of the preceding claims, characterised in that the basic structure also comprises a further layer, in particular a further fabric layer.

4. Press felt according to one of the preceding claims, characterised in that at least some of the longitudinal threads (3, 3.1, 3.2) and / or the transverse threads (4) of the first fabric layer (1) are absorbing threads (6) which absorb laser light of a wavelength which is in the range between 780 nm and 1100 nm in its entirety or to a significant extent.

5. Press felt according to claim 4, characterised in that at least every 10th transverse thread (4), in particular at least every 4th transverse thread (4), preferably every 2nd transverse thread (4) is an absorbent thread (6).

6. Press felt according to one of claims 4 or 5, characterised in that at least every 10th longitudinal thread (3, 3.1, 3.2), in particular at least every 4th longitudinal thread (3, 3.1, 3.2), preferably every 2nd longitudinal thread (3, 3.1, 3.2) is an absorbent thread (6).

7. Press felt according to one of the preceding claims, characterised in that in the first fabric layer (1) and / or the second fabric layer (2), there is no fabric-locking connection at the adjacent crossing points (5) at each crossing point (5) at which the longitudinal threads (3, 3.1) and the transverse threads (4) are connected to one another in a material-locking manner.