Covering and methods for producing a covering

Abstract

A covering (1) for a machine for producing or processing a fibrous web, wherein the covering (1) has a paper side (5) and a running side, comprising a base structure (3) and at least one further structure (2), wherein at least one further structure (2) consists of or comprises a layer of polymer foam (2), and this layer of polymer foam (2) provides the paper side (5) of the covering (1), wherein the at least one layer of polymer foam (2) has an anisotropic pore structure, characterized in that the layer of polymer foam (2) has a bulk density between 100 kg / m³ 3 and 700 kg / m² 3 exhibits.

Description

[0001] The present invention relates to a covering, in particular a press belt for a machine for the production or processing of a fibrous web, according to the preamble of claim 1, and to a method for producing a covering according to the preamble of claim 10.

[0002] In the production of fiber webs, especially in the production of paper webs, an aqueous fiber suspension is usually initially dewatered in the so-called former on a forming screen or between two forming screens, then further dewatered by pressing between two press felts and finally thermally dried.

[0003] For dewatering in presses, press felts are now almost exclusively used. These felts consist of a woven base structure onto which one or more layers of nonwoven fibers are applied. In particular, the paper-contacting side of the press felt is provided by a fine layer of nonwoven fibers.

[0004] Extensive prior art exists for such felts. European patent application EP 2 072 668 A1 serves as an example. In the felts described here, the web material contact surface is also provided by a layer of fibrous material.

[0005] However, although this design has been used for a long time, it does have some disadvantages. Firstly, there is a risk that the high pressure in the press nip will imprint the fiber structure of the nonwoven layer in contact with the paper onto the paper surface. This can lead to a reduction in paper quality due to felt marks. Secondly, it has long been known that after the paper web is dewatered in the press nip, a process called re-wetting occurs. This involves water from the nonwoven layer in contact with the paper returning to the paper web, thus reducing the dewatering effect of the press.

[0006] It is therefore an object of the invention to propose a covering that overcomes or reduces the problems of the prior art.

[0007] This problem is fully solved by a covering, in particular a press belt for a machine for the production or processing of a fibrous web, according to the characterizing portion of claim 1, and by a method for producing a covering according to the characterizing portion of claim 10.

[0008] A covering, in particular a press belt for a machine for the production or processing of a fibrous web, is proposed, wherein the covering has a paper side and a running side. The covering comprises a base structure and at least one further structure. According to the invention, the at least one further structure consists of or comprises a layer of polymer foam, and this layer of polymer foam provides the paper side of the covering.

[0009] Such a layer of polymer foam is permeable due to its porous structure, allowing water from the paper web to flow through it. However, unlike a nonwoven layer, the surface has no fibers or fiber segments that can imprint themselves on the paper surface.

[0010] Further advantageous embodiments of the press belt according to the invention are described in the dependent claims.

[0011] In advantageous embodiments of the invention, the paper-contacting side of the polymer foam layer outside a press nip has a smaller contact area with the paper web than conventional staple fiber nonwoven-based press felts. When compressed in a press nip, however, material components located beneath the uppermost pore layer can also become contact points with the paper web. This can significantly increase the contact area with the paper web, often resulting in a larger contact area than that of conventional press felts with a paper-contacting nonwoven layer.

[0012] Furthermore, it can be advantageously provided that at least one layer of polymer foam has an anisotropic pore structure. In such an anisotropic structure, the shape of at least a large proportion (often more than 50%, or even more than 80%) of the individual pores deviates from the isotropic spherical shape. Thus, in an advantageous embodiment, the pores can have a greater extent in the machine direction of the covering and in the transverse direction of the covering than in the thickness direction. Such a pore structure can be achieved, for example, by compressing a foam layer with an isotropic pore structure. Often, such an anisotropic pore structure allows water to be quickly channeled away from the paper web towards the base structure. The foam layer also typically provides less storage volume as a result.

[0013] In further advantageous embodiments, the covering may include at least one additional functional structure arranged between the base structure and the polymer foam layer or on the side of the base structure facing away from the paper. Such an additional functional structure may, for example, be a layer comprising or consisting of nonwoven fibers. However, the additional functional structure may also be other structures, such as another foam layer, a film, a membrane, or the like. In particularly advantageous embodiments, several additional functional structures may be provided, especially several nonwoven fiber layers.

[0014] In a particularly preferred embodiment of the invention, it can be provided that, under a compressive load of 3 MPa on the covering, the layer of polymer foam is compressed in the thickness direction by more than 30%, preferably by more than 40%, particularly preferably by more than 50%, compared with the initial thickness under a compressive load of 0.1 MPa.

[0015] In particular, it can also be provided that, under a compressive load of 5 MPa or 8 MPa on the covering, the layer of polymer foam is compressed in the thickness direction by more than 30%, preferably by more than 40%, particularly preferably by more than 50%.

[0016] The thickness of a covering, or its corresponding polymer foam layer, can be measured under pressure, for example, by placing a sample under the desired pressure in a computed tomography scanner or other suitable imaging analysis equipment. The resulting images allow for a very precise determination of the thickness of the individual components of the covering.

[0017] A polymer foam layer constructed in this way can be advantageous for use with the covering. The range of 3 MPa to 8 MPa represents the pressure range typically generated in a press nip. If such a covering, together with a paper web laid on it—and possibly another covering—is passed through a press nip, pressure is exerted on the polymer foam layer, compressing it accordingly according to this advantageous embodiment. The specified 0.1 MPa is intended to represent a typical base load that acts on the covering even without the action of the press nip, e.g., due to the weight of the wet paper web, etc. In a particularly preferred embodiment of the covering, the polymer foam layer is designed such that, upon subsequent pressure relief of the covering from 3 MPa (or 5 MPa or 8 MPa) to 0.1 MPa, the polymer foam layer essentially expands back to its original thickness.

[0018] In particularly advantageous configurations of the covering, the combination of compression within a press nip and subsequent expansion after the press nip can increase the dryness of the web and significantly reduce the rewetting described above. This effect was surprisingly observed in the applicant's trials. In this case, compressing the foam layer within a press nip allows the water to drain away more quickly due to the shortened drainage path. Furthermore, the expansion after the press nip alters the pore size and pore volume of the foam layer. This makes it more difficult for water contained in the pores and the base structure to re-enter the web. These two effects, individually or in combination, can lead to a significant increase in the dryness of the web.

[0019] In a further advantageous embodiment, the polymer foam layer can consist of, or comprise, an elastomer, in particular a polyurethane. In particular, a so-called polyurethane soft foam, and preferably a reticulated polyurethane soft foam, can be used. However, according to the invention, other materials can also be used for the polymer foam layer, such as silicone, polyester, polyether, etc.

[0020] In preferred versions, the polymer foam layer can have a basis weight between 60 g / m². 2 and 300 g / m² 2 , up to 500g / m² 2 or more.

[0021] In other preferred embodiments, the polymer foam layer can have a bulk density between 100 kg / m³. 3 and 700 kg / m² 3 exhibit.

[0022] The basic structure of the covering can advantageously comprise or consist of a woven fabric, a non-woven fabric or a permeable film structure.

[0023] In particular, when using a fabric, a single-layer or multi-layer fabric can be used.

[0024] Advantageously, the basic structure can have a permeability of more than 250 cfm, and especially more than 500 cfm. However, basic structures with a lower permeability can also be used in principle.

[0025] Particularly advantageously, the covering can have a permeability between 10 cfm and 250 cfm, especially between 25 cfm and 150 cfm.

[0026] The unit cfm (cubic feet per minute) is commonly used in evaluating the permeability of coverings. 100 cfm corresponds to approximately 0.0472 m³. 3 / s.

[0027] A fabric according to the present invention can be used as a press belt in the press section of a machine for producing a fibrous web – in particular a paper, cardboard, tissue or pulp web. One or more fabrics according to the invention can be used in such a press section.

[0028] With regard to the method, the problem is solved by a method for producing a covering, in particular a covering according to one of claims 1-9, comprising the steps: a) Providing a basic structure, b) Providing at least one further structure, wherein this further structure consists of or comprises a layer of polymer foam c) Connecting at least one further structure and the basic structure in such a way that this further structure provides the paper-contacting side of the covering. d) Compacting the polymer foam layer under the influence of pressure and / or temperature

[0029] The compaction process allows for a wide range of properties of the polymer foam layer to be specifically tailored to the requirements of the covering during its use. Examples, but not limited to, the following are listed below: Adjustment of the thickness of the polymer foam layer Changes in the anisotropy of the pore structure. Compaction allows the thickness-directed expansion of at least a large proportion of the pores to be reduced. This also allows for an adjustment of the permeability of the polymer foam layer. Furthermore, the compressibility or compression hardness and / or elasticity of the polymer foam layer can also be adjusted by compaction. Thus, for example, a wide range of specially adapted coverings can be produced using commercially available standardized polymer foams.

[0030] Another effect that can be achieved through compaction is a further improvement in the surface properties of the paper-contacting side of the covering. This can be achieved by applying pressure and / or temperature, causing the foam layer material to melt superficially. This allows the contacting foam elements or ribs to bond together, creating a material-bonded structure. Surfaces resulting from this process can exhibit increased mechanical stability and abrasion resistance.

[0031] It is also possible to use open-pore foams with larger pore diameters, which are then permanently compressed through compaction. This compression causes cell ribs, which were previously located below the upper pore layer, to act as contact points with the paper web. This results in an increased contact area with the paper web compared to uncompacted foam.

[0032] The compacted foams typically exhibit high permeability with low expansion in the thickness direction.

[0033] Further advantageous embodiments of the method are described in the dependent claims. Compaction can be achieved, for example, by compression at elevated temperature followed by cooling. Cooling can be carried out while maintaining the compaction pressure. In some cases, however, this step can be omitted entirely, or cooling can be carried out under reduced pressure (<90% of the compaction pressure, in particular <75% of the compaction pressure).

[0034] Suitable devices for compression include so-called double belt presses, which often feature heating and cooling zones. Optimal temperatures for compaction can vary depending on the application and material. For preferred applications, such as polyurethane flexible foams, the required temperatures can range between 150°C and 200°C.

[0035] Particularly when compaction occurs after bonding the foam layer to the base structure, another advantageous compaction method is to run the covering through a calender. The calender can have a roller nip, with at least one roller being heated. Advantageously, the heated roller is often the one that comes into contact with the foam layer.

[0036] Furthermore, it may be advantageous to provide that the procedure additionally includes the following step: b1) Providing an additional functional structure and arranging this additional functional structure between the basic structure and the layer of polymer foam or on the paper-facing side of the basic structure.

[0037] As already discussed in the description of the covering, several additional functional structures can also be incorporated. These additional functional structures can be a non-woven fiber layer, a film, a membrane, another foam layer, or similar materials.

[0038] Various methods are possible for joining the base structure to the polymer foam layer and, if applicable, further functional layers. For example, the connection can be made by needling, bonding, or welding, in particular laser transmission welding, as well as a combination of different suitable methods.

[0039] The steps of the proposed method do not necessarily have to be carried out in a fixed order. In particular, the compaction of the polymer foam layer can take place before or after joining the at least one additional structure to the base structure. Multiple compactions of the polymer foam layer are also possible. It is particularly advantageous that compaction can be performed both before and after joining.

[0040] Typically, suitable, commercially available polymer foams, such as polyurethane flexible foams, have thicknesses in the range of 2.5 mm to 9 mm, particularly between 3.5 mm and 7 mm. Such foams can be used as starting material and then further compacted in the process according to the invention.

[0041] In particularly preferred embodiments of the process, the thickness of the polymer foam layer after compaction is less than 30%, preferably less than 25%, and particularly preferably less than 20% of the initial thickness of the foam layer.

[0042] Regarding the determination of the foam thickness, it should be noted that this – unlike the determination of the compressibility and elasticity of the covering – is determined without pressure, i.e., without applying a base load of 0.1 MPa.

[0043] If the polymer foam layer is compacted after being attached to the base structure, it is possible that parts of the foam layer may penetrate or be pressed into depressions in the supporting structure. This portion is not used to determine the thickness of the foam layer.

[0044] In further advantageous embodiments, the polymer foam can have a pore density between 10 and 80 pores per inch (PPI), preferably between 30 and 60 PPI, before compaction.

[0045] PPI is an internationally accepted unit of measurement for the permeability of porous materials and describes a linear structural density.

[0046] In further advantageous embodiments, the polymer foam has a bulk density between 15 and 60 kg / m³ before compaction. 3 , preferably between 25 and 35 kg / m² 3 on.

[0047] The invention will be further explained below with reference to schematic figures. Fig. 1a, Fig. 1b, Fig. 1c and Fig. Figure 2 shows two embodiments of a covering according to the invention.

[0048] In Fig. Figure 1 shows a rough schematic of the structure of a covering 1, which can be used as a press belt 1, but also as a forming belt 1. In the embodiment shown here, the covering 1 comprises a fabric 3, which provides the basic structure 3. A layer of polymer foam 2 is attached to this basic structure 3. This layer can, for example, consist of a polyurethane soft foam. This layer of polymer foam 2 also provides the paper-contacting side of the covering 5. The pores 4 of the layer of polymer foam 2 are in the Fig. The covering shown in 1 is anisotropic. This can be achieved, for example, by compacting a standard polymer foam, which typically has isotropic pores, through a compaction step using pressure and / or temperature. This changes not only the thickness of the foam layer 2 but also the shape of the pores 4. They are deformed in the thickness direction.

[0049] An exemplary example will be used to illustrate a possible manufacturing process for a covering such as in Fig. Figure 1 will be shown and explained. In this example, a woven base structure 3 is provided first. This can be woven from polyester filaments. Additionally, a foam in the form of a reticulated polyurethane soft foam is provided. In this example, it has a thickness of 4 mm and a pore density of 45 PPI. Laser transmission welding is a suitable method for joining the layers of polymer foam 2 to the base structure 3. In this example, a NIR line laser with a wavelength of 940 nm is used. This was pressed with a joining pressure of approximately 20 N / cm. It is particularly advantageous for laser transmission welding if the polymer foam 2 absorbs the laser light completely or partially (usually the absorption is > 70%), while the base structure 3 is completely or largely transparent to the laser light.In this example, this was achieved by coloring the polymer foam, using an anthracite-colored foam. By choosing a polyester base fabric, the laser light was able to first penetrate the base structure 3 and was then absorbed by the polymer foam. This generated the heat necessary for welding at the joint between the base structure 3 and the foam layer 2. This is a common principle in laser transmission welding.

[0050] The laminate thus bonded was then compacted under pressure at a temperature of approximately 190°C.

[0051] The Fig. 1b and Fig. Figure 1c shows a covering according to the invention, which in addition to the features described in Fig. The components described in 1a may also have one or more additional functional structures 6, 6a, 6b. These are implemented here as layers of nonwoven fibers, but other functional structures such as films, membranes, foam layers, or similar materials are also conceivable. The covering in Fig. 1b has a fiber fleece layer 6 which is arranged between the base structure 3 and the foam layer 2.

[0052] The in Fig. The covering shown in 1c has three layers of fiber fleece 6, 6a, 6b, where the individual layers of fiber fleece in this example can comprise fibers of different fineness.

[0053] To further illustrate this, the following example 1 shows a possible manufacturing process for a press belt, as described in Fig. Figure 1c is shown and explained in more detail. Although the described manufacturing process is advantageous, the invention is not limited to this exemplary process.

[0054] In Example 1, a pressed felt was first produced consisting of a woven fabric 3 and three layers of nonwoven fiber 6, 6a, 6b made of a polyamide material. The nonwoven layers were bonded to the woven fabric 3 by needling. This felt had a basis weight of 1100 g / m². 2 and a permeability of approximately 62.8 CFM. Furthermore, an open-cell polyurethane (PU) flexible foam was used. This flexible foam had a basis weight of 120 g / m². 2The polyurethane (PU) foam had a thickness of 4 mm and a pore density of 30 PPI. In Example 1, the PU foam was anthracite-colored and absorbed NIR laser radiation in the 940 nm range. The foam layer was joined using NIR laser transmission welding. For this purpose, the laser was directed at the running side of the belt. The polyamide material of the original felt was almost transparent to the laser light, but was absorbed by the foam layer at the interface with the nonwoven fiber layer, generating heat. This melted the material of at least one of the two contacting layers, resulting in the layers being welded together. The welding process was further enhanced by applying a certain amount of joining pressure to the felt-foam sandwich. This joining pressure was achieved by applying 25 N of pressure to the felt-foam sandwich, which was supported on a solid surface, using the laser's roller optics with a line width of 31 mm.In this example, the laser's power output was 1,350 W.

[0055] After the entire press band was welded, the foam layer was compacted. This compaction was carried out at a line load of 6 kN / m and a temperature of 195°C. These values ​​can be adjusted depending on the application. A typical temperature range for compaction is between 150°C and 210°C. The line load can also be chosen higher or lower.

[0056] The resulting press strip from Example 1 had a basis weight of 1229 g / m². 2 and a permeability of 63.7 CFM. Due to compaction, the pores of the soft foam layer exhibited an anisotropic structure. Under a load of 3 MPa, the foam layer was compressed by more than 30%.

[0057] In experiments conducted by the applicant, a surprisingly higher dryness content was achieved in a paper using the press belt described in Example 1 than with comparable felts without a paper-contacting foam layer 2.

[0058] Fig. Figure 2 shows a schematic representation of a covering according to the invention, in which the basic structure 3a is provided by a permeable film structure 3a. The film structure 3a can be a single, perforated film. Alternatively, such a film structure can itself be composed of a laminate of different films. The connection between the foam layer 2 and the basic structure 3a can be achieved, for example, by welding or by bonding.

Claims

[1] A covering (1) for a machine for the production or processing of a fibrous web, wherein the covering (1) has a paper side (5) and a running side, comprising a base structure (3) and at least one further structure (2), wherein at least one further structure (2) consists of or comprises a layer of polymer foam (2), and this layer of polymer foam (2) provides the paper side (5) of the covering (1), wherein the at least one layer of polymer foam (2) has an anisotropic pore structure, characterized by , that the layer of polymer foam (2) has a bulk density between 100 kg / m³ 3 and 700 kg / m² 3 exhibits. [2] Covering (1) according to claim 1, characterized by , that more than 50% of the pores deviate from the isotropic spherical shape and have a greater extent in the machine direction of the covering (1) and in the transverse direction of the covering (1) than in the thickness direction. [3] Covering (1) according to claim 1 or 2, characterized by , that the covering (1) comprises at least one additional functional structure (6, 6a, 6b) which is arranged between the basic structure (2) and the layer of polymer foam (2) and / or on the paper-away side of the basic structure (3), wherein this additional functional structure (6, 6a, 6b) in particular comprises or consists of nonwoven fibers. [4] Covering (1) according to any one of the preceding claims, characterized by , that when the covering (1) is subjected to a compressive load of 3 MPa, the layer of polymer foam (2) is compressed by more than 30% in the thickness direction, compared to the initial thickness when subjected to a compressive load of 0.1 MPa. [5] Covering (1) according to claim 4, characterized by , that upon subsequent pressure relief of the covering (1) from 3MPa to 0.1 MPa the layer of polymer foam (2) essentially expands back to its original thickness. [6] Covering (1) according to any one of the preceding claims, characterized by that the layer of polymer foam (2) consists of or comprises an elastomer, in particular a polyurethane. [7] Covering (1) according to any one of the preceding claims, characterized by , that the basic structure (2) comprises or consists of a fabric, a mat or a permeable film structure. [8] Method for producing a covering (1), in particular a covering (1) according to any one of claims 1-7, comprising the steps: a) Providing a basic structure (3), b) Provide at least one further structure (2), c) Connecting at least one further structure (2) and the basic structure (3), characterized by , that the further structure (2) consists of or comprises a layer of polymer foam (2), wherein the layer of polymer foam (2) has a bulk density between 100 kg / m³ 3 and 700 kg / m² 3exhibits and provides the paper-contacting side (5) of the covering (1), and the method also includes the step: d) Compacting the layer of polymer foam (2) under the influence of pressure and / or temperature. [9] Method according to claim 8, characterized by , that the procedure additionally includes the step: b1) Providing an additional functional structure (6, 6a, 6b) and arranging this additional functional structure (6, 6a, 6b) between the basic structure (3) and the layer of polymer foam (2) and / or on the paper-away side of the basic structure (3). [10] Method according to claim 9, characterized by , that the additional functional structure (6, 6a, 6b) comprises or consists of nonwoven fibers. [11] Method according to any one of claims 8 to 10, characterized by, that the compaction of the polymer foam layer (2) takes place before and / or after the bonding of the polymer foam layer (2) with the base structure (3). [12] Method according to any one of claims 8 to 11, characterized by , that the layer of polymer foam (2) is compacted to less than 30% of the initial thickness by compaction. [13] Use of a covering (1) according to any one of claims 1 to 7 as a press belt (1) in the press section of a machine for producing a fibrous web.

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