Method for producing kraft paper

Abstract

The invention relates to a method for producing kraft paper, having the following steps: forming a fibrous web; shaking the fibrous web; pressing the previously partially dewatered fibrous web in a pressing section (5); further drying the fibrous web in a drying section (6); and winding up the paper web formed from the fibrous web in a winding station (7), wherein the fibrous web is embossed in an embossing station (84) after being dried in the drying section (6), and the fibrous web has a dry content of at least 85 wt.% when the fibrous web enters the embossing station (84). The invention also relates, inter alia, to an embossing station and an installation for producing kraft paper.

Description

In one aspect, the present invention relates to a method for producing kraft paper. In further aspects, the invention relates to an embossing station for a kraft paper production system, a kraft paper production system, and use of an embossing station. Various methods for producing kraft paper are known in the prior art. Kraft paper is a specific paper type that is characterised, among other things, by the fact that it has a particularly high strength and is used for various packaging applications due to this property, such as the production of paper bags. In addition to strength, other parameters are also important depending on the specific application of the kraft paper, such as extensibility or air permeability. Kraft papers that are embossed or have an embossed pattern are known in the state of the art. These are typically regular or irregular embossments that give the kraft paper a predetermined surface structure with which visual or functional effects can be achieved. According to prior art methods, such embossed kraft papers are produced by subjecting the still moist fibrous web to an embossing step. Typically, an embossing station is provided for this purpose, which is arranged the dryer section of a system for the production of kraft paper. Here, the fibrous web can have a dry content of between 60 wt.% and 80 wt.%. However, performing the embossing step at this stage of the method may cause the fibrous web to wrinkle, which is disadvantageous in terms of process stability and may optionally also cause the fibrous web to tear (web break). In particular, an object of the present invention may thus be seen in overcoming these and other disadvantages of the prior art. The invention thus preferably relates to a method for producing kraft paper, in particular sack kraft paper, in which, in a first step, a fibrous web running in the machine flow direction is formed. This takes place in particular by applying a fibrous suspension to a wire section by means of a headbox. A press section, in which mechanical dewatering and simultaneous compaction of the fibrous web is performed, is arranged downstream of the wire section. This may typically be performed with multiple press nips, the first of which is usually designed as a suction press with double felt guides and the others as roll presses or shoe presses, which have either double or single felt guides. The mechanically pressed fibrous web may be subjected to one or more drying processes in order to achieve the desired dry content. Before, between or after one or more drying processes, further production and / or processing steps may be provided, for example compacting the fibrous web or calendering the fibrous web. In particular, the fibrous web may be compacted between two drying steps, i.e. after a pre-drying step in a pre-dryer section and before an after-drying step in an after-dryer section. This may increase the extensibility of the paper in the machine direction. This step may, for example, be performed in a so-called compacting apparatus, such as, for example, a Clupak device. The embossing of the fibrous web may in particular be performed in an embossing station and after the final drying step, wherein the embossing may be performed with thermo rolls and / or embossing rolls. After the last processing step, i.e. embossing, the finished paper web may be wound up on the reeling unit and then further processed to the desired finished roll width. In particular, the dry content of the fibrous web may be at least 85 wt% when it enters the embossing station or is embossed. Surprisingly, it was discovered that embossing at such a high dry content can reduce or completely prevent creasing of the fibrous web, while still achieving a high-quality embossing result. This is particularly the case if the extensibility of the kraft paper, in particular according to ISO 1924-3:2005, exceeds 5%, preferably exceeds 7.5%, in the cross direction and optionally also in the machine direction. The embossing pattern created on the fibrous web may be designed in a variety of ways. In particular, the embossing pattern is formed by a plurality of embossments, which may be connected to each other or separated from each other by unembossed planar portions. Typically, the individual embossments have small dimensions, namely an embossing depth of up to 0.3 mm, a length of up to 0.9 mm and a width of at most 0.6 mm. An exemplary embossing pattern consists of pyramidal embossments with a rectangular base, wherein the embossing depth is about 0.3 mm. The dimensions of the base may be about 0.9 mm (in the machine direction) x about 0.6 mm (in the cross direction). Neighbouring embossments in the cross direction of the paper may form embossment rows extending parallel or at a slight angle (for example up to 5°) to the cross direction. The embossments of consecutive embossment rows in the machine direction may have a distance of about 2 mm to 3 mm. Optionally, the embossments of consecutive embossment rows in the machine direction are not directly above each other, but are offset in the cross direction. The offset may be about 0.6 mm. Optionally, embossing may be performed at ambient temperature, i.e. without heating or cooling the embossing roll or another part of the embossing station. Typically, the ambient temperature is between 20°C and 40°C, depending on local conditions. During embossing, small amounts of material may be removed from the fibrous web, and may accumulate on the embossing rolls and impair the further embossing process. It may therefore be provided that the embossing roll is cleaned continuously or at certain intervals during the process. Such cleaning may be performed by a cleaning brush which is passed over the outer surface of the embossing roll. Optionally, a cleaning agent, such as a water-air mixture, may also be applied to the surface of the embossing roll in order to improve the cleaning effect in conjunction with the cleaning roll. In particular, the invention also relates to an embossing station for a system for the production of kraft paper. The embossing station may have an embossing roll and a counter roll. In particular, the embossing profile is defined by the surface structure of the embossing roll. Optionally, the counter roll may have a smooth but elastic surface. The hardness of the surface of the counter roller, measured according to Pusey & Jones (ISO 6123-1), may be between 25 and 35. To clean the surface of the embossing roll, the embossing station may comprise a cleaning brush and optionally also an application or spraying apparatus for applying the cleaning agent. The cleaning brush may be arranged to be movable so that the brush can be moved back and forth parallel to the axis of rotation of the embossing roll. A guide and a drive may be provided for this purpose, for example. Further, the embossing station may have any of the features described in the context of the method or another aspect of the present invention. Optionally, the invention also relates to the use of such an embossing station in the production of kraft paper. Optionally, the invention also relates to a system for the production of kraft paper, i.e. a kraft paper production system, comprising an embossing station as described herein. In addition to the embossing station, such a kraft paper production system may comprise at least the following system components: headbox, wire section, press section, dryer section, reeling unit. Optionally, the dryer section may comprise multiple separate dryer sections. Additionally, a compacting apparatus (for example a Clupak unit) may be provided. In particular, the embossing station is arranged downstream of the dryer section and upstream of the reeling unit. The invention optionally also relates to a kraft paper as such that is embossed, in particular dry-embossed. It should be noted that in all aspects of the invention, any features described in relation to other aspects may be provided. For example, the embossing station may have features described in relation to the method according to the invention and vice versa. In particular, a method for the production of kraft paper is described, which comprises the following steps: - forming a fibrous web extending in the machine direction by continuously applying a fibrous suspension to a wire section by a headbox, - shaking the fibrous web transversely to the machine direction, i.e., in the cross direction, using a shaking apparatus, - pressing the fibrous web, which has been partially dewatered in the wire section, in a press section, - further drying of the fibrous web in a dryer section, - winding the paper web formed from the fibrous web in a reeling station. Optionally, it is provided that, after drying in the dryer section, the fibrous web is embossed in an embossing station. Optionally, it is provided that the fibrous web has a dry content of at least 85 wt% when it enters the embossing station. Optionally, it is provided that the fibrous web is provided with an embossing structure during embossing, which is formed from an arrangement of embossments, wherein the embossments have a height of at least 0.1 mm, in particular between 0.1 mm and 2 mm. Optionally, it is provided that the embossments have a rectangular, spherical-segmentshaped, pyramidal or prismatic geometry. Optionally, it is provided that neighbouring embossments are directly adjacent to one another in the machine direction and in the cross direction of the fibrous web. Optionally, it is provided that neighbouring embossments are spaced apart by a planar portion of the fibrous web, wherein the distance between neighbouring embossments in the machine direction of the fibrous web is at least 0.5 mm, in particular between 0.5 mm and 5.0 mm, and / or wherein the distance between neighbouring embossments in the cross direction of the fibrous web is at least 0.5 mm, in particular between 0.5 mm and 5.0 mm. Optionally, it is provided that the embossing station comprises an embossing roll, wherein the embossing roll is equipped with an embossing profile on its surface. Optionally, it is provided that the embossing profile of the embossing roll is cleaned, in particular during embossing, by a cleaning brush, wherein a cleaning agent, in particular a water-air mixture, is applied to the embossing profile. Optionally, it is provided that the cleaning brush rotates about an axis of rotation aligned substantially parallel to the axis of rotation of the embossing roll, and / or in that the cleaning brush is moved back and forth substantially parallel to the axis of rotation of the embossing roll. Optionally, it is provided that the surface temperature of the embossing roll is between 20°C and 100°C during embossing. Optionally, it is provided that the embossing station is an embossing station with one or more of the features described herein. An embossing station for embossing a fibrous web for a system for the production of kraft paper is also described, wherein the embossing station comprises an embossing roll and a counter roll, wherein an embossing nip is provided between the embossing roll and the counter roll, through which the fibrous web is guided for embossing, wherein the embossing roll is provided with an embossing profile on its surface, and wherein the counter roller has an elastic surface. Optionally, it is provided that the embossing station comprises a cleaning brush for cleaning the embossing profile of the embossing roll. Optionally, it is provided that the embossing station comprises an application apparatus for applying a cleaning agent, in particular an air-water mixture, to the embossing profile of the embossing roll. Optionally, it is provided that the cleaning brush is configured as a brush roll and has a rotary drive for rotating the cleaning brush about an axis of rotation, wherein the axis of rotation of the cleaning brush is arranged substantially parallel to the axis of rotation of the embossing roll. Optionally, it is provided that the cleaning brush has a displacement drive for moving the cleaning brush back and forth substantially parallel to the axis of rotation of the embossing roll. Optionally, it is provided that an adjusting apparatus is provided for adjusting the rolling pressure exerted on the fibrous web in the embossing nip. A system for the production of kraft paper is also described, which comprises the following system components: - a wire section for forming a fibrous web by continuously applying a fibrous suspension along the machine flow direction by a headbox, - a shaking apparatus for shaking the fibrous web transversely to the machine flow direction, i.e. in the cross direction, - a press section for pressing the fibrous web that has been partially dewatered in the wire section, - a dryer section for drying the fibrous web, - an embossing station arranged between the dryer section and the reeling station, - a reeling station for winding the paper web formed from the fibrous web. Optionally, it is provided that the embossing station is an embossing station with one or more of the features described herein. Optionally, it is provided that the dryer section comprises a pre-dryer section and an after-dryer section, and that a compacting apparatus, in particular a Clupak device, is arranged between the pre-dryer section and the after-dryer section. Use of an embossing station having one or more of the features described herein in a method of producing kraft paper is also disclosed. Optionally, this may be a method having one or more of the features described herein. A kraft paper with dry-embossed embossments is also described. The kraft paper may have an extensibility according to ISO 1924-3:2005 in the cross direction and optionally also in the machine direction exceeding 5%, preferably exceeding 7.5%. Optionally, it is provided that the embossments are pyramidal with a rectangular base. Optionally, it is provided that the embossments are arranged in embossment rows, wherein the embossment rows extend in the cross direction of the kraft paper or at an angle of up to 5° to the cross direction of the kraft paper. Optionally, it is provided that the embossments of neighbouring embossment rows in the machine direction of the kraft paper have an offset in the cross direction of the kraft paper. Further features of the invention become apparent from the claims, the figures and the description of the exemplary embodiment. In the following, the present invention will be discussed in detail with reference to exemplary embodiments. In the figures: Fig. 1a-c show a schematic general view of a kraft paper production system; and Fig. 2 shows a schematic detailed view of an embossing station according to an exemplary embodiment of the invention; Fig. 3a shows a schematic side view of an embossed kraft paper; and Fig. 3b shows a schematic top view of the kraft paper shown in Fig. 3a. Unless otherwise specified, the following features are shown in the figures: Machine flow direction 1, wire section 2, headbox 3, shaking apparatus 4, press section 5, drying section 6, reeling station 7, dewatering wire 8, outer side 9, breast roll 10, first dewatering portion 11, inner side 12, second dewatering portion 13, first dewatering apparatus 14, second dewatering apparatus 15, former strip 16, suction zone 17, topformer dewatering wire 18, pre-dryer section 19, after-dryer section 20, compacting apparatus 21, calendaring station 22, fibrous web travel direction 23, suction box 24, paper reel 25, embossing station 84, embossing roller 85, counter roll 86, embossing nip 87, embossing profile 88, embossment 89, cleaning brush 90, axis of rotation 91 (of the embossing roll 85), axis of rotation 92 (of the cleaning brush 90), embossing depth 93, application apparatus 94, planar portion 95, adjusting apparatus 96, embossment rows 97, cross direction 98, machine direction 99, embossment columns 100. Fig. 1a-c show a schematic general view of a kraft paper production system. The system comprises, in machine direction 1, a headbox 3, a wire section 2, a press section 5, a pre-dryer section 19, a compacting apparatus 21, an after-dryer section 20, a calendaring station 22, and a reeling station 7. The headbox 3 serves in a known manner to apply a fibrous suspension to the continuously circulating dewatering wire 8 of the wire section 2, as a result of which a fibrous web is formed. Throughout the system, the fibrous web is continuously dewatered / dried and passes successively through the above-mentioned system components. Finally, paper reels 25 are formed, which can be transported away and / or fed to further processing steps. The production speed of the system in this exemplary embodiment is about 1100 m / min. The produced paper web has a paper width of about 6.4 m. The functionality of the individual system components is sufficiently known to a person skilled in the field of paper production and will not be explained in detail at this point. An exemplary kraft paper produced using the system shown in Fig. 1 has the following property profile: Property determined in accordance with machine direction (MD) / cross direction (CD) value Grammage ISO 536 -- 80 g / m2 Tensile strength index ISO 1924-3 MD 82 Nm / g CD 70 Nm / g tensile energy absorption index ISO 1924-3 MD and CD 3.5 J / g extensibility ISO 1924-3 MD 8.0% CD 8.3% extensibility centre* ISO 1924-3 CD 8.0% extensibility edge** ISO 1924-3 CD 8.7% Gurley air resistance ISO 5636-5 -- 5 s Cobb60 ISO 535 -- 32 g / m2 Ambertec formation -- -- 0.5 ^(g / m2) *measured at a distance of ±5 cm from the geometric centre of the paper web in its cross direction **measured at a maximum distance of 10 cm from the edge of the paper web in its cross direction In an embodiment according to the invention, the calendering station 22 included in the system according to Fig. 1 may be replaced by an embossing station 84. For example, this may be an embossing station 84 as shown schematically in Fig. 2. The embossing station 84 comprises an embossing roll 85 and a counter roll 86, wherein an embossing nip 87 is arranged between the embossing roll 85 and the counter roll 86. The fibrous web emerging from the dryer section is guided through this embossing nip 87. An embossing structure 88 is provided on the surface of the embossing roll 85, which substantially corresponds to the embossing profile to be formed on the fibrous web. The surface of the counter roll 86 is smooth, but is equipped with an elastic material. In this exemplary embodiment, the elastic material has a Pusey & Jones hardness of about 30. The embossing station 84 comprises an adjusting apparatus 96 via which the position of the counter roller 86 can be adjusted and thus the rolling pressure in the embossing nip 87 can be adapted. The fibrous web emerging from the embossing nip 87 has the final embossing profile. No further active drying step is required and the fibrous web can be rolled up right away. Further, a cleaning assembly is provided in the embossing station 84, which comprises a cleaning brush 90 and an application apparatus 94. The cleaning brush 90 is configured as a rotating brush with a rotary drive. The width of the cleaning brush 90 is smaller than the width of the embossing roll 85. In order to be able to cover the entire width of the embossing roll 85, a drive is also provided to move the cleaning brush 90 back and forth along the axis of rotation 91 of the embossing roll 85. The axis of rotation 91 of the embossing roll 85 extends substantially parallel to the axis of rotation 92 of the cleaning brush. The application apparatus 94 is configured as a spraying device which can spray a water-air mixture as a cleaning agent onto the surface of the embossing roll 85. This improves the cleaning effect of the cleaning brush 90. The embossing structure resulting from such an embossing station 84 may, as shown in Fig. 3a and 3b, be formed by pyramidal embossments 89 with a rectangular base, which are separated from one another by planar portions 95. No embossments 89 are provided in the planar portions 95. The embossing depth 93 of the embossments 89 is about 0.3 mm in this exemplary embodiment. Here, the width of an embossment 89 is about 0.6 mm and the length is about 0.9 mm. The embossments 89 are arranged in embossment rows 97, which are formed by a plurality of neighbouring embossments 89 extending in the cross direction 98 of the paper. The embossment rows 97 are arranged at an angle of about 1° relative to the cross direction 98. The distance between the embossments 89 of two neighbouring embossment rows 97 is about 2.2 mm in this exemplary embodiment. In the machine direction 99 of the paper, successive embossments 89 of two embossment rows 97 have an offset, i.e. the embossments 89 are not arranged directly above one another in the machine direction 99, but are offset at an angle. In this exemplary embodiment, the offset is about 0.6 mm. This results in embossment columns 100 that extend at an angle to the machine direction 99.

Claims

1. A method for producing kraft paper comprising the following steps:- forming a fibrous web extending in the machine direction (1) by continuously applying a fibrous suspension to a wire section (2) by a headbox (3),- shaking the fibrous web transversely to the machine direction, i.e., in the crossdirection, using a shaking apparatus (4),- pressing the fibrous web, which has been partially dewatered in the wire section (2), in a press section (5),- further drying of the fibrous web in a dryer section (6),- winding the paper web formed from the fibrous web in a reeling station (7), characterised- in that, after drying in the dryer section (6), the fibrous web is embossed in an embossing station (84), and- in that the fibrous web has a dry content of at least 85 wt% when it enters the embossing station (84).

2. The method according to claim 1, characterised in that during embossing, the fibrous web is provided with an embossing structure formed from an arrangement of embossments (89) wherein the embossments (89) have a height (93) of at least 0.1 mm, in particular between 0.1 mm and 2 mm.

3. The method according to claim 1 or 2, characterised in that the embossments (89) have a rectangular, spherical-segment-shaped, pyramidal or prismatic geometry.

4. The method according to any one of claims 1 to 3, characterised- in that neighbouring embossments (89) are directly adjacent to one another in the machine direction and in the cross direction of the fibrous web, or- in that neighbouring embossments (89) are spaced apart by a planar portion of the fibrous web, wherein the distance between neighbouring embossments (89) in the machine direction of the fibrous web is at least 0.5 mm, in particular between 0.5 mm and 5.0 mm, and / or wherein the distance between neighbouring embossments (89) in the cross direction of the fibrous web is at least 0.5 mm, in particular between 0.5 mm and 5.0 mm.

5. The method according to any one of claims 1 to 4, characterised in that the embossing station (84) comprises an embossing roll (85), wherein the embossing roll (85) is equipped with an embossing profile (88) on its surface.

6. The method according to claim 5, characterised in that the embossing profile (88) of the embossing roll (85) is cleaned, in particular during embossing, by a cleaning brush (90), wherein a cleaning agent, in particular a water-air mixture, is applied to the embossing profile (88).

7. The method according to claim 6, characterised in that the cleaning brush (90) rotates about an axis of rotation (92) aligned substantially parallel to the axis of rotation (91) of the embossing roll (85), and / or in that the cleaning brush (90) is moved back and forth substantially parallel to the axis of rotation (91) of the embossing roll (85).

8. The method according to any one of claims 1 to 7, characterised in that the surface temperature of the embossing roll (85) is between 20°C and 100°C during embossing.

9. The method according to any one of claims 1 to 8, characterised in that the embossing station (84) is an embossing station (84) according to any one of claims 10 to 14.

10. An embossing station for embossing a fibrous web, for a system for the production of kraft paper, wherein the embossing station (84) comprises an embossing roll (85) and a counter roll (86), wherein an embossing nip (87) is provided between the embossing roll (85) and the counter roll (86), through which the fibrous web is guided for embossing, wherein the embossing roll (85) is provided with an embossing profile (88) on its surface, and wherein the counter roller (86) has an elastic surface (88).

11. The embossing station according to claim 10, characterised in that the embossing station (84) comprises a cleaning brush (90) for cleaning the embossing profile (88) of the embossing roll (85).

12. The embossing station according to claim 11, characterised in that the embossing station (84) comprises an application apparatus (94) for applying a cleaning agent, in particular an air-water mixture, to the embossing profile (88) of the embossing roll (85).

13. The embossing station according to any one of claims 11 or 12, characterised in that the cleaning brush (90) is configured as a brush roll and has a rotary drive for rotating the cleaning brush (90) about an axis of rotation (92), wherein the axis of rotation (92) of the cleaning brush (90) is arranged substantially parallel to the axis of rotation (91) of the embossing roll (85).

14. The embossing station according to any one of claims 11 to 13, characterised in that the cleaning brush (90) has a displacement drive for moving the cleaning brush (90) back and forth substantially parallel to the axis of rotation (91) of the embossing roll (84).

15. The embossing station according to any one of claims 11 to 14, characterised in that an adjusting apparatus (96) is provided for adjusting the rolling pressure exerted on the fibrous web in the embossing nip (87).

16. A system for producing kraft paper comprising the following system components: - a wire section (2) for forming a fibrous web by continuously applying a fibrous suspension along the machine flow direction (1) by a headbox (3),- a shaking apparatus (4) for shaking the fibrous web transversely to the machine flow direction (1), i.e. in the cross direction,- a press section (5) for pressing the fibrous web that has been partially dewatered in the wire section (2),- a dryer section (6) for drying the fibrous web,- an embossing station (84) arranged between the dryer section (6) and the reeling station (7),- a reeling station (7) for winding the paper web formed from the fibrous web, characterised in that the embossing station (84) is an embossing station according to any one of claims 10 to 15.

17. The system according to claim 16, characterised in that the dryer section (6) comprises a pre-dryer section (19) and an after-dryer section (20), and in that a compacting apparatus (21), in particular a Clupak device, is arranged between the pre-dryer section (19) and the after-dryer section (20).

18. Use of an embossing station according to any one of claims 10 to 15 in a method for the production of kraft paper, in particular in a method according to any one of claims 1 to 9.

19. A kraft paper with dry-embossed embossments (89), wherein the kraft paper has an extensibility according to ISO 1924-3:2005 in the cross direction and optionally also in the machine direction exceeding 5%, preferably exceeding 7.5%.

20. Kraft paper according to claim 19, characterised- in that the embossments (89) are pyramidal with a rectangular base, and / or- in that the embossments (89) are arranged in embossment rows (97), wherein the embossment rows (97) extend in the cross direction (98) of the kraft paper or at an angle of up to 5° to the cross direction (98) of the kraft paper, and / or- in that the embossments (89) of neighbouring embossment rows (97) in the machine direction (99) of the kraft paper have an offset in the cross direction (98) of the kraft paper.

Images