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

[0001] Process for producing kraft paper

[0002] The present invention relates, in one aspect, to a method for producing kraft paper. In further aspects, the invention relates to an embossing station for a kraft paper production plant, a kraft paper production plant, and the use of an embossing station.

[0003] Various processes for producing kraft paper are known in the art. Kraft paper is a special type of paper characterized, among other things, by its particularly high strength. Due to this property, it is used for various packaging applications, 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 stretchability or air permeability.

[0004] Kraft papers provided with an embossing or embossed pattern are known in the prior art. These are typically regular or irregular embossings that impart a predetermined surface structure to the kraft paper, with which optical 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 in the drying section of a kraft paper production plant. The fibrous web can have a dry matter content of between 60% and 80% by weight.

[0005] However, such an arrangement of the embossing step within the process can lead to the fibrous web forming wrinkles, which is disadvantageous in terms of process stability and can possibly also cause the fibrous web to tear (web break).

[0006] In particular, an object of the present invention can therefore be seen in overcoming these and other disadvantages of the prior art.

[0007] The invention therefore preferably relates to a process for producing kraft paper, in particular sack kraft paper, in which, in a first step, a fibrous web running in the machine direction is formed. This is carried out, in particular, by applying a fibrous suspension to a wire section by means of a headbox.

[0008] Following the wire section, a press section is typically arranged, where mechanical dewatering with simultaneous compaction of the fibrous web can take place. This can typically be achieved with several press nips, the first of which is usually designed as a suction press with double felt guides, and the others as roller presses or shoe presses with either double or single felt guides.

[0009] The mechanically extruded fibrous web can be subjected to one or more drying processes to achieve the desired dry content. Further manufacturing and / or processing steps, such as compressing or calendering the fibrous web, can be performed before, between, or after one or more drying processes.

[0010] Compressing the fibrous web can occur, in particular, between two drying steps, i.e., after a pre-drying step in a pre-drying section and before a post-drying step in a post-drying section. This can increase the paper's extensibility in the machine direction. This step can be performed, for example, in a so-called compression device, such as a Clupak device.

[0011] The embossing of the fibrous web can be carried out in particular in an embossing station and after the last drying step, whereby the embossing can be carried out with thermal rollers and / or with embossing rollers.

[0012] After the final processing step, i.e. embossing, the finished paper web can be wound up on the reel and then further processed to the desired finished roll width.

[0013] In particular, the dry content of the fibrous web can be at least 85% by weight when it enters the embossing station or is embossed. Surprisingly, it has been discovered that embossing at such a high dry content can reduce or completely prevent wrinkling of the fibrous web, while still achieving a high-quality embossed result. This is particularly the case when the extensibility of the kraft paper, in particular according to ISO 1924-3:2005, in the transverse direction and optionally also in the machine direction is greater than 5%, preferably greater than 7.5%.

[0014] The embossed pattern created on the fibrous web can take on a variety of forms. In particular, the embossed pattern is formed from a large number of impressions that can be connected to one another or separated from one another by unembossed flat sections. Typically, the individual impressions have small dimensions, namely an embossed 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 embossed pattern consists of pyramid-shaped impressions with a rectangular base, wherein the embossed depth is approximately 0.3 mm. The dimensions of the base can be approximately 0.9 mm (in the machine direction) x approximately 0.6 mm (in the cross direction). Embossments adjacent to one another in the cross direction of the paper can form rows of impressions that can run parallel or at a slight angle (e.g., up to 5°) to the cross direction.The indentations of consecutive rows of indentations in the machine direction can be spaced approximately 2 mm to 3 mm apart. In some cases, the indentations of consecutive rows of indentations in the machine direction may not be directly superimposed, but rather offset in the transverse direction. The offset can be approximately 0.6 mm.

[0015] If necessary, embossing can be carried out at ambient temperature, i.e., without heating or cooling the embossing roller or any other part of the embossing station. The ambient temperature is typically between 20°C and 40°C, depending on local conditions.

[0016] During embossing, small amounts of material from the fibrous web can be removed, which can accumulate on the embossing rollers and impair the further embossing process. Therefore, the embossing roller can be cleaned continuously or at specific intervals during the process. Such cleaning can be achieved using a cleaning brush that is moved over the outer surface of the embossing roller. If necessary, a cleaning agent, such as a water-air mixture, can also be applied to the surface of the embossing roller to improve the cleaning effect in conjunction with the cleaning roller.

[0017] The invention also relates in particular to an embossing station for a plant for producing kraft paper. This embossing station can have an embossing roller and a counter roller. The embossing profile is defined in particular by the surface structure of the embossing roller. The counter roller optionally has a smooth, yet elastic, surface. The hardness of the surface of the counter roller, measured according to Pusey & Jones (ISO 6123-1), can be between 25 and 35.

[0018] To clean the surface of the embossing roller, the embossing station can include a cleaning brush and, if necessary, an application or spray device for applying the cleaning agent.

[0019] The cleaning brush can be arranged so that it can be moved back and forth parallel to the rotational axis of the embossing roller. A guide and drive can be provided for this purpose, for example.

[0020] The embossing station may further comprise any features described in connection with the method or another aspect of the present invention.

[0021] Optionally, the invention also relates to the use of such an embossing station in the production of kraft paper.

[0022] Optionally, the invention also relates to a plant for producing kraft paper, i.e., a kraft paper production plant comprising an embossing station as described here. Such a kraft paper production plant may comprise, in addition to the embossing station, at least the following plant components: headbox, wire section, press section, dryer section, and reeling section. Optionally, the dryer section may comprise several separate dryer sections. Additionally, an upsetting device (e.g., a Clupak unit) may be provided.

[0023] In particular, the embossing station is located downstream of the drying section and upstream of the reeling section.

[0024] The invention may further relate to a kraft paper as such having an embossing, in particular a dry-embossed embossing. It should be noted that any features described in connection with other aspects may be provided in all aspects of the invention. For example, the embossing station may have features described in connection with the method according to the invention, and vice versa.

[0025] In particular, a process for producing kraft paper is described, which comprises the following steps:

[0026] - Forming a fibrous web running along the machine direction by continuously applying a fibrous suspension to a wire section through a headbox,

[0027] - Shaking the fibrous web transversely to the machine direction, i.e. in the transverse direction, by a shaking device,

[0028] - Pressing the fibrous web, previously partially dewatered in the wire section, in a press section,

[0029] - further drying of the fibrous web in a drying section,

[0030] - Winding up the paper web formed from the fibrous web in a winding station.

[0031] If appropriate, the fibrous web is embossed in an embossing station after drying in the drying section. If appropriate, the fibrous web has a dry matter content of at least 85% by weight upon entering the embossing station.

[0032] Optionally, it is provided that the fibrous web is provided with an embossed 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 of between 0.1 mm and 2 mm.

[0033] Where appropriate, the indentations may have a rectangular, spherical segment, pyramid or prism geometry.

[0034] Optionally, it is provided that adjacent indentations directly adjoin one another in the machine direction and in the transverse direction of the fibrous web. Optionally, it is provided that adjacent indentations are spaced from one another by a flat section of the fibrous web, wherein the distance between adjacent indentations 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 adjacent indentations in the transverse direction of the fibrous web is at least 0.5 mm, in particular between 0.5 mm and 5.0 mm.

[0035] If necessary, the embossing station may comprise an embossing roller, the embossing roller being provided with an embossing profile on its surface.

[0036] If necessary, it is provided that the embossing profile of the embossing roller is cleaned, in particular during embossing, by means of a cleaning brush, wherein a cleaning agent, in particular a water-air mixture, is applied to the embossing profile.

[0037] Optionally, it is provided that the cleaning brush rotates about a rotation axis aligned substantially parallel to the rotation axis of the embossing roller, and / or that the cleaning brush is moved back and forth substantially parallel to the rotation axis of the embossing roller.

[0038] If necessary, the surface temperature of the embossing roller during embossing is between 20°C and 100°C.

[0039] Where appropriate, the stamping station is a stamping station with one or more of the features described here.

[0040] Also described is an embossing station for embossing a fibrous web for a plant for producing kraft paper. The embossing station comprises an embossing roller and a counterroller. An embossing gap is provided between the embossing roller and the counterroller, through which the fibrous web is guided for embossing. The embossing roller is equipped with an embossing profile on its surface, and the counterroller has an elastic surface. Optionally, the embossing station may comprise a cleaning brush for cleaning the embossing profile of the embossing roller.

[0041] If appropriate, the embossing station may comprise an application device for applying a cleaning agent, in particular an air-water mixture, to the embossing profile of the embossing roller.

[0042] Optionally, it is provided that the cleaning brush is designed as a brush roller and has a rotation drive for rotating the cleaning brush about a rotation axis, wherein the rotation axis of the cleaning brush is arranged substantially parallel to the rotation axis of the embossing roller.

[0043] If necessary, the cleaning brush is provided with a displacement drive for moving the cleaning brush back and forth substantially parallel to the rotational axis of the embossing roller.

[0044] If necessary, an adjusting device is provided to adjust the rolling pressure exerted on the fibrous web in the embossing gap.

[0045] A plant for the production of kraft paper is also described, which includes the following plant components:

[0046] - a wire section for forming a fibrous web by continuously applying a fibrous suspension along the machine direction through a headbox,

[0047] - a shaking device for shaking the fibrous web transversely to the machine direction, i.e. in the transverse direction,

[0048] - a press section for pressing the fibrous web partially dewatered in the wire section,

[0049] - a drying section for drying the fibrous web,

[0050] - an embossing station located between the drying section and the reeling station,

[0051] - a winding station for winding the paper web formed from the fibrous web. Optionally, the embossing station is an embossing station with one or more of the features described herein.

[0052] Optionally, it is provided that the drying section comprises a pre-drying section and a post-drying section, and that an upsetting device, in particular a Clupak device, is arranged between the pre-drying section and the post-drying section.

[0053] Also disclosed is the use of an embossing station having one or more of the features described herein in a process for producing kraft paper. Optionally, this may be a process having one or more of the features described herein.

[0054] A kraft paper with dry-embossed imprints is also described. The kraft paper can have an extensibility according to ISO 1924-3:2005 in the transverse direction and optionally also in the machine direction of greater than 5%, preferably greater than 7.5%.

[0055] Where appropriate, the indentations shall be pyramid-shaped with a rectangular base.

[0056] Where appropriate, it is provided that the embossings are arranged in embossing rows, wherein the embossing rows run in the transverse direction of the kraft paper or at an angle of up to 5° to the transverse direction of the kraft paper.

[0057] If necessary, it is provided that the embossings of embossing rows adjacent in the machine direction of the kraft paper have an offset in the transverse direction of the kraft paper.

[0058] Further features of the invention will become apparent from the patent claims, the figures, and the description of the exemplary embodiment. The present invention will be explained in detail below using an exemplary embodiment.

[0059] They show:

[0060] Fig. 1a-c shows a schematic overall view of a kraft paper production plant; and Fig. 2 shows a schematic detailed view of an embossing station according to an embodiment of the invention;

[0061] Fig. 3a is a schematic side view of an embossed kraft paper; and Fig. 3b is a schematic top view of the kraft paper shown in Fig. 3a.

[0062] Unless otherwise indicated, the following features are shown in the figures: Machine direction 1, wire section 2, headbox 3, jogging device 4, press section 5, dryer section 6, reeling station 7, dewatering wire 8, outer side 9, breast roll 10, first dewatering section 11, inner side 12, second dewatering section 13, first dewatering device 14, second dewatering device 15, forming bar 16, suction zone 17, top former dewatering wire 18, pre-dryer section 19, after-dryer section 20, edging device 21, calendering station 22, running direction of the fibrous web 23, suction box 24, paper reel 25, embossing station 84, embossing roll 85, counter roll 86, embossing gap 87, embossing profile 88, embossing 89, cleaning brush 90, rotation axis 91 (of the embossing roller 85), rotation axis 92 (of the cleaning brush 90), embossing depth 93, application device 94, plan section 95, adjusting device 96, embossing rows 97, cross direction 98, machine direction 99,Imprint columns 100.,

[0063] Fig. 1a-c show a schematic overall view of a kraft paper production plant. The plant comprises, in machine direction 1, a headbox 3, a wire section 2, a press section 5, a pre-dryer section 19, an upsetting device 21, an after-dryer section 20, a calendering station 22, and a winding station 7.

[0064] The headbox 3 serves, in a known manner, to apply a fiber suspension to the continuously rotating dewatering wire 8 of the wire section 2, thereby forming a fiber web. As the system progresses, the fiber web is continuously dewatered or dried, passing successively through the aforementioned system components. Finally, paper rolls 25 are formed, which can be transported away and / or fed to further processing steps.

[0065] The production speed of the system in this exemplary embodiment is approximately 1100 m / min. The resulting paper web has a paper width of approximately 6.4 m.

[0066] The function of the individual plant components is well known to a specialist in the field of paper production and will not be explained in detail here. An example of kraft paper produced using the plant shown in Fig. 1 has the following property profile:

[0067] 'measured in an area ±5 cm from the geometric center of the paper web in its transverse direction

[0068] “measured in an area not exceeding 10 cm from the edge of the paper web in its transverse direction

[0069] In one embodiment of the invention, the calendering station 22 included in the system according to Fig. 1 can be replaced by an embossing station 84. For example, this can be an embossing station 84 as schematically shown in Fig. 2.

[0070] The embossing station 84 comprises an embossing roller 85 and a counter roller 86, with an embossing gap 87 arranged between the embossing roller 85 and the counter roller 86. The fibrous web emerging from the dryer section is guided through this embossing gap 87.

[0071] An embossed structure 88 is provided on the surface of the embossed roller 85, which essentially corresponds to the embossed profile to be formed on the fibrous web.

[0072] The surface of the counter roll 86 is smooth, but covered with an elastic material. In this exemplary embodiment, the elastic material has a Pusey & Jones hardness of approximately 30. The embossing station 84 includes an adjusting device 96 by which the position of the counter roll 86 can be adjusted, thus adjusting the rolling pressure in the embossing gap 87.

[0073] The fibrous web emerging from the embossing nip 87 exhibits the final embossed profile. No further active drying step is required, and the fibrous web can be rolled up directly.

[0074] In the embossing station 84, a cleaning arrangement is also provided, which comprises a cleaning brush 90 and an application device 94.

[0075] The cleaning brush 90 is designed as a rotating brush with a rotary drive. The width of the cleaning brush 90 is less than the width of the embossing roller 85. In order to cover the entire width of the embossing roller 85, a drive is also provided to move the cleaning brush 90 back and forth along the rotational axis 91 of the embossing roller 85. The rotational axis 91 of the embossing roller 85 runs essentially parallel to the rotational axis 92 of the cleaning brush. The application device 94 is designed as a spray device that can spray a water-air mixture as a cleaning agent onto the surface of the embossing roller 85. This improves the cleaning effect of the cleaning brush 90.

[0076] The embossed structure obtained by means of such an embossing station 84 can, as shown in Figs. 3a and 3b, be formed from pyramid-shaped impressions 89 with a rectangular base, separated from one another by flat sections 95. No impressions 89 are provided in the flat sections 95. The embossing depth 93 of the impressions 89 is approximately 0.3 mm in this exemplary embodiment. The width of an impression 89 here is approximately 0.6 mm and the length approximately 0.9 mm.

[0077] The indentations 89 are arranged in indentation rows 97, which are formed by a plurality of adjacent indentations 89 extending in the transverse direction 98 of the paper. Compared to the transverse direction 98, the indentation rows 97 are arranged at an angle of approximately 1°.

[0078] The distance between the impressions 89 of two adjacent rows of impressions 97 is approximately 2.2 mm in this embodiment.

[0079] Consecutive impressions 89 of two impression rows 97 in the machine direction 99 of the paper are offset. The impressions 89 are therefore not arranged directly above one another in the machine direction 99, but rather are offset at an angle. In this exemplary embodiment, the offset is approximately 0.6 mm. This results in impression columns 100 that run at an angle to the machine direction 99.

Claims

Patent claims 1. A process for producing kraft paper comprising the following steps: - forming a fibrous web running along the machine direction (1) by continuously applying a fibrous suspension to a wire section (2) through a headbox (3), - shaking the fibrous web transversely to the machine direction, i.e. in the transverse direction, by a shaking device (4), - pressing the fibrous web previously partially dewatered in the wire section (2) in a press section (5), - further drying of the fibrous web in a drying section (6), - Winding up the paper web formed from the fibrous web in a winding station (7), characterized in that - that the fibrous web is embossed in an embossing station (84) after drying in the drying section (6), and - that the fibrous web has a dry content of at least 85% by weight when entering the embossing station (84).

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

3. Method according to claim 1 or 2, characterized in that the impressions (89) have a rectangular, spherical segment-shaped, pyramid-shaped or prism-shaped geometry.

4. Method according to one of claims 1 to 3, characterized in that - that adjacent impressions (89) are directly adjacent to one another in the machine direction and in the transverse direction of the fibrous web, or - that adjacent impressions (89) are spaced apart from one another by a flat section of the fibrous web, wherein the distance between adjacent impressions (89) in the machine direction of the fibrous web at least 0.5 mm, in particular between 0.5 mm and 5.0 mm, and / or wherein the distance between adjacent impressions (89) in the transverse direction of the fibrous web is at least 0.5 mm, in particular between 0.5 mm and 5.0 mm.

5. Method according to one of claims 1 to 4, characterized in that the embossing station (84) comprises an embossing roller (85), wherein the embossing roller (85) is equipped with an embossing profile (88) on its surface.

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

7. Method according to claim 6, characterized in that the cleaning brush (90) rotates about a rotation axis (92) aligned substantially parallel to the rotation axis (91) of the embossing roller (85), and / or that the cleaning brush (90) is moved back and forth substantially parallel to the rotation axis (91) of the embossing roller (85).

8. Method according to one of claims 1 to 7, characterized in that the surface temperature of the embossing roller (85) during embossing is between 20°C and 100°C.

9. Method according to one of claims 1 to 8, characterized in that the embossing station (84) is an embossing station (84) according to one of claims 10 to 14 10. Embossing station for embossing a fibrous web, for a plant for producing kraft paper, wherein the embossing station (84) comprises an embossing roller (85) and a counter roller (86), wherein an embossing gap (87) is provided between the embossing roller (85) and the counter roller (86), through which the fibrous web is guided for embossing, wherein the embossing roller (85) is equipped with an embossing profile (88) on its surface, and wherein the counter roller (86) has an elastic surface (88).

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

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

13. Embossing station according to claim 11 or 12, characterized in that the cleaning brush (90) is designed as a brush roller and has a rotation drive for rotating the cleaning brush (90) about a rotation axis (92), wherein the rotation axis (92) of the cleaning brush (90) is arranged substantially parallel to the rotation axis (91) of the embossing roller (85).

14. Embossing station according to one of claims 11 to 13, characterized 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 roller (84).

15. Embossing station according to one of claims 11 to 14, characterized in that an adjusting device (96) is provided for adjusting the rolling pressure exerted on the fibrous web in the embossing gap (87).

16. Plant for the production of kraft paper comprising the following plant components: - a wire section (2) for forming a fibrous web by continuously applying a fibrous suspension along the machine direction (1) through a headbox (3), - a shaking device (4) for shaking the fibrous web transversely to the machine running direction (1 ), i.e. in the transverse direction, - a press section (5) for pressing the fibrous web partially dewatered in the wire section (2), - a drying section (6) for drying the fibrous web, - an embossing station (84) arranged between the drying section (6) and the winding station (7), - a winding station (7) for winding up the paper web formed from the fibrous web, characterized in that the embossing station (84) is an embossing station according to one of claims 10 to 15.

17. Plant according to claim 16, characterized in that the drying section (6) comprises a pre-drying section (19) and a post-drying section (20), and that an upsetting device (21), in particular a Clupak device, is arranged between the pre-drying section (19) and the post-drying section (20).

18. Use of an embossing station according to one of claims 10 to 15 in a process for producing kraft paper, in particular in a process according to one of claims 1 to 9.

19. Kraft paper with dry-embossed impressions (89), wherein the kraft paper has an extensibility according to ISO 1924-3:2005 in the transverse direction and optionally also in the machine direction of greater than 5%, preferably greater than 7.5%.

20. Kraft paper according to claim 19, characterized in that - that the impressions (89) are pyramid-shaped with a rectangular base, and / or - that the embossings (89) are arranged in embossing rows (97), wherein the embossing rows (97) run in the transverse direction (98) of the kraft paper or at an angle of up to 5° to the transverse direction (98) of the kraft paper, and / or - that the impressions (89) of adjacent rows of impressions (97) in the machine direction (99) of the kraft paper have an offset in the transverse direction (98) of the kraft paper.

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