Multi-layered paperboard

Abstract

There is provided a multi-layered paperboard comprising a print layer and a back layer, wherein the print layer as well as the back layer comprises AKD or ASA, rosin size and alum, the dry weight ratio between the AKD or ASA and the rosin size in the print layer is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer.

Description

MULTI-LAYERED PAPERBOARDTECHNICAL FIELD

[0001] The present disclosure relates to the field of paperboards, and in particular to multi-layered paperboards.BACKGROUND

[0002] Multi-layered paperboards are used in different applications, in particular in packaging applications. The paperboards is made from at least two layers: a print layer and a back layer. The print layer can also be referred to as a top layer. The layers are attached to each other. A multi-layered paperboard can have more than two layers, such as at least three layers. In such case, the paperboard further has one or more middle layers arranged between the print layer and the back layer.

[0003] One type of packaging where multi-layered paperboards are used is in liquid packaging as liquid packaging board (LPB).

[0004] One problem in paperboard is the creation of deposits. Deposits are visual as stains to the user providing a less appealing appearance. Therefore, paperboard having deposits has to be discarded. It is therefore desirable to reduce the number of deposits.SUMMARY

[0005] An objective of the present disclosure is to provide a multi-layered paperboard with reduced visual deposits.

[0006] Accordingly, the present disclosure provides the following list of itemized embodiments:1. A multi-layered paperboard comprising a print layer and a back layer, wherein the print layer as well as the back layer comprises AKD or ASA, rosin size and alum, the dry weight ratio between the AKD or ASA and the rosin size in the print layer is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer.2. The multi-layered paperboard of item 1, wherein the total content of the AKD or ASA and the rosin size in the print layer is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the print layer.3. The multi-layered paperboard of item 1 or 2, further comprising a middle layer arranged between the print layer and the back layer, and wherein the middle layer comprises AKD or ASA and rosin size.4. The multi-layered paperboard of item 3, wherein the dry weight ratio between the AKD or ASA and the rosin size in the middle layer is at least 1:1, such as at least 1.1:1, such as at least 1.2:1.5. The multi-layered paperboard of item 3 or 4, wherein the total content of the AKD or ASA and the rosin size in the middle layer is 1 - 8 kg / tonne, such as 2 - 7 kg / tonne, based on dry weight of fibres in the middle layer.6. The multi-layered paperboard of any one of the preceding items, wherein the dry weight ratio between the AKD or ASA and the rosin size in the print layer is at least 1:1, such as at least 1.1:1, such as at least 1.2:1, such as at least 1.3:1, such as at least 1.4:1, such as at least 1.5:1.7. The multi-layered paperboard of any one of the preceding items, wherein the total content of the AKD or ASA and the rosin size in the back layer is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the back layer.8. The multi-layered paperboard of any one of the preceding items, wherein the dry weight ratio between the AKD or ASA and the rosin size in the back layer is at least 1:1, such as at least 1.1:1.9. The multi-layered paperboard of any one of the preceding items, wherein the amount of rosin size in the print layer is 1 kg / tonne or less, such as 0.8 kg / tonne or less based on dry weight of fibres in the print layer.10. The multi-layered paperboard of any one of the preceding items, wherein the amount of alum in the print layer is 1.8 kg / tonne or less, such as 1.5 kg / tonne or less, based on dry weight of fibres in the print layer.11. The multi-layered paperboard of any one of the preceding items, wherein at least 80 wt% by dry weight of fibres in the print layer are bleached.12. The multi-layered paperboard of any one of the preceding items, wherein the print layer comprises kraft pulp in an amount of at least 80% by dry weight, such as at least 90% by dry weight.13. The multi-layered paperboard of item 12, wherein the kraft pulp is bleached.14- The multi-layered paperboard of any one of the preceding items, wherein the back layer comprises kraft pulp in an amount of at least 80% by dry weight, such as at least 90% by dry weight.15. The multi-layered paperboard of any one of the items 3-5, wherein the middle layer comprises chemithermomechanical pulp (CTMP) and broke pulp.16. The multi-layered paperboard of item 15, wherein the proportion of CTMP in the middle layer is 5-55% by dry weight, such as 10-55% by dry weight, such as 25- 55% by dry weight, such as 30-50% by dry weight.17. The multi-layered paperboard of item 15 or 16, wherein the proportion of broke pulp in the middle layer is 25-55% by dry weight, such as 30-50% by dry weight.18. The multi-layered paperboard of any one of the items 12-17, wherein the proportion of kraft pulp in the middle layer is 0-35% by dry weight, such as 5-30% by dry weight.19. The multi-layered paperboard of any one of the preceding items, wherein the Parker Print Surf (PPS) roughness measured according to ISO 8791-4:2007 of the print layer of the paperboard is below 3.0 pm, such as below 2.0 pm.20. The multi-layered paperboard of any one of the preceding items, wherein the grammage according to ISO 536:2020 of the paperboard is 150-400 g / m2, such as 180-350 g / m2.21. The multi-layered paperboard of any one of the preceding items, wherein the grammage according to ISO 536:2020 of the print layer is 35-90 g / m2, such as 45-80 g / m2.22. The multi-layered paperboard of any one of the preceding items, wherein the grammage according to ISO 536:2020 of the back layer is 30-70 g / m2, such as 40-65 g / m2.23. The multi-layered paperboard of any one of the preceding items, wherein the grammage according to ISO 536:2020 of the middle layer is 70-150 g / m2, such as So- 130 g / m2.24. The multi-layered paperboard of any one of the preceding items, wherein the density according to ISO 534:2011 of the multi-layered paperboard is 640-820 kg / m3, such as 650-800 kg / m3.25. The multi-layered paperboard of any one of the preceding items, wherein the thickness according to ISO 534:2011 of the multi-layered paperboard is 220-600 pm, such as 250-550 pm.26. The multi-layered paperboard of any one of the preceding items, wherein the geometrical short-span compression test (SCT) index according to ISO 9895:2008 of the multi-layered paperboard is at least 22 Nm / g.27. The multi-layered paperboard of any one of the preceding items, wherein the geometric bending resistance index measured according to ISO 2493-2:2020 of the multi-layered paperboard is at least 9 mNm6 / g3, such as at least 10 mNm6 / g3.28. Use of the multi-layered paperboard according to any one of the preceding items in packaging, such as liquid packaging.29. A method of producing a multi-layered paperboard comprising a print layer and a back layer, said method comprising the steps of:- forming a print layer web from a print layer furnish in a forming section of a full-scale paperboard machine, wherein the print layer furnish comprises AKD or ASA, rosin size and alum wherein the dry weight ratio between the AKD or ASA and the rosin size in the print layer furnish is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer furnish;- forming a back layer web from a back layer furnish in a forming section of a full- scale paperboard machine, wherein the back layer furnish comprises AKD or ASA and rosin size; and- forming a multi-layered paperboard from the print layer web and the back layer web.30. The method of item 29, wherein the total content of the AKD or ASA and the rosin size in the print layer furnish is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the print layer furnish.31. The method of item 29 or 30, wherein the multi-layered paperboard comprises the print layer, the back layer and also a middle layer, wherein the method further comprises the steps:- forming a middle layer web from a middle layer furnish comprising AKD or ASA and rosin size in a forming section of a full-scale paperboard machine; and forming the multi-layered paperboard from the print layer web, the middle layer web and the back layer web so that the middle layer web is arranged between the print layer web and the back layer web.32. The method of item 31, wherein the dry weight ratio between the AKD or ASA and the rosin size in the middle layer furnish is at least 1:1, such as at least 1.1:1, such as at least 1.2:1.33. The method of item 31 or 32, wherein the total content of the AKD or ASA and the rosin size in the middle layer furnish is 1 - 8 kg / tonne, such as 2 - 7 kg / tonne, based on dry weight of fibres in the middle layer furnish.34. The method according to any one of the items 29-33, wherein the dry weight ratio between the AKD or ASA and the rosin size in the print layer furnish is at least 1:1, such as at least 1.1:1, such as at least 1.2:1, such as at least 1.3:1, such as at least 1.4:1, such as at least 1.5:1.35. The method according to any one of the items 29-34, wherein the total content of the AKD or ASA and the rosin size in the back layer furnish is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the back layer furnish.36. The method according to any one of the items 29-35, wherein the dry weight ratio between the AKD or ASA and the rosin size in the back layer furnish is at least 1:1, such as at least 1.1:1.37. The method according to any one of the items 29-36, wherein the headbox pH of the print layer furnish is above 7, such as at least 7.2, such as at least 7.3.38. The method according to any one of the items 29-37, wherein the headbox pH of the back layer furnish is above 7, such as at least 7.1.39. The method according to any one of the items 31-38, wherein the headbox pH of the middle layer furnish is above 7, such as at least 7.1.40. The method according to any one of the items 29-39, further comprising the step of coating the print layer with at least one coating layer.41. The method according to any one of the items 29-40, wherein the multi-layered paperboard is calendered.42. The method of item 41, wherein the line load in the calendering is 30-300 kN / m, such as 30-250 kN / m, such as 30-60 kN / m.43. The method according to any one of the items 29-42, wherein the amount of rosin size in the print layer furnish is 1 kg / tonne or less, such as 0.8 kg / tonne or less based on dry weight of fibres in the print layer furnish.44. The method according to any one of the items 29-42, wherein the amount of alum in the print layer furnish is 1.8 kg / tonne or less, such as 1.5 kg / tonne or less, based on dry weight of fibres in the print layer furnish.DETAILED DESCRIPTION

[0007] As a first aspect of the present disclosure there is provided a multi-layered paperboard comprising a print layer and a back layer, wherein the print layer as well as the back layer comprises AKD or ASA, rosin size and alum, the dry weight ratio between the AKD or ASA and the rosin size in the print layer is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer.

[0008] Deposits, or stains, are problematic from a perspective of appearance. The print layer is the outermost layer of the paperboard facing the consumer, and the visual appearance of the print layer is therefore the most important.

[0009] The deposits can be visible in the finished products and / or are problematic for machinery and production equipment. In any case, it is desirable to reduce the amount of deposits. In case the deposit is visible in the paperboard, the paperboard has to be discarded, in particular if it is a liquid packaging board.

[0010] Moreover, it is important that the function of the paperboard is not impaired. In the case of a liquid packaging board, the resistance to liquids is an important property. One of the functions of internal sizing, such as AKD, ASA and rosin size, is to provide resistance to liquids. Hence, it is not evident that an altered balance between those will not impair the resistance to liquids. As is well-known to the skilled person, AKD is alkyl ketene dimer and ASA is alkenyl succinic anhydride.

[0011] The inventors have realized that a multi-layered paperboard having a print layer, which is the layer visible to the users, with a dry weight ratio of AKD or ASA to rosin size being at least 0.8:1 in combination with the amount of alum being below 2.0 kg / tonne based on dry weight of fibres in the print layer, the deposits in thefinished product is advantageously reduced, while the resistance to liquids is maintained.

[0012] The dry weight ratio between the AKD or the ASA and the rosin size may be even higher than 0.8:1 in the print layer, typically it is at least 1:1, such as at least 1.1:1, such as at least 1.2:1, such as at least 1.3:1, such as at least 1.4:1, such as at least 1.5:1.

[0013] Typically, the total content of the AKD or ASA and the rosin size in the print layer is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the print layer.

[0014] Typically, the dry weight ratio between the AKD or ASA and the rosin size in the back layer is at least 1:1, such as at least 1.1:1.

[0015] The total content of the AKD or ASA and the rosin size in the back layer is typically 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the back layer.

[0016] Typically, the amount of rosin size in the print layer is 1 kg / tonne or less, such as 0.8 kg / tonne or less based on dry weight of fibres in the print layer.

[0017] Typically, the amount of alum in the print layer is 1.8 kg / tonne or less, such as 1.5 kg / tonne or less, based on dry weight of fibres in the print layer.

[0018] Typically, at least 80 wt% by dry weight of fibres in the print layer are bleached. Bleached pulp is preferred over unbleached pulp in the print layer as it is beneficial for reduction of visible deposits.

[0019] The print layer typically comprises kraft pulp in an amount of at least 80% by dry weight, such as at least 90% by dry weight. The proportion of kraft pulp in the back layer may be at least 80% by dry weight, such as at least 90% by dry weight. The print layer typically comprises at least 30% by dry weight, such as at least 50% by dry weight softwood kraft pulp. The print layer preferably comprises a mix of hardwood and softwood kraft pulp, such as at least 20%, such as at least 30% by dry weight hardwood kraft pulp and at least 20%, such as at least 30 wt% by dry weight of softwood kraft pulp. Typically, the kraft pulp in the print layer is bleached.

[0020] The multi-layered paperboard may further comprise a middle layer arranged between the print layer and the back layer, and wherein the middle layercomprises AKD or ASA and rosin size. In the middle layer, the dry weight ratio between the AKD or ASA and the rosin size is typically at least 1:1, such as at least 1.1:1, such as at least 1.2:1. The total content of the AKD or ASA and the rosin size in the middle layer is typically 1 - 8 kg / tonne, such as 2 - 7 kg / tonne, based on dry weight of fibres in the middle layer.

[0021] The middle layer typically comprises chemithermomechanical pulp (CTMP) and broke pulp. CTMP is a bulky mechanical pulp. It is beneficial to include CTMP in the middle layer as it decreases the density, i.e. increases the bulk, of the paperboard. A high bulk is preferred as less fibres are used per gram of paperboard. By designing the paperboard in an I-beam structure with chemical pulp in the outer layers and bulky mechanical pulp in the middle layer, strength properties are provided. Within the content of the present disclosure, CTMP also includes high- temperature chemithermomechanical pulp (HT-CTMP). The proportion of CTMP in the middle layer is typically 5-55% by dry weight, such as 10-55% by dry weight, such as 25-55% by dry weight, such as 30-50% by dry weight.

[0022] Broke pulp is formed from trimmings and / or produced, but damaged (or otherwise wasted), board. Typically, the proportion of broke pulp in the middle layer furnish is 25-55% by dry weight, such as 30-50% by dry weight. The broke pulp is typically obtained from the paperboard, which means that it has essentially the same fibre composition as the multi-layered paperboard.

[0023] The middle layer may further include kraft pulp. Typically, the proportion of kraft pulp in the middle layer is 0-35% by dry weight, such as 5-30% by dry weight. Inclusion of kraft pulp in the middle layer is beneficial in terms of strength properties, but it is not essential to include kraft pulp. One reason for including kraft pulp, which is relatively strong type of pulp, is to balance the CTMP, which is a relatively weak type of pulp.

[0024] The grammage of the multi-layered paperboard (including any pigmentbased coating) may for example be 150-400 g / m2, such as 180-350 g / m2. The grammage of the print layer (excluding any pigment-based coating) may be 35-90 g / m2, such as 45-80 g / m2. The grammage of the back layer may be 30-70 g / m2, such as 40-65 g / m2. The grammage of the middle layer may be 70-150 g / m2, such as 80- 130 g / m2. Grammage, also referred to as basis weight, is measured according to ISO 536:2020.

[0025] The density of the multi-layered paperboard may for example be 640-820 kg / m3, such as 650-800 kg / m3. Density is measured according to ISO 534:2011.

[0026] The thickness of the multi-layered paperboard may for example be 220- 600 pm, such as 250-550 pm. Thickness is measured according to ISO 534:2011.

[0027] To separate an individual layer from the multi-layered paperboard and thereby facilitate measurements thereon, a FORTUNA Bandknife-Splitting Machine (Type AB 320 E / P) can be used. Such a machine that has been customized for paperboard splitting is commercially available and is used by several major companies in the paperboard field.

[0028] Alternatively, a surface grinding technique can be used to remove all layers but the layer of interest. Such a surface grinding is one of the services that are commercially available at RISE Bioeconomy (formerly Innventia) in Stockholm, Sweden.

[0029] Surface properties of the multi-layered paperboard includes Bendtsen surface roughness. Bendtsen surface roughness is measured according to ISO 8791- 2:2013. The Bendtsen surface roughness of the print layer is typically below 300 ml / min, such as below 150 ml / min. The Bendtsen surface roughness of the back layer is typically below 2000 ml / min, such as below 1500 ml / min. It is preferred that the surface roughness of the print layer is lower than the surface roughness of the back layer since the print layer is used facing the customer. Another surface property is Parker Print Surf (PPS) roughness (pm) measured according to ISO 8791-4:2007. The PPS roughness of the print layer of the multi-layered paperboard is typically below 3.0 pm, such as below 2.0 pm. To provide such surface properties, the multilayered paperboard has typically been calendered.

[0030] The geometrical short-span compression test (SCT) index according to ISO 9895:2008 of the multi-layered paperboard is typically at least 22 Nm / g. Geometric SCT index is calculated as the square root of the product of the SCT index in machine direction (MD) and cross direction (CD). The SCT index defines the compression strength of the multi-layered paperboard and geometrical SCT-index is a mechanical property that benefit from the usage of maple CTMP as particular source for the CTMP.

[0031] The geometric bending resistance index measured according to ISO 2493- 2:2020 of the multi-layered paperboard is typically at least 9 mNm6 / g3, such as at least 10 mNm6 / g3.

[0032] The multi-layered paperboard is not limited to a certain type of paperboard. It can be a containerboard, such as a folding boxboard (FBB) or a liquid packaging board (LPB). It can also be a containerboard, typically a linerboard. Preferably, the multi-layered paperboard is an LPB.

[0033] As a second aspect of the present disclosure there is provided the use of the multi-layered paperboard according to the first aspect in packaging, such as liquid packaging.

[0034] In liquid packaging, liquid products, such as milk, juice, and other beverages are packaged. Such packaging is sold directly to consumers and a both appearance and function is important. A combination of stain-free appearance and ability to hold liquids is beneficial. Such combinations is provided with the present disclosure.

[0035] The examples and embodiments discussed above in connection to the first aspect applies to the second aspect mutatis mutandis.

[0036] As a third aspect of the present disclosure, there is provided a method of producing a multi-layered paperboard comprising a print layer and a back layer, said method comprising the steps of:- forming a print layer web from a print layer furnish in a forming section of a full-scale paperboard machine, wherein the print layer furnish comprises AKD or ASA, rosin size and alum wherein the dry weight ratio between the AKD or ASA and the rosin size in the print layer furnish is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer furnish;- forming a back layer web from a back layer furnish in a forming section of a full- scale paperboard machine, wherein the back layer furnish comprises AKD or ASA and rosin size; and- forming a multi-layered paperboard from the print layer web and the back layer web.

[0037] Typically, the dry weight ratio between the AKD or ASA and the rosin size in the print layer furnish is at least 1.1:1, such as at least 1.2:1, such as at least 1.3:1, such as at least 1.4:1, such as at least 1.5:1.

[0038] Typically, the total content of the AKD or ASA and the rosin size in the print layer furnish is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the print layer furnish.

[0039] Typically, wherein the amount of rosin size in the print layer furnish is 1 kg / tonne or less, such as 0.8 kg / tonne or less based on dry weight of fibres in the print layer furnish.

[0040] Typically, the amount of alum in the print layer furnish is 1.8 kg / tonne or less, such as 1.5 kg / tonne or less, based on dry weight of fibres in the print layer furnish.

[0041] Typically, the dry weight ratio between the AKD or ASA and the rosin size in the back layer furnish is at least 1:1, such as at least 1.1:1.

[0042] Typically, the total content of the AKD or ASA and the rosin size in the back layer furnish is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the back layer furnish.

[0043] The multi-layered paperboard may further comprise a middle layer. In such case, the method further comprises the steps:- forming a middle layer web from a middle layer furnish comprising AKD or ASA and rosin size in a forming section of a full-scale paperboard machine; and- forming the multi-layered paperboard from the print layer web, the middle layer web and the back layer web so that the middle layer web is arranged between the print layer web and the back layer web.

[0044] The dry weight ratio between the AKD or ASA and the rosin size in the middle layer furnish is typically at least 1:1, such as at least 1.1:1, such as at least 1.2:1.

[0045] The total content of the AKD or ASA and the rosin size in the middle layer furnish is typically 1 - 8 kg / tonne, such as 2 - 7 kg / tonne, based on dry weight of fibres in the middle layer furnish.

[0046] The headbox pH of the print layer furnish is typically above 7, such as at least 7.2, such as at least 7.3. Such pH is advantageous for reduction of deposits. Theheadbox pH of the back layer furnish is typically above 7, such as at least 7.1. The headbox pH of the middle layer furnish is typically above 7, such as at least 7.1.

[0047] The furnishes typically contains further additives in addition to the sizing agents, including retention aid(s), alum and starch.

[0048] The method may comprise the step of coating the print layer with at least one coating layer. The at least one coating layer is typically pigment-based. The total coat weight (dry) may be 12-30 g / m2. The print layer may be coated with more than one coating, such as a dual-layer coating or triple-layer coating. Suitable pigments are clay and / or calcium carbonate and / or talc. In case of a dual-layer coating, drying between the application of a first coating layer and the application of a second coating layer can be conducted. For the single layer coating, drying is conducted after coating. Drying is typically performed with non-contact drying, such as IR and / or hot air, or contact drying, such as with drying cylinder(s), or a combination of non-contact and contact drying. The present disclosure is not limited to any particular coating technique and several different types of techniques, can thus be used. The techniques include: blade coating, rod coating, air-knife coating, rotogravure coating and / or curtain coating. All coatings may be applied in-line (also referred to as on-line). In such case, the productivity is increased by eliminating the handling operations linked to off-line treatment and by eliminating, or at least reducing, the amount of waste. Alternatively, the coatings may also be applied off-line.

[0049] Typically, the multi-layered paperboard is calendered. In such case, the line load in the calendering is typically 30-300 kN / m, such as 30-250 kN / m, such as 30-60 kN / m. Calendering can be conducted before and / or after coating, if the paperboard is coated.

[0050] Typically, the print layer furnish and back layer furnish, and optional middle layer furnish, are each provided from separate headboxes on separate forming wires and thereafter combined, for example by couching and / or in the press section of the board machine. Alternatively, all furnishes are provided from a multi-layer headbox directly onto one forming wire.

[0051] In case of a middle layer being formed, the print layer web may be merged with the middle layer web by couching. The back layer web may also be merged with the middle layer web by couching. This is typically done at a couching dryness above10%, such as above 13% of any one of the webs. Couching at such dryness is beneficial in respect to further reduce the strength of the interface between the print layer and the middle layer, and if also used to reduce the strength of the interface between the back layer and the middle layer. Thereby delamination in the middle layer during folding is reduced. Here, couching dryness refers to the dryness of the print layer web at the point of couching. In one embodiment, the print layer web and the middle layer web both have a dryness above 10%, such as above 13%, at the point of couching.

[0052] In case of a two-layered board the print layer web may be merged with the back layer web by couching. This is typically done at a couching dryness above 10%, such as above 13% of any one of the webs. In one embodiment, the print layer web and the back layer web both have a dryness above 10%, such as above 13%, at the point of couching.

[0053] The examples and embodiments discussed above in connection to the first and second aspect applies to the third aspect mutatis mutandis.EXAMPLESManufacturing of 2-layered paperboards

[0054] In a full-scale paperboard machine, 2-layered paperboards having a print layer and a back layer were produced from a print layer furnish and a back layer furnish.

[0055] The pulp composition in the print layer furnish was a mix of bleached softwood kraft pulp (SW kraft) and bleached hardwood kraft pulp (HW kraft). The pulp composition in the back layer furnish was 100 wt% unbleached SW kraft.

[0056] AKD, rosin size and alum were added to both furnishes. During three different time periods lasting from one year to about 8 months each different amounts were added. The addition levels as well as the pH of the top layer furnish in the headbox is presented in Table 1 below.Table 1. Summary of dosages of AKD, rosin size and alum into the top layer furnish as well as the pH of the furnishes in the headbox and dry weight ratios of the additions of AKD and rosin size of the 2-layered paperboards.Manufacturing of 3-layered paperboards

[0057] In a full-scale paperboard machine, 3-layered paperboards having a print layer, a middle layer and a back layer were produced from a print layer furnish, a middle layer furnish and a back layer furnish.

[0058] The pulp composition in the print layer furnish was a mix of 70 wt% bleached softwood kraft pulp (SW kraft) and 30 wt% bleached hardwood kraft pulp (HW kraft). The pulp composition in the back layer furnish was 100 wt% SW kraft. The pulp composition in the middle layer furnish was 35wt% SW kraft, 35wt% CTMP and 30wt% broke pulp.

[0059] AKD, rosin size and alum were added to all three furnishes. During four different time periods lasting about 6 months each different amounts were added. The addition levels as well as the pH of respective furnish in the headbox is presented in Table 2 below.Table 2. Summary of dosages of AKD, rosin size and alum into the furnishes as well as the pH of the furnishes in the headbox and dry weight ratios of the additions of AKD and rosin size of the 3-layered paperboards.

[0060] The print layer was coated with a pigment -based coating with a grammage of about 20 g / m2and the back layer with a pigment-based coating with a grammage of about 6 g / m2. Prior to coating the uncoated paperboards were calendered using a line load of 40-60 kN / m.Evaluation of the multi-layered paperboards

[0061] For each of the boards, the average number of deposits were evaluated. The average number of deposits are those visible from the print layer side of the paperboard and are counted as number of deposits per 10000 / m2. The counting was conducted using an automated vision system from procemex operating on-line in the paperboard machine.

[0062] Moreover, edge-wick test was conducted to measure the board's resistance to liquid absorption along its edges. This test evaluates how well the paperboard prevents moisture from seeping into the edges, which is crucial for maintaining the integrity of liquid packaging. During the test, five pre-conditioned samples each having dimensions of 25x75 mm of each board was submerged in diluted lactic acid (1% lactic acid) for 1 hour. Edgewick is thereafter calculated according to:Edgewick= (Original weight - weight after submergion) / (thickness in |am x total edgelength for the samples).

[0063] Low absorption indicates better moisture resistance, which is important for ensuring that the packaging remains sturdy and leak-proof in real-world conditions. Moreover, the lactic acid simulates conditions that the paperboard might encounter when exposed to acidic foods or beverages. This test, thereby, also assesses how well the material can withstand exposure to weak acids without degrading, swelling, or losing strength.

[0064] In Table 3 below the results from the evaluation of the 2-layered paperboard is presented.Table 3. Results from evaluation of number of deposits and edgewick testing of the 2- layered paperboard

[0065] As seen in Table 3, when the settings were changed from the reference to the inventive examples for the 2-layered paperboards, the average number of deposits were reduced by at least 50% and yet the edgewick was maintained.

[0066] In Table 4 below the results from the evaluation of the 3-layered paperboard is presented.Table 4. Results from evaluation of number of deposits and edgewick testing of the 3- layered paperboard.

[0067] As seen in Table 4, when the settings were changed from the reference to the inventive examples for the 3-layered paperboards, the average number of deposits were reduced by at least 45% and yet the edgewick was maintained.

[0068] Thereby, the number of deposits can be drastically reduced without compromising the function of the paperboard by producing paperboards according to the present disclosure.

Claims

CLAIMS1. A multi-layered paperboard comprising a print layer and a back layer, wherein the print layer as well as the back layer comprises AKD or ASA, rosin size and alum, the dry weight ratio between the AKD or ASA and the rosin size in the print layer is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer.

2. The multi-layered paperboard of claim 1, wherein the total content of the AKD or ASA and the rosin size in the print layer is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the print layer.

3. The multi-layered paperboard of claim 1 or 2, further comprising a middle layer arranged between the print layer and the back layer, and wherein the middle layer comprises AKD or ASA and rosin size.

4. The multi-layered paperboard of claim 3, wherein the dry weight ratio between the AKD or ASA and the rosin size in the middle layer is at least 1:1, such as at least 1.1:1, such as at least 1.2:1.

5. The multi-layered paperboard of claim 3 or 4, wherein the total content of the AKD or ASA and the rosin size in the middle layer is 1 - 8 kg / tonne, such as 2 - 7 kg / tonne, based on dry weight of fibres in the middle layer.

6. The multi-layered paperboard of any one of the preceding claims, wherein the dry weight ratio between the AKD or ASA and the rosin size in the print layer is at least 1:1, such as at least 1.1:1, such as at least 1.2:1, such as at least 1.3:1, such as at least 1.4:1, such as at least 1.5:1.

7. The multi-layered paperboard of any one of the preceding claims, wherein the total content of the AKD or ASA and the rosin size in the back layer is 0.5 - 5 kg / tonne, such as 1 - 4 kg / tonne, based on dry weight of fibres in the back layer.

8. The multi-layered paperboard of any one of the preceding claims, wherein the dry weight ratio between the AKD or ASA and the rosin size in the back layer is at least 1:1, such as at least 1.1:1.

9. The multi-layered paperboard of any one of the preceding claims, wherein the amount of rosin size in the print layer is 1 kg / tonne or less, such as 0.8 kg / tonne or less based on dry weight of fibres in the print layer.

10. The multi-layered paperboard of any one of the preceding claims, wherein the amount of alum in the print layer is 1.8 kg / tonne or less, such as 1.5 kg / tonne or less, based on dry weight of fibres in the print layer.

11. The multi-layered paperboard of any one of the preceding claims, wherein at least 80 wt% by dry weight of fibres in the print layer are bleached.

12. Use of the multi-layered paperboard according to any one of the preceding claims in packaging, such as liquid packaging.

13. A method of producing a multi-layered paperboard comprising a print layer and a back layer, said method comprising the steps of:- forming a print layer web from a print layer furnish in a forming section of a full-scale paperboard machine, wherein the print layer furnish comprises AKD or ASA, rosin size and alum wherein the dry weight ratio between the AKD or ASA and the rosin size in the print layer furnish is at least 0.8:1 and the amount of alum is below 2.0 kg / tonne based on dry weight of fibres in the print layer furnish;- forming a back layer web from a back layer furnish in a forming section of a full- scale paperboard machine, wherein the back layer furnish comprises AKD or ASA and rosin size; and- forming a multi-layered paperboard from the print layer web and the back layer web.

14. The method of claim 12, wherein the multi-layered paperboard comprises the print layer, the back layer and also a middle layer, wherein the method further comprises the steps:- forming a middle layer web from a middle layer furnish comprising AKD or ASA and rosin size in a forming section of a full-scale paperboard machine; and forming the multi-layered paperboard from the print layer web, the middle layer web and the back layer web so that the middle layer web is arranged between the print layer web and the back layer web.

15. The method according to claim 12 or 13, wherein the headbox pH of the print layer furnish is above 7, such as at least 7.2, such as at least 7.3.

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