Kraft paper web

Abstract

The invention relates to a kraft paper web made of kraft paper, in particular sack kraft paper, having an elongation at break determined according to ISO 1924-3, wherein the elongation at break of the kraft paper web in the transverse direction is at least 4%; the kraft paper web has a width (101), extending in the transverse direction, of at least 4.0 m; the elongation at break in % in the transverse direction, measured in an edge region (102) of the kraft paper web, is greater than the elongation at break in % in the transverse direction, measured in a central region (103) of the kraft paper web, by at most a factor of 1.3, preferably by at most a factor of 1.2; a normal distance of the edge region (102) to the kraft paper web outer edge (104) running in the machine direction is between 0 cm and 50 cm; and a normal distance of the central region (103) to the kraft paper web outer edge (104) running in the machine direction corresponds to ±25 cm half the width (101) of the kraft paper web. The invention further relates to a method.

Description

[0001] 65462

[0002] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0003] Kraft paper web

[0004] The present invention relates to a kraft paper web and a method for producing a kraft paper web.

[0005] Kraft papers are known in various forms in the prior art. Kraft paper is a special type of paper characterized, among other things, by its particularly high strength, which makes it suitable for various packaging applications, such as the production of paper bags. A key parameter important for the typical applications of kraft paper is its elasticity.

[0006] Kraft papers with high elongation (e.g., an elongation at break of more than 4% in both the machine and transverse directions) are generally known in the art. While such elongation in the machine direction can typically be achieved by applying micro-creping (e.g., in a so-called Clupak unit), the transverse elongation is typically controlled by adjusting the energy input during the refining of the kraft pulp. The drying of the paper web during the paper machine also has a known influence on elongation, particularly in the transverse direction. Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0007] The material produced in an industrial paper machine is typically in the form of an elongated paper web with a width of 4 meters or more. For transport and further processing, the paper web is usually wound onto reels, known as drums.

[0008] It is known that the elongation of kraft papers varies across the width of the paper web, especially when dealing with kraft papers that have high transverse elongation. Typically, the elongation is lowest in the center of the paper web, while it increases towards the edges.

[0009] For example, investigations by the present inventors revealed that a conventional sack kraft paper web with a width of approximately 6 m can exhibit a center-to-edge variation in elongation at break of up to 60%, when the transverse elongation at break in the center is approximately 8%. The transverse elongation at break in an edge region of this exemplary paper web is therefore up to approximately 13%.

[0010] Within the scope of the present invention, it was found that such a high center-edge variation in elongation at break has numerous disadvantages with regard to the further processing properties of the paper, including processability, printability, coating capability and appearance of the kraft paper.

[0011] A general objective of the invention can now be seen as creating a kraft paper web that has improved processing properties compared to the prior art.

[0012] It has now been surprisingly discovered that this problem can be solved, among other things, by using kraft paper webs with lower center-to-edge variation in elongation at break. Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0013] This may result in further advantages, which could be selected from one or more of the following examples:

[0014] - Uniform strength: More uniform elongation across the entire width of the paper web provides more consistent strength. Sack kraft paper is frequently used for packaging heavy or bulky items, and any variation in strength can create weak points that are prone to tearing or breaking.

[0015] - Processing efficiency: During the converting process, where a paper roll is unwound and processed into smaller rolls or sheets, uniform elongation properties can prevent problems such as wrinkling, misalignment, or breakage. This can ensure smooth operation of the converting equipment and reduce downtime and waste. - Print and coating quality: In applications where the kraft paper is printed or coated, uniform elongation ensures that the paper maintains consistent tension. This results in better adhesion of prints and coatings, which in turn leads to higher-quality end products.

[0016] - Uniform appearance: In applications where the appearance of the packaging is important, such as branded bags or packaging, uniform stretchability can help to ensure a consistent appearance without distortions or defects.

[0017] - Predictable performance: By reducing variations in stretchability, the paper's performance characteristics (such as flexibility, durability, and elasticity) can be more predictable. This can be crucial for manufacturers and end users who need to rely on consistent behavior for their specific applications.

[0018] - Reduced material waste: Uneven stretching can lead to increased material waste due to defects and rejects. By minimizing differences in stretching, the overall yield of each paper web or roll is maximized, making the production process more cost-efficient and sustainable. Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0019] The invention therefore relates in one aspect to kraft paper or a kraft paper web as such. Furthermore, the invention relates to a method found that enables the production of kraft paper.

[0020] This section describes in particular a kraft paper web with high elongation at break in the transverse direction (also referred to as CD, for cross direction), for example of more than 4%, which exhibits a low center-edge variation in elongation at break.

[0021] In all aspects described here, the elongation at break can be determined according to ISO 1924-3.

[0022] The elongation at break (CD) in a peripheral area of ​​the paper web may be at most 30% greater than the elongation at break (CD) in a central area of ​​the paper web. The stated percentage of the center-edge variation refers to the relative (percentage) difference between the respective elongation at break values. In other words, the elongation at break (CD) in a peripheral area of ​​the paper web is at most 1.3 times the elongation at break (CD) in a central area of ​​the paper web.

[0023] The aforementioned edge region of the paper web can have a normal distance of 0 cm to 50 cm from an outer edge of the paper web. The aforementioned central region can lie within a range of ±25 cm from the center of the paper web, wherein the center of the paper web has a normal distance to both outer edges of the paper web that corresponds to half the width of the paper web.

[0024] The width of the paper web is defined in particular by the normal distance between two outer edges of the paper web running in the machine direction.

[0025] The paper web naturally has two edge regions, as defined here. The slight difference in elongation at break between the central region and the edge region described here can occur in both edge regions. Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0026] Kraft paper may contain cationic starch, in particular bulk starch, in a content of at least 4 kg / t (dry weight). This quantity refers specifically to the dry weight of the pulp contained in the kraft paper.

[0027] The kraft paper may contain a sizing agent.

[0028] The pulp used to produce the kraft paper web may be treated by low-consistency and high-consistency refining. The energy input for low-consistency refining can range from 0 kWh / t to 150 kWh / t. The energy input for high-consistency refining can range from 100 kWh / t to 400 kWh / t.

[0029] The pulp used to produce the kraft paper web can have a Schopper-Riegler value of 10 °SR to 15 °SR in the unmilled kraft pulp.

[0030] The pulp used to manufacture the kraft paper web is preferably kraft pulp, i.e., in particular pulp obtained by chemical digestion of wood material using the sulfate process.

[0031] The wood material can be softwood or it can contain a certain proportion of hardwood.

[0032] A kraft paper web with a low center-edge variation in elongation at break can be obtained, for example, by subjecting the fiber web produced in a paper machine to impact drying in at least one, preferably several, such as two or three, impact drying sections, in which the free shrinkage distance of the fiber web in the impact drying section(s) is at least 3 m.

[0033] Impact drying is typically achieved by deflecting the fiber web around heated cylindrical rollers and simultaneously exposing it to a stream of hot air. Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0034] Pressure is applied to achieve drying. Advantageously, the temperature in cylinder drying is so high that the evaporating water creates a gas or vapor cushion, allowing the fiber web to glide over the cylinder surface. This enables the fiber web to shrink freely within an impact drying section.

[0035] Impact drying differs from contact drying, which is also known in the production of kraft paper, in that the fiber web is not pressed against the outer circumference of a drying cylinder by drying screens or similar pressing devices.

[0036] Typically, a plant for the production of kraft paper includes several drying units arranged in series (e.g. contact drying cylinders with or without impact drying hoods), with the entirety of the drying units being referred to as the drying section.

[0037] The drying section can be divided into several sections, with different sections typically separated in the machine direction by a further device for processing the fiber web. A typical example where several drying sections are provided is a plant used, in particular, for the production of sack kraft paper. According to the prior art, a first drying section, referred to as the pre-drying section, is provided, followed by a compression device for compressing the fiber web. After passing the compression device, the fiber web is guided into a second drying section, referred to as the post-drying section. If the compression device is not used in such a plant, the fiber web can be guided past the compression device by a bypass device, in which essentially no treatment of the fiber web takes place.Even then, the division into pre-drying section and post-drying section exists.

[0038] Drying section sections, which are used in particular as post-drying sections and are arranged in the machine direction after a compression device, can be supplied by Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0039] have a contact drying area and an impact drying area, with the impact drying area usually following directly on top of the contract drying area.

[0040] It is known that the impact drying section comprises several (e.g., two or three) impact drying sections through which the fiber web is guided. The impact drying sections are typically arranged between two drying cylinders with an associated drying screen, the drying screen pressing the fiber web against the outer circumference of the drying cylinder in the pressure zone.

[0041] Each drying roller with its associated drying screen can be described as a contact drying section. In a contact drying section, the drying screen rests against the outer circumference of the drying roller, particularly in sections, thus forming a pressure zone in which the fiber web is pressed, or can be pressed, against the outer circumference of the drying roller.

[0042] The present inventors have now discovered that a kraft paper with advantageous properties can be obtained if the free shrinkage length of the fiber web in the impact drying section(s) is at least 3 m.

[0043] This allows for improved free shrinkage of the fiber web during the drying process, which ultimately enables improved control of the transverse elongation properties of the produced kraft paper and, in particular, a reduced center-edge variation in elongation at break.

[0044] The free shrinkage length is, in particular, the length that a fiber web travels from exiting the pressure zone located upstream of it in the machine direction to entering the pressure zone located downstream in the machine direction. Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0045] Typically, no equipment is provided in this area to protect the fiber web against the outer circumference of drying rollers or...

[0046] Pressing cylindrical rollers, as is the case in the contact drying area.

[0047] An impact drying section comprises, for example, two or more impact drying units, each of which includes a heated cylindrical roller that heats the fiber web from one side and a drying hood that blows hot air onto the other side of the fiber web. This allows for the application of heat to the fiber web from both sides.

[0048] The temperature of the heated cylinder roll is particularly high, exceeding 100°C, for example between 110°C and 160°C, so that the moisture escaping from the fiber web creates a vapor cushion. Due to this vapor cushion, the fiber web hovers, at least in sections, above the cylinder rolls, thus enabling particularly advantageous free paper web shrinkage.

[0049] In the direction of machine travel, between the two impact drying units (i.e., cylindrical rollers with drying hoods), a further, also heated, drying roller can be provided, which may or may not be equipped with a drying hood. This further drying roller is not assigned a drying screen or any other pressure device.

[0050] Such a drying section can have several impact drying sections in succession, configured as described here, wherein the separation of the impact drying sections within the impact drying area can be formed by contact drying sections comprising a drying cylinder with an associated drying screen. In particular, the drying cylinders in the contact drying sections are also heated, but at a lower temperature compared to the rollers or cylinders in the impact drying section.

[0051] If necessary, the drying cylinders in this area can also be unheated to avoid causing additional cross-shrinkage of the paper web.65462

[0052] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0053] In the direction of the machine, prior to the impact drying section, a contact drying section is provided, comprising drying cylinders with associated drying screens but no impact drying equipment. This drying section serves primarily to stabilize the fiber web after treatment in, for example, a shrinking unit (e.g., a Clupak unit). This stabilization function specifically consists of enabling wrinkle-free operation and centering the fiber web within the system. In the subsequent impact drying section, the fiber web shrinks due to the drying process. The dry content of the fiber web in the impact drying section is typically between 65% and 85%.

[0054] Each drying screen follows the outer circumference of the respective drying cylinder within a contact area, with the contact area comprising a certain segment of the outer circumference of the drying cylinder. The greater the length of the contact area, the longer the contact time of the fiber web with each individual drying cylinder and the stronger the drying effect.

[0055] To vary the contact pressure with which the fiber web is pressed against the drying cylinder via the drying screen, at least one of the guide rollers used to guide the drying screens can be designed as a movable tension roller. This tension roller is arranged such that its displacement changes the tension exerted on the drying screen.

[0056] Typically, each drying cylinder in the contact drying section has a separately controllable drive. This allows for the adjustment of different rotational speeds for the drying cylinders, which can offer further advantages in terms of train design. For example, it is possible for the rotational speed of successive drying cylinders to increase steadily, which is not possible in drying section sections with only one drying screen and without separately controllable drives.

[0057] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0058] The drying sieves typically do not have their own drives; instead, they are driven by the drying cylinders.

[0059] The drying cylinders can be heated separately, with the heating taking place from the inside. For this purpose, the drying cylinders are equipped with a steam supply system through which saturated steam (water) can be introduced. The steam pressure can be, for example, up to 12 bar.

[0060] In the contact drying section, which is arranged upstream of the impact drying section, at least one, preferably three, drying cylinders and a corresponding number of drying screens are provided. The drying cylinders of the contact drying section are arranged directly one after the other, meaning that no other drying devices, i.e., devices that exert a significant drying effect on the fiber web, are provided between them.

[0061] This also describes a drying section, in particular a post-drying section, for drying a fibrous web, for a plant for the production of kraft paper, wherein the drying section comprises an impact drying area which has at least one impact drying section, in particular three impact drying sections, wherein each impact drying section is bounded before and after it in the machine direction by a drying cylinder, to which a drying screen is assigned, which follows the outer circumference of the drying cylinder in a pressure area and in particular presses a fibrous web passing through the drying section against the drying cylinder.

[0062] If necessary, the impact drying section is designed such that a fiber web running through the drying section is positioned between the outlet of the pressure area arranged upstream in the machine direction and the 65462

[0063] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0064] The entry point of the pressure area, which is arranged downstream in the direction of machine travel, passes through a free shrinkage section of at least 15.0 m.

[0065] Optionally, the impact drying section includes at least one impact drying device, wherein an impact drying device includes a cylindrical roller around which the fiber web is guided, and wherein a drying hood is associated with the cylindrical roller which blows hot air onto the surface of the fiber web.

[0066] If necessary, the impact drying section is provided for to be free of drying screens.

[0067] If necessary, the fiber web is provided for to lie freely against the outer circumference of the cylindrical rollers in the impact drying section.

[0068] Optionally, it is provided that the impact drying section comprises at least two, in particular three, impact drying devices, and that the impact drying section comprises a further drying cylinder, wherein the further drying cylinder is arranged in the machine direction between the two cylinder rollers.

[0069] If necessary, it is provided that a drying hood is assigned to the additional drying cylinder, or that the additional drying cylinder is designed without a drying hood.

[0070] Optionally, it is provided, characterized in that the impact drying area follows a contact drying area in the machine direction, wherein at least two, in particular three, drying cylinders are provided in the contact drying area arranged one after the other in the machine direction, and wherein each drying cylinder is assigned an endlessly rotating drying screen which follows the outer circumference of the respective drying cylinder in a pressure area.65462

[0071] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0072] A plant for the production of kraft paper is also described, comprising the following plant components:

[0073] - a sieve section for forming a fiber web by continuously applying a fiber suspension along the machine direction via a headbox, - a shaking device for shaking the fiber web transversely to the machine direction, i.e. in the transverse direction,

[0074] - a press section for pressing the fiber web that has been partially dewatered in the sieve section,

[0075] - a drying section for drying the fiber web,

[0076] - a winding station for winding the paper web formed from the fiber web.

[0077] The drying section may include a drying section as described here.

[0078] If necessary, the drying section is provided for to include a pre-drying section and a post-drying section, and that the post-drying section of the plant is a drying section as described here.

[0079] If necessary, a compression device, in particular a Clupak device, is provided for between the pre-drying section and the post-drying section.

[0080] Furthermore, a process for the production of kraft paper or a kraft paper web is described, comprising the following steps:

[0081] - Forming a fiber web running along the machine direction by continuously applying a fiber suspension to a screen section via a headbox,

[0082] - Shaking the fiber web perpendicular to the machine direction, i.e. in the transverse direction, by a shaking device,

[0083] - Pressing the fiber web, which was previously partially dewatered in the sieving section, in a

[0084] Press section, 65462

[0085] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0086] - further drying of the fiber web in a drying section,

[0087] - Winding the paper web formed from the fiber web in a winding station, wherein the drying section comprises an impact drying area which has at least one impact drying section, in particular three impact drying sections, wherein each impact drying section is bounded before and after it in the machine direction by a drying cylinder to which a drying screen is assigned, which follows the outer circumference of the drying cylinder in a pressure area and in particular presses a fiber web passing through the drying section against the drying cylinder.

[0088] If necessary, the fiber web passes through a free shrinkage section in the impact drying section, which is arranged between the exit of the pressure area located upstream in the machine direction and the entrance of the pressure area located downstream in the machine direction, wherein the free shrinkage section has a length of at least 15.0 m.

[0089] If necessary, the running speed of the fiber web is to be at least 600 m / min.

[0090] If necessary, it is provided that cylindrical rollers in the impact drying section are heated to a temperature of at least 110°C.

[0091] If necessary, drying hoods in the impact drying section are heated to a temperature of at least 160°C, possibly up to 400°C.

[0092] If applicable, it is provided that the fiber web has a dryness content of at least 55 wt.%, in particular at least 65 wt.%, upon entering the drying section and / or that the fiber web has a dryness content of at most 95 wt.%, in particular at most 85 wt.%, upon exiting the drying section.65462

[0093] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0094] The use of a dry section with one or more of the features described herein, in particular a dry section according to the invention, in a process for producing kraft paper, preferably in a process with one or more of the features described herein, in particular in a process according to the invention, is also described.

[0095] The invention may relate to a kraft paper web made of kraft paper, in particular sack kraft paper, with an elongation at break determined according to ISO 1924-3, wherein the elongation at break of the kraft paper web in the transverse direction is at least 4% and wherein the kraft paper web has a transverse width of at least 4.0 m.

[0096] It is preferably provided that the elongation at break in % measured in the transverse direction in an edge region of the kraft paper web is greater by a factor of at most 1.3, preferably by a factor of at most 1.2, than the elongation at break in % measured in the transverse direction in a central region of the kraft paper web, wherein the edge region has a normal distance of between 0 cm and 50 cm to an outer edge of the kraft paper web running in the machine direction and wherein the central region has a normal distance to an outer edge (104) of the kraft paper web running in the machine direction which corresponds to ±25 cm half the width of the kraft paper web.

[0097] If necessary, the elongation at break, measured in % at both edge areas of the kraft paper web, is greater by a factor of at most 1.3, preferably by a factor of at most 1.2, than the elongation at break, measured in % in the middle area of ​​the kraft paper web.

[0098] If applicable, the kraft paper is provided for to be formed from kraft pulp containing softwood fibers and, if applicable, hardwood fibers, the softwood fibers optionally being derived from spruce and / or pine wood, which may be 65462

[0099] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0100] have a fiber length of at least 2.1 mm, and wherein the hardwood fibers, if present, may be of birch origin.

[0101] If applicable, the kraft paper is required to contain starch in a proportion of at least 4 kg / t-atro.

[0102] Optionally, the kraft paper may contain a sizing agent, such as at least one alkenyl succinic anhydride and / or at least one alkylated ketene dimer.

[0103] If applicable, the kraft paper is made from kraft pulp that has undergone low-consistency refining and high-consistency refining.

[0104] If applicable, it is provided that the energy input for low-consistency grinding is between 0 kWh / t and 150 kWh / t, and / or that the energy input for high-consistency grinding is between 50 kWh / t and 400 kWh / t.

[0105] If applicable, the kraft paper is made from kraft pulp with a Schopper-Riegler value of 10 °SR to 15 °SR in the unmilled kraft pulp.

[0106] If necessary, the kraft paper web is rolled onto a drum and has a length of at least 10,000 m.

[0107] The invention may also relate to a method for producing kraft paper, in particular a kraft paper web as described herein.

[0108] Preferably, the procedure should include the following steps:

[0109] - Forming a fiber web running along the machine direction by continuously applying a fiber suspension to a screen section through a headbox, 65462

[0110] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0111] - if necessary, shaking the fiber web perpendicular to the machine direction, i.e. in the transverse direction, by a shaking device,

[0112] - Pressing the fiber web, which was previously partially dewatered in the sieving section, in a press section,

[0113] - further drying of the fiber web in a drying section,

[0114] - Winding the kraft paper web formed from the fiber web in a winding station,

[0115] wherein the drying section comprises an impact drying area which has at least one impact drying section, in particular three impact drying sections, wherein each impact drying section is bounded before and after it in the machine direction by a drying cylinder to which a drying screen is assigned which follows the outer circumference of the drying cylinder in a pressure area and in particular presses a fibrous web passing through the drying section against the drying cylinder.

[0116] It is also preferred that the fiber web passes through a free shrinkage section in the impact drying section, which is arranged between the exit of the pressure area arranged upstream in the machine direction and the entrance of the pressure area arranged downstream in the machine direction, wherein the free shrinkage section has a length of at least 3 m.

[0117] If necessary, the fiber web is to be moved at a speed of between 600 m / min and 1,200 m / min.

[0118] If necessary, the fiber suspension is provided to contain Kraft pulp that has been subjected to low-consistency and high-consistency milling.

[0119] If applicable, the energy input for low-consistency grinding is intended to be between 0 kWh / t and 150 kWh / t, and / or the energy input for high-consistency grinding is intended to be between 50 kWh / t and 400 kWh / t.65462

[0120] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0121] Further features of the invention can be seen from the patent claims, the figures and the description of the exemplary embodiment.

[0122] The present invention will now be explained in detail using an exemplary embodiment.

[0123] They show:

[0124] Fig. 1a-c shows a schematic overall view of a kraft paper manufacturing plant according to a first embodiment;

[0125] Fig. 2 shows a schematic detail view of a drying section according to a second embodiment of the invention; and

[0126] Fig. 3 shows a schematic view of a section of a kraft paper web according to an exemplary embodiment.

[0127] Unless otherwise indicated, the following features are shown in the figures: Machine direction 1, screen section 2, headbox 3, shaking device 4, press section 5, drying section 6, winding station 7, dewatering screen 8, pre-drying section 19, post-drying section 20, upsetting device 21, calendering device 22, paper roll 25, contact drying section 70, impact drying section 71, drying cylinder 72, drying screen 73, pressure area 74, guide roller 75, tension roller 76, impact drying device 77, drying hood 78, cylinder roller 79, further drying cylinder 80, fiber web 81, displacement direction 82, impact drying section 83, width 101, edge area 102, center area 103, outer edge 104.

[0128] Figures 1a-c show a schematic overall view of a kraft paper manufacturing plant according to a first embodiment of the invention. The plant comprises, in the machine direction 1, a headbox 3, a wire section 2, a press section 5, a pre-drying section 19, a compression unit 21, a post-drying section 20, a calendering unit 22, and a winding station 7. The calendering unit 22 can alternatively be configured as an embossing unit.

[0129] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0130] The headbox 3 serves, in a known manner, to apply a fiber suspension onto the continuously circulating dewatering screen 8 of the screen section 2, thereby forming a fiber web. As the fiber web progresses through the system, it is continuously dewatered or dried and passes successively through the aforementioned system components. Finally, paper rolls 25 are formed, which can be transported away and / or fed into further processing steps.

[0131] The production speed of the system in this embodiment is approximately 1100 m / min. The resulting paper web has a paper width of approximately 6.4 m.

[0132] The function of the individual plant components is sufficiently known to a person skilled in the field of paper production and will not be explained in detail here. The features of the plant according to the invention relate in particular to the final drying section 20.

[0133] Figure 2 shows an enlarged view of a drying section according to a second embodiment of the invention. The drying section according to this embodiment can be used in the system shown in Figure 1 to produce kraft paper.

[0134] The drying section shown here comprises a contact drying area 70, followed by an impact drying area 71. In Fig. 2, only the last part of the contact drying area 70 is shown.

[0135] The impact drying area 71 comprises three impact drying sections 83, also referred to as impact drying groups, each of which comprises two impact drying devices 77.

[0136] Each impact drying unit 77 in turn comprises a drying hood 78 and a cylindrical roller 79 associated with this drying hood 78. The fiber web 81 passing through an impact drying unit 77 is guided over the cylindrical roller 7965462

[0137] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0138] The air is deflected and blown with hot air through the drying hood 78, thus undergoing drying. Furthermore, the cylindrical roller 79 is heated so that the moisture escaping from the fiber web 81 creates a gas cushion, preventing the fiber web 81 from being in direct contact with the outer surface of the cylindrical roller 79, at least in sections.

[0139] Between the two impact drying units 77, a further drying cylinder 80 is provided, which has a drive but no drying screen. The further drying cylinder 80 is also heated.

[0140] The beginning of each impact drying section 83 is defined by the point at which the fiber web 81 exits the pressure area 74, which, viewed in the machine direction 1, is located directly above or in front of the impact drying section 83.

[0141] The end of each impact drying section 83 is in turn defined by the point at which the fiber web 81 enters the pressure area 74, which, viewed in the machine direction 1, lies directly below or after the impact drying section 83.

[0142] The free shrink length is the length traversed by the fiber web 81 between its exit from the pressure section 74 located upstream in the machine direction 1 and its entry into the pressure section 74 located downstream in the machine direction 1. In other words, the free shrink length is the length along which the fiber web 81 travels in the machine direction 1 between two drying cylinders 72 with drying screens 73. Along the free shrink length, the fiber web has the opportunity to shrink.

[0143] In the present embodiment, the free shrinkage length is approximately 17.5 m. Figures 1b and c.65462 show a complete drying section according to the invention, also used as a post-drying section 20.

[0144] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0145] Here it can be seen that the fiber web is guided directly from the compression device 21 into the post-drying section 20, which begins with a sequence of three drying cylinders 72 with separate drying screens 73. These three drying cylinders 72 form the contact drying area 70 of the post-drying section 20 according to the invention.

[0146] In terms of detailed structure and function, the drying section shown in Fig. 1 is similar to that shown in Fig. 2.

[0147] In one embodiment of a method according to the invention, the fiber web exiting the compression device 21 has a dry content of about 65% and is introduced into a post-drying section 20 designed according to the invention.

[0148] In the impact drying units 77, the cylindrical rollers 79 are heated to a temperature of approximately 110-160°C, so that the fiber web 81 is also heated and the moisture escaping from the fiber web 81 creates a vapor cushion that forms between the fiber web 81 and the outer surface of the cylindrical rollers 79. This allows free shrinkage as the drying of the fiber web 81 progresses.

[0149] The resulting kraft paper exhibits a multitude of advantageous properties. The kraft paper produced according to the described embodiment has the following exemplary property profile: 65462

[0150] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0151] >

[0152]

[0153] *measured in an area at a distance of ±5 cm from the geometric center of the paper web in its transverse direction

[0154] **measured within a range of no more than 10 cm from the edge of the paper web in its transverse direction

[0155] The kraft paper according to this embodiment consists of 100% virgin softwood fibers, which were processed by low- and high-consistency refining. The energy input for low-consistency refining was 90 kWh / t and for high-consistency refining it was 350 kWh / t. The Schopper-Riegler value of the pulp in the unrefined pulp was 12 °SR.

[0156] The kraft paper contains alkenyl succinic anhydride as a sizing agent in a content of approximately < 1 kg / t atro and starch in a content of approximately 6 kg / t atro, each based on the total weight of the pulp contained in the kraft paper.

[0157] Fig. 3 shows a schematic view of a section of a kraft paper web, as described above.65462

[0158] Mondi AG, Marxergasse 4A, 1030 Vienna (AT)

[0159] The kraft paper web has two outer edges 104 running in the machine direction 1. The normal distance between the outer edges 104 corresponds to the width 101 of the paper web. Edge regions 102 run in the area of ​​the outer edges 104, and a central region 103 runs in the center of the paper web.

[0160] The edge areas 102 have a width of approximately 50 cm in the transverse direction of the paper web, starting from the respective outer edge 104 of the paper web.

[0161] The central area 103 runs in a strip approximately 50 cm wide in the center of the paper web, namely at a distance of ± 25 cm from the center of the paper web, wherein the center of the paper web has the same normal distance from both outer edges 104, which corresponds to half the width 101 of the paper web.

[0162] As can be seen from the table above, the center-edge variation of the elongation at break is less than 10%, since the elongation at break (CD) in the center is 1038.0% and the elongation at break (CD) in the edge is 1028.7%.

Claims

65462 Mondi AG, Marxergasse 4A, 1030 Wien (AT) Patent claims 1. Kraft paper web made of kraft paper, in particular sack kraft paper, with an elongation at break determined according to ISO 1924-3, wherein the elongation at break of the kraft paper web in the transverse direction is at least 4% and wherein the kraft paper web has a transverse width (101) of at least 4.0 m, characterized in that the elongation at break in % measured in the transverse direction in an edge region (102) of the kraft paper web is greater by a factor of at most 1.3, preferably by a factor of at most 1.2, than the elongation at break in % measured in the transverse direction in a central region (103) of the kraft paper web, wherein the edge region (102) has a normal distance of between 0 cm and 50 cm to an outer edge (104) of the kraft paper web extending in the machine direction and wherein the central region (103) has a normal distance to an outer edge (104) of the kraft paper web extending in the machine direction which is ±25 cm half the width (101) corresponds to the kraft paper web.

2. Kraft paper web according to claim 1, characterized in that the elongation at break measured in % in both edge regions (102) of the kraft paper web is greater by a factor of at most 1.3, preferably by a factor of at most 1.2, than the elongation at break measured in % in the middle region (103) of the kraft paper web.

3. Kraft paper web according to claim 1 or 2, characterized in that the kraft paper is formed from kraft pulp containing softwood fibers and optionally hardwood fibers, wherein the softwood fibers optionally originate from spruce and / or pine wood, which optionally have a fiber length of at least 2.1 mm, and wherein the hardwood fibers, if present, optionally originate from birch wood.

4. Kraft paper web according to any one of claims 1 to 3, characterized in that the kraft paper contains starch in a proportion of at least 4 kg / t (dry weight). Mondi AG, Marxergasse 4A, 1030 Vienna (AT) 5. Kraft paper web according to one of claims 1 to 4, characterized in that the kraft paper contains a sizing agent, such as at least one alkenyl succinic anhydride and / or at least one alkylated ketene dimer.

6. Kraft paper web according to one of claims 1 to 5, characterized in that the kraft paper is formed from kraft pulp which has been subjected to low-consistency refining and high-consistency refining.

7. Kraft paper web according to claim 6, characterized in that the energy input during low-consistency refining is between 0 kWh / t and 150 kWh / t, and / or that the energy input during high-consistency refining is between 50 kWh / t and 400 kWh / t.

8. Kraft paper web according to one of claims 1 to 7, characterized in that the kraft paper is formed from kraft pulp with a Schopper-Riegler value of 10 °SR to 15 °SR in the unmilled kraft pulp.

9. Kraft paper web according to one of claims 1 to 8, characterized in that it is rolled onto a drum and has a length of at least 10,000 m.

10. Method for producing kraft paper, in particular a kraft paper web according to any one of claims 1 to 9, characterized in that the method comprises the following steps: - Forming a fiber web running along the machine direction (1) by continuously applying a fiber suspension to a screen section (2) through a headbox (3), - if necessary, shaking the fiber web transversely to the machine direction, i.e. in the transverse direction, by a shaking device (4), - Pressing the fiber web, which was previously partially dewatered in the sieve section (2), in a press section (5), - further drying of the fiber web in a drying section (6), Mondi AG, Marxergasse 4A, 1030 Vienna (AT) - Winding the kraft paper web formed from the fiber web in a winding station (7), wherein the drying section (6) comprises an impact drying area (71) which has at least one impact drying section (83), in particular three impact drying sections (83), wherein each impact drying section (83) is bounded before and after it in the machine direction (1) by a drying cylinder (72) to which a drying screen (73) is assigned, which follows the outer circumference of the drying cylinder (72) in a pressure area (74) and in particular presses a fibrous web passing through the drying section against the drying cylinder (72), characterized in that the fiber web passes through a free shrink section in the impact drying section (83), which is arranged between the exit of the pressure area (74) arranged upstream in the machine direction (1) and the entrance of the pressure area (74) arranged downstream in the machine direction (1), wherein the free shrink section has a length of at least 3 m.

11. Method according to claim 10, characterized in that the fibrous web is moved at a speed of between 600 m / min and 1,200 m / min.

12. Method according to claim 10 or 11, characterized in that the fibrous suspension contains Kraft pulp which has been subjected to low-consistency milling and high-consistency milling.

13. Method according to claim 12, characterized in that the energy input during low-consistency grinding is between 0 kWh / t and 150 kWh / t, and / or that the energy input during high-consistency grinding is between 50 kWh / t and 400 kWh / t.

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