Method and forming belt for producing a fibre material web

Abstract

A method for producing a structured fibrous web, in particular a tissue web, includes bringing a fibrous suspension into contact with at least one structured forming belt and dewatering by using at least one dewatering element, in particular a suction element. The at least one structured forming belt includes a layer of polymer foam providing a paper-contacting side of the structured forming belt. The structure of the foam layer is at least partially transferred to the fibrous web. A structured forming belt and a machine for producing a fibrous web are also provided.

Claims

1. A method for producing a structured fibrous web or a tissue web, the method comprising the following steps: providing at least one structured forming belt including a layer of polymer foam providing a paper-contacting side of the at least one structured forming belt, the pores in the layer of polymer foam having a pore density of less than 30 PPI; bringing a fibrous suspension into contact with the at least one structured forming belt; using at least one dewatering element or a suction element to dewater the fibrous suspension; and at least partly transferring a structure of the foam layer to the fibrous web.

2. The method according to claim 1, which further comprises carrying out the transferring step by transferring at least part of the structure of the layer of polymer foam to the fibrous web as a pore structure of the layer of polymer foam.

3. The method according to claim 1, which further comprises carrying out the transferring step by transferring at least part of the structure of the layer of polymer foam to the fibrous web as an external structure having been incorporated in the layer of polymer foam.

4. The method according to claim 3, which further comprises incorporating the external structure in the layer of polymer foam by embossing, branding, etching, cutting, or punching.

5. A structured forming belt or clothing for a machine for producing a fibrous web or a tissue web, the structured forming belt comprising: a paper-contacting side; a backing side; a support structure; and at least one layer of polymer foam providing said paper-contacting side being suitable for transferring a structure to the fibrous web, said at least one layer of polymer foam having a pore density of less than 30 PPI.

6. The structured forming belt according to claim 5, wherein said structure is a uniform or non-uniform structure.

7. The structured forming belt according to claim 5, wherein said at least one layer of polymer foam has an embossed, branded, etched, cut, or punched external structure.

8. The structured forming belt according to claim 5, wherein said at least one layer of polymer foam is formed of or includes an elastomer or a polyurethane.

9. The structured forming belt according to claim 5, wherein said at least one layer of polymer foam is formed of or includes polyamide, polyester, or polyethylene.

10. The structured forming belt according to claim 5, wherein said at least one layer of polymer foam has an anisotropic pore structure.

11. The structured forming belt according to claim 5, wherein said at least one layer of polymer foam is adhesively bonded, welded or NIR transmission welded to said support structure.

12. The structured forming belt according to claim 10, wherein compression of the at least one layer of polymer foam results in pores having the anisotropic pore structure that are deformed in a thickness direction of said at least one layer of polymer foam.

13. A machine for producing a fibrous web or a tissue web, the machine comprising at least one structured forming belt according to claim 5.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) The invention will be further explained hereunder by means of schematic, not-to-scale drawings in which:

(2) FIG. 1 shows an embodiment of a structured forming belt according to the invention;

(3) FIG. 2 schematically shows the formation of the fibrous web on the forming belt;

(4) FIG. 3 shows a fragment of the surface of a roller for transferring an external structure to a layer of polymer foam;

(5) FIG. 4 shows a view of a structured forming belt according to the invention.

DESCRIPTION OF THE INVENTION

(6) The construction of a potential embodiment of the structured forming belt 1 is shown in FIG. 1. The structured forming belt 1 in the embodiment shown here comprises a woven fabric 3 which makes available the support structure 3. A layer of polymer foam 2 is fastened to said support structure 3. Said layer can be composed of a polyurethane soft foam, for example. Said layer of polymer foam 2 also makes available the paper-contacting side 5 of the structured forming belt 1. The pores 4 of the layer of polymer foam 2 in the clothing shown in FIG. 1 are anisotropic. This can be implemented, for example, in that a standard polymer foam which usually has isotropic pores has been compacted by way of a compacting step by means of pressure and/or temperature. On account thereof, apart from the thickness of the foam layer 2, the shape of the pores 4 is also modified. Said pores 4 are deformed in the thickness direction.

(7) A potential production method for a structured forming belt as is shown in FIG. 1 is to be explained by way of an exemplary example. In the example, a woven support structure 3 is first made available. Said support structure 3 is woven from polyester filaments. Moreover, a foam, for example in the form of a reticulated polyurethane soft foam, is made available. Said foam in the example has a thickness of 4 mm and a pore density of 45 PPI. However, a polymer foam having a pore density of less than 45 PPI, in particular also of less than 30 PPI, can also be advantageously used.

(8) Laser transmission welding represents a suitable method for connecting the layers of polymer foam 2 to the support structure 3. A NIR laser having a wavelength of 940 nm is used in the example. Said NIR laser was pressed thereonto at a joining pressure of approx. 20 N/cm. In laser transmission welding it is particularly advantageous for the polymer foam 2 to fully or partially absorb the laser light while the support structure 3 is fully or largely transparent to the laser light. This was achieved in the example by dying the polymer foam, an anthracite-colored foam being used herein. On account of the choice of a basic woven fabric of polyester, the laser light was able to first penetrate the support structure 3 and said laser light was thereafter absorbed by the polymer foam.

(9) The heat required for welding was thus generated at the connection location between support structure 3 and foam layer 2. This is a principle that is usual in laser transmission welding.

(10) The laminate thus connected was then compacted under pressure at a temperature of approx. 190 C. The resulting clothing 1 had a permeability of 400 CFM at a thickness of 1.07 mm (measured at 6 kPa pressure). The proportion of the support structure 3 in the example herein was 0.49 mm, the proportion of the foam layer 2 was 0.58 mm. At an initial thickness of the foam of 4 mm, said foam was compacted by the method to 14.5% of the initial thickness thereof.

(11) At a pressure of 50 kPa, the laminate 1 was compressed to 0.91 mm, wherein the thickness of the foam layer 2 was 0.42 mm. At this pressure, the foam layer was thus further compressed by 27%. When releasing the pressure to 6 kPa, the foam layer expanded again to the initial thickness thereof (within the range of measuring accuracy).

(12) The formation of the fibrous web on the forming belt from FIG. 1 is schematically illustrated in FIG. 2. The process is intended to explain the creation of a structured fibrous web in an exemplary manner. A suspension having fibers 6, in particular cellulosic fibers 6, herein is applied to the structured forming belt 1. The dewatering in FIG. 2 is performed from the top to the bottom, that is to say that the water first runs through the layer of polymer foam 2 and then through the support structure 3. The dewatering process can be supported by a dewatering element (not illustrated in FIG. 2), for example a suction box, which is disposed on that side of the forming belt 1 that faces away from the paper. The fibers 6 in this process are deposited on the paper-contacting side 5 of the forming belt 1. This paper-contacting side 5 is made available by the layer of polymer foam 2. On account of the pore structure of the polymer foam 2, some of the fibers 6 by way of comparatively large pores 4 can invade the layer of polymer foam 2 and are deposited in said pores 4. In this way, the structure of the forming belt 1, in particular the pore structure of the layer of polymer foam 2, is at least in part transferred to the fibrous web. This effect of the deposition of fibers in the pore structure can be supported by an appropriate choice of the polymer foam. In general, foams having a pore density of less than 45 PPI, in particular of less than 30 PPI, can thus be advantageous. However, depending on the suitability of the fibrous material (fiber length, degree of fibrillation), foams having other pore densities can also be successfully used.

(13) A structured fibrous web which has been produced by means of a method according to the invention can have great advantages, for example in terms of thickness and porosity, in relation to a comparable non-structured fibrous web. On account of the greater thickness, fibrous webs having a lower mass per unit area which nevertheless have all desired product characteristics can also be produced. On account of the saving in terms of fibrous material that can thus be achieved, the method is also very advantageous in economic terms.

(14) By way of the test result hereunder it is to be illustrated as a way of example which effects can be achieved by a structured fibrous web produced according to the invention as compared to a web that is formed on a conventional SSB screen:

(15) TABLE-US-00001 Mass per Porosity Forming belt unit area Thickness Density (Bendtsen) 1. SSB 86.2 [g/m.sup.2] 132 [m] 0.653 [g/cm.sup.3] 421 [ml/min] screen 2. Structured 81.5 [g/m.sup.2] 149 [m] 0.547 [g/cm.sup.3] 955 [ml/min] forming belt

(16) The increased thickness at a lower mass per unit area as well as the significantly increased porosity of the structured product are particularly conspicuous herein.

(17) FIG. 3 shows a fragment of the surface of a roller for transferring an external structure to a layer of polymer foam 2. The roller in the example shown here has both raised structural elements 11a, 11 b which are embossed in a layer of polymer foam 2. Moreover, the fragment in FIG. 3 has a multiplicity of structural elements 10 which are embodied as round depressions in the roller surface. These structural elements are transferred as raised elements to the foam layer 2.

(18) In the case of the example shown in FIG. 3, both the roller surface as well as the raised structural elements 11a, 11b are embodied from metal. However, it can also be provided that said raised structural elements 11a, 11b are fully or partially composed of a non-heat-conducting material, for example of a polymer. While transferring the structural elements 10, 11a, 11 b in principle can be performed in a separate operating step prior to or subsequent to the production of the clothing on the polymer foam 2 by means of such a or similar roller, the transfer is however often advantageously performed conjointly with compacting the foam layer. In this way, one process step in the production of the clothing can be dispensed with. Moreover, no additional devices are required for this transfer.

(19) FIG. 4 finally shows a heavily enlarged view of a structured forming belt 1 according to the invention. The view is made onto the paper-contacting side of the forming belt 1. The layer of polymer foam 2 can be seen, and the underlying woven fabric 3 of the support structure 3 can be seen through the pores 4. In the process of sheet formation, fibers 6 will be deposited both on the webs 7 of polymer material as well as fully or partially penetrate the pores 4 of the layer of polymer foam. The diameter d of such a pore in FIG. 4 is approx. 1 mm. However, in other advantageous forming belts, smaller pores, for example having diameters of 750 m, 500 m or less, or else larger pores having diameters of 1.5 mm, 2 mm or more, can also be used. The pore sizes in a structured forming belt will usually have a certain distribution.