Kraftliner paper and method for its production

Abstract

The invention relates to Kraftliner paper consisting of fibrous material and optionally one or more additives, wherein the fibrous material contains eucalyptus wood fiber in an amount of between 80 wt.-% and 100 wt.-%, preferably between 90 wt.-% and 100 wt.-%, based on dry mass of the fibrous material, wherein the average fiber length of the eucalyptus wood fiber is between 0.60 mm and 0.80 mm, in particular between 0.65 mm and 0.75 mm, and wherein the average fiber width of the eucalyptus wood fiber is between 10 μm and 20 μm, in particular in between 10 μm and 18 μm. The invention also relates to a method for the production of Kraftliner paper.

Description

[0001] 64291 / MB

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

[0003] Kraftliner paper and method for its production

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

[0005] Kraftliner paper generally relates to a high-strength paper product that is primarily used in the production of corrugated cardboard, used for the packing of products, such as fruits and vegetables. Kraftliner paper is produced using wood fibers that are processed through the Kraft process, a chemical pulping method that separates lignin from cellulose fibers, thereby producing a durable and resilient paper. The process results in fibrous material, which allows to manufacture a paper product that is highly resistant to mechanical stress, making it ideal for packaging applications where strength and durability are critical. If the fibrous material is present in the form of an aqueous suspension, it is also referred to as pulp. A suspension of fibrous material or fibers made by chemical pulping is therefore referred to as chemical pulp, and a suspension of fibrous material or fibers made by Kraft pulping may be referred to as Kraft pulp.

[0006] Kraftliner paper is typically a two-layer product, but in can also have only one layer or more than two layers, such as three layers. It is characterized among other properties by its high strength and ability to withstand compression, which are essential qualities for maintaining the integrity of packaging during transportation and storage. Additionally, it often possesses superior printability and surface properties, enabling it to serve as a reliable medium for branding and product information on packaging. 64291 / MB

[0007] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0008] White Top Kraftliner (WTKL) is a specific example of Kraftliner paper, which is typically predominantly or entirely made from Kraft pulp. WTKL is typically a multi-layer paper product, which is composed of a cover layer made with bleached fiber and at least a further layer made with unbleached fiber.

[0009] Generally, the Kraft pulping process is applied for hardwood and softwood. Nevertheless, softwood is often preferred for the production of Kraft papers, such as Kraftliner paper, because softwood is generally known to have longer fibers compared to hardwood. According to accepted theory, these longer fibers interlock more effectively, contributing to stronger paper products, which are desirable in packaging materials.

[0010] Kraftliners have therefore traditionally been made with high percentages of softwood, with the final paper typically consisting of around 40-100% softwood and the remainder being composed of, e.g., hardwood fibers, recycled fibers, or the like.

[0011] Recently, hardwood pulp ( / .e., pulp containing short fibers) has been proposed for use in the production of Kraft paper, such as Kraftliner paper. This is particularly due to the increased availability I lower cost of hardwood (depending on geography), as well as the typically higher production efficiency due to, e.g., lower resin content of hardwood as compared to softwood, allowing higher machine speeds and lower COD (chemical oxygen demand) in the wastewaters. However, this transition to hardwood has implied giving up certain strength properties, such as burst resistance.

[0012] In relation to this development, for example, WO 2019 / 126848 A1 and WO 2022 / 160024 A1 disclose Kraftliner papers, which comprise a fibrous composition of 100% short chemical fiber from eucalyptus pulp. Typical Kraftliner papers on the market, which include a lower fraction of eucalyptus fiber, may provide a higher burst resistance index than those containing 100% short chemical fiber from eucalyptus pulp described in the prior art.

[0013] It has now been surprisingly found that improved mechanical properties of Kraftliner paper can be obtained if the fiber contained in the paper consists of a high amount of 64291 / MB

[0014] Mondi AG, Marxergasse 4A, 1030 Wien (AT) hardwood fibers, in particular eucalyptus fibers, in addition with certain conditions being met.

[0015] One possibility to obtain Kraftliner paper with improved properties is the provision of a specific selection of fiber parameters, such as length and width. Another, alternative, possibility is that the hardwood fibers are mixed with a small amount of softwood fibers, such as in an amount of up to 10 wt.-%, such as described below, which surprisingly provides beneficial mechanical characteristics of the Kraftliner paper. It has also been surprisingly found that instead of said softwood fibers, fibers from trees of the genus Acacia can be used in the Kraftliner paper, despite being hardwood fibers.

[0016] The eucalyptus fibers may have a length of between 0.60 mm and 0.80 mm, such as between 0.65 mm and 0.75 mm, or between 0.65 mm and 0.73 mm; and a width of between 10 pm and 20 pm, such as between 10 pm and 18 pm or between 12 pm and 16 pm, in particular between 12.4 pm and 15.8 pm; length and width being determined in an L&W Fiber Tester Code 912 (ABB Asea Brown Boveri Ltd) in particular in accordance with ISO 16065-2:2014. The length and width specifically relate to average length and width, in particular to number average length and width, as determined in accordance with ISO 16065-2:2014. The parameters fiber length and width, as well as optionally fines, may be measured from a suspension of the fibers (pulp) between the refiner and headbox within a papermaking process.

[0017] This way, a Kraftliner paper with a burst index of up to 5.10 kPa.m2 / g or even up to 5.50 kPa.m2 / g can be obtained, exceeding the parameters known in the prior art with high hardwood fiber Kraftliner paper and lying in the range of known conventional Kraftliner paper with an approximately equally mixed softwood I hardwood fiber. An additional advantage of the disclosed Kraftliner paper may be a reduction in the power demand as compared to high-softwood Kraftliner paper, given the fact that hardwood, such as eucalyptus wood, typically presents lower moisture compared with softwood, such that less water is required to be evaporated during the papermaking process.

[0018] The Kraftliner paper produced using such fibers can be a one-ply paper, but preferably it is a two- or three-ply paper. The Kraftliner paper may also be a multi-ply paper. 64291 / MB

[0019] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0020] The eucalyptus fiber may generally comprise or consist of fiber obtained from Eucalyptus spp., in particular one or more of the following: Eucalyptus dunnii, Eucalyptus benthamii, Eucalyptus grandis, Eucalyptus macarthurii, Eucalyptus nitens, Eucalyptus urophylla, Eucalyptus smithii. The eucalyptus fiber may additionally or alternatively comprise or consist of fiber from Eucalyptus hybrids, such as one or more of the following: Eucalyptus grandis x Eucalyptus nitens, Eucalyptus grandis x Eucalyptus urophylla, Eucalyptus urophylla x Eucalyptus grandis.

[0021] Optionally, the eucalyptus fiber may comprise or consist of fiber obtained from one or more of the following: Eucalyptus grandis x Eucalyptus nitens, Eucalyptus grandis x Eucalyptus urophylla, Eucalyptus dunnii. The fiber may essentially consist of fiber obtained from these eucalyptus species and / or hybrids, meaning that a certain amount of fiber obtained from other species and / or hybrids may be contained, such as less than 25 wt.-%, less than 15 wt.-%, or less than 5 wt.-%, based on the dry weight of the fiber.

[0022] The eucalyptus fiber may comprise or consist of fiber from trees grown in temperate and / or subtropical climate. The term “temperate” may contain the following sub-classes: cold temperate cold (<14°C, such as 10-14°C or 12-14°C); cold temperate warm (14- 16°C); warm temperate cool (16-18°C); warm temperate warm (18-20°C). The term “subtropical” may designate a temperature of 20-24°C. Generally, the mentioned temperatures may refer to annual average temperatures. In addition, in climates encompassed by the term “temperate”, the lowest temperature may be -3°C in case of sub-class cold temperate cold, and 0°C in case of any other sub-class mentioned here.

[0023] In a specific embodiment, the eucalyptus fiber may comprise or consist of fiber obtained from one or more of the following:

[0024] - Eucalyptus grandis x Eucalyptus nitens grown in one or more of the following climates: cold temperate cold, cold temperate warm, warm temperate cool

[0025] - Eucalyptus grandis x Eucalyptus urophylla grown in one or more of the following climates: warm temperate warm, subtropical

[0026] - Eucalyptus dunnii grown in one or more of the following climates: cold temperate warm, warm temperate cool, warm temperate warm 64291 / MB

[0027] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0028] The fibrous composition may also contain a certain fraction of other fibers than those derived from eucalyptus. These other fibers may be softwood fibers, such as from pine, fir, and / or spruce.

[0029] The softwood fibers may partially or fully be replaced by fibers from trees of the genus Acacia (acacia fibers). Such fibers may present economic advantages as compared to softwood fibers while maintaining the mechanical strength of the Kraftliner paper. Without wishing to be bound to this theory, the present inventors assume that this may be attributed to the specific length-to-thickness ratio in acacia fibers. In particular, although acacia fibers are short, it presents longer fibers and has thinner cell walls than eucalyptus, which in combination with eucalyptus may provide beneficial mechanical properties, in particular burst strength, of the paper.

[0030] It may generally be envisaged that the content of other fibers (in particular softwood fibers and / or acacia fibers) in the fibrous composition is up to 20 wt.-%, such as up to 10 wt.-%, based on the total dry mass of the fibrous composition in the Kraftliner paper.

[0031] In particular, it is therefore disclosed a Kraftliner paper, which has a fibrous composition that consists of at least 90 wt.-%, such as at least 91 wt.-%, 92 wt.-%, or 93 wt.-%, in particular at least 94 wt.-% or 95 wt.-% fiber derived from hardwood and up to 96 wt.-%, 97 wt.-%, or 98 wt.-%, in particular up to 99 wt.-% fiber derived from hardwood, in particular from eucalyptus wood, with the remainder of the fibrous material in said fibrous composition being derived from softwood and / or acacia.

[0032] Generally, the fibrous composition is derived from pulp that is produced via chemical pulping, such as Kraft pulping. This applies to the eucalyptus wood fiber and / or to other wood fiber disclosed herein. In specific cases, the fibrous composition contains no or only a minor fraction, such as below 2 wt.-%, below 1 wt.-%, or below 0.5 wt.-%, of fiber derived from or processed by other pulping methods, such as mechanical pulping.

[0033] In particular, the term “fibrous material” refers to the complete dry material fraction of the Kraftliner paper that is derived from wood pulp. Specifically, this material fraction 64291 / MB

[0034] Mondi AG, Marxergasse 4A, 1030 Wien (AT) apart from the fibers contains fines up to a certain amount and other constituents derived from the wood material, from which the fibers obtained, such as lignin.

[0035] The Kraftliner paper may also contain one or more additives, which are generally well known in the art. Additives are typically not encompassed by the meaning of the terms “fibrous material” or “fibrous composition”. In other words: The total dry mass of the Kraftliner paper disclosed herein may be the sum of the dry mass of fibrous material and the dry mass of optional additives. The dry mass of the Kraftliner paper may particularly be referred to as grammage, determined in accordance with ISO 536:2019.

[0036] Additives may be bulk additives, which are added to the pulp prior to the actual papermaking process, i.e., prior to forming a sheet-like web from the pulp, or they may be surface additives, which are added to a surface of the sheet-like web comprising the the fibrous material after its formation.

[0037] An exemplary additive is starch, such as bulk starch and / or surface starch.

[0038] The fiber may be bleached fiber, unbleached fiber or a mixture of unbleached fiber.

[0039] As already mentioned, the Kraftliner paper may consist of two or more plies. In this case, two or more plies may have the same or different composition. Except otherwise mentioned, features disclosed herein typically relate to the average composition of the Kraftliner paper, i.e., the average, in particular the mass-based average, over all plies.

[0040] A multi-ply Kraftliner paper may consist at least of a layer of unbleached fiber and a layer of bleached fiber. Therefore, different layers of the Kraftliner paper may be provided, which have different lignin contents, as particularly expressed by the Kappa number determined in accordance with Tappi T236 OS-76.

[0041] In particular, the Kraftliner paper may contain a layer of white Kraft paper (also referred to as “top layer”) and a layer of brown Kraft paper (also referred to as “bottom layer”). 64291 / MB

[0042] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0043] The white Kraft paper may be obtained by using fiber, which has been bleached in a bleaching process using chlorine dioxide and a peroxide, in particular hydrogen peroxide, as bleaching agents.

[0044] In a multi-ply Kraftliner paper, the fibrous composition of the layers may be the same or different. However, it is preferred if the fibrous compositions of different layers are different.

[0045] Specifically, the Kraftliner paper may comprise or consist of two layers, namely a top layer and a bottom layer.

[0046] The top layer may be white Kraft paper and / or the bottom layer may be brown Kraft paper.

[0047] The top layer may consist of fibrous material and optionally one or more additives, wherein the fibrous material contains bleached eucalyptus wood fiber in an amount of between 90 wt.-% and 100 wt.-%, such as between 95 wt.-% and 100 wt.-%, based on dry mass of the fibrous material. The fibrous material of the top layer may be completely or essentially completely derived from unbleached eucalyptus wood fiber. The remainder of dry mass of the fibrous material (if present) in the top layer may be softwood fiber and / or acacia fiber, being bleached or unbleached softwood fiber and / or bleached or unbleached acacia fiber.

[0048] The base layer may consist of fibrous material and optionally one or more additives, wherein the fibrous material contains unbleached eucalyptus wood fiber in an amount of between 60 wt.-% and 100 wt.-%, such as between 70 wt.-% and 98 wt.-%, or between 80 wt.-% and 98 wt.-%, based on dry mass of the fibrous material. Generally, the amount of softwood fiber or acacia fiber in the base layer may be at least 2 wt.-%, such as at least 5 wt.-%, or at least 10 wt.-%. It may be up to 40 wt.-%, such as up to 30 wt.- %, up to 25 wt.-%, up to 10 wt.-% or up to 6 wt.-%. This fraction of fiber may also be a mixture of softwood fiber and acacia fiber. The softwood fiber and / or acacia fiber in the bottom layer may be bleached or unbleached softwood pulp and / or bleached or unbleached acacia fiber. 64291 / MB

[0049] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0050] The grammage of the top layer, in particular determined in accordance with ISO 536:2019, may be between 50 g / m2and 150 g / m2, such as between 70 g / m2and 130 g / m2

[0051] The grammage of the bottom layer, in particular determined in accordance with ISO 536:2019, may be between 30 g / m2and 140 g / m2, such as between 45 g / m2and 120 g / m2.

[0052] The grammage of the top layer, in particular determined in accordance with ISO 536:2019, may be between 40% and 70%, such as between 50% and 65%, of the total grammage of the Kraftliner paper, in particular determined in accordance with ISO 536:2019.

[0053] The grammage of the base layer, in particular determined in accordance with ISO 536:2019, may be between 30% and 60%, such as between 35% and 50%, of the total grammage of the Kraftliner paper, in particular determined in accordance with ISO 536:2019.

[0054] The additives in different layers may be the same or different in amount and / or type.

[0055] A two-ply Kraftliner papers may have a top ply that consists of fibrous material and optionally one or more additives, wherein the fibrous material contains eucalyptus wood fiber in an amount of between 95 wt.-% and 100 wt.-%, such as 100 wt.-%, based on dry mass of the fibrous material, and a bottom ply that consists of fibrous material and optionally one or more additives, wherein the fibrous material contains softwood fiber, acacia fiber, or a mixture thereof, in an amount of between 5 wt.-% and 60 wt.-%, based on dry mass of the fibrous material, the remainder of the fibrous material being eucalyptus wood fiber, wherein the average composition of the fibrous material in top and bottom layer is as described herein.

[0056] The fiber in the bottom ply may be unbleached fiber or at least partially unbleached fiber. 64291 / MB

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

[0058] It has been surprisingly found that the same or similar fibrous material composition of different layers may be provide a higher ply bond of the layers.

[0059] Adding the softwood and / or acacia fiber in the bottom ply may avoid the additional degradation of the long fibers that comes with the bleaching processes. It is therefore preferable that the softwood and / or acacia fiber in the bottom ply is unbleached. In this case, those fibers also conserve more lignin, which is assumed to aid in the fiber bonding, further improving the overall strength of the paper.

[0060] A Kraftliner paper is disclosed consisting of fibrous material and optionally one or more additives, wherein the fibrous material contains eucalyptus wood fiber in an amount of between 80 wt.-% and 100 wt.-%, preferably between 90 wt.-% and 100 wt.-%, based on dry mass of the fibrous material.

[0061] It may be provided that the average fiber length of the eucalyptus wood fiber is between 0.60 mm and 0.80 mm, in particular between 0.65 mm and 0.75 mm, and that the average fiber width of the eucalyptus wood fiber is between 10 pm and 20 pm, in particular in between 10 pm and 18 pm, such as in between 12 pm and 16 pm, wherein the fiber width and the fiber length are determined in accordance with ISO 16065- 2:2014.

[0062] It may be provided that the fibrous material additionally contains softwood fiber, in particular fiber from coniferous trees such as pine, spruce, or fir, and / or acacia fiber, wherein preferably the remainder to 100 wt.-% based on dry mass of the fibrous material is softwood fiber and / or acacia fiber.

[0063] It may be provided that the fibrous material is derived from chemical pulp, in particular Kraft pulp.

[0064] It may be provided that the fibrous material consists of between 90 wt.-% and 98 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber. 64291 / MB

[0065] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0066] It may be provided that the fibrous material consists of between 91 wt.-% and 97 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber.

[0067] It may be provided that the fibrous material consists of between 92 wt.-% and 96 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber.

[0068] It may be provided that the fibrous material consists of between 93 wt.-% or 94 wt.-% and 96 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber.

[0069] It may be provided that the Kraftliner paper consists of two layers, or wherein the Kraftliner paper consists of three layers, and / or that the Kraftliner paper has a grammage of between 115 g / m2and 250 g / m2, as determined in accordance with ISO 536:2019.

[0070] It may be provided that a first layer contains or consists of at least partially bleached fibrous material, wherein a second and an optional third layer contains or consists of unbleached fibrous material.

[0071] It may be provided that the Kraftliner paper has a fines content of between 10 % and 18 %, as determined in accordance with ISO 16065-2:2014, and / or that the Kraftliner paper has a tensile index of between 35 Nm / g and 65 Nm / g, as determined in accordance with ISO 1924-3:2005.

[0072] It may be provided that the Kraftliner paper comprises or consists of two layers, wherein a first layer has a Kappa number of 10 mL KMnO4 / g and 20 mL KMnO4 / g, as determined in accordance with Tappi T236 OS-76.

[0073] It may be provided that a second layer has a Kappa number of 20 mL KMnO4 / g and 80 mL KMnO4 / g, as determined in accordance with Tappi T236 OS-76. 64291 / MB

[0074] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0075] It may be provided that the Kraftliner paper has a tensile stiffness of between 800 kN / m and 1 ,500 kN / m in machine direction (MD), as determined in accordance with ISO 1924-3:2005.

[0076] It may be provided that the Kraftliner paper has a tensile stiffness of between 250 kN / m and 500 kN / m in transverse direction (CD), as determined in accordance with ISO 1924- 3:2005.

[0077] It may be provided that the Kraftliner paper has a whiteness of between 85 % and 99 %, as determined in accordance with ISO 11475:2004.

[0078] It may be provided that the Kraftliner paper has a burst index of at least 4.5 kPa.m2 / g, such as between 4.5 kPa.m2 / g and 5.50 kPa.m2 / g, as determined in accordance with ISO 2758:2014.

[0079] It may be provided that the Kraftliner paper contains surface starch in an amount of up to 20 kg / t, or that the Kraftliner paper contains no surface starch.

[0080] It may be provided that the Kraftliner paper comprises or consists of a top layer and a bottom layer, wherein composition of the fibrous material of the top layer is different from the composition of the fibrous material of the bottom layer; and wherein the fibrous material of the top layer contains bleached eucalyptus wood fiber in an amount of between 90 wt.-% and 100 wt.-%, such as between 95 wt.-% and 100 wt.- %, preferably in an amount of 100 wt.-%, based on dry mass of the fibrous material; and wherein the fibrous material of the bottom layer contains softwood fiber, acacia fiber, or a mixture thereof, in an amount of between 1 wt.-% and 40 wt.-%, preferably in an amount of between 2 wt.-% and 26 wt.-%, based on dry mass of the fibrous material, the remainder optionally being unbleached eucalyptus wood fiber. 64291 / MB

[0081] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0082] Also disclosed is a method for producing a Kraftliner paper, in particular a Kraftliner paper as described herein, in a paper-making machine, the method comprising the following steps:

[0083] - providing of eucalyptus wood fiber, wherein the average fiber length of the eucalyptus wood fiber is between 0.60 mm and 0.80 mm, in particular between 0.65 mm and 0.75 mm, and wherein the average fiber width of the eucalyptus wood fiber is between 10 pm and 20 pm, in particular in between 10 pm and 18 pm, such as in between 12 pm and 16 pm, wherein the fiber width and the fiber length are determined in accordance with ISO 16065-2:2014,

[0084] - forming a first fiber suspension, which contains the eucalyptus wood fiber in an amount of between 80 wt.-% and 100 wt.-%, preferably between 90 wt.-% and 100 wt.- %, based on dry mass of the total fibrous material contained in the first fiber suspension,

[0085] - feeding the first fiber suspension to a forming section of the paper-making machine for partial de-watering of the suspension and for forming a first intermediate sheet material comprising the fibrous material from the first fiber suspension,

[0086] - further de-watering the formed intermediate sheet material in a press section and in a drying section of the paper-making machine to obtain the Kraftliner paper,

[0087] - winding the formed Kraftliner paper onto a reel in a reel section of the paper-making machine.

[0088] It may be provided that the method further comprises the following steps:

[0089] - forming a second fiber suspension, which contains softwood fiber, acacia fiber, or a mixture thereof, in an amount of between 1 wt.-% and 40 wt.%, preferably between 2 wt.-% and 26 wt.-%, based on dry mass of the total fibrous material contained in the first fiber suspension, the remainder optionally being eucalyptus wood fiber as described herein,

[0090] - feeding the second fiber suspension to a forming section of the paper-making machine for partial de-watering of the suspension and for forming an second intermediate sheet material comprising the fibrous material from the second fiber suspension, and 64291 / MB

[0091] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0092] - overlaying the formed first and second intermediate sheets to form a two-ply intermediate sheet material, such that the intermediate sheet material comprises or consists of the first and second intermediate sheet materials.

[0093] It may be provided that the first fiber suspension additionally contains softwood fiber, in particular fiber from coniferous trees such as pine, spruce, or fir, and / or acacia fiber, wherein preferably the remainder to 100 wt.-% based on dry mass of the total fibrous material contained in the first fiber suspension is softwood fiber and / or acacia fiber.

[0094] It may be provided that the softwood fiber in the second fiber suspension is fiber from coniferous trees such as pine, spruce, or fir.

[0095] It may be provided that the sheet material, after having been de-watered in the press section and in the drying section, is impregnated with surface starch.

[0096] It may be provided that at least a part of the fibrous material, in particular the eucalyptus wood fiber used for forming the first fiber suspension, is bleached using chlorine dioxide and hydrogen peroxide as bleaching agents.

[0097] It may be provided that, if present, the eucalyptus wood fiber contained in the second fiber suspension is unbleached.

[0098] Further features are disclosed in the appended claims, as well in the description of the application examples. Below described are application examples of specific embodiments of the Kraftliner paper as disclosed herein, as well as comparative examples.

[0099] In all examples, the properties are determined according to the standards as listed in Table 1 below, except otherwise indicated. 64291 / MB

[0100] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0101] Table 1: Standards used for the determination of parameters.

[0102] Example 1

[0103] In this example, which is according to the present disclosure, a White Top Kraftliner paper comprising two layers has been prepared.

[0104] A first layer, also called top layer, contains a fibrous composition, which has 100 wt.-% eucalyptus wood fiber. A second layer, also called bottom layer, contains a fibrous composition, which has approx. 90 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% (7.e. , approx. 10 wt.-%) being fiber derived from pinewood. The average composition of the fibrous material is approx. 95 wt.-% eucalyptus wood fiber, the remainder being pinewood fiber.

[0105] The top layer consists of bleached fiber, whereas the bottom layer consists of unbleached fiber.

[0106] The fiber is derived from eucalyptus wood of Eucalyptus grandis x Eucalyptus urophylla hybrids grown in warm temperate cool climate conditions in eastern South Africa.

[0107] The properties of the eucalyptus fibers used in both layers in the production of this Kraftliner paper are given in Table 2. 64291 / MB

[0108] Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0109] Table 2: Parameters of fibers according to Example 1.

[0110] Surface starch is applied in the production of the Kraftliner paper to the surface of the top layer in an amount of approx. 15 kg / t dry weight of the paper.

[0111] The results of some properties of the White Top Kraftliner paper according to this exemplary embodiment are listed in Table 3 below.

[0112] Table 3: Properties of the White Top Kraftliner paper according to Example 1.

[0113] Example 2

[0114] In this example, which is according to the present disclosure, a White Top Kraftliner paper comprising two layers has been prepared.

[0115] The White Top Kraftliner paper has essentially been prepared as described in Example 1 , with the only difference that the pinewood fiber has been replaced by fiber derived from trees of the genus Acacia.

[0116] The White Top Kraftliner paper of Example 2 showed similar characteristics as compared to the White Top Kraftliner paper of Example 1 . 64291 / MB Mondi AG, Marxergasse 4A, 1030 Wien (AT)

[0117] Comparative Example 1

[0118] For this comparative example, the information contained in EP 4286585 A1 has been used. Here, a White Top Kraftliner paper comprising three layers is described, in which the layers comprise 100% short chemical eucalyptus fibers.

[0119] The ranges of physical-mechanical properties of short eucalyptus chemical fibers that are acceptable to be used in base and middle layers in the production of this Kraftliner paper are given in Table 6. No standard for the determination of these parameters is mentioned in EP 4286585 A1 .

[0120] Table 6: Acceptable parameters of fibers according to Comparative Example 1.

[0121] The results of properties of White Top Kraftliner paper with 100% short chemical eucalyptus fiber with application of surface starch (30 kg / t paper), taken from Table 4 of EP 4286585 A1 , are listed in Table 7 below.

[0122] Table 7 Properties of the White Top Kraftliner paper according to Comparative Example 1.

[0123] Comparative Example 2

[0124] In this comparative example, a White Top Kraftliner paper comprising two layers has been prepared, which is according to a current product from the applicant’s production 64291 / MB Mondi AG, Marxergasse 4A, 1030 Wien (AT) facility. The top layer contains a fibrous composition, which has 100 wt.-% bleached eucalyptus wood fiber. The bottom layer contains a fibrous composition, which has from 50 wt.-% unbleached eucalyptus wood fiber, the remainder to 100 wt.-% being fiber derived from pinewood. On average, the eucalyptus wood fiber content of this Kraftliner paper is approx. 75 wt.-%.

[0125] Starch is applied in the production of the Kraftliner paper as surface starch in an amount of approx. 15 kg / t dry weight of the paper.

[0126] The results of the most important properties of the White Top Kraftliner paper according to this exemplary embodiment are listed in Table 8 below.

[0127] Table 8: Properties of the White Top Kraftliner paper according to Comparative Example 2.

[0128] Comparison of the results

[0129] The results show that the Kraftliner paper according to the invention provides improved mechanical properties as compared to prior art Kraftliner paper. In particular, the burst index of the inventive Kraftliner paper is significantly higher than the burst index of Kraftliner paper made of 100 wt.-% eucalyptus pulp as well as of Kraftliner paper with a lower amount of eucalyptus pulp.

[0130] This finding is not only surprising, but also poses a significant improvement in the field of Kraftliner paper production, as it allows the production of Kraftliner paper with higher mechanical strength at lower production costs as compared to the prior art.

Claims

64291 / MBMondi AG, Marxergasse 4A, 1030 Wien (AT)Claims1 . A Kraftliner paper consisting of fibrous material and optionally one or more additives, wherein the fibrous material contains eucalyptus wood fiber in an amount of between 80 wt.-% and 100 wt.-%, preferably between 90 wt.-% and 100 wt.-%, based on dry mass of the fibrous material, characterized in that the average fiber length of the eucalyptus wood fiber is between 0.60 mm and 0.80 mm, in particular between 0.65 mm and 0.75 mm, and in that the average fiber width of the eucalyptus wood fiber is between 10 pm and 20 pm, in particular in between 10 pm and 18 pm, such as in between 12 pm and 16 pm, wherein the fiber width and the fiber length are determined in accordance with ISO 16065- 2:2014.

2. The Kraftliner paper according to claim 1 , wherein the fibrous material additionally contains softwood fiber, in particular fiber from coniferous trees such as pine, spruce, or fir, and / or acacia fiber, wherein preferably the remainder to 100 wt.-% based on dry mass of the fibrous material is softwood fiber and / or acacia fiber.

3. The Kraftliner paper according to claim 1 or 2, wherein the fibrous material is derived from chemical pulp, in particular Kraft pulp.

4. The Kraftliner paper according to any of claims 1 to 3, wherein the fibrous material consists of:- between 90 wt.-% and 98 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber, or- between 91 wt.-% and 97 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber, or- between 92 wt.-% and 96 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber, or- between 93 wt.-% or 94 wt.-% and 96 wt.-% eucalyptus wood fiber, the remainder to 100 wt.-% being softwood and / or acacia fiber.64291 / MBMondi AG, Marxergasse 4A, 1030 Wien (AT)5. The Kraftliner paper according to one of claims 1 to 4, wherein the Kraftliner paper consists of two layers, or wherein the Kraftliner paper consists of three layers, and / or wherein the Kraftliner paper has a grammage of between 115 g / m2and 250 g / m2, as determined in accordance with ISO 536:2019.

6. The Kraftliner paper according to claim 5, wherein a first layer contains or consists of at least partially bleached fibrous material, and wherein a second and an optional third layer contains or consists of unbleached fibrous material.

7. The Kraftliner paper according to one of claims 1 to 6, wherein the Kraftliner paper has a fines content of between 10 % and 18 %, as determined in accordance with ISO 16065-2:2014, and / or wherein the Kraftliner paper has a tensile index of between 35 Nm / g and 65 Nm / g, as determined in accordance with ISO 1924- 3:2005.

8. The Kraftliner paper according to one of claims 1 to 7, wherein the Kraftliner paper comprises or consists of two layers, wherein a first layer has a Kappa number of 10 mL KMnO4 / g and 20 mL KMnO4 / g, as determined in accordance with Tappi T236 OS-76, and / or wherein a second layer has a Kappa number of 20 mL KMnO4 / g and 80 mL KMnO4 / g, as determined in accordance with Tappi T236 OS- 76.

9. The Kraftliner paper according to one of claims 1 to 8, wherein the Kraftliner paper has a tensile stiffness of between 800 kN / m and 1 ,500 kN / m in machine direction (MD), as determined in accordance with ISO 1924-3:2005, and / or wherein the Kraftliner paper has a tensile stiffness of between 250 kN / m and 500 kN / m in transverse direction (CD), as determined in accordance with ISO 1924-3:2005.

10. The Kraftliner paper according to one of claims 1 to 9, wherein the Kraftliner paper has a whiteness of between 85 % and 99 %, as determined in accordance with ISO 11475:2004.64291 / MBMondi AG, Marxergasse 4A, 1030 Wien (AT)11 . The Kraftliner paper according to one of claims 1 to 10, wherein the Kraftliner paper has a burst index of at least 4.5 kPa.m2 / g, such as between 4.5 kPa.m2 / g and 5.50 kPa.m2 / g, as determined in accordance with ISO 2758:2014.

12. The Kraftliner paper according to one of claims 1 to 11 , wherein the Kraftliner paper contains surface starch in an amount of up to 20 kg / t, or wherein the Kraftliner paper contains no surface starch.

13. The Kraftliner paper according to one of claims 1 to 12, wherein the Kraftliner paper comprises or consists of a top layer and a bottom layer, wherein composition of the fibrous material of the top layer is different from the composition of the fibrous material of the bottom layer and- wherein the fibrous material of the top layer contains bleached eucalyptus wood fiber in an amount of between 90 wt.-% and 100 wt.-%, such as between 95 wt.-% and 100 wt.-%, preferably in an amount of 100 wt.-%, based on dry mass of the fibrous material, and- wherein the fibrous material of the bottom layer contains softwood fiber, acacia fiber, or a mixture thereof, in an amount of between 1 wt.-% and 40 wt.-%, preferably in an amount of between 2 wt.-% and 26 wt.-%, based on dry mass of the fibrous material, the remainder optionally being unbleached eucalyptus wood fiber.

14. A method for producing a Kraftliner paper, in particular a Kraftliner paper according to one of claims 1 to 13, in a paper-making machine, the method comprising the following steps: a. providing of eucalyptus wood fiber, wherein the average fiber length of the eucalyptus wood fiber is between 0.60 mm and 0.80 mm, in particular between 0.65 mm and 0.75 mm, and wherein the average fiber width of the eucalyptus wood fiber is between 10 pm and 20 pm, in particular in between 10 pm and 18 pm, such as in between 12 pm and 16 pm, wherein the fiber width and the fiber length are determined in accordance with ISO 16065-2:2014,64291 / MBMondi AG, Marxergasse 4A, 1030 Wien (AT) b. forming a first fiber suspension, which contains the eucalyptus wood fiber in an amount of between 80 wt.-% and 100 wt.-%, preferably between 90 wt.-% and 100 wt.-%, based on dry mass of the total fibrous material contained in the first fiber suspension, c. feeding the first fiber suspension to a forming section of the paper-making machine for partial de-watering of the suspension and for forming a first intermediate sheet material comprising the fibrous material from the first fiber suspension, d. further de-watering the formed intermediate sheet material in a press section and in a drying section of the paper-making machine to obtain the Kraftliner paper, e. winding the formed Kraftliner paper onto a reel in a reel section of the paper-making machine.

15. The method according to claim 14, wherein the method further comprises the following steps:- forming a second fiber suspension, which contains softwood fiber, acacia fiber, or a mixture thereof, in an amount of between 1 wt.-% and 40 wt.%, preferably between 2 wt.-% and 26 wt.-%, based on dry mass of the total fibrous material contained in the first fiber suspension, the remainder optionally being eucalyptus wood fiber provided in step (a),- feeding the second fiber suspension to a forming section of the paper-making machine for partial de-watering of the suspension and for forming an second intermediate sheet material comprising the fibrous material from the second fiber suspension, and- prior to step (d), overlaying the formed first and second intermediate sheets to form a two-ply intermediate sheet material, such that the intermediate sheet material processed in step (d) comprises or consists of the first and second intermediate sheet materials.

16. The method according to claim 14 or 15, wherein the first fiber suspension additionally contains softwood fiber, in particular fiber from coniferous trees such as pine, spruce, or fir, and / or acacia fiber, wherein preferably the remainder to 10064291 / MBMondi AG, Marxergasse 4A, 1030 Wien (AT) wt.-% based on dry mass of the total fibrous material contained in the first fiber suspension is softwood fiber and / or acacia fiber, and / or wherein softwood fiber in the second fiber suspension is fiber from coniferous trees such as pine, spruce, or fir.

17. The method according to any of claims 14 to 16, wherein the sheet material, after having been de-watered in the press section and in the drying section, is impregnated with surface starch.

18. The method according to any of claims 14 to 17, wherein at least a part of the fibrous material, in particular the eucalyptus wood fiber used for forming the first fiber suspension, is bleached using chlorine dioxide and hydrogen peroxide as bleaching agents.

19. The method according to any of claims 15 to 18, wherein, if present, the eucalyptus wood fiber contained in the second fiber suspension is unbleached.

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