Kraft top liner and method

Abstract

The invention relates to a Kraft top liner having a top ply and a bottom ply, wherein the grammage of the bottom ply is between 60% and 80%, in particular between 65% and 75%, of the total grammage of the Kraft top liner, and wherein the total grammage of the Kraft top liner is at least 220 g / m2, in particular from 220 g / m2 to 340 g / m2 The invention also relates to a method and a Kraft top liner produced by this method.

Description

[0001] 64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) Kraft top liner and method

[0002] The present invention pertains to a Kraft top liner, a method for the production of a Kraft top liner, and a Kraft top liner obtainable by this method, as defined in the independent claims. Generally, the invention relates to certain advancements in the field of containerboard materials, with a particular focus on Kraft top liners.

[0003] Kraft top liners are paper products, which are used as components in the production of corrugated fiberboard as the outer facings that provide both structural integrity and aesthetic appeal to packaging solutions.

[0004] According to the Confederation of European Paper Industries (CEP I; European list of corrugated base papers, 6thEd., Updated in Oct. 2022), a Kraft top liner is defined as a recycled fiber-based paper product with a top layer predominantly made from virgin wood pulp and an additional bottom layer essentially made of recycled fibers. While the top layer enhances the liner's strength properties and provides a superior printing surface, thereby improving the overall performance and appearance, the bottom layer imparts cost-effectiveness and environmental benefits to the product.

[0005] Following the generally applicable CEPI definition mentioned above, a Kraft top liner is additionally characterized by some property requirements, including a sized grade, which when measured by the 1 -minute COBB test, has values typically in the range from 25 g / m2to 45 g / m2. In addition, a Kraft top liner has to fulfill certain mechanical64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) requirements. Those are a minimum burst index or a minimum SCT-CD index; namely a burst index of at least 2.8 when measured in accordance with ISO 2758; a burst index of at least 2.9 when measured in accordance with ISO 2759; or a SCT-CD (short-span compressive test in cross direction) index of at least 14.5 measured in accordance with ISO 9895. These specifications ensure that the Kraft top liner possesses the necessary mechanical properties to perform effectively in packaging applications.

[0006] Typical Kraft top liners available on the market have a grammage of up to 200 g / m2or only slightly above, which in the past has been considered sufficient in relation to mechanical stability of the final product, while posing no major problems for producibil ity of such materials.

[0007] The present inventors have now found that Kraft top liners with higher grammages can unexpectedly elevate mechanical stability and versatility of this type of paper product, combined with serving a still un-met market need.

[0008] The present invention therefore particularly relates to a Kraft top liner as such, which has a grammage of at least 220 g / m2, such as at least 250 g / m2, in particular in combination with a brown top layer. The grammage is particularly determined in accordance with ISO 536:2019, when described herein.

[0009] In a related aspect of the invention, it has been found that such Kraft top liner may be obtained in an efficient and reproducible manner, if the bottom ply of the Kraft top liner, which typically makes up about 70% of the total grammage of the Kraft top liner, is produced in a paper machine with a wire section, which is particularly equipped with a breast roll shaker.

[0010] A breast roll shaker generally describes a device that brings the breast roll of the wire section of a paper machine into a controlled oscillating movement in the cross direction with a specified frequency and amplitude.

[0011] Without wishing to be bound to this theory, the present inventors assume that the breast roll shaker, which is known to allow an improved distribution of the fibers upon applying64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) the pulp onto the wire section, also aids in the de-watering process of the pulp. This way, the high-grammage web for the bottom ply can be de-watered efficiently before proceeding to further sections of the paper machine, thereby improving processability.

[0012] As the use of a breast roll shaker may also impart the paper product with unique properties, the invention furthermore relates to a Kraft top liner, which is obtainable or obtained by such method.

[0013] In a preferred instance, the Kraft top liner described herein is a Kraft top liner in accordance with the above-mentioned CEPI standards.

[0014] It has at least two or, preferably, exactly two plies, which plies are made by de-watering suspensions consisting of water, a fibrous composition, and optional additives, which are particularly non-fibrous. The fibrous composition typically contains or consists of pulp, namely virgin wood pulp in case of the Kraft top liner’s top ply, and recycled pulp I fiber in case of the Kraft top liner’s bottom ply.

[0015] As mentioned above, the Kraft top liner particularly is a brown Kraft top liner, which can be characterized by the fact that the Kappa number of the fibrous composition, which is used to prepare the top ply, is at least 70 as determined in accordance with ISO 302:2015. The Kappa number is a commonly used measure for the residual lignin content in pulp, apart from the visual appearance of the product affecting the mechanical properties of the material.

[0016] In an optional variant of the invention, at least part of the fibrous composition, which is used to prepare the top ply, is never-dried pulp. Generally, in the context of the present invention, the term never-dried pulp relates to pulp, which has never been subjected to a drying process after pulping, such as a drying process that resulted in the pulp having a water content of less than 30 wt.-% in relation to the total dry mass of the solid material. It is assumed that never-dried pulp may aid in imparting beneficial mechanical properties to the Kraft top liner.64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) The fibrous composition used for the preparation of the bottom layer may for example be recycled Kraft fiber or recycled semichemical fiber. In a specific embodiment, said fibrous composition is OCC (old corrugated cardboard) fiber, which is particularly obtained from material as defined in EN 643:2014 class 1.05 including all sub-classes.

[0017] A specific characteristic of the Kraft top liner as described herein may be that the grammage of the top ply is lower than the grammage of the bottom ply. In a typical, but not necessarily limiting case, the grammage of the top ply is about 30% of the total grammage, while the grammage of the bottom ply is about 70% of the total grammage.

[0018] The invention particularly relates to a Kraft top liner having a top ply and a bottom ply,

[0019] - wherein the top ply is made from a suspension consisting of: water; a first fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 90 wt.-%, such as at least 95 wt.-%, of the dry weight of the first fibrous composition is virgin wood pulp, wherein the Kappa number of the first fibrous composition is at least 70 determined accordance with ISO 302:2015,

[0020] - wherein the bottom ply is made from a suspension consisting of: water; a second fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 80 wt.-%, such as at least 90 wt.-%, of the dry weight of the second fibrous composition is recycled fiber, in particular recycled Kraft fiber and / or recycled semichemical fiber,

[0021] - wherein the grammage of the bottom ply is between 60% and 80%, in particular between 65% and 75%, of the total grammage of the Kraft top liner.

[0022] It is preferably provided that the total grammage of the Kraft top liner is at least 220 g / m2, in particular from 220 g / m2to 340 g / m2, determined in accordance with ISO 536:2019.

[0023] It may be provided that the virgin wood pulp in the first fibrous composition is never-dried pulp.64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) It may be provided that the virgin wood pulp in the first fibrous composition is pulp derived from softwood, optionally with an addition of pulp derived from hardwood, such as in an amount of up to 40 wt.-%, particularly up to 20 wt.-%, in relation to the dry weight of the first fibrous composition.

[0024] It may be provided that the total grammage of the Kraft top liner is from 250 g / m2to 330 g / m2determined in accordance with ISO 536:2019.

[0025] It may be provided that the total grammage of the Kraft top liner is from 230 g / m2to 300 g / m2, such as from 250 g / m2to 280 g / m2, determined in accordance with ISO 536:2012.

[0026] It may be provided that the recycled fiber in the second fibrous composition comprises or consists of OCC fiber.

[0027] It may be provided that the bottom ply is produced in a paper machine using a wire section with a breast roll shaker.

[0028] It may be provided that the top ply and / or the bottom ply contain starch, in particular in the form of surface starch and / or bulk starch.

[0029] It may be provided that the Kappa number of the first fibrous composition is at least 75, such as at least 80, in particular at least 85, determined accordance with ISO 302:2015.

[0030] The invention particularly also relates to a process for producing a Kraft top liner, comprising the following steps:

[0031] - producing a first web for the provision of a top ply of the Kraft top liner from a suspension consisting of: water; a first fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 90 wt.-%, such as at least 95 wt.-%, of the dry weight of the first fibrous composition is virgin wood pulp, wherein the Kappa number of the first fibrous composition is at least 70 determined accordance with ISO 302:2015,64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) - producing a second web for the provision of a bottom ply of the Kraft top liner from a suspension consisting of: water; a second fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 80 wt.-%, such as at least 90 wt.-%, of the dry weight of the second fibrous composition is recycled fiber, in particular recycled Kraft fiber and / or recycled semichemical fiber,

[0032] -joining the first web and the second web for forming a two-ply web, and drying the two-ply web to obtain the Kraft top liner,

[0033] - wherein the grammage of the bottom ply is between 60% and 80%, in particular between 65% and 75%, of the total grammage of the Kraft top liner.

[0034] Preferably, it is provided

[0035] - that the second web is produced by applying the suspension containing the second fibrous composition to a wire of the wire section of a paper machine, which wire particularly has a breast roll shaker,

[0036] and

[0037] - that the total grammage of the Kraft top liner is at least 220 g / m2, in particular from 220 g / m2to 340 g / m2, determined in accordance with ISO 536:2012.

[0038] It may be provided that the breast roll shaker operates at a frequency of from 4,0 Hz to 9,0 Hz.

[0039] It may be provided that the breast roll shaker operates at an amplitude of from 4,0 mm to 32 mm, preferably from 4,0 mm to 11 mm.

[0040] The invention particularly also relates to a Kraft top liner obtainable via a process as described herein.64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) Further features of the invention can be derived from the claims and from the exemplary embodiment, which will be described hereinafter. This exemplary embodiment only serves illustrative purposes, without aiming to limit the scope of the invention as defined by the claims.

[0041] In this exemplary embodiment, a brown Kraft top liner has been produced in a paper machine with two parallel-running wires.

[0042] For the preparation of the top ply, a suspension has been prepared by mixing water, virgin never-dried softwood pulp with a Kappa value of approx. 73, sizing emulsion, Aluminum sulfate, silica sols, bentonite, and descaler.

[0043] For the preparation of the bottom ply, a suspension has been prepared by mixing water, OCC fiber, sizing emulsion, Aluminum sulfate, glyoxalated polyacrylamides, silica sols, and bentonite.

[0044] The suspension for the top ply was applied to the first wire and partially de-watered to form a first web, while the suspension for the bottom ply was applied to the second wire and partially de-watered to form a second web.

[0045] The second wire was equipped with a breast roll shaker, so to oscillate the breast roll in cross direction with a predetermined frequency and amplitude. In this exemplary embodiment, the breast roll shaker was operated at a frequency of approx. 8,0 Hz and an amplitude of approx. 6 mm.

[0046] After forming the two webs in the wire section, they were merged and subjected to further processing to yield a Kraft top liner with a grammage of approx. 275 g / m2

[0047] The two suspensions were applied to the respective wire section in such amounts to yield a Kraft top liner with the top ply constituting approx. 30% of the total grammage of the liner and the bottom ply constituting approx. 70% of the total grammage of the liner.

Claims

64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) Claims1. A Kraft top liner having a top ply and a bottom ply,- wherein the top ply is made from a suspension consisting of: water; a first fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 90 wt.- %, such as at least 95 wt.-%, of the dry weight of the first fibrous composition is virgin wood pulp, wherein the Kappa number of the first fibrous composition is at least 70 determined accordance with ISO 302:2015,- wherein the bottom ply is made from a suspension consisting of: water; a second fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 80 wt.-%, such as at least 90 wt.-%, of the dry weight of the second fibrous composition is recycled fiber, in particular recycled Kraft fiber and / or recycled semichemical fiber,- wherein the grammage of the bottom ply is between 60% and 80%, in particular between 65% and 75%, of the total grammage of the Kraft top liner, characterized in that the total grammage of the Kraft top liner is at least 220 g / m2, in particular from 220 g / m2to 340 g / m2, determined in accordance with ISO 536:2019.

2. The Kraft top liner according to claim 1 , wherein the virgin wood pulp in the first fibrous composition is never-dried pulp.

3. The Kraft top liner according to claim 1 or 2, wherein the virgin wood pulp in the first fibrous composition is pulp derived from softwood, optionally with an addition of pulp derived from hardwood, such as in an amount of up to 40 wt.-%, particularly up to 20 wt.-%, in relation to the dry weight of the first fibrous composition.

4. The Kraft top liner according to any of claims 1 to 3, wherein the total grammage of the Kraft top liner is from 250 g / m2to 330 g / m2determined in accordance with ISO 536:2019.64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) 5. The Kraft top liner according to any of claims 1 to 3, wherein the total grammage of the Kraft top liner is from 230 g / m2to 300 g / m2, such as from 250 g / m2to 280 g / m2, determined in accordance with ISO 536:2012.

6. The Kraft top liner according to any of claims 1 to 4, wherein the recycled fiber in the second fibrous composition comprises or consists of OCC fiber.

7. The Kraft top liner according to any of claims 1 to 6, wherein the bottom ply is produced in a paper machine using a wire section with a breast roll shaker.

8. The Kraft top liner according to any of claims 1 to 7, wherein the top ply and / or the bottom ply contain starch, in particular in the form of surface starch and / or bulk starch.

9. The Kraft top liner according to any of claims 1 to 7, wherein the Kappa number of the first fibrous composition is at least 75, such as at least 80, in particular at least 85, determined accordance with ISO 302:2015.

10. A process for producing a Kraft top liner, comprising the following steps:- producing a first web for the provision of a top ply of the Kraft top liner from a suspension consisting of: water; a first fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 90 wt.-%, such as at least 95 wt.-%, of the dry weight of the first fibrous composition is virgin wood pulp, wherein the Kappa number of the first fibrous composition is at least 70 determined accordance with ISO 302:2015,- producing a second web for the provision of a bottom ply of the Kraft top liner from a suspension consisting of: water; a second fibrous composition; optionally, particularly non-fibrous, additives; wherein at least 80 wt.-%, such as at least 90 wt.-%, of the dry weight of the second fibrous composition is recycled fiber, in particular recycled Kraft fiber and / or recycled semichemical fiber,-joining the first web and the second web for forming a two-ply web, and drying the two-ply web to obtain the Kraft top liner,- wherein the grammage of the bottom ply is between 60% and 80%, in particular between 65% and 75%, of the total grammage of the Kraft top liner,64578 / MB / - Mondi AG, Marxergasse 4A, 1030 Wien (AT) characterized in- that the second web is produced by applying the suspension containing the second fibrous composition to a wire of the wire section of a paper machine, which wire particularly has a breast roll shaker, and- in that the total grammage of the Kraft top liner is at least 220 g / m2, in particular from 220 g / m2to 340 g / m2, determined in accordance with ISO 536:2012.

11. The Kraft top liner according to claim 10, wherein the breast roll shaker operates at a frequency of from 4,0 Hz to 9,0 Hz, and / or wherein the breast roll shaker operates at an amplitude of from 4,0 mm to 32 mm, preferably from 4,0 mm to 11 mm.

12. A Kraft top liner obtainable via a process according to claim 10 or 11.